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

During the Laramide orogeny (80–40 Ma) in southern Arizona, a series of weakly–strongly peraluminous granites (± muscovite ± garnet) were emplaced. These rocks have been interpreted to be crustal melts derived from local basement rocks, and we refer to them as the southern Arizona anatectic suite. This suite is distinct from dominantly metaluminous continental arc magmatism in the region. Intrusions are commonly manifest as small plutons and sill/dike networks emplaced in the middle crust and are frequently found in the exhumed footwalls of Oligocene–Miocene metamorphic core complexes. Here, we report on and characterize 60–50-Ma (new zircon U-Pb ages) granites in the Pinaleño-Jackson Mountain core complex, referred to as the Relleno granite, and evaluate its potential melt sources. These rocks are contemporaneous with other granites in the region like the Wilderness Suite. Basement rocks in the Pinaleño Mountains include 1.6–1.1-Ga (meta)igneous lithologies including diabase and amphibolite that occur as enclaves. The Relleno granite displays strongly evolved εNd_(i) values (-9.4 to -11.8), but moderately evolved zircon εHf_(i) values (-4.7 to -7.9). We interpret Nd-Hf isotopic decoupling to be related to disequilibrium melting of crustal rocks with incomplete zircon dissolution. Relleno granite zircons include inherited cores from ca. 1.6–1.4 Ga consistent with melting or assimilation of Proterozoic granitoids. Moderately evolved ⁸⁷Sr/⁸⁶Sr_(i) values (0.706 to 0.709) and relatively low δ¹⁸O values are consistent with melting and assimilation of mafic crust, including diabase and amphibolite enclaves. Zircon saturation temperatures (714–792 °C), Ti-in-zircon thermometry (673–751 °C), and O isotope equilibration temperatures (< 825 °C) indicate a range of possible magmatic temperatures of the Relleno granite. Perple_X modeling suggests melts derived from country rock can produce compositions similar to the Relleno granite at these temperatures. These results helps constrain melting conditions in the middle crust during the Laramide orogeny and suggests that magmatism distinctive from the continental arc was widespread in southern Arizona with variable geochemical and isotopic characteristics which may reflect partial melting and assimilation of regionally diverse country rock.