NEW SHRIMP U-PB ZIRCON DATA FROM THE CENTRAL VIRGINIA PIEDMONT

New SHRIMP U-Pb zircon ages reveal details of the Paleozoic amalgamation and tectonic genesis of the central Virginia Piedmont, and contributes to our knowledge of the geologic framework of the Central VA Seismic Zone. From west to east, the epicentral area of the 2011 Mineral, VA earthquake consists of: (1) early Paleozoic Potomac terrane (PT) rocks west of the Chopawamsic fault; (2) Middle Ordovician Chopawamsic Fm (CF) volcanic arc rocks, stitched together with the PT across the fault by the Silurian Ellisville Granodiorite (EG); (3) Quantico Fm (QF) successor-basin deposits; (4) metagranitoids and amphibolite east of the QF, assigned to the Ta River Metamorphic Suite (TRMS) and correlated regionally with the CF; and (5) metagranitoid and amphibolite of the Elk Hill Complex, which is located between the TRMS and the Lakeside fault and the Goochland terrane rocks and the Spotsylvania high-strain zone to the east.

Felsic gneiss interpreted as a meta-volcanic rock at the base of the QF yielded a zircon crystallization age of 447 ± 2 Ma, statistically identical to a ~448 Ma age reported elsewhere for QF felsic gneiss in northern VA. Two samples of metagranitoids in the TRMS yielded ages of 442 ± 4 Ma and 457 ± 3 Ma with Precambrian inheritance. The ~442 Ma metagranitoid is similar in age to the EG. The 457 Ma metagranitoid overlaps in age with the metagranitoid from the Elk Hill Complex, which yielded an age of 453 ± 3.5 Ma with evidence of post 350 Ma disturbance in the latter.

These data reveal important implications for the Central VA Piedmont Paleozoic lithotectonic framework: (1) felsic volcanic gneiss is stratigraphically below <427 Ma QF quartzite; which yield a detrital zircon age population of ~450 Ma (Holm-Denoma et al. 2016), suggesting non-Laurentian provenance from the east; (2) metagranitoid in the TRMS, previously thought correlative with the ~330 Ma Falmouth Intrusive Suite, has at least two older ages, both younger than CF rocks (470–465 Ma) west of the QF, suggesting that the QF may straddle a Paleozoic fault or unconformity; (3) the mineral assemblage and age of the ~441 Ma metagranitoid east of the QF indicate that it may be an eastern facies of a more widespread EG complex in the Central VA Piedmont; and 4) at its northernmost end, the ductile Paleozoic Lakeside fault separates hornblende-bearing metagranitoids of similar age.