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  • Author or Editor: Katalin Judik x
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K-Ar age data of illite-K-white mica-rich, <2 µm grain size fraction samples were determined on various lithotypes of the Paleozoic-Mesozoic, very low- to low-grade metamorphic complex, the Jurassic ophiolitic mélange and the Cretaceous-Paleocene sedimentary sequence of Mt. Medvednica, Croatia. K-Ar ages of the high-temperature anchizonal-epizonal Paleozoic-Mesozoic complex scatter around ca. 110 Ma for slates, phyllites and marbles with phyllite intercalations, while they are significantly younger (ca. 80 Ma) for metavolcanoclastic rocks that are devoid of detrital K-white mica. Beside the Cretaceous (Alpine) K-Ar radiometric age data, no evidence of a possibly older, Variscan metamorphic event was detected. In the Jurassic ophiolitic mélange and the Cretaceous-Paleocene sequence the obtained mixed isotopic age data do not provide reliable estimates for the age of the diagenetic alterations. In the Paleozoic and Mesozoic formations from the Bükk, Szendrõ and Uppony Mts. (NE Hungary) and in the Paleozoic rocks series from the Internal Dinarides, similar Alpine (Cretaceous) K-Ar ages were determined. These data may suggest metamorphic processes related to the subduction of the Dinaridic oceanic crust beneath Eurasia accompanied by compressional crustal thickening.

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New mineral paragenetic, illite Kübler index, chlorite “crystallinity”, apparent crystallite thickness, lattice strain, and K-white mica geobarometric data proved that the Eoalpine (Paleozoic-Mesozoic) metamorphic complex was affected by medium-pressure, high-temperature anchizonal regional metamorphism, whereas the Jurassic ophiolitic mélange and the Late Cretaceous-Paleocene sedimentary sequence of Mt. Medvednica were diagenetically altered. Mineral chemical investigations carried out on phyllosilicate flakes found in various microstructural positions revealed complete mineral chemical homogenization of chlorite and K-white mica of selected slate samples from the Eoalpine (Paleozoic-Mesozoic) metamorphic complex. One possible explanation of this feature is an Alpine (Cretaceous) regional metamorphic event with polyphase deformational history. Variscan low-temperature metamorphism overprinted by an Alpine (Cretaceous) event, with temperatures at least as high as those of the Variscan one, may be an alternative, although more complicated explanation. However, no isotope geochronological evidence supports this assumption. At present only one metamorphic event can be detected. Its physical conditions were ca. 350-400 °C on the basis of illite Kübler index and chlorite Al(IV) empirical thermometers and 3-4 kbar using K-white mica b cell dimension measurements.

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