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Archean S-Type granites: petrology, geochemistry and geochronology of the Lekkersmaak and Willie plutons, Kaapvaal Craton, South Africa

Marc Poujol 1 Justine Jaguin 1 Jean-François Moyen 2, 3 Philippe Boulvais 1 Jean-Louis Paquette 3 
3 LMV-ENSMSE - Laboratoire Magmas et Volcans
Mines Saint-Étienne MSE - École des Mines de Saint-Étienne, SPIN, CNRS - Centre National de la Recherche Scientifique : UMR6524, LMV - Laboratoire Magmas et Volcans : UMR 6524
Abstract : S-type granites correspond to reworking of pre-existing continental material and form by partial melting of (meta)sediments. Early-to-mid Archean S-type granites are rare and are more frequently found, usually as relatively small intrusions, during the Neoarchean and the Archean-Proterozoic transition. In the context of Archean geology, their paucity is therefore significant, in that it matches the uncommon nature of processes during this period. In this study, we focus on the Late Mesoarchean Willie pluton and Neoarchean Lekkersmaak pluton, which crop out to the south of the Murchison Greenstone Belt in the Northern Kaapvaal Craton, South Africa. These intrusions represent the oldest S-type plutons known so far in the region. We constrain their petrogenesis by documenting their petrology, mineralogy (mineral composition), geochemistry (major and trace elements, Sr, Nd and O isotopes) and zircon U-Pb geochronology. The large Lekkersmaak and smaller Willie plutons represent two granites sharing many geochemical characteristics; they are both peraluminous, Ms-bearing S-type granites although they are more sodic and less potassic than typical S-type granites. The Willie granite was emplaced 2816 ± 9 Ma ago while the Lekkersmaak yielded an emplacement age of 2771 ± 2 Ma. Therefore, S-Type magmatism in the area occurred twice at 2.82 Ga and then 40 Myr later at 2.77 Ga, involving comparable immature metasedimentary sources and conditions of partial melting. Finally, a sample from the Lekkersmaak pluton devoided of S-type features as it is muscovite-free and aluminum poor was emplaced 2734 ± 11 Ma ago. This confirms that the magmatic activity south of the Murchison Greenstone Belt was episodic as stated in previous studies.
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Marc Poujol, Justine Jaguin, Jean-François Moyen, Philippe Boulvais, Jean-Louis Paquette. Archean S-Type granites: petrology, geochemistry and geochronology of the Lekkersmaak and Willie plutons, Kaapvaal Craton, South Africa. South African Journal of Geology, Geological Society of South Africa, 2021, 124 (1), pp.87-110. ⟨10.25131/sajg.124.0004⟩. ⟨insu-02958915⟩



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