Muztaghata Dome Miocene Eclogite Facies Metamorphism: A Record of Lower Crustal Evolution of the NE Pamir

The Pamir gneiss domes represent the most extensive exposure of mid to lower crustal rocks in the Himalayan-Tibetan orogen north of the India-Asia suture zone. Unlike other domes in the Central and Southern Pamir, the Muztaghata dome stands out due to its higher metamorphic grade, more complex structural elements, and variable timing of metamorphism. In order to unravel the P-T-t history of the Muztaghata dome and better constrain the timing of peak metamorphism, we applied petrologic modeling in concert with geochronology to samples from the structure. The Muztaghata gneiss dome is composed of a structurally higher metapelite-dominated terrane in the west and a structurally lower orthogneiss terrane in the east. Our results from the western terrane indicate high-pressure eclogite facies peak conditions of similar to 800 degrees C/22 kbar at similar to 25-20 Ma. Zircon grains from metapelitic samples from the western terrane also yield Early Jurassic metamorphic U-Pb ages with REE signals that indicate coeval garnet growth. Our results from the eastern terrane record high-pressure amphibolite facies peak conditions of similar to 650 degrees C/14 kbar at similar to 24-20 Ma, noticeably lower than the structurally higher western terrane indicating structural juxtaposition during Miocene exhumation. Peak metamorphic conditions from the eastern terrane indicate depths below the current Moho, supporting the interpretation that the Early Miocene Pamir crust was thicker than present. This was followed by rapid exhumation from depths of similar to 75-80 km and partial westward collapse of the Pamir after 20 Ma, possibly driven in part by regional lithospheric delamination.