Metamorphism of magnesium-altered felsic volcanic rocks from Bergslagen, central Sweden. A transition from Mg-chlorite- to cordierite-rich rocks

Abstract

The ca. 1.9 Ga old felsic volcanic rocks of the Bergslagen province have been affected by synvolcanic hydrothermal Mg-metasomatism. A spatial relationship exists between Mg-alteration zones and calc-silicate/carbonate banded-iron mineralizations and a genetic link is suggested. This is supported by the occurrence of the Mg-rich tourmaline dravite, which exists in the Mg-alteration zones as well as in the wall-rocks to iron-oxide mineralizations. Following later regional metamorphism and deformation, these rocks occur as large zones of Mg-rich schists parallel to sub-parallel with the regional foliation (S1). The dravite crystals have been affected by this regional deformation. A second deformational episode folded S1 along steeply dipping fold axes. Locally, this resulted in a crenulation cleavage and the development of a new generation of micas (S2) crosscutting S1. In the westernmost areas the Mg-schists contain a fine-grained network of Mg-chlorite and muscovite with little or no preferred orientation, indicating low pressures and low-grade metamorphic conditions. The grade of recrystallization and deformation increase towards the east and a boundary between low- and medium-grade metamorphic terrains is crossed. The metamorphic boundary is marked by the disappearance of chlorite and the appearance of cordierite in the Mg-schists. Porphyroblasts of Mg-chlorite or cordierite grew across S1 and cordierite porphyroblasts are transected by S2 micas. This indicates that the peak of regional metamorphism occurred between the two deformational episodes. Retrogression of cordierite to chlorite and muscovite is confined to the vicinity of crosscutting S2 micas.