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  • 1
    Electronic Resource
    Electronic Resource
    A Tertiary age for Alpine high-pressure metamorphism in the Gran Paradiso massif, Western Alps: a Rb–Sr microsampling study (2004)
    Oxford, UK : Blackwell Science Inc
    Journal of metamorphic geology 22 (2004), S. 0 
    Details
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The timing of high-pressure (HP) metamorphism in the internal basement massifs of the Western Alps has been contentious. In the Gran Paradiso massif silvery micaschists, thought to have developed from granitic precursors, contain assemblages indicative of pressures in excess of 18 kbar at 500–550 °C. This paper presents unique geochronological data for the paragenesis of the silvery micaschist HP assemblage. Rb–Sr microsampling of an apatite–phengite pair thought to have remained closed to Rb–Sr exchange since the HP paragenesis formed has yielded an age of 43.0 ± 0.5 Ma. Greenschist retrogression occurred after 36.3 ± 0.4 Ma, probably in the interval 36–34 Ma. The localised disturbance of the Rb–Sr system in phengite, apatite and allanite during retrogression means that only in situ microsampling could obtain meaningful ages from these rocks. The new data indicating a Tertiary age for HP metamorphism in the Gran Paradiso massif agree with recent data for other internal basement massifs in the Western Alps. A model fitting the Gran Paradiso massif into the Western Alpine framework is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1525-1314.2004.00512.x
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  • 2
    Electronic Resource
    Electronic Resource
    Strontium isotopic equilibration during metamorphism of tillites from the Ogcheon Belt, South Korea (1985)
    Springer
    Contributions to mineralogy and petrology 90 (1985), S. 346-352 
    Details
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The NE/SW trending Ogcheon Belt formed during the Mesozoic by deformation and metamorphism of a sedimentary sequence which includes “tillite” formations of late Precambrian or Lower Palaeozoic age. In this study Rb-Sr isotopic data are reported from cm-scale sampling of clasts and matrix in these “tillites”, which underwent metamorphism at grades ranging from the biotite zone to the kyanite zone. Sr-isotopic disequilibrium between clasts and matrix persists up to the highest grade but in four out of five rocks the matrix samples approximate to isochrons (MSWD's〈12). The mechanism for this selective isotopic equilibration is considered to be related to the metamorphic reactions in the matrix, which were probably accompanied by a substantial fluid flow. The matrix “isochrons” for three of the rocks, together with separated white micas from associated schists, suggest that metamorphism took place in the Trias, approximately 200 Myr ago. Metamorphism was followed by slow cooling and biotite closure temperatures were not reached until 190-150 Myr. — These results suggest that where high fluid/rock ratios can be demonstrated on independent petrological grounds, the analysis of small-scale whole-rock samples may allow effective Rb/Sr dating of metamorphism.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00384713
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  • 3
    Electronic Resource
    Electronic Resource
    Thermal history of the Sonnblick Dome, south-east Tauern Window, Austria: Implications for heterogeneous uplift within the Pennine basement (1993)
    Springer
    International journal of earth sciences 82 (1993), S. 667-675 
    Details
    ISSN: 1437-3262
    Keywords: Tauern Window, eastern Alps ; Isotopic mineral datating ; Thermal history ; Tectonic displacement
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The Sonnblick Dome is one of several domal structures affecting the interface between basement and cover within the Pennine Zone of the Tauern Window in the eastern Alps. Rb-Sr isotopic data, comprising 19 biotite and 22 white mica ages from variably deformed granitic gneisses, provide new evidence of the thermal and tectonic history of the dome and its relationships with other parts of the south-east Tauern Window. White mica ages generally cluster between 26 and 30 Ma although there are values up to 82 Ma, which appear to reflect incomplete equilibration during Tertiary metamorphism under low amphibolite facies conditions; six closely spaced samples from an intensely sheared ‘gneiss lamella’ are more tightly grouped between 26 and 27.6 Ma and provide the best estimate of the age of syntectonic crystallization. Biotite ages are systematically younger, ranging from 19 to 23.5 Ma, reflecting closure during post-metamorphic cooling. Sonnblick Dome and the Hochalm Dome approximately 20 km further east, where closure of Rb-Sr in biotite did not occur until 16.5 Ma; the metamorphic peak here is also probably younger, possibly as late as 22 Ma. The Sonnblick Dome was formed before 27 Ma and the deformation style had changed to extension before biotite closure by 19 Ma. In contrast, rapid updoming in the Hochalm Dome was previously dated at 16.5 Ma and the differences in thermal history can be linked to differences in deformation history. Overall the geochronological data from the south-east Tauern Window demonstrate the heterogeneity of thermal history on a geographical scale of 10 km and emphasize the importance of tectonic displacements in controlling temperature within orogenic belts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00191494
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  • 4
    Electronic Resource
    Electronic Resource
    Pre-Alpine history of the pennine zone in the Tauern Window, Austria: U-Pb and Rb-Sr geochronology (1981)
    Springer
    Contributions to mineralogy and petrology 77 (1981), S. 262-266 
    Details
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Zircon ages from major lithologies of the Zentralgneis suggest that much of the Variscan magmatism in the Tauern Window is older than previously suggested. In the southeast Tauern Window a tonalite has been dated at 314±7 m.y. and a granodioritic biotite augen gneiss at 313±10 m.y. Two granodiorites from the Granatspitzkern yielded zircon data consistent with a similar age. These zircon data require re-interpretation of some previously published Rb-Sr whole rock ages and raise the possibility that Alpine metamorphism caused more widespread disturbance of Rb-Sr whole rocks than commonly supposed. Rb-Sr data on fabric-forming white micas from two banded gneisses give ages close to 220 m.y., indicating the foliation in these rocks is pre-Alpine and has not been greatly affected by Alpine recrystallisation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00373541
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  • 5
    Electronic Resource
    Electronic Resource
    Metamorphic evolution of the Sesia-Lanzo Zone, Western Alps: time constraints from multi-system geochronology (1996)
    Springer
    Contributions to mineralogy and petrology 126 (1996), S. 152-168 
    Details
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  The Sesia-Lanzo Zone is a polymetamorphic unit containing Hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the Alpine orogeny. Different parts of the unit record different stages on the P-T-deformation evolution, allowing multi-system isotopic studies to unravel the precise timing of the metamorphic history. New Rb–Sr white mica and U–Pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60–70 Ma. This substantially alters the common view of early- to mid-Cretaceous eclogite facies metamorphism in this unit. The new results are more consistent with the established geotectonic framework for the Alpine orogeny, since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension. It is also more concordant with recent studies of other units that demonstrate post-Cretaceous high-pressure metamorphism. Step-heated 40Ar–39Ar analysis of phengites yields good plateaux giving ages older than the corresponding Rb–Sr age. Such anomalously high ages indicate the presence of radiogenic argon-rich fluids in the grain boundary network under the fluid/pressure conditions acting during this high-pressure metamorphic event. The U–Pb sphene ages are variable in polymetamorphic rocks, and show inheritance of older Pb or sphene crystals into the high-pressure event. Two monometamorphic assemblages yield concordant ages at 66±1 Ma, reflecting crystallisation of the eclogite facies assemblage. The Gneiss Minuti Complex (GMC) lies structurally below the Eclogitic Micaschists, and its pervasive greenschist facies fabric yields tightly clustered Rb–Sr white mica ages at 38–39 Ma. This greenschist event did not affect the majority of the EMC. The 40Ar–39Ar ages of micas formed at this time were very disturbed, whereas micas surviving from an earlier higher pressure assemblage had their 40Ar–39Ar system reset. The greenschist event did not strongly affect U–Pb systematics in Hercynian age sphenes, suggesting that the GMC did not uniformly suffer an eclogite facies metamorphism during the Alpine cycle, but was juxtaposed against the EMC later in the orogeny. This model still requires that the locus of deformation and metamorphism (and possibly fluid flux) moved outboard with time, leaving the Sesia-Lanzo basement as a shear-bounded unreactive block within the orogenic wedge.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004100050241
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  • 6
    Electronic Resource
    Electronic Resource
    Strontium isotope distribution in a regionally metamorphosed granite from the Zentralgneis, south-east Tauernfenster, Austria (1971)
    Springer
    Contributions to mineralogy and petrology 32 (1971), S. 274-288 
    Details
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The effect of Alpine (∼40 my) low amphibolite facies metamorphism on strontium isotopes in the Variscan (∼250–300 my) granitic complex of the Zentralgneis was investigated by analysing samples of a leucogranite which forms small intrusions into a tonalite. The leucogranite has Rb/Sr ratios up to 30. Two muscovites and three biotites underwent complete reequilibration during the metamorphism. The ages of 12–20 my are consistent with published K-Ar ages on micas from the same area, and reflect post-metamorphic cooling. Six whole-rock samples define an isochron with an age of 244±7 my (λ=1.47×10−11yr−1). Three further samples, collected within 30 cm of contacts with the tonalite, plus 1 cm sawn slabs of leucogranite and tonalite from either side of a contact fit a line with 10% greater slope; this line is interpreted as a mixing line, resulting from contamination of the leucogranite, near its contacts, with strontium from the enclosing tonalite. The time of contamination is uncertain but petrological evidence suggests it is the result of the metamorphism. Neither leucogranite whole rocks nor minerals from the tonalite, at contacts and in a xenolith, show any evidence for preferential mobilisation of radiogenic strontium in the course of metamorphism.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00373345
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  • 7
    Electronic Resource
    Electronic Resource
    A Sm-Nd isotopic study of the South Harris Igneous Complex, the Outer Hebrides (1983)
    Springer
    Contributions to mineralogy and petrology 82 (1983), S. 91-98 
    Details
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Sm-Nd geochronology may be used to bracket the age of metamorphism in rocks which are difficult to date by other methods. By coupling whole rock Sm-Nd analyses of the principal members of the South Harris Igneous Complex, with Sm-Nd mineral isochrons on two anothositic gabbros, the age of granulite facios metamorphism has been defined. Whole rock analyses of three pairs of closely spaced samples of the anorthosite give consistent ages averaging 2.18±0.06 Gyr, but in general the data from the anorthosite do not define an isochron as a result of variable contamination of the evolving magma chamber. Whole rock data on the tonalite indicate that it is younger than 2.06 Gyr; its mean TCHUR age is 1.86±0.05 Gyr. Garnet-pyroxene-amphibole-plagioclase mineral isochrons on two anorthosite samples give identical 1.87±0.04 Gyr ages which date cooling after the high pressure granulite facies metamorphism. Together with the tonalite whole rock data this defines the age of that metamorphism and confirms Dearnley's original assignment of an early Laxfordian age.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00371178
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  • 8
    Electronic Resource
    Electronic Resource
    Uranium-lead isotopic evidence from zircons for lower Palaeozoic tectonic activity in the Austroalpine Nappe, the Eastern Alps (1980)
    Springer
    Contributions to mineralogy and petrology 71 (1980), S. 283-288 
    Details
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract U/Pb isotopic data for two zircon suites are presented: (1) a pre D 1 tonalite gneiss gives an age of 443 ± 16 13 m.y. (2) a post D 1 leucogranite gneiss gives 427 ± 11 10 m.y. Both zircon suites contain a minor inherited pre-Cambrian component. The data confirm a lower Palaeozoic age for the major, fabric-forming D 1 event. Hercynian metamorphism which was sufficient to reset muscovite K/Ar ages to ca. 290 m.y. had little effect on the zircons. The possible importance of lower Palaeozoic orogeny in southern Europe and the uncertainties in regional palaeogeography are emphasised.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00371670
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  • 9
    Electronic Resource
    Electronic Resource
    Pb-isotope evidence for the timing of episodic mineralization in the Llanrwst and Llanfair-Talhaiarn orefields, North Wales (1996)
    Springer
    Mineralium deposita 31 (1996), S. 93-97 
    Details
    ISSN: 1432-1866
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Galenas from the Llanrwst and Llanfair-Talhaiarn orefields in North Wales have a restricted range of Pb-isotope values. M1 and M2 mineralizations at Llanrwst are isotopically separate in 206Pb/204Pb versus 207Pb/204Pb space, while the isotopic composition of the M3 mineralization exceeds the combined M1 and M2 range. The lead source for the Llanrwst orefield is believed to be the underlying Ordovician volcanic arc material. Data from the related Llanfair-Talhaiarn orefield suggest lead was derived from the host Silurian sediments in addition to the Ordovician basement. The time separation of the M1 and M2 veins at Llanrwst has been calculated using μ and ω data from Dixon et al. (1990) at 11 Ma and 15 Ma respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00225399
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  • 10
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    Pre-Alpine history of the pennine zone in the Tauern Window, Austria: U-Pb and Rb-Sr geochronology (1981)
    Springer
    Details
    Publication Date: 1981-10-01
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
    Published by Springer
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