新疆尾亚蛇绿混杂岩与镁铁质麻粒岩地球化学特征及构造意义  

Geochemical Features and Tectonic Significances of Weiya Ophiolitic Mlange and Mafic Granulite, Xinjiang

作　　者：陈希节[1] 舒良树[1] 马绪宣 

机构地区：[1]南京大学内生金属矿床成矿机制研究国家重点实验室,地球科学与工程学院,南京210093

出　　处：《高校地质学报》2012年第4期661-675,共15页Geological Journal of China Universities

基　　金：国家重点基础研究发展计划"中亚造山带大陆动力学过程与成矿作用"(2007CB411301);南京大学矿床研究国家重点实验室项目(2008-I-01)资助

摘　　要：在中天山北缘断裂带的哈密尾亚地区出露有蛇绿混杂岩和镁铁质麻粒岩。蛇绿混杂岩呈近东西带状分布，在约2km宽度范围内见有蛇纹岩、辉绿岩、辉长岩以及硅质岩等，均呈构造岩块裹挟在强烈剪切的泥砂质基质中镁铁质麻粒岩呈透镜状出现在片岩和片麻岩、花岗质片麻岩中，并被后期岩浆岩侵入。岩石地球化学测试数据表明，辉长岩与辉绿岩成分相近，以富MgO，贫Na2O和K20为特征，Na2O〉K2O。TiO2含量在1．45％-1．66％之间，平均为1．55％（9组）REE配分曲线呈平坦型，LREE轻微富集，HFSE分异差，不相容元素较富集。总体上具有富集型洋脊玄武岩（EMORB）地球化学特征。镁铁质麻粒岩的Si02含量为45％～47％，K20＋Na20为0．26％～0．79％，CaO为22．9％～24．3％，A1203含量介于10．9％．13．9％之间。在TAS图中，落在亚碱性辉长一辉绿岩区，TiO：含量在1．18％～1．47％之间，与岛弧和活动大陆边缘玄武岩相当，其原岩为不成熟岛弧环境下形成的拉斑玄武岩。在微量元素上，5组镁铁质麻粒岩均有较高的∑REE，REE配分曲线呈平坦型，LREE富集。在主量和微量元素构造环境判别图解中，9件辉长岩一辉绿岩和5件麻粒岩分别落在EMORB和岛弧拉斑玄武岩（IAT）区域。综合区域地质、岩石学和地球化学研究结果，认为尾亚东的镁铁质一超镁铁质岩石组合属于构造肢解的EMORB型蛇绿岩成员，形成于古洋盆初始裂解阶段的洋中脊构造环境，可能是属于古天山洋盆的岩石圈残片。尾亚西的镁铁质麻粒岩其原岩形成于不成熟的岛弧环境。它们的存在表明中天山北缘曾发生古板块俯冲消减机制，其分布位置可以代表古天山洋闭合后的缝合线。The ophiolitic mélange and mafic granulites are outcropped in the Weiya segment which is near to the northern boundary fault of the Central Tianshan belt. The sub-E-W-striking ophiolitie mélange 2 km wide is located about 8 km east of the Weiya railway station. It is composed of various blocks such as serpentinite, diabase, gabbro and siliceous rocks. All these blocks are flattened parallel to the regional metamorphic foliation and thus had likely experienced the same deformation as their country rocks （sandy and muddy phyllites and schists）. The mafic granulites are situated about 8 km west of Weiya and occur in the schists and gneisses as lenticular blocks, which were intruded by later stage granitic plutons. The petrological and geochemical data suggest that the geochemical features of the diabase and gabbro of the ophiolitic mélange are similar, characterized by highMgO contents （6.78%）, low NazO＋K20 （0.52%） contents and Na2O 〉 K2O. The TiO2 contents are between 1.45%-1.66% （1.55% on average of nine samples）, which are similar to ones of the typical EMORB-type basalt and displays the REE distribution curves characterized by a flat type and lower ∑REE with no or slight Eu anomaly. They exhibit distinct features in trace element spider diagrams characterized by slight fractionation of high field strength elements （HFSE） and enrichment in incompatible elements. The mafic granulites have SiO2 contents of 45%-47%, lower （K20＋Na20） contents of 0.26%-0.79%, CaO contents of 22.9%-24.3% and A1203 contents of 10.9%-13.9%. All the projections on the TAS diagrams are located in the subalkline gabbro-diabase area. The contents of TiO2 （1.18%-1.47%） are similar to island-arc or active continental margin basalt. The mafic granulites display geochemical features of immature island-arc tholeiite. Their REE distributions are characterized by a flat type and lower ∑REE and with no or slight Eu anomaly. The trace element spider diagrams show features of slight fractionation of high field

关 键 词：蛇绿混杂岩 镁铁质麻粒岩 地球化学 构造演化 中天山北缘断裂 哈密尾亚 

分 类 号：P548[天文地球—构造地质学] P595[天文地球—地质学]