Effects of fluid flow, cooling and deformation as recorded by⁴⁰Ar/³⁹Ar, Rb–Sr and zircon fission track ages in very low- to low-grade metamorphic rocks in Avalonian SE Cape Breton Island (Nova Scotia, Canada)

Abstract

40 Ar/ 39 Ar in situ UV laser ablation of white mica, Rb-Sr mineral isochrons and zircon fission track dating were applied to determine ages of very low-to low-grade metamorphic processes at 3.5 ± 0.4 kbar, 280 ± 30°C in the Avalonian Mira terrane of SE Cape Breton Island (Nova Scotia). The Mira terrane comprises Neoproterozoic volcanic-arc rocks overlain by Cambrian sedimentary rocks. Crystallization of metamorphic white mica was dated in six metavolcanic samples by 40 Ar/ 39 Ar spot age peaks between 396 ± 3 and 363 ± 14 Ma. Rb-Sr systematics of minerals and mineral aggregates yielded two isochrons at 389 ± 7 Ma and 365 ± 8 Ma, corroborating equilibrium conditions during very low-to low-grade metamorphism. The dated white mica is oriented parallel to foliations produced by sinistral strike-slip faulting and/or folding related to the Middle-Late Devonian transpressive assembly of Avalonian terranes during convergence and emplacement of the neighbouring Meguma terrane. Exhumation occurred earlier in the NW Mira terrane than in the SE. Transpression was related to the closure of the Rheic Ocean between Gondwana and Laurussia by NW-directed convergence. The 40 Ar/ 39 Ar spot age spectra also display relict age peaks at 477-465 Ma, 439 Ma and 420-428 Ma attributed to deformation and fluid access, possibly related to the collision of Avalonia with composite Laurentia or to earlier Ordovician-Silurian rifting. Fission track ages of zircon from Mira terrane samples range between 242 ± 18 and 225 ± 21 Ma and reflect late Palaeozoic reburial and reheating close to previous peak metamorphic temperatures under fluid-absent conditions during rifting prior to opening of the Central Atlantic Ocean.

Zircon fission‐track ages from Newfoundland—A proxy for high geothermal gradients and exhumation before opening of the Central Atlantic Ocean

“…Our new ZFT ages from central Newfoundland are in the same range as our ZFT results from Nova Scotia (Willner et al., ; Figures and ). In Nova Scotia, these ages are also considerably younger than the white mica 40 Ar/ 39 Ar ages (396–363 Ma; Willner et al., ) and up to 4,700 m thick Carboniferous sedimentary cover (~360 to 305 Ma) in the adjacent Sydney basin (Boehner & Giles, ; Figure ). Considering the maximum thickness of the cover, reheating to ≥220°C requires a palaeo‐geothermal gradient exceeding 40°C/km.…”

“…For correlation of known FT ages, we compiled all available thermochronological data from the Central Atlantic conjugate margins (Figure B). Areas which cooled from temperatures of ≥220°C such as central Newfoundland (this work) and Cape Breton Island (Willner et al., ; Figure ) are aligned parallel to the late Triassic–early Jurassic rift zone and the final break‐up line (Figure A, Ba–c) constituting a horst of major uplift in the western denuding rift shoulder. All available fission‐track data in Atlantic Canada (Figure Ba–d) show maxima at 200–240 Ma coinciding with the onset of sedimentation in rift basins (Schlische et al., ) and the CAMP event representing major uplift, exhumation, and cooling before break‐up.…”

“…(). Inserted are ZFT ages (Ma; red) of basement rocks in central Newfoundland (this work) and SE Cape Breton Island (Willner et al., ) and of sedimentary rocks in the Minas Basin (Ravenhurst et al., ). AFT ages (Ma; blue) of basement and sedimentary rocks are compiled from Arne et al.…”

“…Zircon fission‐track (ZFT) ages of Appalachian metamorphic rocks in Nova Scotia (Willner et al., ) revealed ages up to 150 Ma younger than the overlying Lower Carboniferous sedimentary cover of the late Palaeozoic Maritimes Basin. These data indicate a period of cooling before opening of the Atlantic Ocean, supporting similar conclusions drawn from several apatite fission‐track (AFT) studies in Atlantic Canada (e.g., Hendriks, Jamieson, Willett, & Zentilli, ; Ryan, ; Ryan & Zentilli, ).…”

Influence of deformation and fluids on Ar retention in white mica: Dating the Dover Fault, Newfoundland Appalachians

“…394 Ma. This is compatible with the conclusions of Willner et al (2015), who have argued that the Neoacadian (400-365 Ma) arrival of Meguma terrane to composite Laurentia along a dextral transpressional boundary produced a conjugate sinistral shear zone that facilitated escape of the Mira terrane of Cape Breton Island, and Newfoundland Avalonia to the NW (present day coordinates). This conjugate system was short lived, as a Devonian shear reversal is recorded in Newfoundland, forming the narrower and lower temperature, NE-striking dextral Dover Fault by 385 Ma.…”