Geochemistry and Sr–Nd–Pb isotope characteristics of Miocene basalt–trachyte rock association in transitional zone between the Outer Western Carpathians and Bohemian Massif

Abstract

Representative samples covering all petrographic types of the Miocene subvolcanic (dykes and sills) basalt– trachyte rock association from the Uherský Brod area (UB, Moravia) in the Czech Republic were studied. The UB area is associated with the Klippen Zone in the neighbourhood of the contact of the Carpathian–Pannonian Block with the Bohemian Massif. The petrographic and geochemical features of the subvolcanic rocks from the UB area only partly resemble those of the calc-alkaline rocks in a similar structural position in the coeval Pieniny area in Poland as well as of the transitional calc-alkaline rocks from the Central Slovakian volcanic field. The Miocene subvolcanic rocks were generated by melting of the variably metasomatised sub-lithospheric source and subsequent fractional crystallisation of the primary basaltic magma. The compositional characteristics of this rock series straddle the alkaline and subalkaline volcanic rock fields, while the rocks from the Pieniny area and from the Central Slovakian volcanic field are predominantly calc-alkaline andesites. The distribution of incompatible elements such as U, Th, REE, Nb, Ta, Sr, Ba as well as LaN/YbN ratios and Sr–Nd–Pb isotope composition of the rocks from the UB and Pieniny areas differ substantially, whereas the geochemical patterns of the UB rocks are slightly similar to those of the transitional rocks from the Central Slovakian volcanic field. The modest contents of incompatible trace elements in the andesitic rocks from the Pieniny area share several characteristics of the prevalent calc-alkaline andesitic rocks from the Carpathian–Pannonian region. Basaltic–trachytic series from the UB area are characterised by higher ɛNd (~2.5) and lower 87Sr/86Sr (~0.704) and 207Pb/204Pb (~15.65) initial values in comparison with the calc-alkaline rocks from the Carpathian–Pannonian region including the Pieniny area. Nevertheless, the basaltic–trachytic rocks from the UB, richer in some of the incompatible trace elements (e.g., U, Th, REE, Sr and Ba) partly resemble (including Sr–Nd–Pb isotopes) the Na-alkaline Miocene volcanic rock series of the NE shoulder of the Cheb–Domažlice Graben in the Bohemian Massif.