EARLY-PALEOGENE MAGMATISM IN THE PINALEÑO MOUNTAINS, ARIZONA: EVIDENCE FOR CRUSTAL MELTING OF DIVERSE BASEMENT ASSEMBLAGES DURING THE LARAMIDE OROGENY

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 in the footwall of the Pinaleño-Jackson Mountain metamorphic core complex of southeast Arizona, called the Relleno suite. Zircon U-Pb geochronology indicates that the suite was emplaced from 58–52 Ma and consists of multiple cross-cutting intrusive phases. Zircon Lu/Hf isotope geochemistry, whole rock Sr, and Nd isotope geochemistry, and mineral O isotope geochemistry was used to investigate the source of these rocks and evaluate whether they are related to crustal anatexis. Average initial zircon εHf values of the suite range from -4.7 to -7.9, initial whole rock εNd and ⁸⁷Sr/⁸⁶Sr values range from -9.4 to -11.9 and 0.7064 to 0.7094, and quartz δ¹⁸O_VSMOW values range from 6.8‰ to 9.4‰. Isotopic and geochemical data of these rocks are consistent with derivation from and assimilation of mafic (meta)igneous rocks, at deep crustal levels and is supported by thermodynamic melt modelings of partial melting of Proterozoic igneous rocks similar to those exposed in the Pinaleño Mountains. In comparison to other Laramide-age anatectic granites in southeast Arizona, those exposed in the Pinaleño Mountains are temporally similar but present compositional and isotopic differences which reflect melting and assimilation of different and/or a more diverse range of lithologies, producing distinct mineralogical and isotopic characteristics. The results suggest that crustal melting 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.