INTERMEDIATE VOLCANICS IN A BIMODAL SETTING: SILURIAN ANDESITES OF THE FOX ISLANDS, ME

Silurian volcanic rocks occur in the northern part of Vinalhaven Island and along the south shore of North Haven Island (the Fox Islands) in Penobscot Bay, Maine. They overlie the fossiliferous Lower (?) Silurian Ames Knob Formation and are intruded by the Vinalhaven granite pluton (420 +/-1 Ma; David Hawkins, pers. comm.), one of several plutons of the Coastal Maine Magmatic Province produced by the repeated injection of mafic magma into a silicic chamber at a shallow crustal level. The younger members of the volcanic pile - the Vinalhaven Rhyolite (Newton, J.L., 1999) and underlying Vinalhaven Diabase (Klemetti, E.W., 1999) - resemble the rocks of the pluton in their apparently bimodal character, though the "diabase" is geochemically a basaltic andesite to andesite and has petrographic features suggesting it is a hybrid. Older volcanics in this Silurian sequence are the felsic tuffs in the Polly Cove Formation (named by Ollie Gates, 2001, and the stratigraphic equivalent of the Ames Knob), the overlying Thorofare Andesite, and the tuffs and tuff breccias of the lower part of the Seal Cove Formation (Gates, O., 2001; Szramek, L., 2002). The Thorofare forms a 750-meter-thick pile of gray to reddish-brown intermediate lava flows and breccias on either side of the Thorofare strait and on small islands within it. The fragmental rocks are dominated by laharic breccias but also consist of autobreccias, bedded tuff breccias, and rare airfall tuffs. Breccia clasts are mainly andesite and mudstone, with locally abundant felsic volcanic fragments that may be the equivalent of volcanics in the Upper Cambrian Castine and Ellsworth formations to the north. Lower greenschist metamorphism and local hydrothermal alteration have largely obscured the primary mineralogy of the flow units in the Thorofare. The least altered samples from it preserve an intergranular fabric of mostly andesine and clinopyroxene; on a Winchester and Floyd (1977) classification diagram using immobile trace elements, they plot mostly as andesites with overlap into the fields of basaltic andesite and dacite. Most flow samples are in the IAT (island arc tholeiite) field of a Mullen-type (1983) tectonic discrimination diagram, suggesting that they formed before consolidation of the peri-Avalonian arc systems with the Laurentian mainland.