Ages (K-Ar) et durées d’édification subaérienne des îles de Moorea, Raiatea et Maupiti (Société, Polynésie française)

Abstract

Calderas are one of the main structural Moorea (Fig. la) is located 20 km to features that characterize the Society Islands, the NW qf Tahiti. The shield-building a group of islands related to hot-spot type volcanic activity (Blanchard et al., 1981, Brousse and Berger, 1985). Therefore, it is important to bracket the ages of these collapses. Two main stages characterize these within-plate oceanic volcanoes (Wilson, 1989). The ,first is the shield-building stage, which comprises the main part of the volcano and which ends with the formation of a central caldera or by lateral collapse(s) During the so-called post-shield building stage, lavas fill the caldera or are erupted onto the flanks of the volcano after the collapse.We have carried out a geochronological study of the islands of Moorea, Raiatea and Maupiti to bracket the ages of the calderas, and to estimate the duration of a) the build-up of the subaerial parts of the shield and b) the post-shield building stage. For this, representative samples of each stage of building were selected on the basis of revised stratigraphie data, and then duted using the Cassignol K-Ar technique. Moorea (Fig. la) is located 20 km to the NW of Tahiti. The shield-building stage is represented by the Tohiea volcano, the post-shield stage by the Papetoai massif (Blanchard, 1978; Bellon and Blanchard, 1981; Blanchard et al., 1981; Le Dez, 1996). The published ages ,for Moorea (Dymond, 1975; Duncan and McDougall, I976; Diraison, 1991) bracket the subaerial volcanic activity between 1.90 and 1.55 Ma. Raiatea (Fig. lb), located 215 km NW of Tahiti, consists of two coalescent structures (Blais et al., 1997); a basaltic shield volcano and the Temehani ridge made up of trachytic flows that represent the postshield stage. The existing ages ,for the subaerial part of Raiatea range between 3.17 and 2.39 Ma (Duncan and McDougall, 1976 ; Diraison, 1991). Maupiti, 310 km NW of Tahiti, is the remnant part of a shield volcano intruded by post-shield dylkes. Previous geochronological work (Duncan and McDougall, 1976: Diraison, 1991) determined age values scattered between 4.79 and 4.05 Ma. Highly accurate radiometric dating has been conducted on IO samples ,from Moorea, 14 from Raiatea and 4.from Maupiti, using the Cassignol technique of K-Ar dating (Cassignol and Gillot, 1982; Gillot and Cornette, 1986). All the results are reported in Table 1. The oldest dated subaerial lava front Moorea has an age of 1.72 ± 0.03 Ma, which is used to define the lower limit subaerial shield-building stage. This stage ended at 1.51 ± 0.02 Ma. The samples from the post-shield stage have ages scattered between 1.53 ± 0.02 and 1.36 ± 0.02 Ma This yield durations of 0.21 ± 0.05 Ma and 0.17 ± 0.04 Ma, respectively, ,for the shield-building (emerged part) and post-shield building stages. We propose an age of 1.52 ± 0.03 Ma ,for the caldera collapse. The basaltic lavas from Raiatea, which characterize the shield-building stage, have ages ranging from 2.75 ± 0.03 to 2.52 ± 0.03 Ma, which yields a duration of 0.23 ± 0.06 Ma for the emerged part of the stage. The post-shield stage extends from 2.54 ± 0.03 to 2.44 ± 0.03 Ma, which corresponds to a duration of 0.10 ± 0.06 Ma. The age of the collapse is estimated to be around 2.53 ± 0.03 Ma. For Maupiti, the ages obtained for the shield-building stage range between 4.51 ± 0.05 and 4.47 ± 0.05 giving a duration 0.04 ± 0.10 Ma. The post-shield dyke has an age of 4.21 ± 0.04 Ma. The durations calculated above are underestimated because a part of the subaerial massif which cannot be estimated, is currently below sea level following subsidence of the massif: Therefore, only the youngest part of the shield-building stage can be sampled. The full duration of the subaerial part of the shield-building stage may be estimated by the use of the Age (Ma) vs. Distance (km) diagram (Fig. 2). Following Brousse and Leotot (1988), we consider a drift speed of Il.1 cm/y for the Pacific plate. The most recent emerged island of the Society lineament is Mehetia. The oldest dated subaerial lava has an age of 0.31 Ma (White and Duncan, 1996). We use this age to define the origin of line A in Fig. 2. According to this mode], the subaerial volcanic activity started 1.48, 1.85, 3.55 and 4.45 Ma ago at Tahiti, Moorea, Raiatea and Maupiti, respectively. The Tahiti and Moorea model ages are slightly older but not significantly different from the radiometric ages. This approach validates the radiometric ages. For Raiatea, the model age for the beginning of the subaerial activity (3.55 Ma) is significantly older than the radiometric age (2.75 Ma). This can be explained by a greater subsidence effect. Nevertheless, if we take into account the published ages for Huahine (Duncan and McDougall, 1976; Roperch and Duncan, 1990; Diraison, 1991), Tahaa (Duncan and McDougall, 1976; Gisbert, 1989; Diraison, 1991) and Bora Bora (Duncan and McDougall 1976; Diraison, 1991), we observe that the model ages are not significantly different from the radiometric ages. We propose that the volcanic activity in Raiatea is connected with the volcanic activity in Huahine and Tahaa. On Figure 2, it appears that the age of the oldest subaerial lava of Raiatea corresponds to the boundary between the shield and post-shield stages of Huahine and Tahaa. A relationship between the volcanic activities of Moorea and Tahiti is also proposed. The beginning of the subaerial activity of Tahiti coincides with the end of activity at Moorea.