LATE JURASSIC MAGMATISM IN XURU TSO BATHOLITH, CENTRAL LHASA SUBTERRANE: PRODUCTS OF SOUTHWARD SUBDUCTION OF BANGONG-NUJIANG NEO-TETHYAN OCEANIC SEAFLOOR?

The Lhasa Terrane is widely considered an archetype of a continental arc and continent–continent collision zone, which experienced the Jurassic–Cretaceous Lhasa–Qiangtang collision marked by the BNSZ and resulted in widely exposed Mesozoic magmatic rocks. However, the interpretations of the petrogenesis and geodynamic setting of the central Lhasa subterrane during the late-Jurassic remain poorly constrained.

The enclaves can provide important information on the nature of the source regions, and the evidence of interaction between continental crust and mantle. This study presents new zircon U-Pb age, whole-rock geochemistry, and zircon Hf isotopic data for the host granite and dioritic enclaves from the Xuru Tso area. The zircon dating for the host granites yields U-Pb age of 155.1 ± 0.7Ma, which is identical to that of the dioritic enclave (155.7 ± 0.7Ma). There is a good linear evolution trend between the enclaves and the granites in Harker diagrams, which indicates magma mixing .The host rocks are metaluminous to slightly peraluminous and high K calc-alkaline I-type granites, showing enrichment of large ion lithophile elements and light rare-earth elements relative to high field-strength elements. Their negative zircon ε_Hf(t) values (-16.6 - -6.6) indicate that they were derived from antaxis of ancient Lhasa terrane crust. The dioritic enclaves are metaluminous calc-alkaline and exhibit overall negative but significantly higher zircon ε_Hf(t) values (-8.9 - -3.8) in comparison with the host granites, suggesting that these enclaves may document the input from mantle lithosphere.

Combined with the published chronological and geochemical data of the central and northern Lhasa subterrane, these observations indicate that the late-Jurassic granites from Xuru Tso were formed by the magma mixing between mantle-derived melts and crust-derived silicic melts originating from ancient crust beneath central Lhasa subterrane, and can be best explained as reflecting the southward subduction of Bangong-Nujiang Neo-Tethyan oceanic seafloor.