Amphibole crystallization as a tool for monitoring complex mixing between basaltic and latitic magmas: The Fazenda Lagoas monzonitic stock, Borborema Province, Brazil

Abstract

Amphibole is the dominant mafic in the quartz monzonite and microgranular dioritic enclaves of the Fazenda Lagoas stock (623 ± 5 Ma), located in the Macururé Domain, in the central region of the Sergipano Orogenic System. Structures and textures in the rocks studied indicate the presence of several pulses of basaltic magma (diorite enclaves) in the latitic magma (quartz monzonites). The magmatic amphibole crystals correspond to pargasite, edenite, tschermakite and Mg-hornblende. The chemical evolution of amphibole crystals occurs through an increase in Si, Mg, vacancy in the A site, and a decrease in Al_total, K, Na, Fe³⁺ Ca, Ti (substitutions of edenite, tschermakite, and pargasite). Reduction in temperature, pressure, water solubility in magmas, and increase in oxygen fugacity indicate the evolution of the Fazenda Lagoas plutonic system. Amphibole thermobarometry made it possible to calculate the depth and temperature of the earliest interaction between basaltic and latitic magmas at a depth of 27 km, with latitic magma temperatures between 967 and 940 °C. Subsequently, there were several contributions of basaltic magma until the magma chamber at Fazenda Lagoas was ca. 17 km deep. Fazenda Lagoas stock is one of the shoshonite intrusions of the Macururé Domain, the one that has a record of the deepest hydrated basaltic magma, probably at the base of the ancient continental crust of the Sergipano Orogenic System.