MAGMA MINGLING IN THE PASSADUMKEAG RIVER PLUTON, MAINE

The Passadumkeag River pluton is a reverse zoned, Devonian-age granitic pluton located in east-central Maine. The pluton is part of the Bottle Lake Complex, which lies between the towns of Lincoln and Topsfield. Ayuso (1984) divided the pluton into a rim facies (71 to 77 % SiO2) and an inner core facies (67 to 72 % SiO2). Abundant mafic xenoliths (50 to 63 % SiO2) were distributed throughout the core and become rarer towards the rim. Along the western margin of the pluton a recent cut has exposed new outcrop not previously available for study. Within the exposure, dark circular blobs of medium-fine grained diorite (62-63% SiO2) are randomly dispersed throughout a light colored course-grained granite (71-73% SiO2). The granite contains abundant quartz, biotite, and k-spar as well as less common plagioclase. The diorite contains abundant plagioclase, hornblende, and biotite as well as less common quartz and k-spar. Both field and thin section observations indicate a distinct, non-gradational contact between the blobs and the host rock. The smaller grain size of the dioritic blobs also indicates rapid cooling. These observations and the field relations are consistent with the co-mingling of a small amount of hot, dioritic magma with a much larger volume of cooler granitic magma. The silica content of the granite (71-73% SiO2) is consistent with Ayuso’s description of the rim facies rocks. The silica content of the dioritic blobs (62-63%) is inconsistent with Ayuso’s description of the rim or core facies of the pluton, indicating that these blobs represent a different magma from any previously described. At this point in the study there are insufficient data to establish the origin of or any genetic relationship between the diorite and granite. However, the results clearly indicate the presence of a magma with much lower SiO2 contents than those represented by Ayuso’s (1984) core facies. The composition of the diorite magma falls within the upper end of Ayuso’s (1984) mafic xenoliths. If both the diorite blobs and mafic xenoliths represent the same magma (or type of magma) then such magmas have extended further into the rim facies then originally thought. Such magmas may be volumetrically significant which would suggest that the degree of compositional zonation within the pluton may be more extreme than originally thought.