Serra Verde granitic stock: a calc-alkaline granitic magmatism in the Rio Grande do Norte Domain, NE, of Borborema Province

Abstract

The Serra Verde granitic pluton is located in the central-eastern portion of the Seridó belt, intruded into metasedimentary and orthogneiss rocks of the Seridó Group and Caicó Complex respectively. It exhibits a geometry en cornue, elongated in the NESW direction, and was emplaced during the brasiliano orogeny. Its composition is monzogranitic with fine and homogeneous texture, whose dominant mineral paragenesis (more than 80% modal) consists of K-feldspar, plagioclase and quartz. The main mafic mineral is biotite, whereas amphibole, titanite, epidote, opaque, allanite, zircon and apatite represent the accessory mineralogy. We also characterized a late-magmatic paragenesis consisting of chlorite, granular epidote, carbonate and muscovite, related to CO2 and H2 O rich fluids. Chemically, the rocks of the Serra Verde Pluton can be classified as metaluminous to peraluminous, of calc-alkaline affiliation, with high contents of Na2 O (> 4.2%), Sr (> 400 ppm) and Ba (> 700 ppm), and low levels K2 O (≤ 3.7%), TiO2 (< 0.3%), Rb (< 80 ppm) and Y (≤ 28 ppm). Micropetrografic evidences (mineral paragenesis and microtexture) indicate that the evolution of the magma occurred in conditions of moderate to high fO2 , above of FMQ buffer. Thermobarometric data obtained from the geochemistry of minor and trace elements, as well as CIPW normative data, suggest a final crystallization pressure (minimum) for the Serra Verde Pluton around 3-5 kbar with the liquidus temperature around 800°C while the solidus temperature is between 700-660°C. These data are consistent with those obtained by several authors in various other Neoproterozoic granitoids from the Rio Grande do Norte Domain. Based on the analysis of petrographic and geochemical data, a crustal magma source for the Serra Verde pluton is suggested. The quartz-dioritic to tonalitic orthogneiss from the basement complex, are likely candidates for this source. Regarding to other calcium alkaline plutons from the Rio Grande do Norte Domain, the Serra Verde pluton is more enriched in SiO2 and Na2 O and more depleted in Fe2 O3 (T), MgO and TiO2