engleski

Polyphase metamorphic evolution of the Mid-Bosnian Schist Mts. (MBSM) Paleozoic complex from the Central Dinarides (Bosnia and Herzegovina)

Paleozoic allochthonous disrupted complexes from the Internal Dinarides, representing the south- eastern continuation of the Paleozoic complexes of the adjacent Eastern Alps, are included in the Noric- Bosnian and Betic-Serbian zones (FLUGEL, 1990). The Paleozoic complex of the MBSM, with its surface area of about 3000 km2 is one of the largest Paleozoic terrane of the Dinarides. It is included in the Pannonian-Durmitor nappe (PAMIC & JURKOVIC, 2001). Rocks of the predominant Variscan complex are represented by fossiliferous Devonian carbonate platform sequence which is conformably underlain by afossiliferous metaclastic rocks and cherts of presumed Silurian and Ordovician age. In these rocks are included large masses of penecontemporaneous metarhyolites with pyroclastics and subordinate metamafic volcanics. These Variscan rocks are disconformably overlain by post-Variscan, mainly Permian formations grading into Scythian sedimentary rocks. Systematic sampling for radiometric dating was carried out on best exposed profiles in the northeastern part of the MBSM (Busovaca area), their central parts (the Fojnica area) and their southeasternmost part (the Bradina area). Based on K-Ar whole rock and mineral measurements on 30 samples, four geochronological age groups can be distinguished which are comprised within an interval of about 300 Ma. a) The highest whole rock age of 343&plusmn ; ; 13 Ma was measured on a phyllite indicating Early Carboniferous main Variscan deformation as well as the pre- Variscan age of protolithic formations. b) Ages of the second group obtained on metadiabases and orthogreenschists ranging from 287.8&plusmn ; ; 11.1 to 268.7&plusmn ; ; 10.2 Ma record Early Permian tectonomagmatic activity. On these rocks decreased K-Ar ages of 247&plusmn ; ; 9.5 and 238.4&plusmn ; ; 9.2 Ma, probably due to Ar loss, were measured as well. c) More common Eoalpine (Early Cretaceous) K-Ar ages of the third group obtained on metarhyolites range mainly between 120.7&plusmn ; ; 4.6 and 91.8&plusmn ; ; 4.7 Ma. d) The most common measured ages of the fourth geochronological group are spanned in the interval between 50.4&plusmn ; ; 4.9 and 34.9&plusmn ; ; 1.4 Ma that correspond to Early/Middle Eocene. Beside this an Ar-Ar plateau age of 31.2&plusmn ; ; 0.3 Ma was measured on hyalophane. A realistic closure temperature of 253°C has been determined for the hyalophane: this datum postdates metamorphism and is regarded as a cooling age (BALOGH et al., 1999).

metamorphism; Central Dinarides; Paleozoic complex; K-Ar; Ar-Ar

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