U-Pb e Sm-Nd em Ortognaisses Riacianos e Estaterianos nos Terrenos Rio Piranhas-Seridó e Jaguaribeano, Província Borborema, Brasil

The Portalegre shear zone separates the Rio Piranhas-Seridó and the Jaguaribeano geological terranes in the Northern Domain of the Borborema Province. Banded gneisses of the Caicó and the Jaguaretama Complexes represent the basements of these terranes. U-Pb analyses yielded ages of 2,193 ± 16 Ma for the Jaguaretama Complex, which is similar to those obtained for the Caicó Complex. T DM Sm-Nd ages for these basement complexes showed Neoarchean values (ca. 2.5 Ga), with εNd(t) values close to the CHUR composition. Elongated bodies were found intruding into the rocks of the Jaguaretama Complex. These foliated bodies with elongated phenocrysts are described as augen gneisses of the Serra do Deserto Suite. These gneisses yielded crystallization ages of 1,777 ± 8 Ma, confirming an extensional period during the Statherian, which is well represented in the Jaguaribeano Terrane by A-type plutonic rocks. Augen gneisses with similar textures, the so called Poço da Cruz Suite, are found intruding the Caicó Complex in the Rio Piranhas-Seridó terrane; however, the ages found throughout the domain are predominantly Rhyacian (ca. 2.2 Ga) and present calc-alkaline affinities. These findings indicate that the augen gneisses, which are widely distributed in these two terranes, have different signatures and are temporally well separated. Furthermore, in agreement with previous geophysical data suggesting a division of terranes marked by the Portalegre Shear Zone, it also demonstrates that even though the Caicó and the Jaguaretama Complexes have similar ages, similar structural evolution and metamorphism under similar pressure and temperature conditions, they exhibit marked differences in composition.A zona de cisalhamento Portalegre separa os terrenos geológicos Rio Piranhas-Seridó e Jaguaribeano do Domínio Setentrional da Província Borborema, com gnaisses bandados dos Complexos Caicó e Jaguaretama representando os respectivos embasamentos destes terrenos. Análises U-Pb forneceram idades de 2.193 ± 16 Ma para os ortognaisses do Complexo Jaguaretama, resultados muito similares às determinadas para o Complexo Caicó. Idades T DM Sm-Nd nesses Complexos forneceram valores neoarqueanos (ca. 2,5 Ga), com εNd(t) muito próximos ao CHUR. Intrudindo as rochas do Complexo Jaguaretama, destacam-se corpos alongados, foliados e com pórfiros estirados, denominados de Suíte Serra do Deserto. Estes forneceram idades de cristalização a 1.777 ± 8 Ma, ratificando um período extensional que ocorreu no Estateriano, bem representado no terreno Jaguaribeano por sequências vulcanossedimentares e plutônicas Tipo-A. Augen gnaisses com texturas similares, denominados de Suíte Poço da Cruz, são encontrados intrudindo o Complexo Caicó no terreno Rio Piranhas-Seridó, porém, as idades registradas por todo este domínio são predominantemente Riacianas (ca. 2,2 Ga) e de afinidades cálcio-alcalinas, indicando que estes augen gnaisses possuem diferentes assinaturas e estão bem separados no tempo. Além disso, além dos dados geofísicos prévios que sugerem esta divisão de Terrenos marcada pela Zona de Cisalhamento Portalegre, os Complexos Caicó e Jaguaretama exibem diferenças composicionais marcantes. Corpos de anfibolitos alongados e concordantes a subconcordantes com o bandamento do Complexo Caicó e importante contribuição sedimentar na constituição do Complexo Jaguaretama com importantes intercalações de mármores são algumas destas diferenças

Neoarchean, Rhyacian and Neoproterozoic units of the Saquinho region, eastern Rio Piranhas-Seridó domain, Borborema Province (northeastern Brazil): implications for the stratigraphic model

Aeromagnetic, petrographic and isotopic data coupled with information obtained from a deep stratigraphic hole (440 m), shallow drillings (<100 m), and field data from the surroundings of the Saquinho iron mine in the Rio-Piranhas-Seridó domain of the Borborema Province, northeastern Brazil, allowed to identify a Neoarchean unit while providing a better understanding of the stratigraphy of the Jucurutu Formation, the host of the iron mineralization, and its basement. The drill hole allowed differentiating three compartments (upper, intermediate and lower). The upper compartment consists of paragneisses/schists, marbles and banded iron formations (top to bottom, respectively) correlated to the Jucurutu Formation (Seridó Group). The marbles provided δ13C and δ18O values ranging from 2.4 to 10.4‰ and from -9.7 to -5.3‰, respectively, which are compatible with those of marine-glacial carbonates and correlated to those of the Jucurutu Formation. The intermediate compartment consists of amphibole-bearing biotite augen gneiss, varying in composition from alkali feldspar granite to syenogranite, with U-Pb zircon dating (LAMMC-ICPMS) of 2210 ± 13 Ma, correlating it with the Rhyacian augen gneisses described in the literature. Finally, the lower compartment consists of a sequence composed mainly of microaugen gneisses, fine banded gneisses and metamafic-metaultramafic rocks (Saquinho Complex). The U-Pb zircon dating (LAM-MC-ICPMS) provided Neoarchean ages of 2512 ± 3 Ma and 2501 ± 3 Ma, respectively, for microaugen gneisses and metamafic-metaultramafic rocks, which were considered as crystallization ages of the protoliths. In the lower part of the drill hole, disseminated sulfides (pyrite, chalcopyrite, and more rarely pyrrhotite) occur, especially in the metamafic-metaultramafic lithotypes, which added to the Neoarchean age of the lithotypes, may represent new economic/metallogenic targets for the Seridó region. The geological model proposed for the iron formations of the Saquinho area involves the interaction of fluids with the Neoarchean mafic rocks, which percolated brittle and/or ductile/brittle structures