Abstract
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 $742 \pm 2 Ma (2\sigma)$. 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 $765 \pm 7 Ma (2\sigma)$ 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 $\epsilon_{Nd}$ values ranging from -2.0 to 2.8 and Mesoproterozoic $T_{DM}$ model ages of 112...
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