Late silurian volcanism in coastal Maine: The Cranberry Island series

The Cranberry Island series is a Silurian (424 +/- 1 Ma) volcanic successio n that crops out on three islands along the southeastern coast of Maine. It is part of the coastal Maine magmatic province and represents bimodal magm atism that occurred on a composite crustal block before or during accretion to Laurentia. The series consists of similar to 1.8 km of felsic pyroclast ic rocks and felsic lava flows overlain by similar to 0.8 km of basaltic tu ffs and basaltic lava flows, The lower and upper members of the Cranberry I sland series are interpreted, respectively, as (1) felsic pyroclastic rocks and lavas erupted from a mingled felsic and mafic magma reservoir and (2) phreatomagmatic basaltic rocks overlain by effusive basaltic flows, Some of the basaltic inclusions hosted by the felsic pyroclastic rocks may be chil led magmatic enclaves, suggesting that droplets of basaltic magma were entr ained by felsic magma prior to eruption. The compositionally layered pluton s of the Cadillac Mountain intrusive complex to the north are likely candid ates, in both age and composition, for the magma bodies from which the Cran berry Island series erupted. Mafic rocks of the Cranberry Island series do not show the enrichment in large ion lithophile elements and the depletion in high field strength elements diagnostic of subduction-related magmas. Th e bimodal character of magmatism, the lack of strong are geochemical signat ure, and the rarity of andesites in the Cranberry Island series and through out the coastal Maine volcanic belt argue against an origin in a compressio nal setting. Alternative settings for magmatism include (1) backarc extensi on, (2) crustal extension associated with large-scale transcurrent faulting , and (3) extension-related magmatism associated with rifting of the coasta l Maine volcanic belt from another continental margin.