The Early to early Late Jurassic magmatic arc of the lower Colorado River region of southern California and southwest Arizona spanned ∼30 m.y., from ca. 190 to 158 Ma. The arc-type volcanic and plutonic rocks interacted extensively with the Proterozoic Mojave Province crust and show evidence for geographic-based age and compositional changes. The region lies adjacent to an amagmatic gap in the Jurassic arc of the southwest United States, near the western terminus of proposed Late Jurassic basins formed in conjunction with the opening of the Gulf of Mexico, and near, but to the north of, the projection of the trace of the sinistral Late Jurassic Mojave-Sonora megashear where it crossed from northern Mexico into the United States. Quartz-phyric dacitic to rhyolitic pyroclastic and locally hypabyssal rocks of the Dome Rock sequence were emplaced in two broad time periods, one between 190 and 185 Ma and the second between 173 and 158 Ma. Three compositionally expanded pluton units constituting the Kitt Peak–Trigo Peaks superunit were emplaced in the mid- to upper crust between 173 and 158 Ma, broadly contemporaneous with the younger phase of explosive volcanism. The compositionally expanded plutonic rocks consist of three informally named temporally and compositionally distinct magmatic units, from oldest to youngest, the Araz Wash diorite, the Middle Camp porphyritic granodiorite, and the Gold Rock Ranch granite. Each unit was emplaced over 4–6 m.y. periods of time. Dioritic rocks dominate the older Araz Wash diorite unit (173–169 Ma), granodiorite dominates the Middle Camp porphyritic granodiorite unit (167–163 Ma), and granite dominates the Gold Rock Ranch granite unit (163–158 Ma). Shortening and regional metamorphism throughout the lower Colorado River region accompanied emplacement of the Gold Rock Ranch granite unit. A Late Jurassic(?) mafic-felsic dike swarm forms the youngest magmatic unit in the region. The Jurassic magmatic history in the lower Colorado River region ended in the early Late Jurassic at ca. 158 Ma. Termination of magmatism in the Late Jurassic in the lower Colorado River region is distinct from adjacent parts of the arc to northwest in the Mojave Desert region or to the southeast in southern Arizona, where Late Jurassic magmatism continued to at least 146 Ma. At this time in the Late Jurassic, the Mojave-Sonora megashear had cut through the arc to the south of the lower Colorado River region, where degradation of the arc is recorded in sedimentary rocks now composing the lower parts of the McCoy Mountains Formation, the Winter-haven Formation, and informally named rocks of Slumgullion.