中天山南缘乌瓦门蛇绿岩形成构造环境  

Tectonic setting of the Wuwamen ophiolite at the southern margin of Middle Tianshan Belt

作　　者：董云鹏[1] 周鼎武[1] 张国伟[1] 张成立[1] 夏林圻[2] 徐学义[2] 李向民[2] 

机构地区：[1]西北大学大陆动力学教育部重点实验室 [2]中国地质调查局西安地质矿产研究所,西安710054

出　　处：《岩石学报》2005年第1期37-44,共8页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划(973)(2001CB409801)国家自然科学基金(40472115)中国地质调查局(200113000022)项目联合资助

摘　　要：中天山南缘乌瓦门蛇绿混杂岩主要由构造岩块和混杂基质两部分组成,构造岩块主要包括:由变质橄榄岩、辉长岩、玄武岩组成的蛇绿岩残块、中天山基底变质岩系的斜长角闪岩和片麻岩构造块体以及来源于南天山泥盆系的大理岩残块;混杂基质主要为强烈剪切变形的绿泥石英片岩、绢云石英片岩、二云母石英片岩、千枚岩和变砂岩。变质橄榄岩主要为蛇纹石化的橄榄岩,SiO2、TiO2、Al2O3和CaO含量相对较高,而MgO含量相对较低。总体特征类似于二辉橄榄岩。稀土元素总量低,是球粒陨石稀土元素总量的0.3-0.5倍,以强烈亏损LREE为特征。乌瓦门玄武岩属于拉斑系列,主量元素显示低Al2O3、高TiO2、MgO,贫K2O、P2O5,Na2O>K2O性状,并以低∑REE、LREE亏损、高场强元素不分异为特征,类似于N-MORB。同时,部分岩石样品在高场强元素地球化学性状类似的基础上,具有不同程度的LILE、Th富集和Nb、Ta亏损,以及Pb富集特征,并有Zr的轻度低谷,显示岩浆源区曾遭受不同程度的俯冲带流体交代作用的影响。综合分析认为,乌瓦门蛇绿岩形成于弧后盆地环境。On the basis of synthetic studies of geology and geochemistry, an ophiolitic m lange in Wuwamen area was discovered that emplaced in the boundary fault separating the Southern Tianshan and Middle Tianshan belt. It is composed of different tectonic blocks with different lithological features and ages, and chaotic matrixes. The blocks contacting each other by a shear zone or chaotic matrix mainly include ophiolite slices, amphibolite and gneiss blocks of metamorphic basement of Middle Tianshan belt, and Devonian marble blocks from Southern Tianshan Belt. The ophiolite is composed of meta-peridotites, gabbros and basalts slices. Geochemical analysis suggests that the composition of these peridotites being similar to that of the Iherzolite in a mid-ocean ridge with high SiO2, TiO2, Al2O3, CaO contents, and low MgO content. Additionally, it is characterized by low SigmaBEE, and depleted in LREE intensively. Meanwhile, the basalts from the Wuwamen ophiolitic melange show high TiO2 and MgO content, low Al2O3, K2O, P2O5 content, and Na2O > K2O compositional features. In addition to, the geochemical characteristic of low SigmaREE, depleted in LREE and unfractionated in most HSF elements (Zr, Hf, Sm, Y and Yb) indicates that the basalts are similar to a normal mid-ocean basalt (N-MORB). Above all, some samples are showing distinguishable enrichment in large-ion lithophile (LIL) elements, Th and Pb contents, and depletion in Nb and Ta contents. It is suggest that the magma source was influenced by input of the subduction components. Base on all above evidences and regional geological data, it is reasonable suggest that the Wuwamen ophiolite was formed in a back-arc basin setting which have been existed along the southern margin of the Middle Tianshan Belt.

关 键 词：蛇绿混杂岩 地球化学 弧后盆地 天山造山带 

分 类 号：P588.125[天文地球—岩石学]