ALBERT

Description: The Spanish region of Galicia is situated in the extreme north-western part of the country due North of Portugal and West of Asturias. It is bordered by the Atlantic Ocean to the West and by the Bay of Biscay to the North (see fig. 1). The area under investigation concerns the western provinces of La Coruña and Pontevedra mainly. Apart from early reconnaissance work by Schulz (1858), Barrois (1892), Sampelayo (1922), Lotze (1945), Carlé (1945), Navarro and del Valle (1959) the area is at present being investigated and mapped on a scale of 1:50.000 by López de Azcona, Parga Pondal and their associates for the Instituto Geológico y Minero de España. So far nine sheets and explanatory memoirs have been published between 1948 and 1956. Parga Pondal has also published a geological sketch map on a scale of 1:400.000 and an explanatory note of the province of La Coruña in 1956, and since 1931 he has contributed substantially to the knowledge of Galician geology in a series of papers concerning petrological, mineralogical, tectonic and sedimentological aspects of it. Between 1955 and 1959 de Sitter and Zwart conducted geological research by the Department of Structural and Applied Geology of the University of Leyden in the area between Lage and Malpica. Summaries of their results appeared in 1955 and 1957, while one of their associates, Insinger, published a short account of his work in the vicinity of Mugía in 1961.

Description: Discordant bedding is being reported with increasing frequency from rocks of almost any grade of metamorphism. It is, usually without much discrimination, cited as evidence of sedimentary origin or even as an indication of the sequence of deposition. In the present paper examples are given of discordant structures simulating both deltaic and torrential cross lamination but on closer inspection obviously due to differential stretching and variable orientation of cleavage in a thinly laminated quartzphyllite which has been isoclinally folded and subsequently flattened in the bedding plane. Microfabric analysis of micaceous cleavage planes can be a valuable aid in the distinction between discordant structures of sedimentary and deformational nature, and the more so the higher their degree of recristallisation and neomineralisation provided that these processes are strictly nimetic. Workers in the metamorphic field are cautioned to be on the alert for similar features of pseudo discordant bedding which is undoubtedly common in flattened and sheared rocks of variable competence such as the phyllonites and granulites, many of which have been derived from igneous rocks with a primary planar structure. No reliable recognition of top- and bottomsets can be made where discordant bedding has been affected by deformation along existing planes of weakness such as the lamination surfaces.

Description: Recent contributions by penologists of Leiden University to the present state of knowledge concerning crystalline Galicia are summarized. An enumeration of the most important rock-types is given and their grouping in geotectonic units is attempted. The structural and metamorphic history is outlined and shown to comprise a variety of pre-hercynian (pre-Cambro-ordovician) as well as hercynian and post-hercynian elements. It is argued that originally geosynclinal rocks of the pre-hercynian orogen (frequently attaining the eclogite, granulite or charnockite metamorphic facies) were intruded by Cambro-ordovician granitic rocks, and were subsequently incorporated in the hercynian orogen, where they were re-deformed, predominantly to N-S trending isoclinal folds, and re-metamorphosed, locally reaching a wet anatextitic climax in the cordieriteamphibolite facies of Winkler. The hercynian orogenic cycle is claimed to have terminated in W. Galicia with further episodes of tectonization and recrystallization, and with retrograde metamorphism and granitic intrusions of an increasingly high-level nature. A synoptic table, presenting a tentative correlation of geological events in crystalline Galicia, is appended.

Description: La région que j’ai étudiée et relevée est située dans les Alpes françaises, à environ 4° longitude est et 50°15’ latitude nord, et à environ 2000 mètres d’altitude. Elle fait partie de la chaîne dite „chaîne de Belledonne” qui s’étend des massifs de Beaufort et du Grand Mont dans le N N E jusqu’à ceux du Taillefer et de la Mure dans le S S W. Cette chaîne forme du point de vue géographique comme du point de vue géologique le prolongement méridional de la chaîne des Aiguilles Rouges et de la chaîne du Mont Blanc. Celles-ci font partie de la zone externe des massifs cristallins des Alpes, dont le prolongement en Suisse est connu sous le nom de „massifs centraux des Alpes”. On entend par là les chaînes anté-alpines, et principalement hercyniennes des Alpes, constituées de roches du Houiller d’une part, de formations antérieures d’autre part, et recouvertes enfin de séries d’âges mésozoïque et permien. Dans le sud, cette chaîne hercynienne se divise de nouveau en deux branches, dont la plus orientale, celle des Grandes Rousses, aboutit à la partie déversée vers l’est, dans le massif du Pelvoux. Taillefer et la Mure forment le pivot le plus avancé de cet arc, et en effet ce dernier massif disparaît partiellement vers le S W sous le manteau sédimentaire du Dévoluy (cf. fig. 1). Le massif de Belledonne proprement dit, avec son point culminant: le Grand Pic de Belledonne, qui atteint à quelques mètres près les 3000, forme le tronçon central de cette chaîne et se trouve à quelques kilomètres à l’est de Grenoble dans le département de l’Isère. Il est limité à l’ouest par l’Isère et le Drac; au sud par la Romanche; à l’est par l’Eau d’Olle et au nord par la vallée de Laval et le Col de la Coche. Les Lacs Robert sont à peu près situés au centre du Massif de Belledonne, dans un cirque encoché dans le ralliement méridional de la chaîne occidentale à la chaîne principale du massif. Cette chaîne principale, comprise entre le Jasse Bralart au N et le Petit Vent au S, limite la région levée à l’est, tandis que la Botte et le lac Achard la limitent au sud. A l’ouest elle s’arrête au pied de la Croix de Chamrousse, le pivot méridional, et du Grand Eulier, le contrefort septentrional de la chaîne secondaire; et au nord audessus de la Prairie de l’Oursière. Cette région a une largeur de 3 km et une longueur de 4 km environ. Le Grand Sorbier dans la chaîne principale, avec ses 2522 m. en est le point culminant. L’impraticabilité du terrain fixa des frontières plus ou moins naturelles au lever. En particulier le flanc E de la chaîne principale, qui domine la vallée de Baton, à part quelques sentiers, n’est pas accessible sans danger, à cause du mauvais état de la roche schisteuse. Il en est de même pour la pente occidentale du Grand Eulier et du Casserousse, tandis que le flanc méridional du Petit Vent et de la Botte est un des versants de la gorge profonde de 2000 m. où coule la Romanche. C’est à cela qu’est dû le nombre restreint d’observations faites sur quelques parties de la périphérie.

Description: Historical notes on Galician geology, and on the work of the Leiden University petrology team in particular, are first provided. This is followed by an introduction to the geology of Galicia with emphasis on its crystalline basement and upper mantle inliers. Six lithotectonic units are distinguished: 1) the Variscan granitic rocks and migmatites. 2) the Palaeozoic supracrustal rocks and dismembered meta-ophiolites, 3) the blastomylonitic graben between Malpica and Tuy, 4) the Lalin and Forcarey Units, 5) the Ordenes basin and its mesocatazonal peripheral belt, 6) the predominantly mafic-ultramafic catazonal complex at Cabo Ortegal. The supracrustal history of western Galicia in late Precambrian and early Palaeozoic times, as inferred from neighbouring areas, is briefly outlined and is compared with the igneous, tectonic and metamorphic evolution of the upper mantle and lower crustal rocks contained in the polymetamorphic basement complexes (1—4), which have sustained high-pressure and high-temperature metamorphism under a flow-folding regime prior to the Variscan orogeny. Several models proposed for the development of the basement complexes in the northern Hesperian Massif are briefly discussed. It is considered most likely that an early Palaeozoic rift system caused by mantle plume diapirism, and accompanied by deepseated thermal metamorphism, lower crustal recycling, updoming of the crust and incipient sea floor spreading, was closely followed by Variscan low-pressure metamorphism, migmatization and granite emplacement under an intermittently compressive and dilatational tectonic regime. Finally, the probability of a Precambrian orogenic crust in western Galicia is briefly explored.

Description: The Spanish region of Galicia is situated in the extreme north-western part of the country due North of Portugal and West of Asturias. It is bordered by the Atlantic Ocean to the West and by the Bay of Biscay to the North (see fig. 1). The area under investigation concerns the western provinces of La Coru\xc3\xb1a and Pontevedra mainly.\nApart from early reconnaissance work by Schulz (1858), Barrois (1892), Sampelayo (1922), Lotze (1945), Carl\xc3\xa9 (1945), Navarro and del Valle (1959) the area is at present being investigated and mapped on a scale of 1:50.000 by L\xc3\xb3pez de Azcona, Parga Pondal and their associates for the Instituto Geol\xc3\xb3gico y Minero de Espa\xc3\xb1a. So far nine sheets and explanatory memoirs have been published between 1948 and 1956. Parga Pondal has also published a geological sketch map on a scale of 1:400.000 and an explanatory note of the province of La Coru\xc3\xb1a in 1956, and since 1931 he has contributed substantially to the knowledge of Galician geology in a series of papers concerning petrological, mineralogical, tectonic and sedimentological aspects of it. Between 1955 and 1959 de Sitter and Zwart conducted geological research by the Department of Structural and Applied Geology of the University of Leyden in the area between Lage and Malpica. Summaries of their results appeared in 1955 and 1957, while one of their associates, Insinger, published a short account of his work in the vicinity of Mug\xc3\xada in 1961.

Description: In Galicia occur several polymetamorphic complexes that contain mafic and ultramafic rocks. Mafic volcanics and gabbros are situated in or near the complexes. An episode of metamorphism and granitization encompassed the generation of the volcanics and gabbros. Gravity surveys revealed the existence of deep roots below polymetamorphic complexes. Geochronological investigations yielded Palaeozoic ages from about 500 Ma for the generation of ultramafic rocks and calcalkaline granite series, till ages of around 300 Ma for postkinematic alkaline and calcalkaline granites. A model involving mantle-plume diapirism and rejuvenation of the lower crust is proposed for the Early Palaeozoic evolution of the continental lithosphere of Galicia.\nAn excursion route is given, leading along 22 exposures of mostly mafic and ultramafic rocks, to be visited in 4 1/2 days.

Description: Historical notes on Galician geology, and on the work of the Leiden University petrology team in particular, are first provided. This is followed by an introduction to the geology of Galicia with emphasis on its crystalline basement and upper mantle inliers.\nSix lithotectonic units are distinguished: 1) the Variscan granitic rocks and migmatites. 2) the Palaeozoic supracrustal rocks and dismembered meta-ophiolites, 3) the blastomylonitic graben between Malpica and Tuy, 4) the Lalin and Forcarey Units, 5) the Ordenes basin and its mesocatazonal peripheral belt, 6) the predominantly mafic-ultramafic catazonal complex at Cabo Ortegal.\nThe supracrustal history of western Galicia in late Precambrian and early Palaeozoic times, as inferred from neighbouring areas, is briefly outlined and is compared with the igneous, tectonic and metamorphic evolution of the upper mantle and lower crustal rocks contained in the polymetamorphic basement complexes (1\xe2\x80\x944), which have sustained high-pressure and high-temperature metamorphism under a flow-folding regime prior to the Variscan orogeny. Several models proposed for the development of the basement complexes in the northern Hesperian Massif are briefly discussed. It is considered most likely that an early Palaeozoic rift system caused by mantle plume diapirism, and accompanied by deepseated thermal metamorphism, lower crustal recycling, updoming of the crust and incipient sea floor spreading, was closely followed by Variscan low-pressure metamorphism, migmatization and granite emplacement under an intermittently compressive and dilatational tectonic regime. Finally, the probability of a Precambrian orogenic crust in western Galicia is briefly explored.

Description: La r\xc3\xa9gion que j\xe2\x80\x99ai \xc3\xa9tudi\xc3\xa9e et relev\xc3\xa9e est situ\xc3\xa9e dans les Alpes fran\xc3\xa7aises, \xc3\xa0 environ 4\xc2\xb0 longitude est et 50\xc2\xb015\xe2\x80\x99 latitude nord, et \xc3\xa0 environ 2000 m\xc3\xa8tres d\xe2\x80\x99altitude. Elle fait partie de la cha\xc3\xaene dite \xe2\x80\x9echa\xc3\xaene de Belledonne\xe2\x80\x9d qui s\xe2\x80\x99\xc3\xa9tend des massifs de Beaufort et du Grand Mont dans le N N E jusqu\xe2\x80\x99\xc3\xa0 ceux du Taillefer et de la Mure dans le S S W. Cette cha\xc3\xaene forme du point de vue g\xc3\xa9ographique comme du point de vue g\xc3\xa9ologique le prolongement m\xc3\xa9ridional de la cha\xc3\xaene des Aiguilles Rouges et de la cha\xc3\xaene du Mont Blanc. Celles-ci font partie de la zone externe des massifs cristallins des Alpes, dont le prolongement en Suisse est connu sous le nom de \xe2\x80\x9emassifs centraux des Alpes\xe2\x80\x9d. On entend par l\xc3\xa0 les cha\xc3\xaenes ant\xc3\xa9-alpines, et principalement hercyniennes des Alpes, constitu\xc3\xa9es de roches du Houiller d\xe2\x80\x99une part, de formations ant\xc3\xa9rieures d\xe2\x80\x99autre part, et recouvertes enfin de s\xc3\xa9ries d\xe2\x80\x99\xc3\xa2ges m\xc3\xa9sozo\xc3\xafque et permien. Dans le sud, cette cha\xc3\xaene hercynienne se divise de nouveau en deux branches, dont la plus orientale, celle des Grandes Rousses, aboutit \xc3\xa0 la partie d\xc3\xa9vers\xc3\xa9e vers l\xe2\x80\x99est, dans le massif du Pelvoux. Taillefer et la Mure forment le pivot le plus avanc\xc3\xa9 de cet arc, et en effet ce dernier massif dispara\xc3\xaet partiellement vers le S W sous le manteau s\xc3\xa9dimentaire du D\xc3\xa9voluy (cf. fig. 1). Le massif de Belledonne proprement dit, avec son point culminant: le Grand Pic de Belledonne, qui atteint \xc3\xa0 quelques m\xc3\xa8tres pr\xc3\xa8s les 3000, forme le tron\xc3\xa7on central de cette cha\xc3\xaene et se trouve \xc3\xa0 quelques kilom\xc3\xa8tres \xc3\xa0 l\xe2\x80\x99est de Grenoble dans le d\xc3\xa9partement de l\xe2\x80\x99Is\xc3\xa8re. Il est limit\xc3\xa9 \xc3\xa0 l\xe2\x80\x99ouest par l\xe2\x80\x99Is\xc3\xa8re et le Drac; au sud par la Romanche; \xc3\xa0 l\xe2\x80\x99est par l\xe2\x80\x99Eau d\xe2\x80\x99Olle et au nord par la vall\xc3\xa9e de Laval et le Col de la Coche.\nLes Lacs Robert sont \xc3\xa0 peu pr\xc3\xa8s situ\xc3\xa9s au centre du Massif de Belledonne, dans un cirque encoch\xc3\xa9 dans le ralliement m\xc3\xa9ridional de la cha\xc3\xaene occidentale \xc3\xa0 la cha\xc3\xaene principale du massif. Cette cha\xc3\xaene principale, comprise entre le Jasse Bralart au N et le Petit Vent au S, limite la r\xc3\xa9gion lev\xc3\xa9e \xc3\xa0 l\xe2\x80\x99est, tandis que la Botte et le lac Achard la limitent au sud. A l\xe2\x80\x99ouest elle s\xe2\x80\x99arr\xc3\xaate au pied de la Croix de Chamrousse, le pivot m\xc3\xa9ridional, et du Grand Eulier, le contrefort septentrional de la cha\xc3\xaene secondaire; et au nord audessus de la Prairie de l\xe2\x80\x99Oursi\xc3\xa8re. Cette r\xc3\xa9gion a une largeur de 3 km et une longueur de 4 km environ. Le Grand Sorbier dans la cha\xc3\xaene principale, avec ses 2522 m. en est le point culminant. L\xe2\x80\x99impraticabilit\xc3\xa9 du terrain fixa des fronti\xc3\xa8res plus ou moins naturelles au lever. En particulier le flanc E de la cha\xc3\xaene principale, qui domine la vall\xc3\xa9e de Baton, \xc3\xa0 part quelques sentiers, n\xe2\x80\x99est pas accessible sans danger, \xc3\xa0 cause du mauvais \xc3\xa9tat de la roche schisteuse. Il en est de m\xc3\xaame pour la pente occidentale du Grand Eulier et du Casserousse, tandis que le flanc m\xc3\xa9ridional du Petit Vent et de la Botte est un des versants de la gorge profonde de 2000 m. o\xc3\xb9 coule la Romanche. C\xe2\x80\x99est \xc3\xa0 cela qu\xe2\x80\x99est d\xc3\xbb le nombre restreint d\xe2\x80\x99observations faites sur quelques parties de la p\xc3\xa9riph\xc3\xa9rie.