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  • Zircon dating  (3)
  • Crustal evolution  (2)
  • 1
    Electronic Resource
    Electronic Resource
    Evidence from zircon dating for existence of approximately 2.1 Ga old crystalline basement in southern Bohemia, Czech Republic (1993)
    Springer
    International journal of earth sciences 82 (1993), S. 42-50 
    Details
    ISSN: 1437-3262
    Keywords: Zircon dating ; Sphene dating ; Bohemian Massif ; Orthogneisses ; Czech Republic
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Zircon ages are reported for three Moldanubian amphibolite grade orthogneisses from the southern Bohemian Massif obtained by conventional U/Pb analyses. For two of these orthogneisses, conventional U/Pb data are supported by ion microprobe single zircon ages or single grain evaporation data. The amphibolite grade orthogneisses, occurring in three small tectonic lenses within the Varied Group close to the South Bohemian Main Thrust, are of tonalitic, granodioritic or quartz dioritic composition. Conventional bulk size fraction and ion microprobe analyses of nearly euhedral zircons from a metatonalite, erroneously interpreted as a metagreywacke in a previous study, yielded an upper Concordia intercept age of 2048 ± 12 Ma. The well preserved euhedral grain shapes of the zircons suggest crystallization from a magmatic phase, and the upper Concordia intercept age is now interpreted as reflecting a magmatic event at that time. The age of this rock is compatible with the conventional zircon data and the (207Pb/206Pb)* single grain evaporation result from two further orthogneisses providing intrusion ages of 2 060 ± 12, 2 104 ± 1 and 2 061 ± 6 Ma, respectively. For one sample a concordant U/Pb age for sphene of 355 ± 2 Ma defines the age of amphibolite facies metamorphism. The upper Concordia intercept ages of three orthogneisses constitute the first direct evidence for the presence of early Proterozoic crust under the supracrustal cover in the southern part of the Bohemian Massif.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00563269
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  • 2
    Electronic Resource
    Electronic Resource
    Zircon dating of North Bohemian granulites, Czech Republic: further evidence for the Lower Carboniferous high-pressure event in the Bohemian Massif (1996)
    Springer
    Geologische Rundschau 85 (1996), S. 154-161 
    Details
    ISSN: 0016-7835
    Keywords: Key words HP granulites ; Zircon dating ; Rutile dating ; Lower Carboniferous ; N Bohemia ; Czech Republic
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  U–Pb zircon and rutile multigrain ages and 207Pb/206Pb zircon evaporation ages are reported from high-pressure felsic and metapelitic granulites from northern Bohemia, Czech Republic. The granulites, in contrast to those from other occurrences in the Bohemian Massif, do not show evidence of successive HT/MP-LP overprints. Multigrain size fractions of nearly spherical, multifaceted, metamorphic zircons from three samples are slightly discordant and yield a U–Pb Concordia intercept age of 348±10 Ma, whereas single zircon evaporation of two samples resulted in 207Pb/206Pb ages of 339±1.5 and 339±1.4 Ma, respectively. A rutile fraction from one sample has a U–Pb Concordia intercept age of 346±14 Ma. All ages are identical, within error, and a mean age of 342±5 Ma was adopted to reflect the peak of HP metamorphism. Because rutile has a lower closing temperature for the U–Pb isotopic system than zircon, the results and the P–T data imply rapid uplift and cooling after peak metamorphism. The above age is identical to ages for high-grade metamorphism reported from the southern Bohemian Massif and the Granulite Massif in Saxony. It can be speculated that all these granulites were part of the same lower crustal unit in early Carboniferous, being separated later due to crustal stacking and subsequent late Variscan orogenic collapse.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00192073
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  • 3
    Electronic Resource
    Electronic Resource
    Zircon dating of North Bohemian granulites, Czech Republic: further evidence for the Lower Carboniferous high-pressure event in the Bohemian Massif (1996)
    Springer
    International journal of earth sciences 85 (1996), S. 154-161 
    Details
    ISSN: 1437-3262
    Keywords: HP granulites ; Zircon dating ; Rutile dating ; Lower Carboniferous ; N Bohemia Czech Republic
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract U-Pb zircon and rutile multigrain ages and 207Pb/206Pb zircon evaporation ages are reported from high-pressure felsic and metapelitic granulites from northern Bohemia, Czech Republic. The granulites, in contrast to those from other occurrences in the Bohemian Massif, do not show evidence of successive HT/MPLP overprints. Multigrain size fractions of nearly spherical, multifaceted, metamorphic zircons from three samples are slightly discordant and yield a U-Pb Concordia intercept age of 348 ± 10 Ma, whereas single zircon evaporation of two samples resulted in 207Pb/206Pb ages of 339 ± 1.5 and 339 ± 1.4 Ma, respectively. A rutile fraction from one sample has a U-Pb Concordia intercept age of 346 ± 14 Ma. All ages are identical, within error, and a mean age of 342 ± 5 Ma was adopted to reflect the peak of HP metamorphism. Because rutile has a lower closing temperature for the U-Pb isotopic system than zircon, the results and the P-T data imply rapid uplift and cooling after peak metamorphism. The above age is identical to ages for high-grade metamorphism reported from the southern Bohemian Massif and the Granulite Massif in Saxony. It can be speculated that all these granulites were part of the same lower crustal unit in early Carboniferous, being separated later due to crustal stacking and subsequent late Variscan orogenic collapse.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00192073
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  • 4
    Electronic Resource
    Electronic Resource
    Precambrian basement around Wadi Halfa, Sudan: a new perspective on the evolution of the East Saharan Craton (1994)
    Springer
    International journal of earth sciences 83 (1994), S. 564-577 
    Details
    ISSN: 1437-3262
    Keywords: Precambrian ; Geochronology ; Geochemistry ; Crustal evolution ; Africa
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract This paper provides new geochemical and isotopic data on the evolution of the western foreland to the Nubian shield of north-east Africa. There is abundant evidence for early to middle Proterozoic crust west of the River Nile, but this was severely affected by the Pan-African (≈ 500–900 Ma) orogenic cycle. The results are reported of Rb-Sr whole rock and zircon evaporation geochronological studies and whole rock Sm-Nd and feldspar Pb isotopic analyses for four rock units around Wadi Halfa in northernmost Sudan. These results indicate the presence of heterogeneous pre-Pan-African crustal components, preserved in mylonitic gneisses and in conglomerates that unconformably overlie the gneisses. Several episodes of crust formation, inferred from zircon ages, are preserved in the gneisses : 2.6, 2.4, 2.0, 1.7, 1.2 and 0.72 Ga. Nd model ages for the same units are invariably older than the zircon ages, yet still record a predominantly late Archaean and Palaeoproterozoic history, with depleted mantle model ages between 1.3 and 2.8 Ga. The earliest recorded Pan-African magmatic event is about 720 Ma and dates the beginning of collisional deformation. A younger Pan-African volcanic sequence (≈ 650 Ma) has isotopic compositions of Sr and Nd compatible with derivation from late Prote rozoic asthenospheric mantle. A ≈ 530 Ma anorogenic ‘A-type’ granite also has isotopic compositions suggesting derivation from a primitive source. The inferred tectonic evolution began with rifting to form an oceanic re-entrant. This was followed by subduction leading to collision at about 700 Ma, accompanied by post-orogenic rifting at about 650 Ma.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01083228
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  • 5
    Electronic Resource
    Electronic Resource
    Precambrian basement around Wadi Haifa, Sudan: a new perspective on the evolution of the East Saharan Craton (1994)
    Springer
    International journal of earth sciences 83 (1994), S. 564-577 
    Details
    ISSN: 1437-3262
    Keywords: Precambrian ; Geochronology ; Geochemistry ; Crustal evolution ; Africa
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract This paper provides new geochemical and isotopic data on the evolution of the western foreland to the Nubian shield of north-east Africa. There is abundant evidence for early to middle Proterozoic crust west of the River Nile, but this was severely affected by the Pan-African (≈ 500–900 Ma) orogenic cycle. The results are reported of Rb-Sr whole rock and zircon evaporation geochronological studies and whole rock Sm-Nd and feldspar Pb isotopic analyses for four rock units around Wadi Halfa in northernmost Sudan. These results indicate the presence of heterogeneous pre-Pan-African crustal components, preserved in mylonitic gneisses and in conglomerates that unconformably overlie the gneisses. Several episodes of crust formation, inferred from zircon ages, are preserved in the gneisses : 2.6, 2.4, 2.0, 1.7, 1.2 and 0.72 Ga. Nd model ages for the same units are invariably older than the zircon ages, yet still record a predominantly late Archaean and Palaeoproterozoic history, with depleted mantle model ages between 1.3 and 2.8 Ga. The earliest recorded Pan-African magmatic event is about 720 Ma and dates the beginning of collisional deformation. A younger Pan-African volcanic sequence (≈ 650 Ma) has isotopic compositions of Sr and Nd compatible with derivation from late Prote rozoic asthenospheric mantle. A ≈ 530 Ma anorogenic ‘A-type’ granite also has isotopic compositions suggesting derivation from a primitive source. The inferred tectonic evolution began with rifting to form an oceanic re-entrant. This was followed by subduction leading to collision at about 700 Ma, accompanied by post-orogenic rifting at about 650 Ma.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00194162
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