Journal of Petrology Advance Access originally published online on July 25, 2005
Journal of Petrology 2005 46(12):2527-2568; doi:10.1093/petrology/egi064
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Lithospheric Mantle Evolution during Continental Break-Up: The West Iberia Non-Volcanic Passive Margin
1 LABORATOIRE MAGMAS ET VOLCANS, CNRS UMR 6524, UNIVERSITÉ BLAISE PASCAL ET OPGC, 5 RUE KESSLER, 63038 CLERMONT-FERRAND, FRANCE
2 DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI PAVIA AND CNR-IGG, VIA FERRATA 1, 27100 PAVIA, ITALY
3 ENS LYON, LABORATOIRE DE SCIENCES DE LA TERRE, 46 ALLÉE D'ITALIE, 69364 LYON, FRANCE, NOW AT CENTRE DE RECHERCHES PÉTROGRAPHIQUES ET GÉOCHIMIQUES (CRPG-CNRS), 54501 VANDOEUVRE-LÈS-NANCY CEDEX FRANCE
RECEIVED MARCH 15, 2004; ACCEPTED JUNE 15, 2005
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONTHE GALICIA MARGIN AND...SAMPLE DESCRIPTIONSANALYTICAL TECHNIQUESMAJOR AND TRACE ELEMENT...Sr-Nd ISOTOPIC GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Ultramafic (lherzolites, metasomatized peridotites, harzburgites, websterites and clinopyroxenites) and mafic igneous (basalts, dolerites, diorites and gabbros) rocks exposed at the sea-floor along the West Iberia continental margin represent a rare opportunity to study the transition zone between continental and oceanic lithosphere. The igneous rocks are enriched in LREE, unlike North Atlantic MORB. A correlation between their 143Nd/144Nd isotopic composition and Ce/Yb ratio suggests that they originate from mixing between partial melts of a depleted mantle source similar to DMM and of an enriched mantle source which may reside within the continental lithosphere. Clinopyroxenes and amphiboles in the ultramafic rocks are LREE depleted and have flat HREE patterns with concentrations higher than those of abyssal peridotites. Clinopyroxenes in the harzburgites are less LREE depleted but have lower HREE concentrations. The clinopyroxenes in the Galicia Bank (GB) lherzolites have radiogenic Nd (143Nd/144Nd ranging from 0·512937 to 0·513402) and unradiogenic Sr (87Sr/86Sr ranging from 0·702100 to 0·702311) isotopic ratios similar to, or higher than, DMM (Depleted MORB Mantle) whereas the clinopyroxenes in the Iberia Abyssal Plain websterites have low-Nd isotopic compositions (143Nd/144Nd ranging from 0·512283 to 0·512553) with high-Sr isotopic ratios (87Sr/86Sr ranging from 0·704170 to 0·705919). Amphiboles in Galicia Bank lherzolites and diorites have Nd–Sr isotopic compositions (143Nd/144Nd from 0·512804 to 0·512938 and 87Sr/86Sr from 0·703243 to 0·703887) intermediate between those of the clinopyroxenes from the Galicia Bank and the Iberia Abyssal Plain, but similar to the clinopyroxenes in the 5100 Hill harzburgite (143Nd/144Nd = 0·512865 and 87Sr/86Sr = 0·703591) and to the igneous rocks (143Nd/144Nd ranging from 0·512729 to 0·513121 and 87Sr/86Sr ranging from 0·702255 to 0·705109). The major and trace element compositions of cpx in the Galicia Bank spinel lherzolites provide evidence for large-scale refertilization of the lithospheric upper mantle by MORB-like tholeiitic melts. The associated harzburgites did not undergo partial melting during the rifting stage, but, in earlier times, probably during, or even before, the Hercynian orogeny. Iberia Abyssal Plain websterites are interpreted as high-pressure cumulates formed in the mantle. Their high Sm/Nd ratios (from 0·43 to 0·67) coupled with very low-Nd isotopic compositions are best explained by a two-stage history: formation of the cumulates from the percolation of enriched melts long before the rifting, followed by low-degree partial melting of the pyroxenites, accounting for their LREE depletion. This last event probably occurs during the rifting episode, 122 Myr ago. The isotopic heterogeneities observed in the ultramafic rocks of the Iberia margin were already present at the time of the rifting event. They reflect a long and complex history of depletion and enrichment events in an old part of the mantle, and provide strong arguments for a sub-continental origin of this part of the upper mantle.
KEY WORDS: Iberia margin; mantle peridotites; igneous rocks; petrology; geochemistry
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONTHE GALICIA MARGIN AND...SAMPLE DESCRIPTIONSANALYTICAL TECHNIQUESMAJOR AND TRACE ELEMENT...Sr-Nd ISOTOPIC GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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The study of continental passive margins provides important insights into ocean-opening processes. A detailed knowledge of these margins is, therefore, essential for understanding sea-floor spreading and mantle dynamics (Boillot, 1981
). Together with information provided by the petrology of pre-, syn- and post-rift sediments, and by the structural arrangement of tilted fault blocks in the brittle zone, the characteristics and origin of the igneous rocks associated with passive continental margins, as well as the characteristics of the underlying upper mantle, are essential data needed to describe the structural transition between continental and oceanic domains, and to develop geodynamic models of ocean formation. The occurrence of two main types—non-volcanic vs volcanic—of passive margin (e.g. Eldholm et al., 1995) emphasizes the importance of mantle processes in continental break-up, particularly the degree of partial melting and the efficiency of lithospheric thermal erosion.
Passive margins and associated continent–ocean transition zones represent the oldest part of the oceanic domain; as a consequence, they are generally hidden. They may be covered by sedimentary layers up to 10 km thick (e.g. the west-Atlantic north-American margin; Diebold et al., 1988) or by volcanic rocks (seaward dipping reflector sequences or SDRS, e.g. the 5 km thick sequence on the SE Greenland passive margin; Larsen et al., 1994). Therefore, the deeper zones of the margins—continental crust, oceanic crust and upper mantle—commonly escape direct observation by dredging and drilling. The characteristics of these deeper zones are known only from geophysical investigations (e.g. Boillot, 1981). The occurrence of magnetic anomalies allows the limits of the oceanic crust to be identified. On the other hand, gravity studies and the variation in seismic wave velocity beneath the margins allow us to describe the geometrical relationships between sediments, continental crust, oceanic crust and mantle. On the basis of such investigations, several different models of rifting and continental break-up have been proposed, among these the classic model of McKenzie (1978). This pure-shear extension model is characterized by the symmetrical deformation of the continental crust and lithospheric mantle with respect to the axial zone of the rift, which is marked by asthenospheric bulging.
Wernicke (1985), based on his work in the Basin and Range Province of the western United States, proposed a continental rifting and spreading model based on simple extensional shear mechanisms. His model emphasizes the role of large, simple-shear, detachment faults, which are strongly asymmetric structures, and, especially, of asthenospheric bulging shifted with respect to the rift axis by several tens of kilometres. Consequently, the upper mantle reaching the rift floor is typically thinned old continental lithosphere rather than newly formed oceanic lithosphere such as might form by the cooling of the asthenosphere intruded during extension.
Wernicke's model has been applied for example to the Southern Red Sea (Voggenreiter et al., 1988), as well as to some sections of the former passive margin of the Tethys Ocean which are still recognizable in the Alps (Lemoine et al., 1987; Florineth & Froitzheim, 1994; Froitzheim & Manatschal, 1996; Manatschal & Nievergelt, 1997). It has also been proposed for the East Atlantic passive margin, along the western coast of Spain and Portugal, where detailed geological and geophysical studies have been performed during the last 25 years (e.g. Boillot et al., 1987, 1988; Beslier et al., 1993; Sawyer et al., 1994; Beslier et al., 1995; Boillot et al., 1995; Abe, 2001; Hébert et al., 2001; Whitmarsh et al., 2001). In few of these studies, however, has the nature of the mantle rocks beneath the rift structures been completely characterized. This study provides new geochemical data for the Galicia Margin and Iberia Abyssal Plain mantle peridotites and igneous rocks. These data constrain the origin and evolution of the upper mantle peridotites exhumed during the pre-rift to oceanization stages of the Central North Atlantic, as well as the petrogenesis of the igneous rocks emplaced during the initiation of continental break-up and subsequent sea-floor spreading.
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THE GALICIA MARGIN AND THE IBERIA ABYSSAL PLAIN |
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The East Atlantic, Iberia, non-volcanic passive margin is covered by a thin sequence of sediments (Sibuet, 1992
) and, thus, provides a very favourable area in which to study the ocean–continent transition. For this reason, a number of investigations have been performed in this zone, including three ODP Legs (Leg 103: Boillot et al., 1987, 1988; Leg 149: Sawyer et al., 1994; Leg 173: Whitmarsh et al., 1998) and two diving cruises of the submersible Nautile (Galinaute I and II: Boillot et al., 1988, 1995).
The oceanic crust to the west of the continental margin is clearly identified by the 120·4 Ma MØ magnetic anomaly (Srivastava et al., 1990), shown in Fig. 1. The continental edge, to the east, is characterized by several tilted fault blocks, based on bathymetry and seismic reflection profiles (Sibuet et al., 1987; Thommeret et al., 1988; Thomas et al., 1996; Whitmarsh et al., 1996). The continental nature of these blocks has been established from dredge, core and dive samples (Groupe Galice, 1979; Boillot et al., 1987, 1988).
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Mantle peridotites are exposed at the sea-floor between these two well characterized domains. Dredging of the 5100 Hill (Fig. 1; Boillot et al., 1980
) suggests that they occur in a series of tilted blocks detected by geophysical investigations, drilling and diving (Boillot et al., 1980, 1987; Beslier et al., 1993; Sawyer et al., 1994; Whitmarsh et al., 1996). They form a N–S trending ridge almost 500 km long and parallel to, but eastwards of, the MØ magnetic anomaly (Fig. 1). The track of this ultramafic ridge is lost southwards, beneath the Tago Abyssal Plain, but it can be identified on the Gorringe Bank from which peridotites and serpentinites have been collected (Auzende et al., 1979). This ultramafic ridge is a distinctive feature of the west Atlantic passive margin but does not have any equivalents elsewhere in the world.
The peridotites exhibit a high-temperature foliation and subsequent low-temperature deformation textures associated with serpentinization and calcitization. These are interpreted to have developed during mantle upwelling related to successive phases of continental rifting (Evans & Girardeau, 1988; Girardeau et al., 1988; Beslier et al., 1988). The peridotites are associated with several different generations of magmatic rocks (e.g. Féraud et al., 1988; Kornprobst et al., 1988; Boillot et al., 1995; Seifert et al., 1997; Charpentier et al., 1998). These include amphibole-diorites, which occur as diffuse and very thin (several centimetres thick) sheets parallel to the foliation of the peridotites, but which have typically experienced very little high-temperature deformation. It is believed that the emplacement of these sheets was synchronous with the end of the high-temperature deformation stage in the peridotites (122·0 ± 0·6 Ma; Féraud et al., 1988). There are also weakly deformed (at low temperature) gabbros and pyroxenites (122·1 ± 0·3 Ma; Schärer et al., 1995); late-stage dolerite dykes cross-cut all the earlier structures. All these rocks are covered by basaltic lava flows that are older than the MØ magnetic anomaly. As a result, the emplacement of the peridotites is constrained to be synchronous with the end of the rifting stage.
On the basis of mineralogical data, the peridotites from the ultramafic ridge were initially considered as fragments of sub-continental lithospheric mantle (Boillot et al., 1980; Kornprobst et al., 1981; Kornprobst & Tabit, 1988) rather than originating from the asthenosphere; this opinion was also supported by a single Nd isotopic ratio obtained on a Cr-diopside from a harzburgite sampled at the 5100 Hill (143Nd/144Ndmeasured = 0·512865 ± 16; Charpentier et al., 1998) which was significantly lower than the average DMM value of 0·51313 (Su & Langmuir, 2003). According to this hypothesis, subsequently supported by further studies (e.g. Cornen et al., 1999; Hébert et al., 2001) and in agreement with the analogue modelling of Brun & Beslier (1996), the peridotites would have been exhumed from the deep, plastic part of the lithospheric mantle during the rifting process. However, on the basis of the occurrence of plagioclase in the peridotites, coupled with the evidence for limited melt extraction (about 10%), other workers (Evans & Girardeau, 1988) have suggested that the upper mantle in this area originated from the asthenosphere; if correct, the peridotites from the ultramafic ridge would represent the earliest oceanic lithosphere of the North Atlantic region.
The chemical and isotopic characteristics of the igneous rocks associated with the peridotites are rather variable. Some gabbros have N-MORB-like trace element and Sr–Nd isotope characteristics (Seifert & Brunotte, 1996; Seifert et al., 1997; Schärer et al., 2000) and could have crystallized from melts which originated in the asthenosphere. Other gabbros and pyroxenites, as well as the dolerites and basalts, are transitional between compositions relatively enriched in incompatible elements, and more depleted compositions, closer to N-MORB (Kornprobst et al., 1988; Charpentier et al., 1998; Cornen et al., 1999). The parental magmas could, thus, have been due to more or less pronounced interaction between the sub-continental lithosphere and melts extracted from the asthenosphere (Charpentier et al., 1998).
To resolve the petrogenesis of the exhumed ultramafic rocks, a detailed study was carried out on peridotites and pyroxenites from the West Iberia ultramafic ridge. Because of the intense serpentinization of most of the samples, this study focused only on the less altered rock-forming minerals: clinopyroxene, amphibole and plagioclase from peridotites, pyroxenites and diorites. These minerals were analysed by electron microprobe for major elements, by ICP-MS for trace elements, and by thermal ionization and plasma source mass spectrometry for Sr–Nd isotopic geochemistry. The peridotite samples analysed are all from the Galicia Margin ultramafic ridge (dives 26, 28, 30, 32, 33, 35 and 36 of Galinaute II cruise: Boillot et al., 1995; dredge H78DR24 on the 5100 Hill: Boillot et al., 1980). One of the websterites comes from Galinaute II, dive 28; the other websterites, as well as a clinopyroxenite, were cored at site 897, ODP leg 149, cores 64, 65, 66 and 67 (Sawyer et al., 1994). Amphibole diorites were sampled during dives 10, 32 and 37 of Galinaute I and II (Boillot et al., 1988, 1995). In addition, magmatic rocks (basalts, dolerites, pyroxenites and gabbros) collected during the Galinaute I and II campaigns (dives 12, 13, 14, 15, 17, 21, 22, 31, 33 and 34) were studied for the composition of their clinopyroxenes and for their REE contents and Sr–Nd isotopic composition. (Details of sample locations are given in Table 1 and Figs 1, 2 and 3.)
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SAMPLE DESCRIPTIONS |
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Except in some favourable cases, small-scale submarine geological structures are not easy to observe, neither from a submersible nor by drilling. Additionally, most ultramafic and igneous samples are rather extensively altered, which makes precise textural and modal studies of the rocks difficult. As a result, petrological descriptions of submarine samples cannot be as accurate as they are for rocks collected at the Earth's surface.
Ultramafic rocks: peridotites and pyroxenites
Although generally extensively altered and cross-cut by late serpentinite and carbonate vein networks, these rocks clearly exhibit porphyroclastic textures typical of ultramafic tectonites. The porphyroclasts (up to 1 cm long; of orthopyroxene, clinopyroxene and spinel) are more or less elongated, parallel to the main foliation of the rock (Girardeau et al., 1988; Beslier et al., 1988). When present, olivine forms a fine-grained matrix with a mosaic (or sometimes mylonitic) texture; the almost complete transformation of olivine into serpentinite makes the textural description of the matrix effectively impossible. Orthopyroxene is also altered but the cores of some crystals are still preserved and exhibit thin clinopyroxene exsolution lamellae. In contrast, clinopyroxene, spinel and brown amphibole have apparently resisted this alteration. When present, secondary plagioclase is either fresh, or altered to brownish argillaceous products. In addition to serpentine and carbonate, the late-stage low-temperature mineral association also includes amphibole (tremolite–actinolite), chlorite and magnetite. For the description of the ultramafic samples, three localities have been distinguished (Fig. 1): the Galicia Bank (GB), the 5100 Hill and the Iberia Abyssal Plain (IAP).
On the Galicia Bank, two main rock types have been identified among the 23 ultramafic samples that have been collected. The first type is represented by spinel peridotites; most of these rocks contain green or brown–green spinel and pale green clinopyroxene, the latter forming 5–10% of the modal mineralogy of the rocks, which can, therefore, be classified as lherzolites. One sample (33·06) is clinopyroxene-poor (less than 5%), contains brown spinel and is classified as a harzburgite. One of the lherzolite samples (28·02) contains a sheet of spinel websterite which lies parallel to the main high-temperature foliation of the surrounding peridotite. This sheet is about 2 cm thick and is made of deformed subhedral green clinopyroxene crystals (50%), several millimetres in size, associated with anhedral green spinel (15%); the orthopyroxene appears completely altered in thin section but was identified after sample crushing. This websterite layer is quite similar to those observed in most orogenic lherzolites, which are generally considered to be high-pressure cumulates left behind from percolating melts (Kornprobst, 1969; Fabriès et al., 1991).
The second major rock-type from the Galicia Bank is represented by amphibole-bearing lherzolites, referred to subsequently as metasomatized lherzolites. In these samples, amphibole occurs as interstitial, weakly deformed, crystals, as small brown rims around clinopyroxene or spinel, and as spindle-shaped lamellae and blebs within the clinopyroxene; the latter look like exsolution features but are more consistently interpreted as reaction products. Except for sample 36·02, which is plagioclase-free, all the metasomatized peridotites from the Galicia Bank contain secondary plagioclase associated with brown amphibole which form coronites around spinel (Fig. 4f). The deep brown to black spinel is strongly corroded; in some instances, it is directly surrounded by plagioclase, whereas amphibole makes up the outer rim of the coronite; in other instances, the brown amphibole defines an inner rim and is surrounded by plagioclase or its alteration products. A careful thin section study did not detect any plagioclase, nor its alteration products, outside of these coronites, in contrast to mantle rocks in which plagioclase has crystallized as a consequence of melt impregnation processes (Nicolas, 1986; Rampone et al., 1997). The amphibole–plagioclase coronite around spinel precludes an origin attributable only to the destabilization of spinel (+opx +cpx) as a consequence of decompression. The reaction observed clearly occurred in an open system involving the percolation of a hydrous fluid, or melt, accounting for amphibole crystallization. The close association of plagioclase with corroded dark spinel and the lack of plagioclase in the amphibole-bearing sample 36·02 indicate, however, that the percolating fluid, or melt, was not, by itself, the cause of plagioclase crystallization in the rocks. Instead, the fluid or melt could simply have acted as a flux which induced the spinel destabilization reaction.
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At the 5100 Hill site, a number of peridotite samples have been collected by dredging (H78DR24), drilling (leg 103, hole 637A) and by submersible (Galinaute I, dive 4). All these rocks are highly altered. Clinopyroxene-poor (less than 5%), these peridotites are, therefore, classified as harzburgites, and contain either brown spinel, or dark spinel + plagioclase associations in which plagioclase forms narrow elongated, coronites around the spinel. This feature was interpreted as occurring due to the destabilization of the opx + cpx + sp assemblage due to decompression (Evans & Girardeau, 1988
; Kornprobst & Tabit, 1988). Plagioclase has also been described, forming veins in some samples (Girardeau et al., 1988); these veins are very narrow (less than 0·5 mm) and are discontinuous and dispersed, in contrast to the plagioclase networks described as a melt impregnation feature by Rampone et al. (1997) in ophiolitic peridotites. For the above reasons, these plagioclase veins in the 5100 Hill peridotites are simply considered as parts of elongated coronites around spinel, tangentially cut by the thin section plane. In the Iberia Abyssal Plain, very small peridotite samples have been recovered from two ODP sites (holes 897 and 899) of Leg 149. Three peridotites were collected from an ultramafic breccia on the top of site 899. These are clinopyroxene-poor or even clinopyroxene-free (or, at least, clinopyroxene is not observed in the thin sections) harzburgites containing dark brown elongated spinel, sometimes surrounded by a thin plagioclase rim. Plagioclase and its alteration products are not observed outside of these coronites around spinel. The scarcity or apparent lack of clinopyroxene account for the fact that so little plagioclase crystallized around spinel by decompression of these harzburgites. Olivine is well preserved but is cross-cut by numerous microfractures, making the relatively coarse mosaic texture (grain size about 2 mm) difficult to observe. Orthopyroxene, as relatively small crystals (3 mm large), does not make up more than 10% of the mode.
On the top of site 897, irregular pyroxenite lenses, several to 10 cm thick, are disseminated in a darker serpentinite matrix, within about 5 m of the drill-core (Sawyer et al., 1994). The ultramafic matrix is completely serpentinized and does not contain any primary peridotitic phases, except for dark spinel surrounded by thin altered, plagioclase rims. The pyroxenites are mainly spinel–websterites in which the clinopyroxene is fresh and contains thick (10–20 µm) exsolution lamellae of orthopyroxene (Fig. 4e). Opx, which is significantly serpentinized in these rocks, contains thin (<1 µm thick) cpx lamellae. Brown spinel is abundant (10–15% of the mode), always surrounded by large (up to 1 mm) zones elongated in the foliation, made of plagioclase and olivine (less than 5%) in apparent textural equilibrium (Fig. 4c and d). One of these pyroxenite lenses in the serpentinites is a fresh porphyroclastic clinopyroxenite (Fig. 4a and b); it contains approximately 60% cpx, 35% spinel + plagioclase + olivine intergrowth (olivine representing about 5% of the whole rock). Similar to the GB websterite (28·02), these pyroxenites are considered to be high-pressure cumulates left by melts infiltrating the surrounding peridotites. In both websterites and the clinopyroxenite, olivine occurs only in association with plagioclase around spinel. The large amount of plagioclase in these websterites and clinopyroxenite led Cornen et al. (1999) to consider that the feldspar crystallized from a liquid which impregnated the pyroxenites before the development of the high-temperature foliation. The textural features, the lack of any plagioclase veins in the rocks, as well as mineralogical and chemical observations (see below) lead us to consider once again that this association is a typical result of the subsolidus crystallization of plagioclase in clinopyroxene- and spinel-rich assemblages, the role played by the impregnating melt or fluid being nil or extremely small.
Igneous rocks
The peridotites of the ultramafic ridge are intruded or overlain by different kinds of igneous rocks that were emplaced at different times. These rocks are described in the order of their time of emplacement.
Diorites
Diorite veins, 1 to several centimetres thick, have been observed in the Galicia Bank (dives 10, 32 and 37), parallel to the foliation of the host amphibole-bearing peridotite (Beslier et al., 1988; Féraud et al., 1988). They often display a porphyroclastic texture resulting from high-temperature recrystallization. Nevertheless, the presence of ductiley deformed dykes coexisting with undeformed ones suggests that the injection of magma must have been syn- to post-tectonic (e.g. Beslier et al., 1988). These rocks contain plagioclase (An35), brown amphibole and ilmenite and have been dated at 122·0 ± 0·6 Ma (39Ar/40Ar; Féraud et al., 1988). Except for the pyroxenite layer in the GB peridotites, the diorites represent the earliest igneous products identified on the Iberia margin. Their association with the amphibole-bearing peridotites is to be emphasized. In mylonitic zones (e.g. sample 37·08), amphibole-bearing dykes have been tectonically dispersed within the surrounding peridotites.
Pyroxenite
A layered pyroxenite was sampled at dive site 14, just above a serpentinite (Figs 2 and 3). This rock is slightly altered and consists mainly of clinopyroxene, subordinate orthopyroxene and plagioclase. The coarse granular primary texture is partially overprinted by weak schistosity (Beslier et al., 1988). Opx and cpx look like cumulus phases, whereas plagioclase is interstitial. This pyroxenite, with its still recognizable igneous structure and texture, was emplaced after the diorite veins described above, and crystallized from a silica-rich melt, as demonstrated by the occurrence of opx in the cumulus assemblage.
Gabbros
Gabbros have been observed in a single occurrence, almost all along the cross-section explored during dive 34 (Figs 2 and 3). Eight samples were collected from an outcrop which is about 600 m high. The texture is coarse, porphyroclastic to granuloblastic, due to the development of a weak foliation which, frequently, does not completely obliterate the original gabbroic cumulate texture. Cpx appears as porphyroclasts (up to 1 cm in size), more or less recrystallized into much smaller neoblasts (0·3–0·4 mm). Plagioclase is the main constituent by volume (70–80%); the initial magmatic crystals are almost completely recrystallized in the granuloblastic matrix and the neoblasts do not exceed 0·5 mm in size. Fresh olivine was not observed, but some clusters of secondary hydrous phases (serpentinite and chlorite) may represent former olivine crystals. Interstitial brown amphibole may be due to the late magmatic crystallization of the intercumulus liquid.
Dolerites
Dolerite samples were collected from breccias, or from dykes cross-cutting the peridotites (dives 14, 28, 33). These rocks have ophitic textures and, in one single occurrence (14·07), a porphyritic texture with cpx and ol phenocrysts. The main mineral phases are plagioclase and clinopyroxene, along with accessory brown amphibole, ilmenite and apatite. Biotite occurs in one sample (14·07), as well as sub-idiomorphic dark brown spinel. Generally, the dolerites exhibit well developed, secondary, low-temperature mineral associations involving mainly epidote, clinozoisite, chlorite and actinolite.
Basalts
Basalts were sampled at dive sites 12, 13, 15, 17, 21, 31 and 34, from lava flows, pillowed or not, or from isolated pillows dispersed on the sea-floor. The basalts generally have a porphyritic texture, with up to 20% of large plagioclase phenocrysts (up to 2 cm long). Cpx and ol microphenocrysts (1 or 2 mm across) also occur, as well as an ilmenite–titanomagnetite association. The originally glassy matrix is extremely fine-grained. Apart from a surprisingly fresh hyaloclastite (sample 34·09), and one basalt (17·09), all these rocks are strongly altered, their matrix and the olivine microphenocrysts being completely transformed into low-temperature hydrous phases. Plagioclase phenocrysts are partially altered to clay-minerals and adularia, whereas cpx is the only mineral phase which remains unaltered in almost every sample.
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ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONTHE GALICIA MARGIN AND...SAMPLE DESCRIPTIONSANALYTICAL TECHNIQUESMAJOR AND TRACE ELEMENT...Sr-Nd ISOTOPIC GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Major element compositions of the minerals were obtained using a CAMECA SX100 electron microprobe at the University of Clermont-Ferrand. The accelerating voltage during analysis was 15 kV, and current intensity was adjusted to 15 nA. Counting time was 20 s for Cr, Ni and Ti, and 10 s for all the other elements. The analytical error is between 1 and 5% for concentrations higher than 1%, and between 5 and 10% for concentrations lower than 1%.
The trace element composition of the magmatic rocks and of cpx and/or amphibole from some of the samples was analysed by solution ICP-MS, on a Fisons PQ2+ at the University of Clermont-Ferrand. For the minerals, a few milligrams were used. The analytical precision is better than 10% for all the elements (1
), and very often better than 5%, especially for the REE. Accuracy was checked by analysing the international standard BHVO and is better than 10% (1) for all the elements. Before trace element and isotope analysis, the minerals were leached using the following procedure: 30 min in HCl 2·5N, 15 min in HF 5%, 30 min in HCl 2·5N and 30 min in distilled water. Minerals were put in an ultrasonic bath for the last 5 min of each step and were rinsed with distilled water between each step.
When it was difficult to separate individual minerals in large amounts, the trace element composition of individual grains was obtained on polished thin sections (100 µm thick) by laser ablation ICP-MS at the CNR–Institute of Geosciences and Earth Resources, Section of Pavia. The laser probe consists of a double focusing sector field analyser (Finnigan Mat Element) coupled with a Q-switched Nd:YAG laser source (Quantel Brilliant) operating at 266 nm. Helium was used as carrier gas and was mixed with Ar downstream of the ablation cell. Most of the analyses were performed with a 40 µm diameter laser beam and energy of 0·7 mW. For the plagioclases, which are relatively poor in trace elements, the laser beam was enlarged to 80 µm and energy increased to 4 mW. Quantification was done using the SRN NIST 612 glass as an external standard, with 44Ca as an internal standard. Data reduction was done using the GLITTER software package (www.es.mq.edu.au/gemoc/glitter). Precision and accuracy (both better than 10%) were assessed from repeated analyses of the BCR2-g standard. The reported values correspond to the average of several analyses (up to five analyses from different grains) from the same thin section. Full details of the analytical parameters and quantification procedures can be found in Tiepolo et al. (2003).
Sr and Nd isotope analyses were performed on whole-rocks or on optically pure separated mineral grains (approximately 50 mg for amphibole, 100 mg for clinopyroxene and 200 mg for plagioclase). Nd and Sm concentrations were determined by isotope dilution. Elements to be analysed were separated on chromatographic columns (AG50X4, TRU-spec and Ln-spec). For Sr, all the samples were analysed by thermo-ionization mass spectrometry after loading on a single Ta filament. 87Sr/86Sr values were normalized to 86Sr/88Sr of 0·1194. The average value of the NBS987 standard Sr isotope composition measured during this study is 0·710228 ± 17 (n = 6). Some of the Sm–Nd analyses were also performed by thermo-ionization mass spectrometry at the University of Clermont-Ferrand using triple Ta–Re filaments. 143Nd/144Nd values were normalized to 146Nd/144Nd of 0·7219. The average value of the LaJolla standard Nd isotope composition measured during the course of this work is 0·511832 ± 7 (n = 4). The other analyses were obtained by multi-collector ICP-MS on a VG plasma 54 at the Ecole Normale Supérieure of Lyon. The analytical methods for these analyses have been described in Luais et al. (1997).
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MAJOR AND TRACE ELEMENT GEOCHEMISTRY OF MAIN MINERAL PHASES AND IGNEOUS ROCKS |
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TOPABSTRACTINTRODUCTIONTHE GALICIA MARGIN AND...SAMPLE DESCRIPTIONSANALYTICAL TECHNIQUESMAJOR AND TRACE ELEMENT...Sr-Nd ISOTOPIC GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Major element composition of minerals in the ultramafic rocks
Olivine
In most samples, olivine is replaced by serpentine and carbonates and is impossible to analyse. Thus, no data are reported for the GB spinel–lherzolites. The analysed olivines (Table 2 and Electronic Appendix 1, available at http://www.petrology.oupjournals.org) show a range of compositions in agreement with the nature of the host-rock. In the IAP harzburgites, the Fo-content is high (90·1–91·9), although significantly lower in the metasomatized lherzolites from GB (88·7–90·0). In the IAP pyroxenites, the olivine Fo-content is lower still (85·5–89·1). The NiO-content ranges from 0·26 to 0·38 wt %, except in the IAP clinopyroxenite (0·15
NiO 0·25%) and in the websterite 897C 64R05 090–100 cm in which the rather Fo-poor olivine (85·5) is also poor in nickel (NiO = 0·23 wt %).
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Orthopyroxene
The opx also varies in composition according to the nature of the host-rock. The Mg* ratio (= Mg/(Mg + Fe + Mn) cations proportions) ranges from 0·90 to 0·92 in the harzburgites, and from 0·89 to 0·90 in the spinel–lherzolites (Table 3 and Electronic Appendix 1). In the IAP websterites, as well as in the GB metasomatized peridotites, Mg* is rather heterogeneous, even in a single crystal, and ranges from 0·86 to 0·90 and from 0·88 to 0·91, respectively. The En-content in opx is fairly well correlated with the Fo-content in the associated olivine (R2 > 0·91; Fig. 5a). This suggests that the harzburgites have experienced a much greater degree of melt extraction than the lherzolites. The opx alumina content is also variable, ranging from 6·0 to 1·7 wt %, depending on the rock-type (Table 3), and decreasing from core to rim of the individual porphyroclasts. Although the Al-content behaviour with respect to Mg* is rather unclear in the IAP websterites (not shown), a negative correlation (R2 > 0·68) is evident for the opx in the peridotites, due to increasing melt extraction from lherzolites to harzburgites (Fig. 5b); the opx in the metasomatized peridotites from GB defines part of this correlation but also show decreasing Al-content without any significant variation in the Mg*-number (Fig. 5b); this is probably related to sub-solidus re-equilibration with secondary amphibole.
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Clinopyroxene
The clinopyroxenes analysed are mainly diopside or Mg–Ca-rich augite (Table 4 and Electronic Appendix 1). Their Mg* ratios are variable according to the nature of the host-rock, with the highest values recorded in the harzburgites (0·91–0·94) and the lowest in the pyroxenites (0·85–0·91). Cpx from the lherzolites have intermediate Mg*-number ranging from 0·89 to 0·92 (Fig. 5c). The metasomatized peridotites from GB are characterized by cpx with a large range of Mg* values, intermediate between the harzburgite and the pyroxenite clinopyroxene. A positive correlation appears between cpx and opx Mg*-numbers from harzburgites, sp-lherzolites and pyroxenites (Fig. 5c); in contrast, a negative correlation is observed between cpx and opx from the GB metasomatized peridotites; this may be related to subsolidus Mg–Fe exchange between cpx and the associated amphibole. Some specific elements, such as Ti, Na, Cr and Al, also provide significant information. A rough positive correlation (R2 = 0·5, not shown) between Cr and Mg* in cpx provides supporting evidence that the harzburgites are depleted with respect to the lherzolites, and could represent residues after melt extraction; in contrast, the iron-rich websterites and the clinopyroxenite appear to be relatively fertile (i.e. able to provide a significant amount of partial melt at relatively low T) with respect to the associated peridotites. Taking into account the behaviour of Na, Cr and Ti (Fig. 5e and f), things look a bit more complicated. In the GB sp-lherzolites and associated websterite, the cpx is Na- and Ti-rich (1·46
Na2O wt % 2·20; 0·33 TiO2 wt % 1·23) and compares with the most Na- and Ti-rich cpx in any orogenic lherzolite (Kornprobst et al., 1981; Kornprobst & Tabit, 1988); therefore, melt extraction from these rocks must have been extremely limited. On the contrary, the GB spinel lherzolites could have experienced strong modal re-enrichment by melt impregnation before the development of the high-temperature foliation. The associated GB websterite may represent a cumulate of that melt.
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Cpx in the harzburgites from GB and 5100 Hill (0·7
Na2O 1·48; 0·07 TiO2 0·45) are much poorer in Na than cpx in the sp-lherzolites, but still contain incompatible Na associated with Al in the jadeite molecule, together with compatible Na associated with the refractory Na–Cr kosmochlor molecule (Fig. 5f; see discussion in Kornprobst et al., 1981, 1982); therefore, these harzburgites did not experience a very high degree of melt extraction or were significantly re-enriched in Na after partial melting, but not as much as the associated lherzolites. Due to their high Cr-content, the IAP harzburgite cpx contain very little or no incompatible Na (as Na–Cr in cpx can even have negative values; Fig. 5e); so, instead of kosmochlor, Cr is incorporated into a chromium-bearing Ca-Tschermak type molecule X2CrAlSiO6. This is generally observed in the abyssal peridotites that have experienced a high degree of melt extraction (Kornprobst et al., 1981). However, in contrast with cpx in abyssal peridotites, the cpx in the IAP harzburgite contains a significant amount of Ti (0·32 TiO2 wt % 0·48) instead of almost 0 (Fig. 5e). The relatively iron-rich cpx from the IAP websterites and clinopyroxenite are also characterized by low-Na (0·12 Na2O wt % 0·65), and relatively high Ti- (0·18 TiO2 wt % 1·12) contents. In addition, cpx in the clinopyroxenite is also extremely poor in Cr (0·05–0·18 Cr2O3 wt %). Al and Cr are negatively correlated in cpx from IAP websterites (Fig. 5d). This feature has already been described in mantle peridotites (Rampone et al., 1993) and interpreted as being due to subsolidus plagioclase crystallization.
Spinel
In the ultramafic rocks from the Iberia margin, the spinel composition depends again on the bulk composition of the host-rock (Table 5 and Electronic Appendix 1). The Cr* number of the spinel is linearly correlated (R2 = 0·86, not shown) with the Mg*-number of the associated cpx in the GB sp-lherzolites and websterite, as well as in the harzburgites (GB, 5100 Hill and IAP). This is in agreement with the observations above, about the compositional variation of associated ol, opx and cpx in these rocks, reflecting an increasing amount of melt extraction from lherzolites to harzburgites. In contrast, spinel in the IAP websterites and clinopyroxenite, as well as in the metasomatized peridotites from GB, does not fit with this correlation. Instead, the sp in these latter rocks is strongly enriched in Cr for a moderate increase in Mg*-number in the associated cpx. On the other hand, Ti- and Cr-contents do not show any correlation in the spinel from the sp-lherzolites and harzburgites (Fig. 6a); the Cr-content increases from lherzolites to harzburgites, as expected as a consequence of a partial melting process; Ti-contents of the sp are extremely low in these rocks, except for a few analyses (five) of spinel in three different GB lherzolites (30·01, 30·02 and 35·03) which have relatively high Ti-contents (up to 0·46 TiO2 wt %). In contrast, Ti- and Cr-contents are fairly well correlated (R2 > 0·72) in the IAP websterites taken as a whole (Fig. 6a), although extremely low in the associated clinopyroxenite. Such enrichment of both Cr and Ti in spinel from the ultramafic rocks is related to secondary subsolidus crystallization of plagioclase (Kornprobst & Tabit, 1988; Rampone et al., 1993), as observed in thin section; furthermore, the equation of this correlation is almost identical to the model correlation calculated for the progressive removal of (Mg,Fe)Al2O4 from spinel (Fig. 6b); therefore, it is considered, especially in the IAP websterites, that the spinel compositional variations are strictly related to the subsolidus crystallization of plagioclase. The Ti- and Cr-concentrations in the spinel from the metasomatized peridotites are extremely dispersed, which is in agreement with both metasomatism and low-pressure recrystallization experienced together by the mineral paragenesis in these rocks.
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Plagioclase
Fresh and analysable plagioclase has only been found in the pyroxenites from the IAP and in the metasomatized peridotites from GB (Table 6 and Electronic Appendix 1). In the pyroxenites, the plagioclase is highly calcic (88·5
An 96·8), with slightly higher An-contents than recorded by Evans & Girardeau (1988) in the plagioclase bearing harzburgites from 5100 Hill (An81·5 and An73·7). Except for one single relatively albite-rich composition (An = 57·4) recorded from the highly serpentinized sample 37·08, the plagioclase in the metasomatized peridotites is also An-rich (An85–91). Although most plagioclases in the pyroxenites are almost totally devoid of the orthoclase component, the more albitic plagioclase (37·08) contains 0·47 wt % of K2O (i.e. Or = 2·7); this may signify that the fluid (or melt) phase responsible for the crystallization of amphibole in the metasomatized peridotites was also sufficiently Na- and K-rich to allow the crystallization of relatively alkaline secondary plagioclase in the coronites around the spinel. Furthermore, in one single sample (36·04), K-feldspar (Or99·6–Ab0·4) has been analysed in a deeply altered coronite; such an extremely potassic composition fits more with a hydrothermal phase (adularia), rather than with high-temperature crystallization.
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Amphibole
Amphibole was only observed in the metasomatized peridotites from GB, either associated with clinopyroxene and spinel, or as isolated crystals within the matrix. These amphiboles are Mg-pargasites (0·77
Mg* 0·91), with highly variable Ti- and Cr-contents (0·13 TiO2 wt % 5·47; 0·0 Cr2O3 2·43), even in a single sample (Table 7 and Electronic Appendix 1). No correlation appears between the concentrations of these two elements. This signifies that the amphibole did not crystallize directly from a percolating fluid or melt, but resulted from metasomatic reactions during which the spinel was destabilized.
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Major element composition of minerals in the igneous rocks
Olivine
Fresh olivine was only observed in one basalt (17·09) and as microphenocrysts in the hyaloclastite (34·09). In the latter, the Fo-content is fairly constant (78·0 ± 0·5), whereas in basalt 17·09, the Fo-content ranges from 78·0 in the microphenocrysts to 58·0 in the matrix.
Pyroxene
Orthopyroxene has only been observed in the layered pyroxenite from dive 14 (14·05); it is a bronzite (0·78 Mg* 0·80) very Al-rich (Al2O3 up to 7 wt %) and Ca-poor (0·25 CaO 0·55 wt %). In this rock, the cpx composition is also characterized by a relatively low Mg*-number (0·82–0·85), high Al-content (8·0 Al2O3 9·0 wt %), as well as a relatively high Na-content (0·80–1·22 Na2O wt %); these features, as well as the obvious cumulus texture of the host rock, make this pyroxenite quite different from the IAP porphyroclastic websterites and clinopyroxenite. In the gabbros, clinopyroxene is an augite (0·81 Mg* 0·86), but much poorer in Al and Na than the clinopyroxene in the pyroxenite. In the dolerites and basalts, all clinopyroxenes are augite with variable Mg*-ratios (0·83–0·45) depending on the degree of crystallization (data can be found in Electronic Appendix 2, available at http://www.petrology.oupjournals.org). Clinopyroxene phenocrysts (in basalt 13·01 and dolerite 14·07) are Mg-rich and have compositions close to that of clinopyroxene in the gabbro. The behaviour of Ti with respect to the degree of differentiation (Ti vs Mg*; Fig. 7a) is variable. In sample 13·01, the trend is close to that of continental alkali basalts (high Ti-content with a relatively low Mg*-number). In contrast, the clinopyroxene in the dolerites shows little or no Ti-enrichment, for a large range of Mg*. This is a typical behaviour of clinopyroxene compositions in continental tholeiites from the Atlantic margins, which consist mostly of quartz–tholeiites (e.g. Bertrand, 1991). Clinopyroxenes from the other GB basalts exhibit trends between these two extremes, as also do the Mid-Atlantic ridge olivine–tholeiites. Although the Ti-content in clinopyroxene, as a function of the degree of differentiation, could be related to several factors (e.g. the activity of silica in the melt, the oxygen fugacity and the cooling rate; Bertrand, 1991), the variation of Si-content in clinopyroxene with respect to Mg*-number (Fig. 7b) suggests that the activity of silica in the melt has been a deciding factor.
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Amphibole
Brown amphibole is a significant constituent of the diorite veins observed in some peridotite outcrops (dives 10, 32, 37). In these rocks, the amphiboles are pargasites much richer in Fe (0·50
Mg* 0·60) than those in the metasomatized peridotites, as well as being relatively poor in silica (40–42 wt % of SiO2 instead of 42–46%). The amphibole is also rather rich in TiO2 compared with that in the peridotites (4–5% instead of 0·0–5·0%) and almost completely free of Cr (Cr2O3 < 0·02% instead of 0·0–2·6%). It is clear that the amphiboles in the metasomatized peridotites did not crystallize directly from the same type of melt as represented by the diorite veins, but more probably were due to the chemical re-equilibration of a percolating melt or fluid with the ultramafic assemblage. The amphibole from the diorites tectonically dispersed within peridotites in mylonitic zones (e.g. 37·08) have intermediate compositions between the amphibole in the diorites and those in the mestasomatized peridotites (Fig. 7c). This may represent the way by which the igneous amphibole has been chemically transformed in a Mg- and Cr-rich, as well as Ti-poor, environment, by re-equilibration with the minerals from the peridotites, especially during the subsolidus recrystallization of spinel-bearing associations. Brown amphibole in the dolerites, as phenocrysts or interstital crystals, is relatively constant in composition and differs somewhat from both amphiboles in the dioritic veins and metasomatized peridotites, mainly in terms of its intermediate Mg*-number (0·72–0·76), and lower alumina content (9·0% Al2O3 instead of 9·5–15·5%).
Plagioclase
Plagioclase is present in all igneous rocks. In the diorite veins, it is rather rich in albite (An36–40), and poor in the orthoclase endmember (0·3 Or 1·2). In the pyroxenite 14·05, the interstitial plagioclase is more calcic (An57–59; Ab41–43; Or0–0·5). In the basalts and dolerites, the plagioclase phenocrysts are not optically zoned and show very little compositional variation (An80–88–Ab12–20), except for a very thin margin (<0·03 mm) much richer in the albite component (Ab up to 30). The Or-content is extremely low. The plagioclase laths in the dolerites and microliths in the basalts exhibit relatively large compositional variations (An60–40; Ab59–34; Or0·7–1·0) with continuous normal zoning.
Fe–Ti oxides
The basalts show the common association ilmenite + titanomagnetite, usually as very finely grained crystals, now oxidized, in the matrix. Microprobe analyses have been possible in only one sample (13·06). The average composition of the Fe–Ti oxide is close to Ülvospinel 80–Magnetite 20, indicating relatively low oxygen fugacity conditions (logfO2 = –10) for temperatures between 1050 and 1100°C (Lindsley, 1976). In the dolerites, the ilmenite is either the only titaniferous phase or appears together (but not in equilibrium) with a microcrystalline secondary titanite phase. The ilmenite in the dolerites is Mg-poor and Mn-rich; its hematite content is very low or nil. If these compositions are primary, they would indicate more reducing conditions than those recorded in basalt 13·06.
Trace element composition of minerals in ultramafic rocks
Due to heavy alteration of most samples, trace element analyses were carried out (in situ or after mineral separation) on minerals rather than on whole-rocks. The data are listed in Tables 8, 9 and 10.
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Clinopyroxene
The cpx have low REE contents, with flat HREE patterns, and are all relatively depleted in LREE (Fig. 8). Cpx from the GB metasomatized peridotites are less depleted in LREE for the same HREE content compared with cpx from the IAP websterites. Some samples have peculiar cpx compositions: cpx in the harzburgite 33·06 from the GB and in the clinopyroxenite 897C 66R04 067–070 cm from the IAP have low REE content with flat patterns and only slight depletion in LREE. They are quite similar to cpx in harzburgite H78DR24 from the 5100 Hill. Note that among the IAP samples, the cpx from the harzburgite is less depleted in LREE than those from the websterites. Most of the analysed cpx have REE concentrations higher than those of abyssal peridotites (Johnson and Dick, 1992
).
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Most other trace elements are present in very low abundances and are often below the detection limit of the ICP-MS or LA–ICP-MS techniques. However, the cpx from the metasomatized samples have high Zr- and Hf-concentrations (Zr between 40 and 75 ppm, whereas other cpx have Zr-contents ranging from 2·5 to 32 ppm). The cpx in the lherzolites from the GB, as well as the 5100 Hill harzburgite, have the highest Sr-concentrations (from 18 to 60 ppm), whereas in the IAP websterites, they have very low Sr-contents, below 10 ppm. The metasomatized samples contain cpx with variable Sr-contents (from 5 to 22 ppm).
Plagioclase
This mineral is often altered and, thus, difficult to analyse for its trace element concentrations. Data have only been obtained for several pyroxenites (six websterites and one clinopyroxenite) from the IAP. The plagioclases contain low amounts of REE, are slightly enriched in LREE compared with the HREE and all have a strong positive Eu anomaly (Fig. 9). All the REE patterns show depletion in La compared with Ce; this is less significant in the clinopyroxenite, which has lower REE concentrations than the websterites. Associated with the high Eu-content, all the plagioclases have high Sr-concentrations, with values ranging from 12 up to 100 ppm. Plagioclase in the dioritic vein 37·07 has a much higher Sr- and Ba-content, and also higher concentrations of REE.
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Amphibole
All the amphiboles analysed have similar flat REE patterns, with a slight depletion in LREE (Fig. 10). Only the amphibole from the metasomatized peridotite 32·05 and from the diorite 37·08 shows LREE enrichment. In all the samples in which cpx and amphibole have been analysed, both minerals have similar REE patterns and abundances, which is what we could expect if these minerals are in chemical equilibrium (Chazot et al., 1996
). The Sr-content of these amphiboles is variable, ranging from 32 to 150 ppm. Amphiboles from the dioritic veins have REE patterns similar to the amphiboles in the peridotites but are four to ten times more enriched in REE. They also have a more pronounced negative Eu anomaly.
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Trace element composition of whole igneous rocks
Most of the igneous rocks collected on the Galicia Bank (GB) have experienced hydrothermal alteration. The whole-rock analyses have loss on ignition values up to 9·63% and cannot, therefore, be considered as representative of the original composition of the samples, especially from the point of view of K2O- and Na2O-contents. Only two whole-rock analyses (basalt 17·09 and hyaloclastite 34·09) provide useful data. The basalt is an alkali-rich olivine tholeiite, whereas the hyaloclastite is a quartz tholeiite. Although almost all elements are more or less mobile during alteration processes (e.g. Humphris and Thompson, 1978
; Bienvenu et al., 1990), the rare-earth elements, as well as some trace elements such as Nb, Zr, Hf, Ta, Cr, are considered stable enough to be used as petrogenetic indicators in altered basalts. In the particular case of the basalts and dolerites of the GB, rather low Rb-concentrations (13–20 ppm) that are considered as an alteration index (Bienvenu et al., 1990), together with regular REE patterns (Fig. 11), allow us to infer that the mobility of these trace elements has been minimal. Moreover, the comparison, for three different samples (basalt 13·06 and dolerites 14·08 and 28·11), of whole-rock and clinopyroxene Nd-isotopic ratios (Table 11) shows rather close values (0·513039 ± 35 vs 0·513121 ± 19; 0·512964 ± 14 vs 0·513033 ± 9 and 0·512947 ± 08 vs 0·512973 ± 07, respectively), even within the uncertainty bracket when recalculated to an age of 120 Ma. All these observations suggest to us that it is acceptable to use the trace element data for petrological purposes in the study of the igneous rocks of the Galicia Bank.
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Rare-earth element data have been obtained for 13 basalts and six dolerites, as well as for one gabbro (34·06) and the pyroxenite 14·05. Other trace element data are available for eight basalts and four dolerites (Table 12). Pyroxenite 14·05, which has a high Cr- and Ni-content, appears to be a pyroxene cumulate, whereas the large positive Eu anomaly suggests that the gabbro is a plagioclase cumulate. Therefore, these two rocks will not be considered further in this section. The basalts and dolerites display a large range of variation from the point of view of their trace element contents.
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REE patterns
The basalts and dolerites from GB exhibit a large variety of REE patterns (Fig. 11); some of these have moderately positive or negative Eu anomalies that can be related to plagioclase fractionation or accumulation. Except for one single sample (17·09), the patterns are characterized by La-enrichment factors with respect to chondrites, which are always lower than 50, in agreement with their tholeiitic bulk-rock compositions. All the dolerites are LREE enriched, whereas most basalts have REE patterns ranging from moderately LREE-depleted (12·02; 12·07; 15·03) to moderately LREE-enriched (13·04; 13·06; 15·01; 21·03; 31·01; 34·10); flat patterns also occur (21·02). Such a variety of patterns at a relatively small scale along the Galicia Bank was also described at ODP drill-holes 558 and 561 (leg 82; Bougault et al., 1985
; Rideout & Schilling, 1985), along the equatorial Mid-Atlantic ridge (Hannigan et al., 2001), as well as the southern Mid-Atlantic ridge (Le Roux et al., 2002). This was interpreted to reflect heterogeneity of the mantle source and, from this point of view, the basalts and dolerites of the GB resemble those of T-MORB (T for transitional), the composition of which is in between that of depleted N-MORB (N for normal) and enriched E-MORB (Schilling et al., 1983; Schilling, 1986; Dosso et al., 1993; Schilling et al., 1994; etc.). Sample 17·09 is quite different, having a strongly LREE-enriched pattern (La enrichment factor = 135).
Trace element behaviour
Some incompatible elements display very good positive correlations with each other (R2 up to 0·85), which can be ascribed either to fractional crystallization or to variations in source composition and degree of partial melting. The latter is preferred due to the lack of any correlations between incompatible and compatible elements (e.g. Cr-content) and differentiation indexes (e.g. whole-rock Mg*).
Since the pioneering work of Pearce & Cann (1973), a great number of diagrams using a variety of trace elements have been developed to assign basalts to specific tectonic environments or characteristic mantle sources. According to a number of these plots (e.g. Fig. 12), all the basalts and dolerites from the Galicia Bank have tholeiitic compositions intermediate between depleted N-MORB and enriched E-MORB of the Mid-Atlantic ridge system, and, to a first approximation, could be considered as transitional MORB (T-MORB) (Schilling et al., 1983; Schilling, 1986). Actually, the behaviour of Ti and Si, with respect to Mg*, in the clinopyroxene (Fig. 7) of these rocks, leads us to conclude that the basalt compositions can effectively be considered as close to those of T-MORB. In contrast, the dolerites have higher silica contents, which make them more similar in composition to continental tholeiites. Although the hyaloclastite 34·09 was not found in place, its quartz–tholeiite composition provides some support to this interpretation.
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Sr–Nd ISOTOPIC GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONTHE GALICIA MARGIN AND...SAMPLE DESCRIPTIONSANALYTICAL TECHNIQUESMAJOR AND TRACE ELEMENT...Sr-Nd ISOTOPIC GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Ultramafic rocks
Only samples with fresh minerals were selected for Sr- and Nd-isotopic analysis. Even after this selection, it is always difficult to determine whether the minerals have been affected by hydrothermal alteration, which can preferentially modify their Sr-isotopic composition. For these reasons, Nd isotopes are often more reliable because they are less sensitive to such interaction. Sr- and Nd-isotopic data have been obtained for clinopyroxene and, in some cases, amphibole and plagioclase, in nine lherzolites, one websterite and one harzburgite from the GB, seven websterites and one clinopyroxenite from the IAP, and one harzburgite from the 5100 Hill. The data show large variations and are presented in Table 13 and in Fig. 13. Clinopyroxenes in the lherzolite and websterite samples from the Galicia Bank have the least radiogenic Sr- and the most radiogenic Nd-isotopic compositions, with present-day 87Sr/86Sr values ranging from 0·702100 to 0·702445 and 143Nd/144Nd values ranging from 0·512937 to 0·513402. The harzburgite 33·06 has a very low Nd-isotopic ratio (0·512489) but no Sr-isotopic composition was obtained for this sample. Clinopyroxenes from the IAP rocks have more radiogenic Sr- and less radiogenic Nd-isotopic compositions, with present-day 87Sr/86Sr values ranging from 0·704170 to 0·705919 and 143Nd/144Nd values ranging from 0·512283 to 0·512665 (Fig. 13). Clinopyroxene from the 5100 Hill harzburgite (sample H78DR24) has an isotopic composition intermediate between these two groups (87Sr/86Sr = 0·703591 and 143Nd/144Nd = 0·512865). For the two IAP samples in which plagioclase has been analysed, the Nd-isotopic composition is very similar to that of the associated clinopyroxene, but the Sr-isotopic composition is more radiogenic, probably reflecting hydrothermal alteration.
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Amphiboles have been analysed in one metasomatized rock (36·02) and three dioritic veins from the GB. They have isotopic compositions very similar to the clinopyroxenes in the 5100 Hill sample, intermediate between those from the GB and those from the IAP (Fig. 13).
Igneous rocks
The magmatic rocks (pyroxenite, gabbro, basalts and dolerites) have rather homogeneous isotopic compositions with 143Nd/144Nd ratios ranging from 0·512729 to 0·513121 (Table 11). Although these samples are rather altered by hydrothermal processes, the Nd isotopic ratio seems to be significant. Sr isotopic compositions are also relatively homogeneous (87Sr/86Sr ranging from 0·703243 to 0·703775 for most samples) but with four samples (basalts 12·02 and 13·04; dolerites 14·02 and 14·08) having elevated Sr isotopic values (>0·7046, Fig. 13). These latter values probably reflect the fact that Sr is more sensitive than Nd to hydrothermal alteration: in dolerite 14·08, the cpx has an Sr isotopic ratio (0·703775 ± 33) lower than the host rock (0·704578 ± 14), in agreement with this interpretation. However, in dolerite 28·11, the cpx and whole-rock have quite similar Sr-isotopic ratios (0·703309 ± 7 and 0·703304 ± 8, respectively). Age-corrected Sr-isotopic values (120 Ma) are not significantly different and do not affect these observations. The whole-rock Nd-isotopic composition of the basalts and dolerites is roughly similar to that of the amphibole in the dioritic veins and in the metasomatized peridotite 36·02, as well as of clinopyroxene in the harzburgite from the 5100 Hill (sample H78DR24). Plot of the initial (recalculated at 120 Ma) 143Nd/144Nd-isotope composition vs Ce/Yb (Fig. 14) shows that the dolerites, as a whole, are derived from slightly more enriched sources than the basalts, the latter being more enriched than the depleted MORB-source mantle (according to the data of Su & Langmuir, 2003).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONTHE GALICIA MARGIN AND...SAMPLE DESCRIPTIONSANALYTICAL TECHNIQUESMAJOR AND TRACE ELEMENT...Sr-Nd ISOTOPIC GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Processes controlling the diversity of the ultramafic rocks
The ultramafic rocks sampled on the West Iberia margin range from lherzolites to harzburgites, websterites and clinopyroxenites. As generally observed in studies of abyssal peridotites (Johnson et al., 1990
; Johnson & Dick, 1992; Hellebrand et al., 2001), all the samples contain cpx depleted in LREE. This may signify that they have been affected by partial melting events, before or during their emplacement along the margin. However, although the harzburgites from GB, 5100 Hill and the IAP contain Cr-rich spinel as well as Mg-rich and Na-poor cpx, in agreement with more or less significant melt extraction during their history, the enriched Nd-isotopic compositions of their cpx is rather far, especially for the GB harzburgite (143Nd/144Nd value in Table 13), from what can be expected for abyssal peridotites (Fig. 13). In contrast, the sp-lherzolites from GB contain cpx whose Nd-isotope composition is similar to that of the abyssal peridotites (Fig. 13) but exhibit mineral chemistry characteristics (Al-rich spinel and Na–Ti-rich cpx), which make these lherzolites extremely fertile. Therefore, these particular lherzolites did not really melt, or very little, nor did the associated websterites. On the other hand, the IAP websterites and clinopyroxenites are characterized by Cr-rich spinel and Na-poor but Fe-rich cpx. These latter are significantly more fusible than their Mg-rich analogues from the surrounding harzburgites, thus clearly documenting that the IAP pyroxenites have been emplaced as high-pressure cumulates within the harzburgites after the partial melting event these latter underwent and before the high-temperature deformation that these ultramafic rocks experienced altogether. Furthermore, all cpx from IAP pyroxenites are strongly depleted in terms of LREE and have fairly enriched Sr–Nd isotopic signatures. What could be the role played by late-stage metasomatic processes in determining the geochemical diversity exhibited by these ultramafic rocks? Are the two main secondary phases developed in these rocks, namely amphibole and plagioclase, related to such a metasomatism?
Emplacement of diorite veins and the amphibole-bearing peridotites
The amphibole diorite veins which cross-cut some GB peridotites may be related to the episode of melt percolation which induced metasomatism in some of the peridotites. The effect on the modal mineralogy of the peridotite of this melt percolation involves the crystallization of amphibole, both as interstitial crystals and as irregular bundles in cpx. No interstitial plagioclase is associated with the interstitial amphibole but a secondary plagioclase + amphibole assemblage does occur as coronas around corroded spinel (Fig. 4f). As noted above, it is believed that the plagioclase may be due to the decompression-related destabilization of spinel—a reaction possibly triggered by the percolation of melt or fluid through the rock. The relatively Na-rich composition of the plagioclase in these coronas could be tentatively related to the infiltrating melt itself. The cryptic metasomatic effect of melt or fluid percolation is clearly shown by the trace element characteristics of the cpx in the amphibole-bearing peridotites; these are much richer in incompatible elements than the cpx from the surrounding spinel lherzolites (Fig. 15a and b). Due to the intergrowth between amphibole and cpx in these samples, the effect of the melt infiltration on the Nd-isotope composition of the cpx is not known. If the growth of amphibole in the metasomatized peridotites is directly related to the intrusion of the diorite veins, then it must have partially re-equilibrated with the host rock, to account for its much lower enrichment in incompatible elements relative to the amphibole in the diorite (Figs 10, 15a and b and Table 9). This was suggested above, based on the major element chemistry of the amphiboles (see above).
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Plagioclase in peridotites and pyroxenites
Plagioclase is also a secondary phase in some harzburgites (5100 Hill, IAP) as well as in the IAP pyroxenites. In the harzburgites, plagioclase is strictly restricted to thin coronas around corroded spinel; the effect of the high-temperature deformation frequently causes the coronas to be elongated in the foliation. However, plagioclase has never been observed as veins which can be definitely related to an impregnation process for their significant continuity and length. The associated spinel is enriched in both Cr and Ti, whereas cpx is enriched in Ti and Cr and depleted in Na. This suggests that the dominant mechanism to produce plagioclase in these rocks was the subsolidus destabilization of the assemblage opx + cpx + sp; the role played by melt or fluid percolation in this process was probably fairly limited. However, the abundance of some incompatible elements in cpx may suggest limited fluid circulation in the plagioclase-bearing harzburgites, and especially in the IAP harzburgite. These clinopyroxenes have Ce and Zr (and most of the REE) contents somewhat higher than cpx from the plagioclase-free harzburgite (e.g. 33·06 from GB); however, they do not significantly differ from the average cpx composition in the peridotites of the whole Iberia margin, except for the much more rich cpx from the metasomatized peridotites (Fig. 15a and b).
The IAP pyroxenites contain a significant amount of plagioclase (up to 5% by volume). Moreover, in contrast to observations from the plagioclase-bearing harzburgites, the plagioclase-rich areas show some continuity in the rocks. Nevertheless, plagioclase is always closely associated with corroded spinel grains and small subhedral olivine crystals, which represent one of the products of the subsolidus reaction (Fig. 4a–d). As emphasized above, spinel is enriched in both Ti and Cr (Fig. 6), whereas the Na-depleted clinopyroxene is enriched in Cr and depleted in Al (Fig. 5d). These features seem inconsistent with a melt percolation event. Additionally, cpx from the IAP websterites are relatively poor in some incompatible elements (Ce and Zr, Fig. 15), and cpx from the clinopyroxenite even more. This makes it extremely unlikely that this has been a large-scale impregnation of the peridotites by incompatible element-enriched silicate melts (such as those described in GB: diorites, dolerites and basalts) during their recent pre-rift and syn-rift history. The relatively depleted characteristics of the pyroxenites could reflect the percolation of depleted melts; however, such an hypothesis is in agreement with neither the iron-rich composition of their cpx nor with their enriched Nd-isotopic signatures. The subsolidus origin of the plagioclase, well documented by textural features, is also supported by evidence for equilibrium between cpx and plagioclase in terms of their Ce-contents (Fig. 16) and their similar enriched Nd-isotopic compositions (Fig. 13).
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Incompatible elements in the clinopyroxene of the ultramafic rocks: products of discrete fluid percolation?
Although, based on the arguments presented above, the crystallization of plagioclase within the peridotites and websterites is believed to be mostly, or even totally, due to low-pressure subsolidus recrystallization, the amphibole-bearing peridotites from the GB clearly indicate that the Iberia margin peridotites experienced a late-stage, pervasive, metasomatism at a regional scale. The strongest evidence for this is the emplacement of the amphibole-diorite dykes. The incompatible element concentrations in both amphibole and cpx from the ultramafic rocks show that cryptic metasomatism probably also occurred in all the peridotites and pyroxenites from the Iberia margin. By comparison with cpx from typical abyssal peridotites (e.g. Johnson et al., 1990
; Johnson & Dick, 1992), the cpx from the GB and IAP peridotites and pyroxenites appears to be rather rich in incompatible elements (Fig. 15a and b). Only cpx from some abyssal peridotites of the Romanche Fracture Zone (group II, Seyler & Bonatti, 1997) have incompatible element contents matching those of cpx from the Iberia margin ultramafic rocks. According to Seyler & Bonatti (1997), these ‘Group II’ peridotites have been significantly contaminated by a percolating melt. Moreover, when the incompatible element behaviour of both abyssal peridotites and ultramafic rocks from the Iberia margin is considered, a clear correlation between Zr and Ce is observed. As shown in Fig. 15, these elements are well correlated in cpx, but the correlation also includes amphibole from the metasomatized peridotites and from the diorites. If the composition of the diorite amphibole is assumed as the hypothetical metasomatic melt, then the amphibole-bearing peridotites can be regarded as the products of modal metasomatism induced by this contaminant, which also likely caused cryptic metasomatism in the remaining peridotites and websterites from the Iberia margin. As a result, the latter are characterized by enrichment in both Zr and Ce relative to the uncontaminated abyssal peridotites.
Partial melting
The circulation of silicate melts through, or their extraction from, a portion of the mantle should be registered by the chemical composition of the constituent minerals of the affected mantle and, particularly, by the trace element signatures of the clinopyroxenes in the residual peridotites. The REE composition of clinopyroxenes can be used to estimate whether they are compatible with their host rocks being residual peridotites after partial melting. Among our samples, only the lherzolites and harzburgites can be considered as the residues of a partial melting event. The case of the IAP websterites will be discussed subsequently.
In their modelling of the peridotites from the Central Indian Ridge, Hellebrand et al. (2002) discussed in great detail the constraints on the choice of the model parameters, such as the trace element composition and the modal mineralogy of the source rock. These parameters can be adjusted to provide a best fit to the data if they fall in a reasonable range, compatible with the constraints obtained from natural samples.
Detailed modelling to constrain quantitatively the generation of the mantle rocks from the Iberia margin is beyond the scope of this paper. Our calculations are not aimed at providing an accurate partial melting model, as too many parameters remain unconstrained (e.g. the chemical and modal composition of the source), but at verifying whether the observed compositions are in qualitative agreement with an origin of the peridotites as refractory residues after partial melting. For the partition coefficients of trace elements between minerals and silicate melt, we used the compilation of Suhr et al. (1998). The trace element composition of the mantle source rock, as well as its modal mineralogy and the melting mode, are given in Table 14. The values for the source composition are very close to the primitive mantle composition of Hofmann (1988) for the HREE, but slightly depleted in LREE; the values for the melting mode are similar to those used by Hellebrand et al. (2002), but the initial cpx mode was slightly lowered (from 17 to 14%). These small changes have been introduced to better account for the modal mineralogy and chemistry of our mineral and whole-rock samples.
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In the model, we calculated the composition of the residual whole-rock after fractional melting of a lherzolitic source, and then the composition of the clinopyroxene in equilibrium with this residue. The results of the calculations are shown in Fig. 17.
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The clinopyroxenes of the GB lherzolites can be modelled by 0·5–3% melt extraction from the original protolith in the spinel stability field. The fit can be improved by using slightly higher trace element concentrations in the source, but this does not change the main conclusion of this modelling that the chemical composition of the lherzolites is due to the extraction of a very low-degree melt. This conclusion reinforces the interpretation based on the mineral chemistry and Sr–Nd isotopic data reported above, that the spinel lherzolites did not melt significantly during the rifting stage.
As previously shown, cpx from the GB and the 5100 Hill harzburgites have lower REE contents compared with those in the lherzolites (Fig. 8). The HREE content of the cpx in the harzburgites can be accounted for by the same model as used previously for the lherzolites, but with higher melt fractions (between 5 and 10%, Fig. 17). However, the LREE and MREE compositions of the cpx are clearly not matched by this model. The nearly flat REE patterns at low REE contents displayed by these harzburgites (Fig. 17) suggest that after a partial melting event, they have subsequently been re-enriched by melt percolation, as discussed above.
Nature of the mantle along the Iberia margin
Many arguments have been put forward to demonstrate that the emplacement of the peridotite ridge along the Iberia margin is contemporaneous with the end of continental rifting and the beginning of oceanic crust formation. This ridge is located just to the east of the first magnetic anomaly in the Atlantic Ocean (Srivastava et al., 1990), and to the west of the last tilted continental blocks, and, thus represents a transition zone between the continental and the oceanic lithosphere. It is important to understand the origin and evolution of this part of the mantle and especially to decipher whether it is a part of the newly formed oceanic lithosphere (the first oceanic mantle) or if it represents a piece of the old continental lithospheric mantle.
The Sr- and Nd-isotopic compositions of the peridotites and the pyroxenites analysed in this study support a subcontinental mantle origin for at least a large part of this ultramafic ridge. The Nd-isotopic compositions show a large scatter in the 143Nd/144Nd versus 147Sm/144Nd diagram (Fig. 18). The fact that no clear linear trend is observed in this diagram has two important implications: (1) the isotopic heterogeneity does not occur due to mixing between only two different mantle components; (2) the isotopic data do not define an isochron, so that the isotopic heterogeneity was not created during a single partial melting event, or the partial melting event was not important enough to erase the initial heterogeneity of the mantle. Indeed, the calculated initial (120 Ma) Nd-isotopic ratios show the same range of variation as depicted by the present-day values (Fig. 19), indicating that this part of the mantle was already heterogeneous at the time of rifting.
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The plagioclase-free lherzolites are the most useful samples to address the issue of the origin of this part of the mantle. In all the samples, the cpx have 143Nd/144Nd ratios equal to or higher than (Fig. 18) that of the Depleted MORB Mantle (DMM). However, all the cpx have higher 147Sm/144Nd ratios than DMM. These isotopic data allow us to constrain the origin of this part of the mantle and especially the timing of the melting event responsible for the LREE depletion.
Several characteristics of the composition of the minerals from the GB lherzolites allow us to conclude that these samples did not undergo partial melting during Atlantic rifting. This is reinforced by the isotopic composition of the cpx from these lherzolite samples. If such a melting event had occurred during continental break-up (i.e. 120 Ma) in an asthenospheric mantle source with the Nd-isotopic composition of DMM, the residual peridotites, represented by the sampled lherzolites, would have present-day 143Nd/144Nd values higher than DMM, in agreement with their high Sm/Nd ratios. This is clearly not the case for most of the samples, so we can rule out this hypothesis. The calculated temporal evolution of the Nd-isotopic composition of these samples, depicted in Fig. 19, shows that 120 Myr ago, this part of the mantle had a heterogeneous Nd-isotopic composition with some 143Nd/144Nd ratios lower than the lower limit of the DMM. On the other hand, if the melting event occurred a long time before the rifting, the source would have had a very low Nd-isotopic composition, e.g. close to the CHUR composition at 500 Ma to account for the measured 147Sm/144Nd and 143Nd/144Nd.
In the harzburgites, the low Ti-content of the clinopyroxenes indicates that these samples are residues from a partial melting event (Fig. 5e). In the two harzburgites which have been analysed for their trace element and Nd-isotopic composition (33·06 and H78DR24), the clinopyroxenes have a Sm/Nd ratio similar to the depleted mantle value (Workman & Hart, 2005), but lower 143Nd/144Nd values relative to the DMM component. The similarity of the Sm/Nd ratio in the cpx from the harzburgites and the depleted mantle implies that they evolved isotopically in the same way since at least the opening of the Atlantic Ocean 120 Myr ago, as no younger magmatic event likely associated with a change of this ratio has yet been discovered in the area.
As for the spinel-free lherzolites, the Nd-isotopic composition of the two harzburgites at the time of the rifting is not compatible with their origin as refractory residua after partial melting of a depleted mantle source (Fig. 19). The cpx in these samples have higher Ce/Sm and lower Sm/Nd ratios than the cpx from the lherzolites, and lower initial (120 Ma) Nd-isotope ratios. These results can only be explained by a complex history during which the samples probably experienced enrichment a long time ago in the continental lithosphere to acquire such low Nd-isotope ratios. Furthermore, they underwent partial melting to form residual harzburgites depleted in the most incompatible elements. The trace element ratios, as well as the high Na2O-content of the clinopyroxenes, suggest that the harzburgites were re-enriched more recently, probably by melt percolation associated with the rifting.
The clinopyroxenes from the IAP websterites and the clinopyroxenite (sample 897C 66R04 067–070 cm) have the most radiogenic Sr-isotope values and the least radiogenic Nd-isotope ratios (Fig. 13). Moreover, they have highly variable but, in most cases, high Sm/Nd ratios, sometimes higher than cpx from the Galicia Bank lherzolites (Fig. 17). The origin of these websterites and the clinopyroxenite is difficult to assess based only on the geochemical data presented here. A genetic link between the websterites and melt circulation during the proto-Atlantic oceanic crust formation has nevertheless been advocated for a websterite sample from the Gorringe Bank on the basis of its major and trace element composition (Serri et al., 1988). However, the Nd-isotopic composition of the IAP websterites 120 Myr ago was already very low (Fig. 19)—lower than the chondritic value at that time, and thus far lower than the isotopic composition of the lherzolites. These isotopic compositions confirm the suggestion made earlier that the websterites represent high-pressure cumulates formed in the continental lithospheric mantle a long time before rifting. The decoupling between the Nd-isotope composition and the high Sm/Nd values of clinopyroxenes indicates that these samples experienced a Sm/Nd fractionation event during the youngest period of their evolution. A partial melting event, related to the uprising of the mantle during Atlantic rifting 
120 Myr ago, can account for the high Sm/Nd of these samples.
The magmatic rocks sampled in the GB area have Sr- and Nd-isotope compositions intermediate between those of the GB lherzolites and the IAP websterites (Fig. 13). As a whole, the basalts have higher Nd-isotope ratios than the dolerites, for a similar Sr-isotopic composition. The basalts with the highest 143Nd/144Nd ratios (12·07, 13·01 and 15·03) are similar to the less radiogenic Mid-Atlantic Ridge (MAR) basalts (Ito et al., 1987). All the other samples have lower Nd-isotope ratios than the MAR basalts. The 143Nd/144Nd values of the magmatic rocks are negatively correlated with their Ce/Yb ratios (Fig. 14). This correlation indicates that the trace element variations occurring in the magmatic rocks are not fully ascribable to partial melting or crystal fractionation processes. This correlation is probably due to an interaction between two different sources in the mantle. One source is similar to the DMM; the other one has a low Nd-isotope ratio and a high Ce/Yb ratio and represents an enriched source possibly located in the subcontinental lithospheric mantle. Melts produced by the partial melting of the IAP websterites can represent such an enriched contaminant. The correlation between 143Nd/144Nd and Ce/Yb may thus reflect interaction between asthenospheric magmas similar to the MAR basalts and the overlying lithospheric mantle during their ascent to the surface.
The Sr- and Nd-isotopic composition of the magmatic rocks is very similar to the composition of both the metasomatic amphiboles analysed in the GB lherzolite 36·02 and the amphiboles from the dioritic veins (Fig. 13). These two kinds of amphiboles may have formed during the migration of silicate melts from their source towards the near surface, where they are now expressed as basalts and dolerites.
The range of Sm–Nd isotopic compositions exhibited by the Iberia margin peridotites and pyroxenites is very similar to the isotopic composition of the peridotites from the Lherz and Beni Bousera orogenic massifs of S. Spain and N. Africa (Mukasa et al., 1991; Pearson et al., 1993; Fig. 18). Clinopyroxenes from the GB samples also have Sr- and Nd-isotopic compositions in the same range as the samples from Lanzo (Bodinier et al., 1991) and the External Liguride Units (Rampone et al., 1995). These peridotitic massifs, among others, have been interpreted as portions of the continental lithospheric mantle brought up to the surface during orogenic events and give a good indication of the isotopic composition of the subcontinental mantle beneath Europe at the time of these orogenic events. This mantle obviously had a long and complex history of depletion and subsequent enrichment events leading to significant heterogeneity in the isotopic compositions of the constituent minerals. The similarity between these isotopic compositions and the values measured in the minerals of most of the mantle samples from the Galicia margin is a strong argument for a subcontinental rather than an oceanic origin for this part of the mantle at the time of rifting. Partial melting events, as well as melt percolation related to Atlantic rifting, have accentuated these heterogeneities in terms of major and trace elements, leading to the variety of mantle rocks sampled along the margin. These late events have also obliterated evidence constraining the age of formation of the pyroxenites in the mantle.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONTHE GALICIA MARGIN AND...SAMPLE DESCRIPTIONSANALYTICAL TECHNIQUESMAJOR AND TRACE ELEMENT...Sr-Nd ISOTOPIC GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Although only a few samples have been studied, compared with the regional extent of the area under consideration, the petrological and geochemical data presented above bring some new insights about mantle behaviour during the rifting of the West Iberia passive margin. The ultramafic and igneous rocks sampled have very heterogeneous compositions which reflect a complex evolution that, in part, occurred during rifting and continental break-up. However, scrutinizing all the available petrological and geochemical data suggests that some of the characteristics of the Iberia Margin mantle were acquired before the beginning of rifting and reflect older magmatic and/or metasomatic events. Figure 20 represents, schematically, a simplified view of the different events which have affected this part of the mantle and which are summarized below.
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The GB spinel lherzolites: syn-rift contamination of the lithospheric mantle by MORB-like melts
The major and trace element compositions of cpx in the GB spinel lherzolites provide evidence for large-scale modal and cryptic metasomatism of the lithospheric mantle by MORB-like tholeiitic melts. The websterite lens (28·02) is a likely candidate to represent a cumulate from such a percolating melt. This event took place at the beginning of rifting, before the development of the HT foliation. At that time (i.e.
120 Ma), the parental magma which formed the websterite had Nd-isotopic composition similar to that of the depleted mantle (DMM), source of Atlantic MORB. Although largely refertilized by such melt infiltration, resulting in a mineral assemblage with up to 10% Na- and Ti-rich cpx, during their ascent towards the surface during continental break-up, the GB spinel lherzolites did not experience significant melt extraction, nor low-pressure recrystallization in the plagioclase stability field. This suggests fast ascent and cooling of this upper mantle sector. As a consequence, the associated harzburgites (33·06, and H78DR24 at 5100 Hill), which possess petrological features indicative of significant melt extraction, are considered not to have undergone partial melting during the rifting stage, but much earlier, probably during, or even before, the Hercynian Orogeny. The rather enriched geochemical characteristics of these harzburgites (higher LREE-enrichment and lower Nd-isotopic ratios relative to the associated lherzolites) may have also been inherited from this earlier stage, perhaps caused by interaction with migrating melts after the partial melting episode.
The IAP pyroxenites: syn-rift partial melting of older enriched mafic cumulates
The IAP pyroxenites have paradoxical geochemical characteristics; their cpx are very LREE depleted, but are isotopically enriched, with very low 143Nd/144Nd ratios. Such features are probably related to a two-stage history: (1) percolation of mantle peridotites (the surrounding harzburgites) by enriched melts from which the pyroxenites formed as cumulus products; (2) subsequent partial melting of the pyroxenites, accounting for their LREE depletion. Similar observations on some Beni-Bousera pyroxenites led Pearson et al. (1993) to similar conclusions. In the particular case of the IAP, based on their very low present-day Nd-isotopic ratios and their high Sm/Nd ratios, the partial melting stage should have occurred as late as possible in their history in order to prevent radiogenic in growth of Nd. Therefore, the IAP pyroxenites probably experienced melt extraction during the rifting, associated with continental break-up, caused by adiabatic decompression and conductive heating from upwelling asthenosphere and/or the emplacement of melts extracted from the asthenosphere. Although highly LREE depleted, the IAP pyroxenites contain rather iron-rich pyroxenes; therefore, they are still relatively fertile and cannot have experienced such high-degree partial melting as the surrounding harzburgites. This implies that the harzburgites represent residues of a partial melting event which occurred before the emplacement of the associated pyroxenites, and consequently before the rifting stage. It is concluded that the IAP pyroxenites were derived from the solidification of enriched melts within peridotites which have been rendered refractory during an earlier partial melting event. These melts could be subduction-related, as inferred for Balmuccia (Ivrea Zone), where mantle peridotites were also intruded by isotopically enriched melts (Voshage et al., 1987, 1990). Thus, the IAP pyroxenites and associated harzburgites could represent relics of Hercynian (or even older) continental mantle lithosphere, which was more or less deeply reactivated during Atlantic rifting. Relative to the GB peridotites, the IAP websterites were maintained at relatively high temperature. This high thermal regime allows these samples to undergo substantial low-pressure recrystallization during their uprising to the surface. In contrast, the spinel–lherzolite assemblage was kept metastable due to fast cooling on the rocks.
Partial melting and hybridization
A variety of igneous rocks were emplaced along the Iberia continental margin, during Mesozoic continental break-up, before the onset of steady-state sea-floor spreading marked by the first magnetic anomaly MØ, at 120·4 Ma. The earliest melts are inferred to have partially refertilized the GB spinel–lherzolites during the early stages of rifting, and have MORB-like geochemical signatures, similar to those of IAP and GB gabbros (Seifert & Brunotte, 1996; Seifert et al., 1997; Schärer et al., 2000). This emphasizes the role of partial melting of asthenospheric mantle during continental rifting.
A range of mafic igneous rocks, including diorites, pyroxenites, gabbros, dolerites and basalts, were emplaced from the end of the high-temperature deformation of peridotites to the end of rifting (Malod et al., 1993). All of these rocks are olivine tholeiites and quartz–tholeiites with variable, but relatively enriched, geochemical characteristics which are inferred to be due to mixing of melts from a DMM-like source with those from an enriched source with a composition more similar to the E-MORB end-member. The lack of Pb-isotope data, due to the extremely low Pb-contents of the clinopyroxenes, prevents more precise inferences about the nature of this enriched source. However, the influence of a deep, enriched asthenospheric mantle source (hotspot or plume) on the melt compositions is unlikely, as no subsequent influence of such a hotspot is known in this particular sector of the Atlantic. On the other hand, the relatively silica-rich composition of some of the igneous rocks (e.g. evidenced by the composition of clinopyroxene from the dolerites and by the whole-rock chemistry of the hyaloclastite 34·09) is unlikely to be related to high-degree melting of garnet or spinel peridotite, as no significant volumes of magmatic rocks are present in the area. We consider that the enriched end-member forms part of the Iberia margin continental lithosphere, which is partly represented by the IAP ultramafic rocks. Support for this interpretation is provided by the evidence for partial melting of initially enriched pyroxenites—a process that may also account for the formation of silica-rich melts if it occurred under low-pressure conditions.
Melt transfer-related modal and cryptic late-stage metasomatism
Late-stage metasomatism is evidenced in the GB peridotites by the crystallization of amphibole porphyroblasts at the very end of the HT deformation. This modification of the mantle rocks is spatially related with the emplacement of amphibole-bearing diorite dykes. Moreover, amphibole in the peridotites has geochemical characteristics (REE and Nd-isotopic ratio) similar to that in the diorites and, more generally, to the most enriched melts which formed dykes and lava flows in the area. Partial melting of the deep continental lithosphere, evidenced by the IAP websterites, and hybridization with melts from the asthenosphere may also have caused modal metasomatism in neighbouring mantle units. Most probably, this melt percolation was also associated with a more general cryptic metasomatism characterized, in almost all ultramafic rocks along the Iberia Margin, by relatively high incompatible element contents in clinopyroxenes.
The Iberia Margin ultramafic ridge: continental lithosphere or proto-oceanic lithosphere?
Isotopic heterogeneities in the ultramafic rocks of the Iberia margin were mostly acquired before the onset of Mesozoic rifting event and are as variable as those observed in peridotite massifs such as Beni-Bousera or Lherz. They reflect a long and complex history of depletion and enrichment events in an old part of the Earth's upper mantle, and provide strong arguments for a subcontinental origin for this part of the upper mantle.
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ACKNOWLEDGEMENTS |
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We thank the French CNRS, IFREMER and the crews of the MS Nadir and its submersible Nautile for invaluable help provided during the dive cruises Galinaute I and II. We are especially indebted to Gilbert Boillot, chief scientist on board, and to the ODP curator who provided us with several samples from the Iberia Abyssal Plain. Valuable discussions occurred both onboard and onshore, with Jacques Girardeau, Urs Schairer, Marie-Odile Beslier, Jean-Pierre Brun and Philippe Vidal. Thanks go to Michèle Veschambre, Françoise Vidal, Chantal Bosq and Pierro Bottazzi for their help during the acquisition of chemical and isotopic data. Elisabetta Rampone, Réjean Hébert, Eric Hellebrand and Mike Roden, as well as Editor Marjorie Wilson, are warmly acknowledged for their careful and especially constructive reviews which considerably improved the quality of the manuscript.
* Corresponding author. Telephone: 33 47 3346759. Fax: 33 47 3346744. E-mail: G.Chazot{at}opgc.univ-bpclermont.fr
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TOPABSTRACTINTRODUCTIONTHE GALICIA MARGIN AND...SAMPLE DESCRIPTIONSANALYTICAL TECHNIQUESMAJOR AND TRACE ELEMENT...Sr-Nd ISOTOPIC GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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