THE EFFECTS OF LITHOLOGY, BULK CHEMISTRY AND MODAL COMPOSITION ON ILLITE CRYSTALLINITY - A CASE-STUDY FROM THE BAKONY MTS, HUNGARY

Modal and bulk chemical compositions, illite and chlorite ''crystallinity'' indices (IC and ChC), white mica b(o) and vitrinite reflectance values of a Palaeozoic variegated elastic sequence have been determined. The strongly varying IC values are inadequate for the determination of metamorphic zones. Based on statistical analyses of the factors which might affect IC, the presence of paragonite, even in very small quantities as a discrete phase and/or as mixed-layers with illite-muscovite, proved to be the decisive factor. In contrast, no significant correlations were found between the modal composition of the main phases, bulk chemical composition (including Fe3+/Fe2+ ratio) and IC, ChC, or white mica b(o) values. Thus, neither lithofacies (granulometric), nor redox conditions had any effect on ''crystallinity'' and b(o) parameters. Chlorite crystallinity indices and vitrinite reflectance values, together with IC values of selected samples, indicate ''anchizonal'' conditions. The distribution of white mica b(o) values indicates that this Hercynian metamorphism was of low-pressure type.