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From Cadomian magmatic arc to Rheic ocean closure: The geochronological-geochemical record of nappe protoliths of the Münchberg Massif, NE Bavaria (Germany)

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Abstract

The Münchberg Massif in northeastern Bavaria, Germany is an allochthonous metamorphic nappe complex within the Saxothuringian Zone of the Variscan orogen. From top to bottom it consists of four major units: Hangend-Serie, Liegend-Serie, Randamphibolit-Serie and Prasinit-Phyllit-Serie, which show an inverted metamorphic gradient of eclogite- to amphibolite-facies (top) to greenschist-facies (bottom) and are separated from each other by thrust faults. New geochemical and U-Pb zircon data indicate that the four units host metasedimentary and meta-igneous rocks which were formed at different time and in distinct geotectonic settings during the evolution of the Saxothuringian terrane between 550 and 370 Ma. Mafic and felsic protoliths of the Hangend-Serie result from a bimodal magmatism in an evolved oceanic to continental magmatic arc setting at about 550 Ma. These rocks represent relics of the Cadomian magmatic arc, which formed a cordillera at the northern margin of Gondwana during the Neoproterozoic. The Liegend-Serie hosts slivers of granitic orthogneisses, emplaced during magmatic events at c. 505 and 480 Ma, and Early Palaeozoic paragneisses, with our samples deposited at ≤ 483 Ma. Ortho- and paragneisses were affected by an amphibolite-facies metamorphic overprint at c. 380 Ma. Granite emplacement and sediment deposition can be related to the separation of the Avalonia microterrane from the northern Gondwana margin. Amphibolite protoliths of the Randamphibolit-Serie emplaced at c. 400 Ma. They show MORB to E-MORB signatures, pointing to their formation along an oceanic spreading centre within the Rheic ocean. Mafic igneous rocks in the Prasinit-Phyllit-Serie emplaced at nearly the same time (407–401 Ma), but their calc-alkaline to tholeiitic character rather suggests formation in an intra-oceanic island arc/back arc system. This convergent margin lasted for about 30 Ma until the Late Devonian, as is suggested by a maximum deposition age of 371 Ma of associated phyllites, and by metamorphic Ar-Ar ages of 374–368 Ma. The timing of the different magmatic and sedimentary events in the Münchberg Massif and their plate tectonic settings are similar to those estimated for other Variscan nappe complexes throughout Europe, comprising the French Massif Central and NW Spain. This similarity indicates that the Münchberg Massif forms part of a European-wide suture zone, along which rock units of different origin were assembled in a complex way during the Variscan Orogeny.

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... The Cadomian-Avalonian orogen is the result of the succession of variable geodynamic events occurring along the northern margin of the West African craton (Gondwana). These events include the formation of a continental magmatic arc from ca. 750 to 570 Ma, back-arc basin closure followed by arc-continent collision from ca. 570 to 540 Ma, and major granitoid magmatism at ca. 540 Ma (Linnemann et al. 2008(Linnemann et al. , 2014Koglin et al. 2018). During the Cambro-Ordovician (ca. ...
... During the Cambro-Ordovician (ca. 510-470 Ma), a widespread extension event, leading to the separation of continental micro-blocks (Ibero-Armorica) from the northern Gondwana margin, induced an important magmatism related to the emplacement of various granitoids and volcanic rocks ( Fig. 1) (Ballèvre et al. 2012(Ballèvre et al. , 2013Kroner and Romer 2013;Villaseca et al. 2016;Koglin et al. 2018). Subduction of oceanic then continental domains in the EVB occurred until ca. ...
... Nance and Murphy 1994;Dabard et al. 1996;Ballèvre et al. 2001;Brun et al. Zeh et al. 2001;Linnemann et al. 2008Linnemann et al. , 2014Koglin et al. 2018). This hypothesis is supported by the vertical array in the εHf(t) vs. age diagram (array CA in Fig. 10d), reflecting the interaction of crustal melts formed by reworking of Archean-to-Paleoproterozoic crust of the West African craton (εHft down to − 22), and mantle-derived melts (εHft up to + 13). ...
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Muscovite peraluminous granites (MPGs) form by partial melting of the continental crust and can be related to metalliferous deposits such as tin, tungsten and uranium(U). Metal enrichment in MPGs commonly results from fractional crystallization, but the metal contents of the source play a major role for their fertility. Between ca. 320 and 300 Ma (Late Carboniferous), the French Armorican Variscan belt was intruded by numerous U-fertile MPGs that contain inherited zircon grains with a wide range of ages from Archean-to-Carboniferous. U-Pb and Hf isotopic data of zircon grains from Brioverian-to-Carboniferous sediments, Cambrian-to-Early Carboniferous granitoids and Late Carboniferous MPGs indicate that the crust of the Armorican Massif is made up by detritus mainly derived from the West African craton (3500-1600 Ma; TDM = 3.8-2.3 Ga), Grenvillian belt (1200-900 Ma; TDM = 2.7-1.2 Ga) and Avalonian-Cadomian belt (800-550 Ma; TDM = 2.5-0.8 Ga), and that the crust was affected by magmatic events at 510-470 Ma (TDM = 1.6-0.6 Ga), 410-330 Ma (TDM = 1.6-1 Ga) and 320-300 Ma. Furthermore, they reveal that the Late Carboniferous MPGs were mainly formed by partial melting of Brioverian sediments with Cambro-Ordovician and Devonian-Carboniferous granitoids, which are all genetically linked with each other and characterized by Th/U < 4. The new data suggest that the U-fertile MPGs result from multiple reworking of U-rich Brioverian sediments, deposited ca. 550 Ma ago on the northern margin of Gondwana, and partially molten during several Paleozoic events, causing a successive increase in U content in the middle-upper crust.
... The structural uppermost Allochthonous Unit of the Münchberg complex was affected by HP-LT metamorphism at 390 Ma and was thrusted over the Saxo-Thuringian unit at 340 Ma (Kreuzer et al., 1989;Klemd, 2010;Koglin et al., 2018; Table 1). The uppermost allochthonous unit represents an inverted metamorphic gradient of four contrasting nappes or series (from top to bottom): (i) the structurally uppermost "Hangend-Serie" comprises hornblende-bearing gneiss, amphibolite, leucocratic gneiss, and eclogite lenses that represent ca. ...
... The uppermost allochthonous unit represents an inverted metamorphic gradient of four contrasting nappes or series (from top to bottom): (i) the structurally uppermost "Hangend-Serie" comprises hornblende-bearing gneiss, amphibolite, leucocratic gneiss, and eclogite lenses that represent ca. 550-480 Ma bimodal magmatic protoliths (Koglin et al., 2018). The (ii) "Liegend-Serie" comprises muscovite-and biotite-bearing paragneiss and orthogneiss. ...
... The (ii) "Liegend-Serie" comprises muscovite-and biotite-bearing paragneiss and orthogneiss. The protoliths are Neoproterozoic-Lower Ordovician Gondwana shelf sedimentary rocks sourced from the Cadomian magmatic arc that were intruded by Cambrian felsic magmatic rocks (Koglin et al., 2018). The (iii) "Randamphibolit Serie" consists of tholeiitic amphibolite, marble, calcsilicate rock, and meta-tuff. ...
... The Münchberg nappe pile shows inverted metamorphic zonation ( Fig. 1): from bottom to top, it comprises Paleozoic sedimentary rocks of the Bavarian lithofacies (anchimetamorphic), the Prasinit-Phyllit-Serie (greenschist facies), Randamphibolit-Serie (amphibolite facies), Liegendserie (amphibolite facies), and Hangendserie (Behr et al., 1982;Klemd, 2010). A large part of the Hangendserie consists of amphibolites and felsic orthogneisses that are thought to represent bimodal magmatism at ca. 550 Ma within a Cadomian arc terrane (Koglin et al., 2018). The numerous eclogite occurrences are located in the lower part of the Hangendserie and in the Hangendserie-Liegendserie boundary, and viewed either as parts of the Hangendserie (e.g., Matthes et al., 1974) or as tectonic bodies (e.g., Stettner, 1960). ...
... The Liegendserie contains metasediments (deposition age ≤ 483 Ma; Koglin et al., 2018) and orthogneisses (ca. 505 and 480 Ma old protoliths; Koglin et al., 2018), but also metagabbros with largely preserved igneous minerals that are partly replaced by an amphibolite facies paragenesis along grain boundaries (Matthes and Seidel, 1977;Bosbach et al., 1991). ...
... The Liegendserie contains metasediments (deposition age ≤ 483 Ma; Koglin et al., 2018) and orthogneisses (ca. 505 and 480 Ma old protoliths; Koglin et al., 2018), but also metagabbros with largely preserved igneous minerals that are partly replaced by an amphibolite facies paragenesis along grain boundaries (Matthes and Seidel, 1977;Bosbach et al., 1991). It has been debated whether the metagabbros are genetically related to the eclogites, but were subducted to different depths and therefore underwent a different metamorphic evolution. ...
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Eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif record a complex succession of igneous, hydrothermal and metamorphic processes. The geodynamic setting related to the protolith formation and the impact of different types of fluid-rock interactions have been uncertain up to now. We use major and trace element chemistry as well as oxygen isotopes to disentangle the geochemical signatures related to the different stages of the rocks' history. In the Münchberg Massif, dark eclogites (kyanite-free; Fe-Ti-MORB signature) are distinguished from light eclogites (kyanite-bearing; higher Mg#, Al2O3, and Cr; lower incompatible element contents; positive Eu anomalies; MORB to arc basalt signature). The δ18O values for both types (+5.0 to +10.8‰) are equal to, or higher than those of MORB. Amphibolite facies metagabbros have a more enriched, almost OIB-like trace element signature and high δ18O values (+9.4 to +10.3‰). Good linear correlations between fluid-immobile elements throughout the eclogite types confirm their derivation from a common, N-MORB to E-MORB-like parental magma. We interpret the light eclogites as former plagioclase-rich cumulates and the dark eclogites as their complementary differentiates. This relationship is partly obscured by variable degrees of magma contamination by sediments, which also affected the metagabbros. However, the metagabbros originated from a more enriched mantle source than the eclogites. Following intrusion, the eclogites were subjected to hydrothermal alteration under the influence of seawater, as indicated by positive correlations between Li, B, Sb, and δ18O. Metamorphic fluid-rock interactions appear to be mostly of limited extent, probably due to the lack of lawsonite dehydration as a fluid source. Nevertheless, the contents at least of some fluid-mobile elements, such as LILE, Li, and Pb, were probably modified during the subduction-exhumation cycle of the eclogites. The crustal contamination of the protolith magmas argues against derivation of the eclogites and metagabbros from typical oceanic crust. Instead, a rift-drift transition setting related to the opening of the Rheic or Saxothuringian Ocean seems most likely. The eclogites and metagabbros, alongside with similar rocks in the Mariánské Lázně complex and other resembling high-pressure massifs, may record different stages of this rift-drift transition.
... The structural uppermost Allochthonous Unit of the Münchberg complex was affected by HP-LT metamorphism at 390 Ma and was thrusted over the Saxo-Thuringian unit at 340 Ma (Kreuzer et al., 1989;Klemd, 2010;Koglin et al., 2018; Table 1). The uppermost allochthonous unit represents an inverted metamorphic gradient of four contrasting nappes or series (from top to bottom): (i) the structurally uppermost "Hangend-Serie" comprises hornblende-bearing gneiss, amphibolite, leucocratic gneiss, and eclogite lenses that represent ca. ...
... The uppermost allochthonous unit represents an inverted metamorphic gradient of four contrasting nappes or series (from top to bottom): (i) the structurally uppermost "Hangend-Serie" comprises hornblende-bearing gneiss, amphibolite, leucocratic gneiss, and eclogite lenses that represent ca. 550-480 Ma bimodal magmatic protoliths (Koglin et al., 2018). The (ii) "Liegend-Serie" comprises muscovite-and biotite-bearing paragneiss and orthogneiss. ...
... The (ii) "Liegend-Serie" comprises muscovite-and biotite-bearing paragneiss and orthogneiss. The protoliths are Neoproterozoic-Lower Ordovician Gondwana shelf sedimentary rocks sourced from the Cadomian magmatic arc that were intruded by Cambrian felsic magmatic rocks (Koglin et al., 2018). The (iii) "Randamphibolit Serie" consists of tholeiitic amphibolite, marble, calcsilicate rock, and meta-tuff. ...
Article
Global tectonic and climatic models for the Permian-Triassic boundary (PTB) are highly debated. One of the most disputed topics is the temperature increase associated with CO2 emissions generated by the Siberian Trap volcanism and its potential influence on chemical weathering and associated variations in sediment fluxes. By integrating crustal architecture, plate modelling, structural kinematics, and two climatic models, we reconstruct the drainage evolution of Variscan tectonostratigraphic units from the SE portion of the Germanic Basin, from which we also extract the parameters necessary to calculate sediment flux across a time-scale of 28 Ma (Guadalupian-Lower Triassic). We reconstruct the sedimentary response to climatic and tectonic perturbations using Quantitative Provenance Analysis (QPA) and integrate compositional data into a sedimentological framework, paleodrainage and paleoclimatic models. Raman heavy mineral analysis, as well as geochronology and geochemistry of detrital apatite, zircon, and rutile, document variation in drainage lithologies and sediment flux which are controlled by regional extensional tectonics and increasingly humid conditions at the PTB. The sedimentary successions of the SE Germanic Basin record climatic perturbations on a 104 years timescale, while the effects of tectonics are visible on a 106 years timescale. The interplay of climate, tectonics and lithology, and their effects on sediment production and drainage evolution resulted in changes in sediment flux from 2.3 Mt./yr during the Guadalupian (Capitanian), to 3.80 Mt./yr in the Lopingian (Changhsingian) to 7.44 Mt./yr at the end of the Lower Triassic (Olenekian). The multifaceted workflow provided in this study represents the first step towards more precise reconstructions of sediment routing systems in deep-time and provides the first ground-truthed quantification of sediment flux across the Permian-Triassic Boundary.
... Our results are compared with the existing data from NW Iberia and the Münchberg Massif, which shed light on the tectonic affinity and broader context of the GSM within the northern peri-Gondwana realm (e.g. Fuenlabrada et al., 2020;Koglin et al., 2018). Finally, we propose a new geodynamic model for the Eo-Variscan HP/HT event and emplacement of Devonian ophiolites in the western Sudetes. ...
... Hajná et al., 2018;Hajná et al., 2017) and in the Cabo Ortegal and Órdenes Complexes in the NW Iberia (Albert et al., 2015;Díaz Garcia et al., 2010). These sedimentary sequences were recently considered to be part of the Variscan Upper Allochthon Martínez Catalán et al., 2020) and are mainly characterized by immature turbiditic or flysch-like pelite and graywacke protoliths (Díaz García et al., 2010;Fuenlabrada et al., 2010;Koglin et al., 2018 -Suárez et al., 2007;Ordóñez Casado et al., 2001;Scherer et al., 2002). In the nappe pile of the NW Iberia, this "upper allochthonous unit" occurs above an ophiolite dated at c. 400 Ma and interpreted as a "middle allochthonous unit" (Martínez Catalán et al., 2020). ...
... The sedimentary basins located on the northern Gondwana margin would have been filled with different proportions of old crustal and arc detritus, depending on source availability during deposition. The common flysch-like graywacke sequences and high abundance of Ediacaran to Cambrian populations close to the MDAs supplemented by more or less abundant Paleoproterozoic to Archean populations of the GSM, the Münchberg Massif (Koglin et al., 2018) and NW Iberia (Albert et al., 2015;Díaz García et al., 2010;Fernández-Suárez et al., 2014;Fuenlabrada et al., 2020;Fuenlabrada et al., 2010) indicates their position in proximity to a plate margin, i.e. close or inside the arc complexes terrane (Figs. 15 and 16a), such as intra-arc or back-arc basins, which have been filled by uplifted nearby sources (Cawood et al., 2012). ...
Article
The Góry Sowie Massif (GSM) in Poland represents a Devonian high-pressure-ultrahigh-pressure (HP-UHP) ter-rane dominated by paragneiss, with subordinate orthogneiss, metabasite and felsic granulite. Whole-rock geo-chemistry of four migmatitic paragneiss and five granulite samples from the northern part of the GSM was combined with U-Pb-Lu-Hf isotopic data on zircon in order to constrain the sedimentary provenance, tectonic setting and paleogeographic location of the sedimentary paleo-basins now forming the GSM. Granulite and paragneiss protoliths are geochemically analogous to flysch-like graywacke and point either to a continental arc or an active continental margin setting dominated by Cambrian felsic arc detritus. Detrital zircon age spectra record a dominant Ediacaran to Cambrian population (493-600 Ma) with rare clusters of Palaeoproterozoic to Neoarchean age (1900-2100 Ma and 2400-2700 Ma). These reflect a peri-Gondwana provenance in the vicinity of the Trans-Saharan belt and place firm constraints on the age of sedimentary protolith of paragneisses at the middle to late Cambrian. Except one sample, the felsic granulites show only late Cambrian depositional ages. The zircon overgrowth rims give two age clusters, one at c. 397-402 Ma that possibly records a HP metamorphic event, and the other at c. 379-393 Ma that probably records the timing of a high-temperature event including migmatization during exhumation. This tectono-thermal event was coeval with emplacement of Devonian ophiolitic rocks that surround the GSM. The GSM rock association, degree and timing of metamorphism and co-eval ophiolite emplacement resemble closely the Galicia-Trás-os-Montes middle and upper allochthonous units in NW Iberia and in the Münchberg Massif in Germany. We suggest that all these massifs were in close spatial proximity before the Devonian and may form part of a single, now tectonically dismembered, terrane or archipelago , which was located along the northern periphery of the Gondwana margin during the Cambro-Ordovician.
... Comparable nappes have been detected in the western extension of the MGCZ (Armorican Massif; Faure et al. 2010) and in the nappe pile of the Münchberg Massif (Fig.1, insert map: MüMa). Cadomian basement with calc-alkaline intrusions at 550 Ma of the "Hangendserie" of the Münchberg Massif (Koglin et al. 2018) could be compared with the 540 Ma metagranites (Dörr & Stein 2019) of the Cadomian basement of the West Odenwald. The Early Palaeozoic basement of the Münchberg Massif, with Ordovician intrusions and metasediments of the "Liegendserie" and "Prasinit-Phyllit-Serie" (Koglin et al. 2018) is probably comparable with the Early Palaeozoic basement of the core of the East Odenwald (Unit IV). ...
... Cadomian basement with calc-alkaline intrusions at 550 Ma of the "Hangendserie" of the Münchberg Massif (Koglin et al. 2018) could be compared with the 540 Ma metagranites (Dörr & Stein 2019) of the Cadomian basement of the West Odenwald. The Early Palaeozoic basement of the Münchberg Massif, with Ordovician intrusions and metasediments of the "Liegendserie" and "Prasinit-Phyllit-Serie" (Koglin et al. 2018) is probably comparable with the Early Palaeozoic basement of the core of the East Odenwald (Unit IV). A Devonian metamorphic event around 375 Ma is also present in the Münchberg Massif Kreuzer et al.1989, Koglin et al. 2018 and references therein). ...
... The Early Palaeozoic basement of the Münchberg Massif, with Ordovician intrusions and metasediments of the "Liegendserie" and "Prasinit-Phyllit-Serie" (Koglin et al. 2018) is probably comparable with the Early Palaeozoic basement of the core of the East Odenwald (Unit IV). A Devonian metamorphic event around 375 Ma is also present in the Münchberg Massif Kreuzer et al.1989, Koglin et al. 2018 and references therein). Frasnian U-Pb ages on zircon (Th/U <0.1) and titanite from eight ortho-and paragneisses ( Fig. 15a: red numbers) date a highgrade metamorphic event at around 375 Ma (Dörr & Stein 2019;Kirchner & Albert 2020;Lippolt 1986;Todt et al. 1995;Dörr et al. 2021). ...
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Coeval subduction and collision at the end of the Variscan Orogeny (Odenwald, Mid-German Crystalline Zone, Germany) Stein, E., Dörr, W., Helm, J., Schastok, J. & Velledits, F. (2022): Coeval subduction and collision at the end of the Variscan Orogeny (Odenwald, Mid-German Crystalline Zone, Germany). – Z. Dt. Ges. Geowiss., 173: 211–236, Stuttgart. Abstract: The Mid-German Crystalline Zone (MGCZ) is an integral part of the European Variscides. It is a heterogeneous tectono-metamorphic terrane between Armorica (Gondwana) and the Old Red Continent (Laurussia). At the Devonian�Carboniferous boundary the Frankenstein Gabbro (Unit I) from the northern Odenwald (MGCZ; with low 87Sr/86Sri value of 0.7035, positive εNd value of +3.8) intruded at 362 ±9 Ma at shallow level (4 km depth) into the Upper Devonian (375 ±5 Ma, Frasnian) olivine-gabbro and wehrlite of an island arc or into thin continental crust. Ar-Ar plagioclase cooling ages at 359 ±3 Ma prove an exhumation of the gabbro to near-surface level by the end of the Devonian (Kirsch et al. 1988). The calc-alkaline gabbro, diorite, and granodiorite to tonalite of the central part of the West Odenwald (Unit II) with low values of 7Sr/86Sri (0.7047) and positive εNd values (+0.8 to -0.3) form part of an Early Carboniferous (354 ±4 Ma to 337 ±2 Ma) subduction-related complex of plutonic rocks. Granodiorite and quartz monzonite to granite (346 ±1 Ma to 337 ±2 Ma) of a high-K calc-alkaline to shoshonitic series with higher values of 87Sr/86Sri (0.7076 to 0.708) and lower values of εNd (-2 to -4) of the southern part of the West Odenwald (Unit III) are coeval with the subduction-related plutons of Unit II and could be explained by a higher amount of crustal contamination of the melt during continent collision. The southern part of the Odenwald collided first with Laurussia. During the coeval subduction and collision the tectonic units of the Odenwald occupied different positions in the NW active continental plate margin of Armorica. The final tectonic contact between the units of the West Odenwald occurred at 323 Ma (cooling below 300 °C) at the end of the Variscan collision but before the Upper Carboniferous (Bashkirian, 320 Ma) extension formed the coal-bearing Saar-Nahe Basin which rests on top of the Mid-German Crystalline Zone. In comparison to other plutonic complexes of the Variscan chain most of the Carboniferous plutons of the West Oden�wald contain a high amount of inherited zircons. These were incorporated during the magma generation from a lower crust which is similar to the Silurian to Lower Devonian gneiss of the East Odenwald (Unit IV, Böllstein Odenwald) or an equivalent of it. Since the Lower Tournaisian, a Silurian to Emsian basement has formed the lower crust of the West Oden�wald
... This suggests that while a single parental magma may be valid for the Münchberg Massif eclogite, the MLC eclogite is better explained by the aforementioned bimodal setting. In fact, an equivalent of the supra-subduction eclogite in the MLC may be present in the Münchberg Massif in the form of the more basic rocks of the Hangend Series at the top of the nappe pile (Koglin et al., 2018;Fig. 9). ...
... Correlation between the Variscan metamorphic evolution of these allochthonous complexes with the MLC and other Mid-Devonian metamorphic complexes of the Bohemian Massif is firmly established (e.g., Martínez Catalán et al., 2020Catalán et al., , 2021. Within the Bohemian complexes, both Koglin et al. (2018) and Tabaud et al. (2021) present evidence for active margin sedimentation until the late Cambrian. ...
... Rifting, associated with the opening of the Iapetus Ocean, transported Baltica to equatorial latitudes (Fig. 11b). However, during Fig. 9. Comparison of geochemical data from MLC and Münchberg Massif (Koglin et al., 2018;Pohlner et al., 2021). While eclogite from both complexes appear to have both mantle-derived and supra-subduction members in the Nb/Yb vs Th/Yb classification diagram (Pearce, 2008); the supra-subduction-type (Nb/Nb* <1) from the Münchberg Massif are characterised by increasing Eu anomaly (Eu/Eu*) and decreasing total REE contents. ...
Article
Mid-Devonian high-pressure (HP) and high-temperature (HT) metamorphism represents an enigmatic early phase in the evolution of the Variscan Orogeny. Within the Bohemian Massif this metamorphism is recorded mostly in allochthonous complexes with uncertain relationship to the major tectonic units. In this regard, the Mariánské Lázně Complex (MLC) is unique in its position at the base of its original upper plate (Teplá-Barrandian Zone). The MLC is composed of diverse, but predominantly mafic, magmatic-metamorphic rocks with late Ediacaran to mid-Devonian protolith ages. Mid-Devonian HP eclogite-facies metamorphism was swiftly followed by a HT granulite-facies overprint contemporaneous with the emplacement of magmatic rocks with apparent supra-subduction affinity. New Hf in zircon isotopic measurements combined with a review of whole-rock isotopic and geochemical data reveals that the magmatic protoliths of the MLC, as well as in the upper plate Teplá-Barrandian Zone, developed above a relatively unaltered Neoproterozoic lithospheric mantle. They remained coupled with this lithospheric mantle throughout a geological timeframe that encompasses separate Ediacaran and Cambrian age arc magmatism, protracted early Paleozoic rifting, and the earliest phases of the Variscan Orogeny. These results are presented in the context of reconstructing the original architecture of the Variscan terranes up to and including the mid-Devonian HP-HT event.
... This is further supported by the presence of numerous >1800 Ma old zircon and a prominent age gap between 1800 and 1000 Ma (Figs. 10l, o), which excludes a Baltican/ East Avalonian source area (e.g., Zeh and Gerdes, 2010). The zircon age spectrum of KK6 from the eastern Odenwald unit IV is similar to that of other Variscan rocks from the Saxothuringian and/or Mid-German Crystalline Zone such as, for example, samples from the Frankenwald (Höhn et al., 2018), Münchberg Massif (Bahlburg et al., 2010;Koglin et al., 2018), central Sudetes (Mazur et al., 2015), and the southern Spessart (Kirchner and Albert, 2020), Ruhla (Brotterode Formation; Gerdes and Zeh, 2006) and Hohnsdorf basement areas (Zieger et al., 2020) (for sample locations see Fig. 2). All these samples were interpreted by the respective authors to represent relics of Cadomian arc rocks that formed initially at the northern margin of Gondwana in the late Ediacaran to early Cambrian. ...
... lower plate Rhenohercynian material with a solely Baltica-derived cover (e.g., Krohe, 1996;Oncken, 1997;Eckelmann et al., 2014;Will et al., 2015) but may indicate the presence of exotic Saxothuringian rocks with Gondwanan (Armorican/West African) affinities (Dörr and Stein, 2019;Kirchner and Albert, 2020). Possibly, there is a genetic link to the Münchberg Massif (Bahlburg et al., 2010;Koglin et al., 2018) in the east and/or the Massif Central and the Armorican Massif to the west. Further provenance studies should help to further clarify this matter. ...
Article
New zircon U-Pb-Hf-O isotope, whole rock geochemical and Sr-Nd-Pb isotope geochemical data of Variscan felsic to intermediate rocks from the Odenwald-Spessart basement, Mid-German Crystalline Zone, Germany are presented. Peraluminous, high-K calc-alkaline S-type granite in the eastern Odenwald basement (Group 1 rocks) formed by partial melting of the lower crust at c. 425 Ma. Their high-K calc-alkaline composition indicates the presence of sedimentary and/or metasomatized lithospheric mantle material during formation of the melts. The protolithic melts of metaluminous, medium- to high-K calc-alkaline I-type granodiorite and diorite of the Spessart and western Odenwald basement as well as the Neustadt (Odenwald) outlier (Group 2 rocks) formed by partial melting of the upper mantle at c. 340 Ma. The melt source was enriched through previously subducted Mesoproterozoic sedimentary material. Eastern Odenwald basement leucocratic gneiss have zircon with ages ranging from 2803 Ma to 336 Ma and main age peaks between 620 and 570 Ma and 360 and 330 Ma. The presence of numerous >1800 Ma old zircon and a prominent age gap between 1800 and 1000 Ma imply a Gondwanan zircon source and indicate the presence of Cadomian material in the Odenwald-Spessart basement. Group 2 diorite is exposed over a distance of at least 60 km from the easternmost Spessart to the westernmost Odenwald and is expected to underlie the eastern Odenwald basement. We suggest that Group 2 diorite formation was related to the presence of a mantle plume, which was also responsible for the widespread Carboniferous magmatism and the associated high-temperature metamorphism in the Odenwald-Spessart basement and other areas of the Variscan orogen.
... Fig. 9j-k) and yield a 206 Pb/ 238 U weighted mean age of 400.8 ± 8 Ma (MSWD = 3.1) (Fig. 6c). While this population overlaps in age with Devonian detrital zircons in the Variscan Saxoturingian Prasinit-Phyllit-Serie (Koglin et al., 2018), the fact that it is only detected in recrystallized rims suggests it is record of minor lead loss from the c. 470-450 Ma zircon age population during the Variscan orogeny (Fig. 6c). ...
... While, the Paleoproterozoic population was likely derived from a West African Craton source south of the peri-Gondwanan Belt and that comprises Archaean to Paleoproterozoic basement (Fig. 10). The U-Pb zircon age spectrum of the Laion gneiss is typical of Neoproterozoic sedimentary sequences throughout the Saxothuringian Zone (Koglin et al., 2018) and of several Neoproterozoic metasedimentary units of the Moldanubian Zone (Kober et al., 2004), NW Spain (Albert et al., 2015) and Morocco (Abati et al., 2012). ...
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Zircon U–Pb LA-ICPMS ages were obtained from three metasedimentary and two metavolcanic samples from the Monte Cavallino (South Tyrol) and the Cima Vallona (Carnic Alps) tectono metamorphic groups from the eastern South Alpine crystalline basement in NE Italy. These analyses were performed to constrain the maximum depositional ages of the South Alpine domain, and to compare the spatial and temporal provenance variations with those of adjacent terranes. The detrital zircon dataset from the metasedimentary rocks (416 grains) yield populations with age peaks at 2.7–2.9 Ga, 1.8–2.1 Ga, 1.2-0.85 Ga, and 0.65-0.45 Ga, with maximum depositional ages ranging from the latest Neoproterozoic to the Silurian. The metavolcanic zircon dataset (209 grains) documents the presence of a two Ordovician volcanic events in the South Alpine domain. The detrital zircon dataset implies that the clastic units of the South Alpine crystalline basement were (a) deposited on the peri-Gondwanan active continental margin and (b) were originally sourced from the Proterozoic and Paleozoic units of NW Gondwana and hence should no longer be considered as exotic elements. The age spectra of the three metasedimentary units highlight differences between the Ediacaran basement gneiss, the Ordovician greywacke, and the Silurian metaconglomerate, suggesting up-section age variations due to a temporal change in provenance. Collectively, these new detrital zircon U–Pb ages imply that the clastic units within the South Alpine domain recorded sedimentation at c. 550 Ma on the peri-Gondwanan active continental margin, followed by rift related continental and marine sedimentation in a back-arc basin setting until at least the Silurian. The South Alpine domain ultimately rifted off from Gondwana due to back-arc spreading, and subsequently underwent Variscan metamorphism during accretion onto the Laurussia margin, which started at c. 380 Ma and lasted until at least c. 320 Ma.
... The intrusion age of 542 Ma of the metagranites from the Odenwald fits with that of granodiorites of the Saxothuringian Zone (Kröner et al. 2001;Dörr et al. 2002;Zelazniewicz et al. 2004. Slightly (3%) older gneiss (calc-alkaline plutonic rocks) intruded at ca. 552 Ma into a Cadomian basement in the Münchberger nappe of the 1 3 Saxothuringian Zone (Koglin et al. 2018). Subductionrelated plutons with Late Ediacaran ages could also be detected in the Moldanubian Zone yielding U-Pb zircon ages at 549 ± 6 Ma and 555 ± 12 Ma (Teipel 2003). ...
... In the Münchberger nappe, Ar-Ar and K-Ar ages on muscovite and hornblende of 372-390 Ma Söllner et al. 1981) are common. In paragneiss of the Münchberger nappe, Koglin et al. (2018) recently discovered metamorphic zircons with an age at 390 ± 3 Ma. The Cadomian basement of the West Odenwald displays similar Ediacaran and Devonian magmatic and metamorphic events such as the Münchberger nappe and could also be interpreted as a nappe. ...
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Detrital zircon age spectra of Ediacaran paragneiss from the Rheic suture between the Rhenohercynian Zone and Saxothuringian Zone suggest that they originated from different parts of peri-Gondwana. The paragneiss from the Northern Phyllite Belt displays an age spectrum of detrital zircons with a high amount of Neoproterozoic (82%) and Mesoproterozoic zircons (11%) typical for Amazonian provenance, whereas the spectrum from the metagreywacke of the Odenwald (Mid-German Crystalline Zone) shows a Mesoproterozoic age gap which is correlated with the West African Craton. The metagreywacke of the Odenwald contains 20% Paleoproterozoic and 32% Archean zircons, whereas the paragneiss of the Northern Phyllite Belt (Wartenstein Crystalline) contains only 6% Paleoproterozoic and no Archean zircons. The paleoposition of the basement of Northern Phyllite Belt was proximal to the Avalonian magmatic arc of the London–Brabant high. The Armorican metagreywacke of the Odenwald occupied a distal position to a Neoproterozoic magmatic arc, probably in a back-arc basin related to the West African Craton. Such a U–Pb age spectrum of detrital zircons together with a Mesoproterozoic age gap is typical for sediments of Armorica in Europe during the Ediacaran to Carboniferous. Neoproterozoic igneous rocks extruded at 566 ± 2 Ma forming a volcano-clastic sequence of the Cadomian magmatic arc which is the wall rock of the Silurian to Carboniferous plutons of the entire West Odenwald. This is the first occurrence of an extensive Cadomian crystalline basement in the Mid-German Crystalline Zone. Metagranite dykes crosscut the foliation of the Cadomian para- and orthogneiss at 542 ± 3 Ma. The deformation and migmatization of the Cadomian basement are bracketed between 566 and 542 Ma. A similar late Cadomian event is known from the Bohemian Massif and the Armorican Massif. Large Cadomian plutons with an age around 540 Ma, like that of the northern Odenwald, are common for Armorica. Silurian to Devonian granitoids (434 ± 4 Ma, 411 ± 5 Ma) are witness to an active margin along the northern boundary of Armorica. The Cadomian basement of the Odenwald together with the Palaeozoic granitoids is overprinted by a high-grade metamorphism at 384 ± 4 Ma (U–Pb on zircon) and cooled down below ca. 500 °C at 370 Ma (K–Ar ages of amphibole; U–Pb on titanite). Such a combination of late Cadomian and early Variscan ages could be correlated with the Münchberger Nappe, the Tepla–Barrandian Unit, Central Armorican Domain and the Massif Central.
... Below, the Liegend Series consists of metasediments, orthogneisses, metagabbros and serpentinites and also includes eclogites. The igneous protoliths have ages of 525-480 Ma (Koglin et al. 2018), the same as those of the Iberian Upper Allochthon, and the eclogites point to equivalence with the high-P/ high-T units, with peak pressure conditions roughly dated in Münchberg at 400 Ma and bearing record of subsequent amphibolite-facies metamorphism at 380 Ma. ...
... Below, the Rand Amphibolite and Phyllite-Prasinite units, formed around 400 Ma (Koglin et al. 2018), and metamorphosed at c. 365 Ma (Kreuzer et al. 1989), are probable equivalents of the Early Devonian ophiolites of the Middle Allochthon, as their features fit those of the Purrido and Moeche units, respectively. The structural succession of the Frankenberg klippe consists of a low-grade volcanosedimentary complex and the gneiss, amphibolite and prasinite units (Rötzler et al. 1999;Klemd 2010). ...
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The NW Iberian Allochthon and the Teplá-Barrandian and Moldanubian zones represent the internal parts of the Variscan belt in their respective domains. A correlation based on the lithological association, protolith ages, metamorphic evolution, detrital zircon age spectra and tectonic setting is attempted between the NW Iberian Massif and SE Bohemian Massif in order to check whether they could have formed part of the same allochthonous stack. The Galicia-Trás-os-Montes Zone of the Iberian Massif and the internal zones of the Bohemian Massif include from bottom to top a Parautochthon and Lower Allochthon representing the outer edge of the northern Gondwana margin, an oceanic Middle Allochthon with Cambro-Ordovician and Early Devonian ophiolites and an Upper Allochthon interpreted as a peri-Gondwanan terrane. Early Variscan, subduction-related high-pressure metamorphism characterizes many of the allochthonous units, with ages younging from the structurally upper to the lower units from 400–385 Ma to 370–360 Ma, respectively. High- and ultrahigh-pressure metamorphism occurred also in the Saxothuringian Autochthon at 360–340 Ma, but not in the NW Iberian Autochthon. The different behavior of the Autochthon in the Iberian and Bohemian massifs accounts for their distinct evolutions from 360 Ma onward. We conclude that the Upper Allochthon was a unique peri-Gondwanan terrane, whereas the Middle Allochthon represents units of the same peri-Gondwanan ocean, opened at the Cambro-Ordovician boundary, and having recorded localized renewed activity in the Silurian–Early Devonian. No other oceans separated the Lower Allochthon, Parautochthon and Autochthon.
... 1: Geologische Karte des Untersuchungsgebietes mit Lage der beprobten Gebiete (Profil Schorgasttal, Granite des Fichtelgebirges, die Probenorte der Vulkanite und Diabase der paläozoischen Schichtenfolge sind als gelbe Dreiecke dargestellt) und korrespondierender Ausschnitt aus der Karte der Erdmagnetfeldanomalien (hochpassgefilterte Karte mit Grenzwellenlänge 40 km) in Nordost Bayern. gendserie, Hangendserie) unterschiedlicher Alter und Protolithe besteht (Koglin et al., 2018). Im Bereich der Münchberger Masse zeigt sich insbesondere am Südwestrand eine ausgeprägte magnetische Anomalie (Abb. ...
... Eine lokale Erhöhung der Werte durch das Auftreten von Magnetit wurde in den feinkörnigen Randamphiboliten registriert. Protolithe der Randamphibolite sind Basalte mit MORB Signatur(Koglin et al., 2018). Das ferrimagnetische Verhalten könnte daher auf primäre Bildung von Fe-Oxiden zurückgeführt werden. ...
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Magnetic field maps display distinct anomalies for NE Bavaria. In this study we test several rock types, such as volcanic intercalations in paleozoic rock units, metamorphic rocks of the Münchberg Massif and granites of the Fichtelgebirge for their possible contribution to the magnetic anomalies. In some of the volcanic rocks (diabase, pillow lavas) high magnetic susceptibilities of up to 50 * 10-3 SI have been recorded. Magnetite has been identified as ferrimagnetic component. Despite an overall overprint during the Variscan orogeny magnitudes of remanent magnetization reach up to 0.9 A/m. Thus these rock types can be considered as cause for localized magnetic anomalies within the Paleozoic metasedimentary units. The tested rock types from the Münchberg Massif show paramagnetic behavior and low magnetic susceptibility, except for some zones with fine-grained, magnetite-bearing amphibolite. A contribution of these units to the distinct magnetic anomalies along the western margin of the exposed basement is therefore unlikely. Granite types of older and younger intrusive complexes of the Fichtelgebirge all show low magnetic susceptibility in the range of 0.3 to 0.04 * 10-3 SI. A major contribution of these granites to magnetic anomalies in the area of the Fichtelgebirge can be ruled out.
... 474-456 Ma, Helbing andTiepolo, 2005) and the Münchberg Massif of the Saxo-Thuringian Zone (ca. 485-478 Ma, Koglin et al., 2018). ...
Article
Alkaline granitic dikes intruding the metasedimentary mantle and orthogneiss cores of the Aston and Hospitalet domes of the Axial Zone of the Pyrenees are subjects of a laser ablation ICP-MS U-Pb zircon geochronology study. The age spectra recorded by detrital, magmatic xenocrystic and inherited zircons reveal a more complex, nearly continuous Paleozoic magmatic history of the Variscan basement of the Pyrenees than previously known. Inherited and detrital zircons of Mesoarchean, Paleoproterozoic to Ediacaran ages attest to the Peri-Gondwana location of the Cambrian sediments that later form the metamorphic core of the Variscan Pyrenees. The youngest magmatic zircon ages fall into the late Carboniferous and earliest Permian, ranging from ca. 306–297 Ma, and represent the emplacement ages of the dikes and small granite intrusions. The age spectra of magmatic xenocrystic zircons contain several maxima, middle (475–465 Ma) and late Ordovician (455–445 Ma), early (415–402 Ma) and late Devonian (385–383 Ma), early (356–351 Ma) and middle Carboniferous (ca. 328 Ma). Middle Ordovician and middle Carboniferous ages are obtained from xenocrystic zircons that were assimilated from the rocks the dikes intruded, the Aston and Hospitalet orthogneisses and the Soulcem granite. The presence of early-mid Carboniferous magmatic zircons in several samples lends further support to a wide-spread early Variscan magmatic activity in the central Pyrenees. The other age peaks do not have equivalent igneous or metaigneous rocks in the central Axial Zone, but are thought to be present in the Pyrenean crust, not exposed and yet to be identified. The diversity of Ordovician, Devonian and Carboniferous up to Permian magmatic ages indicates polyphase emplacement of intrusive bodies during pre-Variscan and Variscan orogenies. The source of the heat for the Devonian to early-mid Carboniferous magmatic activity remains elusive and may involve intracontinental subduction zone, lithospheric-scale shearing or a mantle plume (TUZO).
... During Ediacaran times, to the north of UN1, there is a consensus to consider this was the place of the peri-Gondwanan terranes (e.g. von Raumer et al., 2003;Giacomini et al., 2006;Drost et al., 2011;Garfunkel, 2015) with an extensive Cadomian arc running from Amazonia up to North Africa and beyond (Meert and Torsvik, 2003) that last until 540 Ma with a major sinistral shear belt along the northern boundary of West Africa Fernández-Suárez et al., 2002;Koglin et al., 2018). The consequence of the Cadomian accretion would be to have deport the continental margin to the north, the actual north-Gondwana margin as far as the Phanerozoic is concerned (see also Drost et al., 2011). ...
Article
The Trans-Saharan Belt is one of the most important orogenic systems constitutive of the Pan-African cycle, which, at the end of the Neoproterozoic, led to the formation of the Gondwana Supercontinent. It is marked by the opening and closing of oceanic domains, collision of continental blocks and the deformation of thick synorogenic sedimentary basins. It extends from north to south over a distance of 3000 km in Africa, including the Nigerian Shield and the Tuareg Shield as well as their counterparts beneath the Phanerozoic oil-rich North- and South-Saharan sedimentary basins. In this study, we take advantage of potential field methods (magnetism and gravity) to analyze the crustal-scale structures of the Tuareg Shield terranes and to track these Pan-African structures below the sedimentary basins, offering a new, >1000 km extent. The map interpretations are based on the classical potential field transforms and two-dimensional forward modeling. We have identified geophysical units and first-order bounding lineaments essentially defined owing to magnetic and gravimetric anomaly signatures. In particular, we are able to highlight curved terminations, which in the Trans-Saharan context have been still poorly documented. We provide for the first time a rheological map showing a categorization of contrasted basement units from the south of the Tuareg Shield up to the Atlas Belt. These units highlight the contrasted rheological behavior of the Tuareg tectonostratigraphic terranes during (i) the northerly Pan-African tectonic escape characteristic of the Trans-Saharan Belt and (ii) the North Sahara basin development, especially during intraplate reworking tied to the Variscan event. The discovery of a relatively rigid E-W oriented unit to the south of the Atlas system, and on which the escaping Pan-African terranes were blocked, offers a new perspective on the structural framework of the north-Gondwana margin. It will help to understand how occurred the rendezvous of the N-S oriented Pan-African terranes and the E-W oriented Cadomian peri-Gondwanan terranes.
... During Ediacaran times, to the north of UN1, there is a consensus to consider this was the place of the peri-Gondwanan terranes (e.g. von Raumer et al., 2003;Giacomini et al., 2006;Drost et al., 2011;Garfunkel, 2015) with an extensive Cadomian arc running from Amazonia up to North Africa and beyond (Meert and Torsvik, 2003) that last until 540 Ma with a major sinistral shear belt along the northern boundary of West Africa Fernández-Suárez et al., 2002;Koglin et al., 2018). The consequence of the Cadomian accretion would be to have deport the continental margin to the north, the actual north-Gondwana margin as far as the Phanerozoic is concerned (see also Drost et al., 2011). ...
Thesis
L’analyse des données magnétiques et gravimétriques de la partie nord de la ceinture trans-saharienne a permis de proposer une carte de compartimentalisation géophysique et rhéologique des structures crustales et de visualiser les terranes du bouclier Touareg sur plus de 1000 km au nord, sous les bassins sédimentaires sahariens et plusieurs aspects ont pu être discutés. Un pseudo réseau de quatre générations successives de paléovallées et incisions a été mis en évidence dans la succession glaciaire de l’Ordovicien supérieur au NE du bassin d’Illizi sur la base des données sismiques de haute résolution. Pour chaque incision, la géométrie ainsi que les faciès sismiques de leur remplissage ont été déterminés. Un parallélisme entre la distribution de certaines paléovallées et l'orientation des anomalies magnétiques a été observé, mais aucune relation stratigraphique entre ces structures n'a été identifiée sur les sections sismiques. Un réseau complexe de vallées tunnel glaciaires du Pléistocène en mer du Nord a été identifié sur la base de données aéromagnétiques à haute résolution. Une analyse magnétique détaillée a été réalisée en combinant plusieurs méthodes magnétiques. A la fin, des modèles magnétiques synthétiques 2D ont été calculés pour les incisions ordoviciennes, appliqué pour le cas du bassin d’Illizi. Les résultats obtenus montrent que leur détection magnétique est possible, si toutes fois un levé magnétique à haute résolution serait disponible.
... Zulauf et al. 1997;Dörr et al. 1998Dörr et al. , 2002Venera et al. 2000;Drost et al. 2004) as well as the Moldanubicum (Vrána & Kröner 1995;von Quadt 1997;Friedl et al. 2004;Schulmann et al. 2005) (Fig. 8). Magmatic complexes of similar age are also widespread in the Saxothuringicum, Schwarzburg Anticline and Münchberg Massif (Kemnitz et al. 2002;Linnemann et al. 2007;Höhn et al. 2018;Koglin et al. 2018), Fichtelgebirge and Erzgebirge (Tichomirowa et al. 2001;Košler et al. 2004;Mingram et al. 2004), Krkonoše-Jizera Complex (Oliver et al. 1993;Kröner et al. 2001;Oberc-Dziedzic et al. 2010) and Orlica-Sńiezṅik Dome Turniak et al. 2000;Mazur et al. 2010) (Fig. 8). In contrast, such rocks are not found in the Brunovistulicum, except for a few rare occurrences in the Silesicum (Desná Dome; Kröner et al. 2000) (Fig. 8). ...
Article
The Kouřim Unit represents one of the largest pre-Variscan metaigneous complexes in the Bohemian Massif and a geochronological, whole-rock geochemical and Sr–Nd isotopic study was conducted in order to better understand the magmatic evolution of the Early Palaeozoic Gondwana margin. Five orthogneisses give U–Pb zircon ages ranging from 492 ± 4 Ma to 484 ± 2 Ma. Two leucogranites give U–Pb zircon ages of 500 ± 4 Ma and 485 ± 2 Ma, interpreted to be inherited from their orthogneiss host rock. Two samples from the metasedimentary host rock are dominated by Neoproterozoic–Cambrian detrital zircons. The abundance of zircon inheritance in the orthogneisses and whole-rock Sr–Nd isotopic composition imply an origin from relatively matured continental crustal material. The subalkaline, subaluminous–slightly peraluminous and high-K calc-alkaline arc-like geochemical signature of the orthogneisses is interpreted as inherited from the recycled Cadomian metasedimentary source and both the magmatic and metasedimentary rocks are correlated with similar occurrences in the adjacent Moldanubicum and Teplá–Barrandian Unit. The Late Cambrian–Early Ordovician magmatic activity is linked to crustal anatexis, which was likely initiated by thermal and gravitational relaxation of the thickened Cadomian arc-type crust, followed by lithospheric thinning assisted by far-field forces. The extensional event led to the formation of a passive margin associated with the opening of the Rheic Ocean.
... The Devonian thermal event from 370 to 390 Ma is widespread in crystalline basement of the Central European Variscides. Devonian monazite and zircon U-Pb ages are known from the allochthonous units of the Saxothuringian Zone (Münchberger nappe pile metamorphic grown zircons at 390 ± 3 Ma; Koglin et al. 2018), from the Moldanubian Zone (Teplá-Barrandian Unit and the Mariánské Lázně Complex; 380 to 387 ± 3 Ma, isotope dilution thermal ionization mass spectrometry (ID-TIMS) (Timmermann et al. 2004(Timmermann et al. , 2006) and from the Central Armorican Domain (Armorican Massif, Schulz, 2013). ...
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Abstract 17 High-pressure low-temperature metamorphic sediments of the Phyllite-Quartzite unit sensu stricto and the Talea Ori group are investigated in the field, microstructurally and by U-Pb dating of detrital zircons to shed light on their paleogeographic origin. Zircon age spectra with ages > 450 Ma of the Phyllite-Quartzite unit sensu stricto indicate a paleogeographic origin at the northern margin of East Gondwana. In contrast, the lower stratigraphic, siliciclastic formations of the Talea Ori group show a high number of well-rounded Cambrian to Early Carboniferous aged zircons and a Neoproterozoic zircon age spectrum with East Gondwana affinity. Based on the comparison of zircon age data, a possible distal sediment source is the Sakarya Zone at the southern active margin of Eurasia. To reconcile the zircon data with the geological observations we propose different alternative models, or a combination of these, including sediment transport from the Sakarya Zone and/or a westerly source towards the northern margin of Gondwana as well as terrane-displacement of the Sakarya Zone. Also, a paleogeographic origin of the Talea Ori group at the southern active margin of Eurasia cannot be excluded. This alternative, however, would not be consistent with the usually assumed association of the Talea Ori group to the Plattenkalk unit characterized by a paleogeographic origin at the northern margin of Gondwana.
... The latter hypothesis could be more plausible when we also consider low-grade metasediments such as those outcropping at the margin of the Münchberg metamorphic complex. There, phyllites occur in the prasinite-phylliteseries, a nappe of this complex, showing K-Ar and Ar-Ar ages of metamorphism between 374 and 366 Ma (Koglin et al., 2018). Thus, the low-grade metamorphism was contemporaneous to the HP metamorphism of the micaschists and suggests that the low-grade metasediments were not overridden by thick Avalonian crust in contrast to the Elstergebirge micaschists. ...
Article
A chloritoid-garnet-bearing micaschist from the southern part of the Elstergebirge was studied to better understand the Variscan orogenic evolution in the Saxothuringian zone of the northwestern Bohemian Massif. Based on the textural relations and compositions of minerals, especially of zoned garnet and potassic white mica, a P-T path was reconstructed using contoured P-T pseudosections. The U-Th-Pb dating of monazite in the micaschist was undertaken with the electron microprobe. The micaschist experienced P-T conditions along a clockwise path between 16 kbar at 510 °C and 5 kbar at 555 °C followed by isobaric heating to about 600 °C. Monazite ages range between 315 and 480 Ma with the most prominent maxima and side maxima at 346.0±1.1 (2σ), 357.3±2.3, and 368.3±1.7 Ma. Ages older than 380 Ma were related to detrital monazite pointing to a Devonian sedimentary protolith. Other ages around 325 Ma were assigned to the isobaric heating by nearby post-tectonic granites. The high-pressure event, being the result of the collision of Laurussia and Gondwana after closure of the Rheic Ocean, occurred in the Late Devonian. The exhumation to 15–20 km (5 kbar) ended probably in the Early Carboniferous. The high-pressure micaschists from the Fichtelgebirge to the Erzgebirge crystalline complexes are suggested to represent a single nappe within a metamorphic nappe pile. This nappe is composed of metasedimentary slices, which experienced different peak pressures rather than representing a coherent crustal section.
... The events were geodynamically linked to the Eovariscan orogenic phase due to initial Laurussia-Gondwana collision, also initiated in the pre-KW (hassi-jamieae) interval ( Fig. 5B; fig. 2 in Averbuch et al., 2005). In the context, Koglin et al. (2018) reported the Frasnian (~375 Ma) stacking of the four Bavarian nappe units in accretionary wedge of the Rheic Ocean. ...
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Although the prime causation of the Late Devonian Frasnian–Famennian (F–F) mass extinction remains conjectural, such destructive factors as the spread of anoxia and rapid upheavals in the runaway greenhouse climate are generally accepted in the Earth-bound multicausal scenario. In terms of prime triggers of these global changes, volcanism paroxysm coupled with the Eovariscan tectonism has been suspected for many years. However, the recent discovery of multiple anomalous mercury enrichments at the worldwide scale provides a reliable factual basis for proposing a volcanic–tectonic scenario for the stepwise F–F ecological catastrophe, specifically the Kellwasser (KW) Crisis. A focus is usually on the cataclysmic emplacement of the Viluy large igneous province (LIP) in eastern Siberia. However, the long-lasted effusive outpouring was likely episodically paired with amplified arc magmatism and hydrothermal activity, and the rapid climate oscillations and glacioustatic responses could in fact have been promoted by diverse feedbacks driven by volcanism and tectonics. The anti-greenhouse effect of expanding intertidal–estuarine and riparian woodlands during transient CO2-greenhouse spikes was another key feedback on Late Devonian land. An updated volcanic press-pulse model is proposed with reference to the recent timing of LIPs and arc magmatism and the revised date of 371.9 Ma for the F–F boundary. The global changes were initiated by the pre-KW effusive activity of LIPs, which caused extreme stress in the global ecosystem. Nevertheless, at least two decisive pulses of sill-type intrusions and/or kimberlite/carbonatite eruptions, in addition to flood basalt extrusions on the East European Platform, are thought to have eventually led to the end-Frasnian ecological catastrophe. These stimuli have been enhanced by effective orbital modulation. An attractive option is to apply the scenario to other Late Devonian global events, as evidences in particular by the Hg spikes that coincide with the end-Famennian Hangenberg Crisis.
... This absence is caused by its supposed consumption in intra-oceanic subduction that had developed during the closure of the Variscan ocean(s) (Díaz García et al., 1999;Sánchez Martínez et al., 2007;von Raumer et al., 2013;Arenas et al., 2016) and that might be related to the generation of new supra-subduction-type oceanic lithosphere (Sánchez Martínez et al., 2007). However, alternative explanation suggest that this closure is due to the subduction beneath Avalonia or Laurussia (Fuenlabrada et al., 2020;Koglin et al., 2018). On a regional scale, the Central-Sudetic Ophiolites were supposedly formed above a subduction zone, which involved also the subducted Ordovician passive margin represented by units belonging to the Saxothuringian Zone and the subduction wedge represented by the high grade Góry Sowie Massif. ...
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The Variscan Central-Sudetic Ophiolites (CSO) are located in the Sudetes, the NE segment of the Bohemian Massif. The Devonian CSO display highly depleted, harzburgite mantle sections containing gabbroic dykes and local occurrences of mostly isotropic, large gabbroic bodies as well as volcanic rocks. The ultramafic rocks locally show melt percolation-derived clinopyroxene-olivine aggregates and chromitites. The low REE composition and depletion in LREE relative to HREE of the clinopyroxene as well as the chromite Cr# and Mg# values are typical for phases formed from refractory melts occurring in the supra-subduction zone environment. The gabbroic bodies consist of differently evolved, mostly cumulate rocks, while the volcanic rocks form a relatively monotonous basalt sequence. The trace element compositions of both the plutonic and volcanic rocks display depleted N-MORB affinities, their derivation from a refractory mantle source is further reflected by previously published depleted mantle-like Sr-Nd isotopic compositions. The ultramafic and mafic members of the CSO show greenschist- to lower amphibolite-facies metamorphic overprints. The occurrence of depleted harzburgites and isotropic gabbros forming the mantle and the crustal sections of the CSO, respectively, indicate intermediate- to slow-spreading conditions and a heterogeneous structure of the oceanic lithosphere, untypical for layered ophiolites. The geochemical characteristics of the melt-percolation phases of the ultramafic rocks, the bulk-rock compositions of the mafic rocks and the homogeneous volcanic sequences suggest a formation of the CSO in a mature back-arc basin setting, lacking subduction-derived contamination. Spreading rate conditions and geochemical affinities of the CSO are similar to those of other Devonian ophiolites localized along the Variscan Belt, for instance the Upper Ophiolitic Units in the NW Iberian Massif (Spain), thereby confirming the general absence of N-MORB type lithosphere in the European Variscan Orogen.
... The European Variscan belt is a collisional orogen resulting from the convergence between Gondwana-derived terranes and Baltica during the Upper Palaeozoic (e.g. Franke, 2000;von Raumer et al., 2002;von Raumer et al., 2003;Linnemann et al., 2010;Jastrzębski et al., 2013;Stampfli et al., 2013;Koglin et al., 2018). This convergence led to the closure of a number of oceanic domains (i.e. ...
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Zircon U-Pb SIMS dating combined with in-context (in thin section) monazite and xenotime U + Th-total Pb dating was used to clarify the Palaeozoic evolution of the ‘cold’ Chopok granite (Nízke Tatry Mountains, Slovakia). Four distinct zircon, monazite and xenotime age domains testify to a prolonged evolution from igneous formation to multi-stage metasomatism and hydrothermal overprinting. The geological interpretation of age patterns from ‘cold’ granites, expected to have low zircon saturation temperatures (<800 °C) and relatively high amounts of zircon inheritance, requires special care, especially for what concerns proper attribution of zircon inheritance and igneous growth ages. These issues can be resolved using zircon saturation temperatures (TZrn) as proxy for the amount of zircon inheritance in combination with the temperature differences between TZrn and the granite solidus. In this respect, the Chopok granite is an atypical ‘cold’ granite. Due to TZrn being substantially lower (ca. 80 °C ± 50 °C) than the granite solidus temperature, practically no zircon inheritance was found. The zircon age data indicates that the Chopok granite is a product of an Early Ordovician (475.8 ± 3.3 Ma) magmatic event, corresponding with the widespread Early Palaeozoic magmatism recorded throughout the European Variscan belt. This is further corroborated by the phosphate mineral ages. The post-magmatic activity recorded in the U-Pb systematics of the zircon and phosphates overgrowths can be related to the different phases of the evolution of the Variscan orogen. Early Carboniferous (ca. 352 Ma) metasomatism documents the main Variscan orogenic event, whereas the Permo-Triassic age (ca. 255 Ma) reflects thermo-tectonic activity associated with large-scale crustal extension, contemporaneous with the initial continental leading to the break-up of Pangea.
... 447-405 Ma) (Kebede et al. 2005). However, dominantly felsic peraluminous magmatism and crustal anatexis at ca. 490-450 Ma are well-recorded in the Moldanubian Domain and allochthonous units of the Saxo-Thuringian Domain of the Bohemian Massif (Friedl et al. 2004;Teipel et al. 2004;Sagawe et al. 2016;Koglin et al. 2018;Soejono et al. 2019). Comparable felsic peraluminous magmatism and migmatites yielding ages of ca. ...
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Coupled U–Pb and Lu–Hf LA-ICP-MS detrital and igneous zircon data were obtained from metasedimentary sequences (Kaserer Formation, Schmirntal Quartzite, Seidlwinkel Formation, Bündnerschiefer Basin, Riffler Basin) of the western Tauern Window (Eastern Alps). Results show maximum deposition ages between the Late Permian and the Triassic, indicating protracted sedimentation and magmatism between the Late Paleozoic and the Mesozoic. The Lu–Hf fingerprint shows a change from subchondritic to variable subchondritic to suprachondritic compositions at ca. 290 Ma, possibly documenting the transition from Late Paleozoic Variscan post-collisional processes to intracontinental extension. Lithospheric thinning and magmatic underplating may explain the observed Hf isotopic evolution as the result of mixing of crustal and mantellic sources. From a paleogeographical perspective, results confirm that the Tauern Window was situated between Alpine basement units (South Alpine, Austroalpine and External Massifs) and the Bohemian Massif during the Permian–Triassic.
... The early subduction event was characterized by ca. 400-390 Ma imbrications of HP nappes with intraoceanic arc assemblages well preserved in ophiolitic sequences in Iberia (Sanchéz Martínez et al., 2007), southern Armorica (Cartier and Faure, 2004) and the Münchberg massif (Koglin et al., 2018) (Fig. 1a). In contrast, the later subduction-collisional event is characterized by the development of granulite-migmatite domes, which enclose boudins of ultra-HP and ultra-high-temperature (UHT) mantle fragments, most spectacularly exposed within the Bohemian Massif Additional structural elements are from Chlupáčova et al. (1975), Zulauf (2001) and Peřestý et al. (2017). ...
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Previously unrecognized mafic and felsic plutonic rocks, formerly interpreted as an anatectic part of the amphibolite-eclogite Mariánské Lázně Complex (MLC, Bohemian Massif) were examined together with a possible upper-crustal equivalent represented by the Čistá pluton intruding continental crust farther east. These plutonic rocks were studied by whole-rock geochemistry (major and trace elements, Sr-Nd isotopes) and zircon U-Pb geochronology. The MLC magmatic rocks range from pyroxene-amphibole gabbro to trondhjemite and oligoclasite. The Čistá pluton consists of porphyritic granitoids. The calc-alkaline nature of these rocks, relative enrichment of fluid-mobile elements (including large-ion lithophile elements), strong fractionation of light rare earth elements over heavy rare earth elements and depletion of high field strength elements are evidence for active margin magmatism. U-Pb zircon dating indicates that the magmatism was Mid-Devonian in both the MLC (ca. 385 Ma) and Čistá pluton (ca. 390 up to ca. 365 Ma). Both magmatic units show inherited cores probably linked to recycling of crustal material. Sr-Nd isotopic compositions indicate that the gabbro formed from a depleted MORB mantle source composition with a small contribution of a subduction and crustal component. A three-stage geodynamic model is proposed: 1) ca. 390–380 Ma recycling of a high-pressure accretionary wedge and migration of arc melts into more distal parts of the upper continental plate, 2) ca. 380–370 Ma compression and thickening of the accretionary wedge-arc edifice, and 3) ca. 370–365 Ma development of a giant east-dipping detachment that reworked the whole edifice in the west and also involved emplacement of the supracrustal Čistá pluton along antithetic west-dipping shear zone in the east. The studied magmatic rocks record a Mid-Devonian magmatic arc related to the formation of a subduction wedge stacked beneath the upper plate that was followed by formation of a second and independent Carboniferous continental arc further east.
... In the Münchberger nappe Ar-Ar and K-Ar ages on muscovite and hornblende of 372,390 Ma (Kreuzer and Seidel 1989;Söllner et al. 1981) are common. In paragneiss of the Münchberger nappe Koglin et al. (2018) recently discovered metamorphic zircons with an age at 390 ± 3 Ma (Fig. 2B, MüMa in blue), which fits with the metamorphic zircons of the Cadomian basement of the West Odenwald (Unit II). ...
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U–Pb age spectra of detrital zircons related to the East European Platform could be traced in paragneiss through the whole Mid-German-Crystalline Zone (Variscides, Central Europe) from the Odenwald via the Spessart to the Ruhla crystalline forming an exotic unit between Armorica and Laurussia. The depositional ages of the paragneiss are defined by the youngest age of the detrital zircons and the oldest intrusion ages as Ordovician to Silurian. The Ediacaran dominated age spectrum of detrital zircons from the paragneiss of the East Odenwald suggests the latter to be derived from the shelf of the East European Platform (Baltica), which was influenced by the 1.5 Ga old detritus delivered from a giant intrusion (Mazury granitoid, Poland). The detrital zircon age spectrum of the lower Palaeozoic paragneiss of the East Odenwald and sandstone of the northern Holy Cross Mountains are identical. The pure Sveconorwegian spectrum of the lower Palaeozoic quartzite from the Spessart, (Kirchner and Albert Int J Earth Sci 2020) and the Ruhla (Zeh and Gerdes Gondwana Res 17:254–263, 2010) could be sourced from Bornholm and southern Sweden. A U–Pb age spectrum with 88% Palaeozoic detrital zircons from a volcano-sedimentary rock of the East Odenwald is interpreted to be derived from a Silurian magmatic arc (46%), which was probably generated during the drift of the Mid-German-Crystalline Zone micro-continent to the south. A tentative plate tectonic model of Mid-German-Crystalline Zone is presented taking into account (a) the East European Platform related age spectra of the detrital zircons (b) the Ordovician to Silurian depositional age of the metasediments (c) the Silurian and Early Devonian intrusion age of the plutonic and volcanic rocks and (d) the U–Pb ages of the Middle Devonian high-grade metamorphism. The East European Platform-related part of the Mid-German-Crystalline Zone is interpreted as a micro-continent, which drifted through the Rheic Ocean to the south and collided with the Saxothuringian (Armorican Terrane Assemblage) during the Early Devonian. Such large-scale tectonic transport from the northern continent to the southern continent is also known from the SW Iberia, where Laurussia-related metasediments of the Rheic suture zone are explained by a large scale tectonic escape (Braid et al. J Geol Soc Lond 168:383–392, 2011).
... Our new zircon U-Pb ages (ca. 530 Ma) show that the emplacement ages of S-type metagranite coincided with global Early Cambrian magmatism, which was generated during the Cadomian orogeny belt (e.g., Linnemann et al., 2000Linnemann et al., , 2008Nance et al., 2010;Yılmaz Ş ahin et al., 2014;Moghadam et al., 2015;Beyarslan et al., 2016;Koglin et al., 2018). ...
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The Fe isotope systematics of subducted lithologies are crucial for the understanding of redox-dependent mass transfer in subducting slabs, with consequences for the compositions of arc magmas and of the deep mantle. We investigated eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif to unravel whether their Fe isotope compositions are dominated by the igneous/sedimentary protolith signature, by low-temperature seawater alteration, or by later fluid-rock interactions during the subduction-exhumation cycle. Although the eclogites are thought to be derived from a continental rather than oceanic setting (possibly a rift-drift transition stage), they have mid-ocean ridge basalt (MORB)-like major and trace element compositions. They are often moderately oxidized compared to MORB (Fe³⁺/ΣFe = 0.06 to 0.30). Their δ⁵⁶Fe values (+0.00 to +0.17‰; mean + 0.08 ± 0.01‰) mostly resemble those of MORB (+0.07 to +0.17‰). The metagabbros, which are derived from a more enriched mantle source than the eclogites, yielded heavier δ⁵⁶Fe values (+0.09 to +0.22‰) similar to ocean island basalts, whereas those of the paragneisses (+0.03 to +0.10‰) are typical for pelitic sediments. It appears that the Fe isotope compositions of the igneous protoliths are largely preserved and little if any Fe was mobilized during the diverse fluid-rock interaction stages. The parental magma of the eclogites was probably somewhat isotopically lighter than similarly differentiated MORB magmas, perhaps due to the presence of metasomatized, isotopically light peridotites in the subcontinental lithospheric mantle (SCLM) source. Although it is possible that δ⁵⁶Fe values were slightly modified during seawater alteration and/or metamorphic fluid-rock interactions in some of the eclogites, the impact of fluid-rock interactions on the major element compositions of the eclogites appears to be small. Furthermore, the scarcity of metamorphic veins in the Münchberg Massif argues against significant Fe mobilization in the slab. We suggest that continental eclogites tend to retain their magmatic δ⁵⁶Fe values throughout the subduction-exhumation cycle, whereas δ⁵⁶Fe values of oceanic eclogites may often be dominated by seafloor alteration with potential local modifications in the slab close to fluid channels. The remarkable robustness of the Fe isotope compositions of continental eclogites suggests that they may be used to reconstruct protolith mantle source properties despite the complex post-magmatic history.
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A combined U–Pb zircon geochronological and whole-rock isotopic and geochemical study has been carried out on high-grade orthogneiss, meta-basite, and meta-sediments from the Erzgebirge. The results indicate multiple pulses of Ediacaran–Ordovician magmatism in a transitional volcanic-arc to rift-basin setting. Orthogneiss from high-pressure nappes exhibit a step-like pattern of inherited zircon ages and emplacement ages of 500–475 Ma. In contrast, granite gneiss from the medium-pressure core of the Erzgebirge is characterised by three pulses of magmatism in the Early Cambrian, Late Cambrian, and Early Ordovician. A trend of decreasing Th/U ratios in zircon is observed to c.500 Ma, after which significant increases in the trend and variability of the data is inferred to mark the transition from arc-related to rift-related magmatism. Sediments deposited in the Early Cambrian have continental island arc affinity. Major detrital peaks in the Ediacaran and subordinate Tonian, Palaeoproterozoic, and Neoarchaean data are consistent with an Avalonian-Cadomian Arc and West African Craton derivation. The Early Cambrian sediments were locally reworked by a thermal event in the Ordovician resulting in leucocratic banding and recorded in Ordovician zircon rims characterised by systematically lower Th/U ratios. Ptygmatically folded leucocratic bands containing Ordovician zircon rims, associated with low Th/U ratios, are further observed in the granite gneiss core of the Erzgebirge. Variscan ages are rare, except in a fine-grained high-pressure micaschist, which contains exclusively small, structure-less, zircon with a weighted mean age of 350 ± 2 Ma. These data, along with a re-evaluation of previously published data, have been interpreted as the product of flattening subduction during the Early Cambrian; followed by the opening of slab windows in the Late Cambrian; and finally delamination in the Early Ordovician. Delamination of the orphaned slab led to asthenospheric upwellings triggering extension, bimodal magmatic pulses, recycling of fertile crust, high-temperature metamorphism, and cratonisation of relatively young crust.
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The non-cylindrical character of the Variscan Belt is established via a description of its most salient geological characteristics and correlations among the main and relatively minor massifs from Bohemia to Morocco on the basis of a bibliographical synthesis. Using the firmly established tectonostratigraphic domains, the continuity of Variscan zones is discussed, as well as the western termination of the one that occupies most of the central part of the belt. This zone is characterized by its allochthony, and breaks the continuity of the zonation, that is, the cylindrism of the belt. A proposal is made to call this zone the Mid-Variscan Allochthon and its wedge shape in map view is explained performing a step by step restoration of late Variscan deformation. Once the geometry of the belt is interpreted by a succession of regionally significant deformation events, a kinematic reconstruction is proposed. It is based on recent developments on the knowledge of Paleozoic plate tectonics and intended to show that the deformation model is compatible with current paleogeographical developments.
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Geological evidence, supported by biogeographical data and in accord with palaeomagnetic constraints, indicates that “one ocean” models for the Variscides should be discarded, and confirms, instead, the existence of three Gondwana-derived microcontinents which were involved in the Variscan collision: Avalonia, North Armorica (Franconia and Thuringia subdivided by a failed Vesser Rift), and South Armorica (Central Iberia/Armorica/Bohemia), all divided by small oceans. In addition, parts of south-eastern Europe, including Adria and Apulia, are combined here under the new name of Palaeo-Adria, which was also Peri-Gondwanan in the Early Palaeozoic. Oceanic separations were formed by the break-up of the northern Gondwana margin from the Late Cambrian onwards. Most of the oceans or seaways remained narrow, but – much like the Alpine oceans – gave birth to orogenic belts with HP-UHP metamorphism and extensive allochthons: the Saxo-Thuringian Ocean between North and South Armorica and the Galicia-Moldanubian Ocean between South Armorica and Palaeo-Adria. Only the Rheic Ocean between Avalonia and peri-Gondwana was wide enough to be unambiguously recorded by biogeography and palaeomagnetism, and its north-western arm closed before or during the Emsian in Europe. Ridge subduction under the northernmost part of Armorica in the Emsian created the narrow and short-lived Rheno-Hercynian Ocean. It is that ocean (and not the Rheic) whose opening and closure controlled the evolution of the Rheno-Hercynian foldbelt in south-west Iberia, south-west England, Germany, and Moravia (Czech Republic). Devonian magmatism and sedimentation set within belts of Early Variscan deformation and metamorphism are probably strike-slip-related. The first arrival of flysch on the forelands and/or the age of deformation of foreland sequences constrains the sequential closure of the Variscan seaways (Galicia-Moldanubian in the Givetian; Saxo-Thuringian in the Early Famennian; Rheno-Hercynian in the Tournaisian). Additional Mid- to Late Devonian and (partly) Early Carboniferous magmatism and extension in the Rheno-Hercynian, Saxo-Thuringian and Galicia-Moldanubian basins overlapped with Variscan geodynamics as strictly defined. The Early Carboniferous episode was the start of episodic anorogenic heating which lasted until the Permian and probably relates to Tethys rifting.
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The East Massif Central (EMC), France, is part of the internal zone of the Variscan belt where late Carboniferous crustal melting and orogenic collapse have largely obliterated the pre- to early-Variscan geological record. Nevertheless, parts of this history can be reconstructed by using in-situ U-Th-Pb-Lu-Hf isotopic data of texturally well-defined zircon grains from different lithological units. All the main rock units commonly described in the EMC are present in the area of Tournon and include meta-sedimentary and meta-igneous rocks of the Upper Gneiss Unit (UGU) and of the Lower Gneiss Unit (LGU), as well as cross-cutting Variscan granitoid dikes and a heterogeneous granite coring the major Velay dome. Herein we demonstrate that the UGU and the LGU have markedly distinct zircon records. The results of this study are consistent with deposition of the protoliths of the paragneisses within a back-arc basin that was located adjacent to the Arabian-Nubian shield and/or the Saharan Metacraton during the late Ediacaran and collected detritus from the Gondwana continent. At ~ 545 Ma some of these sedimentary rocks were affected by a first melting event that formed the protoliths of the LGU orthogneisses, those of which subsequently remelted at ca. 308 Ma to form the Velay granite-migmatite dome. Protoliths of the UGU result mainly from a bimodal rift-related magmatism at ~ 480 Ma, corresponding to melting of the Ediacaran sediments and depleted mantle. Zircon rims from the UGU additionally provide evidence for a metamorphic/migmatitic overprint during the Lower Carboniferous (~ 350–340 Ma). Finally, several generations of granite dikes of which inherited zircons display characteristics of both the UGU and the LGU were protractedly emplaced from ~ 322 Ma to ~ 308 Ma, the youngest of which being coeval with the formation of the Velay dome. Our data further show that the vast majority of crustal material ultimately involved in the Variscan orogeny, which forms the present-day basement in the EMC, was derived from a sedimentary mixture of various components from the Gondwana continent deposited in Ediacaran times, with no evidence for the involvement of an older autochthonous crust.
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New U-Pb dating on zircon yielded ca. 470 Ma ages for the granitoids from the Lévézou massif in the southern French Massif Central. These new ages do not support the previous interpretation of these granitoids as syn-tectonic intrusions emplaced during the late Devonian-early Carboniferous thrusting. The geochemical and isotopic nature of this magmatism is linked to a major magmatic Ordovician event recorded throughout the European Variscan belt and related to extreme thinning of continental margins during a rifting event or a back arc extension. The comparable isotopic signatures of these granitoids on each side of the eclogite-bearing leptyno-amphibolitic complex in the Lévézou massif, together with the fact that they were emplaced at the same time, strongly suggest that these granitoids were originally part of a single unit, tectonically duplicated by either isoclinal folding or thrusting during the Variscan tectonics.
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SUMMARY: Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (≤1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (≤2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by
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The Careón unit consists of imbricate tectonic sheets of serpentinite, metagabbro, and diabase dikes interpreted as a dismembered ophiolitic sequence. Zircons from leucogabbro intruding serpentinites point to an igneous crystallization at 395 ± 3 Ma, in accord with previous results. Combined with a reported 377 ± 1 Ma Ar/Ar cooling age, this date implies that ophiolite generation occurred shortly before its tectonic emplacement. Most samples are enriched in light rare earth elements (REE) relative to heavy REEs and show distinct fractionation of the heavy REEs. Enrichments in Th and La relative to Nb are ubiquitous, while negative anomalies of Zr and Ti occur in most samples. Initial εNd values (+6.4 to +9.1), even for samples with high light REE/heavy REE ratios, point to a time-integrated mantle source strongly depleted in Nd relative to Sm, and preclude significant contamination of mafic melts during their ascent through the crust. Combined trace element and Nd-isotope data favor an intraoceanic, suprasubduction-zone setting, where hydrous fluids and silicate melts metasomatized a wedge of depleted mantle and triggered its partial melting. On the basis of magmatic affinities, age constraints, and broad tectonometamorphic context, ophiolite generation is interpreted to reflect oceanic spreading above a subduction zone dipping away from a passive continental margin. Suprasubduction extension might have been associated with subduction hinge retreat caused by instability of old, cold oceanic lithosphere entering the subduction zone. It is suggested that this occurred in the context of early stage arc collision documented at that epoch in the Variscan realm.
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In tectonic maps of Variscan Europe, allochthonous pieces of Cadomian basement clearly stand out with their predominant metabasic to ultrabasic elements, the so-called exotic terranes with ophiolites. Most of these domains are observed in basements of the Central Iberian Allochthone, the South Armorican domain, the nappe structures of the French Massif Central, the Saxothuringian Zone and the Bohemian Massif. Similar relics can be recognized in many Alpine basement areas, and correlations with supposedly more autochthonous basements, such as the Ossa Morena Zone and the Central Iberian basement, can be envisaged. All of these relics are thought to represent the interrupted trace of a former continuous or discontinuous structure, characterized by the presence of ocean-derived proto-Rheic rock suites. These can be interpreted as pieces of former magmatic arcs of Ediacaran to Cambrian age accreted to the Gondwana margin, which later were scattered as allochthonous units during the Variscan plate-tectonic processes. The presence of similar rock suites of Ordovician age in the Alpine realm is explained by the accretion of exotic China-derived basements and their collision with the Gondwana margin during the opening of the Rheic Ocean.
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Geochronological U-Pb (LA-ICP-MS), geochemical and isotopic data from metavolcanic felsic rocks of the Canigó and Cap de Creus massifs in the Eastern Pyrenees provide evidence of an Ediacaran magmatic event lasting 30 Ma in NE Iberia. These data also constrain the age of the Late Neoproterozoic succession in the Cap de Creus massif, where depositional ages range from 577 to 558 Ma, and in the Canigó massif, where the data (575 to 568 Ma) represent minimum ages. The geochemistry of the felsic rocks indicates that they were formed in a back-arc environment and they record a fragment of a long-lived subduction-related magmatic arc (620 to 520 Ma) in the active northern Gondwana margin. The homogeneity shown by all these crustal fragments along this margin suggests that the individualization of the Pyrenean basement from the Iberian Massif started later, probably during its transition from an active to a passive margin in Cambro-Ordovician times.
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This study focuses on the provenance of the uppermost unit of the Upper Allochthon of the Variscan belt with combined U–Pb and Lu–Hf zircon (LA-ICPMS) and Sm–Nd whole-rock analyses. This unit is represented in the Cabo Ortegal Complex (NW Iberia) by the metasiliciclastic Cariño Gneisses which overlap ophiolitic units that represent perigondwanan oceanic domains. The data set indicates a maximum depositional age of c. 510 Ma and a c. 1.73 Ga Sm–Nd model age, typifying a late or post Pan-African (or Cadomian) and Eburnean events, which entailed abundant input of juvenile material involving broad mixing with older crustal sources. The Mesoproterozoic activity is scarce and scattered and therefore unlikely to represent a major crust generation pulse in the source area of the siliciclastic unit. The data set also records an Archean orogenic pulse in its source area followed by a long lasting crust reworking process, where the Eburnean juvenile materials intruded. These data are interpreted as indicative of a West Africa Craton provenance, where the siliciclastic series from which the Cariño Gneisses are derived were probably deposited in a back-arc type basin where detritus was mostly sourced by the continent rather than the magmatic arc.
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The chemical composition of metamorphosed siliciclastic rocks in the Orlica-Śnieżnik Dome (Bohemian Massif) identifies the main sources for the Neoproterozoic [the Młynowiec Formation (MF)], Early Cambrian [the Stronie Formation (SF)] and Late Cambrian/Early Ordovician [the Goszów quartzites (GQ)] sediments. The MF developed from erosion of a Cadomian magmatic arc along the northern Gondwana margin. The variegated SF, with supra-subduction affinities, shows chemical characteristics pointing to erosion of the freshly exhumed Cadomian orogen and detritus deposition in the back-arc basin. The very different chemical features of the GQ indicate deposition in a basin sited on a passive continental margin. The explanation proposed for the observed changes in chemical composition involves three main stages: (1) The pre ~540 Ma evolution of an active continental margin and related back-arc basin ceased with the collision and accretion of the magmatic arc to the Gondwana margin; (2) Early Cambrian rift to drift transition (540–500 Ma) and development of a depositional basin filled with detritus derived from remnants of the magmatic arc; (3) Peri-Gondwana break-up leading to the formation of shallow-water passive margin depositional basins filled with quartz-rich detritus resembling Early Ordovician Armorican quartzites known from other parts of the Variscan Belt.
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Young rhyolites and associated lavas and magmatic enclaves from the Katmai-Novarupta volcanic system (Alaskan Peninsula), and the Crater Lake and Medicine Lake volcanic system (Cascades) were analyzed for U and Th isotope abundances, as well as major and trace element concentrations, to investigate the time-scales of the processes that lead to rhyolite generation in continental arcs. Basalts and basaltic andesites typically migrate from the mantle to the surface within several thousand years. Variations in (230 Th)/(232 Th) and (238 U)/(232 Th) ratios with SiO2 concentrations in intermediate lavas appear to result from crystal fractionation combined with assimilation of recently crystallized magmas. These data also suggest that ∼104-105 years of mafic magmatism are required at a volcanic center to generate silicic andesites and dacites. Rhyolite genesis involves varying proportions of crystal fractionation of intermediate magmas and assimilation of crust. The near-equilibrium (238 U /(230 Th) ratios for all of the rhyolites suggest an average time since U was last fractionated from Th for the constituents making up these rhyolites of > 105 years. Therefore, the residence times of continental magmas and their entrained crystals appear to increase by a minimum of 2-3 orders of magnitude with increasing SiO2 concentrations from basalt to rhyolite.
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A means of relating geochemical concentrations to existing sandstone classification schemes is based on three chemical parameters: the CiO2/Al2O3 ratio, the Fe2O3/K2O ratio, and the Ca content. The Ca content is used to differentiate noncalcareous from calcareous sandstones and shales and to separate siliciclastic from carbonate rocks. Sandstones are classified the same by this scheme as by petrographic analysis about 84% of the time, and shales are effectively discriminated from sandstones. -from Author
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This paper is focused on a compilation of known data from the low-grade metamorphosed rocks of the Ediacaran period in the German part of the Saxo-Thuringian Zone at the northeastern margin of the Bohemian Massif. The geotectonic setting during the formation of Ediacaran rock units is characterized by Cadomian orogenic processes from c. 650-540 Ma at the periphery of the West African Craton. The basin development during that time is characterized by the formation of a Cadomian backarc basin with a passive margin, and the outboard sitting Cadomian magmatic arc originated at c. 570-560 Ma. This arc-marginal basin system was formed on stretched continental crust in a strike-slip regime and reflects an active-margin setting in a style similar to the recent West Pacific. The backarc basin was closed between c. 560-540 Ma by the collision of the Cadomian magmatic arc with the cratonic hinterland: this resulted in the closure of the backarc basin and the formation of a Cadomian retroarc basin. Collision of an oceanic ridge with the Cadomian Orogenic Belt led to a slab break-off of the subducted oceanic plate resulting in an extreme heat flow, and a magmatic and anatectic event culminating at c. 540 Ma that was responsible for the intrusion of voluminous granitoid plutons.
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Stratigraphically well-defined volcanic rocks in Palaeozoic volcano-sedimentary units of the Frankenwald area (Saxothuringian Zone, Variscan Orogen) were sampled for geochemical characterisation and U–Pb zircon dating. The oldest rock suite comprises quartz keratophyre, brecciated keratophyre, quartz keratophyre tuff and basalt, formed in Upper Cambrian to Tremadocian time (c. 497– 478 Ma). Basaltic volcanism continued until the Silurian. Quartz keratophyre shows post-collisional calc-alkaline signature, the Ordovician–Silurian basalt has alkaline signature typical of continental rift environments. The combined datasets provide evidence of Cambro-Ordovician bimodal volcanism and successive rifting until the Silurian. This evolution very likely resulted from break-up of the northern Gondwana margin, as recorded in many terranes throughout Europe. The position at the northern Gondwana margin is supported by detrital zircon grains in some tuffs,with typical Gondwana-derived age spectra mostly recording ages of 550–750 Ma and minor age populations of 950–1100 and 1700–2700 Ma. The absence of N-MORB basalt in the Frankenwald area points to a retarded breakoff of the Saxothuringian terrane along a continental rift system from Uppermost Cambrian to Middle Silurian time. Geochemical data for a second suite of Upper Devonian basalt provide evidence of emplacement in a hot spot-related ocean-island setting south of the Rheic Ocean. Our results also require partial revision of the lithostratigraphy of the Frankenwald area. The basal volcanic unit of the Randschiefer Formation yielded a Tremadocian age and, therefore, should be attributed to the Vogtendorf Formation. Keratophyre of the Vogtendorf Formation, previously assigned to the Tremadoc, is most likely of Upper Devonian age.
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The South Armorican Shear Zone (SASZ), in the French Armorican Variscan belt, is a lithospheric wrench fault that acted during the Late Carboniferous as a transition zone between two distinct domains: a thickened domain to the south affected by extension and crustal magmatism, and a weakly thickened domain to the north subjected to dextral wrenching and crust- and mantle-derived magmatism. The Pontivy-Rostrenen complex is a composite intrusion emplaced along the SASZ. To the south, the complex is made of leucogranites whereas, to the north, monzogranites outcrop together with small intrusions of quartz monzodiorite. U-Pb dating of magmatic zircon by LA-ICP-MS reveal that most magmatic rocks were emplaced at ca. 315 Ma (between 316.7 ± 2.5 Ma and 310.3 ± 4.7 Ma), excepted a late leucogranitic intrusion that was emplaced at 304.7 ± 2.7 Ma. The leucogranites (- 4.8 < εNd (T) < 2.1; presence of Archean to Paleozoic inherited zircon) are strongly peraluminous (A/CNK > 1.1) and formed by partial melting of metasediments and peraluminous orthogneisses. The monzogranite (- 4.0 < εNd (T) < - 3.2; scarce Paleozoic inherited zircon) is moderately peraluminous (1 < A/CNK < 1.3) and formed by partial melting of an orthogneiss with a probable metaluminous composition whereas the quartz monzodiorite (- 3.2 < εNd (T) < - 2.2; no inherited zircon) is metaluminous (0.7 < A/CNK < 1.1) and formed by partial melting of a metasomatized lithospheric mantle. The evolution of the magmas was controlled by fractional crystallization, magma mixing and/or peritectic mineral entrainment. At the scale of the Armorican Variscan belt, crustal partial melting, to the south of the SASZ, was triggered by lithospheric thinning and adiabatic decompression during extension. Conversely, to the north, asthenosphere upwelling during strike-slip deformation and subsequent slab tearing, as suggested by tomographic data, induced the melting of both the crust and the mantle fertilized during previous subduction events. This process is likely not exclusive to the Armorican Massif and may be applied to other regions in the Variscan belt, such as Iberia.
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NW Iberia includes a rather complete section of a Variscan suture, where different terranes with continental or oceanic affinities appear with clear structural relationships. Three groups of terranes, namely Upper, Ophiolitic and Basal units and a frontal tectonic mélange appear in Galicia, in Cabo Ortegal, Órdenes and Malpica-Tui complexes. They constitute a huge allochthonous pile thrust over the Iberian parautochthonous and autochthonous domains, which represent the section of the Gondwanan margin that escaped continental subduction during the Variscan cycle. Considering the allochthonous character of the nappe pile and the strong deformation associated to the Variscan collision, there are problems to identify the original tectonic setting of the terranes and thence, it is difficult to reconstruct the paleogeographic context during the Variscan and pre-Variscan times in detail. Key features to perform any model for the Variscan convergence should consider the existence of two different high-P metamorphic events (dated at c. 400 and 370 Ma, respectively), separated in time for the generation of mafic-ultramafic sequences at c. 395 Ma which constitute the most common ophiolites described in the Variscan suture.
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Abstract The Variscides of Europe and N-Africa are the result of the convergence of the plates of Gondwana and Laurussia in the Paleozoic. This orogen is characterized by the juxtaposition of blocks of continental crust that are little affected by the Variscan orogeny. These low strain domains principally consist of Neoproterozoic/Cambrian Cadomian basement overlain by volcano-sedimentary successions of an extended peri-Gondwana shelf. These Cadomian blocks are separated by high strain zones containing the record of subduction-related processes. Traditionally the high strain zones are interpreted as sutures between one or more postulated lithospheric microplates sandwiched between the two major plates. Paleobio-geographic constraints in combination with geochemical and isotopic fingerprints of the protoliths, however, imply that the Variscides are the result of the exclusive interaction of the two plates of Gondwana and Laurussia. Here we explain the Variscan orogen in a two plate scenario, reasoning that the complexity of the Variscan orogen (multitude of high-grade metamorphic belts, compositional diversity of coeval magmatism, and arrangement of foreland basins) is the result of the distribution of crustal domains of contrasting rheological properties. Post-Cadomian rifting along the Cadomian–Avalonian belt, which culminated in the opening of the Rheic Ocean, resulted in vast coeval intracontinental extension and the formation of extended peri-Gondwana shelf areas, namely the Avalonian shelf and the Armorican Spur to the north and south of the evolving Rheic Ocean, respectively. Both shelf areas affected by heterogeneous extension consist of stable continental blocks separated by zones of thinner continental crust. During Variscan collisional tectonics the continental blocks behave as unsubductable crust, whereas the thinner continental crust was subductable and came to constitute the high strain domains of the orogen. The variable interplay between both crustal types in space and time is seen as the principal cause for the observed sequence of orogenic processes. The first collisional contact along the convergent Gondwana–Laurussia plate boundary occurred between Brittany and the Midland microcraton causing the early Devonian deformation along the Anglo-Brabant Fold Belt. This process is coeval with the initiation of continental subduction along the Armorican Spur of the Gondwana plate and the formation of back arc and transtensional basins to both sides of the Armorican Spur (e.g., Lizard, Rheno-Hercynian, Careón, Sleza) on the Laurussia plate. As further subduction along this collision zone is blocked, the plate boundary zone between the Gondwana and Laurentia plates is reorganized, leading to a flip of the subduction polarity and a subduction zone jump outboard of the already accreted blocks. The following Devonian–Early Carboniferous subduction accretion process is responsible for the juxtaposition of additional Cadomian blocks against Laurussia and a second suite of high-pressure rocks. The final collision between Gondwana and Laurussia is marked by an intracontinental subduction event affecting the entire internal zone of the orogen. Subduction stopped at 340 Ma and the following isothermal exhumation of the deeply subducted continental crust is primarily responsible for Late Variscan high-temperature metamorphism and cogenetic voluminous granitic magmatism. During this final transpressional stage the irregular shape of the Variscan orogen was established by the highly oblique motion of the decoupled lithospheric blocks (e.g. Iberia and Saxo-Thuringia). Rapid overfilling of synorogenic marine basins in the foreland and subsequent folding of these deposits along vast external fold and thrust belts finally shaped the Variscides, feigning a relatively simple architecture. In terms of plate tectonics, the model places the opening of the Paleotethys in the Devonian with a rotational axis of the spreading center just east of the Variscan orogen. The movement of Gondwana relative to Laurussia follows small circle paths about this axis from 370 to 300 Ma. As a consequence of the incomplete closure of the Rheic Ocean after the termination of the Variscan orogeny, Gondwana decoupled from the European Variscides along the dextral Gibraltar Fault Zone. The relative motion between Gondwana and Laurussia after 300 Ma is associated with a shift of the rotational axis to a position close to the Oslo Rift, and is related to the opening of the Neotethys and the evolution of the Central European Extensional Province. The Permian convergence of Gondwana and Laurussia led to the final Permian collisional tectonics along the Mauritanides/Alleghanides. The assembly of the “Wegenerian” Pangea is complete by the end of the Paleozoic.
Article
As laid out in Chapters V.B.1 and V.B.2, the parautochthonous Thuringian fades of the Saxothuringian Belt has been overthrust from the SE by nappes, which have survived in the core of the Vogtland Synform. These are the tectonic klippen of Münchberg, Wildenfels, and Frankenberg (in order from W to E; see Fig. 1). The klippen represent, in fact, erosional remnants of one or several nappe piles, which were originally more extensive, and possibly laterally continuous with each other. The tectonic sequence in each of these klippen is characterized by inversion of the stratigraphy and of the metamorphic grades. The lower nappes comprise (in order from bottom to top): proximal Early Carboniferous flysch, Silurian/Devonian bedded chert, and a volcanosedimentary sequence of largely Ordovician age. These very-low-grade, fossiliferous sequences are overlain by a number of metamorphic thrust sheets. This chapter describes the lithology, tectonic sequence, and internal structure of the metamorphic nappes in the klippen, and of their probable equivalents at the northwestern margin of the Moldanubian Region, from which they are derived. We set out from the Münchberg klippe, which is the largest, most differentiated and best studied example of its kind, and then briefly discuss the closely related units. Details on the metamorphic evolution are available in the contribution by Blümel (Chap. V.C.2)
Article
The French concept of the so-called leptynitic-amphibolitic complexes was created by Forestier in 1961. Originally, it was purely descriptive and applied to bimodal acid-basic associations in high grade metamorphic series of the Massif central, commonly associated with minor amounts of carbonates and garnet peridotites. Since then, several aspects have been emphasized by successive authors: 1) the associated basic and ortho- or paraderived acid rocks must contain relict high-pressure assemblages such as eclogites and/or HP granulites; 2) the L.A.C. are bimodal magmatic series affected by at least one metamorphic event of variable intensity; 3) the L.A.C. are typical of orogenic mobile zones; 4) the L.A.C. are litho-tectonic formations emphasizing major ductile deep-seated shear-zones. Consequently, the L.A.C. are no longer precisely defined and any attempt to do using a single criterion can lead to erroneous geological interpretations. -from English summary
Article
Field relations, mineral textures, and mineral compositions for pegmatite segregations in eclogites from Weissenstein (Munchberg Massif, Germany) indicate that partial melting has occurred during isothermal decompression of the eclogite from 25 kbar to about 10 kbar. The pegmatites have a leuco-tonalitic bulk composition. The matrix of the pegmatite is rich in quartz and plagioclase with abundant graphic intergrowths. The complex intergrowth textures and compositional relationships record at least four stages of mineral reaction and growth in the pegmatites during rapid uplift of the eclogites. The P-T path for pegmatite formation has been constrained by experimental data (from the literature) on melting in tonalitic systems and zoisite-clinozoisite phase relations. The results suggest that partial melting during decompression may be a common process in eclogite terranes, leading to high-pressure pegmatites, with low-Fe epidote minerals as magmatic phases. -from Authors
Article
With reference to the general character of the Variscan foldbelt a key problem within the Saxothuringian Zone is the question of the nature of the metamorphic Munchberg Massif in NE Bavaria. Several new lines of evidence are brought to bear on the problem. The Munchberg Massif is regarded as a pile of nappes. In Carboniferous time nappes of Palaeozoic sedimentary and volcanic rocks, either non-metamorphic or of low grade, evolved and were then overriden by the nappes of high-grade crystalline rocks. The probable root zone of the nappes is at the boundary between the Saxothuringian and Moldanubian Zones, now some 50 km away to the S. Such an interpretation probably applies to all the 'Zwischengebirge' present to the NE along the strike and may extend to include the Gory Sowie (Eulengebirge) in W Poland. The evidence would not support interpretation in terms of a conventional B (Benioff) subduction of oceanic crust. A possible mechanism involving A (Ampferer) intraplate subduction is briefly discussed.-from Authors
Granitoids as categorized by tectonic environment are 1) island arc granitoids (IAG), 2) continental arc granitoids (CAG), 3) continental collision granitoids (CCG), 4) post-orogenic granitoids (POG), 5) rift-related granitoids (RRG), 5) rift-related granitoids (RRG), 6) continental epeirogenic uplift granitoids (CEUG), and 7) oceanic plagiogranites (OP). Of these, the IAG, CAG, CCG, and POG are considered orogenic granitoids, and the RRG, CEUG, and OP are considered anorogenic granitoids. The discrimination of granitoids is based on the major-element chemistry. Various discrimination plots are presented which sequentially discriminate the different tectonic environments. The proposed discrimination scheme is applied to the Proterozoic granitoids of the midcontinent of the United States. It is shown that the Arbuckle granitoids are not anorogenic as previously thought. -from Authors
Article
Sandstones of the Palaeozoic turbidite sequences of eastern Australia show a large variation in their major element geochemistry, reflecting the distinct sedimentary provenance and tectonic setting of each suite. A close correlation exists between the geochemical composition of sandstones and tectonic settings of sedimentary basins. The nature of the continental margin and oceanic basins can be deciphered on the basis of the major element composition of sandstones. The most discriminating parameters are described.-from Author
Article
Eclogitic relics testifying to one or more early events of high-pressure metamorphism are present in different tectonic units of the NE Bavarian crystalline basement forming the north-western margin of the Bohemian Massif. Eclogites and their amphibolized derivatives are most frequent in the Munchberg nappe complex where they form constituents of the uppermost crystalline nappe, the so-called Hangendserie. -from Authors
Article
Seven analysed samples of the augengneiss of the Lower Series of the Munchberg Gneiss-Massif yield an age of 482 + or - 20 m.y., indicating the intrusion of the granitic protolith. A Rb/Sr whole-rock isochron obtained on metasediments of the Lower Series (473 + or - 22 m.y.) reflects the Sr-isotopic homogenization during a Caledonian thermal event. Estimates of the time of sedimentation based on the Rb/Sr whole-rock analyses of the metasediments of the Lower Series yield a maximum age of 740 m.y. The age of intrusion of the Weickenreuth metagranodiorite could be limited from 456 + or - 18 m.y. to 540 + or - 16 m.y. Rb/Sr whole-rock analyses have been carried out on acid, hornblende-free layers of the Upper Series in order to find out the time of the last Sr-isotopic equilibration. The age of 421 + or - 14 m.y. is interpreted as Sr-isotopic homogenization during diagenesis or low grade metamorphism in course of the early Variscan cycle in the Munchberg Gneiss-Massif. The paragneisses of the Upper Series were deposited during the Lower Palaeozoic. -I.Kb.
Conference Paper
Geochronological U-Pb (LA-ICP-MS) data from metavolcanic felsic rocks of the Canigó and Cap de Creus massifs in the Eastern Pyrenees provide evidence of an Ediacaran magmatic event lasting 30 Ma in NE Iberia. The data constrain the age of the Late Neoproterozoic succession in the Cap de Creus massif, where depositional ages range from 577±3 Ma (near the base) to 558±3 Ma (near the top), and in the Canigó massif, 575±4 Ma to 568±6 Ma (all from the middle of the succession, which could thus be older at the base). Geochemical and isotopic data indicates that the rocks were formed in a back-arc environment and record a fragment of a long-lived subduction-related magmatic arc (620 to 520 Ma) in the active northern Gondwana margin. The homogeneity shown by all these crustal fragments along this margin suggests that differentiation between the Pyrenean basement and the Iberian Massif arose later, probably during its transition from an active to a passive margin in Cambro-Ordovician times.
Article
Zusammenfassung Für Teile der Habachformation in den Hohen Tauern wird erstmals eine biostratigraphische Altersbestimmung vorgelegt. Sie stützt sich auf Mikrofossilvorkommen in Phylliten ("Ha-bachphylliten") des Habachtals und in gebänderten Metasedi-menten (Muskovit-Chlorit-Biotit-Albit-Schiefer und Graphit-Quarzite) aus der Basisschieferfolge am Osthang des Felber-tals westlich vom Brentling. Die Habachphyllite enthalten Acritarchen, die für den Zeit-raum Oberriphäikum bis Untervendium bezeichnend sind. Die Basischieferfolge hat nur Reste von coccoiden Cyanobakte-rien geliefert. Diese erlauben keine genaue s!ratigraphische Einordnung, denn Cyanobakterien dieses Typs sind im ge-samten Porterozoikum weit verbreitet. Ähnliche Spektren wie in den Habachphylliten haben sich in Phylliten aus der Prasinit-Phyllit-Serie am Südrand der Münchberger Gneismasse gefunden, dort mit Acritarchen des unteren Vendiums.
Article
Results from laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) and isotope dilution (ID) analyses of minerals and rocks from a single outcrop of the Venetia Klippe of the Limpopo Belt indicate that the U–Pb and Hf isotope system homogenized on the decimetre scale under amphibolite-facies conditions of ⩽645 ± 25 °C and ⩽7.0 ± 1.1 kbar, i.e. in the presence of an aqueous fluid phase. For a metabasite sample, homogenization is supported by isotope analyses of metamorphic zircon, garnet, and whole rock, which yield a six-point Lu–Hf isochron age of 2039.7 ± 3.4 Ma, with initial 176Hf/177Hf of 0.28126 ± 0.00001, and a U–Pb zircon age of 2042 ± 10 Ma. The occurrence of a few inherited magmatic zircon cores with ages up to 2705 Ma, and with significantly lower initial 176Hf/177Hf of 0.28112, however, indicate that homogenization was incomplete. For a chlorite–biotite–garnet schist isotope homogenization is reflected by within error identical zircon and monazite U–Pb ages of 2045 ± 10 Ma and 2041 ± 8 Ma, respectively, and by a zircon-garnet-whole rock Lu–Hf isochron age of 2083 ± 63 Ma, with an initial 176Hf/177Hf of 0.28140 ± 0.00003. Contemporaneous formation of metamorphic zircon, monazite and garnet in the chlorite schist is not only supported by the isotope data, but also by chlorite inclusions in all three minerals, and by inclusions of metamorphic zircon in garnet. The inclusion textures and the identical initial 176Hf/177Hf support the conclusion that metamorphic zircon grains precipitated from an aqueous fluid phase, after dissolution of zirconium-bearing phases elsewhere, followed by a major HFSE transport, and Hf isotope homogenization. This fluid perhaps was Ca-bearing, as is suggested by the fact that garnet in the schist sample is the only Ca-bearing phase, and that metamorphic monazite, dating the metamorphic peak, is partially replacement by apatite. The fact that the metamorphic zircon rims in the metabasite sample have significantly lower initial 176Hf/177Hf than expected from the Lu–Hf isotope analyses of relic zircon cores and whole rock additionally hint that Ca-HFSE-bearing fluids infiltrated from surrounding quartzo-feldspathic gneisses, which had much lower 176Hf/177Hf at the time of metamorphism.
Article
The Cadomian Orogen in the NE Bohemian and the northern Armorican Massifs shows a distinct orogenic zoning from recent NW to SE consisting of (i) an outboard sitting continental crustal unit comprising Neoproterozoic rocks associated with c. 2.0 Ga old Icartian Basement, (ii) a magmatic arc and a back-arc basin, (iii) a foreland or retro-arc basin, and (iv) the passive margin of the back-arc basin. New U-Pb zircon ages of detrital zircon of Neoproterozoic to Fortunian siliciclastics from the Schwarzburg Antiform in the Saxo-Thuringian Zone (NE Bohemian Massif) identify the West African Craton as the hinterland for the Cadomian Orogen as demonstrated by zircon populations dated at 1.8-2.2, 2.5-2.7, 3.0-3.1, and 3.4-3.5 Ga. The dominant zircon population (c. 50-70% in each sample) is derived from a Cadomian magmatic arc in a time slice of c. 570-750 Ma. The magmatic activity of the Cadomian arc stopped at c. 570 Ma. Closure of the back-arc basin by arc-continent collision occurred between c. 570 and 542 Ma under the formation of a foreland (retro-arc) basin. A short-living remnant basin existed between c.542 and 540 Ma. Granitoid plutonism at 539-540 Ma documents the final pulse of the Cadomian Orogeny. Hf isotope compositions, calculated epsilon Hf-i values and Tom model ages for detrital and magmatic zircon show that during the c. 180 Ma long Cadomian magmatic arc activity juvenile arc magmas were contaminated by recycling of Eburnian and Archaean crust. Mixing with an evolved continental crust is always present. The inferred geotectonic setting is a continental magmatic arc during the Neoproterozoic developed on a stretched Archaean and Palaeoproterozoic (Eburnian) crust. In the West African crustal evolution it can be demonstrated that during Eburnian orogenic processes (c. 1.8-2.2 Ga) in most cases a 2.5-3.4 Ga old basement became reworked. Archaean 2.5-2.9 Ga magmas remelted a 3.0-3.4. Ga crust. Zircon grains with an age of 3.0-3.1 and 3.4 Ga are derived from juvenile magmas. Two zircon grains dated at 2779 22 and 3542 +/- 28 Ma imply reworking of pre-existing Eoarchean to Hadean crust and show T-DM model ages of 3.98 and 4.29 Ga, respectively.
Article
Abundant zircon grains were found within a metasomatic chlorite–corrensite (trioctahedral regularly interstratified chlorite/smectite) shell that surrounds rodingite from the Jordanów–Gogołów (JG) serpentinite massif (Sudetic ophiolite, Bohemian Massif). The zircon grains bear two types of primary fluid inclusions containing either liquid CO2 or aqueous solutions. The inclusions were used to assign the P–T conditions of the zircon formation (270–300 °C, ca. 1 kbar), using the crossed-isochore method. Abraded single zircon grains give a U–Pb age of 400+4/−3 Ma, which is interpreted as the age of zircon crystallization. This zircon formed during rodingitization, i.e., a metasomatic process directly connected with the serpentinization and thus provides the opportunity of direct dating of the early serpentinization process.The zircon age from the Jordanów–Gogołów massif is consistent with the crystallization age of metagabbro protoliths from the Sudetic ophiolite as determined by Oliver et al. [J. Geol. Soc. (Lond.) 150 (1993) 355]. A Ce anomaly, combined with the mineralogical record of greenschist facies metamorphism of the mafic rocks from Sudetic ophiolite, suggests that the Sm–Nd whole-rock isotopic data of Pin et al. [Lithos 21 (1988) 195] (353±21 Ma) possibly characterize the age of later metamorphic event.
Article
This paper describes late Cambrian dikes and Early Ordovician volcano-sedimentary successions of the Prague Basin, Bohemian Massif, to discuss the timing and kinematics of breakup of the northern margin of Gondwana. Andesitic dikes indicate minor E–W crustal extension in the late Cambrian, whereas the Tremadocian to Dapingian lithofacies distribution and linear array of depocenters suggest opening of this Rheic Ocean rift-related basin during NW–SE pure shear-dominated extension. This kinematic change was associated with the onset of basic submarine volcanism, presumably resulting from decompression mantle melting as the amount of extension increased. We conclude from these inferences and from a comparison with other Avalonian–Cadomian terranes that the rifting along the northern Gondwana margin was a two-stage process involving one major pulse of terrane detachment in the early Cambrian and one in the Early Ordovician. While the geodynamic cause for the former phase remains unclear, but still may include effects of Cadomian subduction (roll-back, slab break-off), isostatic rebound, or mantle plume, the incipient stage of the latter phase may have been triggered by the onset of subduction of the Iapetus Ocean at around 510 Ma, followed by advanced extension broadly coeval (Tremadocian to Darriwilian) in large portions of the Avalonian–Cadomian belt. Unequal amounts of extension resulted in the separation and drift of some terranes, while other portions of the belt remained adjacent to Gondwana.
Article
In the Münchberg Massif in the Variscan foldbelt of southern Germany two varieties of eclogite are known which are intercalated with amphibolite-facies meta-igneous and meta-sedimentary rocks: a dark kyanite-free and a lighter colored kyanite-bearing type. Kyanite-free eclogites, which are discussed here, have a major and trace element composition which suggests derivation from ocean-floor basalts with melt to cumulate compositions. Internal Sm-Nd isochrons (clinopyroxene-amphibole-garnet) and one Rb-Sr isochron (clinopyroxene-amphibole-mica) yield eclogitization ages in the range of 380 to 395 Ma. Thus, the age of eclogitization is only marginally higher ( < 15 Ma) than the age of amphibolite-facies metamorphism in the Münchberg Massif as derived from K-Ar ages of amphiboles and micas from metasediments and meta-igneous rocks. A seven point whole-rock Sm-Nd isochron for one eclogite body results in an age of 480 +/- 23 Ma with an initial εNd of 8.7 +/- 0.6 and is likely to record the age of igneous formation of the eclogite protoliths. Sr isotopic compositions back-calculated to that time are anomalously high and variable if compared to Nd isotopes. This can be explained by alteration with an aqueous or fluid phase with high 87Sr/86Sr, most likely seawater, either during igneous formation in an oceanic rift environment or subduction-related eclogitization. In addition, some eclogites show a marked enrichment of incompatible, immobile elements and plot far below the whole-rock Sm-Nd isochron. These features are ascribed to the presence of an evolved crustal component, probably acquired during extrusion of the basaltic protoliths by mixing with country-rock gneisses.
Article
The Corredoiras orthogneiss belongs to the intermediate pressure upper units of the Órdenes Complex (Variscan belt, NW Spain), mainly composed by granodioritic orthogneisses, with small bodies of tonalitic orthogneisses, amphibole-rich orthogneisses and metagabbronorites. In this work we study their chemical and isotopic composition, to gain insight into the linkage between plate tectonics and magmatism and to improve the knowledge of the paleogeographic evolution of the European Variscan Belt.Granodioritic and tonalitic orthogneisses range from intermediate to felsic rocks, with K2O/Na2O ratios≈1, typical of calc-alkaline rocks, and high Na2O content, characteristic of I-type granites. Metagabbronorites are basic rocks, but some of them are contaminated by interaction with the felsic magmas, showing enrichment in SiO2, Na2O and K2O. All Corredoiras metaigneous rocks are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE), resulting in a high LILE/HFSE ratio. These geochemical features are the most characteristic of magmas related to subduction zones; furthermore all orthogneisses display significant negative anomalies in Ta, Nb and Zr, which together with their low contents in Y and Yb match up with granitoids generated in volcanic arcs or subduction zones. SHRIMP U-Pb zircon dating provides a concordia age of 492±3Ma. Granodioritic orthogneiss has negative εNd(492 Ma) values (−2.2 to −3.6) and high (87Sr/86Sr)i ratios (0.707 to 0.708), on the other hand tonalitic orthogneisses and metagabbronorites have positive εNd(492 Ma) (1.0 to 2.4) and low (87Sr/86Sr)i (0.703 to 0.705), suggesting that granodioritic orthogneisses have a clear crustal influence in their generation, whereas tonalitic orthogneisses and metagabbros can be related to basic magmas extracted from the mantle or from a basic lower continental crust.The Corredoiras chemical characteristics permit us to interpret that this rocks were probably generated in an ensialic island arc and may represent a peri–Gondwanan fragment drifted away to open the Rheic Ocean.HighligthsWe study the geochemical characteristics of Corredoiras metaigneous unit (NW Iberian Massif).This unit has affinity with igneous continental arc association.Nd and Sr values show a mixture between juvenile material and melting of an old crust.Corredoiras magmatic rocks were generated during Late Cambrian – Early Ordovician age in an arc inner zone.
Article
Three distinct tectonic regimes were identified for felsic and intermediate volcanic rocks using published datasets from twenty-six different geographical locations around the world. The three well-defined tectonic regimes include oceanic arcs, active continental margins and within-plate volcanic zones. This subdivision is based on concentrations and ratios of the incompatible trace elements Ta, Th and Yb as geochemical tectonic discriminants. The separation of tectonic regimes is demonstrated on two discriminant diagrams, where the three zones are separated by ca. 45 degrees diagonal lines on one, and by horizontal lines on the other. The ca. 45 degrees trends of the boundaries between tectonic provinces on a Ta/Yb versus Th/Yb diagram are due to the similar incompatibility of Th and Ta relative to the somewhat lower incompatibility of Yb. On a Th/Ta versus Yb diagram, the three tectonic zones are separated by horizontal lines; datasets within individual zones have characteristic Th/Ta values, ca. 1-6 for within-plate volcanic zones! >6-20 for active continental margins, and >20-90 for oceanic area. These discriminant diagrams can be successfully used to identify the tectonic environments of intermediate and felsic volcanic rocks, and to evaluate the tectonic history of a region.
Article
The mid-European segment of the Variscides is a tectonic collage consisting of (from north to south): Avalonia, a Silurian-early Devonian magmatic arc, members of the Armorican Terrane Assemblage (ATA: Franconia, Saxo-Thuringia, Bohemia) and Moldanubia (another member of the ATA or part of N Gondwana?). The evolution on the northern flank of the Variscides is complex. Narrowing of the Rheic Ocean between Avalonia and the ATA occurred during the late Ordovician through early Emsian, and was accompanied by formation of an oceanic island arc. By the early Emsian, the passive margin of Avalonia, the island arc and some northern part of the ATA were closely juxtaposed, but there is no tectonometamorphic evidence of collision. Renewed extension in late Emsian time created the narrow Rheno-Hercynian Ocean whose trace is preserved in South Cornwall and at the southern margins of the Rhenish Massif and Harz Mts. Opening of this ‘successor ocean’ to the Rheic left Armorican fragments stranded on the northern shore. These were later carried at the base of thrust sheets over the Avalonian foreland. Closure of the Rheno-Hercynian Ocean in earliest Carboniferous time was followed by deformation of the foreland sequences during the late lower Carboniferous to Westphalian. Closure of narrow oceanic realms on both sides of Bohemia occurred during the mid- and late Devonian by bilateral subduction under the Bohemian microplate. In both these belts (Saxo-Thuringian, Moldanubian), continental lithosphere was subducted to asthenospheric depths, and later partially obducted. Collisional deformation and metamorphism were active from the late Devonian to the late lower Carboniferous in a regime of dextral transpression. The orthogonal component of intra-continental shortening produced an anti-parallel pair of lithospheric mantle slabs which probably joined under the zone of structural parting and became detached. This allowed the ascent of asthenospheric material, with important thermal and rheological consequences. The strike slip displacements were probably in the order of hundreds of kilometres, since they have excised significant palaeogeographic elements.
Article
Two bimodal meta-igneous complexes from the southern Massif Central provide important tectonic constraints on the initial stage of the Palaeozoic orogen. The metabasites may be subdivided into two groups according to the distribution of the Large Ion Lithophile (LIL) and High Field Strength (HFS) elements: N-type MORBs with low LIL/HFS ratios and subduction-type basalts with high LIL/HFS ratios. These associations suggest a complex tectonic setting such as an extensional zone or a back-arc basin above a subduction zone. The great volume of felsic rocks with low incompatible trace element contents indicates partial melting of an underlying continental crust. The association of terrigeneous sediments with N-type MORBs, subduction-type basalts and high-silica peraluminous rhyolites argue against an intra-oceanic setting, and suggest that the area was one of ensialic crustal tension during middle Ordovician. The two complexes are identified as remnants of ensialic back-arc basins rather than relicts of major oceanic sutures. Such marginal basins could have been located in a mainly continental domain, such as the Armorican microplate.