Sylvain Blais

Age progression along the Society hotspot chain (French Polynesia) based on new unspiked K-Ar ages

New K-Ar dates of volcanic rocks from five of the nine islands of the Society Archipelago (Moorea, Huahine, Raiatea, Bora Bora and Maupiti), confirm a Pacific plate velocity of around 11 cm/a during the last 4.3 m.y. These new data allow us to analyse the age-distance relationship along the chain and to evaluate possible temporal variations in the activity of the Society hotspot. A clear increase of ages is observed along the linear chain away from the present Society hotspot location. The time-space relationship between Taiarapu, Tahiti-Nui and Moorea can be explained by a simple hotspot model. Nevertheless, the simple fixed hotspot model assuming constant Pacific plate velocity may need adjustments to fully explain the age progression along the Archipelago. The slight departures from a linear age distribution can be explained by changes in Pacific plate motion which occurred at 5 and 3 Ma. In addition, the contemporaneous magmatic activities in the pairs Bora-Bora/Tahaa, Raiatea/Huahine, Maiao/Moorea require additional lithospheric control on magma transport. Combined with the hotspot activity, lithospheric loading may have produced extension and triggered volcanism along already existing fractures linking paired islands. The most likely model for the Society chain, proposed by McNutt [1998], involves a plume originating from a wide deep thermally anomalous zone (the Pacific Superswell) as a rising diapir (hotspot of secondary type according to the classification of Courtillot et al. [2003]). It melted during ascent and ponded beneath the Pacific plate to form short linear island chains showing rather good age vs. distance correlations.

The island of Maupiti : the oldest emergent volcano in the Society hot spot chain (French Polynesia)

The island of Maupiti is the top of an eroded volcanic shield surrounded by a large lagoon and coral reef. The most important part of this shield is made up of alkali basaltic lavas grading upward into hawaiitic flows. The shield-building event is dated at ca. 4.51 ± 0.04 Ma. A few mugearitic and benmoreitic late dykes were emplaced at 4.21 ± 0.04 Ma. These chronological data are consistent with a Pacific plate motion of 11 cm/year with respect to a fixed Society plume. The Maupiti lava series results from the effects of fractional crystallisation of a parental alkali basalt magma displaying a prominent EM II isotopic and trace element signature, in many ways similar to that of the youngest Society hot spot lavas. This feature suggests that the geochemical signature of the Society plume did not change through time.

Géologie, géochimie et géochronologie de l'île de Bora Bora (Société, Polynésie française)

Geology, geochemistry and geochronology of Bora Bora island (Society islands, French Polynesia). Bora Bora island, located at the northwestern edge of the Society archipelago represents the upper part of a shield volcano made up of a pile of meter-thick alkali basalt flows and rare hawaiites. The summit caldera (4.4 km in diam- eter) is still recognisable and a southwestern tilting of the volcanic edifice led to a relative uplift of the northern and eastern parts of the volcano. Bora Bora lavas derive from low partial melting degrees of an EMII mantle source the composition of which was influenced by continental materials. The subaerial volcanic activity corresponding to the presently emerged portion of the edifice took place between 3.45 and 3.10 Myr. Such ages are consistent with the formation of the island by being above the Society hot spot considered as fixed and a Pacific plate motion of 11 cm·yr -1 .

Geological and petrologic evolution of Huahine island (Society archipelago, French Polynesia) : an unusual intraoceanic shield volcano

Huahine (Leeward Islands, Society Archipelago) is composed of two islands, Huahine Nui and Huahine Iti, separated by the shallow Port Bourayne and Maroe bays and surrounded by a common lagoon. The two islands, however, belong to a single basaltic and trachybasaltic shield volcano, the emerged part of which was constructed during a very short period, between 2.65 and 2.52 Ma. The volcano is made of composite basaltic flows belonging to three distinct petrogenetic types, which derive from low degrees of partial melting of heterogeneous mantle sources. This building stage lead to the formation of a central caldeira. Then, a WSW-ENE trending graben formed separating Huahine Nui from Huahine Iti. As a consequence, Huahine differs from most of the other Polynesian islands which display large collapse structures opened toward the sea. After a period of inactivity of at least 0.25 m.y., magmatic activity resumed, leading to the emplacement of five trachyphonolitic intrusions along N-S trending deep regional fractures. These lavas, which do not result from the fractional crystallization of the shield basalts, are considered as derived from the melting of a deep intrusive network of dykes.