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Petrología de la unidad eclogítica del complejo de Cabo Ortegal (NW de España)
... GPa, 780-800 C), eclogite (1.8-2.2 GPa, 700-800 C), granulite (1.4-1.7 GPa, 740-835 C) and gneiss (approximately 1.5 GPa, 700 C) (Ábalos et al., 2003;Gil Ibarguchi et al., 1990;Mendia, 2000;Puelles et al., 2005;Tilhac et al., 2016Tilhac et al., , 2017. The age of peak metamorphism (approximately 382-389 Ma U-Pb on titanite and zircon; SHRIMP U-Pb on zircon and internal Sm-Nd isochrons on Cpx-Grtwhole-rock), recorded in eclogites and granulites, corresponds to the formation and accretion of the HP sheets of the units of the COC at sub-Moho depths (Ábalos et al., 2003;Ord oñez Casado et al., 2001;Peucat et al., 1990;Santos Zalduegui et al., 2002). ...
... The eclogitic unit of the COC forms one of the largest known continuous eclogite outcrops with a ridge 100-700 m thick and 17 km long in the direction N-NE to S-SW (Ábalos, 1997). Detailed petrographic, mineralogical, geochemical, structural and thermobarometric data of eclogites are provided byÁbalos et al. (2010), Gil , Mendia (2000) and Vogel (1967). Based on the consistency of isotopic and geochemical characteristics, the whole eclogite massif has an oceanic crustal origin (Bernard-Griffiths et al., 1985;Peucat et al., 1990), representing different stages of N-MORB tholeiitic series with variable degrees of differentiation (Ábalos et al., 2010;Mendia, 2000). ...
... Detailed petrographic, mineralogical, geochemical, structural and thermobarometric data of eclogites are provided byÁbalos et al. (2010), Gil , Mendia (2000) and Vogel (1967). Based on the consistency of isotopic and geochemical characteristics, the whole eclogite massif has an oceanic crustal origin (Bernard-Griffiths et al., 1985;Peucat et al., 1990), representing different stages of N-MORB tholeiitic series with variable degrees of differentiation (Ábalos et al., 2010;Mendia, 2000). Consequently, besides the most abundant variety, termed as N-MORB or massive eclogite, kyanite-rich deformed and ferrotitaniferous eclogites can be further distinguished (Gil Ibarguchi et al., 1990;Mendia, 2000). ...
Primary multiphase fluid inclusions (MFI) were studied in one eclogite and two granulites from the Cabo Ortegal Complex (COC, NW‐Spain) by means of Raman imaging, SEM‐EDS and FIB‐SEM. Complementary, secondary MFI in pyroxenites from COC were also investigated. MFI hosted in eclogite and granulites occur along growth zones or in 3D clusters in garnet porphyroblasts suggesting a primary origin at high‐pressure (HP) metamorphic conditions. The mineral assemblage of MFI is mainly composed of Fe‐Mg‐Ca‐carbonates and phyllosilicates ± graphite ± quartz ± corundum ± pyrite ± apatite ± rutile and a fluid phase composed of nitrogen ± methane ± carbon‐dioxide. The mineral proportions vary among the lithologies. Dominant carbonates and hydrous silicates are interpreted as step‐daughter minerals (crystals formed in the MFI after entrapment as a result of fluid‐host interaction), whereas apatite, quartz and rutile are considered in part as accidentally trapped minerals since they also occur as crystal inclusions together with MFI in each rock type. Quartz and corundum occur together in MFI in ultramafic granulite and are regarded as step‐daughter minerals in this lithology. These observations suggest that the MFI are products of post‐entrapment reactions of a homogeneous COHN fluid system with the host mineral. Thermodynamic calculations in the CaFMAS‐COHN system confirmed that bulk composition of the MFI in eclogite is similar to the host garnet+COHN composition except for a potential lost of H2O. Carbonation and hydration reaction between the host (i.e., garnet or pyroxene) and the fluid inclusion results in the consumption of all CO2 and part of the H2O from the fluid phase producing Ca‐Fe‐Mg‐carbonates and hydrous step‐daughter minerals, mostly pyrophyllite and chlorite. Nitrogen content of the originally trapped COHN fluid in eclogite was estimated to have a maximum value of 10 mol% at peak HP conditions and 30–40 mol% at retrograde conditions that is within the range of the observed MFI in the residual fluid (13–68 mol%). Pseudosection modelling confirmed the stability of the phase assemblage in the MFI in a specific low pressure, low temperature stability field (between 300–400 °C at pressures < 1 GPa), caused by H2O and CO2‐consuming reactions possibly in a single step. Our findings indicate that such processes in the exhuming HP units may play a role in global nitrogen and carbon cycling as well as potentially contributing to nitrogen and methane supply to subsurface‐surface environments during devolatilization in the forearc regions of convergent plate margins.
... The metasedimentary rocks of the eclogitic Banded Gneisses Formation resemble those of the Sobrado Unit and both of them show an extensive migmatization and abundant leucosomes. PT conditions for the eclogites of the eclogitic Banded Gneisses Formation have been calculated by means of thermobarometry (Mendía, 2000;Mendía, Gil Ibarguchi, & Ábalos, 2001). These authors distinguish three types of eclogites: common eclogites, ferrotitaniferous eclogites and kyanite-bearing eclogites. ...
... The conventional methods used to study these rocks include the garnet-clinopyroxene geothermometer (Berman, Aranovich, & Pattison, 1995;Ellis & Green, 1979;Krogh, 1988;Powell, 1985), the garnet-phengite geothermometer (Green & Hellman, 1982), the garnet-amphibole geothermometer (Perchuk, 1991;Powell, 1985), the garnet-biotite geothermometer (Lavrent'eva & Perchuk, 1981) and the jadeite content in clinopyroxene to estimate minimum pressures (Holland, 1980(Holland, , 1990. Mendía (2000) and Mendía et al. (2001) concluded that the most reliable results are provided by the calibrations of Powell (1985) for the common eclogites, and of Ellis and Green (1979) for the ferrotitaniferous eclogites and kyanite-bearing eclogites. They obtained PT conditions of 770-800°C and >17.9 kbar for the metamorphic peak. ...
... A metamorphic event, under pressures higher than those calculated for the residual paragneisses (star in Figure 14), is broadly preserved in the mafic rocks of the HP-HT Upper Units. PT calculations for this HP event in the eclogitic basal layer of the eclogitic Banded Gneisses Formation yielded conditions of 22 kbar and 800°C (Mendía, 2000;Mendía et al., 2001). Point 'M' in Figure 14 represents these PT conditions and has been included in the diagram aiming at tracing the most likely full path for the studied HP-HT Upper Units, from an initial stage under HP-LT conditions (paths 'A' and 'B'; Figure 14) through HP granulite facies conditions (Point 'M' and star; Figure 14) and down to amphibolite facies conditions (decompression + cooling path after star; Figure 14). ...
The Upper Units of the allochthonous complexes of the NW Iberian Massif constitute a terrane with continental affinity. They represent the vestiges of a Cambrian magmatic arc developed in the periphery of Gondwana (West African Craton) which was involved in the Devonian Variscan collision, undergoing high‐pressure, high‐temperature metamorphism. This includes ultramafic rocks, high‐pressure mafic rocks (eclogites and granulites) and high‐pressure migmatitic paragneisses. The latter rocks show an extensive migmatization with the leucosomes oriented parallel to the regional foliation. The migmatitic paragneisses are composed of garnet, kyanite, biotite, quartz, plagioclase, K‐feldspar, rutile and Ti‐hematite. Thermodynamic modelling using the measured bulk composition in the NCKFMASTHO system indicates metamorphic peak conditions of ~15 kbar and ~800‐835ºC, followed by a significant cooling. The prograde evolution is assessed by means of a melt reintegration approach, using the composition of the garnet and its inclusions. An appropriate composition of liquid is added to the measured bulk composition to emulate the pre‐melting bulk composition. Modelling of this melt‐reintegrated composition allows to identify a colder high‐pressure episode below ~500ºC. Zircon crystals extracted from the leucosomes show overgrowths crystallized from the partial melt at c. 389 Ma (U‐Pb system). The P‐T‐t path proposed reveals a subduction of the peri‐Gondwanan arc‐derived section down to mantle depths. An isobaric heating stage occurred as a result of residence at great depths and/or inception of a transient oceanic basin at c. 395 Ma. The ensuing near‐isothermal exhumation occurred due to the extension related to the inception of the basin, reaching the thermal peak shortly before c. 389 Ma. Subsequent cooling is related to the underthrusting of colder oceanic and transitional crust below the HP‐HT Upper Units.
... Both ensembles share the same protoliths, but the HP/HT units reached eclogite-(A Capelada unit) and high-pressure granulitefacies conditions (Fornás, Belmil, Melide, Sobrado and Cedeira units). The Devonian metamorphic event (Gil Ibarguchi et al. 1999;Mendía 2000) was followed by decompression and partial melting together with the development of a penetrative mylonitization in the amphibolite facies. The retrograde amphibolite-facies metamorphism has been dated at ca. 390-380 Ma (Dallmeyer et al. , 1997Valverde Vaquero and Fernández 1996;Gomez Barreiro et al. 2006). ...
... The most distinctive rocks in the HP/HT units are serpentinized ultramafic rocks, variably migmatised paragneisses, garnetclinopyroxene-bearing granulites and eclogites retrogressed to the amphibolite-facies (Vogel 1967;Hubregtse 1973). Gil Ibarguchi et al. (1990) and Mendía (2000) described three main eclogitic types: common eclogites, Fe-Ti eclogites after Fe-Ti gabbros and Al-Mg eclogites derived from troctolitic gabbros. Peak pressures have been estimated in kyanite-bearing Al-Mg-rich eclogites in ca. ...
... Peak pressures have been estimated in kyanite-bearing Al-Mg-rich eclogites in ca. 2.3 GPa at 750-800°C (A Capelada unit; Mendía 2000). Tholeiitic gabbros show a quite complete textural evolution, from undeformed types with almost pristine igneous textures including clinopyroxene + orthopyroxene + plagioclase ± olivine, through coronitic gabbros and metagabbros, to high-pressure granulites with granonematoblastic textures and without any relict of the previous stages . ...
Various segments of Variscan crust are currently exposed in Iberia in response to successive tectonic events during the Variscan orogeny itself and subsequent extensional and compressive events during the Alpine cycle, all accompanied by surface erosion, and collectively contributing to their exhumation. We review the main characteristics and geodynamic contexts of the metamorphic complexes developed in Iberia during the Variscan cycle, which include: (i) LP-HT complexes associated to the Cambrian-Early Ordovician rift stage; (ii) HP-LT complexes associated to subduction; and (iii) syn-to-post-collisional, MP and LP/HT complexes from the hinterland to the foreland fold-and thrust belts. All the above contexts are illustrated with case studies. Finally, a review of Variscan metamorphism in the Pyrenees and Catalan Coastal Ranges, located far away from the Rheic suture is also presented.
... In some sectors of that layer, pre-metamorphic mingling processes are still recognizable (Fig. 21f), as well as some lenses of augengneiss similar to those observed in the IP Upper Units. Mendia (2000) described three main eclogitic types: common eclogites, Fe-Ti eclogites after Fe-Ti gabbros, and Al-Mg eclogites derived from troctolitic gabbros. The high-P mineral assemblage in the later (Grt + Cr-Omp + Qtz + Ky + Zo + White mica + Rt) formed at c. 23 kbar (Mendia, 2000). ...
... Mendia (2000) described three main eclogitic types: common eclogites, Fe-Ti eclogites after Fe-Ti gabbros, and Al-Mg eclogites derived from troctolitic gabbros. The high-P mineral assemblage in the later (Grt + Cr-Omp + Qtz + Ky + Zo + White mica + Rt) formed at c. 23 kbar (Mendia, 2000). The gabbroic protoliths of these eclogites were dated at c. 491-495 Ma (U-Pb in zircon; Ordóñez Casado et al., 2001;Albert et al., 2013), while the granitic protoliths of the orthogneisses yielded ages of c. 484-506 Ma (U-Pb in zircon; . ...
... P-T paths contemporary to S 2 depict a drastic isothermal exhumation. D 1 mineral assemblages with Grt + Cpx + Zo + Rt were replaced by more hydrated phases, such as Grt + Hbl + Czo + Spn (Vogel, 1967;Gil Ibarguchi et al., 1990;Arenas, 1991;Mendia, 2000). The stretching lineation associated with S 2 trends NNE-SSW and D 2 kinematic indicators consistently indicate top-to-the-North sense of shearing (Ábalos et al., 1994, 1996). ...
NW Iberia includes a rather complete section of a Variscan suture, where different terranes with continental or oceanic affinities appear with clear structural relationships. Three groups of terranes, namely Upper, Ophiolitic and Basal units and a frontal tectonic mélange appear in Galicia, in Cabo Ortegal, Órdenes and Malpica-Tui complexes. They constitute a huge allochthonous pile thrust over the Iberian parautochthonous and autochthonous domains, which represent the section of the Gondwanan margin that escaped continental subduction during the Variscan cycle. Considering the allochthonous character of the nappe pile and the strong deformation associated to the Variscan collision, there are problems to identify the original tectonic setting of the terranes and thence, it is difficult to reconstruct the paleogeographic context during the Variscan and pre-Variscan times in detail. Key features to perform any model for the Variscan convergence should consider the existence of two different high-P metamorphic events (dated at c. 400 and 370 Ma, respectively), separated in time for the generation of mafic-ultramafic sequences at c. 395 Ma which constitute the most common ophiolites described in the Variscan suture.
... However, a later origin related to exsolution of Ti from garnet (Bishop et al. 1978) during retrogression cannot be excluded since these inclusions are more common in corroded garnet crystals. The presence of kyanite, which is characteristic of eclogite facies metamorphism in metabasites, is remarked although it is much more scarce in these rocks than in the Concepenido eclogites (Mendia 1996). Vogel (1967) already noticed this difference and considered that kyanite in the Bacariza Fm was only present in pegmatoid veins and therefore, not to be related to an earlier eclogite facies stage. ...
... The results of this study agreee with earlier ones on that clinopyroxenes from the Bacariza Fm show lower XJd and higher XAc than clinopyroxenes from the Concepenido eclogites in the COC (Vogel 1967;Arenas and Peinado 1984;Gil Ibarguchi et al. 1990, Mendia 1996 (Fig. 5). However, this difference is not necessarily to be related to different metamorphic conditions but it could be also constrained by difference in whole rock composition between the Concepenido eclogites and the granulites from the Bacariza Fm. ...
... However, this difference is not necessarily to be related to different metamorphic conditions but it could be also constrained by difference in whole rock composition between the Concepenido eclogites and the granulites from the Bacariza Fm. Most eclogites in the Concepenido band show higher M number and low CaO/Na2O radio than the metabasites here studied, according to data from Vogel (1967), Gravestock (1992) and Mendia (1996). This last author distinguishes different types of eclogites, among which those that are enriched in Fe-Ti show simillar M number to the Bacariza rocks but have much more lower CaO/Na2O ratio. ...
... They occur in separately mapped units ( Fig. 1 and Plate 1) and both are tectonites (Ábalos, 1997). Based upon petrographic and geochemical criteria, Mendia (1996) distinguished three eclogite varieties: "common or N-MORB eclogites" (~70% of the outcrops), "ferro-titaniferous eclogites" (<5% of the outcrops), and "kyanite-bearing eclogites" (~25% of the outcrops). The two eclogite types cited fi rst constitute the "massive eclogite," whereas the "kyanite-bearing eclogites" correspond to the "deformed eclogites" cited above. ...
... All these eclogite types appear to be genetically related on geochemical grounds. Their isotopic and geochemical signatures (Bernard-Griffi ths et al., 1985) suggest that the protoliths might represent products of different stages of a normal-mid ocean ridge basalt (N-MORB) tholeiitic series with variable degrees of differentiation (Mendia, 1996; Table 1). The U-Pb analyses on zircon have enabled the dating of the igneous protoliths of eclogite at 476 ± 13 Ma (Schäfer et al., 1993). ...
... Garnet and omphacite constitute >80%-90% of the total mineral content. Detailed petrographic descriptions of these rocks were given by Vogel (1967), Gil Ibarguchi et al. (1990), Mendia (1996), and Mendia et al. (2001). ...
High-pressure, sheath-like fold-nappes made of normal-mid ocean ridge basalt (N-MORB) eclogites, high-pressure volcanic-arc granulites, arc-root lithospheric mantle peridotites, and shallow crustal volcanosedimentary gneisses were amalgamated at deep realms of an Eohercynian orogenic channel and currently crop out in the Cabo Ortegal Complex. Compressional seismic-wave velocity measurements determined in eclogite at confining pressures of 600 MPa range from 8.4 to 7.3 km/s, whereas anisotropy varies between 0.7% and 3.2%. Shear-wave velocities range from 4.8 to 4.2 km/s, and the respective anisotropies vary between 0.1% and 3.4%. The Poisson's ratios are low (0.25-0.265), independent of the degree of retrogression or deformation. Seismic velocities and anisotropy are related to the fabric of omphacite, and calculated eclogite anisotropy is not symmetrical with respect to the tectonic fabrics. Calculated seismic-reflection coefficients induced by lithological, structural, and rock petrofabric variations of eclogites show that these can be identified as reflectors imprinting a particular, penetrative, and dipping fabric to discrete seismic-profile segments of paleosubduction settings.
... The primary structure of the upper ensemble is related to the earlier, highest-pressure and highest temperature tectonic events [cf. Girardeau et al., 1989;Gil Ibarguchi et al., 1990, 2000Girardeau and Gil Ibarguchi, 1991;Á balos et al., 1994, 1996Á balos, 1997]. Either Precambrian [Ries and Shackleton, 1971;Vogel and Abdel Monem, 1971;Engels et al., 1974;Marques et al., 1992Marques et al., , 1996Santos et al., 1995, early Paleozoic [Kuijper et al., 1982;Peucat et al., 1990;Fernández Suárez et al., 2002] or middle-late Paleozoic ages Ordóñez et al., 2001] have been proposed for these events. ...
... [15] Eclogites form a 100-700 m thick and 20 km long outcrop (Figures 2 and 3). Detailed petrographic, mineralogical and geochemical descriptions are provided by Vogel [1967], Gil Ibarguchi et al. [1990], Mendia [1996], and Á balos [1997]. They are composed of garnet, omphacite (both constitute 75 -90% of the total mineral content), quartz, zoisite and rutile. ...
... They are composed of garnet, omphacite (both constitute 75 -90% of the total mineral content), quartz, zoisite and rutile. Combined petrographic and deformation-related criteria permit the eclogite outcrop to be described as composed of two principal eclogite types [Á balos et al., 1994]: ''massive eclogite'' (E1, roughly the ''common or N-MORB'' and ''ferro-titaniferous eclogites'' of Mendia [1996]; Figure 5e), and ''deformed eclogite'' in shear zones (E2, the ''kyanite-bearing eclogites'' of Mendia [1996]; Figure 5f ). ...
High-pressure metamorphic fold-nappes and ductile thrusts occur in the Cabo Ortegal upper allochthons. They involve arc-root lithospheric mantle peridotites, high-pressure volcanic-arc granulites, normal mid-oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high-pressure gneiss units. Confluence of units from such disparate tectonic settings occurred at an eo-Hercynian convergent plate margin. Their internal deformation, juxtaposition, and high-pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations. Fold nappes exhibit a remarkable sheath-like form and contain mineral and stretching lineations defined by high-pressure mineral assemblages subparallel to fold axes. We argue that these structures outline the internal organization of high-pressure sheets at deep tectonic realms of an oblique subduction/collision orogenic channel.
... The Upper Member of the mafic rocks (100-200 m) is basically formed of eclogites, but also of fi negrained dark garnet amphibolites, retrogressed after eclogites-granulites (Vogel, 1967), especially in the northwestern outcrops of the Cabo Ortegal Complex. The most common type of eclogite contains Omp, Grt, Zo, Qtz, Amph and Rt, ('Concepenido-type eclogites' of Vogel, 1967), but kyanite-bearing eclogites (Vogel, 1967; Gil Ibarguchi et al., 1990; Mendia, 1996) and Fe-Ti rich eclogites have also been observed (Mendia, 1996). The Ky-bearing eclogites are mainly exposed as a thin band of less than 25 m thickness, along the contacts with the under-and overlying rocks. ...
... The Upper Member of the mafic rocks (100-200 m) is basically formed of eclogites, but also of fi negrained dark garnet amphibolites, retrogressed after eclogites-granulites (Vogel, 1967), especially in the northwestern outcrops of the Cabo Ortegal Complex. The most common type of eclogite contains Omp, Grt, Zo, Qtz, Amph and Rt, ('Concepenido-type eclogites' of Vogel, 1967), but kyanite-bearing eclogites (Vogel, 1967; Gil Ibarguchi et al., 1990; Mendia, 1996) and Fe-Ti rich eclogites have also been observed (Mendia, 1996). The Ky-bearing eclogites are mainly exposed as a thin band of less than 25 m thickness, along the contacts with the under-and overlying rocks. ...
... From major-and trace element geochemistry Van Calsteren (1978) proposed for the eclogites a continental quartz-normative tholeiite parentage. However, most authors lately agree on a N-type MORB protolith at least for the common eclogites (Bernard-Griffiths et al., 1985; Peucat et al., 1990; Gravestock, 1992; Mendia, 1996). The most common omphacite CPO in Cabo Ortegal is an S-type, with point maxima for b[010] and girdle distributions for c[001] (Fig. 19). ...
... The Upper Member of the mafic rocks (100-200 m) is basically formed of eclogites, but also of fine-grained dark garnet amphibolites, retrogressed after eclogites-granulites (Vogel, 1967), especially in the northwestern outcrops of the Cabo Ortegal Complex. The most common type of eclogite contains Omp, Grt, Zo, Qtz, Amph and Rt, ('Concepenido-type eclogites' of Vogel, 1967 ), but kyanitebearing eclogites (Vogel, 1967; Gil Ibarguchi et al., 1990; Mendia, 1996) and Fe-Ti rich eclogites have also been observed (Mendia, 1996). The Ky-bearing eclogites are mainly exposed as a thin band of less than 25 m thickness, along the contacts with the under-and overlying rocks. ...
... The Upper Member of the mafic rocks (100-200 m) is basically formed of eclogites, but also of fine-grained dark garnet amphibolites, retrogressed after eclogites-granulites (Vogel, 1967), especially in the northwestern outcrops of the Cabo Ortegal Complex. The most common type of eclogite contains Omp, Grt, Zo, Qtz, Amph and Rt, ('Concepenido-type eclogites' of Vogel, 1967 ), but kyanitebearing eclogites (Vogel, 1967; Gil Ibarguchi et al., 1990; Mendia, 1996) and Fe-Ti rich eclogites have also been observed (Mendia, 1996). The Ky-bearing eclogites are mainly exposed as a thin band of less than 25 m thickness, along the contacts with the under-and overlying rocks. ...
... From major-and trace element geochemistry Van Calsteren (1978) proposed for the eclogites a continental quartznormative tholeiite parentage. However, most authors lately agree on a N-type MORB protolith at least for the common eclogites (Bernard-Griffiths et al., 1985; Peucat et al., 1990; Gravestock, 1992; Mendia, 1996). ...
... In any case, the main tectonothermal event, the high-P granulite and eclogite facies metamorphism occurred later, and has been dated by U-Pb and 40 Ar/ 39 Ar methods between at least 425 and 390 Ma (SCHÄFER ET AL., 1993; SANTOS ZALDUEGUI ET AL., 1996; ORDÓÑEZ CASADO ET AL., 2001; FERNÁNDEZ-SUÁREZ ET AL., 2002c; GÓMEZ BARREIRO . This high-P, Silurian to Early Devonian metamorphic event, which implied subduction (GIL IBARGUCHI ET AL., 1999), was followed by decompression and partial melting and then, successively, by a penetrative mylonitization in the amphibolite facies, recumbent folding, and thrusting in the greenschist facies (VOGEL, 1967;MARCOS ET AL., 1984;GIL IBARGUCHI ET AL., 1990;ARENAS, 1991;GIRARDEAU AND GIL IBARGUCHI, 1991;MENDIA ARANGUREN, 2000). The retrograde amphibolite-facies metamorphism has been dated at 390-380 Ma (DALLMEYER VALVERDE VAQUERO AND FERNÁNDEZ, 1996;. ...
... • The 425 (or older)-390 Ma ages of metamorphism obtained in the upper units reflect a compressional event of (pre?-) Silurian to Early Devonian age that created a thick metamorphic pile whose deep parts registered pressures of 1.8 GPa or higher (GIL IBARGUCHI MENDIA ARANGUREN, 2000). Thickening of the upper units and the subduction of some of them probably reflect their underthrusting following accretion to a large continental mass, probably Laurussia. ...
... MENDIA ARANGUREN (2000) described three main types of eclogites within this band: i) common eclogites of basaltic composition, ii) Mg-and Alrich cumulates of troctolitic composition and iii) scarce ferrotitaniferous eclogites. P-T calculations in these eclogites indicate peak metamorphic conditions at 780-800 °C and 2.2 GPa (MENDIA ARANGUREN, 2000), comparable to those in the Banded Gneisses. ...
... In any case, the main tectonothermal event, the high-P granulite and eclogite facies metamorphism occurred later, and has been dated by U-Pb and 40 Ar/ 39 Ar methods between at least 425 and 390 Ma (SCHÄFER ET AL., 1993; SANTOS ZALDUEGUI ET AL., 1996; ORDÓÑEZ CASADO ET AL., 2001; FERNÁNDEZ-SUÁREZ ET AL., 2002c; GÓMEZ BARREIRO . This high-P, Silurian to Early Devonian metamorphic event, which implied subduction (GIL IBARGUCHI ET AL., 1999), was followed by decompression and partial melting and then, successively, by a penetrative mylonitization in the amphibolite facies, recumbent folding, and thrusting in the greenschist facies (VOGEL, 1967;MARCOS ET AL., 1984;GIL IBARGUCHI ET AL., 1990;ARENAS, 1991;GIRARDEAU AND GIL IBARGUCHI, 1991;MENDIA ARANGUREN, 2000). The retrograde amphibolite-facies metamorphism has been dated at 390-380 Ma (DALLMEYER VALVERDE VAQUERO AND FERNÁNDEZ, 1996;. ...
... • The 425 (or older)-390 Ma ages of metamorphism obtained in the upper units reflect a compressional event of (pre?-) Silurian to Early Devonian age that created a thick metamorphic pile whose deep parts registered pressures of 1.8 GPa or higher (GIL IBARGUCHI MENDIA ARANGUREN, 2000). Thickening of the upper units and the subduction of some of them probably reflect their underthrusting following accretion to a large continental mass, probably Laurussia. ...
... MENDIA ARANGUREN (2000) described three main types of eclogites within this band: i) common eclogites of basaltic composition, ii) Mg-and Alrich cumulates of troctolitic composition and iii) scarce ferrotitaniferous eclogites. P-T calculations in these eclogites indicate peak metamorphic conditions at 780-800 °C and 2.2 GPa (MENDIA ARANGUREN, 2000), comparable to those in the Banded Gneisses. ...
... Rare rutile-and ilmenite-rich mafic granulites, containing dark green pyroxene and reddish garnets, also occur. In view of the chemical composition of their constituent minerals (Table 1), these rocks might represent metamorphosed Fe-and Ti-rich igneous protoliths equivalent to those found in high-pressure terranes elsewhere and also in the eclogite unit of Cabo Ortegal (Galán and Marcos, 1997;Mendia, 2000). Trondhjemitic veins (Fig. 3a), likely representing unrooted mobilisates produced during decompression of the Bacariza Formation, are not uncommon. ...
... Rare pyroxene in garnet preserves the highest jadeite content of all the granulites studied (Q 59-73 Jd 21-32 Ae 6-11 ). This is distinctly lower, however, than that of omphacite from nearby eclogites Mendia, 2000). In the mylonitic granulites, primary clinopyroxene is Q 67-88 Jd 7-30 Ae 0-11 (cf. ...
... Once the granulites reached the maximum burial depths at ca. 790 8C, 1.6 GPa, they were accumulated in a subduction conduit and subsequently underplated by other units (either of upper crustal and lithospheric mantle origin) with varying metamorphic imprints (occasionally reaching 2.2 GPa in the case of eclogites; cf. Mendia, 2000). ...
The high-pressure Bacariza granulite formation comprises various lithostratigraphic units of granulite orthogneisses, ultramafic, Mg-rich mafic, intermediate and common mafic granulites, as well as of more exotic intercalations. Mineral assemblages in equilibrium in ultramafic- to intermediate rocks contain garnet, clinopyroxene and plagioclase, with different amounts of zoisite/clinozoisite, kyanite, quartz, scapolite, rutile and ilmenite depending on the granulite lithotype, whereas granulite orthogneisses contain garnet, phengite, biotite, K-feldspar, antiperthitic plagioclase, quartz and rutile as primary phases. Thermobarometry of these rocks supports the existence of a high-pressure metamorphism for which near-peak P–T conditions have been estimated at ca. 790 °C and 1.6 GPa. The preserved fabrics and structures enable us to relate the metamorphism to coeval polyphasic deformational processes. Dynamic retrogression began under still high-pressure granulite facies conditions (1.4 GPa and ca. 740 °C) and is postdated by symplectitization (1.3 GPa and ca. 715 °C). Subsequent retrogression under medium pressure amphibolite facies conditions at similar temperature was either widespread and static or localized and dynamic as a result of intense deformation partitioning during uplift. Loading/heating and subsequent decompression/cooling are related to a single cycle in a subduction conduit setting. This study suggests that high-pressure granulite metamorphism might not be as uncommon in the high-pressure metamorphic series as previously thought. Moreover, it might constitute a diagnostic feature of convergent lithospheric settings, whether or not associated with eclogite facies metamorphism in adjacent units.
... The protolithologies for the two upper units (HP-HT and MP) are considered to be equivalent, but the intensity of metamorphism and deformation prevents us from recogniz-ing the original sedimentary and igneous characteristics [6]. The most abundant lithologies are paragneisses and meta-mafic-ultramafic rocks such as garnet and clinopyroxene granulites and eclogites or their retrograde metamorphic equivalents such as amphibolites and greenschist [44][45][46]. The Fornás unit, where the Touro deposit is located, is characterized by the abundance of amphibolites hosted by the paragneisses. ...
... The first, during Ordovician times (±480 Ma), must have been largely transtensional, with the depositing of siliciclastic sediments in a back-arc extensional basin setting, including basaltic volcanism and the formation of the Volcanic Massive Sulfide deposits. Many authors [4][5][6][7][8][9][10]12,13,[41][42][43][44][45][46][47][48][49][50] describe the general characteristics of this geological setting. The VMS bodies are mostly hosted in the basalts but quite frequently also in the siliciclastic sediments, as can be seen in the sections and in the 3D model (Figures 7 and 8). ...
The Touro volcanogenic massive sulfide (VMS) deposit is located in the NW of the Iberian Variscan massif in the Galicia-Trás-os-Montes Zone, an amalgamation of several allochthonous terrains. The Órdenes complex is the most extensive of the allochthone complexes, and amphibolites and paragneisses host the deposit, characterized as being massive or semimassive (stringers) sulfides, mostly made up of pyrrhotite and chalcopyrite. The total resources are 103 Mt, containing 0.41% copper. A 3D model of the different orebodies and host rocks was generated using data from 1090 drill core logs. The model revealed that the structure of the area is a N–S-trending antiform. The orebodies crop out in the limbs and in the hinge zone. The mineralized structures are mostly tabular, up to 100 m in thickness and subhorizontal. Based on the petrography, geochemistry and the 3D model, the Touro deposit is classified as a VMS of the mafic-siliciclastic type formed in an Ordovician back-arc setting, which was buried and metamorphosed in Middle Devonian.
... In the Upper Allochthon, the main, high-P and high-T metamorphism is characterized by high-P granulite and eclogite facies dated at 400-390 Ma (Schäfer et al. 1993;Santos Zalduegui et al. 1996;Ordóñez Casado et al. 2001;Fernández-Suárez et al. 2007). This event implied Early-Middle Devonian subduction (Gil Ibarguchi et al. 1990Mendia Aranguren 2000), and eventually accretion to a large continental mass (Laurussia?) or another terrane Ballèvre et al. 2014). The age of the high-P and high-T event and its geodynamic meaning has been a subject of debate (Gómez Barreiro et al. 2006), and a limited number of exhumative fabrics show Late Silurian-Early Devonian ages, which may suggest an older (pre-Variscan) age for the high-P and high-T metamorphism (Gómez Barreiro et al. 2006;Arenas et al. 2013). ...
... The retrograde amphibolite-facies metamorphism has been dated at 390-380 Ma (Dallmeyer et al. , 1997; Gómez Barreiro et al. 2006). Subsequently, the Upper Allochthon underwent mylonitization under lower amphibolite facies conditions, followed by recumbent folding and thrusting under greenschist facies conditions (Vogel 1967;Marcos et al. 1984;Gil Ibarguchi et al. 1990; Girardeau and Gil Ibarguchi 1991;Mendia Aranguren 2000). ...
The Variscan deformation in the Iberian Massif is related to the large-scale plate tectonic scenario that drove to the destruction of the Rheic and other intervening oceans, to finally form the Pangea Supercontinent. The Northern Iberian Massif structure consists in an East-vergent orogenic wedge developed at the footwall of a rootless oceanic suture. The collisional architecture of this wedge has been strongly modified by extensional tectonics in the hinterland and orocline formation affecting the whole domain. The Southwestern Iberian Massif transect contains two orogenic sutures cropping out at both boundaries of the OMZ and shows a general transpressive character of the whole collisional evolution, as well as an Early Carboniferous transtensional/extensional stage that gave way to flysch sedimentation, voluminous bimodal magmatism and oblique left-lateral extensional shearing.
... In the Upper Allochthon, the main, high-P and high-T metamorphism is characterized by high-P granulite and eclogite facies dated at 400-390 Ma (Schäfer et al. 1993;Santos Zalduegui et al. 1996;Ordóñez Casado et al. 2001;Fernández-Suárez et al. 2007). This event implied Early-Middle Devonian subduction (Gil Ibarguchi et al. 1990Mendia Aranguren 2000), and eventually accretion to a large continental mass (Laurussia?) or another terrane Ballèvre et al. 2014). The age of the high-P and high-T event and its geodynamic meaning has been a subject of debate (Gómez Barreiro et al. 2006), and a limited number of exhumative fabrics show Late Silurian-Early Devonian ages, which may suggest an older (pre-Variscan) age for the high-P and high-T metamorphism (Gómez Barreiro et al. 2006;Arenas et al. 2013). ...
... The retrograde amphibolite-facies metamorphism has been dated at 390-380 Ma (Dallmeyer et al. , 1997; Gómez Barreiro et al. 2006). Subsequently, the Upper Allochthon underwent mylonitization under lower amphibolite facies conditions, followed by recumbent folding and thrusting under greenschist facies conditions (Vogel 1967;Marcos et al. 1984;Gil Ibarguchi et al. 1990; Girardeau and Gil Ibarguchi 1991;Mendia Aranguren 2000). ...
The Variscan deformation in the Iberian Massif is related to the large-scale plate tectonic scenario that drove to the destruction of the Rheic and other intervening oceans, to finally form the Pangea Supercontinent. The Northern Iberian Massif structure consists in an East-vergent orogenic wedge developed at the footwall of a rootless oceanic suture. The collisional architecture of this wedge has been strongly modified by extensional tectonics in the hinterland and orocline formation affecting the whole domain. The Southwestern Iberian Massif transect contains two orogenic sutures cropping out at both boundaries of the OMZ and shows a general transpressive character of the whole collisional evolution, as well as an Early Carboniferous transtensional/extensional stage that gave way to flysch sedimentation, voluminous bimodal magmatism and oblique left-lateral extensional shearing.
... Cabo Ortegal Complex: Four eclogites have been collected from the MORB-derived eclogitic band, where maximum P-T conditions have been estimated at 25-28 kbar and 780-800 °C (Mendia, 2000). Sample CO-13-2 is a ferro-titaniferous eclogite from the center of the eclogite unit with the following paragenesis: Cpx + Grt + Rt + Qtz + Amph + (Zrn) ± (Ap) ± white mica ± Zo. ...
... The thermometer of Watson et al. (2006) was modified by Tomkins et al. (2007) to take into account the effects of pressure. For the thermometer of Tomkins we have applied the maximum pressure conditions that have been reported for these rocks from previous thermobarometric studies (Mendia, 2000;Puelles et al., 2005;Rodríguez Aller, 2005 As pointed out be Meinhold (2010), for temperatures above 570 °C , there is a significant departure between the thermometer of Zack (2004) and that of Watson et al. (2006). In the case of the Malpica-Tui eclogite (MT9519), the thermometer of Zack and the thermometer of Tomkins et al. (2007) give temperatures of 712-715 °C , which are an overestimation from the peak temperature of ca. ...
A set of rutiles from eclogites and granulites within the Cabo Ortegal and Malpica-Tui complexes has been analyzed by EMPA for Ti, Cr, Al, Fe, Nb, Zr, Si and V, in order to test the application of the Zr-in-rutile thermometry and characterize their protoliths. These are zircon-bearing high-pressure rocks with well-constrained P-T conditions from previous studies. Rutiles from Fe-Ti eclogite and pyrigarnite at Cabo Ortegal are Cr-rich (>2000 ppm) and have Cr/Nb ratios >1, while Ky-eclogites have rutile with different Cr, Nb and V compositions indicating derivation from diverse protoliths. Despite their chemical variation, the rutiles from Cabo Ortegal rocks show remarkable homogeneity in Zr contents (470±42 ppm), being richer in Zr than the rutiles from Malpica-Tui (ca. 303±30 ppm). The Zr-in-rutile thermometry shows that temperatures can be significantly over-and underestimated depending on the thermometer applied, and caution needs to be exercised in their use. In the case of the HP/HT rocks of Cabo Ortegal the thermometers of Zack et al. (2004) and Tomkins et al. (2007) provide more reasonable temperatures (777±11 o C); while in the Malpica-Tui HP/IT-eclogite, it is the thermometer of Watson et al. (2006) that appears to work better (644±9 o C). Resumen: Se han determinado mediante microsonda electrónica los contenidos de Ti, Cr, Al, Fe, Nb, Zr, Si y V en rutilos de eclogitas y granulitas de los complejos de Cabo Ortegal y Malpica-Tui a fin de evaluar la termómetría de circonio en rutilo y caracterizar los protolitos. Son rocas de alta presión con condiciones P-T bien establecidas en estudios previos. Los rutilos de las eclogitas ferro-titaníferas y las pirigarnitas de Cabo Ortegal son ricos en Cr (>2000 ppm) y tienen ratios Cr/Nb >1, mientras que los de las eclogitas con distena muestran contenidos diferentes de Cr, Nb y V indicando distintos protolitos. A pesar deesta variabilidad, los rutilos de Cabo Ortegal tienen un contenido en Zr muy homogéneo (470±42 ppm) y superior a los de Malpica-Tui (303±30 ppm). La termometría de Zr en rutilo muestra que las temperaturas pueden sub-y sobrestimarse significativamente dependiendo del termómetro aplicado. En el caso de las rocas de AP/AT de Cabo Ortegal los termómetros de Zack et al. (2004) y Tomkins et al. (2007) proporcionan unas temperaturas coherentes (777±11 °C), mientras que para la eclogita AP/IT de Malpica-Tui es el termómetro de Watson et al. (2006) el que parece funcionar mejor (644± 9 °C).
... Also locally, the D 2 high-strain zones show garnet or amphibole lineations with scattered patterns (Fig. 2b). The mineral assemblages (M 2 ) associated with this dominant D 2 deformation indicate retrogression from eclogite to amphibolite facies during the imbrication of the whole stack (Gil- Fernández, 1997;Mendia 2000). The following is a description of the main lithological units within the D 2 imbricate complex at the Masanteo peninsula. ...
... This D 2 fabric developed in both mafic and migmatitic gneisses and characterizes such contacts. Peak metamorphic conditions during the eclogite stage have been constrained by Mendia (2000) at 800 • C and 2.2 GPa. ...
High-grade, highly deformed gneisses crop out continuously along the
Masanteo peninsula and constitute the upper part of the lower crustal
section in the Cabo Ortegal nappe (NW Spain). The rock sequence formed by
migmatitic quartzo-feldspathic (qz-fsp) gneisses and mafic rocks records the
early Ordovician (ca. 480–488 Ma) injection of felsic dioritic/granodioritic dykes at the base of
the qz-fsp gneisses, and Devonian eclogitization (ca. 390.4 ± 1.2 Ma),
prior to its exhumation. A SE-vergent ductile thrust constitutes the base of
quartzo-feldspathic gneissic unit, incorporating mafic eclogite blocks within
migmatitic gneisses. A NW-vergent detachment displaced metasedimentary
qz-fsp gneisses over the migmatites. A difference in metamorphic pressure of
ca. 0.5 GPa is estimated between both gneissic units. The
tectono-metamorphic relationships of the basal ductile thrust and the normal
detachment bounding the top of the migmatites indicate that both discrete
mechanical contacts were active before the recumbent folding affecting the
sequence of gneisses during their final emplacement. The progressive
tectonic exhumation from eclogite to greenschist facies conditions occurred
over ca. 10 Ma and involved bulk thinning of the high-grade rock sequence in
the high pressure and high temperature (HP–HT) Cabo Ortegal nappe. The necessary strain was accommodated by the
development of a widespread main foliation, dominated by flattening, that
subsequently localized to a network of anastomosing shear bands that evolved
to planar shear zones. Qz-fsp gneisses and neighbouring mafic granulites
were exhumed at > 3 mm yr−1, and the exhumation path involved a
cooling of ∼ 20 °C/100 MPa, These figures are
comparable to currently active subduction zones, although exhumation P–T trajectory and ascent rates are at the hotter and slower end in comparison
with currently active similar settings, suggesting an extremely ductile
deformation environment during the exhumation of qz-fsp gneisses within a
coherent Cabo Ortegal nappe.
... Finally, the ultramafic rocks include a heterogeneous ensemble of harzburgites, dunites and garnet pyroxenites for which an oceanic affinity has been proposed (Girardeau and Gil Ibarguchi 1991). The peak metamorphic conditions attained by the HP-HT unit range from 9 to 18 kbar and 700 to 850°C (Gil Ibarguchi et al. 1990;Arenas and Martı´nez Catala´n 1993;Mendia 1996;Gala´n and Marcos 2000). ...
... 300 m) located in the basal part of the metasedimentary succession (Fig. 2). Thermobarometric determinations in these basal eclogites suggest that eclogite-facies peak conditions were reached at 750-800°C and 20-23 kbar (real pressure; Mendia 1996). The sample is a fine-to medium-grained porphyroblastic gneiss with a high-temperature mineral assemblage that lacks primary muscovite. ...
... Los datos sobre composición química de las eclogitas (incluyendo tierras raras e isótopos de Sr y Nd) indican que se trata probablemente de un fragmento de litosfera oceánica metamorfizada (BERNARD-GRIFFITHS et al., 1985;PEUCAT et al., 1990;GRAVESTOCK, 1992;MENDIA, 1996MENDIA, , 2000. Los diferentes tipos de eclogitas podrían estar relacionados genéticamente desde el punto de vista geoquímico. ...
... Teniendo en cuenta los datos de edad disponibles para los distintos episodios descritos es posible precisar la trayectoria P-T-deformación con una dimensión temporal. Los protolitos de las eclogitas, rocas básicas de afinidad MORB con diversos grados de evolución magmática (MENDIA, 1996(MENDIA, , 2000 se habrían formado hace unos 480-510 Ma, al igual que los de otras rocas básicas del Complejo afectadas por el metamorfismo de alta presión. Podría tratarse de un segmento de corteza oceánica desarrollado durante el Paleozoico Inferior correspondiente en términos generales al océano situado entre Gondwana y Laurentia. ...
There are presented the results of the detailed petrologic study of the eclogites that form the eclogitic unit of the Cabo Ortegal Complex establishing a model for its metamorphic evolution. From the petrologic viewpoint three types of eclogites have been recognised: common, with distene and ferrotitaniferous, defining their outcrop and structural characteristics. The eclogitic unit is formed by several slices of common eclogites limited by important shears areas in which the eclogites with distene mainly outcrop. There have been recognised and quantified (P-T conditions) the different stages in the metamorphic evolution of these rocks, which have been related to the available structural and geochronological data in order to obtain a global idea of the P-T-time-deformation evolution followed by the eclogites. The data obtained indicate conditions of 780-800 OC and c. 22 kbar for the metamorphic maximum associated to the first stage of recognised deformation, D1, related to a subduction process. Afterwards, it occured a second deformation episode, still under eclogitic conditions, at 660 - 700 OC and 20 kbar, related to the stacking of eclogite slices and probably to the beginning of tectonic exhumation of these rocks. The retrograde evolution is characterised by a first stage of decompression under nearly isothermal conditions with a subsequent more important cooling. During this period D3 and D4 phases took place under conditions extensional and related to the final emplacement of the complex. The uplift of the rocks of the eclogitic unit should have been mainly tectonic thus allowing a considerable preservation during the subsequent low-grade metamorphic-deforming events. The evolution of the eclogitic unit constitutes an example of a piece of oceanic crust subducted up to the depths superior to 70 km with formation of an orogenic wedge related to the tectonic superposition of the different structural units forming the superior alloctonous of the complex. It is considered that this process marked the beginning of the Hercynian Orogenesis in this sector about 390-400 Ma.
... However, the high-P granulite and eclogite facies metamorphism is dated, by U-Pb and 40 Ar/ 39 Ar methods, between 425 and 390 Ma [30,32,37,54,66,73]. This event involved subduction [36], and was successively followed by decompression, partial melting, penetrative amphibolite-facies mylonitization, which was dated at 390-380 Ma [20,21,37], recumbent folding, and thrusting [35,42,52]. ...
... Deformation and metamorphism progressed eastward and their imprint is reflected in the progressive younging of metamorphic ages from the upper to the basal allochthonous units. In the upper units, 425-390 Ma ages reflect Silurian to Early Devonian compression that created a thick metamorphic pile whose deep parts registered pressures of 1.8 GPa [35,52]. An accretionary wedge, developed at the Laurussia margin ( Fig. 3ce), involved thickening, underthrusting and subduction of the upper units. ...
Ophiolites of different Paleozoic ages occur in North-West (NW) Iberia in a rootless suture representing the remnants of the Rheic Ocean. Associated allochthonous terranes in the hanging- and foot-walls of the suture derive from the former margins, whereas the relative autochthon corresponds to the Paleozoic passive margin of northern Gondwana. The Paleozoic tectonic evolution of this part of the circum-Atlantic region is deduced from the stratigraphical, petrological, structural and metamorphic evolution of the different units and their ages. The tectonic reconstruction covers from Cambro-Ordovician continental rifting and the opening of the Rheic Ocean to its Middle to Upper Devonian closure. Then, the Variscan Laurussia–Gondwana convergence and collision is briefly described, from its onset to the late stages of collapse associated with the demise of the orogenic roots.
... Los datos sobre composición química de las eclogitas (incluyendo tierras raras e isótopos de Sr y Nd) indican que se trata probablemente de un fragmento de litosfera oceánica metamorfizada (BERNARD-GRIFFITHS et al., 1985;PEUCAT et al., 1990;GRAVESTOCK, 1992;MENDIA, 1996MENDIA, , 2000. Los diferentes tipos de eclogitas podrían estar relacionados genéticamente desde el punto de vista geoquímico. ...
... Teniendo en cuenta los datos de edad disponibles para los distintos episodios descritos es posible precisar la trayectoria P-T-deformación con una dimensión temporal. Los protolitos de las eclogitas, rocas básicas de afinidad MORB con diversos grados de evolución magmática (MENDIA, 1996(MENDIA, , 2000 se habrían formado hace unos 480-510 Ma, al igual que los de otras rocas básicas del Complejo afectadas por el metamorfismo de alta presión. Podría tratarse de un segmento de corteza oceánica desarrollado durante el Paleozoico Inferior correspondiente en términos generales al océano situado entre Gondwana y Laurentia. ...
Se presentan los resultados obtenidos a partir del estudio petrológico detallado de las eclogitas que componen la unidad eclogítica del Complejo de Cabo Ortegal estableciéndose un modelo para la evolución metamórfica de la misma. Desde el punto de vista petrológico se han reconocido tres tipos de eclogitas: comunes, con distena y ferrotitaníferas, definiéndose las características de afloramiento y estructurales de las mismas. La unidad eclogítica está compuesta por diversas láminas de eclogitas comunes limitadas por importantes zonas de cizalla en las que afloran principalmente las eclogitas con distena. Se han reconocido y cuantificado (condiciones P-T) los distintos estadios en la evolución metamórfica de estas rocas, los cuales se han relacionado con los datos estructurales y geocronológicos disponibles para obtener una idea global de la evolución P-Tdeformación- tiempo que han seguido las eclogitas. Los datos obtenidos indican condiciones de 780-800 °C y c. 22 kbar para el máximo metamórfico asociado a la primera fase de deformación reconocida, D1, relacionada con un proceso de subducción. Posteriormente se produjo una segunda fase de deformación en condiciones todavía eclogíticas, a 660 - 700 °C y 20 kbar, relacionada con el apilamiento de las unidades y probablemente con el comienzo de la exhumación tectónica de estas rocas. La evolución retrógrada se caracteriza por un primer estadio de descompresión casi isotérmica con un posterior enfriamiento más importante. Durante este periodo tuvieron lugar las fases de deformación D3 y D4 en condiciones extensionales y relacionadas con el emplazamiento final del complejo. El ascenso de las rocas de la unidad eclogítica habría sido fundamentalmente tectónico lo que permitió una preservación considerable durante los posteriores eventos metamórfico-deformativos de menor grado. La evolución de la unidad eclogítica constituye un ejemplo de un fragmento de corteza oceánica subducida hasta profundidades superiores a 70 km con formación de una cuña orogénica relacionada con la superposición tectónica de las diferentes unidades estructurales que componen el alóctono superior del complejo. Este proceso se considera que marcó el comienzo de la Orogénesis Hercínica en este sector hace unos 390-400 Ma.
... GPa, 740-870 C, respectively. [25][26][27][28] Percolation of hydrous (i.e., COHN) fluids causing multistage fluid-rock interactions occurred not only at the peak but also during retrograde metamorphic evolution of the COC. 29,30 The studied MFI trapped in garnet of eclogite and granulite ( Figure 2) are always primary (trapped during garnet growth) and represent a COHN fluid being homogeneous at entrapment, in some cases accidentally co-entrapped with minerals (e.g. ...
In this study, we performed high resolution 3D Raman mapping on primary multiphase fluid inclusions from a granulite and an eclogite of the Cabo Orte-gal Complex (NW Spain), the latter representing a former subduction zone environment. Several microns-sized inclusions are great targets to test the advantages and limitations of 3D Raman mapping. We show how optical and Raman scattering properties of the phases inside the inclusions can affect the veracity of both 2D and 3D Raman maps. As an independent control, 3D Raman models were compared with focused ion beam-scanning electron microscopy submicron-scale slicing of three previously 3D Raman mapped inclusions, allowing us to optimize Raman spectroscopy data evaluation and provide volume proportion calculations. We found that a non-optimized evaluation procedure might result in even an order of magnitude over-or underestimation of phase volume properties. We propose that correcting the Raman maps even with empirically determined Raman cross sections of the phases inside the inclusions can vastly improve the quality of the maps. The results provide valuable input data for thermodynamic modeling and draw implications for fluid/rock interactions. K E Y W O R D S 3D Raman mapping, multiphase fluid inclusions, Raman cross section, volume percentages
... A general 89 HP/UHP-HT event occurring not before 390-400 Ma has been proposed by Arenas et al. (2014), taking into 90 account regional data from different allochthonous complexes in NW Iberia and France. Those ages imply (Basterra et al., 1989;Gil Ibarguchi et al., 1990;Mendia, 2000;Albert et al., 2012). ...
Abstract. Zircon crystal texture and Th / U ratio have been used as a watertight argument when interpreting U-Pb ages. The wide, and sometimes indiscriminate, use of those gauges could result into misinterpretation of the geological meaning of U-Pb data. A case study is presented here where zircons from a controversial polymetamorphic eclogite unit were analyzed with SHRIMP. Both U-Pb and trace element (TE) data were collected for each point. The combination of TE and structural arguments indicates that zircon was part of the eclogite facies mineral assemblage at 390 Ma. However, using Th / U ratio and CL textures lead to a different interpretation. Our results suggest that in complex orogenic scenarios and extreme environments well-known techniques (CL) and geochemical relationships (Th / U) must be used in combination with TE data and structural relationships as provenance/process gauges. While geochronology provides accurate isotope relationships, their temporal dimension must rely on structural and petrological evidence.
... BMD, Boimorto detachment; CD, Corredoiras detachment; QD, Queimada detachment However, the main tectonothermal event in the Órdenes and Cabo Ortegal complexes, the high-P and high-T granulite and eclogite facies metamorphism that characterize the group occurred later, at 400-390 Ma (Schäfer et al. 1993;Santos Zalduegui et al. 1996;Ordóñez Casado et al. 2001;Fernández-Suárez et al. 2002. This Early-Middle Devonian metamorphic event, also identified in the Bragança Complex (Mateus et al. 2016), and which implied subduction, was followed by decompression and partial melting and then, successively, by a penetrative mylonitization in the amphibolite facies, recumbent folding, and finally thrusting under greenschist facies conditions (Vogel 1967;Marcos et al. 1984Marcos et al. , 2002Gil Ibarguchi et al. 1990;Arenas 1991;Girardeau and Gil Ibarguchi 1991;Mendia Aranguren 2000). The retrograde amphibolite-facies metamorphism has been dated at 390-380 Ma (Dallmeyer et al. , 1997Valverde Vaquero and Fernández 1996;Gómez Barreiro et al. 2006). ...
The NW Iberian Allochthon represents the suture of a peri-Gondwanan, Paleozoic ocean formed around the Cambro-Ordovician boundary but having registered renewed spreading during the Lower Devonian. The ocean separated a piece of continental crust detached from NW Gondwana from the northern and northwestern parts of present Africa. The separation was a consequence of slab roll-back of a wide, subducting oceanic lithosphere around 500 Ma. Early Variscan plate convergence closed the oceanic domain involving three subduction events between 400–365 Ma, and building an accretionary prism formed by the detached continental fragment, the new oceanic lithosphere and the external margin of Gondwana. The change of regime from subductive to collisional occurred at the Devonian-Carboniferous transition and was responsible for the emplacement of the Allochthon above the Central Iberian Autochthon.
... A complete review of scientific studies on the XPCO was also made. To date, several dissertations, articles, and scientific books have been published (Arenas 1988; Monterrubio et al. 1992;Mendía 2000;Marcos et al. 2002;Puelles 2004;Verde-Vilanova 2009), as well as descriptive and didactic material of a different type (Lahuerta-Mouriño and Lucas-Domínguez 1990; Ábalos et al. 2000;Martínez-Catalán et al. 2010;Arán-Ferreiro et al. 2014;Canosa 2015). The memoirs of the geological maps made in recent decades by the Geological and Mining Institute of Spain provided complementary technical information of interest. ...
Recently, a project has been set in motion to declare the Cape Ortegal complex (NW Iberian Peninsula), a geological reference in Spain and Europe, as a geopark. This initial phase has the purpose of reflecting and discussing the benefits of creating a geopark as a tool to foment education in geodiversity, geotourism, and scientific debate. We define geo-singularities as those elements of a geopark that stand out the most to a tourist due to their processes, forms, compositions, or mere presence in situ, being able to do so in a scientific, spiritual or sensitive manner, among others. Geo-singularities are, therefore, of first-rate geotourism value. This case report focuses on the Teixidelo valley-fault geo-singularity, whose main attraction is the movement of its base (> 30 m in the last 70 years). We relied on field observations and literature review and benefited from discussions during conference workshops, meetings, and personal interviews with local associations, the local community, and municipal administrators. Thanks to this information, we were able to conduct our analysis towards the development of a collective reflection exercise about tourism on a global, regional, and local level, capable of providing insight to politicians and local organizations in the management of geotourism at the Cape Ortegal complex, as well as questioning if the geopark is a good strategy for the member municipalities. This analysis and collective interpretation allowed us to determine the potentialities, dangers, and challenges of the Teixidelo valley-fault as a geotourism asset in the framework of the Cape Ortegal complex. The current tourism situation, within a global context, is characterized by a significant increase in tourists who have experienced different geological assets. In many cases, this translated into a massification of places that were not prepared to receive so many tourists. We have identified two types of massification: projected (places that the Administration promotes without a proper management and protection plan) and another random one (the popularity of a place for a specific and unforeseeable reason). Geo-singularities are a strategic tool that can become a showcase piece for a geopark. The type of management strategy applied will largely determine the future geo-conservation of the geological values of the study area. Based on this, a first action plan was designed aimed at municipal governments and local associations. The reflections, approaches, and proposals carried out in this case report can be extrapolated to other territories, whether or not they are geoparks.
... A CASE STUDY FROM THE NW IBERIAN MASSIF 1,2 López-Carmona, A., 1,2 Gutiérrez-Alonso, G., 3 Floor, P., 3 Arps, C.E.S., 3 Kriegsman, L., 3 Gill, A. D.E. Vogel can be considered one of the "fathers" of the development of high-grade petrology in Europe. ...
D.E. Vogel can be considered one of the „fathers“ of the development
of high-grade petrology in Europe. This contribution
aims to be a tribute to his research in the Cabo Ortegal Complex,
(NW Iberian Massif).
The Iberian Variscan orogen is one of the key geological
realms to study the latest Precambrian and Paleozoic continental
evolution. During more than 30 years (50s-80s), the prolific
geologists from the “Leiden School” (The Netherlands) developed
several studies in this area. These studies helped to lay the
foundations of the geology in northern Spain, and their investigations
currently remain a benchmark. In this context, the seminal
petrological work done by D.E. Vogel in the NW Iberian Massif
has not been revised since 1967. Thus, no modern metamorphic
studies have been completed in such a strategic eclogite bearing
terrane, which includes some of the best-preserved and most representative
high-pressure (HP) outcrops of the western European
Variscan belt.
The Cabo Ortegal Complex includes a high-pressure granulite-
facies unit (14-.6 kbar, 740-840°C; Puelles et al., 2005)
tectonically sandwiched between an eclogite-facies unit (22
kbar, 750-800°C; Mendía Aranguren, 2000) and an ultra-mafic
(peridotite) unit (Vogel, 1967). D.E. Vogel petrological collection
covers the three units and is almost entirely curated (thin
sections and hand samples) by the Naturalis Biodiversity Center.
One of the most interesting samples of this collection is a typical
kyanite-garnet-clinopyroxene eclogite with fine-grained coronas
and symplectites. Similar rocks have been described in other HP
terranes such as e.g. the Western Gneiss Region (Johansson and
Möller, 1986) and the Eastern Alps (Vrabec et al., 2012; Schorn
and Diener, 2017).
We aim to evaluate to what extent the eclogite to retroeclogite
transition is driven by hydration through characterizing the
H2O dependence in the origin of the symplectitic/coronitic textures
developed during exhumation, including a comprehensive
description of the textural relations and its link to the whole P−T
evolution (Fig. 1).
... The main tectonothermal event, the high-P granulite and eclogite facies metamorphism, occurred later, however, and has been dated at 400-390 Ma (Schäfer et al. 1993;Santos Zalduegui et al. 1996;Ordóñez Casado et al. 2001;Fernández-Suárez et al. 2007). This high-P Early-Middle Devonian metamorphic event, which implied subduction (Gil Ibarguchi et al. 1999), was followed by decompression and partial melting (Fig. 5a) and then successively by a penetrative mylonitization in the amphibolite facies, recumbent folding and thrusting under greenschist facies conditions (Vogel 1967;Marcos et al. 1984;Gil Ibarguchi et al. 1990;Girardeau & Gil Ibarguchi 1991;Mendia Aranguren 2000). The retrograde amphibolite-facies metamorphism has been dated at 390-380 Ma (Dallmeyer et al. 1991(Dallmeyer et al. , 1997Valverde Vaquero & Fernández 1996;Gómez Barreiro et al. 2006). ...
A correlation between allochthonous units exposed in the NW Iberian Massif and the southern Armorican Massif is carried out based on lithological associations, structural position, age and geochemistry of protoliths and tectonometamorphic evolution. The units on both sides of the Bay of Biscay are grouped into Upper, Middle and Lower allochthons, whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the Parautochthon. The Lower Allochthon represents a fragment of the outermost edge of Gondwana that underwent continental subduction shortly after the closure of a Palaeozoic ocean which, in turn, is represented by the Middle Allochthon. The latter consists of supra-subduction ophiolites and metasedimentary sequences alternating with basic, mid-ocean ridge basalt (MORB)-type volcanics, with inheritances suggesting the proximity of a continental domain. Seafloor spreading began at the Cambro-Ordovician boundary and oceanic crust was still formed during the Late Devonian, covering the lifetime of the Rheic Ocean, which is possibly represented by the Middle Allochthon. The opening of the oceanic domain was related to pulling apart the peri-Gondwanan continental magmatic arc, which is represented by the Upper Allochthon.
... The Variscan belt is the result of the collision between Gondwana, Laurussia and an unconstrained number of microplates resulting in the supercontinent Pangea during the Devonian and Carboniferous (Nance et al., 2010;Domeier and Torsvik, 2014). The earliest Variscan deformation in Iberia is interpreted to have occurred prior to c. 400 Ma (Dallmeyer and Gil-Ibarguchi, 1990;Quesada, 1991;Mendía-Aranguren, 2000;Fernández-Suárez et al., 2007;Gómez-Barreiro et al., 2007;Martínez-Catalán et al., 2009). Continental collision started at ca. 365-370 Ma (Dallmeyer et al., 1997;Rodríguez et al., 2003;López-Carmona et al., 2014) with the underplating of the Gondwanan margin below Laurussia, giving rise to an eastward (in present day coordinates) migration of deformation and related syn-orogenic sedimentation (Dallmeyer et al., 1997). ...
The Cantabrian orocline is a large structure that bends the Variscan orogen of Western Europe in NW Iberia. The extensively studied kinematics of its core, the foreland of the orogen, indicates that the structure is secondary, i.e. acquired after the formation of the orogenic edifice. However, the extent of the Cantabrian orocline away from its core is under debate. In this paper we study the kinematics of the Cantabrian orocline beyond the foreland. We collected and analyzed samples from the northern and central parts of the Truchas syncline, which provides new data within the hinterland of the orogen in NW Iberia. The analysis of 320 samples shows a late Carboniferous remagnetization with an E to NE declination and shallow downward inclinations. These results suggest a counterclockwise rotation of ~60º and peri-equatorial but still southern hemisphere latitude for Iberia during the uppermost Carboniferous-Early Permian. This rotation fits with the expected kinematic evolution of the Truchas syncline if it indeed was part of the Cantabrian orocline.
... 425 Ma age for retrogressive amphibolitefacies foliation in high-P and high-T units, suggest an even older age for the high-P and high-T metamorphism (Dallmeyer et al., 1997;Gómez Barreiro, 2004;Gómez Barreiro et al., 2006). This compressional event of Silurian to Early Devonian age produced a thick metamorphic pile, and the deep parts register pressures of 1.8 GPa or higher Mendia Aranguren, 2000). Thickening of the upper units and the subduction of some of them probably refl ect their underthrusting following accretion to a large continental mass, either Baltica or Laurentia (Fig. 5). ...
... Martínez Arenas et al., 2014). Evidence of subduction beneath the Laurussian (northern) margin of the Rheic Ocean prior to c. 400 Ma is interpreted as the earliest record of ocean closure (Gil Ibarguchi et al., 1990;Mendía Aranguren, 2000;Fernández-Suárez et al., 2006;Gómez Barreiro et al., 2007;Martínez Catalán et al., 2009). By c. 395 Ma, an increase in the inferred rate of convergence, and evidence for coupling of the subducting slab to the Gondwana margin, is interpreted to reflect northward subduction of an oceanic ridge in the Rheic Ocean (Woodcock et al., 2007;Gutiérrez-Alonso et al., 2008). ...
... An early high-P granulite to eclogite facies metamorphism was followed by decompression and partial melting and then, successively, by a penetrative mylonitization in the amphibolite facies, followed by recumbent folding and thrusting in the greenschist facies (Vogel, 1967;Marcos et al., 1984;Gil Ibarguchi et al., 1990;Arenas, 1991;Girardeau and Gil Ibarguchi, 1991;Martínez Catalán and Arenas, 1992;Mendia, 2000). U-Pb dating of zircon, monazite, titanite, and rutile reveal the presence of an Early Ordovician metamorphic event ca. ...
A high-pressure and high-temperature unit exposed in the core of the Sobrado tectonic window (Ordenes Complex, northwestern Iberian Massif) contains a lithological ensemble, of continental and/or arc affinity, affected by Early Ordovician metamorphism; the unit reached highest conditions in the high-pressure granulite facies and in the eclogite facies. Alternating with metasediments, map-scale bands of metabasites include a large number of metagabbro bodies ranging from several meters to 2 km in length. The metagabbros were variably affected by the regional metamorphism. Well-preserved gabbros with almost intact igneous textures and mineralogy can be found, as well as metagabbros showing diverse reactional coronitic textures, and highly recrystallized types with no preserved igneous features and entirely transformed into granoblastic rocks. The variety of textures and assemblages in the coronas is interpreted to reflect the prograde reaction history of the metagabbros. Three groups of coronas have been distinguished: (I) Amp + Opx; (II) (Amp) + (Cpx) + Opx + Grt; (III) (Amp) + Cpx + Opx + Grt (subordinate phases in parentheses). The development and evolution of these coronas follow a complex prograde reaction history, the general characteristics of which are described. Detailed thermobarometry, carried out in groups II and III coronas, together with inferred pressure-temperature conditions for group I coronas, show that the corona textures were generated during a drastic and virtually isothermal increase in pressure. Highest pressure-temperature conditions, registered in group III coronas, reached 660-770 °C and 13-17 kbar. Subduction of a relatively thin slice of continental or arc-related crust below a mantle wedge, either with an intact thermal structure or heated and thermally softened in a relatively inner part of a volcanic arc, is suggested as the probable dynamic setting to explain the pressure-temperature path found in the metagabbros.
... The estimated metamorphic peak conditions range from 842-884 °C for 1.4-1.6 GPa (Bacariza-type eclogites, Galán and Marcos, 1999) to 770-800 °C for 1.4-1.7 GPa (Concepenido-type eclogites, Mendia, 1996). ...
... Catalan et al. 2007;Arenas et al. 2014). The first record of ocean closure is indicated by evidence of subduction beneath the Laurussian (northern) margin of the Rheic Ocean prior to c. 400 Ma (Ibarguchi et al. 1990;Mendía Aranguren 2000;Fernandez-Suarez et al. 2007;Gómez Barreiro et al. 2007;Catalan et al. 2009). By c. 395 Ma, an increase in the inferred rate of convergence, and indications of coupling of the Gondwana margin to a subducting slab, is thought to reflect northward subduction of the Rheic mid-oceanic ridge (Woodcock et al. 2007;Gutierrez-Alonso et al. 2008). ...
Seven samples of Siluro-Devonian sedimentary rocks from the Cantabrian and Central Iberian zones of the Iberian Variscan belt have been investigated for provenance and contain four main age populations in variable relative proportion: Ediacaran–Cryogenian ( c . 0.55–0.8 Ga), Tonian–Stenian (0.85–1.2 Ga), Palaeoproterozoic ( c . 1.8–2.2 Ga) and Archaean ( c . 2.5–3.3 Ga). Five samples contain very minor Palaeozoic (Cambrian) zircons and six samples contain minor but significant zircons of Middle and Early Mesoproterozoic (Ectasian–Calymmian, 1.6–1.8) age. These data highlight the transition from an arc environment to a stable platform following the opening of the Rheic Ocean. Variations in detrital zircon populations in Middle–Late Devonian times reflect the onset of Variscan convergence between Laurussia and Gondwana. The presence of a high proportion of zircons of Tonian–Stenian age in Devonian sedimentary rocks may be interpreted as (1) the existence of a large Tonian–Stenian arc terrane exposed in the NE African realm (in or around the Arabian–Nubian Shield), (2) the participation, from the Ordovician time, of a more easterly alongshore provenance of Tonian–Stenian zircons, and (3) an increase in the relative proportion of Tonian–Stenian zircons with respect to the Ediacaran–Cryogenian population owing to the drift of the Avalonian–Cadomian ribbon continent, or the progressive burial of Ediacaran–Cryogenian rocks coeval with the denudation of older source rocks from the craton interior.
Supplementary material
Tables with the analytical data and the geochronological results are available at http://www.geolsoc.org.uk/SUP18812 .
... The first group, i.e. the Champtoceaux and Malpica-Tuy eclogite, does not have an oceanic origin. On the other hand, the Vendée and Limousin eclogite, which has a rather clear oceanic origin, apparently belongs to the same high-P belt as the Braganca eclogite (Portugal) and the Cabo Ortegal eclogite (Galicia: see Mendía-Aranguren, 2000). ...
... The main lithologic and structural features of the units that make up the Cabo Ortegal Complex were described fi rst by Vogel (1967), Engels (1972), and Arenas (1988). Other works that more recently have described geological aspects of the complex are those of Santos Zalduegui (1995), Fernández (1997), Azcárraga (2000), Mendia (2000), Castiñeiras (2003), and Puelles (2003). ...
Two Paleozoic ophiolites of different age mark the Variscan suture in the Cabo Ortegal Complex (northwestern Iberian Massif). The Moeche and Purrido alloch-thonous ophiolites are structurally located between an exotic terrane of continen-tal affi nity (basal units), below, and another terrane with arc affi nity (upper units), above. The mafi c rocks of the Moeche unit are greenschist with a composition typical of tholeiitic basalt. Their rare earth element (REE) concentrations range from 12 to 31 times the chondritic abundances, with almost fl at chondrite-normalized REE patterns ([La N /Yb N ] avg = 0.83), and they have no signifi cant Eu anomalies. According to their contents in some immobile trace elements, the geochemical composition of these metabasalts seems to be transitional between normal mid-ocean-ridge basalt (N-MORB) and island-arc tholeiites. Their abundances in the most immobile trace elements with the highest discriminating power (Th, Nb, Ce, Zr, Ti, and Y), normal-ized to an average tholeiitic N-MORB, depict quite fl at patterns close to the average N-MORB concentrations, although they are slightly enriched. A small negative Nb anomaly points to an origin linked to a subduction zone. The Purrido unit consists of amphibolites with compositions also equivalent to tholeiitic basalt. REE compositions range from 6 to 22 times chondritic abundance, with essentially fl at chondrite-normalized REE patterns ([La N /Yb N ] avg = 0.99) and very small positive Eu anomalies ([Eu/Eu*] avg = 1.08). The abundance of some trace *Current address: Departamento de Petrología y Geoquímica, Facultad de Geología, Universidad Complutense, C./José Antonio Novais, nº 2, 28040 Madrid, Spain; s.sanchez@geo.ucm.es.
... During their exhumation, the HP-HT units were affected by intense shearing that generated a pervasive mylonitic fabric with regional distribution, followed by the development of large recumbent folds and thrusts (Marcos et al., 1984;Ábalos et al., 2003Puelles et al., 2009). P-T conditions of the eclogite facies metamorphism developed in the HP-HT units were estimated at 780-800 °C and 22 kbar, using classical thermobarometric techniques on the mineral assemblages from the main layer of eclogites ( Fig. 3) in the Cabo Ortegal Complex (Mendia, 2000). This is a ca. ...
The tectonothermal evolution of the eclogite facies gneisses of the Cabo Ortegal Complex has been investigated using new field and petrologic studies. This gneissic unit is included in the high-pressure and high-temperature (HP-HT) upper units of the allochthonous complexes of NW Iberia, a Cambrian arc-derived peri-Gondwanan terrane emplaced above ophiolitic units considered to represent the Rheic Ocean suture. Two detailed cross-sections performed in excellent exposures on the Cantabrian coast allows a clear understanding of the lithological constitution and deformations affecting the unit. The eclogitic gneiss unit is constituted by metasedimentary gneisses with abundant mafic inclusions, felsic rocks and rare ultramafic inclusions. The felsic rocks range from centimetric leucosomes to metric bodies and most of them were generated during extensive partial melting affecting the unit. The first tectonic fabric detected in the unit is a rarely preserved S1 foliation developed during subduction (ca. 400-390 Ma; 22 kbar). A drastic and fast exhumation (ca. 390 Ma; 12-10 kbar) generated a regional mylonitic foliation which represents the most prominent structural feature in the HP-HT upper units. The consecutive generation of extensional detachments, recumbent folds and a basal thrust occurred during the change from continental-type subduction to the underthrusting of ophiolitic units. These consecutive structures are probably related and the consequence of a long and pronounced exhumation of the subducted complex. The tectonothermal evolution of the eclogite facies gneisses of the Cabo Ortegal Complex represents an illustrative example of the intricate history of these deeply subducted units, which are relatively frequent in the suture zone of the Variscan Belt. It is important to remark that the structural, metamorphic and geochronological patterns in the evolution of this gneissic unit follows almost perfectly the predictions based in numerical experiments about the exhumation of ultra-high-P complexes. Consequently it represents a direct confirmation of these models.
... Thus, HP rocks are common in the allochthonous complexes of the Galicia-Tras-os-Montes Zone in NW Iberia, where the eclogite stage shows a wide range of P-T conditions. High-temperature eclogites appear in the La Capelada unit of Cabo Ortegal Complex (780 ± 25°C and 22 ± 1 kbar) followed by a granulite stage at 800 ± 50°C and 15 ± 1 kbar (Mendía, 2000;Ábalos et al., 2003). Ultramafic layers record similar high-pressure equilibrium conditions at c. 800°C and 16.5 kbar (Girardeau and Gil Ibarguchi, 1991) (Fig. 12a). ...
The rocks at Tenzuela represent the only known outcrop of HP metabasites in the internal sectors of the Iberian Belt, in the vast Central Iberian Zone (CIZ). These rocks appear as lenses of meta-tholeiites of continental affinity intruded within the Neoproterozoic metasedimentary sequences. The mafic types have high Th/Yb and Ta/Yb ratios that suggest, combined with initial εNd values ranging from + 4.4 to + 6.0, derivation from a slightly enriched mantle source. SHRIMP zircon data indicate an Early Ordovician age (206Pb/238U mean age of 473 ± 2 Ma) for this magmatic event. However, the U–Pb data do not yield a constrained age for the high-pressure metamorphic event undergone by the metabasites, which nevertheless can be estimated at just before 335 Ma, during the Variscan collision. The HP metabasites rarely overpass peak pressure conditions of 15 kbar, and thus are remarkably lower than those of eclogites related to oceanic subduction in ophiolite- or allochthonous complexes from NW Iberia. On the other hand, these pressures are similar to those of the metabasites intercalated in orthogneisses along the Coimbra–Córdoba Shear Zone, in the boundary with the Ossa-Morena Zone (OMZ). In this regard, the new geochronological data set together with already published ages shows that the subduction-related Iberian eclogites (allochthonous complexes) are older (Lower to Middle Devonian) than the HP metabasites from more intracontinental settings, mostly recrystallized at lower HP conditions during the Visean. Thus, the studied metabasites should form part of an inner, diachronous belt of HP rocks within the European Variscan orogen, far away from the main subduction front. Nevertheless, the suggested geodynamic scenario needs future confirmation by accurate geochronological data for the HP metamorphism.
... Peak conditions in mafic rocks were at a minimum 1.2–1.4 GPa and 770–876 C; the higher pressure and lower temperature correspond to eclogites sensu stricto (Gil Ibarguchi et al., 1990; Mendia, 1996; Galán and Marcos, 2000). Peak conditions in quartzofeldspathic gneisses were 1.5–1.6 ...
The upper tectonic unit of the Cabo Ortegal Complex (northwest Spain) comprises an
ordered rock sequence of ultramafic and mafic rocks and quartzofeldspathic gneisses. The
entire assembly forms part of a transitional mafic crust. In an early Paleozoic subduction
episode, most of this crust was metamorphosed at eclogite facies and high-pressure granulite
facies conditions. Field and microstructural observations suggest that this metamorphism
occurred simultaneously with deformation, producing a pervasive tectonic fabric
that developed under a strain regime of bulk coaxial deformation. Downdip extension of
the crust in the subduction zone is inferred to have thinned the rock sequence. The relatively
high pressures and high temperatures (770–900 8C and .1.2–1.7 GPa) during
deformation at peak conditions, in addition to the significant thinning of the units, suggest
a tectonic setting of ductile slab breakoff as the cause of the intense and widespread
deformation.
... The HP–HT units were subjected to HP metamorphism at ca. 400 Ma, which is interpreted to reflect subduction (Schäffer et al. 1993; Santos Zalduegui et al. 1996; Ordóñez Casado et al. 2001; Fernández-Suárez et al. 2002b Roger and Matte 2005; Gómez Barreiro et al. 2006) and a HT metamorphic event at ca. 395 Ma, which is generally attributed to their exhumation (Dallmeyer et al. 1997; Valverde Vaquero and Fernández 1996; Gómez Barreiro et al. 2006; Fernández-Suárez et al. 2007 ). The ultramafic lithologies have been variably interpreted as imbricated subducted oceanic slabs (Gil Ibarguchi et al. 1990, Mendía 2000), residual harzburgites related to ophiolitic complexes (Santos Zalduegui et al. 1996; Santos et al. 2002), exhumed subcontinental mantle (Peucat et al. 1990; A ´ balos et al. 1996), subducted back-arc ophiolites (Peucat et al. 1990), or the crust–mantle interface beneath a magmatic arc (Moreno et al. 2001). Finally, the structurally overlying intermediate pressure (IP) units are characterized by greenschist-facies metasedimentary and metavolcanic successions that are thought to represent the vestiges of a dissected magmatic arc (Díaz Gutiérrez-Alonso et al. 2000). ...
Cited By (since 1996): 12, Export Date: 11 October 2012, Source: Scopus, CODEN: CJESA, doi: 10.1139/E08-019, Language of Original Document: English, Correspondence Address: Murphy, J. B.; Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada; email: bmurphy@stfx.ca, References: Ábalos, B., Azcárraga, J., Gil Ibarguchi, J.I., Mendia, M., Santos Zalduegui, J.F., Flow stress, strain rate and effective viscosity evaluation in a high-pressure metamorphic nappe (Cabo Ortegal, Spain) (1996) Journal of Metamorphic Geology, 14, pp. 227-248;
... Plagioclase is a very common product of the first stage of mineral reactions. It should be noted that the band of eclogites that has been mapped inFig. 1 (from Marcos et al., 2002) and also by previous authors (Vogel, 1967; Engels, 1972; Mendia, 1996) includes both the eclogites and the products of their retrogradation at different stages. ...
The Concepenido eclogites are part of an ordered rock sequence in the upper tectonic unit of the Cabo Ortegal Complex (NW Spain). They form a continuous and about 200 m thick layer separating other mafic rocks from quartzo-feldspathic gneisses. The whole sequence has been deformed and metamorphosed at high pressure and high temperature during the development of an early Paleozoic subduction zone. In this study, microfabrics of these eclogites are quantitatively analysed, in particular crystallographic preferred orientations (CPO) in omphacite and garnet measured by electron back-scatter diffraction (EBSD). From field observations and from detailed microstructural analysis we estimate that the high pressure deformation, which produced the crystallographic and shape fabrics in omphacite, has occurred under a strain regime close to bulk coaxial flattening. Both omphacite lineations and omphacite c[001] concentrations scatter in geographical coordinates and do not show a single preferred orientation that would indicate a predominant direction of extension. We suggest that the high pressure and high temperature environment during deformation, the coaxial nature of strain and the significant thinning of the layer and rock sequence may relate to flattening deformation of a subducting slab with extension slightly preferred in a down-dip direction.
... The earliest evidence of oceanic closure is recorded by northward subduction of the Rheic Ocean along the Laurussian margin, the onset of which occurred before ca. 400 Ma based on the age of the earliest metamorphism (Dallmeyer and Gil Ibarguchi, 1990;Mendía Aranguren, 2000;Fernández-Suárez et al., 2007;Gómez Barreiro et al., 2006;Martínez-Catalán et al., 2009). Subduction of the Rheic mid-ocean ridge by 395 Ma (Woodcock et al., 2007;Gutiérrez-Alonso et al., 2008a) is thought to have caused an increase in the convergence rate and the coupling of both oceanic margins. ...
The Rheic Ocean, which separated Laurussia from Gondwana following the closure of Iapetus, is arguably the most important ocean of the Palaeozoic. Its suture extends from Mexico to Turkey and its closure produced the climactic Variscan–Alleghanian–Ouachita orogeny that assembled the supercontinent, Pangaea.Following protracted Cambrian rifting that represented a continuum from Neoproterozoic orogenic processes, the Rheic Ocean opened in the Early Ordovician with the separation of several Neoproterozoic arc terranes from the continental margin of northern Gondwana. Separation occurred along the line of a former Neoproterozoic suture following the onset of subduction in the outboard Iapetus Ocean. The timing of rift–drift transition and drive for subsequent spreading was likely governed by slab pull, accounting for the rapid rate (8–10 cm/yr) at which the Rheic Ocean widened.During the Ordovician, the ocean broadened at the expense of Iapetus and attained its greatest width (~ 4000 km) in the Silurian, by which time Baltica had sutured to Laurentia and the Neoproterozoic arc terranes had accreted to Laurussia, closing Iapetus in the process. Closure of the Rheic Ocean began in the Devonian and was facilitated by northward subduction beneath southern Baltica and southward subduction beneath northwest Gondwana. Closure was largely complete by the Mississippian as Gondwana and Laurussia sutured to build Pangaea, North Africa colliding with southern Europe to create the Variscan orogen in the Devonian–Carboniferous, and West Africa and South America suturing to North America to form the Alleghanian and Ouachita orogens, respectively, during the Carboniferous–Permian.The Rheic Ocean consequently plays a dominant role in the basement geology of southern Europe, in the Appalachian–Ouachita orogeny of North America, and in the Palaeozoic sedimentary, structural and tectonothermal record from Middle America to the Middle East. With its closure, the ocean brought about the assembly of Pangaea and brought the Palaeozoic Era to an end.
The NW Iberian Allochthon and the Teplá-Barrandian and Moldanubian zones represent the internal parts of the Variscan belt in their respective domains. A correlation based on the lithological association, protolith ages, metamorphic evolution, detrital zircon age spectra and tectonic setting is attempted between the NW Iberian Massif and SE Bohemian Massif in order to check whether they could have formed part of the same allochthonous stack. The Galicia-Trás-os-Montes Zone of the Iberian Massif and the internal zones of the Bohemian Massif include from bottom to top a Parautochthon and Lower Allochthon representing the outer edge of the northern Gondwana margin, an oceanic Middle Allochthon with Cambro-Ordovician and Early Devonian ophiolites and an Upper Allochthon interpreted as a peri-Gondwanan terrane. Early Variscan, subduction-related high-pressure metamorphism characterizes many of the allochthonous units, with ages younging from the structurally upper to the lower units from 400–385 Ma to 370–360 Ma, respectively. High- and ultrahigh-pressure metamorphism occurred also in the Saxothuringian Autochthon at 360–340 Ma, but not in the NW Iberian Autochthon. The different behavior of the Autochthon in the Iberian and Bohemian massifs accounts for their distinct evolutions from 360 Ma onward. We conclude that the Upper Allochthon was a unique peri-Gondwanan terrane, whereas the Middle Allochthon represents units of the same peri-Gondwanan ocean, opened at the Cambro-Ordovician boundary, and having recorded localized renewed activity in the Silurian–Early Devonian. No other oceans separated the Lower Allochthon, Parautochthon and Autochthon.
High-grade highly deformed gneisses crop out continuously along the Masanteo peninsula in the Cabo Ortegal nappe (NW Spain). The rock sequence formed by quartzo-feldspathic gneisses and mafic rocks records two partial melting events: during the Early Ordovician (ca. 480–488 Ma.), at the base of the Qz-Fsp gneisses, and immediately after eclogization (ca. 390.4 ± 1.2 Ma), during its early Variscan exhumation. Despite the strain accumulated during their final exhumation in which a pervasive blastomylonitic S2 foliation was developed, primary sedimentary layering in Qz-Fsp gneisses is well preserved locally at the top of the sequence. This first stage of the exhumation process occurred in ~ 10 Ma, during which bulk flattening of the high-grade rock sequence was accommodated by anastomosing shear bands that evolved to planar shear zones. Strain was progressively localized along the boundaries of the migmatitic Qz-Fsp gneisses. A SE-vergent ductile thrust constitutes the base of gneisses, incorporating eclogite blocks-in-matrix. A NW-vergent detachment placed the metasedimentary Qz-Fsp gneisses over the migmatitic Qz-Fsp gneisses. A difference in metamorphic pressure of ca. 0.5 GPa is estimated between both gneissic units. The high-grade deformation reduced substantially the thickness of the gneissic rock sequence during the process of exhumation controlled by change in the strain direction and the progressive localization of strain. The combined movement of the top detachment and basal thrust resulted in an extrusion of the migmatites within the nappe, directed to the SE in current coordinates.
The Corredoiras orthogneiss belongs to the intermediate pressure upper units of the Órdenes Complex (Variscan belt, NW Spain), mainly composed by granodioritic orthogneisses, with small bodies of tonalitic orthogneisses, amphibole-rich orthogneisses and metagabbronorites. In this work we study their chemical and isotopic composition, to gain insight into the linkage between plate tectonics and magmatism and to improve the knowledge of the paleogeographic evolution of the European Variscan Belt.Granodioritic and tonalitic orthogneisses range from intermediate to felsic rocks, with K2O/Na2O ratios≈1, typical of calc-alkaline rocks, and high Na2O content, characteristic of I-type granites. Metagabbronorites are basic rocks, but some of them are contaminated by interaction with the felsic magmas, showing enrichment in SiO2, Na2O and K2O. All Corredoiras metaigneous rocks are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE), resulting in a high LILE/HFSE ratio. These geochemical features are the most characteristic of magmas related to subduction zones; furthermore all orthogneisses display significant negative anomalies in Ta, Nb and Zr, which together with their low contents in Y and Yb match up with granitoids generated in volcanic arcs or subduction zones. SHRIMP U-Pb zircon dating provides a concordia age of 492±3Ma. Granodioritic orthogneiss has negative εNd(492 Ma) values (−2.2 to −3.6) and high (87Sr/86Sr)i ratios (0.707 to 0.708), on the other hand tonalitic orthogneisses and metagabbronorites have positive εNd(492 Ma) (1.0 to 2.4) and low (87Sr/86Sr)i (0.703 to 0.705), suggesting that granodioritic orthogneisses have a clear crustal influence in their generation, whereas tonalitic orthogneisses and metagabbros can be related to basic magmas extracted from the mantle or from a basic lower continental crust.The Corredoiras chemical characteristics permit us to interpret that this rocks were probably generated in an ensialic island arc and may represent a peri–Gondwanan fragment drifted away to open the Rheic Ocean.HighligthsWe study the geochemical characteristics of Corredoiras metaigneous unit (NW Iberian Massif).This unit has affinity with igneous continental arc association.Nd and Sr values show a mixture between juvenile material and melting of an old crust.Corredoiras magmatic rocks were generated during Late Cambrian – Early Ordovician age in an arc inner zone.
The fabric and the anisotropy of magnetic susceptibility of the Cabo Ortegal eclogite (NW Spain) are studied. These mafic rocks were metamorphosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogite slices and deformed eclogite in shear zones have bulk magnetic susceptibilities of 31 to 82 · 10−5 S.I. and 28 to 75 · 10−5 S.I., respectively. The paramagnetic mineral fraction is the principal magnetic susceptibility carrier. This fraction includes notably garnet and clinopyroxene as matrix minerals, and ilmenite and rutile as accessory constituents. Though magnetic anisotropy degree varies between 3.1 % and 6.6%, variations of this parameter in each rock type are marked. In the deformed eclogite, magnetic lineation (Kmax) and the pole to the magnetic foliation (Kmin) are coaxial and coincident with macroscopic petrofabric elements (foliation and lineation). In the massive eclogite, the magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of anisotropy. The anisotropy of magnetic susceptibility is interpreted as being due to the crystallographic preferred orientation and spatial organisation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformation is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry.
A correlation between allochthonous units exposed in the NW Iberian Massif and the southern Armorican Massif is carried out based on lithological associations, structural position, age and geochemistry of protoliths and tectonometamorphic evolution. The units on both sides of the Bay of Biscay are grouped into Upper, Middle and Lower allochthons, whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the Parautochthon. The Lower Allochthon represents a fragment of the outermost edge of Gondwana that underwent continental subduction shortly after the closure of a Palaeozoic ocean which, in turn, is represented by the Middle Allochthon. The latter consists of supra-subduction ophiolites and metasedimentary sequences alternating with basic, mid-ocean ridge basalt (MORB)-type volcanics, with inheritances suggesting the proximity of a continental domain. Seafloor spreading began at the Cambro-Ordovician boundary and oceanic crust was still formed during the Late Devonian, covering the lifetime of the Rheic Ocean, which is possibly represented by the Middle Allochthon. The opening of the oceanic domain was related to pulling apart the peri-Gondwanan continental magmatic arc, which is represented by the Upper Allochthon.
The high pressure mafic granulites of the Bacariza Formation outcrop in the two uppermost structural units of the Cabo Ortegal Complex (La Capelada unit and Cedeira unit) were separated by a Variscan thrust. In both cases, they appear as heterogeneous metabasites in normal contact between ultramafic rocks and other more homogeneous and less differentiated metabasic rocks, also affected by catazonal metamorphism. The main difference between the mafic granulites in the two units is the degree of deformation, which is higher in the underlying Cedeira unit. Petrologie and mineralogical data indicate that the high-pressure (HP) granulites (Gt-Cpx ± Amp-Pl ± Qtz ± Scp-Rt ± Ilm-Czo) are already retrograde (M2 Stage), post-dating an earlier eclogite facies metamorphism (Ml Stage) characterised by the mineral associations: Gt-Cpx ± Amp ± Ky ± Qtz-Rt and Gt-Cpx ± Amp ± Qtz ± Zo-Rt. The main structure relatted to the exhumation processes is the development of a general mylonitic foliation that, although initiated in granulite facies conditions, was mainly equilibrated in amphibolite facies (M3 Stage). This foliation was affected by isoclinal folds, which led to the formation of the Variscan thrusts responsible for the present stacking position. Thrust conditions were transitional between amphibolite and greenschist facies (M4 Stage). Thermobarometric data point to different P—T exhumation paths in the two units. Estimated P—T conditions were higher in La Capelada unit during Ml (P ≥ 13 kbar; 860 °C) and M2 (15 kbar; 800 °C) than in the Cedeira unit (Ml: P ≥ 11 kbar, 770 °C; M2: 12 kbar; 750 °C). Temperatures for the M3 stage were comparable (720 °C) in both units but rocks from the Cedeira unit show a much bigger drop in pressure. This resulted in an isothermal decompression type path for the Cedeira unit, while both P and T decreased more steadily in La Capelada rocks. These were always located at deeper level than the Cedeira rocks before the Variscan stacking. The difference in the two paths is related to different exhumation rates; higher in rocks from the Cedeira unit than in those from La Capelada. Exhumation processes coeval with underthrusting, and a different location of the rocks with respect to the main shear zone responsible for the exhumation would account for the distinct paths.
U‐Pb (zircon) crystallization ages of 52 late‐Variscan granitoid intrusions from NW Iberia (19 from new data, 33 from previous studies) constrain the lithospheric evolution of this realm of the Variscan belt of Western Europe and allow assessment of the relationship between oroclinal development and magmatism in late‐Carboniferous‐early Permian times. The U‐Pb ages, in conjunction with a range of geological observations, are consistent with the following sequence of events: (i) oroclinal bending starts at 310–305 Ma producing lithospheric thinning and asthenospheric upwelling in the outer arc of the orocline accompanied by production of mantle and lower crustal melts; (ii) between 305 and 300 Ma, melting continues under the outer arc of the orocline (Central Iberian Zone of the Iberian Variscan belt) and mid‐crustal melting is initiated. Coevally, the lithospheric root beneath the inner arc of the orocline thickened due to progressive arc closure; (iii) between 300 and 292 Ma, foundering of the lithospheric root followed by melting in the lithospheric mantle and the lower crust beneath the inner arc due to upwelling of asthenospheric mantle; (iv) cooling of the lithosphere between 292 and 286 Ma resulting in a drastic attenuation of lower crustal high‐temperature melting. By 285 Ma, the thermal engine generated by orocline‐driven lithospheric thinning/ delamination had cooled down beyond its capability to produce significant amounts of mantle or crustal melts. The model proposed explains the genesis of voluminous amounts of granitoid magmas in post‐orogenic conditions and suggests that oroclines and similar post‐orogenic granitoids, common constituents of numerous orogenic belts, may be similarly related elsewhere.
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