The origin of post-Paleozoic magmatism in eastern Paraguay

The ages of magmatic rocks are crucial for understanding of the geodynamic relationships among different magmatic events. Between the compressional Andean and the extensional Atlantic systems, Paraguay has been the site of six main taphrogenic events since the end of Paleozoic times. Other than the Paraná flood tholeiites (133-134 Ma; Early Cretaceous, Hauterivian), new high-precision 40 Ar/ 39 Ar ages show that other alkaline magmatism of various types occurred, namely sodic magmatism at 241.5 ± 1.3 Ma (Middle Triassic, Anisian), 118.3 ± 1.6 Ma (late Early Cretaceous, Aptian), and 58.7 ± 2.4 Ma (Paleocene); and potassic magmatism at 138.9 ± 0.7 (Early Cretaceous, Venginian) and 126.4 ± 0.4 Ma (Early Cretaceous, Barremian). The main geochemical characteristics of the sodic alkaline rock types are systematic Nb-Ta positive anomalies and Sr-Nd isotopes trending to the bulk Earth or the depleted mantle components, contrasting with potassic rocks and tholeiitic basalts that show negative Nb-Ta anomalies and Sr-Nd isotopes trending to the enriched mantle components. The Pb isotope versus Sr-Nd systematics confirm the distinction between potassic rocks enriched in "high-radiogenic" Sr and low in "less radiogenic" Nd-Pb and sodic rocks ranging from depleted components to bulk Earth and transitional to the Paraná flood tholeiites. The occurrence of alkaline, both sodic and potassic (and carbonatititic), and tholeiitic magmatism in the whole Paraná-Angola-Etendeka system, and even in the Andean system, implies appropriate lithospheric sources to generate the various types of magmatic rocks. Therefore, any hypothesis of an asthenospheric plume origin is not compelling; rather, possibly such a plume provided a thermal perturbation and/or a decompressional environment, and possibly mantle sources were driven by Precambrian melts that contaminated and veined the lithosphere. A decompressional environment is inferred as a possible mechanism driven by differential rotation of different subplates in the South America and south Africa plates.