浙西毛坦地区基性岩墙地球化学特征及其构造意义

江南造山带中新元古代基性岩墙广泛发育，其成因和构造背景的研究对认识江南造山带的构造过程和华南板块新元古代构造演化具有重要意义。对出露于江南造山带东北段的浙西毛坦地区新元古代基性岩墙进行了系统的地球化学研究。结果表明:基性岩墙的岩性主要为辉绿岩和少量辉长岩，岩相学及地球化学特征显示其主要属拉斑玄武岩系列，稀土元素特征还显示了拉斑玄武岩到高铝玄武岩的过渡；在原始地幔标准化微量元素蛛网图上，Nb、Ta、Ti负异常显示了岛弧拉斑玄武岩的特征，但在MgO-FeO^*-Al₂O₃、Ti/100-Zr-3Y、TiO₂-FeO^T/MgO、Zr/Y-Zr、Ti-Zr、Zr/4-2Nb-Y和Th/Yb-Ta/Yb等图解上，基性岩墙样品大都落在洋岛、洋中脊和板内玄武岩区域，表明基性岩墙形成于伸展构造背景，主要为板内裂谷环境，且在岩浆上升侵位过程中不同程度混染了地壳物质；岩石的w(Th)/w(Ta)值及w(La)/w(Yb)值均较低，结合构造环境判别图解和岩石的拉斑玄武岩属性等，显示基性岩墙具有与地幔柱相关的岩石地球化学特征。这些特征显示，浙西毛坦地区的基性岩墙可能是Rodinia超大陆在新元古代因地幔柱活动导致软流圈上涌及部分熔融，使基性岩浆沿着由裂解作用形成的区域张裂构造侵入所形成的。

Geochemical Characteristics and Their Tectonic Implications of the Mafic Dykes in Maotan Area of Western Zhejiang,China

The Neoproterozoic mafic dykes are widespread in Jiangnan orogen. The study on genesis and tectonic setting has significant implications for the tectonic process of Jiangnan orogen and Neoproterozoic tectonic evolution in South China plate. A systematic geochemical study on Neoproterozoic mafic dykes from Maotan area of western Zhejiang at the northeastern part of Jiangnan orogen was carried out. The results show that mafic dykes are mainly diabases and a small amount of gabbros; they are tholeiitic series according to petrography and geochemical characteristics; the REE characteristics show the transition from tholeiitic series to high-alumina series; the negative Nb, Ta, Ti anomalies on primitive mantle-normalized trace element spider diagram show the characteristics of island arc tholeiite, but the mafic dyke samples mainly fall in tectonic environment areas of ocean island, mid-ocean ridge and intraplate basalt on the diagrams of MgO-FeO^*-Al₂O₃, Ti/100-Zr-3Y, TiO₂-FeO^T/MgO, Zr/Y-Zr, Ti-Zr, Zr/4-2Nb-Y and Th/Yb-Ta/Yb, etc.; the mafic dykes are formed on extensional tectonic setting, mainly on the intraplate rift environment, and suffers crustal contamination during ascending emplacement to varying degrees; all the w(Th)/w(Ta) and w(La)/w(Yb) of the mafic dykes are relatively low; combined with the tectonic environment discrimination diagrams of the rocks and their tholeiitic properties, the mafic dykes show the mantle plume characteristic in geochemistry. The above characteristics indicate that the mafic dykes are formed by the intrusion of the basic magma along the regional fractures, caused by the rifting of crust; the magma is generated by partial melting of lower mantle during the asthenosphere upwelling; it is related to Neoproterozoic plume event and the breakup of Rodinia supercontinent.