GEOLOGY AND PETROLOGY OF TUBUAI (AUSTRAL ISLANDS, FRENCH-POLYNESIA)

Tubuai island, famous for its HIMU geochemical signature, is made up o f two separate volcanoes. The western Hanareho volcano is a crescent-s haped remnant of a largely collapsed caldera wall formed ca. 9.5 Ma ag o. Its lavas range from nephelinites to peralkaline phonolites through xenolith-rich phonolitic tephrites. The eastern Herani volcano is old er (12.4 to 9.9 Ma) and its lavas evolve with increasing incompatible element contents from alkali basalts to basanites and analcite-bearing tephrites. Major and trace element data for Tubuai basalts are consis tent with a model of time-decreasing degrees of partial melting of an isotopically homogeneous HIMU mantle source, generating successively a lkali basalts, basanites and nephelinites.