engleski

Determination of thermal alterations in samples from the Radlovac formation (Slavonian Mts.) using illite and chlorite "crystallinity" indices

The Kübler index (KI), also known as illite "crystallinity" index (IC) (Kübler, 1967), is defined as the calibrated full width at half maximum (FWHM) of the X-ray powder diffractometric first basal reflection of illite-muscovite. It has been widely used for determining diagenetic and low-grade temperature metamorphic zones (anchizone - epizone) for rocks that lack metamorphic index minerals. Recently, measurements of chlorite "crystallinity" (ChC) based on the FWHM of the 001 and 002 basal reflections of chlorite have also been applied successfully to determine thermal changes that affected rocks during their geological history. Both methods were used for the present study of metamorphic stage of rocks, mostly phyllites and slates, from the Radlovac formation situated in the Papuk, Mts. Slavonia. Sixteen samples were studied with X-ray powder diffraction method (XRPD) to determine the modal composition and the degree of thermal alterations that affected these rocks. K-white mica b cell dimension was also measured on samples that represent various lithotypes from the Radlovac formation in order to characterize the pressure range of metamorphism. The modal composition of the rocks studied contain mostly illite-muscovite, chlorite, plagioclase (albite) and quartz, with very low percentage of calcite and other clay minerals (phlogopite, kaolinite, montmorillonite ). Illite and chlorite "crystallinity" indices show that the thermal alterations (regional metamorphism) range from the upper anchizone (approximately 250 °C) to the low temperature part of the epizone (300 °C), thus samples are slightly metamorphosed. Due to the variable b data (8.992 - 9.044 &Aring ; ) the determination of pressure conditions of the studied area is questionable, especially because only fine metaclastic rocks with adequate modal composition are suitable for semiquantitative pressure indication.

illite crystallinity; chlorite crystallinity; Radlovac formation; metamorphism

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