Structural studies in the pre-Cambrian of western Greenland. Part II. Geology of Tovqussap nunâ

Abstract

This work deals with the structural analysis of a small area, Tovqussap nunâ, in the southern part of the Sukkertoppen district, West Greenland (fig. l). This unusually well-exposed area is built up of pre-Cambrian granulite and amphibolite facies rocks. After an introduction, in which an outline of the mapping method is given and the petrographic nomenclature discussed, about a hundred selected rock specimens are described. This is followed by a detailed account of the geological map (Plates 1 and 2). A special rock division, the diorites, is given a more thorough treatment on account of their considerable chronological significance. The structural analysis begins with an elucidation of the geometric relations in eastern Tovqussap nunâ. With the help of Wulff net stereograms, structural contour maps and profiles, an analysis is carried out on this area's complicated structures. This analysis has revealed three superimposed fold phases. The experience thus obtained was used in the analysis of central and western Tovqussap nunâ. Through the results of the analysis it is reckoned that the high-metamorphic Tovqussaq rocks belong to an approximately thousand metres thick supracrustal series comprising five major concordant stratigraphic units (PI. 4). This series was first folded by flat-lying isoclinal folds with roughly NNW axes (the Midterhøj phase, see table 1 and fig. 78). This folding is assumed from analogies to have taken place during an early stage of the metamorphic development (slate/schist facies). The structures belonging to this first phase were later refolded on approximately ENE axes in large recumbent folds with amplitudes which can exceed 10 km. This fold phase (the Smalledal phase) was probably accompanied by amphibolite facies metamorphism. Refolding of the large western nappe during the last stage of the Smalledal phase is thought to have created the conditions for a later diapirism. This diapirism expressed itself by a transport of plastic material towards the top of the dome within the single rock units (the gneisses). The diapirism seemingly occurred during a change in the tectonic movement pattern leading to renewed refolding, this time on SE to S plunging axes under granulite facies conditions. The structures which were developed during this renewed refolding (the Pâkitsoq phase) show a distinct dependence on the form of the earlier structures. At the end of the Pâkitsoq phase a subphase (the Langø sub-phase) can be distinguished, during which steep foliation planes were superimposed on the earlier formed folds. The formation of the late kinematic diorites can be shown to have taken place in the antiformal hinge zones of pyribolite layers where movement along steep foliation planes has facilitated the ascent of salic material migrating from the underlying gneiss of the fold cores. The postkinematic diorite dykes and aplites, which also are regarded as having been formed metasomatically, represent an intermediate period during which the Tovqussaq district was subjected to tensional stresses. The originally planar aplites make it possible to distinguish yet another movement phase, which locally has left a strong imprint on the detailed structural picture, though it has only brought about small changes in the form of the major structures. This posthumous phase took place under amphibolite facies conditions and led to a retrograde metamorphism of the older granulite facies rocks. Where the hypersthene gneisses were only subjected to weak penetrative movements they were made over to hornblende-biotile-bearing 'purple gneisses', whereas where penetrative movements were more intense, they recrystallised into light coloured biotite gneisses. The granulites on Langø and Tugdlerunarssuit were also formed during the posthumous phase, on account of a local higher Mg/Fe ratio which permitted the preservation (or formation anew) of these rocks' present granulite facies mineral assemblage in spite of PT conditions which otherwise would have led to the formation of typical amphibolite facies minerals (as in the light coloured biotite gneisses). This retrograde metamorphism resulted also in local granitisation due to a redistribution of the mobile granite material under the prevailing tectonic conditions. Towards the close of the posthumous phase para- to postcrystalline mylonites and faults were developed. These are believed to represent the last traces of orogenic phenomena in the area. Later followed the intrusion of two groups of basic dykes, separated in time by a regional system of NE-trending wrench faults. This cratogenic stage of the area's evolution has been dealt with separately in an earlier publication (Berthelsen and Bridgwater, 1960). The structural evolution under consideration here is interpreted as belonging to a single geological cycle. The different fold phases are taken to represent successive structural events in which the rocks of the area have been involved during their passage through different tectonic levels under the mountain chain. This kinematic analysis enables one to follow the Tovqussaq rocks through an early sinking stage and a later rising stage. The deformation of the Tovqussaq district probably took place during the Ketilidian orogeny which was established by E. Wegmann in SW. Greenland. In the closing chapter the progressive metamorphism is discussed first, after which the mineral facies position of the Tovqussaq rocks is dealt with. Furthermore, the results of some trace element analyses are discussed. The author is of the opinion that the present content of trace elements in the rocks may only partly reflect their original distribution in the parent rocks. Even Zr, which is usually regarded as a very stable element during metamorphism, is thought to have undergone a real redistribution. A high content of Ni and Cr in certain calc-silicate rocks is considered to bear witness of a migration of these elements from the surrounding gneisses during skarn-metasomatism. With regard to the origin of the ultrabasic rocks, the author disputes H. Sørensen’s theory that they were segregated from hypersthene amphibolites (H. Sørensen, 1953). The ultrabasic composition of these rocks is considered by the author to date from the geosynclinal stage, where they could have been either siliceous dolomites or, perhaps more likely, ultrabasic ophiolites of extrusive origin. The basic rocks (pyribolites and gabbro-anorthosites) could have formed from extrusives (or early formed sills) or marly and limy sediments. Their present relatively high content of Ni and Cr indicates however that the former is the more probable. As the basic rocks are quantitatively important the enclosing gneisses cannot have provided Ni and Cr in sufficient quantity to account for their present concentration in the pyribolites and gabbro-anorthosites without a migration of these elements over extraordinarily long distances. Since the gneisses and granulites in Tovqussaq and in the surrounding areas show a genetic connection to definite sedimentogenous rocks, it is considered that the quartz dioritic to granitic rocks are metamorphosed and metasomatised greywackes and schists. In connection with this petrogenetic discussion the author has introduced some general considerations, amongst others concerning what role anatectic-processes have played during granulite and amphibolite facies metamorphism. By a comparison of the structures in non-metamorphic sediments with those in high-metamorphic Greenland gneisses, it is concluded that the introduction of anatectic processes to account for the complicated gneiss structures is redundant. The discussion concludes with some remarks on the formation of dome structures in gneiss areas in the light of the results from the kinematic analysis of the Tovqussaq dome.