A revised interpretation of Cambrian and Ordovician rocks in the Bourinot belt of central Cape Breton Island, Nova Scotia

Abstract

N ew U-Pb (zircon) data from the northern Boisdale Hills show that rhyolite on Long Island and syenogranite o f the nearby Mount Cameron pluton have similar ages o f 505 ± 3 Ma and 509 ± 2 Ma, respectively. These ages are similar to those known or inferred from some felsic plutonic units elsewhere in central Cape Breton Island and suggest that latest Cambrian to earliest Ordovician igneous activity was widespread in the Bras d'Or terrane. The new dates also suggest that stratigraphic and structural relationships in the Boisdale Hills are more complex than previously interpreted. The dated rhyolite and other volcanic and sedimentary rocks in the northern Bourinot belt in the Boisdale Hills were previously included in undivided Middle Cambrian Bourinot Group. In the southern Bourinot belt, the Bourinot Group was subdivided into the Eskasoni, Dugald, and Gregwa formations. The Eskasoni Formation is dominantly a bimodal volcanic suite with petrological characteristics indicative o f origin in a continental within-plate tectonic setting. Its present contacts, both with adjacent older metamorphic and plutonic rocks and with the apparently overlying fossiliferous Dugald and volcanogenic Gregwa formations, are faulted. Our new mapping demonstrates that the Eskasoni, Dugald and Gregwa forma tions can be extended into the central Bourinot belt where they were previously undivided. However, continuity cannot be demonstrated between these Middle Cambrian units and the dated volcanic and associated sedimentary units in the northern Bourinot belt. Hence, assuming that both U-Pb and fossil ages are correct, our interpretation is that the Upper Cambrian-Lower Ordovician Northern Boisdale Hills volcanic unit is younger than the Bourinot Group, although petrochemical data suggest that it formed in a similar tectonic regime. The presence in the Bourinot belt o f fauna characteristic o f the Acado-Baltic faunal province appears to tie the Bras d'Or terrane to other Avalonian (peri-Gondwanan) terranes. However, the Bras d'Or terrane differs from the adjacent Mira terrane which includes Lower as well as Middle and Upper Cambrian units and lacks volcanic and plutonic rocks o f this age. D e nouvelles donndes U-Pb (zircon) provenant de la partie nord des collines Boisdale montrent que la rhyolite sur l'lle Long et le syenogranite du pluton du mont Cameron k proximite ont des Ages similaires, de 505 ± 3 Ma et 509 ± 2 Ma respectivement. Ces ages sont similaires a ceux connus ou supposes pour quelques unites plutoniques felsiques ailleurs dans le centre de l'ile-du-Cap-Breton, et suggerent que l'activite ignee de la fin du Cambrien au debut de l'Ordovicien etait tepandue dans le terrain de Bras d'Or. Ces nouvelles datations sugg&rent aussi que les relations stratigraphiques et structurales dans les collines Boisdale sont plus complexes que ne le montraient les interpretations anterieures. La rhyolite datee et d'autres roches volcaniques et sedimentaires de la partie nord de la ceinture de Bourinot dans les monts Boisdale etaient auparavant...

Palaeozoic within-plate volcanic rocks in Nova Scotia (Canada) reinterpreted: isotopic constraints on magmatic source and palaeocontinental reconstructions

“…More recently, Lin (1993) and Lynch, Tremblay & Rose (1993) have shown that the Ordovician-Silurian Aspy rocks lie unconformably upon late Proterozoic Bras d'Or rocks, implying that they were part of a single terrane by Ordovician time. Helmstaedt & Tella (1973) reported pebbles of Bras d'Or lithologies in the Cambro-Ordovician overstep sequence, an observation which suggests that the Mira and Bras d'Or terranes were either adjacent (White et al 1994), or amalgamated (Keppie & Dallmeyer, 1989) during Cambrian time. Barr & Hegner (1992) used neodymium isotopic data from Keppie & Dallmeyer, 1989) and terranes in Cape Breton Island according to Barr & Raeside (1989 Proterozoic and Palaeozoic felsic rocks to suggest that different magma sources lay beneath the various terranes in Cape Breton Island throughout Proterozoic and early Palaeozoic times.…”

Superposed Neoproterozoic and Silurian magmatic arcs in central Cape Breton Island, Canada: geochemical and geochronological constraints

“…1b). These magmatic arc rocks are overlain by Middle Cambrian-Lower Ordovician rocks that include bimodal, within-plate, rift-related volcanic rocks dated at 505 ± 3 Ma (White et al 1994;Keppie et al 1997). This brackets the change from subduction to rifting between 545 and 505 Ma.…”

“…Over the past decade, these two views have converged. Thus, White et al (1994) have concluded that the Avalonian overstep sequence of Cambrian age does indeed unconformably overlie their Bras d'Or and Mira terranes. However, they still believe that the two terranes were separate, because Middle Cambrian volcanic rocks in the Bras d'Or terrane are nearly absent in the Mira terrane.…”

Connecting Cape Breton Island and Newfoundland, Canada: Geophysical Modeling of pre-Carboniferous 'Basement' Rocks in the Cabot Strait Area

“…The ca. 500 Ma plutons are of granitic composition and may have formed by crustal melting during post-orogenic uplift or during periods of localized extension within the terrane, perhaps linked to the initial stages of separation from Gondwana in the Middle to Late Cambrian (White et al 1994;van Staal and Barr 2012). The variation in level of exposure in Bras d'Or terrane is consistent with the Bras d'Or terrane having been thrust over the Aspy terrane; subsequent erosion thus exposes deeper levels of the terrane near its boundary with the Aspy terrane (Barr et al 1995;Lin 2001).…”