Abstract

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.

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