四川理塘地区二叠纪洋岛型岩石组合的识别及其构造意义:来自岩石学、地球化学和年代学证据

Identification of the Permian ocean island rock association from the Litang Area in Sichuan Province and its tectonic significance:Evidence from petrology,geochemistry and geochronology

甘孜-理塘蛇绿混杂岩带是西南"三江"构造带的重要组成部分,曾经历了古特提斯构造体系的演化,形成了完整的"沟-弧-盆体系"。本文是首次在甘孜-理塘蛇绿混杂岩带内发现有达森隆洼岩组洋岛型岩石组合,通过详细的岩石学、地球化学、年代学研究表明,达森隆洼岩组主要由枕状玄武岩、灰岩、硅质岩组成,玄武岩SiO_2含量为44.18%～48.82%,富TiO_2含量为2.71%～3.02%,∑REE=141.03×10^(-6)～164.94×10^(-6),轻稀土元素较重稀土元素富集,相对富集Rb、Th、U,亏损Ba、K、Sr,无明显的Nb和Ta异常,稀土元素配分曲线和微量元素蛛网图与典型洋岛玄武岩曲线相似。古生物化石显示该套洋岛型岩石组合沉积时代为二叠纪,锆石U-Pb同位素测年获得枕状玄武岩形成年龄为中二叠世。通过本次研究表明,甘孜-理塘洋盆至少在早二叠世时期就已发育有成熟洋盆。

The Ganzi-Litang ophiolite melange belt,which is an important part of the 6Three-River,orogenic belt in southwest of China,had experienced the evolution of paleo-tethys and formed one complete trench-arc-basin system.This study firstly reports the discovery of Dasenlongwa petrofabric ocean island rock association occurring in the Ganzi-Litang ophiolite melange belt.Detailed petrology,geochemistry and geochronology study shows that the ocean island rock is composed of pillow basalt,limestone and siliceous,and the basalt has SiO2 of 44.18%-48.82%and high TiO2 of 2.71%-3.02%.The rock is characterized by a total REE content of 141.03×10^-6-164.94×10^-6,enrichment of LREE relative to HREE,relative enrichment of Rb,Th and U,and depletion of Ba,K and Sr,no distinct anomalies of Nb and Ta.Its REE patterns and the trace element spider diagram are similar to that of typical OIB.Paleontological fossil shows that the sedimentary time of this oceanic island rock is Permian.The SHRIMP II U-Pb dating of zircon from the pillow basalt yields an age of the middle Permian.This research suggests that the Ganzi-Litang oceanic basin had been mature at least in early Permian.