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Determination of Thermal Alterations in Metapelites from the "Radlovac Formation" (Slavonian Mts.) Using Illite Kübler Index, Chlorite "Crystallinity" and Vitrinite Reflectance: Preliminary Results

"Radlovac Formation", also named in literature as Radlovac metamorphic complex, comprises sub-greenschist facies rocks, mainly slates, metasandstones, metaconglomerates and phyllites, locally intruded by metadiabases and metagabbros. At present, a wide set of age constraints based on paleontological evidences, K-Ar radioisotopic age data and the pre-Alpine structural position of the unit, point to the Variscan very-low grade metamorphism (JAMIČIĆ, 1988 ; PAMIĆ & LANPHERE, 1991). Illite Kübler index ("crystallinity") (KIC) (KÜBLER, 1968), is defined as the full width at half maximum (FWHM) of the first basal X-ray powder diffraction maximum of illite or muscovite measured in Δ °2Θ . KIC has been widely used for determining diagenetic and low-grade metamorphic zones (anchizone – epizone) for fine-grained rocks that lack typical metamorphic index minerals. In the last few decades, measurements of chlorite "crystallinity" (ChC), based on the full width of half maximum of the 001 and 002 basal “ reflections” of chlorite, have been also successfully applied to determine thermal changes that affected rocks during their geological history (ÁRKAI, 1991). Since the occurrence of other phyllosilicates, such as detrital K white mica, paragonite, margarite and pyrophillite, may often affect the 10 Å ; peak profile, the petrogenetic interpretation of the KIC data has to be carried out with caution. Thus, simultaneous measurements of IC and ChC are recommended (ÁRKAI et al., 1995). Vitrinite reflectance is a simple and widely used optical technique for assessing the level of thermal maturity of organic matter (vitrinite) and often shows good correlation with illite Kübler index and chlorite "crystallinity" data (KISCH, 1987). Values of vitrinite reflectance increase with depth in response to increasing temperature in an irreversible way. Illite and chlorite "crystallinity" indices and vitrinite reflectance data were used for the present evaluation of metamorphic grade of rocks, mostly phyllites, metasandstones, metaconglomerates and slates from the "Radlovac Formation" situated in Radlovačka rijeka valley, Mt. Papuk. Six samples were studied by X-ray powder diffraction method (XRPD) to determine the mineral composition and the degree of thermal alteration that affected these rocks. The mineral composition of the studied rocks comprises illite and/or muscovite, chlorite, plagioclase and quartz, with low quantity of calcite. Percentage of organic carbon (%TOC) was determined in order to eliminate the samples that are not suitable for vitrinite reflectance measurements (Table 1). Vitrinite particles were found in four samples in very low quantity. Only one sample was suitable for vitrinite reflectance measurements (Figure 1a), while vitrinite in other three samples was fine-grained, thus not suitable for the analyses (Figure 1b). Illite and chlorite "crystallinity" indices (Table 1), obtained on the <2μ m fractions, show that thermal alterations (regional metamorphism) range from the upper anchizone (approximately 250°C) to the low temperature part of epizone (300°C), thus all samples are slightly metamorphosed. The results of vitrinite reflectance measurements on sample RA-12 indicates meta-anthracite coal rank and shows good correlation with KIC and ChC data. References: ÁRKAI, P. (1991): Chlorite crystallinity: an empirical approach and correlation with illite crystallinity, coal rank and mineral facies as exemplified by Palaeozoic and Mesozoic rocks of northeast Hungary.-J. Metamorphic Geol., 9, 723-734. ÁRKAI, P., SASSI, P. F., SASSI, R. (1995): Simultaneous measurements of chlorite and illite crystallinity: a more reliable tool for monitoring low- to very low grade metamorphisms in metapelites. A case study from the Southern Alps (NE Italy).- Eur. J. Mineral., 7, 1115-1128. JAMIČIĆ, D. (1988): Tectonics of the Slavonian Mts. (in Croatian).- PhD Thesis, University of Zagreb. KISCH, H. J. (1987): Correlation between indicators of very low-grade metamorphism.- In: Frey, M. (ed.): Low-temperature metamorphism, Blackie and Son Ltd, Glasgow, 227-301. KÜBLER, B. (1968): Évaluation quantitative du métamorphisme par la cristallinité de l'illite.- Bull. Centre Rech. Pau-SNPA, 2, 385-397. PAMIĆ, J. & LANPHERE, M. (1991): Hercynian granites and metamorphic rocks from Papuk, Psunj, Krndija and surrounding basement of the Pannonian Basin (northern Croatia) (in Croatian).- Geologija, 34, 81-253.

Radlovac Formation ; Croatia ; very-low grade metamorphism ; XRPD ; illihlorite te Kübler index ; chlorite

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