川西甘孜-理塘结合带洋岛型玄武岩地球化学、年代学特征及其地质意义

甘孜-理塘结合带作为三江特提斯造山带与松潘-甘孜造山带的重要组成部分，发育大量的洋岛-海山岩石组合，洋岛玄武岩的存在标志着洋盆已发育成熟的洋壳.木里地区的洋岛-海山岩石组合由"基性火山岩+碳酸盐岩"构成，对基性火山岩的地球化学特征分析表明：基性火山岩SiO₂含量为40.17%~49.19%，TiO₂含量1.77%~4.86%，Al₂O₃含量9.67%~15.39%，MgO含量3.85%~17.75%，全碱含量（K₂O+Na₂O）0.87%~6.33%，属碱性玄武岩系列.稀土总量∑REE值较高，为106.21×10^-6~378.83×10^-6，（La/Yb）_N比值为9.23~39.41，为轻稀土富集型，Eu、Ce无异常，配分模式为右倾型.微量元素表现出大离子亲石元素Rb、Ba、Th、K富集，而亏损高场强元素Nb、Zr.稀土元素及微量元素特征与标准洋岛型玄武岩相近，源区为软流圈地幔石榴石橄榄岩，且部分经历了岩浆结晶分异，形成于洋岛台地环境.玄武岩的锆石U-Pb定年结果为218.96~221.71 Ma，表明洋岛型玄武岩形成于晚三叠世中期.

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.