Geochronology and K/Rb Ratios of an Anorthosite Near Parry Sound, Ontario

Abstract

<p> The primary purpose of this study was to date the last metamorphism of a metanorthosite near Parry Sound, Ontario, by the Rb-Sr method on the mineral phases: biotite, hornblende and plagioclase. Due to problems with the mass spectrometer, the prepared samples could not be run. </p> <p> K/Rb ratios on biotites and plagioclase give a calculated K/Rb whole rock ratio for the mafic rich bands of about 230, consistent with the main trend of igneous rocks. The plagioclase K/Rb ratio, however, is much higher, and the anorthosite body as a whole may have a much higher K/Rb ratio, consistent with oceanic tholeiites and achondritic meteorites and indicating a possible lower crustal or mantle origin. </p>