Geochronological constraints on the evolution of a suture: the Ossa-Morena/Central Iberian contact (Variscan Belt, south-west Iberian Peninsula)

Geol Rundsch (1995) 84:375-383 © Springer-Verlag 1995

A. Azor • F. Bea • F. Gonzfilez Lodeiro J. F. Simancas

Geochronological constraints on the evolution of a suture: the Ossa-Morena/Central Iberian contact (Variscan Belt, south-west Iberian Peninsula)

Received: 3 May 1994 / Accepted: 30 December 1994

Abstract One of the main tectonic boundaries of the Variscan Belt in the Iberian Peninsula is the Ossa- Morena/Central Iberian contact. This contact is marked by a highly deformed unit (Central Unit) which re- corded an initial high-pressure/high-temperature metamorphic evolution. Rb-Sr whole-rock isotopic data from three gneissic bodies cropping out in the Central Unit yield two Late Proterozoic ages (690+ 134 and 632 + 103 Ma) and an early Palaeozoic age (495 + 13 Ma), which we interpret as protolith ages. The two Late Proterozoic orthogneisses show initial 87Sr/ 86Sr ratios typical of mantle-derived materials or those with significant mantle participation (87Sr/S6Sr> 0.709). These new radiometric data, together with ages previously published and the structural evolution of the Central Unit, lead to the conclusions that: (1) there are magmatic protoliths of Late Proterozoic and Early Pal- aeozoic ages; (2) the metamorphic evolution of this area, including the high-pressure event, belongs to the Variscan orogenic cycle; (3) the deformations observed affect the rocks of the entire Central Unit, accordingly they are post-Ordovician, i.e. Variscan; and (4) consequently, the Ossa-Morena/Central Iberian contact is interpreted here as a Variscan suture.

Key words IBERIAN Peninsula • Variscan suture • Rb-Sr whole-rock isotopic data • Protolith ages • Orthogneisses

introduction

The European Variscan Belt shows its most complete geotraverse in the Iberian Peninsula (Fig. la). Accord- ing to stratigraphic, metamorphic, magmatic and tectonic criteria, several zones have been distinguished in this geotraverse (Julivert et al. 1972; Farias et al. 1987). Recognition of noteworthy Late Precambrian magmatic activity and Late Precambrian unconformities in some areas of the belt indicates the existence of a Cad- omian orogenic cycle. The Importance of the Variscan overprint on the Cadomian structures and metamorphism has been the subject of much discussion in recent years. This controversy has basically focused on the Ossa-Morena Zone and its contact with the Central Iberian Zone.

This study aims to determine the age of orthogneisses that crop out within the Ossa-Morena/Central Iberian contact, as well as the age of the main deformation and metamorphism in this contact. For this pur- pose, we present here new Rb-Sr age determinations and discuss them together with previously published geochronological data. Our discussion is restricted to a highly deformed and metamorphosed unit, the so- called Central Unit in this study, that has been considered either as a Cadomian suture (Quesada 1991; Aba- los et al. 1991) or as a Variscan suture (Burg et al. 1981; Matte 1991; Azor et al. 1993, 1994).

Antonio Azor ([IN) • Fraticisco Gonzfilez Lodeiro J. Fernando Simancas Departamento de Geodin~imica, Universidad de Granada, Campus de Fuentenueva S/N, E-18002 Granada, Spain. Tel.: 34 58 27 28 83. Fax.: 34 58 24 33 52. E-mail: [email protected].

Fernando Bea Departamento de Mineralogfa y Petrologia, Universidad de Granada, Avenida de Fuentenueva S/N, E-18002 Granada, Spain

Geological setting of the Ossa-Morena/Central Iberian contact

The boundaries between the different zones defined in the Iberian Massif (Julivert et al. 1972) varies from suture-type contacts to palaeogeographical changes. The boundary between the South Portuguese and Ossa- Morena zones is a Variscan suture (Bard and Moine 1979; Munhfi et al. 1986; Crespo-Blanc and Orozco 1991). The Ossa-Morena/Central Iberian contact is a highly deformed and metamorphosed band, known as

376

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Fig. la Sketch of the Variscan Belt in the Iberian Peninsula showing the main zones. Oblique ruling indicates the Ossa-More- na/Central Iberian contact (Central Unit in this study). Rectangle shows the area mapped in Fig. lb. b Geological map of the Ossa- Morena/Central Iberian contact showing the location of the suites of samples studied in this work. ARG, suite of samples Arroyo Argalldn; CAR, suite of samples Cardenchosa; HIG, suite of samples Higuera de Llerena; and RIS, suite of samples Riscal

the Coimbra-C6rdoba Shear Zone (Burg et al. 1981). The existence of an initial eclogite-facies metamorphic event has led us to consider this contact as a suture. However, the controversy is focused on the age of this suture, namely, is it Cadomian or Variscan ?

In the Ossa-Morena/Central Iberian contact, we have been able to distinguish three different units that

registered contrasted tectonometamorphic evolutions (Azor et al. 1994). From north-east to south-west these units are (Fig. lb): the border of the Central Iberian Zone (Northern Unit); a central gneissic and pelitic band (Central Unit); and the border of the Ossa-More- na Zone (Southern Unit).

The Northern Unit has a Palaeozoic stratigraphy characteristic of the Central Iberian Zone, but the Up- per Precambrian lithologies are identical to those of the Ossa-Morena Zone. This unit was affected by intense deformation and evolved under low-temperature/intermediate- or low-pressure metamorphic conditions. It is separated from the Central Unit by a northeasterly dipping contact that corresponds to an important fault. This fault (the Matachel Fault), which is parallel to the principal foliation of the Central Unit, is the final stage

377

of a shearing affecting the whole Central Unit (Azor 1994; Azor et al. 1994). The Southern Unit consists mainly of sedimentary sequences which registered low- pressure metamorphism, associated with kilometric- scale folds and ductile shearing with top to the south- east sense of movement. This unit is separated from the Central Unit by a semi-brittle left-lateral fault, with downthrow of the north-eastern block (Azuaga Fault). The late Azuaga Fault obliterates the main contact between the Central Unit and the Southern Unit. This main contact may have been a right-lateral thrust be- fore the semi-brittle left-lateral movement took place (Azor 1994; Azor et al. 1994). The Central Unit, on which this study is focused, is described in detail her- eafter.

Lithology of the Central Unit

The Central Unit comprises aluminous schists with beds of metasandstone and quartzite, in which numer- ous orthogneisses and amphibolites, occasionally garnet-bearing, are intercalated. These various lithologies form a roughly monoclinal succession in which the top is located to the north-east.

Three main types of orthogneiss can be distinguished: 1. AmphiboIic orthogneisses These are medium- to

coarse-grained gneisses, sometimes with centimetric- scale feldspar phenocrysts, containing alkaline amphibole (arfvedsonite or riebeckite) and sometimes alkaline pyroxene (aegirine-augite). Their geochemical features are consistent with those of peralkaline granitoids (Ribeiro and Floor 1987, Azor 1994; Azor et al. in prep.). These orthogneisses crop out as NW- SE elongated bodies. One of them has been sampled for radiometric dating (suite CAR, Fig. lb). Other peralkaline bodies (Almendralejo and Aceuchal orthogneisses), located to the north-west of the mapped area in Fig. lb, have been previously investi- gated (Garcfa Casquero et al. 1985; Ochsner et al. 1992; Ochsner 1993).

2. Leucocratic orthogneisses These are medium- or coarse-grained gneisses with feldspar phenocrysts up to 1-2 cm; aplitic and pegmatitic facies are also found. The largest body is the Ribera del Fresno peraluminous orthogneiss (Chacdn et al. 1980), which crops out in a NW-SE trending band, 15-20 km 2 in extent (Fig. lb). Another very similar orthogneissic body is known as Las Minillas (Del- gado Quesada 1971) (Fig. lb). Both these bodies have been the object of previous radiometric investi- gation (Garcia Casquero et al. 1985; Ochsner et al. 1992; Ochsner 1993).

3. Biotitic augen gneisses They are coarse-grained gneisses, in which large feldspar phenocrysts up to 7-8 cm are found in a dark, biotite-rich matrix. These gneisses alternate with medium- to fine- grained orthogneisses and ultramylonitic gneisses.

The most important outcrop is a 200 m wide band extending from Higuera de Llerena to Arroyo del Riscal (Fig. lb). This body has also been sampled for this study (suites RIS and HIG, Fig. lb). The orthogneisses and amphibolites are more abun-

dant in the middle and lower parts of the sequence, whereas metapelites predominate in the upper part. These metasediments were named the Atalaya Forma- tion by previous workers and their depositional age is unknown, as no fossils have been found.

Tectonometamorphic evolution of the Central Unit

The structure of the Central Unit is the result of the superposition of three phases of deformation. The first phase is responsible for an L-S fabric that developed in a process dominated by heterogeneous shearing. The foliation has an average NW-SE strike and very variable dips due to the superposition of the two other phases of deformation. The stretching lineation has a NW-SE trend with a very gentle plunge to the north- west or south-east. Shear criteria indicate top to the north-west movement (left-lateral when foliation dips steeply). This main phase of deformation began in high- to intermediate-grade metamorphic conditions and ended in low-grade conditions. Biotite-garnet assemblages are found in the north-eastern part of this unit. Somewhat further to the south-west, biotite-garnet-staurolite and biotite-garnet-sillimanite-(kyanite) assemblages are found and, in the extreme south-west, biotite-garnet-sillimanite-(K-feldspar) and biotite-garnet-kyanite-(K-feldspar) assemblages are common. Pet- rographic analysis reveals that these parageneses are syn-kinematic with respect to the main foliation. These parageneses indicate pressures of 6-8 kbar and maxi- mum temperatures of 600-650 ° C (Azor et al. 1994).

In some garnet-bearing amphibolite bodies, eclogite assemblages retrograded to amphibolite or greenschist facies can be recognized. Eclogite parageneses consist of garnet, jadeite-rich clinopyroxene, rutile, zoisite and quartz. Most of the primary clinopyroxenes appear as symplectitic intergrowths of diopside and albite. Rutile shows progressive transformation to ilmenite and then titanite. Large green hornblende crystals surround garnet and diopside-albite intergrowths. Abalos et al. (1991) estimated temperatures of 685-700° C and pressures of over 15 kbar for the eclogite facies metamorphism, which pre-dates the main foliation. As there is no indication of high-pressure metamorphism in the Northern Unit, the contact between the Central Unit and the Northern Unit therefore presents a strong metamorphic gap in pressure and temperature. This fact, together with the geometry and kinematics of shear deformation in the Central Unit, which is contempora- neous with retrogression of the eclogite assemblages, indicates that this contact is the hanging wall of a syn- metamorphic extensional shear zone that has a strong strike-slip component (Azor 1994; Azor et al. 1994).

378

Table 1 Summary of metamorphism and protolith isotope ages available on the Central Unit

Rock(s) Age (Ma) Reference Method

Protolith ages Amphibolitized basic rock Cardenchosa orthogneiss Arroyo Argall6n orthogneiss Hig. Llerena and Riscal orthogneiss Almendralejo orthogneiss Almendralejo orthogneiss Almendralejo orthogneiss Aceuchal orthogneiss Aceuchal orthogneiss Ribera del Fresno orthogneiss Ribera del Fresno orthogneiss Ribera del Fresno orthogneiss Ribera del Fresno orthogneiss Minillas orthogneiss

Metamorphism ages Granulitie gneiss Calc-silicate gneiss Calc-silicate gneiss Several amphibolites Several gneissic rocks Several gneissic rocks Two gneissic rocks Several gneissic rocks

611 + 17/- 12 Sch/~fer (1990) U-Pb on zircons 690 + 134 This study Whole-rock Rb-Sr 632+103 This study Whole-rock Rb-Sr 495 + 13 This study Whole-rock Rb-Sr 475 + 9 Garcfa Casquero et al. (1985) Whole-rock Rb-Sr 474 + 9 / - 6 Ochsner (1993) U-Pb on zircons 471 + 16/- 10 Ochsner et al. (1992) U-Pb on zircons 477 + 5 / - 4 Ochsner (1993) U-Pb on zircons 474 + 7 / - 6 Ochsner et al. (1992) U-Pb on zircons 423+8 Garcia Casquero et al. (1985) Whole-rock Pb-Sr 556 + 159/- 65 Ochsner (1993) U-Pb on zircons 542 + 80/- 47 Ochsner et al. (1992) U-Pb on zircons 470-475 Schfifer (unpublished) SHRIMP on zircons 474 Schiller (unpublished) SHRIMP on zircons

427 + 45 418-+2 350 358-378 331-339 337-392 322 + 5 and 325 + 9 330-335

Schfifer (1990) Sch/~fer (1990) Gebauer (1993, pers. comm.) Ouesada and Dallmeyer (1994) Ouesada and Dallmeyer (1994) Garcfa Casquero et al. (1988) Garcfa Casquero et al. (1988) Blatrix and Burg (1981)

Sm-Nd on garnets Sm-Nd on garnets SHRIMP on zircons 4°Ar/39Ar on amphiboles 4°Ar/39Ar on muscovites Rb-Sr muscovite whole rock K-Ar on muscovites 4°Ar/39Ar on biotites

The second phase of deformation gave rise to folds with subhorizontal NW-SE trending axes and an axial plane surface dipping 30-45 ° to the south-west. Low- grade metamorphic conditions prevailed during this phase. The third phase of deformation caused open upright folds approximately coaxial with the two pre- ceding phases.

Previous geochronological data on the Central Unit

Until now, only a few radiometric studies have been under taken on the Central Unit. These data are sum- marized in the following (see Table 1).

Magmatic protoli th ages

The oldest protoli th age of 611+ 17/-12 Ma has been obtained on an amphibolite facies metabasite with the U-Pb method on zircons (Sch~ifer 1990; Schiller et al. 1991). Other protoli th ages have turned out to be Early Palaeozoic. Garc/a Casquero et al. (1985) have investi- gated two orthogneissic bodies from the Central Unit using the Rb-Sr method on whole rock. For the peralkaline Almendrale jo orthogneiss, they obtained an age of 475 + 9 Ma, whereas for the peraluminous Ribera del Fresno orthogneiss, they determined an age of 423 +38 Ma. The same two igneous bodies were later dated by U-Pb on zircons (Ochsner et al. 1992; Ochsner 1993). The Almendralejo orthogneiss gave very similar results (471 + 16/-10 Ma and 474+ 9/-6 Ma) to the Rb- Sr dating. On the contrary, the Ribera del Fresno or-

thogneiss revealed imprecise older ages (542+80/-47 and 556 + 159/-65 Ma), which may reflect either a mix- ture of two zircon populations (Precambrian zircon xe- nocrysts and zircons grown during the formation of the granitic protolith) or post-emplacement Pb loss that occurred during the early Variscan high-pressure metamorphism. Recently, U-Pb sensitive high resolution ion microprobe (SHRIMP) analysis of monozircons from Las Minillas and Ribera del Fresno orthogneisses has provided ages of c. 474 and 470-475 Ma, respectively (Gebauer personal communication, cited in Eguiluz et al. 1993; Schiller unpublished data, cited in Ochsner 1993). For the Aceuchal orthogneiss, Ochsner et al. (1992) and Ochsner (1993) have obtained two U-Pb zircon ages of 474 + 7/--6 and 477.1 + 5/-4 Ma, respectively. Similar ages were obtained in peralkaline intrusive bodies cropping out immediately to the north-east and south-west of the Central Unit (466 + 12 Ma, Priem et al. 1970; 482 + 16 Ma, Lancelot and Allegret 1982).

In summary, ages available on the igneous protoliths seem to indicate the existence of (1) some Late Prote- rozoic magmatic bodies and (2) more abundant peralkaline or peraluminous intrusives, essentially Ordovi- cian in age (Table 1).

Metamorphism ages

One characteristic feature of the Central Unit is the existence of early high-pressure and high-temperature metamorphism. This metamorphism produced firstly eclogitic mineral assemblages, preserved in the basic rocks of this unit, and then granulitic mineral assem-

379

blages, mainly preserved in the acid rocks in which the basic rocks are interbedded. The only radiometric dating of this high-pressure event was obtained on a granulitic gneiss (Sm-Nd method on garnets); the resulting age of 427 + 45 Ma is thought to be the age of the granulitic metamorphism (Schiller 1990; Schiller et al. 1991). This age agrees with the date of 418 + 2 obtained by the U-Pb method on zircons from a calc-silicate gneiss of the Central Unit (Schiller 1990; Schiller et al. 1991). However, it is probable that the aforementioned ages may be modified towards a slight rejuvenation (early Variscan), according to recent U-Pb SHRIMP datings of eclogites, carried out later by the same workers (Ge- bauer 1993, personal communication; Sch~ifer unpublished data, cited in Ochsner 1993).

Dates for the medium- and low-grade metamorphism show a large time span (Table 1). 4°Ar/39Ar radiometric ages of hornblende concentrates from Cen- tral Unit amphibolites are in the range 358-378 Ma (Quesada and Dallmeyer 1994). Rb-Sr muscovite/ whole-rock ages from gneissic rocks of this unit, which are believed to have the same closure temperature as the Ar system in hornblende (~500 °C), range from 392 to 337 Ma (Garcfa Casquero et al. 1988). A lower closure temperature (300-400 ° C) is given for the Ar system in muscovite and biotite and the K-Ar system in muscovite. 4°Ar/39Ar radiometric dating of muscovite concentrates from gneissic rocks has given ages of 331-339 Ma (Quesada and Dallmeyer 1994). 4°Ar/39Ar radiometric dating of biotite concentrates from gneissic rocks provided ages of 330-335 Ma (Blatrix and Burg 1981). K-Ar radiometric dating of muscovite concentrates from gneissic rocks provided ages of 322 + 5 and 325 + 9 Ma (Garc/a Casquero et al. 1988).

In summary, geochronological data available for the Central Unit indicate a Silurian or Early Devonian age for the high-pressure/high-temperature metamorphism and a Late Devonian to Early Carboniferous age for retrogression to medium- and low-grade metamorphic conditions.

Sample location and description

The location of the four suites of samples studied in the Central Unit is shown in Fig. lb.

The suite of samples CAR comes from a 20 m thick body that is continuous along a 2 km strike. In hand specimens it is a coarse-grained amphibolic gneiss with 1-2 cm feldspar porphyroclasts. Its mineral composi- tion is K-feldspar, plagioclase, quartz, arfvedsonite amphibole, titanite and epidote. In some samples (e.g. CAR-6), K-feldspar porphyroclasts are strongly stretched; in other samples (e.g. CAR-l), the mylonitic deformation has produced an important grain size re- duction, giving way macroscopically to a dark grey facies.

The suite of samples ARG was collected in a highly deformed sequence of coarse-grained gneisses, migma-

titic gneisses and ultramylonitic gneisses. In this suite, a certain variety of lithologies (principally leucocratic orthogneisses) is combined with a variable degree of deformation. This suite seems to belong to a volcano-sedimentary sequence. Minor basic rocks (eclogitic garnet- bearing amphibolites) are intercalated, probably repre- senting dykes; they have not been included in the analysed samples. In the orthogneissic rocks of this suite, large K-feldspar porphyroclastic crystals and polycrys- talline plagioclase aggregates appear surrounded by a mylonitic foliation marked by quartz ribbons, biotite, muscovite and feldspars. In some samples garnet is present.

The suite of samples HIG comes from the surroundings of Higuera de Llerena and is made up of biotitic augen gneisses, mylonitic and ultramylonitic gneisses. These three types of gneisses come from similar igneous protoliths, their differences being mainly due to the degree of deformation. Minor metasedimentary rocks also appear in this sequence; they have not been included in the suite of samples analysed. The mineral assemblage of these rocks consists of K-feldspar, plagioclase, quartz, red-brown biotite and garnet. The suite of samples RIS is made up of similar rocks as suite HIG. The biotitic augen gneisses present large K-feldspar porphyroclasts, plagioclase, quartz, garnet, red- brown biotite, muscovite and fibrolitic sillimanite. These two suites come from a continuous cartographic band (Fig. lb).

Analytical procedure

Large homogeneous samples (weight ~5-10 kg) were collected for this study. Samples were reduced to a grain size of less than 1 cm in a crusher with hardened steel jaws. Analytical powders were obtained by grind- ing about 50 g of each crushed sample in a tungsten car- bide jar until the grain size was less than 25 txm.

87Sr/86Sr ratios were determined by a VG sector 54 thermal ionization mass spectrometer at McMaster University, Hamilton, Canada, after sample dissolution and chromatographic separation of Sr using conven- tional methods. Long time stability measured through repeated analyses of the NBS-987 standard has better than + 0.004 rel.% at the two standard deviation level. 85Rb/SSSr ratios were directly determined on a PE SCIEX ELAN-5000 ICP mass Spectrometer after con- ventional sample dissolution. STRb/86Sr ratios were calculated through the expression: 87Rb/86Sr=SSRb/ SaSr x3.2321, assuming 87Rb/86Sr=0.38600 and 88Sr/ 86Sr=8.3732. Precision calculated on ten replicates of the same sample was better than + 1.2 rel.% at the two standard deviation level.

Fitting of isochrons was performed in two ways. For the suites of samples HIG and RIS, we used model 3 of York (1969) using the program from Ludwig (1992). For the suites of samples A R G and CAR, which have undergone strong metamorphic differentiation, fitting

380

Table 2 S7Sr/S6Sr and 87Rb/86Sr ratios of samples analysed in this work

87Sr/86Sr 87Rb/86Sr

Arroyo Argall6n orthogneiss ARG-1 0.711104 0.512 ARG-2 0.710603 0.652 ARG-3 0.708723 0.500 ARG-4 0.718955 1.576 ARG-5 0.721773 1.992 ARG-6 0.713432 0.830 ARG-7 0.70677 0.276 ARG-8 0.706681 0.170 ARG-11 0.709453 0.302

Cardenchosa orthogneiss CAR-1 0.703906 0.089 CAR-2 0.704464 0.151 CAR-3 0.704967 0.190 CAR-4 0.704343 0.111 CAR-5 0.704424 0.118 CAR-6 0.704439 0.136

Higuera de Llerena orthogneiss HIG-1 0.723163 1.481 HIG-2 0.73279 3.001 HIG-3 0.73597 3.572 HIG-4 0.73427 3.124 HIG-5 0.808831 13.894 m HIG-6 0.779281 9.541

Riscal orthogneiss RIS-1 0.744752 5.416 RIS-2 0.873151 23.008 RIS-3 0.783554 10.148 RIS-4 0.717934 1.083 RIS-6 0.715531 0.673 RIS-7 0.713807 0.257 RIS-8 0.713159 0.360 b RIS-9 0.716241 0.916 RIS-10 0.71329 0.337

of isochrons was performed using the local isotopic equilibrium or free-line model of Cameron et al. (1981), which was developed to interpret imperfectly fitted Rb-Sr isochrons from polymetamorphic terranes. According to these workers, this method is particularly suitable for the rational assessment of the primary metamorphic or protolithic age of a given suite, despite the scattered distribution on Sr evolution diagrams.

Results

The analytical results are given in Table 2 and Fig. 2. Samples of the suite CAR, in spite of their narrow

S7Rb/86Sr and 87Sr/86Sr ranges, yield an isotopic age of 690+ 134 Ma (MSWD =16.2), with initial S7Sr/86Sr of 0.7031 + 0.0012 (Fig. 2a). Such a low ratio, together with the A-type granite chemistry of the samples (Azor 1994; Azor et al. in prep.), indicates that they derive essentially from mantle sources.

The suite A R G shows a narrow range of S7Sr/a6Sr ratios and a wide scatter in the Rb-Sr plot (Fig. 2b), which, considering the heterogeneous migmatitic-ultra-

0.706

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mylonitic structure of the samples, is probably related to open-system behaviour during high-grade metamorphism. The whole-rock Rb-Sr isotopic data of this suite (Fig. 2b) fit at 632+ 103 Ma (MSWD =349), with an initial 87Sr/86Sr ratio of 0.7052-+ 0.0014. This initial aVSr/ 868r ratio is not as low as that obtained for suite CAR,

381

but still indicates a considerable proportion of mantle- derived materials, which is consistent with the geochemical features of the samples, close to those of I- type granites.

The suites HIG and RIS, which correspond to peraluminous S-type granites, belong to the same orthogneissic body (Fig. lb). For this reason, they have been plotted together in an 87Sr/86Sr-g7Rb/g6Sr diagram, ob- taining an age of 495+ 13 Ma (MSWD =431) (Fig. 2c), with an initial SVSr/86Sr of 0.7109, thus indicating a sub- stantial crustal component. This age is very similar to those obtained with separate plots for the two suites (489-+ 10 Ma for the Higuera de Llerena orthogneiss and 496 + 14 Ma for the Riscal orthogneiss).

The MSWD values obtained were in the three cases more than 2.5 and therefore 87Sr/a6Sr - 87Rb/86Sr plots may be considered as errorchrons (Brooks et al. 1972; Wendt and Carl 1991). This could be due to the intense deformation and metamorphic differentiation of the rocks studied, but the strong coherence of our data with previous determinations, as well as with geological and structural observations, give support to the obtained ages.

We can therefore conclude that the suites of samples studied define two groups of ages. The suites CAR and A R G give Late Proterozoic ages and a low initial 87Sr/ 86Sr ratio. The suites HIG and RIS, in contrast, give an Early Palaeozoic age and have a high initial 87Sr/S6Sr ratio.

Discussion

In the following discussion, we take into account previous geochronological data as well as those provided in this study.

According to the available data, there are igneous protoliths of two different age groups in the Central Unit. The oldest protoliths are Late Proterozoic: c. 611 Ma for an amphibolite (Sch~ifer 1990), c. 632 Ma for heterogeneous migmatitic-ultramylonitic gneisses (suite ARG) and c. 690 Ma for a peralkaline orthogneiss (suite CAR). We infer that these ages give evidence for the presence of some Late Proterozoic protoliths in the Central Unit. It is important to note the existence of Late Proterozoic protoliths of both basic and acid affinities and, within the acid group, the existence of peralkaline magmas is reported for the first time.

The second group of igneous bodies is of Early Pal- aeozoic age. Most radiometric dates give Ordovician ages: c. 475 Ma (Garcfa Casquero et al. 1985), c. 471 Ma and c. 474 Ma (Ochsner et al. 1992; Ochsner 1993), c. 489 Ma (suite HIG) and c. 496 Ma (suite RIS). These intrusive bodies are acid and peralkaline or peraluminous. Outside the Central Unit, but close to it, similar peralkaline orthogneiss bodies of the same age can be found: c. 466 Ma (Priem et al. 1970) and c. 482Ma (Lancelot and Allegret 1982).

The dominant peralkaline character of the intrusive bodies within and in the surroundings of the Central Unit, strongly suggests a geodynamic rift setting during the Early Palaeozoic. The stratigraphic evolution of the northern part of the Ossa-Morena Zone and the southern part of the Central Iberian Zone shows progressive break-up of a carbonate platform from the Early Cam- brian (Lifi~n and Quesada 1990). This evolution is in accordance with the geodynamic interpretation pro- posed here. Orthogneisses with the same characteristics and age are also found in the north-western Iberian Peninsula (Malpica-Tuy Unit), where they have been related to a rifting process during the Early Palaeozoic (Pin et al. 1992). A similar magmatic event related to Early Palaeozoic rifting is not only documented in the Iberian Massif, but also all along the Variscan Belt (e.g. Pin and Lancelot 1982; D6rr et al. 1992). The presence of a Late Proterozoic peralkaline body could likewise be interpreted as related to a rifting stage.

All of the orthogneisses from the Central Unit have undergone the same structural evolution, as they have been affected by the same tectonic events as the meta- pelitic host rocks. This fact is of crucial importance for establishing the age of the deformations in the Central Unit.

Radiometric data available in the Central Unit indicate a completely Variscan metamorphic evolution. The high-pressure/high-temperature event has been dated at c. 427 Ma, i.e. Silurian (Sch~ifer 1990; Schiller et al. 1991), but this high-pressure event could be slight- ly younger according to new U-Pb SHRIMP datings of eclogites (Gebauer 1993 personal communnication; Sch~ifer unpublished data, cited in Ochsner 1993). Con- sequently, the age of the early subduction stage should be Silurian or Early Devonian. Retrogression into medium- to low-grade metamorphic conditions occurred during the Late Devonian to Early Carboniferous: available cooling ages range from c. 390 to c. 320 Ma (Blatrix and Burg 1981; Garcia Casquero et al. 1988; Quesada and Dallmeyer 1994). The main deformation of the Central Unit, reflected as an L-S fabric penetrat- ing the entire unit, took place under high-temperature/ intermediate-pressure conditions, and was responsible for the exhumation of this unit (Azor 1994; Azor et al. 1994). Consequently, this exhumation took place between the Late Devonian and Early Carboniferous. Such a tectono-metamorphic evolution and its timing are classical in the innermost high-grade metamorphic zones of the Variscan Belt (e.g. Pin and Peucat 1986; Costa 1992).

Outside the Central Unit, in the whole Ossa-Morena Zone and the southern border of the Central Iberian Zone, Late Proterozoic unconformities and magmatism are found (Sfinchez Carretero et al. 1989). These data seem to demonstrate the existence of a Cadomian orogenic evolution. However, within the Central Unit where Late Proterozoic protoliths have been found, the Variscan evolution has completely obliterated all possible Cadomian structures and metamorphism. All ra-

382

diometric ages available on metamorphism are Varis- can and it was not possible to identify any relic pre- Variscan tectonic fabric.

Conclusions

In this study, we corroborate the existence of Late Pro- terozoic protoliths in the Ossa-Morena/Central Iberian contact (Central Unit). The presence of peralkaline orthogneisses in the Central Unit suggests a rifting epi- sode during the Late Proterozoic.

The Ordovician age of two other orthogneissic bodies is also established. These new data, in addition to previous radiometric datings, reassert the importance of igneous intrusions during the Early Palaeozoic. The peralkaline character of most of these intrusions lead us to interpret this magmatic activity as being related to a rifting process during the Early Palaeozoic.

Available radiometric ages on the metamorphism of the Central Unit range from Silurian (early high-pressure/high-temperature metamorphism) to Early Car- boniferous (subsequent medium- and low-grade metamorphic evolution), which means that the complete metamorphic evolution that can be recognized in the area studied is related to the Variscan Orogeny.

In the Central Unit, all penetrative deformations observed in the metasedimentary rocks also affect the Or- dovician intrusive bodies. For this reason, it is not possible to assign any of these deformations to a Cadomian orogenic cycle.

The tectonometamorphic evolution described here indicates that the Central Unit is an orogenic suture. Radiometric ages support the hypothesis that the Cen- tral Unit constitutes a Variscan suture in the Iberian Peninsula.

Acknowledgements We acknowledge Christine Laurin for im- proving our English manuscript. Comments and suggestions by Sylvie Costa have greatly improved the presentation and content of this work. Financial support by the CICYT (Spain) PB.90/ C0860/C03/01 Project.

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Geochronological constraints on the evolution of a suture: the Ossa-Morena/Central Iberian contact (Variscan Belt, south-west Iberian Peninsula)