Partial melting of HP–UHP felsic gneiss in the South Altyn Tagh reveals the rapid exhumation of a deeply subducted slab

Abstract

The partial melting of HP–UHP rocks plays a crucial role in facilitating the exhumation of deeply subducted rocks. However, accurately determining the P–T–t conditions of the initial melting and subsequent crystallization is often challenging, which can hamper our understanding of the relationship between partial melting and exhumation. We present a detailed investigation of granitic veins in the (U)HP felsic gneiss in the eastern South Altyn Tagh (SAT), northwest China. These granitic veins have similar whole rock geochemical characteristics to those of the felsic gneiss: high K₂O (5.01–9.95 wt%) and Rb contents, and low Sr contents, with negative correlations between Rb/Sr ratios and Ba or Sr contents. This suggests that the veins were generated by muscovite-dehydration melting of the felsic gneiss. Garnet crystals in the felsic gneiss contain many titanite and multiphase solid inclusions in their mantles. Using the compositions of the garnet mantles and granitic veins, alongside pseudosection modelling of the felsic gneiss, we estimate that the initial melting conditions of the felsic gneiss occurred at 2.57–2.61 GPa and 1000 °C, just above the solidus. In situ UPb dating of titanite inclusions in the garnet mantles yields an initial melting age of ∼485 Ma. Furthermore, anatectic zircon grains in the granitic veins yield UPb ages of 488–484 Ma. Using Ti-in-zircon thermometry and ¹⁷⁶Lu/¹⁷⁷Hf-in-zircon geobarometry, we estimate that the anatectic zircon grains crystallized at 647–872 °C and 0.16–0.82 GPa. The estimated P–T–t path from the initial melting of the felsic gneiss to the crystallization of the granitic veins indicates rapid exhumation from ∼2.6 to 0.16–0.82 GPa. This exhumation can be attributed to partial melting, which weakened the rocks and enabled their ascent to shallower depths. Moreover, the potassic melts derived from partial melting of the felsic gneiss provide new insights into the genesis of potassic granitoids in subduction zones.