238U/206Pb SHRIMP, 40Ar/39Ar geochronologic, and geochemical data from the Central Virginia Piedmont

The Central Virginia Seismic Zone (CVSZ) is an area of historically increased earthquake activity that is of particular importance due to its proximity to many well-populated regions in the Virginia Piedmont. Notably, the epicentral region of the 2011 magnitude (M)5.8 earthquake in Mineral, Virginia falls within the CVSZ (Horton and Williams, 2012), making this region relevant not just historically, but also in more recent times. Thus, the U.S. Geological Survey is focusing on detailed bedrock mapping of the central Virginia Piedmont to better understand bedrock and fault geometry, fault history, and seismic risk potential. Though the central Virginia Piedmont has been mapped to some extent in previous studies (Hopkins, 1960; Hughes, 2014; Pavlides, 1981, 1989, 1990; Spears, 2011), this mapping effort builds upon existing maps and provides new geochronologic and geochemical data. These new data supplement mapping efforts by providing age constraints and provenance signatures on rock units within the central Virginia Piedmont that otherwise did not have these, which provides insight into the geologic history of the Virginia Piedmont as a whole.

Seven samples for zircon sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) U-Pb dating are presented here, five of which are samples of or granitic intrusions into the Chopawamsic Formation. The remaining two SHRIMP-RG samples are felsic gneiss, tonalite intruding the Elk Hill Complex, and biotite-muscovite migmatitic paragneiss. Zircon was extracted from each of these samples through conventional mineral separation techniques (rock crushing, milling, Wilfley table, Frantz electromagnetic separation, heavy liquids; expanded methods are available in this metadata, in the Processing Step section), then hand-picked under a binocular microscope using both incident and transmitted light. These zircon grains were mounted in epoxy, polished through to expose halfway through the zircon, then analyzed under both backscatter electron (BSE) and panchromatic cathodoluminescence (CL) modes in a Hitachi SU5000 field emission scanning electron microscope. These BSE and CL images were used to select zircon spot analysis locations for two SHRIMP-RG analytical sessions in 2016 and 2017. The same analytical setup was used in each session and consisted of 9 (2017) or 10 (2016) peak location measurements in 5 cycles on the USGS/Stanford single collector SHRIMP-RG using a spot diameter of ~20 micrometers and a depth of ~1 micrometer. Zircon standard R33 (419 Ma; Black et al., 2004) was mounted in epoxy along with the sample zircon to use for correcting 206Pb/238U ages for elemental fractionation. This R33 standard was run after every fourth unknown analysis. Additionally, the MADDER concentration standard (Barth and Wooden, 2010) was mounted with each set of unknowns for the purpose of standardizing U and Th concentrations.

Zircon data was reduced using SQUID 2 (Ludwig, 2009), then plotted using Isoplot 3.75 (Ludwig, 2012). Weighted averages were calculated for each sample using the 206Pb/238U ratio (or the 207Pb/206Pb ratio, in the case of ages older than 1,300 Ma). All weighted average ages are reported at two standard deviations from the mean. Each age was then plotted on a Wetherill or Tera-Wasserburg (Spencer et al., 2016) concordia plot to determine concordance.

Major and trace element geochemical analyses were conducted on four samples between 2017 and 2018 by Activation Laboratories, Ltd. for the USGS, using both fusion inductively coupled plasma (FUS-ICP) and fusion mass spectrometry (FUS-MS).

New 40Ar/39Ar geochronologic data is also presented for two samples. Muscovite was separated from these two samples using conventional mineral separation techniques (jaw crusher, disk mill, Wilfley table, Frantz electromagnetic separator) and analyzed by argon geochronology at the U.S. Geological Survey Bascom ARgon Dating (BARD) Laboratory, located within the Florence Bascom Geoscience Center in Reston, VA.

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