The Petrology of Moinian Calc-silicate Gneisses from Fannich Forest, and their Significance as Indicators of Metamorphic Grade

Thin strips of calc-silicate gneiss occurring within the Moinianpelites of Fannich Forest were analysed and examined petrologically.They usually consist of zoisite, plagioclase, quartz, calcicalmandine, and hornblende or biotite, and although rich in calcium,are depleted in magnesium and potassium. In some of the rocksstudied, biotite is replacing hornblende, and zoisite is replacingplagioclase, and the timing of these reactions, and the finalstate of equilibrium achieved, is apparently controlled by boththe CaO/Al₂O₂ ratio in the rock, and the metamorphic grade.The reactions observed in the calc-silicates indicate that thefinal metamorphic event in Fannich was retrogressive. Comparisonwith calc-silicates from Morar showing similar retrogressivereactions hints that a widespread retrogressive metamorphicevent may have affected a large area of the Moinian of westernScotland.