COMPLEX ALKALINE MAGMA EVOLUTION AND EMPLACEMENT HISTORY OF THE MIOCENE MONOGENETIC ZEBIN VOLCANO (CZECH REPUBLIC)

A detailed petrographic and geochemical study of the mid-Miocene Zebín Hill tuff cone located in the Jičín volcanic field, Czech Republic, is combined with field observations to constrain alkaline magma evolution within a well preserved monogenetic volcanic system in the Eger Rift. The central part of Zebín Hill consists of a main conduit plug associated with numerous smaller dikes and blob like intrusions emplaced into phreatomagmatic pyroclastic deposits. Selected samples representing the main volcanic plumbing features were studied petrographically to distinguish variations in texture, grain-size, and mineral proportion among individual intrusions. Petrographic observations were further supported by quantitative chemical analyses of individual minerals, conducted on Tescan MIRA 3GMU fitted with SDD X-Max 80mm2 EDS detector and AZtecEnergy software. The main plug and dikes proximal to the main plug consist of coherent pure basanite, whereas peripheral dikes are of mingled character (basanitic and trachytic magma batches). The unusual mingled character of the dikes are confirmed by textures, mineral assemblages, reverse zoning of clinopyroxenes in hybrid veinlets (Ti rich cores ; Mg rich rims) as well as a wide spectrum of feldspar compositions displaying a continuous trend from sanidine to labradorite. Comparison of these new data with other petrographic and geochemical studies of volcanoes within the Jičín volcanic field suggest the presence of a zoned magma chamber underneath Zebín Hill as no other trachytic or phonolitic bodies are known within the region. This conclusion is inconsistent with previous interpretations of magma evolution within monogenetic volcanic systems as even apparently small monogenetic volcanoes can be associated with compositionally complex magma feeding and storage systems. In this case, a basanitic volcano, with an original size not exceeding 1 km across, formed from magma chamber with complex differentiation and mingling processes.