Successive midcrustal, high-grade metamorphic events provide insight into Mid-Archean mountain-building along the SE margin of the proto-Kaapvaal craton
In this study, we investigate the evolution of continental crust at the Paleoarchean to Mesoarchean boundary by documenting the deposition, deep burial, and partial melting of metasedimentary gneisses along the SE margin of the proto–Kaapvaal craton. Successive high-grade metamorphic events recorded by the gneisses between ca. 3.23 and 3.07 Ga not only coincided with the timing of subduction-accretion in the adjacent Barberton greenstone belt at ca. 3.23–3.22 Ga, but also with discrete pulses of potassic granitic magmatism during differentiation and consolidation of the newly assembled lithosphere. Mineral equilibria modeling demonstrates that the granulites evolved along pressure-temperature ( P - T ) paths similar to those documented for metamorphism in modern collisional orogens. High-temperature deformation at ca. 3.11–3.07 Ga during uplift of the terrane was coaxial with the main NE-SW–trending structural grain of the Barberton greenstone belt and reflects the regional NW-SE shortening and NE-SW orogen-parallel extension exhibited by the younger potassic granites. We reconcile these features with Mesoarchean terrane assembly in Barberton via two NW-dipping subduction zones. The trace of the first is represented by the main terrane boundary within the Barberton greenstone belt; the second resulted in accretion, burial, and high-temperature metamorphism along the SE margin of the Kaapvaal craton, with the granulites providing valuable insight into the mid- to lower-crustal response to what appears to have been a protracted accretionary orogenic event.