Crustal history of the Mesoproterozoic basement of the San Rafael Block (Mendoza)

Abstract

The present isotopic data are the result of a study of basement rocks of the San Rafael Block, located at the Mendoza Province, Argentina. Two areas were sampled, the type section of the Cerro La Ventana Formation known as Leones-Ponón Trehué-La Estrechura region and ductile-deformed rocks of the El Nihuil Mafic Unit. The Rb–Sr, Sm–Nd, Pb–Pb, and U–Pb methodologies were applied in order to constrain the basement age, tectonic history and to discuss relationships with equivalent Mesoproterozoic units located along the Cuyania terrane in the proto-An- dean Gondwana margin. The Rb–Sr isochron yielded an age of 1148 ± 83 Ma, with initial ratio of 0.70292 indicat- ive of a slightly evolved Mesoproterozoic source. The Sm–Nd isochron indicates an age of 1228 ± 63 Ma, whereas TDM ages ranges between 1.23–1.64 Ga with εNd in between −0.94 and +4.7 recording a source less evolved than CHUR for the time of crystallization. In Pb/Pb diagrams the samples plot similarly to rocks from the basement of Cuyania terrane. Zircon U–Pb data obtained from tonalitic facies (Leones River type section) yielded a crystalliza- tion age of 1214.7 ± 6.5 Ma (ID-TIMS). In situ U–Pb (LA-ICP-MS) zircon data carried out at two different laborat- ories on tonalitic orthogneisses (El Nihuil Mafic Unit) record an intercept at 1256 ± 10 Ma plotted in a concordia diagram, whereas in a Tera-Wasserburg diagram the age is of 1222 ± 6.9 Ma. Therefore, a Mesoproterozoic age for the basement of the San Rafael Block is here confirmed; this ca. 1.2 Ga age is found elsewhere within the Cuyania allochthonous terrane.