The Appinite-Migmatite Complex of Sanabria, NW Iberian Massif, Spain

The Sanabria appinitic rocks and host migmatites form an unusual,non-peri-batholithic complex in which all the typical membersof the appinite suite are present. It differs from most appiniticcomplexes in the deeper level of emplacement and the close temporaland spatial association with migmatites. Consequently, manyin situ relationships that resulted from the invasion of maficmagma into a crustal anatectic zone are extremely well preserved.The complex shows unequivocal relations between members of theappinitic suite and between these and migmatites derived byanatexis of a gneissic formation (Ollo de Sapo gneiss). Theserelations point to derivation of monzodiorites and biotite dioritesby hydrous basalt fractionation combined with fluid-assistedmelting of the crustal rocks surrounding the appinitic intrusions.This hydrous basic magma may be derived from an enriched regionof the mantle associated with subduction. Petrogenetic modelshave been tested using a combination of field relations andgeochemical data. Despite the complexity of the processes involved,it is concluded that water played an important role in the petrogenesisof the intermediate and mafic magmas. Reaction between monzodioritemelts and the host migmatites was responsible for the generationof a range of intermediate rocks within the complex. The needfor water to facilitate magma generation in both the mantleand the crust suggests that melting is linked with subduction.This interpretation has important implications because appiniticmagmatism may be considered as indicative of subduction processesinvolved not only in the generation of the mafic end-membersof the suite, but also in the generation of batholiths withwhich the appinitic rocks are spatially and temporally associated. KEY WORDS: appinite; monzodiorite; migmatite; Variscan orogen; Iberian massif     

The Central Iberian arc, an orocline centered in the Iberian Massif and some implications for the Variscan belt

An arcuate structure, comparable in size with the Ibero-Armorican arc, is delineated by Variscan folds and magnetic anomalies in the Central Iberian Zone of the Iberian Massif. Called the Central Iberian arc, its sense of curvature is opposite to that of the Ibero-Armorican arc, and its core is occupied by the Galicia-Trás-os-Montes Zone of NW Iberia, which includes the Rheic suture. Other zones of the Iberian Massif are bent by the arc, but the Ossa-Morena and South Portuguese zones are not involved. The arc formed during the Late Carboniferous, at final stages of thermal relaxation and collapse, and an origin related with right-lateral ductile transpression at the scale of the Variscan belt is proposed. The Central Iberian arc explains the width of the Central Iberian Zone, clarifies the position of the allochthonous terranes of NW Iberia, and opens new perspectives for correlations with the rest of the Variscan belt, in particular, with the Armorican Massif, whose central zone represents the continuation of the southwest branch of the arc detached by strike-slip tectonics.     

Experimental Constraints on Hercynian Anatexis in the Iberian Massif, Spain

We have studied experimentally the melting relationships ofthe Ollo de Sapo gneiss (OSG), an important crustal protolithfor the Iberian leucogranites, of possible volcanoclastic origin.The results of this study are compared with previously determinedPTt paths, allowing us to interpret the mechanisms of meltingand granitoid production during the Hercynian orogenic cycle.Phase relationships determined in fluid-absent experiments indicatethat the OSG is a fertile source for peraluminous leucogranites.The slope of the fluid-absent solidus is strongly controlledby the breakdown of Ms in the presence of Qtz, Pl and Kfs. Thissolidus curve has a positive slope ranging from dP/dT = 30 bar/°Cat low P (<6 kbar) to dP/dT = 70 bar/°C at higher P (6–15kbar). The relationships between the Ms vapour-absent solidusand the PTt metamorphic paths in different sectors of the Iberianmassif have two important implications: (1) melt productivityis strongly favoured at low P; (2) anatexis in the Iberian massifprobably took place by decompression associated with crustalthinning and extension. These results are in agreement withthe relationships between granite production and tectonic deformationphases observed in the Iberian massif. Our results emphasizethat anatexis is a process that is strongly controlled bothby the phase relationships of the crustal protoliths and bythe thermal structure of the continental crust. Consequently,one must be careful when assigning potential crustal protolithsto particular granite associations exclusively on the basisof geochemical comparisons. KEY WORDS: anatexis; Hercynian orogen; Iberian massif     

Geochronological constraints on the evolution of a suture: the Ossa-Morena/Central Iberian contact (Variscan Belt,south-west Iberian Peninsula)

One of the main tectonic boundaries of the Variscan Belt in the Iberian Peninsula is the Ossa-Morena/Central Iberian contact. This contact is marked by a highly deformed unit (Central Unit) which recorded an initial high-pressure/high-temperature metamorphic evolution. Rb-Sr whole-rock isotopic data from three gneissic bodies cropping out in the Central Unit yield two Late Proterozoic ages (690 ± 134 and 632 ± 103 Ma) and an early Palaeozoic age (495 ± 13 Ma), which we interpret as protolith ages. The two Late Proterozoic orthogneisses show initial ⁸⁷Sr/⁸⁶Sr ratios typical of mantle-derived materials or those with significant mantle participation (⁸⁷Sr/⁸⁶Sr > 0.709). These new radiometric data, together with ages previously published and the structural evolution of the Central Unit, lead to the conclusions that: (1) there are magmatic protoliths of Late Proterozoic and Early Palaeozoic ages; (2) the metamorphic evolution of this area, including the high-pressure event, belongs to the Variscan orogenic cycle; (3) the deformations observed affect the rocks of the entire Central Unit, accordingly they are post-Ordovician, i.e. Variscan; and (4) consequently, the Ossa-Morena/Central Iberian contact is interpreted here as a Variscan suture.     

Variscan tectonics in the Iberian Pyrite Belt, South Portuguese Zone

This paper aims to discuss the structural evolution of the Iberian Pyrite Belt during the Variscan Orogeny. It provides new structural data, maps and cross sections from the eastern part of the Iberian Pyrite Belt. Regional geology of the South Portuguese Zone and lithostratigraphy of the Iberian Pyrite Belt are first briefly summarised. Three roughly homoaxial deformation phases are distinguished, and are mainly characterised by south-verging multi-order folds, axial planar cleavages and thrusts. Three structural units are distinguished: the La Puebla de Guzmán and Valverde del Camino antiforms are rooted units related to the propagation of southward-directed thrust systems that may branch onto the lower décollement level of the South Portuguese Zone; El Cerro de Andévalo is a structurally higher unit, mainly composed of allochthonous D1 thrust nappes. No evidence of sinistral transpression has been found in the transected cleavage and the strike of S3 with respect to S2. Better evidence of transpression is the moderately to steeply westerly plunging folds that show S-type asymmetry in down-plunge view. Variscan deformation in the Iberian Pyrite Belt is defined as the combination of a dominant southwards shear and a sinistral E-shear caused by oblique continental collision between the South Portuguese plate and the Iberian Massif.     

P-T path determinations in the Tormes Gneissic Dome, NW Iberian Massif, Spain

The Tormes Gneissic Dome (TGD, NW sector of the Iberian Massif, Spain) is a high-grade metamorphic complex affected by a major episode of extensional deformation (D2). The syn-D2 P–T  path of the Lower Unit of the TGD was deduced from the analysis of reaction textures related to superimposed fabrics developed during exhumation, analysis of mineral zoning and thermobarometric calculations. It comprises an initial phase of decompression, determined using the tweequ thermobarometric technique, from 6.4–8.1 kbar at 735–750 °C (upper structural levels) and 7.2 kbar at 770 °C (lower structural levels) to 3.3–3.9 kbar and 645–680 °C. This evolution is consistent with the observed sequence of melting reactions and the generation of garnet- and cordierite-bearing anatectic granitoids. The later part of the syn-D2 P–T  path consisted of almost isobaric cooling associated with the thermal re-equilibration of the unit in the new structural position. This segment of the P–T  path is recorded by assemblages with And +Bt+Ms and Ms+ Chl +Ab related to the later mylonitic S2 fabrics, which indicate retrogression to low-amphibolite and greenschist facies conditions.     

Paleomagnetism of the Central Iberian curve's putative hinge: Too many oroclines in the Iberian Variscides

The Variscan mountain belt in Iberia defines a large “S” shape with the Cantabrian Orocline in the north and the Central Iberian curve, an alleged orocline belt of opposite curvature, to the south. The Cantabrian Orocline is kinematically well constrained, but the geometry and kinematics of the Central Iberian curve are still controversial. Here, we investigate the kinematics of the Central Iberian curve, which plays an important role in the amalgamation of Pangea since it may have accommodated much of the post-collisional deformation. We have performed a paleomagnetic study on Carboniferous granitoids and Cambrian limestones within the hinge of the curve. Our paleomagnetic and rock magnetic results show a primary magnetization in the granitoids and a widespread Carboniferous remagnetization of the limestones. Syn-kinematic granitoids show ca. 70° counter-clockwise rotations consistent with the southern limb of the Cantabrian Orocline. Post-kinematic granitoids and Cambrian limestones show consistent inclinations but very scattered declinations suggesting that they were magnetized coevally to and after the ~ 70° rotation. Our results show no differential rotations between northern, southern limb and the hinge zone. Therefore, we discard a late Carboniferous oroclinal origin for the Central Iberian curve.     

Petrogenesis of a late-Variscan rhyodacite at the Ossa Morena-Central Iberian zones boundary,Iberian Massif,Central Portugal: Evidence for the involvement of lithospheric mantle and meta-igneous lower crust

A late-Variscan rhyodacite is exposed at the contact between the Ossa Morena Zone and the Central Iberian Zone of the Iberian Massif, Central Portugal. Dykes of rhyodacite intruded the Série Negra Unit and the Sardoal Complex that are part of the Cadomian basement. The igneous crystallization age of the rhyodacite (308 ± 1 Ma) was obtained on igneous monazite by the ID-TIMS U-Pb method. It is broadly coeval with the emplacement of late-Variscan granitoids during the last deformation phase of the Variscan Orogeny (ca. 304–314 Ma) and with the development of the large late-Variscan strike-slip shear zones (ca. 307 Ma). The rhyodacite samples are calc-alkaline, show identical composition and belong to the same magmatic sequence. The rhyodacite isotopic signatures (Sm-Nd and δ¹⁸O) are consistent with depleted-mantle juvenile sources and the contribution of the meta-igneous lower crust. The input of mantle juvenile sources is related to Variscan reactivation of lithospheric fractures. The inherited Neoproterozoic (ca. 619 Ma) and Mesoproterozoic (ca. 1054 Ma) zircon ages, are similar to those of the Central Iberian Zone. This suggests that lower crust of the Central Iberian Zone was involved in the magma generation of the rhyodacite. Coeval late-Variscan magmatic rocks display a larger contribution from ancient crustal components, which may be attributed to the smaller volume and faster cooling rate of the rhyodacite and consequent lower melting of the crust. Mixing of juvenile mantle-derived melts with melts from the lower continental crust was followed by fractional crystallization of garnet and amphibole that remained in the source. Fractional crystallization of plagioclase, biotite, quartz and zircon occurred in shallower magma chambers. Fractional crystallization of zircon was not significant.     

Chert in the Iberian Pyrite Belt

Since lenses of chert are common within the volcano-sedimentary succession hosting the massive sulphide deposits of the Iberian Pyrite Belt (Spain and Portugal), we examined numerous chert occurrences, both petrographically and geochemically, to test their possible value for massive sulphide exploration. The chert is found at two main lithostratigraphic levels (upper and lower) that are also interpreted as massive-sulphide bearing. In both cases the chert is located at the top of acidic volcanic sequences or in the associated sediments; we have not been able to observe the relationships between massive sulphides and chert, but some of the large orebodies of the Province (Lousal, La Zarza, Tharsis, Planes-San Antonio body of Rio Tinto, Neves) are described as being locally capped by chert facies. Four main types are recognized among the chert and associated facies: (1) red hematitic chert?±?magnetite; (2) radiolarian and/or sedimentary-textured (conglomeratic) chert with hematite and/or Mn oxides; (3) pale sulphidic chert; (4) rhodonite and/or Mn carbonate?±?magnetite facies. In the Spanish part of the Province the radiolarian chert is confined to the upper level; the distribution of the other types appears to be haphazard. The hydrothermal origin of the South Iberian chert is shown by its high Fe-Mn and low Co-Ni-Cu contents. The presence of small positive Ce anomalies indicates a shallow marine environment (shelf or epicontinental sea), which is consistent with the volcanological and sedimentological data. The chert was emplaced below the sea floor through chemical precipitation and/or through alteration and replacement of the country rock, residual traces of which are ghost phenocrysts and high Al, Ti and rare earth contents. Macro- and microscopic relationships indicate that the oxide facies (hematite?±?magnetite) formed first, probably providing a protective insulating cover against the marine environment and enabling an evolution towards sulphide facies; a phase of Mn?carbonate and silicate + quartz?±?chlorite + sulphides appears to be even later. It was not possible, through discrimination, to isolate a chert that could be considered as representing a lateral marker of massive sulphides; moreover, both field observations and geochemical data seem to indicate a relative independence of this siliceous sulphide hydrothermal activity from the hydrothermal activity giving rise to the massive sulphides. Such is also indicated by the lead isotopic signature of the chert, which is appreciably more radiogenic than that of the massive sulphides; the lead enrichment in the sulphidic chert facies indicates the participation of a different source (sediments, sea water) from that of the massive sulphides. The hypothesis of an independent hydrothermal “chert” event can thus be envisaged, wherein the chert reflects submarine low-temperature hydrothermal activity that is most apparent during a “break” within the volcano-sedimentary succession and which may locally have competed with the high-temperature hydrothermal activity giving rise to the massive sulphides. The interest of the chert thus rests in its palaeodynamic significance, as a marker of periods of volcanic quiescence, and in its possible role as a protective insulating cap favourable to the deposition of massive sulphides.     

Low-Pressure Metamorphism in the Sierra Albarrana Area (Variscan Belt,Iberian Massif)

A low-pressure metamorphic zonation ranging from biotite tomigmatite zones occurs in the Sierra Albarrana area (VariscanBelt of southwestern Iberian Peninsula) in uppermost Precambrianto Lower Palaeozoic metasedimentary rocks. The principal deformationin this area is related to a major ductile shear zone whosecentral part is localized immediately to the southwest of theSierra Albarrana Quartzites. The metamorphism is synchronouswith respect to this deformation. The metamorphic zones aresymmetrically distributed with respect to the Sierra AlbarranaQuartzites. Pressure–temperature (P–T) conditionsare 3.5–4 kbar and range from 400°C (biotite zone)to 500°C (staurolite–garnet zone) up to 650–700°C(migmatite zone). We have not detected pressure variations alongthe different metamorphic zones. Relic kyanite is observed inthe form of inclusions in andalusite within veins in the lower-gradepart of the staurolite–andalusite zone. The low-pressuremetamorphism of the Sierra Albarrana area arises from a two-stagehistory including moderate crustal thickening followed by subsequentlocalization of deformation in a transcurrent shear zone duringpeak P–T conditions. Channelized fluid flow within themajor ductile shear zone may have contributed to the heat budgetof the low-pressure metamorphism. KEY WORDS: fluid flow; Iberian Massif; low-pressure metamorphism; shear zone; Sierra Albarrana area     

Lower Cretaceous charophytes from the Serrania de Cuenca,Iberian chain: Taxonomy,biostratigraphy and palaeoecology

The assemblages of fossil charophyte fructifications from the La Huérguina Formation at the La Huérguina stratotype and the Las Hoyas section, along with some other samples from isolated outcrops are composed of Atopochara trivolvis var. triquetra, Globator maillardii var. trochiliscoides, Globator maillardii var. biutricularis var. nov., Clavator harrisii var. reyi, Ascidiella cruciata and Mesochara harrisii. Some reworked utricles of Ascidiella iberica var. iberica were also found. The in situ assemblage belongs to the Cruciata-Paucibracteatus Biozone, which is Late Barremian–Early Aptian in age. These results confirm that the entire deposition of the La Huérguina Formation took place within this biozone and not earlier, as previously thought. The top of the unit can be limited to the uppermost Barremian with biostratigraphic data from ostracods. Globator maillardii var. biutricularis var. nov. is defined as the end form for the Globator lineage according to present knowledge. It is characterised by its unique utricle morphology, showing a basal ring representing a second, external utricular layer. This layer shows a primitive structure, reminiscent of G. maillardii var. mutabilis, in contrast to the internal utricle, which is more derived and similar to G. maillardii var. trochiliscoides. In comparison with other non-marine formations of the same age in the Iberian Chain and in Europe, the charophyte assemblages from the La Huérguina Formation appear to be relatively poor and monotonous, suggesting that some of the species found elsewhere never reached this part of the basin owing to the brief development of non-marine facies there. The main differences in composition between the samples studied are indicative of the palaeoecological conditions. Atopochara trivolvis triquetra was found to be dominant in shallow lacustrine facies and Globator maillardii var. trochiliscoides and var. biutricularis are associated with temporary lakes from a well-drained palustrine area. Clavator harrisii and Mesochara harrisii thrived on floodplains whereas Ascidiella cruciata grew in palustrine environments with significant edaphic activity.     

Geology and genesis of the Aznalcóllar massive sulphide deposits, Iberian Pyrite Belt, Spain

The Aznalcóllar mining district is located on the eastern edge of the Iberian Pyrite Belt (IPB) containing complex geologic features that may help to understand the geology and metallogeny of the whole IPB. The district includes several ore deposits with total reserves of up to 130 Mt of massive sulphides. Average grades are approximately 3.6% Zn, 2% Pb, 0.4% Cu and 65?ppm Ag. Mined Cu-rich stockwork mineralizations consist of 30?Mt with an average grade of 0.6% Cu. Outcropping lithologies in the Aznalcóllar district include detrital and volcanic rocks of the three main stratigraphic units identified in the IPB: Phyllite-Quartzite Group (PQ), Volcano-Sedimentary Complex (VSC) and Culm Group. Two sequences can be distinguished within the VSC. The Southern sequence (SS) is mainly detritic and includes unusual features, such as basaltic pillow-lavas and shallow-water limestone levels, the latter located in its uppermost part. In contrast, the Aznalcóllar-Los Frailes sequence (AFS) contains abundant volcanics, related to the two main felsic volcanic episodies in the IPB. These distinct stratigraphic features each show a different palaegeographic evolution during Upper Devonian and Lower Carboniferous. Massive sulphides occur in association with black shales overlying the first felsic volcanic package (V_A1) Palynomorph data obtained from this black shale horizon indicate a Strunian age for massive sulphides, and consequently an Upper Devonian age for the V_A1 cycle. Field and textural relationships of volcanics suggest an evolution from a subaerial pyroclastic environment (V_A1) to hydroclastic subvolcanic conditions for the V_A2. This evolution can be related to compartmentalizing and increasing depth of the sedimentary basin, which may also be inferred from changes in the associated sediments, including black shales and massive sulphides. Despite changes in the character of volcanism, the same dacitic to rhyolitic composition is found in both pyroclastic and subvolcanic igneous series. The main igneous process controlling chemical variation of volcanics is fractional crystallization of plagioclase (+accessories). This process took place in shallow, sub-surface reservoirs giving rise to a compositional range of rocks that covers the total variation range of felsic rocks in the IPB. The Hercynian orogeny produced a complex structural evolution with a major, ductile deformation phase (F₁), and development of folds that evolved to thrusts by short flank lamination. These thrusts caused tectonic repetition of massive and stockwork orebodies. In Aznalcóllar, some of the stockwork mineralization overthrusts massive sulphides. These structures are cut by large brittle overthrusts and by late wrench faults. The original geometric features of massive sulphide deposits correspond to large blankets with very variable thicknesses (10 to 100?m), systematically associated with stockworks. Footwall rock alteration exhibits a zonation, with an inner chloritic zone and a peripheral sericitic zone. Silicification, sulphidization and carbonatization processes also occur. Hydrothermal alteration is considered a multi-stage process, geochemically characterized by Fe, Mg and Co enrichment and intense leaching of alkalies and Ca. REE, Zr, Y and Hf are also mobilized in the inner chloritic zones. Three ore types occur, both in stockworks and massive sulphides, named pyritic, polymetallic and Cu-pyritic. Of these, Cu-pyritic is more common in stockworks, whereas polymetallic is prevalent in massive sulphides. Zoning of sulphide masses roughly sketches a typical VHMS pattern, but many alternating polymetallic and barren pyritic zones are probably related to tectonics. Although the paragenesis is complex, several successive mineral associations can be distinguished, namely: framboidal pyritic, high-temperature pyritic (300?°C), colloform pyritic, polymetallic and a late, Cu-rich high-temperature association (350?°C). Fluid inclusion data suggest that hydrothermal fluids changed continuously in temperature and salinity, both in time and space. Highest Th and salinities correspond to inner stockworks zones and later fluids. Statistic population analysis of fluid inclusion data points to three stages of hydrothermal activity, at low (<200?°C), intermediate (200–300?°C) and high temperatures (300–400?°C). ³⁴S values in massive sulphides are lower than in stockwork mineralization suggesting a moderate bacterial activity, favoured by the euxinoid environment prevailing during black shale deposition. The intimate relation between massive sulphides and black shales points to an origin of massive sulphides by precipitation and replacement within black shale sediments. These would have acted both as physical and chemical barriers during sulphide deposition. Hydrothermal activity started during black shale deposition, triggered by a rise in thermal gradient due to the ascent of basic magmas. We suggest a three-stage genetic model: (1) low temperature, diffuse fluid flow, producing pyrite-bearing lenses and disseminations interbedded with black shales; locally, channelized high-T fluid flow occurs; (2) hydrothermal cyclic activity at a low to intermediate temperature, producing most of the pyritic and polymetallic ores, and (3) a late high-temperature phase, yielding Cu-rich and Bi-bearing mineralization, mainly in the stockwork zone.     

Blue amphiboles,metamorphic regime and plate tectonic modelling in the Iberian Pyrite Belt

Riebeckite-arfvedsonite amphiboles occur in very low-grade metamorphosed doleritic sills at various localities within the Iberian Pyrite Belt. The alkali-amphiboles grew during sub-solidus hydrothermal spilitization of basalt associated with submarine massive sulphide ore formation. The riebeckite-arfvedsonite is only very rarely preserved, being converted to albite-chlorite during regional metamorphism. In the South Portuguese zone Hercynian regional metamorphic grade increases in a northward direction from zeolite facies south of the Pyrite Belt through prehnite-pumpellyite facies to the greenschist facies in its northernmost zone. Compositional and mineralogical data indicate a geothermal gradient in the order of 40–50 °C/km.Volcanism in the Pyrite Belt is essentially representative of a bimodal association of twoleiitic to alkalic basalt and dacite/rhyolite. Geochemical data for the Pyrite Belt mafic meta-volcanics contrast with available data for subduction related volcanic suites in orogenic belts but exhibit similarities with the basaltic members of basalt-rhyolite associations found in areas of extensional tectonics. It is proposed that the Iberian Pyrite Belt volcanism represents magmatic activity in an intra-continental basin undergoing rifting during the late Devonian and lower Carboniferous times. On leave from: Mineralogia e Geologia, Faculdade de Ciencias, Lisboa-2, Portugal     

Diagenetic K-feldspar pseudomorphs in the Triassic Buntsandstein sandstones of the Iberian Range, Spain

Early diagenetic K-feldspar in the Triassic Buntsandstein of the Iberian Range (Spain) occurs as pseudomorphs after detrital K-feldspar (Or<93) and plagioclase (Ab<96). These pseudomorphs are chemically pure (Or>99), untwinned, commonly heavily clouded by vacuoles and tiny inclusions, dark-luminescing and are composed of numerous fine euhedral crystals of K-feldspar. The latter property suggests that the pseudomorphs form via dissolution of detrital K-feldspar and plagioclase and precipitation of authigenic K-feldspar. X-ray diffraction analysis shows that the authigenic K-feldspar is intermediate microcline.     

Basalts, underplated gabbros and pyroxenites record the rifting process of the West Iberian margin

Summary New petrological data on magmatic rocks obtained from the Iberia Abyssal Plain and from the Gorringe Bank, combined with those already known on the Galicia Bank, allow to better constrain the formation of the West Iberian Margin. These three zones were sampled along East-West transects of the ocean-continent transition, immediately West or at the foot of the last tilted continental block of the margin.These sections expose similar lithological successions including scarce basalts, locally pillowed, resting directly on peridotites (predominant) which themselves include layers of pyroxenites and rare lenses of gabbros and gabbroic differentiates. The latter are locally strongly sheared and metamorphosed e.g. to chlorite schists.At the Galicia Bank the exposed undepleted lherzolites are considered to be typical of a sub-continental lithospheric mantle environment. The mantle sections exposed at both the Iberia Abyssal Plain and the Gorringe Bank are clearly different. The presence of plagioclase-bearing websterite lenses and of small alkaline pyroxenites within the predominant harzburgites, is unknown in pure oceanic environments and supports their lithospheric sub-continental origin.Along each transect, the mineralogy of the gabbroic series documents various depths of emplacement and deformation. The highest pressures near 0.8 GPa ( 24 km depth) at Galicia Bank and Iberia Abyssal Plain are estimated from Al-rich pyroxenes and from the occurrence of metamorphic spinels in the sheared gabbros (Iberia Abyssal Plain). These gabbros are interpreted as deriving from underplated magmas at the base of a slightly thinned continental crust. The lower pressures ( 0.6 GPa) registered in the Gorringe Bank gabbroic rocks and in some reexamined gabbros from the Galicia Bank, suggest the existence of successive intrusions during the stretching process.The few basalts sampled on top of the exposed peridotites of the sea floor are inferred to be among the first post-rift extrusive magmas. They differ slightly from the North to the South of the margin: enriched and transitional tholeiites occur at Galicia Bank and in the Iberia Abyssal Plain, transitional and depleted tholeiites at the Gorringe Bank. The trace elements and the isotope data available show that the basalts and the gabbros derived from similar heterogeneous mantle sources (10.3 Nd_T 3.6). The behavior of some incompatible elements (Nb in particular) documents locally either a possible fractionation of rutile in shallow lithospheric mantle or some contamination by the continental crust. A very slow lithospheric stretching, assisted by the intrusion, underplating, and the shearing of a few gabbroic sills, followed by the unroofing of heterogeneous subcontinental mantle can account for the formation of the entire West Iberian margin.

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Résumé De nouveaux résultats obtenus à partlr de l'étude des roches magmatiques dans la plaine Abyssale Ibérique et sur le Bane de Gorrlnge permettent avec ceux obtenus sur le Bane de Galice de mieux contraindre la formation de la marge Ouest Ibérique. Ces trois zones ont été échantillonnées selon des coupes Est-Ouest de la transition océan-continent, à partir du pied on immédiatement à l'Ouest des derniers blocs continentaux basculés.La croûte de chacun de ces secteurs est composée de rares basaltes, localement en coussin, reposant directement sur des péridotites (prédominantes) qui elles-mêmes contiennent des niveaux de pyroxénites et de rares lentilles de gabbros localement différenciés. Ces derniers sont souvent fortement cisaillés et métamorphises et parfois transformés en schistes chloriteux.Les péridotites fertiles (lherzolites) du Bane de Gallce sont considérées comme représentatives d'un manteau lithosphérique sous-continental. Cette origine est étendue aux sections mantelliques à dominante harzburgitique de la plaine Abyssale Ibérique et du Bane de Gorringe. En effet elles exposent de rares pyroxénites alcalines et d'abondantes lentilles de webstérite à plagioclase, inconnues en domaine purement océanique.La minéralogie des séries gabbroïques indique des conditions variées de mise en place des magmas. Les pyroxènes riches en aluminium du Bane de Galice et de la Plaine Abyssale Ibérique et la présence locale de spinelle dans les gabbros cisaillés (Plaine Abyssale Ibérique) supposent une cristallisation sous des pressions proches de 0.8 GPa ( profondeur de 24 km). Ces gabbros ne peuvent done pas avoir cristallisé sous une croûte océanique normale et sont interprétés comme provenant de magmas sous-plaqués à la base d'une croûte continentale faiblement amincie. Quelques gabbros du Bane de Gallce et ceux du Bane de Gorringe ont enregistré des conditions de cristallisation plus superficielles (à molns de 18 km), donc au sein de péridotites situées sous une croûte continentale plus mince ou inexistante. Dans un processus d'étirement crustal la mlse en place de ces gabbros apparaît ainsi échelonnée avec des venues précoces dans le cas de la Plaine Abyssale Ibérique et plus tardives dans le cas du Banc de Gorringe.Les basaltes reposant directement sur les péridotites du fond océanique correspondent aux premiers magmas extrusifs post-rift. Ils different sensiblement du Nord au Sud de la marge: depuis des tholeiites enrichies et transitionnelles exposees sur le Banc de Gallce et dans la Plaine Abyssale Ibérique, jusqu' à des tholeiites transitionnelles et appauvries sur le Banc de Gorringe. Les éléments traces et les données isotopiques disponibles montrent que les basaltes et les gabbros dérivent de sources mantelliques similaires et hétérogènes (10.3 Nd_T 3.6). Le comportement de quelques éléments incompatibles (Nb en particulier) témoigne localement d'un possible fractionnement de rutile à faible profondeur dans le manteau lithosphérique on d'une contamination par de la croûte continentale. Un amincissement lithosphérique très lent, partiellement aidé par l'injection, le sous-plaquage et le cisaillement de quelques sills de gabbro et suivi de la dénudation d'un manteau sous-continental hétérogène, rend compte de la formation de l'ensemble de la marge Ouest Ibérique.

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With 11 Figures     

Tertiary compressional deformation of the Iberian plate

The Tertiary deformation of the Iberian plate is here interpreted as the result of changes in the coupling between the Iberian–African plates. During the early stages of the Africa/Iberia subduction (Palaeocene), deformation was confined at the Betic plate boundary. From the Eocene, during the collision in the southern plate margin, compressional deformation delocalized and distributed throughout the Iberian plate. First, in the Pyrenees, where the main stage of thrusting occurred during the Late Eocene – Early Oligocene. Then (mainly Oligocene – Late Miocene), in the inner part of the Iberian plate, forming basement uplifts in the Iberian Chain and the Central System, in correspondence of pre-existing (Mesozoic and Variscan) structures. Finally, during the decay of compression inside the Iberian plate, extension took place the Mediterranean margin and the Alboran Sea.     

Conditions of kaolin illitization in the Permo-Triassic sandstones from the SE Iberian Ranges, Spain

Kaolin is a widespread authigenic clay mineral in the Permo-Triassic sandstones from the marginal areas of the SE Iberian Basin. However, relatively more extensive illitization of kaolin occurs in the SW basin margin compared with the slight occurrence in the NE margin. SEM analysis of sandstones reveals that illite replaced small kaolinite crystals while blocky dickite remained unaltered. Kaolin illitization is suggested to take place in relation to the maximum burial depth reached during Late Cretaceous post-rift stage. Vitrinite reflectance data denotes a maximum burial temperature of 118  °C and 144 °C in the NE basin margin and in the SW basin margin, respectively. Thus, the extent of illitization in the SW margin is attributed to the higher T_peak reached by the Permo-Triassic succession. Regarding the lack of K-feldspars observed in the sandstones and interbedded mudrocks, the source of K⁺ is mainly related to the alteration of detrital mica.     

A morphotectonic study of the Central System,Iberian Peninsula

The Central System (CS) or Spanish Central System forms part of two of the five morphotectonic mesoblocks that make up the Intermediate macroblock (number 6) of the Iberian plate. The combination of geological, geophysical and geomorphological data, used in accordance with the Rantsman methodology (1979), served as the basis for obtaining a regional analysis. The cartography obtained shows territorial units (4 macroblocks, 4 mesoblocks, 35 blocks, 85 microblocks and 162 nanoblocks), morphotectonic alignments (quantity/order: 3/2, 4/3, 5/23, 6/48, 7/93 and 8/164) and morphotectonic knots (quantity/order: 1/2, 5/3, 35/4, 85/5, 162/6, 324/7, 816/8). The number of delimited morphostructures increases from the central part towards the east. At the block level, one may distinguish a transverse differentiation of the territorial units and alignments, which is interpreted as an expression of the region’s lithospheric heterogeneity. There is a close relationship between the morphostructures and seismicity, indicating that greater activity occurs in the blocks of the eastern and northeastern sectors.     

History of vegetation during the Holocene in the Courel and Queixa Sierras,Galicia, northwest Iberian Peninsula

Palynological studies undertaken in the mountainous regions of the northwest of the Iberian Peninsula are few in number and have been concerned largely with the second half of the Holocene. New pollen data from two Galician sierras, the Courel and Queixa Sierras, provide a 10000‐yr record of vegetation and climate change. In the Courel Sierra before 9750 yr BP, Laguna Lucenza (1420 m a.s.l.) reflects a period of open landscape covered by Poaceae and heliophilous plants, which may be correlated with the Younger Dryas. The onset of the Holocene is characterised by the expansion of oak woodland, prior to 9300 yr BP, following a short phase of birch along with the gradual decline of pine. The oak values reach a peak at 8350 ± 80 yr BP. Towards 8800 yr BP Corylus begins to expand, followed by Alnus (7500 yr BP) and Ulmus. During this period, the Fraga pollen assemblage (Queixa Sierra, 1360 m a.s.l.) indicates Betula woodland surrounding the site, masking the regional predominance of oak. After 5000 yr BP there is a gradual decrease in arboreal pollen values in both Sierras. Castanea appears in Laguna Lucenza (Courel Sierra) at 4075 ± 75 yr BP. There is widespread deforestation during the last 4000 yr. During this period the presence of large quantities of microcharcoal particles points to the occurrence of fire. The reduction in forest is associated with the arrival of cultivation at 4000 yr BP at low altitudes in the Queixa Sierra. At higher altitudes the first agricultural activity is dated at later than 2000 yr BP. This coincides with the first record of cereal cultivation at high altitude in the Courel Sierra. Copyright © 2000 John Wiley & Sons, Ltd.