新疆北部富蕴县沙尔布拉克玻安岩的地球化学特征及构造意义

Geochemical characteristics of the Shaerbulake boninites and their tectonic significance, Fuyun County, northern Xinjiang, China

详细的地球化学研究表明,新疆北部富蕴县沙尔布拉克中泥盆统北塔山组的辉石安山岩具有中等的SiO2含量(52.13%～60.45%);高MgO(3.91%～9.31%)和相容元素(包括Ni、Cr、Co等)含量,以及高Mg#和Al2O3/TiO2比值;低TiO2含量(0.21%～0.35%);同时具有U型稀土元素分布模式;并强烈亏损高场强元素。这些特征均与玻安岩的地球化学特征一致,应属于玻安岩,形成于前弧环境。沙尔布拉克玻安岩的存在表明,古亚洲洋在早—中泥盆世向南发生了洋壳俯冲作用。

The petrologic and geochemical studies on the pyroxene andesites in Middle Devonian Beitashan Group in Saerbulake, Fuyun County, northern Xinjiang have been carried out. The pyroxene andesites have porphyritic texture. The main porphyritic crystals are uralitized or epidotized pyroxenes and slate plagioclase. The matrix consists of acicular hornblendes and a few slate micro plagioclases. The rocks have moderate SiO2 (52.13% ～ 60.45% ), low TiO2 (0.21% ～ 0.35% ), high MgO (3.91% ～ 9.31% ) and CaO (4.66% ～ 9.46% ) contents with high Mg# and Al2O3/TiO2 (38.9～ 77.3) ratios. The compatible elements (such as Ni, Cr and Co) contents are high with strong HFSE (Ti, Zr, Hf, Y and Yb) depletion. The MORB normalized spider diagram shows obviously negative Nb, Ta, Zr and Hf anomalies and positive Sr anomaly. The total REE contents are low with depletion of middle REE in the samples. Their REE pattern belongs to U type. Therefore, the petrologic and geochemical characteristics of pyroxene andesites in Middle Devonian Beitashan Group in Saerbulake are very similar to those of boninite. They are boninite series rocks, and formed in forearc environment. The existence of boninite in Shaerbulake means that there was not only a northward subduction of the Paleo Asian Ocean beneath the Siberian Plate, but also a southward subduction beneath the Kazakhstan Junggar Plate in Early and Middle Devonian.<P>