OCCURRENCE OF TRACHYTES IN THE SUBSTRATUM OF FANGATAUFA ATOLL, GAMBIER LINEAR-CHAIN, FRENCH-POLYNESIA

Alkaline trachytes containing sodic plagioclase, sanidine, phlogopite, zircon and apatite phenocrysts are described from the volcanic substr atum of Fangataufa atoll, which was previously considered as exclusive ly basaltic or hawaiitic. They occur as very altered dykes intruding s ubmarine alkali basalt lava flows beneath the lagoon (Mitre 3 drill ho le). They bring evidence of the occurrence of crystal fractionation pr ocesses leading from alkali basalts to trachytic residual liquids, in a probably short-lived magma chamber.