Significance of MORB-derived Amphibolites from the Aracena Metamorphic Belt, Southwest Spain

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 2 PAGES 235-260 1996

ANTONIO CASTRO1*, CARLOS FERNANDEZ1, JESUS D. DE LA ROSA1, INAKI MORENO-VENTAS1 ANDGRAEME ROGERS2

'DEPARTAMENTO DE GEOLOOlA, UNIVERSIDAD DE HUELVA, CAMPUS DE LA RAB1DA, 21819 HUELVA, SPAIN"SCOTTISH UNIVERSITIES RESEARCH AND REACTOR CENTRE, EAST KILBRIDE, GLASGOW G75 0O_U, UK

Significance of MORB-derivedAmphibolites from the AracenaMetamorphic Belt, Southwest SpainThe Aracena metamorphic belt, in the southwest IberianMassif, is characterized by the presence of MORB-derivedamphibolites and continental rocks deformed and meta-morphosed during the Hercynian orogeny. Geochemical relation-ships of these amphibolites indicate the existence of a multiplefractionation process from a set of parental magmas, implyingthe existence of a multi-chamber system beneath the ridge wherethe basalt protolith was extruded. Neodymium isotopic ratios aretypical of MORB, and oxygen isotopes indicate that theseamphibolites have been derived from the uppermost part of theoceanic crust Thermal evolution, revealed from the study ofchemical variations in the amphibole chemistry, is interpreted asresulting from subduction in a low-pressure regime in which thethermal structure of the continental hanging-wall played animportant role. This continental wall was previously heated bysubduction of a slab window resulting from migration of a triplejunction along the continental edge during plate convergence.Three petrologic arguments support this tectonic model Theseare: (1) the low-pressure inverted metamorphic gradient ofamphibolites of the oceanic domain; (2) the high-temperature-low-pressure metamorphism of the continental hanging wall;(3) the early intrusion of boninites into the continental domain.

KEY WORD& amphiboliU; bomrdtts; Hercynian btlt; inverted meta-morphic gradient; slab-window

INTRODUCTIONAmphibolites are rocks of high petrologic interest asthey are normally derived from igneous protoliths oftholeiite affinity that in many cases are recognized asfragments of old oceanic crust. Furthermore, amphi-bolitic complexes are of special relevance in the studyof the thermal evolution of ancient orogenic belts,

such as the Appalachian belt of North America,where the study of the metamorphism of amphibo-litic rocks has resulted in several important referenceworks by Laird (1980), Laird & Albee (1981) andSpear (1982), among others, which have establishedthermal constraints on the conditions of meta-morphism over a wide range of amphibole stability.In a similar way, the petrogenesis and thermal evo-lution of the amphibole-bearing units of the Aracenamassif, Spain, have been investigated. The Aracenamassif is a linear, high-temperature metamorphicbelt (Bard, 1969) which marks an important suturezone in the Hercynian fold belt of Western Europe.The massif was extensively mapped by Bard (1969),but many questions related to the metamorphismand petrogenesis of the amphibolites have remainedunsolved for more than 20 years. Bard reported theexistence of an anomalously high-temperature meta-morphic gradient. The Aracena area was mentionedby Miyashiro (1973, p. 176) as an example of a low-pressure metamorphic belt in which the transversedistance from the thermal axis, in granulite facies, tothe biotite isograd is only 3—10 km. This is a ubi-quitous feature of the Aracena massif, and the originof this ultra-high-temperature gradient hasimportant implications for the evolution of thesouthern sector of the Hercynian belt of Europe.However, the correct identification of this meta-morphic gradient is difficult to determine becausemore than one episode of metamorphism and de-formation has affected the Aracena massif; hencedetailed studies are necessary to constrain the P-Tconditions of the main metamorphic event. Detailedsampling and mapping has been carried out during

•Corresponding author. i Oxford University Preu 1996

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 2 APRIL 1996

the last few years, and the main results of this studyare presented here. Two problems are addressed inthis study: (1) the petrogenesis of the amphibolites;(2) the thermal evolution during the main stage ofprograde metamorphism and deformation. On onehand, whole-rock geochemistry, including major andtrace elements, and oxygen isotopes of the amphibo-lites are used to identify the nature of the igneousprotolith. On the other hand, microprobe analyses,from plagioclases and ferromagnesian phases such asamphibole and pyroxene, are used to constrain P-Tconditions. Furthermore, a detailed mapping and astructural study of a sector of the Aracena meta-morphic belt has clarified the deformational historyrelated to the thermal events that affected the areaduring the Hercynian orogeny. The results of thiswork provide important insights of crustal evolutionand plate dynamics in the past.

REGIONAL SETTING OF THEARACENA METAMORPHICBELTThe Aracena metamorphic belt (AMB) constitutesthe southernmost outcrop of high-grade rocks in theOssa-Morena zone (OMZ), one of the main unitsforming the Iberian Massif, in the west of the Euro-pean Hercynides (Fig. 1). This zone of the IberianMassif is characterized by alternating linear meta-morphic and plutonic belts (Bard, 1969) followingthe regional structural trend. The southernboundary of the OMZ is marked by an east-west-oriented shear zone [the South Iberian shear zone ofCrespo-Blanc & Orozco (1988)] developed mainlywithin the southern part of an oceanic sheet (Ace-buches amphibolites; Bard, 1969; Bard & Moine,1979; Dupuy et al., 1979) but also affecting animbricate sequence of terrigenous sediments, serpen-tinites and metabasites (the Pulo do Lobo zone),interpreted as an accretionary prism (Eden, 1991).The Pulo do Lobo zone is located to the south of theamphibolite band and has been overthrust by them.These amphibolites are traditionally assigned to theOMZ, and constitute the southernmost limit of theAracena metamorphic belt. Although their meta-morphic and structural characteristics are inagreement with this somewhat rigid division, clearlythe complex tectono-thermal evolution of this unitmust be deciphered before a complete, moredynamic picture can emerge. Following the seminalwork by Bard (1969), the geological exploration ofthe AMB remained neglected for many years, untilmore recent studies recognized the exceptional geo-tectonic significance of this sector of the European

Hercynides (e.g. Crespo-Blanc & Orozco, 1988;Eden, 1991; Quesada et al., 1994).

METAMORPHIC DOMAINSAND STRUCTURE OF THEARACENA METAMORPHICBELTThe main feature of the Aracena metamorphic belt(AMB) is the linear disposition of metamorphiczones, which in some cases are bounded by ductilefaults which post-date the main episode of de-formation and metamorphism. These zones havedistinctive lithological, structural and metamorphiccharacteristics and, though they can be integratedwithin a unique process of crustal reactivationduring the Hercynian event, they can be consideredseparately so as to simplify the descriptions. Weprefer to refer to these zones as domains, as theyshow considerable internal coherence as a result of acommon process of deformation and metamorphism.Two main domains can be distinguished in the AMB(Fig. 2): (1) the southern (oceanic) domain, com-posed of amphibolites and mafic schists interpretedas derived from metamorphism of an oceanic crust(Dupuy et al., 1979); (2) the northern (continental)domain, composed of continental pelitic and calc-silicate gneisses with intercalated marbles, amphibo-lites (the Rellano amphibolites) and mafic granu-lites.

The oceanic domain (OD) is composed primarilyof amphibolites with subordinate metadolerites andmafic schists. These amphibolites outcrop as a long(>100 km), narrow (~1 km) band with a rougheast-west azimuth and dipping >50° to the north.This band is disrupted by late, brittle strike-slipfaults (Fig. lb). To the north, the amphibolitic bandis in contact with the continental domain of theAMB. To the south, it overlies the Pulo do Lobozone, recently interpreted as an accretionary prismas mentioned above. The present thickness of theamphibolitic pile is ~600 m in the studied area.

Three main deformational phases can be identifiedin this domain. The first phase (OD-Z))), coincidentwith the main metamorphic stage, is very pene-trative and is responsible for the development of ametamorphic banding and foliation. The intensity ofthis fabric and the grain size increase towards thetop. Structural analysis points to a non-coaxialdeformation regime for OD-Z)), and reveals acomplex kinematic frame, with a top-to-the-souththrust component, and a sinistral strike-slip com-ponent (Fig. 3). During the second phase (OD-.D2)ductile shear-zones are developed which have par-

236

CASTRO et al. MORB-DERIVED AMPHIBOLITES, SW SPAIN

Mainly Precambrian rocks of medium-to high-grade metamorphism

50 km

, 9°W

High-grade metamorphicrocks of the AMB

Paleozoic rocks of low- to verylow-grade metamorphism

6°W

[ j Post-Hercynian coverI ' - ' j Permian sediments and volcanic rocks+*Jl Hercynian igneous acid rocks***1 Hercynian igneous basic rocks

Metasediments and volcanics(South Portuguese zone)

10 km Aracena metamorphic belt| Medium to low-grade rocks

| High-grade rocks

| Amphibolites

Continental domain

Oceanic domainFig. 1. (a) Sketch of the louthernmojt part of the Iberian Mauif, ihowing the diitribution of metamorphic belu in the Oisa—Morenazone [division by Julivert tt al. (1974) and Quesada (1991)]. The inset depicts the location of thu area in the context of the westernEuropean Hercynides. (b) Simplified geological map of the Aracena metamorphic belt and adjacent areai [division after Crapo-Blanc

(1987)].

ticularly affected the bottom of the amphibolite pile,where > 150 m of mylonites can be recognized(Crespo-Blanc & Orozco, 1988). However, an ana-stomosing network of thinner shear bands traversesthe entire amphibolite pile, isolating fish-shapedportions of less deformed rock. The OD-Z>2 developsa retromorphism over the amphibolites to green-schist facies (mafic schists with actinolite-chlorite),which overprints several facies of the previous meta-

morphism. The kinematics of the second deforma-tional phase is similar to that of the first one, butwith a stronger strike-slip component (Fig. 3). Athird, ductile phase of deformation (OD-fi3) pro-duced the shear zones which locally appear at theboundary between the oceanic and continentaldomains. These are centimetre- to metre-scale shearbands, producing a strong mylonitization and retro-morphism of high-grade amphibolites. Locally, they

237


AMB-Continental domain

PlagQ Sp

Hb Plag Cd Gar

Fig. 2. Cross-section of the southern part of the Aracena metamorphic belt. Metamorphic isograds are represented as thick discontinuouslines. 1, Pulo do Lobo metascdimenta; a, masjive quartzites. 2, Act—Hb—Ep amphibolites; a, Act—Chi schists related to the second phaseof deformation and metamorphiim. 3, Hb amphibolites; a, Act-Chl schiit (same meaning as in 2); b, Hb—Cpx amphibolites. 4, Calc-silicates with occasional metabasites. 5, Pelitic granulites and gneisses. 6, Mafic granulites. 7, Marbles. 8, Rellano amphibolites. 9,

Leucocratic gneisses.

penetrate the continental domain, leaving intact theoriginal, pre-shearing contact between the amphibo-lites and continental rocks. The kinematic indicatorsin the shear zone (Fig. 3) suggest a top-to-the-souththrust motion.

The complex succession that characterizes thecontinental domain was folded and metamorphosedduring the main phases of the Hercynian orogeny. Inthe southern part of the CD metamorphism reachedgranulite facies at low pressure (Bard, 1969). Incontrast, towards the north, the CD is characterizedby the presence of a medium- to low-grade meta-morphism affecting a terrigenous series withabundant acidic and basic metavolcanics. This ter-rigenous—volcanic composite series overlies Cam-brian carbonates (Aracena dolomites) and these inturn are situated over Precambrian terrigenous series(La Umbria series).

The first deformational phase in the continentaldomain (CD-Di) generated a regional, penetrative,axial plane foliation associated with large, south-wards vergent folds. The original attitude of bothfoliation and folds is difficult to ascertain as the laterdeformations produced complex interference pat-terns, with local reorientation of the previous struc-tures. Nevertheless, a subhorizontal or gently north-dipping position can be deduced from cartographicarguments. Coeval with the first deformational phase

is a low-/" metamorphism whose thermal peak,represented by low-pressure—ultra-high temperature(~900°C and 3 kbar) gneisses, is late with respect tothe main phase of folding. It is important to notethat the metamorphic zonation within the CD isasymmetrical. The highest temperature zone occursin the south, in contact with the amphibolites of theoceanic domain. This high-T zone, with pelitic andmafic granulites and high-temperature amphibolites(Rellano amphibolites), defines a band parallel tothe oceanic domain but oblique to the structuraltrend of the CD, in such a way that the main litho-logies appear metamorphosed at different degreesfrom east to west. Towards the north, the low-/"metamorphism evolves from granulite to amphi-bolite and finally to greenschist facies. Severalductile shear zones indicating either strike-slip ornormal senses of motion appear across the CD.

ROCK TYPES AND FIELDRELATIONSHIPSMetabasites of the oceanic domainThe oceanic domain of the AMB is composed of athick pile of amphibolites, which have very fewexotic components such as metasomatic nodules andsome pelitic layers associated with a late episode of

238

CASTRO it aL MORB-DERIVED AMPHIBOLITES, SW SPAIN

(a) N (b) N

(0 N

Fig. 3. Structural diagrams of the AMB. Equal-area, lower-hemi-sphere projectioru. Contour densities of poles to foliation. Uniformdistribution: 3a. Dots: mineral or mylonitic lineation. (a) Firstphase, oceanic domain. Contour interval: l-4o\ Number of folia-tion poles: n = 49. Mineral lineation: n = 9. (b) Second phase,oceanic domain. Contour interval: 1-65(7. Mylonitic foliation:n = 47. Stretching lineationi: n = 32. (c) Third phase, oceanicdomain and second phase, continental domain. Triangles: myloni-tic foliation, n= 16. Dots: mineral and stretching lineations, n= 10.(d) First phase, continental domain. Contour interval: 1 "280".

Foliation: n = 285. Mineral lineation: n = 47.

deformation and retrogression. Previous work byBard & Moine (1979), as well as the data includedin this paper on the geochemistry of these amphibo-lites, clearly supports the idea that they representmetamorphosed tholeiitic basalts of MORB affinity.This feature gives the AMB a special significance inthe geotectonic interpretation of the Hercynianchain of Iberia. Several geotectonic models havebeen proposed (e.g. Abalos et aL, 1991; Bard, 1992;Quesada et aL, 1994) in recent years, but noneaccounts for the complex metamorphic evolution ofthe amphibolite pile.

The following rock units can be distinguishedaccording to textural and mineralogical criteria: (1)amphibolites; (2) mafic schists; (3) metadolerites.These rock units are disposed in parallel bands,elongated following the regional trend of structures(east-west to NW-SE). Within the group of amphi-bolites, two main types may be distinguishedaccording to common assemblages: (1) normalamphibolites, composed of amphibole—plagioclase,which dominate the oceanic domain of the AMB; (2)quartz-rich amphibolites, which are interlayered

with the former and are composed of quartz—diopside-plagioclase—amphibole. Figure 4 shows thetypical aspect of layered amphibolites with quartz-rich facies intercalated with normal amphibolites.

In zones with low-grade metamorphism and lowdeformation there are bands in which a relict mag-matic texture may be recognized. These are meta-dolerites that may contain relict phenocrysts ofaugite and plagioclase. These metadolerites appearas lens-shaped bodies (10-30 cm in size) bounded byanastomosing shear bands. They are mainly locatedtowards the south of the pile. The foliation of theOD-Z)[ is parallel to a characteristic layering inwhich fine-grained amphibolites alternate withmedium- to coarse-grained amphibolites. The onlydifference between the two types is the grain size andnot the composition as in the case of the layeringshown in Fig. 4, and referred to above.

Figure 5 shows the distribution of facies andstructures in a typical section across the amphibolitepile near the locality of Almonaster. The mainepisode of metamorphism is associated with OT)-D\,indicating a top-to-the-south directed thrusting, anda sinistral strike-slip component, as based on meso-and microstructures observed in the rocks. Thisgrade of metamorphism increases towards the north,reaching the transition to the granulite facies nearthe contact with the continental domain. As themain foliation dips towards the north, the meta-morphism appears inverted, with the amphibolite—granulite facies transition at the top of the meta-morphic pile. This is a peculiar feature of the AMBand will be discussed below.

The mafic schists are chlorite-actinolite—albiteschists developed in discrete shear bands affectingthe pre-existing amphibolites. They represent thesecond episode of metamorphism and deformation(OD-.D2). The most important feature of thisshearing phase is that it occurred under low-gradeconditions producing a local retrogression in theamphibolites. In places, hornblende amphibolitebands are preserved. As the main objective of thisstudy is to analyse the first episode of meta-morphism, these mafic schists are not discussed.

Metabasites of the continental domainIntercalated within the marbles and calc-silicates ofthe continental domain there are tabular bodies (10-200 m thick) of very coarse-grained amphibolites,called the Rellano amphibolites. Hornblende crystalsare normally > 2 cm in length. Occasionally, inirregular pockets, the major length may reach > 10cm. These coarse-grained amphibolites are isotropic

239


Fig. 4. Mesoscopic aspect of layered amphibolites trom the Hb zone.

240

CASTRO el at. MORB-DERIVED AMPHIBOLITES, SW SPAIN

LegendContinental Domain

}•—I—̂—| PelKic granulKes

Oceanic Domain

Inverted metamorphic gradient

Vertical scale (m)6 0 0 -

550-1Samples

Hbamphibolites

Act-Hb-Epamphibolites

]*.•';•] Hb-Cpx amphibolites

| .^r j Leucocratic amphibolites

I I Medium to fine-grained,I 1 banded amphibolites

• Fine-grained, homogeneousamphibolites

Retromorphism

* £ £ • ^ S _ : Shear zones

Vein-filling

_ Leucocratic veinsEpidote-bearing veinsand nodulesPlagioclase veins

F=77i "Pulo do Lobo" metasedimentsIXvMi (greenschist facies)

Almmaster \la Real ^

I km

Location of the column

/ /

>^-'.'./.', ContinentateJ3^ •• : domain•:

1 1 Hb amphibolites

E3 Act-Hb-Ep amphibolites

\i*j£yyyyyyyyyy

Fig. 5. Schematic profile of amphibolitei from the oceanic domain near the village of Almonajter la Real (see inset for location).

rocks, not affected by the main episode of regionaldeformation of the continental domain.

Other metabasites outcropping in the continentaldomain are mafic granulites which appear as kilo-metre-sized, irregular bodies crosscutting the mainfoliation of the leucocratic and pelitic gneisses withinwhich they are enclosed. These rocks are always iso-tropic, with no apparent foliation. They constitutepremetamorphic intrusions of mafic magmas that arelate with respect to the main episode of regionaldeformation of the continental domain, but earlierthan the metamorphic peak. These relationships are

important in interpreting the tectonothermal evo-lution of the AMB.

PETROGRAPHYThe normal amphibolites of the oceanic domainare generally composed of plagioclase—amphibole ±clinopyroxene ± sphene ± epidote ± ilmenite ±magnetite ± quartz + apatite, with amphibole andplagioclase constituting >90 vol.% of the rock.Despite such compositional homogeneity there areimportant variations in grain size. In general,

241


coarse-grained amphibolites ( > 4 mm) are moabundant near the top of the pile in the Hb-C]zone, where they appear as narrow bands (5-50 crintercalated with medium-grained (1—4 mramphibolites. The contacts are sharp and there ano compositional differences between these altenating bands. Medium-grained (1-4 mm) amphiblites dominate the metamorphic pile in the Hb zoiand evolve towards the south, in the Act-Hb zonto fine-grained ( < 1 mm) amphibolites alternatingnarrow bands (2—20 cm) with zones of retrogressicwhere the amphibolites have been transformed inchlorite—actinolite schists. Independent of the grasize, the amphibolites have a well-developed metmorphic foliation parallel to the granulometnclayering. This feature, together with the lack ofcompositional differences between layers, suggeststhat the foliation and layering developed at the sametime during the main episode of metamorphism anddeformation (OD-Di), the coarse-grained bandsperhaps representing zones richer in fluids comparedwith adjacent zones poorer in fluids that developedhigh-nucleation density textures.

Table 1 summarizes the main petrographic varia-tions with distance in the normal amphibolites fromthe oceanic domain. In the Hb—Cpx zone, at the topof the amphibolite pile (Figs 2 and 5), the commonassemblage is plagioclase-hornblende-clinopyroxenewith subordinate sphene, ilmenite and magnetite.Plagioclase shows a continuous zoning ranging fromAnjo at the rim to Arigj at the core. The amphiboleis brown to green hornblende occasionally zonedwith a brown core and a dark green rim. The grainboundaries between this amphibole and the clino-pyroxene (diopside) are sharp with no signs ofreaction between the phases. Locally, diopside mayshow reaction textures to an actinolitic amphibole,probably related to the retrogression that affectedthe amphibolite pile.

The change in the colour of the amphiboles whichdefine the main foliation is related to the amphibolecomposition, which, in turn, reflects temperature.This feature was first recognized by Bard (1969).Amphiboles evolve from a yellowish green colour atthe bottom through dark green to brown at the topof the pile. The change from green to brown isroughly coincident with the appearance of diopside.This change is mainly related to the titaniumcontent of the amphibole, but other compositionalvariations are also involved, as revealed bymicroprobe analyses. These changes clearly indicatean increase of metamorphic temperature from thebottom to the top of the amphibolite pile.

Another petrographic feature of the amphibolitesin the Act-Hb and Hb zones of the OD is the pre-

100 urn

Fig. 6. Back-scattered electron images (Z-contrast) showing thezoning in plagioclase with relict cores. The brighter zones areremnants of calcic cores as revealed by the BSE (Z-contrast)image. Such zones are free of amphibole inclusions, these beingabundant in the darker zones, which represent the new plagioclase

formed in the coune of metamorphism.

sence of relict igneous phases such as plagioclase andaugite, supporting the igneous nature of the amphi-bolite protolith. Relict plagioclases are characterizedby the presence of oscillatory zoning in the cores ofporphyroblasts. These normally appear fragmentedand contoured by the main foliation defined byamphibole and new plagioclase. Figure 6 shows thetypical aspect of one of these relict plagioclases.

The Rellano amphibolites appearing in the con-tinental domain are characterized by the very coarsegrain size and the dark brown colour of the amphi-boles. The texture is typically granoblastic withabundant polygonal junctions between plagioclasesand between plagioclase and amphibole. They arecomposed of brown hornblende—plagioclase ±sphene ± clinopyroxene ± ilmenite.

Finally, the mafic granulites of the CD are char-acterized by the assemblage clinopyroxene—ortho-pyroxene—plagioclase—quartz + ilmenite i magnetiteand the retrograde phases biotite and amphiboledeveloped as a corona reaction after orthopyroxene.These granulites are not deformed and show atypical granoblastic texture between ortho- andclinopyroxene. Plagioclase normally appears ascentimetre-sized, poikiloblastic crystals enclosing thepyroxenes. Though these rocks normally have meta-morphic textures, many facies retain their originaligneous textures, the mineral assemblage beingessentially the same. These igneous facies areimportant, as they indicate the igneous provenanceof these mafic bodies.

MINERAL CHEMISTRYPlagioclase, amphibole and pyroxene have been

242

CASTRO el aL MORB-DERIVED AMPHIBOLTTES, SW SPAIN

Table 1: Summarized petrography of normal amphibolitesfrom the oceanic domain, showing the mainvariations across a complete section of the amphibolite pile

Plag.Hornb.

CpxActin.QuartzGarnet

EpidoteChlor.Sphene

Magnet.Ilmenite

Om £ 52m

Hb-Cpx zone y* *','

Continuous zoning A/

Brown to green y Green

Diopsidc A

/

/Scarce or absent ,/

/Only in pods with Cpx /

\

'',

. 260m 480m

Hbzone ^ Act zone

No zonmg /,Reltct (iftneoai) crvstals ^

/Bluish green to yellowish green y

/ Relict igneous mgite

Scarce, only in mm bands y

In mm bands ^ Abundant

\

/

analysed by electron probe microanalysis (EPMA)to constrain P-T conditions during metamorphism.Pressure may be estimated from the composition ofpyroxene in equilibrium with quartz in the quartz-bearing amphibolites. Plagioclase—amphibole pairsare used to identify equilibrium conditions, andfinally the chemistry of amphibole may be useful toobtain some information on the P-T trajectories.Zoned amphiboles have been analysed to constrainP-T variations during metamorphism in the con-tinental domain and to compare these variationswith the results obtained from similar zoning pat-terns in amphiboles of the oceanic domain.

Representative samples of amphibolites from theoceanic and continental domains have been analysedby EPMA, mainly for amphibole, plagioclase andclinopyroxene. Several samples of the Rellanoamphibolites from two distinct localities of the con-tinental domain have been collected. A set of 24samples has been collected from amphibolites of theoceanic domain in a 550 m traverse perpendicular tothe foliation, from the Act-Hb zone to the Cpx-Hbzone (Fig. 5, Table 1). Quartz amphibolites havebeen excluded from this study to keep the composi-tional variable constant. These, have a very homo-geneous whole-rock composition (Table 2), and

variations in the amphibole compositions may berelated to changes in the P-T conditions.

Bard (1969) divided the Aracena metamorphicbelt into three main domains based on the colour ofamphiboles in metabasites, including amphibolitesappearing in the continental domain. Variationsfrom green-blue through green to brown occur inthe oceanic domain in a section of ~500 m, denotinga very high thermal gradient. The study of thesevariations is critical in understanding the meta-morphic evolution of the AMB.

Selected samples were analysed by EPMA(Cameca Camebax) at the University of Oviedo(Spain) and by EPMA (JEOL Superprobe 733) atthe University of St Andrews (Scotland). Operatingconditions were similar in both laboratories, with aprobe current of 20 nA and an accelerating voltageof 15 kV. Standards were in both cases a combin-ation of pure metals, oxides and minerals, with theapplication of ZAF correction procedures. Theresults were checked, giving a high degree of repro-ductivity between the two laboratories. Specialemphasis was placed on amphiboles but other co-existing phases such as plagioclases and pyroxeneshave been also analysed to determine the pressureand temperature conditions during metamorphism.

243


Table 2: Average chemical composition of normalamphibolites and typical MORB

Rock type:

SiOjTiO2

AbO,FejO,FeOFUJOJT

MgOMnOCaONajOK20P2O

LOITotal

Normal amphibolrte

Av.

49-281-17

16-43——9-077-220-17

11-562-830-180-141-01

9904

Max.

61-501-58

2060——11-308-38023

12-703-660-540-213-10

Min.

47-80053

14-50——6-605 190-116232-340-010-050-46

1

49-301-80

15-202-40800

—8-300-17

10-802-600-240-21

—9902

2

49-431-62

15-972097-55

—8-500-18

10 732 870-180-15

—99-27

1, Oceanic tholeiite (Hyndman, 1972); 2, amphibolite (Spear, 1981).

Amphibole

Representative amphibole analyses are listed inTable 3. The schemes of Robinson et al. (1982) wereused for formula normalization. The calculation ofthe oxidation state in amphiboles is crucial for acomplete estimation of site occupancy, especially theNa occupancy in the M4 site. Any procedure basedon stoichiometric constraints is inaccurate, as theamphibole formula may have vacancies in the A siteand in the eight-fold coordinated M4 site, makingthe stoichiometric recalculation dependent onassumptions on site occupancy. It is apparent thatthe ferric:ferrous iron ratio is strongly dependent onthe_/o, of the system and it may be evaluated if theoxygen fugacity is determined in other com-plementary ways.

The experimental work of Spear (1981) estab-lished a correlation between the oxygen fugacity andthe composition of amphiboles by running severalexperiments on MORB-derived amphibolites at dif-ferent oxygen fugacities fixed by the three bufferswustite-magnetite (WM), quartz-fayalite-magnetite(QFM) and haematite-magnetite (HM). The resultsare not of general applicability, as bulk compositionand temperature condition the / o ? in the system.However, in our case these experimental data can beused as a good guide for ferric iron estimations in the

amphibole molecule for the following reasons: (1)the rock used by Spear (1981) in the experiments isvery close in composition to the amphibolites of thisstudy (Table 2); (2) the P-T conditions of theexperimental work by Spear (1981) are comparablewith the P-T range of the studied amphibolites.Cation proportions in the structural formulae, inde-pendent of the ferric iron recalculation, comparedwith the amphibole compositions obtained on threedifferent buffers in Spear's (1981) experiments,indicate that the Aracena amphiboles crystallizedunder conditions of fOl around the QFM buffer.Furthermore, these amphibolites are derived from aMORB protolith and most terrestrial basalts haveoxygen fugacity fixed around the QFM buffer(Carmichael & Nicholls, 1967; Blundy et al., 1991),conditions that possibly were maintained during themetamorphic process.

Moreover, as reported experimentally by Moody& Jenkins (1983), oxygen fugacity determines thestability of some minor minerals such as sphene,epidote, ilmenite, magnetite, rutile and haematite inmafic systems, epidote and haematite being char-acteristic of the most oxidizing regimes. In theseexperimental runs (Moody & Jenkins, 1983), theassemblage sphene-ilmenite-magnetite was foundwhen oxygen fugacities were around the QFM andnickel-nickel oxide (NNO) buffers. At the QFM

244

CASTRO el al. MORB-DERIVED AMPHIBOLITES, SW SPAIN

Table 3: Representative microprobe analyses ofamphiboles

Rock type: Cpx-Hb amphibolitaj Hb amphibolites Act-Hb amphibolrta Rsllano

Sample:

SI02

TiO2

AJjO,FejOa*FeOMnOMgOCaONa2OK20Total

•Fe2O =

893-22-9 893-24-12 893-25-3

47-391007 07202

12-760-33

12-1811-920 880-33

95-89

45-891-168372-19

13-840-26

11-8812-110-72008

96-49

12-5% FeO,.

44-641-688942-24

14-140-21

11-5011-981-740-11

97-18

893-28-4

46051-558691-98

12-51033

12-3011-881-04012

96-46

893-30-9

45-581-198482-33

14-720-44

11-1311-801180-09

96-94

893-31-5

47-251-167-24228

14-410-30

11-9711-791-040-O6

97-50

893-39-8

47-280737022-14

13-480-31

120212-330-620-66

96-60

893-40-2

48-141-447-792-06

12-980-26

12-9110-66

1-11009

97-45

893-41-1

52-560-163-371-459-170-29

15-5412-560-160-10

95-37

89227-4

47-630-878-201-98

12-46028

13 8211-771 -61007

98-69

89227-7

46-711-278061-96

12-390-20

13-6611-451-58008

97-24

89227-10

44-151-669-632-13

13-450-35

11-9511-812-100 1 7

97-31

buffer, sphene occurs as a low-temperature productwhereas ilmenite-magnetite are high-temperatureproducts. The common oxide assemblage of theAMB amphibolites is ilmenite—magnetite, ilmenitebeing very poor in titano-haematite (<5% mol).Sphene is present in all the amphibolites from theoceanic domain. According to these constraints, wehave adopted a recalculation procedure based on afixed proportion of ferric iron of 12-5% of the totaliron, the composition of the amphiboles bufferedaround the QFM buffer, measured with the probe,for all the amphiboles studied in this paper,according to the experimental data of Spear (1981).The quartz-rich amphibolites and other metabasitesof the AMB probably do not fit this constraint, asthey have compositions that are different from thatof the experimental runs of Spear (1981). Usingthese constraints for the ferric iron recalculation, anapproach can be made to the cation allocations inthe structural sites of the amphibole molecule fol-lowing the scheme of Robinson et al. (1982). Thisprocedure allows one to determine the occupancy inthe A site and the application of the classificationscheme of Leake (1978) modified by Pe-Piper (1988)presented in Fig. 7.

The best correlation is between Ti and tetrahedralAl, denoting the relative importance of the Ti-Tschermak (Ti-Ts) exchange Al^TiMg-jSi-j. Also,there is a good correlation between Al and A-siteoccupancy, denoting the edenite-type (Ed) exchangeAlIVNa(A)Si_iD-i. These two exchanges accountfor most of the four-fold co-ordinated Al. Anassessment of the relative importance of the glauco-phane exchange (Gl) is difficult as Na(M4) is

0.8

0.6

0.4

0.2

S

I•

• •Actinolhkhornblende

J_

Hornblende

Actinolite

-H-6.0 6.5

Oceanic domain

O Cpx-Hb zoneo Hbzone+ Aa-Hbione* Zoned crystal

7.0 7.5Si

Continental domain

ffl Zoned cryital (Molarei)* Zoned cryjtal (Sin C.)

8.0

Fig. 7. Lcakc'i (1978) classification diagram for amphiboles[modified by Pe-Piper (1988)] plotting amphiboles from theoceanic and continental domains of the Aracena metamorphicbelt. (Note that pargajitc and edenite dominate the compositionof the cores of zoned amphiboles from the Rellano amphibolites.)

dependent on the ferric-iron recalculation. Conse-quently, its relative importance is poorly constrainedby this method of cation correlation. It can,however, be estimated by calculation of the glauco-phane and other molecules in the amphibole spacecomposition (see the Appendix).

Figure 8a shows a molecular plot relating the

245


* Spew (1982) <* Tmauki a al. (1984) t » Spear (1981)

Fig. 8. (a) Molecular plots relating the additive componenttrcmolite (Tr) to the end-member edenite (Ed). Thae moleculeiare calculated from algebraic transformation of the amphibolespace composition (see Appendix). For comparative purposes, theamphiboles from the experimental rum at 3 kbar by Spear (1981)have been included in the plots (encircled crosses). Other symbolsas in Fig. 7. (b) Baricentric projection of the analysed amphibolesin the space Tr-Gr-Pg (Ed + Ts) through the FcMg-i interchangeand from PI (AnM). Curve 4 marks the compositional evolution ofthe Aracena amphiboles (symbols as in Fig. 7). Other amphibolesare plotted for comparative purposes; these are: 1, experimentaldata at 3 kbar of Spear (1981); 2, zoned amphiboles of Trzicnskiltd. (1984); 3, zoned amphiboles of Spear (1982). Isotherms andisobars are tentatively traced following the experimental data ofSpear (1981) as a guide. The model reproduces the anomalouscooling with increasing pressure referred to by Trzienslri tt ai.

(1984).

the AMB amphiboles, indicating that the confiningpressure was < 3 kbar in both the oceanic and con-tinental domains of the AMB. Furthermore, thecompositional variation displayed by a single zonedcrystal is coincident with the variation displayed byamphiboles from different samples collected acrossthe amphibolite pile (Fig. 8a).

Figure 8b is a baricentric projection of theamphibole—plagioclase space composition, projectedfrom PI (Anso), Ti-Ts and through the FeMg_iinterchange. As Ed and Ts are strongly dependenton temperature (e.g. Spear, 1980, 1981; Blundy &Holland, 1990) they are plotted together as a par-gasite molecule in the right-hand corner. Gl isplotted in the upper corner; as this molecule isstrongly dependent on pressure, the figure may betaken as a distorted P—T diagram in which it is pos-sible to identify the shape of any P—T path linkingdata points from a zoned amphibole or from a set ofsamples from a graded massif. Isotherms and isobarsare tentatively traced taking as a reference the con-stant-pressure experimental data at 3 kbar of Spear(1981). The path depicted by the Aracena amphi-bolites is nearly horizontal and within the low-pressure region of the diagram. For comparativepurposes, other paths obtained from zoned amphi-boles from other metamorphic belts are also shown.This diagram reproduces the effect of increasingpressure during cooling reported by Trzienski et al.(1984) and deduced from a zoned crystal.

Zoned amphibole crystals from the oceanic andcontinental (Rellano amphibolites) domains displaya characteristic P—T pattern on the baricentric pro-jection Tr—Gl—(Ed + Ts). According to this pattern,zoned amphiboles record a first episode of increasingtemperature marked by an appreciable increment inthe Ed + Ts content, and a second episode of coolingdenoted by an increase in the Tr content. Both epi-sodes occurred at low pressure, this being the moresalient feature of the thermal evolution of theAracena metamorphic belt.

molecules of Ed calculated for the Aracena amphi-boles to the additive component tremolite (Tr). Thisfigure confirms the importance of the Ed-type sub-stitution in our amphiboles, implying that the maincompositional variation is related to temperature(Blundy & Holland, 1990). The Gl content is nearlyconstant for amphiboles which equilibrated at dif-ferent temperatures. Also included in this plot arethe experimental data of Spear (1981) for amphi-boles synthesized at temperatures from 551 to 763°Cat 3 kbar constant pressure. The Gl contents of theseexperimental amphiboles are higher than those of

Amphibole—plagioclase relationshipsPlagioclase is, together with amphibole, thedominant phase in the amphibolites of the oceanicdomain. As indicated in Table 1, two distinct typesof plagioclase may be distinguished according to thegrain size and composition. Type I plagioclase hasthe same grain size (1-4 mm) as coexistingamphibole, with which it shows polygonal contacts.This indicates the existence of textural, and probablychemical, equilibrium between the two phases, asthey grew at the same time under the same P—Tconditions. This type shows slight changes in cotn-

246

CASTRO el at. MORB-DERIVED AMPHIBOLITES, SW SPAIN

position (An42—An^o) from the Hb—Act zone to theCpx—Hb zone. Type II are large crystals (1~4 mm)characterized by the presence of an anorthite-rich(AngQ-jo), igneous core with oscillatory zoning. Thiscore is free of amphibole inclusions. The rim has acomposition identical to that of the matrix plagio-clase at around Ao^o-so- Figure 6 shows a back-scat-tered electron (BSE) scanning electron microscopeimage illustrating these relationships betweenplagioclase and amphibole. It can be appreciatedthat the amphiboles defining the foliation are inter-grown with plagioclase of the matrix as well as withthe outer rim of the type II plagioclases. This impliesthat these outer rims re-equilibrated during thecourse of metamorphism as part of the plagioclase isderived by 'retrogression' of an igneous, anorthite-rich plagioclase. The matrix plagioclases have beenanalysed with the probe near the contacts withadjacent amphibole crystals. Representative analysesare listed in Table 4. The relationships betweencoexisting amphibole and plagioclase indicate thatequilibrium only existed at the local scale, as pairsfrom the same sample have distinct slopes. Thisobserved local equilibrium is probably related to thecompositional variability of relict plagioclases.Amphibole may grow in two different local situa-tions in a reduced rock volume: (1) amphibolegrown together with plagioclase during a progradereaction after a lower-grade assemblage (e.g. Act-Ab—Ep); (2) amphibole grown by a reaction withrelict plagioclase and other lower-grade amphibole.In this case, as the plagioclase is richer in An thanthe expected metamorphic plagioclase, the equili-brium composition may be locally influenced by ananomalously high activity of Ca, producing anamphibole with a very low Na occupancy in M4.

ClinopyroxeneClinopyroxene is an abundant phase in the normalamphibolites of the oceanic domain at the top of thepile (Fig. 5). However, in the quartz-rich amphibo-lites of the Hb zone, clinopyroxene is a veryabundant mineral and occasionally the only ferro-magnesian phase. For instance, the leucocratic bandsof Fig. 4 are very rich in clinopyroxene and quartzwhereas the adjacent normal amphibolites are free ofclinopyroxene. This fact is in agreement with theexperimental data reported by Spear (1981), fromwhich the temperature of the clinopyroxene-inreaction is lowered if quartz is in excess in thesystem. Representative analyses of pyroxenes fromthese rock types are listed in Table 5.

The composition is very clustered around diopsidethat is poor in jadeite. For the pyroxenes coexistingwith quartz in the lcucocratic amphibolites, thequartz-jadeite equilibrium may be used to estimatepressures, as both phases are in textural equilibriumand crystallized together during the main episode ofdeformation. According to the calibration ofHolland (1980), and for a temperature of ~750°C(the clinopyroxene-in temperature), a maximumpressure of ~2-5 kbar has been estimated.

WHOLE-ROCK CHEMISTRYA selection of 28 samples has been made from thedifferent facies that constitute the oceanic domain ofthe AMB: 22 samples correspond to normal amphi-bolites characterized by the common assemblageamphibole—plagioclase, and 6 samples correspond tothe quartz-rich amphibolites appearing as centri-metre-sized bands alternating with the normal facies.

Table 4: Representative microprobe analyses ofplagioclases

Rock type:

Sample:

SiO2

AljO3

FeO,MnOMgOCeONa,0K20Total

Cpx-Hb amphibolites

893-22-10 893-24-14

58-7025-41

0-370-000-008-097-000-10

99-66

565927030-170000089-865-790-04

99-65

893-25-4

53-6828660-23002000

11-954-400-O4

99-14

Hb amphibolites

893-28-5

650327 000-120000-06

10-345-53001

98-16

893-30-8

59-3125060-170-000-107-467-140-07

9938

893-31-4

59-4125-200-270000-067-546-94005

99-47

Act-Hb amphibolites

893-39-9

54-9127090-3B0-000-00

10-535-230-16

98 32

893-40-4

57-7325-980-110010-039-136-64005

9986

893-41-3

570125-850-130-010-009-196-870-06

99-12

247


Table 5: Representative microprobe analyses of pyroxenes

Rock type:

Sample:

SiO2

no,AJ2O,

FoOMnOMgOCaONa2OK2OTotal

Normal amphibolites

893-22-2

52-610060-900-937-450-25

12-6924-660-29000

99-62

893-22-4

51-930-211 -511-417-600-27

12-5123-30

0-490-00

9923

893-22-14

62380221-380009-100-30

12-2723-82000000

99-46

Q-amphibo)rt(

M72

52-750070-900-009-380-39

12-7323-050-49001

99-76

»

M76

52-670-081-020-009-730-43

12-7623-26

0-490-00

100-43

M78

52-390-100-960009-310-42

12-9523-18

0-580-00

9988

'Recalculated.

For comparative purposes, representative samplesfrom other metabasites appearing in the continentaldomain have been included in this study. These are(1) the Rellano amphibolites that alternate withmarbles and calc-silicates (two samples), and (2) themafic granulites (five samples) appearing in thehigh-temperature zone. These samples have beenanalysed for major and trace elements. Severalrepresentative samples have also been analysed foroxygen and neodymiun isotopes, with the aim ofidentifying the igneous nature or otherwise of thesemetabasitic rocks, as well as the nature of the sourceand any process related to their evolution.

Major and trace (Ba, Nb, Rb, Sr, Y and Zr) ele-ments were determined by X-ray fluorescence(XRF) spectrometry at XRAL Laboratories ofCanada. The lower reporting limit for major ele-ments is 0-01% and for trace elements is 10 p.p.m.Precision for major element analyses is ± 2 % and fortrace elements is less than ± 5 % . Inductively coupledplasma mass spectrometry (ICP-MS) at XRALLaboratories was used for determination of REE, Thand U. The detection limits are 0-1 p.p.m. for La,Ce, Pr, Nd, Sm, Gd, Tb, Dy, Er, Tm, Yb, Th andU; and 0-05 p.p.m. for Eu, Ho and Lu.

Major and trace elementsA list of selected chemical analyses is given in Table6. Figure 9 shows the silica variation diagrams.Normal amphibolites have major element signaturestypical of tholeiitic basalts, whereas the quartz-richamphibolites are richer in SiO2, TiC>2, P2O5 andNa2O but poorer in MgO and CaO. The Rellanoamphibolites, intercalated within sediments of thecontinental domain, are very similar in composition

(with the exception of silica) to the normal amphi-bolites of the oceanic domain. The samples from themafic granulites—the pre-metamorphic intrusions ofthe continental domain—are very different in com-position from the other mafic rocks of the AMB (Fig.9). The salient feature of these mafic granulites is thehigh MgO content (up to 15 wt%) at high values ofsilica (>50 wt%) . These values are typical ofboninites, derived through the partial melting of anextremely depleted lithospheric mantle [seeCrawford tt al. (1981)]. All these metabasic rocksderive from the metamorphism of sub-alkaline mag-matic rocks, as evidenced by the total alkalis to silica(TAS) ratio.

The contents of trace elements of petrogeneticinterest, to be compared with typical MORBs usingthe normalization base of Pearce (1983), are listed inTable 6. The normal amphibolites have low contentsof immobile incompatible elements typical of oceanictholeiites (MORB). These values are extremely lowin the mafic granulites for most of the selected ele-ments. These relationships are well illustrated in theMORB-normalized spider diagrams (Fig. 10)(Pearce, 1983). Compared with MORB these maficgranulites have higher contents of Rb, Ba and K butlower concentrations of the high field strength ele-ments (HFSE). The main geochemical features ofthese mafic granulites are the high MgO contents(up to 15-9%) and high silica contents (>50%).They are really high-Mg andesites with many simi-larities with the typical boninites as defined byCrawford et al. (1989).

Normal amphibolites from the oceanic domainhave patterns similar to N-MORB for someimmobile elements, but are enriched with respect toN-MORB in other elements such as Th and Nb.

248

CASTRO et al. MORB-DERTVED AMPHIBOLITES, SW SPAIN

Table 6: Whole-rock analyses of representative samples from metabasites of the Aracena metamorphicbelt

Rock type:Sample:

SiO2

TiO2

AljO3

FejO*MgOMnOCaONa20K2OPjOB

LOITotal

Normal amphlbolitesA L A 8 1 0

49-90-68

18 36-926-680-13

1123280-10051

9824

Tract ehments (pj>.m.)

CrVScNlCoCuZnBaRbSrYThZrHfNbLaCePrNdSmEuGdTbDyHoErTmYbLu

33013424-7

1153670350-226

n.d.16030

4 989

2 8n.d.

5-914

n.a.82-30-9

n j .

0-5n.a.n.a.n.a.n j .

2-90-47

ALB 17 1

49-31-33

14-510-27-540-16

1182 80070-140-75

98-69

29028541-3724211-535-3

n.d20

18118

n.d83

1-921

513

n.a1030 9

n.a.0-6

n.an.an.an.a

2 50-39

ALB183

47-91-14

16-59278290-16

1182-670-110-130-7

98 67

29021833-1

1274448956-214

n.d.158

190-9

902-4

265-3

14n.a.102-81-2

n j .

0-5n j .

n j .

n.a.n j .

2 20-36

89322

49-11 28

14-810-28-380-19

12-62560060-130-77

10007

33025034632936-334-6

n.d18

191220-7

931-6

115

14-31-88-72 91-073 20-63-90 822-40-3230-34

89326

4881-58

15-610-68-320-2

12-32-450-O10-18046

100-5

28016135933536-647-3

n.d.23

18231

n.d.139

3-4n.d.

4-615-52 2

11-541-44-40 85-31-093-10-530-43

89327

50-11-13

16-19 68-290-19

12 33040010-120-77

100-65

620209

368433

9 432-3

nJ .18

19824

nxi.88

2 413

3-812 81-79-13-11-173-60-64-30 882 60-42-40-35

89339

51-51 -15

16-88-225-470-2

12-72-920-540-17046

100-13

27415525792747-357-3

16934

30525

n j .

143n.a.n.d.

13-631-43 9

174-61-384-90-85-11033-10-42-70-39

CO 2

4871-42

15-89-486820-16

112-750-420-18105

97-77

170196

34-3763758-2639111

20930

1-489

2-7197-4

18-32-8

13-34-11 -514-20-84-812-80-42-50-35

CO 6

47-61-3619-88-665 960-1310-62-420-410-261-5987

6620327-9313453350-8n.d18365n.dn.d541-4n.d6 316-22-1103-11-1830-53-30-631-80 21-4022

n.d., not determined; n.a., not analysed.

249


Table 6: continued

Rock typo:

Sample:

SiO2

TiO2

AljOjFe20j,MgOMnO

CaONa2OK2OP2O6

LOITotal

Q-amphiboIrt

ALA89

5641-79

14-51152-54

0-185-384-940070 6 50-2

98-15

es

ALB 173

53-1216

15-411

2690-128-25-140060 66

0-198-63

ALB191

52-32-59

13-613 63060-187-764-290060-630-15

98-22

Relleno

CO 8

43-81-59

18-7

8-727-780-1

14-81 57

0-420031-1

98-61

A29422A

45-21-44

18-5

9-088 280-12

12-12 41-010-04

M B97-88

Granulhss

89316

53-10-368O9808

150-21

13-21 090-750060-46

10004

89318

510-437-838 49

15-9

0-2114-50-90-59006

0-3199-79

83321

56-60-652

16-56-877 050-169-210-65

0-660-141-85

99-69

A2946

51-40-4069-83

8-111-80-19

14-40-980-660030-55

97-94

A2942

50-10-53

15-19-249-930-29-771071-620-150-95

98-13

Trtce ahmenti (p.pjn.)

CrVScNlCoCuZnBaRbSrYThZrHfNbLaCePrNdSmEuGdTbDyHoErTmYbLu

816317-69

25296

56n.d10

17477

4-5505

122725260

n.e.34

93-1

n.a.1-7

n.a.n.a.n.a.n.a.

8-11-29

529725-91026

6-627-434

n.d.210

916-4

43711

1318-746

n.a.33

8 93-1

n.a2

n.a.n.a.n.a.n.a.

8-71-35

42131248

82815-330-343

n.d.19475

4-1397

8 95122-659-6

8-642-713-63-86

14-72-6

16-73-54

10-11-49-21-37

6927646-568497743-265

n.d.347

18n.d.46

1-620

3-810-1

1-68291-12280-63-20671-80-21-50-21

8534847703637-461-8

26320

297190-4

761-294-9

12-81-99-430-973-20-63-60-732-10-31-80-24

n.a11940

31854

129662

n.d42

15714

n.e.23

n.a17

5-2

15-329-230-730-53-10-581-60 21-30-19

17015946

30957

12649-9

n.d36

16017

n an.d

0-4124-8

162-2

10-43-10-653-20-53 30 651-90 31-6023

14012014962568-130-1

15830

26097-3

866-6

191126

2-79-92-30-662-40-320351

0-10-90-12

58020658

1193313 847-6

52121

14641

155

1-536-3

18-93-2

17-46-41-16-91-37-91-614-40-83-50-5

55012931

1633113-269-9

73349

25821

1-543

1-168-8

19-62-7

123-50-86

3-50-6

3-90-792-40-42 20-36

n.d., not determined; n.a., not analysed.

250

CASTRO et aL MORB-DERTVED AMPHIBOLITES, SW SPAIN

3

2

1

0

25

20

15

105

15

12

9

6

320

15

10

5

015

10

5

TIO2

• o

. A12O3

oo

o°3*

n°

-

o°_

F e 2 O 3

<>

. MgO

<>

•

•

o

•

*

o

o

o

0

0

%

oo

-

o ° •

CaO

o° o *o

oo

Na2O ,° °o°o

* m *

o ° o o

40 45 50 55 SiO2

Fig. 9. Harkcr diagram* of the amphibolites from the oceanicdomain. Solid circles: normal amphibolites. Open circJea: quartz-rich amphibolites. For comparative purposes other metabasitesfrom the continental domain have been represented. These arethe El Rcllano amphibolites (diamonds) and the mafic granulites

(asterisks).

These features are intermediate between N-MORBand P-MORB (Saunders, 1984), suggesting thatthese amphibolites were derived from transitional-type MORBs.

On a chondrite-normalized plot (Fig. 1 1), most ofthe amphibolites from the oceanic domain display anearly flat pattern with no Eu anomaly. Twosamples display enrichment of LREE over HREEand no Eu anomaly. The quartz-rich amphiboliteshave a nearly flat pattern identical to that of thenormal amphibolites but displaced to higher values.These patterns are characteristic of transitional tho-leiites (T-MORB), which have intermediate featuresbetween normal and primitive MORBs (Saunders,1984).

Neodymium isotopesTo determine the source nature of the studied meta-basites (amphibolites and granulites) we haveselected a set of representative samples from theserock types for Sm-Nd isotopes. Apart from thenature of the source region, the Nd isotope data willsupply important information on the timing ofmagma generation in the ridge, from pairs of frac-tionation-related facies of amphibolites.

Five amphibolite samples have been analysed forSm and Nd isotopes. Two samples are quartz-richamphibolites collected from two separated layersalternating with normal amphibolites (Fig. 4) in theHb-zone of the amphibolite unit. The other threesamples correspond to normal amphibolites, two ofwhich form pairs with the above-mentioned samplesof quartz-rich amphibolites.

Nd isotope ratios were determined at SURRC,East Kilbride. Nd was separated from whole-rockpowders using techniques described by Barbero et al.(1995), except that a modified procedure wasadopted for the separation of Ba from the REE. Thedried down bulk REE fraction collected from thecation exchange columns was dissolved in 3 MHNO3 and loaded onto a column containing 2 ml ofEichrom Sr Spec resin (100-150 fim particle size).The REE were eluted with 3 M HNO3, whereas Bawas retained on the column. Nd was separated fromthe other REE using the temperature-controlledanion exchange procedure of Barbero et al. (1995).No further purification was necessary. Samples wererun on a VG Sector 54-30 thermal ionization massspectrometer in multi-dynamic mode. 143Nd/144Ndratios were corrected for mass fractionation using14€Nd/144Nd = 0-7219. During the course of thisstudy the Johnson & Matthey Nd standard gavel43Nd/144Nd = 0-511500± 10 (2 SD).

251


100 -3

10 -

1 -E

O 0.104

0.01

(a)

—i—i—i—i—i—Sr K Rb Ba Th

—I 1 1 1 1 1 1 1 1 1—NbCe P Zi Sm Ti Y Yb Sc Cr

Amphiboliteso AL B 25 1• CO2a 89322• 89326* 89339a CO6

Q-Amphibolites7 ALB 19 1T ALA87+ AB-7x ALB 17 2

100 -g (b)

10 —

0.1

RellanoAmphiboliteso CO 8. A29422A

GranulitesD 89316• 89318* 89321A A2946v A2942

Sr K Rb Ba Th NbCe P Zr Sm Ti Y Yb Sc Cr

Fig. 10. Rock-MORB normalized ipider-diagrams with representative samples of metabasites from the Aracena metamorphic belt.

Table 7 shows the Sm and Nd concentrations andthe Nd isotope ratios of these samples. For everypair, the Nd isotope ratio of the quartz-rich amphi-bolite is lower than that of the respective normalamphibolite and the tie-lines linking every pair areparallel to one another (Fig. 12). This behaviour isin agreement with the fractionation process deducedfrom the whole-rock chemistry. The slope of thesetie-lines may be related to the age of the fraction-ation process; however, this is poorly constrainedbecause of the large errors resulting from the narrowrange in the Sm/Nd ratios. Initial eNd fall between+ 9-2 and + 7-9, taking a minimum reference age of~350 Ma according to the " A r ^ A r ages (Dall-meyer et al., 1993) obtained for the metamorphism ofthe Aracena amphibolites. These values are typicalof MORB (DePaolo, 1988).

Oxygen isotopes

The use of oxygen isotopes allows us to recognize theoriginal position of the amphibolite slab in theoceanic crust. It is well documented from oxygendata of the Samail ophiolite in Oman (Gregory &Taylor, 1981) that only the uppermost part (2-3km) of the oceanic crust is affected by seawatermetasomatism. Oxygen isotope data from repre-sentative samples of the oceanic domain are listed inTable 8. These amphibolites display a wide range inthe <518O (V-SMOW normalization) value from3-7%o to 8-8%o. This variation occurs in samples col-lected from the same outcrop and with minor differ-ences in composition. It is typical of thehydrothermally altered oceanic crust, which mayvary from 20%o to 20%o (Spooner et al., 1974;

252

CASTRO tt aL MORB-DERTVED AMPHIBOLITES, SW SPAIN

B•cCo1

100 -q

50 —

10

5 —

Amphiboliteso ALB25 1• CO 2o 89322• 89326A 89339» CO6

Q-Amphibolites' ALB 19 1» A L A 8 7+ AB-7x ALB 17 2

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

B

O

I

100

50 -

10 —

5 —

_

-

—

_

a ^ ~ " * \ A A- '

0 O^ "°

1 1 1 1La Ce Pr Nd

^ — ^ ^ —

i i i i i i i

Sm Eu Gd Tb Dy Ho Er

RellanoAmphiboliteso CO 8

- - T . • A29422A

V "

^ ^ ^ i Granulites_ _ _ ^ ^ D 89316

^~"~-* • 89318A 89321A A29467 A2942

I iYb Lu

Fig. 11. Rock-chondrite normalized spider-diagrami with representative samples of metabasites from the Aracena metamorphic belt.Normalizing valua from Nakamura (1974).

Table 7: Neodymium isotope analyses from selected samples of the amphibolite unit

Rock type

Normal amph.

Normal amph.

Normal amph.

Q-amph.

Q-amph.

Sample

ALA8-10

ALB17-1

ALB18-3

ALA8-9

ALB17-3

CHUR

Sm (p.p.ra)

2-3

3 0

2 8

9 0

8 9

Nd (p.p.m.)

8-0

10-0

10-0

340

330

147Srn/14*Nd

0 1738

0-1814

0-1693

0-1600

0-1631

0-1967

143Nd/l44Nd

0-613022

0-513073

0-512981

0-612965

0-512995

0-512638

11

17

12

7

6

(143Nd/144Nd)Mo

0-512624

0-512657

0-512593

0-512598

0-512621

0-512187

*Nd 3GO

+8-5

+9-2

+7-9

+ 8 0

+85

0-0

253


Table 8: Oxygen isotopes ofamphibolitesfrom the Aracenametamorphic belt

Sample

CO28932289328893328934289339AL9228CO6ALB 17 2

Assemble Ba

Hb-PHI-MtCpx-Hb-PMI-Mt-Zf-ApHb-PMI-Mt-ApHb-PMI-MtHb-PMI-Mt-EpHb-PHI-Mt-EpHb-PMI-MtHb-PHI-MtH b-PI-CH MVIt-Sph-Ap-Zr

Rock type

Hb amphiboliteCpx-Hb amphiboliteHb amphiboliteHb amphiboliteHb amphiboliteHb amphiboliteHb amphiboliteHb amphiboliteQ-emphibolrte

<518O

8-14-73-73-97-68 88 87-35-5

2

0.51312

0.51308

0.51304

0.51300

0.51296

0.51292

Normal amphibohtesO ALA 8 10

O ALB 17-1

A ALB 18 3

Q-ajnphibohtes• ALA 8 9• ALB 17 3

651 ± 199 Ma

' * 630 ±217 Ma

I.HOl

014 0 16 0.18 0.2

Fig. 12. l4SNd/144Nd vi U7Sm/144Nd diagram plotting selectedsamples of amphibolitei and quartz-rich amphibolites (see text).

Gregory & Taylor, 1981), although this wide rangeis reduced in the course of subduction (Hoefs, 1987),as the fluids expelled during prograde reactions tendto homogenize the resulting amphibolite pile.

DISCUSSIONPetrogenesis of metabasitesBoth REE and ENJ data strongly support an originfrom mid-ocean ridge magmatism for the amphibo-lites from the Aracena metamorphic belt. If we con-sider jointly the three normal amphibolites analysedfor Nd isotopes, they display a good straight corre-lation (Fig. 12). The meaning of this correlation isnot very well constrained by this study. However, itmay be the result of an isotope homogenization thatoccurred in the mantle source 1-14 Ga ago, the ageobtained taking this correlation as an isochron. Thisimplies that the isotope homogenization occurredaround 600 Ma before the partial melting event that

produced the oceanic crust. This time interval is inagreement with the estimations by DePaolo (1988)based on the small total range of the eNd data, within12 units, and the expected percentages of partialmelting from the peridotite source.

The relationships between normal amphibolitesand quartz-rich amphibolites are complex, so, onone hand, the relative enrichment in the majoroxides is compatible with a fractionation processfrom a tholeiite composition, although, on the otherhand, the samples are not aligned on a single, con-tinuous trend similar to a liquid-line of descent. Bycontrast, both groups show separated but paralleltrends for most of the major oxides (Fig. 9). Asquartz-rich amphibolites and normal amphibolitesare associated in layers within the amphibolite pileof the oceanic domain, we have sampled severalpairs from selected outcrops. It is interesting to notethat the lines linking pairs of amphibolites fromseparated outcrops are parallel, as depicted in Fig.13a and b for the cases of T1O2 and P2O5, respect-ively. This behaviour is identical for several traceelements, as depicted in Fig. 13c and d for the casesof a typical refractory element (Cr) and an incom-patible immobile element (Zr), respectively. Thisbehaviour supports the existence of a multiple frac-tionation process from a set of parental magmas. Itimplies the existence of a multi-chamber systembeneath the ridge; every magma chamber having acomposition similar to the adjacent chambers, andall the chambers undergoing similar fractionationprocesses to separated closed systems.

This fractionation process accounts for the REEenrichment of the quartz-rich amphibolites. Analternative mechanism for REE enrichment in theseamphibolites, based on differential rates of partialmelting, is inconsistent with the transitional char-acter (T-MORB) of these melts. A low rate of

254

CASTRO et al. MORB-DERTVED AMPfflBOLITES, SW SPAIN

(a) 3 (b) 1

50 55 60 45

Sid SidFig. 13. Silica variation plots for major and trace elements, showing the relationships between normal and quartz-rich amphibolites.

Tie-Iinej link pairs of samples from two adjacent bands.

partial melting for the peridotite source wouldproduce REE-rich melts but with a non-flat pattern;this will be positive or negative depending on thenormal or primitive character of the source. Figure14 shows the Aracena metabasites plotted on a Ce,/YbB vs Yb, (n denotes chondrite-normalized values)diagram with the main fields of normal, transitionaland primitive MORBs. Most of the normal amphi-bolites from the oceanic domain plot in the field ofT-MORB and only a few plot near the field of P-MORB. The quartz-rich amphibolites plot outsidefrom the MORB fields and away from the partialmelting trend calculated for a garnet lherzolitesource (Saunders, 1984).

The variations observed in the (518O values may bedue to distinct processes. On one hand, the increasein <518O is parallel to the increase in mobile incom-patible elements. It may be the result of hydro-thermal alteration during seawater interaction. Thisalteration may affect the upper 2-3 km of theoceanic crust (McCulloch & Taylor, 1980; Gregory& Taylor, 1981), giving rise to very heterogeneousdistributions of the altered zones, as these are relatedto fluid circulation through fractures in the basaltpile (Peacock, 1993). These altered zones are

enriched in mobile incompatible elements dissolvedin the seawater. The newly formed minerals resultingfrom the alteration process record a high 8 O valuein equilibrium with the seawater. Other relict

10

0.1

Cen

Source

/Yb

p.m.

i

n

1$N

I

^ ^ P - t y p e MORB

S^^-type MORB

-type MORB

10 100Cer

Fig. 14. Ce/Yb vi Yb (normalized values) showing the dispositionof the Aracena metabasites compared with the MORB types(symbols as in Fig. 9). The fields for N-, P- and T-type MORBare drafted from single data point compiled by Saunders (1984).

255


minerals, not affected by this process, retain theoriginal low S O values, giving rise to the observedheterogeneity, which depends on the intensity of thealteration process. On the other hand, alteration byfluids related to the retrograde metamorphism andthe interaction with continental sediments, duringthe late exhumation of the amphibolite pile, alsomay cause the 518O value to increase but with noparallel enrichment with respect to mobile large ionlithophile elements (LILE). These data are inagreement with an oceanic provenance of theamphibolites, and indicate that they come from themetamorphism and late exhumation of theuppermost part of an oceanic crust. This is crucialfor the proposed tectonic model, as other possiblemodels, such as obduction of an oceanic slab, areruled out.

The amphibolites interlayered with marbles andcalc-silicates in the continental domain (the Rellanoamphibolites) are very similar in composition to thenormal amphibolites of the oceanic domain. Theyare remnants of an initial stage of rifting before thedevelopment of the ocean represented by theamphibolites of the oceanic domain. The mantlesource for both types of basalts was probably thesame, as indicated by the geochemical signaturesmentioned above.

The mafic granulites, intruded as pre-meta-morphic bodies into the pelitic migmatites of thecontinental domain, represent andesite magmas ofboninite affinity. Though many of these rocks have ametamorphic texture, some facies retain their ori-ginal igneous textures. These igneous facies are veryrich in low-Ca pyroxenes. The metamorphism ofthese igneous rocks is essentially a textural change,as the mineralogy and whole-rock chemistry are notmodified. The facies with typical igneous textureshave no signs of being igneous cumulates, so weconclude that the geochemical signatures come fromthe andesite magma from which they crystallized.However, the contents in LILE, though being lowerthan for typical MORB (Fig. 10), are slightly higherthan for typical boninites. This feature is moremarked for mobile LILE, but also is appreciable forsome HFSE (Fig. 10). These aspects may result fromthe influence of fluids released by the subducting slabthat metasomatized the harzburgite source region forboninite magmatism [see Hickey & Frey (1982) andCrawford et al. (1989)].

The fluids released from the down-going slabmetasomatize the adjacent lithospheric mantlewedge, transforming this region of the sterile mantlelithosphere into a potentially fertile harzburgite.Cessation of subduction may be a way to promote athermal rebound, in this metasomatized wedge,

reaching the wet peridotite solidus and producingthe boninite magmatism. Another mechanism toinduce the thermal rebound is the subduction of aridge with the consequent creation of a slab-freewindow (Rogers & Saunders, 1989; Hole et al.,1991). In the Crawford et al. (1981) model, thereactivation of the lithospheric mantle is induced bythe ascent of a peridotite diapir beneath the sub-duction-related volcanic arc. The proximity ofboninite rocks to the oceanic domain in the AMB, aswell as the continental nature of the host into whichthese magmas intruded, are arguments against anyrelation with a volcanic-arc setting. Cessation ofsubduction is unlikely because of the preservation ofan inverted metamorphic gradient in the oceanicdomain of the AMB, for which it is necessary thatsubduction continues after the main episode ofmetamorphism.

Thermal evolution of the oceanic domainThe absence of garnet in the amphibolites from theoceanic domain, together with the absence of epidotein the hornblende—plagioclase facies, indicates thatthese rocks equilibrated at low pressures. Thepressure estimated from the jadeite content in clino-pyroxene is <2'5 kbar as calculated for the quartz-rich amphibolites, in which the quartz-jadeite-albiteequilibrium may be applied. This pressure is coin-cident with that obtained by comparison with theexperimental data of Spear (1981). The equilibriumtemperature varies across the amphibolite pile,increasing from south to north, from the greenschist-amphibolite facies transition (~550°C) up to theamphibolite-granulite facies transition (Fig. 2). Thetemperature of this amphibolite-granulite faciestransition is estimated at ~ 780°C according to theexperimental data of Spear (1981). This variation intemperature occurs across a narrow band of no morethan 500 m, implying a metamorphic gradient of~460°C/km. Because the metamorphic foliation, thecontacts of the amphibolite pile and the isograds diptowards the north, this gradient is inverted; that is,with the temperature increasing upwards. An alter-native explanation, implying tectonic inversion, byfolding and/or stacking of the amphibolite pile maybe ruled out, based on structural data. These includethe differences in tectonic style between the oceanicand continental domains: the axial traces of foldswithin the continental domain are crosscut by thecontact with the oceanic domain, in which the mainphase of deformation is by shearing. Both episodes ofdeformation occurred at the same time and may bekinematically compatible with the same tectonicprocess, but the amphibolite slab is not geometrically

256

CASTRO tt dL MORB-DERIVED AMPHIBOLITES, SW SPAIN

linked with inverted limbs of folds in the continentaldomain. The second metamorphic episode is associ-ated with discrete shear bands developed within theamphibolite pile, but especially concentrated at thesouthern and northern margins of the pile. Duringthis episode of deformation (OD-Z)2), retrogressionof the pre-existing assemblages to greenschist faciesoccurred.

Tectonic modelThe inverted metamorphic gradient of the oceanicdomain is the typical thermal structure found insubduction-related complexes such as the Pelona-Orocopia complex of the Western Cordillera inCalifornia (Graham & England, 1976; Peacock,1987). The inverted metamorphic gradient can be aconsequence of dynamic advection related to sub-duction, and shear heating along the subductionthrust. Both factors contribute to heat transfer fromthe hanging wall to the oceanic slab.

However, the inverted metamorphic gradient ofthe AMB is greater than those reported in sub-duction-related complexes [see Peacock (1987)].Another difference between these complexes and theAMB is the high pressure (> 8 kbar) compared withthe low-pressure regime of the AMB. The thermalstructure of the continental hanging wall, at the timeof subduction, may be responsible for the low-pressure, inverted-gradient metamorphism experi-enced by the slab. This implies that the continentalhanging wall must be heated immediately beforeslab subduction. Evidence for this anomalous heatingof the continental block is documented by the low-pressure—ultra-high-temperature regime observedover a narrow and linear band in the continentaldomain of the AMB. Furthermore, the presence ofpre-metamorphic intrusions of Mg-rich andesitemagmas with boninite affinities in the CD is evi-dence for the partial melting of the lithosphericmantle beneath the continental margin at the time ofsubduction.

Geochemical and petrogenetic features, as well asthe thermal evolution of the AMB suggest the fol-lowing scenario (Fig. 15):

(1) Subduction of an oceanic lithosphere beneaththe CD. Dehydration of the down-going slabinduced the metasomatism of the overlying litho-spheric mantle (Fig. 15a).

(2) The ridge subduction deduced from this study,with the development of a slab-free window, impliesthe migration of the triple junction along the edge ofthe CD. It promoted the upwelling of the underlyingasthenosphere, and a thermal rebound developing ahigh-T belt in the upper plate above the slab

(a)oceanic crust continental crust

mantle lithosphere

asthenosphere -(slab-free window)'

oceanic crust;

(c)oceanic crust

mantle lithosphereinverted

AMB-continental

crust

asthenosphere(slab-free window) * - -̂

Fig. 15. Schematic tectonic model for the Aracena metamorphicbelt. The migration of a triple junction along the edge of theGondwana plate (a) produces the consumption of an intermediateoceanic plate, the opening of a slab window, and the upwelling ofthe underlying asthenosphere. As a result, a high-T belt developsin the continental margin, the AMB (b). Bodies with dot pattemirepresent boninitic intrusions. Subduction of the western plateunder the thermal anomaly creates an inverted metamorphicgradient in the top of the oceanic slab. In a later stage (c), theupper slice of the subducting slab is incorporated into the conti-nental margin. The process involves the downwards migration ofthe plate boundary, and the partial subduction of the accretionaryprism, overthrust by the amphibolite slice. As a consequence, agreenschist-facies retromorphism develops in the amphibolite pile.

window. This increase in temperature inducedpartial melting of the metasomatized lithosphericmantle wedge under the continental margin, withthe genesis of boninite magmatism (Fig. 15b).

(3) Hot subduction of the western plate under thethermal anomaly created an inverted metamorphicgradient in the top of the oceanic slab (Fig. 15c).

(4) In a later stage, the upper slice of the sub-

257


ducting slab was accreted to the base of the con-tinental margin. The process involved the downwardmigration of the plate boundary and the partialunderthrusting of the accretionary prism, producingthe uplift rate required to preserve the invertedmetamorphic gradient [see Peacock (1987)].

CONCLUSIONSThe Aracena metamorphic belt represents a low-pressure-high-temperature subduction complex inwhich metamorphism of the subducting oceanic slabwas induced from the previously heated continentalhanging wall. This anomalous heating occurred as aconsequence of thermal rebound associated withsubduction of the ridge and the creation of a slab-free window. This tectonic model is supported bythree petrological features: (1) the low-pressureinverted metamorphic gradient of amphibolites ofthe oceanic domain; (2) the high-temperature-low-pressure metamorphism of the continental hangingwall; (3) the early intrusion of boninites into thecontinental domain. These features confer a specialrelevance to the Aracena metamorphic belt. Theymay be used as indirect criteria for recognizingancient subduction zones in which the typical sig-natures such as blueschist metamorphism are absent.The Aracena metamorphic belt may be taken as amodel example for atypical subduction settings inancient orogenies.

ACKNOWLEDGEMENTSThis paper considerably benefited from the con-structive criticisms by Dr T. Rushmer and Dr R.Rudnick. This study is part of the AROCHE (Ana-lisis de la Rama Oeste de la Cadena HercinicaEuropea) project funded by the Spanish Ministry ofEducation and Science (CICYT-DGICYT, ProjectPB91-0600), by the Andalusian Government (Pro-jects PAI-4018 and PAI-4108) and the University ofHuelva.

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Robinson, P., Spear, F. S., Schumacher, J. C , Laird, J., Klein,C , Evans, B. W. & Doolan, B. L., 1982. Phase relations ofmetamorphic amphiboles: natural occurrences and theory.Mineralogical Society of America, Reviews in Mineralogy 277, 1—277.

Rogers, G. & Saunders, A. D., 1989. Magnesian andesites fromMexico, Chile and the Aleutian Islands: implications for mag-matiim associated with ridge—trench collisions. In: Crawford, A.J. (ed.) Baniniles and Related Rocks. London: Unwin Hyman, pp.416-445.

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RECEIVED JANUARY 23, 1995REVISED TYPESCRD7T ACCEPTED SEPTEMBER 11, 1995

APPENDIX: THE AMPHIBOLECOMPOSITION SPACEThe main problem in investigating the amphibole compositionspace is to find a set of linearly independent phase componentswith which all the possible chemical changes in the amphibolemolecule may be modelled. For calcic amphiboles, such as thosefound in the Aracena amphibolites, the composition space can be apriori modelled with the additive component tremolite (Tr) andseveral molecules that result from the application of the exchangecomponents, as shown in Table Al.

Table Al

Phase component Exchangevector

End-member molecule

NaCa2MgBSi7AIO22(OH)2AI2Si_i

TremoliteEdeniteTschermakiteGlaucophane AINaMg_iCa_i Na2Mg3AI2Sis022(0H)2Ti-Tschermakite AI2TiSL2Mg-i Ca2Mg4TiAI2Sia022(0H)2

The composition space is defined by the oxides SiO2—A^Oj—FeO—MgO—TiOj-CaO—Na2O. The space can be condensedthrough FeMg_,, resulting in a six-dimension condensed space(SAFTCN) in which the above-mentioned exchange componentsand the additive component Tr can be modelled. In theory, anycalcic amphibole can be expressed in terms of these end-membersand the Tr additive component by algebraic transformation of thecomposition space. However, a sixth molecule or exchange com-ponent must be added to proceed with the algebraic transforma-tion. A plagiodaie component must be used, and it is justified asmost of the changes in the amphibole composition are paralleledby changes in the plagiodase composition through the plagiodaseexchange that operates between GI—Ts and Ab—An [see Spear(1980, 1981)].

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However, if the plagioclase exchange vector NaSiCa_|Al_, iiconiidered as a component of the amphibole space composition,the component matrix cannot be transformed, as there is a lineardependence between Gl and PI, or between Ts and PI. No otherexchange component can be added, as the space is constrained forcalcic and calcosodic amphiboles, occupancy in the M4 lite beingdominated by Ca (at >90%). The proportion of the MgCa_i ex-change, reduced as a consequence of the cummingtonite—hom-blende solvus, does not occur in the amphiboles of the Aracenaamphibolites. The plagiodase exchange relating Gl and Ts inamphiboles is coupled by the same exchange as that relating Aband An in plagiodases. In other words, the plagioclase exchangein the amphibole molecule in relation to changing P and 7" is onlypossible if a parallel change in the anorthite content of theplagioclase occurs. Thii implies the existence of a net-transfer re-action defined by the plagioclase exchange

2(NaAlSi3Oa) + Ca2Mg3Al4Si6O22(OH)2albite tschermakite

= Na2Mg3Al2Si8O22(OH)2glaucophane

2(CaAl2Si2Oa).anorthite

The existence of this net-transfer reaction is of great relevance,as it implies changes in volume and entropy and can be used inthermometry and barometry (Spear, 1980). For our purposes, itmeans diat plagioclase must be considered as an 'external com-ponent' of the amphibole molecule. The proportion of plagioclasein the amphibole molecule must be zero if the assumed anorthitecontent satisfies chemical and thermodynamic equilibrium withinthe amphibole molecule. In our case, we take as a reference aplagioclase of 50 mol % in anorthite constant composition (themost usual composition of the analysed samples) and introduce itinto the composition space together with the additive componentTr and the end-members Gl, Ts, Ti-Ts and Ed. The input matrixand the transformation matrix are listed in Table A2. If an am-phibole has a negative value for the plagioclase content it meansthat the assumed composition of Any) is not the real compositionat equilibrium. This fixed composition may appear as under-estimated (negative) for high-temperature amphiboles or over-estimated (positive) for low-temperature amphiboles.Furthermore, as the Ts, Ti-Ts and Ed exchanges are stronglydependent on temperature (Spear, 1981; Blundy & Holland,1990) and Gl on pressure, the variations in both molecules may beused as a guide to determine the shape of the P—T path.

Table A2: Input and inverted matrix for the amphibole space composition

Input matrix

8-02 03 00 00 02 0

Inverted matrix

00630-250

- 0 0 6 30000

-0-2500-250

8 00 05 00 02 00 0

0094-0-208

0-3230000

- 0 0 4 2-0-292

6 04 03 00 02 00 0

0-125-0-167

0-20800000-167

-0833

6-02-04 01 02 00 0

0063-0-250-0-563

1000-0-250

0-250

7 01 05 00 02 01 0

-0-563-0083-0-271

00000-5831083

2-51-50 00 00-50-5

GlTrTs

Ti-TsEd

PKAnso)

-

SAFTCN

-0031-0-542-0-385

00000-7920542

SAFTCN

GlTrTs

Ti-TsEd

PKAnso)

260

Significance of MORB-derived Amphibolites from the Aracena Metamorphic Belt, Southwest Spain

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