Geochemistry of the Fellingsbro type granites, South Central Sweden

Abstract

The Fellingsbro granite sensu stricto is a massive coarse-grained rock characterized by red, angular microcline megacrysts up to 7 cm long. The Lisjö granite is of the same type as the Fellingsbro granite but is situated 20km to the NE. Both intrusions belong to a belt of c. 1.78 Ga old granites coinciding with the major so-called Central Swedish Gravity Low. Gravity studies suggest that the Fellingsbro and Lisjö granites rose from a huge granite ridge through structurally controlled root zones, forming mushroom-shaped intrusions. These porphyritic granites have the geochemical characteristics (e.g. high Y, Nb, Zr and HREE) of granites generated in 'within-plate' or tensional rifting environments. Even-grained granites of approximately the same age occurring in heterogeneous mixtures of granites, pegmatites, xenoliths and partly assimilated country rocks e.g. in the Fellingsbro and Låsen areas, probably represent minimum melt type rocks that have not moved far from their source. The even-grained interior of the Lisjö granite and the contemporaneous Dingtuna and Pingstaberg granites, were generated by differentiation of granites with similar characteristics as the porphyritic Fellingsbro granite. Recently published U-Pb zircon datings have demonstrated that the Småland-Värmland granite-porphyry Belt has approximately the same age as the granites in this study. We speculate that both groups were emplaced in a tensional environment on the continental side of a subduction related calc-alkaline belt, remnents of which may be preserved in the SW Swedish Gneiss Region. The Fellingsbro and Lisjö granites were generated more towards the interior of the pre-existing c. 1.89 Ga old continent than the Småland-Värmland granites.