前寒武纪豆荚状铬铁矿中矿物包裹体的FIB-TEM精细研究

Precambrian ophiolite (ophiolitic mélange) is a typical record of plate movement in the early Earth. The podiform chromite and its mineral inclusions contain some key information about subduction-collision, crust-mantle material cycle and so on. They are important objects in plate tectonics and geodynamics research. In recent years, not only ultrahigh-pressure deep-source minerals such as diamond and stishovite were recovered from Phanerozoic ophiolitic chromite, but also crustal minerals such as ancient zircons with Precambrian ages were also recovered, which indicates that there was deep subduction and crust-mantle material circulation in Phanerozoic. However, no similar information has been reported in Precambrian podiform chromite. In this application, mineral inclusions in Precambrian podiform chromite from Miaowan, Yangtze Craton and Jianping, North China Craton are taken as research objects. Their morphology, composition, structure and types are comprehensively analyzed to explore whether there are deep-source or unusual mineral inclusions in Precambrian podiform chromites, which can provide new research perspectives and basic data about the onset time and style of Precambrian plate tectonics. In addition, different characteristics of inclusions in Phanerozoic, Proterozoic and Archean podiform chromites are compared that plate movement styles in different geological periods can be deduced. Furthermore, the similarities and differences of mantle convection and plate movement characteristics in different geological periods are discussed, to deepen the understanding of plate tectonic evolution.

前寒武纪蛇绿（混杂）岩是地球早期存在板块运动的标志记录，其中特征的豆荚状铬铁矿及其矿物包裹体可携带俯冲-碰撞、壳幔物质循环等关键信息，是板块构造和地球动力学研究的重要对象。近年来显生宙蛇绿岩铬铁矿中不仅发现有金刚石、斯石英等超高压深源矿物，同时还找到前寒武纪古老锆石等壳源矿物，表明显生宙存在板块深俯冲和壳幔物质循环作用。然而尚无研究报道前寒武纪豆荚状铬铁矿中存在类似信息。本申请以扬子克拉通庙湾和华北克拉通建平前寒武纪豆荚状铬铁矿中矿物包裹体为研究对象，对其进行形态、成分、结构和种类等特征的综合分析，探索前寒武纪豆荚状铬铁矿中是否含有深源或不寻常矿物包裹体，为前寒武纪板块构造启动时限和运动样式提供新的研究视角和基础数据。此外对比显生宙、元古宙、太古宙豆荚状铬铁矿包裹体的各类特征，推演不同地质历史时期板块运动样式，探讨不同地质历史时期地幔对流和板块运动特征的异同，从而加深对板块构造演化的认识。

本项目聚焦不同地质历史时期板块构造运动差异。研究对象为前寒武纪豆荚状铬铁矿及其中矿物包裹体。豆荚状铬铁矿是前寒武纪蛇绿（混杂）岩中的特征矿物，蛇绿（混杂）岩是地球早期存在板块运动的标志记录。通过对铬铁矿进行主微量元素分析，对其中矿物包裹体进行形态、成分、结构和种类等特征的综合分析，可解析其中携带的俯冲-碰撞、壳幔物质循环等关键信息。通过探索前寒武纪豆荚状铬铁矿中是否含有深源或不寻常矿物包裹体，确定前寒武纪板块构造启动时限和运动样式，加深对前寒武纪板块构造机制的理解。本项目目前获得以下新认识：.1.首次在华北克拉通新太古代遵化豆荚状铬铁矿中找到了金红石的超高压矿物相TiO2 II，其形成压力条件在7.5 GPa以上。通过与显生宙豆荚状铬铁矿中矿物种类和形态对比，以及全球相关地质背景研究，推断在新太古代存在现代板块构造运动；.2.通过研究庙湾蛇绿岩内方辉橄榄岩和纯橄榄岩中铬铁矿主量和原位微区微量元素数据，显示其与俯冲过程强烈相关，表明庙湾蛇绿岩形成于俯冲带上环境；.3.遵化豆荚状铬铁矿的主微量元素特征以及计算得到的母熔体元素特征，加上寄主橄榄岩具有高Mg#值的特征表明，遵化豆荚状铬铁矿是由岩浆作用形成的，包括在俯冲带上环境中的玻安质熔体和方辉橄榄岩之间熔-岩反应。.综上，本项目对前寒武纪豆荚状铬铁矿的主微量元素特征以及铬铁矿内矿物包裹体进行识别研究，表明前寒武纪蛇绿岩通常形成于俯冲带上环境，由熔-岩反应形成。在新太古代遵化豆荚状铬铁矿中同时找到了超高压矿物包裹体和常见壳源矿物包裹体，推断在新太古代就存在现代板块构造运动。6篇论文发表在Earth-Science Reviews, Precambrian Research, Lithos, Ore Geology Reviews, Journal of Earth Science和地球科学等国内外知名期刊上。