Brod, and 5 co-authors, reported in 2013, that the Tapira complex is an alkaline silicate carbonatite belonging to the kamafugite-carbonatite association in the Late-Cretaceous Alto Paranaíba Igneous Province (APIP). It is dominated by coarse-grained plutonic rocks (bebedourite e a phlogopite-, apatite-, and perovskite-rich clinopyroxenite e with subordinated dunites, wehrlites, carbonatites and phoscorites). The plutonic rocks are crosscut by fine-grained ultramafic alkaline rocks (phlogopite picrites, bebedouritic dikes) and fine-grained carbonatites. Both types of dike-rocks show petrographic evidence of the coexistence of immiscible silicate and carbonatite liquids, such as carbonate ocelli present in the silicate rocks and, more rarely, silicate ocelli within carbonatites.
A detailed geochemical study of the rock types in the complex, with emphasis on the fine-grained varieties, showed that whilst some rocks may be related to each other through crystal fractionation (e.g. phlogopite picrites and bebedouritic dikes), others display anomalous trace-element behavior that cannot be readily explained by the fractionation of a particular phase or combination of phases. The authors interpreted such anomalous geochemical signatures as produced by silicate carbonate liquid immiscibility, on the basis of available experimental data on partition coefficients between coexisting immiscible liquids. The immiscibility signatures comprise: (a) decoupling of geochemical pairs, such as Nb-Ta and Zr-Hf; (b) rotation of REE patterns, which cross over the patterns of the primitive liquids; and (c) matching and opposite enrichment-depletion trace elements relationships in spider diagrams of conjugate immiscible liquids. The authors suggested that, once established, such geochemical signatures are very difficult to erase during the subsequent petrogenetic evolution processes, which may result in superimposed conflicting signatures.
The authors concluded that carbonatite silicate liquid immiscibility processes are likely to produce several types of easily recognizable geochemical fingerprints,
such as:
a) decoupling of geochemical pairs like Nb-Ta and Zr-Hf, whereby the Nb/Ta chondrite-normalized ratio will increase, and Zr/Hf will vary in the carbonatite branch, with little or no visible effect on the silicate conjugate.
b) Anticlockwise rotation of REE patterns in the silicate branch, and clockwise rotation of REE patterns in the carbonatite branch, with La/Lu ratios decreasing in the former and increasing in the latter, comparatively to the primitive liquids. This will typically generate crossover REE patterns.
c) Bulk enrichment of Zr and Hf in the silicate branch, and a corresponding bulk decrease in these elements in the carbonatite branch, generating mirrored anomalies in spider diagrams.These features may also be present for other elements, such as Th, Rb, K, P, and the REE.
d) Strong Ta depletion in the carbonatite branch.
It should be noted that the appearance of these geochemical signatures depends on composition, temperature and pressure of the system, as well as they may superimpose one another when liquid immiscibility is recurrent during magma evolution. The available field, petrographic, chemical, mineralogical, and isotopic evidence indicate that the Tapira complex, similarly to many other alkaline carbonatite complexes worldwide, resulted of a complex interplay of several different petrogenetic processes, such as liquid immiscibility, crystal fractionation, and degassing/ metasomatism. Interestingly, once established, the geochemical signatures of liquid immiscibility are not easily erased by subsequent processes.
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