PETROLOGIC AND TEMPORAL CHARACTERISTICS OF GRENVILLE-AGE OROGENESIS, BLUE RIDGE PROVINCE, VIRGINIA

New data obtained through detailed geologic mapping, petrologic investigations, and SHRIMP and TIMS U-Pb isotopic analyses of zircon and monazite indicate that basement rocks of the Blue Ridge province in northern Virginia preserve evidence of episodic magmatism and deformation spanning nearly 150 m.y. Field relations, mineral assemblages, textures, and geochemical compositions indicate that most of the rocks have igneous protoliths. Granitic gneisses and high-silica charnockites dated at 1,155-1,145 Ma collectively define the earliest episode of regional magmatism. This activity was followed by a 70 m.y. period of tectonic quiescence during which only the Marshall Metagranite of about 1,110 Ma is known to have been emplaced. Widespread magmatic activity resumed at 1,080-1,070 Ma with emplacement of granitic plutons that were deformed prior to intrusion of granite, leucocratic granite, and low-silica charnockite at 1,065-1,050 Ma. Isotopic analyses of monazite and overgrowths on concentrically zoned zircon indicate a final episode of thermal disturbance at about 1,020-1,000 Ma; however, rocks of this age have not been recognized. Most rocks exhibit compositional features similar to A-type granites, including high Zr and Ga/Al; all rocks have high FeO/(FeO + MgO) characteristic of granites and charnockites of tholeiitic affinity. The occurrence of strongly metaluminous, compositionally unevolved charnockitic magmas near the end of the igneous cycle suggests that episodic replenishment of melts occurred during the orogenic period. The three episodes of magmatism recognized in the Blue Ridge correspond temporally to three pulses of magmatic activity that predated and were synchronous with the Ottawan orogeny in the Adirondacks. Moreover, the 1,070-1,065 Ma episode of ductile deformation observed in Virginia occurred within the period of regional Ottawan contraction documented in the Adirondacks. Such regional correlation of magmatic and deformational events, based on field relations and precise U-Pb geochronology, underscores the widespread nature of orogenic processes that occurred during the culminating period of Grenvillian tectonism and illustrates the possibility of establishing a regional chronology of Grenville-age orogenesis throughout eastern North America.