TACONIAN OROGENESIS AND ALLEGHANIAN OVERPRINTING IN CENTRAL VIRGINIA PIEDMONT AS REVEALED BY NEW 40AR/39AR DATA

As part of a coordinated regional study of the epicentral region of the 2011 M5.8 Mineral, Virginia earthquake, new detailed geologic mapping and ⁴⁰Ar/³⁹Ar thermochronological data from amphibole and white mica in the central Virginia Piedmont have helped to delineate the relative roles of the Taconian and Alleghanian orogenies in this region. The NE-striking lithologic belts here record the accretion of the Ordovician Chopawamsic volcanic arc to Laurentia and intrusion of the Ordovician-Silurian Ellisville granodiorite as a stitching pluton across a 2-3 km wide, ultramafic-bearing suture zone that is interpreted as the pre- to early-metamorphic Chopawamsic fault zone (CFZ) and mélange zone III of the Mine Run Complex. Southeast of the Ellisville pluton and its southwest-extending neck, upper greenschist- to amphibolite-facies metavolcanic/volcaniclastic rocks of the Chopawamsic Formation yield amphibole plateau and correlation ages of ~300-310 Ma, indicative of cooling from a tectonothermal event (>500^oC) during the Alleghanian orogeny. Northwest of the Ellisville neck and southeast of the Mountain Run fault zone (MRFZ), an amphibolite within the Ordovician suture zone/CFZ yields an amphibole correlation age of ~450 Ma, whereas nonfoliated hornblende diorite of the Ordovician Green Springs pluton yields an amphibole plateau age of 451±2 Ma and a nearby phyllite yields a white mica plateau age of 445±2 Ma. These ages indicate that Taconian tectonic and intrusive fabrics in this area did not experience reheating above 500^oC (diorite) or 350^oC (phyllite) in the Alleghanian. White mica from phyllites within the MRFZ yield disturbed spectra suggesting that original Taconian fabrics experienced later thermal resetting and/or recrystallization. The boundary between thermally undisturbed Taconian rocks and those to the southeast that experienced an Alleghanian tectonothermal overprint appears to lie in the area of the Ellisville neck, which is cut by shear zones interpreted as Alleghanian in age, and the ultramafic-bearing suture zone/CFZ just to the northwest of the neck, which is the surface projection of a prominent pair of listric reflectors in the I-64 seismic line.