Avalonia of the northern Appalachian orogen includes clastic sedimentary rocks that span the Ediacaran – Cambrian boundary, including the Global Boundary Stratotype Section and Point at Fortune Head, Newfoundland. Detrital muscovite becomes abundant in sedimentary strata near and above the GSSP. Single-crystal 40Ar/39Ar total-fusion analyses for detrital muscovite sampled from these strata in Newfoundland and the Mira terrane of Nova Scotia yield ages that range from ca. 540-634 Ma, dominated by single modes that are skewed to younger ages. Laser incremental heating 40Ar/39Ar age spectra for individual muscovite crystals are typical of 40Ar-loss, with ages for initial increments as young as ca. 400 Ma that generally increase through the measurements, defining plateau ages with an average of 637.1±4.7 Ma. WDS maps show K and Al loss selectively along interlayer (001) zones and grain boundaries now occupied by aluminous, metamorphic chlorite. Loss of K and 40Ar was linked to regional, low-grade metamorphism during late Ediacaran transpression, overprinted by metamorphism during late Silurian-Devonian juxtaposition of Avalonia with Ganderia and Meguma. The detrital muscovite source is inferred to have been aluminous rocks in the Avalonian crust metamorphosed during ca. 650 collision of Avalonia with the West African Craton. The oldest muscovite ages coincide with the end of the Marinoan glacial stage, which could indicate a causal relationship between the termination of glaciation and initial mid-crustal cooling of the source terrane through closure temperatures for muscovite. Exhumation and erosion during extension and basin development through the Ediacaran-Cambrian transition provided abundant detrital muscovite.
Barr et al. (Wed,) studied this question.