Host: The Japanese Association of Mineralogists, Petrologists and Economic Geologists
Anomalously large chemical variations in muscovite-paragonite and muscovite-celadonite systems are observed in white micas from the Piemonte calcschists in the Chisone valley area, internal western Alps. The petrographical and chemical observations on white mica strongly suggest that most mica crystals with high Na/K ratio in the chlorite zone are of detrital origins and were derived from the pre-Alpine high-temperature metamorphic sequence such the Caledonian and/or Variscan. The continuous compositional variations from 0.00 to 0.78 in Na/(Na+K) ratio are due to that the part having various mixtures of submicroscopic muscovite and paragonite crystal aggregates were analyzed by EPMA. In the rutile zone, the paragonite content of white micas are less than 20%, suggesting that the white micas have been homogenized in the Alpine metamorphism even if the detrital white micas existed. Metamorphic mica is still very heterogeneous. The total range in Si content becomes wider with increasing of metamorphic grade: 3.22-3.39 pfu for the chlorite zone, 3.07-3.45 pfu for the chloritoid zone and 3.06-3.59 pfu for the rutile zone. This clearly indicates that some mica grains have disequilibrium chemistry with other metamorphic primary phases, suggesting that the micas have experienced significant retrogressive chemical reactions in cooling and exhumations of the host schists.
