HEATin´EUROPE – Variscan orogenic vs. anorogenic polyphase heating

Studies in all Variscan orogenic belts have revealed magmatic and HT/LP metamorphic events which are incompatible with the collisional evolution of the Variscides. U–Pb datings on zircon and monazite from granitoid intrusions, felsic and mafic volcanism, granulites and migmatites have yielded age clusters between ca. 360 and 270 Ma, i.e., before, during and after Variscan collision. In foreland fold and thrust belts (FTB), these clusters are represented by K/Ar ages of anchizonal metapelites and fine-grained volcanic ashes. They may occur in zones cutting across sutures and even in areas outside the Variscan orogen (Scotland).

            Diachronous collisional deformation and metamorphism are best documented by progradation of the synorogenic flysch wedge and tectonic deformation in its wake (datd by K/Ar on clavage). Some pre-orogenic heat pulses have survived in the external parts of FTB, where syn- and post-orogenic heating were weak.

            Anorogenic age clusters are best explained by the activity of mantle plumes, which have repeatedly heated and softened the crust, thus permitting formation of Devonian and Carboniferous marine basins even within crystalline parts of Variscides and preventing formation of Tibetan-style plateaus. These processes are best explained by the position of Europe over the plume generation zone TUZO (after Tuzo Wilson), from the early Carboniferous onwards, which is responsible for the notorious high-T events in pre-Mesozoic rocks of entire Europe.

Franke et al. (2011) http://doi.org/10.1007/s00531-010-0512-7,

Franke (2014) http://doi.org/10.1007/s00531-014-1014-9,

Franke et al. (2017) http://dx.doi.org/10.1016/j.gr.2017.03.005