U-PB GEOCHRONOLOGY OF GRENVILLIAN AND LATE JURASSIC LOWER CRUSTAL XENOLITHS IN THE SHENANDOAH VALLEY, VA

Late Jurassic (~150 Ma) and Eocene (~48 Ma) volcanics in the Shenandoah Valley, VA, contain metasedimentary and igneous lower crustal xenoliths. The analysis of these xenoliths allows for vertical profiling of the crust and mantle and provides important constraints on the structure and geological history of the Eastern North American Margin (ENAM). U-Pb ages were determined for xenoliths from three locations at the LaserChron Center at the University of Arizona. Xenoliths were cut from dike material, and zircons crystals were separated and hand selected for analysis using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). At two locations, Arey Farm and 5 Springs, only garnet-sillimanite-paragneiss xenoliths were found. The wide age ranges of 883-1161 Ma (5 Springs) and 938-1258 Ma (Arey Farm) from these xenoliths strongly suggest detrital populations of Grenville age and further detrital age dating is needed. At the third location, Vulcan Quarry, two distinct age populations were found: 151.7±1.8 Ma (MSWD = 0.50) and a less abundant population ranging from ~950-1215 Ma. The late Jurassic igneous xenoliths correspond in age with a pulse of magmatism at ~150 Ma in the Shenandoah Valley. The age of the Vulcan Quarry dike is unknown, but we hypothesize that the igneous xenoliths are cogenetically related to this magmatism that post dates the rifting of Pangaea by 50 Ma. The zircons from metasedimentary xenoliths have no discernible rims of post-Grenville age. This implies there has not been resetting by subsequent orogenic events in the Appalachians, even in the location which also contains ~150 Ma xenoliths. The gneissic xenoliths at 5 Springs and Arey Farm record ultrahigh temperatures (max T > 900°C; Helsley 2013). Future work to relate the Grenvillian xenoliths to known Grenvillian basement rocks exposed to the east in the Blue Ridge will provide regional context of the composition and age of the lower crust in ENAM.