Continental Crustal Growth Processes Recorded in the Gangdese Batholith, Southern Tibet

The continental crust in the overriding plate of the India-Asia collision zone in southern Tibet is characterized by an overthickened layer of felsic composition with an underlying granulite-eclogite layer. A large data set indicates that this crust experienced magmatism from 245 to 10 Ma, as recorded by the Gangdese Batholith. Magmatism was punctuated by flare-ups at 185−170, 90−75, and 55−45 Ma caused by a combination of external and internal factors. The growth of this crust starts with a period dominated by fractional crystallization and the formation of voluminous (ultra)mafic arc cumulates in the lower crust during subduction, followed by their melting during late-subduction and collision, due to changes in convergence rate. This combined accumulation-melting process resulted in the vertical stratification and density sorting of the Gangdese crust. Comparisons with other similarly thickened collision zones suggests that this is a general process that leads to the stabilization of continental crust. The Gangdese Batholith records the time-integrated development of the world's thickest crust, reaching greater than 50 km at 55-45 Ma and greater than 70 km after 32 Ma. The Gangdese Batholith records three magmatic flare-ups in response to distinct drivers; the last one at 55−45 Ma marks the arrival of India. Magmatism was first dominated by fractional crystallization (accumulation) followed by crustal melting: the accumulation-melting process. Accumulation-melting in other collision zones provides a general process for vertical stratification and stabilization of continental crust.