Southeastern Section - 50th Annual Meeting (April 5-6, 2001)

Paper No. 0
Presentation Time: 9:20 AM


TOLLO, Richard P., Department of Earth and Environmental Sciences, George Washington Univ, Washington, DC 20052 and ALEINIKOFF, John N., U.S. Geol Survey, Denver, CO 80225,

New geologic mapping, petrologic studies, and SHRIMP U-Pb dating of zircons have established an initial framework for the timing of Grenville-age tectonic events in the Virginia Blue Ridge from Thornton Gap south to Wolftown. The earliest recognized period of magmatism and deformation and the highest grades of metamorphism occurred prior to about 1,150 Ma and are represented by strongly foliated, metaluminous to mildly peraluminous, garnet + Opx-bearing gneiss that occurs only as inliers within younger, metaluminous, high-silica charnockite and farsundite. Emplacement of the charnockite and farsundite is dated at 1,149 ± 6 Ma by SHRIMP analyses of oscillatory-zoned cores in zircons. These rocks were intruded during a period of abundant granitic magmatism that appears to have ended by about 1,140 Ma. Inliers of geochemically evolved, strongly foliated, porphyroblastic granite gneiss that occur near Fletcher and contain zircons with magmatic cores dated at 1,073 ± 9 Ma indicate that granitic magmatism resumed following a nearly 70 m.y. hiatus; truncation of gneissic foliation by younger charnockite indicates that associated deformation terminated by about 1,050 Ma. The youngest recognized period of magmatism involved a possibly synchronous bimodal group of plutons including peraluminous biotite ± garnet-bearing, leucocratic granitoids dated at 1,065 ± 6 Ma (Old Rag Granite) and strongly metaluminous, low-silica farsundite and quartz jotunite dated at 1,051 ± 7 Ma. Temporal calibration of these magmatic and deformation events indicates that Grenville-age orogenesis in the central Virginia Blue Ridge involved an extended interval characterized by episodic emplacement of compositionally distinct, largely granitic magmas punctuated by at least two periods of regional deformation. These data underscore the episodic nature of orogenic processes associated with amalgamation of Rodinia and establish an initial basis for regional correlation with Mesoproterozoic tectonic events that occurred elsewhere along the Laurentian margin.