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Péridotites et serpentinites du complexe ophiolitique de la Nouvelle-Calédonie.

Abstract : The New Caledonia ophiolite (Southwest Pacific) is one of the world's largest (500 km long, 50 km wide and 2 km thick). It consists of a large continuous massif occurring in the south of the island and some smaller isolated klippes located along the West coast.The peridotites are mostly harzburgitic, except on the northernmost massifs which are also composed of lherzolites. Emplaced during Eocene, the ophiolite is thrust upon the magmatic Poya terrane, composed of basalts from midocean ridges, back arc basins and ocean islands. This nappe was tectonically accreted from the subducting plate prior and during the obduction of the ultramafic nappe. Considering petrological, geochemical and mineralogical approaches together with the development of new analytical techniques, our study shows that the New Caledonia ophiolite has been affected during its evolution by multiple magmatic, metamorphic and alteration processes. The geochemical analysis of the peridotites show that the ophiolite recorded two successive melting processes: (1) a first in a ridge environment during the opening of the South Loyalty Basin from Late Cretaceous to Paleocene. This melting process formed the oceanic basalts of the Poya terrane and their associated residue, the lherzolites composing the northernmost massifs; (2) a second during Eocene in a forearc environment, leading to the genesis of boninitic melts and highly depleted peridotites (i.e. harzburgites) that form the bulk of the ophiolite. The occurrence of both styles of melting regime within the same ophiolite can only be explained by a forced inception of subduction at or near an active spreading center. At the same time of these magmatic events, our results show that the peridotites have been serpentinized in the different environments in which the ophiolite has evolved. We highlight four episodes: (1) formation of lizardite by interaction of the lherzolites with the seawater in a ridge environment during the opening of South Loyalty Basin, (2) formation of lizardite in the harzburgites by metasomatic fluid circulations derived from the subducting slab, during Eocene subduction; (3) formation of antigorite by metasomatic fluid circulations associated with the rapid isothermal exhumation of the metamorphic Diahot terrane, responsible of heat advection below the ophiolite, and (4) formation of chrysotile during the obduction by meteoric fluid circulation. Finally, after complete obduction (at ~ 34 Ma), our results show that the ophiolite has experienced a strong supergene alteration due to tropical conditions. This alteration is manifested by a laterization process leading to the leaching of silica, magnesium and rare-earth elements in peridotite from the superficial part of the ophiolite. These elements are transported by the percolation of meteoric fluids to the bottom of the ophiolitic sequence in which they accumulate and reprecipitate by supersaturation forming magnesite, amorphous silica and talc. Thus, thanks to the development of new analytical methods, we show that it is possible to trace the evolution of an ophiolite, from its formation in depth to its alteration at the Earth surface.
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Submitted on : Monday, August 16, 2010 - 5:35:53 PM
Last modification on : Thursday, November 19, 2020 - 3:54:26 PM
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Marc Ulrich. Péridotites et serpentinites du complexe ophiolitique de la Nouvelle-Calédonie.. Géochimie. Université de Nouvelle Calédonie, 2010. Français. ⟨NNT : 2010NCAL0046⟩. ⟨tel-00509848⟩

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