L’île de Moorea (Société) : édification rapide d’un volcan-bouclier polynésien

Abstract

Moorea (Society Islands) is a roughly triangular shaped island (16 km long and 12 km wide) which represents the summit of an eroded alkali-basaltic shield volcano (Fig. 1). It lies on 4000m-deep Pacific oceanic trust of Maastrichtian age and reaches an elevation of 1207 m above sea level at Mt. Tohiea. The morphological particularity of the island lies in the presence of a Sharp peak (Mt. Rotui) close to the centre of the caldera (Fig. 2). The Moorea volcanic suite ranges from predominant alkali basait to trachyphonolite (with a single sample of quartz-normative trachyte) through hawaiite and very abundant mugearite and benmoreite (Fig. 3 and Table I). Geology and chronology. The Mt. Tohiea shield volcano, which was built-up between 1.72 and 1.51 Ma, is made up of five units that, by order of emplacement, are: (i) the Afareaiatu lahar breccias, (ii) the Belvédère zeolitized basaltic flows, (iii) a thick pile of composite basaltic lava flows belonging to the Mouaputa Formation, which makes up most of the Tohiea shield, (iv) the Mt. Rotui benmoreite units, and finally (v) the uppermost Tohiea columnar-jointed basaltic fzows, which were emplaced within depressions on the flanks of the main shield. The northern part of the shield collapsed immediately at the end of the building stage, leaving an isolated remnant forming Mt. Rotui (Fig. 4). We interpret this event as a gravity collapse involving a décollement of the northern shield units at the level of the lahar breccias (Fig. Sa). This décollement was possibly triggered by the intrusion of numerous sills and dykes below the future caldera ,floor (Fig. 5b). Three mass+ were emplaced along the caldera rim immediately after the collapse event: the Paopao (1.52 Ma) and Paveo Point basaltic volcanoes and the more complex Papetoai massif (1.53 to 1.47 Ma) made up of benmoreite and trachyphonolite. Volcanic activity in Moorea ended 1.36 Ma ago with the intrusion of the Paopao trachyte. Petrology The dominant rock type on Moorea is an alkali basalt characterized by considerable enrichment in the most incompatible elements (Fig. 6) and fractionation of light rare earth elements (LREE) with respect to heavy, rare earth elements (HREE), indicative of the occurrence of residual garnet in the source. Intermediate and evolved lavas are char- acterized by increasing concentrations of the most incompatible trace elements together with selective depletions in Ba, Sr, P and Ti with respect to basalts (Fig. 6). These features are consistent with a ,fractional crystallization process in volving separation of olivine, clinopyroxene, calcic plagioclase, Fe-Ti oxides, apatite and ,finally anorthoclase. Crystal .fractionation accounts ,for most of the progressive increases in incompatible ele- ments (Fig. 7), but some mugearites and benmoreites that are abnormally rich in transition elements (e.g. Cr, Fig. 7) display textural evidence for magma mixing: they contain mafic glass globules together with numerous xenocrysts of basaltic origin (Mg-rich olivine, Cr-bearing spinel and diopside). The particularly high abondance of intermediate lavas (mugearite and benmoreite) in the Moorea alkalibasalt suite is likely due to magma mixing events which followed the refilling of the upper reservoir, at ,first containing trachyphonolitic magmas, by uprising basaltic magmas (Fig. 8).