Variolites of the Paleoproterozoic Hekpoort Formation (Transvaal sub-basin, Kaapvaal craton): Multistage undercooling textures?

Abstract

Globular structures in volcanic and hypabyssal rocks called varioles (or spherulites) have been described from a wide variety of places and rock types including komatiites, basalts, dacites, rhyolites as well as alkaline dykes, but the processes leading to their formation are still debated. Here, we present a field, petrological and geochemical study of variole-bearing lavas (variolites) in the Paleoproterozoic volcanic Hekpoort Formation in the southern part of the Transvaal sub-basin of South Africa. Variolites were only observed in a restricted area, corresponding to a ≥3.5 km long basin or depression. The variolites are locally up to 50 m thick and correspond to a succession of one-meter-thick lava flows. Most of the flows consist of two distinct layers, with an upper layer rich in spherical varioles (1–32 cm in diameter), overlying a layer generally devoid in varioles but often showing vertical jointing. Both layers show randomly oriented acicular textures of centimetre-sized skeletal clinopyroxene crystals (Al-rich augite) that precede the formation of varioles. The varioles themselves are mainly composed of radiating acicular augite (1 μm in diameter), the crystallization of which caused diffusion of incompatible elements, which became enriched in the groundmass around the varioles. The whole rock geochemistry of the Hekpoort variolites is indistinguishable from that of non-variolitic Hekpoort lavas with the same MgO content. This excludes liquid immiscibility as the origin of the varioles. The acicular texture (of Al-rich augite) in combination with the incompatible element enrichment around the varioles suggests that both skeletal and acicular augite are related to undercooling rather than devitrification. Sub-aqueous deposition is thought to be the cause of undercooling.