Metamorphic evolution of calc-schists in the Central Alps, Switzerland

摘要

Calc-schists of the Mesozoic metasedimentary units in the Central Alps, between the Simplon fault, the Bergell intrusion, the Gotthard massif and the Insubric line, are investigated to assess their metamorphic evolution and to gain a better understanding of scapolite stability and its phase relations. These rocks have only experienced the Mesoalpine metamorphic event. The abundance of silicate minerals in the calc-schists results in a series of characteristic mineral assemblages with increasing metamorphic grade. In the outer part of the Central Alps at lower amphibolite facies conditions, mineral assemblages containing clinozoisite and plagioclase are diagnostic. In areas of intermediate metamorphic grade, clinozoisite and plagioclase are replaced by scapolite-bearing assemblages and K-feldspar begins to form. In the upper amphibolite facies region around Bellinzona clinopyroxene is observed as an additional phase within the scapolite-bearing assemblages. The evolution of mineral assemblages observed from the outer to the central part of the Central Alps can also be identified within individual higher grade rocks. Phase diagram calculations show a good agreement with the observed mineral assemblages and their evolution, helping to validate the new thermodynamic model for scapolite that was used. They allow estimation of the temperature of peak metamorphism. However, the thermodynamic modelling leads to the conclusion that such carbonate-bearing rocks are not well suited to constrain the pressure of metamorphism.