DETAILED STRATIGRAPHY AND GEOCHEMISTRY OF LOWER MOUNT ROGERS FORMATION METAVOLCANIC UNITS EXPOSED ON ELK GARDEN RIDGE, VA

The Lower Mount Rogers Formation (LMRG) is described by Rankin (1989) as a late Precambrian sequence of intercalated basalts, volcanogenic sediments with minor rhyolite. We have chosen to examine the sequence of LMRG units exposed along Elk Garden Ridge, a high shoulder between the summits of Whitetop Mtn. and Mount Rogers in the Mount Rogers National Recreation Area in SW Virginia. This sequence represents an upthrust block of LMRG units enclosed by exposures of Whitetop and Wilburn Rhyolites, and offers the opportunity to examine a thick section representing early eruptive activity in the ancient Mount Rogers volcanic center in a single location.

In the field, progressive lithologic changes can be observed walking up-section along Elk Garden Ridge that are indicative of changes in lava volumes, lava compositions, and eruptive environments. A thick sequence of aphyric and amygduloidal pillow basalts comprise the lower part of the section, grading upward into sheet flow basalts, and massive basalts with distinctive 1-2 cm swallow-tail plagioclase phenocrysts. Up section, eruptive products transition into rhyolitic ignimbrites and ash and lapilli tuffs. Boulders of cobble conglomerates near the middle of the sequence suggest short periods of relative quiescence. Massive rhyolite flows (e.g., the Fees rhyolite of Rankin, 1989) are not evident in the section, suggesting local differences in eruptive products and eruptive styles across the outcrop area during the early history of this volcanic field.

Petrographically the rocks reflect the regional greenschist facies metamorphic conditions with chlorite and epidote as major phenocrysts in the basalts, strong oxidation of the rhyolites and tuffs (to a purple/red appearance) and unakite-like mineralization among basalt pillows Compositionally all of the rocks in the Elk Garden sequence are strongly enriched in alkali metals, with elevated Na₂O and K₂O contents, and high TiO₂ in the basalts, consistent with “anorogenic” magmatism associated with the earliest stages of Iapetan rifting.