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. 
References:
Alexander Jr., E.C., Mickelson, G.M., Lanphere, M.A., 1978. MMhb-1: a new 40Ar – 39Ar dating standard. 4th International Conference on Geochronology, Cosmochronology and Isotope Geology. U.S. Geological Survey Open File Report 78-701, pp. 6 – 8.
Barth, A.P., and Wooden, J.L., 2010, Coupled elemental and isotopic analyses of polygenetic zircons from granitic rocks by ion microprobe, with implications for melt evolution and the sources of granitic magmas: Chemical Geology, v. 277, no. 1-2, p. 149–159.
Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mudil, R., Campbell, I.H., Korsch, R.J., Williams, I.S., and Foudoulis, C., 2004, Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards: Chemical Geology, v. 205, no. 1-2, p. 115–140.
Carter, M.W., McAleer, R.J., Holm-Denoma C.S., Spears, D. B., Regan, S.P., Burton, W. C., and Evans, N.H., 2020, Geochronologic age constraints on tectonostratigraphic units of the central Virginia Piedmont, USA, U.S. Geological Survey Professional Paper 1861, 28, doi:10.3133/pp1861
Gehrels, G., 2012, Detrital zircon U-Pb geochronology—Current methods and new opportunities, in Busby, C., and Azor, A., eds., Tectonics of sedimentary basins—Recent Advances: Hoboken, N.J., Wiley-Blackwell Publishing p. 47–62.
Hopkins, H.R., 1960, Geology of western Louisa County, Virginia: Ithaca, Cornell University, Ph.D. dissertation, 98 p.
Horton, J.W., Jr., and Williams, R., 2012, The 2011 Virginia earthquake—What are scientists learning?: Eos, v. 93, no. 33, p. 317–318.
Hughes, K.S., 2014, Does the Chopawamsic fault represent the main Iapetan suture in the Southern Appalachian orogen? Geology, geochemistry, and geochronology of the western Piedmont of Virginia and insight into local intraplate seismicity: Raleigh, North Carolina State University, Ph.D. dissertation, 320 p., 1:24,000-scale plate.
Ludwig, K.R., 2009, SQUID 2, A user’s manual (rev 2.50): Berkeley Geochronology Center Special Publication 5, 110 p.
Ludwig, K.R., 2012, User’s manual for Isoplot 3.75—A geochronological toolkit for Microsoft Excel: Berkeley Geochronology Center Special Publication 5, 75 p.
Nier, A.O., 1950, A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium: Physical Review, v. 77, no. 6, 
p. 789–793.
Pavlides, L., 1981, The central Virginia volcanic-plutonic belt—An island arc of Cambrian(?) age: U.S. Geological Survey Professional Paper 1231-A, 34 p.
Pavlides, L., 1989, Early composite mélange terrane, central Appalachian Piedmont, Virginia and Maryland; Its origin and tectonic history, in Horton, W., and Rast, N., eds., Mélanges and olistostromes of the U.S. Appalachians: Geological Society of America Special Paper 228, p. 135–194.
Pavlides, L., 1990, Geology of part of the northern Virginia Piedmont: U.S. Geological Survey Open-File Report 90–548, scale 1:100,000.
Spears, D.B., 2011, Geology of the Lakeside Village quadrangle, Virginia: Virginia Division of Geology and Mineral Resources Publication 177, scale 1:24,000.
Spencer, C.J., Kirkland, C.L., and Taylor, R.J., 2016, Strategies towards statistically robust interpretations of in situ U–Pb zircon geochronology: Geoscience Frontiers, v. 7, no. 4, p. 581–589.
Staudacher T. H., Jessberger E. K., Dorflinger D., and Kiko J., 1978, A refined ultrahigh-vacuum furnace for rare gas analysis. J. Phys. E: Sci. Instrum. 11, p. 781–784
Wetherill, G.W., 1956. Discordant uranium-lead ages, I. Transactions - American Geophysical Union 37 (3), 320-326.

Tags: 40ar-39ar, bascom-argon-dating-lab, central-virginia-seismic-zone, cvsz, fbgc, florence-bascom-geoscience-center, geochronology, geoscientificinformation, shrimp, u-s-geological-survey, usgs, usgs-64ee4b5ed34e4b6d8ab70fbd, zircon-u-pb
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