An evaluation of element mobility in the Modderfontein ultramafic complex, Johannesburg: Origin as an Archaean ophiolite fragment or greenstone belt remnant?

Abstract

The Johannesburg Dome -a tonalite-trondhjemite-granodiorite (TTG)-dominated terrane in the central Kaapvaal Craton -contains a suite of ultramafic-mafic complexes that are concentrated largely along its southern rim. These > 3.3 Ga ultramafic-mafic complexes have recently been reinterpreted as fragments of an Archaean ophiolite (Anhaeusser 2006a), challenging a longstanding hypothesis whereby the complexes represent the intruded remnants of an Archaean greenstone belt. As with similar interpretations of ultramafic-mafic units in other Archaean cratons, the ophiolite hypothesis is used as evidence in favour of Phanerozoic-style plate tectonic processes having operated in the Archaean, with this geodynamic regime the prevailing explanation for the rocks and structures displayed by the Kaapvaal Craton. Through detailed new geological mapping of the scarcely studied Modderfontein Complex, alongside petrography, bulk-rock geochemistry and mineral chemistry, we here assess the validity of both hypotheses. Moreover, having experienced amphibolite-facies metamorphism and substantial hydrothermal alteration, we assess the degree of element mobility experienced by the Modderfontein Complex and discuss the implications for subsequent geodynamic interpretations. The 1 km 2 area mapped comprises separate northern and southern domains, with the former dominated by homogenous serpentinite that contains irregularly-shaped chromitite lenses, and the latter comprising coarsely-layered peridotite, pyroxenite, gabbro and amphibolite. The data indicate that the Modderfontein Complex has experienced significant mobility of Pd, the fluid-mobile lithophile elements (e.g., Ba, Rb and Cs) and potentially some elements generally considered immobile. Mobility of Pd is restricted to chromitite lenses, where Pd was originally hosted by sulphide mineral phases (e.g., pentlandite). This element was immobile in all other Modderfontein lithologies, where it is hosted by nano-scale PGM, demonstrating that PGE mobility is, in-part, controlled by the host phase(s). Moreover, based on a variety of petrographic and geochemical characteristics, including PGE mineralogy and spinel mineral chemistry, it is considered unlikely that the Modderfontein Complex represents an ophiolite

Impact melt rocks from the Late Paleocene Hiawatha impact structure, northwest Greenland

“…Anomalous PGE ratios observed in the PGE‐enriched silicate melt rocks from the Hiawatha impact structure (averages of [Rh/Pt] N = 1.37, [Rh/Ru] N = 0.32 and [Pd/Pt] N = 3.23) are unusual for terrestrial rocks, but common for fractionated iron meteorites. Terrestrial candidate sources for the origin of the high (Rh/Pt) N values are depleted mantle, some komatiitic lavas or rare sulfide‐rich chromite rocks (Guice et al., 2019; Holwell & McDonald, 2006; Maier et al., 2003) with only the latter realistically likely to produce the accompanying high (Rh/Os) N and (Rh/Ir) N values. No occurrences of such lithologies are known from the exposed foreland adjacent to the Hiawatha impact structure (Nutman et al., 2008).…”

Forearc Variability and the Geochemical Diversity of Suprasubduction Zone Ophiolites: Insights From the Leka Ophiolite Complex, Norway

“…Linear correlations between elements considered immobile during high-temperature metamorphism (Y, Yb, Nb, Zr) are used to determine the most immobile element in the sample suite (method outlined in Guice et al, 2018Guice et al, , 2019. For the LOC samples, Y and Yb have a strong linear correlation (R 2 = 0.99), indicating minimal relative fractionation due to secondary alteration.…”