RECYCLING OF CONTINENTAL CRUST CAPTURED IN XENOLITHS

Xenoliths that erupted at 11 Ma in the SE Pamir of Tajikistan illuminate what happens when crust founders into the mantle. U-Pb and trace-element laser-ablation split stream inductively coupled plasma mass spectrometry of zircon shows that the igneous and metasedimentary xenoliths were likely derived from the crustal section into which they were intruded: the Jurassic–Cretaceous Andean-style magmatic arc and its Proterozoic–Mesozoic host rocks along the southern margin of Asia. The xenoliths reached temperatures of 1000–1050°C and depths of 90 km--significantly greater than the 70 km Moho--most probably during foundering of the lower crust and uppermost mantle. Spatially heterogeneous recrystallization of zircons was extensive, yielding a range of dates down to 11 Ma. The zircons show distinct changes in Eu anomaly, Lu/Gd ratio, and Ti concentrations that are interpreted to indicate garnet growth and minimal heating at 22−20 Ma, and then 200–300°C of heating, ~25 km of burial, and alkali−carbonate melt injection at 14−11 Ma. These changes are interpreted to coincide with: i) heat input due to Indian slab breakoff at ~22‒20 Ma; ii) rapid thickening and foundering of the Pamir lithosphere at 14‒11 Ma, prior to and synchronous with collision between deep Indian and Asian lithospheres beneath the Pamir.