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Metasedimentary rocks from southeastern slopes of Mt. Medvednica, (Zagorje-Mid-Transdanubian zone, Croatia), conclusions from investigation of chloritoid schists

The investigation of chloritoid schist is a part of the ongoing research of metasedimentary rocks from Mt. Medvednica. Mt. Medvednica is a part of the Zagorje-Mid-Transdanubian zone, which has transitional features of main Alpine and Dinaridic tectonostratigraphic units. Zagorje-Mid-Transdanubian zone is marked by the Balaton fault on the north and on the southeast by the Zagreb-Zemplin fault zone, which is also considered to be the northwest border of the Dinarides. The Zagorje-Mid-Transdanubian zone also separates the Alcapa and Tisia, two major basement blocks of the Pannonian Basin (TOMLJENOVIĆ, 2000). Samples were collected from Stari potok creek (SE slopes of Mt. Medvednica) and a geological column was drawn. Samples are categorized as chloritoid-chlorite-quartz-muscovite schist, chlorite-muscovite-quartz schist, chlorite-muscovite-quartz-chloritoid schist and intercalations of marble. Accessory minerals in chloritoid schist are tourmaline, zircon, apatite, monazite, xenotime and rutile. The chloritoid schist found in the upper part of the column was analyzed in more detail. X-ray diffraction (XRD), X-ray fluorescence (XRF), laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), scanning electron microscopy (SEM), electron microprobe and microstructural investigations were used to determine the chemical composition, microstructural features and morphology of zircons of chloritoid schist. PERPLE_X software package was used for the pressure-temperature calculations (CONNOLLY, 2005). Two deformational events (syn- and post-metamorphic) were recognized. The primary foliation is a result of the syn-metamorphic event and the second deformational event is recognized in the form of flaser structure, where the mineral grains in cleavages are translated, fractured and rotated. Chlorite-chloritoid thermometry (VIDAL et al., 1999) and intersection of phengite, chlorite and chloritoid isopleths in the MnNCKFMASHTO pseudosection suggests relict high-pressure metamorphism at approx. 13-14 kbar and 410-440°C and equilibration of chlorite and white mica inside greenschist facies P-T conditions at approx. 8 kbar and 350-400°C.Thermometry of chlorite and chloritoid gave unexpectedly high temperatures over 500°C which are subject of further research. The chemical analyses of chloritoid schist 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). 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). These results indicate that acid rocks could be a possible protolith. Whole rock composition of La–Th–Sc and Th–Sc–Zr/10 suggests that possible depositional environment could be continental arc (BHATIA & CROOK, 1986). Zircon typology suggests that there could be at least two sources of zircon grains. One source is possibly magmatic with rounded detrital zircons and another source could be of volcanic origin with typically idiomorphic zircon grains (PUPIN, 1980). Argillaceous sandstones, probably derived from acid magmatic rocks, are very likely the source rocks of the chloritoid schist. Such material, with possible contribution of volcanic material, intercalated with carbonates was probably deposited in the continental arc environment. Carbonate intercalations could indicate deposition in a marine environment. References: BHATIA, M. R. & CROOK, K. A. W. (1986): Trace element characteristics of greywackes and tectonic setting discrimination of sedimentary basins.– Contrib. Mineral. Petrol., 92, 181-193. PUPIN, J. P. (1980): Zircon and Granite Petrology.– Contrib. Mineral. Petrol., 73, 207-220. TOMLJENOVIĆ, B. (2000): Zagorje Mid-Transdanubian Zone.– Vijesti Hrvatskog geološkog društva, 37/2, 27-29, Zagreb. VIDAL, O., GOFFE, B., BOUSQUET, R., PARRA, T., (1999): Calibration and testing of an empirical chloritoid-chlorite Mg-Fe exchange thermometer and thermodynamic data for daphnite.– J. of Metamorph. Geol., 17, 25-39. CONNOLLY, J.A.D. (2005): Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation. – Earth and Planet. Sci. Letters, 236, 524-541.

chloritoid schist; zircon; Mt. Medvednica; metapelites

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