Geochemistry, Petrogenesis and Geodynamic Relationships of Miocene Calc-alkaline Volcanic Rocks in the Western Carpathian Arc, Eastern Central Europe

Abstract

We report major and trace element abundances and Sr, Nd and Pb isotopic data for Miocene (16� 5^11 Ma) calc-alkaline volcanic rocks from the western segment of the Carpathian arc. This volcanic suite consists mostly of andesites and dacites; basalts and basaltic andesites as well as rhyolites are rare and occur only at a late stage. Amphibole fractionation both at high and low pressure played a significant role in magmatic differentiation, accompanied by highpressure garnet fractionation during the early stages. Sr^Nd^Pb isotopic data indicate a major role for crustal materials in the petrogenesis of the magmas.The parental mafic magmas could have been generated from an enriched mid-ocean ridge basalt (E-MORB)type mantle source, previously metasomatized by fluids derived from subducted sediment. Initially, the mafic magmas ponded beneath the thick continental crust and initiated melting in the lower crust. Mixing of mafic magmas with silicic melts from metasedimentary lower crust resulted in relatively Al-rich hybrid dacitic magmas, from which almandine could crystallize at high pressure. The amount of crustal involvement in the petrogenesis of the magmas decreased with time as the continental crust thinned. A striking change of mantle source occurred at about 13 Ma. The basaltic magmas generated during the later stages of the calc-alkaline magmatism were derived from a more enriched mantle source, akin to FOZO. An upwelling mantle plume is unlikely to be present in this area; therefore this mantle component probably resides in the heterogeneous upper mantle. Following the calc-alkaline magmatism, alkaline mafic magmas erupted that were also generated from an enriched asthenospheric source. We propose that both types of magmatism were related in some way to lithospheric extension of the Pannonian Basin and that subduction played only an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase of extension by melting of metasomatized, enriched lithospheric mantle and were contaminated by various crustal materials, whereas the alkaline mafic magmas were generated during the post-extensional stage by low-degree melting of the shallow asthenosphere. The western Carpathian volcanic areas provide an example of long-lasting magmatism in which magma compositions changed continuously in response to changing geodynamic setting.