新疆西准噶尔萨尔托海蛇绿混杂岩形成时代及构造环境分析

萨尔托海蛇绿混杂岩是西准噶尔地区达拉布特蛇绿混杂岩带的重要组成部分,其形成时代和构造环境颇具争议。通过LA-ICP-MS锆石微区原位U-Pb同位素测年,获得其中辉长辉绿岩的 ²⁰⁶Pb/²³⁸U 加权平均年龄为388.8±1.1 Ma,确定该蛇绿混杂岩形成于中泥盆世。岩石地球化学分析表明,纯橄岩(蛇纹岩)属于低Si、低碱、高Mg的变质橄榄岩,REE呈∑REE 偏低的平坦型分布模式,Cr、Ni和Co含量较高,变玄武岩属于高Ti、富Mg、富Fe、偏碱的拉斑系列,REE分布模式呈现 ∑REE 偏低、Eu异常不明显的平坦型曲线,微量元素分布模式为K、Cs、Th、Nb和P相对亏损的平坦型曲线,Zr/Nb比值低,总体表现出与E-MORB相似的特征。综合判断认为,萨尔托海蛇绿混杂岩形成于大陆裂谷向洋盆转化的构造环境,类似于"红海式"洋盆,为进一步深入认识新疆地区众多蛇绿混杂岩的形成构造环境提供了新思路。

The age and tectonic setting of Sartokay ophiolitic mélange in western Junggar,Xinjiang

For Sartokay ophiolitic mélange,as an important part of the Dalabute ophiolite in western Junggar of Xinjiang,it is widely controversial on its formation age and the tectonic setting.High resolution LA-ICP-MS in situ dating yields the zircon ²⁰⁶Pb/²³⁸U weighted average age of 536.4±3.6 Ma from the gabbro-diabase in Sartokay ophiolitic mélange,by which we believe that this ophiolitic mélange formed in Middle Devonian.The geochemical characteristics prove that the Dunite(Serpentinite)belongs to the metamorphic peridotite with low Si,low alkali,and high Mg.The REE distribution pattern of the dunite is generally flat with low ∑REE,high Cr,Ni and Co.The meta-basalt belongs to the subalkalictholeiite series rich in Ti,Mg and Fe,and the REE distribution pattern shows a flat-type with lower ∑REE,and without obvious Eu anomaly.The trace element distribution of the meta-basalt is also flat,with relatively depleted K,Cs,Th,Nb and P,lower Zr/Nb ratio.All these results indicate that Sartokay ophiolitic mélange generally resembles the E-MORB type.Comprehensively,we consider that Sartokay ophiolitic mélange formed in a tectonic setting where a continental rift was transforming into an ocean basin,which may be similar to the "Red Sea type".This study provides a new clue to the further understanding of the tectonic settings of the ophiolites in Xinjiang.