Foundering Triggered by the Collision of India and Asia Captured in Xenoliths

Abstract

AbstractXenoliths that erupted in the SE Pamir of Tajikistan from 1000 to 1050°C and 90 km depth illuminate what happens when crust founders into the mantle. 40Ar/39Ar dating of minerals from the xenoliths and volcanic host rocks of the shoshonitic Dunkeldik pipe and dike field indicates eruption at 11.2 ± 0.2 Ma. 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. Recrystallization of these 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 compatible with 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 and (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.