Abstract:
Anatectic veins containing the Be minerals khmaralite and beryllian sapphirine as primary phases (or surinamite derived therefrom) are associated with Mg–Al-rich paragneisses at three localities in the ultrahigh-temperature Napier complex, Antarctica, a unique Be mineralization in the granulite facies. Likely precursors of the paragneisses are volcaniclastic deposits that were hydrothermally altered by heated seawater prior to metamorphism. Regular distribution of Be among minerals in the paragneisses suggests an approach to equilibrium with Be greatly concentrated in sapphirine (25–3430?ppm Be) or cordierite (560–930?ppm Be) relative to plagioclase An53–66 (14–43?ppm Be) > cores of coarse-grained orthopyroxene (0·7–29?ppm Be) > coronitic orthopyroxene (0·4–14?ppm Be) ≈ sillimanite (0·1–26?ppm Be) ≈ plagioclase An18–33 (0·6–15?ppm Be) > biotite (0·06–8?ppm Be) > K-feldspar, quartz, garnet (0·05–0·7?ppm Be). Sapphirine-bearing paragneisses have average Be concentrations, 4·9 ± 2·4?ppm (13 samples), about twice that of typical pelites, whereas paragneisses lacking sapphirine and primary cordierite have only 2·9 ± 2·1?ppm Be (12 samples), implying some loss of Be during metamorphism. The likely source rocks for the Be-rich melts were biotitic rocks lacking the Be sinks sapphirine and cordierite. These gneisses were probably less competent than the sapphirine-bearing gneisses, so the melts were drawn to the latter and collected in spaces opened during deformation and boudinage of the more competent paragneisses. Fractionation of the melts concentrated Be to the extent that Be minerals could crystallize. The final result was Be-mineralized anatectic veins hosted by relatively Be-rich sapphirine-bearing paragneisses.
