art07.pdf - GEOGACETA 46 2009 La fbrica de las rocas...

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27 GEOGACETA, 46, 2009 Copyright ® 2009 Sociedad Geológica de España / www.geogaceta.com La fábrica de las rocas miloníticas de la Zona de Cizalla de Los Llanos (Calzadilla de los Barros, Badajoz) Mylonite fabrics in Los Llanos Shear Zone (Calzadilla de los Barros, Badajoz) Alberto Jiménez-Díaz (1) , Ramón Capote (1) , Rosa Tejero (1) , Rosario Lunar (2) , Lorena Ortega (2) , Serafín Monterrubio (3) , Casimiro Maldonado (4) y David Rodríguez (4) (1) Departamento de Geodinámica, Facultad de Ciencias Geológicas, Universidad Complutense, 28040 Madrid. [email protected]; [email protected]; [email protected] (2) Departamento de Cristalografía y Mineralogía, Facultad de Ciencias Geológicas, Universidad Complutense, 28040 Madrid. [email protected]; [email protected] (3) Escuela Politécnica Superior, Universidad de Salamanca, 49022 Zamora. [email protected] (4) Río Narcea Nickel S.A., Zafra, 06300 Badajoz. ABSTRACT The Los Llanos Shear Zone bounds the Cabeza Gorda and Zafra units located in the Ossa-Morena Zone (SW Iberian Massif). Within the Cabeza Gorda unit two peridotitic massifs crop out, named Sierra Cabeza Gorda and Cerro Cabrera. The mylonitic rocks consist of metamorphic and peridotitic rocks extensively deformed that exhibit a penetrative foliation. Mylonitic fabrics are characterized by s and d structures and S-C and S-C’ band. Some examples show a crenulation cleavage that points out the existence of at least two deformation stages. Kinematic criteria are widespread, but dextral-reverse movement prevails both in outcrop and thin section. Los Llanos Shear Zone orientation is NW-SE, similar to variscan structures trend in the area, and deforms precambrian and paleozoic rocks. Los Llanos Shear Zone represents a dextral-reverse structure of Variscan age. Key words: Mylonites, peridotitic massifs , shear zone, Calzadilla de los Barros Geogaceta, 46 (2009), 27-30 ISSN: 0213683X Introducción En la Zona de Ossa-Morena, en el SO del Macizo Ibérico, afloran varios cuerpos de rocas ultramáficas entre las que se en- cuentran los macizos peridotíticos de Calzadilla de los Barros. La naturaleza ofiolítica de estas peridotitas fue puesta de manifiesto por Monterrubio (1991), a par- tir del estudio de las cromitas contenidas en ellas, y representan parte de una litosfera oceánica. Hasta las investigacio- nes con objetivos mineros de Río Narcea Nickel S.A., las relaciones entre las rocas peridotíticas y las rocas precámbricas y paleozoicas no habían sido estudiadas en detalle. La integración de datos geológicos, geofísicos y geoquímicos ha mostrado el carácter cabalgante del con- tacto entre las peridotitas y las rocas encajantes en el borde sur del macizo Ce- rro Cabrera, y la existencia más al sur de una zona de cizalla denominada «Zona de Cizalla de los Llanos», a favor de la cual se produce una intensa deformación.
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