A Study of Plagioclase Feldspars in Low-Grade Metamorphic Rocks from the Madoc Area of Southeastern Ontario

Abstract

A brief description of a low grade regionally metamorphosed region in the Madoc area in Southern Ontario is given. Plagioclase compositions from metamorphosed basic rocks of this region were studied with the intention of finding co-existing plagioclases in the region of peristerite solvus. X-ray diffraction was found useful in determining the presence of two co-existing plagioclase feldspars. Two peaks occurred in the region of the 131 and 131 peaks if this was the case. Compositional determinations for albitic feldspars were found inaccurate as the angular separation (Δ29) between the 131 and 131 peaks was apparently reduced indicating plagioclase composition less than An0. For anorthite rich feldspars the angular separation between 131 and 131 gave relatively accurate compositions. The electron microprobe was used·to determine the plagioclase compositions of samples from the Madoc area. Results compared well for anorthite rich feldspars determined by both X-ray diffraction and. electron microprobe techniques. Only one rock was found to contain two plgioclases related to a peristerite solvus. A zoned nature appears to exist. Albite and oligoclase coexist, the oligoclase grading into andesine. Also, albitic plagioclase is associated with epidote inclusions; a diffusion relationship probably existing between the two. With increased metamorphic grade, epidote disappears and the anorthite content of plagioclase increases.