Planar deformation features in quartz from impact-produced polymict breccia of the Xiuyan crater, China

Abstract

The 1.8km-diameter Xiuyan crater is an impact structure in northeastern China, exposed in a Proterozoic metamorphic rock complex. The major rocks of the crater are composed of granulite, hornblendite, gneiss, tremolite marble, and marble. The bottom at the center of the crater covers about 100m thick lacustrine sediments underlain by 188m thick crater-fill breccia. A layer of polymict breccia composed of clasts of granulite, gneiss, hornblendite, and fragments of glass as well as clastic matrix, occurs near the base, in the depth interval from 260 to 295m. An investigation in quartz from the polymict breccia in the crater-fill units reveals abundant planar deformation features (PDFs). Quartz with multiple sets of PDFs is found in clasts of granulite that consist of mainly quartz and feldspar, and in fine-grained matrix of the impact-produced polymict breccia. A universal stage was used to measure the orientation of PDFs in 70 grains of quartz from five thin sections made from the clasts of granulite of polymict breccia recovered at the depth of 290m. Forty-four percent of the quartz grains contain three sets of PDFs, and another 40% contain two sets of PDFs. The most abundant PDFs are rhombohedron forms of , and with frequency of 33.5, 22.3, and 9.6%, respectively. A predominant PDF form of in quartz suggests a shock pressure >20GPa. The occurrence of PDFs in quartz from the polymict breccia provides crucial evidence for shock metamorphism of target rocks and confirms the impact origin of this crater, which thus appears to be the first confirmed impact crater in China. © The Meteoritical Society, 2011.