The coast of Maine hosts several Silurian (~424-421 Ma) volcanic successions, including the Cranberry Island series (Seaman et al., 1999),the Eastport series (Gates and Moench, 1981; Llamas and Hepburn, 2013), and the Vinalhaven series (Gates, 2001; Wobus et al. 2006). Isle au Haut, an island in Penobscot Bay, also hosts a volcanic series and its parent layered plutonic suite, dated at 424±1 Ma (Chapman,1996). The Isle au Haut volcanic series consists, from bottom to top, of flow-banded rhyolite, lithic lapilli tuff, and then three sequences of welded rhyolitic lapilli tuff grading into basaltic enclave-rich tuff breccias, topped with an enclave-free tuff breccia, a debris flow, a surge deposit, and a final enclave-bearing crystal lithic tuff. This volcanic succession indicates that the eruptive sequence began with an effusive phase, possibly associated with eruption of degassed rhyolite along a ring fracture complex, followed by a series of explosive events. The three welded tuffs, each overlain by a basaltic enclave-rich deposit, suggest that a stratified, bimodal magma chamber repeatedly developed a felsic-over mafic stratigraphy prior to eruption. Chapman (1996) proposed that the Isle au Haut silicic magma chamber was periodically injected with basaltic magma, consistent with the interpretation of the volcanic succession. Injections of basalt into the silicic chamber may have triggered the three rhyolitic ignimbrite eruptions. Data are insufficient to speculate on the gaps in time between the three eruptions. Although the areal extent of the ignimbrite sheets cannot be determined, their thickness suggests that the volcanic succession preserved on Isle au Haut records a series of major silicic explosive eruptions consistent with more recent caldera-forming eruptions.

Chapman, 1996, Ph.D. Dissertation, Univerity of Massachusetts; Gates and Moench, 1981, USGS Professional Paper 1184; Gates, Olcott. 2001. Bedrock geology of North Haven and Vinalhaven Islands. [Augusta]: Maine Geological Survey; Llamas and Hepburn, 2013, Geol. Soc. Am Bull., Seaman et al., 1999, Geol. Soc. Am Bull., 111, 686-708; Wobus et al., Northeastern Geology, 2006, 28, 342-357