Formation of Paleoarchean continental nuclei during a tectonic transition from plateau to delamination: Evidence from the Barberton granitoid-greenstone terrane, South Africa

Whether Earth’s earliest continental nuclei formed in subduction-related (plate tectonics) or thick, plateau-like (not related to plate tectonics) regimes remains controversial. Resolving this debate hinges on elucidating the petrogenesis of the earliest tonalite-trondhjemite-granodiorite (TTG) suites, specifically the nature of their basaltic sources and physico-chemical conditions (pressure-temperature) during partial melting. Here, we investigated the Paleoarchean Steynsdorp Complex—the oldest TTG complex in South Africa’s Barberton granitoid-greenstone terrane (BGGT). Zircon U-Pb dating confirms the main magmatic phase at ca. 3.51 Ga and identifies a previously unrecognized ca. 3.22 Ga magmatic episode within the same complex. Thermodynamic modeling indicates that both the TTG phases were derived from partial melting of hydrous basaltic sources but at distinct pressures: 1.1–1.3 GPa for the ca. 3.51 Ga phase and 0.9–1.1 GPa for the ca. 3.22 Ga phase. Isotopically, the ca. 3.51 Ga TTGs yield mantle-like δ18O and εHf(t) values tracking a ca. 3.7–3.55 Ga source and reflecting melting within a thickened mafic plateau. In contrast, the ca. 3.22 Ga TTGs exhibit a distinct decoupled isotopic signature characterized by elevated δ18O (up to 7.0‰) and highly positive εHf(t) values (up to +5). Combined with elevated Mg# and Cr concentrations, these geochemical characteristics indicate that the ca. 3.22 Ga magmatism resulted from subduction-driven tectonic thickening followed by lower crustal delamination, which induced asthenospheric upwelling and subsequent hybridization of juvenile mantle-derived melts with supracrustal-derived melts. Our results suggest a fundamental transition from stable plateau growth at ca. 3.51 Ga to a dynamic subduction-delamination regime at ca. 3.22 Ga, which may ultimately drive the formation and stabilization of the BGGT continental nuclei.