P-T-t-D paths of the North Himalayan metamorphic rocks: Implications for the Himalayan orogeny

- The schists of the Yardoi gneiss dome record three major stages of metamorphism and deformation.
- The schists have a clockwise P-T-t-D path, started at ca. 48 Ma and lasted to ca. 16 Ma.
- The initiation of India-Asia collision in the eastern Himalaya was at ca. 50 Ma.
- Tectonic discontinuity is present in the upper structural level of the GHS.

Metamorphic P-T-t-D paths have been determined in the Yardoi gneiss dome of the eastern Himalaya that place new constraints on the genesis of the North Himalayan Gneiss Dome and tectonic evolution of the Himalayan orogen. Our study shows that the schists from the Yardoi dome underwent at least three stages of metamorphism and deformation. The first stage of prograde metamorphism is represented by the core of porphyroblastic garnet, showing distinct growth compositional zoning, and hosting mineral inclusions of biotite, muscovite, quartz and ilmenite; the curved inclusion trails define the first stage of deformation foliation (S1). The second stage of peak metamorphism is represented by the rim of porphyroblastic garnet and the aligned matrix minerals of biotite, muscovite, plagioclase, quartz, kyanite and staurolite, which define the main deformation foliation (S2). The third stage of retrograde metamorphism is characterized by occurrence of biotite, plagioclase, muscovite and sillimanite within the distributed shear bands, which define the late deformation foliation (S3). Phase equilibrium modeling reveals that Yardoi schists witnessed a peak metamorphism of upper amphibolite-facies under P-T conditions of 7-8 kbar and 615-665 °C, and records a clockwise P-T-t-D path with a prograde process of both temperature and pressure increase and subsequent retrogression of isothermal decompression. The zircon U-Pb dating indicates that the prograde process occurred during the Middle Eocene of 48-36 Ma and the retrograde metamorphism probably lasted to ca. 16 Ma. The present result shows that the formation of the Yardoi schists was related to the subduction and exhumation of Indian crust, the initial collision of Indian and Asian continents occurred at ca. 50 Ma, the northeastern margin of the Indian continent was shallowly underthrusted beneath the Asian continent, and distinct tectonic discontinuity is present in the Greater Himalayan Sequence.