Metamorphic processes preserved in early Archean supracrustal rocks of the Barberton Greenstone Belt, South Africa

Abstract

AbstractThe Palaeoarchean supracrustal rocks of the Barberton Greenstone Belt (BGB) consist of hydrothermally altered volcanic and silicified sedimentary rocks that form the dramatic topography in the Barberton Mountain Land of Mphumalanga Province in South Africa and Swaziland. Previous metamorphic studies are limited and used mainly empirical chlorite geothermometry in the BGB sequence regionally, or Raman thermometry, both proposing a narrow range in metamorphic conditions of around 320°C regional for the BGB. This study demonstrates that a range of petrological and thermodynamic modelling techniques are required in an integrated approach to unravel the very low- to medium-grade metamorphic conditions preserved in the oldest part of the BGB. The study investigates low-temperature metamorphic processes in the Komati, Hooggenoeg, Kromberg and Mendon formations of the c. 3530–3298 Ma Onverwacht Group. Chlorite thermodynamic modelling indicates metamorphic conditions of between 250 and 445°C, in the c. 3482 Ma Komati Formation, but uncertainties on these conditions are large (c. 80 and 100°C) due to the protoliths being mostly ultramafic in composition with uncertainty in XFe3+ estimates. Raman thermometry on carbonaceous material in the c. 3472 Ma volcano-sedimentary Middle Marker chert (HC1) at the base of the Hooggenoeg Formation indicates metamorphic conditions of between 281 and 301°C, lower than in previous studies. Raman thermometry on a carbonaceous chert (HC5) in the 3472–3432 Ma Hooggenoeg Formation in conjunction with δ18O oxygen isotope values on underlying metabasalts, indicate metamorphic temperature conditions of less than 340°C, preserved in the Hooggenoeg Formation. Application and critical evaluation of chlorite thermodynamic modelling results confirm the presence of an inverted metamorphic field gradient beneath a fuchsite–chlorite–quartz shear zone in the c. 3334 Ma Kromberg Formation. Thermodynamic modelling using a di-octahedral mica model indicates that retrograde metamorphism in the Kromberg ultramafic shear zone occurred along a kyanite-type geothermal gradient, providing evidence in support of tectonic thrusting at c. 3227–3223 Ma. In the c. 3298 Ma Mendon Formation seawater alteration involving metasomatism occurred at conditions of T = 150°C with log aCO2(aq) of −2.74. This study sheds new light on the metamorphic history of the BGB supracrustal sequence and early Earth metamorphic processes. It also demonstrates that combining different petrological and thermodynamic methods across the BGB supracrustal stratigraphy allows a robust critical assessment of metamorphic conditions for the first time, with implications for the early Archean geodynamic evolution of the BGB.