Abstract

From three Rügen boreholes (G14, Rügen 5, Loissin 1) across the Caledonian deformation front four Lower Palaeozoic slate samples and three detrital muscovite concentrates from associated sandstones have been analysed by 40Ar/39Ar analysis. Microprobe analysis of the detrital muscovites show homogeneous, but distinct chemical compositions for each concentrate. Particulate organic matter reflectance and illite crystallinity indicate that all the analysed whole–rock samples have experienced anchizonal metamorphic conditions.

40Ar/39Ar analysis of the samples from the G14 and Loissin 1 wells give almost identical results. Muscovite concentrates define plateau ages of c. 830 Ma and show an isotopic rejuvenation of low–temperature increments around 440 Ma. Whole–rock slate samples give plateau ages of c. 440 Ma and c. 427 Ma. Isotopic rejuvenation in the low–temperature increments prior to 350 Ma in the Loissin 1 sample can be attributed to the Permo–Carbonifereous volcanism which is known from this borehole. In contrast, samples from the borehole Rügen 5 show a different isotope pattern. A muscovite concentrate defines a plateau age of 609 Ma. Two whole rock slate samples give internally discordant but identical 40Ar/39Ar spectra with consistent isotopic rejuvenation from high– to low–temperature increments from 510 Ma to 420 Ma.

While for the G14 and Loissin 1 sandstone samples a Baltica–derived provenance can be assumed, greywackes of Rügen 5 indicate a Cadomian–influenced Perigondwanan detrital source. According to the results of whole–rock slate samples the main Caledonian tectonothermal event in NE–Germany occurred around the Ordovician–Silurian boundary or in the early Silurian (450–425 Ma). From these data the Rügen Palaeozoic appears as an allochthonous Caledonian nappe complex which has been thrust much further onto the Baltic shield than previously thought.

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