PETROLOGIC AND GEOCHEMICAL EVOLUTION OF THE LATE NEOPROTEROZOIC RAZOR RIDGE VOLCANIC CENTER IN SOUTHWESTERN VIRGINIA

The Mount Rogers area of SW Virginia contains a well-preserved sequence of late Neoproterozoic (ca. 760 Ma) volcanic rocks associated with crustal extension. Rhyolites occur within three fault-bounded slices interlayered with greenstones and metasedimentary lithologies. This petrologic study focuses on the previously uncharacterized Razor Ridge area and seeks to clarify possible relationships with rhyolites of the other slices.

The Razor Ridge volcanic center is comprised of metamorphosed high-silica rhyolites interlayered with greenstones and terrestrial sedimentary deposits. Although metamorphosed and deformed at greenschist-facies conditions, original textures and features remain recognizable in the rhyolites. The Razor Ridge rhyolites are divided into four petrologic units (Units A-D), distinguished on the basis of texture, modal mineralogy, geochemical composition, and field relations. All rhyolites are porphyritic, typically oxidized, and contain alkali feldspar + quartz +/- plagioclase in varying amounts and proportions. Two rhyolites contain textural evidence for pyroclastic origin; Unit A displays recrystallized, compressed pumice and Unit B contains well-preserved accretionary lapilli.

All rocks in the Mount Rogers area display A-type compositional affinity and are strongly ferroan (tholeiitic). Rhyolites of Unit A display the most chemical heterogeneity and are the most chemically evolved; Unit B rhyolites display the most unevolved compositions. Trends exhibited by Sr, Eu/Eu*, and Ga/Al suggest that feldspar fractionation influenced this evolution. Rhyolites at both the Razor Ridge and nearby Mt. Rogers centers appear to share broadly similar mineral assemblages, chemical compositions, and associated rock types. Each center also contains rhyolites of both effusive and pyroclastic origin.

Due to the probable age of the rocks, their regional setting, association with mafic dikes and immature sedimentary deposits, and overall A-type characteristics, the tectonic setting for this volcanism was likely extensional. Modern analogs for the Razor Ridge center include Yellowstone or the East African Rift, both of which display compositional bimodality but differ in eruptive mechanisms and products.