Subducted Front of the Indian Continental Crust Beneath the Tibetan Plateau in the Early Eocene

The immense Tibetan Plateau is a masterpiece of the ongoing India-Asia collision. The timing of the collision is a critical parameter to reconstruct the plateau evolution, but has been not well understood yet. Here, we report a newly discovered Eocene (53.8 Ma) shoshonitic intrusion with the input of Indian continental material in the Yangbajing area of the Lhasa Terrane, south Tibet. The reverse Pb-207/Pb-206 zoning exhibited by feldspars, the linear isotopic arrays between whole-rock Os-187/Os-188 and 1/Os-188, the in-situ feldspar Pb-208/Pb-204 and Pb-206/Pb-204 values, and the in-situ zircon epsilon(Hf)(t) values and delta O-18 values strongly demonstrate a binary interaction process for the generation of the Yangbajing shoshonitic intrusion. Our Os isotopes further illustrate that the interaction process was dominated by recycled crustal melt assimilating minor mantle melt. In addition, different isotopic systems consistently point to an end-member represented by the underthrust crust slice of Indian continent, which indicates that the Indian lithosphere must have already subducted beneath the Lhasa Terrane at 53.8 Ma. This is also supported by the abrupt isotopic shift toward the enriched values and the composition variation toward the high potassium contents in the subsequent Tibetan magmatism, corresponding to an increasing involvement of the Indian continental crust into the upper plate magmatism. Therefore, the Yangbajing shoshonitic intrusion is one of the earliest known magmatic evidence for the onset of the India-Asia collision. Plain Language Summary The timing of the ongoing India-Asia collision is a critical parameter to reconstruct the plateau evolution. Here, we report a newly discovered Eocene shoshonitic intrusion with the input of Indian continental material in the Yangbajing area of the Lhasa Terrane, south Tibet. The reverse Pb-207/Pb-206 zoning exhibited by feldspars, the linear isotopic arrays between whole-rock Os-187/Os-188 and 1/Os-188, the in-situ feldspar Pb-208/Pb-204 and Pb-206/Pb-204 values, and the in-situ zircon epsilon(Hf)(t) values and delta O-18 values strongly demonstrate a binary interaction process for the generation of the Yangbajing shoshonitic intrusion. Our Os isotopes further illustrate that the interaction process was dominated by recycled crustal melt assimilating minor mantle melt. In addition, different isotopic systems consistently point to an end-member represented by the underthrust crust slice of Indian continent, which indicates that the Indian lithosphere must have already subducted beneath the Lhasa Terrane at 53.8 Ma.