Low-Pressure Metamorphism in the Sierra Albarrana Area (Variscan Belt, Iberian …hera.ugr.es/doi/15012025.pdf · 2005-03-18 · JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 PAGES 35–64

JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 PAGES 35–64 1997

Low-Pressure Metamorphism in theSierra Albarrana Area (Variscan Belt,Iberian Massif )

A. AZOR1∗ AND M. BALLEVRE2

1DEPARTAMENTO DE GEODINAMICA, UNIVERSIDAD DE GRANADA, FACULTAD DE CIENCIAS,

CAMPUS FUENTENUEVA S/N, E-18002 GRANADA, SPAIN2LABORATOIRE DE PETROLOGIE–GEOCHIMIE, GEOSCIENCES RENNES (UPR–CNRS 4661), UNIVERSITE RENNES I,

F-35042 RENNES CEDEX, FRANCE

RECEIVED FEBRUARY 2, 1996 REVISED TYPESCRIPT ACCEPTED AUGUST 2, 1996

A low-pressure metamorphic zonation ranging from biotite to mig- orogenic belts, including the Variscan Belt of Southwestmatite zones occurs in the Sierra Albarrana area (Variscan Belt of Europe. De Yoreo et al. (1991) presented a review of somesouthwestern Iberian Peninsula) in uppermost Precambrian to Lower possible tectonic settings of low-pressure metamorphism.Palaeozoic metasedimentary rocks. The principal deformation in Those workers considered low-pressure metamorphismthis area is related to a major ductile shear zone whose central part to be that in which peak temperature is above 500°Cis localized immediately to the southwest of the Sierra Albarrana and pressure does not exceed that of the aluminiumQuartzites. The metamorphism is synchronous with respect to this silicate (Als) triple point. The following four tectonicdeformation. The metamorphic zones are symmetrically distributed situations were envisaged for the low-pressure meta-with respect to the Sierra Albarrana Quartzites. Pressure–temperature morphism (De Yoreo et al., 1991): (1) magmatic arcs;(P–T) conditions are ~3·5–4 kbar and range from ~400°C (2) regions of crustal extension; (3) continent–continent(biotite zone) to 500°C (staurolite–garnet zone) up to 650–700°C collision zones where thermal relaxation occurs after(migmatite zone). We have not detected pressure variations along crustal thickening; (4) regions with significant fluxes ofthe different metamorphic zones. Relic kyanite is observed in the aqueous fluids, especially those located above subductionform of inclusions in andalusite within veins in the lower-grade part zones.of the staurolite–andalusite zone. The low-pressure metamorphism of This paper is concerned with the description of a high-the Sierra Albarrana area arises from a two-stage history including temperature–low-pressure facies series terrane (the Sierramoderate crustal thickening followed by subsequent localization of Albarrana area, Variscan Belt, southwestern Iberian Mas-deformation in a transcurrent shear zone during peak P–T conditions. sif ) (Figs 1 and 2). In this area, a metamorphic zonationChannelized fluid flow within the major ductile shear zone may from the biotite zone up to the migmatite zone developedhave contributed to the heat budget of the low-pressure metamorphism. at low pressure (Garrote, 1976; Gonzalez del Tanago &

Peinado, 1990). As will be shown here, the metamorphiczonation is not related to post-thickening thermal re-laxation but rather exhibits a roughly symmetric pattern

KEY WORDS: fluid flow; Iberian Massif; low-pressure metamorphism; around a major ductile shear zone. In addition, syn-shear zone; Sierra Albarrana area metamorphic magmatism is not recognized. For these

reasons, the low-pressure metamorphism of the Sierra Al-barrana area may provide a good example of moderate

INTRODUCTION crustal thickening followed by significant heat advectionby aqueous- or silicate-rich fluids along a major ductileRegions of low-pressure and medium- to high-

temperature metamorphism are widespread in many shear zone.

∗Corresponding author. Telephone: (34) 58 24 29 00. Fax: (34) 58 2433 52. e-mail: [email protected] Oxford University Press 1997

JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 JANUARY 1997

Fig. 1. (a) Geologic map of the southwestern Iberian Massif showing the major zones. (b) Geologic map of the Ossa–Morena–Central IberianZones boundary in which the Sierra Albarrana area is located.

metasedimentary rocks. The lithostratigraphic sequenceGEOLOGICAL SETTING OF THEcan be divided into three groups of rocks separated by

SIERRA ALBARRANA AREA tectonic contacts (Azor, 1994; Azor et al., 1994).The Sierra Albarrana area is located in the southwestern The first group consists of Upper Proterozoic to Lowerpart of the Iberian Massif, immediately to the southwest of Cambrian rocks, in which three formations can be dis-a crustal-scale shear zone known as the Badajoz–Cordoba tinguished. These are, from bottom to top (i.e. fromShear Zone (Burg et al., 1981) (Fig. 1a and 1b). This southwest to northeast), as follows:ductile shear zone is the boundary between two major (1) A volcanosedimentary succession of Upperzones of the Iberian Massif: the Central Iberian Zone to Proterozoic age (Vendian), known as the Malcocinadothe northeast and the Ossa–Morena Zone to the south- Formation (Fricke, 1941; Delgado Quesada, 1971).west. The Badajoz–Cordoba Shear Zone represents one (2) A Lower Cambrian succession of limestones andof the sutures of the Variscan Belt (e.g. Burg et al., 1981;

dolostones with slate intercalations, known as theMatte, 1991; Azor, 1994; Azor et al., 1994). The SierraPedroche Formation (Linan, 1978; Azor, 1994).Albarrana area is separated from the Badajoz–Cordoba

(3) A Lower Palaeozoic (probably Lower to MiddleShear Zone by a subvertical semibrittle left-lateral faultCambrian) monotonous succession of slates andknown as the Azuaga Fault (Fig. 1b).metagreywackes with some quartzite intercalations,known as the Villares Formation (Linan, 1978; Azor,1994).Pre-Carboniferous rocks of the Sierra

Albarrana area The second group of rocks crops out in a bandLithostratigraphic sequence located to the northeast of the first group and separatedThe area studied (Figs 1b and 2) is made up mainly of from it by a late brittle fault known as the Onza Fault

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AZOR AND BALLEVRE LP METAMORPHISM IN SIERRA ALBARRANA

Fig. 2. Geologic map and cross-section of the Sierra Albarrana area. The various lithologic units are described in the text. 1, MalcocinadoFormation; 2, Pedroche Formation; 3, Villares Formation; 4, Albariza Micaschists; 5, migmatitic gneisses with minor amphibolites; 6, SierraAlbarrana Quartzites; 7, migmatitic gneisses, micaschists and metagreywackes; 8, Lower Carboniferous undeformed sediments of the Valdeinfiernobasin; 9, Badajoz–Cordoba Shear Zone; 10, Los Ojuelos Gabbro; 11, La Cardenchosa granite; AB, geologic cross-section; SP, principal foliation;SC, crenulation foliation. The shear zone mainly corresponds to the migmatitic gneisses located to the southwest of the Sierra Albarrana

Quartzites. Its location has been indicated on the cross-section (see text for further explanation).

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(Figs 1b and 2) (Azor, 1994). This second group, lineation and perpendicular to the foliation, whichindicate a non-coaxial component for the principalknown as the Albariza Micaschists, is made up of adeformation. The shear criteria recognized [see Simpsonsuccession of schists and metasandstones with am-& Schmid (1983) and Hanmer & Passchier (1991) for aphibolite intercalations, the age of which is probablyreview] are S–C structures, asymmetric tails in feldspathicLower Palaeozoic (Azor, 1994).veins and rotation of the foliation in synkinematic garnet,The third group of rocks crops out to the northeast ofandalusite and staurolite porphyroblasts. These criteriathe second one (Fig. 2). The contact between these twoconsistently indicate a dextral sense of movement (Azor,groups of rocks coincides with a narrow band of intensely1994; Azor et al., 1994).deformed rocks, interpreted as a ductile shear zone

The age of the principal phase of deformation and(Azor, 1994; Azor et al., 1994). Three formations can bemetamorphism must be post-Lower Palaeozoic, as itdistinguished in this group of rocks. They are, fromaffects rocks that are palaeontologically dated as Lowerbottom to top, as follows:Palaeozoic (Marcos et al., 1991). 40Ar/39Ar radiometric(1) Migmatitic gneisses with minor amphibolite,dating (Dallmeyer & Quesada, 1992) indicates ages ofmetagreywacke and quartzite intercalations.390 Ma for amphiboles and 360–350 Ma for muscovites.(2) Feldspathic quartzites with intercalated pa-Dallmeyer & Quesada (1992) have suggested that theseragneisses, schists, metagreywackes and amphibolitesages record a thermal rejuvenation during the Variscan(Sierra Albarrana Quartzites, Delgado Quesada, 1971).orogeny of minerals that grew during a Late PrecambrianThese quartzites are of Lower Palaeozoic age according(i.e. Cadomian) event. However, because the Palaeozoicto palaeontological evidence (Marcos et al., 1991). Theage of some rocks of this area has been documented,outcrop area of these quartzites is a small ridge knownand because only one tectonothermal event has beenas the Sierra Albarrana. Way-up criteria at the southwestidentified (see below), we interpret these ages as coolingand northeast contacts of this succession indicate a stra-ages after an Upper Devonian–Lower Carboniferoustigraphic top to the northeast.metamorphism (i.e. Variscan in age).(3) Migmatitic gneisses and schists with minor quartzites

A second phase of deformation can be recognized inand metagreywackes.the Sierra Albarrana area. It consists of folds that affectthe above structures and locally generate a subverticalcrenulation cleavage (Fig. 2). This phase is almost coaxialStructurewith the principal deformation phase and is responsibleThe pre-Carboniferous rocks of the Sierra Albarranafor the present steep dips of the principal foliation and

area are affected by a principal phase of penetrativefor the folds affecting the southern end of the Sierra

deformation responsible for the development of tight Albarrana Quartzites (Fig. 2).kilometre-scale upright folds and a ductile shear zone The latest Variscan deformation was an episode oflocated immediately to the southwest of the Sierra Al- brittle faulting that took place during the Carboniferousbarrana Quartzites (Fig. 2). Southwest of the shear zone, and generated the low-angle normal fault located to thethe fabric developed is generally planar and constitutes northwest of Sierra Albarrana and several steeply dippingthe axial planar foliation of the folds recognized (Fig. left-lateral faults (Fig. 2). The Onza Fault separating the2). The ductile shear zone mainly coincides with the Albariza Micaschists from the Villares Formation belongsmigmatitic gneisses located to the southwest of the Sierra to the latter set and, in addition to the left-lateral move-Albarrana Quartzites. Within this zone, the fabric is ment, displays a component of downthrowing of thestrongly planar and linear. Northeast of the shear zone, southwestern block.the intensity of the planar–linear fabric progressivelydecreases and kilometre-scale folds related to this de-formation are recognized in the Sierra Albarrana Quartz-

The Valdeinfierno basinites (Fig. 2).In the whole area at issue here, the principal foliation Undeformed and unmetamorphosed sediments of Lower

is subvertical or steeply dipping in its present orientation Carboniferous age [Upper Tournaisian to Lower Viseanand has a NW–SE strike. The stretching lineation is according to Wagner (1978), Garrote & Broutin (1979)subhorizontal or gently plunging to the southeast or and Roldan (1983)] rest unconformably on low-gradenorthwest, except in the southeastern end of the area schists and slates immediately to the northwest of thestudied, where plunges of 40–70° to the southeast are Sierra Albarrana, in the Valdeinfierno basin (Figs 1b andfound (Fig. 3). 2). This basin is bounded by a synsedimentary normal

Within the ductile shear zone and immediately to the fault, and the pebbles from the conglomeratic layerssouthwest and northeast, different types of shear criteria within the basin record the progressive exhumation of

the metamorphic rocks of the Sierra Albarrana areacan be observed in sections parallel to the stretching

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Fig. 3. Structural map of the Sierra Albarrana area showing strike and plunge of the stretching lineation (arrows). The Sierra AlbarranaQuartzites and the Valdeinfierno basin are depicted by the same pattern as in Fig. 2. The dotted line represents the external limit of the

metamorphic contact aureole associated with La Cardenchosa granite.

(Roldan, 1983; Gabaldon et al., 1983; Gabaldon & Que- IDENTIFICATION OFsada, 1986; Azor, 1994).

METAMORPHIC ZONESIn the area at issue here, metapelitic rocks constitute themajor part of the stratigraphic sequence (see GeologicalLa Cardenchosa graniteSetting). We have been concerned with assemblages from

The area studied is bounded to the east by La Car- metapelites alone and have examined about 300 thindenchosa granite (Figs 1b and 2). The intrusion postdates sections. The distribution of low-variance assemblages asthe regional ductile deformation and metamorphism of well as some higher-variance ones is shown in Fig. 4.the Sierra Albarrana area and is associated with a contact The distribution of aluminium silicate polymorphs isaureole (Garrote, 1976; Garrote & Sanchez Carretero, shown in Fig. 5. The relationships between the different1979). The age of La Cardenchosa granite with respect metamorphic phases and the deformation history areto the Valdeinfierno basin is not known yet, but some shown in Fig. 6. To constrain the P–T conditions in theworkers have suggested that the granite is younger (i.e. different metamorphic zones, low-variance assemblagesNamurian–Westphalian) than the Lower Carboniferous (i.e. those with three or more AFM phases) were studiedsediments (Delgado Quesada et al., 1985). with the electron microprobe. Table 1 shows the location

In summary, the tectonothermal evolution of the area of the 15 samples studied and their mineralogy.studied is two-fold. An intense ductile deformation as-sociated with low-pressure metamorphism occurred dur-ing a major dextral shearing. The exhumation of the

Biotite zonemetamorphic rocks took place during the Lower Car-boniferous, thus being synchronous with sedimentation Pelitic rocks from the biotite zone are slates or fine-grained

schists. These rocks generally show a slaty cleavage thatwithin the Valdeinfierno basin.

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Table 1: Mineral parageneses and location of the samples analysed; the location of the samples on map is

shown in Figs 4 and 5; the metamorphic zone of each sample is also indicated

Sample UTM coordinates Kfs Ms Grt St Chl And Sil P Sil F Ilm Met. zone

AA-166 30S TH 885113 St–Grt

AA-175 30S TH 795184 St–Grt

AA-53 30S TH 901202 St–Grt

AA-85 30S TH 818184 St–Grt

AA-137 30S TH 905169 St–Grt

AA-171 30S TH 859140 St–Grt

AA-223 30S TH 925136 St–Grt

AA-233 30S TH 889127 St–Grt

AA-80 30S TH 820195 Sil

AA-138 30S TH 896154 Sil

AA-139 30S TH 899156 Sil

AA-141 30S TH 894170 Sil

AA-18 30S TH 891152 Mig

AA-20 30S TH 879158 Mig

AA-104 30S TH 864190 Mig

Sil P, prismatic sillimanite; Sil F, fibrolitic sillimanite; Mig, migmatite zone. Other mineral abbreviations after Kretz (1983).

constitutes the principal foliation. Locally, a crenulation foliation, which is generally linear and discontinuous butat high angles with respect to the external foliation (Fig.cleavage overprints the principal foliation. The mineral

parageneses of these rocks are, in addition to quartz and 7). Because the sense (clockwise) and the amount (about20–30°) of rotation of the internal foliation is constantmuscovite, Bt + Chl or Bt. Accessory minerals are

ore minerals and tourmaline. Chloritoid was found by at the scale of the thin section, garnet growth appears tobe synkinematic, although affected by late rotation. InGarrote (1976) and Contreras et al. (1984), but its exact

location is not known. the most altered samples, garnet is transformed alongmargins and fractures to chlorite, biotite and Fe-oxides.

Staurolite crystals up to 1 cm in size contain quartzand ilmenite inclusions, defining an internal foliation. In

Staurolite–garnet zone most cases (e.g. sample AA-175), the internal foliation isPelitic rocks from the staurolite–garnet zone are medium- rotated with respect to the external foliation, althoughto fine-grained schists in which garnet, biotite and at both foliations are in continuity at the border of thetimes staurolite can be recognized in hand specimen. In porphyroblasts (Fig. 8a). These relationships indicate thatthe field, the principal foliation is a slaty cleavage or staurolite porphyroblasts are rotated with respect to theschistosity. The principal foliation is occasionally affected matrix.by a later crenulation cleavage and is generally the first Two types of biotite can be recognized: (1) elongatedpenetrative deformation as the sedimentary layering can crystals which, together with muscovite, define the fo-be recognized. The lowest-variance assemblages from liation, and (2) up to 1 mm porphyroblasts with thethis zone are Ms + Grt + St + Bt and Ms + Grt + cleavage parallel or oblique to the principal foliation (e.g.Bt + Chl, but higher-variance assemblages such as Ms samples AA-166 and AA-222). In the latter case, the+ Grt + Bt are also observed. Common accessory shape of the porphyroblasts and the angle between theminerals are ilmenite, zircon and tourmaline. Chloritoid foliation and the {001} cleavage indicate a dextral shearhas also been reported in this zone (Garrote, 1976; sense (Fig. 7). The biotite porphyroblasts are altered toContreras et al., 1984). chlorite. If a late deformation is present, biotite por-

Garnet (0·1–2 mm) normally forms euhedral or sub- phyroblasts are kinked when their cleavage is oblique tohedral inclusion-free crystals, although sometimes it has the principal foliation.inclusions of quartz and minor ilmenite. Most garnet Some chlorite grains are parallel to the foliation, whichgrains have pressure shadows. In some samples (e.g. can be taken to suggest that chlorite was in equilibrium

with garnet, muscovite and biotite. Chlorite is also foundsample AA-222), quartz inclusions define an internal

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Fig. 4. Distribution of AFM-mineral assemblages in the Sierra Albarrana area. Andalusite-in, sillimanite-in and migmatite-in isograds havebeen marked (dashed lines). Other symbols are as in Fig. 3. All the samples analysed are numbered. These numbers correspond to the numbering

in the text, tables and other figures. Other samples cited are also located.

within pressure shadows around garnet (Fig. 7), replacing Staurolite forms euhedral to subhedral porphyroblastsgarnet and biotite, or as porphyroblasts overgrowing the with an internal foliation marked by quartz inclusions.foliation (Fig. 8b). Staurolite also includes euhedral ilmenite and garnet. In

some cases (e.g. sample AA-171) the internal foliation iscontinuous with the external foliation (i.e. the principalfoliation), and staurolite porphyroblasts are rotated withStaurolite–andalusite zonerespect to the matrix. In other samples (e.g. samples AA-This zone is defined by the coexistence of staurolite and85 and AA-223) the internal foliation is a crenulatedandalusite (Figs 4–6). Metapelitic rocks from this zonecleavage, recording an earlier stage of development ofare fine- to medium-grained schists containing por-the foliation now observed in the matrix. In these cases,phyroblasts of staurolite (up to 2 cm) and andalusitetransposed microfolds can be seen in the matrix, showing(up to 5 cm). Crystals of biotite, muscovite and garnetthat the principal foliation corresponds to a crenulationgenerally do not exceed 2 mm. The principal foliationcleavage. Andalusite appears as subhedral porphyroblastsin these schists is mainly a schistosity, although in somewith numerous quartz, muscovite, biotite and ilmenitecases a crenulation cleavage is observed. Locally, a mil-inclusions sometimes defining an internal foliation. As inlimetre-spaced crenulation cleavage overprints the prin-the staurolite porphyroblasts, the internal foliation iscipal foliation. The most abundant assemblage is Ms +sometimes a crenulated cleavage (e.g. sample AA-233),Grt + St + Bt + And. Some pelitic rocks, however,which we interpret as having recorded the earliest stagescontain higher-variance assemblages such as Ms + Grtof foliation development within the matrix. Euhedral+ Bt, Ms + Grt + St + Bt or Ms + Grt + Bt +garnet is occasionally included in andalusite, but stauroliteAnd. Accessory minerals are ilmenite, zircon, tourmaline

and apatite. inclusions are not observed.

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Fig. 5. Distribution of aluminium silicate polymorphs in the Sierra Albarrana area. Andalusite-in, andalusite-out and sillimanite-in regionalisograds have been depicted (dashed lines). Distribution of aluminium silicate polymorphs in the contact aureole of La Cardenchosa Granite is

also shown. Sample location is indicated with numbers as in Fig. 4. Other symbols are as in Fig. 3.

The fact that the principal foliation is either a schistosity sample AA-223), large muscovite grains develop aroundthe andalusite porphyroblasts.or a crenulation cleavage can be interpreted as evidence

Chlorite develops in the more altered samples at thefor polymetamorphism with two different generations ofexpense of biotite and garnet. It is also present as por-staurolite and andalusite porphyroblasts. Nevertheless,phyroblasts up to 1 mm in size that overgrow the principalthis is not the case, as this fact can be related to thefoliation (e.g. samples AA-85 and AA-223). The growthexistence of a non-coaxial component in the principalof the chlorite porphyroblasts postdates the crystallizationdeformation as indicated by the development of a planar–of the other phases as well as the principal deformation.linear fabric and shear criteria. In this regard, the su-

Textural equilibrium between staurolite, andalusite,perposition of different fabrics in a shear zone is fairlybiotite, garnet, plagioclase and muscovite is observed. Allcommon owing to the rotation affecting all surfaces andthese phases are synkinematic with respect to the principallines included in it (Ramsay & Huber, 1987). If the anglefoliation as deduced from pressure shadows and internalwith the boundaries of the shear zone is adequate,foliation–external foliation relationships in staurolite, an-earliest developed fabrics can be folded during progressivedalusite and garnet porphyroblasts.deformation and the principal foliation finally observed

would be a crenulation cleavage. In other cases, thefabric would not rotate, owing to its initial orientation

Sillimanite zonewith respect to the boundaries of the shear zone, andthe final foliation observed would be a schistosity. The sillimanite zone is marked by the disappearance of

Garnet and biotite present the same textural char- staurolite (staurolite-out isograd), which seems to coincideacteristics as in the staurolite–garnet zone. Muscovite on a regional scale with the incoming of sillimanite

(sillimanite-in isograd) in most metapelites (Figs 4–6).crystals are parallel to the foliation. In some samples (e.g.

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Fig. 6. Sequence of synkinematic phases within metapelites, late to postkinematic phases in metapelites, and mineral assemblages observedwithin veins in the different metamorphic zones.

Andalusite porphyroblasts are still present in a narrow Andalusite porphyroblasts have characteristics similarto those of the staurolite–andalusite zone, and sometimesband adjacent to the staurolite–andalusite zone (Fig. 5).

The modal amount of garnet decreases upgrade from contain garnet pseudomorphs (Fig. 9). Prismatic sil-limanite progressively develops either within fractures inthe sillimanite-in isograd.

Metapelites from the sillimanite zone are fine- to the andalusite porphyroblasts (sample AA-141, Fig. 9) oras aggregates between andalusite grains (sample AA-139).medium-grained schists in which fibrolitic sillimanite and

biotite are recognized in hand specimen. The principal Fibrolitic sillimanite is sometimes present in small clusters(sample AA-80).foliation is a schistosity occasionally affected by a cren-

ulation cleavage. The most abundant assemblages in this Garnet is less common than in the staurolite–andalusitezone and appears as euhedral to subhedral inclusion-freezone are Ms + Grt + Bt + And + Sil and Ms + Bt

+ Sil. The three-phase AFM assemblage Ms + Grt + crystals of several microns to 1 mm in size. Reddishbrown biotite and muscovite lamellae define the principalBt + Sil is occasionally present. Accessory minerals are

tourmaline, apatite, zircon and ilmenite. Relic kyanite is foliation. Some muscovite grains develop around an-dalusite (sample AA-141). Synkinematic phases with re-observed as inclusions within biotite crystals in a rare

cordierite-bearing layer (Gonzalez del Tanago & Peinado, spect to the principal foliation are sillimanite, biotite,garnet, plagioclase and muscovite.1990).

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which grow over the foliation, although sometimes theyappear deformed. Fibrolitic sillimanite is present withinall phases (Fig. 10a). Accessory minerals are tourmaline,apatite and zircon. The lack of ilmenite is also char-acteristic of this zone (Fig. 6).

Sample AA-104 is slightly different from the othersamples of this zone, because it shows a textural equi-librium between biotite, K-feldspar, prismatic sillimanite,muscovite, quartz and tourmaline (Fig. 10b): the grainsof the different phases have approximately the same sizeand are in mutual contact. Some sillimanite grains havebeen replaced by fine-grained muscovite aggregates.

DISTRIBUTION OF METAMORPHICFig. 7. Subhedral garnet with internal foliation marked by quartz ZONESinclusions (sample AA-222, staurolite–garnet zone). The internal fo-

A metamorphic zonation (Figs 4–6) ranging from theliation is rotated with respect to the principal foliation indicating adextral sense of shearing (arrows); the angle between {001} cleavage biotite zone to the migmatite zone developed during thein biotite porphyroblasts and the principal foliation also indicates a principal deformation in the Sierra Albarrana area. This

dextral sense of shearing. Garnet diameter is 1·28 mm.zonation has been noted in previous studies (Laurent,1974; Garrote, 1976; Gonzalez del Tanago & Peinado,

Migmatite zone 1990; Gonzalez del Tanago, 1993). According to theseworkers, the metamorphic zones are roughly symmetricalMetapelites from the migmatite zone show a gneissic

layering defined by millimetre-scale alternating mi- with respect to the Sierra Albarrana Quartzites and themetamorphic grade decreases towards the southwest ascaceous and quartzofeldspathic domains. The quartzo-

feldspathic domains have a coarse-grained granitic min- well as towards the northeast up to the chlorite zone.This simple pattern requires modification in the light oferalogy and form essentially continuous layers or small

boudinaged pods. The micaceous domains are made our data. Because late granite intrusion and brittle faultinghave modified the distribution of metamorphic zones, itup by biotite, fibrolitic sillimanite and at times minor

muscovite. Fibrolitic sillimanite sometimes forms elong- is only in the central part of the area studied where theinitial distribution is preserved. We shall now turn toated whitish nodules up to 1·5 cm in size parallel to the

stretching lineation. We interpret these textural char- discussion of the relevance of the late modifications beforeconsidering the relationships between the distribution ofacteristics as being due to partial melting. In this zone,

most rocks are of quartzitic or quartzofeldspathic com- metamorphic zones and the ductile deformations.Three kinds of late modifications are recognized:position, thus making it difficult to map the boundaries

of the zone, as partial melting affects only the metapelitic (1) Structural investigations to the southwest of theSierra Albarrana reveal the existence of a left-laterallithologies. To the southwest of the Sierra Albarrana

Quartzites, the migmatite zone extends to the contact brittle fault with downthrowing of the southwestern block(the Onza Fault, Azor, 1994). This kinematics is consistentwith the Albariza Micaschists. Migmatites are sometimes

present within the Sierra Albarrana Quartzites, where with the distribution of the metamorphic zones: thebiotite zone is located to the south of the Onza Fault, inthey represent interbedded metapelites. Finally, mig-

matites are largely developed to the northeast of the the downthrown block, whereas higher-grade zones (fromstaurolite–garnet up to the migmatite zones) can beSierra Albarrana Quartzites, the zone boundary being

located within the formation of migmatitic gneisses and observed to the north of the fault, i.e. in the upthrownblock (Fig. 4). The southeasternmost part of the meta-schists.

In most migmatitic metapelites (e.g. samples AA-18 morphic domain is cut across by the Onza Fault, asshown by the occurrence of garnet-bearing micaschistsand AA-20), complex relationships between coexisting

phases can be observed. Large grains of K-feldspar south of the fault. These latter rocks can be interpretedas high-variance assemblages belonging to the staurolite–contain euhedral inclusions of biotite and, more rarely,

fibrolite. Matrix biotite is strongly corroded by fibrolitic garnet zone, or as assemblages from a garnet zone,transitional between the biotite and staurolite–garnetsillimanite (Fig. 10a). Myrmekitic intergrowths are fre-

quently present at the contact between plagioclase and zones. The garnet-bearing micaschists have not beenidentified in the southwestern part of the area studiedK-feldspar. Muscovite grains constitute either small cor-

roded grains or large (up to 1 cm in size) porphyroblasts probably because they are cut across by the Onza Fault.

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Fig. 8. (a) Photomicrograph of euhedral staurolite and garnets from sample AA-175 (staurolite–garnet zone). Internal foliation in staurolite ismarked by quartz inclusions and is continuous with the external foliation, which is defined by biotite and muscovite. Rotation of the internalfoliation in staurolite indicates a sinistral sense of shearing in the photograph, which, in map view, corresponds to a dextral sense of shearing.Plane-polarized light. Diameter of the garnet located in the lower left corner of the photograph is 1·24 mm. (b) Photomicrograph of chloriteporphyroblasts from sample AA-170 (staurolite–garnet zone). Chlorite porphyroblasts postdate the principal foliation, which is defined by biotiteand muscovite. Staurolite and garnet are also present in this sample. Plane-polarized light. The chlorite porphyroblast located to the left in the

photograph is 0·36 mm wide.

45


Fig. 9. Garnet pseudomorph included in andalusite porphyroblast from sample AA-141 (sillimanite zone). Prismatic sillimanite appears alongfractures in the andalusite porphyroblast. Garnet pseudomorph is made up of quartz, muscovite and biotite. Cross-polarized light. Garnet

pseudomorph diameter is 1·46 mm.

(2) The metamorphic zonation of the Sierra Albarrana between the direction and plunge of the stretching lin-eation (Fig. 3) and the shape of the isograds (Figs 4 andarea is cut across towards the northwest by a low-angle5). When the stretching lineation is subhorizontal, thebrittle normal fault (Figs 1b and 4), which is responsibleisograds run parallel to the lithologic boundaries and tofor the location of the Lower Carboniferous Valdeinfiernothe shear zone (e.g. the andalusite-in and sillimanite-inbasin (Azor, 1994). The rocks in the hangingwall of theisograds to the southwest of the Sierra Albarrana). Infault belong to the Villares Formation, corresponding toaddition, the sillimanite-in and migmatite isograds closemetamorphic conditions of the biotite zone.where the plunge of the stretching lineation sharply(3) The contact aureole associated with La Car-increases.denchosa granite cuts across the regional metamorphic

zonation (Garrote, 1976) (Fig. 4). The development ofhornfelses largely overprints the regional metamorphiczonation. Assemblages belonging to the staurolite–garnetzone and lower-grade ones, if ever present, have been DISTRIBUTION AND MINERALreplaced by contact metamorphic assemblages. Con-

ASSEMBLAGES OF VEINS ANDsequently, no isograd associated with the regional meta-morphism can be mapped near La Cardenchosa granite, PEGMATITESand the northern boundary of the staurolite–andalusite Veins are especially abundant in the Sierra Albarranazone probably corresponds to the external limit of the area. The mineral content and the spatial distribution ofcontact aureole (Fig. 4). these veins are summarized in Figs 6 and 11, respectively.

The distribution of metamorphic zones is well pre- These data are consistent with previous observationsserved in the central part of the area studied, i.e. on both (Garrote et al., 1980; Ortega Huertas et al., 1982).sides of the Sierra Albarrana. At the sample scale, mineral Only quartz veins are present in the biotite and stau-growth is synchronous with the principal foliation. This rolite–garnet zones. The amount and size of these veinsmeans that the Sierra Albarrana area does not constitute increase in the staurolite–andalusite zone, where they maya thermal dome postdating the development of the prin- also contain large crystals of muscovite and andalusite (upcipal foliation, as proposed by Quesada & Munha (1990). to 10 cm). In one locality (Era de la Charneca, Fig. 11),

a large number of mineral assemblages can be observed.At a regional scale, a close correlation can be observed

46


Fig. 10. (a) Photomicrograph of fibrolitic sillimanite related to biotite from sample AA-18 (migmatite zone). (Note sillimanite needles surroundingbiotite and included in K-feldspar.) Plane-polarized light. Width of the field is 1·4 mm. (b) Photomicrograph of Ms–Kfs–Bt–Sil assemblage fromsample AA-104 (migmatite zone). Apparent textural equilibrium between the different phases is shown. Plane-polarized light. Width of the field

is 1·4 mm.

Most veins contain various combinations of quartz, mus- andalusite–muscovite, as well as a minor amount of eithergarnet or staurolite. Interestingly, a few kyanite crystalscovite and andalusite, especially andalusite–quartz and

47


Fig. 11. Distribution of coarse-grained veins and pegmatites. It should be noted that they are preferentially distributed in the higher-grade partof the area, and that their mineralogy is consistent with the metamorphic grade of the enclosing metapelites. Other symbols are as in Fig. 2.

are observed as inclusions within andalusite (sample AA- The volume and the orientation of the veins aredependent on both the metamorphic grade and the415, Fig. 12). These observations are consistent with

previous reports (Abad Ortega, 1993; Gonzalez del Tan- nature of the host rocks. Veins are larger in the higher-grade zones (up to 500 m long and 70–100 m thick)ago, 1993) and show that kyanite is a relic phase.

Veins containing quartz, muscovite and andalusite compared with the lower-grade ones (up to 10–20 mlong and 1 m thick). On the overall outcrop scale, theypartly replaced by sillimanite are present in the lower-

grade part of the sillimanite zone (e.g. sample AA- parallel the strike of the foliation when the enclosingrocks are slates or schists, even if their margins locally276). In the higher-grade part of the sillimanite zone,

aluminium silicate is not present in the veins, which cut across the foliation. Veins elongated perpendicularor subperpendicular to the strike of the foliation are onlyessentially consist of coarse-grained quartz, plagioclase,

muscovite and tourmaline crystals. Biotite is a minor observed within the Sierra Albarrana Quartzites or withinthe quartzites interlayered within the gneissic rocks loc-phase in some veins.

In the migmatite zone, pegmatite veins are extremely ated to the southwest of the Sierra Albarrana (e.g. Cerrode la Sal, Fig. 11). Nevertheless, many of the quartz andabundant and are essentially made up of quartz, albite,

orthoclase and tourmaline. Graphic intergrowths be- pegmatitic veins are deformed (i.e. foliated, folded and/or boudinaged) whereas others are undeformed. Thistween K-feldspar or plagioclase and quartz are frequent.

In addition, some veins contain giant biotite and beryl indicates that the veins are probably synchronous withthe principal deformation and metamorphism.crystals (up to 1 m). The mineralogy of the pegmatites

has been dealt with in a number of recent studies (e.g. In summary, the volume of veins increases with themetamorphic grade and their mineralogical content isGonzalez del Tanago, 1991; Abad Ortega, 1993; Abad

Ortega et al., 1993; Abad Ortega & Nieto, 1995a, 1995b). closely related to the paragenesis in the metapelitic host

48


Fig. 12. Kyanite relic included within andalusite from veins in Era de la Charneca (staurolite–andalusite zone).

rocks. These data suggest that they derive from local mol %) contents. Microprobe analyses reveal two typessubsolidus dehydration reactions or melting reactions, of variations: (1) zoning patterns within individual grainsdepending on the grade. Alternatively, at least the peg- and (2) systematic variations of the chemistry of garnetmatitic veins could be related to a granitic intrusion rims between different metamorphic zones.located below the higher-grade part of the metamorphic Zoning profiles are shown in Fig. 13. Most garnets fromdomain. These two possibilities will be considered in the staurolite–garnet and staurolite–andalusite zones aremore detail in the final discussion on the origin of the characterized by a decrease in MnO and an increase inmetamorphism. FeO and MgO contents from core to rim. Garnets from

the sillimanite zone are unzoned (Fig. 13, sample AA-138) or show flat profiles with an increase in MnO anda decrease in MgO contents at the rims (Fig. 13, sampleMINERAL CHEMISTRY OFAA-80). This evolution of zoning profiles with increasing

COEXISTING PHASES grade is similar to the one documented by DempsterThe chemical composition of the phases in selected (1985) in a medium-pressure metamorphic series, and issamples was analysed with an electron microprobe interpreted in a similar way [see also Tracy (1982),(Microsonde Ouest, Brest, France) using the PAP cor- Loomis (1983) and Chakraborty & Ganguly (1990)].rection. Analytical conditions were 15 kV accelerating Garnets from the staurolite–garnet and staurolite–voltage, 15 nA sample current and 6 s counting time. andalusite zones preserve growth zoning. In the sil-Standards were albite (Na), orthoclase (K), corundum limanite zone, more efficient diffusion appears to have(Al), wollastonite (Ca, Si), forsterite (Mg), ilmenite (Mn, erased the growth zoning, and there is evidence for aTi), Fe2O3 (Fe) and ZnS (Zn). Zn was detected with the slight down-temperature reequilibration of garnet rims.La ray, and all others using the Ka ray. The spessartine content of cores decreases from the

staurolite–garnet zone (~15 mol %) to the sillimanitezone (~8–10 mol %), but remains relatively high, which

Garnet suggests that garnet is stabilized by MnO. The pyropecontent increases from the staurolite–garnet zone (5 molThe garnets analysed (see Table 2 and Fig. 13) are%) to the sillimanite zone (~10 mol %). Accordingly,essentially almandine (64–82 mol %)–spessartine (5–27the Mg/(Mg + Fe) ratio slightly increases from themol %)–pyrope (4–10 mol %) solid solutions with low to

very low grossularite (0–6 mol %) and andradite (0–2 staurolite–garnet (0·06–0·07) to the staurolite–andalusite

49


Table 2: Representative garnet analyses and structural formulae on a

basis of 12 oxygens and 8 cations; almandine, spessartine, pyrope,

grossular, andradite and uvarovite percentages are indicated; for each

sample, core and rim analyses are distinguished and the metamorphic

zone is indicated

Sample: AA-166 AA-175 AA-137 AA-223 AA-80

Met. zone: St–Grt St–Grt St–And St–And Sil

rim rim rim rim core

SiO2 36·84 36·95 36·73 37·32 37·00

TiO2 0·02 0·00 0·00 0·00 0·00

Al2O3 20·96 20·87 20·91 20·80 20·86

Cr2O3 0·01 0·11 0·05 0·12 0·00

FeO 33·01 35·40 37·14 32·73 34·45

MnO 6·84 3·43 2·44 4·53 4·81

MgO 1·32 2·34 1·85 2·23 2·65

CaO 1·61 1·52 1·78 1·86 0·93

Na2O 0·03 0·05 0·01 0·00 0·00

K2O 0·00 0·00 0·00 0·00 0·02

Total 100·64 100·64 100·90 99·59 100·71

Structural formulae on a basis of 12 oxygens and 8 cations

Si 2·985 2·975 2·961 3·024 2·976

AlIV 0·015 0·025 0·039 0·000 0·024

AlVI 1·987 1·956 1·948 1·987 1·955

Ti 0·001 0·000 0·000 0·000 0·000

Cr 0·001 0·007 0·003 0·007 0·000

Fe3+ 0·011 0·037 0·049 0·000 0·045

Fe2+ 2·227 2·348 2·456 2·218 2·273

Mn 0·470 0·234 0·167 0·311 0·328

Mg 0·159 0·280 0·222 0·269 0·317

Ca 0·140 0·131 0·154 0·161 0·080

Na 0·005 0·007 0·002 0·000 0·000

K 0·000 0·000 0·000 0·000 0·002

Almandine 74·33 78·45 81·91 74·96 75·83

Spessartine 15·68 7·81 5·55 10·50 10·93

Pyrope 5·32 9·37 7·41 9·09 10·58

Grossular 4·25 2·92 3·41 5·20 1·17

Andradite 0·40 1·23 1·62 0·00 1·49

Uvarovite 0·02 0·23 0·10 0·25 0·00

Mg/(Mg + Fe) 0·07 0·11 0·08 0·11 0·12

(~0·12) zones, and shows the same value in the sillimanite ZnO contents are very low (0–0·4 and 0·1–1·1 wt %,respectively). No zoning has been observed in staurolite.zone as in the staurolite–andalusite zone.The staurolite is slightly more magnesian [Mg/(Mg +Fe) ratio of ~0·15–0·17] than the coexisting garnet.

StauroliteBiotiteThe structural formulae of staurolite have been calculated

on a basis of 46 oxygens. Staurolite composition is very Typical compositions of biotites in the different meta-morphic zones are shown in Table 4. Most of the biotitessimilar in all the samples studied (Table 3). MnO and

50


Fig. 13. Compositional zoning in garnets from the Sierra Albarrana metapelites. Each profile extends from rim to rim, through the core of the garnet.

51


Table 3: Representative staurolite analyses and structural

formulae on a basis of 46 oxygens; the metamorphic zone

of each sample is indicated

Sample: AA-175 AA-137 AA-223

Met. zone: St–Grt St–And St–And

SiO2 27·97 27·91 27·26

TiO2 0·51 0·42 0·40

Al2O3 53·44 53·82 55·10

Cr2O3 0·00 0·05 0·00

FeO 13·81 14·97 13·02

MnO 0·17 0·10 0·34

MgO 1·61 1·51 1·46

ZnO 0·00 0·12 0·74

CaO 0·00 0·00 0·08

Na2O 0·08 0·00 0·01

K2O 0·00 0·00 0·02

Total 97·59 98·90 98·44

Structural formulae on a basis of 46 oxygens

Si 7·778 7·704 7·527

Al 17·525 17·517 17·941

Ti 0·106 0·087 0·083

Cr 0·000 0·011 0·000

Fe2+ 3·213 3·456 3·008

Mn 0·040 0·023 0·080

Mg 0·669 0·621 0·599

Zn 0·000 0·024 0·152

Ca 0·000 0·000 0·022

Na 0·044 0·000 0·006

K 0·000 0·000 0·008

29·375 29·445 29·427

Mg/(Mg + Fe) 0·17 0·15 0·17

analysed, especially those from the staurolite–garnet zone, Chlorites in these three samples have the same com-position.show low K2O contents, probably owing to a partial

alteration to chlorite. Within the staurolite–andalusitezone, biotite inclusions in andalusite are altered to alesser degree than matrix biotites, as shown by the higher MuscoviteK2O contents. Biotites from the staurolite–andalusite

Most of the samples studied contain primary muscovite,zone are slightly more magnesian than biotites from theexcept some of the migmatite zone. No compositionalsillimanite and migmatite zones (Table 4 and Fig. 14).difference can be established between the crystals parallelTi contents increase with metamorphic grade (Fig. 14)to the foliation, those included within andalusite, andregardless of whether ilmenite is present or not.those appearing as porphyroblasts. Their celadonite con-tents are consistently low or very low, as Si contents varybetween 3·03 and 3·10 per formula unit (p.f.u.) (Table

Chlorite 4). In most samples, MgO and FeO contents range from0·3 to 0·6 and from 0·7 to 1·5 wt %, respectively.Chlorite is present in samples AA-85 (Sta–Grt zone) and

AA-175 (Sta–And zone) as porphyroblasts cutting across Muscovites from sample AA-104 are richer in total FeO(2·22–2·74 mol %) and poorer in Al2O3, which indicatesthe foliation (Fig. 8b) and in sample AA-166 (Sta–Grt

zone) as an alteration product of biotite and garnet. that most of the iron is ferric rather than ferrous. The

52


Table 4: Representative biotite and muscovite analyses and structural formulae on a basis of 11 oxygens; the

metamorphic zone of each sample is indicated

Sample: AA-223 AA-80 AA-138 AA-18 AA-104 AA-166 AA-137 AA-138 AA-18 AA-104

Met. zone: St–And Sil Sil Mig Mig Grt St–And Sil Mig Mig

Mineral: Bt Bt Bt Bt Bt Ms Ms Ms Ms Ms

SiO2 35·94 34·99 35·99 35·15 35·28 46·50 47·35 46·68 45·32 52·74

TiO2 1·43 2·44 2·08 3·73 2·96 0·26 0·00 0·00 0·94 0·00

Al2O3 20·18 19·58 19·14 18·96 18·33 36·37 37·21 35·93 35·41 28·66

Cr2O3 0·21 0·04 0·05 0·00 0·00 0·02 0·02 0·03 0·10 0·01

FeO 20·46 21·51 21·39 20·00 21·09 1·52 0·68 0·73 0·90 2·22

MnO 0·12 0·04 0·15 0·04 0·04 0·02 0·06 0·00 0·12 0·00

MgO 8·74 7·66 8·07 7·58 8·11 0·49 0·39 0·37 0·55 2·58

CaO 0·03 0·00 0·00 0·00 0·00 0·03 0·00 0·00 0·00 0·13

Na2O 0·15 0·24 0·38 0·13 0·14 1·10 1·94 1·32 0·47 0·16

K2O 8·65 8·74 8·15 9·87 10·28 8·90 7·96 9·99 11·16 9·41

Total 95·93 95·23 95·40 95·46 96·22 95·20 95·61 95·05 94·97 95·90


Si 2·709 2·681 2·737 2·687 2·697 3·072 3·087 3·098 3·039 3·450

AlIV 1·291 1·319 1·263 1·313 1·303 0·928 0·913 0·902 0·961 0·550

AlVI 0·503 0·449 0·454 0·395 0·349 1·904 1·948 1·909 1·839 1·660

Ti 0·081 0·141 0·119 0·214 0·170 0·013 0·000 0·000 0·047 0·000

Cr 0·013 0·003 0·003 0·000 0·000 0·001 0·001 0·002 0·005 0·001

Fe2+ 1·290 1·379 1·361 1·278 1·349 0·084 0·037 0·041 0·051 0·121

Mn 0·008 0·002 0·010 0·003 0·002 0·001 0·003 0·000 0·007 0·000

Mg 0·982 0·875 0·915 0·864 0·924 0·048 0·038 0·037 0·055 0·251

Ca 0·002 0·000 0·000 0·000 0·000 0·002 0·000 0·000 0·000 0·009

Na 0·022 0·035 0·056 0·020 0·021 0·141 0·245 0·170 0·061 0·020

K 0·832 0·854 0·791 0·963 1·003 0·750 0·662 0·846 0·955 0·786

7·734 7·738 7·708 7·736 7·819 6·944 6·935 7·004 7·019 6·848

Mg/(Mg + Fe) 0·43 0·39 0·40 0·40 0·41

paragonite content of the muscovites ranges from 5 to three samples (AA-18, AA-104 and AA-138) and itscomposition approximates Or90Ab10 (Table 5).30 mol %. Fine-grained, secondary muscovites growing

at the expense of prismatic sillimanite (sample AA-104)or K-feldspar (sample AA-138 ) have higher Si contents(up to 3·45 p.f.u.) and, accordingly, higher MgO contents. Andalusite and sillimanite

Minor components were not detected in andalusite andsillimanite. Only very low contents in Fe2O3 (0–0·8 wt%) are present.Feldspars

Plagioclases from the staurolite–garnet and staurolite–andalusite zones are generally oligoclase (samples AA-

Oxides137 and AA-175), but nearly pure albite is also observed(samples AA-53 and AA-223) (Table 5). It is not known Primary oxides in the samples studied are ilmenites withwhether these albitic compositions result from the late MnO contents between 0·28 and 2·83 mol %. In thealteration of more calcic primary plagioclases, or rep- matrix, most of the ilmenites are altered to a TiO2-richresent compositions across the peristerite gap. phase, most probably anatase as reported by Hebert &

Plagioclases from the sillimanite zone have anorthite Ballevre (1993) from staurolite-bearing micaschists in thecontents equal to or higher than 18–20 mol %, except Cadomian Belt of northern Brittany. Ilmenites included

in andalusite, staurolite and garnet are normally notin sample AA-104. Primary K-feldspar was analysed in

53


assemblage sequences, combined with Schreinemakers’rules (e.g. Hensen, 1971; Harte, 1975; Thompson, 1976;Harte & Hudson, 1979; Pattison & Harte, 1985; Pattison& Tracy, 1991).

A crucial point in the interpretation of the mineralassemblages from the Sierra Albarrana area is the locationof some reactions with respect to the And–Sil equilibrium.No general agreement has been reached in depictingphase relations in low-pressure metapelites based strictlyon thermodynamic databases. In addition, comparisonwith natural assemblages is plagued by departures fromthe model KFMASH system owing to the incorporationof non-KFMASH components, especially CaO and MnOwithin garnet and possibly also Fe2O3 and TiO2 intobiotite. In this regard, a good example is shown by thecommon occurrence of the assemblage Grt–St–Bt–And–Ms–Qtz, where Grt is stabilized by the in-corporation of MnO (see Giaramita & Day, 1991;Pattison & Tracy, 1991; Symmes & Ferry, 1992; Droop& Harte, 1995; Pitra & Guiraud, 1996).

Fig. 14. Ti content (p.f.u.) vs Mg/(Fe + Mg) diagram for biotites of Despite these difficulties, the petrogenetic grid of Pat-the three higher-grade zones. The progressive increase in Ti content tison & Tracy (1991) agrees well with the sequence offrom the staurolite–andalusite zone to the migmatite zone should be

mineral reactions deduced from our own observations,noted.and is used below. Figure 16 shows part of the KFMASHgrid of Pattison & Tracy (1991), but some modifications

altered. In the samples belonging to the migmatite zone have been made to take into account the additional(AA-18, AA-20, AA-104), no ore minerals are present, constituent MnO. Assuming that MnO enters only intoprobably because all the TiO2 in the rock was in- garnet, some invariant points and equilibrium curves arecorporated into biotite (Fig. 14). displaced with respect to their position in the KFMASH

grid. In particular, the invariant point I1 will be displacedwithin the andalusite stability field (Fig. 16). A sequenceof continuous and discontinuous model reactions can bePHASE RELATIONS AND REACTIONreconstructed with increasing grade.

HISTORYThe observed mineral assemblages, their relationshipswith respect to the deformation history and their meas-ured chemical compositions are used to decipher thereaction history. Because the studied rocks invariably The entrance in the staurolite–garnet zonecontain quartz and muscovite or K-feldspar, we will use Because the transition between the biotite zone and thethe AFM projection (Thompson, 1957) to depict phase staurolite–garnet zone is obscured by the Onza Fault,compatibilities (Fig. 15). A vapour (Vap) phase (or a melt the metamorphic reactions responsible for that transitionphase in the case of the migmatite zone) is assumed to cannot be established accurately. The appearance ofbe in excess. Because MnO is an essential constituent garnet is most probably related to the continuousfor garnet (e.g. Giaramita & Day, 1991; Symmes & Ferry, FeMnMg reaction1992), it is taken into account in the AFM projection(Fig. 15). (1)Chl + Ms + Qtz = Grt + Bt + Vap.

Phase relations in low-pressure metapelites have beeninvestigated using essentially two methods. Some workers The entrance of staurolite is related to the continuoushave calculated stable KFMASH reactions using ex- FeMnMg reactionperimentally based internally consistent thermodynamicdatasets for the mineral end-members, combined with (2)Grt + Chl + Ms + Qtz = St + Bt + Vapestimated solution models (e.g. Spear & Cheney, 1989;Powell & Holland, 1990; Dymoke & Sandiford, 1992; or to the continuous FeMg reactionXu et al., 1994). Other workers have constructed reactiongrids based on repeated occurrences of natural mineral (3)Chl + Ms + Qtz = St + Bt + Vap.

54


Table 5: Representative plagioclase and K-feldspar analyses and structural formulae on

a basis of eight oxygens; the metamorphic zone of each sample is indicated

Sample: AA-137 AA-223 AA-80 AA-138 AA-18 AA-18

Met. zone: St–And St–And Sil Sil Mig Mig

Mineral: Pl Pl Pl Kfs Pl Kfs

SiO2 63·35 68·94 62·01 64·09 61·86 64·37

TiO2 0·02 0·05 0·00 0·03 0·00 0·00

Al2O3 22·90 19·46 23·88 19·33 23·96 18·68

Cr2O3 0·00 0·04 0·00 0·09 0·02 0·14

FeO 0·00 0·03 0·00 0·14 0·00 0·00

MnO 0·04 0·01 0·00 0·01 0·04 0·00

MgO 0·02 0·06 0·00 0·00 0·00 0·01

CaO 4·03 0·13 5·32 0·00 5·43 0·08

Na2O 8·76 11·90 8·69 0·29 8·75 1·50

K2O 0·00 0·06 0·15 15·19 0·17 15·48

Total 99·12 100·68 100·04 99·17 100·22 100·26


Si 2·814 2·994 2·749 2·970 2·741 2·970

AlIV 1·186 0·996 1·248 1·030 1·252 1·016

AlVI 0·013 0·000 0·000 0·027 0·000 0·000

Ti 0·001 0·002 0·000 0·001 0·000 0·000

Cr 0·000 0·001 0·000 0·003 0·001 0·005

Fe2+ 0·000 0·001 0·000 0·005 0·000 0·000

Mn 0·002 0·000 0·000 0·000 0·001 0·000

Mg 0·001 0·002 0·000 0·000 0·000 0·001

Ca 0·192 0·006 0·253 0·000 0·258 0·004

Na 0·755 1·002 0·747 0·026 0·752 0·134

K 0·000 0·003 0·009 0·898 0·010 0·911

4·963 5·008 5·005 4·961 5·014 5·042

Ca/(Ca + Na) 0·20 0·00 0·25 0·26

These reactions account for the disappearance of primary, The sillimanite-in isogradsynkinematic chlorite in the staurolite–garnet zone and Staurolite breakdown (i.e. the entrance within the sil-the decrease in spessartine content within garnet at limanite zone) took place either by means of the modelincreasing grade. continuous reaction

(5)St + Ms + Qtz = Als + Bt + Vap

or the model discontinuous reactionThe andalusite-in isograd

(6)St + Ms + Qtz = Grt + Bt + Als + VapThe beginning of the staurolite–andalusite zone is definedby the appearance of andalusite. AFM-phase com- as suggested by the AFM topologies seen in the staurolite–patibilities (Fig. 15) suggest that the model KFMASH andalusite and sillimanite zones (Fig. 15). At map scale,discontinuous reaction the staurolite-out isograd coincides with the sillimanite-

in one (Figs 4 and 5), whereas andalusite is still re-(4)St + Chl + Ms + Qtz = Bt + And + Vapcognizable in the lower-grade part of the sillimanite zone.

was responsible for andalusite formation. Garnet may Two models are compatible with these observations:not have been involved in the andalusite-producing re- (1) The staurolite-breakdown reactions (5) and (6) couldaction, as suggested by its euhedral shape when included have occurred within the andalusite stability field, with

the transformation of andalusite into sillimanite takingwithin andalusite.

55


Fig. 15. Al2O3–(FeO+MnO)–MgO projection from quartz, muscovite and vapour of coexisting minerals in the Sierra Albarrana metapelites.For the sample AA-223, in which the four-phase AFM assemblage Grt–St–Bt–And is present, an FeO–MnO–MgO projection from aluminiumsilicate, quartz, muscovite and vapour is also shown. For the migmatite zone (sample AA-18) the projection has been done from K-feldspar in

instead of muscovite.

56


Fig. 16. Petrogenetic grid for a portion of the KFMASH system [slightly modified after Pattison & Tracy (1991)] showing the sequence ofcontinuous (light lines bounding Fe-richer end-member reactions) and discontinuous reactions (heavy lines) with increasing grade. Equilibriabetween aluminium silicate polymorphs are also indicated with light dashed lines. Dashed arrow indicates the proposed P–T conditions for thestaurolite–andalusite zone (left), sillimanite zone (centre), and migmatite zone (right). Melting reactions in the NKASH system after Thompson

& Algor (1977). (See text for further explanations.)

place during a later temperature increase. In this model, most probably due to reaction (5) or (6) rather than toreaction (7), which would explain the coincidence ofa Grt–Bt–And zone must be observed before the firststaurolite disappearance with sillimanite appearance.appearance of sillimanite. This is not the case in our

The sillimanite zone is also marked by a strong decreasestudy. Indeed, Grt–Bt–And assemblages are observed inin the modal abundance of garnet, which may be relatedsome metapelites (Fig. 4) but Grt stabilization is due toto the model continuous equilibriumits high MnO content.

(2) An alternative model assumes that reactions (5) and(8)Grt + Ms = Als + Bt + Vap.(6) took place in the sillimanite stability field. In that

case, the replacement of andalusite by sillimanite must The same reaction could also explain the garnet pseudo-have taken place before the staurolite breakdown. Be- morphs observed in some andalusite porphyroblasts (Fig.cause the Gibbs energy difference between andalusite 9).and sillimanite is extremely small, the reaction

(7)And = Sil

Melting reactionsis kinetically sluggish. On the contrary, the dehydrationreactions (5) and (6) are associated with large Gibbs The migmatite zone is characterized by the widespreadenergy differences, which means that they are kinetically occurrence of sillimanite and K-feldspar instead of mus-

covite and the large amount of leucosome segregationseasier (see Pattison, 1992). Thus, sillimanite growth is

57


(up to 20–30 % in volume), which are thought to derive 1988; Powell & Holland, 1988) is suspect in the rocksfrom in situ partial melting. studied, because the CaO and MnO contents in garnet

The model reaction are very low and high, respectively. This fact by itselfaccords well with low-pressure conditions (e.g. Hebert &

(9)Ms + Ab + Qtz = Als + Kfs + Vap Ballevre, 1993). Owing to the low anorthite content ofplagioclase, calculated pressure values are erratic, exceptmay account for the development of Kfs + Sil but notin the sillimanite zone where values range from 2·4 toof liquid (L). The onset of partial melting is therefore2·7 kbar. Previous work in metapelites from the sillimaniteprobably due to one of the following three model re-zone gives P estimates of 4·9±0·5 kbar using the Hodgesactions:& Spear (1982) calibration of the garnet–plagioclase–Al2SiO5–quartz geobarometer (Gonzalez del(10)Ms + Kfs + Ab + Qtz + Vap = LTanago & Peinado, 1990). Pressure estimates within

(11)Ms + Ab + Qtz + Vap = L + Als garnet-bearing amphibolites from the staurolite–andalusite zone give values of the order of 4·3±0·5 kbar

(12)Ms + Qtz + Ab = Als + Kfs + L. (Gonzalez del Tanago & Arenas, 1991). These valuesare not significantly different from our own estimate ofThe coincidence of Kfs–Sil and migmatites provides4·0±0·5 kbar (see below).good evidence that dehydration-melting of muscovite

[reaction (12)] largely controlled partial melting. Furtherindications in favour of this model are the following. Thelack of Kfs in most metapelites before the migmatite Geothermometryzone indicates that production of partial melts through

The geothermometer based on FeMg exchange betweenreaction (10) is unlikely. Moreover, the large volume ofgarnet and biotite (Thompson, 1976; Ferry & Spear,leucosome suggests that fluid-present partial melting [i.e.1978; Williams & Grambling, 1990) has been applied toreactions (10) and (11)] is unlikely. Finally, the lackthe rocks studied. The main problem to be faced withof garnet within migmatitic metapelites indicates thatthis geothermometer relates to the fact that biotite istemperatures were not high enough to lead to de-partially altered to chlorite in most samples.hydration-melting of Ms + Bt + Qtz or Bt + Sil +

Using the less altered biotite compositions and garnetQtz.rims for the staurolite–garnet and staurolite–andalusiteSome of the rocks studied (e.g. sample AA-104) showzones, and garnet cores for the sillimanite zone, garnet–the four-phase assemblage Ms + Qtz + Kfs + Sil inbiotite geothermometry yields increasing temperaturesapparent textural equilibrium (Fig. 10b). This texturefrom the staurolite–garnet zone (500±50°C) to the sil-suggests that either the H2O activity was buffered by thelimanite zone (600±50°C) (Fig. 17). In individualfour-phase assemblage or, alternatively, infiltrating H2O samples, no systematic variation between the differentmaintained a high H2O activity.calibrations of the geothermometer is observed, exceptIn most rocks, especially those presenting evidence ofin sample AA-166, where the spessartine content ofpartial melting (e.g. sample AA-18), primary muscovitegarnet is higher than in the other samples and theis generally lacking, which suggests that fluid-absentcalibration of Williams & Grambling (1990) gives higherpartial melting took place by reaction (12). In these rocks,temperatures than the calibrations of Thompson (1976)fibrolitic sillimanite develops at the expense of biotiteand Ferry & Spear (1978). The trend in estimated tem-(Fig. 10a), and large muscovite porphyroblasts with in-peratures is generally in accordance with the sequenceclusions of sillimanite needles overgrow the foliation butof mineral assemblages tied to the petrogenetic grid (Figsare also slightly deformed (undulose extinction, kink-16 and 17).folds). These textures are consistent with an increase in

H2O activity at the end of the deformation history, whichcan be taken to result from the crystallization of nearbymelts or, alternatively, from fluid infiltration during cool- P–T HISTORYing from peak conditions.

The sequence of mineral assemblages in the studiedarea is characteristic of the andalusite–sillimanite type ofMiyashiro (1961), found at pressures below the aluminiumsilicate triple point. Several attempts have been made to

P–T CONDITIONS clarify relative pressure in such domains (e.g. Hietanen,Geobarometry 1967; Carmichael, 1978; Pattison & Tracy, 1991). On

the basis that (1) andalusite is found at lower grades thanThe use of the plagioclase–garnet–Al2SiO5–quartz geo-barometer (Newton & Haselton, 1981; Koziol & Newton, sillimanite, (2) staurolite breakdown probably occurred in

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Fig. 17. Diagram showing temperature estimates obtained with the garnet–biotite geothermometer. Three calibrations of this geothermometerhave been used: T: Thompson (1976); FS: Ferry & Spear (1978); WG: Williams & Grambling (1990). The progressive temperature increase

from the staurolite–garnet zone to the sillimanite zone should be noted.

the sillimanite stability field and (3) migmatite formation del Tanago, 1993), where it is usually overgrown byis due to muscovite breakdown, the Sierra Albarrana andalusite (Fig. 12). Second, kyanite is also reported inmetamorphism corresponds to facies series 2b of Pattison some cordierite-bearing schists from the higher-grade& Tracy (1991). As stated above, it is implicitly assumed rocks (Gonzalez del Tanago & Peinado, 1990). Accordingthat the succession of isograds in the Sierra Albarrana to Gonzalez del Tanago & Peinado (1990), kyanite occursarea records an isobaric section through the crust. The as relic crystals enclosed within biotite. The rock containsquestion that then arises is whether this pattern [‘the an assemblage consisting of quartz, plagioclase, K-feld-metamorphic field gradient’ of England & Richardson spar, cordierite and prismatic sillimanite. These ob-(1977)] results from a nearly isobaric heating–cooling servations clearly indicate that the aluminium silicatecycle (as in most contact aureoles), or from the pre- succession in the Sierra Albarrana area is first kyanite,servation of peak-temperature assemblages within rocks then andalusite and finally sillimanite.having undergone a clockwise or anticlockwise P–T loop. The relic character of kyanite in the Sierra AlbarranaThis question is an important one, because it has sig- area precludes the possibility of an anticlockwise P–Tnificant implications for the origin of the low-pressure path, similar to the one described in Mount Isametamorphism in the Sierra Albarrana area. One clue (Reinhardt, 1992). Another explanation for kyanite oc-is provided by the kyanite occurrences, which are dis- currence is that the rocks studied have been submittedcussed in detail below. to a clockwise P–T path. If this was the case, severe

constraints on the shape of the P–T path can be derivedfrom the observed assemblages. Specifically, the P–T

path has to enter the andalusite stability field before theSignificance of kyanitefirst aluminium silicate producing reaction in metapelites,Kyanite has been reported in some rocks from thei.e. reactions (4) and (5) (Fig. 16). This explanation isstaurolite–andalusite and sillimanite zones (Garrote,consistent with the occurrence of kyanite overprinted by1976; Gonzalez del Tanago & Peinado, 1990; Abadandalusite in veins, and with the absence of kyanite inOrtega, 1993; Gonzalez del Tanago, 1993). Despitemetapelites, which indicates that in these rocks the firstthe fact that kyanite is unusually common in similaraluminium silicate producing reaction took place in themetamorphic series [see review by Pattison & Tracyandalusite stability field. To sum up, we consider that the(1991)], its significance is not well understood.few kyanite occurrences reported in the Sierra AlbarranaTwo types of kyanite occurrences are known in thearea record the earliest part of the P–T history. They arearea studied. First, kyanite is found in veins in the

staurolite–andalusite zone (Abad Ortega, 1993; Gonzalez compatible with either a nearly isobaric heating–cooling

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be determined with any certainty. If we assume that a P

variation did actually occur, such variation must havebeen lower than the uncertainties and/or real dis-placement of the invariant points owing to minor com-ponents (i.e. only of the order of 0·5–1 kbar).

Given that the rocks studied were sediments depositedon the Earth’s surface and because the synmetamorphicdeformation is compressional, we suggest that the mostprobable P–T path involves a clockwise cycle with a veryslight increase in pressure during heating, followed bycooling at slightly decreasing pressures (Fig. 18). PeakP–T conditions therefore may define a metamorphic fieldgradient which is nearly isobaric or presents a very gentlepositive slope. The location of the aluminium silicate triplepoint (Kerrick, 1990; Bohlen et al., 1991; Hemingway et

al., 1991; Pattison, 1992; Holdaway & Mukhopadhyay,1993) and of the invariant point I3 roughly constrainpressures around 3·5±0·5 kbar in the staurolite–andalusite zone and 4±0·5 kbar in the migmatite zone,corresponding to a burial depth of the order of 10–12Fig. 18. Simplified P–T diagram for the Sierra Albarrana area. The

horizontal, dashed line shows the ‘metamorphic field gradient’. Two km.clockwise P–T paths are proposed for the staurolite–andalusite (St–And)and migmatite (Mig) zones. It should be noted that the P–T path ofthe staurolite–andalusite zone first enters the kyanite stability field, thenthe andalusite stability field, in accordance with the occurrence of

DISCUSSION AND CONCLUSIONkyanite relics within andalusite. The location of the aluminium silicatetriple point and the melting reactions are taken from Holdaway & Main constraintsMukhopadhyay (1993) and Thompson & Algor (1977), respectively.

Any model which seeks to account for the low-pressuremetamorphism in the Sierra Albarrana area should takeinto account the main conclusions of this study, whichare summarized below.cycle or a clockwise path characterized by a very slight

The metamorphism in the Sierra Albarrana area isincrease or decrease in pressure at increasing tem-synchronous with the principal deformation, which isperatures (Fig. 18).characterized by upright folds with subhorizontal axesassociated with a steeply dipping foliation and with asubhorizontal stretching lineation. The intensity of this

P–T path deformation increases towards the migmatitic gneisseslocated to the southwest of the Sierra Albarrana Quartz-Two isobaric paths can reproduce the sequence of mineral

assemblages observed. They are parallel and located on ites. This strongly deformed band is interpreted here as amajor ductile shear zone. Stratigraphic, palaeontologicaleach side of the invariant point I1′. The higher-pressure

path (Fig. 16) is in closer agreement with the data and geochronological data constrain the tectono-meta-morphic evolution of this area to be Variscan in age.available, provided that sillimanite growth would have

occurred owing to reaction (5) or (6). Determining The regional metamorphism is characterized by in-creasing grade towards the central part of the area, givingwhether P varies across metamorphic boundaries, how-

ever, presents greater difficulties. No definitive argument way to a roughly concentric pattern of isograds aroundthe Sierra Albarrana Quartzites. The shear zone mainlyin favour of a P variation across strike has been found.

In particular, observed assemblages within the migmatite corresponds to the southwestern part of the migmatitezone. To the southwest of the shear zone, metamorphiczone imply that pressure must have been lower than that

of the invariant point I3. In principle, the relative position grade progressively decreases through the sillimanite,staurolite–andalusite, staurolite–garnet and biotite zones.of invariant points I1 and I1′ on the one hand, and I2

and I3 on the other hand (Fig. 16), might indicate The highest metamorphic grade (migmatite zone) de-velops to the northeast of the shear zone in a broad bandwhether a P increase or decrease has occurred from

the staurolite–andalusite zone to the migmatite zone. centred around the Sierra Albarrana Quartzites. To thenortheast of the migmatite zone, metamorphic gradeHowever, because the exact P–T location of the invariant

points is dependent on the MnO content within garnet decreases through the sillimanite and staurolite–andalusite zones.(I1 and I1′) and H2O activity (I2 and I3), P variation cannot

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The metamorphism developed at relatively low pres- development of a transcurrent shear zone, which issures (~3·5–4 kbar) and temperatures ranging from synchronous with the peak P–T conditions; and a third~500°C in the staurolite–garnet zone to ~650°C in the stage accounting for the exhumation of the Sierra Al-migmatite zone. No pressure difference can be found barrana metamorphic rocks, during the sedimentation inacross the studied area, suggesting an almost isobaric the Valdeinfierno basin (Lower Carboniferous).section through the metamorphic terrane. Rare kyanite In trancurrent shear zones, the heat source for low-relics are interpreted as belonging to the earliest part of pressure metamorphism has been claimed to be shearthe prograde history. Therefore, no evidence for a heating and/or heat advection by channelized fluid flowmedium- or high-pressure metamorphism before the low- or synkinematic magmatic intrusions (e.g. Hanmer et al.,pressure event has been found. 1982; Leloup & Kienast, 1993). As regards the Sierra

Voluminous coarse-grained veins and pegmatites occur Albarrana area, the model proposed here is liable to twothroughout the area, and their mineralogy is consistent variants:with the metamorphic grade in the enclosing rocks, (1) The initial phase of thickening is associated withranging from quartz–andalusite–muscovite in the prograde, dehydration, reactions at depth. The largestaurolite–andalusite zone to quartz–orthoclase– amount of H2O-rich fluid resulting from these de-albite–tourmaline in the migmatite zone. hydration reactions could have been channelled during

the second phase along the shear zone, and this resultsin heat advection along the shear zone.

(2) One cannot preclude that the studied area is locatedPossible modelsjust above a synkinematic intrusion of granitoid com-Taking into consideration the above constraints, severalposition, and that the post-metamorphic exhumation hasmodels for explaining the low-pressure metamorphismnot been sufficient to unroof this magmatic body. Underin the studied area are possible. These models are nowthis hypothesis, the numerous pegmatite veins wouldbriefly discussed.provide evidence for the existence of a granitoid body atAs a first model, the low-pressure metamorphism coulddepth. The large amounts of fluid released during theirresult from a thermal perturbation related to a largecrystallization were probably channelized along the shearnumber of granitoid intrusions at mid- to upper-crustalzone.depths (e.g. Barton & Hanson, 1989). Unfortunately,

Further research needs to be carried out to determineplutonic rocks are relatively scarce or absent in the studiedthe possible contribution of both the fluid flow and thearea, i.e. south of the Badajoz–Cordoba shear zone (Fig.magmatic intrusion to the heat budget, but the Sierra1). Moreover, when present, granitoid intrusions (theAlbarrana area provides valuable insights into the de-Cardenchosa granite) postdate the low-pressure meta-velopment of low-pressure metamorphism in terranes notmorphism of the Sierra Albarrana area. Consequently,previously submitted to a higher-pressure event.this simple model is not appropriate for the studied area.

The second model for the development of low-pressuremetamorphism involves late-orogenic extension aftercrustal thickening (e.g. Reinhardt & Kleemann, 1994;Escuder Viruete et al., 1994). This model does not fitwell with the characteristics of the Sierra Albarrana area. ACKNOWLEDGEMENTSIn particular, because no medium- or high-pressure relics

We gratefully thank Marcel Bohn for technical assistancehave been found, the studied area should be located inwith microprobe work. Jean L. Sanders and Franciscothe upper plate, i.e. above a detachment zone. In thisGonzalvez Garcıa considerably improved the Englishcase, the principal foliation should be gently dipping,version of the manuscript. Michel Lautram drew somewhich is not observed in the Sierra Albarrana area.of the figures. Mario Sanchez Gomez is kindly thankedTherefore, this tectonic setting can be ruled out on thefor computer assistance in the preparation of some figures.basis of both structural and petrological data.Many thanks are due to D. R. M. Pattison and S. HarleyIn a third model, the low-pressure metamorphism of thefor their detailed and critical review of the manuscript.Sierra Albarrana area could be related to a transcurrentDiscussions with J. F. Simancas and F. Gonzalez Lodeirocrustal-scale shear zone. This is consistent with the strainwere very helpful in clarifying some aspects of the geo-pattern observed (steeply dipping foliation and sub-logical history of the area studied. A.A. received financialhorizontal stretching lineation). A more elaborated ver-support from the CICYT (Spain), Projects PB-90/sion of this model results from a combination of theC0860/C03/01 and PB 93/1149/C03/01. M.B. grate-following processes: an initial stage of moderate crustalfully acknowledges the hospitality of the Residence ‘Car-thickening, accounting for the burial of the Lower

Palaeozoic sediments; a second stage marked by the men de la Victoria’ at the University of Granada.

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Low-Pressure Metamorphism in the Sierra Albarrana Area (Variscan Belt, Iberian …hera.ugr.es/doi/15012025.pdf · 2005-03-18 · JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 PAGES 35–64