Petrogênese e evolução magmática da Suíte Sanukitóide Rio Maria, Terreno Granito – Greenstone de Rio Maria, Cráton Amazônico

Abstract

The Archean sanukitoid Rio Maria suite yielded zircon ages of ~2.87 Ga and is exposed in large domains of the Rio Maria Granite-Greenstone Terrane, southeastern Amazonian craton. It is intrusive in the greenstone belts of the Andorinhas Supergroup, in the Arco Verde, Mariazinha, and Caracol tonalite, and Mogno trondhjemite. Archean potassic leucogranites, Água Fria trondhjemite, and the Paleoproterozoic granites of Jamon Suite are intrusive in the rocks of the Rio Maria suite. The dominant rocks have granodiorite to subordinate monzogranitic compositions, with minor proportions of intermediate quartz-diorites or quartz-monzodiorites rocks, in addition to mafic end members occurring as layered rocks or as enclaves. The Rio Maria suite has clear geochemical characteristics of Sanukitoids rocks (high Mg#, elevated Cr and Ni contents, LREE enrichment, and high Ba and Sr contents relative to typical calc-alkaline series). The significant geochemical contrasts between the occurrences of the granodiorites in different areas suggest that this unit corresponds in fact to a granodioritic suite of rocks derived from similar but distinct magmas. In spite of their broad geochemical similarities, granodiorites, intermediate rocks, and mafic enclaves show some significant differences in their REE patterns and in the behavior of Rb, Ba, Sr, and Y. The granodiorites and intermediate rocks are not related by fractional crystallization and the internal evolution of intermediate rocks were leaded by fractionation of amphibole + biotite ± apatite, whereas granodiorites evolved by fractionation of plagioclase + amphibole ± biotite. The layered rocks should have been derived from the granodiorite magma by accumulation of 50% of amphibole (dark layer) and 30% of amphibole ± plagioclase accumulation (gray layer). Modeling and geochemical data suggest that mafic enclave and granodiorite magmas were originated at different depths and should have mingled during their ascent and final emplacement and a limited interaction would explain the relatively uniform geochemical behavior of each rock variety and the distinct trends displayed by their rocks in different modal and geochemical diagrams. These contrasts between granodiorites and mafic enclaves are reflected in the behavior of the Sr and Y, which are generally seen as good indicators of the pressure of melt formation. The behavior of these elements, observed in different sanukitoid rocks from Archean terranes worldwide, indicates that the geochemical and modal contrasts observed between the granodioritic (granodiorites) and monzonitic (mafic enclaves) sanukitoid series are a general feature of these rocks and their origin is strongly dependent of the pressure of magma generation and, as a consequence, that the nature of the series could indicate the approximate depth of formation of its magma. The petrogenesis of the Rio Maria suite requires melting of a modified mantle extensively metasomatized by addition of about 30% TTG-like melt to generate the granodiorite (21% of melt) and intermediate magmas (24% of melt), and ~20% TTG-like melt in the case of mafic enclave magma (9% of melt). Our modeling results indicate that an active subduction tectonic setting was present in the Rio Maria terrane in between 2.98 to 2.92 Ga to generate the TTG magmas and the proposed metasomatism of the mantle by these magmas, before the melting process responsible for the origin of the sanukitoid magmas. A tectonothermal event at ~2.87 Ga, possibly related to a mantle plume, causing the partial melting of the metasomatized mantle and generating the Rio Maria sanukitoid magmas. In the rocks of the Rio Maria suite, the mineral assemblage is dominated by amphiboleplagioclase-biotite and epidote minerals, all of inferred magmatic origin, pyroxenes being notably absent. Textural and compositional criteria indicate that amphibole is a principal mineral on the liquidus of all the Rio Maria rocks. To derive crystallisation conditions, the phase assemblages, proportions and compositions of the natural rocks were compared with experimental works carried out on similar magma compositions. The comparison shows that the parental magmas were water-rich, with more than 7 wt% dissolved H2O near liquidus, with crystallisation temperature in the range 950-680°C. The Mg/(Mg+Fe) ratios of both amphibole and biotite indicate fO2 conditions in the range NNO + 0.5 up to NNO + 2.5, therefore pointing to both water-rich and oxidizing conditions for sanukitoid magmas. Analyses of amphibole aluminium content in cumulate rocks, indicate in addition a high pressure crystallisation stage, around 700-1000 MPa, prior to emplacement in the upper crust at around 200 MPa. Sanukitoid magmas share therefore two of the principal characteristics of modern arc magmas, elevated redox sate and volatile contents, which suggest that they may have formed in a geodynamic environment broadly similar to present-day subduction zones.