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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
    Pyroxenite xenoliths from Marsabit (Northern Kenya): evidence for different magmatic events in the lithospheric mantle and interaction between peridotite and pyroxenite
    (2009)
    Kaeser, Benjamin
    ;
    Olker, Bettina
    ;
    Kalt, Angelika 
    ;
    Altherr, Rainer
    ;
    Pettke, Thomas
    Garnet-bearing and garnet-free pyroxenite xenoliths from Quaternary basanites of Marsabit, northern Kenya, were analysed for microstructures and mineral compositions (major and trace elements) to constrain the thermal and compositional evolution of the lithospheric mantle in this region. Garnet-bearing rocks are amphibole-bearing websterite with ~5–10 vol% orthopyroxene. Clinopyroxene is LREE-depleted and garnet has high HREE contents, in agreement with an origin as cumulates from basaltic mantle melts. Primary orthopyroxene inclusions in garnet suggest that the parental melts were orthopyroxene-saturated. Rock fabrics vary from weakly to strongly deformed. Thermobarometry indicates extensive decompression and cooling (~970–1,100°C at ~2.3–2.6 GPa to ~700–800°C at ~0.5–1.0 GPa) during deformation, best interpreted as pyroxenite intrusion into thick Paleozoic continental lithosphere subsequently followed by continental rifting (i.e., formation of the Mesozoic Anza Graben). During continental rifting, garnet websterites were decompressed (garnet-to-spinel transition) and experienced the same P–T evolution as their host peridotites. Strongly deformed samples show compositional overlaps with cpx-rich, initially garnet-bearing lherzolite, best explained by partial re-equilibration of peridotite and pyroxenite during deformation and mechanical mingling. In contrast, garnet-free pyroxenites include undeformed, cumulate-like samples, indicating that they are younger than the garnet websterites. Major and trace element compositions of clinopyroxene and calculated equilibrium melts suggest crystallisation from alkaline basaltic melt similar to the host basanite, which suggests formation in the context of alkaline magmatism during the development of the Kenya rift.
  • Publication
    Accès libre
    Detrital, metamorphic and metasomatic tourmaline in high-pressure metasediments from Syros (Greece): intra-grain boron isotope patterns determined by secondary-ion mass spectrometry
    (2008)
    Marschall, Horst R.
    ;
    Altherr, Rainer
    ;
    Kalt, Angelika 
    ;
    Ludwig, Thomas
    The boron isotopic composition of zoned tourmaline in two metasediments from the island of Syros, determined by secondary-ion mass spectrometry (SIMS), reflects the sedimentary and metamorphic record of the rocks. Tourmaline from a silicate-bearing marble contains small (≤20 μm) detrital cores with highly variable δ11B values (−10.7 to +3.6‰), pointing to a heterogeneous protolith derived from multiple sources. The sedimentary B isotopic record survived the entire metamorphic cycle with peak temperatures of ~500°C. Prograde to peak metamorphic rims are homogeneous and similar among all analysed grains (δ11B ≈ +0.9‰). The varying δ11B values of detrital cores in the siliceous marble demonstrate that in situ B isotope analysis of tourmaline by SIMS is a potentially powerful tool for provenance studies not only in sediments but also in metasediments. A meta-tuffitic blueschist bears abundant tourmaline with dravitic cores of detrital or authigenic origin (δ11B ≈ −3.3‰), and prograde to peak metamorphic overgrowth zones (−1.6‰). Fe-rich rims, formed during influx of B-bearing fluids under retrograde conditions, show strongly increasing δ11B values (up to +7.7‰) towards the margins of the grains. The δ11B values of metamorphic tourmaline from Syros, formed in mixed terrigenous–marine sediments, reflect the B signal blended from these two different sources, and was probably not altered by dehydration during subduction.
  • Publication
    Accès libre
    Partitioning and budget of Li, Be and B in high-pressure metamorphic rocks
    (2006-09-15)
    Marschall, Horst R.
    ;
    Altherr, Rainer
    ;
    Ludwig, Thomas
    ;
    Kalt, Angelika 
    ;
    Gméling, Katalin
    ;
    Kasztovszky, Zsolt
    Partitioning and budget of Li, Be and B in high-pressure metamorphic rocks from the island of Syros (Greece) were studied, using secondary ion mass spectrometry, inductively coupled plasma optical emission spectrometry and prompt gamma neutron activation analysis. Partitioning between coexisting mineral phases was found to be rather constant and independent of element concentrations. For several mineral pairs, apparent partition coefficients vary in a narrow range, while concentrations vary by more than an order of magnitude. Hence, it was possible to establish sets of inter-mineral partition coefficients for Li, Be and B among 15 different high-pressure minerals. This data set provides important information on the behaviour of the light elements in different lithologies within subducting slabs from the onset of metamorphism to the eclogite stage. It is essential for modelling trace-element and isotope fractionation during subduction and dehydration of oceanic crust.
  • Publication
    Métadonnées seulement
    Aluminous granulites from the Pulur complex, NE Turkey : a case of partial melting, efficient melt extraction and crystallisation
    (2004)
    Topuz, Gültekin
    ;
    Altherr, Rainer
    ;
    Kalt, Angelika 
    ;
    Satır, Muharrem
    ;
    Werner, Olaf
    ;
    Schwarz, Winfried H.
    In the Pulur complex, NE Turkey, a heterogeneous rock sequence ranging from quartz-rich mesocratic gneisses to silica- and alkali-deficient, Fe-, Mg- and Al-rich melanocratic rocks is characterized by granulite-facies assemblages involving garnet, cordierite, sillimanite, ilmenite, ±spinel, ±plagioclase, ±quartz, ±biotite, ±corundum, rutile and monazite. Textural evidence for partial melting in the aluminous granulites, particularly leucosomes, is largely absent or strongly obliterated by a late-stage hydrothermal overprint. However, inclusion relations, high peak P–T conditions, the refractory modes, bulk and biotite compositions of the melanocratic rocks strongly support a model of partial melting. The melt was almost completely removed from the melanocratic rocks and crystallised within the adjacent mesocratic gneisses which are silica-rich, bear evidence of former feldspar and show a large range in major element concentrations as well as a negative correlation of most elements with SiO2. Peak conditions are estimated to be ≥800 °C and 0.7–0.8 GPa. Subsequent near-isothermal decompression to ~0.4–0.5 GPa at 800–730 °C is suggested by the formation of cordierite coronas and cordierite–spinel symplectites around garnet and in the matrix. Sm–Nd, Rb–Sr and 40Ar/39Ar isotope data indicate peak conditions at ~330 Ma and cooling below 300 °C at ~310 Ma.
  • 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.
  • Publication
    Accès libre
    Contrasting P-T conditions recorded in ultramafic high-pressure rocks from the Variscan Schwarzwald (F.R.G.)
    (1995)
    Kalt, Angelika 
    ;
    Altherr, Rainer
    ;
    Hanel, Michael
    This paper presents mineralogical and textural data as well as thermobarometric calculations on ultramafic high-pressure rocks from the Variscan basement of the Schwarzwald (F.R.G.). The rocks form small isolated bodies within low-pressure / high-temperature gneisses and migmatites. The results of this study constrain contrasting P-T evolutions for four garnet-bearing ultramafic high-pressure rocks. Two magnesian garnet-spinal peridotites sampled near the southern margin of the Central Schwarzwald Gneiss Complex (CSGC) were equilibrated at 670-740° C and 1.4-1.8rGPa. These P-T conditions are similar to those recorded by eclogites intercalated in the same basement unit. Two garnet websterites sampled from the northern part of the CSGC have comparatively low Mg/(Mg+Fe) and low Cr and Ni abundances and are interpreted as former cumulates. These rocks most probably experienced an initial high-temperature stage within the spinel peridotite stability field, followed by re-equilibration at 740-850° C / 3.2-4.3rGPa and subsequent recrystallization at lower pressures. Further petrologic studies have to reveal whether ultramafic high-pressure rocks of the Schwarzwald can generally be assigned to these two groups which are mainly defined by contrasting peak pressures.