Saxothuringian Zone Research Articles

Contraints on the evolution of the Mid German Crystalline Rise — a study of outcrops west of the river Rhine

Several small outcrops along the western Rhinegraben escarpment expose rocks which represent the western prolongation of the so-called Mid-German Crystalline Rise. This basement ridge separates the Rhenohercynian and Saxothuringian zones of the Variscan belt of Europe and thus marks the boundary between the external and the internal zones. The variable rock association includes an orthogneissamphibolite complex, weakly deformed low grade sediments (?Devonian and Visean), and a number of different syn- to post-orogenic granodioritic to granitic intrusives, all crosscut by Late Lower Carboniferous undeformed lamprophyric dikes and unconformable overlain by Permian sediments and volcanics.

Crustal-scale balanced cross sections through the Variscan fold belt, Germany: the central EGT-segment

Abstract This paper presents crustal-scale, geometrically and isostatically balanced cross sections through the Rhenohercynian and Saxothuringian zones of the Variscan fold belt of Germany. The cross sections are based on surface geological data and on results of the DEKORP deep reflection profiles. They were constructed by means of computer-assisted, iterative forward modelling. The Rhenohercynian zone, corresponding to the Externides of the Variscan belt, developed out of a some 300 km wide rifted continental margin sedimentary prism, limited to the south by the narrow Giessen oceanic basin. During Carboniferous imbrication this shelf sequence is stripped of its basement above one or more decollements and shortened by some 150 km. Closure of the Giessen ocean and emplacement of the ophiolite bearing Giessen nappe entailed a minimum additional 200 km of crustal shortening. The structure of the model correlates well with the present outcrop pattern of rock units, their metamorphic history, the seismic and magnetotelluric architecture of the crust, and its present regional distribution of thickness. The deeper crust beneath the Rhenish Massif is probably devoid of extremely weak decollement horizons and pronounced rheological stratification. In the southward adjacent Saxothuringian zone, nappe development involving the stacking of crustal slices during Middle Devonian to Visean times accounts for a further 170 km of crustal shortening above an infracrustal, partly blind thrust system with flat-ramp-geometry. Total crustal shortening in the analyzed profiles is estimated to be of the order of 520 km; this figure is a conservative value. Prominent uncertainties include the unknown importance of crustal wrenching, the state of strain within thrust units, the distribution of strain within the lower crust, and major changes in thrusting directions during the course of orogenic history.

Magnetic anomaly pattern of Central Europe

The magnetic anomalies represented on our map of Central Europe reflect the geological units and these anomalies are tentatively correlated with known geological structures. In Scandinavia strong magnetic anomalies caused by crystalline rocks of the East European Platform have been observed. Their southern boundary corresponds to the Elbe Line, the border between the mobile part of the East European Platform and the Dutch-German Platform. Here only some broader anomalies can be found and are caused by a magnetic basement (magnetization M = 2 A/m) under a 12–15 km thick layer of sediments. A southern margin of the Dutch-German Platform can be assumed along the W-E “Northern German Line” which is marked by local magnetic anomalies. Younger magmatic events are presumed to be the sources of these anomalies. The Rhenohercynian is located south of the Dutch-German Platform. Small NE-striking anomalies indicate a weak magnetic basement ( M = 0.35 A/m). A magnetic relief varying between 9 and 15 km is obtained by relief calculations. West of the Rhenohercynian anomalies of relatively strong amplitudes prevail. These correspond to the Brabant Massif, the eastern boundary of which presumably influences the tectonically controlled anomalies of Erkelenz and Lingen. There is no magnetic indication of a continuation of the Brabant Massif into the Rhenohercynian. The Mid-German Crystalline Rise and the Saxothuringian zone of the Variscan belt are situated south of the Rhenohercynian. Both areas show strong magnetic anomalies. The southwestern border forms an arch around the Brabant Massif and ends at the Paris Basin Anomaly. This magnetic zone finds its equivalent in a magnetic belt, the Donau Anomaly, which can be followed to the Massif Central in the southwest. This is located in the middle of the Moldanubian, which is characterized by a relatively undisturbed magnetic field. The strong anomalies in the Saxothuringian and Donau magnetic belts point to an accumulation of magmatic rocks. These might be fragments of oceanic crust or ophiolites marking previous subduction or obduction zones. Another explanation may be the intrusion of predominantly basic material during former crustal thinning or stretching. With the exception of the Saxothuringian and Donau Anomaly magnetic belts there is no indication of oceanic crust in the magnetic anomaly map.

Paleopositions of the Saxothuringian (Northern Vosges, Pfalz, Odenwald, Spessart) in Variscan times: paleomagnetic investigation

In the Saxothuringian zone, thermal demagnetization of 581 specimens of Late Devonian-Early Carboniferous gabbros, diorites, granodiorites, gneisses and schists yields different, time dependent paleomagnetic directions. The oldest C n directions (VGP — northern Vosges: 19°N, 222°E; Spessart: 27.5°N, 227°E) were emplaced during crystallization of the Visean plutonics, in the time range 340–330 Ma. C p directions (N.V: 47°N, 241°E), mostly characterized by low unblocking temperatures, are interpreted as slightly younger overprints or as resultant vectors of C n and present day directions. The B directions (northern Vosges: 14°N, 132°E; Odenwald: 6°N, 128°E) exist as primary magnetizations in Middle-Late Carboniferous rhyolites and as overprint in the Late Devonian-Early Carboniferous plutonics. The A′ directions (Sp: 28°N, 175°E; Od: 26°N, 168°E) correspond to overprints emplaced later in the Late Carboniferous. The A directions (northern Vosges: 49°N, 155°E; Pfalz: 45°N, 154°E; Odenwald: 48°N, 151°E; Spessart: 47°N, 165°E) are consistent with the Permian-Early Triassic European directions and are present as primary directions in Permian volcanics. The Late Carboniferous-Permo-Triassic overprints are mainly characterized by high unblocking temperatures due to the presence of secondary hematite. The deviations from C to B and from B to A′ are interpreted as due to two successive block rotations, the first 90° clockwise or counterclockwise, the second 40° clockwise, and the deviation from A′ to A to a 2500 km northward drift of the Saxothuringian.

Lead isotope and Pb-Pb model age determinations of ores from Central Europe and their metallogenetic interpretation

The paper presents lead isotope data from 211 samples from Phanerozoic lead-bearing ore occurrences in Central Europe, particularly from the Southern part of the former German Democratic Republic. The data are interpreted in terms of Amov's dynamic model of continuous lead isotope evolution. The relationships between thoro-genic and urano-genic model ages and the source of lead in different regional units are discussed. We observed differences in lead isotope evolution in the Hercynian internides and externides. Within the Moldanubian and Saxothuringian zones we distinguish five main lead-bearing ore associations: (1) Cambrian, stratiform base metal (Hermsdorf-Waldsassen; 206Pb/ 204Pb=17.50–17.70), (2) Devonian, vein type Sb-bearing, metamorphogene (neumuhle-Hartmannsdorf; 17.80–18.00), (3) Upper Carboniferous-Permian, polymetallic, including tin, vein type (Kutna hora-Freiberg-Altenberg; 18.00–18.20), (4) Triassic (-Jurassic), Pb−Ba, vein type (Střibro-Halsbrucke; 18.20–18.60), (5) Cenozoic, polymetallic, vein type, riftogene (Roztoky-Banska Stiavnica; 18.80–19.10). Pb isotope characteristics from ores of the Montagne Noire and the Brioude-Massiac district correspond to this subdivision. Ore associations from the Rheno-Hercynian zone display higher 207Pb/204Pb ratios which can be explained by more evolved and less metamorphosed source rocks. Mineralizations of the eastern Harz (Strasberg-Neudorf) belong to the Permian association, those from the western Harz (Clausthal-Bad Grund) to the Triassic-Jurassic. Because of Pb isotope agreement the stratabound Rammelsberg and the vein bound Ramsbeck-I mineralization are presumed to be isogenetic. Pb isotope identity of distinct mineralizations in the basement zone (Halsbrucke-Bad Grund) and in the Triassic sediments (Gorny Slask-Mechernich-Bleiglanzbanke) suggests a strong genetic coherence. Pb isotope conformity between the Upper Carboniferous-Permian-Triassic ore associations and Hercynian postkinematic granitoids, and lamprophyric rocks, also favours a close relationship. Pb isotope and other data indicate crustal sources. As the age of the ore associations decreases, crustal influences generally increase, apart from the Roztoky mineralization.

Crustal features of some main suture zones along the European Geotraverse

The European Geotraverse (EGT) crosses along a 4000 km profile from the North Cape to Tunisia the following main suture zones: the Tornquist-Teisseyre zone between the Baltic Shield and the Variscan realm, the transition zones between Rhenohercynian and Saxothuringian as well as between Saxothuringian and Moldanubian zones in the Variscan part of central Europe, and the collision zone between the European continent and the Adriatic microplate.

Age of intrusion and rapid cooling of the Frankenstein gabbro (Odenwald, SW-Germany) evidenced by40Ar/39Ar and single-zircon207Pb/206Pb measurements

The Frankenstein gabbro complex in the northern Odenwald/FR Germany is one of the Hercynian plutonic bodies of the Saxothuringian zone in the Variscan belt.87Sr/86Sr isotopic investigations on pyroxene,40Ar/39Ar age determinations on hornblendes, plagioclases, biotites and pyroxenes and207Pb/206Pb dating on single zircon crystals have been carried out.

Geological interpretation of DEKORP 2-S: A deep seismic reflection profile across the Saxothuringian and possible implications for the Late Variscan structural evolution of Central Europe

A 250 km long deep seismic section—the first of its kind in West Germany's DEKORP Project—is interpreted in terms of late Variscan collisional tectonics. The structural model used is based on the intracontinental thrust tectonics concept of modern collision orogens. The crustal profile across the Saxothuringian Zone seems to indicate north-facing ramps, antiformal stacks and culminations within the crystalline basement of the internal Mid European Variscides that have been only partially overprinted by Permo-Mesozoic crustal stretching.

The evolution of the South Hunsrück and Taunus Borderzone

The main fabric development of the South Hunsrück and Taunus Borderzone (HTB) occurred during Late Variscan homoaxial backfolding of an Early Variscan stack of thrust sheets in a narrow WSW-ENE trending zone, several hundred kilometres long. Rotation and uplift of the southern Rhenohercynian Zone was accompanied by the subsidence of the southward adjacent, northernmost parts of the Saxothuringian Zone, and the development of the Saar-Nahe Basin.During the latest Carboniferous changing stress conditions gave rise to the development of normal faults along the HTB and an extensive tholeiitic and rhyolitic volcanism. Contemporaneous uplift of the Hunsrück is reflected by coarse elastics in the Saar-Nahe Basin. Subsequently the HTB became covered by latest Carboniferous, Early Permian and Mesozoic sediments. Cretaceous uplift of the Rhenish Shield caused their erosion.The faultzone separating the Rhenohercynian Hunsrück and Taunus from the Permo-Carboniferous Saar-Nahe Basin is characterised by reverse and normal faults. Conjugate shear, indicated by slickensides on steep oblique fracture planes, as well as slight folding and small scale thrusting in post-Variscan sediments, may be attributed to compressional foreland deformation, reactivating Variscan joint fabrics and fault systems. Timing of these deformations is ambiguous and could partly be related to dextral shear during the Cenozoic subsidence of the Rhine Graben.Cenozoic sediments of the Upper Rhine graben overstep the northeastern parts of the HTB. Seismic investigations indicate that the HTB is associated with a system of southeast hading listric planes, extending beneath the Moho. Occasional dip-slip earthquakes occur along these fractures.

Sr isotope variation in vein barites from the NE Bavarian Basement: Relevance for the source of elements and genesis of unconformity-related barite deposits

Vein-type barite mineralizations from the Saxothuringian and Moldanubian zone of the NE Bavarian Basement have similar87Sr/86Sr ratios. The different rock types of both tectonic regions are of no influence on those ratios, so that no distinction between barites from the Moldanubian and Saxothuringian zone can be made. The87Sr/86Sr data point to a crustal derivation of ore-bearing fluids rather than to a homogeneous mantle source. Carbonates associated with barite in these veins display similar ratios. The87Sr/86Sr ratios of barites vary with host rocks. From the enclosing wall rocks towards the granitic core zone the ratios tend to increase, while a continuous decrease along with plunge of fold axes (e.g. Berga Anticline) of the host anticline is demonstrated for the Frankenwald vein-type barite occurrences. Based on these isotope data a model on unconformity related barite deposits is discussed for this area.

Antimoniferous mineralization from the Mid-European Saxothuringian Zone: mineralogy, geology, geochemistry and ensialic origin

In the North Bavarian Basement (FR Germany) as well as in the Thuringian Forest (GDR) both of which belonging to the Saxothuringian Zone antimony mineralizations have been worked up to the early fifties. Mineralogical and geological investigations led to the following classification of antimony ore deposits and -concentrations: Antimony in the black shales was concentrated during late diagenetic cementation along with copper. The sulpho-salt-bearing stibnite veins in the metamorphic country rocks of the granites are genetically associated with the granitic intrusions nearby. This is proved by trace element chemistry and structural as well as mineralogical features of those veins. U/Pb age dating of contemporaneous pitchblende from the »polymetallic uranium paragenesis« from Hoehensteinweg uranium deposit yielded a late Variscan age of formation for these veins of type 2. Early Paleozoic rocks (metavolcanics) are suggested to have been the parent material for these granite-related Sb concentrations. However the monotonous Sb veins are more akin to the basic protore in deeper crustal sections. The thermal aureole of a deep-seated heat source is preserved by the Ag/Sb ratio of galena in Pb-Zn veins (type 4), which shows a marked variation along the SW plunging Berga Anticline from increased Ag/Sb ratios near the core of the anticline towards reduced values in mining sites more peripheral to this fold structure. Pb isotopes of stibnite as well as Pb sulphides point to a crustal derivation of both elements. An in-situ re-deposition and leaching of Sb from the wall rocks as well as antimoniferous black shales may be ruled out. It has to be emphasized that intra-crustal mass movements (A subduction) and the late Vanscan igneous activity are the major factors controling release of Sb from the Early Paleozoic low metal concentrations within metavolcanic rocks and subsequent discharge of Sb-bearing fluids within joints and fractures related to late Variscan tectonic movements. A schematic exploration concept is outlined.

The Variscan Belt in Central Europe: Main structures, geodynamic implications, open questions

The Variscan Belt of Europe originated from the confrontation of northern Europe and Gondwana, with intervening pre-Variscan blocks (Cadomian and older: Armorica, Tepla-Barrandean, Moravo-Silesian). Though a strike-slip component cannot be excluded, geological evidence suggests a subduction/collision process. Newly discovered features include basaltic rocks with MORB affinities, high-pressure metamorphic rocks, trench deposits, and large-scale allochthonism, which also affects the pre-Variscan basement, and has often led to an inversion of the metamorphic isogrades. Large-scale thrusting is evident at all crustal levels, and is also observable by geophysical methods. The structure of the Variscan crust is best described as horizontal-tectonic layering. Late Ordovician through Carboniferous convergence closed one Rhenohercynian and one Saxo-thuringian basin to the north, and at least one Mediterranean basin to the south of the pre-Variscan blocks. Subduction occurred from the north as well as from the south down under the dorsal core region. The exact timing and the driving mechanism of this bilateral activity remains a problem. Likewise uncertain are the exact limination of the pre-Variscan blocks, and the existence and width of oceanic areas. In the Saxothuringian Zone and in the south-facing part of the Moldanubian Zone, there is good evidence of early Palaeozoic rifting and of late Ordovician through Carboniferous plate convergence. Palaeomagnetic and palaeoclimatic constraints, however, suggest only a limited extent of oceanic basins in these areas, while they appear to allow a wider ocean at the Rhenohercynian-Saxothuringian boundary, which is relatively unobtrusive from the geological point of view. Research during the past decade has produced a wealth of information on the Variscan Fold belt in Europe. The resulting geodynamic models, however, are extremely diverging. Evidently, it is not the time yet for definitive statements. The present paper intends to review the state of knowledge, to deduce such basic items of a geodynamic model as appear reasonably well documented, and to point out the most urgent open questions. Stress will be laid on Central Europe, which offers a complete cross section through the northern flank of the Variscan Belt, and where we have carried out most of our studies during the last years.

Late events in the tectonic history of the Saxothuringian zone

Summary The Saxothuringian zone lies between the Moldanubian block (largely consolidated in late Precambrian time) to the south and the Rhenohercynian zone to the north. It is characterized by exotic blocks of relatively high-grade metamorphic rocks set among very low-grade Palaeozoic sequences. These ‘Zwischengebirge’ (Münchberg, Wildenfels, Frankenberg) were formerly interpreted as metamorphic ‘diapirs’. Recent investigations have led to a revival of the nappe concepts previously proposed by Suess, Wurm, Kossmat and others. The Münchberg complex is a pile of later Proterozoic to early Palaeozoic volcanic and sedimentary rocks, some now at advanced states of metamophism, 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 Münchberg pile of nappes. Tectonic deformation of the Palaeozoic sequence began with the production of tight to isoclinal, recumbent, NW-facing folds, accompanied and outlasted by subhorizontal thrusting. It was at this time that the nappe-like tectonic units (already in their metamorphic state) were emplaced. An F 2 refolding produced open, upright to SE-facing folds. A study of illite crystallinity has indicated the significance of a transverse zone, which was the locus of enhanced heat flow throughout the time of deformation, and has confirmed that a metamorphic inversion was introduced when the (relatively strongly metamorphic) wildflysch was thrust over the autochthonous Devonian and Carboniferous. The Saxothuringian zone shows the closest approach to an alpino-type character found in the northern part of the Variscides. Basin development, deformation and metamorphism are best explained in terms of a model based on horizontal tectonism.

Variscan wildflysch and nappe tectonics in the Saxothuringian Zone (Northeast Bavaria, West Germany)

Variscan wildflysch and nappe tectonics in the Saxothuringian Zone (Northeast Bavaria, West Germany)

Chemical variation in Hercynian basalts relative to plate tectonics

The geochemistry of Hercynian basalts is reviewed with special reference to SW England. Hercynian basalts have the following chemical features: (1) relative enrichment in incompatible elements, (2) fractionated rare earth element (REE) patterns ranging from mild to strongly light-REE enriched, and (3) generally low large-ion-lithophile (LIL)/high field strength (HFS) element ratios. High and variable alkali contents and K/Rb ratios are indicative of low-grade alteration. Basalts are predominantly tholeiites, although alkali basalts may be developed on some trough margins. Basalts from different sedimentary troughs within the main Hercynian tectonic zones are characterized by specific incompatible element abundances and ratios. These variations are interpreted in terms of variable fractional crystallization, partial melting and mantle heterogeneity. Hercynian volcanism was typically bimodal with basic and acid products characterizing the Rheno-Hercynian and Saxo-Thuringian zones. Activity in the Moldanubian zone was more variable with a high proportion of calc-alkali andesites. Two systematic chemical changes throughout the Devonian and Carboniferous have been recognized, with both light/heavy REE and LIL/HFS element ratios increasing with time. These features may relate to the introduction of LIL-enriched fluids derived by dehydration of subducted lithosphere that caused progressive metasomatism of the overlying mantle wedge. Chemical discrimination of tectonic setting demonstrates that most of the basalts typify the continental intra-plate environment, although early Devonian basalts produced in the initial stages of continent rifting have some of the chemical features of incompatible element enriched oceanic basalts. An ensialic back-arc basin model underlain by a shallow northward-dipping subduction zone provides a possible explanation for the spatial and temporal variation in the Hercynian volcanic rocks.

The intermediate massifs in the Saxothuringikum (German Variscan orogene)

The problematic crystalline complexes of the Saxon intermediate massif and of the gneiss massif of Münchberg in NE Bavaria, which appear abruptly in the Paleozoic series of a large synclinorium of the Saxothuringian zone in the German Variscan mountains, can be interpreted in the light of modern data and methods as pre-Variscan intruded laccolithic complexes which were altered into gneiss and sheared out divergently as autochthonous klippes in Variscan time.

The state geological documentation centre (Geofond)

The most complete Ediacaran (terminal Neoproterozoic) to Early Cambrian record in the Saxo-Thuringian Zone is provided by the Cadomian-deformed Ediacaran Rothstein Formation and the unconformably overlying Early Cambrian Zwethau Formation in the Torgau–Doberlug Syncline (TDS). Conglomerates and greywackes of the marine Rothstein Formation are of continental magmatic arc provenance and record detrital zircon SHRIMP ages that indicate a Late Cryogenian to Early Ediacaran (700–580 Ma) age for the arc source and its emplacement into Palaeoproterozoic (2000 Ma) crust. Tuffaceous intercalations record extrabasinal eruptions of evolved calc-alkaline lavas that point to ongoing Cadomian continental arc magmatism. SHRIMP zircon ages from a tuffaceous layer date this volcanic activity and the formation's deposition as Late Ediacaran (566 ± 10 Ma). Close correlation in provenance and age to other Ediacaran units of the Saxo-Thuringian Zone points to a common palaeogeographic setting in the Avalonian–Cadomian belt. Detrital and inherited zircon ages, and Nd-isotopic ratios from these Ediacaran siliciclastic rocks suggest a position on the active margin of Gondwana near the West African craton.The Early Cambrian Zwethau Formation records the erosion of the underlying Ediacaran rocks followed by the successive evolution of: (i) a carbonate-dominated subtidal ramp with calcimicrobial–archaeocyathan buildups, (ii) a shallow-subtidal to intertidal mixed ramp with peritidal sediments, and oolite shoal complexes, and (iii) a more siliciclastic depositional environment. Calcimicrobial–archaeocyathan buildups, oolite shoals, and sulfate nodules in intertidal sediments indicate warm, arid to semi-arid climatic conditions and a palaeogeographic setting in equatorial to sub-equatorial latitudes. Archaeocyaths, the trilobite taxon Dolerolichia, and the sedimentary facies assemblages compare closely to those of the “Mediterranean” Early Cambrian, constraining the palaeogeographic setting to the European, sub-equatorial, western Gondwana shelf realm. Archaeocyaths record a mid-Early Cambrian (Middle Issendalenian/Ovetian) age, making these the oldest Cambrian sediments in both the Saxo-Thuringian Zone and the Bohemian Massif. The Middle Cambrian lithological and palaeontological records of the TDS closely resemble those of the Frankenwald area of the Saxo-Thuringian Zone as well as those of other fragments of the western Gondwana shelf in Spain, Morocco, and Bohemia.Archaeocyaths, Early Cambrian trilobites, and correlations with Morocco suggest Cambrian deposition commenced at ca. 520 Ma, such that the Cadomian unconformity represents a time gap of about 35–55 Ma (latest Ediacaran to Early Lower Cambrian). This gap, which is related to the Cadomian orogeny and so common to all areas of the Cadomian belt, obscures some of palaeobiological, palaeogeographic, and climatic change that marks the Precambrian–Cambrian transition.