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This paper present new U–Pb geochronological data (LA-ICP MS on zircon) of granitoids from the Moroccan Meseta Variscan belt. Zircons from the Oulad Ouaslam granodiorite in the Jebilet massif in the Western Meseta highlight a peak of magmatic activity at 335.2 ± 0.8 Ma. In the Eastern Meseta, the ages obtained from several granitoids facies from the High Moulouya magmatic complex restrained its magmatic activity to the early Visean: 339.6 ± 1.9 Ma for the Perdreaux granite, 337 ± 1.6 Ma for the calc-alkaline «grey» granite and 343.1±1.2 Ma for the El Hassir granodiorite. Similar late Tournaisian – early Visean ages were obtained for the Zekarra granodiorite (348.1 ± 2.3 Ma) and the Merguechoum granite (345.2 ± 2.8 Ma) in the Horsts belt. This pre-orogenic Eovariscan magmatic activity is contemporaneous with the opening of early Carboniferous intracontinental basins in the whole Northwest Africa but its relationship with Eovariscan deformation typifying the Eastern Meseta remains temporally unclear. Then, the late Carboniferous ages of 308.2 ± 2.7 Ma obtained for the Beni Snassen monzogranite, 303.5 ± 1.7 for the Tanncherfi granodiorite and 305.2 ± 1.4 for the Boudoufoud granite in the Horsts belt show that they all are synchronous with the main late Carboniferous convergent Variscan event in Northwest Africa. This tectonic and magmatic tempos contrast with the one described in the European Variscan belt, where the syn-collisional magmatism forms coevally with the Northwest African Eovariscan event, re-interpreted as a possible extensional event. We proposed that the early Carboniferous intracontinental basins and magmatism in Northwest Africa are due to the propagation of the Paleotethys ocean within the Gondwana continent. In contrary, the late Carboniferous – early Permian tectono-magmatic activity could reflect the docking of the east Gondwana margin with the Iberian branch of the European Variscan belt.

Comprehensive set of seismic and potential field data from the whole European Variscan belt is used to interpret the structure and evolution of the European Variscides as defined by Martínez Catalán et al. (2021). The gravity data show the presence of high amplitude, short-wavelength gravity anomalies correlated with the outcrops of eclogites, ultramafic rocks and ophiolites delineating the main body of the Mid-Variscan Allochthon (MVA) and the Devonian Mid-Variscan suture (MVS). The medium amplitude and elongated long-wavelength gravity highs, aligned parallel to the Variscan structural grain, correspond to the low-grade Proterozoic rocks of the MVA and Devonian arc – back-arc system. On the other hand, the short wavelength negative gravity anomalies developed in the central part of the belt coincide with Carboniferous (330–310 Ma) per- to meta-aluminous magmatic bodies. The magnetic data show two belts correlated with Carboniferous Rhenohercynian and Devonian Mid-Variscan magmatic arc granitoids. The Rhenohercynian and Mid-Variscan subduction systems are also well-imaged by moderately dipping primary reflectors in reflection seismic lines. Younger moderately dipping reflectors in the upper-middle crust coincide with outcrops of Carboniferous detachments, limiting granite plutons and core complexes along-strike the core of the Variscan orogeny. Deep crustal reflectors are considered as an expression of lower crustal flow resulting from extensional re-equilibration of the previously thickened Variscan crust. A P-wave velocity logs synthesis shows a high-velocity cratonic crust surrounding a thin Variscan orogenic crust defined by low-velocity lower and middle crusts. The latter crustal type coincides with regional outcrops of 330–310 Ma per- to meta- aluminous granitoids and associated gravity lows along-strike the belt. All these data are used to define the primary polarity of Devonian subduction systems defining the European Variscan belt (Schulmann et al., 2022) and discuss the Carboniferous extension forming specific structure of the Variscan crust. This geodynamic evolution is integrated into a paleomagnetically constrained model of the movements of continental plates and intervening oceans (Edel et al., 2018; Martínez Catalán et al., 2021).REFERENCES:Catalan, J.R.M., Schulmann, K. and Ghienne, J.F., 2021. The Mid-Variscan Allochthon: Keys from correlation, partial retrodeformation and plate-tectonic reconstruction to unlock the geometry of a non-cylindrical belt. Earth-Science Reviews, 220, 1–65.Edel, J.B., Schulmann, K., Lexa, O. and Lardeaux, J.M., 2018. Late Palaeozoic palaeomagnetic and tectonic constraints for amalgamation of Pangea supercontinent in the European Variscan belt. Earth-science reviews, 177, 589-612.Schulmann, K., Edel, J.B., Catalán, J.R.M., Mazur, S., Guy, A., Lardeaux, J.M., Ayarza, P. and Palomeras, I., 2022. Tectonic evolution and global crustal architecture of the European Variscan belt constrained by geophysical data. Earth-Science Reviews, 234, p.104195.

The allochthonous complexes of the Galicia-Trás-os-Montes Zone (GTMZ) of the NW Iberian Massif consist of an ensemble of peri-Gondwanan terranes and ophiolitic units stacked during the Variscan orogeny. The Middle Allochthon also known as the ophiolitic complex represents the variscan suture of one or more peri-Gondwanan oceans, and includes Cambro-Ordovician to Lower Devonian units. In the allochthonous Morais Complex (Trás-os-Montes, Portugal), the ophiolitic complex comprises four structural units, which from bottom to top are Macedo de Cavaleiros, Pombais, Izeda-Remondes and Morais-Talhinhas.The two first units are quite similar to each other and consist of greenschists and metapelites, with metabasites dominating in Pombais and metapelites in Macedo de Cavaleiros. No age data are available for these two units. Their structural position is comparable to that of the Cambro-Ordovician Vila de Cruces Unit in the Órdenes Complex, but also to that of the Lower Devonian Moeche Unit in the Cabo Ortegal Complex, both in Galicia.The Izeda-Remondes and Morais-Talhinhas units mostly consist of fine grained amphibolites associated with deformed gabbros, mafic cumulates and serpentinized ultramafics. The Izeda-Remondes Unit is structurally the lower and the older of the two, dated by Pin et al. (2006) around 447 ± 24 Ma (Sm-Nd whole rock isochron). The upper ophiolitic Morais-Talhinhas Unit was also dated by Pin et al. (2006) giving U-Pb ages of 405 ± 1 Ma and 396 ± 1 Ma.This contribution brings new geochronological and geochemical data from the Middle Allochthon providing new understanding of the history of the suture of the Morais allochthonous complex. Zircons have been collected for LA-MC-ICP-MS U–Pb analyses in two felsic intrusions in the Izeda-Remondes Unit giving concordant ages ranging from 422 ± 4 Ma to 432 ± 4 Ma. These ages together with new and previous whole rock geochemical data obtained from basic and felsic igneous samples from the ophiolitic complex are interpreted to date and reflect the formation of igneous protoliths in an oceanic ridge setting forming part of the Rheic oceanic realm, during Silurian to Devonian. The mantle source for the basic rocks of all four units is similar to that of N-MORB with some influence from a subduction zone.REFERENCE:Pin, C., Paquette, J. L., Ábalos, B., Santos, F. J., & Gil Ibarguchi, J. I. (2006). Composite origin of an early Variscan transported suture: Ophiolitic units of the Morais Nappe Complex (north Portugal). Tectonics, 25(5).Acknowledgements: Spanish Ministry of Science and Innovation, project PID2020-117332GB-C21.

Arcuate trace of large structures characterizes many mountain chains. The Variscan Belt is not an exception, and depicts one of the tightest oroclines in the world, the Ibero-Armorican Arc. In addition, the belt features a few more open arcs in the Eastern Moroccan Meseta, Central Iberia, the French Massif Central and the Bohemian Massif. All Variscan arcs are considered oroclines or secondary oroclines according to definitions by Weil and Sussman (2004) and Johnston et al. (2013) respectively. They are also essentially late orogenic features, but their timing and deformation mechanisms differ. Models explaining their origin have been proposed for some individual arcs, and are generally controversial.This contribution aims at interpreting the ensemble of Variscan arcs paying attention to their age relative to previous orogenic features as well as to those associated with arc development. Such features include first order structures, metamorphism and plutonism, as well as magnetic and gravimetric anomalies. Development of the arcs is viewed as somehow related with late Variscan dextral transpression provoked by the relative displacement of Laurussia to the East relatively to Gondwana during the Pennsylvanian and early Permian (325-290 Ma; Arthaud and Matte, 1977; Shelley and Bossière, 2000; Martínez Catalán et al., 2021). But several mechanisms operated to form the arcs, the most important of them being ductile transcurrent shearing, indentation and shortening perpendicular and parallel to the orogenic trend. These mechanisms acted at different time intervals and their participation or relative importance varies for each arc, as well as their involvement in the development of the structural, metamorphic and igneous features and in the geophysical characteristics.REFERENCES:Arthaud, F. and Matte, P. 1977. Late Paleozoic strike-slip faulting in southern Europe and northern Africa: result of a right-lateral shear zone between the Appalachians and the Urals. Geological Society of America Bulletin, 88, 1305-1320.Johnston, S.T., Weil, A.B. and Gutiérrez-Alonso, G. 2013. Oroclines: Thick and thin. Geological Society of America Bulletin, 125 (5-6), 643-663.Martínez Catalán, J.R., Schulmann, K. and Ghienne, J.F. 2021. The Mid-Variscan Allochthon: Keys from correlation, partial retrodeformation and plate-tectonic reconstruction to unlock the geometry of a non-cylindrical belt. Earth-Science Reviews, 220, 103700, 1-65.Shelley, D. and Bossière, G. 2000. A new model for the Hercynian Orogen of Gondwanan France and Iberia. Journal of Structural Geology, 22 (6), 757-776.Acknowledgement: Spanish Ministry of Science and Innovation, project PID2020-117332GB-C21.

The Barrovian type metamorphism affecting the Precambrian microcontinents of peri-Siberian tract of the Central Asian Orogenic Belt is mostly dated indirectly on zircon from (syn-tectonic) magmatic rocks as Late Proterozoic – Ordovician. However, in-situ monazite geochronology in micaschists and migmatite gneisses at the northern part of the Precambrian Baidrag block, central Mongolia, revealed that the Baikalian Late Proterozoic – Early Cambrian cycle overprints an earlier Tonian phase of metamorphism. The apparent Barrovian-type zoning ranging from garnet, staurolite, kyanite to kyanite/sillimanite migmatitic gneisses is thus false and points to hidden metamorphic discontinuities and mixed metamorphic histories from different times. Therefore, to decipher and interpret the record of different tectono-metamorphic events it is necessary to unravel complete P–T–t paths from individual samples. Two localities with Tonian-age monazite show anticlockwise P–T paths: 1) Grt−Sil−Ky gneiss records burial to the sillimanite stability field (~720°C, 6.0 kbar) followed by burial to the kyanite stability field (~750°C, 9 kbar) and, 2) The Grt−St schist records burial to the staurolite stability field (~620°C, 6 kbar), further followed by almost isothermal burial (~590°C, 8.5 kbar). Based on monazite textural position, internal zoning, and REE patterns, the time of prograde burial under a thermal gradient of 27–32°C/km is estimated at c. 890−853 Ma and further burial under a geothermal gradient of 18–22°C/km is dated at c. 835−815 Ma. On the other hand three localities with Late Proterozoic to Cambrian monazite ages show clockwise metamorphic paths at variable P–T gradients: 3) P–T conditions of the Grt schist reaches ~5 kbar and 500 °C and 4) the Grt−St−Ky schist reaches conditions of 9 kbar and 670 °C, indicating burial under a geothermal gradient of 20–26 °C/km. 5) Grt–Sil gneiss shows peak of 6–7 kbar and 700–750 °C, indicating melting conditions at 30–32 °C/km gradient. Monazite included in porphyroblasts and in the matrix indicate that these P–T conditions reached under variable geothermal gradient were semi-contemporaneous and occurred between 570 and 520 Ma. By correlation with published zircon ages of 600–530 Ma from granitoid magmatic rocks we suggest that the areas with higher geothermal gradient may be explained by closer vicinity of magmatic intrusions. These P−T and geochronology data from a continuous Barrovian metamorphic section suggest that anticlockwise P−T evolution from c. 930 to 750 Ma can be interpreted as a result of thickening of peri-Rodinian supra-subduction extensional and hot edifice. This metamorphic event was followed by a clockwise P−T evolution from c. 570 to 520 Ma possibly related to shortening of the northern Baidrag active margin and incipient collision with with peri-Siberian continental mass further north.

Movement of a large volume of granitic melt is an important factor in the compositional differentiation of the continental crust and the presence of melt in rocks profoundly influences their rheology. Different mechanisms controlling melt migration through crust were proposed. We suggest that pervasive melt flow, analogous to reactive porous melt flow in mantle, could be possibly one of them. It is generally accepted that migration of felsic melts in continental crust starts with short distance pervasive microscopic flow into segregation veins which extract melt. However, we show that pervasive melt flow may be a regional mode of melt migration in continental crust. In such scenario, melt driven by deformation passes pervasively along grain boundaries through the whole rock volume. And the term pervasive melt flow is used for grain-scale, diffuse, porous and reactive flow of felsic silicate melt through rocks. This is effectively an open-system process that thoroughly reworks the resident rock mass. Through-flow of melt destroys pre-existing fabrics and the original chemical and isotopic nature of the protolith. Melt segregation is inefficient and protolith become isotropic granite-like, with partly preserved relics of the original, without ever containing more than a few melt percent at any time. The fabric and geochemical nature of these granites encapsulates the complex history of hybridization.In order to decipher duration of pervasive melt migration we used precise U-Pb monazite ID-TIMS (isotope dilution thermal ionization mass spectrometry) and U-Pb monazite Laser Ablation Split Stream (LASS) geochronology in combination with monazite chemistry as well as U-Pb zircon SHRIMP geochronology. Monazite reveal continuous chemical equilibration with passing melt. They are getting progressively enriched in HREE and depleted in Eu. Monazites in the least affected rock preserve original magmatic zoning in Th and U, in contrast to more with melt equilibrated rock types, where this zoning is lost. Data for each migmatite type reveal similar date spread for both cores and the Y-rich well defined rims of single monazite grains, indicating a disconnect between U-Pb dates and chemical zoning. There is also no correlation between U-Pb ages and Yb/Gd ratio. This suggest perturbance of the isotopic system. We interpret these random distribution within-grain date variations as a result of dissolution-reprecipitation reactions between monazite grains and melt. During the coupled dissolution-reprecipitation radiogenic Pb was redistributed within the grain. This is supported by dissolution of apatite into silicate melts that stabilizes monazite during migmatitization, preventing their dissolution but not reaction with passing melt. Redistribution of radiogenic Pb resulted in meaningless individual ages from different migmatite types, but gave overall duration of the thermal event – pervasive melt flow. Duration of pervasive melt flow was dated 8-10myr. This suggest that porous flow of silicate melts in continental crust is a process which can operate over a long time and impacts on the rheology of the crust during orogeny.

The Central Asian Orogenic Belt (CAOB) is a Paleozoic accretionary-collisional orogen located at the eastern Pangea in between the Siberian Craton to the north and the North China and Tarim cratons to the south. Several contradictory geodynamic models were proposed to explain the tectonic assemblage: oroclinal bending and strike-slip duplication of a giant intraoceanic arc or a progressive lateral accretion of linear continental and oceanic terranes towards the Siberian Craton. However, none is generally accepted. A multidisciplinary and multiscale approach integrating potential field analysis and modelling provides new insights into understanding the crustal structures beneath the CAOB.First, we present a synthesis of the previous geophysical studies, which constitute the constraints for the modelling. Second, based on global gravity and magnetic anomaly grids, the large-scale statistical analysis of their lineaments reveals the distribution of the contrasting tectonic zones. Then, the topography of the Moho is determined by 3D forward modelling of the GOCE gravity gradients, which is then integrated into 2D and 3D crustal scale models of southern and central Mongolia. A geodynamic model is derived from the resulting crustal architectures. Thus, the combination of these methods allows us to: (1) unravel the existence and distribution of suspect terranes in accretionary systems; (2) correlate the contrasting tectonic zones with the gravity and magnetic signals and the thickness of the crust, thereby revealing the inheritance of Paleozoic and Mesozoic orogenic history; and (3) determine the significance and possible origin of the major anomalies, which are related to tectonic processes such as lower crustal relamination, presence of deep-seated fault zones and sutures, or delimitation of main tectonomagmatic domains. Finally, with the case study of Central Mongolia, we demonstrate the real benefit and the significant progress, which can be achieved by using potential field analysis combined with seismic receiver function and geological analyses.

The Dunhuang block in NW China preserves Archean to Paleoproterozoic basement rocks that are exposed alongside Paleozoic magmatic and metamorphic rocks. Although both subduction-accretion and collisional processes have been proposed for the formation of Paleozoic metamorphic rocks, links between their metamorphic ages, P−T evolution and deformational history remains ambiguous. Here we present zircon and in-situ monazite U−Pb geochronology linked to P−T modelling of metapelites from the Hongliuxia belt in the southern Dunhuang block. Oriented inclusion trails in garnet from metapelites reveal rare relics of an S1 fabric. The earliest continuous metamorphic fabric is an originally steep N-S striking foliation S2. This fabric was further reworked by upright folds F3 associated with development of an ubiquitous steep, mainly south-dipping, E-W striking axial planar foliation S3. The Bt−Ms−St−Pl−Qz−Tur−Ilm assemblage forming inclusions in garnet is assigned as the D1-M1a event whereas the foliation S1b in metapelites is associated with Grt–Ky–St–Bt–Ms–Pl–Qz–Rt assemblage. The Grt−Ky−St aligned parallel to the S2 matrix in low-strain domains are considered as remnants of a dismembered M1 assemblage, while the S2 foliation is characterized by the Grt–Sil–Bt–Pl–Qz–Rt–Liq in high-strain domains. The S3 foliation is associated with the Grt–Sil–Bt–Ms–Pl–Qz–Kfs–Chl–Ilm assemblage. Altogether, metapelites record similar clockwise P–T evolution an early prograde (M1a) stage starting at 4.5–5 kbar and 500–550°C, metamorphic peak (M1b) stage at ~8 kbar and 700–725°C, decompressional heating to ~6 kbar and ~750°C (M2) and a retrograde stage to 4.5–5.5 kbar and 500–550°C (M3). Zircon U−Pb geochronological investigations suggest that metapelites from the basement record metamorphic ages of 1847 ± 11 Ma and 404 ± 15 Ma. In-situ U–Pb dating of monazite combined to monazite trace-element composition analysis further suggest that the rock burial most likely started at c. 410 Ma, peak-P conditions M1b were reached at 400–395 Ma, M2 heating occurred at c. 390 Ma and M3 retrogression occurred between c. 384 and 353 Ma. The D1-M1 burial event reflects either underthrusting of the basement below the supra-subduction active margin system or propagation of the deformation front to the south of the Dunhuang block. The D2-M2 event is a consequence of thermal relaxation following crustal thickening, possibly accompanied by convective lithospheric thinning, whereas D3-M3 reflects exhumation during shortening of the system. Combined with the available regional data, it is suggested that the Devonian multi-stage tectono-metamorphic evolution described in the study area corresponds to a polyphase Andean-type deformation of the active margin of the Dunhuang block. Such a process can be regarded as a response to a progressive relocation of the Dunhuang block alongside with the Tarim-North China Collage in the Devonian. Funding: This research is part of the project No. 2021/43/P/ST10/02996 co-funded by the National Science Centre and the European Union Framework Program for Research and Innovation Horizon 2020 under the Marie Skłodowska-Curie grant agreement No. 945339, as well as the President’s International Fellowship Initiative for Postdoctoral Researchers of the Chinese Academy of Sciences, grant No. 2021PC0013.

Origins of early Paleozoic metabasites (granulites and amphibolites) and their host metasedimentary rocks in the Dunhuang block, NW China, are addressed by new geochronological and geochemical data. The metabasites show back-arc basalt–like geochemical features, marked by high Zr/Nb ratios and Zr-Hf troughs, but they can be classified into two groups based on their dissimilar protolith ages and distinct Nd signatures. Most group I metabasaltic rocks were emplaced before 455 Ma and possess high Ba/Nb ratios (11.46–224), low (Nb/La)PM (0.10–0.71), and negative whole-rock εNd(t) values (−12.7 to −2.7), whereas group II rocks have protolith ages around 445 Ma, low Ba/Nb ratios (0.70–22.93), low (Nb/La)PM (0.78–1.51), and less evolved whole-rock Nd isotopic features (εNd[t]: −2.0 to +2.7). It is proposed that group I metabasites originated from an enriched lithospheric mantle, while group II metabasites were derived from the depleted asthenospheric mantle. The metasedimentary rocks received detritus mainly from the neighboring Cambrian magmatic arc, and they are compositionally similar to active-margin sediments. Metamorphic zircon U-Pb ages ranging 462–422 Ma from the investigated rocks together with prominent magmatism further suggest high-grade metamorphism prevailing during the Late Ordovician–early Silurian. Based on these data, a Cascadia-type evolution is proposed involving an Ordovician–early Silurian suprasubduction stretching of the Cambrian active continental margin, which culminated with mantle upwelling. Recent paleogeographic reconstructions support the evolution and assembly of interior Proto–Tethys-Ran oceanic and continental plates, including the Dunhuang block, between 510 and 440 Ma, followed by Panthalassan subduction beneath the Tarim–North China continental assemblage at 440–430 Ma.

Previously unrecognized mafic and felsic plutonic rocks, formerly interpreted as an anatectic part of the amphibolite-eclogite Marianske Lazně Complex (MLC, Bohemian Massif) were examined together with a possible upper-crustal equivalent represented by the Cista pluton intruding continental crust farther east. These plutonic rocks were studied by whole-rock geochemistry (major and trace elements, Sr-Nd isotopes) and zircon U-Pb geochronology. The MLC magmatic rocks range from pyroxene-amphibole gabbro to trondhjemite and oligoclasite. The Cista pluton consists of porphyritic granitoids. The calc-alkaline nature of these rocks, relative enrichment of fluid-mobile elements (including large-ion lithophile elements), strong fractionation of light rare earth elements over heavy rare earth elements and depletion of high field strength elements are evidence for active margin magmatism. U-Pb zircon dating indicates that the magmatism was Mid-Devonian in both the MLC (ca. 385 Ma) and Cista pluton (ca. 390 up to ca. 365 Ma). Both magmatic units show inherited cores probably linked to recycling of crustal material. Sr-Nd isotopic compositions indicate that the gabbro formed from a depleted MORB mantle source composition with a small contribution of a subduction and crustal component. A three-stage geodynamic model is proposed: 1) ca. 390–380 Ma recycling of a high-pressure accretionary wedge and migration of arc melts into more distal parts of the upper continental plate, 2) ca. 380–370 Ma compression and thickening of the accretionary wedge-arc edifice, and 3) ca. 370–365 Ma development of a giant east-dipping detachment that reworked the whole edifice in the west and also involved emplacement of the supracrustal Cista pluton along antithetic west-dipping shear zone in the east. The studied magmatic rocks record a Mid-Devonian magmatic arc related to the formation of a subduction wedge stacked beneath the upper plate that was followed by formation of a second and independent Carboniferous continental arc further east.

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How and at which thermal conditions the convergence between the Chinese Altai and East Junggar operated remain poorly understood. This issue is addressed in the current study by focusing on the timing and petrogenesis of syntectonic granite dykes from the representative areas of Fuyun (convergent front) and Kalasu-Aletai (Chinese Altai interior). It is shown that Fuyun and Kalasu-Aletai dykes are fractionated I- and S-type granites, with zircon and monazite U-Pb ages of 300−291 Ma and 281−265 Ma, respectively. Geochemically, the Fuyun dykes have lower contents of aluminous (ASI: 0.97−1.13) and light rare earth element-enriched features, while the Kalasu-Aletai dykes have ASI = 1.01−2.17 and show overall flat rare earth element patterns with tetrad effects. The Fuyun dykes exhibit less evolved Sr-Nd isotopic characteristics (87Sr/86Srinitial: 0.7039−0.7048, εNd(t): + 5.7 to + 6.1) with respect to those of the Kalasu-Aletai dykes (87Sr/86Srinitial: 0.6978−0.7183, εNd(t): −7.6 to +3.0). The Fuyun and Kalasu-Aletai dykes are geochemically compatible with isotopically less evolved East Junggar arc components and heterogeneous Ordovician wedge sediment of the Chinese Altai, respectively, implying genetic links. We propose that the late Paleozoic Chinese Altai−Junggar convergence created a local perturbation of weak mantle beneath the southern Chinese Altai, causing partial melting of the underthrusting East Junggar and the overriding Altai components successively. The resulting magmas were emplaced along northward propagating syn-tectonic tensional fractures perpendicular to the Chinese Altai−East Junggar deformation front that serves as an excellent indicator of the convergent-shortening process.

Flow of partially molten crust is a key contributor to mass and heat redistribution within orogenic systems, however, this process has not yet been fully understood in accretionary orogens. This issue is addressed in a Devonian migmatite-granite complex from the Chinese Altai through structural, petrological, and geochronological investigations presented in this study. The migmatite-granite complex records a gradual evolution from metatexite, diatexite to granite and preserves a record of two main Devonian phases of deformation designated D1 and D2. The D1 phase was subdivided into an early crustal thickening episode (D1B) and a later extensional episode (D1M) followed by D2 upright folding. The D1M episode is associated with anatexis in the deep crust. Vertical shortening, associated with D1M, gave rise to the segregation of melt and formation of a sub-horizontal layering of stromatic metatexite. This fabric was reworked by the D2 deformation associated with the migration of anatectic magma in the cores of F2 antiforms. Geochronological investigations combined with petro-structural analysis reveal that: (1) D1M partial melting started probably at 420–410 Ma and formed sub-horizontal stromatic metatexites at ∼30 km depth; (2) The anatectic magma accumulated and migrated when a drainage network developed, as attested by the pervasive formation of massive diatexite migmatites, at 410–400 Ma; (3) Soon after, massive flow of the partially molten crust from orogenic lower to orogenic upper crustal levels, assisted by the interplay between D2 upright folding and magma diapirism, led to migmatite-granite emplacement in the cores of regional F2 antiforms that lasted until at least 390 Ma; (4) a terminal stage was manifested by the emplacement of 370–360 Ma granite dykes into the surrounding metamorphic envelope. We propose that Devonian anatexis assisted by deformation governed first the horizontal and then the vertical flow of partially molten orogenic lower crust, which drove crustal flow, mass redistribution, and crustal differentiation in the accretionary system of the Chinese Altai.

The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known. In contrast, accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood. To fill this gap, we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt, where numerous data testify for Meso- to Neo-proterozoic magmatic reworking. The tectono-metamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic-early Cambrian (Baikalian) cycle. However, we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block, previously considered as a typical example of the Baikalian metamorphic belt. This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist. Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at ~720 °C and 6.0 kbar followed by further burial to the kyanite stability field at ~750 °C and ~9 kbar and decompression to ~650 °C and ~8 kbar. The Grt-St schist records initial burial to the staurolite stability field at ~620 °C and 6 kbar, followed by further burial to ~590 °C and 8.5 kbar. The monazite data yield a continuum of 207Pb-corrected 238U/206Pb dates of ca. 926−768 Ma in the Grt-Sil-Ky gneiss, and ca. 937−754 Ma in the Grt-St schist. Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial to 6.0 kbar under a thermal gradient of 27-32 °C/km is estimated at ca. 890−853 Ma. It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18-22 °C/km dated at ca. 835−815 Ma. The late monazite recrystallization at ca. 790 Ma is related to decompression. Additionally, monazite with dates of ca. 568−515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint. Metamorphic zircon rims with Th/U ratios of ~0.01-0.06 in Grt-Sil-Ky gneiss with 877±7 Ma age, together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism. The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice - probably of back-arc or arc type. This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri- Rodinian subduction of the Mirovoi Ocean. The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly, followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.

The ribbon-like Altai accretionary sedimentary wedge, representing the SW exteriors of the the Tuva-Mongol Orocline, suffered important Devonian and Permian deformation, metamorphism and melting. The last Permian deformation was associated with massive lower crustal melting, granulitization and lateral lower crustal flow of anatectic material. This lateral transfer was controlled by upwelling of the mantle below the extended parts of the crust. The subsequent Permian shortening led to development of a series of crustal scale detachment folds cored by migmatite-magmatite complexes and surrounded by weakly metamorphosed rocks in marginal synforms. The current study aims to understand the geometry, kinematics and dynamics of such large scale folding in the Chinese Altai during compression of thermally softened crust confined in the Tuva-Mongol Orocline. In such a setting, the angle of convergence is progressively increasing during collision, as the curvature of the orocline increases. To visualize and quantify this process, we employed analog modeling by using paraffin wax for ductile lower crust and sand-cenosphere mixture for brittle upper crust. The model domains (60cm×70cm×3cm) are preheated for 15 hours to attain a stable initial thermal and rheological gradient. The base of the models sustains the temperature at 51 °C (the melting point for the paraffin wax) while the top part of the model is heated to 48 °C by convective air. Strain in the models is quantified from the top view using the stereoscopic digital image correlation system from Lavision GmbH. The models are shortened by movement of indenter wall driven by a step-motor. Three series of experiments were designed to simulate the above detachment folds. In the first series of models, the indenter wall is perpendicular to the shortening direction. In the second scenario, the indenter wall is initially obliquely oriented to the shortening direction. As for last scenario, the angle of convergence α (defined as the angle between the plate motion vector and the plate boundary) is continuously increased from initial 60° to 90°. This last mode mimics the effect of the closing orocline confining the thermally softened crust. All models display progressive development of an array of folds with crestal grabens that are cored by molten and partially molten wax. We describe how the style of folding, degree of strain partitioning and distribution of transcurrent movements differ between the modes of convergence.

In the westernmost part of the Variscan belt, petro-structural observations, mineral equilibria modelling and U-Pb age determination were carried in the Jebilet massif (Morocco), in order to precise the deformation–metamorphism of NW Africa during Late Paleozoic Variscan events. A first episode corresponds to the emplacement of the main Oulad Ouaslam intrusion at 335.2 ± 0.8 Ma (U-Pb LA-ICP MS on zircon) within a Visean intracontinental basin. A second episode correspond S to SE convergence resulting into nappe stacking and upright folding in the supracrustal level together with the progressive development of a sub-horizontal metamorphic foliation around the pre-heated pelitic country rocks of the intrusion up to the sillimanite zone. Crd-And growth during this event in metapelite indicate indicating heating and increase of pressure up to 600–625 °C and 1.6–2.0 kbar. The timing of this sequence in metapelite is constrained by the U-Pb monazite age of 323.4 ± 3.6 Ma, interpreted as minimum age of metamorphic peak conditions, ca.

The Variscan belt in NW Africa represents an intraplate orogen, which mainly developed from Late Carboniferous (ca. from the Bashkirian: 323–315 Ma) to the end of Cisuralian (283–273 Ma), in response to the closure of an oceanic domain between NW Gondwana and Laurentia. The orogenic events were accompanied by an important magmatic activity, distributed across the whole northern Morocco, north of the South Meseta Fault Zone which separates the Meseta domain in the north from the Anti-Atlas domain to the south, where no magmatic activity was recorded at that time. Moreover, an older magmatic phase occurred during the Late Devonian-Early Carboniferous however the associated geodynamic context remains under debate.In northern Morocco, existing geochronological results for these magmatic activities are mainly obtained using Rb-Sr methods (whole rock or on different minerals). However, recent U-Pb datings on different massifs in Morocco have revealed older ages. Therefore, Rb-Sr ages should be considered with caution.In this contribution, we present new U-Pb geochronological data (LA-ICP MS on zircons) for several plutonic massifs of northern Morocco from the Western (Jebilet) and Eastern Meseta. In the later, we provide new datings on 3 magmatic facies in the High Moulouya massif, but also for the Boudoufoud, Beni Snassen, Tanncherfi, Zekkara and Merguechoum massifs. These new data reinforce the idea that two main magmatic events must be properly separated in northern Morocco. First, a clear Early Carboniferous event, possibly beginning as soon as the end of Late Devonian, is recorded within both Western and Eastern Meseta, and this magmatic event ends at around 330 Ma. Afterwards, from ca. 310–305 Ma to ca.280–275 Ma, the second magmatic event is also expressed across both Western and Eastern Meseta. While this second magmatic event is clearly related to the Variscan orogenic events, the unclear geodynamic context for our new Early Carboniferous ages allows the re-opening of the discussion about the « Eovariscan » phase in NW Africa. Finally, these two magmatic phases are compared to the magmatic records from West Europe Variscan belt.

The concept of detachment folding was developed between the '60s and '80s and generally describes displacement and buckling deformation of a competent layer above a weak, usually low viscosity horizon during tectonic shortening. From this definition, and based on the Biot-Ramberg theory, it is clear that geometrical parameters of such folds depend on contrasting rheology in both layers or on the rheological gradient in a complex multilayer. These systems were originally studied in association with the thin-skinned deformation and salt tectonics, and recently with regards to large-scale lithospheric deformation. In the latter, the rapidly heated lower crust is partially melted and a thin melt layer at the MOHO depth serves as the detachment horizon during collision and shortening. Our experimental work contributes to the understanding of the geometrical, kinematic and dynamic behaviour of such types of detachment folds as this deformation process strongly depends on a thermally dependent rheological gradient and nonlinear shortening velocity. The natural prototype for our models is for example the Chandmann or Bugat metamorphic domes in central Asia (CAOB). Our aim is to parametrize the style of such crustal-scale detachment folds depending on the rheological properties of the layered crust and the thermal gradient.For this purpose, we developed an apparatus for thermal analogue models capable of producing thermal gradients and programmable shortening. Paraffin wax is used as the analogue for the partially molten lower crust. The advantage of this material is, that it reproduces the temperature-controlled rheological stratification of the crust in hot orogens with a melt layer at the bottom (at Moho) superposed by partially molten crust. The upper crust is represented by a granular mixture of low-density cenosphere particles and silica sand, respectively. To keep the models properly dynamically scaled, we take into account the relationship for progressively decelerated plate convergence in the orogens. With increasing of both, basal heating and shortening rate, the folds' finite geometry converges to a system of pseudo-symmetric folds, cored by various amounts of the melt with respect to their position in the fold sequence. The dynamics of the fold amplification also depends on the position in the array of folds and is described by four evolutionary steps; initial perturbation, amplification with the melt inflow into axial zones of the folds, locking and simple vertical extrusion. With decreasing intensity of basal heating, the total melt amount is lower, deformation is more localized and converges to brittle-ductile coupling. Typical products are thrust systems on a local scale or pop-up structures on a large scale. Melt is localized in form of small fingers underneath the pop-up structures. Relatively colder and slower models display homogeneous thickening.A higher degree of heating results in melt redistributed along the axial planes of folds. Analysis of the layer interfaces curvature, paths and tortuosity of the material particles in these high-temperature experiments (based on resultant displacements calculated by the PIV method) also revealed asymmetrical evolution of the P-T-t paths for associated limbs of the pseudo-symmetrical folds.

The tectonometamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by Baikalian Late Proterozoic – Early Cambrian cycle related to amalgamation of Proterozoic oceanic and continent fragments to Siberain landmass. Here we present in-situ monazite geochronology linked to P−T modelling of micashischsts and migmatite gneisses at the northern part of the Precambrian Baydrag block (central Mongolia) previously considered as a part of Baikalian metamorphic belt. Garnet-sillimanite-kyanite gneiss records first burial to the sillimanite stability at ~725 °C and 6.5 kbar, followed by burial to the kyanite stability at ~650 °C and ~8 kbar. The garnet-staurolite schist records burial to the staurolite-stability at ~620 °C and 6 kbar, followed by a nearly isothermal burial to ~580 °C and 9 kbar. The monazite data yield a continuum of 207Pb-corrected 238U/206Pb dates of c. 926−768 Ma in the Grt−Sil−Ky gneiss, and c. 937−754 Ma in the Grt-St schist. Based on monazite textural positon and internal zoning, the time of prograde burial and peak under a thermal gradient of 28–32 °C/km is estimated at c. 870−890 Ma. It is not clear whether such high grade conditions prevailed until a phase of further burial under a geothermal gradient of 18–22 °C/km and dated at 800−820 Ma. Additionally, monazite with dates of c. 568−515 Ma occurs as whole grains or as rims with sharp boundaries on Grenvillean monazite in Grt-St schist testifying for minor Baikalian overprint. Metamorphic zircon rims with Th/U ratio ~0.01–0.06 in Grt−Sil−Ky gneiss with 877 ± 7 Ma age, together with lower intercepts of zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high grade metamorphism. The P−T and geochronology data show anticlockwise P−T evolution from c. 930 to 750 Ma which is interpreted as a result of thickening of supra-subduction extensional and hot edifice – probably of back arc or arc type. This kind of prograde metamorphism was so far described only on the northern part of the Tarim block and interpreted as a result of initiation of peri-Rodinian subduction of Mirovoi Ocean. Here, we further discuss geodynamic consequences of a unique discovery of Tonian metamorphism in term of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly followed by strong mechanical coupling potentially related to onset of Rodinia splitting.

During orogenic processes continental crust experiences significant partial melting. Repeated thermal pulses or fluctuation in fluid content can even cause multiple anatectic events that result in complex intrusion suits. The Vosges Mountains (NE France) reveal two chronologically and geochemically distinct tectono-magmatic events. An early major pulse of Mg‒K magmatism was followed ten millions years later by development of a magma-rich detachment zone and intrusion of Central Vosges Granite forming a felsic MASH zone. This MASH zone is characterized by the production of a large quantity of anatectic melts that interacted with the older Mg‒K granites and surrounding granulites and metasedimentary rocks. We aim to understand how such hybridization processes impact on the crustal rocks rheology, deformation as well as its geochemistry and geochronology. Three different granite varieties were distinguished: (i) the older Mg‒K granite end-member that is coarse-grained with a high proportion of feldspar phenocrysts, zircon U-Pb ages of 340 Ma and specific geochemical signature; (ii) Medium-grained type has a smaller amount of phenocrysts and shows advanced brecciation where fine-grained Pl+Kfs+Qtz form discontinuous corridors to an interconnected network surrounding fractured phenocrysts. Its geochemical signature suggests that this represents a mixing of Mg−K and Central Vosges granites, as confirmed by the presence of both inherited (340 Ma) and younger (330‒310 Ma) zircon domains; (iii) Isotropic medium-grained granite that shows geochemical signature typical for the Central Vosges Granite in which younger zircon domains (310‒320 Ma) dominate over inherited xenocrysts (340 Ma). These three granite varieties represent different stages of magma hybridization by the break up of the older Mg‒K granite by the younger Central Vosges Granite magmas. The interaction between new melt and previously crystallized granitoids results in variety of granite textures, fabrics, chemical compositions, isotopic signatures and deformational behavior. In summary, the resulting signature is result of interplay of melt transfer and interaction in the MASH zone.

A multidisciplinary approach integrating potential field analysis with geological and geochemical data provides new insights into the understanding of the crustal structure and evolution of the Mongolian collage. Magnetic and gravity data demonstrate the inconsistency between the geologically defined terranes and the geophysical domains in the southwestern part of the Mongolian collage. The combination of potential field analysis and modelling with whole rock geochemistry and isotopic mapping of Carboniferous–Permian granitoids indicates the presence of a homogeneous lower crust composed of a felsic to intermediate juvenile material beneath geophysically heterogeneous upper crust. This feature is interpreted as a result of a trench-directed lower crustal emplacement of an arc type crust underplating deformed Paleozoic oceanic crust. The potential field data also confirmed the occurrence of two orthogonal late Devonian and Permian–Triassic deformation upper crustal fabrics at the scale of the southwestern Mongolian collage. The prominent magnetic highs correspond to the tectono-metamorphic domains and magmatic provinces. The gravity anomalies highlight a periodicity of the signal correlating with alternating Permian–Triassic high and low strain zones, forming a zone of major deformation wrapping around the hinge of Mongolian orocline. The geometry and kinematics of dextral and sinistral transpressive faults are explained to result from the reactivation of Permian–Triassic deformation zones in the Cenozoic stress field.

Cambrian age volcanic arc-related rocks crop-out in high-grade metamorphic complexes marking the principal Devonian age suture zone in the Bohemian Massif. The age and tectonic setting of these rocks are established from whole-rock geochemical and isotopic data from basic-intermediate rocks transformed to eclogite and granulite, and U-Pb and Lu-Hf isotopic data of detrital zircon in associated meta-sediments. Recent works have established that these rocks can be correlated on the basis of their age and tectonic setting as well as their Variscan metamorphic evolution with allochthonous complexes in NW Iberia, and potentially, the whole European Variscan Belt (Martínez Catalán et al., 2020). Detrital zircon spectra from the high-grade metasediments associated with the Cambrian volcanic arc are superficially similar to the classically interpreted ‘West African signature’ with Neoarchean, Paleoproterozoic and late Neoproterozoic maxima. Nonetheless, limited Cryogenian input and shifting of the late Neoproterozoic maxima into the early Cambrian is in contrast to Variscan autochthonous complexes. Moreover, Lu-Hf isotopic data show important contrasts in the source of the Paleoproterozoic detritus. The Variscan autochthon is charecterised by Paleoproterozoic detritus with negative ɛHfi values indicating reworking of an Archean crust. However, the Paleoproterozoic detritus in the Cambrian arc terrane exhibits mostly positive ɛHfi values indicating a juvenile Paleoproterozoic source.In fact, the data from the Cambrian arc terrane show remarkable similarity to detrital zircon data from early Cambrian sediments on the southern margin of the East European Craton (Paszkowski et al., 2021). These sediments incorporate significant juvenile late Neoproterozoic-early Cambrian detritus from an as yet unidentified volcanic arc and their whole-rock geochemical composition is consistent with immature sediments sourced from intermediate igneous rocks. If true, this correlation suggests that the Cambrian arc terrane is a peri-East European Craton assemblage and necessitates a re-evaluation of the role of the East European Craton within the Variscan Orogeny.Catalán, J. R. M., Collett, S., Schulmann, K., Aleksandrowski, P., and Mazur, S., 2020. Correlation of allochthonous terranes and major tectonostratigraphic domains between NW Iberia and the Bohemian Massif, European Variscan belt. International Journal of Earth Sciences, 109, 1105-1131.Paszkowski, M., Budzyń, B., Mazur, S., Sláma, J., Środoń, J., Millar, I.L., Shumlyanskyy, L., Kędzior, A. and Liivamägi, S., 2021. Detrital zircon U-Pb and Hf constraints on provenance and timing of deposition of the Mesoproterozoic to Cambrian sedimentary cover of the East European Craton, part II: Ukraine. Precambrian Research, 362, 106282.

The common view of melt transport in the continental crust involves an initial stage of percolation along grain boundaries, melt segregation into leucosomes and dykes, coalescence of small melt conduits into larger ones and quick nearly vertical melt flow leading to formation of plutons. An entirely different style of melt migration was described in the Bohemian Massif, eastern European Variscan belt. There, a sequence of metaigneous migmatites was described where veins are lacking, leucosomes are rare and relics of melt are spread along grain boundaries. Textural, geochemical and compositional variations in these rocks show that they formed due to equilibration with melt coming from an external source, and that pervasive flow along grain boundaries was the dominant mechanism of melt transport.The question arises, at what conditions this style of melt transport can operate and what consequences the different styles of melt transport have on the crustal-scale tectonics. We address this question by means of a 2D crustal-scale model of two-phase flow using the code ASPECT (aspect.geodynamics.org). The system of pores through which the melt flows is not resolved in our model and it is described only by its permeability. A low permeability describes material with pores along grain boundaries while a high permeability corresponds to a system of leucosomes, dykes or cracksFor different material properties and thermal conditions we obtain different styles of melt migration and characteristics of the modeled crust. The melt can form a diffuse zone in the lower–middle crust, km-scale waves of high melt fraction gathering into sub-vertical channels, or a horizontal zone with high melt fraction in the middle crust. The lower crust is depleted and the middle crust is enriched in incompatible elements, and composition of the middle crust typically shows km-scale variations. The compositional variations are obtained even in the models with low permeability that corresponds to the melt percolation along grain boundaries, in agreement with the characteristics of the Bohemian migmatites.

The Mongolian Altai Zone is a part of the extensive Cambrian–Ordovician accretionary system located at the junction of the Siberian craton to the north and Tarim and North China cratons to the south. It extends approximately 2,000 km from Russia to Mongolia and represents one of the critical elements for reconstructing the early Paleozoic geodynamics of the Central Asian Orogenic Belt (CAOB). The studied section comprises a succession of deformed low- and high-grade metasedimentary rocks characterised by dominant terrigenous components mixed with volcanogenic material. The detrital zircons analysis revealed two separate groups a) more mature siliciclastic sediments (mostly sandstones) with maximum depositional age of Cambrian–Ordovician (ca. 463–489 Ma; zircon U-Pb) and b) more juvenile greywacke type sediments with Ordovician–Silurian (ca. 438–446 Ma; zircon U-Pb) maximum depositional age. U-Pb ages of detrital zircons show Cambrian-Ordovician (εHf(t) values –24.8 to +16.0) and Late Archean to Neoproterozoic source (εHf(t) values –35.5 to +10.4) and are interpreted as derived from the Ikh Mongol continental arc and the Baydrag continent. The greywackes, in addition, contain Silurian detrital zircons, with εHf(t) values from –0.5 to +13, suggesting syn-depositional contribution of juvenile material from a nearby magmatic arc. Both types of sediments are affected by Devonian (ca. 369–382 Ma; zircon U-Pb) metamorphism and magmatism granites, as well as stroingly reworked during the Permian (ca. 271–296; zircon U-Pb) under various metamorphic conditions. Late Devonian granitoids associated with felsic migmatites, and their zircon εHf(t) values from –9.5 to +13.5, indicate extensive melting of the sedimentary pile. A Permian high-temperature metamorphism is associated with granodiorite intrusions (εHf(t) values from –22.0 to +12.6) that contain Devonian zircon xenocrysts, suggesting melting of a Devonian source. The tectonic evolution of the Mongolian Altai Zone can be discretized in four events from which the first two were related to early Paleozoic metamorphic and magmatic evolution. The third one is associated with crustal-scale detachment that exhumed the early Permian migmatite-magmatite core complex in the south. The whole edifice was later affected by significant Permian-Triassic horizontal N-S shortening leading to juxtaposition of contrasting crustal levels thereby forming “apparent” terrane structure of the Mongolian Altai Zone. The whole edifice is interpreted as a Cambrian to Silurian fore-arc, affected by Devonian syn-extensional deep crustal melting. In addition,the Permian anatectic zone is interpreted as a deep part of an inverted continental rift.

Accepted manuscript version, licensed CC BY-NC-ND 4.0. In this study, U–Pb ages and Hf isotopic composition of detrital zircons from the Precambrian metasedimentary autochthon of the Brunovistulian Domain in the eastern Bohemian Massif were investigated to understand the pre-collisional evolution of the eastern periphery of the European Variscan belt. Detrital zircons of the Tonian sequences have mostly Paleoproterozoic to Neoproterozoic ages (c. 2.1–0.9 Ga) and are interpreted as detritus derived from the basement of either Baltica or Amazonia. The mostly positive εHf(t) values (–4 to + 16) indicate a juvenile nature of their magma sources with minor older crustal components. In contrast, the Ediacaran sequences contain dominantly Neoproterozoic zircons (c. 600 Ma) and only rare Paleo- and Mesoproterozoic ages indicate that they were sourced from the adjacent Neoproterozoic magmatic arc with very limited input of recycled cratonic detritus. The large spread of εHf(t) values (–15 to + 13) of the Neoproterozoic zircons suggests significant mixing of mantle-derived magmas with mature crustal material, typical of large continental magmatic arc systems. The zircon age patterns of the Ediacaran sequences, characterized by a dominance of the late Neoproterozoic zircons and limited Mesoproterozoic zircons, are nearly identical to those from the Teplá–Barrandian Unit and Moldanubian Zone, pointing to their similar sources. We consider such age populations as a record of sources actually exposed at the time of deposition, rather than the real provenance signature of the continental basement. The change in detrital zircon U–Pb age and Hf record of the Brunovistulian Domain took place between the early and late Neoproteorozoic, and probably reflects the plate-tectonic reconfiguration from the Rodinia formation/break-up to the evolution of the Gondwana or Baltica active margins. Our data challenge the main arguments for an existence of the Rheic oceanic suture between the Brunovistulian Domain and Moldanubian Zone and allow for an alternative pre-collisional model of the Bohemian Massif as a single Neoproterozoic crustal domain

The NW Iberian Allochthon and the Tepla-Barrandian and Moldanubian zones represent the internal parts of the Variscan belt in their respective domains. A correlation based on the lithological association, protolith ages, metamorphic evolution, detrital zircon age spectra and tectonic setting is attempted between the NW Iberian Massif and SE Bohemian Massif in order to check whether they could have formed part of the same allochthonous stack. The Galicia-Tras-os-Montes Zone of the Iberian Massif and the internal zones of the Bohemian Massif include from bottom to top a Parautochthon and Lower Allochthon representing the outer edge of the northern Gondwana margin, an oceanic Middle Allochthon with Cambro-Ordovician and Early Devonian ophiolites and an Upper Allochthon interpreted as a peri-Gondwanan terrane. Early Variscan, subduction-related high-pressure metamorphism characterizes many of the allochthonous units, with ages younging from the structurally upper to the lower units from 400–385 Ma to 370–360 Ma, respectively. High- and ultrahigh-pressure metamorphism occurred also in the Saxothuringian Autochthon at 360–340 Ma, but not in the NW Iberian Autochthon. The different behavior of the Autochthon in the Iberian and Bohemian massifs accounts for their distinct evolutions from 360 Ma onward. We conclude that the Upper Allochthon was a unique peri-Gondwanan terrane, whereas the Middle Allochthon represents units of the same peri-Gondwanan ocean, opened at the Cambro-Ordovician boundary, and having recorded localized renewed activity in the Silurian–Early Devonian. No other oceans separated the Lower Allochthon, Parautochthon and Autochthon.

The non-cylindrical character of the Variscan Belt is established via a description of its most salient geological characteristics and correlations among the main and relatively minor massifs from Bohemia to Morocco on the basis of a bibliographical synthesis. Using the firmly established tectonostratigraphic domains, the continuity of Variscan zones is discussed, as well as the western termination of the one that occupies most of the central part of the belt. This zone is characterized by its allochthony, and breaks the continuity of the zonation, that is, the cylindrism of the belt. A proposal is made to call this zone the Mid-Variscan Allochthon and its wedge shape in map view is explained performing a step by step restoration of late Variscan deformation. Once the geometry of the belt is interpreted by a succession of regionally significant deformation events, a kinematic reconstruction is proposed. It is based on recent developments on the knowledge of Paleozoic plate tectonics and intended to show that the deformation model is compatible with current paleogeographical developments.

International audience; Metasandstones from early Cambrian to early Carboniferous stratigraphic successions were sampled in the Rehamna massif of the Western Meseta in Morocco. The early Cambrian sample shows a single Paleoproterozoic population at ca. 2 Ga suggesting a local basement source. The Ordovician sample is largely dominated by a Cryogenian-Ediacaran population and minor Paleoproterozoic peaks. The Devonian sample reveals age populations similar to NorthWest African Cambrian to Devonian age spectra indicating that the southern-derived West Gondwana source essentially pertained up to the Devonian. The two early Carboniferous samples show more heterogeneous zircon age spectra with a marked Ediacaran peak [2] accompanied by Paleoproterozoic and Mesoproterozoic sub-peaks indicating important reorganization of the drainage systems. One sample also shows presence of Upper Devonian to early Carboniferous zircon grains, which suggests local magmatic sources associated to the formation of intracontinental extensional basins. The comparison of detrital zircon spectra with paleogeographic reconstructions indicate that the early Carboniferous change in detrital zircon sources can be interpreted in the framework of the opening of the Paleotethys ocean with coeval erosion of orogenic topographies linked to the emplacement of a Mid-Variscan Allochthon, and/or collision of an Avalonian indenter to the north.

International audience; A multidisciplinary approach integrating potential field analysis with geological and geochemical data provides new insights into the understanding of the crustal structure and evolution of the Mongolian collage. Magnetic and gravity data demonstrate the inconsistency between the geologically defined terranes and the geophysical domains in the southwestern part of the Mongolian collage. The combination of potential field analysis and modelling with whole rock geochemistry and isotopic mapping of Carboniferous–Permian granitoids indicates the presence of a homogeneous lower crust composed of a felsic to intermediate juvenile material beneath geophysically heterogeneous upper crust. This feature is interpreted as a result of a trench-directed lower crustal emplacement of an arc type crust underplating deformed Paleozoic oceanic crust. The potential field data also confirmed the occurrence of two orthogonal late Devonian and Permian–Triassic deformation upper crustal fabrics at the scale of the southwestern Mongolian collage. The prominent magnetic highs correspond to the tectono-metamorphic domains and magmatic provinces. The gravity anomalies highlight a periodicity of the signal correlating with alternating Permian–Triassic high and low strain zones, forming a zone of major deformation wrapping around the hinge of Mongolian orocline. The geometry and kinematics of dextral and sinistral transpressive faults are explained to result from the reactivation of Permian–Triassic deformation zones in the Cenozoic stress field.

Melting of the continental crust and subsequent melt transport has been most thoroughly described in the case of metasedimentary rocks. In these rocks segregation and migration of melt occur either through an interconnected network of veins and melt-rich layers (leucosome) or in form of diapirs. For these rocks, porous flow of melt at grain scale is mostly regarded only as a transient stage of separation of melt from the solid rock.An entirely different style of melting and melt transport occurs in the case of felsic metaigneous rocks. We use the example from the Bohemian Massif, the eastern European Variscan belt, where metaigneous migmatites were studied in large detail. Here, melt did not segregate from the solid rock but migrated pervasively along most of the grain boundaries and equilibrated with the host rock. This equilibration resulted in formation of a continuous sequence of texturally, geochemically and compositionally different migmatites.The question arises, what are the conditions and driving forces for this unusual behavior. We attempt to address this question by means of numerical modeling of two-phase flow (i.e. flow of porous solid matrix and melt), using the open-source finite-element ASPECT code (aspect.geodynamics.org). Most previous numerical studies of this process were either purely generic or focused on the melting of the mantle. In order to study this process in crustal conditions, we set up a 2D crustal-scale thermo-mechanical model that includes melting and freezing. We investigate the role of material properties (viscosity, solidus and liquidus temperatures, solid matrix permeability, melt composition) and thermal and velocity boundary conditions, as well as the effect of grid resolution. The results are discussed in terms of realistic parameter values and possible styles of melt migration and deformation of the matrix.

High-pressure granitic orthogneiss of the south-eastern Orlica–Śnieżnik Dome (NE Bohemian Massif) shows relics of a shallow-dipping S1 foliation, reworked by upright F2 folds and a mostly pervasive N-S trending subvertical axial planar S2 foliation. Based on macroscopic observations, a gradual transition perpendicular to the subvertical S2 foliation from banded to schlieren and nebulitic orthogneiss was distinguished. All rock types comprise plagioclase, K-feldspar, quartz, white mica, biotite and garnet. The transition is characterized by increasing presence of interstitial phases along like-like grain boundaries and by progressive replacement of recrystallized K-feldspar grains by fine-grained myrmekite. These textural changes are characteristic for syn-deformational grain-scale melt percolation, which is in line with the observed enrichment of the rocks in incompatible elements such as REEs, Ba, Sr, and K, suggesting open-system behaviour with melt passing through the rocks. The P–T path deduced from the thermodynamic modelling indicates decompression from ~15−16 kbar and ~650–740 ºC to ~6 kbar and ~640 ºC. Melt was already present at the P–T peak conditions as indicated by the albitic composition of plagioclase in films, interstitial grains and in myrmekite. The variably re-equilibrated garnet suggests that melt content may have varied along the decompression path, involving successively both melt gain and loss. The 6–8 km wide zone of vertical foliation and migmatite textural gradients is interpreted as vertical crustal-scale channel where the grain-scale melt percolation was associated with horizontal shortening and vertical flow of partially molten crustal wedge en masse.

International audience; We present a new analogue model of oroclinal buckling, which is based on recent advances in our geological understanding of the Mongolian orocline in the Central Asian Orogenic Belt. The model simulates an amplification of a preexisting, gently arcuated accretionary system. In the analogue models, we established three neighboring subdomains that represent contrasting lithospheric domains separated by a steep “subduction” interface. The shortening of the model is parallel to the pseudo-linear interface between these subdomains and produces two different deformation patterns in the amplified interlimb areas of the orocline. We used analogue modelling techniques to investigate both the flow of a ductile lithosphere and the brittle deformation of an upper crust that is associated with the development of an orocline. The modelling results show that: (1) two types of regional-scale folds develop both of which are upright. The largest are steeply plunging oroclines and the smaller have sub-horizontal axes; (2) the overall deformation distribution displays significant thickening/thinning and exhumation of the lower ductile layers with variations of upward transfer of crustal material; (3) the hinge regions of the two adjacent oroclines studied consist of different materials, one contains oceanic and the other felsic lower crustal rocks. It is found that the geometry of the lithosphere beneath the two regions is very different; (4) the geometry of the orocline is non-cylindrical and exhibits an increasing plunge of fold axes when traced from the upper-crust to deeper lithospheric layers. The resulting model is used to geophysically constrain deep crustal structures and surface deformation patterns in Mongolia.

The Velke Vrbno Dome crops out at the boundary between the Brunovistulian Terrane and the internal parts of the Bohemian Massif. Here, eclogite boudins occur within an Ediacaran volcano-sedimentary sequence. Strong Nb depletion (Nb/Nb* = 0.19–0.82) combined with moderately positive Nd isotopic compositions (eNd(i) = +3.89 – +5.77) is used to argue for emplacement of the eclogite protoliths in a transitional supra-subduction to continental-rift setting. Conversely, heterogeneously enriched large ion lithophile elements and highly radiogenic Sr isotopic ratios (87Sr/86Sr = 0.705–0.720) are interpreted to have been modified following fluid infiltration subsequent to eclogite-facies metamorphism. U–Pb laser ablation inductively coupled plasma mass spectrometry dating of magmatic zircon from the rift-type eclogite indicates Early Cambrian emplacement (c. 535 Ma) following episodic Ediacaran volcanic arc activity. Moreover, a continental setting is emphasized by zircon dating of a mylonitic orthogneiss, revealing a fragment of Palaeoproterozoic (c. 2000 Ma) basement, the first such finding within the Brunovistulian Terrane sensu stricto. The new data from eclogite confirm that rifting in this segment of Gondwana pre-dated the Ordovician opening of the Rheic Ocean and therefore that the suture between Brunovistulia and the rest of the Bohemian Massif likely represents the vestige of an older hyperextended basin or oceanic tract. Supplementary material: Previously unpublished single zircon evaporation ages from Ediacaran orthogneiss from the Velke Vrbno Dome (supplement A); detailed analytical methodology (supplement B); whole rock geochemical data (supplement C); and U-Pb LA-ICP-MS zircon data (supplement D) are available at https://doi.org/10.6084/m9.figshare.c.5233079