西秦岭印支早期美武岩体的岩石成因及其构造意义

Petrogenesis and tectonic implications of the Early Indosinian Meiwu Pluton in West Qinling,central China

对西秦岭合作地区的美武岩体进行了锆石U-Pb定年、地球化学和Sr-Nd同位素组成研究。结果表明,美武岩体的侵位年龄为245～242Ma,属于印支早期。美武岩体为一个复式岩基,主要岩石类型为石英闪长岩、花岗闪长岩和黑云母花岗岩,其中,花岗闪长岩中含有丰富的暗色微粒包体。美武岩体不同岩性单元的化学组成表现出不同的演化趋势。美武花岗闪长岩和黑云母花岗岩的地球化学和Sr-Nd同位素的组成指示美武岩体的岩浆源区主要以壳源物质为主,其中,花岗闪长岩具有较高的Mg#(50～58)、Cr((46～93)×10-6)和Ni((12～50)×10-6)含量,表明有少量幔源物质的加入。石英闪长岩和暗色微粒包体具有高的Mg#(63和62)、Cr(217×10-6和318×10-6)和Ni(61×10-6和110×10-6)的含量,表明其主要来自于幔源岩浆。结合区域地质背景,认为西秦岭中部的美武岩体形成于后碰撞早期的构造背景之下,可能与俯冲的阿尼玛卿洋壳断离作用有关。板片断离模型能较好地解释西秦岭印支早期侵入岩线性分布的特征和西秦岭造山带内中三叠世地壳的快速抬升。

A combined study of U-Pb zircon dating,whole-rock chemical and Sr-Nd isotopic composition was carried out for the Meiwu pluton in the middle part of the west Qinling.U-Pb zircon dating shows that the Meiwu pluton intruded at 245-242 Ma.The Meiwu pluton,a complex batholith,is composed of quartz-diorite,granodiorite and biotite granite,with abundant mafic magmatic enclaves（MMEs） in the granodirite.The different rock types in the Meiwu pluton are characterized by distinct chemical evolution trend.Geochemical and Sr-Nd isotopic composition indicate the magma of the Meiwu granodiorite and biotite granite was dominantly derived from crustal source.However,the granodiorite show relatively high Mg#（50-58）,Cr（217×10-6,（46-93）×10-6） and Ni（（12-50）×10-6）,suggesting a minor contribution from the mantle.The quartz-diorite and mafic magmatic enclaves display high Mg#（63 and 62）,Cr（217×10-6 and 318×10-6） and Ni（61×10-6 and 110×10-6）,respectively,indicating that their magma were dominant mantle-derived.Geochemical and U-Pb zircon ages,combined with regional setting,suggest the Meiwu Pluton in the middle part of the west Qinling resulted from break-off of the subducted Animaque oeanic slab at early stage of post-collisional setting.The slab break-off model can well explain the linear distribution of the Early Indosinian plutons and rapid crustal uplift during the Middle Triassic in the west Qinling.