Abstract

Late Cretaceous–early Tertiary arc magmatism in east-central Sonora includes 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. Relatively 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 the known duration of Late Cretaceous–early Tertiary magmatism in east-central Sonora. Four of the samples yield ages between 73 and 70 Ma, and the other two are 90 and 89 Ma (errors are generally <1 m.y.). All of these ages are older than 65 to 55 Ma K-Ar and U-Pb ages for plutons of the Sonoran batholith in east- central Sonora determined in other studies. Combined, the ages indicate arc 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 arc will require substantial age information from its volcanic rocks.

A component of inherited zircon was detected in five of the six dated samples from the Tarahumara Formation. Although ages calculated from upper concordia intercepts are mostly imprecise, all are consistent with the presence of one or more Proterozoic components. In one sample, a persistent inherited component provided a precise upper-intercept age of 1448 Ma. These small cores surrounded by new zircon 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.

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