Famennian, thermally altered chert exotic clasts from the Jędrzychowice/Ludwigsdorf wildflysch (Sudetes) -stratigraphic and tectonic implications

Abstract

A wildflysch sequence recently recognized in the Görlitzer Schiefergebirge/western Kaczawskie Mts. boundary zone permits the characterization of the westernmost Kaczawa Unit as a chaotic complex. The cherts, many of which contain numerous radiolarians, occur as exotic clasts within the olistostrome deposits of the wildflysch. They are associated with allochthonous blocks of blueschist, andesite pillow lavas and pyroclastic rocks, flysch facies clastic rocks, mudstones (often of black shale lithology) and carbonates. The conodonts extracted from the grey-greenish radiolarite cherts indicate a mid-Famennian age (Early rhomboidea Zone to Early marginifera Zone). The conodont-bearing radiolarite clasts reveal no trace of a tectonometamorphic fabric. Their 4-4.5 conodont colour alteration index (CAI) indicates that the rock underwent low-temperature (250-285°C) thermal alteration. The lack of thermal overprint in the olistostrome matrix allows this alteration to be interpreted as a part of the tectonothermal, post-mid-Famennian evolution of the source area. A number of the exotic blocks of unmetamorphosed volcanic rocks associated with the chert exotics seems to indicate igneous (volcanic) activity as the reason for the chert alteration. The fact that exotic blocks of the Devonian chert have been found as clasts within deposits earlier believed to represent Upper Proterozoic (Cadomian) flysch suggests the need for reassessement of the extent of Cadomian rocks in the westernmost Sudetes.