The thermal structure of continental crust in active orogens: insight from Miocene eclogite and granulite xenoliths of the Pamir Mountains

Rare ultrahigh-temperature–(near)ultrahigh-pressure (UHT–near-UHP) crustal xenoliths erupted at 11 Ma in the Pamir Mountains, southeastern Tajikistan, preserve a compositional and thermal record at mantle depths of crustal material subducted beneath the largest collisional orogen on Earth. A combination of oxygen-isotope thermometry, major-element thermobarometry and pseudosection analysis reveals that, prior to eruption, the xenoliths partially equilibrated at conditions ranging from 815 C at 19 kbar to 1100 C at 27 kbar for eclogites and granulites, and 884 C at 20 kbar to 1012 C at 33 kbar for garnet–phlogopite websterites. To reach these conditions, the eclogites and granulites must have undergone mica-dehydration melting. The extraction depths exceed the present-day Pamir Moho at 65 km depth and suggest an average thermal gradient of 12–13 C km. The relatively cold geotherm implies the introduction of these rocks to mantle depths by subduction or gravitational foundering (transient crustal drip). The xenoliths provide a window into a part of the orogenic history in which crustal material reached UHT–(U)HP conditions, partially melted, and then decompressed, without being overprinted by the later post-thermal relaxation history.