Geochronologic age constraints on tectonostratigraphic units of the central Virginia Piedmont, USA

New geologic mapping coupled with uranium-lead (U-Pb) zircon geochronology (sensitive high-resolution ion microprobe-reverse geometry [SHRIMP-RG] and laser ablation-inductively coupled plasma-mass spectrometry [LA-ICP-MS]) analyses of 10 samples, provides new constraints on the tectonostratigraphic framework of the central Virginia Piedmont. Detrital zircon analysis confirms that the Silurian-Devonian Quantico Formation is a postorogenic successor basin, with zircons derived primarily from Ordovician Chopawamsic Formation volcanic rocks. Detrital zircons from strata of the Long Island syncline, previously mapped as a separate successor basin, have a peri-Gondwanan component distinct from Laurentian-sourced rocks of the Potomac terrane to the west. Volcanism of the Chopawamsic Formation spanned at least 14 million years during the Ordovician. The Chopawamsic Formation contains sheet-like Late OrdovicianSilurian granodioritic and tonalitic intrusions that were once mapped as Carboniferous. Biotite-muscovite migmatitic paragneiss, which borders the Chopawamsic Formation on its southeast side and also occurs east of the Lakeside fault, preserves evidence of Silurian deformation and metamorphism, with a Carboniferous (Alleghanian) overprint. Limited SHRIMP-RG analysis of detrital zircons from this paragneiss yields a Laurentian (Mesoproterozoic) signature, which suggests that the structurally concordant contact between volcanic rocks of the Chopawamsic Formation and paragneiss is either a pre-Alleghanian fault or an unconformity. Introduction Precision uranium-lead (U-Pb) zircon geochronology is a critical tool in conjunction with detailed geologic mapping to provide ages of rock units and their mineral constituents to bracket regional tectonic events. This is particularly true in the Piedmont Province of central Virginia (figs. 1 and 2), where metamorphic rocks in the Appalachian hinterland record tectonic events that are separated by just a few million years (for example, Sinha and others, 2012; Hughes and others, 2013). The regional structure is of special interest because of its relation to the historically active Central Virginia Seismic Zone (CVSZ), including the epicentral region of the 2011 magnitude (M)5.8 Mineral, Virginia, earthquake (Horton and Williams, 2012). To better understand the regional framework for seismic risk assessment, cooperative detailed geologic mapping across nine 7.5-minute quadrangles in the central Virginia Piedmont was conducted by the U.S. Geological Survey and Virginia Department of Mines, Minerals and Energy, Division of Geology and Mineral Resources following the 2011 Mineral, Virginia, earthquake. This study builds on earlier mapping by Pavlides (1981, 1989, 1990) in the northern Virginia Piedmont, Spears (2011) along the James River to the south, and Hopkins (1960) and Hughes (2014) in the epicentral area of the 2011 earthquake. We used both sensitive high-resolution ion microprobereverse geometry (SHRIMP-RG) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology to help answer three outstanding questions in this geologically complex region. (1) Can stratigraphic succession be established within the Ordovician Chopawamsic Formation? (2) When was the onset of the postorogenic successor basin development above or near the volcanic arc represented by the Chopawamsic Formation? (3) What was the timing of deformation and metamorphism in the easternmost 1U.S. Geological Survey. 2Virginia Department of Mines, Minerals and Energy. 3University of Alaska-Fairbanks. 4Center for Sustainable Groundwater. 2 Geochronologic Age Constraints on Tectonostratigraphic Units of the Central Virginia Piedmont, USA Sm ith Ri ve r a llo ch tho n Blu e R idg e Eas tern M ilto n b elt CT