Petrography and geochemistry of coarse- crystalline veins within Vredefort Granophyre, Vredefort impact structure, South Africa

摘要

Vredefort Granophyre dykes are generally considered to be a part of impact melt, which intruded downward from the melt sheet of the Vredefort impact structure (South Africa). This work reports enigmatic textural inhomogeneities found within the Daskop and southeast Holfontein granophyre dykes located in the core of the Vredefort impact structure, and in the Kopjeskraal core-collar granophyre dyke. The heterogeneities attributed to the clast-rich dykes (up to 70 vol% of clasts), however, do not occur within the portions of high clast abundance in these dykes. Instead, they are found within dyke portions with 5 to 10 vol% of clasts. They were not observed in the clast-poor northwest Holfontein dyke. The reported heterogeneities are veins with macroscopically straight and sharp boundaries that are parallel to the strike of the host dyke. The veins are usually from 4 to 20 cm wide and are often differentially eroded with respect to the host rock. Microscopic analyses demonstrate that the veins are coarser-grained than the host granophyre, with the increase in grain size of at least four times. Portable μ-XRF, whole rock WD-XRF and microprobe analyses indicate practically identical composition of the veins to the host granophyre. Slightly higher Ab and K mole % values of plagioclase and Fe-enriched rims of orthopyroxene within the veins suggest lower crystallization temperatures of the veins and/or crystallization over a longer period of time. We interpret the coarse-grained veins as either pegmatitic, typical for felsic intrusive rocks, or being similar to coarse-grained segregation veins reported from basaltic flows, both of which would normally involve late, volatile-enriched residual melts. In either case, the veins probably formed with comparatively small volatile component (≤1 wt% of water) derived from the wall rocks and from the incorporated lithic clasts. The main implication of our study is that the granophyre dykes are inhomogeneous and complicated formation