MEHETIA ISLAND, SOUTH-PACIFIC - GEOLOGY AND PETROLOGY OF THE EMERGED PART OF THE SOCIETY HOT-SPOT

Mehetia is the youngest island of the Society hot-spot chain, representing the summit of a large submarine volcanic edifice (4035 m in height). Its conical shape results from strombolian-type eruptions. Mehetia comprises two major volcanic formations. The ''old edifice'' made up of interbedded pyroclastic rocks and lava flows, forms the main part of the island. The base of its stratigraphic pile is dated at 70,000-75,000 yr B.P. by K/Ar. Younger events (less-than-or-equal-to 3 1,000 yr B.P.) have built up a volcanic cone, with a summit hydromagmatic crater, and emplaced lava flows extending south of the island where they overlie coral reef limestones. Between the two eruptive periods, an erosional phase caused the partial destruction of the old edifice. Six successive stages of construction of the island are described from stratigraphic relations. Mehetia does not show any indication of present-day volcanic or fumarolic activity but should be considered as an active volcano as evidenced by the 1981 volcanoseismic crisis. Mehetia lavas include oceanites, primitive and evolved alkali basalts and basanites together with rare hawaiites and mugearites. Two main petrogenetic processes (i.e. accumulation and crystal fractionation) are inferred from bulk-rock composition and microprobe data on phenocrysts. Oceanites are derived from basalts through accumulation of olivine and clinopyroxene. Major and trace elements trends suggest evolution through crystal fractionation processes involving olivine, clinopyroxene, and Fe-Ti oxides.