Ossa-Morena Zone Research Articles

Geologic map of the Valencia del Ventoso Variscan igneous complex (SW Iberian Massif, Spain): an example of multi-stage intrusion building by contrasted magma compositions

A map of the areal distribution of lithotypes within the Valencia del Ventoso igneous complex (ca. 130 km2) is presented. The complex is located in the southern Ossa-Morena Zone of the Iberian Massif (Variscan Foldbelt, Spain). Field and laboratory data reveal an intricate geological context resulting from multiple, compositionally contrasted, melt pulses emplaced over a short time period. According to field relationships and lithotype characteristics, four main plutonic intrusions and two dyke swarms are discerned within the complex. Firstly, three small (<10 km2) stocks (Medina de las Torres, Valencia del Ventoso, and Pozuelo) were emplaced aligned N40E. These are composed of massive and banded gabbro, gabbrodiorite, diorite, quartzdiorite, quartzmonzodiorite and tonalite. The Valencia del Ventoso main pluton (ca. 100 km2) has been intruded, in order of emplacement, a Basic Unit of gabbro and quartzdiorite in the central area of the complex, and an Acid Unit of quartzmonzodiorite, granodiorite and monzogranite in outermost areas and through the Basic Unit. Finally, two E-W trending dyke swarms were emplaced. The first one, rhyolitic in composition, relates to the most evolved residual liquids of the plutonic association. The second one accounts for a different magmatism, Late Permian to Triassic in age, and includes rhyolite, trachyte, latite, andesite and basalt lithotypes. The geologic map of the Valencia del Ventoso plutonic complex at 1:25,000 scale is presented on a high-resolution digital terrain model base as a reference for field-oriented geologic studies in igneous petrology.

Geobiology of a lower Cambrian carbonate platform, Pedroche Formation, Ossa Morena Zone, Spain

The Cambrian Pedroche Formation comprises a mixed siliciclastic–carbonate succession recording subtidal deposition on a marine platform. Carbonate carbon isotope chemostratigraphy confirms previous biostratigraphic assignment of the Pedroche Formation to the Atdabanian regional stage of Siberia, correlative to Cambrian Series 2. At the outcrop scale, thrombolitic facies comprise ~60% of carbonate-normalized stratigraphy and coated-grains another ~10%. Petrographic point counts reveal that skeletons contribute at most 20% to thrombolitic inter-reef and reef-flank lithologies; on average, archaeocyath clasts make up 68% of skeletal materials. In contrast, petrographic point counts show that skeletons comprise a negligible volume of biohermal and biostromal thrombolite, associated nodular carbonate facies, and ooid, oncoid and peloid grainstone facies. As such, archaeocyathan reefal bioconstructions represent a specific and limited locus of skeletal carbonate production and deposition. Consistent with data from coeval, globally dispersed lower Cambrian successions, our analysis of the Pedroche Formation supports the view that lower Cambrian carbonates have more in common with earlier, Neoproterozoic deposits than with younger carbonates dominated by skeletal production and accumulation.

HYDROTHERMAL PALLADIUM ENRICHMENT IN PODIFORM CHROMITITES OF CALZADILLA DE LOS BARROS (SW IBERIAN PENINSULA)

The serpentinized peridotite massifs of Calzadilla de los Barros (Ossa-Morena zone, Iberian massif) consist of harzburgites and small dunite bodies containing Al-rich chromitite pods. Minor discordant clinopyroxenite veins cut the peridotite massifs. The chromitites show total transformation of the silicate matrix into chlorite and post-magmatic formation of chalcopyrite, millerite, magnetite, and chalcedony. Olivine in harzburgites and dunites is mainly transformed into pargasite. The clinopyroxenites show diopside transformation into epidote, grossular, and titanite and locally into hydrous minerals (chlorite, pargasite, and tremolite) associated with the formation of Cu-Fe-Ni sulfides, suggesting significant hydrothermal alteration. The chondrite-normalized platinum-group elements patterns of the chromitites are very similar to those of the harzburgites and clinopyroxenites, showing an unusual sharply positive slope from Pt to Pd, as well as high Pd/Ir ratios. In the chromitites, the Pd/Ir ratio shows both positive and negative linear correlations with bulk platinum-group element contents. These results suggest that most of the Os, Ir, and Ru were incorporated during the magmatic phase, while Pd and Pt were incorporated during post-magmatic processes of hydrothermal alteration and formation of Cu-Fe-Ni sulfides. This study provides a better understanding of the post-magmatic processes that allow Pd enrichment in ophiolite complexes, and highlights how platinum-group element patterns can provide significant insights into both magmatic and post-magmatic processes.

High-pressure metamorphism in the Early Variscan subduction complex of the SW Iberian Massif

Several units exposed in the boundary area of the Ossa Morena Zone (OMZ) and the South Portuguese Zone (SPZ) preserve petrographic and thermobarometric evidence for an early metamorphic episode (M1), developed under a high-P, low to intermediate-T gradient, related to Early Variscan subduction in the SW Iberian Massif. In the OMZ, these are the Cubito–Moura Unit (Pmin~9.2kbar and T=395±45°C), blueschists from its basal mélange (Pmin~12.4kbar and T=310±11°C), and the underlying Fuenteheridos Group (P=10.9±0.4kbar and T=449±31°C). The equivalent units in the SPZ are the La Minilla Formation (P=8.7±0.4kbar and T=388±16°C) and the lawsonite pseudomorphs-bearing Pulo do Lobo Formation. All these units formed part of an approximately NE verging orogenic wedge (present coordinates) developed by the accretion of subducted slabs of the outermost margin of Gondwana and other elements of the Rheic Ocean realm, from at least the Middle Devonian to the lowermost Tournaisian. High-pressure rocks were subsequently emplaced on more internal zones of the OMZ that only experienced a younger high-T, low to intermediate-P metamorphism (M2). This high-T event was coeval with magmatic activity from the uppermost Devonian to the Middle Mississippian, probably as a consequence of transtensional lithospheric thinning and/or delamination of the lower crust and mantle lithosphere of the Gondwana margin. Pre-Late Devonian synorogenic sedimentation in forearc and back-arc basins of the subduction complex evolved to a Late Devonian–Middle Mississippian foreland basin system in early collisional stages. Finally, a new Middle–Late Mississippian fold-and-thrust belt with opposite (SW) vergence and new foreland basins developed during late collisional stages.

Multi-fractal modelling and spatial Cu-soil anomaly analysis along the southern border of the Iberian Terrane in Portugal

The present study examines a wide Cu-soil geochemistry survey covering an area of ca. 570km2 along the southern border of the Iberian Terrane in Portugal, from Ferreira do Alentejo to Serpa. The surveyed area covers the southern border of the Ossa Morena Zone (OMZ), largely dominated by the Beja Igneous Complex (BIC) and adjoining remnants of the Beja Acebuches Ophiolite Complex (BAOC), where different ore-forming systems are known to occur. The methodology employed combines geostatistical analysis and multi-fractal modelling to compute threshold values and assess the spatial variability of the Cu-soil data set.Values of regional threshold are variable and reflect the prevalence of different rock types occurring at the southern Iberian Terrane: (i) >40ppm when gabbroic (±diorite) rocks of BIC dominate over the country metavolcanic/metasedimentary sequences; (ii) 30–40ppm when a mixing of gabbroic rocks of BIC and BAOC exists; (iii) 20–30ppm when mesocratic to felsic BIC intrusive rocks predominate over gabbroic rocks; and (iv) <20ppm where the Cainozoic sediments cover a significant part of the surveyed area. Different types of Cu-soil anomalies are related to spatially co-existent but distinct (syngenetic or epigenetic/structurally controlled) Cu-sources, which can be distinguished on the basis of local anomalous values (Va) and anisotropy axial ratios (AAR) as follows: (i) Va≤500ppm and 1.8≤AAR≤2.5 trace Cu anomalies developed within gabbroic rocks without significant structural control; (ii) Va≤500ppm and 2.2≤AAR≤2.9 indicate Cu anomalies associated with major shear zones and/or subsidiary structures; (iii) Va≤600ppm and 1.5≤AAR≤3.3 point to very local Cu anomalies that usually define alignments related to strike–slip fault zones; and (iv) Va≤600ppm and 1.8≤AAR≤2.2 correspond to several anomalies following major lithological contacts, namely, between intrusive rocks of BIC and the metamorphic country rock sequences.

Geochemistry and provenance of the Carboniferous Baixo Alentejo Flysch Group, South Portuguese Zone

This work is focused on the turbiditic sediments from the Carboniferous Baixo Alentejo Flysch Group (BAFG) in the South Portuguese Zone, an external zone of the Iberian Variscides. The aim of this work is to constrain the provenance and tectonic setting of these sediments in a context of a complex evolution of SW Iberian Variscides. For this purpose, we performed a systematic study of petrographical and geochemical signatures of greywackes and shales from the three BAFG formations: Mértola, Mira and Brejeira. Major and trace element composition and ratios suggest heterogeneous source area composition for BAFG shales and greywackes. For the oldest Mértola Formation greywackes, source area is dominated by granitoid rocks with minor mafic input. The latter becomes residual in the Mira Formation. The youngest Brejeira Formation greywackes show clear felsic affiliation associated with an increase in recycled components. The shales of all three BAFG formations denote a granodioritic affiliation. Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration (PIA) values suggest moderate weathering in the source areas of Mértola and Mira formations. These indices, together with A–CN–K relations, point out to steady-state weathering conditions in the source areas of both formations. In contrast, both CIA and PIA values for the Brejeira Formation indicate variable conditions of palaeoweathering, from moderate to intense, as a consequence of non-steady-state conditions probably triggered by tectonic instability in the provenance area. Compared to the greywackes, the shales of all three BAFG formations exhibit higher CIA and PIA values, as well as low K2O/Al2O3 (~0.2) and index of compositional variability (<1), reflecting the cumulative effect of multiple cycles of sedimentary recycling and prolonged chemical weathering history. Major and trace element distribution and upper continental crust-normalized multi-element plots suggest that the sediments of BAFG were derived mainly from a continental arc/active margin with minor contribution from old continental crust. Together, our geochemical data are compatible with BAFG sediments derived mainly from SW border of the Ossa Morena Zone (Gondwanan affinity), with possible contribution from an external (Avalonian) source.

Adakitic-like magmatism in western Ossa–Morena Zone (Portugal): Geochemical and isotopic constraints of the Pavia pluton

Granitic rocks are a major component of the Earth's continental crust and occur in a wide variety of tectonic settings. Their chemical and isotopic characterization is crucial to the recognition of the potential sources and mechanisms involved in their generation. In this study, we present the first whole rock chemical and isotopic (Sr–Nd–O) data for the Pavia pluton (328–317Ma), located near the western border of the Ossa–Morena Zone (Évora Massif, Portugal). Major and trace element geochemistry suggests that the different granitic phases composing this intrusive body (enclaves, granites (s.l.) and crosscutting dikes) represent independent magma pulses and the majority is similar to TTGs and adakites. The little Sr–Nd–O isotopic variation, with (87Sr/86Sr)328=0.70428–0.70560, εNd328 ranging between −3.4 and +0.4 and δ18O varying from +5.6‰ to +8.4‰ implies an isotopically similar protolith for all phases. The most viable mechanism for the generation of the Pavia pluton adakitic-like magmatism is assimilation–fractional crystallization of a mantle-derived magma. This mechanism was also invoked to explain the genesis of other plutons within the Évora Massif but they have a distinct chemistry (typical arc calc-alkaline rocks). The chemical differences between them and the Pavia pluton granitic rocks are interpreted as the result of lower degrees of crustal assimilation and higher degrees of contamination of mantle-derived magmas by the sinking slab (after subduction blocking and subsequent slab break-off).

Multifractal analysis of geochemical anomalies: A tool for assessing prospectivity at the SE border of the Ossa Morena Zone, Variscan Massif (Spain)

The aim of this paper is to demonstrate discrimination of geochemical anomalies by application of multifractal analysis techniques to a stream sediment geochemical data (774 samples) from a geologically complex area at the boundary between the Ossa Morena (OMZ) and South Portuguese (SPZ) zones of the Iberian Variscan Massif. In this area, which is poorly investigated up to now, small to medium size iron–oxide–copper–gold (IOCG-type) and PbZnCu mineralizations are known to exist. The use of multifractal techniques highlighted weak anomalies not manifested by using conventional methods. The use of the C–A method has been very helpful for establishing thresholds separate anomalies of elements from the regional background, but local anomalies could not be identified as clearly as with the singularity mapping method. The characterization of selected elements (Fe, Cu, Pb, and Zn) is based on the use of multifractal spectrum curves f(α) and indices such as R, Δα and M (multifractality). These have proved to be very useful for characterizing different geochemical behaviors (e.g. depletion or enrichment of elements) as well as spatial distribution and singularity. The singularity (α) maps of the studied elements show areas more accurately compared to conventional geochemical maps, and reveal that local anomalies are not only spatially associated with known occurrences but also are disposed along trends consistent with structures known to be linked to mineralization. Therefore, singularity maps used as spatial evidence for integration with other layers of spatial evidence in mineral prospectivity mapping to provide a useful input for decision-making about further mineral exploration in the study area.

Cambrian rift-related magmatism in the Ossa-Morena Zone (Iberian Massif): Geochemical and geophysical evidence of Gondwana break‐up

Volcanic rocks of Cambrian age from Zafra (Ossa-Morena Zone, Iberian Massif) are the result of rift processes that affected Cadomian arc units accreted to the NW edge of Gondwana during the Neoproterozoic–Early Cambrian transition. Tephrite to rhyolite volcanics define an alkaline transitional association (Coombs type). Basic–ultrabasic rocks exhibit typical alkaline REE-patterns, strongly enriched in LREE with respect to HREE. Two parental magmas are identified, one with a mantle signature, lack of Nb negative anomaly and εNd500Ma +3.8 to +4.2; another with crustal contribution, minor Nb negative anomaly and εNd500Ma +0.8 to +1.8. Intermediate-acid rocks show variable REE fractionation and share geochemical characteristics of both basic–ultrabasic groups with restricted εNd500Ma +2.2 to 3.1 and general absence of Nb negative anomaly. Basic–ultrabasic melts resulted from different amounts of partial melting of a homogeneous source and segregation at the garnet–spinel transition zone. We argue that the “Hales transition” recently recognized in reflection seismic experiments of SW Iberia might image such a source region. Mantle-derived magmas ponded at the base of the crust and weakly interacted with crustal rocks/melts, whilst intermediate-acid rocks were generated by plagioclase±clinopyroxene±amphibole fractionation. Melt ascent took place through fractures, with limited crustal interaction. Based upon the new geochemical results and complementary cartographic and geophysical data, a model is presented for the Cambrian break-up of North Gondwana due to magma ascent from the mantle.

Geochemical Constraints on the Origin of the Ni–Cu Sulfide Ores in the Tejadillas Prospect (Cortegana Igneous Complex, SW Spain)

AbstractAfter the discovery of the Aguablanca ore deposit (the unique Ni–Cu mine operating in SW Europe), a number of mafic‐ultramafic intrusions bearing Ni–Cu magmatic sulfides have been found in the Ossa–Morena Zone of the Iberian Massif (SW Iberian Peninsula). The Tejadillas prospect is one of these intrusions, situated close to the border between the Ossa–Morena Zone and the South Portuguese Zone of the Iberian Massif. This prospect contains an average grade of 0.16 wt % Ni and 0.08 wt % Cu with peaks of 1.2 wt % Ni and 0.2 wt % Cu. It forms part of the Cortegana Igneous Complex, a group of small mafic‐ultramafic igneous bodies located 65 km west of the Aguablanca deposit. In spite of good initial results, exploration work has revealed that sulfide mineralization is much less abundant than in Aguablanca. A comparative study using whole‐rock geochemical data between Aguablanca and Tejadillas shows that the Tejadillas igneous rocks present a lower degree of crustal contamination than those of Aguablanca. The low crustal contamination of the Tejadillas magmas inhibited the assimilation of significant amounts of crustal sulfur to the silicate magmas, resulting in the sparse formation of sulfides. In addition, Tejadillas sulfides are strongly depleted in PGE, with total PGE contents ranging from 14 to 81 ppb, the sum of Pd and Pt, since Os, Ir, Ru and Rh are usually below or close to the detection limit (2 ppb). High Cu/Pd ratios (9700–146,000) and depleted mantle‐normalized PGE patterns suggest that the Tejadillas sulfides formed from PGE‐depleted silicate magmas. Modeling has led us to establish that these sulfides segregated under R‐factors between 1000 and 10,000 from a silicate melt that previously experienced 0.015% of sulfide extraction. All these results highlight the importance of contamination processes with S‐rich crustal rocks and multiple episodes of sulfide segregations in the genesis of high‐tenor Ni–Cu–PGE ore deposits in mafic‐ultramafic intrusions of the region.

Early carboniferous wrenching, exhumation of high-grade metamorphic rocks and basin instability in SW Iberia: Constraints derived from structural geology and U–Pb and 40Ar–39Ar geochronology

New U–Pb and 40Ar–39Ar geochronology and structural data from high- to medium grade metamorphic shear zones of the Ossa-Morena Zone, and structural data from Early Carboniferous basins (Ossa-Morena Zone and South-Portuguese Zone), place additional constraints on the Variscan tectonics in SW Iberia. A zircon U–Pb age of 465±14Ma (Middle Ordovician) measured on migmatite from the Coimbra–Cordoba shear zone is interpreted as the age of protolith crystallization. This age determination revises the information contained in the geological map of Portugal, in which these rocks were considered to be Proterozoic in age.This paper describes the evolution of Variscan wrench tectonics related to the development of shear zones, exhumation of deep crustal rocks and emplacement of magma in the Ossa-Morena Zone basement. In the Coimbra–Cordoba shear zone (transpressional), migmatites were rapidly exhumed from a depth of 42.5km to 16.6km over a period of ca. 10Ma in the Viséan (ca. 340–330Ma), indicating oblique slip exhumation rates of 8.5 to 10.6mm/yr (Campo Maior migmatites) and 3.2mm/yr (Ouguela gneisses) respectively. In the Évora Massif, the gneisses of the Boa Fé shear zone (transtensional) were exhumed from 18.5 to 7.4km depth in the period ca. 344–334Ma (Viséan), with exhumation oblique slip rates of 2.8 to 4.2mm/yr. At the same time, the Early Carboniferous basins of SW Iberia were filled by turbidites and olistoliths, composed mostly of Devonian rocks. The presence of olistoliths indicates significant tectonic instability during sedimentation with large-scale mass movement, probably in the form of gravity slides. Deformation and metamorphism dated at 356±12Ma, 321±13Ma and 322±29Ma respectively suggests that Variscan wrench movements were active in SW Iberia during the Early Carboniferous for a period of at least 35Ma.

The missing Rheic Ocean magmatic arcs: Provenance analysis of Late Paleozoic sedimentary clastic rocks of SW Iberia

Early Carboniferous turbiditic sedimentary rocks in synorogenic basins located on both sides of the Rheic suture in SW Iberia were studied for provenance analysis. An enigmatic feature of this suture, which resulted from closure of the Rheic Ocean with the amalgamation of Pangea in the Late Carboniferous, is that there are no recognizable mid- to Late Devonian subduction-related magmatic rocks, which should have been generated during the process of subduction, on either side of it. U–Pb LA–ICP-MS geochronology of detrital zircons from Early Carboniferous turbidites in the vicinity of the Rheic suture in SW Iberia, where it separates the Ossa–Morena Zone (with Gondwana continental basement) to the north from the South Portuguese Zone (with unknown/Meguma? continental basement) to the south, reveals the abundance of mid- to Late Devonian (51–81%) and Early Carboniferous (13–25%) ages. The Cabrela and Mértola turbidites of the Ossa–Morena and South Portuguese zones, respectively, are largely devoid of older zircons, differing from the age spectra of detrital zircons in the oldest (Late Devonian) strata in the underlying South Portuguese Zone, which contain abundant Cambrian and Neoproterozoic ages. Mid- to Late Devonian zircons in the Cabrela Formation (age cluster at c. 391Ma, Eifelian–Givetian transition) and Mértola Formation (age clusters at c. 369Ma and at c. 387Ma, Famennian and Givetian respectively) are attributable to a source terrane made up of magmatic rocks with a simple geological history lacking both multiple tectonic events and older continental basement. The terrane capable of sourcing sediments dispersed on both sides of the suture is interpreted to have been completely removed by erosion in SW Iberia. Given that closure of the Rheic Ocean required subduction of its oceanic lithosphere and the absence of significant arc magmatism on either side of the Rheic suture, we suggest: 1) the source of the zircons in the SW Iberia basins was a short-lived Rheic ocean magmatic arc, and 2) given the lack of older zircons in the SW Iberia basins, this short-lived arc was probably developed in an intra-oceanic environment.

North-Gondwana assembly, break-up and paleogeography: U–Pb isotope evidence from detrital and igneous zircons of Ediacaran and Cambrian rocks of SW Iberia

The Estremoz Anticline in the Ossa-Morena Zone (SW Iberia) includes upper Ediacaran detrital rocks unconformably overlain by lower Cambrian detrital and carbonate rocks. The spectra of detrital zircon U–Pb ages dominated by Cryogenian and Ediacaran ages (with a typical gap in Mesoproterozoic ages) of the upper Ediacaran greywackes and lower Cambrian arkosic sandstones indicate provenance from sources placed near or at the North-Gondwana margin. These late Ediacaran and early Cambrian basins developed in a paleoposition close to the West African Craton and related to a long-lived Neoproterozoic magmatism (c. 850–700Ma — Pan-African suture; c. 700–635Ma — early Cadomian arc; and c. 635–545Ma — late Cadomian arc).The rhyolites of the Volcanic–Sedimentary Complex of Estremoz whose stratigraphic position so far has been a controversy, yielded an upper Cambrian age (Furongian) at about 499Ma indicating that carbonate production was episodic in SW Iberia during the Cambrian. This new evidence should be taken into account in the reshaping of paleogeographic reconstruction models that have erroneously insisted on placing Iberia at southerly cold water higher latitudes (>60°S) during the Furongian.

The provenance of Late Ediacaran and Early Ordovician siliciclastic rocks in the Southwest Central Iberian Zone: Constraints from detrital zircon data on northern Gondwana margin evolution during the late Neoproterozoic

U–Pb geochronology of detrital zircon from Late Ediacaran (Beiras Group greywackes) and Early Ordovician (Sarnelhas arkosic quartzites and Armorican quartzites of Penacova) sedimentary rocks of the southwest Central Iberian Zone (SW CIZ) constrain the evolution of northern Gondwana active-passive margin transition. The LA-ICP-MS U–Pb data set (375 detrital zircons with 90–110% concordant ages) is dominated by Neoproterozoic ages (75% for the greywakes and 60% for the quartzites), among which the main age cluster (more significant for Beiras Group greywackes) is Cryogenian (c.840–750 Ma), while a few Mesoproterozoic and Tonian ages are also present (percentages <8%). These two features, and the predominance of Cryogenian ages over Ediacaran ages, distinguish the Beiras Group greywackes (SW CIZ) from the time-equivalent Serie Negra (Ossa-Morena Zone – OMZ), with which they are in inferred contact. The age spectra of the Beiras Group greywackes also reveal three major episodes of zircon crystallisation in the source area during the Neoproterozoic that are probably associated with a long-lived system of magmatism that developed either along or in the vicinity of the northern Gondwana margin at: (1) c. 850–700 Ma – Pan-African suture (not well represented in OMZ); (2) c. 700–635 Ma – early Cadomian arc; and (3) c. 635–545 Ma – late Cadomian arc. Comparison of Neoproterozoic ages and those of the Paleoproterozoic (c. 2–1.8 Ga) and Archean (mainly Neoarchean – 2.8–2.6 Ga, but also older) in the Beiras Group greywackes with U–Pb ages of Cadomian correlatives shows that: (1) SW CIZ, OMZ, Saxo-Thuringian Zone, North Armorican Cadomian Belt and Anti-Atlas) evolved together during the formation of back-arc basins on the northern Gondwana active margin and (2) all recorded synorogenic basins that were filled during the Ediacaran by detritus resulting from erosion of the West African craton, the Pan-African suture and a long-lived Cadomian magmatic arc. Differences in detrital zircon age populations in the greywackes of the Beiras Group (SW CIZ Cadomian basement) and the Serie Negra (OMZ Cadomian basement) are also observed in their respective overlying Early Ordovician quartzites. Since both these SW Iberia Cadomian basements evolved together along the active margin of Gondwana (but sufficiently separated to account for the differences in their detrital zircon content), this continuation of differing zircon populations into the Early Ordovician suggests that the inferred contact presently juxtaposing the Beiras Group and the Serie Negra is not pre-Early Ordovician and so is unlikely to demonstrate a Cadomian suture.

Tectonic escape of a crustal fragment during the closure of the Rheic Ocean: U–Pb detrital zircon data from the Late Palaeozoic Pulo do Lobo and South Portuguese zones, southern Iberia

Abstract: The Pulo do Lobo Zone, which crops out immediately north of the allochthonous South Portuguese Zone in southern Iberia, is classically interpreted as a polydeformed accretionary complex developed along the southern margin of the Gondwanan parautochthon (Ossa–Morena Zone), during the late Palaeozoic closure of the Rheic Ocean. This closure was a major event during the amalgamation of Pangaea. U–Pb laser ablation inductively coupled mass spectrometry dating of detrital zircons from late Palaeozoic Devono-Carboniferous clastic units in the South Portuguese Zone and Pulo do Lobo Zone yield contrasting age populations and attest to the exotic nature of both zones. Detrital zircons from the South Portuguese Zone display populations typical of detritus derived from either Gondwana (Ossa–Morena Zone), or peri-Gondwanan terranes. In contrast, rocks from the Pulo do Lobo Zone contain populations consistent with derivation from Baltica, Laurentia or recycled early Silurian deposits along the Laurentian margin. An example of one such deposit is the Southern Uplands terrane of the British Caledonides. Taken together, these data can be reconciled by a model involving tectonic transport of a crustal fragment that was laterally equivalent to the Southern Uplands terrane between the allochthonous South Portuguese Zone and Gondwana as a result of an early Devonian collision between an Iberian indenter with Laurussia. Supplementary material: U–Pb data tables, concordia diagrams, methods and representative back-scattered electron images are available at http://www.geolsoc.org.uk/SUP18441 .

Constraints on the crustal structure of the internal Variscan Belt in SW Europe: A magnetotelluric transect along the eastern part of Central Iberian Zone, Iberian Massif

[1] The Iberian Massif is the best exposed segment of the European Variscan Belt. It includes relatively well preserved terranes that were accreted by transpression along time and resulted in a number of geotectonic units that formed part of the Late Paleozoic assembly of the Pangaea Supercontinent. In SW Iberia, these units are the Central Iberian Zone (CIZ), Ossa Morena Zone (OMZ), and the South Portuguese Zone (SPZ). A 210 km long NE-SW magnetotelluric profile was carried out through the CIZ, from the OMZ-CIZ boundary toward the north, reaching the Tagus (Cenozoic) basin. Data dimensionality analysis resulted in a suitable 2-D electrical resistivity structure, allowing a 2-D inversion of the data set. Complementary available geophysical data (deep seismic, gravity and aeromagnetic) and a comparison with a detailed geological cross section led us to constrain the interpretation of the 2-D electrical resistivity structure of the CIZ crust. The results show, for the upper crust, the existence of diverse conductive/resistive bodies that correlate well with known geological features (sedimentary basins, faults, granitic plutons, mineralized systems). A mild but steady conductive band is located along the middle and lower crust that is interpreted as a mafic granulite basement. The upper section of this band connects with several elongated shallow conductors, providing further evidence for the existence, in the Central Iberian Zone, of a complex decollement system where the major faults are rooted. Such a crustal architecture is viewed as the northward continuation of the Variscan large-scale structures previously recognized in the southern sectors (OMZ and SPZ).

Tectonic style and structural analysis of the Puebla de Guzmán Antiform (Iberian Pyrite Belt, South Portuguese Zone, SW Spain)

Abstract In the South Portuguese Zone (Iberian Massif), thin-skinned tectonics linked to the collision with the Ossa-Morena Zone produced the inversion of previous extensional basins in Carboniferous times. Its central domain, namely the Iberian Pyrite Belt, underwent two deformation phases at mostly low-grade metamorphic conditions linked to a progressive deformation migrating upwards from a basal detachment and from north to south. The Puebla de Guzmán Antiform is one of the most outstanding cartographic structures in the Iberian Pyrite Belt, representing a imbricate fan thrust system developed during the second regional deformation phase. In the Puebla de Guzmán Antiform, the first deformation phase gave rise to a penetrative slaty cleavage (S 1 ), which is also recognized in the whole Iberian Pyrite Belt and constitutes the main foliation all over the region. Its genesis is possibly linked to the coetaneous development of thrusts at deeper crustal levels and SSW-vergent folds at all levels above these thrusts. First phase structures were deformed by large-scale imbricated thrust systems with lateral (NNE–SSW) and frontal (WNW–ESE) ramps, which constitute the most relevant regional cartographic structures. This second deformation phase generated thrusts, two set of folds with WNW–ESE and NNE–SSW-oriented axes, as well as two related axial plane crenulation cleavages. These relatively brittle to ductile-brittle second phase structures have been identified in many areas of the Iberian Pyrite Belt, and especially in the southern limb of the Puebla de Guzmán Antiform. The second phase thrusts reported from the Puebla de Guzmán Antiform have not been folded according to both the geological map of the area and the analysis of maximum shortening and stretching axes.

Tracing the Cadomian magmatism with detrital/inherited zircon ages by in-situ U–Pb SHRIMP geochronology (Ossa-Morena Zone, SW Iberian Massif)

U–Pb SHRIMP geochronology on zircon extracted from a granite, a sandstone and a quartzite of the SW Iberian Massif (Ossa-Morena Zone), was used in order to investigate the contribution of Late Neoproterozoic (Cadomian) tectonothermal history to the crustal growth of northern Gondwana. The analysed Cambrian rocks were sampled along the southern margin of the Coimbra–Cordoba shear zone (Barquete granite and Crato sandstone), and within this shear zone (Ouguela quartzite). The Barquete granite yielded a crystallization age of 526 ± 4 Ma and evidence for Neoproterozoic and Paleoproterozoic inherited ages. The geochemical signature of the Barquete granite matches the igneous activity of the initial magmatic flooding of the Ossa-Morena Zone during the Early Cambrian. The petrography and geochemical signature of the Crato and Ouguela sandstones indicate that these clastic rocks were mainly derived from felsic igneous rocks. The youngest detrital zircon grain extracted from the Crato sandstone, dated at 552.7 ± 5.5 Ma, has a zircon overgrowth that yielded 532 ± 5.6 Ma. In the Ouguela quartzite, the youngest concordant age yielded 556.4 ± 5.7 Ma, but we also found a zircon overgrowth dated at 536.2 ± 5.6 Ma. The ages of the youngest detrital zircon in each sedimentary rock place an upper limit on the deposition age of Early Cambrian (c. 536–532 Ma). Four main Late Neoproterozoic age clusters at c. 640–638 Ma, c. 612–613 Ma, c. 590–585 Ma and c. 560 Ma were recognized in the populations of detrital zircons from both sandstones. These ages of zircon-forming events seem to represent four successive thermal/magmatic pulses that overlap the Cadomian and Pan-African orogenies. These findings match other results published for the OMZ. Our U–Pb results show that detrital zircons in siliciclastic sediments and inherited zircon in granites are dominated by Neoproterozoic ages and few Paleoproterozoic and Archean ages. Those ages reported in the Ossa-Morena Zone rocks, together with a remarkable lack of Mesoproterozoic ages suggest that the clastic rocks in this peri-Gondwana basin were derived from the West African craton. The large population of Late Cryogenian and Ediacaran ages indicates denudation of the Cadomian basement during the Early Cambrian. A potential source for the detrital and inherited zircons found in this study is a long-lived magmatic arc that is only partly exposed in the SW Iberian Massif but is well represented in other peri-Gondwana regions. The large amount of Cadomian ages obtained in this study (c. 640–560 Ma) reinforces the idea that Cadomian magmatism played a significant role in the continental crustal growth history of Late Neoproterozoic uplift and erosion in Western and Central Europe (Cadomia).

3D geological modelling of a polyphase deformed pre‐Variscan IOCG mineralization located at the southeastern border of the Ossa Morena Zone, Iberian Massif (Spain)

AbstractA previously unrecorded group of stratiform and stratabound‐type magnetite beds (IOCG) has been found at the southeastern border of the Ossa Morena Zone (Variscan Massif of Spain). These mineralizations, hosted in Upper Precambrian‐Lower Cambrian rocks, are located in a transpressive belt, and were strongly deformed by the Variscan Orogeny, thus showing polyphase noncoaxial folding. This folding has been recognized by the interference patterns observed in magnetite beds outcropping in old mine workings. A detailed mapping of several adits shows a Ramsay type 1 ‘egg box’ or dome‐and‐basin interference pattern which arises when both D1 (NW‐SE trending) and D2 (NE‐SW trending) fold axes and axial planes are orthogonal or at a high angle to each other. An analysis of the magnetitic layers using orthogonal thickness and conic functions has been carried out to characterize fold profiles geometry.Deformation is also accommodated by oblique WNW‐ESE trending thrust faults which produce centimetric to metric displacements of the magnetite beds. A 22% shortening in the NE‐SW direction and 24% in the NW‐SE direction has been estimated. Finally, as polyphase folding and thrust structures in three dimensions have a complex geometry, a 3D geological modelling has been carried out to obtain a better insight into the geometry of mineralized magnetite bodies. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Stratigraphic succession, facies and depositional environment of Emsian reefal carbonates in the Ossa-Morena Zone (SW Spain)

The Devonian succession between the Guadiana and Guadalquivir valleys in the Obejo- Valsequillo Domain comprises more than 600 m of shale and sandstone with some interbedded limestone and marl. The most complete reefal sequences of the region are exposed in the Guadamez-2 section, which is located on the western bank of the Guadamez River, near Zalamea de la Serena. It consists mainly of shale and calcareous shale in its lower part and shaly, skeletal and reefal limestone in its upper part. Conodont and brachiopod data indicate that this section spans the interval from the Lochkovian to at least the upper Emsian. Eight microfacies types have been identified in the calcareous facies: A, brachiopod-echinoderm wackestone/packstone; B1, echinoderm grainstone; B2, echinoderm-bryozoan grainstone/packstone; C, echinoderm packstone; D, tabulate coral rudstone; E, brachiopod wackestone/packstone; F, brachiopod grainstone/packstone; and G, stromatoporoid and tabulate coral boundstone/rudstone. Microfacies A, B1, C, E and F represent a sequence of environments from middle platform (A) to tidal flat (F), including shoals (B1) and shallow platform facies. Microfacies B2, D and G represent the development of patch-reefs super - imposed on the shoals.