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Strain localisation and fluid trapping at granite roof: example of the Late-Hercynian Boborás granite and associated Bruès veins system (Galicia, Spain)

Abstract : If plutonism and granite emplacement are particularly well studied, associated fluid circulations within host rocks are poorly understood. For instance, path and final destination of these kinds of fluids are rarely identified and no clear ideas are available concerning their trapping. So, granite roof exposure represents a great deal in studying this problem. We herein present a case-study of Late-Hercynian granite roof affected by a fracture network evolving from early aplito-pegmatitic sills and dykes to late quartz gold-bearing veins. The Boborás granite is assumed to represent one of the syntectonic G3 granite within the Hercynian belt of NW Spain (Galicia). Road cuttings and ancient mining works provide exceptional 3D exposure of the granite roof, Palaeozoic host micaschist, and fracture network called the Bruès vein system. This one is concentrated around granite roof, within a 200 meters thick zone. At the granite contact, micaschist foliation is flat. Sills and low-angle dykes are filled by micro-granite and/or pegmatite. Both are crosscut by high-angle pegmatitic dykes which are frequently bordered by late quartz shear veins. Associated vertical “en echelon” tension gashes filled by quartz are also observed. All these structures are oriented E-W to NE-SW. It may be stressed that sills are sometimes directly linked with granitic bodies. Microtectonics investigations allow to determine that all these structures, since the earlier horizontal ones to the later high-dipping ones, are formed within a homogenous tectonic context characterized by north verging normal motion. Evidences are : i) top of granite, microgranite sills show a N-S submagmatic extension expressed by E-W trending microfractures of plagioclase crystals filled by magmatic quartz; ii) steped wall rock of high-angle microgranite dykes and sigmoidal sills are compatible with northward normal motion; iii) late quartz shear veins parallel to high-angle dykes are developed within north verging normal faults underlined by drag folds in micaschists. Microscopic textures of quartz shear veins are characteristic of syntectonic growth during north-directed normal tectonics. All the structures described above have been interpreted within a continuous long lived tectonic event associated with magmatic and hydrothermal fluid circulations. Because the structures evolved in time and space from near-horizontal to vertical ones, a regional extensional context is suspected to prevail during the Bruès vein system formation. The evolution of the subsequent deformation from submagmatic state through the ductile/brittle one until the brittle one agrees with the cooling of pluton in extensional context. However, this assumption needs to be confirmed in the light of more regional tectonic analyses. The Boborás granite and the Bruès vein system thus constitute a remarkable case study of geometrical and temporal association between open-fill fractures and intrusion. The lack of overprinting by high strain ductile structures (mylonite), makes possible to trace the continuous evolution from submagmatic to solid-state deformation. It also takes up the crucial role of strain localization at granite roof for development of fluid trapping structures.
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https://hal-insu.archives-ouvertes.fr/hal-00107902
Contributor : Nathalie Pothier <>
Submitted on : Thursday, October 19, 2006 - 9:52:53 AM
Last modification on : Monday, June 15, 2020 - 3:52:04 PM

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  • HAL Id : hal-00107902, version 1

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Eric Gloaguen, O. Gerbeaud, Alain Chauvet, Yannick Branquet, Catherine Lerouge, et al.. Strain localisation and fluid trapping at granite roof: example of the Late-Hercynian Boborás granite and associated Bruès veins system (Galicia, Spain). Deformation mechanisms, Rheology and Tectonics, 2003, Saint Malo, France. 1p. ⟨hal-00107902⟩

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