Abstract: In1999,Xu Xi-wei et al.made an expedition to the Jinshajiang suture zone,northern Xizang.During the expedition,several rock samples were collected,among which5are mafic-ultramafic rocks.The five rock samples were collected from depopulated zone in northern Xizang,where no research work has been done before.Petrographical observation,whole-rock chemical analysis,REE and trace elements analysis,isotopic analysis,as well as microprobe analysis of mineral compositions of these samples have been carried out.Among these rock samples4are gabbro,and one is olivine pyroxynite,all of which belong to calcalkaline and alkaline suite,having a Rittman coefficient of 2.8～7.5.The rock samples do not contain any orthopyroxene,and no deformation microstructures are observed within the thin sections,indicating that no significant deformation has experienced by the rocks since their formation. The chemical compositions of the rocks are characterized by high Al₂O₃,FeO,Na₂O+K₂O,and low SiO₂,MgO contents.All of the rock samples have high REE abundance and similar REE pattern characterized by LREE enrichment with very small negative Eu anomaly.The Sr,Sm and Pb isotopes analyses of the samples do not yield ideal isochron,so that the accurate ages of the rocks can't be determined.The rocks studied have high Sr abundance and high⁸⁷Sr/⁸⁶Sr ratio(>0.706 9),as well as high ²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios.The afore-mentioned features of the rocks are significantly different from those of the ophiolite suites in northern Xizang.This may indicate that the rocks studied in this paper do not belong to the rocks of ophiolite suite,and their formation might be related to the back-arc expansion of the Jinshajiang suture zone.All the obtained results may be useful for further study in this region.

Key words: mafic-ultra-mafic rocks, petrology, petrochemistry, isotope composition, Jinshajiang suture zone, northern Xizang

摘要： 对采自藏北无人区西藏金沙江缝合带的基性、超基性岩样品进行了岩石学、岩石化学、稀土和微量元素、同位素等分析测试,以及主要矿物成分的电子探针分析。研究结果表明,这些样品为辉长岩和辉石岩,属于钙碱性至碱性系列,其矿物组成没有斜方辉石,未见变形显微构造特征。其化学成分以高Al₂O₃,FeO,Na₂O+K₂O和低SiO₂,MgO含量为特征。稀土元素配分型式以斜率较大的微弱负铕异常的右倾曲线为特征,LREE富集。它们具有较高的Sr丰度和⁸⁷Sr/⁸⁶Sr比值(>0.706 9)以及高的²⁰⁷Pb/²⁰⁴Pb,²⁰⁸Pb/ ²⁰⁴Pb和²⁰⁶Pb/²⁰⁴Pb比值。上述特征有别于藏北蛇绿岩的基性、超基性岩,表明它们不是蛇绿岩套组合,其形成可能与金沙江缝合带的弧后扩张有关。

关键词: 基性、超基性岩, 矿物成分, 岩石学, 岩石化学, 同位素组成, 金沙江缝合带, 西藏