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

Information about internal variation in a band of foliated graniticrocks in the Pre-Cambrian of northeastern Alberta has been obtainedby mapping variation in biotite refractive index on the basisof 430 determinations distributed throughout the northern portionof the band. In many parts of the area isopleths of the biotiterefractive index parallel the regional foliation, whereas refractiveindex does not vary in other portions of the area. A correlation coefficient of 0?804 between refractive indexand Fe:Mg ratio in seven analysed biotites supports the contentionthat refractive index can be used as a measure of relative Fe:Mgratio in this suite of related biotites. Of the factors thatcontrol the Fe:Mg ratio in biotites, variation in oxygen partialpressure is the most important in this particular geologicalsituation. As a result, the pattern of refractive index variationreflects the pattern of variation in oxygen partial pressureat the time of formation of the biotite. In the light of this relationship, the isopleths that parallelthe foliation in many areas are interpreted as relics of anoriginal system consisting of individual layers that were closedsystems with respect to oxygen partial pressure. The areas ofuniform refractive index have been subject to a continuous vaporphase which equalized oxygen partial pressures throughout thearea. This vapor phase could be of magmatic origin or couldarise during progressive metamorphism. Field relations and chemical and mineralogical variation withinand around this group of rocks are consistent with this interpretationand further suggest that the granitic rocks were derived froma mixed suite of intermediate to basic volcanic rocks, tuffaceousmetasediments, and feldspathic wackes by metamorphic recrystallizationand accompanying homogenization of these materials.