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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 Slcza (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 meta basites. 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 greenschist facies mineral assemblages, whereas some metagabbros 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 sand stones, pelites, cherts, limestones, plutonic rocks (quartzdiorites, 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 (155Gd). 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.*]/[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.206]Pb=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 UPb 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.206]Pb 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 (ICPMS). 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, 3.944.7% MgO, 8.92-9.84% CaO, 2.04-2.76% [Na.sub.2]O, 0.110.34% [K.sub.2]O, 0.99-1.01% Ti[O.sub.2], and 0.15-0.18% [P.sub.2][O.sub.5] (Table 5). The samples of common metavolcanics are also metabasites with slightly lower Si[O.sub.2] contents (48.0651.64%). 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 Si[O.sub.2] 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 (HREE), with slightly negative Eu anomalies (Eu/[Eu.sup.*] = 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.sup.*] = 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.sup.*] = 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.sup.*] = 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]N = 2.69-3.71, Fig. 12d) and without significant Eu anomalies (Eu/[Eu.sup.*] = 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]-[P.sub.2][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]-[P.sub.2][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 calcalkaline 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 for the following facts: 1) the incorporation to the tectonic melange of lithologies derived from the most external margin of Gondwana, previously subducted 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 meta morphic 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 intraRheic 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-subduction 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

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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

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].

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

    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

             Isotopic ratios and 1 [sigma] (absolute) errors

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

    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      11.88454       1.38         0.06048        0.94
    14.1      11.64601       1.40         0.05751        1.12
     5.2      11.31620       1.39         0.06751        1.89
     6.1      11.12346       1.40         0.06467        0.73
    15.2      11.15173       1.36         0.05727        0.70
    15.1      10.92881       1.35         0.05764        0.48

             Isotopic ratios and 1 [sigma] (absolute) errors

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

    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      11.93829       1.38         0.05682        1.72
    14.1      11.64601       1.40         0.05751        1.12
     5.2      11.46819       1.41         0.05670        5.10
     6.1      11.22166       1.40         0.05753        2.31
    15.2      11.15415       1.36         0.05709        0.71
    15.1      10.94201       1.35         0.05665        0.78

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

    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       0.0839        0.0012          519         7
    14.1       0.0859        0.0012          531         7
     5.2       0.0874        0.0012          540         7
     6.1       0.0892        0.0013          551         8
    15.2       0.0898        0.0012          555         7
    15.1       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 2.--SHRIMP U-Th-Pb ANALYSES OF ZIRCONS FROM THE INSUA
GRANITOID GCH-05-8

Table 2.--U-Th-Pb SHRIMP analytical data for zircons from the
metagranitoid GCH-05-8. C, core; R, rim. All errors are 1a. (Table
2 continues in next page)

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)

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

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

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

           Isotopic ratios and 1 [sigma] (absolute) errors

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

   37.1      21.28388       1.06         0.05835        7.78
   20.1      20.72375       0.99         0.04854       12.02
   31.1      20.36914       0.92         0.05196        3.79
    7.1      20.37535       1.29         0.04907        6.04
    7.2      19.84915       0.41         0.05221        1.59
   11.1      13.38147       1.08         0.05643        3.95
   27.1      12.99535       0.34         0.05727        1.39
    1.1      12.98183       0.42         0.05692        0.98
   37.1      21.28388       1.06         0.05835        7.78
    1.2      12.74303       0.52         0.05857        5.03
   32.1      12.67712       1.04         0.05424        3.82
    4.1      12.60974       0.35         0.05779        0.95
   36.1      12.64739       0.48         0.05434        1.77
   24.2      12.59759       0.33         0.05734        1.68
   10.1      12.58683       0.26         0.05786        0.53
   39.1      12.58612       0.47         0.05698        1.13
   19.1      12.56532       0.27         0.05782        0.58
   40.1      12.47296       0.40         0.05829        0.87
   13.1      12.49534       0.53         0.05667        1.54
   25.1      12.46063       0.45         0.05813        0.93
   38.1      12.47273       0.37         0.05724        1.01
   18.1      12.44179       0.40         0.05693        0.86
   14.1      12.41991       0.60         0.05835        1.69
   15.1      12.42398       0.29         0.05715        0.56
   16.1      12.39445       0.42         0.05803        0.85
   30.1      12.41166       0.49         0.05684        2.32
   21.1      12.41413       0.34         0.05603        0.71
    9.1      12.38251       0.32         0.05781        1.17
    6.1      12.39541       0.33         0.05645        0.95
   24.1      12.36061       0.47         0.05766        0.98
    8.1      12.36877       0.29         0.05673        0.59
   22.1      12.35292       0.84         0.05593        5.87
   26.1      12.25665       1.05         0.06056        4.02
   23.1      12.21986       0.47         0.05878        0.91
   40.2      12.22461       0.55         0.05758        2.22
   17.1      12.22528       0.27         0.05751        0.53
   34.1      12.14166       0.50         0.05760        0.97
   12.1      12.11794       0.18         0.05728        0.30
   25.2      11.70922       0.31         0.05757        0.65
   33.1      10.83532       0.95         0.06166        1.75
    5.1      10.85283       1.23         0.05934        5.22
   35.1       9.12257      11.73          --            --
   29.1       7.86511       1.27         0.06474        3.42
   28.1       2.81058       0.65         0.14304        1.27

             Isotopic ratios and
                  1 [sigma]
              (absolute) errors        [sup.206]Pb/
                                        [sup.238]
GCH-05-5   [sup.206]Pb/    [+ or -]        U (d)       age
Anal. #      [sup.238]     1 [sigma]       (Ma)
               U (c)

   37.1       0.0466        0.0005          294         3
   20.1       0.0485        0.0004          305         3
   31.1       0.0491        0.0005          309         3
    7.1       0.0493        0.0006          310         4
    7.2       0.0504        0.0002          317         1
   11.1       0.0747        0.0008          465         5
   27.1       0.0769        0.0003          478         2
    1.1       0.0770        0.0003          478         2
   37.1       0.0466        0.0005          294         3
    1.2       0.0783        0.0005          486         3
   32.1       0.0791        0.0008          491         5
    4.1       0.0792        0.0003          492         2
   36.1       0.0793        0.0004          492         2
   24.2       0.0793        0.0003          492         2
   10.1       0.0794        0.0002          492         1
   39.1       0.0795        0.0004          493         2
   19.1       0.0795        0.0002          493         1
   40.1       0.0801        0.0003          496         2
   13.1       0.0801        0.0004          497         3
   25.1       0.0802        0.0004          497         2
   38.1       0.0802        0.0003          497         2
   18.1       0.0804        0.0003          499         2
   14.1       0.0804        0.0005          499         3
   15.1       0.0805        0.0002          499         1
   16.1       0.0806        0.0003          500         2
   30.1       0.0806        0.0004          500         2
   21.1       0.0807        0.0003          500         2
    9.1       0.0807        0.0003          500         2
    6.1       0.0808        0.0003          501         2
   24.1       0.0809        0.0004          501         2
    8.1       0.0809        0.0002          502         1
   22.1       0.0811        0.0006          503         4
   26.1       0.0813        0.0009          504         5
   23.1       0.0817        0.0004          506         2
   40.2       0.0818        0.0005          507         3
   17.1       0.0818        0.0002          507         1
   34.1       0.0823        0.0004          510         3
   12.1       0.0825        0.0002          511         1
   25.2       0.0854        0.0003          529         2
   33.1       0.0920        0.0009          567         5
    5.1       0.0921        0.0011          568         7
   35.1       0.1027        0.0163          630        95
   29.1       0.1272        0.0017          772         9
   28.1       0.3452        0.0031         2264        22

(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

* Except analysis 28.1 ([sup.207]Pb/[sup.206]Pb age,
[sup.204]Pb corrected for common lead).

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

Table 3. LA-ICP-MS U-Pb analyses of zircons from the metagranitoid
GCH-05-6.

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

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

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

jl13b05    0.20      0.0832       0.0003       0.6522       0.0044
jl13a10    0.23      0.0790       0.0007       0.6240       0.0062
jl13b16    0.24      0.0828       0.0006       0.6537       0.0056
jl13b10    0.28      0.0841       0.0006       0.6638       0.0053
jl13b13    0.27      0.0858       0.0004       0.6786       0.0043
jl13a05    0.27      0.0822       0.0004       0.6513       0.0038
jl13d09    0.30      0.0881       0.0004       0.6990       0.0036
jl13d13    0.29      0.0881       0.0008       0.6992       0.0050
jl13a14    0.18      0.0773       0.0003       0.6148       0.0049
jl13c08    0.31      0.0854       0.0003       0.6802       0.0032
jl13d07    0.28      0.0883       0.0004       0.7034       0.0030
jl13a13    0.23      0.0803       0.0004       0.6398       0.0040
jl13a11    0.22      0.0859       0.0006       0.6848       0.0041
jl13b14    0.27      0.0857       0.0009       0.6838       0.0079
jl13c13    0.25      0.0858       0.0005       0.6852       0.0039
jl13b15    0.25      0.0878       0.0005       0.7017       0.0054
jl13b06    0.31      0.0894       0.0005       0.7152       0.0036
jl13c05    0.24      0.0875       0.0008       0.7005       0.0056
jl13b11    0.29      0.0827       0.0004       0.6629       0.0029
jl13a07    0.33      0.0843       0.0004       0.6754       0.0038
jl13d11    0.27      0.0883       0.0006       0.7079       0.0050
jl13b07    0.23      0.0842       0.0006       0.6767       0.0077
jl13a12    0.24      0.0818       0.0005       0.6579       0.0051
jl13c15    0.26      0.0841       0.0006       0.6760       0.0051
jl13d14    0.25      0.0888       0.0007       0.7141       0.0055
jl13c09    0.29      0.0881       0.0005       0.7091       0.0050
jl13a06    0.24      0.0812       0.0003       0.6549       0.0054
jl13d15    0.27      0.0866       0.0008       0.6998       0.0064
jl13c10    0.22      0.0854       0.0007       0.6905       0.0057
jl13d12    0.32      0.0899       0.0007       0.7281       0.0063
jl13a16    0.26      0.0840       0.0004       0.6813       0.0033
jl13d06    0.28      0.0868       0.0005       0.7048       0.0041
jl13d16    0.26      0.0860       0.0007       0.7002       0.0073
jl13d10    0.26      0.0897       0.0004       0.7380       0.0056

             Isotopic ratios and
Sample       1 [sigma] (absolute)         Ages and 2 [sigma]
                    errors                   errors (Ma)

Anal. #    [sup.207]Pb/   [+ or -]    [sup.206]Pb/   [+ or -]
           [sup.206]U     1 [sigma]   [sup.238]U     2 [sigma]

jl13b05       0.0569       0.0004         515            4
jl13a10       0.0573       0.0005         490            8
jl13b16       0.0572       0.0004         513            7
jl13b10       0.0573       0.0005         520            7
jl13b13       0.0574       0.0003         530            4
jl13a05       0.0575       0.0004         509            5
jl13d09       0.0576       0.0003         544            4
jl13d13       0.0575       0.0003         544           10
jl13a14       0.0577       0.0005         480            4
jl13c08       0.0577       0.0003         529            4
jl13d07       0.0577       0.0003         546            5
jl13a13       0.0578       0.0004         498            5
jl13a11       0.0578       0.0003         531            7
jl13b14       0.0579       0.0005         530           10
jl13c13       0.0579       0.0002         530            5
jl13b15       0.0580       0.0003         542            6
jl13b06       0.0580       0.0001         552            6
jl13c05       0.0581       0.0003         541            9
jl13b11       0.0581       0.0003         512            5
jl13a07       0.0581       0.0004         521            5
jl13d11       0.0581       0.0003         546            7
jl13b07       0.0583       0.0007         521            7
jl13a12       0.0583       0.0004         507            6
jl13c15       0.0583       0.0003         521            7
jl13d14       0.0583       0.0002         548            9
jl13c09       0.0584       0.0003         544            5
jl13a06       0.0585       0.0004         503            4
jl13d15       0.0586       0.0003         536            9
jl13c10       0.0586       0.0004         528            9
jl13d12       0.0587       0.0003         555            8
jl13a16       0.0588       0.0003         520            4
jl13d06       0.0589       0.0003         537            6
jl13d16       0.0591       0.0004         532            9
jl13d10       0.0597       0.0003         554            5

Sample              Ages and 2 [sigma] errors (Ma)

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

jl13b05        510            5           484           30
jl13a10        492            8           500           40
jl13b16        511            7           500           30
jl13b10        517            6           500           42
jl13b13        526            5           504           26
jl13a05        509            5           510           30
jl13d09        538            4           512           20
jl13d13        538            6           512           22
jl13a14        487            6           516           36
jl13c08        527            4           518           24
jl13d07        541            4           518           20
jl13a13        502            5           520           28
jl13a11        530            5           522           24
jl13b14        529           10           524           36
jl13c13        530            5           526           18
jl13b15        540            6           528           22
jl13b06        548            4           528           12
jl13c05        539            7           530           26
jl13b11        516            3           534           24
jl13a07        524            5           534           28
jl13d11        543            6           534           22
jl13b07        525            9           538           54
jl13a12        513            6           540           28
jl13c15        524            6           540           22
jl13d14        547            6           540           20
jl13c09        544            6           542           20
jl13a06        512            7           548           32
jl13d15        539            8           550           20
jl13c10        533            7           552           30
jl13d12        555            7           556           18
jl13a16        528            4           560           20
jl13d06        542            5           562           24
jl13d16        539            9           568           32
jl13d10        561            7           592           24

Sample

Anal. #    % disc

jl13b05    -6.40
jl13a10     2.00
jl13b16    -2.60
jl13b10    -4.00
jl13b13    -5.16
jl13a05     0.20
jl13d09    -6.25
jl13d13    -6.25
jl13a14     6.98
jl13c08    -2.12
jl13d07    -5.41
jl13a13     4.23
jl13a11    -1.72
jl13b14    -1.15
jl13c13    -0.76
jl13b15    -2.65
jl13b06    -4.55
jl13c05    -2.08
jl13b11     4.12
jl13a07     2.43
jl13d11    -2.25
jl13b07     3.16
jl13a12     6.11
jl13c15     3.52
jl13d14    -1.48
jl13c09    -0.37
jl13a06     8.21
jl13d15     2.55
jl13c10     4.35
jl13d12     0.18
jl13a16     7.14
jl13d06     4.45
jl13d16     6.34
jl13d10     6.42

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

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

Table 4.--LA-ICP-MS analyses of detrital zircons from the
metaconglomerate SO-3. (Table 4 continues in next page)

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

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

SO3-109    1.90      0.0746       0.0013        0.5802        0.036
SO3-111    1.78      0.0748       0.0010        0.5817        0.025
SO3-112    1.27      0.0775       0.0009        0.6103        0.022
SO3-110    1.88      0.0778       0.0009        0.6121        0.025
SO3-108    1.91      0.0784       0.0011        0.6178        0.030
SO3-14     0.18      0.0802       0.0009        0.6339        0.010
SO3-17     0.18      0.0809       0.0008        0.6387        0.007
SO3-33     0.15      0.0811       0.0008        0.6411        0.007
SO3-113    0.91      0.0816       0.0014        0.6458        0.033
SO3-43     0.20      0.0839       0.0008        0.6766        0.008
SO3-47     1.67      0.0864       0.0009        0.6926        0.019
SO3-53     0.63      0.0901       0.0009        0.7303        0.008
SO3-55     0.47      0.0914       0.0009        0.7422        0.009
SO3-39     0.94      0.0918       0.0009        0.7537        0.010
SO3-38     0.34      0.0922       0.0009        0.7503        0.008
SO3-35     0.74      0.0930       0.0010        0.7595        0.012
SO3-45     0.31      0.0931       0.0009        0.7651        0.010
SO3-37     0.59      0.0955       0.0009        0.7863        0.011
SO3-46     0.40      0.0958       0.0009        0.7871        0.009
SO3-106    0.55      0.0980       0.0010        0.8112        0.012
SO3-61     0.43      0.0993       0.0011        0.8242        0.014
SO3-22     0.45      0.1004       0.0009        0.8357        0.011
SO3-05     0.06      0.1020       0.0011        0.8560        0.011
SO3-54     0.17      0.1023       0.0011        0.8570        0.013
SO3-19     0.29      0.1161       0.0011        1.0083        0.011
SO3-48     0.55      0.2061       0.0019        2.2864        0.026
SO3-50     0.48      0.2308       0.0021        2.7418        0.034
SO3-16     0.74      0.2350       0.0022        2.8255        0.031
SO3-11     1.55      0.3306       0.0030        5.1321        0.054
SO3-04     1.70      0.3570       0.0032        6.0106        0.059
SO3-57     0.41      0.3361       0.0033        5.3084        0.086
SO3-56     0.73      0.3429       0.0032        5.5141        0.060
SO3-02     2.44      0.3436       0.0036        5.5324        0.075
SO3-42     1.13      0.3447       0.0032        5.5548        0.059
SO3-44     0.75      0.3452       0.0032        5.5701        0.060
SO3-58     0.44      0.3556       0.0039        5.9531        0.089
SO3-60     0.11      0.3670       0.0036        6.2812        0.072
SO3-06     0.63      0.3694       0.0035        6.3556        0.068
SO3-25     0.41      0.3772       0.0032        6.6281        0.062
SO3-31     0.48      0.3736       0.0034        6.5814        0.065
SO3-20     1.15      0.3778       0.0034        6.6559        0.071
SO3-28     1.25      0.3781       0.0034        6.6632        0.065
SO3-08     0.29      0.3797       0.0036        6.7175        0.070
SO3-01     0.52      0.3934       0.0039        7.3393        0.081
SO3-40     0.21      0.4172       0.0087        8.3279        0.291
SO3-107    0.36      0.4219       0.0040        8.6014        0.117
SO3-59     0.59      0.4496       0.0049        9.8997        0.156
SO3-51     0.78      0.4712       0.0046       10.5983        0.122
SO3-24     0.33      0.4833       0.0052       11.2217        0.149
SO3-52     0.70      0.4655       0.0051       10.8578        0.244
SO3-29     0.35      0.5055       0.0045       12.4415        0.119
SO3-18     0.58      0.5077       0.0045       12.6521        0.123
SO3-26     0.45      0.5170       0.0046       13.0905        0.124
SO3-09     0.56      0.5350       0.0051       14.1942        0.151
SO3-23     0.58      0.5502       0.0050       15.2761        0.145
SO3-21     0.15      0.5534       0.0058       15.4041        0.190
SO3-03     0.27      0.5547       0.0049       15.4649        0.151
SO3-07     0.52      0.5979       0.0053       18.6053        0.176
SO3-10     0.67      0.6541       0.0063       23.4129        0.250

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

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

SO3-109       0.0564       0.0036         464            7
SO3-111       0.0564       0.0025         465            6
SO3-112       0.0571       0.0021         481            6
SO3-110       0.0571       0.0024         483            6
SO3-108       0.0571       0.0029         487            7
SO3-14        0.0573       0.0009         497            5
SO3-17        0.0573       0.0007         501            5
SO3-33        0.0574       0.0007         503            5
SO3-113       0.0574       0.0030         506            8
SO3-43        0.0585       0.0008         519            4
SO3-47        0.0582       0.0016         534            6
SO3-53        0.0588       0.0007         556            5
SO3-55        0.0589       0.0007         564            5
SO3-39        0.0596       0.0009         566            5
SO3-38        0.0591       0.0007         568            5
SO3-35        0.0593       0.0010         573            6
SO3-45        0.0596       0.0008         574            6
SO3-37        0.0597       0.0010         588            5
SO3-46        0.0596       0.0008         590            6
SO3-106       0.0600       0.0009         603            6
SO3-61        0.0602       0.0011         610            6
SO3-22        0.0604       0.0009         617            5
SO3-05        0.0609       0.0008         626            6
SO3-54        0.0607       0.0009         628            6
SO3-19        0.0630       0.0007         708            6
SO3-48        0.0805       0.0010         1208          10
SO3-50        0.0862       0.0012         1339          11
SO3-16        0.0872       0.0010         1360          12
SO3-11        0.1126       0.0014         1841          14
SO3-04        0.1221       0.0014         1968          15
SO3-57        0.1146       0.0021         1868          16
SO3-56        0.1166       0.0014         1901          15
SO3-02        0.1168       0.0017         1904          17
SO3-42        0.1169       0.0014         1909          15
SO3-44        0.1170       0.0014         1912          16
SO3-58        0.1214       0.0019         1961          18
SO3-60        0.1241       0.0016         2015          17
SO3-06        0.1248       0.0014         2026          17
SO3-25        0.1275       0.0014         2063          15
SO3-31        0.1278       0.0014         2046          16
SO3-20        0.1278       0.0016         2066          16
SO3-28        0.1278       0.0014         2067          16
SO3-08        0.1283       0.0014         2075          17
SO3-01        0.1353       0.0015         2138          18
SO3-40        0.1448       0.0054         2248          40
SO3-107       0.1479       0.0024         2269          18
SO3-59        0.1597       0.0026         2393          22
SO3-51        0.1632       0.0020         2489          20
SO3-24        0.1684       0.0023         2542          23
SO3-52        0.1692       0.0042         2464          23
SO3-29        0.1785       0.0019         2637          19
SO3-18        0.1808       0.0020         2647          19
SO3-26        0.1837       0.0020         2686          20
SO3-09        0.1924       0.0022         2763          22
SO3-23        0.2014       0.0021         2826          21
SO3-21        0.2019       0.0025         2839          24
SO3-03        0.2022       0.0023         2845          20
SO3-07        0.2257       0.0024         3021          21
SO3-10        0.2596       0.0029         3244          24


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

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

SO3-109        465           23           469           110
SO3-111        466           16           468           73
SO3-112        484           14           497           57
SO3-110        485           15           495           67
SO3-108        488           19           496           83
SO3-14         498            6           505           16
SO3-17         502            4           503           10
SO3-33         503            4           505           11
SO3-113        506           20           507           81
SO3-43         525            5           549           13
SO3-47         534           11           536           40
SO3-53         557            5           558           11
SO3-55         564            5           564           12
SO3-39         570            6           588           14
SO3-38         568            5           569           11
SO3-35         574            7           576           18
SO3-45         577            6           588           13
SO3-37         589            6           593           16
SO3-46         590            5           590           12
SO3-106        603            7           604           16
SO3-61         610            8           612           19
SO3-22         617            6           618           15
SO3-05         628            6           635           12
SO3-54         628            7           630           15
SO3-19         708            6           708           10
SO3-48         1208           8           1209          10
SO3-50         1340           9           1342          12
SO3-16         1362           8           1366           9
SO3-11         1841           9           1842           9
SO3-04         1977           9           1988           8
SO3-57         1870          14           1873          16
SO3-56         1903           9           1905           9
SO3-02         1906          12           1908          11
SO3-42         1909           9           1909           9
SO3-44         1911           9           1911           9
SO3-58         1969          13           1977          13
SO3-60         2016          10           2017           9
SO3-06         2026           9           2026           8
SO3-25         2063           8           2063           7
SO3-31         2057           9           2068           8
SO3-20         2067           9           2068           8
SO3-28         2068           9           2068           8
SO3-08         2075           9           2075           8
SO3-01         2154          10           2168           9
SO3-40         2267          32           2285          33
SO3-107        2297          12           2321          29
SO3-59         2425          15           2453          13
SO3-51         2489          11           2489           9
SO3-24         2542          12           2542          10
SO3-52         2511          21           2550          43
SO3-29         2638           9           2639           7
SO3-18         2654           9           2660           7
SO3-26         2686           9           2686           7
SO3-09         2763          10           2763           8
SO3-23         2833           9           2837           7
SO3-21         2841          12           2842           9
SO3-03         2844           9           2844           7
SO3-07         3022           9           3022           7
SO3-10         3244          10           3245           8

Sample SO-3           Reported age

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

SO3-109      464           7         1.1
SO3-111      465           6         0.7
SO3-112      481           6         3.2
SO3-110      483           6         2.5
SO3-108      487           7         2.0
SO3-14       497           5         1.5
SO3-17       501           5         0.4
SO3-33       503           5         0.6
SO3-113      506           8         0.2
SO3-43       519           4         5.7
SO3-47       534           6         0.3
SO3-53       556           5         0.4
SO3-55       564           5         0.0
SO3-39       566           5         4.0
SO3-38       568           5         0.1
SO3-35       573           6         0.6
SO3-45       574           6         2.5
SO3-37       588           5         0.9
SO3-46       590           6         0.1
SO3-106      603           6         0.2
SO3-61       610           6         0.3
SO3-22       617           5         0.2
SO3-05       626           6         1.5
SO3-54       628           6         0.3
SO3-19       708           6         0.0
SO3-48       1209         10         0.1
SO3-50       1342         12         0.2
SO3-16       1366          9         0.4
SO3-11       1842          9         0.0
SO3-04       1988          8         1.2
SO3-57       1873         16         0.3
SO3-56       1905          9         0.3
SO3-02       1908         11         0.3
SO3-42       1909          9         0.0
SO3-44       1911          9         0.0
SO3-58       1977         13         0.9
SO3-60       2017          9         0.1
SO3-06       2026          8         0.0
SO3-25       2063          7         0.0
SO3-31       2068          8         1.2
SO3-20       2068          8         0.1
SO3-28       2068          8         0.1
SO3-08       2075          8         0.0
SO3-01       2168          9         1.6
SO3-40       2285         33         2.0
SO3-107      2321         29         2.7
SO3-59       2453         13         2.9
SO3-51       2489          9         0.0
SO3-24       2542         10         0.0
SO3-52       2550         43         4.1
SO3-29       2639          7         0.1
SO3-18       2660          7         0.6
SO3-26       2686          7         0.0
SO3-09       2763          8         0.0
SO3-23       2837          7         0.5
SO3-21       2842          9         0.1
SO3-03       2844          7         0.0
SO3-07       3022          7         0.1
SO3-10       3245          8         0.0

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

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

Table 5. Whole rock major and trace element data of the 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      CI-10

Si[O.sub.2]             50.53      49.75      51.33      51.69
[Al.sub.2][O.sub.3]     18.54      18.92      18.46      18.28
[Fe.sub.2][O.sub.3]      9.08       9.57       8.84       8.71
MnO                     0.165      0.289      0.151      0.158
MgO                      4.27        4.7       4.12       3.94
CaO                      9.34       9.23       9.19       9.84
[Na.sub.2]O              2.76       2.04       2.33       2.28
[K.sub.2]O               0.34       0.11       0.26       0.17
Ti[O.sub.2]             1.006      0.992      1.003      0.998
[P.sub.2][O.sub.5]       0.15       0.17       0.17       0.17
LOI (1)                  3.24       3.72       3.34       3.12
TOTAL                   99.42      99.49      99.19      99.36

Sc                         28         29         27         27
V                         189        187        186        197
Cr                         40         70         60         70
Co                         17         20         20         19
Ni                       < 20         20         20         50
Cu                         40        110         50         30
Zn                       < 30        120         80         80
Ga                         17         19         18         17
Rb                          8          3          5          5
Sr                        296        327        314        333
Y                          27       26.9       26.4       24.1
Zr                         93        100        101         95
Nb                        3.1        3.1        3.3          3
Cs                        0.4        0.3        0.4        0.3
Ba                        211         87        179        181
Hf                        2.8        2.8        2.7        2.6
Ta                       0.19       0.19        0.2       0.18
Pb                        < 5          7        < 5          7
Th                       3.02       3.06       3.06       3.03
U                        1.28       1.29       1.28       1.31

La                       12.8         13       12.3       12.7
Ce                       27.3       27.7       25.8         27
Pr                       3.73       3.75       3.52       3.62
Nd                       16.1       15.9       15.9         16
Sm                       4.09       3.82        3.8       3.71
Eu                       1.19       1.14       1.08        1.1
Gd                       4.11       4.16       3.89       3.98
Tb                       0.71       0.69        0.7       0.68
Dy                       4.33       4.24       4.08       3.97
Ho                       0.89       0.87       0.83       0.78
Er                       2.65        2.6       2.47       2.36
Tm                      0.383       0.38      0.362      0.358
Yb                       2.36       2.34       2.25       2.21
Lu                       0.34      0.339      0.322       0.31
[SIGMA] REE             80.98      80.93      77.30      78.78
Eu/Eu *                  0.89       0.88       0.86       0.88
[(La/Sm).sub.N]          1.93       2.10       2.00       2.11
[(Gd/Yb).sub.N]          1.39       1.42       1.38       1.44
[(La/Yb).sub.N]          3.63       3.71       3.66       3.84

Sample                  CI-11      CI-12

Si[O.sub.2]             51.88      51.42
[Al.sub.2][O.sub.3]      18.3      17.84
[Fe.sub.2][O.sub.3]      8.61       8.46
MnO                     0.155      0.203
MgO                      4.03       4.54
CaO                      8.92       9.22
[Na.sub.2]O              2.53       2.35
[K.sub.2]O               0.34        0.2
Ti[O.sub.2]             0.992      0.995
[P.sub.2][O.sub.5]       0.16       0.18
LOI (1)                  3.17       3.41
TOTAL                   99.09      98.82

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

La                       12.3       11.8
Ce                       26.6       25.9
Pr                       3.55       3.41
Nd                       15.4       15.4
Sm                        3.8       3.61
Eu                       1.13       1.08
Gd                       4.04       3.86
Tb                       0.66       0.64
Dy                       3.92       3.84
Ho                       0.78       0.75
Er                        2.4       2.26
Tm                      0.352      0.336
Yb                       2.22       2.13
Lu                      0.311      0.291
[SIGMA] REE             77.46      75.31
Eu/Eu *                  0.89       0.89
[(La/Sm).sub.N]          2.00       2.02
[(Gd/Yb).sub.N]          1.45       1.44
[(La/Yb).sub.N]          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.

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   CE-104

Si[O.sub.2]            50.17     49.5    48.92    48.06    51.64
[Al.sub.2][O.sub.3]    15.03    14.97    14.34    15.38    15.28
[Fe.sub.2][O.sub.3]    11.52     10.5    10.64    11.11    10.83
MnO                    0.179    0.143     0.15    0.154    0.198
MgO                     8.24     8.38      9.4     8.51     7.67
CaO                     5.84      7.8     8.22     9.36     6.04
[Na.sub.2]O              2.7     3.96     3.36     2.74     4.48
[K.sub.2]O              0.02     0.04     0.22     0.05      0.1
Ti[O.sub.2]            0.936     0.91    0.885    0.885    0.706
[P.sub.2][O.sub.5]      0.07     0.08     0.08     0.09     0.05
LOI (1)                 4.77     2.81     2.94     3.21     2.86
TOTAL                  99.47    99.09    99.16    99.55    99.85

Sc                        40       40       38       42       43
V                        288      279      271      273      270
Cr                       260      280      330      320      100
Co                        35       37       39       34       37
Ni                        50       90      110       80       30
Cu                       110       50       50       70       90
Zn                       140       50       60       70       60
Ga                        16       15       16       14       12
Rb                       < 1      < 1        3      < 1      < 1
Sr                        62       90       73      107       32
Y                       21.1     20.1     21.2     23.3     18.2
Zr                        41       40       40       43       30
Nb                       0.6      0.5      0.5      0.5      0.3
Cs                       0.3      0.1      0.4      0.1    < 0.1
Ba                         8       16       29       11       16
Hf                       1.4      1.3      1.2      1.4        1
Ta                      0.02     0.02     0.02     0.01   < 0.01
Pb                       < 5      < 5      < 5      < 5      < 5
Th                       0.2     0.19     0.17     0.24     0.12
U                       0.12     0.12     0.12     0.19     0.15

La                      1.81     1.74     1.81        2     0.86
Ce                      5.21     5.03      5.1     5.36     2.78
Pr                      0.92      0.9     0.92     0.99     0.53
Nd                      5.25     5.06     5.37     5.57     3.32
Sm                      1.79     1.75     1.84     1.95     1.23
Eu                      0.73    0.743     0.83    0.768    0.476
Gd                      2.72     2.65     2.71     3.08     1.99
Tb                      0.53     0.51     0.51     0.59     0.41
Dy                      3.37     3.28     3.33     3.73     2.85
Ho                      0.72     0.69     0.71      0.8     0.63
Er                      2.19     2.06     2.25     2.49     1.95
Tm                     0.328    0.306    0.344    0.372    0.294
Yb                      2.15     2.02     2.18     2.39     1.93
Lu                      0.32    0.299    0.317    0.373    0.308
[SIGMA] REE            28.04    27.04    28.22    30.46    19.56
Eu/Eu *                 1.02     1.06     1.14     0.96     0.94
[(La/Sm).sub.N]         0.62     0.61     0.61     0.63     0.43
[(Gd/Yb).sub.N]         1.01     1.05     0.99     1.03     0.82
[(La/Yb).sub.N]         0.56     0.58     0.56     0.56     0.30

Sample                CE-105   CE-107

Si[O.sub.2]             49.1    49.07
[Al.sub.2][O.sub.3]    15.81    14.78
[Fe.sub.2][O.sub.3]    10.43     9.92
MnO                    0.185    0.171
MgO                      7.3     9.66
CaO                      9.6     8.85
[Na.sub.2]O             3.17     3.09
[K.sub.2]O              0.09     0.12
Ti[O.sub.2]            0.748    0.691
[P.sub.2][O.sub.5]      0.06     0.04
LOI (1)                 2.73     3.19
TOTAL                  99.22    99.58

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

La                      1.18     1.65
Ce                      3.42     3.32
Pr                      0.62     0.67
Nd                      3.82     4.22
Sm                      1.45     1.46
Eu                     0.526     0.57
Gd                      2.34     2.13
Tb                      0.49     0.43
Dy                      3.22     2.93
Ho                       0.7     0.63
Er                      2.15     1.87
Tm                     0.328    0.272
Yb                      2.17     1.73
Lu                      0.33    0.267
[SIGMA] REE            22.74    22.15
Eu/Eu *                 0.88     0.99
[(La/Sm).sub.N]         0.50     0.70
[(Gd/Yb).sub.N]         0.86     0.98
[(La/Yb).sub.N]         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

Table 7. Whole rock major and trace element data of the Espasante
dikes.

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

Sample                CI-1    CI-2     CI-3    CI-4    CI-5     CI-6

Si[O.sub.2]          49.06   49.15    49.07   48.96   48.93    49.24
[Al.sub.2][O.sub.3]  15.21   15.39    15.24   15.27   15.06    15.58
[Fe.sub.2][O.sub.3]  12.41    12.1    12.17   12.27   12.15    11.76
MnO                  0.224   0.228    0.223   0.216   0.225    0.231
MgO                   6.31    6.37     6.39    6.25    6.49     6.16
CaO                   9.58    9.68      9.5    9.42    9.82     10.6
[Na.sub.2]O           0.36    0.32     0.45    0.58    0.29     0.24
[K.sub.2]O            0.03    0.08   < 0.01    0.02    0.02   < 0.01
Ti[O.sub.2]          1.613   1.598     1.63   1.634   1.628    1.566
[P.sub.2][O.sub.5]    0.12    0.15     0.16    0.16    0.15     0.15
LOI (1)               4.18    3.95     4.21    4.42    4.01     3.99
TOTAL                 99.1   99.02    99.04    99.2   98.77    99.52

Sc                      39      39       40      40      40       38
V                      295     294      300     297     302      287
Cr                     110      90      100     110     100      150
Co                      24      33       31      31      32       31
Ni                      40      50       50      50      50       80
Cu                      50      30       60      50      50       40
Zn                      50     130       90      90     120       90
Ga                      17      18       18      17      18       18
Rb                     < 1     < 1      < 1     < 1     < 1      < 1
Sr                     240     280      231     236     286      299
Y                     35.1    32.9     33.4    32.8    40.8     32.1
Zr                      88      89       90      90      91       87
Nb                     1.7     1.6      1.7     1.6     1.7      1.8
Cs                     0.1     0.1      0.2     0.2    < 0.1     0.1
Ba                      29      15       30      20      22       20
Hf                     2.6     2.5      2.7     2.5     2.6      2.5
Ta                    0.09    0.08     0.09    0.08    0.08     0.07
Pb                     < 5       8      < 5       7       7        6
Th                     0.6    0.56     0.61    0.58    0.59     0.54
U                     0.24    0.36     0.24    0.24    0.25     0.22

La                    5.95    5.89      5.7    5.64      10      5.3
Ce                      14    13.8     14.1    13.7    14.1     13.5
Pr                    2.29    2.25     2.24    2.17    2.93     2.11
Nd                    11.4    11.4     11.2    11.2    14.2     10.8
Sm                    3.73    3.62     3.61    3.47    4.25     3.37
Eu                    1.42     1.4      1.4    1.37    1.63     1.33
Gd                    4.75    4.55      4.6     4.6    5.56     4.25
Tb                    0.87    0.85     0.85    0.82    1.04     0.79
Dy                     5.5    5.37     5.38    5.36    6.67     4.99
Ho                    1.15    1.11     1.11    1.13    1.39     1.04
Er                    3.35    3.32     3.28    3.28    4.17     3.21
Tm                   0.502   0.482     0.48   0.476   0.611    0.473
Yb                    3.25    2.98     3.05    2.92    3.78     2.99
Lu                   0.468   0.425    0.438   0.428   0.531    0.418
[SIGMA] REE          58.63   57.45    57.44   56.56   70.86    54.57
Eu/Eu *               1.04    1.06     1.06    1.05    1.03     1.08
[(La/Sm).sub.N]       0.98    1.00     0.97    1.00    1.45     0.97
[(Gd/Yb).sub.N]       1.16    1.22     1.20    1.26    1.17     1.13
[(La/Yb).sub.N]       1.22    1.32     1.25    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

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   CE-94    CE-96

Si[O.sub.2]           54.43   53.96   53.61   51.78   69.76    72.58
[Al.sub.2][O.sub.3]   16.67   15.29   17.36   15.28   14.29    13.84
[Fe.sub.2][O.sub.3]    7.42     7.3    6.37   11.02    3.68        3
MnO                   0.105   0.126   0.113   0.203   0.042    0.034
MgO                    7.07    8.77    6.57    7.28    2.01     1.34
CaO                    7.23    5.86    6.93     4.7    0.67     1.43
[Na.sub.2]O            2.97    3.25    2.44    4.79    6.44     6.15
[K.sub.2]O             0.94    1.57    2.68    0.07    0.51     0.35
Ti[O.sub.2]           0.336   0.271   0.256   0.896   0.361    0.355
[P.sub.2][O.sub.5]     0.04    0.04    0.04    0.06    0.08     0.09
LOI (1)                2.54    2.92    2.89    3.25    1.28     0.87
TOTAL                 99.75   99.36   99.26   99.33   99.12   100.04

Sc                       27      29      31      43      11        7
V                       176     177     261     312      50       40
Cr                      140     160     130      60     140       90
Co                       17      23      21      40       7        5
Ni                       80     120      40    < 20    < 20     < 20
Cu                       20      50    < 10      60      30       30
Zn                     < 30      30      40      50    < 30     < 30
Ga                       13      12      17      17      13       14
Rb                       24      48      91       1      13        8
Sr                      266     105     104      84     101      137
Y                      11.2    11.2    10.4    22.9    18.7       18
Zr                       33      34      37      37      89       87
Nb                      1.1       1     1.8     0.8     3.6      3.5
Cs                      0.7     1.5     2.8   < 0.1     0.4      0.6
Ba                      164     268     378      13     180      131
Hf                      1.1       1       1     1.3     2.4      2.5
Ta                     0.06    0.06    0.13    0.01    0.33     0.33
Pb                      < 5     < 5     < 5     < 5     < 5      < 5
Th                     1.15    1.45    1.68    0.31    4.31     4.23
U                      0.63    0.74    0.96    0.18     2.4     2.48

La                     4.39    4.72    5.88    1.44      12     12.4
Ce                     8.57    8.97    11.8    4.28    24.2     23.1
Pr                     1.14    1.17    1.48    0.78    2.87     2.79
Nd                     4.66    4.59    5.83    4.61      11     10.3
Sm                     1.25    1.24    1.34    1.73    2.36     2.26
Eu                    0.428   0.438    0.51   0.676   0.734    0.606
Gd                     1.49    1.45    1.51    2.74    2.64     2.56
Tb                     0.27    0.26    0.26    0.55    0.47     0.45
Dy                     1.71    1.74    1.64    3.68     2.8     2.71
Ho                     0.37    0.37    0.34    0.79    0.59     0.57
Er                     1.14    1.12    1.04    2.46    1.84     1.77
Tm                    0.173   0.176   0.158   0.386   0.287    0.283
Yb                     1.09    1.13    1.06    2.52    1.91     1.83
Lu                    0.165   0.169   0.163   0.389   0.292    0.277
[SIGMA] REE           26.85   27.54   33.01   27.03   63.99    61.91
Eu/Eu *                0.96    1.00    1.10    0.95    0.90     0.77
[(La/Sm).sub.N]        2.17    2.35    2.71    0.51    3.14     3.39
[(Gd/Yb).sub.N]        1.09    1.02    1.14    0.87    1.10     1.12
[(La/Yb).sub.N]        2.69    2.79    3.71    0.38    4.20     4.53

Sample                 CE-97    CE-98

Si[O.sub.2]            71.44     70.4
[Al.sub.2][O.sub.3]    13.84    14.04
[Fe.sub.2][O.sub.3]     2.44     2.66
MnO                    0.032    0.047
MgO                      1.4     2.06
CaO                     0.53     0.58
[Na.sub.2]O             2.74     3.38
[K.sub.2]O              4.28     3.55
Ti[O.sub.2]            0.473      0.5
[P.sub.2][O.sub.5]      0.14     0.15
LOI (1)                 1.71      1.8
TOTAL                  99.03    99.17

Sc                        10        9
V                         18       16
Cr                       120      100
Co                         2        3
Ni                      < 20     < 20
Cu                      < 10     < 10
Zn                      < 30       40
Ga                        17       18
Rb                       129      110
Sr                        78       62
Y                       43.4     45.3
Zr                       146      163
Nb                      12.2     12.6
Cs                       2.3      3.2
Ba                       938      729
Hf                         4      4.5
Ta                      1.05     1.03
Pb                       < 5       25
Th                      14.5     15.9
U                       7.25     4.26

La                      40.9     44.2
Ce                      76.1     88.5
Pr                      9.35     10.5
Nd                      34.5     39.7
Sm                      6.94     7.97
Eu                      1.28     1.46
Gd                      6.86     8.04
Tb                      1.14      1.3
Dy                      6.52     7.48
Ho                      1.32     1.45
Er                       4.1     4.24
Tm                      0.61    0.629
Yb                      3.72     3.96
Lu                     0.536    0.563
[SIGMA] REE           193.88   219.99
Eu/Eu *                 0.57     0.56
[(La/Sm).sub.N]         3.64     3.42
[(Gd/Yb).sub.N]         1.47     1.62
[(La/Yb).sub.N]         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.
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Author:Arenas, R.; Martinez, S. Sanchez; Castineiras, P.; Jeffries, T.E.; Fernandez, R. Diez; Andonaegui, P
Publication:Journal of Iberian Geology
Article Type:Report
Date:Jan 1, 2009
Words:22568
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