1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield

At least 14 small (1-11 km across) 1.8 Ga Svecofennian post-collisional bimodal intrusions occur in southern Finland and Russian Karelia in a 600-km-long belt from the Aland Islands to the NW Lake Ladoga region. The rocks range from ultramafic, calc-alkaline, apatite-rich potassium lamprophyres to peraluminous HiBaSr granites, and form a shoshonitic series with K2O + Na2O > 5%, K2O/Na2O > 0.5, Al2O3 > 9% over a wide spectrum of SiO2 (32-78%). Although strongly enriched in all rocks, the LILE Ba and Sr and the LREE generally define a decreasing trend with increasing SiO2. Depletion is noted for HFSE Ti, Nb and Ta. Available isotopic data show overlapping values for lamprophyres and granites within separate intrusions and a cogenetic origin is thus not precluded. Initial magmas (Mg# > 65) in this shoshonitic association are considered to be generated in an enriched lithospheric mantle during post-collisional uplift some 30 Ma after the regional Svecofennian metamorphic peak. However, prior to the melting episode, the lithospheric mantle was affected by carbonatite metasomatism; more extensively in the east than in the west. The melts generated in the more carbonate-rich mantle are extremely enriched in P2O5 similar to 4%, F similar to 12,000 ppm, LILE: Ba similar to 9000 ppm, Sr similar to 7000 ppm, LREE: La similar to 600 ppm and Ce similar to 1000 ppm. The parental magma underwent 55-60% fractionation of biotite + clinopyroxene + apatite + magnetite + sphene whereupon intermediate varieties were produced. After further fractionation, 60-80%, of K-feldspar + amphibole + plagioclase + (minor magnetite, sphene and apatite), leucosyenites and quartz-monzonites were formed. In the west, where the source was less affected by carbonatite metasomatism, calc-alkaline lamprophyres (vogesites, minettes and spessartites) and equivalent plutonic rocks (monzonites) were formed. Removal of about 50% of biotite, amphibole, plagioclase, magnetite, apatite and sphene produced peraluminous HiBaSr granites. The impact of crustal assimilation is considered to be low. At about 1.8 Ga, the post-collisional shoshonitic magmatism brought juvenile material, particularly enriched in alkalis, LILE, LREE and F, into the crust. Although areally restricted, the regional distribution of the post-collisional intrusions may indicate that larger volumes of 1.8 Ga juvenile material resides in unexposed parts of the crust. (C) 1998 Elsevier Science B.V. All rights reserved.