P-T constraints on the metamorphic evolution of the Paleoarchean Kromberg type-section, Barberton Greenstone Belt, South Africa

Abstract

Low-grade metabasites and hydrothermally altered ultramafic rocks form most of the Paleoarchean Barberton Greenstone Belt (BGB) of South Africa. However, P-T constraints are sparse and the nature of the greenschist-facies metamorphism is poorly characterized. This study provides new P-T estimates and descriptions of the petrological characteristics of altered mafic-ultramafic rocks across the Kromberg type-section, Onverwacht Group, BGB, from both surface samples and fresh drill core. Application of a chlorite thermodynamic multi-equilibrium calculation and pseudosection modelling, in conjunction with petrographic observations, indicates a wide range in metamorphic conditions from sub-greenschist to the uppermost greenschist facies across the type-section. A central fuchsite-bearing zone containing strong mylonitic fabrics, referred to as the Kromberg Section Mylonites (KSM), records at least two metamorphic events: a high-T, low-P (420 ± 30°C, <3 kbar) metamorphism, and a lower-T event (T = 240-350°C, P = 2·9 ± 0·15 kbar) related to retrograde metamorphism associated with extensional quartz-carbonate veins. Pillow lava metabasalts directly beneath the KSM record the same HT-LP-type metamorphic conditions, whereas metabasalts 90 m and 125 m above the KSM record temperatures of 250-340°C. Lower in the Kromberg section, metamorphic conditions vary from 215-321°C (at 2·9 kbar, mid to lower parts) to very low-grade conditions of 140-209°C in the lowermost Kromberg. Thus, an inverted metamorphic field gradient is documented beneath the KSM. Petrological analysis of the fuchsite-bearing rocks of the KSM indicates that they contain listvenite, a hydrothermal alteration product typically found in mafic-ultramafic tectonic mélange zones of ophiolite terranes. Together with the new metamorphic P-T constraints this means that these rocks are not a product of Archean atmospheric weathering. Rather, the inverted metamorphic field gradient in the Kromberg type-section suggests that the KSM represents a Paleoarchean thrust zone. Cr-spinel compositions in the ultramafic protolith to the KSM listvenites suggest a supra-subduction zone setting. A new geodynamic model is therefore proposed in which the mafic-ultramafic oceanic rocks of the Kromberg type-section were obducted as a thrust nappe pile in a regional transpressional tectonic regime between 3227 and 3230 Ma. © The Author 2012. Published by Oxford University Press. All rights reserved.