The magmatic evolution of the upper --3450 Ma Hooggenoeg Formation, Barberton greenstone belt, Kaapvaal Craton, South Africa

Abstract

This report presents new whole-rock geochemical data on 45 samples from the upper part of the ~3450 Ma Hooggenoeg Formation of the Barberton greenstone belt, Kaapvaal Craton, South Africa. The study attempts to constrain the magmatic evolution of the felsic upper part of the Hooggenoeg Formation. Considerable controversy exists concerning the ideas pertaining to the origin and emplacement of Archaean felsic suites. Resolving these issues will contribute to the understanding of the evolution of Early Archaean greenstone belts, and in particular their tectonic settings and the role of subduction processes in Archaean geology. Detailed mapping and geochemical sampling in a key area of the Buck Ridge on the western limb of the Onverwacht Anticline, was conducted in order to obtain an accurate overview of the different lithological units present in the upper part of the Hooggenoeg Formation. The Hooggenoeg Formation there is characterized by (ultra-) mafic massive and pillowed lavas; a trondhjemitic suite of silicified felsic intrusive and flow banded rocks; and sedimentary chert beds. Veins of felsic, chert and (ultra-) mafic material intrude the belt, in that order. The depositional environment is thought to be a shoaling shallow sea in which the Hooggenoeg Formation has been deposited in a west-block down, listric faulted, synsedimentary setting. The Hooggenoeg Formation felsic rocks were divided into two groups: an intrusive group of interlocking and shallow intrusive rocks, and a porphyritic group of rocks from the veins, based on differences in immobile and incompatible element compositions, This subdivision was not contradicted by major elemental compositions, despite pervasive alteration. Lavas from the upper part of the felsic unit were too altered to be assigned to one of these groups. The intrusive group is related TTG-suite Stolzburg Pluton, which intruded along the southern margin of the Barberton greenstone belt. Melting of an amphibolite or quartz eclogite has been suggested as a probable origin for these high-Al2O3 felsic magmas. (Ultra-) Mafic rocks of the Hooggenoeg Formation were most likely not parental for the felsic rocks. Subduction processes may have played a role in the generation of the felsic rocks, but a tectonic setting for the (ultra-) mafic rocks remains uncertain. The felsic units of the Hooggenoeg Formation are very similar to those of the Panorama Formation of the Early Archaean Coppin Gap greenstone belt of Western Australia. Similarities in geological setting, petrography, and geochemical (trace element in particular) characteristics suggest a possible genetic relation between the two formations and support the theory that a combined continent Vaalbara existed ~3450 Ma.