TIMING OF LOW-TEMPERATURE METAMORPHISM AND COOLING OF THE PALEOZOIC AND MESOZOIC FORMATIONS OF THE BUKKIUM, INNERMOST WESTERN CARPATHIANS, HUNGARY

K-Ar ages of illite-muscovite and fission track ages of zircon and apa tite were determined from various lithotypes of the Bukkium, which for ms the innermost segment of the Western Carpathians. The stratigraphic ages of these Dinaric type formations cover a wide range from the Lat e Ordovician up to the Late Jurassic. The grade of the orogenic dynamo -thermal metamorphism varies from the late diagenetic zone through the 'anchizone' up to the 'epizone' (chlorite, maximally biotite isograd of the greenschist facies). The K-Ar system of the illite-muscovite in the <2 mu m grain-size fraction approached equilibrium only in 'epizo nal' and high-temperature 'anchizonal' conditions. The orogenic metamo rphism culminated between the eo-Hellenic (160-120 Ma) phase connected to the beginning of the subduction in the Dinarides, and the Austrian (100-95 Ma) phase characterized by compressional crustal thickening. No isotope geochronological evidence was found for proving any Hercyni an recrystallization. The stability field of fission tracks in zircon was approached using the thermal histories of the different tectonic u nits. A temperature less than 250 degrees C and effective heating time of 20-30 Ma had only negligible effects on the tracks, whereas total annealing was reached between 250 and 300 degrees C. Apatite fission t rack ages from the Paleozoic and Mesozoic formations show that the upl ift of the Bukk Mountains occurred only in the Tertiary (not earlier t han ca. 40 Ma ago). Thermal modeling based on apatite fission track le ngth spectra and preserved Paleogene sediment thickness data proved th at the Late Neogene burial of the recently exhumed plateau of the Bukk Mountains exceeded 1 km.