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Low-grade metamorphism in the Montagne Noire (S-France): Conodont Alteration Index (CAI) in Palaeozoic carbonates and implications for the exhumation of a hot metamorphic core complex
Wiederer, U.; Konigshof, P.; Feist, R.; Franke, W.; Doublier, M.P. (2002). Low-grade metamorphism in the Montagne Noire (S-France): Conodont Alteration Index (CAI) in Palaeozoic carbonates and implications for the exhumation of a hot metamorphic core complex. Schweiz. Mineral. Petrogr. Mitt. 82(2): 393-407
In: Schweizerische Mineralogische und Petrographische Mitteilungen = Bulletin Suisse de Minéralogie et Pétrographie = Swiss Bulletin of Mineralogy and Petrology. Swiss Society of Mineralogy and Petrology: Frauenfeld. ISSN 0036-7699, more
Peer reviewed article  

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Author keywords
    conodont alteration index (CAI); illite crystallinity (IC); metamorphiccore complex; Variscides; Montagne Noire

Authors  Top 
  • Wiederer, U.
  • Konigshof, P.
  • Feist, R.
  • Franke, W.
  • Doublier, M.P.

    The Montagne Noire is a metamorphic core complex situated in the southern foreland of the Variscan belt in South France. The core of the structure (Zone Axiale) exposes LP/HT gneisses metamorphosed in Carboniferous time. Exhumation of the hot gneisses has imposed amphibolite facies to low-grade metamorphism on the Palaeozoic rocks in the mantle of the Zone Axiale. Individual stratigraphic units can be traced without major changes in sedimentary facies across the metamorphic zonation, from diagenetic to epizonal grades. Therefore, the area is ideally suited for a comparison between different methods of assessing lower metamorphic grades. The conodont alteration index (CAI) analysed in c. 300 samples shows that CAI is not dependant on stratigraphic age, and is not significantly altered by secondary dolomitization. Locally increased CAI values are attributed to alteration by fluids. In samples with CAI greater than or equal to 5, conodont elements show ductile deformation. Comparisons with an earlier set of crystallinity (IC) data (uncalibrated values of ENGEL et al., 1981) reveal a non-linear correlation between CAI and IC, with IC rapidly decreasing in samples with CAI less than or equal to 3. IC values between 0,23 and 0.46, which grossly correspond to the boundaries of the anchizone for Kubler indices, correlate with CAI of c. 4 to 3.
    Although the earlier IC do not permit a satisfactory correlation with CAI, they clearly reflect the same metamorphic zonation and help to reveal the relationships between deformation and metamorphism. Metamorphism decreases away from the Zone Axiale and the metamorphic zonation cuts across the inverted limbs of recumbent fold nappes, Hence, metamorphism must have been acquired after crustal stacking, and represent dynamic contact metamorphism imposed by the rising, hot crystalline core. Accordingly, tectonic structures in areas of high CAI reveal the same extensional kinematics as in the Zone Axiale. The relatively narrow, present-day contours of the Zone Axiale are probably due to cooling and constriction of the extensional window.

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