THE PUNCTUATED SILURIAN AND DEVONIAN IGNIMBRITE FLARE-UP OF COASTAL AND CENTRAL MAINE

The late Silurian (424-419 Ma) and the Silurian-Devonian boundary (~400-410 Ma) were times of large volume bimodal intrusions in Maine, forming, respectively bimodal complexes in the coastal Maine magmatic province, and the central Maine magmatic belt. Of the many bimodal magmatic complexes that appeared during these times, at least five erupted to produce large-volume silicic volcanic eruption sequences1-4 km thick. Four large magmatic complexes in the coastal Maine belt all erupted between 419 and 424 Ma, including, from west to east, the Vinalhaven, Isle au Haut, Cranberry Islands and Eastport 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 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, making it one of the largest silicic caldera eruptions in the global rock record. The basement of both the coastal Maine volcanic belt and the central Maine belt is Gander terrane, which 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 Avalon plate, or delamination of that plate beneath Gander (Llamas and Hepburn, 2013) resulted in decompression melting of the mantle, and partial melting of silicic Gander crust, leading to coastal Maine bimodal magmatism and very large (>1000 km³) scale silicic eruptions. Llamas and Hepburn (2013) showed that the Eastport volcanic series records the transition from pre-425 Ma calc-alkaline collision-related volcanism to post-425 Ma extensional volcanism in the coastal Maine volcanic province. Rocks representing pre-410 Ma calc-alkaline magmatism are absent in central Maine, consistent with an entirely within-plate extensional setting for central Maine belt magmatism.