The West Moose River pluton is one of a series of latest Devonian – Early Carboniferous plutons of granite and gabbro–diorite in the Cobequid Highlands (southern edge of Avalon zone). Intrusion of minor gabbro preceded emplacement of the granite, which is fractured and cut by later mafic dykes, of which some predate and others postdate a Namurian compressive deformation. Minor hybrid phases reflect mixing of mafic and felsic magmas, which are also represented by basalt and rhyolite in the synchronous Fountain Lake Group. The mafic rocks are continental tholeiites reflecting regional Early Carboniferous extension. Later dykes are more fractionated, with high P2O5, TiO2, and other high-field-strength elements. The dykes are pervasively altered, probably by residual fluids from the granite hydrothermally circulated by the hot mafic rocks. Higher Ga/Al, Hf, and Zn content in basalts than dykes suggests upward increase in halogen complexing in the mafic plumbing system. Late-stage fluids concentrated K, Rb, and Cs in the dykes but not the lavas. Alteration in granite includes mafic partings (biotite and Fe–Ti oxides) along fractures, particularly near dykes, and local albitization. Such alteration hinders determination of granite protolith from geochemistry, but mineral composition and the regional enrichment of the granite in Nb, Y, and Ga/Al is similar to that in A-type granites. Alteration associated with the mafic dykes continued for tens of millions of years after pluton emplacement.