Microfabric evolution during metasomatism and deformation, exemplified by the nodular sillimanite gneisses (Bamble lithotectonic domain, South Norway)

Abstract

Proterozoic foliated and nodular sillimanite gneisses from the Bamble lithotectonic domain, South Norway, are analysed to unravel their microfabric evolution with mineral reactions during metasomatism and associated deformation. The nodules form cm-scaled spherical to ellipsoidal sillimanite-quartz aggregates that locally grade into foliated sillimanite gneisses. Independent on their fabric, they record incomplete breakdown reactions of biotite and K-feldspar recorded by muscovite lamellae and associated Fe-oxide needles in biotite and by muscovite-quartz aggregates after K-feldspar. Muscovite is partly replaced by sillimanite. Based on immobile Al, the nodular gneiss forming reactions give excess K, Mg and H2O that may leave the nodular gneiss to form a metasomatic agent and caused regional metasomatism (scapolitisation) in the surrounding rocks. Quartz in the foliated gneisses shows a pronounced shape but no marked crystallographic preferred orientation. There is no indication of major strain accumulation by quartz dislocation creep. Muscovite shows lobate phase boundaries to quartz, which is interpreted as reaction fabric, from the breakdown reactions of K-feldspar and biotite. The nodular and sillimanite gneisses formed during metasomatic mineral reactions, where major elements K, Mg and H2O leave the rock and an Al-rich metasomatic restite remains. We suggest that the metasomatism involved a molar volume loss, where reactions forming muscovite, quartz and sillimanite occurred by incongruent dissolution-precipitation creep at low stresses forming the nodular and foliated gneisses. Our study demonstrates that metasomatism with chemical rock changes and mass transfer associated with incongruent dissolution-precipitation contributed to the observed reaction and deformation microfabric.