THE 420-430 MA BIMODAL VOLCANIC EVENT OF COASTAL MAINE

Four large-volume bimodal plutonic complexes (Vinalhaven, Cadillac Mountain, Isle au Haut, and Gouldsboro), exposed along the southern coast of Maine crystallized within a short (430-420 Ma) time span and each erupted to produce rhyolitic to bimodal volcanic successions. Although much of the volcanic pile produced by the Gouldsboro pluton has been removed by erosion, volcanic successions to >2km thick produced by the other three plutonic complexes attest to the world-class scale of these eruptions. In younger volcanic fields, thicknesses on this scale are typical of pyroclastic eruptions of 1000-2000 km3. The 420-430 Ma bimodal plutonic/volcanic centers of coastal Maine make up a province comparable in both lifespan and size of individual eruptive centers to the Altiplano Puna complex (deSilva et al., 2008), the Southern Rocky Mountain volcanic field (Lipman, 2007), and the Mogollon-Datil volcanic province (Elston, 1984). They represent pulsed eruptions of silicic melt derived largely by crustal anatexis. Younger volcanic centers of this scale and compositional range typically are associated with caldera-style eruptions. The deeper erosional level of the Silurian complexes has generally removed the structural evidence of caldera volcanism. Large scale eruptions of younger silicic ignimbrites evacuate magma batches that accumulated over a few hundred thousand years (de Silva, 2008), and compositional variety in volcanic deposits indicates that significant zonation can develop in magma chambers on that time scale. The 420-430 Ma coastal Maine plutonic/volcanic complexes offer a view of the plumbing systems of supervolcano-producing intrusions that is unavailable in younger complexes. The deep erosional level presents the opportunity to examine both the extent to which magma chambers between closely spaced volcanic centers communicated with one another, and the character of the final magma batches to accumulate in dying bimodal volcanic systems.