Late Silurian volcanism in coastal Maine: The Cranberry Island series

Abstract

The Cranberry Island series is a Silurian (424 ± 1 Ma) volcanic succession that crops out on three islands along the southeastern coast of Maine. It is part of the coastal Maine magmatic province and represents bimodal magmatism that occurred on a composite crustal block before or during accretion to Laurentia. The series consists of ∼1.8 km of felsic pyroclastic rocks and felsic lava flows overlain by ∼0.8 km of basaltic tuffs and basaltic lava flows. The lower and upper members of the Cranberry Island 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 chilled magmatic enclaves, suggesting that droplets of basaltic magma were entrained by felsic magma prior to eruption. The compositionally layered plutons of the Cadillac Mountain intrusive complex to the north are likely candidates, in both age and composition, for the magma bodies from which the Cranberry 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. The bimodal character of magmatism, the lack of strong arc geochemical signature, and the rarity of andesites in the Cranberry Island series and throughout the coastal Maine volcanic belt argue against an origin in a compressional setting. Alternative settings for magmatism include (1) backarc extension, (2) crustal extension associated with large-scale transcurrent faulting, and (3) extension-related magmatism associated with rifting of the coastal Maine volcanic belt from another continental margin.