Sj. Seaman et al., VOLCANIC EXPRESSION OF BIMODAL MAGMATISM - THE CRANBERRY ISLAND CADILLAC MOUNTAIN COMPLEX, COASTAL MAINE, The Journal of geology, 103(3), 1995, pp. 301-311
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.