Mineral chemistry, isotope geochemistry and petrogenesis of niobium-rich rocks from the Catalao I carbonatite-phoscorite complex, Central Brazil

摘要

The Catalao I carbonatite complex is part of the Late-Cretaceous Alto Paranaiba Igneous Province (APIP) and consists of a multi-intrusion body zoned from bebedourite-(pyroxenite)-series rocks at the border to carbonatite- and phoscorite-series rocks at the core. The phoscorites consist of apatite, magnetite, and a Mg-silicate (phlogopite or olivine) and are subdivided into early-stage; olivine-bearing (P1); and more evolved, olivine-lacking rocks, dominated either by apatite (P2) or magnetite (P3). P1 rocks are typical phoscorites, whereas the late-stage P2 and P3 rocks are petrographically classified as nelsonites (apatite-magnetite rocks) and host the Nb + P + Fe mineralization of the Catalao I Complex. Dolomite carbonatite (DC) occurs in association with both P2 and P3 late-stage phoscorite, forming paired phoscorite-carbonatite sets. Mica chemistry changes from phlogopite in phoscorite through phlogopite cores with tetra-ferriphlogopite rims in apatite late-stage phoscorite to tetra-ferriphlogopite in magnetite late-stage phoscorite and dolomite carbonatite, which is similar to the Al-depletion seen in micas in phoscorites from the Kovdor and Sokli Complexes. Apatite from phoscorites is enriched in Si, whereas those from late-stage phoscorites and dolomite carbonatites are enriched in strontium. Core chemistry from both apatite and phlogopite crystals shows a trend consistent with evolution from phoscorite to dolomite carbonatite. Umenites from phoscorite are Mg-rich while those from late-stage phoscorites and dolomite carbonatites tend to consist of nearly pure FeTiO3. REE patterns are highly fractionated and indicate that all rock types are cogenetic. The discrepancy in the radiogenic signature between phoscorites and carbonatites from Catalao I suggests that a petrogenetic process, probably related to fluids, can fractionate Nd isotopes, generating different compositions within the province.