Relationships between the chemical and modal compositions of metapelites from Connemara,Ireland

Multiple regression analysis has been used to obtain expressions for the modal abundances of major pelite phases as a function of rock composition for a suite of staurolite grade rocks from north east Connemara, and for a suite of nearby sillimanite grade pelites of the same range of bulk compositions. The abundances of the principal silicate phases at both grades are controlled by the concentrations of their major components in the rock, except for garnet whose abundance is dominantly controlled by the Ca-content of the rock. Prediction equations have been used to evaluate changes in mineral abundances between the staurolite and sillimanite zones for individual samples, and it is found that breakdown of staurolite, muscovite and some garnet is accompanied by growth of biotite, sillimanite and plagioclase.