Sm/Nd and Lu/Hf isotopes in the Svecofennian rocks of southernmost Finland

Abstract

The granite-rich area surrounded by metaigneous and metasedimentary migmatites around the Hanko peninsula has an unclear history regarding its magmatic origin. The arc complex of Southern Finland hosts several compositionally different early Svecofennian and late Svecofennian granitoids. The area is part of the accretionary orogen that produced both volcanic rocks and their plutonic counterparts (e.g. Väisänen and Mänttäri, 2002). It is unclear how much mantle input the igneous rocks in the area received and how the magmas behaved during this tectonically active period, and if the area is part of a different terrane than the rest of southern Finland, such as the Orijärvi area further north which is suspected to have formed in the back-arc region of the arc complex (Väisänen and Mänttäri, 2002). By using the two isotope systems of Sm/Nd and Lu/Hf, this study aims to provide further clues about the origins and evolution of the magmas in the area. The results from this study suggests a mildly depleted mantle source under Fennoscandia during the older Svecofennian event (1.89-1.87 Ga), which then subsided and allowed for magma homogenization during the younger Svecofennian event (1.84-1.80 Ga). The results from the Hf-analysis point to a fore-arc environment, as there are no signs of typical back-arc-related pelitic sedimentary zircons.