Petrological features of the first Cenozoic alkaline magmatic event recorded in northwestern Uruguay, southern extreme of the Paran ́a basin

Abstract

The Cenozoic mafic alkaline rocks found in northwestern Uruguay, in the southernmost portion of the Paran ́a Basin, comprise three subvolcanic plugs composed of Ne-tephrites. They cut through basalts that make up the Paran ́a Magmatic Province as well as part of the sandstones of the Late Jurassic–Early Cretaceous age. These mafic rocks are characterized by an aphanitic hypocrystalline texture that is occasionally porphyritic with py- roxene and olivine macrocrysts/xenocrysts. We present the first lithogeochemical results that also include the Sr–Nd–Pb isotopes to determine the petrogenetic processes. These mafic rocks are enriched in light rare earth elements (La/LuN = 23.12–28.81) with respect to chondrite, in large-ion lithophile elements (Ba, U, and La), and in high-field-strength elements, such as Ta, Nb, and Zr, with respect to primitive mantle, and have a Mg# of 0.53–0.56. The trace element geochemistry and isotopic data (low radiogenic 206Pb/204Pb and low 87Sr/86Sri between 18.315 and 18.733, 0.70397 and 0.70443, respectively) indicate that the primary magma could be the product of the melting of an enriched mantle source with the possible involvement of metasomatic processes, lacking significant contamination by the continental crust. The K–Ar (whole rock) geochronology conducted on two samples yielded ages of 63.7 ± 2.5 and 51.5 ± 1.7 Ma for this magmatic event