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Recycling of Continental Crust Captured in Pamir Xenoliths


Xenoliths that erupted in the SE Pamir of Tajikistan from 1000–1100°C and 90 km depth are exclusively crustal, providing a means of examining what happens to crust that founders into the mantle. 40Ar/39Ar dating of volcanic minerals indicates an eruption age of 10.0 ± 0.2 Ma. U-Pb + trace-element laser-ablation split stream inductively coupled plasma mass spectrometry of zircon shows that the xenoliths were likely derived from the crustal section into which they were intruded: the igneous xenoliths were derived from the Jurassic–Cretaceous Trans-Himalayan Batholith, and the metasedimentary xenoliths are like the stratigraphic section that hosts the Batholith. Recrystallization of these zircons was extensive, yielding a range of dates down to 10 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, ~40 km of burial, and alkali−carbonate melt injection at 14−11 Ma. These dramatic changes are interpreted to coincide with foundering of the Pamir lower crust caused by tectonic thickening and northward rollback of the Asian slab. These xenoliths provide our only known record of the physical and chemical changes during the foundering continental crust.

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