Les mégacristaux de feldspaths alcalins dans les formations porphyroïdes des massifs cristallins externes de Belledonne et des Grandes Rousses ( Alpes françaises). Recherche de leurs origines à partir de leurs microstrucutres et de leur composition.

Abstract : The porphyroid formations of the Belledonne and Grandes Rousses outer crystalline massifs, in the Western Alps, are of pre·Westphalian age. They are made up of orthogn,eisses. and of granitoid rocks. The alkali feldspar megacrysts, of variable size, are mainly subauromorphic to automorphic. Petrographic study of hand specimens. in conjunction with detailed study of oriented thin sections of the megacrysts, allowed recognition and characterization of the various recognizable textures. These textures chiefly consist of syncrystallization of potash feldspar with albite, of thermal cracks. and of inner crystallizations. They have been used to define the original and evolutionary structural levels of the megacrysts studied. Thus it was generally possible to ascertain the nature of the original porphyroid rock. After being emplaced either provisionally or for good, this original rock may have been either metamorphized, or tectonized, or both. We have therefore been able ta compare the porphyrotd series wirh one another, and to propose a model for their evolution. Four units have thus been identified. They are characterized by megacrysts which give evidence of original granitic of granitoid material being emplaced into a structural level of evolution which was either more or less deep-seated "plutonic", or "subvolcanic". Some of these units contain certain facies which cannot be correlated with the whole of the gneisses studed : this is the case for the Sept Laux porphyroid granite, and for the Cevins orthogneiss. Those facies which can be correlated are grouped according to two geochimical trends, viz. (1) an alkaline trend= Pussy + Montjoie + Alpetta + Lac Blanc (2) a basic trend= Saint Colomban + gneiss amphibolique de Pussy. The first set (1) ,is regarded as due to one single granitic intrusion. Erosion, coupled with mountain-building, allows the intermediate-depth facies to be observed to the North (Pussy + Montjoie). To the South, the Alpetta unit and the Lac Blanc formation are the most deep-seated facies of this intrusion. Set (2) is regarded as consisting of intrusive granodioritic material. The present·time ground surface cuts it at a structural level of intermediate depth. Facies showing a sub-alkaline trend and Pussy amphibole gneiss have aIl been acted upon by a metamorphism of greenschist facies. The Lac Blanc formation is the only one to exhibit volcano-sedimentary features. Albitization of certain megacrysts is not a secondary phenomenon in their evolution. The albite present in the crystals is an albite of syncrystallization. At a given concentration. it indicates a "subvolcanic" structural level.
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Mineralogy. Universite Scientifique et Medicale de Grenoble, 1982. French


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Dominique Bonnet. Les mégacristaux de feldspaths alcalins dans les formations porphyroïdes des massifs cristallins externes de Belledonne et des Grandes Rousses ( Alpes françaises). Recherche de leurs origines à partir de leurs microstrucutres et de leur composition.. Mineralogy. Universite Scientifique et Medicale de Grenoble, 1982. French. <tel-00563221>

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