Some geochemical features of the Upper Cretaceous bimodal volcanic association from the Požeška Gora Mt. in Slavonija (northern Croatia, Yugoslavia)
The first part of the paper deals with the evaluation of all available geological and petrological data on volcanic rocks of bimodal association of the Požeška Gora Mt. They are of Upper Cretaceous, mostly Senonian age, what is proved by geological and radiometric methods. The bimodal volcanic association is made up of approximately the same proportion of alkali feldspar rhyolite and ophitic metabasalt with relict ophitic basalt. Geochemistry of basalts and rhyolites has been studied in detail. On the basis of immobile trace elements, geotectonic setting of the basalts has been considered. Alkali feldspar rhyolites belong to high-silica varieties which contain increased quantities of gold and silver. The rhyolites can be correlated in certain immobile trace element contents with the associated alkali feldspar granites, and both groups of rocks have the same 87Sr/86Sr ratio. These two facts indicate their common origin from the same A-granitoid melt originated in crustal environment. By contrast, basalt melts are of the upper mantle origin. Bimodal volcanic association from Požeška Gora Mt. can be correlated with identical associations in the world. Most of them are genetically related to subduction and subsequent extension processes so that they have their specific geochemical, petrological and genetical features which are discussed in detail.