Mid-crustal melting in Western Yunnan and its influence on extrusion of the Tibetan Plateau

Abstract

The Himalayan-Tibetan system is growing upward and outward, influenced by mid-lower crustal melting and flow. This study offers a missing geochemical perspective on current geophysics, linking mid-crustal melting beneath SE Tibet to the initiation of transcurrent tectonics that have facilitated the ongoing southeastward extrusion of the Tibetan plateau. New zircon U-Pb data reveal a pulse of Miocene felsic magmatism southeast of the Eastern Himalayan Syntaxis. These have high SiO₂, variable but low ɛ_Hf(t) and high δ¹⁸O which strongly suggest metasedimentary anatexis. Moderate La/Yb and Sr/Y, with high ⁸⁷Sr/⁸⁶Sr_(i) and low ɛ_Nd(t) are comparable with mid-lower crust biotite-bearing granitic gneisses. Together with biotite and zircon thermobarometry, these characteristics suggest biotite-dehydration anatexis in the middle crust. Experimental and rheological data suggest melt fractions (> 10%) sufficient to reduce crustal viscosity and allow rheological accommodation for Miocene extrusion of the plateau, and the continued presence of mid-crustal melt is supported by current geophysical data.