PETROLOGY AND U-PB GEOCHRONOLOGY OF THE ROBERTSON RIVER IGNEOUS SUITE, BLUE RIDGE PROVINCE, VIRGINIA - EVIDENCE FOR MULTISTAGE MAGMATISM ASSOCIATED WITH AN EARLY EPISODE OF LAURENTIAN RIFTING

The Late Neoproterozoic (735-702 Ma) Robertson River Igneous Suite inc ludes at least eight plutons ranging in composition from syenogranite to alkali feldspar granite to alkali feldspar syenite. These plutons i ntruded Mesoproterozoic (1.2-1.0 Ga) gneissic basement of the Blue Rid ge anticlinorium in northern and central Virginia during an early epis ode of Laurentian rifting. Robertson River plutons range In compositio n from metaluminous to peralkaline and, relative to other granite type s, exhibit compositional characteristics of A-type granitoids includin g (1) marked enrichment in Nb, Zr, Y, REE (except Eu), and Ga, (2) hig h Ga/Al and FeO(total)/MgO, and (3) depletion of Ba an and Sr. High Ga /Al ratios are particularly diagnostic of the suite and serve as an ef fective discriminant between originally metaluminous and peralkaline b ulk compositions, providing a useful proxy for widely used indicators based on major elements that are prone to remobilization. U-Pb isotopi c analyses of zircons indicate that the suite was emplaced in two puls es, occurring at 735 to 722 and 706 to 702 Ma. Metaluminous magmas wer e emplaced during both pulses, formed most of the main batholith, and fractionated as independent, time-correlative groups. Peralkaline magm as were emplaced only during the final pulse, formed a volcanic center that erupted unknown quantities of rhyolite, and experienced a style of fractionation similar to the metaluminous types. Differences in Ce/ Nb, Y/Nb, and Yb/Ta ratios suggest that the metaluminous and peralkali ne magmas were derived from different sources. The Robertson River Ign eous Suite is part of a regional group of Late Neoproterozoic (760-700 Ma) plutons including at least 20 other A-type granitoid bodies expos ed throughout the Laurentian terrane of Virginia and northwestern Nort h Carolina. Like the Robertson River, most of the other granitoids are metaluminous in composition, typically form multi-intrusive, elongate plutons, and are not geographically constrained to the vicinity of ma jor Neoproterozoic volcanic centers. The Late Neoproterozoic plutons o f the Laurentian Appalachians are similar in lithology, composition, a nd style of emplacement to Miocene granitoids that occur on the southe rn flanks of the present Red Sea rift and that are considered a younge r analog of the Appalachian plutons. However, the Red Sea granitoids w ere emplaced within a few million years of the initial development of oceanic crust whereas the Appalachian plutons appear largely to have p re-dated development of the Iapetus Ocean by about 130 my.