40AR/39AR AGES FOR ANDESITES OF THE GUARACANAL FORMATION, PUERTO RICO

Volcanic rocks along the dissected northern margin of the Caribbean plate formed in a magmatic arc that was active from the Early Cretaceous to the Late Eocene. The plutonic and volcanic rocks of Puerto Rico preserve a considerable range of this geologic history, and share tectonic features and stratigraphy with similar rocks in Cuba and Hispaniola. Many of the geologic age constraints for Puerto Rico are provided through biostratigraphic data, for fauna that are interstratified with volcanic rocks or separated from igneous lithologies by nonconformities and faults. We selected five samples of hornblende bearing andesites from the Guaracanal Formation for radiometric age dating in the Auburn Noble Isotope Mass Analysis Laboratory (ANIMAL). The samples have calc-alkaline geochemical signatures typical of arc volcanic rocks in the region, and are very fresh with only minor deuteric alteration effects. Laser ⁴⁰Ar/³⁹Ar fusion ages for single hornblende phenocrysts from these andesites tend to define Gaussian distributions with single modes of ca. 63 to 65 Ma (with ca. 20 age determinations per sample). Incremental heating analyses (for aliquots of ca. 10 crystals from each sample) result in release spectra that exhibit minor effects of extraneous, non-atmospheric argon and radiogenic ⁴⁰Ar loss, but the higher-temperature portions of the spectra yield plateau ages that are also ca. 63-65 Ma. We interpret these Paleocene to latest Cretaceous ages to represent the timing of andesitic volcanism in the Guaracanal formation. These ages seem slightly older than ages proposed for similar andesites in western Puerto Rico, and appear in agreement with ages proposed for andesites in the El Cobre Group of eastern Cuba. The Guaracanal andesites of the present study are clearly younger than the Campanian ⁴⁰Ar/³⁹Ar ages reported for mafic intrusive rocks of St. Croix by Kappelman et al. (2012; this GSA meeting). Collectively, the new ⁴⁰Ar/³⁹Ar ages for the andesites of the present study emphasize the extent of the Paleocene to latest Cretaceous arc magmatism preserved in the Greater Antilles.