Kyanite-staurolite ortho-amphibolite from the Chapada region, Goias, central Brazil

Universidade de Brasilia, Instituto de Geociencias, Brasilia, Brazil

This study deals with the kyanite-staurolite assemblage from a metabasite (ortho-amphibolite) in the Chapada area of the Neoproterozoic Mara Rosa volcano-sedimentary sequence of Goias state, Brazil. It is closely associated with hydrothermally altered rocks and contains the assemblage hornblende, epidote, quartz, kyanite, staurolite, plagioclase and rutile. The staurolite from the amphibolite shows lower values of SiO ₂ and higher values of MgO, MnO, ZnO and X _mg than those in staurolites from the metasediments of the same area. In the Al-Na-Ca-(Fe+Mg) diagram, the amphibolite plots in the aluminous part of the diagram and the phase volume encloses the area of mafic volcanic bulk compositions. Field relations and compositions in both kyanite-staurolite amphibolite and staurolite are consistent with formation of this amphibolite as a result of metamorphism of metasomatised basalts.

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