Petrogenesis of post-collisional Late Paleozoic volcanic rocks of the Bohemian Massif (Central Europe): Isotopic variations of the lithospheric mantle related to Variscan orogeny

Abstract

Post-collisional volcanic rocks associated with the Variscan orogeny are preserved in relicts of Late-Paleozoic volcano-sedimentary basins in the Bohemian Massif (central Europe). They include both the Late Carboniferous calc-alkaline and the Early Permian tholeiitic suites, which were emplaced in an extensional intracontinental setting. Both suites were derived from an enriched continental lithospheric mantle and underwent early fractionation of mafic silicate minerals. A second period of fractionation occurred within the crust. Modeling indicates that the calc-alkaline rocks fractionated at a greater depth (0.55 GPa), under oxidizing (ΔFMQ +1) and hydrous conditions whereas the tholeiitic rocks fractionated at shallow depth (0.1 GPa), under moderately oxidizing (ΔFMQ +0.7) and drier conditions. During the Variscan orogeny, the lithospheric mantle under the Bohemian Massif underwent wholesale modification, where the mantle was contaminated by subducted Precambrian sediment admixture. The Carboniferous-Permian mantle-derived rocks typically have negative ɛNd(t) and Precambrian depleted mantle model ages (TDM) compared to the equivalent Early Paleozoic rocks of the Bohemian Massif, which have positive ɛNd(t) and younger TDM. Subsequently, prior to the Cenozoic, the lithospheric mantle under the whole Bohemian Massif was rejuvenated as the Cenozoic mantle-derived rocks have positive ɛNd(t) and young TDM.