Abstract

Contemporaneous mafic and felsic magmatism occurs in a variety of tectonic settings where continental crust is invaded by mantle-derived basaltic melt. Bimodal magmatism, including supervolcano-scale eruptions, occurred in both the Coastal Maine magmatic province and the Central Maine magmatic belt during two phases of accretion of Avalonia to the margin of North America in the Late Silurian and Early Devonian. The magmatic complexes of both coastal and central Maine provide an opportunity to examine the storage and eruption settings of basaltic and rhyolitic magma from the base of plutons through the top of their volcanic successions. The coastal and central Maine magmatic provinces represent the southwesternmost part of a >70,000 km2 bimodal igneous province that includes volcanic and plutonic rocks that extend into the Gaspé Peninsula of Quebec, Canada. Taken together, these magmatic belts embody a large igneous province–scale locus of igneous activity. Four complexes in Maine are part of the Coastal Maine magmatic province and erupted between ca. 424 and 420 Ma, producing volcanic successions up to 3 km thick. The fifth large silicic eruptive complex examined here, the Katahdin/Traveler complex of central Maine, is part of the Central Maine magmatic belt, and it produced the ca. 407 Ma Traveler rhyolite, of which ∼3200-m-thick sections are preserved. The three coastal Maine magmatic complexes for which an arguably clear genetic relationship exists between pluton and volcanic succession are smaller than the Katahdin/Traveler system (on the basis of area of the remnant magma chamber of Mount Desert Island [∼500 km2] compared to that of the Katahdin batholith [∼1350 km2]), but they produced volcanic successions from 0.65 (Vinalhaven) to at least 2.2 km (Mount Desert/Cranberry Isles and Isle au Haut) thick. Single ignimbrite thicknesses in these complexes range from 350 m to 860 m. The remaining coastal Maine magmatic complex discussed here, the Eastport series, differs from the other four in that the pluton(s) from which the ∼2.4-km-thick succession originated has (have) not been identified. A detailed integration of gravity and magnetic data indicates that strong positive magnetic anomalies and weaker gravity anomalies support a model of thin felsic igneous rocks underlain by a significant volume of mafic to ultramafic rocks. The dominance of gabbro in the coastal Maine crustal column suggests that crustal extension and intrusion of mantle-derived basalt drove the development of bimodal magmatic complexes. The gabbro:granite ratio in the subsurface of the Coastal Maine magmatic province, and the presence of hornblende, rather than pyroxene, as the main ferromagnesian mineral in the gabbros of the province are consistent with a model of flux of hydrous basalt into the crust of ∼10–2 m3/m2/yr, leading to a ratio of crustal melt to mantle-derived basaltic melt of ∼1:0.25, if the invading basaltic melt was wet (∼5.6 wt% water). The absence of andesites in this setting is likely a consequence of the subduction-related origin of the hydrous basalts, resulting in the generation of dacitic rather than andesitic melts in lower-crustal mush zones, and the eventual extraction of interstitial melts from those dacitic melts in the upper crust to produce the granites and rhyolites that dominate the igneous rocks at the present surface in coastal and central Maine.

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