南天山北部志留系巴音布鲁克组火山-侵入杂岩的形成环境及构造意义

The Tectonic Setting and Implication of Volcanic-Magmatic Complex from the Upper-Silurian Bayinbuluke Formation, Southern Tianshan

南天山北部上志留统巴音布鲁克组火山岩系以中-基性喷出岩为主，并有辉长岩侵入．岩层中爆发相火山碎屑岩极为发育。各火山-侵入杂岩的Zr／Nb=26～47，平均为37，Hf／Ta=11．9～19．9，平均为14．8．暗示其源区与N—MORB相类似，为亏损地幔源。（La／Yb）N值为2．92～5．21、Nb／La比值为0．22～0．33，平均为0．29，显示出LREE和LILE强烈富集，HFSE相对亏损，尤其出现明显的Ta、Nb负异常，具有岛弧岩浆岩的地球化学特征，应形成于岛弧环境。该套火山-侵入岩与中天山南缘蛇绿混杂岩带紧邻，地层时代也与古生代南天山洋的俯冲-消减时代相当，它很有可能是南天山洋盆于晚志留世时发生的洋内俯冲-消减所引发的岛弧火山-岩浆作用的地质记录。

The volcanics of the Upper-Silurian Bayinbuluke Formation in the northern part of the Southern Tianshan are mostly intermidiate-basic in composition, are associated with gabbro and contain abundant pyroclastic rocks. The Zr/Nb values （26～47, averaged 37）, Hf/Ta values （11, 9～19.9, averaged 14.8） of the volcanic-magmatic complexes indicate that, similar to the N-MORB, they have a depleted mantle derivation. Their （La/Yb）N values range from 2.92 to 5.21） and Nb/La values from 0.22 to 0.33, averaged 0.29, showing that they are enriched in LREEs and LII,Es and depleted in HFSEs（especially with apparent negative Nb and Ta abnormalies）, features similar to those of the island arc magmatic rocks and an apparent indication of formation in island arc setting. In space, the Bayinbuluke Formation is near an ophiolitic mélange belt on the southern margin of the Central Tianshan; in time, these strata almost have similar age as the subducting age of the Southern Tianshan Ocean. Thus, it stands a good chance that the formation of these volcanic-magmatic complexes owes to late-Silurian subduction of the Southern Tianshan Ocean.