INVESTIGATING THE PROVENANCE & METAMORPHIC HISTORY OF KYANITE QUARTZITES OF THE VIRGINIA CHOPAWAMSIC-MILTON TERRANE USING TRACE ELEMENT THERMOBAROMETRY AND TRACE ELEMENT ANALYSIS

The Virginia Piedmont is comprised of terranes accreted onto Laurentia during Paleozoic Appalachian orogenesis (Bailey & Owens, 2012). These terranes provide valuable insight into continental growth during Appalachian orogenesis, and yet, critical aspects of their metamorphic history remain poorly understood. The Chopawamsic terrane is an Ordovician volcanic island-arc metamorphosed to amphibolite facies. It contains kyanite quartzites with an unusual assemblage of >95% quartz + kyanite with accessory rutile and zircon (Owens & Pasek, 2007). Previous P-T results indicate temperatures of 600 +/-30 °C and pressures between 6.3 and 6.5 Kbar at Willis Mountain (Cochrane 1986), while TE thermobarometry across the terrane yields conditions in the kyanite stability field, but with much higher-P that previously estimated, (Ingle 2024, AGU abstract). In this contribution, we assess the relevant calibrations available for Ti-in-Qtz (Osborne et al. 2022, Thomas et al. 2010) and Zr-in-Rt (Tomkins et al. 2007, Kohn et al. 2020) geothermometers to determine the upper and lower P-T bounds estimated from TE data in these phases. We further investigate trace elements in kyanite (Cr, Fe, Mg, P, Ti, V, S) and rutile (Nb, Zr, Cr, Fe) via EPMA to elucidate chemical signatures of the protoliths and metamorphic processes/fluids involved in crystallizing these phases.

Kyanite quartzite in the Chopawamsic terrane contains either deep blue or light grey kyanite. Cathodoluminescence (CL) imaging reveals both core (CL-dark) -rim (CL-bright) zoning, patchy zoning, and oscillatory zoning structures, as well as kyanite lacking any notable CL zoning. Trace element analysis reveals a strong correlation between high Cr abundance and CL bright regions in kyanite, and the abundance of other trace elements may provide a way to fingerprint different kyanite CL growth domains. Ti-in-quartz and Zr-in-Rutile trace element thermobarometry reveals amphibolite facies metamorphic conditions. We assess the range of P-T values yielded by the different calibrations for these trace element thermometers. Thermobarometry and trace element analyses will provide context for application of U-Pb petrochronology to constrain the timing of protolith zircon crystallization and timing of metamorphism likely recorded by rutile in kyanite quartzite.