A Study of Shock-Metamorphic Features of Feldspars from the Xiuyan Impact Crater

Abstract

Feldspar is the most abundant mineral in the Earth’s crust and is widely distributed in rocks. It is also one of the most common minerals in meteorites. Shock-metamorphic features in feldspar are widely used to calibrate the temperature and pressure of shock events and can also provide clues for searching for impact craters on Earth. In this study, shocked alkali feldspars in the lithic breccia and suevite from Xiuyan Impact Crater were investigated using polarizing optical microscopes, Raman spectroscopy and electron microprobes to better constrain the shock history of this crater. For this study, feldspar grains occurring in gneiss clasts in the impact breccia and four shock stages were identified, e.g., weakly shocked feldspar, moderately shocked feldspar, strongly shocked feldspar, and whole rock melting. According to the shock classification system for alkali feldspar and felsic rocks, we estimated the shock pressure (SP) and post-shock temperature (PST) histories of these gneiss clasts. Weakly shocked feldspars display irregular fractures and undulatory extinction, and their shock stage is F-S2, which indicates that SP and PST are from ~5 to ~14 GPa and ~100 °C, respectively. Moderately shocked feldspars show planar deformation features and are partially transformed into diaplectic glass, which indicates that the F-S5 shock stage of SP and PST is from ~32 to ~45 GPa and 300–900 °C. Strongly shocked feldspars that occur as vesicular glass indicate a shock stage of F-S6, and the SP and PST are 45–60 GPa and 900–1500 °C, respectively. The whole felsic rock melting occurs as mixed melt glass clast and belongs to the F-S7 stage, and SP and PST are >60 GPa and >1500 °C, respectively.