Patch Migmatites from Västervik, Sweden: Subsolidus or Supersolidus? Reactions, P-T Conditions and Trace Element Distributions

Abstract

The Flecky Gneiss from Västervik, south Sweden, contains a mafic and a felsic mineral segregation, composed of cordierite, biotite, andalusite and sillimanite, and alkali feldspar, plagioclase and quartz, respectively. Despite its similarity to a migmatite, this lithology has always been described as a metamorphic, subsolidus segregation. In this thesis, 38 thin sections of the Flecky Gneiss have been analysed structurally (petrographic microscope) and chemically (Electron Probe Microanalyser) to determine if melting has taken place. These thin sections cover a large part of the Västervik metamorphic region. Major and trace element segregation and zoning were measured for cordierite, alkali feldspar, plagioclase and biotite. Additionally, pressure-temperature conditions were modelled with Perple_X. In multiple thin sections, this investigation resulted in convincing structural indications for melting including phenocrysts, eutectic melt crystallisation and mafic selvedges in the leucosome as well as plagioclase melt remnants in the mesosome. These structural observations vary between thin sections and were not universally observed. Chemical zoning of Ba and Sr indicates a former melt presence during mineral growth and simultaneous growth of the melanosome and the leucosome. This was also observed in thin sections that were structurally less convincing of melt having taken place. Mineralogical assemblages and modelling in Perple_X concluded on two phases of contact metamorphism. Firstly during burial at 200 MPa and 680 - 730 °C and secondly at 300 MPa and 730 °C. These are distinguished by a phase of cooling and cordierite recrystallisation at 300 MPa and 630 °C. This implies a maximum burial depth of 11 km. This concludes that the Västervik Flecky Gneiss locally experienced melt in both heating events and should therefore be regarded as a patchy migmatite. Metamorphic conditions in the Västervik region vary widely on a small spatial scale resulting in a diversity of metamorphic facies offering more opportunities for mineralogical research. Additional research and data collection is yet required to reliably distinguish between other and similar sub- and supersolidus mineral reactions.