George River Suite-Bras D'or Gneiss
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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).