Eocene high grade metamorphism and crustal anatexis in the North Himalaya Gneiss Domes, Southern Tibet

Determination of the timing and geochemical nature of early metamorphic and anatectic events in the Himalayan orogen may provide key insights into the physical and chemical behavior of lower crustal materials during the early stage of tectonic evolution in large-scale collisional belts. The Yardoi gneiss dome is the easternmost dome of the North Himalayan Gneiss Domes (NHGD), and contains three types of amphibolites with distinct mineral assemblage, elemental and radiogenic isotope geochemistry, as well as various types of gneisses. SHRIMP zircon U/Pb analyses on the garnet amphibolite and garnet-bearing biotite granitic gneiss yield ages of nearly peak metamorphism at 45.0±1.0 Ma and 47.±1.8 Ma, respectively, which are 2 to 4 Ma older than the age for partial melting in migmatitic garnet amphibolite (43.5±1.3 Ma). Available data have demonstrated that ultra-high pressure metamorphism in the Tethyan Himalaya occurred at ～55 Ma, and high amphibolite facies to granulite facies metamorphism at 45 to 47 Ma. In addition, partial melting at thickened crustal conditions occurred at 43.5±1.3 Ma, which led to the formation of high Sr/Y ratios two-mica granites. The high-grade metamorphic rocks in the NHGD may represent the subducted front of the Indian continental lithosphere. In large collisional belts, fertile components in crustal materials could melt and form granitic melts with relatively high Na/K and Sr/Y ratios under thickened crustal conditions, significantly different from those formed by decompressional melting during rapid exhumation.