A review of the pre-Permian geology of the Variscan French Massif Central

The Massif Central, like the southern part of the Massif Armoricain, belongs to the North Gondwana margin. The Massif Central consists of a stack of nappes resulting of six main tectonicmetamorphic events. The first one, D0, is coeval with an Early Late Silurian (ca 415 Ma) highpressure (or ultra high-pressure) metamorphism for which the associated structures are poorly documented. The Early Devonian D1 event, responsible for top-to-the-SW nappe displacement, is coeval with migmatization and exhumation of high-pressure rocks around 385-380 Ma. In the northern part of the Massif Central, metamorphic rocks with retrogressed eclogites are covered by Late Devonian undeformed sedimentary rocks. The Late Devonian-Early Carboniferous D2 event is a top-to-the-NW shearing coeval with an intermediate pressure-temperature metamorphism dated around 360-350 Ma. The Visean D3 event is a top-to-the-south ductile shearing widespread in the southern Massif Central. Coevally, in the northern Massif Central, the D3 event corresponds to the onset of synorogenic extension. The next two events, D4 and D5, of Early and Late Carboniferous age, correspond to the syn and late orogenic extensional tectonics, respectively. The former is controlled by NW-SE stretching whereas the later is accommodated by a NNE-SSW stretching. These structural and metamorphic events are replaced in a geodynamic evolution model. in su -0 03 52 69 9, v er si on 1 13 J an 2 00 9 Author manuscript, published in "Comptes Rendus Geoscience 341, 2-3 (2009) 202-213" DOI : 10.1016/j.crte.2008.12.001 2 The possibilities of one or two cycles involving microcontinent drifting, rewelding and collision are discussed. Les grands traits de l’évolution anté-permienne du massif central français Résumé Le Massif central français, comme la partie sud du Massif armoricain, appartiennent à la marge nord-gondwanienne. L’architecture du Massif central consiste en un empilement de nappes synmétamorphes qui résultent de six événements tectono-métamorphiques. Le premier, D0, est synchrone du métamorphisme Silurien de haute à ultra-haute pression dont les structures associées restent très mal connues. L’événement du Dévonien inférieur, D1, responsable de la mise en place du charriage de l’Unité Supérieure des Gneiss (USG) sur l’Unité inférieure des Gneiss avec un mouvement vers le SW, est synchrone à postérieur à la première fusion crustale datée vers 385-380 Ma. Ces migmatites renferment des bloc d’éclogites rétromorphosées en amphibolites. Dans le Nord du Massif central, l’USG est recouverte pas des séries sédimentaires d’âge Dévonien. L’événement D2, d’âge Dévonien terminal-Tournaisien est représenté par un cisaillement ductile vers le NW dans des conditions de moyenne pression-moyenne température. Dans le Sud du Massif Central, l’événement Viséen D3 correspond à un cisaillement vers le SSW qui affecte l’Unité Para-Autochtone et l’Avant-Pays plissé. Dans le Nord du Massif, au même moment, débute l’extension syn-orogénique, contemporaine du magmatisme des Tuffs Anthracifères. Les deux derniers événements, D4 et D5 représentent les phases de dés-épaississement crustal. Le stade D4, daté du début du Carbonifère supérieur (namuro-westphalien) par des plutons leucogranitiques ou monzogranitiques syncinématiques, se caractérise par un étirement NW-SE traduisant l’extension syn-orogénique. Le stade D5, d’âge Carbonifère supérieur (stéphanien), accommode l’extension post-orogénique avec un étirement NNE-SSW. Ces événements sont replacés dans leur cadre géodynamique. A l’Ordovicien inférieur, la séparation du microcontinent Armorica de Gondwana est responsable de l’ouverture de l’Océan Médio-Européen. Au Silurien, la fermeture in su -0 03 52 69 9, v er si on 1 13 J an 2 00 9 3 de cet océan puis la collision Armorica –Gondwana sont controlées par une sudduction sous Armorica. L’évolution dévono-carbonifère est encore discutée. La possibilité d’une seule convergence avec une subduction vers le Nord du Silurien au Carbonifère est envisagée. En revanche, un autre scénario privilégie la subduction vers le Sud de l’océan rhéïque, situé au Nord d’Armorica. Elle serait à l’origine de la formation d’un arc magmatique et du basin marginal de la Brévenne au Dévonien moyen-supérieur. La fermeture de l’Océan rhéique puis la collision Gondwana –Laurussia au Carbonifère inférieur se traduit par les événements D2 et D3. Dans les deux modèles, les événements D4 et D5 représentent les stades extensifs synà post-collisionnels. Introduction In France, the Massif Central is the largest exposed area where Variscan metamorphic and plutonic rocks can be analyzed. The entire massif belongs to the northern Gondwanian margin, which corresponds to the southern continent involved in the Variscan collision. Due to the limited amount of post-Permian reworking, the French Massif Central (FMC) is a suitable place to understand the structure and evolution of the lower plate of the Gondwanian Domain. This article aims to provide an overview of the structure and tectono-metamorphic evolution of the FMC and to discuss the still controversial points related with its geodynamic evolution. The bulk architecture of the Massif The Massif Central, alike the southern part of the Massif Armoricain or the southern Vosges, is a stack of metamorphic nappes [e.g. 40, 29 and enclosed references]. From bottom to top, and South to North, the following units are recognized (Figs. 1, 2). i) The foreland basin develops along the southernmost part of the Massif Central. It is a Middle Carboniferous (Visean-Namurian) turbiditic basin that extends southward below the coastal plain of the Mediterranean Sea and up to the Pyrénées. In the Montagne Noire, proximal turbiditic facies includes kilometer-scale olistoliths of Paleozoic sedimentary rocks coming from the next unit [23]. in su -0 03 52 69 9, v er si on 1 13 J an 2 00 9 4 ii) The Paleozoic Fold-and-Thrust Belt consists of Early Cambrian to Middle Carboniferous weakly or un-metamorphosed sedimentary rocks displaced to the South as thrust sheets or kilometric-scale recumbent folds well developed in the Montagne Noire. The CambrianOrdovician passive margin deposits are unconformably overlain by Early Devonian terrigeneous rocks, followed by a Middle Devonian to Early Carboniferous kilometer thick carbonate platform. The almost complete absence of Silurian rocks and the Early Devonian unconformity argue for an Early Paleozoic tectonic event (at ca 410-400 Ma), which is also recorded to the North in the metamorphic rocks. iii) To the North, the Para-autochthonous Unit overthrusts the Paleozoic sedimentary series. It is formed by greenschist to epidote-amphibolite facies metapelites, quartzites and a small amount of limestone and amphibolite. This Para-autochthonous Unit is widely exposed in southern MCF (Cévennes and Albigeois). In the northern FMC, the Para-autochthonous Unit crops out in tectonic windows surrounded by the Lower Gneiss Unit. Due to metamorphic recrystallizations, fossils are not preserved, and the protolith ages are unknown; some Ordovician plutons (now transformed into orthogneiss) intrude the series and Ordovician detrital zircons are found in some volcani-clastic layers. iv) The Lower Gneiss Unit (LGU) is composed of metagreywackes, metapelites and metarhyolites intruded by numerous Cambrian to Early Ordovician porphyritic alkaline granitoids transformed into augen orthogneiss during the tectonic-metamorphic events. The LGU underwent Middle Devonian metamorphism leading to crustal melting coeval with a ductile shearing dated in the Limousin at 375-370 Ma and attributed to the D1 event (cf below, 21, 30, 38). Moreover, the LGU also experienced a medium pressure/medium temperature metamorphism with a biotitegarnet-staurolite assemblage, and attributed to the D2 event. v) The Upper Gneiss Unit (UGU) forms the overlying nappe. The protoliths of this unit are partly similar to those of the LGU. Moreover, the UGU is also characterized by a bi-modal magmatic association of acidic lavas, tuffs and mafic rocks (basalts, gabbros, rare ultramafic rocks) called the "leptynite-amphibolite complex". The UGU contains high pressure rocks, such as in su -0 03 52 69 9, v er si on 1 13 J an 2 00 9 5 eclogites or HP granulitic orthogneiss, that locally may reach the coesite-eclogite facies [ 35, 39, 61] however, blueschists are very rare in the FMC. The upper part of the UGU consists of Devonian migmatites, with ages around 385-380 Ma, [21, 29, 38] formed by the partial melting of pelitic and quartzo-feldspathic rocks. These rocks contain amphibolite blocks derived from retrogressed eclogites that did not experience partial melting. In the Limousin, some metagabbros and serpentinized ultramafics are considered by some authors to represent ophiolites [e.g.18]. However, it is worth noting that siliceous sediments, i.e. radiolarian cherts or siliceous shales are absent. On the basis of recent work in the Mts du Lyonnais, the UGU might be subdivided into several subunits, such as i) the bimodal quartzo-feldspathic (leptynite-amphibolite) series, ii) a block-in-matrix series with eclogites and peridotites enclosed in a paragneiss matrix, iii) a mafic unit devoid of HP rocks. In the present state of knowledge, it is not possible to provide a general map of these subunits for the entire MCF, therefore the structural map (Fig. 1) does not take into account this subdivision. The units described above represent the classical nappe pile of the FMC in which the metamorphism increases from bottom to top. This architecture is often described as an “Himalayan-type inverted metamorphism”. However other units have to be considered too. vi) The Thiviers-Payzac Unit is the highest tectonic unit of the allochthonous stack in the Massif Central developed in the South Limousin area [35], and extends southward in Rouergue where it is called the St-Sernin-sur-Rance Nappe [34, Fig. 1]. It is formed by Cambrian metagreywackes, rhyolites and quartzites metamorphosed under amphibolite facies conditions, and never experienced the high-pressure metamorphism. The Thiviers-Payzac Unit is lithologically similar to the Para-autochthonous Unit. On the basis of metamorphic and structural observations [92, 32, 58] supported by inferences based on interpretation of seismic data [4], the ThiviersPayzac Unit is interpreted to be allochthonous on the UGU. Emplacement of the Thiviers-Payzac Unit is a result of the second deformation event (D2, cf. below). vii) The Brévenne Unit consists of pillow basalts, diabase dykes, gabbros, ultramafics and siliceous sedimentary rocks) and massive sulfide deposits of Devonian age, interpreted as an in su -0 03 52 69 9, v er si on 1 13 J an 2 00 9 6 ophiolitic nappe [53, 55, 62]. The geochemistry of the mafic rocks is in agreement with a back-arc basin setting for the Brévenne ophiolite [53, 62]. The Brévenne Unit is an ophiolitic nappe metamorphosed to greenschist facies conditions. This unit is separated from the UGU, to the south, by a Middle Carboniferous dextral wrench fault [15, 39]. Some authors suggest an earlier displacement to the NW over pre-Late Devonian gneiss [26, 43]. The age of this early ductile shearing is constained by an Early Visean unconformity [36]. In the South Limousin, some small outcrops of gabbro, mafic metavolcanic rocks (sometimes pillowed), radiolarian cherts, siliceous red shales and Middle Devonian limestones form the Génis Unit. The structural and paleogeographic setting of the Génis Unit is not yet settled. Although sometimes considered as an ophiolitic nappe [e.g. 40], the lack of outcrop continuity allows an interpretation that the Génis Unit might be a Late Devonian-Early Carboniferous olistostrome reworking oceanic rocks similar to those of the Brévenne Unit. vii) The Somme Unit develops only in the NE part of the FMC. It is formed by undeformed and unmetamorphozed volcanic and volcaniclastic rocks and massive sulfide deposits of Middle to Late Devonian age. The magmatic rocks have calc-alkaline geochemical affinities typical of a magmatic arc setting [16]. vii) The “Tuffs anthracifères” series developedt only in the northern FMC, from N. Limousin to Morvan. These terrigeneous rocks (sandstone, shales, conglomerates and coal measures) and felsic magmatic rocks (dacites and rhyolites) are dated as Late Visean (ca 330Ma, 7). These unmetamorphozed rocks unconformably overlie Late Devonian-early Carboniferous plutonic or metamorphic rocks. The “Tuffs anthracifères” series is coeval with the Fold-andThrust Belt. Thus it supports the north-south propagation of the tectonic events throughout the