Alkaline magmatism from Northern Paraguay (Alto Paraguay): A Permo-Triassic Province

Sodic alkaline occurrences, mainly represented by slightly saturated or undersaturated to strongly undersaturated syenites, are found along the Paraguay river at the border of Paraguay and Brazil. They are emplaced into the Alto Paraguay Province, tectonically related to the Tapajos Province and to the Apa High. Mineralogical, petrographical and geochemical features are quite similar to those of the Early Cretaceous complexexs from the Velasco region (Bolivia) and different to those shown by the equivalent rock types from central-western Paraguay. Notably, radiometric data (K/Ar, Rb/Sr and 40Ar-39Ar) give a Permo-Triassic age (240-250 Ma) for the Alto Paraguay alkaline outcrops.