Abstract
General information is presented on ten agpaitic occurrences located in southern Brazil and at the border between Brazil and Paraguay. All the Brazilian agpaitic rocks are Late Cretaceous in age, whereas the Paraguayan ones are older than Early Triassic. The most significant occurrence is Poços de Caldas, the largest alkaline massif in South America. In general, these agpaitic rocks contain mineral assemblages that indicate presence of typical halogen-bearing Na–Ca–HFSE phases, eudialyte-, rinkite- and wöhlerite-group minerals being the most frequent ones. However, these associations are indeed more complex in terms of composition, with accessory phases in some cases consisting of various minerals, including U–Th oxides/silicates, Nb oxides, REE–Sr–Ba bearing carbonates–fluorocarbonates–phosphates–silicates and Zr–Na rich silicates. They usually form late magmatic stage to hydrothermal/deuteric assemblages linked with coarse and fine-grained, mainly silica-undersaturated evolved rocks. Data also indicate significant differences in type, amount and composition of agpaitic minerals in all investigated occurrences.
Highlights
We present a general overview of the main agpaitic alkaline rocks of the southern Brazilian platform and the current knowledge of their exotic accessory mineralogy
In Brazil, alkaline centers occur predominantly dispersed over the eastern flanks of the country, and as several individual bodies along two very distinct NE and NW trends that extend for many kilometers [47,48]
In the state of São Paulo, Brazil, alkaline coastal islands include São Sebastião, the largest island encompassing three isolated syenitic stocks (São Sebastião, Serraria and Mirante), Búzios and Vitória to the northeast, and Monte do Trigo massif to the west
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Rocks as peralkaline nepheline syenites (and phonolites) characterized by complex Zr and Ti minerals, such as eudialyte and rinkite, rather than simple minerals such as zircon and ilmenite [1,2]. Agpaitic mineral assemblages are thought to originate from crystallization of residual magmatic liquids rich in very distinctive elements, such as REE, HFSE, LILE, and volatiles (F, Cl). These assemblages may preferentially form in the late-magmatic and hydrothermal/deuteric stages, as indicated by textural evidence (Table 1). Agpaitic assemblages commonly derive from miaskitic peralkaline igneous rocks of foid syenite composition (mainly nepheline syenites and their fine-grained varieties) formed during final magma differentiation stages [7,8]. We present a general overview of the main agpaitic alkaline rocks of the southern Brazilian platform and the current knowledge of their exotic accessory mineralogy
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