DIVERSE BOROSILICATE ASSEMBLAGES IN ANATECTIC GRANITIC PEGMATITES CUTTING BORON-RICH GRANULITE-FACIES ROCKS OF THE LARSEMANN HILLS, PRYDZ BAY, EAST ANTARCTICA

In the great majority of granitic pegmatites boron is localized exclusively in minerals of the tourmaline group. This is not the case in the Larsemann Hills, where 5 borosilicate minerals appear to be primary phases in granitic pegmatites derived from anatexis of granulite-facies (~7 kbar; ~800-850°C) metasediments and metavolcanogenic rocks. The metamorphic rocks comprise several mappable boron-rich units: (1) biotite plagioclase gneiss with pods of prismatine + cordierite + biotite +/- tourmaline +/- orthopyroxene + minor grandidierite; (2) leucocratic sillimanitic gneiss with abundant grandidierite and prismatine and subordinate tourmaline; and tourmaline metaquartzite. There are two generations of boron-enriched pegmatites: (1) roughly concordant, but locally crosscutting, irregular pods up to 1 m thick (D2-D3 pegmatites) often spatially associated with prismatine-bearing pods and (2) planar veinlets (D4 pegmatites) cutting D2 and D3 structures and not necessarily associated with rocks containing borosilicate. A distinctive feature of both generations of pegmatite is a graphic intergrowth up to 30 cm across of schorl-dravite grains in polygranular quartz. Other borosilicates presumed to be primary are the ferromagnesian minerals prismatine and grandidierite as well as two Al-rich minerals: pale purple, titanian dumortierite (3.7 -5.0 wt% TiO2) in parallel prisms in quartz, and boralsilite, an acicular mineral resembling sillimanite, often in fans, in quartz and microcline near the graphic intergrowths. Boralsilite ranges in composition from Al16.0B5.9Si2.1O37 to Al15.8Mg0.2Fe0.4B4.9Si2.7O37, i.e., it contains 0 to 30% of a werdingite component. It is not found with prismatine or cordierite. Secondary phases include bright-blue, highly pleochroic Ti-poor dumortierite, highly zoned blue foitite-schorl, and andalusite. We attribute the diversity of primary borosilicate minerals to extreme boron enrichment of the pegmatites during anatexis and to minimal loss of boron to wallrocks during crystallization, probably because relatively little fluid was present. Temperatures were sufficiently high and water activities sufficiently low to stabilize not only Al2SiO5 + K-feldspar in lieu of muscovite, but also boralsilite with substantial werdingite in solid solution.