| Summary: | The conversion of basaltic crust into a thick, buoyant felsic crust of tonalite-trondhjemite-granodiorite (TTG) composition has been a crucial event in the Earth’s evolution during the Archaean Eon (4.0–2.5 Ga). The formation of TTGs as the result of the melting of hydrated mafic rocks is well established by geochemical modelling and experimental methods, but field studies are rare. Here, we investigate the migmatite structures and major and trace element geochemistry of Neoarchaean Lake Inari TTG-amphibolite terrain in northern Finland, in the Lapland-Kola Province of Arctic Fennoscandia. The Lake Inari amphibolites are low magnesium and low-titanium subalkaline basalts metamorphosed in the upper amphibolite facies that have flat or slightly LREE-enriched REE patterns. Geochemically, they resemble younger flood basalts. The TTGs have two coeval and intermingled geochemical endmembers, the low- and high-HREE types, which differ in their garnet-compatible element (HREE, Mg, Sc, Y, Co, and Zn) contents reflecting the presence or absence of garnet in their source. The TTG-metabasalt association shows various migmatite structures such as metatexites, metatexite-diatexite transitions and massive diatexites that have formed in response to rheological crustal strength drops, melt SEMR (Segregation, Extraction, Migration, and Redistribution) processes, and syn-anatectic strain. The Lake Inari terrain was a melt-rich crustal horizon with widespread migmatization due to partial melting of a basaltic plateau generated over a mantle plume, supporting a stagnant to sluggish lid tectonic setting for their formation. The Lake Inari terrain formed by migmatization of a basaltic source. Overall, the present denudation level of the terrain mostly represents the metatexite-diatexite transition and diatexite zones. Plateau basalts were converted to TTGs through mid-crustal migmatization, i.e., partial melt SEMR processes controlled by rheological crustal strength drops at different melt fractions and syn-anatectic strain. Peer ...
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