Archean Inari and Rommaeno complexes in Arctic Fennoscandia – windows into the evolution of early continental crust

Northernmost Finland hosts two Archean tonalite-trondhjemite-granodiorite (TTG) complexes, the Inari complex in the east and the Rommaeno complex in the west. In our newly launched research project, “Birth of Continents”, these will serve as key areas in deciphering the mechanisms of the formation of buoyant, felsic crustal material from a basaltic precursor. Existing geochronological data are relatively sparse, but suggest a prolonged, semicontinuous zircon crystallization in excess of 200 Ma within spatially limited areas. Sampling in these extremely remote complexes has been skewed by the sparsity of road networks. Our sampling campaign with a more balanced grid will give a more thorough view into the age distributions within these TTG complexes.

Preliminary geochemical results as well as field evidence point to certain differences between the two complexes. These may reflect e.g. degrees of melting, variable metamorphic grade, or perhaps different present-day erosional levels. With help of Lu-Hf isotopes in zircon, we aim to constrain the pressures of zircon crystallization. Possible differences in the protoliths of the migmatized TTGs will be modeled with a melt reintegration procedure. Ultimately, we hope to develop a compelling model on how the earliest refractory felsic crust evolved from the transient basaltic one.