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Journal of Petrology Volume 42 Number 9 Pages 1621-1642 2001
© Oxford University Press 2001
Cadomian Lower-Crustal Contributions to Variscan Granite Petrogenesis (South Bohemian Pluton, Austria): Constraints from Zircon Typology and Geochronology, Whole-Rock, and Feldspar Pb–Sr Isotope Systematics
1LABORATORY FOR GEOCHRONOLOGY, INSTITUTE OF GEOLOGY, UNIVERSITY OF VIENNA, GEOZENTRUM, ALTHANSTRASSE 14, A-1090 VIENNA, AUSTRIA
2INSTITUTE OF PETROLOGY, UNIVERSITY OF VIENNA, GEOZENTRUM, ALTHANSTRASSE 14, A-1090 VIENNA, AUSTRIA
3GEOLOGICAL SURVEY OF AUSTRIA, RASUMOFSKYGASSE 23, A-1030 VIENNA, AUSTRIA
4INSTITUTE OF GEOLOGY AND PALAEONTOLOGY, UNIVERSITY OF SALZBURG, HELLBRUNNERSTRASSE 34, A-5020 SALZBURG, AUSTRIA
Received April 3, 2000; Revised typescript accepted February 21, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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A hybrid pyroxene-bearing Weinsberg type granitoid of the South Bohemian batholith (Austria) consists of two independent mineral assemblages that were formed during two different magmatic events. The older, inherited assemblage forms unevenly distributed millimetre-sized multi-grain patches of quartz + mesoperthitic alkali feldspar + andesine/bytownite + clinopyroxene (XMg = 0·50–0·54) + orthopyroxene (XMg = 0·35–0·42) ± ilmenite ± accessories. It is interpreted to represent remnants of a mangeritic igneous rock with a superimposed granulite-facies re-equilibration texture characterized by unzoned pyroxenes and plagioclase. The enclosing younger assemblage with alkali feldspar + oligoclase/andesine + quartz + biotite ± accessories crystallized from a biotite-bearing granitic melt with feldspars exhibiting typical magmatic zoning. Coexisting with the inherited assemblage are zircons with a characteristic typology (S23 to D, mean J4). Zircons belonging to the granitic assemblage, on the other hand, show a distinctly different typology (L2 to S5, mean L4) or are anhedral. A Cambrian age of formation and subsequent re-equilibration of the inherited assemblage is inferred from a mean U/Pb and 207Pb/206Pb evaporation age of 523 ± 5 Ma for the J4 zircons. Granitic L4 zircons show a mean 207Pb/206Pb evaporation age of 355 ± 9 Ma, interpreted as the age of zircon growth during a Carboniferous partial melting event in the lower crust. Granite emplacement at 345 ± 5 Ma is inferred from U/Pb analysis of the anhedral zircon population. The comparably low radiogenic common Pb isotope composition of megacrystic alkali feldspars suggests that at least some domains of these crystals are inherited from the older, pyroxene-bearing mineral assemblage. Rb/Sr whole-rock dating is thus severely jeopardized by the presence of the inherited alkali feldspar crystals, leading to widely scattering data points and errorchron ages of no geological significance.
KEY WORDS: Austria; Bohemian Massif; geochronology; granites; Pb–Sr isotopes
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Understanding the processes that govern the evolution of the continental crust from formation to destruction is still a major research goal in Earth sciences. Igneous rocks, mainly granitoids, constitute one of the most essential parts of the continental crust. They usually form by partial melting or assimilation–fractional crystallization (AFC) processes in the lower crust and are subsequently intruded into the middle or upper crust. By doing so they dramatically influence the complete continental crust with respect to, for instance, geochemistry and rheology. Understanding the evolution of continental crust therefore demands comprehension of granitoid origins and petrogenesis.
Granitoids, however, can form exceedingly complex systems, which, even today, are only poorly understood. Unravelling the origin of a given granite still requires a major analytical effort and commonly ends in some contradictory results. Granitoids contain source information in their geochemical and isotopic signature and in their enclaves. Enclaves, in the simplest case, are either endogene (i.e. comagmatic restites or cumulate material) or exogene (i.e. country rocks) and can vary in size from kilometre scale to multi-grain domains or even single crystals. Single-crystal ‘enclaves’ are normally called inherited or xenocrysts. Common xenocrysts in granitoids are, for instance, zircon and monazite, at least their cores. Under certain circumstances, as proposed in this paper, feldspars and pyroxenes can also occur as xenocrysts.
Whole-rock major element, trace element and isotope analyses are the most commonly applied methods for characterization and classification of granitoids. However, they do not necessarily provide relevant answers on the origin of a specific granitoid, especially in the presence of inherited mineral phases. The admixture of large enclaves can be avoided by separation, but single inherited mineral grains will inevitably be included in the whole-rock analysis. Major and trace element systematics is ruled by the major rock-forming minerals, whereas the rare earth elements (REE) are mainly governed by accessories. Therefore, depending on the type of inheritance present, the whole-rock analysis will be compromised, a fact known for a long time but seldom thoroughly appreciated. Isotope geochemists have often dealt with spurious Rb/Sr whole-rock ‘isochron’ age data. These are interpreted as stemming from mixing effects, wall-rock assimilation and incomplete homogenization. The conclusion that non-ideal Rb/Sr whole-rock systematics also implies some effect on the whole-rock geochemistry has not been drawn very often. Any assumptions drawn from such geochemical data thus have to be considered carefully. The importance and significance of zircon inheritance is also long known. But again the influence of zircon inheritance on whole-rock REE systematics has not been fully appreciated.
This paper exemplifies some of the complexities encountered in establishing the geochronology and isotope geochemistry of a Variscan granitoid from the South Bohemian pluton (Bohemian Massif, Austria). Using zircon typology, single-grain U/Pb and 207Pb/206Pb evaporation zircon dating techniques it is shown that fragments of a mangerite of Cambrian age form an inherited phase within a Variscan (Carboniferous) granitoid. The influence of the inherited mineral assemblage on Pb isotope and Rb/Sr whole-rock and alkali feldspar systematics is also discussed.
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GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Bohemian Massif forms the largest continuous exposure of Upper Palaeozoic Variscan consolidated crust in Central Europe. The South Bohemian pluton (Fig. 1) is situated at the southern edge of the Bohemian Massif. It forms a large composite batholith which intruded high-grade metamorphic rocks of the Monotonous Series (Moldanubian zone) during the late Palaeozoic (Frasl & Finger, 1988
; Klötzli et al., 2000). To the west the gradational contact to highly anatectic country rocks (Finger & Clemens, 1995; Klötzli et al., 2000) suggests that here the deepest level of the batholith is exposed. To the north and east, respectively, the contact is clearly intrusive, indicating a higher level of emplacement (Frank et al., 1990; Klötzli et al., 2000).
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, pyroxene-bearing sample localities; •, pyroxene-free samples. Cross-hatched areas are major villages.There is no general agreement on the genesis of the South Bohemian pluton. At least three tectonothermal models have been proposed: (1) Matte (1986)
favoured a simple model of crustal thickening as a result of compressional tectonics leading to widespread melting of the lower crust. (2) Melting induced by mantle upwelling and magmatic underplating caused by post-orogenic collapse was proposed by Montel et al. (1992). (3) Finger & Steyrer (1990) suggested that late Variscan magmatic underplating was triggered by a northward dipping subduction zone along the southern flank of the Variscan fold belt. Regardless of the models, the granitoids of the South Bohemian pluton show no pronounced mantle signature and the melts were essentially produced through anatectic recycling of older, presumably Cadomian, continental crust (Gerdes et al., 1996; Klötzli et al., 2000).
Two main types of magmatic rocks can be distinguished within the South Bohemian pluton (Fig. 1; Scharbert, 1987; Frasl & Finger, 1988; Liew et al., 1989; Vellmer & Wedepohl, 1994; Finger & Clemens, 1995; Koller & Klötzli, 1998; Klötzli et al., 2000): (1) older, syn-orogenic granitoids (
355–335 Ma) comprising minor gabbros, diorites and coarse-grained biotite granitoids with megacrystic alkali feldspar (Weinsberg type granite, together with pyroxene-bearing varieties of Sarleinsbach and hornblende-bearing Rastenberg granodiorite); (2) younger, mainly post-orogenic granitoids (335–300 Ma) comprising medium- to fine-grained biotite granites (named after different occurrences: Mauthausen and Schrems type granites; Freistadt type granodiorite) forming stocks, sills and dykes mainly within the Weinsberg type granite; two-mica or muscovite granites of Eisgarn type and subtypes thereof (Landstein, Címer, Mrákotín); small intrusions of mineralized granites (Nebelstein, Hirschenschlag, Homolka) and pegmatites, aplites and lamprophyres.
Weinsberg type granite
The coarse-grained, biotite-bearing Weinsberg type granite forms the most widespread rock suite of the South Bohemian pluton (Fig. 1). It is characterized by megacrystic alkali feldspar crystals up to 20 cm in size and a marked geochemical heterogeneity. SiO2 ranges from 58 to 74 wt %. K, Zr and Ba abundances are generally high (Vellmer & Wedepohl, 1994; Koller & Klötzli, 1998). The overall character is metaluminous to peraluminous. On the basis of whole-rock geochemistry, both I- and S-type varieties can be identified within the pluton. 87Sr/86Sr initial ratios are 0·707–0·709 and 
Nd ranges from -4 to -5 (Vellmer & Wedepohl, 1994; Klötzli et al., 2000).
On the basis of zircon typology distributions (Stöbich, 1992) and differences in whole-rock geochemistry, Finger & von Quadt (1992) distinguished two domains within the Weinsberg granite. Weinsberg granite I is metaluminous to weakly peraluminous. It is mainly found in the southern and western parts of the pluton. A slightly more peraluminous and more SiO2-rich Weinsberg II variety occurs predominantly in the NE of the pluton.
The Weinsberg granite plutonic suite is interpreted as being derived from lower-crustal partial melting of variably aluminous, biotite-rich gneisses, which started to melt as a consequence of fluid-absent biotite–plagioclase–quartz breakdown (Finger & Clemens, 1995).
Pyroxene-bearing Weinsberg type granite
A hybrid, pyroxene-bearing sub-facies of the Weinsberg granite from the vicinity of Sarleinsbach (Figs 1 and 2) has been interpreted as quartz monzodioritic cumulates within the Weinsberg I domain (Frasl & Finger, 1988; Finger & Clemens, 1995). On the basis of mineral chemistry and element fractionation trends, however, this interpretation cannot be further supported (Koller, 1994a, 1994b). A brief summary of the petrography and petrology of the investigated rocks is given here. Detailed accounts have been given by Koller (1994a, 1994b) and Klötzli & Koller (1998a), and references therein.
Within the pyroxene-bearing granite an inherited paragenesis is represented by unevenly distributed millimetre-sized multigrain patches within a younger host rock (Fig. 3a). This older paragenesis is characterized by a mangeritic mineral assemblage consisting of quartz + mesoperthitic alkali feldspar + andesine/bytownite + clinopyroxene (XMg = 0·50–0·54) + orthopyroxene (XMg = 0·35–0·42) ± ilmenite ± accessories. The mineral assemblage displays a distinct high-temperature equigranular texture interpreted as stemming from granulite-facies equilibration. Plagioclase is only weakly zoned; clinopyroxene and orthopyroxene are unzoned (Fig. 3b). Pyroxenes can partly be replaced by coronitic ferro-tschermakitic hornblende and biotite.
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Orthopyroxene–clinopyroxene thermometry (Andersen et al., 1993
) indicates a post-magmatic re-equilibration temperature of 755 ± 26°C at 0·75 ± 0·10 GPa (plagioclase–clinopyroxene barometry; Newton, 1983) for the older mineral assemblage. A similar temperature range is found by two-feldspar thermometry (Kroll et al., 1993) using preserved mesoperthitic alkali feldspar compositions. The ferro-tschermakitic hornblendes formed between 706 and 665°C (amphibole–plagioclase thermometry; Holland & Blundy, 1994) at 0·69–0·91 GPa (Al-in-amphibole; Anderson, 1996) defining post-magmatic cooling and uplift. Amphibole–plagioclase equilibrium temperatures from magmatic green hornblendes are in the range of 786–659°C (Holland & Blundy, 1994) at 0·36–0·28 GPa (Al-in-amphibole; Anderson, 1996). Re-equilibration during cooling and uplift is indicated by the occurrence of uralitic amphibole, mainly replacing clinopyroxene, with T 685–354°C (amphibole–plagioclase thermometry; Holland & Blundy, 1994) at 0·22–0·17 GPa (Al-in-amphibole; Anderson, 1996). The younger host granite paragenesis (Fig. 3a) consists of alkali feldspar + oligoclase + quartz + biotite ± accessories and exhibits a magmatic texture, i.e. both alkali feldspar and plagioclase show typical magmatic zonation. A myrmekitic reaction zone formed by oligoclase + quartz ± biotite is a common feature around the alkali feldspars of the inherited assemblage (Fig. 3c).
Relative abundances of the inherited mineral assemblage range from a few percent by volume (e.g. sample 2E92/3 Mairhof with 15 vol. %) to practically inheritance-free host rock, where only single mineral grains (zircons, relict pyroxenes) can be attributed to the older mineral assemblage.
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ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Sample selection and preparation
All investigated samples were selected on the basis of petrographic and petrological characteristics. The primary selection criterion was the amount of inherited mineral assemblage as deduced from the abundance of pyroxenes. On this basis, samples with different amounts of inheritance were selected for the zircon typology investigations (see below; 2E92/3 with
30% inheritance, 2E92/7 with 10% inheritance; 2E92/1 and 2E92/6 with intermediate amounts). The typology investigations revealed a bimodal typology distribution, which directly correlates with the amount of inheritance present. U/Pb and 207Pb/206Pb zircon dating was then conducted on sample 2E92/3, which exhibited the highest amount of zircons interpreted as being inherited. The samples used for Rb/Sr whole-rock investigations were taken from a continuing study of the granites of the South Bohemian pluton (WG6/89–WG18/89; S. Scharbert, unpublished data, 2000). They cover the whole range of practically inheritance-free Weinsberg granite (WG7/89) to samples with abundant inheritance (WG17/89). The megacrystic alkali feldspars of sample 01/95 showed the least amount of perthitic exsolution of all investigated samples. Therefore they were interpreted as representing the least altered feldspar composition and were thus chosen for the common Pb investigations. Samples sizes varied between 20 and 40 kg. Samples were washed and repeatedly crushed in a jaw crusher and sieved to grain sizes <1·0 mm. Zircon and alkali feldspar were then separated following standard procedures combining Wilfley table, magnetic and heavy liquid separation and hand picking. Whole-rock samples were taken as representative aliquots of the crushed rock.
Zircon typology investigations
The zircon typology classification (Pupin, 1980) is based on the apparent relative sizes of the zircon prism faces (100) and (110) and pyramid faces (211) and (101), respectively. These relative sizes of either faces are interpreted as being characteristic for distinct physico-chemical conditions during zircon growth. Changes or differences in relative sizes within a zircon population indicate changing or different conditions of crystal growth. The dominant development of (100) is attributed to zircon growth during high-temperature magmatism, whereas the dominant presence of (110) more probably reflects anatectic zircon growth at lower temperatures. The formation of the (211) pyramid is favoured by the dominance of Al over Na + K in the melt. In Na + K-rich melts growth of the (101) pyramid is favoured. The relative size relationships of the crystal faces are plotted in so-called zircon typology plots. On the x-axis the relative size of the (211) vs the (101) pyramid face is plotted, whereas on the y-axis the relative size of the prism faces (100) and (101) is plotted. Zircon having formed under different physico-chemical conditions will plot in different regions of the typology plot.
Zircon classification was determined after random selection of idiomorphic crystals under a binocular microscope. Zircon typology and subgroup classification following the scheme of Pupin (1980) were established by back-scattered electron imaging of at least 200 pre-selected crystals per sample.
Zircon evaporation analysis
Preparatory handling for zircon evaporation analysis and full details of the technique applied have been summarized by Klötzli (1997). Pb isotopic analyses were obtained using a Finnigan MAT 262 multi-collector mass spectrometer equipped with an axial scanning electron microscope–ion counter system. Only high-temperature steps (>1400°C) with 206Pb/204Pb > 50 000 were used for age calculations. The maximum common Pb correction applied results in age shifts of <1 Ma for 206Pb/204Pb = 18·5 and 207Pb/204Pb = 15·64 (most radiogenically enriched common Pb from South Bohemian pluton whole-rock analysis; see Fig. 10, below). For age calculation and error estimates the ‘Isoplot’ software (Ludwig, 1992) was used. Ages and errors are propagated weighted mean values calculated from at least two evaporation steps and at least 20 measured 207Pb/206Pb ratios. Errors reported are 2 SEM.
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U/Pb isotope analysis
Zircon pre-chemistry handling (air-abrasion, cleaning) for all U/Pb analyses followed procedures given by Parrish et al. (1987)
. U and Pb analyses were obtained using a Finnigan MAT 262 multi-collector mass spectrometer equipped with an axial scanning electron microscope–ion counter system.
For three fractions of the investigated sample 2E92/3 (see Table 3, below; fractions 1027 d, e and x) U/Pb analysis followed the procedures given by Parrish et al. (1987). The other six fractions (see Table 3; fractions 1146 A–F) were analysed by a slightly modified vapour digestion procedure originally described by Wendt & Todt (1991). Pb fractionation, derived from NIST SRM 981 and 982 standard measurements, was 0·107% per mass unit (±29%). U fractionation was corrected using U500 standard measurements (0·087% per mass unit; ±17%). U/Pb analyses were controlled by replicate analyses of standard zircon 91500 (Wiedenbeck et al., 1995). External reproducibility for U/Pb analysis based on the 91500 standard measurements is 0·2% for 207Pb/206Pb (0·07494 ± 0·00015) and 0·25% for 206Pb/238U (0·17972 ± 0·00045).
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For samples 1027 and 1146 the total procedural Pb blank was 8 pg and 2 pg, respectively. The corresponding U blank was below 0·1 pg for both methods. For age calculation and error statistics, ‘Rockage’ (Parrish et al., 1987) and the ‘Isoplot/Ex’ software (Ludwig, 1999) were used. Intercept ages are mainly based on a Monte Carlo simulation with 5000 trials. All errors reported are 2 SD. Common Pb corrections were made using alkali feldspar Pb isotopic ratios recalculated to the assumed zircon crystallization age.
U/Pb alkali feldspar analyses followed the procedures of Ludwig & Silver (1977), which were adapted as follows. About 50 mg alkali feldspars per sample were hand picked from a 100–200 µm size fraction and ultrasonically cleaned in acetone and repeatedly rinsed in ultra-pure water and dried. Because only the final alkali feldspar residue isotopic composition was of major interest, only one severe leaching step (5% HF–70% HNO3 mixture of 40:1, at 40°C for 1 h) was applied. Residual alkali feldspar grains were then rinsed twice with ultrapure water in an ultrasonic bath. The wash was added to the leachate. Residual grains were completely dissolved in the same acid mixture as used for the leaching step at 200°C for 24 h. Pb and U were separated with 12 ml micro-columns using normal HCl ion exchange techniques. Isotope dilution and mass spectrometry procedures were identical to those of the zircon U/Pb analyses. The total procedural Pb blank was 8 ng.
Rb/Sr isotope analysis
The whole-rock Rb/Sr analyses followed procedures described by Scharbert (1987); 300 mg of the crushed sample were used. Rb and Sr analyses were obtained using a single-collector Micromass MM30 mass spectrometer. Reported errors are 2 SEM, with the exception of 87Rb/86Sr, which is taken to be 1[USK1]%. The NIST SRM 987 standard during duration of the study was measured at 87Sr/86Sr = 0·71008 ± 0·00007. Total procedural blanks for both elements were 1 ng. For regression calculations and error estimates the ‘Isoplot’ software (Ludwig, 1992) was used.
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Zircon typology investigations
By thin-section and electron-microprobe investigations zircons definitely belonging to the inherited mineral assemblage could be identified. They either form inclusions in orthopyroxene, clinopyroxene and inherited alkali feldspar or are interstitial. As it was impossible to separate inherited remnants from the granitic matrix before mineral separation, an alternative approach was needed to distinguish zircons belonging to the inherited, pyroxene-bearing paragenesis from those zircons belonging to the granitic paragenesis in the mineral separates. A possible correlation between zircon typology distribution and relative amounts of inherited remnants in different samples was envisaged. Four samples with different relative amounts of inheritance were therefore investigated to test the approach (see Fig. 5a, below).
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20% rel. abundance) to light grey (
5% rel. abundance). (a) Four samples of pyroxene-bearing Weinsberg type granite from the vicinity of Sarleinsbach displaying prominent bimodal zircon typology distributions. One typology group plots in the lower part of the diagram comprising subtypes S23 to D (mean J4; zircons 1146 A and 1027 d of Fig.
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In all samples the majority of the crystals (
80%) are anhedral and display complex dissolution and overgrowth phenomena, thus prohibiting proper classification (Fig. 4, zircons 1146 E and 1146 F). Very often, zircons are broken or intergrown parallel to the c-axis. In the remaining 20% of the zircons, two typological groups can be identified (Figs 4 and 5). One group plots in the lower part of the typology diagram (subtypes S23 to D, mean J4; Fig. 4, zircons 1146 A and 1027 d, called J4 group hereafter; Pupin, 1980). These zircons are much less affected by dissolution and overgrowth than the rest of the zircons. The second typology group plots in the upper part of the diagram (subtypes L2 to S5, mean L4; Fig. 4, zircon 1027 e, called L4 group hereafter; Pupin, 1980). These zircons show varying amounts of dissolution and overgrowth. The typological groups found are not correlated with zircon size, elongation, colour or nature of inclusions.
In samples 2E92/1 and 2E92/3 most classifiable zircons belong to the J4 group, whereas in samples 2E92/6 and 2E92/7 the relative amounts of J4 and L4 group zircons are about equal. Sample 2E92/3 from Mairhof was chosen for zircon geochronology investigations as it showed the highest amount of inherited paragenesis (30%) and the most prominent discrimination between the two zircon typology groups.
Zircon evaporation dating
Zircon evaporation dating results are given in Tables 1 and 2, and Figs 6–8. A total of 35 zircons were analysed from sample 2E92/3 Mairhof. Thirty-two crystals showed no common Pb contribution (206Pb/204Pb > 50 000) thus allowing calculation of meaningful minimum 207Pb/206Pb ages. Figure 6 shows the probability density distribution of the age data. Two main probability maxima can be distinguished. Because of their near-Gaussian normal distribution they are interpreted to represent discrete age groups, although a secondary maximum and a small shoulder toward lower ages lead to a slight negative skewness. The first maximum, at 355 ± 9 Ma, is characteristic for the L4 zircon group. The second maximum corresponds to 529 ± 22 Ma and is characteristic for the J4 zircon group. The small secondary maximum on the lower age side of the L4 peak lies at 342 ± 7 Ma. The shoulder on the J4 peak is not identified as having age significance. Additional small secondary maxima are found at 640 Ma, 930 Ma and 1260 Ma. These ages correspond to single zircon grains in both the L4 and J4 groups. No ages older than 1300 Ma were found.
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On a plot of 207Pb/206Pb evaporation age versus the ratio of the relative error on the respective 208Pb/206Pb and 207Pb/206Pb ratios two discrete data clusters can be identified (Fig. 7). One cluster, with a mean relative error ratio of 0·51 ± 0·35, correlates with the J4 zircon group and an age fraction of 490–590 Ma. The second cluster, with a mean relative error ratio of 1·73 ± 1·15, corresponds to the L4 zircon group with ages <380 Ma.
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Three out of the total of 35 zircons showed a pronounced common Pb contribution with 206Pb/204Pb ranging from 270 to 950 (five temperature steps, Table 2). Such high amounts of common Pb do not allow calculation of meaningful 207Pb/206Pb ages. The regression calculation of a secondary 206Pb/204Pb–207Pb/204Pb isochron through the data results in an age value of 564 ± 160 Ma (Fig. 8a). Using an inverse isochron approach, an intercept 207Pb/206Pb age of 527 ± 83 Ma is obtained (Fig. 8b).
Zircon U/Pb dating
Conventional U/Pb analyses from sample 2E92/3 Mairhof are reported in Table 3. Figure 9 shows a Tera–Wasserburg concordia plot of the data. Five highest-quality zircons with J4 typology (zircons 1027 d, 1146 A, B, C and D; see Fig. 4) and three zircons showing different amounts of overgrowth but interpreted to belong to the granitic paragenesis (L4 typology, zircons 1027 e, 1146 E and F; see Fig. 4) were analysed. Additionally, a mixed fraction (1027 X) consisting of five undifferentiated zircons with visible cores was analysed.
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level (see Table
J4 zircons were air-abraded to 50% of the original size, whereas L4 and anhedral zircons were only slightly abraded to 80% to preserve young overgrowths.
Zircon 1027 d yields a concordia age of 522·4 ± 5·4 Ma (Ludwig, 1999). Zircons 1146 A, B and D, and the mixed fraction 1027 X are all highly discordant. A regression through all five points results in a lower intercept age of 517 ± 22 Ma and an upper intercept age of 2098 ± 57 Ma. Excluding the mixed fraction 1027 X from the regression results in poorly defined intercept ages of 519 ± 120 Ma and 2151 ± 370 Ma. Zircon 1146 C plots at a lower age close to the L4 zircons (207Pb/206Pb age of 338 ± 13 Ma).
Zircon 1146 E yields a concordia age of 349·8 ± 2·5 Ma. The other two zircons are highly discordant. Discordia calculation through all three crystals results in intercept ages of 347 ± 37 and 2044 ± 210 Ma. Including the mixed fraction 1027 X in the regression gives within error identical, but better constrained, intercept ages of 346 ± 3 Ma and 1986 ± 19 Ma. Including the J4 zircon 1146 C in the regression results in essentially identical intercept ages of 345 ± 2 Ma and 1985 ± 18 Ma.
Feldspar common Pb data
To obtain reliable common Pb isotopic values, the innermost cores of two large megacrystic alkali feldspars with only minor amounts of perthitic exsolution were analysed from sample 01/95 (Fig. 2, Table 4). U contents for the leachate and the residue in both crystals are very similar, whereas Pb content varies much more. Resulting µ values are between 5·02 and 12·9 for the leachates and 0·65 and 0·44 for the residues. Mean 206Pb/204Pb ratios recalculated to 530 Ma are 18·27 and 18·03 for leachates and residues, respectively, 207Pb/204Pb ratios are 15·58 and 15·60 (Fig. 10). For comparison, alkali feldspar and whole-rock Pb isotope data for granitoids from the South Bohemian pluton and the Schwarzwald (Kober & Lippolt, 1985; U. S. Klötzli & S. Scharbert, unpublished data, 1999) and the high-grade metasediments of the Monotonous Series are also shown (U. S. Klötzli, unpublished data, 2000).
Rb/Sr whole-rock data
The Rb/Sr whole-rock data obtained for seven samples are reported in Table 5. Rb and Sr contents range from 65 to 131 ppm and 347 to 402 ppm, respectively. Rb/Sr varies between 0·169 and 0·367, whereas 87Sr/86Sr is 0·71039–0·71311. On a Rb/Sr isochron plot (Fig. 11), four samples out of seven (excluding 14/89, 16/89 and 17/89) define a quasi-linear array, the regression of which corresponds to an age of 330 ± 28 Ma. The initial 87Sr/86Sr is 0·70807 ± 0·00035.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Zircon typology distribution
Apart from the very abundant anhedral zircons (
80%), two different typological groups can be identified (Figs 4 and 5). The J4 group classifies in the lower part of the zircon typology diagram (subtypes S23 to D; Pupin, 1980); the L4 group plots in the upper part of the diagram (subtypes L2 to S5; Pupin, 1980). The J4 group is most abundant in samples 2E92/1 (Arnreit) and 2E92/3 (Mairhof), whereas in samples 2E92/6 and 2E92/7 (both West Sarleinsbach) the relative amounts of J4 and L4 zircons are about equal. A direct correlation between petrography and zircon typology distribution is possible. Sample 2E92/3 contains the highest amount of inherited remnants and the maximum abundance of the J4 zircons. This coincidence justifies the interpretation that the J4 zircon typology group constitutes part of the inherited mineral assemblage, which can also be postulated from thin-section studies and back-scattered electron imaging. J4 zircons are far less affected by partial dissolution and overgrowth than the rest of the zircons. Apparently, they were protected from interaction with the granitic magma by their host minerals [pyroxenes, feldspars and quartz(?)], further substantiating the interpretation that these zircons represent inheritance. L4 zircons are more abundant in samples from Sarleinsbach (2E92/6, 2E92/7), which also show lesser amounts of inheritance based on petrographic studies. L4 zircons are consequently interpreted as belonging to the granitic mineral assemblage. Increasing amounts of overgrowth on the granitic L4 zircons eventually lead to the formation of the completely anhedral zircons. These are therefore interpreted as belonging solely to the granitic paragenesis. This observation is in line with the more or less complete absence of overgrowths on the J4 zircons, as stated above.
In its southern and eastern part, pyroxene-free ‘normal’ Weinsberg granite (i.e. 22/85 from ESE Königswiesen; Scharbert, 1987) shows a broader overall typology distribution with a slight bimodality (Fig. 5b), designated as Weinsberg granite I by Stöbich (1992). Zircons are inferred to have grown in a poorly homogenized, complex plutonic system in the lower crust (Finger & von Quadt, 1992). A second type of Weinsberg granite is found north of the Weinsberg granite I. This Weinsberg granite II of Stöbich (1992) is characterized by a more homogeneous zircon typology distribution plotting in the upper left part of the zircon typology diagram (i.e. FR-20 from SSE Windhaag, Fig. 5b; Finger & Haunschmid, 1988). This is interpreted as reflecting zircon crystallization in a more homogenized magmatic system at a higher crustal level than that proposed for domain 1 Weinsberg granite (Finger & von Quadt, 1992).
On the basis of the zircon typology distribution and petrogenetic data of Klötzli & Koller (1998a, 1998b) the pyroxene-bearing Weinsberg type granite of Sarleinsbach belongs to the Weinsberg granite I, although the latter does not show the marked bimodal zircon typology distribution as the pyroxene-bearing Weinsberg type granite of Sarleinsbach. This possibly can be attributed to the lack of ‘older’ zircons being shielded from the granitic magma by pre-existing mineral grains, as proposed for the J4 zircons from our study area. Without such a shielding mechanism the former zircon typology distribution is more or less obliterated or even completely destroyed. Therefore, from the zircon typology distribution alone, it can be concluded that the magmatic system that led to the formation of the pyroxene-bearing Weinsberg type granite was not as mature as for the ‘normal’ Weinsberg granite. This may reflect smaller amounts of Variscan partial melt formation at Sarleinsbach than postulated for the rest of the Weinsberg granite (i.e. 30% partial melting of tonalitic lower-crustal material; Vellmer & Wedepohl, 1994). Or it may reflect the existence of distinctly different lower-crustal protolith compositions of the different Weinsberg granite varieties. A similar bimodal zircon typology distribution associated with zircon protection by large alkali feldspar crystals, as found in Sarleinsbach, has been described from the rocks of the Rastenberg granodiorite in the easternmost South Bohemian pluton (Klötzli & Parrish, 1996).
Zircon U/Pb and evaporation ages
Inherited J4 zircons exhibit a well-constrained mean 207Pb/206Pb evaporation age of 529 ± 22 Ma (Fig. 6). The shoulder on the lower age side of the probability peak is attributed to slight resetting and/or overgrowth of J4 zircons during a later Pb loss event (see detailed discussion below). The concordant U/Pb age of zircon 1027 d (522·4 ± 5·4 Ma) and the lower intercept age of the four J4 single zircons and the mixed fraction 1027 X (517 ± 22 Ma) are identical within error to the mean evaporation age found for the J4 zircons (Figs 6 and 9). The poorly defined intercept ages of the discordia regression through the four single zircons (not shown; 519 ± 120 Ma and 2151 ± 370 Ma) will not be considered further. For the J4 zircon group a weighted mean age of 523 ± 5 Ma (Middle to Late Cambrian) is adopted. It is interpreted as the best estimate for the time of J4 zircon crystallization. Assuming a direct relation of the growth of these zircons to the inherited mineral assemblage, the age can be interpreted in two ways: either the age directly dates the formation of the inferred igneous precursor, or it reflects post-magmatic re-equilibration under granulite-facies conditions. On the basis of petrological evidence, the conditions of precursor re-equilibration are estimated at T 755°C and P 0·75 GPa (Klötzli & Koller, 1998b) but no independent information for the age of this re-equilibration can be provided. No P–T data are available for the inferred igneous precursor. On the basis of field relations, two possibilities arise as to when precursor re-equilibration can have happened: (1) shortly after primary magmatic crystallization, in which case the zircon ages would show that the precursor formation is completely unrelated to Variscan magmatism; or (2) before or during early Variscan magmatism in the Carboniferous, in which case the Cambrian ages would then probably date the original igneous formation of the protolith.
The zircon age data alone cannot definitely distinguish which scenario is more likely. The J4 zircon typology (Pupin, 1980) suggests zircon growth in an igneous system. Thus J4 zircon ages could be interpreted as reflecting magmatic formation around 523 ± 5 Ma. In this case re-equilibration of the inherited mineral assemblage would be a later metamorphic event, presumably during the Variscan orogeny. However, we suggest that the re-equilibration is clearly recorded in the prominent crystal homogenization of the J4 zircons observed during evaporation analysis. Evidence for crystal homogenization comes from comparably small ratios of the relative errors on 208Pb/206Pb and 207Pb/206Pb (Fig. 7). Small ratios of relative errors of 208Pb/206Pb and 207Pb/206Pb evaporation data indicate the complete or partial absence of an oscillatory zircon zonation (Klötzli, 1999). Lack of zircon zonation is characteristic of metamorphic zircons or zircons having undergone high-temperature homogenization (e.g. Vavra, 1990; Pidgeon, 1991; Hanchar & Rudnick, 1995). The mean relative error ratio of 0·51 ± 0·35 found for the J4 zircons is intermediate between typical ‘magmatic’ relative error ratios >1 (i.e. 1·73 ± 1·15 for the unaffected granitic L4 zircons) and ‘metamorphic’ relative error ratios <0·5 [for a more detailed discussion, see Klötzli (1999)]. The low relative error ratio suggests strong J4 zircon homogenization during post-magmatic re-equilibration. Zircon homogenization causes Pb loss and therefore a rejuvenation of apparent isotopic ages. Age dispersion caused by incomplete Variscan Pb loss would lead to a continuum of 207Pb/206Pb evaporation ages and U/Pb ages between a primary zircon crystallization age (523 Ma) and Variscan overprinting (350–320 Ma; Karabinos, 1997). However, the discrete near-Gaussian 207Pb/206Pb evaporation age distributions shown in Fig. 6 and the clusters formed by relative error systematics (Fig. 7) contradict such a scenario. The same conclusions can be drawn from the complete absence of any U/Pb ages intermediate between the J4 zircon and L4 zircon discordia. For a magmatic event much older than the Cambrian precursor re-equilibration the same argument as for Variscan re-equilibration holds true. Age dispersion for a Cambrian magmatic event immediately followed by the 523 ± 5 Ma re-equilibration would probably be undetectable within analytical error. We therefore conclude that igneous formation was immediately followed by granulite-facies re-equilibration (dated at 523 ± 5 Ma) within a few million years.
Petrological data further show that the country rocks around Sarleinsbach never experienced Variscan granulite facies P–T conditions required for metamorphic mangerite–charnockite formation (Finger & Clemens, 1995). On the other hand, if a magmatic precursor formation in the Variscan is postulated, the Cambrian zircon ages (520–530 Ma) would imply an extremely homogeneous inheritance. Such a model probably would require formation of the mangeritic precursor rock by overprinting of a single protolith, a very unlikely situation in view of the heterogeneous lithological composition of the Moldanubian zone (Frasl & Finger, 1988; Finger & von Quadt, 1992; Finger & Clemens, 1995) and the lack of any possible protolith lithologies of proven Cambrian age (Klötzli et al., 2000). The only Cambrian ages known are zircon U/Pb ages of 526 ± 4 Ma for eclogite lenses that are interpreted as tectonic slivers within the Monotonous Series (Sallingberg, Lower Austria; O’Brien & Kröner, 1999). At the moment, the relationship between the pyroxene-bearing granitoids from Sarleinsbach and the polymetamorphic Moldanubian basement rocks is enigmatic.
Interpretation of ages for zircons associated with the granitic magma is not straightforward. 207Pb/206Pb evaporation ages for the idiomorphic, high-quality L4 zircons indicate a crystallization age of 355 ± 9 Ma (Fig. 6). The concordant U/Pb age of 350 ± 3 Ma of the anhedral zircon 1146 E and the lower intercept age of 346 ± 3 Ma found for all granitic zircons (Fig. 9) are identical within error to the evaporation age data, although a definitive shift toward younger ages is evident. Two possibilities for the interpretation of the age data have to be discussed. A systematic deviation of ages obtained by the two zircon dating methods can safely be neglected for the following reasons. All reported zircon evaporation analyses show 206Pb/204Pb values >50 000 resulting in age corrections of <1 Ma. Mass spectrometry Pb fractionation is strictly controlled (Klötzli, 1997). It amounts to 1–2 Ma for the given age range but the correction goes in the ‘wrong’ direction leading to higher ages. In this sense, the fractionation correction runs opposite to the common Pb correction. In the present study the two corrections practically cancel out. U/Pb analyses are correct to ±0·2% for the reported 207Pb/206Pb and to ±0·25% for 206Pb/238U, in fact better than the analytical errors on the individual analyses. Recalculated to 345 Ma this is ±4·5 Ma and 0·8 Ma, respectively. The observed trend toward younger ages could be partly explained by a systematic shift from analytical uncertainty alone. However, compared with the published U/Pb ages for the 91500 standard zircon (Wiedenbeck et al., 1995) the mean U/Pb 91500 zircon ages determined during the course of this study are too high by 2 Ma, again enhancing the observed age difference. Common Pb correction of the U/Pb data follows the same systematics as for the evaporation analyses, although 206Pb/204Pb is significantly smaller for the U/Pb analyses. Thus it can be concluded that the observed age difference is not due to analytical factors. A geological explanation has to be found: 207Pb/206Pb evaporation ages of L4 zircons indicate a magmatic formation age of 355 ± 9 Ma. Such Tournaisian ages are found throughout the South Bohemian pluton (Klötzli et al., 2000). For the normal Weinsberg varieties reported zircon ages are 357 ± 9 Ma from Königswiesen (sample 22/85; Klötzli, 1993) and 353 Ma from Plochwald (F. Finger, personal communication, 1994). For the Rastenberg granodiorite zircon U/Pb and 207Pb/206Pb evaporation ages derived from a specific long prismatic S24 population are 353 ± 2 Ma and 353 ± 9 Ma, respectively (Klötzli & Parrish, 1996). All these ages have been interpreted as dating a large-scale pervasive magmatic event in the lower crust, which eventually led to the intrusion of the individual plutons some 10 m.y. later (Klötzli et al., 2000). Obviously, the mean 207Pb/206Pb evaporation age of 355 ± 9 Ma reported here fits well into this scheme, thus extending the limits of this supposed magmatic event well into the westernmost part of the South Bohemian pluton. The question then arises why the actual intrusion event was not readily detected by the zircon evaporation dating. Inspection of the morphology of the investigated zircons provides a possible answer. Only highest quality zircons with a clearly recognizable typology were used for the evaporation dating, whereas for the U/Pb analyses strongly overgrown and therefore anhedral zircons were chosen (compare Fig. 3). Zircon growth, dissolution and overgrowth not only occur early in a magmatic cycle (as early liquidus phase) but are also possible during the later stages (i.e. Evans & Hanson, 1993; Klötzli & Parrish, 1996). It can thus be argued that the euhedral, well-crystallized L4 zircons used for evaporation were formed early in the Variscan magmatic cycle, as stated above. Such zircons were then partly dissolved and/or overgrown during the later plutonic stages, possibly during the intrusion. Analysis of such crystals leads to the observed younger U/Pb ages. Only a rather small but pronounced secondary peak at 342 ± 7 Ma on the lower age side of the 355 Ma probability maximum (zircon 2E93/3-AC, Fig. 6) reflects the imprint of this post-355 Ma event on the high-quality L4 zircons used for evaporation analysis. Thin overgrowths present on other crystals were evaporated during the initial heating steps and were thus not detected.
In view of already published intrusion ages for the South Bohemian pluton and distinct zircon morphology a weighted mean age of 345 ± 5 Ma is taken to date the actual emplacement of the pyroxene-bearing Weinsberg type granite in the vicinity of Sarleinsbach. This interpretation is in line with the relative error systematics of the L4 zircon evaporation data, as discussed above.
Vellmer & Wedepohl (1994) proposed that the Weinsberg granite sensu lato was formed by up to 30% partial melting of tonalitic lower-crustal material at 800–850°C. Finger & Clemens (1995) reported comparable values of >850°C at <0·7 GPa. On the basis of our new age data, this partial melting event led to zircon recrystallization and growth in the Tournaisian at 355 ± 9 Ma. The estimated minimum P–T conditions for this event are 786°C at 0·36 GPa (Klötzli & Koller, 1998b), somewhat lower than the previously reported literature values above. The formation of coronitic ferro-tschermakitic hornblende around clinopyroxene (at 706–665°C and 0·69–0·91 GPa; Klötzli & Koller, 1998b) could also be attributed to this event. Lower temperature and pressure estimates from amphibole–plagioclase equilibrium temperatures for magmatic green hornblendes (659°C at 0·28 GPa; Klötzli & Koller, 1998b), indicating magma cooling and uplift, may be related to magma emplacement, tentatively dated at 345 ± 5 Ma.
The zircon U/Pb and evaporation age information from truly inherited zircon fits into the common inheritance patterns found elsewhere in the Central European Variscides. U/Pb upper intercept ages >1986 Ma (Fig. 9) have been known for some time. They imply the existence of Lower Proterozoic protoliths, which have been repeatedly reworked into the Cadomian and Variscan basement sequences. More remarkable is the 642 ± 21 Ma zircon age of 2E92/3-P (Table 1, Fig. 6). This is identical to Cadomian formation ages found for the Weiterndorf granite gneiss (640 ± 20 Ma, U. S. Klötzli, unpublished data, 2000) and the Spitz gneiss (620 Ma; Friedl et al., 1998), both found further to the east in the polymetamorphic Moldanubian basement series. A similar inheritance of Cadomian intrusive rocks with a magmatic age of 623 ± 22 Ma has been postulated for the Rastenberg granodiorite (Klötzli & Parrish, 1996). All these data imply the widespread incorporation and partial reworking of Cadomian igneous rocks with formation ages of around 620–640 Ma throughout the southeastern part of the Moldanubian zone.
Common Pb data
In three zircons analysed by the evaporation method the amount of common Pb present was substantial, with low 206Pb/204Pb values ranging from 270 to 950. Such high amounts of 204Pb are rarely found in zircon evaporation analysis (Klötzli, 1997). Interestingly, it was not only detected in the low-temperature evaporation steps, as is normal, but even in the high-temperature steps. No difference in typology, size, colour or inclusions present could be used to discriminate these common Pb-bearing crystals from the rest of the zircons.
The evaporation data prove that the three zircons incorporated common Pb in a pervasive manner during crystallization and not during a later event. A calculated secondary isochron age for the common Pb from the three zircons lies at 564 ± 160 Ma (Fig. 8a). The corresponding 207Pb/206Pb intercept age at 204Pb/206Pb = 0 is 527 ± 83 Ma (Fig. 8b). Both age values overlap within error with the inferred re-equilibration age of the inherited mineral assemblage at 523 ± 5 Ma. This strongly indicates that the common Pb was introduced into the crystals during Cambrian zircon growth and not during the Carboniferous, as the granitic L4 typology of two of the three crystals might suggest. The L4 typology of these crystals is therefore regarded as only a thin overgrowth on inherited J4 crystals. This overgrowth was probed during the initial zircon cleaning and/or low-temperature evaporation steps, but analyses did not result in any usable Pb isotopic data. The inherited zircons seem to have completely retained their Cadomian Pb isotopic signature during Variscan magmatism. This lends additional support to the above conclusion that the common Pb was directly incorporated into the crystal lattice, because Pb in zircon inclusions or as micro-cleavage filling would have been mobilized during the Variscan orogeny.
On the basis of petrological investigations it has been proposed that at least some of the large alkali feldspars present in the pyroxene-bearing Weinsberg type granite constitute a part of the former mangeritic mineral assemblage (e.g. high-celsian components distributed in a patchy manner; Koller, 1994b; Klötzli & Koller, 1998b; Koller & Klötzli, 1998). Such domains in alkali feldspars, unaffected by post-equilibration exsolution processes, can therefore be regarded as relicts of an inherited mineral phase in the Variscan granitoid.
When recalculated to 530 Ma (Fig. 10, Table 4) the residues from leached cores from megacrystic alkali feldspar exhibit a comparably non-evolved common Pb isotopic composition. Even the present-day values plot to the left of the proposed field of alkali feldspar and whole-rock Pb isotopic compositions at 355 Ma. The Variscan Pb isotopic composition at 355 Ma is therefore not shown on the plot (being situated between the data points at 530 Ma and the present day). This suggests that the common Pb from the leached feldspars from the pyroxene-bearing Weinsberg type granite must be of a different, probably older, provenance than the common Pb found in the ‘normal’ Weinsberg granite and the accompanying intrusive units. The inference drawn from the zircon Pb systematics that the common Pb found is of Cambrian age therefore seems to be valid. We thus conclude that zircon and alkali feldspar incorporated the same common Pb during the Cambrian magmatic precursor formation or post-magmatic granulite-facies re-equilibration. Consequently, a reference line drawn through the alkali feldspar residue and zircon common Pb data points should result in a secondary Pb/Pb isochron age defining the time of system closure (i.e. magmatic crystallization or post-magmatic re-equilibration of zircon and alkali feldspar). The calculation (dashed line in Fig. 10) results in a age of 410 ± 120 Ma. Neglecting the large error, this young age could imply that some minor amount of more radiogenic Pb component is still present in the leached feldspar cores. Thus, the true Pb isotopic composition of the Cambrian feldspar cores could well be even less radiogenic, enhancing the difference in Pb isotopic composition between Cadomian and Variscan alkali feldspar from the South Bohemian pluton and the Schwarzwald. Corrected 207Pb/206Pb evaporation ages (using 206Pb/204Pb = 18·03, 207Pb/204Pb = 15·59 from alkali feldspar residues) from the three common Pb-bearing zircons are in the range 111–498 Ma with large individual errors. These young ages demonstrate the complex behaviour of common Pb in zircon or, more likely, alkali feldspar during Variscan subsolidus exsolution and albitization. Taking the corrected 498 Ma 207Pb/206Pb evaporation age of 2E92/3-I, 10-12 (Table 2) as indicating the best age estimate for system closure, calculation of the alkali feldspar–zircon secondary isochron results in an age of 497 ± 70 Ma, demonstrating closed-system behaviour of alkali feldspar and zircon since the Cambrian.
The alkali feldspar leachate from sample 01/1/95 shows, for the time of Variscan magmatic activity, a more radiogenic common 206Pb/204Pb composition (recalculated to 355 Ma; Fig. 10) than the common Pb isotopic composition of alkali feldspars from other Variscan granitoids of the South Bohemian pluton and the Schwarzwald (Kober & Lippolt, 1985; U. S. Klötzli & S. Scharbert, unpublished data, 1999). The observed leached Pb more directly reflects the whole-rock Pb isotopic composition characteristic of the igneous rocks of the South Bohemian pluton. Assuming that no unrecognized U-rich inclusions are responsible for the higher amount of radiogenic lead, this can tentatively be attributed to the formation of the perthitic exsolution and the albite veining (Housh & Bowring, 1991; Klötzli & Koller, 1998b), thereby incorporating more radiogenic lead from the total rock reservoir. However, as only one leaching step was applied no definitive distinction between the potential lead reservoirs is possible. Nevertheless, as a whole, the data are interpreted as giving some evidence for partial alkali feldspar recrystallization, overgrowth and subsolidus exsolution during the Variscan magmatic event with incorporation of Variscan whole-rock common Pb into Cadomian alkali feldspar. The leachate from sample 01/2/95 plots far from any reasonable Pb composition and we do not have an explanation for this at present.
Biotite–plagioclase–quartz gneisses, equivalents of the widespread metagreywackes, form possible source rocks for the Weinsberg granite varieties (Finger & Clemens, 1995). No whole-rock or feldspar Pb isotope data are available for these lithologies. On the basis of field evidence and Sr isotope systematics, Scharbert (1998) interpreted the paragneisses of the Monotonous Series as possible protolith lithologies for the easterly-situated main body of the Weinsberg granite. However, the age systematics and Pb isotope information presented here for the paragneisses of the Monotonous Series (Fig. 10; Klötzli et al., 2000, U. S. Klötzli; unpublished data, 2000) rule out such a simple partial melting model. A more complex model has been suggested by Vellmer & Wedepohl (1994), who proposed that the Weinsberg granite sensu lato was formed by 30% partial melting of tonalitic lower-crustal material, triggered by the influx of incompatible-element enriched mantle fluids. If correct, this implies that the apparent whole-rock and feldspar Pb isotope systematics of the granite are governed by at least a three-component system comprising an inherited mangerite source, a lower-crustal source, possibly metasedimentary in origin (Finger & Clemens, 1995; Scharbert, 1998), and a metasomatic mantle source (Vellmer & Wedepohl, 1994). Apart from the actual process of melt generation, some late-stage crustal assimilation during magma ascent and emplacement has also to be taken into account. Therefore, a multi-step–multi-component melting–assimilation process with resulting complex isotopic disequilibrium has to be invoked to explain the observed temporal and isotopic relationships.
Rb/Sr whole-rock data
Compared with ‘normal’ Weinsberg granite values (<317 ppm Sr, >157 ppm Rb, mean Rb/Sr = 1·94, Scharbert, 1987; Frank et al., 1990) the pyroxene-bearing granitoids of Sarleinsbach are substantially enriched in Sr and depleted in Rb (347–402 ppm Sr, 65–131 ppm Rb, mean Rb/Sr = 0·292). The present-day least radiogenic 87Sr/86Sr isotopic composition is therefore less radiogenic (<0·71311) than that of ‘normal’ Weinsberg granite (>0·71575, Scharbert, 1987; Frank et al., 1990). In this respect, they resemble more closely the Rastenberg granodiorite (mean Rb/Sr = 0·261), the Karlstift granite (mean Rb/Sr = 0·264) and the Freistadt granodiorite (mean Rb/Sr = 0·231; Scharbert, 1987; Frank et al., 1990; Klötzli & Parrish, 1996).
Four out of seven whole-rock samples define a quasi-linear array on a Rb/Sr isochron diagram, the regression of which corresponds to an errorchron ‘age’ of 330 ± 28 Ma and an initial 87Sr/86Sr of 0·70807 ± 0·00035 (Fig. 11). The remaining three samples plot on either side of the regression line. Except for a possible link to sample locality, there is no obvious cause for the deviation of these points from the line as neither a geochemical, petrological nor a zircon typological relationship to the Rb/Sr systematics is evident. Inspection of a 1/Sr–87Sr/86Sr diagram reveals that the data points do not represent a simple mixing line, nor can a fractionation trend be deduced from a Rb–Sr diagram. Because of this and considering the large error on the 330 Ma regression line, no meaningful interpretation of the Rb/Sr data is possible. However, Rb/Sr whole-rock data have also to be seen in the light of the proposed inheritance character of some alkali feldspars. It has been shown that the Pb isotopic systematics of at least some parts of the large alkali feldspar megacrysts have not been modified during Variscan magmatism. These alkali feldspar domains, which do not show any signs of perthitic exsolution or albitization, have more or less completely preserved their Pb inventory. If this also holds true for Sr, a certain influence on Rb/Sr whole-rock systematics has to be taken into account. The amount and grade of elemental homogenization in a mineral are controlled by the respective element diffusion in the alkali feldspar crystal lattice, which is strongly influenced by cation charge and ionic radius. Sr is divalent and, under most geological conditions, Pb will most probably be also in the divalent and not in the tetravalent state (Otto, 1966). Therefore the cation charges for Sr and Pb in alkali feldspar most certainly are identical. Ionic radii do not differ significantly (0·121 nm for Sr; 0·129 nm for Pb; Shannon, 1976). In view of these similarities, Sr can also be retained during a high-temperature event, as has been shown for Pb. This is further substantiated by the experimental data of Cherniak & Watson (1992), who reported an effective Sr closure temperature in orthoclase of >600°C (at a cooling rate of 1°C/Ma) for non-exsolved micrometre-sized crystal domains in a fluid-free environment. Such crystals can retain their Sr inventory at the core for several million years at >700°C (Cherniak & Watson, 1992). In view of the influence of alkali feldspar on the Rb/Sr whole-rock systematics (Scharbert, 1998), such closure temperatures seem to be unrealistically high. The shielding effect of the inherited mineral assemblage resulted in the protection of at least the inner part of some alkali feldspar crystals from reaction with the fluid-saturated biotite-bearing granitic melt. Therefore the preservation of the Pb and Sr isotope systematics, respectively, is interpreted as a direct expression of this shielding effect, as discussed for the zircon typology and age systematics. The preservation of celsian-rich crystal domains also supports these findings (Koller & Klötzli, 1998). Taking all the available information into account, the 330 ± 28 Ma Rb/Sr age is best interpreted as a minimum age for granite emplacement or high-temperature cooling.
By comparing the pyroxene-bearing Weinsberg type granite varieties with data reported for the ‘normal’ Weinsberg granite (Fig. 12; Scharbert, 1987; Frank et al., 1990) the following geochemical implications can be drawn. All Sarleinsbach variety samples plot above the 349 ± 4 Ma regression line of Frank et al. (1990), similar to the samples designated as ‘contaminated’ Weinsberg granite by Scharbert (1987) but at considerably lower 87Rb/86Sr. Scharbert (1998) has attributed the deviation of the ‘contaminated’ Weinsberg granite samples from the 349 Ma reference line to the assimilation of metasedimentary wall rocks in different proportions. Wall rocks are thought to be equivalents of the paragneisses of the Monotonous Series. For the pyroxene-bearing Weinsberg type granite samples such a simple assumption is certainly not valid, as has already been demonstrated by the common Pb systematics of the investigated rocks. Again, at least a three-component mixture consisting of precursor inheritance (i.e. in the form of inherited Sr from the alkali feldspars), a mantle-derived metasomatic component and a paragneiss component, as postulated for the Weinsberg granite sensu lato (Scharbert, 1987; Liew et al., 1989; Koller et al., 1993; Koller, 1994a; Vellmer & Wedepohl, 1994; Finger & Clemens, 1995; Koller & Klötzli, 1998), has to be inferred. However, the exact composition, mixing proportions and mechanisms are as yet completely unresolved. Additionally, the initial isotopic homogeneity of at least the mangeritic and metasedimentary components has to be questioned as well, thus further complicating the situation.
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Two virtually independent mineral assemblages can be found in the hybrid, pyroxene-bearing Weinsberg granite variety of Sarleinsbach (Mühlviertel, Austria). The older assemblage stems from a re-equilibrated mangerite source (quartz + alkali feldspar + andesine/bytownite + orthopyroxene + clinopyroxene ± ilmenite ± accessories), whereas the younger crystallized from a biotite-bearing granitic melt (quartz + alkali feldspar + oligoclase/andesine + biotite ± accessories). On the basis of their typology distributions,
20% of the zircons present can be attributed to either the inherited or the granitic mineral assemblage. Zircons belonging to the inheritance show a mean J4 typology, whereas granitic zircons exhibit a mean L4 typology. The remaining 80% are anhedral. The relative proportions of J4 to L4 zircons in the samples directly correlate with the amount of remnant mineral assemblage present. Inherited J4 zircons are remarkably less affected by dissolution and overgrowth than are the granitic L4 zircons. They appear to have been protected from interaction with the granitic melt by their host mineral, further substantiating the interpretation that these zircons are inherited. U/Pb and evaporation dating of J4 zircons provides evidence that this inherited assemblage crystallized during the Cambrian at 523 ± 5 Ma.
Ages derived from granitic L4 and anhedral zircons point to a two-stage magmatic event during the Variscan orogeny in the Carboniferous. A first event dated at 355 ± 9 Ma (L4 zircons), is attributed to the partial melting of Moldanubian lower crust. The second event, at 345 ± 5 Ma, derived from anhedral zircons, constrains the granite emplacement to the middle crust.
The similarity between the non-radiogenic common Pb in megacrystic alkali feldspars and common Pb in some zircons demonstrates that the core regions of some of the alkali feldspars grew during the formation of the inherited mineral assemblage in the Early Palaeozoic in a lower-crustal environment. Subsequently, some of the alkali feldspar cores obviously were not completely resorbed or equilibrated during the Carboniferous partial melting event (reaching temperatures around 800°C at moderate pressure). Similar conclusions have been drawn for large alkali feldspar crystals from the Rastenberg granodiorite (Klötzli & Parrish, 1996).
The 330 ± 28 Ma Rb/Sr whole-rock errorchron age is interpreted as a minimum age for the granite emplacement. However, in the light of the probable preservation of an old Sr component stored in the inherited alkali feldspar crystals, the relevance of Rb/Sr whole-rock data has to be questioned.
Apart from the geochronological information now available, the textural evidence, zircon typology and geochemical fractionation trends rule out the possibility that the pyroxene-bearing mineral assemblage found in the Weinsberg granite varieties of Sarleinsbach is of cumulate origin, as has previously been proposed by Frasl & Finger (1988) and Finger & Clemens (1995). All evidence strongly supports the coexistence of a inherited mineral assemblage and a ‘normal’ granitic assemblage within individual granite samples. Such a finding further implies that whole-rock geochemical and isotope investigations of complex, ‘hybrid’ granitoids do not necessarily provide any unequivocal information on their origin and petrogenesis. This is especially true if inherited mineral phases are part of the rock-forming constituents, i.e. alkali feldspar, which strongly influence major and trace element geochemistry.
The proposed emplacement age of 345 ± 5 Ma for the Weinsberg type granitoids of Sarleinsbach is contemporaneous with the peak of high-T metamorphism in the Moldanubian zone (Petrakakis, 1997; Klötzli et al., 2000). Therefore, the models of Matte (1986) or Finger & Steyrer (1990), who proposed that the onset of partial melting of the Variscan crust occurred 20–30 m.y. after collision and metamorphism, have to be re-evaluated.
Considering the complex petrogenetic and geochronological evolution of the pyroxene-bearing Weinsberg type granitoids of Sarleinsbach, other occurrences of similar, coarse-grained rocks (e.g. granitoids of the Central Bohemian pluton, Black Forest, Vosges Mountains and French Massif Central) should be suspected of possibly bearing similar complexities. Therefore, new geotectonic models of the Variscan and pre-Variscan evolution of Europe should certainly be based on more detailed petrological and geochronological investigations into the inheritance patterns of igneous rocks. Further investigations should also aim at better understanding the influence of inherited mineral assemblages on the REE systematics of their host granitoids. Possibly, some of the petrogenetic and geotectonic implications drawn from such data will have to be reconsidered as well.
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ACKNOWLEDGEMENTS |
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Various financial contributions of the Austrian Science Foundation (P11106-Geo, S47-Geo) are gratefully acknowledged. Invaluable help by discussion, criticism and advice during the course of this study were given by G. Buda, M. Jelenc, T. Kebede and K. Petrakakis. We also thank I. Braun, R. Trumbull, M. Wilson and J. A. Winchester for critical and constructive reviews.
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FOOTNOTES |
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*Corresponding author. Telephone: +43-1-4277-53460. Fax: +43-1-4277-9534. E-mail: urs.kloetzli{at}univie.ac.at
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REFERENCES |
|---|
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Andersen, D. J., Lindsley, D. H. & Davidson, P. M. (1993). QUILF: a Pascal program to assess equilibria among Fe–Mg–Mn–Ti oxides, pyroxenes, olivine and quartz. Computers and Geosciences 19(9), 1333–1350.
Anderson, J. L. (1996). Status of thermobarometry in granitic batholiths. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 125–138.
Cherniak, D. J. & Watson, E. B. (1992). A study of strontium diffusion in alkali feldspar, Na–K feldspar and anorthite using Rutherford backscattering spectroscopy. Earth and Planetary Science Letters 113, 411–425.
Evans, O. C. & Hanson, G. N. (1993). Accessory-mineral fractionation of rare-earth element (REE) abundances in granitoid rocks. Chemical Geology 110, 69–93.[Web of Science]
Finger, F. & Clemens, J. D. (1995). Migmatization and ‘secondary’ granitic magmas: effects of emplacement and crystallisation of ‘primary’ granitoids in South Bohemia, Austria. Contributions to Mineralogy and Petrology 120, 311–326.[Web of Science]
Finger, F. & Haunschmid, B. (1988). Die mikroskopische Untersuchung der akzessorischen Zirkone als Methode zur Klärung der Intrusionsfolge in Granitgebieten—Eine Studie im nordöstlichen oberösterreichischen Moldanubikum. Jahrbuch der Geologischen Bundesanstalt 131(2), 255–266.
Finger, F. & Steyrer, H. P. (1990). I-type granitoids as indicators of a late Palaeozoic convergent ocean–continent margin along the southern flank of the Central European Variscan Orogen. Geology 18, 1207–1210.
Finger, F. & von Quadt, A. (1992). Wie alt ist der Weinsberg Granit? U/Pb versus Rb/Sr Geochronologie. Mitteilungen der Österreichischen Mineralogischen Gesellschaft 137, 83–86.
Frank, W., Hammer, S., Popp, F., Scharbert, S. & Thöni, M. (1990). Isotopengeologische Neuergebnisse zur Entwicklungsgeschichte der Böhmischen Masse: Proterozoische Gesteinsserien und Variszische Hauptorogenese. Österreichische Beiträge zu Meteorologie und Geophysik 3, 185–228.
Frasl, G. & Finger, F. (1988). Führer zur Exkursion der Österreichischen Geologischen Gesellschaft ins Mühlviertel und in den Sauwald. Reihe Exkursionsführer, Mitteilungen der Österreichischen Geologischen Gesellschaft, 30 pp.
Friedl, G., McNaughton, N., Fletcher, I. R. & Finger, F. (1998). New SHRIMP-zircon ages for orthogneisses from the south-eastern part of the Bohemian Massif (Lower Austria). Acta Universitae Carolinae 42(2), 251–252.
Gerdes, A., Wörner, G. & Finger, F. (1996). Mantle sources in Hercynian granitoids? A trace element and isotope study. Journal of Conference Abstracts 1(1), 201.
Giletti, B. J. & Casserly, J. E. D. (1994). Sr diffusion in plagioclase feldspars. Geochimica et Cosmochimica Acta 58, 3785–3793.
Hanchar, J. M. & Rudnick, R. L. (1995). Revealing hidden structures: the application of cathodoluminescence and back-scattered electron imaging to zircon dating from lower crustal xenoliths. Lithos 36, 289–303.
Holland, T. & Blundy, J. (1994). Non-ideal interactions in calcic amphiboles and their bearing on amphibole–plagioclase thermometry. Contributions to Mineralogy and Petrology 116, 433–447.
Housh, T. & Bowring, S. E. (1991). Lead isotope heterogeneities within alkali feldspars: implications for the determination of initial lead isotopic compositions. Geochimica et Cosmochimica Acta 55, 2309–2316.
Karabinos, P. (1997). An evaluation of the single-grain zircon evaporation method in highly discordant samples. Geochimica et Cosmochimica Acta 61(12), 2467–2474.
Klötzli, U. S. (1993). Einzelzirkon-207Pb/206Pb-Datierungen an Gesteinen der südlichen Böhmischen Masse (Rastenberger Granodiorit, Weinsberger Granit). Mitteilungen der Österreichischen Mineralogischen Gesellschaft 138, 123–130.
Klötzli, U. S. (1997). Single zircon evaporation TIMS: method and procedures. Analyst 122, 1239–1248.[Web of Science]
Klötzli, U. S. (1999). Th/U zonation in zircon derived from evaporation analysis: a model and implications. Chemical Geology 158, 325–333.
Klötzli, U. S. & Koller, F. (1998a). Geochronology of pre-Variscan crustal remnants and their granitoid host rocks, South Bohemian Pluton, Austria. Mitteilungen der Österreichischen Mineralogischen Gesellschaft 143, 314–315.
Klötzli, U. S. & Koller, F. (1998b). Sarleinsbach type of Weinsberg granite. In: Breiter, K. (ed.) Genetic Significance of Phosphorus in Fractionated Granites. Excursion Guide. IGCP373. Prague: Czech Geological Survey, pp. 137–140.
Klötzli, U. S. & Parrish, R. R. (1996). Zircon U–Pb and Pb–Pb geochronology of the Rastenberg granodiorite, South Bohemian Massif, Austria. Mineralogy and Petrology 58, 197–214.
Klötzli, U. S., Frank, W., Scharbert, S. & Thöni, M. (2000). The evolution of the SE Bohemian Massif based on geochronological data: a review. Jahrbuch der Geologischen Bundesanstalt 141, 377–394.
Kober, B. & Lippolt, H. J. (1985). Pre-Hercynian mantle lead transfer to basement rocks as indicated by lead isotopes of the Schwarzwald crystalline, SW-Germany. Contributions to Mineralogy and Petrology 90, 162–171.
Koller, F. (1994a). The South Bohemian Pluton—a complex batholith with a multiple intrusion history. Mitteilungen der Österreichischen Mineralogischen Gesellschaft 139, 71–73.
Koller, F. (1994b). Prä-Variszische Krustenreste im Weinsberger Granit und Rastenberger Granodiorit. Mitteilungen der Österreichischen Mineralogischen Gesellschaft 139, 322–324.
Koller, F. & Klötzli, U. S. (1998). The evolution of the South Bohemian Pluton. In: Breiter, K. (ed.) Genetic Significance of Phosphorus in Fractionated Granites. Excursion Guide, IGCP373. Prague: Czech Geological Survey, pp. 11–14.
Koller, F., Scharbert, S. & Höck, V. (1993). Neue Untersuchungsergebnisse zur Genese einiger Granite des südböhmischen Plutons. Mitteilungen der Österreichischen Mineralogischen Gesellschaft 138, 179–196.
Kroll, H., Evangelakakis, C. & Voll, G. (1993). Two-feldspar geothermometry: a review and revision for slowly cooled rocks. Contributions to Mineralogy and Petrology 114, 510–518.
Liew, T. C., Finger, F. & Höck, V. (1989). The Moldanubian granitoid plutons of Austria: chemical and isotopic studies bearing on their environmental setting. Chemical Geology 78, 41–55.
Ludwig, K. R. (1992). Isoplot: a plotting and regression program for radiogenic-isotope data, version 2.57. US Geological Survey, Open-File Report 91-445.
Ludwig, K. R. (1999). Isoplot/Ex, a geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center, Special Publication 1a, 49 pp.
Ludwig, K. R. & Silver, L. T. (1977). Lead-isotope inhomogeneity in Precambrian igneous alkali feldspar. Geochimica et Cosmochimica Acta 41, 1457–1471.
Matte, Ph. (1986). Tectonics and plate tectonic model for the Variscan belt of Europe. Tectonophysics 126, 329–374.[Web of Science]
Matura, A. (1976). Hypothesen zum Bau und zur geologischen Geschichte des kristallinen Grundgebirges von Südwestmähren und dem niederösterreichischen Waldviertel. Jahrbuch der Geologischen Bundesanstalt 119, 57 pp.
Montel, J. M., Marignac, C., Barbey, P. & Pichavant, M. (1992). Thermobarometry and granite genesis: Hercynian low-P, high-T Velay anatectic dome (French Massif Central). Journal of Metamorphic Geology 10, 1–15.[Web of Science]
Newton, R. C. (1983). Geobarometry of high-grade metamorphic rocks. American Journal of Science 283-A, 1–28.[Web of Science]
O’Brien, P. J. & Kröner, A. (1999). Zircon ages of meta-eclogites in the Monotonous Series of the Moldanubian Zone in Austria and their significance. Beihefte zum European Journal of Mineralogy 11(1), 170.
Otto, E. M. (1966). Equilibrium pressures of oxygen over oxides of lead at various temperatures. Journal of the Electrochemical Society 113, 525–527.
Parrish, R. R., Roddick, J. C., Loveridge, W. D. & Sullivan, R. W. (1987). Uranium–lead analytical techniques at the geochronology laboratory, Geological Survey of Canada. Geological Survey of Canada Paper 87-2, 3–7.
Petrakakis, K. (1997). Evolution of Moldanubian rocks in Austria: review and synthesis. Journal of Metamorphic Geology 15, 203–222.
Pidgeon, R. T. (1991). Recrystallisation of oscillatory zone zircon: some geochronological and petrological implications. Contributions to Mineralogy and Petrology 110, 463–472.
Pupin, J. P. (1980). Zircon and granite petrology. Contributions to Mineralogy and Petrology 73, 207–220.[Web of Science]
Scharbert, S. (1987). Rb–Sr Untersuchungen granitoider Gesteine des Moldanubikums in Österreich. Mitteilungen der Österreichischen Mineralogischen Gesellschaft 132, 21–37.
Scharbert, S. (1998). Some geochronological data from the South Bohemian Pluton in Austria: a critical review. Acta Universitae Carolinae 42, 114–118.
Shannon, R. D. (1976). Revised effective ionic-radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallographica A32, 751.
Stacey, J. S. & Kramers, J. D. (1975). Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters 26, 207–221.[Web of Science]
Stöbich, D. M. (1992). Trachtstudien an den akzessorischen Zirkonen des Weinsberger Granits im östlichen Mühlviertel und westlichen Waldviertel. Masters thesis, University of Salzburg, 56 pp.
Vavra, G. (1990). On the kinematics of zircon growth and its petrogenetic significance: a cathodoluminescence study. Contributions to Mineralogy and Petrology 106, 90–99.
Vellmer, C. & Wedepohl, K. H. (1994). Geochemical characterisation and origin of granitoids from the South Bohemian Batholith in Lower Austria. Contributions to Mineralogy and Petrology 118, 13–32.
Wendt, J. I. & Todt, W. (1991). A vapour digestion method for dating single zircons by direct measurement of U and Pb without chemical separation. Terra Abstracts 3, 507–508.
Wiedenbeck, M., Allé, P., Corfu, F., Griffin, W. L., Meier, M., Oberli, I., Quadt, A., Roddick, J. C. & Spiegel, W. (1995). Three natural zircon standards for U–Th–Pb, Lu–Hf, trace element and REE analyses. Geostandard Newsletters 19(1), 1–23.
Zartman, R. E. & Doe, B. R. (1981). Plumbotectonics—the model. Tectonophysics 75, 135–162.[Web of Science]
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