Duration of Late Cretaceous-early Tertiary magmatism in east-central Sonora, Mexico

Late Cretaceous-early Tertiary are magmatism in east-central Sonora include s the Sonoran batholith and the volcanic rocks of the Tarahumara Formation. The Tarahumara Formation consists predominantly of propylitically altered andesitic to dacitic lava, agglomerate, and volcanic breccia, all of local derivation, along with subordinate felsic pyroclastic components. Relativel y thin volcaniclastic intervals with intercalated freshwater limestone that contains Late Cretaceous plant fossils occur within the upper third of the formation. Six new U-Pb zircon ages from the Tarahumara Formation extend t he known duration of Late Cretaceous-early Tertiary magmatism in east-centr al Sonora, Four of the samples yield ages between 73 and 70 Ma, and the oth er two are 90 and 89 Ma (errors are generally <1 m.y.). All of these ages a re older than 65 to 55 Ma K-Ar and U-Pb ages for plutons of the Sonoran bat holith in east-central Sonora determined in other studies, Combined, the ag es indicate are magmatism over at least 35 m.y. Initiation of magmatism in eastern Sonora by 90 Ma is much earlier than predicted by regional trends, which are largely based on geochronology of plutons. It appears that a full history of the regional evolution of this magmatic art will require substa ntial age information from its volcanic rocks. A component of inherited zir con was detected in five of the six dated samples from the Tarahumara Forma tion. Although ages calculated from upper concordia intercepts are mostly i mprecise, all are consistent with the presence of one or more Proterozoic c omponents. In one sample, a persistent inherited component provided a preci se upper-intercept age of 1448 Ma, These small cores surrounded by new zirc on growth must represent material incorporated into the magma chamber prior to eruption. The results record the presence of Laurentian basement 200 km south and east of known outcrops in Sonora.