Evidence of mantle metasomatism and heterogeneity from peridotite inclusions of northeastern Brazil and Paraguay

Textural-petrographic features, major and trace elements and mineral chemistry are reported for spinel-peridotite xenoliths from northeastern Brazil (NE-suite) and Paraguay (PY-suite). Variation trends defined by bulk rock and mineral chemistry are consistent with the effects of variable degree of high-pressure melting for both suites. The PY-suite strongly differs from the NE-suite by its very high contents of incompatible elements. This different whole-rock chemistry is related to the presence of glassy-microcrystalline blebs which are interpreted as remains of pre-existing hydrous phases of metasomatic origin. The difference between the two suites is also recorded in the chemistry of pyroxenes and spinels, which develop along different variation patterns. Although the suites partially overlap in their ranges ofmgopx (Mg/(Fe2+ + Fe3+) ratio in orthopyroxenes), the PY-suite is, on the average, moremgopx rich (residual) than the NE-suite. Within the PY-suite a rough positive correlation exists between degree of residuality and degree of metasomatic effects. The sharp differences between the NE-suite and the PY-suite imply mantle heterogeneity on regional scale, whereas the variability within each suite is essentially related to different degrees of melting and/or metasomatism and imply mantle heterogeneity on local scale.