Pseudotachylitic breccias are the most prominent impact-induced deformation phenomenon in the Vredefort Dome, the eroded central uplift of the 2.02 Ga, originally 250-km-wide Vredefort impact structure in South Africa. Controversy remains about the origin of these melt breccias, and the most popular hypotheses are genesis by (1) shearing (friction melting), (2) shock compression melting, (3) decompression melting immediately after shock propagation through the target or slightly later during the modification phase of cratering, (4) combinations of these processes, or (5) intrusion of allochthonous impact melt. A resolution to this problem requires detailed multidisciplinary analysis in order to characterize the nature of different occurrences of such breccias with the aim of identifying the melt-forming process. Past work has focused mainly on orientation and geometry of Vredefort pseudotachylitic breccia veins, besides a few whole-rock geochemical investigations of mostly decimeter- to tens of meter-sized occurrences, whereas detailed geometric and micro-chemical analysis has not yet been adequately related to microdeformation studies of such melt breccias. Here, we report the results of detailed microchemical analyses of small- to meso-scale pseudotachylitic breccias in a polished 3 × 1.5 m granite slab from a dimension stone quarry in the western core of the Vredefort Dome, supplemented by data for samples from the Rand Granite Quarry in the northern sector of the core.

The veinlets selected for analysis do not provide textural evidence for shearing/faulting. Electron microprobe analysis of pseudotachylitic breccia groundmass and X-ray fluorescence bulk chemical analysis of both pseudotachylitic breccias and their host rocks reveal that pseudotachylitic breccia commonly displays a close chemical relationship to its direct wall rock. If groundmass compositions are corrected for the inherent microclast content, correspondence of breccia groundmass and immediate host rock composition is further enhanced. For small veinlets (<1 mm width), melting appears to have occurred locally, with compositions of melt and immediately adjacent host rock minerals commonly being identical. It is, thus, suggested that larger breccia zones could be sites of pooling of melt generated in places throughout the wider environs of dilational sites. For millimeter-scale veinlets, local melt formation and also a lack of lateral mixing are indicated. In contrast, pseudotachylitic breccia veinlets <1 mm, and quite possibly also some larger veins, could conceivably have formed by shock or decompression melting. As previous shock experimental work has demonstrated, this local melting could have been accomplished with or without a friction component.