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Kalt, Angelika

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Kalt, Angelika
Affiliation principale
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https://libra.unine.ch/handle/123456789/160

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  • Publication
    Accès libre
    Metamorphic evolution of ultrahigh-pressure garnet peridotites from the Variscan Vosges Mts. (France)
    (1996-12-10)
    Altherr, Rainer
    ;
    Kalt, Angelika 
    In the Central Vosges Mts. (France) of the Variscan belt, Mg---Cr garnet peridotite bodies occur within the uppermost tectonometamorphic unit (Leptynitic granulites) as lenses in low-pressure/high-temperature metamorphic rocks. Neglecting late-stage serpentinization, the metamorphic evolution of these rocks was characterized by four stages. During stage I, the rocks were equilibrated at high pressures and temperatures (> 4.9 GPa/ > 950°C, in most cases > 1100°C), either within or near to the stability field of diamond. Stage II is documented by the formation of coronas around relict garnet with the assemblage orthopyroxene ± clinopyroxene ± amphibole + spinel ± plagioclase. During stage III, the remaining garnet was transformed to very fine-grained kelyphite consisting of orthopyroxene + amphibole + spinel ± plagioclase. Small relict garnet grains are preserved in one peridotite only. Stage IV corresponds to the late formation of tremolitic hornblende and chlorite which partially replaced the pseudomorphs after garnet or occur along cracks in the matrix of some rocks. Compositional zoning patterns of pyroxene porphyroclasts suggest that initial decompression was either accompanied by a moderate increase in temperature or nearly isothermal. Garnet breakdown textures and compositions of minerals grown during stages II and III also suggest rapid decompression at still elevated temperatures (1000-720°C).
  • Publication
    Accès libre
    Metamorphic evolution of garnet-spinel peridotites from the Variscan Schwarzwald (Germany)
    (1996)
    Kalt, Angelika 
    ;
    Altherr, Rainer
    Garnet-spinel peridotites form small, isolated, variably retrogressed bodies within the low-pressure high-temperature gneisses and migmatites of the Variscan basement of the Schwarzwald, southwest Germany. Detailed mineralogical and textural studies as well as geothermobarometric calculations on samples from three occurrences are presented. Two of the garnet-spinel peridotites have equilibrated at 680-770vv°C, 1.4-1.8 GPa within the garnet-spinel peridotite stability field, one of the samples having experienced an earlier stage within the spinel peridotite stability field (790vv°C, <1.8 GPa). The third sample, with only garnet and spinel preserved, probably equilibrated within the garnet peridotite stability field at higher pressures. These findings are in line with the distinction of two groups of ultramafic garnet-bearing high-pressure rocks with different equilibration conditions within the Schwarzwald (670-740vv°C, 1.4-1.8 GPa and 740-850vv°C, 3.2-4.3 GPa) which has previously been established (Kalt et al. 1995). The equilibration conditions of 670-770vv°C and 1.4-1.8 GPa for garnet-spinel peridotites from the Central Schwarzwald Gneiss Complex (CSGC) are similar to those for eclogites of the Schwarzwald and also correspond quite well to those for garnet-spinel peridotites from the Moldanubian zone of the Vosges mountains and of eclogites from the Moldanubian s.str. of the Bohemian Massif.