39Ar mineral age constraints

Tectonophysics231(1994)195413

Tectonut~e~a~ evolution of the Ba~ajo~-~or~~b~ shear zone ( SW Iberia) : characteristics and 4oAq/ 39A_r mineral age constraints Cecilia Quesada a, RD. Dallmeyer b o I.T.G.E.,RiosRams23,2k%BII&&d &xaid bDepmtment ofGeokqp,University of Georgia,Athens, GA 3oM12, USA

(ReceivedDecember1,1991;revisedversion accepted May $1992)

Abstract

Field characteristics indicate that the Badajoz-Cordiiba shear zone has had a long, polyphase tectonothermal evolution. ‘%r/3gAr cooling ages recorded within central, ductile sectors of the Badajo~-~rd~ba shear zone range from ca. 370 to 360 Ma (~p~~le~ and 340 to 330 Ma (muscovite). These are interpreted to date ~st-metamo~hic cooling through contrasting temperatures required for intracrystalline retention of argon following a regionally significant Variscan tectonothermal overprint. These results combined with field relationships indicate that the 3adajoz-~rd~ba shear zone experienced at least 15 km of uplift relative to adjacent areas during late Paleozoic sinistral transpression. The data also indicate that individual structural units within the shear zone experienced contrasting uplift histories (both in total amount and rate). The 4oAr/3QArcooling ages indicate complete Variscan rejuvenation of all older intracrystalline argon systems within central sectors of the Badajoz-Cordriba shear zone,

1.Introduction The Central Iberian and Ossa-Morena zones comprise significant portions of the southern Iberian Massif (Fig. 1). The tectonothermal evolution of the intervening structural boundary has been ~ntr~ersia~. Robardet (1976) tentatively located the boundary along the Azuaga Fault which constitutes the southern limit of an extensive tract of myionitic rocks (termed the Badajoz-Cordiba Shear Zone by Laurent and Bladier, 1974). Robardet 0976) documented significant biofacies differences between Paleozoic units ex-

posed south (the Ossa-Morena zone) and north (the Central Iberian zone) of the mylonite zone. These were initiahy ~te~reted to reflect fauna1 provincialism, and used as evidence for significant early and middle Paleozoic palinspastic separation (Robardet, 1976; Paris and Robardet, 1977). This interpretation was generaily accepted by many subsequent workers who considered the intervening structural boundary (the BadajozCord6ba shear zone) to represent a “cryptic” late Paleozoic suture (e.g., Riieiro, 1981; Ribeiro et al., 1983; Matte, 1983, 1986; Juhvert and Matinez, 1987). Subsequent, ~st-~~~ision~ re-

~1951/94/SO7.~ ta 1994 Elsevier Science B.V. All rights reserved SSL?Z0040-1951(93)E0125-~

250 km.

AF -

AZUAGA

FAULT

Fig. 1. Zonal subdivision of the Iberian Massif (modified after Quesada, 1991). 2 = post-Paleozoic rocks (la = in Alpine thrust and fold belts); 2 = Precambrian and Paleozoic successions in Alpine fold belts or with unknown correlations with the Iberian autochthon; 3 = Precambrian and Paleozoic rocks in the Iberian autochthon; 4 = Precambrian and Paleozoic rocks in exotic Iberian terranes (basal ophiolite units in black); 5 = Hercynian Suture; E = Badajoz; C = Cordbba. Inset: previous zonal subdivision of Iberia (modified after Julivert et al., 1974). CZ = Cantabrian zone; WALZ = West Asturian-Leonese zone; CIZ = Central Iberian zone; 0iUZ = Ossa-Morena zone: SPZ = South Portuguese Zone; PB = Pedroches Batholith; BCSZ = Badajoz-Cord6ba shear zone.

AC

C. Quesada,R.D. Dallmeyer/ Tectonophysics231 (1994) 195-213

activation of the Badajoz-Cordoba shear zone within a late PaIeozoic intracontinentai sinistra1 wrench system was suggested by Burg et al. (1981). Recent field and coordinated geochronologic investigations have more clearly defined the tectonic stratigraphy and structura1 evolution within the Badajoz-Cordoba shear zone and adjacent sectors of the Ossa-Morena and CentraI Iberian zones. Results of these studies have questioned inte~retation of the Badajoz-Cordoba shear zones as a hte Puleozoic (Variscan) suture. Instead, the data impIy a compIex polyphase tectonothe~al evolution which included: (1) late Precambrian accretion of the Ossa-Morena zone to the Iberian Massif (Quesada, 1989; Abalos, 1989; Abalos and Eguiluz, 1990a,b; Ribeiro et al., 1990); (2) extensional reactivation in the early Paleozoic (Quesada, 1987; Apalategui et al., 1990; Ribeiro et al., 1990); and (3) late Paleozoic (Variscan) sinistral wrench reactivation (Laurent and Bladier, 1974; Burg et al., 1981; Matte, 1983, 1986; Apalategui et al., 1990; Ribeiro et al., 1990; Quesada, 1990a). Results of a program of 4oAr/3gAr mineral dating in the Badajoz-Cordoba shear zone are presented herein. These together with structural and stratigraphic relationships permit a more complete definition and characterization of the tectonothermal evolution of this regionally significant tectonic boundary than was previously possible. 2. Regional setting of the Bad@oz-Cord6ba shear zone The most prominent structural features within the Badajoz-Cordoba shear zone are a system of sinistral, strike-slip duplexes which developed at various scales during late-stage brittle activity (Fig. 2). These structures isolate tectonic units which dispIay distinctive and contrasting stratigraphic characteristics. Comparison of stratigraphic data from individual structura1 units indicates that five major and distinctive lithotectonic eiements comprise the Badajoz-Cordoba shear zone (Fig. 3). These inciude: (1) three contrasting and imbricated Precambrian successions which display a subhorizontal

197

mylonitic fabric developed parallel to internal thrust faults (Apalategui and Higueras, 1983; Quesada, 1990a); (2) lower Paleozoic (Cambrian through Silurian), rift-related, bimodal plutons with alkaline geochemical characteristics (Lancelot and Ailegret, 1982; SQnchez-Carretero et al., 1990); (3) lower Paleozoic platformal sedimentary successions (Cambrian through Devonian) which unconformably overlie previously deformed Precambrian units (Apalategui et al., 1989); (4) Lower and Upper ~rboniferous sedimentary successions which infilled marine and terrestrial pull-apart basins (Gabaldon et al., 1983, 1985; Gabaldon and Quesada, 1986); and (5) Carboniferous bimodal igneous complexes which in&de both shallow plutonic and volcanic rocks (Pascual and Perez Lorente, 1975; PCrezLorente, 1980; Pascual, 1981; Delgado et al., 1985 Sanchez-Carretero et al., 1990). The three contrasting Precambrian structural units within the Badajoz-Cordoba shear zone are exposed within a central belt of variably mylonitic rocks (Figs. 3 and 5). These are separated by subhorizontal tectonic contacts which are interpreted to be Precambrian ductile thrusts (Quesada, 1990a; Abalos et al., 1990b). The lower structural unit (the Sierra Albarrana Group, Quesada, 1990a) is comprised of platformal and flyschoid metasedimentary rocks. The intermediate-level Precambrian structural unit is represented by isolated exposures of variably retrogressed ultramafic rocks and amphibolite which have been interpreted to represent relics of a fragmented ophiolitic sequence (Quesada, 1989, 1990a). The upper Precambrian structural unit includes (Figs. 4 and 5): (1) a basal rift succession with local structural intercalations of basement (the Azuaga Gneiss Group, Quesada, 1990a,b); (2) a middle-upper Riphean metasedimentary unit (the Serie Negra Group) which is conformably overlain by a graywacke (volcaniclastic) succession; and (3) an unconformably overlying upper Riphean-Vendian volcanic-sedimentary succession of calcalkaline, arc-related rocks. 4oAr/39Ar results discussed herein relate to two of the Precambrian units exposed within the centra1 zone of myIonitic rocks.

Fig. 2. Simplified geological map of the Badajoz-Cordoba shear zone. 1= post-Paleozoic cover,. 2 = main belt of ductile mylonites (mostly Precambrian protoliths); 3 = Precambrian and lower Paleozoic rocks non-penetratively deformed by shearing (mostly brittle); 4 = Lower Carboniferous successions (partly affected by brittle shearing); 5 = Upper Carboniferous Penaroya-Belmez intermontane basin (locally affected by brittle shearing); 6 = Precambrian plutons; 7 = lower Paleozoic (rift-related) plutons; 8 = Carboniferous plutons (partly inftlling tension veins related to shearing); A-B = location of section shown on Pig. 6. Inset: structural sketch map of eastern segments of the Badajoz-Cord6ba shear zone, showing strike-slip duplex development.

* 1 Sample locality

20 km

199

C. Quesada, RD. DaUmeyer/ Tectonophysics 231 (1994) 195-213

AGE

I

5

u PP.

ii! a 0

Low

DEVONIAN SILURIAN ORDOVICIAN

CAMBRIAN

Fig. 3. Schematic illustration of major tectonostratigraphic units within the Badajoz-Cordoba shear zone. UPS, MPS and LPS are upper, middle and lower Precambrian units, respectively. Framed fields represent sedimentary gaps.

+

t

+

+ +

;+

+

+ + +

+ + + +

+ + •t -I

Fig. 4. Sigmoidal dike swarm exposed along northwestern extremities of the Villaviciosa-La Coronada Carboniferous igneous belt.

1 = metamorphic Precambrian successions; 2 = Carboniferous plutonic and subvolcanic rocks; 3 = porphyry dikes (see Fig. 2 for location).

200

C. Quesada, R.D. Dallmeyer / Tectonophysics 231 (1994) 195-213

CONTINENTAL VOLCANIC

BACK

- ARC

ALBARRANA

Fig. 5. Schematic illustration of tectonostratigraphic relationships between various Precambrian successions which occur within the Badajoz-Cordoba shear zone and adjacent areas (from Quesada, 1990a,b). US, MPS and UPS represent lower, middle and upper Precambrian successions, respectively.

3. Structural elements of the Badqjoz-Cord6ba shear zone Two genetically different sets of shear structures may be distinguished within the BadajozCordoba shear zone. These include an older set of sub-horizontal ductile mylonitic fabrics related

to Precambrian thrusts and a younger group of subvertical to steeply dipping, ductile and brittle structures developed during left-lateral shearing (Apalategui et al., 1990). Distinction of these two systems of shear structures is possible because some plutons were intruded across older, flat-lying structures but record the affects of sinistral shear-

Fig. 6. Diagrammatic cross-section across the main ductile mylonitic belt showing superposition of Precambrian successions (partly adapted after Apalategui and Higueras, 1983). I = lower succession (Sierra Albarrana Group); 2 = 600 Ma granites; S = intermediate (ophiolitic) succession; 4 = upper succession (Valencia de las Torres-Cerro Muriano Supergroup); 5 = Lower Carboniferous rocks, 6 = steeply dipping (Hercynian) mylonite zones (ornaments in the Precambrian units parallel to penetrative, old flat-lying fabric; see Fig. 2 for location).

C. Quesada, R.D. Dallmeyer / Tectonophysics 231 (1994) 195-213

ing. Crystallization ages (U-Pb zircon and Rb-Sr whole-rock) recorded by these plutons range between ca. 500 and ca. 440 Ma. (Lancelot and Allegret, 1982; Garcia-Casquero et al., 1985). 3.1. Sub-horizontal mylonitic structures A sub-horizontal mylonitic foliation developed parallel to structural boundaries separating the three contrasting Precambrian lithotectonic units (Figs. 3, 5 and 6; Quesada, 1989, 1990a,b; Quesada et al., 1991). A N-S-stretching lineation locally developed within the mylonitic foliation and indicates a development associated with thrusting. The precise sense of motion is uncertain because N- and S-directed kinematic indicators have been described (e.g., Abalos, 1989; Aba10s and Eguiluz, 1989; Quesada, 1990a,b). A lower limit for the age of this fabric is provided by ca. 600 Ma crystallization ages reported by Schafer et al. (1989) for peraluminous granitic plutons (e.g., Ribera de1 Fresno granite) which intrude the lower structural unit and display sub-horizontal mylonitic fabrics. Low-grade, Lower Cambrian sedimentary successions unconformably overlie Precambrian crystalline rocks in the BadajozCordoba shear zone (Apalategui et al., 1990) and adjacent sectors of the Ossa-Morena zone (Femandez et al., 1983; L&in, 1978). These require initial development of sub-horizontal mylonite fabrics prior to the Early Cambrian. Lithologic units correlative with the Serie Negra Group (uppermost Precambrian structural unit) are exposed north of the main belt of ductile mylonites in the Badajoz-Cordoba shear zone (the Obejo-Valsequillo domain, Fig. 2). These rocks do not appear to have been extensively overprinted by late Paleozoic (Variscan) thermal events because muscovite and hornblende within this area record 4oAr/39Ar plateau cooling ages of ca. 575-550 Ma (Dallmeyer and Quesada, 1992). These cooling ages confirm that a significant late Precambrian tectonothermal event affected at least southern margins of the Iberian autochthon. 3.2. Late Paleozoic structures Tectonothermal characteristics of the Badajoz-Cordoba shear zone are complex in detail

201

but generally constitute a sheared, flattened, and uplifted block relative to adjacent sectors of the Ossa-Morena and Central Iberian zones. Constituent structural “units” within the BadajozCordoba shear zone display contrasting deformational histories and are separated by intervening ductile shear zones or brittle strike-slip faults (Fig. 2). Together, all structural characteristics consistently reflect development within a regional sinistral shear regime. Brittle structures are represented by strike-slip and thrust faults. These define a strike-slip, duplex geometry for the entire shear zone. They steepen with depth to define an asymmetric “flower structure”. Northeastern margins of the shear zone are characterized by a sequence of thrust faults which comprise a belt with oblique (sinistral) components. This belt extends outward from the core of the shear zone and is characterized by a systematically decreasing strain and metamorphic gradient (Apalategui et al., 1990). The boundary of the Badajoz-Cordoba shear zone is located along the frontal thrust of the imbricated sequence (Fig. 2). Extensional structures developed in association with sinistral shearing at all scales within the Badajoz-Cordoba shear zone. Small-scale but widespread extensional structures include extensional crenulation cleavages (ductile) and “en echelon” veining (brittle). These usually occur at high angles (NE-SW) to shear surfaces. Other extensional structures include normal faults which have commonly been reactivated (inverted) as thrust faults. These include marginal faults which border the Upper Carboniferous (lower Westphalian) Pearroya-Belmez coal basin (Fig. 2). Additional evidence for a complex, polyphase structural evolution of the Badajoz-Cordoba shear zone is implied by development of late Paleozoic intermontane basins which probably formed above minor, negative flower structures within an overall regionally positive flower structure. Antithetic (NE-SW) “bookshelf’ extensional faults also occur in marginal areas of the shear zone. Variations in metamorphic grade within the Badajoz-Cordoba shear zone generally follow the regional structural patterns. The central ductile

202

C. Quesada, R. D. Dallmeyer / Tectonophysics 231 (1994) 195-213

zone is characterized by amphibolite-grade metamorphism with some upper level Precambrian units locally displaying relic, higher-grade assemblages (including granulite and eclogite). Distribution of the relict assemblages in comparison with the amphibolite-grade metamorphism recorded in the lower Paleozoic plutons (which cut subhorizontal ductile thrust faults) suggests that the eclogite and granulite assemblages must have developed prior to Precambrian imbrication. This is consistent with the overall character of the penetrative subhorizontal mylonitic foliation which developed during maintenance of amphibolite-grade metamorphic conditions. The present-day metamorphic geometry implies that no crustal levels below those characteristic of the amphibolite grade were exhumed during late Paleozoic transpressional uplift of the BadajozCorddba shear zone. Development of a related late Paleozoic subvertical mylonitic foliation was concomitant with lower amphibolite- to lower greenschist-grade retrogressive metamorphism. This appears to have followed a pressure-temperature path consistent with both cooling and decompression (Abalos, 1990). Metamorphic grade within Paleozoic and Precambrian rocks exposed north and south of the central ductile core of the Badajoz-Cordoba shear zone ranges from lowermost amphibolite to upper anchizone. Metamorphic grade systematically increases inward toward the central ductile core, and lower amphibolite-grade assemblages are only displayed by Precambrian units exposed in the flanking-zones. These are structurally overlain by units characterized by greenschist or anchizone metamorphism. Therefore, a Late Proterozoic tectonothermal event is suggested. This is consistent with ca. 575-550 Ma 4oAr/39Ar ages recorded by minerals within northern flanking zones (Blatrix and Burg, 1981; Dallmeyer and Quesada, 1992).

4. Previous geocbmuology within the BaddozCordi5ba shear zone Isotopic ages reported from the BadajozCordoba shear zone include a 620 Ma U-Pb age

for zircon from granulite (uppermost Precambrian structural sequence) and a 600 Ma U-Pb age for zircon from the Ribera de1 Fresno granitic gneiss of the lower structural unit (Schafer et al., 1989). Both ages were interpreted to reflect initial magmatic crystallization of zircon within protoliths. The 600 Ma age, therefore, provides a lower age limit for development of flat-lying mylonitic fabrics within the Badajoz-Cordoba shear zone. Geochronologic results (U-Pb zircon, Rb-Sr whole-rock, and K-Ar mineral) have also been reported for lower Paleozoic bimodal plutons exposed within the Badajoz-Cordoba shear zone and adjacent sectors of the Ossa-Morena zone (e.g., Priem et al., 1972; Bellon et al., 1979; DuPont et al., 1981; Lancelot and Allegret, 1982; Pereira and Macedo, 1983; Garcia-Casquero et al., 1985, 1988; Lancelot et al., 1985; Galindo et al., 1987; Serrano-Pinto et al., 1987). These range between ca. 500 + 10 and 440 f 10 Ma. Isotopic ages reported for granitic plutons within the Badajoz-Cordoba shear zone range between 482 + 16 Ma (U-Pb zircon, the Alter Pedroso pluton; Lancelot and Allegret, 19821, and 440 _t 6 Ma (Rb-Sr whole-rock isochron, the Portalegre pluton; Priem et al., 1972). Geochronologic results relating to the upper Paleozoic tectonothermal evolution of the Badajoz-Cordoba shear zone include: (1) model Rb-Sr muscovite ages of 355 and 337 Ma (errors not reported, Garcia-Casquero et al., 1988) for various mylonitic rocks; and (2) K-Ar biotite ages of 331 + 6 Ma and 340 f 6 Ma (Abranches et al., 1979; Andrade et al., 19831, for the Ribera de1 Fresno and Aceuchal plutons, respectively.

5. Analytical techniques The techniques used during 4oAr/39Ar analyses of the mineral concentrates generally followed those described in detail by Dallmeyer and Keppie (1987). Optically pure (> 99%) mineral concentrates were wrapped in aluminum-foil packets, encapsulated in sealed quartz vials and irradiated in either the U.S. Geological Survey TRIGA reactor (samples 2-8) or the H-5 position of the

C. Quesada, RD. Dallmeyer / Tectonophysics 231 (1994) 195-213

Ford Reactor at the University of Michigan (sample 1). Variations in the flux of neutrons along the length of the irradiation assembly were monitored with several mineral standards, including MMhb-1 (Samson and Alexander, 1987). The samples were incrementally heated until fusion in a double-vacuum, resistance heated furnace. Temperatures were monitored with a direct-contact thermocouple and are controlled to L-1°C between increments and are accurate to f5”C. Blank-corrected isotopic ratios were adjusted for the effects of mass discrimination and interfering isotopes produced during irradiation. Apparent 4oAr/39Ar ages were calculated from the corrected isotopic ratios using the decay constants and isotopic abundance ratios listed by Steiger and Jgger (1977). Intralaboratory uncertainties are reported, and have been calculated by statistical propagation of uncertainties associated with measurement of each isotopic ratio (at two standard deviations of the mean) through the age equation. Interlaboratory uncertainties are f 1.25-1.5% of the quoted age. Analysis of the MMhb-1 monitor indicates that apparent K/Ca ratios may be calculated through the relationship 0.505 ( f 0.003) x (39Ar/ 37k) corrected (Ford Reactor) or 0.518 ( f 0.005) X (39Ar/37Ar) corrected (TRIGA Reactor). Analyses of the amphibole concentrates have been plotted on 36Ar/40Ar vs. 39Ar/40Ar isotope correlation diagrams using the regression techniques of York (1969). Total-gas ages have been computed for each sample by appropriate weighting of the age and percent 39Ar released within each temperature increment. A “plateau” is herein considered to be defined if the ages recorded by four or more contiguous gas fractions with similar apparent K/Ca ratios, each representing > 4% of the total 39Ar evolved and characterized by generally similar apparent K/Ca ratios (and together constituting > 50% of the total quantity of 39Ar evolved) are mutually similar within a f 1% intralaboratory uncertainty. 6. Results Five hornblende and three muscovite concentrates were prepared from samples collected

203

within the mylonitic inner core of the BadajozCordoba shear zone. Sample localities are indicated in Fig. 2. Coordinates and petrographic descriptions are listed in the Appendix. The eight concentrates have been analyzed with 4oAr/39Ar incremental-release techniques. Analytical data are listed in Tables l-3 and are portrayed as apparent age spectra in Figs. 7 and 8. Apparent K/Ca ratios are relatively small and display considerable intrasample variations in the homblende analyses (Fig. 7). Apparent K/Ca ratios recorded by increments evolved from the muscovite concentrates are very large and show no significant or systematic variations throughout most of the muscovite analyses. The only exceptions are small-volume increments evolved from some concentrates at either the very lowest and/ or the very highest (fusion) experimental temperatures. These likely relate, at least in part, to gas released from trace amounts of optically undetectable mineral contaminants in the muscovite concentrates (including both relatively nonretentive and relatively refractory varieties). 6.1. Hornblende Five hornblende concentrates have been analyzed from variably foliated amphibolite (Aronches and Las Mesas amphibolite units) collected within Precambrian sequences within upper structural units exposed in the central core of the Badajoz-Cord6ba shear zone (the Azuaga Gneiss Complex). The concentrates are marked by considerable variation in the apparent ages recorded by gas fractions evolved at low and high experimental temperatures. These are matched by fluctuations in apparent K/Ca ratios suggesting that experimental evolution of argon occurred from compositionally distinct, and variably retentive, phases. These could be represented by: (1) very minor, optically undetectable mineralogical contaminants in the amphibole concentrates; (2) petrographically unresolvable exsolution or compositional zonation within constituent amphibole grains; (3) minor chloritic replacement of amphibole; and/ or (4) intracrystalline inclusions. In general, > 75% of the total 39Ar evolved from each concentrate was characterized by similar

204

C. Quesada, R.D. Dallmeyer / Tectonophysics

231 (1994) 195-213

Table 1 ?4r/3gAr analytical data for incremental heating experiments on hornblende Cordoba shear zone, southern Iberia Release temp PC)

*Ar/ 3gAr a

36Ar/3gAr a

37Ar/ 3gAr b

concentrates

from the Azuaga Gneiss, Badajoz-

39Ar % of total

%4OAl non-atmos. ’

36Aka (%o)

2.16 1.87 2.44 2.57 1.50 2.69 7.73 11.55 17.35 16.92 14.37 10.84 2.11 2.62 3.28 100.00

64.67 78.28 68.41 77.21 83.57 95.17 89.82 93.58 95.08 94.79 95.33 94.08 84.95 78.25 56.89 89.06

0.36 0.85 1.01 2.34 6.56 32.32 20.37 29.36 35.39 34.77 35.70 30.51 12.50 8.85 4.03 26.02

-

Apparent age (Ma) ’

Aronches Amphibolite

Sample 1: J = 0.006245 550 136.77 625 83.76 700 57.75 730 43.34 175 39.21 795 39.40 820 37.95 845 37.79 870 36.46 920 36.18 945 37.36 970 37.25 1000 44.83 1035 45.24 Fusion 52.25 Total 41.72

0.16397 0.06197 0.06224 0.03409 0.02319 0.00814 0.01625 0.01144 0.00920 0.00959 0.00899 0.01056 0.02596 0.03640 0.07928 0.02160

2.054 1.841 2.192 2.795 5.321 9.202 11.577 11.742 11.385

11.660 11.218 11.267 11.349 11.263 11.171 10.491

358.2 + 5.3

78.76

Total without 550-570, 1000, 1035°C and fusion

794.4 f 24.1 619.8 f 36.3 398.3 + 30.4 343.0 * 43.1 337.1 * 41.1 384.1 + 22.2 351.0 + 8.3 362.9 f 3.6 356.3 + 4.8 352.9 f 3.2 365.1 f 4.6 359.8 f 6.1 387.9 zt 11.3 363.1 f 16.1 309.6 f 30.2 368.3 f 12.8

Las Mesas Amphibolite

Sample 2: J = 0.008472 575 44.55 625 43.01 675 50.58 710 39.61 740 44.20 770 44.37 800 37.67 825 34.98 845 34.95 865 33.25 885 32.20 905 36.61 925 39.28 950 35.94 Fusion 179.59 Total 44.00

0.95733 0.04782 0.08629 0.05523 0.06531 0.06496 0.04609 0.03834 0.03795 0.03258 0.02752 0.03990 0.05160 0.03793 0.51651 0.06537

Total without 575-870°C and fusion

5.129 3.045 5.008 11.262 17.000 19.793 21.044 20.785 20.629 20.631 20.960 21.030 21.092 21.084 21.061 18.274

3.89 5.19 4.18 3.39 2.19 2.57 7.74 9.90 7.36 7.66 16.09 8.74 5.95 10.41 4.74 100.00 73.85

62.89 67.70 50.37 61.07 59.42 60.31 68.32 72.37 72.64 76.02 79.96 72.39 65.48 73.52 15.95 68.10

2.43 1.73 1.58 5.55 7.08 8.29 12.42 14.74 14.78 17.22 20.72 14.34 11.12 15.12 1.11 12.51

385.2+ 398.3 + 353.5 f 338.5 f 365.9 f 372.7 + 360.1 rt 354.7 + 355.5 f 354.2 + 360.2 + 369.8 + 369.9 f 368.8 rt 396.7 f 364.7 +

5.7 6.5 5.9 3.0 6.2 7.1 1.3 2.7 3.4 3.3 1.2 3.1 4.1 3.2 5.8 4.2

360.7 f

2.7

a Measured. b Corrected for post-irradiation decay of 37Ar (35.1 day l/Zlife). ’ [40Aq,,.- (36Ar,,,,X295.511/40Art,t, d Calculated using correction factors of Dalrymple et al. (1981) for the U.S. Geological Survey TRIGA Reactor or Harrison and Fitzgerald (1986) for the Ford Reactor at the University of Michigan: 2a intralaboratory errors.

205

C. Quesada, R.D. Dallmeyer / Tectonophysics 231 (1994) 195-213 Table 1 (continued) Release temu PC)

?4r/?u

a

Sample 4: J = 0.008515 72.33 515 52.74 625 54.38 675 725 38.06 38.49 750 34.17 775 800 30.53 28.88 825 860 28.21 28.68 880 28.85 900 32.63 920 945 37.43 35.97 975 Fusion 165.51 33.84 Total Total without 515-750,920-975°C Sample 6: J = 0.008000 575 132.64 625 136.87 675 65.49 725 35.20 775 31.96 800 33.14 820 29.83 840 29.13 860 30.02 880 29.56 900 45.64 Fusion 73.37 Total 33.92

36&,

39h

a

0.09346 0.06330 0.09358 0.04061 0.04869 0.03223 0.01960 0.01425 0.01057 0.00909 0.01088 0.02527 0.03719 0.03091 0.46558 0.02653

3lh,

39&

2.271 2.335 4.229 11.984 15.787 14.526 13.228 12.348 11.394 11.447 11.409 11.259 11.295 11.332 11.124 11.302

5.200 7.365 9.747 11.179 11.403 11.331 11.539 11.393 11.315 11.256 10.840 10.265 11.193

Total without 575-625, 900°C and fusion Sample 8: J = 0.008415 575 70.81 625 136.33 675 80.48 720 47.67 770 36.86 800 33.23 825 33.20 845 33.30 865 30.63 885 31.28 905 35.23 925 45.27 950 51.12 Fusion 493.12 Total 48.78 Total without 575-675,92.5-950°C

0.20251 0.33262 0.19233 0.07333 0.04238 0.03265 0.03124 0.03042 0.02183 0.01896 0.03506 0.07692 0.08816 1.55238 0.08175 and fusion

39Ar

%Yk

% of total

non-atmos. ’

(o/o)

Apparent age (Ma) d

1.78 3.47 2.23 3.31 2.66 4.19 8.21 13.31 25.08 16.27 10.78 2.81 1.72 3.02 1.18 100.00

62.06 64.88 49.76 70.98 65.90 75.53 84.49 88.83 92.15 93.82 92.01 79.81 73.05 77.12 17.41 84.81

0.66 1.00 1.23 8.03 8.82 12.26 18.36 23.56 29.32 34.25 28.51 12.12 8.26 9.97 0.65 22.52

584.7 461.8 375.0 376.1 356.0 361.3 360.9 358.9 363.1 370.6 370.1 363.9 380.0 385.1 398.4 374.2

36~ca

77.72

and fusion

0.22455 0.30610 0.13356 0.03109 0.01936 0.02254 0.01390 0.00794 0.00806 0.00763 0.06557 0.19892 0.02178

b

1.20 1.42 1.65 4.57 8.81 8.61 12.63 25.01 15.39 19.74 0.72 0.26 100.00

50.29 34.34 40.92 16.43 84.95 82.63 89.32 95.07 95.07 95.41 59.45 21.00 88.86

0.63 0.65 1.98 9.78 16.02 13.67 22.58 39.05 38.17 40.11 4.50 1.40 29.53

94.76

8.875 13.650 31.316 43.960 37.119 31.645 29.824 29.416 29.793 27.880 29.380 28.979 28.578 28.751 31.166

2.35 1.63 2.41 8.12 15.03 12.73 8.21 7.04 15.08 14.14 5.77 2.50 2.83 2.15 100.00 86.12

16.49 28.71 32.50 61.94 74.11 78.60 79.40 80.09 86.74 89.23 77.28 54.93 53.52 7.44 73.18

1.19 1.12 4.43 16.31 23.83 26.36 25.97 26.31 37.12 40.00 22.19 10.25 8.82 0.50 25.48

f f f f f f f f f f f f f f + f

8.6 5.1 12.3 12.6 13.2 5.1 6.1 2.3 1.8 1.9 2.3 8.6 12.3 13.6 13.9 4.8

365.6 f

2.5

773.5 578.1 352.4 353.9 356.9 359.7 358.9 363.5 370.5 369.4 356.6 211.0 370.6

f f f f f f + f f f f f f

10.0 12.6 9.4 2.7 3.4 1.6 1.8 0.9 1.8 0.8 20.6 38.4 2.8

363.2 +

1.8

170.0 517.4 365.5 410.6 381.4 365.0 367.7 371.6 370.3 386.7 378.5 378.6 380.0 493.4 371.2

f f f f f f f f f f f f + f f

378.4 f

24.1 20.1 18.6 5.2 2.3 3.1 2.9 4.1 3.5 2.9 4.3 9.3 15.2 63.4 6.5 3.7

206

C. Quesada, R.D. Dallmeyer / Tectonophysics

231 (1994) 195-213

Table 2 36Ar/40Ar VS. 39Ar/40.h isotope correlations using plateau analytical data from incremental-heating concentrates from the Azuaga Gneiss Complex, Badajoz-C&doba shear zone, southern Iberia Sample

Isotope correlation age (Ma) a

experiments on hornblende

?Ar/seAr intercept h

MSWD

39Ar % of total

Calculated 4?4r/39Ar plateau age (Ma) ’

298.6 + 14.3

1.66

78.76

358.2 f 5.3

316.3 294.2 276.6 329.9

0.53 1.76 2.11

73.85 77.72 94.76

360.7 + 2.7 365.6 f 2.5 363.2 f 1.8

1.05

86.12

378.4 k 3.7

Aronches Amphibolite 362.2 + 3.3

1

Las Mesas Amphibolite 354.1 363.2 365.6 361.1

2 4 6 8

f f f f

3.2 2.3 3.1 4.2

rt f f f

10.9 8.6 12.9 8.7

a Calculated using the inverse abscissa intercept (4”Ar/39Ar ratio) in the age equation. b Inverse ordinate intercept. ’ Table 1.

0 3,

400.

1

,ooe

r-

3632kl6

365.6 t 2 5

4oc

- 330

330-

6

4 300,

A300

0

20

40

CUMULATIVE

so

% 3sAr

eo

100

RELEASED

Fig. 7. 40Ar/39Ar apparent age and apparent K/Ca spectra for hornblende concentrates from the Badajoz-Cordoba shear zone, southern Iberia. 20, intralaboratory uncertainties indicated by vertical width of bars. Experimental temperatures increase from left to right. Total-gas and plateau ages listed on each spectrum (plateau increments delineated with arrows).

C. Quesada,RD. Dallmeyer/ Tectonophysics231 (1994) 195-213 4SO

3 400-

1

331.2*1.3

300

u

400

-

I

334.8fl.O

3oou

1 1

Fig. 8. %r/3gAr apparent age spectra of muscovite cxmcentrates from the Badajoz-Cord6ba shear zone, southern Iberia. Data plotted as in Fig. 7.

207

6.2. Muscovite Two muscovite concentrates (5 and 71 were prepared from variably mylonitic quartzite within the Azuaga Gneiss Complex (the upper Precambrian structural unit). Another (3) was separated from mylonitic Ribera de1 Fresno granite intrusive into the lower structural unit. The three concentrates display nearly concordant 4oAr/39Ar age spectra (Fig. 8) which define plateau ages which range between 331.2 f 1.3 (3) and 339.4 f 1.7 Ma (7). These are interpreted to date the last cooling through temperatures required for intracrystalline retention of argon. Although not fully calibrated experimentally, use of the preliminary data of Robbins (1972) in the diffusion equations of Dodson (1973) indicates muscovite closure temperatures of ca. 375-400°C.

7. Geologic significance apparent K/ Ca ratios, indicating experimental evolution of gas at intermediate and high temperatures occurred from compositionally uniform populations of intracrystalline “sites”. Most of these gas fractions record mutually similar, intrasample apparent “sAr/39Ar ages which correspond to plateaux defining ages ranging between 358.2 f 5.3 (1) and 378.4 f 3.7 Ma (8). The plateau data generally correspond to well-defined %/‘%r vs. 39Ar/40Ar isotope correlations (MSWD < 2.5, Table 2) which yield ages ranging between 354.1 f 3.2 (2) and 365.5 f 3.1 Ma (6). Inverse ordinate intercepts are not greatly different from the 4oAr/36Ar ratio in the present-day atmosphere, and, therefore, do not suggest significant intracrystalline contamination with extraneous argon components. The isotope correlation ages are interpreted to date the last cooling through those temperatures required for intracrystalline retention of argon within constituent hornblende grains. Harrison (1981) indicated that closure temperatures for argon systems within igneous hornblende are not significantly affected by compositional variations. He suggested that values of 500 f 25°C are appropriate for the range of cooling rates likely to be encountered in most geological settings.

All of the analysed samples were collected within the central ductile belt of the BadajozCordoba shear zone. Hornblende in these sequences record ca. 365-355 Ma isotope-correlation ages which indicate that temperatures in excess of ca. 500°C must have been maintained until the latest Devonian or earliest Carboniferous (Toumaisian-DNAG time-scale calibration; Palmer, 1983). Thus, the 4oAr/39Ar results do not allow discrimination between Precambrian and Paleozoic thermal events. The results indicate late Paleozoic transpressional uplift of amphibolite-grade sectors of the Iberian crust in the central mylonite belt. Less significant uplift of initially shallower crustal levels (< ca. 4OO“C) appears to have occurred within marginal structural belts where Precambrian mineral ages are still recorded. Muscovite plateau ages suggest that cooling through ca. 400°C was accomplished by ca. 340330 Ma (Visean). An interval of ca. 20-30 m.y. is thus inferred for the overall uplift and cooling of the central belt (between the retention of argon in hornblende and in muscovite). Assuming a 3O”C/km geothermal gradient (suggested by the Barrovian metamorphic characteristics of these

208

C. Quesada, R.D. Dallmeyer/

Tecronophysics

Table 3 4oAr/39Ar analytical data for incremental heating experiments on muscovite Fresno Granite, Badajoz-C&doba shear zone, southern Iberia Release temp PC)

4oAr/39Ar

a

Ribera de1 Fresno Granite Sample 3: J = 0.008591 575 24.11 610 25.09 670 24.16 700 23.89 750 23.71 800 23.69 850 23.68 Fusion 24.03 Total 23.86 Total without

0.01499 0.00304 0.00098 0.00132 0.00105 0.00099 0.00095 0.00252 0.00139

Sample 550 580 600 625 650 675 710 750 790 820 850 880 Fusion Total

a Measured. b Corrected

0.012 0.006 0.003 0.001 0.001 0.001 0.002 0.004 0.003

0.037 0.002 0.001 0.003 0.001 0.001 0.003 0.000 0.003 0.001

0.05095 0.01580 0.00665 0.00221 0.00165 0.00132 0.00156 0.00161 0.00170 0.00175 0.00179 0.00179 0.00967 0.00237

1.589 0.839 0.317 0.199 0.115 0.261 0.032 0.437 0.197 0.566 0.333 0.202 2.077 0.422

and fusion

for oost-irradiation

from the Azuaga

39Ar % of total

SNAr non-atmos.

1.40 4.06 13.78 9.26 21.67 24.01 20.25 5.58 100.00

81.60 96.40 98.78 98.35 98.67 98.75 98.78 96.88 98.26

’

Gneiss

0.58 2.42 4.54 11.23 21.39 18.88 13.98 19.96 7.02 100.00

0.52 1.14 4.67 6.64 15.52 13.90 18.42 7.47 10.63 6.63 7.99 5.59 0.87 100.00

and Ribera

36Aka (%)

Apparent age (Ma) d

0.22 0.06 0.01 0.02 0.01 0.02 0.04 0.05 0.04

281.7 340.6 336.4 331.6 330.3 330.3 330.3 328.8 330.9

It f f + f f f f +

331.2 f

70.79 94.00 97.32 97.80 97.82 97.95 97.33 97.96 98.04 97.55

0.04 0.01 0.01 0.02 0.02 0.02 0.04 0.01 0.06 0.03

251.9 341.4 342.4 337.1 334.0 332.7 334.7 333.3 336.7 334.3

40.09 82.56 92.98 97.54 98.03 98.35 98.13 98.23 97.92 98.09 97.76 97.93 90.32 97.26

147.3 303.0 352.9 349.3 341.8 340.5 338.6 340.1 339.2 337.7 337.0 338.0 345.6 339.3

6.85 1.44 1.30 2.45 1.89 5.40 0.55 7.40 3.15 8.78 5.07 3.07 5.84 4.29

3.5 2.0 0.8 1.4 1.1 1.2 1.3 2.6 1.6 1.3

1.0

f 17.6 f 13.4 f 4.3 + 1.6 -fr 0.8 f 1.3 f 1.2 * 2.0 + 1.9 k 2.9 * 3.0 f 3.4 f 22.0 f 2.2

339.4 f

86.16

de1

f 14.7 + 2.6 + 2.7 f 1.1 f 0.9 f 0.7 f 0.6 f 1.0 + 1.0 f 1.1

334.8 +

97.00

565 and 600°C

550-625°C

concentrates

94.55

0.02518 0.00540 0.00232 0.00186 0.00183 0.00170 0.00225 0.00170 0.00165 0.00208

7: J = 0.008383 25.35 26.42 27.72 26.13 25.39 25.21 25.11 25.20 25.21 25.03 25.08 25.11 27.86 25.41

Total without

37Ar/ 39Ar b

575 and 610°C

Azuaga Gneiss Sample 5: J = 0.008305 565 25.48 600 26.69 625 25.86 660 25.30 700 25.04 760 24.90 820 25.22 870 24.94 Fusion 25.21 Total 25.14 Total without

36Ar/ 3gAr a

231 (1994) 195-213

1.7

decay of 37Ar (35.1 day l/Zlife).

’ [40Aq,t.- (36A;,~,,,X295.5)l/40Art,It d Calculated using correction factors of Dahymple et al. (1981) for the U.S. Geological Fitzgerald (1986) for the Ford Reactor at the University of Michigan: 2a intralaboratory

Survey TRIGA errors.

Reactor

or Harrison

and

C. Quesada, R.D. Dallmeyer / Tectonophysics 231 (1994) 195-213

rocks), an average uplift rate of ca. 0.15 mm/ yr may be inferred. A difference of about 10 m.y. exists between muscovite cooling ages recorded in southeastern and northwestern segments of the BadajozCordoba shear zone. The same trend is shown by hornblende samples, although the range of ages is more restricted (ca. 5 m.y.1. This regional variation may record a sequential northwestern progression of transpressional uplift within the Badajoz-Cord6ba shear zone. In detail, each individual structural horse within this belt appears to have experienced a unique tectonothermal evolution. This may be exemplified by discussing results from two specific structural units. Samples 3 and 8 were collected within a structural unit that displays little Variscan internal deformation and is comprised of a structural superposition of Precambrian thrust sheets (Abalos, 1989). Sample 8 (Azuaga Gneiss Group amphibolite) yielded an isotope correlation age of 361 f 4 Ma. Sample 3 (muscovite concentrate from the Ribera de1 Fresno granite gneiss collected a few hundred meters structurally below the amphibolite) yielded a plateau age of 331 + 1 Ma. In this structural setting, cooling from 500 to 400°C required 30 m.y., corresponding to average uplift rate of 0.1 mm/ yr. By contrast, two hornblende concentrates (4 and 6) from another structural unit (the Las Mesas Amphibolite, Azuaga Gneiss Group) record isotope-correlation ages of 363 f 2 and 365 * 3 Ma (4 and 6). A muscovite concentrate (7) from paragneiss within the Azuaga Gneiss Group records a plateau cooling age of 339 f 2 Ma. An uplift rate of 0.13 mm/ yr may be deduced for uplift and cooling of this structural unit. During this time interval, unconformably overlying shallow marine Visean successions were deposited (Perez-Lorente, 1978). This, together with the muscovite cooling age, indicates a much more rapid uplift rate between 339 and 330 Ma which resulted in subaerial exhumation. A minimum transpressional uplift of 13 km appears to have characterized this period of about 9 m.y. At an average thermal gradient of 3O”C/km, the average rate of uplift would have been ca. 1.4 mm/yr. Subsequent deformation of the unconformably

209

overlying lower Carboniferous successions was associated with development of pressure-solution cleavage concominant with anchizone metamorphism in the Namurian (Perez-Lorente, 1978). However, subsidence must have been moderate because mineral argon systems within underlying Precambrian rocks record no evidence of rejuvenation. Renewed uplift and subsidence are indicated by lower Westhalian terrestrial coal measures which unconformably overlie both Precambrian and deformed lower Carboniferous successions. 8. Regional implications The Badajoz-Cordoba shear zone records a prolonged, complex tectonothermal evolution. Most recent events were associated with late Paleozoic (Variscan) orogeny in an intracontinental wrench system (Burg et al., 1981). Although the Variscan orogeny may be broadly considered to reflect collision of Gondwana and Laurentia, many uncertainties exist on details related to the existence and character of intervening tectonic elements. Western extremities of the European Variscides appear to reflect collision of an IberoAquitanian “indentor” (perhaps a promontory of northern Gondwana) with a northern continent (e.g., Brun and Burg, 1982; Matte, 1983, 1986; Ribeiro et al., 1990). This led to formation of the Ibero-Armorican Arc and associated structural features which included: (1) large-scale imbrication of frontal areas situated along northern sectors of the Iberian Massif (Fig. 1); and (2) largescale strike-slip faulting along lateral margins (dextral along the Armorican margin and sinistral along the southern Iberian margin). Sinistral slip along the southwestern promontory margin was likely initially accommodated within the margin. Subsequently accomodation was within the suture following closure of an oceanic branch situated south of the promontory (Silva et al., 1990; Quesada, 1991). Other strike-slip systems within adjacent units (the Ossa Morena and south Portuguese zones, Fig. 1) also accomodated sinistral slip. This likely involved reactivation of pre-existing structural elements, such as the BadajozCorddba shear zone.

210

C. Queda,

R.D. DaUmeyer/ Tectonophysics 231 (1994) 195-213

9. AcH~ents

10.3. Sample 3

This research was, in part, supported by a grant (EAR-87203221 from the Crustal Structure and Tectonics Program of the U.S. National Science Foundation to R.D.D. Drs. A. Prave and R.A. Strachan provided thorough, constructive reviews of early drafts of the manuscript.

Coordinates: 6”9’13”W, 38’35’12”N Unit: the Ribera de1 Fresno granite gneiss, intrusive into the lower unit. Structure: medium-grained porphyritic facies adjacent to the northern intrusive contact of the pluton. At this locality this granite gneiss is richer in muscovite and other volatile-rich minerals than the average biotite-bearing granite. Micas grow on a prominent flat-lying mylonitic fabric. Mineralogy: quartz, K-feldspar, plagioclase, muscovite, biotite, garnet. Texture: porphyroblastic. Petrographic classification: mylonitic muscovitebearing granite gneiss.

10. Appendix: sample locations and petrographic characteristics 10.1. Sample 1

Coordinates: 7”16’45”W, 39”7’20”N Unit: the Azuaga Gneiss Group (upper unit) Structure: banded amphibolites with prominent subvertical mylonitic foliation and subhorizontal stretching lineation. Scattered boudins of massive granulite/ eclogite mafic rocks. Mineralogy: hornblende, plagioclase, quartz, garnet, opaque minerals. Texture: nematoblastic growth of amphibole along a mylonitic fabric which surrounds partially broken garnet clasts with plagioclase coronas. Petrographic classification: Amphibolite (mylonitic-granulite). 10.2. Sample 2 Coordinates: 6”2’19”W, 38’25’27”N Unit: the Azuaga Gneiss Group (upper unit) massive mafic granulite boudins Structure: (centimeter-meter scale) surrounded by mylonitic amphibolites with a flat-lying mylonitic foliation. 5 m over basal thrust contact of the Precambrian upper unit which rests onto the intermediate sepentinite unit at this locality. Mineralogy: amphibole, plagioclase, garnet, clinopyroxene, quartz, opaques. Texture: granoblastic with discrete shear bands where amphibole-plagioclase intergrowths define a prominent planolinear fabric. Petrographic classification: mafic granulite (partly retrogressed).

10.4. Sample 4 Coordinates: 7”34’14”W, 38”14’56”N Unit: the Azuaga Gneiss Group (upper unit). Structure: banded amphibolite with a prominent planolinear fabric. Mylonitic foliation is steeply deeping and the stretching lineation phrnges gently northwestward. Mineralogy: hornblende, plagioclase, quartz, opaque minerals (retrogression to colourless amphibole and chlorite occurs along discrete millimetre-scale shear bands). Texture: nematoblastic with discrete shear bands along which grain-size reduction and mineral charges occur. Petrographic classification: amphibolite. 10.5. Sample 5

Coordinates: 5”#‘23” W, 38”23’17”N Unit: the Azuaga Gneiss Group (upper unit) Structure: banded sequence of para- and orthogneisses cut by meter-scale amphibolite horizons (dykes?). Along-strike, steeply deeping mylonitic foliation showing evidence of sinistral strike-slip. Mineralogy: quartz, K-feldspar, plagioclase, biotite, muscovite, garnet, sillimanite. Texture: blastomylonitic. Special interest has the presence of abundant clasts of feldspar and muscovite (sometimes with sillimanite inclusions) en-

C. Quesada, R.D. Dallmeyer / Tectonophysics 231 (1994) 195-213

globed by the fine-grained blastomylonitic matrix. Petrographic classification: blastomylonitic paragneiss. 10.6. Sample 6 Coordinates: 5”32’36”W, 38”11’2S’N Unit: the Azuaga Gneiss Group Structure: meter-scale amphibolite band (dike?) cutting banded gneisses. Locally these amphibolite horizons cross-cut a mylonitic foliation in the gneisses, but they themselves show prominent foliations and stretching lineations. amphibole (80%), plagioclase, Mineralogy: opaque minerals, quartz. Texture: completely annealed nematoblastic fabric. Petrographic classification: amphibolite. 10.7. Sample 7 Coordinates: 5”29’53”W, 38”14’06”N Unit: the Azuaga Gneiss Group Structure: mineralogy and texture as for sample 5. Petrographic classification: blastomylonitic paragneiss. 10.8. Sample 8 Coordinates: 6”17’33”W, 38”39’17”N Unit: the Azuaga Gneiss Group Structure: decimeter-scale banded amphibolites with a flat-lying mylonitic foliation parallel to the basal contact of the unit. Mineralogy: hornblende, plagioclase, quartz, garnet, opaque minerals. Texture: nematoblastic (annealed). Alternating plagioclase rich and poor bands. Petrographic classification: banded amphibolite.

11. References Abalos, B., 1989. Structural geology of the Ribera del Fresno Window (Badajoz-Corddba Shear zone). Rev. Sot. Geol. Esp., 2: 103-112.

211

Abalos, B., 1990. Cinem&a y mecanismos de la deformaci6n en rCgimen de transpresi6n: Evohrcion estructural y metam&fica de la zona de cizalla ductil de BadajozCord6ba. Ph.D. Thesis, Univ. Pais Vasco, Bilbao, 430 pp. Abalos, B. and Eguiluz, L., 1989. Structural analysis of deformed early lineations in black quartzites from the central Badajoz-Cordoba shear zone (Iberian Variscan Fold Belt). Rev. Sot. Geol. Esp., 2: 95-102. Abalos, B. and Eguiluz, L., 1990a. El corredor blastomilonitico de Badajoz-Gxdoba: un complejo orogi%co de subduccion/ colision durante la Orogenia Pan-Africana. CinBmatica, din&mica e historia de levantamiento de1 apilamiento de unidades tect6nicas. Geogaceta, 7: 73-76. Abalos, B. and Eguiluz, L., 1990b. Constitution tectonoestratigrafica de1 corredor blastomilonitico de BadajozCordoba: nueva propuesta de subdivision. Geogaceta, 7: 71-73. Abranches, M.C.B., Canilho, M.H. and Canelhas, M.G.S., 1979. Idade absoluta pelo mCtodo do Rb-Sr dos granitos do Port0 e de Portalegre (Nota preliminar). Bol. Sot. Geol. Port., 21: 239-248. Andrade, A.A.S., Borges, F.S., Marques, M.M., Noronha, F. and Pinto, M.S., 1983. Contribuiqb para o conhecirnento da faixa metamorfica da Foz do Douro (Nota previa). I Congresso National de Geologia de Portugal, Aveiro (abstract). Apalategui, 0. and Higueras, P., 1983. Mapa Geologico de Espaiia, escala 1:50,000, 2a serie (Magna), hoja no. 855 (Usagre). Inst. Geol. Min. Esp., 72 pp. Apalategui, O., Jorquera, A. and Villalobos, M., 1989. Mapa Geologico de Espafia, escala 1: 50,000, 2a, serie (Magna), hoja no. 803 (Almendralejo). Inst. Teen. Geomin. Esp., 75 PP. Apalategui, O., Eguiluz, L. and Quesada, C., 1990. Structure of the Ossa-Morena Zone. In: R.D. Dallmeyer and E. Martinez Garcia (Editors), Pre-Mesozoic Geology of Iberia. Springer-Verlag, Heidelberg, pp. 280-291. Bellon, H., Blachbre, H., Crousilles, M., Deloche, C., Dizsaut, C., Hertrich, B., Prost-Dame, V., Rossi, P., Simon, D. and Tamain, G., 1979. Radiochronologie, evolution tectonomagmatique et implications mCtallog&riques dans les Cadomo-variscides du Sud-Est hesperique. Bull. Sot. Geol. Fr., SCr. 7, 21: 113-120. Blat& P. and Burg, J.P., 1981. 4oAr/39Ar dates from Sierra Morena (Southern Spain): Variscan metamorphism and Cadomian orogeny. Neues Jahrb. Mineral. Monatsh., 10: 470-478. Brun, J.P. and Burg, J.P., 1982. Combined thrusting and wrenching in the Ibero-Armorican arc: a comer effect during continental collision. Earth Planet Sci. Lett., 61: 319-332. Burg, J.P., Iglesias, M., Laurent, P., Matte, P. and Ribeiro, A., 1981. Variscan intracontinental deformation: the Coimbra-Cord6ba shear zone (SW Iberian Peninsula). Tectonophysics, 78: 161-177. Dallmeyer, R.D. and Keppie, J.D., 1987. Polyphase late Paiaeozoic tectonothermal evolution of the southwestern

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