The Cadomian orogeny in the Iberian Massif revisited

A Neoproterozoic suture is exposed at the contact between the Ossa Morena Zone and the Central Iberian Zone, in the Iberian Massif (Central Portugal), the westernmost segment of the European Variscides. Although, the Cadomian magmatic and tectonometamorphic events have been previously documented, their timing is still poorly constrained, particularly in the inner zones of the suture. We used geochronological (ID-TIMS U-Pb) data to establish the sequence of events, isotopic (Rb-Sr, Sm-Nd) data to characterize the magmatic sources and thermodynamic modelling to determine the maximum P-T conditions attained during the Cadomian metamorphism. The first event, in the future Ossa Morena Zone, is the onset of island arc magmatism represented mainly by tholeiites with a MORB signature. Their igneous crystallization age is unknown, but they are older than ca. 539Ma. This magmatic activity was accompanied by deposition of fine-grained sediments in a Neoproterozoic basin. The second event is the evolution of the Cadomian magmatic arc in different stages. The earliest magmatic stage occurs at ca. 692Ma, which is the oldest igneous age known in the Ossa Morena Zone. It is followed by the generation of subalkaline and peraluminous protoliths at ca. 569Ma, with the isotopic signature of old crustal sources. The final phase of the arc magmatism (ca. 548–544Ma) involved mainly partial melting of continental crust. The range of the main magmatic activity must have been between ca. 569Ma and ca. 544Ma as mentioned for other areas in the Ossa Morena Zone. A major metamorphic event, recorded in metamorphic monazite, zircon and titanite at ca. 540Ma, attained upper amphibolite facies conditions close to the transition to granulite facies (7–8kbar and 640–660°C). It represents the continental arc accretion of the Ossa Morena Zone with the Iberian Autochthon passive margin (future Central Iberian Zone). The Early Ordovician rocks (ca. 483–477Ma) were generated from depleted and juvenile sources. These rocks are strongly deformed and with melting features, display metamorphism at amphibolite facies conditions. They are interpreted as related with the Rheic Ocean.

A diverse assemblage of acritarchs, that represents the oldest fossil record of the Iberian Massif, is described from Ediacaran metasedimentary rocks of the Tentudía Formation included in the Ossa Morena Zone stratigraphy (Southwestern Iberia). The microfossils found include Assesserium pyramidalis, Cavaspina sp. A, Ceratosphaeridium sp. A, Dictyotidium sp. A, Multifronsphaeridium sp. A, Schizofusa zangwenlongii, Tanarium megaconicum Tanarium sp. A, and Tanarium ? sp. B, This assemblage is diagnostic of the mainly lower Ediacaran Doushantuo-Pertatataka acritarchs (c. 633—565 Ma.). The lithological diversity of the Tentudía Formation suggests that these Ediacaran sedimentary rocks were deposited in a diversity of environments (platform, slope and submarine fans, including mass transport deposits in a back-arc basin that extended into the southern part of the Central Iberian Zone. The mass transport deposits are represented in the Ossa-Morena Zone by the Salvaleón Olistostrome composed by turbidites including chert clast probably derived from the shelf. The migration of the mass transport deposits into the Central Iberian Zone (Orellana Formation) indicates the migration of the back-arc basin depocenter. Later inversion of the back-arc basin is marked by the deformation of the lower Ediacaran sedimentary rocks Serie Negra Group in the Ossa-Morena Zone and Domo Extremeño Group in the Central Iberian Zone. In the upper Ediacaran-Terreneuvian a carbonate platform (Ibor Group) was established in the southern part of the Central Iberian Zone unconformably overlying the previously deformed Domo Extremeño Group. The most proximal areas in the Ossa-Morena Zone are represented by the Malcocinado Formation which overlies the Serie Negra Group and the distal areas of the southern part of the Central Iberian Zone are represented by distal shelf and slope deposits of the Cijara Formation, which are overlain by mass transport deposits that include carbonate olistolites from the Ibor Group (such as the Fuentes and Membrillar olistostromes).

[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).

Electromagnetic imaging of Variscan crustal structures in SW Iberia: the role of interconnected graphite

Tectonic setting of Cadomian low-pressure metamorphism in the central Ossa-Morena zone (Iberian massif, SW spain)

The structure of a major suture zone in the SW Iberian Massif: the Ossa-Morena/Central Iberian contact

The south-western part of the Iberian Peninsula, including the southern branch of the Iberian Massif, has recently been the subject of several magnetotelluric (MT) studies. This area is made up of three different tectonic terranes: the South Portuguese Zone (SPZ), the Ossa Morena Zone (OMZ) and the Central Iberian Zone (CIZ). The boundaries between these zones are considered to be sutures, which appear as high electrical conductivity anomalies in the MT surveys. The OMZ is characterised by a conductive layer at middle-lower crustal levels. To investigate the continuity of this conductive layer into the CIZ, a new MT profile was carried out. This 75-km long ENE profile goes through the boundary between the OMZ and the CIZ. The results of a two-dimensional magnetotelluric inversion revealed a high-conductivity anomaly in the transition OMZ/CIZ (the so-called Central Unit), which is interpreted as due to interconnected graphite along shear planes. High-conductivity anomalies appeared in the middle crust of the CIZ, whose geometry and location are consistent with the conductive layer previously found in the OMZ, thus confirming the prolongation of the conductive layer into the CIZ. The top of this layer correlated spatially with a broad reflector detected by a seismic profile previously acquired in the same area. This, together with other geological and petrological evidence, points to a common origin for both features.

New SHRIMP U–Pb ages of detrital zircons from Neoproterozoic low-grade metasandstones of the Schist–Graywacke Complex (Central Iberian Zone, Iberian Massif) sampled just below and above the intra-Alcudian unconformity at two selected locations contribute to reconstruct the geodynamic evolution of Iberia during the Cadomian orogeny in the north Gondwana margin. The youngest zircons (i.e., maximum depositional age) in the Lower Alcudian are c. 580–576 Ma, while those in the Upper Alcudian are c. 555–552 Ma. The obtained remarkable time gap of about 21 Ma supports the existence of a tectonic event in between. This event resulted in moderate folding (without related foliation/metamorphism) that verticalized the Lower Alcudian previous to the deposition of the Upper Alcudian. Additional evidence of late Cadomian tectonothermal events elsewhere in Iberia also fit in the interval c. 560–550 Ma. Combined with other geological data, the most probable maximum depositional ages are c. 580–560 Ma for the Lower Alcudian (previous to the late Cadomian folding event) and c. 550–540 Ma for the Upper Alcudian (previous to the deposition of the overlying Pusian Group and Lower Cambrian sandstones and limestones). A comparison of the new zircon age spectra with possible source areas verifies recent studies that point to the Cadomian foreland in the north Gondwana continent affected by the Pan-African orogeny: the West African Craton and/or the Saharan Metacraton. Furthermore, ongoing Cadomian arc-related magmatism in Iberia (c. 605–545 Ma) could have contributed as a local zircon source. The end of the Cadomian activity is marked by a transient stage (ephemeral Lower Cambrian platform) which preceded widespread Cambro–Ordovician rifting of north Gondwana.

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.

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

The boundary between the Central Iberian and Ossa-Morena zones, in south-central Spain, occurs within the Badajoz–Cordoba shear zone. The spatial and temporal relationships of structures across the La Codosera syncline, located within the Central Iberian zone just to the north of the shear zone, demonstrate a progressive increase in strike-slip shear towards the Badajoz shear zone. Within the region of the La Codosera syncline, Precambrian to Cambrian rocks were subject to pre-Ordovician tilting and open folding, accompanied by local faulting and veining, but no cleavage. These, together with overlying Palaeozoic rocks were subsequently affected by Hercynian ductile deformation, which produced steeply inclined folds, cleavage and faulting. Intense left-lateral shearing on the southern limb of the La Codosera syncline produced augen, shear bands, sub-horizontal stretching lineation, boudinage, variably oriented folds and steep faults, whereas the northern limb of the syncline is characterized by folding and thrusting in a transpressional regime, with limited net northward transport. Late Hercynian faulting produced N–S and NE–SW faults which overprint the early structures and granites. These rotate to a NW-SE trend and increase in right-lateral displacement as a result of domino or bookshelf faulting which can be attributed to continued left-lateral motion along the Badajoz shear zone.

Ediacaran and Early Cambrian sedimentary rocks from NW Iberia have been investigated for detrital zircon U–Pb ages. A total of 1,161 concordant U–Pb ages were obtained in zircons separated from four Ediacaran samples (3 from the Cantabrian Zone and one from the Central Iberian zone) and two Lower Cambrian samples (one from the Cantabrian Zone and one from the Central Iberian Zone). Major and trace elements including REE and Sm–Nd isotopes were also analyzed on the same set of samples. The stratigraphically older Ediacaran sequence in the Cantabrian Zone has a maximum sedimentation age of ca. 600 Ma based on detrital zircon content and is intruded by ca. 590–580 Ma granitoids constraining the deposition of this part of the sequence between ca. 600 and 580 Ma. The stratigraphically younger Ediacaran sequence in the Cantabrian Zone has a maximum sedimentation age of ca. 553 Ma. The Ediacaran sample from the Central Iberian Zone has an identical within error maximum sedimentation age of ca. 555 Ma. The detrital zircon U–Pb age patterns are very similar in all the Ediacaran samples from both zones including the main age groups ca. 0.55–0.75 Ga, ca. 0.85–1.15 Ga and minor Paleoproterozoic (ca. 1.9–2.1 Ga) and Archean (ca. 2.4–2.6 Ga) populations. Kolmogorov–Smirnov statistical tests performed on this set of samples indicate that they all were derived from the same parent population (i.e., same source area). The same can be said on the basis of Nd isotopes, REE patterns and trace element concentrations. The two Cambrian samples, however, show contrasting signatures: The sample from the Cantabrian Zone lacks the ca. 0.85–1.15 Ga population and has a high proportion of Paleoproterozoic and Archean zircons (>60 %) and a more negative eNd and higher TDM values than the Ediacaran samples. The Early Cambrian sample from the Central Iberian Zone has the same U–Pb detrital zircon age distribution (based on KS tests) as all the Ediacaran samples but has a significantly more negative eNd value. These data suggest apparently continuous sedimentation in the NW Iberian realm of northern Gondwana between ca. 600 and 550 Ma and changes in the detrital influx around the Ediacaran–Cambrian boundary. The nature and origin of these changes cannot be determined with available data, but they must involve tectonic activity on the margin as evidenced by the angular unconformity separating the Ediacaran and Lower Cambrian strata in the Cantabrian Zone. The absence of this unconformity and the apparent continuity of detrital zircon age distribution between Ediacaran and Cambrian rocks in the Central Iberian Zone suggest that the margin became segmented with significant transport and sedimentation flux changes in relatively short distances. As to the paleoposition of NW Iberia in Ediacaran–Early Cambrian times, comparison of the data presented herein with a wealth of relevant data from the literature both on the European peri-Gondwanan terranes and on the terranes of northern Africa suggests that NW Iberia may have lain closer to the present-day Egypt–Israel–Jordan area and that the potential source of the hitherto enigmatic Tonian–Stenian zircons could be traced to exposed segments of arc terranes such as that described in the Sinai Peninsula (Be’eri-Shlevin et al. in Geology 40:403–406, 2012).

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

Progressive opening of the Rheic Ocean led to the drifting away of one or several ribbon terranes, generally ascribed to Avalonia, and inaugurated a passive margin stage on the newly formed margin of NW Gondwana. In Iberia, which remained on the Gondwanan side of the ocean, the rift to drift transition is recorded in the Ossa Morena Zone in latest Furongian times and migrated towards more internal parts of the margin during the Lower Ordovician. The passive margin stage is characterized by development of open marine platform sedimentation locally punctuated by eruption/intrusion of mainly basaltic, alkaline volcanic rocks, during transient periods of tectonic extension. A progression from outer (Ossa Morena Zone), through intermediate (Central Iberian and West Asturian-Leonese Zone, to inner (Cantabrian Zone) shelf environments can be generally established, although with significant variations related to local tectonic development. The end of the passive margin stage is marked by the formation of syn-orogenic basins, which roughly migrate in the same direction, i.e. from external to internal parts of the margin, as a response to the propagation towards the foreland of the Variscan orogenic wedge.

Variscan syncollisional extension in the Iberian Massif: structural, metamorphic and geochronological evidence from the Somosierra sector of the Sierra de Guadarrama (Central Iberian Zone, Spain)