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

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 melti...