The northern margin of the Grenville Province in western Labrador — anatomy of an ancient orogenic front

Precambrian Research, 22 (1983) 41--73

41

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE N O R T H E R N MARGIN OF THE GRENVILLE PROVINCE IN WESTERN L A B R A D O R - - A N A T O M Y OF AN ANCIENT OROGENIC FRONT

T. RIVERS

Department o f Earth Sciences, Memorial University of Newfoundland, St. John % Newfoundland A I B 3X5 (Canada) (Received June 6, 1982; revision accepted January 18, 1983)

ABSTRACT Rivers, T., 1983. The northern margin of the Grenville Province in western Labrador -anatomy of an ancient orogenic front. Precambrian Res., 22: 41--73. In western Labrador, the northern margin of the Grenville Province, k n o w n as the Grenville Front Tectonic Zone, abuts against the Superior Foreland Zone composed of an Archean gneiss terrain, and against the Churchill Foreland Zone composed of weakly metamorphosed and deformed Aphebian supracrustals. In both terrains, a similar tectonic style is developed in which Aphebian rocks from within the Grenville orogen have been thrust northwards over the adjacent foreland. The Grenville Front is marked by mylonite/ phyllonite zones in the Archean gneisses of the Superior foreland, whereas in the supracrustals it is characterized by cleavage folding. Grenvillian deformation involved 3 episodes, of which two gave rise to northeast trending structural elements and are interpreted to be part of a single protracted event involving northerly translation of rocks from within the orogen towards the foreland. A third deformation comprised large~cale crossfolds, and m a y be related to the formation of a major arcuate thrust salient. Spatially, the northeast-trending structural elements occur in the Grenville Front Tectonic Zone, while the crossfolds dominate in the interior of the orogen. A zone of interference structures related to the two fold trends occurs in the southern part of the Grenville Front Tectonic Zone. The Hudsonian Front (westerly limit of Hudsonian deformation in the Churchill Province) passes through the Churchill Foreland. Zone in the study area. There, Grenvillian structures are superimposed on older Hudsonian structures. Elsewhere in the area studied, however, all deformation and metamorphism of Aphebian rocks is of Grenvillian age. Metamorphic zones in pelitic rocks indicate a progressive increase in grade towards the interior of the Grenville orogen. The occurrence of the mineral pair kyanite -- K-feldspar preceding the assemblage sillimanite--K-feldspar in the prograde sequence indicates that peak metamorphic conditions m a y have exceeded 8 kb and 700°C. Such elevated pressures during metamorphism m a y have been achieved by stacking of thrust slices in the Grenville Front Tectonic Zone. Both deformation and postorogenic uplift were diachronous in the Grenville Province; deformation was initiated in the high-grade core of the orogen and migrated northwards towards the margin, whilst uplift occurred earlier in the margin than in the core.

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43 INTRODUCTION

Western Labrador, located at the junction of the Superior, Churchill and Grenville tectonic provinces of the Canadian shield (Fig. 1) is, together with adjacent eastern Quebec, a region of particular interest with regard to the understanding of the Grenvillian orogeny (ca. 1000 Ma). Much of the Grenville Province in this area is underlain by reworked Aphebian (Lower Proterozoic) rocks directly correlative with those of the Labrador Trough in the adjacent Churchill Province to the north. The resultant 'pinning' of the Grenville Province to the adjacent foreland is critical to tectonic interpretations of the Grenville Front Tectonic Zone (Wynne-Edwards, 1972) and imposes some constraint on the location of ancient plate boundaries in the vicinity of the Grenville Front. Few of the rather limited number of regional studies in the Grenville Province have encompassed the Grenville Front Tectonic Zone, understanding of which is fundamental to any overall interpretation of Grenvillian orogenesis. Work by Wynne-Edwards (1969) on the interior of the orogen in Quebec, while not the first to identify old reworked basement rocks, was important in its clear recognition of the probable wide extent of such thermotectonically overprinted basement in the Grenville Province. Previous studies in or adjacent to the region discussed in the present paper (e.g., Gastil and Knowles, 1957; Jackson, 1962; Knowles, 1967; Dalziel et al., 1969; Seguin, 1973; Roach and Duffell, 1974), generally covered smaller areas, and in some instances reached conflicting conclusions regarding the tectonometamorphic history. The present paper, based on four years of regional mapping and new geochronological data, presents solutions to several outstanding problems and also a coherent model of stratigraphic, structural and metamorphic evolution in the region. While the paper deals mainly with rocks in the Grenville Province and with Grenvillian thermotectonism, it also contains discussion of Archean, Aphebian and Helikian (Middle Proterozoic) rocks of the Churchill and Superior foreland zones, since comparison between these and reworked equivalents in the Grenville orogen conveys important insights into the effects of Grenvillian orogeny.

Fig. I. (A) Geological m a p of western Labrador and eastern Quebec showing location of the study area at the junction of the Superior, Churchill and Grenville Provinces. Structural trends are shown as wavy lines, thrust faults of Hudsonian age with single tooth, of Grenvillian age with double tooth; dot pattern indicates area underlain by Kaniapiskau Supergroup; G F T Z = Grenville Front Tectonic Zone. Inset boxes A, B, C and D correspond to Figures 4, 5, 10 and 7, respectively. (B) M a p of eastern Canada and contiguous U.S.A. with Grenville Province (stippled) and Circum-Ungava Fold Belt composed of Belcher Island Fold Belt (BIFB), Cape Smith Belt (CSB) and Labrador Trough or Labrador Fold Belt ~(LFB): A O, Atlantic Ocean; H B, Hudson Bay. Inset box is area shown in Fig. 1A.

44 SUPERIOR FORELAND ZONE In Labrador the Superior foreland comprises a little studied, predominantly high grade, migmatitic gneiss terrain, intruded by granitoid plutons, which is collectively termed the Ashuanipi Metamorphic Complex. Geological Survey of Canada reconnaissance mapping (Duffell and Roach, 1959; Eade, 1960; Stevenson, 1963, 1964; Fahrig, 1967) indicates that this complex, forming part of the Ungava subprovince of the Superior Province, extends as far west as Hudson Bay and northwards to the Lower Proterozoic Cape Smith Fold Belt (Fig. 1). The Ashuanipi Metamorphic Complex is characterized by variable smallscale intermixture of pyroxene-bearing acid and intermediate to basic gneisses which cannot be subdivided into mappable units. At outcrop scale these migmatites typically comprise two components: a dark, fine- to medium-grained two pyroxene--plagioclase ferromagnesian gneiss (FeO + MgO = 15--18%; CaO = 10--12%, Roach and Duffell, 1968) which generally possesses a weak to well
45 zone, but re-metamorphosed and tectonically reworked during the Hudsonian orogeny at ~ 1750 Ma; and in part a metaplutonic assemblage of tonalite to granite composition (R.J. Wardle, personal communication, 1981). The gneisses are mostly within amphibolite--granulite facies and have strong north to northwesterly structural trends, characteristic of the Hudsonian orogen in this region (Taylor, 1979). The western part of the Churchill foreland comprises part of the Labrador Fold Belt, or Labrador Trough, which is part of the Circum-Ungava Fold Belt (Fig. 1). It incorporates Aphebian (Lower Proterozoic) rocks which lie unconformably on the Archean basement, but which were generally deformed during the Hudsonian orogeny. Aphebian strata of the Labrador Trough are termed the Kaniapiskau Supergroup (Fig. 1), and comprise a lower, predominantly clastic and chemical sedimentary sequence, the Knob Lake Group, and an upper greywacke and basic volcanic assemblage, the Doublet Group. The latter occurs mainly in the central Labrador Trough, north of the present study area, and is not considered further here. The stratigraphy of the Kaniapiskau Supergroup is known in some detail (e.g., Dimroth, 1968, 1971, 1972, 1978; Dimroth et al., 1970; Wardle and Bailey, 1981), and the paleogeography of Aphebian sedimentation is well documented. The sequence began with fluviatile sedimentation in a faultbounded rift, followed by marine incursion and deposition of a deeper water greywacke--shale sequence with subordinate basic volcanics. After infilling of the basin, a shallow-water platformal sequence was deposited which was possibly confined to the west of the Trough. It includes a distinctive assemblage of dolomitic carbonate, orthoquartzite and iron formation. These were followed by renewed deep-water sedimentation with predominant greywacke--shale, pillow basalts and related diabase sills. The cyclical nature of sedimentation and volcanism was noted by Dimroth et al. (1970), and has featured in most subsequent interpretations. Wardle and Bailey (1981) speculate that the Trough comprised a continental shelf--slope--rise triplet on the western margin of a proto-oceanic rift system; however, the nature of the ocean basin is poorly understood. Structure and metamorphism of the Knob Lake Group in the Labrador Trough were described by Dimroth et al. (1970). Metamorphic grade increases from subgreenschist facies in the west to upper amphibolite facies at the eastern margin of the trough (Dimroth and Dressier, 1978; Wardle, 1979a). The Knob Lake Group was folded about north--northwesterly trending axial surfaces and thrust westwards across the Archean basement. Folds are gently to strongly overturned towards the west, again indicating westerly transport. Recent work by R.J. Wardle and the author has shown that the Hudsonian Front (i.e., the westerly limit of Hudsonian deformation) diverges from the Superior craton (Fig. 1), and is transected by the Grenville Front in the region of Evening Lake.

46 K N O B L A K E G R O U P IN T H E G R E N V I L L E P R O V I N C E Stratigraphy of the K n o b Lake Group in the Grenville Province is similar to that in the Labrador Trough and some units can be traced almost continuously across the Grenville front into the interior Grenville orogen (Fig. 2). W

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Grenville Province (Fig. 3) is similar to that presented by Dimroth (1978), Wardle and Bailey (1981) and others for the main Labrador Trough. Quartzofeldspathic schists and gneisses at the base of the sequence in the Grenville Province are considered part of the Attikamagen Formation, and appear to be equivalent to unmetamorphosed greywacke--shale present in the extreme north of the area; the fluviatile Seward Formation (Fig. 2) was

47 not recognized and all sedimentation with the Grenville Province seems to be marine. Equivalent quartzo--feldspathic schists and gneisses may extend at least 200 km east and south, and 300 km west of the map area, implying the existence of a major marine basin of sedimentation in the Grenville TABLE I Table of formations in the Southern Labrador Trough (Grenville Province) and contiguous central Labrador Trough (Churchill Province) Formation

Lithology

Comments

Shabogamo Intrusive Suite

Gabbro, norite, amphibolite diorite, monzonite

E m p l a c e d ~ 1 4 0 0 Ma

Blueberry Lake Group

Felsic v o l c a n i c s a n d v o l c a n i clastic; subvolcanic plutons

Relative age of Sims Formation and Blueberry Lake Group unknown

Sims Formation

Orthoquartzite, quartz pebble conglomerate

Helikian

Aphebian -- Knob Lake Group

Tamarack River Formation

Red sandstone, siltstone conglomerate, limestone

Occurs in foreland zone, n o t r e c o g n i z e d in G r e n v i l l e Province

Menihek Formation

Carbonaceous shale and greyw a c k e ; pelitic a n d semipelitic s c h i s t s a n d gneisses

Sokoman Formation

Silicate, c a r b o n a t e a n d oxide iron formation and ferruglnous cherts

Interbedded with McKay River Formation in Grenville F r o n t T e c t o n i c Z o n e

Nimish Subgroup

Basic v o l c a n i c s , v o l c a n i clastics and subvolcanic plutons

May be equivalent to McKay River Formation

McKay River Formation

Badc volcanics and volcaniclastic s e d i m e n t s ; g r e e n schists and amphibolite

Occurs in Grenville Front T e c t o n i c Z o n e in e a s t o f m a p a r e a , l a r g e l y a b s e n t in west. Interbedded with Denault and Sokoman Form a t i o n s in n o r t h a n d Attikamagen Formation in south

Wishart Formation

Orthoquartzite, chert, minor shale; quartzite, minor m u s c o v i t e schist

In part a littoral equivalent to the Denault Formation

Doily Formation

R e d , g r e e n a n d b l a c k shale a n d m i n o r sillstone

N o t s e e n in G r e n v i l l e Province

Fleming Formation

Chert breccia

Not seen in Grenville Province

Denaul$ Formation

Dolostone, pisoliths and stromatolites common; dolornitic marble

Interbedded with McKay River Formation in Grenville Front Tectonic Zone

Attikamagen Formation

G r e y w a c k e , shale; s e m i p e l i t i c a n d pelitic s c h i s t s a n d gneisses

Lithologically indistinguishable from Menihek Formation

Seward Formation

Arkose, fine conglomerate, fluviatile red beds

Not recognized in Grenville Province

2 - p y r o x e n e b e a r i n g gneisses and migmatites

Marginally reworked in Grenville Province

Archean

Ashuanipi Metamorphic Complex

48

RECONSTRUCTION OF THE SEDIMENTARY BASIN BEFORE DEFORMATION

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Silicate,carbonate and oxide iron fomation.

McKay River Formation

Basic volcanic and volcaniclastic rocks

Wishart Formation

Orthoquartzite, minor pelite

Denault Formation

Carbonate bankond platformal carbonates

Attikamagen Formation

Greywacke and shale

Ashuanipi Metamorphic Complex

High grade gneisses and intrusive rocks

Fig. 3. Palinspastic reconstruction of the Knob Lake Group basin in the Grenville Province. Province during the L ow e r Proterozoic. Within the map area, the K nob Lake G r o u p is interpreted as an onlapping sequence, largely by analogy with w ork in the central Labrador Trough, since m os t contacts observed in the field are tectonic (Fig. 4). Carbonates of the Denault F o r m a t i o n are totally recrystallized and contain no original textures. Wardle and Bailey (1981) describe dolomitic muds, breecias and a m o r e distal reef facies in the central L abrador Trough, an interpretation followed here. The Wishart Form at i on quartzites partly overlie and are in pa r t a lateral facies equivalent of the Denault Form at i on (c.f., Gastil and Knowles, 1957); apparently conformable contacts between the Wishart and the underlying Attikamagen F o r m a t i o n are seen in the Iron Ore C o mp an y mining area on the west side o f Wabush Lake (Fig. 5). T he Wishart F o r m a t i o n is interpreted as a littoral facies.

49 APHEBIAN

KNOB LAKE GROUP Menihek Format*on Sokomon Formation

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The Sokoman Formation (Figs. 2,3) overlies both the Wishart and Denault Formations; sedimentary textures and structures such as ooliths and stromatolites are known from the central Labrador Trough, and have also been noted within the Grenville Province (E. Blaxland, personal communication, 1978), indicating that it is a shallow-water unit; and its spatial relationship to the underlying Denault Formation suggests that it was deposited in a restricted basin bounded by the Archean basement to the north and west and a carbonate (stromatolite?) bank to the south and east. The Sokoman Formation is a typical Proterozoic iron formation, being banded on a variety of scales from centimetres to metres. In the vicinity of the mines, it has been subdivided into lower and upper silicate--carbonate units and an intervening middle oxide unit, but this subdivision cannot be extended much beyond the mine area. Basic volcanic rocks (Nimish Subgroup, Evans, 1978; McKay River formation, Noel, 1982) are locally associated with iron formation in the Grenville Front Tectonic Zone and further north. In the western part of the area, shales and greywackes of the Menihek Formation overlie the Sokoman Formation. Near their mutual contact the Menihek Formation is distinctively graphitic (starved basin facies?), but this is not a feature of the unit as a whole. Generally, the Menihek Formation is lithologically indistinguishable from the Attikamagen Formation. Thus, in the Grenville Province, the platformal marble--quartzite--iron formation sequence of the middle Knob Lake Group is distinctive, despite polyphase deformation and metamorphism, and is important in stratigraphic reconstruction. In the southeastern interior deep-water basin, however, distinction between the upper and lower quartzofeldspathic units of the Attikamagen and Menihek is difficult. In the west, the Aphebian succession is underlain by the Archean Ashuanipi Metamorphic Complex; the nature of the basement to the southeast is unknown. Jackson (1976) indicated the presence of reworked Archean gneisses in this region, but these have not been identified in more recent mapping (Rivers, 1981). HELIKIAN ROCKS

Helikian (middle Proterozoic) rocks, formed between the Hudsonian and Grenvillian orogenies, occur in three units. The Blueberry Lake group outcrops entirely within the Grenville Front Tectonic Zone; the Sims Formation and the Shabagamo Intrusive Suite occur both in the Churchill Foreland Zone, where they are undeformed, and in the Grenville Province where they have undergone Grenvillian orogenesis, thus forming important markers in establishing the nature of progressive Grenvillian deformation and metamorphism.

Fig. 5. Geological map of part o f Grenville F r o n t T e c t o n i c Z o n e near Wabush Lake (box B, Fig. 1A) showing interference o f n o r t h e a s t and n o r t h w e s t structural trends.

52 The Blueberry Lake group (Wardle, 1979b; Brooks et al., 1981), occurring in the eastern part of the study area (east of C, Fig. 1), comprises felsic volcanics, with abundant tufts and possible subvolcanic plutons, and a variety of volcaniclastic sediments. Volcanics from the group are dated at 1540 Ma (Brooks et al., 1981). The Sims Formation (Fahrig, 1967; Ware, 1979) comprises fluviatile arkosic sandstones and minor conglomerates and an overlying thick orthoquartzite. Clastic detritus from the Ashuanipi Metamorphic Complex and the Knob Lake Group can be recognized, and the unconformity between the latter and the Sims Formation is well exposed in several localities (Ware and Wardle, 1979). The Shabogamo Intrusive Suite (Rivers, 1980b) incorporates numerous intrusions of predominantly gabbroic composition, with subordinate pyroxenite and amphibolite. It is also considered to include dioritic and quartz dioritic plutons which are abundant in the interior Grenville Province. Rocks of the Shabogamo Intrusive Suite intrude both the Blueberry Lake group and the Sims Formation. Gabbro of the suite is dated at ~ 1400 Ma by Rb/Sr, Sm/Nd and 4°Ar/39Ar methods (Brooks et al., 1981; Zindler et al., 1981; Dallmeyer, 1982a). Mode of intrusion varies with position in the sedimentary basin (Fig. 3), in the platformal part of the Knob Lake succession, most intrusions are sills, while in the interior basin small stocks and plutons predominate, suggesting that the axis of igneous activity is to the southeast. GRENVILLE FRONT TECTONIC ZONE The term 'Grenville Front Tectonic Zone' was suggested by WynneEdwards (1972) for the northern margin of the Grenville Province, 15--80 km wide, which exhibits a marked negative Bouguer gravity anomaly, and a relatively simple magnetic pattern, both of which contrast with the patterns in the interior of the orogen. He further noted that rocks in the zone are " c o m m o n l y in the granulite facies, with strong northeast-trending foliation and numerous parallel zones of cataclasis and mylonitization. The grade of metamorphism is generally high, and kyanite in the dominant aluminosilicate, in contrast to sillimanite, which is c o m m o n throughout the rest of the Grenville Province", (Wynne-Edwards, 1972, p. 271). Many of these features are exhibited in the Grenville Front Tectonic Zone in western Labrador. However, there are significant differences in character between the front zone in the western part of the area, where it abuts against highgrade gneisses of the Superior foreland, and in the east where the zone is adjacent to low-grade supracrustal rocks of the Churchill foreland. GRENVILLE FRONT TECTONIC ZONE ADJACENT TO SUPERIOR FORELAND The Bruce Lake area {Fig. 4), ~ 40 km north of Labrador City, encompasses part of the western front zone where the Grenville Province abuts the

53 Superior foreland. The structural geology is characterized by northeast trending thrust faults, accompanied in many rocks by well
54

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~++ Basement ~ Axial Trace F2 fold "'/ Axial Trace F3fold ,~ Synform upright,overturned Antiform upright,overturned (4 Thrust Fault Fault

55 large-scale stratigraphic inversion in the Knob Lake Group is related to a major recumbent FG1 fold and associated limb thrust (Rivers and Massey, 1979). Both the penetrative SG1 schistosity and thrust faults are widely deformed by large- and small-scale FG2 folds developed about gently plunging axes which consistently trend northeast parallel to the Grenville Front Tectonic Zone. Coeval axial planar fabrics (SG2) are moderately developed, but generally non-penetrative, typically occurring as a crenulation or differentiated cleavage. Locally, in slates, complete transposition of SG1 into SG2 occurs, but this is thought to be atypical. FG2 folds are consistently overturned to the northwest, indicating that transport was towards the craton, as during FG1. Both the first and second generation Grenvillian structures are deformed by major FG3 folds developed about upright northwest-trending axial surfaces. These folds are developed most strongly towards the interior of the orogen and appear to die out towards the orogenic front, as seen in the Wabush--Labrador City area (Figs. 5, 6). There, northeasterly FG1/FG2 trends dominate in the north, and northwesterly FG3 trends in the south. Major FG2/FG3 fold interference patterns are restricted to an overlap zone, ~ 10--15 km wide, along the southern margin of the Grenville Front Tectonic Zone. Within this zone, the change from the well developed type-1 (Ramsay, 1967) dome and basin interference pattern in the central part of the Wabush--Labrador City area (Fig. 6) to a type-2 mushroom pattern in the north indicates progressive overturning of the FG2 folds northwards. Similar development of FG3 folds and FG2/FG3 fold interference, well defined by gabbro sills of the Shabogamo suite emplaced in Attikamagen schists, occurs in the De Mille Lake area (Fig. 7). In the Wabush--Labrador City area (Fig. 8), the complex outcrop pattern of the quartzite unit is in part related to similar fold interference, but is also related to the lensoid character of the unit, and to infolding of the sedimentary contact with the overlying iron formation. FG1 and FG2 structures are not evident in the interior of the orogen, which is dominated by FG3 folds with wavelengths up to several kilometres; these continue south and west of the present study area (e.g., Franconi et al., 1975), but die out as large-scale features within the Grenville Front Tectonic Zone. However, variably plunging metre-scale FG3 folds, superposed on the limbs of FG2 structures, do occur in the Front Zone, as for example in the pit faces of Iron Ore Company mines west of Wabush Lake, where superposition and age relations can be well documented (Rivers, 1978). The spatial distribution of Grenvillian F2 and F3 structures is summarized schematically in Fig. 9.

Fig. 6. Structural m a p of the area s h o w n in Fig. 5, showing development of d o m e and basin structure.

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,~.:.,~'..m~ ~

vv,-..vv,

~

-

.,vv. . . . . .

--

..... ,- i "

,vvvvv-~

i,~,~ ' ,

57

L

~) !: i:i:i:

iii

\ A\

'\ \

\ \

\

\ \

LEoE=0

~

Wishart Formation-quartzite

///

~. \ b

i "~

\

\

\

\

Axial trace F2 Grenvillian folds overturned syncline,antJcline

~,~

Axial trace Fs Grenv,llion folds upright syncline,anticiine

\ \

~F

~/~ ~

\ ~

\ =:~

\

\

Geologicol contact,onland underwater

~'

Iron ore mines

Fig. 8. Geological map showing the outcrop pattern of Wishart Formation quartzite in Grenville Front Tectonic Zone.

58

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ~ :::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: / i ========================================================= ........... i J ==================================================================================== c

::::::::i::i:i:i:i:i:i:i:i:i:i:i:!:i:i:i:i:i:i:i:i!iiiii!i ~ z~~ ::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::: :::: : : : : : : ~ : :~

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:::::::::::::::::::::::::::::::::::::::::::::::¢ :::::::::::::::::::::::::::::::::::::::::::

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:~:~...~

l

/

~ ~

-" /

" "~•

\ \

/X

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I

~

L/

,-'

r--

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TZ

~ -

, ,,

1

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x

/

? Z ' , - ? ,--,

-"

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/

GF

~ •

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I

~J

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X

\xJ

\2

Fig. 9. Sketch of spatial relationship of F2 and F3 structures in Grenville Front Zone. Dot pattern, Archean basement, G F T Z = Grenville Front Tectonic Zone.

GRENVILLE

FRONT

TECTONIC

ZONE

ADJACENT

TO CHURCHILL

FORELAND

In the Evening Lake area (Fig. 10) the Churchill Province foreland largely comprises low-grade Aphebian metasediments of the Knob Lake Group. The northern part is poorly exposed, and mapping is largely based on air-photo interpretation and the use of Shabogamo suite gabbro sills as marker horizons. In the northwest, shales of the Menihek Formation strike NW -- SE along the prevailing Hudsonian trend, and a few variably plunging major Hudsonian folds have been delineated; these mark the westerly limit of Hudsonian deformation in this region. Plunge variation in the Hudsonian folds is interpreted as the result of refolding by superposed Grenvillian structures, whereby Hudsonian structures acquire a steep plunge within the limb zones of Grenvillian overfolds. These relationships, particularly well developed in the McKay River area, will be described in detail elsewhere. No penetrative Hudsonian or Grenvillian fabric is evident in the northern part of the Evening Lake area. In the central and southern parts, an east- to

Sims Formation

McKoy River Formation Attikomogen For motion

~

~

..........

~,

Anticline (Hudsonian)

Syncline (Hudsonion)

Syncline overturned (Grenvillian)

C

~.,

"

~"

Pig. 10. Geological m a p o f Evening Lake area ( b o x C, Fig. 1 A ) o f Grenville F r o n t Tectonc Z o n e adjacent t o Churchill foreland. N o r t h w e s t structural trends in north o f m a p are ~udsonian, n o r t h e a s t structural trends in southern half o f m a p are Grenvillian.

?

Orientation of cleavage or schistosily (Grenvillian)

"~ Anticline overturned (Grenvillion)

Fault

~~

Geological contact Orientation of bedding

J

-~ I

Sokomon Formation

~

Symbols Geological trend

Menihek Formation

~

~PHEBIAN Knob Lake Group

~

IELIKIAN v - ~ Shobogomo intrusive Suite ~

C

""'\

b D

cn ~v

60 northeast-trending cleavage crosscuts and postdates Hudsonian folds and is thus considered to be of Grenvillian age. Further south still, predominant structural trends, defined by bedding, cleavage and fold axes, are northeasterly and this region is considered to be part of the Grenville Front Tectonic Zone. Quartzites of the Helikian Sims Formation in the central part of the area (Fig. 10) are deformed by major northeast-trending folds and display weak coeval axial-planar fabrics. The folds are of similar style and sense of vergence as FG2 folds further west and are correlated with them. Shabogamo gabbros have been intruded along both the unconformity between Aphebian and Helikian metasediments and within Aphebian strata previously folded during the Hudsonian event. It seems likely that the unconformity was reactivated as a ddcollement surface during Grenvillian orogeny; there is marked disharmony of folding across it, with an angle of "~ 50 ° between Hudsonian fold trends below the unconformity and Grenvillian trends above it. Two major thrust faults indicated in Fig. 10 are postulated on the basis of the abrupt discordance of structures in the north, and on the basis of a metamorphic discordance in the southeast, and it is thought that the entire central part of the area is allochthonous. In general, however, poor exposure does not permit clear documentation of thrust faulting in this area. FG3 crossfolding is not evident in the Evening Lake area. However, immediately to the south, northwesterly trends predominate (Rivers, 1980a) suggesting that FG3 structures are present in that region. S T R U C T U R A L SYNTHESIS

In both areas discussed above, the northern margin of the Grenville Province is marked by a northeast- to east-trending Grenvillian overprint on older structures of the foreland; however, the nature of the overprint varies. In the Archean gneisses, reworking is largely restricted to discrete mylonite/ phyllonite zones, between which the gneisses are little disturbed. More complete reworking, such that the Archean gneisses cannot be petrographically distinguished from schists of the Aphebian Knob Lake Group, may occur further to the southeast, but this is not documented. In the Churchill foreland, however, the most northerly manifestation of the Grenvillian orogeny is a slaty cleavage imposed on, and crosscutting, Hudsonian structures in Aphebian rocks (Rivers, 1982). The contrasting structural response is presumably related to the differing theology of rocks in the foreland zone, consequent upon differing metamorphic state, water contact and pre-Grenvillian layer anisotropy. The dryer crystalline gneisses yielded inhomogeneously along mylonite zones which allowed fluid access and localized development of greenschist-facies assemblages. The supracrustals deformed in a more homogeneous manner involving ductile shortening and resultant cleavage development.

61 In b o t h areas the Grenville Front Tectonic Zone is dominated by northeasterly trending folds and broadly coeval thrust faults, of t w o generations, parallel to the Grenville front. This distinguishes the zone elsewhere (WynneEdwards, 1972) and results presented here suggest that both episodes involved tectonic transport across the foreland, a feature c o m m o n to most orogenic foreland fold/thrust belts. DG1 and DG2 are considered to be phases of a single, essentially continuous, protracted deformational episode in which early thrusting and asymmetrical isoclinal folding was followed by northeast-trending overturned DG2 folds with a similar sense of vergence. Cumulative displacement on the thrusts is unknown; displacement on individual faults is unlikely to be very large, since similar stratigraphic elements occur on both sides. F o r example, the Sokoman iron formation (maximum thickness 300 m?) constrains the amount of displacement on thrusts in the Bruce Lake area (Fig. 4). The change in tectonic style between DG1 and DG2 is presumable a function of changing orientation of the incremental strain axes. In DG1 the maximum principal strain axis (E,) was probably oriented parallel to the dip of the thrust planes (i.e., gently dipping to the southeast). During DG2 the orientation of E1 steepened and lay in the axial surface of DG2 folds, and early lineations parallel to E1 were refolded. A possible reason for the rotation of E1 with time lies in the temporal change of the mode of deformation. Considering the low- to medium-grade cover sequence as one mechanically homogeneous rock package, and the basement gneisses as the other, it is likely that folding and thrusting of the cover sequence proceded without interruption until the thrust faults intersected the underlying basement. Incorporation into the thrust belt of these dry rocks caused the formation of a basement ramp {Fig. 11). Thrust movement in the lower part of the pile became much reduced or locked, and was a c c o m m o d a t e d by subhorizontal ductile shortening (i.e., folding with subvertical extension) in the upper part of the pile, with continued movement producing the observed a s y m m e t r y of F2 folds. The observation that DG2 folds are restricted to the Grenville Front Tectonic Zone suggests that this deformation episode was an 'edge effect' and was not propagated into the interior of the orogen. FG3 folds are more difficult to accommodate into this scheme, as their axial surfaces are approximately perpendicular to those of FG2. However, their subhorizontal axes imply a c o m p o n e n t of lateral horizontal shortening along axes parallel to the strike of the belt, suggesting the possibility of a biaxial compressive stress field. This might be explained by squeezing of the moving thrust slices into a narrow arcuate shaped foreland zone, as is illustrated schematically in Fig. 12 (T.J. Calon, personal communication, 1982). If the models for FG1, FG2 and FG3 folds are correct, the overprinting relationships are largely due to different rates of deformation at different locations in the orogen, and in this case are propagated by the 'edge effect' at the margin of the orogenic belt. Such models conflict with those of Fyson

62

NW

A

SE

~

-L

~-

~

GI

El

-L

~_

..

~-~

o- I

D

o- I

E ' ~

isotherms~

~

,increoslng T

~"~isobars [ mpresi(~ng

63

/

~<(~2C

<

~lC~+

Fig. 12. Schematic diagram for the development of Grenvillian F3 crossfolds. Biaxial compression occurs as the internal part of the omgen is 'squeezed' into an arcuate-shaped foreland zone.

(1971), who explained overprinting relationships by the migration of the locus of deformation from the infrastructure, characterized by recumbent structural styles, to the superstructure which is characterized by upright folds and subvertical fabrics. The involvement of a rheologically distinct basement m a y be a key difference between the area discussed here and those considered by Fyson (1971). METAMORPHISM

Metamorphic grade is low in the Grenville Front Tectonic Zone and increases southeastwards towards the interior of the orogen (Fig. 13). A concomitant change in trend of isograds from northeasterly to northerly is also evident. Metamorphic textures in Aphebian rocks indicate a single prograde metamorphic episode, locally followed by minor retrogression. Peak metamorphic Fig. 11. Schematic model of the development of F~ and F 2 folds during Grenvillian orogeny, o~ = maximum principal compressive stress direction, e~ = maximum principal strain direction. Vertical arrow ffi locus of maximum postorogenic uplift. Isotherms and isobars during Grenvillian metamorphism are schematic -- note peak metamorphic pressures may be achieved just south of Grenville Front Tectonic Zone.

64 minerals either define the main $1 fabric or have that fabric slightly warped around them, indicating attainment of peak metamorphic grade during DG1 deformation. Grade remained high during DG2 (micas commonly recrystallized into polygonal arcs around FG2 fold hinges), but the timing of DG3 with respect to the metamorphic peak is not precisely known (SG3 fabrics are not developed). In the Grenville Front Zone, the isograds may be telescoped by subparallel thrusts, and isograds may also be refolded by later fold generations, in particular by FG3 structures. However, neither of these features is clearly documented. Chlorite--muscovite and chlorite--muscovite--biotite zone assemblages occur in pelites of the Menihek Formation and in mylonite/phyllonite zones related to thrust faults in the Archean gneiss (Fig. 4). Within the thrusts, chlorite, muscovite and biotite define the mylonitic/phyllonitic fabric, indicating that thrusting was at least, in part, coeval with the peak of metamorphism. Away from the thrusts, Archean gneisses are variably retrogressed, with replacement of plagioclase by muscovite--epidote and pyroxerie by biotite--actinolite. Locally a clear distinction between high-grade fox-red Kenoran biotites and low-grade olive-green to brown Grenvillian biotite is evident. Some Grenvillian biotites display crystallographically oriented (sagenitic) exsolved opaques (ilmenite?) indicating replacement of Kenoran biotites. The assemblage chlorite--muscovite--biotite garnet was observed in only two localities defining a very narrow zone. It is succeeded by the appearance of staurolite and kyanite together with garnet in aluminous pelites. This assemblage was recorded in several localities in the mining area west of Wabush Lake (Fig. 5). Staurolite occurs across a zone ~ 5 km wide and is then replaced by the equivalent assemblage kyanite--biotite--garnet in a zone up to 25 km wide to the southeast. The succeeding kyanite--K-feldspar zone marks the passage into conditions b e y o n d the upper stability limit of muscovite in the presence of quartz. K-feldspar occurs as microcline and also quite commonly in perthite and antiperthite. Locally occurring muscovite appears to be either a relic, or a product of late-stage retrogression. The western limit of this zone is poorly defined due to extensive fluvioglacial cover. To the east, replacement of kyanite by sillimanite in the presence of K-feldspar defines the highest grade metamorphic zone. Granitic veins and sweats, composed of quartz, plagioclase and microcline, are common in the three highest grade metamorphic zones (Fig. 13). The veins are typically parallel to the regional fabric and are interpreted as the products of in situ anatexis. The proportion of melt varies from 0 to ~ 70% in the muscovite--biotite--garnet--kyanite zone, probably as a function of bulk composition of the paleosome. In the sillimanite--K-feldspar zone all pelitic rocks are pervasively migmatitic, and the restite fraction predominantly comprises sillimanite, biotite, plagioclase and opaque oxides.

65

Km 0

20 Km

SYMBOLS

ARCHEAN BASEMENT T H R U S T FAULT CH LORITE- M U S C O V I T E - B I O T I T E ISOGRAD C H L O R I T E - M U S C O V I T E - G A R N E T ISOGRAD STAUROLITE - KYANITE- GARNET ISOGRAD MUSOVlTE- KYANITE- BIOTITEG A R N E T ISOGRAD M I N I M U M M E L T ISOGRAD K Y A N I T E - K F E L D S P A R ISOGRAD S I L L I M A N I T E - K.FE LDSPAR ISOGRAD ( Symbols on hlgi'~ grade side of dsograd )

Fig. 13. Metamorphic map, showing mineral zones in pelites, western Labrador. Jackson (1976) considered these highly migmatitic rocks to be part of the Archean Ashuanipi Complex reworked during Grenvillian orogeny. However, no relict Kenoran structures or mineral parageneses are found and the migmatites grade imperceptibly into low-grade schists of the Attikamagen Formation, indicating their c o m m o n origin. Metamorphic P and T conditions can be approximated by considering the appropriate experimentally determined equilibria, which in Fig. 13 have been assembled into a petrogenetic grid for the Labrador pelites. Several calculated positions exist for some of the postulated reactions, and an attempt was made to choose those which best fit the observed distribution of isograds. The location of the invariant print and univariant curves in the A12SiO5 system is after Holdaway (1971). Staurolite-producing reactions have been considered by several authors, and the results indicate that a number of factors including the Mg:Fe ratio and oxygen fugacity may affect the temperature and pressure of reaction. In summarizing their results, Winkler (1979) suggests that these variations are not large, and he draws a

66 rather narrow 'staurolite in' reaction band based on their data. However, Carmichael (1978) points out that location of this curve, in conjunction with the aluminosilicate triple point of Holdaway (1971) is up to 130°C too high, and in conflict with a substantial b o d y of field evidence. Accordingly, the 'staurolite in' and 'staurolite out' reaction curves are taken from St. Onge (1981) and Carmichael (1978), respectively. The reactions which govern the generation of granitic liquids and the upper stability limit of muscovite in the presence of quartz, have been investigated by several authors. Lambert et al. (1969) and Storre and Karotke (1972) demonstrated the existence of an array of melting reactions involving muscovite radiating from an invariant point which they located at ~ 700°C and 5 kb; and Kerrick (1972) discussed the effect of variable P H 2 o o n such reactions. He placed the invariant point at approximately 650°C and 3.75--8 kb, depending upon XH20 (ranging from 1.0 to 0.5, respectively). Recent work has emphasized the effect of plagioclase composition on melting reactions (e.g., Thompson, 1974; Thompson and Algor, 1977; Tracey, 1978), though Carmichael (1978) suggests that the combined effects of the MgO: FeO ratio in micas and the CaO:Na20 ratio in feldspar may approximately cancel each other out, leaving the invariant point virtually coincident with that in the simplified 4 ~ o m p o n e n t (SiO2 --A12 03 --K2 O--H2 O) system. In Fig. 13, the invariant point of Kerrick (1972) for PH20 = 1 has been used. Justification for the latter assumption lies in the proximity of the 'staurolite out' isograd and the 'first appearance of a minimum melt' isograd in the field (Fig. 13). If PH20 is significantly <: 1, the 'staurolite out' isograd will migrate to lower temperatures (though, according to Carmichael (1978, p. 779), this effect is small), while the first appearance of a minimum melt will move to higher temperatures. Using the data of Carmichael (1978) and Kerrick (1972), the intersection of the 'staurolite out' isograd with the first appearance of a minimum melt isograd at XH20 = 0.5 will be at 11--12 kb, resulting in wide separation of the isograds at lower pressures, and in conflict with field evidence. INTERPRETATION OF METAMORPHISM Prograde mineral zoning (Fig. 13) implies an erosion surface P--T curve shown by the dotted pattern in Fig. 14. The baric location of the path is constrained at high temperature by the intersection of the kyanite = sillimanite curve with the biotite + muscovite + albite + quartz = aluminosilicate + K-feldspar + liquid curve. Baric constraint at lower grades is lacking. The occurrence of the mineral pair kyanite--K-feldspar implies metamorphic conditions of ~ 8 kb and 700°C, indicating a depth of burial in the order of 27 km during metamorphism (Fig. 14). This corresponds to bathozone 6 of Carmichael (1978), the deepest zone set up in his scheme. An additional signature of elevated lithostatic pressure during Grenvillian metamorphism is the proximity of the 'staurolite out' isogmd to the isograd

67 Ky'Kf Zone Ky-Stour

I

Ky-mu-go-bi

zoo.

I

9 28

8 24 7

2O ~6 v

"10

:=5

16

®

~.

3

13_ 4 i2

3 8 2

4 I

3so

450

s~o

sso

75o

T e m p e r o t u r e (°C)

Fig. 14. Petrogenetic grid for Labrador pelites. For location o f curves see text. Dotted path is suggested P--T erosion surface curve in map area.

marking the inception of partial melting and genesis of in situ migmatites in pelites (Fig. 13). Kyanite is known to be the characteristic aluminosilicate of the Grenville Front Tectonic Zone (Wynne-Edwards, 1972), but this is the first reported occurrence known to the author of the assemblage kyanite--K-feldspar from the zone. It has, subsequently, also been observed in eastern Labrador (C.F. Gower, personal communication, 1982). Thus, the implication of medium-pressure metamorphism in the front zone is upheld and enhanced by data presented here. A feature apparent from the projected erosion surface P--T path in Fig. 14 is that both temperature and pressure vary across the Grenville Front Tectonic Zone. Variation in the attitudes of isobars and isotherms, as deduced from isograds in regions of great topographic relief, has been discussed by Thompson (1976). He shows that, generally, in an orogenic belt with no later folding of the isograds, isobars and isotherms will not be

68 parallel to each other. Moreover, only in the restrictive case of post-metamorphic uplift without tilting, will the isobars be parallel to the present erosion surface. It is suggested in Fig. 11 that isobars are not parallel to the present erosion surface, but that they decrease north and south away from the south Grenville Front Tectonic Zone. The northward decrease is dictated by the P--T grid (Fig. 11); the southward decrease is predicted, and has yet to be documented. GEOCHRONOLOGY A number of K--Ar radiometric ages from quartzofeldspathic schists and gneisses of the Attikamagen and Menihek Formations have been reported in the last 20 years, e.g., Leech et al. (1963), Wanless et al. (1966, 1974), Fahrig (1967), Seguin (1973) and Jackson (1976). The recorded ages vary between ~ 3520 and 850 Ma. These results are discussed in some detail by Dallmeyer and Rivers ( 1 9 8 3 ) a n d Dallmeyer (1982b). Several are geologically unreasonable (metamorphic age is older than the age of the unit metamorphosed), and others yield inconsistent results (biotite mineral ages older than those of coexisting hornblende). DaUmeyer and Rivers (1983) argue that these spurious results reflect the widespread incorporation of excess argon in the minerals (especially biotite) during Grenvillian metamorphism. Brooks et al. (1981), using the Rb/Sr method, showed that the age of Grenvillian metamorphism of the quartzofeldspathic schists was approximately 1000 Ma ago (970 + 180 Ma, 940 + 110 Ma). The 4°Ar/39Ar method was employed by Dallmeyer and Rivers (1983), who conducted a traverse across part of the metamorphic gradient of the Grenville Front Tectonic Zone. Their results show that 4°Ar/39Ar biotite ages are unreliable, and most reflect a considerable content of excess or non-radiogenic argon, which is inseparable from the radiogenically produced gas. Hornblende ages, on the other hand, appear to be internally consistent (excess argon in hornblende is easily distinguished from radiogenic argon), and show a gradual variation across the front tectonic zone from 968 + 16 Ma in the lower grade portion of the kyanite--biotite-garnet--muscovite zone to 905 + 14 Ma in the upper grade part of the same zone. This was interpreted to reflect diachronous uplift through the blocking temperature for argon in the hornblende crystal lattice (Dallmeyer and Rivers, 1983). Uplift therefore occurred later in the interior of the Grenville orogen than in the Grenville Front Tectonic Zone. CONCLUSIONS The pattern of deformation in the Grenville Front Tectonic Zone is not unlike that in several younger orogenic belts. In areas where the crystalline basement is not present as a buttress, i.e., adjacent to the Churchill Foreland Zone, the Grenville front resembles a foreland fold--thrust belt. Where

69 the crystalline basement is present in the foreland zone, it is clear that it is only marginally involved in the orogeny, and that thrusting is essentially restricted to supracrustal rocks, a rather surprising conclusion in the light of the characteristically catazonal nature of the Grenvillian orogen. Whether this model is widely applicable within the Grenvillian orogen is not known; it would appear to account for the tectonic style of deformation in the Seal Lake Group, central Labrador (Thomas, 1979), and in the Otish Mountains of Quebec (Chown, 1979), both areas containing previously undeformed supracrustal sequences. Davidson (1982), however, reports large-scale basement thrusts in the interior Grenville Province of eastern Ontario, suggesting that other deformation mechanisms may be applicable elsewhere. Mineral assemblages in the study area indicate rather elevated lithostatic pressures during metamorphism, with depths of burial in the range 15--28 km across the metamorphic gradient. Combining the structural interpretation of the Grenville Front Tectonic Zone (Fig. 11) with this information yields a model in which tectonic thickening of the front zone by stacking of thrust slices seems likely. Such a model is also compatible with the negative Bouguer anomaly of the front tectonic zone (Wynne-Edwards, 1972), and suggests that the thickening may be an 'edge effect' as indicated in Fig. 11. Testing of this hypothesis will have to await more complete mapping in the interior of the orogen, together with systematic observation of mineral assemblages, so that bathograds may be drawn on a regional scale. At this stage, it is predicted that bathograd 6 assemblages (Carmichael, 1978) will be restricted to a zone along the southern margin of the Grenville Front Tectonic Zone, as a result of the 'edge effect'. The tectonic model (Fig. 11) implies that orogenesis was diachronous across the Grenville orogen, being initiated in the high-grade metamorphic core and spreading northwards towards the northwest margin of the orogen with time. 4°Ar/S9Ar radiometric data for hornblende (Dallmeyer and Rivers, 1983) indicate that uplift through the blocking temperature for argon occurred earlier in the Grenville Front Tectonic Zone than in the interior of the orogen. Integration of these two pieces of information suggests that erosion of the topographic high caused by the stacking of thrust slices in what is now the front tectonic zone occurred while rocks in the interior of the orogen were still undergoing metamorphism. Thus, metamorphism was both initiated earlier and terminated later in the core zone than at the orogenic margins. Metamorphism and subsequent uplift were both diachronous, but younging in opposite directions. Baer (1981), citing the range in metamorphic ages within the Grenville Province from 700 to 1100 Ma, suggests that these may reflect more than one orogeny within the Grenville Belt (his terminology). However, the progressive change in hornblende ages from the Grenville Front Tectonic Zone to the interior of the orogen in the area studied here implies that differential uplift may be the controlling factor, as was earlier suggested by Harper (1967). However, uplift appears to have been abnormally slow in compari-

7O son w i t h y o u n g e r orogenic belts w h i c h h a v e a half-life of 1 5 - - 2 0 M a (D.M. C a r m i c h a e l , oral c o m m u n i c a t i o n , 1982). T h e a p p l i c a b i l i t y o f P h a n e r o z o i c p l a t e t e c t o n i c m o d e l s is c u r r e n t l y being e v a l u a t e d in m a n y P r o t e r o z o i c terrains. T h e t e c t o n o m e t a m o r p h i c developm e n t d e s c r i b e d h e r e is c o m p a t i b l e with such m o d e l s , b u t until the e x i s t e n c e o f the p u t a t i v e sutures o f D e w e y and B u r k e ( 1 9 7 3 ) and R o n d o t ( 1 9 7 8 ) within the Grenville P r o v i n c e has been evaluated, all such p a r a d i g m s will r e m a i n speculative a n d c i r c u m s t a n t i a l . ACKNOWLEDGEMENTS T h e field w o r k o n w h i c h this p a p e r is based was d o n e while the a u t h o r was e m p l o y e d b y t h e Mineral D e v e l o p m e n t Division of the N e w f o u n d l a n d D e p a r t m e n t o f Mines and E n e r g y . C o n v e r s a t i o n s with m a n y p e o p l e there, b u t especially R i c h a r d Wardle and Charles G o w e r have h e l p e d t o shape m y t h o u g h t s o v e r the last f e w years. I t h a n k J o h n L e w r y , T o m C a l o n and R i c h a r d Wardle f o r critically reviewing an earlier version of the m a n u s c r i p t . W i n s t o n H o w e l l d r a f t e d the diagrams a n d M a u r e e n M o o r e t y p e d t h e m a n u script.

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