THE ORIGIN OF GARNETS FROM THE NORTH JAY AND CAPE COD HILL PLUTONS, WEST-CENTRAL MAINE

The North Jay and Cape Cod Hill plutons (NJ and CCH respectively) are two-mica, peraluminous granite intrusions that outcrop in west central Maine. They are similar petrographically being medium-grained, light grey equigranular granites with biotite + muscovite, microcline, plagioclase and quartz – a typical “S – type” mineral assemblage. In addition, they contain minor amounts of garnet, which occurs sporadically throughout these granites but also within xenoliths of schistose material (surmicaceous enclaves) and small gt + bt knots. Therefore the question addressed in this presentation is whether the garnets are magmatic or inherited xenocrysts?

Three main parameters can be utilized to determine the origin of garnet in granitoids – their crystal habit, composition (including any zoning), and geothermometry. Garnet in the NJ and CCH granites are varied in crystal habit and can be anhedral with numerous inclusions of quartz and feldspar or more euhedral in form. Those that have a more euhedral disposition are often almost totally surrounded by biotite and could represent xenocrystic knots. Initial compositional data from the NJ pluton indicate that these garnets plot close to almandine + spessartine corner on the discrimination diagrams of Krippner et al. (2014). Likewise the more euhedral garnets are compositionally zoned, most notably with regard to Ca and Y concentrations, whereas the anhedral garnets are compositionally homogeneous.

Preliminary geothermometry calculations reveal temperatures < 600⁰C, which would suggest that they are inherited from a metamorphic source rather than magmatic in origin. However, more data will enable us to critically assess if these garnets are xenocrystic and inherited from the disaggregation of surmicaceous enclaves or if some of the garnets actually crystallized from these peraluminous melts.