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Effects of fluid flow, cooling and deformation as recorded by 40Ar/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)

Published online by Cambridge University Press:  11 November 2014

ARNE P. WILLNER ,
SANDRA M. BARR ,
JOHANNES GLODNY ,
HANS-JOACHIM MASSONNE ,
MASAFUMI SUDO ,
STUART N. THOMSON ,
CEES R. VAN STAAL  and
CHRIS E. WHITE
ARNE P. WILLNER*
Affiliation:
Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, 70174 Stuttgart, Germany Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität, 44780 Bochum, Germany
SANDRA M. BARR
Affiliation:
Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada
JOHANNES GLODNY
Affiliation:
Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, D-14473 Potsdam, Germany
HANS-JOACHIM MASSONNE
Affiliation:
Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, 70174 Stuttgart, Germany
MASAFUMI SUDO
Affiliation:
Institut für Erd- und Umweltwissenschaften, Universität Potsdam, Karl-Liebknecht-Str. 24, D-14476 Potsdam-Golm, Germany
STUART N. THOMSON
Affiliation:
Department of Geosciences, University of Arizona, 1040 E. 4th St, Tucson, AZ 85721-0077, USA
CEES R. VAN STAAL
Affiliation:
Geological Survey of Canada, 625 Robson Street, Vancouver, BC V6B 5J3, Canada
CHRIS E. WHITE
Affiliation:
Nova Scotia Department of Natural Resources, PO Box 698, Halifax, Nova Scotia B3J 2T9, Canada
*
†Author for correspondence: arne.willner@rub.de
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Abstract

40Ar/39Ar 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 40Ar/39Ar 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 40Ar/39Ar 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.

Keywords

40Ar/39Ar spot agesRb–Sr mineral isochronszircon fission tracksvery low-/low-grade metamorphismAvaloniaDevonian transpressionRheic Ocean
Type
Original Articles
Information
Geological Magazine , Volume 152 , Issue 5 , September 2015 , pp. 767 - 787
Copyright
Copyright © Cambridge University Press 2014 

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