PETROGENESIS OF AN ASH-FLOW BRECCIA, MOUNT ROGERS, VIRGINIA

The Mount Rogers area of the Blue Ridge province in SW Virginia contains significant accumulations of rhyolite extruded in a late Neoproterozoic extensional tectonic setting. Stratigraphy of the ca. 750 Ma Mount Rogers volcanic center includes four rhyolite units representing early effusive flows followed by ash-flow deposits of the Wilburn Rhyolite. This study examines a distinctive, unsorted breccia that crops out as a lens at the base of the Wilburn directly above the underlying Whitetop Rhyolite. Clasts within the polymictic breccia consist predominantly of rhyolite of Whitetop affinity. Clast sizes range from <1 to about 30 cm. The spherulitic, tuffaceous matrix contains delicate features such as pumice and recycled pyroclasts, and has a phenocryst assemblage identical to that of the Wilburn Rhyolite. These features suggest that the breccia was deposited by a pyroclastic flow during initial outpourings of the Wilburn Rhyolite. Granophyre in the matrix suggests rapid cooling of the source magma chamber. Alternative modes of deposition are inconsistent with observed characteristics of the breccia. A pyroclastic surge origin is unlikely, as the breccia lacks internal stratification. Deposition or reworking by streams or lahars is ruled out because such processes are inconsistent with the presence of fragile components in the matrix. The dominantly matrix-supported character of the breccia is inconsistent with origin by dome or wall collapse. Our results suggest that initial pyroclastic fountaining resulted in localized pyroclastic flows that were likely confined to topographic indentations such as paleovalleys. The Wilburn ash-flow sheet can be divided into at least three distinct horizons (from oldest to youngest): (1) basal lithic breccia, (2) pumice-rich tuff, and (3) moderately to highly welded pumiceous tuff. Fiamme in the highly welded uppermost lithofacies likely formed within the central portion of the thick Wilburn sheet; the higher sections of the sheet were removed by erosion. This model suggests that the high-silica, pyroclastic Wilburn Rhyolite is comparable to culminating deposits at other encratonic silicic centers, such as Yellowstone, and is a result of draining of the uppermost portions of a partially evacuated magma reservoir.