Origin Of Anorthosite in the Adirondacks and in general

The relationships of the Adirondack anorthosite and associated rocks have been reinvestigated. The order of intrusion is interpreted as anorthosite, gabbro, quartz‐syenite, diabase, granite, diabase (Keeweenawan). The evidence is clear that gabbro and quartz‐syenite form dikes in anorthosite, and it is believed that they also show evidences of chilling against anorthosite. The anorthosite‐magma has given rise to profound pyrometasomatic replacements of limestone country rock of a type peculiar to it. The metamorphosed rocks are locally injected by quartz‐plagioclase veinings related to the anorthosite. All the phenomena are consistent in indicating formation of the anorthosite through differentiation from a gabbroic anorthosite‐magma with a distinct content of volatiles. The hypothesis of a rest‐magma of the composition of the quartzsyenite on the one hand, squeezed out from an anorthositic fraction on the other, does not satisfy the requirements of the new data.