Determination of Metamorphic Grade for Two Formations of the Huronian Supergroup: Whitefish Falls Area, Ontario

Abstract

<p>Samples from the Whitefish Falls Area were examined petrographically and chemically to determine the regional grade of metamorphism. Mineralogy from the Lorrain formation (quartzities) and Gowganda Formation (argillites) were apparently incongrous. The resolution of this problem formed the major part of the study.</p> <p>Mineralogically the quartzites contained a kyanitequartz paragenesis with a subordinate kyanite-chloritoidquartz paragenesis. The argillites showed a chlorite-biotite -plagioclase-calcite-quartz paragenesis with only minor epidote present. From textural evidence possible reactions were deduced which were used to set the upper and lower limits for pressure and temperature. These boundaries were 470°C and 3.9 kb., lower limit and 550°C with a variable pressure for the upper limit.</p>

Chemically, whole rock analysis was used to determine the parameters for various chemographic diagrams. AFM, ACF and AKF plots for the argillites showed a very restrictive composition which excluded the almandine field and promoted the biotite field. The bulk composition of the argillites was cited as the main reason for the inhibition of high grade index minerals, thus appearing to be lower grade than their quartzite counterparts. AFM and AKF diagrams for the quartzites verified their mineralogy and proved their high grade nature.