Journal of Petrology Advance Access originally published online on November 11, 2008
Journal of Petrology 2008 49(10):1853-1872; doi:10.1093/petrology/egn049
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Two Distinctive Granite Suites in the SW Bohemian Massif and their Record of Emplacement: Constraints from Geochemistry and Zircon 207Pb/206Pb Chronology
1institute of Geosciences, University of Tübingen, 72074 Tübingen, Germany
2Bayerisches Landesamt Für Umwelt, 95615 Marktredwitz, Germany
3Czech Geological Survey, 15200 Prague 5, Czech Republic
RECEIVED FEBRUARY 15, 2008; ACCEPTED SEPTEMBER 15, 2008
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTING AND PREVIOUS...GRANITES OF THE BAVARIAN...ANALYTICAL METHODSSINGLE-ZIRCON Pb-EVAPORATION...DISCUSSION AND IMPLICATIONSCONCLUSIONSREFERENCES |
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Various types of gneisses, migmatites and granites dominate the outcrops of the Bavarian Forest, in the southwestern Moldanubian Sector of the Bohemian Massif, providing evidence for the geological characteristics of a crustal root zone. Fourteen granite intrusions from this area have been dated by the single-zircon Pb-evaporation technique. Major and trace elements, and Rb–Sr and Sm–Nd isotope compositions of these rocks have also been determined. The 207Pb/206Pb single-zircon ages indicate Carboniferous magma production in only a few million years. The intrusions NE of the Bavarian Pfahl Zone are dated between 328 and 321 Ma, whereas ages between 324 and 321 Ma are obtained in the area SW of this zone. The coincidence between these ages and regional peak-metamorphism in this area (‘Bavarian Phase’) indicates a connection between metamorphism and melt extraction. When the geochemical and isotopic compositions of the granites are examined, supplemented by data from previous geochemical studies, it becomes evident that the Bavarian Pfahl Zone, which runs diagonally across the Bavarian Forest, is a terrane boundary that separates two regions of peraluminous S-type plutons predominantly of crustal anatectic origin but with distinct compositional features. Plutons located in the SW and along the Pfahl Zone (Bavarian Terrane) define a high Ca–Sr–Y suite, whereas plutons from the NE and neighbouring crystalline units (Ostrong Terrane) constitute a low Ca–Sr–Y suite. Granites from the Bavarian Terrane have significantly more radiogenic initial Nd and less radiogenic initial Sr isotope ratios than granites from the Ostrong Terrane, indicative of different source materials. These findings support the idea that the difference in magma composition is linked to the nature of the underlying crust.
KEY WORDS: Bavarian Forest; geochemistry; granite; Pb evaporation; zircon
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PREVIOUS...GRANITES OF THE BAVARIAN...ANALYTICAL METHODSSINGLE-ZIRCON Pb-EVAPORATION...DISCUSSION AND IMPLICATIONSCONCLUSIONSREFERENCES |
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The origin and evolution of granites is a major focus of petrological research. Some of the fundamental aspects of granite petrogenesis include the identification of source components (e.g. mantle, crust or mixtures of both) and granite types. Tackling granite petrogenesis often involves radiometric age-dating (providing the time of emplacement or cooling) as well as geochemical or isotopic fingerprinting. Two key questions are: (1) Was magma formation and emplacement of short (discrete) or long (continuous) duration? (2) Do the granitoids belong to the same or different magmatic suites? In this study we investigate late Variscan S-type granites from the southwestern Moldanubian Sector of the Bohemian Massif, in the Bavarian Forest. Prior to this study the crystallization ages of these granites were largely unknown and the age relations between different intrusions of the same pluton were solely based on field observations. Moreover, existing geochemical and isotopic data were insufficient to determine the characteristic compositional features of the granites and to facilitate possible regional correlations.
For radiometric dating we applied the single-grain Pb-evaporation method, the principles of which were summarized by Kober (1986
, 1987). The potential of this method was validated in a number of subsequent studies (Cocherie et al., 1992; Chapman & Roddick, 1994; Karabinos, 1997; Klötzli, 1997; Dougherty-Page & Bartlett, 1999). As it is not possible to obtain information on the U–Pb systematics of the sample, the evaporation method is based on the premise that concordant lead is released from the sample and that discordant lead is driven out during pre-heating steps. The evaporation method has proven to provide 207Pb/206Pb zircon ages with high analytical precision (<1% age error) that often are concordant with ages derived by other methods (e.g. Affaton et al., 2000; Kröner et al., 2001; Siebel et al., 2003; Görz et al., 2004). In a number of cases the results are compromised by zircon inheritance, and thus age significance can be attached only to ages defined by several overlapping analyses. In spite of the problem of zircon inheritance, the results of the present study reveal a single period of granite formation and, combined with previously published data, they demonstrate the intimate link of the granites with late Variscan regional crustal anatexis.
Hitherto, the granites sensu stricto of the Bavarian Forest have been treated as a more or less coherent group of crustally derived rocks (e.g. Holub et al., 1995). It was frequently stated that certain granite massifs (Kristallgranite, Finsterau) have textural and mineralogical similarities to the Weinsberg-type granite, a coarse-grained biotite granite of the South Bohemian Batholith, Upper Austria, whereas others (Dreisessel–Plöckenstein) resemble the two-mica Eisgarn-type granite (Thiele & Fuchs, 1965). In this study we demonstrate that the granites on both sides of the Pfahl Zone have distinct compositional characteristics, which are most compatible with their derivation from two different basement-terrane units.
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GEOLOGICAL SETTING AND PREVIOUS GEOCHRONOLOGY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PREVIOUS...GRANITES OF THE BAVARIAN...ANALYTICAL METHODSSINGLE-ZIRCON Pb-EVAPORATION...DISCUSSION AND IMPLICATIONSCONCLUSIONSREFERENCES |
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The Bavarian Forest is located in the southwestern Bohemian Massif (Fig. 1) and forms part of the innermost zone of the Variscan orogenic belt in central Europe. It is mainly made up of migmatites, diatexites and gneisses (Teipel et al., 2004
, and references therein). The region belongs to the Moldanubian Sector, which is of (peri-)Gondwanan origin (Winchester et al., 2002, and references therein). Together with other Gondwana-derived terranes (e.g. Armorica, Saxothuringia, Teplá–Barrandia, Moravo-Silesia), Moldanubia was accreted to the northerly situated Laurentia–Baltica during the Variscan orogeny (Devonian–Carboniferous). After final collision and amalgamation of Laurentia–Baltica, Gondwana and intervening basement fragments, the Moldanubian Sector, as the internal domain of this mountain chain, was affected by late Variscan regional metamorphism and crustal anatexis. During this event, large volumes of peraluminous S-type granites and, in places, minor volumes of I-type granites (dioritic to tonalitic rocks) were generated and intruded the metamorphosed Variscan crust.
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The Moldanubian Sector does not form a coherent basement domain. It consists of different units or terranes with different structural and metamorphic evolution (Fiala et al., 1995
). The basement of the Bavarian Forest has often been referred to as the strongly overprinted southwestern margin of the Moldanubian. Fuchs (1976) coined the term ‘Bavarikum’ for this zone, and the term ‘Bavarian Phase’ was used by Finger et al. (2007) to distinguish it from the area further to the NE. In the Bavarian part of the Moldanubian sector the approximately NW–SE dextral strike-slip Pfahl Shear Zone divides the area into the ‘Vorderer Bayerischer Wald’ and the ‘Hinterer Bayerischer Wald’. The names Bavarian Terrane and Ostrong Terrane were used by Fiala et al. (1995) for the areas SW and NE of the Pfahl Zone, respectively. The term ‘terrane’ was applied to discriminate Moldanubian basement units with different geological history separated by major thrusts or faults (Fiala et al., 1995). According to this subdivision the Bavarian Terrane comprises the ‘Vorderer Bayerischer Wald’, whereas the Ostrong Terrane includes the ‘Hinterer Bayerischer Wald’ and Moldanubian rocks further NE in the Southern Palatinate Forest (Fig. 1), in the Czech Republic and in Upper Austria. For a more detailed discussion of the subdivision of the Moldanubian Sector of the Bohemian Massif, the reader is referred to Fiala et al. (1995) and Finger et al. (2007).
Lithologically, most of the Bavarian Terrane consists of various types of para-anatexites (pearl gneisses, diatexites and migmatites) and, to a minor extent, ortho-anatexites. These rocks represent a deep crustal level of the Moldanubian Sector. A higher crustal level, with cordierite-bearing gneisses, biotite–sillimanite gneisses and mica schists is exposed in the Ostrong Terrane. All these rocks re-equilibrated under low-pressure and high-temperature conditions. Calculation of the peak P–T conditions prevailing during late Variscan regional metamorphism and crustal anatexis yielded 800–850°C and 0·5–0·7 GPa (Kalt et al., 1999). The metamorphic peak has been dated at 323–326 Ma by U–Pb zircon and monazite methods (Kalt et al., 2000).
In general, the plutons in the Bavarian Forest are composite bodies that were formed during multiple intrusion events. Until recently, only a limited number of robust or high-resolution age determinations have been made for these rocks. A recent review of existing data was given by Klein et al. (2008). If older, less reliable Rb–Sr whole-rock data are excluded, the granites senso stricto investigated so far (i.e. Kristallgranite, Fürstenstein Pluton, Hauzenberg Pluton, Rinchnach Granite, Patersdorf Granite) yield crystallization ages between 329 and 312 Ma [U–Pb or Pb–Pb ages on zircon and monazite; see Klein et al. (2008) for compilation].
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GRANITES OF THE BAVARIAN FOREST |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PREVIOUS...GRANITES OF THE BAVARIAN...ANALYTICAL METHODSSINGLE-ZIRCON Pb-EVAPORATION...DISCUSSION AND IMPLICATIONSCONCLUSIONSREFERENCES |
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Post-kinematic granite intrusions occur throughout the Bavarian Forest and, at the current level of exposure, cover c. 1000 km2 (Fig. 1). Most of the intrusions have sharp discordant intrusive contacts with the metamorphic country rocks and with other intrusions. However, some granite bodies, such as the Kristallgranite, remained in the migmatite zone and are arguably parautochthonous (Propach, 1989
). Because of the lack of exposure, the exact contact relationships often remain unclear.
Bavarian Terrane
Isotopic characteristics and emplacement ages of the three largest plutons within the Bavarian Terrane (Kristallgranite, Fürstenstein, Hauzenberg) were reported in earlier studies (Propach et al., 2000; Chen & Siebel, 2004; Klein et al., 2008). The coarse-grained Kristallgranite with megacrystic K-feldspar is a widespread (
400 km2) magmatic rock in the Regensburg Forest (Fig. 1). The Fürstenstein Pluton (100 km2) shows a considerable compositional variation from quartz-diorite or tonalite through medium-grained biotite granite to coarse-grained porphyritic two-mica granite. The Hauzenberg Pluton (100 km2) comprises fine- and medium-grained biotite–muscovite granite (Hauzenberg I, Hauzenberg II) and a granodiorite intrusion, mapped and described in detail by Dollinger (1967).
From the Bavarian Terrane we sampled the granites of Lalling, Metten and Sattelpeilnstein (samples Lal, Met and Sat in Fig. 1). These granites comprise fine- to medium-grained varieties (Fig. 2a). The sample from the Lalling Granite (Lal) was collected from a quarry where granite is associated with small volumes of dioritic or tonalitic intrusions. The sample comprises a non-porphyritic, fine- to medium-grained biotite-granite. The Metten Pluton, located close to the Danube Fault, is the largest investigated intrusion of the Bavarian Terrane. The pluton was mapped and subdivided by Schreyer (1967) into five facies, each representing a different intrusion. According to Schreyer's classification, analysed samples comprise the medium-grained Metten facies (Met1, Fig. 2a) and the fine-grained (younger) Luhof facies (Met3). Granites with very similar texture and mineral composition crop out adjacent to the Bavarian Pfahl (Rinchnach Granite, Patersdorf Granite) and were investigated in an earlier study (Siebel et al., 2006b). The small intrusion of Sattelpeilnstein forms an elliptical body in the eastern Regensburg Forest. The collected sample (Sat) is a medium-grained weakly porphyritic biotite-granite.
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In most granite samples from the Bavarian Terrane biotite is more abundant than muscovite. In the Lalling and Sattelpeilnstein granites, muscovite occurs only as fine- to medium-grained secondary mineral. The Met1 rock, however, is a two-mica granite in which large flakes of primary muscovite and biotite occur in almost equal modal proportions.
Ostrong Terrane
In the northern part of the Ostrong Terrane several granite stocks are exposed close to the Runding Fault (Fig. 1). Amongst them, the two largest intrusions, hereafter referred to as Miltach and Arnbruck Granite, were sampled for geochemical and isotopic analyses. Both granites are leucogranites with indistinguishable textural characteristics and are composed of quartz, plagioclase, K-feldspar, biotite and muscovite. The Arnbruck Granite is cut by the Runding Fault (Fig. 1). Further SE, the large Finsterau and Dreisessel Plutons occur along the German–Czech border region. The Finsterau Pluton consists of a coarse-grained porphyritic (alkali-feldspar megacrysts) granite (Finsterau I; Fig. 2b), which dominates the outcrop area. Medium-grained, weakly porphyritic granite (Finsterau II) locally shows sharp contact against Finsterau I (Bauberger, 1981). A fine- to medium-grained two-mica granite cuts the Finsterau granites in several locations. This granite type forms a separate intrusion at and around the top of the Lusen summit and is here referred to as Lusen Granite (Fig. 2c). Samples from all these granites were chosen for analysis.
The Dreisessel–Plöckenstein Pluton, also referred to as the Plech
Pluton by Czech researchers, occupies the area SE of the Finsterau Massif. This pluton shows the most pronounced negative Bouguer gravity anomaly in the Moldanubian Sector (Blí
kovsk & Novotn, 1982) and modelling indicates that the pluton extends to depths greater than 7–8 km below the present surface (Verner et al., 2008). Breiter & Koller (2005) and Breiter et al. (2007) divided the Dreisessel–Plöckenstein Pluton into three main granite facies: (1) the generally equigranular, coarse-grained, locally weakly porphyritic Plöckenstein Granite (two-mica granite) forms the major part of the pluton (Fig. 2d); (2) serial porphyritic, coarse-grained Dreisessel Granite (two-mica granite) forms a north–south elongated body in the western part of the pluton; (3) the hiatal porphyritic Steinberg Granite [biotite-rich granite; the name is after Ott (1988, 1992)] with a medium-grained groundmass forms a crescent-shaped body SW of the Plöckenstein intrusion. The characteristic feature of the Steinberg Granite is the strong alignment of K-feldspar phenocrysts (Fig. 2e) and the high Th concentration (40–90 ppm, Breiter et al., 2007). One sample of each of these granite types was taken for analysis.
Some age data for the Dreisessel–Plöckenstein Pluton have been published previously, including mineral ages in the range 319–309 Ma (Ar–Ar muscovite ages), 324–306 Ma (Rb–Sr muscovite ages) and 324–299 Ma (chemical Th–Pb monazite ages). A more complete geochronological dataset has been given by Breiter et al. (2007).
The southern and western parts of the Dreisessel–Plöckenstein Pluton are mainly composed of coarse-grained muscovite-rich granite that is petrographically identical to the Plöckenstein Granite. It was defined as a separate intrusion, the Haidmühle Granite, by Ott (1992) but was regarded as part of the Plöckenstein Granite by Verner et al. (2008). The Haidmühle Granite is bounded to the SW by the Bavarian Pfahl Zone and a change in magmatic foliation close to the Pfahl Zone is recorded by the magnetic fabric (anisotropy of magnetic susceptibility data, Verner et al., 2008). A dark grey porphyritic biotite granite with K-feldspar phenocrysts (Fig. 2f) is exposed to the west of the Haidmühle Granite. This intrusion is known as the Haidel Granite (Ott, 1988). Samples from the Haidmühle Granite and the Haidel Granite were collected for analysis.
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PREVIOUS...GRANITES OF THE BAVARIAN...ANALYTICAL METHODSSINGLE-ZIRCON Pb-EVAPORATION...DISCUSSION AND IMPLICATIONSCONCLUSIONSREFERENCES |
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Each granite sample (2–5 kg) was crushed and split, and c. 100 g representative material was pulverized in an agate mill for geochemical analysis. Major elements and certain trace elements (Ba, Y, Zr, La, Ce, Yb) were determined on fused lithium tetraborate glass beads by wavelength-dispersive X-ray fluorescence (XRF) techniques. The precision is better than 5–10% for all measured trace elements. Loss on ignition (LOI) was determined gravimetrically at 1050°C. Concentrations of Rb, Sr, Sm and Nd were determined by isotope dilution (see below). Zircons were obtained from the 200–125 and 125–63 µm sieve size fractions using a Wilfley table, a Frantz isodynamic separator and heavy liquids followed by handpicking under a binocular microscope. To study their internal structures, zircon grains were mounted in epoxy resin, polished and observed in cathodoluminescence (CL) using an electron microscope (type LEO 1459). For Pb-evaporation analyses zircons were embedded in a Re evaporation filament and measured using a Finnigan MAT 262 mass spectrometer equipped with a single secondary electron multiplier (SEM). The procedures have been described by Siebel et al. (2003
). From each zircon grain three or four temperature steps were measured and the mean of the 204Pb-corrected radiogenic 207Pb/206Pb ratios from all steps was calculated if the data for each of the steps were concordant within error. The age for several zircons from the same sample is given as a weighted average and the error refers to the 95% confidence level (ISOPLOT, Ludwig, 2003). Repeated measurements on natural zircons from zircon standard 91500 were performed for geologically realistic age and error treatment [some of the results have been published by Chen et al. (2002)].
For Rb–Sr and Sm–Nd isotope analyses, whole-rock powders were spiked with 150Nd–149Sm and 87Rb–84Sr tracer solutions (isotope dilution analyses) prior to dissolution in hydrofluoric acid at 180°C in pressure digestion bombs. Rb, Sr and the light rare earth elements were isolated from each other by standard ion exchange chromatography with a 5 ml resin bed of AG 50W-X12 (200–400 mesh). Nd and Sm were separated on quartz columns using 1·7 ml Teflon powder coated with HDEHP, di-(2-ethylhexyl)-orthophosphoric acid, as cation exchange medium. Isotopic data were obtained in static mode on a Finnigan MAT 262 mass spectrometer. The 143Nd/144Nd ratios were normalized to 146Nd/144Nd = 0·7219, and the 87Sr/86Sr isotope ratios to 86Sr/88Sr = 0·1194. Repeated measurements of the La Jolla Nd standard (n = 12) gave a 143Nd/144Nd ratio of 0·511838 ± 13 (errors are ± 2
of the mean) and 10 analyses of the NBS 987 Sr standard yielded a mean value of 87Sr/86Sr = 0·710248 ± 0·000012, in good agreement with the certified values. Total procedural blanks were <90 pg for Nd and <220 pg for Sr. Two-stage depleted mantle Nd model-ages (TDM) were calculated with depleted present-day parameters 143Nd/144Nd = 0·513151 and 147Sm/144Nd = 0·219 (Liew & Hofmann, 1988) and 
Nd values were calculated using present-day CHUR values of 0·1967 for 147Sm/144Nd (Jacobsen & Wasserburg, 1980) and 0·512638 for 143Nd/144Nd (Goldstein et al., 1984).
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SINGLE-ZIRCON Pb-EVAPORATION DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PREVIOUS...GRANITES OF THE BAVARIAN...ANALYTICAL METHODSSINGLE-ZIRCON Pb-EVAPORATION...DISCUSSION AND IMPLICATIONSCONCLUSIONSREFERENCES |
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Inheritance of older zircon domains poses a general problem during Pb evaporation analyses. Such effects played a major role during this study, as all the investigated granites have S-type compositional characteristics (see Table 1 for the geochemical compositions of the investigated bulk-rock samples from which zircons were extracted). In general, grains with portions of pre-magmatic zircon fragments can be easily identified during isotopic measurement, as they normally yield increasingly higher 207Pb/206Pb ratios, or apparently older ages, for successively higher temperature steps. In such grains, the lower temperature steps yield higher proportions of Pb from magmatic rim zones, whereas in the higher temperature steps the proportion of Pb from inherited zircon cores predominates (see also Klötzli, 1999
).
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To obtain the intrusion age, reproducible 207Pb/206Pb ages for different temperature steps of a given grain, and consistent ages for different grains from the same sample are needed. For such measurements, an inherited zircon contribution can be largely excluded. Given the existence of pre-magmatic zircon fragments, this necessitated the analysis of a relatively large number of zircon grains (in many cases 10 or more grains per sample) to place effective constraints on granite crystallization. A summary of the data for magmatic zircons is given in Table 2.
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Bavarian Terrane
Zircons from the granites belonging to the Bavarian Terrane have specific morphological features. In the Metten Pluton, zircons typically occur as near-spherical grains and show stubby crystal habits with little morphological variation. As seen in CL images (e.g. Fig. 3a and b), only a few grains show well-developed regular oscillatory growth zoning. An explanation for oscillatory zoning in zircons involves unstable chemical gradients around the grain during magmatic crystallization. Diffusion associated with late magmatic processes can obliterate the igneous oscillatory zoning pattern (Connelly, 2000
). It is remarkable that some of the Metten grains are similar in appearance to those observed in high-grade gneisses (Corfu et al., 2003). Such grains might represent overgrowth-free xenocrystic zircons that might have been assimilated into the magma during late-stage crystallization. Zircons from the Lalling, Sattelpeinstein and Miltach granites form elongated idiomorphic grains and show magmatic growth zonation (Fig. 3c–e). In some cases this zonation is part of an older zircon generation. Such grains can be identified if resorption zones between inner and outer zones are developed (Fig. 3d, grains 3 and 4).
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The Pb-evaporation technique applied to three grains of Metten sample Met1 (Metten facies) gives ages around 322 and 328 Ma, with a mean value of 324·2 ± 5·0 Ma (Fig. 4a). Four zircon ages from sample Met3 (Luhof facies) vary between 318 and 326 Ma with a slightly younger mean age of 321·0 ± 3·8 Ma (Fig. 4b). Other grains in both samples yield increasingly radiogenic 207Pb/206/Pb ratios for successively higher heating steps, indicating the presence of pre-magmatic zircon fragments. The 207Pb/206Pb ages of five zircons from Lalling sample Lal are between 315 and 329 Ma with a mean age of 321·6 ± 5·6 Ma (Fig. 4c). 207Pb/206Pb ages from this sample are more scattered than for the other samples, which can be ascribed to the higher contribution of common lead, indicated by high 204Pb/206Pb ratios observed in nearly all grains from the Lalling Granite (Table 2). Admixture of older inherited zircon material was observed in two grains that yielded increasing 207Pb/206Pb ratios as the evaporation temperature was increased from 1380°C through 1400°C and 1420°C to 1440°C. Information provided by these analyses points to a minimum age of >360 Ma for the pre-magmatic zircon component. Four grains of Sattelpeilnstein sample Sat gave ages between 319 and 327 Ma with a weighted mean age of 322·3 ± 3·4 Ma (Fig. 4d). In a fifth grain, increase of 207Pb/206Pb ratios with evaporation temperature suggests inheritance.
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Ostrong Terrane
The CL images of most granites from the Ostrong Terrane have a similar appearance and zircon populations from different samples show similar internal structures. Usually, crystals are prismatic, with aspect ratios up to three, and exhibit well-preserved oscillatory growth zonation. Crystals vary in size from 400 to 200 µm in the Finsterau I Granite (Fig. 3g) and from 250–63 µm in all other granites. Many grains show oscillatory zoning with brightly luminescent centres surrounded by dark rims (e.g. grains in Figs. 3e, i and j). In some grains inner concentric zones were partly dissolved and replaced by darker zircon domains probably during late or post-magmatic stages (Fig. 3e). CL images also reveal that some grains contain cores of older magmatic zircons. Some of the cores can show well-developed pre-magmatic oscillatory zonation. In other grains (e.g. Fig. 3n, grains 2 and 3), zonation within the core is not concentric with respect to the external morphology of the grain, clearly indicating that older (i.e. inherited) grains were resorbed in the magma and subsequently overgrown by a new magmatic rim.
Four zircons from a sample of Miltach Granite (Mi) were vaporized and gave 207Pb/206Pb ages between 319 and 327 Ma, with a weighted mean age of 321·8 ± 3·7 Ma (Fig. 5a). Pre-magmatic zircon material was not detected in zircons from this sample. Zircon grains from the Arnbruck Granite show lead isotopic heterogeneity and yield 207Pb/206Pb minimum ages for the pre-magmatic zircon component around c. 335 Ma, 350 Ma and 410 Ma. Meaningful data with constant 207Pb/206Pb ratios for different temperature steps that are relevant for the determination of the crystallization age were obtained from only two grains. Both grains yield ages of c. 325 Ma and the mean age is 325 ± 2·1 Ma (Fig. 5b). After solidification, the Arnbruck Granite was cross-cut by the Runding Fault (Fig. 1). This resulted in a solid-state overprint of the granite, and the age data demonstrate that this tectonic disruption, which did not cause lateral displacement, was younger than 325 Ma.
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From the Finsterau I Granite (sample Fin I) four grains yield virtually identical 207Pb/206Pb ages of c. 325 Ma, one grain gives an age of c. 329 Ma, and the weighted mean age of all five grains of 325·9 ± 1·9 Ma (Fig. 5c) is regarded as the time of crystallization for this granite. Three grains from sample Fin I were marked by slightly older age components between c. 340 and 350 Ma. Zircons with similar apparent ages, which might have been predominantly formed during the Variscan orogeny, were encountered in the Haidmühle and the Haidel Granites (see below). In the sample from Finsterau II (Fin II) we did not find evidence for zircon inheritance. Six analyses of zircons from this sample yielded 207Pb/206Pb ages between 321 and 327 Ma. The ages agree within analytical uncertainty and yield a weighted average age of 324·1 ± 1·8 Ma (Fig. 5d).
Zircon grains from Lusen sample Lu1 yield a range of old apparent 207Pb/206Pb ages and it seems that this zircon population is dominated by inheritance. Only one grain from this sample gave a Carboniferous age of 323·1 ± 3·5 Ma (Fig. 5e). Four analyses from sample Lu2 yielded homogeneous Pb-isotope compositions for different temperature steps and their total grain 207Pb/206Pb ages range between 320 and 328 Ma. The weighted mean of these four grains is 324·9 ± 3·5 Ma (Fig. 5f). One grain from sample Lu2 yields an older age of c. 340 Ma, which appears to have been derived from admixture of a pre-magmatic zircon domain.
Three zircon grains from a sample of the Dreisessel Granite yield well-defined 207Pb/206Pb ages of c. 327 Ma, which are identical within error, yielding a mean age of 327·1 ± 1·9 Ma (Fig. 6a). Notably, none of the grains show evidence for existence of a pre-magmatic zircon component. Four grains from a sample of the Plöckenstein Granite yield 207Pb/206Pb ages between 321 and 328 Ma (Fig. 6b), and the mean age of 324·8 ± 3·4 Ma is interpreted as the crystallization age of the Plöckenstein Granite. There is little evidence in the data for inheritance, as only one zircon yields a slightly older apparent 207Pb/206Pb age of c. 335 Ma. Four grains from the Steinberg Granite yielded homogeneous Pb-isotope compositions for different heating steps and their ages vary between 325 and 329 Ma, resulting in an average age of 328·1 ± 1·7 Ma (Fig. 6c). Heterogeneous 207Pb/206Pb ratios were measured in five grains and the highest temperature step, which can be regarded as the closest approximation to the minimum age of the inherited zircon component, gave apparent ages of c. 340 Ma, 370 Ma, 460 Ma and 650 Ma. The 207Pb/206Pb ages of three grains from the Haidmühle Granite are identical within error, yielding a mean age of 320·7 ± 1·6 Ma (Fig. 6d) that is interpreted as the magmatic crystallization age of the Haidmühle Granite. Zircons with apparent 207Pb/206Pb ages between 340 and 350 Ma were frequently encountered in this granite. In two other grains from the Haidmühle Granite, apparent ages of c. 370 Ma and c. 630 Ma were measured in the highest temperature steps. Of the zircons selected for Pb-evaporation analyses from the Haidel Granite, four grains yield consistent ages between 321 and 325 Ma, with a mean age of 323·4 ± 2·6 Ma (Fig. 6e). Two crystals give older apparent ages of c. 500 Ma and c. 670 Ma for the high-temperature evaporation steps. These data suggest a contribution from older (pan-African?) crust. Again, two other crystals yield slightly older apparent ages between c. 340 and c. 350 Ma similar to those found in the Haidmühle Granite.
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Implications from geochronology
Despite the complexity of the Pb–Pb systematics encountered in numerous zircon grains from granites within the Bavarian Forest, the Pb–Pb evaporation data on magmatic zircons (i.e. those free of older inheritance) put important constraints on the chronology of pluton emplacement. Concerning the age data of purely magmatic zircon grains (Table 2), there seems to be a broad consistency in emplacement age between the various granites. Single-zircon 207Pb/206Pb evaporation ages of the granites investigated in this study range from 328 to 321 Ma in the Ostrong Terrane and from 324 to 321 Ma in the Bavarian Terrane. Clear regional age differences are virtually lacking. U–Pb and Pb–Pb zircon ages as young as 316–312 Ma have been reported for the Saldenburg and Eberhardsreuth intrusions from the composite Fürstenstein Pluton (Chen & Siebel, 2004
), and these results reveal a younger, late Variscan intrusive event restricted to the southeastern part of the Bavarian Terrane. However, from the current geochronological dataset it is apparent that the majority of the granites intruded within a time span of less than 10 Myr, and this favours the interpretation that melting was initiated by a large regional heat flow event. The new data show that melt production and crustal high-temperature–low-pressure metamorphism in the Bavarian Forest were synchronous processes.
Relics of pre-magmatic (inherited) zircon components were detected in most of the investigated samples. In those zircon grains apparent ages between 340 and 350 Ma were most abundantly observed, particularly in granites from the Ostrong Terrane. This corresponds to the age of Moldanubian granulite metamorphism (e.g. Kröner et al., 2000; Janou
ek et al., 2004) and several large granulite bodies of South Bohemia occur NE of the Dreisessel–Plöckenstein Pluton, close to our study area. Recently, Finger et al. (2007) reported evidence for earlier high-temperature–low-pressure metamorphism at 330–345 Ma in the southwestern Moldanubian sector. Taking the evidence from these studies, some of the pre-magmatic zircons found in the granites could have been derived from Early Carboniferous metamorphic rocks.
The analysed zircons from all the Bavarian granites show low Th/U signatures (from 0·02 to 0·41) with an average value of 0·135 (Table 2). No difference in Th/U between granites from the Bavarian and Ostrong Terranes could be found. The Th/U ratios tend to be distinctly less than the commonly quoted ratios of >0·5 for igneous zircons (Hoskin & Schaltegger, 2003). However, U/Th ratios in zircons from felsic granitic rocks are lower than in more mafic igneous rocks and are in the range of 0·11–1·0 (Ahrens et al., 1967, fig. 1). Very low U/Th ratios are a common feature of differentiated S-type granites and have been reported for many magmatic zircons from granites of the Upper Palatinate Forest (Siebel et al., 2003).
Regional geochemical comparison
Whole-rock geochemical and Nd–Sr isotope data from the samples investigated during this study are presented in Table 1. A comprehensive geochemical and isotopic dataset is now available for granites to the SW and NE of the Pfahl Zone, and in the following discussion we aim to compare the compositional features of the granites from these two regions. Literature data from the Bavarian Terrane, in addition to the data presented in Table 1, are from Chen & Siebel (2004) for the Fürstenstein Pluton, Siebel et al.. (2006a and Siebel et al. in preparation) for granites from the Regensburg Forest and Siebel et al. (2006b) for the Rinchnach and Patersdorf Granites. The latter are closely linked with the Pfahl Zone (Fig. 1). The Patersdorf Granite occurs on both sides of this shear zone and the Rinchnach Granite immediately at its northern side. Both granites have geochemical affinities to the granites SW of the Pfahl Zone and in the following geochemical characterization they will be included in the group of the Bavarian Terrane granites.
Geochemical and Nd–Sr isotopic compositions of the major granite units are also established from studies in western Bohemia and the Southern Palatinate Forest. Data from these regions, which comprise the northward extension of the Ostrong Terrane (Fiala et al., 1995; Finger et al., 2007), come from the Rozvadov Pluton in Western Bohemia (Siebel et al., 1999) and from the Southern Palatinate Forest (Neunburg Granite, Oberviechtach Granite) (Chen et al., 2003). The dataset (Table 1 and literature data) comprises 27 granite samples from 14 intrusions from the Bavarian Terrane and 25 samples from 15 intrusions within the Ostrong Terrane.
Synthesis of the data shows that granites from the Bavarian and the Ostrong Terranes have distinctive geochemical characteristics (Fig. 7). According to the QAP classification, almost all samples plot in the granite field and only few can be classified as granodiorite (Fig. 7a). The other diagrams in Fig. 7 illustrate some fundamental chemical differences between these granites. The granites of the Bavarian Terrane are weakly peraluminous (aluminum saturation index, expressed as molar Al2O3/(K2O + Na2O + CaO), A/CNK, from 1·0 to 1·2) whereas the granites within the Ostrong Terrane are strongly peraluminous with A/CNK values between 1·1 and 1·35 (Fig. 7b). According to the MALI classification (Na2O + K2O – CaO vs silica) proposed by Frost et al. (2001), most granites plot in the calc-alkalic and alkali-calcic fields and the majority of granites from the Ostrong Terrane fit the compositional range of peraluminous leucogranites (Fig. 7c). Potassium exceeds sodium in most samples with large variation in K/Ca ratio (Fig. 7d). For the given dataset, Ca is a more useful discriminator than silica (Fig. 7e) and, in the remaining diagrams of Fig. 7, this is illustrated by plots of major and trace elements vs CaO. Calcium concentration is higher in the granites from the Bavarian Terrane. At the same time, these rocks are higher in other mafic elements (Ti–Fe–Mg) and lower in P (not shown) and less enriched in silica compared with those from the Ostrong Terrane. Amongst the trace elements, the Ca-rich granites contain higher concentrations of Y (Fig. 7j). The Ce/Y ratio, which can serve as an indicator of the fractionation between light and heavy rare earth elements, is low in the granites from the Bavarian Terrane but reaches higher values in granites from the Ostrong Terrane (Fig. 7k). The same relationship is found for the Rb/Sr ratios (Fig. 7l), reflecting a higher degree of differentiation of the granites from the Ostrong Terrane. From the diagrams shown in Fig. 7 there appears to be a fairly clear geochemical discrimination between high Ca–Sr–Y granites of the Bavarian Terrane and low Ca–Sr–Y granites of the Ostrong Terrane. As the only significant exception, the Finsterau I Granite has geochemical features more akin to the granites from the Bavarian Terrane (see also Table 1).
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The Nd and Sr isotope compositions are plotted in Fig. 8. Recalculation of the Nd-isotope data to the emplacement age reveals that the Nd-isotope ratios (
Nd values) of the high Ca–Sr–Y granites are higher (–3 to –7) than the range shown by the low Ca–Sr–Y granites (–5 to –9) (Fig. 8a). Accordingly, the initial Sr isotope ratios are less radiogenic in the high Ca–Sr–Y granites (0·705–0·710) and more radiogenic in the low Ca–Sr–Y granites (0·707–0·730) (Fig. 8b).
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DISCUSSION AND IMPLICATIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PREVIOUS...GRANITES OF THE BAVARIAN...ANALYTICAL METHODSSINGLE-ZIRCON Pb-EVAPORATION...DISCUSSION AND IMPLICATIONSCONCLUSIONSREFERENCES |
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The geochemical evidence presented in the previous section suggests different melting conditions or different source contributions for the granites NE and SW of the Pfahl Zone. The higher Ca and Sr contents of the granites from the Bavarian Terrane could simply be a reflection of a greater amount of plagioclase in their source material. In addition, these rocks require a garnet-free source to explain the low Ce/Y ratios and the high Y concentrations. During production of the low Ca–Sr–Y granites of the Ostrong Terrane, either melting of plagioclase was suppressed or the source was depleted in this mineral. Fractionation of plagioclase would result in depletion of Ca but also of Al; however, depletion of Al is not observed in the dataset (Fig. 7h). Melt extraction from a metapelite source in the presence of residual garnet would be a suitable process for the generation of these rocks. Because Ca and Sr are compatible elements in plagioclase and Y is a compatible trace element in apatite and zircon, the predominance of high Ca–Sr–Y granites in the Bavarian Terrane could reflect some accumulation of these early magmatic minerals supporting a cumulative nature of the magmatic rocks SW of the Pfahl Zone (F. Finger, personal communication). Cumulative magmatic rocks are typically exposed in deeper crustal levels and this is in line with the highly anatectic character of the Bavarian Terrane.
The isotope data give support to the assumption that the sources for the Bavarian granites that intruded the Bavarian Terrane were not identical to the sources for those that intruded the Ostrong Terrane (Fig. 8). Thus a tentative interpretation can be made that the basement must be of different type for the two terranes, with the Pfahl Shear Zone defining a terrane boundary (Fig. 9).
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A zonation in granite composition can also be traced on the Austrian side of the Moldanubian sector: (1) I-type granites (so-called Schlieren granites) are dominant south of the Pfahl Zone in the Mühl Zone sector, which is the prolongation of the Bavarian Terrane into Austria; (2) S-type granites (Eisgarn type granite) are dominant north of the Pfahl Zone (e.g. Frasl & Finger, 1991
; Finger & Clemens, 1995). This zonation was ascribed by Frasl & Finger (1991) to reflect different composition of the pre-anatextic crust or different crustal levels. Thus it appears that this striking spatial zonation is a general feature of the southern Moldanubian sector.
Figure 10 shows that the compositions of the granites are largely consistent with melts derived from experimental partial melting of different metapelite compositions (Patiño-Douce, 1999). These diagrams also suggest that amphibolites and meta-greywackes are less suitable sources for the majority of the granites. Pelitic or semi-pelitic metasedimentary gneisses like the outcropping garnet- and cordierite-bearing rocks of the Ostrong Terrane, which may continue at greater depth, could be regarded as potential sources of the granites within this unit. Such a source would yield sufficient quantities of melt of peraluminous composition (e.g. Koester et al., 2002).
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We have argued elsewhere (Siebel et al., 2006a
) that the granites of the Bavarian Terrane (i.e. Regensburg Forest) were probably derived from more mafic protoliths than the now exposed diatexites of this area. Some involvement of mantle-derived magma was postulated in the petrogenesis of the the Saldenburg and Tittling Granites of the Fürstenstein Pluton (Chen & Siebel, 2004). It is a general observation that small dioritic bodies are more frequently found SE of the Pfahl Zone. Thus it might be argued that additional input of mantle-derived material to the crust of the Bavarian Terrane has played a role in granitoid magma genesis. However, there is little field evidence to suggest that these granites are the products of hybridization between crustal- and mantle-derived magmas. For most of the Bavarian Terrane granites no clear correlation is visible in the Nd vs Ca diagram (Fig. 8a). This argues against two-component mixing between mafic (mantle) and felsic (diatexite) components. In addition, the preponderance of granitic rocks over dioritic rocks in the Bavarian Forest is not compatible with the involvement of larger volumes of mantle-derived magma during granite petrogenesis. For a conclusive judgement, it has to be verified in more detail whether the Moldanubian basement rocks have the isotope composition required to be a potential source.
Crustal melting demands a heat source and, given the synchronism between magmatism and metamorphism, it is likely that metamorphism set the stage for the creation of a melt flow network within the crust, and this aided melt extraction, granite formation and crustal anatexis. It was previously pointed out by Finger & Clemens (1995) that in the Mühl Zone, intrusions of voluminous granites such as the Weinsberg Granite supplied heat and caused large-scale anatexis in neighbouring rocks. Ultimately, this late Variscan ‘hot crustal phase’ could have been triggered by radiogenic intracrustal heating (Gerdes et al., 2000) or delamination of mantle lithosphere (Henk et al., 2000; Finger et al., 2007).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PREVIOUS...GRANITES OF THE BAVARIAN...ANALYTICAL METHODSSINGLE-ZIRCON Pb-EVAPORATION...DISCUSSION AND IMPLICATIONSCONCLUSIONSREFERENCES |
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Single-zircon Pb-evaporation analyses show that granite formation in the southwestern Moldanubian Sector of the Bohemian Massif was synchronous with the peak of high-temperature–low-pressure metamorphism. The 207Pb/206Pb evaporation ages of 14 granite intrusive rocks from the Bavarian Forest are consistent with a short episode of crustal melting between 328 and 321 Ma. Overall, this study demonstrates the feasibility of using the Pb-evaporation method to establish the detailed geochronology of S-type granites despite complexities resulting from inherited zircon material.
Taking bulk-rock geochemical and isotopic characteristics into account, it now appears that the granites from the Bavarian Terrane and the Ostrong Terrane define two distinct granite types, which formed at about the same time but from different source materials. The presence of high- and low-Ca–Sr–Y granites with distinct Nd–Sr signatures confirms earlier suggestions that the Pfahl Shear Zone is a terrane boundary that has juxtaposed two compositionally distinct Variscan basement units (Fig. 9). More work is required to ascertain the compositional differences between the two units.
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ACKNOWLEDGEMENTS |
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We thank G. Bartholomä for zircon separation, Ch. Dekant and H. Schulz for their assistance in providing CL images, and H. Taubald for help during XRF analyses. Financial support from the Bavarian Environment Agency is gratefully acknowledged.
*Corresponding author. Telephone: ++49-7071-29 74 991. Fax: ++49-7071-29 57 13. E-mail: wolfgang.siebel{at}uni-tuebingen.de
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