Biotite Variation as a Guide to Petrogenesis of Granitic Rocks in the Pre-Cambrian of Northeastern Alberta

Abstract

Information about internal variation in a band of foliated granitic rocks in the Pre-Cambrian of northeastern Alberta has been obtained by mapping variation in biotite refractive index on the basis of 430 determinations distributed throughout the northern portion of the band. In many parts of the area isopleths of the biotite refractive index parallel the regional foliation, whereas refractive index does not vary in other portions of the area. A correlation coefficient of 0·804 between refractive index and Fe:Mg ratio in seven analysed biotites supports the contention that refractive index can be used as a measure of relative Fe:Mg ratio in this suite of related biotites. Of the factors that control the Fe:Mg ratio in biotites, variation in oxygen partial pressure is the most important in this particular geological situation. As a result, the pattern of refractive index variation reflects the pattern of variation in oxygen partial pressure at the time of formation of the biotite. In the light of this relationship, the isopleths that parallel the foliation in many areas are interpreted as relics of an original system consisting of individual layers that were closed systems with respect to oxygen partial pressure. The areas of uniform refractive index have been subject to a continuous vapor phase which equalized oxygen partial pressures throughout the area. This vapor phase could be of magmatic origin or could arise during progressive metamorphism. Field relations and chemical and mineralogical variation within and around this group of rocks are consistent with this interpretation and further suggest that the granitic rocks were derived from a mixed suite of intermediate to basic volcanic rocks, tuffaceous metasediments, and feldspathic wackes by metamorphic recrystallization and accompanying homogenization of these materials.