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Universite Du Quebec a Chicoutimi UNIVERSITE DU QUEBEC A CHICOUTIMI CHRONOSTRATIGRAPHÏE ET PETROGRAPHIE DU COMPLEXE GNEISSIQUE DE CHICOUTIMI EN BORDURE DU COMPLEXE ANORTHOSITIQUE DU LAC ST-JEAN par MICHEL HERVET Département des Sciences Appliquées MEMOIRE PRESENTE EN VUE DE L'OBTENTION DE LA MAITRISE EN SCIENCES APPLIQUEES EN GEOLOGIE SEPTEMBRE 1986 UIUQAC bibliothèque Paul-Emile-Bouletj Mise en garde/Advice Afin de rendre accessible au plus Motivated by a desire to make the grand nombre le résultat des results of its graduate students' travaux de recherche menés par ses research accessible to all, and in étudiants gradués et dans l'esprit des accordance with the rules règles qui régissent le dépôt et la governing the acceptation and diffusion des mémoires et thèses diffusion of dissertations and produits dans cette Institution, theses in this Institution, the l'Université du Québec à Université du Québec à Chicoutimi (UQAC) est fière de Chicoutimi (UQAC) is proud to rendre accessible une version make a complete version of this complète et gratuite de cette œuvre. work available at no cost to the reader. L'auteur conserve néanmoins la The author retains ownership of the propriété du droit d'auteur qui copyright of this dissertation or protège ce mémoire ou cette thèse. thesis. Neither the dissertation or Ni le mémoire ou la thèse ni des thesis, nor substantial extracts from extraits substantiels de ceux-ci ne it, may be printed or otherwise peuvent être imprimés ou autrement reproduced without the author's reproduits sans son autorisation. permission. RESUME Le Complexe Gneissique de Chicoutimi (C.G.C.), situé en bordure du Complexe Anorthositique du Lac St-Jean (C.A.L.S.J.), se subdivise en quatre unités lithostructurales majeures: 1) un socle gneissique avec déformations et migmatisations polyphasées d'âge I et II, 2) les plutonites majeures d'âge III constituées de la suite bimodale d'intrusifs "anorthosite-charnockite" accompagnée d'une déformation (Dj) et recoupées par les dykes de métabasite III, 3) les plutons "granitiques" tardi- ou post-tectoniques, 4) le magmatisrae alcalin d'âge IV associé au graben du Saguenay. Dans la région de l'étude, les unités d'âge I sont constituées de gneiss de protolithes pélitiques et volcano- sédimentaires possiblement pré-hudsoniens. Ceux-ci furent affectés d'une première phase de métamorphisme, attribuée à l'orogénie hudsonienne, ayant produit une déformation D^ et une phase de migmatisation estimée à des conditions équivalentes a la courbe de fusion minimum du système granitique, soit c. 650°C à 4 Kb. L'essaim de dykes de métabasite II, constituant 5 à 10 pourcent du socle gneissique, semble relié à l'injection volumineuse de magma basique à la base de la croûte dans un contexte tectono-magmatique anorogénique de rift actif. La seconde phase de métamorphisme, d'âge grenvillien, coïncide avec la déformation Dg. et a produit une migmatisation extensive, par anatexie, des unités du socle dans des conditions T-P estimées autours de 800°C et 6 Kb. Le groupe d'âge III se subdivise en trois périodes: Inférieur, Moyen et Supérieur. Les unités des périodes Inférieur et Moyen sont constituées d'une suite intrusive de composition bi-modale dominée par la mise en place, au niveau structural actuel, du lobe de Chicoutimi du C.A.L.S.J.. La ferrodiorite, trouvée en bordure de ce dernier, ainsi que les injections plus tardives d'enderbite et d'opdalite, constituent le reste de la suite basique. La suite acide est constituée de l'intrusion de la farsundite, pénecontemporaine à la mise en place de l'anorthosite, suivie de l'adamellite à quartz ainsi que des intrusions de granite oeillé et de la mangérite de Chicoutimi. L'adamellite mafique constitue un faciès hybride intermédiaire. Ces suites sont contrôlées par le fractionnement du plagioclase et offrent un patron d'évolution similaire à celui de la suite de Morin sur le diagramme Qz-Pl-Or. La mise en place de l'anorthosite à l'état solide, par diapirisme, a produit une déformation (D,) concordante au contact, localisée de part et d'autre de la bordure et qui se superpose aux déformations grenviliennes. Les unités d'âge III Supérieur sont constituées des dykes de métabasite III syn- et post-cinématiques, ces derniers constituant 5 à 8 pourcent en volume des unités, confinés à une bande large de 2 à 3 Km au contact de l'anorthos" te. Le faciès métamorphique granulitique a opéré durant toute la période d'âge III. La rétromorphose est un phénomène tardif qui offre un développement hétérogène en bordure du contact du C.A.L.S.J.: elle est issue de la combinaison de facteurs litho-tectoniques qui ont produit une rééquilibration des unités au faciès amphibolite. Des déformations tardi-grenviliennes auraient contrôlé l'emplacement des intrusions alcalines et des déformations associées au graben du Saguenay. L'abondance des massifs anorthositiques a la surface de la province de Grenville serait l'aboutissement naturel d'un processus de rééquilibration dynamique infra-crustale à la suite de l'épaississement de la croûte associée à une collision continentale. ABSTRACT The Chicoutimi Gneissic Complex (C.G.C.), located on the Southeast border of the Lac St-Jean Anorthosite Complex (L.S.J.A.C.), is subdivided in four major lithostructural units: 1) A gneissic basement with polyphase deformations and migmatizations of Age groups I and II, 2) The plutonic rocks of Age group III composed of the "anorthosite-charnockite" bimodal intrusive suite associated with the D3 deformation and cross-cut by the metabasite III dykes, 3) The late- to post-tectonic "granitic" plutons, 4) The alkalic magmatism of Age group IV associated with the Saguenay graben. In the study region, the gneisses of pre-Hudsonnian pelitic and volcano-sedimentary protholiths compose the units of Group of Age I. The first metamorphic event, coïnciding with the D ^.deformation, is related to the Hudsonian Orogeny which produced the M j_migmatization at T-P phase conditions estimated to be equivalent to the granite minimum melt curve, about 650°C and 4 Kb. The metabasite II dyke swarm, making up 5 to 10 volume percent of the gneissic basement, seems related to voluminous injection of basic magma at the base of the crust in an anorogenic tectono-magmatic context of active rifting. The second metamorphic event, of Grenvillian age, coïncided with the Da deformation and it is associated with an extensive migmatization, by anatexis, of the basement units at P-T conditions estimated around 800°C and 6Kb. Age group III is subdivided in three periods: Early, Middle and Late. The units of the Early and Middle periods are composed of an intrusive suite of bi-modal composition dominated by the "emplacement" of the Chicoutimi lobe of the L.S.J.A.C. at the present structural level. The ferrodiorite, bordering the latter, and the late injections of enderbite and opdalite, constitute the rest of the basic suite. The acid suite is composed of injections of farsundite penecontemporaneous with the "emplacement" of the anorthosite, followed by quartz adamellite and by mangerite in the Chicoutimi area. An intermediate hybrid faciès is composed of the mafic adamellite. These suites were controlled by plagioclase fractionation and offer an evolutionary trend similar to that of the Morin suite on the Qz-Pl-Or diagram. Solid state "emplacement" of the anorthosite, by diapirism, produced the D3 deformation, concordant to the contact, limited to both sides of the border zone and which is superposed on the Grenvillian deformations. The units of the Late period of Group of Age III are composed of the syn- and post-kinematic metabasite III dykes, the latter constituting 5 to 8 volume percent of the units, confined to a 2-3 Km wide band at the contact of the anorthosite. Granulite grade metamorphic conditions faciès prevaled during all three periods of Age group III. Retromorphism is a late phenomenon which developed heterogenously near the contact zone of the L.S.J.A.C. It stems from a combin'.-ion of litho-tectonic factors which led to a re-equilibration of the units to the amphibolite facies. The location of the alkalic intrusions and the deformation associated with the Saguenay graben may be related to and controlled by late Grenvillian deformation. The abundance of the anorthosite massifs in the Grenville Province could be the natural result of a dynamic infra-crustal re-equilibration process following a crustal thickening associated to a continental collision. TABLE DES MATIERES v page Résumé i Abstract iii Table des matières v Liste des figures • xii Liste des planches • xiv Liste des tableaux xvi Remerciements • •« xviii INTRODUCTION 0.1 Cadre géologique régional 1 0.2 Enoncé du problème et but 2 0.3 Travaux antérieurs 3 0.4 Localisation et méthode de travail 4 0 .5 Plan du mémoire • 6 0.6 Nomenclature • 6 CHAPITRE I : GEOLOGIE DE LA REGION DU SAGUENAY-LAC ST-JEAN 1.0 Introduction 13 1.1 Unités d'âge I 14 1.1.1 Paragneiss, gneiss mafiques et "Granites" I 14 1.1.2 Migmatisation I 15 1.1.3 Déformation I et Métamorphisme 15 1.2 Unités d'âge II 15 1.2.1 "Granites" II et Dykes de métabasite II 16 1.2.2 Migmatisation II 16 1.2.3 Déformation II et Métamorphisme 17 1.3 Unités d'âge III 17 1.3.1 Complexe anorthositique du lac St-Jean (C.A.L.S.J.) 18 1.3.2 Plutonites ceinturant le C.A.L.S.J. et les dykes basiques et acides III 19 1.3.3 Déformation III et Métamorphisme 19 1.4 Unités d'âge IV 20 CHAPITRE II : LE SOCLE GNEISSIQUE D'AGE I ET II 2.0 Introduction 27 2.1 Analyse structurale 27 2.1.1 Résumé et interprétation 30 2.2 Métasédiments 31 vi 2.2.1 Migmatisation I 32 2.2.1.1 Description mégascopique 32 2.2.1.2 Description microscopique : Le mobilisât ...
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