A regional compilation and reinterpretation of geochemical analyses of mafic igneous rocks from the Ordovician Tetagouche Group in northern New Brunswick indicates the presence of at least three mafic assemblages: two groups of basalts and gabbros and one group of gabbros only. The three groups can be separated on the basis of TiO2 and trace-element content. Group A basalts generally have mid-ocean-ridge basalt (MORB)-like compositions, whereas group B basalts indicate a "within-plate" setting (WPB). Group C gabbros intrude into silicic volcanics that underlie the basalts but are rare or absent in the areas underlain by group A and B basalts. The group C gabbros probably represent the earliest stages of mafic igneous activity and have MORB-like compositions or represent a transition between MORB and arc basalts.The association of group A and B basalts with abundant marine sediments, such as thinly bedded turbidites and black and red shales, and the fact that they erupted locally on top of continental-crust-derived silicic volcanics suggest that they represent a marginal-sea or back-arc-basin environment rather than true oceanic (Iapetus) eruptives as has been suggested.The proposed back-arc environment and existing structural data are inconsistent with previously proposed tectonic models and large-scale correlations. Evidence for penetrative pre-Late Ordovician (Taconic orogeny sensu stricto) deformation is lacking in the northern part of the Miramichi Zone. Instead the earliest folding and related thrusting (D1) is ascribed to deformation associated with closure of the back-arc basin in post-middle Caradocian times.