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Pressure-temperature-time evolution in the high-grade metamorphic basement of the Siviez-Mischabel nappe (Val d’Hérens, Val d’Anniviers)

Denomination Géologie Structurale et Alpine
Defense Maîtrise : Univ. Genève, 2014
Abstract The Banded amphibolites, a mafic layer situated at the base of the Adlerflüe Formation in the basement of the Sivez-Mischabel (SM) nappe (Valais, CH), contain local occurrences of eclogite; witnesses of a pre-Alpine high-grade metamorphic event. They have a composition of NMORB enriched in LILE, U, Th, Pb and LREE which are characteristic for a back-arc depositional environment. The pressure-temperature time path was modelled for these eclogite occurrences using the program DOMINO and the garnet-clinopyroxene Fe2+-Mg geothermometer. Peak metamorphic conditions were estimated to be 15-17 kbar and 650-800°C. The retrograde event, responsible for the breakdown of omphacite and the growth of pargasite is estimated at approximatively 8 kbar and 500-600°C, which places it in the amphibolite facies. The age of the youngest zircon within the SOPA (“schistes oeillés à porphyroblastes d’albite”), a metasedimentary unit directly overlying the eclogites, was determined to be 520.9±7.1 Ma which, together with the age of the crosscutting Thyon metagranite (500+3-4 Ma), provides an age window for the deposition of the SOPA. Timing of the emplacement of the Banded amphibolites and related rocks can therefore be constrained to the mid to late Cambrian. The recrystallized domains in zircon of all studied rock types yield coherent ages, with a calculated mean value of 346.58±0.97 Ma. This lower Carboniferous age is consistent with the ultra-high pressure events linked to the Variscan orogeny and is proposed as the age of the eclogitic metamorphism in the Siviez-Mischabel nappe.
Keywords Siviez-Mischabel nappeEclogiteZircon datingVariscan orogenyGarnetHigh pressure metamorphismPhase diagramGeothermometryCambrian
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Research group Petrology and Volcanology
Project Augustin Lombard
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NORMAND, Raphaël. Pressure-temperature-time evolution in the high-grade metamorphic basement of the Siviez-Mischabel nappe (Val d’Hérens, Val d’Anniviers). Université de Genève. Maîtrise, 2014. https://archive-ouverte.unige.ch/unige:87540

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Deposited on : 2016-09-20

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