Northeastern Section - 37th Annual Meeting (March 25-27, 2002)

Volcanic rocks in the Machias-Eastport area of easternmost Maine represent the thickest and best dated Silurian-Early Devonian volcanic succession in the Coastal Volcanic Belt. Trace element and REE data have been obtained for basaltic to intermediate volcanic rocks and rhyolites from the Silurian Dennys, Edmunds and Leighton Fms., the Lower Devonian Eastport Fm. and the post-Acadian Perry Fm. Based on immobile element ratios (Zr/Ti vs. Nb/Y), Silurian rocks form a continuous subalkalic sequence between basalt and rhyolite, while Eastport rocks have a more bimodal character. In general, all the basalts display LREE enrichment patterns. Silurian basalts contrast with Devonian basaltic samples having, in general, a higher enrichment in the LREEs and negative Eu anomalies. La/Yb values in Silurian basalts range from 4 to 7 and are similar to those for calc-alkaline basalts. On MORB-normalized incompatible element diagrams, the Silurian basalts show a stronger enrichment in the incompatible elements and a stronger Nb-Ta anomaly than the Devonian basalts. Trace element tectonic discrimination diagrams indicate the Silurian basalts were generated in an arc environment, while most Devonian samples show a transitional character between arc and continental tholeiites. Silurian rhyolites have a clear arc affinity on granitic discrimination diagrams, while normalized REE patterns are LREE enriched with strong negative Eu anomalies.

The geochemistry of the Machias-Eastport volcanic rocks suggest a change in the tectonic setting during the mid-Paleozoic from arc-related to within plate by the end of the Acadian Orogeny. These results are consistent with recent tectonic models (Bradley et al., 2000; Tucker et al., 2001) for the Acadian Orogeny in which Avalon was the overriding plate migrating northwestward over ancient North America. The Coastal Volcanic Belt was probably formed on the Avalonian margin during plate convergence above a SE-dipping subduction zone. Delamination of the downgoing lithosphere and subsequent decompression led to an extensional tectonic regime by the time Devonian magmatism developed.