Origin and emplacement of two compositional layered ultramafic bodies in the Caledonides of Västerbotten, Sweden

Abstract

The distribution of the main elements in two often more or less serpentinized, olivinitic—peridotitic bodies in the Björkvattnet—Virisen—Fättjarn area of the Swedish Caledonides of Västerbotten County was found to vary in the outcrops according to a compositional layering. This can be illustrated by calculating the normative mineral components of the rock, i.e. olivine and pyroxene. Olivinite/harzburgite interlayering and chromium-rich spinel banding have allowed of field identification of the compositional layering. It is obvious that all the analyses from the two investigated bodies can be plotted within the field of Alpine-type, ultramafic rocks. The ultramafic material is thought to be derived from a non-hydrous, refractory depleted upper mantle. At an early, probably pre-orogenic stage, a change from a peridotitic—dunitic state into a serpentinitic mineralogy occurred. Olivines and pyroxenes were converted into serpentine minerals. The serpentinites protruded, during Ordovician subduction, into a sedimentary, water-rich environment (Köli) and in some cases reached the surface. The serpentinites also protruded into the Seve and, during later metamorphism, came into an environment with a rather high temperature, resulting in dehydration and crystallization of olivine-rich rocks.