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Geochronology, stratigraphy and geochemistry of Cambro-Ordovician, Silurian and Devonian volcanic rocks of the Saxothuringian Zone in NE Bavaria (Germany)—new constraints for Gondwana break up and ocean–island magmatism

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Abstract

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 Lower and Middle Devonian sediments depict a continuation of the sedimentary style of the Silurian with the addition of minor psammitic intercalations. From the Lower Frasnian, bimodal within-plate magmatism Höhn et al. 2018) devel-oped intrusives exclusively within units of the Wrench-and-Thrust Zone (Fig. 5). Large volumes of tholeiitic magmatites are a characteristic feature of the Parautochthonous Unit. ...
... The Allochthonous Unit consists of greenschist facies metasediments (phyllites, phyllitic slates and quartzites) intercalated with shear lenses of Upper Devonian granitoids. The latter are part of an I-to A-type felsic magmatic suite which can be observed across various localities (Gehmlich & Drost 2005;Werner et al. 2005;Höhn et al. 2018) within the Gehmlich et al. (2000), Bartzsch et al. (2008) and Hahn et al. (2010). The differentiation of sedimentary facies begins with the onset of Lower Frasnian magmatism. ...
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The Berga Antiform represents one of the key areas of the Saxo-Thuringian Zone of the Central-European Variscides. The geological record constrains early Palaeozoic marine sedimentation on a post-Cadomian Peri-Gondwana shelf, interrupted by Frasnian magmatism, and eventually followed by Lower Carboniferous synorogenic sedimentation. Variscan tectonics resulted in a heterogeneously deformed structure culminating in the juxtaposition of weakly deformed, very lowgrade magmato-sedimentary lithologies with a pervasively deformed phyllite complex. The available dataset is in conflict with the traditional view explaining the entire Berga Antiform as a relatively simple SE-vergent anticline. By means of 3D modelling techniques and structural geological mapping, we propose a more intricate architecture for the Berga Antiform. Corroborating with the traditional view, the parautochthonous southwestern part can be characterised as an anticline. In contrast, the Variscan phyllite complex in the core of the northeastern segment of the antiform represents an allochthonous unit. The late Variscan (N)NW–(S)SE sinistral transpression affected the region and culminated in the principal antiformal structure. The primary tectonic juxtaposition of the allochthonous and parautochthonous parts of the Berga Antiform is either the result of SW-directed nappe stacking or younger (S)SE-directed thrusting. In our model, the Berga Antiform constitutes an integral part of the Wrench-and-Thrust Zone as the connecting link between the Autochthonous and the Allochthonous Domain of the Saxo-Thuringian Zone. Furthermore, it demonstrates the applicability of geological 3D modelling in regional geology as a tool for the understanding of complex geological phenomena.
... The Cambro-Ordovician protolith ages of the Saxon granulites overlap with U-Pb zircon ages in the range 500-470 Ma obtained throughout the Variscan belt for a late stage of rift magmatism in the northern Gondwana margin (e.g. Díez Fernández, Pereira, & Foster, 2015;Höhn et al., 2018;Kryza & Fanning, 2007;Kusbach et al., 2015;Tichomirowa, Sergeev, Berger, & Leonhardt, 2012). Another age population of the Saxon granulites dated by Sagawe et al. (2016) to c. 406 Ma is of uncertain magmatic or metamorphic origin. ...
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Controversy over the plate tectonic affinity and evolution of the Saxon granulites in a two- or multi-plate setting during inter- or intracontinental collision makes the Saxon Granulite Massif a key area for the understanding of the Palaeozoic Variscan orogeny. The massif is a large dome structure in which tectonic slivers of metapelite and metaophiolite units occur along a shear zone separating a diapir-like body of high-Pgranulite below from low-Pmetasedimentary rocks above. Each of the upper structural units records a different metamorphic evolution until its assembly with the exhuming granulite body. New age and petrologic data suggest that the metaophiolites developed from early Cambrian protoliths during high-Pamphibolite facies metamorphism in the mid- to late-Devonian and thermal overprinting by the exhuming hot granulite body in the early Carboniferous. A correlation of new Ar-Ar biotite ages with publishedP-T-tdata for the granulites implies that exhumation and cooling of the granulite body occurred at average rates of similar to 8 mm/year and similar to 80 degrees C/Ma, with a drop in exhumation rate from similar to 20 to similar to 2.5 mm/year and a slight rise in cooling rate between early and late stages of exhumation. A time lag ofc. 2 Ma between cooling through the closure temperatures for argon diffusion in hornblende and biotite indicates a cooling rate of 90 degrees C/Ma when all units had assembled into the massif. A two-plate model of the Variscan orogeny in which the above evolution is related to a short-lived intra-Gondwana subduction zone conflicts with the oceanic affinity of the metaophiolites and the timescale ofc. 50 Ma for the metamorphism. Alternative models focusing on the internal Variscan belt assume distinctly different material paths through the lower or upper crust for strikingly similar granulite massifs. An earlier proposed model of bilateral subduction below the internal Variscan belt may solve this problem.
... 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. ...
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.
... The Cambro-Ordovician protolith ages of the Saxon granulites overlap with U-Pb zircon ages in the range 500-470 Ma obtained throughout the Variscan belt for a late stage of rift magmatism in the northern Gondwana margin (e.g. Díez Fernández, Pereira, & Foster, 2015;Höhn et al., 2018;Kryza & Fanning, 2007;Kusbach et al., 2015;Tichomirowa, Sergeev, Berger, & Leonhardt, 2012). Another age population of the Saxon granulites dated by Sagawe et al. (2016) to c. 406 Ma is of uncertain magmatic or metamorphic origin. ...
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Controversy over the plate tectonic affinity and evolution of the Saxon granulites in a two‐ or multi‐plate setting during inter‐ or intracontinental collision makes the Saxon Granulite Massif a key area for the understanding of the Paleozoic Variscan orogeny. The massif is a large dome structure in which tectonic slivers of metapelite and metaophiolite units occur along a shear zone separating a diapir‐like body of high‐pressure granulite below from low‐pressure metasedimentary rocks above. Each of the upper structural units records a different metamorphic evolution until its assembly with the exhuming granulite body. New age and petrologic data suggest that the metaophiolites developed from early Cambrian protoliths during high‐pressure amphibolite facies metamorphism in the mid to late Devonian and thermal overprinting by the exhuming hot granulite body in the early Carboniferous. A correlation of new Ar‐Ar biotite ages with published P‐T‐t data for the granulites implies that exhumation and cooling of the granulite body occurred at average rates of ~8 mm/year and ~80 °C/Ma, with a drop in exhumation rate from ~20 to ~2.5 mm/year and a slight rise in cooling rate between early and late stages of exhumation. A time lag of c. 2 Ma between cooling through the closure temperatures for argon diffusion in hornblende and biotite indicates a cooling rate of 90 °C/Ma when all units had assembled into the massif. A two‐plate model of the Variscan orogeny in which the above evolution is related to a short‐lived intra‐Gondwana subduction zone conflicts with the oceanic affinity of the metaophiolites and the timescale of c. 50 Ma for the metamorphism. Alternative models focusing on the internal Variscan belt assume distinctly different material paths through the lower or upper crust for strikingly similar granulite massifs. An earlier proposed model of bilateral subduction below the internal Variscan belt may solve this problem.
... 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). ...
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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.
... Arenas et al. 2016), French Massif Central (e.g. , and Saxothuringian Domain (e.g. Bankwitz et al. 1994;Höhn et al. 2017), where it is related to the breakup of continental microterranes from the northern Gondwanan margin (e.g. Kroner and Romer 2013). ...
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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.
... This rifting event is well known also from several other units within the Saxothuringian Zone and the European Variscides. In the Saxothuringian Zone, magmatism between 505 and 480 Ma is documented by granites and rhyolites (including keratophyre and quartz keratophyre) in the Elbe Zone, Lausitz Anticline, Schwarzburg Anticline, Erzgebirge Anticline, and the Frankenwald area occurring in close vicinity to the Münchberg Massif (for data and locations see: Tichomirova, 2001;Linnemann et al., 2010aLinnemann et al., , 2010bHöhn et al., 2017). Furthermore, it is reflected by bimodal rock suites consisting of felsic volcanics and mostly MORB-type mafic rocks, in the Vesser Zone north of the Schwarzburg Anticline (Bankwitz et al., 1994;Kemnitz et al., 2002), from calc-alkaline to alkaline volcanism in the Teplá-Barrandian Unit (Sláma et al., 2008;Žák et al., 2013), from the "Leptyno-Amphibolitic Complex", forming the base of the Upper Gneiss Unit of the French Massif Central (e.g. ...
Article
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 stratiform Cu-Zn sulfide deposit at Kupferberg in Germany represents Bavaria’s largest historic base metal producer. The deposit is hosted by Early Paleozoic volcano-sedimentary strata at the margin of a high-grade allochthonous metamorphic complex. The present paper reports on the first Cu and S isotope data as well as trace element analyses of pyrite from this unusual deposit. The new data point to syn-orogenic mineralization that was driven by metamorphic fluids during nappe emplacement. Primary Cu ore occurs as texturally late chalcopyrite within stratiform laminated pyrite in black shale in two different tectonostratigraphic units of very low and low metamorphic grade, respectively, that were juxtaposed during the Variscan orogeny. Trace element contents of different pyrite types suggest the presence of at least one hydrothermal pyrite generation (mean Co/Ni = 35), with the other pyrite types being syn-sedimentary/early diagenetic (mean Co/Ni = 3.7). Copper isotope analyses yielded a narrow δ⁶⁵Cu range of −0.26 to 0.36‰ for all ore types suggesting a hypogene origin for the principal chalcopyrite mineralization. The ore lenses in the two different tectonostratigraphic units differ with regard to their δ³⁴S values, but little difference exists between poorly and strongly mineralized domains within a given locality. A genetic model is proposed in which syn-sedimentary/early diagenetic pyrite with subordinate chalcopyrite and sphalerite formed in black shale beds in the two different stratigraphic units, followed by late-tectonic strata-internal, hydrothermal mobilization of Fe, Cu, and Zn during syn-orogenic thrusting, which concentrated especially Cu to ore grade. In agreement with this model, Cu distribution in stream sediments in this region shows distinct enrichments bound to the margin of the allochthonous complex. Thus, Kupferberg can be considered a rare example of a syn-orogenic Cu deposit with the Cu probably being derived from syn-sedimentary/early diagenetic pyrite contained in Early Paleozoic shale units.
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The Variscan Upper Allochthon is a continental-affinity terrane that recorded a Cambrian–Ediacaran magmatic arc generation, a subsequent transition to a passive margin, and a collision-related high-pressure metamorphism during the Devonian–Carboniferous amalgamation of Pangea. The objective of this paper is to decipher which continental margin subducted in the Devonian HP–HT event. To do so, a provenance study is presented using combined U–Pb (n = 613) and Lu–Hf (n = 463) isotopic LA–ICP–MS zircon analyses and Sm–Nd whole–rock (n = 5) determinations. These analyses have been performed on five samples of the Banded Gneisses (Cabo Ortegal Complex, NW Iberia), which forms part of the HP–HT bottom member of the Upper Allochthon. Paleozoic–Neoproterozoic zircon ages (34.7%) have a maximum abundance at 522–512 Ma, peaks at 575, 561, 545 Ma and minor abundance peaks between 780 and 590 Ma, and show from their Lu–Hf compositions a volcanic arc mixing pattern. This arc was probably related to the Cadomian arc system. The Mesoproterozoic population is scarce and scattered (2.8%), and due to its Lu–Hf pattern it is proposed that this population is also West Africa Craton derived. The Paleoproterozoic population (39.6%) is concentrated at 2.07 Ga and it is linked to the Eburnean Orogeny, where depleted mantle derived magmas intruded an Archean craton margin. This craton is represented by the Archean population (22.8%), which is grouped at 2.52 - 2.48 Ga, 2.68 - 2.61 Ga and 3.0 Ga, and shows long term reworking processes and at least two juvenile magma intrusions. These data show that the Variscan Upper Allochthon has a West-African provenance and therefore it strongly suggests that the NW Iberian allochthonous complexes, and their correlative European terranes, are also West-Africa derived. These results allow us to finally clarify that the first high-pressure event, recorded during the eo-Variscan amalgamation of Pangea, was attained by the subduction of the margin of Gondwana under Laurussia.This article is protected by copyright. All rights reserved.
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The Lusatian Massif in the Central European Variscides, composed of Upper Neoproterozoic (c. 570 Ma) greywacke intruded by c. 540 Ma old Cambrian granodiorites (and the somewhat younger Zawidow granodiorites), constitutes a fragment of the Cadomian basement of the Saxo-Thuringian Zone. The Lusatian Massif adheres on the east to the Karkonosze-Izera Massif composed of the c. 500 Ma Izera/Rumburk granites related to the Cambro-Ordovician rifting of the Cadomian basement, and narrow belts of micaschists. Trace-element and Sm-Nd isotope data suggest that the source rocks for the Lusatian greywacke, the Zawidow granodiorite and the Izera/Rumburk granite could have been similar, though not the same. The new SHRIMP U-Pb zircon data for the Zawidow granodiorite reveal, apart from the expected c. 540 Ma ages, three other zircon age populations of around 630 Ma, 600 Ma and 510 Ma, the latest being evidently younger than 540 Ma that was considered as the age of the granodiorite magma emplacement. Zircons with Pb-206/U-238 ages around 510 Ma, and somewhat older (up to c. 538 Ma), have also been reported from the Izera granites. This could mean that the granitic plutonism related to the Cadomian orogenic cycle and the Cambro-Ordovician rifting triggered two or more magmatic pulses during at least c. 30 My. During the prolonged period of igneous activity, the plate-tectonic environment at the Gondwana margin changed from collisional (Cadomian Orogeny), to initial rifting (Cambro-Ordovician).
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Post-Cadomian, Upper Cambrian rifting has been documented by 1. the evidence of bimodal volcanogenic sequences and their geochemical signature (MORBs, partially WPBs); 2. the positive End values that show mantle derivation; 3. the evidence of ore bodies of black smoker type; 4. the sedimentary development of accompanying littoral to marine sequences, controlled by rifting, and 5. U-Pb zircon dating of the magmatic rocks. Detrital components and geochemical discrimination of siliciclastic horizons indicate a granitoid dominated source region that probably is related to the Cadomian basement. Rifting, initiated in the Vesser Zone, reflects the first step of the decay of Cadomia along the peri-Gondwanan margin.
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The core of borehole 1209/78 west of Dober-lug–Kirchhain and south of Herzberg in the Torgau– Doberlug Syncline records an atypical lower part of the Tröbitz Formation with thin limestone horizons. These limestone layers include the remains of a low to moderately diverse fauna with the trilobites Protolenus (Hupeolenus) bergstroemi n. sp., Cambrunicornia saxonica n. sp., Ornamentaspis? aff. todraensis Geyer 1990a, Calodiscus? n. sp., the remains of two undetermined olenelloid? and paradoxidid? species, at least two brachiopods (Tremato-bolus, undetermined acrotretoid), and one hyolith. The fauna clearly suggests a position in the lower Agdzian stage of the West Gondwana chronostratigraphic scheme and correlation with the lowermost to lower Middle Cambrian strata in regions such as the Moroccan Atlas ranges and northern Spain, so the assemblages represent the oldest Middle Cambrian fauna known from the Saxothuringian domain and reconfirm the palaeogeographic position in the Perigondwanan segment. The lithological differences of the fossiliferous cores from those of the typical Tröbitz For-mation and the recorded high-energy conditions indicate high-frequency sea-level changes suggesting that this part of the succession may be a late stage of the subglobally recognizable eustatic sea-level fluctuations at the traditional Lower–Middle Cambrian boundary interval.
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A volcanic tuff 1.0 m above the base of the Triebenreuth Formation in the Franconian Forest provides the first precise and biostratigraphically bracketed date within the traditional Middle Cambrian. The first illustration of fossils from the Triebenreuth Formation in this report and their discussion allow a more highly refined correlation within the Middle Cambrian. A weighted mean 206Pb–238U date of 503.14±0.13/0.25/0.59 Ma on zircons from this subaerial pyroclastic tuff was determined by U–Pb chemical abrasion isotope dilution mass spectrometry (CA-TIMS) techniques. At c. 6.0–7.0 Ma younger than the base of the traditional Middle Cambrian in Avalonia, the new West Gondwanan date from east-central Germany suggests that estimates of 500 Ma for the base of the traditional Upper Cambrian and 497 Ma on the base of the Furongian Series may prove to be too ‘old’. Biostratigraphically well-bracketed dates through most of the Middle Cambrian/Series 3 and below the upper Upper Cambrian/upper Furongian Series do not exist. An earlier determined 494.4±3.8 Ma date from the Southwell Group of Tasmania may actually prove to be a reasonable estimate for the age of the base of the traditional Upper Cambrian. Until high precision dates are determined on the base of the traditional Upper Cambrian and base of the Furongian Series, the rates of biotic replacements and geological developments and the durations of biotic zones in the Middle/Series 3 and Upper Cambrian/Furongian Series remain as ‘best guesses’.
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The basement of the Romanian Carpathians is made of Neoproterozoic to early Paleozoic peri-Gondwanan terranes variably involved in the Variscan orogeny, similarly to other basement terrains of Europe. They were hardly dismembered during the Alpine orogeny and traditionally have their own names in the three Carpathian areas. The Danubian domain of the South Carpathians comprises the Drăgşan and Lainici-Păiuş peri-Amazonian terranes. The Drăgşan terrane originated within the ocean surrounding Rodinia and docked with Rodinia at ∼800 Ma. It does not contain Cadomian magmatism and consequently it is classified as an Avalonian extra-Cadomian terrane. The Lainici-Păiuş terrane is a Ganderian fragment strongly modified by Cadomian subduction-related magmatism. It is attached to the Moesia platform. The Tisoviţa terrane is an ophiolite that marks the boundary between Drăgşan and Lainici-Păiuş terranes. The other basement terranes of the Romanian Carpathians originated close to the Ordovician North-African orogen, as a result of the eastern Rheic Ocean opening and closure. Except for the Sebeş-Lotru terrane that includes a lower metamorphic unit of Cadomian age, all the other terranes (Bretila, Tulgheş, Negrişoara and Rebra in the East Carpathians, Someş, Biharia and Baia de Arieş in the Apuseni mountains, Fagaraş, Leaota, Caraş and Padeş in the South Carpathians) represent late Cambrian–Ordovician rock assemblages. Their provenance, is probably within paleo-northeast Africa, close to the Arabian-Nubian shield. The late Cambrian–Ordovician terranes are defined here as Carpathian-type terranes. According to their lithostratigraphy and origin, some are of continental margin magmatic arc setting, whereas others formed in rift and back-arc environment and closed to passive continental margin settings. In a paleogeographic reconstruction, the continental margin magmatic arc terranes were first that drifted out, followed by the passive continental margin terranes with the back-arc terranes in their front. They accreted to Laurussia during the Variscan orogeny. Some of them (Sebeş-Lotru in South Carpathians and Baia de Arieş in Apuseni mountains) underwent eclogite-grade metamorphism. The Danubian terranes, the Bretila terrane and the Someş terrane were intruded by Variscan granitoids.
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Export Date: 11 October 2012, Source: Scopus, doi: 10.1016/j.gsf.2011.11.008, Language of Original Document: English, Correspondence Address: Nance, R.D.; Department of Geological Sciences, 316 Clippinger Laboratories, Ohio University, Athens, OH 45701, United States; email: nance@ohio.edu, References: Arenas, R., Martinez Catalan, J.R., Sanchez Martinez, S., Fernandez-Suarez, J., Andonaegui, P., Pearce, J.A., Corfu, F., The Vila de Cruces ophiolite: A remnant of the early Rheic Ocean in the Variscan suture of Galicia (northwest Iberian Massif) (2007) Journal of Geology, 115 (2), pp. 129-148. , DOI 10.1086/510645;
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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 istopic 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. -from Authors
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Four metasedimentary zircon populations from different tectonometamorphic units of the Central and the Northern Schwarzwald (Variscan belt, SW-Germany) were investigated using SEM, cathodoluminescence and SHRIMP dating. Despite partially strong modifications of primary internal morphologies during Variscan metamorphism at amphibolite (750C, 0.4–0.6GPa) and granulite-facies conditions (950–1,000C, 1.4–1.8GPa), many grains show well-preserved protolith ages. The detritus indicates a northern Gondwana origin and different Palaeozoic episodes of sediment deposition and consolidation. Two of the studied sediments were deposited in Cambrian/early-Ordovician times and consolidated in positions close to northern Gondwana. Late Ordovician and rare Devonian detritus from sediments of two other tectonometamorphic units indicates much later sedimentation close to the leading edge of Gondwana or a terrane assemblage during northern drift towards Laurussia. Subsolidus growth of new zircon due to Variscan granulite facies metamorphism of one of the tectonometamorphic units is precisely dated at 3352Ma.
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With the aim to link zircon composition with paragenesis and thus metamorphic conditions, zircons from eclogite- and granulite-facies rocks were analysed for trace elements using LA-ICP-MS and SHRIMP ion microprobe. Metamorphic zircons from these different settings display a large variation in trace element composition. In the granulites, zircon overgrowths formed in equilibrium with partial melt and are similar to magmatic zircon in terms of high Y, Hf and P content, steep heavy-enriched REE pattern, positive Ce anomaly and negative Eu anomaly. They are distinguishable from magmatic zircon because of their low Th/U ratio. Independently of whole rock composition, metamorphic zircon domains in eclogite-facies rocks have low Th/U ratio and reduced HREE enrichment and Eu anomaly. In a low grade metamorphic vein, zircon has low Th/U ratio but is extremely enriched in Y, Nb and HREE.
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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.
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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)
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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
Chapter
The Ordovician type sections for global correlation, that have been defined in the last decade, are situated either in Sweden (Baltica continent), North America (Laurentia) or the Yangtze Platform (South China continent). The historical sections in the British Isles have been abandoned and will serve only as regional standards in the future. The Ordovician outcrops and subsurface sections of Central Europe are, compared to the new standards, of limited value for international correlation and for the understanding of the Ordovician world. The Ordovician of Central Europe belongs to various areas with, in general, a very complex tectonic evolution. The localities described here (from Belgium, Germany, the Czech Republic, Poland and the Alpine region) are part of a vast region affected by the Variscan Orogeny and, in palaeogeographical tems, all of these areas belonged to peri-Gondanan terranes, with the exception of the northeastern part of Poland that belonged to the Baltica palaeocontinent. It is today widely accepted that, during the Ordovician, the eastern part of the microcontinent of Avalonia included Belgium, western and northern Germany, and possibly northwestern Poland. It is less clear to what entity the outcrop areas of the Rhenohercynian, Saxothuringian and Moldanubian zones belonged. It is evident that they must be attributed to Gondwana-derived terranes (such as, in palaeogeographical terms, Armorica or the Armorican Terrane Assemblage and Perunica) or to sedimentary basins in the vicinity of the Gondwanan supercontinent. However, it is still not clear whether the different areas were separate microcontinents or simply tectonically separated units (terranes) or different sedimentary basins. Ongoing and future research will possibly provide answers to these questions. Our review includes the Avalonian sequences of Belgium in the northwestern part of the investigated area of Central Europe, continues into western, northern and eastern Germany, and extends into northwestern and southern Poland. The review of the Ordovician of the Saxothuringian and the Moldanubian zones includes the outcrop areas of southeastern Germany, the Czech Republic and southwestern Poland. The Ordovician from the pre-Variscan parts of the Alpine mountain chains of Switzerland, Austria and northern Italy are also briefly discussed. For each individual area, we present the stratigraphical succession, based on the most recent results, and correlate the successions with the modern standard of Ordovician stratigraphy, including the timeslice subdivision of Webby et al. (2004). The most complete successions are those from Belgium (Brabant Massif and Condroz Inlier), from Saxothuringia (Schwarzburg Anticline) and from Bohemia (Prague Basin). Most of the other areas present generally isolated outcrops in a complex tectonic context that cannot easily be integrated in a complete stratigraphical succession. Such areas include the Ardennes (Belgium and western Germany), the Black Forest (Schwarzwald, southwestern Germany), the Ebbe Mountains and isolated outcrops in Hessen (western Germany), the subsurface Ordovician of northern Germany, many of the areas in eastern Germany (such as the Harz Mountains or the Lausitz region, for example), and most areas in Poland, but also the basement of the Alps, which is very poorly understood but includes some Ordovician fragments. Due to the complex tectonic history, most rocks are poorly preserved, and fossils are often absent in the sedimentary successions, which makes a perfect understanding of the stratigraphical succession difficult in many areas. Furthermore the development of the different sedimentary basins is mostly unknown thus far. Sedimentary analyses are absent from many areas, and the precise relationship between the different successions presented in this review remains preliminary. Fossils are absent in many sedimentary units, and the mostly siliciclastic successions, that are now attributed to cold-water environments of high latitudes in the southern hemisphere, provide, compared to the palaeocontinents at low latitudes, only few fossils. An exception is the Prague Basin, an area that is famous for its excellent palaeontological content. The development of carbonate rocks in the latest Ordovician was possibly due to a global warming event. In summary, a high number of micro- and macrofossils have been described from the various successions of the Ordovician of Central Europe, allowing not only international correlation and palaeogeographical attributions, but also a first interpretation of the palaeoenvironment.
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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.
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The description of the Upper Tremadocian fauna from Vogtendorf is used as an opportunity to replace the provisional term ‘Randschiefer-Serie’ of the Bavarian Ordovician of the Frankenwald by the terms Vogtendorf Formation for the lower part with a predominance of volcanic rocks and Gösmes Formation for the upper part in which sediments predominate. The Vogtendorf fauna comprises articulate and inarticulate brachiopods, a paragastropod, trilobites and cystoids. New species arePoramborthis vonhorstigi n. sp.,Ranorthis franconica n. sp.,Parapilekia vogtendorfensis n. sp.,Agerina alkleini n. sp.,Macrocystella greilingi n. sp. andEchinosphaerites henkleini n. sp. Lithology and biofacies indicate shallow marine conditions. The fauna is dominated by Mediterranean elements, but shows relations to Baltica. Closely comparable faunal associations and the joint occurrence ofKvania kvanica (Mergl 1984) are evidence of a correlation in age with the Milina Formation of Bohemia and of a close biogeographical relationship of the Bavarian type of facies with the Barrandian during the Lower Ordovician. Further,Euloma ornatum Angelin 1854 allows direct correlation of the Vogtendorf Formation and indirect correlation of the Milina Formation with the late Tremadocian (Shumardia pusilla Zone of the Alum Shale Formation and the Bjørkåsholmen Formation) of Scandinavia. The absence of any species of the Mílina and Vogtendorf faunas in the Leimitz Shales excludes even a partial correlation in age.
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
Using the U-Pb geochronology of zircon we can understand the growth of and collapse of mountain chains, both recent and ancient. In the high-temperature metamorphic rocks that underlie mountain ranges, zircon may survive from precursor rocks, recrystallize, or grow anew. All these possibilities must be considered in the interpretation of zircon ages. Microtextural characterisation and microanalysis, coupled with considerations of mineral equilibria and trace element distributions between zircon and neighbouring silicate minerals, provide insights into the factors c modification and growth. Zircon ages do not usually corresponding metamorphism but instead provide information on the from high temperatures, including the timing and rates of e he deep roots of mountain chains. on ages.
Article
During early Palaeozoic time the Cadomian basement of the northern margin of Gondwana underwent extensive rifting with the formation of various crustal blocks that eventually became separated by seaways. Attenuation of the continental lithosphere was accompanied by the emplacement of anatectic granites and extensive mafic-dominated bimodal magmatism, often featuring basalts with an ocean crust chemistry. Intrusive metabasites in deep crustal segments (associated with granitic orthogneisses) or extrusive submarine lavas at higher levels (associated with pelagic and carbonate basinal sediments) show a wide range of chemical characteristics dominated by variably enriched tholeiites. Most crustal blocks show the presence of three main chemical groups of metabasites: Low-Titholeiitic metabasalts, Main Series tholeiitic metabasalts and alkalic metabasalt series. They differ in the degree of incompatible element enrichment (depleted to highly enriched normalized patterns), in selected large ion lithophile (LIL) to high field strength element (HFSE) ratios, and abundances of HFSE and their ratios. Both the metatholeiite groups are characterized by a common enrichment of light REE-Th-Nb-Ta. High Th values (or Th/Ta ratios) and associated low ε Nd values (especially in the Low-Ti tholeiitic metabasalts) reflect sediment contamination in the mantle source rather than at crustal levels, although this latter feature cannot be ruled out entirely. The range of chemical variation exhibited is a consequence of the melting of (a) a lithospheric source contaminated by a sediment component (which generated the Low-Ti tholeiites), and (b) a high-level asthenospheric mid-ocean ridge basalt (MORB)-type source that mixed with a plume component (which generated the range of enriched Main Series tholeiites and the alkali basalts). It is considered that a plume played an important role in the generation of both early granites and the enriched MORB-type compositions in the metabasites. Its significance for the initial fragmentation of Gondwana is unknown, but its presence may have facilitated deep continental crust melting and the fracturing into small crustal blocks. The early-mid-Jurassic plume-instigated break-up of the southern Gondwana supercontinent is considered to be a possible tectonic and chemical analogue for Early Palaeozoic Sudetic rifting and its magmatic products.
Article
Saxo-Thuringia is classified as a tectonostratigraphic terrane belonging to the Armorican Terrane Collage (Cadomia). As a former part of the Avalonian-Cadomian Orogenic Belt, it became (after Cadomian orogenic events, rift-related Cambro-Ordovician geodynamic processes and a northward drift within Late Ordovician to Early Silurian times), during Late Devonian to Early Carboniferous continent-continent collision, a part of the Central European Variscides. By making use of single zircon geochronology, geochemistry and basin analysis, geological processes were reconstructed from latest Neoproterozoic to Ordovician time: (1) 660-540 Ma: subduction, back-arc sedimentation and tectonomagmatic activity in a Cadomian continental island-arc setting marginal to Gondwana; (2) 540 Ma: obduction and deformation of the island arc and marginal basins; (3) 540-530 Ma: widespread plutonism related to the obduction-related Cadomian heating event and crustal extension; (4) 530-500 Ma: transform margin regime connected with strike-slip generated formation of Early to Mid-Cambrian pull-apart basins; (5) 500-490 Ma: Late Cambrian uplift and formation of a chemical weathering crust; (6) 490-470 Ma: Ordovician rift setting with related sedimentation regime and intense igneous activity; (7) 440-435 Ma: division from Gondwana and start of northward drift. The West African and the Amazonian Cratons of Gondwana, as well as parts of Brittany, were singled out by a study of inherited and detrital zircons as potential source areas in the hinterland of Saxo-Thuringia.
Article
Analysis of tectonostratigraphic units in the West Sudetes reveals the same geological events as in the areas west of the Elbe Fault Zone: a late Proterozoic (Cadomian) orogenic event, Cambro-Ordovician to Devonian rift-drift, and late Devonian to early Carboniferous subduction-collision. There is no conclusive evidence of an Ordovician orogenic event. Tectonic units in the Sudetes are shown to be related to terranes defined in western parts of the Bohemian Massif. The Lausitz-Izera Block, the Orlica-Snieznik Unit and the Staré Mêsto Belt represent easterly continuations of the Saxo-Thuringian Terrane. The Rudawy Janowickie Unit and the Sudetic Ophiolite contain fragments of the Saxo-Thuringian Ocean. The protoliths of the Görlitz-Kaczawa Unit, the South Karkonosze Unit, the Góry Sowie and the Klodzko Units either belong to the Bohemian Terrane or else were welded onto it during mid-late Devonian metamorphism and deformation. Relicts of the Saxo-Thuringian Foreland Basin are marked by flysch with olistoliths in the Görlitz-Kaczawa Unit and in the Bardo Basin. The spatial array of terranes in and around the Bohemian Massif reveals a disrupted orocline, dissected by dextral transpression along the Moldanubian Thrust. This orocline was formed when central parts of the Variscan belt were accommodated in an embayment of the southern margin of the Old Red Continent.
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.
Article
Most of the existing major- and trace element-based discrimination diagrams are characterized by major defects such as subjective field boundaries, the constant-sum problem, and inadequacy of samples in creating them. Although major advances toward the solution of all these problems have been recently achieved through major element-based discriminant function diagrams, their applicability to old, altered rocks may be questionable. We present new discriminant function diagrams based on immobile trace elements and log-ratio transformation of the data of basic and ultrabasic rocks. These new diagrams are extremely successful in distinguishing the four tectonic settings (island-arc, continental rift, ocean island, and mid-oceanic ridge) and especially the plate margin (island-arc and mid-ocean ridge grouped together) and intra-plate or plate interior (continental rift and ocean island combined together) settings. The overall success rate for these natural log-transformed ratio-based diagrams using five trace elements (La, Sm, Yb, Nb, and Th), i.e., using four ratios ln(La/Th), ln(Sm/Th), ln(Yb/Th), and ln(Nb/Th), varies from 78.8% to 96.4%. Moreover, transitional tectonic settings such as interaction of a mid-ocean ridge with an ocean island or subduction of a ridge can be identified because basalts formed in these settings display linear trends in the present diagrams. Finally, the application of our new discrimination diagrams to altered, metamorphosed rocks from four widely separated areas lends further support regarding the usefulness of our proposal.
Article
Gresens' (1967) method of analysis of changes in volume and concentrations during metasomatism have been applied in many studies of hydrothermal alteration. This paper provides a simple method of solution of Gresens' equations, for both volume (or mass) change and concentration changes, one which requires no significant manipulation of analytical data and is readily accomplished both graphically and on a computer spreadsheet. Gresens' equation is rearranged into a linear relationship between the concentration of a component in the altered rock and that in the original. Simultaneous solution of such equations for all components that show no relative gain or loss of mass defines an "isocon." On a graph of the concentrations in the altered rock against those in the original, an isocon is a straight line through the origin. The slope of the isocon defines the mass change in the alteration, and the deviation of a data point from the isocon defines the concentration change for the corresponding component. As is shown, this can be applied to several stages of alteration simultaneously, and to other kinds of mass transfer such as migmatization.
Article
The actual gains and losses that take place in metasomatic alterations cannot be obtained without a knowledge of the relationship between composition changes and volume changes that accompany the process. A general set of equations is derived that allows calculations of gains and losses in terms of the chemical analyses and the specific gravities of the unaltered and metasomatized rocks or minerals. When comparing two rocks, it is necessary to know or assume knowledge of either the volume change or the geochemical behavior of one component in order to solve the problem. When N minerals are compared in a single alteration equation, N-1 additional facts or assumptions with respect to volume change or geochemical behavior are necessary. It is not necessary to calculate the structural formulas for the minerals. Examples are given of various techniques for solving composition-volume relationships.Trace elements should be evaluated in terms of composition-volume relationships because released trace elements are potential ore sources. Trace element data from individual minerals involved in a suspected reaction may be utilized quantitatively as supporting evidence for the reaction.
Article
article We have examined the provenance and tectonic setting of the Middle Cambrian and Ordovician siliciclastic deposits and associated volcanic rocks of the Bavarian Facies, Franconia, Germany, in the Saxothuringian Zone of the Variscan orogen. The units were deposited on the North African rifted margin of Gondwana and representdeep-waterequivalents oftheshallowmarinematuresandstonesuccessionsoftheareallyextensive Thuringian Facies. U-Pb ages of detrital zircons of the Middle Cambrian Wildensteiner Formation, the Middle Ordovician Plattensandstone of the Randschiefer Series, and the Upper Ordovician Döbra sandstone fall into four distinct age groups: 2.4 to 1.8 Ga (15%), 0.75 to 0.54 Ga (55%) and 0.54 to 0.44 Ga (14%); minor abundances(4%)occuraround1 Ga.ThisagedistributionisconsistentwithanorthernGondwanan derivation, mainly from the Cadomian continental magmatic arc in northern Africa. The c. 2 Ga ages indicate a provenance from Eburnean or equivalent sources on the West Africa craton and in northeast Africa and Arabia. The scarcity of grains of Kibaran age (c. 1 Ga) is characteristic of a derivation from metamorphic and magmatic sources on the Arabian-Nubian Shield, rather than from distant major Kibaran age terrains. The youngest group mainly reflects Late Cambrian to Ordovician rift magmatism widespread in the northern Gondwanan Cadomian terranes. The Hf isotopic compositions of selected dated zircons at the time of their crystallization (εHf(t); T=3.5-0.47 Ga) vary between −30 and +7. Zircons with positive εHf(t) values are almost exclusively restricted to the age group between 0.5 and 0.9 Ga. The Hf isotope data suggest that magmatism associated with the Cadomian continental magmatic arc and post-Cadomian (late Cambrian-Ordovician) marginal rifts in northern Gondwana mainly involved mixing of juvenile magmas with Paleoproterozoic crust.
Article
SHRIMP analyses of detrital zircon cores front high-grade metasediments from the Mid-German Crystalline Rise,,which is situated between Eastern Avalonia in the NW and Saxo-Thuringia in the SE. yield mostly ages of c. 550 Ma and c. 2.06 Ga, with minor c. 1.0 and 2.4-2.9 Ga components. The sedimentary protoliths were deposited during the Late Proterozoic to Early Cambrian, probably prior to the break up of the northern Gondwana margin at 460-500 Ma. These data are consistent with the sediments' high epsilon Nd values (0.9 to -3.0), which are comparable to those of well-documented Late Proterozoic sediments from other parts of Europe, The combined isotopic data suggest derivation of the sediments front at least three distinct crustal source regions. Dominant sources were the Avalonian-Cadomian orogenic belt (c. 45%), situated at the northern margin of Gondwana during the Neoproterozoic, and the West African and/or eastern Amazonian cratons (c. 45%). The Grenvillian belt was a minor source (c. 10%).
Article
This contribution summarizes and brings up to date the recommendations made by the IUGS Subcommission on the Systematics of Igneous Rocks for the classification of volcanic rocks when modal analyses are lacking. The classification is on a non-genetic basis using the total alkali-silica (TAS) diagram, and is as nearly consistent as possible with the QAPF modal classification. The diagram is divided into 15 fields, two of which contain two root names which are separated according to other chemical criteria, giving the following 17 root names: basalt, basaltic andesite, andesite, dacite, rhyolite, trachybasalt, basaltic trachyandesite, trachyandesite, trachyte, trachydacite, picrobasalt, basanite, tephrite, phonotephrite, tephriphonolite, phonolite and foidite. Using Na-K criteria, trachybasalt may be further divided into the sub-root names hawaiite and potassic trachybasalt, basaltic trachyandesite into the sub-root names mugearite and shoshonite, and trachyandesite into the sub-root names benmoreite and latite.
Article
Early Palaeozoic bimodal rift-related magmatism is widespread throughout much of the Variscides of Europe. It is traceable from the Polish Sudetes to NW Iberia. Granitic plutonism generally predates Cambro–Ordovician bimodal magmatism. In the N Bohemian Massif this early Palaeozoic granitic plutonism was generated by partial melting of Cadomian basement, whereas contemporaneous alkali granites with a mantle component are typical of the NW Iberian Massif. Silurian-Devonian mafic magmatism in the N Bohemian Massif, Massif Central and NW Iberian Massif is partly preserved as obducted ophiolites. Compositional diversity displayed by Cambro-Ordovician mafic magmatism can be accounted for by interaction between a spreading centre and an upwelling mantle plume. This indicates that combined tensional forces and mantle plume convection assisted the early Palaeozoic dispersal of terranes from the N Gondwana margin. Continued fragmentation resulted in development of an archipelago of related terranes separated by a network of seaways and formation of oceanic crust.
Article
The abundance and distribution of selected minor and trace elements (Ti, Zr, Y, Nb, Ce, Ga and Sc) in fresh volcanic rocks can be used to classify the differentiation products of subalkaline and alkaline magma series in a similar manner to methods using normative or major-element indices. A number of variation diagrams may be used to distinguish common volcanic rock types in terms of the above elements.As these elements are immobile during post-consolidation alteration and metamorphic processes, this method of rock-type classification may, when applied to metavolcanic rocks, prove more reliable than the commonly used methods that utilize major elements, some of which are known to be mobile.
Article
New LA-ICP-MS U–Pb detrital zircon ages from Ediacaran and Paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the Teplá-Barrandian unit (TBU) in the centre of the Bohemian Massif (Central Europe, Czech Republic). The samples taken span the period from ≤ 635 Ma to ∼ 385 Ma and permit recognition of provenance changes that reflect changes in geotectonic regime. Detrital zircon age spectra of two Ediacaran, one Lower Cambrian and three Upper Ordovician samples resemble the ages known from the NW African proportion of Gondwana, particularly the Trans-Saharan belt, while three rocks from higher Lower Cambrian to Lowermost Ordovician strata contain detritus that may have been derived exclusively from local sources. The age spectrum of the Devonian rock is a combination of the NW Gondwanan and local features. These new findings in combination with a wide range of published data are in agreement with a Neoproterozoic subduction-related setting at the margin of Gondwana followed by a Cambrian/Early Ordovician rifting stage and an Ordovician passive margin setting. Furthermore the data are in favour of a position of the TBU at the Gondwanan margin throughout pre-Variscan times.
Article
In the Marvejols area (Southern french Massif Central), the gneissic Marvejols supergroup is overthrust on the metasedimentary “Série du Lot”, deposited in part prior to 540 Ma. The allochtonous terranes are characterized by the occurrence of a leptyno-amphibolitic group, a complex association of mafic and felsic rocks of igneous and sedimentary derivation. A 480±10 Ma age has been obtained by U-Pb dating of zircons, for the crystallization of both mafic and felsic meta-igneous rocks. These rocks were emplaced during an important extensional tectonics. Relics of eclogites, pyrigarnites, coronite gabbros and HP-trondhjemites are clear evidence for a further HP-HT event dated at 415±6 Ma on zircons from a HP trondhjemite. Subsequently, the Marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345±10 Ma. Rifting and thinning of the continental crust in Cambro-Ordovician times appears to be a major geodynamic feature which could account for the thermal events often referred to the “Caledonian” orogeny. The Silurian (415 Ma) age of the HP episode is clearly older than the main Variscan tectonometamorphic event; it is in agreement with Rb-Sr dating of the Moldanubian granulites and with some radiometric data from Southern Brittany (France). These results point to a compressive phase, probably in a subduction context, in view of the high pressures reached (15–20 Kb), after the Cambro-Ordovician distensive phase. The main final tectono-metamorphic paroxysm (blocking of subduction process and continental collison ?) is not prior to the end of Devonian (340–350 Ma) and is related to the Variscan orogeny s.s
Article
Neoproterozoic rocks in the Saxo-Thuringian part of Armorica formed in an active margin setting and were overprinted during Cadomian orogenic processes at the northern margin of Gondwana. The Early Palaeozoic overstep sequence in Saxo-Thuringia was deposited in a Cambro-Ordovician rift setting that reflects the separation of Avalonia and other terranes from the Gondwana mainland. Upper Ordovician and Silurian to Early Carboniferous shelf sediments of Saxo-Thuringia were deposited at the southern passive margin of the Rheic Ocean. SHRIMP U/Pb geochronology on detrital and inherited zircon grains from pre-Variscan basement rocks of the northern part of the Bohemian Massif (Saxo-Thuringia, Germany) demonstrates a distinct West African provenance for sediments and magmatic rocks in this part of peri-Gondwana. Nd-isotope data of Late Neoproterozoic to Early Carboniferous sedimentary rocks show no change in sediment provenance from the Neoproterozoic to the Lower Carboniferous, which implies that Saxo-Thuringia did not leave its West African source before the Variscan Orogeny leading to the Lower Carboniferous configuration of Pangea. Hence, large parts of the pre-Variscan basement of Western and Central Europe often referred to as Armorica or Armorican Terrane Assemblage may have remained with Africa in pre-Pangean time, which makes Armorica a remnant of a Greater Africa in Gondwanan Europe. The separation of Armorica from the Gondwana mainland and a long drift during the Palaeozoic is not supported by the presented data.
Article
At the northwestern edge of the Hercynian Bohemian Massif (Saxothuringian belt) new U-Pb zircon age data from rift-related magmatic rocks indicate that the initiation of Gondwana break-up in this area started during the Middle to Upper Cambrian. Magmatic rocks from a bimodal, MORB- to within-plate volcanic sequence in the Vesser area are dated between ca. 517 and 501 Ma. The volcaniclastic sequences analysed exhibit basal layers of conglomerates and mature sandstones, which can be correlated with a widespread Gondwana-derived onlap horizon of an uppermost Cambrian/Tremadocian age that links the Vesser area with the Saxothuringian continental basin. The association of the Vesser rocks with the Saxothuringian terrane as part of the Armorican terrane assemblage is further demonstrated by a coeval magmatic development and by identical detrital components which are derived from a common Cadomian basement (white mica with a ca. 539 Ma K-Ar minimum age and inherited zircon signatures). The Vesser unit, situated between the NW margin of the Saxothuringian zone and the Mid-German Crystalline Zone, probably represents a N-facing remnant of an ocean-continent transition of the, or within the, Armorican terrane assemblage and involves sections of the early break-up process at the peri-Gondwanan shelf south of the Rheic ocean.
Article
Cadomian orogenic processes and their continuum to the opening of the Rheic Ocean were modeled by making use of new LA-ICP-MS U–Pb ages from detrital zircons of sedimentary rocks of Late Neoproterozoic (Ediacaran) and Cambro-Ordovician sediments of the Ossa-Morena Zone (Iberian Massif) compared with those from the Saxo-Thuringian Zones (Bohemian Massif). Presented data constrain a diachrony of Cadomian and related geotectonic processes along the northern realm of the Gondwana Supercontinent. Early stage of Cadomian evolution is characterized by a continental magmatic arc at the periphery of the West African Craton and a related back-arc basin opened at c. 590 to 570 Ma. Diacronic arc–continent collision was caused by oblique vector of subduction and started first in the East of Peri-Gondwana at c. 560–570 Ma and resulted at c. 543 Ma in the formation of a short-lived Cadomian retro-arc basin in the Saxo-Thuringian Zone. In contrast, more to the West in the Ossa-Morena Zone, the Cadomian back-arc basin was longer active, at least until c. 545 Ma. In that region, final magmatic pulse of the Cadomian magmatic arc at c. 550 Ma is documented by new zircon data. Closure of the Cadomian back-arc basin and arc–continent collision in the Ossa-Morena Zone occurred between c. 545 Ma and the overall onset of Cambrian plutonism at c. 540 Ma. A mid-oceanic ridge was subducted underneath the Cadomian orogen accompanied by slab break-off of the subducted oceanic plate. Oblique incision of the oceanic ridge into the continent caused the formation of rift basins during the Lower to Middle Cambrian (c. 530–500 Ma). This process continued and finally caused the opening of the Rheic Ocean documented by thick Lower Ordovician siliciclastic sediments and a final magmatic event at c. 490–485 Ma. Opening of the Cambrian rift basin and of the Rheic Ocean again was diachronic and started from the West of Peri-Gondwana and expanded eastward.
Article
Uranium–lead ages obtained by LA-ICP-MS analyses of zircon cores from a high-grade Armorican metasediment from the Mid-German Crystalline Rise, Central Germany, yield results which are identical to, but more precise than those previously obtained by SHRIMP dating. This is mainly due to the fact that SHRIMP analyses are more sensitive than LA-ICP-MS analyses to common Pb contamination on the surface of the grain mount. The new U–Pb ages, in combination with in-situ Hf isotope analyses of zircon, provide the first evidence that detrital zircons within Armorican sediments crystallized in both juvenile and evolved magmatic rocks during the Archaean at 2.7–2.9 Ga, the Palaeoproterozoic at 1.8–2.1 Ga, and the Neoproterozoic/Early Palaeozoic at 500–720 Ma. In addition, zircons were formed at ca. 1.0 Ga by remelting of Palaeoproterozoic crust during the Grenville orogeny. The U–Pb dataset shows an age gap between 1.8 and 1.0 Ga, which is characteristic of Armorican sediments, and indicates that the metasediment protolith is younger than Late Cambrian. In addition, the data support previous conclusions that sediments constituting the Armorican terrane assemblage were derived from three crustal sources. Dominant sources were the Avalonian–Cadomian belt (ca. 45%), situated at the northern margin of Gondwana during the Neoproterozoic, and the West-African and/or eastern Amazonian cratons (ca. 50%). The Grenville belt was a minor source (< 5%). Variation of εHf(t) values of the Neoproterozoic/Early Paleozoic zircons indicates two periods of increased juvenile magma formation, one at 595–575 Ma and a second at 515–500 Ma. The older event is coeval with the formation of the Avalonian–Cadomian magmatic arc, whereas the younger event can be related to the break-up of the northern Gondwana margin in Cambrian/Ordovician times. In between, at around 545 Ma, only recycling of older crustal material took place.
Article
The recently proposed [1] ThHfTa diagram is shown in the light of considerable additional data to be a sensitive indicator of the tectonic environment in which an unknown lava (basic of silicic) was erupted. With the presently available data it is now possible to distinguish calc-alkaline lavas from island arc tholeiites. Some of the fields of the different tectonic environments have been enlarged and the boundaries between the fields modified slightly, but the conclusions drawn by Wood et al. [1] are still supported by the data. It is not possible to discriminate between E-type MORB andtholeiitic within-plate basalts using this diagram, but when used in conjunction with the ZrTiY triangular diagram [23] (as originally suggested [1]) these magma types can be distinguished.The effects of different types of bulk lower and upper crustal contamination of a within-plate alkali basalt on the Th, Hf, Ta and radiogenic isotope concentrations of the residual liquids are calculated in detail. The ratios of ThHfTa are shown to be extremely sensitive to crustal contamination processes. Data published by Thompson and co-workers [2,5] for the Tertiary lavas of Skye, Scotland, are used to illustrate the complexity of crustal contamination and develop a petrogenic model for these lavas. These calculations suggest that the use of isotopic data alone to estimate the extent of crustal contamination a particular lava has suffered is fraught with errors and should be interpreted with care.