The Ben Bullen plutons constitute a small medium-K. gabbro-tonalite-trondhjemite suite spatially and temporally associated with extensive adamellite and granite of the Bathurst Batholith. The mafic composition of the bulk of the Ben Bullen rocks suggests a mantle source for the parent magma. Chemical and petrographic studies indicate that the rocks are closely comparable with medium-K calc-alkaline volcanic suites in island arcs. The Ben Bullen series are M-type granites in the sense of being formed by crystal fractionation of a gabbroic parental magma. Initial crystallization and separation of olivine and pyroxene followed by marked fractionation of hornblende probably drove the differentiating magma along a typical calc-alkaline trend of strong alkali-enrichment. It is possible that the rise of these mantle-derived melts into the crust initiated the crustal melting that produced the associated felsic K-rich granitoids of the Bathurst Batholith.