Ca-K Metasomatism in the System CaO-K₂O-MgO-Al₂O₃-SiO₂-H₂O and Skarn Formation in Pelitic Rocks

Abstract

This study focuses on skarns resulting of the metasomatic transformation of pelitic rocks. The skarns found in the Costabonne Peak area (eastern Pyrenees, France) are first described as an example and then compared with that of other deposits. It is shown that the genetic model which account for the transformation in the Costabonne Peak area may be of general applicability for the understanding of skarn process in pelitic rocks. The skarns are typically zoned: schist (Z0), biotite zone (Z1), amphibole zone (Z2), pyroxene zone (Z3) and garnet zone (Z4). Two superposed successions of transformations are observed: (1) muscovite => feldspar => garnet and (2) biotite => amphibole => pyroxene. These transformations are due to important exchange of CaO and K₂O. Evaluation of mass transfer, phase relations and thermodynamic calculations in the system CaO — K₂O — SiO₂ — MgO — A1₂O₃ — H₂O suggest that the chemical potentials of K₂O and CaO are the most important factors which have to be taken into account in the description of the metasomatic transformations. The succession of zones is shown to take place with increasing μ_CaO. The nature of the feldspar (plagioclase or K-feldspar) depends on the value of μ_K2O relative to μ_CaO. The presence of epidote instead of feldspar in the skarn zoning results from lower temperature conditions.