Prolonged Archaean migmatization and TTG Genesis in the Lake Inari terrain: Insights into stagnant lid tectonics and early continental crust formation

The transformation of basaltic crust into thick, buoyant felsic crust with a tonalite-trondhjemite-granodiorite (TTG) composition was a pivotal process in Earth's evolution during the Archaean Eon (4.03–2.50 Ga). Geochemical modeling and experimental studies strongly support the hypothesis that TTGs formed via the partial melting of hydrated mafic rocks, though field evidence remains scarce. TTGs are categorized into two geochemical types: low-HREE and high-HREE (heavy rare earth elements) TTGs. While their genesis is widely attributed to the partial melting of hydrated basaltic crust, the mechanisms producing the two types remain debated.

This study presents U–Pb zircon ages to investigate the temporal relationships between the two TTG types in the Lake Inari terrain, Fennoscandia shield. Zircon populations from low- and high-HREE TTGs, along with adjacent porphyritic granitoids, indicate a prolonged migmatization phase from 2900 to 2600 Ma. This suggests sustained heat from a stationary mantle plume associated with stagnant or sluggish lid tectonics. The intermingling and parallel ages of high- and low-HREE TTGs imply a shared source rather than distinct tectonic settings.

Prolonged melting episodes of thickened felsic crust produced porphyritic granites between 2650 and 2500 Ma. The ∼1.9 Ga Lapland-Kola orogeny caused minor zircon recrystallization, but did not alter the Archaean migmatite morphology. TTG formation in the Lake Inari terrain peaked around 2.8 Ga, approximately 100 Ma before the proposed assembly of the Kenorland supercontinent.

The bimodal TTG-metabasalt association exhibits diverse migmatite structures, including metatexites, metatexite-diatexite transitions, and massive diatexites. These features formed due to crustal weakening, melt segregation, extraction, migration, and redistribution (SEMR) processes, as well as synanatectic strain. Metabasalts within the terrain likely represent remnants of a basaltic upper ‘lid’ layer. We interpret the Lake Inari terrain as evidence of extensive migmatization in deeper layers of an overthickened basaltic plateau, sustained by a mantle plume and consistent with a stagnant or sluggish lid tectonic setting. Our findings suggest that the partial melting of plateau basalts, producing buoyant TTGs, may have initiated continental evolution