Geochemistry of Silurian-Devonian volcanic rocks in the Coastal Volcanic belt, Machias-Eastport area, Maine: Evidence for a pre-Acadian arc

Abstract

The Coastal Volcanic belt in easternmost Maine consists of Silurian–Devonian volcanic rocks related to the convergence of Avalonia with the leading edge of composite Laurentia just before and during early stages of the Acadian orogeny. New major- and trace-element analyses from the Machias-Eastport area indicate that the Early to Late Silurian volcanic rocks in the Dennys, Edmunds, and Leighton Formations formed in a continental arc environment, in contrast to some earlier interpretations. These rocks have a continuous compositional range from basalt, through andesite, to rhyolite. The mafic rocks in these formations are calc-alkaline and show subduction-related geochemical signatures on various discrimination diagrams, whereas the felsic rocks have geochemical characteristics of volcanic arc granites. In contrast, the thick volcanic deposits of the younger Late Silurian Eastport Formation are bimodal and tholeiitic and have transitional to within-plate geochemical signatures. Thus, the transition over time, as the Acadian seaway closed, from subduction and arc settings to continental extension can been seen in this rock sequence, and it constrains the time of Avalonia’s docking. Regionally, volcanic rocks in the Kingston terrane of southern New Brunswick are likely correlatives of the earlier arc-volcanic rocks (Dennys, Edmunds, and Leighton Formations), while Eastport Formation analogues are found in the coastal Maine magmatic province, Cranberry Island series, and probably the Passamaquoddy Bay volcanic series, which similarly appear to have formed after the transition to an extensional environment occurred, although other extensional settings cannot be entirely ruled out. Basalts in the post-Acadian Perry Formation indicate continental extension was also active in the Late Devonian.