RECONSTRUCTING THE EVOLUTION OF THE PAMIR LOWER CRUST: ZIRCON U-PB AND TRACE-ELEMENT PETROCHRONOLOGY OF UHP/UHT XENOLITHS

A unique suite of eclogite- and granulite-facies crustal xenoliths were erupted from ~1000°C and 90 km depth (20–30 below the modern Moho) at 10 Ma within the Dunkeldik ultrapotassic magmatic field in the Pamir portion of the Cenozoic India-Eurasia collision zone. These ultrahigh-temperature and (near) ultrahigh-pressure xenoliths contain a spectacular record of compositional, thermal, and physical changes that occur during the foundering of lower crust. Due to their abrupt exhumation, the rocks are weakly retrogressed, with metastable high-pressure mineral assemblages that offer an exceptionally clear view of the effects of UHT/(U)HP conditions on crustal rocks. The mineral assemblages range from eclogite to meta-sedimentary compositions. LASS (laser ablation split stream) U-Pb + trace-element analyses on zircon were performed on 30 xenoliths to reconstruct the P-T- t path of the foundering crust. The zircons have oscillatory-zoned cores with mid- to Late Cretaceous U-Pb dates and modest Ti contents compatible with derivation from the continental margin arc developed on the southern margin of Eurasia. They have metamorphic rims with flat, homogeneous CL response, primarily Late Oligocene-Miocene U-Pb dates and elevated Ti contents compatible with temperatures of 950-1150°C. A significant decrease in Y+HREE concentrations and a change from strongly negative to weakly negative Eu/Eu* at 20 Ma is compatible with the initiation of garnet stability and the breakdown of plagioclase at that time. These data suggest that thickening of the Pamir lower crust began at 20 Ma, and led to crustal foundering prior to the eruption of the xenoliths.