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

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-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。

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

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 basalts, 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.