Building up the first continents: Mesoarchean to Paleoproterozoic crustal evolution in West Troms, Norway, inferred from granitoid petrology, geochemistry and zircon U-Pb/Lu-Hf isotopes

Combining geological, petrographic, geochemical and zircon U-Pb and Lu-lif isotopic data on granitoids provides powerful insights into the architecture, growth and geodynamic setting of Precambrian continents. Using such a dataset, we explore ca. 1.3 Ga of continent construction in the West Troms Basement Complex (WTBC), northern Norway, between the Mesoarchean and Paleoproterozoic. The new data show that the WTBC consists of two Archean lithotectonic segments: (1) a northeastern (NE) segment made up by juvenile (epsilon Hf-(t) = +2 to + 3) TTGs, formed between 2.92 and 2.83 Ga by increasingly deeper melting (Sr/Y from < 50 to > 90) of a young mafic crust; (2) a southwestern (SW) segment dominated by 2.71-2.67 Ga, sanukitoid-like (qz-)diorite-granodiorite-granite, derived from mantle-TTGs interactions and unrelated to the NE segment, as shown by positive epsilon Hf-(t) ( + 1 to + 2). The intervening domain comprises 2.74-2.69 Ga sanukitoids and granites with minor 2.96 Ga remnants, and may correspond to either reworked NE segment (epsilon Hf-(t) = -1) or a third segment. The Archean crust was intruded at 1.87-1.86 and 1.80-1.75 Ga by qz-monzonite, monzodiorite and monzo-/syenogranite plutons belonging to the Transscandinavian Igneous Belt (TIB). These show negative epsilon Hf-(t) (-7 to -13) pointing to melting of Archean mafic-intermediate lower crust and enriched lithospheric mantle. These observations are explained by a geodynamic model involving: (i) stabilization of the NE segment as a 3.0-2.8 Ga proto-cratonic nucleus, by formation and repeated melting of a progressively thickening mafic plateau; and (ii) drifting of this proto-craton towards the SW at 2.75-2.65 Ga, leading to passive subduction at its leading edge and formation of the SW segment as an arc-like complex. The subsequent Svecofennian accretionary orogeny (1.97-1.81 Ga) led to over-thickening and delamination of the lower crust and lithospheric mantle, forming the 1.87-1.75 Ga granitoids. A similar transition from intraplate to horizontal tectonics has been recorded in other cratonic domains, yet at different times ranging from Paleoarchean to Paleoproterozoic. This suggests that the onset of plate tectonics is not a globally synchronous event and depends on the regional lithospheric evolution.