The UK preserves a diverse record of Carboniferous to Permian magmatism that occurs in a lower plate setting, relative to the southwards-dipping latest Devonian Rheic-Rhenohercynian suture zone, that is seismically imaged offshore to the south of SW England. The classical model of foreland-propagating thrust systems, immediately following the closure of the Rhenohercynian Ocean, is not consistent with the widespread occurrence of Early Carboniferous syn-rift mafic magmatism immediately north of the suture that persists to Scotland.

The SW England Permian magmatic province includes the Cornubian and Haig Fras batholiths and associated mafic igneous rocks developed over a 20 Ma period in Early Permian. These developed in a post-Variscan extensional regime that brought about exhumation and thinning of the lower plate during reactivation of the Rhenohercynian suture zone (Cornubian Batholith) and Bristol Channel Fault Zone (Haig Fras Batholith). Decompression melting of the mantle during lithospheric extension is the most likely cause but lithospheric delamination cannot be excluded.

North of the Variscan Front, latest Carboniferous to Early Permian magmatism was renewed following Variscan intraplate shortening. Most occurrences are consistent with decompressive melting during N-S extension and/or localised extension during movements on wrench faults.

The overall record of Carboniferous and Permian magmatism across the UK does can be explained by extensional tectonics and decompression melting of lithospheric ± asthenospheric mantle plus, possibly, lithospheric delamination below SW England. Whilst mantle plume involvement has been invoked for magmatism in some areas, it is not required to explain heat flow and magmatic activity.

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

Oxygen isotopes in sedimentary rocks have long been used to reconstruct paleo-surface temperatures. Chert, a microcrystalline form of SiO₂, has also been explored as a paleotemperature proxy, but the factors controlling its isotopic composition remained uncertain. A recent numerical model of silica diagenesis has now linked chert oxygen-isotope ratios directly to paleo-heat flow, opening new opportunities for its application [1].

Diagenetic chert forms when amorphous silica (opal-A) dissolves and reprecipitates, passing through an opal-CT intermediate before becoming microcrystalline quartz. The oxygen-isotope ratios (δ¹⁸O, Δ′¹⁷O) in the resulting chert capture the temperature and fluid composition at the depth of the opal-CT to quartz transition. By simulating that diagenetic transition, the new model relates δ¹⁸O_chert and Δ′¹⁷O_chert to burial temperatures, pore fluid alteration and heat flow.

We use the model to reconstruct paleo-heat flow during prograde silica phase transformation and derive burial rates and pore fluid alteration across the Rhenohercynian basin. Our data reveal paleo-heat flow in the order of 50 to 70 mW m^-2, agreeing with a previous model [2] and high burial rates in the east of up to 80 m Myr^-1, agreeing with the thickness of overlying turbidites. Burial rates and clay mineral content coincide with ¹⁸O-depleted compositions of cherts, i.e compositions in three isotope space that deviate from the expected temperature-equilibrium. This dataset lays the foundation for a new tool to study the thermal- and burial histories of sedimentary basins.

[1] Tatzel, et al. (2022) PNAS 119

[2] Littke et al. (2000) Geol. Soc. Spec. Publ. 179

The Mid-German Crystalline Zone (MGCZ) plays a key role to unravel the complex pre- to syn-Variscan tectono-magmatic evolution in central Europe from the Neoproterozoic to the late Carboniferous. The most complete record of this evolution is preserved in basement rocks of the Bergsträsser Odenwald, even though highly ambivalent. While a large number of (isotope) geochemical and geochronological data and geodynamic models exist for the Variscan evolution of the Bergsträsser Odenwald (between 360 and 320 Ma), information about pre-Variscan events is scarce, although lately published zircon ages constrain pieces of Cadomian basement and Silurian intrusiva. Our new results of zircon U-Pb dating confirm these findings, but additionally point to a widespread occurrence of pre-Variscan intrusive rocks in the Bergsträsser Odenwald, which are known so far in this extent only from the Böllstein Odenwald.

Detrital zircon grains reveal different age spectra, pointing to a wide range of source areas and supply routes of the detritus from source to sink. Some age spectra correlate well with published data from other units of the MGCZ or the Saxo-Thuringian Zone, whereas others are completely different, and unique for the Odenwald. Maximum depositional ages range from the Neoproterozoic (c. 550 Ma) to Early Variscan (c. 355 Ma) and indicate that the Odenwald basement preserves different periods of sediment deposition and a prolonged geodynamic evolution, much longer and complex than previously thought. Our new results have consequences for the interpretation of the evolution of the MGCZ and adjacent areas within the Central European Variscides.

In the Saxo-Thuringian Zone (SXZ) of the Variscides, the strict bipartite subdivision of Paleozoic marine sediments date to the beginning of the last Century. This proposition based on the observation that early Paleozoic lithologies (Bavarian facies; ba) associated with the Münchberg Gneiss Complex in the hanging wall differ significantly from underlying lithologies of the same age. Sediments of the foot wall were ascribed as the (par-)autochthonous Thuringian facies (th) of the former Saxo-Thuringian basin. In such a view, continuous sedimentation of both facies types prevailed until the early Carboniferous (Visean, ~330 Ma). Only the crystalline pile of Münchberg and associated nappes (ba) of the former distal part of the Saxo-Thuringian basin are regarded as northwestward overthrusted allochthonous units. However, such a bipartite model is not able to explain the complex architecture of the SXZ. For example, Early Carboniferous (>340 Ma) Paleozoic nappes in the Vogtland (th), the Nossen-Willsdruff- and the Elbtalschiefergebirge (th+ba) are discordantly overlain by Visean synorogenic sediments (ba+th). The long-lasting (Early Devonian-Carboniferous) accretionary tectonics in the Erzgebirge-Fichtelgebirge Complex (EFC) also precedes the proposed end of continuous sedimentation in the SXZ. Medium- to high-pressure metasedimentary nappes of the EFC exhibit the distinctive geochemical signature of early Ordovician siliciclastic rocks (th). After a concise review of the sedimentary record of the Saxo-Thuringian Zone, we present a tectono-sedimentary model explaining the spatial distribution of the sediments with the formation of the vast Peri-Gondwana shelf eventually followed by prolonged continental accretionary tectonics during the collision of Gondwana with Laurussia.