Magmatic characteristics of the Paleocene Cerro Nevazón region and other Late Cretaceous to Early Tertiary calc-alkaline subvolcanic to plutonic units in the Neuquén Andes, Argentina

Abstract

Voluminous Late Cretaceous–Early Tertiary calc-alkaline magmatism in the northwest part of the Neuquén province, Argentina, is characterized by a NS-trending belt of olivine basalt-augitic andesite volcanic and amphibole-rich diorite subvolcanic facies that were grouped in the Neuquén-Mendoza volcanic province. Of samples associated with skarn mineralization in the Cerro Nevazón area, two were dated by K/Ar in amphibole and one by Ar/Ar in plagioclase; together, they yield ages between 60.1±1.6 and 56.0±1.7 Ma. The igneous rocks contain normally zoned plagioclase (64–74%), amphibole (magnesiohastingsite and edenite; 15–20%), biotite (5–10%), quartz (4–18%), minor enstatite–ferrosilite (Wo 2.5–3En 71–73Fs 24–25), and augite (Wo 42–43En 42–43Fs 14–15), with accessory magnetite, minor ilmenite, apatite, sphene, and traces of zircon. The fractionation of calcic plagioclase+olivine+pyroxene±magnetite±ilmenite is responsible for the evolution of the parental magmas, which originated in approximately 10% melting of a mantle source contaminated by fluids from the oceanic subducted lithosphere, with sparse or no residual garnet. The gabbro rare earth element patterns are similar to those of the Antuco volcano (37°25′S) and other Quaternary basaltic rocks from the southern volcanic zone (CSVZ) of the Andes (37–41°5′S) emplaced in a relatively thin continental crust (≈30 km). Although the Cerro Nevazón granitoids share some geochemical signatures with the Paleogene Caicayén igneous units, also located in the northwestern region of Neuquén, they have different evolutionary styles, as expressed by the lack of amphibole fractionation and less Cs enrichment in the Caicayén rocks. With similar SiO 2 contents, the Upper Cretaceous–Paleocene Campana Mahuida subvolcanic rocks from the same region are richer in incompatible trace elements (K, Rb, Sr, Ba, Nb, La, Ce, Yb, Th, and U) than either the Nevazón or Caicayén units, which suggests that their parental magmas were contaminated with crustal material. The low Fe 2O 3/FeO ratios in the Nevazón igneous rocks, combined with the presence of ilmenite in most rocks and the low Mg/(Mg+Fe +2) ratios in mafic minerals, suggest that the Nevazón magmas evolved under more reduced conditions than did the Caicayén or Campana Mahuida magmas.