Petrogenesis, magmatic evolution of the Morro de São João Alkaline Massif, southeastern Brazil, and implications for the evolution of the South American Platform

The Morro de São João Alkaline Massif, Meso-Cenozoic, is an alkaline intrusion of approximately 10 km², located within the Poços de Caldas – Cabo Frio Alkaline Alignment. The massif consists of metaluminous rocks, ultrabasic to intermediate, potassic to ultrapotassic, SiO₂-undersaturated, and occasionally intersected by hypabyssal rocks, associated or not with pseudoleucite. The rocks of the massif are olivine-free and rich in nepheline, alkali-feldspar, and titanium- and calcium-bearing minerals. The (Dy/Yb)_N vs. (La/Yb)_N ratios suggest that the partial melting responsible for generating the massif occurred at approximately 1% of a garnet-bearing spinel peridotite. Meanwhile, isotopic ratios indicate that this source results from a mixture of up to 71.95 % of the EM I end-member, with influences from DMM, HIMU, and carbonatite. The magmatic evolution of the massif was not driven exclusively by fractional crystallization; the observed mineralogical and lithological diversity reflects complex processes of crustal assimilation and magma mixing. Evidence of these processes is provided by high ratios of ⁸⁷Sr/⁸⁶Sr, Ba/Sr and Rb/Sr, as well as low ratios of P₂O₅/K₂O, in addition to distinctive mineral textures such as compositional zoning in garnet and clinopyroxene crystals, poikilitic textures in amphiboles, and geochemical modeling of trace elements suggesting that the nepheline-monzosyenite, a hybrid member of the massif, contains up to 32% mafic contribution from malignite.