40Ar– 39Ar white mica ages reveal Neoproterozoic/Paleozoic provenance and an Alleghanian overprint in coeval Upper Ordovician–Lower Devonian rocks of Meguma and Avalonia

Abstract

40Ar– 39Ar analyses (IR single grain, total fusion) of white micas from Upper Ordovician–Lower Devonian sedimentary strata in the Meguma (Annapolis section) and Avalon (Arisaig section) terranes of the Canadian Appalachians complement recent detrital zircon studies by providing constraints on local source areas and identifying the age of low temperature (< 350 °C) events along this portion of the northern margin of the Rheic Ocean. In the Annapolis section, pre-depositional muscovite ages range in age from Cambrian to Early Ordovician but occur only in the arenaceous ca. 440 Ma White Rock Formation, reflecting the relatively inert behavior of the quartz sandstone compared to overlying argillaceous rocks. Pre-depositional ages of muscovites from coeval strata in the Arisaig Group range from Cambrian to Middle Ordovician. In younger Arisaig strata, detrital muscovite ages typically range from Late Neoproterozoic–Early Ordovician, but some are similar to the depositional ages of the formations. The detrital muscovites in both Annapolis and Arisaig sections are interpreted to have been derived from neighboring terranes (e.g. Ganderia) during the accretion of Avalonia to Baltica and Laurentia. A remarkable outcome of the study is that many muscovite ages are significantly younger than the depositional age of the strata. They do not record the (Middle–Late Devonian) age of Acadian folding or adjacent intrusions, but instead preserve a precise age of (322.6 ± 2.1 Ma, 16 analyses). Although the sections sampled display no discernable evidence for Carboniferous tectonism, these ages are coeval with documented Alleghanian deformation in both Avalonian and Meguma terrane rocks and we interpret them to reflect distributed fluid flow coeval with dextral shear along the Avalon–Meguma terrane boundary.