Neoproterozoic to Cenozoic magmatism in the central part of the Bohemian Massif (Czech Republic): Isotopic tracking of the evolution of the mantle through the Variscan orogeny

Abstract

Abstract The evolution of the mantle source beneath the Tepla-Barrandian (TBB) and the adjacent southern part of the Saxo-Thuringian (S-STB) and northern part of the Moldanubian (N-MB) blocks of the Bohemian Massif (Czech Republic) is tracked from the Late Neoproterozoic to the Cenozoic in order to examine the coupling between the crust and underlying continental lithospheric mantle (CLM) during the Late Paleozoic Variscan orogeny. In the TBB, Late Neoproterozoic mafic rocks have highly radiogenic Nd but moderately radiogenic Sr isotopes and were derived from a spinel peridotite mantle. These within-plate rocks were emplaced in an intra-oceanic back-arc basin. Early Paleozoic (Ordovician to Early Devonian) mafic rocks are rift/extension-related, enriched in LREE and Nb relative to HREE and were probably derived from a garnet peridotite mantle source. Compared to Neoproterozoic basalts, their ƐNd(t) values are slightly lower but TDM model ages are indistinguishable suggesting derivation from a deeper portion of the same mantle source. Thus, the weak deformation and low-grade metamorphism associated with the Cadomian orogeny did not significantly decouple the crust from the underlying mantle source beneath the TBB. Early Paleozoic rift-related mafic rocks of the S-STB and N-MB have comparable isotopic signatures indicating a similar mantle source. Late Paleozoic mafic magmatism occurred during the extensional collapse phase of the Variscan orogen, in the aftermath of the collision between Gondwana and Laurussia. The TBB and S-STB basaltic rocks are within-plate, and transitional between alkaline and tholeiitic compositions but show a relative depletion of Nb. Their isotopic characteristics contrast with older basalts in that they have negative ƐNd(t) values, high initial 87Sr/86Sr ratios and older TDM model ages (1050 to 1300\u202fMa). Late Paleozoic mantle-derived potassic-ultrapotassic magmas occurring as lamprophyre dikes and small volume high Mg K intrusions in N-MB, S-STB and TBB have similar isotopic characteristics suggesting that all these Late Paleozoic rocks were derived from an old CLM contaminated by fluids or silicic melts derived from subducted Precambrian crustal material or alternatively, the region was underthrust by Gondwanan mantle during the Variscan collision. Cenozoic volcanic rocks in the Bohemian Massif are local representatives of the regionally extensive Cenozoic European volcanic province. The rocks are within-plate, alkaline basalts enriched in LREE and Nb and were derived from garnet peridotite mantle. They are significantly more juvenile than Carboniferous-Early Permian mafic rocks, with higher ƐNd(t) values (typically +3 to +5), lower initial 87Sr/86Sr ratios and significantly younger TDM age. These data suggest significant input from upwelling asthenospheric mantle, implying that at least a part of the Permian CLM mantle was re-fertilized.