Controls of Biotite Zone Mineral Chemistry in Archaean Meta-Sediments near Yellowknife, Northwest Territories, Canada

Archaean meta-sediments near Yellowknife, Canada, exhibit alow-pressure facies series and broad metamorphic zonation rounda central pluton. Meta-pelites and meta-greywackes from thebiotite zone have been studied using analyses of 59 mineralsand 14 rocks. Mineral compositions were controlled by both host-rockcomposition and metamorphic grade. Increased grade commonly imposed on the minerals a progressivecompositional maturation. This involved progressive compositionalchange (especially in meta-greywacke biotites) and/or narrowingof compositional range (particularly in muscovites). Specificeffects of increased grade are as follows. Biotites in meta-greywackesexhibit increased Mg/Fe and Na/K but decreased (Na+K) content.Biotites in meta-pelites change little except for increasedAl^IV/Si. The muscovites display decreasing maximum Si contents,increased Al^VI at the expense of Fe and Mg, and increased Na/K.The chlorites show only a slight general increase in Mg/Fe. Bulk compositional control is manifested in various ways. Insome instances minerals from subtly different rock types areperceptibly distinct; in others the pro-grade maturation trendsof minerals in these rock types are different. Thus Si is moreabundant in meta-greywacke biotites than in meta-pelite biotitesand (Na+Ba+K) content of muscovites is greater in meta-greywackes. The observed features are used to deduce mechanisms of controlby grade and bulk composition and to discuss general circumstancesunder which they operate.