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Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France:structural and 40Ar/39Ar investigations. Aude Gébelin, Maurice Brunel, Patrick Monié, Michel Faure, Nicolas Arnaud To cite this version: Aude Gébelin, Maurice Brunel, Patrick Monié, Michel Faure, Nicolas Arnaud. Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France:structural and 40Ar/39Ar investigations.. Tectonics, American Geophysical Union (AGU), 2007, 26 (2), pp.TC2008. 10.1029/2005TC001822. hal-00104212 HAL Id: hal-00104212 https://hal-insu.archives-ouvertes.fr/hal-00104212 Submitted on 2 Nov 2010 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. TECTONICS, VOL. 26, TC2008, doi:10.1029/2005TC001822, 2007 Click Here for Full Article Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and 40Ar/39Ar investigations Aude Ge´belin,1,2 Maurice Brunel,1 Patrick Monie´,1 Michel Faure,3 and Nicolas Arnaud1 Received 1 April 2005; revised 8 August 2006; accepted 14 September 2006; published 27 March 2007. 40 39 [1] New structural, microstructural, and Ar/ Ar constitute a remarkable natural laboratory to debate these data from the NW Massif Central (France) provide questions. additional constraints on the timing and tectonic [3] The Variscan Belt of western Europe was built setting of late Variscan granite magmatism. Previous between 500 and 250 Ma from the convergence and studies had emphasized the role of late orogenic collision of two main continents, Laurentia-Baltica to extension in the emplacement of granite plutons in the the northwest and Gondwana to the southeast [e.g., Matte, 1991]. Postcollisional intracontinental tectonothermal Limousin region. In contrast, the new data set is events extended over a time span of 100 Myr, between consistent with syntectonic emplacement of magma in 380 and 280 Ma [Matte, 1991]. The Limousin area, in the a dextral simple shear active from 350 to 300 Ma in a northwest part of the Massif Central (France), first experi- transpressional regime. As an alternative hypothesis to enced crustal shortening and thickening [Matte, 1986], late orogenic extension, we propose that magmas followedbyextensionandcrustalthinning[Van den migrated into tensional bridges between active P Driesche and Brun, 1989; Faure et al., 1990; Burg et shears associated with a lithospheric shear zone al., 1990; Faure, 1995]. It is characterized by voluminous comparable to a pop-up structure. The Galician two-mica leucogranites resulting from partial melting of the region, in the western end of the Ibero-Armorican Variscan crustal basement [e.g., Cuney et al., 1990]. These tectonic arc, exhibits major left-lateral ductile shear intrusions are spatially associated with normal faults and zones which can be interpreted as conjugate structures major synmagmatic strike-slip shear zones that merge to the northwest with the South Armorican Shear Zone (SASZ). to the Limousin and Armorican shear zones. Previous studies emphasized the role of Carboniferous late Citation: Ge´belin, A., M. Brunel, P. Monie´, M. Faure, and orogenic extension and normal faults in the emplacement of N. Arnaud (2007), Transpressional tectonics and Carboniferous Limousin granite plutons [Faure, 1989; Faure and Pons, magmatism in the Limousin, Massif Central, France: Structural 40 39 1991; Faure, 1995]. However, new field evidences suggest and Ar/ Ar investigations, Tectonics, 26, TC2008, that strike-slip faults were also closely associated with doi:10.1029/2005TC001822. magma genesis. What are the relationships then between strike-slip and normal faulting contemporaneous with gran- 1. Introduction ites emplacement? [4] The purpose of this paper is to describe the structures [2] Many studies have shown that melts have an influ- and microstructures of rocks affected by ductile strike-slip ence on the rheology and deformation of the continental shearing in the Limousin area, and to present new 40Ar/39Ar crust [e.g., Davidson et al., 1994]. A number of examples geochronological data. These data, collected from the core show a close spatial and temporal relationship between and border of granitic plutons and from wall rocks, show magmas and crustal ductile shear zones [Hutton and that strike-slip faulting started around 350 Ma and ended Reavy, 1992; Davidson et al., 1992; Tikoff and Teyssier, around 300 Ma. The structural study was performed in order 1992; Neves et al., 1996; Tikoff and de Saint Blanquat, to better understand: (1) the deformation mechanisms that 1997]. Field observations lead to the following questions: prevailed during cooling of the granites; (2) the relation- does lithospheric deformation trigger magma formation ships between the tectonic setting and late Variscan granite and ascent? Does magma rheology have an impact on emplacement; and (3) the mechanisms of magma migration the location of the crustal deformation? The Variscan and emplacement. granite plutons in the Limousin region, western Europe, [5] After a thorough structural characterization and argon thermochronology investigations on the Limousin area, comparisons are made with other parts of the Armorican 1Laboratoire Dynamique de la Lithosphe`re, UMR 5573, Universite´ Massif. The new results are then synthesized in a discussion Montpellier II, Montpellier, France. in which we propose that granitic magmatism took place in 2Now at Mining Business Unit, AREVA NC, Ve´lizy, France. a transpressional context where the magmas were emplaced 3Institut des Sciences de la Terre d’Orle´ans, UMR 6113, Universite´ d’Orle´ans, Orle´ans, France. into tensional bridges between active P shears. Some of the implications of this new interpretation are discussed in the Copyright 2007 by the American Geophysical Union. framework of the French Massif Central. Finally, the new 0278-7407/07/2005TC001822$12.00 TC2008 1of27 TC2008 GE´ BELIN ET AL.: MAGMATISM AND TECTONICS TC2008 results are integrated in a general geodynamic model at the 2.2. Limousin Region scale of the Limousin and Armorican massifs. [10] The Limousin region, in the northwest of Massif Central, is a key area to understand Variscan tectonic and 2. Geological Setting and Previous Work magmatic processes because of the voluminous granitic magmatism and its association with strike-slip shear zones. 2.1. Massif Central (France) in the Variscan Belt This region is overlain to the west by the ‘‘Seuil du Poitou’’ [6] The West European Variscan Belt formed as a result Mesozoic series and bounded to the east by the Stephanian of a Himalaya-type continental collision between the ‘‘Sillon Houiller’’ sinistral strike-slip fault (Figure 1). Laurentia-Baltica and Gondwana lithospheric plates [e.g., [11] The Limousin region, hosts two granite types: the Matte, 1991]. In the French Massif Central, the Variscan biotite ± hornblende ± cordierite-bearing granites (Gue´ret orogenic events range from Late Silurian–Early Devonian, type), and the two-mica leucogranites [e.g., Scaillet et al., i.e., 440–400 Ma, which is a period of high-pressure (HP) 1996a, 1996b]. The Gue´ret-type granites were emplaced in metamorphism [Ledru et al., 1989; Santallier et al., 1994; a transpressive setting [Cuney et al., 2001] between 356 and Faure et al., 1997; Matte et al., 1998], up to Late 349 ± 5 Ma (U–Pb zircon age) [Bertrand et al., 2001], at a Carboniferous–Early Permian, i.e., 300 Ma which is the depth of 14 ± 2 km [Freiberger et al., 2001]. These granites deposition time of the late orogenic sediments. are thought to be derived from the interface between [7] Eclogites are associated with felsic and mafic HP metasedimentary and mafic igneous lower crust [Downes granulites [Nicollet and Leyreloup, 1978; Pin and Vielzeuf, and Duthou, 1988; Shaw, 1991]. The Gue´ret Massif 1983] and with spinel and/or garnet lherzolites [Gardien et (Figure 1), emplaced at 356 ± 10 Ma (Rb/Sr on whole al., 1988; Gardien, 1990]. The P-T conditions of the rocks [Berthier et al., 1979]), hosts not only Gue´ret-type eclogite facies metamorphism are estimated to 13–20 kbar granites but also cordierite-bearing anatectic paragneisses. and 650–750°C[Mercier et al., 1991]. The coesite- The unconformity of the ‘‘Visean Tufs Anthracife`res’’ bearing eclogites of eastern Massif Central [Lardeaux et formation [Faure et al., 2002] with the Gue´ret granite al., 2001] were brought to shallow crustal levels during indicates that the granite was exposed at the surface at continental collision. Eclogite exhumation (60 km of 335 Ma. The Gue´ret massif is fault bounded (Figure 1) uplift) took place in a transpressive regime [Lardeaux et [Grolier and Letourneur, 1968; Lerouge,1988]andis al., 2001]. interpreted as an extensional allochton shifted to the south- [8] The Middle Devonian to Early Carboniferous east in Namurian-Westphalian times [Faure and Pons, 1991; recorded continental collision and nappe stacking. Two Faure, 1995]. The granite has subhorizontal NW-SE gneissic units are distinguished in most parts of Massif magnetic foliations

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