TEXTURAL ANALYSIS OF AMPHIBOLITES AND SCHIST FROM THE CENTRAL VIRGINIA SEISMIC ZONE: CONSTRAINING THE PALEOZOIC TECTONIC FRAMEWORK IN A SEISMICALLY ACTIVE REGION

The central Virginia Piedmont in the vicinity of the Central Virginia seismic zone (CVSZ) is composed of allochthonous terranes. In response to the 2011, M_w 5.8 Mineral, VA earthquake, several quadrangles have been mapped at 1:24000 scale in the epicentral area, and amphibolites and schists are being studied to address fundamental questions about these terranes such as timing of accretion and subsequent metamorphism and deformation. Samples were collected from garnet and amphibolite-bearing belts of rock that include, from W to E: (1) slope-rise metasediments of the Byrd Mill formation (informal name); (2) volcanic-arc rocks of the Middle Ordovician Chopawamsic Fm); (3) a synclinorium of Late Ordovician to Devonian (?) Quantico Fm. successor-basin rocks; and (4) amphibolite of the Ta River Metamorphic Suite that correlates with the Chopawamsic Fm. Optical petrography from schists in the Byrd Mill formation, where Alleghanian and Taconian metamorphism overlap, show that garnets overgrow an earlier foliation, but are wrapped by a later foliation defined by chlorite. In a quartz-mica-garnet schist from the Quantico Fm. the dominant foliation is defined by muscovite + biotite and wraps around garnet porphyroblasts with linear inclusion trails orthogonal to this foliation. Garnet-bearing amphibolites collected farther east near the Lakeside and Spotsylvania faults are moderately to strongly foliated and contain euhedral, inclusion-free garnets which overgrow the dominant foliation defined by amphibole.

Preliminary WDS element mapping of garnets from several samples show an increase in Mg with a decrease in Ca towards the rims, which is typically attributed to prograde growth along a clockwise P-T path. Euhedral garnets from the Byrd Mill formation also have narrow Ca-rich rims, perhaps related to thermal effects of the nearby Ellisville pluton or overprinting Alleghanian orogenesis. Continuing work will focus on attributing fabrics and assemblages to the Taconian versus Alleghanian orogenesis by a combination of garnet geothermobarometry and ⁴⁰Ar/³⁹Ar geochronology.