serie NOVA TERRA nº 49

55 fabrics can be associated. The signi fi cance of this metamorphic event is uncertain, although it has been repeatedly dated at c. 490 – 480 Ma ( Abati et al., 1999, 2007 ) and, as in the lower sheet, has been in- terpreted as related to the earlier dynamics of the peri-Gondwanan arc. The generation of granulitic shear zones that affect the large gabbroic massifs at the base of this sheet has been linked to this event ( Abati et al., 2007 ). In this intermediate-pressure upper unit, it is often dif fi cult to recognize the signi fi cance of the tectonothermal evolution subsequent to the development of the dominant regional fabrics, including events associated with the accretion of the arc to Laurussia and those that came later, during the Variscan. As a result, these uppermost units afford an excellent opportunity to investigate the role of the early dynamics of the peri-Gondwanan magmatic arc. The metagreywacke series, which occupies the uppermost structural levels, is particularly well exposed on the coastline, where it outcrops a schistose formation with frequent recumbent folds, intruded by a swarm of diabasic dykes that, in most cases, clearly cut the regional fabrics. The features and internal structure of this metagreywacke formation were fi rst described by Matte and Capdevila (1978) , who identi fi ed the presence of large recumbent folds. These folds make it dif fi cult to create detailed stratigraphic columns of the greywacke formation or estimate its true thickness. 3. Lithological and sedimentological features of the top greywacke series The uppermost series in the intermediate-pressure upper units occupies the central part of the Órdenes Complex ( Fig. 3 ) and is exclusively composed of metasediments intruded by minor gabbroic and diabasic dykes. Intense polyphase deformation precludes precise determination of thickness, but it is roughly estimated to be a maximum of 3 km ( Fig. 4 ). The lower half of this series contains a strong penetrative crenulation cleavage (S 2 ), with syntectonic garnet and biotite porphyroblasts, the paragenesis of which culminates in the Fig. 4. Sketch showing the geology of the top metagreywackes and related metasediments in the Ares region. A general columnar section of these metasediments is shown, together with the distribution of the most important diabase dykes, the position of the samples dated by U – Pb (detrital zircons in two greywackes; Fernández-Suárez et al., 2003 ) and the location of the sections sampled for whole rock geochemistry of the metagreywackes. The age diagram used to deduce the maximum sedimentary age of the greywackes is also included (U – Pb ages for the younger group of detrital zircons). 342 J.M. Fuenlabrada et al. / Gondwana Research 17 (2010) 338 – 351