Age and thermal history of Eo- and Neohimalayan granitoids, eastern Himalaya

Four distinct granitoid suites outcrop across a N-S transect in the eastern Himalaya between the Main Central Thrust (MCT) and the Indus Tsangpo suture. The Arunachal and Tsona leucogranites outcrop, respectively, in the MCT hanging-wall and adjacent the South Tibetan Detachment. Both crystallized at about 19. Ma, and appear to be lateral equivalents of the High Himalayan Leucogranites. In the central Tethyan Himalaya, the Dala granitoids form a suite of undeformed plutons emplaced into deformed Tethyan Himalayan Sequence metasediments. In the northern Tethyan Himalaya, the Yala-Xiangbo granitoids are part of an igneous complex in the core of the Yala-Xiangbo dome. Both the Dala and Yala-Xiangbo granitoids have Eocene magmatic ages (44.1. ± 1.2. Ma and 42. ± 5. Ma, respectively) that are notably older than the vast majority of post-collisional granitoids along the main Himalayan Arc. Pelitic units from the core of the Yala-Xiangbo dome record two episodes of amphibolite-grade metamorphism and monazite growth coinciding with well-documented episodes of Eohimalayan and Neohimalayan metamorphism. The thermochronological histories of the Dala and Yala-Xiangbo units suggest that the eastern Tethyan Himalaya was tectonically quiescent from ca. 40. Ma until ca. 15. Ma. Miocene exhumation of the Dala and Yala-Xiangbo units, and all North Himalayan Domes studied to date, appears best explained by a structural control, such as concurrent north-directed motion on the Great Counter Thrust. We hypothesize that an extension of the Ninety East Ridge may have been responsible for Eocene high heat-flow beneath the eastern Himalaya and explain the localization of magmatism there.