Maritime Sediments and Atlantic Geology V o l . 22 December, 1986 No. 3 Pre-Carbon Iferous TectonostratI graphic Subdivisions of Cape Breton Island, Nova Scotia Sandra M. Barr and Robert P. Raeslde Department of Geology, Acadia University Wolfvllle, Nova Scotia BOP 1X0 Cape Breton Island can be divided into four zones on the basis of contrasting stratigraphy, metamorphism, and plutonism. A Southeastern zone is characterized by late Precambrian volcanism and plutonism, followed by Cambro-Ordovician rift-basin sedimentation and minor volcanism. The Bras d'Or zone to the northwest is underlain by gneissic basement and overlying platformal (carbonate and clastic) sedimentary rocks, intruded by mainly late Precambrian and Qrdovician(?) granitoid rocks. The Highlands zone has a gneissic core flanked by typically lower grade sedimentary and volcanic rocks of probable Precambrian age intruded by diverse and abundant dioritic to granitic plutons ranging in age from Precambrian to Carboniferous. The Northwestern Highlands zone has gneissic basement intruded by varied plutonic rocks including anorthosite and syenite, the latter of Grenvillian age. The nature and significance of the boundaries between these zones are as yet uncertain, but only the Southeastern and Bras d'Or zones are considered to be part of the Avalon Terrane. Des contrastes de stratigraphie. mdtamorphisme et plutonisme permettent de diviser l'ile du Cap-Breton en quatre zones. Dn zone sud-est se distinque par un volcanisme et un plutonisme tardi-prAeambriens auxquels succAdent, au Cambro-Grdovicien, une sAdimentation bassinale de type rift et un faible volcanisme. Au Nord-Ouest, la zone de Bras d ’Or comporte un socle gneissique sur lequel s'est effectuAe une sAdimentation nAritique (terrigAne et A carbonates), le tout injectA de granitoides principalement tardi-prAcambriens et ordoviciens(?). La zone de Highlands englobe un noyau gneissique ceinturA de roches sAdimentaires et volcaniques auxquels s'est typiquement imprimA un mAtamorphisme de faible intensitA et qui sont probablement d'age prAcambrien. Ceux-ci sont totis recoup As par d'abondants plutons dioritiques et granitiques divers dont l'Sge s'Achelonne du PrAcambrien au CaronifAre. La zone de Northwestern Highlands renferme un socle de gneiss recoupA par diverses roches plutoniques dont 1'anorthosite et la syAnite, cette demiAre Atant d'age grenvillien. Le doute subsiste encore en ce qui regarde la nature et la signification des frontiAres entre ces zones; cependant seules la zone sud-est et la zone de Bras d'Or sont considArAes comme appartenant A la laniAre d'Avalon. [Traduit par le journal] INTRODUCTION reconnaissance scale. More recent map­ ping and petrological studies in north­ All of Cape Breton Island has ern Cape Breton Island (Raeside et a l. generally been Included in the Avalon 1986, Barr et a l. 1985a, b, Raeside Zone or Terrane of the Appalachian and Barr this issue, Jamieson and Craw Orogen (e.g. Williams 1978, 1979, 1983) have demonstrated that its O'Brien et a l. 1983, Rast and Skehan geology is distinct from that of 1983, Williams and Hatcher 1983). southeastern, and probably central, However, these interpretations have Cape Breton Island. been based on data from older The purpose of this paper is to geological mapping (e.g. as compiled by propose a subdivision of Cape Breton Keppie 1979), much of which was on a Island into four zones (Fig. 1), which contrast in pre-Carboniferous strati­ MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 22, 252-263 (1986) graphy, metamorphism and plutonism. It Received May 2. 1986 0711-1150/86/030252-12$2.80/0 Revision Accepted September 5. 1986 253 Barr and Raeside Fig. 1. Simplified geological map of Cape Breton Island showing the proposed tectonostratigraphic subdivisions. Map is compiled from various sources referenced in the text. SR, Salmon River; GM, Gillis Mountain; DC, Deep Cove; WBF. Wilkie Brook Fault; RRF, Red River Fault. Carboniferous sedimentary rocks include Horton Group and overlying units. MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 254 SOUTHEASTERN BRAS D'OR HIGHLANDS NW HHES1 t RN_ HUil IU1NDS CARBONIFEROUS CARBONIFEROUS CARBONIFEROUS CARBONIFEROUS Sedimentary rocks Sedimentary rocks Sedimentary rocks II Sedimentary rocks 0EV0N0-CARB0NIFEROUS DEVONO-CARBONIFEROUS DEVONO-CARBONIFEROUS DEVONO-CARBONIFEROUS Sedimentary rocks, Sedimentary & bimodal Sedimentary & bimodal m inor t u f f volcanic rocks volcanic rocks ■ ■ (Fisset Brook Fm) «"5T O /1 Granitoid rocks Granitoid rocks Granitoid rocks ■ s'. Granitoid rocks i l l ORDOVICIAN? ORDOVIClAN-DEVONIAN? TT) Granitoid rocks Granitoid rocks Diorite, tonaTite CAMBRO-ORDOVICIAN CAMBRO-ORDOVICIAN Sedimentary and Sedimentary and volcanic rocks (mainly volcanic rocks B Kelvin Glen Group) 0 (mainly Bourinot Group) LATE HADRYNIAN-CAMBRIAN LATE HADRYNIAN-CAMBRIAN LATE HADRYNIAN-CAMBRIAN »N / I Granitic rocks Diorite to granite Diorite to granite + + (includes Cheticamp pluton) Diorite, tonal1te, 1eucotonalite (in part may be younger) LATE PRECAMBRIAN LATE PRECAMBRIAN LATE PRECAMBRIAN-EARLY PALEOZOIC? PRECAMBRIAN Volcanic and sedimentary rocks Fourchu Group (and Volcanic rocks (Money Point Group, Crowd!s Mtn Syenite, anorthosite, Giant Lake Complex) (Price Point unit) volcanics, Western Highlands monzodiorite m complex, McMillian Flowage Fm) H George R iv e r Group Paragneiss and orthogneiss Quartzofeldspathic and Gneiss (includes Kellys (Cheticamp Lake, Cape North Group, syenitic gneiss (Polletts Mtn, Barachois River, m undifferentiated gneisses) H Cove Brook Group) Lime H i l l ) Legend for Fig. 1. is further suggested that only two of chemical studies of mafic flows, and of these zones can he correlated with the mafic intrusions interpreted to be Avalon Terrane, as defined in eastern cogenetic with the volcanic rocks, Newfoundland and southern New indicate calc-alkalic transitional to Brunswick. The proposal has important tholeiitic affinity (Keppie et a 7. implications for lateral correlations 1979, Macdonald 1983). of terrane s in the northern The Stirling belt to the northwest Appalachians. is apparently more restricted in com­ position and dominated by bimodal flows SOUTHEASTERN ZONE and varied subaqueous pyroclastic and epiclastic rocks containing chert and Southeastern Cape Breton Island is carbonate layers, as well as syngenetic characterized by the Fourchu Group Fe-Zn-Pb-Cu occurrences (Macdonald (Weeks 1954) which consists mainly of 1983). This belt contains much more volcanic rocks, now generally sedimentary material than the coastal metamorphosed to lower greenschist belt and appears less pervasively de­ facies. These rocks occur in five formed and metamorphosed. It was orig­ belts (Fig. 1). The southernmost inally mapped as Cambrian (Weeks 1954) coastal belt serves at present as the but more recent work has indicated a "type area" of the Fourchu Group Precambrian age (Smith 1978, Macdonald because of its excellent coastal ex­ 1983, McMulliri 1984). Smith (1978) posures which have been relatively well referred to part of this belt as the studied (Murphy 1977, Keppie et a l. Giant Lake Complex. 1979, Macdonald 1983). It consists Fourchu Group rocks to the north­ mainly of subaerial pyroclastic rocks west in the East Bay Hills, Sporting and minor mafic to felsic flows. Geo­ Mountain, and Coxheath Hills belts are 255 Barr and Raeside generally less well known, but contain granitoid rocks are generally post- varied pyroclastic rocks and flows tectonic , although locally strongly apparently of calc-alkalic affinity and sheared. Hence it is more probable formed in association with subduetion that the granitoid magmas are younger at a continental margin (Helmstaedt and than the volcanic rocks and represent Telia 1973, Rowan 1985, Sexton 1985). late-orogenic plutonism whereas the Details of stratigraphic relations volcanic rocks were erupted early in within and between these five belts the orogenic cycle. are lacking, making it difficult to These granitoid and metavolcanic compare the rocks directly with those units are overlain unc onf ormably by of the Avalon Terrane of eastern various early Cambrian to Ordovician Newfoundland or elsewhere. Most sedimentary units (Hutchinson 1952, workers have implied correlation with Weeks 1954, Smith 1978, McMullin 1984). much of the Late Hadrynian sequence in Although stratigraphic relations are Newfoundland, from the Love not entirely resolved, Smith (1978) Cove/Harbour Main groups to the Bull assigned most of these units to the Aim/Signal Hill/ Marystown groups (in­ Kelvin Glen Group. The units include ferred to range from about 680 Ma to conglomerate, siltstone, shale, minor less than 600 Ma) and with the Cold- limestone and possibly some volcanic brook Group of southern New Brunswick rocks (Smith 1978). They contain tri- (Rast and Skehan 1983, O'Brien et a l. lobites and brachiopods generally char­ 1983). acteristic of the Atlantic faunal realm The metavolcanic rocks of the (Hutchinson 1952), a fact widely used Fourchu Group are intruded by granitoid in reconstructions of now dispersed plutons which have given Rb-Sr isochron Avalonian terranes (e.g. O'Brien et ages ranging from 544 ± 21 Ma to 577 + al. 1983). 21 Ma (Cormier 1972, 1979; Barr et a l. A group of Devonian granites has 1984a, Sexton 1985). These are gen­ intruded these rocks and the Fourchu erally in agreement with K-Ar ages for Group, forming an arcuate belt from amphiboles from the same units (Sexton Salmon River through Gillis Mountain to 1985, McMullin 1984), except an older Deep Cove (Barr and Macdonald