METAMORPHIC CONSTRAINTS ACROSS A MAFIC-ULTRAMAFIC TECTONITE ZONE BETWEEN THE MENDON AND KROMBERG COMPLEXES, BARBERTON GREENSTONE BELT, SOUTH AFRICA

Field and petrological aspects of sheared rocks in the Ekulindini Shear Zone (ESZ) that separates the Mendon Complex (MC) and the underlying Kromberg Complex (KC) in the Mesoarchean upper Onverwacht Suite, Barberton Greenstone Belt, are presented on the basis of surface geology and new drill core recovered during the Barberton Scientific Drilling Project (BSDP). Metamorphic conditions determined in the ESZ are compared to those in the MC and KC. Petrographic and electron microprobe analysis of ultramafic microlithons preserved in surface and fresh drill core samples from the upper banded part of the ESZ indicate variable development of the assemblage fuchsite + carbonate + quartz ± sulphides, i.e. listvenite. In thin section, fuchsite pseudomorphs in listvenite have replaced pre-existing asymmetric σ-style pyroxene porphyroclasts with σ-shaped foliation tail trajectories indicating that the likely protoliths to these rocks were ultramafic mylonites. Metamorphic temperature estimates were determined from compositions of rare Cr-chlorite grains in a listvenite surface sample and in drill core samples using a recently developed multi-equilibrium thermodynamic calculation in WINTWQ (v. 2.32). The listvenite records temperatures of between T = 405-455 ^oC. Metamorphic conditions in various microdomains of selected drill core thin sections are more variable but some T estimates are similarly high (e.g. T = 465±30 ^oC), particularly in the more deformed parts of microdomains. Undeformed metabasalt units directly above and below the shear zone record metamorphic temperature estimates of between T = 408-110 ^oC and T = 424-123 ^oC for the lower MC and upper KC, respectively. Metabasalt of the lower KC record lower temperatures of T = 357-145 ^oC. This new data thus reveals an inverted metamorphic gradient preserved in KC beneath the ESZ, suggesting thrust repetition across this tectonite.