ERUPTIVE HISTORY AND GEOCHEMISTRY OF THE ISLE AU HAUT VOLCANIC SERIES, COASTAL MAINE

The Isle au Haut volcanic series represents the repeated eruption of a bimodal, stratified magma chamber. New geochemical data have been used to link the volcanic rocks with the Isle au Haut Igneous Complex (IaHIC) and to place the rocks within the context of the coastal volcanic belt. 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 also hosts a volcanic series and an associated layered plutonic suite, dated at 424±1 Ma (Chapman,1996). The Isle au Haut Volcanic Series (IaHVS) is a 3.4 km thick volcanic deposit, with flow-banded rhyolites at the base and three cycles of welded tuffs overlain by basaltic-enclave bearing deposits above. This stratigraphy is interpreted to represent effusive eruptions from ring fractures followed by three cauldera eruption events. Similarities in major, trace, and REE geochemistry suggest a genetic relationship between the IaHIC and the volcanics it intrudes. Major- and trace-element analyses further reinforce the interpretation of a bimodal series and suggest that rare andesitic compositions are the product of mixing felsic and mafic end-member magmas. Based on REE analysis, the rocks have within-plate geochemical signatures and, combined with the bimodal nature of the series, suggest an origin within an extending crustal environment. It has been suggested that the coastal volcanic belt represents the transition from the subduction of the Avalon plate to an extensional setting in the Late Silurian (Llamas and Hepburn, 2013). Within this model, the IaHVS and the nearly synchronous Cranberry Island Series and the Eastport Formation probably erupted after the composite Avalon terrane docked with Ganderia. The IaH volcanic series and the closely associated Cranberry Island series preserve evidence of extension and voluminous explosive bimodal volcanism on the Laurentian margin during the late-Silurian.