Géologie de la région située entre Tigssaluk Fjord et Sermiligârssuk Fjord (partie médiane), SW-Groenland

Abstract

The investigations, the results of which are described in the present work, were conducted during two distinct stages, from 1957 to 1960: 1) the field work, including the drawing of the geological map at a scale of 1: 20000 and the collecting of numerous samples; this was made possible by the organization and facilities put at our disposal by the Geological Survey of Greenland, 2) the examination of the collected material, which was conducted in the laboratories of geology and petrography of the University of Lausanne.

The area which we have studied is composed of an ensemble of gneiss, schists, dykes and massifs of various age and composition; it is situated in the SW part of Greenland, about 30 km NNW of Ivigtut (Fig. 1 and Plate 1). The limits of this territory are the following: to the N, Sermiligârssuk fjord and the glacier of Sioralik ; to the E, a depressed zone occupied by lakes; to the S, Tigssaluk Fjord and a fault-zone oriented approximately E-W; lastly, to the W, an ideal N-S line which marks the western limit of the maps at a 1: 20000 scale of the Geodætisk Institut that we used for the geological map.

No geological investigations had been conducted in the area; the only published information concerns the region as a whole, and is to be found in the works of N. V. USSING (1912), C. E. WEGMANN (1938 and 1947), J. BONDAM (1956) and A. BERTHELSEN (1960 et 1961). Let us also mention the unpublished reports of our companions, the geologists of the Geological Survey of Greenland.

Apart from a few small zones which were examined in detail, the geological mapping of the region was conducted in an exploratory manner with a view to establishing rather a chronology as precise as possible than a detailed map at a 1:20000 scale.

The chronology, the main lines of which we are now going to put forward, represents an attempt at a synthesis based on observations made in our territory as well as in the entire region of Ivigtut; here we have the opportunity of seeing the deep zones of a mountain chain in which are visible traces of all the phenomena which follow each other during the evolution of a “drame-type”, such as H. and G. TERMIER (1956), for example, have described it.

1. - Pre-Ketilidian stage.

This has not been brought to light in our territory, but its presence is possible: in the form of elements contained in the Ketilidian elastic sediments or in the form of tectonic wedges belonging to the old basement and emplaced during the Ketilidian orogeny.

1. - Ketilidian geosynclinal stage.

The following phenomena occur during this stage: clastic and pyroclastic sedimentation, complex ophiolitic volcanism with the following probable succession: pillow-lavas in flows, gabbro-anorthositic sills (?), gabbroic sills, peridotitic sills.

1. - Ketilidian orogenic stage.

During this stage various processes such as regional metamorphism, migmatization and quartzo-feldspathic and sodic metasomatism affect all the rocks accumulated in the geosyncline; these processes have not attained everywhere the same intensity, therefore transforming the volcanic, elastic and pyroclastic series into either more or less homogenized and migmatized gneiss, or into non-migmatized crystalline schists. The map on plate 2 shows the distribution of these various types of rocks. The processes of migmatization and metasomatism are accompanied by orogenic efforts which fold the Ketilidian series in three successive phases forming folds, the axes of which have different directions (Plate 6 and fig. 51); the greenschists on one hand, the various gneisses on the other, react differently to these orogenic stresses and thus form two distinct tectonic levels: the suprastructure and the infrastructure (refer to C. E. WEGMANN and J. HALLER). Among other secondary effects of the tectonic efforts we observed, along certain fault zones, the "reintrusion", probably in a solid state, of the ultramafic sills contained in the geosyncline. Near the end of this second Ketilidian stage, migmatization and metasomatism evolve and form several very localized massifs of autochthonous granites with a potassic tendency, which illustrates quite well the hypotheses of H. H. READ on the evolution of granitic series in mountain chains undergoing formation.

1. - Post-orogenic Kuanitic stage.

This period is marked by an alternation of phases of tension with emplacement of several generations of basic dykes, probably doleritic in composition, and of phases of compression which correspond to the formation of important faults or to the furth er movement of pre-existing ones. The directions of the successive dykes are: E-W, NW-SE, NE-SW (Plate 3).

1. - Sanerutian stage of reactivation.

It is possible that this reactivation of the basement corresponds to the repercussion of the Nagssugtoqidian orogeny visible in the region of Søndre Strømfjord; its manifestations are the following in the Ivigtut region: - a new rise in the regional thermal front, which determines a retrograde metamorphism on all the basement rocks (the Kuanitic dykes are thus transformed into more or less chloritized amphibolites), - weak activity of block-tectonics (further movement of pre-existent faults ), but also of fold-tectonics (formation of small structures a few m in size), - various types of metasomatism, the importance of which varies from one place to

another: formation of quartz, sodic plagioclase, potassic feldspars, tourmaline, emplacement of granites in the form of discordant batholiths (amongst which the Tigssaluk granites) accompanied by various acid, basic and ultrabasic veins and dykes (Plate 4).

1. - Gardar stage of post-orogenic tranquillity.

During this period one again notes an alternation of phases of tension with dyke intrusion and phases of compression with formation of faults or the movement of pre-existent ones (Plates 4, 5, and 7); the succession of intrusions is the following: dykes with a lamprophyric tendency, which are probably transformed, old, basic dykes, dykes with a trachytic tendency, dykes of porphyritic dolerite, doleritic dykes. One can bring to light right through this period hydrothermal and pneumatolytic activity. The magmatic evolution is very complex and it is difficult to determine its mechanism with precision; it appears, however, that the primary magma is of a decidedly alkaline, basaltic tendency.

1. - Post-Gardar period.

Whereas all the phenomena that we have just enumerated took place in the old Pre-Cambrian (Sanerutian = 1590-1600 MY), a few doleritic dykes, very rare in our area, were probably em placed during the Cretaceous-Tertiary period; their direction is always parallel to that of the coast and they are possibly in relation with the big flows of the Plateau basalts in the region of Disko.

1. - Quaternary.

Plate 8 shows the schematic distribution of the recent glacial and fluvio-gla