Svecofennian intra-orogenic gabbroic magmatism: a case study from Turku, southwestern Finland

Using single-grain zircon U-Pb dating by LA-MC-ICPMS and whole-rock geochemistry, we have studied the Palaeoproterozoic gabbroic rocks from Moisio in southwest Finland. Three ages were obtained. The interpreted intrusion age is ~1.86 Ga, which places it in the 1.87±1.84 Ga intra-orogenic period of southern Svecofennia. The other ages, ~1.89 Ga and ~1.83 Ga, are inferred to be inherited and metamorphic ages, respectively. The K, LREE, LILE, Fe-, P-, Ti- and F-concentrations reveal two compositionally distinct groups: (i) an enriched monzogabbro group and (ii) a less enriched gabbro group. The composition of the monzogabbro group resembles the other intra-orogenic intrusions from southern Svecofennia, whereas the unrelated gabbro group is more comparable to the synorogenic rocks in the region. The magma source the monzogabbro experienced a subduction related carbonate metasomatism, induced by sediment subduction and subduction erosion. Evidently, the Moisio monzogabbro represent enriched, mantle derived magmatism in southern Svecofennia as a part of the intra-orogenic igneous activity. The intra-orogenic magmatism is considered to have conveyed considerable amounts of heat from the mantle into the crust contributing to subsequent lateorogenic high-grade metamorphism.a