Química mineral de cristais de biotita em enclaves sieníticos em granitos do Stock Granítico Glória Sul, Domínio Macururé, sul da Província Borborema

Abstract

Biotite is an important constituent in the syenitic microgranular enclaves hosted in granites of the Glória Sul Stock, located in the central region of the Sergipano Orogenic System. These enclaves are centimetric, rounded and ellipsoidal, have finer grain size than the surrounding granites and are composed of microcline, orthoclase, albite-oligoclase, diopside, quartz, hornblende, titanite, apatite, zircon, magnetite, ilmenite, and pyrite. The textures present in the enclaves imply to rapid cooling (fine-grained, acicular apatite and zircon and zonation in plagioclase). The chemical data allowed the brown mica from these enclaves to be classified as Mg-biotite and Fe-biotite. The compositional evolution of the investigated biotite crystals is essentially controlled by two substitutions (VIR2+ + 2 VIAl3+ ═ VI□ + 2 VISi4+ and VIR2+ + VITi ═ 2VIAl3+) that reflect aluminum and titanium decrease. The biotite chemistry of the enclaves indicates magma crystallization of the magnetite series of calcium-alkaline nature. The chemical signature of the crystals studied also indicates that the magma that formed it has a complex evolution and results from the mixture between mantle (trachytic) and crustal (rhyolitic) magma. Thermobarometry indicated that crystallization of biotite in the syenitic microgranular enclaves occurred at approximately 6.3 km between temperatures 788-975 °C. The similarity of the thermobarometric data of biotite from the enclaves and the surrounding granites suggests that these values correspond to that of the physical conditions of the interaction between trachytic (enclaves) and rhyolitic (granite) magmas. The petrogenesis established for the enclaves are coherent with the collisional environment established for the Sergipe Orogenic System.