Petrogenesis of Cenozoic high-Mg (picritic) volcanic rocks in the Ceske stredohori Mts. (Bohemian Massif, Czech Republic)
Abstract
Representative samples of high-Mg (picritic) rocks of Eocene to Oligocene age (38 to 21 Ma) from the České středohoří Mts. (CSM) in the Ohře/Eger Rift were studied and compared with Cenozoic high-Mg rocks from the Bohemian Massif. The picritic rocks of the CSM correspond to picrite, meimechite and basanite to tephrite.
Individual petrographic types are characterised by the association of mafic minerals: magnesian olivine {Fo(86-80)} + clinopyroxene (aluminian diopside) +- amphibole (pargasite/ kaersutite) +- dark mica (ferroan phlogopite). The contents of incompatible elements, such as Nb, Ta, U, Th, the La(N)/Yb(N) ratios, and the overall Sr-Nd-Pb isotope signatures of the high-Mg rocks partly overlap the basanites of the Ústí Formation, also showing a similar age (44-21 Ma).
The Pb isotope signature for all recognised high-Mg rock types is compatible with a variable contribution from an inherited (Variscan) orogenic Pb component preserved in their lithospheric mantle source. Both the high-Mg volcanic rocks from the CSM and from other areas of the Bohemian Massif require the involvement of a heterogenous lithospheric source.
Their parental melts were generated by variable degree of melting of an olivine-rich peridotitic mantle, containing clinopyroxene-amphibole (+- phlogopite) vein assemblages with highly radiogenic Sr and Pb signatures.