ORIGIN OF SMALL GRANITOID BODIES IN THE WESTERN PIEDMONT OF CENTRAL VIRGINIA

The foci of this study are the nature and relationship between three small (<15km²) metamorphosed ca. 470 Ma granitoid bodies: the Plentiful Creek trondhjemite, the Paytes granite, and the Locust Grove tonalite. These bodies are surrounded by the metasedimentary Potomac terrane of the western Appalachian Piedmont of central Virginia and have been previously interpreted as olistoliths derived from the Chopawamsic terrane (Pavlides, 1989). However, recent work shows that a detrital origin of these bodies is highly unlikely because of the lack of any ca. 470 Ma detrital zircons discovered during extensive sampling of the metasedimentary rocks of the Potomac terrane (Hughes et al., 2014). The goal of this study was to scrutinize and interpret geochemical data from samples of the three bodies in order to determine if they share a magmatic origin.

REE (rare earth element) data illustrate a similarity between the Plentiful Creek trondhjemite and the Paytes granite, which are similar to a larger pluton (Goldvein pluton) in the same unit of the Potomac terrane. Samples from the Locust Grove tonalite have generally higher light REE concentrations and generally lower heavy REE concentrations than the two other bodies.

Hafnium data from individual zircons help most to elucidate a shared or dissimilar petrogenesis for the bodies. The Paytes granite yields a large range in εHf values (n=19 analyses) from -27.5 to 0. For all (n=22) but two zircon analyses of the Locust Grove tonalite, there is a very narrow range in εHf values from -8 to -6.5; two anomalous analyses in the Locust Grove tonalite yield εHf values of -27 and -11.5 and may be inherited xenocrysts. For comparison, 17 of 22 zircon analyses from a ca. 470 Ma pluton (Richland Run pluton) in the Chopawamsic terrane yield εHf values between -1.7 and 1.7, a range dissimilar from the granitoid bodies.

In light of these data, we interpret that the Plentiful Creek trondjhemite and Paytes granite— which are within 3 km of each other and are geochemically similar to other plutons in the Potomac terrane—originated from a common magmatic source and were introduced intrusively to the Potomac terrane. The Locust Grove tonalite is unlike other regional plutonic rocks and may have been delivered to its current position tectonically along poorly understood faults within the Potomac terrane.