UNICOI LAVAS IN THE BLUE RIDGE OF SW VIRGINIA

Results from a field and laboratory program to describe, measure and sample nine (9) Unicoi outcrop sections from the central Virginia Blue Ridge southwest to the Tennessee border, expand on earlier works that tended to simplify the Unicoi igneous stratigraphy. The new regional stratigraphic study based on igneous and metamorphic petrology indicates that there are a variety of greenschist facies metamorphic rocks in three separate metamorphosed volcanic and volcanoclastic sequences discernable within the Unicoi along the Blue Ridge from central VA to NE TN. These metavolcanic rocks include a bimodal pyroclastic suite in the lower 100 meters of the Unicoi NE of Roanoke, a predominately felsic pyroclastic suite in the lower 100 meters of the Unicoi SW of Roanoke and a 100 m thick, laterally persistent, continental flood basalt sequence on the SW side of a major fracture zone SW of Roanoke. Relating the line of section to the abrupt change in the amount of transport on the Blue Ridge frontal thrust system SW of Roanoke (Henika, 2001) puts the depocenter in the most southwestern sections a significant distance to the SE of the Blue Ridge. This helps to explain Simpson’s conclusion that the Unicoi in far southwestern Virginia was deposited on attenuated continental crust. Petrographic study of key Unicoi outcrops supports the concept of Rankin (1970) that there were separate volcanic centers with different volcanic stratigraphy northeast and southwest of Roanoke. Newly reported detailed mapping and petrography also supports Rankin’s hypothesis that major concentrations of felsic volcanic rocks are largely restricted to sites at major bends in Appalachian structural trends, particularly one just southwest of Roanoke and one centered on the Grayson Highlands (Mount Rogers) along the Virginia-North Carolina border.