Komatiites of the Mendon Formation: Late-stage ultramafic volcanism in the Barberton Greenstone Belt

Abstract

The Mendon Formation is a newly defined stratigraphic unit composed of interbedded komatiitic lavas and cherty metasedimentary rocks at the top of the Onverwacht Group in the Barberton Greenstone Belt. It lies conformably on a chert at the top of the underlying Kromberg Formation that has been dated at 3,334 ± 3 Ma and beneath the conformably overlying basal Fig Tree Group dated at 3,259 ± 3 Ma. A chert within the Mendon Formation has yielded an age of 3,298 ± 3 Ma. Although rocks of the Mendon Formation crop out in a structurally complex portion of the Barberton Greenstone Belt, several key marker beds and distinctive geochemical traits of individual flows can be used to develop a coherent stratigraphy for the unit. The formation consists of a number of thick komatiitic flow units separated by thin, widely traceable layers of chert representing silicified tuffs, carbonaceous sediments, and chemical deposits. The freshest Mendon komatiites contain relict igneous clinopyroxene and chromespinel. Olivine and glass have been completely altered to chlorite and plagioclase to albite. Micron-scale details of original igneous textures are commonly preserved in even the most altered samples where fine-grained quartz and sericite replaced original igneous minerals soon after deposition. Individual stratigraphic subunits of the Mendon Formation contain sets of flows with distinctive chemical compositions. The variation within each subunit is primarily due to olivine fractionation. Two major compositional groups of komatiites are recognized. Al-depleted komatiites have Al2O3/TiO2 ratios near 10, and other magmatically incompatible and metasomatically immobile element ratios that are distinctly nonchondritic, though similar to komatiites found lower in the Onverwacht Group. Al-enriched komatiites have Al2O3/TiO2 ratios greater than 30, and other incompatible and immobile element ratios distinctly non-chondritic and dissimilar to those of the Al-depleted komatiites. Al-enriched komatiites have been found only rarely in other portions of the Barberton sequence. Al-undepleted komatiites, with nearly chondritic incompatible element ratios are not found in the Mendon Formation, though they are found in the correlative Weltevreden Formation. A preferred mechanism for the formation of both Al-depleted and Al-enriched komatiites in the Mendon Formation requires two stages of melting during the ascent of deeply sourced plumes. At depths between 450 and 650 km majoritic garnet is removed yielding melts with Al-depleted komatiite composition. Continued melting of these plumes would yield Al-enriched komatiite compositions. Byerly, G. R., 1999, Komatiites of the Mendon Formation: Late-stage ultramafic volcanism in the Barberton Greenstone Belt, in Lowe, D. R., and Byerly, G. R., eds., Geologic Evolution of the Barberton Greenstone Belt, South Africa: Boulder, Colorado, Geological Society of America Special Paper 329.