Article

Ar-Ar spectra on minerals from the KTB and related medium-pressure units

Authors:
  • retired (from Federal Geological Survey of Germany, Hannover)
To read the full-text of this research, you can request a copy directly from the authors.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... Lithotype 4 is widespread near the thrust plane of the nappe complexes of the Münchberg Gneiss Complex and the Zone of Erbendorf-Vohenstrauß, a dynamometamorphic event operative between 377 and 372.5 Ma and a maximum temperature of 570°C (Figs. 1 and 3) Tables 4 and 5) (Kreuzer et al. 1993). ...
... All of them have derived from paragneisses (Fig. 10). They are correlated with the formation of Nb-Ta minerals (Coltan) between 303.8 ± 1.9 and 299.6 ± 1.9 Ma and the presence of lamprophyric dykes found all across among the pegmatitic rocks which formed in the time span 306 ± 4-295.1 ± 3 Ma (Kreuzer et al., 1993;Dill et al., 2008bDill et al., , 2009. The link to the afore-mentioned Si metasomatites has been recorded by Takagi et al. (2007). ...
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
... The MMC in northeastern Bavaria is located in the northwestern portion of the Bohemian Massif ( Fig. 1) and forms an oval-shaped complex (~25 9 15 km 2 ), extending from the village of Bad Berneck in the south to the town of Hof in the north. As part of the Saxo-Thuringian Zone (STZ), the metamorphism of the MMC took place during the Variscan orogeny (Gebauer & Gr€ unenfelder, 1979;Kreuzer et al., 1989Kreuzer et al., , 1993Stosch & Lugmair, 1990;Hammerschmidt & Franz, 1992). This orogeny resulted from the movement of Gondwana to the northwest and its collision with Laurussia (Lauren-tia+Baltica) to form Pangaea (Franke, 1989;Linnemann et al., 2003;Massonne & O'Brien, 2003;Massonne, 2005;Linnemann & Romer, 2010;Arenas et al., 2014). ...
... Ar-Ar plateau and isochron ages, determined on phengite from eclogites, yielded 365 AE 7 Ma (Hammerschmidt & Franz, 1992). Kreuzer et al. (1993) obtained an age of 379 AE 1 Ma (Ar-Ar plateau and K-Ar isochron), studying muscovite and hornblende from eclogite. Kreuzer et al. (1989) got different ages (410-345 Ma) for the metamorphism in all four units of the MMC, applying the K-Ar method to biotite, hornblende and muscovite. ...
Article
Small oval-shaped, unshielded monazite grains found in a Variscan garnet-muscovite-bearing mylonitic paragneiss from the Liegendserie unit of the Münchberg Metamorphic Complex (MMC) in the northwestern Bohemian Massif, central Europe, yield only pre-Variscan ages. These ages, determined with the electron microprobe, have maxima at about 545, 520 and 495 Ma and two side-maxima at 455 and 575 Ma, and are comparable with previously determined ages of detrital zircon reported from paragneisses elsewhere in the NW Bohemian Massif. The pressure (P) - temperature (T) history of this mylonitic paragneiss, determined from contoured P-T pseudosections, involved an initial stage at 6 kbar/600 °C, reaching peak P-T conditions of 12.5 kbar/670 °C with partial melting, followed by mylonitization and retrogression to 9 kbar/610 °C. The monazite, representing detrital grains derived from igneous rocks of a Cadomian provenance between 575–455 Ma, has survived these Variscan metamorphic/deformational events unchanged because this mineral has probably never been outside its P-T stability field during metamorphism. This article is protected by copyright. All rights reserved.
... The fracture system occurs also within the granite plutons. Lamprophyric dykes 305-295 in age (Kreuzer et al. 1993) intersect discordant the older dykes indicating again the late-Variscan crustal E-Wextension at about 310 Ma. These dyke events give evidence for early formed deep-seated fractures during the early cooling stage of the granites. ...
Chapter
Full-text available
This chapter documents the fracture process associated with the early cooling stage of felsic magma. Characteristics of pre-exhumation joints include their spatial distribution in granite bodies, their fracture surface morphology, and geological and petrological evidence for the depth of fracture initiation. These characteristics allow inferences about the depth and the time of joint origin in the South Bohemian Pluton. The intrusion levels of currently exposed granites of the pluton were 7.4 km in the northern part and 14.3 km in the southern part. Within the northern Mrákotín Granite (Boršov) early NNE joints propagated while the granite was at a temperature near the solidus, and, in part, magma was still being injected, post-dated by thin granite dykes along NNE joints. Evidence for the pre-exhumation initiation of these joints comes from the geochronological dating of these late-granite dykes (1–2 cm thick) at 324.9 Ma in age, which were creating their own rupture in the rock. The timing of the pluton emplacement at 330–324 Ma and the cooling ages of 328–320 Ma have been given by previous studies. From fluid inclusions within the late-granite dykes that occupy joint surfaces, the trapping depth of the analysed inclusions was calculated to be 7.4 km. Near the solidus H 2 O separates during the crystallization of anhydrous phases. The associated increasing H 2 O pressure can initiate the first cracks and can generate a small portion of new granitic melt, which forces the cyclic fracture propagation together with mobile, low-viscosity ‘residual melt’ input into the fracture. The determination of the intrusion level and time at which the dykes began cooling provide evidence for the joint initiation at a depth of 7.4 km, which was connected with the level and process of final emplacement and early cooling of the Mrákotín Granite long before the main exhumation. At the earliest, the erosion of the upper rock pile, 7.4 km in thickness, started significantly after generation of the early joint sets. The NNE-trending joints are persistent in orientation throughout the South Bohemian Pluton, but the joint-surface morphology varies in all subplutons and occupies all sections of the stress intensity v. crack-propagation velocity curve (Wiederhorn-Bahat curve).
... K–Ar and Ar–Ar mica ages of ca. 325 Ma were also found at some localities around the Leuchtenberg granite (Kreuzer et al. 1989Kreuzer et al. , 1993; Henjes-Kunst, personal communication) and were attributed to the contact metamorphic overprint. The northern Leuchtenberg granite and the associated redwitzites show a northward decrease of the K–Ar biotite and amphibole ages from ~325 to ~300 Ma within a distance of ca. 10 km. ...
Article
Full-text available
Pb-evaporation zircon analyses coupled with a detailed cathodoluminescence (CL) study on the complete series of granitoids from the northern Oberpfalz, NE Bavaria, provide new evidence for the commencement and timing of late-Variscan magmatism. All granitoids analysed in this study were dated before by Rb-Sr and/or K-Ar methods. Investigated samples comprise medium-grained, I-type dioritic rocks (redwitzites), I/S-type granites (Leuchtenberg, Marktredwitz (G1), Zainhammer) and S-type granites (Falkenberg, Liebenstein, Mitterteich, Friedenfels, Steinwald, Flossenbrg, Brnau). The zircon evaporation technique reveals three groups of 207Pb/206Pb ages which are interpreted to represent magmatic crystallisation: (1) ages of 324-321 Ma are found in all analysed redwitzites and in almost all I/S-type granites; (2) the granites of Falkenberg and Liebenstein yield ages of ~315 Ma; (3) ages between 312 and 310 Ma are recorded in the Mitterteich, Friedenfels, Steinwald and Flossenbrg granites. Titanite crystals from different redwitzite bodies yield conventional U-Pb ages of 325-322 Ma, identical to the Pb-evaporation zircon data of these rocks. The S-type granites of groups 2 and 3 contain zircons with relict cores but only a small number of them yield older ages, indicating that some of the cores must have lost their radiogenic Pb. From the geochronological data, we infer that metamorphic conditions of the Variscan crust produced different granite types at different times. The data support a model involving an early period of mantle-related magmatism which postdates the final convergence stage of the Variscan orogen. This magmatic activity was at the same time as the thermal peak of regional metamorphism and is followed by a late period of crustal anatexis, which is probably related to post-collisional extension of the thickened Variscan crust.
... Contact metamorphism occurred especially in the Saxothuringian rocks of the Fichtelgebirge. The final stage of the Variscan magmatic activity is characterized by the intrusion of late-Carboniferous lamprophyres (Kreuzer et al. 1993) and probably Permian dykes of quartz porphyre (Stettner 1981). ...
Article
Full-text available
 Since 1985, apatite fission-track analysis was applied to more than 70 samples from surface outcrops and shallow boreholes at the western margin of the Bohemian massif. Apatite ages were determined by the grain-population method. Additional information from the frequency distributions of fully confined spontaneous tracks was used for modelling of t–T paths in the low-temperature range (<120 °C). Seven zircon samples were dated by the external detector method. Zircon ages between 283 and 215 Ma indicate unroofing during the Permian molasse stage and the Triassic. Tectonic quiescence and slow subsidence prevailed from the Jurassic until the middle Cretaceous. In the basement area south of Weiden, a Mesozoic partial annealing zone (for apatite fission tracks) is now exposed at the surface. Farther north, the basement was affected by stronger Cretaceous and Palaeogene erosion, which yielded cooling ages between 110 and 49 Ma. This second period of post-Variscan denudation was correlated to reverse faulting along the Franconian Line.
... It is logical to assume the late Paleozoic A-type mineralization to have been sourced from the root zone, too (Fig. 14). During the late Devonian, the nappes underwent HP/MP regional metamorphism and were converted into metapegmatites (type G) (Kreuzer et al., 1993). The overthrusting of the allochthonous units, today present in two discrete klippen, the Münchberg Gneiss Complex (MGC) in the N and the Zone of Erbendorf-Vohenstrauβ (ZEV) in the S were accompanied by the emplacement of tectonically induced felsic mobilizates along their eastern boundaries derived from retrograde metamorphic processes (type F) during the waning stages of a MP-HP metamorphic regime (Figs. ...
Article
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).
... 380-400 Ma datiert wurde (u.a. Kreuzer et al., 1993). In den Metabasiten wurden auch Mineralparagenesen einer früheren Hochdruck-Metamorphose nachgewiesen (O'Brien et al., 1992). ...
Article
Full-text available
Zugl.: Giessen, Univ., Diss., 1995. Computerdatei im Fernzugriff.
... Amphibolite-facies metamorphic overprint in the three uppermost units occurred at c. 380 Ma, as constrained by K-Ar and Ar-Ar ages of white micas and hornblende . White micas from the Prasinit-Phyllit-Serie yield consistently younger K-Ar ages of c. 365 Ma (Kreuzer et al., , 1993 and Ar-Ar ages of 368-374 Ma (Kreuzer and Seidel unpubl. data), indicating juxtaposition with the other units of the Münchberg Massif during the Late Devonian. ...
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.
... M.RENÉ (Kreuzer et al., 1993) have dated biotite in the lamprophyre to c. 295 Ma, which corresponds well to an age of 297 Ma (Rb-Sr) for albite granite from a smaller stock at Křížový kámen in the Rozvadov pluton area (Siebel et al., 1996). These analyses suggest that the aplite genesi s can be dated within a range of 315 -295 Ma, l.e. ...
Chapter
Aplites and pegmatites can be correlated with their country rocks using the temporal, the compositional/chemical and physical relationships. The emplacement of pegmatitic rocks began during the early Paleozoic (470–440 Ma) which are encountered as metapegmatites in nappe units in the Bohemian Massif, proper, and in the Ötztal Massif within the Alpine Mts. Range. At the end of the MP-HT metamorphism around 370 Ma pegmatoids came into existence in the allochthonous units. From the core to the margin near the collision zone the granites and the pegmatites get younger. The belong to the Variscan heat event, but a close-up view of some of the pegmatites bear witness of an older cooling age of the muscovite from the HPPP pegmatite than the whole-rock age of the nearby Flossenbürg granite. The pegmatite show different ages as the major-rock-forming silicates and rare-element minerals, e.g. columbite are considered. Considering the element assemblage of the pegmatites and aplites in the Hagendorf-Pleystein-Pegmatite Province reveals that the element has not been derived from one source only. Intracrustal sources may be claimed for Li, F, U, Sn, B, P, As and Mn. Niobium, beryllium and bismuth (?) are of subcrustal derivation. In addition, there are subcrustal - intermediate repositories as it is the case with the Zn, REE and Y. Apart from the granitic intrusive rocks and metamorphic rocks, another group of subcrustal magmatic rocks, (meta)-lamprophyres, has not been drawn the attention to, which they deserve. If the fractionation and zonation in a pegmatite field or province is investigated and an attempt is made to compare these individual pegmatites and aplites with a nearby granite some critical points have to be considered.
Article
Petrochemical and Rb-Sr, K-Ar and Sm-Nd isotopic data presented for the Mitterteich granite provide information on whole rock and mineral compositional characteristics, intrusion and cooling history, and protolith nature and put further constraints on the Variscan magmatic evolution in north-east Bavaria.The compositional characteristics classify the Mitterteich granite as a peraluminous (monzo-)granite (SiO2 67.3–73.5 wt.% ). Values for K2O/Na2O (> 1.2 and Al2O3/(CaO + N2O + K2O) (>1.1) are in the range of S-type granites. The rare earth elements show fractionated chondrite-normalized patterns (La N /Yb N =24–19) with negative Eu anomalies (Eu N /Eu N *=0.35–0.19). The micas have restricted ranges of major element composition, but reveal notable variations in trace element concentrations. Different biotite fractions of single specimens show a trend to lower concentrations of compatible elements in the finer fraction which can be explained as a result of asynchronous growth during the fractionation process. The PT conditions of crystallization of the magma based on muscovite and biotite is 600–640C at 3 kbar. Regression of the whole rock samples gives an isochron corresponding to a 87Rb-87Sr age of 310 7 Ma, initial 87Sr/86Sr of 0.71040.0010 (2 errors) and MSWD =0.03. Muscovite and biotite yield concordant K-Ar ages between 310 and 308 Ma, indicating a fast cooling rate of the granite intrusion. Nd T310values average –4.21.0. Nd model ages of 1.4 Ga suggest a source region of mid-Proterozoic age.The Rb-Sr isochron age and initial Sr ratio of the Mitterteich granite are indistinguishable from those of the adjacent Falkenberg granite, establishing a genetic link. However, the K-Ar mica ages suggest that the Mitterteich granite must have undergone a faster uplift or cooling history than Falkenberg. Confronted with the geochronological record of granite emplacement in north-east Bavaria, the new results substantiate the view of three key periods of magmatic activity around 330–325, 315–305 and 290 Ma.
Article
The type of provenance information obtained from dating detrital grains will depend upon the thermal sensitivity of the isotopic dating method. For detrital zircon, the two methods (fission track and U–Pb) in routine use have widely different thermal sensitivities and therefore may provide quite different provenance information. Zircon fission track data are sensitive to thermal overprinting in the temperature range 200–300°C and therefore have the unique ability to provide source thermal history data. The purpose of this study is to evaluate the suitability of zircon fission track data for provenance studies of the Triassic siliciclastic sediments of central Europe with an emphasis on testing proposed sediment pathways. Zircon fission track data obtained during this study are consistent with a sediment pathway linking Fennoscandia with the central German Basin. In addition, during Carnian times, there must also have been a pathway linking the Schilfsandstein Formation of the German basin with the Lunzersandstein of the Tethyan realm, pointing to a European-wide fluvial transport system. Detrital zircon fission track data are therefore a useful means for discerning sediment source and reconstructing sediment pathways.
Article
Full-text available
&#112The KTB boreholes that were drilled from 1989 to 1994 near Windischeschenbach, NE Bavaria, penetrated rocks of the Zone Erbendorf-Vohenstrauss (ZEV). The upper and lower sections of the 9101m-deep profile are mainly composed of gneiss units (former greywacke sediments) and variegated units (alternating gneisses and metabasites, probably of volcano-sedimentary origin). Metabasic units (amphibolites, metagabbros) constitute mainly the middle section. Geochemical data suggest their derivation from oceanic crust. All rocks are probably pre- to early Ordovician in protolith age. They underwent early Devonian MP metamorphism. Relics of preceeding HP metamorphism are preserved in metagabbros. Felsic to mafic dykes (dominantly Upper Carboniferous lamprophyres) crosscut the metamorphic rocks. The ZEV units are steeply inclined down to the final depth. The main structures (foliation, folds, faults) trend NW–SE. Semibrittle to brittle deformation plays an unexpected important role. In contrast to the conditions of the Mesozoic (mainly Cretaceous/Tertiary) faulting the P–T conditions of MP metamorphism, the cooling ages and the late-Variscan graphite-bearing prehnite–actinolite facies mineralization show no significant depth-dependent gradients. This can be explained by Mesozoic supracrustal stacking. A bundle of reverse faults between 6850 and 7300&#113m belongs to the Franconian lineament and can be correlated with the most prominent seismic reflector (SE1) in the area.
Article
 Paleostress analysis and evaluation of the stratigraphic unconformities reveal an extremely polyphase development of one of the most prominent fault zones of Central Europe, usually known as the Franconian Line (FL). Because the FL is just one fault zone, although it is the most important fault zone within a complex fault system, a more appropriate term is used herein: Western Border fault zone of the Bohemian massif (WBZ). The reconstruction of the paleostress history was carried out by analysing sequences of individual strain increments that belong to the same stratigraphic “units” (e.g., late-Variscan granites, Tertiary basalts). A succession of at least 15 paleostress directions and tectonic regimes have been determined since late-Variscan time.
Article
&#112Leaching experiments (pH=3, T=70&#117C) have been performed on paragneisses and metabasites from the KTB drill cores. The experiments yielded the fraction of total element content that is easily available to acidic aqueous fluids. The intensity of mineralogic alteration in the studied samples is expressed in terms of easily leachable fractions of K or Ba. Its reliability is shown by correlation with a petrographically determined alteration index. For the paragneisses, the easily available fractions of Fe, Mg, Mn, Ca, Na, K, Ba, Co, Cr, Ni, Sr, V, and Y together with the variables whole-rock content and depth of samples were subjected to a factor analysis, the results of which are presented as a five-factor model. Correlation plots of easily leachable element fraction and whole-rock content vs an alteration index (percentage of leachable K) reveal the changes in leachability as a function of intensity of alteration. In a cataclastic boundary between paragneisses and metabasites at 1610&#113m depth, the changes in whole-rock content and leachability of elements are studied in detail and compared with the general trends in alteration. The results point to considerable intercrystalline element redistribution during retrograde metamorphism, with only insignificant element transport on a meter scale. Although considerable fractions of many elements were easily available to acidic fluids (3<pH<4) in the experiments, the whole-rock element content was maintained during retrogression, suggesting that water/rock ratio and fluid flow were low. During alteration K, Ba, Fe, Co, Ni, Cr, and V are immobilized in situ, whereas Sr, Ca, and Y become more leachable. Across the studied, strongly altered, lithologic contact, element transport up to 7&#114m normal to foliation can be traced. Here, Ca, Sr, Mg, Co, Ni, and Cr are slightly enriched, whereas K, Ba, Fe, and Mn are slightly depleted.
Article
&#112Garnet zoning characteristics, reaction microstructures, and thermobarometric studies document a multistage P–T evolution for the paragneisses of the KTB drill core. The prograde heating and burial path led through the sillimanite stability field before it entered the kyanite field close to the P–T peak. The uppermost paragneiss–amphibolite unit (0–560&#114m) reached peak conditions at 650–670&#117C and 7–8&#114kbar. The units below (560–3574&#114m) show higher peak temperatures of 680–720&#117C, with maximum pressures between 8 and 9&#114kbar. This P–T discontinuity implies a tectonic contact, but otherwise, the two sections of the drill core do not show evidence for a separate metamorphic history and probably lay on a common crustal paleogeotherm at peak conditions. Homogenization of Mg, Fe, and Mn in garnet at or near peak conditions points to a significant time interval between burial and subsequent decompression. Accordingly, the prograde metamorphic episode must be significantly older than the 380–370&#114Ma decompression and cooling period, possibly mid-Ordovician. Following nearisothermal decompression with partial breakdown of garnet and growth of sillimanite, a second, previously unrecognized pressure peak was reached at 5.7– 6.7&#114kbar and temperatures of around 590&#117C. A late low-pressure imprint is documented by the local presence of andalusite. The lack of a continuous metamorphic P–T gradient in the drilled crustal section either requires strong postmetamorphic tilting of isograds and/or small-scale intraformational stacking of crustal slices of similar metamorphic grade.
Article
The Leuchtenberg granite (Oberpfalz, NE Bavaria) displays a continuous differentiation trend ranging from mildy peraluminous, coarse-grained, porphyritic biotite granites (BG) to strongly peraluminous, medium- to fine-grained, garnet-bearing muscovite granites (GMG). The Rb–Sr and K–Ar age determinations of whole-rock and mineral samples from the granite and associated intermediate rocks (redwitzites) have revealed two divergent age gradients: Rb–Sr wholerock dates decrease and initial 87Sr/86Sr ratios increase for successively more evolved subsets of the granite. All BG samples (87Rb/86Sr=2–16) yield a date of 3262 Ma with a low initial 87Sr/86Sr ratio of 0.707780.00013 (1), while all GMG samples (87Rb/86Sr=70 to 1000) yield a younger date of 3172 Ma with an enhanced initial 87Sr/86Sr ratio of 0.71460.0039. The K–Ar measurements on biotites and muscovites give closely concordant dates for the GMG (326–323 Ma) and the southern lobe of the BG (324–320 Ma). The northern lobe of the BG, including the redwitzites, shows a well-defined trend of decreasing K–Ar dates from 320 Ma to 300 Ma towards the northwest. Critical consideration of both isotope systems leads to the conclusion that the Rb–Sr system of the GMG was disturbed by a later hydrothermal event. The ca. 326 Ma whole-rock Rb–Sr date for the BG is not in conflict with any of the K–Ar mineral dates and is taken as approaching the crystallization age of the Leuchtenberg granite. The K–Ar age progression within the northern lobe of the BG indicates that this part either cooled down over a protracted period of some 20 Ma or experienced reheating at ca. 300 Ma. The study highlights the potential of combined Rb–Sr and K–Ar dating in deciphering detailed chronology on the scale of a single igneous intrusion.
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
The Zone of Erbendorf-Vohenstrauß (ZEV) on the western margin of the Bohemian Massif was deformed by an Upper Cretaceous intra-plate deformation event. Dextral transpression was caused by the reactivation of pre-existing structures. Using the extensive geological database available, we have constructed a three-dimensional virtual model of the ZEV. The model was deformed in reverse, to remove the effects of the Upper Cretaceous event. This involved moving the hanging wall (the ZEV) in a sinistral transtensive sense northwards above a composite active fault surface composed of two steep faults, perpendicular to each other in strike, and a detachment intersecting both faults at 9.5 km depth. Hanging-wall deformation was accommodated by antithetic inclined shear. Seven kilometres heave of the hanging wall fulfilled the geological constraints. Calculated uplifts range from 2 to 6 km. Deformation is mostly only contained within the ZEV. The hanging-wall deformation above a linked fault system was highly complex, causing rollover above one fault and drag-folding above the other. The most important control on the vertical movement and deformation of the hanging wall was a 30° change in the strike of one of the coupled faults.
Article
SmNd isotopic results are presented for late-and post-tectonic granitoids, comprising peraluminous granites and subordinate metaluminous dioritic rocks (redwitzites) from the Fichtelgebirge (FG) and Northern Oberpfalz (NOP), NE Bavaria. The data, combined with a number of earlier geochromological studies on these granitoids, place severe constraints on the crustal evolution of this region during the Carboniferous and pre-Carboniferous epoch.Redwitzites range in ϵNd(T) from −4 to 0 (inclusive data from P.K. Holl and coworkers). The ϵNd(T)-values of the granites s.s. are restricted to overlapping ranges of −8 to −3 (FG) and −8 to −2 (NOP). In both domains, the older granites (330-325 Ma) are characterized by ϵNd(T)-values of > −4 whilst the younger granites (315-305 and ∼290 Ma) have ϵNd(T)-values of < −4. The diversity observed in Nd isotopic characteristic is interpreted in terms of different source material. The Nd isotopic compositions of the redwitzites exhibit source heterogeneity suggesting mixing between mantle magma and crust. The Nd isotopic features of the older granites are consistent with the magmatic precursors having been generated by partial melting of pre-existing mature crust variably contaminated by mantle material or, alternatively, by melting of chemically less evolved crust resembling paragneisses of teh ZEV structural unit. The origin of the younger granites can be confidently linked to anatexis of common Moldanubian and Saxothuringian metasediments. Two-stage Nd model ages (TDM) of the granitoids are in the range 1.1–1.7 Ga. These ages provide further support of substantial involvement of pre-Phanerozoic crust in the generation of the granitoids.The Leuchtenberg granite and the G2 and G3 granites yield late Variscan SmNd isochron ages which are concordant with previous RbSr data on these granites. By contrast, the Nd isotopic data for Bärnau and Flossenbürg define pseudo-isochrons suggestive of mixing between contrasting felsic melts.
Article
The crustal unit penetrated during the German Continental Deep Drilling Program (KTB), the Zone of Erbendorf-Vohenstrauss (ZEV), comprises an association of metabasites and paragneisses and orthogneisses that underwent high- and medium-pressure metamorphic cycles during the early Palaeozoic. In this summary of the structural, petrological, geochemical and geochronological information from borehole and surface rocks, we show that geological models proposed prior to drilling have had to be significantly modified. Features of the ZEV, such as the dominant NW-SE trending structures, Devonian (>370Ma) medium-pressure, amphibolite facies, metamorphism (0.6-0.8 GPa, ~720°C), and earlier eclogite stage, are directly comparable with those of the nearby Bohemicum unit in western Bohemia. Intervening units, in contrast, exhibit NE-SW trending structures and Carboniferous (315-325 Ma), low-pressure, metamorphism: all units are cut by predominantly posttectonic granites (mostly
Chapter
EinleitungArbeiten zur methodischen Erweiterung Erfassung, Datierung und Interpretation von Ungleichgewichten der Isotopenverteilung von Strontium und NeodymUntersuchungen zur Frage der Anwendbarkeit geochronologischer Methoden Einfluss von Gesteinsdeformation, Rekristallisation und Temperung auf IsotopenverteilungenEinfluss wasserreicher fluider Phasen auf die Isotopenverteilung in GesteinenInterpretation von U-Pb-Altern akzessorischer MonaziteHomogenitätsbereiche und „Trennwände“ für den IsotopenaustauschUntersuchungen zu geochronologischen Fragestellungen einiger ausgewählter Gebiete Charnockite IndiensGebänderte Metasedimente NordwestargentiniensGneise und Relikte einer HP-HT-Metamorphose im zentralen SchwarzwaldAlter detritischer Zirkone im nordwest-mitteleuropäischen Paläozoikum (Rheinisches Schiefergebirge, Ardennen, Brabanter Massiv)Variszische Plutonite des HarzesAlkaligesteine des Urals, RusslandMagmen des KaiserstuhlsMetamorpher Kernkomplex der Insel Thasos, Nord-GriechenlandIsotopengeochemie des Stoffaustauschs in der Fenitaureole des Iivaara-Alkaligesteinskomplexes, FinnlandBeiträge zum Kontinentalen Tiefbohrprogramm der Bundesrepublik (KTB) Die Tiefbohrung und ihr Umfeld„Widersprüche“ in der geochronologischen Information aus der Tiefbohrung und dem UmfeldArbeiten zur Lagerstättengenese Blei-Isotopie von Galeniten aus dem Bergbaugebiet der Anden ZentralperusStrontium-Isotopie hydrothermaler Gangminerale in Lagerstätten WestdeutschlandsBeiträge zur Archäometallurgie Blei-Isotopenuntersuchungen an bronzezeitlichen VerhüttungsproduktenBlei-Isotopie mittelalterlicher GläserLiteraturProjekte des ZLG seit 1976Verzeichnis der Veröffentlichungen mit Ergebnissen des ZLG (ohne Kurzfassungen, Dissertationen und Habilitationsschriften)Verzeichnis der Dissertationen und Habilitationsschriften mit Ergebnissen des ZLGMitglieder der Senatskommission für Geowissenschaftliche Gemeinschaftsforschung Erfassung, Datierung und Interpretation von Ungleichgewichten der Isotopenverteilung von Strontium und Neodym Einfluss von Gesteinsdeformation, Rekristallisation und Temperung auf IsotopenverteilungenEinfluss wasserreicher fluider Phasen auf die Isotopenverteilung in GesteinenInterpretation von U-Pb-Altern akzessorischer MonaziteHomogenitätsbereiche und „Trennwände“ für den Isotopenaustausch Charnockite IndiensGebänderte Metasedimente NordwestargentiniensGneise und Relikte einer HP-HT-Metamorphose im zentralen SchwarzwaldAlter detritischer Zirkone im nordwest-mitteleuropäischen Paläozoikum (Rheinisches Schiefergebirge, Ardennen, Brabanter Massiv)Variszische Plutonite des HarzesAlkaligesteine des Urals, RusslandMagmen des KaiserstuhlsMetamorpher Kernkomplex der Insel Thasos, Nord-GriechenlandIsotopengeochemie des Stoffaustauschs in der Fenitaureole des Iivaara-Alkaligesteinskomplexes, Finnland Die Tiefbohrung und ihr Umfeld„Widersprüche“ in der geochronologischen Information aus der Tiefbohrung und dem Umfeld Blei-Isotopie von Galeniten aus dem Bergbaugebiet der Anden ZentralperusStrontium-Isotopie hydrothermaler Gangminerale in Lagerstätten Westdeutschlands Blei-Isotopenuntersuchungen an bronzezeitlichen VerhüttungsproduktenBlei-Isotopie mittelalterlicher Gläser
Article
Until its final depth of 9101 m the superdeep well KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) drilled through Variscan basement rocks which suffered strong brittle-ductile and brittle deformation during late-and post-Variscan times. Investigations of faults and mineralized tension gashes revealed the following deformation stages: (1) Upper Carboniferous subvertical tension gashes, (2) Upper Carboniferous reverse faults, (3) Cretaceous subhorizontal tension gashes and reverse faults, (4) ?Neogene normal faults. Clear depth-dependent variations of metamorphic conditions of the post-Variscan deformation structures are obvious, whereas the Upper Carboniferous structures show almost no variations with depth. This refers to the metamorphic index minerals as well as to the deformation fabrics in quartz. We explain this considerable lack of gradients as being the result of reactivative reverse faulting that led to considerable vertical thickening of the upper crust. The fault geometry indicates an antiformal stack, the frontal ramp of which was drilled by KTB at 7000 m depth. There is strong evidence that this frontal ramp, referred to as the Franconian Lineament at the surface, rises from a subhorizontal detachment at about 10km depth which corresponds to the brittle-ductile boundary layer of quartz-bearing rocks. The considerable amount of supracrustal vertical thickening above the Franconian Lineament results from repeated movements along this detachment since Upper Carboniferous times. It seems reasonable to suppose that in intraplate tectonic settings the brittle-ductile boundary layer is prone to form a long-lived decoupling horizon along which the upper brittle crust is detached from the viscous middle and lower crust.
Article
Full-text available
The post-Carboniferous crustal evolution of the German Continental Deep Drilling Program (KTB) area, as summarized in this paper, could not be predicted from surface observations: deep drilling was essential for its revelation. The most conspicuous and unexpected feature discovered in the drill hole is the absence of marked gradients with respect to the pre-Carboniferous record. There are no depth-related differences in K-Ar cooling ages of hornblende and white mica, in petrology or in lithology. All metamorphic rocks encountered, both at the surface as well as in the drill hole down to 9100 m depth, were below 300øC from the Carboniferous onward. The late to post-Carboniferous deformation is essentially confined to several fault zones. A major fault zone encountered in the drill hole at 7000 m depth is linked by a prominent seismic reflector to the Franconian Lineament, the surface boundary between Variscan basement and Mesozoic cover. This fault zone probably formed in the late Paleozoic and reactivated as a reverse fault in the Mesozoic. Two important episodes of NE-SW directed shortening by movements along reverse faults took place in the early Triassic and in the late Cretaceous, as indicated by the distribution of apatite and titanite fission-track ages, the sericite K-Ar ages of fault rocks, and the sedimentary record in the adjacent basins. Upper crustal slices were detached at a specific level, corresponding to the approximate position of the brittle-ductile transition in post-Variscan times, and form an antiformal stack that was penetrated by the KTB throughout its entire depth range.
Chapter
Within or at the periphery of the Saxothuringian Basin several isolated basement areas of medium or even high grades are found. These are the Münchberg Mass, the “Zwischengebirge” of Wildenfels and of Frankenberg, the Zone von Erbendorf Vohenstrauß and the Sächsische Granulitgebirge including its Schiefermantel (Fig. 1). The Erzgebirge Zone is incorporated into this chapter here since it may also be allochtonous. The last main metamorphism of most of the allochthonous units is of the MP type except for the Sächsische Granulitgebirge and its Schiefermantel, and some peripheral schists of the Erzgebirge Zone, which show an anomalously high geotherm.
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
The post-Carboniferous crustal evolution of the German Continental Deep Drilling Program (KTB) area, as summarized in this paper, could not be predicted from surface observations: deep drilling was essential for its revelation. The most conspicuous and unexpected feature discovered in the drill hole is the absence of marked gradients with respect to the pre-Carboniferous record. There are no depth-related differences in K-Ar cooling ages of hornblende and white mica, in petrology or in lithology. All metamorphic rocks encountered, both at the surface as well as in the drill hole down to 9100 m depth, were below 300°C from the Carboniferous onward. The late to post-Carboniferous deformation is essentially confined to several fault zones. A major fault zone encountered in the drill hole at 7000 m depth is linked by a prominent seismic reflector to the Franconian Lineament, the surface boundary between Variscan basement and Mesozoic cover. This fault zone probably formed in the late Paleozoic and reactivated as a reverse fault in the Mesozoic. Two important episodes of NE-SW directed shortening by movements along reverse faults took place in the early Triassic and in the late Cretaceous, as indicated by the distribution of apatite and titanite fission-track ages, the sericite K-Ar ages of fault rocks, and the sedimentary record in the adjacent basins. Upper crustal slices were detached at a specific level, corresponding to the approximate position of the brittle-ductile transition in post-Variscan times, and form an antiformal stack that was penetrated by the KTB throughout its entire depth range.
ResearchGate has not been able to resolve any references for this publication.