Displays

The European Variscan belt sharply changes its trend in easternmost Germany and western Poland, where the ENE- to NE-striking structures are replaced by the ESE- to SE-trending ones. The structures of still another, NNE-SSW strike, take the lead, however, along the SE margin of the Bohemian Massif. The Variscan belt seems, thus, to make nearly a U-turn, encircling the Bohemian Massif from the north. This has been explained for almost a century by assuming a 180° oroclinal loop, in which the Rhenohercynian and Saxothuringian tectonostratigraphic zones inarm the core of the Bohemian Massif. According to this classical view, the outermost tectonostratigraphic zone of the Variscan belt, the Rhenohercynian Zone, continues eastward in the deep substratum of the Permian-Mesozoic basin and reappears at the surface along the eastern rim of the Bohemian Massif.

Since the late 1970s an alternative view has gained an increasing attention that postulates a dextral transpressional regime during the final accretion of the Variscan terranes. This transpressional tectonic context is believed to have resulted from sublatitudinal, right-lateral displacements between Gondwana and Laurussia. Near the Carboniferous-Permian boundary, Gondwana decoupled from the newly formed European Variscan belt and proceeded westward, toward the southern edge of the Laurentian segment of Laurussia, owing to the development of the Appalachian subduction system. Concomitantly with the peak of the Alleghanian orogeny during early Permian, the European Variscan belt experienced a crosscut of its major tectonic zones along a set of dextral strike-slip faults.

In this study, we investigate directions and continuity of structural trends in the external zones of the Variscan orogen in Poland and map a foreland extent of Variscan deformations using seismic, gravimetric-magnetic and borehole data. These permit us testing the orocline- vs strike-slip concepts and develop an overall kinematic model for the NE Variscides.

Matched filtering of isostatic gravity, guided by results of spectral analysis, along with other derivatives of gravity and magnetic fields reveal a dominant WNW-ESE-trending pre-Permian structural grain in the external zones of the Variscan belt in Poland. This trend is confirmed by regional distribution of dips in Carboniferous and Devonian strata that were penetrated by boreholes beneath Permian-Mesozoic sediments. Seismic constraints on the position of the Variscan deformation front come from (1) the GRUNDY 2003 seismic experiment, combining wide-angle reflection-refraction measurements with the near-vertical reflection seismics in central Poland and (2) PolandSPAN and POLCRUST-01 deep reflection profiles in SE Poland. The WNW-ESE structural trend in the Variscan foreland is parallel to a set of major strike-slip fault zones in the area that are considered to convey a significant dextral displacement between Laurussia and Gondwana. The revised position of the Variscan deformation front shows a similar, uninterrupted, generally WNW-ESE trend, up to the SE border of Poland, which indicates an initial continuation of the more internal Variscan zones into the area of the present-day Carpathians. The geometry of the Variscan deformation front along with the pattern of the Variscan structural grain are inconsistent with the idea of an oroclinal loop affecting the external, non-metamorphic Variscan belt.

How to cite: Mazur, S., Aleksandrowski, P., Gągała, Ł., Krzywiec, P., Żaba, J., Gaidzik, K., and Sikora, R.: The shape of the Variscan Belt in Central Europe: Strike-slip tectonics versus oroclinal bending, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3867, https://doi.org/10.5194/egusphere-egu2020-3867, 2020.

High- to ultrahigh pressure rocks ((U)HP) from some collisional orogens bear evidences of post collisional heating recorded by a β-shaped pressure–temperature–time (P–T–t) path. The post peak pressure heating segment of the P–T–t path, which can be well developed such as in the Bohemian Massif of the Variscan orogenic belt, occurs after the (U)HP rocks are exhumated from mantle depths to various crustal levels. This process is often explained by geologists as a result of mantle delamination or slab breakoff. Based on a two-dimensional coupled petrological–thermomechanical tectono-magmatic numerical model, we demonstrate that slab rollback during ongoing continental subduction can be considered as a possible mechanism responsible for the effective extraction of (ultra)high pressure metamorphic rocks and their later heating. This slab rollback scenario is further compared numerically with the classical continental collision scenario associated with slab breakoff. The mantle upwelling occurring in the experiments with slab breakoff, which is responsible for the heating of the exhumed crustal material, is not directly related to the slab breakoff but can be caused either by slab bending before slab breakoff or by post-breakoff exhumation of the subducted crust.

How to cite: Sizova, E., Hauzenberger, C., Fritz, H., Faryad, S. W., and Gerya, T.: Late Orogenic Heating: Slab Breakoff or Slab Rollback?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1462, https://doi.org/10.5194/egusphere-egu2020-1462, 2020.

In the easternmost part of the European Variscan collisional belt, the Bohemian Massif, strongly metamorphosed felsic rocks crop out at several locations in a current distance of up to several hundreds of kilometers from the supposed contact of the subducting and overriding plates. These rocks were interpreted to originate from the subducting plate (now the Saxothuringian domain), which means that the orogenic root (the Moldanubian domain) consists of rocks that originate from both upper and lower plate. More specifically, the root domain is composed of (U)HP granulites and orthogneiss, garnet peridotites, eclogites and ultra-potassic plutons that alternate with the less metamorphosed rocks of the upper plate.

Such a process of subduction and emplacement of the subducted crust into the upper plate is called relamination. In order to better constrain the dynamics of relamination, we set up a numerical thermal-mechanical model and compare the modeling results with the data from the Bohemian Massif. The model simulates oceanic and continental subduction and takes into account non-linear visco-plastic rheology, percolation of fluids, melting and melt extraction. For different parameter values, the models show different styles of behavior, namely (i) exhumation of the subducted crust along the plate interface, and (ii) flow of the subducted crust beneath the upper plate and then incorporation into its crust (i.e. relamination).

In the former case, the material records heterogeneous peak metamorphism sampling the conditions along the subduction zone, and cooling during decompression. Similar features are typical for the metamorphic complex in the Saxothuringian domain of the Bohemian Massif.

In the latter case, the typical feature is the development of diapirs that grow from the subducted continental crust, pierce the overlying lithosphere and intrude into the middle crust of the upper plate. We show that growth of such trans-lithospheric diapirs results in a similar rock assemblage as observed in the orogenic root in the Bohemian Massif. The pressure-temperature-time paths obtained in the modeled diapirs mimic those of the Moldanubian granulites. The flow of crustal material through the mantle wedge results into mixing, hydration of the mantle and melting of both materials. Emplacement of the resulting melt into crust can explain formation of the Moldanubian ultra-potassic plutons.

How to cite: Maierová, P., Schulmann, K., Štípská, P., Gerya, T., and Lexa, O.: Trans-lithospheric diapirism documented in the Variscan Bohemian Massif – comparison with numerical models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7355, https://doi.org/10.5194/egusphere-egu2020-7355, 2020.

The granites with I- and S-type affinity in the Variscan segments of the Alpine West-Carpathian edifice belong to the oldest intrusions within the European Variscides. Granites and granodiorites of the West-Carpathian crystalline basement are mostly classified as S-type, whereas tonalities and granodiorites belong to the I-type suite. Both suites probably originated in the volcanic arc setting as product of subduction-related regime in the Galatian superterrane (Broska et al. 2013). The I- and S-type granite bodies were firstly identified in the West-Carpathian Tribeč Core Mountains and the new SHRIMP and CHIME datings recognised their Visean geotectonic overprint. The subduction-related I-type granites show the age span 364-358 Ma followed by the intrusion of the S-type granites dated by SHRIMP on 358 Ma. The bimodal SHRIMP data of a dyke placed within S-type granites show ages 351 Ma and 330 Ma, or primary vs. alteration age. The CHIME age from monazite dating shows 347 Ma because monazite indicate probably early stage of massive granite alteration perhaps during collisional process, younger zircons represents later phase of the event.  CHIME dating of newly formed monazite in greisenised S-type granite gives the age 344 Ma. The granite showing strong greisenization (total degradation of feldspars and formation of quartz - white mica assemblages) is dated by SHRIMP on 355 Ma. The greisenised granite contains abundant tourmaline with high dravitic molecule, Sr-rich apatite and common monazite. Abundant tiny stoichiometrically pure apatite grains in this granite indicate their exsolution from feldspars enriched in phosphorus. The S-type granite dyke from the ridge of the Tribeč Mts gives zircon SHRIMP age 355 Ma and CHIME monazite age 342 Ma. The dating results of the Tribeč granites identified: (1) older Upper Devonian/Lower Mississippian subduction-related I-type tonalites (ca. 364-351 Ma), and (2) S-type granites Middle/Upper Mississippian (Visean) intruding in time span 342-330 Ma reflecting probably of the collisional event in the Variscan orogeny. Dual evolution of the Tribeč Mts. Variscan granitic rocks is partly corroborated by Hf isotopes from the dated zircons with εHf_(t) = +3.5 ~ –2.4 for the older granites, and εHf_(t) = –0.3 ~ –4.9 for the younger ones. The evolution of the I- and S-type granites seems to be rather different from the granite evolution known in the Bohemian Massif and therefore the origin of Variscan hybrid granites from the Western Carpathians we placed on the SW side of Galatian volcanic arc as result of Paleo-Tethys subduction (see Stampfli and Borel, 2002, Stampfli et al. 2013).

Acknowledgments: Support from Slovak Research and Development Agency: APVV SK-KR-18-0008, APVV-14-0278/, APVV-18-0107, and VEGA 2/0075/20 are greatly appreciated.

How to cite: Broska, I., Yi, K., Kohút, M., and Petrík, I.: Visean overprint of the Devonian-Early Carboniferous granites: result of Variscan collisional stage revealed by zircon SHRIMP dating (Tribeč Mts., Western Carpathians), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5160, https://doi.org/10.5194/egusphere-egu2020-5160, 2020.

In the Carpathian–Pannonian region (Pannonian Basin, Hungary and the Apuseni Mts, Romania) several Late Paleozoic magmatic episodes were revealed by zircon U-Pb geochronology. These events were genetically controlled by a post-collisional to extensional tectonic regime and occurred along the European Variscan Orogenic Belt. Detailed geochronological and geochemical information about the products of this magmatism play crucial role in the regional correlation studies which is the main goal of our research.

In the Tisza Mega-unit, including southern Transdanubia and the eastern Pannonian Basin (Hungary) as well as the Apuseni Mts (Romania), Permian felsic (dominantly rhyodacitic-dacitic) ignimbrites are common. In the western–central part of the Apuseni Mts, they are accompanied by basaltic and subordinate andesitic lavas, corresponding to a bimodal volcanic suite. Cogenetic plutonic (granites, diorites, gabbros) and subvolcanic rocks (felsic–intermediate dykes) occur in the SW part of the Apuseni Mts, Highiş massif. Immobile element features (REE patterns and multi-element spider diagrams) are similar for all of the aforementioned rock types, suggesting fractional crystallization from a common or similar source. Zircon U-Pb ages of this cogenetic rock assemblage overlap each other and fall within a ~10 Myr long time-span (269–259 Ma, Guadalupian). In contrast to the previous assumptions, the Permian felsic volcanites in the Tisza Mega-unit are not in connection with the granitoid rocks known in the basement of the eastern Pannonian Basin (e.g., Battonya granite). Based on our new data, the granitoids represent a Variscan (~356 Ma, Mississippian) plutonic body.

The dacitic subvolcanic rocks (dykes) and lavas in the ALCAPA Mega-unit, Central Transdanubia (Hungary) represent an older (~281 Ma, Cisuralian) and geochemically distinct volcanic episode than the magmatism in the Tisza Mega-unit. Associated plutonic rocks, however, are not known in the study area.

Regarding a broader correlation, the zircon U-Pb ages of the studied Permian plutonic and volcanic rocks of the Tisza Mega-unit are significantly younger than the ages of other well-studied parts of the Central European Variscides (e.g., Intra-Sudetic Basin, NE Germany) where much older ages were identified (300–280 Ma). On the other hand, felsic volcanic rocks of the ALCAPA Mega-unit do not differ from the aforementioned parts of the European Variscides in age. Based on whole-rock geochemistry and zircon geochronology, all of the observed Permian magmatic rocks show similarity with the Permian felsic volcanites of the Western Carpathians (Slovakia). Some further assumptions have been raised: (1) felsic volcanic rocks of the Tisza Mega-unit could correlate with similar rocks of the Southern Gemeric (Vozárová et al. 2009) and Silicic Units (Ondrejka et al. 2018) of the ALCAPA Mega-unit, while (2) the studied samples of Central Transdanubia might be in relationship with the felsic volcanites of the Northern Veporic Unit, ALCAPA Mega-unit (Vozárová et al. 2016). This study was financed by NRDIF (K131690).

Ondrejka, M., Li, X.H., Vojtko, R., Putiš, M., Uher, P., Sobocký, T. (2018). Geol Carpath 69(2):187–198.

Vozárová, A., Šmelko, M., Paderin, I. (2009). Geol Carpath 60(6):439–448.

Vozárová, A., Rodionov, N., Vozár, J., Lepekhina, E., Šarinová, K. (2016). Geol Carpath 61:221–237.

How to cite: Szemerédi, M., Lukács, R., Varga, A., Dunkl, I., Seghedi, I., Tatu, M., Pál-Molnár, E., Szepesi, J., and Harangi, S.: Permian magmatism in the Carpathian–Panonnian region (Hungary and Romania): New geochronological and geochemical results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8405, https://doi.org/10.5194/egusphere-egu2020-8405, 2020.

The Paleozoic basement exposed in the External Crystalline Massifs of the Western Alps (ECM) contains numerous relics of Variscan eclogites and high pressure granulites preserved in high grade migmatitic gneisses. These relics are taken to indicate that the ECM underwent an early HP metamorphic stage during the Variscan Orogeny. However, due to the scarcity of recent thermobarometric and geochronological data, the geodynamic significance of this high pressure metamorphism remains unclear. Based on petrological similarities with other eclogite-bearing formations in the European Variscides (especially the “leptyno-amphibolic compex” in the French Variscides), it has been suggested that the high pressure rocks from the ECM mark a mid-Devonian subduction cycle, preceding the main Carboniferous Variscan collisional stage (Fréville et al., 2018; Guillot and Ménot, 2009). This interpretation mostly relies on one mid-Devonian U-Pb zircon age (395±2 Ma) obtained in eclogites from the massif of Belledonne (Paquette et al., 1989), which has been interpreted as the age of eclogitization. However, dating of high pressure granulites in the Argentera Massif (Rubatto et al., 2010) yielded a Carboniferous age (ca. 340 Ma) for the high pressure stage, questioning the previous geodynamical interpretation. We present here the results of a detailed petrological and geochronological investigation of the high grade formation of the Lacs de la Tempête in NE Belledonne, where some of the eclogites dated by Paquette et al. (1989) were sampled. This area exposes mostly high-grade migmatitic metasediments with intercalated lenses of orthogneiss and garnet-bearing amphibolites, preserving locally eclogitic assemblages. Thermobarometric estimations coupling forward pseudosection modelling, Zr in rutile thermometry and garnet growth modelling constrain the minimal P conditions during the high pressure stage at ca. 1.4-1.6 GPa and 700 °C. The early HP assemblage was then strongly overprinted by granulite facies metamorphism at ca. 1.0-1.2 GPa and 750 °C, also recorded in the surrounding metasediments. U-Pb dating of zircon reveals that the eclogites derived from Ordovician protoliths. Zircon overgrowth in the eclogites and the surrounding metasediments constrain the age of HP metamorphism between ca. 350-305 Ma, with no evidence for a Devonian event. Rutile dating in the eclogites supports the late Carboniferous age of metamorphism. The middle-late Carboniferous corresponds to the main period of Variscan nappe stacking in the ECM, following a period of arc magmatism during late Devonian-Tournaisian (ca. 360-350 Ma, Fréville et al., 2018). We therefore suggest that the 350-305 Ma ages recorded in the HP units of the ECM do not correspond to a Devonian subduction, but rather represent the equilibration of orogenic lower crust at HP-MT conditions during the Variscan nappe stacking events, followed by re-equilibration at lower P during late Carboniferous. This evolution presents striking similarities with the high pressure units of the Moldanubian zone in the Bohemian massif (Schulmann et al., 2009). However, deciphering the exact meaning of U-Pb ages in retrogressed eclogites remains a challenge, and further field and petrological investigation is required to produce a consistent history of the Variscan collision in the ECM.

How to cite: Jacob, J., Guillot, S., Rubatto, D., Janots, E., Melleton, J., and Faure, M.: Geodynamic significance of the Variscan eclogites in the External Crystalline Massifs (Western Alps): marker of a subduction or crustal thickening?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7913, https://doi.org/10.5194/egusphere-egu2020-7913, 2020.

In the SE part of the Variscan French Massif Central, the Cévennes area belongs to the para-autochthonous unit of the southern Variscan belt. This area underwent three metamorphic events (Faure et al., 2001).  I) A green schist to low amphibolite facies one (500°C, 4.5Kb Arnaud, 1997) developed in micaschists and quartzites. These rocks were stacked as south-directed nappes during the final stage of the Variscan crustal thickening dated at ca 340 Ma by ⁴⁰Ar/³⁹Ar on biotite (Caron, 1994). This early event was responsible for the flat-lying foliation, the N-S striking stretching lineation, and intrafolial foliation. II) A high temperature event (680°C, 4.5kb Rakib, 1996) dated at ca 325 Ma (⁴⁰Ar/³⁹Ar on two biotites, Najoui et al, 2000) overprinted the early one. On the basis of the mineral assemblages of this event, a NE-ward increase of the T conditions was interpreted as a remote effect of the Velay Dome (Rakib, 1996). III) Finally, the M^t-Lozère and Aigoual-S^t-Guiral-Liron monzogranitic plutons intruded the Cévennes para-autochthonous unit. Monazite and biotite yield U-Pb, and ⁴⁰Ar/³⁹Ar ages at 315-303Ma and 306 Ma , respectively (Brichaud et al. 2008). The pluton emplacement conditions are determined at 695°C, 1.5Kb (Najoui et al, 2000).

We report Raman Spectrometry of Carbonaceous Matter (RSCM) paleotemperature data acquired on more than 100 samples throughout the entire Cévennes area. These show a regional homogeneous thermal distribution with a 535 ± 50 °C mean temperature without any geometric correlation with the nappes structure, nor the granitic intrusions. Moreover, no thermal increase towards the NE can be documented. SW of the Aigoual-S^t-Guiral-Liron massif, our RSCM data document a temperature jumps between the overlying Cévennes micaschists and the underlying epimetamorphic rocks belonging the the Fold-and-Thust belt unit of the French Massif Central.

In order to constrain the age of this regional thermal event, we ⁴⁰Ar/³⁹Ar dated 25 new regionally-distributed syn- and post-folial muscovites by step heating along two N-S cross sections within the Cévennes micaschists series. In areas distant from the plutons, the muscovite yields a ca 325 Ma age interpreted as the one of the HT event recorded by the RSCM measurements. However, young muscovite ages at ca 305Ma are observed around the plutons. We assume that the heat supplied by the plutons reset these muscovites at around 400°C while the organic matter cannot record the contact metamorphic peak lower than the regional one. Moreover, ⁴⁰Ar/³⁹Ar in-situ analyses carried out on 5 mm-sized post folial (but deformed) biotites in the central part of the micaschist series provide ages around 320Ma. The presence of a hidden dome, underneath the Cévennes micaschists, similar to the pre-Velay migmatites exposed in the northern part of the Cévennes area (Faure et al., 2001, Be et al., 2006) is discussed.

How to cite: Montmartin, C., Faure, M., Scaillet, S., and Raimbourg, H.: Coupling RSCM paleothermometry with 40Ar/39Ar analysis in the south Variscan belt (Cévennes, France): new constraints on the late-orogenic metamorphic gradient in an orogen outer domain. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20140, https://doi.org/10.5194/egusphere-egu2020-20140, 2020.

The Greater Caucasus - the complex geological structure of the Caucasus is an integrated part of the Mediterranean (Alpine-Himalayan) collision orogenic belt. The Dizi series is exposed within the Greater Caucasus Southern Slope zone, in the core of Svaneti anticlinorium. It is composed of faunistically dated from the Devonian to the Triassic inclusively thin-striped and crenulated terrigenous deposits, various volcanites and marbles. Despite the well-studied stratigraphy and tectonics of the Dizi series, the issues of metamorphism, unlike the other rocks of the pre-Alpine crystalline basement of the Greater Caucasus are less studied. The rocks of the Dizi series underwent regional metamorphism of the greenschist facies chlorite-sericite sub-facies under a temperature of 300-340°С and pressures of ≈ 2-2.5 kbar. Characteristic mineral assemblages are established on the basis of microprobe analysis of chlorite, K-mica, plagioclase, actinolite, actinolitic hornblende and prehnite. Due to the contact impact of the Bathonian intrusions on the regionally metamorphosed rocks of the Dizi series, various hornfelses, spotted schists and skarns were formed. Composition of minerals of contact-metamorphism - biotite, cordierite, muscovite, plagioclase, cummingtonite, hornblende, chlorite, clinopyroxene, clinozoisite and K-feldspar is determined. According to the results of studies of key mineral assemblages of contact-metamorphosed rocks, three exocontact zones are distinguished, corresponding to the albite-epidote-hornfels, andalusite-biotite-muscovite-chlorite-hornfels and andalusite-biotite-muscovite-hornfels sub-facies conditions. The first zone is marked by the appearance of biotite, muscovite and plagioclase of oligoclase-andesine series in metapelites; hornblende, biotite and clinozoisite in metabasites and amphibole schists; wollastonite and clinozoisite in carbonate-silicate schists. The beginning of the second zone is marked in the appearance of cordierite, corundum in metapelites and of scapolite in metabasites and carbonate-silicate schists. By the disappearance of chlorite in the metapelites, the appearance of cummingtonite in metabasites and garnet in carbonate-silicate schists, a transition to the third zone is established. In the high-temperature part of the last zone, in the metapelites fibrolite is formed. The maximum temperature in the aureole of contact metamorphism is 550⁰С, and the pressure is about 0.5-1 kbar. Due to very low pressure during the re-crystallization of rocks pyralspite garnet is missing in the mineral associations of the Dizi series rocks. Instead of garnet, the association of chlorite-quartz-muscovite appeared. Under the conditions of increasing temperature during the metamorphism a change in the characteristic features of the mineral composition is shown graphically. Based on the accessible data the authors have drawn the contact metamorphism fields on the existing general scheme of facies and subfacies of regional metamorphism.

Acknowledgments: This work was supported by Shota Rustaveli National Science Foundation (SRNSF) [PHDF-19-159, Regional and Contact metamorphism of the Dizi series].

How to cite: Javakhishvili, I., Tsutsunava, T., Shengelia, D., Chichinadze, G., and Beridze, G.: Regional and Contact Metamorphism of the Dizi Series (the Greater Caucasus), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2952, https://doi.org/10.5194/egusphere-egu2020-2952, 2020.

S

Several potassic-alkaline Variscan plutons (330 – 305 Ma) outcrop in Kraishte and Stara planina regions in Bulgaria: Lutskan, Svidnya, 7^th Prestola Monastery, Buhovo-Seslavtsi, potassic syenites west of Shipka and Shipka (from west to east). These magmatic bodies have intermediate to acid compositions and evolve toward peralkaline syenites-granite residual varieties. They present a broad diversity in rock-forming mineralogy reflecting the variations of magma chemistry and conditions of crystallization. Evolution of mafic silicates in the plutons show unique features which enable to discriminate the trend of mineral evolution in each magmatic complex.

Pyroxenes from Svidnya pluton are presented in all facial types. Its compositions cover the entire spectrum from calcic to sodic varieties as pyroxenes evolve from diopside to aegirine. The clinopyroxenes from peralkaline syenite porphyries in Buhovo-Seslavtsi pluton belong to Ca-Na pyroxenes and their compositions vary from Wo₂₅-En₁₃-Fs₁₃-Ac₄₂- to Wo₁₁-En₆-Fs₂-Ac₆₅. Pyroxenes from potassic syenites west of Shipka display limited range and belong to pure diopside, whereas pyroxenes in the peralkaline dykes from Shipka pluton are aegirine-augites. Also, aegirine-augite and aegirine from Svidnya and Buhovo-Seslavtsi are enriched in Ti (TiO₂ up to 6.5 wt. %), while aegirine-augite from Shipka shows high Zr content (ZrO₂ up to 2.9 wt. %), as Ti and Zr enter pyroxene structure via Na(Mg,Fe²⁺)_0.5(Ti, Zr)_0.5Si₂O₆ molecule.

Amphiboles from Lutskan and 7^th Prestola Monastery are low temperature, reflecting their near-solidus stage of crystallization or postmagmatic reequilibration due to the circulation of deuteric fluids. Their composition is winchite - riebeckite, and winchite – barroisite, respectively. In turn, amphiboles from Svidnya complex display a narrow compositional variation from richterites to magnesio-arfvedsonite, and rarely eckermanite. Amphiboles in Buhovo-Seslavtzi complex show broad diversity in their composition as they belong to sodic-calcic and sodic groups. They evolve from ferrobaroisite, ferrowinchite to richterite and potassic-magnesio-arfvedsonite with [A]-site filled by K. Amphiboles from the potassic syenites outcropping west of Shipka are arfvedsonite, characterised with elevated Ti content (up to 4.4 wt. % TiO₂).

Micas from all complexes show limited evolution. In Svidnya, Buhovo-Seslavtsi, Shipka and 7^th Prestola Monastery only biotite is present. Characteristic feature of biotites from Shipka is the elevated fluorine content (up to 5 wt. % F) which coupled with presence of fluorite implies on the F-domination in the fluid phase during the crystallization of the rocks. In Lutskan and in the syenites west of Shipka micas show broad range of variation from phlogopite to biotite.

Acknowledgements: The financial support provided by the NSF (Ministry of Education and Science of Bulgaria) through DH 14/8 project is acknowledged.

How to cite: Dyulgerov, M.: Composition of mafic minerals from peralkaline potassic syenites-granite association from Bulgaria, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3016, https://doi.org/10.5194/egusphere-egu2020-3016, 2020.

The Mt. Papuk area in Croatia is a natural laboratory for studying magmatic and metamorphic processes on exposed igneous and metamorphic rocks that were created during several major orogenic events – pre-Variscan, Variscan and Alpine. Among them, the Variscan orogeny was recognized as the most widespread and the best documented one already in the last century. In recent years research on pre-Variscan and Alpine events led to detailed information on timing and P-T evolution, whereas the Variscan orogeny in the vast area between Bohemian Massif and Mediterranean terranes was just sporadically investigated. The huge gap in Variscan P-T-t data started to be an obstacle for regional paleogeographic reconstructions that can be overcome by studies of the Mt. Papuk area bearing new key informations.

To determine the timing of Variscan event(s), dating with the electron microprobe on monazite and xenotime and the LA-ICP-MS on apatite and zircon was conducted. So far, we extracted a set of geochronological data from four selected type-localities (Šandrovac, Jankovac, Čarugin Kamen, Koturić) with medium- to high-grade gneiss including migmatite in the western part of the Mt. Papuk area using monazite. In addition, a metamorphic P-T-t path was constrained.

The rock specimens show a schistose fabric and a well-preserved mid- to coarse-grained granoblastic texture. Some of them show traces of partial melting. The schistosity is defined by the preferred orientation of elongated feldspar grains, mica (biotite and muscovite)-rich domains and quartz ribbons. K-feldspar and plagioclase are the dominant phases followed by quartz, biotite, white mica and, in some rocks, almandine-rich garnet (65-70% mol.% alm) and staurolite. Zircon, apatite, monazite, rhabdophane, allanite, ilmenite, rutile and titanite are accessory minerals.

Monazite grains are irregular in shape and locally elongated varying in size from ~15-50 μm. They are irregularly distributed within the matrix assemblage enclosed in micas, feldspar, garnet and quartz. Monazite shows a high Ce₂O₃ content (around 28 wt. %). La₂O₃, Nd₂O₃ and ThO₂ contents slightly vary around 13 wt. %, 12 wt. %, and 3.4-5.3 wt. %, respectively. In general, the composition of monazite does not differ significantly among localities with the exception of yttrium. The content of Y₂O₃ is highest (up to 4 wt. %) in monazite from rock samples that show traces of partial melting, revealing a high-T event, and around 2 wt. % in monazite from gneiss.

The weighted average age of 374.1±5.8 Ma (1σ, 95% confidence level, MSWD=0.68, probability of fit=0.993, n=96) fits well with the measurements for each type-locality: 384.5±9.0 Ma (n=15), 373.3±7.6 Ma (n=28), 379.0±10.0 Ma (n=31) and 364.0±24.0 Ma (n=22), respectively. However, probability density histograms reveal discernable groups at 390, 373 and 330 Ma age maxima and point to more than one event during the metamorphic evolution of the Variscan crust. The derived P-T-t path implies a rapid exhumation from a depth of ca. 30 km with a nearly isothermal hairpin-like (“narrow”) clockwise path reaching max. P-T values of 9-9.5 kbar and 610°C with occurrence of melt during exhumation at ~5 kbar and 640°C.

How to cite: Balen, D. and Massonne, H.-J.: Variscan monazite ages and peak metamorphic P-T conditions recorded in gneiss/migmatite from the Pannonian Basin Basement (Mt. Papuk, Croatia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2621, https://doi.org/10.5194/egusphere-egu2020-2621, 2020.

The sources and critical enrichment processes for granite related tin ores are still not well understood. The Erzgebirge represents one of the classical regions for tin mineralization. We investigated the four largest plutons from the Western Erzgebirge (Germany) for the geochemistry of bulk rocks and autocrystic zircons and relate this information to their intrusion ages. The source rocks of the Variscan granites were identified as high-grade metamorphic rocks based on the comparison of Hf-O isotope data on zircons, the abundance of xenocrystic zircon ages as well as Nd and Hf model ages. Among these rocks, restite is the most likely candidate for later Variscan melts.

In contrast to previously published suggestions (Romer and Kroner, 2015; Wolf et al., 2018), we can exclude a substantial role of intense sedimentary weathering as an important control factor for later Sn and W enrichment in granite related ores of the Western Erzgebirge due to the remarkable homogeneous Hf and low O isotopes in granitic zircons that are extremely distinct to all pre-Devonian basement rocks of Saxothuringia. We document a source enrichment from meta-sedimentary rocks (575 Ma) towards metamorphic rocks (340 Ma) were restites from granulite-facies melts are enriched 6–7 times in Sn compared to UCC (upper continental crust). These rocks are also enriched in K, but depleted in Na and Ca, contain abundant muscovite, and are fertile for later melting. Further enrichment of Sn and W occurred during multiple melt production of the older igneous granites (323–318 Ma) leading finally to a general enrichment of Sn (15 times compared to UCC) in the tin granites (315-314 Ma). Multiple melt production did not lead to a very strong enrichment of ore metals in the granites but is probably very important for a general enrichment of Sn and W in the thick granite-rich crust of the Erzgebirge. Efficient leaching by hydrothermal fluids led to a very strong enrichment (up to several orders) of Sn and W in the greisen ore bodies.

References:

Romer, R.L.; Kroner, U. Sediment and weathering control on the distribution of Paleozoic magmatic tin-tungsten mineralization. Mineral. Depos. 2015, 50, 327–338, doi:10.1007/s00126-014-0540-5.

Wolf, M.; Romer, R.L.; Franz, L.; Lopez-Moro, F.J. Tin in granitic melts: The role of melting temperature and protolith composition. Lithos 2018, 310–311, 20–30.

How to cite: Tichomirowa, M., Gerdes, A., Lapp, M., Leonhardt, D., and Whitehouse, M.: The chemical evolution from older (323–318 Ma) towards younger highly evolved tin granites (315–314 Ma)—sources and metal enrichment in Variscan granites of the Western Erzgebirge (Central European Variscides, Germany), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3092, https://doi.org/10.5194/egusphere-egu2020-3092, 2020.

In the Late Carboniferous to Early Permian, post-orogenic processes led to the intrusion of compositionally diverse granitoids and to intense silicic volcanism in Central Europe. In the Lusatian Block, which is situated in the eastern part of the Saxothuringian Zone of the Variscan orogen, the late- to post-Variscan granitoids are subordinate in comparison to the Cadomian basement and late- to post-Variscan volcanic rocks are almost absent. The Lusatian Block is bound towards the NE and the SW by major deep reaching fault zones. Both the granitoid and the volcanic rocks are situated near the boundaries of the block and probably associated with the major NW trending faults of the Elbe Fault Zone (e.g. Hammer et al., 1999, Lisowiec et al., 2014, Oberc-Dziezic et al., 2015). The Elbe Fault Zone is a continental scale zone of crustal weakness that was reactivated with different kinematics at different times (Scheck et al., 2002). 

We acquired new precise CA-ID-TIMS U-Pb zircon ages of the Koenigshain and the Stolpen granites and the volcanics of the Weissig Basin. Our new data show that the Variscan magmatism of the Lusatian Block occurred at two distinct periods, depending on the structures on which they are bound. The age difference between the two groups (12 Myr) is clearly evident in both CA-ID-TIMS and evaporation analyses. Consequently, zircon evaporation data of other granitoid and volcanic rocks that were not dated with CA-ID-TIMS can be assigned to one of the two groups in the Lusatian Block. The new age dating allows comparison of the evolution of the investigated rocks to adjacent Variscan magmatic rocks.

References:

Hammer et al. (1999), Z. geol. Wiss 27, 401-415.

Lisowiec et al. (2014), Acta Geologica Polonica 64 (4), 457-472.

Oberc-Dziezic et al. (2015), Int. J. Earth. Sci. 104, 1139-1166.

Scheck et al. (2002), Tectonophysics 360, 281-299.

How to cite: Käßner, A., Tichomirowa, M., Lapp, M., and Leonhardt, D.: Late- to post-Variscan magmatism in the Lusatian Block occurred during two short episodes: Evidence from zircon dating, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10533, https://doi.org/10.5194/egusphere-egu2020-10533, 2020.

The crystalline basement of the Schladming Nappe, Eastern Alps, is part of the Silvretta-Seckau Nappe system. It consists mainly of ortho- and paragneisses which were intruded by slightly overprinted granites and granodiorites. On top of the basement a sedimentary cover (e.g. Rannach Formation) containing quartzites and meta-conglomerates is usually developed.

In the last decade the Schladming Nappe has not stirred interest as there is no precise geochronological data available and the metagranitoids are assumed to be part of the widespread magmatic intrusions connected to the Variscian orogeny. These general presumptions will be examined by new U/Pb zircon data in order to complete the knowledge of the pre-Alpine and Alpine magmatic and tectonic evolution of the Schladming nappe system. Additionally, major and trace elements geochemistry will provide information on the origin and evolution of the magmatic source.

In order to better define the sedimentary cover sequence a provenance study including dating of detrital zircons is undertaken. By dating these detrital zircons, the minimum deposition ages of the sedimentary precursor rocks as well as information about the paleogeographic positions of these units will be obtained.

How to cite: Haas, I., Kurz, W., Gallhofer, D., and Hauzenberger, C.: New constraints on the pre-Alpine evolution of the Austroalpine basement: A LA-ICP-MS U/Pb zircon study on the Schladming nappe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5654, https://doi.org/10.5194/egusphere-egu2020-5654, 2020.

Orogenic garnet peridotites with associated garnet pyroxenites and eclogites in the (U)HP-(U)HT terranes provide insight into mantle melting and subduction-related metamorphism in collisional orogenic belts. Here we demonstrate that they also represent unique tracers of early subduction processes in the internal part of the European Variscan Belt, where subsequent high-temperature processes affect thermochronometers in crustal rocks. Our study focused on several localities within the Kutná Hora Crystalline Complex (KHCC), a key area for the evolution of the Variscan Bohemian Massif due to its position, evidence for a deep crustal subduction (diamond in granulites) and complete geochronological record.

The mantle rocks show highly variable petrographical and geochemical characteristics reflecting derivation from contrasting mantle sources which have undergone both mantle melting and enrichment due to subduction-related metasomatism.  While the Úhrov lherzolite has trace element and Sr–Nd–Hf composition similar to depleted oceanic asthenospheric mantle, the composition of the Bečváry lherzolite reflects extensive refertilization by basaltic melts associated with Grt±Cpx precipitation. Multiple solid inclusions (MSI) trapped in garnet, dominated by Ti and Fe-Ti oxides (rutile, ilmenite), represent relics of Ti-rich low-degree basaltic partial melt. Minor hornblende/phlogopite and carbonate reflect mantle metasomatism by H₂O±CO₂-bearing fluids. Highly to mildly radiogenic Sr–Nd–Hf–Os isotopic compositions along with negative HFSE anomalies in clinopyroxene indicate only a very small contribution of recycled crustal component. The Doubrava peridotites exhibit marked petrographic variability ranging from harzburgite to composite dunite-wehrlite/olivine-bearing pyroxenite assemblage and contrasting geochemical patterns. This can be best explained by interaction between depleted protolith and SiO₂-undersaturated melt with small proportion of recycled crust (~5 % when subducted oceanic crust is considered). The KHCC eclogites show diverse origins, involving products of high-pressure crystal accumulation from mantle-derived basaltic melts, or a fragment of MORB-like gabbroic cumulate and crustal-derived material both metamorphosed at HT–HP conditions.

The Úhrov peridotite yields Lu–Hf age of 395 ± 23 Ma, interpreted as dating garnet growth based on detailed examination of trace element garnet zoning. By contrast, eclogites yield younger Lu–Hf ages of ~350 and 330 Ma, respectively, representing mixed ages as demonstrated by garnet trace element zoning and a strong granulite-facies overprint.

We propose a refined model for Devonian–Carboniferous evolution of the Bohemian Massif,   with the subduction of the oceanic crust and associated oceanic asthenospheric mantle beneath the Teplá–Barrandian at ~400 Ma related to closure of the Saxothuringian ocean between Gondwana-derived microcontinents. The overlaying lithospheric mantle wedge was refertilized by fluids/melts. Oceanic subduction passed to continental subduction of the Saxothuringian crust (~370–360 Ma?) accompanied by the break-off  of the eclogitized oceanic crust facilitating incorporation of the upwelling asthenospheric mantle into the Moldanubian lithospheric mantle wedge. Subsequent collision and coeval exhumation of mantle and crustal rocks occurred at ~350–330 Ma and might be associated with mixing/mingling of crustal-derived melts and mafic lithologies producing the observed geochemical and geochronological signatures.

How to cite: Kotková, J., Ackerman, L., Čopjaková, R., Sláma, J., Trubač, J., and Dillingerová, V.: Petrogenesis and Lu–Hf dating of (ultra)mafic rocks from the Kutná Hora Crystalline Complex: implications for the Devonian evolution of the Bohemian Massif , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7983, https://doi.org/10.5194/egusphere-egu2020-7983, 2020.

The granulite occurrences from the Moldanubian zone were extensively studied in the last three decades and their metamorphic overprint at high pressures and at UHT conditions are well constrained. However, there are still some discrepancies regarding the prograde PT-path evolution, the genesis of the granulites and the tectonic processes required to produce the proposed PT-paths. Here we present a comprehensive petrological study where we have investigated more than 300 granulite samples from one of the largest occurrences, the Poechlarn-Wieselburg area - Dunkelsteinerwald. Conventional geothermobarometry, garnet zoning pattern, thermodynamic modelling and Zr-in-rutile thermometry on rutile grains enclosed in garnets in felsic and mafic granulites allowed to constrain the prograde as well as the retrograde segments of the PT path. Polycrystalline melt inclusions and high-Ti biotite relics as well as a uniform temperature of approximately 800°C obtained from rutile inclusions (Zr-in-rutile thermometry) in garnet cores disagree with a continuous prograde garnet growth but favour a metastable overstepping of the garnet-in reaction and growth by the peritectic biotite breakdown reaction to garnet and melt within a very narrow PT interval. Subsequent heating to T>1000°C initiated a second stage of garnet growth with a very distinct chemical composition. The preservation of the zoning pattern at these metamorphic conditions clearly document a very short lived process. Diffusion models predict a time span of <5 Ma and cooling rates of 50-60°C/my. Zircon U-Pb ages usually cluster around 340 Ma representing the metamorphic peak. However, in mafic granulites zircon ages from approximately 410 Ma to 340 Ma are obtained indicating either an older formation age for the precursor rock of the mafic granulites or just documenting the occurrence of xenocrysts. We applied a series of coupled petrological–thermomechanical tectono-magmatic numerical model to reproduce our deduced PTt-path that evolved from exhumation of subducted lower crust followed by intense heating at the crust-mantle boundary.

How to cite: Hauzenberger, C., Schantl, P., Sizova, E., Fritz, H., Finger, F., Linner, M., and Müller, T.: Genesis of felsic and mafic HP granulites from the Moldanubian Zone, Lower Austria, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18232, https://doi.org/10.5194/egusphere-egu2020-18232, 2020.

The Moldanubian Zone in Austria is traditionally subdivided into several tectonostratigraphic subunits, which were juxtaposed to their nowadays position during the Variscan orogeny. The Gföhl unit at the highest tectonic position exposes the Moldanubian granulites at the top, underlain by the granitic Gföhl orthogneiss. At its base lies the Raabs unit, a sequence of mafic rocks (amphibolites and sepentinites) accompanied by metasediments. The Drosendorf unit represents a sedimentary sequence mainly consisting of paragneisses, amphibolites and marbles. At the lowest position the Ostrong unit is dominated by low-P paragneisses with local appearances of eclogites.
A comprehensive study along four W–E profiles from the Danube valley (P1) in the south, to the Thaya valley (P4) in the north, revealed a disparate distribution of metamorphic conditions within the Drosendorf and the Gföhl units (Raabs unit and Gföhl orthogneiss). Along P1 several lithologies of the investigated units show similar P–T conditions of 0.8–1.2 GPa and 750–800 °C, followed by a decompression stage to 0.6–0.8 GPa and ~750 °C. Towards the north the temperature within the Drosendorf unit is continuously decreasing to 650–700 °C, at pressure conditions of 0.4–0.8 GPa. P–T conditions for Raabs unit and Gföhl orthogneiss are decreasing as well but are increasing again at P4. At the western end of P4 they reach similar conditions as in P1 (0.6–1.0 GPa and 725–800), but a decrease towards the east can be observed. A slight W–E decreasing trend is also observable in P2 and P3. Th–U–Pb microprobe dating of several metasedimentary and orthogneiss samples resulted in a Carboniferous age (~340 Ma) for metamorphism. At one locality in the south an older monazite generation indicates an incipient collisional metamorphism in the Devonian (~370 Ma).
The observed N–S gradient indicates that the southern parts represent formerly deeper buried lower crustal parts, whereas towards the north middle crustal levels are exposed, which were exhumed in a first stage. In a second stage of exhumation in the northernmost area, the oblique thrusting of lower crustal segment including the Gföhl unit onto the already exhumed lower-middle crustal parts caused the formation of a duplex structure, which is responsible for the present appearance of the area around the Drosendorf window.

How to cite: Sorger, D., Hauzenberger, C. A., Linner, M., Finger, F., and Fritz, H.: Tectonic implications of the metamorphic field gradient in the Austrian Drosendorf and Gföhl units, Moldanubian Zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15679, https://doi.org/10.5194/egusphere-egu2020-15679, 2020.

This work deals with a portion of the Variscan lower to intermediate crust exposed in the Palmi area (SW Calabria, Italy). It mainly consists of amphibole-bearing tonalite and migmatitic paragneiss. The latter shows a peak metamorphic assemblage of biotite, K-feldspar, garnet, sillimanite and cordierite. Gabbros occur as foliated, decimeter-thick layers within the migmatites and as a decametric main body adjacent to the paragneiss. No contacts are exposed between the migmatites and the gabbro body, which is mainly weakly foliated and fine-grained, even though unfoliated, coarse-grained portions rarely occur. The gabbros overall contain plagioclase (An_89-80) frequently developing triple junctions, amphibole, biotite, and accessory zircon + ilmenite ± allanite. Minor quartz is present in the gabbro layers within the paragneiss. Amphibole consists of cummingtonite grading into hornblende on the rims and retains some relic cleavage from a pyroxene predecessor.

Major and trace element mineral data in tandem with U-Pb zircon dating of the gabbro were examined to achieve information about: (i) the chemical effects triggered by the migration of migmatite-related melts into lower mafic crust, and (ii) their relationship with grain size and foliation variation.

U-Pb dating of sector-zoned, magmatic zircon cores from the gabbro body yielded a Carboniferous age of intrusion. Rare thin, homogeneous zircon rims gave Lower Permian ages, which could be related to a thermal event that caused the partial resetting of the U–Pb zircon isotope system and was most likely related to the partial melting of the paragneiss. Mineral geochemistry reveals that the amphibole from the gabbro interlayered with the paragneiss is depleted in Mg#, and enriched in Al and K with respect to the amphiboles from the main body. It also shows a highly evolved REE geochemical signature, thereby suggesting the involvement of a melt with an evolved geochemical signature, rich of Al, Fe, K and incompatible elements. In the main body, amphibole shows decreasing Mg# and increasing K and Al from the coarse- to the fine-grained domains. Amphibole from the fine-grained portions also differs for showing LREE-depleted patterns reflecting crystallization of a LREE-rich phase (i.e., allanite) simultaneously with amphibole. Taken as a whole, parallel patterns and increase of REE and incompatible trace elements contents indicate that the transition from cummingtonite to hornblende did not involve reaction with other minerals or exotic agent, but most likely reflect decrease of temperature conditions associated with the closure of the system.

We propose that anatectic melts from the migmatitic paragneiss migrated and interacted with the gabbro promoting the replacement of precursor mafic minerals (e.g., orthopyroxene) with amphibole (associated with segregation of biotite ± allanite). The migration of the migmatite-related melt governed a geochemical gradient within the gabbros, with the foliated and fine-grained domains recording the strongest modification of the initial compositions. We thus speculate that small grain-size and anisotropy promoted high melt migration, which enabled better interaction with precursor minerals and nucleation of new mineral phases.

How to cite: Renna, M. R., Langone, A., Caggianelli, A., and Prosser, G.: Chemical signature of migmatite-related melts migration in lower mafic crust: mineral geochemistry and zircon dating constraints (Variscan lower crust, SW Calabria, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7029, https://doi.org/10.5194/egusphere-egu2020-7029, 2020.

                The Aiguilles-Rouge Massif (ARM) is one of the Western External Crystallin Massifs (ECM) of the French Alps. Similarly to the other ECMs, the ARM exposes a Variscan basement made of migmatitic ortho- and paragneisses and micaschists that hold metric boudins of retrograded eclogites, amphibolites and serpentinites. Upward, low-grade and weakly metamorphosed Late-Carboniferous terrigenous sediments overly the Variscan basement. Deformation and metamorphism occurred between 330 and 300 Ma. The whole ARM is structured by a main N-S to NE-SW trending and vertical foliation formed in response to a regional dextral transpression. The tectonic significance of the ARM’s high-pressure rocks in the Variscan belt realm as relics of a subduction zone, pieces of crustal root of an orogenic plateau or overpressure phenomenon along a high-strain zone is still highly debated. A question that also remains is how eclogite Pressure–Temperature–time-Deformation history (P–T–t-D path) relates to the metamorphic paths recorded in the surrounding migmatitic rocks. In this contribution we present new structural and microstructural (EBSD data) observations that give us a detailed vision of the partitioning of the crustal scale deformation during Late-Variscan time. Three main deformations, named D1, D2 and D3, have been recognized in the gneissic core of the ARM. D1 is relictual and corresponds to a flat-lying S1 foliation that is only visible in the high grade metasedimentary rocks and preserved in low-D2 strain domains. D1 is associated with a partial melting metamorphic event M1. D2 is characterized by three main orientations of planar fabrics that are oriented in directions N160, N0 and N20. These planar fabrics are interpreted as S2-C2-C2’ related to anastomosed system developed under a bulk dextral transpression. D2 shearing becomes more penetrative toward the NE, where it is associated to local partial melting. D3 corresponds to the development of a flat-lying S3 cleavage together with the folding of vertical D2 foliations. The D3 is linked to a regional vertical shortening, associated to few liquid injections. These partial melting conditions occurring during D1, D2 and D3 deformations may unravel a continuum of these three deformations during a short period of time. Processing of new thermobarometric and LA-ICP-MS U-Pb geochronological data on eclogites, surrounding rocks and migmatites are currently in progress. The new obtained results will be presented in addition to the structural and metamorphic data in order to discuss the P-T-t-D path of the deeply buried metasedimentary rocks, migmatites and preserved eclogites.

How to cite: Vanardois, J., Trap, P., Roger, F., Barou, F., Lanari, P., Marquer, D., Paquette, J.-L., Melleton, J., and Fréville, K.: New deformation, metamorphic and geochronological data on the Aiguilles-Rouges massif (Alpine External Crystallin massifs, France). A reappraisal of the Variscan tectono-metamorphic evolution in the Alpine Western External Crystallin massifs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8143, https://doi.org/10.5194/egusphere-egu2020-8143, 2020.

Post-Variscan early Permian magmatism is widespread in Corsica with mafic dykes emplaced during the extensional tectonic phase which followed the Variscan orogeny. This study focuses on a mafic dyke swarm intruded in the region of Ajaccio (Corsica, France). New U-Pb zircon geochronological data show that these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes have a calc-alkaline affinity, while a few dykes show tholeiitic affinity resembling N-MORB basalts. Calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. We interpret these data as evidence that an enriched mantle source, which was likely formed during Variscan subduction, sourced the calc-alkaline suite, while a depleted mantle component dominates the source of the tholeiitic suite. Notably, coeval Permian mafic intrusive bodies from throughout Corsica and from the Southern, Central and Western Alps display similar ages and geochemical features to the Ajaccio dyke swarm. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt

How to cite: Boscaini, A., Marzoli, A., Davies, J. H. F. L., Chiaradia, M., and Bertrand, H.: Permian post-collisional basic magmatism from Corsica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9633, https://doi.org/10.5194/egusphere-egu2020-9633, 2020.

The Danube River with a length of 2,800 km is the second longest European river after the Volga. As the Danube River crosses multiple sedimentary basins (Vienna, Pannonian, Dacian) its drainage basin covers a variety of geological units of the Alps, Carpathians, Dinarides and Balkans; hence, its tributaries contain a large sedimentary diversity. Detrital zircon (DZ) studies are appropriate for understanding the pattern of orogenic erosion, sediment routing and mixing of different signals during the transport and preservation of the river sediments. This work presents U-Pb geochronology data obtained from modern sediments of seven tributaries in the Lower Danube: Cerna, Topolniţa, Jiu, Olt, Argeş, Ialomiţa and Siret. Additionally, 1 sample was collected from the Danube Delta front.

The studied samples exhibit several main peaks, which are from oldest to newest: (i) Cambro-Ordovician, linked to the backarc basins and island arcs of Peri-Gondwana subduction (600 – 440 Ma); (ii) Lower to Middle Carboniferous from Variscan magmatic and metamorphic rocks (350 – 320 Ma), showing significant values in most analysed samples; iii) Alpine, younger than 100 Ma, most probably related to the Southern Carpathian Late Cretaceous Banatitic arc and to the Neogene volcanism of the Eastern Carpathians and Apuseni Mountains. The obtained ages on the DZ geochronology show downstream mixing, similarly to recent published data focused on the sediment provenance studies (Balintoni et al., 2014; Ducea et al., 2018).

For the Lower Danube western investigated samples, our results show as main source the metamorphic rocks characteristic for the Upper and Lower Danubian tectonic units of the Southern Carpathians (ca. 300 Ma). Some larger tributaries in the eastern (downstream) Lower Danube show temporal disperse peaks on the DZ geochronology, feature probably reflecting successive processes of recycling. Notably, the most representative sources of DZ identified in the samples from easternmost Lower Danube tributaries are the Varistic metamorphites.

The results suggests that the sediments of the western studied tributaries, characterized by small drainage basin, are mainly composed by igneous and metamorphic rocks. The eastern tributaries with larger drainage basins and therefore a much-varied type of rocks show a more complex DZ distribution; probably, only a small amount of DZ grains indicates the “primary” source rock. The sample from the Danube Delta Front yielded a wide DZ distribution, mirroring the huge amount of sedimentary material from various sources belonging to all basins crossed by the Danube.

The financial support for this paper was provided by the Romanian Ministry of Research and Innovation, through the Programme Development of the National System of Research – Institutional Performance, Project of Excellence for Rivers-Deltas-Sea systems No. 8PFE/2018.

References:

Balintoni, I., Balica, C., Ducea, M.N., Hann, H.P. (2014). Peri-Gondwanan terranes in the Romanian Carpathians: A review of their spatial distribution, origin, provenance and evolution. Geoscience Frontiers 5: 395–411.

Ducea, M.N., Giosan, L., Carter, A., Balica, C., Stoica, A.M., Roban, R.D., Balintoni, I., Filip, D., Petrescu, L. (2018). U-Pb detrital zircon geochronology of the Lower Danube and its tributaries; implications for the geology of the Carpathians. Geochemistry, Geophysics, Geosystems, 19(9), 3208-3223.

How to cite: Pojar, I., Capaldi, T. N., Olariu, C., and Melinte - Dobrinescu, M. C.: Detrital zircon geochronology and sedimentary provenance of the Lower Danube River, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5188, https://doi.org/10.5194/egusphere-egu2020-5188, 2020.