PETROGENESIS AND MAGMATIC EVOLUTION FROM THE MORRO DE SÃO JOÃO ALKALINE COMPLEX, SOUTHEASTERN BRAZIL

The Meso-Cenozoic Morro de São João alkaline complex is an association of felsic and mafic alkaline rocks, some of which are cumulative in nature, weakly to strongly silica-undersaturated, ultrabasic to intermediate, potassic, and metaluminous. The complex is located at the eastern end from the Poços de Caldas-Cabo Frio Alkaline Alignment. It intrudes into migmatitic orthogneisses and granitoids from the Cabo Frio Terrain from the Ribeira Belt. This work presents preliminary Sr-Nd-Pb-Hf isotopic data and new U/Pb geochronological data for the complex. The isotopic signatures indicate the complex has a mantle source evolved with parts of crustal contamination. The initial ratios of ¹⁴³Nd/¹⁴⁴Nd _(i) vs. ⁸⁷Sr/⁸⁶Sr _(i) and ¹⁷⁶Hf/¹⁷⁷Hf _(i) vs. ¹⁴³Nd/¹⁴⁴Nd _(i) indicate a mantle reservoir source in the vicinity of EM I. Furthermore, the initial ratios ²⁰⁶Pb/²⁰⁴Pb _(i) vs. ²⁰⁸Pb/²⁰⁴Pb _(i) and ²⁰⁶Pb/²⁰⁴Pb _(i) vs. ²⁰⁷Pb/²⁰⁴Pb _(i) indicate a mantle and orogenic source. The T_DM ages 716 and 735 Ma, correspond with the Brasiliano Orogeny, which, in the present geological context, are collisional events of the Ribeira Orogeny. The U/Pb geochronological data indicate that the magmatic activities that generated the complex occurred in two stages. The first, around 89.7 Ma, involved the generation of a mafic magma chamber with the occurrence of cumulus. The second stage, around 65.9 Ma, involved felsic magmatism. During the evolution of the magma chamber, crustal contributions to the complex involved the assimilation of xenoliths and melts from the host rocks. Disequilibrium textures resulting from the crystallization and hybridization phases provide further evidence that the chamber was developing in an open system despite cumulus features in the complex. These cumulus features result from mixing crustal and interstitial melts in the magma chamber at the time of assimilation.