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

Abstract

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.