SHRIMP U-PB AGES OF DETRITAL ZIRCONS FROM PARAGNEISS IN THE VIRGINIA BLUE RIDGE PROVIDE EVIDENCE FOR LATE MESOPROTEROZOIC-EARLY NEOPROTEROZOIC SEDIMENTATION

Distinctive paragneisses occur as nine isolated bodies (~0.25-5 km long) over a distance of 300 km, and are surrounded by 1.18-1.03 Ga orthogneisses that constitute most of the Mesoproterozoic basement of the Blue Ridge in central and northern Virginia. The poorly exposed, rusty-weathering graphitic rocks consist of alternating layers of quartz-plagioclase, garnet-biotite, and quartzite that locally contain as much as 25 percent modal almandine garnet (≤1 cm in diameter). The mineralogy and texture suggest that the protoliths were impure sandstone or graywacke. Previous researchers interpreted these layered metamorphic rocks to represent inliers of the older crust into which the orthogneisses intruded, despite the lack of cross-cutting relationships or xenoliths within the granitoids.

Detrital zircons (round, frosted, and pitted; wide range of colors; truncated internal oscillatory zoning) were dated by U-Pb isotopic analyses on the USGS/Stanford SHRIMP-RG. Results from four samples indicate that most grains have ages within the range 1.26-1.02 Ga; a small population has older ages of 1.8-1.6 Ga. In all four cases, the paragneisses yielded ages that overlap with, and were younger than, ages of the enclosing orthogneisses. These results suggest that the sedimentary protoliths of the paragneisses postdate and were unconformably deposited on Grenvillian basement. At least three outcrops of paragneiss are unconformably overlain by Neoproterozoic (~700 to 575 Ma) phyllite and graywacke. In the Paleozoic, the Neoproterozoic rocks were metamorphosed at lower greenschist-facies conditions and the paragneiss was retrograded to chlorite schist.

The Blue Ridge paragneisses are likely remnants of the missing rock record of the Mesoproterozoic and Neoproterozoic sedimentary rocks of Rodinia. Grenvillian crust was uplifted, eroded, and sediments were locally deposited, metamorphosed, and eroded, prior to deposition of Neoproterozoic rocks at about 760 to 575 Ma. Preserved Rodinian sedimentary rocks of similar age include the Las Viboras Group in Sonora, Mexico (younger than 1057 Ma), the Middle Run Formation in the subsurface of Ohio (younger than 1012 Ma), and the Torridon Group in Scotland (1060-994 Ma).