A mid-Mesozoic plutonic suite ranging from ultramafic-gabbroic rocks to dioritic rocks and commonly intruded by younger granitoids is widespread in the Klamath Mountains–western Sierra Nevada, California. The ultramafic-gabbroic rocks are clinopyroxene-rich and commonly vary from wehrlite to olivine-hornblende clinopyroxenite and melagabbro. Associated dioritic rocks include biotite–two-pyroxene diorite–monzodiorite, and the granitic rocks typically range from tonalite to granodiorite. These plutonic complexes are coextensive and broadly coeval with a suite of weakly metamorphosed volcanic rocks ranging in composition from basalt to basaltic andesite to andesite. The volcanic rocks are chiefly volcaniclastic and are characterized by several distinct phenocryst assemblages: clinopyroxene + plagioclase (± olivine or hornblende). The plutonic complexes and the associated volcanic rocks are restricted to terranes that lack continental (sialic) crust and that, at least locally, contain juvenile ophiolitic crust generated adjacent to the locus of magmatism. Thus, their petrogenesis may be related to recurrent fracturing and magmatism within a rifted ensimatic arc.