Origem dos megacristais de K-feldspato nos Plutons Barcelona e Monte das Gameleiras, RN

Abstract

The Barcelona and Monte das Gameleiras Plutons, located in the northeast portion of the Borborema Province, São José do Campestre Domain, are characterized by the presence of three facies, with a large domain of porphyritic granites with high-K calc-alkaline chemical signatures. The porphyritic granites of these plutons are marked by the presence of microperthitic K-feldspar megacrysts with a variety of mineral inclusions defining a concentric zonation. This work presents a qualitative and quantitative textural characterization, and mineral chemistry of major, minor and trace element for the K-feldspar megacrysts from the porphyritic granites, as well as the detailed petrographic description of its main inclusions. The results point to an extensive and relatively slow crystallization and cooling interval, which excludes the megacrysts formation by subsolidus processes. The zoning of the megacrysts is marked by the variation of the mineral inclusions amounts of the nuclei to the crystalline edges, as well as by the variation in Na2O (0.54 and 1.15 wt.%). The analysis of the trace elements also reveal variations in Ba (2871.8 and 4406.4 ppm), Rb (261.5 and 329.2 ppm) and Sr (273.4 and 420.2 ppm). These features reflect the concomitant crystallization of plagioclase, quartz and other phases that influence the partitioning of these elements. Quantitative textural analysis using the crystal size distribution (CSD) technique indicates patterns of textural coarsening in both plutons, as well as accumulation for the Barcelona Pluton, and fractionation for the Monte das Gameleiras Pluton.