LITHIUM ISOTOPIC COMPOSITION AND CONCENTRATION OF THE INDIAN CONTINENTAL CRUST

The Himalayan–Tibetan plateau, which is the most outstanding natural laboratory for studying continental orogenesis, is the product of the closure of the Neo-Tethys ocean and the continued convergence of India–Asia since ~65 Ma. Insights into the origin of post-collisional Cenozoic magmatic rocks (e.g., adakitic rocks, potassic and ultrapotassic volcanic rocks) in the Lhasa terrane of the southern Tibetan Plateau are crucial to understanding of the crust–mantle interaction in continental subduction zones. However, it has been pointed to the significance of the different geological end-members, especially the crustal end-members (e.g., Indian upper/lower crust, Tibetan lower crust, juvenile lower crust) in the petrogenesis of the post-collisional potassic–ultrapotassic and ore-forming adakite-like magmatism in southern Tibet. In order to further constrain the processes by which the post-collisional Cenozoic magmatic rocks are generated, we have to adopt a new technological means. In the Earth Sciences, lithium stable isotopes have frequently been utilized for their ability to trace recycling of surface-modified materials to the mantle by subduction processes. Although several features of terrestrial Li isotopic distributions in major reservoirs are clear, the Li isotopic signature of continental crust is a matter of debate. The newly geophysical data confirm an updated model of Argand-type underthrusting of the Indian plate in which the Indian upper crust has been scraped off to form the Himalaya while the Precambrian Indian lower crust has underthrust north as far as the Bangong–Nujiang suture zone (BNSZ). To better constrain the lithium isotopic compositions of Indian upper and lower continental crust, we have undertaken a systematic study of 4 whole-rock leucogranite from Luozha and longzi in Tethyan Himalaya, 14 whole-rock granulite–gneiss from Zhangmu in High Himalaya and 5 whole-rock two-mica granite from Quedang and Dala in Tethyan Himalaya. The lithium contents of the Indian upper and lower continental crust are 22.8~100 ppm and 32.5~83.7 ppm, respectively; and their corresponding δ⁷Li values are 0.9~5.6 ‰ and –4.4~–0.1 ‰, respectively.