The Passa Quatro alkaline complex is one of the main intrusions in the Serra do Mar Cretaceous to Paleogene Igneous Province of southeastern Brazil. It is composed mainly of nepheline syenites and alkali syenites, with minor phonolitic dikes. The dominant felsic phases are potassic feldspar and nepheline, with minor sodic plagioclase in less silica-undersaturated rocks. The main mafic phases are clinopyroxene, amphibole, biotite, and oxides. The wealth of accessory phases includes titanite, eudialyte, astrophyllite–kupletskite, F-disilicates, phosphates, phosphosilicates, and F-REE-carbonates, with their specific ranges of composition. These accessory minerals often mantle zircon and fluorite corroded crystals, evidence of (1) the petrographic transition from miaskitic to agpaitic in the same rock, and (2) a decrease in the activity of fluorine in the coexisting residual magmas, from the stability range of fluorite to that of F-poorer disilicates. There is also a major role, as yet undervalued, for manganese in the phase stability of the various accessory phases in rocks/magmas devoid of magnesium. The three neighboring complexes of Itatiaia, Passa Quatro, and Poços de Caldas have significant differences in the types and amounts of agpaitic minerals in their peralkaline rocks. The Passa Quatro intrusion (with aegirine, titanite, and minor eudialyte) can be considered slightly more silica undersaturated and peralkaline than the nearby Itatiaia complex and has transitional features towards the highly agpaitic nepheline syenites of Poços de Caldas (with aegirine, titanite, eudialyte, and aenigmatite). Eudialyte and titanite represent accessory phases that influence the incompatible element behavior in the residual magmatic compositions (mostly phonolites and peralkaline phonolites) of the Serra do Mar potassic province.

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