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The Palaeoproterozoic Rio Itapicuru Greenstone Belt is a group of volcano sedimentary rocks which host two major gold districts, Fazenda Brasileiro and Fazenda Maria Preta. It is subdivided into three sequences: a basal mafic volcanic unit, overlain by a felsic volcanic unit, and covered by a clastic sedimentary sequence on top. The felsic volcanic sequence is the subject of this study. It is comprised by basalt-andesites, andesites, dacites interleaved with pelitic and chemical sediments with rare pyroclastic rocks, intruded by small subvolcanic dioritic bodies.
The volcanic rocks are porphyritic with hornblende and plagioclase phenocrysts. They are subdivided into two distinct groups, one with adakitic geochemical signature (high Sr/Y, Sr∼900 ppm, Y∼5ppm, (La/Yb)n= 8.30, K2O ∼ 1.8 ppm, Na2O ∼ 4.5), and the other is a classical calc-alkaline arc lavas, with negative Eu and Sr anomalies. The calc-alkaline rocks also have higher values of incompatible elements, such as Y, Yb, Zr and Th and lower values of LILE than the adakites. For the adakites, a slab melt source model is suggested, evidenced by enrichment of LILE relative to HFSE, with garnet + amphibole as the residual phase. The calc-alkaline volcanics can be interpreted as mantle partial melts metasomatised not only by aqueous fluids released from the subducting slab but also by slab melts as evidenced by their slightly higher #Mg, Cr and Ni values. Low pressure fractional crystallization of plagioclase is required to account for the Eu and Sr negative anomalies in the calc-alkaline lavas.
Geochemical modeling shows that the two felsic lava groups cannot be linked to each other by fractional crystallization, although hornblende fractionation occurred in the adakites, and plagioclase in the calc-alkaline.
U-Pb SHRIMP dating indicates a crystallization age of 2,081 ± 9 Ma for the adakitic volcanics. Positive εNd values (2.17-1.82) and inherited zircons (3364, 3017, 3064 Ma) evidence a magma generated at a subduction zone followed by interaction with Archean lithosphere, probably the basement of the Rio Itapicuru Greenstone Belt, hence suggesting a continental margin arc for the origin of the felsic volcanic unit.
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