Large-scale Explosive Silicic Volcanic Eruptions in Maine, USA: Where, When, and Why
Tuesday, 16 December 2014: 5:30 PM
Two magmatic belts in Maine host essentially undeformed, well-preserved Silurian to Devonian volcanic sequences that include thick ash flow tuffs and rhyolitic lava flows. The Coastal Maine volcanic belt consists of at least five bimodal volcanic complexes (419 to 424 +/- 2 Ma) hosting volcanic sequences 1-4 km thick, spanning approximately 160 km of the Maine coastline. Entire cross-sections of the volcanic-plutonic complexes are visible, providing excellent sites to study the volcano/pluton interface. The Central Maine belt also extends approximately 160 km, northeast to southwest, across central Maine, and also hosts several bimodal plutonic/volcanic complexes. Rocks in the Central Maine belt range from 400 to 410 Ma (Hubacher and Lux, 1987; Bradley et al., 1996). The largest complex in the Central Maine belt is the ~407 Ma (Rankin and Tucker, 1995) Katahdin granite and Moxie mafic intrusive complex and the coeval Traveler Rhyolite, a monotonous two-member, 3200-meter-thick pyroclastic succession. In Rankin and Hon (1987), Hon argued that the original volume of the Traveler rhyolite was at least 5000 km3, making it one of the largest silicic caldera eruptions in the rock record. Both the Coastal Maine volcanic belt and the Central Maine belt are on the Gander terrane, a peri-Gondwanan crustal block that accreted to Laurentia during the Salinic orogeny. Accretion of the block was complete by ~421 Ma (Pollock et al., 2012), but by then the Avalon terrane was accreting to Gander. Either back-arc extension associated with subduction of oceanic lithosphere on the leading edge of the Avalonian plate, or delamination of that plate beneath Gander resulted in back-arc extension, decompression melting of the mantle, and partial melting of thick crust. The Central Maine belt, farther inboard of the downgoing Avalonian slab, developed similar bimodal, extension-related magmatism by approximately 410 Ma. Large silicic caldera eruptions developed in these belts as a result of subduction of the Avalon plate beneath a thick Gander plate, production of dry, mantle-derived basalt in extensional settings, and sufficient crustal extension in the wide Gander terrane to permit transport of basaltic melt to shallow crustal levels, where it intruded large silicic magma chambers and initiated supervolcano-scale eruptions.