Carbonatites and primary carbonates in the Rio Apa and Amambay regions, NE Paraguay

Abstract

In the Rio Apa and Amambay regions, north-eastern Paraguay (NPAA), potassic, alkaline-carbonatitic rocks (138–139Ma) predate the eruption of tholeiitic flood basalts (133±1Ma). These rocks, mainly outcropping as dykes or ring-like complexes, intrude a Cambro-Ordovician carbonate platform and Precambrian metamorphic rocks along with their Silurian and Permo-Carboniferous sediments. The main rock-types range from basanite to trachyte and trachyphonolite (and intrusive equivalents) to carbonatite, in addition to glimmeritic and pyroxenitic veins. Geological and geophysical evidence indicate that the NPAA magmatism is related to extensional tectonics, like the Early Cretaceous alkaline-carbonatitic complexes from central-eastern Paraguay (127±1Ma) and the eastern magmatic occurrences of the Paraná Basin.Oxygen and carbon isotope compositions (whole rocks and carbonates) vary from values close to the field of continental lithospheric mantle, or that of primary carbonatites, up to values typical of a hydrothermal environment. An isotope exchange model implies that the main isotope variations, ranging from a magmatic (e.g., 1200°C) to a low temperature (<400°C) environment, involved fluids with CO2/H2O ratios between 0.8 and 1.0. In particular, the OC isotopic variations, in combination with the La vs La/Yb ratios, suggest an increasingly higher level of carbonate in the silicate liquids, with increasing evolution, i.e., basanite→phonotephrite→trachyphonolite–phonolite (trachyte)→carbonatite. SrNd isotopes show that the carbonatites are mantle derived without significant crustal contamination and that they can be related to isotopically enriched sources where newly formed veins (enriched component) and peridotite matrix (depleted component) underwent differing isotopic evolution.TDM model ages for NPAA range from 1.2 to 2.3Ga. Considering that in the whole Paraná Basin isotopically distinct K-alkaline and tholeiitic magmas were generated following the enrichment of the subcontinental mantle mainly between 1.0 and 2.3Ga (Paleo-Mesoproterozoic events), the mantle sources preserved the isotopic heterogeneities over long time periods, suggesting a non-convective, i.e., lithospheric, mantle source beneath different cratonic or intercratonic areas.