Material mixing in shock-induced pseudotachylites, Vredefort impact structure, South Africa

Abstract

Impact-generated pseudotachylites (PTs) have been extensively studied in the Vredefort Impact Structure, South Africa, and have been shown to correspond chemically and isotopically to their host rocks. Such melts are frequently described as forming strictly in situ with lithic clasts derived from the immediate wall rocks. Mixed compositions are rare, where PTs are observed directly along a lithological contact. In this study, we document the presence of vast numbers of granitic clasts within dolerite-hosted PT veins within the Vredefort impact structure. The groundmasses of dolerite-hosted PT veins have two distinct geochemical compositions: (1) mafic, and (2) intermediate. The two compositional melt phases have a sharp contact, indicating immiscibility and/or mingling. Shocked zircon and monazite within the granite clasts enclosed in a dolerite-hosted PT vein indicate shock deformation prior to clast transport into the dolerite. In monazite, we report a new shock microtwin geometry 180°/[104].

The mechanisms by which granitic clasts were emplaced in the dolerite-hosted veins are complex. Previous experimental work has suggested that varying shock velocity in different lithologies may be responsible for material mixing. In addition, the thermal pulse accompanying the shock wave would have caused quartz in the granite to transform into volumetrically larger high-temperature polymorphs, significantly increasing the granite volume compared to the adjacent dolerite. Together with fluctuation of stresses (post-impact reverberations) after the passage of the main shock wave, this volume adjustment initiated brittle fracturing of the dolerite. A negative pressure gradient was generated within the dolerite that allowed granitic material to be injected into the dolerite-hosted melt veins.