Volcanic Expression of Bimodal Magmatism: The Cranberry Island-Cadillac Mountain Complex, Coastal Maine

Abstract

The Cranberry Island series is a thick succession of Silurian volcanic breccias, ash flow tuffs, lava flows, and hypabyssal intrusions comprising the southern margin of Mount Desert Island and the Cranberry Islands of southeastern Maine. The rocks of the series preserve remarkable textures indicative of the simultaneous eruption of mafic and felsic magmas, during both explosive and effusive events. The Cadillac Mountain intrusive complex is an approximately 20-km-wide composite layered mafic to felsic plutonic body that dominates Mount Desert Island immediately north of the outcrop of the Cranberry Island series. The rocks of the Cranberry Island series and those of the Cadillac Mountain intrusive complex are correlative in age and similar in composition. The age of the Cranberry Island series, based on new U-Pb geochronology of zircon, is . The Cadillac Mountain granite, the Somesville granite, and the Southwest Harbor granite, all parts of the Cadillac Mountain intrusive complex, are similar in composition to parts of the Cranberry Island series. The Somesville granite has been dated at and the Cadillac Mountain granite at . These relations suggest that the Cadillac Mountain intrusive complex and the Cranberry Island series may be a deeply eroded, shallow-level pluton and its erupted equivalent, respectively. The volumes of the pluton and of the volcanic succession, the style of volcanism, and the compositional bimodality of the complex are reminiscent of caldera-style volcanism in terranes of crustal extension and thinning.