DEPTH AND TEMPERATURE OF THE EOCENE LITHOSPHERIC MANTLE BENEATH HIGHLAND COUNTY, VA

An abundance of Eocene alkaline igneous dikes, sills, and plugs are exposed in the Shenandoah Valley and Allegheny Front in VA and WV. These igneous bodies provide important petrologic and geochemical constraints on the depth and composition of the lithosphere and asthenosphere under Eastern North America. In this study, we determine the depth and temperature of the lithospheric mantle under Highland County, VA, using mantle xenocryst compositions from a small Eocene picrobasalt located along Rt. 631, northeast of Monterey, VA. The plug contains xenocrysts of clinopyroxene and olivine up to ~1cm in diameter within a groundmass of devitrified glass containing microphenocrysts of plagioclase, olivine, and clinopyroxene. The JEOL Superprobe 8900R electron microprobe at the USGS in Reston, VA, was used to quantify clinopyroxene compositions at 15 kV and 20 nA with WDS, and to identify unknown mineral phases using EDS analyses. Olivine xenocrysts contain Cr-rich spinel, consistent with a mantle origin for these minerals. Clinopyroxene xenocrysts contain silicate melt inclusions, as well as inclusions containing calcite, dolomite, and sulfides. Clinopyroxene and clinopyroxene-melt geothermobarometric equations summarized in Putirka (2008) were used to calculate P and T from compositional data. The whole-rock composition was used to represent the melt composition in equilibrium with crystal cores. The clinopyroxene cores have an average composition of Wo₄₇En₄₄Fs₉ (n=82). The average T calculated for clinopyroxene cores is 1371±38°C, and the average P = 17.5±3 kbar (corresponding to ~53 km). Two clinopyroxene-only barometers (Eqns. 32a and 32b of Putirka 2008) produce P = 14.1±1.8 kbar (~42 km). The T and P recorded by the Highland County clinopyroxene xenocryst cores are significantly higher than those calculated for clinopyroxene xenocrysts cores from the Mole Hill basalt located 55 km to the northeast (T = 1250±19°C and P = 10±2 kbar; Sacco 2011). The P are the same within error if the clinopyroxene-only barometers are used (13±1 kbar for Mole Hill vs. 14.1±1.8 kbar for Highland County). These results suggest an increase in melting temperature towards the southwest and a possible increase in melting depth and/or a southwesterly thickening of the lithosphere across the Valley and Ridge of VA.