一种生长型新锆石的成因探讨——以平果沉积型铝土矿为例
Origin of a New Overgrowth-Type Zircon—An Example from the Pingguo Sedimentary Bauxite

普遍认为锆石是在岩浆或变质作用下形成，但平果铝土矿矿石中的锆石具有部分不同于传统认识的锆石特征，在其形貌上部分锆石核、边部存在明显差异，核部具有一定磨圆，而边部晶型完整，显示典型二元结构特征；利用ICP-MS进行精确U-Pb同位素定年分析，发现锆石U-Pb年龄具有多期、多阶段特征，核部年龄主要集中在寒武纪，锆石边部年龄多表现为二叠纪；经电子探针原位微量元素测试分析，锆石核部显示岩浆锆石特征，而边部显示海相沉积作用特征。同时对研究区矿石的手标本观察及电子显微镜下观察，都未发现任何岩浆热液叠加现象。推测认为，平果铝土矿中的锆石为一种新的生长型锆石，其锆石核部为寒武系地层中的锆石是经过表生搬运而来，而其边部则是二叠纪合山组的沉积和成岩阶段经二次生长所形成。
It is generally suggested that zircon is crystallized from high-temperature magmas or formed by metamorphic processes. However, the zircons from the Pingguo bauxite ores show some characteristics different from those of typical and metamorphic zircons. The core and mantle in some zircons are obvious differences in morphology. The core has a certain degree of roundness, whereas the mantle shows a complete crystal feature, showing typical characteristics of binary structure. Using ICP-MS to carry out accurate U-Pb isotope dating analysis, it is found that the U-Pb age of zircon has the characteristics of multi-phase and multi-phase. The core age is mainly concentrated in the Cambrian, and the edge age of zircon is mainly in the Permian; the electron probe in-situ trace element analysis shows that the core of zircon shows the characteristics of magmatic zircon, while the edge shows the characteristics of marine sedimentation. At the same time, the hand samples of the ore in the study area were observed and the electron microscope under observation, no magmatic hydrothermal superposition phenomenon is found. Based on the results, it is proposed that the zircons from the Pingguo bauxite ores are a new overgrowth-type zircon, within which the core is a supergene transport of Cambrian zircon, and the mantle was formed by the secondary growth during sedimentary and diagenetic stage of the Heshanformation in Permian.