THE COASTAL VOLCANIC BELT (CVB), EASTERNMOST MAINE: GEOCHEMISTRY AND TECTONIC SETTING

The Eastport-Machias area of easternmost Maine contains a thick sequence of well preserved and well dated Silurian and Early Devonian volcanic rocks generally assumed to have peri-Gondwanan affinities. Forty-nine samples of the volcanic members of this sequence, as well as the overlying Perry Fm., have been analyzed for both major and trace elements. The rocks range from basalts to rhyolites and give a bi-modal distribution on an alkalis vs. silica plot as previously noted by Gates and Moench (1981). However, immobile trace element concentrations (Zr/Ti vs. Nb/Y) of the samples indicate these rocks form a continuous sub-alkalic sequence between basalt and rhyolite, with the majority of the more mafic samples lying in the basaltic andesite and andesite fields. Silurian mafic rocks from the Dennys, Edmunds and Leighton Fms. contrast with those from the Early Devonian Eastport and Late Devonian Perry Fms., having generally lower contents of Nb, Zr, Y, Ti and P and higher values of Cr and Ni. The Silurian basaltic to andesitic samples are high-alumina, calc-alkalic rocks that plot in the volcanic arc fields on various tectonic discrimination diagrams. Likewise, the Silurian felsic rocks also indicate volcanic arc affinities on granitic discrimination diagrams. In contrast, the post-Acadian Perry Fm. yields a clear within-plate geochemical signature. However, the tectonic environment in which the volcanics of the Eastport Fm. erupted is more difficult to establish and may be somewhat transitional in nature. While felsic rocks from the Eastport give within-plate signatures, characterization of the mafic rocks is less clear. We interpret the geochemistry of the volcanic rocks of the CVB in the Eastport-Machias area as pointing to volcanic arc activity throughout the Silurian. By Early Devonian, the tectonic setting was changing from an arc to likely one of extension or transtension. By Late Devonian, the CVB was clearly in a within-plate tectonic setting.