Petrography and shock-metamorphic features of impact breccias from the Xiuyan impact crater

Abstract

撞击角砾岩是陨石撞击过程形成的特有岩石种类，是研究撞击成坑过程、陨石坑定年、矿物岩石冲击变质的理想对象。岫岩陨石坑是一个直径1800m的简单陨石坑，坑内有大量松散堆积的撞击角砾岩。本研究通过光学显微镜、费氏台、电子探针、X射线荧光光谱仪、电感耦合等离子质谱仪等分析测试手段，主要研究了岫岩陨石坑撞击角砾岩的岩相学和冲击变质特征，并在此基础上讨论了撞击角砾岩的形成过程和陨石坑的形貌特征。岫岩陨石坑内产出有三种撞击角砾岩，分别是来自上部的玄武质角砾岩和复成分岩屑角砾岩，以及底部的含熔体角砾岩。组成玄武质角砾岩和复成分岩屑角砾岩的碎屑受到的冲击程度较低，仅有少量石英发育面状变形页理，指示不超过20GPa的冲击压力。而组成含熔体角砾岩的碎屑受到了很强的冲击，发育了熔融硅酸盐玻璃、石英面状变形页理、柯石英、二氧化硅玻璃、击变长石玻璃、莱氏石等冲击变质特征，指示的峰值压力超过50GPa。本研究证实了含熔体角砾岩通常产出在简单陨石坑底部，由瞬间坑的坑缘和坑壁垮塌的岩石碎屑与坑底的冲击熔体混合形成。岫岩坑的真实深度是495m，真实深度与直径的比值为0.275，符合简单陨石坑的尺寸特征。陨石坑内的撞击角砾岩中心厚度为188m，与直径之比为0.104，略低于其它简单坑，可能是受丘陵地貌影响导致改造阶段垮塌到坑内的岩石角砾偏少。;Impact breccia is a unique rock type that formed during the meteorite impact process and it is an ideal sample for the studies on the formation process of impact crater, dating impact craters, and shock effects in minerals and rocks. The Xiuyan impact crater is a simple crater with a diameter of 1800m and large amount of impact breccia in it. Here we mainly document the petrology and shock metamorphism of impact breccias from the Xiuyan impact crater through optical microscope, universal-stage microscope, electron probe microanalysis, X-ray fluorescence, and inductively coupled plasma mass spectrometry and then discuss their formation process and the morphology of impact crater. There are three types of impact breccia in the Xiuyan impact crater, which are the basaltic breccia and the polymict lithic breccia from the upper part of impact breccia lens, and the melt-bearing breccia at the bottom. The clasts in the basaltic breccia and the polymict lithic breccia are weakly shocked and only some quartz grains display planar deformation features which indicate shock pressures less than 20GPa. In contrast, the clasts in the melt-bearing breccia are strongly shocked, in which silicate glass, quartz planar deformation features, coesite, silica glass, diaplectic feldspar glass, reidite are observed, which indicate a shock pressure >50GPa. This study confirms that the melt-bearing breccia is usually produced at the bottom of simple impact craters and is formed by mixing rock clasts collapsed from the transient crater rim and wall with shock melt at the bottom. The true depth of Xiuyan impact crater is 495m, and the true depth/diameter ration is 0.275, which is consist with other simple impact craters. The central thickness of impact breccia lens is 188m and the thickness/diameter ration is 0.104, slightly lower than that of other simple impact craters, which may be due to less rock clasts collapsed into the crater in the modification stage that caused by hilly terrain.