Variations in the Concentration of Ta and Nb in a Peraluminous Haplogranite High-Fluorine Melt in Relation to the Change in the Association of Liquidus Phases

Resumo

A series of experiments were carried out in the peraluminous part of the haplogranite high fluorine system SiO₂–Al₂O₃–Na₂O–Li₂O–F at a temperature of 700°C, a pressure of 100 МPa, a water content of 10 wt %, and with the addition of ~1 wt % Ta and Nb. Two types of equilibrium phase associations were obtained: (1) an aluminosilicate melt–topaz–quartz–Ta-Nb phase and (2) an aluminosilicate melt–brine aluminofluorine melt–cryolite. The maximum content of Ta and Nb in the silicate melt of the first type is ~0.29 and 0.35 wt %, respectively. The silicate melt of the second type is characterized by an almost four-fold content of these metals (1.27 and 1.21 wt %, respectively), without reaching saturation by the Ta–Nb phase. A similar jump in the total amount of Ta and Nb in genetically related highly differentiated rocks was observed for topaz- and cryolite-bearing granite massifs in the Piting Province (Brazil).