On the Development of Gneisses and Granites in the Eskilstuna Region to the West of Stockholm

Abstract

The bedrock of the Eskilstuna region immediately to the south of Lake Mälaren (Fig. 1) belongs to a synclinorium within an old Archean range known as the Svecofennides. It comprises two supracrustal series, viz. the basal leptite-hälleflint series, which is mainly volcanic, and the overlying quartzite-graywacke-schist series. These are the oldest Svecofennian rocks encountered, and most of them have changed to gneisses, granite, and granodiorite during the Svecofennian orogenesis. The development of granite and granodiorite did not only involve granitizations in situ but also implied large-scale homogeneizations and mobilizations often on addition of juvenile matter, mineralizers especially.

The older, primorogenic alteration products comprise red and gray gneisses as well as a slightly schistose granite (acid gneiss-granite¹) and a more schistose granodiorite (in Sweden called intermediate gneiss-granite) accompanied by a schistose tonalite (in Sweden classed as basic gneiss-granite). The sedimentary gray gneiss is frequently rich in garnet, cordierite, and sillimanite.

The younger, serorogenic rock alterations involve the development of pegmatitic veins in all inhomogeneous rocks of the region, the gneisses especially. Further more granites were formed, both by granitization in situ and by mobilization of acid matter from the deepest parts of the Svecofennian fold-arcs. These granites are accompanied by late pegmatite dikes.

A table of the Svecofennian petrological sequence in the Eskilstuna region will be found on p. 2.