Mineral chemistry and crystallization conditions of granites emplaced along the Remígio-Pocinhos Shear Zone, NE Brazil: Pilõezinhos Pluton

Abstract

The Pilõezinhos Pluton (PP). comprises a ~100 km² -elongated intrusion in the NE part of the Transversal Domain of the Borborema Province. It is composed of monzogranites and syenogranites, subdivided into two distinct petrographic facies. The PP intrudes the boundary between the dextral Remigio - Pocinhos and sinistral Matinhas shear zones. The main objective of this work is to characterize the conditions of crystallization from the PP granites. The main mineralogy was characterized by petrography and Electron Microprobe mineral chemistry analysis. It consists of quartz; alkali feldspars (microcline and pertite); plagioclase, ranging from oligoclase to andesine; siderophillite-rich biotite; and amphibole, with composition ranging from hastingsite to Fe-tschermakite. The accessory mineralogy comprises titanite, zircon, apatite, oxides, dominantly ilmenite, and allanite grains holding large amount of REE. The mineral chemistry defined that the magma crystallization occurred at temperature ranging from 745 to 1003 °C, defined from the amphibole – plagioclase geothermometer and Zr geothermometer, and pressure from 4.28-5.98 kbar (geobarometer of Al^T in hornblendas). Iron-rich mafic mineralogy and the presence of primary ilmenite suggest crystallization under low fO₂ conditions, below the FMQ buffer. Rebalancing in subsolidus conditions occurs due to the action of late fluids, with temperatures ranging from 505 to 566 ° C, is suggested by the presence of sericite, chlorite and myrmekite. The mineralogical, chemical and structural features suggest that the PP granites represent A₂ -type magmatism, which evolved by fractional crystallization.