Tectonic thickening in stagnant to mobile lid transition facilitated the stabilization of Archean cratons

Abstract

Archean cratons are the oldest domains of tectonically stable continental crust and their formation coincided with the widespread emplacement of potassic granites, which reflects reorganization of crustal composition. However, the factors influencing the shifts in crustal composition and craton stability remain controversial. In this study of ca. 3.51–3.22 Ga trondhjemite-tonalite-granodiorite (TTG) and ca. 3.11–3.10 Ga potassic granites of the eastern Kaapvaal craton, we apply a newly developed zircon oxybarometer-hygrometer and document a progressive increase of magmatic fO₂ values and H₂O contents from ca. 3.51 to 3.22 Ga, peaking at ca. 3.10 Ga with values higher than those of known Archean granitic rocks and similar to modern orogenic granites. Concurrent increases in magma crystallization depths and crustal thickness culminated at 3.11–3.10 Ga. These trends reflect a tectonic transition from stagnant lid (3.66–3.23 Ga) to mobile lid (3.22–3.10 Ga) in eastern Kaapvaal craton, supported by isotopic shifts from juvenile to evolved Hf-O signatures. The combination of thermodynamic and geochemical modeling demonstrates that the widespread ca. 3.11–3.10 Ga potassic granites originated from partial melting of the middle to lower crust in regions of crustal thickening. The Kaapvaal Craton is proposed to be among the earliest cratons that witnessed tectonic thickening in stagnant to mobile lid transition, which might have played a critical role in the earliest stabilization and maturation of cratons.