Petrologia experimental e química mineral das suítes Neoarqueanas Vila Jussara e Planalto, Província Carajás, Amazônia, Brasil

Abstract

In the Carajás Province (CP), during the late stages of the neoarchean (2.75-2.73), in the Sapucaia and Canãa dos Carajás domains granitoids represented by the Vila Jussara and Planalto suites were formed respectively. These suites are metaluminous and have a geochemical affinity with A-type granites and FeO/(FeO+MgO) whole rock ratios that vary from ferroan to magnesian. Chemical-mineralogical studies performed in these neoarchean granites, based on optical microscopy, scanning electronic microscopy, electron microprobe and experimental petrology revealed notable variation between the main mineral phases. Magmatic epidote is a common mineral phase in the reduced, oxidized and magnesian varieties of the Vila Jussara suite, with pistacite contents between 25 and 30% mol. In the Planalto Suite and others neoarchean granites of the CP magmatic epidote is absent. The study of the dissolution kinetics of Archean epidotes of the CP reveals that its formation and stability are directly linked to pressure, temperature and oxygen fugacity conditions, however, its stability is also conditioned by mechanisms of magma rise, emplacement and crystallization, which affected the dissolution intensity of the epidote crystals. Chemical-mineralogical performed on titanites of the Vila Jussara and Planalto Suites and the paleoproterozoic Jamon Suite revealed notable textural and compositional variations. The titanite’s Fe/Al ratios are quite variable in the studied granites, three major titanite groups were distinguished: 1) high Fe/Al ratio (Fe/Al>0.5); 2) moderate Fe/Al ratio (0.5≤Fe/Al≥0.25); and 3) low Fe/Al ratio (Fe/Al<0.25). Furthermore, in general, the obtained data support the major titanite tendency to stabilize under oxidizing conditions, near the nickel-nickel oxide buffer (NNO), however, the occurrence of magmatic titanite in the reduced varieties from the Planalto and Vila Jussara Suites ratifies that its crystallization is possible under conditions near the fayalite-magnetite-quartz buffer (FMQ). Moreover, experimental petrology studies performed on the same neoarchean suites, reveal that the sample MDP-02E, with tonalitic composition, which represents the magnesian oxidized magma from the Vila Jussara suite, exhibits SiO2 content around 60% in the whole rock composition and 61.05% in the experimental glass, while the sample with sienogranitic composition (AMR-116), from the strongly reduced variety of the Planalto Suite, shows SiO2 content of 74.13% in whole rock and 73.17% in glass, pointing out that the initially calibrated experimental conditions approach the natural magmatic conditions. To characterize the crystallization parameters of the tonalitic and sienogranitic magma, nine experiments were performed on the two samples, with the following conditions: pressure ~4 kbar, ƒO2 ~NNO-1.3 (1.3 log unit bellow the NNO buffer) and temperature varying from 850°C to 668°C and water content from 9% to 6% in weight. Experiments with ƒO2 ~NNO+2.4 and temperatures of 800°C to 700°C with similar pressures and water content variations of the reduced experiments were also performed. Subordinately, experiments with pressures of 8 to 2 kbar with variable redox conditions were performed. These experiments show that the tonalite from the Vila Jussara Suite was crystallized at ~4 kbar from a water-rich magma (>5% in weight) in ƒO2 oxidizing conditions, probably between NNO and NNO+1. On the other hand, the experiments performed on the sample with sienogranitic composition from the Planalto Suite show a main paragenesis of Cpx+Fa which substantially diverge from the natural minerals, suggesting that the experiments did not reach the natural conditions.