Isotopic and paleontologic evidence for correlating three volcanic sequences in the Maine coastal volcanic belt

The correlation of the Castine Volcanics, the Thorofare Andesite–Vinalhaven Rhyolite sequence, and the Cranberry Island Series of Shaler (1889), suggested by their similar lithologies, appearance, and structural histories, is supported by the results of Rb-Sr whole-rock isotopic analyses and by the faunal assemblages from old and new fossil localities in the Castine Volcanics, Ames Knob Formation, and Thorofare Andesite. The volcanic rocks are partly Late Silurian, but mostly Early Devonian in age and yield an average radiogenic age of 390 ± 5 m.y. The use of calcite-bearing volcanic samples for whole-rock Rb-Sr dating degrades the method by greatly increasing the uncertainty of the isochron and initial Sr⁸⁷/Sr⁸⁶. Lower to Middle Devonian granitic plutons have initial ratios of Sr⁸⁷ and Sr⁸⁶ similar to those in the volcanic formations. The Castine Volcanics and the Lower Devonian granite of Sedgwick may be comagmatic, but the time interval between the extrusion of the Vinalhaven Rhyolite and its intrusion by the Middle Devonian granite of Vinalhaven Island is too long to support the comagmatic hypothesis.