PLATE REORGANIZATION DRIVES THE FORMATION OF THE GANGDESE BATHOLITH IN SOUTHERN TIBET (Invited Presentation)

Magmatism at convergent plate margins records the growth and reworking of the Earth's continental crust. However, what drives the generation of this magmatic activity, and in particular variations in the rate of generation, remains enigmatic. We address this problem based on chronological and geochemical data of igneous rocks from the Gangdese Batholith in southern Tibet. The batholith developed along the southern Asian margin and formed in an archetypal convergent plate margin through subduction of the Neo-Tethys followed by the transition to India-Asia collision. The batholith was emplaced during 245-10 Ma and records three major flare-ups in magmatic activity at 185-170, 90-75, and 55-45 Ma. Despite lacking subduction rate estimates, the high Ba/Th and low Th/Y ratios of the 185-170 Ma gabbroic rocks and the low Ba/Th and high Th/Y ratios of the ~165 Ma gabbroic rocks, as well as the decreasing zircon d¹⁸O of felsic rocks from 180 to 165 Ma, are likely the result of an initial increase in the subduction rate of the Neo-Tethys at ~180 Ma followed by a slowdown at 170–160 Ma. The increase in the India-Asia convergence rate at 90-84 Ma, the 90 ± 5 Ma high-T charnockites with dry CO₂-rich fluid inclusions and high-T granulite-facies country rocks, suggest that the 90-75 Ma flare-up can be linked with the increased melting of the hydrated mantle wedge due to rapid subduction at 90-84 Ma followed by ridge subduction of the Neo-Tethys at 84-75 Ma. The rapid convergence at 55-51 Ma and the sudden deceleration between India and Asia at ~51 Ma, as well as the extensive magmatism with compositional diversity and increased melting temperature at ~51 Ma along the length of the batholith, suggest that the 55-45 Ma flare-up was governed by high India-Asia convergence rates and subsequent slab breakoff of the Neo-Tethys. The scarcity of magmatism at 130-75 Ma in the Gangdese arc west of ~88°E could be related to the increasing coupling between the Neo-Tethyan and Gangdese lithospheres hindering magma generation. This coupling was caused by the counterclockwise rotations of the Indian plate in response to its separation from Australia at ~130 Ma, from Antarctica at ~100 Ma, and from Africa at ~94 Ma. Comprehensive data from the Gangdese batholith highlights the importance of plate reorganization (i.e., external drivers) in controlling variations in the rates of magmatic activity at convergent plate margins.