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Strain localisation and fabric development in the lower crust is controlled by the active deformation mechanisms. Understanding the driving forces of such deformation aids in quantifying the stresses and rates of the deformation processes. Here we show that diffusion creep plays a major role in deformation of gabbro lenses at upper amphibolite facies conditions. The Kågen gabbro in the North Norwegian Caledonides intruded the Vaddas Nappe at 439 Ma at pressures of 7-9 kbar, temperatures of 650-900°C (depths of ∼26-34 km). The Kågen gabbro on south Arnøya is made up of undeformed gabbro lenses with sheared margins wrapping around them. This contribution analyses the evolution of the microstructures and fabric of the low strain gabbro to high strain margins. Microstructural and textural data indicate that preferential crystal growth of amphibole grains in the extension direction has produced the deformation microstructure and the CPO. Dissolution precipitation creep is inferred to be the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: amphibole, garnet and zoisite. Synchronous deformation and mineral reactions of clinopyroxene suggests mafic rocks can become mechanically weak during the general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. Deformation and metamorphic reaction were both important transformation processes during diffusion creep deformation of the margins of the gabbro lenses. The weakening is directly connected to a transformation process that facilitates diffusion creep deformation of strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation, the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses.

Zircon ages (U–Pb, LA-ICP-MS) obtained from skarns in the Kutna Hora and Svratka units and in the Moldanubian Zone (Bohemian Massif) show significant variations between the lithotectonic units as well as among samples from individual localities. The ages fall between 310 Ma and 3.1 Ga. We suggest that such large spread is a result of both different zircon clastic grain sources and subsequent metamorphic histories. The oldest Neoarchaean to Neoproterozoic ages are interpreted as detrital zircon grains from pre-Cadomian basement. Pronounced age maxima between 600 and 520 Ma (late Neoproterozoic to early Cambrian) were found in skarn samples from the Moldanubian Zone, Svratka and Kutna Hora units. We interpret these as maximum ages of skarn precursor deposition. Apparent Cambrian–Ordovician– Silurian to early Devonian zircon ages were obtained from non-mineralized samples of the Moldanubian Zone and also in the Kutna Hora Unit. This wide temporal interval could reflect maximum ages of skarn precursors related to disintegration of abundant early Palaeozoic magmatic rocks, provided the zircons were not modified by later metamorphism or fluid circulation. The early Carboniferous ages are interpreted to result from strong Variscan HT metamorphism and subsequent circulation of post-metamorphic fluids. Zircons of this age are dominant in skarns with massive magnetite mineralization, while the non-mineralized samples preserve the older ages.

*Corresponding author Skarns in the Svratka Unit, in the neighbouring part of the Moldanubian Zone and in the Kutna Hora Complex were studied with respect to their metamorphic evolution, major- and trace-element geochemistry, oxygen isotopic composition and zircon ages. Skarns form competent lenses and layers in metamorphosed siliciclastic rocks and preserve some early deformation structures and several equilibrium assemblages representing the products of successive metamorphic reactions. The main rock-forming minerals, garnet and clinopyroxene, are accompanied by less abundant magnetite, amphibole, plagioclase, epidote ± quartz. In the Svratka Unit the early prograde M 1 , prograde/peak M 2 , and retrograde M 3 metamorphic stages have been distinguished. Metamorphic conditions in skarns of the Moldanubian Zone are limited to a relatively narrow interval of amphibolite facies. The prograde and retrograde events in the Kutna Hora Complex skarns probably took place under amphibolite-facies conditions. The presence of magnetite and the increasing proportion of the andradite component in the garnet indicate locally increased oxygen fugacity. Skarn geochemistry does not show systematic differences in the skarn composition among the three units. The regional variations are exceeded by differences among samples from individual localities. The Al 2 O 3 /TiO 2 , Al 2 O 3 /Zr, TiO 2 /Nb ratios point to the variable proportion of the detrital material, combined in skarn protoliths with CaO and FeO, the major non-detrital components. The skarns exhibit elevated abundances of Cu, Zn, Sn and As. The Eu/Eu* ratio varies in the range of 0.5–8.6, the total REE contents vary from 8 to 345 ppm. The lowest ΣREE values (< 100 ppm) occur in skarns with magnetite mineralization. The wide intervals of ΣREE and Eu/Eu* values are interpreted to indicate variations in the temperature and redox conditions among layers of the same locality and at various localities. The oxygen isotope compositions of garnets, pyroxenes and amphiboles from skarns of the Svratka Unit exhibit a range of δ 18 O = 0.1 to 4.1 ‰. In situ (laser-ablation ICP-MS) U-Pb dating of zircon from one of the Svratka Unit skarn bodies yielded a wide range of ages (0.5–2.6 Ga), supporting the detrital origin of this zircon population. The skarn protoliths were probably rocks of mixed detrital-exhalative origin deposited on the sea floor . The geological position of skarns, with their structural and metamorphic record, probably reflect tectono-metamorphic evolution shared with that of their host rocks. The geochemical characteristics, including oxygen isotopic compositions and the presence of detrital zircons with a wide range of ages exclude metasomatic, and point to a sedimentary-exhalative mode of origin for the studied skarns.

The boundary between the Saxothuringian and the Tepla–Barrandian zones at the western margin of the Bohemian Massif represents an important tectonic suture of the Central European Variscides. However, most of this boundary is covered by Late Carboniferous and younger sedimentary and volcanic rocks, which prevent direct observation of particular geological units. We present a compilation of geological and depth measurement data from 12,134 exploration boreholes that reached the basement of the volcanic and sedimentary infill in the area of the Eger Graben in the north-western Bohemia, and correlate covered geological units with those exposed on the present-day surface. The resulting compilation reveals the relief of the sedimentary basins basement and interprets the real extent of the basement geological units in the western part of the Bohemian Massif. It also shows the position of the contact between units with the Saxothuringian and the Tepla–Barrandian affinities and suggests the boundary between rocks with Devonian metamorphic record and those metamorphosed during the Early Carboniferous period of the Variscan tectonometamorphic cycle.

U-Pb isotopic data obtained by laser ablation ICPMS analysis of nine zircons with centre to margin oscillatory growth zones from a K-feldspar-rich augen gneiss in the allochthonous Lower Crystalline Nappe of the Erzgebirge domain of the western part of the Bohemian Massif yield a concordia age of 524 ± 10 Ma (2 sigma). This Early Cambrian age represents the time of magmatic crystallization of the protolith of a representative, from near Měděnec, of the allochthonous “Red gneiss” whose igneous nature is shown by the presence of (deformed) xenoliths. Data from TIMS analysis of zircons with variable proportions of unzoned xenocrystic cores surrounded by oscillatory-zoned overgrowths point to magma derivation from upper Proterozoic, or older, rocks. Data obtained for five zircon grains from another “Red gneiss” in the Lower Crystalline Nappe (in the Klinovec anticline) plot below the concordia with the age of the one point that is near concordant being 519 ± 26 Ma (2 sigma). These data, together with internal features of the zircons, are consistent with Early Cambrian granitic plutonism also in this part of the Erzgebirge but with later Pb loss, possibly associated with considerable fluid movement during thrust nappe development. Another sample of a coarse-grained orthogneiss from the autochthonous St Catherine9s dome yielded a significantly younger Early Ordovician age of 480 ± 10 Ma (2 sigma) calculated from eight zircon analyses. However, three zircon grains from the same sample gave a significantly older near-concordant Late Proterozoic age of ca. 620 Ma. Provided that the age difference of ca . 40 Ma between orthogneisses from Měděnec - Klinovec and St Catherine9s dome holds also for other orthogneisses in the Erzgebirge, zircon U-Pb age data could be used to discriminate between allochthonous and autochthonous units in this region. The ca. 25 Ma difference between the Early Cambrian protolith age of the augen gneiss from near Měděnec determined by the laser ablation ICPMS technique and a previously reported older age of 550 ± 9 Ma for a nearby sample determined by the Pb-Pb evaporation technique is accounted for on the basis of the latter not being adequate for dating zircons with a small xenocrystic component. This study demonstrates the importance of high spatial resolution dating techniques, such as SHRIMP or laser ablation ICPMS, in dating zircons with complex growth history that are common in crustally-derived melts.

In garnetiferous micaschists occurring at the northern termination of the Moravian domain, metabasites were found containing relics of eclogite-facies assemblages. In these rocks, minimum pressure conditions of 13 kbar at temperatures of 625–655 °C were followed by decompression to amphibolite-facies conditions. Surrounding metapelites contain Fe-rich chloritoid and Fe-rich staurolite preserved as inclusions in garnet porphyroblasts indicating early PT conditions of 570 °C at pressures of 6–7 kbar. The matrix assemblage shows an increase of temperature and pressure to about 615 °C at 6.5–9 kbar. The PT paths of both metapelite and amphibolitized eclogite are suggested to converge at the peak temperature conditions of the metasediments. The PT evolution of the eclogitic sample and of the surrounding metapelites is different from the PT history of the same rocks occurring in the Moldanubian domain but similar to that of the Moravian metasediments. Thus, the presence of eclogite-facies metamorphism at the eastern margin of the Bohemian massif suggests early subduction and subsequent collision of the Brunovistulian foreland with the internal Variscan orogenic root domain.

In the central part of the Krusne hory (Erzgebirge) Mountains (Saxothuringian domain of the Bohemian Massif) in the Czech Republic, mafic eclogites are associated with a horizon of garnetiferous micaschists. In these rocks, analysis of stable mineral assemblages together with thermodynamic modelling of their evolution shows that the two-phase AFM assemblage garnet-kyanite in the Měděnec area has equilibrated at 640°C and 22 kbar, whereas the two-phase assemblage garnet-chloritoid in the Srni area represents amphibolite-facies conditions. The matrix mineral assemblages show that subsequent decompression is not associated with temperature increase. The observed metamorphic evolution suggests rapid exhumation of the eastern margin of the Saxothuringian domain after its subduction below an already thickened orogenic root.