Growth of early Archaean crust in the Ancient Gneiss Complex of Swaziland as revealed by single zircon dating

Abstract

We present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the Ancient Gneiss Complex (AGC) of Swaziland, southern Africa, that enable us to document a more extended history of early Archaean crustal growth in the eastern Kaapvaal craton than previously documented. The oldest rocks appear to be strongly flattened tonalitic-trondhjemitic orthogneisses dated as 3644 ± 4 Ma (Compston and Kröner, 1988) which are in faulted contact with rocks of the Barberton Greenstone Belt (BGB) in an AGC enclave in northwest Swaziland and which also record subsequent thermal-metamorphic-plutonic events at ≈ 3580, 3500, 3430, 3200 and 3000 Ma. These rocks are tectonically interbanded with ≈ 3200 Ma old granodiorites and K-rich leucogranites, thereby providing evidence for a severe and important post-3.2 Ga deformation event that is also inferred for ≈ 3500 to 3200 Ma old tonalitic to trondhjemitic gneisses in the AGC of northeast Swaziland. In central Swaziland zircons from the oldest tonalitic gneiss of the AGC have an age of 3560 Ma whereas younger additions of granitoid, which also intrude the Dwalile greenstone enclave in the AGC, record ages of ≈ 3450–3460 Ma. Intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology. Chemical and whole-rock isotopic data are consistent with derivation of some AGC gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors, but this is not generally true for the entire AGC. We suggest that at least some gneisses resulted from intracrustal melting, perhaps during crustal thickening events as recorded in granulite parageneses within the AGC. Documentation of 3.5 Ga or older granitoid crust in all major regions of the AGC now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the Barberton greenstone succession was deposited. The combined geochronologic and structural/metamorphic data for the AGC and BGB reveal a complex history of growth and deformation for the eastern Kaapvaal craton that lasted for at least 600 Ma from 3.64 to about 3 Ga ago. The magmatic and tectonic evolution of the AGC was broadly similar to that of the BGB and suggests that early Archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels. Our age data and the structural/metamorphic evolution of the eastern Kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening, uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening.