Spanish Central System Research Articles

The Cretaceous of the Spanish Central System: A review of the lithostratigraphy, palaeontology and sequence stratigraphy, including new educational perspectives

The Southern Cretaceous of the Central System is formed by both terrigenous and carbonate rocks that have been affected by intense dolomitization. They are part of the sediments deposited on the western margin of the Iberian Basin during the Cenomanian–Maastrichtian, the depositional architecture of which has been reconstructed through a detailed sequence stratigraphical study and analysis of its fossil record. It is formed by three large transgressive-regressive cycles in which the carbonate platform moves towards the West, while the terrigenous facies of its base decreases and the overlying clay facies, with gypsum, increases. The integration of sedimentological and palaeontological data, and the identification of a set of parasequences has allowed the differentiation of thirteen depositional sequences. This research has been essential to carry out modern geological mapping and provide society with a model that improves the economic geology and geotechnical exploration of these areas. Furthermore, this scientific progress has also impacted on the interest of society towards the geosciences and, specially, palaeontology. This has widened the scope for new research fields related to scientific outreach for the community and learning opportunities at different educational levels.

The mid‐latitude hydrolaccolith of the Spanish Central System (Southern Europe): A top‐to‐bottom integration of geomatic, geophysical and sedimentary datasets for characterising a singular periglacial landform

AbstractPermafrost study in the Spanish Central System (~41°N) has remained elusive in past decades. Although numerous periglacial features have been described, none has yielded conclusive information about the existence/distribution of permafrost across these mountains. This work focuses on integrating light detection and ranging and unmanned aerial vehicle data with geophysical methods and sedimentary records, along with preliminary thermal information, for the comprehensive study of a frost mound. The singularity of this feature resides in its location, currently dominated by a seasonal frost regime, and its size reaching 59 m long, 22.5 m wide and 4 m high, representing the largest landform of this type in Iberia. From top‐to‐bottom, the internal structure comprises three resistivity units (G1, G2 and G3). The most superficial unit (G1) consists of a sequence of burial soils alternating with sand and silts, with pebbles and gravel at the bottom, followed by a high porous layer that belongs to a landslide deposit (G2), likely composed of gravel and pebbles laying over unit G3 (bedrock). Calibrated 14C ages suggest this landform formed at least ~4300 years ago during the mid‐Holocene. Finally, the preliminary surface thermal map indicates the presence of sectors affected by strong thermal gradients likely driven by water table variations. Our results conclude that the dynamics are primarily controlled by the seasonal frost regime influencing local subsurface drainage, which allows us to classify this landform as a hydrodynamic pingo‐like structure. We also demonstrate that our approach is suitable for assessing the evolution of this type of periglacial landform. Implementing long‐lasting annual monitoring programmes would help to understand local periglacial dynamics better.

Noble and base metal geochemistry of late- to post-orogenic mafic dykes from central Spain

The post-tectonic and post-orogenic mafic rocks from the Spanish Central System (SCS) (Iberian Massif) include dyke swarms of shoshonitic (microgabbros) and alkaline (lamprophyres and diabases) geochemical affinity, which register the nature of the metasomatic lithospheric mantle under central Spain. Such magmas sometimes show a direct (or indirect) relationship with the formation of orogenic and intrusion-related gold deposits, which are relatively abundant in the Iberian Massif. The noble and base metal composition of these intrusions shows Primitive Mantle-normalized patterns characterized by positive Au and Co anomalies and fractionated platinum group elements (PGE): from lower Ir-group PGE (IPGE; Ir–Ru) to higher Pd-group PGE (PPGE; Rh–Pt–Pd). The low contents of PGE, together with the base metal contents of pyrite (which is the dominant sulphide phase in the alkaline dykes), is in accordance with low degrees of mantle partial melting and the early segregation of sulphides during magma differentiation. The scarcity of PGE mineral deposits in the Iberian Massif could be explained in part by the apparent lack of PGE enrichment in the Iberian lithospheric mantle. On the contrary, the positive Au anomaly of the SCS mafic dykes represents relatively high Au contents, similar to (and higher than) those of mafic rocks derived from metasomatized subcontinental lithospheric mantle underlying Au-endowed cratons. Several geochemical features point to subduction-related metasomatism of either oceanic or continental nature as the main source of Au enrichment. The Au re-fertilization of the lithospheric mantle under central Spain makes it a potential source in the formation of gold mineralizations.

Magnitude of formative flows in stream potholes

Although it is generally recognized that geomorphic work is tied to bedrock channel reshaping, the importance of low vs. high flow stages that cause the most geomorphic impact remains unclear. The objective of the research is to study the concept of “formative flow” in bedrock channels and determine, through morphological studies, if those flows have any impact on sculpted features such as potholes and how this relationship relates to various inputs such as flow stages (magnitude and frequency), shear stress, and sediment size. Here, we studied the distribution of the main pothole typologies and tried to understand why potholes are found along bedrock river channels. Specifically, we examined stream potholes from three locations along the Spanish Central System: Alberche, Tietar, and Manzanares rivers. We conducted the research by taking precise geometric measurements, classifying potholes, analyzing flow magnitude and frequency, and using a two-dimensional (2D) hydrodynamic model to assess key variables in Manzanares river. This research demonstrated that bankfull depths completely cover all pothole typologies in all the analyzed sites but are not sufficient to achieve its formative flow depth (FFD). Using a detailed 2D hydrodynamic model in Manzanares river, we discovered that dimensions of cylindrical potholes are closely related to bankfull discharge and that this depth is connected to FFD. Other potholes, such as erosive-compound and erosive-lateral, are historical remnants, and their shapes are not related to any particular FFD and are likely associated with rare events and catastrophic breaks. A collection of laterals that exhibit FFD near bankfull flows appear to represent a part of the recent evolution of a knickpoint. To summarize, it can be inferred from the findings that the utility of morphological analysis in conjunction with the 2D hydrodynamic model is to examine the fraction of erosional/active features to determine the degree of senescence and/or change in natural conditions in a river reach.

Nature and evolution of the lower crust under central Spain: Inferences from granulite xenoliths (Calatrava Volcanic Field-Spanish central system)

So far, the nature and evolution of the lower crust under central Spain have been constrained mainly on the basis of a heterogeneous suite of granulite xenoliths from the Spanish Central System (SCS). In recent years, ultramafic volcanics from the Calatrava Volcanic Field (CVF) have also provided deep-seated crustal xenoliths which have not been studied in detail. Our data, combining mineral, whole-rock and isotopic geochemistry with U–Pb–Hf isotope ratios in zircons from the CVF and SCS xenoliths, highlight the felsic composition of the lower crust under central Iberia. A number of the Calatrava xenoliths represents Variscan igneous protoliths, which are a minor population in the SCS, and were likely formed by crystallisation of intermediate and felsic melts in the lower crust during the Variscan orogeny (leucodiorite protolith age of 314 ± 3 Ma and leucogranite protolith age of 308 ± 2.5 Ma). U–Pb data of metamorphic zircons show that granulite-facies metamorphism mainly occurred from 299 to 285 Ma in both areas. These ages are slightly younger than those of granitic intrusions that could be genetically related to the granulitic residue, which points to a main role of U–Pb isotope resetting in lower crustal zircons during HT or UHT conditions. The zircon U–Pb–Hf isotopic ratios support the idea that the lower crust in central Iberia consists mainly of Ordovician–Neoproterozoic metaigneous and metasedimentary rocks associated with the Cadomian continental arc of northern Gondwana. These rocks provide evidence of mixing between juvenile magmas and an enriched crustal component, ultimately extracted from an Eburnean crust. Other more evolved components present in detrital zircons are likely related to recycling of Archean crust derived from North Africa cratonic terranes.

Habitat differentiation and geographic separation of Isoetes velata populations in central Iberian Peninsula

Populations of Isoetes velata were studied in order to provide more information on their habitat and distribution in the Western and Northwestern Iberian Peninsula. Habitat together with morphology of megaspores were used as diagnostic features. The form I. velata subsp. velata f. lereschii is considered here as a synonym of I. velata subsp. asturicense and therefore only two taxa remain within the species: I. velata subsp. velata and I. velata subsp. asturicense. Intermediate individuals have been found in the Western Central System. The differentiation within the I. velata group appears to be the result of gradual genetic divergence after isolation. Palaeobotanical data confirm the presence of I. velata subsp. asturicense in Late Glacial lake sediments in northwestern Spain. Cluster classification of the soft-water vegetation with Isoetes velata subsp. asturicense revealed two main groups in the Spanish Central System. A single association, the Sparganio angustifolii-Callitrichetum fontqueri which includes Iberian Atlantic stands of Sparganium angustifolium and Isoetes velata subsp. asturicense, is here recognized. Besides, a variant of the association with Eleocharis acicularis is identified in shallow temporary waters in the Western Sierra de Gredos.

Hydrogeochemistry of granitic mountain zones and the influence of adjacent sedimentary basins at their tectonic borders: the case of the Spanish Central System batholith

The hydrogeochemical characteristics of springs in the granites of the Spanish Central System (SCS), a mountain range affected by cortical tectonic structures, are described, along with an investigation of the spring water origin. In springs with variable flow and where they are associated with minor alterations and fractures, water type is Ca-(Na)-HCO3 with low total dissolved solids (TDS; 54–200 ppm) and a neutral or slightly acid pH. In springs that have continuous flow and association with relevant fractures, water type is Na-HCO3, with higher pH and TDS (240–563 ppm). There are five springs with water type Na-(Ca)-Cl and high TDS (780–9,205 ppm) near the SCS’s southern tectonic borders. Within 5,000 years of apparent residence time, Ca-(Na)-HCO3 water progresses to Na-HCO3 type, slightly increasing HCO3 content, losing Ca due to calcite precipitation and ionic exchange, and gaining S due to sulphate reduction. No changes are observed after 20,000 years of apparent age. The influence of internal CO2 is compatible with Na-HCO3 water type characteristics, facilitated by fracturing. Most of the springs yield cold water; thus, a flow depth up to 500 m can be deduced. Only the Messejana-Plasencia fault region shows Na-HCO3-type thermal springs. Chloride type water is a result of the influence of the Tertiary basins’ water moving towards the southern border of the SCS. The reverse fault in such contact, together with the subvertical structures affected by it, are responsible for the springs emerging in the granite. The northern border of the SCS shows the opposite behaviour.

Geochemistry and geochronology of mafic rocks from the Spanish Central System: Constraints on the mantle evolution beneath central Spain

The Spanish Central System (SCS) contains several suites of Palaeozoic mafic igneous intrusions with contrasting geochemical affinity: Ordovician tholeiitic metabasites, Variscan calc-alkaline gabbros (Gb1) and microdiorites (Gb2), shoshonitic monzogabbros (Gb3) and alkaline diabases and lamprophyres (Gb4). Not all of these rocks are accurately dated, and several aspects of their genesis are still poorly understood. We present new whole-rock geochemical data (major and trace elements, and Sr–Nd isotopes), U–Pb and Lu–Hf isotopic ratios on magmatic zircons and 40Ar/39Ar amphibole geochronology results in order to establish a precise chronology for the successive events of magmatism in the SCS, and discuss the nature of their mantle sources. Accurate ages have been determined for the Variscan gabbros (305–294 ​Ma), the microdiorites (299 ​Ma) and the accompanying felsic porphyries (292 ​Ma), the shoshonitic monzogabbros (285 ​Ma), and the alkaline diabases (274 ​Ma) and monzosyenites (271–264 ​Ma). According to this information, the Variscan mafic magmatism would be mainly concentrated in the range of 305–294 ​Ma, with a final manifestation represented by the minor shoshonitic dykes. The alkaline magmatism proved to be slightly older than previously thought and yielded at least two distinct pulses: diabases and lamprophyres–monzosyenites. Zircon Hf isotopes evidence the involvement of depleted and slightly enriched mantle sources. The bulk of the εHf values are in the broad range of −8 to +11, indicative of melting both depleted and enriched mantle regions. The high within-sample Hf isotope variation (up to ~11 epsilon units) shown by samples from the Variscan series (gabbros, microdiorites and monzogabbros) could be explained mainly by hybridisation of magmas derived from heterogeneous lithospheric mantle sources. Pressure estimates indicate that the Variscan mafic magmas were extracted from the lithosphere. The Nd–Hf isotopic composition of these suites of rocks suggests the recycling of pelitic sediments during the Cadomian orogeny. Deeper (asthenospheric) mantle levels were involved in the generation of the alkaline suite, whose anomalous negative εHf values (moderately decoupled with respect to radiogenic Nd) could be associated with subducted oceanic components raised by mantle upwelling associated with lithosphere thinning and extension during the Permian.

Lithospheric image of the Central Iberian Zone (Iberian Massif) using global-phase seismic interferometry

Abstract. The Spanish Central System is an intraplate mountain range that divides the Iberian Inner Plateau in two sectors – the northern Duero Basin and the Tajo Basin to the south. The topography of the area is highly variable with the Tajo Basin having an average altitude of 450–500 m and the Duero Basin having a higher average altitude of 750–800 m. The Spanish Central System is characterized by a thick-skin pop-up and pop-down configuration formed by the reactivation of Variscan structures during the Alpine orogeny. The high topography is, most probably, the response of a tectonically thickened crust that should be the response to (1) the geometry of the Moho discontinuity, (2) an imbricated crustal architecture, and/or (3) the rheological properties of the lithosphere. Shedding some light on these features is the main target of the current investigation. In this work, we present the lithospheric-scale model across this part of the Iberian Massif. We have used data from the Central Iberian Massif Deformation (CIMDEF) project, which consists of recordings of an almost-linear array of 69 short-period seismic stations, which define a 320 km long transect. We have applied the so-called global-phase seismic interferometry. The technique uses continuous recordings of global earthquakes (>120∘ epicentral distance) to extract global phases and their reverberations within the lithosphere. The processing provides an approximation of the zero-offset reflection response of a single station to a vertical source, sending (near)-vertical seismic energy. Results indeed reveal a clear thickening of the crust below the Central System, resulting, most probably, from an imbrication of the lower crust. Accordingly, the crust–mantle boundary is mapped as a relatively flat interface at approximately 10 s two-way travel time except in the Central System, where this feature deepens towards the NW reaching more than 12 s. The boundary between the upper and lower crust is well defined and is found at 5 s two-way travel time. The upper crust has a very distinctive signature depending on the region. Reflectivity at upper-mantle depths is scattered throughout the profile, located between 13 and 18 s, and probably related to the Hales discontinuity.

Cadalso de los Vidrios leucogranite ‘Blanco Cristal’: a widely used heritage stone from Spain

Abstract Cadalso de los Vidrios leucogranite is extracted from the Sierra de Guadarrama (Spanish Central System) under the trade name ‘Blanco Cristal’. It is a fine- to medium-grained, hypidiomorphic and equigranular building stone that has been used in many heritage buildings, such as the Villena Palace (1534). The current annual production of Cadalso de los Vidrios leucogranite is approximately 8000 m 3 , of which 40% is exported around the world. It has been used in places such as the Vieux-Port in Marseille (France), Cork International Airport (Ireland) and the Puri EXIM Bank of Jakarta (Indonesia) among others. It is currently being used in the construction of the iconic tower of Jesus Christ of the Sagrada Familia Basilica, in Barcelona (Spain). Cadalso de los Vidrios leucogranite Blanco Cristal has excellent petrophysical properties, a uniform colour and good resistance to thermal shock and freezing temperatures. These qualities make Blanco Cristal an ideal material for monuments and buildings.

Petrographic and geochemical study of the stone warrior stelae from central Iberia: Linking the geological record and archaeological heritage

AbstractSix warrior stelae from central Iberia (Toledo, Spain) were studied using different geological techniques to discriminate their potential source materials and evaluate the displacement suffered from the quarry sites to their final location. The similar petrographic and geochemical features of the stelae, when compared with other lithological equivalents cropping out in the surrounding area, suggest that most raw materials were quarried less than 10 km from the finds. These materials mainly belong to a set of metasedimentary rocks from the Schist–Graywacke Complex and other Ordovician brownish sandstones, mostly found in the vicinity of the stelae sites. In contrast, the Talavera de la Reina stele, previously used as a statue‐menhir, was engraved on a leucogranite, geochemically similar to some plutonic units in the Navamorcuende area, in the southernmost region of the Spanish Central System batholith. The fact that all stelae and the proposed geological sources were located close to ancient tracks suggests that the raw materials were quarried in connection with these natural routes. The stelae may, therefore, have been landmarks or reference points that signaled transit passages used for trade or grazing, in the framework of continuous territorial expansion during the Late Bronze Age and Early Iron Age.

Li-Na-metasomatism related to I-type granite magmatism: A case study of the highly fractionated La Pedriza pluton (Iberian Variscan belt)

This study describes rare albitites associated with highly fractionated I-type granitic melts, showing a mineral paragenesis that resembles those of high-temperature metasomatized rocks (fenites) associated with alkaline and carbonatitic intrusions. The studied albitites occur as lenticular bodies on the meter scale, hosted in peraluminous granites, and are scattered in a narrow sector next to the peraluminous and highly fractionated I-type La Pedriza leucogranite (LPL) pluton in the Spanish Central System. Albitites show sharp contacts and preserve textural features of the host granites (i.e., isovolumetric metasomatism). These rocks consist of a variably albitized magmatic feldspar framework (70–85 modal %) and interstitial domains where singular Na-Li-Fe3+-rich amphiboles and pyroxenes crystallized with albite or quartz. Titanite, andradite-grossular garnet, Mg-Li-rich mica, magnetite, fluorite, and apatite are metasomatic accessory minerals. Mass balance indicates enrichments of Na2O, FeO, MgO, MnO, Li, F, Be, and some HFSE (high-field-strength elements, such as Zn, Sc, and Sn) and LILE (large-ion lithophile elements) depletion. U-Pb data from metasomatic titanite yield 307 ± 10 Ma, which is coeval with the LPL and within the range of the regional granite emplacement (about 309–298 Ma). The LPL also shows strong compositional affinities (F, Li, Na, Sn, and Zn enrichment) with albitites. Similar Sr-Nd-δD isotopic signatures between albitites and host granites suggest that magmatic fluids might be dominant during metasomatism. However, the heterogeneity of δ18O, which shifts toward lower values in the more distant outcrops, suggests contribution of meteoric in origin low-salinity waters during prograde metasomatism. P-T-fO2 conditions are estimated at 0.2–0.3 GPa, 645–700 °C, and relatively oxidized conditions. We propose that Na-HFSE-flux-rich residual melts of the LPL expelled two immiscible hot liquids: an alkaline low-density liquid and a high-density hydrosilicate liquid (HSL). The low-density fluid dissolved magmatic quartz and provided albitization of the magmatic feldspar framework by a diffusive mechanism. Quartz dissolution created microchannels for infiltration of the relatively low viscosity HSL, forming tubular channels. This fluid was enriched in network modifiers (Li, Na, F, Be) and cations (Ca, Al) released after albitization, and it precipitated in these microchannels to form the new metasomatic paragenesis (quartz, albite, uncommon Li-rich mafic silicates, and accessory minerals).

Holocene vegetation and fire dynamics in the supra-Mediterranean belt of the Gredos Range (central Iberian Peninsula)

High-resolution pollen, macrofossil and charcoal data, combined with multivariate analysis, were used to reconstruct Holocene vegetation and fire dynamics at Manaderos. The studied mire is located in a highly valued location in the supra-Mediterranean belt of the Gredos Range (central Iberian Peninsula). The record started at 1260 calendar years (cal yrs) BP, according to eight radiocarbon dates, with the occurrence of an open Quercus pyrenaica forest and Pinus nigra stands. The palynological data show a dramatic change ∼380 cal yrs BP, when the oak forest was progressively replaced by a maritime pine one. The interpretation of this record is related to an increase in agricultural and livestock activities and changes in the role of fire, thus Pyrenean oak canopy was consumed by fires providing openings for Pinus pinaster, better adapted to fire. The results are compared with other sequences from the Spanish Central System in order to better understand the past dynamics of the main forest constituents and to provide a critical view of the role of fire, anthropic dynamics and climatic events.

Neogene Epeirogeny of Iberia

AbstractThe origin of Iberia's topography is examined by combining gravity, magmatic, topographic, and seismological observations with geomorphic considerations. We have four principal results. First, the highest coherence between free‐air gravity and topography is at wavelengths 250 km where admittance indicates that elastic thickness of Iberia's plate is 20 ± 3 km. These results imply that flexural and subplate support of Iberian topography could be expressed at wavelengths of O(100) km. Second, P‐to‐S receiver functions and simple isostatic calculations indicate that while crustal thickness variations and flexural loading (e.g., as a result of plate shortening) partially explain the elevation of Pyrenean, Betics, Cantabrian, Spanish Central System, and Iberian Chain topography, they fail to explain the elevation of large parts of Iberia. Third, a new full waveform shear wave tomographic model and velocity to temperature conversions suggest that the asthenosphere beneath Iberia is anomalously slow and has excess temperatures of up to 162 ± 14 °C. Simple isostatic calculations indicate asthenospheric support of topography of up to 1 km. Neogene‐Recent (∼23–0 Ma) extrusive magmatism (e.g., Calatrava, Catalan) sit atop many of the slow shear wave velocity anomalies. Finally, biostratigraphic data, combined with inversion of 3,217 river profiles, show that most of Iberia's topography grew during the last ∼30 Ma at rates of up to 0.3 mm/year. Best‐fitting theoretical rivers have a low residual root‐mean‐square misfit (=0.96) and calculated uplift is consistent with an independent inventory of stratigraphic and biostratigraphic observations. We suggest that Neogene‐Recent growth of most of central Iberia's topography was a result of asthenospheric support.

Late-Variscan multistage hydrothermal processes unveiled by chemical ages coupled with compositiona.

The scheelite skarn from Los Santos and the W-Au veins from El Cabaco district, located in the Spanish Central System Batholith (SCSB), are some of the best-known tungsten ore deposits in Spain. Uraninite is an accessory mineral in both deposits, which underwent several hydrothermal flow events. Chemical and textural characteristics, as well as electron microprobe U-Th-Pb uraninite chemical data from the different stages of the skarn and the vein-type mineralizations, are presented here. Based on these data the uraninite was able to be classified into two groups. Group I uraninite has an octahedral habit and occurs as inclusions in K-feldspar relicts of the leucogranite related to Los Santos skarn formation. It shows high Th (6.95 to 8.51wt.% ThO 2 ) and high Rare Earth Elements (REEs) contents (0.55 to 1.38wt.% ∑REE 2 O 3 ). Group II uraninite occurs i) associated to El Cabaco granite, in a greenish selvage-style greisen and its reddish envelope and in the mineralized rimming quartz veins and ii) in Los Santos high-temperature endoskarn and anorthite skarn, where it is associated with U-rich mica. This uraninite type has lower Th and ∑REE 2 O 3 contents than Group-I uraninite. The mineral chemistry and the assemblage and textural relationships suggest that Group-I uraninite is magmatic and the attained U-Th-Pb chemical age of 300±4Ma is interpreted as the magmatic age of the skarn-forming aplite granites in the western part of the SCSB. Group-II uraninite includes two events: i) hydrothermal uraninite, which yields an age of 295±2Ma, dates a strong alkali mobilization and early tungsten deposition and ii) a later hydrothermal process, around 287±4Ma, that resulted in sulfides and late scheelite precipitation and widespread silicification. Finally, the gold deposition is younger than this silicification according to textural criteria. Therefore, W-Au deposits in the western part of the SCSB were formed by superposition of several processes that took place some 15Ma after the skarn-forming granite crystallized. Comparable W, W-Au and U deposits in the Variscan orogenic belt show a similar timing of hydrothermal events, suggesting that the hydrothermal history was controlled by large-scale Late-Variscan tectonic processes.

Does the persistence of sweet chestnut depend on cultural inputs? Regeneration, recruitment, and mortality in Quercus- and Castanea-dominated forests

Quercus secondary forests show a gradual transition toward mixed forests, with sweet chestnut ( Castanea sativa ) becoming increasingly abundant in the western Spanish Central System. Additionally, in chestnut-dominated stands, it shows a certain resistance to competitive displacement by Quercus pyrenaica . Our results partially refute the traditional view that C. sativa is unable to recruit in the absence of cultural inputs. Sweet chestnut, Castanea sativa, is a component of European broadleaf forests and is one of the most managed trees. Due to a reduction in cultural inputs, chestnut-dominated stands tend to be invaded by other species, and it is unclear how chestnut is able to persist in natural mixed forests. Our work aimed to identity the main factors that limit the establishment of C. sativa and to analyze the recruitment and mortality processes of C. sativa trees. The age, growth ring patterns, regeneration density, and the spatial structure of trees and saplings in 11 plots in the Spanish Central System were analyzed. Chestnut seedling density increased with C. sativa basal area, but transition toward the sapling stage appeared limited owing to light availability. In Quercus pyrenaica secondary forests, sparse canopies did not constrain chestnut regeneration, and in old chestnut stands, C. sativa showed a certain resistance to competitive displacement. By contrast, mixed young coppices showed a high mortality, most likely due to competition with other vigorous resprouters. Quercus secondary forests showed a gradual transition toward mixed forests with sweet chestnut becoming increasingly more abundant. In old stands, C. sativa is likely to persist under a gap-phase mode of regeneration. Our results partially refute the traditional view that C. sativa is unable to recruit in the absence of cultural inputs.

Las Hoyas in the “dehesa” of Cañal (Salamanca, Spain), an ancient sedimentary gold mining excavation

The site known as Las Hoyas, in the municipal area of Pelayos (Salamanca), is a large open-cast mining pit of great antiquity, where sedimentary gold was exploited. Its typology corresponds to that used in traditional Roman mining, but nearby ruins belong to a Visigothic hamlet. The presence of several Roman settlements, roads and bridges in the area, joined to ruins with names of possible Roman origin, not yet studied, suggest the mine was active during the height of the Roman Empire. The sediments bearing the gold are continental of Cenozoic age (Oligocenic) and fill a tectonic groove of Alpine origin confined by SW-NE main faults and NW-SE secondary faults. The source area is the close reliefs placed to the SE, which are the first buttresses of the Spanish Central System.

Magnetic susceptibility of Variscan granite-types of the Spanish Central System and the redox state of magma

Magnetic susceptibility (MS) has been measured in Variscan granites from central Spain. They yield values in the order of 15 to 180µSI units for S- and I-type granites, indicating that both types belong to the ilmenite series. Only samples from magnetite-bearing leucogranites from the I-type La Pedriza massif show high MS values, in the order of 500-1400µSI, reflecting the presence of this ferromagnetic mineral. Mineral chemistry of magmatic Fe-rich minerals (mainly biotite) suggests similar oxidation values for both granite types. MS values change in highly fractionated granites accordingly either with the presence of rare new Fe-oxide phases (some I-type leucogranites) or with the marked modal amount decrease of Fe-rich minerals (I- and S-type leucogranites). The redox state in highly fractionated granite melts is mostly controlled by magmatic processes that modify redox conditions inherited from the source region. Thus, the occurrence of magnetite or ilmenite in granites is primarily controlled by the oxidation state of the source material but also by the differentiation degree of the granite melt. The presence of magnetite in some Variscan I-type leucogranites might be a consequence of crystal fractionation processes in a more limited mafic mineral assemblage than in S-type granite melts.

Basic Ordovician magmatism of the Spanish Central System: Constraints on the source and geodynamic setting

New geochemical and geochronological data obtained from metabasites intrusive into pre-Floian metasedimentary and metaigneous rocks of the Spanish Central System (Revenga and El Caloco sectors) represent a complex pre-Variscan magmatic event. Analysed samples exhibit tholeiitic affinity but relatively high incompatible element contents. These rocks can be distinguished from similar tholeiitic SCS metabasites because they represent more primitive liquids with higher metal (Cr, Ni) and LILE contents and display a slightly enriched isotopic (Sr, Nd) composition. These data point to the involvement of several mantle sources including lithospheric sections with a crustal imprint. Two samples yield Ordovician U–Pb zircon intrusion ages of 473.1 (+3.8/−6.8) and 453.3±4.6Ma. This episode of basic magmatism is not coetaneous with the abundant Cambrian–Ordovician felsic orthogneisses of the Central Iberian Zone and likely represents a rifting context (Rheic Ocean opening) which started about 477Ma. A comparison with Lower to Middle Paleozoic magmatic rocks from other west European terranes implies a heterogeneous evolution from the Ediacaran to Middle Paleozoic in the northern margin of Gondwana. While western terranes (e.g., Armorican Massif, Saxo–Thuringian Zone, Ossa–Morena Zone) display monotonous shelf sedimentation and magmatic quiescence from the Upper Ordovician to Variscan collision, eastern terranes (e.g., Central Iberian Zone, Corsica–Sardinia, Alps, Pyrenees) exhibit magmatism of contrasting geochemical affinity, including basic alkaline and tholeiitic series, indicative of a more complex tectonic evolution.

The dialectic between deciduous and coniferous forests in central Iberia: A palaeoenvironmental perspective during the late Holocene in the Gredos range

Pollen analyses supported by fifteen AMS 14C dates from Fuente de la Leche (1382 m) and Fuente del Pino Blanco (1343 m) peat sequences, in the Gredos range (central Spain), are used to reconstruct the late Holocene vegetation history in this mountain region. These results are compared with other sequences from the Spanish Central System in order to better understand the past dynamics of the main forest constituents and to provide a critical view of the dialectic between conifers and deciduous forests. The vegetation record at Fuente de la Leche starts at 3000 cal yr BP, with the occurrence of rather closed forests composed of Pinus sylvestris and Betula. Pinus sylvestris was the dominant pollen taxa for almost the entire period. However, during the last 850 years, Pinus sylvestris forests were progressively replaced by Quercus pyrenaica woodlands and grasslands, as a result of a high pastoral pressure, until forming mixed woodlands ∼400 years ago. The interpretation of Fuente del Pino Blanco record is that dramatic changes started about 265 years ago. Prior to this date, an open canopy woodland included species (Alnus glutinosa, Quercus pyrenaica) whose distributions no longer exist in the studied area. We suggest that the increase in agricultural (rye and other cereals) and livestock activities forced a change in the role of fire in the supramediterranean belt of the Gredos range, thus Pyrenean oak canopy was consumed by fires, providing openings for Pinus pinaster and broom communities better adapted to fire.