东准噶尔玛因鄂博地区增生楔中基性混杂岩块地球化学特征及构造环境探讨

东准噶尔玛因鄂博地区发育一典型的小型增生楔, 增生楔中辉长岩岩块具有高铝(Al2O3:14.38%～16.33%), 高钛(TiO2:1.81%～2.46%), 贫钾(K2O:0.17%～0.63%)的地球化学特征。相对富集大离子亲石元素(Sr、K、Rb、Ba), 高场强元素(Nb、Ta、Zr、Hf)没有明显的异常, 源区具有类似于洋脊玄武岩的特征。基性岩块具有类似于E-MORB和OIB的稀土元素配分形式, 没有出现Nb、Ta、Ti的明显亏损, 说明其形成过程中可能没有受到或受到较弱的消减带影响, 综合上述认识, 认为夹于增生楔中的此类基性岩块体很可能来源于洋壳俯冲过程中增生的海山残片;根据玛因鄂博增生楔物质组成特征以及前人的相关年代学证据表明该增生楔形成于古生代弧后盆地环境, 在晚泥盆世之前形成俯冲增生杂岩体。

Geochemical Characteristics and Tectonic Environment of the Accretionary Wedges Mafic Mingle Rocks in Mayin'ebo Region,East Junggar

A typical small accretionary wedge develops in Mayin'ebo region, east Junggar. Gabbro rock in the accretionary wedge block has high Al2O3(14.38%-16.33%), high TiO2(1.81%-2.46%) and poor K2O(0.17%-0.63%)geochemical characteristics. It is relatively enriched in LILE(Sr, K, Rb, Ba), and has no obvious anomaly in HFSE(Nb, Ta, Zr, Hf) content, moreover, the material source is identical with oceanic ridge basalt. Mafic rocks have similar E-MORB and OIB REE pattern, and no significant loss of Nb, Ta, Ti appeared, indicating that the process of its formation may not have been or weakly affected by the subduction zone. Comprehensively, such mafic rock of accretionary wedge is likely to come from the oceanic crust subducted seamount fragments proliferation process. According to material composition characteristics and previous geochronology evidence, Mayin'ebo accretionary wedge was formed in the post-Paleozoic accretionary wedge arc basin, the subduction accretionary complex was formed before the late Devonian.