The České středohoří Mts. represent the largest and most complicated relict of volcanic complex within the Ohře (Eger) Rift forming an integral part of the Central European Volcanic Province of Cenozoic age. The rift-related (extrusive and intrusive) alkaline volcanic series are exclusively of mantle-derived origin. Using the stratigraphic model of effusive basaltic rock superposition in the central part of the České středohoří Mts. (CAJZ et al. 1999), that relates the volcanics to the Lower Formation (LF), Upper Formation (UF), Uppermost Formation (UMF) and the Group of Late Miocene Intrusives (GLMI), geochemical characteristics were established for the individual formations. Only trachybasaltic rocks of the UF substantially differ from mutually geochemically indistinguishable basanitic rock groups comprising the LF, UMF and GLMI formations. Geochemical characteristics of the LF rocks indicate the influence of several different sources on the parental magma. The trace element and isotopic data, and modelling of magma evolution during assimilation-fractional crystallisation (AFC) process imply, that rocks of the LF, UMF and GLMI represent more primitive magma derived from depleted mantle and influenced by HIMU-like component. Rocks of the UF crystallised from more evolved magma that formed from the primitive one by the AFC process, where the upper crust was assimilated and clinopyroxene, olivine and in later stages also Fe-oxides fractionated. Two magmatic chambers were probably present beneath the Ohře Rift during the Tertiary; the deeper one closely associated with the plume component and the shallower one in deeper parts of the upper crust. According to the K-Ar dating volcanic activity in the LF, UF and UMF overlapped in the time lasting from 44 Ma to 21 Ma. Both chambers must have been active simultaneously. Intrusion of the GLMI was younger, starting at ca 13 Ma. Geochemical similarity of this rock group to the LF and UMF points to reactivation of the deeper chamber. Comparing the studied Tertiary rocks to the Permo-Carboniferous volcanics of the Bohemian Massif, a similarity was found indicating a common source material and existence of the HIMU-like source already in the Permo-Carboniferous time. Derivation of the initial magma at that time from depleted mantle is confirmed by the Nd-model ages of the most primitive samples.
ASJC Scopus subject areas
- Geochemistry and Petrology