Crustal Evolution and Metallogenesis in Selected Areas of the Indian Shield - A Monograph

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CRUSTAL EVOLUTION AND METALLOGENESIS IN SELECTED AREAS OF THE INDIAN SHIELD A MONOGRAPH based on w ork done un der a Research Program m e at D epartm ent o f G eology, P residency C ollege, C alcutta w ith fin ancial support

SX-.

of the U niversity G rants Com m ission, New Delhi

iN D IA N S O C IE T Y OF EARTH S C IE N C E S C A LC U TTA ,

IN D IA

NOVEMBER 1984

CRU STAL EVOLUTION AND M ETALLOGENESIS IN SELECTED A R E A S OF THE INDIAN SHIELD

A MONOGRAPH based on w o rk done under a Research Program m e at D epartm ent of G eology, P residency C ollege, C alcutta w ith fin a n cia l su p p o rt o f the U niversity G rants C om m ission, New Delhi

NOVEMBER 1984

INDIAN

SOCIETY

OF EARTH

SCIENCES

CALCUTTA, INDIA

PREFACE The orig in and evolution o f the E arth’s crust is a fascinating su b je ct that has stim ulated much sp e cu la tio n and debate since early 19th century. The rapid accum ulation of data in various fields of geology, geoph ysics and geochem istry in the last 30 years has c o n trib u te d much to o ur understanding of the physical and chem ical nature of the E arth’s cru st and o f the processes by which it has evolved. It is now generally agreed that partial m elting o f the Earth’s in terior p roduced m agm as w hich m oved to the surface and produced the first continen tal crust. M aterials eroded from the co ntinen ts have been added to the continental m argins, w hich, to gethe r w ith the m agm atic m aterials com ing up from the m antle, led to the lateral grow th of the continen ts. E pisodic additions of p a rtia lly m elted m antle m aterials to d iffe re n t levels o f the co n tin e n ­ tal crust led to th ic k e n in g o f the crust. System atic studies on the crustal e volution of given regions o f the c o n ti­ nents need (a) a ccu m ulatio n of a vast am ount of geolog ical, geoche m ica l and geophysical data fo r that region, and (b ) c ritic a l co n sid e ra tio n o f relevant experim ental and theoretical studies to p re d ic t behaviour o f m aterials in different situa tion s w ith in the crust and the upper m antle. That is w hy viable models fo r crustal evolution could not be put fo rw a rd until around the late sixties and early seventies. A p lethora of such m odels re la tin g to cru sta l evolution of the shield areas in G reenland, South A frica , A u stra lia and North Am erica cam e up w ith in the last fifteen years. In India, som e very general models fo r crustal e volu tion o f the Indian shield have been put forw ard, but no system atic regional studies s p e c ific a lly to d e cip h e r the m anner and history of crustal evolution was done until the present p ro je c t was taken up by the Departm ent o f G eology, P residency C ollege, C alcutta. However, o f late som e com m endable and system atic w o rk along sim ila r lines have been ca rrie d out in the Karnataka region of the Indian shield. For over 30 years now, m em bers o f the fa cu lty o f the G eology Dept, of Presidency C ollege have been carrying o u t extensive researches in the fie ld s of petrology, geochem istry, geoch ro n o lo g y, stru ctu ra l geology, stra tig ra p h y and econom ic geology o f the S inghbhum region o f B ih a r and O rissa as w ell as parts of C hhotanagpur, Rajasthan and Eastern G hats region o f O rissa and Andhra Pradesh. These studies w ere done m ainly through in d ividua l efforts or in sm all groups. By the m id-seventies, m uch valuable data on the basic geo­ logic and geoche m ical fra m ew ork o f these regions w ere co lle cte d through such studies. Studies by the G e ologica l Survey, N ational G eophysical Research Institute and several u n iversity scie n tists had also added m uch valuable geo­ logical, geochem ical and geo ph ysical data. Against th is backg ro un d , the fa cu lty m em bers o f the D epartm ent o f Geology, Presidency C ollege, C alcutta d ecided in 1978 to make use o f a g ra n t from the U niversity G rants C om m ission, New Delhi to ca rry o u t an in-depth study of the various problem s relating to crustal evolution of (a ) Singhbhum Keonjhar region of B ih a r and Orissa, (b ) selected areas o f the Eastern Ghats Belt of O rissa and A nd hra Pradesh and (c ) selected areas of the P recam brian mobile belt of Rajasthan. These areas, betw een them selves, illu stra te the major tectono-ge ochem ical regim es o f the Indian shield. For exam ple, the region of South S inghbhum -K eonjhar represents in the m ain an A rchaean G ranite-G reenstone b e lt w ith the iron ore bearing basins on e ith e r side o f a central b a th o lith ic com plex. In the course of the present program m e of study,

< H ) relics of 3.8 Ga old crust have been found w ithin the b a th o lith ic com plex. The north S inghbhum P roterozo ic m obile b e lt has many ch a ra cte ristics of Phanerozo ic geosynclines, but w ith many unique features too, such as the S inghbhum Shear Zone. The Eastern G hats G ranulite B elt is ch a ra cte rise d by th e Khondalite-C harnockite a ssocia tion as w ell as several m ajor lineam ents w hich have served as loci fo r in tru sio n s o f gabbro-anorthosite, u ltram afic and a lka lin e (in c lu d in g ca rb o n a tite ) plutons. The Rajasthan P roterozoic m obile b e lt w ith successive m ajor d e fo rm atio n al episodes and w ith fault-bounded te c to n ic s represents an othe r te cton o -g e och em ical regim e. The p a rtic ip a n ts o f this U G C -supported D epartm ental research program m e have w orked as a cohesive team fo r a period o f five years from S eptem ber 1978 to A ugust 1983. In the course o f th is study they have co lle cte d a huge volum e of new data and m ade in te rp re ta tio n s on the structure, petrology, geochem istry, stra tig ra p h y as w ell as som e s ig n ific a n t (th o u g h fe w ) data on g e o ch ro n o lo g y and m etallogen y o f parts of these regions, the broad aim being to determ ine the m anner and history of e vo lution o f the co n tinen tal cru st of these re g io n s ; some of these results have already com e o u t in reputed jo u rn a ls. However, it was fe lt th a t a com prehensive and w ell-docum ented a cco u n t o f these studies should be pub lishe d under one cover, so as to serve as a reference m aterial fo r m ore intensive stu die s on the various facets of the crustal e volution o f the Indian sh ield as a w hole. A c c o rd in g ly , the p a rticip a n ts o f the p ro je c t requested the Indian S o ciety o f Earth S ciences to arrange fo r p u b lica tio n of a M onograph on the sub je ct. Left to itself, it w ould not have been possible fo r th e ISES to accede to th is request. F ortunately, th e D epartm ent of S cience & Technology, Govt, o f India, w as kind enough to m eet the m ajor part o f the c o s t o f p ro d u ctio n o f such a M onograp h, so as to enable the S ociety to undertake th is pu b lica tio n . The S ociety is indeed grate ful to the Dept, o f S cience & Technolog y fo r th e ir generou s gesture. The M onograph co nsists of seven a rticle s by the p a rticip a n ts of the UGCsupp orted research program m e— three o f them relate to the problem s o f crustal evolution o f the S inghbh um region, one each is on th e Eastern G hats and C entral R ajasthan, one a rtic le relates to m etallogeny in a p a rt o f the fam ous Singhbhum C o pp e r Belt, w hile the last a rticle attem pts a b rie f com parative sum m ary o f the tre n d s o f crustal evolution in the three provinces of the Indian shield. If only the p u b lic a tio n o f this M onograph w ould generate su fficie n t interest am ong the Indian earth scie n tists to undertake m ore intensive and m ore thorough studies on th e cru sta l e volution o f d iffe re n t parts o f Indian shield, our S ociety w ould feel am ply rew arded. P u b lica tion o f th is M onograph w ith in a reasonably sh o rt tim e was made p ossible throu gh the u n tirin g efforts of Dr. S. L. Ray and Shri J. S. Ray, who spent m uch o f th e ir va lu a b le tim e in seeing the m anuscript th ro u g h the press.

D epartm ent of G eology, Presidency C ollege, C alcutta. Novem ber, 1984

A. K. SAHA Chairm an, Indian S ociety o f Earth S ciences.

CONTENTS STUDIES ON CRUSTAL EVOLUTION OF

THE

SINGHBHUM-ORISSA

IRON-ORE

A . K . S A H A . S. G H O SH , D . D A S G U P T A ,

CRATON

K . M U K H O P A D H Y A Y and S. L . R A Y THE CHAKRADHARPUR GRANITE GNEISS COMPLEX OF WEST SINGHBHUM,

P. K . B A N D O P A D H Y A Y and S. S E N G U P T A

BIHAR PETROLOGY AND TECTONIC

SETTING OF THE LAVAS OF DALMA VOLCANIC

M AN AS K. CH AKRABO RTI

BELT, EASTERN INDIA

and M IH IR K . B O SE MAGMATO-TECTONIC EVENTS IN THE EASTERN GHATS GRANULITE BELT OF OF THE INDIAN SHIELD

M IH IR K . BO SE, J Y O T I S A N K A R R A Y and M O N O ] M A I T R A

SOME ASPECTS OF THE TECTONIC EVOLUTION OF THE DELHI SUPERGROUP

P. K . G A N G O P A D H Y A Y ,

OF ROCKS IN CENTRAL RAJASTHAN

A . G A N G O P A D H Y A Y and A . L A H I R I COPPER MINERALISATION AT RAKHA MINES, OF THE ORES

AND

METAMORPHISM

P. K . G A N G O P A D H Y A Y and M . K . S A M A N T A

SUMMARY OF THE TRENDS OF CRUSTAL SHIELD

BIHAR

EVOLUTION IN PARTS OF INDIAN

A . K. SA H A

STUDIES ON CRUSTAL EVOLUTION OF THE SINGHBHUM-ORISSA IRON-ORE CRATON A. K. SAHA, S. GHOSH, D. DASGUPTA & K. MUKHOPADHYAY

Department o f Geology, Presidency College, Calcutta and S. L. RAY

Department o f Mining and Geology, Bengal Engineering College, Howrali

ABSTRACT C ritical structural, stratigraphic, geochronologic and geochem ical studies by the authors and their co-w orkers in dica te that a roughly tria n g u la r region, bounded by the arcuate Copper Belt Thrust zone in the north and the Sukinda Thrust in the south, represents an ancient cratonic block with ‘G reenstone— Granite assem blage’. In this craton, crustal growth started around c. 4.0 Ga and the last m ajor orogenic cycle closed c. 2.9 Ga. The pattern of early crustal developm ent in this craton differs in several ways from that in other known early Archaean terrains of Greenland, South Africa and southern India, viz. (a) evidence of sial form ation starting prior to 3.8 Ga, (b ) rapid thicke ning o f the continental crust to at least 30 km by 3.2 Ga, (c ) absence o f kom atiites and rarity of ultramafics, and (d ) existence of > ‘3.8 Ga supracrustals with strong petrographic and geochem ical resem blance with Phanerozoie supracrustals. However, the Archaean granitic members of this cia to n are rem arkably sim ila r to those of other Archaean terrains. Geochemical studies including REE and LIL elem ents suggest that (a) the O lder M etam orphic G roup (O M G ) orthoam phibolites ( > 3 . 8 Ga) were derived by partial m elting of a K-poor basalt (l.K T ) o r of a mantle peridotite with relatively high concentration of LIL elements, while the para-am phibolites of the same group were derived by weathering of mafic rocks with relatively high concentration of REE-enriched minerals, (b ) the OMG tonalite (3.775 Ga) was derived by partial m elting of am phibolite which had a short residence time near base of the crust, (c ) the Singhbhum G ranite magmas (c. 3.0 Ga) were formed ip two distinct modes:— ( i) partial m elting of am phibolite for the magmas of phases I and II, and (ii) partial m elting of a siliceous garnet-bearing granulite fo r magmas of the phase III units, and (d ) the Mayurbhanj G ranite magmas (c. 2.1 Ga) were generated by 70-80% fractional crystallisation of a granodiorite parent. Trend Surface analysis based on areal variations of the m ineralogic parameters and m ultivariate Factor analysis based on both petrological and geochem ical param eters have been used to bring out the details o f the crystallisation history of the OMG tonalite-gneiss and also of several members of the Singhbhum G ranite batholith. A model of crustal developm ent starting with form ation of a ‘s in k’ zone in the thin ultra m afic/ mafic crust at 4.0 Ga through successive accretion of mafic and sialic m aterials derived from upper mantle as well as recycling o f crustal materials, is developed and illustrated schem atically.

INTRODUCTION The existence of an old c ra to n ic b lo ck in the Precam bian tra c t o f the S inghbhum region was firs t postulated by S arkar and Saha in 1959 (S arkar and Saha 1959). It was recognised that ro cks of the Iron Ore orogenic cycle co n sistin g m ainly o f the Iron Ore G roup low -grade m etasedim ents, meta-

vo lca n ics and m afic intrusives (w h ich are intruded and g ranitised by the S inghbhum G ranite b a th o lith ic com plex) are separated by the 200 km long C opper B elt Thrust Zone from the younge r m etasedim ents and meta­ basics o f the S inghbhum o ro g e n ic c y cle in the north and east. On the basis of very m eagre g e o ch ro n o lo g ic data then available,

Monograph on Crustal Evolution, Indian Society of Earth Sciences, p. 1

74. 1984.

2

A. K. SAHA AND OTHERS

it was presum ed that the clo sing dates of the tw o o ro g e n ic cycles w ere c. 2000 Ma and 900 Ma, respectively. M ore d eta ile d s tru ctu ra l, s tra tig ra p h ic and g e o c h ro n o lo g ic studies have been c a rrie d out since then by S arkar, Saha and co-w orkers (T a ble 1 ). It is now established

Figure 1 : Regional geological map of SinghbhumOrissa Iron O re craton and neighbourhood ; the craton includes the Singhbhum G ranite Complex. Map com piled from published geological maps of the G eological Survey of India, Sarkar and Saha (1977, 1983) as w ell as unpublished data with A. K. Saha. 1 — O lder M etam orphic G roup ; 2. Supracrustal rocks of Iron Ore Group and Eastern Ghats Belt ; 2A. Proterozoic supracrustals ; 3. Unclassified gneisses ; 4. Singhbhum G ranite Complex, including the O lder Metamor­ phic tonalite-gneiss ; 5. Bonai G ranite ; 6 . Nilgiri Granite ; 7. Mayurbhanj 'Granite : 8. Gondwanas ; 9. Tertiary and Quaternary.

that a roughly tria n g u la r region bounded by the arcuate C o pper B elt T hrust Zone in the north and the S ukinda Thrust in the south, represents an a n cie nt c ra to n ic blo ck with ‘G reenstone-G ranite’ assem blage (la st m ajor o ro ge nic c ycle closed c. 2900 M a ), w hich is surrounde d on the east and north by relatively high-grade m etam orphic Satpura Belt (clo sin g date, c. 900 M a) and in the south by the g ra n u lite facies Eastern Ghat

B elt (clo sin g date, c.1500 M a ). Ttiis ancient c ra to n ic b lo ck has been designated as the “ Singhbhum -O rissa Iron Ore cra to n ” (S arkar and Saha 1983 ; Saha and Ray 1984) (Fig. 1). REGIONAL STRUCTURE AND STRATIGRAPHY The m ajor part o f this region is occu p ie d by the S inghbhum G ranite b a th o lith ic com ­ plex (Figs. 2 & 3 ) w hich o ccupies a northsouth elongated tra c t of about 8,000 sq. km. R elics of the oldest recognisable form ation in this area, viz., the O lder M etam orphic G roup (O M G ), con sistin g of m edium -grade p e litic schists, arenites, calc-schists, paraand ortho-am phibolites, o c c u r w ithin this b a th o lith ic com plex, the type area being a 200 sq. km. area to the w est o f Cham pua (2 2 r 0 4 ': 85 4 0 '). The O ld e r M etam orphics are intruded and partly granitised by an extensive suite o f b io tite (-hornblend e) tonalite-gneiss grading to tro n d h je m ite w hich occupies a 900 sq km tra c t in the westcentral part of the b a th o lith ic com plex. Num erous other sm all relics o f the tonalitetro n d h je m ite o ccu r th ro u g h o u t the b a th o li­ th ic com plex. The closing date of m etam or­ phism of both the OMG m etam orphics as w ell as the tonalite-tron dhjem ite is c. 3200 Ma as indicated by tw o Rb-Sr w hole rock isochrons and num erous K-Ar and Rb-Sr m ineral dates (cf. Sarkar and Saha 1977; Sarkar et at 1979). A nine-point Sm-Nd isochron of the tonalite-tron dhjem ite has indi­ cated an age o f 3775±89 Ma w hich probably represents the crysta llisa tio n age fo r the to n a lite (Basu et at 1981) (T ables 2A, 2B ), although Rb-Sr w hole rock isochron dates and K-Ar m ineral dates indicate that the tonalite-gneiss and the OMG rocks were m etam orphosed between 3100 and 3200 Ma. Since the to nalites are synkinem atically intruded into the OMG m etam orphics, this w ould mean th a t the OMG sedim ents were laid dow n on an a n cient ocean flo o r well before 3.8 Ga ago. The rest of the batholith is made up of at least 12 m agm atic bodies o f b io tite granod io rite -g ra n ite (S inghbhum G ranite proper) em placed in three d is tin c t but closely related

3

CRUSTAL EVOLUTION : SINGHBHUM-ORISSA TABLE 1 GENERALISED CHRONOSTRATIGRAPHIC SUCCESSION OF THE IRON-ORE

CRATON

OF

SINGHBHUM-ORISSA (MODIFIED FROM

SARKAR AND SAHA 1983).

Newer Dolerile dykes and sills

(ca. 1600-950 Ma)

Mayurbhanj Granite G abbro-anorthosite Ultram afic intrusions Kolhan Group

(ca. 2000-2100 Ma)

(ca. 2100 Ma)

------------------ U nconform ity Jagannathpur Lavas,

Dhanjori-Sim lfpal lavas

Malangtoli Lavas

Quartzite, conglom erate

(ca. 2100-2200 Ma) Dhanjori Group

Singhbhum

G roup :

Pelitic and arenaceous metasediments

------------i------Unconform ity — ■-----------------N ilgiri Granite, Singhbhum G ranite (3000-2900 Ma) “ Bonai Granite ---------------Iron

Ore Orogeny

^ Epidiorites (intrusives) Upper shales with sandstones and volcanics Iron Ore Group

Banded hematite jasper with iron ore

(3200-3100 Ma)

Tuffs, act'd volcanics and tuffaceous shales Mafic lavas with tuffs Sandstone and conglom erate

(lo ca l)

------------------ U nconform ity --------------------(Folding and m etamorphism 3200 Ma) Older M etam orphic tonalite-trondhjem ite (ca. 3800 Ma) -----------O lder M etam orphic Orogeny —

Older M etamorphic Group

(?4.0 Ga)

Mafic intrusives : ortho-am phibolites,

metagabbro ;

Calc-magnesian m etasediments : calc-gneiss, calc-schist. hornblende schist ; muscovilebiotite schist, quartzite, quartz schist.

phases (Rb-Sr w hole rock isochron date of c. 2950 Ma) plus a num ber o f patches of OMG ortho-am phibolites w hich have been m etasom atically granitised. The b a th o lith ic complex is enclosed on all sides by the Iron Ore Group (IO G ) of low -grade m etam orphic

sedim ents (in c lu d in g BHJ and banded iron fo rm a tio n s ), acid-interm edia te and m afic vo lca n ics as w ell as m afic sill-like intrusives. These rocks are B A B ' te ctonites w ith a general NNE-SSW tre n d in g first-generation, low -plunging fo ld s w hose attitudes have been

LEGEND O .H ,O L D E R

M E T A M O R P H IC C R O U P

O .M .C .,O L D E R M ETA M O R P H IC C N E IS 5 1 C ,IRON ORE G R O U P tL A V A S .B .H .J . AN O S H A L E S ) S C , S IN G H B H U M

G R A N IT E

C S , C H A IB A S A

FORM ATION

O S .O H A L B H U M

FO R M ATIO N

O H , DHANJOR1 S A N D S T O N E -C O N G L O M E R A T E * L A V A O L , O ALM A

LAVA

J l , JA G A N N A T H P U R L A V A K ,K O L H A N

G RO UP

C , C A B B R O -A N O R T H O S IT E N C , N IL C IR I G R A N IT E P O S S IB L Y R E L A T E D TO S IN G H B H U M G R A N IT E !? ) G R A N IT E S

R ELATED

O R O G E N IC

CYCLE

S Y NFO RM O F F IR S T

TO S IN G H B H U M

G EN E R A TIO N

A N T IF O R M OF F IR S T G E N E R A T IO N S Y N C IIN O R IU M

—

O F F IR S T

G E N E R A T IO N

A N T IC L IN O R IU M OF F IR S T

G E N E R A TIO N

SY NFO RM OF SE C O N O

G E N E R A T IO N

A N T IF O R M OF S E C O N D

GENERATIO N

THRUST

ANO W RENCH

FA U LT

UNEARS

A N D FO R M A TIO N B O U N D A R IE S . SC ALE 0

10 KM

Figure 2 : G eotectonic-cum -geological map of Singhbhum, Keonjhar, Mayurbhanj Districts and adjacent regions based on ERTS (LANDSAT) imagery and ground data, showing linears, geological boundaries, axial traces of folds of different generations, Copper belt thrust (CBT) and Sukinda thrust (after Sarkar & Saha, 1983, fig. 1).

CRUSTAL EVOLUTION : SINGHBHUM-ORISSA

S

Figure 3 : G eological map of Singhbhum -Orissa Iron Ore Craton com piled from published g e o io g x a l maps of the G eological Survey of India, Sarkar and Saha (1977), Saha & Ray (1984) as w ell as unpublshed data by the authors and co-workers. The map shows the Singhbhum G ranite along with its twelve magmatic units viz. I. Saraikella — Jorapokhar — Tiring, II. Haludpukur Chapia, III. Rajnagar — Kuyali, IV. K a lik a p u r— Matku, V. Daiima, VI. Gorumahisani, VII. R a ira n g p u r— Kuldiha, VIII. Garmaria — Khorband — M a lu k a — Jashipur — Karanjia, IX. Hatgamaria, X. Bara Nanda, X!. K e o n jharg arh— Bhaunra, XII. M anda — A s a n a — Besoi. Few im portant locations (so lid squares) are BD— Bahalda, BS— Besoi, CMP— Champua, GN-— G orumahisani, HP— Haiudpukhur, KJ— Karanjia,, K P —Kalikapur, KY— Kuyali, PS— Parsala, ON— O nlajori, RP— Rairangpur and SA— Saraikela.

rotated round to a near-parallelism w ith the boundary of S inghbhum G ranite ba tholith in the north and in the south. Two na rrow and long roof-pendants of the Iron Ore G roup

rocks extend into the batholith ; one from the northern b order through Patka (2 2 r 35' : 8 6 f 14') and the other from the eastern border through M ailum G hati (2 2 c2 9 ': 86 1 0 '). The

6

A. K. SAHA AND OTHERS TABLE 2A Rb-Sr AND Sm-Nd AGE DATA FROM THE ARCHAEANS OF SINGHBHUM REGION (AFTER SARKAR AND SAHA 1977); Rb-Sr AGES RECALCULATED USING ,\(“ 7 Rb) - 1.4 2x10 n a - ' .

Serial

Sample

No.

No.

1.

Sample Description

J - 1

Age

M uscovite from m uscovite-perthite pegm atite in Singhbhum G ranite ; Jorapokhar (mean of

in Ma

3136

3 determ inations) 2.

G - 3

M uscovite

from

m uscovite-perthjte

pegmatite

3240

in OMG tonaiile-gneiss near O nlajori (mean of 3 determ inations) 3.

A K /2 1 4

4.

AK/161

B iotito from Champua

5.

J P / 1-5

Biotite granite

Biotite from OMG biotite tonalite-gneiss near Ghampua (mean of 2 determ inations)

5 samples

6.

OMG

biotite tonalite-gneiss near

(whole rock)

isochron, Singh­

bhum Granite, Jorapokhar

O N /1-6 6 samples

Biotite tonalite— granodiorite from OMG gneiss, O nlajori (W R) isochron

3035

2956

2888 ±200 (87Sr/86Sr) r = .711 ± .009 3113185 (87sr / 8«sr) / = .703 ±.003

7.

8.

S R /103 S R .'581 SR.•'607(1) S R /607 (4)

)

S R /103 SR/581 SR .'607(1) SR .'607 (2) O N /1-5

1

( ( '

Biotite tonalite-gneisses from OMG gneiss, near Champua (WR) Isochron

9 point (WR) Sm-Nd isochron of OMG gneiss samples from Champua and O nlajori

3132 ± 8 5 (87Sr/86Sr) r = .7018 ±.0003

3775 ± 8 9 ( i« N d /'« < N d ), = .50798 ±.00007

II

m agm atic m em bers o f the S inghbhum G ra­ nite contain num erous xe n o liths of the Iron Ore G roup e p id io rite , q u a rtiz ite (in c lu d in g BH J) and local p h yllite (Saha 1972; S arkar and Saha 1977). The Iron Ore G roup of rocks are intruded by three o th e r b a th o lith ic g ra n ite bodies, viz., the N ilg iri G ranite in the east (S arkar and Saha 1977), the Bonai G ranite in the west (Jones 1934) and the M ayurbhanj G ranite (Saha 1975, Saha et « a~>.

3 Serial

Sample

No.

No.

Sample Description

1.

J ,1 0 /6 2

M uscovite from schist (O M G)

2.

AK/281

M uscovite

from

muscovite-quartz near Champua

Age

plagioclase

m uscovite-quartz-plagioclase

in

Ma

2880 ± 6 0

3070 ±300

schist (O M G ) near Champua 3.

A K /7 5

Hornblende from hornblende-plagioclase-quartz schist (O M G) near Champua

3169 ±180

4.

C /1 /6 3

Hornblende from hornblende-plagioclase-quartz schist (O M G ) near Champua

3101 ± 4 0

5.

A K /102

Hornblende

3169 ±6 3

from

hornblende-plagioclase-

quartz schist (O M G ) near Champua 6.

SR /548

Hornblende from OMG am phibolite near Champua

3243 ±7 0

7.

SR /576

Hornblende from OMG am phibolite near Champua

3146

8.

A K /10 9

B iotite from biotite tonalite-gneiss (O M G ) near

3216 ± 9 6

Champua 9.

A K /21 4

Biotite from biotite tonalite-gneiss (O M G ) near Champua Hornblende from the same sample

3318 ±99 3175 ± 9 0

10.

A K / 161

Biotite from biotite tonalite-gneiss (O M G ) near Champua

3074 + 60 } 31 15 ±90 J

11.

A K /B 1 8 8 /6 3

Hornblende f rom OMG orthom etam orphic horn­ blende schist enclave in Singhbhum Granite near Khorband

3210± 175

12.

A K /51N

Hornblende from OMG hornblende schist in c lu ­

3060 ±105

sion in Bahalda hornblende granodiorite 13.

G— 3

M uscovite from m uscovite-perthite in OMG biotite tonalite, O nlajori

14.

J— 1

Muscovite from muscovite pegmatite Singhbhum Granite, Jorapokhar

15.

P /83

pegmatite within

B iotite from biotite granodiorite, Dalima unit of of Singhbhum Granite

lavas (called the D hanjori lavas) was laid down mainly under subm arine co nd ition s. Both these basins are ch aracterized by the presence of a basal sandstone-conglom erate. In the southern S im lipal Basin there are two

3480 ± 5 0 > 3250 ± 9 0 ) 2929 3040 3050±90

J 3

’ 714 ±5 8

o ther sandstone horizons w ith in the lavas, but in the northern D hanjori Basin there is no such interbedded sandstone. Probably o f about the same age are the Jagannathpur, M alangtoli and related m afic lavas along the

8

A. K. SAHA AND OTHERS

western flank o f the S inghbhum G ranite B atholithic Com plex. In som e of these western basins, the o u tpo u rin g o f the lavas was preceded and fo llow ed by the deposi­ tion of the Kolhan G roup (cf. Dunn 1940) of sandstone-conglom erate o verlain by shales and lim estones. This period of lava e ru p tio n s a cco m ­ panied by sedim entation appears to have been follow ed clo sely by a period of maficultram afic intru sio ns (m ain ly s ills and stocks) over w id e areas in th e craton. The ultram afics w hich are sm all bodies (som e of w hich carry c h ro m ite d e p o sits) in trude into the Iron O re G roup in the west, south­ east as well as in the north o f the b a th o lith ic com plex (cf, Jones 1 9 34 ; Dunn and Dey 1942; C h a krab o rty et al 1980) and also locally the S inghbhum G ranite b ath olith itself (cf. Bose and G oles 1 9 7 0; Saha et ul 1972). Then fo llow ed the in trusio n of a suite of g abbro-a northosite w ith c h a ra c te ris tic vanadiferous and tita n ife ro u s m agnetite (cf. Dunn and Dey 1942) in the eastern part of the craton. This was fo llo w e d clo se ly by the intrusion of the M ayurbhanj G ranite batholith. The youn ge st P recam brian lith o -u n it of the craton is the N ew er D olerite w hich form s the c h a ra c te ris tic dyke swarm (m ainly NNEtre n d in g ) w ith in all the b a th o lith ic com plexes as w ell as in the m etam orphites ^houghout the craton (D unn and Dey 1942 ; Saha et «l 1973). The arcuate C opper B elt ‘T h ru st’ Zone \J9hich form s the northern bound ary o f this cra to n ic b lo c k is sub-vertical in the extrem e south-east but dips at 40°-50" to w a rd s north over the greater part o f its tren d from Kanyaluka (2 2 “ 2 8 ': 8 6 °3 1 ') to Lotapa har (2 2°35/ : 85°3 2') ; but fa rth e r w est th is fa u lt zone is m ostly of the nature o f a sub-vertical fa u lt along w hich th e northern b lo ck has been upthrow n relative to the Iron Ore craton. Along the S ukinda Thrust w hich runs approxim ately east-w est and dip s at m ode­ rate to steep angles to the south the Iron Ore craton has been d ow nth ro w n relative to the Eastern Ghats belt in the south. The ancient S inghbhum -O rissa Iron-O re blo ck

owes its preservation to its being sandw itched betw een the tw o m ajor thrusts along its tw o boundaries. The te c to n ic d istinctivene ss o f the Singh­ bhum -O rissa Iron Ore craton is clearly m ark­ ed out in the Landsat im agery of the region (S arkar and Saha 1977). A persistent set of close-spaced photo-linears, tre n d in g N 25° E to N 30°E is a ch a ra cte ristic feature th ro u g h o u t the craton, but is absent in the S atpura P rovince to th e north and in the Eastern Ghats Province to the south. These linears w hich are independent o f the strike of the rocks, are spaced only a few hundred m etres apart and are best seen over rocky ridges where such ridges intersect the linears at high angle. Such linears m aintain a constant trend over all the rock form ations in clu d in g the Archaeans, the Kolhan G roup, Jagann athpur Lavas, D hanjori G roup, M ayur­ bhanj G ranite and the part of the Chaibasa form ation w hich lies west o f the C opper Belt thrust zone. The photo-linears stop abruptly at the C opper B elt thrust zone. These must be related to the basem ent “ g ra in ” o f the Iron Ore craton (Fig. 2 ). STRUCTURE AND GEOCHEMISTRY OF THE OLDER METAMORPHIC GROUP AND TONALITE GNEISSES An intensive structural, pe tro lo g ica l and g eochem ical study o f an area of 800 sq km around Cham pua (22°04'N : 8 5 °4 0 'E ), the type area o f the O lder M etam orphic G roup (O M G ) and the associated to nalites (O M T G ), has been ca rrie d out. Structural Analysis and Field Relations of the O M G and the O M TG Rocks

The OMG m etam orphites w hich co nsist of m edium -grade m etam orphosed pelites w ith bands of calc-m agnesian m etam or­ phites (ortho- and para-am phib olites), quartz schists w ith local anthop hyllite schist, cumm ingtonite quartzite and talc-trem olite schist show marked structural unity w ith the to nalite gneisses (O M TG ) w hich occupy large areas to the east and south of C ham ­ pua (Fig. 4 ). The planar structures in these rocks a r e :—

> CRUSTAL EVOLUTION : SINGHBHUM-ORISSA

A. K. SAHA AND OTHERS

STRUCTURAL MAP OF THE CHAMPUA REGION (PARTS OF KEONJHAR 8 SINGHBHUM DISTRICTS). EASTERN INDIA < OMTG

OMTG

OMTG

O M TG

OMTG

lA D U B A

GANGPUR

PATAJAINT tom tg

OMTG

OMTG

OMTG

OMTG IAINTGARH

OMTG

OMTG

OMTG OMTG

OMTG KUOAGADI OMTG

OMTO

/

OMTG

OMTG

OMTG

OMTG

OMTG

OMTG

OMTG

OMTG

OMTG

OMTG

OMTG POTRA OMTG O M T3 OMTO

OMTG

OMTG

'Mf

K A L IK A F P R A S A O .

4S

,e

OMTG

OMTG

6Q 25 85.

OMTG

*

OMTTG

O M TG

om tg\

3of=° • 47 OMTG

OMTG

OMTG

7

eo

OMTG

PAR SALA

"'V'

/-A > M T G

'OMTG OMTG

OMTG 3

OMTG A

**

4 KM .

S C ALE

OMTG

OMTG

OMTGQ

Legend OMTG

'OMTG

Q uartz b re c c ia OMTG

OMTG

OMTG

[ N0D ]

Newer d o le rite

J—-

Kolhan sandstone / conglom erate

J

CZhU I KG 1 ---------1

OMTG

I

PG

1

Basic lavo K o ro n jia

Gronlte

Palaoponga G ra n ite

) I

J

Singhbhum Gran, L (-^BG )

L QR j

Q uartz r e * f

[0MTG~|

O lder M etam orphic Tonalite — gneiss

(OM TG)

M etagabbro Older M etam orphic

L iL j

Q u a r tz itc / Q u a r t z schist

| CM 1 --------- 1

C 5 cm ) w ith decussate structure have developed and coarsening of grain size due to therm al m etam orphism has taken place.

W ithin the granite, a b o ut 25-50 m east of the tuff-Kolhan sandstone co nta ct, a 60 m x

Thus the overall field evidence in the Deo Hver section indicates that before intrusion

■H

CRUSTAL EVOLUTION : SINGHBHUM-ORISSA

of the Singhbhum G ranite, dipping (30 -45 ) IOG conform ably overlying the regionally m etam orphosed

the w est to WNWro cks w ere unhighly fold e d and OMG rocks.

that many o f the pegm atite and a p lite in je c ­ tions w ith in the OMTG rock are also truncated by the e n closing p o rp h yritic g r a n ite ; in the latter another set of pegm a­ tite and aplite in je ctio n s occur.

.--------, I * . * I P E G M A TITE

|---------- , P ALA SP O N G A |______ | POR PH YR ITIC G R A N IT E (SIN G H B H U M G R A N ITE

l» « I OMG TONALITE G NE IS S

1 1^

T A P L IT IC IE U C O

G R N ITE

1 - w t F O L IA T IO N

Figure 7 : Equal-area projection diagram (after S. N. Sarkar, P. R. Paul and S. Majumdar, un­ published data) ; jr Si poles of Kolhans and BHJ exposures and rafts in Deo river section ; mean Si for Kolhan dips 8 ' /N .2 8 8 ', and mean Si plane for BHJ and tuff dips 32" /N .292 s. Modal Sj corresponding to poles (mean — centred circles) in OMG rafts dips 5 T /S W .

Near the b order regions of the main OMTG area, in je ctio n s o f the ad jacen t Singhbhum G ranite are lo ca lly abundant. Along the northern b o rd e r region o f the OMTG near Juldiha there are extensive in ­ jections of le u co cra tic g ra n ite /g ra n o d io rite , the latter being very sim ilar to the a d jo inin g Karanjia m em ber o f the S inghbhum G ranite. Towards south, about 14 km w est o f Jhom pra (2 1 °5 0 ': 85 35') there is a zone of mixed granitic rocks w here the OMTG, the Palas­ ponga porphyrite granite (a variant of the Keonjhargarh unit of S inghbhum G ranite) along with injection s of a p litic le ucogran ite and pegm atite o ccu r togethe r. C ritic a l field observations (Figs. 8A, B ) cle arly indicate that the OMTG occu rs as xe n o liths w ithin the Palasponga p o rp h y ritic gran ite and the foliation of the p o rp h y ritic gra n ite swerves around the x e n o lith s ; margins of the OMTG

are clearly rounded by magmatic corrosion. An Interesting feature of these exposures is

Figure 8A : Sketch map o f an exposures 3 km west of the Palasponga showing blocks o f OMG tonalite gneiss enclosed in po rphyritic Palasponga granite, both rock types being intruded by aplite and pegmatite.

r - r —i

I.

.

I PEG M A TITE

,--------- , l«

»

V /'/A

I OMG T O N A LITE G N E IS S

,---------- 1 PALASPO NG A I______ I PORPHYRITIC GRANITE

r rr r r r ,

(SINGHBHUM

G R A N ITE )

L v . : vl A P LITIC LEUCO GRANITE

FOLIATION

Figure 8 B : Relations among various kinds of g ra nitic rocks in a mixed zone, 3 km west of Palasponga.

A. K. SAHA AND OTHERS

12

Petrology and G eochem istry of the O lder M etam orphic G roup

The type area fo r the O lder M etam orphic G roup around C ham pua consists do m inantly of p e litic schists w ith several th ic k bands of para-am phibolites, relative ly thin bands of quartzite and quartz-se ricite schist, and local bands o f a n th o p h yllite schist, and are intruded by sill-like m afic intrusives {n o w m etam orphosed to am p h ib o lite and horn­ blende s c h is t). The m edium -coarse grained m uscoviteb io tite s c h is t consists o f quartz, b io tite and m uscovite to g e th e r w ith a sm all pro p o rtio n o f opaque ores and rare sillim a n ite and g a r n e t; subhed ra l sm all to u rm aline and e lo n g a te d -e llip tic a l zirco n (lig h t bro w n) are occasiona l accessories. The quartzite bands lo c a lly carry a green co lo ured m uscovite ; in a few rocks, such as near G odori, the green m uscovite constitutes a bout 15 to 20 p erce n t by volum e of the r o c k ; the flakes are 0.3 to 0,6 cm across. A che m ical analysis (T able 3) of the green m uscovite in dica ted th at it is chrom ian m uscovite ra ther than fuchsite.

TABLE 3 CHEMICAL ANALYSIS OF THE GREEN MUSCOVITE FROM OMG QUARTZITE, NEAR GODORI Si Os

48.29

AliOn Cr^Os

35.67

FessOs

0.55

MgO

1.55

CaO

0.65

Na20 K>0 H?0-

1.75

0.25

4.92 3.30 Total

96.93

S tru ctural form ula o f th is m ineral com es to— (,C a .O0 N a . 45 K . 8 4,) ( A l 4. 09 C r . 03 F e s -0fi

M g .g ,) (S in.46 A l l .g4)(O H )2.8g Some o f the quartz sch ists are quartzm agnetite schists and quartz-m agnetitecu m m ing ton ite schists. In one lo c a lity west

of Karanjia, cu m m ingtonite ( M g :F e = 4 5 : 55) is the d om inant m ineral together with m agnetite and little quartz. M ajor and trace elem ents in five OMG m uscovite-biotite schists have been deter­ mined and com pared with the average p e litic ro ck (cf. Shaw 1954, 1956). The average com position o f the p e litic schists o f the present area is broadly sim ila r to that o f the average p e litic rocks (T able 4 ), but the OMG p e litic schist has s lig h tly higher S i0 2, K.,0 and K .O / Na.,0 ratio, and low er TiO„, CaO and A l20 3 ; other m ajor oxides are slig h tly low er o r more or less sim ila r to the average m edium -grade m e tapeiitic rock. As regards trace elem ent com p o sitio n the Cham pua schist is a ppreciab ly rich e r in Cr, Ni & Co and to some extent in V, Ga and appreciab ly poorer in Sr, Li, Y and Z r com pared to the average m e tapeiitic ro ck of Shaw (1954, 1956). The G owganda a rg illite (early Pro­ te ro zo ic) of O ntario (Y oung 1969) has som ew hat higher ALO ,, MgO and low er SiO., than those in the OMG p e litic schists (Table 4 ). U and Th data on a few p e litic schists indicate that U tends lo rem ain fa irly constant w ith increasing S i% , but Th tends to decrease w ith increasing S i% . The slig h t enrichm ent of the OMG pelites in Cr and Ni and its im poverishm ent in Ti seem to be ch a ra cte ristic of many A rchaean sedim ents (N aqvi et al 1978; Taylor 1979). The calc-magnesian m etam or/jfiites w hich cover considerab le areas in the NW & eastern parts o f the type area consist of calc-gneiss (b a n d e d ), hornblende schist (sch isto se ) and am phibolite (nearly mas­ siv e ). Because o f intim ate association of the three varieties, it has not been possible to map them separately. All the three varieties are com posed m ainly of am phibole, plagioclase and quartz to gethe r w ith m inor and varying p ro portion s of clinopyroxene, epidote-zoisite, ca lc ite and opaque ores. C ritica l study of the petrography of the OMG calc-m agnesian m etam orphites has indicated that they fall into Iw o d istin ct groups viz. m assive to schistose am phibolites and banded am phibolites. The form er group of rocks is characterised by coarse-

MAJOR

AND

TRACE

TABLE 4 ELEMENTS IN THE OMG 0IOTITE-MUSCOVITE SCHIST AND QUARTZITE/ QUARTZ-SCHIST ROCKS ALONG

WITH AVERAGE ANALYSES OF SI. No. Sp. No

1

2

3

SR

_

SR

SR

621

501

500/1

B Ba Be Co Cr Cs Cu Ga La Li Ni Pb Rb Sc Sr Th U V Y Zr

170 > 200 0 axis NE-SW; AP2v NE-SW

MIHIR

F3S cross fo ld ' causing culm ination and depression of F2 ; dom e and basin type interference. Fi iso clin ica l antiform plunges to­ wards north (in the NE sector). APi = E-W, M.L. plunges steeply towards SW. F2 Open fold plunges steeply to the south. APj == N-S.

Tectonic remnant of layered igneous body (re-crystallised and deform ed).

Ramaduari, Sankaran, Selvan and W indley 1975.

Chimakurti, Prakasam dt., A.P.

Gabbro, hornblende syenite, m elanofelsic syenite, nepheline syenite etc.

Quartzite, pelitic gneiss also migmatite.

Country rocks affected by folds plunging 5 0 ’ towards 76°, fold structure is mimetic in syenite body (Plunging 58° towards 6 0 ").

T e c h n ic a lly controlled intrusions, brodly late tectonic.

Bhattacharyya, Bose and Dutta, 1982.

Koraput, Orissa.

Alkali gabbro, calc, alkali syenite, perthite syenite, nepheline syenite, a la skitf.

Khondalite, calc, gneiss and granulites.

Development of antiform al and synformal tight (reclined type) folds in country r o c k s ; also m im etic in intrusives. So parallel to Si A ntiform al axis 48° towards 114°. Synformal axis ; 46" towards 136°.

Perthosite nepheline syenite, alaskite.

Garnetilerous biotite gneiss, hypersthene granulites, am phibolite.

APs s NE-SW (on S i). Fs== Cross fold. M.L. plunges 20° towards (on Syenite).

6.

7.

dt.,

Bose 1970.

Synformal c lo s u re ; convergence of the intrusive at d e p th ; may be tec­ h n ic a lly controlled emplacement.

Sarkar

1970.

MAITRA

218°

T e c h n ic a lly controlled instrusions.

MONOJ

8 . Kunavaram, Khammam dt., A.P.

Salem

RAY AND

G ranite gneisses, am phibolites.

Sittampundi, Tamil Nadu.

K. BOSE, JYOTISANKAR

Gabbro, eclogite (gra nu lite), pyroxenite, charno.ckite, anorthositic gabbro-

5.

TABLE

SI. No.

Locality

2

1

Igneous emplacement 3

1

( C o n td .)

Country rocks

S tructure

Remarks

Source of data

4

5

6

7 Basu and Bose 1970.

10.

Kondapalli, A.P.

Pyroxenite, gabbro, norite, leuconorite, anorthosite, tonalite.

Charnockites, migmatites etc.

Country rocks show S„ parallel to Si. Fold on country rocks plunges gently towards ENE or WSW.

Post orogenic emplocement, layered type intrusion ; unaffected by folding.

Bose and Bose 1977.

11.

Elchuru, Prakasam dt., A.P.

Mafic alkaline rocks, syenites, lamprophyres and carbonatites.

Highly deformed quartzo feldspathic m igm atitic gneisses.

Arcuate form of alkalic intrusion. Host rocks reflect interference of tw o phases of folding with small angular divergence between the ir fold axes.

Syn-to late-kinema­ tic, controlled by fold movements, resulting in phacolithic geometry.

Bose and Nag 1980. Sarkar 1981.

12.

Khammam, Khammam dt., A.P.

Tonalite-granodiorite-granite suite, linked by differentiation history.

Country rocks occu r as xenolithic mafic bands (deform ed) within the graniw toid suite.

Igneous intrusions free of any structural d e fo rm a tio n ; country rocks affected by at least two phases of deformation.

13.

Angul, Orissa.

Anorthosite, norite, gabbro, acid charnockite.

Khondalite, leptynite, calc, silicate gneiss, granulite, quartzite etc.

The fold axis of the m ajor synformal fold on Si plunges 56° towards 145°.

Krishna dt..

Post-tectonic calc-alkaline emplacement,

Mattra 1981, Maitra 1984 (in Press).

Igneous emplacement and deformation closely related.

Bhattacharyya and De 1964.

GHATS

M oulding of the prevailing structure caused... by em place­ ment of the alkalic pluton.

EASTERN

S„ parallel to Si found, Fold affected country rocks, also mi­ metic in intrusives. Fold non-cylindrical, plunges 57 towards 29°. M.L. plunges at high angle to­ ward NNE.

IN THE

Gneisses and granulities.

EVENTS

Ferror syenites, different varities of nepheline syenite.

MAGMATO-TECTONIC

Sivamalai, Coimbatore dt., Tamil Na3u.

9.

Table— 1 (Contd.)

SI. No.

Locality

Igneous emolacement

2

1

Country rocks

Structure

Remarks

4

5

6

Source of data

Visakapatnam, A.P.

Gabbro, norite, hornblende norite, biotite-norite and hypersthene norite.

Garnet-sillim anite gneisess, garnetbiotite-sillimanite gneiss, quartzite and calc, granulite.

S„ parallel to Si found, another fold plunging towards SE. M.L. plunges 45° towards SE.

Intrusive and country rocks closely related.

Sriram das and M urthy 1975.

15.

Kadavur, Tiruchirapally, Tamil Nadu.

A northositic rocks, mafic rocks.

Quartzite, am phi­ bolite, calc, gneisses.

Fi = Eight reclined fold, plunges steeply towards SE. APi : ENE— WSW. (open symform) co-axiai with F;. A P -: N-S. : Fj open asymmetric folds, axis E-W.

Igneous intrusion corresponding to PreFi, Syn-Fi and post-Fs episodes.

Sarkar and Bose 1979.

16.

Oddanchatram, Madurai dt., Tamil Nadu.

Anorthosite with subordinate gabbro anorthosite.

Meta sedirr.entaries, basic granulites, migmatites.

Country rocks constituted a NNE plunging antiform al fold.

Anorthosite reeks constitute a dome shaped labradorite pluton emplaced as a phacolith.

Narshima Rao 1977.

MIHIR

14.

K. BOSE, JYOTISANKAR RAY AND MONOJ

Note :

Fi of one area may not be the same deform ation causing Fi in another area. may not be operative all over the belt.

In other word deform ation causing Fi in different areas

MAITRA

Fi = First fo ld ; F i= S e c o n d fo ld ; F s=T hird fo ld ; APi = Axial plfine o f the first fo ld ; AP2= A x ia l plane of the second fo ld ; AP3= A x ia l plane of the third fo ld ; S„ = Primary b e d d in g ; Stff= Primary igneous la ye rin g ; St = F oliatio n/ Schistosity ; M.L. = M ineral lineation.

MAGMATO-TECTONIC EVENTS IN THE EASTERN GHATS

complex is strongly co ntrolle d by fold move­ ments in country rocks and define c o n co r­ dant arcuate geom etry of p hacolith. It is striking that Eastern G hats Pre­ cambrian belt, which is know n to be c h a ra c­ terised by intensive deform ation , accom ­ modates some igneous m em bers apparently free of deform ation h istory and m etam or­ phism. Some of apparen tly post- to latetectonic em placem ents so far studied are gravity-differentiated m afic bodies but pos­ sibly also include some calc-alka line g ra n i­ toid suite. The areas w here such deform a­ tion free intrusions have been studied are: 1. Strongly differentiate d m afic sheets of K ondapalli-G angineni h ill range, Krishna d istrict, A.P. (B ose and Bose 1982).

TABLE

123

2. Funnel shaped d ifferentiate d body o f C him akurti area, Prakasam d istrict, A.P. (D atta 1978, B hattacharyya 1980). 3. T o n a lite -gran odiorite-granite suite of Kham m am area, Khammarn d istrict, A.P. (M a itra 1981). A ccepting the sequence o f te c to n ic de­ form ation and associated m agm atism as out­ lined above, d e cip h e rin g of the sequence of events could be attem pted. In absence of adequate g e o ch ro n o lo g ica l data fo r the key litho-units it w ill re fle ct the broad g e olog ical history o f the P recam brian terrain rather than a true stra tig ra p h ic colum n. A pro­ posed P recam brian ch ro n o lo g y in the Eastern Ghats g ra n u lite belt, envisaged from the w ork ca rrie d o u t in d iffe re n t sec­ tors, is presented in Table 2 .

2

GENERALISED PRECAMBRIAN CHRONOLOGY IN EASTERN GHATS HIGH GRADE BELT

Archaean Relationships in Windley and Smith (1976).

High

Grade

Regions,

Precambrian Chronology in Eastern Ghats Granulite Belt (as proposed in this co n trib u tio n )

Oldest : Formation of gneissic basement Deposition of supracrustal rocks (basic volcanics, pelitic schists, gneisses, marbles and quartzites) Interthrusting of supracrustals and basement

G ranite-granodiorite-tonalite (basem ent) ( 3000 Ma ? later rea ctivited ). Layered A northosite-am phibolite m etaplutonite s Ultram afic-m afic (basem ent com ponent).

Intrusion of layered ultram afic-gabbro-anorthositic complexes, Intrusion of tonalite, granodiorite

Formation of supracrustal rocks. Regional folding ( = h ig h grade m etam orphism ) Folding metamorphism m igm atisation (R e a ctiva tio n ? of ba sem ent/supracrustals)

Youngest : Deformation and metamorphism of tonalite-granodiorite to give rise to high grade gneisses

G ranite (Alaskite, pink and grey granites, pegm atites) Cross fo ld in g ; Massif anothosite (Ca. 1300 Ma) Alkaline plutons, carbonatites, Gravity differentiated layerel mafic intrusions, Post-to late-tectonic tonalite-granodiorite-granite emplacement.

124

MIHIR K. BOSE, JYOTISANKAR RAY AND MONOJ MAITRA TABLE CHEMICAL ANALYSIS

3

OF MAFIC MINERALS FROM GABBRO COMPLEX (SPECIMEN NO. S-55)

OF

TORAPPADI

Result in Wt. % C linopyroxene

O rthopyroxene

Hornblende

FeO (to ta l) CrjOs MnO NiO MgO CaO Na20 K2O

53.838 .107 1.468 4.387 .143 .155 — 15.606 23.961 .323 —

1.156 13.940 .151 .214 — 29.019 .241 .154 —

49.917 .437 8.569 6.561 .534 .098 .060 18.183 12.280 1.470 .011

Total

99.988

99.90

98.12

SiO. TiO j

AI2O3

Si Al Al Ti Fe+s Cr Mg Ni Fe ( .026 3

1.968 1 .032

\J

.037 .003

.017

^

.004 .853

.004 1.547

i

2.000

- 2.001 .135 .005 .941 .023

—

.417 .006 .009

y

7.108 } i .982 '

2.000

8.00

.438 ^ .046 j .059 3.811 .007

2.010

L 5 144 f |

.771

,

.012 1.850 , .401 I 2.253 )

.011

2.253

Number of ions on basis of six oxygens (fo r CPX & OPX) and on the ba e is'o f twenty three oxygens (fo r hornblende). EMP Analysis available by courtesy of Dr. D. Saha, Royal School of Mines, London.

PETROLO G ICAL RESUME OF THE EM PLACEMENTS T orappadi Layered C om plex, T am ilnadu An ultra m a fic-m afic layered com plex com ­ prising pyroxenite, m ela-gabbro, m ela-norite, gabb ro and norite at T orappad i has been re­ ported (Ray and Bose 1984). The pyroxenite is com posed ch ie fly o f orth op yro xe ne with sub ordina te am ount of pale-green am phibole a n d /o r clin o p yroxe n e . The m afic units co nsist of p lag io cla se (a ndesine to bytownite) and pyroxene as the m ajor m inerals w ith co n sid e ra b le am ount o f am phibole. EMP analysis o f representa tive clino-and orthopyroxe ne as w ell as am phibole from the m afic m em ber (g a b b ro ) o f Torappadi has been presented in Table 3. T orappadi com plex has been em placed w ith in a g ra n ite -g ra n o d io rite -to n a lite terrain. A nalyses of some representa tive co u n try rocks have been fu rnished in Table 3A.

TABLE

3A

CHEMICAL ANALYSES OF THE TONALITETRONDHJEMIEE GENESSES FROM TORAPPADI NORTH ARCOT, TAM IL NADU Sr. No. Sp. No. SiOs AljO:i Fe,'0:l FeO MgO CaO Na^O K2O TiO:! PsOs MnO H2O (to ta l) LOI Total

1 619

2 790

69.46 11.92 1.08 2.33 0.75 4.04 4.16 1.73 1.40 0.32 0.04 4.00 n.d. 99.98

75.99 10.95 0.29 2.50 0.40 1.46 3.35 2.17 0.08 0.32 0.02 n.d. 2.34 99.87

1. Hyperthene-tonalite 3. Hyperthene-tonalite n.d. = not determined.

3 810

4 147

70.80 T2.78 0.55 1.10 0.69 3.59 5.24 1.73 0.15 0.34 0.18 2.40 n.d. 99.55

73.30 11.75 0.41 0.87 0.11 2.75 5.06 3.18 0.06 0.27 0.01 2.21 n.d. 99.98

2. Trondhjem ite-gneiss 4. Tonalite A na lyst: J. S. Ray

125

MAGMATO-TECTONIC EVENTS IN THE EASTERN GHATS

Fresh plagioclase fra ctio n s were sepa­ rated from the m afic and m ela-m afic units of Torappadi as well as from that of adjacen t smaller layered bodies of M elnachipattu, by repeated operations in isodynam ic separa­ tor. Partial chem ical analyses of these plagioclase sam ples are presented in Table 4. TABLE

4

CHEMICAL ANALYSIS OF PLAGIOCLASE SAMPLES FROM MAFIC AND MELA-MAFIC UNITS OF TORAPPADI AND ADJACENT AREAS (data in Wt % ) Sr. No. Sp. No. NajO KaO CaO

1 S-1

2 1-15

3 S-25

4 611

5 583'

5.14 0.98 10.21

5.62 0.87 9.53

2.40 0.12 16.00

4.50 0.61 12.00

2.25 0.69 15.48

Equivalents feldspar com position (W t) % ) 43.49 5.79 50.65

Ab Or An 1. 2. 3. 4. 5.

47.55 5.14 47.28

20.30 0.71 79.37

38.07 3.60 59.52

19.01 4.08 76.87

Calcic labradorite, southern mela-gabbro unit, Torappadi. Labradorite, southern mela-gabbro unit, Torappadi. Calcic bytownite, Gabbro unit (m id d le ), Torap­ padi. __ Bytownite, Gabbro layer, M elnachipattu. Bytownite, Gabbro layer (northern b lo ck), Torappadi.

M afic-ultram afics of Ten-M udiyanur, Tamil Nadu

The ro ck types of Ten-M udiyanur area were studied in three d iffe re nt sub-sectors and named after nearby villa ge s w h ich are as fo llo w s : (a)

A lla p p a n u r: Near to the v illa g e Allappanur the igneous com plex, e llip tic a l in shape, com prises m agnesite bearing u ltra m afic at the core fo llow ed outw ard by gabbro. In the northern extrem ity o f the com plex, exposure o f c o rru n d u m syenite has been located. The m afic un it form s the envelope of the ultra m afic w ith o u t any ch ille d contact. The rocks o f the co m ple x possibly crysta llise d from a s in g le m agm a under­ g o in g fra c tio n a l crysta llisa tio n .

(b) M u d iy a n u r: Near to the villa g e M udiyanur a few arcuate lenses of m etam or phosed u ltram afic rocks have been m apped. These bodies com prise talctre m o lite -a ctin o lite schists. (c) Pum a/ai: Pumalai hill is m ainly made up o f gra n ite gneiss. A round this, arcuate rim s of u ltram afic and m afic rocks are found to occur.

C him alpahad C om plex, Kham m am , A.P.

The C him plapahad co m plex com prises m etaanorthosite, g ra n u lite and striped a m p h ibolite (F ig. 4 ). A ll o f them were prim a rily co-m agm atic and differentiate d under high w ater pressure from a parent m agm a of th o le iite co m p o sitio n (G upta 1977). Few representative chem ical analyses of rocks are given in T able 5. The C him al­ pahad com plex was em placed as a sheet TABLE

5

CHEMICAL ANALYSIS OF THE CHIMALPAHAD AND TALLADA AM BHIBOLITE AND GARNETIFEROUS BASIC GRANULITE SI. No. Sp. No.

1 P-120

2 146B

3 14

SiO^ TiOA l20:t Fe,0:, FeO MgO CaO Na-0 K ,0 MnO P2O; H20 (to ta l) CO,

50.51 0.35 16.87 4.17 7.50 6.87 9.65 1.72 0.51 0.03 0.12 1.43 —

49.31 0.42 13.70 1.70 10.07 12.41 7.87 1.47 0.37 0.01 1.66 —

51:08 0.81 14.10 5.65 7.16 6.91 10.70 1.25 0.50 0.04 0.01 0.88 0.59

49.97 0.89 14.36 3.48 10.62 6.67 9.42 1.92 0.93 0.18 0.12 1.42 —

Total

99.73

98.99

99.68

99.98

—

4 482

SI. No.‘1 = Thinnly striped am phibolite of chim alpahad SI. No. 2 = Am phibolite of Chimalpahad SI. No. 3 = G ranulite of Chim alpahad SI. No. 4= Am phibolite o f Tallada (ana­ lyst : S. Roy M oulik) Source of d a ta : Gupta (1977) for SI. Nos. 1, Roy M oulik (1982) for Sf. No. 4.

A n a ly s t: S. Gupta

2

and 3

12 6

MIHIR K. BOSE, JYOTISANKAR RAY AND MONOJ MAITRA

like body w hich suffered fra c tio n a l c ry s ta lli­ sation and now show s c o nspicu o u s alternate layers of highly c a lc ic p la g iocla se (average norm ative com p o sitio n A b ig . a a A n ^ . ^ O iV js vide Table 6 ) and horn blen d e w ith sub­ o rdinate am ount of pyroxene, garnet, scapolite and zoisite.

ch ie f constituen ts in the suite are a lka iic fe ld sp a r (p e rth ite ), pyroxene (fe rro a u g ite ), am phibole, b io tite and nepheline. The diffe re n tia tio n index in the suite shows gradual increase from the m elanofelsic syenite to nepheline syenite through horn­ blende syenite. C hem ical analysis fo r these rocks have been given in Table 7. The

TABLE 6 TABLE CHEMICAL ANALYSES OF PLAGIOCLASES OF CHIMALPAHAD ANORTHOSITES SI. No. Sp. No. CaO% Na20 % K -0%

1 151

2 69A

3 10

15.58 2.17 0.44

15.60 2.07 0.41

14.84 2.02 0.14

Equivalent feldspars

(w t% ) Average

An Ab Or

78.98 18.75 2.27

Source of data :

79.83 17.87 2.30

80.96 18.42 0.62

79.92 18.34 1.73

A n a ly s t: S. Gupta Gupta (1977)

The C him alpahad com plex is associated w ith sm aller c o n c o rd a n t sa te llite bands. They are associated with m afic-granulites ( = o liv in e g a b b ro ) and local m ela-ultram afics located tow ards the base o f the com plex. The m etaanorthosite com plex is sp atially associated w ith extensive am ph ibolite s w hich on chem ical basis correspond to tholeite. A new and s ig n ific a n t observation from th is terra in is progressive increase in the grade of m etam orphism to w a rd s ESE i.e. to w a rd s the main Eastern G hats range as indicated by key assem blages o f m afic cou n try rocks.

C him akurti a lkaiic suite, A.P.

The a lk a iic suite o f rocks exposed about 16 kms. w est o f C him aku rti in Prakasam dis­ tric t, A.P., co m p rise s m elanofelsic syenite, hornblende syenite and ne phe line syenite in the sequence of crysta llis a tio n (Bhattacharyya et al 1981, vide Fig. 9 ). M elano­ fe lsic syenite, the o ld e st m em ber appears as d is tin c t inclu sio ns w ith in o the r syenites. The

7

CHEMICAL ANALYSES OF ERRAKONDA SYEN­ ITES, CHIMAKURTI AREA, PRAKASAM DISTRICT, ANDHRA PRADESH

1 SiOiTiOAlaO.i Fe-0:i FeO MnO MgO CaO Na-iO K20 PaOs LOI Total

2 ____

3

57.48 1.00 18.63 3.25 5.72 0.02 0.31 3.69 4.24 4.98 0.26 0.48

56.03 tr 23.49 0.80 1.57 0.56 0.32 2.02 6.49 6.39 0.75 1.30

58.62 0.16 23.80 0.49 0.39 0.03 tr 1.45 7.29 6.51 0.34 0.35

100.06

99.77

99.43

1. M elanofelsic Syenite of Chim akurti alkaiic suites, A n a ly s t: B. P. Gupta 2. Hornblende syenite. Analyst': S. K. Bose and H. S. Datta 3. Nepheline synite, A n a ly s t: S. K. Bose and H. S. Datta Source of d a ta ; Bhattacharyya et. al. (1981)

alkalies, alum ina as also a g p aicity show steady increase in d ica tin g advancing stages of diffe re n tia tio n of a parent syenitic magma. The d ifferent mem bers of the site are linked by progressive diffe re n tia tio n but the a lkaiic suite does not show spatial association w ith any m afic mem ber to w hich its parentage can be attributed. Hence the parent m agma from w hich the C him akurti syenite crystallised seems to have been p rim a rily a m afic syenitic magma w hich was generated at the closing phase o f deform ation history in the belt. In vie w of p e tro g ra p h ic types and m ineralogy of the rocks, the Chim akurti

MAGMATO-TECTONIC EVENTS IN THE EASTERN GHATS

alkalic suite has strik in g resem blance to Sivamalai alkalic assem blages (Bose 1971). Absence of older m afic m em ber in both the suites is striking. Both the em placem ents faithfully mimic the stru ctu ra l geom etry of

W * V . N o 2 0 +k 2 o Figure 10: Al-O.i versus N ajO -i-K jO diagram showing the tho leiitic nature of the mafic rocks in the Kondapalli-Gangineni igneous suite. Ultramafic cumulates shown by crosses and mafic rocks shown by solid circles.

the country rocks. R ock asso ciation s of Koraput suite in O rissa and KunavaramRudramkota suite of A.P., on the o th er hand are very sim ilar. This sim ila rity is also evident in terms of trace-elem ent d strib u tio n in the two suites (Bose, M aitra and G hosh Roy 1984), The broad fold geom etry locally developed at the northern end o f Kunvaram body (S a rkar 1970) is d ifferent from that o f K oraput em placem ent. The axial trends o f folds in the tw o terrains though oriented m utually at rig h t angle, both the em placem ents appear to be te c to n ica lly controlled (late kin em atic) guided by avail­ able structural fram e w ork. E lchuru alka­ line com plex in A.P., however, is apparently distinctive from all these a lka line e m place­ ments in rock chem istry (B ose and Nag 1980: Bose, Mitra and Ghosh Roy, 1984). It, is possible that the a lkaline suites o f Eastern Qhats may have a little tim e gaps between

127

th e ir em placem ents accom panie d by change in magma chem istry. K ondap aili-G angineni suite, A.P. The area around K o ndap alli was pre­ viously studied by K rishna Rao (1964) and Leelanandam (1967,, 1972), but none of them could establish th e existence o f welldeveloped g ra vity-differentia ted igneous com plex in the hill range (B ose and Bose 1982). The K ondap alli-G angineni igneous com plex, K rishna d istrict, A.P. com prises a group of d ifferentiate d igneous sheetintrusions representing fra c tio n a tio n of parental th o le iite m agm a (Fig. 10). The m ajor m em bers of the su ite reveal a com ­ p ositiona l spectrum from pyroxenite (w ith sub-ordinate du n ite -ch ro m itite b a n d s), through m ela-gabbronorite, g a b b ro n o rite interlayered w ith anorthosites to m ore felsic m em bers viz. quartz d io rile and tro n d h je m ite (Fig. 11). C om positional layers in the m afic and u ltram afic zones o f the com plex are the result of crysta l-liq u id m ovem ents as evi­ denced by m ajor and trace-elem ent va ria ­ tions (T ables 8 and 9) and te x tu re s in TABLE CHEMICAL

ANALYSES OF THE GANGINENI SUITE 1

2

51.43

0.15 —

49.08 0.75 6.07 2.25 10.83 0.20 19.62 10.35 0.58 0.12 0.07 0.85

100.10

100.67

Sr. No. Si 0-2 TiOa AluOn FeaOn FeO MnO MgO CaO Na O KjO P?Os H2O Total

8

—

6.66 3.60 6.10 0.16 30.65 1.12 0.23 —

3 50.64 0.04 29.56 0.65 0.97 0.03 0.94 13.13 2.98 0.65 0.02 — 99.61

KONDAPALLI-

4

5

50.33 1.02 16.67 1.74 9.33 0.15 7.20 10.31 2.12 0.36 0.12 —

65.05 0.09 22.18 0.69 0.62 0.01 0.56 5.64 3.90 0.69 0.35

99.35

99.75

—

1. Enstatolite, average of 2 rocks, 2. Pyroxenite, (W ebsterite) average o f 4 rocks, 3 Anorthosite, average o f 5 rocks. 4. G abbronorite, average of 6 rocks, 5. Felsic member, average of 3 rocks, A n a ly s t: S. K. Bose Source of data": Bo$e and Bose (1982)

128

MIHIR K. BOSE, JYOTISANKAR RAY AND MONOJ MAITRA

Figure 11 : G eological map of a part of Kondapalli hill range, showing the nature of occurrence of the igneous intrusions. Inset index map shows the location of the investigated area.

the suite. The igneous com plex o f Kondapalli-G angin eni is undeform ed and unm eta­ m orphosed and the re fore d is tin c tiv e am ong the anortho site bearing igneous com plexes of the Eastern G hats P recam brian belt. The obvious defo rm atio n free c h a ra cte r of K ond ap alli suite may be a n ticip a te d to be due to em placem ent along lim bs o f regional folds. However, the im m ediately associated co u ntry rocks record as m any as three phases of deform ation, none o f w hich is signatured on the igneous com plex. AS m entioned above, the m ajor m embers of the d ifferen tiate d in tru sio ns have sig n i­ fic a n t spatial and s tra tig ra p h ic re la tions as observed in d iffe re n t bodies viz. ultram afics tow ards the flo o r and acid m em bers on the lop o f the sheets (F ig. 12). The ro ck types co m prisin g the w hole suite o f the area may be listed as b e lo w : Felsic type

Q uartz-diorite, tonalite, tro n d h je m ite

M afic and ultrabasic type

:

Mela gabbro-norite, gabbro-n orite anorthosite, fe ld sp a th ic pyroxenite

Figure 12: Diagrammatic section showing different litho-m embers of the Kondapalli suite. G = Gravity, M - d ir e c tio n of movement of magma, C = direc­ tion of crystal sinking. 1. G ranolites (country ro cks), 2. Ultram afics, 3. G abbronorites, 4. Felsic members, A = Anorthosites.

MAGMATO-TECTONIC EVENTS IN THE EASTERN GHATS

129

TABLE 9 TRACE ELEMENT CONTENTS (AVERAGE AND RANGE*) AND ELEMENT RATIOS IN MEMBERS OF KONDAPALLI-3ANGINENI SUITE br. No.

1

Ba

30

2

3

4

108

425

208.8

(10-350) Co

50

44

(30-1200)

750

< 10-30

1040

25

Nl

•

800

28.1

90

Sr

.


e origin o f the m etaanorthosite gabbros and garneiiferous granulites of the Sittam pundi Complex, Madras, India. J. Geol. Soc. 1nd., 16, 391-408. Krishna Rao, J. S. R. 19 64 : C hrom ite from Konda­ palli, Krishna dist., A.P. Econ. Geol., 59, 678-683. Leelanandam, C. 1967 : O ccurrence of anorthosites from charnockite area of Kondapalli, AndJira PraPradesh. Bull. Geol. Soc. India, 4, 5-7. Leelanandam, C. 1972 ; An anorthosite layered com ­ plex near Kondapalli, A. P., Q. J. Geol. Min. Met. Soc. Ind., 44, 105-107. Maitra, M. 1981 : Petrological studies of the area around Khammam, Khammam district, A.P. M.Sc. thesis, Cal. Univ. (un pu b.). Maitra, M. 19 84 : On the to n a litic suite of Khammam, A.P.— a study of m ineral correlation as petrogenetic clue. J. Geol. Soc. Ind. (in press). Nag, S. 1979 : On the alkaline com plex of Elchuru, Prakasam district, Andhra Pradesh, Sci. Cult. 45, 207-209. Nag, S., Smith, T. E. and Chakravorty, P. S. 1983 : A lkaline basic intrusive of Elchuru, Prakasam district, India, Proc. Ind. Acad. Sci. (Earth & Planet S ci.), 9 2 (1 ), 73-80. Narasimha Rao, P. 1977 : Petrology and geochemistry of the anorthosite suite of Oddanchatram, Madurai dist. Tam il Nadu, India (A bst.). Symp. Archaean geochem istry : The origin and evolu­ tion of the Archaean continental crust, Hyderabad, 71-73.

Bose, M. K. and Nag, S. 1980 . Petrology of the alka­ line suite of Elchuru, Prakasam district, Andhra Pradesh. Proc. Ind. Acad. Sci. (Earth Planet S ci.), 8 9 (3 ), 383-401.

Ramaduari, S., Sankaran, M., Selvan, T. A. and Windley, B. F. 1975 : The stratigraphy and struc­ ture o f the Sittam pundi com plex, Tamil Nadu, India. J. Geol. Soc. Ind. 1 6 (4 ), 409-414.

Bose. S. K. and Bose, M. K. 1982 : Petrology and geochem istry of the Kondapalli-G angineni igneous complex, Krishna district, Andhra Pradesh. Ind. J. Earth Sci., 9 (2 ), 150-166.

Ray, J. S. and Maitra, M. 1984 : Some aspects of the Eastern Ghats Precam brian belt and their bearing on crustal evolution : a review. (A bst.). Sem. on crustal evolution of Ind. shield and its bearing on metallogeny, Ind. Soc. Earth Sci., 22-23.

Bose, M. K., Maitra, M., and Ghosh Roy, A. K. 1984': Inter element relation in some alkaline suites of the Eastern Ghats Precambran belt. Proc. Ind. Acad. Sci. (Earth Planet Sci.) 93. Datta , H. S. 1978 : Petrology o f Errakonda hill, Chim akurthi, West o f Chim akurthi, Prakasam dis­ trict, Andhra Pradesh. M.Sc. thesis, Cal. Univ. (unpub.). Emslie, R. F. 1978: Anorthosite massifs, rapakivi granites and late Proterozoic rifting o f North America, Precamb. Res. 7, 61-98.

Ray, J. S. and Bose, M. K. 1984 ; Observations on Torappadi layered com plex, North Arcot district, Tamil Nadu. (A b st.). Proc. Ind. Sci. Congr., 71st. sess., 37-38. Ray, S. and Bose, M. K. 1975 : Tectonic and petro­ logic evolution of the Eastern Ghats Precambrian belt. Chayanica G eologica, 1, 1-13. Roy, Shyamal 19 7 6 : Studies on khondalites and related rocks in parts o f Orissa. M.Sc. thesis, Cal. Univ. (U npub.).

MAGMATO-TECTONIC EVENTS IN THE EASTERN GHATS Roy Moulik, S. 1982 : Structure and petrology o f the area north of Tallada, Khammam district, A.P. M.Sc. thesis, Cal. Uinv. (U n pu b.). Sarkar, A. varam, special suites,

1970 : Geology of the area around KunaKhammam district, Andhra Pradesh with reference to the petrology of the syenite M.Sc. thesis, Cal. Univ. (unpub.).

Sarkar, A. 1981 : S tructure and tectonom agm atic evolution of the Elchuru alkaline com plex, Andhra Pradesh, India. Proc. 4th GEOSEA conference. Manila, Phillipines, 131-137. Sarkar, A., Bhanumathi, L. and Balasubrahmanyan, M. N. 1977 : Petrology, geochem istry and geo­ chronology of the C hilka lake igneo.us complex, Orissa State, In d ia : an appraisal. (A bst.). Symp.

133

Archaean G eochem istry ; the origin and evolution of Archaean continental crust, Hyderabad, 71-73. Sarkar, A. and Bose, M. K. 1978 : O bservations on the Kadavur igneous com plex, Tiruchirapalli, Tamil Nadu. Ind. J. Earth Sci. 5 (2 ), 194-199. Sriramdas, A. and Murthy, M. S. 1975 : Lithology and structure of the Eastern Ghats o f Visakapatnam, Andhra Pradesh. J. Geol. Soc. Ind. 16, 188-192. Vemban, N. A., Subramaniam, K. S., Gopalakrishnan, K. and Venkata Rao, V. 1977 : M ajor fa u lts /d is ­ lo ca tio n s/lin e a m e n ts of Tamil Nadu. Geol. Surv. Ind. Misc. pub. 31, 53-56. W indley, B. F. and Smith, J. V. 1976 ; Archaean high grade com plexes and modern continental mar­ gins. Nature 260, 671-675.

SOME ASPECTS OF THE TECTONIC EVOLUTION OF THE DELHI SUPERGROUP OF ROCKS IN CENTRAL RAJASTHAN

P. K. GANGOPADHYAY, A. GANGOPADHYAY and A. LAHiRI

Department of Geology, Presidency College, Calcutta. ABSTRACT The lith ostratig raph io units of the Delhi Supergroup are exposed in Central Raja­ sthan in a NNE-SSW trending linear fold belt. Field mapping of the selected lo calities in Central Rajasthan suggests that although disposition of the lithounits have been struc­ turally con tro lled during the second phase of deform ation of the Delhi Supergroup, the rocks bear im prints of at least three phases of deform ation. G eochem ical observations of the lithounits across the 'Delhi S ynclinorium ’ suggest that the earth’s crust con­ stituting the Delhi Supergroup show fluctuating chem ical com position. Petrochemical observations of the intrusive bodies, both the basic rocks occu rring at the basal part of the Delhi Supergroup and the post-Delhi granites and pegmatites, indicate the eruption of the basic magma at a converging plate ju nctio n and generation of granite magma by partial m elting of the crustal m aterial at the plate margin.

INTRODUCTION An area of ap p ro xim a te ly 2,500 sq. kms. was selected in central R ajasthan for study o f the pattern of crustal evo lu tio n in th is c ri­ tical P recam brian terra in o f NW India (Fig.

1 ). In o rd e r to arrive at a synthesis o f s tru c ­ ture and stra tig ra p h y o f the region several critic a l sections have been studied in detail so as to obtain c om p le te cross-sections of the Delhi S ynclin orium and the Banded Gneissic C om plex b orde din g the S ynclinorium , both to the east and to the west. These in­ ve stig atio n s involved studies on several as­ pects of the d iffe re n t ro c k types, nam ely structure, stratig ra ph y, petrology, geo­ chem istry and g eo ch ro n o lo g y. The o b je c ­ tives of the in vestig atio n s w ere o rie nte d to determ ine :— 1. S tructu ral and lith o lo g ic a l m apping in selected lo c a litie s so as to obtain a crosssectional view of the D elhi S ynclinorium . 2. E xam ination o f the c o n ta c t region betw een Delhi S up ergroup o f ro cks and

Banded G neissic C om plex both in the eastern flank and the w estern flank of the S yncli­ norium . 3. Status of the d iffe re n t g ra n itic bodies o ccu rrin g w ithin the Delhi m etasedim ents and their relation to orogenesis. 4. Status of the basic rocks w hich cons­ titu te an im portant part o f the Delhi Super­ group and their bearing on te c to n ic evolu­ tion of the region. 5. A m odel show ing like ly mode of evo­ lution of this platform al regim e and surroun­ ding P recam brian terrain. G eology of R ajasthan

selected

terrains

of

Central

In order to obtain a cle a r p icture of this linear fold belt tw o cross-sections, both of Barr-Sendra region and Phulad-Deogarh region, were chosen (vide Fig. 1 ). Besides, in o rd e r to study the nature of w eak zone, the areas like Jassakhera and Sham garh were also chosen. T h e b rie f ge o lo g ica l features are discussed in Table 1.

Monograph on Crustal Evolution, Indian Society o f Earth Sciences, p. 134-160, 1984.

TECTONIC EVOLUTION : DELHI SUPERGROUP

Figure

1:

Map showing location

of the selected areas of investigation.

STRUCTURE AND STRATIGRAPHY OF THE DELHI SUPERGROUP T hrougho ut C entral Rajasthan it is found that the D elhi S u pergrou p of ro cks show three phases o f deform ation. The structural-cum -lithological m apping on the scale of 1.5 cm to 1 km for B arr region and PhuladDeogarh region is represented in Fig. 2a and b. The salient features fo r the stru ctural episodes in this part are as fo llo w s :— 1. trending)

135

The first set of fold, F, (NNE-SSW is cha ra cterised by a tig h t, long-

lim bed, iso clin a l fold w hich is rootless and intra fo lia l in style (Fig. 3 a ). The re crysta l­ lisation is very m eagre. 2. The second set of fold, F„ is co-axial with F, and represented by open, disharm onic p o lyclin a l fold. The co-axiality bet­ ween F, and F 2 p ro d u ce the hook-shaped pattern o f interference (Fig. 3 b ). This phase o f fo ld in g episode is the dom inant one and is ch a ra cte rise d by an axial plane schistosity, S... This -recrystal­ lisation sch isto sity is the dom inant schisto-

136

P. K. GANGOPADHYAY, A. GANGOPADHYAY AND A. LAHIRI TABLE 1 GEOLOGY OF SELECTED REGIONS IN CENTRAL RAJASTHAN

Localities or cross section

Lithology

Geochem ical outline

Barr-Sendra cross-section

The Delhi Supergroup com mences w ith Barr conglom erate at its lowest level and succeeded upwards by arkose, m ica schists and subsequently by im pure marbles.

Arenaceous and siliceous Alwar Group gradually changing to impure calcareous-cum argillaceous Ajabgarh Group proceeding from Barr towards Sendra

Jassakhera area

NW o f the fault FF represented by migmatised micas c h is t; SE of the fault represented by heterogeneous assemblages.

Shamgarh area

The Delhi Super­ group characterised by brecciated marble and PreDelhi rocks and the Augengneiss, consisting of augens of pink coloured felds­ pars.

Phulad-Deogarh cross-section

The Delhi Super­ group represented by basic and ultrabasic rocks at its lowest level, succoeded upwards by im pure m arble of Ajabgarh Group.

Any Remarkable feature (i) Alw ar Group very much thinned com pared to Ajabgarh Group here. (ii) Presence of a meta­ m orphic banding occu r­ ring parallel to axial plane of second Folds, F2.

O ccurrence of high grade (sillim anite gar­ net) rock along fault.

(a) The area occurs at the heart of straight shear zone, marked by the presence of mylonites within this zone. (b ) The Pre-Delhis dow ndip striping ral lineation.

Basal Alwar Group represented by low-K-tholeiites succeeded upwards by im pure cal­ careous rocks of Ajabgarh Group with varying am ounts o f Feo, Mgo & Tio;.

sity th ro u g h o u t central R ajasthan (vid e Fig. 1). It co ntro ls the iith o fo rm a tio n a l boundary and the re su lt of te c to n ic s th a t co n tro ls the effusion o f acid phases represented by postDelhi granites and pegm atites. On this NNE-SSW tren d ing plane of sch isto sity the

show mine­

(a ) Regionally the arkosic and conglom eratic rocks of Alwar Group repla­ ced by the th ick sequ­ ence of meta basic and ultrabasic rocks at Phulad. (b) The straight junction (FF fault junction, Heron 1953) showing the evidence of remnants of basic rocks later m ylonitised and also characterised by rever­ sal of metamorphism of lower Am phibolite facies at its west to Alm andine high grade at fts east.

m ineral lineation and the pebble elongation co lle ctive ly referred to as L., lineation show the rake variation either NE-ward or SWward. An e ffective generation during this phase o f deform ation is the constructive phase of m etam orphism w ith in the Delhi Su-

P. K. GANGOPODHYAY, A. GANGOPADHYAY AND A. LAHIR1

Pigure 3a : Photograph of tigh t long lim bed isoclinal told, Fi w ithin Kham blighat para-am phibolite horizon.

Figure 3 b : Photograph showing the hook shaped inter­ ference pattern in Piplight trem olite-schist.

TECTONIC EVOLUTION : DELHI SUPERGROUP

P. K. GANGOPODHYAY, A. GANGOPADHYAY AND A. LAHIRI

Figure 3c : Photom icrograph showing the m ylonites folded by Fj.

Figure 3d : Photograph of the rotated xenolith of marble within Borwar granite ; G-granite, X-xenolith of marble.

TECTONIC EVOLUTION : DELHI SUPERGROUP

137

TECTONIC EVOLUTION : DELHI SUPERGROUP

L EGEN D

Gr

JETPU R A

MS

M IC A S C H IS T

G R A N IT E

Nm

NANONA M ARBE

MK

M A L N IK I- B A R A T R E M O U T E

Kqt

K A M O O R IB O R A Q U A R T Z IT E

K ts

C A LC S C H IS T

K A M O O R IB O R A T R E M O L IT E A C T IN O L IT E

CALC S C H IS T

B s*

B A R R G A R N E TIFE O U S M IC A S C H IS T

Beg

B A R R C O N G LO M E R A T E

Gn

G N E IS S

/ 70

STRIKE 8 DIP BEDDING FIRST GENERATION STRUCTURE A X IS OF FOLO

F)

SECOND GENERATION STRUCTURES

/ bo

STRIKE 6. DIP OF SCHISTOSITY S j AXIS OF FOLO F j

/* 5

LIN6ATION L 2 PEBBLE ELONGATION, I , IH IR P GENEBATinN S T R U C T u nr

jfM

Figure

2 a : S tructura l-cum -lith olog ical map of Barr area.

pergroup. The tim e-relation betw een d efor­ mation and re crysta llisa tio n of m inerals is determined fo r the B arr-Sendra region (Lahiri, unpub. M.Sc. Thesis, 1978) and given in Table 2. It is found from the ta b le that the most effective constru ctive phase o f m etam orphism is represented by the D 2 phase of d eform a­ tion. Map pattern (vide Fig. 2b) also sug­ gests that this phase of defo rm a tion may be guiding te cton ics fo r the shearing (as re­ presented by FF fault ju n c tio n in PhuladDeogarh cross-section of the 'D elh Syn­ clin o riu m ’ vide Fig. 2 b ). In Phulad-D eogarh 18

AXIS OF FOLO F3

region the reversal o f m etam orphism , so that o rig in a lly a low -m edium grade of metamor* phism (g re e n sch ist-lo w e r a m p h ib o lite facies of m etam orphism ) at the w estern part o f the shear ju n c tio n at phulad is changin g to higngrade (alm andine-high grade) o f m etam or­ phism at the eastern part of the shear ju n ctio n at Deogarh, is also supposed to be generated during th is phase. 3. The th ird is characterised by schistosity, S,, on plane (vide Figs. 2a

phase of fo ld in g episode broad open w arps of SE-NW trending axial & b ).

138

73* so'

INSET MAP

0

DELHI SUPER GROUP SANDED GNEISS CO M PLEX (2 3

bask: rocks

egg

LOCATION OF SAMPLES FOR GEOCHRONOLOGY 55'

g n e is s

AND

E H ] DELHI M ETASE ( 2 3 AN JAN A DIMENTS GRANITE _____1 % and mol. AI,,0:, / ( Na.,0 + K ,0 + C a O )> 1% belong to S-type (Table 9) ; the granites 9

GEOCHEMICAL CRITERIA (CHAPPELL AND WHITE 1974) AS APPLIED FOR IDENTIFCATION

OF THE GENETIC TYPES OF POST-DELHI GRANITES

Mol. AM ^/N a.-O + K>0 + CaO

Localities

Borwar Jetpura C. 1 (Chang Type-1) C. 2 (Chang Type-2) Kunaija Phulad

Norm ative C o run dum '%

_

NaaO %

1.46 1.90

1.99

2.33 4.95

1.40

—

4.9

1.51 1.66 2.62

— 1.53 5.59

N.D.— Net- determined.

4.8 3.22 0.42

k 2o

%

Genetic type

4.10 0.69

S-type S-type

3.9

S-type

4.0 2.75 2.03

N.D. S-type S-type

157

TECTONIC EVOLUTION : DELHI SUPERGROUP

Pi

Figure 17a:

are derived material.

Plots of S-type granites in N iggli's a l - a l k - c / f m diagram J -J e tu p ra , type-1 C.2 - Chang type-2, K-Kunaija, Ph-Phulad.

from

sedim entary

parent

It is found that except som e m inor varia­ tions such as Type II (C .2 ) of Chang, all the granites are of S-type. This is also re­ flected in N ig g li's a l-alk-C /fm and ( + T )( - T ) - G diagram s (Fig. 17a & b ). All the granites are in the a rg illa ce o u s fie ld except­ ing type-ll of C hang area. So partial m elting of the crustal m aterial is a very plausible explanation fo r the genesis of th a t granite suite R egionally that p ictu re may be com ­ pared from the gra nite of Seoli area (K hetri C opper B e lt). Pandian and Varm a (1983) studied the genesis o f the granite. They are also of the same opinion. The trend o f crustal e volution that could be established in the selected pa rt o f Cen­ tral Rajasthan is show n d ia g ra m m a tica lly (Fig. 18), and may be sum m arised as fo llo w s :— Form ation o f 2,500-2,000 m.y.

Bo - Borwar, C.1 —Chang,

a stable cra to n around on w hich a NNE-SSW

—

?

—JK | O

o

fl. *0 O 0

tn

Figure 17 b: Plots of S-type granites in N iggli’s ( + T ) ( - T ) - C diagram J -J e tp u ra , Bo - Borwar, P h -P h u la d , K -K u n a ija , C. 1 - chang type -1 C. 2 - chang, type 2.

tre n d in g linear platform basin evolved. In this basin the orthoqua rtzite-carbonate association of th e Delhi S upergrou p were laid dow n in shallow to m oderate depth co n d itio n s The basin was subjected to

P. K. GANGOPADHYAY, A. GANGOPADHYAY AND A. LAHIRI

W

W

t a

B asic Rocks

L~~-I Ajobgarh Facies Rocks

Figure 18: Plate te cto n ic model for evolution of the Delhi Supergroup.

TECTONIC EVOLUTION : DELHI SUPERGROUP

pulses of deform ation, reach ing the cu lm ina­ tion around D„ phase o f deform ation (w hich is responsible fo r the NNE-SSW fo ld s of the Delhi) w hich w itnessed late synkinem atic to post-kinem atic gra nite intrusion around 850750 m.y. The basem ent-cover contact (original un con fo rm ity) was su b jected to movement and evolved as a steep d ip p ing dislocation zone all along the eastern margin of the D elhi S yn clino riu m . The emplacemen t o f g ra n itic m aterials continued up to the post-te cton ic stages around 9001000 m.y. ACKNOWLEDGEMENTS The fin a ncial assistance from the U niver­ sity G rants C om m ission is th a n kfu lly acknowledged. The authors express sin­ cere g ratitude to Prof. A. K. Saha, Head of the Dept, of G eology fo r p ro vidin g labora­ tory fa cilitie s at the D epartm ent. They are also thankful to Prof. D. M ukhopadhyay, Dr. K. P. Ghosh, Dr. S. S engupta and Sri Tapas B hattacharya fo r active p a rtic ip a tio n and stim ulating d iscussion during field work. A num ber o f M.Sc. G eology students cf Presidency C ollege also p a rticip a te d in collecting data from selected areas. S incere thanks are due to Sri S. K. M oitra of Fort Gloster Industries, Howrah fo r c a rryin g out the entire chem ical w ork and to Dr. V. Khiltova of the In stitu t of P recam brian Geology and G eochrono logy, Leningrad for dating some of the sam ples. The authors are also grateful to the Postm asters of Sendra and D eogarh and S tation M aster of Phulad for th e ir help in fin d in g accom m oda­ tions in the field. REFERENCES Choudhary, A. K., Gopalon, K. and Sastry, C. A. 1984 : Present status of the geochronology of the Precambrian rocks of Rajasthan. Tectonophysics, 105, 131-140. C rjw ford, A. R. 1970 : The Precam brian geochrono­ logy of Rajasthan and Bundel Khand, northern India. Can J. Earth Sci., 7 (1 ), 91-110. Chappell, B. W. and White, A. J. R. 1974 : Two con­ trasting granite types. Pacific Geol., 8, 173-174. Floyd, P. A. and W inchester, J. A. 1975: Magma type and tectonic setting discrim inant using im ­ mobile elements Earth and Planet. Sci. Lett., 27, 211-218,

159

Gangopadhyay, P. K. 1972 : Structure and tectonics of Alw ar region in NE Rajasthan, India with special preference to Precambrian stratigraphy. 24th IGC, Sec. 2, 118-124. Gangopadhyay, P. K. and Lahiri, A. 1983 : Barr con­ glom erate : 'Its recognition and significance in the stratigraphy of the Delhi S upergroup in Central Rajasthan’. J. Geol. Soc. Ind., 24 (11 ), 562-570. Glassley, W. 1974 ; G eochem istry and tecton ics of the Crescent volcanic rocks, O lym pic Peninsula, W ashington. Bull. Geol. Soc. Amer., 85, 785-794. Green, D. H. 1975 : Genesis of Archaean peridotitic m a g m a s: constraints o f Archaean geothermal gradients and tectonics. Geology, 3, 215-225. Green, D. H., Nicholls, L. A., Viljoen, M. and Viljoen, R. 1975 : Experim ental dem onstration of the existence of p e rid o titic liquids in earliest Archaean magmatism. Geology, 3, 11-14. Glikson, A. V. and Sheraton, J. W. 1972 : Early Pre­ cam brian trond hjem itic suits in W. Australia and NW Scottland and geochem ical evolution of Shield. Earth and Planet. Sci., 17, 227-242. Heron, A. M. 1953: The Geology of Central Rajputana. Mem. Geol. Surv. Ind., 79, 399 P. Hess, H. H. 1960 : S tillw ater igneous com plex Mon­ tana. Geol. Soc. Amer., Mem., 80, pp. 74. Irvine, T. N. and Baragar, W. R. A. 1 9 7 f? ' A guide to the chem ical classification of com m on volcanic rocks. Can. J. Earth Sci., 8, 523-548. Leake, B. E. 1964 : The chem ical d istinctio n between ortho-' and para-am phibolites. J. Petrol., 5, 238254. Macdonald, G. A. and Katsura, T. 1964 : Chemical com position of Howaiian lavas. J. Petrol., 5, 83133. Miyashiro, A. and Shido, F. 1975 : T holeiitic and calc-alkaline series in relation to the behaviour of titanium , vanadium, chrom ium and nickel. Amer., J. Sci., 275, 265-277. Moores, E. M. 1969 : Petrology and structure of the Vourinos O ph iolitic Com plex of northern Greece. Geol. Soc. Amer., Spec, paper, 118, pp. 74. Naha, K. and Halyburton, R. V. 1974 : Late stress system from conjugate fold s and kink bands in the main Raialo Syncline, Udaipur district Raja­ sthan, India. Geol. Soc. Amer. Bull. 85, 251-256. Orville, P. M. 1969 : A model for m etam orphic differentiation origin of thin layered am phibolites. Amer. J. Sci., 267, 64-86. Pearce. J. A. 1975: Basalt geochem istry using to investigate past tecton ic environm ents on cypras. Tectonophysics. 25, 41-67. Pandian Sundara, M. and Varma, O. P. 19 83 : Evolu­ tion of the massive granites In th e copper Belt, Rajasthan : Im plication in regional correlation. A discussion. Ind. J. Earth Sci., 1 0 (2 ), 232-236. Petlijohn, F. J. 1975: Harpar and Bros.

Sedim entary Rocks,

3rd Ed.

160

P. K. GANGOPADHYAY, A. GANGOPADHYAY AND A. LAHIRI

Pearce. J. A. and Cann, J. R. 1973L) T ectonic setting of basic volcanic rocks determ ined using trace elem ents analysis. Earth & Planet. Sci. Lett., 19, 290-300. Pearce, T. H., Goram, B. E. and Birkett, T. C. 1975 : The TiO ;— KiO PaOs diagram : a method of dis­ crim inating between oceanic and non-oceanic basalts. Earth & Planet. Sci. Lett., 24. 419-426. Pearce, T. H., Goram, B. E. and Birkett, T. C. 1977,; The relationship between m ajor element chem istry and tectonic environm ent of basic and inter­ mediate volcanic rocks. Earth & Planet. Sci. Lett., 36, 121-132. Ray, S. K. 1974 : Structural history of the Saladipura pyrite pyrrhotite deposit and associated rocks, Khetri copper Belt, Rajasthan. J. Geol. Soc. Ind., 15, 227-238.

Sychanthavong, S. P. and Desai, S. D. 1977 : Proto­ plate tectonics con tro lling the Precambrian de­ form ations and m etallogenetic epochs of NorthWest Peninsular India. Mineral. Sci. Engg., 9 (4 ), 218-236. Sen, S. 1981 : Proterozoic plate tectonics in the evolution of crust and location of m etalliferous deposits, Rajasthan. Quart. J. Geol. Min. Met. Soc. Ind., 53(32 4), 162-185. Tuttle, O. F. and Bowen, N. L. 1958 i> O rigin of granite in the light of experimental studies in the system NaAISinO 8--K A IS i.iO s — SiO;— HaO. Geol. Soc. Amer. Mem,, 74, 153 p. W inchester, J. A . and Floyd, P. A. 1977 : Geo­ chem ical discrim ination of different magma series and th e ir differentiation products using imm obile elements. Chem. Geol., 20, 325-343.

I

COPPER M INERALIZATION AT RAKHA MINES, BIHAR AND METAMORPHISM OF THE ORES P.K. GANGOPADHYAY and M. K. SAMANTA

Department of Geology, Presidency College, Calcutta. ABSTRACT The copper ore bodies in the Rakha Mines occur in various form s like dissem ina­ tions, stringers, massive sheets and lenses within the 'granular ro ck’ (quartz-chloriteblotite-schist) and in qu artz-chlorite schist. Surface and subsurface mapping of country rocks and selected ore sheets suggest that the phases of deform ation and metamorphism of the ores and the country rocks are coeval. A notable feature is that the ore bodies are flattened and elongated, recalling tongue shape with long axis nearly parallel to the penetrative and consistent lineation in the country rocks, which in dica te exten­ sion of the rock mass in the nearly 'dow ndip' direction in the Rakha Mines Valley. The metamorphism of the ore bodies , has been investigated using 'etch tech niq ue’ with reagents like H2O2 : N H 4 0H , chrom ic acid and HNO?. The deform ation features and tex­ tures of the ore bodies have been discussed from the results obtained.

INTRODUCTION During the course of structural m apping of the Dhanjori basin b orde ring th e northern limit of outcrop of the S inghbhum G ranite, the Rakha Mines area was selected fo r

Figure 1 :

detailed study (Fig. 1 ). Both surface and and subsurface studies w ere ca rrie d out on the co p p e r ore bearing rocks, The ore bodies in the Rakha M ines o ccu r in a WNW-ESE tre n d in g narrow valley boun-

Map showing main physiographic features of Rakha mines area. The copper mines are located between "H anging w a ll" and "F oo t w a ll” quartzites.

Monograph on Crustal Evolution, Indian Society of Earth Sciences, p. 161-171, 1984. 21

P. K. GANGOPADHYAY AND M. K. SAMANTA

162

ded by "h a n g in g w a ll” and “ footwaM" quarliz ite ridges o f the Rakha Form ation of the D hanjori G roup. The co p p e r ore bodies p refe ra bly o c c u r tow ards the southern side of the valley, close to the ‘‘fo o tw a ll” quartzite. The m ajor host ro ck is “ granular ro c k " (q u a rtz-ch lo rite -b io tite s c h is t). Tow ards the northw estern part of this valley, the same “ gra n u la r ro c k ” and quartzite con­ tain ra d io a ctive m inerals w here Jaduguda uranium m ine is situated. The co p p e r ore bodies o f the Rakha M ines are deform ed and m etam orphosed along w ith the country rocks. The purpose o f the study of the ores is tw o fold ; firstly, to find out w hether the ore m inerals are coevally m etam orphosed w ith the host rock environm ent, and secondly, to determ ine th e ir rela tion w ith deform ation.

(a) S tructures of : tight, long-lim bed, infirs t generatra fo lia l folds ( F ,), oftio n ten reclined in s ty le ; associated m ineral lineration L, and axial place schistosity, S, : m ainly on m inor scale. (b ) S tructures of : m oderately open to losecond geca lly tig h t folds (F.,), neration puckers and k in k s ; as­ sociated crenulation cleavage, S „ ; F, usually occu rs at high angle to F, ; m ainly on m inor or m a croscopic scale. (c ) S tructures of : open warps and folds th ird genera- ( F J ; some are devetio n loped on large scale.

Three g eneratio ns of structures have been recorded in the co u n try ro cks and the ore bodies at Rakha. These a r e :—

OCCURRENCE OF ORE BODIES The ore bodies in Rakha Mines area are p rin c ip a lly dissem inations, stringers and

S T h LO O E 6 T h LOOE

lC m :

4 0 M , H O R IZ O N T A L V A R T IC A L

i

Th L E V E L

S T h LE V E L

Figure 2 :

Diagram showing the shape of ore-bodjes,

C o p p e r M in e r a l i z a t i o n AT

massive sheet-like, tabular and lensoid in form. The dissem inations and string ers of ore bodies are mainly confined to gran u­ lar rock” (quartz-chlorite-bio tite schist) whereas sheeWike, tab ula r and lensiod massive ore shoots occu r in quartz-chlorite schist. Strikingly, the ore bodies are not found in quartz-biotite sch ist and m ylo nitic rocks. “ Granular ro c k " is the m ajor p ro d u c­ tive host rock in Rakha M ines area. The subsurface m apping has been done at the 3rd (120m), 4th (160m ) and 5th (200m ) levels, 5th and 6th lode drives and the as­ sociated cross-cut regions of the Mines. The strike and dip of the ore bodies are more or less conform able w ith those of schistosity, S,. Where later cleavage, Sa is nearly

N

r a k h A m i n e s , b Ih a A

163

parallel to Sj because of close fo ld in g of the earlier schistosity, dispersion of cop p e r ore along S., occurs. S ubsurface m apping of the selected ore bodies by the present authors, to gethe r w ith maps com piled by the H industhan C opper Ltd., indicate that the ore bodies are fla t­ tened and elongated, sim ulating tongue shape (Fig. 2 ). The long axis of the ore body extends nearly parallel to the “ downd ip ” m ineral lin cation. The rake o f the m ineral lineation on S, along the Rakha Mines Valley is m axim um and ranges from 70° to 80 . The ore shoots are also folded by NNE-SSW tre n d in g broad w arps, F „ into antiform s and synform s. In places the shoots bifurcate or coalesce (vide Fig. 2 ).

S SCHISTOSfTY, Si

NET- LIKE OftE

Figure 3 : Diagram showing relation of ore stringers and structural elements, (a) Streaky ore stringers parallel to schistosity. Si. (b ) Stringers folded by Fz. (c ) Folded ore stringers in underground mines at 4th level, (d ) Two parallel stringers connected by another.

164

P. K. GANGOPADHYAY AND M. K. SAMANTA

The s trin g e rs o f ore bodies show evidence of later fo ld ing and re m o b iliza tio n . In m ost cases streaky ore strin ge rs are parallel to schistosity, S t (Fig. 3 ). In places these s tring e rs are fold ed along w ith S, by Fx (Fig. 3 ). The s trin g e rs may even be tig h tly folded w ith axial planes parallel to crenulation cleavage S ,2 (Fig. 3 ). It is also noted th a t the stringers, w hich o c c u r along S2, may be con ne cte d by veinlets across S2, suggestin g local late m o b ilisa tion of ore (Fig. 3 ).

PETROGRAPHY OF ORE Ore m ineral assem blage, relation

texture

and

tim e

Ore m ic ro s c o p ic studies of series of selected p o lish ed sectio ns (fro m 3rd, 4th and 5th levels, 5th and 6th lode d rive s) were ca rried out. The. purpose o f study was, firs tly to exam ine w h ethe r there is any levelw ise va ria tio n o f m ineral assem blage, and secondly, to make a careful textural study fo r de term ining the tim e relation, deform a­ tion and crysta lliza tio n sequences of the ore m inerals. The m etam orphism o f the sulp­ hide ores has been studied by stru ctu re etchin g w ith d iffe re n t reagents. The fo llo w ­ ing m inerals have been identified . C halcopyrite, pyrite, m agnetite and covellite are the m ost com m on m inerals o f the present ore assem blage but pyrrho tite, mackinaw ite, ch a lco cite , vio la rite , m olybdenite and arsenopyrite are also present in m inor to trace q uantities. Q uartz, c h lo rite and sericite, in varying p ro p o rtio n s, c o n stitu te the gangue. C h a lco p yrite is gene ra lly the d o ­ m inant m ineral in the ores fo llo w e d by pyrite, m agnetite and co ve llite . The study reveals tha t there is no rem ark­ able m ine ra lo gica l va ria tio n e ith e r ve rtica lly or la terally in the ore bodies. The textu ra l features and tim e o f crys­ talliza tio n o f the ore m inerals are stated below (cf. Figs. 4, 5 and 6 ). C h a lc o p y rite : This is the m ajor m ineral of the ore body usually o c c u rrin g as irre g u la r aggregates o f grains, som etim es fra ctu re d

(Fig. 4 ). In places the interphase boundary is stra ig h t but may not reach into trip le ­ p o in t ju n c tio n (Fig. 6 ). Som etim es the ch a lco p yrite o ccu rs as irregular vein w ithin associated silica te gangue m inerals. All of the above features suggest that the chal­ co p yrite is later than s ilica te gangue. Fea­ tures of tw inning of ch a lco p yrite as revealed by stru ctu ra l e tching w ill be discussed later. Pyrite : Three im portant m odes o f o c c u r­ rence have been observed. These are :— (a) P orphyrobla stic c u b ic p yrite ‘island' as well as aggregates flo ating in ch a lco p yrite (Figs. 4 and 5 ). These are m ostly pitted and som etim es fra ctu re d (Fig. 5 ). Such type of pyrite, associated w ith silica te and m agne­ tite, show s m utual boundary relation (Fig. 5 ). (b ) Pyrite o f the second type occu rs as large irre g u la r mass w ith finger-like grow th w ith in gangue. G row th of the small cu b ic crystals w ith irregular boundary has been o b ­ served w ithin such irre g u la r mass o f pyrite (Fig. 4 ). (c ) Pyrite of the th ird type occurs as irre g u la r m obilized veins and elongated grains along the boundary of chalcop yrite (Figs. 4 and 5 ). It has been observed that the floating pyrite cubes along the boundary of ch a lco p yrite is m obilized to form thin film of pyrite w hich covers the boundary of chal­ copyrite (Fig. 4 ). The relation of first type o f pyrite with ch a lco p yrite and m agnetite suggests that p yrite is e a rlie r than ch a lco p yrite but could be sim ultaneous or later than m agnetite. The grow th o f pyrite crystals o f the second and th ird types are later a n d /o r sim ultane­ ous w ith chalcopyrite. M a g n e tite : 'Is la n d s ’ o f large euhedral to subhedral grains o c c u r w ithin chalcopy­ rite (Figs. 4 and 6 ). In places the bound­ ary is partly covered w ith gangue and partly w ith ch a lco p yrite associated w ith thin covel­ lite. In all cases the stra ig h t as well as curved bold o u tlin e of m agnetite suggests that m agnetite is e a rlie r than chalcopyrite and covellite.

COPPER MINERALIZATION AT RAKHA MINES, BIHAR

165

T0 s c A

I t

■ IC m

LEGENO 1 71 s i l i c a t e

PYRRHO TITE

oangue

f t [ ' ' I CH ALC O P YR ITE m a g n e t it e

r ^ r - r i

00

I

a r s e n o p y r it e

p y r it e

M A C K IN A W IT E

chalcoci te

|

covel l i te

1 1 [ V| M O LY B D E N IT E

Figure 4 : (a) 'Island' of cubic pitted pyrite floating in irregular chalcopyrite. Irregular pyrite with subgrain form ation surrounded by silicate gangue. (b ) Aggregate of cubic pyrite and euhedral to sub-hedral pyrrhotite floating in irre gular chalcopyrite. (c ) Floating cu b ic pyrite in chalcopyrite is m obilised and arranged along chalcopyrite boundary. Elongated pyrite also occur along the same boundary, (d) 'Island' of magne­ tite partly associated with silicate gangue and partly associated with chalcopyrite. C ovellite occurs as irregular mass in chalcopyrite and along the boundary of magnetite.

C ovellite : Two d ifferen t m odes of o c c u r­ rence o f co ve llite have been observed :— (a) C ovellite usually o c cu rs as th in irre­ g u la r veins o f 0.6 mm. to 0.018 mm. th ic k ­ ness, m ostly along a n d /o r w ith in c h a lco p y­

rite, in som e cases along the boundary of m agnetite, ch a lco p yrite and gangue (Figs. 4, 5 and 6 ). W here c o v e llite veins are slig h ­ tly th ic k e r it ‘ca u g h t u p ’ the s ilica te frag­ ments w ithin the veins (Fig. 5 ). The

P. K. GANGOPADHYAY AND M. K. SAMANTA

166

0 s

c A L

E

I Cm

Figure pyrite. within pyrite gated

5 ; (a) Irregular veins of covellite with ‘caught up' silicate fragments occu r within chalco(b) Branching nature of thin covellite w ithin chalcopyrite. A cubic pyrite also occurs chalcopyrite and associated partly with covellite. (c ) Branching covellite within chalco­ and thin film s of covellite along silicate gangue. Pyrite along the silicate boundary is elon­ and fractured, (d ) A successive rim of cha lco cite follow ed by covellite along the boun­ dary o f chalcopyrite ore. The whole mass occurs within silicate gangue.

branching nature o f the cove llite veins are also rem arkable (Figs. 5 and 6 ). A ll these features suggest a replacem en t texture fo r covellite. A classic exam ple o f re place­ ment texture has been observed under mi­ croscope w here c h a lc o p y rite core is e n c ir­ cled by c h a lc o c ite fo llo w e d by co vellite (Fig. 5 ). This feature suggests th a t c h a lc o ­ pyrite is e a rlie r than c h a lo c ite and co ve llite is the latest am ong these three.

Chalcocite : M ostly occurs as veins. In places irregular fine aggregate o ccu rs along the ch a lco p yrite boundary.

Pyrrhotite : The d is trib u tio n o f pyrrhotite in the ore is very lim ited. At places it form s aggregates of euthedral to subhedral g ra in s ; elsew here it occu rs as irre g u la r patches w ith­ in and as rim around ch a lco p yrite (Figs. 4 and 6 ). The feature suggests that pyrrhotite crysta lliza tio n largely overlapped th a t of (b ) C ovellite o f the second type occurs c h a lco p yrite and outlasted it. as thin irre g u la r patches w ithin c h a lc o p y rite (Fig. 4 ). Som etim es they form aggregates M o ly b d e n ite : This is not abundant in o f such irre gu la r sm all patches. ores. G enerally it o ccu rs w ith in the chalco-

167

COPPER MINERALIZATION AT RAKHA MINES, BIHAR

Figure 6 : (a) Radiating angular edges of m olybdenite m ostly within chalcopyrite silicate gangue. Thin irregular pyrite and co ve llite also occur within chalcopyrite. molybdenite with notched edges occu r in chalcopyrite. (c ) Euhedral arsenopyrite pyrite and associated with gangue. Irregular veins of covellite in chalcopyrite. (d ) veinlets and oriented needles of M ackinawite within chalcopyrite.

pyrite partly associated w ith gangue. The radiating h a bit w ith a n gular edges o f m oly­ bdenite flakes is very c h a ra c te ris tic (Fig. 6 ). The radiating notches are usually pointed within ch a lco p yrite than in gangue (Fig. 6 ). At places the m olybdenite con tain s fine in­ clusion of s ilic a te gangue. Som etim es the slightly elongated flakes have large num ber of notches (Fig. 6 ). Arsenopyrite : There are very few o c c u r­ rences as euhedral to subhedral grains w ithin c h a lc o p y rite (Fig. 6 ). Som etim es these are partly associated w ith gangue. P rism atic shape o f the crystal is c h a ra cte ris­

and partly in (b ) Elongated within chalco­ Thin irregular

tic here. O ccurrences of arsenopyrite sug­ gest that it is earlier than ch a lcopyrite. M a c k in a w ite : M ackinaw ite is present alm ost in all observed polished sec­ tions as a m inor phase. It o ccu rs m ainly as sm all veinlets, oriented needles and flames in ch a lco p yrite (Fig. 6 ). These oriented exsolved lam ellae have curved boundaries. O p tica lly these are greyish pink w ith higher po lish in g hardness. V io la rite : This com prises very 'in sig n ifi­ cant parts o f the ores. This o ccu rs as sub­ hedral grains as well as patchy mass w ithin ch a lcopyrite.

168

P. K. GANGOPADHYAY AND M. K. SAMANTA

The sequence o f m ineral fo rm a tion is reflected by the textural re la tion sh ip of ore m inerals. Based on the available textural relatiions am ong the o re m inerals as detailed above, the pa ra ge n e tic sequence of the tion (D ,, D;, D ,) is presented below.

Time ________________________ ____ y D,

D2

Ds

M agnetite Pyrite A rsenopyrite

METAMORPHISM OF ORE MINERALS In recent years' the study of m etam or­ phism o f sulphide ores has been based not only on experim ental w ork but also on scru­ tiny of natural o ccu rre n ce s involving the ore bodies and th e ir host rocks (c f C lark and K elly 1973; A tkinson 1974; R ickard and Z w eifel 1975; R oscoe 1975). The sulphide ores o f the present area, w hich o c c u r in a m etam orphic environm ent, have been studied m icro sco p ica lly using d iffe re n t etch reagents in order to understand the nature o f m etam orphism of these ores. An attem pt has been m ade to evaluate these results in the lig h t of te c to n ic evolution o f the region.

Pyrrhotte C h a lc o p y rite .............. ...................................... ?

M aterials and M ethods

C halco cite

The m inerals exam ined from d ifferent levels o f Rakha M ines are ch a lco p yrite and pyrite because these m inerals are the m ost abundant phases in the ore.

7 C ovellite V iolarite

.....................................

M olybdenite

....................................

M ackinaw ite

.....................................

The reagents used fo r etching and the results obtained are briefly sum m arised in T able 1.

TABLE 1 RESULTS OF ETCHING Mineral (1) (a) Chalcopyrite.

Reagent

Significant etch textures

H2O2 : NH4OH ( 1 : 1 ) ; for 75 seconds.

Four sets of twins ; ( i) thick, lance— & (ii) olate to spindle shaped twin lam ellae on (110) and (102). (iii) Thin straight twins ( 112 ).

(b) Chalco­ pyrite.

(2) Pyrite.

Chrom ic acid (1 : 10 m ixture of potassiumdichrom ate and con. H aSOJ for 10 minutes. HNO, (2 min. deep etch, 1 min. weak etch) follow ed by the application of 6M HC! for 1 minute.

References of principles

used

Clark and Kelly 1975: Experi­ mental deform ation of chalco­ pyrite to 2,000 Bars and 500°C ; Roscoe 1975': Experim ental de­ form ation of natural chalcopyrite at tem peratures up to 3000°C over the strain rate range 10~2 to 1 0 - 6 Sec - '.

(iv ) "C hevron type” polysynthetic deform ation twin (112).

A tkinson 1974: Experimental deform ation of polycrystalline galena, chalcopyrite and pyrrhotite.

Only one set of deformadeform ation twin on (110). Thick lamellae are con­ cave but tw in lamellae are convex.

Sarkar and Deb 1974: Metam or­ phism of sulphides of the Singh­ bhum copper belt.

Porphyroblastic individuals represent m icrom osaics. Subgrains are coarser with bold outline.

Rickard and Zweifel 1976: Genesis of Precam brian sulphide ores, Skellefte District, Sweden.

P. K. GANGOPADHYAY AND M. K. SAMANTA

COPPER MINERALIZATION AT RAKHA MINES, BIHAR

Figure 7 a ; Photom icrograph showing lanceolatic tw ins on (110) and (102), which show abrupt changes in thickness of twins on (11 2). These twins are in chalco­ pyrite with H2O2 ; NH t OH etching.

Figure 7 b : Photom icrograph showing irregular de­ form ation twin of ‘Chevron type- on (112) twin lamellae (b la ck) in chalcopyrite with H , 0 , ; NH ,OH etching. The broad irregular lam ellae are orginal (110) twins.

COPPER MINERALIZATION AT RAKHA MINES, BIHAR

169 I

Com m ents on deform ation features

1.

C halcopyrite— w ith H 20., : N H 40 H etching.

dral to subhedral p o rp h yro b la stic individua ls represent m icrom osaics. In all cases the trip p le -ju n ctio n s of subgrains are not s trictly at 12 0 “.

Four d ifferent sets of form a tion tw ins have been identified and reported here fo r the firs t from the S inghbhum belt. These are oriented in three sets in grain individuals. A detailed d e sc rip tio n of the deform ation tw ins are listed below :

The deform ation features such as sliplines, deform ation tw ins, d isto rte d tw in boun­ daries in ch a lco p yrite and developm ent of m icrom osaics in p yrite suggest that the ore m inerals are deform ed and they suffered (i) and ( ii) First and second types are m etam orphism along w ith the co u n try rocks. From experim ental study on ch a lco p yrite (c f thick, lanceolate to spindle-shaped tw ins C lark and Kelley 1973; A tkinson 1974; lam ellae on ( 1 1 0 ) and ( 10 2 ) respectively Roscoe 1975) it had been suggested that (Fig. 7 a ). These lam ellae are brow n in about 10 0 ’ C deform ation tw in n in g becom es co lo u r developed w ith in yellow groundm ass. an im portant m echanism and the strength These are p in chin g and sw elling, and abrupt of ch a lco p yrite is su b sta n tia lly reduced. change in thickness. Spindle-shaped lam el From the m icro sco p ic study o f etched pyrite llae are best observed where pointed ends of grains from ores in S kellefte d istrict, Sweden, ( 1 1 0 ) spindle-shaped tw in lam ellae term i­ R ickard and Zw eifel (1975) suggested th a t nate against anothe r set of deform ation tw in the ores had annealed and subsequently (112 ? ). In all cases it can be observed suffered various degrees of regional metathat the ( 1 1 0 ) and ( 10 2 ) lam ellae are not m ophism . The present study of deform a­ continuo us from grain to grain but these end tion tw ins in ch a lco p yrite and m icrom osaics ab ru ptly along grain boundary. The o rie n ­ in pyrite could, therefore, suggest th a t the tation o f ( 1 1 0 ) and ( 10 2 ) lam ellae is d iffe ­ ores in Rakha area have been su bjected to rent in differen t grains. m etam orphism above 100 'C i.e., under the ( iii) Third type o f tw in s are extrem ely g reenschist fa cie s o f regional m etam or­ thin stra ig h t tw ins w hich pass throu gh the phism . The m ain phase o f crysta llisa tio n la n ceo tic tw ins. The thin tw ins fo llo w the of th e ore m inerals is between D, and D , same ( 1 1 2 ) tw in law as deform ation tw ins, phases o f d e form ation w hich affected the but they are never polysynthetic. (vide ore bodies and the country rocks alike. Fig. 7a). (iv )T h e fo u rth set of tw ins are on (1 1 2 ). These are very ch a ra c te ris tic type o f poly­ syn the tic deform ation tw inn in g. These are c o ntin uo us “ chevron ty p e ” w ith long and sh ort lim bs. The lim bs grad ua lly th icken tow ards hinges. Except “ chevron typ e ” the th ic k ( 1 1 2 ) deform ation tw in w ith irre­ g u la r h a bit is not com m on (Fig. 7 b ). (b ) Chalcopyrite— w ith chrom ic acid O nly one set of deform ation tw in on (110) developed w ith etching by ch ro m ic acid. The th ic k e r lenses are concgve but thin lam ellae are convex in character. These end along c h a lc o p y rite boundary. 2.

P yrite— w ith H N O : H C I Etched pyrite grains show tha t the euhe22

Further, S arkar and Deb (1974) sug­ gested that grains in single phase agregates show conspicuo us polygonal outline. Angle close to 120 at “ trip le -ju n ctio n p o in ts” w ould suggest te xlu ra l equ ilib riu m . But in the present sudy, ch a lco p yrite shows no sub­ grain form ation and the grain ju n ctio n s in single phase aggregates are not s trictly at 1 2 0 ‘. For pyrite m icrom osaics, the sub­ grains are also not stric tly at an angle of 120'. It is seen that the subhedral quartz and m agnetite grains o ccu r in intim ate as­ so cia tio n w ith ch a lco p yrite and pyrite and a typical g ra n o b la stic texture is developed. The above evidences indicate there is indeed a tenden cy fo r e qulibrium am ong the co­ e xisting m inerals but this has only been achieved locally.

P. K. GANGOPADHVAY AND M. K. SAMANTA

170

DISCUSSION ON EM PLACEM ENT OF ORE BODIES There are several theories regarding the genesis of the co p p e r and associated sul­ phide ores along the “ S inghbhum C opper B e lt” . (i) The first, proposed by Dunn (1937), supports an ep ig e n e tic hydrotherm al model favouring d e riva tio n of the ore m aterials from the g ra n itic ro cks of the shear zone, p a rticu ­ larly the soda-granite su ite of rocks. Resi­ dual liq u id s from these rocks are considered to have bocom e e n rich ed in potash, lime, m agnesia, and iron by base exchange with already co n solida te d g ra n itic rocks. These liq u id s later s p lit into tw o ore-bearing flu id s — an e a rlie r o xide fra c tio n from w hich the apatite-m agnetite veins form ed and a later sul­ phide fra c tio n from w hich the c o p p e r sul­ phide veins form ed.( ii) The second view proposed by Banerji (1962) and also his co-w orkers (Talapatra 1 968; Ghosh 1972) favours a metasom atic hydrotherm al o rig in fo r these depo-

NW

flu id s gave c h lo ritiza tio n soda-granite uranium and form ed.

rise to zones of b iotitization , and s e ricitiza tio n around the bodies w ithin w hich copper, apatite-m agnetite ore deposits

(iii) The th ird view proposed by Sarkar et al (1971) and S arkar and Deb (1974) re­ lates to the copper sulphide deposits w hich are considered to represent exhalative volcan o g e n ic layered type deposits. The copper su lp h id e deposits o rig inated as co ncentra­ tions w ithin the vo lca n ic rocks of the shear zone and were th e re a fte r m etam orphosed w ith little o r no m igration or later concen­ tration. In this co n te xt the fo llo w in g points re­ garding the copper ore bodies at Rakha are s u g g e s tiv e : (i) The ore bodies are restricted mainly to one persistent stra tig ra p h ic horizon called the “ g ra nular ro c k ” (quartz-chlorite-bio tite s c h is t). In spite of deform ation, in cluding fo lding, the ore bodies have been located w ithin this lith o stra tig ra p h ic unit.

SE

Tftn. SCALE Figure 8 : Sketch of deltaic current bedding in 'granular ro c k ’ occu rring in the Rakha Mines Valley.

sits. The ore d e p o s it o f the shear zone have been m obilized from the v o lca n o g e n ic rocks o f that zone by a lb ite-rich m etasom atic flu id s perm eating th ro ug h them . The flu id s them selves becom e e n riche d in Fe, Mg, Ca, Co, Ni, Cr, V, Mn, Ti and H.,0. Thus en­ riched in basic elem ents and w ater, these

(ii) C urrent bedding has been observed w ithin the “ g ra nular ro c k ” in the Rakha M ines valley (F ig. 8 ). The foresets show asym ptotic bottom and truncated top. These rocks o c c u r just above the 'foot-w all granite'. (iii) The ore bodies are m ainly dissem i­ nated and strin g e r-like but solid (m assive)

COPPER MINERALIZATION AT RAKHA MINES, BIHAR

ore bands also o c c u r ; all are parallel to schistosity S,. The stringers and solid bands are folded by Fz and F.t. (iv) M etam orphism of sulphide ore bodies and the country rocks, ranging from greenschist to low er am phibolite facies, is coeval in time and space. (v) The study of a few u ndergro und ore bodies and the host rocks did not give any indication of noticeable wall ro ck a lteration. The above observation suggests th a t a genetic association o f the ore bodies w ith vocanogenic-sedim entary sequence of co u n ­ try rocks in the C opper B elt does exist. However, the ore m aterials have been m obi­ lized and relocalised under stru ctu ra l co n ­ trol (vide Fig. 3 ). ACKNOWLEDGEMENTS The aulhors are grateful to Prof. A. K. Saha, Head of the D epartm ent, P residency C ollege for the fa cilitie s o f the la b orato ry study and field study. H industan C opper Lim ited gave us the o p p o rtu n ity to v is it the under­ ground mines. Dr. A. K. G hosh of G eology Department, C alcutta U niversity, kin d ly helped the authors in prepa ring etched sam­ ples of ores and th e ir study. REFERENCES Atkinson, B. K. 1974 : Experimental deform ation of polycrystalline galena, chalcopyrite and pyrrhotite. Inst. Min. Metal. Trans. Sec., B ./8 3 B. 19B. 28. Banerjee, A. K. 1962 ; Cross folding, m igm atization and ore localization along part of the Singhbhum

171

Shear Zone, south of Tatanagar, Bihar, India. Econ. Geol., 57. Banerji. A. K. 1974 : Role of basic volcanism in copper m ineralization along the Singhbhum Shear Zone, Bihar, Eastern India. Fourth IAGOD Symp. Verna, 1, 208-214. Banerji, A. K. 1981 : O re Genesis and its relationship to volcanism , tectonism , granitic activity and metasomatism along the Singhbhum Shear Zone, Eastern India. Econ. Geol., 76, 905-912. Clark, B. R. and Kelly, W. C. 1975: Sulphide de­ form ation studies : Experim ental deform ation of pyrrhotite and sphalerite to 2,000 Bars and 50Q°C. Econ. Geol., 68, 332-352. Clark, B. R. and Kelly, W. C. 1975 : Sulphide De­ form ation studies : III : Experim ental deform ation of chalcopyrite to 2,000 Bars and 500"C. Bull. Soc. Econ. Geol., 70, 431-453. Dunn, J. A. 1937 : Mineral deposits of Eastern Singh­ bhum and surrounding areas. Mem. Geol. Surv. Ind., 69, 1-279. Ghosh, A. K. 1972 : Trace element geochem istry and genesis of the copper ore deposits of the Singh­ bhum Shear Zone, eastern India. M ineralium Deposita, 7, 292-313. Rickard, D. T. and Zweifel, H. 1975 : Genesis of Precam brian sulphide ores, Skellefte District, Swe­ den. Econ. Geol., 70, 255-273. Roscoe, W. E. 1975 : Experim ental Deform ation of natural C halcopyrite at tem peratures up to 300°C over the strain rare range 10~2 to 10-® s e c - '. Econ. Geol., 70, 454-472. Sarkar, S. C., Deb, M. and Roy Chowdhury, K. 1971 : Sulphide ore m ineralization along Singhbhum Shear zone, Bihar. .S o c. M ining Geologists, Japan, Spec. Issue, 3, 226-234. Sarkar, S. C. and Deb, M. 1974: M etam orphism of sulphides of Singhbhum Copper Belt, India— the evidence from the Ore Fabric. Econ. Geol., 69, 1282-1293. Talapatra, A. K. 1968 : Sulphide m ineralization asso­ ciated with m igm atization in the southeastern part o f Singhbhum Shear Zone, Bihar, India. Econ. Geol., 63, 156-165.

S U M M A R Y OF TH E TR E N D S O F C R U S TA L E V O L U TIO N IN PARTS OF TH E IN D IA N S H IE LD

A. K. SAHA D epartm ent o f Geology, Presidency College, Calcutta

In the case o f the S inghbhum -O rissa Iron Ore craton, an inte grate d plausible model of the crustal e vo lutio n a ry history could be derived out o f the available struc­ tural, geochem ical and g e o ch ro n o lo g ica l data. But the relevant info rm a tion s relating to the Eastern G hats B elt and the C entral Rajas­ than B elt studied under the present P roject are still now fragm e n ta ry and no integrated model fo r th e ir c ru s ta rd e v e lo p m e n t can yet be b u ilt up. However, it is clea r that the three regions d iffe r s ig n ific a n tly in respect of the m anner and history of crustal evolu­ tion. In the S inghbhum -O rissa craton, the crustal g row th since 4.0 Ga appears to have been b ro a dly ce n trifug al, w ith gradual thicke n in g o f the cru s t to alm ost its full de­ velopm ent by 2.9 Ga. C rustal develop­ ment during the P rote ro zo ic appears to have been only m inor at least horizo nta lly. The entire craton is c o nside re d to have develop­ ed over a “ sin k-zo ne " of the th in p rim ordial m afic-ultram afic crust, induced by the “ visco us-d ra g ” o f the u nd erlying asthenosphere. S uccessive stages of unde rp la ting of the crust (s ta rtin g from 4.0 Ga) by m antle-derived m afic m elts and partial mel­ ting of such underpla ted m aterials p roduced salic and m afic intrusives and v o lca n ics led to th icke n in g of the cru st— there was also at least tw o m ajor stages o f basin form ation, once > 3.8 Ga ago (w hen the OMG sedi­ ments a n d ? v o lc a n ic s a ccu m u la te d ) and again 3.2-3.1 Ga ago, (w hen the Iron Ore G roup sedim ents and v o lc a n ic s a ccu m u­ la te d ). In the Eastern Ghats, crustal de­ velopm ent d uring the A rchaean and the Pro­ Monograph on Crustal Evolution,

te ro zo ic have been equally im p o rta n t; seve­ ral deep faults appear to have played a m ajor role in the crustal developm ent. A lthough paucity o f g e o ch ro n o lo g ic data precludes any system atic attem pt at recon­ stru ctio n o f the crust-building events in the Eastern Ghats, w ith the data co llected under this P roject, considered along with the re­ sults of previous w ork, a tentative sequence of crustal developm ent has been proposed. On the presum ed basem ent (com prising ton a litic ro c k s /la y e re d m a fic-ultram afics/layered c a lc ic anotho sisties) accum ulated the supracrustals now represented by khondalites and associated granulites ; the chem ical data o f khonda lites are strongly suggestive of th e ir vo lca n o g e n ic origin. Deep-seated catazonal m etam orphism of the belt was at least lo ca lly accom panied by anatexis of the basem ent and supracrustals ; extensive crus­ tal dom ing and riftin g o f the stabilised belt appears to have led to em placem ent o f massif-type anorthosite s and late-tectonic alkaline rocks. P ost-tectonic em placem ents under more or less quiescen t co n d itio n o f this belt led to developm ent o f some gravity-differen­ tiated m afic bodies. In C entral Rajasthan, little can be said a b out crustal developm ent during the A r­ chaean, but the area studied represents pri­ m arily P roterozoic supracrustals form ed over an A rchaean basem ent w here m etam or­ phism , m ultiphase deform ation and igneous in je ctio n s o ccu rre d along continent-ocean convergen t plate boundary. A com parative Table 1.

study

Indian Society of Earth Sciences, p. 172-173, 1984.

is

attem pted

in

173

SUMMARY OF THE TRENDS OF CRUSTAL EVOLUTION TABLE

1

COMPARISON OF THE MANNER AND HISTORY OF CRUSTAL EVOLUTION IN THE SINGHBHUMORISSA CRATON, PARTS OF EASTERN GHATS AND CENTRAL RAJASTHAN BELTS EASTERN GHATS BELT

SINGHBHUM-ORISSA CRATON 1.

2.

3.

O nly the shallow and interm e­ diate crustal depths are ex­ posed. Continental crust development started around 4.0 Ga and was nearly com plete both in depth, and lateral extents by about 2.9 Ga.

1.

Deeper parts of the crust are exposed.

1.

Interm ediate parts of the crust are exposed.

2.

Crustal developm ent was par­ tly Archaean and partly Pro­ terozoic.

2.

M ajor developm ent of the crust apparently took place during Proterozoic.

M ajor Archaean events :

3.

Development of layered maficultram afic sheets, and layered meta-anorthosite (re lics of the prim ordial (? ) oceanic litho­ sphere) and o f tonalite. De­ velopm ent of supracrustals re­ presented by the khondalite series and associated orthoand para-granulites along faultguided basins.

3.

No definite Archaean rocks exposed ; fhe basement, how­ ever, appears to be made up largely of Archaean (o r early P roterozoic) sia lic crust.

4.

During Proterozoic extensive crustal dom ing occurred, lead­ ing to em placem ent of the massif-type anoi'ihosites and alkaline em placements, deve­ lopm ent of lineam ent systems probably along old fault systems.

4.

This

Development of continental crust apparently started in a 'sink’ zone of the prim itive thin b a sa litic/u ltra m a fic crust, with successive stages of un­ derplating of the crust by mantle-derived m afic melts, partial m elting of such under­ plated m aterials to produce salic and mafic intrusives and v o lc a n ic s ; sedim entation and volcanism in elongate basins, once around 4.0 Ga and again in 3.2-3.1 Ga. 4.

CENTRAL RAJASTHAN

Proterozoic events : Fringe re­ gions of the Archaean craton in east and north were affected by intra-plate subduction along the so-called Copper Belt thrust zone; also, two prom inent basins (D hanjori and Sim lipal basins) developed in the east and were filled with a succession of ortho-quartzite and mafic lavas and then gently to m oderately folded ; several isolated basins (m afic lavas and * Kolhan Group sedim ents) de­ veloped in the region in be­ tween Singhbhum G ranite and and Bonai G ranite massifs. M id-Proterozoic NNE-SSW arch­ ing of the central part of the craton opened up fractures which were filled by mantlederived Newer d o le rite dyke swarm.

region

is essentially a linear platform al sedim entary basin, subse­ quently deform ed and meta­ morphosed in several phases and intruded by synkinem atic S-type granites. Intrusion of low-K tholeiites occurred along the margin with a subducting oceanic plate in the west.

Proterozoic

INDIAN SOCIETY OF EARTH SCIENCES COUNCIL

C hairm an: Vice-C hairm an:

Dr. A. K. Saha

Secretary :

Dr. A. K. B anerji E d ito r:

Asst. S ecretary:

Sri M. B. C hakraborty

Sri S. N. Sen, Sri Benoy C hakraborti, Sri Santanu De

EDITORIAL

ADVISORY

Dr. S. B hatia, Chandigarh Sri R.

Sri Jyo tisa n ka r Ray

Dr. Pradip Kum ar G angopadhyay

Treasurer: M em bers:

Dr S. K. Deb

BOARD

Dr. T. K. R oychow dhury, Calcutta

N. Bose, Calcutta

Dr. S. K. Sen, Kharagpur

Dr. S. Chanda, Jadavpur

Dr. S. Sengupta, Kharagpur

Dr. V.

Dr. R. Srinivasan, Bangalore

K. Gaur, Hyderabad

Dr. N. C. G hosh, Patna

Dr. R. K. Lai, Varanasi

Dr. S. K. Ghosh, Jadavpur

Dr. R. Vaidyanadhan, W altair

EDITORIAL

C h airm a n:

Dr. A. K. Saha, Calcutta

E d ito r:

M anaging E d ito r:

Dr. Pradip K. G angopadhyay, Calcutta

Dr. S. K. Deb, Calcutta

ASSOCIATE

Dr. A. K. Ghosh, Calcutta

BOARD

EDITORS

Sri A nis Kum ar Ray, Calcutta

Dr. S. L. Ray, How rah

FOREIGN

Dr. S. B hattach arji, Brooklyn. U.S.A.

•

CORRESPONDENTS

Prof. N. D. C hatterjee, Bochum. W. Germany

Published by Dr. Pradip Kumar Gangopadhyay on behalf o f the Indian Society o f Earth Sciences, Department o f Geology, Presidency College, Calcutta-700 073 and printed by him at Manasi Press, 73, S isir Bhaduri Sarani, Calcutta-700 006.