14-4 Zircon Geochronology of New Intrusive Bedrock Samples from the Appalachian Piedmont of Northwest Washington, District of Columbia

Session: New Research in the Appalachian-Ouachita Orogen: Integrated studies from the Foreland to the Hinterland

Presenting Author:

Nikita Kepezhinskas

Authors:

Kepezhinskas, Nikita P.¹, Kamenov, George D.², Foster, David A.³, Johnson, Elizabeth Ann⁴, Iseri, David A.⁵, McGuinness, Brendan⁶, Dunlap, Daniel⁷, Fahim, Ali⁸

Abstract:

The central Appalachian Piedmont in the District of Columbia hosts a variety of metamorphic and igneous rocks. Previous work in this region has provided detailed reconstructions of lithological (Darton, 1950; Southworth and Denenny, 2005) and geochronological data, including detailed age dates for intrusive rocks (Aleinikoff et al., 2002; Sinha et al., 2012). We present new, initial zircon U-Pb geochronology results for a previously unmapped intrusion in northeast Washington, District of Columbia. Drill core of the intercepted intrusion has a grey, phaneritic texture that is macroscopically composed of quartz + plagioclase + amphibole ± biotite. Samples for geochronological analysis were obtained from a depth of 14.07 to 44.33 meters in diamond core drill hole, which was collared vertically at 38° 56’ 47.55” N, 77° 01’ 53.64” W. A majority of concordant zircon core (n=10) and rim (n=4) zones range from a minimum age of 935 ± 21 Ma to a maximum age of 1420 ± 20 Ma. One concordant zircon rim (n=1) records a ²⁰⁶Pb / ²³⁸U age of 445 ± 20 Ma. Backscatter electron imaging of zircon grains depicts variable zonation, with generally strongly visible zones corresponding to Grenville-age zircon cores. This initial geochronological data set suggests a statistically significant population of inherited, Grenville-age zircons that were entrained into a melt in the Silurian to Ordovician. The young zircon rim age is within error of regional, syntectonic plutons such as the Falls Church tonalite (469 ± 6 Ma; Aleinkoff et al., 2002), Kensington tonalite (463 ± 8 Ma; Aleinkoff et al., 2002), and the Norbeck tonalite (449 ± 7 Ma, 468 ± 7 Ma; Aleinkoff et al., 2002; Sinha et al., 2012).

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