Banca de DEFESA: RENATO CARLOS VIEIRA SANTIAGO

LITHOGEOCHEMISTRY, GEOCHRONOLOGY AND TECTONO-METAMORPHIC EVOLUTION OF MIGMATITIC ORTHOGNAISSES OF THE DOMO OF ITABAIANA, SERGIPE

Itabaiana dome; orthogneisses; Archean TTGs; geochronology; Mesoarchean

The Itabaiana Dome is a geological unit that represents the outcrop of the Archean basement in the southern portion of the Borborema Province. It is surrounded by lithotypes of the Vaza Barris Domain, one of the lithostratigraphic units of the Sergipan Orogenic System (SOS). This SOS unit has been the object of study for more than 50 years of geological research in the area. It is referred to on geological maps as a gneissic-migmatitic complex formed by migmatized orthogneisses, with rare intercalations of amphibolitized mafic levels. According to older work, the complex was reworked and uplifted during the Neoproterozoic deformation process that established the SOS. Previous petrographic studies have characterized these orthogneisses as rocks that underwent different stages of deformation. They present both preserved primary features, such as a banded microstructure typical of orthogneisses and migmatitic features (which reflect various stages of anatexis), and microstructures associated with ductile-brittle shear processes. In these rocks, microstructures of mylonite and other more specific ones, such as augen and flaser, are present. They are essentially composed of K-feldspar (microcline), plagioclase, quartz, biotite, and/or hornblende, besides garnet, titanite, apatite, and zircon as accessory terms. Chlorite and epidote eventually appear as secondary minerals. The modal compositions of the samples, when plotted on the QAP diagram, appear predominantly in the tonalite and granodiorite fields. According to the chemical analyses performed on 20 samples, the migmatitic orthogneisses are sodium-rich rocks (K2O/Na2O < 0.5) typically belonging to the normal calcium alkaline series, with some samples tending toward the high-potassium calcium alkaline series. The analyses revealed the existence of two groups distinct from each other, mainly with respect to K2O contents (Type I: K2O > 2.5; Type II: K2O < 2.5). According to analysis of the REE patterns and multi-elemental diagrams, the two groups have significant negative anomalies of Th-U, Ta-Nb, and Ti, in addition to low Y contents, presenting a strongly fractionated pattern, with enrichment in LREE and depletion in HREE. Only one group shows positive Sr anomaly and concavity in the HREE spectra, also features typical of Archean TTGs. It is suggested, then, that both have an affinity with TTGs and that the group more enriched in K2O (Type II) suffered some kind of crustal participation in its genesis. Some geochronological analyses were initially made, which showed that the origin of these rocks was associated with magmatic and tectonic-metamorphic events that occurred during the Mesoarchean. The initial geochronological analyses provided two U-Pb ages in zircon (LA-ICP-MS) of 2729 ± 12 Ma and 2737.4 ± 5.6 Ma, both made in the clear portion (leucosome) of migmatitic orthogneiss samples. And a U-Pb age in zircon (SHRIMP) of 2878 ± 11 Ma was made in the dark portion of migmatitic (paleosome) orthogneiss sample. The older age was interpreted as an age of protolith crystallization. The younger ages evidence a reworking/migmatization of these rocks during a tectonothermal event in the Mesoarchean. A geochronological detailing done later corroborated the idea that in the period between 2.9-2.8, there would have been a large high-grade tectonometamorphic event that gave rise to these rocks. In addition to this, we found the oldest rock in the SOS. This is a basic granulite (metagabronorite), which showed U-Pb zircon ages (SHRIMP) around 2933 Ma. These ages were compared with ages found in other possibly related geological units, such as the Santa Luz and Uauá Complex, which represents the northeastern portion of the Serrinha Block and the Ntem Complex, which represents the Archean basement in the northern portion of the Congo Craton. This comparison was also made with some geochemical data. The similar behavior of some samples and also the similarity in some ages suggest that the Itabaiana Dome and the Ntem Complex constituted a single continental mass during the Mesoarchean. The thermobarometric conditions associated with the origin of these rocks were verified by a mineral chemistry study using conventional geothermobarometric methods and P-T pseudosection. Using the integrated geothermobarometer, temperatures between 650.9 - 690.7 °C and pressures between 6.08 - 7.42 kbar were calculated. The P-T pseudosection shows a range of 680-750 °C and 4.5-8.0 kbar. The estimated P-T conditions for migmatitic orthogneisses are consistent with those of solidus with fluid presence for tonalitic assemblages. This condition may be associated with migmatization of these rocks during the tectono-metamorphic events between 2.8 and 2.7 Ga.