Litologia i geneza zmetamorfizowanych skał osadowych i wulkanicznych jednostki Chełmca (Góry Kaczawskie)

Zdzisław Baranowski, Anna Haydukiewicz, Ryszard Kryza, Stanisław Lorenc, Andrzej Muszyński, Zdzisława Urbanek

Abstract


Based on detailed drill core studies from fourteen boreholes (up to 1500 m deep) and on field observations, the Chełmiec tectonic unit in the northern part of the Kaczawa Mts appears to contain two types of tectonostratigraphic elements. The first is fragments of a stratigraphic sequence, composed mainly of dark muddy slates (metamudstones) and variegated laminated silty-clayey slates (both considered as Ordovician), and of volcaniclastic rocks, greenstones and dia-bases. The second element is represented by mélange bodies which consist of dark muddy slates (matrix) enclosing fragments of various lithologies: dark siliceous and graphitic slates, light siliceous slates, quartzites, greywackes, variegated silty-clayey slates etc. (probably Upper Devonian or Lower Carboniferous).
Due to the lack of biostratigraphic evidence, the stratigraphic subdivision is based on lithological criteria. Using e.g. the rule of superposition and analysis of lithological contacts and sedimentary and volcanogenic structures, the stratigraphic succession was defined, and three informal lithostratigraphic units were distinguished: (a) an association of metamudstones and diabases, (b) an association of metavolcaniclastic rocks (both within the stratigraphic sequence), and (c) a mélange association. The dark metamudstones and variegated silty-clayey slates of the association of metamudstones and diabases are interpreted as turbidites. The volcaniclastic rocks, of clearly epiclastic character, were delivered episodically from marginal parts of the basin or volcanic heights by denser turbiditic currents and other types of gravity flows. Simultaneously, volcanic activity occurred within the basin itself producing basaltic lavas (now observed as subvolcanic diabases and effusive greenstones), which geochemically correspond to recent mildly alkaline within-plate basalts. The geotectonic setting of the basin is difficult to define precisely but the sequence was probably emplaced in an outer fan or in a basin at a continental margin.
The mélange represents a later stage of the evolution of the Kaczawa Complex. Most probably, it was deposited from gravity flows and slumps in a trench or on a trench slope. Its origin is thought to have been connected within the formation of a Variscian accretionary prism in Late Devonian and Early Carboniferous times.
The rock complex of the Chełmiec Unit experienced several stages of deformation during the Variscian orogeny. The first event resulted in a system of thrusts (and associated folds?) and it was related to (or partly preceeded by) the formation of mélange. It is likely that deformation at this stage (and in particular in its later phase) took place under blueschist facies conditions. The second deformation event, probably under greenschist facies conditions, was associated with folding which steepened the earlier foliation and produced new asymmetric folds. The third deformation event, partly under semi-brittle/brittle conditions, is responsible for new thrusts cutting the earlier structures and the formation of large open folds, such as the Bolków-Wojcieszów antiform.
The deformation of the rocks of the Chełmiec Unit was associated with greenschist facies metamorphism. In general, primary sedimentary and volcanic structures are well preserved in the rocks of this unit which often seem to have suffered weaker deformation and metamorphism than that observed in other units of the Kaczawa Mts. No clear evidence of the early high-pressure episode which is widespread in other tectonic units of the Kaczawa Complex has been found in the Chełmiec Unit so far.

Keywords


sedimentary succession, volcanic rocks, volcaniclastic rocks, mélange, Kaczawa complex, Sudetes,Variscan orogeny.

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