SINGLE CRYSTAL 40AR/39AR STUDY OF ALKALI FELDSPARS SUBJECT TO A LOW-GRADE METAMORPHIC OVERPRINT: A CASE STUDY OF MT. ROGERS RHYOLITE

Rhyolites of the Mt. Rogers Fm. of southwestern Virginia formed in a ca. 760 Ma eruptive event associated with rifting of the Laurentian margin, and later underwent at least one episode of low-grade regional metamorphism between the latest Neoproterozoic and mid-Paleozoic. We have undertaken a combined mineralogic, petrologic and ⁴⁰Ar/³⁹Ar geochronologic study of alkali feldspar phenocrysts in a sample from the Wilburn member of the Mt. Rogers rhyolite to investigate their record of post-eruptive and subsequent low-grade metamorphic evolution. The sample is a dark, exceptionally fine-grained rock that presumably reflects devitrification of an original obsidian with abundant 1-2 mm phenocrysts. Alkali feldspars retain relatively pristine morphologies, twinning, and occasional relict melt inclusions. Elemental mapping shows feldspars to be dominated by domains of nearly pure albite or nearly pure microcline, in irregular to polygonal patches up to ca. 0.25 mm across. Feldspar crystal fragments roughly 1.0 to 0.5 mm across were chosen for irradiation and incremental heating ⁴⁰Ar/³⁹Ar analysis in the ANIMAL facility. Most individual crystal fragments yield single-crystal apparent age spectra that are complex, dominated by release of ‘excess’ argon, with minimum ages as young as ca. 330 Ma. However, some crystal fragments yield simple spectra little affected by extraneous argon, with ages that begin at ca. 250 Ma and monotonically increase to ca. 333 Ma. The crystals characterized by complex spectra have much lower yields of ³⁹Ar_K than the crystals with the simple spectra, and these two spectral types are interpreted to represent crystal fragments with a higher proportion of albite (complex) vs. crystal fragments richer in microcline (simple). We interpret the alkali feldspars to have undergone redistribution of major elements during Visean-stage Alleghanian metamorphism. The larger microcline-rich patches are interpreted to have been at temperatures appropriate for significant ⁴⁰Ar diffusion from the approximate timing of metamorphism until final argon retention in the Permian. The timing of this event in the Mt. Rogers massif supports the inference that regional, Visean-stage Alleghanian metamorphic effects are present throughout Western Blue Ridge and Talladega Belt of the southern Appalachians.