Temporal Emplacement Sequence of the South Nam Xe Carbonatites (northwest Vietnam)

South Nam Xe carbonatites are located in northwest Vietnam and include calcioand ferro-carbonatite dikes. This investigation on their petrography, mineralogy and whole rock chemistry aims to constrain temporal emplacement sequence of the carbonatites during their evolution. The calciocarbonatites are supposed to be formed in the first or second stage due to massive coarse-grained texture with an assemblage of calcite, typical magmatic alkaline silicates (aegirine, arfvedsonite), biotite, fluorapatite and magnetite. Their calcites show a high CaO/(MgO + Fe2O3 + MnO) ratio and a predominance of SrO over MnO (SrO = 3.81-3.98 wt.%; MnO = 0.66-0.78 wt.%). Rare earth elements (REE) tend to participate in rock-forming minerals rather than in isolated REE minerals. The ferrocarbonatites are composed of magmatic and hydrothermal varieties and assumed to be formed in the third and/or fourth stage. Major minerals of the former include zoned ankerite, Sr-rich calcite, subhedral feldspar crystals, phlogopite and magnetite; fluorapatite, monazite and REE carbonates are minor resulting in a moderate REE concentration of 43,200 ppm. Meanwhile, the latter is predominant by syntax-texture REE fluorcarbonates and (Ba,Sr) sulphates. Further, the highest REE concentration (163,900 ppm) of the rock coupled with abundance of volatile minerals (fluorite, fluorcarbonates, sulphides) and O enrichment in the calcites (δOV-SMOW = 12.01-13.26‰) is probably attributed to hydrothermal subjection in the last stage.