Abstract

BALLESTEROS-CAMARO, Carlos Andrés  and  RIOS-GUERRERO, Johanna. Mineral chemistry of biotites in gneisses from the Rio Urubú Metamorphic Suite outcropping in the Serra Repartimento, Central Guyana Domain in the Amazonian Craton, Brazil: petrogenetic implications. bol.geol. [online]. 2019, vol.41, n.1, pp.97-116. ISSN 0120-0283.  https://doi.org/10.18273/revbol.v41nl-2019005.

Orthogneisses of the Paleoproteorozoic basement from the Rio Urubú Metamorphic Suite (RUMS), related to the post-transamazonic event, are widely distributed in the Central Guyana Domain (CGD) in the Amazonian Craton, northeastern Brazil. In this study, it was investigated the petrography, mineralogy and chemistry of biotites within the biotite-hornblende gneisses from the RUMS. It was also determined the formation conditions of these biotites and discuss the significance of these minerals in terms of petrogenesis. It can be classified through the results of petrography and mineral chemistry biotites into two distinct groups: 1) primary biotite classified as Bt-IA, less deformed, igneous appearance preserved and with no mineral orientation; 2) primary biotite with a tendency to re-equilibrated biotite, classified as Bt-IB, with low to moderate evidence of deformation and showing the main foliation of the rock and the gneissic structure, and being consistent with the tectono-metamorphic processes associated with shear zones that affected the CGD. Microprobe data show that the total Mg contents in less deformed biotite (Bt-IA) are higher, classified as Mg-biotites, while the deformed biotite (Bt-IB) presents higher Fe content and are classified as Fe-biotites. Results for strictly primary biotite indicate that the rocks of the RUMS are associated to granites originated from an I-type magma and calc-alkaline geochemical affinity, where these biotites crystallized at temperatures (T) de 720°C - 760°C, with oxygen fugacity (fO2) conditions of -12.71 and -13.66, Fe/(Fe + Mg) ratios ≥ 0.50, showing crystallization for these rocks around the NNO buffer, suggesting reducing conditions for the parental magma and also indicating a typical crust source for the magmas that contributed to the formation of these granites.

Keywords : Rio Urubú Metamorphic Suite; Mineral chemistry; Biotite; oxygen fugacity; Parental magma.

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