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El complejo esquisto‐grauwáckico y el paleozoico en el centro‐oeste español
... This complex is surrounded by several granite plutons with contacts ranging from concordant to intrusive (Fig. 2). The Schist–Greywacke Complex (SGC) has been documented (e.g., Díez Balda 1980, Rodríguez Alonso 1985, Beetsma 1995) in the northwestern part of the Iberian Central System. It comprises Neoproterozoic – Lower Cambrian metasedimentary rocks considered as a possible protolith of the Peña Negra complex, as well as of many other granitic complexes in central Iberia (Ramírez & Grundvig 2000). ...
... All are unconformably overlain by Ordovician–Silurian metasedimentary rocks (Díez Balda 1980, Nozal & Robles 1988). Two different main units, described in the SGC (Rodríguez Alonso 1985), can also be distinguished throughout the Central Iberian Zone. The Lower Unit is lithologically monotonous and occupies the western part of the PNAC area (containing low-grade metamorphic assemblages), whereas the Upper Unit outcrops mainly toward the northwestern part and constitutes a varied lithological succession that includes a more intensely metamorphosed and tectonized nucleus (Martinamor– Castellanos Antiform) in which orthogneiss and granites intruded the pre-Ordovician formations. ...
... The Lower Unit is lithologically monotonous and occupies the western part of the PNAC area (containing low-grade metamorphic assemblages), whereas the Upper Unit outcrops mainly toward the northwestern part and constitutes a varied lithological succession that includes a more intensely metamorphosed and tectonized nucleus (Martinamor– Castellanos Antiform) in which orthogneiss and granites intruded the pre-Ordovician formations. The whole stratigraphic sequence is interpreted as representing continuous siliciclastic sedimentation with the development of a turbiditic facies, which evolved upward into a mixed siliciclastic rock – carbonate slope-platform sedimentation , within an area of relative tectonic instability associated with volcanic activity (Rodríguez Alonso 1985, Vidal et al. 1994, Rodríguez Alonso & Palacios 1994, Rodríguez Alonso & Alonso Gavilan 1995). The Lower Unit is generally devoid of marker beds and is composed of alternating sandstones and mudstones , with some intercalation of conglomerates and disordered levels. ...
Estudio petrológico del Complejo anatéctico de Peña Negra (Sierra de Gredos, España) A large anatectic complex of Hercynian age, the Peña Negra complex, crops out in the central-north area of the Sierra de Gredos, Avila batholith, central Spain. Migmatites and anatectic granitic rocks were generated during the Upper Carboniferous from a Neoproterozoic – Lower Cambrian schist–greywacke complex with minor interlayered orthogneisses. Granitic rocks appear as subhorizontal sheets of megacrystic granodiorite and as lenticular bodies of leucogranite. These formed by in situ anatexis and correspond to melt–restite segregations under two different conditions, namely high melt-fraction in the case of the granodiorite, and low melt-fraction in the case of the leucogranites. The leucogranites display large enclaves of sillimanite-rich material, representing the restitic fraction. Granodiorite was generated as consequence of crustal thickening under conditions of T ≈ 750 to 800°C and P ≈ 4 kbar, coinciding with the second phase of the Hercynian orogeny. After this event, an extensional episode involved smaller degrees of decompression-induced melting, and generated small batches of cordierite leucogranites from an already depleted source. Anatexis was enhanced, in both cases, by shear structures and by the existence of a fertile protolith.
... The uppermost parts of this sequence are lower Cambrian in age. This sedimentary succession has been subdivided into different units (Fig. 2b) by several authors, and the following are the Ediacaran parts of the succession: Lower Unit and the lower part of the Upper Unit (Rodríguez Alonso 1985; Rodríguez-Alonso et al. 2004); the lower part of the Monterrubio Formation (Díez Balda 1986); and units Int J Earth Sci (Geol Rundsch) 123 ...
... The Cambrian succession in the CIZ lays conformably over the Ediacaran deposits described above and is composed by the upper part of the SGC, the Tamames Sandstones and the Tamames Limestone (correlated with the Láncara Formation). The Cambrian rocks of the SGC have also been subdivided into different units (Fig. 2b ) by several authors as follows: Cambrian strata comprise the highest part of the Upper Unit (Rodríguez Alonso 1985; Rodríguez-Alonso et al. 2004); part of the Monterrubio and Aldeatejada formations (Díez Balda 1986); and units V to XII (Valladares et al. 1998Valladares et al. , 2000Valladares et al. , 2002 ) which are indicated inFig. 2b. ...
Ediacaran and Early Cambrian sedimentary rocks from NW Iberia have been investigated for detrital zircon U–Pb ages. A total of 1,161 concordant U–Pb ages were obtained in zircons separated from four Ediacaran samples (3 from the Cantabrian Zone and one from the Central Iberian zone) and two Lower Cambrian samples (one from the Cantabrian Zone and one from the Central Iberian Zone). Major and trace elements including REE and Sm–Nd isotopes were also analyzed on the same set of samples. The stratigraphically older Ediacaran sequence in the Cantabrian Zone has a maximum sedimentation age of ca. 600 Ma based on detrital zircon content and is intruded by ca. 590–580 Ma granitoids constraining the deposition of this part of the sequence between ca. 600 and 580 Ma. The stratigraphically younger Ediacaran sequence in the Cantabrian Zone has a maximum sedimentation age of ca. 553 Ma. The Ediacaran sample from the Central Iberian Zone has an identical within error maximum sedimentation age of ca. 555 Ma. The detrital zircon U–Pb age patterns are very similar in all the Ediacaran samples from both zones including the main age groups ca. 0.55–0.75 Ga, ca. 0.85–1.15 Ga and minor Paleoproterozoic (ca. 1.9–2.1 Ga) and Archean (ca. 2.4–2.6 Ga) populations. Kolmogorov–Smirnov statistical tests performed on this set of samples indicate that they all were derived from the same parent population (i.e., same source area). The same can be said on the basis of Nd isotopes, REE patterns and trace element concentrations. The two Cambrian samples, however, show contrasting signatures: The sample from the Cantabrian Zone lacks the ca. 0.85–1.15 Ga population and has a high proportion of Paleoproterozoic and Archean zircons (>60 %) and a more negative ε
Nd and higher T
DM values than the Ediacaran samples. The Early Cambrian sample from the Central Iberian Zone has the same U–Pb detrital zircon age distribution (based on KS tests) as all the Ediacaran samples but has a significantly more negative ε
Nd value. These data suggest apparently continuous sedimentation in the NW Iberian realm of northern Gondwana between ca. 600 and 550 Ma and changes in the detrital influx around the Ediacaran–Cambrian boundary. The nature and origin of these changes cannot be determined with available data, but they must involve tectonic activity on the margin as evidenced by the angular unconformity separating the Ediacaran and Lower Cambrian strata in the Cantabrian Zone. The absence of this unconformity and the apparent continuity of detrital zircon age distribution between Ediacaran and Cambrian rocks in the Central Iberian Zone suggest that the margin became segmented with significant transport and sedimentation flux changes in relatively short distances. As to the paleoposition of NW Iberia in Ediacaran–Early Cambrian times, comparison of the data presented herein with a wealth of relevant data from the literature both on the European peri-Gondwanan terranes and on the terranes of northern Africa suggests that NW Iberia may have lain closer to the present-day Egypt–Israel–Jordan area and that the potential source of the hitherto enigmatic Tonian–Stenian zircons could be traced to exposed segments of arc terranes such as that described in the Sinai Peninsula (Be’eri-Shlevin et al. in Geology 40:403–406, 2012).
... Two subunits have been classically distinguished in the Schist and Greywacke complex (Fig. 9a). The lower unit is mainly composed of lutites and sandstones, with minor contents of microconglomerates and volcanoclastic layers (Rodríguez Alonso, 1985;Álvarez Nava et al., 1988;San José et al., 1990;Valladares et al., 1998). The upper unit is predominantly pelitic, with subordinate amounts of sandstones, limestones and volcanic and volcanoclastic layers (Díez Balda, 1980;San José, 1983;Rodríguez Alonso, 1985;Álvarez Nava et al., 1988;Pieren, 2000). ...
... The lower unit is mainly composed of lutites and sandstones, with minor contents of microconglomerates and volcanoclastic layers (Rodríguez Alonso, 1985;Álvarez Nava et al., 1988;San José et al., 1990;Valladares et al., 1998). The upper unit is predominantly pelitic, with subordinate amounts of sandstones, limestones and volcanic and volcanoclastic layers (Díez Balda, 1980;San José, 1983;Rodríguez Alonso, 1985;Álvarez Nava et al., 1988;Pieren, 2000). Pelites and sandstones are geochemically homogeneous across the Central Iberian Zone (e.g., Valladares et al, 2002;Ugidos et al., 2003a). ...
One of the most intriguing characteristics of the northern (Iberia) and southern (Puna) Gondwana margins is the presence of large volumes of Late Cambrian–Early Ordovician magmatic rocks with ferrosilicic composition, i.e., rocks with high iron and silica contents (FeO > 4.0 wt.%, SiO > 63 wt.%) for very low contents in calcium (CaO < 1.5 wt.%). Geological and geochemical features, as well as experimental results, show that ferrosilicic magmas resulted from near-total melting (80–90%) of crustal sources of metagreywacke and charnockite affinities, possibly derived from Neoproterozoic volcanoclastic sediments and/or their granulite facies equivalents, under very high temperatures (1000 °C–1200 °C) and at pressures of 1.0 to 2.0 GPa. A plausible tectonic setting for this peculiar magmatism is a back-arc region subjected to extension, with the ferrosilicic magmas ascending from a deep cold diapir or mantle wedge plume. Rifting in the back-arc progressed until the aperture of an ocean basin (the Rheic ocean) in the northern margin of Gondwana, but became aborted in Argentina.
... Various interpretations have been given to the depositional environment of the Schist and Graywacke Complex, ranging from sedimentation on a passive margin (Ugidos et al., 1997(Ugidos et al., , 2003Valladares et al., 2000) to (Cadomian) syn-orogenic flysch on the footwall to the Cadomian suture with the OMZ, which is interpreted to lie along the ancestral Badajoz-Córdoba Shear Zone ( Fig. 1) (Quesada,1990a(Quesada, ,b, 1997. The Upper and Lower sequences both contain interbedded debris flows, glaciomarine diamictites (Pieren Pidal, 2000) and minor calc-alkaline volcanic rocks (Rodríguez Alonso, 1985;Rodríguez Alonso and Alonso Gavilán, 1995;Rodríguez Alonso et al., 2004). The Upper sequence reaches into the lowermost Cambrian and includes some carbonate and phosphate rocks (Ortega Gironés and González Lodeiro, 1986). ...
... The depositional age of the Schist and Graywacke Complex is interpreted to range from 600 to 540 Ma (Vidal et al., 1999). Both sequences are characterized by conglomerates that contain black chert cobbles which are thought to be derived from the Serie Negra metasedimentary succession of the OMZ (Rodríguez Alonso, 1985;Pieren Pidal, 2000). If correct, this interpretation implies a geographic linkage of the OMZ and CIZ at least since the Ediacaran. ...
Sm–Nd isotopic analyses of Palaeozoic sedimentary and igneous rocks in the southwest Iberian Massif (western end of the European Variscan Belt) are presented in order to unravel its complex poly-orogenic evolution during the closure of the Rheic Ocean and the amalgamation of Pangea. The Gondwanan margin in southwest Iberia SW Iberia is subdivided into the Ossa Morena and Central Iberian zones, separated by the Badajoz–Córdoba Shear Zone which represents a cryptic suture zone between these terranes. The relationships between these terranes, and between units preserved within the suture zone (e.g. the Sierra Albarrana Group) during the Palaeozoic and Neoproterozoic are controversial.Sm–Nd isotopic studies of representative sedimentary sequences covering the entire pre-Variscan record of the Ossa Morena and Central Iberian zones show very similar characteristics from the uppermost Ediacaran onwards. These data indicate that their accretion to one another must have been completed by the Late Neoproterozoic–Ediacarin that time (an event assigned to Cadomian orogeny) and that they never separated substantially from each other since that time. The Sm–Nd isotopic composition of the Sierra Albarrana Group metasedimentary rocks is similar to that of the pre-Cadomian sequences of the Ossa Morena Zone (Serie Negra), suggesting derivation from a common source. The common provenance of the Palaeozoic sequences in the two zones is identical to that of the pre-Cadomian Serie Negra of the Ossa Morena Zone, which in accordance with the data presented herein and published U–Pb zircon data indicates a West African affinity.
... Estas series metasedimentarias, constituidas fundamentalmente por pizarras y grauvacas, fueron denominadas Complejo Esquisto Grauváquico (CEG) en Portugal y área de Salamanca-Cáceres y más modernamente Alogrupo Domo Extremeño. Están afectadas por las fases de deformación pertenecientes a las orogenias Cadomiense y Hercínica (Gumiel y Campos, 2012) que van acompañadas de metamorfismo y plutonismo (Rodríguez Alonso, 1985) al que estarían ligados los batolitos de granitos alcalinos de Plasenzuela y Trujillo. ...
Geological characterization of the settlement of Villasviejas del Tamuja (Cáceres): A geological study in the settlement of Villaviejas del Tamuja is carried out with the aim of providing the knowledge and characteristics of the geological basemente. By electrical tomography, from the different profiles at different depths, it has been possible to establish the boundary between the archaeological and geological layer and to define the geometry of the original slate substrate. In depth, a possible granite intrusion has been identified under the slates that would have generated the bulge and also a contact aureole that would mark the compositional discordance. With these data, the geological knowledge of the study area is covered both in surface and in depth, enabling future research. Palabras clave: Geología, tomografía eléctrica, arqueología, Villasviejas del Tamuja
... Torrowangea reportados mas não figurados por Martín Herrero et al. (1990). Rodríguez Alonso (1986, Fig. 3) figura icnofósseis encontrados perto do rio Azaba que necessitam de um exame mais detalhado para identificação. SL -San Lorenzo de Calatrava, pequenos fósseis de concha (Anabarella), Vidal et al. (1999). ...
The record of late Ediacaran and lower Cambrian trace fossils in the Central Iberian Zone, Iberian Peninsula, is reviewed. The evolution in trace fossil morphologies is comparable to that documented worldwide, with domination of simple trace fossils such as Gordia and Helminthoidichnites in late Ediacaran rocks and the appearance of more complex trace fossil morphologies in Cambrian rocks, including Treptichnus pedum, Rusophycus and Psammichnites. There are possible latest Ediacaran treptichnids (but not Treptichnus pedum), although the precise placement of the Ediacaran-Cambrian boundary in the Central Iberian Zone on the basis of trace fossils is complicated by a general upward-shallowing trend that may confound environmental and evolutionary signals. Resumo: O registo de icnofósseis do Ediacariano superior e do Câmbrico inferior da Zona Centro-Ibérica é revisto. A evolução dos padrões icnológicos é comparável com aquela documentada um pouco por todo o mundo, com o domínio de icnofósseis simples tais como Gordia e Helminthoidichnites nas rochas do Ediacariano superior e o aparecimento de formas mais complexas nas rochas câmbricas, incluindo Treptichnus pedum, Rusophycus e Psammichnites. Existem possivelmente treptichnídeos ediacarianos (mas não Treptichnus pedum), embora o posicionamento preciso do limite Ediacariano-Câmbrico na Zona Centro-Ibérica, tendo como base os icnofósseis, é complicada pela tendência para a prevalência de fácies marinhas progressivamente menos profundas para o topo da sequência, o que poderá sobrepor sinais ambientais àqueles de significado evolutivo.
... The current position of the orthogneisses intrusive into the N-CIZ metasediments could be indicative of transport and burial (subduction) of the SGC below the N-CIZ sequences (Fig. 12). The presence of pre-Variscan folding in the SGC, indicated by an angular discordance with the later Paleozoic sedimentary sequences (e.g., Rodríguez Alonso, 1985), is consistent with an orogenic setting. This angular discordance between Neoproterozoic to Early Cambrian rocks and lowermost Ordovician strata has long been known (e.g., Schmidt-Thomé, 1945), and recent geochronological data pointing to a maximum sedimentation age of 550 to 541 Ma for the uppermost Neoproterozoic sequences (Talavera et al., 2013;Orejana et al., 2015) support a late Early to Middle Cambrian age for this crustal thickening and the complex succession of tectonic, metamorphic and magmatic events that occurred in the CIZ at that time (Fig. 13). ...
Metagranitic orthogneisses are abundant in the Central Iberian Zone (CIZ). This felsic magmatism has a highly peraluminous composition (A/CNK = 1.07–1.62) defining a typical S-type granite character, common in crustal thickening environments. The studied Spanish Central System (SCS) orthogneisses yield Late Cambrian to Early Ordovician U–Pb zircon ages (496 to 481 Ma), overlapping with the available literature data (mostly from 477 to 500 Ma). These orthogneisses are intrusive into metasedimentary sequences from the northern CIZ that have been recently dated at about 536 Ma. Late Ediacaran inherited zircons are common in the SCS orthogneisses (10% to 75% of the total zircon population). Most inheritance ranges from Neoproterozoic to Late Mesoproterozoic in age (0.52 to 1.25 Ga) and shows marked positive ԐHft values (>+5). This long period of Proterozoic juvenile input is only recognized in the metasedimentary rocks of the Schist–Greywacke Complex, outcropping in the southern CIZ. The proposed linkage between the southern CIZ metasediments (as sources) and the studied orthogneisses is reinforced by their similar Nd isotopic signatures (from − 2.81 to − 4.95) and the highly peraluminous character of the orthogneisses. The intrusion of this felsic magmatism within the northern CIZ, having been generated by melting of the more distal southern CIZ metasediments, together with their recycled crustal origin, suggests crustal thickening of the northern Gondwana margin during a period of flat subduction. The orthogneisses define a large linear S-type magmatic belt cropping out for over 650 km from central Spain to Galicia. The ~ 35 Ma delay between sedimentation and granite intrusion is a typical time interval for crustal thickening models. This thickening stage evolved toward a passive margin setting, allowing the deposition of the siliciclastic Ordovician series which covered the previous terranes. Minor Floian-aged tholeiitic magmatism, giving rise to scarce metabasite outcrops in the SCS, probably postdates thickening and marks this tectonic change in central Iberia.
... The current position of the orthogneisses intrusive into the N-CIZ metasediments could be indicative of transport and burial (subduction) of the SCG below the N-CIZ sequences (Fig. 12). The presence of pre-Variscan folding in the SGC, indicated by an angular discordance with the later Paleozoic sedimentary sequences (e.g., Rodríguez Alonso, 1985), is consitent with an orogenic setting. This angular discordance between Neoproterozoic to Early Cambrian rocks and lowermost Ordovician strata has long been known (e.g, Schmidt-Thome, 1945), and recent geochronological data pointing to a maximum sedimentation age of 550 to 541 Ma for the uppermost Neoproterozoic sequences (Talavera et al., 2013;Orejana et al., 2015) support a late Early to Middle Cambrian age for this crustal thickening and the complex succession of tectonic, metamorphic and magmatic events that occurred in the CIZ at that time (Fig. 13). ...
... Although the geometry of the pre-Ordovician folds are often considered as open folds (e.g. Diez Balda, 1986), sometimes with box-fold geometry (Ribeiro, 1974), high angular relations (Ribeiro, 1974; Dias da Silva, 2014), or even reverse limbs situations (Rodríguez Alonso, 1985) have been found. Recent work in northern Portugal (Marão and Poiares sectors) shows the absence of the Iberian unconformity and that the strongly deformed zones induced by Toledanian deformation are restricted to narrow bands bounded by wide regions of negligible deformation. ...
One of the main features of the Central-Iberian Zone is the relation between the lower Ordovician and the Cambrian or Upper Proterozoic rocks (the "Sardic deformation"; Lotze, 1956) which is marked by two unconformities: the upper Iberian one between the Armorican Quartzite Formation and underlying early Ordovician units and the Toledanian unconformity between these units and pre-Ordovician basement (Gutiérrez-Marco et al., 2002). The latter recognition that the Sardic unconformity in Sardinia type locality have a younger intra-Ordovician age, led to consider that Toledanian unconformity designation should be used instead of Sardic one (Gutiérrez-Marco et al., 2002). Due to the long use of "Sardic deformation" expression in Iberian and to the possibility of diachronism between Iberia and Sardinia such expression continue to be used it in a sensu latu (Romão et al., 2005).
The genetical mechanisms for these unconformities are still highly debatable: general extension along the northern Gondwana passive margin (e.g. Dias da Silva, 2014), or a transient inversion event (Romão et al., 2005)? To choose between models is difficult because the pre-Ordovician structures are poorly known.
Although the geometry of the pre-Ordovician folds are often considered as open folds (e.g. Diez Balda, 1986),
sometimes with box-fold geometry (Ribeiro, 1974), high angular relations (Ribeiro, 1974; Dias da Silva, 2014), or even reverse limbs situations (Rodríguez Alonso, 1985) have been found. Recent work in northern Portugal (Marão and Poiares sectors) shows the absence of the Iberian unconformity and that the strongly deformed zones induced by Toledanian deformation are restricted to narrow bands bounded by wide regions of negligible deformation. Thus, most of the Toledanian unconformity appears as disconformities, with angular unconformities restricted to narrow bands: overlying the Toledanian unconformity a Lower Ordovician conglomerate with pebbles of pre-Ordovician rocks is pervasive. Previous regional geometry, mainly the common reverse limbs prior to the Variscan deformation, can't be explained only by rotation of blocks above normal faults as often considered. However it should be the expected relation in a situation of inversion tectonic. This compressive event could have been a transient inversion in the northern Gondwana Cambro-
Ordovician global extension (Romão et al., 2005). The Toledanian deformation increase towards south where the iberian unconformity is also strong.
... Constituye una alternancia métrica a decimétrica de lutitas y grauvacas, de tipo turbidítico, de varios miles de metros de espesor reconocido, y de la que no se conoce el muro. Esta unidad se corresponde con las Pizarras del Guadiana y el Alcudiense inferior indiferenciado (Herranz et al., 1977) (Rodríguez Alonso, 1985;Pieren, 2000). Se interpretan estos materiales como un depósito de zonas canalizadas y de talud al que llegan aportes procedentes de la denudación de los materiales precámbricos de la ZOM, como lo atestiguan los abundantes cantos exóticos de cuarcitas negras que acompañan a los cantos de cuarzo bien redondeados que han sido retrabajados claramente en medios continentales que se estaban erosionando y que con toda probabilidad estaban asociados la cadena Cadomiense situada en la ZOM, lo que explicaría la naturaleza de los aportes, y la distribución de los niveles conglomeráticos. ...
... Moreover, it should be noted that Ordovician to Silurian metasedimentary rocks were deposited unconformably over these Neoproterozoic domains, likely covering the original boundary. The major difference with the subdivision of Lotze (1945) is the inclusion of the Las Hurdes-Sierra de Gata massif (area a in Fig. 1) in the S-CIZ (micro)-terrane taking into consideration that the stratigraphy, petrology and geochemical data of these metasedimentary rocks indicate a SGC character (e.g., Rodríguez Alonso, 1985;Ugidos et al., 1997). This means that the Lotze's boundary should be shifted to the north ( Fig. 1). ...
... ; Garaa de figuerola, Franco & Castro, 1983;Ribeiro et d., 1983;Rodriguez Alonso, 1985; Dkz Balda, 1986; Ferreira et ul., 1987; Mpez-Plaza & Martinez Catalh, 1987) can be stated as follows. ...
A method is proposed for adjusting the mass balance to characterize quantitatively the behaviour of minerals in anatexis. The method is based on an unconstrained simple mixing model that can be expressed as:
image
where B , A 0 , and A 1‐ n , are compositional vectors of segregate, source rock and source minerals, respectively. The most important concepts are: (1) degree of partial fusion: F MM = 1/ a 0 ; (2) mineral fractionation index:
image
and (3) plagioclase differentiation index:
image .
For a given mineral, the MFI values have the following meaning: (a) MFI <0: residual phase originated, at least partly, as a product of incongruent melting; (b) 0 > MFI <1: preferential retention in the residue; (c) MFI = 1: identical modal fraction in source and melt; (d) a 0 > MFI > 1: preferential incorporation into the segregate, and (e) MFI > a 0 : external contribution to the anatectic system defined by a 0 A 0 . To test the method and illustrate its use, it was applied to two real problems of partial melting in the Peña Negra Anatectic Complex (Central Spain). The first is a very simple case of segregation of a diktyonitic neosome from an orthogneiss through partial melting located in vertical shear zones. This process is characterized by: (1) F MM = 0.51; (2) active incorporation of K‐feldspar, plagioclase and biotite into the segregate; (3) disequilibrium melting of plagioclase; (4) residual behaviour of quartz and ilmenite. The second case concerns the formation of a cordierite‐bearing granite from granodioritoid diatexites through an anatectic process, whose most salient characteristics are: (1) F MM = 0.45; (2) incongruent melting of biotite; (3) residual behaviour of plagioclase, which melted with a PDI of 1.22; (4) preferential incorporation of quartz into the segregate; (5) total extraction of K‐feldspar from the residue.
... From one orthogneiss and three high-grade metasediments, we separated zircons and studied them by cathodoluminescence imaging and single-grain stepwise evaporation 207 Pb/ 206 Pb analyses, checking a few grains by ion probe with the U-Pb method. We also used major-element data from another 173 samples of low-grade metasediments, most from the same region and others from all over central Iberia, collected from the literature (Rodríguez Alonso 1985; Nä gler 1990; Beetsma 1995; Nä gler et al. 1995; Tassinari et al. 1996; Ugidos et al. 1997a Ugidos et al. , 1997b); in addition to major-element data, trace-element data were reported for 80 of the samples, 57 had published Sr isotopes, and 40 had published Nd isotopes (tables 1, 2). ...
The Central Iberian Zone comprises low-grade metamorphic zones alternating with Variscan thermal domes appar-ently equilibrated at the same pressure. The Variscan thermal domes are composed of authochtonous to allochtonous granites and anatectic complexes that represent remnants of the granite source layer. The anatectic complexes consist of ∼90% high-grade metasediments and ∼10% Cadomian (Ma) felsic metaigneous rocks, the latter being 543 6 extremely rare in low-grade metamorphic areas. The high-grade metasediments form part of the Schist-Greywacke Complex, the dominant formation of central Iberia, but compared with equivalent rocks from low-grade metamorphic areas, they are more fertile (67% vs. 31% of haplogranitic component), have a higher heat production (2.7 vs. 1.5 mW m 3), are isotopically more juvenile (0.7108 vs. 0.7179 87 Sr/ 86 Sr 350 Ma ; 3.3 vs. 3.7 540 Ma [Nd]), and contain younger pre-Variscan zircons (m.yr. old). These differences reveal that the high-grade metasediments were derived 544 14 from ∼540-m.yr.-old felsic igneous rocks, whereas the low-grade metasediments were derived from recycling older sedimentary rocks. A model of 1D transient geotherms for the postcollisional crust of central Iberia reveals that, after 25–30 m.yr. of thermal maturation, the high-grade metasediments of the source layer can melt if they were buried in the middle crust. The model provides a sensible explanation for the most relevant features of Variscan magmatism in this area, in particular, (1) the ∼50-m.yr. time span recorded by the source layer zircons; (2) the lack of subduction-related magmas and the scarcity of mantle magmas; (3) the lack of almost any intrusive activity during the first ∼30 m.yr. after crustal thickening, followed by (4) the sudden formation of huge volumes of crustally derived granite magmas, which (5) happened simultaneously all over central Iberia and (6) marked the beginning of the gravitational collapse of the orogen. Therefore, we propose that the Variscan thermal domes of central Iberia occurred where the Paleozoic sequence of the Schist-Greywacke Complex derived from a Cadomian felsic igneous protolith because of the elevated heat production and fertility of the metasediments.
... In the Central Iberian Zone, the Lower Ordovician sediments consist of siliciclastic shallow marine shelf deposits "Armorican Quartzites" and associated metapelites (e.g., Julivert and Martínez, 1987;Oliveira et al., 1992;Díez Montes et al., 2004). The Lower Ordovician unconformably oversteps a Neoproterozoic-Early Cambrian detrital sequence, known as the "Schist-Greywacke Complex" (e.g., Teixeira, 1955;Rodríguez-Alonso, 1985;Vidal et al., 1994). This angular unconformity is associated with important thickness changes in the Lower Ordovician sequences that are interpreted to reflect the existence of paleo-reliefs or syn-sedimentary troughs triggered by extensional events (McDougall et al., 1987). ...
The Central Iberian–Ossa-Morena transition zone (SW Iberian Massif) represents a segment of the northern Gondwana margin with a long geodynamic evolution, characterized by the superposition of Cadomian and Variscan events. The Early Ordovician is mainly represented by porphyritic felsic volcaniclastic rocks (the Urra Formation) that pass up into a siliciclastic sediments typical of the Central Iberian Zone (Lower Ordovician Armorican Quartzite Formation). The Urra Formation unconformably overlies the previously deformed and metamorphosed Ediacaran sediments of the Série Negra (with Ossa-Morena Zone paleogeographic affinity). New SHRIMP zircon data obtained from the Urra Formation volcaniclastic rocks indicate an Early Ordovician age (206Pb/238U ages ranging from 494.6 ± 6.8 Ma to 488.3 ± 5.2 Ma) for this magmatic event. The inherited zircon cores indicate the presence of multicycle protoliths with different Precambrian ages: Neoproterozoic (698–577 Ma), Paleoproterozoic (2.33 Ga) and Paleoarchean (3.2–3.3 Ga). There is a noticeable lack of Meso- to Neoarchean and Mesoproterozoic ages. The data support the hypothesis that the volcaniclastic rocks were derived by partial melting of Cadomian basement (linked to a West African Craton provenance). The Urra Formation volcaniclastic rocks have rhyolitic to dacitic compositions, are peraluminous and similar to calc-alkaline high-K series suites elsewhere. Isotopic signatures present a wide range of values (87Sr/86Sr)t = 0.7085–0.7190, more restricted εNdt (− 2.65 to − 0.35) and δ18O = 9.63–10.34‰, compatible with magmas derived from crustal rocks, including portions of the lower crust. Some samples show disturbance of the Rb–Sr system as shown by unrealistic values for (87Sr/86Sr)t < 0.703, probably due to Variscan deformation and metamorphism. The volcaniclastic rocks with a significant sedimentary contribution (upper unit) are distinguished from the others by the lowest values of εNdt (− 5.53 to − 4.85). The geochemical data are compatible with an orogenic geodynamic environment. However, the “orogenic” signature can be considered to represent, in part, an inherited feature caused by melting of the Cadomian basement which also has calc-alkaline affinities. The Early Ordovician crustal growth and associated magmatism, represented by the Urra felsic volcaniclastic rocks and associated calc-alkaline granitoids, diorites and gabbros, can be interpreted in terms of the underplating and temporal storage of mantle-derived magmas as the potential source for the “orogenic melts” that were intruded during Early Paleozoic extension. This record of Early Ordovician magmatism has striking similarities with other correlatives from the Iberian, Bohemian and Armorican massifs that are discussed in this paper. This comparison reinforces the probable existence of a large-scale crustal melting process linked to a significant episode of extension on the northern Gondwana margin that probably resulted in the birth of the Rheic Ocean.
... pers.), admitiendo que el conjunto de la «Serie de El Alamo» constituya un grupo con significado Mtoestratigráfico y conelacionable. De este modo, el paragneis de grano fino sería correlacionado con los tramos detríticos de la Unidad Inferior de RODRÍGUEZ ALONSO (1985). La formación de gneises grandulares podría quedar, así, integrada en esta Unidad, o bien, representar realmente el zócalo granítico infrayacente. ...
The studied area is located in the Spanish sector of The Chaves-Miranda do Douro hercynian antiform (central western Spain). From a structural point of view, an interference model of folding between phase II, with an associated flat foliation, and phase III with folds of subvertical axial planes is inferred. A lithostratigraphical sequence is proposed, from botton to top: biotitic glandular gneiss-leucocratic glandular gneiss (po�Óhyroblasts becoming scarce)-fine grained paragneiss-pelitic gneiss and metapelites with quarzitic, metatonalitic and metabasitic interbedded rocks. A petrographie, and in some cases, geochemical, characterization of all these rocks is made. This suggests a probable P-T-t metamorphic path, from a greater pressure metamorphism (with staurolite relicts) to a second one, with sillimanite, cordierite, or andalusite. During this latter metamorphism (late phase III) anatectic conditions are attained, and frequently gneisses progress to metatexites and diatexites. Thus, it is possible to infer some kind of protolith-anatexite correspondence such as paragneiss-fine grained granite and pelitic rocks-porphyritic granite. EI área estudiada forma parte del sector español del antiforme de fase III Chaves-Miranda do Douro. Desde un punto de vista estructural se propone un modelo de interferencia de pliegues entre la fase II, que lleva asociada una esquistosidad subhorizontal, y la fase III de pliegues de planos axial subvertical. Se propone también una columna litoestratigráfica con diversas faciès de gneises glandulares que aumentan su carácter leucocrático hacia el techo y disminuyen la proporción de porfidoblastos, para pasar después a los «paragneises de grano fino». La parte más alta de la secuencia está representada por rocas predominantemente pelíticas a las que están asociadas especialmente granitos porfídicos. Los procesos metamórficos son abordados a partir de una amplia descripción mineralógica-textural, sugiriendo una trayectoria P-T-t que se inicia con un metamorfismo de mayor presión (con relictos de estaurolita) y se sucede con otro metamorfismo que conduce a la anatexia, representado por sillimanita y cordierita, o andalucita, según los casos. En el análisis del proceso anatéctico (tardi fase III) se hace énfasis en las posibles correspondencias protolitoanatexita, aplicado, en especial, a los paragneises y granitos de grano fino, así como a las metapelitas/gneises fémicos y granitos porfídicos.
An updated stratigraphic subdivision of the Ediacaran and Terreneuvian in the Alcudia valley and the Toledo Mountains, Central Iberian Zone, is documented here. The Lower Alcudian-Domo Extremeño Supergroup is subdivided, from bottom to top, into the conformable Guadiana (La Coronada and Sta. Mª de Zújar formations) and Campanario (Botija, Monroy and Orellana formations) groups. The supergroup contains biostratigraphically non-significant bacterial acritarchs and dubious fossils, but its Ediacaran age is constrained by detrital zircon analyses. This siliciclastic sedimentary package is unconformably overlain by either the siliciclastic-carbonate Ibor Group (Castañar, Villarta and Arrocampo formations) or the siliciclastic Cíjara Formation. The fossil content of the former group includes Sabelliditids, Vendotaenids, macrophytes, bioaccumulations and reefs rich in Cloudina, Sinotubulites, Protolagena and stromatolites and thrombolites, and a wide diversity of ichnofossils; whereas the Cíjara Formation has yielded bacteria attributed to Bavlinella and Palaeogomphosphaeria, and simple trace fossils (Gordia, Helminthoidichnites and treptichnids). The Ediacaran-Cambrian boundary, based on ichnofossils, lies at the base of the Arrocampo Formation and the uppermost part of the Cíjara Formation. In the Alcudia valley, the Ibor Group is subdivided into another three-fold, lithologically equivalent subdivision, known as the Tamujar, Hinojosas (rich in ichnofossils, such as treptichnids, Monomorphichnus, Psammichnites and Taphrelminthopsis) and Cabezarrubias (including Bergaueria and Planolites) formations. Another unconformity, intra-Fortunian in age, marks the tops of the Ibor Group and the Cíjara Formation, wich are subsequently overlain by the San Lorenzo Formation and some megabreccia beds, respectively. Overlying the slope-related Fuentes, Membrillar and Navalpino megrabreccia beds, the Terreneuvian-Cambrian Series 2, heterolithic Pusa Formation is subdivided into thre members, the middle one characterized by the record of phosphate ore deposits (e.g., the Fontanarejo Bed). The formation is shale dominated and has yielded the body-fossil Beltanelliformis and numerous ichnofossils (Monomorphicnus and Treptichnus; lower member), sponge spicules associated with thrombolites (middle member), and Scenella, trilobites, archaeocyaths and ichnofossils (Dactyloidites, Psammichnites, Rusophycus and Teichichnus; upper member). Where the megabreccia beds are absent, the Ibor Group is unconformably overlain by the San Lorenzo and Fuentepizarra formations, the latter containing phosphatic limestone interbeds rich in skeletonized microfossils, such as Anabarella, which represents the Fortunian-Cambrian Stage 2 boundary interval.
This report contains research work developed in the areas of deposit of water-courses in dehesas of Extremadura (Guadalperalón basin, Cáceres) with the aim of establishing, through analysis of the sedimentary sequence, conditions that have controlled processes of erosion-sedimentation in the basin over time and the possible influence of man through his different management of land use.
The Central-Iberian Zone of the Hesperian Massif was defined by Julivert et al. (1972)2 as a composite of Galaico-Castilian and East Lusitanian-Alcudian Zones of Lotze (1945), because, in spite of the obvious differences that both have from the structural and metamorphic-magmatic point of view, common stratigraphic characteristics (particularly Lower Ordovician unconformity) show that both regions had a close sedimentary history and a jointly liable palaeogeographic evolution, but with very significant peculiarities in each of them.
Geolodía Salamanca 2014 es una iniciativa de divulgación de la geología relacionada con la profesión
del geólogo a través de una de sus facetas más atrayente para el público en general, las
excursiones de campo. También se quiere dar a conocer nuestro patrimonio geológico para que se tome conciencia de la importancia y necesidad de protegerlo.
Itinerario geológico por el espacio Natural de Las Quilamas, cuyo objetivo es dar a conocer los tesoros y divulgar los puntos de interés geológico que entrañan estas sierras del sur de la provincia de Salamanca.
Lower-Middle Cambrian regional stages of the Iberian Peninsula (Cordubian, Ovetian, Marianian, Bilbilian, Acadoparadoxides and Solenopleuropsidae stages) are now reconsidered in the light of new stratigraphic and palaeontological (trace fossils, trilobites and archaeocyaths) studies. These stages are re-described with selected stratotypes and chronostratigraphic boundaries. The names Leonian and Caesaraugustian are formally proposed to replace the provisional stages of Acadoparadoxides and Solenopleuropsidae, respectively. A correlation chart of the Lower-Middle Cambrian lithostratigraphic units from the Iberian Peninsula is also presented. Two regessive-transgressive cycles are proposed to explain the lithological and palaeontological peculiarities through the Lower-Middle Cambrian sequence in the Iberian Peninsula.
The Pe˜na Negra anatectic complex (central Spain) was formed from a pelitic protolith low in boron (3–80 ppm), by dehydration reactions, melting and fluid loss, which expelled much of the boron; residual B is present in sillimanite and muscovite. Migmatite melanosomes and restites (sillimanite + biotite + ilmenite) are relatively richer in B (mean 29 ppm), while the leucosomes are poorer (13 ppm). Late shears, fractures and zones of retrograde alteration have permitted some reintroduction of B. Experiments demonstrated that B cannot be removed from these rocks by heating or by leaching, indicating that very little is present in superficial adsorption and that the element occurs in mineral structures.
Petrographic, geochemical and field studies in low grade metamorphic areas (Ciudad Rodrigo-Hurdes-Sierra de Gata domain, CRHSG, central-western Spain) show that Neoproterozoic-Lowermost Cambrian series in the Central Iberian Zone (CIZ) record two kinds of provenance sources including: (1) detrital material derived from recycled orogens and (2) a Cadomian coeval juvenile contribution that governs their isotopic signature. Evidence of magmatism contemporaneous with Neoproterozoic-Cambrian sedimentation is provided by the presence of coherent, massive volcanic rocks (metabasalts, metaandesites, and metarhyolites), volcaniclastic shales, sandstones, conglomerates and breccias. The appearance of volcanogenic lithic fragments and crystals mixed in different proportions with siliciclastic constituents and also present within calcareous components in the sedimentary succession, reinforces this evidence. Although most of the selected volcanic and volcaniclastic samples appear to show tholeiitic affinity, some of them display calc-alkaline affinity. Different trace element ratios, such as Sm/Nd, Nb/Yb and Ta/Yb, suggest a magmatic evolution in the same tectonic setting. The geochemical results reported here support the existence of an active geodynamic setting as a direct contributor to the synsedimentary and magmatic content of the Neoproterozoic–Lowermost Cambrian successions in the CIZ. In particular, the relatively high εNd (T)values and the high range of f Sm/Nd ratios are consistent with an active margin during the Neoproterozoic–Early Cambrian. The existence of tectonic activity is also confirmed by the presence of synsedimentary deformation and volcanic rocks. All of these traits favour a geodynamic model in which the Iberian Cadomian segment represented in the CIZ would have been part of an active northern margin of Gondwana, with an associated magmatic arc and related basins during Neoproterozoic–Lower Cambrian times. A proposed link between the Ossa Morena and the Central Iberian Zones might account for late Cadomian pull-apart basins developed on both sides of the magmatic arc, sharing the same scenario and involving similar magmatic activity during the Neoproterozoic–Cambrian transition.
This paper describes the results of 77 chemical analyses of groundwater from springs and wells in the «Complejo Esquisto Grauváquico» and other paleozoic materials from the south of Salamanca province. The types of groundwater are very diverse, usually bicarbonate dominant, with TDS (Total Dissolved Solids) between 0.6 and 16 milimoles/l. Salinity is shown to be greater in the drilled wells than in the dug wells and springs, which confirms the existence of deep flow. Principal Component Analysis indicates that the chemical composition of groundwater must be the result of multiple reactions between the water and the minerals that occur in parallel, with none of the reactions being specially predominant. The organization and geographic distribution of sorne of the elements suggest the existence of regional flow systems.
Se presentan los resultados de 77 análisis químicos de aguas subterráneas procedentes de manantiales, pozos excavados y sondeos en el Complejo Esquisto Grauváquico y otros materiales paleozoicos al Sur de la provincia de Salamanca. Se trata de aguas muy dispares, en general bicarbonatadas mixtas, con TSD (total de sólidos disueltos) entre 0,6 y 16 milimoles/l. Se pone de manifiesto una mayor salinidad de las aguas de sondeos en relación con las de pozos y manantiales, lo que demuestra la existencia de una circulación a cierta profundidad. El análisis de componentes principales muestra que la composición química de las aguas debe ser el resultado de múltiples reacciones entre el agua y los minerales que se producen en paralelo, sin que destaque ninguna en especial. La distribución geográfica ordenada de algunos elementos obliga a plantear como posible la hipótesis de la existencia de un flujo regional.
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