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 isotopic data are presented to constrain the timing of rift basin formation and the source of concurrent magmatism. U-Pb data for three concordant zircon fractions from the stratigraphically highest metarhyolite within the GMF yield an age of 742 ± 2 Ma (2σ). This age is interpreted 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 Mountain metagranite. A U-Pb age of 765 ± 7 Ma (2σ) for Brown Mountain metagranite, based on five zircon fractions, places a maximum limit for the onset of rift basin formation. Volcanic rocks of the GMF have initial εNd values ranging from -2.0 to 2.8 and Mesoproterozoic TDM model ages of 1124 to 1392 Ma. Nd evolution trends for felsic volcanic and plutonic rocks of the Crossnore Complex in North Carolina and southern Virginia are similar to those for samples of Mesoproterozoic basement rock. Pb isotopic data for felsic volcanic and plutonic rocks indicate that Mesoproterozoic crust is a possible source of the magmatism, as ²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb values plot along a similar crustal growth curve. The GMF belongs to an early stage of abortive, Neoproterozoic rifting (c. 750 Ma) along the eastern margin of Laurentia. Early Paleozoic passive margin development, associated with the development of Iapetus Ocean, cannot be directly related to Neoproterozoic volcanism and rifting in the Grandfather Mountain basin. We suggest that the 750 Ma rift sequences preserved along both margins of Laurentia are similarly linked to the breakup of Rodinia and the breakout of Laurentia as a relatively discrete land mass.