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Eoalpine low-temperature metamorphism in the Paleozoic complex of Mt. Medvednica, Croatia - mineral chemical and microstructural characterizations

Mineral chemical studies were carried out on K-white mica and chlorite flakes, situated in different microstructural positions of various rock types from the Paleozoic (Silurian-Devonian) complex of Mt. Medvednica, Croatia thrusted over the Ophiolite melange complex of the Dinarides. This unit consisting of predominantly slates, metavolcanics and orthogreenschists was affected by Eoalpine (122-110 Ma) very low- to low-grade (KI=0.28+0.03 D=430 Angstroms) and medium pressure (K-white mica b=9.007+0.192 Angstroms) regional metamorphism according to illite and chlorite crystallinity, Voigt method and K-white mica geobarometric investigations. Modal composition of slates in general is characterized by K-white mica, chlorite, quartz, albite and/or paragonite and minor hematite and/or pyrite and rutile. In slates two penetrative foliations are observed, one is a bedding-parallel syn-metamorphic foliation (S0-1), later crenulated along axial plane cleavage (S2). Similarly to the slates crenulation cleavage (S2) developed in the phyllosilicate-rich intercalations in phyllosilicate-rich, banded marbles (cipollino) and chlorite-rich schistose metavolcanics. K-white mica and chlorite stacks are situated parallel to S0-1 and S2 foliation planes. These and other presumably relict flakes as well as matrix forming fine-grained phyllosilicates were investigated using electron microprobe. In various rock types, contrary to all expectations, mineral chemical differences were not recognized either in chlorites or in K-white micas situated in different microstructural positions. Consequently, if more than one metamorphic events had affected the different rock types of the Paleozoic unit of Mt. Medvednica, as it was supposed before, their P and T conditions could have not been significantly different from each-other, and/or the later metamorphic event could have caused complete mineral chemical homogenization. Chlorite-muscovite thermobarometer of Vidal and Parra (2000) was used besides the various empirical chlorite-Al(IV) thermometers, phyllosilicate crystallinity indices and K-white mica b geobarometry in order to evaluate the pressure and temperature conditions of different rock types from the unit. The obtained pressure (3-3.5 kbar) and temperature (350-400º C) data are in agreement with the results of the above mentioned methods.

mineral chemistry; chlorite; K-white mica; microstructural investigations; thermobarometry

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