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Tectónica extensional Varisca no Maciço de Évora (Zona de Ossa-

Morena): o corte geológico de Valverde

Variscan extensional tectonics in the Évora Massif (Ossa-Morena

Zone): the Valverde cross-section

Pereira, M. F.(1), Chichorro, M.(1), Silva, J.B.(2) (1) Departamento de Geociências, Centro de Geofísica de Évora, Universidade de Évora, Apt.94, 7001-554 Évora, Portugal (2) Departamento de Geologia, Faculdade Ciências, Universidade de Lisboa, Portugal E-mail: mpereira@uevora.pt

SUMÁRIO Estudos recentes desenvolvidos no corte geológico de Valverde permitiram definir uma coluna estratigráfica aproximada, que é de forma aceitável, correlacionável com outras secções da Zona de Ossa-Morena, onde formações datadas do Câmbrico têm sido interpretadas com estando associadas a rifting. O metamorfismo e deformação registados nesta zona de cisalhamento intenso representam um exemplo do efeito de uma tectónica extensional Varisca no Maciço de Évora

Palavras-chave: Orogenia Varisca, tectónica extensional, metamorfismo de alto-grau a intermédio, Maciço de Évora, Zona de Ossa-Morena

SUMMARY Recent studies allowed the definition of a lithostratigraphic column in the Valverde cross-section, which is reasonable, correlated with other Cambrian age rocks from the Ossa-Morena Zone interpreted to be related with rifting events. The metamorphism and deformation recorded on this high strain shear zone represents an example of the effects of Variscan extensional tectonics on the Evora Massif.

Key-words: Variscan orogeny, extensional tectonics, high-medium grade metamorphism, Évora Massif, Ossa-Morena Zone

Introduction and Geological setting Extensional tectonics is presently recognized in the Variscan belt of Western Europe, as the lithospheric process responsible for the deposition of Carboniferous sediments. Evidences for such process are also found in the ductile crust by synmetamorphic deformation and contemporaneous emplacement of granitoids plutons. Located at the westernmost domains of the Ossa-Morena Zone, as part of the so-called Montemor-Ficalho sector [1], the Évora Massif [2] is a wide area of the Alentejo region that extends along approximately 75 km from Montemor-o-Novo to Évora. Thirty-seven years ago the region of Évora was covered by a 1/50 000 scale geological map made by the Geological Survey of Portugal [3]. This previous

work revealed the presence of different pre-Cenozoic geological units. Here were distinguished: Hercynian igneous rocks (granitoids, gabbros and diorites), Cambrian and older crystallophyllian series (mica schists, green schists, amphybolites, quartzites, marbles and leptinites), Granitoid gnaisses and migmatites (with a wide variety of textures and with granodioritic, quatzdioritic and granitic composition) and Hornfels (with calc-silicate, amphibolitic and pelitic nature) [e.g., 1]. The obtained information led to the recognition that the crystallophyllian series were affected by deformation and regional metamorphism before migmatization and emplacement of the undeformed granitoids (ca. 304 Ma, Rb-Sr/biotite; from the quartzdiorite of Évora [3]). Migmatization was interpreted to be linked with the first events of the Hercynian granitization and also contemporaneous with two deformation events (ca. 323 Ma, Rb-

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Sr/biotite; from a gneiss sampled in the Montemor-o-Novo – Arraiolos road [3]). Hercynian granitoids and the migmatization were both responsible for the occurrence of the mapped hornfels. This geological map of Évora represents, nearby Valverde (6 km towards the west of Évora) a N-S trending boundary between Granitoid gneisses and migmatites and Hornfels, which is cut by Late-Hercynian granitoids [3]. Recent, detailed mapping developed during the last four years in the Herdade da Mitra (Valverde), during the field geology practice lectures of the University of Évora (Geological Engineering and Biology-Geology graduate courses), helped to improve the knowledge of this area of the Évora Massif. The first schematic cross-section for this area of Valverde and geochemical studies made on amphibolites were presented recently by [4-5]. This works attempts to summarize the available data by focusing on the stratigraphy of the Cambrian that have experienced an important Carboniferous extensional tectonics, under high-grade metamorphic conditions (transition amphibolite facies - migmatization). The Valverde cross-section Along this cross-section, with approximately 500 m, it is possible to map an irregular boundary between undeformed coarse-grained granitoids and older sedimentary and igneous rocks affected by penetrative deformation and metamorphosed to the amphibolite facies - migmatization. This metamorphic basement comprises from West towards the East, migmatites, felsic gneisses and granitoids, mica schists with calc-silicate rocks and banded, fine-grained and coarse-grained amphibolites. Although the rocks are strongly deformed the map-scale units do not appear to have been changed and an approximate stratigraphic column could be constructed (Fig.1). The structure displayed by the metamorphic rocks is characterized by a well-developed steeply dipping to subvertical foliation linked to a dip-slip stretching lineation (unusually in the Évora Massif, where the sense of movement is generally slightly dipping; [4]). The gneisses, migmatitic gneisses and associated granitoids From West towards the East, it is possible to observe that the migmatitic gneisses and associated granitoids gave progressively place to orthogneisses with centimetre- to m-scale veins of granitoids. These rocks are very similar to the calc-alkaline gneisses (leptinites s.l.) which were studied in the Santiago do Escoural cross-section [6].

Figure 1- Simplified lithostratigraphic column for the Valverde cross-section, showing the distribution of deformation and metamorphism. U-Pb SHRIMP results on zircons gave a protolith age of 505±5 Ma (Cambrian, following the ISC [7]) [8]. Based on zircon metamorphic overgrowths it is possible to suggest a long-lived thermal from Tournaisian to Visean. However, newly growth monazite gave a precise slightly younger Carboniferous age (322±6 Ma, Serpukhovian, following the ISC [7]) for the high-grade metamorphism which affected these rocks [8]. The protholit of the migmatitic gneisses are partially melted quartz-feldspatic igneous rocks. The limit between the migmatitic gneisses, associated granitoids and felsic gneisses is not always clear, but when it is seen, it is transitional. Nebulitic, sheeted and foliated structural varieties locally with melt veins and few and very stretched mafic rock fragments characterize these sheared high-grade rocks. In migmatitic gneisses biotite, feldspar and quartz are nearly equidimensional and oriented more or less randomly or formed a gneissosity (conspicuous by the presence of schlieren structures and elongated and isolated rock fragments). Gneisses are strongly foliated and lineated rocks with granoblastic texture defined by the preferred orientation of biotite, quartz and feldspar crystals. Melt veins observed in migmatitic gneisses and gneisses tend to be parallel or discordant to foliation. The foliation trending is generally uniform but admits slightly sinuous deflections in two perpendicular directions. C’-type extensional shear planes also developed, which created locally dilatant surfaces filled with melt. These structures indicate

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sense of shear with downward movement of the eastern block (Fig.2).

Figure 2- Structural sketch of the Valverde cross-section (not to scale; see Fig.1 for legend). Lower-hemisphere equal-area plot of the main mesoscale structures. Detail of a c’-type extensional shear planes filled with melt and boudinaged more competent layers.

The mica schists with calc-silicate rocks and amphibolites Biotite-feldspar-rich schists that represent immature sediments with calc-alkaline signature (similar to the Ediacaran-Early Cambrian basin sediments; [6]), dominate the eastern part of the Valverde cross-section. Their foliation and lineation are marked by biotite and stretched quartz. Mica schists include meter- to-hundred m-long and meter-width intercalations of epidote-garnet-rich calc-silicate rocks. These rocks show a well developed foliation formed by dynamically recrystallized calcite, epidote and growth of garnet and vesuvianite. Millimeter- to cm-width carbonate-garnet-epidote-rich layers occur associated with thin layers of biotite-amphibole-rich schists within banded amphibolites, which display a VAB signature [5]. These fine-grained amphibolites display a well-developed foliation mainly marked by the elongated prismatic amphiboles and ribbons of quartz and plagioclase. At mesoscale, it is possible to observe that they are affected by boudinage in two perpendicular directions, one of them parallel to the dip-slip stretching lineation. Associated with these mica schists also occur coarse-grained amphibolites and fine-grained dark amphibolites with N-MORB signature [5]. The described amphibolites form meter- to-hundred m-long and meter-width elongated outcrops surrounded by mica schists. Ribbons of dynamically recrystallized fine-grained plagioclase and hornblende which surrounds elongated amphibole prismatic blasts and sigmoidal-shape aggregates (sometimes with pyroxene) indicate that these rocks were deformed under medium to high metamorphic grade. The stretching lineation is defined by plagioclase ribbons and elongated amphibole blasts and aggregates. Several microstructures as asymmetric mantle-porphyroclasts, subgrain structures, S-C structures and c’-type extensional cleavage consistently indicate the downward movement of the eastern block. At mesoscale, centimeter- to m-long and centimeter-width intercalations of fine-grained amphibolites, with E-MORB signature [6], also appear within the mica schists. They are often affected by boudinage in two perpendicular directions (one of them parallel to the dip-slip stretching lineation) and are locally folded. Discussion Cambrian rift-related sequence The approximate lithostratigraphic column defined for Valverde is reasonable correlated with other Cambrian age rocks from the Ossa-Morena Zone that have been interpreted to be related with rifting events [9]. The U-Pb SHRIMP zircon age (505±5 Ma) obtained for the igneous protolith confirms the

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existence of an important magmatic event probably associated with rifting during the Cambrian. The VAB geochemistry displayed by the banded amphibolites may reflect the contaminated character of these rocks (para-amphibolites/reworked tuffs?) that include very thin layers of biotite- and/or amphibole-rich mica schists. This suggests a close-temporal relation between basic magmatism and sedimentation. The MORB-type amphibolites could represent intrusions on a volcanic-sedimentary sequence dominated by immature siliciclastic sediments with calc-silicate intercalations. They are also well-represented in the Arraiolos and Barragem do Caia [4]. Carboniferous (Visean) extensional tectonics developed under high-medium grade metamorphic conditions The field relations and textural analysis of deformed rocks are consistent with generation of in situ partial melting during low-pressure amphibolite facies - migmatization conditions, synchronous with extensional shearing. The prograde path towards the West is inferred by the increasing abundance of melt veins (foliation-parallel or discordant), increasing grain-size and decreasing gneissosity. Deformation acting throughout grain growth characterizes the strongly foliated and lineated rocks. Amphiboles and plagioclase (amphibolites), biotite and quartz (mica schists) and quartz, biotite and feldspar (migmatitic gneisses and felsic gneisses) are oriented parallel to foliation and lineation. Several meso- to micro-scale kinematic indicators consistently indicate the downward movement of the eastern block. The U-Pb SHRIMP monazite age data confirms that the geological characteristics observed in the Valverde Cambrian deformed rocks have resulted from a Visean (322±6 Ma) high thermal gradient and a strong thinning caused by syn-metamorphic shearing. We conclude that the metamorphism and deformation recorded in Valverde define an extensional high strain shear zone formed in the context of the Variscan orogeny.

Acknowledgements This work is a contribution for the IGCP Project 497 and CGL2004-068068-C04-02/BTE. References [1] Oliveira, J.T., Oliveira, V., Piçarra, J.M., 1991, Traços gerais da evolução tectonoestratigrafica da Zona de Ossa Morena em Portugal, Cuadernos Laboratorio Xeologico Laxe, Coruña 16, p. 221-250 [2] Carvalhosa, A., 1983. Esquema geológico do

Maciço de Évora. Comunicações dos Serviços Geológicos de Portugal 69, 201-208. [3] Carvalhosa, A., Galopim de Carvalho, A.M., Matos Alves, C. A., Pina, H.L., 1969, Carta Geológica de Portugal, Noticia Explicativa da Folha 40-A (Évora), Serviços Geológicos de Portugal, scale 1:50 000. [4] Pereira, M.F., Silva, J.B., Chichorro, M., 2003. Internal structure of the Évora Massif: The Évora High-grade metamorphic terrains and the Montemor-o-Novo Shear Zone (Ossa-Morena Zone, Portugal). Geogaceta 33, 71-74. [5] Pereira, M.F., Chichorro, M., Santos, J.F., Moita, P., Silva, J.B., 2004, Geochemistry of lower Paleozoic anorogenic basic rocks from the Évora Massif (Western Ossa-Morena Zone, Portugal), Geogaceta 35, p. 87-91. [6] Pereira, M.F., Chichorro, M., Linnemann, U., Eguiluz, L., Silva, J.B., 2006, Inherited arc signature in Ediacaran and Early Cambrian basins of the Ossa-Morena Zone (Iberian Massif, Portugal): paleographic link with European and North African Cadomian correlatives. Precambrian Research 144: 297-315 [7] Gradstein, F.M., Ogg, J.G., Smith, A.G., Bleeker, W., Lourens, L.J., 2004. A new Geological Time Scale with special reference to Precambrian and Neogene. Episodes 27 82), 83-100 [8] Chichorro et al., in prep. [9] Sanchez-Garcia, T., Bellindo, F., Quesada, C., 2003. Geodynamic setting and geochemical signatures of Cambrian-Ordovician rift-related igneous rocks (Ossa-Morena Zone, SW Ibéria). Tectonophysics 365: 233-255.