Pegmatites in South-eastern Brazil
Résumé
Two pegmatite groups are represented in southeastern Brazil. The first group, resulting from fractional crystallization, is linked to syn-tectonic granites (580 Ma - deformation event D ) and the second one is associated with another partial melting event of the crust which simultaneously produced porphyritic leucogranites [(520-500 Ma)- later deformation event D2] and pegmatites themselves. The Fe/Mn ratios of tourmalines, micas and columbite-tantalites decrease continuously from simple pegmatites to gem quality elbaite-rich ones. The Fe/Mn ratio has been used as a qualitative fractionation index which seems to reflect the regional zoning of the first pegmatites group around a hidden granite body. The Fe/Mn ratio values correlate negatively with Na and Li contents in tourmalines and mica. The REE, Nb, Co and Zn contents in tourmalines, micas, and columbite-tantalites are controlled by their mineral assemblages.