Evolution of authigenic and detrital K-micas at the boundary between anchimetamorphism and low-temperature metamorphism during the Cretaceous tectono-metamorphic cycle in the Western Carpathians

Microprobe data on authigenic and detrital K-micas from sedimentary-volcani c rocks of the Upper Paleozoic-Mesozoic complexes of the Western Carpathian s, which were metamorphosed to the anchizone, epizone, and chlorite-sericit e-subfacies grades during the Early Alpine cycle, indicate that the composi tion of the minerals is different at each of the three metamorphic zones. I n the upper part of the anchizone (similar to 250 degrees C), the authigeni c K-micas are illite-phengite with (K + Na) from 0.7 to 0.95 f.u. With the approach of the epizone boundary (250-300 degrees C), illite gradually give s way to highly crystalline muscovite-phengite with (K + Na) from 0.85 to 1 .0 f.u., which remains stable throughout the chlorite-sericite subfacies (3 00-350 degrees C). In the anchizone, detrital biotite either retains its pr imary, "granitic" high-temperature composition or is pseudomorphed by anoma lous black-brownish weakly pleochroic metastable Ti-phengite and Ti-chlorit e, whose high TiO2 concentrations (up to 3.5-4.0 wt %) are inherited from t he replaced biotite. These Ti-micas occur throughout the epizone. In additi on to the above-mentioned minerals, the matrix of the rocks contains "norma l" low-Ti chlorite and phengite. With the transition to the chlorite-serici te subfacies, Ti-bearing micas decompose and are replaced by phengite-chlor ite-leucoxene aggregates. Hence, the presence of the metastable products of biotite decomposition, along with newly formed equilibrium minerals in the rock matrix, suggests that the anchimetamorphic processes did not attain c omplete equilibrium, a feature that makes them different from normal metamo rphism. The detrital muscovite retains its primary composition over the int erval of the three metamorphic grades and compositionally corresponds to th e high-temperature mice in granites: in contrast to the authigenic phengite , it is poor in (Mg + Fe), saturated with (K + Na), and has a high Na/(Na K) ratio. It is only in the upper portion of the chlorite-sericite subfaci es, near the metamorphic biotite isograde, that the detrital muscovite beco mes fully dissolved in the phengite matrix, or the compositions of the musc ovite and phengite become equalized. The high ME, Fe, and Si concentrations typical of phengite of the Phn + Chl + Kfs + Qtz assemblage make it possib le to roughly estimate the pressure by analogy with the Phn + Br + Kfs + Qt z assemblage, which is used in phengite geobarometry.