PETROGRAPHIC ANALYSIS OF METASANDSTONES IN THE RIFT-RELATED MOUNT ROGERS FORMATION, SW VIRGINIA

The Mount Rogers Formation (MRF) in southwest Virginia documents volcanism and sedimentation during an initial, aborted rifting of the eastern Laurentian margin of Rodinia ~ 760 Ma. The lower MRF rests unconformably on the 1.1-1.2 Ga Grenville basement, and is overlain by 758 (+/- 12) Ma rhyolites of the upper MRF. Interbedded bimodal volcanic and clastic sedimentary rocks, now metamorphosed to low grade, comprise the lithology of the lower MRF. We interpret the metasedimentary rocks as having been deposited as alluvial fans in down-dropped basins associated with the aborted rift. Metasandstones of the lower MRF are poorly sorted and contain quartz, feldspars, and abundant lithic clasts, most derived from the underlying basement granitoids or the intraformational volcanic rocks. Our petrographic analysis of the metasandstones includes point-counting and plotting the results on QFL and QmFLt diagrams. The results show that the rocks plot in the transitional arc/recycled orogen fields rather than the expected basement uplift field. We believe that this discrepancy is due to the local derivation and high degree of immaturity of the sandstones, thus increasing the lithic component plotted on the ternary diagrams. Rhyolite lithic clasts are particularly affected by subsequent foliation development and shearing deformation that occurred during low grade metamorphism, making them difficult to distinguish from the matrix. We are currently working to refine our methodology for use with the more deformed samples.