SPINEL CHEMISTRY AND PETROGENETIC PROCESSES IN THE TAPIRA ALKALINE-CARBONATITE COMPLEX, MINAS GERAIS, BRAZIL

Abstract

The Tapira complex, SE Brazil, is part of the Late-Cretaceous Alto Paranaíba Igneous Province (APIP), and consists of a Silicate Plutonic-rock Series (SPS, comprising wehrlites, bebedourites and syenites) and subordinate carbonatites. Two major units of coarse-grained ultramafic rocks (B1 and B2) and five groups of carbonatites (C1 to C5) occur in the complex The plutonic rocks are crosscut by ultramafic fine-grained dykes (phlogopite picrites and bebedouritic dykes). Spinel composition is intimately related to magma evolution, reflecting both crystal fractionation and liquid immiscibility processes. Spinel-group minerals evolve from chromite through ulvöspinel to magnetite with magma fractionation and high-Cr spinel is a reliable indicator of the most primitive rocks in the complex (phlogopite picrites). Spinel chemical variations agree well with other (field, petrographic and geochemical) evidence, which indicate magmatic evolution from B1 to B2 in the SPS and from C1 to C5 in the carbonatites. Spinels from some Tapira wehrlites cannot be attributed to fractionation from phlogopite picrites or bebedouritic magmas. These wehrlites are more likely to have been produced by crystal fractionation from unevolved immiscible carbonate-rich liquids, rather than from their silicate-rich counterparts.