The geology and geochemistry of the Sterkspruit intrusion, Barberton Mountain Land, Mpumalanga province

Abstract

The Sterkspruit Intrusion, in the south-western portion of the Barberton greenstone belt, is a sill-like body containing rocks of gabbroic to dioritic composition. It is hosted by a sequence of komatiitic basalts and komatiites of the Lower Onverwacht Group. The intrusion is considered unique in this area in that it lacks ultramafic components and has no affinities with the surrounding mafic- to- ultramafic lavas. The gabbroic suite also contains an unusual abundance of quartz, and the chill margin shows an evolved quartz-normative, tholeiitic parental magma. Based on petrographic and geochemical evidence, the intrusion can be subdivided into four gabbroic zones and a quartz diorite, which is an end product of a differentiating magma. The chill margin records an MgO content of 4.8%, an Mg# of 42, an Si02 value of 52.5% and a normative plagioclase composition of An44. The sill-like nature of the body, indicated by geochemical trends, and the steep sub-vertical layering, point to a body that has been tilted along with the surrounding lavas. This constrains the relative age of the body to be older than the emplacement of the Kaap Valley and Nelshoogte Plutons, which caused the regional deformation observed in the Nelshoogte Schist Belt. A sequence of tholeiitic to andesitic basalts from the Kromberg Formation in the Upper Onverwacht Group, have compositions which can be correlated with the Sterkspruit Intrusion. It is possible that this body represents a subvolcanic magma chamber, which acted as a feeder to tholeiitic lavas higher up in the volcanic sequence. The Sterkspruit Mafic Dyke Swarm intruded the southern Nelshoogte Schist Belt and displays intra-dyke chemical variation.