华北克拉通前寒武纪豆荚状铬铁矿的特征识别：对地球动力学的启示

Ophiolite-hosted podiform chromitites and peridotites contain information on ancient mantle compositions as well as magmatic and tectonic processes in subduction zones. Because of new discoveries related to the mineralogy, geochemistry and structure of these bodies, the study of ophiolites is again an emerging frontier topic in plate tectonics, as well as the evolution and composition of oceanic mantle. In recent years, numerous UHP minerals, highly reduced phases and crustal-type minerals have been recovered from both podiform chromitites and peridotites of Phanerozoic ophiolites. These great contributions indicate that crustal material were subducted and recycled into the mantle and it can be an indicator of the operation of plate tectonics and subduction of crustal material to the deep mantle. The applicants have discovered mantle peridotites with disseminated chromites in a mélange zone in the Zunhua and Jianping areas of the North China Craton with textures and mineralogies very similar to those of Phanerozoic ophiolites. The age of this mélange zone is not precisely determined but it is cut by granites with ages of 2.5 Ga, suggesting formation in the Neoarchean or earliest Proterozoic. We propose to analyse the mineral chemistry，microstructure and microfabrics of chromite and composition of mineral inclusions within chromite. The results will be helpful for us to understand the Precambrian mantle composition and plate tectonics at that time. Our proposed study of these chromitites, especially their mineral chemistry, geochemistry and mineral inclusions, will allow a detailed comparison with similar rocks in both Phanerozoic and Precambrian age. The results should provide a better understanding of mantle compositions and the nature of plate tectonics on different periods of earth history.

蛇绿岩型豆荚状铬铁矿和地幔橄榄岩保存古老地幔组成、俯冲带岩浆活动等重要信息，对其矿物学、地球化学和显微结构的新探索，是板块构造和大洋地幔组成演化的前沿课题。近年来显生宙蛇绿岩豆荚状铬铁矿和地幔橄榄岩中找到大量超高压、高还原态以及地壳矿物，表明壳源物质参与了板块运动和地幔对流循环过程。华北克拉通河北遵化和辽宁建平蛇绿岩质构造混杂岩带被2.5Ga花岗岩切穿，表明其形成年龄可能为新太古代或早元古代，而其中蛇绿岩型地幔橄榄岩和侵染状铬铁矿与显生宙蛇绿岩结构特征相似，且铬铁矿中存在与显生宙蛇绿岩类似的矿物包体。因此拟将显生宙豆荚状铬铁矿研究方法引入前寒武铬铁矿研究，对其进行地球化学、矿物化学，矿物包裹体等研究，并与显生宙铬铁矿的特征对比。该研究有助于理解前寒武地幔组成和构造特征，可探讨前寒武与显生宙俯冲和地幔对流作用的异同，从而认识板块构造在不同地质时期的特征。

蛇绿岩内的豆荚状铬铁矿和地幔橄榄岩保存有古老地幔组成、俯冲带岩浆活动和构造演化过程等重要信息。近年来对豆荚状铬铁矿矿物学、地球化学和结构的新探索，使蛇绿岩研究再次成为板块构造研究和地幔组成演化的前沿课题。近年来在显生宙蛇绿岩型豆荚状铬铁矿中找到大量超高压矿物、高还原态矿物以及地壳矿物，表明壳源物质参与了地幔对流循环，可作为板块运动作用的标志，但前寒武纪尤其是太古宙豆荚状铬铁矿的相关研究还不够充分。..申请人通过研究华北克拉通河北遵化地区蛇绿混杂带中地幔橄榄岩及其中的铬铁矿，详细分析和识别豆荚状铬铁矿及其中的矿物包裹体的矿物化学、显微结构特征，发现其具有与典型显生宙蛇绿岩豆荚状铬铁矿相似的矿物和矿物包裹体，观察到该地区豆荚状铬铁矿中有指示超高压变质作用的矿物和矿物包裹体存在，同时含有典型的地壳起源的矿物。这些观察表明遵化太古宙豆荚状铬铁矿可能经历了俯冲再循环过程，为太古宙存在板块运动和深俯冲过程提供了新的证据。此外，通过研究元古代扬子克拉通庙湾蛇绿岩中铬铁矿的矿物化学、地球化学和矿物包裹体特征，对比显生宙同前寒武（元古代和太古宙）豆荚状铬铁矿特征，我们认为前寒武纪的板块构造同显生宙以来的板块构造基本类似，板块构造的演化具有连续性。