Ž . Lithos 54 2000 63–81 www.elsevier.nlrlocaterlithos Tectonic setting of post-collisional magmatism in the Palaeoproterozoic Svecofennian Orogen, SW Finland Markku Vaisanen a, ) , Irmeli Manttari b , Leo M. Kriegsman a , Pentti Holtta b ¨¨ ¨ ¨ ¨ ¨ a Department of Geology, UniÕersity of Turku, FIN-20014 Turku, Finland b Geological SurÕey of Finland, PO Box 96, FIN-02151 Espoo, Finland Received 5 July 1999; accepted 7 April 2000 Abstract 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. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Mantle enrichment; Magmatism; Heat sources; Low pressure; Metamorphism; Granulites 1. Introduction Southwestern Finland was a key area when Seder- Ž . holm 1934 laid the groundwork for the understand- ing of the connection between orogenic processes ) Corresponding author. Present address: Geological Survey of Finland, PO Box 96, FIN-02151 Espoo, Finland. Fax: q 358-2- 0205502113. Ž . E-mail address: markku.vaisanen@gsf.fi M. Vaisanen . ¨¨ and magmatism. He divided the episodic formation of magmatic rocks into four groups, which are now Ž . often called early orogenic synorogenic , late oro- genic, post-orogenic and anorogenic rocks. In the Palaeoproterozoic Svecofennian Orogen in south- western Finland, the early orogenic magmatic rocks Ž . 1.89–1.87 Ga comprise foliated I-type tonalites, granodiorites and gabbros; late orogenic rocks Ž . 1.84–1.81 Ga are S-type high-K granites; post-oro- Ž . genic rocks 1.815–1.77 Ga comprise volumetri- cally minor bimodal mafic rocks and granites; anoro- 0024-4937r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0024-4937 00 00018-9