M. Francisco Pereira

Variscan intra-orogenic extensional tectonics in the Ossa-Morena Zone (Évora-Aracena-Lora del Rı́o metamorphic belt, SW Iberian Massif): SHRIMP zircon U-Th-Pb geochronology

Abstract Following a Middle–Late Devonian (c. 390–360 Ma) phase of crustal shortening and mountain building, continental extension and onset of high-medium-grade metamorphic terrains occurred in the SW Iberian Massif during the Visean (c. 345–326 Ma). The Évora–Aracena–Lora del Rı́o metamorphic belt extends along the Ossa–Morena Zone southern margin from south Portugal through the south of Spain, a distance of 250 km. This major structural domain is characterized by local development of high-temperature–low-pressure metamorphism (c. 345–335 Ma) that reached high amphibolite to granulite facies. These high-medium-grade metamorphic terrains consist of strongly sheared Ediacaran and Cambrian–early Ordovician (c. 600–480 Ma) protoliths. The dominant structure is a widespread steeply-dipping foliation with a gently-plunging stretching lineation generally oriented parallel to the fold axes. Despite of the wrench nature of this collisional orogen, kinematic indicators of left-lateral shearing are locally compatible with an oblique component of extension. These extensional transcurrent movements associated with pervasive mylonitic foliation (c. 345–335 Ma) explain the exhumation of scarce occurrences of eclogites (c. 370 Ma). Mafic-intermediate plutonic and hypabyssal rocks (c. 355–320 Ma), mainly I-type high-K calc-alkaline diorites, tonalites, granodiorites, gabbros and peraluminous biotite granites, are associated with these metamorphic terrains. Volcanic rocks of the same chemical composition and age are preserved in Tournaisian–Visean (c. 350–335 Ma) marine basins dominated by detrital sequences with local development of syn-sedimentary gravitational collapse structures. This study, supported by new U–Pb zircon dating, demonstrates the importance of intra-orogenic transtension in the Gondwana margin during the Early Carboniferous when the Rheic ocean between Laurussia and Gondwana closed, forming the Appalachian and Variscan mountains.

Peralkaline and alkaline magmatism of the Ossa-Morena zone (SW Iberia): Age, source, and implications for the Paleozoic evolution of Gondwanan lithosphere

The Ossa-Morena zone in SW Iberia represents a section of the northern margin of West Gondwana that formed part of a Cordilleran-type orogenic system during the Neoproterozoic (Cadomian orogeny). The crustal section in this zone preserves the record of rifting that led to the opening of the Rheic Ocean in the early Paleozoic and the collision of Gondwana and Laurussia in the late Paleozoic (Variscan orogeny). We present U-Pb zircon data from three alkaline to peralkaline syenites that intruded Neoproterozoic and Cambrian strata and give crystallization ages ranging between ca. 490 Ma and 470 Ma. Lu/Hf isotopic data from these zircons give positive initial e Hf values (0 ≤ e Hf(t) ≤ +11.5) that approach the model values for the depleted mantle at the time of crystallization. This suggests that a significant proportion of the magma was derived from the mantle, with limited mixing/assimilation with crustal-derived melts. Alkaline/peralkaline magmatic suites of similar age and chemical composition intruded other sections of the northern margin of West Gondwana and along the boundaries of the continental blocks that today make up Iberia. These blocks are further characterized by the presence of high-pressure metamorphic belts that formed during accretion and subsequent collision of peri-Gondwanan domains against Laurussia during the Devonian and Carboniferous (Variscan orogeny). Our U-Pb and Lu-Hf data set indicates that during the Cambrian–Ordovician transition, lithosphere extension reached a stage of narrow intracontinental rifting, where deeply sourced magmas, probably coming from the lower crust and/or the upper mantle, intruded continental upper crust across various sections of previously stretched crust. We propose that necking of the Gondwana lithosphere into several continental microblocks with fertile mantle beneath them compartmentalized extension (multiblock model), which favored the onset of early Paleozoic peralkaline and alkaline magmas. The boundaries of microblocks represent zones of inherited crustal weakness that were later reactivated during the late Paleozoic as major accretionary faults related to the amalgamation of Pangea during the Variscan orogeny. Our dynamic model provides an explanation for the unusual spatial relationship between peralkaline and alkaline igneous provinces (usually shallow in the crust) and the occurrence of high-pressure rocks. Our observations suggest that Cordilleran-type orogens subjected to extension after long-lived subduction can develop wide continental platforms that feature multiple continental blocks. In addition, the formation of sequenced high-pressure belts in collisional orogens can be explained as the ultimate consequence of multiple necking events within continental lithosphere during previous collapse of a Cordilleran-type orogen.

Zircon geochronology of intrusive rocks from Cap de Creus, Eastern Pyrenees

New petrological and U–Pb zircon geochronological information has been obtained from intrusive plutonic rocks and migmatites from the Cap de Creus massif (Eastern Pyrenees) in order to constrain the timing of the thermal and tectonic evolution of this northeasternmost segment of Iberia during late Palaeozoic time. Zircons from a deformed syntectonic quartz diorite from the northern Cap de Creus Tudela migmatitic complex yield a mean age of 298.8±3.8 Ma. A syntectonic granodiorite from the Roses pluton in the southern area of lowest metamorphic grade of the massif has been dated at 290.8±2.9 Ma. All the analysed zircons from two samples of migmatitic rocks yield inherited ages from the Precambrian metasedimentary protolith (with two main age clusters at c . 730–542 Ma and c . 2.9–2.2 Ga). However, field structural relationships indicate that migmatization occurred synchronously with the emplacement of the quartz dioritic magmas at c . 299 Ma. Thus, the results of this study suggest that subduction-related calc-alkaline magmatic activity in the Cap de Creus was coeval and coupled with D 2 dextral transpression involving NNW–SSE crustal shortening during Late Carboniferous – Early Permian time ( c . 299–291 Ma). Since these age determinations are within the range of those obtained for undeformed (or slightly deformed) calc-alkaline igneous rocks from NE Iberia, it follows that the Cap de Creus massif would represent a zone of intense localization of D 2 transpression and subsequent D 3 ductile wrenching that extended into the Lower Permian during a transitional stage between the Variscan and Cimmerian cycles.

Potential sources of Ediacaran strata of Iberia: a review

Advances in stratigraphy, geochemistry and U-Pb detrital zircon geochronology from Ediacaran strata of Iberia allow for the improved characterisation of crustal growth in the North Gondwana active margin. The formation of Cadomian magmatic arcs and associated back-arc basins that took place in the North Gondwana active margin was a long-term process. Iberia has been placed in the Cadomian belt in currently accepted palaeogeography reconstructions at c. 570–560 Ma, based on the characteristics of Ediacaran strata. The Ediacaran strata of Iberia with outstanding geochemical homogeneity are distributed across three zones of Iberia: (1) Narcea Slates in the Cantabrian and West Asturian Leonese zones (maximum depositional age of c. 600 and 553 Ma); (2) Schist–Greywacke Complex (Lower Series) in the Central Iberian Zone (maximum depositional age of c. 578 to 550 Ma); and (3) Série Negra in the Ossa-Morena Zone (maximum depositional age of c. 590–545 Ma). Pre-Cryogenian detrital zircons found in the Ediacaran strata of Iberia seem to be related to distal sources distributed across three main areas of North Gondwana inland. The oldest detrital zircons probably derive from distal sources such as the West African craton, the Trans-Saharan belt and the Arabian–Nubian Shield, in view of the increase in distance from sedimentary basins. The West African craton is the most likely source for Archean and Palaeoproterozoic detrital zircons, while the Trans-Saharan belt and the Arabian–Nubian shield could provide a source for Tonian and Mesoproterozoic grains. The youngest zircon ages (c. 630–545 Ma), which make up the dominant population in the Ediacaran strata of Iberia, are probably derived from proximal sources as would be the Cadomian magmatic arc system, not excluding the contribution of the Pan-African orogen.

Development of local orthorhombic fabrics within a simple-shear dominated sinistral transpression zone: the Arronches sheared gneisses (Iberian Massif, Portugal)

Abstract The Coimbra-Cordoba shear zone (Iberian Massif), characterized by simple-shear dominated sinistral transpression, exposes several outcrops of strongly sheared peralkaline gneisses surrounded by mica schists and amphibolites. These gneisses are included in the Arronches Tectonic Unit, a thick unit of mylonitic rocks with a steep foliation and an associated gently plunging stretching lineation parallel to the fold axes. Strain partitioning is testified by widely spaced anastomosing shear bands around less-strained domains and by the existence of different shearing domains ranging from relatively ‘less-strained’ and coarse-grained mylonites to highly strained and fine-grained ultramylonites. Three shearing domains defined by textural and structural changes resulted from progressive deformation and increasing strain, which leads to increased mylonitization of gneisses. This is revealed by the increased modal percentage of the matrix and the decreased percentage of porphyroclasts, accompanied by evolution from orthorhombic to monoclinic fabrics: Conjugate Shearing Domain (CSD), Intermediate Sinistral Domain (ISD), and Sinistral Domain (SD). This contribution shows that in a simple-shear sinistral dominated transpression zone with a well-developed and widespread monoclinic fabric, it is possible to find mechanical conditions to produce local orthorhombic fabrics. In the Arronches gneisses a local strain regime exists in apparent contradiction with the bulk deformation regime.

A new model for the Hercynian Orogen of Gondwanan France and Iberia: discussion

The study by Shelley and BossieAre (2000) is an important contribution to the discussion concerning the Ibero±Armorican arc (IAA) generation model. This model comes as one of a sequence of previous ideas already published in several papers (Bard, 1971; Matte and Ribeiro, 1975; Lefort and Ribeiro, 1980; Ribeiro et al., 1980; Burg et al., 1981; Brun and Burg, 1982; Julivert, 1987; Ribeiro et al., 1990; Dias and Ribeiro, 1995; Ribeiro et al., 1995; Silva, 1997). A common feature of the interpretations in these papers is the great importance attributed to two major transcurrent faults: the dextral Porto±Tomar shear zone (PTSZ) and the sinistral Tomar±Badajoz±Cordoba shear zone (BCSZ), to explain the extension of the Iberian structures into the Armorican Massif in the form of trace the arcuate shape of the IAA. The large-scale migration, rotation and accretion of microplates controlled by oblique interactions have been a matter of controversy since researchers applied different kinematic models to reconstruct such global tectonic movements. For example, Badham (1982) interpreted the Hercynides as the result of dextral interaction between Europe and Africa and considered the arcuate geometry of the IAA as a secondary structure. In contrast, Matte (1986) compared the present example of the Himalayas with the structures of the IAA in terms of an indentation of crustal fragments, and proposed the progressive development of the symmetrical outward lateral escape of blocks in France and Iberia. In this model, the indentor migration caused opposing strike-slip movements on either side represented by the major transcurrent faults of the Armorican Massif and the Iberian Massif. This geometry and kinematics were considered to be syngenetic with the formation of the arcuate structure of the IAA. However, the main geologic lines linking these western European Hercynian massifs through major shear zones are still questionable (e.g. Lefort, 1989). In this comment we believe that the regional approach regarding the transcurrent faults of Hercynian Iberia (BCSZ and PTSZ) cannot be considered to be well established. In the following text we will comment on signi®cant omissions concerning available work of the last decade and also focus our discussion on ®eld data and the structural interpretation of Hercynian Iberia (Portugal and Spain).

Comment on “Geodynamic evolution of the SW Europe Variscides” by António Ribeiro et al.

[1] Ribeiro et al. [2007] have presented a geodynamic view of the SW Iberia Variscides based on data from Portugal. Their treatment of already published data is commendable, and the knowledge gained will surely encourage the discussion of the SW Europe Variscides. However, in our opinion, Ribeiro et al.’s modeling and interpretation of the Ediacaran–Lower Ordovician ( 560–470 Ma) geodynamic evolution are of limited value. In this regard, they based their analysis of the Ossa-Morena Zone (OMZ) on assumptions which are contradicted by recent published data. Ribeiro et al. ignored recent progress in the OMZ Ediacaran-Ordovician stratigraphy and, as a consequence they misunderstood the structure of domains overprinted by strong Variscan (Carboniferous) deformation and metamorphism. [2] The first arguable point of Ribeiro et al.’s [2007] work is the possibility of occurrence of a likely Grenville inlier in the OMZ. Until now, no rocks older than the Ediacaran have ever been proven to exist in the OMZ (Portugal and Spain). The oldest dated rocks are Ediacaran felsic dykes with 623 ± 3Ma (U-Pb TIMS/zircon, Loma del Aire rhyolites [SanchezGarcia et al., 2007]). The most representative Ediacaran rocks constitute the Serie Negra succession [Eguiluz et al., 2000; Pereira et al., 2006b]. Regardless of the Cambrian and Carboniferous high-grade metamorphism with anatexis, zircons from the Ediacaran Serie Negra metasediments preserve older cores and metamorphic overgrowths [OrdonezCasado, 1998; Fernandez-Suarez et al., 2002; Chichorro, 2006;Chichorro et al., 2006;Pereira et al., 2008; Linnemann et al., 2008]. The obtained ages for the metamorphic overgrowths are mainly Carboniferous, but also Ediacaran, Cambrian and Ordovician. U-Pb SHRIMP and LA-ICP-MS dating on detrital zircons from the Serie Negra metasediments indicate maximum age of deposition ranging from 560 to 540Ma [Ordonez-Casado, 1998; Fernandez-Suarez et al., 2002; Pereira et al., 2006a, 2008; Linnemann et al., 2008]. A common feature of all investigated samples is an ‘‘age gap’’ between 1.7 and 1.0 Ga. This age gap with no sign of Grenvillian zircon forming events is a characteristic of a Cadomian/West African provenance and distinguishes Cadomia/West Africa from Amazonia and Baltica. These Ediacaran sediments represent the infill of basins related to dismantling of a Cadomian continental arc [Pereira and Chichorro, 2004; Pereira et al., 2006b, 2007; Linnemann et al., 2008]. Ribeiro et al.’s [2007] statement that the Ediacaran calc-alkaline plutons are spatially limited to the northern margin of the Coimbra-Cordoba shear zone is not correct. It was recognized as plutonism of Ediacaran age in the northern margin of this major Variscan shear zone with 580 Ma (Pb-Pb Kober/zircon, Aljucen granodiorite [Talavera et al., 2008]), 573 ± 14 Ma (U-Pb SHRIMP/ zircon, Valle de la Serena porphyritic granitoid [OrdonezCasado, 1998]) and 555 Ma (Sm-Nd/garnet, MeridaMontoro gabbro-diorite [Bandres et al., 2002]) but also toward the south with 552 ± 10 Ma (U-Pb SHRIMP/zircon; Ahillones granite [Ordonez-Casado, 1998]). The Cadomian back-arc basin was active longer, at least until 545Ma. The final magmatic pulse of the Cadomian magmatic arc at 550 Ma (coeval with the crystallization of the Ahillones granite) is documented by newU-Pb LA-ICP-MS zircon data [Pereira et al., 2006a; Linnemann et al., 2008]. Closure of the Cadomian back-arc basin and final events of arc-continent collision in the OMZ occurred probably between 545 Ma and the overall onset of Cambrian intracontinental riftrelated plutonism at 530 Ma [Sanchez-Garcia et al., 2003, 2008]. [3] The second debatable point is the assertion of Ribeiro et al. [2007] that continental rifting on the Cambrian platform of northern Gondwana started around the Middle Cambrian. New zircon U-Pb zircon dating indicates an early igneous event with calc-alkaline signature and peraluminous tendency in the OMZ during the Lower Cambrian ( 530–515 Ma) associated with rift-related carbonate and siliciclastic deposition: 532 ± 4 Ma (U-Pb TIMS/zircon, Mina Afortunata leucogranite [Sanchez-Garcia et al., 2008]), 530 ± 3 Ma (U-Pb SHRIMP/zircon, Bodonal porphyroid [Romeo et al., 2006]) and 529–527Ma (U-Pb SHRIMP/zircon, Alcacovas and Santiago do Escoural orthogneisses [Chichorro, 2006; Chichorro et al., 2008]). This igneous event that seems to represent last residual melts of high-temperature, zirconundersaturated mafic magmas later affected by crustal contamination, while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids, is interpreted as the result of an extensional tectonic process accompanied by strong thermal rise [Sanchez-Garcia et al., 2003, 2008; Chichorro et al., 2008]. This situation was probably connected to an underlying mantle plume or simply TECTONICS, VOL. 28, TC4009, doi:10.1029/2008TC002430, 2009 Click Here for Full Article

The role of strain localization in magma injection into a transtensional shear zone (Variscan belt, SW Iberia)

This study deals with the interaction between deformation and magmatism in mid- to deep-crustal domains. The relation is analysed between migmatites and shear zones and the spatial distribution of leucogranitoid veins and dykes running through a footwall migmatite system, and reaching a transtensional shear zone operated under amphibolite- to greenschist-facies metamorphic conditions (Boa Fé shear zone, Variscan belt, SW Iberia). Statistical results show that the frequency of width and spacing of the leucogranitoid dykes conform to power-law distributions comparable with observations in volcanic systems. The fractal geometry of the distribution of leucogranitoid dykes highlights the development of a dense framework of thinner weakly or non-mineralized veins and dykes formed at higher nucleation/growth ratios in the footwall migmatite system that contrasts with the emplacement of thicker dykes associated with strongly mineralized thinner veins within the shear zone. The volume of injected leucogranitoid dykes in the shear zone is lower as compared with the footwall and is comparable with an expanding footwall shear zone with non-coaxial flow and volume increase. The Boa Fé shear zone seems to form a physical barrier to the transport of magma to the hanging wall.

Detrital provenance of the Upper Triassic siliciclastic rocks from southwest Iberia: a review

BackgroundIn southwestern Iberia the Upper Triassic successions of Lusitanian, Alentejo and Algarve basins records the fragmentation of Pangaea in Permian–Triassic during which the paleogeography of Iberia was dominated by a series of coalescing, alluvial–deltaic wedges and axial braided rivers.PurposeIn this study, we discuss the potential sediment sources of the Lusitanian, Alentejo and Algarve basins based on detrital zircon-age spectra, suggesting that Iberia occupied a central position Iberia in Pangaea during late Triassic.MethodsConventional sedimentary petrography and paleocurrent measurements of previous works was combined with recently published detrital zircon U–Pb geochronology of the Upper Triassic siliciclastic rocks of southwest Iberia to shed light on the detrital provenance record.ResultsZircon age populations found in the Upper Triassic strata of the Lusitanian, Alentejo and Algarve basins is dominated by Neoproterozoic (33–76%) and Paleoproterozoic (12–15%) grains. The most important differences are the dominance of Devonian–Carboniferous (33%) zircon in the Alentejo basin and the greater representativeness of Permian–Carboniferous (6%) zircon in the Lusitanian basin.ConclusionThe deposition in these Upper Triassic basins of Portugal is marked by variability in sedimentary sources, involving the denudation and local-scale directions of sediment transport from the Iberian basement with possible additional supplies derived from outside present-day Iberia. The Upper Triassic successions evolved separately with the detrital transport being probably controlled by local drainage systems, and occupying a central position in Pangaea just before the opening of the Central Atlantic Ocean.ResúmenEn esta revisión, combinamos la petrografía sedimentaria convencional y las mediciones de paleocorrentes obtenidas de trabajos previos de las rocas sedimentarias siliciclásticas del Triásico Superior del suroeste de Iberia, con datos de geocronología U-Pb de circón detrítico recientemente publicados por nosotros. El objetivo del trabajo es mejorar el conocimiento sobre la procedencia de las rocas sedimentarias siliciclásticas del Triásico Superior de las cuencas Lusitaniana, del Alentejo e del Algarve. En las poblaciones de circón detrítico de las cuencas del Triásico Superior del suroeste de Iberia predominan las edades del Neoproterozóico (33-76%) y Paleoproterozóico (12-15%). Las diferencias más importantes son: el predominio de granos del Devónico-Carbonífero (33%) en la cuenca del Alentejo y la mayor representatividad de granos del Permiano-Carbonífero (6%) en la cuenca Lusitaniana. La deposición en estas cuencas del Triásico Superior de Portugal se caracteriza por la variabilidad de las fuentes sedimentarias, que probablemente implicó la denudación y dirección local del transporte sedimentario desde el zócalo ibérico (zonas Centro Ibérica, de Ossa-Morena, Sul Portuguesa y do Pulo do Lobo) con posibles suministros adicionales procedentes de regiones actualmente localizadas fuera de Iberia (Terreno Meguma, Nueva Escocia). Las sucesiones del Triásico Superior evolucionaron por separado, siendo el transporte de sedimentos probablemente controlado por sistemas de drenaje locales.