The link between migmatites and S-type granites in the Turku area, southern Finland

Abstract

Metamorphism and compositions of metasedimentary migmatites and related small-scale granites are investigated in the Turku area (SW Finland). Phase assemblages and mineral compositions suggest peak P– T-conditions of ∼800 °C and 6 kbar. The portion of melt produced in the migmatites by dehydration melting is estimated to be ∼20 wt.%. The partial melt is K 2O-rich and water undersaturated (∼4 wt.% H 2O). The leucosomes and small-scale granites have almost the same proportions of felsic minerals, but the mafic minerals (Grt, Crd, Bt) are enriched in the granites along with the contents of TiO 2, FeO tot, MgO and HFSE. The leucosomes have low total REE values and strongly positive Eu-anomalies. The REE patterns of the granites are characterized by relatively high LREE, low HREE and distinct negative Eu-anomalies. The distribution of major elements and trace element signatures suggests that leucosomes and granites are related as cumulates and evolved residual melts, respectively. The decimeter to meter-scale leucosomes are interpreted to be the product of crystal fractionation, which occurred during near-isobaric cooling, and the granites are the (crystallized) residual melts which could move further on and form larger bodies within and outside of the migmatite complex.