Petrogênese, evolução magmática do complexo alcalino do morro de São João, sudeste do Brasil, e suas implicações na evolução da plataforma Sul-Americana

Abstract

The Morro de São João Alkaline Complex, Meso-Cenozoic, is an alkaline intrusion of approximately 10 km², located within the Poços de Caldas – Cabo Frio Alkaline Alignment and intruded into the Cabo Frio Terrane, Ribeira Orogeny. The complex 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 Morro de São João are nepheline-rich, alkali-feldspar (Or97,08Ab2,02An0,90), and titanium- and calcium-bearing minerals. The (Dy/Yb)N vs. (La/Yb)N ratios suggest that the partial melting responsible for generating the complex 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 % of the EM I end-member, with influences from de DMM (14 %), and HIMU (14 %).The magmatic evolution of the complex 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 the 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 complex, contains up to 32% mafic contribution from malignite. Magnetometric signatures reveal that the complex exhibits up to eight magnetic anomalies in the subsurface, interpreted as multiple intrusions with distinct signatures. Additionally, the processing of magnetometric data revealed that, at the time of crystallization, the complex was subject to the global phenomenon of geomagnetic reversal. This finding, together with isotopic signatures, supports the hypothesis of the existence of a mantle plume providing heat for the genesis of the complex.and influence of a mantle plume in the genesis of the complex.