岫岩陨石坑石英的冲击变质特征

Shock-Metamorphic Features of Quartz in Xiuyan Crater

直径1.8 km的岫岩陨石坑位于辽宁省岫岩县苏子沟镇。坑区基岩为下元古界变质岩,由变粒岩、片麻岩、角闪岩、透闪岩和大理岩等岩石组成。坑内充填的撞击角砾岩石英颗粒呈现典型的冲击变质特征,其中包括沿着石英(0001)、{10 11}、{10 12}和{10 13}等方向发育的多组面状变形页理,以及石英发生相转变形成二氧化硅玻璃和柯石英。产出在二氧化硅玻璃中的针状和粒状柯石英表明从二氧化硅熔体中结晶形成。石英面状变形页理的发育特点限定冲击压力高达35 GPa,而二氧化硅熔体玻璃的存在表明冲击压力高达50 GPa。当压力释放和温度升高,二氧化硅熔体首先被形成。随着压力进一步释放到2.5～13 GPa,柯石英从二氧化硅熔体中结晶析出。岫岩陨石坑中石英面状变形页理和柯石英的存在提供了矿物冲击变质的诊断性证据。

The 1.8-km-diameter Xiuyan crater is located in Xiuyan County of Liaoning Province.The basement rock of the crater is made up of the Early Proterozoic metamorphic rocks composed of granulite,gneiss,hornblendite,tremolite marble,marble and so on.Quartz from the impact breccia filling in the inner crater shows typical features of shock metamorphism,including the planar deformation features（PDFs） along the crystallographic orientation of quartz on（0001）,{1011},{1012} and {1013} and so on,as well as the occurrence of silica glass and coesite formed through phase transformation of quartz.The needle-like and granular coesite occurring in silica glass is indicative of crystallization from silica melt.The features of PDFs in quartz indicate a shock pressure up to 35 GPa,whereas the occurrence of silica melt glass has undergone a shock pressure up to 50 GPa.As soon as pressure released and temperature increased,silica melt was firstly formed.As pressure dropped to the regime of 2.5-3 GPa,coesite crystallized from silica melt.The existence of coesite and PDFs in quartz provides diagnostic evidence for shock metamorphism of minerals in Xiuyan Crater.