Mineralogy of Lamprophyres and Mafic Enclaves Associated with the Paleoproterozoic Cara Suja Syenite, Northeast Brazil

Abstract

The Paleoproterozoic Cara Suja Syenite in northeastern Brazil is composed mainly of coarse-grained silica-saturated syenites with minor mafic rocks, such as syenitic autoliths and mafic micro-granular enclaves. Lamprophyric and hybrid dikes occur in this massif and crosscut the volcano-sedimentary country rocks. Syenitic autoliths are medium-grained syenites associated with mafic mineral segregations in the host syenites. Mafic microgranular enclaves are elliptic or rounded, finegrained mesocratic syenites. Lamprophyric dikes are porphyritic rocks with large biotite phenocrysts and amphibole agglomerations in a fine-grained groundmass consisting mostly of alkali-feldspar. Hybrid dikes contain abundant feldspar xenocrysts. Petrographic and field relations indicate mingling of syenitic and mafic magmas. Lamprophyres, autoliths, and mafic microgranular enclaves, as well as the host syenites, have an alkaline potassic-ultrapotassic affinity and a trace-element signature of magmas produced by the partial melting of a subduction-modified mantle. Mineral chemistry data show preserved clinopyroxene compositions only in syenitic autoliths. They are Al2O3- and TiO2-poor diopsides with slight variation in wollastonite contents (44 to 50 wt%), generally rimmed by late- or post-magmatic actinolite. The amphibole of lamprophyres and mafic microgranular enclaves is Mg-hornblende, characterized by high Si (6.95 to 7.49 atoms per formula), high F (up to 1.13 wt%) and low TiO2 (0.22 to 0.50 wt%). Micas are Mg-biotite and phlogopite with large amounts of opaque oxides and titanite inclusions. Phlogopite of lamprophyres, of mafic microgranular enclaves, and of autoliths has very similar compositions, with Si values around 5.9 atoms per formula, Mg/(Mg+Fe+2) ratio ranging from 0.65 to 0.70, and with low TiO2 (below 1.1 wt%) and high F (2.1 to 4.2 wt%) contents. The general overlap and compositional similarities of mafic minerals from lamprophyres and mafic microgranular enclaves indicate that they are co-magmatic and evolved under similar conditions. The textural features and composition of Mg-hornblende and phlogopite in these rocks suggest that they are primary phases re-equilibrated under crustal conditions during magma ascent.