The volcanic and sedimentary rocks of the Passamaquoddy Bay (PB) area of southeastern New Brunswick are part of the Silurian–Devonian Coastal Volcanic Belt (CVB), an extensive belt of bimodal volcanic rocks. The PB sequence is 4 km thick, has four cycles of mafic and felsic volcanism, and is intruded by mafic dykes at all levels. There are two ages of dykes, those related to the Late Silurian PB volcanism (PB dykes) and Mesozoic dykes (the Minister Island Dyke) related to the opening of the North Atlantic. The PB mafic dykes are subalkalic basalt to basaltic andesite, within-plate tholeiites. The dykes are moderately to highly evolved (Mg# = 66.6 to 26.6), with trends of major and trace elements typical of the fractionation of olivine, pyroxene, plagioclase, and ilmenite. The PB mafic dyke swarm comprises over 155 dykes which represent a greater range of compositions than the associated flows, suggesting that they give a more complete representation of the Late Silurian PB mafic magmas. They exhibit incompatible element characteristics best accounted for by crustal contamination. The dykes plot on a linear array away from mantle mixing lines between depleted and enriched mantle sources and toward the composition of the PB felsic units, suggesting that these felsic units are representative of partial melts and fractionates of the source contaminate. The variable TiO2 contents (1.2–4.3 wt.%) and incompatible element ratio trends plotted against a fractionation index suggest that mantle metasomatism, either fluid or melt derived, may also have influenced the mantle source of the dykes. The dykes dip steeply and have a relatively consistent strike to the north. Most dykes range in thickness from 0.5 to 2 m, but range up to 9 m. The single orientation of the dykes, along with their chemical characteristics and volume, and association with a bimodal intraplate volcanic sequence, are consistent with an extensional tectonic setting. Constraints of the regional geology suggest that this extension was associated with convergence, perhaps in a back-arc setting.