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The Mongolian micro‐continental fragments play an important geodynamic role in the Palaeozoic evolution of the Central Asian Orogenic Belt. However, the original provenance of individual blocks within the Nuna/Columbia and Rodinia supercontinents is not well constrained. In this work, we present zircon U–Pb and Lu–Hf isotopic data from the Mesoproterozoic to Neoproterozoic metamorphosed cover of the Baidrag Block in central Mongolia. These data reveal important maxima in the Neoarchean, Palaeoproterozoic and locally Neoproterozoic. Although the majority of the new data can be attributed to local sources within the basement of the Mongolian micro‐continental fragments, an important contribution of juvenile 2.0 Ga zircons is linked to the western margin of the Siberian Craton, and 1.5 Ga zircons are either directly derived from North Australia or indirectly recycled from sediments in NW Laurentia. [ABSTRACT FROM AUTHOR]

The general configuration of the main continental blocks in the Gondwana supercontinent and the Ediacaran–early Paleozoic tectonic evolution of its northern margin are widely accepted. However, reconstruction of the original positions and the question of potential separation of the Gondwana-derived crustal segments that are now included in the Variscan and Alpine orogenic belts remain controversial. The Western Carpathians, part of the Alpine–Carpathian belt, represents an important crustal segment broken-off from northern Gondwana and later incorporated into both the Variscan and Alpine collisional orogens. The earliest tectonic evolution and paleogeography of the pre-Variscan basement of the Western Carpathians remains poorly known, due to insufficient age data and intense polyphase tectonometamorphic overprints, both Variscan and Alpine. This paper provides new results of U–Pb dating and Hf isotopic analysis of detrital zircons as well as a whole-rock geochemical study from metavolcanic-sedimentary basement units of the Western Carpathians. The obtained age spectra suggest that the sedimentary succession was supplied dominantly by Ediacaran (c. 600 Ma) zircons, with a relatively minor role for Stenian–Tonian (c. 1.2–0.9 Ga) and Paleoproterozoic cratonic (c. 2.2–1.8 Ga) zircons. The mixed Hf isotopic signature (ε Hf(t) values ranging from −20 to +12) of the Ediacaran zircons indicates substantial mixing of mantle-derived magmas with mature crustal material, typical of continental magmatic arcs. In contrast, the mostly negative ε Hf(t) values (−15 to +4) of the cratonic zircons suggest recycling of an older continental crust. The presumably youngest part of the sequence is also characterised by high proportion of early Paleozoic zircons with generally negative ε Hf(t) values (−10 to −2). The zircon U–Pb age spectra, Hf isotopic patterns and whole-rock geochemical signatures of the studied Western Carpathians sequences are interpreted as reflecting deposition at a progressively developing Cambrian–Silurian passive margin setting. The West Carpathian data have been correlated with a comprehensive detrital zircon U–Pb age and Hf isotope data set compiled from possible source areas and other Gondwana-derived units to test the possibility of their primary linkages. These correlations indicate strong similarity in both zircon U–Pb age spectra and Hf isotopic compositions to other parts of the Ediacaran (Cadomian) continental magmatic arc. Older, cratonic sources are linked to the Saharan or East African parts of northern Gondwana, whereas the early Paleozoic detritus must represent a local volcanic source. Taken together, our new data from the Western Carpathians provide constraints for a new paleogeographic model of the northern African part of the Gondwana passive margin. • Cambrian–Silurian formation of Western Carpathians in the Gondwana passive margin. • Detrital zircon ages and Hf isotopic data record temporal variations in sources. • Affinity of the Western Carpathians to the central Saharan segment of Gondwana. • Provenance shift from the Saharan province to the East African province. • New paleogeographic reconstruction of the African part of Gondwana margin. [ABSTRACT FROM AUTHOR]

Interpretation of the deep geological and tectonic setting is a key step for safety assessment of the potential sites for a deep geological repository of radioactive waste. However, the conventional geological mapping is difficult in the agricultural landscape with lacking rock outcrops. To address this task, we combined geological and hydrogeological mapping, morphostructural analysis and geophysical methods to construct a new geological map and a 3D model of the Březový potok site in the southwestern Czech Republic. They provide unique constraints for the understanding of the subsurface geological and tectonic architecture. The results are compared with the archive geological data and the usefulness of individual geophysical methods to detect the lithological contacts and tectonic zones is discussed. We present this multi-disciplinary research strategy as a workflow for detailed geological characterization, which can be successfully applied on other sites with similar geological environment. [ABSTRACT FROM AUTHOR]

The primary relationships and character of the boundaries between principal lithotectonic domains in the Mongolian tract of the Central Asian Orogenic Belt (CAOB) are still poorly constrained. This brings much uncertainty in understanding of the orogeny configuration and the complete accretionary history. The plutonic Khuurai Tsenkher Gol Complex and the mainly metasedimentary Bij Group represent associated medium- to high-grade basement complexes exposed in the Hovd Zone close to its boundary with the Lake Zone in western Mongolia. The Khuurai Tsenkher Gol Complex is composed of variously deformed acid to basic magmatic rocks intimately associated with the metamorphosed sedimentary and volcanic rocks of the Bij Group. Results of our field work, new U–Pb zircon ages and whole-rock geochemical data suggest an existence of two separate magmatic events within the evolution of the Khuurai Tsenkher Gol Complex. Early to Mid-Ordovician (476 ± 5 Ma and 467 ± 4 Ma protoliths) normal- to high-K calc-alkaline orthogneisses, metadiorites and metagabbros predominate over Mid-Silurian (430 ± 3 Ma) tholeiitic–mildly alkaline quartz monzodiorites. Whereas the geochemical signature of the former suite unequivocally demonstrates its magmatic-arc origin, that of the latter quartz monzodiorite suggests an intra-plate setting. As shown by Sr–Nd isotopic data, the older arc-related magmas were derived from depleted mantle and/or were generated by partial melting of juvenile metabasic crust. Detrital zircon age populations of the metasedimentary rocks together with geochemical signatures of the associated amphibolites imply that the Bij Group was a volcano-sedimentary sequence, formed probably in the associated fore-arc wedge basin. Moreover, our data argue for an identical provenance of the Altai and Hovd domains, overall westward sediment transport during the Early Palaeozoic and the east-dipping subduction polarity. The obvious similarities of the Khuurai Tsenkher Gol Complex (Hovd Zone) with the neighbouring Togtokhinshil Complex (Lake Zone) suggest that both magmatic complexes originally belonged to the same magmatic arc, related to the Palaeo-Asian subduction system. The geodynamic cause of the later, within-plate magmatic pulse is unclear, but was probably still related to the effects of retreating subduction (slab window/ocean ridge subduction or back-arc lithosphere thinning). The Khuurai Tsenkher Gol Complex was subsequently separated from the western margin of the Lake Zone and imbricated into the Hovd Zone mélange. It is proposed that the Lake/Hovd zones boundary in the study area represents a younger deformation zone rather than a true terrane boundary/suture. This could be a general feature of the suture zones within this part of the CAOB. [ABSTRACT FROM AUTHOR]

Cadomian magmatic complexes of the Brunovistulian Domain crop out at the eastern termination of the Bohemian Massif. However, the age, nature and geotectonic affinity of some of pre-Variscan (meta-)igneous rock complexes from this domain are still unknown. Geochronological and geochemical study of the granitic rocks across the Brunovistulian Domain reveals new information about the timing and nature of this magmatic activity originally situated along the northern margin of Gondwana. Zircon U-Pb data (601 ± 3 Ma, Brno Massif; 634 ± 6 Ma, paraautochtonous core of the Svratka Dome; 568 ± 3 Ma, Bíteš orthogneiss) from the allochtonous Moravicum indicate the prolonged magmatic activity within the Brunovistulian Domain during the Ediacaran. The major- and trace-element and Sr-Nd isotopic signatures show heterogeneous geochemical characteristics of the granitic rocks and suggest a magmatic-arc geotectonic setting. The two-stage Depleted Mantle Nd model ages ( c. 1.3-2.0 Ga) indicate derivation of the granitic rocks from a relatively primitive crustal source, as well as from an ancient and evolved continental crust of the Brunovistulian Domain. These results constrain the magmatic-arc activity to c. 635-570 Ma and provide a further evidence for a long-lived (at least c. 65 Myr) and likely episodic subduction-related magmatism at the northern margin of Gondwana. The presence of granitic intrusions derived from variously mature crustal sources at different times suggests heterogeneous crustal segments to having been involved in the magmatic-arc system during its multistage evolution. [ABSTRACT FROM AUTHOR]

New field, petrographic, geochemical and geochronological data from the Togtokhinshil Complex (western margin of the Lake Zone, Central Asian Orogenic Belt) reveal the presence of two separate Early Palaeozoic magmatic pulses: Mid-Ordovician and Late Devonian. The Mid-Ordovician (c. 460 Ma) magmatism produced various types of mafic plutonic rocks (gabbro-diorite suite) emplaced into the volcanosedimentary sequence of the Baatar Fm. Gabbros to gabbrodiorites have metaluminous and mostly low to normal-K calc-alkaline or possibly tholeiitic character. The majorand trace-element as well as Nd isotopic signatures suggest its magmatic-arc geotectonic setting and imply the derivation either exclusively from the mantle or in part from the juvenile mafic crust. The Late Devonian (c. 376 Ma) granite suite intruded the rocks of both the gabbro-diorite suite and the Baatar Fm. Granites are subaluminous and have a high-K calc-alkaline to shoshonitic character. Their magmatic arc-like geochemical signature is interpreted as inherited from the recycled arc-related crustal source. The granite suite seems a product of extensive mantle heat-induced crustal anatexis. The Depleted Mantle Nd model ages (c. 0.75 Ga) indicate derivation of the magmatic rocks of the both suites from relatively juvenile, and geochemically immature, sources. These results provide a clear evidence of the Mid-Ordovician arc-related magmatism in the western part of the Lake Zone. They thus prolong the assumed period of magmatic-arc activity and constrain onset of accretionary processes in this part of the Central Asian Orogenic Belt. The gabbro-diorite suite represents either part of a long-lived magmatic-arc or, more likely, a member of multiple island-arc system along the western margin of the Lake Zone. Newly described Late Devonian magmatism in the Togtokhinshil Complex provides further evidence of tectono-thermal event that was widespread in the Central Asian Orogenic Belt. The geodynamic cause for this event remains uncertain, but effects of lithospheric thinning and/or asthenospheric mantle upwelling are considered the most likely. [ABSTRACT FROM AUTHOR]

[Display omitted] • Western juvenile and eastern mature Ordovician arcs intruded the Altai wedge. • Late Ordovician–Silurian volcanic gaps were related to Altai wedge thickening. • An early Devonian back-arc formed in the western part of the Altai wedge. • Back-arc extension migrated to the east in the late Devonian. • This diachronous evolution resulted from lateral variations in subduction modes. The early Palaeozoic volcano–sedimentary evolution of the Khovd and Gobi-Altai zones of the Mongol–Altai Domain was investigated by quantitative petrographic analysis of sediments, whole-rock geochemistry of volcanic rocks, and U–Pb zircon dating. Three evolutionary stages were identified in the Khovd Zone: (1) Cambrian–mid Ordovician basaltic (Nb/Yb = 0.09–1.25) and rhyodacitic (Nb/Yb = 4.36) volcanism and sedimentation of immature greywackes in the fore-arc basin in front of the juvenile continental arc, (2) late Ordovician flysch-type sedimentation accompanied by a volcanic gap coincided with Barrovian metamorphism in footwall sequences reflecting a compressional stage, and (3) Silurian–Devonian within-plate bimodal volcanism (with a strong asthenospheric imprint; Nb/Yb = 4.45–20.34) followed by mature siliciclastic deposition interpreted as transformation of the former fore-arc region into a back-arc domain associated with HT metamorphism. In the Gobi-Altai Zone the three stages were: (1) Cambro–Ordovician marginal greywacke–sandstone succession adjacent to a mature continental arc (mafic rocks Nb/Yb = 0.71–4.36), (2) Silurian siliciclastic sedimentation associated with migration of the volcanic gap and Barrovian metamorphism, and (3) Lower Devonian deep-marine sedimentation followed by shallow-marine carbonate sedimentation, initially accompanied by calc-alkaline basalts (Nb/Yb = 0.74–4.57) and andesites to rhyolites (Nb/Yb = 0.69–4.87), evolving in the mid–late Devonian to within-plate alkali basalts (Nb/Yb = 11.05) thus reflecting back-arc basin development. These temporal and spatial variations in the Mongol-Altai Domain are interpreted as a diachronous sequence of retreating and advancing subduction modes instead of juxtaposition of suspect terranes as proposed in previous studies. [ABSTRACT FROM AUTHOR]

[Display omitted] • Contrasting detrital zircon age and Hf isotopic patterns of Brunovistulian basement. • Rodinia internal or peripheral position and Baltica/Amazonia sources in Tonian. • Ediacaran deposition in a back-arc basin with active magmatic arc dominant source. • Transition from Rodinia formation/break-up to Gondwana or Baltica active margin. • Model of the Bohemian Massif as a single Neoproterozoic crustal domain. 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 ε Hf(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 ε Hf(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. [ABSTRACT FROM AUTHOR]

A polyphase tectono-metamorphic evolution of the Mongolian tract of the Central Asian Orogenic Belt (CAOB) is inferred from a structural, petrological and geochronological study in the Hovd Zone at the NE periphery of the Altai Domain. Four tectonic events are recorded here. A relict sub-vertical fabric (S1) is associated with the growth of garnet cores and staurolite, typical of MP–MT Barrovian-type metamorphism (M1). This fabric was isoclinally folded and overprinted by sub-horizontal foliation (S2) associated with growth of garnet rims, sillimanite and cordierite, typical of LP–HT metamorphism (M2). The P–T path inferred from petrology and thermodynamic modelling shows that the peak conditions reached 590–610 °C and c. 5.8 kbar (M1) and were followed by decompression to 4–5 kbar associated with heating to 630–690 °C (M2). Subsequently, the studied region was strongly reworked by NW–SE trending upright folds (F3) locally associated with development of muscovite- and chlorite-bearing cleavage (S3). The region is weakly affected by post-metamorphic D4 overprint. Monazite in-situ electron-microprobe dating was complemented by LA–ICP-MS U–Pb dating, revealing three distinct texturally defined age groups at c. 455, 435 and 385 Ma. The ages at c. 455 Ma are related with monazite included in garnet cores, the age group at c. 435 Ma with monazite in garnet rims and in S2 foliation, while the ages of c. 385 Ma come mostly from monazite in chlorite–muscovite-bearing bands. Based on the monazite textural position and REE patterns, these ages are correlated with the tectono-metamorphic events D1–D3, reflecting short-lived Late Ordovician shortening, prolonged Middle Silurian–Early Devonian extension and Late Devonian–Early Carboniferous compression, respectively. Alternation of shortening and extension periods is interpreted as being caused by tectonic switching, i.e. repeated alternation of advancing and retreating subduction regimes. The latest Permian event resulted from the collision of the Mongolian orogenic collage with the Tarim and North China cratons in the south and caused rotation of previous fabrics to the current configuration. [Display omitted] • Late Ordovician Barrovian metamorphism and crustal thickening. • Middle Silurian–Early Devonian LP–HT metamorphism and crustal scale extension. • Late Devonian–Early Carboniferous horizontal compression. • Early Palaeozoic tectonic switching. • Permian rotation related to oroclinal bending. [ABSTRACT FROM AUTHOR]

The recently opened Bukov underground research facility is used by the Czech national authority for radioactive waste repository for underground research and safety assessment purposes as part of the Czech deep geological repository (DGR) development programme. The laboratory, which is both site-specific and generic at the same time, is located at a depth of 550 m below ground level in the former Rožná uranium mine in the southeast of the Czech Republic in crystalline rocks of the Bohemian Massif. During the laboratory construction, a multidisciplinary approach was used in order to document and visualize the environment for the following experimental programme. The obtained data are important for understanding the geological environment and the rock characteristics of the Bohemian Massif in the process of deep repository planning and siting. The multidisciplinary approach included petrology, structural geology, geochemistry, geochronology, geotechnical parameters and excavation damage zone description, as well as 3D visualization of the rock environment and laboratory itself. New field observations reveal the Cambrian–Ordovician volcano-sedimentary origin of the studied rocks that have been affected by two-phase reworking during the Variscan orogeny. The rock mass is of high to very high strength and low permeability and the rock mass properties are mainly influenced by tectonic structures. Above all, the presented multidisciplinary approach enables unified and comparable evaluation of such facilities as well as the potential locations of the future DGR. • URF Bukov represents newly opened hybrid-type URF, unique in the Central Europe. • DGR planning and siting process in crystalline rocks in Bohemian Massif. • Quality and stress state assessment of the rock mass within underground laboratory. • Rock mass properties are mainly influenced by structural features. • Multidisciplinary approach enables unified and comparable evaluation of various sites. [ABSTRACT FROM AUTHOR]

The Brno Massif is the largest exposed part of the Brunovistulicum (eastern Bohemian Massif) representing Precambrian basement incorporated into the Central European Variscan Belt. Two well-known Cadomian granodiorite complexes of magmatic-arc origin are separated by N–S-trending belt of mafic rocks previously compared to ophiolite. This so-called Central Basic Belt is formed by a slightly metamorphosed volcanic part (Metabasite Zone) in the east and dominantly plutonic Diorite Zone in the west. Our new geological, geochemical and isotopic data including U–Pb zircon dating reveal two distinct Precambrian magmatic events within the Central Basic Belt preceding the Cadomian arc. The geochemical signatures of the dominant late Tonian (c. 730 Ma) tholeiitic basalts ( ε Nd 725 = + 7.8 to + 6.7) in the Metabasite Zone suggest a direct derivation from a mantle source in an extensional setting. Also, the associated sporadic rhyolitic lavas and tuffs are primitive, showing a short mean crustal residence ( ε Nd 725 = + 6.0 and + 5.7; T DM Nd .2stg ~ 0.9 Ga). By contrast, the Cryogenian (c. 650 Ma) magmatism of the Diorite Zone clearly demonstrates features of a magmatic-arc origin. Rather primitive whole-rock geochemistry and radiogenic Nd isotopic signature ( ε Nd 655 values typically falling between + 7 and + 6) show that this arc was either intraoceanic, or developed on recently accreted, immature mafic crust. Based on all the available data, three successive tectono-magmatic stages have been identified in the Brno Massif in the Neoproterozoic times (c. 730–600 Ma), as products of a single long-lived, multi-stage subduction system spanning nearly full Neoproterozoic supercontinent cycle from the break-up of Rodinia to the assembly of Pannotia. [ABSTRACT FROM AUTHOR]

[Display omitted] • Cambrian sediments from Ukraine contain detrital input from a cryptic active margin. • This active margin input is confirmed from new whole-rock geochemical data. • Zircon U-Pb and Lu-Hf data strongly correlates with data from Variscan allochthon. • Further correlations with Avalonia and eastern parts of Baltica also identified. • Highlight significant role of Baltica active margins in Ediacaran-Cambrian. New whole-rock geochemical data indicate changing provenance of sedimentary rocks deposited on the southern margin of Baltica in the late Neoproterozoic to early Paleozoic. The oldest part of the sequence is characterized by highly weathered detritus sourced from a mafic igneous source, inferred to be the nearby Volyn Large Igneous Province (L.I.P.), in an intracontinental rift (aulacogen) setting. Neoproterozoic–Cambrian boundary sequences are weakly weathered and sourced from intermediate to felsic rocks in an active margin setting whereas the youngest parts of the sequence exhibit significant quartz accumulation consistent with deposition of recycled material in a passive margin setting. These data support published U-Pb and Lu-Hf zircon data from the same sedimentary sequence where the Neoproterozoic-Cambrian boundary strata are characterized by anomalously high Ediacaran–Cambrian zircon contribution with juvenile isotopic composition. Taken together, these data indicate a cryptic late Neoproterozoic–early Cambrian active margin along the periphery of southern Baltica. Based on a correlation of similarly aged foreland basin and accretionary complexes, and links to paleomagnetically constrained plate reconstructions it is hypothesized that this detritus was sourced from the Avalonian micro-continent and a Cambrian arc terrane that is today preserved in high-grade metamorphic complexes of the European Variscides. These findings have potentially profound implications for our understanding of the supercontinent cycle that links the Rodinia to Pangea supercontinents, in particular the role played by Baltica-derived terranes in Paleozoic collisional orogeny. [ABSTRACT FROM AUTHOR]

Understanding the distribution of brittle structures throughout a rock mass is one of veryimportant factors for safety assessment of underground facilities, e.g., deep geologicrepositories for spent nuclear waste. In spite of the importance of geological data, in theinitial stages of the evaluation process it is often not feasible to perform extensive drillingcampaigns or geophysical measurements. To address the problem, we have developed amethodology which allows us to construct Discrete Fracture Network (DFN) models offractured rock mass using limited surface outcrop data. In order to fully exploit the information potential of rock outcrops, an enhanced procedureof structural data acquisition based on 3D photogrammetry has been proposed. First,using high-resolution digital images obtained in the field, realistic 3D models ofoutcrops are reconstructed and placed in correct geographical position and orientation.Subsequently, fractures are visualised via faces of the triangulated mesh and their spatialcharacteristics, such as trace length, dip, and strike can be extracted. The greatest benefit ofthe adopted approach is that properly oriented, reconstructed and geographicallyemplaced outcrop model can be used repeatedly as the need for more structural dataarises. The acquired structural-geological data are used to construct 3D DFN models of thefractured rock mass. The DFN modelling employs common statistical distributions offracture size (power law distribution) and orientation (Fisher distribution). In order tohonour the field observations, fracture volume density (P30) and the parametersof the fracture size distribution are obtained by minimizing objective functions,which quantify the discrepancy between the observations and model reproductionsat the outcrops. Once the model parameters are identified, any number of DFNrealizations can be created by a random process respecting the adopted statisticaldistributions. The algorithms for DFN identification and reconstruction have beenimplemented using Python programming language in an in-house specialized softwareDFraM. This work is a result of Radioactive Waste Repository Authority, Czech Republicproject "Research support for safety assessment of a deep geological repository". [ABSTRACT FROM AUTHOR]