The Asunción-Sapucai-Villarrica graben (ASV) in Eastern Paraguay at the westernmost part of the Paraná Basin was the site of intense magmatic activity in Mesozoic and Tertiary times. Geological, petrological, mineralogical, and geochemical results indicate that the following magmatic events are dominant in the area: (1) tholeiitic basalt and basaltic andesites, flows and sills of low- and high-titanium types; (2) K-alkaline magmatism, where two suites are distinguished, that is, basanite to phonolite and alkali basalt to trachyte and their intrusive analogues; (3) ankaratrite to phonolite with strong Na-alkaline affinity, where mantle xenoliths in ultramafic rocks are high- and low-potassium suites, respectively. The structural and geophysical data show extensional characteristics for ASV. On the whole, the geochemical features imply different mantle sources, consistently with Sr-Nd isotopes that are Rb-Nd enriched and depleted for the potassic and sodic rocks, respectively. Nd model ages suggest that some notional distinct “metasomatic events” may have occurred during Paleoproterozoic to Neoproterozoic times as precursor to the alkaline and tholeiitic magmas. It seems, therefore, that the genesis of the ASV magmatism is dominated by a lithospheric mantle, characterized by small-scale heterogeneity. 1. Introduction Velázquez et al. [1] presented a structural analysis of the central segment of the “Asunción Rift,” mainly based on the previous papers related to the Eastern Paraguay magmatism in general, and to the Asunción-Sapucai-Villarrica graben (ASV) in particular, and also based on extensive field data collected earlier on the dyke swarms cropping out in the area. However, some aspects as, for example, the close association in space of potassic and sodic alkaline rock-types with tholeiitic dykes and flows (both of high- and low-Ti types; cf. [2]), have not been discussed in detail by the above authors. This paper focuses on general aspects of the magmatism from the western margin of the Paraná-Angola-Etendeka system (PAE), where tholeiitic flows and dykes (Early Cretaceous, both of high-Ti and low-Ti types; cf. Figure 1(a) and [3]) are associated in time and space with a wide variety of alkaline rock-types (both potassic and sodic) and carbonatites. Figure 1: (a) Sketch map of the Paraná-Angola-Etendeka system [ 3] where the arrows indicate the occurrences of the main dyke swarms. The basaltic lavas are subdivided into broad high- and low-Ti groups and late-stage rhyolites (yellow fields). EP: Eastern Paraguay. (b) Main lineaments in the
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