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

Abstract

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 formations, which contrasts with previous reports. Additionally, we report for the first time that the composition of the granophyre is corresponding to dacite.

Geophysical characterization of the Daskop granophyre dyke and surrounding host rocks, Vredefort impact structure, South Africa

“…; Kovaleva et al. 2018c) can be explained as resulting from the deposition of the clasts at the bottom of the dyke due to gravitational settling or via some melt flow mechanism through a low‐viscosity, possibly superheated melt (Wielicki et al. ).…”

“…1997a), local variability in clast content has been noted (Kovaleva et al. 2018c), and both Reimold et al. () and Therriault et al.…”

“…Another recent study showed a relatively high abundance of inhomogeneously distributed clasts within the dyke (Kovaleva et al. 2018c). A possible interpretation for the inhomogeneous clast distribution is that the clasts are concentrated along one side of the dyke due to gravitational settling on the lower side of a dipping melt intrusion (Huber and Kovaleva ).…”

“…; Kovaleva et al. 2018c). Accessory minerals include K‐feldspar (sanidine to orthoclase), clinopyroxene, biotite, amphibole, magnetite, ilmenite, spinel, and zircon (Therriault et al.…”

“…1), with generally homogeneous composition regardless of the location of the dykes (Therriault et al 1996). Despite the generally homogeneous chemical composition of the dykes (Nel 1927;Reimold et al 1990;Therriault et al 1997a), local variability in clast content has been noted (Kovaleva et al 2018c), and both Reimold et al (1990) and Therriault et al (1996) found that where mafic intrusions are crosscut by the dykes, the granophyre composition is more mafic.…”