DETRITAL MUSCOVITE FROM THE EDIACARAN-CAMBRIAN BOUNDARY ZONE IN ATLANTIC CANADA: AGE, PROVENANCE, AND EFFECTS OF LOW-GRADE METAMORPHISM ON 40AR/39AR SYSTEMATICS

In addition to its characteristic Neoproterozoic units, Avalonia in the northern Appalachian orogen includes clastic sedimentary sequences that span the Ediacaran – Cambrian boundary, including the Global Boundary Stratotype Section and Point (GSSP) at Fortune Head, Newfoundland. Abundant and coarse detrital muscovite appears abruptly a few metres below the GSSP (and in equivalent sections in New Brunswick and Cape Breton Island) and persists throughout the Cambrian stratigraphy. The muscovite from these sections yields single-crystal ⁴⁰Ar/³⁹Ar total-fusion age distributions with prominent modes at ca. 630-620 Ma, and some ages skewed to the Devonian, much younger than permitted by the depositional age. WDS maps of K-Mg-Fe-Al show that K and Al were stripped selectively from detrital muscovite crystals as certain interlayers were replaced by an aluminous, metamorphic chlorite. Laser incremental heating ⁴⁰Ar/³⁹Ar age spectra for individual muscovite crystals (n=24) yield spectra typical of superimposed ⁴⁰Ar-loss, with plateau ages of ca. 630-640 Ma and an implied timing for a loss event of ca. 425-400 Ma. These results are consistent with an ‘all or nothing’ metamorphic reaction mechanism for loss of K and ⁴⁰Ar from the interlayer sites of muscovite, with subordinate loss of ⁴⁰Ar by volume diffusion. The timing of this loss event is consistent with regional, low-grade metamorphism during Silurian-Devonian (Acadian) juxtaposition of Avalonia and Ganderia. The original muscovite source rock was likely a medium- to high-grade metapelite metamorphosed and deformed at 650-630 Ma, or coarse-grained aluminous granite or granitic pegmatite. These aluminous muscovite-bearing rocks were likely a major component of the still-unknown source area for Cambrian sediments deposited in Avalonia. This enigmatic source was exposed or became accessible very abruptly and very close to the end of the Ediacaran. The large volume of quartz-rich sedimentary rocks and abundance of detrital muscovite imply a large continental source area and alluvial systems throughout the Cambrian that are not compatible with rock units currently exposed in Avalonia or elsewhere in the Appalachian orogen.