PRELIMINARY RESULTS FROM FIELD MAPPING AND PETROLOGIC ANALYSIS OF GRENVILLE-AGE BASEMENT ROCKS, WASHINGTON AND WOODVILLE 7.5'-QUADRANGLES, BLUE RIDGE PROVINCE, VIRGINIA

Basement within the core of the Blue Ridge anticlinorium in northern Virginia includes a diverse array of igneous and meta-igneous rocks that preserve evidence of an extended period of Grenvillian orogenesis. Nevertheless, improvements in understanding of the geologic history of the Blue Ridge have traditionally lagged because of a lack of detailed bedrock mapping. In recent years, however, mapping at 1:24,000 scale within and around Shenandoah National Park has begun to provide a sound framework for geological modeling. The area for this study was previously mapped only in reconnaissance at 1:100,000 scale and thus represents a significant gap in our understanding of local basement geology. The area is lithologically and structurally complex containing multiple lithologies of different ages that are likely to collectively span the entire known period of Grenvillian magmatic activity in the Blue Ridge. Field mapping, in combination with petrologic and geochemical studies, are being undertaken in preparation for geochronologic investigations in order to evaluate the role of these rocks in Blue Ridge orogenesis. Rocks in the quadrangle include a diverse array of lithologies including granitic gneisses, charnockites, and leucogranitoids. Rocks can be divided on the basis of fabrics into two groups; (1) gneissic and foliated rocks that predate deformation, and (2) weakly foliated to nonfoliated rocks that postdate deformation. Within the oldest group of rocks, field relations indicate that gneisses were intruded by foliated leucogranitoids forming contact zones marked by xenoliths of the former within the latter. Because the foliated leucogranitoids are similar to the oldest known rocks in the northern Blue Ridge, the invaded gneisses are likely to represent heretofore unrecognized ancient crust. Orthopyroxene + amphibole charnockites include both foliated and nonfoliated types that postdate the foliated leucogranitoids. Youngest rocks include weakly foliated to nonfoliated leucogranitoids (with or without garnet) that correlate regionally to post- tectonic granites that are about 1060 Ma in age. The exclusively granitic composition of rocks in the quadrangle is indicative of the northern Blue Ridge and reflects the dominant role of felsic magmatism in local construction of the Grenville orogen.