Geology, geochemistry and isotopic characteristics of the Archaean Kaap Valley pluton, Barberton Mountain Land, South Africa

Abstract Formed approximately 3500 Ma ago, the Kaap Valley tonalite pluton is one of the oldest and the largest of a suite of tonalite-trondhjemite gneiss plutons in the immediate environs of the Barberton greenstone belt. Although similar in many respects to the other members of this suite, the Kaap Valley pluton is distinctively more mafic in composition, containing hornblende instead of biotite as its dominant mafic mineral, and is enriched in rare earth elements. These characteristics are not compatible with an origin simply involving a greater degree of partial melting than that required to form more felsic trondhjemites from the same precursor. The parental magma to the Kaap Valley tonalite is considered, rather, to originally have been a trondhjemitic magma enriched in minor phases such as zircon and apatite, and possibly also dissolved CO 2 vapour, which has been contaminated by a mafic component. The form and composition of hornblendes in the tonalite are inconsistent with a xenocrystic origin by incomplete digestion of surrounding mafic metavolcanic country rocks and, consequently, the contamination is inferred to have taken place prior to magma emplacement. Possible mechanisms include magma mixing and complete assimilation at depth of mafic rocks. A subdued form of crystal fractionation occurred during solidification of this magma, resulting in the segregation of discrete hornblende and hornblende + biotite tonalite phases. The tonalite was subsequently diapirically emplaced into its present position, perhaps about 3200 Ma ago, a process which resulted in deformation of the surrounding greenstone belt successions. Diapirism also perturbed the original, probably sub-horizontal, compositional zoning, the present geometry of which can best be explained in terms of an orthogonal pattern of intersecting structural lineaments.