CRUSTAL STRUCTURE AND LATE PALEOZOIC SUPERVOLCANO-SCALE ERUPTIONS IN MAINE, USA

Four coastal Late Silurian and one inland Early Devonian magmatic province in present-day Maine produced supervolcano (~1000 km³ and greater)-scale explosive felsic volcanic eruptions. Rocks from each coastal complex (Isle au Haut, Vinalhaven, Cranberry Islands, and Eastport) have been dated at approximately 424-420 Ma. Thicknesses of single ignimbrites range from 350 m to ~1 km and total volcanic succession thicknesses are up to 3 km thick. The central Maine eruption of the Katahdin batholith at ~ 408 Ma produced the ~5000 km³ Traveler Rhyolite. Gravity and magnetic data show that the coastal Maine complexes are underlain by dominantly mafic rocks. In addition, most of the complexes are bimodal with sub equal felsic and mafic volcanic rocks, and felsic volcanism was not preceded by andesitic eruptions. These characteristics are suggested to be consequences of the tectonic setting and the character of mantle-derived basalt that provided the heat source for crustal melting and the trigger for eruptions. Extension and thinning of continental crust, and large-scale intrusion of basaltic melt produced by decompression melting of upwelling mantle was the result of subduction and possible delamination/slab break-off of the Avalon plate beneath a thick Gander plate. Infusions of dry basaltic melt into magma chambers of water-rich (4-7 wt%) rhyolitic melt derived from partial melting of thinned continental crust resulted in mingling and mixing of the two melts, and possible initiation of catastrophic eruptions. The mafic plutons that surround and dominate the lower reaches of the granitic plutons of coastal Maine have hornblende as the dominant ferromagnesian mineral rather than pyroxene. This suggests that these mafic melts may have been initially dry and that they were not water-saturated at their high intrusion temperature. As they cooled and crystallized, the residual melt became more hydrous and ultimately pyroxene crystals reacted with the hydrous melt to produce amphibole (Beard, 2017). This scenario is consistent with the suggestion that the basaltic melts that provided the heat for crustal anatexis and large scale volcanism both in coastal Maine and central Maine were dry, hot melts derived from partial melting of the upper mantle, possibly as a result of one or more episodes of delamination of the downgoing slab.