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Métamorphisme hercynien et antéhercynien dans le bassin du haut-Allier (Massif Central français)

Abstract : The zoneographic method (ZM) was proposed by J. Jung and M. Roques during the 1940's and used by french-speaking authors for the interpretation of metamorphic regions of crystalline schists. Two main trends were advocated: (1) metamorphic degree (2) migmatization. --(1) Isometamorphic zones were defined on a metapelite type-sequence of "ectinites" (rocks affected only by topochemical recrystallization) as : X, unmetamorphic zone - Y2, upper micaschist zone (chlorite-sericite schists) - Y1, lower micaschist zone (biotite-muscovite micaschists) - Z2, upper gneiss zone (biotite-muscovite gneisses) - Z1, lower gneiss zone (biotite-sillimanite gneisse) - U, ultra-low gneiss (orthoclase-cordierite gneisses). --(2) By contrast to ectinites, migmatites, viewed as non-topochemically affected by metasomatism, are feldspar-rich, mainly alkaline gneisses where major mineral species are biotite, muscovite, cordierite, sillimanite, garnet. Their pattern is regularly layered for the embrechite rock-type or more disturbed (nebulite, arterite, agmatite...) leading to the anatexite rock-type. Anatexite and embrechite may coexist anywhere in the type-sequence, nevertheless embrechites are more frequent in the upper migmatite levels. Around the town of Brioude, in the french Massif Central basement massif, the crystalline schists of the Allier river upper basin (ARUB) are surrounded by younger, Hercynian granite massifs of Carboniferous age (Margeride to the S; Velay to the E; Livradois-Forez to the N) and overlain to the W by the Cainozoic to Recent basalt flows from the Cantal volcano. The highly metamorphic ARUB rock-types comprise a major felsic component with kyanite-cordierite-sillimanite gneisses and micaschists, where various formations were mapped (pl. 3: ~150 km x 100 km), plus a number of ultramafic outcrops (pl. 4) and ore occurences (pl. 5: Sb, U, fluorite). Based on the ZM, the basement of the ARUB is interpreted as composed of two major units. The NW-SE trending Massiac Synclinorium (SM, pl. 1), W of Brioude, is diagnosed as exhibiting an early zoneography-I comprising a Z1-zone. This "ectinite-I" massif, possibly issued from a Precambrian protolith, has also been affected by a "migmatite-I" event, with various anatexites plus an anatectic granite in the deeper part of the Z1-zone. The SM key-formation is the Brivadois Leptyno-Amphibolic Group ( where Brivadois < Brioude) with more than 80% of leptynites (= alkali feldspar-rich felsic gneisses, primitively igneous rocks) alternating with subordinate mafic rocks (peridotite to amphibolite). The SM is encircled to the SW, SE and NE by the Desges-Senouire-Doulon synclinorium (pl. 2) comprising Y1-, Z2-, Z1-zones of the zoneography-I with a similar related migmatite-I event. Later, this second synclinorium has been affected by an Hercynian (~Early Carboniferous in age) event of deformation and lower grade metamorphism (chlorite-sericite schists of the Y2-zone), related to a zoneography-II and associated to a migmatite-II event.
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Submitted on : Monday, December 17, 2012 - 4:08:55 PM
Last modification on : Wednesday, November 29, 2017 - 9:54:20 AM
Long-term archiving on: : Sunday, December 18, 2016 - 2:41:34 AM

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François-Hubert Forestier. Métamorphisme hercynien et antéhercynien dans le bassin du haut-Allier (Massif Central français). Géochimie. Faculté des Sciences de l'Université de Clermont-Ferrand, 1961. Français. ⟨NNT : ⟩. ⟨tel-00765853⟩

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