Chemical-memory of biotite crystals preserves the temperature and pressure conditions of the mixing process between trachytic and rhyolitic magmas in the Glória Sul stock, Sergipano orogenic System, Northeast Brazil

Abstract

Biotite is an important constituent in the microgranular syenitic enclaves of the Glória Sul stock, located in the central region of the Sergipano Orogenic System, Northeast Brazil. These enclaves are centimeter-sized, rounded, and ellipsoidal, finer-grained than the surrounding granites. They are composed of microcline, orthoclase, albite-oligoclase, diopside, quartz, hornblende, titanite, apatite, zircon, epidote, magnetite, ilmenite, thorite, and pyrite. The textures present in the enclaves indicate mixing between magmas and rapid cooling of the trachyte magma that formed the microgranular enclaves (fine-grained texture, acicular apatite and zircon, blade-shaped biotite). The studied biotite exhibits a magmatic texture and is chemically classified as Fe- and Mg-biotite, particularly those with TiO2 > 2.3%. The chemical evolution of biotite is controlled by two substitutions (VI R2+ + 2 VI Al3+ ═ VI □ + 2 VI Si4+ and VI R2+ + VI Ti ═ 2 VIAl3+) reflecting a decrease in titanium and an increase in aluminum. The chemical data of biotite crystals indicate that they crystallized from I type magma, with calc-alkaline affinity, suggesting that the rocks of Gloria Sul stock were formed by the interaction between mantle- and crustal-derived magmas. Thermobarometry of magmatic biotite (enclaves and granites) provided crystallization temperatures between 900 and 800 °C and a depth of 36 to 13 km. The interaction between trachytic (enclave) and rhyolitic (granite) magmas in Glória Sul stock at occurred at about 26 km depth and at a temperature about 800 °C.