The identification of the Eocene magmatism and tectonic significance in the middle Gangdise magmatic belt, Nanmulin area, Tibet

The Nanmulin rock is located in the southern Gangdise center. In this study, the authors conducted systematical zircon LA-ICP-MS U-Pb dating and whole-rock geochemical analysis for the monzonitic granite in Nanmulin. The zircon U-Pb analyses demonstrate that the crystallization and emplacement age of the plutons is 50.24 ±0.68Ma, suggesting Eocene magmatic activities. It is an important part of the Gangdise batholith. Geochemical characteristics show that Rittmann index(σ) is 1.66~1.94, implying calc-alkaline series. The Al₂O₃ varies in the range of 11.82%~12.45%, whereas A/CNK varies in the range of 1.11~1.15, suggesting peraluminous characteristics. The granite is characterized by high SiO₂(77.4%~78.18%, averagely 77.82%), high K (shoshonite series). Rb, Th, U and LREE are enriched but Nb, Ta, Zr and HREE are depleted, suggesting arc-related or earth crust geochemical affinities. Sm/Nd varies from 0.45 to 0.57, averagely 0.53, suggesting the deep source characteristics of magma. (La/Yb)_N is high, averagely 9.25, suggesting enrichment of the LREE and depletion of HREE, with clear negative Eu anomaly. In summary, the process of granite petrogenesis resulted from subduction or breakoff of the Tethyan Ocean crust experiencing dehydration. This action resulted in partial melting of overlying mantle wedge and caused upward migration of basaltic magma near Moho. Huge thermal baking forces caused partial melting of the lower crust (clay-rich or argillaceous rocks). Magma mixing occurred from both basaltic magma and felsic magma. Nanmulin granite with MME was formed. This study provides isotope age evidence on the time of the closure of the Tethyan Ocean and Indian-Asian continental collision. Moreover, it enriches the diagenetic model and geochemical characteristics of Gangdise rock.