ULTRAHIGH TEMPERATURES RECORDED IN LOWER CRUSTAL XENOLITHS FROM THE SHENANDOAH VALLEY, VA

Petrographic observations and Zr-in-rutile thermometry of xenoliths found within Late Jurassic dikes were used to constrain the phase assemblage and temperature of the lower crust under Eastern North America. Samples were collected from three locations in western VA: 5 Springs, Vulcan Quarry, and Arey Farm. Mineral phases were identified using EDS and Raman analyses. Metasedimentary gneisses from 5 Springs and Arey Farm contain quartz + alkali feldspar + garnet + rutile + sillimanite + zircon ± apatite ± graphite with retrograde chlorite, ilmenite, monazite, sulfides, and barite. A metasedimentary xenolith from Vulcan Quarry consists of feldspar + quartz + biotite + graphite + zircon + rutile with minor barite. Igneous xenoliths from Vulcan Quarry contain feldspar + pyroxene + apatite + magnetite + complex Nb oxide. Quartz inclusions surrounded by radial fractures within garnet were found in the metasedimentary gneiss xenoliths. Two types of rutile inclusions are present within garnet: exsolved needles (< 5 µm wide) and larger (~20 µm) subhedral grains. Zr concentrations of subhedral rutile inclusions in garnet and groundmass rutile were measured by EMPA. Preliminary comparison of the mineral assemblage of the gneissic xenoliths with the pseudosection of Ague 2013 (Geology, 41 271-274) constrains minimum P > 7.25 kbar for 840 °C <T<1000 °C. The presence of sillimanite restricts maximum P <14.5 kbar in this temperature range. The Zr-in-rutile geothermometer of Tomkins 2007 (Geology, 25, 703-713) in the β-quartz field was used to calculate temperatures assuming P = 10 kbar. The Vulcan Quarry biotite gneiss xenolith records temperatures between 655 - 784°C. The Arey Farm and 5 Springs xenoliths have calculated temperature ranges of 718 – 984°C and 637- 912°C, respectively, that extend into the UHT (ultrahigh temperature) field. UHT have only been reported from one other location in the Eastern US (Ague 2013). The metasedimentary xenoliths have Grenvillian detrital zircon ages (Rossi 2013) with no evidence for growth in subsequent orogenic events. The UHT recorded in the xenoliths may have been produced by prolonged (>800 Ma) deep burial and partial melting of the lower crust.