REVISITING THE GEOCHEMISTRY OF VOLCANIC ROCKS FROM THE COASTAL VOLCANIC BELT IN THE MACHIAS-EASTPORT AREA: EVIDENCE FOR A PRE-ACADIAN ARC?

The origin of the Coastal Volcanic Belt in eastermost Maine is closely related to the convergence and early stages of the Acadian collision between Avalonia and the leading edge of composite North America during the mid-Paleozoic. Siluro-Devonian volcanic rocks of the Machias-Eastport area (eastermost Maine) display systematic variation trends in their major and trace element concentrations, suggesting that fractional crystallization played an important role in the differentiation of the suite. While Middle Silurian Dennys, Edmunds, and Leighton Fm. rocks have a continuous compositional range from basalt, through andesite, to rhyolite, the Late Silurian Eastport Fm. is more bimodal. Calc-alkaline Dennys, Edmunds, and Leighton Fm. mafic rocks have lower Fe₂O_3T, Nd, and Zr, and higher Cr and Ni concentrations than the tholeiitic Eastport and post-Acadian Upper Devonian Perry Fm. mafic samples. On chondrite-normalized REE patterns and trace-element discriminant diagrams the Dennys, Edmunds, and Leighton Fms. show a subduction-related signature, while a within-plate geochemical signature characterizes Eastport and Perry Fm. rocks. These results are consistent with recent models in which the Coastal Volcanic Belt is a continental arc on the margin of Ganderia’s trailing edge that formed during the Middle Silurian as a result of west-directed pre-Acadian subduction of the oceanic lithosphere that separated Avalon and Ganderia. This compressional model likely changed by the Late Silurian into an extensional or transtensional tectonic setting that may have resulted from either the plate boundary switching to a transpressional regime, the formation of a back arc basin due to progressive shallowing of the subducting slab, or delamination of the downgoing lithosphere and subsequent decompression.