ASSESSING VOLCANISM IN THE MOUNT ROGERS FORMATION, SW VA: STRATIGRAPHIC PLACEMENT OF THE BEARPEN RHYOLITE

The Mount Rogers Formation (MRF) in southwest Virginia represents intra-continental rifting along the eastern Laurentian margin of Rodinia that occurred ~760-750 Ma. This event preceded the opening of the Iapetus Ocean at ~ 565 Ma. The MRF noncomformably rests atop Grenville basement (1.3 – 1.0 Ga) and is divided into two parts. The upper MRF consists of rhyolites and ash flow sheets and is divided into three units: the older Buzzard Rock Rhyolite, the middle Whitetop Rhyolite, and the younger pyroclastic Wilburn Rhyolite. The lower MRF consists of interlayered bimodal volcanics, including the Fees Rhyolite, along with clastic sedimentary alluvial fan deposits. Based on our recent mapping we have defined a previously unrecognized unit that we call the Bearpen rhyolite for its location on Bearpen Ridge. Compared to the other units in the upper and lower MRF, the Bearpen contains abundant large K-feldspar phenocrysts that are in concentrated clusters. The matrix of the unit is dark maroon to gray and flows around the phenocrysts. Quartz and minor plagioclase also occur as phenocrysts within the unit. Fiamme and possible flow banding occur in some outcrops. This project is focused on mapping the contacts of the Bearpen rhyolite with the other units in the MRF and describing its petrographic characteristics, in order to determine its placement in the overall stratigraphy.

Petrographically, the Bearpen rhyolite strongly resembles the Wilburn Rhyolite. However, our mapping shows that stratigraphically, the Bearpen rhyolite occurs between the Fees Rhyolite in the lower MRF and the Whitetop Rhyolite in the upper MRF. As well, our U-Pb zircon age date of 756 Ma +/- 3.1 Ma for the Bearpen rhyolite does not overlap with the 749 Ma +/- 3 Ma age of the Wilburn (Tollo et al., 2012). This new information suggests the Bearpen is stratigraphically within the lower MRF but formed from processes similar to those that created the ash flow sheets of the Wilburn rhyolite.