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Le granite du Manaslu marqueur de la subduction et de l'extension intracontinentales himalayennes

Abstract : The Manaslu granite (Central Nepal) allows to propose a model of emplacement of a magmatic body in an intracontinental collision and extensional context and to study the relationships between thermal, anatectic and tectonic processes in the Himalayan range from 30 to 15 Ma. The metamorphic study of the contact aureole suggests a depth of emplacement of the granite between 18-21 km for the base to 9-13 km at the top with a temperature of 550 ± 50°C. The occurence of magmatic H20-fluid in the aureole has been demonstrated by stable O and H isotope analysis. The great thickness of the upper continental crust implies the piling up of nappes above the Tethyan Sedimentary Series, before granite emplacement. Based on a structural, microstructural and magnetic (AMS) study of the granite and the surrounding rocks, we propose a model of emplacement of the Manaslu granite controled by:(i) East-West dextral shearing and lateral expansion toward the East and the North-East of the successive magmatic pulses, in an extensional fractures, oblique to the shear direction. New 40Ar/39Ar ages from the upper contact aureole compare with others available data are consistent with the crystallization of the pluton at around 25 Ma followed by a - 6 Ma period of cooling at rates on the order of 20°C/m.y.; cooling then appears to have accelerated to rates on the order of 100°C/Ma sometime between 16 and 19 Ma. We speculate that this rapid cooling may be related to tectonic denudation by normal fauiting above the granite. This 18 ± 1 Ma period of extension known ail long the Himalayan range could be responsible of a new magmatic event in the Manaslu area. A geochemical study of major, trace-elements and Rb/Sr isotopic study of the Manaslu granite allows to propose a new model of melting of the High Himalayan Cristalline: low-fraction melting of metapelites under fluid-absent conditions (XH20 < 0.3) follows by high degree of partial-melting of metagreywackes under water-rich conditions (XH20 > 0.7). The ages, the modalities of emplacement of the High Himalayan leucogranites and the pressure-temperature conditions in the High Himalayan Crystalline suggest a younger activation of the MCT toward the East, compatible with the counter-clockwise of lndia and eastward extrusion of Tibet during the India-Asia collision since 25 Ma.
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Submitted on : Monday, August 13, 2012 - 9:17:08 AM
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  • HAL Id : tel-00723696, version 1



Stéphane Guillot. Le granite du Manaslu marqueur de la subduction et de l'extension intracontinentales himalayennes. Pétrographie. Université Joseph-Fourier - Grenoble I, 1993. Français. ⟨tel-00723696⟩



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