Northeastern Section - 36th Annual Meeting (March 12-14, 2001)

The Wallamatogus Pluton, located in the Coastal Maine Magmatic Province, exhibits systematic textural variation in grain size, mineralogy, and phenocryst abundance (alkali-feldspar) from the margin to the center of the pluton. Three facies have been defined: 1.) outermost, seriate, coarse-grained facies, 2.) porphyritic medium- to fine-grained facies, 3.) innermost, fine-grained equigranular facies. These facies are concentrically arranged and contacts between them are gradational. Muscovite and biotite are common in all facies; the outer 2 facies contain garnet and tourmaline. High strain zones occur locally, and in thin section mica and plagioclase grains are kinked; garnet and feldspar grains are fractured and separated; quartz exhibits recrystallization and commonly fills cracks in feldspar phenocrysts. Different muscovite forms are distinguished using petrographic and compositional data. Primary muscovite is coarse-grained, euhedral to subhedral, resorbed perpendicular to cleavage, and has the following compositional ranges (wt%): TiO2 0.34-1.12; Al2O3 34.04-36.16; Na2O 0.16-0.75; FeO 1.05-1.39; MgO 0.51-0.89. Secondary muscovite is fine-grained, subhedral, embayed within feldspars, and has the following compositional ranges (wt%): TiO2 0.01-0.33; Al2O3 31.46-35.69; Na2O 0.16-0.50; FeO 1.00-3.45; MgO 0.38-1.49. Muscovites in micaceous enclaves are medium- to fine-grained, euhedral with clean terminations, and has the following compositional ranges (wt%): TiO2 0.75-1.16; Al2O3 34.55-35.79; Na2O 0.41-0.59; FeO 1.06-1.29; MgO 0.58-0.89. Muscovite presumably related to cordierite alteration is medium-grained, subhedral to anhedral, intergrown with biotite, and has the following compositional ranges (wt%): TiO2 0.00-0.02; Al2O3 33.39-34.91; Na2O 0.27-0.38; FeO 1.53-1.98; MgO 0.94-1.27. Geothermometry on garnet-biotite pairs record temperatures of 275 to 650°C using Ferry and Spear’s (1978) calibration. Temperatures decrease systematically with increasing Mn content. Temperatures from garnets with the lowest Mn content (~30 mol% spessartine) record a temperature of 650°C, which may reflect magmatic conditions during crystallization. Low temperatures (200 to 450°C) are associated with high strain zones (e.g. fractured garnet) and may reflect a post-crystallization event of fluid-mineral reactions related to significant deformation.