Geology of the anorthosite massif in Chester County, Pennsylvania

The major objectives of this study have been to distinguish any variation within the anorthosite and to determine its age relationship with the adjacent rocks. Two distinct facies comprise the pre-Cambrian Honeybrook anorthosite: (1) a relatively pure anorthosite facies, and (2) a chilled border facies which is more mafic than the pure anorthosite facies. Quartz-monzonite of pre-Cambrian age has enveloped and penetrated the Honeybrook anorthosite, causing severe alteration in the anorthosite and forming a hybrid rock. The anorthosite is also intruded by a pre-Cambrian pegmatite dike. Metadiabase dikes of pre-Cambrian age cut both the anorthosite and quartz-monzonite. The structure of the anorthosite is doubtful. The meager data suggests a possible domical body. The primary magma yielding the anorthosite complex is assumed to have had the composition of gabbroic anorthosite. An upper chilled border of this composition developed. By gravity stratification during crystallization the mafic crystals settled to form gabbro and the plagioclase crystals remained suspended to form anorthosite.