The Mechanic Settlement Pluton, located at the northern margin of the Caledonian Highlands in southern New Brunswick, is composed of rocks ranging from ultramafic (lherzolite, plagioclase-bearing lherzolite) through mafic (mainly olivine gabbronorite and gabbro) to intermediate (quartz diorite and monzodiorite). Spatial distribution of these lithologies, textural features, and geochemistry are consistent with evolution of a tholeiitic mafic parent magma by crystal fractionation processes, with some evidence for magma mingling between evolved gabbroic and quartz dioritic magmas. The dioritic rocks form most of the southwestern (upper?) part of the pluton, whereas the varied gabbroic rocks with ultramafic layers form the northeastern part. U–Pb (zircon) dating of a quartz diorite sample from the southwestern part of the pluton indicates crystallization at 557 ± 3 Ma. Amphibole and phlogopite in two lherzolite samples from the northeastern part of the pluton gave 40Ar/39Ar dates of 550 ± 5 and 539 ± 5 Ma, respectively, indicating that the pluton cooled rapidly through the closure temperature for amphibole, with subsequent slower cooling to the time of phlogopite closure. The pluton is interpreted to be the intrusive equivalent of basaltic units in the host Coldbrook Group, analogous to granitic plutons elsewhere in the Caledonian Highlands which appear to be the intrusive equivalents of felsic volcanic rocks in the group. These plutonic and volcanic rocks represent a major, short-lived (ca. 560–550 Ma), dominantly bimodal igneous event, apparently related to late Precambrian extension within the Avalon terrane of southern New Brunswick.