Southern Central Iberian Zone Research Articles

Pentaradiate diploporites (Echinodermata) from the Spanish Middle Ordovician and their taxonomic significance

New Middle Ordovician specimens of pentaradiate diploporite blastozoans from the southern Central Iberian Zone, Spain, show that Oretanocalix is an aristocystitid diploporite with five ambulacra; all other aristocystitid genera have 2?4 ambulacra. The laterally elongate oral frame is composed of eight plates, with five, facet-bearing, circumorals (COO) and three, non-facet-bearing, periorals (POO). This necessitates a new interpretation of the aristocystitid oral area and diagnosis of the family Aristocystitidae. Previous interpretations assumed four COO and four POO. Weathered examples lack oral cover plates and show that plates CO1 and CO4, which support ambulacra D and B, respectively, do not reach the inner margin of the peristome. They also reveal abundant diplopores, which were covered by a thin epistereom in life. Diplopores grew at plate sutures and were incorporated into both adjacent plates as the plates enlarged. Oretanocalix julioi n. sp. extends the stratigraphic range of Oretanocalix back to the basal Dobrotivian ?low in the Upper Darriwilian. The genus Batalleria is a sphaeronitid diploporite with food grooves that branched within the peristome. The branching pattern reflects Lovén’s law and is: AR, BL, CR, DL, ER, where A?E denote Carpenter’s ambulacra and L (left) and R (right) denote on which side of the ambulacrum the first branch lies. Each branch leaves the peristome by a separate ambulacral orifice. No other sphaeronitids have food grooves that branch within the peristome and all other sphaeronitids have one ambulacral orifice per ambulacrum. Thus, a new diagnosis of sphaeronitids is necessary.

Revisión de la sucesión de la Sierra de la Zarzuela: último registro del Ediacárico-¿Fortuniense inferior? del Gran Anticlinorio Extremeño central, Zona Centroibérica meridional, Macizo Ibérico, España

En la Zona Centroibérica meridional se reconoce la sucesión más completa de rocas del Ediacárico tardío que afloran en el sinclinal de la Sierra de la Zarzuela, discordantes sobre el Supergroup del Alcudiense inferior-Domo Extremeño. El objetivo de esta contribución es estudiar y redefinir un marco litoestratigráfico regional para las rocas del “Alcudiense inferior” de este área relacionada con el Grupo de Ibor.

Origin of cordierite-bearing monzogranites from the southern Central Iberian Zone – Inferences from the zoned Sierra Bermeja Pluton (Extremadura, Spain)

Voluminous peraluminous (biotite, biotite–cordierite and cordierite–muscovite-bearing) granitic plutons were emplaced in the southern part of the Central Iberian Zone (Iberian Massif) during the waning stages of the Variscan Orogeny. The role of the mantle and the nature of the crustal sources involved in the generation of these remarkable granites are still a matter of discussion. To address these issues, a detailed study of the Sierra Bermeja Pluton, a ~60 km2 cordierite-bearing monzogranite intrusion representative of the regional Nisa–Alburquerque–Los Pedroches Magmatic Alignment, has been carried out. This pluton was formed by an inward sequence of three distinct but near contemporaneous magma pulses: Outer Unit (OU), Middle Unit (MU) and Inner Unit (IU). The nested pluton is cut by a number of syn-plutonic vaugneritic dykes, while lamprophyres were distinctly later.The age of the Pluton has been established at ca. 306 Ma (309 ± 3 to 304 ± 3 Ma) by LA-ICP-MS U–Pb dating of zircon and monazite. The peraluminosity (A/CNK = 1.01–1.33) and P2O5 contents (0.19–0.35 wt%) of the monzogranitic units increase inwards (OU → MU → IU). This is accompanied by an overall decrease in TiO2, MgO, CaO, and FeOt, and most trace-elements, including REE. The three units display similarly low 87Sr/86Sr306Ma ratios (0.7033–0.7046) but decreasing εNd306Ma (OU: ~ −2.7, MU: ~ −3.8, IU: −7.5 to −4.7). Vaugneritic rocks have homogeneous composition with similar 87Sr/86Sr306Ma ratios (~0.7043) to those of the monzogranites but higher εNd306Ma (~ +1.1) values. Whole-rock geochemical data imply a partial melting of mainly Cambro–Ordovician felsic metaigneous rocks (OU) with increasing proportion of Neoproterozoic–Lower Cambrian metasediments of the Schist–Greywacke Complex (MU) that formed, almost exclusively, the source of the IU. Thermobarometric data suggest that melting processes progressed upward through the geological record and magma emplacement took place at <3 kbar, with crystallization temperatures of 710–810 °C. Geochemical modelling implies that each monzogranitic unit could have evolved independently at the emplacement level by a 20–26% fractional crystallization of a mineral assemblage constituted by Kfs + Pl > Bt >> Ap + Mnz + Xtm. The present results could be extensible to other cordierite-bearing granites of the southern Central Iberian Zone although forthcoming experimental work and detailed comparisons are required to confirm such extrapolation.

Petrography and geochemistry of late- to post-Variscan vaugnerite series rocks and calc-alkaline lamprophyres within a cordierite-bearing monzogranite (Sierra Bermeja Pluton, southern Iberian Massif)

The Sierra Bermeja Pluton (southern Central Iberian Zone, Iberian Massif) is a late-Variscan intrusive constituted by cordierite-bearing peraluminous monzogranites. Detailed field mapping has allowed to disclose the presence of several NE–SW trending longitudinal composite bodies, formed by either aphanitic or phaneritic mesocratic rocks. According to their petrography and geochemistry these rocks are categorized as calc-alkaline lamprophyres and vaugnerite series rocks. Their primary mineralogy is characterized by variable amounts of plagioclase, amphibole, clinopyroxene, biotite, K-feldspar, quartz and apatite. Broadly, they show low SiO 2 content (49–56wt.%), and high MgO+FeO t (10–17wt.%), K 2 O (3–5wt.%), Ba (963–2095ppm), Sr (401–1149ppm) and Cr (87–330ppm) contents. Field scale observations suggest that vaugneritic rocks and lamprophyres would constitute two independent magma pulses. Vaugneritic dioritoids intruded as syn-plutonic dykes, whereas lamprophyres were emplaced after the almost complete consolidation of the host monzogranites. In this way, vaugnerite series rocks would be an evidence for the contemporaneity of crustal- and mantle-melting processes during a late-Variscan stage, while lamprophyres would represent the ending of this stage.

Superimposed folding and W-Sn vein-type mineralisation in the Central Iberian Zone associated with late-Variscan oroclinal buckling: A structural analysis from the Regoufe area (Portugal)

The Cantabrian orocline is a major orocline that bends the Variscan belt of Western Europe. Despite a wealth of studies, its timing and relationship with the late-Variscan folding stages in the Iberian Massif are still debated. This study provides an integrated structural analysis of the Variscan fold generations and associated W-Sn bearing vein systems within the southern Central Iberian Zone (Regoufe, Portugal), giving insight into their kinematic relationship with oroclinal buckling. Two superimposed fold generations with mutually parallel, steep to vertical fold axes have been identified. Isoclinal F1 folds with E-W striking axial planes formed during coaxial shortening and are spatially localised. F3 folds with NW-SE axial planes are associated with late-Variscan transpressional deformation and constrain the structural outline of the major folds. Temporal and kinematic considerations indicate that vertical-axis, asymmetric F3 folding corresponds with the expected deformation style within the southern limb of the Cantabrian orocline. The Regoufe area is characterised by two types of W-Sn vein-type mineralisation. Granite-hosted hydrothermal quartz veins show a concentric distribution and are associated with the stress regime during granite emplacement. The vein system within the metasedimentary host rock, however, is emplaced along a regional, subhorizontal cross-fold joint system. This cross-fold joint system developed orthogonal to the subvertical F3 fold axes, and is interpreted to be coeval with oroclinal buckling. These results impact the understanding of (i) the relative significance and geometry of the main deformation stages in the Iberian massif, reconciling the timing and kinematics of F3 regional folding with oroclinal buckling, and (ii) the kinematic relationship between late- to postorogenic deformation and W-Sn vein-type mineralisation.

The late-Variscan peraluminous Valdepeñas pluton (southern Central Iberian Zone)

The Valdepenas pluton is the easternmost outcrop of the Caceres-Valdepenas magmatic alignment (southern Central Iberian Zone). This massif is constituted by a cordierite-bearing porphyritic monzogranite and may be grouped within the so-called “Serie Mixta” granitoids. The Valdepenas monzogranite is of magnesian [FeO t /(FeO t +MgO)~0.76], alkali-calcic [(Na 2 O+K 2 O)–CaO=7.8–8.5] and peraluminous (A/CNK=1.14–1.20). Multielemental- and REE-normalized patterns are comparable to those of similar rocks in the Nisa- Alburquerque-Los Pedroches magmatic alignment, and slightly differ from those of the Montes de Toledo batholith, both in the southern Central Iberian Zone. The U-Pb zircon age of 303±3Ma is consistent with the late-orogenic character of the intrusion and is in accordance with most of the granitic peraluminous intrusions in the southern Central Iberian Zone. 86 Sr/ 87 Sr 300Ma ratios (0.707424–0.711253), eNd 300Ma values (-5.53 to -6.68) and whole-rock major and trace element compositions of the studied rocks, suggest that the parental magma of the Valdepenas monzogranite could derive from a crustal metaigneous source. The U-Pb ages (552–650Ma) of inherited zircon cores found in Valdepenas monzogranite samples match those often found in Lower Paleozoic metavolcanics and granitic orthogneisses of Central Iberia and, furthermore, point to Upper Neoproterozoic metaigneous basement rocks as possible protoliths at the magma source. Based on the solubility of monazite in peraluminous melts, the estimated emplacement temperature of the studied monzogranite is 742–762oC. The results obtained in this work would contribute to a better understanding of the origin of the “Serie Mixta” granitoids.

A ~565 Ma old glaciation in the Ediacaran of peri-Gondwanan West Africa

In the Cadomian orogen of the NE Bohemian Massif and of SW Iberia, a post-Gaskiers glacial event dated at c. 565 Ma has been detected. Such Ediacaran-aged glaciomarine deposits occur in the Weesenstein and Clanzschwitz groups of the Saxo-Thuringian zone (Bohemia) and in the Lower Alcudian group of the southern Central Iberian zone (Iberia). Both areas are parts of Cadomia situated in the Western and Central European Variscides. Glaciomarine sedimentary rocks are characterized by such features as dropstones, flat iron-shaped pebbles (“Bugeleisen-Geschiebe”), facetted pebbles, dreikanters, and zircon grains affected by ice abrasion. For age and provenance determination, LA–ICP–MS U–Pb ages (n = 1124) and Hf isotope (n = 446) analyses were performed. The maximum age of the glaciomarine deposits within a Cadomian back-arc basin based on U–Pb analytics resulted in the youngest detrital zircon populations showing ages of 562–565 Ma and of c. 566–576 Ma old zircon derived from granitoid pebbles within the diamictites. The youngest age recorded was 538–540 Ma based on zircon from the plutons which had intruded the previously deformed Ediacaran metasedimentary rocks. Previously described glaciomarine diamictites of Cadomia (Weesenstein, Clanzschwitz, and Orellana diamictites) are most definitely younger than the c. 579–581 Ma Gaskiers glaciation in Newfoundland (Gaskiers) and in SE New England (Squantum). We propose the term Weesenstein–Orellana glaciation for this new Ediacaran glacial event, named after the most relevant regions of exposure. Palaeogeographically, these glaciomarine diamictites and related sedimentary deposits lie on the periphery of the West African Craton (western peri-Gondwana), and evidence has been provided by detrital zircon U–Pb ages and their Hf isotope composition. Correlation with similar glaciomarine deposits in the Anti-Atlas (Bou Azzer) and Saudi Arabia suggests a continued distribution of post-Gaskiers glacial deposits along the Gondwana margin of Northern Africa. The Weesenstein–Orellana glaciation correlates in part with the Shuram–Wonoka δ13C anomaly.

Insights into petrogenesis of the Jálama pluton (Central Iberian Zone, western Spain)

ABSTRACTThe Jálama pluton (JP) is a Variscan peraluminous granitoid that intruded into low-grade metasediments from the Central Iberian Zone (CIZ). It comprises a sillimanite-bearing two-mica monzogranite in the inner zone, followed by a tourmaline-bearing two-mica monzogranite, and a marginal tourmaline-muscovite leucogranite in the northern half of the pluton. Microgranitoid enclaves and metasedimentary xenoliths occur locally in monzogranites. The change in rock type from the central monzogranite to the marginal leucogranite corresponds to decreasing TiO2, MgO, FeO, CaO, Sr, Ba, Zr, and ΣREE, and increasing SiO2, Na2O, P2O5, Rb, Li, Cs, Ta, Sn, and W. Fe/(Fe+Mg) ratios in biotite, muscovite and tourmaline increase with increasing Fe/(Fe+Mg) in bulk rock, suggesting an important control of the bulk-rock composition on mineral chemistry. The high peraluminosity, the low CaO and high P contents, as well as the similarity of ε(Nd)300 values in both the granites and metasediments of the southern CIZ constitute strong evidences for a crustal origin of the granite suite, probably by melting of these metasedimentary rocks. Field and petrographic observations, together with mineralogical and geochemical data, suggest that assimilation and mingling/mixing acted in concert with fractional crystallization during the formation of the JP. These processes may also have been important in the evolution of other granitoids from this region.

Zircon Hf signatures from granitic orthogneisses of the Spanish Central System: Significance and sources of the Cambro-Ordovician magmatism in the Iberian Variscan Belt

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 481Ma), overlapping with the available literature data (mostly from 477 to 500Ma). These orthogneisses are intrusive into metasedimentary sequences from the northern CIZ that have been recently dated at about 536Ma. 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 1.25Ga) 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 650km from central Spain to Galicia. The ~35Ma 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.

Lithospheric velocity model across the Southern Central Iberian Zone (Variscan Iberian Massif): The ALCUDIA wide-angle seismic reflection transect

A P wave seismic velocity model has been obtained for the Central Iberian Zone, the largest continental fragment of the Iberian Variscan Belt. The spatially dense, high-resolution, wide-angle seismic reflection experiment, ALCUDIA-WA, was acquired in 2012 across central Iberia, aiming to constrain the lithospheric structure and resolve the physical properties of the crust and upper mantle. The seismic transect, ~310 km long, crossed the Central Iberian Zone from its suture with the Ossa-Morena Zone to the southern limit of the Central System mountain range. The energy generated by five shots was recorded by ~900 seismic stations. High-amplitude phases were identified in every shot gather for the upper crust (Pg and PiP) and Moho (PmP and Pn). In the upper crust, the P wave velocities increase beneath the Cenozoic Tajo Basin. The base of the upper crust varies from ~13 km to ~20 km between the southernmost Central Iberian Zone and the Tajo Basin. Lower crustal velocities are more homogeneous. From SW-NE, the traveltime of PmP arrivals varies from ~10.5 s to ~11.8 s, indicating lateral variations in the P wave velocity and the crustal thickness, reflecting an increase toward the north related with alpine tectonics and the isostatic response of the crust to the orogenic load. The results suggest that the high velocities of the upper crust near the Central System might correspond to igneous rocks and/or high-grade metamorphic rocks. The contrasting lithologies and the increase in the Moho depth to the north evidence differences in the Variscan evolution.

Ediacaran–Cambrian paleogeography and geodynamic setting of the Central Iberian Zone: Constraints from coupled U–Pb–Hf isotopes of detrital zircons

The combination of U–Pb geochronology and Hf isotopes in detrital zircons represents a valuable tool in the characterization of sediment sources and paleographic reconstructions. This analytical methodology has been applied to seven Neoproterozoic to Lower Cambrian metasedimentary rocks from the northern and southern domains of the Central Iberian Zone (CIZ), which represent part of the Cadomian basement in the Iberian Massif. The samples collected in the southern domain (belonging to the Schist–Greywacke Complex) yield Ediacaran maximum depositional ages (582–550Ma), whereas samples from northern areas range from 588 to 536Ma, respectively in the Talavera and the Guadarrama sectors. A relatively large fraction of zircon grains gave anomalously young ages (∼500–320Ma), probably due to post-depositional lead loss induced by thermal overprints. Zircon U–Pb age distribution provide similar broad age ranges in all cases: abundant Ediacaran–Late Cryogenian zircons (540–700Ma), and less abundant ages in the following ranges: 700–850Ma, 880–1100Ma, 1900–2250Ma and 2400–2800Ma. However, the southern CIZ samples display an important juvenile input at ca. 1000–1075Ma, 800Ma and in the range 700–570Ma, which contrasts with the mostly recycled nature of the northern CIZ zircons. The Ediacaran juvenile signatures can be derived from the nearby Cadomian arc, as sedimentation likely occurred in a back-arc setting, but Early Cryogenian and Stenian juvenile peaks are more characteristic of basement rocks from NE Africa, so the CIZ was likely situated near the Saharan Metacraton and the Arabian–Nubian Shield during Late Ediacaran. On the other hand, the predominance of evolved components in the younger northern CIZ sediments implies important geodynamic modifications in northern Gondwana at the Ediacaran–Cambrian boundary. The differences between southern and northern CIZ metasedimentary rocks can be interpreted as a transition from a back-arc scenario to development of new basins after the arc collision against the main continent. In this context, the arc signature would decline and the influence of hinterland more mature regions would increase.

Hydrothermal phosphate vein-type ores from the southern Central Iberian Zone, Spain: Evidence for their relationship to granites and Neoproterozoic metasedimentary rocks

Hydrothermal quartz–apatite veins, called “Iberian-type”, occur exclusively in the southern Central Iberian Zone. This study presents a multidisciplinary approach leading to the mineralogical, fluid inclusions, and geochemical characterization of these veins from two representative areas, Logrosán and Belvís–Navalmoral, in order to establish their relationship with nearby phosphorous-rich granites and apatite in Neoproterozoic metasediments. The mineral assemblage of these veins comprises quartz, apatite, minor sulfides, dolomite, Fe–Mg-carbonates and Fe–Mn-oxides. Three texturally different types of apatite have been recognized in the veins: apatite I showing prismatic habit associated with minor sulfides, apatite II occurring as white fibrous radial crystal aggregates called “dahllite”, and apatite III as grayish to greenish hexagonal crystals. Hydrothermal apatite from veins (H-apatite) is enriched in Sr and depleted in Mn–Y–REE–Th–U–Pb compared to magmatic apatite (M-apatite) from the granitic plutons. However, trace element compositions of apatite from metasedimentary phosphorous-rich levels or nodules (S-apatite) of the Schist–Greywacke Complex show similar characteristics to the H-apatite, although the H-apatite stands out for its relatively high Sr-contents. This relative Sr enrichment in H-apatite is interpreted as inherited from both phosphate in Neoproterozoic metasediments (S-apatite) and carbonate levels. REE, fluid inclusion and stable isotope data are consistent with a long episode of hydrothermal activity implying cooling and dilution processes while interacting with phosphate-rich shale and carbonate beds in the SGC. Fluid inclusion study undertaken on hydrothermal apatite and quartz reveals the presence of aqueous low salinity fluids (0.2–6.7wt.% NaCl equiv.) at moderate to low Th (125–350°C). All available data point at a recycling event of the southern CIZ metasediments (the SGC) during post-Variscan hydrothermal fluid circulation as the more plausible origin of the phosphate vein-type mineralizations.

Regional structure of the southern segment of Central Iberian Zone (Spanish Variscan Belt) interpreted from potential field images and 2.5 D modelling of Alcudia gravity transect

This study presents a structural interpretation of potential field data at the southern segment of the Spanish Central Iberian Zone, where little is known about its geometry and physical properties in depth. We used ground gravity and aeromagnetic data from the Spanish Geological Survey (IGME) databases, together with a gravity survey recently acquired along the Alcudia deep seismic reflection profile, and also petrophysical data from rock samples, in part specifically acquired for this work. The aim is to characterize with potential fields the depth structure and physical properties of a complex area, focusing on features of the upper crust where the potential fields can provide a useful insight, as these features appear to be transparent in the images of seismic surveys carried up to date. After building a 2.5 D model of the magnetic susceptibility and density distribution for the whole crust with the constraints of the Alcudia seismic transect, relationships between lower and upper crustal structures can be better established, and surface geophysical maps allow estimating the lateral extensions of the main structures characterized by the potential field modelling. Interpreted gravity and magnetic Alcudia features consist of: 1) long-wavelength anomalies due to changes in crustal thickness and structure, identified along the Alcudia seismic reflection profile; some crustal boundaries and the seismic Moho have been slightly modified in the potential field model to fit the observed data; and, 2) short wavelength anomalies related to upper crustal features as large NW–SE faults and folds, and plutonic alignments. Northern and southern Alcudia segments show relevant lower-middle crust shortening structures underneath extensive Variscan plutons (Mora, Pedroches) and folded basins (Pedroches). Potential field imaging and modelling of these structures have provided a better understanding of the subsurface geology, particularly in non-reflective areas, unravelling the deep structure of the southern Central Iberian Zone.

Imaging the crustal structure of the Central Iberian Zone (Variscan Belt): The ALCUDIA deep seismic reflection transect

ALCUDIA is a 230 km long, vertical incidence deep seismic reflection transect acquired in spring 2007 across the southern Central Iberian Zone (part of the pre‐Mesozoic Gondwana paleocontinent) of the Variscan Orogen of Spain. The carefully designed acquisition parameters resulted in a 20 s TWTT deep, 60–90 fold, high‐resolution seismic reflection transect. The processed image shows a weakly reflective upper crust (the scarce reflectivity matching structures identified at surface), a thick, highly reflective and laminated lower crust, and a flat Moho located at 10 s TWTT (∼30 km depth). The transect can be divided into three segments with different structural styles in the lower crust. In the central segment, the lower crust is imaged by regular, horizontal and parallel reflectors, whereas in the northern and southern segments it displays oblique reflectors interpreted as an important thrust (north) and tectonic wedging involving the mantle (south). The ALCUDIA seismic image shows that in an intracontinental orogenic crust, far from the suture zones, the upper and lower crust may react differently to shortening in different sectors, which is taken as evidence for decoupling. The interpreted structures, as deduced from surface geology and the seismic image, show that deformation was distributed homogeneously in the upper crust, whereas it was concentrated in wedge/thrust structures at specific sectors in the lower crust. The seismic image also shows the location of late Variscan faults in spatial association with the lower crustal thickened areas.

Lower Devonian faunas and palynomorphs from the Dornes Syncline (Central Iberian Zone, Portugal): stratigraphical and paleogeographical implications

In the Dornes syncline, southern Central Iberian Zone, Portugal, the uppermost levels of the Serra do Luacao Formation up to now have been considered to be of Pridoli-Lochkovian? age because they lie immediately under the Dornes Formation, which is of Pragian age and is well defined stratigraphically. New paleontological elements including benthic faunas and palynomorphs allow us to confirm that the top of the Serra do Luacao Formation is securely Lochkovian, for upper Lochkovian strata are present although relatively thin. The transition from the Serra do Luacao to the Dornes formations and their lithostratigraphical relationship are now clear: the terrigenous succession belongs to the Serra do Luacao Formation and the limestones represent the Dornes Formation. The benthic faunas clearly have North-Gondwanan affinities.

U–Pb dating of granodiorite and granite units of the Los Pedroches batholith. Implications for geodynamic models of the southern Central Iberian Zone (Iberian Massif)

The first U–Pb geochronological results on the magmatic alignment of the Los Pedroches batholith are presented. The batholith is composed of a main granodioritic unit, several granite plutons and an important acid to basic dyke complex, all of them intrusive after the main Variscan regional deformation phase, D1, along the boundary between the Ossa-Morena and Central Iberian zones (SW Iberian Massif). Zircons from samples on both extremes of the granodiorite massif record nearly simultaneous magmatic crystallization at ca. 308 Ma, while the emplacement of granite plutons was diachronic between 314 and 304 Ma. The U–Pb results combined with new field and textural observations allow to better constrain the age of Variscan deformations D2 and D3 across the region, while the age of D1 remains imprecise. Transcurrent D2 shearing-tightening of D1 folds occurred around 314 Ma (lower Westphalian) in relation to the emplacement of the first granitic magmas. D3 faults and shear bands bearing a strong extensional component developed at ca. 308 Ma (upper Westphalian), associated to the intrusion of the main granodiorite pluton (granodiorite) of the batholith. Together with available geochemical and geophysical information, these results point to the Variscan reactivation of lithospheric fractures at the origin and subsequent emplacement of hybrid magmas within this sector of the Massif.

The Puente Génave-Castelo de Vide shear zone (southern Central Iberian Zone, Iberian Massif): geometry, kinematics and regional implications

Abstract This paper is concerned with the existence and the salient characteristics of an extensional shear zone (the Puente Génave-Castelo de Vide shear zone) along 400 km in the southern Central Iberian Zone. This shear zone is almost parallel to the previous Variscan structures, but it is slightly cross-cut by the discontinuous late Carboniferous Alburquerque-Pedroches batholithic alignment (which has been considered by some authors to be a fundamental feature for the sub-zonation of the Iberian Massif). The shear zone dips towards the S or SW, showing a top-to-the-S sense of movement. Early ductile deformation was concentrated along a band of andalusite-rich carbonaceous shales in the shear zone, but this can also be recognized in the footwall, particularly in the eastern sector, where the southern border of the (pre- to syn-kinematic) Santa Elena stock was intensely sheared; later intrusion of dyke-swarms agrees with the extensional stress field. Finally the shear zone appears brecchiated and bounded by two brittle faults. From a regional point of view, the shear zone described separates rocks with some differences regarding their stratigraphic, igneous and structural characteristics. These differences, and their precise limits, have been the basis for numerous subdivisions of this part of the Iberian Massif during the last decades. It is our contention that if any subzonation of the southern Central Iberian Zone is to be made (namely eg : Obejo-Valsequillo Domain, Lusitan-Marianic Domain), its northern boundary should be the Puente Génave-Castelo de Vide shear zone. As for the southern boundary, it is located in the Badajoz-Córdoba suture that separates the Central Iberian Zone from the Ossa-Morena Zone.

Relationships between very low‐grade metamorphism and tectonic deformation: examples from the southern Central Iberian Zone (Iberian Massif, Variscan Belt)

We have studied the syn‐kinematic very low‐grade metamorphism in a polyphase Variscan deformed region using X‐ray diffraction techniques. Two phases of regional metamorphism are related to their respective episodes of penetrative deformation in the southern Central Iberian Zone. The data obtained suggest that the rocks did not reach metamorphic equilibrium, but strain favoured the progress of mineral reactions in the more deformed parts. The first deformation is Devonian in age and consists in a heterogeneous ductile shearing coeval with large‐scale recumbent folding that were produced under high‐anchizone to epizone metamorphic conditions. The heterogeneity of the shearing originated strain gradients that can be said to enhance the growth of new minerals and the illite polytype transformation in the highly strained overturned limb of the preserved pile of recumbent folds, but illite crystallinity remained constant throughout the structure. The second deformation is Mid‐Carboniferous in age and consists in an upright folding that took place under late diagenesis to low‐anchizone metamorphic conditions. The distribution of mineral parageneses and illite crystallinity across one of the upright folds suggests that strain gradients favoured the metamorphic reaction progress from the hinge (low strain) towards the limbs (high strain). Other characteristics of the region such as a metamorphic gap associated with an unconformity at the base of the Lower Carboniferous rocks, or cryptic contacts aureoles surrounding volcanic intercalations and a large granitic batholith, are also studied.

Evolution of the Western European continental crust: implications from Nd and Pb isotopes in Iberian sediments

This paper presents Smz.sbnd;Nd and geochemical data on fine-grained sediments of the Iberian Massif, Spain, together with new and published Uz.sbnd;Pb results of selected sedimentary rocks. Profiles from two major zones (Cantabrian and Central Iberian Zone) have been investigated. Both profiles exhibit almost complete stratigraphic successions from uppermost Precambrian to Lower Carboniferous. Paleozoic shallow-depth platform sediments are essentially unmetamorphosed; the Precambrian syn-orogenic succession underwent a metamorphic overprint up to biotite zone. 147 Sm 144 Nd ratios range from 0.0988 to 0.1457 and Nd contents from 6 to 112 ppm. ϵ Nd[today]-values vary from −16.5 to −7.6, Pb contents from 1.7 to 97 ppm. Samples with high ϵ Nt[ T] ( > −3) are found to be low in μ 2[330Ma] ( < 9.75). Mean Nd crustal residence ages ( T DM) are 1.92 Ga (Cantabrian Zone) and 1.96 Ga (Central Iberian Zone). As juvenile Cadomian crust is present in the Iberian Massif, the oldest components have to be significantly older than 2 Ga. Within the European Variscides comparably old ( T DM) were so far only described from the Armorican Massif, France. An active margin of the Gondwana continent is the most probable provenance of the uppermost Precambrian strata as indicated by their Cadomian age, syn-orogenic facies and juvenile components. Precambrian as well as samples from the Precambrian-Cambrian boundary gave ϵ Nt[ T]-values > −4.8, significantly higher than average Paleozoic rocks. Contributions of juvenile Cadomian crust to the syn-orogenic sediments must be in the order of 30–50% to fit the Nd and Pb data. There is no evidence for any addition of mafic material according to major or trace elements, or to Sm Nd ratios. Lower Cambrian samples are already below −7.3 in ϵ Nd[ T]. Thus, compared to the total pre-existing crust, contribution of juvenile crust was very small. ϵ Nd[ T]-values of almost all samples ranging in stratigraphic age from 515 to 410 Ma are strikingly negative ( −12.9 to −10.1) and thus indicate the absence of newly differentiated continental crust in the provenance (s). Only the Tremadocian of the southern Central Iberian Zone gave high ϵ Nt[ T]-values ( −4.0 to + 1.0). Again there is no evidence for addition of mafic material to the strata. As Precambrian and Tremadocian ϵ Nt[ T] peaks are separated by the lowermost Cambrian of ϵ Nt[ T] > -7, juvenile crust may have been added related to the pre-Ordovician unconformity (Sardic phase). During the Middle Paleozoic (410–330 Ma), ϵ Nt[ T]-values increase from — − 12 to ∼ − 7, indicating a continuous contribution of juvenile continental crust of ∼20%. As major terranes of the Iberian Massif were not amalgamated until Late Paleozoic, it can be assumed that the respective juvenile crust was formed within the regime of one or several small subduction zones. Nd isotope data of this study compared with published results of Brittany, France, suggest that the Medio-North ArmoricanCentro-Iberian domain persisted from uppermost Precambrian to the Variscan.

New agnostina (Trilobita) from the Llanvirn (Ordovician) of Spain

Se estudian los trilobites agnóstidos procedentes de nueve localidades, siete de ellas nuevas para estas faunas, situadas en las pizarras del Llanvirn inferior de la parte meridional de la Zona Centroibérica (Montes de Toledo, Villuercas y región de Almadén). Se determina la presencia de los géneros Geragnostella y Geragnostus, y se describen tres nuevas especies: Geragnostella gilcidae n. sp., Geragnostus hispanicus n. sp. y Geragnostus ninhursagae n. sp. Se proponen también los nuevos nombres Geragnostus pilleti nom. nov. y Geragnostus abdullaevi nom. nov. para designar, respectivamente, las especies G. howelli PILLET, 1978, y G. elongatus ABDULLAEV, 1972, consideradas ambas homónimos objetivos primarios de otras especies definidas con anterioridad.