SCHWEIZ. MINERAL. PETROGR. MITT. 82, 393–407, 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 by Ulrich Wiederer 1 , Peter Königshof 1 , Raimund Feist 2 , Wolfgang Franke 3 and Michael P. Doublier 4 Abstract 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 ≥ 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 ≤ 3. IC values between 0.23 and 0.46, which grossly correspond to the boundaries of the anchizone for Kübler 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 metamor- phic zonation and help to reveal the relationships between deformation and metamorphism. Metamorphism de- creases 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 meta- morphism 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. Keywords: conodont alteration index (CAI), illite crystallinity (IC), metamorphic core complex, Variscides, Montagne Noire. 1 Forschungsinstitut und Naturmuseum Senckenberg, Senckenberganlage 25, D-60325 Frankfurt a. Main. <againhe@web.de> and <pkoenigs@sng.uni-frankfurt.de> 2 Institut des Sciences de l’Évolution, Lab. de Paléontologie, Université de Montpellier II, Place E. Bataillon, F-34095 Montpellier Cedex 05. <rfeist@isem.univ.montp2.fr> 3 Institut für Geowissenschaften der Universität, Senckenbergstr. 3, D-35390 Gießen. <wolfgang.franke@geolo.uni- giessen.de> 4 Institut für Geologie und Paläontologie der Universität, Senckenberg-Anlage 32, D-60325 Frankfurt a. Main. <Michael.P.Doublier@geolo.uni-giessen.de> 1. Introduction The Montagne Noire is situated on the southern flank of the Variscan Belt in South France (e.g., MATTE, 1991). It contains a low pressure meta- morphic core with anatectic gneisses and granites, overlain and flanked by Palaeozoic metasedi- ments (Fig. 1). The low metamorphic grade of the Palaeozoic cover and the presence of Carbonifer- ous flysch sediments indicate an external position within the orogen, comparable with, e.g., the ex- ternal massifs of the western Alps. Astonishingly, the high-grade core has yielded Carboniferous metamorphic and magmatic ages. The geodynam- ic context of this “hot external massif” is still a matter of controversy.