东昆仑西段夏日哈木榴辉岩原岩属性及陆(弧)陆碰撞

摘要

东昆仑西段夏日哈木榴辉岩呈透镜状分布于古元古代金水口群中,榴辉岩的SiO2含量47.4％～50.78％,Mg#值为40～62,表明其原岩为分异程度较高的基性岩,属于拉斑玄武岩系列.基于岩石Nb的含量,将夏日哈木榴辉岩分为两类——富Nb型榴辉岩(Nb＞7×10-6)和正常榴辉岩(Nb＜7×10-6).富Nb型榴辉岩具有贫SiO2、MgO、Al2O3,富集FeOt、TiO2的特点;稀土元素配分曲线具有右倾的OIB特点,与古生代富Nb玄武岩和Seki玄武岩微量元素特征类似;微量元素具有弱的Nb、Ta、Ti的负异常,与古生代富Nb玄武岩特征类似.正常榴辉岩具有贫FeOt、TiO2,富集SiO2、MgO、Al2O3的特点;稀土元素配分曲线具有弱右倾的E-MORB特点,微量元素具有弱的Nb、Ta、P的负异常,U、Sr的正异常,类似于Okinawa板内弧后盆地玄武岩微量元素特征.富Nb型榴辉岩的εNd(0)和εNd(t=571Ma)值分别为0～4.55、-0.88～7.26,正常榴辉岩的εNd (0)和εNd(t=571Ma)值分别为2.2～5.19、3.17～6.22,两者均位于加里东HP-UHP变质带和大别-苏鲁HP-UHP变质带榴辉岩Sm-Nd同位素组成范围内.在微量元素图解上,样品显示具有地壳混染的趋势.由此可知,夏日哈木榴辉岩的原岩可能是具有陆内弧后盆地玄武岩性质的岩石,源区受到俯冲板片熔体的改造,岩浆过程中同化混染一定量的下地壳.根据现有的年代学资料,夏日哈木榴辉岩峰期变质可能发生在晚奥陶世-志留纪.夏日哈木榴辉岩可能是在弧后盆地消亡过程中,弧陆发生俯冲碰撞的结果.%Xiarihamu eclogites occur as lenses in the Proterozoic Jinshuikou Group at the western part of East Kunlun.The eclogites have SiO2 content varying from 47.4％ to 50.78％ and Mg# values from 40 to 62,implying that protoliths of the eclogites were highly fractionated mafic rocks and suggesting that they belong to tholeiite series.This study divides the Xiarihamu eclogites into two groups in the light of Nb contents,Nb-enriched eclogites (Nb＞7 × 10-6) and normal eclogites (Nb＜7× 10-6).The Nb-enriched eclogites are characterized by low SiO2,MgO,Al2O3 and high FeOt,TiO2.The chondrite-normalized REE patterns of the Nb-enriched eclogites display OIB patterns consistent with Paleozoic Nb-enriched basalts (NEB) and Seki island arc basalts.In additional,the primitive mantle-normalized trace elements spider diagrams have weakly negative anomalies of Nb,Ta,Ti,consistent with Paleozoic Nb-enriched basalts (NEB).The normal eclogites (Nb＜7× 10-6) are characterized by low FeOt,TiO2 and high SiO2,MgO,Al2O3.The REE patterns of normal eclogites display E-MORB patterns;the spider diagrams show weakly negative anomalies of Nb,Ta,P and positive anomalies of U,Sr,consistent with continental back arc basin basalt (BABB) like Okinawa BABB.εNd (0) and εNd (t=571Ma) isotopic contents of the Nb-enriched eclogites are 0～4.55 and-0.88～7.26 and those of the normal eclogites are 2.2～5.19,3.17～6.22,both plotted into the field of HP-UHP belts of Caledonian orogeny and Sulu-Dabie orogeny.Trace element diagrams also show that the samples have been assimilated and contaminated.Therefore,the protoliths of the Xiarihamu eclogites could be interpreted to be derived from intracontinental BABB with the origin reconstructed by subduction-related crust and assimilation-contamination during magmatism.According to the published age data,it can be concluded that high-pressure metamorphic event in the North Kunlun terrane occurred in the Late Ordovician-Silurian and the Xiarihamu eclogite is probably the result of metamorphism owing to an arc-continent collision during closure of the back-arc basin.