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

Nick J. Gardiner, Jack A. Mulder, Christopher L. Kirkland, Tim E. Johnson and Oliver Nebel
Palaeoarchaean TTGs of the Pilbara and Kaapvaal cratons compared; an early Vaalbara Supercraton evaluated
South African Journal of Geology (March 2021) 124 (1): 37-52

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 epsilon Hf(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 epsilon Hf(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.


ISSN: 1012-0750
EISSN: 1996-8590
Serial Title: South African Journal of Geology
Serial Volume: 124
Serial Issue: 1
Title: Palaeoarchaean TTGs of the Pilbara and Kaapvaal cratons compared; an early Vaalbara Supercraton evaluated
Affiliation: University of St. Andrews, School of Earth and Environmental Sciences, Saint Andrews, United Kingdom
Pages: 37-52
Published: 202103
Text Language: English
Publisher: Bureau for Scientific Publications, Pretoria, South Africa
References: 4
Accession Number: 2021-043817
Categories: Solid-earth geophysicsGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. chart, 1 table, geol. sketch maps
S27°00'00" - S18°00'00", E18°00'00" - E30°00'00"
S21°00'00" - S21°00'00", E119°30'00" - E119°30'00"
Secondary Affiliation: Monash University, AUS, AustraliaCurtin University, AUS, Australia
Country of Publication: South Africa
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute. Abstract, Copyright, Geological Society of South Africa. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202114
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