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Petrographic features of chloritoid schist from southeastern slopes of Mt. Medvednica, (Zagorje-Mid-Transdanubian zone, Croatia)

In the frame of detail mapping of parametamorphic rocks on the southeastern slopes of Mt. Medvednica (Zagorje-Mid-Transdanubian zone, Croatia), samples of the chloritoid schists from the upper part of geological column were analyzed in more detail. This study is a part of preliminary research that included XRD, XRF, ICP-MS, SEM, electron microprobe and microstructural characterization of chloritoid schists in order to determine their petrography, microstructural features, mineral assemblages, phase composition, whole-rock and mineral chemical composition and morphology of accessory minerals (zircon typology). The samples can be categorized as chlorite-muscovite-quartz schist, chloritoid-chlorite-quartz-muscovite schist, chlorite-muscovite-quartz-chloritoid schist and interbeds of marble. Accessory minerals in chloritoid schist are tourmaline, zircon and rutile. Microstructural features show two deformational events, the sinmetamorphic and postmetamorphic events. The latter deformational event is recorded in the development of flazer structure, where the mineral grains in cleavages are translated, fractured and rotated. The whole-rock chemical analyses show high concentrations of SiO2 (74.79 wt. %), K2O (2.5 wt. %), Al2O3 (13.22 wt. %), and low values of MgO (0.99 wt. %) and CaO (0.08 wt. %).These results indicate that acid rocks could be a possible protolith. The REE distribution normalized to chondrite shows higher LREE to HREE concentrations ((La/Yb)N=5.68, (La/Sm)N=3.05, (Gd/Yb)N=1.21), while the Eu anomaly has a low value (Eu/Eu*=0.7). Such metamorphic mineral assemblage is characteristic for low-grade metamorphism. The chlorite-chloritoid geothermometer gives metamorphic temperature values of approximately 450°C. The source rocks of the chloritoid schist are argillaceous sandstones, derived from acid magmatic rocks, interbeded with carbonates. Carbonate interbeds indicate deposition in a marine environment. The morphology of zircons shows that the source for protolith is of granitoid composition, while their weak roundness indicates a short transport.

chloritoid; schist; Medvednica; Croatia

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