Tectonic setting of post-collisional magmatism in the Palaeoproterozoic Svecofennian Orogen, SW Finland

Five bimodal post-collisional intrusions in southwestern Finland have been investigated. Geochemically, the mafic rocks are shoshonitic monzodiorites, which are highly enriched in Fe, P, Ti, F, LREE and in incompatible trace elements. The felsic rocks are garnet bearing peraluminous, S-type anatectic granites. New data on the mafic and the felsic intrusions yielded the same U–Pb zircon age of 1815 Ma. Therefore, the mafic and felsic intrusions are coeval but not cogenetic. Narrow contact metamorphic aureoles around the mafic intrusions contain garnet–orthopyroxene bearing assemblages, and thermobarometry indicates an intrusion depth of at least 15 km. Hence, there was little or no unroofing after peak regional metamorphism at 4–6 kbar. The geochemical characteristics of the mafic rocks suggest that they were derived from subcontinental lithospheric mantle that was previously enriched by fluids released during Svecofennian subduction. It is suggested here that hot upwelling asthenosphere convectively removed subcontinental lithospheric mantle and triggered partial melting of the enriched parts of the mantle. Uprising mafic melts increased the already high temperatures at mid-crustal levels and caused granulite facies metamorphism, crustal anatexis and production of granitic melts.