Leucosome-forming small-scale geochemical processes in the metapelitic migmatites of the Turku area, Finland

Abstract

Selected outcrops of the metapelitic migmatites of the Turku area (southwestern Finland) have been investigated for their petrographic and chemical properties. Three different leucosome types could be distinguished: leucosome sills, in-situ leucosomes, and composite leucosomes. Almost all leucosomes are rich in Or-dominated alkali feldspar, only a few composite leucosomes show plagioclase-rich layers in addition. All leucosomes are of very similar composition (except the rare plagioclase-rich ones) and assumed to be generated by dehydration melting. The melt proportion formed by this reaction at peak metamorphic conditions (800°C, 5–6 kbar) is estimated from experimental data to be approximately 20 wt.%. The different leucosome types are considered to be the result of various degrees of melt segregation and transportation: the in-situ leucosomes were formed almost in place, the composite leucosomes are the outcome of segregation and transportation of partial melts over relatively short distances (<1 m), and the leucosome sills represent the most mobile melt fraction. A comparison of REE patterns of alkali feldspar–quartz-rich leucosomes and restite assemblages reveals low overall REE contents in the leucosomes with distinct positive Eu-anomalies. The Eu is inherited from the feldspars of the source rock while the light and heavy REE remained in accessory minerals and garnet in the restite. When compared to the metapelitic source rock, the restites are enriched in HREE depending on their modal garnet abundance.