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-ferriphlog opite in carbonatites and associated alkaline silicate rocks from the Brazi lian carbonatite complexes of Jacupiranga, Tapira and Catal (a) over tildeo are presented. A wide textural variety of mica is found, ranging from prim ary magmatic crystals to late-stage metasomatic phases. Primary micas from the carbonatites and from the associated plutonic silicate rocks follow dis tinct evolution paths with magma differentiation. In all three complexes, m icas from the silicate rocks evolve from phlogopite towards annite, althoug h 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. Metas omatic micas bridge the gap between primary micas from silicate-rocks and f rom carbonatites. The complete phlogopite-tetra-ferriphlogopite series is r eported from the Catal (a) over tildeo and Tapira complexes. This study sho ws that micas from carbonatite complexes may span a wide compositional rang e, largely overlapping the fields of micas from several types of alkaline u ltrapotassic rocks, especially regarding Ti and Al contents. The use of phl ogopite composition and evolution to discriminate between different types o f alkaline rocks should be undertaken with caution. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.