Printer Friendly

The basal tectonic melange of the Cabo Ortegal Complex (NW Iberian Massif): a key unit in the suture of Pangea/La melange tectonica basal del Complejo de Cabo Ortegal (NW del Macizo Iberico): una unidad clave en la sutura de Pangea.

1. Introduction

The development of thick tectonic melanges is documented in several orogenic belts, but in general they are rather unusual units associated to first order tectonic contacts. These common melanges may have igneous and sedimentary components, but in normal cases all the lithologies involved in the mixing unit can be identified in other terranes represented in the same region. In other words, the most typical melanges show a tectonic origin but do not include exotic elements. Although they can provide important information about the tectonic history of a given part of a belt, they supply only limited information about the origin of the terranes involved. Conversely, large ophiolitic melanges are less common and they can reach kilometre-scale thickness and extended continuity. They are characterized by the presence of a serpentinite matrix which surrounds tectonic blocks or slices of very varied lithologies. These ophiolitic melanges can provide important data about the tectonic setting of the terranes present in the orogenic belt, because they usually include exotic elements that may record tectonothermal histories unknown in the region outside the melange unit. Several melanges typically contain high-P rocks that are not represented in the surrounding terranes. In other cases, a given ophiolitic assemblage only exists inside the tectonic melange and never appears as an independent unit with regional distribution. In these situations, the large ophiolitic melanges include the only accessible information about terranes with a very exotic nature which are unrecognised outside the mixing unit (MacPherson et al., 2006; Federico et al., 2007). These observations, together with the intensity of the deformation as well as the mantle components, lead to the interpretation of large ophiolitic melanges as paleo-subduction zones, the most accepted tectonic setting for the generation of large serpentinite melanges (Gerya et al., 2002; Federico et al., 2007; Osmaston, 2008). In the most typical cases, the tectonic melange must be generated in the upper part of a subduction zone, because the development of the serpentinitic matrix implies the hydration of the mantle wedge by percolation of ascending fluids from the subducting slab (Gerya et al., 2002). The development of serpentinite melanges is not possible after the dehydration of the slab, and only melanges with a peridotite matrix can be considered from a theoretical perspective. These melanges, if they really exist, are much more uncommon, because the rheology of anhydrous peridotite probably inhibits the development of the tectonic mixing. Different dynamic models have been suggested to explain the precise mechanism involved in the generation of large ophiolitic melanges (Osmaston, 2008). Recent numerical models have proposed that water loss from the subducting plate produces a low-viscosity serpentinite channel in the overlying mantle wedge, where a forced return flow of subducted material is established (Gerya et al., 2002; Stockhert and Gerya, 2005; Federico et al., 2007).

Large ophiolitic melanges can be considered as markers of plate boundaries, and their distribution in orogenic belts is generally restricted to suture zones. However, their development may not been uniform throughout geological time. Many cases of ophiolitic melanges have been described in circum-Pacific belts (Hirauchi et al., 2008; Kato and Saka, 2003) and in different Cenozoic orogens, such as in the Alps (Federico et al., 2007) or in the Himalayas (Maheo et al., 2006; Guilmette et al., 2008). However, the references to Paleozoic ophiolitic melanges are more unusual and the presence of these mixing units in Proterozoic belts is rare (see. Hefferan et al., 2002; Zhang et al., 2008). In the Caledonian Belt of southern Scotland, Kawai et al. (2008) have recently described two thick units of ophiolitic melanges included in the Ballantrae Ophiolite; these melanges were generated during convergence between Avalonia and Laurentia and the consequent closure of the Iapetus Ocean. In the European Variscan Belt, references to large ophiolitic melanges are very rare. However, one of these melanges occurs at the base of the Cabo Ortegal Complex, in the NW of the Iberian Peninsula (Arenas et al., 2007b, 2008), but it has not been described in detail until now. This melange is involved in the terrane assemblage of the NW Iberian Massif, which is mainly included in the so-called allochthonous complexes of Galicia-Tras-os-Montes. These terranes are considered far-travelled allochthonous units emplaced during the closure of the Rheic Ocean, in the last stages of the Pangea assembly. Therefore, the allochthonous complexes of NW Iberia preserve an excellent section of the Pangea suture. This paper presents a detailed description of the basal ophiolitic melange of the Cabo Ortegal Complex, the Somozas Melange, and the geochronology and geochemistry of its most characteristic lithologies. An interpretation of the origin of this important mixing unit, in the context of the convergence and final collision between Gondwana and Laurussia, will be finally discussed.

2. Geological setting

The European Variscan Belt is a Devonian-Carboniferous orogen generated during the progressive collision between Gondwana and Laurussia following the closure of the Rheic Ocean (Matte, 1991; Martinez Catalan et al.,, 2007). This orogen can be mapped between the SW of the Iberian Peninsula and the Bohemian Massif, following a curvilinear outline, even though it is affected by some large oroclinal folds. However, the belt probably continues to the east of the Carpathians Arc but its precise location is unknown (Oczlon, 2006). The axial zone of the Variscan Belt is characterized by the presence of several allochthonous complexes comprising exotic terranes with ophiolites and high-P metamorphic rocks (Fig. 1; Arenas et al., 1986). As a whole, these exotic terranes delineate the complex Pangea suture in Europe, which it is rootless and transported inside the allochthonous complexes towards more external regions of the Variscan Belt. In the NW of the Iberian Massif, this suture zone occurs within several allochthonous complexes that are folded into a regional synformal structure. These complexes are remnants of a gigantic pile of nappes, and they contain a representative section of the terranes involved in the most internal part of the suture. In Galicia, NW Spain, the Cabo Ortegal and Ordenes complexes and the Malpica-Tui Unit define a WNW-ESE continuous section across the suture zone (Fig. 2). Considering this section, it is possible to recognize three main exotic terranes included in the allochthonous complexes. These are, from top to bottom, the upper units, the ophiolitic units and the basal units (Fig. 2).

[FIGURE 1 OMITTED]

The upper units contain a variety of metasedimentary and igneous rocks, including ultramafic rocks, dated at c. 520-500 Ma, affected by metamorphism ranging between the greenschist and the eclogite facies. Figure 3 shows the most important lithologies involved in the upper units in the Cabo Ortegal Complex; the Ordenes Complex may include an even greater lithological variety (Martinez Catalan et al., 2002). The upper units are an arc-derived terrane with peri-Gondwanan provenance (Fernandez-Suarez et al., 2003), characterized by a polymetamorphic tectonothermal evolution. A first intermediate pressure metamorphic event, dated at c. 490-480 Ma (Abati et al., 1999, 2007; Fernandez-Suarez et al., 2002), is related to the dynamics of the magmatic arc developed at the Gondwanan margin. Subsequently, the rifting of the arc from the continental margin and its northward drifting is considered coeval with that of the Avalonia microcontinent (Gomez Barreiro et al., 2007; Murphy and Gutierrez Alonso, 2008). The final accretion of the arc to the southern margin of Laurussia caused a high-P and highT metamorphic event, identified in the lower part of the upper units of the Cabo Ortegal and Ordenes complexes (Figs. 2 and 3). This high-P event has been dated at c. 390 Ma in the eclogites of the Cabo Ortegal Complex (Ordonez Casado et al., 2001), and at c. 410-390 Ma in the mafic granulites of the Ordenes and Cabo Ortegal complexes (Fernandez-Suarez et al., 2007).

The NW Iberia ophiolitic units, as it is also the case for the rest of ophiolites involved in the Variscan Belt, were generated within the Rheic Ocean domain, and they supply information about the opening and closure of this ocean. Two main ophiolitic assemblages, which are characterized in the field as paired ophiolitic units, can be identified in NW Iberia; the lower ophiolitic units and upper ophiolitic units (Figs. 2 and 3). The lower ophiolitic units consist of a thick pile of greenschists with intercalations of schists and phyllites, and more scarce layers of orthogneisses and ultramafic rocks. The chemical composition of the mafic rocks is characteristic of arc tholeiites, and the protolith age obtained in one of the orthogneisses is c. 500 Ma. These ophiolites were probably generated in a back-arc setting during the first stages of the Rheic Ocean opening (Arenas et al., 2007a). The upper ophiolitic units are consist of metagabbros, metadiabases, amphibolites and ultramafic rocks, dated at c. 395 Ma. The best preserved sections in these ophiolites were described in the Careon Ophiolite (SE of the Ordenes Complex), which exhibit a lithological assemblage representative of a supra-subduction zone ophiolite (Diaz Garcia et al., 1999). This ophiolite neither contains volcanic rocks nor a sheeted dike complex, but it shows frequent doleritic dikes intruding at any level of the gabbroic or ultramafic section, which is considered to be indicative of an extensional context. Sanchez Martinez et al. (2007) have proposed that these ophiolites, as other equivalent ophiolites in the Variscan Belt like the Lizard (SW England) or Sleza (Poland) ophiolites, were generated within an intraoceanic subduction zone which dipped to the north and removed the old and cold N-MORB type lithosphere of the Rheic Ocean. The new oceanic crust generated in this supra-subduction zone context has a composition of arc tholeiites. It represents the last oceanic lithosphere generated inside the Rheic Ocean domain, just slightly before its closure due to the onset of the collision between Gondwana and Laurussia. The accretion time of the Careon-type ophiolites below the high-P upper units is estimated at 380 Ma (Dallmeyer et al., 1997).

The basal units are constituted by schists, paragneises and metagreywackes, and a variety of orthogneisses, frequently very abundant in these units, amphibolites and eclogites. Two igneous series that are different in age can be distinguished in the basal units: a first series with calc-alkaline affinity dated at c. 492 Ma, and a younger series with alkaline-peralkaline composition with protolith ages at c. 472 Ma (see Abati et al., 2009). The basal units show a pervasive high-P and low to intermediate-T metamorphic event and were accreted to the orogenic wedge below the ophiolitic units. Therefore, they are interpreted as a fragment of the most external Gondwanan margin subucted below the orogenic wedge developed in the southern margin of Laurussia (Arenas et al., 1995, 1997; Martinez Catalan et al., 1996). The basal units record the oldest Variscan deformation recognized in the European margin of Gondwana, associated to the final stages of the Pangea assembly. Recent [sup.40]Ar/[sup.39]Ar and U-Pb isotopic dating suggests that the subduction of the Gondwanan margin and the coeval high-P metamorphism took place at c. 370 Ma (Rodriguez et al. 2003; Abati et al., 2009). The basal units are thrust over a thick allochthonous series of metasedimentary and volcanic rocks, namely the Parautochthon or Lower Allochthon, which has been also described as Schistose Domain. This series is not included in the allochthonous complexes because is similar to the autocthonous sequences of the Central-Iberian Zone, and is not exotic in nature. However, it can be distinguished from the Central Iberian Zone by the higher detrital character of its sedimentary series and by the abundance of felsic volcanics. The chronology of the Schistose Domain may be different between regions, and its stratigraphy and structure are poorly known. However, recent paleontological and U-Pb geochronological data suggest an Early to Middle Ordovician age for the Schistose Domain located below the Cabo Ortegal Complex (Valverde-Vaquero et al., 2005).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

In the NW Iberian Massif, the Somozas Melange is the only tectonic melange identified to date. This melange is similar to a typical serpentinite melange where a mantle wedge is involved in the mixing unit, and its characteristic structural position at the base of the exotic terranes, allows us to consider this unit as an important feature of Variscan convergence and a local manifestation of the Pangea suture. The Somozas Melange appears in the leading edge of the allochthonous pile of NW Iberia (Figs. 2 and 3), which advanced from West to East (present coordinates) (Martinez Catalan et al., 2007). The melange is located at the contact between the allochthonous terranes and the units of the Gondwana margin that do not show high-P metamorphism, which consequently were not subducted below the southern margin of Laurussia.

3. Structure and rock types of the melange unit

Based on the discontinuous character of its lithologies at regional scale, the Somozas Melange was firstly described as a fragmented ophiolite, (Arenas, 1985; Arenas et al., 1986). Marcos et al. (2002) pointed out the equivalence of this unit with a tectonic melange. This melange crops out discontinuously in the eastern part of the Cabo Ortegal Complex, in the core of upright late antiforms (Figs. 2 and 3). The type section and the best exposures are located to the west of the Somozas village, where the melange unit appears in the core of two antiforms. This region was mapped in detail to investigate the lithology and internal structure of the melange (Fig. 4). The melange unit underlies the Moeche and Espasante units (lower ophiolitic units and basal units, respectively), cutting the contact between both units at a high angle as can be observed in the western limb of the Somozas Antiform. The melange unit gently dips to the west and disappears below the Cabo Ortegal Complex, with no more recurrences in the NW Iberian Massif. This is the unit with the lowest structural position in the allochthonous complexes, but its existence is limited to the leading edge of the allochthonous pile.

The Somozas Melange contains two different subunits (Fig. 4). The upper unit has a rather variable thickness reaching up to 800 m to the South of the Moeche village, and can be classified as a typical ophiolitic melange where a highly sheared matrix of serpentinites surrounds tectonic blocks and slices of variable size and continuity (Fig. 5). The smallest tectonic blocks are one metre or so in length. The largest blocks in the melange are kilometers in length. The most common rocks in the melange are gabbros, diabases, granitoids and volcanic rocks. Large tectonic blocks of high temperature metamorphic rocks also occur. Moreover, intercalations of phyllites and phyllonites occur, whereas tectonic blocks and slices of sandstones, conglomerates and marbles are less common (Fig. 4). The lower subunit may attain thickness of 1000 m and is a melange with a matrix of ocher-colored phyllites or blue phyllonites surrounding tectonic blocks and slices of the lithologies involved in the ophiolitic melange. This subunit was formed later than the ophiolitic melange and it represents a complex imbrication zone between the ophiolitic melange and a metasedimentary unit. The entire Somozas Melange is thrust over the Schistose Domain and hence is emplaced over series that belong to the external Gondwanan margin.

The igneous rocks involved in the Somozas Melange do not generally preserve their primary mineralogy. Only a few rare metagabbros contain igneous clinopyroxene and orthopyroxene partially replaced by hornblende. The igneous phases are replaced by low temperature, or more rarely medium temperature, metamorphic minerals, developing mineral assemblages typical of the greenschist or amphibolite facies. This alteration was hydrothermal in origin, with an almost perfect preservation of the original igneous textures in areas where subsequent deformation was weak. The pervasive deformation inside the tectonic blocks and slices is very heterogeneous because the serpentinite matrix is preferentially sheared and this feature favors a low internal deformation in many of the large tectonic blocks and slices. The deformation and regional metamorphism associated with melange formation and its later thrusting affect igneous rocks that previously underwent oceanic hydrothermal metamorphism.

The submarine volcanic rocks include lava flows, broken pillow breccias, submarine breccias, close-packed pillow lavas and hyaloclastites. The textures and the original mineralogy deduced from the hydrothermal phases, suggest basaltic and basaltic andesite compositions. The porphyritic types are abundant and contain many millimetre-sized pseudomorphs of plagioclase phenocrysts and less common pseudomorphs of mafic phenocrysts, all of them comprised of hydrothermal phases. The broken pillow breccias can include complete and undeformed pillow lavas up to 1 m in diameter, with chilled margins and blastoporphyritic cores. The submarine breccias can show a variably recrystallized dark hyaloclastitic matrix, that in scarce outcrops may preserve remnants of shards. The volcanic rocks are intruded by abundant diabase dykes, but primary contacts with plutonic or sedimentary rocks are not exposed. Coarse to medium grained gabbros can appear both in monolithological slices or showing intrusive relationships with granitoids and diabases. The granitoids are fine to medium grained with well preserved primary textures, with compositions of diorites, quartz-diorites, tonalites and granodiorites. K-feldspar bearing types are almost absent, but there is a single tectonic block consisting of a K-feldspar-rich granitoid with a monzogranitic composition. In the metagranitoids, the primary plagioclase is typically replaced by albite and epidote-clinozoisite, whereas the primary mafic minerals are replaced by chlorite, amphibole or brown (or more rarely) green stilpnomelane. Highly sheared serpentinites are the most abundant rock type in the ophiolitic melange, they seldom preserve primary igneous minerals but the presence of a chromium-rich spinel is almost pervasive.

[FIGURE 4 OMITTED]

The high temperature tectonic blocks contain a diversity of highly sheared tonalitic orthogneisses and metabasites. The metabasic rocks include common amphibolites and zoisite and rutile-rich amphibolites. There are no precise thermobarometric data for these rocks, but they contain characteristic types of amphiboles and a degree of recrystallization that enables their distinction from the other mafic igneous rocks involved in the melange. The presence of tectonic blocks with contrasted metamorphic conditions is common in many large ophiolitic melanges (Federico et al., 2007). However, as discussed below, the protolith age obtained with U-Pb geochronology in one of the high-T orthogneisses suggests the correlation of these rocks with those of the basal units of the allochthonous complexes, probably with the Espasante Unit, rather than with the igneous rocks involved in the melange. Therefore, it is suggested that these high-T rocks were incorporated to the melange as metamorphic rocks derived from a terrane accreted in an upper position in the orogenic wedge.

[FIGURE 5 OMITTED]

As it is the case in other ophiolitic melange, the tectonic blocks involved in the Somozas Melange have a contrasting metamorphic evolution. Most of them exhibit green-schist facies mineral assemblages, whereas some meta-gabbros exhibit a mineralogy characteristic of the low-T part of the amphibolite facies (Arenas, 1985). Moreover, the metahyaloclastitic matrix of some submarine breccias contains paragonite, garnet and kyanite, and fragments in the breccia itself may contain almandine garnet growing around ilmenite aggregates. These data confirm the inclusion in the ophiolitic melange of tectonic blocks derived from different depths, some of them with a metamorphic evolution probably developed under a high-P gradient, which is consistent with the generation of the tectonic melange in a subduction zone.

The most abundant metasediments in the melange are black or dark-blue phyllonites. Moreover, there are also common ocher-colored phyllites that may appear with a fine schistosity previous to the generation of the melange. These metasediments are considered as tectonic blocks and slices that escaped from the strong shearing associated with melange formation. Tectonic blocks of sandstones, conglomerates and marbles also occur. The tectonic blocks of metacarbonates have a thickness ranging between 1 m and tens of metres. The metacarbonates have a saccharoidal texture, are white to grey in colour and are intensely deformed. However, Van der Meer Mohr (1975) described some fauna that suggest an age that is younger than Middle Ordovician. Conglomerates and marbles similar to those included in the Somozas Melange have not been described neither in the Parautochthonous series nor in other units of the allochthonous complexes. Hence, it is clear that they have exotic nature and uncertain origin. Moreover, a direct correlation between the pelitic metasediments in the Somozas Melange and the metasediments from the upper part of the Parautochthonous below the Cabo Ortegal Complex cannot be established. In this way, it can be pointed out that on top of the Parautochthon several levels of high-silica rhyolites are observed, while their presence inside the Somozas Melange has not been proven (Fig. 4).

4. U-Pb zircon geochronology

4.1. Sample selection and analytical techniques

In order to determine the age of the igneous and sedimentary lithologies involved in the Somozas Melange, U-Pb zircon dating has been performed on 4 representative samples: one orthogneiss from a large high-T tectonic block (sample GCH-05-11); two metagranitoids involved in the ophiolitic melange (samples GCH-05-8 and GCH-05-6); and one conglomerate also included in the ophiolitic melange (sample SO-3). The location of these samples is shown in the map of the Cabo Ortegal Complex (Fig. 3). The detailed map of the Somozas Antiform (Fig. 4) also shows the location of the three samples coming from that region.

Sample GCH-05-11 is a tonalitic orthogneiss collected in the small village of Gradoy. It belongs to a tectonic block of orthogneisses included between phyllonites in the lower melange subunit (Fig. 4). It is a medium grained orthogneiss with a cataclastic fabric that apparently developed after an earlier fabric of granoblastic character, and consists of quartz, albitic plagioclase, biotite, chlorite, sericite, epidote-clinozoisite, ilmenite, pyrite, apatite and zircon. The cataclasis occurred at medium temperature, during the retrogression of a previous mineral assemblage developed at higher temperature.

Sample GCH-05-8 is a barely deformed metagranitoid from the Insua region. It belongs to a tectonic block that includes granitoids, gabbros and diorites which appears surrounded by phyllites and phyllonites. The intrusive relationships between the igneous lithologies of this tectonic block are not clear. The metagranitoid dated by U-Pb geochronology has tonalitic composition, shows a fine grained granular texture and it is affected by a low-T metamorphism. The mineral composition is quartz, albitic plagioclase, chlorite, stilpnomelane, white mica, epidote-clinozoisite, ilmenite, pyrite, apatite and zircon.

Sample GCH-05-6 is a metagranitoid collected in the small village of Ferreiras. It is part of a monolithologic tectonic block included in serpentinites with hundred of meters-size continuity (Fig. 4). It is a medium grained, moderately deformed rock with blastogranular texture and monzogranitic composition. Contains quartz, plagioclase, K-feldspar, biotite, chlorite, white mica, epidoteclinozoisite, ilmenite, pyrite, apatite and zircon.

Sample SO-3 is a metaconglomerate collected near the little village of Ferreiras, in an old serpentinite quarry. It is part of a 7 m thick tectonic block included between mylonitic serpentinites (Fig. 4). The metaconglomerate is poorly deformed and contains cm-size pebbles of sandstones, pelites, cherts, limestones, plutonic rocks (quartz-diorites, tonalites and granitoids) and volcanic rocks (basalts, andesites, dacites and glass fragments). This metaconglomerate shows a low temperature (greenschist facies) metamorphic recrystallization.

U-Th-Pb analyses of zircon in samples GCH-05-11 and GCH-05-8 were conducted on the Bay SHRIMP-RG (Sensitive High Resolution Ion Microprobe-Reverse Geometry) operated by the SUMAC facility (USGS-Stanford University) during two analytical sessions in February and October 2006. Zircon separation was carried out at the Universidad Complutense (Madrid) following standard techniques, including crushing, pulverizing, Wilfley table, sieving, magnetic separator and methylene iodide. The zircons were handpicked under a binocular microscope and mounted on a double-sided adhesive on glass slides in 1 x 6 mm parallel rows together with some chips of zircon standard R33 (Black et al., 2004). After being set in epoxy resin, the zircons were ground down to expose their central portions by using 1500 grit wet sandpaper, and polished with 6 [micro]m and 1 [micro]m diamond abrasive on a lap wheel. Prior to isotopic analysis, the internal structure, inclusions, fractures and physical defects were identified with transmitted and reflected light on a petrographic microscope, and with cathodoluminescence (CL) on a JEOL 5800LV electron microscope (housed at USGS-Denver). After the analysis, secondary electron images were taken to locate the exact position of the spots. Analytical procedures for zircon dating followed the methods described in Williams (1997). Secondary ions were generated from the target spot with an [O.sup.2]- primary ion beam varying from 4-6 nA. The primary ion beam produced a spot with a diameter of ~25 microns and a depth of 1-2 microns for an analysis time of 8-10 minutes. Twelve peaks were measured sequentially in a single collector: [sup.90][Zr.sub.2][sup.16]O, [sup.204]Pb, background (0.050 mass units above [sup.204]Pb), [sup.206]Pb, [sup.207]Pb, [sup.208]Pb, [sup.238]U, [sup.248]Th[sup.16]O, [sup.254]U[sup.16]O, [sup.166]Er[sup.16]O, [sup.172]Yb[sup.16]O, [sup.180]Hf[sup.16]O. One additional peak was included in the second session ([sup.155]Gd). Five scans were collected, and the counting time for [sup.206]Pb was increased according to the Paleozoic age of the samples to improve counting statistics and precision of the [sup.206]Pb/[sup.238]U age. Before collecting the data, the primary beam was rastered for 90-120 seconds over the area to be analyzed. The concentration of U was calibrated using zircon standard CZ3 (550 ppm U; Pidgeon et al., 1995), and isotopic compositions were calibrated against R33 ([sup.206]Pb*/[sup.238]U = 0.06716, equivalent to an age of 419 Ma, Black et al., 2004) which was analyzed every four analyses. Data reduction follows the methods described by Williams (1997), and Ireland and Williams (2003), and SQUID (version 1.08) and ISOPLOT (version 3.00) software (Ludwig, 2002, 2003) were used. All the ages, except one, are younger than 1 Ga, so they are reported based on [sup.206]Pb/[sup.238]U ratios corrected from common Pb using the [sup.207]Pb method. The oldest age is reported based on its [sup.204]Pb-corrected [sup.206]Pb/[sup.207]Pb isotopic ratio. The Pb composition used for initial Pb corrections ([sup.204]Pb/[sup.206]Pb=0.0554, [sup.207]Pb/[sup.206]Pb=0.864 and [sup.208]Pb/[sup.206P]b=2.097) was estimated using the Stacey and Kramers (1975) model. Analytical results are presented in Tables 1 and 2.

U-Th-Pb analyses of zircon in sample GCH-05-6 were conducted at the Natural History Museum of London using the analytical technique of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) during one analytical session in September 2005. Zircon separation was carried out at the Universidad Complutense (Madrid) following the standard techniques described in the previous samples. Zircons were set in synthetic resin mounts, polished and cleaned in a HN[O.sub.3] ultrasonic bath and polished to expose equatorial sections. Analytical instrumentation, analytical protocol and techniques, data reduction, age calculation and common Pb correction are as described by Jeffries et al. (2003). Concordia age calculations, and creation of concordia plots, were performed by using ISOPLOT (version 3.00) software (Ludwig, 2003). Analytical results are presented in Table 3.

U-Th-Pb analyses of zircon in sample SO-3 were conducted at the GEMOC Key Centre, Macquarie University, using a UV laser ablation system (Norman et al., 1996) coupled to an Agilent 4500, Series 300 ICP-MS. Zircon separation was carried out at the Universidad Complutense (Madrid) following the standard techniques described in the previous samples. ICP-MS operating conditions, data acquisition parameters, analytical protocol and data processing methodology are the same as those specified by Martinez Catalan et al. (2008). Concordia age calculations, and creation of concordia plots, were performed by using ISOPLOT (version 3.00) software (Ludwig, 2003). Analytical results are presented in Table 4.

4.2. U-Pb results

Sample GCH-05-11 (Gradoy orthogneiss)

Zircons from sample GCH-05-11 are small, blocky, idiomorphic grains with light yellow color. Under cathodoluminescence (Fig. 6a), broad homogeneous weakly luminescent areas are evident in most of the cores. These areas are mantled by variably thick oscillatory zones that are separated by thin luminescent bands, suggesting different stages of zircon precipitation during the evolution of the magma (Corfu et al., 2003). Some discontinuous non-luminescent rims can also be observed.

Twenty-three analyses performed in 21 zircon grains from the Gradoy orthogneiss were aimed either at homogeneous areas or at oscillatory zones, and only one non-luminescent rim was thick enough to place a spot. Excluding the seven oldest analyses based on their reverse discordance (analyses 14.1, 15.1, 15.2 and 16.1) or high common Pb (analyses 5.1, 5.2 and 6.1), and the six youngest due to Pb loss, a weighted mean [sup.206]Pb/[sup.238]U age of 485 [+ or -] 6 Ma is obtained, with a mean square of weighted deviation (MSWD) of 1.6 (Fig. 7). This age is interpreted as the best estimate for the crystallization of the igneous protolith of the orthogneiss.

Sample GCH-05-8 (Insua granitoid)

In sample GCH-05-8, zircons are mainly colorless, clear, euhedral prismatic grains and broken prisms with preserved faces. Some tan, clear, and subrounded to multifaceted equant grains, typical of metamorphic environments (Corfu et al., 2003) are present. CL images show different internal textures in the zircons, disregarding their morphology (Fig. 6b). Zircons with a homogeneous domain are poorly luminescent and are commonly surrounded by thin irregular non-luminescent rims. Some cores have luminescent oscillatory zoning, bordered by irregular, thin rims. Other zircons display complex internal structures with combined oscillatory and sector-zoned cores variably resorbed and mantled by a luminescent domain, which can be in turn surrounded by a discontinuous non-luminescent rim.

The forty-four analyses carried out in 40 grains from the Insua granitoid are divided according to their age into inherited, magmatic and Variscan. The inherited age population includes all the analyses older than 505 Ma. The oldest age corresponds to a rim in a rounded grain (28.1) that yields a discordant (15%) [sup.207]Pb/[sup.206]Pb age of 2264 [+ or -] 22 Ma. Another individual analysis from a moderately luminescent core (29.1) yields a [sup.206]Pb/[sup.238]U age of 772 [+ or -] 9 Ma. In grain 35.1, an age of 630 Ma is obtained, but this result has been rejected due to the high common Pb content. Two analyses from weakly luminescent oscillatory cores give ages of 568 and 567 Ma. Four of the six youngest ages in the inheritance population are obtained from non-luminescent cores and rims, and are rejected due to their high U content (U>2100 ppm, grains 12.1, 17.1 and 25.2) or high common Pb (>0.50%, analysis 40.2). Two remaining analyses from luminescent cores yield an age of 506 [+ or -] 2 and 510 [+ or -] 3 Ma (grains 23.1 and 34.1, respectively). The magmatic age population comprises 28 analyses ranging from 465 to 505 Ma. The five youngest ages are rejected due to their high U content (analysis 1.2), high common Pb (analyses 1.2, 11.1 and 27.1), probable presence of inclusions (analyses 1.2 and 2.1) or mixed domains (analyses 1.1, 11.1 and 27.1). The next seven youngest analyses yield a mean age of 492 [+ or -] 1 Ma. However, these spots should be rejected due to the possibility of Pb loss because the secondary electron images taken after the SHRIMP session show that they hit small fractures that were not visible in the reflected light images used during the analytical session. Two older analyses are also discarded due to their big error (spot 26.1) and high common Pb (spot 22.1). The remaining 14 analyses in this population yield a mean age of 499 [+ or -] 1 Ma (MSWD = 0.79), which is considered the best estimate for the crystallization of this metagranitoid (Fig. 8). The Variscan age population is constituted by five analyses that yield a mean age of 311 [+ or -] 11 Ma, with a MSWD of 16 (Fig. 8). This small number of analysis suggests that the Variscan ages could be the result of Pb loss. However, the possibility that this age could represent metamorphic zircon recrystallization or new zircon growth cannot be ruled out.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Sample GCH-05-6 (Ferreiras granitoid)

Forty-eight analyses were performed on 46 zircon grains from the granitoid sample GCH-05-6. Fourteen of those were rejected based on its discordance higher than 10%. The 34 selected analyses yielded [sup.206]Pb/[sup.238]U ages ranging between 480 [+ or -] 4 and 555 [+ or -] 8 Ma (Table 3). When plotted in the concordia diagram, they constitute a continuous cluster, and it is possible to calculate a concordia age where there is maximum density of overlapping ellipses, obtaining a result of 527 [+ or -] 2 Ma (MSWD = 0.64; Fig. 9). The same result is also obtained by calculating the average age of the whole set of selected data. The features of the zircon crystals and their low Th/U ratios (0.18-0.33, see Table 3) are compatible with an igneous origin. Therefore, the Cambrian age obtained for the U-Pb dating of this granitoid is interpreted as its protolith age.

Sample SO-3 (Ferreiras conglomerate)

Sixty analyses were performed in zircon grains from the sample SO-3, of which only concordant or nearly concordant (<10% discordant) data were considered for interpretation of detrital zircon age. U-Pb and Pb-Pb ratios and ages for the 59 selected analyses are given in Table 4. The reported ages used to plot the population histogram (Fig. 10b) are the [sup.207]Pb/[sup.206]Pb ages for zircons older than 1.0 Ga and [sup.206]Pb/[sup.238]U for those younger than 1.0 Ga. This is because [sup.207]Pb/[sup.206P]b ages become increasingly imprecise below 1.0 Ga due to the change of the concordia slope. The most important population of zircons (40.7% of the analyzed grains) is represented by 24 concordant and subconcordant analyses with U-Pb Middle Ordovician to Neoproterozoic ages ranging between 464 [+ or -] 7 and 628 [+ or -] 6 Ma (Table 4, Fig. 10), with the maximum density around 500 and 660 Ma. The second significant population is comprised of 15 concordant and subconcordant analyses with [sup.207]Pb/[sup.206]Pb ages between 1842 [+ or -] 9 and 2075 [+ or -] 9 Ma, and the maximum density around 1900 and 2070 Ma. There are also a few clusters of analyses (16 in total) of Palaeoproterozoic and Archaean ages, three analyses of Mesoproterozoic ages ranging from 1209 [+ or -] 10 to 1366 [+ or -] 9 Ma and a single Neoproterozoic analysis of 708 [+ or -] 6 Ma (Table 4, Fig. 10).

The youngest zircon dated from this conglomerate sample is concordant with an age of 464 [+ or -] 7 Ma. Taking into account that this unit only developed low grade metamorphism it could be possible to interpret this datum as the maximum depositional age of the sediments. However considering the statistical uncertainty of a single analysis, we favour a lower to middle Ordovician time interval for the sedimentation as the zircon population increases dramatically during this period (Fig. 10b). Zircons in the age range of 500-750 Ma which constitute the main population in this sample (Fig. 10), correspond to the Cadomian-Avalonian-Pan-African events (Fernandez-Suarez et al., 2002; Linnemann et al., 2004) and they are lacking in sediments with a provenance from the Baltica craton. The presence of a population with Palaeoproterozoic ages in the interval of c. 1800-2200 Ma (Fig. 10), together with the absence of the Mesoproterozoic population between 900-1100 Ma, which appears in sediments derived from the Amazonia Craton (Nance and Murphy, 1994), is typical of a West-African provenance (Eburnian events). On the other hand, the population of Archaean ages (Fig. 10b) can be related to Liberian events from Africa (Fernandez-Suarez et al., 2002). In conclusion, these age populations indicate that the deposition of these conglomerates was adjacent to the Gondwana margin.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

5. Geochemistry of the igneous rocks

A set of 27 samples of the most representative igneous rocks from the Somozas Melange were collected in order to study their geochemical composition. These rocks are typical members of the ophiolitic melange, and they include: 13 samples corresponding to different types of metavolcanic rocks, some of them clearly submarine, 6 diabase dikes and 8 different types of gabbros, diorites and granitoids. The milling of these samples was performed at the Universidad Complutense de Madrid and they were analysed at the Activation Laboratories Ltd. (Actlabs) in Canada. The digestion procedure was the lithium metaborate/tetraborate fusion, and the analytical technique used to measure the elemental concentrations was inductively coupled plasma mass spectrometry (ICP-MS). The results obtained appear in Tables 5 to 8.

[FIGURE 10 OMITTED]

5.1. General geochemical features and classification

Two main groups of metavolcanic rocks can be distinguished in the Somozas Melange, submarine volcanic rocks whose most representative outcrops are located in the Espasante locality, in the coast section (Fig. 3), and common metavolcanic rocks. The 6 samples corresponding to the first group are characterized by Si[O.sub.2] contents ranging between 49.75 and 51.88 that allow their classification as basic rocks. The compositional ranges of the rest of the major elements for them vary: 17.84-18.92% [Al.sub.2][O.sub.3], 8.46-9.57% [Fe.sub.2][O.sub.3](T), 0.15-0.29% MnO, 3.94-4.7% MgO, 8.92-9.84% CaO, 2.04-2.76% [Na.sub.2]O, 0.11-0.34% [K.sub.2]O, 0.99-1.01% Ti[O.sup.2], and 0.15-0.18% [P.sup.2][O.sub.5] (Table 5). The samples of common metavolcanics are also metabasites with slightly lower Si[O.sub.2] contents (48.06-51.64%). Regarding the rest of the major elements, their composition is lower in [Al.sub.2][O.sub.3], CaO, [K.sub.2]O, Ti[O.sub.2], [P.sub.2][O.sub.5] (14.34-15.81%, 5.84-9.6%, 0.02-0.22%, 0.69-0.94%, 0.04-0.09%, respectively) and higher in [Fe.sub.2][O.sub.3](T), MgO and [Na.sub.2]O (9.92-11.52%, 7.3-9.66%, 2.7-4.48%, respectively) compared to that of the submarine volcanic rocks (Table 6).

The diabase dikes appear in the same outcrops of Espasante where the analyzed submarine volcanic rocks were taken. These dikes show clear intrusive relationships with all the submarine volcanic rocks, including the lava flows, the submarine breccias and the pillow lavas. The compositional range of the diabase dikes is very narrow and they have compositions of mafic rocks (48.93-49.24% Si[O.sub.2]) with contents in major elements ranging: 15.06-15.58% [Al.sub.2][O.sub.3], 11.76-12.41% [Fe.sub.2][O.sub.3](T), 0.22-0.23% MnO, 6.16-6.49% MgO, 9.42-10.6% CaO, 0.24-0.58% [Na.sub.2]O, 0.01-0.08% [K.sub.2]O, 1.57-1.63% Ti[O.sub.2], and 0.12-0.16% [P.sub.2][O.sub.5] (Table 7).

One of the plutonic rocks has a Si[O.sub.2] content typical of mafic rocks (sample CE-99, 51.78%), three have intermediate compositions (samples CE-92, CE- 93 and CE-95; 53.61-54.43% Si[O.sub.2]), and the rest are felsic granitoids with SiO2 content ranging between 69.76-72.58%. The metabasite sample can also be distinguished from the samples of intermediate composition according to its lower contents in CaO and [K.sub.2]O, and higher [Fe.sub.2][O.sub.3] (T), MnO, [Na.sub.2]O, Ti[O.sub.2] and [P.sub.2][O.sub.5] (Table 8).

Secondary processes such as hydrothermal alteration, metamorphism or deformation may have alterated the primary compositions of the rocks. Therefore, the chemical classification of the melange samples uses a combination of diagrams based in mobile (silica and alkalis, Fig. 11a) and immobile (Ti, Zr, Nb and Y, Fig. 11b) elements. According to them, all the investigated samples of the Somozas Melange have compositions typical of subalkaline rocks. It is possible to chemically distinguish quite clearly the two different types of metavolcanic rocks, given that the Espasante submarine metavolcanics can be classified as basaltic andesites whereas the common metavolcanic rocks are more similar to basalts. This difference is more marked in their immobile trace element contents. Same differences exist between the mafic and the intermediate plutonic rocks; a sample of gabbro has compositions equivalent to a basalt and the intermediate rocks have compositions compatible with basaltic andesites. The felsic granitoids have compositions equivalent to that of rhyodacites.

The submarine metavolcanics have total rare earth element ([SIGMA]REE) contents ranging between 75 and 81 ppm, and concentrations between 9 and 40 times the chondritic abundances (Nakamura, 1974). They show fractionated chondrite-normalized REE patterns ([[La/Yb].sub.N] = 3.63-3.84, Fig. 12a) typical of calc-alkaline rocks, enriched in light REE (LREE) compared to the heavy REE ([summation]REE), with slightly negative Eu anomalies (Eu/Eu* = 0.86-0.89; calculated according to Taylor and MacLennan, 1985). The common metavolcanics have very lower [SIGMA]REE, ranging between 20 and 30 ppm and concentrations between 3 and 12 times the chondritic abundances. Their chondrite-normalized REE patterns (Fig. 12b) are parallel although depleted compared to that corresponding to a typical N-MORB according to Pearce and Parkinson (1993). These are almost flat for the HREE ([[Gd/ Yb].sub.N] = 0.82-1.05) and depleted on LREE compared to the HREE ([[La/Sm].sub.N] = 0.43-0.70), without significant Eu anomalies (Eu/Eu* = 0.88-1.14). The samples of diabase dikes have total REE contents ranging between 55 and 71 ppm, and concentrations between 12 and 30 times the chondritic abundances. They show relatively flat normalized REE patterns, although slightly enriched in LREE relative to HREE ([[La/Yb].sub.N] = 1.19-1.78, Fig. 12c), and without significant Eu anomalies (Eu/Eu* = 0.88-1.14). The gabbroic sample has total REE contents in the same range than the common volcanics (27 ppm). It shows concentrations between 4 and 11 times the chondritic abundances and its normalized REE pattern (Fig. 12d) is almost flat for the HREE ([[Gd/Yb].sub.N] = 0.87) and depleted in light LREE compared to the HREE ([[La/Sm].sub.N] = 0.51), without a significant Eu anomaly (Eu/Eu* = 0.95), very similar to those of the common metavolcanics. The intermediate plutonic rock samples, although having similar total REE to the gabbroic sample (27-33 ppm, from 4 to 18 times the chondritic abundances), have fractionated REE patterns an enrichment of the LREE compared to the HREE ([[La/Yb].sub.N] = 2.69-3.71, Fig. 12d) and without significant Eu anomalies (Eu/Eu* = 0.96-1.10). These are comparable to submarine volcanics, although they are more depleted, suggesting they are less evolved lithologies. Regarding the granitoids, it is possible to distinguish two of them with low REE contents (62-64 ppm, from 7 to 38 times the chondritic abundances) and the other two more enriched (194-220 ppm, from 16 to 134 times the chondritic abundances), that are also the most evolved according to their higher Ti/Nb and Zr/Y ratios. All of them have fractionated chondrite-normalized REE patterns that are steeper in the case of the most enriched samples ([[La/Yb].sub.N] = 4.20-4.53, in samples CE-94 and CE-96, and 7.35-7.46 in samples CE-97 and CE-98; Fig. 12e), with a significant negative Eu anomaly, most marked in the REE-enriched samples.

5.2. Tectonic setting

The most useful geochemical discrimination diagrams to determine the tectonic setting of the Somozas Melange rocks are Ti-Zr-Y (Pearce and Cann, 1973), Th-Hf-Ta (Wood, 1980), MnO-Ti[O.sub.2]-P2[O.sub.5] (modified after Mullen, 1983), Ta/Yb-Th/Yb (Pearce, 1983) and Ta-Yb (Pearce et al. , 1984), which are essentially based on immobile trace and major elements. The last two diagrams are suitable for felsic rocks, specially the Ta-Yb diagram that was specifically designed to identify granitoid types. In the Ti-Zr-Y, the melange samples plot forming two distinguished groups; the first, including the common volcanics, dikes and the gabbro sample is located in or near the field of island-arc tholeiites, and the second group, consisting in the submarine metavolcanics and the intermediate plutonic rocks, is located in the calc-alkali basalts field (Fig. 13a). The Th-Hf-Ta diagram allows the most accurate discrimination of subduction related rocks. All the samples collected from the Somozas Melange plot in the field corresponding to rocks generated in destructive plate margins (Fig. 13b), due to their low Ta contents. It is possible to distinguish between samples with tholeiitic affinity (common metavolcanics, dikes and gabbro) which have Hf/Th [greater than or equal to] 4.2, and samples with calc-alkaline affinity (submarine metavolcanics, intermediate plutonic rocks and acid granitoids) characterized by Hf/Th [less than or equal to] 0.96. Their projection in the MnO-Ti[O.sub.2]-P2[O.sub.5] diagram confirms this origin for the mafic and intermediate samples of the Somozas Melange as they are located near the apex corresponding to supra-subduction zone rocks (Fig. 13c). All the mafic and intermediate samples with Ta/Yb ratios higher than 0.01 plot in the subduction-related field of the Ta/Yb-Th/Yb diagram far from the mantle array (Fig. 13d). All of them show Th/Yb ratios higher than typical N-MORB, but compatible with that of island-arc tholeiites in the case of the diabase dikes (Th/Yb = 0.156-0.2). The ratios of the common volcanics and the intermediate plutonic rocks are even higher (Th/Yb = 1.055-1.585), indicating calc-alkaline affinity, and their Ta/Yb ratios are intermediate between those typical of rocks generated in oceanic arcs and active continental margins (Ta/Yb = 0.053-0.123). The Ta-Yb diagram confirms the origin in a supra-subduction zone for the Somozas Melange acid granitoids, showing that their compositions are similar to that of volcanic arc granites (Fig. 14e).

Trace element abundance diagrams normalized to the average composition of rocks of a typical dynamic origin have been plotted for each group of samples to determine more accurately their tectonic setting. The average N-MORB composition (Pearce, 1996) has been the normalizing factor used to plot all the melange samples, except for the felsic granitoids, which were normalized to the ORG composition (Pearce et al., 1984). Both the compositional range of each lithological type and their average composition are represented in Figure 14. As can be observed, a quite clear resemblance exists between the patterns corresponding to the samples of submarine metavolcanics and the intermediate plutonic rocks, and between those corresponding to the common metavolcanics, dikes and gabbro. The two first lithological types are characterized by strongly fractionated trace element patterns (Fig 14a, d and e) with a marked Nb anomaly, although the intermediate plutonic rocks are more depleted in all the elements, suggesting that they are less evolved lithologies. Both submarine volcanics and intermediate rocks are strongly enriched in Th compared to N-MORB. The metavolcanics are also enriched in Ce, slightly depleted in Ti, but with similar concentrations in Nb, Zr and Y compared to the typical N-MORB composition (Fig. 14a and e). The intermediate rocks are depleted in Nb, Zr, Ti and Y compared to the N-MORB and have similar Ce contents (Fig. 14d and e). The significant Nb anomaly present in all the samples clearly indicates an origin in a subduction zone environment, whereas the strong fractionation of their trace element patterns is typical of calc-alkalic rocks, which suggest that they were probably generated in an evolved volcanic arc. Common volcanics, diabase dikes and gabbro are characterized by trace element patterns essentially parallel to that of N-MORB, except for their marked Nb anomaly (Fig. 14b, c and e). The common metavolcanics are the most depleted samples, with lower contents in Nb, Ce Zr, Ti and Y than those of the average N-MORB (Fig. 14b and e). The diabase dikes have higher contents in trace elements than the common metavolcanics. In relation to N-MORB they show similar abundance of Zr, Ti and Y, but they are enriched in Th and Ce and depleted in Nb (Fig. 14c and e). The gabbro has a trace element pattern very similar to that of the common metavolcanics (Fig. 14d and e), although slightly enriched in Th and Nb than these samples. The geochemical features of these three lithologics, such as their trace element contents similar to N-MORB, together with their little fractionated patterns indicate their tholeiitic affinity, whereas their marked Nb anomaly suggests a subduction-related origin. The granitoids are characterized by strongly fractionated trace element patterns and generally depleted trace element abundances compared to ORG, except for the strong enrichment in Th, and some samples with slight enrichment in Ce. The most significant feature of their patterns is a pronounced negative Ta and Nb anomaly, which together with their low contents in Y and Yb are typical of granitoids generated in volcanic arcs or subduction zones.

[FIGURE 11 OMITTED]

In summary, according to the information provided by the trace elements with the most immobile behaviour and the highest discriminating power, submarine metavolcanics, intermediate plutonic rocks and granitoids of the Somozas Melange are classified as calc-alkaline rocks which were generated in a supra-subduction zone setting, probably during the mature stages of the evolution of a volcanic arc. However, common metavolcanics, diabase dikes and gabbro show geochemical features compatible with that of island arc tholeiites, also related to the activity of a subduction zone.

6. The origin of the Somozas Melange

6.1. Interpretation of the U-Pb and geochemical data

Whole rock geochemical data show that two igneous series with different composition are represented in the Somozas Melange. Both suites contain plutonic and volcanic members: an igneous series with calc-alkaline composition and another one equivalent to island-arc tholeiites. It is difficult to clarify the relative chronology of both series and their possible regional coexistence, as they are restricted to the tectonic blocks and slices involved in the melange. However, there are key exposures in the coastal section, around the Espasante village (Fig. 3), suggesting that both series shared a common paleogeographic origin, but they were probably formed at different times. These outcrops define a thick tectonic slice constituted by calc-alkaline volcanic rocks with broken pillow breccias, close-packed pillow lavas and lava flows, intruded by a set of diabase dikes with compositions of island-arc tholeiites (Arenas and Peinado, 1981; Arenas, 1985). Moreover calc-alkaline rocks, either mafic or felsic, which are rather common in the ophiolitic melange, are not observed to intrude island arc tholeiitic volcanic rocks. If these relationships are typical of the complete assemblage of igneous rocks in the ophiolitic melange, these rocks may be remnants of a mature calc-alkaline volcanic arc that was affected by extension in a later stage and intruded by a new magmatic suite with the chemistry of island-arc tholeiites. The new U-Pb geochronology presented in this contribution includes the dating of two granitoids with calc-alkaline affinity (GCH-05-8 and GCH-05-6), and their ages strongly suggest that a mature volcanic arc was active during a great extent of the Cambrian (c. 500-527 Ma; Figs. 8 and 9). Geochronological data obtained from the conglomerate SO-3 suggest that the activity in this arc spanned the interval between the Ediacaran and the Early Ordovician. However, the youngest activity in the arc was probably residual because there are few detrital zircons with this age. Considering the age populations of the detrital zircons in this conglomerate, the activity in the volcanic arc represented in the Somozas Melange probably occurred in a peri-Gondwanan setting, which is in agreement with the data obtained in similar rocks in the NW Iberian Massif and in the Bohemian Massif (Fernandez Suarez et al., 2002; Linnemann et al., 2004).

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

The overall structure and evolution of the peri-Gondwanan arc preserved in the Somozas Melange is similar to that presented in Figure 15, based on a model for the Lau Basin-Tonga Trench region (Hawkins, 2003). The model shows a mature calc-alkaline volcanic arc of Cambrian age, with the onset of the extensional activity in the arc resulting in the opening of intra-arc basins which were rapidly filled up with sediments as magmatism changed from calc-alkaline to island-arc tholeiites. According to previous data on the context and chronology for the opening of the Rheic Ocean (Murphy et al., 2006; Arenas et al. , 2007a), it is acepted that continuous extension in the margin of Gondwana and the final rifting and the drift of Avalonia and related minor terranes, including fragments of the peri-Gondwanan arcs, finally caused the opening of this oceanic domain. In the NW of the Iberian Massif, the upper units of the allochthonous complexes contain igneous rocks with calc-alkaline and island-arc tholeiite affinities (Andonaegui et al., 2002; Castineiras, 2005), with a chronology similar to that of the calc-alkaline rocks from the Somozas Melange (c. 520-500 Ma). These units have been repeatedly interpreted as a fragment of a peri-Gondwanan arc rifted and finally drifted away from the main continent during the opening of the Rheic Ocean (Abati et al., 1999, 2007; Gomez Barreiro et al. , 2007; Murphy and Gutierrez Alonso, 2008). The new whole rock geochemistry and U-Pb geochronology data included in this contribution suggest an equivalence between both calc-alkaline series, which are interpreted to have been generated in the same Cambrian peri-Gondwanan volcanic-arc system.

6.2. Origin of the high-T tectonic blocks

A common characteristic to many ophiolitic melanges is the presence of tectonic blocks with contrasting metamorphic conditions (Federico et al., 2007; Kawai et al., 2008). In this context, the presence in the Somozas Melange of high-T tectonic blocks with orthogneisses and amphibolites may be explained by the incorporation in the mixing zone of rocks subducted to different depths that finally reached the low-viscosity serpentinite channel which forced their return. However, our data suggest that this straightforward interpretation may not apply in this case. The orthogneisses and amphibolites included in the high-T tectonic blocks show a tectonothermal evolution similar to some of the lithologies forming part of the Espasante Unit (Figs. 3 and 4), representing the basal units in the Cabo Ortegal Complex. Moreover, the UPb age obtained for the Gradoy orthogneiss (c. 485 Ma; Figs. 4 and 7) suggests affinity to the basal units of the allochthonous complexes, where the granitic magmatism is consistently younger (492-472 Ma) than in the upper units (520-500 Ma).

The basal units of the Cabo Ortegal Complex include the allochthonous terrane located on top of the Somozas Melange, and they have been repeatedly interpreted as the most external margin of Gondwana subducted at the onset of the Variscan deformation. Even though the Somozas Melange underlies the contact between the Moeche and Espasante units with out-of-sequence relationships (see the geological cross sections in Figs. 2 and 3), the basal units are those located in the lowest structural position in the terrane pile above the melange zone and they are apparently involved in the generation of the melange. The basal units were affected by high-P metamorphism at 370 Ma (Rodriguez et al., 2003; Abati et al., 2009), followed by a pronounced exhumation. The high-T tectonic blocks derived from the basal units are mixed in the melange with lithologies affected by lower grade metamorphism, which suggests that they were incorporated in the melange after the high-P event of c. 370 Ma, when the subducted margin of Gondwana experienced important rates of exhumation. The age of 370 Ma should be considered a maximum age limit for the generation of the Somozas Melange.

[FIGURE 14 OMITTED]

6.3. Origin of the melange unit and the assembly of Pangea

The identification of the high-T tectonic blocks as elements derived from the basal units of the allochthonous complexes, incorporated to the melange after the c. 370 Ma subduction event and after important exhumation of the subducted margin, is important because it suggests that the Somozas Melange represents a huge mixing unit directly located below the southern margin of Laurussia. The same conclusion can be inferred from the structural position of the melange, which is located below the basal units of the allochthonous complexes and therefore in a more external position in the belt. Based on this evidence, it is not possible to relate the Somozas Melange with the main subduction zone which affected the most external margin of Gondwana at the onset of the Variscan deformation. However, the relationship of the large ophiolitic melanges with first order subduction zones have been clearly documented. It is therefore necessary to consider the existence of a secondary subduction zone developed behind the subducted margin of Gondwana, closer to the continent. This subduction zone was apparently active only after the development of a pronounced decompression of the previously subducted continental margin (Fig. 16).

[FIGURE 15 OMITTED]

The geochemistry of igneous rocks and the U-Pb geochronology included in this contribution suggest that the ophiolitic melange contains remnants of a Cambrian volcanic arc of peri-Gondwanan provenance. A similar volcanic arc does not exist in the most proximal sectors of the Gondwanan margin, presently located in the foreland zones of the belt. However, as has been discussed before, this volcanic arc shows identical characteristics to the arc-derived terrane located in the upper units of the allochthonous complexes, above the ophiolites generated during the closure of the Rheic Ocean, which define the main suture of the Variscan Belt (Fig. 16). The model purported to explain the origin of the Somozas Melange should account fot the following facts: 1) the incorporation to the tectonic melange of lithologies derived from the most external margin of Gondwana, previously sub-ducted and affected by pronounced decompression; 2) the incorporation in the melange of remnants of a Cambrian peri-Gondwanan arc, which in NW Iberia only has equivalence in the arc-derived terrane located in the upper units of the allochtonous complexes; 3) the generation of the melange in a secondary subduction zone with activity after 370 Ma.

[FIGURE 16 OMITTED]

[FIGURE 17 OMITTED]

Figure 17 contains a comprehensive model explaining the most probable tectonic setting for the generation of the Somozas Melange. It shows the distribution of allochthonous terranes in the southern margin of Laurussia, also to the south of Avalonia, which is firstly characterized by the accretion of a peri-Gondwanan terrane with volcanic-arc affinities and Cambrian age. U-Pb geochronological data, obtained in the upper units of the allochthonous complexes, for the high-P and high-T metamorphic event simultaneous to the accretion of this arc, suggest an age in the range 410-390 Ma (Ordonez Casado et al., 2001; Fernandez-Suarez et al., 2007). The accretion of this arc coincided with the beginning of the contraction in the Rheic Ocean. The final stages of the closure of this ocean were probably preceded by oblique convergence between Gondwana and Laurussia. Previously, an intra-Rheic subduction zone would have removed most of the old and cold lithosphere of the Rheic Ocean, generating in the Middle Devonian (c. 395 Ma) the supra-sub-duction zone ophiolites typical of the European Variscan Belt (Diaz Garcia et al., 1999; Sanchez Martinez et al., 2007). The subduction of the most external margin of Gondwana should have started before 370 Ma, because this is the obtained age in the basal units of Galicia for the high-P metamorphism associated to this event (Rodriguez et al., 2003; Abati et al., 2009). This oblique subduction marks the beginning of the deformation in the most external margin of Gondwana, representing the first real Variscan deformation and metamorphism identified in the basement of western Europe. The progression of the oblique convergence and subduction was coeval with the exhumation of previously subducted continental sections, according to the process described by Platt (1986), and with the probable generation of a new secondary frontal subduction zone (Fig. 17). This new subduction zone represents the dynamic setting for the generation of the Somozas Melange, and hence the place for the mixing of tectonic blocks derived from the basal units (high-T tectonic blocks) and the remnants of a Cambrian peri-Gondwanan volcanic-arc similar to that exposed in the upper units of the allochthonous complexes. The continuation of the convergence derived in a transition towards an intracontinental setting and the blocking of the activity in the secondary subduction zone. The deformation advanced towards the most external zones of the belt, favoring the accretion of the Parautochthon to the orogenic wedge, probably representing a restricted basin located between the volcanic-arc system and the continent, and finally the accretion of the autochthonous domain.

The suggested model is compatible with the terrane distribution in the NW of the Iberian Massif and also with the overall structure of the orogenic wedge in this sector of the belt. It also allows to explain one of the most enigmatic aspects in the Somozas Melange, such as the incorporation to the mixing unit of the remnants of a terrane with volcanic-arc affinity. In the NW Iberian Massif, this terrane does not exist below the suture zone defined by the ophiolitic units. The model also explains the existence in the melange of high-T tectonic blocks. Finally, it also presents a dynamic context for the generation of the tectonic melange related to the activity of an important subduction zone, a characteristic in most ophiolitic melanges. The Somozas Melange is connected to one of the most important contacts developed in the basement of western Europe during the assembly of Pangea. Its continuation could be expected across the French Massif Central and the Bohemian Massif, where the allochthonous complexes described in NW Iberia can be recognized (Martinez Catalan et al., 2007). The identification of equivalent units in these regions will enable further correlation of the allochthonous terranes involved in the Pangea suture. However, a similar melange has not yet been described in the rest of the Variscan Belt.

Acknowledgements

Financial support for this research has been provided by Spanish project CGL2007-65338-CO2-01/BTE (Ministerio de Ciencia e Innovacion). The authors thank Jose Ramon Martinez Catalan and Javier Fernandez Suarez for field cooperation and mineral separation, respectively. Juan Gomez Barreiro and Wayne Premo are kindly acknowledged for their assistance during the SHRIMP analytical sessions as well as the staff from the Denver Microbeam Laboratoire (USGS) and the SUMAC facility. SSM especially acknowledges the analytical facilities provided by the Natural History Museum of London through financial support of the European Union Synthesys Project. This study is also a contribution to the IGCP 497, "The Rheic Ocean: Origin, evolution and correlatives". Brendan Murphy and Jean Paul Liegeois are kindly acknowledged for insightful reviews of the manuscript.

Received: 26/01/09 / Accepted: 27/05/09

References

Abati, J., Dunning, G.R., Arenas, R., Diaz Garcia, F., Gonzalez Cuadra, P., Martinez Catalan, J.R., Andonaegui, P. (1999): Early Ordovician orogenic event in Galicia (NW Spain): evidences from U-Pb ages in the uppermost unit of the Ordenes Complex. Earth and Planetary Science Letters, 165: 213-228.

Abati, J., Castineiras, P., Arenas, R., Fernandez-Suarez, J., Gomez-Barreiro, J., Wooden, J. (2007): Using SHRIMP zircon dating to unravel tectonothermal events in arc environments. The early Palaeozoic arc of NW Iberia revisited. Terra Nova, 19: 432-439.

Abati, J., Gerdes, A., Fernandez-Suarez, J., Arenas, R., Whitehouse, M.J., Diez Fernandez, R. (2009): Magmatism and early-Variscan continental subduction in the northern Gondwana margin recorded in zircons from the basal units of Galicia, NW Spain. Geological Society of America Bulletin. In press.

Andonaegui, P., Gonzalez del Tanago, J., Arenas, R., Abati, J., Martinez Catalan, J.R., Peinado, M., Diaz Garcia, F. (2002): Tectonic setting of the Monte Castelo gabbro (Ordenes Complex, northwestern Iberian Massif): Evidence for an arc-related terrane in the hanging wall to the Variscan suture. In: J.R. Martinez Catalan, R.D. Hatcher Jr., R. Arenas, F. Diaz Garcia (eds.), Variscan-Appalachian Dynamics: the building of the Late Paleozoic Basement. Geological Society of America Special Paper, 364: 37-56. Arenas, R. (1985): Evolucion petrologica y geoquimica de la unidad aloctona inferior del complejo metamorfico basico-ultrabasico de Cabo Ortegal (Unidad de Moeche) y del Silurico paraautoctono, Cadena Hercinica Iberica (NW de Espana). Tesis Doctoral. Universidad Complutense de Madrid: 543 p.

Arenas, R., Peinado, M. (1981): Presencia de pillow-lavas en las metavolcanitas submarinas de las proximidades de Espasante, Cabo Ortegal, NW de Espana. Cuadernos de Geologia Iberica, 7: 105-119.

Arenas, R., Gil Ibarguchi, J.I., Gonzalez Lodeiro, F., Klein, E., Martinez Catalan, J.R., Ortega Girones, E., Pablo Macia, J.G. de, Peinado, M. (1986): Tectonoestratigraphic units in the complexes with mafic and related rocks of the NW of the Iberian Massif. Hercynica, II: 87-110.

Arenas, R., Rubio Pascual, F.J., Diaz Garcia, F., Martinez Catalan, J.R. (1995): High-pressure microinclusions and development of an inverted metamorphic gradient in the Santiago Schists (Ordenes Complex, NW Iberian Massif, Spain): evidence of subduction and syn-collisional decompression. Journal of Metamorphic Geology, 13: 141-164.

Arenas, R., Abati, J., Martinez Catalan, J.R., Diaz Garcia, F., Rubio Pascual, F.J. (1997): P-T evolution of eclogites from the Agualada Unit (Ordenes Complex, NW Iberian Massif, Spain): Implications for crustal subduction. Lithos, 40: 221-242.

Arenas, R., Martinez Catalan, J.R., Sanchez Martinez, S., Fernandez-Suarez, J., Andonaegui, P., Pearce, J.A., Corfu, F. (2007a): The Vila de Cruces Ophiolite: A remnant of the Early Rheic Ocean in the Variscan suture of Galicia (Northwest Iberian Massif). Journal of geology, 115: 129-148.

Arenas, R., Sanchez Martinez, S., Castineiras, P., Fernandez Suarez, J., Jeffries, T. (2007b): Geochemistry and geochronology of the ophiolite involved in the Somozas melange: new insights on the birth of the Rheic Ocean. In: R. Arenas, J.R. Martinez Catalan, J. Abati, S. Sanchez Martinez (eds.), The rootless Variscan suture of NW Iberia (Galicia, Spain). The International Geoscience Programme, IGCP 497. Galicia Meeting 2007. Field trip guide & Conference abstracts. Publicaciones del Instituto Geologico y Minero de Espana: 151-153.

Arenas, R., Sanchez Martinez, S., Castineiras, P., FernandezSuarez, J., Diez Fernandez, R., Jeffries, T.E. (2008): The basal tectonic melange of the Cabo Ortegal Complex (NW Spain): Rock assemblages, involved terranes and paleogeographic scenario for the suture of Pangea. In: P. Konigshof, U. Linneman (eds.), From Gondwana and Laurussia to Pangaea: Dynamics of oceans and supercontinents. The International Geoscience Programme, IGCP 497 and IGCP 499. 20th International Senckenberg-Conference & 2nd Geinitz-Conference. Abstracts and Programme: 19-21.

Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mundil, R., Campbell, I.H., Korsch, R.J., Williams, I.S., Foudoulis, C. (2004): Improved [sup.206]Pb/[sup.238]U microprobe geochronology by the monitoring of a trace-element-related matrix effect, SHRIMP, ID-TIMS, ELA-ICP MS and oxygen isotope documentation for a series of zircon standards. Chemical Geology, 205: 115-140.

Castineiras, P. (2005): Origen y evolucion tectonotermal de las unidades de O Pino y Carino (Complejos Aloctonos de Galicia). Nova Terra, 28: 279 p.

Corfu, F., Hanchar, J.M., Hoskin, P.W.O., Kinny, P. (2003): Atlas of zircon textures. In: J.M. Hanchar, P.W.O. Hoskin (eds.). Zircon. Mineralogical Society of America, Washington. Reviews in Mineralogy and Geochemistry, 53: 468-500.

Dallmeyer, R.D., Martinez Catalan, J.R., Arenas, R., Gil Ibarguchi, J.I., Gutierrez Alonso, G., Farias, P., Aller, J., Bastida, F. (1997): Diachronous Variscan tectonothermal activity in the NW Iberian Massif: evidence from 40Ar/39Ar dating of regional fabrics. Tectonophysics, 277: 307-337.

Diaz Garcia, F., Arenas, R., Martinez Catalan, J.R., Gonzalez del Tanago, J., Dunning, G.R. (1999): Tectonic evolution of the Careon Ophiolite (northwest Spain): a remnant of oceanic lithosphere in the Variscan Belt. Journal of Geology, 107: 587-605.

Federico, L., Crispini, L., Scambelluri, M., Capponi, G. (2007): Ophiolite melange zone records exhumation in a fossil subduction channel. Geology, 35: 499-502.

Fernandez-Suarez, J., Corfu, F., Arenas, R., Marcos, A., Martinez Catalan, J.R., Diaz Garcia, F., Abati, J., Fernandez, F.J. (2002): U-Pb evidence for a polymetamorphic evolution of the HP-HT units of the NW Iberia Massif. Contributions to Mineralogy and Petrology, 143: 236-253.

Fernandez-Suarez, J., Diaz Garcia, F., Jeffries, T.E., Arenas, R., Abati, J. (2003): Constraints on the provenance of the uppermost allochthonous terrane of the NW Iberian Massif: Inferences from detrital zircon U-Pb ages. Terra Nova, 15: 138-144.

Fernandez Suarez, J., Arenas, R., Abati, J., Martinez Catalan, J.R., Whitehouse, M.J., Jeffries, T.E. (2007): U-Pb chronometry of polymetamorphic high-pressure granulites: An example from the allochthonous terranes of the NW Iberian Variscan belt. In: R.D. Hatcher Jr., M.P. Carlson, J.H. McBride, J.R. Martinez Catalan (eds.), 4-D Framework of Continental Crust. Geological Society of America Memoir, 200: 469-488.

Gerya, T.V., Stockhert, B., Perchuk, A.L. (2002): Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics, 21 (6), Art No. 1056.

Gomez Barreiro, J., Martinez Catalan, J.R., Arenas, R., Castineiras, P., Abati, J., Diaz Garcia, F., Wijbrans, J.R. (2007): Tectonic evolution of the upper allochthon of the Ordenes complex (northwestern Iberian Massif): Structural constraints to a polyorogenic peri-Gondwanan terrane. In: U. Linneman, R.D. Nance, P. Kraft, G. Zulauf (eds.), The evolution of the Rheic Ocean: From Avalonian-Cadomian active margin to Alleghenian-Variscan collision. Geological Society of America Special Paper, 423: 315-332.

Guilmette, C., Hebert, R., Dupuis, C., Wang, C, Li, Z. (2008): Metamorphic history and geodynamic significance of highgrade metabasites from the ophiolitic melange beneath the Yarlung Zangbo ophiolites, Xigaze area, Tibet. Journal of Asian Earth Sciences, 32: 423-437.

Hawkins, J.W. (2003): Geology of supra-subduction zones--Implications for the origin of ophiolites. In: Y. Dilek, S. Newcomb (eds.), Ophiolite concept and the evolution of geological thought. Geological Society of America Special Paper, 373: 227-268.

Hefferan, K.P., Admou, H., Hilal, R., Karson, J.A., Saquaque, A., Juteau, T., Bohn, M.M., Samson, S.D., Kornprobst, J.M. (2002): Proterozoic blueschist-bearing melange in the AntiAtlas Mountains, Morocco. Precambrian Research, 118: 179-194.

Hirauchi, K., Tamura, A., Arai, S., Yamaguchi, H., Hisada, K. (2008): Fertile abyssal peridotites within the Franciscan subduction complex, central California: Possible origin as detached remnants of oceanic fracture zones located close to a slow-spreading ridge. Lithos, 105: 319-328.

Ireland, T.R., Williams, I.S. (2003): Considerations in zircon geochronology by SIMS. In: J.M Hanchar, P.W.O. Hoskin (eds). Zircon. Mineralogical Society of America, Washington. Reviews in Mineralogy and Geochemistry, 53: 215-241.

Jeffries, T., Fernandez-Suarez, J., Corfu, F., Gutierrez-Alonso, G. (2003): Advances in U-Pb geochronology using a frequency quintupled Nd:YAG based laser ablation system (lambda = 213nm) and quadrupole based ICPMS. Journal of Analytical Atomic Spectrometry, 18: 847-855.

Kato, K., Saka, Y. (2003): Kurosegawa terrane as a transform fault zone in southwest Japan. Gondwana Research, 6: 669686.

Kawai, T., Windley, B.F., Shibuya, T., Omori, S., Sawaki, Y., Maruyama, S. (2008): Large P-T gap between Ballantrae blueschist/garnet pyroxenite and surrounding ophiolite, southern Scotland, UK: Diapiric exhumation of a Caledonian serpentinite melange. Lithos, 104: 337-354.

Le Maitre, R.W., Bateman, P., Dudek, A., Keller, J., Lameyre Le Bas, M.J., Sabine, P.A., Schmid, R., Sorensen, H., Streckeisen, A., Wooley, A.R., Zanettin, B. (1989): A classification of igneous rocks and glossary of terms. Blackwell Scientific Publications, Oxford: 193 p.

Linnemann, U., McNaughton, N.J., Romer, R.L., Gehmlich, M., Drost, K., Tonk., C. (2004): West African provenance for Saxo-Thuringia (Bohemian Massif): Did Armorica ever leave pre-Pangean Gondwana?--U/Pb-SHRIMP zircon evidence and the Nd-isotopic record. International Journal of Earth Sciences, 93: 683-705.

Ludwig, K.R. (2002): SQUID 1.02, a user's manual. Berkeley Geochronology Center Special Publication, 2: 17 p.

Ludwig, K.R. (2003): ISOPLOT/Ex, version 3, A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication, 4: 71 p.

MacPherson, G.J., Giaramita, M.J., Phipps, S.P. (2006): Tectonic implications of diverse igneous blocks in Franciscan melange, Northern California and southwestern Oregon. American Mineralogist, 91: 1509-1520.

Maheo, G., Fayoux, X., Guillot, S., Garzanti, E., Capiez, P., Mascle, G. (2006): Relicts of an inra-oceanic arc in the SapiShergol melange zone (Ladakh, NW Himalaya, India): implications for the closure of the Neo-Tethys Ocean. Journal of Asian Earth Sciences, 26: 695-707.

Marcos, A., Farias, P., Galan, G., Fernandez, F.J., Llana-Funez, S. (2002): Tectonic framework of the Cabo Ortegal Complex: A slab of lower crust exhumed in the Variscan orogen (northwestern Iberian Peninsula). In: J.R. Martinez Catalan, R.D. Hatcher Jr., R. Arenas, F. Diaz Garcia (eds.), VariscanAppalachian Dynamics: the building of the Late Paleozoic Basement. Geological Society of America Special Paper, 364: 143-162.

Martinez Catalan, J.R., Arenas, R., Diaz Garcia, F., Rubio Pascual, F.J., Abati, J., Marquinez, J. (1996): Variscan exhumation of a subducted Paleozoic continental margin: The basal units of the Ordenes Complex, Galicia, NW Spain. Tectonics, 15: 106-121.

Martinez Catalan, J.R., Diaz Garcia, F., Arenas, R., Abati, J., Castineiras, P., Gonzalez Cuadra, P., Gomez Barreiro, J., Rubio Pascual, F. (2002): Thrust and detachment systems in the Ordenes Complex (northwestern Spain): Implications for the Variscan-Appalachian geodynamics. In: J.R. Martinez Catalan, R.D. Hatcher Jr., R. Arenas, F. Diaz Garcia (eds.), Variscan-Appalachian Dynamics: the building of the Late Paleozoic Basement. Geological Society of America Special Paper, 364: 163-182.

Martinez Catalan, J.R., Arenas, R., Diaz Garcia, F., Gonzalez Cuadra, P., Gomez-Barreiro, J., Abati, J., Castineiras, P., Fernandez-Suarez, J., Sanchez Martinez, S., Andonaegui, P., Gonzalez Clavijo, E., Diez Montes, A., Rubio Pascual F.J., Valle Aguado, B. (2007): Space and time in the tectonic evolution of the northwestern Iberian Massif: Implications for the Variscan belt. In: R.D. Hatcher Jr., M.P. Carlson, J.H. McBride, J.R. Martinez Catalan (eds.), 4-D Framework of Continental Crust. Geological Society of America Memoir, 200: 403-423.

Martinez Catalan, J.R., Fernandez-Suarez, J., Meireles, C., Clavijo, E.G., Belousova, E., Saeed, A. (2008): U-Pb detrital zircon ages in synorogenic deposits of the NW Iberian Massif (Variscan belt). Interplay of Devonian-Carboniferous sedimentation and thrust tectonics. Journal of The Geological Society, 165: 687-698.

Matte, Ph. (1991): Accretionary history and crustal evolution of the Variscan belt in Western Europe. Tectonophysics, 196: 309-337.

Mullen, E.D. (1983): MnO/TiO2/P2O5: a minor element discriminant for basaltic rocks of oceanic environments and its aplications for petrogenesis. Earth and Planetary Science Letters, 62: 53-62.

Murphy, J.B., Gutierrez-Alonso, G., Nance, R.D., FernandezSuarez, J., Keppie, J.D., Quesada, C., Strachan, R.A., Dostal, J. (2006): Origin of the Rheic Ocean: rifting along a Neoproterozoic suture? Geology, 34: 325-328.

Murphy, J.B., Gutierrez-Alonso, G. (2008): The origin of the Variscan upper allochthons in the Ortegal Complex, northwestern Iberia: Sm-Nd isotopic constraints on the closure of the Rheic Ocean. Canadian Journal of Earth Science, 45: 651-668.

Nakamura, N. (1974): Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary condrites. Geochemica et Cosmochimica Acta, 38: 757-775.

Nance, R.D., Murphy, J.B. (1994): Contrasting basement signatures and the palinspastic restoration of peripheral orogens: example from Neoproterozoic Avalonian-Cadomian belt. Geology, 22: 617-620.

Norman, M.D., Pearson, N.J., Sharma, A.A., Griffin, W.L. (1996): Quantitative analysis of trace elements in geological materials by laser ablation ICPMS: instrumental operating conditions and calibration values of NIST glasses. Geostandards Newsletter, 20: 247-261.

Oczlon, M.S. (2006): Terrane map of Europe. Gaea Heidelbergensis, 15.

Ordonez Casado, B., Gebauer, D., Schafer, H.J., Gil Ibarguchi, J.I., Peucat, J.J. (2001): A single Devonian subduction event for the HP/HT metamorphism of the Cabo Ortegal complex within the Iberian Massif. Tectonophysics, 332: 359-385.

Osmaston, M.F. (2008): Basal subduction tectonic erosion (STE), butter melanges, and the construction and exhumation of HP-UHP belts: The Alps example and some comparisons. International Geology Review, 50: 685-754.

Pearce, J.A. (1983): Role of the sub-continental lithosphere in magma genesis at active continental margins. In: C.J. Hawkesworth, M.J. Norry (eds.), Continental basalts and mantle xenoliths. Shiva, Nantwich: 230-249.

Pearce, J.A. (1996): A users guide to basalt discrimination diagrams. In: D.A. Wyman (ed.), Trace Element Geochemistry of Volcanic Rocks: Applications for Massive Sulphide Exploration. Short Course Notes. Geological Association of Canada, 12: 79-113.

Pearce, J.A., Cann, J.R. (1973): Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth and Planetary Science Letters, 19: 290-300.

Pearce, J.A., Harris, N.B.W., Tindle, A.G. (1984): Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25: 956-983.

Pearce, J.A., Parkinson, I.J. (1993): Trace element models for mantle melting; application to volcanic arc petrogenesis. In: H.M. Prichard, T. Alabaster, N.B.W. Harris, C.R. Neary (eds.), Magmatic processes and plate tectonics. Geological Society Special Publications, 76: 373-403.

Pidgeon, R.T., Furfaro, D., Kennedy, A.K., Nemchin, A.A., van Bronswjk, W. (1995): Calibration of zircon standards for the Curtin SHRIMPII. U.S. Geological Survey Circular, 1107: 251.

Platt, J.P. (1986): Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks. Geological Society of America Bulletin, 97: 1037-1053.

Rodriguez, J., Cosca, M.A., Gil Ibarguchi, J.I., Dallmeyer, R.D. (2003): Strain partitioning and preservation of [sup.40]Ar/[sup.39]Ar ages during Variscan exhumation of a subducted crust (Malpica-Tui complex, NW Spain). Lithos, 70: 111-139.

Sanchez Martinez, S., Arenas, R., Diaz Garcia, S., Martinez Catalan, J.R., Gomez Barreiro, J., Pearce, J. (2007): The Ca reon Ophiolite, NW Spain: supra-subduction zone setting for the youngest Rheic Ocean floor. Geology, 35: 53-56.

Stacey, J.S., Kramers, J.D. (1975): Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planetary Science Letters, 26: 207-221.

Stockhert, B., Gerya, T.V. (2005): Pre-collisional high pressure metamorphism and nappe tectonics at active continental margins: a numerical simulation. Terra Nova, 17: 102-110.

Taylor, S.R., McLennan, S.M. (1985): The continental crust: its composition and evolution. Blackwell, Oxford: 328 p.

Valverde-Vaquero, P., Marcos, A., Farias, P., Gallastegui, G. (2005): U-Pb dating of Ordovician felsic volcanism in the Schistose Domain of the Galicia-Tras-os-Montes Zone near Cabo Ortegal (NW Spain). Geologica Acta, 3: 27-37.

Van der Meer Mohr, C.G. (1975): The Palaeozoic strata near Moeche in Galicia, NW Spain. Leidse Geologische Mededelingen, 49: 33-37.

Williams, I.S. (1997): U-Th-Pb geochronology by ion microprobe: not just ages but histories. Economic Geology, 7: 1-35.

Winchester, J.A., Floyd, P.A. (1977): Geochemical discrimination of different magma series and their differentiation products using inmobile elements. Chemical Geology, 20: 325-343.

Wood, D.A. (1980): The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province. Earth and Planetary Science Letters, 50: 11-30.

Zhang, Q., Wang, C.Y., Liu, D., Jian, P., Qian, Q., Zhou, G., Robinson, P.T. (2008): A brief review of ophiolites in China. Journal of Asian Earth Sciences, 32: 308-324.

R. Arenas * (1), S. Sanchez Martinez (1), P. Castineiras (1), T.E. Jeffries (2), R. Diez Fernandez (3), P. Andonaegui (1)

(1) Departamento de Petrologia y Geoquimica and Instituto de Geologia Economica (CSIC-UCM) Facultad de Ciencias Geologicas, Universidad Complutense de Madrid, 28040 Madrid, Spain.

* corresponding author: arenas@geo.ucm.es (2) Department of Mineralogy. The Natural History Museum. Cromwell Road. London SW7 5BD UK. (3) Departamento de Geologia. Universidad de Salamanca. 37008 Salamanca, Spain
TABLE 1.- SHRIMP U-Th-Pb ANALYSES OF ZIRCONS FROM THE GRADOY
ORTHOGNEISS GCH-05-11

              Common                           [sup.232]
GCH-05-11    [sup.206]                           Th/
Anal. #       Pb (%)     U (ppm)   Th (PPm)    [sup.238]

21.1          0.449       1408       492         0.36
17.1          0.068       1209       473         0.40
20.1          0.186        643       211         0.34
1.1           0.138       1154       381         0.34
18.1          0.067        314        73         0.24
8.1           0.096        841       274         0.34
19.1          0.276        597       165         0.29
12.1          <0.001       417        97         0.24
2.1           <0.001      1457       482         0.34
9.1           0.142        726       210         0.30
4.1           0.027       1566       530         0.35
3.1           0.041       1418       509         0.37
7.1           0.009       1607       667         0.43
13.1          <0.001      1012       277         0.28
10.1          <0.001       550       149         0.28
11.1          <0.001       979       314         0.33
16.1          <0.001       845       263         0.32
5.1 R         0.340        797       245         0.32
14.1          <0.001       598       201         0.35
5.2 C         1.135       1101       364         0.34
6.1           0.749       1418       413         0.30
15.2 C        <0.001      1747       738         0.44
15.1 R        <0.001      2999       847         0.29

             [sup.238]              [sup.207]
                U/                     Pb/
GCH-05-11    [sup.206]   [+ or -]   [sup.206]   [+ or -]
Anal. #       Pb (a)     1[sigma]     Pb (a)     1[sigma]

21.1         18.03137      1.41      0.05703       0.95
17.1         14.41643      1.42      0.05604       0.91
20.1         14.31522      1.40      0.05706       1.22
1.1          14.17749      1.38      0.05678       0.88
18.1         13.83705      1.51      0.05647       1.79
8.1          13.82558      1.40      0.05671       1.00
19.1         13.10161      1.40      0.05876       1.17
12.1         13.08823      1.58      0.05622       1.47
2.1          13.00829      1.36      0.05657       0.74
9.1          12.89165      1.39      0.05788       1.05
4.1          12.77682      1.36      0.05706       0.72
3.1          12.74427      1.36      0.05721       0.75
7.1          12.74628      1.36      0.05695       0.79
13.1         12.61773      1.37      0.05662       0.89
10.1         12.53871      1.40      0.05629       1.19
11.1         12.47006      1.37      0.05590       0.88
16.1         11.97462      1.38      0.05705       0.94
5.1 R        11.88454      1.38      0.06048       0.94
14.1         11.64601      1.40      0.05751       1.12
5.2 C        11.31620      1.39      0.06751       1.89
6.1          11.12346      1.40      0.06467       0.73
15.2 C       11.15173      1.36      0.05727       0.70
15.1 R       10.92881      1.35      0.05764       0.48

                [sup.238]               [sup.207]
                   U/                     Pb/
GCH-05-11       [sup.206]   [+ or -]    [sup.206]   [+ or -]
Anal. #          Pb (b)     1[sigma]     Pb (b)     1[sigma]

21.1            18.04867      1.41      0.05626       1.11
17.1            14.42345      1.42      0.05564       0.98
20.1            14.31522      1.40      0.05706       1.22
1.1             14.18240      1.38      0.05650       0.90
18.1            13.83705      1.51      0.05647       1.79
8.1             13.82558      1.40      0.05671       1.00
19.1            13.12878      1.40      0.05708       1.56
12.1            13.10068      1.58      0.05544       1.56
2.1             13.01165      1.36      0.05636       0.77
9.1             12.89607      1.39      0.05760       1.08
4.1             12.78078      1.36      0.05681       0.74
3.1             12.74743      1.36      0.05701       0.78
7.1             12.74628      1.36      0.05695       0.79
13.1            12.62259      1.37      0.05631       0.96
10.1            12.53871      1.40      0.05629       1.19
11.1            12.47006      1.37      0.05590       0.88
16.1            11.97462      1.38      0.05705       0.94
5.1 R           11.93829      1.38      0.05682       1.72
14.1            11.64601      1.40      0.05751       1.12
5.2 C           11.46819      1.41      0.05670       5.10
6.1             11.22166      1.40      0.05753       2.31
15.2 C          11.15415      1.36      0.05709       0.71
15.1 R          10.94201      1.35      0.05665       0.78

             [sup.206]                  Pb/
               Pb/                   [sup.238]
GCH-05-11    [sup.238]   [+ or -]   U (d) age
Anal. #       U (c)      1[sigma]      (Ma)

21.1          0.0552      0.0008    346    5
17.1          0.0693      0.0010    432    6
20.1          0.0697      0.0010    435    6
1.1           0.0704      0.0010    439    6
18.1          0.0722      0.0011    450    7
8.1           0.0723      0.0010    450    6
19.1          0.0761      0.0011    473    6
12.1          0.0764      0.0012    475    7
2.1           0.0769      0.0011    477    6
9.1           0.0775      0.0011    481    7
4.1           0.0782      0.0011    486    6
3.1           0.0784      0.0011    487    6
7.1           0.0784      0.0011    487    6
13.1          0.0793      0.0011    492    7
10.1          0.0798      0.0011    495    7
11.1          0.0803      0.0011    498    7
16.1          0.0836      0.0012    517    7
5.1 R         0.0839      0.0012    519    7
14.1          0.0859      0.0012    531    7
5.2 C         0.0874      0.0012    540    7
6.1           0.0892      0.0013    551    8
15.2 C        0.0898      0.0012    555    7
15.1 R        0.0916      0.0013    565    7

(a) Uncorrected ratios.

(b) Radiogenic lead [sup.204] Pb corrected for common lead.

(c) Radiogenic lead [sup.207] Pb corrected for
common lead

(d) [sup.207] Pb corrected for common lead.

Table 1.- U-Th-Pb SHRIMP analytical data for zircons from the
orthogneiss GCH-05-11. C, core; R, rim. All errors are 1[sigma].
Tabla 1.- Datos analiticos U-Th-Pb (SHRIMP) de los circones del
ortogneis GCH-05-11. C, centro; R, borde. Todos los errores son
1[sigma].

TABLE 2.- SHRIMP U-Th-Pb ANALYSES OF ZIRCONS FROM THE INSUA
GRANITOID GCH-05-8 Tabla 2.- Datos analiticos U-Th-Pb (SHRIMP) de
los circones del metagranitoide GCH-05-8. C, centro; R, borde.
Todos los errores son 1a. (La Tabla 2 continua en la pagina
siguiente)

           Common                          [sup.232]
             [sup                            Th/
GCH-05-5   .206]Pb                         [sup.238]
Anal. #      (%)     U (PPm)   Th (PPm)       U

37.1        1.802      256        77         0.31
20.1        2.561      317       240         0.78
31.1        0.400      241        74         0.32
7.1         0.340      119       167         1.46
7.2         0.198     1811        7          0.00
11.1        0.340      135       123         0.94
27.1        0.419     1252       327         0.27
1.1 C       0.074      792       111         0.14
37.1        1.802      256        77         0.31
1.2 R       2.484     3657       733         0.21
32.1        0.152      137        45         0.34
4.1         0.175     1200       275         0.24
36.1       <0.001      632        96         0.16
24.2 R      0.189     1583       329         0.21
10.1        0.084     2173       525         0.25
39.1        0.004      713        97         0.14
19.1        0.117     2042       552         0.28
40.1 R      0.218      976       237         0.25
13.1        0.133      511        90         0.18
25.1 C      0.094      723       114         0.16
38.1        0.117     1105       220         0.21
18.1       <0.001      932       189         0.21
14.1        0.357      408        71         0.18
15.1       <0.001     1822       477         0.27
16.1        0.122      827       142         0.18
30.1        0.457      834       149         0.18
21.1       <0.001     1275       358         0.29
9.1         0.291     1714       552         0.33
6.1         0.075     1609       231         0.15
24.1 C      0.020      658        83         0.13
8.1        <0.001     1687       360         0.22
22.1        1.138      377        66         0.18
26.1        0.105      135        54         0.41
23.1        0.111      671        85         0.13
40.2 C      0.616      820       126         0.16
17.1        0.002     2239       667         0.31
34.1       <0.001      564        36         0.07
12.1       <0.001     6105       2161        0.37
25.2 R     <0.001     2175       584         0.28
33.1        0.149      144        86         0.62
5.1         0.366      93         53         0.59
35.1       29.921      533       479         0.93
29.1        0.127      74         20         0.27
28.1        3.197      268       106         0.41

                  Isotopic ratios and 1[sigma] (absolute) errors

            [sup.238]                                        [sup.238]
               U/                   [sup.207]                   U/
GCH-05-5    [sup.206]   [+ or -]    Pb/[sup      [+ or -]    [sup.206]
Anal. #      Pb (a)     1[sigma]   .206]Pb (a)   1[sigma]     Pb (b)

37.1        21.06184      0.92       0.06666       1.90      21.28388
20.1        20.09512      0.81       0.07303       2.15      20.72375
31.1        20.27289      0.91       0.05576       2.65      20.36914
7.1         20.21830      1.26       0.05530       2.79      20.37535
7.2         19.79708      0.41       0.05432       0.90      19.84915
11.1        13.33790      1.06       0.05907       2.11      13.38147
27.1        12.95107      0.33       0.06004       0.63      12.99535
1.1 C       12.97634      0.42       0.05726       0.83      12.98183
37.1        21.06184      0.92       0.06666       1.90      21.28388
1.2 R       12.45275      0.47       0.07703       0.71      12.74303
32.1        12.61519      1.02       0.05822       2.00      12.67712
4.1         12.59997      0.34       0.05842       0.67      12.60974
36.1        12.61491      0.47       0.05644       0.92      12.64739
24.2 R      12.57869      0.33       0.05855       1.51      12.59759
10.1        12.58924      0.26       0.05770       0.51      12.58683
39.1        12.58477      0.47       0.05707       1.05      12.58612
19.1        12.56269      0.27       0.05799       0.54      12.56532
40.1 R      12.46358      0.40       0.05890       0.78      12.47296
13.1        12.47165      0.53       0.05821       1.06      12.49534
25.1 C      12.46407      0.45       0.05791       0.88      12.46063
38.1        12.45961      0.37       0.05809       0.93      12.47273
18.1        12.43954      0.40       0.05708       0.78      12.44179
14.1        12.39338      0.60       0.06009       1.17      12.41991
15.1        12.42455      0.29       0.05711       0.56      12.42398
16.1        12.39188      0.42       0.05820       0.83      12.39445
30.1        12.34937      0.48       0.06092       1.52      12.41166
21.1        12.40702      0.34       0.05650       0.68      12.41413
9.1         12.35538      0.31       0.05959       0.68      12.38251
6.1         12.37415      0.33       0.05784       0.63      12.39541
24.1 C      12.36436      0.47       0.05742       0.92      12.36061
8.1         12.36325      0.29       0.05709       0.57      12.36877
22.1        12.19195      0.76       0.06654       1.44      12.35292
26.1        12.29280      1.02       0.05817       2.01      12.25665
23.1        12.22728      0.47       0.05829       0.91      12.21986
40.2 C      12.15208      0.54       0.06240       0.88      12.22461
17.1        12.22667      0.27       0.05742       0.52      12.22528
34.1        12.14734      0.50       0.05722       0.98      12.14166
12.1        12.11794      0.18       0.05728       0.30      12.11794
25.2 R      11.70491      0.31       0.05787       0.61      11.70922
33.1        10.85459      0.95       0.06022       1.77      10.83532
5.1         10.81721      1.19       0.06201       2.20      10.85283
35.1        6.82647       0.53       0.30558      27.49      9.12257
29.1        7.85339       1.25       0.06597       2.09      7.86511
28.1        2.80398       0.65       0.14503       1.18      2.81058

               Isotopic ratios and 1[sigma] (absolute) errors

                       [sup.207]               [sup.206]
                         Pb/                     Pb/
GCH-05-5   [+ or -]    [sup.206]   [+ or -]    [sup.238]   [+ or -]
Anal. #    1[sigma]     Pb (b)     1[sigma]     U (c)      1[sigma]

37.1         1.06      0.05835       7.78       0.0466      0.0005
20.1         0.99      0.04854      12.02       0.0485      0.0004
31.1         0.92      0.05196       3.79       0.0491      0.0005
7.1          1.29      0.04907       6.04       0.0493      0.0006
7.2          0.41      0.05221       1.59       0.0504      0.0002
11.1         1.08      0.05643       3.95       0.0747      0.0008
27.1         0.34      0.05727       1.39       0.0769      0.0003
1.1 C        0.42      0.05692       0.98       0.0770      0.0003
37.1         1.06      0.05835       7.78       0.0466      0.0005
1.2 R        0.52      0.05857       5.03       0.0783      0.0005
32.1         1.04      0.05424       3.82       0.0791      0.0008
4.1          0.35      0.05779       0.95       0.0792      0.0003
36.1         0.48      0.05434       1.77       0.0793      0.0004
24.2 R       0.33      0.05734       1.68       0.0793      0.0003
10.1         0.26      0.05786       0.53       0.0794      0.0002
39.1         0.47      0.05698       1.13       0.0795      0.0004
19.1         0.27      0.05782       0.58       0.0795      0.0002
40.1 R       0.40      0.05829       0.87       0.0801      0.0003
13.1         0.53      0.05667       1.54       0.0801      0.0004
25.1 C       0.45      0.05813       0.93       0.0802      0.0004
38.1         0.37      0.05724       1.01       0.0802      0.0003
18.1         0.40      0.05693       0.86       0.0804      0.0003
14.1         0.60      0.05835       1.69       0.0804      0.0005
15.1         0.29      0.05715       0.56       0.0805      0.0002
16.1         0.42      0.05803       0.85       0.0806      0.0003
30.1         0.49      0.05684       2.32       0.0806      0.0004
21.1         0.34      0.05603       0.71       0.0807      0.0003
9.1          0.32      0.05781       1.17       0.0807      0.0003
6.1          0.33      0.05645       0.95       0.0808      0.0003
24.1 C       0.47      0.05766       0.98       0.0809      0.0004
8.1          0.29      0.05673       0.59       0.0809      0.0002
22.1         0.84      0.05593       5.87       0.0811      0.0006
26.1         1.05      0.06056       4.02       0.0813      0.0009
23.1         0.47      0.05878       0.91       0.0817      0.0004
40.2 C       0.55      0.05758       2.22       0.0818      0.0005
17.1         0.27      0.05751       0.53       0.0818      0.0002
34.1         0.50      0.05760       0.97       0.0823      0.0004
12.1         0.18      0.05728       0.30       0.0825      0.0002
25.2 R       0.31      0.05757       0.65       0.0854      0.0003
33.1         0.95      0.06166       1.75       0.0920      0.0009
5.1          1.23      0.05934       5.22       0.0921      0.0011
35.1        11.73         -           -         0.1027      0.0163
29.1         1.27      0.06474       3.42       0.1272      0.0017
28.1         0.65      0.14304       1.27       0.3452      0.0031

            [sup.206]
              Pb/
            [sup.238]
GCH-05-5     U (d)      age
Anal. #       (Ma)

37.1          294        3
20.1          305        3
31.1          309        3
7.1           310        4
7.2           317        1
11.1          465        5
27.1          478        2
1.1 C         478        2
37.1          294        3
1.2 R         486        3
32.1          491        5
4.1           492        2
36.1          492        2
24.2 R        492        2
10.1          492        1
39.1          493        2
19.1          493        1
40.1 R        496        2
13.1          497        3
25.1 C        497        2
38.1          497        2
18.1          499        2
14.1          499        3
15.1          499        1
16.1          500        2
30.1          500        2
21.1          500        2
9.1           500        2
6.1           501        2
24.1 C        501        2
8.1           502        1
22.1          503        4
26.1          504        5
23.1          506        2
40.2 C        507        3
17.1          507        1
34.1          510        3
12.1          511        1
25.2 R        529        2
33.1          567        5
5.1           568        7
35.1          630       95
29.1          772        9
28.1          2264      22

(a) Uncorrected ratios.
(b) Radiogenic lead 204Pb corrected for common lead.
(c) Radiogenic lead 207Pb corrected for common lead.
(d) 207Pb corrected for common lead
* Except analysis 28.1 (207Pb/206Pb age, 204Pb corrected for
common lead).

TABLE 3.- LA-ICP-MS U-Pb ANALYSES OF ZIRCONS FROM THE FERREIRAS
GRANITOID GCH-05-6.

Tabla 3. Datos analiticos U-Pb (LA-ICP-MS) de los circones del
metagranitoide GCH-05-6.

                                Isotopic
     Sample      ratios and 1[sigma] (absolute) errors

                 [sup.206]b/   [+ or -]   [sup.207]Pb/
Anal. #   Th/U   [238.sup]U    1[sigma]    [sup.235]U

jl13b05   0.20     0.0832       0.0003       0.6522
jl13a10   0.23     0.0790       0.0007       0.6240
jl13b16   0.24     0.0828       0.0006       0.6537
jl13b10   0.28     0.0841       0.0006       0.6638
jl13b13   0.27     0.0858       0.0004       0.6786
jl13a05   0.27     0.0822       0.0004       0.6513
jl13d09   0.30     0.0881       0.0004       0.6990
jl13d13   0.29     0.0881       0.0008       0.6992
jl13a14   0.18     0.0773       0.0003       0.6148
jl13c08   0.31     0.0854       0.0003       0.6802
jl13d07   0.28     0.0883       0.0004       0.7034
jl13a13   0.23     0.0803       0.0004       0.6398
jl13a11   0.22     0.0859       0.0006       0.6848
jl13b14   0.27     0.0857       0.0009       0.6838
jl13c13   0.25     0.0858       0.0005       0.6852
jl13b15   0.25     0.0878       0.0005       0.7017
jl13b06   0.31     0.0894       0.0005       0.7152
jl13c05   0.24     0.0875       0.0008       0.7005
jl13b11   0.29     0.0827       0.0004       0.6629
jl13a07   0.33     0.0843       0.0004       0.6754
jl13d11   0.27     0.0883       0.0006       0.7079
jl13b07   0.23     0.0842       0.0006       0.6767
jl13a12   0.24     0.0818       0.0005       0.6579
jl13c15   0.26     0.0841       0.0006       0.6760
jl13d14   0.25     0.0888       0.0007       0.7141
jl13c09   0.29     0.0881       0.0005       0.7091
jl13a06   0.24     0.0812       0.0003       0.6549
jl13d15   0.27     0.0866       0.0008       0.6998
jl13c10   0.22     0.0854       0.0007       0.6905
jl13d12   0.32     0.0899       0.0007       0.7281
jl13a16   0.26     0.0840       0.0004       0.6813
jl13d06   0.28     0.0868       0.0005       0.7048
jl13d16   0.26     0.0860       0.0007       0.7002
jl13d10   0.26     0.0897       0.0004       0.7380

                             Isotopic
     Sample    ratios and 1[sigma] (absolute) errors

                 [+ or -]   [207.sup]Pb   [+ or -]
Anal. #   Th/U   1[sigma]   [206.sup]Pb   1[sigma]

jl13b05   0.20    0.0044      0.0569       0.0004
jl13a10   0.23    0.0062      0.0573       0.0005
jl13b16   0.24    0.0056      0.0572       0.0004
jl13b10   0.28    0.0053      0.0573       0.0005
jl13b13   0.27    0.0043      0.0574       0.0003
jl13a05   0.27    0.0038      0.0575       0.0004
jl13d09   0.30    0.0036      0.0576       0.0003
jl13d13   0.29    0.0050      0.0575       0.0003
jl13a14   0.18    0.0049      0.0577       0.0005
jl13c08   0.31    0.0032      0.0577       0.0003
jl13d07   0.28    0.0030      0.0577       0.0003
jl13a13   0.23    0.0040      0.0578       0.0004
jl13a11   0.22    0.0041      0.0578       0.0003
jl13b14   0.27    0.0079      0.0579       0.0005
jl13c13   0.25    0.0039      0.0579       0.0002
jl13b15   0.25    0.0054      0.0580       0.0003
jl13b06   0.31    0.0036      0.0580       0.0001
jl13c05   0.24    0.0056      0.0581       0.0003
jl13b11   0.29    0.0029      0.0581       0.0003
jl13a07   0.33    0.0038      0.0581       0.0004
jl13d11   0.27    0.0050      0.0581       0.0003
jl13b07   0.23    0.0077      0.0583       0.0007
jl13a12   0.24    0.0051      0.0583       0.0004
jl13c15   0.26    0.0051      0.0583       0.0003
jl13d14   0.25    0.0055      0.0583       0.0002
jl13c09   0.29    0.0050      0.0584       0.0003
jl13a06   0.24    0.0054      0.0585       0.0004
jl13d15   0.27    0.0064      0.0586       0.0003
jl13c10   0.22    0.0057      0.0586       0.0004
jl13d12   0.32    0.0063      0.0587       0.0003
jl13a16   0.26    0.0033      0.0588       0.0003
jl13d06   0.28    0.0041      0.0589       0.0003
jl13d16   0.26    0.0073      0.0591       0.0004
jl13d10   0.26    0.0056      0.0597       0.0003

                                      Ages and 2
     Sample                      [sigma] errors (Ma)

                 [206.sup]Pb/   [+ or -]   [sup.207]Pb/   [+ or -]
Anal. #   Th/U   [238.sup.U]    2[sigma]    [235.sup]U    2[sigma]

jl13b05   0.20       515           4           510           5
jl13a10   0.23       490           8           492           8
jl13b16   0.24       513           7           511           7
jl13b10   0.28       520           7           517           6
jl13b13   0.27       530           4           526           5
jl13a05   0.27       509           5           509           5
jl13d09   0.30       544           4           538           4
jl13d13   0.29       544           10          538           6
jl13a14   0.18       480           4           487           6
jl13c08   0.31       529           4           527           4
jl13d07   0.28       546           5           541           4
jl13a13   0.23       498           5           502           5
jl13a11   0.22       531           7           530           5
jl13b14   0.27       530           10          529           10
jl13c13   0.25       530           5           530           5
jl13b15   0.25       542           6           540           6
jl13b06   0.31       552           6           548           4
jl13c05   0.24       541           9           539           7
jl13b11   0.29       512           5           516           3
jl13a07   0.33       521           5           524           5
jl13d11   0.27       546           7           543           6
jl13b07   0.23       521           7           525           9
jl13a12   0.24       507           6           513           6
jl13c15   0.26       521           7           524           6
jl13d14   0.25       548           9           547           6
jl13c09   0.29       544           5           544           6
jl13a06   0.24       503           4           512           7
jl13d15   0.27       536           9           539           8
jl13c10   0.22       528           9           533           7
jl13d12   0.32       555           8           555           7
jl13a16   0.26       520           4           528           4
jl13d06   0.28       537           6           542           5
jl13d16   0.26       532           9           539           9
jl13d10   0.26       554           5           561           7

                         Ages and 2
     Sample         [sigma] errors (Ma)

                 [sup.207]Pb/   [+ or -]
Anal. #   Th/U   [206.sup]Pb    2[sigma]   % disc

jl13b05   0.20       484           30      -6.40
jl13a10   0.23       500           40       2.00
jl13b16   0.24       500           30      -2.60
jl13b10   0.28       500           42      -4.00
jl13b13   0.27       504           26      -5.16
jl13a05   0.27       510           30       0.20
jl13d09   0.30       512           20      -6.25
jl13d13   0.29       512           22      -6.25
jl13a14   0.18       516           36       6.98
jl13c08   0.31       518           24      -2.12
jl13d07   0.28       518           20      -5.41
jl13a13   0.23       520           28       4.23
jl13a11   0.22       522           24      -1.72
jl13b14   0.27       524           36      -1.15
jl13c13   0.25       526           18      -0.76
jl13b15   0.25       528           22      -2.65
jl13b06   0.31       528           12      -4.55
jl13c05   0.24       530           26      -2.08
jl13b11   0.29       534           24       4.12
jl13a07   0.33       534           28       2.43
jl13d11   0.27       534           22      -2.25
jl13b07   0.23       538           54       3.16
jl13a12   0.24       540           28       6.11
jl13c15   0.26       540           22       3.52
jl13d14   0.25       540           20      -1.48
jl13c09   0.29       542           20      -0.37
jl13a06   0.24       548           32       8.21
jl13d15   0.27       550           20       2.55
jl13c10   0.22       552           30       4.35
jl13d12   0.32       556           18       0.18
jl13a16   0.26       560           20       7.14
jl13d06   0.28       562           24       4.45
jl13d16   0.26       568           32       6.34
jl13d10   0.26       592           24       6.42

disc%= percent discordance calculated from [207.sup]Pb/[206.sup.Pb]
and [206.sup.Pb]/[238.sup.]U ages (negative values: reversely
discordant analyses).

TABLE 4.- LA-ICP-MS U-Pb ANALYSES OF DETRITAL ZIRCONS FROM THE
FERREIRAS CONGLOMERATE SO-3.

Tabla 4.- Datos analiticos U-Pb (LA-ICP-MS) de los circones
detriticos del metaconglomerado SO-3.
(Esta tabla continua en la pagina siguiente)

                            Isotopic ratios
 Sample SO-3         and 1[sigma] (absolute) errors

                 [206.sup]Pb/   [+ or -]   [207.sup]Pb/
Anal. #   Th/U    [238.sup]U    1[sigma]    [235.sup]U

SO3-109   1.90      0.0746       0.0013       0.5802
SO3-111   1.78      0.0748       0.0010       0.5817
SO3-112   1.27      0.0775       0.0009       0.6103
SO3-110   1.88      0.0778       0.0009       0.6121
SO3-108   1.91      0.0784       0.0011       0.6178
SO3-14    0.18      0.0802       0.0009       0.6339
SO3-17    0.18      0.0809       0.0008       0.6387
SO3-33    0.15      0.0811       0.0008       0.6411
SO3-113   0.91      0.0816       0.0014       0.6458
SO3-43    0.20      0.0839       0.0008       0.6766
SO3-47    1.67      0.0864       0.0009       0.6926
SO3-53    0.63      0.0901       0.0009       0.7303
SO3-55    0.47      0.0914       0.0009       0.7422
SO3-39    0.94      0.0918       0.0009       0.7537
SO3-38    0.34      0.0922       0.0009       0.7503
SO3-35    0.74      0.0930       0.0010       0.7595
SO3-45    0.31      0.0931       0.0009       0.7651
SO3-37    0.59      0.0955       0.0009       0.7863
SO3-46    0.40      0.0958       0.0009       0.7871
SO3-106   0.55      0.0980       0.0010       0.8112
SO3-61    0.43      0.0993       0.0011       0.8242
SO3-22    0.45      0.1004       0.0009       0.8357
SO3-05    0.06      0.1020       0.0011       0.8560
SO3-54    0.17      0.1023       0.0011       0.8570
SO3-19    0.29      0.1161       0.0011       1.0083
SO3-48    0.55      0.2061       0.0019       2.2864
SO3-50    0.48      0.2308       0.0021       2.7418
SO3-16    0.74      0.2350       0.0022       2.8255
SO3-11    1.55      0.3306       0.0030       5.1321
SO3-04    1.70      0.3570       0.0032       6.0106
SO3-57    0.41      0.3361       0.0033       5.3084
SO3-56    0.73      0.3429       0.0032       5.5141
SO3-02    2.44      0.3436       0.0036       5.5324
SO3-42    1.13      0.3447       0.0032       5.5548
SO3-44    0.75      0.3452       0.0032       5.5701
SO3-58    0.44      0.3556       0.0039       5.9531
SO3-60    0.11      0.3670       0.0036       6.2812
SO3-06    0.63      0.3694       0.0035       6.3556
SO3-25    0.41      0.3772       0.0032       6.6281
SO3-31    0.48      0.3736       0.0034       6.5814
SO3-20    1.15      0.3778       0.0034       6.6559
SO3-28    1.25      0.3781       0.0034       6.6632
SO3-08    0.29      0.3797       0.0036       6.7175
SO3-01    0.52      0.3934       0.0039       7.3393
SO3-40    0.21      0.4172       0.0087       8.3279
SO3-107   0.36      0.4219       0.0040       8.6014
SO3-59    0.59      0.4496       0.0049       9.8997
SO3-51    0.78      0.4712       0.0046      10.5983
SO3-24    0.33      0.4833       0.0052      11.2217
SO3-52    0.70      0.4655       0.0051      10.8578
SO3-29    0.35      0.5055       0.0045      12.4415
SO3-18    0.58      0.5077       0.0045      12.6521
SO3-26    0.45      0.5170       0.0046      13.0905
SO3-09    0.56      0.5350       0.0051      14.1942
SO3-23    0.58      0.5502       0.0050      15.2761
SO3-21    0.15      0.5534       0.0058      15.4041
SO3-03    0.27      0.5547       0.0049      15.4649
SO3-07    0.52      0.5979       0.0053      18.6053
SO3-10    0.67      0.6541       0.0063      23.4129

                          Isotopic ratios
 Sample SO-3      and 1[sigma] (absolute) errors

                 [+ or -]   [207.sup]Pb/   [+ or -]
Anal. #   Th/U   1[sigma]   [206.sup.]Pb   1[sigma]

SO3-109   1.90    0.036        0.0564       0.0036
SO3-111   1.78    0.025        0.0564       0.0025
SO3-112   1.27    0.022        0.0571       0.0021
SO3-110   1.88    0.025        0.0571       0.0024
SO3-108   1.91    0.030        0.0571       0.0029
SO3-14    0.18    0.010        0.0573       0.0009
SO3-17    0.18    0.007        0.0573       0.0007
SO3-33    0.15    0.007        0.0574       0.0007
SO3-113   0.91    0.033        0.0574       0.0030
SO3-43    0.20    0.008        0.0585       0.0008
SO3-47    1.67    0.019        0.0582       0.0016
SO3-53    0.63    0.008        0.0588       0.0007
SO3-55    0.47    0.009        0.0589       0.0007
SO3-39    0.94    0.010        0.0596       0.0009
SO3-38    0.34    0.008        0.0591       0.0007
SO3-35    0.74    0.012        0.0593       0.0010
SO3-45    0.31    0.010        0.0596       0.0008
SO3-37    0.59    0.011        0.0597       0.0010
SO3-46    0.40    0.009        0.0596       0.0008
SO3-106   0.55    0.012        0.0600       0.0009
SO3-61    0.43    0.014        0.0602       0.0011
SO3-22    0.45    0.011        0.0604       0.0009
SO3-05    0.06    0.011        0.0609       0.0008
SO3-54    0.17    0.013        0.0607       0.0009
SO3-19    0.29    0.011        0.0630       0.0007
SO3-48    0.55    0.026        0.0805       0.0010
SO3-50    0.48    0.034        0.0862       0.0012
SO3-16    0.74    0.031        0.0872       0.0010
SO3-11    1.55    0.054        0.1126       0.0014
SO3-04    1.70    0.059        0.1221       0.0014
SO3-57    0.41    0.086        0.1146       0.0021
SO3-56    0.73    0.060        0.1166       0.0014
SO3-02    2.44    0.075        0.1168       0.0017
SO3-42    1.13    0.059        0.1169       0.0014
SO3-44    0.75    0.060        0.1170       0.0014
SO3-58    0.44    0.089        0.1214       0.0019
SO3-60    0.11    0.072        0.1241       0.0016
SO3-06    0.63    0.068        0.1248       0.0014
SO3-25    0.41    0.062        0.1275       0.0014
SO3-31    0.48    0.065        0.1278       0.0014
SO3-20    1.15    0.071        0.1278       0.0016
SO3-28    1.25    0.065        0.1278       0.0014
SO3-08    0.29    0.070        0.1283       0.0014
SO3-01    0.52    0.081        0.1353       0.0015
SO3-40    0.21    0.291        0.1448       0.0054
SO3-107   0.36    0.117        0.1479       0.0024
SO3-59    0.59    0.156        0.1597       0.0026
SO3-51    0.78    0.122        0.1632       0.0020
SO3-24    0.33    0.149        0.1684       0.0023
SO3-52    0.70    0.244        0.1692       0.0042
SO3-29    0.35    0.119        0.1785       0.0019
SO3-18    0.58    0.123        0.1808       0.0020
SO3-26    0.45    0.124        0.1837       0.0020
SO3-09    0.56    0.151        0.1924       0.0022
SO3-23    0.58    0.145        0.2014       0.0021
SO3-21    0.15    0.190        0.2019       0.0025
SO3-03    0.27    0.151        0.2022       0.0023
SO3-07    0.52    0.176        0.2257       0.0024
SO3-10    0.67    0.250        0.2596       0.0029

 Sample SO-3         Ages and 1[sigma] errors (Ma)

                 [206.sup]Pb/   [+ or -]   [207.sup]Pb/
Anal. #   Th/U    [238.sup]U    1[sigma]    [235.sup]U

SO3-109   1.90       464           7           465
SO3-111   1.78       465           6           466
SO3-112   1.27       481           6           484
SO3-110   1.88       483           6           485
SO3-108   1.91       487           7           488
SO3-14    0.18       497           5           498
SO3-17    0.18       501           5           502
SO3-33    0.15       503           5           503
SO3-113   0.91       506           8           506
SO3-43    0.20       519           4           525
SO3-47    1.67       534           6           534
SO3-53    0.63       556           5           557
SO3-55    0.47       564           5           564
SO3-39    0.94       566           5           570
SO3-38    0.34       568           5           568
SO3-35    0.74       573           6           574
SO3-45    0.31       574           6           577
SO3-37    0.59       588           5           589
SO3-46    0.40       590           6           590
SO3-106   0.55       603           6           603
SO3-61    0.43       610           6           610
SO3-22    0.45       617           5           617
SO3-05    0.06       626           6           628
SO3-54    0.17       628           6           628
SO3-19    0.29       708           6           708
SO3-48    0.55       1208          10          1208
SO3-50    0.48       1339          11          1340
SO3-16    0.74       1360          12          1362
SO3-11    1.55       1841          14          1841
SO3-04    1.70       1968          15          1977
SO3-57    0.41       1868          16          1870
SO3-56    0.73       1901          15          1903
SO3-02    2.44       1904          17          1906
SO3-42    1.13       1909          15          1909
SO3-44    0.75       1912          16          1911
SO3-58    0.44       1961          18          1969
SO3-60    0.11       2015          17          2016
SO3-06    0.63       2026          17          2026
SO3-25    0.41       2063          15          2063
SO3-31    0.48       2046          16          2057
SO3-20    1.15       2066          16          2067
SO3-28    1.25       2067          16          2068
SO3-08    0.29       2075          17          2075
SO3-01    0.52       2138          18          2154
SO3-40    0.21       2248          40          2267
SO3-107   0.36       2269          18          2297
SO3-59    0.59       2393          22          2425
SO3-51    0.78       2489          20          2489
SO3-24    0.33       2542          23          2542
SO3-52    0.70       2464          23          2511
SO3-29    0.35       2637          19          2638
SO3-18    0.58       2647          19          2654
SO3-26    0.45       2686          20          2686
SO3-09    0.56       2763          22          2763
SO3-23    0.58       2826          21          2833
SO3-21    0.15       2839          24          2841
SO3-03    0.27       2845          20          2844
SO3-07    0.52       3021          21          3022
SO3-10    0.67       3244          24          3244

  Sample SO-3       Ages and 1[sigma] errors (Ma)

                 [+ or -]   [207.sup]Pb/   [+ or -]
Anal. #   Th/U   1[sigma]   [206.sup]Pb    1[sigma]

SO3-109   1.90      23          469          110
SO3-111   1.78      16          468           73
SO3-112   1.27      14          497           57
SO3-110   1.88      15          495           67
SO3-108   1.91      19          496           83
SO3-14    0.18      6           505           16
SO3-17    0.18      4           503           10
SO3-33    0.15      4           505           11
SO3-113   0.91      20          507           81
SO3-43    0.20      5           549           13
SO3-47    1.67      11          536           40
SO3-53    0.63      5           558           11
SO3-55    0.47      5           564           12
SO3-39    0.94      6           588           14
SO3-38    0.34      5           569           11
SO3-35    0.74      7           576           18
SO3-45    0.31      6           588           13
SO3-37    0.59      6           593           16
SO3-46    0.40      5           590           12
SO3-106   0.55      7           604           16
SO3-61    0.43      8           612           19
SO3-22    0.45      6           618           15
SO3-05    0.06      6           635           12
SO3-54    0.17      7           630           15
SO3-19    0.29      6           708           10
SO3-48    0.55      8           1209          10
SO3-50    0.48      9           1342          12
SO3-16    0.74      8           1366          9
SO3-11    1.55      9           1842          9
SO3-04    1.70      9           1988          8
SO3-57    0.41      14          1873          16
SO3-56    0.73      9           1905          9
SO3-02    2.44      12          1908          11
SO3-42    1.13      9           1909          9
SO3-44    0.75      9           1911          9
SO3-58    0.44      13          1977          13
SO3-60    0.11      10          2017          9
SO3-06    0.63      9           2026          8
SO3-25    0.41      8           2063          7
SO3-31    0.48      9           2068          8
SO3-20    1.15      9           2068          8
SO3-28    1.25      9           2068          8
SO3-08    0.29      9           2075          8
SO3-01    0.52      10          2168          9
SO3-40    0.21      32          2285          33
SO3-107   0.36      12          2321          29
SO3-59    0.59      15          2453          13
SO3-51    0.78      11          2489          9
SO3-24    0.33      12          2542          10
SO3-52    0.70      21          2550          43
SO3-29    0.35      9           2639          7
SO3-18    0.58      9           2660          7
SO3-26    0.45      9           2686          7
SO3-09    0.56      10          2763          8
SO3-23    0.58      9           2837          7
SO3-21    0.15      12          2842          9
SO3-03    0.27      9           2844          7
SO3-07    0.52      9           3022          7
SO3-10    0.67      10          3245          8

  Sample SO-3     Reported age

                 Age    [+ or -]
Anal. #   Th/U   (Ma)   1[sigma]   % disc

SO3-109   1.90   464       7        1.1
SO3-111   1.78   465       6        0.7
SO3-112   1.27   481       6        3.2
SO3-110   1.88   483       6        2.5
SO3-108   1.91   487       7        2.0
SO3-14    0.18   497       5        1.5
SO3-17    0.18   501       5        0.4
SO3-33    0.15   503       5        0.6
SO3-113   0.91   506       8        0.2
SO3-43    0.20   519       4        5.7
SO3-47    1.67   534       6        0.3
SO3-53    0.63   556       5        0.4
SO3-55    0.47   564       5        0.0
SO3-39    0.94   566       5        4.0
SO3-38    0.34   568       5        0.1
SO3-35    0.74   573       6        0.6
SO3-45    0.31   574       6        2.5
SO3-37    0.59   588       5        0.9
SO3-46    0.40   590       6        0.1
SO3-106   0.55   603       6        0.2
SO3-61    0.43   610       6        0.3
SO3-22    0.45   617       5        0.2
SO3-05    0.06   626       6        1.5
SO3-54    0.17   628       6        0.3
SO3-19    0.29   708       6        0.0
SO3-48    0.55   1209      10       0.1
SO3-50    0.48   1342      12       0.2
SO3-16    0.74   1366      9        0.4
SO3-11    1.55   1842      9        0.0
SO3-04    1.70   1988      8        1.2
SO3-57    0.41   1873      16       0.3
SO3-56    0.73   1905      9        0.3
SO3-02    2.44   1908      11       0.3
SO3-42    1.13   1909      9        0.0
SO3-44    0.75   1911      9        0.0
SO3-58    0.44   1977      13       0.9
SO3-60    0.11   2017      9        0.1
SO3-06    0.63   2026      8        0.0
SO3-25    0.41   2063      7        0.0
SO3-31    0.48   2068      8        1.2
SO3-20    1.15   2068      8        0.1
SO3-28    1.25   2068      8        0.1
SO3-08    0.29   2075      8        0.0
SO3-01    0.52   2168      9        1.6
SO3-40    0.21   2285      33       2.0
SO3-107   0.36   2321      29       2.7
SO3-59    0.59   2453      13       2.9
SO3-51    0.78   2489      9        0.0
SO3-24    0.33   2542      10       0.0
SO3-52    0.70   2550      43       4.1
SO3-29    0.35   2639      7        0.1
SO3-18    0.58   2660      7        0.6
SO3-26    0.45   2686      7        0.0
SO3-09    0.56   2763      8        0.0
SO3-23    0.58   2837      7        0.5
SO3-21    0.15   2842      9        0.1
SO3-03    0.27   2844      7        0.0
SO3-07    0.52   3022      7        0.1
SO3-10    0.67   3245      8        0.0

disc%= percent discordance calculated from [207.sup]Pb/[206.sup]Pb
and [206.sup.Pb]/[238.sup.U] ages.

TABLE 5.- WHOLE ROCK MAJOR AND TRACE ELEMENT DATA
OF ESPASANTE SUBMARINE VOLCANIC ROCKS.

Tabla 5. Analisis quimicos de elementos mayores y traza
de las rocas volcanicas submarinas de Espasante.

Sample                CI-7    CI-8    CI-9

Si[O.sub.2]           50.53   49.75   51.33
[Al.sub.2][O.sub.3]   18.54   18.92   18.46
F[e.sub.2][O.sub.3]   9.08    9.57    8.84
MnO                   0.165   0.289   0.151
MgO                   4.27     4.7    4.12
CaO                   9.34    9.23    9.19
[Na.sub.2]O           2.76    2.04    2.33
[K.sub.2]O            0.34    0.11    0.26
Ti[O.sub.2]           1.006   0.992   1.003
[P.sub.2][O.sub.5]    0.15    0.17    0.17
LO[I.sup.1]           3.24    3.72    3.34
TOTAL                 99.42   99.49   99.19

Sc                     28      29      27
V                      189     187     186
Cr                     40      70      60
Co                     17      20      20
Ni                    < 20     20      20
Cu                     40      110     50
Zn                    < 30     120     80
Ga                     17      19      18
Rb                      8       3       5
Sr                     296     327     314
Y                      27     26.9    26.4
Zr                     93      100     101
Nb                     3.1     3.1     3.3
Cs                     0.4     0.3     0.4
Ba                     211     87      179
Hf                     2.8     2.8     2.7
Ta                    0.19    0.19     0.2
Pb                     < 5      7      < 5
Th                    3.02    3.06    3.06
U                     1.28    1.29    1.28

La                    12.8     13     12.3
Ce                    27.3    27.7    25.8
Pr                    3.73    3.75    3.52
Nd                    16.1    15.9    15.9
Sm                    4.09    3.82     3.8
Eu                    1.19    1.14    1.08
Gd                    4.11    4.16    3.89
Tb                    0.71    0.69     0.7
Dy                    4.33    4.24    4.08
Ho                    0.89    0.87    0.83
Er                    2.65     2.6    2.47
Tm                    0.383   0.38    0.362
Yb                    2.36    2.34    2.25
Lu                    0.34    0.339   0.322
[SIGMA] REE           80.98   80.93   77.30
Eu/Eu *               0.89    0.88    0.86
[(La/Sm).sub.N]       1.93    2.10    2.00
[(Gd/Yb).sub.N]       1.39    1.42    1.38
[(La/Yb).sub.N]       3.63    3.71    3.66

Sample                CI-10   CI-11   CI-12

Si[O.sub.2]           51.69   51.88   51.42
[Al.sub.2][O.sub.3]   18.28   18.3    17.84
F[e.sub.2][O.sub.3]   8.71    8.61    8.46
MnO                   0.158   0.155   0.203
MgO                   3.94    4.03    4.54
CaO                   9.84    8.92    9.22
[Na.sub.2]O           2.28    2.53    2.35
[K.sub.2]O            0.17    0.34     0.2
Ti[O.sub.2]           0.998   0.992   0.995
[P.sub.2][O.sub.5]    0.17    0.16    0.18
LO[I.sup.1]           3.12    3.17    3.41
TOTAL                 99.36   99.09   98.82

Sc                     27      28      28
V                      197     182     201
Cr                     70      70      50
Co                     19      19      19
Ni                     50      20     < 20
Cu                     30      40      50
Zn                     80      80      120
Ga                     17      17      17
Rb                      5      11       5
Sr                     333     314     307
Y                     24.1    24.8    24.2
Zr                     95      104     98
Nb                      3      3.2      3
Cs                     0.3     0.8     0.4
Ba                     181     197     182
Hf                     2.6     2.9     2.7
Ta                    0.18     0.2    0.18
Pb                      7       5       9
Th                    3.03    3.08    2.86
U                     1.31    1.37    1.28

La                    12.7    12.3    11.8
Ce                     27     26.6    25.9
Pr                    3.62    3.55    3.41
Nd                     16     15.4    15.4
Sm                    3.71     3.8    3.61
Eu                     1.1    1.13    1.08
Gd                    3.98    4.04    3.86
Tb                    0.68    0.66    0.64
Dy                    3.97    3.92    3.84
Ho                    0.78    0.78    0.75
Er                    2.36     2.4    2.26
Tm                    0.358   0.352   0.336
Yb                    2.21    2.22    2.13
Lu                    0.31    0.311   0.291
[SIGMA] REE           78.78   77.46   75.31
Eu/Eu *               0.88    0.89    0.89
[(La/Sm).sub.N]       2.11    2.00    2.02
[(Gd/Yb).sub.N]       1.44    1.45    1.44
[(La/Yb).sub.N]       3.84    3.70    3.70

(1) Loss on ignition.
Oxides are in weight percent (%). Trace and rare earth elements
are in parts per million (ppm).
The element concentrations expressed with the < sign are below
detection limit.

TABLE 6.- WHOLE ROCK MAJOR AND TRACE ELEMENT DATA
OF COMMON VOLCANIC ROCKS.

Tabla. 6. Analisis quimicos de elementos mayores y traza
de las rocas volcanicas comunes.

Sample                CE-100   CE-101   CE-102   CE-103

Si[O.sub.2]           50.17     49.5    48.92    48.06
[A1.sub.2][O.sub.3]   15.03    14.97    14.34    15.38
[Fe.sub.2][O.sub.3]   11.52     10.5    10.64    11.11
MnO                   0.179    0.143     0.15    0.154
MgO                    8.24     8.38     9.4      8.51
CaO                    5.84     7.8      8.22     9.36
[Na.sub.2]O            2.7      3.96     3.36     2.74
[K.sub.2]O             0.02     0.04     0.22     0.05
Ti[O.sub.2]           0.936     0.91    0.885    0.885
[P.sub.2][O.sub.5]     0.07     0.08     0.08     0.09
LO[I.sub.1]            4.77     2.81     2.94     3.21
TOTAL                 99.47    99.09    99.16    99.55

Sc                      40       40       38       42
V                      288      279      271      273
Cr                     260      280      330      320
Co                      35       37       39       34
Ni                      50       90      110       80
Cu                     110       50       50       70
Zn                     140       50       60       70
Ga                      16       15       16       14
Rb                     < 1      < 1       3       < 1
Sr                      62       90       73      107
Y                      21.1     20.1     21.2     23.3
Zr                      41       40       40       43
Nb                     0.6      0.5      0.5      0.5
Cs                     0.3      0.1      0.4      0.1
Ba                      8        16       29       11
Hf                     1.4      1.3      1.2      1.4
Ta                     0.02     0.02     0.02     0.01
Pb                     < 5      < 5      < 5      < 5
Th                     0.2      0.19     0.17     0.24
U                      0.12     0.12     0.12     0.19

La                     1.81     1.74     1.81      2
Ce                     5.21     5.03     5.1      5.36
Pr                     0.92     0.9      0.92     0.99
Nd                     5.25     5.06     5.37     5.57
Sm                     1.79     1.75     1.84     1.95
Eu                     0.73    0.743     0.83    0.768
Gd                     2.72     2.65     2.71     3.08
Tb                     0.53     0.51     0.51     0.59
Dy                     3.37     3.28     3.33     3.73
Ho                     0.72     0.69     0.71     0.8
Er                     2.19     2.06     2.25     2.49
Tm                    0.328    0.306    0.344    0.372
Yb                     2.15     2.02     2.18     2.39
Lu                     0.32    0.299    0.317    0.373
[SIGMA] REE           28.04    27.04    28.22    30.46
Eu/Eu *                1.02     1.06     1.14     0.96
[(La/Sm).sub.N]        0.62     0.61     0.61     0.63
[(Gd/Yb).sub.N]        1.01     1.05     0.99     1.03
[(La/Yb).sub.N]        0.56     0.58     0.56     0.56

Sample                CE-104   CE-105   CE-107

Si[O.sub.2]           51.64     49.1    49.07
[A1.sub.2][O.sub.3]   15.28    15.81    14.78
[Fe.sub.2][O.sub.3]   10.83    10.43     9.92
MnO                   0.198    0.185    0.171
MgO                    7.67     7.3      9.66
CaO                    6.04     9.6      8.85
[Na.sub.2]O            4.48     3.17     3.09
[K.sub.2]O             0.1      0.09     0.12
Ti[O.sub.2]           0.706    0.748    0.691
[P.sub.2][O.sub.5]     0.05     0.06     0.04
LO[I.sub.1]            2.86     2.73     3.19
TOTAL                 99.85    99.22    99.58

Sc                      43       44       46
V                      270      281      248
Cr                     100      170      430
Co                      37       36       36
Ni                      30       40      100
Cu                      90       70       70
Zn                      60       40       50
Ga                      12       14       12
Rb                     < 1       1        2
Sr                      32       23      109
Y                      18.2      20       19
Zr                      30       30       25
Nb                     0.3      0.3      0.3
Cs                    < 0.1    < 0.1     0.1
Ba                      16       22       29
Hf                      1        1       0.8
Ta                    < 0.01   < 0.01   < 0.01
Pb                     < 5      < 5      < 5
Th                     0.12     0.14     0.07
U                      0.15     0.17     0.06

La                     0.86     1.18     1.65
Ce                     2.78     3.42     3.32
Pr                     0.53     0.62     0.67
Nd                     3.32     3.82     4.22
Sm                     1.23     1.45     1.46
Eu                    0.476    0.526     0.57
Gd                     1.99     2.34     2.13
Tb                     0.41     0.49     0.43
Dy                     2.85     3.22     2.93
Ho                     0.63     0.7      0.63
Er                     1.95     2.15     1.87
Tm                    0.294    0.328    0.272
Yb                     1.93     2.17     1.73
Lu                    0.308     0.33    0.267
[SIGMA] REE           19.56    22.74    22.15
Eu/Eu *                0.94     0.88     0.99
[(La/Sm).sub.N]        0.43     0.50     0.70
[(Gd/Yb).sub.N]        0.82     0.86     0.98
[(La/Yb).sub.N]        0.30     0.36     0.64

(1) Loss on ignition.
Oxides are in weight percent (%). Trace and rare earth elements
are in parts per million (ppm).

The element concentrations expressed with the < sign are below
detection limit.

TABLE 7.- WHOLE ROCK MAJOR AND TRACE ELEMENT DATA
OF ESPASANTE DYKES

Tabla 7. Analisis quimicos de elementos mayores y traza
de los diques de Espasante.

Sample                CI-1    CI-2     CI-3

Si[O.sub.2]           49.06   49.15   49.07
[Al.sub.2][O.sub.3]   15.21   15.39   15.24
[Fe.sub.2][O.sub.3]   12.41   12.1    12.17
MnO                   0.224   0.228   0.223
MgO                   6.31    6.37     6.39
CaO                   9.58    9.68     9.5
[Na.sub.2]O           0.36    0.32     0.45
[K.sub.2]O            0.03    0.08    < 0.01
Ti[O.sub.2]           1.613   1.598    1.63
[P.sub.2][O.sub.5]    0.12    0.15     0.16
LO[I.sub.1]           4.18    3.95     4.21
TOTAL                 99.1    99.02   99.04

Sc                     39      39       40
V                      295     294     300
Cr                     110     90      100
Co                     24      33       31
Ni                     40      50       50
Cu                     50      30       60
Zn                     50      130      90
Ga                     17      18       18
Rb                     < 1     < 1     < 1
Sr                     240     280     231
Y                     35.1    32.9     33.4
Zr                     88      89       90
Nb                     1.7     1.6     1.7
Cs                     0.1     0.1     0.2
Ba                     29      15       30
Hf                     2.6     2.5     2.7
Ta                    0.09    0.08     0.09
Pb                     < 5      8      < 5
Th                     0.6    0.56     0.61
U                     0.24    0.36     0.24

La                    5.95    5.89     5.7
Ce                     14     13.8     14.1
Pr                    2.29    2.25     2.24
Nd                    11.4    11.4     11.2
Sm                    3.73    3.62     3.61
Eu                    1.42     1.4     1.4
Gd                    4.75    4.55     4.6
Tb                    0.87    0.85     0.85
Dy                     5.5    5.37     5.38
Ho                    1.15    1.11     1.11
Er                    3.35    3.32     3.28
Tm                    0.502   0.482    0.48
Yb                    3.25    2.98     3.05
Lu                    0.468   0.425   0.438
[SIGMA] REE           58.63   57.45   57.44
Eu/Eu *               1.04    1.06     1.06
[(La/Sm).sub.N]       0.98    1.00     0.97
[(Gd/Yb).sub.N]       1.16    1.22     1.20
[(La/Yb).sub.N]       1.22    1.32     1.25

Sample                CI-4    CI-5     CI-6

Si[O.sub.2]           48.96   48.93   49.24
[Al.sub.2][O.sub.3]   15.27   15.06   15.58
[Fe.sub.2][O.sub.3]   12.27   12.15   11.76
MnO                   0.216   0.225   0.231
MgO                   6.25    6.49     6.16
CaO                   9.42    9.82     10.6
[Na.sub.2]O           0.58    0.29     0.24
[K.sub.2]O            0.02    0.02    < 0.01
Ti[O.sub.2]           1.634   1.628   1.566
[P.sub.2][O.sub.5]    0.16    0.15     0.15
LO[I.sub.1]           4.42    4.01     3.99
TOTAL                 99.2    98.77   99.52

Sc                     40      40       38
V                      297     302     287
Cr                     110     100     150
Co                     31      32       31
Ni                     50      50       80
Cu                     50      50       40
Zn                     90      120      90
Ga                     17      18       18
Rb                     < 1     < 1     < 1
Sr                     236     286     299
Y                     32.8    40.8     32.1
Zr                     90      91       87
Nb                     1.6     1.7     1.8
Cs                     0.2    < 0.1    0.1
Ba                     20      22       20
Hf                     2.5     2.6     2.5
Ta                    0.08    0.08     0.07
Pb                      7       7       6
Th                    0.58    0.59     0.54
U                     0.24    0.25     0.22

La                    5.64     10      5.3
Ce                    13.7    14.1     13.5
Pr                    2.17    2.93     2.11
Nd                    11.2    14.2     10.8
Sm                    3.47    4.25     3.37
Eu                    1.37    1.63     1.33
Gd                     4.6    5.56     4.25
Tb                    0.82    1.04     0.79
Dy                    5.36    6.67     4.99
Ho                    1.13    1.39     1.04
Er                    3.28    4.17     3.21
Tm                    0.476   0.611   0.473
Yb                    2.92    3.78     2.99
Lu                    0.428   0.531   0.418
[SIGMA] REE           56.56   70.86   54.57
Eu/Eu *               1.05    1.03     1.08
[(La/Sm).sub.N]       1.00    1.45     0.97
[(Gd/Yb).sub.N]       1.26    1.17     1.13
[(La/Yb).sub.N]       1.29    1.77     1.19

(1) Loss on ignition.
Oxides are in weight percent (%). Trace and rare earth elements
are in parts per million (ppm).

The element concentrations expressed with the < sign are below
detection limit.

TABLE 8.- WHOLE ROCK MAJOR AND TRACE ELEMENT DATA
OF GABBROIC ROCKS AND GRANITOIDS

Tabla 8. Analisis quimicos de elementos mayores y traza
de las rocas gabroicas y granitoides.

Sample                CE-92   CE-93   CE-95   CE-99

Si[O.sub.2]           54.43   53.96   53.61   51.78
[Al.sub.2][O.sub.3]   16.67   15.29   17.36   15.28
[Fe.sub.2][O.sub.3]   7.42     7.3    6.37    11.02
MnO                   0.105   0.126   0.113   0.203
MgO                   7.07    8.77    6.57    7.28
CaO                   7.23    5.86    6.93     4.7
[Na.sub.2]O           2.97    3.25    2.44    4.79
[K.sub.2]O            0.94    1.57    2.68    0.07
Ti[O.sub.2]           0.336   0.271   0.256   0.896
[P.sub.2][O.sub.5]    0.04    0.04    0.04    0.06
LOI (1)               2.54    2.92    2.89    3.25
TOTAL                 99.75   99.36   99.26   99.33

Sc                     27      29      31      43
V                      176     177     261     312
Cr                     140     160     130     60
Co                     17      23      21      40
Ni                     80      120     40     < 20
Cu                     20      50     < 10     60
Zn                    < 30     30      40      50
Ga                     13      12      17      17
Rb                     24      48      91       1
Sr                     266     105     104     84
Y                     11.2    11.2    10.4    22.9
Zr                     33      34      37      37
Nb                     1.1      1      1.8     0.8
Cs                     0.7     1.5     2.8    < 0.1
Ba                     164     268     378     13
Hf                     1.1      1       1      1.3
Ta                    0.06    0.06    0.13    0.01
Pb                     < 5     < 5     < 5     < 5
Th                    1.15    1.45    1.68    0.31
U                     0.63    0.74    0.96    0.18

La                    4.39    4.72    5.88    1.44
Ce                    8.57    8.97    11.8    4.28
Pr                    1.14    1.17    1.48    0.78
Nd                    4.66    4.59    5.83    4.61
Sm                    1.25    1.24    1.34    1.73
Eu                    0.428   0.438   0.51    0.676
Gd                    1.49    1.45    1.51    2.74
Tb                    0.27    0.26    0.26    0.55
Dy                    1.71    1.74    1.64    3.68
Ho                    0.37    0.37    0.34    0.79
Er                    1.14    1.12    1.04    2.46
Tm                    0.173   0.176   0.158   0.386
Yb                    1.09    1.13    1.06    2.52
Lu                    0.165   0.169   0.163   0.389
[SIGMA] REE           26.85   27.54   33.01   27.03
Eu/Eu *               0.96    1.00    1.10    0.95
[(La/Sm).sub.N]       2.17    2.35    2.71    0.51
[(Gd/Yb).sub.N]       1.09    1.02    1.14    0.87
[(La/Yb).sub.N]       2.69    2.79    3.71    0.38

Sample                CE-94   CE-96    CE-97    CE-98

Si[O.sub.2]           69.76   72.58    71.44     70.4
[Al.sub.2][O.sub.3]   14.29   13.84    13.84    14.04
[Fe.sub.2][O.sub.3]   3.68      3       2.44     2.66
MnO                   0.042   0.034      3      0.047
MgO                   2.01     1.34     1.4      2.06
CaO                   0.67     1.43     0.53     0.58
[Na.sub.2]O           6.44     6.15     2.74     3.38
[K.sub.2]O            0.51     0.35     4.28     3.55
Ti[O.sub.2]           0.361   0.355    0.473     0.5
[P.sub.2][O.sub.5]    0.08     0.09     0.14     0.15
LOI (1)               1.28     0.87     1.71     1.8
TOTAL                 99.12   100.04   99.03    99.17

Sc                     11       7        10       9
V                      50       40       18       16
Cr                     140      90      120      100
Co                      7       5        2        3
Ni                    < 20     < 20     < 20     < 20
Cu                     30       30      < 10     < 10
Zn                    < 30     < 30     < 30      40
Ga                     13       14       17       18
Rb                     13       8       129      110
Sr                     101     137       78       62
Y                     18.7      18      43.4     45.3
Zr                     89       87      146      163
Nb                     3.6     3.5      12.2     12.6
Cs                     0.4     0.6      2.3      3.2
Ba                     180     131      938      729
Hf                     2.4     2.5       4       4.5
Ta                    0.33     0.33     1.05     1.03
Pb                     < 5     < 5      < 5       25
Th                    4.31     4.23     14.5     15.9
U                      2.4     2.48     7.25     4.26

La                     12      12.4     40.9     44.2
Ce                    24.2     23.1     76.1     88.5
Pr                    2.87     2.79     9.35     10.5
Nd                     11      10.3     34.5     39.7
Sm                    2.36     2.26     6.94     7.97
Eu                    0.734   0.606     1.28     1.46
Gd                    2.64     2.56     6.86     8.04
Tb                    0.47     0.45     1.14     1.3
Dy                     2.8     2.71     6.52     7.48
Ho                    0.59     0.57     1.32     1.45
Er                    1.84     1.77     4.1      4.24
Tm                    0.287   0.283     0.61    0.629
Yb                    1.91     1.83     3.72     3.96
Lu                    0.292   0.277    0.536    0.563
[SIGMA] REE           63.99   61.91    193.88   219.99
Eu/Eu *               0.90     0.77     0.57     0.56
[(La/Sm).sub.N]       3.14     3.39     3.64     3.42
[(Gd/Yb).sub.N]       1.10     1.12     1.47     1.62
[(La/Yb).sub.N]       4.20     4.53     7.35     7.46

Samples CE-92, CE-93 and CE-95 are gabbros type I, sample CE-99
is a gabbro type II, and samples CE-94, CE-96, CE-97 and CE-98
are granitoids.

(1) Loss on ignition.

Oxides are in weight percent (%). Trace and rare earth elements
are in parts per million (ppm).

The element concentrations expressed with the < sign are below
detection limit.
COPYRIGHT 2009 Universidad Complutense de Madrid
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2009 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Arenas, R.; Sanchez Martinez, S.; Castineiras, P.; Jeffries, T.E.; Diez Fernandez, R.; Andonaegui, P
Publication:Journal of Iberian Geology
Article Type:Report
Date:Jul 1, 2009
Words:22882
Previous Article:Mesozoic Terrestrial Ecosystems and Biota.
Next Article:A fern-bennettitalean floral assemblage in Tithonian-Berriasian travertine deposits (Aguilar Formation, Burgos-Palencia, N Spain) and its...
Topics:

Terms of use | Privacy policy | Copyright © 2018 Farlex, Inc. | Feedback | For webmasters