Caracterização do maciço Santa Clara no município de Cujubim (RO) com base em litogeoquímica, geocronologia e estudos isotópicos

Abstract

The Santa Clara Intrusive Suite in the Rondônia Tin Province (SW Amazonian Craton) comprises the Santa Clara, Oriente Velho, Oriente Novo, Manteiga-Sul, Manteiga-Norte, Jararaca, Carmelo, Primavera and das Antas massifs. The rocks of the Santa Clara Intrusive Suite are emplaced in the Jamari Complex, an association of ortho and paragneisses which underwent several and complex metamorphic processes. Characteristics observed during both geological mapping and petrographic analyses allowed, for the first time, to subdivide the Santa Clara Massif (SCM) granitoids into five different facies: porphyry, equigranular, fine-grained facies, pyterlitic and wiborgitic facies. These lithotypes comprise hornblende-biotite granites and biotite granites that range from intermediate to acids, and show compositions similar to syenogranites and monzogranites. Geochemical analyses suggest that these granitoids may be divided into three different magmas, considering field aspects and geochemical characteristics. Therefore, the less evolved magma is represented by porphyritic and equigranular facies, and the most evolved magma comprises both fine-grained and pyterlitic facies. The wiborgitic facies representd the other magma, and is geochemically different from all the others facies. Geochemical analyses also show that the granitoids of the Santa Clara Massif are subalkaline, have very low MgO contents and have metaluminous to slightly peraluminous character. Trace elements patterns show that these granitoids have high contents of incompatible elements (Rb, Zr, Y, Ta, Ce) and REE, with exception of Eu. Moreover, they are also slightly enriched in LILE, strongly depleted in elements such as Sr and Ti, and slightly depleted in Ba, pointing out the importance of plagioclase and titanite fractioning during the evolution of these magmatic liquids. Negative anomalies of Nb, along with other geochemical features, suggest the participation of crustal material in the magmatic processes responsible for these granitoids generation. The lithotypes have typical characteristics of ferroan A-type granites, and FeOt/MgO ratios ranging from 4.27 to 26.22 indicate that these are fractionated felsic granites. REE patterns show a remarkable enrichment in LREE along with negative Eu anomaly. Tectonic discriminant diagrams for the Santa Clara Massif granitoids suggest that these are intraplate granitoids, A2-type, that is, related to post-collisional/post-orogenic settings.The isotopic characteristics observed for the Santa Clara Massif granitoids suggest that these were generated through partial melting of a preexistent lower crust. The U-Pb ages between 1,07 e 1,06 Ga are compatible with a magmatism taken place during the final stages of the supercontinent Rodinia agglutinations and the final stages of the Sunsás-Aguapeí Orogenic Cycle. It is also suggested that the Santa Clara Massif represents the coalescence of three different granitic intrusions, which comprise the magmas described above.