BERYLLIUM IN THE NAPIER COMPLEX, ANTARCTICA: A TRACER FOR DETERMINING THE RELATIVE TIMING OF PARTIAL MELTING, DEFORMATION AND ULTRAHIGH-TEMPERATURE METAMORPHISM IN THE LOWER CRUST

Beryllium provides an important constraint in determining the timing of anatexis relative to deformation and metamorphism in metamorphic complexes containing sapphirine, a major carrier of Be. Sapphirine is a widespread constituent of pelitic granulites in the Napier Complex of Enderby Land, East Antarctica, where it was stable with quartz at the peak of ultrahigh-temperature (UHT) metamorphism (>1000 ºC). The presence of 30-3000 ppm Be in sapphirine could explain the retention of Be at UHT in Napier metapelites, which average 3.5 ± 2.5 ppm Be (cf. 3 ppm for upper crust). Bodies of pegmatitic material presumed to be anatectic are found throughout the Napier Complex and are commonly associated with boudinage, e.g., cm-sized masses filling boudin necks and pods up to 1 m long filling spaces opened by breaking of more competent layers. Boudinage accompanied deformation associated with the peak of UHT metamorphism; nonetheless, many of the pegmatites crosscut UHT fabrics and must be somewhat younger. Because sapphirine sequesters Be, it is not likely that melts enriched in Be could form under conditions where sapphirine + quartz was stable in the pelitic host rocks; instead Be enrichment would be expected only during anatexis of rocks in which sapphirine was breaking down to Be-poor phases. Subsequent to the UHT event in the Napier, sapphirine + quartz was no longer stable, and sapphirine became separated from quartz by coronas of sillimanite, orthopyroxene and garnet, or was consumed entirely. Beryllium released by sapphirine breakdown then became available for incorporation in melt because orthopyroxene and sillimanite are Be-poor (0.1-10 ppm Be). Thus, melting more likely followed peak UHT conditions by a short interval, possibly during an episode of heating associated with continued deformation. Under these conditions (~900 ºC), the primary Be carrier to crystallize from melt was a sapphirine phase, which developed into khmaralite upon cooling if its BeO > 1.7 wt%. Sapphirine/khmaralite is separated from quartz by coronas of sillimanite ± corundum + garnet ± orthopyroxene ± Be phases (surinamite, musgravite, chrysoberyl) > 2 mm thick in pegmatite pods and > 0.1 mm thick in boudin-neck fillings. These coronas may represent reaction of sapphirine/khmaralite with melt prior to crystallization of quartz.