Canadian Journal of Earth Sciences New U-Pb age constraints on the geological history of the Ganderian Bras d'Or terrane, Cape Breton Island, Nova Scotia Journal: Canadian Journal of Earth Sciences Manuscript ID cjes-2018-0248.R2 Manuscript Type: Article Date Submitted by the 30-Jan-2019 Author: Complete List of Authors: van Rooyen, Deanne; Cape Breton University, Mathematics, Physics and Geology Barr, Sandra; Department of Earth and Environmental Science, White, Chris;Draft Nova Scotia Department of Energy and Mines Hamilton, Michael A.; University of Toronto, Department of Earth Sciences detrital zircon, tectonic evolution, Bras d'Or terrane, Appalachian orogen, Keyword: Cape Breton Island Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? : https://mc06.manuscriptcentral.com/cjes-pubs Page 1 of 64 Canadian Journal of Earth Sciences 1 New U-Pb age constraints on the geological history of the Ganderian Bras d'Or terrane, 2 Cape Breton Island, Nova Scotia 3 4 D. van Rooyen1, S.M. Barr2, C.E. White3 and Hamilton, M.A.4 5 6 1. Department of Mathematics, Physics, and Geology, Cape Breton University, Sydney, NS, B1P 7 6L2, Canada 8 [email protected] 9 10 2. Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia 11 DraftB4P 2R6, Canada 12 [email protected] 13 14 3. Nova Scotia Department of Energy and Mines, Halifax, Nova Scotia B3J 2T9, Canada 15 [email protected] 16 17 4. Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada M5S 3B1, 18 Canada 19 [email protected] 20 21 Corresponding author: 22 Deanne van Rooyen, Department of Mathematics, Physics, and Geology, Cape Breton 23 University, Sydney, NS, B1P, e-mail: [email protected]; Telephone: (902) 563-1384 https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 2 of 64 24 ABSTRACT 25 The northern Appalachian orogen preserves evidence of a complex history of amalgamation of 26 terranes with both Laurentian and Gondwanan affinities. The Ganderian Bras d'Or terrane of 27 central Cape Breton Island is not well-represented elsewhere in the orogen and its relationship to 28 other Ganderian terranes is enigmatic, particularly with respect to its pre-Neoproterozoic history. 29 The Boisdale Hills and Kellys Mountain areas contain the oldest metamorphic rocks in the Bras 30 d’Or terrane. Quartzite units in the Boisdale Hills have detrital zircon populations with ages 31 ranging from 3.2 Ga to ca. 1 Ga. Paragneiss units from the Kellys Mountain area contain Meso- 32 to Neoproterozoic detrital zircons, in which the youngest grains indicate that the maximum 33 depositional age is <600 Ma. The detrital zircon populations of rocks from both areas are 34 consistent with Gondwanan provenanceDraft for the protoliths, most likely the Amazonian craton. 35 New U-Pb dates for subduction-related dioritic to granodioritic plutons in the Boisdale Hills 36 yielded ages of 560 Ma to ca. 540 Ma. Sedimentary, bimodal volcanic and plutonic rocks from 37 the Bourinot belt in the Boisdale Hills and related plutons in the Kellys Mountain area have ages 38 of ca. 510 – 490 Ma and are interpreted to have formed during extension related to separation of 39 Ganderia from Gondwana. The southeastern Bras d’Or terrane preserves rocks formed in Pan- 40 African subduction zones on a former passive margin of Gondwana as well as rocks formed 41 during the initial stages of rifting of Ganderia from Gondwana and the subsequent opening of the 42 Rheic Ocean. 43 Key words: Cape Breton Island, detrital zircon, geochronology, tectonic evolution, 44 Appalachians 45 46 https://mc06.manuscriptcentral.com/cjes-pubs Page 3 of 64 Canadian Journal of Earth Sciences 47 Introduction 48 Cape Breton Island exposes a highly compressed cross-section of the northern 49 Appalachian orogen, where geological components representing Laurentia and three accreted 50 Gondwana-derived terranes are exposed across ca. 140 km (Fig. 1, inset). The Laurentian Blair 51 River Inlier forms the northwesternmost component of the island, the Ganderian Aspy and Bras 52 d’Or terranes form the central area, and the Avalonian Mira terrane forms the southern part (Fig. 53 1, inset; Barr et al. 1996; Hibbard et al. 2006, 2007). The Bras d’Or terrane is the most 54 enigmatic of these terranes, particularly with respect to its Proterozoic history and affinity, and 55 its relationship to other Ganderian and Avalonian parts of the northern Appalachian orogen (e.g., 56 Barr and White 1996; Barr et al. 1996a; Dostal et al. 1996; Barr et al. 1998; Keppie and Dostal 57 1998; Murphy et al. 1999; Barr et al. 2014Drafta, b). In contrast to other parts of Ganderia, which are 58 dominated by lower Paleozoic rocks, the Bras d'Or terrane consists primarily of Neoproterozoic 59 and Cambrian metamorphic and plutonic rocks, unconformably overlain by mainly 60 Carboniferous sedimentary rocks (Fig. 1) (Raeside and Barr 1990; White et al. 2016). It has been 61 correlated with rare Neoproterozoic inliers in Ganderian terranes in Newfoundland (Rogers et al. 62 2006; Zagorevski et al. 2007; Zagorevski et al. 2010), and with the Brookville terrane in southern 63 New Brunswick to which it shows most geological similarity (Barr and Raeside 1989; Barr and 64 White 1996; White and Barr 1996; Barr et al. 1998). 65 This study is focused on U-Pb dating of detrital and igneous zircon in metasedimentary 66 and plutonic rocks in the Boisdale Hills and Kellys Mountain areas of the southeastern Bras d’Or 67 terrane (Figs. 1, 2). The detrital zircon data presented here enhance the existing small detrital 68 zircon database for the Bras d’Or terrane (Keppie et al. 1998; Barr et al. 2003; White et al. 2016) 69 and enable more detailed comparison of high- and low-grade metasedimentary units in terms of https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 4 of 64 70 depositional age and provenance. These rocks are the oldest in the Bras d’Or terrane, and are 71 useful to assess the provenance of pre-Appalachian-cycle rocks in Ganderia and the location of 72 Ganderia prior to its accretion to Laurentia. In younger units, the proximity of Laurentia after 73 the beginning of Pangaean amalgamation tends to obscure the smaller pre-Appalachian zircon 74 populations by providing detrital material from a range of source areas. This study, therefore, 75 does not have the challenge of discriminating between Laurentia-derived zircon of a particular 76 age (e.g., Grenvillian) from zircon of similar age from other part of the same orogen. In addition 77 to the detrital zircon data, this study includes U-Pb ages from five plutonic units which give 78 evidence for a more protracted pre-Appalachian cycle plutonic history in the Bras d'Or terrane 79 than previously recognized. 80 Draft 81 Geological setting 82 By the early 2000s, the 1970s' division of the Appalachian orogen into Humber, 83 Dunnage, Gander, Avalon, and Meguma zones (Williams 1978) had been replaced by divisions 84 that recognized their origins either in Laurentia or peripheral to Laurentia versus origins in 85 Gondwana or peripheral to Gondwana (e.g., Hibbard et al. 2006). The former Dunnage zone was 86 subdivided into peri-Laurentian and peri-Gondwanan parts, the latter peripheral to the Ganderian 87 part of Gondwana that was interpreted to have been linked to Amazonia whereas in many 88 reconstructions Avalonia and Meguma are linked to West Africa (e.g., Hibbard et al. 2007; van 89 Staal and Barr 2012; van Staal et al. 2012). In the new interpretations, the Ordovician Taconic 90 orogeny resulted from subduction and closures of seaways adjacent to Laurentia, whereas the 91 Penobscottian orogeny in a similar time frame represented subduction and closure of seaways 92 adjacent to Ganderia (Zagorevski et al. 2010; van Staal and Bar 2012). The Silurian Salinic https://mc06.manuscriptcentral.com/cjes-pubs Page 5 of 64 Canadian Journal of Earth Sciences 93 orogeny resulted from closure of the Iapetus Ocean and juxtaposed Ganderia with Laurentia and 94 peri-Laurentian terranes. Subsequently, in the late Silurian and Devonian, closure of the Rheic 95 Ocean and related seaways, which had formed when Ganderia, Avalonia, and Meguma separated 96 from Gondwana in the early Paleozoic, resulted in the Acadian and Neoacadian orogenies and 97 the construction of the composite Laurentian margin. This composite margin was subsequently 98 deformed by collisional and transcurrent tectonics during closure of Rheic Ocean remnants in the 99 Carboniferous Alleghanian orogeny to form the supercontinent Pangaea (e.g., van Staal and Barr 100 2012). 101 Unravelling the various Laurentian and Gondwanan components in the Appalachian 102 orogen has been challenging, in large part because of limited exposures of their Precambrian 103 infrastructure and overprinting by youngerDraft metamorphic, magmatic, and tectonic events. Hence 104 areas such as the Bras d'Or terrane where such rocks are exposed with limited Paleozoic 105 overprint are invaluable in interpreting provenance within Gondwanan terranes. The Bras d’Or 106 terrane is also valuable because it has been independently linked with the Gondwanan margin as 107 summarized by Pollock et al. (2012) and Domier (2015) through Cambrian Gondwanan faunal 108 provinces which gradually shift towards Laurentian provinces during the Ordovician and Silurian 109 as a result of the opening of the Rheic ocean (Cocks and Fortey 1982) and paleomagnetic data 110 which have been interpreted to place the terrane close to the West African part of Gondwana 111 during the Cambrian (Johnson and Van der Voo 1985). 112 The Neoproterozoic metamorphic rocks of the Bras d'Or terrane include small, isolated 113 areas of low-pressure amphibolite-facies gneiss, and larger areas of greenschist-facies (and in 114 places upper greenschist-facies to lower amphibolite-facies) quartzite, marble, meta-greywacke, 115 and minor volcanic rocks (Fig.