Compression-extension transition and crustal evolution in the early India-Eurasia collision zone

The early India-Eurasia collision induced widespread tectonic deformation and vigorous magmatism throughout southern Tibet, yet the dynamic connections between magmatism and crustal evolution in the collision zone remain controversial. Here, we present new integrated datasets comprising zircon U-Pb geochronology, in situ zircon Hf-O isotopes, whole-rock major and trace element geochemistry, and bulk Sr-Nd isotopes for the Kazi volcanic sequence within the Paleocene–Eocene Pana Formation in the Namling area, southern Lhasa Terrane. Zircon U-Pb dating constrains synchronous eruption of bimodal basaltic-andesites and rhyolite lavas at ca. 48 Ma. Geochemical and isotopic results indicate that the basaltic-andesite magmas originated from subduction-metasomatized hybrid mantle sources, with insignificant crustal contamination and petrogenesis controlled by two-stage fractional crystallization. The contemporaneous rhyolites exhibit typical geochemical signatures of A-type granites, generated by low-pressure dehydration melting of juvenile felsic crustal materials. Consistently depleted Sr-Nd-Hf-O isotopic compositions reveal a predominant juvenile mantle contribution to the magmatic system. The newly identified ca. 48 Ma bimodal magmatism documents the complete breakoff of the Neo-Tethyan oceanic slab and marks a tectonic transition from compression to extension in southern Tibet. This magmatic event records a prominent Phanerozoic crustal growth episode on the Tibetan Plateau, providing robust constraints on the dynamic evolution of tectonic switching and crustal evolution in global collisional orogens.