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 (original) (raw)

2002, Precambrian Research

Numerous greenstone remnants occur in the granitoid-dominated terrane to the south of the Barberton greenstone belt. These have generally been regarded as part of the lowermost formations of the Onverwacht Group on the basis of lithological correlations and the contact relationships with the intrusive Stolzburg trondhjemite pluton. The greenstone remnants consist of metamorphosed mafic and ultramafic metavolcanic sequences, with associated minor sedimentary units. The sediments consist of thin chert and banded iron formation layers that are interbanded with the cyclic (ultra) mafic volcanic units, as well as an up to 10m-thick clastic sedimentary unit that contains well-preserved primary sedimentary features such as trough cross-bedding. Coarse-grained portions of these metasediments are moderately metamorphosed impure arkoses consisting mainly of quartz, microcline, plagioclase and diopside and which contain up to 4.5 wt% K O. SHRIMP ion 2 microprobe and conventional U-Pb dating of detrital zircons reveal ages at 3536, 3526, 3479 and 3442 Ma, respectively, indicating that two protoliths for these sediments predate the formation of the bulk of the Barberton greenstone belt. The oldest age estimate is identical to an age reported for a tectonic wedge of tonalitic gneiss within the Theespruit Formation from the southern portion of the greenstone belt. A minimum age of 3431 11 Ma for the formation of the metasediments is given by a trondhjemite gneiss that locally intrudes the greenstone remnant. Thus, these metasediments were deposited between c. 3442 and 3431 Ma, contemporaneously with erosion of spatially associated older granitoid rocks. Portions of the clastic metasediments that are quartz-bearing, but not quartz-rich, show well-defined thin wavy bedding and compositional banding. In these areas the peak-metamorphic mineral assemblage is commonly diopside + andesine + garnet + quartz, with a garnet core composition of Xgrss = 0.60, Xalm = 0.319, Xpyp = 0.023 and Xspss = 0.06. 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. Retrogression is marked by actinolitic rims around peak metamorphic magnesiohornblende cores in the metamafic rocks, and by a second generation of grunerite that occurs as fibrous aggregates rimming orthopyroxene in the iron formation. Average P-T calculations for the peak-metamorphic mineral assemblages in all these rock types vary between 650-700ºC and 7.0-11.0 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 terrane that formed a basement to the Barberton greenstone belt at the time of the peak metamorphic event. The age of the amphibolite facies metamorphism of rocks correlated with the Fig Tree Group from the Schapenburg schist belt further south is c. 3230 Ma. The continuation of the tectonic fabric from Schapenburg into the greenstone remnants investigated in this study suggests that the rocks in both these areas may be recording the mid-to lower-crustal response to the main compressional event in the Barberton area at c. 3230 Ma.