Mineral chemistry and crystallization conditions of Granitic Barcelona Pluton, extreme northeast of the Borborema Province, Northeastern Brazil

Abstract

The Barcelona Granitic Pluton (BGP) is located at the extreme northeastern portion of the Borborema Province, west of São José do Campestre Domain, and is presumed to be older Ediacaran. The BGP is formed by three distinct petrographic/textural facies, the dominant porphyritic granite facies, from the examination micropetrographic, associated with mineral chemistry analyses. The porphyritic granite facies (biotite monzogranite) has plagioclase (oligoclase with An_25-20%), K-feldspar (perthitic microcline) and quartz always >70 % modal. The amphibole is a hastingsite with moderate Mg/(Mg + Fe) ratios, crystallization pressures between 5,0-6,0 kbar, and 700°C temperature, ending with 3,0-5,0 kbar and 500°C in the ternary granite eutectic. The Al^T content of the amphibole, the pistacite values Ps = 28 in the epidote, and the magnetite martitization process are indicative of a moderate to high ƒO₂ environment. The biotites have a trend with a slight tendency to the annite pole, denoting rebalancing in a subsolidus/ tardi-magmatic stage. In magmatic series’ discriminant diagrams the biotites behave like those of sub-alkaline geochemical affinity, consistent with the medium to high K calc-alkaline of porphyritic granites. Plagioclase, K-feldspar and allanite crystals zonation is indicative of the importance of the fractional crystallization process in the evolution/crystallization of the porphyritic facies of this pluton. The action of fluids rich in CO₂ and H₂ O promotes late transformation of biotite and feldspars, as well as generation of myrmekites. The presence of aggregates of small myrmekite crystals of plagioclase with contact angles of 120° indicates eutectic equilibrium crystallization.