Revaluation of the crystallization conditions of ediacaran alkaline granites in the Rio Piranhas-Seridó and São José do Campestre Domains, Borborema Province, NE-Brazil

Abstract

The crystallization conditions (P, T, ƒO₂) of alkaline granites (A-type) within the Rio Piranhas-Seridó and São José do Campestre domains (NE Borborema Province) were revaluated based on the simultaneous application of multiple geobarometers, geothermometers and crystallization modelling. The studied bodies are the Caxexa, Serra do Algodão, Serra do Boqueirão, Olho D’Água, Japi, and Serra Negra do Norte plutons, and the Flores stock. The results show emplacement pressures between 3.7 and 3.7 kbar (~11–14 km at depth), except for the Japi pluton, with an average pressure of 5.3 kbar (~20 km at depth). Crystallization modelling yield liquidus temperatures in the range between 885–917 and 818ºC for the Flores stock, and solidus between 660–700ºC. Aegirine-augite, magnesio-ferri-hornblende and biotite crystallize between 813–822, 728–751, and 750–789ºC, respectively. Among the accessories, the geothermometer Zr-in-titanite yields temperatures of about 780ºC (maximum of 830ºC) for the Caxexa pluton. Crystallization took place under essentially oxidizing conditions for most of the plutons (+1.0 < Δ_QFM < +2.1), except for the Flores stock, formed under slightly more reduced conditions (Δ_QFM ≈ 0.0). Plutons related to regional shear zones are alkali-richer and, on average, deeper, reinforcing the role of structural control on the emplacement and chemical contrast among these granites. The new estimates are more accurate and contribute to the better understanding of the evolution of the granitic alkaline magmatism within the setentrional portion of the Borborema Province.