Article

Saxothuringian Basin: Exotic metamorphic nappes: Stratigraphy, structure and igneous activity

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Abstract

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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... The Randschiefer Formation experienced subgreenschist facies metamorphic conditions. The greenschist facies Prasinit-Phyllit Formation is composed of low-grade metamorphosed silty shales as well as mafic to intermediate volcanic rocks (Franke et al. 1995). Recent U-Pb zircon dating of these mafic rocks supports a Devonian age of approximately 400 Ma (Koglin et al. 2014). ...
... The lower contact of the Münchberg nappe complex is undoubtedly tectonic, which is reflected not only by the spatial distribution of geological units and structural evidence but also by magnetotelluric data (Ritter et al. 1999). Emplacement of the Münchberg nappe complex is estimated to have taken place in several stages between 380 and 320 Ma (Franke et al. 1995). ...
... This indicates the absence of stratiform, syngenetic chalcopyrite prior to faulting and, together with the Cu isotopic evidence presented here, suggests that the principal chalcopyrite crystallization took place as an accompanying effect of synorogenic thrust faulting. The 40°dip of both units is the result of the adjustment of the shale-dominated, incompetent Randschiefer Formation to the bowl shape of the nappe complex, which moved in southwesterly direction (greenschist facies D3 deformation phase after Franke et al. 1995). In addition, the concentration of anomalously high Cu domains along the southwestern margin of the Münchberg nappe complex (Fig. 8a) seems to support fluid migration in this direction. ...
Article
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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.
... The Variscan belt of Europe is a collage of microplates with a bilateral symmetry (see Fig. 1 and the latest review in Franke et al. 1995). The internal part of the northern flank is controlled by the active margin of the Tepla-Barrandian terrane, which has overthrust the Saxothuringian foreland towards the northwest. ...
... Metamorphism of the rocks in the klippen was brought about by grossly southeastward subduction under Cadomian crust at the NW margin of the Tepla-Barrandian. The high-grade rocks were brought back Franke et al. 1995) to the surface in a tectonic regime dominated by dextral transpression (Franke et al. 1992a). On their ascent towards the WNW, they sequentally accreted Saxothuringian rocks of increasingly lower metamorphic grade and younger stratigraphic age, all of which are now assembled in the inverted Mu¨nchberg pile. ...
... The Mu¨nchberg klippe consists of a nappe stack (Fig. 2) recently summarized by Franke et al. (1995). Un-stacking of the nappes leads to the palinspastic sequence proposed in Fig. 3: 1. Hangend-Serie (upper sequence; unit no. ...
Article
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 The Saxothuringian flysch basin, on the north flank of the Central European Variscides, was fed and eventually overthrust by the northwestern, active margin of the Tepla-Barrandian terrane. Clast spectra, mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge. The earliest clastic sediments preserved are of early Famennian age (ca. 370 Ma). They are exposed immediately to the NW of the suture, and belong to the par-autochthon of the foreland. Besides ultramafic (?ophiolite) material, these rocks contain clasts derived from Early Paleozoic continental slope sediments, originally deposited at the NW margin of the Saxothuringian basin. These findings, together with the paleogeographic position of the Famennian clastics debris on the northwestern passive margin, indicate that the Saxothuringian narrow ocean had been closed by that time. Microprobe analyses of detrital hornblendes suggest derivation from the “Randamphibolit” unit, now present in the middle part of the Saxothuringian allochthon (Münchberg nappes). Detrital zircons of metamorphic rocks formed a little earlier (ca. 380 Ma) indicate rapid recycling at the tectonic front. The middle part of the flysch sequence (ca. early to middle Viséan), both in the par-autochthon and in the allochthon, contains abundant clasts of Paleozoic rocks derived from the northwestern slope and rise, together with debris of Cadomian basement, 500-Ma granitoids and 380 Ma (early Variscan) crystalline rocks. All of these source rocks were still available in the youngest part of the flysch (c. middle to late Viséan), but some clasts record, in addition, accretion of the northwestern shelf. Our findings permit deduction of minimum rates of tectonic shortening well in excess of 10–30 mm per year, and rates of exhumation of ca. 3 mm/a, and possibly more.
... The Variscan massifs have been investigated by different authors (Matte et al., 1990;Franke et al., 1995;McKerrow et al., 2000) ( Fig. 1a). An overview of the uplifted crystalline basement block, including the major graphite and impsonite occurrences (also called metagraphite see, e.g., René, 2017) in the Bohemian Massif has been given by Dill et al. (2008) (Fig. 1b). ...
... The southern zone consists of the Moldanubian Zone, which reflects the autochthonous part of the basement and the Bohemicum (also called Teplá-Barrandian zone after Weber and Behr, 1983), which is exposed in some klippens such as the ZEV (Erbendorf-Vohenstrauß Zone) and the MM (Münchberg Gneiss Complex), remnants of a former coherent nappe complex overthrusted onto the Saxothuringian Zone. The autochthonous part of the Moldanubian is made up of magmatic and clastic rocks, which were converted into high-grade gneisses granulites and orthogneisses (Franke et al., 1995;McKerrow et al., 2000). ...
Article
Along the western edge of the Bohemian Massif, SE Germany, graphitic carbon occurs in metabasic rocks plus calcsilicates, metabiolites, and paragneisses (graphite I), in pegmatites (graphite II) and in mineralized structure zones (semigraphite and impsonite) The current studies unveiled these graphitic carbon compounds are strikingly different with regard to their age and temperature of formation: Graphite I ( 324 Ma, 570 to 625 °C), graphite II (3173 Ma, > 400°C), semi-graphite (305 Ma, 225 to 400°C), impsonite (< 298 4 Ma, 100 to 363 °C). Semi-graphite takes a special position among these graphitic carbon compounds because it links the different carbon modifications with regard to its age of formation, its structural position and its S- and C isotopes that point to a mantle and crustal influence on its formation in contrast to graphite (graphite I : crustal, graphite II : mantle) and impsonite (mantle). Semi-graphite precipitated in a fault zone which evolved from a zone of strong felsic mobilization in metabasic rocks spawning K-Na feldspar-quartz pegmatoids/aploids. During conversion of a pre-existing zone of felsic mobilization into a brittle shearzone not only carbonaceous matter but also Ni-, Pb-, Cu-, Zn-, As-, Fe-, Hg- and Mo sulfides were concentrated. The semi-graphite-bearing mineralized zone is located near rare element pegmatites hosting graphite flakes. The C-bearing systems are useful pathfinders to locate structurebound mineral deposits hosting U or base metals in the Variscan orogen. The structurebound metalliferous semi-graphite mineralization in metabasic rocks can be taken as a reference type of dual-source hydrocarbon immigration into fault zones, syn- to postkinematically relative to the fault movement. The model can be applied to host rocks undergoing retrograde medium- to very-low-grade stage dynamo-metamorphic conditions. To elucidate the complex history of the various types of graphite and metamorphosed bitumen a multidisciplinary approach has been taken involving petrographic and geological field mapping combined with drill core examination, petrographic and ore microscopy supplemented by electron microprobe, X-ray diffraction and scanning electron microscopy with EDX, micro-Raman spectroscopy in addition to classical coal petrographic studies, and inorganic geochemistry of major and minor elements and isotope (carbon and sulfur) chemical analysis followed up by a statistical treatment of the various chemical datasets.
... In the Sudetes, in the Kaczawa segment of the Saxothuringian Basin, intra-plate alkali basalts occurred around 502-486 Ma and were followed by E-to N-MORB metabasalts associated with graptolite-bearing Silurian shales (Urbanek et al., 1995) and with tholeiitic basalts associated with conodont-bearing upper Devonian sediments . Devonian basaltic magmatism also occurred at the southwestern margin of the Lausitz-Izera Massif (Ješted Mts.) and in the Elbtal Schiefergebirge (see review in Franke et al., 1995). ...
... And we consider that an analogous situation applies to the Early-Late Devonian extension of Saxothuringia and the basaltic magmatism within the Saxothuringian Basin. Thus, this extensional episode was coeval with high pressure to medium pressure metamorphism at the opposite margins of the Saxothuringian Basin, as dated in NE Bavaria (Münchberg Klippe) at 400-380 Ma (review in Franke et al., 1995;Scherer et al., 2002) and in the West Sudetes at 360 Ma (Maluski and Patočka, 1997). Furthermore, the timing of the high-pressure events is seen to be very similar in other Sudetic fragments of the Armorican Terrane Assemblage, such as the Góry Sowie Massif of the Bohemian Terrane (review in Franke and Żelaźniewicz, 2000), which faces the Saxothuringian Basin. ...
Article
In the Lausitz–Izera Massif of the Saxothuringian terrane, part of the central European Variscides, 515–480Ma metagranites have been intruded by a swarm of basic veins that have within-plate alkali basalt geochemistry, or, rarely, normal mid-ocean ridge basalt (N-MORB) geochemistry. Based on their spatial orientation, distribution in the region and structural relationships to the host metagranites, these basic veins could be grouped into three types that displayed six varieties of host–vein contacts. Zircons extracted from eight of the basic veins revealed that there were two populations: clear zircons and brown zircons. Each type were subjected to both single grain conventional U–Pb analysis and to sensitive high-resolution ion microprobe (SHRIMP) U–Pb analysis. The clear grains yielded ages>480Ma, clustering at ~500Ma; the brown grains yielded ages between ~390 and 365Ma, clustering at ~370Ma. The colourless zircons possess an oscillatory zoned structure and rare earth element (REE) distribution pattern that suggests they come from the host granites and from deeper Neoproterozoic granitoid basement. However, the basic host rocks show almost no obvious contamination by felsic material. The turbid brown grains, which are dark under cathodoluminescence, also seem to be mainly magmatic because of their internal structures, high Th/U ratios and REE pattern. When the zircon results are integrated into the regional geology, an ~30m.y. period of episodic, intra-plate, predominantly alkaline basic magmatism at the passive margin of the Saxothuringian Terrane is suggested, with maximum activity at ~370Ma during an extensional regime. Combining the factors of the relatively high thermal gradient during early metamorphism of the metabasites (~450°C, 2kbar), the geology of Saxothuringia and the regional structural data, we propose a plume model of “hot fingers” type whereby the basic magmatism of the Izera region was due to repeated Devonian intrusions derived from garnet-bearing enriched (OIB type) and less commonly depleted asthenosphere sources. The waning stages of this activity were coeval with the early deformation of the Izera granite host–basic vein system and matched by rifting and extensional widening of the Saxothuringian Basin in its Izera–Kaczawa segment.
... The greenschist-facies Prasinit-Phyllit Formation emanated from silty shales as well as mafic to intermediate volcanic rocks (Franke et al. 1995). U-Pb zircon age data of the mafic rocks support a Devonian age of approximately 400 Ma (Koglin et al. 2014). ...
... The distribution of Cu in stream sediments of northeast Bavaria shows a strong connection between anomalous Cu-enrichment and the margin of the Münchberg nappe complex. The high number of positive Cu-anomalies (including Kupferberg) at the southwestern margin likely reflects the movement of the allochthonous complex (greenschist-facies D3-deformation-phase after Franke et al. 1995) in southwesterly direction. ...
Conference Paper
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New data on pyrite chemistry, petrology and most importantly Cu and S isotope data reveal a two-stage genetic model for the Kupferberg Cu-Zn deposit in eastern Germany, with the principal Cu mineralization being related to the expulsion of syn-orogenic fluids from beneath a high-grade metamorphic nappe complex. The deposit consists of several sulfide lenses bound to Early Palaeozoic volcano-sedimentary strata at the margin of the allochthonous Münchberg nappe complex. The main Cu ore is texturally late chalcopyrite, mostly within pyrite-rich carbonaceous and calcareous shale in two different tectonostratigraphic units that became juxtaposed during the Variscan Orogeny. 65 Cu range of-0.26 to 0.36 ‰ for all ore types suggesting a hypogene origin for the principal chalcopyrite mineralization. 34 S values of the ore lenses in the two different tectonostratigraphic units differ distinctly, whereas little difference exists between strongly mineralized ore types and local pyrite-rich beds. This indicates a strata-internal mobilization from pyrite-rich shales that became overthrust by the allochthonous nappe complex. Syn-tectonic fluid expulsion is further indicated by several positive Cu-anomalies along the southwestern margin of the nappe complex. The recognition of syn-orogenic Cu mineralization below a high-grade nappe complex raises the possibility of similar, potentially economic base metal deposits to occur at the base of metamorphic nappe complexes also elsewhere in the world.
... 350 Ma HP rocks (eclogites and granulites) in the Orlica-Ś nieynik unit is not typical of the Saxothuringian domain proper, but fits well with the Moldanubian domain. In the Saxothuringian domain, the occurrences of HP rocks and intense 340 Ma L-MP/HT metamorphism are restricted to the Saxonian Granulite massif and the Mqnchberg, Wildenfels and Frankenberg massifsthe three latter units otherwise interpreted as dexotic metamorphic nappesT by Franke et al. (1995), and also to the Erzgebirge area, which is recently also considered as a nappe pile resulting from longdistance westward thrusting (Konopásek et al., 2001) and, therefore, not representing the Saxothuringian proper. Nevertheless, due to scarcity of conclusive data, the affiliation of the Orlica-Ś nieynik unit is still a matter of debate, its Saxothuringian affinities being recently defended by Franke and Ż elaźniewicz (2002), following their earlier concepts (Franke et al., 1993;Franke and Ż elaźniewicz, 2000). ...
Article
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The Sudetes in southwestern Poland and northern Bohemia expose a Palaeozoic collage of the northeastermost extremity of the Variscan belt. One of presumed terrane boundaries occurs in the Orlica Mts, along the contact between the phyllite-amphibolite complex (Neoproterozoic?) of the Nové Město unit of probable Teplá-Barrandian affinities in the SW and the orthogneiss (Early Ordovician protolith) and mica-schist (Neoproterozoic?) complexes of the Orlica-Śnieżnik unit in the NE, most likely representing the Moldanubian terrane. The synmetamorphic structural evolution of both adjacent units comprised four deformation events. The Nové Město rocks have recorded an early stage of ductile, top-to-the SE thrusting (D1), which must have resulted in an early juxtaposition of the both terranes within a nappe pile. The original overthrust contact was subsequently modified by a dextral shear-dominated event (D2), which produced a 1–2 km wide shear zone at the boundary of both units, in which earlier fabric elements were overprinted and mostly obliterated. This deformation brought the lower-grade Nové Město rocks into present-day strike-slip contact with those of the regionally uplifted, higher-grade Orlica-Śnieżnik unit. Postdating this juxtaposition are E–W trending folds F3, which affect both the adjacent units. The contact shear zone also contains record of a late, top-to-the SW, semi-brittle, normal-slip displacement (D4), downthrowing the Nové Město unit. The peak, amphibolite facies metamorphic conditions were attained in the Orlica-Śnieżnik unit and in the contact shear zone prior to the D2 event. They corresponded to maximum temperatures and pressures of ca. 600 °C and 10 kbar, respectively. The D1 thrusting event in the phyllites of the Nové Město unit took place under greenschist facies conditions (ca. 350 °C and 4.5 kbar) and its effects mostly survived the later amphibolite-facies metamorphism. These structural relationships are equivalent to those observed elsewhere in the Bohemian Massif on the Teplá-Barrandian/Moldanubian terrane boundary. The characteristic features of this boundary are a high metamorphic gradient but lack of metamorphic inversion; the occurrence of mid-Carboniferous stitching plutons; and the importance of mid-Carboniferous deformation that resulted in significant downwards movement of the low-grade Teplá-Barrandian terrane relative to the uplifted high-grade and hot lower to middle crust of the Moldanubian terrane. The downthrow of the Teplá-Barrandian occurred on ductile shear zones showing either down-dip-slip kinematics or that of transfer strike-slip, as was the case with the Orlica Mts., both kinematics being associated within the same, regional-scale, linked shear zone extensional system. The extensional crustal collapse must have been preceded by a SE-directed crustal stacking achieved through overthrust emplacement of the Teplá-Barrandian on top of the Moldanubian terrane.
... 1-9) and the Moldanubian zone which encompasses a great variety of rocks including the rocks under consideration (10)(11)(12)(13)(14)(15). The Moldanubian Zone is made up of an autochthonous unit ( Fig. 11-8) called the Moldanubicum sensu stricto and an allochthonous one (Fig. 1, 10-11) named the Teplá-Barrandian zone or in Bavaria as Zone of Erbendorf-Vohenstrauß "ZEV" (Malkovsky, 1979;Behr, 1983, Stettner, 1992;Franke et al., 1995). The Late Variscan granitic rocks resembling the pegmatitic with regard to their basic chemical and mineralogical compositions are exposed in large complexes of S-type granites in a NNW-SSE trending zone and are arranged in order of their age of intrusion ( Fig. 1 -in parts 4, [5][6][7][12][13][14]: Leuchtenberg (LEU), Falkenberg (FAL) Flossenbürg Granites (FLO) with its eastern outlier at Bärnau (BAR) (Voll, 1960;Forster 1965;Forster and Kummer, 1974;Steiner, 1986). ...
Article
In some areas of the Variscan orogen felsic mobilizates (pegmatitic and aplitic rocks) are closely associated with stratiform and stockwork-like bodies enriched in Ca minerals (e.g. wollastonite, diopside-hedenbergite s.s.s., grossular-spessartine s.s.s., siderite..) and bodies aligned to them similar in structure but abundant in quartz, plagioclase and mica. Geological mapping and lithochemical studies are the tools to decipher the nature of these crystalline rocks which are common to the Hagendorf-Pleystein Pegmatite Province, SE Germany, and present in many ensialic orogens elsewhere. Geological and chemical data suggest paired belts of a restite-mobilization system. The Ca and Si metasomatites are different from calcareous metasediments and quartzites elsewhere in the SE German basement devoid of mobilizates (parent rocks: limestones and cherts). Mobilization conducive to this paired belt of metasomatites involved silica mobilized from a deep level of the crust as a result of metamorphic-metasomatic alteration of Precambrian to Early Paleozoic metagreywackes during retrograde metamorphism from HP to LP metamorphism around 680–600 °C. The arrangement of mobilizates and restites in the field has been denominated as metamorphic differentiation sensu lato. The zone of silica mobilization is transitional into a zone of pegmatoids and aploids that overlaps with another one characterized by rocks derived from Ca metasomatism the footwall facies of which developed in the range 750–400 °C while in the hanging wall metamorphic rocks of rare-element pegmatites 570–430 °C occurred. The intensity of Ca metasomatism diminishes from the footwall to the hanging wall rocks and reflects a subcrustal impact. These investigations call attention among exploration geologists and petrologists to an alternative origin of “metasilica” and “metacarbonate” rocks being encountered in a zoned arrangement with felsic mobilizates (pegmatitic and aplitic rocks). The current study also raises the question “Quo vadis” pegmatology? It is an amendment to the mainstream geoscientific handling of pegmatitic rocks as “..texturally complex igneous rocks” genetically linked to granitic plutons (see review of London (2018) in Ore Geology Reviews). Taking a holistic approach can give us a reality check and prevent pegmatology from converting into a one-way street (granites-only) that eventually ends up in a dead-end street. The field evidence is the litmus test for all our models created in the laboratory and on the PC. There is no ore geology without field geology. To get access: Your personalized Share Link: https://authors.elsevier.com/a/1YVxT_3s85-rFt
... younger than the estimated age for the opening of the Rheic Ocean (ca. 490-500 Ma; Pin and Carme, 1987;Ménot et al., 1988;Franke, 1995;Kemnitz et al., 2002;Nance et al., 2002;Timmermann et al., 2006;Arenas et al., 2007;Linnemann et al., 2007;Kryza and Pin, 2010;Pedro et al., 2010). Notwithstanding, it is widely accepted that this rifting event is related to a large gravitational pull on the Gondwana margin provided by subduction of the Iapetus-Tornquist lithosphere. ...
Article
The basal units of the allochthonous complexes of NW Iberia are used to examine the Lower Paleozoic geodynamic evolution of the northern Gondwana margin. These units represent the most external continental margin and the sequence of major magmatic events that affected them has been dated. Isotopic dating and field data highlight the existence of two magmatic pulses, dated at 489±4 Ma (granodiorites) and 474±3 Ma (alkali-granites), and a slightly younger alkaline/peralkaline pulse, dated at ca. 470–475 Ma (alkaline and peralkaline granites). Their framing into the regional background has allowed us to explore the major lithosphere-scale processes developed at the Gondwana periphery at that time, as well as to conceive a consistent model for the opening of the Rheic Ocean that reconciles the timing of sea opening and back-arc extension with the timing of intracontinental rifting. The sequence of events is framed in a Cambrian and Ordovician peri-Gondwanan subduction setting where we also explore how subduction may be linked to coeval intraplate magmatism far inboard of the arc–trench. This contribution discusses how such a scenario can be traced in basement areas through a modern analog perspective.
... 1. Although most of the crust involved in the collision process was of the same immature Cadomian type as in the FMC, Cadomian arc-type crustal segments are equally found (e.g. in the Armorican Massif: Chantraine et al. 1994; or in the MuÈ nchberg Nappes: Franke et al. 1995b). ...
Article
The French Massif Central (FMC) represents the whole West European Variscan (WEV) belt, in terms of both the geodynamic evolution and the metallic content. Thus, a study of the metallogenic evolution of the FMC may elucidate the conditions that allow the mineralisation of a collision belt, since recent collision belts, e.g. the Himalayas or the Alps show that mineralisation does not necessarily result from the collision process. The Palaeozoic history of the FMC is divided into three geodynamic stages unevenly involved from the metallogenic view point. The Eo-Variscan stage (Cambrian to Silurian) was not important; the Meso-Variscan stage (Devonian-Early Carboniferous) was of limited importance; and most of the mineralisations formed during the Neo-Variscan stage (Late Carboniferous-Early Permian). In addition, some more mineralisation was produced during the Mesozoic because of the thermal reactivation linked with the Alpine orogenies.
... For details the reader is referred to, e.g., Schwan (1976), Franke (1984) and Behr et al. (1984). Comprehensive descriptions of the lithostratigraphy, structure and metamorphic evolution are given by, e.g., Blümel (1995) and Franke et al. (1995). ...
Article
Mylonitic gneisses from the Münchberg Massif contain single grains (type I) and polycrystalline aggregates (type II) of garnet displaying a distinct elongation parallel to a macroscopic lineation which is interpreted as the result of ductile deformation. Lattice-preferred orientations of quartz (textures) symmetrical to the macroscopic foliation and lineation and the lack of rotational microfabrics indicate that the bulk deformation was pure shear at least during the latest strain increments. Garnet textures measured by EBSD together with microprobe analyses demonstrate that these two structural types of garnet can be related to two different processes of ductile deformation: (1) For the single grains stretching can be attributed to diffusion creep along grain boundary zones (Coble creep). The related mass transfer is indicated by the fact that primary growth zones are cut off at the long faces of the grains while the related strain shadow domains do not show comparable chemical zoning. Pressure solution and precipitation suitable to produce similar structures can be largely ruled out because retrogressive reactions pointing to the presence of free hydrous fluids are missing. (2) For the polycrystalline garnet aggregates consisting of cores grading into fine-grained mantles, dislocation creep and associated rotation recrystallization can be assumed. Continuous lattice rotation from the core to the outer polycrystalline rim allow a determination of the related dominant slip systems which are {100}<010> and equivalent systems according to the cubic lattice symmetry. The same holds for garnets which appear to be completely recrystallized. For this type of fine-grained aggregates an alternative nucleation model is discussed. Due to penetrative dislocation glide in connection with short range diffusion and the resulting lattice rotation, primary growth zones are strongly disturbed.
... In case of an asynchronous and spatially separated formation of N-and E-MORBs, the enriched metabasalts might have been emplaced also in an intra-oceanic (OIB) setting. Whatever the exact setting was, it is very likely that the metabasites of the Randamphibolit-Serie represent remnants of the Rheic oceanic crust (Linnemann, 2007;Žák and Sláma, 2017), and not of the Saxothuringian ocean, which according to the model of Franke et al. (1995Franke et al. ( , 2017 formed a separate oceanic basin during the Palaeozoic. ...
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.
... The Variscan basement consists of polymetamorphic gneisses and magmatic rocks which developed from the Precambrian through the lower Carboniferous (Stein, 1988;Franke et al., 1995;Franke and Stein, 2000;Linnemann, 2003;Kroner and Hahn, 2004). In the aftermaths of the Variscan orogeny granites and granodiorites were intruded into this basement during the Late Carboniferous and Early Permian. ...
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Geomagnetic (magnetic susceptibility), geoelectrical (resistivity and conductivity, self-potential/SP, induced polarization/IP), and radiometric measurements (gamma radiation of K, U, and Th) are well adapted to the needs and wants of geoscientists and exploration geologists, in particular, who widely use them in wireline tools and for ground surveys. Miniaturization of technical components resulted in the production of handheld devices which enable field geologists to an improving of the mineralogical and chemical database and the efficiency of the routine work in the field or at a drill site. The tools and devices used by a field geologist are categorized and presented in a tripartite set. The tools for routine field work with “hammer and laptop” belong to the A-level kit. The handheld devices under consideration constitute an intermediate level (B-level) to enhance the mineralogical and chemical database using physical methods. More advanced level applications make use of short-wave infra-red mineral analyzers or portable X-ray fluorescence devices (C-level). Handheld tools are designed for one-dimensional (cross-sectioning) and two-dimensional (mapping) surveys as well as drill core and cuttings examinations during terrain analysis. They can be operated in foot-borne surveys by one field geologist and the obtained data interpreted without an exuberant computing capacity. In the present overview, siliciclastic rocks and residual deposits have been singled out for their good response to the afore-mentioned methods. Their practical use is demonstrated by means of some case histories, each standing for a particular sedimentary lithology and discussed in combination with similar applications from literature: mixed-type (calcareous) siliciclastic rocks from Mesozoic–Cenozoic basins, SE Germany; residual argillaceous/kaolin deposits on top of granites of the Variscan basement, SE Germany; Neogene red-bed sediments from the promontory of the Tien Shan, East Uzbekistan; and Cretaceous gray-bed sediments with coal seams from the Baganuur basin, Central Mongolia. Cobweb diagrams, histograms, ternary diagrams, simple x-y plots and x-y plots in combination with spider diagrams have proved to be the most suitable ways when it comes to combine the data obtained from various methods and to illustrate these results for further interpretation on screen. The geophysical methods are discussed as to their strong and weak points to cater for a solution in three important subject matters of applied and genetic sedimentology: (1) constraining the redox regime, (2) determination of the lithology and mineralogy, (3) and provenance analysis and lithostratigraphy. Magnetic methods have proved to be useful for all objectives (1, 2, and 3), gamma spectrometry can successfully be applied for objectives 1 and 2, and micro-resistivity contributes significantly to solutions of objective 2. Magnetic and gamma spectrometric methods do not need any direct contact with the sedimentary rocks and therefore can be correlated with equivalent airborne surveys and are less depending on the wetability of the substrate and climate than the geoelectrical methods under study. The final goal of this review is to create a matrix of applicability of the methods and enable the field geologist to select the most suitable type of geophysical measurement or combination of tools for a solution to one of the three issues as a function of the sediment types under study.
... The crystalline rocks of the NE Bavarian Basement pertain to two geodynamic zones, the Saxothuringian Zone, represented by Ordovician and Cambrian rocks at the northern margin of the study area (Fig. 1c, d -9) and the Moldanubian zone which encompasses a great variety of rocks and hosts the pegmatitic and aplitic rocks (Fig. 1c, d -1 to 8, 10 to 15). The Moldanubian Zone is made up of an autochthonous unit (Fig. 1c, d -1 to 8) called the Moldanubicum sensu stricto and an allochthonous one (Fig. 1c, d -10 to 11) named the Teplá-Barrandian zone or in Bavaria as Zone of Erbendorf-Vohenstrauβ "ZEV" (Malkovský, 1979;Weber and Behr, 1983;Stettner, 1992;Franke et al., 1995). Their rocks are of Neoproterozoic through Early Paleozoic age. ...
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The geology of pegmatite systems encompasses lithology, shape, and structure while the chemistry of major and trace elements is indicative of the ore composition; both are the “pillars” of the CMS classification scheme (Chemical composition-Mineral assemblage-Structural geology) for barren and rare-metal pegmatites, including their granitic affiliates. The term Variscan-type has been coined to describe a style of formation linked to the ensialic orogens and a timebound mineralization sandwiched between the Caledonides and the Alpides. The primary formation covers the time from the Neoproterozoic through the Permian and ends with a hydrothermal phase waning eventually in the supergene alteration and is subdivided into three stages: (1) from diatectic to metatectic gneisses, (2) from metapegmatites, metamorphic pegmatoids to thrusting, (3) from the crust to the mantle and from barren to rare metal pegmatites. This evolution is characterized by a retrograde metamorphism from HP/MP to LP regimes. The tabular and stock-like pegmatitic, aplitic and granitic rocks in autochthonous and allochthonous units are grouped into 8 types (A–H) based on the above qualifiers of the CMS scheme. On a large scale, felsic mobilizates are accumulated by mimetic (facsimile) crystallization in anticlines with the most effective traps encountered where the directions of great circle plunges cut each other at almost right angle (stereonet analysis). The term “mobilizates” is used to describe felsic mobile components in the crust which migrated to a different extent from the site of their formation. On a small scale, where southward-dipping planar architectural elements are cut across by deep-seated lineaments the “temperature depression” of the retrograde system occurs and rare-metal pegmatites are located. This subhorizontal plane is correlated with a gently dipping Moho and vertical lineamentary fault zones with bulges of the Moho (chemical contour map analysis). Spider diagrams whose element contents are normalized to a reference paragneiss are categorized into 4 chemical patterns: (1) circular patterns (= metamorphic mobilizates, magmatic mobilizates), (2) necking-down patterns (= different degrees of fractionation), (3) lens-shaped patterns (= wall rock alteration), (4) stellate pattern (= different degrees of fractionation and mixing of fluids). The marker assemblages among the major elements are: Si-Fe-P: metamorphic to magmatic (sub)crustal mobilizates, K-Na-Al: metamorphic mobilizates, Ti-Mg: restites of metamorphic and magmatic mobilizates, Ca: remnant in the exocontact of pegmatitic systems, Mn: marker of depth-pressure The marker assemblages among the minor elements are: As-Bi: HT hydrothermal-metamorphic fluids, Cu-Ni-Mo: hydrothermal-deep-seated +(ultra)basic sources, U-Zn: hydrothermal-deep-seated sources, Pb: LT hydrothermal, Nb-Ba-Rb: pegmatitic fractionation-Ba (early)-Rb (late), Zr: restites of metamorphic mobilization + fractionation, REE: metamorphic mobilizates. The marker to discriminate hypogene and supergene kaolinization are: (1) hypogene (Ca- Mg out, Zn-Cu-Bi-Rb-Nb in), (2) supergene (Zr-Ti in).
... The lithological units in the study area are part of the Saxothuringian Zone and the northern part of the Moldanubian Zone extending along the western edge of the Bohemian Massif, which is the core complex of the Central European Variscides (Franke, 1995;Franke et al., 1995). The oldest lithologies and structures are representative of a rift basin that evolved from the Cambrian, in parts as early as late Proterozoic, through the Ordovician (Linnemann, 2003;Kroner and Hahn, 2004). ...
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New plate-tectonic reconstructions of the Gondwana margin suggest that the location of Gondwana-derived terranes should not only be guided by the models, but should also consider the possible detrital input from some Asian blocks (Hunia), supposed to have been located along the Cambrian Gondwana margin, and accreted in the Silurian to the North-Chinese block. Consequently, the Gondwana margin has to be subdivided into a more western domain, where the future Avalonian blocks will be separated from Gondwana by the opening Rheic Ocean, whereas in its eastern continuation, hosting the future basement areas of Central Europe, different periods of crustal extension should be distinguished. Instead of applying a rather cylindrical model, it is supposed that crustal extension follows a much more complex pattern, where local back-arcs or intra-continental rifts are involved.
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A model based on new structural and geochemical data is presented. It unifies the structural history of the Izera, Rudawy Janowickie and Kaczawa complexes with the Fore Sudetic block, despite their current separation by the Intra-Sudetic and Marginal Sudetic faults. Above the granitoid Izera, Kowary and Wadroze gneisses, at the base of the structural sequence, the ductile Kowary shear zone marks the basal decollement of the Swierzawa thrust sheet, comprising often highly strained metasediments associated with enriched tholeiitic and alkaline metabasites. Above it, the ductile Kaczorow shear zone, corresponding to the main mylonitic zone within the Leszczyniec shear zone in the Rudawy Janowickie Complex, marks the base of the Dobromierz thrust sheet, characterized by voluminous MORB-like meta-tholeiites and minor metasediments in the higher parts of the Rudawy Janowickie and Kaczawa complexes and the Pyszczynska Hill area of the Fore-Sudetic Block. In the east the Sleza ophiolite and the Gory Sowie Block override the entire nappe stack. Kinematic fabrics in the major and related shear zones indicate D 1 compressional transport towards the northwest, followed by minor D 2 extensional movements. The thrust stack was deformed during D 3 by southwest verging folds, was subsequently intruded by post-orogenic granites, and later disrupted by the Intra-Sudetic, Marginal Sudetic and associated faults.
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A part of the deep seismic near normal-incidence Munchberg-Vogtland-Erzgebirge (MVE) observations located in the Saxo-Thuringian Belt at the northern rim of the Bohemian Massif has been reprocessed using the method of amplitude preserving 3D prestack depth migration. The advantage of this method is its ability to deliver a quantitative and geometrically correct image of the crustal reflectivity without preference of certain dips. The application of a prestack migration method is preferable in complex areas where diffractions and reflections with alternating dips are expected. Along the reprocessed part of the profile, location and dip of many reflective elements have considerably changed compared to the poststack migration [Z. Geol. Wiss. 22 (1994) Appendix 3.4]. The Franconian Line (FL), a major post-Variscan fault, is imaged down to I I km depth and its fault throw can be assessed at 6 kin. Observations of reflecting elements in the prolongation of the FL and a significant change of reflectivity give rise to the idea of an extent of the FL into the lower crust. High reflection amplitudes in the Vogtland area are interpreted within the scope of fluids ascending from a magmatic body at the crust-mantle boundary.
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New magnetotelluric data from the Münchberg Gneiss complex in Southern Germany reveal a zone of extremely high electrical conductivity. 1-D modelling of the data is justified in the period range 0.01 to 10 s. At least three layers are required to explain the steepness of the apparent resistivity curves, and the best-fitting models comprise four layers with successively higher conductivities. The layers of highest conductivity at depths between 2.2 and 3.6 km correlate with pronounced bands of high seismic reflectivity (profile DEKORP 85-4N). The Münchberg complex is today widely recognized as a tectonic klippe, consisting of rocks whose metamorphic and stratigraphic order is inverted rather than overturned. The material was transported into its present position by predominantly horizontal tectonic forces along shear zones. We interpret the high conductivity and high reflectivity as remnants of this transport process.
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The Early Palaeozoic East Krkonoše Complex (EKC) situated in the central West Sudetes, NE Bohemian Massif, is a volcano-sedimentary suite containing abundant mafic and felsic volcanics metamorphosed to greenschist facies. The trace element distribution patterns and Nd isotope signatures (ENd500 = + 3.1 to + 6.6) of the metabasites (metabasalts) indicate that they may be related to a rising mantle diapir associated with intracontinental rifting. At the early stage, limited melting of an upwelling asthenosphere produced alkali basalts and enriched tholeiites which compositionally resemble oceanic island basalts. A later stage of rifting with larger degrees of melting at shallower depths generated tholeiitic basalts with E-MORB to N-MORB characteristics. The values of (87Sr/86Sr)i = 0.706 and ENd500 = − 5 ±1 of the porphyroids (metarhyolites) as well as the lack of rocks with intermediate compositions suggest that the felsic rocks were formed by a partial melting event of continental crust triggered by mantle melts. The geochemistry of the EKC bimodal metavolcanics and their association with abundant terrigenous metasediments suggest that the felsic–mafic volcanic suite was generated during intracontinental rifting. This process, widespread in Western and Central Europe during the Early Palaeozoic, is evidence of large-scale fragmentation of the northern margin of the Gondwana supercontinent. Copyright
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Multidisciplinary studies of geotransects across the North European Plain and Southern North Sea, and geological reexamination of the Variscides of the North Bohemian Massif, permit a new 3-D reassessment of the relationships between the principal crustal blocks abutting Baltica along the Trans-European Suture Zone (TESZ). Accretion was in three stages: Cambrian accretion of the Bruno–Silesian, Lysogory and Malopolska terranes; end-Ordovician/early Silurian accretion of Avalonia; and early Carboniferous accretion of the Armorican Terrane Assemblage (ATA). Palaeozoic plume-influenced metabasite geochemistry in the Bohemian Massif explains the progressive rifting away of peri-Gondwanan crustal blocks before their accretion to Baltica. Geophysical data, faunal and provenance information from boreholes, and dated small inliers and cores confirm that Avalonian crust extends beyond the Anglo-Brabant Deformation Belt eastwards to northwest Poland. The location and dip of reflectors along the TESZ and beneath the North European Plain suggest that Avalonian crust overrode the Baltica passive margin, marked by a high-velocity lower crustal layer, on shallowly southwest-dipping thrust planes forming the Heligoland–Pomerania Deformation Belt. The ‘‘Variscan orocline’’ of southwest Poland masks two junctions between the Armorican Terrane Assemblage (ATA) and previously accreted crustal blocks. To the east is a dextrally transpressive contact with the Bruno–Silesian and Malopolska blocks, accreted in the Cambrian, while to the north is a thrust contact with easternmost Avalonia, deeply buried beneath younger sedimentary cover. In the northeast Bohemian and Rhenohercynian Massifs Devonian ‘‘early Variscide’’ deformation dominated by WNW and NW-directed thrusting, records closure of Ordovician–Devonian seaways between detached ‘‘islands’’ of the ATA and Avalonia.
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Tectonic zones and palaeogeographic units (terranes) in the German segment of the Variscides correlate with equivalents in the Sudetes at the NE margin of the Bohemian Massif. This correlation defines an arcuate structure with an opening angle of about 90°. The structure is truncated to the SE by a crustal scale, NE-trending fault zone with dextral transpression, the Moldanubian Thrust (MT). The arc cannot have been formed by northeastward indentation of the Bohemian Massif, since there is no evidence of a fault zone on the NW flank of the notional indenter, and little evidence for northeastward tectonic transport. Kinematic and age constraints on the main fault zones instead suggest that the structural array was formed by a complex sequence of events. Northwestward displacement along the margin of the East European Platform (EEP) with clockwise rotation was followed by large southwestward movements along the Moldanubian Thrust, and renewed northwestward displacement along the SW margin of the East European Platform.
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Mafic blueschists of the East Krkonoše Complex (EKC, Krkonoše-Jizera Block, central West Sudetes) are glaucophane-bearing metabasites partly overprinted to greenschist facies assemblages. Major and trace element concentrations and Sm-Nd radiogenic isotopes were studied with the aim to evaluate the chemical affinities of their igneous protoliths of Early Ordovician age (485 ± 4 Ma, [1]), and draw inferences on their paleotectonic setting. The distribution of immobile incompatible trace elements patterns (e.g., Th/Nb from 0.07 to 0.1) and Nd isotope signatures (?Ndi = +5.9 to +7.7) of most mafic samples correspond to those of modern basalts of MORB—and OIB-types, respectively, free of significant interactions with materials from the continental crust. Basalts of this compositional diversity may be generated during the early stages of sea-floor spreading, but may also build sea-floor volcanoes (seamounts) situated close to mid-ocean ridges. One sample displays evidence for significant crustal contribution (Th/Nb = 0.4, ?Ndi = + 3.2), while another one, with deep Nb anomaly (Nb/Nb*= 0.33) combined with LREE depletion (Sm/Nd = 0.2018) and strongly radiogenic Nd isotope signature (?Ndi = + 7.4), is reminiscent of supra-subduction lavas formed in the absence of recycling of sediments of continental provenance. The igneous precursors of the EKC mafic blueschists were extracted from diverse mantle sources, that probably reflect a range of tectonic environments: incipient sea-floor spreading subsequent to lithospheric break-up heralded by bimodal volcanism [2], oceanic seamounts and, possibly, fragments of supra-subduction oceanic crust. The occurrence of such a heterogeneous igneous assemblage, combined with the presence of HP-LT metamorphic overprinting, suggests that all these mafic bodies were scrapped off subducted oceanic crust and its attached passive margin, and incorporated into an accretionary complex during the final closure of an oceanic domain, in Late Devonian - Early Carboniferous times.
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Multidisciplinary studies undertaken within the EU-funded PACE Network have permitted a new 3-D reassessment of the relationships between the principal crustal blocks abutting Baltica along the Trans-European Suture Zone ( TESZ ). The simplest model indicates that accretion was in three stages: end-Cambrian accretion of the Bruno-Silesian, Łysogóry and Małopolska terranes; late Ordovician accretion of Avalonia, and early Carboniferous accretion of the Armorican Terrane Assemblage ( ATA ), which had coalesced during Late Devonian — Early Carboniferous time. All these accreted blocks contain similar Neoproterozoic basement indicating a peri-Gondwanan origin: Palaeozoic plume-influenced metabasite geochemistry in the Bohemian Massif in turn may explain their progressive separation from Gondwana before their accretion to Baltica, although separation of the Bruno-Silesian and related blocks from Baltica during the Cambrian is contentious. Inherited ages from both the Bruno-Silesian crustal block and Avalonia contain a 1.5 Ga ‘Rondonian’ component arguing for proximity to the Amazonian craton at the end of the Neoproterozoic: such a component is absent from Armorican terranes, which suggests that they have closer affinities with the West African craton. Models showing the former locations of these terranes and the larger continents from which they rifted, or to which they became attached, must conform to the above constraints, as well as those provided by palaeomagnetic data. Hence, at the end of the Proterozoic and in the early Palaeozoic, these smaller terranes, some of which contain Neoproterozoic ophiolitic marginal basin and magmatic arc remnants, probably occurred within the end-Proterozoic supercontinent as part of a ‘Pacific-type’ margin, which became dismembered and relocated as the supercontinent fragmented.
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A synthesis of published and new data is used to interpret the Sudetic segment of the Variscan belt as having formed by the accretion of four major and two or three minor terranes. From west to east the major terranes are (1) Lusatia - Izera Terrane, exposing Armorican continental basement reworked by Ordovician plutonism and Late Devonian - Carboniferous collision, showing Saxothuringian affinities; (2) composite Gory Sowie - Klodzko Terrane characterized by multistage evolution (Silurian subduction, mid- to late Devonian collision, exhumation and extension, Carboniferous deformational overprint), with analogues elsewhere in the Bohemian Massif, Massif Central and Armorica; (3) Moldanubian (Gfohl) Terrane comprising the Orlica - Snieznik and Kamieniec massifs, affected by Early Carboniferous high-grade metamorphism and exhumation and (4) Brunovistulian Terrane in the East Sudetes, set up on Avalonian crust and affected by Devonian to late Carboniferous sedimentation, magmatism and tectonism. The main terranes are separated by two smaller ones squeezed along their boundaries: (1) Moravian Terrane, between the Moldanubian and Brunovistulian, deformed during Early Carboniferous collision, and (2) SE Karkonosze Terrane of affinities to the Saxothuringian oceanic realm, sandwiched between the Lusatia - Izera and Gory Sowie - Klodzko (together with Tepla - Barrandian) terranes, subjected to high pressure-metamorphism and tectonized during Late Devonian - Early Carboniferous convergence. The Kaczawa Terrane in the NW, of oceanic accretionary prism features, metamorphosed and deformed during latest Devonian - Early Carboniferous times, may either be a distinct unit unrelated to closure of the Saxothuringian Ocean or represent a continuation of the SE Karkonosze Terrane.
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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.
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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.
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The Mariánské-Lázně Complex is a Cambro-Ordovician terrane of oceanic affinity tectonically emplaced between the Saxothuringian Zone and Teplá-Barrandian Unit, NW Czech Republic. It forms a SE-dipping allochthonous body that comprises the largest contiguous exposure of metamorphosed basic and ultrabasic lithologies in the Bohemian Massif. Petrographic evidence indicates that a significant proportion of protoliths underwent eclogite facies metamorphism (570 to 720 °C, 1.44 to 2.10 GPa), followed by an increase in temperature (up to around 800 °C) and a subsequent widespread retrograde amphibolite facies event (550 to 680 °C, 0.75 to 1.20 GPa). New major and trace element geochemical analyses of metamorphosed basic and ultrabasic lithologies indicate that they exhibit geochemical characteristics attributable to a sea floor origin. The metabasites were generated at a spreading centre that interacted with deep-seated upwelling mantle asthenosphere. Separate, independently fractionating basic melt batches existed: these were derived from depleted and enriched asthenosphere and depleted sub-continental lithosphere sources. Geochemical correlation of the Mariánské-Lázně Complex with other early Palaeozoic metabasic provinces facilitates comparison of metabasic lithologies occuring in tectonically dislocated nappe pile thrust sheets, and allows delineation of important suture zones in the European Variscides.
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The Saxonian Granulites represent a major exposure of high-pressure rocks within the mid-European Variscan belt. The granulites emerge in an extensional dome structure beneath a low-grade Paleozoic cover. The boundary between the granulites and their cover is a crustal-scale shear zone with transport top to the SE, juxtaposing high-pressure (HP) granulites against greenschist-grade rocks. Seismic reflection and refraction profiling reveal that the granulite dome and its western continuation up to the SW margin of the Bohemian Massif are underlain by a reflective layer up to 1 s two-way time (TWT) thickness (~3.5 km), with P wave velocities V(p) generally above 6.0 and up to 7.0 km/s (probably a sheet of metabasic rocks). This layer exhibits a NE trending antiformal structure, in line with the granulite antiform, with an apex at ~1.2 s TWT. The outcrop of felsic granulite forms a local cap on the NE part of this high-velocity layer. A magnetotelluric survey has revealed high resistivity in the upper crust and a zone of high conductivity under the high-velocity layer, in the middle and lower crust, terminating ~10 km to the south of the granulite outcrop. Similar high-grade rocks occur in the Erzgebirge antiform SE of the Saxonian Granulites, but their exhumation was later followed by grossly westdirected stacking with medium-pressure and low-pressure rocks, followed by backthrusting toward the SE and late open folds. Isotopic data both from the Saxonian Granulites and the Erzgebirge indicate HP metamorphism ~360-370 Ma, followed by a granulite stage at 350-340 Ma. This is entirely incompatible with the record of low-grade sediments overlying the crystalline rocks, which document subsidence and marine sedimentation lasting until ~330 Ma. This paradox is explained by tectonic underplating, differential thinning of the hanging wall lithosphere, and extensional unroofing of the high-grade rocks derived from one of the subduction zones adjacent towards the NW and SE. Tectonic underplating and exhumation of the granulites must have occurred under the floor of a marine basin.
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Multidisciplinary studies of geotransects across the North European Plain and Southern North Sea, and geological reexamination of the Variscides of the North Bohemian Massif, permit a new 3-D reassessment of the relationships between the principal crustal blocks abutting Baltica along the Trans-European Suture Zone (TESZ). Accretion was in three stages: Cambrian accretion of the Bruno–Silesian, Lysogory and Malopolska terranes; end-Ordovician/early Silurian accretion of Avalonia; and early Carboniferous accretion of the Armorican Terrane Assemblage (ATA). Palaeozoic plume-influenced metabasite geochemistry in the Bohemian Massif explains the progressive rifting away of peri-Gondwanan crustal blocks before their accretion to Baltica. Geophysical data, faunal and provenance information from boreholes, and dated small inliers and cores confirm that Avalonian crust extends beyond the Anglo-Brabant Deformation Belt eastwards to northwest Poland. The location and dip of reflectors along the TESZ and beneath the North European Plain suggest that Avalonian crust overrode the Baltica passive margin, marked by a high-velocity lower crustal layer, on shallowly southwest-dipping thrust planes forming the Heligoland–Pomerania Deformation Belt. The ‘‘Variscan orocline’’ of southwest Poland masks two junctions between the Armorican Terrane Assemblage (ATA) and previously accreted crustal blocks. To the east is a dextrally transpressive contact Palaeozoic amalgamation of Central Europe: new results from recent geological and geophysical investigations.
Article
The Polish Sudetes on the NE margin of the Bohemian Massif comprise a complex mosaic of pre-Permian basement units, traditionally included in the Variscides. A hypothesis of significant Caledonian orogenesis in this area originated in the 1920s, was subsequently rejected, and then was recently revived in models which invoked Early Palaeozoic to Early-Mid Devonian subduction and continental collision along a proposed extension of the Tornquist suture zone. We reassess the evidence invoked in support of the Caledonian orogeny, such as supposed regional pre-Upper Devonian unconformity, Ordovician bimodal magmatism and radiometric, palaeontological, palaeomagnetic and structural data, and suggest these are either inconclusive or misinterpreted. On the other hand, the Sudetes record Mid?-Late Devonian blueschist metamorphism followed by an Early Carboniferous regional high temperature event, widespread Late Devonian/Early Carboniferous flysch/molasse sedimentation and abundant granite intrusion in the Carboniferous to Early Permian. We discuss the usage of the term ‘Caledonian’ in a plate tectonic context and suggest it should not be used simply to denote Early to Mid-Palaeozoic tectonic activity. The tectonic evolution of the Sudetes was temporally different from, and resulted from convergence of different crustal domains than that of the British-Scandinavian-Pomeranian Caledonides. The Sudetic Palaeozoic sequences most probably developed on Armorican Neoproterozoic crust and in adjacent oceanic(?) domains and, therefore, the Sudetes form part of the Variscan orogenic belt.
Article
The Saxonian Granulites represent a major exposure of high-pressure rocks within the mid-European Variscan belt. The granulites emerge in an extensional dome structure beneath a low-grade Paleozoic cover. The boundary between the granulites and their cover is a crustal-scale shear zone with transport top to the SE, juxtaposing high-pressure (HP) granulites against greenschist-grade rocks. Seismic reflection and refraction profiling reveal that the granulite dome and its western continuation up to the SW margin of the Bohemian Massif are underlain by a reflective layer up to 1 s two-way time (TWT) thickness (~3.5 km), with P wave velocities Vp generally above 6.0 and up to 7.0 km/s (probably a sheet of metabasic rocks). This layer exhibits a NE trending antiformal structure, in line with the granulite antiform, with an apex at ~1.2 s TWT. The outcrop of felsic granulite forms a local cap on the NE part of this high-velocity layer. A magnetotelluric survey has revealed high resistivity in the upper crust and a zone of high conductivity under the high-velocity layer, in the middle and lower crust, terminating ~10 km to the south of the granulite outcrop. Similar high-grade rocks occur in the Erzgebirge antiform SE of the Saxonian Granulites, but their exhumation was later followed by grossly westdirected stacking with medium-pressure and low-pressure rocks, followed by backthrusting toward the SE and late open folds. Isotopic data both from the Saxonian Granulites and the Erzgebirge indicate HP metamorphism ~360-370 Ma, followed by a granulite stage at 350-340 Ma. This is entirely incompatible with the record of low-grade sediments overlying the crystalline rocks, which document subsidence and marine sedimentation lasting until ~330 Ma. This paradox is explained by tectonic underplating, differential thinning of the hanging wall lithosphere, and extensional unroofing of the high-grade rocks derived from one of the subduction zones adjacent towards the NW and SE. Tectonic underplating and exhumation of the granulites must have occurred under the floor of a marine basin.
Article
Pegmatitic rocks are very coarse-grained basement rocks abundant in quartz, feldspar or/and mica, in places, endowed either with mega crystals of the aforementioned rock-forming minerals or rare-element minerals. Pegmatites are treated in this study together with aplitic rocks, which are compositionally similar to pegmatites but strikingly different from them by their fine-grained texture. Rocks of the granitic suite take an intermediate position between the two and, locally, they are transitional into both end-member types, emphasized in the denomination by supplements such as aplite granite or pegmatitic granite. A similar scenario can be reported for syenitic and, less frequently, for granodioritic through dioritic rocks which are found to be associated in time and space with pegmatites and aplites.
Article
LA-MC-ICP-MS U-Pb zircon ages and whole-rock geochemical data obtained from volcanic rocks erupted in the northern margin of Gondwana provide new insights on the polyphase magmatic evolution of the NW Iberian domain during the establishment of passive margin conditions in Lower Paleozoic times. The U-Pb data show crystallization ages of ca. 455 Ma for two calc-alkaline rhyolites sampled in the Upper Parautochthon of the eastern Galicia – Trás-os-Montes Zone (GTMZ) and for an intraplate basalt intruded into Middle Ordovician slates of the autochthonous series of the Central Iberian Zone (CIZ). Together with previous data, the ages obtained reveal a periodic magmatic activity across the northern Gondwana margin during the Lower Paleozoic, which is comparable to that observed in NE Iberia and in other massifs of the Mediterranean realm. Both geochronological and geochemical data reinforce paleontological and stratigraphic evidences for paleogeographic proximity between these domains, and contribute to the recognition of extensional-related magmatism along the northern margin of Central Gondwana associated with the opening of the Rheic Ocean.
Article
The Carboniferous Period is closely linked with the Variscan Orogeny which dominated many aspects of Carbonifeous basin development. In addition, tectonic, magmatic and metamorphic processes were closely linked with the ongoing orogenic activity (Figs 9.89 & 9.90). Compared with other orogens, the Variscan Orogeny had an unusally complex evolution and many aspects are the subject of active debate (Franke 2000). However, its development and architecture have become much clearer since the identification and correlation of plate tectonic elements (terranes, sutures, ophiolites, magmatic arcs, foreland fold-and-thrust belts and foreland basins), and the integration of palaeomagnetic and geophysical data with high-precision dating (Timmerman 2004). In general terms, the Variscan Orogeny may be considered the result of Devonian-early Carboniferous accretion onto the southern margin of Laurussia of various Gondwanaderived terranes or microplates (often composite) of predominantly Neoproterozoic (Cadomian/pan-African) crust, together with their Neoproterozoic-Silurian passive margin successions and accreted Ordovician-Devonian island arcs (Franke 1989; Ziegler 1990; Matte 1991). A number of distinct zones have been recognized across Variscan Europe, each of which shows a different sedimentary, magmatic and tectonomagmatic history (Figs 9.89 & 9.90). These zones can be defined as palaeogeographically coherent units that broadly retained their material integrity throughout the Variscan Orogeny.
Chapter
Pegmatitic rocks are not randomly distributed across the Variscan/Hercynian basement in Central Europe. The evolution of pegmatites s.l. in the course of a complex orogeny of Meso-Europe took rather long, from the Devonian (419 Ma) through the Permian (252 Ma). In terms of structural geology and geodynamics, pegmatitic deposits primarily occur in ensialic Variscan-type orogens (calc-alkaline) with a thickened crust and a preponderance of thrusting and nappe stacking. In Rift-type settings (alkaline) a strong subcrustal impact is evident and as reactivated/reworked pseudopegmatites in Alpine-type orogens (calc-alkaline) these deposits developed during the initial stages when the crustal section was still rather thick. Both types pertain to the marginal ensimatic settings. They left their hallmarks to some extent also within the Central European Variscides and at its southern edge in the Alpine-Carpathian Orogen. The geodynamic units subjected to very-low-grade- to low-grade stage metamorphism at the margin of the Central European Variscides are barren with regard to pegmatites and aplites. Pegmatoids with minor B-(Li)-P-REE-U-Be mineralization occur along a suture zone extending across the present-day continents. It resulted from the late Variscan closure of the Rheic Ocean between Gondwana and Laurussia with remnants of an arc-related plutonism. Within allochthonous metamorphic complexes and nappes barren feldspar-quartz pegmatoids plus metapegmatites developed. Further south another part of this former coherent nappe also contains a small Be-Nb-P mineralization. Within the Subfluence zone, marked by continent-continent collision and thickening of the crust pegmatite, granite- pegmatite (miarolitic), pegmatite-aplite and pegmatoid abundant in B, Be, F, Li, Sn, U, P and As are encountered. Heading further to the core zone of the Variscan orogen, strong diapthoresis and shearing in the contact zone between the Saxothuringian and Moldanubian zones sensu lato favored the emplacement of pegmatite and aplite enriched in B, P, Be, Nb, As, Zr and F. High grade metamorphic rocks in an autochthonous position with a protolith mainly of Proterozoic age exist in the core zone. At the margin they are overthrusted onto adjacent geodynamic units and penetrated by multiple intrusions. The Hagendorf-Pleystein Pegmatite Province is located near the root zone for the nappe complexes thrusted onto the north-western geodynamic realms. Pegmatites and aplites with minor pegmatoids of the Hagendorf-Pleystein Pegmatite Province show the most varied concentration of rare elements in pegmatitic and aplitic rocks in this crustal section (B-P-REE-Nb/Ta-Li-Sc-Zn-Be). In some parts in core zone pegmatites can also be observed associated with skarns. Variscan lithologies were incorporated into the Alpine orogen and reactivated during the Alpine orogeny at the southern edge of the Meso-Europe. They contain granitic pegmatites, meta-pegmatites, pegmatoids and pseudo-pegmatites (B-Be-P-Nb-U-F-As-Li-Sn-REE-U). By quality this element assemblage is not very much different from that of the neighboring Variscan parent rocks. The suite of pegmatitic and aplitic mineral deposits is associated with mineral deposits of non-pegmatitic origin. They include thrustbound deposits (Au-As-Sb-(Hg)-Fe-Cu-Pb-Zn), plutonic/granite-related deposits (Sn-W-Mo-Pb-Ag-Zn-(In)-Cu-U), and unconformity-related (U-Pb-Zn-F-Ba). While the deposits can at least in parts structurally and compositionally related to the various types of pegmatites and aplites, stratabound deposits are mainly marker deposits for geodynamic units prone to aplitic or pegmatitic rocks in an ensialic orogen (SMS > > VM FeS-Cu-Zn, SEDEX Fe deposits, black-shale –hosted U-Cu-Mo-Sb-Zn-REE (low-grade-large-tonnage) and graphite). As an exception from this rule, the two last-mentioned mineralization with organic compounds can be considered (see geophysical surveys).
Article
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 well-known Variscan basement areas of Europe contain relic terranes with a pre-Variscan evolution testifying to their peri-Gondwanan origin (e.g., relics of Neoproterozoic volcanic arcs, and subsequent stages of accretionary wedges, backarc rifting, and spreading). The evolution of these terranes was guided by the diachronous subduction of the proto-Tethys oceanic ridge under different segments of the Gondwana margin. This subduction triggered the emplacement of magmatic bodies and the formation of backarc rifts, some of which became major oceanic realms (Rheic, paleo- Tethys). Consequently, the drifting of Avalonia was followed, after the Silurian and a short Ordovician orogenic event, by the drifting of Armorica and Alpine domains, accompanied by the opening of the paleo-Tethys. The slab rollback of the Rheic ocean is viewed as the major mechanism for the drifting of the European Variscan terranes. This, in turn, generated a large slab pull force responsible for the opening of major rift zones within the passive Eurasian margin. Therefore, the µrst Middle Devonian Variscan orogenic event is viewed as the result of a collision between terranes detached from Gondwana (grouped as the Hun superterrane) and terranes detached from Eurasia. Subsequently, the amalgamated terranes collided with Eurasia in a second Variscan orogenic event in Visean time, accompanied by large-scale lateral escape of major parts of the accreted margin. Final collision of Gondwana with Laurussia did not take place before Late Carboniferous time and was responsible for the Alleghanian orogeny.
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Eclogites are intercalated in gneisses in the central and western Erzgebirge. Petrographically, they show a regional zonation from E (Floha Syncline) to W (Oberwiesenthal) with respect to the primary mineralogy, retrograde reactions and textures. Probable quartz pseudomorphs after coesite are described as inclusions in omphacite and garnet in the Floha Syncline rocks. New data are given on the composition of the eclogite assemblage. For the Floha Syncline eclogites, geothermobarometry indicates an equilibrium at about 700-800°C and ≥13-16 kbar with the higher temperature values in the east. A significantly lower temperature (about 650°C) was obtained for an eclogite from the Oberwiesenthal area. Some aspects contribute to an "in situ' model rather than a "foreign' one at least for the high-temperature ecologites of the Floha Syncline. The present results suggest that nappe tectonics is an essential feature in the geological evolution of the Erzgebirge whereas an overthrusting towards the west is favoured. -Author
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The metamorphic sequences of the Saxonian Erzgebirge were thoroughly overprinted by a Variscan medium-pressure event under amphibolite facies conditions. However, eclogitic relics documenting an older high-pressure event are widespread. P-T conditions of the eclogite-facies metamorphism systematically decrease, over a distance of 50 km, from about >29 kbar/850C, in the central part, to 20–24 kbar/650C, in the westernmost part of the Erzgebirge crystalline complex. A distinct gap in P-T conditions exists between the central and the western Erzgebirge coinciding with the fault zone of the Flha syncline. Therefore, the eclogitebearing sequences are assumed to represent at least two different nappe units. The lower-grade eclogite assemblages in the western Erzgebirge display a continuous metamorphic zonation with a gradual decrease of peak metamorphic temperatures towards the west. Assemblages formed in the stability field of coesite and thus indicating a regional ultra-high pressure metamorphism, are restricted to the central Erzgebirge, where they are widespread in the eclogites, but also present in metaacidic country rocks. The same high-temperature/high-pressure conditions, testifying to a burial of at least 100 km, were independently recorded for the ultramafic garnet pyroxenites associated with the eclogites of the central Erzgebirge. Mineral relics included in the eclogite phases and mineral assemblages formed by retrograde reactions permit reconstruction of the prograde and retrograde P-T paths in the different parts of the Erzgebirge crystalline complex.
Article
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Phengites from eclogites and pegmatites (3T, 2M1, coarse-grained and recrystallized) of the Mnchberg Massif (Weissenstein and Oberkotzau) have been dated by the 40Ar/39Ar method. 3T-micas from the eclogites yielded plateau and isochron ages of 3657 Ma. 2M1-micas show disturbed degassing spectra. Micas from pegmatites show a slight excess Ar component, with an isochron age of 353 to 3513 Ma. An age component of approximately 300 Ma was also detected. In combination with age values from the literature, the cooling history of the Mnchberg Massif from eclogite-facies conditions (390 Ma) to cooling below 350C (350 Ma) is documented. The age component of 300 Ma is attributed to a low-grade stage of mineral growth accompanied by a transitional ductile-brittle deformation. The petrological effects include formation of pumpellyite-prehnite-facies minerals, frequently precipitated in microcraks and cleavage planes of earlier formed minerals. This stage has to be seen in conjunction with the intrusions of the Fichtelgebirge granite.
Chapter
By the time the transition zone between the Saxothuringian and the Moldanubian units in northeastern Bavaria was selected as a possible location for the German continental deep-drilling project (KTB), great differences with respect to the interpretation of the geological evolution of the area were obvious, differences which had important consequences for the geochronological evolution of this area. Stettner (1975,1979) assumed that the high-grade Moldanubian gneisses were metamorphosed during Cadomian time, whereas the Saxothuringian sediments of Early Paleozoic age were supposed to have been subjected to an Early Hercynian reworking. In contrast, Schreyer (1966) proposed that the Saxothuringian and the Moldanubian units had suffered the same low-pressure metamorphism, which would mean that at least the last metamorphic overprinting of both units should be of Hercynian age. This was confirmed by structural studies, which showed a continuous structural development from the Moldanubian into the Saxothuringian (Stein 1987). The structural model of the transition zone between Moldanubian and Saxothuringian is also in agreement with the petrological studies of Wagener-Lohse and Blümel (1984) and Blümel (this Vol.).
Chapter
The Variscan basement at the Continental Deep Drilling Site (KTB) Oberpfalz on the western margin of the Bohemian Massif is composed of three polyphase-deformed structural units: the Saxothuringian, the Moldanubian and MP to HP metamorphic nappe complexes of the Zone of Erbendorf-Vohenstrauß (ZEV) and the Münchberg Massif (MM). The boundary between the Saxothuringian und Moldanubian, which is interpreted as a cryptic suture, is represented by the northwestern rim of the HT-mylonite belt of the Zone of Tirschreuth-Mähring (ZTM). This forms part of a formerly active continental margin, whereas the Saxothuringian terrane represents a formerly passive continental margin. Deformation of the Moldanubian active continental margin already began during the oceanic subduction stage and was therefore a longer-lasting, more penetrative deformational event than in the Saxothuringian, where the whole Variscan deformation is related only to the collisional stage. The klippen of the ZEV and MM form part of a previously coherent nappe complex which was originally connected with the western part of the Bohemian (Zone of Tepla-Taus, ZTT) and now covers the Saxothuringian-Moldanubian suture. The Erbendorf Line near Erbendorf is a nappe boundary. Its interpretation as suture zone can no longer be upheld. The Variscan crustal structure evolved during multiphase deformation in which two main events can be distinguished. The older events up to D3 are characterized by NW-directed tectonic transport. This is revealed by the seismic crustal structure which is characterized by dominantly SE-dipping reflectors in all NW-SE-directed reflection seismic profiles. Based on reflection seismic data, D2-backfolding and backthrusting of the Saxothuringian seems to be restricted to higher crustal levels and is interpreted as having resulted from crustal wedging and ramp tectonics. In the roof of a crustal wedge foreland-dipping backthrusts and antivergent folds develop, whereas foreland-directed overthrusting continues inside and below the wedge. The younger event is characterized by NW-trending and SW-vergent D4-folding and thrusting and merges into late-orogenic dextral wrench faulting and extensional tectonics. The ZEV nappe forms a D4-syncline, in which the rocks have been preserved from erosion. The D4-deformation, which increases in intensity to the south, documents a reorientation of the regional stress field at the southwestern rim of the Bohemian Massif. This is interpreted on the basis of geophysical and geological data as resulting from a late-collisional indentation process. Variscan crustal convergence was probably much stronger than previously assumed. Initial; oceanic subduction zones have been completely overridden by continental crust. Therey, the complexly deformed cryptic sutures seen at the present outcrop level of the crystalline basement have lost their contact woth the previous subduction zone. During collision, terranes previously separated by oceanic crust were thus thrust over one another, so that suture zones widely separated before collision are now closely neighboring or oevrridden by other terranes. The latter is presumably the case at the northwestern margin of the Bohemian Massif, where the Bohemian terrane overthrust the Saxothuringian-Moldanubian suture zone.
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
Article
The recognition of nappes in the Saxothuringian Zone, together with tectonic fabrics and structures in the Moldanubian Zone, combine to suggest a mobilistic concept for the geotectonic developement of these regions. The origin of the Saxothuringian basin can be referred to an Upper Proterozoic to Ordovician rifting process. Metamorphosed sedimentary and basic igneous rocks found along the western margin of the Moldanubian Zone and in the Tepla-Parrandean block, conventionally interpreted as part of a Cadomian cycle, might well have been part of a contiguous Saxothuringian/N-Moldanubian rift basin. This basin unterwent tectonic deformation and metamorphism in Devonian and Lower Carboniferous time. The existence of a Cadomian event is uncertain. Padiometric indications of an Ordovician thermal event can be explained, in the Saxothuringian Zone, as representing rift-metamorphism. Similar data from the southwestern part of the Moldanubian Zone cannot, at present, be reconciled with geological observations. The medium-pressure metamorphic rocks of the Münchberg and related crystalline massifs are the result of Devonian overthrusting and crustal thickening. Later, they were thrust over lower grade Palaeozoic rocks, and this tectonic pile overrode, in Lower Carboniferous time, the area of the later Fichtelgebirge/Erzgebirge antiform, thus effecting regional metamorphism in that area. Certain rock units in the nothwestern part of the Moldanubian Zone and in the Tepla/Barrandean area probably are equivalents of the medium-grade Münchberg rocks. This is suggested by close affinities in their lithology, metamorphic facies, and tectonic fabrics. The arrangement of thrust zones within the Bohemian massif might also be related to tectonic shortening and to the rotation of blocks during the Variscan orogeny.
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.
Article
Some K/Ar ages are presented for a suite of minerals for which Rb/Sr ages had already been determined. About 50% of the biotites contain excess Ar and show specific gas-release patterns in stepwise-heating experiments. Undisturbed ages document a cooling of the Munchberg gneiss, NE Bavaria, from 50 to 300oC in the interval 400-350 m.y. ago. The Rb-Sr isochron for two whole-rocks and their respective clinopyroxenes gives a formation age of 449 + or - 37 m.y. for the WeiSS enstein and Martinsreuth eclogites; the 87Sr/86Sr initial ratio is 0.7039 + or - 0.0001. Sm-Nd analyses give epsilon0 Nd values of +11.2 + or - 0.6 (WeiSS enstein) and +5.0 + or - 0.6 (Martinsreuth), indicating a typical MORB precursor for the WeiSS enstein eclogite, whereas that of Martinsreuth is more probably derived from island arc or oceanic basalts. A medium-P metamorphism affected the lower sequence of gneisses at approx 435 m.y. As the system cooled down, a medium-T deformation took place approx 370 m.y. ago.-R.A.H.
Chapter
The crustal structure and the geotectonic development of the Variscan Belt of Europe have been controversial for al long time. Early in this century, F.E.Suess (1912), Kossmat (1927) and others had published thrust-tectonic concepts for the Variscan Belt, which are nothing less but anticipations of modern structural concepts stimulated by plate tectonics. The target area of the present KTB project served, in this early phase of research, as an outstanding example of horizontally directed tectonism. Since this time, the Variscan basement of NE Bavaria has occupied a key role in the ensuing geotectonic debate.
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
The formation of eclogites and felsic granulites of the Moldanubian basement of the Oberpfalz (area of Winklarn, NE Bavaria) has been dated at 424 Ma using the SmNd method on minerals (garnet, zircon, rutile) and whole rock. Only estimates on the age of the tholeiitic protoliths of the eclogite boudins are possible from the scattered U-Pb zircon data and from SmNd systematics. Both methods suggest protolith ages around 1 Ga and indicate, together with the major-, trace- and rare-earth element data, extraction of the tholeiitic melts from a suboceanic mantle with an initial ϵNd of + 7.5.Based on the geochronological data of three typical Moldanubian cordierite-bearing paragneisses which surround the Winklarn area in distances of 30–60 km, tectonic contacts to the low-pressure country rocks of the eclogites are probable as these paragneisses yielded Ordovician ages for their amphibolite-facies metamorphism and post-Pan-African ages for the deposition of their sedimentary precursors.Low-pressure-high-temperature metamorphism at 323 Ma fully reset monazites in the granulites. This widespread and synchronous Carboniferous overprinting of the Moldanubian basement is probably also responsible for the variable opening of UPb zircon, RbSr whole-rock and SmNd mineral systems.A terrane or microcontinent model still appears most suitable to explain the Ordovician, Silurian, Devonian and Carboniferous high-pressure (subduction) events detected all over the European Variscan belt. In this model, oceanic basins between Gondwana-derived microcontinents were successively closed since the Ordovician, causing differently old subduction zone-related high-pressure metamorphisms.
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
Zusammenfassung Inhalt: Die Frage wird erörtert, ob und inwieweit petrographische und petrochemische Charakteristika für die Rotgneisgruppe des Erzgebirges gegeben werden können. Dabei werden die Bestände der einzelnen Verbreitungsbereiche dieser Magmengruppe nebeneinandergestellt. Der Stand der Forschungsaufgabe wird erörtert.
Article
The Moldanubian-Saxothuringian boundary∗ marks a prominent subduction zone in the internal parts of the European Variscides. In the Oberpfalz area deep reflection seismic profiles show a high density of various reflectors mainly in the upper crust indicating a complex polyphase deformation during the collision process.Seismic structures and related data from surface geology suggest that the Moldanubian-Saxothuringian collision consisted of two main phases. The first phase is marked by extensive NW-directed overthrusting of Moldanubian onto Saxothuringian crust along SE-dipping master décollements, some of which can be traced down to the Moho. During an advanced stage progressive overthrusting led to the development of wedge structures associated with SE-directed backthrusting and backfolding in the upper structural levels. Subsequently some of these wedges and their related décollements were folded to form the main NE-striking anticlines. The first phase of crustal imbrication correlates with a regional low-pressure-high-temperature metamorphism (at about 320 Ma) decreasing in grade northwestward. The second phase is characterized by a reorientation of the main direction of crustal shortening from NW-SE to SW-NE which is interpreted as the result of the N-directed indentation of the Vindelician terrane into the Moldanubian. This event is recorded by conjugate systems of overthrusting and subsequent strike-slip faulting in the Oberpfalz and the Bavarian Forest and the Black Forest-Vosges area. In the area of the southern German block between these basement outcrops the indentation front is marked by the arc-shaped axes of gravimetric and magnetic anomalies and Permo-Carboniferous troughs.
Article
Variations in quartz c-axis fabrics in a mylonite hand specimen correlate with variations of microstructure. Large feldspar clasts in the quartz-rich groundmass have caused heterogeneous deformation on the microscale and different deformation paths undergone by the different regions are postulated to have caused the variations in c-axis fabrics. The different microstructures found in the regions are believed to be related to sympathetic variation in strain rate and the degree of local strain heterogeneity.The microstructures reflect a combination of dynamic recovery and dynamic recrystallization processes. Although the importance of these processes is acknowledged it is thought that fabrics have developed in relation to the deformation process, rather than from the effects of recrystallization modifying pre-existing or co-developing deformation fabrics. Although diffusional processes were important, deformation may have taken place primarily as the result of the conservative component of dislocation motion within the subgrain interiors.Simulations of fabric development were attempted using a program based on the Taylor—Bishop—Hill analysis. The observed asymmetric c-axis fabrics could arise in several ways. Only the implications of two-stage histories are explored in these simulations. It is suggested that fabrics develop in relation to the kinematic framework and that the orientation of the elements of a pattern of preferred orientation are sensitive to the last stages of deformation. Observations from the mylonite strongly support the idea of a late stage comparatively homogeneous deformation that has imprinted itself on the developed fabrics.
Article
The Saxothuringian zone is characterized by exotic blocks of relatively high-grade metamorphic rocks set among very low-grade Palaeozoic sequences. The Munchberg complex is a pile of later Proterozoic to early Palaeozoic volcanic and sedimentary rocks, some now at advanced states of metamorphism, in which both stratigraphic sequences and metamorphic grades appear in inverted order. These rocks rest upon a Carboniferous wildflysch, which, in its turn, rests upon an autochthonous Devonian and, locally, a Lower Carboniferous sequence. The flysch material, like the nappes above, was derived from sources in the SE. Special features of the sedimentary facies, the tectonic deformation, and the state of very low-grade metamorphism, combine with the evidence of a well-developed thrust at the base of the wildflysch sequence to suggest that this sequence should be treated as the lowest tectonic unit in the Munchberg pile of nappes. The Saxothuringian zone shows the closest approach to an alpino-type character found in the N part of the Variscides. Basin development, deformation and metamorphism are best explained in terms of a model based on horizontal tectonism. -from Author
Article
Variscan events are interpreted in terms of a geodynamic process of long duration. It began in the early Palaeozoic, possibly in the late Precambrian in some regions, with widespread rifting of the continental lithosphere. Granulite facies metamorphism, widely in evidence in the Ordovician and Silurian, coincides in time with igneous activity and with continuous accumulation of sediment at the surface. That association is taken to indicate continental rift metamorphism above anomalous regions of the mantle. Folding and metamorphism of what is now regarded as the basement began early in the Devonian. By Upper Devonian at the latest wide areas of crystalline basement had been deeply exposed by erosion. The orogenic crustal shortening which began early in Devonian time induced intensive development of nappe tectonics involving the basement rocks. This resulted in deep-reaching crustal imbrications, especially well shown at the Moldanubian-Saxothuringian zone and Saxothuringian-Rhenohercynian zone boundaries, which evolved to carry crystalline basement rocks towards their foreland regions over distances greater than 100 km. During the course of these nappe developments folding of the adjacent sedimentary troughs proceeded. A geodynamic model of the northern flank of the central European Variscan orogen is presented. (Rb/Sr data are given using λRb 87 = 1.42 × 10 -11 y -1. The error limits are taken from the original papers, original data are given in brackets.
Article
The Variscan belt of Europe resulted from the collision of Africa with Baltica and intervening microplates. In early Paleozoic time, Baltica was probably separated from the areas farther south by a North-German/Polish ocean, whose trace is now under younger cover in the northern part of continental Europe. To the south followed an assembly of microplates with African affinities: a Cadomian (Pan-African) orogen (550 to 600 Ma); traces of the Late Ordovician glaciation (South Pole in the Sahara); and, correspondingly, relatively high paleolatitudes. Some of these units also contain fragments of much older crust (up to 2.5 Ga). These microplates were separated from Africa during an important rifting episode in Cambro-Ordovician time. Rifting is reflected in several belts with spilite-keratophyre volcanism, thick sedimentary sequences, and a thermal event at deeper crustal levels. Small yet widespread occurrences of MOR-type basalts (now mostly eclogites) suggest that rifting attained at least the narrow-ocean stage. To the south of the North- German/Polish ocean, in Ordovician time, the following elements can be recognized (Fig. 1): mid-European microplate; Saxothuringian rift/narrow ocean; Tepla/Barrandian microplate; Barrandian/Moldanubian basin, which is possibly continued westward in the Southern Vosges/Black Forest basin; Vindelician microplate; Massif Central/ Western Alps narrow ocean; and southern Europe, possibly separated by an additional basin from Gondwanaland. Closure of all these basins was accompanied by northward drift of the African assembly, during Late Ordovician through Carboniferous time. Opening of the Rhenohercynian basin within the mid-European plate, during Devonian time, contrasts with the general trend of convergence. The Rhenohercynian basin possibly opened when the mid-European plate overrode the spreading-center of the pre-Caledonian ocean farther north, or alternatively, due to back-arc spreading related to northward subduction of oceanic crust farther south. Subduction zones developed in each of these basins. They exhibit a bilateral symmetry, with the Moldanubian block acting as a median zone of structural parting. Subduction of oceanic crust started in Late Ordovician-Early Devonian time (depending on the region), and was largely terminated by Late Devonian or Early Carboniferous time; convergence continued during Late Carboniferous time and involved the subduction of continental crust. Crustal shortening has brought about alpinotype nappe-thrusting, commonly resulting in an inversion of early Variscan, pressure-dominated metamorphic facies. The tectonic assemblages were welded together in Carboniferous time by extensive lowpressure metamorphism and widespread post-tectonic granites.
Article
The central gneiss complex of the Mnchberg Massif consists of the Liegendserie at the base and the Hangendserie at the top. Metagabbros are found in the Liegendserie; eclogites occur in the Hangendserie. New isotope data revive the discussion whether a genetic relationship exists between metagabbros and eclogites in the Mnchberg Massif. It is possible to relate the two types of rocks to the same protoliths by variable degrees of crustal contamination and magmatic accumulation. Therefore, a common magmatic origin may be assumed. Both the metagabbros and the eclogites were affected by amphibolite-facies metamorphism. The amphibolitization of the metagabbros was a prograde metamorphic event. Increasing temperature is indicated by inverse zonation of recrystallized plagioclase and increasing Mg/Fe ratios in garnet from core to rim. Geothermobarometry yields a temperature of 600C and a pressure of about 11 kbar for the peak of metamorphism. In contrast, the eclogites underwent a first high-pressure stage at a minimum pressure of 14 kbar and a temperature estimated at 600C and were subsequently overprinted under amphibolite-facies conditions at 10 kbar/700C. A common magmatic origin of metagabbros (Liegendserie) and eclogites (Hangendserie) of the Mnchberg Massif can no longer be discarded. However, the converging P-T-t paths reflect a different geodynamic evolution of the Liegendserie and the Hangendserie after magmatism and before amphibolite-facies metamorphism.
Article
Three garnet-biotite pairs and eleven garnet-cordierite-biotite triplets from the Steinach aureole (Oberpfalz, North-East Bavaria) were analyzed using an electron probe microanalyzer.
Article
In the Bavarian margin of the Moldanubian basement of the Bohemian Massif, retrograded eclogitic metabasites are found within cordierite gneisses associated with garnet-free serpentinised peridotites. The primary eclogite assemblage Gar + Omph + Qz + Rut ± Ky is rarely retained. Typically, garnet (Pyr12–37, Alm38–56, Gro14–29) is prograde zoned with thin retrograde rims and a kelyphitic corona of orthopyroxene + plagioclase (rare) or Mg hastingsite + plagioclase (An60) (common). Primary omphacitic clinopyroxene (Jd25–35) is preserved within garnet. Groundmass omphacite (Jd15–25), always showing an early exsolution of plagioclase, is usually replaced by a vermicular Di + Plag (An20) symplectite itself overgrown by a diablastic hasting̊site + plagioclase (An40) aggregate. Quartz is rimmed by hypersthene with an outer margin of diopside. Kyanite—never preserved in the metabasites—is altered anisochemically to a symplectite of hercynite + anorthite; commonly only anorthite aggregates remain.Geothermobarometric studies indicate equilibration around 710 ° C, minimum pressure > 15 kb for the primary eclogite assemblage. The granulite facies overprint occurred at probably slightly higher temperatures but lower pressures with later retrogression into the amphibolite fades as mostly shown by the surrounding gneisses.The P-T-t path preserved within these metabasic rocks is that expected from a region having undergone crustal thickening by subduction and crustal imbrication. Their present-day surface exposure underlain by crust of normal thickness would indicate the presence of a basal thrust complex that has driven under the imbricate crustal pile aiding uplift and erosion: a geometry confirmed by recent geophysical studies.
Article
The Alps have been interpreted traditionally in terms of north- or northwest-directed convergence between Africa and Europe1, with Adria (the continental underpinnings of the Adriatic sea and northern Italy) acting as a promontory of the African continent2. This has been supported by recent analyses of Africa/Europe relative plate motions3–5, which indicate dominantly north–south-directed convergence during the Tertiary phase of intracontinental shortening. We show here, however, that kinematic indicators from Alpine tectonites are difficult to reconcile with this motion history (Figs 1 and 2), and are explained best by roughly west-northwest-directed Adria–Europe motion since about 70 million years ago, combined with an arc-normal pattern of motion driven by local body forces. This implies that Adria has moved independently of both Africa and Europe since at least the Late Cretaceous.
Article
The mineral assemblages in the eclogites and meta-sediments of the Mnchberg gneiss complex suggest minimum pressures of about 15 kbar, and temperatures of 60030 C for the eclogite-facies metamorphism. Both rock types underwent a subsequent amphibolite-facies metamorphism at almost the same temperature range. In the Weissenstein borehole the eclogites and meta-sediments show an intimate interlayering on a centimetre scale. Contacts between the two rock types are often sharp and untectonized suggesting in-situ metamorphism of the eclogites. The following features indicate that the gneisses were subjected to eclogite-facies metamorphism:1. Phengite relics and phengite inclusions in garnet contain up to 3.45 Si per formula unit while amphibolite facies phengites have considerably lower Si-contents (3.0–3.25 p.f.u.). 2. Omphacite relics occur in the form of Na-augite (6–9 mol% jadeite)-oligoclase symplectites. 3. Garnets with up to 40 mol% of the pyrope component occur. The geological and mineral-chemical data indicate that large volumes of crustal material have been buried to depths possibly exceeding 70 km. The preservation of primary eclogitic textures as well as symplectitic textures in the presence of a fluid phase, are indicative of a rapid decompression. This would suggest a tectonic uplift (e.g. underthrusting) as is also indicated by the virtually constant temperature range during the uplift.
Article
The petrography, mineral chemistry and petrogenesis of a sample from the Weissenstein eclogite, Bavaria, Germany, has been investigated. The total mineral assemblage comprises garnet, clinopyroxeneI+II, quartz, amphiboleI+II, rutile, phengite, epidote/allanite, plagioclase, biotite, apatite, pumpellyite, titanite (sphene), zircon, alkali feldspar and calcite. Textural observations combined with geothermobarometry (Fe/Mg distribution between clinopyroxene/garnet and phengite/garnet; jadeite-content of omphacite, Si-content of phengite, and An-content of plagioclase) provide indications of two different stages in the metamorphic evolution of the rock. The main phengitequartz-eclogite mineral equilibration occurred at minimum P=13–17kbar, minimum T=62050 C; the retrograde symplectite stage (clinopyroxeneII, amphiboleII, biotite, plagioclase) occurred at P total between 12 and 8.5 kbar.Reactions of the symplectite stage are:(1) phengite (core) + Na2Oaq + CaOaq=phengite (rim) + biotite + plagioclase + K2Oaq + H2O (2) phengite (core) + clinopyroxeneI + Na2Oaq=phengite (rim + biotite + plagioclase + amphiboleII + SiO2 + K2Oaq + CaOaq + H2O (3) clinopyroxeneI + SiO2 + K2Oaq + H2O=clinopyroxeneII + plagioclase+amphiboleII + Na2Oaq + CaOaq The phengite decomposition produces H2O, whereas the clinopyroxene decomposition consumes H2O.The estimated P-T-conditions for the Weissenstein eclogite are in the same order of magnitude as those for other eclogite bodies from the Alps and Caledonides believed to be related to subduction processes.
Article
Complex reaction textures in coronitic metagabbros and retrograded eclogites of the KTB pilot and an adjacent drilling provide evidence for a multistage metamorphic history in the Variscan basement of the NW Bohemian Massif. The eclogites show complete metamorphic recrystallization leaving no textural or mineral relics of their igneous precursors. In contrast, textural relics of the igneous protolith are still preserved in the metagabbros where the metamorphic overprint under high pressure conditions achieved only partial replacement of the initial assemblage plagioclase + augite + amphibole (+olivine or orthopyroxene?) + ilmenite to form the eclogite facies assemblage garnet + omphacite + kyanite + zoisite + quartz+rutile. The garnets in the metagabbros occur in the typical necklace fashion at the borders between the original plagioclase and mafic phase domains. In the same rocks, omphacite formed by a topotactic reaction mechanism replacing igneous augite as well as in smaller grains at the margins of the texturally igneous clinopyroxene where it occurs without fixed orientation with respect to the relict phase. Both eclogites and metagabbros show a partial breakdown under high pressure granulite (transitional to high pressure amphibolite) facies conditions during which omphacite broke down to vermicular symplectites of diopside + plagioclase. A later pervasive medium pressure metamorphism under amphibolite facies conditions led to the development of assemblages dominated by hornblende + plagioclase+titanite: phases prevailing in the overwhelming majority of the surrounding metabasites. Subsequent vein-associated retrogression produced minerals typical of the greenschist to zeolite facies. All metamorphic stages may be represented in a single thin section but although the overall reaction sequence is apparent, the obvious disequilibrium in the rocks makes the use of conventional geothermobarometry difficult. However, calculations made by assuming an approach to domainal equilibrium show that both the eclogite facies and early breakdown occurred above 10 kb. As the metamorphic unit hosting these particular metabasites is generally characterized by pressures below 10 kb these results have important implications for understanding the tectonometamorphic evolution of the region. The relationship between the studied rocks and other units in the NW Bohemian Massif exhibiting a multistage metamorphic evolution is discussed and possible tectonic models evaluated.
Article
Bereits vor der Intrusion des Leuchtenberger Granits waren seine Rahmen-gesteine unter den Bedingungen der Amphibolit-Fazies regionalmetamorph eingeformt worden, wobei sich folgende Paragenesen bildeten: Muscovit + Biotit + Granat ± Sillimanit ± Staurolith (+ Quarz + Plagioklas) Muscovit + Biotit ± Kalifeldspat (+ Quarz + Plagioklas). Die Kontaktmetamorphose führt in den äußeren Bereichen der Aureole zur Paragenese Muscovit + Andalusit + Cordierit + Biotit (+ Quarz + Plagioklas), die der Hornblende-Hornfels-Fazies entspricht. In der inneren Kontaktzone werden die p-t-Bedingungen einer höhergradierten Hornfelsfazies erreicht, die sich in der Paragenese ± Kalifeldspat + Sillimanit + Cordierit ± Almandin + Biotit (+ Quarz + Plagioklas) dokumentiert. Die zonale Anordnung der Mineral-Paragenesen im Kontakthof läßt sich durch quantitative Verbreitungskarten anschaulich machen (150 Modalanalysen aus 59 Fundpunkten). Phasenbeziehungen und Mineralreaktionen werden anhand von AKF- und AFM-Diagrammen diskutiert, für deren Aufstellung 19 Mineralanalysen neu ausgeführt wurden. Durch den Vergleich mit derzeit verfügbaren experimentellen Unterlagen lassen sich die p-t-Bedingungen im Steinacher Kontakthof abschätzen. Danach ist ein Druckbereich von 1,5–3 kbar am wahrscheinlichsten. Mit der Bildung der höchst-gradierten Hornfelse waren 550° C sicher überschritten, während nach dem Jaeger-Modell 700° C als alleroberste Temperaturgrenze anzusehen ist. In der innersten Kontaktzone muß man mit geringen O2-Partialdrucken rechnen.
Article
The metagabbro-amphibolite sequences in the KTB pilot hole contain intercalations of talc-chlorite-amphibole felses (or ''hosbachites''), which show transitional contacts to the adjacent metagabbros. The hosbachites are characterized by relics of a primary igneous texture and still contain igneous minerals like clinopyroxene, biotite and pseudomorphs after olivine, while brown Ca-amphibole was presumably formed in a late-magmatic stage. The geological, textural, mineralogical and geochemical evidence indicates that the hosbachites were derived from ultramafic cumulates, differentiated from a basaltic magma, either in the inner parts of dolerite sills or in small gabbro intrusions. A pervasive metamorphic overprint under medium-pressure, amphibolite-facies conditions which was accompanied by penetrative deformation led to assemblages with green Ca-amphibole +/- anthophyllite +/- cummingtonite +/- tremolite/actinolite + clinochlore + talc + olivine + ilmenite +/- Cr-bearing spinel + sulfides. Phase relationships are consistent with a prograde P-T path leading to the formation of anthophyllite from olivine + talc at peak metamorphic temperatures of 640-700-degrees-C, at assumed pressures of 8-10 kbar, similar to those derived from mineral assemblages in the adjacent metabasites and metasediments. High-pressure relics locally present in coronitic metagabbros and retrograded eclogites of the KTB pilot hole were not recognized in the hosbachites. A retrograde overprint under greenschist-facies conditions led to the total replacement of igneous or metamorphic olivine by aggregates of antigorite + magnetite, chloritization of biotite and the formation of late tremolite/actinolite.
Article
Der rote Gneis des Erzgebirges wird aufgefaßt als echter Granit mit zugehörigem Kontakthof und magmatischem Gefolge, der aus einer älteren Periode stammt und in die variskische Vergneisung mit einbezogen wurde. Damit wird auch die bisherige Ansicht über die Zeitfolge der variskischen Intrusionen im Erzgebirge unsicher. Das Problem der massenhaften vorvariskischen Granitkonglomerate wird neu beleuchtet.
Article
Rb-Sr isotopic analyses of whole-rocks and biotite and U-Th-Pb analyses of zircon and monazite reveal regional metamorphic events for the Ordovician (Caledonian metamorphism) and the Carboniferous (Variscan=Hercynian orogeny), both accompanied by anatexis. The extent of the Caledonian and Variscan anatexis, however, cannot be evaluated, so far, because the field petrographic criteria are not sufficient to distinguish clearly between early and late Paleozoic anatexites. Evidence for a Precambrian metamorphism has not been found. Rb-Sr whole-rock isochrons obtained on leucosomes and melanosomes of partially molten paragneisses are interpreted as a minimum age of the second, early Variscan anatexis. The alternative explanation of the isochrons as a result of local Sr isotopic redistribution without a melt involved is considered less likely. Concordant and nearly concordant zircon ages (318–335 m.y.) of a coarse-grained granite and of diatexites are regarded as evidence for an intensive early Variscan granitization and palingenesis. Concordant zircon ages of diorite dykes, crosscutting the anatexites, establish a lower time limit of 309–312 m.y. for the Variscan anatexis. Rb-Sr ages of biotite (310-290 m.y.) indicate the end of the Variscan metamorphism. Estimates of the time of sedimentation or diagenesis based on Rb-Sr whole-rock analyses for some metasediment series in the north of the area yield maximum ages of 550 m.y., provided that Rb and Sr migration did not exceed substantially the extent of the outcrops (30–500 m) between the time of diagenesis and the Ordovician metamorphism. Otherwise, an upper limit of 2000–2300 m.y., which is the primary age of detrital zircon populations, can be established. Zircon populations of paragneisses and their anatectic derivatives were separated into size and shape fractions. From morphologic studies and U-Pb isotopic analyses, they were found to be composites of young concordant (318–325 m.y.) and old, highly discordant zircon components, with more than fifty per cent of young crystals in some anatexites. The apparent ages of the composites are 320–750 m.y. The U concentrations of the newly formed crystals can be higher, equal, or lower than those of the inherited zircon component. Some peculiarities in the concordia plot of the zircon data of paragneisses and migmatites (curved pattern; inversion of the generally observed systematics with respect to U concentration, grain size, degree of discordance) are interpreted as the result of polyepisodic disturbances of the inherited crystals in connection with new zircon growth. In the concordia diagram, the data points of the individual zircon grains containing inherited components appear to plot in band or wedgelike areas, and not on lines as the patterns of size fractions of the same zircon populations could pretend. Consequently, ages obtained by extrapolation of the regression curves to the concordia are not necessarily meaningful and require verification by other methods.
Article
In order to evaluate the usefulness of the UPb zircon suite method to date eclogitic rocks, zircon populations have been analyzed from two eclogites and one cogenetic metagabbro. Samples were taken from the Münchberg Gneiss Massif (northeast Bavaria) containing the largest B-type eclogite body [1] of the non-Alpine part of Central Europe.All nine analyzed grain size and magnetic fractions plot linearly on a U/Pb evolution diagram and yield intercept ages at 525−31+40 m.y. and 380−22+14 m.y., respectively, indicating a Lower to Middle Devonian eclogitization of a Cambrian tholeiitic protolith. The degree of discordance of the zircon fractions of the eclogites cannot be correlated with either U content, grain size and/or magnetic properties. Instead, new formation of zircon in the course of transformation of the host rocks into eclogite seems to be the governing factor for the degree of discordance. This can be shown to hold true for zircons of the relictic metagabbro which show — in contrast to those of the eclogites — only few signs of new formation and which are therefore much less discordant than the zircon populations of the eclogites (ca. 20% vs. 80%).From the presence of numerous eclogitic layers interbedded with paragneisses it is concluded that the premetamorphic source rocks must have been basaltic lava flows and/or tuffs [2]. In this case the upper intercept age does not only reflect the time of formation of the tholeiitic source rocks but also indicates the time of deposition of the metasedimentary country rocks in the Cambrian. Thus, the UPb zircon suite method applied to such rocks could be an extremely useful tool to unravel depositional ages of highly metamorphosed metasediments.RbSr analyses have been carried out on the post-eclogitic mineral paragenesis, hornblende and phengite, in order to find out whether or not it belongs to the same Acadian cycle (ca. 380 m.y.) as the high-pressure assemblage. Hercynian ages could be expected as the Palaeozoic metasediments surrounding the Münchberg Gneiss Massif were metamorphosed about 320 m.y. ago. However, the data — together with a muscovite—whole-rock age of the immediate country rocks — indicate that hornblende and phengite were formed immediately after formation of garnet and omphacite in the course of the Acadian metamorphic cycle.An initial Sr isotopic ratio of 0.7031 can be calculated for the tholeiitic protolith of the eclogites which is in agreement with a primary magmatic mantle origin.
Article
A crystallization age of 432−10+20 Ma has been determined for the “La Borie” eclogite (Haut-Allier, France), by U-Pb dating on zircons. This age is yielded by the upper intercept with the Concordia curve. In agreement with recent dates obtained in the southern part of the Massif Central, in Brittany and in Vendée, these data indicate that the high-pressure/high-temperature metamorphic phase representative of the pre-Variscan evolution of the western Europe, took place at the Silurian/Ordovician boundary. This high-pressure/high-temperature metamorphism could be significant of subduction processes more or less synchronous, preceding the main continental collision responsible of the Variscan belt (s.s.)AbstractL'aˆge de cristallisation de l'éclogite de La Borie (Haut-Allier, France) aétédéterminéà432−10+20 Ma par la méthode U-Pb sur zircons. Cetaˆge est obtenu par intercept supérieur dans le diagramme Concordia. En accord avec les résultats récents obtenus dans le Sud du Massif Central, en Bretagne et en Vendée, ces données indiquent que la phase de métamorphisme HP-HT caractéristique de l'évolution antéhercynienne de l'Europe occidentale se placeàla limite Ordovicien-Silurien. Ce métamorphisme HP-HT pourrait correspondreàdes mécanismes de subduction relativement synchrones, précédant la phase de collision continentale responsable de l'édification de la chaiˆne hercynienne sensus stricto.