Geochemistry and evolution of subcontinental lithospheric mantle in central Europe:: Evidence from peridotite xenoliths of the Kozakov volcano, Czech Republic

Neogene basanite lavas of Kozkov volcano, located along the Lusatian fault in the northeastern Czech Republic, contain abundant anhydrous spinel lherzolite xenoliths that provide an exceptionally continuous sampling of the upper two-thirds of central European lithospheric mantle. The xenoliths yield a range of two-pyroxene equilibration temperatures from 680 degrees C to 1070 degrees C, and are estimated to originate from depths of 32-70 km, based on a tectonothermal model for basaltic underplating associated with Neogene rifting. The sub-Kozkov mantle is layered, consisting of an equigranular upper layer (32-43 km), a protogranular intermediate layer that contains spinelpyroxene symplectites after garnet (43-67 km), and an equigranular lower layer (6770 km). Negative correlations of wt TiO2, Al2O3, and CaO with MgO and clinopyroxene mode with Cr-number in the lherzolites record the effects of partial fusion and melt extraction; Y and Yb contents of clinopyroxene and the Cr-number in spinel indicate similar to 5 to similar to 15% partial melting. Subsequent metasomatism of a depleted lherzolite protolith, probably by a silicate melt, produced enrichments in the large ion lithophile elements, light rare earth elements and high field strength elements, and positive anomalies in primitive mantle normalized trace element patterns for P, Zr, and Hf. Although there are slight geochemical discontinuities at the boundaries between the three textural layers of mantle, there tends to be an overall decrease in the degree of depletion with depth, accompanied by a decrease in the magnitude of metasomatism. Clinopyroxene separates from the intermediate protogranular layer and the lower equigranular layer yield Nd-143/Nd-144 values of 0.51287-0.51307 (epsilon(Nd)=+4.6 to 8.4) and Sr-87/Sr-86 values of 0.70328-0.70339. Such values are intermediate with respect to the Nd-Sr isotopic array defined by anhydrous spinel peridotite xenoliths from central Europe and are similar to those associated with the present-day low-velocity anomaly in the upper mantle beneath Europe. The geochemical characteristics of the central European lithospheric mantle reflect a complex evolution related to Devonian to Early Carboniferous plate convergence, accretion, and crustal thickening, Late Carboniferous to Permian extension and gravitational collapse, and Neogene rifting, lithospheric thinning, and magmatism.