Comment on 40Ar/39Ar age constraints on low-grade metamorphism and cleavage development in the Transvaal Supergroup (central Kaapvaal craton, South Africa): implications for the tectonic setting of the Bushveld Igneous Complex (South African Journal of Geology, 109, 393 410), by Alexandre et al. (2006)

Abstract

Alexandre et al. (2006) recently presented the results of a detailed structural study of brittle-ductile deformation, which could, as they state, be seemingly coincident with regional low-grade metamorphism of the pre-2.15 Ga Transvaal Supergroup strata between the Vredefort impact structure of ~2.02 Ga age (Kamo et al. , 1996; Gibson et al. , 1997) and the Bushveld Complex that was intruded at ~2.06 Ga (Walraven, 1997; Buick et al. , 2001). Alexandre et al. (2006) also performed six 40Ar/39Ar step-heating dating experiments with synkinematic white mica specimens. The results comprise ages of ~2.15 Ga and 2042.1 ± 2.9 Ma, the latter derived from “plateaus and pseudo-plateaus, as well as cumulative probability statistics” ( ibid ). These authors suggest that “at ~2.04 Ga the rocks of the Transvaal Supergroup in the central [Kaapvaal] craton were a part of a more extensive fold-and-thrust belt, named here [ibid] the Transvaalide fold-and-thrust belt ”. Alexandre et al. (2006) admit the difficulties to explain the ~2.15 Ga phase (although they gave a try), but assign the time at ~2.04 Ga to “orogenic activity at the margins of the craton”. Alexandre et al. (2006) conclude (p.402) that “The age of 2042.1 ± 2.9 Ma is based on unambiguous plateau and pseudo-plateaus ([their] Figure 6), and therefore can be considered a robust and reliable one…”. They proceed to discuss that this age is clearly distinguishable from the ages of the Bushveld magmatic event and the Vredefort impact event, and that a definite event at that time was also supported by other chronological results for rocks from the entire region between the Limpopo Mobile Belt and the Vredefort impact structure. In the light of these authors’ conclusion based on the age of 2042.1 ± 2.9 Ma, it appears reasonable to scrutinize the argon chronological results …