Abstract:

Nyainqentanglha Early Eocene intermediate-basic intrusive rocks consist of amphibole gabbro and monzodiorite. The zircon LA-MC-ICPMS U-Pb dating for the amphibole gabbro defines a crystallizationage of (49.92±0.24) Ma. The intermediate-basic intrusive rocks are high-K calc-alkaline to shoshonite series based on the SiO₂-K₂O diagram. They contain high total REE with enrichment of LREE and LILE (Rb, Th, K), and depletion of HFSE (Nb, Ta, Ti), showing features of magmatic rocks in island arc or active continental margin. They also show relatively high initial ⁸⁷Sr/⁸⁶Sr values of 0.708 77-0.713 71, low ε_Nd values from -6.9 to -6.1, old modal age of 1 329-1 995 Ma (T_DM), and no correlation for ε_Nd with MgO and Nb/La, which indicates that they come from an enriched mantle metasomatized by ancient continental crust. REE simulation suggests that the forming processes for the intermediate-basic intrusive rocks were controlled by low tomoderate partial melting of the enriched mantle. The dynamic background for the intermediate-basic intrusive rocks are likely linked to breakoff of Neo-Tethyan slab followed by upwelling of asthenospheric materials from the slab window.

Key words: Nyainqentanglha, intermediate-basic intrusive rocks, enriched mantle, asthenosphere, partial melting, geochemistry