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  • 1995-1999  (5)
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  • 1
    Digitale Medien
    Digitale Medien
    Alpine and pre-Alpine magmatism in the root-zone of the western Central Alps (1996)
    Springer
    Contributions to mineralogy and petrology 123 (1996), S. 138-158 
    Details
    ISSN: 1432-0967
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie
    Notizen: Abstract  The highest grade of metamorphism and associated structural elements in orogenic belts may be inherited from earlier orogenic events. We illustrate this point using magmatic and metamorphic rocks from the southern steep belt of the Lepontine Gneiss Dome (Central Alps). The U-Pb zircon ages from an anatectic granite at Verampio and migmatites at Corcapolo and Lavertezzo yield 280–290 Ma, i.e., Hercynian ages. These ages indicate that the highest grade of metamorphism in several crystalline nappes of the Lepontine Gneiss Dome is pre-Alpine. Alpine metamorphism reached sufficiently high grade to reset the Rb-Sr and K-Ar systematics of mica and amphibole, but generally did not result in crustal melting, except in the steep belt to the north of the Insubric Line, where numerous 29 to 26 Ma old pegmatites and aplites had intruded syn- and post-kinematically into gneisses of the ductile Simplon Shear Zone. The emplacement age of these pegmatites gives a minimum estimate for the age of the Alpine metamorphic peak in the Monte Rosa nappe. The U-Pb titanite ages of 33 to 31 Ma from felsic porphyritic veins represent a minimum-age estimate for Alpine metamorphism in the Sesia Zone. A porphyric vein emplaced at 448±5 Ma (U-Pb monazite) demonstrates that there existed a consolidated Caledonian basement in the Sesia Zone.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s004100050147
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  • 2
    Digitale Medien
    Digitale Medien
    U-Pb age and lead isotopic characterization of Au-bearing skarn related to the Andorra granite (central Pyrenees, Spain) (1995)
    Springer
    Mineralium deposita 30 (1995), S. 374-383 
    Details
    ISSN: 1432-1866
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie
    Notizen: Abstract Auriferous skarns are associated with post- and late-kinematic Hercynian granites that intruded into Cambro-Ordovician to Devonian sediments of the central Pyrenees. We determined the age of the Andorra granite and the associated skarn at 305 ± 3 Ma by U-Pb dating titanite from the endo-skarn. The sulfur isotopic composition from sulfides in the skarn (Cardellach et al. 1992) shows a significant variation with isotopically light sulfur (δ34S ≈ +3) in the barren skarns and heavy sulfur (δ34S ≈ +11) in the gold-bearing skarns. Outwards, it increasingly resembles sulfur from arsenopyrite disseminations in the Cambro-Ordovician sediments. The lead isotopic composition from sulfides of the skarns is very homogeneous (206Pb/204Pb = 18.410, 207Pb/204Pb = 15.699, 208Pb/204Pb = 38.574) in contrast to the one in gold-bearing arsenopyrite veins and in arsenopyrite disseminations in the sediments (e.g. 206Pb/204Pb varies from 18.54 to 30.36). Combined, sulfur and lead isotope data indicate that the lead in the skarn is dominantly derived from the granite, whereas the sulfur is derived at variable portions from both the granite and the sediments.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00202280
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  • 3
    Digitale Medien
    Digitale Medien
    Scapolite: a tracer for the initial lead isotopic composition in sulfide deposits with later additions of radiogenic lead (1996)
    Springer
    Mineralium deposita 31 (1996), S. 134-139 
    Details
    ISSN: 1432-1866
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie
    Notizen: Abstract The initial lead isotopic composition of metamorphosed and tectonically reworked sulfide deposits is not always preserved, as sulfides easily change their lead isotopic composition through incorporation of lead derived from external fluids or redistribution and recrystallization of the deposit. Sulfide trace-lead and in cases even galena-lead from such deposits may show exceedingly radiogenic lead isotopic compositions. Thus, the initial lead isotopic composition has to be estimated from other minerals. Scapolite, which is a common phase in alteration haloes associated with epigenetic sulfide deposits in northern Sweden, has very low uranium-contents. Therefore, its trace-lead contents could preserve the initial isotopic composition of the ore-forming fluids. As scapolite is more resistant to recrystallization, it is more likely to reflect the original lead isotope signature of the deposit. This is illustrated using scapolite and sulfides from the Pahtohavare Cu-Au deposit in northern Sweden, which is hosted by Palaeoproterozoic mafic tuffites and graphitic schists and was affected by a mild thermal metamorphism during the Caledonian orogeny.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00225404
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  • 4
    Digitale Medien
    Digitale Medien
    Crystal-chemical and genetic controls of U-Pb systematics of columbite-tantalite (1996)
    Springer
    Mineralogy and petrology 57 (1996), S. 243-260 
    Details
    ISSN: 1438-1168
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie
    Beschreibung / Inhaltsverzeichnis: Zusammenfassung Columbit-Tantalit kann bedeutende Mengen Uran aufnehmen and schließt Blei beinahe vollständig aus. Deshalb ist das Mineral potentiell zur U-Pb Datierung geeignet. Im allgemeinen ergeben Diskordias gut definierte obere Schnittalter. Doch in einigen Fällen ergibt das Kolumbit-Tantalit U-Pb System aufgrund von gestreuten order invers diskordanten Daten, welche keinen systematischen Zusammenhang mit der Mineralchemie oder dem Fraktionierungsgrad des Pegmatites zeigen, schlecht bestimmte Schnittalter. Rekristallisation oder Einschlüsse bedingen im allgemeinen keine anomale oder gestreute U-Pb Systematik, ausser in Fällen wo die Einschlüsse Uraninit oder sekundäre Na, Ta-Phasen sind. Uraninit, dessen Auftreten mit dem lokalen Nebengestein und den Redoxbedingungen während der Pegmatitkristallisation zusammenzuhängen scheint, kann als offenes System eine inverse Diskordanz verursachen, welche von sekundären Nb, Ta-Phasen, die Pb und U von Uraninit in ihre Kristallstruktur einbauen, geerbt wird.
    Notizen: Summary Columbite-tantalite can accommodate considerable amounts of uranium and excludes lead almost completely. It is thus potentially suited for U-Pb dating. Discordias generally yield closely constrained upper intercept ages. Yet, in a few cases, the U-Pb system of columbite-tantalite gives poorly constrained ages due to excess scatter or inverse discordance. These anomalous features do not show any relation with gross mineral chemistry or pegmatite fractionation. Recrystallization or the presence of exsolutions and inclusions generally do not result in anomalous and scattered U-Pb systematics, unless the inclusions are uraninite or secondary Nb, Ta-bearing phases. Open-system behavior of uraninite, whose occurrence seems to be related to the local host-rock-influenced redox conditions during pegmatite crystallization, often results in inverse discordance. Secondary Nb, Ta-bearing phases that accept Pb and U derived from uraninite inclusions into their structure may inherit this inverse discordance.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01162361
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  • 5
    Digitale Medien
    Digitale Medien
    Geology of the Bozdag area, central Menderes massif, SW Turkey: Pan-African basement and Alpine deformation (1998)
    Springer
    Geologische Rundschau 87 (1998), S. 394-406 
    Details
    ISSN: 0016-7835
    Schlagwort(e): Key words Pan-African orogeny ; U ; Pb dating ; Ductile deformation ; Shear sense indicators ; Menderes massif ; Turkey ; Thrusting
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie
    Notizen: Abstract The Menderes massif consists of a Precambrian Core Series that preserves evidence for a polymetamorphic history and a Paleozoic/Mesozoic Cover Series that experienced only the Alpine tectonometamorphic evolution. Structural, petrographic, and geochronologic investigations in the central Menderes massif demonstrate that (a) part of the metamorphic and structural evolution of the Precambrian basement is older than the undeformed 551±1.4-Ma-old Birgi metagranite, and (b) inferred Alpine fabrics overprinting the Cover Series largely have the same attitudes as the old structures in the much older Core Series. The inferred Alpine fabrics include both contractional and extensional structures. Contraction under greenschist to amphibolite facies conditions resulted in the imbrication of the Core and Cover Series and generated an inverted metamorphic sequence by north-directed thrusting. During Alpine extension, most of the south-dipping thrust faults were reactivated as extensional shear zones under decreasing greenschist facies conditions.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s005310050218
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