Syntectonic crustal melting and high-grade metamorphism in a transpressional regime, Variscan Massif Central, France Aude Gébelin a,b,c, ⁎, Françoise Roger a , Maurice Brunel a a Université Montpellier II, Géosciences Montpellier, CNRS-UMR 5243, CC060, Place E.Bataillon, 34095 cedex5 Montpellier, France b Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA c Institut für Geologie, Leibniz Universität Hannover, 30167 Hannover, Germany abstract article info Article history: Received 25 June 2008 Received in revised form 6 March 2009 Accepted 24 March 2009 Available online 31 March 2009 Keywords: Crustal melting High-grade metamorphism Transpression U/Pb dating Variscan French Massif Central Hot collisional orogens are characterized by abundant syn-kinematic granitic magmatism that profoundly affects their tectono-thermal evolutions. Voluminous granitic magmas, emplaced between 360 and 270 Ma, played a visibly important role in the evolution of the Variscan Orogen. In the Limousin region (western Massif Central, France), syntectonic granite plutons are spatially associated with major strike–slip shear zones that merge to the northwest with the South Armorican Shear Zone. This region allowed us to assess the role of magmatism in a hot transpressional orogen. Microstructural data and U/Pb zircon and monazite ages from a mylonitic leucogranite indicate synkinematic emplacement in a dextral transpressional shear zone at 313±4 Ma. Leucogranites are coeval with cordierite-bearing migmatitic gneisses and vertical lenses of leucosome in strike–slip shear zones. We interpret U/Pb monazite ages of 315±4 Ma for the gneisses and 316±2 Ma for the leucosomes as the minimum age of high-grade metamorphism and migmatization respectively. These data suggest a spatial and temporal relationship between transpression, crustal melting, rapid exhumation and magma ascent, and cooling of high-grade metamorphic rocks. Some granites emplaced in the strike–slip shear zone are bounded at their roof by low dip normal faults that strike N–S, perpendicular to the E–W trend of the belt. The abundant crustal magmatism provided a low- viscosity zone that enhanced Variscan orogenic collapse during continued transpression, inducing the development of normal faults in the transpression zone and thrust faults at the front of the collapsed orogen. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The generation of abundant granitic magma, in part by crustal anatexis, is a characteristic feature of “hot” collisional orogens. However, the mechanisms of partial melting and the resulting consequences for the tectonic evolution of such orogens are still debated, partly due to the commonly deep levels of the crust at which these processes occur. Thus, the Variscan belt which exposes vast amounts of granitic intrusions, migmatitic complexes, and numerous scattered outcrops of granulite facies rocks offers a unique opportu- nity to study the role of partial melting during orogeny (Pin and Vielzeuf, 1983; Gapais et al., 1993; Brown and Dallmeyer, 1996; Vanderhaeghe et al., 1999; Ledru et al., 2001; Brown, 2004). In the French Massif Central and southern Brittany, Variscan high- grade rocks located south of the Galicia–South Brittany suture (Fig. 1a) and associated with strike–slip shear zones (Fig. 1b) represent an excellent target to understand the thermomechanical processes associated with wrench zones that develop in a transpressional regime. Consequently, the role of partial melting and magma storage in the crust during orogeny can be evaluated by investigating the timing and structural relationships of magma generation and emplacement with first-order tectonic features such as major shear zones. Transpressional regimes give rise to complex geometries in orogenic belts and the finite strain patterns observed in such orogens cannot be related to plate kinematics in a straightforward fashion. This is particularly important in now deeply eroded orogens like the Variscan belt of central Europe. Vanderhaeghe and Teyssier (2001) proposed that the presence of a layer of low-viscosity rocks in the middle crust could play a significant role in plate-scale deformation of thickened crust and changes in mid- crustal rheology have subsequently been correlated with rock uplift in orogenic belts (Liu and Shen, 1998; Meissner and Mooney, 1998; Godin et al., 2006; Hodges, 2006; Groome et al., 2008; Rey and Coltice, 2008). Moreover, the relative timing of changes in crustal buoyancy due to the combined effects of crustal thinning and partial melting may be a critical parameter in reconstructing the topographic evolution of eroded orogens (Mulch et al., 2007; Mulch and Chamberlain, 2007). Evaluating the thermal and structural history of “hot” orogenic crust deformed and/or exhumed in transpression is therefore a key aspect of understanding oblique tectonic regimes. Tectonophysics 477 (2009) 229–243 ⁎ Corresponding author. Tel.: +49 511 7623883; fax: +49 511 7622172. E-mail address: gebelin@geowi.uni-hannover.de (A. Gébelin). 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.03.022 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto