桂西那坡基性岩地球化学：峨眉山地幔柱与古特提斯俯冲相互作用的证据

摘要

The Napo area in the western Guangxi province, South China, is located in the southwestern margin of the South China Block, and to the north of the North Vietnam Block. The Upper Permian mafic rocks well crop out in the west-southwest of Napo County, and are composed of layered, sub-volcanic diabases and orbiculite. Geochemically, the Napo mafic rocks can be subdivided into two groups: high-Ti group (TiO2>2.8%, Ti/Y>500) and low-Ti group. The high-Ti group is mainly composed of alkaline basalts, and the low-Ti group of tholeiite. The high-Ti rocks have lower SiO2, MgO, higher FeOt, P2O5 than the low-Ti ones. The high-Ti group shows an enrichment of LILE, HFSE, and significant fractionation between LREE and HREE, akin to ocean island basalts (OIB), suggestive of an affinity with the Emeishan high-Ti basalts. In contrast, the low-Ti rocks display relatively higher SiO2, MgO, lower FeOt, P2O5, enriched LILE, depleted Nb, Ta, and relatively flat REE patterns, indicative of island-arc geochemical signatures. The trace element ratios and the tectonic discrimination diagrams indicate that the Napo high-Ti mafic rocks were most likely derived from the enriched OIB mantle source, while the low-Ti group showed the transitional features from OIB- to island arc-like signatures. Integrating the geochemistry and regional geological background, we suggest that the Napo high-Ti mafic rocks were probably related to the Emeishan mantle plume magmatism; however, the low-Ti mafic rocks were possibly produced by interaction between the subducting Paleotethyan slab and the Emeishan mantle plume.%华南板块西南缘、越北地块以北桂西那坡县城以西及西南一带发育一套晚二叠世基性岩,由层状、似层状次火山岩相辉绿岩、辉绿玢岩及球状岩组成。根据岩石地球化学特征,那坡基性岩可划分为高Ti(TiO2>2.8%和Ti/Y >500)和低Ti两部分。高Ti基性岩为碱性玄武岩,而低Ti基性岩为拉斑玄武岩。与低Ti基性岩相比,高Ti基性岩整体具有相对较低的 SiO2、MgO和较高的 FeOt、P2O5,轻、重稀土分馏明显,富集大离子亲石元素(LILE)和高场强元素(HFSE),显示出似OIB地球化学特征,与峨眉山高Ti玄武岩具高度亲缘性；低Ti基性岩具有相对较高的SiO2、MgO和较低的FeOt、P2O5,稀土配分曲线较平坦,富集LILE,严重亏损HFSE(Nb、Ta),与岛弧玄武岩地球化学特征类似。从微量元素比值及相关图解对岩浆源区和构造环境判别,那坡高Ti基性岩来自富集OIB地幔源区,而低Ti基性岩兼具OIB和岛弧岩浆源区的过渡特征。结合岩石地球化学特征及区域地质背景,认为那坡高Ti基性岩可能为峨眉山地幔柱岩浆作用的产物,低Ti 基性岩为古特提斯俯冲与峨眉山地幔柱共同作用的产物,揭示了那坡地区晚二叠世同时受到峨眉山地幔柱和古特提斯俯冲相互作用的影响。