Palaeoarchaean TTGs of the Pilbara and Kaapvaal cratons compared; an early Vaalbara supercraton evaluated

Abstract

The continental crust that dominates Earth’s oldest cratons comprises Eoarchaean to Palaeoarchaean (4.0 to 3.2 Ga) felsic intrusive rocks of the tonalite-trondhjemite-granodiorite (TTG) series. These are found either within high-grade gneiss terranes, which represent Archaean mid-continental crust, or low-grade granite-greenstone belts, which represent relic Archaean upper continental crust. The Palaeoarchaean East Pilbara Terrane (EPT), Pilbara Craton, Western Australia, and the Barberton Granite-Greenstone Belt (BGGB), Kaapvaal Craton, southern Africa, are two of the best exposed granite-greenstone belts. Their striking geological similarities has led to the postulated existence of Vaalbara, a Neoarchaean-Palaeoproterozoic supercraton. Although their respective TTG domes have been compared in terms of a common petrogenetic origin reflecting a volcanic plateau setting, there are important differences in their age, geochemistry, and isotopic profiles. We present new zircon Hf isotope data from five granite domes of the EPT and compare the geochemical and isotopic record of the Palaeoarchaean TTGs from both cratons. Rare >3.5 Ga EPT evolved rocks have juvenile εHf(t) requiring a chondritic source. In contrast, younger TTG domes developed via 3.5 to 3.4 and 3.3 to 3.2 Ga magmatic supersuites with a greater range of εHf(t) towards more depleted and enriched values, trace element signatures requiring an enriched source, and xenocrystic zircons that reflects a mixed source to the TTGs, which variously assimilates packages of older felsic crust and a more juvenile mafic source. EPT TTG domes are composite and record multiple pulses of magmatism. In comparison, BGGB TTGs are less geochemically enriched than those of the EPT and have different age profiles, hosting coeval magmatic units. Hafnium isotopes suggest a predominantly juvenile source to 3.2 Ga northern Barberton TTGs, limited assimilation of older evolved crust in 3.4 Ga southern Barberton TTGs, but significant assimilation of older (Hadean-Eoarchaean) crust in the ca. 3.6 Ga TTGs of the Ancient Gneiss Complex. The foundation of the EPT is younger than that for the oldest components of the Eastern Kaapvaal. Although the broader prevailing Palaeoarchaean geologic framework in which these two cratons formed may reflect similar a geodynamic regime, the superficial similarities in dome structures and stratigraphy of both cratonic terranes is not reflected in their geochemical and age profiles. Both the similarities and the differences between the crustal histories of the two cratons highlights that they are formed from distinct terranes with different ages and individual evolutionary histories. Vaalbara sensu lato represents typical Palaeoarchaean cratonic crust, not in the sense of a single homogeneous craton, but one as diverse as the continents are today.

Secular Evolution of Continents and the Earth System

“…Taken together, the igneous and sedimentary successions therefore record breakup of larger cratonic masses. The inferred late Archean supercratons, variously termed Superia, Sclavia, Kenor, and Vaalbara, disaggregated into the smaller constituent cratons we recognize today; for example, Pilbara, Kaapvaal, Yilgarn, Zimbabwe, and Superior cratons, the Aldan and Anabar shields in Siberia, and the Archean blocks of Baltica and India (Bleeker, 2003; Bleeker & Ernst, 2006; Cheney, 1996; Davey et al., 2020; Gardiner et al., 2021; Söderlund et al., 2010; Williams et al., 1991).…”

Evidence for Protracted Intracrustal Reworking of Palaeoarchaean Crust in the Pilbara Craton (Mount Edgar Dome, Western Australia)

“…In contrast, Gardiner et al [33,34,81] offered an alternative interpretation where the earliest continental crust in the Pilbara area was extracted from a depleted mantle reservoir between 3.7 and 3.5 Ga. In their model, the Palaeoarchaean Supersuites of the Mount Edgar Dome derived partly from this ancient reservoir through intracrustal remelting and partly from minor juvenile additions at the time of magma crystallisation.…”

“…Smaller coloured symbols represent analyses of inherited cores. The two "DM" lines represent the evolution of the depleted mantle that separated from a chondritic mantle either at 4.56 Ga or at 3.8 Ga (e.g., [25]), and with a present-day εHf = +17 [81]: NC: new crust evolution line from Ref. [82].…”

Use of multispectral remote sensing data to map magnetite bodies in the Bushveld Complex, South Africa: a case study of Roossenekal, Limpopo.

“…Magnetite is a crucial tool for paleomagnetism studies, as it carries the dominant magnetic signature in most igneous, metamorphic and sedimentary rocks (Wingate, 1998). Additionally, magnetite is also mined for its economic importance in the iron and steel industry (Legodi & de Waal, 2007).…”