Le volcan Mojanda (Equateur) : deux appareils contemporains aux dynamismes éruptifs, développements et caractères géochimiques différents

Deux centres volcaniques majeurs (Mojanda et Fuya Fuya) constituent ce qui était considéré comme le volcan Mojanda. Après la construction de volcans de base essentiellement laviques, les développements et dynamismes éruptifs de ces deux appareils contemporains diffèrent. Au Mojanda, un nouveau cône est formé par des coulées et des tephra basiques. Au Fuya-Fuya, l'activité est constamment en relation avec un magmatisme acide : deux cycles commençant chacun par une éruption plinienne cataclysmale rhyolitique et se poursuivant par une activité des dômes y sont remarquables. Ils sont suivis par un gigantesque évènement de type St Helens, dont la caldera d'avalanche est ensuite occupée par un dernier complexe de dômes, encore actif à l'Holocène. Ces deux terminaux de développement sont en relation avec deux lignées magmatiques aux signatures géochimiques différentes. (Résumé d'auteur

Les adakites et les andésites riches en Mg du volcan Pichincha (Equateur) : témoins de la fusion de la croûte océanique et de la métasomatose adakitique sous la zone volcanique nord des Andes

Situatedin the fore-arc of the Northern Volcanic Zone (NVZ) of the Andes in Ecuador, Pichincha volcano is an active edifice where have been erupted unusual magmas as adakites and high-Mg andesites. The particular geodynamic setting of the ecuadorian margin (i.e. the flat subduction of the Carnegie Ridge) suggests that thermo-barometric conditions for the partial melting of the oceanic crust are accomplished beneath this volcano. Pichincha adakites possess all the geochemical and isotopic characteristics of slab melts described in various other arc settings. High-Mg andesites with geochemical characteristics close to those of adakites present strong enrichments in MgO that suggest that, once they were produced by ca. 10 % partial melting of the downgoing subducted slab, some adakites en route to the surface strongly interacted with the peridotitic mantle wedge. Adakitic magmas could then represent, as in many other arcs where slab melting occurs, the principal metasomatic agent of the mantle in the NVZ in Ecuador

La collision ride des Loyaute/Arc des Nouvelles Hebrides (Pacifique Sud-Ouest)

The collision between the 22 degree S salient of the Loyalty Ridge and the southern New Hebrides Arc began some 300,000 years ago. Nevertheless, the ridge probably influences, since 1 to 2 Ma, the subduction at the southern arcuate New Hebrides Trench through its action on the bulge of the downgoing plate. The morphology of the ridge, its deep root and relative buoyancy affect the flexure and the subduction of the downgoing plate in several ways. The proposed effects of this arc-ridge collision may be traced on a regional scale: in front of the collision zone, the southern New Hebrides active volcanic line is weakly developed, the intra-arc troughs are absent, and the Benioff zone is short. On the nearby north Fiji basin, the offset of the southern active spreading ridge could also be a consequence of this collision

Annotated record of the detailed examination of Mn deposits from the GEORSTOM III NORD (R/V Le Noroit) cruise stations

The D'Entrecasteaux Zone (Southwest Pacific) is an arched submarine horst- and graben structure, which extends from the northern end of the New Caledonia ridge to the western border of the New Hebrides island arc. A review of the bathymetry, seismic-reflection data, paleomagnetism, gravimetry, seismology and DSDP data available for this area is combined with a study of basaltic samples dredged along the horsts of this regional fracture zone. These basalts show strong petrographic and chemical affinities with MORB. Their fissiontrack ages range between 56 Ma (Paleocene-Eocene boundary) and 36 Ma (Eocene-Oligocene boundary). It is suggested that the D'Entrecasteaux Zone represents the northern arcuate extension of the northeast-dipping Eocene subduction/obduction zone, located along the New Caledonia/Loyalty Islands ridge, while its present morphology appeared from post-obduction extensional movements, resulting in a progressive uplift of basaltic ocean floor since Middle Miocene times