Early Paleogene Magmatism in the Pinaleño Mountains, Arizona: Evidence for Crustal Melting of Diverse Basement Assemblages during the Laramide Orogeny

Abstract

Abstract Granitic rocks, interpreted to be related to crustal melting, were emplaced into regions of thickened crust in southern Arizona during the Laramide orogeny (80–40 Ma). Laramide-age anatectic rocks are exposed as plutons, sills, and dike networks that are commonly found in the exhumed footwalls of metamorphic core complexes. This study investigates newly discovered exposures of granodioritic–leucogranitic rocks from three intrusive phases in the footwall of the Pinaleño–Jackson Mountain metamorphic core complex of southeastern Arizona, called the Relleno suite. Zircon U–Pb geochronology indicates that the suite was emplaced from 58 to 52 Ma. Zircon Lu/Hf isotope geochemistry, whole-rock Sr and Nd isotope geochemistry, and mineral O isotope geochemistry were used to investigate the source of these rocks and evaluate whether they are related to crustal anatexis. Average zircon εHf(t) values of the suite range from −4.7 to −7.9, whole-rock εNd(i) and 87Sr/86Sr(i) values range from −9.4 to −11.8 and 0.7064 to 0.7094 respectively, and quartz δ18OVSMOW values range from 6.8 to 9.4 ‰. Isotopic and geochemical data of these rocks are consistent with derivation from and assimilation of intermediate–mafic (meta)igneous rocks, at deep crustal levels, and are supported by thermodynamic melt models of Proterozoic igneous rocks equivalent to those exposed in the Pinaleño Mountains. In comparison with other Laramide-age anatectic granites in SE Arizona, those exposed in the Pinaleño Mountains are temporally similar but present compositional and isotopic differences that reflect melting and assimilation of different lithologies, producing distinct mineralogical and isotopic characteristics. The results suggest that crustal melting during this interval was not limited to metasedimentary protoliths and may have affected large portions of the deep crust. The early Paleogene Relleno suite in the Pinaleño Mountains strengthens the relationship between crustal melting and regions of thickened crust associated with the Sevier and Laramide orogenies.