岫岩陨石坑撞击角砾岩组成及冲击变质特征研究

Impact craters are a kind of circular structure formed on solid planetary surface by hypervelocity impacts of asteroids, meteoroids and comets. Impact craters are widely distributed on Earth. The investigations of impact craters are of important scientific significance on the formation and evolution of planets in the solar system, the correlation between the impact events and the major catastrophic events in the geological history of Earth, economic resource deposits at the terrestrial impact structures, as well as the physical and chemical behaviors of rocks and minerals under shock-induced high pressures and temperatures. The impact breccia unit is a most important part of the impact structure, and it records so many information of the impact body and shock-metamorphic history of the shocked target rocks. Therefore, the study of impact breccias plays a key role in the impact crater sciences. Although the Xiuyan impact crater has been recently recognized by the international impact crater scientific community, we know little of the impact breccias in the Xiuyan impact crater and a great deal of research on that subject still need to be done. This work will investigate in detail the shock-metamorphic features and geochemical features, especially PGEs content, of the impact breccias in the Xiuyan crater, and will provide critical evidences for further understandings on the geological features and the genetic mechanism and evolution of the Xiuyan crater. At present, the research level of impact craters in our country is rather weak in comparison to that of the western countries. This study can help to further understand the Xiuyan impact crater, and to promote the impact crater research in China.

陨石坑是小行星、流星体或彗星等超高速撞击固体星球表面而形成的凹坑或环状地质构造。陨石坑是地球表面广泛存在的一类地质构造，陨石坑研究在太阳系天体的形成和演化、地球历史上重大环境变迁事件（生物群体灭绝）、矿产资源、矿物的高温高压变化等领域均有重要意义。撞击角砾岩是陨石坑一种重要的地质和构造单元，记录着陨石撞击体以及靶岩物质冲击变质历史的信息，因此撞击角砾岩研究在陨石坑研究中占有很大的比重。岫岩陨石坑是我国获得国际科学界承认的一个撞击坑地质构造，但该坑撞击角砾岩的研究尚未系统开展。本项目拟对岫岩陨石坑中撞击角砾岩的类型和分布特征开展系统研究，分析撞击角砾岩的冲击变质特征和地球化学特征、特别是铂族元素地球化学特征，为深入了解该坑的地质特征、成因机制和形成历史提供依据。我国的陨石坑研究相对于西方国家的水平还较为薄弱，该项研究有助于进一步完善岫岩陨石坑的研究内容，推动我国的陨石坑研究。

本项目对岫岩陨石坑的撞击角砾岩进行了矿物学、岩石学和地球化学研究。主要研究成果如下：（1）岫岩陨石坑的撞击角砾岩分为三类，分别是石质单相角砾岩、石质多相角砾岩和含熔体角砾岩；（2）石质单相角砾岩和石质多相角砾岩分布在角砾岩单元上部，冲击变质程度较弱，仅发现少量石英发育PFs和PDFs。而含熔体角砾岩分布在角砾岩单元下部，冲击变质程度较强，发现有石英PDFs、石英玻璃、柯石英、长石玻璃、莱氏石、角闪石熔融、方解石熔融重结晶等多种冲击变质特征；（3）撞击角砾岩的PGEs丰度属于地壳岩石的正常水平，没有混入陨石撞击体的组分。本研究有助于认识陨石撞击过程的温度和压力历史以及简单陨石坑的形成过程。