A contribution to the geological investigations in the region of Ivigtut, SW Greenland

Abstract

As part of the research project carried out by G.G.U., the study of the territory situated to the NW of Ivigtut, between the fjords of Tigssalûp ilua and Sermiligârssuk, was undertaken. This peneplain, where the effects of quaternary erosion, in particular glacial abrasion are important, is geologically part of the Canado-Greenlandic shield. Since the investigations of C.E. Wegmann (1938), the geological history of this region has been roughly divided into an orogenic phase, during which the Ketilidian chain was born, and a phase of tension with blocktectonics and emplacement of dykes (Gardar period). Recent studies have brought to light two intermediary periods, the Kuanitic period, when the intrusion of dolerites took place, and the Sanerutian period, during which transformation of the pre-existent rocks occurred, due to a rise of the thermal front. During the Ketilidian period, a mountain chain was formed, in which one may distinguish a migmatized infrastructure and a suprastructure composed of an ectinite series. The principal differences between these two units are in structure and lithology. The macroscopic and microscopic petrology of these rocks is reviewed. The suprastructure includes pelitic, basic, quartzo-feldspathic, ferruginous and magnesian schists, which have recrystallized in the first two greenschist subfacies according to F.J. Turner and J. Verhoogen (1960). The infrastructure is characterized by various gneisses (banded, streaky, homogeneous) of which only one member rises out of the ordinary, i.e. the gabbro-anorthosites; the origin of these curious rocks is strongly debated. The apparition of a garnet in these gneisses suggests slightly deeper metamorphic facies. As a whole these rocks represent probably the transformation of the Arsuk group (C.E. Wegmann, 1938). Rust-zones may have various origins. In the greenschists they are probably pyrite-bearing sedimentary layers. In the gneiss they are more often related to crush-zones. Pegmatites are mainly of the simple quartzo-feldspathic type. Zoning, pinchand-swell structure, ptygmatic microfolding are current phenomena. Complex pegmatites containing minerals of pneumatolytic or hydrothermal origin are less frequent. Most of these were probably formed by slow diffusion of acid material in zones of weakness. Whether it be in the infrastructure or in the suprastructure, there exist lenses of ultrabasic rocks, especially serpentinites or talcschists. Reaction phenomena in a contact zone with a pegmatite are described. Several arguments are put forward in favour of the hypothesis of a sill emplaced in the fresh and water-filled sediments of the geosyncline. All these rocks underwent intense folding which may be divided into three phases: 1) a N-S to NNE pre-migmatitic phase. 2) a major WNW migmatitic phase. 3) a NE to ENE mainly post-migmatitic phase. There is no evidence of a considerable break between the different phases. During the first one, small steep folds were formed, the style of which suggests a predominance of sehistose material. The second phase is characterized by the formation of a synclinal suprastructure and migmatitic up-doming in the infrastructure, resulting in big folds, more or less recumbent, separated by steep synclines. Metamorphism attained maximum intensity. Where both phases were very active, crossfolding and wild-folding occurred. An example of the use of the Wulf-net in unravelling one of these problems is given. The effects of the third phase are not very visible in this area since they involve semi-plastic refolding, twisting and bending on a very large scale. Shearing of the suprastructure may be the main result. Microclinization seems also to belong to this phase. The result of Ketilidian tectonics is the individualization of a suprastructure and an infrastructure, separated generally by a structural break. The polymetamorphic character of Ketilidian rocks is summed up, the various transformations through metamorphism succintly described, and the close relationship between migmatization and regional metamorphism affirmed. Migmatization and its relationship to tectonics reveal some interesting features. A complete migmatitic front may be traced from infrastructure to suprastructure. The close association between its emplacement and the WNW folds is quite evident. The distribution of the migmatitic facies in the recumbent anticline, NNE of Angmassivit, demonstrates this fact particularly well. Migmatization and the second phase of folding are clearly contemporaneous. Where resistant barriers (quartzitic horizons) exist, the arrested migmatitic front is concordant with the primary bedding of the suprastructure. Element migration and metasomatism are reviewed. It is shown that sodium has migrated farther than potassium, which remains localized in deep zones. Calcium migration accounts greatly for the formation of basic oligoclase, amphibole, calcite and epidote. Hydrothermal veins and pellicles on joint-planes are frequent. Their main constituents are magnetite, biotite and chlorite. During the Kuanitic period, three generations of tholeitic dolerites were emplaced in three distinct directions. Their width and density of distribution can vary considerably. A rise in the thermal front characterizes the Sanerutian period. All the preexistent rocks undergo low-grade metamorphism. In particular, the Kuanitic Dykes are transforrned into metadolerites, according to the process described by J. Sutton and J. Watson (1951). Migmatitic and metasomatic introduction of material is insignificant. Several fault-systems and crush-zones have been distinguished. Absolute age determinations suggest correlation between the Sanerutian reactivation and the Nagssugtôqidian orogeny, the remains of which are to be found between Søndre Strømfjord and Disko Bugt, also on the W coast, but further N (A. Berthelsen, 1961). The Gardar period begins by the emplacement of lamprophyric dykes. A comparison is established between these dykes and the hornblende-lamprophyres of the Skærgaard region on the E coast of Greenland. In both cases they derive from a basaltic magma enriched in volatiles (mainly alkali-compounds and H20). The Brown Dykes constitute the most important group of the hypabyssal complex of the Gardar period. Three generations are distinguished, solely on the III Geological Investigations in the Region of Ivigtut 7 basis of their direction, which varies between ENE and NE, the first being that of the oldest dykes (BDo), the second, that of the last (BD2). Intrusion phenomena are noted: swarm, bayonet, "en echelon", disposition etc. Moreover, one observes variations of grain-size (chilled-margins, gabbro-pegmatites etc.), of texture (mainly ophitic), and of petrology (acid, basic segregations, "pegmatitoïdes", transversal variations; in composition, they go from quartz dolerites to troctolitic dolerites). They contain phenocrysts, xenocrysts, gneissic or anorthositic xenoliths. A few considerations on the very unlikely existence of a magmatic chamber with great horizontal extent are followed by the description of contact phenomena with effect on the encasing rock, a homogeneous gneiss, which is locally transformed into granophyre; the process is described with some detail. The youngest member of this hypabyssal complex, a group of trachytes, displays quite a wide diversity. In fact this field denomination includes types going from undersaturated (phonolite, tinguaite) to oversaturated (calco-alkaline rhyolite). While a process of differentiation readily explains the petrogenesis of most of these dykes, in the case of a rock with free silica, a certain amount of contamination must have occurred. Faulting has played a very important part in this period. Two main faultsystems determine the great WNW fjords and the lateral NNE fjords. The displacements of the major system are considerable. Moreover, there exist numerous other fractures of smaller extent. Some rare pneumatolytic mineralizations are reviewed. A great lapse of time separates the end of the Gardar and the emplacement of two generations of dolerites, more or less parallel to the coast. These hypabyssal rocks are to be compared to a similar swarm on the E coast. They are probably related to the Plateau basalts further N and came up through parallel fractures, caused by a (Tertiary?) coastal flexure.