George River Suite-Bras D'or Gneiss

STRUCTURAL ANALYSIS OF A POTENTIAL PERI-GONDWANAN

DETACHMENT: GEORGE RIVER SUITE-BRAS D’OR GNEISS

CONTACT RELATIONS IN THE CREIGNISH HILLS, CAPE

BRETON, NOVA SCOTIA

A thesis presented to

the faculty of

the College of Arts and Sciences of Ohio University

In partial fulfillment of the

requirements for the degree

Master of Science

Zachary R. Wessel

June 2004 This thesis entitled

STRUCTURAL ANALYSIS OF A POTENTIAL PERI-GONDWANAN

DETACHMENT: GEORGE RIVER SUITE-BRAS D’OR GNEISS

CONTACT RELATIONS IN THE CREIGNISH HILLS, CAPE

BRETON, NOVA SCOTIA

BY

ZACHARY R. WESSEL

has been approved for

the Department of Geological Sciences

and the College of Arts and Sciences by

R. Damian Nance

Professor of Geological Sciences

Leslie A. Flemming

Dean, College of Arts and Sciences Wessel, Zachary R. M.S. June 2004. Geological Sciences

Structural Analysis of a Potential Peri-Gondwana Detachment: George River Suite-Bras d’Or Gneiss Contact Relations in the Creignish Hills, Cape Breton, Nova Scotia. (104p.)

Director of Thesis: R. Damian Nance

Late Neoproterozoic ductile shear zones that juxtapose low-grade over high-grade assemblages are characteristic features of parts of the peri-Gondwanan terranes of the

Canadian Appalachians. One such ductile shear zone, in the Creignish Hills of Cape

Breton Island, Nova Scotia, brings low-grade platformal metasedimentary rocks of the

George River Suite into contact with underlying high-grade rocks of the Bras d’Or

Gneiss. The low-grade assemblage includes quartzite, marble, schist and phyllite with interlayered felsic volcanogenic units and mafic flows, whereas the high-grade unit comprises low-pressure, high-temperature gneisses and migmatites, including pelitic paragneisses of likely volcanogenic origin. The contact between the two assemblages is defined by a broad mylonite zone that envelopes the high-grade rocks in the form of a

WNW-plunging antiform. The structural dome is in fault contact with Carboniferous strata to the east and south, and is unconformably overlain to the north. Kinematic indicators within the mylonites, including asymmetric porphyroclasts, fractured veins, S-

C fabrics and folded mylonitic foliation, suggest a broadly top-to-the-southeast (dextral) sense of shear, while the presence of gneissic granitoid sheets that are broadly concordant but locally cross-cut and are folded about the mylonitic foliation, suggest that mylonitization was accompanied by partial melting and syntectonic intrusion. Monazite from the gneisses and zircon from the granitoid sheets have yielded near-identical U-Pb crystallization ages of ca. 550 Ma. Juxtaposition of low-grade over high-grade assemblages in several peri-Gondwanan basement blocks in central Cape Breton Island suggests that the folded ductile shear zone exposed in the Creignish Hills is part of a regional low-angle detachment. Similar ductile shear zones with easterly components of shear and low-angle pre-Carboniferous orientations also place low-grade over high-grade rocks in southern New Brunswick and the Cobequid Highlands of mainland Nova Scotia. Dated at ca. 565-540 Ma and ca. 605

Ma, respectively, they suggest repeated late Neoproterozoic detachment within the peri-

Gondwanan arc. In the Cobequid Highlands, detachment was synchronous with arc magmatism and has been attributed to pull-apart basin development in response to oblique subduction. In southern New Brunswick and Cape Breton Island, detachment broadly coincides with the termination of arc magmatism and may reflect diachronous ridge-trench collision.

Approved: R. Damian Nance

Professor of Geological Sciences Acknowledgements

I would like to extend a thank you to Dr. J. Duncan Keppie and Dr. J. Brendan Murphy for their help in my field research and access to otherwise unattainable manuscripts. I would also like to thank Dr. Alan Collins for thoughts and ideas about my research and field methods. To my committee members, Dr. Douglas Green and Dr. David Schneider, I would like to thank you for you’re input and time. Last but not least, I would like to thank my advisor and mentor Dr. R. Damian Nance for his countless hours of help and guidance, for without him this project would not have been completed. vi

Table of Contents

Abstract Acknowledgements List of Figures vii List of Plates viii List of Tables ix I. Introduction 10 Peri-Gondwanan Terranes of Maritime Canada 10 II. Geology of Cape Breton Island 17 Terrane Characteristics 17 Terrane Correlation 22 III. Geology of the Creignish Hills 28 George River Suite 28 Quartzites 30 Greenstone 30 Marbles 34 Schists 34 Phyllite 38 Bras d’Or Gneiss 38 Quartzite and Marble 40 Biotite Gneiss 40 Cordierite-Sillimanite-Andalusite Gneiss 43 Plutons 44 River Denys Pluton 44 Skye Mountain Gabbro 44 Skye Mountain Granite 47 Contact zone between the High- and Low-Grade Units 52 Mylonite 54 Granite Sheets 54 Amphibolite 58 Structural Geometry and Kinematics 58 Shear Bands and S-C Fabrics 60 Fractured Grains and Veins 60 Asymmetric Porphyroclasts 60 Asymmetrically Folded Mylonitic Foliation 65 IV. Discussion 72 Neoproterozoic ductile shear zones of Maritime Canada 73 Cobequid Highlands 75 Southern New Brunswick 77 Implications 81 V. Conclusions 86 References 88 vii

List of Figures

Figure Page

1 Geologic map showing suspect terranes of New England and Maritime Canada 11 2 Diagram showing the evolution of Neoproterozoic arc-related terranes 13 3 Simplified geologic map of Cape Breton Island 20 4 Three possible tectonic models explaining the current distribution of Late Proterozoic volcanic rocks in the Avalon terrane of Maritime Canada 25-26 5 Model for terrane assembly in Cape Breton Island 27 6 Map showing important locations in the eastern Creignish Hills 29 7 Geologic Map of the eastern Creignish Hills 31 8 Diagram of the antiform exposed in the eastern Creignish Hills 53 9 Photomicrograph showing a S-C fabric 61 10 Photograph of a fractured vein 61 11 Photograph of fractured quartz vein 62 12 Photograph of fractured quartz grain 63 13 Asymmetric porphyroclast 64 14 Photomicrograph of an asymmetric porphyroclast 64 15 Asymmetrically folded mylonite foliation, Z-Folds 67 16 Asymmetrically folded mylonite foliation, S-Folds 67 17 Stereographic projection of northern limb of antiform 68 18 Stereographic projection of southern limb of antiform 69 19 Stereographic projection of unfolded limbs 70 20 Stereographic projection of poles to foliation for north and south limbs 71 21 Simplified geologic map of Maritime Canada 74 22 Stereographic projection of structural data from the Cobequid Highlands 78 23 Stereographic projection and structural map of southern New Brunswick 80

viii

List of Plates

Plates Page

1 Photograph of quartzite 32 2 Photomicrograph of quartzite 32 3 Photograph of greenstone 33 4 Photomicrograph of greenstone 33 5 Photograph of marble 35 6 Photomicrograph of marble 35 7 Photographs of schist 36 8 Photomicrographs of schist 37 9 Photograph of phyllite 39 10 Photograph of biotite gneiss 41 11 Photomicrographs of biotite gneiss 42 12 Photograph of River Denys pluton 45 13 Photomicrographs of River Denys pluton 46 14 Photograph of Skye Mountain Gabbro 48 15 Photomicrographs of Skye Mountain Gabbro 49 16 Photograph of Skye Mountain Granite 50 17 Photomicrographs of Skye Mountain Granite 51 18 Photographs of mylonite 55 19 Photomicrographs of mylonite 56 20 Highly deformed, severely folded, granitic sheet 57 21 Photomicrograph of granitic sheet contained within the shear zone 57 22 Photomicrographs of amphibolite contained within the shear zone 59 ix

List of Tables

Table Page

1 Structural data used to construct stereographic projections for the north and south limbs of the ductile shear zone 100 2 Composition of high-grade, intrusive and sheared units 101 3 Composition of low-grade units 102 4 Outcrop information for locations Z-A through Z-30 103 5 Outcrop information for locations Z-31 through Z-67 104 10

I. Introduction

The Creignish Hills of Cape Breton Island, Nova Scotia, make up one of several basement blocks in which rocks of the Neoproterozoic George River Suite and Bras d’Or

Gneiss project through Carboniferous cover (Keppie et al., 2000). The boundary between these two suites of rock constitutes a high-grade/low-grade contact that can be used to test the supposition that extensional detachment complexes may constitute an important component of the Neoproterozoic of Maritime Canada (Murphy et al., 2000a).

In order to provide such a test, this study presents a structural-kinematic analysis of the contact in the Creignish Hills, the results of which are then compared to those from other

Neoproterozoic high-grade/low-grade contacts in the Cobequid Highlands of mainland

Nova Scotia (Nance and Murphy, 1990) and in southern New Brunswick (Nance and

Dallmeyer, 1994).

Peri-Gondwanan Terranes of Maritime Canada

Along the southeastern margin of the Appalachian-Caledonide orogen, from the

Florida subsurface to the Cadomian massifs of Armorica and Bohemia, lies a collection of suspect terranes that have been traditionally associated with the eastern (Gondwanan) margin of the Early Paleozoic Iapetus Ocean (e.g., Nance and Thompson, 1996). These so-called peri-Gondwanan terranes are defined on the basis of their Early Paleozoic fauna and by their Neoproterozoic evolution along an active continental margin. The largest and most extensive of these terranes is Avalonia, which comprises about half of the orogen’s width and occurs along most of the orogen’s southeastern flank from the type area in the Avalon Peninsula of Newfoundland to New England (Fig. 1, Murphy et al.,

1999). 11

Figure 1. Terranes in the New England and Maritime Canada portion of the northern Appalachian orogen (modified from Barr et al., 1998). BRC, Blair River Complex; G, Gander Terrane. 12

Paleontologic, paleomagnetic, and isotopic data indicate that Avalonia originated as one of a number of terranes along the South American and northwest African periphery of Gondwana. Nance et al. (2002) argue that Avalonia accreted to Laurentia in the Late

Ordovician on the basis of faunal evidence (Williams et al., 1995), paleomagnetic data

(Trench and Torsvik, 1992), and isotopic linkages (Murphy et al., 1996).

Neoproterozoic-Cambrian paleomagnetic data, Nd isotopic data, and Cambrian faunal affinities document Gondwanan connections (van der Voo, 1993; Keppie et al., 1997,

Keppie and Ramos, 1999), while detrital zircon populations match sources in Amazonia

(Nance and Murphy, 1994; Keppie et al., 1998a; Keppie and Ramos, 1999). Hence,

Avalonia is thought to have consolidated in the Neoproterozoic and subsequently rifted from the Gondwanan margin in the Late Cambrian-Early Ordovician (Prigmore, et al.,

1997), drifting northward across the Iapetus Ocean throughout Ordovician time

(McNamara et al., 2001) to amalgamate with Laurentia in the Late Ordovician (Keppie et al., 1991).

Avalonia consequently has two important tectonostratigraphic components, one relating to its original tectonic and paleogeographic setting, and the other reflecting its subsequent history of dispersal and accretion to North America (Murphy et al., 1995).

However, this study focuses on only the former. The peri-Gondwanan evolution of

Avalonia has 3 main elements (Fig. 2): (1) the development of an arc from juvenile oceanic origin to an active continental margin setting, (2) its transition by way of continental transform activity to a shallow-marine platform, and (3) rifting from

Gondwana (Nance et al., 2002). Its history as part of an active plate margin may be subdivided into a fragmentary, early extensional arc stage (740-630 Ma), 13

Figure 2. Diagram showing the evolution of Neoproterozoic arc-related terranes (a) The main phase of arc activity in Avalonia and other peri-Gondwanan terranes is attributed to subduction beneath the margin of Gondwana. (b) Main phase activity terminated diachronously with the development of a transform fault, possibly due to ridge trench collision. (c) Separation of West Avalonia from the Gondwana margin by the Early Ordovician (after Murphy and Nance, 2002). 14

a main extensional arc phase (630-570 Ma), and a late phase that shows diachronous transition from transpressional arc to intracontinental transform environments (590-540

Ma; Keppie and Ramos, 1999).

The Late Neoproterozoic (ca. 630-540 Ma) tectonothermal evolution of Avalonia, to which the bulk of the preserved record belongs, records the transition from ensialic arc- related magmatism and volcanic arc basin development, to localized intra-continental rifting, to a shallow-marine platformal setting (Murphy and Nance, 2002). Subduction- related orogenic activity is characterized both locally and regionally by ca. 630-570 Ma low-grade calc-alkalic and tholeiitic volcanic rocks, cogenetic plutons, and broadly coeval synorogenic volcanogenic turbidites deposited in a variety of intra-arc and inter- arc basins (Murphy et al., 1995). Finally, the transition to a shallow marine platform setting is marked by a switch to magmatism of more rift-related bimodal character that accompanied the deposition of continental redbeds (e.g. Landing, 1996; Barr et al.,

1998), and was followed by the development of an uppermost Neoproterozoic-Lower

Paleozoic overstep sequence containing fossils of the Acado-Baltic (Avalonian) faunal province (e.g. Landing, 1996). Hence, the general history of Avalonia includes prolonged (740-570 Ma) Neoproterozoic subduction and its transition, without an intervening continental collision, to an Early Cambrian platformal environment (Nance et al., 2002). As the termination of subduction did not relate to continent-continent collision, the ocean bordering the arc must have remained as a border to the Acado-Baltic platform (Murphy and Nance, 2002). Because of this, the evolution of Avalonia and other peri-Gondwanan terranes and their locations on the Gondwanan margin has provided important constraints on continental reconstructions for the Neoproterozoic and 15

Early Paleozoic (Murphy and Nance, 2002). However, although the history of Avalonia is well documented, the extent of the terrane is controversial because rocks of Avalonian

(Neoproterozoic) age have been widely recognized within parts of the Appalachian central mobile belt, and because some areas traditionally assigned to Avalonia have been reinterpreted as separate terranes (Barr and Kerr, 1997).

In Cape Breton Island and southern New Brunswick, for example, Barr and White

(1996) and White and Barr (1996) respectively distinguish the Mira and Caledonia terranes, which they consider representative of Avalonia sensu stricto comparable with the type area in eastern Newfoundland, from the Bras d’Or and Brookville terranes, which they consider to be separate peri-Gondwanan terranes comparable with rocks of the Hermitage Flexure in southern Newfoundland (Barr et al., 1995). The Bras d’Or terrane comprises low-pressure gneiss, low- to high-grade platformal metasediments, minor volcanic rocks, abundant plutonic rocks, and Cambro-Ordovician and Devonian-

Carboniferous volcanic and sedimentary rocks (Raeside and Barr, 1990). The Brookville terrane comprises 4 major components: (1) Neoproterozoic orthogneisses and paragneisses, (2) platformal metasediments, (3) minor volcanic units, and (4)

Neoproterozoic to Cambrian plutons, (White and Barr, 1996). Barr and White (1996) suggest the Neoproterozoic plutonic rocks, dated at ca. 540 Ma (U-Pb zircon; Bevier and

Barr, 1990), to be of different origin than lithologically similar units in the Caledonia terrane where plutonic units have been dated at ca. 620 Ma (U-Pb zircon; White et al.,

1990b). Based on this evidence, the Bras d’Or and Brookville terranes are thought to have originated along different parts of the Gondwanan margin than the Mira and

Caledonia terranes (Avalonia sensu stricto), and to have separated from Gondwana and 16

moved across Iapetus as separate fragments, amalgamating to Laurentia at different times

(Barr et al., 2003). 17

II. Geology of Cape Breton Island

The geology of Cape Breton Island has traditionally been interpreted from one of two contrasting viewpoints. The first contends that all of Cape Breton Island forms part of

Avalonia, while the second advocates the island to be a complete cross-section of the

Appalachian orogen from Avalonia to the Laurentia margin. Proponents of the latter view (e.g. Barr et al., 1995; Barr and White, 1996) subdivide the island into four terranes, the Blair River (interpreted to represent the Laurentian margin), Aspy, Bras d’Or, and

Mira, the amalgamation of which did not occur until the Devonian. Only two of these terranes, the southernmost Mira and Bras d’Or terranes, are considered to be peri-

Gondwanan, and only the Mira terrane is considered to be part of Avalonia (Barr et al.,

1990). Proponents of the first view, on the other hand, attribute geological differences to varying levels of exposure within a composite Avalonia (Murphy et al., 1990b; Keppie et al., 1991, 1992; Lynch, 1996). Over the past decade, these two views have converged

(Keppie et al., 2000), and while Avalonia is considered to include all of Cape Breton

Island in most early correlations, its use is restricted to the southernmost Mira and Bras d’Or terranes on most recent compilations.

Terrane Characteristics

The Blair River terrane forms the northeastern corner of Cape Breton Island, where

Mesoproterozoic basement is overlain by Late Devonian and Carboniferous rocks. The terrane comprises mainly gneissic rocks of Grenvillian age metamorphosed, at least in part, to the granulite facies, but extensively overprinted by amphibolite and locally greenschist facies conditions (Barr et al., 1995; Miller et al., 1996). The terrane consists of amphibolite-granulite gneisses dated at 1217 Ma or older and 978 +6/-5 Ma with 18

minor marble, quartzite, and schist in contact with plutons which range in composition from gabbro and anorthosite dated at 996 +6/-5 Ma to syenite dated at 1080 +5/-3 Ma (U-

Pb zircon, Miller et al., 1996). These basement rocks are intruded by granite dated at 435

+7/-3 Ma (U-Pb zircon) and diorite with a cooling age of 417 ± 6 Ma (Ar-Ar hornblende,

Miller et al., 1996). Muscovite from the marbles records an Ar-Ar cooling age of 428 ± 7

Ma (Miller et al., 1996). The terrane is generally regarded as an autochthonous part of the Laurentian Grenville Belt (Barr et al., 1995) although it preserves no record of a

Cambrian-Ordovician passive margin succession like that of the Laurentian margin, nor does it record the Ordovician (Taconian) tectonometamorphic event that terminated passive margin development. Instead, the Grenvillian rocks have been affected by extensive Silurian orogenic activity and amphibolite facies overprinting at ca. 425 Ma

(U-Pb titanite, Barr et al., 1995, 1998) that may be related to accretion (Keppie et al.,

1996). The inlier is bounded to the east by the Wilkie Brook fault zone and to the southwest by the Red River fault zone, both of which are zones of extensive mylonite development and greenschist facies semiductile faulting (Barr and Raeside, 1986; Barr et al., 1995).

The Aspy terrane underlies most of western and central Cape Breton Island and is characterized by a variety of Ordovician to Silurian, low- to high-grade metasedimentary and metavolcanic rocks that have been intruded by extensive suites of mainly Early

Silurian to Late Devonian (433 +7/-4 Ma to 375 +5/-4 Ma U-Pb zircon) granitic rocks

(Dunning et al., 1990; Barr et al., 1995). The metamorphic rocks of the Aspy terrane are bounded to the east by the Eastern Highlands Shear Zone, and to the southwest by the 19

Cheticamp pluton (550 ± 8 Ma, U-Pb zircon; Dunning et al., 1990) and associated diorite and gneiss (Barr et al., 1995).

The Bras d’Or terrane comprises most of central Cape Breton where it includes distinctive Neoproterozoic gneisses and metasedimentary rocks, abundant dioritic to granitic rocks, and Cambro-Ordovician and Devonian-Carboniferous volcanic and sedimentary rocks (Dunning et al., 1990; Barr et al., 1995). Precambrian rocks of the

Bras d’Or terrane consist mainly of high-grade (amphibolite facies) paragneisses and low-grade platformal metasedimentary rocks (Raeside and Barr, 1990) that are intruded by ~580-550 Ma calc-alkaline plutons associated with volcanic rocks (Dostal et al.,

1996). Collectively the high- and low-grade rocks have been termed the Bras d’Or

Gneiss and George River Metamorphic Suite, respectively. Outcrops of these gneiss complex/platformal metasedimentary rock associations occur in basement blocks that project through the Carboniferous cover of the Bras d’Or terrane (Keppie et al., 2000).

These include the Creignish Hills, Boisdale Hills, North Mountain, portions of the Cape

Breton Highlands, and Kelly’s Mountain (Fig. 3). Pre-middle Devonian components of the terrane are fourfold: (1) low-pressure, high-temperature gneisses; (2) generally low- grade Proterozoic clastic-volcanic-carbonate units; (3) Neoproterozoic to Cambrian intrusive units, and (4) local Ordovician and Devonian intrusive units (Raeside and Barr,

1990). Precambrian rocks of the Bras d’Or terrane are locally overlain by Cambro-

Ordovician volcanic and sedimentary rocks, which contain Acado-Baltic fauna indicative of Avalonia (Landing, 1994). The southern boundary of the Bras d’Or terrane is mostly covered by Devonian and Carboniferous strata but is inferred to extend through the Bras 20

Figure 3. Simplified geologic map of Cape Breton showing high-grade (orange), and low-grade (green) metamorphic rocks, and areas of Precambrian basement (gray) in the central (Bras d’Or) and southern (Mira) terranes. 21

d’Or Lakes and, on land, to separate low-grade volcanic sequences of the Mira terrane from Bras d’Or-type gneisses and platformal successions (Barr and Raeside, 1989).

The Mira terrane forms the southeastern portion of Cape Breton Island and, like other parts of Avalonia, consists of several Neoproterozoic volcanic-sedimentary-plutonic assemblages overlain by fossiliferous Cambrian strata of the Acado-Baltic realm (Barr,

1993; Bevier et al., 1993). Southeastern most Cape Breton Island is characterized by the

Fourchu Group, which consists mainly of volcanic rocks with U-Pb crystallization ages of 680-575 Ma, now generally metamorphosed to lower greenschist facies (Barr and

Raeside, 1986). However, the terrane has been shown to be composite, in the sense that the Neoproterozoic assemblages occur in separate, northeast trending, fault bounded belts

(Barr et al., 1995). Neoproterozoic volcanic rocks of the Fourchu Group outcrop in five belts informally called the Coastal, Stirling, East Bay Hills, Coxheath Hills, and Sporting

Mountain belts (Fig. 3), separated by younger, mainly sedimentary units (Barr et al.,

1990). Recent geologic studies have shown that the ages of the volcanic rocks vary from ca. 680 Ma in the Stirling belt to ca. 620 Ma in the East Bay Hills, Coxheath Hills, and

Sporting Mountain belts, and ca. 575 Ma and possibly younger in the Coastal belt (Barr,

1993). The geochemistry of mafic rocks in the Fourchu Group is characteristic of subduction-related lavas, and assuming that the volcanic suites are related to the same subduction zone, progressive compositional changes in their geochemistry have been taken to suggest a northwest dipping subduction zone with a trench located southeast of the Coastal belt (Keppie and Dostal, 1991). 22

Terrane Correlation

The four terranes of Cape Breton Island have been correlated with those in

Newfoundland. Thus, Barr et al. (1998) interpret the Blair River terrane to be correlative with inliers of Grenvillian basement in the Humber Zone of western Newfoundland, the central Aspy and Bras d’Or terranes to be correlative with parts of the Central Mobile

Belt (Dunnage and Gander zones), and the Mira terrane to be correlative with the Avalon terrane east of the Dover and Hermitage Bay faults. Similarly, the southernmost Bras d’Or and Mira terranes have been correlated with terranes in southern New Brunswick.

Thus, Samson et al. (2000) have compared the Bras d’Or terrane with the Brookville terrane of southern New Brunswick based on a variety of geological similarities, and consider the isotopic composition of the Mira terrane to be indistinguishable from that of the Caledonia terrane of southern New Brunswick. The Caledonia terrane has also been correlated with parts of the Cobequid and Antigonish Highlands of northern mainland

Nova Scotia (Keppie et al., 1991; Keppie and Dostal, 1991; Murphy et al., 1990b, 1995), although Raeside and Barr (1990) claim that no obvious correlations can be made between the rocks of the Bras d’Or terrane and those of the Nova Scotia mainland.

Based on broad correlations of the Avalonian geology of Maritime Canada, in which rocks of magmatic arc affinity predominate in Cape Breton Island and southern New

Brunswick, whereas basinal facies dominate in mainland Nova Scotia, Keppie and Dostal

(1991) proposed three tectonic models to account for their present geographical distribution: (1) a Neoproterozoic geography with two northwest-dipping subduction zones, one beneath Cape Breton Island and the second beneath southern New Brunswick, between which lay an intra-arc basin (Fig. 4a), (2) a Neoproterozoic geography in which 23

southern New Brunswick and Cape Breton Island were laterally continuous with a single northwest-dipping subduction zone between them that, during the Paleozoic, was offset along the Minas and Chignecto faults (Fig. 4b), and (3) a Neoproterozoic geography in which a single northwest-dipping subduction zone was offset by two E-W transform faults located along the Minas Fault and Scatarie Ridge (Fig. 4c).

Barr et al. (1998), however, reject such broad correlations and propose a model for

Cape Breton Island in which terrane amalgamation does not occur until the Devonian

(Fig. 5). To emphasize differences in the peri-Gondwanan geology of Cape Breton they point out that the Neoproterozoic plutons of the Bras d’Or and Mira terranes have different ages, (~565-555 Ma and ~620 Ma respectively), and that there is no evidence that the Mira terrane was affected by the thermal events seen in the Bras d’Or terrane, suggesting that the Bras d’Or terrane has undergone a different thermal history to that of the Mira terrane. The Cambro-Ordovician rocks in the two terranes also show differences: (1) there is no evidence of magmatism, metamorphism or deformation in the

Mira terrane during the Cambrian or Ordovician, whereas the Bras d’Or terrane shows evidence for all three of these processes, and (2) a continuous section from Precambrian volcanic and sedimentary rocks through Cambrian to Early Ordovician sedimentary rocks is present in the Mira terrane, whereas in the Bras d’Or terrane the Paleozoic rock section begins in the Middle Cambrian (Barr et al., 1995). However, both sedimentary successions contain Acado-Baltic fauna by which Avalonia is defined (Keppie, 1989).

Nevertheless, in their view, portions of the Bras d’Or and Mira terranes now in juxtaposition were not joined in the Cambrian to Early Ordovician, but rather remained separate peri-Gondwanan terranes until they were linked by strike-slip movement in the 24

Devonian (Barr et al., 1995). The model for the amalgamation of Cape Breton put forward by Barr et al. (1998) proposes that the Bras d’Or terrane represents part of a peri-

Gondwanan continent with a southeast-dipping continental margin subduction zone to the northwest (present coordinates, Fig. 5a). The development of a back-arc basin, due to this subduction, may have resulted in the separation of the developing Aspy terrane from the Bras d’Or terrane by the Early Silurian (Fig. 5b). Closure of the Iapetus Ocean subsequently resulted in the closure of the back-arc region and the overriding of

Laurentia by both the Aspy and Bras d’Or terranes in the Devonian (Figs. 5c and d). 25 26

Figure 4. Three tectonic models proposed to explain the current distribution of Neoproterozoic volcanic rocks in Avalonia of Maritime Canada (after Keppie and Dostal, 1991). CH, Caledonian Highlands; C, Cobequid Highlands; AH, Antigonish Highlands; CBI, Cape Breton Island. 27

Figure 5. Model for terrane assembly in Cape Breton showing (a) the Neoproterozoic magmatic arc forming ~580 Ma plutons in the Bras d’Or terrane, (b) development of back-arc basin between the Bras d’Or and Aspy terranes, (c) collision of Aspy and Bras d’Or terranes creating Eastern Highlands shear zone (EHSZ), and (d) promontory-promontory collision resulting in the stacking of terranes (modified from Barr et al., 1998). BRI, future Blair River Inlier. 28

III. Geology of the Creignish Hills

The Creignish Hills are located in south-central Cape Breton, to the west of

Whycocomagh and on the north shore of Bras d’Or Lake (Figs. 3 & 6). In addition to the

Neoproterozoic Bras d’Or Gneiss and the George River Suite, the Creignish Hills expose a variety of Neoproterozoic granitoid plutons that range in composition from gabbro to granite and range in crystallization from 580 to 550 Ma (Keppie et al., 1998a). These plutons intrude the metasedimentary and metavolcanic rocks of the George River Suite, which were multiply deformed, metamorphosed and intruded by syn-tectonic granitic sheets dated at 551 ± 1 Ma (Dallmeyer and Keppie, 1993; Keppie et al., 2000). The plutons also intrude a mylonitic shear zone that separates the George River Suite from the

Bras d’Or Gneiss. These Neoproterozoic rocks are faulted against the Carboniferous

Windsor Group to the south and the Carboniferous Horton Group to the east. They are unconformably overlain by the Horton Group to the north.

George River Suite

The low-grade units of the Neoproterozoic George River Suite include quartzite, marble, schist and phyllite with occasional interlayered felsic volcanogenic units and mafic flows (Campbell, 1990; Dallmeyer and Keppie, 1993; Keppie and Dostal, 1998;

Keppie et al., 1998a). The quartzites and marbles are highly deformed but essentially monomineralic rocks, whereas the schist and phyllites are fine grained and contain a strong foliation defined by greenschist facies assemblages. All of these rocks have been deformed first by isoclinal folds associated with thrusts and an axial planar foliation developed under greenschist facies metamorphic conditions, and subsequently by several 29

Figure 6. Map of the eastern Creignish Hills showing geographic names and sample locations (dot = thin section, x = hand sample) referred to in the text. Inset shows location of field area in Cape Breton Island. 30

generations of open to tight folds and kink bands (Keppie and Dostal, 1998; Keppie et al.,

1998a). Exposure of the George River Suite rocks is limited to minor road cuts and stream valleys in the Skye Mountain area, so the distribution of this unit is poorly constrained. The proposed outcrop pattern (Fig. 7) is based on the extrapolation of field data as well as previous research. Names used as reference locations for the description of units can be seen in Figure 6.

Quartzites

The quartzite (Plate 1) is fine to medium-grained, massive, and dark gray in color with some variations from light gray to black. Tightly folded quartz veins are present in some outcrops. Although quartz makes up the majority of the unit, plagioclase, K-feldspar, and micas are also present in minor amounts (Armitage, 1989). Campbell (1990) has reported occurrences of matrix-supported fragments that include polycrystalline quartz, albite, orthoclase and varying amounts of biotite, muscovite, chlorite, epidote, tourmaline, sericite, apatite, zircon, and opaque minerals. Chlorite, sericite and epidote appear to be the common alteration products. Sample Z-19 (Plate 2) contains quartz, biotite, muscovite and plagioclase with minor accessory minerals of which the majority are too fine to be identified. In hand specimen minor biotite grains can be identified but the majority of the grains are quartz. Exposures of quartzite occur to the northeast of

River Denys, in Blue’s Brook, and in the tributaries to Blue’s Brook (Fig. 7).

Greenstone

The greenstone is dark greenish gray and very fine-grained (Plate 3). The rock locally displays relict volcanic textures such as flattened vesicles (Campbell, 1990). Numerous small veinlets of calcite and a few small veins of zoned and sutured quartz are also 31

Figure 7. Geologic map of the eastern Creignish Hills showing the location of the George River Suite and Bras d’Or Gneiss, the ductile shear zone between these two units, the Silurian Skye Mountain and Neoproterozoic River Denys plutons, and the geologic contacts with the Carboniferous Windsor and Horton Groups (based on field mapping and data from Campbell, 1990; Lynch and Brisson, 1996; and Keppie et al., 1998b). 32

Plate 1. Photograph of quartzite from Blue’s Brook (sample Z-19).

Plate 2. Photomicrograph (crossed polars) of sample Z-19 (field of view 3.5 mm). 33

Plate 3. Photograph of greenstone from River Denys (sample Z-63, field of view 4 cm).

Plate 4. Photomicrograph (plain light) of sample Z-63 (field of view 3.5 mm). 34

present, as can be seen in sample Z-63 (Plate 4). The greenstone is composed mainly of chlorite, and actinolite, with small amounts of epidote, albite, and opaque minerals.

However, the majority of the rock (about 80-90%) is chlorite. Exposures of this lithology are limited to the southwest section of the field area on the northeast side of River Denys

(Fig. 7).

Marbles

The marble is generally massive and gray in color, and commonly contains white streak-like calcite veins (Plate 5). The marble is composed mainly of calcite with minor impurities. Sample Z-28 (Plate 6) contains mostly calcite grains with minor amounts of quartz. The lithology has undergone intense folding and faulting (Campbell, 1990). It is locally sheath folded northwest of River Denys, just outside the field area (Keppie, personal communication), and displays folded veins in the western portion of the field area (Plate 5). The best outcrops are located in the northwestern portion of the map area, just west of Kewstoke, and others occur southwest of River Denys (Fig. 7). Although marble is not well exposed in the field area, it makes up a large portion of the George

River Suite in other areas of the Bras d’Or terrane and plays an important role in the interpretation of the suite as having been deposited in a platformal environment.

Schists

The schist is typically bluish-gray to dark gray and fine- to medium-grained in outcrop

(Plate 7). It is predominantly psammitic and contains mostly quartz, muscovite, biotite and plagioclase with minor amounts of chlorite, epidote and tremolite/actinolite.

Numerous hand samples were collected and many thin sections were cut in order to determine the mineralogy of the schist. Sample Z-15 (Plate 8) is typical of the majority 35

Plate 5. Photograph of marble (sample Z-28) from far west of map area (see Figure 7).

Plate 6. Photomicrograph (crossed polars) of sample Z-28 (field of view 3.5 mm). 36

Plate 7. Photographs (outcrop and close up) of schist from just west of map area (sample Z-64). 37

Plate 8. Photomicrograph (plain light and crossed polars) of schist from Blue’s Brook tributary (sample Z-15, field of view 3.5 mm). 38

of the thin sections, showing quartz, plagioclase, muscovite, epidote and minor amounts of garnet. However, the mineralogy and grain size vary somewhat depending on sample location. Grain size coarsens closer to the shear zone with an increase in the amount and size of mica grains. The lithology has a well-defined schistosity that appears to be equivalent to a slaty cleavage seen in the phyllites. Laminations within the lithology vary in thickness from 0.05 mm to 1.5 cm across. Exposures are found throughout the field area, occurring in the Soapstone Mine area, the upper portion of Blue’s Brook and Blue’s

Brook tributary, and in the stream south of Kewstoke (Fig. 7).

Phyllite

The phyllites vary in color from light gray to light brown (Plate 9). They are fine- grained, essentially quartz-muscovite-biotite rocks with small amounts of chlorite.

Sample Z-67, which exemplifies this lithology, is extremely fine-grained, finely laminated and very incompetent. Attempts to cut a thin section were unsuccessful, but a slight greenish color suggests the presence of chlorite. Laminations vary from 0.05 mm to 1.0 mm in thickness, and outcrops are limited to the headwaters of the stream to the west of Blue’s Brook and north of River Denys (Fig. 7).

Bras d’Or Gneiss

The Neoproterozoic Bras d’Or Gneiss is a low-pressure, high-temperature migmatic unit (Raeside and Barr, 1990) that is thought to be the high-grade equivalent of the

George River Suite. The Bras d’Or Gneiss generally consists of paragneisses (quartzite, marble, and gneiss with both nonvolcanogenic and volcanogenic components) and orthogneisses (Raeside and Barr, 1992; Keppie et al., 1998a). The Bras d’Or Gneiss 39

Plate 9. Photograph of phyllite from western side of Skye Mountain (sample Z-67). 40

in the Creignish Hills is dominated by migmatites and pelitic paragneisses with well- defined mineralogical banding. Mineral assemblages are those of the amphibolite facies and include quartz, plagioclase, muscovite, biotite, hornblende and minor amounts of apatite and chlorite. The reported occurrence of andalusite and sillimanite within the gneisses (Keppie et al., 1998a) suggest low-pressure/high-temperature metamorphism.

The gneiss can be divided into two groups based on compositional variations, but will be referred to as the Bras d’Or Gneiss since one of the groups is small and exposures are uncommon. All units of the Bras d’Or Gneiss have undergone intense deformation and exhibit isoclinal folds and sheath folds associated with thrusts and an axial-planar foliation refolded by several generations of open to tight folds and kink bands (Keppie et al., 1998a).

Quartzite and Marble

The quartzite typically occurs as massive beds interlayed with the marble. Campbell’s

(1990) petrographic analysis revealed the lithology to be composed of 90% quartz with a fabric defined by the alignment and concentration of tremolite, muscovite, and some sillimanite, andalusite and calcite. The marble is generally a massive, pure white rock interlayed with the quartzite. The lithology is generally coarse-grained and contains minor impurities whose presence gives rise to color variations. Both lithologies are exposed on the north side of Indian River across from the stream where sample Z-30 was collected (Figs. 6 and 7).

Biotite Gneiss

The biotite gneiss (Plate 10) ranges in color from grayish-brown to dark gray and is medium-grained with a well-developed granoblastic texture (Plate 11). The composition 41

Plate 10. Photograph of biotite gneiss from Skye Mountain Road (sample Z-1). 42

Plate 11. Photomicrographs (plain light and crossed polars) of biotite gneiss from Skye Mountain Road (sample Z-4, field of view 3.5 mm). 43

is typically quartz, plagioclase (andesine), potassium feldspar (microcline), biotite, muscovite and accessory tourmaline, opaques, zircon and apatite. Sample Z-4 (Plate 11), which is representative of the biotite gneiss, shows that chlorite and sericite are common alteration products. Garnet is also present in some of the more southerly outcrops of the gneiss. Alternating biotite-rich and biotite-poor layers define the gneissic banding and contain quartz as equigranular and xenoblastic crystals. The biotite-poor layers are also host to the majority of the xenoblastic, poorly twinned, and unzoned plagioclase and almost all of the potassium feldspar. Exposures of this lithology are interlayered with cordierite-sillimanite-andalusite gneiss in the eastern portion of the field area between the

Trans-Canada Highway and Indian River, east of Skye Mountain, and on McAskill Brook

(Fig. 7).

Cordierite-Sillimanite-Andalusite Gneiss

The cordierite-sillimanite-andalusite gneiss is very similar to the biotite gneiss in hand specimen and the defining variations in composition are best seen in thin section.

Samples of this lithology were collected but no thin sections were cut. Armitage (1989) and Campbell (1990) state that the unit is similar to the biotite gneiss in that it is composed of quartz, plagioclase, muscovite and biotite with accessory tourmaline, zircon and opaques, but also contains cordierite, sillimanite and andalusite. Chlorite and sericite are again the common alteration products and the lithology is weakly to moderately well banded. The cordierite grains, most of which have been altered to sericite, are xenoblastic and occur in the quartz-feldspar bands while andalusite and sillimanite grains also occur as xenoblasts in the more quartzo-feldspathic layers, but have undergone far less alteration to sericite (Campbell, 1990). Outcrops of this rock are limited to the 44

Indian River area in the northeast section of the field area (Fig. 7). Campbell’s (1990) assessment of the unit, especially the occurrence of cordierite, sillimanite and andalusite places tight P-T constraint (2-3 Kbar and 550-750o C) on the metamorphic history of the

Bras d’Or Gneiss in the eastern Creignish Hills on the basis of reactions between cordierite, sillimanite, andalusite, biotite, and garnet.

Plutons

River Denys Pluton

The River Denys pluton has yielded a crystallization age of 540 ± 3 Ma (U-Pb zircon;

Keppie et al., 2000). The pluton is a tonalite-diorite consisting of tonalite gradational to quartz diorite and diorite with minor granodiorite and quartz monzodiorite (White et al.,

1990a). The pluton is generally massive but exhibits ductile-brittle deformation along its northern margin suggesting that it is late syntectonic (Keppie et al., 2000). The tonalite- diorite is gray, fine- to medium-grained and exhibits subidiomorphic and inequigranular textures (Plate 12). It consists of amphibole (hornblende), plagioclase, and biotite altered to actinolite and chlorite as illustrated by sample Z-61 (Plate 13). Other, less common alteration products include sericite and epidote. The pluton also contains minor amounts of quartz and microcline and accessory minerals that include apatite, opaques and zircon

(Plate 13). It is located to the southwest of River Denys and outcrops along the west bank of the river (Figs. 6 and 7) as well as on the east side of Glencoe Road.

Skye Mountain Gabbro

The Skye Mountain pluton is a Silurian gabbro-diorite with a crystallization age of

438 ± 2 Ma (U-Pb zircon; Keppie et al., 1998b). The pluton is undeformed and intrudes the high-grade Neoproterozoic Bras d’Or Gneiss, cutting across its structural fabrics, at 45

Plate 12. Photograph of River Denys pluton on the north side of Glencoe Road (sample Z-61; dime for scale). 46

Plate 13. Photomicrographs (plain light and crossed polars) of sample Z-61, from the River Denys pluton (field of view 3.5 mm). 47

the northeastern end of the Creignish Hills (Keppie et al., 1998b). The gabbro-diorite is light gray to tan, medium to coarse-grained and anhedral inequigranular (Plate 14). As exemplified by sample Z-54, the gabbro-diorite contains plagioclase, amphibole

(hornblende) and biotite with accessory apatite, zircon and sphene (Plate 15). Fine veinlets of calcite and epidote are also present. Alteration products include sericite, epidote and chlorite. Associated with the pluton is a more mafic olivine pyroxenite that is medium- to coarse-grained and composed of augite, hornblende, relict olivine phenocrysts (altered to iddingsite), accessory opaques and alteration products of epidote and chlorite (Campbell, 1990). Exposures of the gabbro-diorite are limited to stream valleys on the north and northeast side of Skye Mountain (Fig. 7).

Skye Mountain Granite

The Skye Mountain granite does not have a well defined crystallization age, but muscovites sampled in the contact aureole with the Bras d’Or Gneiss record well-defined

40Ar/39Ar plateau ages of ~455 and ~441 Ma (Keppie et al., 2000). However, the presence of Skye Mountain-like granitic dykes in the gabbro-diorite suggest either: (1) that the granitic pluton is younger than the Skye Mountain gabbro-diorite, or (2) that the granite was penecontemperaneous with the gabbro-diorite (Keppie et al., 2000). The granite is light pink and coarse-grained with a xenomorphic inequigranular texture (Plate

16). It is made up of quartz, plagioclase, alkali feldspar, minor biotite and muscovite with accessory apatite, zircon, titanite and opaque minerals as illustrated by sample Z-A

(Plate 17). Although the pluton is relatively unaltered, chlorite, sericite and epidote are present in minor amounts. The pluton is massive and its contacts cut across the 48

Plate 14. Photograph of the Skye Mountain Gabbro (sample Z-54) from the north side of Skye Mountain (field of view 3 cm). 49

Plate 15. Photomicrographs (plain light and crossed polars) of sample Z-54 from the Skye Mountain Gabbro (field of view 3.5 mm). 50

Plate 16. Photograph of Skye Mountain Granite from top of Skye Mountain along the south side of Skye Mountain Road (sample Z-A, field of view 3 cm). 51

Plate 17. Photomicrographs (plain light and crossed polars) of sample Z-A from the Skye Mountain Granite (field of view 3.5 mm). 52

composite foliation in the country rock, indicating that it is post-tectonic (Keppie et al.,

2000). Outcrops are limited to the top of Skye Mountain, to the side of Skye Mountain

Road and to the stream valley flowing from the top of the mountain to the north (Fig. 7).

The contact of the pluton with the surrounding gneiss can be traced along the road on the basis of color changes, but elsewhere outcrop is limited, making the size and shape of the pluton difficult to determine. The Skye Mountain granite also cuts across the ductile shear zone between the Bras d’Or Gneiss and the George River Suite.

Contact Zone between High- and Low-grade Units

The ductile shear zone separating the low-grade George River Suite from the high- grade rocks of the Bras d’Or Gneiss is a broad, ~10 to 30 m, quartzo-feldspathic mylonitic zone dated on the basis of synkinematic granite sheets at 551 ± 1 Ma (U-Pb zircon, Keppie et al., 1998a). Keppie et al. (1998a) additionally obtained a near-identical

U-Pb ages of ca. 550 Ma from monazite in the Bras d’Or Gneisses. The mylonite zone is a moderately dipping structure, which envelopes the structurally lower Bras d’Or Gneiss in the form of a near-isoclinal overturned antiform that plunges gently west-northwest

(Fig. 8). Lying within the mylonitic foliation is a locally strong quartzo-feldspathic mineral lineation that generally plunges gently northwest on the northern limb of the antiform and moderately northwest on the southern overturned limb. The protomylonitic muscovite granite sheets intrusive into psammitic gneisses of the Bras d’Or Gneiss on

Skye Mountain are inferred to be syntectonic because, although generally parallel to and containing the foliation in the host rock, they locally cut across the foliation and are folded about it (Keppie et al., 1998a). Narrow, discontinuous bands of late-syntectonic 53

Figure 8. Three-dimensional sketch showing the plunge, direction of plunge and top-to- the-southeast (dextral) sense of shear of the folded ductile shear zone exposed in the eastern Creignish Hills. 54

amphibolite are interfingered with the granitic sheets and the mylonite within the shear zone. The presence of ca. 550 Ma syntectonic granite sheets not only provides timing constraints for mylonitization, but also suggests that mylonitization was broadly coeval with the low-pressure/high-temperature metamorphism of the Bras d’Or Gneiss as is suggested by the field relations. The broad synchroneity of intrusion and deformation is reflected in the Neoproterozoic igneous rocks, which show fabrics that range from foliated to massive, reflecting their syntectonic to post-tectonic intrusion relative to local deformation (Keppie et al., 2000).

Mylonite

The mylonite zone, which ranges in thickness from ~10 to 30 m, (10 to 20 m on the southern limb and 20 to 30 m on the northern limb), separates the George River Suite from the Bras d’Or Gneiss and is typically light brown to gray in outcrop, fine-grained, with larger quartz ribbons showing dextral shear sense (Plate 18). In thin section, representative sample Z-17 contains quartz, plagioclase, biotite and muscovite (Plate 19).

Although not seen in thin section, epidote and chlorite are evident in hand specimen and assumed to be retrogradational alteration products. Shear sense determined in both thin section and outcrop, based on the kinematic indicators discussed in the following section, are consistently top-to-the-southeast (dextral). Outcrops of the mylonite are limited to

McAskill Brook and the stream running north off Skye Mountain (Fig. 6).

Granite Sheets

In outcrop, the granite sheets (Plate 20) are similar in appearance to the Skye

Mountain granite except that they have related granite veins that are highly deformed. 55

Plate 18. Photographs of mylonite from north side of Skye Mountain (sample Z-17; quarter for scale). 56

Plate 19. Photomicrographs (plain light and crossed polars) of mylonite sample Z-17 (field of view 3.5 mm). 57

Plate 20. Intensely folded granitic sheet from north side of Skye Mountain (quarter for scale).

Plate 21. Photomicrograph (crossed polars) of granitic sheet contained within the shear zone (sample Z-8/1, field of view 3.5 mm). 58

In thin section, representative sample Z-8/1 is predominantly composed of quartz, plagioclase and biotite with accessory opaque minerals and zircon (Plate 21). Alteration products are mostly clinozoisite and epidote with minor amounts of chlorite. The granite sheets are best exposed where sample Z-8b was obtained, on the north side of Skye

Mountain (Fig. 6). Although too small to map individually, all outcrops of the granite sheets lie within the ductile shear zone shown on Figure 7.

Amphibolite

The amphibolite is generally dark gray, medium-grained, and massive to poorly foliated in outcrop. In thin section, representative sample Z-9/1 contains hornblende, plagioclase, quartz, opaque minerals and minor amounts of biotite, microcline and titanite

(Plate 22). Minor amounts of actinolite and chlorite are also present in sample Z-9/1.

The lack of deformation in hand sample and thin section suggests that the amphibolite is post-tectonic. Outcrops are limited to the area of sample Z-8b (Fig. 6). Like the granite sheets the amphibolites are too small to be included in the geologic map but lie within the mylonite zone (Fig. 7).

Structural Geometry and Kinematics

Kinematic indicators within the ductile shear zone separating the George River Suite from the Bras d’Or Gneiss include asymmetric porphyroclasts, fractured grains, fractured veins and S-C fabrics on both the northern and southern limb of the antiform that the shear zone defines. In addition, the mylonitic foliation is asymmetrically folded. The kinematics are well defined in both outcrop and thin section and suggest a broadly top-to- the-southeast (dextral) sense of shear. However, in the northern limb, it is the low-grade

George River Suite that moves southeast over the high-grade Bras d’Or Gneiss, whereas 59

Plate 22. Photomicrograph (plain light and crossed polars) of amphibolite contained within the shear zone (sample Z-9/1, field of view 3.5 mm). 60

in the overturned southern limb, it is the Bras d’Or Gneiss that moves southeast over the

George River Suite.

Shear Bands and S-C Fabrics

Where developed, shear bands are spaced evenly across outcrops and within thin sections. They are subparallel and deflect a pre-existing metamorphic foliation in the mylonite. The bands show a consistent top-to-the-southeast (dextral) sense of shear.

S-C fabrics (Fig. 9) with the same shear sense occur within the ductile shear zone on both the northern and southern limb of the antiform, and also appear in the contact aureole with the Bras d’Or Gneiss and the schist of the George River Suite.

Fractured Grains and Veins

Fractured grains and veins occur at both outcrop and thin section scale. Fractured quartz veins/ribbons like those of sample Z-8 show a dextral sense of shear similar to that recorded by other kinematic indicators (Figs. 10 and 11). Fractured grains in thin sections of the mylonite on both the north and south limb of the antiform show the same sense of shear on a much smaller scale (Fig. 12). Good examples of fractured grain/vein kinematics occur on the northern limb of the antiform in the stream running north off

Skye Mountain, but often can be seen in the McAskill Brook outcrops on the antiforms southern side (Fig. 6).

Asymmetric porphyroclasts

Mylonitic σ- and δ-type porphyroclasts range in size from 3 millimeters (thin section) to 3 centimeters (outcrop). Sense of shear is consistently top-to-the-southeast (dextral) as illustrated by sample Z-35 (Fig. 13) and thin section Z-53 (Fig. 14). Examples are limited to the mylonite and adjacent gneiss in the northern limb of the shear zone. 61

Figure 9. Photomicrograph (crossed polars) showing S-C fabric developed in the mylonite on the north side of Skye Mountain (field of view 3.5 mm).

Figure 10. Photograph of fractured vein in mylonite showing dextral shear sense. 62

Figure 11. Photograph of fractured quartz vein in mylonite showing dextral shear sense. 63

Figure 12. Photomicrograph (plain light and crossed polars) of a fractured quartz grain in mylonite from the north side of Skye Mountain (sample Z-8, field of view 3.5 mm). 64

Figure 13. Asymmetric porphyroclast (δ-structure) from north side of Skye Mountain (sample Z-35; Canadian dollar for scale).

Figure 14. Photomicrograph (plain light) of an asymmetric porphyroclast (σ-structure) from north side of Skye Mountain (sample Z-53, field of view 3.5 mm). 65

Asymmetrically Folded Mylonitic Foliation

Throughout the shear zone the mylonitic foliation is locally asymmetrically folded

(Figs. 15 and 16). These tight to isoclinal structures locally show sheath like geometries and are additionally defined by otherwise discordant granitoid sheets that are weakly to strongly foliated parallel to the mylonitic foliation, suggesting that mylonitization was accompanied by partial melting and synkinematic granitic veining. Plotted stereographically, the asymmetry of these folds again suggests top-to-the-southeast shear sense and the axes of these folds define partial great circle girdles that broadly parallel the general orientation of the mylonitic foliation on both the northern and southern limbs of the antiform (Figs. 17 and 18). According to their sense of asymmetry the folds are distributed into two groups separated by a planar separation angle, the bisector of which broadly parallels the mineral lineation. The sense of shear derived from the fold asymmetry matches the top-to-the-southeast movement recorded in other kinematic indicators and again documents motion of the George River Suite in this direction over the Bras d’Or Gneiss on the northern limb (Fig. 17), and southeastward movement of the gneiss over the George River Suite on the southern limb (Fig. 18). However, if the overturned antiform is unfolded in such a manner as to maintain the regional plunge of the structure, both the north and south limbs record movement of the George River Suite southeastward over the Bras d’Or Gneiss (Fig. 19) like that recorded in the northern limb.

All kinematic indicators are therefore consistent with broadly top-to-the-southeast movement of the low-grade George River Suite over the structurally underlying Bras d’Or Gneiss along a low-angle surface that, in the Creignish Hills, dips gently northwest when the overturned antiformal structure is unfolded. The fold axis is defined by poles to 66

the mylonitic foliation on both the northern and southern limbs of the antiform (Fig. 20) and plunges gently west-northwest at 48/292. The fold defined by the Neoproterozoic ductile shear zone is interpreted as a large-scale example of the smaller asymmetrical folds that fold the mylonitic foliation. The direction of plunge of the major fold axis suggests the overturned antiformal structure is an S-fold. The unfolding of this structure consequently requires counterclockwise rotation of the northern limb and clockwise rotation of the southern limb, which brings the low-grade George River Suite over the high-grade Bras d’Or Gneiss and is consistent with top-to-the-southeast sense of shear.

The data used to create the stereographic projections are shown in Table 1. 67

Figure 15. Asymmetrically Z-folded mylonite foliation from south side of Skye Mountain (sample Z-12; field of view 30 cm).

Figure 16. Asymmetrically S-folded mylonite foliation from south side of Skye Mountain (sample Z-13). 68

Figure 17. Stereographic projection of fold axes in asymmetrically folded mylonitic foliation in the northern limb of the antiformal shear zone showing top-to- the-southeast sense of shear. Curved arrows show sense of fold rotation, dots are mineral lineations, and straight arrows are kinematics based on sense of shear indicators. 69

Figure 18. Stereographic projection of fold axes in asymmetrically folded mylonitic foliation in the southern overturned limb of the antiformal shear zone showing top-to-the-southeast sense of shear. Curved arrows show sense of fold rotation, dots are mineral lineations, and straight arrows are kinematics based on sense of shear indicators. 70

Figure 19. Stereographic projection of fold axes and asymmetries from Figures 17 and 18 showing top-to-the-southeast sense of shear after antiformal structure in ductile shear zone is unfolded about its fold axis. (North limb black, south limb red). 71

Figure 20. Poles to foliation for north (black squares) and south (red dots) limbs of antiformal structure in ductile shear zone. 72

IV. Discussion

Examination of the ductile shear zone separating the low-grade George River Suite from the high-grade Bras d’Or Gneiss in the eastern Creignish Hills suggests generally southeastward movement of the former over the latter. The ductile shear zone is marked by a ~10 to 30 m thick mylonite zone that is exposed on both the north and south sides of

Skye Mountain. Syn- and post-tectonic plutons constrain the time of mylonitization to ca. 550 Ma, which is broadly synchronous with the amphibolite facies metamorphism of the Bras d’Or Gneiss (Keppie et al., 2000). Hence, movement on this low-angle regional tectonic boundary was accompanied by low-pressure/high-temperature metamorphism, partial melting, and the intrusion of synkinematic granitoid sheets, and placed low-grade metasedimentary rocks over high-grade gneisses in a manner consistent with extensional detachment.

The Creignish Hills form one of several peri-Gondwanan basement blocks that project through the Carboniferous cover of central Cape Breton. These inliers of high- temperature/low-pressure gneiss and associated low-grade platformal metasedimentary rocks occur throughout central Cape Breton (Raeside and Barr, 1990) and include the

Boisdale Hills, North Mountain, portions of the Cape Breton Highlands, and Kelly’s

Mountain (Fig. 3). The juxtaposition of the low-grade George River Suite against the high-grade Bras d’Or Gneiss in a number of these blocks suggests that the folded ductile shear zone exposed in the Creignish Hills is part of a regional structure that repeatedly intersects the present erosion surface (Keppie et al., 2000). This, in turn, suggests that the structure’s enveloping surface is a low-angle tectonic boundary along which the high- grade rocks occur in structural domes (Keppie et al., 1998a). Tizzard’s (2002) structural 73

analysis of the southeastern Cape Breton Highlands confirms the shallow dip of the contact between the Neoproterozoic basement and overlying Carboniferous units.

In the Boisdale Hills, some 40 km east of the map area, 585-540 Ma supra-subduction zone magmatic rocks like those exposed in the eastern Creignish Hills are overlain by a

Cambrian-Ordovician succession that includes Middle Cambrian to Lower Ordovician rift-related volcanic rocks dated at 505 ± 3 Ma (White et al., 1994). This suggests a change from arc-related to rift-related magmatism between 540 and 505 Ma (Keppie et al., 2000), the onset of rifting providing a potential cause for extensional detachment.

However, movement on the tectonic boundary between the George River Suite and Bras d’Or Gneiss is contemporaneous with arc-related intrusive rocks dated at 553 ± 2 Ma (U-

Pb, Keppie et al., 2000; White et al., 2003). This suggests that the broadly southeastward detachment on the ductile shear zone preceded the onset of Early Paleozoic rifting and was coincident instead, with the termination of Neoproterozoic arc activity.

Neoproterozoic ductile shear zones of Maritime Canada

The Creignish Hills have not only been compared to similar areas in central Cape

Breton, but also to areas in northern mainland Nova Scotia and southern New Brunswick.

The Bass River Complex in the eastern Cobequid Highlands of mainland Nova Scotia

(Fig. 21) occupies a narrow belt between the Late Paleozoic Cobequid and Rockland

Brook dextral faults (Miller et al., 1996) and is thought to form part of the

Neoproterozoic metamorphic infrastructure of Avalonia in the Northern Appalachians

(Nance and Murphy, 1990; Doig et al., 1991, 1993). Two lithological units within the complex, the Neoproterozoic Great Village River Gneiss and the platformal metasedimentary Gamble Brook Formation, have been broadly correlated with the Bras 74

Figure 21. Map of Maritime Canada showing areas of peri-Gondwanan basement, Neoproterozoic-Early Paleozoic rocks, gneiss complex-platformal metasedimentary associations, and the timing and sense of movement of Neoproterozoic ductile shear zones in southern New Brunswick, northern mainland Nova Scotia and the Creignish Hills rotated to restore associated Carboniferous rocks to the horizontal. 75

d’Or Gneiss and George River Suite, respectively (Keppie et al., 1998a). Late

Neoproterozoic sequences in the Cobequid Highlands are dominated by turbidites interbedded with bimodal volcanic rocks that show arc and back-arc geochemical affinities (Pe-Piper and Piper, 1989; Murphy et al., 1990a). They are generally considered to be the product of volcanic arc basin deposition and were deformed and metamorphosed under greenschist facies conditions before being intruded by Late

Neoproterozoic plutons (Murphy et al., 1997).

In southern New Brunswick, Neoproterozoic and Cambro-Ordovician rocks characteristic of Avalonia outcrop in the Caledonia Highlands on the north shore of the

Bay of Fundy (Bevier and Barr, 1990). Tectonically juxtaposed against these rocks, the

Brookville Gneiss and adjacent Green Head Group represent a gneissic complex- platformal metasedimentary rock association (Dallmeyer et al., 1990) similar to that seen in the Creignish Hills. Both units have undergone deformation and metamorphism, and both have been intruded by Neoproterozoic-Cambrian plutons (Dallmeyer and Nance,

1992; White et al., 2002), in a similar fashion to the same association in both the

Cobequid Highlands and Creignish Hills. In order to better understand the relationship between the Creignish Hills, the Cobequid Highlands, and southern New Brunswick, the structural geology of the latter two areas is discussed in more detail below.

Cobequid Highlands

The Bass River Complex of the Cobequid Highlands includes an amphibolite facies unit, the Great Village River Gneiss, and a greenschist facies unit, the Gamble Brook

Formation (Nance and Murphy, 1990). The Great Village River Gneiss is composed of orthogneiss, paragneiss, amphibolite and schist (Murphy, 2002). Nance and Murphy 76

(1990) describe the unit as including massive to quartz-plagioclase-layered, hornblende amphibolites, hornblende-bearing granitoid orthogneisses, and biotite/garnet-rich, psammitic paragneisses. The Gamble Brook Formation consists of two units: (1) a structurally lower orthoquartzite and arkosic quartzite with interlayered biotite- muscovite-garnet psammitic and pelitic schist, and (2) a structurally higher pelitic unit dominated by biotite-muscovite and biotite-garnet schist and phyllite with minor quartzite and psammitic schist (Murphy, 2002). The contact between the Great Village River

Gneiss and the Gamble Brook Formation is a ductile shear zone that trends E-W and dips south at an average of 52o. The shear zone is characterized by mylonitization, the development of S-C fabrics, local tectonic interleaving of the Great Village River Gneiss and Gamble Brook Formation, syntectonic intrusion of granite gneiss, and the development of several generations of cogenetic, small-scale isoclinal folds (Nance and

Murphy, 1990). The syntectonic granite gneiss has yielded a U-Pb crystallization age of

605 ± 5 Ma (Doig et al., 1991). The Neoproterozoic plutons that intrude the Bass River

Complex include post-tectonic monzogranite, which intrudes the Gamble Brook

Formation and has yielded a U-Pb crystallization age of 612 ± 4 Ma (Doig et al., 1991).

Three major Precambrian deformational episodes have affected the Bass River

Complex, one of which relates to the ductile shear zone separating the high- and low- grade units. Nance and Murphy (1990) have divided the latter deformation into two progressive phases that record the development and subsequent folding of the mylonitic fabric within the ductile shear zone. A prominent LS tectonite fabric developed during the first phase of deformation ranges from an amphibolite facies metamorphic foliation to an intense mylonitic schistosity, and contains an associated mineral lineation defined by a 77

strong dimensionally preferred orientation of hornblende and flattened quartzo- feldspathic augen. Kinematic indicators that include S-C fabrics and asymmetric augen

(σ-structures) yield consistent senses of ESE-directed shear (Fig. 22a). Tight to isoclinal folds in the mylonitic foliation that developed during the second phase of deformation mimic this shear sense and are considered to be the product of a single progressive deformational event (Fig. 22b). Contact relations between the Great Village River Gneiss and Gamble Brook Formation are consequently interpreted to be the product of heterogeneous ductile shear involving continuous, oblique movement towards the east- southeast at ca. 610-600 Ma along the E-W trending shear zone.

Southern New Brunswick

In southern New Brunswick, the Brookville Gneiss is composed mostly of low- pressure/high-temperature (cordierite ± andalusite), locally migmatic, quartz-feldspar- biotite ± hornblende paragneiss with minor calc-silicate horizons that are cut by tonalitic to granodioritic orthogneisses, amphibolite dikes, and younger, undeformed granitic pegmatites (Dallmeyer et al., 1990; White and Barr, 1996). The Green Head Group is a platformal metasedimentary sequence comprising marbles, dolomites, quartzites and minor pelitic rocks (Nance and Dallmeyer, 1994). Both units are intruded by ca. 550-537

Ma plutons that form part of an I-type calc-alkalic suite of medium- to coarse-grained, locally foliated megacrystic monzogranites, granodiorites, tonalities and diorites (White et al., 1990b; Dallmeyer and Nance, 1992; White et al., 2002). The Green Head Group and Brookville Gneiss have been affected by several episodes of ductile deformation and metamorphism and are tectonically separated by a major high-angle ductile shear zone 78

Figure 22. Equal area stereographic projections of structural elements associated with the development of the ductile shear zone separating the Great Village River Gneiss from the Gamble Brook Formation in the Cobequid Highlands. Sense of fold asymmetry (large curved arrows) and sense of shear indicators imply oblique slip with normal and sinistral components towards the ESE (from Nance and Murphy, 1990). 79

(MacKay Highway shear zone) that trends northeast, dips southeast and brings the amphibolite facies rocks of the Brookville Gneiss into contact with the greenschist facies lithologies of the Green Head Group.

The rocks on both sides of the shear zone record a history of progressive deformation with strike-slip and compressional components similar to that in both the Creignish Hills and Cobequid Highlands. At least three distinct phases of deformation are recorded in the lithologies of the Green Head Group, the first two of which are associated with the

Neoproterozoic-Cambrian development of the Mackay Highway shear zone (Nance and

Dallmeyer, 1994). Two deformational phases associated with the shear zone are also found in the Brookville Gneiss. The first of these phases is dominated by dextral shear while the second phase shows compressional deformation dominated by NW-directed shortening. Constraints on the timing of these deformational phases include: (1) an upper limit based on a 605 Ma U-Pb zircon crystallization age for the orthogneiss protolith

(Bevier et al., 1990), (2) a lower limit of 510-500 Ma based on Ar-Ar muscovite plateau ages from schists of the Green Head Group and a late tectonic pegmatite that intrudes the

Brookville Gneiss (Nance and Dallmeyer, 1994), and (3) a U-Pb titanite age of 564 ± 6

Ma and Ar-Ar hornblende ages at ca. 540 Ma from the Brookville Gneiss, that are thought to date its amphibolite facies metamorphism (Bevier et al., 1990; Dallmeyer et al., 1990). The first deformation produced a planar foliation and schistosity in the Green

Head Group and a gneissic foliation and mineral lineation in the Brookville Group, while the second folded these fabrics, producing close-to-tight, upright asymmetric structures in the Green Head Group, and tight to isoclinal asymmetric folds in the Brookville Gneiss

(Fig. 23). The sense of shear recorded in both the Brookville Gneiss and Green Head 80

Figure 23. Structural map and equal-area stereographic projections of structural data for the MacKay Highway shear zone that separates the Green Head Group form the Brookville Gneiss in southern New Brunswick (from Nance and Dallmeyer, 1994). 81

Group is predominantly dextral and, as in the Cobequid Highlands, the deformational episodes are considered phases of a progressive deformational event that is attributed to the transpressional development of the McKay Highway shear zone in the interval ca.

565-540 Ma.

Implications

The attitude of the ductile shear zones separating low-grade and high-grade assemblages in southern New Brunswick (Nance and Dallmeyer, 1994) and the Cobequid

Highlands (Nance and Murphy, 1990) have almost certainly been influenced by younger tectonics. Evidence of this lies in the attitude of Carboniferous strata that locally overlie high-grade/low-grade complexes unconformably in both the Cobequid Highlands and southern New Brunswick. In southern New Brunswick, the contact is a near-vertical northeast-trending structure along which the Green Head Group tectonically underlies the

Brookville Gneiss with respect to which the gneiss has moved northeast, more-or-less parallel to strike. However, the existence of near-vertical westward-younging

Carboniferous strata, of the Albert Formation along the Smith Creek fault, on strike with the MacKay Highway shear zone to the northeast (Pickerill et al., 1985), suggests that the shear zone was initially a much lower angle structure that carried the Green Head Group east-northeast over the Brookville Gneiss. Available geochronological evidence constrains this movement to ca. 565-540 Ma.

In the Cobequid Highlands, the ductile shear zone is a moderately south-southeast- dipping structure along which the Gamble Brook Formation has moved east-southeast with respect to the Great Village River Gneiss. To what extent the attitude of the contact has been modified by younger deformational events is uncertain. However, an inferred 82

unconformity with the Carboniferous Nuttby Formation is mapped as gently dipping

(Murphy et al., 2000b), suggesting that any subsequent modification to the attitude of the ductile shear zone has been minor. Rotation of the moderately southeastward dipping

Carboniferous Nuttby Formation to the horizontal, however, yields a lower angle shear zone along which downward movement of the Gamble Brook Formation relative to the

Great Village River Gneiss likely records primary eastward-directed extensional detachment of the former at ca. 605 Ma.

Similarly, relationships between the ductile shear zone separating the low-grade

George River Suite form the high-grade Bras d’Or Gneiss and Carboniferous strata in the

Creignish Hills suggests steepening of the ductile shear zone by younger tectonic activity.

Evidence of the original relationship between the Neoproterozoic basement and overlying

Carboniferous units exists in the southeastern Cape Breton Highlands, where Tizzard

(2002) has demonstrated that the contact between the two is shallow. In the Creignish

Hills, rotation of the gently westward dipping Carboniferous Creignish Formation

(Horton Group) to the horizontal results in a very gently northwesterly dipping ductile shear zone with a southeasterly component of low-grade over high-grade shear at ca. 550

Ma.

Like that exposed in the Creignish Hills, the ductile shear zones separating low-grade and high-grade assemblages in southern New Brunswick and the Cobequid Highlands were accompanied by partial melting of the high-grade units and synkinematic granitoid veining. In addition, both record easterly components of movement for the low-grade assemblages relative to the high-grade gneisses - east-southeast at ca. 605 Ma in the

Cobequid Highlands and toward the northeast at ca. 565-540 Ma in southern New 83

Brunswick - for data rotated to restore associated Carboniferous strata to the horizontal.

This raises the possibility of large scale and repeated detachment within the peri-

Gondwanan terranes of Maritime Canada.

Given that motion coincides with active arc magmatism within these terranes, possible mechanisms for such detachment include: (1) collapse of the peri-Gondwanan magmatic arc, or (2) metamorphic core complex development perhaps in response to ridge-trench collision, the mechanism by which arc magmatism is believed to have been terminated

(Murphy et al., 1999). Extensional structures in the upper crust of back-arc basins include features interpreted to illustrate detachments that signify simple-shear extension on a basin wide level (Barker et al., 2001). Other sites of preferential extension in arc settings occur in transtensional pull-apart segments of arc-related strike-slip zones, and in the upper part of subduction-accretion prisms (Karig, 1971a, 1971b; Dewey, 1980, 1988).

Subduction rollback and/or upper plate retreat may also lead to focused extension along the volcanic axis where the lithosphere is thinnest and weakest and where the arc crust is thickest (Lister et al., 1984; Dewey, 1988; Schellart et al., 2002). Assessment of these potential mechanisms is best addressed by examining modern analogues for the evolution of Avalonia and other peri-Gondwanan terranes during the late Neoproterozoic.

There are striking similarities between the Late Neoproterozoic to Early Paleozoic record of Avalonia and the Late Paleozoic to Cenozoic history of western North America

(Nance et al., 2002). Murphy and Nance (1989) proposed that the main phase of

Avalonian magmatism occurred as a result of oblique subduction, and led to the development of an extensional magmatic arc and a variety of volcanic arc basins.

Subsequently, the interaction of a continental margin transform system with the 84

subduction zone resulted in the termination of subduction. In order to account for the diachronous cessation of arc volcanism, Nance (1986), Murphy et al., (1999) and Keppie et al., (2000) proposed a mechanism of ridge-trench collision. Likewise, the evolution of western North America involved the development of a major continental margin subduction zone and its transition to a continental margin transform zone (Burchfiel et al., 1992). Studies in the internides of the North American Cordillera have produced widespread evidence of extension postulated to signify gravitational collapse of the orogen while the overall tectonic setting was convergent (Hodges and Walker, 1992).

Hence, the processes that produced ductile shear zones in western North America in the

Cenozoic may be similar to those that formed the ductile shear zones in Maritime Canada in the Neoproterozoic, both being responses to extensional detachment, via gravitational collapse, during a period of plate convergence.

The occurrence of ductile shear zones separating low-grade over high-grade assemblages in the Cobequid Highlands, southern New Brunswick, and the Creignish

Hills provides evidence of extensional detachment at ~ 605 Ma, 565-540 Ma, and 550

Ma, respectively (Fig. 23). Kinematic analysis of the Great Village River Gneiss and the continental affinity of the nearby mafic volcanic rocks (Pe-Piper and Murphy, 1989) suggest that the detachment in the Cobequid Highlands was associated with the development of a small rift or pull-apart basin (Nance and Murphy, 1990). In southern

New Brunswick, ductile shear is contemporaneous with the emplacement of abundant gabbroic to granitic plutons that are interpreted to belong to a compositionally expanded comagmatic I-type suite formed in a Late Neoproterozoic to Cambrian continental margin subduction zone (White et al., 2002). Ductile shear in the Creignish Hills is 85

similarly contemporaneous with arc-related intrusive rocks, suggesting that southeastward detachment preceded Early Paleozoic rifting, and instead, coincided with the termination of subduction. Hence, in both southern New Brunswick and the

Creignish Hills, detachment was likely linked to gravitational collapse of the peri-

Gondwanan magmatic arc. The development of these ductile shear zones may therefore be a response to the diachronous cessation of subduction beneath Avalonia that is interpreted to be the result of ridge-trench collision along the margin of Gondwana. 86

V. Conclusions

The ductile shear zone exposed in the Creignish Hills of central Cape Breton Island forms the contact between the low-grade platformal metasedimentary George River Suite and the high-grade Bras d’Or Gneiss, both of which are of Late Neoproterozoic age. The shear zone comprises a ~10 to 30 meter thick mylonite zone that envelopes the high- grade rocks in the form of a north-northwesterly plunging antiform. Syn- and post- tectonic plutons, and intrusive synkinematic granitoid sheets constrain the time of ductile shear to ca. 550 Ma. Kinematic indicators show a consistent dextral shear sense towards the southeast for both limbs of the antiform. Carboniferous strata are in fault contact to the south and east, and, to the north, unconformably overlie the high-grade/low-grade assemblage at a shallow angle.

Similar ductile shear zones are exposed in the Cobequid Highlands of mainland Nova

Scotia, between the Great Village River Gneiss and Gamble Brook Formation, and in the

Brookville terrane of southern New Brunswick, between the Brookville Gneiss and Green

Head Group. Plutons coeval with these shear zones comprise part of a peri-Gondwanan arc that formed on the margin of Gondwana during the Late Neoproterozoic and later became amalgamated to Laurentia.

Rotation of associated Carboniferous units to the horizontal in the Creignish Hills, southern New Brunswick, and the Cobequid Highlands produces low-angle shear zones, each of which show an easterly component of ductile shear. Movement of the George

River Suite over the Bras d’Or Gneiss in the Creignish Hills occurred in a southeasterly direction at ca. 550 Ma; movement of the Gamble Brook Formation over the Great

Village River Gneiss in the Cobequid Highlands occurred in an east-southeast direction at 87

ca. 605 Ma; and movement of the Green Head Group over the Brookville Gneiss in southern New Brunswick was toward the northeast at ca. 565-540 Ma. In each of these areas movement of low-grade platformal metasedimentary units over the high-grade gneisses is consistent with extensional detachment, suggesting repeated low-angle detachment occurred within the peri-Gondwanan arc in the Late Neoproterozoic. In the

Cobequid Highlands, extensional detachment has been attributed to the transtensional opening of a pull-apart basin in response to oblique subduction (Nance and Murphy,

1990). In the Creignish Hills and southern New Brunswick, however, detachment coincides with the termination of arc plutonism. In these areas detachment is likely to reflect gravitational collapse of the peri-Gondwanan magmatic arc in response to termination of subduction, core complex development in association with ridge-trench collision, or a combination of these processes. 88

REFERENCES

Armitage, A. E. 1989. Geology and petrology of the crystalline rocks of the

Whycocomagh area Cape Breton Island, Nova Scotia. B.S. Thesis, Acadia University,

Wolfville, NS, Canada, 120p.

Barker, D. H. N., Christeson, G. L., and Austin, J. A. 2001. Crustal structure of an active

back-arc basin at the rift-drift transition: Bransfield Strait, Antarctica. EOS, Trans.

AGU, v. 82 (47), Fall Meeting Supplement, Abstract T51E-08, p. 1244.

Barr, S. M. 1993. Geochemistry and tectonic setting of Late Precambrian volcanic and

Plutonic rocks in southeastern Cape Breton Island, Nova Scotia. Canadian Journal of

Earth Sciences, v. 30, p. 1147-1154.

Barr, S. M., and Raeside, R. P. 1986. Pre-Carboniferous tectonostratigraphic

subdivisions of Cape Breton Island, Nova Scotia. Maritime Sediments and Atlantic

Geology, v. 22, p. 252-263.

Barr, S. M., and Raeside, R. P. 1989. Tectono-stratigraphic terranes in Cape Breton

Island, Nova Scotia: Implications for the configuration of the northern Appalachian

orogen. Geology, v. 17, p. 822-825.

Barr, S. M., and White, C. E. 1996. Contrasts in Late Precambrian-Early Paleozoic

tectonothermal history between Avalon Composite Terrane sensu stricto and other

possible peri-Gondwanan terranes in southern New Brunswick and Cape Breton

Island, Canada. In Nance, R. D., and Thompson, M. D., eds., Avalonian and Related

Peri-Gondwanan Terranes of the Circum North Atlantic: Boulder, Colorado,

Geological Society of America Special Paper 304, p. 95-108.

Barr, S. M., and Kerr, A. 1997. Late Precambrian plutons in the Avalon terrane of New 89

Brunswick, Nova Scotia, and Newfoundland. In Sinha, A. K., Whalen, J. B., and

Hogan, J. P., eds., The Nature of Magmatism in the Appalachian Orogen: Boulder,

Colorado, Geological Society of America Memoir 191, p. 45-74.

Barr, S. M., Dunning, G. R., Raeside, R. P., and Jamieson, R. A. 1990. Contrasting U-Pb

ages from plutons in the Bras d’Or and Mira terranes of Cape Breton Island, Nova

Scotia. Canadian Journal of Earth Sciences, v. 27, p. 1200-1208.

Barr, S. M., Raeside, R. P., Miller, B. V., and White, C. E. 1995. Terrane evolution and

accretion in Cape Breton Island, Nova Scotia. In Hibbard, J. P., van Staal, C. R., and

Cawood, P. A., eds., Current Perspectives in the Appalachian-Caledonian Orogen:

Geological Association of Canada, Special Paper 41, p. 391-407.

Barr, S. M., Raeside, R. P., and White, C. E. 1998. Geological correlations between

Cape Breton Island and Newfoundland, northern Appalachian orogen. Canadian

Journal of Earth Sciences, v. 35, p. 1252-1270.

Barr, S. M., Davis, D. W., Kamo, S., and White, C. E. 2003. Significance of U-Pb

detrital zircon ages in quartzite from peri-Gondwanan terranes, New Brunswick and

Nova Scotia, Canada. Precambrian Research, v. 126, p. 123-145.

Bevier, M. L., and Barr, S. M. 1990. U-Pb age constraints on the stratigraphy and

tectonic history of the Avalon terrane, New Brunswick, Canada. Journal of Geology,

v. 98, p. 53-63.

Bevier, M. L., White, C. E., and Barr, S. M. 1990. Late Precambrian U-Pb ages for the

Brookville Gneiss, southern new Brunswick, Journal of Geology, v. 98, p. 955-965.

Bevier, M. L., Barr, S. M., White, C. E., and Macdonald, A. S. 1993. U-Pb

geochronological constraints on the volcanic evolution of the Mira (Avalon) terrane, 90

southeastern Cape Breton Island, Nova Scotia. Canadian Journal of Earth Sciences, v.

30, p. 1-10.

Burchfiel, B. C., Cowan, D. S., and Davis, G. A. 1992. Tectonic overview of the

Cordilleran Orogen in the western United States. In Lipman, P.W., and Zoback, M. L.,

eds., Cordilleran Orogen; conterminous U.S.: Geological Society of America, p. 407-

479.

Campbell, J. E. 1990. The geology of the northeastern Creignish Hills, Cape Breton

Island, Nova Scotia. M.Sc. Thesis, Acadia University, Wolfville, NS, Canada, 170p.

40 39 Dallmeyer, R. D., and Nance, R. D. 1992. Tectonic implications of Ar/ Ar mineral

ages from late Precambrian-Cambrian plutons, Avalon composite terrane, southern

New Brunswick, Canada. Canadian Journal of Earth Sciences, v. 29, p. 2445-2462.

40 39 Dallmeyer, R. D., and Keppie, J. D. 1993. Ar/ Ar Mineral Ages from the Southern

Cape Breton Highlands and Creignish Hills, Cape Breton Island, Canada: Evidence

for a polyphase tectonothermal evolution. The Journal of Geology, v. 101, p. 467-

482.

Dallmeyer, R. D., Doig, R., Nance, R. D., and Murphy, J. B. 1990. 40Ar/39Ar and U-Pb

mineral ages from the Brookville Gneiss: Implications for terrane analysis and

evolution of Avalonia “basement” in southern New Brunswick. Atlantic Geology, v.

26, p. 247-257.

Dewey, J. F. 1980. Epidicity, sequency and style at convergent plate boundaries. In

Strangway, D. W. ed., The Continental Crust and its Mineral Deposits: Toronto,

Ontario, Geological association of Canada, Special Paper 20, p. 553-573. 91

Dewey, J. F. 1988. Extensional collapse of origins. Tectonics, v. 7, p. 1123-1139.

Doig, R., Murphy, J. B., and Nance, R. D. 1991. U-Pb geochronology of late

Proterozoic rocks of the eastern Cobequid Highlands, Avalon composite terrane,

Nova Scotia. Canadian Journal of Earth Sciences, v. 28, p. 504-511.

Doig, R., Murphy, J. B., and Nance, R. D. 1993. Tectonic significance of the late

Proterozoic Economy River Gneiss, Cobequid Highlands, Avalon composite terrane,

Nova Scotia. Canadian Journal of Earth Sciences, v. 30, p. 474-479.

Dostal, J., Keppie, J. D., Cousens, B. L., and Murphy, J. B. 1996. 550-580 Ma

magmatism in Cape Breton Island (Nova Scotia, Canada): The product of NW-

dipping subduction during the final stage of amalgamation of Gondwana. Precambrian

Research, v. 76, p. 93-113.

Dunning, G. R., Barr, S. M., Raeside, R. P., and Jamieson, R. A. 1990. U-Pb zircon,

titanite, and monazite ages in the Bras d’Or and Aspy terranes of Cape Breton Island,

Nova Scotia: Implications for igneous and metamorphic history. Geological Society

of America Bulletin, v. 102, p. 322-330.

Hodges, K. V., and Walker, J. D. 1992. Extension in the Cretaceous Sevier orogen,

North American Cordillera. Geological Society of America Bulletin, v. 104, p. 560-

569.

Karig, D. E. 1971a. Origin and development of marginal basins in the western Pacific.

Journal of Geophysical Research, v. 76, p. 2542-2561.

Karig, D. E. 1971b. Structural history of the Marianna island arc system. Geological

Society of America Bulletin, v. 82, p. 323-344.

Keppie, J. D. 1989. Northern Appalachian terranes and their accretionary history. In 92

Dallmeyer, R. D. ed., Terranes in the Circum-Atlantic Paleozoic Orogens: Boulder,

Colorado, Geological Society of America, Special Paper 230, p. 159-192.

Keppie, J. D., and Dostal, J. 1991. Late Proterozoic tectonic model for the Avalon

terrane in Maritime Canada. Tectonics, v. 10, p. 842-850.

Keppie, J. D., and Dostal, J. 1998. Birth of the Avalon arc in Nova Scotia, Canada:

Geochemical evidence for ~700-630 Ma back-arc rift volcanism off Gondwana.

Geological Magazine, v. 135, p. 171-181.

Keppie, J. D., and Ramos, V. A. 1999. Odyssey of terranes in the Iapetus and Rheic

oceans during the Paleozoic. In Ramos, V. A., and Keppie, J. D., eds., Laurentia-

Gondwana Connections before Pangea: Boulder, Colorado, Geological Society of

America Special Paper 336, p. 267-276.

Keppie, J. D., Nance, R. D., Murphy, J. B., and Dostal, J. 1991. Northern Appalachians:

Avalon and Meguma terranes. In Dallmeyer, R. D., and Lecorche, J. P., eds., The

West African Orogens and Circum-Atlantic correlatives: Berlin, Germany, Springer-

Verlag, p. 315-333.

Keppie, J. D., Dallmeyer, R. D., and Krogh, T. E. 1992. U-Pb and 40Ar/39Ar mineral

ages from Cape North, northern Cape Breton Island: Implications for accretion of the

Avalon composite terrane. Canadian Journal of Earth Sciences, v. 29, p. 277-295.

Keppie, J. D., Dostal, J., Murphy, J. B., and Nance, R. D. 1996. Terrane transfer between

eastern Laurentia and western Gondwana in the Early Paleozoic: Constraints on

global reconstructions. In Nance, R. D., and Thompson, M. D., eds., Avalonian and

Related Peri-Gondwanan Terranes of the Circum North Atlantic: Boulder, Colorado,

Geological Society of America Special Paper 304, p. 369-380. 93

Keppie, J. D., Dostal, J., Murphy, J. B., and Cousens, B. L. 1997. Paleozoic within-plate

volcanic rocks in Nova Scotia (Canada) reinterpreted: Isotopic constraints on

magmatic source and paleocontinental reconstructions. Geological Magazine, v. 134,

p. 425-447.

Keppie, J. D., Davis, D. W., and Krogh, T. E. 1998a. U-Pb geochronological constraints

on Precambrian stratified units in the Avalon composite terrane of Nova Scotia,

Canada: Tectonic implications. Canadian Journal of Earth Sciences, v. 35, p. 222-

236.

Keppie, J. D., Dostal, J., Davis, D. W., and Horton, D. A. 1998b. Earliest Silurian supra-

subduction magmatism in central Cape Breton Island. Atlantic Geology, v. 34, p.

113-120.

Keppie, J. D., Dostal, J., Dallmeyer, R. D., and Doig, R. 2000. Superposed

Neoproterozoic and Silurian magmatic arcs in central Cape Breton Island, Canada:

Geochemical and geochronological constraints. Geological Magazine, v. 137, p. 137-

153.

Landing, E. 1994. Precambrian-Cambrian global stratotype ratified and a new

perspective of Cambrian time. Geology, v. 22, p. 179-182.

Landing, E. 1996. Avalon: Insular continent by the latest Precambrian. In Nance, R.

D., and Thompson, M. D., eds., Avalonian and Related Peri-Gondwanan Terranes of

the Circum North Atlantic: Boulder, Colorado, Geological Society of America

Special Paper 304, p. 29-63.

Lister, G. S., Banga, G. and Feenstra, A. 1984. Metamorphic core complexes of

Cordilleran type in the Cyclades, Aegean Sea, Greece. Geology, v. 12, p. 221-225. 94

Lynch, G. 1996. Tectonic burial, thrust emplacement, and extensional exhumation of the

Cabbot nappe in the Appalachian hinterland of Cape Breton Island, Canada.

Tectonics, v. 15, p. 94-105.

Lynch, G., and Brisson, H. 1996. Bedrock geology, Whycocomagh (11F/14), Geological

Survey of Canada, Open File 2917, Scale 1:50,000.

McNamara, A. K., Mac Niocaill, C., van der Pluijm, B. A., and van der Voo, R. 2001.

West African proximity of the Avalon Terrane in the latest Precambrian. Geological

Society of America Bulletin, v. 113, p. 1161-1170.

Miller, B. V., Dunning, G. R., Barr, S. M., Raeside, R. P., and Jamieson, R. A. 1996.

Magmatism and metamorphism in a Grenvillian fragment: U-Pb and 40Ar/39Ar ages

from the Blair River complex, northern Cape Breton Island, Nova Scotia, Canada.

Geological Society of America Bulletin, v. 108, p. 127-140.

Murphy, J. B. 2002. Geochemistry of the Neoproterozoic metasedimentary Gamble

Brook Formation, Avalon terrane, Nova Scotia: Evidence for a rifted-arc environment

along the West Gondwana margin of Rodinia. The Journal of Geology, v. 110, p.

407-419.

Murphy, J. B., and Nance, R. D. 1989. Model for the evolution of the Avalonian-

Cadomian belt. Geology, v. 17, p. 735-738.

Murphy, J. B., and Nance, R. D. 2002. Sm-Nd isotopic systematics as tectonic tracers:

An example from West Avalonia in the Canadian Appalachians. Earth-Science

Reviews, v. 59, p. 77-100.

Murphy, J. B., Keppie, J. D., Dostal, J., and Hynes, A. J. 1990a. The geochemistry and

petrology of the late Precambrian Georgeville Group: A volcanic arc rift succession

in the Avalon terrane of Nova Scotia. In D’Lemos, R. S., Strachan, R. A., and Topley, 95

C. G. eds., The Cadomian Orogeny, Geological Society Special Publication No. 51, p.

383-393.

Murphy, J. B., Keppie, J. D., Nance, R. D., and Dostal, J. 1990b. The Avalon composite

terrane of Nova Scotia. In Strachan, R .A., and Taylor, G. K., eds., Avalonian and

Cadomian geology of the North Atlantic. Glasgow, Scotland, Blackie, p. 195-213.

Murphy, J. B., Nance, R. D., Keppie, J. D., Dostal, J., and Cousens, B. L. 1995. Odyssey

of West Avalonia: Isotopic constraints for Late Proterozoic III-Early Silurian

paleogeography. In Hibbard, J. P., van Staal, C. R., and Cawood, P. A., eds., Current

Perspectives in the Appalachian-Caledonian Orogen: Geological Association of

Canada, Special Paper 41, p. 227-238.

Murphy, J. B., Keppie, J. D., Dostal, J., Waldron, J. W. F., and Cude, M. P. 1996.

Geochemical and isotopic characteristics of early Silurian clastic sequences in the

Antigonish Highlands, Nova Scotia, Canada: Constraints on the accretion of Avalonia

in the Appalachian-Caledonide Orogen. Canadian Journal of Earth Sciences, v. 33, p.

379-388.

Murphy, J.B., Keppie, J. D., Davis, D., and Krogh, T. E. 1997. Regional significance on

new U-Pb age data for Neoproterozoic igneous units in Avalonian rocks of northern

mainland Nova Scotia, Canada. Geological Magazine, v. 134, p. 113-120.

Murphy, J. B., Keppie, J. D., Dostal, J., and Nance, R. D. 1999. Neoproterozoic-Early

Paleozoic evolution of Avalonia. In Ramos, V. A., and Keppie, J. D., eds., Laurentia-

Gondwana Connections before Pangea: Boulder, Colorado, Geological Society of

America Special Paper 336, p. 253-266.

Murphy, J. B., Strachan, R. A., Nance, R. D., Parker, K. D., and Fowler, M. B. 2000a. 96 Proto-Avalonia: A 1.2-1.0 Ga tectonothermal event and constraints for the evolution

of Rodinia. Geology, v. 28, p. 1071-1074.

Murphy, J. B., Pe-Piper, G., Nance, R. D., Turner, D., and Piper, D. J. W. 2000b.

Geology, Eastern Cobequid Highlands, Nova Scotia, Geological Survey of Canada,

Open File 3703, scale 1:50,000.

Nance, R. D. 1986. Precambrian evolution of the Avalon terrane in the Northern

Appalachians: A review, Maritime Sediments and Atlantic Geology, v. 22, p. 214-

238.

Nance, R. D., and Murphy, J. B. 1990. Kinematic history of the Bass River Complex,

Nova Scotia: Cadomian tectonostratigraphic relations in the Avalon terrane of the

Canadian Appalachians. In D’Lemos, R. S., Strachan, R. A., and Topley, C. G., eds.,

The Cadomian Orogeny, Geological Society Special Publication No. 51, p. 395-406.

Nance, R. D., and Dallmeyer, R. D. 1994. Structural and 40Ar/39Ar mineral age

constraints for the tectonothermal evolution of the Green Head Group and Brookville

Gneiss, southern New Brunswick, Canada: Implications for the configuration of the

Avalon composite terrane. Geological Journal, v. 29, p. 293-322.

Nance, R. D., and Murphy, J. B. 1994. Contrasting basement isotopic signatures and

palinspastic restoration of peripheral orogens: Examples from the Neoproterozoic

Avalonia-Cadomia belt. Geology, v. 22. p. 617-620.

Nance, R. D., and Thompson, M. D., 1996. Avalonian and related peri-Gondwanan

terranes of the circum-North Atlantic: An introduction. In Nance, R. D., and

Thompson, M. D., eds., Avalonian and Related Peri-Gondwanan Terranes of the

Circum North Atlantic: Boulder, Colorado, Geological Society of America Special

Paper 304, p. 1-7. 97 Nance, R. D., Murphy, J. B., and Keppie, J. D. 2002. A Cordilleran model for the

evolution of Avalonia. Tectonophysics, v. 352, p. 11-31.

Pe-Piper, G., and Murphy, J. B. 1989. Petrology of the Late Proterozoic Folly River

Formation, Cobequid Highlands, Nova Scotia: A continental rift sequence within a

volcanic arc. Atlantic Geology, v. 25, p. 143-151.

Pe-Piper, G., and Piper, D. J. W. 1989. The late Hadrynian Jeffers Group, Cobequid

Hills, Avalon Zone of Nova Scotia: A back-arc complex. Geological Society of

America Bulletin, v. 101, p. 364-376.

Pickerill, R. K., Carter, D. C., and St. Peter, C. 1985. The Albert Formation – oil shales,

lakes, fans, and deltas. In Pickerill, R. K., Mawer, C. K., and Fuffe, L.R., eds.,

Fredericton 85, Field Excursions Volume II: Geological Association of Canada and

Mineralogical Association of Canada, Excursion 6.

Prigmore, J. K., Butler, A. J., and Woodcock, N. H. 1997. Rifting during separation of

Eastern Avalonia from Gondwana: Evidence from subsidence analysis. Geology, v.

25, p. 203-206.

Raeside, R. P., and Barr, S. M. 1990. Geology and tectonic development of the Bras

d’Or suspect terrane, Cape Breton Island, Nova Scotia. Canadian Journal of Earth

Sciences, v. 27, p. 1371-1381.

Samson, S. D., Barr, S. M., and White, C. E. 2000. Nd isotopic characteristics of terranes

within the Avalon zone, southern New Brunswick. Canadian Journal of Earth

Sciences, v. 37, p. 1039-1052.

Schellart, W. P., Lister, G. S., and Jessell, M. W. 2002. Analogue modeling of arc

98

backarc deformation in the New Hebrides arc and North Fiji Basin. Geology, v. 30, p.

311-314.

Tizzard, A. M. 2002. Structural geology and basement-cover relations in the

southeastern Cape Breton Highlands, Nova Scotia: Preliminary results. Atlantic

Geology, v. 38, p. 204.

Trench, A., and Torsvik, T. H. 1992. The closure of the Iapetus Ocean and Tornquist

Sea: New paleomagnetic constraints. Journal of the Geological Society of London, v.

149, p. 867-870.

White, C. E., and Barr, S. M. 1996. Geology of the Brookville terrane, southern New

Brunswick, Canada. In Nance, R. D., and Thompson, M. D., eds., Avalonian and

Related Peri-Gondwanan Terranes of the Circum North Atlantic: Boulder, Colorado,

Geological Society of America Special Paper 304, p. 133-147.

White, C. E., Barr, S. M., and Campbell, R. M. 1990a. Petrology of the Creignish Hills

pluton, Cape Breton Island, Nova Scotia. Atlantic Geology, v. 26, p. 109-123.

White, C. E., Barr, S. M., Bevier, M. L., and Deveau, K. A. 1990b. Field relations,

petrography, and age of plutonic units in the Saint John area of southern New

Brunswick. Atlantic Geology, v. 26, p. 259-270.

White, C. E., Barr, S. M., Bevier, M. L., and Kamo, S. 1994. A revised interpretation of

Cambrian and Ordovician rocks of the Bourinot belt of central Cape Breton Island,

Nova Scotia. Atlantic Geology, v. 30, p. 123-142. 99

White, C. E., Barr, S. M., Miller, B. V., and Hamilton, M. A. 2002. Granitoid plutons of

the Brookville terrane, southern New Brunswick: Petrology, age, and tectonic setting.

Atlantic Geology, v. 38, p. 53-74.

White, C. E., Barr, S. M., and Ketchum, J. W. F. 2003. New age controls on rock units in

pre-Carboniferous basement blocks in southwestern Cape Breton Island and adjacent

mainland Nova Scotia. In Mineral resources Branch, Report of Activities 2002; Nova

Scotia Department of Natural Resources, Report 2003-1, p. 163-178.

Williams, S. H., Harper, D. A. T., Neuman, R. B., Boyce, W. D., and Mac Niocaill, C.

1995. Lower Paleozoic fossils from Newfoundland and their importance in

understanding the history of the Iapetus Ocean. In Hibbard, J., van Staal, C., and

Cawood, P., eds., Current Perspectives in the Appalachian-Caledonian Orogen:

Geological Association of Canada, Special Paper 41, p. 115-126. van der Voo, R. 1993. Paleomagnetism of the Atlantic, Tethys and Iapetus Oceans.

Cambridge, United Kingdom, University Press, 411 p. 100

Northern Limb Southern Limb Unrotated Rotated Unrotated Rotated Foliation Mineral Minor Minor Foliation Mineral Minor Minor Lineations Fold Axes Fold Axes Lineations Fold Axes Fold Axes Z-Folds Z-Folds Z-Folds Z-Folds 010/50 W 07/299 31/356 26/002 057/76 N 03/288 26/010 46/344 021/59 NW 08/264 28/001 23/008 062/39 N 07/289 27/013 50/344 036/53 NW 21/259 22/012 23/024 073/50 N 14/276 22/014 55/344 038/71 NW 25/295 27/019 29/027 078/62 N 27/294 21/016 48/349 039/44 NW 26/274 22/024 27/030 083/74 N 39/292 25/020 53/349 040/47 NW 26/292 24/030 30/034 084/44 N 28/024 50/355 042/84 NW 31/253 30/033 32/037 085/59 N 046/38 N 31/254 23/035 28/040 085/70 N 065/71 N 32/267 36/038 31/044 090/52 N 071/58 N 34/254 S-Folds S-Folds 091/59 N S-Folds S-Folds 174/40 W 39/249 42/280 21/301 17/260 26/282 176/39 W 40/289 34/276 22/300 12/256 20/278 40/315 40/274 23/297 08/253 26/282 41/268 47/272 20/293 19/250 16/271 42/267 34/266 28/289 08/248 26/268 42/268 45/264 27/284 43/313 39/263 19/284 49/342 45/250 29/273 50/260 36/245 20/268 52/281 40/242 24/264 72/011 28/240 15/263 72/320

Table 1. Strike and dip of foliation, unrotated and rotated plunge and direction of plunge of minor fold axes, and mineral lineations for the northern and southern limbs of the antiform. 101

Sample # Major Minerals Accessories Alteration Rock Name Bras d'Or Gneiss Z-D1 qtz, plag, kspar, mus, bio opq, zr, tour ser biotite gneiss Z-4 qtz, plag, kspar, mus, bio opq, ap, zr, tour chl, ser biotite gneiss Z-27 qtz, plag, kspar, mus, bio opq, ap chl biotite gneiss Z-31 qtz, plag, mus, bio opq, zr, tour, cor, sil, and chl, ser, ep cor-sil-and gneiss Z-53 qtz, plag, kspar, mus, bio opq, gar chl, ser biotite gneiss Intrusive Units Z-A qtz, plag, kspar, bio opq, ap, tit, zr chl, ep, ser granite Z-54 plag, hbl, bio ap, zr, sph, cal chl, ep, ser gabbro-diorite Z-61 qtz, plag, kspar, bio, hbl opq, ap, zr chl, ep, ser tonalite-diorite Shear Zone Rocks Z-8 qtz, kspar, mus, bio opq, zr mylonitic gneiss Z-8/1 qtz, plag, bio opq, zr clin, ep, chl granite Z-9/1 hbl, plag, opq qtz, bio, kspar, tit, act chl amphibolite Z-17 qtz, plag, bio, mus too fine to identify ep, chl mylonite Z-35 qtz, kspar, mus, bio ap, opq chl mylonitic gneiss Z-40 qtz, kspar, mus, bio ap, opq chl mylonitic gneiss Z-42 qtz, kspar, mus, bio ap, opq chl mylonitic gneiss Z-44 qtz, kspar, mus, bio ap, opq chl mylonitic gneiss Z-45 qtz, kspar, mus, bio ap, opq chl mylonitic gneiss Z-47 qtz, kspar, mus, bio ap, opq chl, ep, ser mylonitic gneiss

Table 2. Mineral assemblages of high-grade, intrusive and shear zone rocks. 102

Sample # Major Minerals Accessories Alteration Rock Name George River Suite Z-6a qtz, plag, kspar, mus, bio opq chl schist Z-15 qtz, plag, mus opq, gar ep schist Z-19 qtz plag, bio, opq, zr, tour chl, ep, ser quartzite Z-22 qtz, plag, kspar, mus, bio opq chl schist Z-28 cal qtz, opq marble Z-32 qtz, plag, kspar, mus opq, zr chl, ep schist Z-33 qtz, plag, kspar, mus opq chl, ep schist Z-34 qtz, plag, kspar, mus opq, zr chl, ep schist Z-48 qtz, plag, kspar, mus, bio opq, zr ser schist Z-57 qtz, plag, kspar, mus, bio opq, zr, tour chl, ep schist Z-63 chl, act opq, ep, plag, cal, qtz greenstone Z-64 qtz, plag, kspar, bio opq, tour chl, ep schist Z-65 qtz, plag, kspar, bio opq, act chl, ep schist

Table 3. Mineral assemblages of low-grade rocks.

Definition of abbreviations act - actinolite kspar - k-feldspar and - andalusite mus - muscovite ap - apatite opq - opaque bio - biotite plag - plagioclase cal - calcite qtz - quartz chl - chlorite ser - sericite clin - clinozoisite sil - sillimanite cor - cordierite sph - sphene ep - epidote tit - titanite gar - garnet tour - tourmaline hbl - hornblende zr - zircon

Note: Definitions for abbreviations used in tables 1-3. 103

Outcrop Foliation Mineral Outcrop Oriented Thin Number Direction Lineation Location Sample Section Z-A N 57.59 W 11.83 x x Z-D1 N 57.55 W 13.91 x x Z-1 073/35 N 14/276 N 57.34 W 11.03 x Z-2 152/43 E N 57.50 W 10.94 x Z-2a 089/44 N 27/294 N 57.58 W 10.85 Z-3 121/34 NE N 57.73 W 11.74 x Z-4 052/42 N 39/292 N 57.65 W 11.74 x x Z-5 065/65 N N 57.41 W 13.76 Z-6 074/79 N N 57.42 W 13.65 x x Z-7 N 58.83 W 09.76 x Z-8 042/84 N N 57.74 W 12.50 x x Z-8a 084/39 N 08/264 N 57.75 W 12.36 Z-8b 084/34 N 21/259 N 57.75 W 12.36 Z-8c 041/80 N N 57.75 W 12.36 Z-8d 173/87 E N 57.75 W 12.36 Z-8/1 N 57.75 W 12.40 x Z-9 N 57.76 W 12.35 x Z-9a 089/25 N 07/299 N 57.77 W 12.31 Z-9b 025/67 NW N 57.77 W 12.31 Z-9/1 N 57.77 W 12.31 x x Z-10 036/69 NW 50/260 N 57.77 W 12.34 Z-11 041/66 N N 58.08 W 14.02 Z-12 106/21 N 07/289 N 56.94 W 09.73 Z-13 094/10 N 03/288 N 56.97 W 09.84 x Z-14 062/39 N N 56.96 W 09.86 Z-15 078/62 N N 56.10 W 11.65 x x Z-16 108/48 N N 56.25 W 11.83 Z-17 084/44 N N 56.98 W 09.82 x x Z-18 091/59 N N 55.97 W 11.88 Z-19 085/59 N N 56.02 W 11.90 x x Z-20 090/52 N N 56.12 W 12.12 x Z-21 057/76 N N 59.00 W 11.38 Z-22 086/46 N N 58.99 W 11.36 x x Z-23 128/54 NE N 58.77 W 11.42 Z-24 077/65 N N 58.82 W 11.44 x Z-25 114/80 N 52/281 N 58.88 W 10.45 x Z-26 174/80 N 49/342 N 58.89 W 10.46 Z-27 083/73 N N 58.71 W 10.38 x x Z-28 N 58.28 W 15.19 x x

Table 4. Sample information for locations Z-A through Z-30. 104

Outcrop Foliation Mineral Outcrop Oriented Thin Number Direction Lineation Location Sample Section Z-29 136/89 NE 72/320 N 58.83 W 09.38 x Z-30 073/71 N N 58.44 W 09.50 x Z-31 082/72 N 72/011 N 58.78 W 09.40 x x Z-32 070/76 N N 58.46 W 13.37 x x Z-33 080/78 N N 58.20 W 12.96 x x Z-34 082/65 N N 58.11 W 12.87 x x Z-35 010/50 W 43/313 N 57.95 W 12.72 x x Z-36 005/30 W 26/292 N 57.94 W 12.72 Z-37 040/47 NW 34/254 N 57.94 W 12.71 Z-38 021/59 W 31/254 N 57.93 W 12.70 x Z-39 020/59 W 31/253 N 57.94 W 12.69 Z-40 036/53 NW 42/267 N 57.91 W 12.65 x x Z-41 009/35 W 41/268 N 57.91 W 12.65 Z-42 039/44 NW 42/268 N 57.90 W 12.66 x x Z-43 038/71 NW N 57.91 W 12.60 Z-44 071/58 N 39/249 N 57.90 W 12.61 x x Z-45 176/39 W 26/274 N 57.83 W 12.59 x x Z-46 174/40 W N 57.83 W 12.59 Z-47 046/38 NW 32/267 N 57.87 W 12.55 x x Z-48 065/71 N N 57.37 W 13.79 x x Z-49 N 57.33 W 13.79 x Z-50 065/79 N N 57.25 W 13.79 x Z-51 065/74 N N 57.19 W 13.79 Z-52 108/72 N 25/295 N 58.84 W 10.37 Z-53 066/52 N 40/289 N 58.61 W 10.38 x x Z-53a 067/44 N N 58.60 W 10.38 x Z-54 N 58.38 W 11.23 x x Z-55 073/50 N N 56.41 W 11.71 x Z-56 083/74 N N 56.61 W 11.79 Z-57 085/70 N N 56.59 W 11.73 x x Z-58 088/79 N N 58.64 W 11.79 Z-59 136/90 N 58.87 W 10.74 Z-60 112/90 40/315 N 58.82 W 10.98 Z-61 N 55.15 W 15.90 x x Z-62 N 55.15 W 16.85 Z-63 098/68 N N 55.61 W 13.94 x x Z-64 158/67 N N 58.18 W 15.52 x x Z-65 165/61 W N 57.12 W 16.23 x x Z-66 161/59 W N 57.19 W 16.33 x Z-67 079/44 N N 57.46 W 14.07 x

Table 5. Sample information for locations Z-31 through Z-67.

Note: The outcrop locations are contained within latitude N 45o55.00’ and longitude W 61o17.00’, and therefore the locations are recorded in minutes and seconds.