Petrology and Structure of the Bethel Area, Maine

Abstract

Sillimanite-grade metasedimentary rocks of the Lower Devonian Littleton Formation and plutonic rocks of the New Hampshire magma series underlie the Bethel quadrangle of western Maine. The exposed Littleton Formation is a series of gneisses and schists about 8500 feet thick including two units, the “pyrrhotitic gneiss” and the “inter-bedded sillimanite schists and quartz-mica schists,” separately mappable from the main quartz-biotite-sillimanite gneiss. Igneous rocks include the Songo Granodiorite, the Speckled Mountain Quartz Monzonite, and the 280-million-year old granitic pegmatite, all of the New Hampshire magma series, and mafic dikes, the youngest rocks in the quadrangle, tentatively classified with the Mississippian(?) White Mountain magma series. The writer, by measuring zircon grains and calculating hydraulic equivalents in light minerals, ascertained that the gneisses and schists were derived from coarse siltstones of a graywacke series. The structure is exceedingly complex in detail, but in general the metasedimentary rocks form a series of northeast-striking, doubly plunging anticlines and synclines. The large, domelike Albany pluton of Songo Granodiorite and countless smaller bodies of igneous rock cut the folds. The Albany pluton occupies the southeastern third of the quadrangle and is cut by pegmatite and mafic dikes. In places the metasedimentary rocks exhibit textures and structures that some ascribe to granitization. The writer believes that these features formed by metamorphic segregation. Chemical analyses of the rocks indicate that little, if any, metasomatism took place during metamorphism. The Speckled Mountain Quartz Monzonite locally exhibits a distinct layering that resembles the relict bedding of meta-sedimentary rocks. The writer considers the possibility that this rock has been metasomatized and mobilized but concludes that it probably represents the consolidation product of a highly contaminated magma.