Phlogopite and tetra-ferriphlogoyite from Brazilian carbonatite complexes:: petrogenetic constraints and implications for mineral-chemistry systematics

The results of a mineral-chemistry study of phlogopite and tetra-ferriphlogopite in carbonatites and associated alkaline silicate rocks from the Brazilian carbonatite complexes of Jacupiranga, Tapira and Catal (a) over tildeo are presented. A wide textural variety of mica is found, ranging from primary magmatic crystals to late-stage metasomatic phases. Primary micas from the carbonatites and from the associated plutonic silicate rocks follow distinct evolution paths with magma differentiation. In all three complexes, micas from the silicate rocks evolve from phlogopite towards annite, although in Jacupiranga they are more Al-rich. Among the micas crystallising from carbonatite liquids, those from Jacupiranga are typically Al- and Mg-rich, while in Tapira and Catal (a) over tildeo they are extremely Al-poor. Metasomatic micas bridge the gap between primary micas from silicate-rocks and from carbonatites. The complete phlogopite-tetra-ferriphlogopite series is reported from the Catal (a) over tildeo and Tapira complexes. This study shows that micas from carbonatite complexes may span a wide compositional range, largely overlapping the fields of micas from several types of alkaline ultrapotassic rocks, especially regarding Ti and Al contents. The use of phlogopite composition and evolution to discriminate between different types of alkaline rocks should be undertaken with caution. (C) 2001 Elsevier Science Ltd. All rights reserved.