PETROGRAPHY AND GEOCHEMISTRY OF THE MIXER POND PLUTON, SOUTH-CENTRAL MAINE

The Mixer Pond pluton (MPP) is essentially a granitoid orthogneiss, located in South-Central Maine. The pluton crops out as an elongate body, approximately 10 km in length but only 1 km at its widest, within a migmatized shear zone. It is extremely variable texturally and mineralogically which, along with the uneven distribution of exposures, adds complexity to the interpretation of its original nature.

Recent field and petrographic studies of the MPP reveal its complex petrography. Portions are weakly foliated with moderately preserved igneous textures, while other portions exhibit strong gneissic banding with distinct compositional layers at the outcrop scale. In mica poor rocks, gneissic banding consists of elongated microcline grains adjacent to granular quartz bands. Also leucocratic layers of microcline and quartz alternate with mm thick layers of biotite and amphibole. The relative volume of biotite and amphibole varies significantly. It is also compositionally diverse, ranging from garnet bearing leucocratic microcline - rich granites to rocks of granodioritic or quartz dioritic composition, which locally contain pyroxene. This variation occurs both along and across strike. Multiple pegmatite bodies further contribute to the overall heterogeneity of the MPP.

The textural and mineralogical diversity of the MPP may reflect the original internal compositional variation produced during the igneous phase of the intrusion. This could have been produced by a combination of multiple magmatic pulses, in situ fractionation or repeated emplacement events forming a composite pluton. Later metamorphism and deformational events may have further complicated the field relationships. New major and minor element data for MPP will allow us to assess these models and elucidate its the original nature.