Phoscorites of the Salitre I complex: Origin and petrogenetic implications

The Salitre I complex is a kidney-shaped alkaline-carbonatite-phoscorite complex part of the Late Cretaceous Alto Paranaíba Igneous Province, central Brazil. This complex is a key example where the phoscorite-carbonatite association was preserved from metasomatism by late-stage fluids and, thus, provides an opportunity to address the genesis of phoscorites. Salitre I is composed of two main coalescing intrusions: a) a northern one dominantly containing perovskite bebedourites (B1), with swarms of ring dykes of carbonatites and phoscorites and, b) a southern one containing melanite bebedourites (B2). Salitre I phoscorites can be subdivided into P1 and P2 based on modal abundance, mineral texture, chemistry and mode of emplacement. P1 phoscorites contain essential olivine, phlogopite and apatite, with accessory magnetite and traces of perovskite whereas P2 phoscorites are composed of the same essential phases but also contain carbonate-rich pockets, accessory pyrochlore and rare phlogopite. Carbonate C-O isotopes and whole-rock Sr-Nd show that Salitre I phoscorites crystallized from mantle-derived parental magmas, similar to other APIP carbonatite complexes. However, chemical and textural discrepancies between P1 and P2 suggest derivation from different magma sources and through distinct processes. For instance, P1 Al- and Ti-rich phlogopite, accessory perovskite and general trace element geochemical behaviour can be associated with the evolution of B1 perovskite bebedourites, i.e. genesis through fractional crystallization from a bebedourite magma. P2 phoscorites, on the other hand, have their evolution linked to a magmatic phase that underwent melt immiscibility, as suggested by trace element fractionation. Additionally, P2 contains pyrochlore, which is generally lacking in bebedourites. Therefore, whereas P1 phoscorites might have derived from a bebedourite magma, P2 phoscorites were generated from a carbonated magma enriched in phosphate and iron oxides.