VOLCANIC EXPRESSION OF BIMODAL MAGMATISM - THE CRANBERRY ISLAND CADILLAC MOUNTAIN COMPLEX, COASTAL MAINE

The Cranberry Island series is a thick succession of Silurian volcanic breccias, ash flow tuffs, lava flows, and hypabyssal intrusions compr ising the southern margin of Mount Desert Island and the Cranberry Isl ands of southeastern Maine. The rocks of the series preserve remarkabl e textures indicative of the simultaneous eruption of mafic and felsic magmas, during both explosive and effusive events. The Cadillac Mount ain intrusive complex is an approximately 20-km-wide composite layered mafic to felsic plutonic body that dominates Mount Desert Island imme diately north of the outcrop of the Cranberry Island series. The rocks of the Cranberry Island series and those of the Cadillac Mountain int rusive 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 424 Ma +/- 1 Ma. The Cadillac Mountain granite, the Somesv ille granite, and the Southwest Harbor granite, all parts of the Cadil lac Mountain intrusive complex, are similar in composition to parts of the Cranberry Island series. The Somesville granite has been dated at 424 Ma +/- 2 Ma and the Cadillac Mountain granite at 419 +/- 2 Ma. Th ese relations suggest that the Cadillac Mountain intrusive complex and the Cranberry Island series may be a deeply eroded, shallow-level plu ton and its erupted equivalent, respectively. The volumes of the pluto n and of the volcanic succession, the style of volcanism, and the comp ositional bimodality of the complex are reminiscent of caldera-style v olcanism in terranes of crustal extension and thinning.