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

MARTA FRANCHINI; LEOPOLDO LÓPEZ-ESCOBAR; ABEL ISIDORO SCHALAMUK; LARRY MEINERT

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2003 vol. 16 p. 399 - 421

0895-9811

Voluminous Late Cretaceous?Early Tertiary calc-alkaline magmatism in the northwest part of the Neuque´n 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 Neuque´n-Mendoza volcanic province. Of samples associated with skarn mineralization in the Cerro Nevazo´n 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 (Wo2.5 ? 3En71 ? 73Fs24 ? 25), and augite (Wo42 ? 43En42 ? 43Fs14 ? 15), with accessory magnetite, minor ilmenite, apatite, sphene, and traces of zircon. The fractionation of calcic plagioclase þ olivine þ 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 (378250S) and other Quaternary basaltic rocks from the southern volcanic zone (CSVZ) of the Andes (37?41850S) emplaced in a relatively thin continental crust (<30 km). Although the Cerro Nevazo´n granitoids share some geochemical signatures with the Paleogene Caicaye´n igneous units, also located in the northwestern region of Neuque´n, they have different evolutionary styles, as expressed by the lack of amphibole fractionation and less Cs enrichment in the Caicaye´n rocks. With similar SiO2 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 Nevazo´n or Caicaye´n units, which suggests that their parental magmas were contaminated with crustal material. The low Fe2O3/FeO ratios in the Nevazo´n igneous rocks, combined with the presence of ilmenite in most rocks and the low Mg/(Mg þ Feþ2) ratios in mafic minerals, suggest that the Nevazo´n magmas evolved under more reduced conditions than did the Caicaye´n or Campana Mahuida magmas.