Mesoproterozoic magmatism and deformation in the northern Blue Ridge, Virginia and Maryland: Application of SHRIMP U-Pb geochronology and integrated field studies in the definition of Grenvillian tectonic history

Geologic mapping of Mesoproterozoic lithologies and foliations, and sensitive high-resolution ion microprobe (SHRIMP) U-Pb crystallization ages of 43 samples of orthogneisses and metagranitoids from the northern Blue Ridge establish new subdivisions: group 1 (1183-1144 Ma), group 2 (1143-1111 Ma), and group 3 (1078-1028 Ma). Protoliths of group 1 were metamorphosed at amphibolite- to granulite-facies conditions and strongly deformed between ca. 1153 and ca. 1144 Ma. Metagranitoids of groups 2 and 3 were emplaced continually for another 115 m.y. and display only local effects of diminishing deformation events. Ages of zircon overgrowths overlap temporally with igneous crystallization ages of group 3, but continued until ca. 960 Ma.The geochronologic sequence defined by rocks in the allochthonous Blue Ridge corresponds temporally with events documented elsewhere in eastern North America. Here, magmatism and the main deformation event occurred at the same time as the Shawinigan orogeny; magmatism with lesser deformation occurred at the same time as the Ottawan orogenic phase of the Grenvillian orogeny. The Neoproterozoic Robert son River batholith separates distinct orthogneisses and metagranitoids geographically. This plutonic body is interpreted to have intruded a Mesoproterozoic boundary that was reactivated during extension from 760 to 700 Ma. Despite the overprinting effects of Neoproterozoic extensional and Paleozoic contractional deformation events, which characteristically impart northeasterly trends to structural features, the majority of the group 1 rocks display prominent, high-grade foliations that trend to the northwest, reflecting the typical Mesoproterozoic tectonic grain observed in massifs from New Jersey to Vermont, as well as in the Adirondacks.