Petrochronology of Himalayan ultrahigh-pressure eclogite

The timing and nature of the India-Asia collision, Earth's largest ongoing continent-continent collisional orogen, are unclear. Ultrahigh-pressure metamorphism of Indian continental margin rocks is used as a proxy for initial collision because it indicates subduction of India. Records of this metamorphism are preserved only at Kaghan Valley (Pakistan) and Tso Morari (Ladakh, India), separated by ~500 km and having published ages of peak pressure of 46.2 ± 0.7 Ma and 53-51 Ma, respectively. The apparent ~6 m.y. age difference may reflect multiple subduction events, a large promontory along the former Indian margin, or inadequate constraints on the time of peak pressure recrystallization at Tso Morari. We present 108 coupled, in situ U/Th-Pb and rare earth element (REE) analyses of zircons in two Tso Morari eclogites to obtain age and petrologic information. The ages range from ca. 53 Ma to 37 Ma, and peak at ca. 47-43 Ma. Flat heavy REE slopes and the absence of an Eu anomaly are compatible with eclogite-facies zircon (re)crystallization. This (re)crystallization probably occurred at ultrahigh pressure, because 64% of the analyses are from zircon included in ultrahigh- pressure garnet and omphacite. These results are consistent with those from Kaghan Valley, and suggest that a single, protracted ultrahigh-pressure metamorphic event occurred contemporaneously across much of the orogen, following initial contact of the Indian and Asian continents at ca. 51 Ma or later. © 2013 Geological Society of America.