Petrogenesis of mica-amphibole-bearing lamprophyres associated with the Paleoproterozoic Morro do Afonso syenite intrusion, eastern Brazil

Abstract

Mica-amphibole-lamprophyres, identified as vogesites, are associated with the Paleoproterozoic Morro do Afonso syenite intrusion in northeastern Brazil. The lamprophyres occur mainly as dykes that crosscut the syenitic rocks and occasionally as enclaves. Lamprophyric rocks are formed by the early magmatic paragenesis amphibole-clinopyroxene-apatite-phlogopite-ilmenite; feldspars are found in the groundmass. Near liquidus amphibole is edenite, close to the boundary with pargasite, which is enriched in alkalis relative to the other amphiboles (Mg-hornblende and actinolite). Clinopyroxene is diopside, and inclusions of phlogopite are analyzed in both clinopyroxene and amphibole phenocrysts. The chemical evolution of the mafic minerals is consistent with increasing oxygen fugacity during late magmatic stages. Whole-rock geochemical data suggest a metaluminous, ultrapotassic parental liquid, with silica saturation close to the limit of undersaturation. Trace element concentrations, such as enrichment in large ion lithophile and strong depletion of some high field-strength elements, indicate a mantle source that was partially modified by a subduction event. In this metasomatic mantle, it is important to emphasize the strong enrichment of light rare-earth elements, which is higher than those typically associated with basaltic rocks from active continental margins, and corresponding concentrations similar to those determined in lamproitic rocks. Major element modeling shows that fractional crystallization and magma flow segregation are the main petrogenetic processes involved in the magmatic evolution of lamprophyre magma, and it is possible to generate syenite magma by these mechanisms.