OCEANIC ISLAND BASALTS IN GARZE-LITANG JUNCTION ZONE, WESTERN SICHUAN PROVINCE: Geochemistry, Geochronology and Geological Implication

A large number of oceanic island-seamount rock assemblages are developed in Ganze-Litang junction zone-an important part of Sanjiang Tethyan Orogen and Songpan-Ganze Orogen. The existence of oceanic island basalts (OIB) indicates that mature oceanic crust has developed in the ocean basin. The oceanic island-seamount rock assemblages in Muli area are composed of basic volcanic rocks and carbonate rocks. The geochemical analysis of basic volcanic rocks shows the SiO₂ content of 40.17%-49.19%, TiO₂ 1.77%-4.86%, Al₂O₃ 9.67%-15.39%, MgO 3.85%-17.75% and total alkali (K₂O+Na₂O) 0.87%-6.33%, respectively, belonging to alkali basalt series. The basalts are characterized by high ∑REE (106.21×10^-6-378.83×10^-6), (La/Yb)_N of 9.23-39.41, LREEs enrichment, without Eu and Ce anomalies, right-dipping distribution patterns, enriched LILEs (Rb, Ba, Th and K), and depleted HFSEs (Nb and Zr). The characteristics of REEs and trace elements are similar to those of standard OIBs, with the provenance of garnet peridotite from asthenosphere mantle, some undergoing magmatic crystallization differentiation and forming in oceanic island platform environment. The zircon U-Pb dating result of basalts (218.96-221.71 Ma) reveals that the OIBs were formed in middle Late Triassic.