Svecofennian post-collisional shoshonitic lamprophyres at the margin of the Karelia Craton: Implications for mantle metasomatism

Petrographic, geochemical, and geochronological data from shoshonitic lamprophyres from the North Savo region, eastern Finland, and the NW Ladoga region, northwest Russia are presented. The two areas are found ~250 km apart along a roughly 450 km long palaeosuture between the Archaean Karelia Craton and the Proterozoic Svecofennian Domain. Two of the samples fromNorth Savo are kersantites,while the remaining ten samples (from both North Savo andNWLadoga) are minettes. Two morphologically different populations of zircon were identified from these lamprophyres and analysed by secondary ionisation mass spectrometry. Concordant analyses of inherited crustal zircon xenocrysts gave both Archaean and Proterozoic 207Pb/206Pb ages. In the NW Ladoga region, these are the first reported Archaean ages from south of the Meijeri Thrust zone. Large, homogeneous and relatively U- and Th-depleted zircons are interpreted as xenocrysts from the metasomatised mantle.  U–(Th)–Pb analyses of these zircons resulted in ages of 1790 ± 3 Ma, 1784 ± 4 Ma, 1785 ± 5 Ma and 1781 ± 20 Ma, representative of the timing of dyke emplacement. Geochemical characteristics of the dykes indicate that the source mantle experienced at least two types of metasomatic enrichment. The first of these was caused by a hydrous alkaline silicate melt, resulting in enrichment in SiO2, Al2O3, FeO, K2O, LILE and LREE. The second was caused by a carbonatitic melt that further enriched the source area in Ba, Th, U, Sr, P and LREE. Infiltration of melt along fractures in the mantle wedge probably resulted in a vein assemblage of biotite – (potassic) amphibole – apatite± carbonate within unreacted mantle peridotite. The most likely source of the metasomatising melts was frompartial melting of subducted carbonate-rich pelitic sediment. ©