Petrology and geochemistry of the tapira alkaline complex, minas gerais state, Brazil

Abstract

The Tapira alkaline complex is the southernmost of a series of carbonatite- bearing intrusions occurring in the Alto Paranaíba region, western Minas Gerais State, Brazil. Together with kamafugites, lamproites and kimberlites, these complexes form part of the Late-Cretaceous Alto Paranaíba Igneous Province (APIP). The Tapira igneous complex is emplaced into rocks of the Late-Proterozoic Brasilia mobile belt, adjacent to a major cratonic area (the Sāo Francisco craton).The complex is formed by the amalgamation of several intrusions, comprising mainly ultramafic rocks (wehrlites and bebedourites), with subordinate syenite, carbonatite and melilitolite. At least two separate units of ultramafic rocks (B1 and B2) and five episodes of carbonatite intrusion (CI to C5) are recognised. The plutonic rocks are crosscut by fine-grained ultramafic and carbonatite dykes. Two varieties of ultramafic dykes are recognised: phlogopite-picrites are the most primitive rocks in the complex; low-Cr dykes are more evolved, and typically lack olivine. The ultramafic dykes are carbonate-rich, and may contain carbonate ocelli, indicating that immiscibility of carbonatite liquid occurred early in the evolution of the complex. The ultramafic dykes are chemically similar to the APIP kamafugites. The primitive Tapira magmas underwent some differentiation in the crust, before their final emplacement. Crystal fractionation from the phlogopite-picrite magma may have produced olivine and chromite-rich cumulates, but these rocks are under- represented in the complex. Crystal fractionation from low-Cr dykes may have produced the bebedourites. The Tapira complex contains examples of carbonatites that originated by either liquid immiscibility or crystal fractionation. These contrasting petrogenetic mechanisms have produced distinct geochemical and mineralogical signatures, which have been used to pinpoint specific events in the evolution of the complex, and to test the consanguinity of carbonatites and associated silicate rocks.