Early Devonian tectonic evolution of the Ganderia domain in the Chaleur Bay Synclinorium of northern New Brunswick: evidence from felsic volcanic rocks of the Dalhousie Group

Lower Devonian (ca. 418–407 Ma) volcanic rocks of the Dalhousie Group of the Gander Zone from the northern mainland Appalachians of northern New Brunswick are part of an overstep unit (the Middle Paleozoic Matapedia cover sequence) deposited across the accreted vestiges of the early Paleozoic Iapetus Ocean (proto-Atlantic Ocean). These shallow marine to subaerial mainly bimodal volcanic rocks were deposited in a post-collisional extensional setting. The felsic melts were generated by anhydrous melting of associated basalts in the lower continental crust with melting initiated by heat from rising underplated mafic magma. The ɛ Nd (t) values of rhyolites are positive (+ 3.45 – + 4.7) and similar to those of associated basalts (+3.9 - + 4.2). Likewise, their depleted model mantle ages ranging from ∼610 to ∼770 Ma are similar to those of basalts (∼700–790 Ma) and interpreted to represent enrichment ages associated with ancient Neoproterozoic subduction. The origin of the volcanic rocks of the Dalhousie Group is probably related to post-Salinic relaxation, extension, and breakoff of the Acadian Seaway slab and marks the transition to the Acadian orogenic cycle.