Secular isotopic variation in lithospheric mantle through the Variscan orogen: Neoproterozoic to Cenozoic magmatism in continental Europe

The evolution of the mantle source beneath a terrane is recorded by the Sm-Nd isotopic systematics of mafic rocks over extended periods of time. The evolution of the mantle beneath the Variscan massifs of continental Europe is tracked in order to examine the coupling between the crust and underlying continental lithospheric mantle during the late Paleozoic Variscan orogeny. Late Neoproterozoic and early Paleozoic (Cambrian-Devonian) extension-related mafic rocks have ε_Nd(t) values similar to depleted mantle and so were probably derived from juvenile mantle sources. Thus, weak deformation and low-grade metamorphism associated with the ca. 540 Ma Cadomian orogeny did not change the mantle source beneath the Variscan massifs. In contrast, late Carboniferous-Permian mafic magmatism, which erupted in the aftermath of the collision between Gondwana and Laurussia, is characterized by negative ε_Nd(t) values and significantly older depleted mantle model ages (T_DM = 1800-800 Ma), suggesting derivation from a lithospheric mantle contaminated by late Paleozoic subduction. This characteristic is shared by most Variscan massifs, and it is consistent with the vast tracts of allochthonous peri-Gondwanan terranes exposed within the orogen, which suggest regional crust-mantle decoupling during Variscan collision. The regionally extensive Cenozoic European Volcanic Province has higher ε_Nd(t) values (+5 to +7) and significantly lower T_DM ages (ca. 500-100 Ma), suggesting that the continental lithospheric mantle was refertilized.