Volcano-tectonic and geochemical evolution of an oceanic intra-plate volcano: Tahiti-Nui (French Polynesia)

Abstract

Geological mapping, accurate K/Ar dating and geochemical analyses of lavas allow a detailed reconstruction of the geological history of Tahiti-Nui Island (French Polynesia). The exposed volcanic activity is first characterized by the construction of a main shield from 1.4 Ma to 870 ka, with a maximum aerial eruptive rate around 2 km3/kyr. Lavas from this early building stage are alkaline, slightly silica-undersaturated, with 87Sr/86Sr and 143Nd/144Nd compositions rather constant and close to the enriched mantle II type. Vent locations were first concentrated along a main E–W rift zone, which was responsible for the lateral collapse of the northern and southern flanks of the main shield, around 0.87 Ma ago. The subsequent activity was first restricted to the northern depression, corresponding to an eruptive rate of about 5 km3/kyr in the period 850–760 ka. Significant variations in La/Sm, 87Sr/86Sr and 143Nd/144Nd occur in lavas erupted immediately after the main northern landslide, indicating a sudden increase in the extent of partial melting likely caused by the decompression subsequent to collapse. However, the later activity declined, and the lavas exhibit a gradual change toward strongly silica-undersaturated basanites, likely indicating a decreasing extent of partial melting of the upper mantle. The evolution of radiogenic isotope ratios over time indicates a change in the source toward more depleted compositions, until around 500 ka. Post-erosion volcanic activity, following an apparent hiatus of 240 kyr, exhibits similar major and trace element and isotope compositions. The volcano-structural and geochemical evolution of Tahiti-Nui and the overall alkaline character of the lavas from other eruptive complexes of the Society alignment suggest a relative weakness (low temperature and low eruptive rate) of the Society plume compared to the Hawaiian hot-spot.