PROVENANCE OF CONGLOMERATES IN THE MOUNT ROGERS FORMATION, SW VA, AND IMPLICATIONS FOR THE NEOPROTEROZOIC GEOLOGIC HISTORY IN THE BLUE RIDGE

Polymict conglomerates in the Neoproterozoic lower Mount Rogers Formation (MRF), SW VA, are evidence of the topographic relief and dynamic processes associated with intracontinental rifting of the Rodinian supercontinent at ca. 750-760 Ma. Located in the Blue Ridge of SW VA, the MRF rests on Mesoproterozoic basement (~1.3-1.0 Ga) along a sheared unconformity. It is divided into two parts. The upper MRF is dominated by rhyolite flows and pyroclastic rocks, and is divided into three volcanic members. The lower MRF comprises bimodal volcanic rocks, including the Fees Rhyolite Member, interlayered with conglomerate and arkosic sandstone. Recent U-Pb zircon dates of MRF rhyolites suggest eruption of all units over a relatively short period of time (7-10 m.y.). Although the absolute age of siliciclastics in the lower MRF is uncertain, field relationships and the principle of superposition suggest that these rocks are slightly older than the rhyolites of the upper MRF, but younger than the ~753 Ma Fees Rhyolite.

The lower MRF conglomerates are dominated by cobble to boulder-sized clasts of granitoids and rhyolite. Granitoid clasts are assumed to derive from the underlying basement. Many of the rhyolite clasts are identical to the Fees Rhyolite in phenocryst assemblage (Qz + Afs + Pl) and texture, providing a maximum age of deposition for the conglomerate. However, field observations have shown distinct variation in clast mineralogy. Maroon rhyolite clasts containing aggregates of alkali feldspar and plagioclase, and only minor quartz, may derive from the upper MRF Buzzard Rock Member. Others such as foliated rhyolite with a greenish matrix are of uncertain affinity, and may represent older volcanics that have since eroded away. Clast samples were collected for geochronological and petrological analysis, in order to assess their provenance and address the following questions: Are the conglomerates younger than one or more of the upper MRF rhyolites? Were there multiple stages of conglomerate deposition during eruption of the MRF sequence? Finally, there is little geological record of deposits formed between ca. 1000-760 Ma in the southern Appalachians. If the geochronology indicates pre-MRF rhyolite clasts, these may provide a link to geological processes that occurred within this age gap.