Origin of mafic magmas beneath northwestern Tibet; constraints from 230Th-238U disequilibria

[1] 238U-230Th disequilibria and Sr, Nd, and Pb isotopic and chemical data for young (<120 ka) trachyandesites from the Ashikule Basin (AKB) in northwestern Tibet provide constraints on the origin of magmas produced within this region of continental collision. Compared to lavas from both continental and oceanic settings, the AKB samples show large excesses of 230Th with respect to 238U (up to (230Th)/(238U) = 1.36). Partial melting of garnet-bearing lithologies (garnet peridotite, garnet pyroxenite, or eclogite) could be responsible for these 230Th excesses and could plausibly occur either in the lithospheric mantle or in the lower crust. Small porosities (<0.4%) and slow melting rates (<104 kg m3 yr1) are required in the case of a garnet peridotite residue, although larger porosities (up to 18%) and melting rates (>103 kg m3 yr1) are permitted in the case of an eclogitic residue; hydrous metasomatic phases, if present, would lower these limits further. The source of the AKB magmas has probably been enriched in incompatible elements relative to bulk Earth since at least the middle Proterozoic (>1 Ga), likely via metasomatism by a relatively dry silicate melt, and, if the ultimate source of the lavas is mantle lithosphere, concentrations of incompatible trace elements in the source could be similar to those of xenoliths and massifs. The 230Th-238U disequilibria provide additional information to evaluate the methods previously proposed to explain melt generation beneath northern Tibet. The measured 230Th enrichments are uncharacteristic o