Minette mafic microgranular enclaves and their relationship to host syenites in systems formed at mantle pressures: major and trace element evidence from the Piquiri Syenite Massif, southernmost Brazil

摘要

Mafic microgranular enclaves in the ultrapotassic Piquiri Syenitic Massif (611 + - 1 Ma) in southern Brazil represent a minette magma mingled with a syenitic one, both produced from similar mantle sources and emplaced in a post-collisional setting of Neo-proterozoic age. The minette magma is compositionally close to typical minettes and high-SiO_2 lamproites, with relatively high contents of LREE, Cs and Rb. It is slightly silica-undersaturated, ultrapotassic and metaluminous, with K_2O/Na_2O ratios around 2-3, and about 4-7 wt percent K_2O. The Piquiri minettes contain K-clinopyroxene and py-rope, which are interpreted to have crystallized under pressures about 5 GPa. Whole-rock and mineral chemistry indicate that the most suitable source for the minette magma is clinopyroxene-phlogopite-apatite-amphibole-sulphide + - garnet mantle veins, under pressures of about 5 GPa and melting temperatures over 1,000 deg C. Fractional melting is admitted in order to explain the extremely high Rb, Cs and LREE contents of the minette melt, and is consistent with its estimated rheological behavior. The syenitic host-rock parental magma was produced from a similar source, probably at lower pressures, and the co-mingling probably occurred still at large depth, under pressures around 3 GPa. Rheological and geochemical considerations support a model based on fractional melting of a veined mantle which had been metasomatized during previous (760-700 Ma) ocean-plate consumption. The subduction-related metasomatism in the source is indicated by low LREE/(Nb or Ta) ratios, high Nb/Ta and U/Th ratios, and low Ti contents. The compositional similarity and close spatial and temporal association of minette and syenitic magmas can be explained by their common source region. Compared to typical lamproitic magmatism, the major difference is that the Piquiri minette magmas are derived from a subduction-modified source.