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Pétrologie des laves d'un volcan intraplaque océanique : le Karthala, île de la Grande-Comore (R.F.I. des Comores)

Abstract : Grande-Comore is the largest island of an archipelago in the Mozambique channel. Il lies on crust which appears to be of oceanic nature, according to previous studies. Karthala is the main shield volcano of this island, and the only one with historical activity. This has occured along two rift zones and at their intersection in the summit caldeira. The product of older volcanism form the Badjini at the south end of Grande-Comore. The low undersaturation and the compositions of the lavas can be explained as resulting essentially from a same degree of partial melting of a single source where phlogopite and garnet are more or less residual. A geophysical model based on the melt production rate of plumes, as well as the comparison with high-pressure experimental data, confirm a high pressure of partial melting between 24 and 33 kbar. Subalkaline and transitional lavas of different building stages of Karthala have various parageneses : olivine basalts, oceanites, ankaramites and plagioclase phyric basalts, which shows that various differentiation processes exist. The evolution of lavas compositions allow us to show clearly,the existence of two distinct magmatic chambers. One lies at about 30 km depth (10 kbar), where a more or less wehrlitic fractionating assemblage early enriched basaltic liquids in incompatible elements. The other is more superficial, and lies near the base of the volcano basis, there occurs principally oceanitic and clinopyroxenitic fractionating, and gabbroic at less scale, leading to liquids of hawaiitic and trachy-basaltic composition.
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Submitted on : Friday, November 23, 2012 - 3:09:44 PM
Last modification on : Thursday, February 25, 2021 - 11:40:24 AM
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  • HAL Id : tel-00756691, version 1

Citation

Franck Desgrolard. Pétrologie des laves d'un volcan intraplaque océanique : le Karthala, île de la Grande-Comore (R.F.I. des Comores). Volcanologie. Université Joseph-Fourier - Grenoble I, 1996. Français. ⟨tel-00756691⟩

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