High-pressure intermediate-temperature metamorphism in the southern Barberton granitoid—greenstone terrain, South Africa: A consequence of subduction-driven overthickening and collapse of Mid-Archean continental crust

Abstract

The Early- to Mid-Archean Barberton granitoid-greenstone terrain of South Africa consists of the low-grade metamorphic Barberton greenstone belt that is surrounded by a composite granitoid-gneiss domain. High-grade greenstone lithologies along the southern margin of the belt and within the adjacent tonalite-trondhjemite-granodiorite (TTG)-greenstone domain record evidence of high-P intermediate-T metamorphism, with estimates of 9-12 kbar at 650-700 °C, 6 kbar at 490 °C, and 7.4 kbar at 580 °C documented from different areas of the domain. Pseudosection modeling of metamorphic assemblages suggests that the domain experienced a high-P clockwise P-T path, involving burial to depths of 35-45 km and heating to peak temperatures while at depth, followed by exhumation along a near-isothermal decompression path. Metamorphism coincides with the main accretionary event in the Barberton greenstone belt at ca. 3.23 Ga. The trajectory of the P-T path and the depths of burial are consistent with the subduction of this continental fragment during the early stages of collision. Exhumation of the domain occurred by extensional detachment faulting during the subsequent collapse of the overthickened orogen. The high-grade TTG-greenstone domain represents a section through the deeper segments of a Mid-Archean collisional belt. The P-T path preserved in these rocks may indicate the presence of relatively cold and rigid continental crust that was able to sustain crustal overthickening before the onset of gravitational collapse.