Granitoid plutons in peri-Gondwanaan terranes of Cape Breton Island, Nova Scotia, Canada: new U-Pb (zircon) age constraints

Abstract

Granitoid plutons are a major component of pre-Carboniferous rocks in Cape Breton Island and knowledge of the time and tectonic setting of their emplacement is crucial for understanding the geological history of the island, guiding exploration for granite-related economic mineralization, and making along-orogen correlations. The distribution of these plutons and their petrological characteristics have been used in the past for recognizing both Laurentian and peri-Gondwanan components in Cape Breton Island, and for subdividing the peri-Gondwanan components into Ganderian and Avalonian terranes. However, ages of many plutons were assumed on the basis of field relations and petrological features compared to those of the relatively few reliably dated plutons. Seventeen new U–Pb (zircon) ages from igneous units reported here provide enhanced understanding of the distribution of pluton ages. Arc-related plutons in the Aspy terrane with ages of ca. 490 to 475 Ma likely record the Penobscottian tectonomagmatic event recognized in the Exploits subzone of central Newfoundland and New Brunswick but not previously recognized in Cape Breton Island. Arc-related Devonian plutonic activity in the same terrane is more widespread, continuous, and protracted (445 Ma to 395 Ma) than previously known. Late Devonian magmatism in the Ganderian Aspy terrane is similar in age to that in the Avalonian Mira terrane (380 to 360 Ma) but the tectonic settings are different. In contrast, magmatic activity in the Bras d’Or terrane is almost exclusively arc-related in the Late Ediacaran (580 to 540 Ma) and rift-related in the Late Cambrian (520 to 490 Ma). The new data support the terrane distinctions previously documented.

Petrology, age, and tectonic setting of the Park Spur pluton, Aspy terrane, central Cape Breton Highlands, Nova Scotia, Canada

“…This new concordia age is interpreted to be more reliable than the previous concordia age of 379.1 ± 3.5 Ma reported for the PSP by Barr et al (2018), which used only three grains in the concordia age calculation. The new age is within error of the U-Pb zircon TIMS age of 375 +5/-4 Ma reported by Dunning et al (1990) for the Black Brook Granitic Suite (BBGS) and supports earlier assumptions that the PSP and Canal plutons are related to the BBGS.…”

“…The recognition of the Aspy terrane as distinct from Bras d'Or terrane to the south and east was based in part on differences in petrological characteristics of their plutonic components juxtaposed at the Eastern Highlands shear zone Raeside 1986, 1989). Decades of subsequent work has confirmed this distinction, now supported by U-Pb zircon ages which demonstrate that the plutonic rocks of the Aspy terrane in the central and eastern Cape Breton Highlands are mainly Silurian and Devonian in contrast to late Ediacaran-Cambrian magmatism in the Bras d'Or terrane (e.g., Barr et al 2018;this study). It is now also recognized that rocks formed at different crustal levels have been juxtaposed along major shear zones within the Aspy terrane, and not just at its boundary with the Bras d'Or terrane.…”

“…The central part of the Aspy terrane is characterized by a complex assemblage of both low-and high-grade metamorphic rocks of early Paleozoic age (e.g., Barr et al 1992; Barr and White 2017a, b). They are intruded by Ordovician through Devonian plutonic suites with compositions that include I-type, S-type, and A-type emplaced in volcanic arc, collisional, and extensional settings, respectively (e.g., Barr et al 2018). Recent and on-going work in the Aspy terrane has provided new age constraints and clarified relationships among the metamorphic units (e.g., McCarron 2020; Grant et al 2019;Grant 2020;, 2019Sombini dos Santos et al 2020;Sunatori 2022;White et al 2018).…”

“…In contrast, large areas of the Aspy terrane contain lower metamorphic grade sedimentary and volcanic rocks formed in Silurian to early Devonian subduction settings (e.g., Barr and Jamieson 1991;Barr et al 2018;White et al 2018). The original relationship among these contrasting low-and high-grade areas is not yet clear, but activation and reactivation of major shear zones within the Aspy terrane has clearly played a major role (e.g., Piette-Lauzière 2022; Sunatori 2022; this study).…”

Long-lived association between Avalonia and the Meguma terrane deduced from zircon geochronology of metasedimentary granulites

“…related magmatism (460 Ma to 440 Ma); and (5) pre-collision arc-related magmatism from 440 Ma to 425 Ma (Fig. 4) 4,13,27,33,34,36,40–46 . Regional magmatism likely ended by ~420 Ma as there is a dearth of Lochkovian zircons (417 ± 9 Ma to 411 ± 9 Ma) in the metasedimentary granulites and Arisaig Group sedimentary rocks compared to the Pridolian (430 Ma to 420 Ma) and Emsian (407 ± 2.8 Ma to 397.5 ± 2.7 Ma) populations.…”