Metabasic rocks in the Varied Group of the Moldanubian Zone, southern Bohemia – their petrology, geochemical character and possible petrogenesis
Abstract
Metabasic rocks are important constituent of the Chýnov and Český Krumlov units in the Varied Group (Moldanubian Zone, S Bohemia). The dataset is dominated by EMORB-like tholeiite basalts spanning from Early Palaeozoic melting of a depleted mantle source (Eps500 = +8.6 to +9.4; TDM = 0.43-0.50 Ga).
Composition of the remaining samples reflects upper crustal contamination (Eps500 = +3.1 to +1.3, progressive enrichment in Th, as well as depletion in Nb, P and Ti). A much smaller group of amphibolites is characterised by steep REE patterns and high HFSE contents (Nb, Ta, Zr and P).
It is of an OIB affinity, with parental alkali basalt generated by a low degree of partial melting of a deep, garnet-bearing asthenospheric mantle source (Eps500 = +4.5 to +6.1). Overall, the likely tectonic setting was attenuated lithosphere, subjected to an Early Palaeozoic extension, leading eventually to fragmentation of the northern Gondwana margin.
The minor OIB component was probably added by a rising mantle plume.