TIMING AND TECTONIC SETTING OF LARGE-SCALE EXPLOSIVE SILICIC VOLCANIC ERUPTIONS IN COASTAL AND CENTRAL MAINE, USA

Coastal and central Maine were the sites of numerous bimodal intrusions and at least five large silicic volcanic eruption sequences that produced 1-4 km-thick volcanic successions between ~407 (Traveler rhyolite eruption of central Maine) and 419-424 my (coastal Maine eruptions). The resulting volcanic successions are essentially undeformed and include thick ash flow tuffs and rhyolitic lava flows. The coastal Maine volcanic belt consists of at least four bimodal plutonic/volcanic complexes including the Vinalhaven, Isle au Haut, Cranberry Islands and Eastport successions. These complexes are nearly 100% exposed, from the floor of the plutons to the preserved tops of the volcanic successions. 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 that erupted from the Katahdin magma chamber (Hon, 1980; Rankin and Hon, 1987). Hon (1980) estimated that the original volume of the Traveler rhyolite was at least 5000 km³, which would make it one of the largest silicic caldera eruptions in the rock record. Both the coastal Maine volcanic belt and the central Maine belt are in 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).By that time 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 silicic Gander crust. The central Maine belt, farther inboard of the downgoing Avalonian slab, developed similar bimodal, extension-related magmatism by approximately 410 Ma .In these extensional settings, silicic partial crustal melts accumulated at shallow crustal levels, commonly coexisted with extension-related basaltic magma, and produced super-eruption (>1000 km³) scale events.