Geochemical and microstructural evidence for in situ formation of pseudotachylitic Sudbury breccia by shock-induced compression and cataclasis

Abstract

Pseudotachylitic breccias occur in the crater floor of large impact structures. Their origin is widely attributed either to shock melting and cataclasis during propagation of an impact shock wave, or to frictional melting and cataclasis during large slip displacements along crater collapse superfaults. By contrast, some models propose that the breccia matrices are allochthonous superheated melts from overlying impact melt sheets or from sites within the crater floor, which leaked into dilational fractures during collapse of the crater. In an effort to test these models, the compositions of pseudotachylite Sudbury breccias in the South Range of the Sudbury impact structure, Canada, were here compared to those of its associated impact melt sheet, the Sudbury Igneous Complex (SIC). The studied breccias occur along the contact between sandstone and a gabbro stock, and additional breccia bodies are located solely within these two lithologies. Major, trace element and lead isotope systematics suggest that the contact breccias are derived by mixing of comminuted gabbro and sandstone and that breccias hosted exclusively by sandstone or gabbro have compositions similar to their host rocks. The SIC has distinct chemical and isotopic compositions which argue strongly against the involvement of the impact melt sheet in the makeup of the breccias. Quartz and plagioclase in clasts and matrix of the breccias are transected by multiple microfractures and they have pervasive mosaic textures characterized by rolling and mottled undulatory extinction of micron-sized subgrain-like domains and new grains. These textures and strong local host control on chemical and clast composition of the breccias suggest that the breccias formed in situ by shock-induced compression and cataclasis of their host rocks during propagation of the shock wave.