Repery tektonicznego rozwoju prekambru Dolnego Śląska

Abstract

GUIDE POINTS IN THE TECTONIC DEVELOPMENT OF THE LOWER SILESIA PRECAMBRIAN

Summary

The paper is a reply to critical comments on my ideas expressed by H. Teisseyre. The comments as well as H. Teisseyre's concepts were published in his paper entitled ''The Precambrian in the Polish part of the Sudetes" (Kwartalnik Geologiczny, v. 12, pp. 749-775). In reply to the criticism and H. Teisseyre's views I am setting forth the following conclusions: 1.With the exception of the vicinity of Zgorzelec over the Sudetes area a gap comprising the Eocambrian exists at the top of the Proterozoic series. The gap is preceded by Early Assyntian movements. The Eocambrian series was folded during the Late Assyntian movements together with its substratum. 2. Over the area where Proterozoic series, particularly the schist series, occur in juxtaposition with the Lower Palaeozoic beds, one may expect a time gap despite the geometric conformity often met with, which shows features of a secondary peneconcordance. Consequently no evidence exists for assuming a depositional continuity between the crystalline Izera unit and the epimetamorphic Early Palaeozoic of the Kaczawskie Mountains and the southern part of the Karkonosze Mountains, as well as between the Proterozoic crystalline rocks in the vicinity of Imbramowice and the Fore-Sudetic Lower Palaeozoic. There also exists a gap beetwen the Moldanubian series of the Sowie Mountains block and the Proterozoic. 3. The occurrence of the Izera leucogranites and the Rumburk granites pebbles - both formed by homogenization of the Izera gneisses - within the Ordovician deposits warrants the conclusion that the evolution of the series of both the regions named is not due to a single folding episode. Prior to the deposition of the Lower Palaeozoic the Izera gneisses had been folded twice viz. in the course of Early Assyntian disturbance (the SW vergence) and during the Late Assyntian movements (superposed folds with S vergence). 4. Likewise, the presence of the Early Assyntian pebbles in the Devonian rocks near Strzelin shows that the former ones have undergone at least one folding more than the latter ones deformed during the Early Bretonian disturbance. 5. Various crystalline units of the Sudetes can be compared with each other on the basis of similarities of the rock series, especially their parent primitive material and the state attained during the main deformation. The subsequent diaphtoresis and homogenization are the additional factors. On such grounds the following tectogenes of different ages have been distinguished: the Moldanubian one represented by the characteristic Sowie Mountains gneisses, the Early Assyntian one built of a series of mica schists and gneisses, the Late Assyntian one comprising the Eocambrian Lusatia greywackes, and the Early Variscan one consisting of slightly metamorphosed or unmetamorphosed Lower Palaeozaic rocks. The depositional sequence terminated prior to Late Devonian time. The grey mica schists occurring along the northern edge of the Izera gneisses by no means can be classified into the Eocambrian. 6. It does not seem justified to classify two different series into one tectogene, the only grounds being that the lineations in both series are parallel to each other - e.g. in the Lower Palaeozoic of the Kaczawskie Mountains as compared to some zones in the Izera gneisses or mica schists and gneisses of the Eastern Karkonosze Mountains. Also the Devonian Jegłowa series and the Strzelin gneisses, the first major folding (Early Assyntian) of which preceded the deposition of the Jegłowa beds, show similar trends of their lineations. The parallelism of the lineations in two different tectogenes is due to structural patterns that are analogous though of different age. 7. The gabbro-diabase massifs which occur around the Sowie Mountains block are related to the close of the Assyntian disturbance - the only period during which disjunctive movements operated simultaneously along three sides of this tectonic block. 8. There is no evidence of Caledonian movements in the crystalline series east of the Sowie Mountains block. An overprint of Palaeozoic movements related to the folding of the Lower Palaeozoic series of the southern Karkonosze Mountains seems likely in the northern part of the Bystrzyckie Mountains crystalline body. 9. The view according to which the Silurian is represented in the Eastern Karkonosze Mountains seems unlikely on the grounds of palaeogeographic relations in the Western Sudetes. The Eastern Karkonosze mica schists gradually pass into the Kowary gneisses and they do not overlie these gneisses discordantly. They form the bedrock of the Leszczyniec unit thrust from the east prior to Palaeozoic time. 10. The Izera gneisses are not a polygenetic formation only in some parts due to granitization of the series of Proterozoic schists. Neither geologic nor petrographic evidences are available which would be indicative of the Rumburk granites and Zawidów granodiorites as the parent rocks from which the Izera gneisses would have been formed; the Rumburk granites and the Zawidów granodiorites originated owing to recrystallization of various parts of these gneisses mainly. 11. The usage of symbols B₁, B₂, B₃ for B-lineations in all the crystalline bodies of Lower Silesia does not seem rational, because lineations represented by the same symbols and occurring in various units are usually different in both their style and age. 12. The name ''Late Saxon" tectonics should not be used as a name bearing any time meaning, if only because no such terms as "Early Saxon" or "Middle Saxon" are precisely defined. The principal characteristic points in the tectonic development of the Lower Silesia Precambrian can be also looked for in the Early Palaeozoic when denudation processes reached down to the Precambrian. The main guide points are as follows: a) formation of a system of Moldanubian folds, b) formation of the Early Assyntian tectogene, c) deposition of the Eocambrian series preserved in Lusatia, and its folding prior to deposition of the Lower Cambrian over the area (the Late Assyntian disturbance); at this time the Early Assyntian folds were thoroughly remodelled, particularly in the Izera Mountains and subsequently the gneisses were subject to homogenization, d) the deposition at Lower Palaeozoic beds continued up to Middle Devonian time; at this time the rocks of the deep zones of the Early Assyntian tectogene were submitted to erosion e) the Early Assyntian gneisses of the eastern branch of the tectogene in question were overlain by the Lower Devonian deposits folded together with their, substratum during the Early Bretonian disturbance. Approximately at this time the Lower Palaeozoic-Devonian series of the Western Sudetes was folded. Any views that do not take into account the afore-listed principal guide points present a distorted picture of the evolution of the Sudetic Precambrian.