LATE-STAGE (CA. 745-740 MA) FELSIC VOLCANISM IN THE MOUNT ROGERS AREA, SW VA: IMPLICATIONS FOR ABORTED RIFTING AND ONSET OF GLACIATION

In southwest Virginia, along the border of North Carolina and Tennessee, is a relic of a Neoproterozoic failed rift-arm that formed on the (present-day) eastern Laurentian margin as the Rodinian supercontinent began to destabilize and break apart. A bimodal volcanic suite, the Mount Rogers Formation (MRF), erupted onto the rifted crust ~760 -750 million years ago. In recent field research, we have identified a younger, late-stage rhyolitic complex, erupted ~745-740 Ma. The Rocky Hollow complex (RHC), as we have informally designated this unit, is comprised of porphyritic rhyolite, crystal and lithic rhyolitic tuff, and volcaniclastic conglomerate, with associated coarse arkosic sandstone. Basaltic dikes of uncertain age intrude the complex. In this study, we describe in detail the lithologies and textures of the Rocky Hollow complex using hand-sample and optical petrography. In addition, we will compare whole-rock geochemistry of the RHC rhyolites to the main MRF rhyolites.

The Rocky Hollow complex appears to fill a transitional period between the volcanics of the MRF and the overlying glaciogenic Konnarock Formation, the latter of which has been interpreted as deposited during the 717-635 Ma Sturtian “Snowball Earth" glaciation. Apparent interlayering of the RHC with the lower part of the Konnarock Formation calls this interpretation into question.