AGE AND GEOCHEMICAL CHARACTERISTICS OF BIMODAL MAGMATISM IN THE NEOPROTEROZOIC GRANDFATHER MOUNTAIN RIFT BASIN

The Grandfather Mountain Formation (GMF) is one of several remnants of Neoproterozoic intracratonic rifting and associated magmatism in the southern Appalachian Blue Ridge province. New U-Pb zircon ages and iso topic data are presented to constrain the timing of rift basin formati on and the source of concurrent magmatism. U-Pb data for three concord ant zircon fractions from the stratigraphically highest metarhyolite w ithin the GMF yield an age of 742 +/- 2 Ma (2 sigma). This age is inte rpreted as the time of crystallization, dating an episode of volcanism and sedimentation during the latter stages of rift basin development. The GMF rests nonconformably on Mesoproterozoic basement and Brown Mo untain metagranite. A U-Pb age of 765 +/- 7 Ma (2 sigma) for Brown Mou ntain metagranite, based on five zircon fractions, places a maximum li mit for the onset of rift basin formation. Volcanic rocks of the GMF h ave initial epsilon(Nd) values ranging from -2.0 to 2.8 and Mesoproter ozoic T-DM model ages of 1124 to 1392 Ma. Nd evolution trends for fels ic volcanic and plutonic rocks of the Crossnore Complex in North Carol ina and southern Virginia are similar to those for samples of Mesoprot erozoic basement rock. Pb isotopic data for felsic volcanic and pluton ic rocks indicate that Mesoproterozoic crust is a possible source of t he magmatism, as Pb-206/Pb-204-Pb-207/Pb-204 values plot along a simil ar crustal growth curve. The GMF belongs to an early stage of abortive , Neoproterozoic rifting (c. 750 Ma) along the eastern margin of Laure ntia. Early Paleozoic passive margin development, associated with the development of Iapetus Ocean, cannot be directly related to Neoprotero zoic volcanism and rifting in the Grandfather Mountain basin. We sugge st that the c. 750 Ma rift sequences preserved along both margins of L aurentia are similarly linked to the breakup of Rodinia and the breako ut of Laurentia as a relatively discrete land mass.