Decoupling of paired elements, crossover REE patterns, and mirrored spider-diagrams: fingerprinting liquid immiscibility in the Tapira Alkaline-Carbonatite Complex, SE Brazil.

BROD, J. A.; T.C. JUNQUEIRA BROD; J.C. GASPAR; I.A. PETRINOVIC; S. DE CASTRO VALENTE; A. CORVAL

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2012

0895-9811

the Late-Cretaceous Alto Paranaíba Igneous Province (APIP). It is dominated by coarse-grained plutonicrocks (bebedourite e a phlogopite-, apatite-, and perovskite-rich clinopyroxenite e with subordinateddunites, wehrlites, carbonatites and phoscorites). The plutonic rocks are crosscut by fine-grained ultramaficalkaline rocks (phlogopite picrites, bebedouritic dikes) and fine-grained carbonatites. Both types ofdike-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. Adetailed 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. phlogopitepicrites and bebedouritic dikes), others display anomalous trace-element behaviour that cannot bereadily explained by the fractionation of a particular phase or combination of phases. We interpret suchanomalous geochemical signatures as produced by silicateecarbonate liquid immiscibility, on the basis ofavailable experimental data on partition coefficients between coexisting immiscible liquids. Theimmiscibility signatures comprise: (a) decoupling of geochemical pairs, such as NbeTa and ZreHf; (b)rotation of REE patterns, which cross over the patterns of the primitive liquids; and (c) matching andopposite enrichment-depletion trace elements relationships in spider diagrams of conjugate immiscibleliquids. We suggest that, once established, such geochemical signatures are very difficult to erase duringthe subsequent petrogenetic evolution processes, which may result in superimposed conflictingsignatures.