Tourmaline-group minerals in the Naipa Li-Cs-Ta granitic pegmatite group, Mozambique: tracers of magmatic to post- magmatic evolution trends

摘要

Tourmalines from the Naipa Li-Cs-Ta granitic pegmatite group and metasomatized adjacent host rock were analyzed by electron microprobe to establish their chemical evolution within the zoned pegmatites, to distinguish between magmatic and hydrothermal tourmalines, and to explain their mode of formation and the origin of the individual crystal zoning. A compositional trend beginning with magmatic schorl from the line rock, schorl, elbaite and 'fluor-elbaite' from the inner intermediate zone and pockets in this zone, followed by elbaite and 'fluor-elbaite' from core zones record a differentiation sequence, but Ca content increases in this sequence. These tourmalines formed from a strong fractionation of an (Na, Al, Li, B)-rich pegmatite, which had Ca at the time of emplacement.