Origin and Intensive Parameters in the Crystallization of Ultrapotassic Syenites: The Serra Do Man Pluton, Northeastern Brazil

Abstract

The late Neoproterozoic ultrapotassic Serra do Man pluton intruded supracrustal schists and igneous rocks of the Alto Pajeu tectonostratigraphic terrane, in the Borborema province of northeastern Brazil. This quartz-alkali feldspar syenite is composed of microcline, albite, normally zoned aegirine-augite, winchite (formed after pyroxene), titanite and apatite, with the major-element geochemistry in consonance with early fractionation of pyroxene-titanite-apatite assemblage. Chondrite-normalized REE-patterns are LREE-enriched (La/Lu ∼ 20) with discrete negative Eu anomaly, and primitive mantle-normalized elemental concentration diagrams display enhanced negative Nb, P and Ti anomalies that are also observed in patterns of similar rocks in NE Brazil. Whole-rock d 18O values are rather high (avg. +8.7% SMOW) and in the +7.8 to +8.6% SMOW range when clinopyroxene-corrected. A five-point whole-rock Rb-Sr isochron yields an age of 580±28 Ma with high Sr o (0.7108), which together with eNd (0.58 Ga) value of -17 and t chur of 2.3 Ga allow hypothesizing a Paleoproterozoic enriched-mantle source for the syenites in the ultrapotassic province of the Alto Pajeu terrane. The aegirine-augite, titanite, and quartz assemblage attests to a high oxygen fugacity prevailing during magmatic crystallization. The almost complete absence of primary hydrated phases assures that the magma was rather anhydrous, and its viscosity, at the liquidus temperature, was relatively high. Density is slightly higher than expected for syenitic magmas (2.8 g/cm 3), being explained by a relatively high iron content of the magma. Geothermobarometry suggests shallow emplacement and temperatures of 700-750°C and 850-900°C, respectively, for the solidus and liquidus of the magma.