Prekambr w polskiej części Sudetów

Abstract

PRE-CAMBRIAN IN THE POLISH PART OF THE SUDETESSummaryThe gneissic block of the Sowie Mts., situated in the central part of the Sudetes and of their foreland, represents the oldest area of the region considered. The block is of triangle outline (Figs. 1 and 2), built up mainly of plagioclase para-gneisses and migmatites, derived apparently from an Archaean supracrustal complex. These rocks are accompanied by microcline-plagioclase augen gneisses, amphibolites, granulites, small granite bodies arranged concordantly, and some other crystalline rocks. The tectonics of the gneissic complex of the Sowie Mts. has recently been investigated by W. Grocholski (1961, 1967), who has distinguished three main “b” lineations found in this highly folded complex. He has classified these lineations as B0, B1 and B2, the B0 lineation being thought to be the oldest one. According to W. Grocholski, all the three sets of linear structures are of Pre-cambrian age. It should also be stressed here that the block of the Sowie Mts. suffered a considerable remodellnig along its three marginal parts, when the Variscan movements began to operate. This can be observed mainly in the eastern part of the block, where large masses of mylonites and cataclasites were formed due to mechanical trantsformation of gneissic material. The fold pattern of the Sudetes and of their foreland is arranged tangentially to the three margins of the Sowie Mts. triangle (Fig. 2). The pattern consists of three main arches the northern, southern and eastern ones. These fold arches are built up mainly of different metamorphic members referred to Pre-Cambrian, old Palaeozoic or Devonian. However, the stratigraphy of the old crystalline sequences is in most cases rather arbitral. In the northern arch the Eocambrian (Algonkian) sequence is represented by Lusatia greywackes and Radzimowice beds, the latter being known from the Kaczawskie Mts. only. The Radzimowice beds are built up principally of various phyllites, mainly dark grey or black in colour. These consist of quartz, sericite and carbonates, as well as of changing amount of albite. Graphitic pigment, more or less abundant, is usually present. Meta-phtanites, quartzites and quartzite schists are subordinate constituents of the rock. The Pre-Cambrian of the southern arch of the Sudetic fold pattern is represented by a supracrustal member that chiefly consists of mica schists, para-gneisses, amphibolites, crystalline limestones, erlans and other rocks. The rock sequence is accompanied by microcline-plagioclase gneisses thought to be related to the Assyntic tectogenic processes. In Poland, the latter rocks are represented by the Izerskie gneisses that constitute the north-western part of the Karkonosze block (Fig. 1). The Izerskie gneisses are, according to M. Kozłowska-Koch (1965) of polygenetic character. In part, they were formed due to granitization of mica schists, in part, as a result of gneissification of some granitoid intrusions. Traces of granitized hornfelses are observed, as well. Some Pre-Cambrian rock assemblages are exposed in the eastern Sudetic arch on vast areas. In the region of Śnieżnik - Lądek, the Pre-Cambrian is represented by the Stronie series. This is a supracrustal assemblage for the most part composed of mica schists end paragneisses. The rocks are accompanied by amphibolites, quartzites, erlans, crystalline limestones and some other rocks. The regional granitization that took place within the Stronie series resulted in formation of different types of microcline-plagioclase gneisses (K. Smulikowski, 1957, 1960). Two main groups of gneisses, differing both in structural feature and in texture, may be distinguished in the region under consideration: Gierałtów gneisses and Śnieżnik gneisses. The former are fine-grained, laminated, streaky or migmatic rocks. Locally, they are associated with granulites, or contain small amphibolites lenses, or small eclogite bodies. The latter that belong to a group known as Śnieżnik gneisses, are represented by coarse-grained flaser and augen gneisses. The rocks of these two groups occur in both thick and thin layers and are arranged alternately where large masses of the Śnieżnik gneisses are in contact with the Gierałtów ones. Both gneissic assemblages are thought to be related to the Assyntic tectogenic processes, although direct evidences are lacking here. The NW trending zone of Niemcza exhibits numerous various rocks, most of them referred rather to Pre-Cambrian. In the western part of the zone, considerable masses of mylonites are found, derived from the Sowie Mts. gneisses, and produced during several periods of high deformations. The deformations were a result of the thrusting of the Sowie Mts. block towards the east. Close to the mylonites, there occur blasto-mylonitic mica schists, accompanied by graphite schists, quartzite schists and quartzites. All these rocks are thought by H. Dziedzic to be rather of Pre-Cambrian age (1961, 1965). Nevertheless, slates have been found to occur in the Niemcza zone, revealing spore flora characteristic of the Visean/Namurian time (H. Dziedzic and T. Górecka, 1965). These are intercalated between older crystalline member rather concordantly, this being an evidence that in the Niemcza zone the younger Variscan movements were also highly intense. Metamorphic rocks that encase the intrusion of the Strzelin granite are composed of both Pre-Cambrian and Devonian rocks. The rocks referred commonly to Pre-Cambrian consist of mica schists and paragneisses, accompanied by quartzites, amphibolites, erlans and crystalline limestones. Another series of quartzites, quartzite schists, meta-conglomerates and phyllites has been described as Lower, perhaps also as Middle Devonian in age (J. Oberc, 1966). Both sequences are folded concordantly with one another. The main deformation of the metamorphic sequence in the Sudetes may be thought as Variscan in age. In many regions it may have been accomplished in a time period between the Caledonian and Variscan epochs of folding. In certain cases, the high folding processes began already at the Caledonian time, and then developed during several Varisean phases. In various regional unit, however, the period of main movements could have been different (compare H. Brause 1965 and H. Jaeger 1963). The gneissic block of the Sowie Mts. seems to be the only region in which the main deformation can be considered as Pre-Cambrian. Two, or rather three distinct lineations, differing in age and in structural development, are found to occur in all the metamorphic assemblages of the Sudetes. According to this author they may be classified as B1, B2 and B3 sets of minor structures respectively. The set B1 is everywhere the most prominent one, and represents the most differential assemblage of the minor structures. It runs parallel to the major structural unit and may be interpreted as belonging to the main deformation. In the old crystalline series related to Pre-Cambrian, the set may be polygenetic in character. Some minor structures formed during the Pre-Cambrian deformation, or at the time when the Assyntic movements were in operation. They could have been remodelled and supplemented during the subsequent high folding. Where both structures are arranged parallely to each other, the older minor structures can hardly be discerned from the younger ones. The lineations B2 and B3 are younger than the B1 set of the minor structures. They trend transversally or obliquely to the B1 lineation and the major fold structures; however, they are parallel arranged to the younger dislocations here. The development of the B2 and B3 sets of these structures is rather monotonous. They are represented only by folds that range from microscopic ones up to several metres along the cross section line.