Late Triassic Paleo-Oceanic Basement Framework of the Peninsular Ranges Batholith of Baja California; New Evidence from the El Arco-Calmalli District

Leon Silver and colleagues at Cal Tech recognized more than 40 years ago that the Peninsular Ranges batholith was constructed almost entirely outboard of cratonal North America based on tracer isotope patterns of batholithic rocks comparable to those of intra oceanic island arcs. Exposure of Peninsular Ranges prebatholithic framework however, especially in the western “Alisitos arc” part of the batholith, is limited. At the southern end of the batholith, prebatholithic basement rocks crop out beneath the Alisitos cover in the El Arco-Calmalli district. Here we report a Late Triassic zircon U-Pb crystallization age of 220 Ma for high level gabbro associated with the El Arco-Calmalli ophiolite succession, which in turn is intruded and overlain by Middle Jurassic arc magmatic rocks that include the El Arco porphyry copper deposit. The El Arco-Calmalli geology correlates with Triassic-Jurassic geology of the Vizcaino-Cedros region farther outboard on the margin, as well as basement geology of the Guerrero terrane of mainland Mexico farther inboard. Detrital zircon data suggest that Guerrero terrane and Vizcaino-Cedros basement has been in proximity to North America cratonal rocks throughout its history. Age, stratigraphic, and petrologic data from Mexico thus support previously proposed forearc rifting models developed for the U.S. sector of the Cordilleran orogen that interpret early Mesozoic ophiolite assemblages as autochthonous or parautochthonous forearc lithosphere constructed outboard of the Mesozoic continental margin arc, and which subsequently served as host to the Cretaceous Peninsular Ranges batholith.