Abstract

A small, meta-granitic clast with a crosscutting microcrystalline vein was found enclosed within the Vredefort Granophyre. Semi-quantitative chemical analyses of the granophyre, granitic clast, and the vein were obtained in situ, demonstrating affinity to granitic composition for the vein. Microstructural and geochemical evidence is consistent with the vein representing pseudotachylite derived from granite.

Within the granitic clast, evidence of high-grade metamorphism with preferential melting of biotite was observed. Brittle, ductile and shock deformation microstructures were found in quartz grains in the granite ground mass, including mosaicism, undulatory extinction, non-planar and curviplanar fractures and planar deformation features (PDFs) with crystallographic orientations {1121} and {2241}. Similar deformation features were observed in quartz clasts within the vein. Pods of in situ pseudotachylitic material have been documented in foliated granite around the vein.

The vein suggests that a generation of pseudotachylite was evolved and already solidified when the granophyre dyke intruded. Therefore, a minimum age of pseudotachylite emplacement is suggested by the presence of the pseudotachylite vein within the granophyre.

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