Parental Magma Characterization of Salitre Cumulate Rocks (Alto Paranaíba Alkaline Province, Brazil) as Inferred from Mineralogical, Petrographic, and Geochemical Data

Abstract

The Alto Paranaiba alkaline province (∼82 Ma), tectonically associated with a NW-trending linear structure bordering the São Francisco craton, consists of a wide variety of igneous forms and magma types. It includes several alkaline-carbonatite complexes, some of which are quite important economically. The Salitre complex is represented by two interconnected oval-shaped bodies, N-S-aligned and variable in size, emplaced into Proterozoic metasedimentary rocks. It consists mainly of syenites showing clear evidence of fenitization, ultramafic rocks (clinopyroxenites, mica-bearing clinopyroxenites, locally referred to as bebedourites, dunites, glimmerites, and per-ovskitites), radial dikes of trachytes/tinguaites, and carbonatites as small veins and also forming an elongated plug, approximately 500 m2 in area, associated with the Salitre I intrusion. In a few cases the carbonatitic rocks grade into phoscorites because of an increase in magnetite content. Texturally, the ultramafites are described as adcumulates, but meso- and orthocumulate rock types also are found. Clinopyroxene, olivine, phlogopite, perovskite, and apatite represent the main cumulus phases, sphene being less common. Intercumulus material consists of the same minerals in addition to melanite. Fresh rocks are rarely present at the surface; thus the entire set of data derives almost entirely from borehole samples. Incompatible-element distributions normalized to primordial mantle for the different ultramafic rock types exhibit many similarities, showing patterns clearly dominated by the abundance of perovskite and phlogopite. Syenitic rocks and carbonatites generally are less enriched than the cumulates. Distribution patterns of REE display LREE enrichment and strong LREE/HREE fractionation typical of alkaline suites; higher REE concentrations are related to the perovskitic rocks. Smooth, almost linear patterns are indicated for all the rock types excluding the syenites (which show a characteristic U-shaped distribution curve, probably the result of the removal of sphene and amphibole). It should also be noted that the dunitic rocks in general are more fractionated than the clinopyroxenitic ones. On the basis of texture and mineralogy, the Salitre ultramafites are interpreted as being formed by accumulative processes. No distinctive layering is evident in the rocks at the borehole scale. The crystallization sequence of the main rock-forming minerals of the ultramafites shows perovskite and opaques (Cr-spinel) as early phases, followed by olivine, clinopyroxene, and finally phlogopite; the last mineral is clearly a two-generation phase, the latest being related genetically to the fenitization processes. Geological, petrographic, and mineralogical evidence for the Salitre rocks indicates a highly complex origin involving multistage crystallization processes from an initial ultrapotassic magma source. This may explain the absence of ijolitic rocks over the entire Alto Paranaíba province, which is contrary to the situation in other alkaline complexes of southern Brazil (e.g., Jacupiranga, Juquiá). Carbonatitic rocks are interpreted as resulting from an unmixing process that developed during the evolution of the parental magma.