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.     

Element mobility associated with syn-metamorphic shear zones near Scotchport, NW Mayo, Ireland

Abstract Three major blastomylonitic synmetamorphic (epidote amphibolite to mid amphibolite facies) shear zones are seen on the NW coast of the Mullet Peninsula in NW Mayo. These shear zones occur at the contacts of major structural units and in an imbricated slice where rocks of the Erris Complex are deformed and chemically modified. Chemical changes associated with individual shear zones have been deduced by comparing the compositions of various gneisses both within and adjacent to the shear zones. Compositional changes are different in the constituent rock-types within each unit and many elements normally considered immobile have been selectively mobilized within the shear zones. Little evidence of wholesale metasomatic introduction of components into these shear zones was found to accompany the selective mobilization.