Boron- and phosphate-rich rocks in the Larsemann Hills, Prydz Bay, East Antarctica: Tectonic implications

Granulite-facies paragneisses enriched in boron and phosphorus are exposed over c. 15 × 5 km² in the Larsemann Hills, Antarctica. The most widespread are biotite gneisses containing centimetre-sized prismatine crystals, but tourmaline metaquartzite and borosilicate gneisses are richest in B (676-19 700mg/g or 0.22-6.34 wt%; B2O3). Chondrite-normalized rare-earth element (REE) patterns give two groups: (1) LaN. 150, Eu*/Eu, 0.4, which comprises most apatite-bearing metaquartzite and metapelite, tourmaline metaquartzite, and Fe-rich rocks (up to 2.3 wt%; P₂O₅); (2) LaN, 150, Eu*/Eu. 0.4, which comprises most borosilicate and sodic leucogneisses (2.5-7.4wt%; Na₂O). Enrichment in boron and phosphorus is attributed to premetamorphic hydrothermal alteration, either in a rifted, most likely marine basin or in a mud volcanic system located inboard of a c. 1000 Ma continental arc that was active along the leading edge of the Indo-Antarctic craton. This margin developed before collision with the Australo-Antarctic craton (c. 530 Ma) merged these rocks into Gondwana and sutured them into their present position in Antarctica. Rocks lithologically similar to those in the Larsemann Hills include prismatine-bearing granulites in the Windmill Islands, Wilkes Land, and tourmaline- quartz rocks, sodic gneisses and apatitic iron formation in the Willyama Supergroup, Broken Hill, Australia.