Source composition, fractional crystallization and magma mixing processes in the 3.48–3.43Ga Tsawela tonalite suite (Ancient Gneiss Complex, Swaziland) – Implications for Palaeoarchaean geodynamics

Abstract The ca. 3480–3430 Ma Tsawela Gneiss (TG) is a well-preserved coarse-grained diorite to tonalite suite within the Ancient Gneiss Complex of Swaziland, eastern Kaapvaal craton. These gneisses are texturally and compositionally distinct from the hosting oldest components of the ca. 3200–3660 Ma TTG-type Ngwane Gneisses (NG). Major and trace elements, in combination with whole-rock hafnium-neodymium isotopic data, were analyzed in the TG and in three samples of ca. 3450 Ma grey NG to constrain sources and magmatic processes. High-field-strength element data (HFSE) were combined with U-Pb SHRIMP II ages and Hf-in-zircon data for key samples to constrain their ages and petrogenesis. In c…

Supracrustal gneisses in southern Swaziland: a basalt-sandstone assemblage of the upper Mozaan Group deformed in the Neoarchaean

Generation of early Archaean grey gneisses through melting of older crust in the eastern Kaapvaal craton, southern Africa

Abstract We report zircon ages, Hf-in-zircon isotopes as well as whole-rock geochemistry and Hf–Nd isotopic systematics for Palaeoarchaean grey gneisses of the Ancient Gneiss Complex of Swaziland, the oldest components of the Kaapvaal craton, southern Africa. The Hf-in-zircon isotopic compositions in these compositionally heterogeneous, multicomponent, migmatitic gneisses are highly variable, even in the oldest zircons dating back to 3.66 Ga, suggesting growth of zircon from melts representing a mix of juvenile and anatectic material derived from differentiated continental crust of Eoarchean to late Hadean age. In contrast, the initial Nd and Hf whole-rock isotopic compositions are frequent…

The Nhlangano gneiss dome in south-west Swaziland – A record of crustal destabilization of the eastern Kaapvaal craton in the Neoarchaean

Abstract The Nhlangano granite-gneiss in south-west Swaziland, dated at 2.98 Ga, forms an elliptical dome of remobilised, migmatitic crust. Strongly deformed supracrustal rocks wrap around or form synformal keels infolded with subdomes of Nhlangano Gneiss and represent Mesoarchaean Pongola Supergroup strata at amphibolite- to granulite-facies metamorphic grade. Amphibolites represent the metamorphosed equivalents of Nsuze Group basalts and andesites on the basis of similar trace element and Nd isotope data. Garnetiferous Mahamba Gneiss, dated at c. 2.96 Ga, represents the metamorphosed equivalents of felsic to intermediate volcanic rocks and minor pelitic interflow sedimentary units of the …

The Chirwa dome: granite emplacement during late Archaean thrusting along the northeastern margin of the Zimbabwe craton

The Chirwa dome in northeast Zimbabwe is situated at the boundary between the Zimbabwe craton and Archaean gneisses of the Zambezi mobile belt. This circular granite intrusion has long been regarded as Archaean granite which was remobilized during Pan-African times and emplaced as a mantled gneiss dome into a Proterozoic metasedimentary sequence. Field mapping, structural work and new zircon dates indicate that the Chirwa dome and surrounding rocks underwent a very different history. The supracrustal sequence was deposited and moderately deformed between ~2613 and ~2601 Ma ago, as bracketed by a provenance date and an intrusive date, respectively. Shortly after deposition, the sequence was …

Archaean Granitoid–Greenstone Geology of the Southeastern Part of the Kaapvaal Craton

The southeastern Kaapvaal Craton is a Palaeoarchaean granitoid–greenstone terrain. Supracrustal rocks are dominated by metamorphosed mafic–ultramafic volcanic rocks intercalated with minor felsic volcanic and chemical sedimentary rocks, including carbonaceous chert and minor iron formation. Siliciclastic sedimentary rocks are rare. The greenstones occur in the Schapenburg and Dwalile fragments close to the Barberton greenstone belt, the Assegaai, De Kraalen, Witrivier and Commondale fragments in the vicinity of Piet Retief, and the Nondweni and Ilangwe greenstone belts together with several smaller fragments in the southern part of the craton. The greenstones are locally in tectonic contact…

High-temperature metamorphism and crustal melting at ca. 3.2 Ga in the eastern Kaapvaal craton, southern Africa

Abstract The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2 Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66–3.20 Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2 Ga granulite-facies assemblage in a metagabbro of the AGC of central Swazi…

The onset of deep recycling of supracrustal materials at the Paleo-Mesoarchean boundary.

Abstract The recycling of supracrustal materials, and in particular hydrated rocks, has a profound impact on mantle composition and thus on the formation of continental crust, because water modifies the physical properties of lithological systems and the mechanisms of partial melting and fractional fractionation. On the modern Earth, plate tectonics offers an efficient mechanism for mass transport from the Earth's surface to its interior, but how far this mechanism dates back in the Earth's history is still uncertain. Here, we use zircon oxygen (O) isotopes to track recycling of supracrustal materials into the magma sources of early Archean igneous suites from the Kaapvaal Craton, southern …

Late Archaean foreland basin deposits, Belingwe greenstone belt, Zimbabwe

Abstract The c. 2.65 Ga old sedimentary Cheshire Formation of the Belingwe greenstone belt (BDB), central Zimbabwe, has been studied in detail for the first time to shed some light on the much debated evolution of this classical belt. The Cheshire Formation rests sharply on a mafic volcanic unit (Zeederbergs Formation) and comprises a basal, eastward-sloping carbonate ramp sequence built of shallowing-upward, metre-scale sedimentary cycles. The cycles strongly resemble Proterozoic and Phanerozoic carbonate cycles and might have formed by small-scale eustatic sea level changes. The top of the carbonate ramp is represented by a karst surface. The carbonates are overlain by and grade laterally…

Hafnium-Neodymium isotope, trace element and U-Pb zircon age constraints on the petrogenesis of the 3.44–3.46 Ga Dwalile greenstone remnant, Ancient gneiss Complex, Swaziland

Abstract The geodynamic setting of the 3.44–3.46 Ga Dwalile greenstone remnant (DGR) of the Ancient Gneiss Complex (Swaziland) and its relationship to other Greenstone belts of similar age is investigated in this contribution. We present Hf-Nd isotope, major and trace element data for komatiites, komatiitic basalts, basalts, basaltic andesites as well as felsic volcanic and sedimentary rocks, all metamorphosed at amphibolite-facies grade. Major and trace element compositions of metabasalts and metakomatiites indicate crustal contamination. The best-preserved samples yield 176Lu-176Hf and 147Sm-143Nd isochron ages of ca. 3500 and 3460 Ma, respectively, in line with U-Pb zircon ages of interl…

Coupled silicon-oxygen isotope fractionation traces Archaean silicification

International audience; Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa.The δ30Si and δ18O values of bulk rock increase with increasing amount of silicification from unsilicified basalts (-0.64‰ 30Si 18O 30Si and δ18O values as high as + 0.81‰ and + 15.6‰, r…

Cyclicity of Triassic to Lower Jurassic continental red beds of the Argana Valley, Morocco: implications for palaeoclimate and basin evolution

Abstract Cyclical playa deposits form a prominent part of the continental clastic succession of the Argana Valley, Western High Atlas of Morocco. The red beds formed in Triassic to Lower Jurassic times during rifting of the North American and African plates. Detailed stratigraphic work revealed asymmetrical, metre-scale cycles in mudstone-dominated successions that constitute the intermediate and upper portion of the basin fill. Sedimentary cycles commonly comprise ephemeral lake shales at the base, playa mudflat mudstones in the intermediate part, and both fluvial and aeolian sandstones at the top. Cycles of the Aglegal Member (T4) are mainly characterized by analcime-rich playa mudflat de…