PETROLOGY AND GEOCHRONOLOGY OF GRENVILLE-AGE MAGMATISM, BLUE RIDGE PROVINCE, NORTHERN VIRGINIA

New mapping and results from petrologic and geochronologic studies indicate that Mesoproterozoic basement rocks in the core of the northern Blue Ridge anticlinorium are dominated by compositionally diverse felsic gniesses and granitoids, including abundant opx-bearing charnockites. Collectively, these rocks preserve an extended record of igneous and metamorphic processes associated with Grenvillian orogenesis. Igneous crystallization ages obtained by U-Pb SHRIMP isotopic analyses of zircon and monazite indicate that protolithic magmas were emplaced at about 1.18–1.16, 1.12–1.08, and 1.06–1.05 Ga. Field relations indicate that a major episode of deformation occurred at 1078–1064 Ma; ages of overgrowths on zoned zircons suggest metamorphic recrystallization at about 1020–1000 Ma. Although leucocratic granitoids and charnockites were emplaced during each interval, the latter are most abundant during the first magmatic period. Contemporaneous charnockite and leucogranite emplaced during the final interval were derived from different sources and are not petrologically related. Only rocks of > 1060 Ma age are foliated to gneissic. All rocks display tholeiitic affinity and trace-element concentrations indicative of derivation from heterogeneous sources. Post-orogenic (1050 ± 8 Ma) charnockite exhibits A-type geochemical affinity; all other meta-igneous rocks are compositionally transitional between A-types and fractionated I-types. The predominance of granitic basement rocks in the northern Blue Ridge is similar to features documented in other North American Grenville-age terranes. However, the apparent lack of rocks of calc-alkaline affinity and >1200 Ma age contrasts with the magmatic record of the Adirondacks, Green Mountains, and possibly the New Jersey Highlands. Blue Ridge basement represents composite orogenic crust developed through repeated emplacement of crustally derived magmas that both pre- and post-dated the main episode of local Mesoproterozoic compression.