Fluid-fluxed melting in the Himalayan orogenic belt: Implications for the initiation of E-W extension in southern Tibet

Abstract

The geochemistry of granite is largely controlled by physical and chemical parameters that are closely linked to tectonic processes in evolving orogenic belts. Therefore, temporal changes in the geochemical compositions of granites could be used to infer critical shifts in tectonic processes. The Himalayan leucogranites are crustal anatexis products, providing a case to formulate petrogenetic models for granites and test tectonic models. From west to east, in the High Himalaya and the Tethyan Himalaya, two groups of leucogranites are derived from fluid-absent melting (Group A) and fluid-fluxed melting of muscovite (Group B), respectively. In the Cona and Mount Everest areas, Group B granites crystallized at 26−10 Ma, and Group A granites formed at 19−13 Ma. Group B granites have higher CaO, Sr, Ba, Zr, Hf, Th, Sr/Y, Zr/Hf, and Th/U, and lower Rb, Nb, Ta, U, Rb/Sr, and 87Sr/86Sr than those in Group A granites. These geochemical differences highlight the role of deep-origin fluids and the dissolution control of the accessory phases on the geochemical compositions in silicic magma systems. Field and microstructural observations show that E-W extension occurred synchronously with the granite intrusion derived from fluid-fluxed melting. Elevated heat flow accompanying the E-W extension could dehydrate hydrous minerals and release fluids from deep-seated crust (e.g., Lesser Himalayan Sequence). Such fluids could flux and melt the metasedimentary rocks within the High Himalaya and produce Group B granites. Together with literature data, from the Lhasa terrane to the Himalayan belt, E-W extensions in Tibet may have initiated as early as 26 Ma.