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 southeastern continuation of the Paleozoic complexes of the adjacent Eastern Alps, are included in the Noric-Bosnian and Betic-Serbian zones (FLÜGEL 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 (PAMIĆ & JURKOVIĆ 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 pencontemporaneous 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 (Busovača area), their central parts (the Fojnica area) and their southeastermost 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 343ą13 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 287.8ą11.1 to 268.7ą10.2 Ma record Early Permian tectonomagmatic activity. On these rocks decreased K-Ar ages of 247ą9.5 and 238.4ą9.2 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 120.7ą4.6 and 91.8ą4.7 Ma. d) The most common measured ages of the fourth geochronological group are spanned in the interval between 50.4ą4.9 and 34.9ą1.4 Ma that correspond to Early/Middle Eocene. Beside this an Ar-Ar plateau age of 31.2ą0.3 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). The presented radiometric ages indicate polyphase metamorphic evolution of the MBSM Paleozoic complex. K-Ar age of 343 Ma represents evidence that the main Variscan deformation took place during the Early Carboniferous. It also indicates a pre-Carboniferous (Ordovician(?)-Silurian(?)-Devonian) age of protolithic formations. Permian radiometric ages (287.8-268.7 Ma) were related to postcollisional strike-slip faulting and exhumation. Eoalpine radiometric ages (120.7-91.8 Ma) reflect metamorphic overprint which was probably related to the emplacement, i.e., the stacking of Paleozoic-Triassic nappes which took place after the first Late Jurassic/Early Cretaceous obduction of the Dinaridic ophiolites (PAMIĆ et al. 2001). This interpretation implies that the Paleozoic-Triassic nappes derived from the NE margin of the Dinaridic Tethys. Early/Middle Eocene radiometric ages reflect the metamorphic overprint which was related to the final Alpine deformation, i.e., the incorporation of the MBSM Paleozoic complex in the present structure of the Dinarides. The lowermost radiometric ages (31 Ma) obtained on Alpine vein minerals are related to postcollisional strike-slip faulting. HRVATOVIĆ (1996) recognized three different deformational phases within MBSM, however undated and thus not correlative to the radiometric age groups defined afore. The MBSM Paleozoic complex is correlative to the Austroalpine Paleozoic units not only in lithology and stratigraphy but also in polymetamorphic evolution. This is indicated by its 1) Ordovician, Silurian and Devonian Paleotethyan generation of protolithic formations, 2) Early Carboniferous main Variscan deformation, 3) Post-Variscan volcanism and exhumation, and 4) Eoalpine and Mesoalpine metamorphic overprints as the result of dispersion and dismembering of the Variscan Europe by Mesozoic/Tertiary tectonism (FRANK 1987, NEUBAUER & SASSI 1993, NEUBAUER et al. 1999). The MBSM can be correlated with the low-grade Paleozoic units, especially with the Carnic Alps or Northern graywacke zone or Gurktal nappe from various tectonostratigraphic units of the Austroalpine realm.

Ar-Ar; K-Ar; polyphase metamorphic evolution; Paleozoic complex; Central Dinarides

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