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Pétrogenèse de laves différenciées en contexte intraplaque océanique et hétérogénéité géochimique au niveau du point chaud des Marquises (Polynésie Française) : étude des îles de Ua Pou et de Nuku Hiva

Abstract : The study of hot-spot related magmas is essential for the understanding of the structure and the compositions of mantle plumes and their interactions with the oceanic lithosphere. Until the early 1990's, the geological knowledge of the Marquesas Islands was based on exploratory field studies. Thus, geochemical modellings of the Marquesan plume published between 1986 and 1993 were only supported by very partial sampling. These modellings have strongly underestimated the interactions between plume-derived magmas and the oceanic lithosphere and did not adress the complex origins of evolved lavas (trachytes and phonolites), which are sometimes very abundant in the Marquesas. Since 1995, more focused studies have shown that geochemical trends can vary considerably from one island to the other, reflecting the small scale heterogeneity of the plume together with complex lithosphere-asthenosphere interactions. The mapping programme of the Central Marquesas islands (Nuku Hiva, Ua Huka and Ua Pou) initiated by the French BRGM in 2000 has allowed their structural study together with a detailed sampling which we have used for petrologic and geochemical investigations. The Marquesas archipelago lies on an oceanic crust formed between 53 and 49 Ma at the Pacific-Farallon spreading centre and later strongly thickened beneath the central part of the archipelago where the Moho reaches depths of 15 to 20 km. Thermal models allow to test the hypothesis of rapid emplacement of a thick mafic root underplated during recent (5.8-0.4 Ma) surface hotspot magmatism. The predicted thermal subsidence declines rapidly within the first three million years, from 250 m/m.y. to 50 m/m.y. Marine terraces throughout the Marquesas are exposed at +2 m elevation and are attributed to the last interglacial highstand at 125 ka. No differential subsidence is observed between the oldest and the youngest islands. These observations are inconsistent with thermal subsidence following a recent underplating event and support rather the hypothesis of an older thickening of the Marquesas crust due to the edification of a near-axis plateau. Ua Pou is long known for its exceptionally abundant phonolites. Our first systematic sampling of this volcanic island leads to the estimation of its emerged volume to 27.5 km3 including 18 km3 of phonolites (65 %) which were emplaced during two distinct stages, 2 km3 of other intermediate and evolved lavas (8 %) and 4.5 km3 of mafic lavas (27 %). Ua Pou island displays an alkaline series ranging from basanites to phonolites with a marked bimodal distribution together with a Daly gap evidenced by the lack of phonotephrites. Major and trace element data coupled with Sr-Nd-Pb isotope analyses and K-Ar ages allow to discuss the complex petrogenetic history of Ua Pou. At ca. 4 Ma, the partial melting of a mantle plume displaying a young HIMU signature produces the Ua Pou tholeiites. Magmatic activity resumes at ca. 2.9 Ma when melting of an heterogeneous mantle with a prominent EM II + HIMU signature source produces primitive basaltic melts. Those melts are either emplaced at depth or at the surface, or evolve by fractional crystallisation towards tephrites. During the same stage, re-melting at depth of this basanitic material produces tephriphonolitic magmas, leaving an amphibole-rich residuum. These magmas evolve by closed system fractional crystallisation towards phonolitic liquids, which can undergone contamination by seawater. Whereas the emplacement of these two phonolitic types is still operating, more or less concomitantly with the production of basanitic magmas, additional open system processes are involved between 2.6 et 2.4 Ma. They lead either to tephriphonolites and phonolites contaminated by plagiogranitic-type materials with an HIMU signature or to phonolites by extreme fractionation coupled with assimilation of oceanic crust materials with a DMM signature. Ua Pou island can thus be regarded as an unusual example of an intraoceanic suite in which the prominence of evolved lavas reflects their origin from partial melting of mafic precursors followed by crustal contamination rather than from fractional crystallisation. Nuku Hiva island is composed of two nested volcanoes : the external shield volcano (Tekao volcano) formed by olivine tholeiites and the inner volcano of Taiohae showing an almost continuous alkaline series ranging from basalts to trachytes. Mafic Taiohae lavas derive from a mantle source more enriched than that of the shield volcano tholeiites, affected by time-decreasing melting rates. Intermediate lavas (hawaiites and mugearites) of the inner Taiohae volcano derive from the basalts through fractional crystallisation under high water pressure, dominated by the fractionation of amphibole (up to 25% of the cumulate). The origin of evolved lavas (benmoreites and trachytes) is constrained by their isotopic signature which is strongly different from that of basaltic-intermediate lavas of the Taiohae volcano. Their origin can be ascribed either to 1) assimilation of oceanic material with a strong EM II signature coupled with fractional crystallisation, or to 2) partial melting at depth of mafic precursors not found at the surface of the island. In Nuku Hiva island intraoceanic suite, magmatic evolution is thus mainly controlled by fractional crystallisation processes under high water pressure, while the origin of evolved lavas evidences complex lithosphere-asthenosphere interactions. Our preferred plume-hotspot model accounting for the Marquesas geochemical heterogeneity is that of a very heterogeneous plume containing randomly distributed small-sized domains with either a subducted oceanic crust signature (HIMU) and a terrigeneous sediment signature (EM II). The DMM signature of the Marquesas magmas would be acquired in a later stage and might either reflect the melting of the base of the oceanic lithosphere above the plume, or interactions between the ascending plume-derived magmas and this lithosphere. Variations of partial melting rates and the interaction between basaltic liquids and the oceanic crust within intracrustal reservoirs can also account for the magmatic heterogeneity within a single island. Although these effects are not documented for the whole set of Marquesas islands, most of the geochemical heterogeneity of their lavas is likely to reflect the mixing in various proportions of EM II, HIMU and DMM source endmembers.Postulating that the plume only contains the EM II and HIMU components and that DMM is the unique component of the oceanic lithosphere, a mixing model between these three components allows to estimate 1) the proportions of each end-member in the plume-derived liquids and 2) the mixing pattern between these liquids and the oceanic lithosphere, leading to estimate the interactions between the plume and the oceanic lithosphere. The DMM contribution appears dominant for the islands of Eiao, Nuku Hiva, Ua Huka and Hiva Oa. That of EM II is dominant for the akaline lavas of Ua Pou and that of HIMU for Fatu Hiva and Ua Pou tholeiites. For most of the Marquesas islands but the two latters ones, the contribution of DMM is quite important, a feature which may indicate either 1) that the DMM end-member is present within the plume, and/or 2) that assimilation of depleted DMM material modifies considerably the composition of the plume liquids. This last hypothesis is consistent with the existence of an oceanic plateau in the Marquesan substratum, the rocks of which having a prominent DMM signature would interact with the plume-derived magmas.
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Christelle Legendre. Pétrogenèse de laves différenciées en contexte intraplaque océanique et hétérogénéité géochimique au niveau du point chaud des Marquises (Polynésie Française) : étude des îles de Ua Pou et de Nuku Hiva. Géochimie. Université de Bretagne occidentale - Brest, 2003. Français. ⟨tel-00008677⟩

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