Metamorphic soles from the Yarlung-Tsangbo ophiolites: Rapid subduction initiation and exhumation

Metamorphic soles within ophiolites are typically considered as the primary evidence of subduction initiation and are crucial for understanding the formation mechanism of ophiolites. Recently, high-temperature metamorphic soles have been discovered in the Yarlung-Tsangbo ophiolites from southern Tibet. In this study, metamorphic sole samples (mafic garnet-bearing granulite) collected from the Bairang section of the Yarlung-Tsangbo ophiolites were systematically analyzed using petrology, U-Pb geochronology of accessory minerals, and coupled modeling techniques, including phase equilibrium, garnet diffusion, and thermo-mechanical models. Petrological observations reveal that the mafic granulite records four stages of metamorphism. In the fresh granulite samples, the mineral inclusions in the garnet core represent the early metamorphic residues, despite the composition of the garnet core being re-equilibrated at the peak stage. The matrix mineral assemblages and the garnet core compositions represent the peak metamorphic stage, while the retrograde metamorphic stage is recorded by garnet rims and symplectites. Most of the samples underwent late metasomatism at sub-greenschist facies. Phase equilibrium modeling indicates that the rock experienced a clockwise P-T path, recording P-T conditions of ca.1.6 similar to 1.7GPa and 829 similar to 863 degrees C at the peak stage, and subsequent near-isothermal decompression conditions of ca.0.9 similar to 1.1GPa and 823 similar to 923 degrees C. The temperature and pressure of 306 similar to 324 degrees C/ < 0.3 GPa of the sub-greenschist assemblages were estimated using phase relations. U-Pb geochronology and temperature calculation results of titanite indicate that the age at the prograde stage of the metamorphic sole is 132.4 +/- 4.8Ma and 133.9 +/- 5.9Ma. Rutile recorded late cooling ages of 126.8 +/- 6.2Ma and 132.3 +/- 3.1Ma, which overlap with the titanite age within uncertainty. The geochronological data suggests that the time scale of metamorphic soles from protolith formation to subduction and exhumation is very short. The garnet diffusion and thermo-mechanical model calculations are also consistent with the geochronological results. The modeling of the entire metamorphic process is only 2.6 similar to 2.8 Myr, and exhumation occurs at a very fast rate of 6.7cm/yr. The results of this study, in conjunction with previous data on the overlying ophiolites, suggest that the formation of the Yarlung-Tsangbo ophiolites is extremely complex and may involve a combination of mid-ocean ridge and subduction-related environments. We highlight that some initial mid-ocean ridge basalts are involved in a cycle of deep subduction, generation of subduction-related melts, and the final formation of supra-subduction-zone rocks. These subduction-related rocks, along with the mid-ocean ridge basalts that were not subducted, form the present Yarlung-Tsangbo ophiolites.