GEOLOGY AND GEOCHEMISTRY OF NUKU-HIVA, MA RQUESAS - TEMPORAL TRENDS IN A LARGE POLYNESIAN SHIELD-VOLCANO

Nuku Hiva, one of the largest shield volcanoes in French Polynesia, wa s built up largely between 4.8 and 3.7 Ma We present a geological sket ch map of the island showing three nested calderas opened southward, t he origin of which is attributed to submarine gravity landslide collap ses. The emergent part of the Tekao shield is made up of thin tholeiit ic flows mostly emplaced between 4.8 and 4.5 Ma, overlain by transitio nal basalts, alkali basalts and hawaiites. The main caldera collapse e vent is dated at 4.05 +/- 0.10 Ma. It was immediately followed by the construction of the Taiohae volcano which exposes an alkalic suite ran ging from basalts to trachytes. Major and trace element data document a rapid transition from tholeiites to alkali basalts, which we relate to time-decreasing degrees of melting of a garnet Iherzolite source. T he isotopic Sr, Nd, Pb variability of Nuku Hiva basalts, and especiall y of the Tekao shield tholeiites, may reflect small-scale heterogeneit ies in a plume of dominantly EMII-HIMU composition.