MESOPROTEROZOIC TO LATE PALEOZOIC GEOLOGY ALONG THE BLUE RIDGE PARKWAY FROM JAMES RIVER TO BEARWALLOW GAP, CENTRAL VIRGINIA

Detailed geologic mapping and preliminary SHRIMP U-Pb zircon analyses of Mesoproterozoic rocks exposed along a ~40 km transect of the Blue Ridge Parkway reveal text-book geologic relationships that provide new insight into the complex geologic history of the Grenvillian orogeny in this region and are useful for educational outreach.

In this central part of the Shenandoah massif, Mesoproterozoic basement rocks consist of an older suite (1200-1142 Ma) of strongly foliated orthopyroxene (opx)-bearing orthogneisses and a younger suite (1060-1004 Ma) of less deformed mostly opx-bearing leuco-granitoids. Garnetiferous leucogneiss (~1200 Ma) and lineated granitoid gneiss (~1155 Ma) are cut by undeformed dikes of porphyritic metagranitoid (~1054 Ma) that have fine-grained chilled margins. These dikes are likely related to a larger pluton of megacrystic metagranitoid (~1060 Ma), which contain xenoliths of foliated dioritic gneiss (~1142 Ma). The megacrystic metagranitoid is cut by fine-grained hypidiomorphic opx-bearing metagranitoid (~1004 Ma). Thin dikes of pegmatitic, locally tourmaline-bearing quartz-feldspar leucogranitoid cut the ~1004 Ma metagranitoid, but a xenolith of finer-grained leucogranitoid (~1023 Ma) suggests there are multiple leucogranitoid-forming magmatic events.

West-trending, Late Mesoproterozoic (?) garnet-graphite-quartz-feldspar paragneiss folded within older orthogneiss is cut by north-striking vertical pegmatites of unknown age. These rocks and structures are overlain by bedded arkose above a high-angle unconformity of late Neoproterozoic-Cambrian age. Amphibolite-facies foliation in megacrystic metagranite (~1060 Ma) is openly folded and transected by northeast-striking greenschist-facies cleavage associated with the late Paleozoic Alleghanian orogeny.