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

Abstract

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 ‘fl uor-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. ‘Fluor-liddicoatite’ occurs in pockets from the core zones. The most calcic composition corresponds to (Ca0.80Na0.18 0.02)∑1.0 (Li1.81Al1.14Mn0.03Fe0.02)∑3.0Al6(Si5.89Al0.11)∑6.0 O18(BO3)3 (OH3.27F 0.73)∑4.0. ‘Fluor-liddicoatite’ formed because Ca conservation and migration took place in the magma. Schorl crystals from the line rock are oscillatory zoned, indicating that crystal growth rates were faster than cations could diffuse through liquid. Some individual crystals of schorl and ‘fl uor-elbaite’ or only of ‘fluor-elbaite’ from the inner intermediate zone and pockets in this zone show progressive zoning, attributed to fractional crystallization. Other crystals of ‘fluor-elbaite’ from these pockets and core zone are reversely zoned due to nucleation and growth of evolved cores followed by back-reaction with the more primitive bulk magma. The main substitution mechanism in schorl and ‘fluor-elbaite’ is (Li1.5Al1.5)(Fe2+)-3 at the Y site, which is controlled by fractional crystallization, but the ( Al) (NaR2+)-1 substitution is also important. The (Ca0.5 0.5)Na-1 substitution occurs in ‘fluor-liddicoatite’. Hydrothermal ‘fl uor-elbaite’ from replacement units in the inner intermediate zone is Fe-, Na- and Ca-poor. The two coupled substitutions (Li1.5Al1.5)(Fe2+)-3 at the Y site and ( Al) (NaR2+)-1 took place. Hydrothermal fluor-dravite from the metasomatized chlorite phyllite adjacent to the granitic pegmatite group resulted from the mixing of magmatic-hydrothermal fl uids carrying Na, Al, Fe, Li, F and B with a meteoric fl uid that has interacted with this Fe-Mg-bearing pelitic country rock. The main substitution in this tourmaline is FeMg–1.