阿尔泰造山带南缘中泥盆世苦橄岩及其大地构造和岩石学意义

Middle Devonian Picrites of South Margin of Altay Orogenic Belt and Implications for Tectonic Setting and Petrogenesis

阿尔泰造山带南缘中泥盆世苦橄岩位于北塔山组地层的下部,其上依次为玄武岩和安山岩.3种岩性共同的特点是贫钛、富铁,具Nb和Ta的负异常以及高场强元素的丰度与MORB相当,具有典型的岛弧火山岩系的特点,是准噶尔洋板块向南西俯冲的结果.苦橄岩和玄武岩的Zr/Nb和Sm/Nd比值与MORB相当,表明其源区为亏损的MORB源.然而玄武岩的Ti/V和Zr/Sm比值均高于苦橄岩,而且玄武岩的稀土元素配分曲线呈平缓型,而苦橄岩则显示出低的稀土总量以及弱富集轻稀土型,指示了玄武岩是被从俯冲的洋壳释放的流体交代的含角闪石的尖晶石橄榄岩的地幔源区低程度部分熔融形成的,苦橄岩则是在高温条件下被流体交代过的石榴石橄榄岩高程度熔融的产物.安山岩则可能是榴辉岩部分熔融形成的.

Middle Devonian picrites of the south margin of Altay orogenic belt occur in the lower part of the Beitashan Fm., overlain by basalts and andesite. They are characterized by a depletion of Ti and enrichment of Fe, negative anomalies of Nb and Ta with abundance of HFSE as MORB, exhibiting typical island arc magmas. They may result from the southwestward subduction of the Junggar ocean plate. The Zr/Nb ratios of the picrites and basalts resemble the MORB, suggesting MORB-like sources. However, Ti/V and Zr/Sm ratios are higher than those of picrites, and the basalts display flat-type REE-chondrite patterns whereas the picrites are characterized by lower total REE concentrations and a slight enrichment of light REE. All suggest that the basalts were generated by lower partial melting of amphibole-bearing spinel peridotite, which was metasomatized by fluids released from the subducted oceanic crust, and the picrites resulted from the higher degree of partial melting of metasomatized garnet peridotite under high temperatures. The andesite may have resulted from the partial melting of eclogites.