The formation of metasomatic reaction veins in dolomitic marble roof pendants in the Bergell Intrusion (Province Sondrio, northern Italy)

Abstract

At its eastern margin the Berge11 intrusion contains large numbers of well exposed dolomitic marble roof pendants and inclusions. Symmetrically zoned reaction veins occurring in these dolomites are related to a regional fracture system which was possibly formed during the cooling and uplift of the intrusion. The vein thicknesses range from a few millimeters to a maximum of 0.6 m. Mineral assemblages found in individual zones of the veins include: calcite + olivine, calcite -I-diopside, calcite + amphibole, calcite + antigorite, calcite + talc. Biotite may be present in all zones. The zone sequences found in different veins show a large variation. The most abundant vein type consists of a single but internally banded amphibole + calcite zone. The internal zonation is caused by discontinuous changes in the amphibole composition. Ironand aluminium-rich amphiboles are found close to the vein center, whereas pure tremolite occurs at the metasomatic front. These discontinuous composition profiles across amphibolite + calcite zones lead to the conclusion that the transport process for many of the veins was infiltration metasomatism. Comparing modal abundances of vein minerals with model vein rocks it is suggested that MgO was conserved during vein formation, and the veins were formed essentially by Si0:-metasornatism. The composition of the vein minerals suggest however that iron and aluminium were transported as well. From phase relations among the different zone assemblages, it may be concluded that vein formation occurred over a pressure-temperature interval (cooling-uplift interval) but that the formation of an Individual vein was an isothermal isobaric process occurring in the presence of gradients in a s ioaq and small gradients in X002 in the vein forming fluid. Geometric relations suggest that the veins may have grown by both deposition of material along an opening central fracture and reaction of a Si0 2-rich aqueous fluid with the adjacent dolomite.