Geology and Geochemistry of Molybdenum Deposits in the Qinling Orogen, P R China 9811648697, 9789811648694

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Modern Approaches in Solid Earth Sciences

YanJing Chen · Franco Pirajno · Nuo Li · XiaoHua Deng · YongFei Yang   Editors

Geology and Geochemistry of Molybdenum Deposits in the Qinling Orogen, P R China

Modern Approaches in Solid Earth Sciences Volume 22

Series Editors Yildirim Dilek, Department of Geology and Environmental Earth Sciences, Miami University, Oxford, OH, USA Franco Pirajno, The University of Western Australia, Perth, WA, Australia Brian Windley, Department of Geology, The University of Leicester, Leicester, UK

Background and motivation Earth Sciences are going through an interesting phase as the traditional disciplinary boundaries are collapsing. Disciplines or sub-disciplines that have been traditionally separated in the past have started interacting more closely, and some new fields have emerged at their interfaces. Disciplinary boundaries between geology, geophysics and geochemistry have become more transparent during the last ten years. Geodesy has developed close interactions with geophysics and geology (tectonics). Specialized research fields, which have been important in development of fundamental expertise, are being interfaced in solving common problems. In Earth Sciences the term System Earth and, correspondingly, Earth System Science have become overall common denominators. Of this full System Earth, Solid Earth Sciences – predominantly addressing the Inner Earth - constitute a major component, whereas others focus on the Oceans, the Atmosphere, and their interaction. This integrated nature in Solid Earth Sciences can be recognized clearly in the field of Geodynamics. The broad research field of Geodynamics builds on contributions from a wide variety of Earth Science disciplines, encompassing geophysics, geology, geochemistry, and geodesy. Continuing theoretical and numerical advances in seismological methods, new developments in computational science, inverse modelling, and space geodetic methods directed to solid Earth problems, new analytical and experimental methods in geochemistry, geology and materials science have contributed to the investigation of challenging problems in geodynamics. Among these problems are the high-resolution 3D structure and composition of the Earth’s interior, the thermal evolution of the Earth on a planetary scale, mantle convection, deformation and dynamics of the lithosphere (including orogeny and basin formation), and landscape evolution through tectonic and surface processes. A characteristic aspect of geodynamic processes is the wide range of spatial and temporal scales involved. An integrated approach to the investigation of geodynamic problems is required to link these scales by incorporating their interactions. Scope and aims of the new series The book series “Modern Approaches in Solid Earth Sciences” provides an integrated publication outlet for innovative and interdisciplinary approaches to problems and processes in Solid Earth Sciences, including Geodynamics. It acknowledges the fact that traditionally separate disciplines or sub-disciplines have started interacting more closely, and some new fields have emerged at their interfaces. Disciplinary boundaries between geology, geophysics and geochemistry have become more transparent during the last ten years. Geodesy has developed close interactions with geophysics and geology (tectonics). Specialized research fields (seismic tomography, double difference techniques etc), which have been important in development of fundamental expertise, are being interfaced in solving common problems. Accepted for inclusion in Scopus. Prospective authors and/or editors should consult one of the Series Editors or the Springer Contact for more details. Any comments or suggestions for future volumes are welcomed.

More information about this series at http://www.springer.com/series/7377

YanJing Chen • Franco Pirajno • Nuo Li • XiaoHua Deng • YongFei Yang Editors

Geology and Geochemistry of Molybdenum Deposits in the Qinling Orogen, P R China

Editors YanJing Chen School of Earth and Space Sciences Peking University Haidian, Beijing, China Nuo Li Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography Chinese Academy of Sciences Urumqi, China

Franco Pirajno Centre for Exploration Targeting The University of Western Australia Crawley, WA, Australia XiaoHua Deng Exploration Targeting Beijing Institute of Geology for Mineral Research Chaoyang, Beijing, China

YongFei Yang Chengdu Center China Geological Survey Chengdu, Sichuan, China Responsible Series Editor: F. Pirajno

ISSN 1876-1682 ISSN 1876-1690 (electronic) Modern Approaches in Solid Earth Sciences ISBN 978-981-16-4869-4 ISBN 978-981-16-4871-7 (eBook) https://doi.org/10.1007/978-981-16-4871-7 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Abstract Molybdenum is a widely used metal in modern industry. China is the largest Mo producer and resource holder in the world. Qinling Orogen is China’s largest Mo metallogenic and producing province. It contains a reserve of >6 Mt Mo in metal from about 30 deposits of different genetic types known in the world, including 6 giant systems with each having >0.5 Mt Mo. This book is the first treatise of available data and viewpoints obtained from the geological and geochemical studies, as well as our new understanding of the Mo deposits in Qinling Orogen which was finally formed during the continental collision between the Yangtze and North China Cratons, following the closure of the northernmost paleo-Tethys. In Chap. 1, we introduce the tectonic framework and history, and the geologic natures of important tectonostratigraphic terranes and their boundaries. The Triassic tectonic setting of Qinling Orogen is considered analogous to the present-day Mediterranean Sea, rather than a

syn-collision to post-collision regime as previously suggested. In Chap. 2, we introduce Epizonogenism to fulfill a conceptual gap of the geological processes that occurred at depths of 0.5 Mt) and tens of small (3-Ga geologic evolution including significant orogenic events corresponding to the assembly and breakup of the Kenor, Nuna, Rodinia, Gondwana, and Pangaea supercontinents. Unique tectonic location, longtime evolution, and complex geological processes endow the Qinling Orogen with various mineral systems including, Mo deposits. Four important but controversial issues are clarified in this chapter: (1) the Early Precambrian crystalline basement in southern North China Craton is the mosaic amalgamation of multiple terranes or blocks that independently developed before 2.05 Ga. (2) The Xiong’er and Xiyanghe groups coevally developed in the continental arc and continental rift, respectively. (3) Triassic Qinling Orogen was analogous to the present Mediterranean Sea, accommodating transition from oceanic plate subduction to continental collision, but not a post-collisional extension. (4) The Yanshanian Orogeny in Qinling Orogen was a tectonic transition from syn-collisional, through post-collisional to intracontinental orogenies, resulting in crustal uplift, granitic magmatism, and hydrothermal mineralization. It was affected by the far-field impact from the Pacific plate subduction since ~130 Ma.

Y. Chen (*) School of Earth and Space Sciences, Peking University, Beijing, China e-mail: [email protected] N. Li Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, China F. Pirajno Centre for Exploration Targeting, The University of Western Australia, Crawley, WA, Australia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Chen et al. (eds.), Geology and Geochemistry of Molybdenum Deposits in the Qinling Orogen, P R China, Modern Approaches in Solid Earth Sciences 22, https://doi.org/10.1007/978-981-16-4871-7_1

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Keywords Yanshanian orogeny · Indosinian orogeny · Paleo-Tethys ocean · North China craton · Yangtze craton · Early Precambrian basement · Xiong’er group · Tectonic framework

1.1 1.1.1

Introduction Tectonic Location and Framework

The Qinling-Dabie and Alps-Himalaya mountains are two good representatives of continental collision orogenic belts in the world. Compared to the Himalayas which is now still in the syn-collisional orogenesis stage, the Qinling Orogen has experienced from pre-collision, syn- to post-collision, through to intra-continental regimes. It is an ideal area to provide insight into the characteristics and evolution of structural deformation, magmatism, fluid flow, and mineralization in continental collision orogenesis. The Qinling Orogen is located in the core of Central China Orogenic Belt (CCOB, Fig. 1.1a) which extends roughly east to west across mainland China, i.e., eastwardly from Pamir, through West Kunlun, Altyn, East Kunlun/Qilian, Qinling, Dabie, to the Sulu orogenic belts. The CCOB was formed from the final closure of the northernmost Paleo-Tethys Ocean, followed by the collision of the united North China-Alax-Tarim continent with the segments separated from the Gondwana Land, such as the Yangtze Craton (YC) and Qaidam Block (Chen et al. 2014; Chen and Santosh 2014). It is a transitional zone from the Paleo-Asia domain to the Tethys domain (Huang and Chen 1987; Fig. 1.1b). As indicated by the age of the oldest lithologic unit in the area, it witnessed a complex geological evolution of >3-Ga, including formation of continental crust, terrane amalgamation and continental

Fig. 1.1 Tectonic outline of mainland China (a; reprinted from Chen et al. 2014, Copyright 2014 John Wiley & Sons) and Asia (b; modified after Sengor and Natal’in 1996, Copyright 1996 Cambridge University Press), showing the location of Qinling Orogen

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Fig. 1.2 Tectonic framework of the Qinling Orogen, showing the distribution of major ore deposits. (Reprinted from Chen et al. 2009b Copyright 2009 China Academic Journal Electronic Publishing House)

collision, supercontinent assembly and breakup, opening and closure of oceans, slab subduction and break off, and intracontinental tectonic deformation (Chen et al. 2004). Its western portion is an intensely compressional setting with a remarkable thickened crust and lithosphere, whereas its eastern end (Sulu orogenic belt) is an extensional setting with thinned crust and lithosphere, which makes the CCOB a key area to understand the mechanisms of tectonic compression, extension and transition from compression to extension. The Qinling Orogen is unique for accommodating both western compression and eastern extension and is thus a key to explore the issues mentioned above (Zhang et al. 2001; Ernst et al. 2008). It is also an ideal region to understand the cycles of supercontinent assembly and breakup (Zhao et al. 2004a; Chen et al. 2009b). The Qinling Orogen was eventually formed by the Mesozoic collision between the South China and North China continents. The Mesozoic collisional processes occurred only in a domain defined by the Longmenshan-Dabashan Fault to the south and the Sanmenxia-Baofeng (San-Bao) Fault to the north (Chen et al. 1990b), which are considered as the Main Boundary Thrust (MBT) and Reverse Boundary Thrust (RBT), respectively (Fig. 1.2; Chen 1996). We consider that the domain between the San-Bao and Longmenshan-Dabashan faults belongs to the Qinling Orogen (Chen and Fu 1992). The Shang-Dan and Mian-Lue fault belts have been identified as the mid-Paleozoic and Triassic suture zones, respectively (Zhang et al. 2001; Dong et al. 2011), which, together with the Precambrian Luanchuan suture (Hu et al. 1988; Jia et al. 1988; Deng et al. 2013a, 2013b), divide the Qinling Orogen into four tectonic units southward termed the Huaxiong block (reactivated southern margin of the North China Craton), the North Qinling Accretion Belt (NQL), the South Qinling Fold Belt (SQL) and the Songpan Fold Belt (foreland fold-and-thrust belt) at the northern margin of the Yangtze Craton (Chen and Santosh 2014; Li et al. 2015b, 2018) (Fig. 1.2).

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The Luanchuan suture has been considered as the boundary between the North China Craton (NCC) and the Qinling orogenic belt or geosyncline, it records the northward subduction of the late Paleoproterozoic Kuanping oceanic plate beneath NCC during 1800-1600 Ma (Hu et al. 1988; Jia et al. 1988; Chen et al. 1992; Zhou et al. 2002a; Deng et al. 2013a, 2013b). The Shang-Dan suture is indicated by the ophiolitic melanges of Neoproterozoic to Early Paleozoic times, separating the South Qinling orogenic belt and the North Qinling accretion belt (Huang and Chen 1987; Hu et al. 1988; Xu 1992; Zhang et al. 2001). It serves as the border between the Laurasia and Gondwana supercontinents (Chen and Santosh 2014; Zhou et al. 2016). The Mian-Lue Suture, marked by ophiolite suites aged from Late Paleozoic to Triassic, is the youngest suture zone recording the termination of the ancient oceanic basins (North arm of the Paleo-Tethys) in Qinling Orogen and the onset of the latest collisional orogeny between North China and South China continents (Zhang et al. 1995, 2001; Li et al. 1996; Yin and Nie 1996; Zhu et al. 1998; Chen and Santosh 2014).

1.1.2

Inventory of Main Ore Types and Commodities

Longtime and complex geological evolution makes Qinling Orogen well-endowed in ore deposits of many genetic types and metal commodities. These deposits are generally unique in their genesis, world-class, and giant in resources (Fig. 1.2). With seven giants and a group of large and medium deposits, the eastern Qinling molybdenum belt (HuaXiong Block) ranks as the largest molybdenum province in the world (Li et al. 2007b; see Chap. 2). In the past three decades, >20 economically important Ag-Pb-Zn vein systems (with a proven reserve of >30 t of Ag metal) were discovered and prospected in the eastern Qinling Orogen, forming one of the most important Ag-Pb-Zn provinces in China, and also the first and largest orogenic-type Ag-Pb-Zn metallogenic province in the world (Chen et al. 2009b; Zhang et al. 2016). The HuaXiong Block is the second largest gold-producing district and orogenic-type gold province next to the Jiaodong Peninsula (northeastern Shandong Province) in China (Chen et al. 2009b). The western SQL (including southern Shaanxi, eastern Gansu, and northern Sichuan) is the second largest Carlin-type and Carlin-like gold province in the world (Chen et al. 2004), containing the Yangshan giant gold deposit (> 300 t Au). As exemplified by the Gongguan-Qindonggou Hg-Sb orefield (Zhang et al. 2014b), the Qinling Orogen is an important part of Qinling-Western Asia Hg-Sb ore belt, which is one of three global Hg-Sb ore belts (Tu and Ding 1986). The South Qinling orogenic belt contains the Shaanxi-Gansu SEDEX-type Pb-Zn belt, as represented by the Changba deposit, and the E-Shaan-Chuan Ba (witherite/ barite) belt which ranks as the largest Ba province in China. In addition, the Qinling Orogen has several significant magmatic-type Cu-Ni systems as represented the Zhouan and Jianchaling deposits (Wang et al. 2016; Mi et al. 2009), and VMS-type Cu-Zn
Au systems as exemplified by the Shuidongling and Liziyuan deposits, but enigmatically lacks important porphyry Cu-Au mineral systems.

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Nevertheless, the Qinling Orogen is currently the most important metallogenic province in China, called “China’s Gold Belt”. The unique geotectonic position, complex geological phenomena, and the worldclass giant mineral systems of the Qinling Orogen have attracted the attention of many geologists. Geologists investigated the geological bodies in detail, accurately described geological phenomena, and the genesis of associated mineral systems. Therefore, the Qinling Orogen has played a major role as the cradle of Chinese geological sciences and researchers for more than 100 years. Li et al. (1978) firstly discussed the tectonic evolution of Qinling Orogen in light of the theory of plate tectonics, which led the geological study of Qinling Orogen to have entered a new stage. Since 1978, the tectonic framework, structural architecture, and geological evolution of the Qinling Orogen were more reasonably understood, providing a powerful base for mineral exploration and ore geology studies. As revealed by tectonic reconstruction and structural analysis, the Qinling Orogen was developed through multi-style and multistage events of plate subduction and continental collision, thereby accomodating complex southward shallow-level overthrusts, coupled with northward deep-level underthrusts, including northward subduction of the Yangtze continental lithosphere beneath the Qinling Orogen (Zhang et al. 2001). Geochemical studies demonstrated that the Qinling Orogen is geochemically heterogeneous, with blocks or terranes displaying contrasting geochemical characteristics (Zhang et al. 2002a). The Mesozoic syn-collision crustal detachments and intracontinental subduction caused strong crust thickening and uplift (Xu et al. 1986), i.e., mountain-building, followed by the post-collisional extension. Geophysical surveys proved that the structural architecture of Qinling Orogen is an asymmetric mushroom-shaped fan; and the upper crust, lower crust, lithospheric mantle, and asthenosphere show different trends and characteristics, forming a flyover-like architecture (Yuan 1996; Zhang et al. 2001; Song et al. 2018). Available paleomagnetic data (Zhu et al. 1998) suggest that lateral crustal shortening between YC and NCC was intense in the period of Triassic-Jurassic, and ended at ~150 Ma, followed by a severe extensional thinning in the period of 150–105 Ma (Chen and Santosh 2014). At about 127 Ma, or 130–125 Ma, tectonic deformation remarkably changed, with the ductile shear zones being changed from compressional thickening to extensional pull-apart type (Zhang et al. 1998), accompanied by the emplacement of large granite domes. The extensional pull-apart type shear zones cut the syn-orogenic granites, whereas compressional thickening shear zones were intruded by the syn-orogenic granites, which can be observed in the Xiaoqinling goldfield (Zhang et al. 1998; Li et al. 2011a). The orogen hosts mineral systems of many genetic types and commodities (Hu et al. 1988), but the hydrothermal mineralization was mostly concentrated in the Jurassic-to-Cretaceous transition (Chen and Fu 1992; Chen et al. 2000a, 2007, 2009b; Mao et al. 2011; Li et al. 2018). Several questions or issues still remain open or controversial (Chen et al. 2009b), of which the most prominent four are: (1) the age and formation process of the Early Precambrian crystalline basement in the southern margin of the North China Craton. (2) the tectonic setting of Xiong’er Group is debated between continental arc (Hu et al. 1988; Jia et al. 1988; Chen et al. 1992; Zhao et al. 2002a; Zhao et al.

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2004a; Zhao et al. 2009; He et al. 2009, 2010; Deng et al. 2013a, 2013b), continental rift (Sun et al. 1985; Zhang et al. 2001; Zhang et al. 2002a), mantle plume-related (Zhao et al. 2002b; Peng et al. 2008). (3) the Triassic tectonic background is interpreted as post-collision (Zhang et al. 2001; Zhang et al. 2002a; Li et al. 1994; Sun et al. 2002), the coexistence of and transition from oceanic plate subductionrelated continental arc to continental collision (Chen et al. 2009b; Chen 2010; Jiang et al. 2010; Dong et al. 2011, 2012; Liu et al. 2011a), or a pre-collisional oceanic plate subduction-related continental arc or an Andean-type continental margin (Li et al. 2007b; Li et al. 2015b; Ni et al. 2012; Chen and Santosh 2014). (4) The mountain-building associated crustal uplift, granitic magmatism, and hydrothermal mineralization in Qinling Orogen most intensively occurred in the Yanshanian periods (Jurassic and Cretaceous), but the Yanshanian tectonic background was associated with syn-collisional, post-collisional and/or intracontinental orogenies, anorogenic, or decratonization, subcontinental lithosphere destruction and thinning, or mantle plume-related and other explanations. It has not been well discussed how to determine the beginning or ending time of a continental collision orogeny, and what can be used as geological indicators. In the following sections, we briefly introduce the geological evolution of Qinling Orogen, lithologic associations and formation processes of main tectonic units of the orogen, and more thoroughly focusing on the four abovementioned key issues.

1.2 1.2.1

Formation and Geology of Qinling Orogen Outline

Earth’s geological history is a magnum opus of both gradual and sudden changes as outlined in Fig. 1.3 (Gradstein et al. 2004; Chen and Chen 2016). Earth’s geological history is subdivided in terms of major geological events (Condie 2016), including endogenic and/or exogenic episodes induced by Earth's internal as well as extraterrestrial energy. The evolution or occurrence of these events is cycling, pulsating, and irreversible (Zhai and Santosh 2011, 2013; Lu et al. 2016). Exogenous geological processes dominate the supergene environment changes, often manifested as unique sedimentary assemblages, geochemical features, drastic changes in life evolution, such as the “Environmental Catastrophe” or "Great Oxidation Event (GOE) around 2.3 Ga (Chen 1990; Holland 2002; Tang and Chen 2013; Chen and Tang 2016; Young 2014, 2019; Tang et al. 2016; Chen et al. 2019). Environmental changes often resulted from extraterrestrial events, such as the impact of planetesimals and asteroids (Glikson and Pirajno 2018) and the periodic motion of the Galaxy (Milky Way). Two types of events can occur either independently, or couple with each other, or present reciprocally. Definition of the time and nature of these events is a key to understand the geological evolution of the Earth through time.

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Fig. 1.3 Key events in and subdivision of Earth’s evolutionary history. (Modified from Gradstein et al. 2004c Copyright 2004 International Union of Geological Sciences; Chen and Chen 2018 Copyright 2018 China Academic Journal Electronic Publishing House; Cohen et al. 2013 Copyright 2013 The ICS International Chronostratigraphic Chart)

The Qinling Orogen experienced at least two major environmental changes and a series of tectonothermal episodes (Fig. 1.3). Two global events occurred around 2300 Ma and 720–542 Ma, respectively. The former is the boundary of Siderian and Rhyacian periods and is suggested as the Archean-Proterozoic boundary (Chen et al. 1994) or the boundary between Transition Eon and Proterozoic Eon (Gradstein et al. 2004). The latter is the Neoproterozoic Oxidation Event associated with the

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Fig. 1.4 Ages of zircons from Triassic granites (a) and Cretaceous sediments (b) and Hf isotopes of detrital zircons (c). (Reprinted from Zhou et al. 2016 Copyright 2016 Elsevier)

Cryogenian Glaciation Event (CGE; Young 2019) or Snowball Earth Event, subsequently followed by Ediacaran emergence of metazoan fossils and Cambrian explosion (Planavsky et al. 2014). Tectonothermal processes resulted from changes in lithosphere plate motion, mantle plume activity, plate subduction, continental collision, and supercontinent break-up. They are often recorded by sedimentary unconformities, such as the unconformity between the Xiong’er Group and its underlying lithostratigraphic units, as recorded by the assembly of the Nuna (or Columbia) supercontinent (Zhao et al. 2002a; Piper 2015). Major tectonothermal events are regionally synchronous and diachronous and generally cyclic and pulsating, accompanied by intensive magmatism. As revealed by a combination of detrital zircon U–Pb ages and Lu–Hf isotope signatures of the Early Cretaceous Donghe Group intermountain molasses in the SQL (Zhou et al. 2016), the zircon U-Pb ages cluster around 2600–2300, 2050–1800, 1200–700, 650–400 and 350–200 Ma, corresponding to the Kenor, Nuna, Rodinia, Gondwana and Pangaea supercontinent assemblies, respectively (Figs. 1.4 and 1.5). These events are locally called ~2550-Ma Shipaihe Movement

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Fig. 1.5 Comparison of εHf(t) signature in zircons from the Donghe Group and the lithologies in the Qinling Orogen and adjacent areas. (Reprinted from Zhou et al. 2016 Copyright 2016 Elsevier). The boxes and their abbreviations (K, S, C1, C2, R, G, P) showing the εHf(t) domains of the Donghe Group. (a) North China Craton, (b) Yangtze Craton, and Bikou Terrane, (c) North Qinling Orogen, (d) South Qinling Orogen

(i.e., Orogeny), ~1850-Ma Zhongyue Movement, ~1000-Ma Jinning Movement, ~430-Ma Caledonian Movement. and ~200-Ma Indosinian Movement, respectively (see next section; Hu et al. 1988; Chen and Fu 1992). The youngest age peak of 350–200 Ma reflects the magmatism related to subduction and closure of the MianLue oceanic plate, followed by the Triassic–Jurassic collision between the YC and the amalgamated QL–NCC continent (QL ¼ Qinling ¼ SQL + NQL). The interval of 208–145 Ma between the sedimentation of the Donghe Group and the youngest age of detrital zircons was coeval with the continental collision between the YC and the QL-NCC continental plates in the Jurassic (Fig. 1.6; Chen and Santosh 2014; Zhou et al. 2016). The Donghe Group sediments could only have been sourced from Late Paleozoic-Triassic strata and Triassic granites in SQL. Zircon grains represent the source of the Late Paleozoic-Triassic clastic sediments. The populations of zircon ages and εHf(t) values of the Donghe Group sediments display a complex source comprising components of NCC, YC, NQL. and SQL, but mainly recycled from the

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Fig. 1.6 Schematic cartoons showing tectonic evolution of the Qinling Orogen and adjacent areas. (Modified from Zhou et al. 2016 Copyright 2016 Elsevier). Abbreviations: HX HuaXiong Block; NQL Northern Qinling accretion belt; SQL Southern Qinling microcontinental massif; CAOB Central Asia orogenic belt

NQL and NCC (Fig. 1.5), suggesting that the SQL was a part of the amalgamated QL-NCC continental plate, rather than an isolated microcontinent, during the Devonian–Triassic (Fig. 1.6). From these data, Zhou et al. (2016) drew out several important conclusions (Fig. 1.6): (1) The detrital zircons with ages of >400 Ma are generally round in shape suggesting a long-distance transport, and were mainly derived from the NQL-NCC continent, indicating that the SQL was connected with the NQL-NCC plate before Devonian; (2) Detrital zircons with ages of 340–208 Ma are euhedral to subhedral in shape, and are interpreted to have been sourced from the adjacent Triassic igneous rocks generated in the subduction-related continental arc in the Qinling Orogen; (3) The temporal interval between the

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beginning of sedimentation (ca. 145 Ma) of the Early Cretaceous Donghe Group and the youngest age of detrital zircons (208 Ma) implies a magmatic gap in the southwestern Qinling Orogen during 208–145 Ma. This gap matches well with the Jurassic post-subduction continental collision between the YC and the Qinling-NCC plates (Chen and Santosh 2014). As can be seen from Fig. 1.6, the post-Silurian tectonic evolution of the Qinling Orogen and its relationship with NCC and YC are reasonably well understood. As to the Neoproterozoic and Early Paleozoic time, the tectonic relationships between the NQL and NCC, and between the SQL and YC have been well documented, but the relationship between the NQL-NCC the South China (YC-SQL) plates is still unclear. The understanding of the pre-Neoproterozoic tectonic evolution of Qinling Orogen is generally uncertain. The pre-Neoproterozoic tectonic evolution in the SQL and the northern margin of the Yangtze Craton is not fully discussed because of limited exposures of pre-Neoproterozoic terranes and associated rocks. In NQL, the pre-Neoproterozoic rocks are more widespread and thus investigated in detail, leading to many models being suggested to illustrate the tectonics of NQL and its adjacent NCC, of which the rifting processes and magmatic arc are most popular. The southern NCC is characterized by Precambrian rocks that record well the Precambrian geological evolution (Chen et al. 2009b; Zhai and Santosh 2011). Therefore, in this book, the chronostratigraphic chart drawn from the southern NCC is shown in order to discuss the geotectonic evolution of the Qinling Orogen.

1.2.2

Formation and Geotectonic Evolution of the Qinling Orogen

The formation and geotectonic evolution of the Qinling Orogen can be divided into three mega-cycles: (1) formation of Early Precambrian basements of the southern NCC (SNCC) and the northern YC before 1800 Ma; (2) marginal accretion and deformation of the ancient North China and South China continents during 1800–200 Ma; and (3) continental collision to post-collisional tectonic evolution after 200 Ma. This long-term process includes several great tectonothermal events: (1) rapid crustal growth along the southern NCC and northern YC associated with the supercontinent Kenor assembly in Neoarchean; (2) continental collision and supercontinent Nuna assembly in the Orosirian (2050–1800 Ma); (3) amalgamation and divergence of terranes/blocks associated with the assembly and breakup of supercontinent Rodinia in early Neoproterozoic; (4) collision between NQL-NCC and SQL-YC associated with supercontinent Gondwana assembly and the Caledonian Orogeny; (5) northward subduction of the MianLue oceanic plate associated with the closure of the Paleo-Tethys and the Pangea supercontinent assembly; and (6) the collision between the North China and Yangtze continents and postcollisional tectonics.

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Kenor and Nuna Supercontinents

Before 1800 Ma, i.e., Early Precambrian, the North China and Yangtze cratons were formed through the assembly of blocks or terranes, respectively. The blocks and terranes usually have independent evolving history and lithologic association. In the Early Precambrian, the high geothermal gradient and rapid convection of heat and materials resulted in high-speed movement and interaction of blocks and terranes, including underthrusting, rifting, and strike-slip processes, or in other words, rapid convergence and/or divergence, which reflect a pan-Wilson cycle (Chen and Fu 1992). The Archean (>2500 Ma) magmatism was intense and extensive, developing volcanic-sedimentary sequences typical of greenstone belts. During 2700–2500 Ma, continental crust grew rapidly with a large scale emplacement of granitoids (Zhai and Santosh 2011; Liu et al. 2002, 2004; Li et al. 2015a; Zhai et al. 2016); and the amalgamation of blocks and terranes which led to the assembly of the Kenor supercontinent (Kerrich et al. 2000; Condie 2016). During this period, most blocks or terranes of the NCC were formed, and thus Zhai et al. (2016) argued that the Eastern Block and Western Block, or the NCC had taken their shape. In the SNCC, the HuaXiong, SongJi and ZhongTiao blocks had appeared as continental nuclei (Fig. 1.7; Sect. 1.3). In the northern margin of the Yangtze Craton, the continental nuclei are represented by the gneisses within the Yudongzi Group (Wu et al. 2012, 2014). In the early Paleoproterozoic, i.e., 2500–2050 Ma or the Siderian plus Rhyacian periods, the terranes of continental crust became stable, and the cratonic basins become common; meanwhile, the hydrosphere-atmosphere system rapidly changed from reducing to oxidizing (Chen 1988, 1990), this is the Great Oxidation Event (GOE) of Holland (2002). The GOE was considered as a worldwide environmental catastrophe at ~2300 Ma, including the earliest global Huronian Glaciation Event (Tang and Chen 2013; Young 2013, 2014, 2019), positive excursion of δ13Ccarb (Lomagundi-Jatuli Event; Melezhik et al. 1999; Tang et al. 2011, 2013), widespread appearance of stromatolites, red beds, evaporites, and phosphorites (Chen 1988, 1990; Tang and Chen 2013; Chen and Tang 2016). This unique process caused the formation of giant BIF-type iron and magnesite deposits. Continents and/or cratons assembled to form Supercontinent Nuna in the Orosirian (2050–1800 Ma), associated with subduction, continental collision, basin closure, deformation, and metamorphism. In the Orosirian, the crystalline basements of the NCC and YC were pieced together through terrane amalgamation. At least three blocks were identified in SNCC, i.e., the HuaXiong, SongJi, and ZhongTiao blocks (Chen et al. 1988, 1990b, 1991a; Zhao et al. 2005). The Early Precambrian volcanic and sedimentary rocks were metamorphosed to greenschist to granulite facies, forming three types of lithologic associations: (1) greenstone belts dominated by metamorphosed mafic rocks, represented by the Dangzehe Group in HuaXiong block and Junzhao Group in SongJi block; (2) khondalite series, mainly composed of graphite-bearing gneisses,

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Fig. 1.7 Outcrop and tectonic framework of Precambrian metamorphic basement in NCC. (Modified from Chen et al. 1998 Copyright 1998 China Academic Journal Electronic Publishing House; Zhao et al. 2005 Copyright 2005 Elsevier; Wan et al. 2006 Copyright 2006 Elsevier; Zhai and Santosh 2011 Copyright 2011 Elsevier). Note:The southern boundary of NCC is the Luanchuan fault, which is 80 km north to the Xinyang city, instead of the Shang-Dan Fault that is mistaken as the southern boundary of the NCC by a few geologists. The Shang-Dan Fault, i.e., the final suture of the northernmost paleo-Tethy as well as the suture between the North China and South China continents or plates, is south to the Xinyang city

sillimanite-garnet-quartz schist, BIFs and marbles, formed from carbonaceous and Al-rich fine-grained clastic rocks (shales) and chemical sediments (carbonate, chert, iron hydroxide, and phosphorite), exemplified by the Shuidigou Group in HuaXiong Block and the Huangtuyao Group in northern NCC; and (3) cratonic or platformtype sedimentary sequences, upwardly consisting of conglomerates, sandstones, shales and chemical sediments dominated by carbonates, such as the Songshan Group in Songji Block, Zhontiao Group in Zhontiao Block and Hutuo Group in Wutai Shan (Chen et al. 2019). These associations are generally low-grade metamorphosed up to greenschist facies due to the protection from their unconformably underlying crystalline basement rocks.

1.2.2.2

Rodinia Supercontinent Assembly

The time period from 1850 to 850 Ma is named “Boring Billion” (Condie 2016 and references therein), due to the worldwide stability of the main cratons. In this period, the NCC (an Early Precambrian continent) expanded southward, with the NQL

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Fig. 1.8 Tectonic framework of North Qinling Accretion Belt and southern North China Craton. (Modified from Li et al. 2007b Copyright 2007 China Academic Journal Electronic Publishing House)

(North Qinling accretionary belt) being formed between the Luanchuan and ShangDan faults (Fig. 1.8). The northern margin of the Yangtze Craton was accreted by the South Qinling microcontinent and Bikou terrane. In the Statherian (1800–1600 Ma: the latest period of Paleoproterozoic), the ancient Kuanping oceanic plate subducted northward beneath the NCC, along the Luanchuan fault, leading to the HuaXiong block being a magmatic arc characterized by the accumulation of Xiong’er Group volcanic rocks. Meanwhile, a passive or failed rift arm, marked by the Xiyanghe Group bimodal volcanic rocks, developed between the SongJi Block to the east and the Zhongtiao (or Ordos) Block (including Zhongtiao terrane) to the west (Fig. 1.8). The Kuanping Group, containing ophiolite mélange, occurs between Luanchuan and Waxuezi faults, is interpreted to be an 1850–1600 Ma accretionary complex (Jia et al. 1988; Chen et al. 1992; Sect. 1.6). The Longwangzhuang A-type granite intruding the Taihua Supergroup and the Xiong’er Group on the north side of the Luanchuan Fault yielded a zircon U-Pb age of 1625
16 Ma (Lu et al. 2003), suggesting that the Paleoproterozoic continent-arc system ended at ~1625 Ma. Subsequently, the Central Qinling Terrane, marked by the Qinling Group gneisses, collided with the NCC at the transition from Paleoproterozzoic to Mesoproterozoic. In the Mesoproterozoic (1600–1000 Ma: Calymmian + Ectasian + Stenian), tectonic extension in the southern margin of ancient North China continent resulted in the deposition of the Guandaokou Group and the lowest portion of Taowan Group along the suture belt between the Central Qinling terrane and the HuaXiong block

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Fig. 1.9 Spatial distribution of Mesoproterozoic sediments in southern North China Craton. (Modified from Hu et al., 1988 Copyright 1988 Nanjing University Press)

Fig. 1.10 Mesoproterozoic sedimentary basins and tectonic settings in southern North China continent. (Modified from Chen and Fu 1992 Copyright 1992 China Seismological Press)

(Figs. 1.9 and 1.10). The Ruyang Group developed in a fault-depression along the San-Bao Fault. The Guandaokou and Ruyang groups are interpreted to have been deposited in the fore-arc and back-arc basins, respectively, given that the Xiong’er Group was formed in a continental arc (Jia et al. 1988; Hu et al. 1988; Chen et al. 1992; Zhao et al. 2002a). The Guandaokou and Ruyang groups are overlain by the Stenian (1200–1000 Ma) Luanchuan Group carbonaceous carbonate-shale-chert and the Luoyu Group clastic sedimentary successions, respectively. The Wufoshan Group, which is supposed to correlate with the Ruyang Group in age, accumulated as intracontinental molasses in the SongJi Block (Figs. 1.9 and 1.10). The volcanic intercalations (with maximum thickness of 134 m) within the Gaoshanhe Formation (Guandaokou Group) and Yunmengshan Formation (Ruyang Group) yielded Rb-Sr isochron ages of 1394
42 Ma and 1267 Ma, respectively (Lü et al. 1993 and references therein). This suggests that the Guandaokou and Ruyang groups belong to the Ectasian in age, whilst the Luanchuan and Luoyu groups are of the Stenian age. These ages are similar to a detrital zircon 207Pb/206Pb age of 1344
13 Ma obtained

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Fig. 1.11 Precambrian terranes in northern Yangtze Craton and South Qinling Folding Belt. (Modified from Jia et al. 1988 Copyright 1988 China Academic Journal Electronic Publishing House)

from the Early Cretaceous sediments in SQL (Zhou et al. 2016), suggesting that the middle Mesoproterozoic magmatism weakly occurred in the southern margin of the North China continent. At the transition from Mesoproterozoic to Neoproterozoic, i.e., 1100–850 Ma, along with the amalgamation of the Rodinia supercontinent, the North China continent pale (NCP) collided with the Central Qinling terrane. This event is part of the Grenvillian orogeny. In this event, the Qinling, Guandaokou, Luanchuan, and Taowan groups were metamorphosed, deformed, and intruded by collisional and post-collisional granites with ages of 980–930 Ma (Zhang et al. 2004; Chen et al. 2006). The Songshuou ophiolitic suite yields an Sm-Nd isochron age of ~1030 Ma, corresponding to the Grenvillian Movement (Dong et al. 2008). A number of Precambrian (mainly 1850–850 Ma) metamorphic terranes, such as Douling, Ankang, Bikou, and Wudang terranes, were recognized on the northern margin of YC and SQL (Fig. 1.11). During the period of 1850–1400 Ma, the northern YC separated into several terranes represented by the Hannan, Huangling, and Shennongjia terranes, forming a passive continental margin spotted with continental terranes. During 1400–850 Ma, these terranes came together again, forming the Yangtze Craton. Meanwhile, magmatic arc terranes, such as Suixian, Tongbai, Douling, Wudang, Ankang, and Foping, developed in and amalgamated to the northern margin of the Yangtze Craton, forming the Precambrian basement of SQL (Jia et al. 1988; Hu et al. 1988; Chen and Fu 1992), called the “South Qinling micro-continent” by Zhang et al. (2001). Tectonic deformation, metamorphism, and magmatism associated with the Grenvillian or Jinning Orogeny occurred in Yangtze Craton and its surrounding terranes and accretion complexes, which is regarded as a distinction between the YC and NCC (Zheng 2003). Obviously, these Precambrian terranes formed the metamorphic basement of the northern margin of the Yangtze Craton and the “South Qinling Micro-continent”.

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1.2.2.3

17

Supercontinent Rodinia Breakup and Gondwana Assembly

After the Grenvillian Orogeny, the supercontinent Rodinia broke up following the post-collision extension, which caused the opening of the Paleo-Tethys Neoproterozoic-Early Paleozoic oceanic basins in Tethyan tectonic domain. In the late Tonian (850–720 Ma), the southeastern margin of the Yangtze Craton broke up (Zhang et al. 2008a, 2008b); numerous granitic and mafic intrusive rocks developed in the western and northwestern margin of the Yangtze Craton (Zhou et al. 2002b; Li et al. 2003a, 2003b; Sun et al. 2009; Lu et al. 2005; Ling et al. 2006). For example, the igneous rocks from the Bikou Group in the Bikou terrane yielded SHRIMP zircon U-Pb ages of 840–776 Ma (Yan et al. 2003); and the sedimentary rocks from the Bikou Group yielded a detrital zircon U-Pb age of ~805 Ma (Sun et al. 2009). The zircon U-Pb age of the ore-bearing spilite in the Huachanggou gold deposit is ~800 Ma (Lin et al. 2013); whilst the granite porphyry and albite porphyry in the Jianchaling nickel deposit yield zircon U-Pb ages of 859
26 Ma and 844
26 Ma, respectively (Dai et al. 2014). The Maotang Group of Douling terrane and Yaolinghe Group of Wudang terrane are volcanic-sedimentary sequences aged 800-700 Ma. The εHf(t) values of the Tonian zircons show a wide range and a complex source consisting of recycled old crust material and juvenile crust. This proves that crustal growth and recycling of old crustal materials in the northern Yangtze plate were both quite intensive. In NQL and SNCC, intensive post-collisional extension resulted in the emplacement of alkaline granites and mafic rocks. For example, the Shuangshan syenites in Fangcheng county, Henan province, yielded a zircon U-Pb age of 844.3
1.6 Ma (Bao et al. 2008); and gabbros intruding the Luanchuan Group, Luanchuan County yielded a zircon U-Pb age of 830
6 Ma (Wang et al. 2011c). Along with the breakup of the supercontinent Rodinia, the Erlangping oceanic basin opened (Fig. 1.12). From Neoproterozoic to Early Paleozoic (650–400 Ma), the northern margin of the South China plate (SCP ¼ YC + SQL) acted as a passive continental margin locally crosscut by rifts or limited sea (Fig. 1.12), as exemplified by the Zhou’an CuNi-PGE deposit in the Suixian Terrane (Henan). The Zhou’an deposit is hosted in a strongly altered mafic-ultramafic complex, which yielded a zircon U-Pb age of

Fig. 1.12 The Neoproterozoic-Early Paleozoic tectonic framework of Qinling Orogen. (Reprinted from Chen and Fu 1992 Copyright 1992 China Seismological Press)

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641.5
3.7 Ma (Yan et al. 2012) and was interpreted as a fragment of ophiolite suite (Wang et al. 2006; Mi et al. 2009). The ShangDan oceanic plate is subducted beneath the North China plate (NCP ¼ NQL + NCC), and a trench-arc-basin system developed in the southern margin of the North China plate (Fig. 1.12). The Yangqigou-Chenyangping ophiolite mélange zone intermittently occurs along the Shang-Dan fault zone, which records the subducted Shang-Dan oceanic plate. The Central Qinling terrane is characterized by the development of the island arc-type volcanic-sedimentary rocks within the Qinling Group. The Erlangping back-arc basin was infilled with the Erlangping Group characterized by pillow basalts between Wazixue and Zhu-Xia faults. At the end of the Early Paleozoic, the South China plate (YC + SQL) collided with the North China plate, i.e., the combination of the Gondwana and Laurasia continents in the Qinling Orogen. This collision caused the closure of the protoTethys, including the Erlangping back-arc basin and the ShangDan ocean, and the formation of Northern Qinling Accretion Belt and the assembly of terranes in the northern margin of the Yangtze Craton to form the Caledonian basement of the Southern Qinling Fold Belt. This understanding is supported by the following evidence: (1) the Piaochi and Huichizi granite batholithes formed at ~495 Ma and 434–421 Ma, respectively (Hu et al. 1988; Wang et al. 2009); (2) several pyroclastic units in the Neoproterozoic stratigraphic sequence widely occur in the Yangtze Craton and its southeastern and northwestern accretionary belts, and show stable stratigraphic horizon and thickness, yielding zircon U-Pb ages of 663–555 Ma (Zhou et al. 2004; Zhou 2016; Zhang et al. 2005, 2008b; Li et al. 2011b; Mao et al. 2013). The debris of these tuffaceous rocks are suggested to have originated from the volcanism in the northern Qinling magmatic arc (Zhou et al. 2016); (3) many metamorphic zircons from the Qinling Group yielded ages of 450–400 Ma; and molybdenite from an orogenic Mo-dominated polymetal deposit hosted in the Erlangping Group yielded Re-Os isochron age of 429 Ma (Li et al. 2009); (4) paleomagnetic data show that NCC was most close to the YC in Silurian in the Paleozoic (Fig. 1.13; Zhu et al. 1998); (5) in YC and SQL the Devonian-Triassic sequences show strikingly weaker metamorphism and tectonic formation than the Early Paleozoic and earlier rocks. For example, the Maotang Group (Neoproterozoic), Taiyangding Group (Cambrian), and Meiziya Group (Early Silurian) in SQL are all are greenschist facies metamorphic rocks. In addition, unconformity and sedimentary interruption can be recognized between the Early Paleozoic and Late Paleozoic stratigraphic units. Therefore, it is generally accepted that a great tectonic event, i.e., Caledonian Orogeny, occurred in the Silurian.

1.2.2.4

Opening and Closure of Paleo-Tethys and Supercontinent Pangea Assembly

As the northernmost branch of the Paleo-Tethys, the MianLue Ocean began to open in the Devonian. In the Devonian-Triassic, the Yangtze Craton and the Southern Qinling micro-continent (a merger of several Precambrian terranes) plunged under

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Fig. 1.13 Paleomagnetic data showing the relative position of NCC and YC in the Phanerozoic. (Modified from Zhu et al. 1998 Copyright 1998 China Academic Journal Electronic Publishing House)

the Paleo-Tethys Ocean, and thus the Devonian-Triassic strata consist of carbonates, cherts, shales, turbidites, and rare evaporites, but lack volcanic rocks or tuffs. The Qinling Orogen is also poor in Late Paleozoic granitoids, but has widespread Triassic I-type granites (dominated by granodiorite). These facts indicate that the Paleo-Tethyan oceanic plate (MianLue oceanic plate) subducted northward beneath the SQL-NQL-NCC continental plate along the Mian-Lue Suture in Triassic. The Triassic Qinling Orogen was an active continental margin, i.e., a trench-magmatic arc system (Fig. 1.14; Chen 2010; Chen and Santosh 2014; Li et al. 2015b; Li and Pirajno 2017), rather than a syn- to post-collisional orogenic belt (Li et al. 1996; Sun et al. 2002). The MianLue Ocean was completely closed in the Triassic, followed by the continental collision between the YC and SQL-NCP. Simultaneously or later, the south arm of the Paleo-Tethys Ocean closed, the Indochina-Qiangtang block collided with the Tarim, Qaidam, Aba, and Yangtze continents or blocks, forming the collisional orogenic belt including West Kunlun, Altyn, East Kunlun, Animaqin, Songpan-Litang, Youjiang-Ailaoshan mountains (Xu et al. 2012; Zhong et al. 2000), resulting in the assembly of supercontinent Pangaea. The abovementioned conclusions are supported by the following evidence: (1) The youngest age peak of the detrital zircons from the Donghe Group range

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Fig. 1.14 Tectonic background of the Qinling Orogen in Triassic. (Reprinted from Chen and Santosh 2014 Copyright 2014 John Wiley & Sons)

from 340 to 208 Ma, concentrating around 250-208 Ma (Fig. 1.4), matching up with the amalgamation time of the supercontinent Pangaea. These zircon grains show the features of magmatic origin (Zhou et al. 2016); (2) Triassic granites widely occur in Qinling Orogen (Li et al. 2015b and its reference) and demonstrate the characteristics of magmatic arc (Jiang et al. 2010; Chen 2010; Chen and Santosh 2014) although they were interpreted to have formed in a post-collisional setting (Zhang et al. 2001; Yang et al. 2006; Zhu et al. 2011); (3) Previous geochronological studies on the ophiolite fragments and the Heigouxia volcanic rocks along the Mian-Lue suture showed that the MianLue Ocean survived at least between 350 and 245 Ma (Lai et al. 2008) and then terminated at ~200 Ma (Chen 2010 and references therein); (4) The paleomagnetic data show that the MianLue Ocean closed westward in a zipperfashion way (Fig. 1.15).

1.2.2.5

Continental Collision and Intracontinental Tectonism

The paleolatitudes for the YC and NCC were not connected prior to the Early Jurassic, suggesting that the YC and NCC became closer during the PermianTriassic and finally connected in the Early Jurassic (Figs. 1.13, 1.15 and 1.16). This means that these two continents were always separated by an ocean which narrowed gradually and closed finally in the Early Jurassic. A systematic latitude discrepancy (about 4–7 ) exists between the eastern (Yuexi) and western (Fengzhou) reference points from the Late Permian to Late Triassic (Zhu et al. 1998), indicating that the two continents connected westward and completely amalgamated at the Triassic-Jurassic transition (Fig. 1.15; Chen and Santosh 2014), along with the westward termination of the MianLue Ocean.

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Fig. 1.15 Relative position of Yangtze and North China cratons around the Mesozoic. (Reprinted from Zhu et al. 1998 Copyright 1998 China Academic Journal Electronic Publishing House)

After the Triassic closure of the MianLue Ocean, the YC began to collide with the QL-NCC plate in the time of 205–195 Ma (Figs. 1.13 and 1.16b). In the period of 195–150 Ma, the YC continuously drifted northward and underthrusted beneath the QL-NCC plate, causing the paleolatitude of the YC was more northerly than that of the QL-NCC plate (Fig. 1.13a,b), which is anomalous compared to the scenarios in the time spans of pre-Jurassic and post-100-Ma. Obviously, this phenomenon resulted from the strong continental collision in the Jurassic, which accommodated a set of intracontinental or A-type subductions of various levels, scales, and styles, causing lithosphere and crust shortening, thickening, and uplifting, i.e., mountainbuilding (Fig. 1.16c), as also revealed by the data from the reflection seismic survey (Yuan 1996). This understanding is strongly supported by the similar paleomagnetic declinations of the YC and NCC during the Jurassic, and further enhanced by the same paleomagnetic declinations of the YC and NCC during the Early Cretaceous (Fig. 1.13c,d). After the culmination of crustal shortening and thickening at about ~150 Ma, the lithosphere began to extend and thin due to the elastic rebound character of the compressed solid material (Fig. 1.16d). In the Early Cretaceous, the anomalous discrepancy in paleolatitudes between the YC and NCC was rapidly reduced to the normal scenario (Figs. 1.13a,b and 1.16e), i.e., the NCC is placed north of the YC. The geological process that led to the reduction in the paleolatitude anomaly can be explained by using the post-collisional extension model (Chen et al. 2009b; Chen 2010). For example, the commonly-observed extensional structures, fault-controlled depressions infilled with red beds and shoshonitic volcanic rocks (Chen and Fu 1992; Chen and Santosh 2014). Along with the tectonic evolution of collisional orogen, extensive Yanshanian (Late Mesozoic) granitoids were emplaced, with an age range of 108–160 Ma (Li et al. 2018), mainly subdivided into three pulses: 108–125, 125–140, and 140–160 Ma. There is a marked variation

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Fig. 1.16 Tectonic evolution of the Qinling Orogen in Mesozoic

between granitoids aged pre- and post-125 Ma in terms of spatial distribution, petrology, geochemistry, and isotopic features. The older suite displays a northward younging trend if taking the MianLue suture as the datum line. It is composed of quartz diorite-granodiorite-monzogranite-granite association, and widespread between 109 E and 112 E. The younger suite occurs only in the easternmost part of the orogen (between 111 E and 113 E), and is dominated by monzogranite, granite, and syenogranite. Compared with the older suite, the younger suite is featured by evolved composition (with higher silica but lower Al2O3, FeOT, MgO, and CaO contents) and witnesses more involvement of juvenile crust. It has a much

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shallower source as evidenced by low Sr/Y ratios and pronounced Sr, Ba, and Eu negative anomalies. In the process, the Qinling Orogen was also affected by interaction with the Indian-Australian, Pacific, and Eurasian plates through far-field impact, showing alternating extrusion and extension. The orogenic belt showed eastward tectonic escape, compressional extrusion in the west, and extensional collapse in the east. Therefore, the deeply-seated granitoids and high-grade metamorphic rocks represented by UHP eclogites are more widely outcropped in the eastern part of the Qinling Orogen.

1.2.3

Major Geologic Events in the Qinling Orogen

The crustal growth and tectonic evolution of the Qinling Orogen are cyclical and pulsating (Hu et al. 1988). Each cycle includes the beginning and ending of a geological event (orogenic or geotectonic event usually called “movement” by Chinese geologists) which is taken as the criterion of geologic time division (Fig. 1.17). The major geologic events and lithostratigraphic division in the Qinling Orogen are illustrated in Fig. 1.16 and briefly addressed from early to late as shown below.

1.2.3.1

The ~3000 Ma Qingyanggou Orogeny

The oldest lithologic unit in the Qinling Orogen is the Shipaihe Complex exposed in the Songshan Terrain, SongJi Block. It is mainly composed of metadiorite, TTG suite, and migmatitic gneisses and yields an Rb-Sr isochron age of 2997 Ma (Chen and Fu 1992). The Yuyao Complex in the Jishan Terrain, SongJi Block is considered to correspond with the Shipaihe Complex, which is mainly composed of migmatites, migmatitic gneisses, and TTGs, and aged 2890 Ma as dated by whole-rock Rb-Sr isochron method (Hu et al. 1988; Chen et al. 1989b, 1990a). Obviously, the Shipaihe and Yuyao complexes indicate that the continental crust rapidly formed during the period of 2890–3000 Ma, leading to the emergence of continental nuclei in southern NCC. In these complexes, enclaves or relics of high-grade supracrustal rocks can be observed, especially at the Qingyanggou area, Junzhao town, Dengfeng county, Henan province. These supracrustal enclaves show chemical compositions similar to komatiite, with higher MgO contents compared to the mafic rocks of the unconformably-overlying Junzhao Group (Chen et al. 1988, 1990a). Therefore, the enclaves of supracrustal rocks in the Shipaihe and Yuyao complexes are considered as the residues of the Qingyanggou greenstone belt, developed before 3000 Ma (Chen et al. 1988). Consequently, the tectonothermal event characterized by the intrusion of Shipaihe TTGs and the destruction and partial melting (migmatization) of the Qingyanggou greenstone belt, termed Qingyanggou Orogeny or Movement, must have occurred at ~3000 Ma, being the boundary of Neoarchean and Mesoarchean (Wang and Li 1991).

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Fig 1.17 Major geological events and lithostratigraphic division of Qinling Orogen. (Modified from Chen and Fu 1992 Copyright 1992 China Seismological Press)

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1.2.3.2

25

~2550 Ma Shipaihe Orogeny

The Shipaihe Orogeny was termed after the Shipaihe unconformity represented by the basal conglomerate association along the Shipaihe Creek in Junzhao town, Dengfeng county, Henan province (Chen et al. 1989b, 1990a). The basal conglomerate association occurs at the contact zones at the base of Junzhao Group with the Shipaihe Complex in Songshan Terrain, and with the Yuyao Complex in Jishan Terrain (Hu et al. 1988; Chen et al. 1989b). The Junzhao Group was metamorphosed to greenschist to amphibolite facies, clearly lower than the amphibolite-to-granulite facies metamorphosed Shipaihe and Yuyao complexes (Chen et al. 1990a). It is clear that the Shipaihe unconformity records a great tectonothermal event. This unconformity was also recognized in the HuaXiong Block: (1) In Lushan Terrain, the thick-bedded leptites (felsic gneisses or/and granulites) at the bottom of Dangzehe Group overlying on the kyanite-containing iron formation of the Beizi Group by an angular unconformity; (2) in Wuyang Terrain, a 463-m-thick leptite bed at the bottom of the Dangzehe Group (Tieshanmiao Fm.) overlying on the Beizi Gp. (Zhaoanzhuang Fm.); and (3) in the Xiong’er Terrain, the komatite lavas of Dangzehe Gp. (Shibangou Fm.) overlying the migmatitic gneiss of the Beizi Gp. (Caogou Fm.) by an angular unconformity. All the isotopic ages have been obtained from the Dangzehe Gp. (HuaXiong Block) and Junzhao Gp. (SongJi Block) are not older than 2.55 Ga, but mainly range from 2.55 to 2.35–2.3 Ga; whereas the rocks underlying the Junzhao and Dangzehe groups yielded isotope ages older than 2.55 Ga. It is worth stressing that the rocks below the unconformity were subjected to a higher degree of partial melting than the rocks above the unconformity, resulting in the formation of migmatites. In addition, in the SongJi Block the Shipaihe and Yuyao complexes show much higher metamorphic grades (high amphibolite to granulite facies) than their unconformably overlying Junzhao Group (greenschist to low amphibolite facies) (for detail see Chen et al. 1989b, 1990a). The abovementioned phenomena indicate that the unconformity represents a great tectonothermal event, termed Shipaihe Orogeny or Movement, which occurred around 2550 Ma. The Shipaihe Orogeny was characterized by a rapid generation of continental crust, temporally coeval with the worldwide continental crust growth event in Neoarchean, associated with the Kenor supercontinent assembly.

1.2.3.3

~2300 Ma Great Oxidation Event or Guojiayao Orogeny

Guo (1987), Sun et al. (1985) and Chen et al. (1988) identified or affirmed the smallangle unconformity or disconformity between the Angou Group and the Junzhao Group in Junzhao and Angou areas in SongJi Block, which is called Guojiayao unconformity (Hu et al. 1988). This unconformity presents as the overlying of the Jiangxian Group on the Sushui Complex in the Zhongtiao Block, and the covering of the Xiaoshan Group on the Tianyemiao Complex in the Xiaoshan Terrain in the HuaXiong Block. The Angou, Jiangxian, and Xiaoshan groups are predominated by

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bimodal volcanic rocks metamorphosed to greenschist facies, with the metamorphic degree being clearly lower than the underlying Junzhao Group and Sushui and Tiaoyemiao complexes, indicating that the unconformity represents a tectonothermal event, i.e., Guojiayao Orogeny (Hu et al. 1988; Chen et al. 1988). The time of this event was constrained at about 2300 Ma, because the pegmatite dikes intruding the Junzhao Group, overlain by the Angou Group, yielded mica K-Ar ages range 2200–2299 Ma (Hu et al. 1988 and references therein). The understanding of the Guojiayao Orogeny is supported by a huge pile of isotope ages obtained after 1990 (Sun et al. 1991; Liu et al. 2015) In the HuaXiong Block, except for the Xiaoshan Terrain, the Dangzehe Group which mainly consists of amphibolite, amphibolite gneiss, and biotite gneiss) is covered by the Shuidigou Group khondalite series comprising marbles, sillimanitegarnet-quartz gneisses, graphite-bearing gneisses, amphibole gneisses, diopsidegarnet granulites, and banded iron formations (BIFs). Differences in the rock assemblages, geochemical signatures are significant, recording a global environmental change or a great oxygenation event (Chen 1988, 1990; Baker and Fallick 1989a, 1989b; Chen and Fu 1991). This event occurred globally at about 2300 Ma (Chen et al. 1989a, 1991b, 1994, 1996, 2000b; Tang et al. 2008, 2009, 2011, 2013, 2016; Chen and Tang 2016; Tang and Chen 2013) and was called Great Oxidation Event (GOE; Holland 2002). In the Lushan Terrain, the contact between Dangzehe Group and Shuidigou Group is a 2150 Ma) is unconformably overlain by the slightlymetamorphosed Xiong’er Group (3.3 Ga No

Reprinted from Chen et al. 1988 and references therein Copyright 1998 China Academic Journal Electronic Publishing House

amphibolite gneiss, and minor felsic schist. Some rocks yield compositions comparable to komatiite. Such supracrustals rocks are similar to those from the Sargur greenstone belt, India, and the Sebakwian greenstone belt, Zimbabwe, in terms of their occurrence, rock assemblage, and ages (Table 1.2; Goodwin 1981). Hence, they are considered as residual of the primary greenstone belt and called the Qingyanggou-type greenstone belt (Chen et al. 1988). Since the primary greenstone belt is representative of the earliest supracrustal rocks of the continent, they should also represent the most primitive crustal components (Goodwin 1981). The ever known primary greenstone belts mainly consist of ultramafic and mafic rocks, indicating a primitive crust was less evolved (Glikson 1976; Chen et al. 1988). Till now, the origin of the primitive crust has still been debated. Available data from space crafts, remote sensing, and comparative planetary research support a planetesimals accumulation or impact origin (Ouyang 1989, 1990, 1991; Glikson and Pirajno 2018), i.e., the primitive crust or the primary greenstone was formed by a number of accretionary planetesimals impacts.

1.3.3

Beizi-Type Greenstone Belt and Shipaihe Complex: Continental Nuclei

The accretion of planetesimals constituted the primitive crust, resulting in the earth being inhomogeneous with regard to composition, radioactive element concentration, and residual energy in three dimensions. This heterogeneity will inevitably result in energy and substance exchange, namely convection and diffusion. They caused extensive volcanism on the surface of the earth, and also led to a wide range of small-scale divergence and convergence. In the divergent area, the partial melting of the planetesimal-related mantle occurred, producing abundant ultramafic-mafic

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Table 1.3 Comparison of the Beizi-type greenstone belts Location Occurrence Petrology

Sequence

Age Basement

Beizi, HuaXiong block NWW-trending anticlines, 5–60 km long Komatiite, peridotite, basalt, dacite, shale, pelite, anorthosite, BIF Cyclic ultramaficmafic-felsic volcanic rocks ! pelite and BIF 3.0–2.55 Ga No basement, unstable crust

Kolar, India Peninsula 10–50 km long belts

Barberton, South Africa Small to medium synclines

Komatiite, peridotite, basalt, anorthosite; minor quartzite, pelite & BIF Volcanic rock ! pelite and BIF

Komatiite, basalt, dacite, rhyolite, shale, quartzite, greywacke, conglomerate cyclic ultramafic volcanic rocks ! felsic volcanic rocks and sediments >3.4–3.2 Ga No basement, unstable crust

>3.1 Ga No basement, unstable crust

Reprinted from Chen et al. 1988 and references therein Copyright 1998 China Academic Journal Electronic Publishing House

magma and thus a new primitive oceanic crust (e.g., the Beizi Group). In the convergent area, the primitive upper mantle and lower crust partially melted, resulting in upward migration and accumulation of intermediate to felsic magmas in the upper crust, forming the primitive continental crust (e.g., the Shipaihe Complex). The above-mentioned tectonothermal processes lead to the formation of oceanic and continental crusts. Further divergence of the oceanic area formed a new oceanic crust, whereas continental convergence resulted in the establishment of a new continental crust. Such kind of tectonic-magmatic system is similar to modern plate tectonics (i.e., the Wilson cycle). However, due to the high geothermal gradient of the earth at that time, its scale or effect was limited, and the convergence and divergence were much faster and more common. It is comparable to the “pan-Wilson cycle”, and hence named as assembling and dispersal cycles (Chen 1990; Chen and Fu 1992). During 3-2.55 Ga (the Shipaihe cycle), the Taihua Composite Terrane was located in the divergent area and well developed with mafic and ultramafic volcanic rocks (the Beizi greenstone belt). The Beizi-type greenstone belt contains abundant komatiites that are similar to those from the Barberton greenstone belts in South Africa and the Kolar greenstone belt in India (Table 1.3), representing the primitive oceanic crust. At the same time, the Songji Block was located in a convergence zone and developed with intermediate to felsic intrusions with magmatic arc-affinity, such as the trondjemite, tonalite, granodiorite, and diorite of the Shipaihe Complex. Their intrusion destroyed the Qingyanggou-type greenstone belt, inducing metamorphism and partial melting (migmatites). Consequently, some Qingyanggou-type greenstone was included in the Shipaihe Complex. If so, the Shipaihe Complex with granite or upper crust affinity may represent a continental nucleus or terrane, and possibly formed in a primitive magmatic arc setting.

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The Junzhao and Dangzehe Greenstone Belts

The ca. 2550 Ma Shipaihe Orogeny, along with the convergence of the Kenor supercontinent, led to amphibolite to granulite facies metamorphism and variable partial melting of the Shipaihe Complex and the Beizi Group greenstone belt. It also caused the out-growth of the continental nuclei in the SongJi Block. The Beizi Group of the Taihua Composite Terrane underwent matamorphism and partial melting, showing some affinity to modern island arc in lithological composition. After the Shipaihe Orogeny, during 2550–2300 Ma (Neoarchean-Siderian) or the Transition Eon (2600–2300 Ma) suggested by Gradstein et al. (2004), the Taihua Composite Terrane was in a divergent setting, as exemplified by the Dangzehe-type greenstone belt overlapping previous Beizi Group (Table 1.4). Because the Beizi Group is similar to an island arc in lithological composition, the related magmatism shows affinity to the oceanic rift, which is composed of bimodal volcanic rocks dominated by komatiite-bearing basalts, followed by minor andesite (Fig. 1.20). Undoubtedly, the basalt was produced by partial melting of the mantle, while the dacite by partially melting of the Beizi Group, or partly by fractional crystallization of the basalt. During 2550–2300 Ma, the Songji Block was in a magmatic arc setting. The Junzhao Group volcanic-sedimentary formation, i.e., the Junzhao type greenstone belt, was developed on the basement of the Shipaihe Complex (Table 1.4). It is typified by a calc-alkaline assemblage of basalt, basaltic andesite, andesite, dacite, and rhyolite. Basalt and basaltic andesite are predominant, accompanied by subordinate andesite but no ultramafic rocks (Fig. 1.20). Basalts and basaltic andesite result from partial melting of subducted oceanic crust, while andesite and rhyolite may be generated from the melting of the Shipaihe Complex. Both the Junzhao and the Dangzehe groups are greenstone belts developed on metamorphic basement, belonging to the secondary greenstone belts (Table 1.4) as defined by Glikson (1976). BIFs (BIF ¼ banded iron formation) were developed in their upper part, similar to the Dharwar greenstone belt in India. Compared to the Junzhao Group, the Dangzehe Group has more mafic composition and linear distribution. It also underwent much higher grade metamorphism and partial melting, indicating an unstable upper crustal basement. It is worth noting that, in the Xiaoshan Terrain of the Huaxiong Block, contemporaneous rocks are the Tianyemiao migmatitic gneiss and migmatite. They are very different from the Dangzehe or Junzhao groups, but similar to the Sushui Group of Zhongtiao Block (Sun et al. 1991). Therefore, we propose that before 2300 Ma, the Xiaoshan terrain was part of the Zhongtiao Block instead of the Huaxiong Block (Fig. 1.22).

2.55–2.3 Unconformably overlying an unstable basement composed of >2.55 Ga Beizi Group

Age (Ga) Basement age, nature and contact

Junzhao Gp Songji block Belts with length of 20–60 km, dome or open fold Basalt, dacite, andesite, rhyolite, graywacke, conglomerate, pelite, qaurtzite, BIF, carbonate Basal conglomerate–sandstone ! mafic-felsic volcanic rocks ! shallow marine sediments 2.55–2.3 Unconformably overlying a stable basement composed of >2.55 Ga Shipaihe Complex 2.6–2.3 Unconformably overlying a stable basement composed of ~3.5 Ga Peninsular Gneissess

Dharwar Gp India Peninsula Open structure, belts up to 400 km long Graywacke, conglomerate, qaurtzite, BIF, basalt, andesite, rhyolite, volcanic rocks Basal shelf sediments ! turbidite + volcanic rocks

2.7–2.5 Unconformably overlying a stable to unstable basement composed of >3.5 Ga Shabani Gneisses

Bulawayan Gp Zimbabwe Open structure composed of small to medium-size linear folds Basalt, dacite, rhyolite, andesite, komatiite, qaurtzite, conglomerate, graywacke, pelite, BIF Cyclic ultramafic ! felsic volcanic rocks ! sediment

Reprinted from Chen et al. 1988 and references therein Copyright 1998 China Academic Journal Electronic Publishing House

Sequence

Petrology

Dangzehe Gp Huaxiong block Belts with length of 5–50 km, linear fold Basalt, komatiite, andesite, dacite, graywacke, conglomerate, qaurtzite, BIF, carbonae, pelite Conglomerate– sandstone ! mafic–felsic volcanic rocks ! chemical sediment

Greenstone Location Occurrence

Table 1.4 Comparison of the secondary greenstone belts

1 Geological Evolution of Qinling Orogen 39

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Fig. 1.20 SiO2 histograms of the Dangzehe and Junzhao greenstone belts. (Reprinted from Chen and Fu 1992 Copyright 1992 China Seismological Press) Fig. 1.21 SiO2 histogram of the Xiaoshan Group volcanic rocks. (Reprinted from Chen and Fu 1992 Copyright 1992 China Seismological Press)

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1.3.5

41

Rhyacian Stratigraphic Unit and the Divergence of Xiaoshan Terrane

At the end of or slightly after the Junzhao and Dangzehe groups, i.e., ~2300 Ma, the Guojiayao Orogeny occurred in SNCC. At the same time, the global geological environment changed significantly, with the atmosphere and hydrosphere changing from reducing to oxidizing (Chen 1988, 1990), i.e., the Great Oxidation Event (GOE; Holland 2002). After the GOE, the geothermal gradient and the atmospheric temperature decreased. The lithosphere became relatively stable, and carbonatebearing strata were extensively developed. Volcanism became less intensive, whereas large-scale mafic dikes and sheets were emplaced. The period between 2300 and 2050 Ma is called Rhyacian. At that time, the Taihua Composite Terrane was well developed with the Shuidigou Group khondalite series, which are important hosts of graphite, sillimanite, and BIF iron deposits. The protolith was considered as carbonaceous carbonates, siliceous rocks, mudstones, interlayered with minor bimodal alkalic olivine basalt and rhyolite (Chen et al. 1988; Ji and Chen 1990). Contemporanous rocks, i.e., the Xiaoshan Group from the Xiaoshan Terrane (Chen et al. 1989a), the Angou group from the SongJi Block (Sun et al. 1985; Guo 1987), and the Jiangxian Group from the Zhongtiao Block (Sun et al. 1991), are comparable. They are all dominated by bimodal volcanic rocks developed in a rifting setting (Fig. 1.21), interlayered with minor clastic rocks, BIFs, and carbonates. The volcanic rocks yield isotopic ages of 2300–2050 Ma (Hu et al. 1988; Li et al. 2015a and references therein). It is important to note that, the Xiaoshan Group is comparable to the Angou Group from the Songji Block and the Jiangxian Group from the Zhongtiao Block in terms of their greenschist facies assemblage, but different from the Shuidigou Group high amphibolite to granulite-facies rocks from the Taihua Composite Terrane (Table 1.1). These bimodal volcanic rocks from different blocks/terranes of the SNCC display consistent isotopic ages of 2250–2100 Ma, which are consistent with the ages of the Hutuo Group from Wutai Shan, inner NCC as well as other cratonic Rhyacian volcanic rocks worldwide (Tang and Chen 2013; Chen et al. 2018). This implies that the Rhyacian bimodal volcanic rocks in the Xiaoshan, Songji, and Zhongtiao blocks were formed in the same rifting system, which possibly caused the separation of the Xiaoshan Terrane to form the Zhongtiao Block (Fig. 1.22). Hence, in SNCC, the ~2050-Ma Songyang Orogeny terminated the development of Rhyacian khondalite series and rifting-related bimodal volcanic rocks, as well as the GOE.

1.3.6

Orosirian Stratigraphic Unit and Cratonization

During the Orosian period (2050–1800 Ma), the Songyang Orogeny (ca. 2050 Ma) and subsequent Zhongyue Orogeny (ca. 1850 Ma) led to the amalgamation of the

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Fig. 1.22 Paleoproterozoic divergence and convergence of terranes or blocks in SNCC. (Modified from Chen and Fu 1992 Copyright 1992 China Seismological Press)

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Xiaoshan Terrane and the Taihua Composite Terrane, forming the Huaxiong Block (Fig. 1.22b,c). Its further amalgamation with the Zhongtiao and SongJi blocks constitutes the Precambrian basement of SNCC. Such orogenies also brought the amphibolite-granulite facies metamorphism of the Taihua Composite Terrane, as well as the greenschist facies metamorphism of the Xiaoshan terrane and the Zhongtiao and SongJi blocks. After the Songyang Orogeny, the rocks older than 2.1 Ga commonly underwent metamorphism and the upper crust became more felsic. A mature, crystalline basement was formed, and the cratonization came to an end. In the Songji and the Zhongtiao blocks, the Songshan and Zhongtiao groups were deposited respectively, which are characterized by assemblages of quartz conglomerates and sandstone (named as “Songshan quartzite”). They were typical of cratonic basin formation, indicating high crustal maturity. By contrast, there was little or no sedimentation in most parts of the Huaxiong Block (e.g., Wuyang, Lushan, Xiong’ershan, Xiaoqinling). Limited intermountain molasses (i.e., the Tietonggou Group) outcropped in the northern Xiaoshan and the Bayuan area of the Lishan terrane. It also exhibits high crustal maturity, as revealed by rock assemblages of quartz conglomerate, quartz sandstone, etc. The ca. 1850 Ma Zhongyue Orogeny was a global orogenic event, which resulted in the convergence of the Nuna supercontinent, the ending of independent evolution of different terranes or blocks in SNCC, and the metamorphism and deformation of the Orosirian tectonostratigraphic unit. To sum up, the crystalline basement of the SNCC was entirely formed at the ~1850-Ma Zhongyue Orogeny.

1.4 1.4.1

Tectonic Setting of Xiong’er and Xiyanghe Groups: Application of Differentiation Index Preamble

The Xiong’er Group and/or Xiyanghe Group (occasionally combined into the Xiong’er Group) outcrops in the SNCC, with a thickness from 3.0 to 7.6 km and an inferred areal extent of about 60,000 km2 (Fig. 1.23; Hu et al. 1988; Chen and Fu 1992). It is a volcanic sequence slightly metamorphosed, locally deformed, unconformably overlying the pre-Orosirian basement (Taihua Supergroup), and overlain by the Mesoproterozoic sedimentary succession of the Guandakou and Ruyang Groups (Chen and Fu 1992). The Xiong’er Group comprises basaltic andesite, andesite, dacite, and rhyolites (Sun et al. 1981, 1985; Hu et al. 1988; Jia 1987; Jia et al. 1988; He et al. 2009). Available SHRIMP and LA-ICP-MS zircon U-Pb age data indicate that the Xiong’er Group volcanic rocks erupted intermittently over protracted intervals from 1.84 Ga, through 1.78–1.75 Ga, to 1.65 Ga, and 1.45 Ga, but the major phase of the volcanism occurred at 1.78–1.65 Ga (Sun et al. 1991; Zhao et al. 2009; He et al. 2009; Cui et al. 2011). The Xiong’er Group is generally

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Fig. 1.23 Spatial distribution of the Xiong’er Group, SNCC. (Reprinted from Chen and Fu 1992 Copyright 1992 China Seismological Press)

accepted as Statherian volcanic rocks formed after the assembly of the Columbia Supercontinent (Rogers and Santosh 2002, 2009; Zhao et al. 2002a, 2004a; Santosh 2010; Meert and Santosh 2017), related to the outward growth of the NCC (Chen et al. 1992; Deng et al. 2013a, 2013b; Li et al. 2014, 2015) or breakup of the Columbia Supercontinent (Peng et al. 2007, 2008). The tectonic setting of the Xiong’er/Xiyanghe Group is debated as having been associated in a continental arc (Hu et al. 1988; Jia et al. 1988; Chen et al. 1992; Zhao et al. 1994; Zhao et al. 2002a, 2004a, 2009; He et al. 2009; Deng et al. 2013a, 2013b), continental rift (Sun et al. 1981, 1985; Zhang et al. 2001) or mantle plume (Zhao et al. 2002b; Peng et al. 2008; Peng 2010). It is peculiar that these contrasting models are drawn from the same or a similar geological, petrological and geochemical dataset, which strongly calls for a reassessment of the current understanding of the linkage between volcanic associations and tectonic settings. In this section, we will introduce these models, and attempt to setup a new link between volcanic associations and tectonic settings, which may help to understand the tectonic setting of the Xiong’er Group volcanic rocks. We also develop a tectonic model of coexisting continental arc and passive rift to illustrate the Statherian magmatism in SNCC.

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1.4.2

Tectonic Models of the Xiong'er and Xiyanghe Groups

1.4.2.1

Rift or Mantle Plume?

45

Sun et al. (1981) first recognized the bimodal features of the Xiong’er Group (Xiyanghe Group is included) and proposed that the Xiong’er Group volcanic rocks were formed in a failed arm (aulacogen) of a three-armed rift system. The model is supported by the following facts: 1. The volcanic rocks of the Xiong’er and Xiyanghe groups are distributed in a triangular area (Fig. 1.24a), with the Luanchuan, Jiangxian-Tongguan, and Luoyang-Wuyang faults being their southern, northwestern, and northeastern boundaries, respectively. The isopachs of the volcanic rocks strike NNE, with the Xiong’er-Xiaoshan area as the center. Such an outcropping scenario of the volcanic rocks resulted from a three-arm rifting system (Fig. 1.24b), of which two arms evolved into the Kuanping Ocean (Fig. 1.24c). 2. As addressed by Sun et al. (1981, 1985), most samples of volcanic rocks plot in the area of alkaline series on the (Na2O + K2O) versus (Na2O/K2O) diagram. The non-alkalic volcanic rocks display the features of the Fenner-type tholeiite series on the correlation diagrams of SiO2, TiO2, and TFeO (total FeO) with increasing TFeO/MgO; their TiO2 contents are higher than the island-arc tholeiites at a given TFeO/MgO ratio. The K2O concentrations are generally higher than 1.5%, and also higher than those of island-arc tholeiites. 3. The Na2O/K2O ratios of the volcanic rocks decrease with increasing SiO2 contents. Moreover, the decreasing of Na2O/K2O ratios drastically changed

Fig. 1.24 Spatial distribution of the Statherian (Xiong’er and Xiyanghe Gps.) volcanic rocks (a; Modified from Sun et al. 1985 Copyright 1985 China Metallurgical Industry Press) and the hypothetic tectonic setting evolution (b and c)

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Table 1.5 Na2O/K2O ratios of Xiong’er Group volcanic rocks SiO2 (%) 68

Xiong’er Gp., Henan Range/average 0.59–2.16/1.26 0.60–3.65/1.34 0.35–0.70/0.56 0.02–0.16/0.09

Xiong’er Gp., Shaanxi Range/average 1.30–2.47/1.86 0.88–1.84/1.51

Kuanping Gp. Range/average 3.41–14.5/8.60

0.32–0.34/0.33

0.41-1.40/0.80

Reprinted from Sun et al. 1981 Copyright 1981 China Academic Journal Electronic Publishing House

around SiO2 ¼ 65%, i.e., the Na2O/K2O ratios of the volcanic rocks with SiO268% (Table 1.5), which is the “bimodal” system of Sun et al. (1981, 1985). Sun et al. (1981) first confirmed the bimodal signature of the volcanic rocks of the Xiong’er Group, although they did not show a histogram of differentiation index (DI) or SiO2 content. They successfully concluded that the two classes of volcanic rocks could not originate from magmatic differentiation and that the felsic class originated from partial melting of the continental crust, whilst the rocks with SiO270.00 27 10.34

378 samples of Yan (1985) SiO2 content (wt%) Num. 46.60–52.20 32 52.22–59.89 199 60.14–64.44 47 64.82–70.28 70 70.64–78.24 30

% 8.46 52.65 12.43 18.52 7.94

Reprinted from Chen and Fu 1992 Copyright 1992 China Seismological Press

3.

4.

5.

6.

with minor rhyolite and trace basalt (Table 1.7), which seems to be a unimodal volcanic association similar to those formed in magmatic arcs. Ultramafic rocks, which are common in a volcanic association formed in a rift or mantle plume setting, have not been observed yet in the Xiong’er Group. This feature of lithologic association of the Xiong’er Group has the consensus of many geologists (Chen et al. 1992; Zhao et al. 1994; He et al. 2009; Wang et al. 2010a) although they interpret the tectonic setting in contrasting ways (Sect. 1.4.2.3). Jia et al. (1988) reported a histogram for the differentiation indices (DI) of the volcanic rocks from the Xiong’er Group (Fig. 1.25). The histogram shows the unimodal feature similar to the Cascades continental arc, but contrasting to the Afar (Ethiopia) triple junction rift system (Fig. 1.25). Therefore, they argued that the Xiong’er Group is a unimodal association dominated by andesites formed in a continental arc. These authors also proposed that the Kuanping oceanic plate subducted northward beneath the southern margin of the North China Craton, resulting in the formation of the Xiong’er Group (including Xiyanghe Group) volcanic rocks in a continental arc setting. According to Yan (1985), the volcanic rocks of the Xiong’er Group yield Peacock alkaline-lime index of 52.6 and Rittmann index (σ) clustering 1.8–4, suggesting an alkaline-calc or a calc-alkaline series enriched in alkalis. The σ values calculated by Jia et al. (1988) range from 1.5 to 3.6 (Table 1.8), suggesting that the rocks belong to the calc-alkaline series. The σ values decrease gradually with increasing SiO2 contents (Table 1.8), implying a continental basement underlying the Xiong’er Group volcanic rocks (Hu et al. 1988). On the discrimination diagrams of SiO2—(Na2O + K2O), Hydman AFM, Miyashiro TiO2—TFeO/MgO, Pearce Er/Y—Er and Varne Nb—Ba and La— Ba, the Xiong’er Group volcanic rocks plot in the field of the island or continental arc or show calc-alkaline trends (for detail see Jia et al. 1988). The K2O contents and the ratios of K2O/Na2O, K2O/SiO2, (K2O + Na2O), and (K2O + Na2O)/Al2O3 of the Xiong’er Group volcanic rocks increase northward (Table 1.9, Fig. 1.26; Jia et al. 1988). This petrochemical polarity seems to accord well with the tectonic model of northward oceanic plate subduction beneath the SNCC (Fig. 1.27).

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Fig. 1.25 The DI histograms for volcanic rocks from Xiong’er Group in SNCC (a; Reprinted from Jia et al. 1988 Copyright 1988 China Academic Journal Electronic Publishing House) and typical tectonic settings in the world (b–e; Reprinted from Martin and Piwinskii 1972 Copyright 1972 John Wiley & Sons)

Table 1.8 The Rittmann Index (σ) of the Xiong’er Group Lithology SiO2 (wt%) σ

Basalt 44.0–53.5 3.59
1.67

Andesite 53.5–62.0 3.07
1.17

Dacite 62.0–70.0 2.46
0.75

Rhyolite >70.0 1.50
0.66

Reprinted from Jia et al. 1988 Copyright 1988 China Academic Journal Electronic Publishing House

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Table 1.9 Chemical composition of the Xiong’er Group K2O (%) Na2O (%) Al2O3 (%) SiO2 (%) K2O/Na2O Na2O/SiO2 K2O/SiO2 K2O + Na2O (%) (K2O + Na2O)/Al2O3

South Luoning (n ¼ 23) 3.22 2.63 13.64 63.14 1.22 0.42 0.051 5.85 0.429

Xiashi (n ¼ 11) 3.26 2.58 13.77 61.51 1.26 0.42 0.053 5.84 0.424

Jiyuan (n ¼ 43) 3.79 2.94 13.65 59.22 1.29 0.50 0.064 6.73 0.493

Reprinted from Jia et al. 1988 Copyright 1988 China Academic Journal Electronic Publishing House Fig. 1.26 Northward variation in petrochemical composition of Xiong’er Group (The location of S Luoning, Xiashi and Jiyuan shown in Fig. 1.23)

It is clear that the continental arc tectonic model is applicable to most of the characteristics of the Xiong’er Group, but the following data have been not well interpreted yet:

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Fig. 1.27 Tectonic model for the Xiong’er Group. (Modified from Deng et al. 2013a Copyright 2013 Elsevier)

1. If the bimodal feature of the Xiong’er Group revealed by Sun et al. (1981, 1985) is unlikely. If a continental arc accommodates a drastic change of K2O/Na2O ratios around the SiO2 contents of 62–66%. 2. Given that the volcanic rocks of the Xiong’er and Xiyanghe groups belong to the same continental arc, the parameters, such as K2O/SiO2, (K2O + Na2O), K2O/ SiO2, (K2O + Na2O)/Al2O3 and K2O/Na2O, should progressively change northward. From Table 1.9 and Fig. 1.26 we can see that the volcanic rocks from the Southern Luoning and Xiashi areas belong to the Xiong’er Group and show very similar values, but they remarkably differ from the Xiyanghe Group volcanic rocks from the Jiyuan area, suggesting that the Xiong’er and Xiyanghe groups are possibly formed in different tectonic settings. 3. The SiO2 contents in volcanic rocks from the Xiyanghe Group at Jiyuan area are lower than those from the Xiong’er Group at Southern Luoning and Xiashi areas (Figs. 1.23 and 1.26). The Xiyanghe Group has significantly more basalts and basaltic andesites than the Xiong’er Group (Zhang et al. 1985). These phenomena disagree with the active continental margin model. 4. The spatial distribution of the Xiong’er and Xiyanghe groups show somewhat a triangular shape, which does not match up well with the continental arc model. Sun et al. (1981) defined that the isopachs (thickness-contours) of the volcanic rocks strike NNE, which disaccords with a continental arc model where the volcanic rocks should be NNW-trending and parallel to the trench (i.e., the Luanchuan Fault).

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Coexistence of Continental Arc and Passive Rift

As addressed above, the lithology and tectonic setting of the Statherian volcanic rocks (Xiong’er and Xiyanghe Gps.) have been hotly debated between two contrasting viewpoints: (1) a bimodal association developed in continental rift system (e.g., Sun et al. 1981, 1985), and (2) a unimodal association generated in a continental arc (e.g., Jia et al. 1988). Chen et al. (1992) carefully investigated the sampling locations and lithologies reported in previous studies and noted that all the geologists who focus on the Xiyanghe Group agreed to a bimodal volcanic association formed in a rift setting (e.g., Zhang et al. 1985; Guan et al. 1988; Yang 1990; Sun et al. 1991), whereas the researchers who mainly study the Xiong’er Group tend to consider a unimodal association developed in a continental arc (e.g.Yan 1985; Hu et al. 1988; Jia et al. 1988), except for a few researchers who insist that the Statherian volcanic rocks are a rift-setting bimodal association (Zhang et al. 2002a; Wang et al. 2010a). Therefore, it is still needed to examine whether the Xiong’er Group is bimodal or unimodal, considering that there is a consensus that the Xiyanghe Group represents a rift-setting bimodal volcanic association with less felsic components than the Xiong’er Group (Table 1.9; Fig. 1.26). Chen and Fu (1992) conducted a geologic profile survey in the Xiong’er Terrane and collected and analyzed 20 samples. Their DI, and K2O, MgO, (K2O + Na2O), and (MgO + CaO) values clearly show bimodalities (Fig. 1.28). The left and right peaks occur in DI ranges of 40–65 and 80–90, respectively, with the left peak being bigger than the right (Fig. 1.28e). Seventeen samples of the left peak have SiO2 contents of 49.77–60.05%, belonging to andesites or basaltic andesites. Three

Fig. 1.28 Geochemical bimodalities of the Xiong’er Group. (Reprinted from Chen and Fu 1992 Copyright 1992 China Seismological Press)

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Fig. 1.29 Histograms of SiO2% and DI of the Xiong’er Group. (Reprinted from Zhao et al. 1994 Copyright 1994 China Academic Journal Electronic Publishing House)

Fig. 1.30 Histogram of SiO2 contents and lithologic association. (Reprinted from Wang et al. 2010a Copyright 2010 Elsevier)

samples of the right peak have SiO2 contents of 64.0%, 66.50%, and 72.25%, respectively, corresponding to dacites and rhyolites. The bimodality of the Xiong’er Group (taking DI ¼ 70 as criteria) also appeared in the DI histogram (Fig. 1.28f) reported by Jia et al. (1988), though it was ever neglected. Zhao et al. (1994) and Wang et al. (2010a) collected 836 and 1032 analyses of the Xiong’er Group volcanic rocks, respectively, and powerfully proved their bimodality (Figs. 1.29 and 1.30). The bigger left peak comprises andesites ad basaltic andesites; the smaller right peak consists of dacites and rhyolites. Zhao et al. (1994) also recognized that the Xiong’er group is a bimodal volcanic sequence with two peaks around the DI values of 52 and 85, or the SiO2 contents of 57% and 70%. To sum up, the Xiong’er Group is definitely a bimodal volcanic association formed in a continental arc, rather than the unimodal as addressed in previous studies; the bimodality of Xiong’er Group is dominated by andesites, contrasting

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Fig. 1.31 Coexisting continental arc and passive rift in SNCC in Statherian

to the bimodality dominated by basalts that originate from the mantle plume or continental rift settings. In terms of the lithology, geochemistry, and spatial distribution of the Xiong’er and Xiyang groups, Chen and Fu (1992) proposed that the Statherian volcanic rocks in SNCC developed in a unique setting of coexisting magmatic arc and passive rift (Fig. 1.31). The main supporting factors are introduced below: 1. The basement structure of SNCC favors coexisting continental arc and passive rift. As addressed above, the crystalline basement of SNCC was finally formed by the terrane amalgamation during Orosirian orogenesis. The suture zones were easily evolved into a rift or an extensional basin, such as the San-Bao Fault between the HuaXiong and SongJi and Zhongtiao blocks, and the San-Jin Fault between the Zhongtiao and SongJi blocks (Figs. 1.18, 1.22 and 1.23). In the Statherian period, the San-Bao Fault was a compressional shear zone, and the San-Jin Fault was an extensional zone. Such a stress field definitely favors the development of continental arc in the HuaXiong Block, and a passive rift between the Zhongtiao and SongJi blocks (Fig. 1.23). 2. The regional geology supports the coexistence of a continental arc and a passive rift. The ophiolite slice-containing Kuanping Group and the andesite-dominated Xiong’er Group yield similar isotope ages (Tables 1.10, 1.11) and occur in the south and north sides of the Luanchuan Fault (Figs. 1.8 and 1.24), respectively, which is an analogue of the modern Andean trench-arc system. Considering that the subduction stress and speed were not equally-distributed along the trench, it could be envisaged that the NNE-directed subduction most intensely occurred at Xiong’er-Xiaoshan area, which caused an NNE-directed passive rift along the San-Jin Fault in the north side of the San-Bao Fault. The passive rift is nearly perpendicular to the striking of the continental arc. Therefore, this tectonic setting resulted in the NNE-trending isopachs of the Xiong’er Group at Xiong’erXiaoshan area, and the development of the Xiyanghe Group along the San-Jin fault (Fig. 1.23). 3. The Statherian volcanic rocks (Xiong’er and Xiyanghe groups) outcrop in an “inverted T-shape” (Fig. 1.23), instead of a triangle area (Sun et al. 1981). The Xiong’er Group occurs only in a WNW-trending arc-shaped belt between the Luanchuan and San-Bao faults. The Xiyanghe Group develops in an NNE-trending belt along the San-Jin Fault, but it is limited to the north of the

1 Geological Evolution of Qinling Orogen

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Table 1.10 Isotope ages for the Xiong’er Group No 1

15

Sample geology and location Dacite porphyry, Xushan Fm., Zhongtiaoshan Dacite porphyry, Xushan Fm., Zhongtiaoshan Rhyolite, Xushan Fm., Zhongtiaoshan Volcanic rock, Xushan Fm., Zhongtiaoshan Basaltic andesite, Xushan Fm., Zhongtiaoshan Augite diorite intruding Majiahe Fm., Xiong’ershan Augite diorite intruding Majiahe Fm., Xiong’ershan Rhyolite porphyry intruding Majiahe Fm., Xiong’ershan Rhyolite porphyry intruding Majiahe Fm., Xiong’ershan Diorite intruding Xiong'er Gp, Xiongershan Diabase intruding Taihua SGp, Xiongershan Rhyolite porphyry intruding Jidanping Fm., Waifangshan Rhyolite porphyry intruding Majiahe Fm., Waifangshan Basaltic andesite, Xushan Fm., Xiaoshan Dacite, Xushan Fm., Waifangshan

16

Rhyolite, Xushan Fm., Waifangshan

17

Rhyolite, Jidanping Fm., Xiaoshan

18 19

Rhyolite, Jidanping Fm., Waifangshan Dacite, Jidanping Fm., Xiong’ershan

20

Andesite, Majiahe Fm., Xiaoshan

21

Yanyaozhai subvolcanic rock, Xiong’er Gp., Xiongershan Xiong’er Group basaltic andesite, Shaanxi

2 3 4 5 6 7 8 9 10 11 12 13 14

22

Sample and method zircon U-Pb concordia SHRIMP zircon U-Pb Zircon U-Pb upper intercept Rock Rb-Sr isochron (n¼9) SHRIMP zircon U-Pb Zircon U-Pb concordia (n ¼ 3) LP-ICPMS zircon U-Pb (n ¼ 24) Single zircon U-Pb concordia LP-ICPMS zircon U-Pb (n ¼ 10) Zircon SHRIMP U-Pb Zircon SHRIMP U-Pb Zircon SHRIMP U-Pb Zircon SHRIMP U-Pb Zircon SHRIMP U-Pb Zircon SHRIMP U-Pb Zircon LA-ICP-MS U-Pb Zircon LA-ICP-MS U-Pb Zircon LA-ICP-MS U-Pb Zircon SHRIMP U-Pb Zircon LA-ICP-MS U-Pb SHRIMP U-Pb Zircon LA-ICP-MS U-Pb

Age (Ma) 1826
32 1840
14 1829
17 1635
6 1767
47 1761
16 1550–3080 1959
44 1685–2745 1789 + 26/ 20 1773
37 1800
16 1776 + 20/ 19 1783
20 1783
13 1778
8 1778
5.5 1751
14 1445
14/ 1450
31 1778
6.5 1781
12 1810
41

Reference Sun et al. (1991) Sun et al. (1991) Sun et al. (1991) Sun et al. (1991) He et al. (2009) Zhao et al. (2001) Zhao et al. (2001) Zhao et al. (2001) Zhao et al. (2001) Zhao et al. (2004b) Zhao et al. (2004b) Zhao et al. (2004b) Zhao et al. (2004b) He et al. (2009) He et al. (2009) He et al. (2009) He et al. (2009) He et al. (2009) He et al. (2009) He et al. (2009) Liu et al. (2011b) Huyan and Lu (2016) (continued)

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Table 1.10 (continued) No 23 24 25 26 27 28 29

Sample geology and location Guandaokou quartz syenite porphyry, Xiong'er Gp., Xiaoshan Yanyaozhai syenogabbro, Xiong'er Gp., Xiong’ershan Miaoling aegirine-augite syenite, Xiong'er Gp., Xiong’ershan Quartz diorite intruding Xiong'er Gp., Xiaoshan Quartz diorite intruding Xiong'er Gp., Xiaoshan Granite porphyry intruding Xiong'er Gp., Xiaoshan Rhyolite porphyry intruding Xiong’er Gp., Waifangshan

Sample and method Zircon U-Pb

Age (Ma) 1731
29

Zircon U-Pb

1750
65

Zircon U-Pb

1644
14

Baddeleyite SIMS U-Pb Zircon SIMS U-Pb

1789.4
3.5 1778
12

Zircon SIMS U-Pb

1786.4
7.7

Zircon LA-ICP-MS U-Pb

1763
15

Reference Ren et al. (2000) Ren et al. (2000) Ren et al. (2000) Cui et al. (2010) Cui et al. (2010) Cui et al. (2010) Wang et al. (2010a)

Sanmenxia city (Fig. 1.23). These two belts meet at a nearly right angle in the vicinity of Sanmenxia, where the center of the supposed triangle should be, but here neither the Xiong’er Group nor the Xiyanghe Group has been observed yet. This indicates that these two groups likely do not connect with each other, and do not belong to one volcanic association developed in the same tectonic setting; instead, they possibly belong to two isolated volcanic associations developed in distinctive tectonic settings. Hence, such an “inverted-T-shape” distribution pattern most likely resulted from the coexistence of a continental arc and a continental rift. 4. The Mesoproterozoic topographic feature and sedimentation suggest a coexistence of continental arc and rift (Fig. 1.9). The Mesoproterozoic Guandaokou Group developed in a fore-arc basin on the south side of the Xiong’er Group, while the Ruyang group developed in the back-arc basin on the north side of the Xiong’er Group along the San-Bao Fault. The Ruyang Group also developed an NNE-trending belt along the San-Jin Fault or the border of Henan and Shanxi provinces. This spatial distribution scenario of the Mesoproterozoic strata shows that the area developed with the Xiong’er Group was generally a WNW-trending uplift belt, whereas the area developed with the Xiyanghe Group was then a depression belt or a trough (rift). 5. The petrology and geochemistry of the Xiong’er Group (Yan 1985; Jia et al. 1988; Hu et al. 1988; Chen et al. 1992; He et al. 2009) show typical features of andesite-dominated volcanic association developed in magmatic arcs, which sharply differ from worldwide rift-setting volcanic rocks characterized by basalt predominance and andesite shortage. The high K2O contents notated by Sun et al. (1985) could be related to the regional high-K2O background in China, particularly, in SNCC. As shown in Table 1.9, the K2O contents of basalt, andesite, and dacite in the basement underlying the Xiong’er Group are generally higher than

1 Geological Evolution of Qinling Orogen

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Table 1.11 Isotope ages for the Kuanping Group 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Sample lithology Greenschist Plagioclase amphibolite Biotite quartz schist Biotite quartz schist Biotite quartz schist Biotite quartz schist Biotite quartz schist greenschist Plagioclase amphibolite Metamorphic basalt Plagioclase amphibolite Plagioclase amphibolite Plagioclase amphibolite Plagioclase amphibolite greenschist Plagioclase amphibolite Metamorphic basalt Biotite quartz schist Biotite quartz schist Biotite quartz schist Albite- actinolite schist Plagioclase amphibolite Greenschist

Sample zircon zircon zircon zircon zircon zircon biotite Whole rock Amphibole Whole rock Amphibole Amphibole Whole rock Amphibole Whole rock Whole rock Whole rock Whole rock Whole rock Whole rock Whole rock Whole rock zircon

Method U-Pb U-Pb U-Pb U-Pb U-Pb Pb-Pb K-Ar Rb-Sr K-Ar Rb-Sr K-Ar K-Ar Sm-Nd K-Ar Sm-Nd Sm-Nd Rb-Sr Rb-Sr Rb-Sr Rb-Sr Sm-Nd Sm-Nd U-Pb

Age (Ma) 1827
11 1753
14 1974 1741 1681 1730 1872 1704 1516 1411
30 1404 1393 1382
30 1250 1085
44 1153
28 1442 1089 1021 1004 1142
18 986
169 943
6

Reference Li (2002) He et al. (2007) Zhang (1987) Zhang (1987) Zhang (1987) Zhang (1987) Zhang (1987) Zhang (1987) Zhang (1987) Gao et al. (1989) Jia et al. (1988) Dong (1986) Pei et al. (1997) Dong (1986) Zhang et al. (1991) Zhang et al. (1991) Wang (1987) Wang (1987) Wang (1987) Wang (1987) Zhang et al. (1994) Zhang and Zhang (1995) Diwu et al. (2010)

the same rocks in the world. This possibly reflects a high-K2O mantle in SNCC. The volcanic rocks with SiO268%) and high K2O/Na2O ratio. If so, the bimodality of the Xiong’er Group is typical of continental arcs, which is further enhanced by the DI-histogram for the Cascade Range in North America, to be further discussed in Sect. 1.4.3. 6. As mentioned in Sect. 1.4.2.2, the K2O, Na2O, and (K2O + Na2O) contents and the K2O/Na2O, Na2O/SiO2, and (Na2O + K2O)/Al2O3 ratios of the Xiong’er Group do not show remarkable differences among different areas, but they clearly differ from those of the Xiyanghe Group, indicating that these two groups belong to different volcanic formations formed in different tectonic settings Compared to the Xiong’er Group, the Xiyanghe Group is more mafic and alkaline, and more

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Table 1.12 Strontium isotope composition of the Xiong’er and Xiyanghe groups Samples and locations Yingou, Wuyang, Henan (by Beijing Institute of Geology, Chinese Academy of Sciences) 8 samples from Jidanping Fm, Yuanqu, Shanxi

Age (Ma) 1710
74

(87Sr/86Sr)I 0.712

951
56

0.7101
7

9 samples from Xushan Fm., Jiangxian, Shanxi

1635
6

0.7072

7 samples from Xushan Fm., Yuanqu

1459
48

0.7071
6

4 samples from Majiahe Fm., Yuanqu

1439
35

0.7069
4

11 samples from Xushan & Majiahe Fms.

1454
36

0.7070
4

4 spilite samples from Shanxian, Henan

1650
26

0.7061
11

1 spilite sample from Shanxian, Henan

(1650)

0.7059

1 sample from Jinduicheng, Shaanxi

(1650)

0.7034

References Hu et al. (1988) Qiao et al. (1985) Sun et al. (1991) Qiao et al. (1985) Qiao et al. (1985) Qiao et al. (1985) Huang and Wu (1990) Huang and Wu (1990) Huang and Wu (1990)

likely was developed in a continental rift. The Xiyanghe Group contains more basalt, basaltic andesite, and rhyolite than the Xiong’er Group, and shows bimodal features similar to those of continental rifts. Zhang et al. (1985) confirmed that the pillow lavas of the Xiyanghe Group in the Yuanqu area were mainly composed of olivine-basalt and basalt, which must be generated from the partial melting of the mantle in a rifting setting. As mentioned above, basalt and olivine basalt have not been identified in the Xiong’er Group. 7. It is well accepted that the Xiong’er and Xiyanghe groups were formed in the period of 1800–1600 Ma, although the SHRIMP or LA-ICPMS zircon U-Pb isotope ages could be up to 1849 Ma (Sun et al. 1991), and low to 1450
31 Ma (He et al. 2009). Available (87Sr/86Sr)I values cluster 0.7059–0.7072 (Table 1.12), which are identical to the values (0.703–0.710) of the igneous rocks from magmatic arcs. Two values are higher than 0.710, of which one is attached with an unacceptable age, and the other is reported without full geological and analytical information. If these two data are geologically meaningful, they possibly represent the magma sourced from the continental crust. It is clear that the tectonic model of coexisting continental arc and rift accords with the characteristics of the Xiong’er and Xiyanghe groups. The model can help to understand a series of puzzling issues which could not be explained only using continental arc or rift models.

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1.4.3

59

Linking Igneous DI Population with Tectonic Settings

From the above, we can see that the Xiong’er Group is a bimodal volcanic association (Sun et al. 1981, 1985; Guan et al. 1988; Chen and Fu 1992; Zhao et al. 1994; Wang et al. 2010a), although several researchers considered it is unimodal (Yan 1985; Jia 1987; Jia et al. 1988; Hu et al. 1988). Its tectonic setting is controversially interpreted to be a continental rift (Sun et al. 1981; Wang et al. 2010a), continental arc (Yan 1985; Jia et al. 1988; Hu et al. 1988; Chen and Fu 1992; Zhao et al. 2002a, 2004a, 2009; He et al. 2009), lagging post-subduction arc (Zhao et al. 1994), or a mantle plume (Zhao et al. 2002b; Pirajno and Chen 2005; Pirajno 2013; Peng et al. 2007, 2008; Peng 2010). Such contrasting tectonic models drawn from similar geological, petrological and geochemical datasets obtained from the same volcanic association strongly call for a reassessment of the current understanding of the linkage between volcanic associations and tectonic settings.

1.4.3.1

Igneous Differentiation Index (DI) as an Indicator of Tectonic Setting

Petrology, mineralogy, element, and isotope geochemistry of igneous rocks are popularly used in the study of tectonic settings and evolution. In general, petrology, mineralogy, and element geochemistry record the features of igneous rocks and magmatic processes; whilst the isotope ratios, particularly Nd and Hf isotopes, record the nature of both the igneous rocks and their sources. Sometimes, the Nd and Hf isotopes record the ancestral nature of igneous rocks, instead of the igneous rocks themselves. For example, most of the Phanerozoic rocks from North Xinjiang show positive εNd(t) and εHf(t) values, and thus indicate a source of the depleted mantle (Wu et al. 2017a, b, and references therein). In this situation, petrology, mineralogy, and element geochemistry may be a more accurate tracer of the genesis of igneous rocks. The differentiation index (DI) must be a powerful genetic indicator because it comprehensively links the petrology, mineralogy, and main element composition of igneous rocks. The concept of Differentiation Index (DI) was introduced by Thornton and Tuttle (1960) to quantitatively describe the differentiation degree of a crystallizing silicate magma system. According to the Bowen reaction series (c.f. Thornton and Tuttle 1960), the final end-member product of crystallization differentiation of any silicate magma is the SiO2-NaAlSiO4-KAlSiO4 system, and that the final minerals must be exclusively some of the six minerals of quartz, albite, orthoclase, leucite, nepheline, and kalsilite. The DI is defined as the sum of these six minerals and reflects the mafic or the silicic (felsic) nature of igneous rocks. The DI value of the end-member SiO2NaAlSiO4-KAlSiO4 system is 100, and of pure pyroxenite is zero. Generally, the mafic (basalt), intermediate (andesite) and felsic (rhyolite) rocks generally yield DI values of 70, respectively.

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The DI is a useful tool to holistically investigate the lithologic association of an igneous suite, particularly, a volcanic sequence (Condie 1982). Martin and Piwinskii (1972) made DI histograms for the Cascades continental arc, the Aleutians island arc, the East African (Afar) continental rifts, and the Iceland oceanic ridge, and thus linked the DI populations of volcanic rocks with their tectonic settings. The unimodal volcanic rocks (only one peak in DI histogram) dominated by andesite are related to the island and/or continental arcs, i.e., compressional settings; whereas the bimodal (two peaks) volcanic rocks composed mainly of basalts and rhyolites are related to extensional or rifting settings. Since then the DI values of volcanic rocks have been widely used to trace tectonic settings (e.g., Condie 1982; Jia et al. 1988; Hu et al. 1988; Chen and Fu 1992).

1.4.3.2

Continental and Island Arcs

In the DI histograms of volcanic rocks developed under different tectonic settings (Fig. 1.25), the Xiong'er Group and the Cascades volcanic sequences show similar bimodal scenarios. They both have a big peak in the range of DI ¼ 35–70, a small peak of DI>70, and a valley around DI ¼ 70 (Fig. 1.25a,b). As known, the Cascades volcanic rocks were formed in a typical continental arc in Cordillera Mts., North America, indicating that the Xiong’er Group is also a volcanic sequence developed in a continental arc. The volcanic rocks in the Aleutians island arc have a high peak in the range of DI ¼ 35–70 (Fig. 1.25c), implying that they are dominated by intermediate or andesitic rocks. Compared to volcanic rocks in the continental arcs (Cascades or Xiong’er Group), the island arc (Aleutians) volcanic rocks do not show a peak in the DI range of >70; and moreover, the peak of DI ¼ 35–70 slightly shift towards the left (Fig. 1.25c). This suggests that the volcanic rocks in the island arc are more mafic than those in the continental arcs. It can be concluded that the peak of DI ¼ 35–70 is related to oceanic plate subduction, or as an indicator of oceanic plate subduction-related magmatic arcs; and possibly, the DI>70 peak is an indicator of the continental crust basement beneath the magmatic arcs.

1.4.3.3

Continental and Oceanic Rifts

The volcanic rocks in continental rift (Afar triple junction in the East African Rift System; EARS, see overview in Pirajno 2009) and oceanic rift or mid-ocean ridge (Iceland) show a peak of DI70, considering their shoshonite affinity.

1.4.3.5

Volcanic DI Histograms of Various Tectonic Settings

As shown above, the volcanic sequences overlying a continental crust basement, exemplified by the Afar rift system, the Iceland oceanic rift with the continent remnant, and the Xiong’er and Cascades continental arcs, have a peak in the range of DI>70, no matter their tectonic settings are extensional or compressional; whereas the volcanic rocks in areas without the continental crust (e.g., the Aleutians) do not have such a peak. Accordingly, we here consider that the volcanic sequence

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Fig. 1.32 SiO2 histogram of Jurassic volcanic rocks in Dabie Shan. (Reprinted from Dai et al. 2003 Copyright 2003 China Academic Journal Electronic Publishing House)

Fig. 1.33 Genesis and DI-population of igneous rocks in various tectonic settings

with a DI>70 peak develops in a tectonic setting with continental crust basement and that the DI>70 peak is an indicator of the partial melting or involvement of continental crust basement underlying volcanic rocks (Fig. 1.33; Table 1.13). In oceanic rifts (e.g., Iceland), island arcs (e.g., Aleutians), and continental arcs (Cascades and Xiong’er), where the oceanic crust was involved in magmatism, the volcanic sequences have a peak of DI ¼ 35–70 (Fig. 1.25). By contrast, in continental rifts (Afar), where no oceanic crust was involved, the volcanic rocks do not have such a peak. Consequently, the volcanic associations with a peak of

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Table 1.13 Links between igneous DI populations and tectonic settings

Andesitic and felsic Andesitic

Peak DI value 70

Tectonic setting Mid-ocean ridge Oceanic island or oceanic rift Mid-ocean ridge with continental segment

Magma source Asthenosphere Asthenosphere, oceanic crust Asthenosphere, oceanic crust, continental crust

Lithology Mafic: MORB Mafic (OIB), minor andesitic Mafic, felsic and andesitic

Continental rift

Asthenosphere, continental crust Oceanic crust, continental crust, enriched mantle Oceanic crust, enriched mantle Continental crust

Mafic, felsic

Continental arc Island arc Continental collision orogen

Example Pacific ridge ? Hawaii or Ontong Java Iceland

Afar Cascades, Xiong’er Aleutians Dabie Shan

DI ¼ 35–70 can be related to the involvement of the oceanic crust, and the peak of DI ¼ 35–70 is an indicator of partial melting of the oceanic crust (Fig. 1.33). In continental and oceanic rifts, represented by the Afar in Ethiopia and Iceland, respectively, where magmatism is characterized by the partial melting of the mantle, the volcanic sequences have a peak of DI70, as exemplified by Iceland. Thus, the continental and oceanic rifts will generate bimodal volcanic rocks of mafic-felsic and mafic-andesitic types (Fig. 1.33; Table 1.13); the oceanic rift with remnant continental segment, i.e., Iceland, develops trimodal type with mafic-andesitic-felsic volcanic rocks (Fig. 1.33). It is inferred that the mid-ocean ridge, where is almost lacking overriding crust, develops the mafic-type unimodal volcanic rocks (Fig. 1.33; Table 1.13), known as MORB. The volcanic rocks in an island or continental arcs are dominated by andesitic rocks and show a high peak of DI ¼ 35–70, originating from partial melting of the subducted oceanic plate, or of the enriched mantle wedge resulted from oceanic plate subduction dehydration. Upward emplacement of these magmas causes local geothermal anomalies and partial melting of the overriding crust. The induced partial melting of overriding continental or oceanic crusts generates DI>70 or DI ¼ 35–70 volcanic rocks in the continental (e.g., Cascades and Xiong’er Group) or island (e.g., Aleutians) arcs, respectively. Therefore, the volcanic rocks in island arcs are generally andesitic-type unimodal, but in continental arcs are bimodal andesitic-felsic (Fig. 1.33; Table 1.11). In continental collision orogeny, both the subducted and overriding slabs are continental crust, and thus the igneous rocks generated during collisional orogeny are generally felsic (Chen et al. 2017a, b; Yang and Wang 2017) and show the felsictype unimodality (Fig. 1.33). Figure 1.33 and Table 1.13 summarize the DI population types of igneous rocks developed in different tectonic settings. It shows that bimodal volcanic association can occur in both continental arcs and continental rifts, i.e., in both compressional and extensional tectonic settings. It must be emphasized that the peaks in DI histogram refer to the whole igneous associations, instead of an individual igneous body (volcanic body or intrusion). For instance, we suggest that mantle melting produces volcanic rocks with a peak of DI0.5 Mt Pb + Zn each) at Changba, Lijiagou, Bafangshan, Sifangshan, Erlihe, and Yindongzi, occurs between the Shang-Dan and Shanyang faults (Fig. 1.34). These Pb-Zn deposits are

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Table 1.14 Indosinian (mainly Triassic) ore deposits in the Qinling Orogen Deposit/ Province Metal Sample geology Epizonogenic hydrothermal Dahe/Henan HgAltered rock Sb ChangbaPbReworked Lijiagou/ Zn SEDEX-type ores Gansu Erlihe/Shaanxi PbReworked Zn SEDEX-type ores

Dashui/Gansu

Au

Method

Age (Ma)

Reference

Whole rock Rb–Sr isochron Sphalerite Rb-Sr isochron

199
5

Lu et al. (2008) Mao et al. (2012)

Pyrite Re-Os isochron

226
17

Reworked SEDEX-type ores Post-ore diorite porphyry

Sphalerite Rb-Sr isochron Zircon LA-ICP-MS U-Pb

220.7
7.3

Post-ore diorite porphyry

Zircon SHRIMP U-Pb

221
3

Hanging-wall granodiorite Hanging-wall granodiorite Diorite dyke in the footwall Diorite dyke in the footwall Footwall pyroclastic rock Granite stock

Zircon SHRIMP U-Pb Zircon (U-Th)/He

213.3
2.5

Zircon (U-Th)/He

210.8
4.9

Apatite (U-Th)/He

211.4
6.3

Zircon SHRIMP U-Pb SHRIMP zircon U-Pb SHRIMP zircon U-Pb SHRIMP zircon U-Pb Quartz 40Ar/39Ar plateau Quartz 40Ar/39Ar isochron Zircon SHRIMP U-Pb Muscovite 40Ar/39Ar plateau Muscovite 40Ar/39Ar plateau Plagioclase 40 Ar/39Ar plateau

212.6
4.8

Granite stock Granitic dike Liba/Gansu

Au

Auriferous quartz vein Auriferous quartz vein Altered pre-ore granitic porphyry Altered pre-ore granitic porphyry Altered pre-ore granitic porphyry Altered pre-ore diorite

222.4
5.2

214
2

210.9
4.8

213.7
2.7 215.1
2.5 210.2
1.6 210.6
1.3 205.0
3.5 221.9
1.0 216.4
1.6 216.8
1.4 227.6
1.4

Zhang et al. (2011a) Hu et al. (2012) Wang et al. (2011a) Zhang et al. (2011a) Zeng et al. (2013) Zeng et al. (2013) Zeng et al. (2013) Zeng et al. (2013) Zeng et al. (2013) Han et al. (2014) Han et al. (2014) Han et al. (2014) Feng et al. (2003) Feng et al. (2003) Zeng et al. (2012) Zeng et al. (2012) Zeng et al. (2012) Zeng et al. (2012) (continued)

1 Geological Evolution of Qinling Orogen

77

Table 1.14 (continued) Deposit/ Province

Baguamiao/ Shaanxi

Metal

Au

Sample geology Altered pre-ore diorite Altered pre-ore diorite Mineralized slate Ore Quartz vein

Metamorphic hydrothermal Liziyuan/ AuPre- to syn-ore Gansu Ag porphyry aanqiao/ Shaanxi Huachanggou/ Shaanxi Yindonggou/ Hubei

Xujiapo/Hubei

Dongtongyu/ Shaanxi No. 15 vein/ Henan Zhangjiaping/ Henan Taoyuan/ Henan Shanggong/ Henan

Miaoling / Henan Beiling/Henan

Method Hornblende 40 Ar/39Ar plateau Biotite 40Ar/39Ar plateau Muscovite 40Ar/39Ar plateau Galena U–Pb isochron Quartz 40Ar/39Ar plateau

Age (Ma) 214.2
1.1 216.0
1.5 216.4
1.3 222.1
3.5 232.6
1.6

Zircon LA-ICP-MS U-Pb

229.2
1.2

Zircon LA-ICP-MS U-Pb SHRIMP U-Pb

242.0
0.8

Fluid inclusion Rb– Sr isochron Muscovite K–Ar

205
6

Ag

Pre-ore granite hostrock Xenotime/monazite in ores Quartz in Ag-quartz vein Altered wallrock

AgAu Au

Ag-Au-bearing quartz veins Altered wallrock

Muscovite 40Ar/39Ar plateau Calamite K–Ar

231
2

Au

Altered wallrock

Biotite K–Ar

224

Au

Altered wallrock

Biotite K–Ar

211.5

Au

Au-bearing vein

208.2

Au

Altered wallrock

K-feldspar Rb–Sr isochron Muscovite K–Ar

Au

Ore

208

Au

Altered wallrocks

Pyrite 40Ar/39Ar plateau Sericite K–Ar

Au

Rb–Sr isochron

242
11

Au

Au-quartz vein

Quartz 40Ar/39Ar plateau Quartz 40Ar/39Ar plateau Quartz 40Ar/39Ar isochron

223
25

Au

Early stage altered rocks Early stage quartz vein Au-quartz vein

Au Au Ag

209
5

216

218

238
5

211

246–180 216

Reference Zeng et al. (2012) Zeng et al. (2012) Zeng et al. (2012) Lu et al. (2008) Lu et al. (2008) Yang et al. (2012a) Zhu et al. (2010) Zhou et al. (2014b) Chen et al. (2004) Chen et al. (2004) Yue et al. (2014) Chen et al. (2004) Chen et al. (2004) Chen et al. (2004) Lu et al. (2008) Lu et al. (2008) Lu et al. (2008) Lu et al. (2008) Chen et al. (2008a) Chen et al. (2008a) Lu et al. (2008) Lu et al. (2008) (continued)

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Table 1.14 (continued) Deposit/ Province Dazhaoyu/ Henan Dahu/Henan

Metal Au

Sample geology Au-quartz vein

Age (Ma) 244
61

Molybdenite

Method Pyrite 40Ar/39Ar plateau Re–Os model age

MoAu

Molybdenite

Re–Os model age

223.7
2.6

Molybdenite

Re–Os model age

232.9
2.7

Molybdenite

Re–Os isochron

218
41

Hydrothermal monazite Ores

SHRIMP U-Th–Pb

216
5

Molybdenite Re–Os isochron

232
11

Molybdenite Re–Os isochron Molybdenite Re–Os model age Molybdenite Re–Os model age Molybdenite Re–Os model age Molybdenite Re–Os model age Molybdenite Re–Os model age Molybdenite Re–Os model age Molybdenite Re–Os model age Molybdenite Re–Os model age Molybdenite Re–Os model age Molybdenite Re–Os model age Molybdenite Re-Os weighted mean age

239
13

Majiawa/ Henan

AuMo

Qianfanling/ Henan Daxigou/ Henan

Mo

Ores

Mo

Ores Ores

Maogou/ Henan

Mo

Ores Ores Ores

Zhifang/Henan

Mo

Ores Ores Ores Ores Ores Ores

223.0
2.8

235.0
3.3 215.0
3.1 230.9
3.3 231.6
3.5 238.8
3.2 235.0
3.3 233.8
3.4 237.7
3.4 237.4
3.3 235.0
3.3 243.8
2.8

Badagou/ Henan

Mo

Ores

Molybdenite Re-Os weighted mean age

246
10

Xiangchungou/ Henan

Mo

Ores

Molybdenite Re-Os weighted mean age

246.0
1.1

Reference Lu et al. (2008) Li et al. (2007a) Li et al. (2007a) Li et al. (2007a) Li et al. (2008) Li et al. (2011a) Wang et al. (2010b) Gao et al. (2010) Gao et al. (2013) Gao et al. (2013) Gao et al. (2013) Gao et al. (2013) Gao et al. (2013) Gao et al. (2013) Gao et al. (2013) Gao et al. (2013) Gao et al. (2013) Gao et al. (2013) Deng et al. (2016) Deng et al. (2017) Deng et al. (2017) (continued)

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Table 1.14 (continued) Deposit/ Province Magmatic hydrothermal Maotang/ Henan Wenquan/ Gansu

Metal

Sample geology

Method

Age (Ma)

Reference

Au

Altered breccia pipe Porphyry-type ores Biotite monzogranite Biotite monzogranite Porphyritic monzogranite Porphyritic monzogranite Granitic poprhyry

Pyrite 40Ar/39Ar plateau Molybdenite Re–Os isochron Biotite K–Ar

223
8

Zircon LA-ICP-MS U-Pb Zircon LA-ICP-MS U-Pb Zircon LA-ICP-MS U-Pb Zircon LA-ICP-MS U-Pb Molybdenite Re-Os isochron Molybdenite Re-Os isochron

223
3

Zircon LA-ICP-MS U-Pb Zircon LA-ICP-MS U-Pb Zircon LA-ICP-MS U-Pb Zircon LA-ICP-MS U-Pb Zircon LA-ICP-MS U-Pb

210.8
5.0

Lu et al. (2008) Zhu et al. (2009) Zhu et al. (2009) Cao et al. (2011) Cao et al. (2011) Zhu et al. (2011) Zhu et al. (2011) Dai et al. (2015) Zhang et al. (2015) Jiang et al. (2010) Liu et al. (2011a) Liu et al. (2011a) Liu et al. (2011a) Dong et al. (2012) Zhang et al. (2015) Zhang et al. (2015) Zhang et al. (2015) Xiao et al. (2014) Xiao et al. (2014)

Mo

Xinpu

Mo

Quartz vein ores

Guilingou

Mo

Molybdenite ore

Granite porphyry Granite porphyry Granite porphyry Granite porphyry Granite porphyry

Liyuantang/ Shaanxi

Mo

214.4
7.1 223–226

225
3 216.2
1.7 217.2
2.0 197.0
1.6 195.5
5.0

222
1 208
2 209
2 201.6
1.2

Granite porphyry

Zircon LA-ICP-MS U-Pb

199
1.4

Granite porphyry

Zircon LA-ICP-MS U-Pb

201
3.1

Granite porphyry

Zircon LA-ICP-MS U-Pb

198
11

Porphyry

Zircon LA-ICP-MS U-Pb Molybdenite Re–Os isochron

210.1
1.9

Porphyry-type ore

200.9
6.2

(continued)

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Table 1.14 (continued) Deposit/ Province Huanglongpu/ Shaanxi

Huangshui’an/ Henan

Metal MoPb

Mo

Sample geology Altered carbonatite dyke

Method Molybdenite Re–Os isochron

Age (Ma) 221

Altered carbonatite dyke Altered carbonatite dyke

Molybdenite Re–Os average Molybdenite Re–Os isochron

221.5
0.3 208.4
3.6

Reference Huang et al. (1995) Stein et al. (1997) Cao et al. (2014)

Modified from Li and Pirajno 2017 Copyright 2017 Elsevier

hosted in the Devonian clastic-carbonate-barite-chert strata. They were previously assigned as SEDEX-type in origin (Qi and Li 1993b; Wang et al. 1996), but now popularly attributed to MVT Pb-Zn systems based on their Triassic isotope ages (Mao et al. 2012). The sphalerite from the Changba-Lijiagou and the Erlihe deposits yield Rb-Sr isochron ages of 222.4
5.2 Ma (Mao et al. 2012) and 220.7
7.3 Ma (Hu et al. 2012), respectively, which are consistent with a Re-Os isochron age of 226
17 Ma dated for pyrite from the Erlihe deposit (Zhang et al. 2011a). Sedimentary-hosted epizonogenic (low-temperature) hydrothermal U and Hg-Sb deposits are also present in the Qinling Orogen. The U deposits cluster in the Larima and Liba Au fields (Chen et al. 2004); the Hg-Sb deposits are represented by the Xunyang Hg-Sb field (Fig. 1.34; Zhang et al. 2014b) and those in the Jinlongshan Au field (Fig. 1.34; Zhang et al. 2006, 2014a). Their precise mineralization ages are still debated and vary between Indosinian (mainly Triassic) and Yanshanian (Jurassic-Cretaceous) due to the scarcity of robust isotope ages. The Qinling Orogen is well endowed with Triassic metamorphic hydrothermal or orogenic-type deposits (Groves et al. 1998; Chen 2006; Pirajno 2009, 2013) including Au, Ag, Mo, Cu, and Pb-Zn. Triassic orogenic-type Au mineralization in the Huaxiong Block (Table 1.14) is represented by the Dongtongyu Au deposit, with an alkali feldspar Rb-Sr isochron age of 208.2 Ma. Hydrothermal sericite from the Taoyuan Au deposit yields a K-Ar age of 211 Ma, and the hydrothermal quartz at the Beiling Au deposit yields a 40Ar/39Ar age of 216.04 Ma (Table 1.14). The Shanggong Au deposit also yields Triassic isotope ages (Fig. 1.34; Chen et al. 2008a). Triassic orogenic-type Mo(-Au) deposits have been recently recognized and exemplified by the Dahu deposit (Fig. 1.34), which yields a molybdenite Re-Os isochron age of 218
41 Ma (Li et al. 2008), and a hydrothermal monazite SHRIMP U-Th–Pb age of 216 Ma (Li et al. 2011a). The nearby Majiawa Mo (Au) deposit yields a molybdenite Re-Os isochron age of 231
11 Ma (Wang et al. 2010a, b). These Triassic deposits usually show an overprinting by the Yanshanian hydrothermal event. Moreover, all the known orogenic-type Mo deposits in Huaxiong Block were formed in the Triassic. For example, the Zhifang, Badaogou, Xiangchungou, and Qianfanling deposits in the Waifangshan area are proved to be of Triassic age based on molybdenite Re-Os isochron dating (Table 1.14). In the SQL, the Xujiapo Au deposit and Yindonggou Ag-Au-Pb-Zn deposits (Fig. 1.34)

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were constrained to have formed in the Triassic (Table 1.14; Chen et al. 2004). The Huachanggou Au deposit is located in the Mian-Lue Suture (Fig. 1.34) and is dated to have formed at 209
5 Ma using SHRIMP U-Pb method on the xenotime and monazite in the ores (Zhou et al. 2014b). The Liziyuan Au-Ag and the Ma’anqiao Au deposits are located at the northern margin of the SQL and have been constrained to be of the Triassic age (Zhu et al. 2010; Yang et al. 2012a). Triassic magmatic hydrothermal deposits in Qinling Orogen include porphyry, porphyry-breccia, porphyry-skarn, intrusion-related quartz vein, and the uncommon carbonatite-hosted types. The Maotang deposit in the northeastern sector of the SQL is a Triassic porphyry-breccia pipe Au system, with a pyrite 40Ar/39Ar age of 222.95
7.58 Ma (Lu et al. 2008). The Huanglongpu and Huangshui’an carbonatite-hosted Mo(Pb) deposits in the Huaxiong Block yield molybdenite Re-Os age of 221.5
0.3 Ma (Stein et al. 1997) and 208.4
3.6 Ma (Cao et al. 2014), respectively. In the Qinling Orogen, porphyry Mo systems (including skarns and quartz veins associated with porphyry stocks) were mainly formed in the Yanshanian Orogeny, although several Indosinian deposits/occurrences have also been discovered recently. For instance, in northwest SQL, the Wenquan Mo deposit yields molybdenite Re-Os age of 214.4
7.1 Ma (Zhu et al. 2009), which are identical to the zircon U-Pb ages of 216–223 Ma for the ore-causative porphyry (Zhu et al. 2011; Cao et al. 2011). In the central SQL, Guilingou, Yueheping, Xinpu, Daxigou, Shentaogou, and Liyuantang also yield Indosinian mineralization age (Table 1.14), although their Mo reserves are limited. Li and Pirajno (2017) proposed substantial differences between Indosinian and Yanshanian porphyry Mo systems. 1. The Yanshanian porphyry Mo systems mainly cluster in the Huaxiong Block, with lesser in the NQL (i.e., Shimengou and Qiushuwan). Indosinian porphyry systems, however, have only been recognized in the west part of the SQL (Fig. 1.34). 2. The ore-causative intrusions responsible for the Yanshanian systems are dominated by shallow porphyries of K-feldspar granite, biotite granite, and monzogranite (Chen et al. 2000a; Li et al. 2007b). By contrast, the Indosinian ore-causative intrusions are largely monzogranite, and can either be small granite porphyry stocks (e.g., Liyuantang) or part of multistage complexes (e.g., Wenquan, Yanzhiba). 3. The Indosinian Mo-bearing intrusions have lower K2O, but higher Al2O3, MgO, CaO, and Na2O contents than Yanshanian counterparts, although they both show affinity to high-K calc-alkaline to shoshonite, metaluminous to peraluminous series. Indosinian intrusions do not display positive Y anomalies which are commonly observed in Yanshanian intrusions, but they both show similar REE and trace element patterns. Their whole rock Sr-Nd and zircon Lu-Hf isotope features vary greatly, indicating different sources (Jiang et al. 2010; Cao et al. 2011; Zhu et al. 2011; Dong et al. 2012; Li et al. 2012a, 2018). 4. Hydrothermal alteration associated with Yanshanian porphyry Mo systems is dominated by intensive potassic alteration and silicification that can even extend

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well into the wall rocks (e.g., the Donggou and Jinduicheng Mo deposits). By contrast, the Indosinian Wenquan Mo deposit exhibits extensive argillic alteration, although the Mo mineralization is associated with the potassic and silicic alteration. 5. The Yanshanian porphyry Mo systems display typical magmatic fluid inclusion assemblages of CO2, aqueous plus daughter mineral-bearing inclusions (Li et al. 2012b). A limited fluid inclusion study, however, reveals that gangue minerals from the Wenquan deposit contain only aqueous inclusions (Han 2009). Bulk molecule and ion compositions further reveal a much lower content of CO2 but higher H2O (Wang et al. 2012) than those of Yanshanian counterparts. Thus, the Indosinian porphyry Mo systems, although sharing some similarities with the Yanshanian porphyry Mo systems, do exhibit unusual features, particularly in terms of economic importance, spatial distribution, ore-causative intrusions, the CO2-poor ore-forming fluids, and intense propylitic and argillic alteration.

1.5.3.2

Spatio-Temporal Distribution and Tectonic Evolution

Mineral systems are temporally associated with major tectonothermal events and specific geodynamic settings. Therefore, a specific type of mineral system can be used as a powerful probe or insight into the geodynamic evolution of the ore-hosting terranes (Pirajno 2016; Chen et al. 2008b). The spatial distribution and geological nature of the pre- and post-Triassic mineral systems also provide important constraints on the Triassic or Indosinian tectonic setting of the Qinling Orogen. Late Paleozoic mineral systems in the Qinling Orogen are dominated by SEDEX-type Pb-Zn deposits hosted in Devonian strata, including the Changba-Lijiagou, Bijiashan, Dengjiashan, Luoba, Qiandongshan, Bafangshan, Yinmusi, Erlihe, and Yindongzi deposits (Qi and Li 1993; Wang et al. 1996). They are proven to have undergone later reworking (Wang et al. 2011b), and some of them are of Indosinian age (Zhang et al. 2011a; Hu et al. 2012; Mao et al. 2012), suggesting an Indosinian tectonic overprinting. The Indosinian tectonic event was followed by a more intense Yanshanian magmaticmetallogenic episode (Li et al. 2007b; Chen et al. 2007, 2009a, b; Mao et al. 2008) which has been widely considered to have occurred under syn- to post-collisional settings, especially the tectonic transition from collisional compression to extension. This shows that the Triassic Qinling Orogen accommodated an oceanic closure through oceanic plate subduction and that the Triassic mineral systems were formed mainly in an active continental margin. Mineral deposits of different genetic types formed in the Triassic (Indosinian), including Carlin-type Au, orogenic-type Au, Ag and Mo, porphyry-breccia pipe Au, porphyry-type Mo, and carbonatite-hosted Mo deposits, which show a distinct spatial distribution: (1) The Au-Ag-Pb-Zn deposits occur in the south, and Au-Mo deposits in the north; and (2) the epizonogenic hydrothermal deposits occur only to the south of the Shang-Dan Fault, whilst the magmatic hydrothermal deposits occur

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mainly to the north of the Shanyang Fault (Fig. 1.34). In terms of the Mo mineral systems, they also display a specific spatial distribution, whereby the known porphyry Mo systems are located in the western part of the South Qinling Orogen, where contemporaneous granitic intrusions are common (Li et al. 2015b). The Mo-mineralized carbonatite systems, as well as the orogenic quartz lode systems, are located in the northeastern part of the Qinling Orogen. Temporally, orogenic Mo deposits were the earliest (mainly between 220 and 250 Ma), followed by carbonatite veins (mainly between 205 and 225 Ma) and porphyry Mo systems (mainly between 195 and 220 Ma). In view of the fact that both the orogenic and porphyry system were mainly formed under a compressional/transpressional environment in continental collision or subduction-related accretion orogenic processes (Groves et al. 1998; Goldfarb et al. 2001; Chen 2006), whereas carbonatite and associated mineralization commonly occurred in extensional settings and related continental rifting (Pirajno 2015), such a complex metallogenic scenario and spatial distribution can be meaningfully interpreted by using a northward B-type subduction along the MianLue Fault during the Triassic, together with a northward increase in post-ore weathering erosion (Fig. 6; Zhou et al. 2016). Specifically, the tectonic setting in the northeastern Qinling Orogen changed from compression/transpression (220–250 Ma) to extension (205–225 Ma). The paleo-Tethys possibly terminated before 220 Ma in eastern Qinling Orogen and Dabie Shan, but still survived to ~200 Ma in southwestern Qinling Orogen.

1.5.4

Concluding Remarks

1. The Triassic marine strata are widespread in the western SQL and the Songpan Fold Belt, and conformably overlie the Devonian-Permian sequence, suggesting that the Paleo-Tethys Ocean still survived in the Triassic. This understanding accords well with the paleomagnetic data which show that the YC and the NCC were independent and separated by an ocean before the Triassic but eventually amalgamated during the transition from the Late Triassic to the Early Jurassic (Fig. 1.13, Sect. 1.2). 2. The Triassic magmatites in the Qinling Orogen clearly display lithological and geochemical polarities from south to north. These polarities resemble those of typical active continental margins, but cannot be interpreted as rifting or syn- to post-collision tectonic settings. Carbonatites and A-type granitoids commonly occurred in extensional or rifting settings appear in SNCC, which may suggest a transition from compression/transpression (>220 Ma) to extension (90% of the Mo metal being resourced from porphyry and porphyry-skarn deposits aged 160–105 Ma (Stein et al. 1997; Li et al. 2007b; Mao et al. 2008). The Xiaoqinling-Xiong’ershan region in the northernmost part is the second largest gold province in China, with the main gold deposits having formed in the Jurassic to Early Cretaceous (Chen and Fu 1992; Mao et al. 2002; Yang et al. 2003; Chen et al. 2007; Zhu et al. 2015). A similar metallogenic scenario has also been identified in lode Ag-Pb-Zn deposits (Li et al. 2013b, 2017; Cao et al. 2015). However, the Yanshanian tectonic setting and evolution in Qinling Orogen is still open (Hu et al. 2005, 2006; Liu et al. 2013; Heberer et al. 2014; Dong et al. 2016). In this section, we present a comprehensive synthesis of information mainly from the Yanshanian (Jurassic to Cretaceous) granitoids, including their types, spatial and temporal distribution, petrological and geochemical characteristics. The results allow the identification of two distinct pulses of Yanshanian magmatism that show a sharp distinction in spatial distribution, petrography, and geochemistry, with 130–125 Ma (~128 Ma) marking a major divide. The Yanshan Orogeny in the Qinling Orogen was an integrated effect of collisional compression-to-extension transition, the back-arc extension related to paleo-Pacific subduction, and/or a Cretaceous Pacific superplume event.

1.6.1

Geology and Geochemistry of the Yanshanian Granitoids

In the Qinling Orogen, there are widespread Late Jurassic-Early Cretaceous granitoids in the central and eastern parts, with little or no volcanic equivalents (Fig. 1.40). They not only have a great bearing on constraining the geodynamic evolution after

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Fig. 1.40 Distribution of Yanshanian granitoids in the Qinling Orogen. (Reprinted from Li et al. 2018 Copyright 2018 Elsevier). Main faults/sutures: (1) San-Bao Fault; (2) Luanchuan Fault; (3) Waxuezi Fault; (4) Zhu-Xia Fault; (5) Shang-Dan Suture; (6) Shanyang Fault; (7) Mian-Lue Suture; (8) Longmenshan-Dabashan Fault

the continental collision, but are also important from an economic view as many of them host important metallic mineralization. For example, the Huashan I, Wenyu, and Niangniangshan granite plutons carry coeval orogenic-type gold mineralization exemplified by the Wenyu, Dongchuang, and Yangzhaiyu deposits (Jiang et al. 2009; Mao et al. 2002; Zhou et al. 2014a, 2015). Some small granite porphyries host porphyry or porphyry-skarn Mo mineral systems represented by the Jinduicheng, Nannihu, Shangfanggou, Donggou, and Yuchiling deposits (Li et al. 2012a, b, 2013a; Yang et al. 2012b, 2013, 2015). The onset of Yanshanian (Jurassic and Cretaceous) granitic magmatism followed the Early Yanshanian (160–200 Ma) magmatic hiatus (Li et al. 2015a, b), and occurred in three pulses around 140–160, 128–140, and 108–128 Ma (Fig. 1.41). These Yanshanian granitoids can be also divided into the mid-Yanshanian (128–160 Ma) and late Yanshanian (108–128 Ma) epochs, because the magmatic rocks with ages of 128–140 Ma and 140–160 Ma show indistinguishable geochemical characteristics (Li et al. 2018). The rocks range in composition from granodiorite, quartz diorite, granite, monzogranite to syenogranite, with the mineral assemblage dominated by K-feldspar, plagioclase, and quartz. Biotite is the most common ferromagnesian mineral, followed by amphibole and muscovite. Cordierite has not been reported in any pluton yet. Li et al. (2018) further grouped these granitoids into amphibole-bearing granitoids (containing amphibole
biotite as ferromagnesian mineral), biotite-bearing granitoids (biotite occurring as the only ferromagnesian mineral), and muscovite-bearing granitoids (containing muscovite
biotite as ferromagnesian mineral), and proposed that coexistence of

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Fig. 1.41 Spatial distribution (a), time framework (b), ages versus longitude (c), and age versus distance away from the Mian-Lue Suture (d) for Yanshanian granitoids in Qinling Orogen. (Modified from Li et al. 2018 Copyright 2018 Elsevier). Abbreviations: AG amphibole-bearing granitoids; BG biotite-bearing granitoids; MG muscovite-bearing granitoids; MME mafic microgranular enclave; Pluton abbreviations: BBS Babaoshan; BLP Balipo; BSG Baishagou; BSY Baishiya; BZS Banzhusi; CG Chigou; DG Donggou; DP Daping; FNS Funiushan; GG Gaogou; HBL Huangbeiling; HH Houhe; HLP Huanglongpu; HPG Haopinggou; HS I Huashan I; HS II Huashan II; HSM Huoshenmiao; HY Heyu; JDC Jinduicheng; LJS Laojunshan; LMG Leimengo; LNS Laoniushan; LWG Longwogou; LSG Lengshuigou; LT Lantian; MHG Muhuguan; MaL Mangling; Mli Miaoliang; ML Miaoling; MLG Mulonggou; NJW Niujiawan; NNH Nannihu; NNS Niangniangshan; NT Nantai; PZG Puzhengou; QYG Qiyugou; SBG Shibaogou; SF Shangfang; SJW Shijiawan; SMG Shimengou (Nangou); SY Shuangyuangou; SYG Shiyaogou; TB Taibai; TDG Tudigou; TGP Taoguanping; TSM Taishanmiao; WG Wagou; WY Wenyu; XG Xigou; XGF Xiaguanfang; XHK Xiaohekou; XMH Xiaomeihe; YG Yanggou; YJG Yinjiaogou; YJ Yuanjiagou; YK Yuku; YZ Yaozhuang; YZJ Yuanzijie

amphibole- and biotite-bearing phases in single pluton may have a common petrogenesis (e.g., Niangniangshan and Babaoshan) or require the variable contribution of juvenile crustal rocks in the source (e.g., Huoshenmiao). Besides, some pluton contains mafic microgranular enclaves (MME) (e.g., Lantian, Laoniushan, Huashan I, Niangniangshan). Yanshanian granitoids generally have high SiO2, low MgO, and Mg# values (Figs. 1.42 and 1.43), and show typical features of the continental crust, such as enrichment of LILE, Pb, and LREE, and depletion of HFSE and HREE (Figs. 1.44 and 1.45). They display high (87Sr/86Sr)i ratios and negative εNd(t) and εHf(t) values that suggest recycling of Precambrian lower continental crust (Fig. 1.46). Their SrNd-Hf isotopic data have provincial characters, which may be a reflection of regional differences in the compositions of their source rocks, and hence of the deep crust (Li et al. 2018). Zircon saturation temperature (Tzr) (Watson and Harrison 1983) of granitoids in the area is generally low. Among 50 plutons, 43 yields weighted average Tzr values

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Fig. 1.42 Total alkali versus SiO2, K2O versus SiO2, A/NK versus A/CNK, and Mg# versus SiO2 diagrams for the Yanshanian granitoids. (Reprinted from Li et al. 2018 Copyright 2018 Elsevier). Abbreviations: AG amphibole-bearing granitoids; BG biotite-bearing granitoids; MG muscovitebearing granitoids; MME mafic microgranular enclave

lower than 800 C; 7 exhibits relatively higher temperatures but still not surpass 860 C (Li et al. 2018). In addition, 40 of the 64 plutons investigated show zircon inheritance. Formation of such low-temperature magma appears to require fluid influx in addition to dehydration melting involving biotite or hornblende (Miller et al. 2003).

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Fig. 1.43 Harker plots of major elements in the Yanshanian granitoids. (Modified from Li et al. 2018 Copyright 2018 Elsevier). Abbreviations: AG amphibole-bearing granitoids; BG biotitebearing granitoids; MG muscovite-bearing granitoids; MME mafic microgranular enclave

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Fig. 1.44 Sr vs Rb, Ta vs Nb, and Hf vs Zr diagrams for the Yanshanian granitoids. (Modified from Li et al. 2018 Copyright 2018 Elsevier). Abbreviations: AG amphibole-bearing granitoids; BG biotite-bearing granitoids; MG muscovite-bearing granitoids; MME mafic microgranular enclave

1.6.2

Differences Between the Mid- and Late Yanshanian Granitoids

The mid- and late Yanshanian granitoids show remarkable differences in spatial distribution, petrology, geochemistry and isotopic features (Li et al. 2018). The mid-Yanshanian (128–160 Ma) granitoids widely occur between 109 E and 112 E. They are dominated by biotite-bearing and amphibole-bearing granitoids, and may contain MME (e.g., Lantian, Laoniushan, Shibaogou, Muhuguan, Taoguanping, Xigou, and Mangling). Minor muscovite-bearing phases can be observed in the Taoguanping and Xigou plutons. By contrast, late Yanshanian intrusions are restricted to the easternmost part of SNCC and NQL (between 112 E and 113 E) (Fig. 1.40). Most of the rocks are biotite-bearing granitoids, although the Funiushan pluton contains minor amphibole (Gao et al. 2013) whereas the Shimengou granite porphyry contains minor muscovite (Yang et al. 2010).

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Fig. 1.45 Primitive mantle-normalized trace element patterns and chondrite-normalized REE patterns for the Yanshanian granitoids. (Modified from Li et al. 2018 Copyright 2018 Elsevier). Abbreviations: AG amphibole-bearing granitoids; BG biotite-bearing granitoids; MG muscovitebearing granitoids; MME mafic microgranular enclave

Compared with the mid-Yanshanian rocks, the late Yanshanian granitoids are featured by much evolved composition, with higher silica but lower Al2O3, FeOT, MgO, and CaO contents (Figs. 1.42 and 1.43). In terms of whole rock Sr-Nd and zircon Lu-Hf isotopic data, they witness more involvement of juvenile crust as evidenced by their higher εNd(t) and εHf(t) but lower TDM2(Nd) and TDM2(Hf) (Fig. 1.46). The trace-element features are also different in many respects (Figs. 1.44 and 1.45). For instance, they have higher Y, Rb, Nb, Ta, Zr, and Hf, but lower Sr, Ba contents. They mostly show more clear Eu-depletion that indicates a shallower source depth of 10 km, corresponding to the mesozonal-to-hypozonal class of the crustal continuum model. N. Li (*) · J. Yao Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, China e-mail: [email protected] X. Deng Beijing Institute of Geology for Mineral Resources, Beijing, China Z. Ni College of Geosciences, China University of Petroleum, Beijing, China F. Pirajno Centre for Exploration Targeting, The University of Western Australia, Crawley, WA, Australia e-mail: [email protected] Y. Chen School of Earth and Space Sciences, Peking University, Beijing, China e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 Y. Chen et al. (eds.), Geology and Geochemistry of Molybdenum Deposits in the Qinling Orogen, P R China, Modern Approaches in Solid Earth Sciences 22, https://doi.org/10.1007/978-981-16-4871-7_6

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Keywords Orogenic-type Mo deposit · Quartz vein · Quartzofeldspathic vein · Metamorphic devolatilization · Backarc compression-extension alternating · Syncollisional compression-to-extension transition

6.1

Introduction

Orogenic gold deposits, as originally suggested by Bohlke (1982) and subsequently proposed by Groves et al. (1998), are the main source of gold reserves in the world (Goldfarb et al. 2001), and their geological features, geochemical characteristics, and tectonic settings are well documented (Groves et al. 1998; Kerrich et al. 2000; Goldfarb et al. 2005, 2014). Orogenic-type is a class of structurally controlled lode deposits formed by metamorphic fluids that are enriched in CO2. Therefore, Chen and co-authors (Chen et al. 2004; Chen 2006b, 2013; Pirajno 2009; Zhang et al. 2014) proposed the potential of orogenic mineral systems, i.e., Ag, Cu, Pb–Zn and Mo, and a new crustal continuum model showing vertical element zonation (Fig. 6.1), based on the geochemical similarities between Au and other metals. In the new crustal continuum model, the structurally controlled lode deposits are distinguished between metamorphic hydrothermal (orogenic) and epizonal hydrothermal types (Chen et al. 2014), which have been all assigned into orogenic class in previous studies (e.g., Groves et al. 1998). The salient features of orogenic-type mineral systems (Kerrich et al. 2000; Chen 2006b) include the following: (1) the deposits were formed at convergent plate margins in accretionary and collisional orogens; (2) the locations of the ore bodies are controlled by shear zones or faults; (3) the ore-forming fluids are metamorphogenic in origin and generally of low salinity and CO2-rich with variable CH4; (4) the mineralization temperature and depth show a wide range from 220 to 500
C and from 5 to 20 km, respectively (Fig. 6.1); (5) the mineralization is coeval with a major orogenic event. According to the above-mentioned geological characters or distinctive criteria, many orogenic-type mineral systems have been identified in China, including the Tieluping Ag deposit (Chen et al. 2004, 2005b), Weishancheng Ag–Au belt (Zhang et al. 2013), Lengshuibeigou Pb–Zn (Qi et al. 2007), Wangpingxigou Pb–Zn (Yao et al. 2008), and Yindonggou Ag–Au–Pb–Zn deposits (Yue et al. 2013, 2014) in the Qinling Orogen; the Bainaimiao Cu–Au (Li et al. 2007c) and Huogeqi Cu–Pb–Zn– Fe deposits (Zhong et al. 2012, 2013) in the eastern part of the Central Asian Orogenic Belt; the Sarekuobu Au (Zhang et al. 2014), Tiemurt Pb–Zn (Zhang et al. 2012), Wulasigou Cu (Zheng et al. 2012), Qiaxia Cu deposits (Zheng et al. 2014), and the Mengku Fe deposit (Wan et al. 2012) in the Altay Orogen in NW China; the W–Mo deposit in the Caledon Orogen in Norway (Larsen and Stein 2007); the Kupferberg Cu–Zn deposit in Germany (Höhn et al. 2017); and the Zinvinjian Cu–Pb–Zn–Au deposit in the Sanandaj–Sirjan metamorphic belt in north-western Iran (Asadi et al. 2018). Very important in the context of this chapter is that the orogenic-type Mo deposits have been identified in the Qinling Orogen,

6 Metamorphic Hydrothermal (Orogenic-Type) Systems

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Fig. 6.1 Crustal continuum model showing vertical element zonation of orogenic-type mineral systems. (Slightly modified from Chen 2013. Copyright 2013 China Academic Journal Electronic Publishing House)

such as the Zhifang and Qianfanling Mo deposits in the Wifangshan Mo-quartz vein cluster (Chen 2006b, 2013; Deng et al. 2008, 2014b, 2016, 2017), the Longmendian Mo deposit in the Xiong’ershan area (Li et al. 2011b, 2014), and the Dahu Au–Mo deposit in the Xiaoqinling gold field (Li et al. 2011a; Ni et al. 2012, 2014). The Qinling Orogen located between the North China Craton and the Yangtze Craton (see Chap. 1) is an important metallogenic belt in China (Zhai and Santosh 2013; Li et al. 2015) and hosts one of the world’s most important Mo provinces, with a combined total reserve of ~6 Mt. Mo metal (Chen et al. 2000; Li et al. 2007b; Mao et al. 2011). These Mo deposits are mainly associated with Mesozoic porphyritic intrusions (Hu et al. 1988; Stein et al. 1997; Mao et al. 2011; Zhang et al. 2011b) and are classified as porphyry-type (e.g., Jinduicheng, Donggou, and Yuchiling) (Li et al. 2012b, d, 2013) or porphyry-skarn-type (e.g., Nannihu and Shangfanggou) (Yang et al. 2012, 2013a) types. In the last decade, a number of vein-type Mo deposits were discovered, such as the Dahu (Li et al. 2011a; Ni et al. 2012), Longmendian (Li et al. 2011b, 2014), Zhaiwa (Deng et al. 2013a, c), and Zhifang (Deng et al. 2014b) quartz-vein systems and the Tumen fluorite veins (Deng et al. 2013b, 2014a). Some of the Mo-containing quartz veins share many geological features with those of

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mesothermal or orogenic-type deposits (Chen 2006b), and the Dahu Au–Mo deposit (Li et al. 2011a; Ni et al. 2012, 2014) and the Zhifang Mo deposit (Deng et al. 2014b, 2016) have been recognized as orogenic-type Mo. The Qianfanling Mo deposit in the Waifangshan area shares similar geological, geochemical, and geochronological features with the Zhifang deposit and may also be a member of orogenic-type Mo, though originally it was considered as a magmatic system (Gao et al. 2013). The Longmendian quartzofeldspathic veins were classified into a migmatitichydrothermal system (Li et al. 2014) and is described in this chapter, considering the transition and overlap between migmatization and high-grade metamorphism. Orogenic Mo deposits in the Qinling Orogen display common features as below: 1. They are located in the northeastern part of the Qinling Orogen, i.e., the East Qinling molybdenum belt (abbreviated to EQMB; Chen et al. 2000), especially in the Xiaoqinling (Dahu Au-Mo), Xiong’ershan (Longmendian Mo), and Waifangshan (Zhifang Mo) area (Fig. 6.2). 2. The orogenic Mo deposits were formed in continental collision or subductionrelated accretion orogenies (Fig. 6.3). The Longmendian Mo deposit was formed at ~1850 Ma (Li et al. 2011b), associated with the suturing of the continental blocks within the North China Craton and their incorporation into the Columbia supercontinent, and thus ascribed as continental collision setting. The Triassic orogenic Mo deposits, including the 21841 Ma Dahu (Li et al. 2008a), the 243.8  2.8 Ma Zhifang (Deng et al. 2016), and the 23913 Ma Qianfanling (Gao et al. 2010) deposits, synchronized the northward subduction of the MianLue Ocean and occurred at the transitional zones between continental arc and back-arc basin (Li et al. 2011a, 2015; Ni et al. 2012; Deng et al. 2016). 3. Hydrothermal alteration includes potassic alteration (mainly occurring as K-feldspar), sericitization, fluoritization, and carbonation. The Mo orebodies are hosted mainly by potassic alteration (Gao et al. 2013; Deng et al. 2014b; Ni et al. 2014), instead of the commonly observed sericitization for orogenic Au deposits worldwide (Groves et al. 1998; Kerrich et al. 2000; Goldfarb et al. 2001). 4. The ore-forming fluids are low-salinity, CO2-rich metamorphic, with fluid inclusions dominated by CO2-H2O and aqueous populations (Gao et al. 2013; Deng et al. 2014b; Ni et al. 2014). Nevertheless, minor daughter mineral-bearing inclusions are observed in the middle-stage quartz and interpreted as a product of fluid boiling (Deng et al. 2014b; Ni et al. 2014). 5. Ore-forming temperatures show a wide range from 210 to 550
C, and estimated trapping pressure of the Longmendian, Zhifang, and Dahu deposits is up to 265 MPa, 285 MPa, and 331 MPa, respectively (Deng et al. 2014b; Li et al. 2014; Ni et al. 2014). Such P-T conditions are higher than the majority of orogenic Au deposits, corresponding to the hypozonal end-member of the orogenic deposits (Chen 2006b; also see Chap. 2). As a powerful example, the Mo mineralization is deeper than the Au orebodies in the Dahu Au–Mo deposit (Ni et al. 2014).

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Fig. 6.2 Regional geology and distribution of orogenic Mo deposits in the Qinling Orogen. (Modified from Li et al. 2007b. Copyright 2007 China Academic Journal Electronic Publishing House)

Fig. 6.3 Plate tectonic model for orogenic Mo deposits, showing two favorable settings exemplified in Qinling Orogen. (Modified from Chen et al. 2014. Copyright 2014 John Wiley & Sons)

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The Waifangshan Mo-Quartz Vein Cluster Introduction

In the Waifangshan area of the EQMB, more than ten quartz vein-type Mo deposits have been discovered through geochemical survey by the No. 5 Team of Henan Bureau of Geological Exploration for Non-ferrous Metals (Fig. 6.4; Table 6.1), including the Zhifang, Fantaigou, Qianfanling, Xiangchungou, Kangjiagou, Badaogou, Shitishang, Tulingcun, Maogou, and Daxigou deposits. These veins yield an estimated reserve of more than 0.1 Mt. Mo metal and expected resources up to 0.5 Mt. in total, constituting the Waifangshan Mo-quartz vein cluster (Chen

Fig. 6.4 Geological map of the Waifangshan Mo-quartz vein cluster. (Reprinted from Deng et al. 2016. Copyright 2016 Elsevier)

Gangue mineral/ alteration Metallic minerals Ore texture

Orebody size Occurring state

Main orebodies Vein shape

Qz, Kfs, Rt, Fl, Carb, Ser, Brt, Ap, Mnz Py, Mo, Gl, Cpy, Sp Mosaic, exsolution, cloudy, relict, armour

Rhyolite, porphyry, Xiong’er Gp. NE-, NW-, and minor NS-, EWtrending K1, K2, K3, K4, K5, K6 Lenticular, tabular 100–28000.39– 10.8680–550 m Dip NE-NNE, at an angle of 8–39


Host rock

Controlling faults

Zhifang Mo 8 Kt 0.12% Mo

Deposit Metal Resource Grade

Broken, star-like, skeleton, relict

Py, Mo, Gl, Sp, Cpy, Po, Hem Broken, skeleton, cove-bay, star-like, exsolution

Py, Mo, Gl, Cpy, Sp

Py, Gl, Mo, Cpy, Sp, Bn, Sch, Wolf, Hem Myrmekitic, relict, covebay

Qz, Kfs, Carb, Chl, Epi

>1000 m long, >10 m thick Dip NNE, at an angle of 25
Qz, Kfs, Rt, Carb, Fl, Ser, Chl

500–1600  0.3–5.0  44–240 m Dip NE, at an angle of 3–26


Stratiform, tabular

NE-trending faults

Daxigou Mo–Au 13 Kt 0.087% Mo; 0.11–1.6 g/ t Au Rhyolite, Xiong’er Gp.

K4, K5, K7, K8, K9, K10, K11 Lenticular, tabular

5 orebodies

EW- and NW-trending detachments

Rhyolite, porphyry, Xiong’er Gp.

Xiangchungou Mo >10 Kt 0.219% Mo

Qz, Kfs, Rt, Carb, Fl, Chl, Ser

NE- and NW-trending shear zone HK1, WK1, EK1, EK2 Stratiform, lenticular, tabular 630–980  2.41– 3.54  300–350 m Dip NE, at an angle of 22–40


Dacite, Xiong’er Gp.

Badaogou Mo 7.6 Kt 0.124% Mo

Py, Mo, Gl, Cpy

Lenticular, tabular 380
C and 300–360
C, respectively. The low-Th peak overlaps with the majority of Th values obtained from the middle-stage minerals, which likely records the overprinting of the middle-stage hydrothermal process (Fig. 6.10a). The Th values for the middle-stage minerals range mainly from 250 to 370
C. Some inclusions with Th above 380
C suggest a middle-stage outgrowth of the earlystage quartz core (Fig. 6.10b), and thus, inherit the signatures of the early-stage hydrothermal process. Similarly, the FIs in middle-stage minerals with Th below 240
C reflect the influence of the late-stage fluid flow; whereas four inclusions in late-stage minerals with Th above 250
C represent the transition of the middle-stage fluid (Fig. 6.10c). Therefore, it is concluded that the early-stage or initial ore-forming fluids are generally of low salinity and high temperature (>380
C) (Fig. 6.11). The middle-stage fluids are characterized by CO2-rich, moderate temperature (250370
C), and widely variable salinities (Fig. 6.11). The fluids in the late stage are characterized by low-salinity, low-temperature (