Metamorphism of the granite–greenstone terrane south of the Barberton greenstone belt, South Africa: an insight into the tectono-thermal evolution of the ‘lower’ portions of the Onverwacht Group

Abstract

This paper reports new geochronological and metamorphic data from Onverwacht Group greenstone remnants exposed in the granitoid-dominated terrain to the south of the Barberton greenstone belt. The greenstone remnants consist of metamorphosed mafic and ultramafic metavolcanic sequences, with minor sedimentary units that comprise thin chert and banded iron formation layers, interbanded with the (ultra) mafic volcanic units, as well as an up to 8 m-thick clastic sedimentary unit that contains well-preserved primary sedimentary features such as trough cross-bedding. Coarse-grained portions of the clastic sediments contain up to 4.5 wt.% K 2O and represent metamorphosed impure arkoses. SHRIMP and conventional U–Pb dating of detrital zircons reveal dates ranging between ca . 3521 and 3540 Ma, indicating that at least two protoliths for these sediments predate the formation of the bulk of the Barberton greenstone belt. A minimum age of 3431±11 Ma for the formation of the metasediments is given by a trondhjemite gneiss that intrudes the greenstone remnant. Thus, these metasediments were deposited between ca. 3521 and 3431 Ma, contemporaneously with the erosion of spacially associated older, and presumably potassium-rich, granitoid rocks. Other portions of the clastic metasediments have a mafic affinity and are characterised by the peak-metamorphic mineral assemblage diopside+andesine+garnet+quartz. This assemblage, and garnet in particular, is extensively replaced by epidote. Peak-metamorphic mineral assemblages of magnesio−hornblende+andesine+quartz, and quartz+ferrosilite+magnetite+grunerite have been recorded from adjacent amphibolites and interlayered iron formation units, respectively. In these rocks, retrogression is marked by actinolitic rims around peak metamorphic magnesio–hornblende cores in the metamafic rocks, and by a second generation of grunerite that occurs as fibrous aggregates rimming orthopyroxene in the iron formation. PT calculations, using a variety of geothermometers and barometers, for the peak-metamorphic mineral assemblages in all these rock types vary between 650 and 700 °C and 8 and 11 kbar. This implies a tectonic setting comparable to some modern orogenic belts and that the granite–greenstone terrane investigated in this study possibly represents an exhumed mid- to lower-crustal terrane that formed a ‘basement’ to the Barberton greenstone belt at the time of the peak metamorphic event at ca. 3230 Ma.