Paragenesis of secondary Ca–Al silicates during hydrothermal alteration of Proterozoic granitic rocks (SE Sweden)

The texture and chemical compositions of secondary Ca–Al silicates (prehnite, pumpellyite, epidote, and titanite) and of related minerals (chlorite, sericite, K-feldspar, albite, Fe-oxides, fluorite, and calcite) are used to discuss their formation conditions during interaction of Proterozoic granitic rocks from SE Sweden with hydrothermal fluids. The paragenetic sequence of the Ca–Al silicates is suggested to have been accomplished at ca. 200–350°C and was controlled by small-scale variations in the chemistry of the hydrothermal fluids, such as a_Ca²⁺/a_H⁺ ratio and f_O2 and f_CO2. These variations in fluid chemistry occurred mostly in closed systems, which involved the redistribution of elements released from the alteration of the magmatic minerals, primarily plagioclase, biotite, and amphibole and inducing porosity modifications. Accordingly, results obtained in this study should have implications for the role of hydrothermal alterations on element redistribution in the continental crust and for suitability of granitic rocks as repositories for radioactive wastes.