A geochemical contribution to the discussion about the genesis of impact-related pseudotachylitic breccias: Studies of PTB in the Otavi and Kudu Quarries of the Vredefort Dome support the“In Situ Formation”hypothesis

Abstract

This work is aimed at contributing to the understanding of the formation of massive pseudotachylitic breccias (PTB) in impact structures. In the past this has been debated as being due to either melting of locally available country rocks or a combination of injection of impact melt from a higher level of the impact structure plus assimilation of other, including locally derived, material. Two occurrences of massive PTB in the Otavi and Kudu quarries of the outer crystalline core of the Vredefort Dome (South Africa) have been investigated petrographically and chemically. As shown in many previous studies of PTB, lithic and mineral clast populations only support derivation of PTB from local precursor material (granitic gneiss and amphibolite in the case of Kudu Quarry and various granitoids and a dolerite/amphibolite component in Otavi Quarry). The new major and trace element chemical systematics of melt rock and possible local precursors are fully consistent with this petrographic finding: in both cases PTB chemistry is readily explained by derivation from directly adjacent country rock, or mixtures of locally occurring granitoids and amphibolite. Harmonic least-squares mixing (HMX) calculations also do not indicate that additional components, such as a Vredefort Granophyre-like impact melt intrusive phase, contributed to the formation of these PTB. Absence of evidence (such as significant displacements along PTB developments) for the origin of PTB by friction melting along significant faults/shear zones is also recorded. As melting immediately after shock wave propagation (end of early compression stage of cratering), which is widely considered as the genetic process for shock vein formation in impact-affected rocks, including meteorites, does not apply to formation of such voluminous PTB, only local melting due to rapid decompression upon central uplift formation, followed by melt pooling in dilation sites, can be called upon to satisfactorily address PTB formation.