Geochronology and Geochemistry of Lower Crustal Xenoliths: Exploring the Formation of the Lower Crust Beneath Central Mongolia

Abstract:

Central Mongolia is far removed from any active margin yet it is marked by the anomalously high Hangay Mountains. Near Tariat on the flanks of the Hangay range we recovered lower crustal xenoliths from the Shavaryn-Tsaram Quaternary basaltic breccia pipe. Two-pyroxene Fe-Mg exchange thermometry indicates the xenoliths equilibrated at 840 ± 30ºC. Previous studies indicated pressure ranges between 12.5 and 15.5 kbar for samples with similar equilibration temperatures from the same locality (Stosch et al., 1995). Abundant zircon in the samples dated by U-Pb laser ablation ICP-MS show a wide range of ages from 50 to 300 Ma with a broad peak around 200 Ma. Each of the seven dated samples shows a similar spread of ages with a maximum age range of 200 Ma for individual xenoliths. The new data suggest that the lower crust in the Tariat area formed in the Late Paleozoic to early Mesozoic during the accretion of the Central Asian Orogenic Belt (CAOB), which is consistent with arc like geochemical signatures of whole-rocks and with common Permo-Triassic Sm-Nd and U-Pb isotope ages for magmatic rocks exposed on the surface in the region. Whether the younger Mesozoic zircon ages reflect later metamorphic events or result from diffusive Pb loss is unclear. Based on elevated Th/U ratios the latter may be more likely, though more work is needed to fully resolve the origin of the zircons. Early Mesozoic whole-rock Sm-Nd model ages for the suite of xenoliths are broadly consistent with the zircon U-Pb data. Stachnik et al. (2014) indicated that the high elevations of the Hangay region could be isostatically supported by a thick crust. Our new data suggests the lower crust, and by inference the high topography, formed in the Late Paleozoic to Early Mesozoic during the formation of the CAOB.