Platinum-group-element distribution in subcontinental mantle: evidence from the Ivrea Zone (Italy) and the Betic – Rifean cordillera (Spain and Morocco)

Platinum-group-element distribution in subcontinental mantle: evidence from the lvrea Zone (Italy) and the Betic - Rifean cordillera (Spain and Morocco)

Giorgio Garuti, Massimo Oddone, and Jose Torres-Ruiz

Abstract: The six platinum-group elements (PGE's) Os, Ir, Ru, Rh, Pt, and Pd and Au were analyzed by instrumental neutron-activation analysis after nickel sulfide fire assay, in peridotites and dyke rocks from the orogenic ultramafic massifs of the Ivrea Zone in the Italian western Alps (Baldissero, Balmuccia, Finero) and the Betico-Rifean cordillera in southern Spain and northern Morocco (Ronda, Beni Bousera). The peridotites are considered as variably depleted, and reenriched low lithosphere, whereas the dyke rocks represent polybaric derivatives of basaltic melts (pyroxenites and gabbros), most coming from the underlying asthenosphere. The peridotites have total PGE content in the range 8.6-54.7 ppb, while mantle-normalized patterns generally grade from nearly flat and PGE rich, in less depleted lherzolites, to negative and PGE poor, in residual harzburgites and dunites. Dyke rocks have total PGE's in the range

. 5.4-250 ppb and positive mantle-normalized patterns. Negative anomalies of Ir-Pt are frequently observed in dykes, indicating that both metals were probably retained in the mantle source of these melts. Most of the peridotites display positive anomaly of Au, and in some case are enriched in Ru, Rh, and Pd, but exhibit the same negative anomalies in Ir and Pt as the dykes. These features are ascribed to reintroduction of noble metals into the residual mantle by reaction with the basaltic melts that generated the dykes, or alternatively by recycling of "dyke material" during further partial melting of the host mantle. The role of the sulfide phase as carrier of the recycled PGE is stressed by clear interelemental correlation in peridotites from the Ivrea Zone. Present data provide evidence that zones of PGE enrichment can originate this way in the subcontinental mantle, and may constitute a potential reservoir for noble metal fertile volcanism in continental rift systems.

RCsumC : Les six iliments du groupe du platine (EGP), Os, Ir, Ru, Rh, Pt et Pd, et l'iliment Au ont i t6 analysis par l'analyse par activation neutronique instrumentale aprtts I'essai pyrognostique sur les sulfures de Ni, des piridotites et roches de dyke des massifs oroginiques d'ultramafites, dans la Zone d'Ivrie des Alpes occidentales de 1'Italie (Baldissero, Balmuccia, Finero) et de la Cordillttre betico-rifaine de 1'Espagne miridionale et du Maroc septentrional (Ronda, Beni Bousera). Ces peridotites sont considirees comme des roches de la lithosphttre infirieure qui furent appauvries a divers degris et ri-enrichies, tandis que les roches de dyke reprisentent des dkrivis de magmas basaltiques form& sous de multiples pressions (pyroxinites et gabbros), provenant generalement de l'asthinosphkre sous-jacente. La teneur totale des EGP dans les piridotites varie de 8,6 a 54,7 ppb, alors que les courbes normalisies du manteau passent gentralement de quasi horizontales et riches en EGP, pour lherzolites moins appauvries, nigatives et pauvres en EGP, pour les harzburgites rtsiduelles et les dunites. La teneur totale des EGP dans les roches de dyke varie de 5,4 a 250 ppb et les courbes normalisees du manteau sont positives. Les anomalies nigatives de Ir-Pt sont frequemment observies dans les dykes, ce qui signifie que ces deux mitaux furent probablement retenus dans la source mantellique de ces magmas. La majoriti des piridotites montrent une anomalie positive de l'eliment Au, et dans certains cas elles sont enrichies en Ru, Rh, Pd, mais elles presentent les memes anomalies nigatives de Ir et Pt que dans le cas des dykes. Ces particularitis sont attribuees a une ri-introduction des mitaux nobles dans le manteau risiduel, soit par une riaction avec les magmas basaltiques qui ont engendre les dykes, ou bien par le recyclage du (( matiriel des dykes D durant une itape plus avancie de la fusion partielle du manteau encaissant. Le r6le qu'a joui la phase sulfurie comme porteur des EGP recycles est t tayi par une correlation inter-iliments claire dans les peridotites de la Zone d'lvrie. Les donnies prisenttes ici dimontrent que les zones d'enrichissement en EGP peuvent avoir kt6 formies de cette f a ~ o n dans un manteau subcontinental, et peuvent constituer un riservoir potentiel pour le volcanisme fertile en mitaux nobles dans les systttmes de rift continental. [Traduit par la ridaction]

I Received February 16, 1996. Accepted November 28, 1996.

I G. Garuti.' Dipartimento di Scienze della Terra, Universita di Modena, Via Santa Eufemia, 19, 41 100-Modena, Italia. M. Oddone. Dipartimento di Chimica Generale, Universita di Pavia, Viale Taramelli, 12, 27100-Pavia, Italia. J. Torres-Ruiz. Departamento de Mineralogia y Petrologia, Universidad de Granada, Fuentenueva s/n, 18010-Granada, Espafia.

I ' Corresponding author (e-mail: [email protected]).

Can. J. Earth Sci. 34: 444 -463 (1997) O 1997 NRC Canada

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Introduction

Mantle uplift associated with continental rifting is usually accompanied by adiabatic partial melting and production of mafic melts that under certain circumstances become carriers of noble metals such as the platinum-group elements (PGE's) and Au. Although not particularly concentrated in noble metals, these mafic melts represent the natural precursors to the formation of economic deposits at crustal levels (Naldrett 1981). Concentration and distribution of PGE's and Au in the subcontinental mantle source of these magmas are poorly constrained, and largely unknown is the behaviour of these elements during generation and upwards intrusion of deeply derived magmas. Available data only concern part of the noble metals, mainly Ir, Pt, Pd, and Au, and mantle rocks from a limited number of occurrences, mainly mantle xenoliths (e.g., Jagoutz et al. 1979; Mitchell and Keays 1981; Morgan et al. 198 1 ) and ultramafic orogenic massifs (e.g., Garuti et al. 1984; Lorand 1989; Lorand et al. 1993). The aim of the present work is to provide information on distribution of the six PGE's and Au in orogenic ultramafic massifs of the Ivrea Zone (western Italian Alps) and the Betic- Rifean cordillera (southern Spain and northern Morocco), which are believed to represent subcontinetal mantle (Nicolas and Jackson 1972).

Analytical notes

The six PGE's (Ru, Rh, Pd, Os, Ir, Pt) and Au were determined by instrumental neutron-activation analysis (INAA) after the nickel sulfide button preconcentration step, at the University of Pavia, using a 250 kW TRIGA Mark I1 reac- tor. All samples were powdered in an agate mill. Nickel sulfide (stoichiometry Ni3S2) beads weighing from 4 to 6 g were obtained by fusion at 1000°C from sample aliquots ranging between 10 and 30 g. Beads were dissolved in hot concentrated HCl, and the solution was filtered, collecting the insoluble PGE's and Au on filter paper. The residue was washed with water and ethanol, then dried and sealed in a polyethylene bag for irradiation. Sensitivity for PGE's and Au was in the range 0.006 -0.01 5 ppb. Precision, based on repeated analyses of the UMT-1 international standard, was f 10%. Other details of the INAA method are reported in Oddone et al. (1994). Major and minor elements were analyzed at the University of Granada using X-ray fluorescence, except for total S, which was determined by a LECO instru- ment, with sensitivity of 5 ppm, and precision of about f 5% at a concentration of 100 ppm. Detection limits were <20 pprn for the minor elements (Na, P, K, Ti), < 10 pprn for Cr and Ni, and generally <0.5 pprn for all the other trace elements, except for Zr and Cu (< 1 pprn).

Geological setting

The orogenic peridotite massifs of the present investigation are those of Baldissero, Balmuccia, Finero (Ivrea Zone), Ronda, and Beni Bousera (Betic - Rifean cordillera), They belong to the group of orogenic lherzolites in the western Mediterranean interpreted by Nicolas and Jackson (1972) as obducted fragments of subcontinental mantle. Although currently debated, this assumption is supported by the fact that the mantle rocks occur in close association with crustal,

high-grade metamorphic terrains. In addition, the orogenic peridotites have textural and petrologic analogies with basalt- born xenoliths, and display characteristics of the lithospheric mantle (Menzies and Dupuy 1991), mainly isotopic and elemental heterogeneity and disequilibrium, multiple light rare earth element (LREE) enrichment. Dyke rocks are present in all the investigated massifs, ranging from a few centimetres to metres in thickness. They represent either polybaric derivatives of basaltic melts intruded upwards from the underlying asthenosphere (exotic), or early fractionation products of magmas formed in situ by partial melting of the host body (indigenous). The mantle massifs of the Ivrea Zone and the Betic - Rifean cordillera were upthrusted to the surface during the Alpine orogeny; however, they underwent different preorogenic histories at the base of the continental crust.

Ivrea Zone The orogenic massifs of the Ivrea Zone are relatively small (3- 12 km2). 'They crop out along a major tectonic linea- ment (Insubric line) separating the southern Alps crystalline basement, unaffected by Tertiary metamorphic overprint, from the Austrides and Pennides domains. Effects of low-grade metamorphism or later serpentinization are nearly absent. Pressure and temperature conditions of mantle equilibration vary from the spinel -1herzolite facies up to the granulite - amphibolite transition zone (Ernst 1978; Garuti et al. 1978, 1980; Shervais 1979). Petrography of the major phases suggests that the mantle in the Ivrea Zone underwent variable degrees of partial melting, increasing from the Baldissero Iherzolite, moderately depleted by the extraction of about 4.5 % mid-ocean-ridge basalts (M0RB)-equivalent magma, to the harzburgite -dunite of Finero, representing the residue after separation of about 18 % melt (Hartmann and Wedepohl 1993). The Balmuccia massif comprises lherzolites, and subordinate harzburgites and dunites, displaying degrees of melting intermediate to that of Baldissero and Finero. Chemical and mineralogical characters of Balmuccia and Finero peridotites reflect metasomatism by asthenospheric and crustal components, producing irregular reenrichment of the depleted mantle. The Balmuccia peridotite displays evidence of metasomatic reaction with incompletely extracted melts (Garuti et al. 1984), or with magmas injected from an underlying mantle source, en route to the surface (Shervais and Mukasa 1991). The residual harzburgite of Finero appears to have been metasomatized by water-rich fluids of possible crustal origin, causing crystallization of abundant phlogopite and amphibole, as well as irregular reintroduction of incompatible elements (Exley et al. 1982; Hartmann and Wedepohl 1993).

Dyke rocks are exceptionally abundant at Balmuccia, where they form a closely woven network occupying up to more than 5% of the total rock volume. Exotic dykes vary from concordant to discordant, and can be divided into at least two groups representing distinct events of emplacement (Shervais and Mukasa 1991). The older suite consists of Cr-diopside websterites that appear frequently deformed, boudinaged, and partially melted. The younger suite is com- posed of less deformed Al-augite websterites and gabbros. In the field, Al-augite websterites are seen to grade into gabbros by an increase of modal plagioclase along strike. Capedri

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et al. (1977) suggested that Al-augite websterites and gabbros might have formed by fractionation of high-Mg basaltic magmas at pressure between 13 and 18 kbar (1 kbar = 100 MPa). Indigenous dykes were also recognized in Balmuccia, consisting of a late generation of discordant and undeformed glimmerite veins (Shervais and Mukasa 1991). The residual mantle of Finero is veined with bronzitite dykes, apparently undeformed and frequently containing phlogopite and (or) amphibole. They are considered as indigenous, possibly derived from a late melting episode related to the fluid metasomatism of the peridotite.

Betic - Rifean cordillera The orogenic peridotite massifs of Ronda ( - 300 km2) and Beni Bousera (-70 km2) are among the most extended in the world. They are located in the arc-shaped Betic -Rifean cordillera surrounding the Alboran Sea in the western Mediterranean. Although affected by variable degrees of serpentinization up to 50% by volume, the mantle peridotites have retained primary assemblages varying from lherzolite to harzburgite, and minor dunite, interpretable as residues after small to moderate partial melting (Kornprobst 1969). A distinctive feature of the ultramafic massif of Ronda is its concentric zoning showing an inner zone of plagioclase lherzolite facies peridotites, grading into spinel lherzolite and garnet lherzolite zones outwards. The Beni Bousera massif shows similar zonation, although the plagioclase lherzolite facies is absent. A controversy surrounds the existence of a true garnet lherzolite facies in these massifs. According to Obata (1980), the garnet peridotites of Ronda represent a relict of high-pressure equilibrated mantle, preserved by rapid cooling, at the margins of an uplifting mantle diapir. The inner zones, consisting of spinel lherzolite and plagioclase lherzolite facies peridotites, formed by equilibration under decreasing pressure of the hotter interior of the diapir, in the last stages of uplift. Contrary to this model is the interpretation that garnet is not part of the deep-mantle assemblage, but possibly crystallized during ductile flow and dispersion of garnet pyroxenite layers at relatively shallow depth (Kornprobst 1969; Dickey 1970; Schubert 1982). Trace-element and isotope geochemistry reported from Ronda support the assumption that subcontinental lithosphere was the source region of this massif (Reisberg and Zindler 1986; Reisberg et al. 1989). More recently, Remaidi et al. (1 99 1) and Gervilla and Remaidi (1993) observed that trace-element variation in peridotites and pyroxenites, from a small area at the spinel lherzolite - plagioclase lherzolite boundary of the Ronda massif, might be explained by reaction between residual mantle and percolating asthenospheric melts, at relatively low pressure. This episode would relate to the development of a kilometre-scale system of porous-melt flow in the upper mantle, at the base of thinning continental lithosphere.

Dyke rocks from Ronda and Beni Bousera display great variability of mineral assemblages and structural relations, which is at the origin of discrepancies in the petrography classifications reported in the literature. Two broad cate- gories are present: (I) bright green chromian pyroxenites, mainly websterites and clinopyroxenites and (2) dark brown to gray dykes, varying from garnet pyroxenites to spinel pyroxenites and olivine gabbros (Dickey 1970; Obata 1980).

Van der Wal and Vissers (1996) reported different types of chromian pyroxenites (category 1) occurring in all petrologic domains of the Ronda massif, and ranging from discordant to concordant. Among the other types, they described strongly boudinaged "Cr-pyroxenite," set parallel to the foliation of the peridotites, in the plagioclase lherzolite zone. We have observed concordant boudinage and lens structure in Cr-pyroxenites also from the spinel lherzolite zone of both Ronda and Beni Bousera, clearly reflecting the same deformation style as that of the peridotite host. Other concordant, but relatively undeformed Cr-rich pyroxenites in the spinel lherzolite and garnet lherzolite facies were interpreted by Garrido et al. (1993) as a replacement of preexisting garnet websterites or spinel websterites by reaction with percolating melts, thus at relatively low pressure. These data would suggest that more than one generation of Cr-pyroxenite dykes must actually exist at Ronda.

Dykes of category 2 are distributed within the massif in accordance with the petrologic zoning. Garnet pyroxenites occur in the garnet lherzolite and spinel lherzolite zone (subfacies ariegite), whereas spinel pyroxenites predominate in the spinel lherzolite zone (subfacies seiland). Both types are described as concordant (Dickey 1970; Obata 1980; Suen and Frey 1987), but we have constantly observed low-angle discordance with the foliation of the host peridotite. The olivine gabbros occur exclusively in the plagioclase lherzolite zone, and are always oblique to the tectonic foliation (Van der Wal and Vissers 1996; this study), although they are reported as concordant by Dickey (1970), Obata (1980), and Suen and Frey (1987). According to Suen and Frey, the olivine gabbros, plagioclase -garnet clinopyroxenites, garnet clinopyroxenites, and garnet websterites formed in this sequence by fractional crystallization and cumulus of exotic, high-Mg melts, which were probably generated by adiabatic partial fusion of an original garnet lherzolite at a pressure higher than 30 kbar.

Data in the literature clearly are controversial, suggesting that any simple interpretation of the origin of dykes at Ronda and Beni Bousera is questionable, because their geochemical typology, structural relations, and chronology of emplacement have not as yet been established.

Petrography and geochemistry of the analyzed material

Thirty-four peridotites and 30 dyke rocks have been studied in thin polished section and analyzed for major and trace elements, including S, Ni, Cu, PGE, and Au. Tables 1 and 2 summarize provenance and main petrographic characters of the analyzed samples, and analytical data are reported in Tables 3 and 4.

Peridotites The prefixes Sp, Plg, and Gnt in Table 1 refer to spinel, plagioclase, and garnet, respectively, and have been used to indicate the facies of provenance for peridotites from Beni Bousera and Ronda; no indication is given for samples from the Ivrea Zone, as all three massifs pertain to the spinel lherzolite facies. Classification of textures follows that of Mercier and Nicolas (1975), using the textural position of spinel with respect to enstatite and olivine as an indicator of

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Table 1. Petrography of mantle peridotites.

Sample Locality Lithology Main mineralogy Texture

BB- 1 Beni Bousera Sp lherzolite 01 -opx -cpx - sp Granular BB-5 Beni Bousera Sp lherzolite 01 -opx -cpx - sp Foliated BB-8 Beni Bousera Sp lherzolite 01 -opx -cpx - sp Granular BB-2 Beni Bousera Sp harzburgite 01 -opx - (cpx) - sp Porphy roclastic BB-6 Beni Bousera Sp harzburgite Ol -opx -sp Porphy roclastic BB-7 Beni Bousera Sp harzburgite 01 - opx - (cpx) - sp Porphyroclastic RO- 1 Ronda Plg lherzolite 01 - opx -cpx - sp - plg Porphyroclastic RO-25 Ronda Plg lherzolite 01 - opx -cpx - sp - plg Porphyroclastic RO-27 Ronda Plg lherzolite 01 -opx -cpx- sp -plg Granular RO-29 Ronda Plg lherzolite 01 -opx - cpx - sp - plg Granular RO-28 Ronda Plg harzburgite 01 -opx - (cpx) - sp - plg Granular RO-6 Ronda Sp lherzolite 01-opx-cpx-sp Granular RO-8 Ronda Sp lherzolite 01 - opx - cpx - sp Granular RO-5 Ronda Sp harzburgite 01 -opx - sp Granular RO- 19 Ronda Sp harzburgite 01 -opx - sp Granular RO-7 Ronda Sp dunite 01 - (opx) - sp Granular RO- 15 Ronda Gnt lherzolite 01 - opx - cpx - sp - ho Foliated RO-20 Ronda Gnt lherzolite 01 -opx -cpx -sp-gnt Porphyroclastic RO-9 Ronda Gnt harzburgite 01 -opx - (cpx) - sp Granular RO- 12 Ronda Gnt harzburgite 01 -opx - (cpx) - sp Foliated RO- 13 Ronda Gnt harzburgite 01 -opx - (cpx) - sp Foliated M0892 Baldissero Lherzolite 01-opx-cpx-sp Protogranular I M0894 Baldissero Lherzolite 01 -opx-cpx-sp Protogranular I BD 1397 Baldissero Harzburgite Ol -opx - (cpx) - sp Protogranular I M 0 8 Balmuccia Lherzolite 01-opx-cpx-sp Granular M 0 9 Balmuccia Lherzolite 01 -opx-cpx-sp Granular BM 1382b Balmuccia Lherzolite 01-opx-cpx-sp Granular BM 1385 Balmuccia Lherzolite 01 - opx - cpx - sp - ho Porphyroclastic PB-10 Finero Harzburgite 01 -opx - ho- phl -sp Equigranular PB- 17 Finero Harzburgite 01-opx-phl-sp Equigranular PB2 1 Finero Harzburgite Ol - (opx) - (ho) - sp Equigranular PR 1373 Finero Harzburgite 01 -opx - (phl) - sp Equigranular PB3 Finero Dunite Ol -opx - phl - sp Equigranular PB12 Finero Dunite Ol -(opx) -(ho) -sp Equigranular

Notes: 0 1 , olivine; opx, orthopyroxene; cpx, clinopyroxene; sp, spinel: gnt, garnet; plg, plagioclase; ho, hornblende; phl, phlogopite.

the petrologic evolution and grade of deformation of mantle samples.

The primary protogranular (I) is the most primitive mantle texture characterized by the exclusive relation of the spinel with enstatite, frequently in the form of exsolution. It was observed only in the Baldissero massif (samples M0892, M0894, BD 1397), and is considered as typical of slightly depleted mantle that underwent a single melting cycle. The other peridotites from both the Ivrea Zone and the Betic - Rifean cordillera exhibit granular, porphyroclastic, or foliated textures of secondary type in which the spinel also occurs interstitial to and included in olivine. These textures result from recrystallization of mantle during repeated cycles of plastic flow and partial melting, and consistently are found in moderately to strongly depleted lherzolites and harzubur- gites. The term granular is used here without any genetic connotation and indicates peridotites, lacking of clear porphyroclastic texture, or strong flattening of mafic minerals defining the foliation (foliated texture). Although isoorienta- tion is usually visible in granular peridotites, in the field it

is marked by the alignment of minerals such as spinel and (or) plagioclase. This is the case of many samples from the plagioclase lherzolite facies of Ronda, in which the foliation is marked by elongation of the plagioclase aggregates set parallel to each other, but usually immersed in a relatively coarse granular matrix of mafic minerals. Plagioclase is tex- turally associated with Cr-spinel in coronitic assemblages. The spinel harzburgite BB7 from the northwest boundary of Beni Bousera shows strongly mylonitized texture, possibly related to plastic flow and shearing in the marginal zones of the massif. The equigranular texture is characterized by coarse grain, weak deformation, and triple points among crystals. It is considered a result of late recrystallization of mantle, not followed by plastic flow. It is typical of the phlogopite-bearing harzburgites and dunites from Finero, which have recrystallized under the action of fluids. The fol- lowing samples were collected in contact with or close to dyke rocks: BM 1385 lherzolite in contact with the BM1384 Cr-diopside websterite; RO 12 harzburgite 50 cm from R 0 1 1 garnet gabbro; R015 lherzolite close to R014 ariegite;

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Table 2. Petrography of dyke rocks.


Sample

BB-3 BB-4 RO-3 RO-4 RO- 17 RO- 18 RO-2 RO-10 RO-14 RO- 16 RO-2 1 RO- 1 1 RO-22 RO-26 RO-320 BD- 1395 BD- 1396 BD- 1387 BD- 1388 MO-384 BM- 1382a BM- 1383 BM- 1384 BM- 1386 MO-368 MO-380 MO-349 MO-367 PR- 1370 PR- 1374

Locality

Beni Bousera Beni Bousera Ronda Ronda Ronda Ronda Ronda Ronda Ronda Ronda Ronda Ronda Ronda Ronda Ronda Baldissero Baldissero Baldissero Baldissero Balmuccia Balmuccia Balmuccia Balmuccia Balmuccia Bal muccia Balmuccia Balmuccia Balmuccia Finero Finero

Lithology Main mineralogy Setting

Cr-websterite Garnet gabbro Cr-websterite Cr-websterite Cr-websterite Cr-websterite Ariegite Ariegite Ariegite Ariegite Ariegite Garnet gabbro Garnet gabbro Olivine gabbro Gabbro Cr-dp websterite Cr-dp websterite Al-aug websterite Al-aug websterite Cr-dp websterite Cr-dp websterite Cr-dp websterite Cr-dp websterite Glimmerite Al-aug websterite Al-aug websterite Gabbro Gabbro Orthopyroxenite Orthopyroxenite

Opx - cpx - sp Plg - cpx - gnt Opx - cpx Opx - cpx - sp Opx - cpx - ho Opx - cpx Cpx - gnt - plg Cpx - gnt - plg Cpx - gnt Cpx - gnt Cpx - gnt - plg Plg-cpx-gnt Plg - cpx - gnt Plg - cpx - 01 Plg - cpx Opx-cpx-sp-01 Opx-cpx-sp-01 Cpx - opx - sp Cpx - opx - sp Opx-cpx-sp-ol Opx - cpx - sp - 01 Opx-cpx-sp-01 Opx-cpx-sp-01 Cpx - phl Cpx - opx - sp - plg Cpx -opx -sp - (ho) Plg-cpx-opx- ho-sp Plg-cpx-opx-ho Opx - (01 - sp) Opx - 01 - phl

Concordant Discordant Subconcordant Concordant Concordant Concordant Discordant Discordant Discordant Discordant Discordant Discordant Discordant Discordant Discordant Subconcordant Subconcordant Discordant Discordant Concordant Concordant Concordant Concordant Discordant Discordant Discordant Discordant Discordant Subconcordant Subconcordant

Notes: Cr-dp, Cr diopside; Al-aug, Al augite. Other abbreviations as in Table 1 .

R 0 2 0 lherzolite close to R 0 2 2 garnet gabbro; and BB2 harzburgite in contact with BB3 websterite and both about 5 0 cm from BB4 garnet gabbro.

The primitiveness index (PI = (Si02 + A1203 + CaO + FeO)/(FeO + MgO) molar) of the peridotites (Table 3) pro- vides a rough estimate of the modal percent of clinopyroxene and minor plagioclase and varies continuously between 1.03 and 0.65, decreasing from lherzolites to harzburgites and dunites. The index bears positive correlation with T i 0 2 ( r = 0.78), N a 2 0 ( r = 0.71), and other analyzed incompatible elements such as Y ( r = 0.69), G a ( r = 0.81) , Sc ( r = 0.91), and V ( r = 0.92) in both groups of peridotites from the Ivrea Zone and Betic - Rifean cordillera. These trends are consistent with increasing degree of melt extraction by partial melting, but in some samples from the Ivrea Zone, M 0 9 from Balmuccia, and PB3, PBIO, and PB17 from Finero, they may reflect reenrichment of residual mantle by the postulated reaction with silicate melts (Shervais and Mukasa 1991) o r metasomatic crustal fluids (Exley et al. 1982; Hartmann and Wedepohl 1993). In particular, the Finero harzburgites and dunites provide examples of the latter case. They d o not contain clinopyroxene and the relatively high values of PI and incompatible elements (Na, K, Sr , Ba, Zr) of some samples are related with the abundant crystallization of phlogopite and pargasite. The loss on ignition (LOI) values in the Finero

peridotites generally correlate positively with K 2 0 , and reflect increasing modal amounts of the hydrous silicates. Anomalous enrichment in incompatible elements with respect to PI also is visible in a few peridotites of the Betic -Rifean cordillera, for example, R b in plagioclase lherzolite R 0 2 9 , Zr and Y in dunite R 0 7 , and Ba in spinel lherzolite BB 1 and plagioclase lherzolite R 0 2 5 . These anomalies are sporadic and weak and they d o not obviously correlate with any petrologic feature of the samples (mineralogy, lithology, facies of provenance, sample location, and degree of serpentinization). Most peridotites from the Betic-Rifean cordillera (except plagioclase lherzolites R 0 2 5 and R 0 2 7 ) have LO1 > 2 wt. % due to increasing degree of serpentinization.

Dyke rocks The nomenclature of Capedri et al. (1977) and Shervais and Mukasa (1991) has been used for dyke rocks from the Ivrea Zone. However, due to the considerable uncertainty con- cerning the classification of dyke rocks from Ronda, we have used the general terms Cr-websterite for the green chromian pyroxenites, ariegite for all the garnet-bearing clinopyroxenites, and garnet gabbro and olivine gabbro for the corresponding lithotypes.

Mineralogies of Cr-diopside websterites from the Ivrea Zone and Cr-websterites from the Betic-Rifean cordillera

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Garuti et al

are quite similar, characterized by a variable proportion of Cr-rich diopside and orthopyroxene, and olivine, spinel, and hornblende are common accessories. The Cr-websterites BB3 (from the spinel lherzolite zone of Beni Bousera), R04 (from the plagioclase lherzolite zone of Ronda), and M01382a (from Balmuccia) have similar structural setting, occurring as tiny concordant lenses in the mantle tectonites. They appear as having been formed by boudinage of early dykes emplaced before the deformative event that produced the porphyroclastic foliation of the host peridotites. The Cr- websterite BB3 is cut at low angle by the undeformed garnet gabbro BB4. The other samples of Cr-websterites from Ronda represent concordant dykes and veins from the spinel lherzolite and garnet lherzolite facies.

Ariegites and garnet gabbros from Ronda were collected from the spinel lherzolite and garnet lherzolite zones. As mentioned earlier, they generally crosscut the peridotites at a low angle. Both the ariegites and garnet gabbros are fine grained. In our samples we observed a continuous transition from one lithotype to the other by reciprocal modal variation of clinopyroxene and plagioclase. Plagioclase in ariegites is typically interstitial and frequently shows reaction with clinopyroxene in the form of spinel simplectite. Garnet typically exhibits kelyphitic aureolas. These reaction features are indicative of mineral instability and were possibly determined by circulation of high-temperature fluids (in the late magmatic stage ?). Olivine gabbro R026 and gabbro R0320 are from the plagioclase 1 herzolite zone.

The Al-augite websterites from Baldissero and Balmuccia are distinguished by Cr-poor, Al-rich clinopyroxene associated with green hercynite. Plagioclase and hornblende are usually present as accessory minerals. Gabbroic dykes of Balmuccia are characterized by abundant hornblende, whereas green hercynite is accessory, frequently occurring as an exsolution simplectite intergrown with clinopyroxene. The Al-augite websterite M0368 is seen to grade into gabbro M0367 along strike. The glimmerite vein from Balmuccia and the orthopyroxenites from Finero typically are coarse grained and contain abundant phlogopite.

The differentiation index (DI = FeOl(Fe0 + Mg0)) of dyke rocks (Table 4) mostly reflects change of the FeIMg ratio in mafic silicates being a function of the degree of differentiation of the parental magma. Generally, A1203, Ti02, CaO, and Na20 positively correlate with DI, account- ing for variations in modal abundance of orthopyroxene, clinopyroxene, and plagioclase. The Cr-websterites from the orogenic massifs examined in this work have DI in the range 0.14 -0.25 and high Cr203 content mainly due to incorpora- tion of Cr in diopside. They chemically resemble similar rocks occurring as dykes in many other alpine-type peridotites and as xenoliths in alkali basalts, throughout the world (Dickey 1970; Shervais and Mukasa 1991). The dykes of the Al-augite suite from the Ivrea Zone and the ariegites and garnet gabbros from Ronda and Beni Bousera are characterized by higher contents of A1203, Ti02, and Na20 and values of DI between 0.19 and 0.48. Their major and trace element correlation trends indicate fractionation from relatively evolved parent magmas probably derived from a different source with respect to the Cr-diopside websterites. This is supported by isotopic data in the dykes from Balmuccia (Shervais and Mukasa 1991). The late-emplaced glimmerite

veins from Balmuccia and the orthopyroxenite dykes from Finero have extremely primitive composition (DI = 0.16), with A1203, TiO,, and Na20 contents even lower than those of the Cr-diopside websterites, but appear enriched in many incompatible elements such as K, Rb, and Ba, which are related to the presence of phlogopite. These characters, coupled with the lack of synkinematic deformation, support the conclusion that parent magmas of glimmerites and orthopyroxenites in Balmuccia and Finero derived from further melting of the host peridotite, probably under the action of fluids.

Sulfur, copper, and nickel relationships

Peridotites Relations of total S with PI and LO1 are presented in Figs. la and lb. The S content of fresh peridotites (LO1 < 2 wt.%) ranges from 24 to 354 ppm and shows positive, sharply linear correlation with PI (Fig. la). The trend apparently indicates that the sulfide component is strongly incompatible during partial melting and that it can survive intensive melting (Garuti et al. 1984; Lorand 1989, 1993). A relevant point is that the analyzed suite of mantle rocks comprises spinel facies peridotite residua, spinel lherzolites reenriched by reaction with basaltic melts, lherzolites equilibrated in the plagioclase facies, and even phlogopite-rich, residual peridotites that experienced contamination by crust-derived fluids under lithospheric conditions. All these rocks conform to the general trend of S - PI variation and show consistent change of the sulfide mineral assemblage, comparable with those of many orogenic lherzolites (Garuti et al. 1984; Lorand 1989), suggesting that the present-day S-PI relationships are the result of equilibration reached by the sulfide component during partial melting and other large-scale processes affecting the mantle during its evolution in the lithosphere at high temperatures and pressures. A significant influence of any other low-temperature process on the sulfide component in the unaltered peridotites examined can be excluded. Weakly to strongly serpentinized peridotites from the Betic - Rifean cordillera (LO1 > 2 wt. %) exhibit irregular scattering of the S -PI correlation (Fig. la), which apparently is due to sulfur enrichment (291 - 1200 ppm) during serpentinization, as suggested by the S -LO1 positive correlation (Fig. lb). Microscopic study of the serpentinized peridotites of Ronda and Beni Bousera (Lorand 1985; this study) shows that primary sulfides are frequently reduced, forming native Cu from chalcopyrite and Fe - Ni alloys from pentlandite, while S is removed and brought into solution. At the same time, secondary nickel and iron sulfides are deposited along cracks and fissures deriving by sulfurization of silicate Ni and Fe released by serpentinization of olivine.

The primary sulfides in upper-mantle orogenic peridotites of Ivrea Zone, Betic-Rifean cordillera, and Pyrenees are dominated by Ni and Cu phases, mainly pentlandite and chalcopyrite, with minor amounts of pyrrhotite (Garuti et al. 1984; Lorand 1985, 1989; this study). However, total Cu only is a function of the sulfide abundance in rocks, whereas Ni is largely controlled by modal olivine. Because of their different mineralogical distributions, Cu and Ni tend to behave as incompatible and compatible elements, respectively, in partial-me1 ting and metasomatic processes. This accounts for the antithetical correlation of Ni (r = -0.69)

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Tab

le 3

. C

hem

ical

ana

lyse

s of

man

tle

peri

doti

tes.

Sam

ple:

B

B1

BB

5 B

B8

BB

2 B

B6

BB

7 R

01

R

02

5

R0

27

R

02

9

R0

28

R

06

R

08

R

05

R

01

9

R0

7

R0

15

L

ithol

ogy:

Sp

-lhz

Sp

-lhz

Sp

-lhz

Sp

-hz

Sp-h

z Sp

-hz

Plg-

lhz

Plg-

lhz

Plg-

lhz

Plg-

lhz

Plg-

hz

Sp-l

hz

Sp-l

hz

Sp-h

z Sp

-hz

Sp-d

n G

nt-l

hz

PI

Si0

2

Ti0

2

A12

03

CaO

Fe

OtO

' M

nO

MgO

N

a20

K

2O

p205

L

O1

0s

Ir

R

u R

ll F't Pd

A

u S C

o N

i C

u Z

n Sc

v C

r G

a R

b S

r Y

Z

r B

a Pb

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Tab

le. 3

(co

ncl

ude

d).

2

El

Sam

ple:

R

020

R0

9

R01

2 R

013

M08

94

M08

92

BD

1397

M

08

M

09

B

M13

82b

BM

1385

PB

10

PB17

PB

21

PR13

73

PB3

PB12

L

ithol

ogy:

G

nt-l

hz

Gnt

-hz

Gnt

-hz

Gnt

-hz

Lhz

L

hz

Hz

Lhz

L

hz

Lhz

L

hz

Hz

Hz

Hz

Hz

Dn

Dn

-

- - -

-

PI

0.89

0.

78

0.78

0.

75

0.81

0.

93

0.75

0.

83

1.03

0.

90

0.83

0.

81

0.82

0.

75

0.70

0.

69

0.67

Si

O,

44.7

7 44

.19

45.1

6 43

.36

44.9

5 46

.68

45.3

5 45

.39

46.1

1 46

.14

44.6

3 44

.46

45.8

0 45

.74

44.8

4 43

.48

43.7

1 T

iO,

0.17

0.

10

0.07

0.

04

0.06

0.

09

0.01

0.

07

0.18

0.

08

0.07

0.

13

0.10

0.

04

0.02

0.

03

0.03

A

120,

3.

38

2.21

1.

58

1.44

2.

56

3.01

0.

96

2.53

4.

59

2.76

2.

44

3.14

2.

35

1.15

0.

57

1.11

0.

56

CaO

3.

43

2.44

1.

76

2.38

2.

44

3.25

1.

17

2.56

5.

72

3.67

2.

86

2.25

1.

51

0.80

0.

25

1.04

0.

53

FeO

tot

9.24

8.

39

8.44

8.

97

8.10

8.

83

8.74

8.

36

7.37

8.

16

8.93

7.

83

8.33

8.

02

8.43

8.

01

8.38

M

nO

0.17

0.

16

0.15

0.

16

0.15

0.

15

0.15

0.

15

0.13

0.

07

0.16

0.

13

0.13

0.

14

0.15

0.

12

0.15

M

gO

38.6

4 42

.47

42.8

3 43

.63

41.6

9 37

.90

43.6

1 40

.92

35.6

9 38

.90

40.8

8 41

.41

41.0

0 44

.03

45.7

4 45

.85

46.6

2 N

a20

0.

18

0.03

0.

00

0.00

0.

04

0.08

0.

00

0.01

0.

22

0.21

0.

03

0.21

0.

08

0.00

0.

00

0.13

0.

00

K2O

0.

01

0.00

0.

00

0.00

0.

00

0.00

0.

01

0.00

0.

00

0.00

0.

00

0.40

0.

69

0.06

0.

01

0.19

0.

01

p20,

0.

02

0.01

0.

01

0.01

0.

01

0.00

0.

00

0.01

0.

00

0.00

0.

00

0.03

0.

01

0.01

0.

00

0.03

0.

02

LO1

2.76

6.

09

7.40

5.

56

0.02

0.

71

4.44

0.

04

0.13

0.

00

0.00

0.

21

0.39

0.

17

0.00

0.

04

0.00

0

s

5.60

6.

30

5.50

6.

30

14.2

0 12

.30

14.7

0 5.

68

5.46

5.

50

8.14

7.

60

8.50

6.

40

3.70

5.

68

7.30

Ir

2.

60

3.55

2.

66

3.56

4.

92

4.37

3.

31

2.43

2.

29

2.40

3.

33

4.32

5.

38

2.30

1.

31

3.36

3.

00

Ru

3.90

2.

70

5.20

5.

60

6.30

6.

23

6.30

6.

70

8.32

6.

40

5.60

4.

60

2.12

3.

50

1.50

4.

11

2.46

Rh

2.

70

2.00

1.

35

1.10

3.

20

3.30

1.

40

3.80

3.

12

3.10

2.

40

0.97

1.

50

1.30

0.

90

1.35

0.

88

Pt

10.3

0 4.

60

3.90

4.

90

16.8

0 15

.40

13.5

0 9.

90

10.1

0 6.

70

5.30

2.

80

2.40

3.

80

1.30

2.

70

2.50

Pd

8.

30

1.90

7.

10

6.30

9.

30

9.50

6.

70

12.4

0 13

.40

11.2

0 9.

12

9.46

1.

80

3.70

6.

70

0.88

4.

10

Au

11.4

0 31

.10

15.3

0 7.

10

6.10

5.

90

1.20

36

.80

33.4

0 7.

20

7.40

25

.40

45.2

0 80

.30

14.7

0 4.

70

97.5

0 S

578

904

800

769

140

261

549

142

354

256

172

128

125

77

50

46

24

Co

87

93

94

96

105

101

93

103

87

94

100

102

93

106

113

149

111

N i

1739

19

46

2046

19

47

2131

19

45

2030

19

76

1705

19

15

1978

22

48

2143

24

50

2599

31

50

2533

C

u 26

20

13

26

25

25

11

2

1 39

48

26

2

1 5

4 5

6 4

Zn

47

43

46

45

45

47

34

45

56

44

44

48

37

42

43

5 3

44

Sc

13

10

8 9

12

13

6 13

23

15

12

11

8

7 7

5 V

68

5 6

49

54

5 7

70

3 7

68

10

9 82

67

5 6

4

1 35

2 9

2 9

C

r 21

71

2304

24

41

2274

24

77

2519

23

83

2499

36

77

2879

24

90

2356

23

71

2673

28

73

2161

24

63

Ga

3 2

2 2

3 3

0 3

5 4

2 2

1 0

1 1

Rb

5 1

1 1

0 0

4 3

7 5

3 10

2 8

0

0 0

Sr

14

7 3

2 0

0 20

7 0

9 1

1 44

19

5

1 2

Y

4 3

3 2

4 4

0 3

7 4

5 3

1 2

2 2

Zr

11

12

10

3 8

5 6

8 13

8

9 15

9

12

6 8

Ba

3 2

3 2

1 3

5 7

13

23

15

12

17 1

132

7 6

4

0

Pb

5 6

4 4

8 6

7 6

7 8

10

11

6 7

8 5

- N

otes

: S

p-lh

z, s

pine

l Ih

erzo

lite;

Sp-

hz,

spin

el h

arzb

urgi

te;

Plg-

lhz,

pla

gioc

lase

Ihe

rzol

ite;

Plg-

hz,

plag

iocl

ase

harz

burg

ite;

Sp-d

n, s

pina

l du

nite

; G

nt-l

hz,

garn

et I

herz

olite

; G

nt-h

z, g

arne

t 4

harz

burg

ite;

Lhz

, Ih

erzo

lite;

Hz,

har

zbur

gite

; D

n, d

unit

e. T

he p

rim

itive

ness

ind

ex (

PI =

mol

ar (

SiO

, +

A12

0, +

CaO

+ F

eO)/

(FeO

+ M

gO))

mai

nly

indi

cate

s va

riat

ion

in m

odal

clin

opyr

oxen

e an

d z

;rl

plag

iocl

ase.

Maj

or o

xide

s an

d lo

ss o

n ig

nitio

n (L

OI)

in w

t. %

, PG

E a

nd A

u in

ppb

, an

d ot

her

trac

e el

emen

ts i

n pp

m.

0

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e n e w n m m m a m 9 ~ ~ 9 " 7 ~ ? 9 9 8 8 9 ~ ~ 9 m ~ o O W O ~ W O ~ ~ ~ O O O ~ O - ~ ~ - - a m m n b b a z n m - w e a e s 3 e z w m n ~ a e w - a m

@ 1997 NRC Canada

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Tab

le 4

(co

nclu

ded)

.

Sam

ple:

B

D-1

395

BD

-139

6 B

D-1

387

BD

-138

8 M

03

84

B

M 1

382a

B

M13

83

BM

1384

M

03

68

M

03

80

M

03

49

M

0367

B

M 1

386

PR-1

370

PR

- 137

4 L

ithol

ogy:

C

r-w

eb

Cr-

web

A

l-w

eb

Al-

web

C

r-w

eb

Cr-

web

C

r-w

eb

Cr-

web

A

l-w

eb

Al-

web

G

ab

Gab

G

limm

O

pxite

O

pxit

e

DI

Si0

2

Ti0

2

A12

03

FeO

tot

MnO

MgO

C

aO

Na,

O

K2O

'2O

5 L

O1

0s

Ir

R

u R

h Pt

Pd

A

u S

C

o N

i C

u Z

n Sc

v C

r G

a R

b S

r Y

Zr

Ba

@

Pb

- 6

3 8

7 7

9 7

4 8

8 8

2 7

8 7

\D

\D

4

Not

es:

Cr-

web

, C

r-di

opsi

de w

ebst

erit

e an

d C

r-w

ebst

erit

e; A

l-w

eb.

Al-

augi

te w

ebst

erit

e; G

nt-g

ab,

garn

et g

abbr

o; 0

1-ga

b, o

livi

ne g

abbr

o; G

ab,

gabb

ro;

Arg

, ar

iegi

te;

Gli

mm

, gl

imm

erit

e; O

pxit

e,

orth

opyr

oxen

ite.

D

iffe

rent

iati

on i

ndex

DI

= F

eO/(

FeO

+ M

gO).

Maj

or e

lem

ents

and

LO

1 in

wt.

%,

PG

E a

nd A

u in

ppb

. an

d ot

her

trac

e el

emen

ts in

ppm

. n

Can

. J. E

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Fig. 1. Mantle peridotites. (a) Variation of the sulfur content as a function of primitiveness index (PI = (SiO, + A1,0, + CaO + FeO)I(FeO + MgO) molar). Fresh peridotites (loss on ignition (LOI) < 2 wt. %; S < 400 ppm) define a magmatic trend of positive linear correlation. (b) Increase of sulfur with LOI. Serpentinized peridotites (LO1 > 2 wt. % ; S > 290 ppm) experienced S addition during low-temperature alteration. (c) Positive variation of CulNi as a function of PI. 0, Ronda; o, Beni Bousera; +, Finero; a, Baldissero; H, Balmuccia.

1200T

0

(a)

and Cu (r = 0.78) with PI in the mantle peridotites analyzed in the present work. As a consequence, Cu/Ni shows positive correlation with PI and decreases from lherzolites to harzburgites and dunites (Fig. lc), reflecting the increasing

residual character of the mantle peridotites. Metasomatized lherzolites from Balmuccia display abnormal Cu enrichment. The Cu/Ni versus PI variation trend also includes the partly serpentinized peridotites from Ronda and Beni Bousera, indicating that serpentinization caused intense mobilization of sulfur but not significant change in total Ni and Cu, which apparently maintain original relations with the magmatic parameters.

Dyke rocks The S content of dyke rocks varies from 84 to 2128 ppm, showing distinct correlation trends with differentiation (DI) in the Ivrea Zone and the Betic-Rifean cordillera.

Orthopy roxenite dykes from Finero and the glimmerite vein from Balmuccia cluster at low values of S and DI, and appear unrelated to the other dyke rocks in the Ivrea Zone (Fig. 2a). The Cr-diopside websterites and the Al-augite websterites and gabbros define negative S-DI correlation trends, indicating that the amount of segregated sulfide liquid decreases with differentiation, possibly due to increasing S solubility in the parental magma during upwards percolation. This means that heat loss, which normally causes S saturation of the silicate melt, is partly counterbalanced by (1) the increment with differentiation of FeO content (Haughton et al. 1974) and, probably more effectively, (2) decrease of total pressure (Wendlandt 1982), which both favour S solubility. Gorbachev and Kashircheva (1986) have shown that the effect of pressure inverts at about 15 kbar. If this is the case, fractionation of Cr-diopside websterites and Al-augite websterites and gabbros in the Ivrea mantle must have occurred under regimens of decreasing pressure (Shervais and Mukasa 1991) in a region probably much deeper than 45 km.

The S - DI correlation in dykes from both Ronda and Beni Bousera (Fig. 2b) is roughly negative only for the ultramafic websterites, whereas the S content appears to increase with fractionation in gabbros and ariegites. This positive correlation can be explained as an effect of heat loss and fractionation at pressures lower than 15 kbar. However, in garnet gabbros and ariegites from Ronda part of the sulfide phase is associated with kelyphitic coronas, suggesting that some sulfide could have been introduced into these rocks via circu- lating fluids responsible for the reactions.

Most dyke rocks have Nils < 2, suggesting that, like Cu, the bulk of Ni is incorporated in the sulfide phase. There is no systematic study of the sulfides in dyke rocks of the Ivrea Zone and Betic - Rifean cordillera; however, we have observed that pyrrhotite, pentlandite, and chalcopyrite are the main constituents of individual sulfide grains encountered as primary dissemination in the rocks. Generally, pyrrhotite is very poor in ultramafic pyroxenite dykes with DI < 0.18, and its modal abundance increases with increasing DI. The total-rock Cu/Ni ratio reflects variation in modal pentlandite and chalcopyrite of the sulfide, and increases with differentiation (Fig. 2c) in the dyke samples as a whole. The Cr-diopside websterites from the Ivrea Zone define a separate trend. The ariegites and gabbros from Betic - Rifean cordillera have the highest Cu/Ni ratio, although total Cu and Ni are both very low because the sulfide phase is dominated by pyrrhotite.

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Platinum-group-element and gold geochemistry

Total-rock element abundance The total PGE content of the analyzed peridotites averages out at 29.7 ppb, approaching the primitive mantle estimate of 30.2 ppb (Barnes et al. 1988), but it covers a wide range from 8.6 to 54.7 ppb. Generally lower PGE contents are found in strongly depleted harzburgites and dunites compared with lherzolites, although a strong regional control was observed. In the primitive mantle of Baldissero, PGE contents decreases slightly from 54.7 ppb in lherzolite to 45.9 ppb in harzburgite. The more evolved and contaminated lherzolites of Balmuccia range from 33.9 to 42.7 ppb, and the residual, metasomatized harzburgites of Finero are the most PGE depleted (15.4-29.7 ppb). The PGE content is rather constant, and remarkably close to primitive mantle abundances in the plagioclase lherzolites of Ronda (27.4 - 33.7 ppb), but it varies greatly in samples from the spinel and the garnet facies (16.5 -34.8 ppb). In Beni Bousera, total PGE content irregularly decreases from lherzolites (25 ppb) to harzburgites (17.3). Mylonitic harzburgite BB7 is characterized by the lowest PGE content among the mantle rocks (8.6 ppb).

The average Au content (18.7 ppb) of the analyzed peridotites is remarkably higher than the primitive mantle estimate (1.2 ppb). It is relatively low (1.2-6.1 ppb) in Baldissero and Beni Bousera, but it varies greatly in Ronda and Balmuccia (2.2-36.8 ppb), reaching its maximum concentrations in the metasomatized peridotites of Finero (4.7 - 97.5 ppb).

The dyke rocks as a whole exhibit practically the same average PGE content as the primitive mantle (30.7 ppb), but are enriched in Au (average = 27.3 ppb) and less refractory metals (PdIIr up to 40). In the Ivrea Zone, total PGE content decreases from the early-emplaced Cr-diopside websterites (35.1 - 5 1.4 ppb) to the younger Al-augite websterite and gabbros (8.6 - 27.8 ppb) , and late glimmerite veins ( 14.2 ppb) . In the Betic - Rifean cordillera, two suites of dyke rocks are found, distinguished by high (65.4 -250.4 ppb) and low (5.4 - 26.2 ppb) PGE contents, respectively. The PGE abundance in these dykes does not strictly correlate with lithology, although the highest PGE concentrations ( 109 - 250 ppb) are observed in the strongly deformed, possibly older Cr-websterites (R04, BB3), whereas in ariegite (R014) and gabbros (R022, R0320) they range from 63 to 77 ppb. The high total amounts of PGE in the PGE-rich dykes are mainly due to high Pt and Pd. It is interesting to note that enrichment in Pt and Pd systematically occurs in mantle peridotites BB2, R015, and R020 collected adjacent to PGE-rich dykes, whereas harzburgite RO 12 close to the PGE-poor R 0 1 1 garnet gabbro exhibits normal PGE contents.

Interelemental correlation Because of their differential behaviour in petrologic processes, the PGE's are conventionally divided into two groups characterized by different solubility in silicate magmas and overall geochemical behaviour: the Ir group (IPGE = Ir + 0 s + Ru), being refractory, scarcely soluble in silicate melts, and having siderophile character, and the Pd group (PPGE = Pd + Rh + Pt), having lower melting point,

Fig. 2. Dyke rocks. (a) Ivrea Zone. Variation of the sulfur content as a function of differentiation index (DI = FeOl(Fe0 + MgO)). Tie lines illustrate the negative correlation between S and degree of differentiation. B, Cr-diopside websterite; A , Al-augite websterite and gabbros; +, orthopyroxenite and glimmerite. Samples M0368 and M0367 pertain to the same dyke. (b) Betic-Rifean cordillera. The correlation between S and DI is slightly negative in Cr-websterite (0) and positive in gabbros (0) and ariegites (A). (c) Increase of CulNi with differentiation in dyke rocks from both localities. Symbols as in (a) and (b).

higher solubility in silicate melts, and chalcophile character (Barnes et al. 1985). These factors are believed to be at the origin of PGE fractionation in nature. They control interelemental covariation patterns, and are emphasized by varia-

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Fig. 3. Mantle peridotites. Variation of PdIIr as a function of PI. 17, Ronda; 0, Beni Bousera; +, Finero; @, Baldissero; H, Balmuccia.

tion of the Pd/Ir ratio, taking the two elements as typical of their respective groups. As Pd and Ir are concentrated at the same levels in primitive mantle (Barnes et al. 1988), the Pd/Ir ratio should be lower than unity in residual mantle material, decreasing with increasing degree of partial melting.

The analyzed mantle peridotites as a whole display relatively good correlation (r > 0.50) for the pairs 0s-Ir, 0s-Pt, Pd-Ru, Pd-Rh, Pd-Pt, and Pt-Rh. In general, the PPGE's correlate positively with PI (r = 0.54), Cu (r = 0.50), and some incompatible elements (V, Sc, Ga), whereas the refractory IPGE's do not. If three samples from Beni Bousera showing extreme negative anomalies of Ir or Pd are excluded, the Pd/Ir ratio shows positive, although not strong, correlation with PI (r = 0.47), decreasing from about 3 in lherzolites to < 1 in harzburgites and dunites (Fig. 3). Three lherzolites from Balmuccia (M08, M09, M01382b), one harzburgite from Finero, and one lherzolite from Ronda (RO 15) display anomalous Pd/Ir ratios (4.06 - 5.85) at given PI, due to Pd enrichment. In peridotites of the Iwrea Zone, the degree of correlation with PI is very high for Pd (r = 0.74), Rh (r = 0.74), and even Ru (r = 0.75), whereas it is only r = 0.4.9 for Pt.

Correlation of PGE's with S, in the peridotites as a whole, is prevented, mainly because of the irregular addition of S during alteration, whereas PGE's are not mobilized sub- stantially. However, in the 14 unaltered peridotites (12 from the Ivrea Zone and 2 from Ronda), S correlation with total PGE is relatively strong (r = 0.69) on average, but it is below the level of significance for the siderophile 0 s and Ir, and progressively increases from r = 0.58 to r = 0.82 in the order Pt, Rh, Pd, and Ru (Fig. 4). Differences in correlation with S would indicate that, in mantle peridotites of the Ivrea Zone, Ru, Rh, and Pd are controlled by evolution of the sulfide phase, whereas 0 s and Ir are not, and Pt has intermediate behaviour. This probably is due to the fact that 0 s and Ir might reside within phases other than the sulfide phase (silicates, oxides, and alloys), whereas Pt may partition between sulfide and other phases such as Pt - Fe alloys. This conclusion is supported by the finding of a small grain of

Pt - Fe - Ni alloy associated with residual nickel sulfide and chromian spinel in the Balmuccia peridotite (Garuti et al. 1984). The Au content of peridotites does not show any obvious geochemical correlation.

Dyke rocks as a whole display good correlation (r = 0.60 -0.90) among all the PGE's except for 0 s . This coherent behaviour suggests that the PGE's are controlled by the sulfide component. Variation of total PGE's with S and of Pd/Ir with DI in dyke rocks is presented in Figs. 5a and 5b.

In the Cr-diopside websterites of the Ivrea Zone, total PGE's exhibit a linear, positive correlation with S (r = 0.83), and Pd/Ir increases with DI from 15 to 45. Positive correlation with S confirms that overall fractionation of PGE's in Cr-diopside websterites of Ivrea Zone was strongly controlled by the sulfide liquid segregating step by step from the parent magma during uplift. However, the increase of the Pd/Ir ratio with differentiation indicates that part of the Ir might have been extracted by fractionation of olivine and (or) spinel. No obvious correlation involving the PGE's is visible in the other dykes of the Ivrea Zone, although the Pd/Ir ratio increases with differentiat ion within the same dyke, passing from the Al-augite websterite M0368 to the spinel gabbro M0367 (Fig. 5b).

Total PGE's exhibit negative correlation with S in Betic - Rifean samples (Fig. 5a), and the Pd/Ir ratio does not correlate with degree of fractionation (DI) except for the early-emplaced and strongly deformed Cr-websterites BB3 and R04. The old Cr-websterites also are the most S-poor and PGE-rich dykes, whereas S-rich and PGE-poor ariegites R02 and R016 are at the opposite end of the trend. The negative shape of PGE-S correlation suggests progressive dilution of PGE's at increasing sulfur content consistent with variation of the magma to sulfide mass ratio (R factor of Campbell and Naldrett 1979) in these dykes. This could be related either with a sudden drop of sulfur solubility during fractionation of the ariegite and garnet gabbro dykes at Ronda or, alternatively, with the addition of sulfur by metasomatic reaction involving the dykes at some post- magmatic stage. The finding of abundant Fe-rich sulfides associated with the development of kelyphite coronas around garnet, in the most S-rich dykes, may support this latter conclusion.

The Au behaves compatibly during fractionation, cor- relating positively with Mg (r = 0.59), Ni (r = 0.68), Cu ( r = 0.65), and Cr (r = 0.62) and negatively with DI ( r = 0.52), A1203 (r = 0.58), and Na20 (r = 0.46).

Mantle-normalized distribution pat terns Distribution patterns of PGE's, Au, Ni, and Cu normalized to the primitive mantle composition (see the MN column in Table 5 for normalization values) according to Barnes et al. (1988) are presented for peridotites and dyke rocks from each of the investigated massifs (Figs. 6- 10). Variations are visible from one massif to another and even within the same body, reflecting the covariation trends previously described. If we omit the PGE-poorest (8.6 ppb) harzburgite BB7, which exhibits unexplainable negative anomalies in Ru and Pd, some important points emerge from the comparative analysis of the mantle-normalized patterns: ( I ) Mantle- normalized patterns of peridotites grade from nearly flat or slightly PPGE enriched in lherzolites to clearly negative and

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Fig. 4. Mantle peridotites. Variations of individual PGE's as a function of total S in unaltered rocks (LO1 < 2 wt. %). Correlation coefficient r increases in the order 0 s - Ir - Pt - Rh - Pd - Ru, indicating increasing control of the sulfide phase.

Os, ppb (r = 0.11) Ru, ppb (r = 0.82)

Ir, ppb (r = 0.05) Rh, ppb (r = 0.64)

Pt, ppb (r = 0.58)

generally PGE depleted in residual harzburgites and dunites. (2) Exotic dykes from the Ivrea Zone are variably enriched in PPGE, Au, and Cu, but frequently display negative anomalies of Ir and Pt. Indigenous dykes from Finero have distinctive PGE patterns with no significant Ir anomaly, and weak Pt excess. (3) Lherzolites from Balmuccia clearly reflect the same Ir-Pt negative anomaly of exotic dykes. (4) In Ronda and Beni Bousera, peridotites in contact with PGE-rich dykes are variably enriched in Pt and Pd. (5) A variable positive anomaly of Au is present in the peridotites.

Pd, ppb (r = 0.75)

These points retlecting the complex petrologic history of the investigated mantle material, and heterogeneity of the mantle at a very small scale, can be discussed and possibly interpreted in terms of partial melting or metasomatic reactions with basaltic melts and crustal-derived fluids.

Effects attributable to partial melting Effects of partial melting on peridotite composition are well illustrated by the observed differential correlation of metals with PI value. The general positive correlation of PPGE and

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Fig. 5. Dyke rocks. (a) Variation of total PGE content as function of S. (b) Variation of Pdlir with the differentiation index (DI) for the Ivrea Zone (solid symbols) and the Betic-Rifean cordillera (open symbols). B, Cr-diopside websterite; A , Al-augite websterite and gabbro; +, orthopyroxenite and glimmerite; U, Cr-websterites; 0 , ariegite; A , gabbro. Relevant trends are shown by tie lines and field. See text for explanation.

total PGE, ppb

Cu with PI opposite to the relation of IPGE and Ni with PI, as well as reduction of the PdIIr ratio from normal lherzolites to dunites (Fig. 3), is indicative of fractionation between the two groups of metals during partial melting. This determines a morphological change of the mantle-normalized patterns from flat to negative going from relatively undepleted lherzolites to the most residual harzburgites and dunites. Compara- ble negative slopes of mantle-normalized patterns would be expected in residual peridotites when residuum-melt bulk partition coefficients (Dredmelt) calculated for Ir, Rh, Pt, Pd, and Cu in natural mafic magmas (Barnes and Picard 1993) are applied. The bulk DreslmeIt value decreases from 6.3 to 0.21 for PGE in theorder Ir-Rh-Pt-Pd, andisO.l for Cu, indicating that the PPGE's are less compatible with respect to the IPGE's, and Cu is less compatible than PPGE's during partial melting processes.

The reason for the differential behaviour between PPGE and IPGE is debated still, and not completely understood. In equilibrium partial melting models, based on the assumption that Ir and Pd (taken as typical of their group) as well as Cu reside in the sulfide component of the primitive mantle, a strong control on fractionation should be exerted by differences in partition coefficients (D) of metals between sulfide and silicate melt (Barnes et al. 1985; Peach et al. 1990). Since experimental D ~ ~ ~ ~ l " b a l u e s are of the same order of magnitude for Ir and Pd (Stone et al. 1990; Bezmen et al. 1994; Fleet et al. 199 1 ; Crocket et al. 1992), no significant PGE fractionation is to be expected this way. Therefore, the assumption that all of the PGE's are incorporated in the sulfide phase of primitive mantle cannot account for the PGE fractionation observed in the present case.

Other mechanisms to produce PGE fractionation during partial melting are based on the concept that IPGE's might have been present originally in the mantle source as refractory phases, or they might have been retained in the residuum forming restitic minerals instead of entering the melts. For example, the possibility of accommodating Ir in olivine, spinel, or alloys is known, and could well account for fractionation of this metal by partial melting (Crocket 1979; Naldrett et a1. 1979; Barnes et al. 1985). Consistent with this conclusion, we have here observed that Ir and 0 s have no correlation with S, being not controlled by the sulfide component in the upper mantle.

This argument also accounts for the observed negative anomalies of Ir and Pt in dyke rocks. These anomalies, particularly pronounced in dykes of the Cr-diopside suite of the Ivrea Zone (Table 5; Figs. 6, 7), indicate that they could have been derived by partial melting of peridotites containing refractory alloys (Amossk et al. 1990), causing substantial amounts of Ir and Pt to be retained in the asthenospheric mantle source of the parent melts, whereas the other PGE's, including the refractory Os, were extracted to a greater extent. In dykes of Ronda and Beni Bousera, the negative anomaly occurs only in Ir, whereas it is weakly depressed in Pt and even becomes a slight positive anomaly in the PGE-rich dykes (Figs. 9, 10).

As the PGE's in dyke rocks are assumed to be carried by the sulfide, and in consideration of the very high DPGE values between sulfide liquid and silicate melt (Stone et al. 1990; Bezmen et a1 . 1994; Fleet et al. 199 1 ; Crocket et al. 1992), mantle-normalized patterns of the dykes actually should reflect those of their parent magmas at the moment of sulfide segregation. The PGE patterns of Cr-websterites in the Ivrea Zone have weakly positive slope (Figs. 6, 7), similar to those of komatiitic lavas, and suggest that the original magmas probably formed by high degree of partial melting. Although these magmas must have suffered limited PGE fractionation during uprising, as indicated by variation of the PdIIr ratio versus DI (Fig. 5), their strong depletion in Ir and Pt seems to be an original feature. Iridium and Pt positive anomalies are therefore expected to exist in the mantle source of the Cr-diopside parent magma, a character that was not observed in any of the analyzed residual peridotites.

The indigenous orthopyroxenite dykes of Finero are distinguished from the other dykes of the Ivrea Zone because of their lack of Ir anomaly and slight positive anomaly in Pt (Fig. 8). Significantly, their PGE profiles are complemen-

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Table 5. Mass balance for the mixing reaction lherzolite-Cr-websterite in the Ivrea Zone.

Notes: MN, primitive mantle estimate (Barnes et al. 1988); BD-lhz, average composition of the primitive Baldissero Iherzolite; Cr-web, average composition of Cr-diopside websterites; BM-lhz, average composition of the metasomatized Balmuccia Iherzolite; Pt/Pt* = (Pt/8.3)/[ (Rh/ 1 .6)(Pd/4.4)j1'; Ir/Ir* = (lr/4.4)/[ (Os/4.2)(Ru/5.6) 1

"Weight fraction of BD-lhz/Cr-web involved in the mixing reaction.

Fig. 6. Mantle-normalized patterns for peridotites and dyke Fig. 7. Mantle-normalized patterns for peridotites and dyke rocks from the Baldissero massif (Ivrea Zone). 11, lherzolite rocks from the Balmuccia massif (Ivrea Zone). 0, lherzolites (M0892); n, lherzolite (M0894); @, harzburgite (BD1397); (M08, M09, BM1382b, BM 1385); @, glimmerite vein dyke field, variation range for Cr-diopside websterites (BD1395, (BM1386); dyke field, variation range for Cr-diopside BD 1396) and Al-augite websterites (BD 1387, BD 1388). websterites (M0384, BM 1382a, BM 1383, BM 1384), Al-augite

100, websterites (M0368, M0380), and gabbros (M0349, M0367).

dyke field

tary to that of the residual harzburgite host, suggesting cogenetic relationships.

Effects possibly due to metasomatic reactions Compared with pristine asthenospheric mantle, the Baldis- sero lherzolite is enriched in certain PGE's and Au having mantle ratios close to 1 for Ir, Ru, Ni, and Cu, whereas 0 s is in excess by a factor of 3, and Rh, Pt, and Pd are variably enriched by factors around 2. Gold also is in excess, although having the lowest concentrations in the Ivrea Zone. Mantle-normalized patterns, however, display a very low degree of PGE fractionation (PdIIr = 1.9-2.2), in agreement with the primitive geochemical character of this mantle (Hartmann and Wedepohl 1993). Compared with Baldissero, the Balmuccia lherzolite displays evidence of intensive inter-

I dyke field> 10

action with the intruding dykes, which is well documented by systematic changes in isotopic and trace element composition (Shervais and Mukasa 199 1). Noble metals have also been affected. The contents of Os, Ir, Pt, and Ni are lower in Balmuccia than in Baldissero, consistent with the higher degree of melt extraction at Balmuccia, but both Ru and Rh appear undepleted, and Pd, Au, and Cu are even in excess (PdIIr = 2.7 -5.9). This supports the interpretation that the mantle residuum after partial melting was reenriched possibly by metasomatic reaction with the intruding dykes. This hypothesis has been tested by plotting average compositions of the lherzolites of Baldissero and Balmuccia and the Cr- diopside websterites as mantle-normalized patterns (Fig. 1 1). The Balmuccia profile is roughly intermediate to the profiles

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Fig. 8. Mantle-normalized patterns for peridotites and dyke rocks from the Finero massif (Ivrea Zone). o, dunites (PB3, PB12); @, harzburgites (PB10, PB17, PB21, PR1373); dyke field, variation range for orthopyroxenite dykes (PR 1373, PR 1 374).

100 T A

Fig. 9. Mantle-normalized patterns for peridotites and dyke rocks from the Ronda massif (Betic - Rifean cordillera). 0, peridotites from the plagioclase zone (R01, R025, R027, R028); A , peridotites from the garnet zone (R09, R012, R013, R015, R020); 0, peridotites from the spinel zone (R05, R06, R07, R08, R019); PGE-rich dykes, variation range for samples R04, R014, R022, and R0320; PGE-poor dykes, variation range for samples R03, R017, R018, R02, R010, R016, R021, R o l l , and R026.

loo T

of Baldissero and the Cr-websterites, clearly reflecting the same Ir and Pt negative anomalies of the dykes. Mass balance calculation presented in Table 5 indicates that amounts variable from 10 to 30% of Cr-diopside websterite component should have been added to the primitive Baldissero composition to approach the Pd and Au contents of the metasomatized lherzolite samples of Balmuccia. Apparently, reaction with the dykes preferentially reintroduced Pd, Au, Cu, and probably Ru and Rh into the mantle residuum after substantial PGE depletion. Correlation among the PGE's, Cu, and S supports the fact that the elements were contributed to the lherzolite via sulfide liquids possibly at the time of dyke intrusion at depth, or when the early Cr-diopside dykes were

Fig. 10. Mantle-normalized patterns for peridotites and dyke rocks from the Beni Bousera massif (Betic-Rifean cordillera). 0, peridotites from the spinel zone (BB1, BB2, BB5, BB6, BB8); m, PGE-rich Cr-websterite (BB3); A , gabbro (BB4).

loo I

Fig. 11. Comparison among average, mantle-normalized compositions of primitive lherzolite (Baldissero; o) , metasomatized lherzolite (Balmuccia; +), and Cr-diopside websterite (A) dykes listed in Table 5.

loo T

involved in subsequent partial melting of the mantle, and the melts were incompletely extracted, leaving abundant sulfide in the source (Garuti et al. 1984). The effect of this metasomatic process is pervasive and clearly visible, even far away from the contact with dyke rocks.

Appreciable Pt and Pd enrichment are seen in peridotites some 10 cm from PGE-rich dykes in both the massifs of Ronda and Beni Bousera. This is attributed to transfer of the two metals from the dykes to the surrounding mantle peridotite, possibly transported by sulfide melts or sulfide-rich fluids. Because of the extremely high PGE to S ratio in the PGE-rich dykes, transfer of only small volumes of sulfide would have produced significant PGE enrichment in the peridotites. The fact that the metasomatism is not pervasive and affects only the adjacent wall rock, with little effect farther away, suggests that it probably occurred at the time of dyke

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emplacement, and was spatially limited, depending on the low mantle to magma mass ratio and (or) relatively low pressure (i.e., depth) at which the melts were intruded. This type of PGE metasomatism does not appear to be related to, or affected by, the postulated system of melt percolation by porous flow (Remaidi et al. 199 1).

The positive anomaly of Au relative to pristine asthenospheric mantle (Barnes et al. 1988) is a distinctive feature of the analyzed peridotites. The anomaly in the mantle of Ronda, Balmuccia, and Finero is much more intense than that observed in noble metal metasomatized mantle below oceanic rifting zones (Lorand et al. 1993). Mass balance calculation indicates that silicate melt producing the early dyke system could be a reliable candidate for Au carrier, at least in the case of Balmuccia and possibly Ronda. But metasomatic reaction with this kind of melt certainly cannot account for the exceptional Au content of some residual harzburgites and dunites of Finero. Here, the Au anomaly is frequently accompanied by an increase of the PdIIr ratio up to 5, and in some cases an increase of Cu. Enrichment in these relatively incompatible metals is in sharp contrast with the high degree of depletion of Finero, and the lack of any correlation with petrologic parameters does not allow any conclusive considerations to be drawn with regard to the nature of the possible mechanism of addition. We believe that enrichment in Pd - Au -Cu coupled with low contents of Ru -Rh -Pt can hardly be attributed to normal silicate melts produced by partial melting of mantle. More likely, metasomatism of these incompatible metals might be related to the postulated fluid contamination suffered by the Finero mantle during its emplacement into the granulitic crust (Exley et al. 1982). It can be noted that positive Au anomalies in mantle peridotites of the Ivrea Zone increase from the primitive lherzolite of Baldissero to Balmuccia and Finero, parallel to the degree of depletion and crustal interaction of the massifs. Progressive uplifting of mantle into the continental crust, as testified by a decrease of equilibration pressure and temperature (Ernst 1978; Garuti et al. 1978) and by microstructural evolution, is accompanied by increasing metasomatic contamination from the crust, revealed by isotopic and trace element reequilibration (Hartmann and Wedepohl 1993). We believe that crustally derived fluids might have been important as contaminant agents and were probably responsible for progressive reintroduction of some incompatible noble metal. Certainly the crust was not the source of the transition metals we are dealing with, but it provided the fluids that acted as the vehicle for mobilization, transport, and redepositions of the metals within the mantle.

Conclusions

The investigation of orogenic peridotites from the ultramafic massifs of the Ivrea Zone and the Betic-Rifean cordillera indicates that subcontinental lithospheric mantle is hetero- geneous in its distribution of PGE's and exhibits variable but almost ubiquitous enrichment in Au. Heterogeneity is probably the result of the superimposition of different processes, such as partial melting, metasomatism induced by mantle- derived magmas, and contamination by crustally derived fluids.

The most primitive mantle of Baldissero has PGE and Au

content nearly double that of the pristine asthenosphere. Similar enrichment has been reported from the lithosphere - asthenosphere transition zone in incipient oceanic rifting and has been interpreted as a result of interaction between the restitic mantle and percolating MORB-type melts (Lorand et al. 1993). The origin of this feature at Baldissero is difficult to explain by reaction with silicate melt, in consideration of the primitive nature of the Baldissero lherzolite, which does not display typical metasomatic characteristics. The high noble metal content therefore would appear as an original signature of this mantle, possibly derived from limited recycling of crustal material followed by rehomogen- ization and stabilization at depth, before the subcontinental emplacement (Hartmann and Wedepohl 1993). None of the mantle peridotites of the Betic-Rifean group of samples is similar to the Baldissero lherzolite. On average they are more depleted in PGE's, and closer to the primitive mantle estimate. Relative abundances and distribution of PGE's in the different petrologic zones of Ronda would suggest that the plagioclase lherzolites have retained the most primitive PGE signature in the area. The most important variations observed are related to different degrees of partial melting suffered by the mantle.

The PGE's appear to be fractionally removed from mantle during partial melting processes. Among other factors, the presence of specific PGE's in refractory phases within the mantle source of magmas or their capacity to enter restitic minerals during melting processes would exert a major control on fractionation.

Present data indicate that metasomatism and contamination of depleted mantle do occur in subcontinental regions and can produce reenrichments in noble metals. Metasoma- tism related to upwards percolation of basaltic melts coming from the underlying asthenosphere causes enrichment in Ru - Rh - Pd and Au, associated with negative Ir - Pt anomalies. The effects of metal metasomatism may be pervasive (Balmuccia, Ivrea Zone) or confined to the proximity of crosscutting dykes (Ronda, Beni Bousera). This was probably due to the large volume of melt that was injected into the Balmuccia mantle compared with that of Ronda and Beni Bousera, or possibly to remelting of large amounts of early- emplaced dykes at Balmuccia.

The residual mantle peridotite of Finero displays the lowest PGE content. It suffered crustal-fluid contamination that probably produced further melting. It would appear from indigenous dyke rocks, formed during this last melting episode, that contaminating fluids were able to remove Ir and Pt from strongly refractory residua, possibly under the action of high water activity. The increasing positive anomaly of Au and minor Pd seems to accompany progressive equilibration of lithospheric mantle with the deep granulitic crust below continents. Zones of metasomatic reenrichment in the subcontinental mantle may represent a potential reservoir for noble metal and Cu fertile volcanism in continental rift systems.

Present data confirm that PGE's are mostly controlled by the evolution of the sulfide phase in dyke rocks and metasomatized mantle. If we take the degree of correlation with S as a measure of chalcophilicity versus siderophilicity, the Pt should be grouped with 0 s and Ir (low affinity for S), and the Ru with Rh and Pd (high affinity for S). This is not con-

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sistent with the proposed subdivision into IPGE's (0s - Ir - Ru) and PPGE's (Rh - Pt - Pd), based on decreasing melting point. As with the rare earth elements, coherent behaviour is apparent among the light PGE's (Ru -Rh - Pd), which display an incompatible and chalcophile tendency, and the heavy PGE's (0s - Ir - Pt), which have more refractory behaviour. This is in agreement with the experiments of Fleet and Stone (1991), who found that PGE's fractionate between alloys and sulfide liquid according to their weight rather than melting point, 0 s - Ir - Pt preferentially concen- trating in the alloys and Ru -Rh - Pd in the sulfides.

Acknowledgments

The present investigation was financially supported by the Italian Ministero della Universita e della Ricerca Scientifica e Tecnologica (grant 1992) and the Spanish Direccion General de Investigacion Cientifica y Tecnologica (project PB9210922). Drs. Fernando Gervilla, Carlos Garrido, and Kamal Targuisti el Khalifi are acknowledged for their help in the field and useful discussions. Mrs. Federica Zaccarini is thanked for the computer-elaboration of data and graphics. We are grateful to Dr. Steven B. Shirey for his constructive criticisms and comments that greatly improved the paper.

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Documents

Platinum-group-element distribution in subcontinental mantle: evidence from the Ivrea Zone (Italy) and the Betic – Rifean cordillera (Spain and Morocco)