西昆仑柯岗蛇绿岩地质地球化学特征及构造意义

Geological, geochemical features and structure significance of Kegang ophiolite, West Kunlun

柯岗蛇绿岩位于青藏高原北缘,与北部的奥依塔格蛇绿岩、南部的库地蛇及苏巴什绿岩组成青藏高原“第五缝合带”。本文对柯岗蛇绿岩中辉长岩进行了岩石学、地球化学及锆石LA-ICP-MS分析。柯岗辉长岩SiO2含量为43.65%-49.29%,在TAS分类图解上,样品主要落在辉长岩区域,与野外及镜下观察一致。柯岗辉长岩稀土元素总量较低,∑REE范围在12.12-45.45μg/g,经球粒陨石标准化的REE分布模式显示,3个样品显示轻稀土略亏损特征, Eu＊为正异常,平均为1.34,1个样品显示轻稀土富集特征, Eu＊为0.86,为岩浆分异结晶不同时期的产物。柯岗辉长岩锆石Th/U范围为0.28-0.67,且具有典型岩浆锆石的震荡环带结构或扇形结构特征。22颗锆石206Pb/238U加权平均年龄为（488.8±2.6） Ma, MSWD=0.81,代表柯岗蛇绿岩套中辉长岩结晶年龄。通过柯岗蛇绿岩中辉长岩地球化学特征和构造环境判别图解,并结合前人资料,认为柯岗蛇绿岩形成环境为岛弧或者弧前环境。

Kegang ophiolite is located at the northern Qingzang Plateau. It is part of the Fifth Suture Zone of Qingzang Plateau with Oytag, Kudi and Subashi ophiolite. This paper reports petrology, geochemistry, and zircon LA-ICP-MS analyses of gabbro in Kogang ophiolite. The major elements show that the distribution of SiO2 from 43.65%-49.29%, and in the TAS classification diagram, the samples plot in gabbro area, which is consistent with the field and microscopy observations. The total REE abundances are low, ranging between 12.12-45.45 μg/g, three samples show slightly enrichments in LREE and depletions in HREE, while other samples shows depletions in LREE and enrichments in HREE. Eu＊ shows slightly positive anomaly in LREE enriched samples, with an average of 1.34, and reflect weak magmatic plagioclase fractional crystallization, while in the LREE depleted samples, the Eu＊ is 0.86, and reflects strong magmatic plagioclase fractional crystallization. In the cathodoluminescence images, the zircons in Kegang gabbro show crystal morphology and clear oscillatory zoning, which together with the high Th/U （0.28 to 0.67）, indicate magmatic origin. 22 single grain zircon U-Pb yielded a weighted mean 206Pb/238U age of （488.8±2.6） Ma （MSWD = 0.81）, which represents the crystallizing age of the gabbro. Based on the geological, geochemical characteristics and previous research results, we believe that Kegang ophiolite was formed in an island arc or forearc environment.