The Tectonics of the Canadian Shield 9781487574161

Citation preview

THE TECTONICS OF THE CANADIAN SHIELD

THE ROY AL SOCIETY OF CANADA Special Publications 1. The Grenville Problem.

Edited by

2. The Proterozoic in Canada. 3. Soils in Canada.

Edited by

JAMES

Edited by ROBERT

F.

4. The Tectonics of the Canadian Shield.

E.

JAMES

THOMSON

E.

GILL

LEGGET

Edited by

JOHN S. STEVENSON

THE TECTONICS OF THE CANADIAN SHIELD

THE ROY AL SOCIETY OF CANADA SPECIAL PUBLICATIONS, NO. 4 Edited by John S. Stevenson

PUBLISHED BY THE UNIVERSITY OF TORONTO PRESS IN CO-OPERATION WITH THE ROYAL SOCIETY OF CANADA 1962

@

UNIVERSITY OF TORONTO PRESS

1962

Printed in Canada

Reprinted in 2018 ISBN 978-1-4875-8560-0 (paper)

PREFACE

were presented as a symposium which formed part of the programme of the annual meeting of Section IV ( Geological Sciences) of the Royal Society of Canada held at McGill University, Montreal, Quebec, on June 6 and 7, 1961. The President of Section IV, J. E. Gill, presided as Chairman of these sessions. The Programme Committee for the McGill meetings consisted of: J . S. Stevenson (Chairman), T. H . Clark, and J. E. Gill, assisted by D. M. Baird, Honorary Secretary of Section IV. The recorded history of the symposium goes back to October 1959 when a Programme Committee of Section IV under the chairmanship of J. E. Hawley met to discuss the 1960 meeting. Since great interest had been expressed in a symposium on the tectonics of the Canadian Shield, a preliminary outline of such a symposium was drawn up by Y. 0. Fortier and C. H. Stockwell. Though it was not possible to hold the symposium at the 1960 meeting, it was decided to have such a programme at the 1961 meeting. The interest that was shown when the tectonics symposium was first suggested has continued throughout the presentation of the programme and the assembling of papers, and the Editor would like to thank all the speakers and authors for their enthusiastic co-operation. Appreciation is also due to Dr. Hawley, Dr. Fortier, Dr. Stockwell, and Dr. Gill for their special assistance; without their help, it would have been impossible to publish this volume. J .S.S. THE PAPERS IN THIS VOLUME

V

CONTENTS

Preface

V

Contributors

lX

Introduction A Tectonic Map of the Canadian Shield

JOHN s. STEVENSON, F.R.s.c.

3

c. H. STOCKWELL, F.R.s.c.

6

On the Relation of Metal Occurrences to Tectonic Divisions of the Canadian Shield

A.H. LANG, F.R.s.c.

16

Yellowknife-Nonacho Age and Structural R. A. BURWASH Relations

AND H. BAADSGAARD

22

Structural Pattern of the Precambrian Shield in Northeastern Alberta and Mica Age-Dates from the Andrew Lake District JOHN D. GODFREY AND H. BAADSGAARD

30

Major Faults in Western Part of Canadian Shield with Special A. R. BYERS, F .R.s.c. Reference to Saskatchewan

40

Tectonics of the Canadian Shield in Northern Manitoba H. D. B. WILSON, F.R.S.C., AND W. C. BRISBIN

Extent of the Huronian System between Lake Timagami and Blind River, Ontario JAMES E. THOMSON,

F.R.s.c.

76

R. N. PARKINSON

90

D. F. HEWITT, F.R.S.C.

102

Operation Overthrust Some Tectonic Features of the Grenville Province of Ontario

60

Tectonics of Part of the Grenville Subprovince F. FITZ OSBORNE, F.R.S.C., in Quebec

AND MARCEL MORIN

118

Tectonics of Regions Bordering the Ungava Stable Area ROBERT BERGERON, JEAN BERARD, AND LEOPOLD GELINAS

144

Some Aspects of Phanerozoic Epeirogenic and Orogenic Events that Involve Precambrian Rocks R. J. W. DOUGLAS, F.R.S.C., ANDS. DUFFELL

vii

149

Vlll

CONTENTS

The Tectonics of the Canadian Shield and Adjoining Sedimentary Basins in Relation to Oil and Gas Occurrences J . c . SPROULE, F .R.s .c . 162 The Effect of New Orogenetic Theories upon Ideas of the Tectonics J . Tuzo WILSON, F .R. s.c. 174 of the Canadian Shield

CONTRIBUTORS

Department of Geology, University of Alberta, Edmonton,

H. BAADSGAARD,

Alberta. JEAN BERARD,

Quebec Department of Natural Resources, Quebec, P.Q.

ROBERT BERGERON,

Quebec Department of Natural Resources, Quebec, P.Q.

w. c. BRISBIN, Department of Geology, University of Manitoba, Winnipeg, Manitoba. R. A. BURWASH,

Department of Geology, University of Alberta, Edmonton,

Alberta. Department of Geology, University of Saskatchewan, Saskatoon, Saskatchewan.

A. R. BYERS,

R. J.

s.

w.

Geological Survey of Canada, Ottawa, Ontario.

DOUGLAS,

DUFFELL,

Geological Survey of Canada, Ottawa, Ontario.

LEOPOLD GELINAS,

Quebec Department of Natural Resources, Quebec, P.Q.

JOHN D. GODFREY,

Geology Division, Research Council of Alberta, Edmon-

ton, Alberta. D. E. HEWITT,

Ontario Department of Mines, Queen's Park, Toronto,

Ontario. A.H. LANG,

Geological Survey of Canada, Ottawa, Ontario.

MARCEL MORIN,

Quebec Department of Natural Resources, Quebec, P.Q.

F. FITZ OSBORNE,

Department of Geology, Laval University, Quebec, P.Q.

R. N. PARKINSON,

Geologist, Hunting Survey Corporation, Toronto, Ontario.

J·

c.

SPROULE,

J.

C. Sproule and Associates Ltd., Calgary, Alberta.

s. STEVENSON, Department of Geological Sciences, McGill University, Montreal, Quebec.

JOHN

ix

CONTRIBUTORS

X

c.

H. STOCKWELL,

Geological Survey of Canada, Ottawa, Ontario.

JAMES E. THOMSON,

Ontario Department of Mines, Queen's Park, Toronto,

Ontario. H. D. B. WILSON,

Manitoba.

Department of Geology, University of Manitoba, Winnipeg,

J · TUZO WILSON,

Ontario.

Department of Physics, University of Toronto, Toronto,

THE TECTONICS OF THE CANADIAN SHIELD

INTRODUCTION John S. Stevenson, F.R.S.C.

THE CANADIAN PRECAMBRIAN SHIELD has been the subject of two earlier symposia and Special Publications of the Royal Society: The Grenville Problem, published in 1956, and The Proterozoic in Canada, published the following year. These considered much of the geology of the Shidd and serve as valuable background for the present study, a volume that considers, for the Shield as a whole, geological events not only of the Precambrian but in part also of the Phanerozoic (post-Precambrian time) . In the history of geological investigation of the Precambrian in Canada, the Grenville area was one of the first to be studied, and although many of the problems relating to the Grenville were discussed in the Grenville volume, special attention has been given in two papers in this volume to the tectonics of the Grenville structural province. One paper deals with the Grenville in Ontario and emphasizes, with descriptive examples, contrasts in tectonic style of structural features formed in the catazone with that of the mesozone and epizone. The other paper deals with the Grenville of Quebec, and in this paper the authors divide the Quebec portion of the Grenville province into two parts, and discuss the different tectonic patterns, kinds of plutonic rocks, and grades of metamorphism in these two parts. Much of the fundamental geological approach to tectonics, an approach based on concepts that are the result of recent detailed field work in many map-areas, is seen not only in the Grenville papers, but also in other papers, especially one concerned with a detailed analysis and interpretation of the tectonics of the four elongated basins or geosynclines bordering the Ungava nucleus or stable area, and one establishing a general pattern of fault structures in the Saskatchewan portion of the Shield. The writer of the latter paper looks forward to the day when, if compatibility of deformation in other areas of the Shield is shown, "it may eventually be possible to postulate a system of primary geotectonic forces which operated near the close of the Precambrian in North America." Current efforts towards a better understanding of Precambrian stratigraphy are fundamental to a better understanding of Precambrian tectonics, and it is gratifying to see in two of the papers in this symposium an appreciation of the necessity for a clearer distinction between rock-stratigraphic and time-stratigraphic nomenclatures. Since the authors of these two papers were members of the Precambrian Committee of the American Commission on Stratigraphic Nomenclature, it

3

4

JOHN S. STEVENSON

is to be hoped that the code as set out by the Commission1 will be used

increasingly to clarify the thinking of workers in the Precambrian and to help to evaluate correlations or lack of correlations not only within but also from one structural province to another. The help in understanding Precambrian tectonics that may be obtained from studies of Phanerozoic sedimentation and tectonics is seen in two of the papers in this volume. One paper discusses in detail the tectonic behaviour during Phanerozoic time as reflected in the sedimentary record of the cratonic cover. The authors of this paper also present evidence for Phanerozoic igneous activity at several places within the Shield. A closely related paper considers the relation of the structural history of tectonic belts, within the sedimentary basins in and adjacent to the Precambrian craton, to problems of the generation and later accumulation of petroleum and natural gas. The photogeological method of investigation has been used extensively by the writer to study and evaluate a number of structural features discussed. The important contribution that can be made by a compilation of aerial photography and geology over very large areas in revealing interesting regional structural anomalies in the Canadian Shield may be seen in the paper on Operation Overthrust. The use of methods of investigation from disciplines other than geology is seen in the paper on Manitoba, in which the writers have used statistical structural diagrams and point-count analyses of geologic maps to define, quantitatively, differences in structural character and petrographic composition between the Superior and Churchill structural provinces, with particular attention to the boundary area between the two provinces or "blocks." In the Proterozoic volume J. E. Gill, in speaking of geochronology, said, "The contributions made to date by workers in this field are very important, and they promise even more important revelations in the future." 2 The accuracy of his forecast is reflected in the present volume by the fact that, of the fourteen papers presented, six make extensive use of geochronology in studying the tectonic history of the Canadian Shield. In several of the papers there is evidence of the striking correlation between age characteristics and the similarities of structural features that were originally used by J. E. Gill, J. T. Wilson, M. E. Wilson, and others to outline their provinces of the Shield. Potassium-argon dates on micas, together with detailed field observations, have been used for dating "older granite" in an area of metamorphosed rocks intruded by "younger granite" in the southeastern District of Mackenzie, and have also been used, together with aeromagnetic and ground surveys, as part of a long-term programme of geological work in the northeastern Alberta part of the Shield. On a wider scale one paper makes extended use of isotopic methods of age determination, again using potassium-argon dates to subdivide the rocks of the Shield on the basis of age of lAmer. Assoc. Pet. Geol., 45, 5 (1961) , 645-65

2J. E. Gill (ed.), The Proterozoic in Canada, Royal Society of Canada, Sp. Pub. 2

(Toronto: University of Toronto Press, 1957), p. 188.

INTRODUCTION

5

folding, igneous intrusion, and accompanying metamorphism, all in relation to structural processes and the dating of orogenies. This paper also shows the progress being made in the preparation of a Tectonic Map of Canada, a co-operative effort of the Geological Survey of Canada, the Geological Association of Canada, and the Alberta Society of Petroleum Geologists. The Precambrian Shield is Canada's greatest source of metals and hence it is fitting that one paper in this volume is concerned directly with metals. This paper demonstrates the rather striking degree of correspondence between the distribution of metals and the major subdivisions of the Shield, and the writer concludes that the distribution of the metals is related to local geological history rather than to original differences in the earth's crust. In addition to its basic scientific implications, this observation would certainly have important implications in the direction of mineral exploration in the Shield. In the last paper in this volume the discussion goes beyond the limits of the Canadian Shield for, as the writer puts it, "we must explain its tectonics as part of the global phenomena." This involves dealing with the thickness of the entire crust and its composition, the development of continents, and speculation concerning the possible separation, during their long history, of adjacent provinces of the Canadian Shield. Together with the two previous symposia on the Canadian Shield, this symposium on its tectonics may be viewed as a promising progress report. Of particular note is the wide variety of approaches successfully being used to contribute to our more complete understanding of the over-all geology of the Canadian Shield.

A TECTONIC MAP OF THE CANADIAN SHIELD C. H. Stockwell, F.R.S.C.

ABSTRACT

The preparation of a new Tectonic Map of Canada is being undertaken jointly by the Geological Association of Canada, the Alberta Society of Petroleum Geologists, and the Geological Survey of Canada. A preliminary, much simplified map of the Canadian Shield is presented here. Isotopic age determinations indicate the presence of three major orogenic periods and this information, when considered in relation to main unconformities, permits the rocks to be mapped according to their age of folding, metamorphism, or intrusion. Areas involved in more than one period of deformation are differentiated where possible and areas of flat-lying or relatively unfolded rocks are distinguished from those affected by major orogenies. A NEW TECTONIC MAP OF CANADA is being prepared jointly by the Geological Association of Canada, the Alberta Society of Petroleum Geologists, and the Geological Survey of Canada. The Survey plans to publish the map on a scale of 1: 5,000,000 and expects that it will serve also as a contribution towards the compilation of a World Tectonic Map under the auspices of the International Geological Congress. The work is still in progress and the present account and accompanying sketch map are preliminary and are presented for the purpose of promoting discussion. The principles used in making the map follow in part those of the 1956 Tectonic Map of the U.S.S.R. as explained by Shatzki and Bogdanoff ( 195 7). Their major map-units are based chiefly on the age of last intensive folding when a geosynclinal region was converted into a craton. Also mapped are unconformably overlying cratonic cover rocks which are generally unfolded and unmetamorphosed. Folded geosynclinal regions are subdivided on the basis of lithology and unconformities into lower, middle, and upper structural phases, the last generally forming marginal or interior troughs composed of material derived from the geosyncline as its mountain building begins. The principles were worked out chiefly in their study of Phanerozoic (post-Precambrian) fold belts. The general principle of mapping rocks according to their age of folding can be applied successfully to the Canadian Precambrian Shield and, although the detailed subdivision of geosynclinal regions into three structural phases is not attempted, the main features of the tectonic history can nevertheless be shown. It is possible to distinguish cratonic regions of three widely different ages and each is overlain unconformably by remnants of flat-lying

Published with the permission of the Director, Geological Survey of Canada. 6

A TECTONIC MAP OF THE CANADIAN SHIELD

7

rocks. Recognizable geosynclinal belts, rather than being subdivided into three structural phases, are merely divided, where applicable, into two parts, one folded and the other unfolded. Rocks involved in more than one period of deformation are distinguished, where possible, from those that have been folded only once. The subdivision of rocks of the Shield on the basis of age of folding, introduction of granitic material, and accompanying metamorphism has become possible with the advent of isotopic methods of age determination and the application of these methods in a reconnaissance fashion over the greater part of the 1,771,000 square miles of the Shield. The dated samples are representative of common rock types, mostly granitic rocks and high grade, thoroughly recrystallized, metamorphic rocks and, although widely spaced and pointing to many unsolved problems, are thought to give a good over-all picture of the main orogenic features. The conclusions are based chiefly on about 215 potassium-argon dates on biotite and muscovite. The ages were determined in the laboratories of the Geological Survey of Canada and the details for most of them have already been reported (Lowden, 1960, 1961 ) . It has not yet been possible to give adequate consideration to many dates published by other workers but, as far as is known, these are in substantial agreement with our own. The potassium-argon ages, with some anomalous exceptions, are thought to be sufficiently accurate for the present purpose. They have been checked against a few Pb-Ur ages on uraninite and thorianite as standards and appear to be only slightly too young. Also Goldich ( 1961) and Aldrich ( 1960) in their studies of similar Precambrian rocks in Minnesota and adjacent parts of Ontario find good agreement between the K-Ar and Rb-Sr methods. In the Sudbury area, however, the discrepancies are great ( Fairbairn, 1960) . CLASSIFICATION AND NOMENCLATURE

Some considerations regarding nomenclature and time-classification, which are basic requirements in the preparation of the tectonic map, have already been presented ( Stockwell, 1961 ) and only a brief summary need be given here. Following Gill ( 1948, 1949), J. T. Wilson ( 1949) , and others, the Shield is divided into a number of structural provinces (Fig. 1). The K-Ar ages permit correlation from one province to another and show, for the most part, a rather impressive grouping around 2500, 1700, and 950 million years, indicating three main orogenic periods. These are called, respectively, the Kenoran, the Hudsonian, and the Grenville orogenies. Because of analytical inexactness and geological uncertainties such as possible loss of argon, a considerable latitude of say + 150 m.y. must be allowed in estimating the age of the peak or of the time interval from the beginning to the end of an orogeny. The Kenoran orogeny (K-Ar age 2500 ± 150 m.y.) is defined as the last period of intensive folding, metamorphism, and intrusion

8

C. H. STOCKWELL

FIGURE

1.

Main structural provinces of the Canadian Shield.

that is typically developed throughout the major part of the Superior province. It is also well represented in the Slave province. Similarly, the Hudsonian (K-Ar age 1700 ± 150 m.y. ) is the last important orogeny that is typically developed throughout the major part of the Churchill province. The Hudsonian also affected the southern part of the Bear province and the Penokean fold belt. The Grenville orogeny (K-Ar age 950 ± 150 m.y., Pb-Ur age about 1000 m.y.) is prevalent throughout the Grenville province. In many areas, especially in parts of the Superior, Churchill, and Grenville provinces, geological evidence indicates the presence of superimposed orogenies but only the youngest is indicated by the potassium-argon method. In the preparation of the tectonic map, as in other studies, it is useful to have a time-classification of the rocks of the geological column. This requires knowledge not only of the time of intrusion of igneous rocks but also the time of deposition of sedimentary and volcanic sequences. The approximate age of crystallization of unmetamorphosed igneous rocks can be determined directly from their primary micas but most sedimentary and volcanic rocks of the Shield are metamorphosed and it is only the age of metamorphism that is determined by the potassium-argon method. However, the age of deposition of some such assemblages can be bracketed between maximum and minimum limits by their relation to intrusive rocks, overriding metamorphism, and unconformably underlying igneous and metamorphic rocks. In the Canadian Shield each of the three orogenic periods is followed by a profound unconformity and the orogenies, when considered in relation to the unconformities, form the basis for a natural, four-fold time-classification.

A TECTONIC MAP OF THE CANADIAN SHIELD

9

The four divisions are here called the Archaean, and the Lower, Middle, and Upper Proterozoic. Further time subdivisions may eventually be possible but it is felt that assigning names to them should be postponed until they serve the useful purpose of interregional correlation. Also the three divisions of the Proterozoic should eventually be given geographic or other names especially if they become useful for international or intercontinental correlation. The Archaean, as here defined, includes granitic and other igneous rocks that were intruded during the Kenoran orogeny and it also includes all older rocks. The Lower Proterozoic includes all sedimentary, volcanic, and igneous rocks deposited or intruded during the time interval between the Kenoran and Hudsonian orogenies as well as those igneous rocks intruded during the Hudsonian. Similarly, the Middle Proterozoic comprises rocks formed during the interval between the Hudsonian and Grenville orogenies as well as those of the Grenville. The Upper Proterozoic comprises Precambrian rocks that are younger than the Grenville orogeny. A main twofold classification, whether or not designated by the names Archaean and Proterozoic, is retained because it is a natural one and is useful in discussing very large units just as, for example, are the words Precambrian and Phanerozoic. The classification, Archaean and Proterozoic, has long been used by the Geological Survey of Canada and, much to the credit of early workers, no major change in former concepts is now needed. The agreement with former usage is indicated by the fact that assemblages such as the type Keewatin, Couchiching, Timiskaming, Abitibi, Pontiac, Rice Lake, and Yellowknife fall in the Archaean. The Huronian, Animikie, Knob Lake, Great Slave, and Snare assemblages, though not necessarily contemporaneous, fall in the Lower Proterozoic. The Dubawnt, Hornby Bay, and at least part of the Coppermine River are Middle Proterozoic. The Double Mer is probably Upper Proterozoic although it could be younger. Many other assemblages have not been bracketed but have only been given a minimum age in some cases and a maximum in others; for example, the Amisk and Missi are Lower Proterozoic or Archaean, the Grenville series is Middle Proterozoic or older, and the Athabasca formation is Middle Proterozoic or younger. ( Credit for dating the Animikie is given to Goldich [1961] who classifies it as Middle Precambrian, but according to the nomenclature here used it becomes Lower Proterozoic.) THE TECTONIC MAP

The nomenclature defined above for the three major orogenic periods and for the time divisions of the geological column are used in the accompanying tectonic map ( Fig. 2). The legend, in effect, consists of three separate legends, one for each orogeny and its associated unfolded rocks. Because of its small scale, the map shows only the gross tectonic features. Many details which will be shown on the final map are omitted; for example, fold axes, trends of gneissic structure, magnetic and gravity trends,

6'o

HUDSON

I

BAY

i

I UNFOLDED ROCKS ON THE HUDSON JAN g:: Cratonic cover; 9a, Middle Proterozoic;

I:: I ~

9b, Middle Proterozoic or younger

Flanking rocks, Middle and Upper Proterozoic

HUDSONIAN OROGENY (la te Lower Proterozoic) Late Lower Proterozoic granitic rocks and undifferentiated gneisses; may include some older granitic rocks involved in the Huds onion

I+ 7++1 ~

6a , Lower Proterozoic rocks folded during the

~ Hudsonion ; 6b,Proterozoic rocks, age of

folding uncertain

Pre·lote Lower Proterozoic rocks, folded during the Hudsonian UNFOLDED ROCKS ON THE KENORAN

~ . Crotonic cover; 4a , Lower Proterozoic; 4b, Middle ~ Proterozoic ; 4c, Lower Proterozoic or younger

Ea ~

Flanking rocks; 3a, Lower Proterozoic; 3b, Lower and Middle Proterozoic; 3c, Lower Proterozoic or younger

KEN ORAN OROGENY (late ArchaeanJ

jv ;JL-1

B

Lale Archcean granitic rocks and undifferentiated gneisses; includes some older granitic rocks involved in the Kenoron Archceon rocks folded during the Kenoron

FIGURE

2.

Tectonic map

UNFOLDED ROCKS ON THE GRENVILLE

1:: i~·::1

Cratonic cover; Upper Proterozoic or younger

GRENVILLE OROGENY (late Middle Proterozoic)

~ / Late Middle Proterozoic granitic rocks, ~ onorthosite, and undifferentiated gneisses

Pre-late Middle Proterozoic rocks folded during the Grenville Lower Proterozoic rocks folded during the Hudsonian and refolded during the Grenville Archcean rocks folded during the Kenoran and refolded during the Grenvi//e Boundary of Shield

• • • • • • ~

Scale of Miles 0

p.. \ \.

..:,

v

V

"' V

2

'

-1 ~

of the Canadian Shield.

200

300

,,_ ~ \

V

...,

100

..J

400

,c

12

C. H. STOCKWELL

faults, ultrabasic and alkaline intrusive rocks, and patterns formed by diabase dykes. The main structural units shown in Figure 2 will become more evident if the map is hand coloured; for example, red for units 1 to 4, orange for units 5 to 9, and yellow for units 10 to 14; the patterns serve to distinguish the subdivisions. By giving the unfolded rocks the same colour as their basement, the successive development of the stable cratons is emphasized. ( By using similar colour schemes for the map of the whole of Canada, the stable basement beneath the relatively undisturbed Phanerozoic cover rocks of the Interior Plains, St. Lawrence and Hudson Bay lowlands, and of the Arctic, are related to the exposed Shield and at the same time are contrasted with the mobile belts of the Cordilleran, Appalachian, and Innuitian regions.) As shown on the map, the Kenoran orogeny is well represented throughout almost the whole of the Superior province and in much of the known parts of the Slave province. In the Superior province especially, geological evidence, such as the presence of granite boulders in conglomerate and areas of granite beneath unconformities, indicates that some of the rocks were involved in more than one orogeny, the Kenoran being the youngest, but the older events have not been clearly distinguished by isotopic dating nor have they been correlated by geological methods. The sedimentary and volcanic rocks ( 1 ) and the granitic rocks ( 2) may therefore be of more than one age. Virtually unfolded flanking deposits ( 3), which are characterized by gentle, outward, monoclinal dips, unconformably overlie the Kenoran, and are found at several places on the borders of the Superior and Slave provinces. Some of these, as bracketed by isotopic dating, fall within the Lower Proterozoic ( 3a) ; others include the Lower Proterozoic Animikie and the Keweenawan ( 3b) ; still others, whose minimum age has not yet been determined, are dated only as Proterozoic ( 3c). These unfolded deposits have apparently been protected by the stable basement on which they lie. They pass farther out into folded rocks of the same age, as described in the subsequent discussion of the Hudsonian orogeny. Generally flat-lying cover rocks are represented by part of the Lower Proterozoic Cobalt group (4a), by the Keweenawan of the Lake Nipigon area (4b), and by scattered small remnants of sediments in northern Quebec which are classified only as Proterozoic or younger ( 4c) . The Hudsonian orogeny affected most of the rocks throughout the Churchill province as well as those of the southern part of the Bear province and those of the Penokean belt. Its effect, in the form of metamorphism and granitic intrusions, locally penetrated the older cratons of the Superior and Slave provinces, as indicated symbolically on the map. The main part of the Churchill province evidently had a long and complex history for, as in the Superior, unconformities are present and granites of more than one age are known but the whole appears to have been overridden by the Hudsonian. The sedimentary and volcanic rocks ( 5 ) are evidently of more than

A TECTONIC MAP OF THE CANADIAN SHIELD

13

one age but only the minimum has been determined and all are therefore classified as merely pre-Hudsonian. However, the rocks ( 6) that flank much of the Superior and Slave provinces are Proterozoic in age and have been deposited unconformably on the deeply eroded surfaces of the two older cratons. Excepting those parts with gentle, outward, monoclinal dips as already described, the flanking rocks have been folded to various degrees of intensity. In most belts ( 6a) the folding took place during the Hudsonian orogeny but rocks of the Belcher and Bathurst belts ( 6b), because they are dated only as Proterozoic, could have been folded at a later time. Excepting the Snare group of the Bear province the flanking fold belts are not normally cut by bodies of granite but a few small ones are found. Fold axes within the belts parallel the basement contact and the belts as a whole are characteristically asymmetrical in cross-section, becoming more highly folded away from the basement, locally with increasing grade of metamorphism, until they either pass into the much more highly metamorphosed and granitized rocks of the main part of the Churchill province or are separated from them by a major fault. At least two of the belts, those of Great Slave Lake and the Labrador trough, appear to be intercratonic troughs, a structure demanding that the rocks beyond their outer boundaries are basement rocks, not necessarily Archaean, · which, like the fold belts themselves, were overridden by the Hudsonian orogeny. It is to be noted that at localities where the flanking deposits are missing the more highly metamorphic and granitic rocks of the Hudsonian are in direct contact with those of the Kenoran as, for example, along the boundary between the Superior and Churchill provinces in Manitoba and between the Slave and Churchill provinces northeast of Great Slave Lake. The Hudsonian basement of folded and granitic rocks of the Bear province are overlain unconformably to the north by almost unfolded rocks ( 8) which have a gentle, outward, monoclinal dip. They include the Hornby Bay and Coppermine groups of Middle to Upper Proterozoic age. Remnants of flat-lying cover rocks unconformably overlie those involved in the Hudsonian orogeny and include the Dubawnt group (9a) which is Middle Proterozoic and the Athabasca formation ( 9b) , which is Middle Proterozoic or younger. Other cover rocks not shown on the map include the EtThen group which overlies the folded Great Slave group and the Sims formation which overlies the folded Knob Lake group of the Labrador trough. The Grenville orogeny is typical of the northeasterly trending Grenville province and is also represented by an intrusion of syenite which has penetrated the Kenoran rocks on the north shore of Lake Superior. The northwest boundary of the Grenville province, called the Grenville front, is a fault in some places but elsewhere is a metamorphic or intrusive contact. It truncates the easterly trending Penokean fold belt, the easterly trending formations of the Superior province, the Misstassini rocks, and the southerly

14

C.H. STOCKWELL

trending Labrador trough. Rocks of the older belts have been traced by geological means for a short distance across the Grenville front but have not been traced farther because of inadequate mapping and the complexity of the overriding Grenville orogeny. Consequently, only small areas of Archaean rocks involved in the Kenoran ( 10) and of Lower Proterozoic rocks involved in the Hudsonian ( 11) can be indicated as having been refolded during the Grenville. Elsewhere the mapped areas of sedimentary and volcanic rocks ( 12) may be classified only as older than the Grenville orogeny. Flat-lying cover rocks on the Grenville are represented by the Double Mer sandstone ( 14) which is Upper Proterozoic or younger. GENERAL COMMENTS

The age determination programme, being of a reconnaissance nature, and depending almost entirely on only the potassium-argon method, has probably given an overly simplified picture of the tectonic history of the Shield and, as the work continues, complications will become more apparent and some of the present problems will be solved. Many of the unmetamorphosed cover rocks, for example, have been dated as only younger than the basement on which they lie but many are cut by diabase dykes which, when dated, will give their minimum age. At present, the general impression is that many of the cover rocks have been deposited soon after the stabilization of their basement rocks and have remained virtually undisturbed through subsequent orogenies that affected other regions. Only two diabases have been dated in the laboratories of the Geological Survey, the Keweenawan at 1000 m.y. and the Nipissing at 2095 m.y., a surprising difference which opens up an interesting field for further study. A comprehensive study of diabase dykes of the Shield is being undertaken by the Geological Survey. Much work remains to be done in an effort to date the age of deposition of metamorphosed sedimentary and volcanic rocks. Without this information the concept of cycles commencing with deposition and ending with folding accompanied by intrusions and metamorphism is difficult to apply in many parts of the Shield especially where rocks may have been involved in more than one orogeny. Likewise, the theory of continental growth requires further study. For example, the Archaean rocks of the Superior and Slave provinces are flanked in many places by belts of Proterozoic rocks, an arrangement which would be strongly suggestive of continental growth were it not for the fact that the age of deposition of rocks farther out is generally unknown. For example, the metamorphic rocks that lie beyond the Proterozoic belts of the Churchill province have been dated only as older than the Hudsonian orogeny (i.e., older than late Lower Proterozoic) . Owing to the great length of Lower Proterozoic time they could therefore still be Proterozoic in age and the theory of continental growth would be supported, but if they are the same

A TECTONIC MAP OF THE CANADIAN SHIELD

15

age as the rocks of the Superior and Slave provinces the theory would be refuted. The need for precise dating of the age of deposition of sedimentary and volcanic rocks is obvious. REFERENCES ALDRICH, L. T. and WETHERILL, G. W. (1960). Rb-Sand K-A ages of rocks in Ontario and northern Minnesota. J. Geophys. Res. 65, 1; 337-40. FAIRBAIRN , H . W., HURLEY, P. M., and PINSON, W. H. (1960). Mineral and rock ages at Sudbury-Blind River, Ontario. Dept. of Geology and Geophysics, Massachusetts Institute of Technology, Eighth Annual Progress Report, 7-42. GILL, J.E. (1948). Mountain building in the Canadian Precambrian Shield. 18th Int. Geol. Congress, pt. XIII, 97-104. - - - ( 1949). Natural divisions of the Canadian Shield. Trans. Roy. Soc. Canada, Sect. IV, Series 3, 43: 61-9. GOLDICH, SAMUEL S., NIER, ALFRED 0., BAADSGAARD, HALFDAN, HOFFMAN, JOHN H ., and KRUEGER, HAROLD W. ( 1961) . The Precambrian geology and geochronology of Minnesota. University of Minnesota, Minnesota Geological Survey, Bull. 41. LowDEN, J. A. ( 1960). Age determinations by the Geological Survey of Canada. Report 1: Isotopic ages. Geol. Surv. Canada, Paper 60-17. - - - ( 1961) . Age determination by the Geological Survey of Canada. Report 2: Isotopic ages. Geol. Surv. Canada, Paper 61-17. SHATZKI, N. S., and BoGDANOFF, A. A. ( 1957)·. Explanatory notes on the Tectonic Map of the U .S.S.R. and adjoining countries (translated by Theodore Shabad and C. Muromcew). Moscow, State Scientific and Technical Publishing House of Geologic and Conservation Literature. STOCKWELL, C. H . ( 1961) . Structural provinces, orogenies, and time classification of rocks of the Canadian Precambrian Shield. In Lowden, J. A. ( 1961). WILSON, J. T . (1949) . Some major structures in the Canadian Shield. Trans. Can. Min. Met., 52: 231-42.

ON THE RELATION OF METAL OCCURRENCES TO TECTONIC DIVISIONS OF THE CANADIAN SHIELD A.H. Lang, F.R.S.C.

ABSTRACT

A preliminary study of the distribution of Canadian metal occurrences was completed recently. The writer was asked to compare the results with the subdivisions of the Canadian Shield proposed by Stockwell in this symposium. The distribution of metals shows some correspondences and some differences with those divisions. The most marked correspondences are with the Bear, Slave, and Grenville provinces. Several relationships to smaller areas and belts that probably can be regarded as tectonic subprovinces will also be discussed.

DURING THE LAST THREE YEARS the Geological Survey of Canada has been experimenting with the compilation of metallogenic maps as aids to the study of metallogenic provinces. The writer was asked to summarize for this symposium the information so obtained in order to indicate the degree of correspondence with the tectonic divisions advocated by Gill ( 1948, 1949), J. T. Wilson ( 1949), and others, and further refined and modified by Stockwell in the preceding paper. Many geologists have contributed to knowledge of the distribution of metals in particular areas and regions, and Jolliffe (1952), Gill (1952), and Lord ( 1951 ) began the definition of metallogenic provinces in parts of Canada. To all these, and to several colleagues in the Geological Survey who compiled maps for individual metals, the writer is indebted for helpful information. NATURE AND SCOPE OF STUDY Maps were compiled for beryllium, chromium, placer gold, lode gold, iron and titanium, lithium, manganese, mercury, molybdenum, nickel and cobalt, niobium, tin, tungsten, uranium, and vanadium. One for platinum was already available. Compilations begun for copper, lead, and zinc will not be completed for some time. The aim is to plot all known occurrences ( or areas containing several or many occurrences) carrying more than "trace" amounts of the metal concerned, to subdivide the occurrences according to principal geological types, to indicate the mineral concerned, and to provide references to further information. In the case of iron, only selected Published by permission of the Director, Geological Survey of Canada. 16

RELATION OF METAL OCCURRENCES

17

occurrences were shown. The main source of data was the economic geology files of the Geological Survey, containing information on roughly 30,000 occurrences and mines. Other records and indexes, and selected federal and provincial reports and maps, were also used. To date, only the maps for beryllium ( 1045A-M2), iron ( 1045A-M4), molybdenum ( 1045A-M3), and uranium ( 1045A-Ml) have been published. The writer collated the data from most of the above-mentioned maps in an attempt to learn the extent to which the information available at present permits generalizations regarding metallogenic provinces and subprovinces based on more than one metal. Because the large number of metals being treated complicated depiction on a single map, one map for the principal metals produced in Canada and another for minor metals were prepared. Because it was considered desirable, at least for a preliminary composite study, to omit widely scattered occurrences only areas containing three or more occurrences of a particular metal were plotted. The principal areas containing occurrences of copper, lead, and zinc were added from selected references, but this part of the study is not as complete or accurate as the remainder. Iron was not dealt with because of the ubiquitousness of iron in small quantities. Molybdenum was not treated because small occurrences of it are so common in several areas that they tended to obscure the illustration of other minor metals. Iron and molybdenum were, however, considered in the report. Areas containing occurrences of native silver regarded as of primary origin were included because they may have significance distinct from other forms of silver deposits. Occurrences and areas of occurrences were plotted empirically, without reference to geological or tectonic features, but because of the small scale used ( 1 inch to 120 miles to permit comparisons with the Geological Map of Canada) the outlining of areas was generalized. For comparison, a map was compiled outlining geological and tectonic divisions, and physiographic divisions where those were lacking. These divisions were based only on published data; some revision could now be made in the light of this symposium. The following deals with the part of the study applicable to the Canadian Shield. Because most of the information has been published in more detail in a recent report ( Lang, 1961 ) it is only summarized briefly here, particular reference being devoted to matters that most clearly indicate correspondence or lack of correspondence with Stockwell's divisions. A few lantern slides were made from critical sections of the maps, parts of the divisional boundaries being added; these are not reproduced in the record of the symposium because the maps are available in the above-mentioned report. BEAR PROVINCE

About ten years ago geological and metallogenic divisions of the Great Bear Lake and Great Slave Lake regions were outlined by Jolliffe and Lord with boundaries and names similar to those of the present discussion.

18

A.H.LANG

Although many mineral occurrences have been discovered in these regions in the meantime, they have served only to indicate the accuracy of the earlier generalizations; indeed, it was largely this fact that encouraged the writer to try to bring the information up to date and to extend the study to other parts of the country. One of the most distinctive divisions is that now called the Bear province, which lies northward from the north arm of Great Slave Lake and eastward from Great Bear Lake. Its most characteristic metal is uranium, hundreds of occurrences of pitchblende having been found over a large part of the province. Few occurrences of other metals have been discovered in the southern part of the province, but the part east of Great Bear Lake is complex, several deposits containing cobalt, nickel, copper, native silver, and gold, as well as a little lead, zinc, and bismuth. Farther to the northeast the most characteristic metal is copper. SLAVE PROVINCE

The typical metal of the Slave province is gold, which has been found in more than one thousand places widely distributed throughout the province. Relatively few occurrences of other metals are known in its western part. The eastern part of the province contains a few deposits of lead, zinc, and copper. A few occurrences of lead, cobalt, nickel, and native silver have been found along the north shore of the east arm of the Slave Lake, and a short distance inland, but these metals, which are characteristic of the area south of the Slave province, are not known to any extent in the remainder of the Slave, therefore they seem to mark a narrow transitional belt rather than metals that should be considered as characteristic of the Slave. Virtually no uranium has been found except in this transitional belt. The eastern part of the Slave province is noteworthy for several relatively small areas containing abundant occurrences of minor metals, mainly in pegmatites. These comprise tungsten, lithium, beryllium, niobium, tantalum, and tin. CHURCHILL PROVINCE

The Churchill province is large and complex. It includes several areas that are fairly distinct metallogenically and which correspond to tectonic subdivisions described by Stockwell and others. Space does not permit listing here all the metals found in some of these subdivisions, but the main features are summarized. A subprovince that Jolliffe called the East Arm, in which copper, lead, zinc, uranium, nickel, cobalt, and native silver have been discovered, corresponds with belts of monoclinal and folded strata of Proterozoic types. Farther south, many of these metals have been found in another belt of folded Proterozoic rocks, in the Nonacho area. North and east of Lake Athabasca is another belt of that kind containing many occurrences of the metals mentioned above, except native silver; a few occurrences of molybdenum, platinum, tin, titanium, mercury, and silver are also known.

RELATION OF METAL OCCURRENCES

19

The numerous and diversified metallic occurrences of the Athabasca belt are in marked contrast to a large area south of Lake Athabasca, which is underlain by flat cover rocks in which virtually no metals have been discovered; it must be admitted, however, that this area is largely covered by overburden. Still farther south is a large complex region containing the La Ronge, Flin Flon, Herb Lake, Lynn Lake, Thompson and other mineral areas mainly producing copper, zinc, nickel, and gold and containing many occurrences of these and other metals. The distribution of these occurrences agrees fairly well with northeasterly trending belts of folded Proterozoic or older strata. SUPERIOR PROVINCE

The map illustrating the distribution of principal metals shows considerable, but far from complete, differences between the neighbouring parts of the Churchill and Superior provinces. All the principal metals are represented in the southeastern part of the Churchill, but only gold is prominent in the northwestern part of the Superior; several occurrences of copper and nickel have, however, been found there as well. Farther south, in the Bird River area and its vicinity, is a remarkably complex region that probably represents a metallogenic subprovince. Here numerous occurrences of lithium, beryllium, tin, tungsten, and other minor metals, mainly associated with pegmatites, have been discovered, as well as occurrences of gold, nickel, cobalt, chromium, copper, lead, and zinc. This region of diversified metals bears several resemblances to the one in the southeastern part of the Slave province. Other similarities between the Slave and the western part of the Superior can be suggested: each has gold as the predominant metal in its northern part; each has a large section farther south containing pegmatitic deposits and numerous minor metals as well as gold; occurrences of pitchblende and native silver in the Port Arthur area are, in a way, analogous to occurrences immediately south of the Slave province, but the distance between the Bird River and Port Arthur areas is too great to make this analogy convincing. The southern part of the Superior province, extending through much of Ontario and Quebec, has been Canada's greatest source of metals. Because it is impossible in a short paper to mention all the important mining districts in this region, only certain features of the territory nearer to the Grenville boundary are discussed. Many of the largest orebodies are or were in a wide belt extending from Sudbury to Chibougamau, and these have been found in three subdivisions of the Superior. Many gold, copper, and zinc deposits are in easterly-trending belts of folded strata of Keewatin and Timiskaming types, as at Noranda, Bourlamaque, Porcupine, Kirkland Lake, and Chibougamau. The deposits at Cobalt and Gowganda are or were in flatlying cover rocks. Sudbury and Blind River are in the Penokean belt of folded Proterozoic and possibly older strata. It is interesting to note that the largest producers of nickel, copper, zinc, native silver, and cobalt are quite

20

A.H. LANG

close to the Grenville "front" although this generalization does not apply to the main gold camps-Porcupine and Kirkland Lake. Although minor occurrences of the other metals mentioned are widespread, there appears to be too great a concentration of large deposits near the Grenville to be merely coincidental. It seems impossible to explain this concentration definitely at present because of difficulties in dating the age or ages of metallization as adequately as has been done for the orogenies. The information available suggests pre-Grenville ages for the deposits. If this is so, perhaps the Grenville orogeny dissipated or recycled deposits that previously existed along the continuations of the mineral belts in the Superior. If, on the other hand, some or all of the deposits in the Sudbury-Chibougamau territory were formed in Grenville or post-Grenville times, they could have been influenced by structural or other conditions related to the Grenville orogeny. What appears to be an important structural feature related mainly to gold deposits can be seen on the Geological Map of Ontario issued recently by the Ontario Department of Mines. It is marked by two granitic plutons and several bands of metamorphosed sedimentary and volcanic strata, forming a more or less elliptical structure having its long axis parallel with that of the Sudbury basin, but being much larger than it. Its periphery includes the Porcupine, Munroe, Ramore, Kirkland Lake, Matachewan, and Swaze gold areas (Satterly, 1958) . GRENVILLE PROVINCE

The Grenville province is an important source of metals, although not rivalling the Superior. With the exception of titanium most of the metals produced are also found inlarge quantities in the Superior, but many of the deposits are of different geological types. The Grenville is noted for ilmenite deposits associated with anorthosites; for metasomatic magnetite and titaniferous magnetite deposits, many of which also carry vanadium ; and for numerous occurrences of uranium and other metals in pegmatites and metasomatic deposits. Areas containing uranium, gold, copper, nickel, lead, zinc, niobium, and beryllium were outlined on the maps, many of these areas overlapping. The niobium deposits mentioned above are in pegmatites and related deposits. Niobium deposits of another kind, associated with alkaline-carbonate rock complexes, and commonly with relatively small circular structures, have been found on islands in Lake Nipissing, and are aligned in a belt of such deposits extending from the Monteregian Hills in the St. Lawrence Lowlands province to the Chapleau and Michipicoten areas in the Superior province. The relationship with the Monteregian Hills suggests that this belt may be post-Grenville in age, and that dates for these deposits would be of interest. It should be noted, however, that niobium has also been reported ( Rowe, 1958) from syenite near Marathon, Ontario, which is on the continuation of this general belt, and that an age determination for this syenite suggests the Grenville orogeny (Fairbairn et al., 1959; Pinson et al., 1960).

RELATION OF METAL OCCURRENCES

21

CONCLUSIONS

The study outlined so briefly above is regarded only as of the most preliminary nature, but it shows patterns that even in their present incomplete forms appear to foreshadow the actual distribution. Most of the patterns seem to correspond fairly well with the tectonic provinces and subprovinces and to support the accuracy of the divisions proposed by Stockwell. It is not necessary for metallogenic patterns to correspond with tectonic or other geological patterns because the metallogenic patterns discernible at present may be related to original variations in the distribution of metals in the earth's crust which transgress tectonic boundaries. The degree of correspondence in the Canadian Shield, however, seems to indicate that the distribution of metals there is at least largely related to the geological history of a particular segment of the Shield, rather than to fundamental differences in the original crust. REFERENCES FAI-RBAIRN, H. W., BuLLWINKEL, H.J., PINSON, W. H., and HURLEY, P. M. (1959). Age investigation of syenites from Coldwell, Ontario. Proc. Geo!. Assoc. Canada, 11: 141-4. GILL, J. E. ( 1948). The Canadian Precambrian Shield. In Structural geology of Canadian ore deposits. Can. Inst. Min. Met., Jubilee vol., 20-48. - - - (1952). Mountain building in the Canadian Precambrian Shield. 18th Int. Geo!. Cong., pt. XIII, 97-104. - - - ( 1949) . Natural divisions of the Canadian Shield. Trans. Royal Soc. Canada, Sect. IV, Series III, 43: 61-9. JOLLIFFE, A. W. (1952). The north-western part of the Canadian Shield. 18th Int. Geo!. Cong., pt. XIII, 141-9. LANG, A. H. ( 1961). A preliminary study of Canadian metallogenic provinces. Geo!. Surv. Canada, Paper 60-33. LoRD, C. S. ( 1951). Mineral industry of District of Mackenzie, Northwest Territories. Geo!. Surv. Canada, Mem. 261. PINSON, W. H., and STAFF ( 1960). Age measurements of the Grenville metamorphism near Dolbeau, Province of Quebec. Dept. of Geology and Geophysics, Massachusetts Institute of Technology, Eighth Annual Progress Report, 277-9. RowE, R . B. ( 1958) . Niobium ( columbium) deposits of Canada. Geo!. Surv. Canada, Econ. Geo!. Series no. 18, 96. SATTERLY, J. (1958) . Geological map of the Province of Ontario. Ont. Dept. Mines, Map 1958B. STOCKWELL, C. H. ( 1962) . A tectonic map of the Canadian Shield. In John S. Stevenson (ed), The Tectonics of The Canadian Shield. Royal .Society of Canada Special Publication no. 4, Toronto, University of Toronto Press, 6-15. WILSON, J. T. (1949). Some major structures in the Canadian Shield. Trans. Can. Inst. Min. Met., 3: 231-42.

YELLOWKNIFE,NONACHO AGE AND STRUCTURAL RELATIONS R. A. Burwash and H. Baadsgaard

ABSTRACT

In southeastern District of Mackenzie, granites have been mapped as pre- and post• Nonacho series. The basal Nonacho conglomerate contains boulders giving K/ A dates on muscovite of 2400 million years. The underlying gneissic complex, which is highly chloritized, gives biotite dates of 1800 million years, and probably represents a basement of Yellowknife age reactivated during the Churchill orogeny. The Nonacho-granite contact is mapped as unconformable where a basal conglomerate is present, intrusive where the basal beds are metarkose. The Nonacho series and the Great Slave group were both deposited on a shelf of the Yellowknife craton, which may have extended as far south as Lake Athabasca.

REGIONAL STUDIES of the Precambrian basement of Western Canada have been in progress at the University of Alberta for a number of years (Burwash, 1951, 1957, 1958, 1959; Garland and Burwash, 1959). As the potassium-argon dating method was applied to an increasing number of regionally distributed samples, it became important to determine whether areas of anomalous age determinations were likely to occur within large areas of apparently uniform age. The critical problem was the ability of the potassium-argon dating method to identify areas of "older granite" in a regionally metamorphosed terrain cut by "younger granite." In 1958 a study was made of the southern part of the Nonacho sedimentary belt and the adjacent granitic rocks. Reconnaissance 4-mile mapping of the Nonacho Lake and Taltson Lake areas by Henderson (1939), and the Fort Smith area by Wilson ( 1941 ) , indicated the presence of granite of two ages in contact with metasedimentary rocks of the Nonacho series. This area thus seemed to offer the possibility of dating "older granite" in an area intruded by "younger granite." THE THEKULTHILi PROBLEM

Field Relations Thekulthili Lake lies near lat. 61 • N, long. 110° W, and occupies parts of the Nonacho Lake, Taltson Lake and Fort Smith map-areas. Along the east shore of Thekulthili Lake two areas of older granite and two areas of younger granite have been mapped ( Fig. 1). In his discussion of Protero22

23

YELLOWKNIFE-NONACHO RELATIONS

TAL TSON LAKE

12420, 22so 1

LEGEND MIL ES

0

(D- LOCALITY No. ~

FIGURE

II 8 50 I

20

AGE (M.Y.)

YOUNGER GRANITE

C:::J

NONACHO SERIES

I&

TAZIN

SERIES

~

OLDER

GRANITE

~

GRANITE- UNDIFFERENTIATED

1.

zoic granitic intrusions in the western Canadian Shield, Henderson ( 1948) states that the pre-Nonacho granitic rocks "cannot be differentiated from the post-Nonacho granites except where their unconformable relation with the Nonacho sediments can be established." The main objective of the 1958 field programme was the sampling of the pre- and post-Nonacho granite bodies for potassium-argon dating. Early in the field work it became apparent that the primary mafic minerals in the granites along the contact with the Nonacho series have been almost universally altered to chlorite and epidote. Neither of these minerals is useful for potassium-argon dating. At two localities gneissic granitic rocks were found which contain unaltered biotite. Locality 1 is a

24

R. A. BURWASH AND H. BAADSGAARD

small body of gneiss occurring within the Nonacho sedimentary belt immediately north of Salkeld Lake. Locality 2 is two miles south and two miles east of the north end of Thekulthili Lake. As field work progressed southward along the east shore of Thekulthili Lake, the absence of datable minerals in the granitic rocks focused attention more sharply on the nature of the contact between the Nonacho series and the adjacent crystalline complex. From observation of the contact, three salient features emerged: ( 1 ) In many places the contact appears to be transitional from arkose or quartzite to siliceous granitic rock, with no distinct boundary. The complete sequence of the transition is always obscured by areas covered with glacial drift or muskeg. ( 2) In a few places, conglomerates containing rounded granite boulders with a maximum diameter of two feet, are in immediate contact with the gneissic complex. The contact is gently dipping, irregular, and has the appearance of an unconformity. ( 3) On the basis of the mafic mineral assemblage, the metamorphic grade is the same on both sides of the contact. The dominant mafic minerals are chlorite and epidote, both in the metasedimentary belt and in the adjacent granites. Since the granites are medium-grained and possess normal igneous texture, the greenschist fades assemblage must represent retrograde metamorphism. The best exposure of . the contact between the conglomerate and the underlying gneissic complex, seen during our study, is at lat. 61 ° 08' N, long. 109° 57' W. At this locality (3) the dominant boulder type is faintly foliated pink granite. Other boulders are vein quartz and plagioclase-orthoclase-quartz-chlorite gneiss. The matrix of the conglomerate is quartz-feldspar-chlorite-sericite schist, with poorly developed schistosity. The underlying gneissic complex is plagioclase-orthoclase-quartz-chlorite gneiss. The attitude of the contact is N 85 ° E, dipping 30° S. The foliation of the gneiss is N 65 ° E, dipping 35 ° S. No structures within the gneissic complex were visibly truncated by the contact, and no granite was seen to intrude the conglomerate. Both gneiss and conglomerate are cut by quartz veins and fresh diabase dykes. At locality 4, two miles north of the mouth of Sparks River, a small, elongate body of hornblende-plagioclase gneiss was found within the granitic terrain. The body of gneiss is cut by quartz-feldspar veins, along which the hornblende is largely altered to chlorite and epidote. The mineralogy and texture suggest that the hornblende-plagioclase gneiss was originally a basic inclusion in the granitic complex, metamorphosed in the almandine amphibolite fades. Subsequent retrograde metamorphism, of which the quartzfeldspar vein is the visible evidence, produced sericitic alteration of the plagioclase and chlorite-epidote alteration of the hornblende. On the east shore of Thekulthili Lake, one-half mile south of the mouth of Sparks River, an outcrop of Nonacho conglomerate was found to contain boulders of muscovite pegmatite and muscovite granite. These were the only boulders found within the Nonacho conglomerate which contained unaltered micas suitable for potassium-argon age determinations. The rock

25

YELLOWKNIFE-NONACHO RELATIONS

underlying this conglomerate at locality 5 is a mottled chlorite-epidoteplagioclase-quartz rock of uncertain origin. Age Determinations The results of potassium-argon age determinations made on samples collected at localities 1, 2, 4, and 5 are given in Table I. The dates fall into two distinct groups. The two northern localities ( 1 and 2) give values ranging from 1790 to 1850 million years. The two southern localities give distinctly higher values, ranging from 2240 to 2420 million years. TABLE I THEKULTHILI LAKE AGE DETERMINATIONS Loe. No. Sample Location and Rock Type 1 1 2 4 5 5

Salkeld Lake 61°26'N, 109°47'W Biotite-sericite gneiss Same sample as above N. Thekulthili Lake-2 mi. east 61°19'N, 109°49'W Biotite gneiss Sparks River mouth-2 mi . north 61 °06'N 109°57'W Hornble~de-plagioclase gneiss Sparks River mouth-½ mi. south 61 °04'N, 109°57'W Muscovite pegmatite boulder Same bed as above Muscovite granite boulder

K2O per cent

A40/K4o

Age (m.y.)

sericite

10 .77

0.177

1790

biotite

9 .32

0 . 178

1800

biotite

8 .34

0 . 186

1850

hornblende

1.37

0 . 253

2240

muscovite

9 .28

0.294

2420

muscovite

10 .64

0.259

2260

Mineral

Decay constants: Xe= 0.589Xl0-10 yr-1, Xfl = 4.76X10-10 yr- 1, K40 / K = 0.0118 atomic per cent abundance.

From the small body of gneissic granite north of Salkeld Lake sericite and biotite were separated from the same rock sample. The values of 1790 and 1800 million years from these two mineral separates are a close check on the value of 1850 million years obtained from the biotite gneiss sampled eight miles to the south. The dates at these two localities most likely represent the time of the period of regional metamorphism which locally developed the greenschist facies mineral assemblage in the Nonacho sedimentary belt. The values of 2240 million years determined for the hornblende from the gneissic inclusions in the granitic complex, and 2260 and 2420 million years for the muscovites from the boulders in the conglomerate are relict dates from an earlier metamorphic and intrusive cycle. The time of crystallization of the hornblende and muscovite was probably 2500 to 2600 million years ago. The lower figures given by the potassium-argon method may be "survival values" ( Goldich, Nier, Baadsgaard, Hoffman, and Krueger, 1961 ) intermediate between the true values and the time of subsequent metamorphism.

26

R. A. BURWASH AND H. BAADSGAARD

In the regional setting of the western Canadian Shield, the older dates are related to the Yellowknife geologic province. The younger dates correspond closely to the time of the Churchill orogeny given by K-Ar dating of micas (Burwash, 1958; Lowdon, 1960). GEOLOGICAL HISTORY OF THE NoNACHO LAKE AREA

On the basis of field observations and physical age determinations the sequence of geological events in the Nonacho Lake, Taltson Lake, and Fort Smith map-areas may be summarized as follows: ( 1) Prior to 2500 million years ago sedimentation and volcanism deposited a sequence of greywackes and slates, probably with interbedded basic volcanics. ( 2) Intrusion and metamorphism at 2500 million years produced in this area a complex of schists, gneisses, and granitic rocks similar to that now exposed in the Yellowknife area. ( 3) Erosion removed most of the sedimentary rocks of low metamorphic grade, forming a peneplain on the gneissic and granitic complex. Local relief on the erosion surface was probably of the order of several hundred feet. ( 4) Sedimentation of the Nonacho series was initiated by the deposition of a basal conglomerate in areas of lowest elevation. As sedimentation proceeded the higher slopes and ridges were mantled with arkosic sediments. Some 10,000 feet or more of coarse elastic sedimentary rocks were deposited in this area. Calcareous sedimentary rocks formed a minor part of the sequence. ( 5) Approximately 1800 to 1900 million years ago a regional increase in temperature raised the pre-Nonacho basement and the lower part of the Nonacho series into the greenschist and epidote-amphibolite fades of metamorphism. Locally the temperature was high enough to cause recrystallization ( or at least complete argon loss) in the biotite gneisses. Most of the pre-Nonacho gneisses and granites were altered to orthoclase-quartz-chlorite-epidote rocks. The unconformity at the base of the Nonacho series remained recognizable where marked by a basal conglomerate. Where the basal Nonacho sedimentary rocks had been arkose, metamorphism produced a gradational contact resembling an intrusive or granitization phenomenon. ( 6) After 1800 million years, erosion has removed sediments of the Nonacho series from most areas except a downfaulted and gently folded trough, paralleling the regional folding of this portion of the Churchill orogenic belt. REGIONAL OCCURRENCE OF THE PRE-NONACHO UNCONFORMITY

To the north and west of Thekulthili Lake an unconformity, formed on a basement complex of Yellowknife age, is found in the east arm of Great Slave Lake. On northern Simpson Island a muscovite pegmatite was dated at 2480 million years. The pegmatite is part of the granitic terrain mapped by Stockwell ( 1936) as underlying the sedimentary rocks of the Great Slave group. The sedimentary formations are in turn cut by a syenite dyke dated

27

YELLOWKNIFE-NONACHO RELATIONS

• 2490, 2590

YELLOWKNIFE CRATON f' •2530

,2450 2500

NONACHO SHELF

DISTRICT

ALBERTA

·1 ·· - ·· - ·- ·· - ··- · - ·- - ·· - . I FIGU'RE

OF MACKENZIE

SASKATCHEWAN

MILES

o--==---- -~~=100 2.

at 2200 million years. The deposition of the Great Slave group must thus have taken place between 2480 and 2200 million years ago. Following emplacement of the pegmatite peneplanation may have required a hundred million years, and deposition of the Great Slave group a similar span of time. A period of alkalic plutonism, distinct from either the Yellowknife or Churchill orogeny, is indicated. The value 2200 million years obtained for biotite from the syenite, while similar to the date obtained for hornblende in the pre-Nonacho gneiss, is probably not a "survival value." Dating of granite, pegmatite, and gneiss from several localities within the Yellowknife geologic province provides a basis for comparison of the ages given for the Thekulthili Lake and Simpson Island samples. Data for these localities are given in Table II.

28

R, A. BURWASH AND H, BAADSGAARD

TABLE II GREAT SLAVE LAKE AND YELLOWKNIFE AGE DETERMINATIONS AK No. 191 140 124

121 122 193 143 144

Sample Location and Rock Type Simpson Island, Great Slave Lake* 61°46'N, 112°42'W Pegmatitic granite Simpson Island, Great Slave Lake 61°46'N, 112°42'W Syenite dyke Kam Lake, Yellowknife Bay 62°26'N, 114°24'W Biotite granite Redout Lake, Gordon Lake south 62°47'N, 113°03'W Biotite-hornblende gneiss Redout Lake, Gordon Lake south 62°46'N, 113°02'W Pegmatitic granite Prosperous Lake 62°40'N, 114°11'W Muscovite-biotite granite Laverty Lake, Walmsley Lake 63°56'N, 108°40'W Muscovite-biotite granite Same sample as above

Mineral

K2O per cent

A40/K4o

Age (m.y.)

muscovite

10.55

0.304

2480

biotite (+ chlorite)

4.02

0.247

2200

biotite

7 .84

0 . 295

2440

biotite

8.18

0.297

2450

biotite

9.07

0.309

2500

10.76

0.315

2530

7.59

0.306

2490

10 . 12

0 .331

2590

muscovite biotite muscovite

*References to map-areas: Great Slave Lake, Stockwell (1936); Yellowknife Bay, Jolliffe (1942); Gordon Lake south, Henderson (1941); Prosperous Lake, Jolliffe (1946); Walmsley Lake, Folinsbee (1952).

The values of 2440, 2450, 2500, and 2490 million years for biotites, and 2530 and 2590 million years for muscovite, correspond closely to previous published dates (Lowdon, 1960). Where muscovite and biotite were separated from the same rock, muscovite gave a slightly higher age value. The occurrence of an older basement complex underlying the Great Slave and Nonacho series of sediments, with the grade of regional metamorphism apparently increasing to the south, suggests the possibility that the Tazin complex of northwestern Saskatchewan and adjacent District of Mackenzie may contain elements of both the basement complex and the metamorphosed sediments of the Nonacho shelf. Whether a reactivated basement of Yellowknife age is exposed or can be recognized in the Lake Athabasca area remains a problem. Application of Rb-Sr and U-Pb dating techniques mav clarify this problem. ACKNOWLEDGMENTS

The authors wish to acknowledge support for field work given by the General Research Fund, University of Alberta. The programme of age dating is supported by the National Research Council and the Geological Survey of Canada. Mass spectrometry is done in the Department of Physics, with the assistance of G. L. Cumming and H. R. Krause. R. A. Burwash received invaluable assistance in the field from R. S. Taylor. We also wish

YELLOWKNIFE-NONACHO RELATIONS

29

to pay tribute to the earlier field work in the area by J. F. Henderson. His recognition of the basal Nonacho unconformity provided the incentive for the current research. REFERENCES

BURWASH, R. A. (1951) . The Precambrian under the central plains of Alberta. Unpublished M.Sc. thesis, Dept. of Geology, University of Alberta. - - - (1957) . Reconnaissance of the subsurface Precambrian of Alberta. Bull. Amer. Assoc. Pet. Geo!., 41: 70-103. - - - (1958). Age of the Precambrian basement. J. Alberta Soc. Pet. Geo!., 6: 214-17. - - - ( 1959) . Pre-Beltian basement in southern Alberta and adjacent British Columbia (abstract) . Bull. Geo!. Soc. Amer., 70 : 1576. FoLINSBEE, R. E. (1952). Walmsley Lake, N.W.T. Geo!. Surv. Canada, Map 1013A. GARLAND, G. D ., and BuRWASH, R. A. ( 1959) . Geophysical and petrological study of the Precambrian of central Alberta, Canada. Bull. Amer. Assoc. Pet. Geo!., 43 : 790-806. GoLDICH, S. S., NIER, A. 0 ., BAADSGAARD, H., HOFFMAN, J. H ., and KRUEGER, H . W. ( 1961). The Precambrian geology and geochronology of Minnesota. Minn. Geol. Survey, Bull. 41. HENDERSON, J. F. ( 1939a) . Taltson Lake, N.W.T . Geo!. Surv. Canada, Map 525A. (1939b) . Nonacho Lake, N.W.T. Geo!. Surv. Canada, Map 526A. - - - (1941) . Gordon Lake south, N.W.T. Geo!. Surv. Canada, Map 645A. - - - ( 1948). Extent of Proterozoic granitic intrusions in the western part of the Canadian Shield. Trans. Roy. Soc. Canada, Sect. IV, Series 3, 42: 41-54. JOLLIFFE, A. W. (1942). Yellowknife Bay, N.W.T . Geo!. Surv. Canada, Map 709A. - - - (1946) . Prosperous Lake, N.W.T. Geol. Surv. Canada, Map 868A. LowooN, J. A. ( 1960). Age determinations by the Geological Survey of Canada. Geol. Surv. Canada, Paper 60-17. STOCKWELL, C. H . ( 1936) . Eastern portion of Great Slave Lake, N.W.T. Geo!. Surv. Canada, Map 377A. WILSON, J. T . (1941) . Fort Smith, N.W.T. Geo!. Surv. Canada, Map 607A.

STRUCTURAL PATTERN OF THE PRECAMBRIAN SHIELD IN NORTHEASTERN ALBERTA AND MICA AGE,DATES FROM THE ANDREW LAKE DISTRICT John D. Godfrey and H. Baadsgaard

ABSTRACT

The regional fold and fault pattern of about 3600 square miles of the Shield in northeastern Alberta has been outlined with the aid of vertical aerial photographs and information from ground work in a limited part of the area. Results of an aeromagnetic survey fit into the structural framework and also correlate very well with the distribution of major rock type~ based on the ground work. Potassium-argon dates have been determined on micas selected from the major rock types of the mapped area and fall in the range 1. 7 to 1.8 billion years. STRUCTURAL PATTERN

The regional fold and fault pattern of about 3600 square miles of the Precambrian Shield in northeastern Alberta has been outlined with the aid of vertical aerial photographs (Godfrey, 1958) and with information from groundwork in a limited part of the area. Two structurally distinct regions can be recognized, the one to the north and the other to the south (Fig. 1) . In the northern region three northerly aligned major zones of weakness are referred to as the Allan, Warren, and Rutherford faults ( Fig. 1 ) . Two synclines lie between these three faults, although the westerly fold is not too clearly defined. The centrally located fold, including the three minor northward-pitching folds to the south, is shown to have a more complex form, approximating that of a synclinorium. These structures are related to a terrain underlain by mixed rock types of plutonic and metasedimentary rock associations. In contrast, the southern region is lacking in major folds and faults of the type found to the north. Instead, the dominantly massive plutonic rocks seem to have reacted as a rigid structural unit and have developed two sets of faults and fractures in response to stresses. The major folds and faults of the northern region form a structural configuration that is harmonious with a particular stress pattern. However, an intervening anticline in the vicinity of the Warren fault is needed to complete the structural picture and it appears that such a fold may have been displaced when the fold limbs were broken by high-angle shears. An easterly 30

31

TUE ANDREW LAKE DISTRICT

110•

60"

u

u 0 N

0

w _j

·20 .

I

I

c::=::i • 20 to -40

=

- 40to-60

-

3.

~~ ✓

.•·

-c1

IJRE

4-• '

-~ e.=~"s:~ ......

I·

,.o ~ ··

-·- -

...... /~

,,,----

---)

_,..-

,:---

.

-·~

Trend line map of area between lat. 54° N and 57° N, and long. 94° Wand 102° W. K/Ar ages shown by circle and date in millions of years.

TECTONICS IN NORTHERN MANITOBA

63

so that the structure producing the gravity anomalies is probably Precambrian in age. The gravity high and related gravity low coinciding with thrust faults and peridotite intrusions has suggested the obvious analogy with an island arc or alpine mountain system. The present paper is a more detailed study of the structure of the Churchill and Superior blocks and the boundary area between the two blocks. A trend map ( Fig. 3) has been prepared by plotting trends of the strike of formations, bedding, gneissosity, and magnetic and electromagnetic anomalies. This trend map has been used to prepare statistical structural diagrams of the various geological units. The area selected for the trend map lies between 54° and 57° N lat. and 96° and 102° W long. Statistical studies have also been made of the distribution of rock types, and the nature of the sedimentation in the Churchill and Superior blocks. The area has been broken down into four principal geological units on the basis of these studies. The four units are: ( 1 ) the Superior block which lies to the southeast of the limit of the greenstone belts ( Fig. 3) ; ( 2) the Churchill block which lies to the northwest of the regional gravity low; ( 3) the fault zone coinciding with the gravity low; and ( 4) the gneissic zone lying between the fault zone and the limit of the greenstone belts. The Superior block in general has an east-west trend. Its western limit has been placed at the line shown on Figure 3 entitled "limit of greenstone belts." This limit of greenstone belts has been drawn on the trend map from the individual geological map-sheets which show that the great greenstone belts of the Superior block end abruptly along this line and that the rocks further to the west consist of granite and granitic gneiss. The Churchill block in the area studied appears to have two principal trends. North of lat. 55 ° N the principal trend is east-west; south of lat. 55 ° N the common trend is northeast-southwest. The trend map appears to indicate that these two trends are complex limbs of a tightly folded structure, and that faults striking east-west occur near the axial region of the major fold. This major fold may be only part of a much larger fold system extending throughout the Churchill block; in northern Saskatchewan the rocks generally trend to the northeast and the rocks in the Seal River area of northern Manitoba trend east-west. The fault zone which coincides with the gravity low trends northeastsouthwest and apparently cuts both the Superior and the Churchill trends. Most of the trends in the gneissic zone are parallel to the principal trend of the Superior block. On the northwest side of the gneissic zone the trends are cut off abruptly by the strongly gneissic fault zone. The southeast border of the gneissic zone appears to be fault controlled in part but may be partly metamorphic. Near Cross Lake, at approximately long. 98° W and lat. 54°35' N the greenstone sedimentary belt is sharply truncated by the gneissic zone. However, further to the northeast along the limit of greenstone belts, near lat. 55° N, it is apparent that the trends of the greenstone belts pass directly into the gneissic zone without any offset or change

64

H. D. B. WILSON AND W. C. BRISBIN

in strike. The rock types of the Superior block become the granitic gneisses and granites of the gneissic belt. The Bear Lake greenstone belt is a good example. Further again to the northeast, where the gravity high crosses latitude 56° N, a greenstone-sedimentary belt almost completely crosses the gneissic zone and is still recognizable near Split Lake where the belt intersects the gravity low and the fault zone. The Nelson River gravity high coincides generally with the gneissic zone. The peak of the high is approximately at the centre of the zone. The trend map also shows that the gravity high is not related to surface exposures and the cause of the high must lie deep within the crust or mantle. The Kl Ar ages determined by the Geological Survey of Canada and the Department of Geology and Geophysics at the Massachusetts Institute of Technology have been plotted on the trend map. All K/Ar ages determined thus far in northern Manitoba in the Churchill block are in the 1600 to 1800 million year range. The dates in the map-area in Figure 3 in the Superior block are not as consistently in the 2400 to 2600 million year range as they are in the remainder of the Superior province of the shield. Near the Nelson River gneissic zone the ages tend to be intermediate between the ages of the Churchill and Superior blocks, although the data are still very meagre. One date of Churchill age lies well within the Superior block in Figure 3 and another date of similar age occurs further east beyond the limit of the map. These dates may represent outlying activity of Churchill age, but their significance is not yet known because of the sparseness of data and geological mapping. The structure of the boundary between the Churchill and Superior blocks at the northeast corner of Figure 3 is not known because of insufficient data. The changing trends of gravity and magnetic data show obvious complications. However, if the general trend of the Nelson River fault and gneissic zones is extended across Hudson Bay it lines up directly with the Cape Smith-Wakeham Bay belt where similar age relations and associated gravity anomalies occur. FREQUENCY DIAGRAMS OF TREND LINES

The structural differences indicated on the trend line map are more apparent when analysed quantitatively. Figure 4 shows circular histograms of trend lines for eleven subdivisions of the area studied. These frequency diagrams are not an attempt to deduce the actual structure of the areas for they are based on trend lines that are two dimensional. Rather, they are an attempt to deduce where structural differences may exist. A more precise treatment involving the third dimension and statistical plots of attitudes on equal area nets is considered in the next section of the paper. The total area was initially subdivided using obvious geologic boundaries, that is, into the Churchill and Superior blocks, the fault zone, and the gneissic zone. The Churchill and Superior blocks were further subdivided on

,,~~-

·••r--

l

•O,!)•

=-••-i--

-=

WW&

WwwmwW

WWWWWWWW

·- .. I

:1o

1 :

/,

I

,

....ua

" ~~ ,; ~

' I

11

,_

~

11

•

l'

I

I

J

"fl,'1,,

~~..\ * \~.,, "·0===-- =----=---------: v ___________ ~_JL __ /1 - - - - ,r----------,, •• ," ., I

I

I'

I

---------

::

,, '

11

I1~

II

V

~---

:

'

I

I

....____

!Ii

,.

11

I

II

I/

....____

'I

FIGURE

'

"

..........

##

.,"

....____

C,

// '-'II

1/

'

C":, C":,

,

It

I/ '•Ot.

SUPERIOR

BLOCK

200 poles·

GNEISSIC ZONE

128 poles

Equal area diagrams of poles to attitudes for major geological units ( lower hemisphere) .

The net of the Churchill block shows a broad girdle of points following a great circle upon which there is only one concentration of points above 5 per cent. This concentration is produced by a high frequency of attitudes striking approximately east-west and dipping approximately 45 ° to the north. Another possible maximum is produced by attitudes striking approximately N 25 ° E and dipping close to vertically. The pole to the great circle girdle plunges 40° in a direction N 20° E. A number of structural interpretations can be made of this diagram. The pattern could be produced

68

H. D. B. WILSON AND W. C. BRISBIN

by a series of folds either overturned or asymmetric towards the southeast and tilted so that the axes now plunge to the northeast. The plunge would correspond to the position of the pole to the great circle along which the points lie. On the other hand, a series of folds overturned to the south and cross-folded could produce this same pattern. Both of these tentative interpretations suggest overturning towards the south. A final answer to the precise nature of the structure of the Churchill block and the Superior block must await sufficient detailed mapping so that this and other types of structural analysis can be carried out in detail on smaller subdivisions of the blocks. The important point to recognize during this preliminary examination is that there is a fundamental general difference in structure between the blocks. A comparison of the net of the fault zone with those of the Churchill and Superior blocks reveals how the fault zone truly separates the two blocks. The attitudes in the fault zone are predominantly vertical and strike northeast-southwest. The actual dips of the faults are not known but the gneissosity and bedding within the zone suggest steep dips. Thirty per cent of the poles plotted from the fault zone dip vertically, 30 per cent dip to the northwest, and 40 per cent dip to the southeast. The equal area net of the gneissic zone confirms what was suspected from the trend line map and the frequency diagrams of the trend lines; the gneissic zone structure is almost identical to that of the Superior block and it is reasonable to conclude that the gneissic zone was originally part of the Superior block. Figure 6 deals with a more detailed structural analysis of the southwestern portion of the Churchill block. Within this area the structure of the predominantly volcanic Flin Flon rocks is compared with the structure of the predominantly sedimentary Kisseynew rocks. The boundary between the two areas has been chosen to follow the "Kisseynew lineament." Both fabric patterns are similar in that they show a broad scatter of points around a single 7 per cent maximum. The concentrations, however, have different orientations. The Kisseynew rocks have a high frequency of attitudes striking west to northwest and dipping 25 ° to 45 ° northeast whereas the Flin Flon rocks have an attitude maximum striking north-northeast with an almost vertical dip. The pattern for the Kisseynew rocks is very similar to the pattern for the Churchill block as a whole ( Fig. 5). The change in structure across the "Kisseynew lineament" may be explained in various ways. The change may be due to a major unconformity or a major fault so that the rocks on either side of the lineament are not related. On the other hand, the change may take place across the axial region of a major regional fold, such as that suggested in the discussion of the trend line map. If this last interpretation is correct the Kisseynew rocks would form the northern limb, and the Flin Flon rocks the southern limb of the major fold. The axial plane of such a fold would strike slightly north of east and dip

69

TECTONICS IN NORTHERN MANITOBA I 2°

99•

100•

KISSEYNEW ROCKS 224 poles

I

,

I,

,

I

,I

I ,,

,

0 FLIN FLON ROCKS 160poln

FIGURE 6.

CONTOUR INTERVAL· I',

LJ

5•10',

Equal area diagrams of poles to attitudes for Kisseynew and Flin Flon rocks ( lower hemisphere) .

to the northwest; the axis would plunge to the northeast. The volcanic rocks of the Flin Flon-Herb Lake area at the core of the fold would be flanked to the north and to the southeast by the predominantly sedimentary rocks of these areas. RocK TYPES IN THE SUPERIOR AND CHURCHILL BLOCKS

The geological map of Manitoba suggests that the over-all rock composition of the Superior and Churchill block differs. Point-counts of the geological maps were made to determine the petrographic composition of the area studied ( Fig. 3). Table I shows the data for the various map-areas. The names of the map-areas refer to the names of the four-mile map-sheets of the National Topographic Series. The numbers following the map names refer to Figure 7 where the sheets are numbered in horizontal rows starting

TABLE I PETROGRAPHIC COMPOSITION OF MANITOBA MAP-AREAS Map-Area

Basic Volcanics and Mm. Equiv.

Churchill Block Granville (1) Uhlman L. (2) Kississing (5) Nelson House (6) Cormorant L. (9) Wekusko L. (10)

10 .0 4.5 7 .8 1 .8 33.9 21.8

Superior Block Sipiwesk (7) Knee L. (8) Cross L. (11) Oxford House (12)

13.0 16.2 7.3 11.5

Rhyolite

-

1.0 2 .3

-

0 .3

-

2.8

Metamorphosed Sediments

Migmatite (sedimentary)

Granite

Gabbro

9 .2 13.5 36 .6 24 . 7 7.7 36.8

31.8 9.7 21 .8 37.9 6.6

1.5

-

47 .5 72.3 32.4 35.6 44 .0 36.8

1.5 3.5 2 .2 6.4

-

85 .5 79.2 90 .5 78.1

0.5

Peridotite

-

1.4

-

6 .6 2 .3

0 .2

-

0 .3

0.8

0.4

-

-

TECTONICS IN NORTHERN MANITOBA

41 ·0 10.0 4 1.

r---

_J

I

I

23.2 4 5 72.31

.

I

_____ .JI

-----,

I

62.6 1.8 37.9:

I

KEY

~.

r-----,

,~ I

~

I

1sEO GST. GR I

I

L _____ J I

FIGURE

7.

=

Petrographic composition of map areas. Sed. sedimentary rocks; gst. volcanic rocks; gr. granitic rocks.

=

=

72

H. D. B. WILSON AND W. C. BRISBIN

with number one at the upper left-hand corner and ending with number twelve at the lower right-hand corner. The actual areas counted are shown in Figure 7 by dashed lines. The data are summarized in Figure 7. In this figure the sedimentary rocks have been combined with the sedimentary migmatites to give the per cent of sedimentary rocks although the migmatite contains considerable granitic as well as sedimentary material. Table II shows the average composition of the Superior block and the Churchill block corrected for the total areas involved on the various mapsheets. For comparison purposes point-counts were made of the graniteprimitive system complex of central Southern Rhodesia and of the Svecofennidic and Karaldic schist belts of southwest Finland ( Simonen, 1960, Fig. 1 ) . The maps used for the tabulation were the Geological Map of Southern Rhodesia, 1946 edition; and the map entitled "Pre-Quaternary Rocks of Finland," 1960 edition, published by the Geological Survey of Finland. TABLE II PETROGRAPHIC COMPOSITION OF PRECAMBRIAN BLOCKS

Basic volcanics Rhyolite Sedimentary rocks Migmatite Granite Gabbro Peridotite

Superior Block

Southern Rhodesia

Churchill Block

Southwest Finland

11.7 1.0 4.0 0 .2 82.6 0 .3 0.2

10.6*

8.7 0.1 19 .3 23.2 47.5 1.2

5 .5

3.3 85.3 0.8

23.3 11.0 58.8 1.4

*Includes all volcanic rocks.

Tables I and II and Figure 7 show that the Churchill and Superior blocks have a different petrographic composition in addition to their different structure. The Superior block is composed largely of granite and has a high volcanic and low sedimentary content. The Churchill block, on the other hand, has less granite, a very high sedimentary content, and a low volcanic content. These data can only be considered first approximations and undoubtedly these figures will change considerably with more detailed mapping and with more careful separation of the rock units. However, little doubt exists that these differences are significant, because the areas in the two blocks were mapped in a similar manner and in many cases by the same geologist, so that such radically different results must represent a true difference in petrographic composition. Table II shows comparisons of the Churchill and Superior blocks with the central shield of Southern Rhodesia and the Precambrian of southwestern Finland. The senior author has observed that the type of structure that occurs in Southern Rhodesia is similar to that of the Superior block whereas the type of structure that occurs in southwestern Finland is comparable to that of the Churchill block. The table shows that the rock

TECTONICS IN NORTHERN MANITOBA

73

composition of the Superior block and the Southern Rhodesia central shield are similar, and the compositions of the Churchill block and southwestern Finland are also similar. It is also of interest to recognize that the granite and metamorphic rocks of the Superior and central Rhodesian blocks have a similar age of about 2500 million years. Those of the Churchill and southwest Finland areas are also similar with dates in the 1600 to 1800 million year range. All dates were determined by the K/ Ar method. These similarities in age, composition, and structure may be purely coincidental, but in the event that they are fundamental they should be studied in other Precambrian shields of the earth. Many differences of opinion concerning generalizations of igneous and metamorphic geology probably result from trying to compare different blocks such as the Superior in Canada with the Svecofennidic in Finland. CONTRAST OF THE SEDIMENTARY ROCKS IN THE CHURCHILL AND SUPERIOR: BLOCKS

Petti john ( 1943) showed convincingly that the Archaean sedimentary rocks in the Canadian shield were almost entirely composed of greywacke and slate. He was actually describing the sediments of the Superior block although at that time the Superior block was not recognized as such. The sedimentary rocks that occur in the Churchill block, however, are obviously considerably different to those in the Superior block. Most of the mapping in the Churchill block is not detailed enough for point-counts measurements of the various sedimentary rock types. However, the maps of the Lynn Lake area ( Milligan, 1960) and the Batty Lake map-area ( Robertson, 1953) are mapped in detail and the various types of sedimentary rocks have been differentiated. Point-counts were made of these map-areas to determine sedimentary composition and the results are shown in Table III. These areas are typical of the type of sedimentation in the TABLE III NATURE OF CHURCHILL BLOCK SEDIMENTARY ROCKS

Per cent of Total Sediments Lynn Lake area arkose and quartzite conglomerate greywacke Kisseynew gneiss Batty Lake map-area quartzite ferruginous shale limestone greywacke calcareous shale

46.2 1.3

26 .9 25.6 100.0

36.7 2.7 2.7

42.7 15.2 100.0

74

H. D. B. WILSON AND W. C. BRISBIN

Churchill block in Manitoba. All of the sedimentary rocks are considerably metamorphosed, but it is obvious that quartzite, arkose, and limestone are much more common than they are in the Superior block as described by Pettijohn. The greywacke mapped by Robertson may not be a typical eugeosynclinal type. Chemical analyses published by Robertson ( 1953) show that the greywacke contains Ji more alumina than normal eugeosynclinal greywacke. The K2O /Na2O ratio is also considerably greater than unity, instead of less than unity as in the typical eugeosynclinal greywackes described by Middleton (1960), and Pettijohn (1943). It seems possible, therefore, that the metamorphosed sedimentary rocks mapped as greywacke in the Batty Lake map area may be metamorphosed shales. It may be concluded that the Churchill sedimentary rocks differ greatly from those of the Superior block. The composition of many of the Churchill sediments is not typical of the eugeosynclinal environment but, instead, is typical of a relatively stable continental shelf. CONTRAST OF THE INTRUSIVE IGNEOUS ROCKS IN THE CHURCHILL AND SUPERIOR BLOCKS An attempt was made to investigate the composition of the intrusive rocks in the area studied, but not enough analyses were available to determine the chemical affinities. The structure of the granitic intrusive rocks differs in the two blocks. In the Churchill block the intrusions are syntectonic and have the same strike and dip as the sedimentary rocks, even where the dips are flat. In contrast, in the Superior block, structures within the intrusions generally have a vertical or near vertical dip and batholithic intrusions are much more common. The intrusions in Southern Rhodesia and in southwestern Finland resemble those of the Superior and Churchill provinces respectively. MEAN GRAVITY ANOMALIES OF THE CHURCHILL AND SUPERIOR BLOCKS The mean gravity values for the Churchill and Superior blocks were computed from the Dominion Observatory Gravity Anomaly Map of Canada ( 195 7) using the values listed on the map as "the mean Bouguer anomaly per degree square." The total area covered in this analysis was bounded by the U .S.A.-Canada border to the south, the limits of the Dominion Observatory gravity work to the north, long. 100° to the west and long. 92° to the east. The average Bouguer anomaly value for the Superior block was computed to be -32.5 milligals; for the Churchill block -51.6 milligals. Using these values the average depth to the base of the crust was determined for each block from curves published by Woolard ( 1959) showing the relationship of the Bouguer gravity anomaly to the depth to the Mohorovicic discontinuity. The average crustal thickness of the Superior block was determined to be 34 km; that of the Churchill block 36 km.

TECTONICS IN NORTHERN MANITOBA

75

These figures suggest that the older Superior block may have been eroded to a depth approximately 2 km greater than the younger Churchill block. This difference in the depth of erosion may account, in part, for the difference in rock type and structure that exists between the blocks. However, it seems unlikely that the erosion of only 2 km of material could cause such changes. CONCLUSIONS

A complete interpretation of the geology and tectonics of this area is dependent on the results of work that is continuing in the area at the present time. However, certain conclusions based on the observations presented in this paper may be stated. ( 1) The boundary between the Superior and Churchill blocks is a fault zone. ( 2) The northwest edge of the Superior block ( the gneissic zone) is a metamorphic-igneous complex with the structural trend of the Superior block. Age dating indicates that some of the dates in this zone are of Churchill ( Hudsonian1 ) age. ( 3) The structure, sedimentation, and igneous activity of the Churchill and Superior blocks are different. The Churchill sedimentary rocks may have been deposited as shelf deposits on an oceanic crust at the edge of the Superior continent or they may lie unconformably on metamorphosed Superior rocks. ( 4) Folding within the Churchill portion of the area studied appears to be overturned towards the Superior block. This observation suggests the development of a structural pattern similar to that in the Appalachian and Cordilleran regions in which overturning and thrusting are towards the stable continent. REFERENCES INNES, M. J. S. ( 1960) . Gravity and isostasy in Northern Ontario and Manitoba. Ottawa: Publications of the Dominion Observatory, 21, 6. MIDDLETON, G. V. ( 1960). Chemical composition of sandstones. Bull. Geol. Soc. Amer., 71: 1011-26. MILLIGAN, G. C. ( 1960). Geology of the Lynn Lake district, Province of Manitoba. Mines Branch Publication 57-1. PETTIJOHN, F. J. (1943). Archaean sedimentation. Bull. Geo!. Soc. Amer., 54: 925-72. ROBERTSON, D.S. (1953). Batty Lake map area. Geo!. Surv. Canada, Mem. 271. SIMONEN, AHTI ( 1960). Pre-Quaternary rocks in Finland. Bull. Comm. Geo!. de Finlande, no. 191. WILSON, H. D. B., and BRISBIN, W. C. ( 1961) . Regional structure of the Thompson• Moak Lake nickel belt. Bull. Can. Inst. Min. Met. ( in press). WooLARD, G. P. ( 1959) . Crustal structure from gravity and seismic measurements. J. Geophys. Res., 64, 10 : 1521-44. 1 Term

proposed by C . H. Stockwell, this symposium.

EXTENT OF THE HURONIAN SYSTEM BETWEEN LAKE TIMAGAMI AND BLIND RIVER, ONT ARIO James E. Thomson, F.R.S.C.

ABSTRACT

Throughout the last decade geologists of the Ontario Department of Mines have been remapping the belt of sedimentary rocks that lies between Lake Timagami and Blind River. This investigation has corroborated the conclusion of earlier workers that the Huronian system (Proterozoic) is separated from the pre-Huronian systems (Archaean) by a great angular and erosional unconformity. Intensive mineral exploration from 1953 to 1958 located radioactive pebble conglomerate in the Mississagi formation at or near the base of the Huronian at many places along this belt. These discoveries have been of great assistance in locating the Huronian-pre-Huronian boundary in greatly deformed parts of the area. Going westward from Lake Timagami the Mississagi and other formations of the Huronian sequence have been traced at intervals to Lake Wanapitei, thence north of the Sudbury basin through the Milnet and Cartier areas, then south through the Agnew Lake area and westward to the main Blind River-Elliot Lake basin. Contrary to some earlier interpretations, there is no good evidence of Huronian formations south of Sudbury or anywhere between the Murray (Worthington) fault and the north shore of Lake Huron, The sedimentary rocks south of the Murray fault are greatly deformed and broken into blocks by regional faulting so that stratigraphic correlation is uncertain. The field relationships suggest a pre-Huronian age (i.e., Sudbury group) for most of this sedimentary unit. However, because Huronian and pre-Huronian sedimentary formations cannot be separated by lithology alone, and because it is sometimes difficult to locate major unconformities in highly deformed areas, the term "unclassified Precambrian" would best describe the stratigraphic and correlational status of this unit pending further detailed studies. INTRODUCTION

This paper is the sequel to previous discussions (Thomson 1953, 1957a) on the age and correlation of the sedimentary rocks in the SudburyEspanola area. Five seasons of field work by geologists of the Ontario Department of Mines have been completed since the last of these discussions of the subject was presented and much new information is available. A vast amount of exploration by mining companies and prospectors was carried out between 1953 and 1958 along this sedimentary belt in the search for uranium deposits. This information, which has been assembled and published (Thomson, 1960), sheds considerable light on the geological problems involved. The object of this paper is to show that the main Huronian basin can 76

-- V

,''

,-,

--'

8 --

-~'

\ff

✓. -"'

F10URE

,J ---,----Garson

•• ·.•

0~ L.

./ / : • • :" .