CONTRASTING MAGMA TYPES, PROCESSES AND TIMING OF THE LEXINGTON PLUTON(S), NORTHWESTERN MAINE

The Lexington pluton outcrops in northwestern Maine and stitches together metasedimentary rocks of Ordovician to Devonian age that were assembled during the Acadian orogeny. It outcrops over an area of 325 km² and is often displayed as having three units – a northern, central and southern lobe, each of differing composition.

The northern unit is a biotite + hornblende + titanite medium grey colored, equigranular granodiorite; a metaluminous I-type mineralogy, confirmed by A/CNK ratios of < 1. The central unit is a megacrystic, biotite + muscovite (and rare garnet) quartz monzonite to granite that has abundant microgranular mafic enclaves. The megacrysts are perthitic K feldspar and can be up to 20cm long with numerous plagioclase, biotite and muscovite inclusions arranged in concentric zones. They are aligned and sometimes concentrated into feldspar “log-jams” suggesting a flow origin. The mafic enclaves are fine-grained hornblende and biotite diorites. They are aligned parallel with the megacrysts, some of which have been “captured” by the enclaves to give them an inherited porphryitic texture. They have lobate contacts with their granitic host suggesting a coeval relationship. The southern unit is a finer grained, mineralogic equivalent of the central unit but contains smaller feldspar phenocrysts (1 -3 cm) and lacks mafic enclaves. Therefore, the central and southern units have an S-type mineralogy with A/CNK ratios > 1.1.

The central and southern units are dated at ~ 404Ma (Solar et al. 1998), and evidence suggests that magma mixing played a significant role during the crystallization of these granitoids. However, the northern unit is much younger at 365Ma (Tomascak et al. 2005) and associated with a major burst of magma generation in the lower crust.