Clast size distribution and quantitative petrography of shocked and unshocked rocks from the El'gygytgyn impact structure

Abstract

AbstractA variety of quantitative petrographic methods, such as the study of clast size distribution (CSD), have been used in the study of melt rocks and have, for example, led to validation of the melting origin of pseudotachylites in paleoseismic faults, the estimate of the energy involved in melting and crushing processes, the distinction between seismic and aseismic faults, and understanding the crystallization process in volcanic rocks. Recently, quantitative petrography was applied to distinguish impact melts from pristine mare basalts on the Moon. Here, we apply this approach to impact lithologies formed in a volcanic target. The El'gygytgyn structure, an 18 km‐diameter and 3.58 Ma old impact crater in north‐eastern Chukotka, Arctic Russia, represents the only known impact crater on Earth mainly excavated in siliceous volcanic rocks. The structure was recently drilled in the framework of an ICDP project, providing fresh samples of suevite and other impact breccias, which can be compared with samples from the unshocked target. As the target is mostly composed of rhyolitic‐dacitic ignimbrites and tuffs, impact melt clasts are almost indistinguishable from the unshocked volcanic clasts in the absence of shock evidence. We show here that geometric characterization provides a reproducible technique for quantitative description of impact lithologies. Although the studied suevite reveals a high local variability in the evaluated geometric parameters, an overall homogenization of these parameters occurs. Furthermore, quantitative petrography allows the classification of unshocked to slightly shocked volcanic clasts included in the suevite.