Lithogeochemistry and geochronology of the migmatitic orthogneisses from the Itabaiana Dome, Sergipe, Brazil: a typical tonalite–trondhjemite–granodiorite suite?

Abstract

The Itabaiana Dome is inserted in the Vaza Barris Domain of the Sergipano Belt, in the state of Sergipe, Brazil. It comprises an gneissicmigmatitic complex composed of migmatized orthogneisses, with interbedded amphibolite levels. The set was reworked and uplifted during the Neoproterozoic deformation process this range. Petrographic studies show that the orthogneisses are tonalite to granodiorite composition, with features from the protomylonitics to mylonitic. The lithogeochemistry shows a strong sodic characteristic of these rocks, which were analyzed in two different group, considering their rare-earth elements (REE)’s pattern and K₂O content. The lithotypes with the contents of K₂O < 2,5% have a term of calcium alkaline series of low K to normal calcium alkaline. However, the ones with content K₂O ≥ 2.5% consist in placed terms in the interface of a normal calcium alkaline series, with a high K calcium alkaline series. The two groups pose a negative anomaly of Th-U, Ta-Nb and Ti with low content of Y. They exhibit a strong fractional pattern, with light rare earth enrichment and depletion of heavy rare earths. Only one group shows a positive anomaly of Sr and the spectrum concavity of heavy rare earths, which are typical features of the Archean tonalite–trondhjemite–granodiorite — Archean TTGs. It is suggested that both has affinity with TTGs and the group that is more enriched in K₂O received a type of crustal participation in its gneiss. The geochronological analyses determined age of 2729 ± 12 Ma (LA-ICP-MS), which is translated as the age of the rock crystallization of orthogneisses.