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 Autor(en): Wiederer, Ulrich / Königshof, Peter / Feist, Raimund Objekttyp: Article Zeitschrift: Schweizerische mineralogische und petrographische Mitteilungen = Bulletin suisse de minéralogie et pétrographie Band (Jahr): 82 (2002) Heft 2: Diagenesis and Low-Grade Metamorphism PDF erstellt am: 11.10.2021 Persistenter Link: http://doi.org/10.5169/seals-62372 Nutzungsbedingungen Die ETH-Bibliothek ist Anbieterin der digitalisierten Zeitschriften. Sie besitzt keine Urheberrechte an den Inhalten der Zeitschriften. Die Rechte liegen in der Regel bei den Herausgebern. Die auf der Plattform e-periodica veröffentlichten Dokumente stehen für nicht-kommerzielle Zwecke in Lehre und Forschung sowie für die private Nutzung frei zur Verfügung. 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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', Peter Königshof', Raimund Feist2, Wolfgang Franke3 and Michael P. Doublier4 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 stratigraphie 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 stratigraphie 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 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. Keywords: conodont alteration index (CAI), illite crystallinity (IC), metamorphic core complex, Variscides, Montagne Noire. 1. Introduction Palaeozoic cover and the presence of Carboniferous flysch sediments indicate an external position The Montagne Noire is situated on the southern within the orogen, comparable with, e.g.. the flank of the Variscan Belt in South France (e.g.. external massifs of the western Alps. Astonishingly, Matte, 1991). It contains a low pressure the high-grade core has yielded Carboniferous metamorphic core with anatectic gneisses and granites, metamorphic and magmatic ages. The geodynam- overiain and flanked by Palaeozoic metasedi- ic context of this "hot external massif' is still a ments (Fig. 1). The low metamorphic grade of the matter of controversy. 1 Forschungsinstitut und Naturmuseum Senckenberg, Senckenberganlage 25, D-60325 Frankfurt a. Main. <[email protected]> and <[email protected]> 2 Institut des Sciences de l'Évolution, Lab. de Paléontologie, Université de Montpellier II, Place E. Bataillon, F-34095 Montpellier Cedex 05. <[email protected]> 3 Institut für Geowissenschaften der Universität, Senckenbergstr. 3, D-35390 Gießen. <[email protected] giessen.de> 4 Institut für Geologie und Paläontologie der Universität, Senckenberg-Anlage 32, D-60325 Frankfurt a. Main. <[email protected]> 394 U. WIEDERER. P. KÖNIGSHOF, R. FEIST. W. FRANKE AND M. DOUBLIER Apart from this geodynamic problem, the wana margin (e.g., Piqué et al., 1994). In most of Montagne Noire offers ideal conditions for the area, Late Ordovician and Silurian deposits methodical metamorphic studies, since a laterally are missing. The Devonian oversteps the early consistent sequence of Palaeozoic sediments Palaeozoic sequences with an angular (quartzites, greywackes, pelites, radiolarian cherts unconformity of 10-20°, which might be interpreted to and carbonates) may be traced across the reflect an orogenic event. In fact, Gebauer and metamorphic zonation, from diagenesis into amphibo- Grünenfelder (1976) have reported U-Pb ages lite facies. Earlier petrological studies have on detrital zircon fractions and Rb-Sr whole rock revealed a concentric zonation of low pressure ages from metamorphic rocks of the Zone Axiale metamorphism centred around the gneissic core spanning between 445 + 10 Ma and 417 ± c. 35 Ma, (Demange, 1985). We have started a multidisci- which they take to indicate a "Caledonian" plinary metamorphic study in the Palaeozoic metamorphic event. However, the pre-Devonian rocks mantle, in order to compare the records of different on the flanks of the Zone Axiale do not show a methods, and to provide additional separate folding or cleavage. Besides, the constraints on the tectono-metamorphic evolution. "Ecailles de Cabrières" of the southeastern Montagne This paper presents a survey of the conodont Noire (see below) contain a stratigraphie alteration index (CAT) in Devonian limestones, sequence without any angular unconformity. comparisons with an earlier study on illite crystal- There is biostratigraphic evidence for Early linity (IC), and tectonic consequences. Ordovician, Late Ordovician and Silurian rocks (Dreyfuss, 1948), the latter being conformably overlain by the Devonian (Feist and Schönlaub, 2. Geological setting 1974). On the northern flank of the Montagne Noire, the sedimentary sequence is restricted to 2.1. STATIGRAPHIC RECORD the Cambrian through Early Ordovician with the exception of one isolated occurrence of fossilifer- The Palaeozoic sequence of the Montagne Noire ous Silurian rocks (Centene, 1977). These starts with a thick pile of Early Cambrian carbonates features rule out a major "Caledonian" orogenic and Middle Cambrian through Early Ordovi- event. The isotopic data cited above require cian clastic sediments deposited at the N-Gond- confirmation by more reliable methods. rig 3a Nappes of the southern flank I 1 Pardailhan Nappe 1 I Mont Peyroux I 1 and Minervois Nappes Northern flank Zone Axiale 1^1 Faugères Nappe I», »I Pre-Variscan granites FAN Stephanian EE Schistes X irrm Parautochthon 1 ortho- and Variscan granites L J deformed Palaeozoic rocks II paragneisses 'fffi Écailles de Cabrières Fig. 1 Simplified geological map of the Montagne Noire, with insets showing the position of areas sampled for CAI (see Figs. 2 and 3). CONODONT ALTERATION INDEX AND METAMORPHIC EVOLUTION 395 Apart from a basal clastic member, the Devonian marbles (X6 X9 X10 and X15), partly associated sequence is entirely composed of carbonates. with black meta-cherts (X, and X15, see Depositional environments show a clear trend of Bogdanoff et al., 1984), which remind the upward deepening, from sabkha-type early diage- Tournaisian and Early Viséan lithologies of the less netic dolomites in the Pragian towards hemipe- metamorphosed units further south (e.g., Engel lagic nodular limestones in the Late Eifelian et al., 1981). through to the basal Carboniferous (Feist, 1985). The "Par-Autochthon", structurally above and These limestones are overlain by radiolarian to the S of the Schistes X, contains clearly identifiable cherts (Tournaisian to Mid-Viséan) and a thick Ordovician, Devonian and Carboniferous sequence of synorogenic, deep water clastic sequences. The repetition of conspicuous protolith sediments (flysch) of Late Viséan to Early