Compositional evolution of tourmaline-supergroup minerals from granitic pegmatites in the Larsemann Hills, East Antarctica

In the Larsemann Hills, Prydz Bay, East Antarctica, granulite-facies metasedimentary units containing tourmaline, prismatine and grandidierite are cut by three generations (D 2 , D 3 and D 4 ) of granitic pegmatites, several of which are also unusually enriched in B-rich phases, including tourmaline (schorl – dravite – foitite solid solutions), prismatine, grandidierite, werdingite, boralsilite and dumortierite. Tourmaline can be used to gain information about the crystallization history of the pegmatites. Six microstructural varieties of tourmaline have been recognized: 1) primary tourmaline in a graphic intergrowth with quartz, 2) tourmaline overgrowths on tourmaline in the graphic intergrowth, 3) prisms of tourmaline with oscillatory zoning, 4) tourmaline that has replaced boralsilite, 5) tourmaline that has replaced prismatine, and 6) chaotically zoned tourmaline. Tourmaline compositions evolved as crystallization proceeded in the D 2 and D 3 pegmatites (considered together) and in the D 4 pegmatites, resulting in an increase in X -site vacancy, a decrease in Ti and F contents, an increase in Al at the ( Y + Z ) sites, and a decrease in the ratio Mg/(Mg + Fe). The first three changes are consistent with decreasing temperature. However, oscillatory zoning is characteristic of open systems, where there had been a mixing from different chemical sources; we consider it more likely that X -site vacancy and F content varied as a function of fluid composition rather than temperature per se. On the basis of the compositional variation and microstructural relations and comparison with the retrograde path inferred for the host rocks, we suggest that the pegmatites evolved as temperatures decreased from 700–750°C, 3–4.5 kbar when the graphic tourmaline + quartz crystallized, through 600–700°C, 3–4 kbar, the stability range considered appropriate for boralsilite, to below 600°C at~3 kbar for secondary tourmaline and dumortierite, conditions consistent with the presence of secondary andalusite.