Palaeomagnetism of the early Palaeoproterozoic, volcanic Hekpoort Formation (Transvaal Supergroup) of the Kaapvaal craton, South Africa

Abstract

The Kaapvaal craton (South Africa) was the host of several major magmatic events during the Palaeoproterozoic, including the volcanic Hekpoort and Ongeluk Formations. Their possible comagmatic origin is the subject of a long debate. We performed a palaeomagnetic study of the Hekpoort Formation to be compared with the available palaeopole of the Ongeluk Formation, but also to contribute to the apparent polar wander path of the Kaapvaal craton. Characterization of magnetic mineralogy by three-axis thermal demagnetization of isothermal remanent magnetization and magnetic susceptibility versus temperature points out magnetite as the main remanence carrier in most samples.Five magnetic components were identified in total, of which the least stable (HKE) near parallels the present geomagnetic field. At higher levels of demagnetization (above 400 • C), two components (HKD and HKC) are identified as thermoviscous overprints likely related to the Karoo large igneous province (LIP) and a magmatic event which occurred between the emplacement of the ∼2055 Ma Bushveld Complex and HKD (possibly linked to the Umkondo LIP), respectively. This LIP is known to be associated with extensive remagnetization. The second most stable component HKB was also revealed at higher steps of thermal and alternative-field treatment. The HKB palaeopole (latitude = 28.4 • N and longitude = 54 • E) is similar to those reported from the Bushveld Complex (∼2055 Ma) and the Vredefort impact structure (∼2023 Ma). A potentially primary remanence direction (HKA; declination = 337 • , inclination = 80 • and α = 6.2 • ) was identified in most sites during the highest levels of thermal demagnetization. Note that the HKA pole position (latitude = −44 • N and longitude = 40 • E) is significantly different from the palaeopole for the Ongeluk Formation (latitude = −0.5 • N and longitude = 107 • E). Although, the primary nature of HKA is supported by positive fold and reversal tests, we cannot exclude the possibility that this component represents an overprint. HKA is, however, most likely older than ∼2.0 Ga given its anteriority to HKB components.

Low‐Fe(III) Greenalite Was a Primary Mineral From Neoarchean Oceans

“…There had been speculation that primary iron silicates could form in deeper-water and more alkaline environments as ferrous iron and silica reacted, but these minerals were not thought to be the major primary iron phase in BIFs (e.g., Beukes & Gutzmer, 2008;Klein, 1974). However, primary magnetic remanences that should have been recorded in original hematite precipitates, if they stayed below 600°C, have never been found in the Australian Hamersley Group and South African Transvaal Supergroup BIFs (Abrajevitch et al, 2014;de Kock et al, 2009;Humbert et al, 2017;Li et al, 1993)-highly suggestive of hematite being emplaced secondarily. Additionally, with the probable elevated silica levels of the Archean ocean (Maliva et al, 2005;Siever, 1992;Stefurak et al, 2014), the mineralization of iron silicates is thermodynamically predicted (Bethke, 2002) from highly reducing oceans with <10 À56 atm of O 2 ( Figure S7).…”

“…Iron silicates such as minnesotaite, greenalite, and stilpnomelane have long been identified in BIFs, but the consensus view categorized these as secondary minerals or derived from volcanic input (Beukes & Gutzmer, 2008;Fischer & Knoll, 2009;Haugaard et al, 2016;Klein, 2005). However, primary magnetic remanences that should have been recorded in original hematite precipitates, if they stayed below 600°C, have never been found in the Australian Hamersley Group and South African Transvaal Supergroup BIFs (Abrajevitch et al, 2014;de Kock et al, 2009;Humbert et al, 2017;Li et al, 1993)-highly suggestive of hematite being emplaced secondarily. However, primary magnetic remanences that should have been recorded in original hematite precipitates, if they stayed below 600°C, have never been found in the Australian Hamersley Group and South African Transvaal Supergroup BIFs (Abrajevitch et al, 2014;de Kock et al, 2009;Humbert et al, 2017;Li et al, 1993)-highly suggestive of hematite being emplaced secondarily.…”

Petrology, physical volcanology and geochemistry of a Paleoproterozoic large igneous province: The Hekpoort Formation in the southern Transvaal sub-basin (Kaapvaal craton)

“…1). Humbert et al (2017), have documented the effects of the Bushveld LIP and the VIS on most of the Hekpoort rocks in the southern Transvaal sub-basin.…”

“…Variolitic lava flows, of up to 30 m in thickness and over 1 km of strike, can locally be found at the basal portion of the Hekpoort Formation near Fochville, around and in the town of Wedela (Table 1; Humbert et al, 2017;Humbert et al, in revision a, b). These flows present at least two distinct layers per flow.…”

“…5s). Humbert et al (2017) interpreted these structures as being related to a complex process of super-cooling whereby the skeletal clinopyroxenes were formed first, followed by acicular clinopyroxene, thereby defining the varioles, and provoking diffusion of several elements that poorly fit into the clinopyroxene lattice (Gardner et al, 2014) Finally, the remaining glass underwent devitrification, forming the finely crystalline matrix observed as the groundmass.…”