JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 PAGES 35–64 1997 Low-Pressure Metamorphism in the Sierra Albarrana Area (Variscan Belt, Iberian Massif ) A. AZOR1∗ AND M. BALLÈVRE2 1 DEPARTAMENTO DE GEODINÁMICA, UNIVERSIDAD DE GRANADA, FACULTAD DE CIENCIAS, CAMPUS FUENTENUEVA S/N, E-18002 GRANADA, SPAIN 2 LABORATOIRE DE PÉTROLOGIE–GÉOCHIMIE, GÉOSCIENCES RENNES (UPR–CNRS 4661), UNIVERSITÉ 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 migmatite zones occurs in the Sierra Albarrana area (Variscan Belt of southwestern Iberian Peninsula) in uppermost Precambrian to Lower Palaeozoic metasedimentary rocks. The principal deformation in this area is related to a major ductile shear zone whose central part is localized immediately to the southwest of the Sierra Albarrana Quartzites. The metamorphism is synchronous with respect to this deformation. The metamorphic zones are symmetrically distributed with respect to the Sierra Albarrana Quartzites. Pressure–temperature (P–T) conditions are ~3·5–4 kbar and range from ~400°C (biotite zone) to 500°C (staurolite–garnet zone) up to 650–700°C (migmatite zone). We have not detected pressure variations along the different metamorphic zones. Relic kyanite is observed in the form of inclusions in andalusite within veins in the lower-grade part of the staurolite–andalusite zone. The low-pressure metamorphism of the Sierra Albarrana area arises from a two-stage history including moderate crustal thickening followed by subsequent localization of deformation in a transcurrent shear zone during peak P–T conditions. Channelized fluid flow within the major ductile shear zone may have contributed to the heat budget of the low-pressure metamorphism. Regions of low-pressure and medium- to hightemperature metamorphism are widespread in many orogenic belts, including the Variscan Belt of Southwest Europe. De Yoreo et al. (1991) presented a review of some possible tectonic settings of low-pressure metamorphism. Those workers considered low-pressure metamorphism to be that in which peak temperature is above 500°C and pressure does not exceed that of the aluminium silicate (Als) triple point. The following four tectonic situations were envisaged for the low-pressure metamorphism (De Yoreo et al., 1991): (1) magmatic arcs; (2) regions of crustal extension; (3) continent–continent collision zones where thermal relaxation occurs after crustal thickening; (4) regions with significant fluxes of aqueous fluids, especially those located above subduction zones. This paper is concerned with the description of a hightemperature–low-pressure facies series terrane (the Sierra Albarrana area, Variscan Belt, southwestern Iberian Massif ) (Figs 1 and 2). In this area, a metamorphic zonation from the biotite zone up to the migmatite zone developed at low pressure (Garrote, 1976; González del Tanago & Peinado, 1990). As will be shown here, the metamorphic zonation is not related to post-thickening thermal relaxation but rather exhibits a roughly symmetric pattern around a major ductile shear zone. In addition, synmetamorphic magmatism is not recognized. For these reasons, the low-pressure metamorphism of the Sierra Albarrana area may provide a good example of moderate crustal thickening followed by significant heat advection by aqueous- or silicate-rich fluids along a major ductile shear zone. ∗Corresponding author. Telephone: (34) 58 24 29 00. Fax: (34) 58 24 33 52. e-mail: [email protected] Oxford University Press 1997 fluid flow; Iberian Massif; low-pressure metamorphism; shear zone; Sierra Albarrana area KEY WORDS: INTRODUCTION 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 Iberian Zones boundary in which the Sierra Albarrana area is located. metasedimentary rocks. The lithostratigraphic sequence can be divided into three groups of rocks separated by tectonic contacts (Azor, 1994; Azor et al., 1994). The first group consists of Upper Proterozoic to Lower Cambrian rocks, in which three formations can be distinguished. These are, from bottom to top (i.e. from southwest to northeast), as follows: (1) A volcanosedimentary succession of Upper Proterozoic age (Vendian), known as the Malcocinado Formation (Fricke, 1941; Delgado Quesada, 1971). (2) A Lower Cambrian succession of limestones and dolostones with slate intercalations, known as the Pedroche Formation (Liñán, 1978; Azor, 1994). (3) A Lower Palaeozoic (probably Lower to Middle Cambrian) monotonous succession of slates and metagreywackes with some quartzite intercalations, known as the Villares Formation (Liñán, 1978; Azor, 1994). The second group of rocks crops out in a band located to the northeast of the first group and separated from it by a late brittle fault known as the Onza Fault GEOLOGICAL SETTING OF THE SIERRA ALBARRANA AREA The Sierra Albarrana area is located in the southwestern part of the Iberian Massif, immediately to the southwest of a crustal-scale shear zone known as the Badajoz–Córdoba Shear Zone (Burg et al., 1981) (Fig. 1a and 1b). This ductile shear zone is the boundary between two major zones of the Iberian Massif: the Central Iberian Zone to the northeast and the Ossa–Morena Zone to the southwest. The Badajoz–Córdoba Shear Zone represents one of the sutures of the Variscan Belt (e.g. Burg et al., 1981; Matte, 1991; Azor, 1994; Azor et al., 1994). The Sierra Albarrana area is separated from the Badajoz–Córdoba Shear Zone by a subvertical semibrittle left-lateral fault known as the Azuaga Fault (Fig. 1b). Pre-Carboniferous rocks of the Sierra Albarrana area Lithostratigraphic sequence The area studied (Figs 1b and 2) is made up mainly of 36 AZOR AND BALLÈVRE 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, Malcocinado Formation; 2, Pedroche Formation; 3, Villares Formation; 4, Albariza Micaschists; 5, migmatitic gneisses with minor amphibolites; 6, Sierra Albarrana Quartzites; 7, migmatitic gneisses, micaschists and metagreywackes; 8, Lower Carboniferous undeformed sediments of the Valdeinfierno basin; 9, Badajoz–Córdoba 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). 37 JOURNAL OF PETROLOGY VOLUME 38 (Figs 1b and 2) (Azor, 1994). This second group, known as the Albariza Micaschists, is made up of a succession of schists and metasandstones with amphibolite intercalations, the age of which is probably Lower Palaeozoic (Azor, 1994). The third group of rocks crops out to the northeast of the second one (Fig. 2). The contact between these two groups of rocks coincides with a narrow band of intensely deformed rocks, interpreted as a ductile shear zone (Azor, 1994; Azor et al., 1994). Three formations can be distinguished in this group of rocks. They are, from bottom to top, as follows: (1) Migmatitic gneisses with minor amphibolite, metagreywacke and quartzite intercalations. (2) Feldspathic quartzites with intercalated paragneisses, schists, metagreywackes and amphibolites (Sierra Albarrana Quartzites, Delgado Quesada, 1971). These quartzites are of Lower Palaeozoic age according to palaeontological evidence (Marcos et al., 1991). The outcrop area of these quartzites is a small ridge known as the Sierra Albarrana. Way-up criteria at the southwest and northeast contacts of this succession indicate a stratigraphic top to the northeast. (3) Migmatitic gneisses and schists with minor quartzites and metagreywackes. NUMBER 1 JANUARY 1997 lineation and perpendicular to the foliation, which indicate a non-coaxial component for the principal deformation. The shear criteria recognized [see Simpson & Schmid (1983) and Hanmer & Passchier (1991) for a review] are S–C structures, asymmetric tails in feldspathic veins and rotation of the foliation in synkinematic garnet, andalusite and staurolite porphyroblasts. These criteria consistently indicate a dextral sense of movement (Azor, 1994; Azor et al., 1994). The age of the principal phase of deformation and metamorphism must be post-Lower Palaeozoic, as it affects rocks that are palaeontologically dated as Lower Palaeozoic (Marcos et al., 1991). 40Ar/39Ar radiometric dating (Dallmeyer & Quesada, 1992) indicates ages of 390 Ma for amphiboles and 360–350 Ma for muscovites. Dallmeyer & Quesada (1992) have suggested that these ages record a thermal rejuvenation during the Variscan orogeny of minerals that grew during a Late Precambrian (i.e. Cadomian) event. However, because the Palaeozoic age of some rocks of this area has been documented, and because only one tectonothermal event has been identified (see below), we interpret these ages as cooling ages after an Upper Devonian–Lower Carboniferous metamorphism (i.e. Variscan in age). A second phase of deformation can be recognized in the Sierra Albarrana area. It consists of folds that affect the above structures and locally generate a subvertical crenulation cleavage (Fig. 2). This phase is almost coaxial with the principal deformation phase and is responsible for the present steep dips of the principal foliation and for the folds affecting the southern end of the Sierra Albarrana Quartzites (Fig. 2). The latest Variscan deformation was an episode of brittle faulting that took place during the Carboniferous and generated the low-angle normal fault located to the northwest of Sierra Albarrana and several steeply dipping left-lateral faults (Fig. 2). The Onza Fault separating the Albariza Micaschists from the Villares Formation belongs to the latter set and, in addition to the left-lateral movement, displays a component of downthrowing of the southwestern block. Structure The pre-Carboniferous rocks of the Sierra Albarrana area are affected by a principal phase of penetrative deformation responsible for the development of tight kilometre-scale upright folds and a ductile shear zone located immediately to the southwest of the Sierra Albarrana Quartzites (Fig. 2). Southwest of the shear zone, the fabric developed is generally planar and constitutes the axial planar foliation of the folds recognized (Fig. 2). The ductile shear zone mainly coincides with the migmatitic gneisses located to the southwest of the Sierra Albarrana Quartzites. Within this zone, the fabric is strongly planar and linear. Northeast of the shear zone, the intensity of the planar–linear fabric progressively decreases and kilometre-scale folds related to this deformation are recognized in the Sierra Albarrana Quartzites (Fig. 2). In the whole area at issue here, the principal foliation is subvertical or steeply dipping in its present orientation and has a NW–SE strike. The stretching lineation is subhorizontal or gently plunging to the southeast or northwest, except in the southeastern end of the area studied, where plunges of 40–70° to the southeast are found (Fig. 3). Within the ductile shear zone and immediately to the southwest and northeast, different types of shear criteria can be observed in sections parallel to the stretching The Valdeinfierno basin Undeformed and unmetamorphosed sediments of Lower Carboniferous age [Upper Tournaisian to Lower Visean according to Wagner (1978), Garrote & Broutin (1979) and Roldán (1983)] rest unconformably on low-grade schists and slates immediately to the northwest of the Sierra Albarrana, in the Valdeinfierno basin (Figs 1b and 2). This basin is bounded by a synsedimentary normal fault, and the pebbles from the conglomeratic layers within the basin record the progressive exhumation of the metamorphic rocks of the Sierra Albarrana area 38 AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA Fig. 3. Structural map of the Sierra Albarrana area showing strike and plunge of the stretching lineation (arrows). The Sierra Albarrana Quartzites 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. (Roldán, 1983; Gabaldón et al., 1983; Gabaldón & Quesada, 1986; Azor, 1994). IDENTIFICATION OF METAMORPHIC ZONES In the area at issue here, metapelitic rocks constitute the major part of the stratigraphic sequence (see Geological Setting). We have been concerned with assemblages from metapelites alone and have examined about 300 thin sections. The distribution of low-variance assemblages as well as some higher-variance ones is shown in Fig. 4. The distribution of aluminium silicate polymorphs is shown in Fig. 5. The relationships between the different metamorphic phases and the deformation history are shown in Fig. 6. To constrain the P–T conditions in the different metamorphic zones, low-variance assemblages (i.e. those with three or more AFM phases) were studied with the electron microprobe. Table 1 shows the location of the 15 samples studied and their mineralogy. La Cardenchosa granite The area studied is bounded to the east by La Cardenchosa granite (Figs 1b and 2). The intrusion postdates the regional ductile deformation and metamorphism of the Sierra Albarrana area and is associated with a contact aureole (Garrote, 1976; Garrote & Sánchez Carretero, 1979). The age of La Cardenchosa granite with respect to the Valdeinfierno basin is not known yet, but some workers have suggested that the granite is younger (i.e. Namurian–Westphalian) than the Lower Carboniferous sediments (Delgado Quesada et al., 1985). In summary, the tectonothermal evolution of the area studied is two-fold. An intense ductile deformation associated with low-pressure metamorphism occurred during a major dextral shearing. The exhumation of the metamorphic rocks took place during the Lower Carboniferous, thus being synchronous with sedimentation within the Valdeinfierno basin. Biotite zone Pelitic rocks from the biotite zone are slates or fine-grained schists. These rocks generally show a slaty cleavage that 39 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 JANUARY 1997 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 cleavage overprints the principal foliation. The mineral parageneses of these rocks are, in addition to quartz and muscovite, Bt + Chl or Bt. Accessory minerals are ore minerals and tourmaline. Chloritoid was found by Garrote (1976) and Contreras et al. (1984), but its exact location is not known. foliation, which is generally linear and discontinuous but at high angles with respect to the external foliation (Fig. 7). Because the sense (clockwise) and the amount (about 20–30°) of rotation of the internal foliation is constant at the scale of the thin section, garnet growth appears to be synkinematic, although affected by late rotation. In the most altered samples, garnet is transformed along margins and fractures to chlorite, biotite and Fe-oxides. Staurolite crystals up to 1 cm in size contain quartz and ilmenite inclusions, defining an internal foliation. In most cases (e.g. sample AA-175), the internal foliation is rotated with respect to the external foliation, although both foliations are in continuity at the border of the porphyroblasts (Fig. 8a). These relationships indicate that staurolite porphyroblasts are rotated with respect to the matrix. Two types of biotite can be recognized: (1) elongated crystals which, together with muscovite, define the foliation, and (2) up to 1 mm porphyroblasts with the cleavage parallel or oblique to the principal foliation (e.g. samples AA-166 and AA-222). In the latter case, the shape of the porphyroblasts and the angle between the foliation and the {001} cleavage indicate a dextral shear sense (Fig. 7). The biotite porphyroblasts are altered to chlorite. If a late deformation is present, biotite porphyroblasts are kinked when their cleavage is oblique to the principal foliation. Some chlorite grains are parallel to the foliation, which can be taken to suggest that chlorite was in equilibrium with garnet, muscovite and biotite. Chlorite is also found Staurolite–garnet zone Pelitic rocks from the staurolite–garnet zone are mediumto fine-grained schists in which garnet, biotite and at times staurolite can be recognized in hand specimen. In the field, the principal foliation is a slaty cleavage or schistosity. The principal foliation is occasionally affected by a later crenulation cleavage and is generally the first penetrative deformation as the sedimentary layering can be recognized. The lowest-variance assemblages from this zone are Ms + Grt + St + Bt and Ms + Grt + Bt + Chl, but higher-variance assemblages such as Ms + Grt + Bt are also observed. Common accessory minerals are ilmenite, zircon and tourmaline. Chloritoid has also been reported in this zone (Garrote, 1976; Contreras et al., 1984). Garnet (0·1–2 mm) normally forms euhedral or subhedral inclusion-free crystals, although sometimes it has inclusions of quartz and minor ilmenite. Most garnet grains have pressure shadows. In some samples (e.g. sample AA-222), quartz inclusions define an internal 40 AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA Fig. 4. Distribution of AFM-mineral assemblages in the Sierra Albarrana area. Andalusite-in, sillimanite-in and migmatite-in isograds have been 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 garnet and biotite, or as porphyroblasts overgrowing the foliation (Fig. 8b). Staurolite forms euhedral to subhedral porphyroblasts with an internal foliation marked by quartz inclusions. Staurolite also includes euhedral ilmenite and garnet. In some cases (e.g. sample AA-171) the internal foliation is continuous with the external foliation (i.e. the principal foliation), and staurolite porphyroblasts are rotated with respect to the matrix. In other samples (e.g. samples AA85 and AA-223) the internal foliation is a crenulated cleavage, recording an earlier stage of development of the foliation now observed in the matrix. In these cases, transposed microfolds can be seen in the matrix, showing that the principal foliation corresponds to a crenulation cleavage. Andalusite appears as subhedral porphyroblasts with numerous quartz, muscovite, biotite and ilmenite inclusions sometimes defining an internal foliation. As in the staurolite porphyroblasts, the internal foliation is sometimes a crenulated cleavage (e.g. sample AA-233), which we interpret as having recorded the earliest stages of foliation development within the matrix. Euhedral garnet is occasionally included in andalusite, but staurolite inclusions are not observed. Staurolite–andalusite zone This zone is defined by the coexistence of staurolite and andalusite (Figs 4–6). Metapelitic rocks from this zone are fine- to medium-grained schists containing porphyroblasts of staurolite (up to 2 cm) and andalusite (up to 5 cm). Crystals of biotite, muscovite and garnet generally do not exceed 2 mm. The principal foliation in these schists is mainly a schistosity, although in some cases a crenulation cleavage is observed. Locally, a millimetre-spaced crenulation cleavage overprints the principal foliation. The most abundant assemblage is Ms + Grt + St + Bt + And. Some pelitic rocks, however, contain higher-variance assemblages such as Ms + Grt + Bt, Ms + Grt + St + Bt or Ms + Grt + Bt + And. Accessory minerals are ilmenite, zircon, tourmaline and apatite. 41 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 JANUARY 1997 Fig. 5. Distribution of aluminium silicate polymorphs in the Sierra Albarrana area. Andalusite-in, andalusite-out and sillimanite-in regional isograds 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 or a crenulation cleavage can be interpreted as evidence for polymetamorphism with two different generations of staurolite and andalusite porphyroblasts. Nevertheless, this is not the case, as this fact can be related to the existence of a non-coaxial component in the principal deformation as indicated by the development of a planar– linear fabric and shear criteria. In this regard, the superposition of different fabrics in a shear zone is fairly common owing to the rotation affecting all surfaces and lines included in it (Ramsay & Huber, 1987). If the angle with the boundaries of the shear zone is adequate, earliest developed fabrics can be folded during progressive deformation and the principal foliation finally observed would be a crenulation cleavage. In other cases, the fabric would not rotate, owing to its initial orientation with respect to the boundaries of the shear zone, and the final foliation observed would be a schistosity. Garnet and biotite present the same textural characteristics as in the staurolite–garnet zone. Muscovite crystals are parallel to the foliation. In some samples (e.g. sample AA-223), large muscovite grains develop around the andalusite porphyroblasts. Chlorite develops in the more altered samples at the expense of biotite and garnet. It is also present as porphyroblasts up to 1 mm in size that overgrow the principal foliation (e.g. samples AA-85 and AA-223). The growth of the chlorite porphyroblasts postdates the crystallization of the other phases as well as the principal deformation. Textural equilibrium between staurolite, andalusite, biotite, garnet, plagioclase and muscovite is observed. All these phases are synkinematic with respect to the principal foliation as deduced from pressure shadows and internal foliation–external foliation relationships in staurolite, andalusite and garnet porphyroblasts. Sillimanite zone The sillimanite zone is marked by the disappearance of staurolite (staurolite-out isograd), which seems to coincide on a regional scale with the incoming of sillimanite (sillimanite-in isograd) in most metapelites (Figs 4–6). 42 AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA Fig. 6. Sequence of synkinematic phases within metapelites, late to postkinematic phases in metapelites, and mineral assemblages observed within veins in the different metamorphic zones. Andalusite porphyroblasts are still present in a narrow band adjacent to the staurolite–andalusite zone (Fig. 5). The modal amount of garnet decreases upgrade from the sillimanite-in isograd. Metapelites from the sillimanite zone are fine- to medium-grained schists in which fibrolitic sillimanite and biotite are recognized in hand specimen. The principal foliation is a schistosity occasionally affected by a crenulation cleavage. The most abundant assemblages in this zone are Ms + Grt + Bt + And + Sil and Ms + Bt + Sil. The three-phase AFM assemblage Ms + Grt + Bt + Sil is occasionally present. Accessory minerals are tourmaline, apatite, zircon and ilmenite. Relic kyanite is observed as inclusions within biotite crystals in a rare cordierite-bearing layer (González del Tanago & Peinado, 1990). Andalusite porphyroblasts have characteristics similar to those of the staurolite–andalusite zone, and sometimes contain garnet pseudomorphs (Fig. 9). Prismatic sillimanite progressively develops either within fractures in the andalusite porphyroblasts (sample AA-141, Fig. 9) or as aggregates between andalusite grains (sample AA-139). Fibrolitic sillimanite is sometimes present in small clusters (sample AA-80). Garnet is less common than in the staurolite–andalusite zone and appears as euhedral to subhedral inclusion-free crystals of several microns to 1 mm in size. Reddish brown biotite and muscovite lamellae define the principal foliation. Some muscovite grains develop around andalusite (sample AA-141). Synkinematic phases with respect to the principal foliation are sillimanite, biotite, garnet, plagioclase and muscovite. 43 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 JANUARY 1997 which grow over the foliation, although sometimes they appear deformed. Fibrolitic sillimanite is present within all phases (Fig. 10a). Accessory minerals are tourmaline, apatite and zircon. The lack of ilmenite is also characteristic of this zone (Fig. 6). Sample AA-104 is slightly different from the other samples of this zone, because it shows a textural equilibrium between biotite, K-feldspar, prismatic sillimanite, muscovite, quartz and tourmaline (Fig. 10b): the grains of the different phases have approximately the same size and are in mutual contact. Some sillimanite grains have been replaced by fine-grained muscovite aggregates. DISTRIBUTION OF METAMORPHIC ZONES Fig. 7. Subhedral garnet with internal foliation marked by quartz inclusions (sample AA-222, staurolite–garnet zone). The internal foliation is rotated with respect to the principal foliation indicating a dextral sense of shearing (arrows); the angle between {001} cleavage in biotite porphyroblasts and the principal foliation also indicates a dextral sense of shearing. Garnet diameter is 1·28 mm. A metamorphic zonation (Figs 4–6) ranging from the biotite zone to the migmatite zone developed during the principal deformation in the Sierra Albarrana area. This zonation has been noted in previous studies (Laurent, 1974; Garrote, 1976; González del Tanago & Peinado, 1990; González del Tanago, 1993). According to these workers, the metamorphic zones are roughly symmetrical with respect to the Sierra Albarrana Quartzites and the metamorphic grade decreases towards the southwest as well as towards the northeast up to the chlorite zone. This simple pattern requires modification in the light of our data. Because late granite intrusion and brittle faulting have modified the distribution of metamorphic zones, it is only in the central part of the area studied where the initial distribution is preserved. We shall now turn to discussion of the relevance of the late modifications before considering the relationships between the distribution of metamorphic zones and the ductile deformations. Three kinds of late modifications are recognized: (1) Structural investigations to the southwest of the Sierra Albarrana reveal the existence of a left-lateral brittle fault with downthrowing of the southwestern block (the Onza Fault, Azor, 1994). This kinematics is consistent with the distribution of the metamorphic zones: the biotite zone is located to the south of the Onza Fault, in the downthrown block, whereas higher-grade zones (from staurolite–garnet up to the migmatite zones) can be observed to the north of the fault, i.e. in the upthrown block (Fig. 4). The southeasternmost part of the metamorphic domain is cut across by the Onza Fault, as shown by the occurrence of garnet-bearing micaschists south of the fault. These latter rocks can be interpreted as high-variance assemblages belonging to the staurolite– garnet zone, or as assemblages from a garnet zone, transitional between the biotite and staurolite–garnet zones. The garnet-bearing micaschists have not been identified in the southwestern part of the area studied probably because they are cut across by the Onza Fault. Migmatite zone Metapelites from the migmatite zone show a gneissic layering defined by millimetre-scale alternating micaceous and quartzofeldspathic domains. The quartzofeldspathic domains have a coarse-grained granitic mineralogy and form essentially continuous layers or small boudinaged pods. The micaceous domains are made up by biotite, fibrolitic sillimanite and at times minor muscovite. Fibrolitic sillimanite sometimes forms elongated whitish nodules up to 1·5 cm in size parallel to the stretching lineation. We interpret these textural characteristics as being due to partial melting. In this zone, most rocks are of quartzitic or quartzofeldspathic composition, thus making it difficult to map the boundaries of the zone, as partial melting affects only the metapelitic lithologies. To the southwest of the Sierra Albarrana Quartzites, the migmatite zone extends to the contact with the Albariza Micaschists. Migmatites are sometimes present within the Sierra Albarrana Quartzites, where they represent interbedded metapelites. Finally, migmatites are largely developed to the northeast of the Sierra Albarrana Quartzites, the zone boundary being located within the formation of migmatitic gneisses and schists. In most migmatitic metapelites (e.g. samples AA-18 and AA-20), complex relationships between coexisting phases can be observed. Large grains of K-feldspar contain euhedral inclusions of biotite and, more rarely, fibrolite. Matrix biotite is strongly corroded by fibrolitic sillimanite (Fig. 10a). Myrmekitic intergrowths are frequently present at the contact between plagioclase and K-feldspar. Muscovite grains constitute either small corroded grains or large (up to 1 cm in size) porphyroblasts 44 AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA Fig. 8. (a) Photomicrograph of euhedral staurolite and garnets from sample AA-175 (staurolite–garnet zone). Internal foliation in staurolite is marked by quartz inclusions and is continuous with the external foliation, which is defined by biotite and muscovite. Rotation of the internal foliation 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 chlorite porphyroblasts from sample AA-170 (staurolite–garnet zone). Chlorite porphyroblasts postdate the principal foliation, which is defined by biotite and 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 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 JANUARY 1997 Fig. 9. Garnet pseudomorph included in andalusite porphyroblast from sample AA-141 (sillimanite zone). Prismatic sillimanite appears along fractures 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 area is cut across towards the northwest by a low-angle brittle normal fault (Figs 1b and 4), which is responsible for the location of the Lower Carboniferous Valdeinfierno basin (Azor, 1994). The rocks in the hangingwall of the fault belong to the Villares Formation, corresponding to metamorphic conditions of the biotite zone. (3) The contact aureole associated with La Cardenchosa granite cuts across the regional metamorphic zonation (Garrote, 1976) (Fig. 4). The development of hornfelses largely overprints the regional metamorphic zonation. Assemblages belonging to the staurolite–garnet zone and lower-grade ones, if ever present, have been replaced by contact metamorphic assemblages. Consequently, no isograd associated with the regional metamorphism can be mapped near La Cardenchosa granite, and the northern boundary of the staurolite–andalusite zone probably corresponds to the external limit of the contact aureole (Fig. 4). The distribution of metamorphic zones is well preserved in the central part of the area studied, i.e. on both sides of the Sierra Albarrana. At the sample scale, mineral growth is synchronous with the principal foliation. This means that the Sierra Albarrana area does not constitute a thermal dome postdating the development of the principal foliation, as proposed by Quesada & Munhá (1990). At a regional scale, a close correlation can be observed between the direction and plunge of the stretching lineation (Fig. 3) and the shape of the isograds (Figs 4 and 5). When the stretching lineation is subhorizontal, the isograds run parallel to the lithologic boundaries and to the shear zone (e.g. the andalusite-in and sillimanite-in isograds to the southwest of the Sierra Albarrana). In addition, the sillimanite-in and migmatite isograds close where the plunge of the stretching lineation sharply increases. DISTRIBUTION AND MINERAL ASSEMBLAGES OF VEINS AND PEGMATITES Veins are especially abundant in the Sierra Albarrana area. The mineral content and the spatial distribution of these veins are summarized in Figs 6 and 11, respectively. These data are consistent with previous observations (Garrote et al., 1980; Ortega Huertas et al., 1982). Only quartz veins are present in the biotite and staurolite–garnet zones. The amount and size of these veins increase in the staurolite–andalusite zone, where they may also contain large crystals of muscovite and andalusite (up to 10 cm). In one locality (Era de la Charneca, Fig. 11), a large number of mineral assemblages can be observed. 46 AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA Fig. 10. (a) Photomicrograph of fibrolitic sillimanite related to biotite from sample AA-18 (migmatite zone). (Note sillimanite needles surrounding biotite and included in K-feldspar.) Plane-polarized light. Width of the field is 1·4 mm. (b) Photomicrograph of Ms–Kfs–Bt–Sil assemblage from sample 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, muscovite and andalusite, especially andalusite–quartz and andalusite–muscovite, as well as a minor amount of either garnet or staurolite. Interestingly, a few kyanite crystals 47 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 JANUARY 1997 Fig. 11. Distribution of coarse-grained veins and pegmatites. It should be noted that they are preferentially distributed in the higher-grade part of 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 AA415, Fig. 12). These observations are consistent with previous reports (Abad Ortega, 1993; González del Tanago, 1993) and show that kyanite is a relic phase. Veins containing quartz, muscovite and andalusite partly replaced by sillimanite are present in the lowergrade part of the sillimanite zone (e.g. sample AA276). In the higher-grade part of the sillimanite zone, aluminium silicate is not present in the veins, which essentially consist of coarse-grained quartz, plagioclase, muscovite and tourmaline crystals. Biotite is a minor phase in some veins. In the migmatite zone, pegmatite veins are extremely abundant and are essentially made up of quartz, albite, orthoclase and tourmaline. Graphic intergrowths between K-feldspar or plagioclase and quartz are frequent. In addition, some veins contain giant biotite and beryl crystals (up to 1 m). The mineralogy of the pegmatites has been dealt with in a number of recent studies (e.g. González del Tanago, 1991; Abad Ortega, 1993; Abad Ortega et al., 1993; Abad Ortega & Nieto, 1995a, 1995b). The volume and the orientation of the veins are dependent on both the metamorphic grade and the nature of the host rocks. Veins are larger in the highergrade zones (up to 500 m long and 70–100 m thick) compared with the lower-grade ones (up to 10–20 m long and 1 m thick). On the overall outcrop scale, they parallel the strike of the foliation when the enclosing rocks are slates or schists, even if their margins locally cut across the foliation. Veins elongated perpendicular or subperpendicular to the strike of the foliation are only observed within the Sierra Albarrana Quartzites or within the quartzites interlayered within the gneissic rocks located to the southwest of the Sierra Albarrana (e.g. Cerro de la Sal, Fig. 11). Nevertheless, many of the quartz and pegmatitic veins are deformed (i.e. foliated, folded and/ or boudinaged) whereas others are undeformed. This indicates that the veins are probably synchronous with the principal deformation and metamorphism. In summary, the volume of veins increases with the metamorphic grade and their mineralogical content is closely related to the paragenesis in the metapelitic host 48 AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA 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 subsolidus dehydration reactions or melting reactions, depending on the grade. Alternatively, at least the pegmatitic veins could be related to a granitic intrusion located below the higher-grade part of the metamorphic domain. These two possibilities will be considered in more detail in the final discussion on the origin of the metamorphism. mol %) contents. Microprobe analyses reveal two types of variations: (1) zoning patterns within individual grains and (2) systematic variations of the chemistry of garnet rims between different metamorphic zones. Zoning profiles are shown in Fig. 13. Most garnets from the staurolite–garnet and staurolite–andalusite zones are characterized by a decrease in MnO and an increase in FeO and MgO contents from core to rim. Garnets from the sillimanite zone are unzoned (Fig. 13, sample AA138) or show flat profiles with an increase in MnO and a decrease in MgO contents at the rims (Fig. 13, sample AA-80). This evolution of zoning profiles with increasing grade is similar to the one documented by Dempster (1985) in a medium-pressure metamorphic series, and is interpreted in a similar way [see also Tracy (1982), Loomis (1983) and Chakraborty & Ganguly (1990)]. Garnets from the staurolite–garnet and staurolite– andalusite zones preserve growth zoning. In the sillimanite zone, more efficient diffusion appears to have erased the growth zoning, and there is evidence for a slight down-temperature reequilibration of garnet rims. The spessartine content of cores decreases from the staurolite–garnet zone (~15 mol %) to the sillimanite zone (~8–10 mol %), but remains relatively high, which suggests that garnet is stabilized by MnO. The pyrope content increases from the staurolite–garnet zone (5 mol %) to the sillimanite zone (~10 mol %). Accordingly, the Mg/(Mg + Fe) ratio slightly increases from the staurolite–garnet (0·06–0·07) to the staurolite–andalusite MINERAL CHEMISTRY OF COEXISTING PHASES The chemical composition of the phases in selected samples was analysed with an electron microprobe (Microsonde Ouest, Brest, France) using the PAP correction. Analytical conditions were 15 kV accelerating voltage, 15 nA sample current and 6 s counting time. Standards were albite (Na), orthoclase (K), corundum (Al), wollastonite (Ca, Si), forsterite (Mg), ilmenite (Mn, Ti), Fe2O3 (Fe) and ZnS (Zn). Zn was detected with the La ray, and all others using the Ka ray. Garnet The garnets analysed (see Table 2 and Fig. 13) are essentially almandine (64–82 mol %)–spessartine (5–27 mol %)–pyrope (4–10 mol %) solid solutions with low to very low grossularite (0–6 mol %) and andradite (0–2 49 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 JANUARY 1997 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 TiO2 0·02 0·00 0·00 0·00 37·00 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 Total 0·00 0·00 0·00 0·00 0·02 100·64 100·64 100·90 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 Almandine 74·33 Spessartine 0·000 0·000 0·000 0·002 78·45 81·91 74·96 75·83 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 zone as in the staurolite–andalusite zone. ZnO contents are very low (0–0·4 and 0·1–1·1 wt %, respectively). No zoning has been observed in staurolite. The staurolite is slightly more magnesian [Mg/(Mg + Fe) ratio of ~0·15–0·17] than the coexisting garnet. Staurolite Biotite The structural formulae of staurolite have been calculated on a basis of 46 oxygens. Staurolite composition is very similar in all the samples studied (Table 3). MnO and Typical compositions of biotites in the different metamorphic zones are shown in Table 4. Most of the biotites 50 AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA 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 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 JANUARY 1997 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 27·26 SiO2 27·97 27·91 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 K 2O 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 0·17 0·15 0·17 Mg/(Mg + Fe) analysed, especially those from the staurolite–garnet zone, show low K2O contents, probably owing to a partial alteration to chlorite. Within the staurolite–andalusite zone, biotite inclusions in andalusite are altered to a lesser degree than matrix biotites, as shown by the higher K2O contents. Biotites from the staurolite–andalusite zone are slightly more magnesian than biotites from the sillimanite and migmatite zones (Table 4 and Fig. 14). Ti contents increase with metamorphic grade (Fig. 14) regardless of whether ilmenite is present or not. Chlorites in these three samples have the same composition. Muscovite Most of the samples studied contain primary muscovite, except some of the migmatite zone. No compositional difference can be established between the crystals parallel to the foliation, those included within andalusite, and those appearing as porphyroblasts. Their celadonite contents are consistently low or very low, as Si contents vary between 3·03 and 3·10 per formula unit (p.f.u.) (Table 4). In most samples, MgO and FeO contents range from 0·3 to 0·6 and from 0·7 to 1·5 wt %, respectively. Muscovites from sample AA-104 are richer in total FeO (2·22–2·74 mol %) and poorer in Al2O3, which indicates that most of the iron is ferric rather than ferrous. The Chlorite Chlorite is present in samples AA-85 (Sta–Grt zone) and AA-175 (Sta–And zone) as porphyroblasts cutting across the foliation (Fig. 8b) and in sample AA-166 (Sta–Grt zone) as an alteration product of biotite and garnet. 52 AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA 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 TiO2 1·43 2·44 2·08 3·73 2·96 0·26 0·00 0·00 0·94 52·74 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 K 2O 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 Structural formulae on a basis of 11 oxygens 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 0·43 0·39 0·40 0·40 0·41 Mg/(Mg + Fe) paragonite content of the muscovites ranges from 5 to 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. three samples (AA-18, AA-104 and AA-138) and its composition approximates Or90Ab10 (Table 5). Andalusite and sillimanite Minor components were not detected in andalusite and sillimanite. 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 AA137 and AA-175), but nearly pure albite is also observed (samples AA-53 and AA-223) (Table 5). It is not known whether these albitic compositions result from the late alteration of more calcic primary plagioclases, or represent compositions across the peristerite gap. Plagioclases from the sillimanite zone have anorthite contents equal to or higher than 18–20 mol %, except in sample AA-104. Primary K-feldspar was analysed in Oxides Primary oxides in the samples studied are ilmenites with MnO contents between 0·28 and 2·83 mol %. In the matrix, most of the ilmenites are altered to a TiO2-rich phase, most probably anatase as reported by Hébert & Ballèvre (1993) from staurolite-bearing micaschists in the Cadomian Belt of northern Brittany. Ilmenites included in andalusite, staurolite and garnet are normally not 53 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 JANUARY 1997 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 mineral assemblages from the Sierra Albarrana area is the location of some reactions with respect to the And–Sil equilibrium. No general agreement has been reached in depicting phase relations in low-pressure metapelites based strictly on thermodynamic databases. In addition, comparison with natural assemblages is plagued by departures from the model KFMASH system owing to the incorporation of non-KFMASH components, especially CaO and MnO within garnet and possibly also Fe2O3 and TiO2 into biotite. In this regard, a good example is shown by the common occurrence of the assemblage Grt– St–Bt–And–Ms–Qtz, where Grt is stabilized by the incorporation of MnO (see Giaramita & Day, 1991; Pattison & Tracy, 1991; Symmes & Ferry, 1992; Droop & Harte, 1995; Pitra & Guiraud, 1996). Despite these difficulties, the petrogenetic grid of Pattison & Tracy (1991) agrees well with the sequence of mineral reactions deduced from our own observations, and is used below. Figure 16 shows part of the KFMASH grid of Pattison & Tracy (1991), but some modifications have been made to take into account the additional constituent MnO. Assuming that MnO enters only into garnet, some invariant points and equilibrium curves are displaced with respect to their position in the KFMASH grid. In particular, the invariant point I1 will be displaced within the andalusite stability field (Fig. 16). A sequence of continuous and discontinuous model reactions can be reconstructed with increasing grade. Fig. 14. Ti content (p.f.u.) vs Mg/(Fe + Mg) diagram for biotites of the three higher-grade zones. The progressive increase in Ti content from the staurolite–andalusite zone to the migmatite zone should be noted. altered. In the samples belonging to the migmatite zone (AA-18, AA-20, AA-104), no ore minerals are present, probably because all the TiO2 in the rock was incorporated into biotite (Fig. 14). PHASE RELATIONS AND REACTION HISTORY The observed mineral assemblages, their relationships with respect to the deformation history and their measured chemical compositions are used to decipher the reaction history. Because the studied rocks invariably contain quartz and muscovite or K-feldspar, we will use the AFM projection (Thompson, 1957) to depict phase compatibilities (Fig. 15). A vapour (Vap) phase (or a melt phase in the case of the migmatite zone) is assumed to be in excess. Because MnO is an essential constituent for garnet (e.g. Giaramita & Day, 1991; Symmes & Ferry, 1992), it is taken into account in the AFM projection (Fig. 15). Phase relations in low-pressure metapelites have been investigated using essentially two methods. Some workers have calculated stable KFMASH reactions using experimentally based internally consistent thermodynamic datasets for the mineral end-members, combined with estimated solution models (e.g. Spear & Cheney, 1989; Powell & Holland, 1990; Dymoke & Sandiford, 1992; Xu et al., 1994). Other workers have constructed reaction grids based on repeated occurrences of natural mineral The entrance in the staurolite–garnet zone Because the transition between the biotite zone and the staurolite–garnet zone is obscured by the Onza Fault, the metamorphic reactions responsible for that transition cannot be established accurately. The appearance of garnet is most probably related to the continuous FeMnMg reaction Chl + Ms + Qtz = Grt + Bt + Vap. (1) The entrance of staurolite is related to the continuous FeMnMg reaction Grt + Chl + Ms + Qtz = St + Bt + Vap (2) or to the continuous FeMg reaction Chl + Ms + Qtz = St + Bt + Vap. 54 (3) AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA 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 Met. zone: St–And St–And Sil Sil Mig AA-18 Mig Mineral: Pl Pl Pl Kfs Pl Kfs SiO2 63·35 68·94 62·01 64·09 61·86 TiO2 0·02 0·05 0·00 0·03 0·00 64·37 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 Structural formulae on a basis of 8 oxygens 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 0·20 0·00 0·25 Ca/(Ca + Na) 0·26 The sillimanite-in isograd These reactions account for the disappearance of primary, synkinematic chlorite in the staurolite–garnet zone and the decrease in spessartine content within garnet at increasing grade. Staurolite breakdown (i.e. the entrance within the sillimanite zone) took place either by means of the model continuous reaction St + Ms + Qtz = Als + Bt + Vap (5) or the model discontinuous reaction The andalusite-in isograd St + Ms + Qtz = Grt + Bt + Als + Vap (6) The beginning of the staurolite–andalusite zone is defined by the appearance of andalusite. AFM-phase compatibilities (Fig. 15) suggest that the model KFMASH discontinuous reaction as suggested by the AFM topologies seen in the staurolite– andalusite and sillimanite zones (Fig. 15). At map scale, the staurolite-out isograd coincides with the sillimanitein one (Figs 4 and 5), whereas andalusite is still recognizable in the lower-grade part of the sillimanite zone. Two models are compatible with these observations: (1) The staurolite-breakdown reactions (5) and (6) could have occurred within the andalusite stability field, with the transformation of andalusite into sillimanite taking St + Chl + Ms + Qtz = Bt + And + Vap (4) was responsible for andalusite formation. Garnet may not have been involved in the andalusite-producing reaction, as suggested by its euhedral shape when included within andalusite. 55 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 JANUARY 1997 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 aluminium silicate, 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 AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA Fig. 16. Petrogenetic grid for a portion of the KFMASH system [slightly modified after Pattison & Tracy (1991)] showing the sequence of continuous (light lines bounding Fe-richer end-member reactions) and discontinuous reactions (heavy lines) with increasing grade. Equilibria between aluminium silicate polymorphs are also indicated with light dashed lines. Dashed arrow indicates the proposed P–T conditions for the staurolite–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, a Grt–Bt–And zone must be observed before the first appearance of sillimanite. This is not the case in our study. Indeed, Grt–Bt–And assemblages are observed in some metapelites (Fig. 4) but Grt stabilization is due to its high MnO content. (2) An alternative model assumes that reactions (5) and (6) took place in the sillimanite stability field. In that case, the replacement of andalusite by sillimanite must have taken place before the staurolite breakdown. Because the Gibbs energy difference between andalusite and sillimanite is extremely small, the reaction And = Sil most probably due to reaction (5) or (6) rather than to reaction (7), which would explain the coincidence of staurolite disappearance with sillimanite appearance. The sillimanite zone is also marked by a strong decrease in the modal abundance of garnet, which may be related to the model continuous equilibrium Grt + Ms = Als + Bt + Vap. (8) The same reaction could also explain the garnet pseudomorphs observed in some andalusite porphyroblasts (Fig. 9). (7) Melting reactions is kinetically sluggish. On the contrary, the dehydration reactions (5) and (6) are associated with large Gibbs energy differences, which means that they are kinetically easier (see Pattison, 1992). Thus, sillimanite growth is The migmatite zone is characterized by the widespread occurrence of sillimanite and K-feldspar instead of muscovite and the large amount of leucosome segregations 57 JOURNAL OF PETROLOGY VOLUME 38 (up to 20–30 % in volume), which are thought to derive from in situ partial melting. The model reaction Ms + Ab + Qtz = Als + Kfs + Vap may account for the development of Kfs + Sil but not of liquid (L). The onset of partial melting is therefore probably due to one of the following three model reactions: (10) Ms + Ab + Qtz + Vap = L + Als (11) Ms + Qtz + Ab = Als + Kfs + L. (12) JANUARY 1997 1988; Powell & Holland, 1988) is suspect in the rocks studied, because the CaO and MnO contents in garnet are very low and high, respectively. This fact by itself accords well with low-pressure conditions (e.g. Hébert & Ballèvre, 1993). Owing to the low anorthite content of plagioclase, calculated pressure values are erratic, except in the sillimanite zone where values range from 2·4 to 2·7 kbar. Previous work in metapelites from the sillimanite zone gives P estimates of 4·9±0·5 kbar using the Hodges & Spear (1982) calibration of the garnet– plagioclase–Al2SiO5–quartz geobarometer (González del Tanago & Peinado, 1990). Pressure estimates within garnet-bearing amphibolites from the staurolite– andalusite zone give values of the order of 4·3±0·5 kbar (González del Tanago & Arenas, 1991). These values are not significantly different from our own estimate of 4·0±0·5 kbar (see below). (9) Ms + Kfs + Ab + Qtz + Vap = L NUMBER 1 The coincidence of Kfs–Sil and migmatites provides good evidence that dehydration-melting of muscovite [reaction (12)] largely controlled partial melting. Further indications in favour of this model are the following. The lack of Kfs in most metapelites before the migmatite zone indicates that production of partial melts through reaction (10) is unlikely. Moreover, the large volume of leucosome suggests that fluid-present partial melting [i.e. reactions (10) and (11)] is unlikely. Finally, the lack of garnet within migmatitic metapelites indicates that temperatures were not high enough to lead to dehydration-melting of Ms + Bt + Qtz or Bt + Sil + Qtz. Some of the rocks studied (e.g. sample AA-104) show the four-phase assemblage Ms + Qtz + Kfs + Sil in apparent textural equilibrium (Fig. 10b). This texture suggests that either the H2O activity was buffered by the four-phase assemblage or, alternatively, infiltrating H2O maintained a high H2O activity. In most rocks, especially those presenting evidence of partial melting (e.g. sample AA-18), primary muscovite is generally lacking, which suggests that fluid-absent partial melting took place by reaction (12). In these rocks, fibrolitic sillimanite develops at the expense of biotite (Fig. 10a), and large muscovite porphyroblasts with inclusions of sillimanite needles overgrow the foliation but are also slightly deformed (undulose extinction, kinkfolds). These textures are consistent with an increase in H2O activity at the end of the deformation history, which can be taken to result from the crystallization of nearby melts or, alternatively, from fluid infiltration during cooling from peak conditions. Geothermometry The geothermometer based on FeMg exchange between garnet and biotite (Thompson, 1976; Ferry & Spear, 1978; Williams & Grambling, 1990) has been applied to the rocks studied. The main problem to be faced with this geothermometer relates to the fact that biotite is partially altered to chlorite in most samples. Using the less altered biotite compositions and garnet rims for the staurolite–garnet and staurolite–andalusite zones, and garnet cores for the sillimanite zone, garnet– biotite geothermometry yields increasing temperatures from the staurolite–garnet zone (500±50°C) to the sillimanite zone (600±50°C) (Fig. 17). In individual samples, no systematic variation between the different calibrations of the geothermometer is observed, except in sample AA-166, where the spessartine content of garnet is higher than in the other samples and the calibration of Williams & Grambling (1990) gives higher temperatures than the calibrations of Thompson (1976) and Ferry & Spear (1978). The trend in estimated temperatures is generally in accordance with the sequence of mineral assemblages tied to the petrogenetic grid (Figs 16 and 17). P–T HISTORY The sequence of mineral assemblages in the studied area is characteristic of the andalusite–sillimanite type of Miyashiro (1961), found at pressures below the aluminium silicate triple point. Several attempts have been made to clarify relative pressure in such domains (e.g. Hietanen, 1967; Carmichael, 1978; Pattison & Tracy, 1991). On the basis that (1) andalusite is found at lower grades than sillimanite, (2) staurolite breakdown probably occurred in P–T CONDITIONS Geobarometry The use of the plagioclase–garnet–Al2SiO5–quartz geobarometer (Newton & Haselton, 1981; Koziol & Newton, 58 AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA Fig. 17. Diagram showing temperature estimates obtained with the garnet–biotite geothermometer. Three calibrations of this geothermometer have 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 is due to muscovite breakdown, the Sierra Albarrana metamorphism corresponds to facies series 2b of Pattison & Tracy (1991). As stated above, it is implicitly assumed that the succession of isograds in the Sierra Albarrana area records an isobaric section through the crust. The question that then arises is whether this pattern [‘the metamorphic field gradient’ of England & Richardson (1977)] results from a nearly isobaric heating–cooling cycle (as in most contact aureoles), or from the preservation of peak-temperature assemblages within rocks having undergone a clockwise or anticlockwise P–T loop. This question is an important one, because it has significant implications for the origin of the low-pressure metamorphism in the Sierra Albarrana area. One clue is provided by the kyanite occurrences, which are discussed in detail below. del Tanago, 1993), where it is usually overgrown by andalusite (Fig. 12). Second, kyanite is also reported in some cordierite-bearing schists from the higher-grade rocks (González del Tanago & Peinado, 1990). According to González del Tanago & Peinado (1990), kyanite occurs as relic crystals enclosed within biotite. The rock contains an assemblage consisting of quartz, plagioclase, K-feldspar, cordierite and prismatic sillimanite. These observations clearly indicate that the aluminium silicate succession in the Sierra Albarrana area is first kyanite, then andalusite and finally sillimanite. The relic character of kyanite in the Sierra Albarrana area precludes the possibility of an anticlockwise P–T path, similar to the one described in Mount Isa (Reinhardt, 1992). Another explanation for kyanite occurrence is that the rocks studied have been submitted to a clockwise P–T path. If this was the case, severe constraints on the shape of the P–T path can be derived from the observed assemblages. Specifically, the P–T path has to enter the andalusite stability field before the first aluminium silicate producing reaction in metapelites, i.e. reactions (4) and (5) (Fig. 16). This explanation is consistent with the occurrence of kyanite overprinted by andalusite in veins, and with the absence of kyanite in metapelites, which indicates that in these rocks the first aluminium silicate producing reaction took place in the andalusite stability field. To sum up, we consider that the few kyanite occurrences reported in the Sierra Albarrana area record the earliest part of the P–T history. They are compatible with either a nearly isobaric heating–cooling Significance of kyanite Kyanite has been reported in some rocks from the staurolite–andalusite and sillimanite zones (Garrote, 1976; González del Tanago & Peinado, 1990; Abad Ortega, 1993; González del Tanago, 1993). Despite the fact that kyanite is unusually common in similar metamorphic series [see review by Pattison & Tracy (1991)], its significance is not well understood. Two types of kyanite occurrences are known in the area studied. First, kyanite is found in veins in the staurolite–andalusite zone (Abad Ortega, 1993; González 59 JOURNAL OF PETROLOGY VOLUME 38 NUMBER 1 JANUARY 1997 be determined with any certainty. If we assume that a P variation did actually occur, such variation must have been lower than the uncertainties and/or real displacement of the invariant points owing to minor components (i.e. only of the order of 0·5–1 kbar). Given that the rocks studied were sediments deposited on the Earth’s surface and because the synmetamorphic deformation is compressional, we suggest that the most probable P–T path involves a clockwise cycle with a very slight increase in pressure during heating, followed by cooling at slightly decreasing pressures (Fig. 18). Peak P–T conditions therefore may define a metamorphic field gradient which is nearly isobaric or presents a very gentle positive slope. The location of the aluminium silicate triple point (Kerrick, 1990; Bohlen et al., 1991; Hemingway et al., 1991; Pattison, 1992; Holdaway & Mukhopadhyay, 1993) and of the invariant point I3 roughly constrain pressures 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 10–12 km. Fig. 18. Simplified P–T diagram for the Sierra Albarrana area. The horizontal, dashed line shows the ‘metamorphic field gradient’. Two 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 of the staurolite–andalusite zone first enters the kyanite stability field, then the andalusite stability field, in accordance with the occurrence of kyanite relics within andalusite. The location of the aluminium silicate triple point and the melting reactions are taken from Holdaway & Mukhopadhyay (1993) and Thompson & Algor (1977), respectively. DISCUSSION AND CONCLUSION Main constraints Any model which seeks to account for the low-pressure metamorphism in the Sierra Albarrana area should take into account the main conclusions of this study, which are summarized below. The metamorphism in the Sierra Albarrana area is synchronous with the principal deformation, which is characterized by upright folds with subhorizontal axes associated with a steeply dipping foliation and with a subhorizontal stretching lineation. The intensity of this deformation increases towards the migmatitic gneisses located to the southwest of the Sierra Albarrana Quartzites. This strongly deformed band is interpreted here as a major ductile shear zone. Stratigraphic, palaeontological and geochronological data constrain the tectono-metamorphic evolution of this area to be Variscan in age. The regional metamorphism is characterized by increasing grade towards the central part of the area, giving way to a roughly concentric pattern of isograds around the Sierra Albarrana Quartzites. The shear zone mainly corresponds to the southwestern part of the migmatite zone. To the southwest of the shear zone, metamorphic grade progressively decreases through the sillimanite, staurolite–andalusite, staurolite–garnet and biotite zones. The highest metamorphic grade (migmatite zone) develops to the northeast of the shear zone in a broad band centred around the Sierra Albarrana Quartzites. To the northeast of the migmatite zone, metamorphic grade decreases through the sillimanite and staurolite– andalusite zones. cycle or a clockwise path characterized by a very slight increase or decrease in pressure at increasing temperatures (Fig. 18). P–T path Two isobaric paths can reproduce the sequence of mineral assemblages observed. They are parallel and located on each side of the invariant point I1′. The higher-pressure path (Fig. 16) is in closer agreement with the data available, provided that sillimanite growth would have occurred owing to reaction (5) or (6). Determining whether P varies across metamorphic boundaries, however, presents greater difficulties. No definitive argument in favour of a P variation across strike has been found. In particular, observed assemblages within the migmatite zone imply that pressure must have been lower than that of the invariant point I3. In principle, the relative position of invariant points I1 and I1′ on the one hand, and I2 and I3 on the other hand (Fig. 16), might indicate whether a P increase or decrease has occurred from the staurolite–andalusite zone to the migmatite zone. However, because the exact P–T location of the invariant points is dependent on the MnO content within garnet (I1 and I1′) and H2O activity (I2 and I3), P variation cannot 60 AZOR AND BALLÈVRE LP METAMORPHISM IN SIERRA ALBARRANA The metamorphism developed at relatively low pressures (~3·5–4 kbar) and temperatures ranging from ~500°C in the staurolite–garnet zone to ~650°C in the migmatite zone. No pressure difference can be found across the studied area, suggesting an almost isobaric section through the metamorphic terrane. Rare kyanite relics are interpreted as belonging to the earliest part of the prograde history. Therefore, no evidence for a medium- or high-pressure metamorphism before the lowpressure event has been found. Voluminous coarse-grained veins and pegmatites occur throughout the area, and their mineralogy is consistent with the metamorphic grade in the enclosing rocks, ranging from quartz–andalusite–muscovite in the staurolite–andalusite zone to quartz–orthoclase– albite–tourmaline in the migmatite zone. development of a transcurrent shear zone, which is synchronous with the peak P–T conditions; and a third stage accounting for the exhumation of the Sierra Albarrana metamorphic rocks, during the sedimentation in the Valdeinfierno basin (Lower Carboniferous). In trancurrent shear zones, the heat source for lowpressure metamorphism has been claimed to be shear heating and/or heat advection by channelized fluid flow or synkinematic magmatic intrusions (e.g. Hanmer et al., 1982; Leloup & Kiénast, 1993). As regards the Sierra Albarrana area, the model proposed here is liable to two variants: (1) The initial phase of thickening is associated with prograde, dehydration, reactions at depth. The large amount of H2O-rich fluid resulting from these dehydration reactions could have been channelled during the second phase along the shear zone, and this results in heat advection along the shear zone. (2) One cannot preclude that the studied area is located just above a synkinematic intrusion of granitoid composition, and that the post-metamorphic exhumation has not been sufficient to unroof this magmatic body. Under this hypothesis, the numerous pegmatite veins would provide evidence for the existence of a granitoid body at depth. The large amounts of fluid released during their crystallization were probably channelized along the shear zone. Further research needs to be carried out to determine the possible contribution of both the fluid flow and the magmatic intrusion to the heat budget, but the Sierra Albarrana area provides valuable insights into the development of low-pressure metamorphism in terranes not previously submitted to a higher-pressure event. Possible models Taking into consideration the above constraints, several models for explaining the low-pressure metamorphism in the studied area are possible. These models are now briefly discussed. As a first model, the low-pressure metamorphism could result from a thermal perturbation related to a large number of granitoid intrusions at mid- to upper-crustal depths (e.g. Barton & Hanson, 1989). Unfortunately, plutonic rocks are relatively scarce or absent in the studied area, i.e. south of the Badajoz–Córdoba shear zone (Fig. 1). Moreover, when present, granitoid intrusions (the Cardenchosa granite) postdate the low-pressure metamorphism of the Sierra Albarrana area. Consequently, this simple model is not appropriate for the studied area. The second model for the development of low-pressure metamorphism involves late-orogenic extension after crustal thickening (e.g. Reinhardt & Kleemann, 1994; Escuder Viruete et al., 1994). This model does not fit well with the characteristics of the Sierra Albarrana area. In particular, because no medium- or high-pressure relics have been found, the studied area should be located in the upper plate, i.e. above a detachment zone. In this case, the principal foliation should be gently dipping, which is not observed in the Sierra Albarrana area. Therefore, this tectonic setting can be ruled out on the basis of both structural and petrological data. In a third model, the low-pressure metamorphism of the Sierra Albarrana area could be related to a transcurrent crustal-scale shear zone. This is consistent with the strain pattern observed (steeply dipping foliation and subhorizontal stretching lineation). A more elaborated version of this model results from a combination of the following processes: an initial stage of moderate crustal thickening, accounting for the burial of the Lower Palaeozoic sediments; a second stage marked by the ACKNOWLEDGEMENTS We gratefully thank Marcel Bohn for technical assistance with microprobe work. Jean L. Sanders and Francisco Gonzálvez Garcı́a considerably improved the English version of the manuscript. Michel Lautram drew some of the figures. Mario Sánchez Gómez is kindly thanked for computer assistance in the preparation of some figures. Many thanks are due to D. R. M. Pattison and S. Harley for their detailed and critical review of the manuscript. Discussions with J. F. Simancas and F. González Lodeiro were very helpful in clarifying some aspects of the geological history of the area studied. A.A. received financial support from the CICYT (Spain), Projects PB-90/ C0860/C03/01 and PB 93/1149/C03/01. 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