TESTING AGE RELATIONSHIPS IN THE MOUNT ROGERS FORMATION, SW VIRGINIA, THROUGH PROVENANCE ANALYSIS OF CONGLOMERATES

The ~750-670 Ma Mount Rogers Formation (MRF) in SW Virginia was formed by intracontinental rifting during initial breakup of Rodinia. The MRF, which unconformably overlies 1.0-1.3 Ga intrusive rocks formed during the assembly of Rodinia, is divided into two units: the upper MRF, which is dominated by rhyolites, and the lower MRF, which consists of bimodal volcanic rocks intermixed with coarse-grained sedimentary rocks (arkose and conglomerate). Although the absolute age of the lower MRF is uncertain, field relationships and the principle of superposition have led researchers to interpret these rocks as older than the upper MRF. New isotopic ages reported by Tollo et al. (2012), however, show that the age of lower MRF rhyolite overlaps with ages of rhyolites in the upper MRF, and they suggested the volcanism occurred over a short period of time. This leads us to reconsider the previous interpretations of the age relationships between volcanic and clastic sedimentary rocks in the lower MRF.

Conglomerates in the lower MRF are dominated by pebble- to boulder-sized clasts of rhyolite and granitic rocks, which have been assumed to represent the underlying lower MRF rhyolite and basement, respectively. While the rhyolite clasts are similar to the underlying rhyolite of the lower MRF, recent petrographic work suggests that multiple rhyolite types are present in the clast assemblage. To assess the variation in clast types, we conducted detailed clast counts of six conglomerate outcrops from various areas in the lower MRF. These outcrops were similar in the fact that the majority of the composition was made up of rhyolite and granitoid clasts. However the rhyolite clasts vary in matrix color and phenocryst assemblage. The most prominent rhyolite type consists of K-feldspar and quartz phenocrysts in a dark gray matrix. Other less abundant types have red, maroon, or green matrix, and some contain K-feldspar and plagioclase phenocrysts with lesser quartz. These observations suggest that there is more than one source of rhyolite clasts. Pink granitoid clasts were found throughout all of the conglomerates counted, whereas white granitoid was much less abundant. A major difference between the conglomerate outcrops appears to be in the abundance of minor clast types such as sandstone, black slate, and vein quartz.