TS13.3

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.

How to cite: Leprêtre, R., Chopin, F., El Houicha, M., Tabaud, A. S., Schulmann, K., Barbarand, J., Míková, J., and Chebli, R.: U-Pb geochronology of granitoids from northern Morocco: a two-step tale for the evolution of the Variscan domain in NW Africa, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12505, https://doi.org/10.5194/egusphere-egu22-12505, 2022.

The crystalline basement of the Sardinia block is made up of an almost complete segment of the Variscan belt. Along a SW-NE transect of roughly 200 km, it is possible to observe the structure of the chain from the shallowest to the deepest domains, starting from an anchimetamorphic external zone in the SW of the Island, to a green-schist facies nappe zone in the center and to a medium to high-grade metamorphic inner zone in northern Sardinia. The exceptional exposure of the chain in Sardinia makes it an essential piece for the reconstruction of the pre-Variscan geodynamics and the Paleozoic terranes puzzle.

In several reconstruction of the pre-Variscan paleogeography, Sardinia is considered a whole single block that experienced, since Cambrian times, several geodynamic settings as part of the northern Gondwana margin, before being involved in the Variscan Orogeny during lower Carboniferous. As stated by previous Authors, the most relevant pre-Variscan geodynamic events recorded in Sardinia occurred during the Lower-Middle Ordovician, when the Sardinian block was located close to a subduction zone where a volcanic arc developed. According to this interpretation, the external, nappe and inner zones acted as back-arc, arc and fore-arc, respectively, belonging to the same lithospheric block.

The main evidences of Ordovician tectonics and volcanic activity are a folding event that affect only the Cambrian-Lower Ordovician successions and an angular unconformity related to the folds sealed by continental and tidal deposits in the external zone and by calc-alkaline volcanic products in the nappe zone.

The review of the paleontological, stratigraphic, magmatic and structural data highlights significant discrepancies between the external and nappe zones, suggesting that these domains did not share the same geodynamic setting and, possibly, paleogeographic position during the Ordovician, implying they drew close and amalgamated only in Variscan times.

This hypothesis is supported by the different ages of the unconformities, Upper Ordovician in the nappe zone and Middle Ordovician in the external zone, and the extent of the stratigraphic gap, long-lasting in the external zone. Furthermore, the activity of the volcanic arc in the nappe zone is contemporaneous to the continentalization and erosive processes in the external zone, that is totally devoid of magmatism and volcano-sedimentary deposits. The Upper Ordovician succession in the external zone define a rift that evolve to a passive margin, whereas in the nappe zone the onset of a passive margin is marked by a nonconformity above the volcanic arc. Note that also the faunas show remarkable differences between the external and nappe zones. Finally, a different paleogeographic position is suggested by the Hirnantian glaciomarine deposit in the external zone, lacking in the nappe zone.

The recognition that the Sardinian block consisted of two distinct terranes before the Variscan Orogeny, entails alternative correlations and an adjustment of the arrangement of the now scattered Variscan terranes. In particular, the external and nappe zones should be located in different positions in order to fit the proper geodynamic setting in the paleogeographic reconstruction at the snapshot time.

How to cite: Cocco, F., Loi, A., and Funedda, A.: Ordovician geodynamics of the Sardinia block: a key for the reconstruction of the pre-Variscan paleogeography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4984, https://doi.org/10.5194/egusphere-egu22-4984, 2022.

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. <15 Ma younger than the emplacement of the intrusion. This very moderate thickening is followed by WNW convergence, sub-orthogonal to the first one, affecting all the previous structures and continuing probably up to the Cisuralian, similar to neighboring massifs. The first episode coincides with emplacement of magma within Visean intracontinental basins at the western tip of the ongoing opening Palaeo-Tethys Ocean, whereas the monazite dating document for the first time the precocious onset, during Late Serpukhovian–Early Bashkirian of the convergence in the NW African Variscan segment.

How to cite: Chopin, F., Simon, M., Schulmann, K., Štípská, P., El Houicha, M., Tabaud, A.-S., Leprêtre, R., Chebli, R., Bosch, D., and Míková, J.: LP-HT metamorphism in the Jebilet Massif (Moroccan variscan belt): from Paleothetys opening to Pangea formation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9227, https://doi.org/10.5194/egusphere-egu22-9227, 2022.

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 ɛHf_i values indicating reworking of an Archean crust. However, the Paleoproterozoic detritus in the Cambrian arc terrane exhibits mostly positive ɛHf_i 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.

How to cite: Collett, S., Schulmann, K., Mazur, S., Tabaud, A.-S., and Soejono, I.: The Cambrian volcanic arc terrane: a peri-East European Craton assemblage in the heart of the European Variscan Belt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2483, https://doi.org/10.5194/egusphere-egu22-2483, 2022.

The basement of West Siberian plate has mosaic architecture and consists of the Paleozoic fold belts and supposed Precambrian massifs. The Frolov-Krasnoleninsky region is located in the central part of the West Siberian basin (near the town of Khanty-Mansiysk) and occupies the key position in its structure.  The junction of the Uralian fold belt, Kazakhstan superterrane, and Irtysh-Zaysan fold zone is located there. However, the pre-Jurassic tectonic evolution of this area is still poorly constrained. Based on the new representative data on drill cores from deep boreholes and geophysical data, whole-rock geochemistry and U-Pb ages, we reconstructed the main stages of the geological history of this area:

• In the Neoprotertozoic the ancient metamorphic complexes of the Krasnoleninsky and Aprel’sky uplifts were formed. Based on the detrital zircons ages and correlation with similar rocks in the nearby fold belts, we suggest that this stage terminated about 600-540 Ma.
• In the Early Paleozoic volcano-terrigenous rocks were accumulated mainly in the island arc tectonic setting. This stage ended with the emplacement of gabbro-granitic plutons, fold deformations and amalgamation of the Kazakhatan superterrane with the Krasnoleninsky block in the beginning of Silurian (446-440 Ma).
• In the Devonian – Early Carboniferous carbonate and overlying mainly clastic sediments were accumulated. The first manifestation of supra-subduction volcanic activity and granitic intrusions are identified as Late Devonian-Early Carboniferous. At the end of Early – beginning of Middle Carboniferous fold deformations related to the closing of back-arc basin took place.
• In the Middle – Late Carboniferous differentiated volcanic series and coeval granodiorite massifs of the continental arc tectonic setting were formed (310-307 Ma). This stage corresponds to the convergence of continental masses of Siberian and East European platforms and Kazakhstan superterrane and the closing of relic oceanic basins between them.
• The Early-Middle Permian is a time of collision, manifested by large granitic plutons in the Krasnoleninsky uplift and other areas of the West Siberia and Uralian fold belt (290-260 Ma). At this stage, main structural features of the basement of West Siberian plate were formed.
• In the Late Permian – Early Triassic West Siberia was an area of wide-scale rifting, caused by the post-collisional extension and the mantle plume ascent. Thick sequences of the contrasting basaltic and felsic lavas were erupted. Volcanic piles are overlain by clastic coal-bearing sediments of the Triassic and, possibly, Early Jurassic age.

How to cite: Latyshev, A., Panchenko, I., Kulikov, P., Smirnova, M., Fedyukin, I., and Gusev, I.: Pre-Jurassic tectonic evolution of the central part of West Siberian basin: the new data from the Frolov-Krasnoleninsky region, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9378, https://doi.org/10.5194/egusphere-egu22-9378, 2022.

Introduction. The Neplyuevka pluton is situated in the Chelyabinsk region of the Southern Urals, in the western part of the Eastern Uralian megazone. The area of pluton is 20×14 km. The Neplyuevka intrusion is comprised of 4 phases: 1) gabbro and diorites, 2) quartz diorites and granodiorites, 3) adamellites, 4) leukogranites. The granitoids intrude the terrigenous rocks of the Lower Ordovician. The overlying terrigenous deposits of the Upper Visean contain granitic lithoclasts. The isotope Rb-Sr ages of the rocks comprising the Neplyuevka pluton lie in the range of 346-340 Ma. The variations in the isotopic ages are consistent with the order of individual phases: 346 Ma for the 2nd phase, 342 for the 3rd phase and 340 for the 4th phase. So, according to the isotope ages, the evolution of the pluton took at least 6 Ma. The pluton also contains Cisuralian leucogranites (278 Ma).

Materials and methods. We have studied the zircon grains extracted from 4 specimens: 1 – granodiorites of the 2nd phase, 2 and 3 – adamellites from the 3rd phase, 4th – leucogranites of the 4th phase. The specimens themselves have been collected from the exact locations and the same rock types as previous sampling for Rb-Sr isotope dating. We have studied the morphology of the grains and their internal structure using cathodoluminescence imaging. The U-Pb dating was performed at the Russian Geological Research Institute (VSEGEI) using SHRIMP-II.

Results and discussion. The zircon grains from the granodiorites and adamellites are represented by transparent and light-yellow, idiomorphic, bipyramidal-prismatic crystals, 200-600 μm in length. The crystals are characterized by medium intensity of luminosity with apparent medium-contrast coarse and fine oscillatory zonation. By their obtained isotopic ages the zircons can be divided into 2 populations 1) 334-342 Ma and 2) 354-356 Ma. The isotopic ages of the 1st population are close to those obtained by Rb-Sr dating.

The formation of the Neplyuevka complex in the beginning of the Carboniferous signifies an important event in the geodynamic evolution of the Southern Urals – the fast transition from island arc magmatism, which continued during the whole Devonian, to the rift magmatism, which ceased only in the middle of the Visean. The termination of island arc magmatism is usually explained by the slab delamination at the Devonian-Carboniferous boundary, while the initiation of the rifting is attributed to the oblique collision between the Paleo-Ural island arc with the Laurussia in the middle of the Tournaisian. The obtained data allows us to reevaluate the age of the rifting initiation, assigning it to the Devonian-Carboniferous boundary.

Financial support. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009. Interpretation of U-Pb data was carried out within the framework of the government assignment of IGEM RAS.

How to cite: Borisenko, A., Tevelev, A., Sobolev, I., Pravikova, N., Kazansky, A., Koptev, E., Kosheleva, I., and Zak, J.: The first results of U-Pb dating of the granitoids of the Neplyuevka pluton(The Southern Urals), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4058, https://doi.org/10.5194/egusphere-egu22-4058, 2022.

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.

How to cite: Guy, A., Schulmann, K., Soejono, I., Holzrichter, N., Lexa, O., and Munschy, M.: Structures and geodynamics of the Mongolian tract of the Central AsianOrogenic Belt constrained by potential field analyses, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2045, https://doi.org/10.5194/egusphere-egu22-2045, 2022.

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.

How to cite: Štípská, P., Peřestý, V., Soejono, I., Schulmann, K., Kylander Clark, A. R. C., Aguilar, C., Racek, M., Novotná, N., Hanžl, P., and Lexa, O.: PTt history from kyanite-sillimanite migmatites and garnet-staurolite schists from the Bayankhongor area, Mongolia indicates suprasubduction switching from extension to compression during Rodinia assembly, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2390, https://doi.org/10.5194/egusphere-egu22-2390, 2022.

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 (ε_Hf(t) values –24.8 to +16.0) and Late Archean to Neoproterozoic source (ε_Hf(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 ε_Hf(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 ε_Hf(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 (ε_Hf(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.

How to cite: Sukhbaatar, T., Lexa, O., Schulmann, K., Aguilar, C., Štípská, P., Wong, J., Jiang, Y., Míková, J., and Zhao, D.: Paleozoic geodynamics and architecture of the Mongolian Altai Zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4701, https://doi.org/10.5194/egusphere-egu22-4701, 2022.