Abstract
The rocks of the Qagssimiut area, South Greenland, belong, apart from a few late dolerite dykes, to the Precambrian shield. Their history was found to be very complex. An orogenic period affected formations mainly composed of quartzites and amphibolitic schists which now occupy but a very small percentage of that area. These rocks, having been submitted to at least two periods of deformation (during which a set of folds with a NW~SE trend, and a major, younger NE-SW set were developed), were subsequently intruded by massifs and plugs of ultrabasic material, which are very much more widely preserved. Widespread granitisation then occurred; in particular, the pre-existent rocks were (partly) transformed into granite, which is by far the principal component of the Qagssimiut area. The processes apparently took place in tranquil conditions. Basic intrusion also takes the form of sheets and dykes, concentrated mainly in the southern parts of the area. Some of these sheets are conspicuously veined and replaced by granite or aplite. The striking characteristics of this phenomenon, which involves static replacement and progressive consumption of basic material by acid, are described in some detail. The contrast between basic and acid material is remarkable, as is the alternation of phases of basic intrusion and granitisation, which occurred under conspicuously stable conditions. The emplacement of small massifs of homogeneous, pink or white microgranite preceded the Gardar period, characterised by the intrusion of dykes (lamprophyres, dolerites, trachytes, porphyries) and block-faulting. A long hiatus separated the end of the Gardar period from the next event, the (Tertiary?) intrusion of two generations of dolerite dykes. They are fresh and never faulted. Quaternary phenomena are mainly related to the activity of the ice-cap, whose traces are ubiquitous.
Highlights
Mais elle ne correspond pas aux phenomt'mes decrits par BERTHELSEN : en effet, les filons kuanitiques furent certainement transformes, dans la region d'Ivigtut, au cours de l'orogenese ketilidienne
Quoique la presqu'ile de Panerfait soit un element structural important, il reste isole, separe du reste par une zone de failles NW-SE, dont le deplacement total est inconnu, mais certainement considerable
U. 41495: relations ophitiques nettes entre les plagioclases et les grains subidiomorphes de pyroxene souvent remplaces par une hornblende commune; la presence de petits cristaux de hornblende dans de plus grands pourrait provenir de plusieurs phases de cristallisation; zonage normal des feldspaths de 60 il. 32 % An
Summary
La region decrite dans le present travail est situee au SW du Groenland, il. peu pres il. mi-chemin entre Ivigtut et Julianehåb. Elle est constituee d'un groupe d'iles disposees en un triangle rectangle, dont 1'hypotenuse est le fjord de Sermilik (ou Sermitsialik), alors que les deux cates de 1'angle droit passent par 60°42' de latitude et 47°05' de longitude. Le village de Qagssimiut est bati sur Pile du meme nom, et, pour des raisons pratiques, nous avons pris ce nom pour definir toute la region. Les iles se compteraient par centaines si 1'on prenait en consideration les plus petites d'entre elles; les plus grandes depassent 15 km 2 Elles sont distribuees au hasard, sauf le long de la cate S de Sermilik, ou elles sont bien alignees. L'interieur des iles est malheureusement pourvu d'une vegetation buissonneuse, et les affleurements eux-memes peuvent etre tapisses de lichens au point que la nature de la roche est difficile il. L'interieur des iles est malheureusement pourvu d'une vegetation buissonneuse, et les affleurements eux-memes peuvent etre tapisses de lichens au point que la nature de la roche est difficile il. determiner
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