西藏西南部休古嘎布蛇绿岩的成因:岩石学和地球化学证据  

ORIGIN OF THE XIUGUGABU OPHIOLITE MASSIF,SW TIBET:EVIDENCE FROM PETROLOGY AND GEOCHEMISTRY

作　　者：徐德明[1] 黄圭成[1] 雷义均[1] 

机构地区：[1]宜昌地质矿产研究所,湖北宜昌443003

出　　处：《大地构造与成矿学》2007年第4期490-501,共12页Geotectonica et Metallogenia

基　　金：中国地质调查局资源评价项目"西藏雅鲁藏布江西段铬铁矿资源远景调查"(1212010530108)

摘　　要：休古嘎布蛇绿岩块位于雅鲁藏布缝合带（YZSZ）西段，主要由地幔橄榄岩和侵入其中的基性岩墙所组成。基性岩墙具有弧后盆地地球化学亲缘性，其LREE亏损，（La／Yb）N为0．39～0．55；具有明显的Nb、Ti负异常及Sr、Ba正异常。Sr、Nd同位素特征表明它们起源于亏损地幔，并且受到了板片析出流体的影响。4个基性岩墙的Sm-Nd同位素样品获得内部等时线年龄为126．2±9．1Ma（MSWD=0．44）。地幔橄榄岩具有汤勺形和U形两组REE分布型式，显示出不同程度部分熔融和地幔交代作用的特征，具有弧一盆体系地球化学亲缘性。第一组橄榄岩的LREE弱富集或近于平坦，尖晶石的Cr^#值低而且变化不大，部分熔融程度较低（15％-20％），可能形成于弧后扩张盆地；第二组橄榄岩的LREE明显富集，尖晶石的Cr^#值高而且变化较大（0．4～0．77），部分熔融程度较高（25％～30％），并经历了强烈的交代富集作用，可能与洋内岛弧有关。The Daba-Xiugugabu ophiolite massif is situated in the western portion of the Yalung Zangbo Suture Zone （YZSZ） , composed mainly of mantle peridotites and of mafic dikes that intruded the mantle peridotites. The mafic dikes show similar geochemical features to back-arc basin basahs, characterized by LREE depletion with （La/Yb）N between 0.39 to 0.55, and clear negative anomalies of Ti and Nb and positive anomalies of Sr and Ba. Sr and Nd isotope systematics indicates an origin from a depleted upper mantle source modified by fluid released from the subducting oceanic slab. Sm-Nd isotopic data of four mafic dike samples yielded an isochron age of 126.2 ± 9. 1Ma （ MSWD = 0.44）, representing the emplacement age of the mafic magma. The mantle peridotites have two types of REE patterns： spoon-shaped and U-shaped. The spoon-shaped peridotites display slightly LREE-enriched to roughly flattened patterns and have low and limited range of spinel Cr^# values, features of mantle peridotites in back-arc extentional basins. Such trace element characteristics reflect lower degrees of partial melting （15% -20% ） and moderate to low extent of metasomatism. By contrast, the peridotites having U-type REE patterns show significant LREE enrichment, and have higher and variable spinel Cr^# values （0.40 - 0.77） , implying that they experienced higher degrees of partial melting （25% -30% ） and the following extensive incompatible element enrichment by melt/fluid metasomatism. Combined geochemical data of the mafic and ultramafic components of the Daba-Xiugugabu ophiolite massif suggests that the ophiolites were probably formed in an intraoceanic arc.

关 键 词：蛇绿岩 地球化学 SR ND同位素 弧盆体系 西藏 

分 类 号：TE121.1[石油与天然气工程—油气勘探] P542[天文地球—构造地质学]