Geochemistry and geodynamics of a Late Cretaceous bimodal volcanic association from the southern part of the Pannonian Basin in Slavonija (Northern Croatia)

In this paper we present petrological and geochemical information on a bimo dal basalt-rhyolite suite associated with A-type granites of Late Cretaceou s age from the South Pannonian Basin in Slavonija (Croatia). Basalts and al kali-feldspar rhyolites, associated in some places with ignimbrites, occur in volcanic bodies that are interlayered with pyroclastic and fossiliferous Upper Cretaceus sedimentary rocks. The petrology and geochemistry of the b asalts and alkali-feldspar rhyolites are constrained by microprobe analyses , major and trace element analyses including REE, and radiogenic and stable isotope data. Basalts that are mostly transformed into metabasalts (mainly spilites), are alkalic to subalkalic and their geochemical signatures, par ticularly trace element and REE patterns, are similar to recent back-are ba salts. Alkali-feldspar rhyolites have similar geochemical features to the a ssociated cogenetic A-type granites, as shown by their large variation of N a2O and K2O (total 8-9%), very low MgO and CaO, and very high Zr contents r anging between 710 and 149 ppm. Geochemical data indicate an amphibole Iher zolite source within a metasomatized upper mantle wedge, with the influence of upper mantle diapir with MORB signatures and continental crust contamin ation. Sr incorporated in the primary basalt melt had an initial Sr-87/Sr-8 6 ratio of 0.7039 indicating an upper mantle origin, whereas the Sr-87/Sr-8 6 ratio for the alkali-feldspar rhyolites and associated A-type granites is 0.7073 indicating an apparent continental crust origin. However, some othe r geochemical data favour the idea that they might have mainly originated b y fractionation of primary mafic melt coupled with contamination of contine ntal crust. Only one rhyolite sample appears to be the product of melting o f continental crust. Geological and geodynamic data indicate that the basal t-rhyolite association was probably related to Alpine subduction processes in the Dinaridic Tethys which can be correlated with recent back-are basins . The difference in geological and isotope ages between the bimodal basalt- rhyolite volcanism with A-type granite plutonism (72 Ma) and the final syki nematic S-type granite plutonism (48 Ma) can be taken as a lifetime of the presumed BARE system of the Dinaridic Tethys. Remnants of this presumed sub duction zone can be traced fur 380 km along the surrounding northernmost Di narides.