新疆天山地区榆树沟-铜花山蛇绿岩特征和构造背景

The Yushugou-Tonghuashan ophiolites in Tianshan, Xinjiang,and their tectonic setting

榆树沟-铜花山蛇绿岩出露于南天山北缘,属哈萨克斯坦-伊犁板块和塔里木板块之间的缝合带。蛇绿岩已被构造肢解,主要由超镁铁岩、堆晶岩、熔岩类组成。超镁铁岩以方辉橄榄岩为主,显示典型的亏损地幔岩特征,橄榄石为富镁型,Fo为90;斜方辉石为顽火辉石,En为90,单斜辉石含量少;副矿物铬尖晶石属低铬型,Cr#值(=Cr/(Cr+Al)×100)16～28,Mg#值(=Mg/(Mg+Fe)×100)63～75,反映了深海橄榄岩特征。归一化后榆树沟超镁铁岩MgO含量38.84%～44.53%,Al2O3为1.51%～3.63%,CaO为0.42%～5.77%,成分近于大洋二辉橄榄岩;铜花山的超镁铁岩叠加碳酸岩化,LREE强烈富集,可能经历了俯冲洋壳流体改造。熔岩类在榆树沟和铜花山均有较大规模产出,其中榆树沟玄武岩为主,铜花山安山岩和英安岩较多。熔岩多已遭受绿片岩相海底热液蚀变。榆树沟玄武岩的REE含量总体比铜花山的低,稀土配分模式均为轻稀土富集型;玄武岩的微量元素特征表明其源区可能遭受过流体作用影响。铜花山三个高镁火山岩化学成份具有SiO2(32%～36%)含量很低、MgO(20.12%～28.50%)含量高、K2O+Na2O(0.06%～0.46%)含量小于2%的特征,综合分析可归为苦橄岩类。堆晶岩通常以构造小岩块产在超镁铁岩块中或基性熔岩中,以辉长岩为主,有少量橄榄辉石岩等,岩石蚀变、构造变形强烈。堆晶岩的成分接近玄武岩,存在LREE富集和略亏损平坦两种类型。岩石地球化学特征表明榆树沟和铜花山的蛇绿岩形成于MOR构造环境,但具有受俯冲带流体改造特征,并伴生有岛弧火山岩。

The Yushugou-Tonghuashan ophiolite,outcropped in the northern part of the southern Tianshan between Yili plate and Tarim plate,has tectonically broken into ultramafic rocks,cumulates and volcanic lavas.The ophiolite is chiefly composed of harzburgite,one kind of typical depleted peridotite.The olivines are forsterite （Fo is about 90）,enstatite （En is about 90） and a few clinopyroxene.The accessory mineral chrome spinel bears the Cr#=Cr/（Cr＋Al）×100 value of 16~28,the Mg#=（Mg＋Fe）×100 value of 63~75,suggesting a low chromium and abyssal peridotite features.After the anhydration,the contents of MgO （38.84%~44.53%）,Al2O3 （1.51%~3.63%） and CaO （0.42%~5.77%） of Yushugou ultramafic rocks are similar to the oceanic lherzolite.The Tonghuashan ultramafic rocks occurred in carbonatite bearing the LREE enrichment might be produced by certain subduction fluid alteration.The volcanic lavas occur widely here,in Yushugou is mainly basalt while in Tonghuashan is andesite and dacite.Most of the volcanic lavas have undergone the moderate greenschist seafloor hydrothermal alteration.REE contents of Tonghuashan basalt are higher that of Yushugou,REE patterns are LREE enrichment type.The trace element compositions of basalt indicate they may suffer from the fluid influence.There are three high-Mg volcanic rock samples,according to the various features like the extreme low SiO2 content （32%~36%）,high MgO content （20.12%~28.50%） and the value of K2O＋Na2O （0.06%~0.46%） less than 2%,which can be judged as picrite.The cumulate,generally locates in the ultramafic rocks or mafic lavas as small blocks,is mostly gabbro with less olivine pyroxenite and with significant alteration and structural deformation,which is similar to the basalt in composition and with two type LREE patterns including enriched and slightly depleted.According to the geochemical features,the Yushugou-Tonghuashan ophiolite was produced in Mid-Ocean Ridge （MOR） environment,with certain subduction fluid alteration and was associated with the generation of island-arc volcanic rocks.