Metamorphism and exhumation of the basal parts of the Barberton greenstone belt, South Africa: Constraining the rates of Mesoarchaean tectonism

Abstract

The Paleo- to Mesoarchaean Barberton granitoid-greenstone terrain of South Africa consists of two main components: the low-grade metamorphic supracrustal greenstone sequence of the Barberton greenstone belt in the north and a high-grade metamorphic granitoid-gneiss terrain to the south. The boundary between the two different domains corresponds to the highly tectonized, amphibolite-facies rocks of the Theespruit Formation that occur along the margins of the greenstone belt. These rocks record high-P, low-T peak metamorphic conditions of 7.4±1.0kbar and 560±20°C that are very similar to estimates from other areas of the high-grade terrain and were attained during the main phase of terrain accretion in the greenstone belt at 3230Ma. In contrast, the greenstone sequence ca. 4km to the north only records low greenschist-facies metamorphism, indicating that a metamorphic break of ca. 18km exists between the high-grade terrain and the greenstone belt. The main phase of deformation in the Theespruit Formation was initiated under peak metamorphic conditions and continued during retrogression. Retrograde P–T estimates and mineral reactions indicate that retrogression involved near-isothermal decompression of ca. 4kbar prior to cooling into the greenschist-facies, suggesting that the fabric in these rocks is an exhumation fabric that accommodated the juxtaposition of the high-grade terrain against the greenstone belt. Geochronological constraints, combined with the depths of burial indicate that exhumation of the high-grade rocks occurred at rates of 2–5mm/a and are comparable to the exhumation rates of crustal rocks in younger orogenic environments. The extremely low apparent geothermal gradients of ca. 20°C/km that are recorded in the high-grade terrain are inconsistent with models of a hotter and weaker crustal environment in the Archaean. Rather, the depths of burial and structural integrity of this terrain suggest that the Mesoarchaean crust was cold and rigid enough to allow tectonic stacking and crustal overthickening and had a rheology similar to modern continental crust.