Journal of StructuralGeology, Vol. 17, No. 12, Pp. 1689 to 1706,1995 Copyright @ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0191-8141/95 s9.5o+o.oo 0191-8141(95)ooo62-3 Granite emplacement during contemporary shortening and normal faulting: structural and magnetic study of the Vkiga Mbssif (NW Spain) T. ROMAN-BERDIEL,*t E. L. PUEYO-MORER*$ and A. M. CASAS-SAINZ* *Departamento de Geologfa, Facultad de Ciencias, Universidad de Zaragoza, 5ooo9 Zaragoza, Spain TGCosciences Rennes, Universite de Rennes, 1.35042 Rennes Cedex, France $Institut de Citincies de la Terra ‘Jaume Almera’, C.S.I.C., 08028 Barcelona, Spain (Received I September 1994; accepted in revised form 16 May 1995) Abstract-The Veiga Massif belongs to the talc-alkaline series of Hercynian granitic rocks of the Ibero- Armorican arc. The Veiga granodiorite intruded during the Upper Carboniferous into the core of the WNW- ESE N-verging ‘0110 de Sapo’ antiform, formed by Precambrian and Palaeozoic metasediments. Internal fabrics show that magma intrusion was contemporary with shortening. Measurements of feldspars orientations and anisotropy of magnetic susceptibility (AMS) throughout the granite are consistent and indicate a foliation striking WNW-ESE (parallel-to-folding), with a constant dip of 75-85”N. The zonation of bulk low-field susceptibility is related to mineral content and indicates a more basic composition at the southern and western borders. The difference in elevation between outcrops (more than 600 m) allows us to infer the three-dimensional attitude of granite fabrics throughout the Massif. Syn-magmatic fabric folds are preserved in the inner part of the igneous body. The highest degree of magnetic anisotropy is observed in areas located near the bottom and top of the intrusion. At the scale of the Massif, foliation is convergent toward the bottom of the intrusion, along a line located at its northern border, where the magma source is interpreted to be located. In the western border of the Massif, the presence of C and S structures indicates that magma cooling was coeval with movement of the Chandoiro fault, a N-S striking normal fault with a N290E hanging wall displacement direction. These results indicate that emplacement of the Veiga granite is coeval with NNE-SSW shortening and with an WNW-ESE extension direction, parallel to the trend of the late folds. INTRODUCTION Mechanisms of granitoid intrusion have been exten- sively discussed in the geological literature (Pitcher 1979, Bateman 1984, Castro 1987, Wickham 1987, Miller et al. 1988, Hutton 1988, Kukowski & Neuge- bauer 1990, Paterson & Fowler 1993). Ascent of magma can occur in different tectonic regimes. Granites intruded in extensional, thrust and wrench regimes can be found in the Hercynian Massif of Iberia related to different stages in the evolution of the Variscan orogen (Brun & Pons 1981, Courrioux 1983, Courrioux et al. 1986, Mpez-Plaza & Gonzalo 1986, Castro 1986, Cor- retgC et al. 1989, Aranguren 8z Tubia 1992). The difficul- ties for clearly identifying the mechanism of intrusion lie in the very nature of igneous rocks: the methods com- monly used in structural geology are very useful to study the effects of ductile deformation, but magmatic folia- tion is hard to measure. Determination of the anisotropy of the magnetic sus- ceptibility (AMS) is demonstrated to be a powerful tool for determining fabrics in igneous rocks (Heller 1973, Hrouda 1982, Par& 1988, Guillet et al. 1983). Some attempts have been made to use the intensity of mag- netic susceptibility as an indicator to determine petro- graphic types (Gleizes et al. 1993, Leblanc et al. 1994). This paper aims to study the conditions of emplace- ment of the granodioritic Veiga Massif and the defor- mation that affects it. The Veiga Massif is an igneous body intruded in the northern part of the Hercynian belt of Spain, between the Central Iberian and the Western Asturian-Leonese zones (Fig. 1). The main objective of this work is to analyse the relationship between fabric orientation in granites, the magnetic fabric and continu- ous ductile deformation (taking into account the impor- tance of three-dimensional analysis in the study of mag- matic bodies). We also attempt to correlate the emplacement and deformation of the Veiga granitoid within the frame of the tectonic evolution of the north- ern Iberian Variscan belt. The methodology used in this work includes analysis of magnetic properties of igneous rocks and kinematic analysis of ductile planar and linear fabrics. GEOLOGICAL SETTING The Iberian Massif is part of the European Hercynian Orogen. It is classically divided into five zones according to stratigraphic, petrologic and structural features (Lotze 1945, Julivert et al. 1972) (Fig. 1): Cantabrian, Western Asturian-Leonese, Central-Iberian, Ossa- Morena and South-Portuguese. The structure of the northern part of the Iberian Hercynian belt is characterized by a westward-facing arc, formed by several stacked thrust units. Several tectonic stages have been distinguished, most of them corresponding to an .o,verall east-west shortening (Bard et al. 1973). The first stages formed recumbent folds and east-verging thrusts; deformation continued forming $6 11:,2-0 1689