Quantitative analyses of F and Cl in apatite : Implication for origin of apatite in meta-ultramafic rock from Tonagh Island in Napier Complex, East Antarctica

The Napier Complex, East Antarctica is one of the oldest Archaean terrane in the world. Tonagh Island is located in the southern end of Amundsen Bay. Mafic gneisses and meta-ultramafic rocks are exposed on Tonagh Island. The meta-ultramafic rock that is located at the northeastern part of the island cuts the layers or foliations of neighboring quartzo-feldspathic gneisses, suggesting that the meta-ultramafic rock was originally intrusive rock. The meta-ultramafic rock consists mainly of olivine, clinopyroxene, orthopyroxene and hornblede. Subhedral apatite is included in olivine. Major and trace element compositions of the meta-ultramafic rock resemble those of komatiitic rocks from the Archaean greenstone belts. Chondrite normalized REE pattern for the meta-ultramafic rock shows enrichment in LREE. Quantitative analyses of F and Cl were done for the apatite in the meta-ultramafic rock. F and Cl contents of the apatite revealed that the apatite was directly crystallized from komatiitic melt. An apatite is regareded as a main container for LREE in the upper mantle. Both LREE-enriched and -depleted types of komatiitic rocks are exposed on the Archaean continents. Therefore, an apatite component may play an important role to understand the petrogenesis of Archaean komatiitic rocks.