Age and origin of anorthosites, charnockites, and granulites in the Central Virginia Blue Ridge: Nd and Sr isotopic evidence
Rb-Sr isotopic data for anorthosites, charnockites, ferrodioritic to quartz monzonitic plutons, and high-grade gneisses of the Blue Ridge of central Virginia show evidence of post-emplacement metamorphism, but in some cases retain Grenville ages. The Pedlar River Charnockite Suite yields an isochron age of 1021 +/-36 Ma, (initial 87Sr/86Sr ratio of 0.7047 +/-6), which agrees with published U-Pb zircon ages. Five samples of that unit which contain Paleozoic mylonitic fabrics define a regression line of 683 Ma, interpreted as a mixing line with no age significance. Samples of the Roseland Anorthosite Complex show excessive scatter on a Rb-Sr evolution diagram probably due to Paleozoic (475 m.y.) metamorphism. Data from the ferrodioritic to quartz monzonitic plutons of the area yield an age of 1009 +/-26 Ma (inital ratio=0.7058 +/-4), which is in the range of the U-Pb zircon ages of 1000-1100 Ma. The Stage Road Layered Gneiss yields an age of 1147 +/-34 Ma (initial ratio of 0.7047 +/- 5). Sm-Nd data for the Pedlar River Charnockite Suite reflect a pre-Grenville age of 1489 +/-118 Ma (e{open}Nd=+6.7 +/-1.2). Data for the Roseland Anorthosite Complex and the ferrodioritic to quartz monzonitic plutons yield Grenville isochron ages of 1045 +/44 Ma (e{open}Nd=+1.0 +/-0.3) and 1027 +/-101 Ma (e{open}Nd=+1.4 +/-1.0), respectively. Two Roseland Anorthosite samples plot far above the isochron, demonstrating the effects of post-emplacement disturbance of Sm-Nd systematics, while mylonitized Pedlar River Charnockite Suite samples show no evidence of Sm-Nd redistribution. The disparity of the Sm-Nd age and other isotopic ages for the Pedlar River Charnockite Suite probably reflects a Sm-Nd "source" age, suggesting the presence of an older crust within this portion of the ca. 1 Ga old basement. ?? 1984 Springer-Verlag.