Fabric evidence for granodiorite emplacement with extensional shear zones in the Variscan Gredos massif (Spanish Central System) Juan Díaz-Alvarado a , Carlos Fernández a, * , Manuel Díaz-Azpiroz b , Antonio Castro c , Ignacio Moreno-Ventas c a Departamento de Geodinámica y Paleontología, Universidad de Huelva, E-21071 Huelva, Spain b Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, 41013 Seville, Spain c Departamento de Geología, Universidad de Huelva, E-21071 Huelva, Spain article info Article history: Received 21 July 2011 Received in revised form 25 June 2012 Accepted 26 June 2012 Available online 6 July 2012 Keywords: Magmatic fabric SPO Granodiorite Deformation Variscan belt abstract Three granitoid bodies in the central part of the Gredos massif (Spanish Central System batholith) are tabular, around 1 km in thickness, and intruded into a migmatitic middle crust during the D3 defor- mation phase of the Variscan Orogeny. Petrologically, they are composed of Bt-granodiorite and Crd- monzogranite, and they show varying abundance of large (cm-scale) feldspar megacrysts. A detailed study of the shape preferred orientation (SPO) magmatic fabric defined by these megacrysts, together with a kinematic analysis of the structures due to interactions between them, and the measurement of quartz c-axis fabrics in migmatites and granitoids, suggests that granitic magma and country rocks were mechanically coupled during deformation. The emplacement took place along large-scale, extensional shear zones active during the first stages of the D3 phase. The shape of the SPO ellipsoids varies from constrictional at the centre of the granitic bodies, to flattening or even oblate at their external contacts with the migmatitic host rocks. The favoured interpretation of this spatial fabric variation is the over- printing of the emplacement fabrics by a constrictional tectonic regime associated with the growth of tabular magma chambers along extensional detachments, followed by shear zone development commonly at the top of the granitic bodies. The entire structure was later folded during the last stages of the D3 phase. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The study of the processes operating during the emplacement and structural evolution of the large masses of granitoid magmas in the continental crust has drawn the attention of petrologists and structural geologists since the pioneering work of Hans Cloos (e.g., Cloos, 1925). Considerable efforts during the second half of the past century identified the importance of the tectonic regime on the segregation, ascent and emplacement of granite magma (e.g., Brown, 1994; Bouchez et al., 1997; Brown and Solar, 1999; Castro et al., 1999; Petford et al., 2000; and references therein). The mechanical behaviour of magmas, the structure, kinematics and rheology of their host rocks, and the stress state of the crust were found to be essential parameters controlling the final shape of plutons and batholiths, and the generation and evolution of fabrics within them (e.g., Berger and Pitcher, 1970; Castro, 1987; Clemens and Mawer, 1992; D’Lemos et al., 1993; Petford et al., 1993; Brown, 1994; Collins and Sawyer, 1996; Clemens et al., 1997; Vigneresse et al., 1999; Petford, 2003; among many others). Modern research about magmatic fabrics follows three main lines. First, the development and application of new techniques for the geometrical and kinematic characterisation of magmatic fabrics. This line includes the identification of the criteria to distinguish between magmatic and solid-state fabrics in granites (Paterson et al., 1989; Bouchez et al., 1992), the 2D and 3D deter- minations of shape fabrics (e.g., Launeau and Robin, 2005; Higgins, 2006), the application of AMS techniques (e.g., Jelinek, 1981; Bouchez, 1997), and the integrated use of classical field techniques, microstructural and geochronological studies (e.g., Kratinová et al., 2007). Numerical and analogue experiments are also now offering new perspectives on the development of magmatic fabrics and * Corresponding author. E-mail address: fcarlos@uhu.es (C. Fernández). Contents lists available at SciVerse ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg 0191-8141/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsg.2012.06.012 Journal of Structural Geology 42 (2012) 74e90