Petrological evidence for crustal melting, unmixing, and undercooling in an alkali-calcic, high-level intrusion: the late Sveconorwegian Vinga intrusion, SW Sweden

Abstract

The subvolcanic Vinga intrusion is a quartz jotunite to monzogranite located in the outermost part of the Goteborg archipelago, SW Sweden. It is one of a number of post-kinematic, late Sveconorwegian intrusions in the region. Rocks in the intrusion display complex, but well preserved igneous textures, including complexly zoned plagioclase megacrysts (cellular) and skeletal/acicular crystals of apatite, zircon, ilmenite and pyrite. The intrusion has high contents of Σ Fe, Ti, P, K, F and high field strength elements, and contains abundant and diverse enclaves (xenolithic, restitic and co-mingled). The cellular plagioclase may represent partially melted and re-equilibrated restitic plagioclase. The skeletal crystals formed due to undercooling and rapid crystallisation in the magma when it was brought to upper crustal levels. Ion-microprobe analyses of skeletal zircons from the intrusion yield a concordia age of 951 ± 7 Ma. The protolith of the parental magma is inferred to have been intermediate, meta-igneous and was partially molten under high temperature conditions as revealed from pyroxene thermometry (>1000 °C) on restitic pyroxenes. Miarolitic cavities, in combination with pressure estimates of ≤2 kbar from Al-in-hornblende barometry, indicate an epizonal emplacement depth at final crystallisation temperatures of around 750–800 °C. Zircon saturation occurred late, during rapid cooling below 900 °C, resulting in skeletal morphologies. The magma evolution started with early separation of restitic pyroxenes, plagioclase and Fe–Ti oxides at midcrustal depths, followed by fractionation. The heat source of melting was related to mafic underplating. Evolved parts of this mafic magma were co-mingled with the Vinga magma.