Outline
Comptes Rendus

High-pressure, low-temperature metamorphism in Alpujarride Units of southeastern Betics (Spain)
Comptes Rendus. Géoscience, Volume 334 (2002) no. 11, pp. 857-865.

Abstracts

We present the first occurrences of high-pressure, low-temperature ferro-magnesiocarpholite-bearing mineral assemblages associated to quartz segregations in the Alpujarride units of southeastern Betics (Sierra de Almagro, Sierra de los Pinos and Sierra Cabrera). Thermobarometric results show that the carpholite-bearing rocks underwent the same PT conditions in the three outcrops, i.e. 8–10 kbar, 350–400 °C. Metamorphic and structural data allow us to conclude that these rocks belong to the same Alpujarride unit. In the Sierra de Almagro, tectonic units with carpholite-bearing rocks overlie low-pressure, low-temperature Alpujarride units, then forming a stack with an inverted tectono-metamorphic sequence, as observed in the central and western part of the Alpujarride complex. The preservation of carpholite-bearing assemblages in these rocks implies that no significant temperature increase occurred during the exhumation history.

Nous rapportons les premières occurrences d'associations métamorphiques de haute pression et basse température à ferro-magnésiocarpholite incluses dans des ségrégations synfoliaires de quartz dans les schistes des unités Alpujarrides, situées dans le Sud-Est de la chaı̂ne Bétique (Sierra de Almagro, Sierra de los Pinos et Sierra Cabrera). Les estimations des conditions thermobarométriques montrent que ces associations à carpholite ont subi les mêmes conditions métamorphiques dans ces trois ensembles, autour de 350 à 400 °C et de 8 à 10 kbar. Les données métamorphiques et structurales permettent de montrer que ces roches appartiennent à la même unité Alpujarride. Dans la Sierra de Almagro, les unités à carpholite sont superposées á des unités Alpujarrides métamorphisées à basse température et basse pression, caractérisant ainsi le métamorphisme inverse déjà observé dans les Alpujarrides centrales et occidentales. L'excellente préservation des associations minérales de haute pression dans ces unités à carpholite suggère que les températures sont restées modérées au cours du retour des roches vers la surface.

Metadata
Received:
Accepted:
Published online:
DOI: 10.1016/S1631-0713(02)01787-X
Keywords: thermobarometry, carpholite, Alpujarrides, southeastern Betics, Spain
Mot clés : thermobarométrie, carpholite, Alpujarrides, Sud-Est de la cordillère Bétique, Espagne
Guillermo Booth-Rea 1; José Miguel Azañón 1; Bruno Goffé 2; Olivier Vidal 3; José Miguel Martı́nez-Martı́nez 1

1 Dpto de Geodinámica, Universidad de Granada, Facultad de Ciencias, 18076 Granada, Spain
2 Laboratoire de géologie, UMR 8538 CNRS, École normale supérieure, 24, rue Lhomond, 75231 Paris cedex 05, France
3 LGCA, CNRS–université Joseph-Fourier, 1381, rue de la Piscine, BP 53, 38041 Grenoble cedex 09, France
@article{CRGEOS_2002__334_11_857_0,
     author = {Guillermo Booth-Rea and Jos\'e Miguel Aza\~n\'on and Bruno Goff\'e and Olivier Vidal and Jos\'e Miguel Mart{\i}́nez-Mart{\i}́nez},
     title = {High-pressure, low-temperature metamorphism in {Alpujarride} {Units} of southeastern {Betics} {(Spain)}},
     journal = {Comptes Rendus. G\'eoscience},
     pages = {857--865},
     publisher = {Elsevier},
     volume = {334},
     number = {11},
     year = {2002},
     doi = {10.1016/S1631-0713(02)01787-X},
     language = {en},
}
TY  - JOUR
AU  - Guillermo Booth-Rea
AU  - José Miguel Azañón
AU  - Bruno Goffé
AU  - Olivier Vidal
AU  - José Miguel Martı́nez-Martı́nez
TI  - High-pressure, low-temperature metamorphism in Alpujarride Units of southeastern Betics (Spain)
JO  - Comptes Rendus. Géoscience
PY  - 2002
SP  - 857
EP  - 865
VL  - 334
IS  - 11
PB  - Elsevier
DO  - 10.1016/S1631-0713(02)01787-X
LA  - en
ID  - CRGEOS_2002__334_11_857_0
ER  - 
%0 Journal Article
%A Guillermo Booth-Rea
%A José Miguel Azañón
%A Bruno Goffé
%A Olivier Vidal
%A José Miguel Martı́nez-Martı́nez
%T High-pressure, low-temperature metamorphism in Alpujarride Units of southeastern Betics (Spain)
%J Comptes Rendus. Géoscience
%D 2002
%P 857-865
%V 334
%N 11
%I Elsevier
%R 10.1016/S1631-0713(02)01787-X
%G en
%F CRGEOS_2002__334_11_857_0
Guillermo Booth-Rea; José Miguel Azañón; Bruno Goffé; Olivier Vidal; José Miguel Martı́nez-Martı́nez. High-pressure, low-temperature metamorphism in Alpujarride Units of southeastern Betics (Spain). Comptes Rendus. Géoscience, Volume 334 (2002) no. 11, pp. 857-865. doi : 10.1016/S1631-0713(02)01787-X. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/S1631-0713(02)01787-X/

Version originale du texte intégral

Version abrégée

1 Introduction

Le domaine crustal d'Alboran est le socle du bassin marin d'Alboran [17,18,23]. Il est constitué, de haut en bas, par les complexes Malaguide, Alpujarride et Nevado-Filabride [9,10]. Ces deux derniers ensembles ont été métamorphisés durant l'orogenèse alpine dans les conditions du faciès schistes bleus ou éclogitiques dans les premiers stades de l'évolution métamorphique [4,7,22,24,30,39]. Le complexe Malaguide a, en revanche, seulement été affecté par un métamorphisme alpin de bas degré [26,31].

Les associations métamorphiques à ferro- et magnésiocarpholite ont été décrites dans les unités Alpujarrides de la chaı̂ne Bétique centrale et occidentale [3–5,24] ainsi que dans les Sebtides [15]. Ces associations représentent des conditions de haute pression et basse température. Dans cet article, nous présentons les premières occurrences d'associations à carpholite dans la partie orientale de la chaı̂ne Bétique, comme preuves de l'extension de ces conditions de haute pression, dans un secteur de la chaı̂ne où le métamorphisme n'était pas décrit.

2 Contexte géologique

L'ensemble Alpujarride actuel résulte des charriages pré-aquitaniens [7,11] ou aquitaniens [4], avec empilement d'unités de haut degré métamorphique sur des unités de bas degré. Cette succession inverse est une structure caractéristique de la chaı̂ne Bétique, qui démontre l'existence de charriages post-métamorphiques [4,7,11]. Cet empilement a été fortement étiré durant le Miocène inférieur et moyen par des failles normales cassantes plates, de telle sorte que les contacts entre les unités sont maintenant des contacts extensifs [19,23,26–28].

Dans la région étudiée, le complexe Alpujarride est constitué par au moins trois unités tectoniques (Almagro, Almanzora et Variegato), superposées dans l'ordre ascendant [37,38]. Les unités d'Álmagro et d'Almanzora affleurent seulement dans le Nord du secteur étudié, dans la Sierra de Almagro. L'unité de Variegato, qui a été définie par Simon [37] dans la Sierra de Almagro, a été corrélée avec les unités Alpujarrides présentes dans la Sierra Cabrera [34].

Les carpholites ont été trouvées dans les schistes à grains fins de la Sierra de Almagro (échantillon w.alm.10), dans des klippes de l'unité de Variegato reposant directement sur l'unité d'Almagro. Dans nombre de ces affleurements, l'unité de Variegato est formée de plusieurs écailles (deux ou trois), n'ayant chacune pas plus de 250 m d'épaisseur et constituées de la succession de schistes noirs graphiteux attribués au Paléozoı̈que, de schistes clairs du Permo-Trias et de carbonates du Trias [36]. Ces écailles, affectées par la tectonique extensive néogène, n'ont pas de continuité latérale.

Dans la Sierra Cabrera, les carpholites n'existent que localement, dans des affleurements situés aux limites sud-est et ouest des Alpujarrides, dans les restes de deux ou trois écailles de la base du complexe Alpujarride (échantillon w.mjc.1, Fig. 1). Vers le nord-est, dans la Sierra de los Pinos, on a également trouvé des carpholites (échantillon w.jv.1, Fig. 1) dans des schistes à grains fins, dans des écailles extrêmement amincies de l'unité de Ramonete [2] située à la base du complexe Alpujarride, dans une position structurale correspondant à celle de l'unité de la Sierra Cabrera.

Figure 1

Geological map of the study area.

Carte géologique de la région étudiée.

3 Pétrographie et thermobarométrie

Les roches à carpholite étudiées sont des schistes et des quartzites contenant fréquemment des veines de quartz en forme de lentilles synfoliaires, parfois plissées en plis isoclinaux. L'association minérale dans ces veines est carpholite + chlorite + pyrophyllite + quartz ± phengite (Fig. 2). Les carpholites constituent, dans ces veines, des fibres blanches à vert pâle, de quelques centimètres ou décimètres de long, ou des microfibres de quelques centaines de microns, incluses dans le quartz. Les chlorites, associées à la pyrophyllite, forment de longs cristaux parallèles aux fibres de carpholite, suggérant ainsi un équilibre entre les deux minéraux. Elles constituent également, en association avec des micas blancs, des textures de remplacement tardif de la carpholite (Fig. 2B).

Figure 2

A. Quartz vein from fine-grained schists of the Variegato unit with Mg-carpholite fibres altered to pyrophyllite and wKm. B. Quartz vein of the Variegato unit, lowest thrust sheet with Mg-carpholite microfibres (Mg Car), altered to a chlorite (Chl) and wKm assemblage. Abbreviations: pyr, pyrophyllite; wKm, white K mica; Mg Car, Mg carpholite; Chl, chlorite.

A. Microphotographie de fibres de Mg-carpholite dans une veine de quartz des schistes à grains fins de l'unité de Variegato, partiellement remplacées par des associations à pyrophyllite et micas blancs. B. Microphotographie de fibres de Mg-carpholite dans une veine de quartz des schistes à grains fins de l'unité de Variegato, remplacées par des chlorites et micas blancs dans l'écaille la plus basse. Abréviations : pyr, pyrophyllite ; wKm, mica blanc K ; Mg Car, Mg carpholite ; Chl, chlorite.

Pour estimer les conditions métamorphiques correspondant à la croissance de ces associations à carpholite, nous avons utilisé la méthode des multi-équilibres [14], qui permet de calculer la position dans l'espace PT des différentes réactions intervenant entre les minéraux présents dans l'association. Pour effectuer ces calculs dans le système KMASH (K2OMgOAl2O3SiO2H2O) incluant toutes les phases décrites ci-dessus, nous avons utilisé le logiciel TWEEQ [14] et les modèles d'activité déterminés par Vidal et al. [40] pour les carpholites, Vidal et al. [41] pour les chlorites, Parra et al. [32] pour les micas blancs et Berman [13] pour les autres phases. La proportion des pôles purs dans les minéraux le long de la solution solide ferro-magnésienne est exprimée sous la forme XMg et XFe, avec XMg=Mg/(Mg+Fe2++Mn) et XFe=Fe2+/(Mg+Fe2++Mn). Les compositions représentatives des minéraux index, comme les carpholites, les micas blancs K et les chlorites, sont présentées dans les Tableaux 1, 2 et 3, respectivement.

Table 1

Representative mineral analyses and structural formulae (see text) for carpholite in samples from the Alpujarride fine-grained schists. See text for the Si and Fe3+ calculation. Activity of Mg-carpholite is calculated as acar=(XMg)(XAl)2(XOH)4 [40].

Analyses et formules structurales représentatives des carpholites des schistes à grains fins des Alpujarrides orientales. Voir texte pour le mode de calcul de Si et Fe3+. L'activité de la Mg-carpholite est calculée selon la formule acar=(XMg)(XAl)2(XOH)4 [40].

Mineral Carpholite
Sample w.jrv.1 w.jrv.2 w.mjc.1 w.mjc.2 w.alm.10 w.mjc.1
analysis 69 75 79 84 17 88
SiO2 39.12 39.57 39.58 39.18 41.08 39.04
Al2O3 32.18 32.50 31.62 32.56 32.98 32.54
FeO 7.30 8.43 8.48 8.16 7.76 8.41
MnO 0.13 0.19 0.09 0.14 0.15 0.26
MgO 8.56 8.72 8.53 8.19 8.59 8.49
F 0.93 1.09 0.94 0.75 0.68 0.70
Summation 88.70 90.75 89.41 89.27 91.39 89.67
Si 1.885 1.852 1.878 1.861 1.909 1.839
Al 1.915 1.859 1.849 1.895 1.907 1.866
Fe3+ 0.085 0.141 0.151 0.105 0.093 0.134
Fe2+ 0.478 0.484 0.492 0.510 0.489 0.491
Mn 0.010 0.014 0.007 0.010 0.011 0.019
Mg 0.512 0.502 0.502 0.479 0.500 0.489
F 0.092 0.104 0.092 0.072 0.065 0.067
X Mg 0.512 0.502 0.502 0.479 0.500 0.489
a Mg 0.479 0.454 0.453 0.446 0.469 0.449
Table 2

Representative mineral analyses and structural formulae for white K micas from the Alpujarride fine-grained schists.

Analyses et formules structurales représentatives des micas blancs des échantillons des schistes à grains fins des Alpujarrides orientales.

Mineral White K micas
Sample w.alm.10 w.alm.10 w.alm.10 w.mjc.1 w.jv.1 w.jv.1
analysis 20 5 6 42b B43 B58
SiO2 49.58 46.97 46.73 46.93 48.68 48.65
TiO2 0.07 0.07 0.07 0.16 0.08 0.12
Al2O3 36.38 34.95 34.74 35.38 35.90 34.76
FeO 1.37 1.80 2.04 2.14 1.62 1.86
MnO 0.00 0.02 0.00 0.04 0.04 0.05
MgO 0.85 0.59 0.62 0.46 0.48 0.54
CaO 0.00 0.09 0.08 0.01 0.02 0.00
Na2O 1.09 1.28 1.30 0.84 0.79 0.68
K2O 8.85 8.73 8.71 9.10 8.94 8.53
F 0.98 0.06 0.06 0.21 0.21 0.17
Sum. 98.19 94.50 94.29 95.06 96.55 95.19
Si 3.158 3.129 3.124 3.114 3.158 3.196
Ti 0.003 0.003 0.004 0.008 0.004 0.006
Al 2.731 2.743 2.737 2.767 2.745 2.692
Al IV 0.839 0.868 0.872 0.878 0.838 0.798
Al VI 1.892 1.876 1.866 1.888 1.907 1.894
Fe2+ 0.073 0.100 0.114 0.119 0.088 0.102
Fe3+ 0.000 0.000 0.000 0.000 0.000 0.000
Mn 0.000 0.001 0.000 0.002 0.002 0.003
Mg 0.081 0.059 0.062 0.045 0.047 0.053
Ca 0.000 0.007 0.006 0.001 0.002 0.000
Na 0.134 0.166 0.168 0.109 0.100 0.087
K 0.719 0.742 0.743 0.770 0.740 0.715
Table 3

Representative mineral analyses and structural formulae for chlorite from the Alpujarride fine-grained schists.

Analyses et formules structurales représentatives des chlorites des échantillons des schistes à grains fins des Alpujarrides orientales.

Mineral Chlorite
Sample analysis w.alm.10 w.alm.10 w.jv.1 w.jv.1 w.mjc.1 w.mjc.1
12 23 56 B55 B17 41b
SiO2 28.54 28.57 27.36 26.68 31.58 25.57
TiO2 0.01 0.03 0.10 0.00 0.07 0.06
Al2O3 24.92 25.25 26.07 25.69 33.71 23.88
FeO 11.39 11.46 15.61 18.73 10.35 17.31
MnO 0.02 0.19 0.04 0.04 0.17 0.20
MgO 24.71 24.27 19.60 16.66 11.96 17.80
Sum. 89.62 89.81 88.95 87.94 87.91 84.95
Si 2.735 2.707 2.671 2.679 2.956 2.657
Ti 0.001 0.002 0.007 0.000 0.005 0.004
Al 2.848 2.820 2.999 3.040 3.719 2.923
Al IV 1.263 1.292 1.322 1.321 1.039 1.339
Al VI 1.584 1.528 1.678 1.719 2.680 1.585
Fe2+ 0.848 0.908 1.274 1.573 0.810 1.504
Fe3+ 0.00 0.00 0.00 0.00 0.00 0.00
Mn 0.000 0.015 0.003 0.003 0.013 0.018
Mg 3.398 3.428 2.852 2.494 1.669 2.756
R2+ 4.246 4.352 4.129 4.070 2.492 4.278
Oct. Sum. 5.848 5.884 5.833 5.810 5.180 5.881
X Mg 0.80 0.79 0.69 0.61 0.67 0.65

Les conditions ainsi estimées pour les schistes à grains fins contenant les carpholites se situent, pour la pression, entre 8 et 10 kbar, pour des températures variant de 330 à 460 °C (Fig. 3). Le fait que les carpholites soient très bien préservées suggère que le retour vers la surface, après le pic en pression, s'est effectué sans augmentation sensible de la température [40].

Figure 3

A. Thermobarometric results for the Alpujarride carpholite bearing fine-grained schists (samples w.alm.10, w.jv.1 and w.mjc.1. B. TWEEQ PT diagram for equilibrium between the phases Mg-carpholite + chlorite + wKm + water + quartz {four independent reactions} (sample w.alm.10, chlorite analysis 12).

A. Résultats des calculs thermobarométriques pour les schistes à grains fins à carpholite des Alpujarrides orientales (échantillons w.alm.10, w.jv.1 et w.mjc.1). B. Diagramme PT calculé avec le logiciel TWEEQ PT pour des équilibres entre Mg-carpholite + chlorite + wKm + eau + quartz {quatre réactions indépendantes} (échantillon w.alm.10, chlorite analyse 12).

4 Discussion et conclusions

Les conditions métamorphiques identiques dans tous ces schistes à grains fins contenant des carpholites, dans les trois secteurs étudiés, nous incitent à les inclure dans une même unité tectonique du complexe Alpujarride. Cette unité, que nous appellerons unité de Variegato, en suivant la nomenclature de Simon [37], devait ainsi constituer une partie unique et homogène de la pile tectonique. Les résultats présentés ici montrent que les conditions de métamorphisme atteintes dans les séries permo-triasiques de l'unité de Variegato sont celles du faciès schistes bleus et qu'elles sont suivies par une décompression sans réchauffement. Ces résultats contredisent les connaissances antérieures sur cette région du Sud-Est de la chaı̂ne Bétique, qui sous-estimaient les conditions du métamorphisme en les classant dans le bas degré du faciès schistes verts [42] ou à la limite du faciès schistes bleus [8]. Ces travaux admettaient aussi une rééquilibration thermique, ce qui n'est pas compatible avec la préservation de la carpholite [40].

Ces écailles de l'unité de Variegato reposent sur le Permo-Trias de l'unité d'Almagro, dont les conditions de métamorphisme sont estimées autour de 4 kbar et 300 °C, conditions qui sont celles du faciès schistes verts [35]. Cette superposition anormale, associée à une inversion de la séquence métamorphique, suggère que ces ensembles ont été affectés par une tectonique en contraction tardi- à post-métamorphique, avant la tectonique extensive cassante qui a eu lieu durant le rifting néogène.

1 Introduction

The Gibraltar Arc constitutes the western end of the Alpine peri-Mediterranean orogenic system. It is a highly arched orogenic system, formed by the Betics, Rif and Tell Chains, which are connected through the Gibraltar Straits. The internal part of the Arc is formed by the Alboran Crustal Domain, which presently constitutes the basement of both the Alboran Sea and the Neogene basins that outcrop in the Internal Betics [17,18,23]. The Alboran Crustal Domain is formed, in ascending order, by the Nevado-Filabride, the Alpujarride/Sebtide [21] and the Malaguide/Ghomaride [16] complexes, and by the Dorsal and Predorsal units [9,10]. However, other authors interpret the Nevado-Filabride complex as a domain that constituted the distal parts of the Iberian margin, together with oceanic crust, during the Mesozoic [29]. The two lowest complexes underwent plurifacial metamorphism during the Alpine orogeny, registering first a high-pressure metamorphic event under eclogite or blueschist facies, then an intermediate or low PT gradient metamorphic event under granulite, amphibolite or greenschist facies [4,7,22,24,30,39]. Meanwhile, the Malaguide Permo-Triassic sequences only underwent anchizonal metamorphism or diagenesis [26,31].

High-pressure, low-temperature metamorphic mineral assemblages including Mg-carpholite have been described in the Alpujarride units outcropping in the western and central Betics [3,5,7,24] and in the Sebtides [15]. After this high-pressure metamorphic event, these Alpujarride units followed an isothermal exhumation PT path [3,4,7,22,39]. On the other hand, the Alpujarride units outcropping in the southeastern Betics have been described as reaching only lower-greenschist metamorphic facies [42], or lower-blueschist facies, like the Triassic metabasites of the Almanzora unit, in the Northern Sierra de los Filabres, which include sodic blue amphiboles, grown under PT conditions of approximately 7 kbar and 400 °C [8]. Bakker et al. [8] also described a late-heating event that affects the southeastern Betic Alpujarride units, during their exhumation path. Do these differences between the eastern and the western–central Alpujarride units really exist?

In this paper, we present the first occurrences of ferro-magnesiocarpholite-bearing assemblages in several of the Alpujarride complex outcrops of southeastern Betics, proving that carpholite occurs in some of the Alpujarride units at the scale of the Betic Chain.

2 Geological setting

In the Betic Cordilleras, the Alpujarride complex includes different nappes with similar lithostratigraphic sequences, although with variable thickness. When complete, an Alpujarride unit is typically formed, in ascending order, by gneisses and dark graphite schists (Palaeozoic protholites), light-coloured fine-grained schists and quartzites (Permo-Triassic protholites) and a thick Triassic carbonate sequence [1,5,20]. The Alpujarride complex constitutes the remnants of a pre-Aquitanian or Aquitanian thrust stack [4,7,11], which shows higher-grade metamorphic rocks overlying lower-grade ones. This inverted stacking order is a common feature of the Betics, which proves the existence of a late to post-metamorphic thrusting event [4,6,7,11]. However, this thrusting event has been denied by other authors, who consider that after the high-pressure metamorphic event, the Alboran Domain was exhumed towards the surface by ongoing extensional tectonics [26,33,42]. The resulting thrust stack was submitted to extension during the Lower and Middle Neogene by brittle low-angle normal faults, so that the tectonic units are now bound by extensional contacts [5,12,19,23,27,28]. Shortening occurred again during the Upper Neogene [43], forming large east–west-trending antiforms [28] that constitute in particular the present-day mountain ranges of the studied area of southeastern Betics (Fig. 1).

In the investigated area, the Alpujarride complex seems to be formed by three tectonic units, in ascending order the Almagro, Almanzora and Variegato units [37,38], although Sanz de Galdeano and Garcı́a-Tortosa [36] have proposed that the Almagro and Almanzora units constitute a single unit, based on their great stratigraphical similarity. The Almagro and Almanzora units only crop out in the north of the study area (Sierra de Almagro), while the Variegato unit [37,38] would be also represented in the southern outcrops (Sierra Cabrera) [34].

The ferro-magnesiocarpholite-bearing fine-grained schists of Sierra de Almagro are found in klippes of the Variegato unit, above the Almagro unit (sample w.alm.10). Although the Variegato unit has been considered as a single tectonic unit [37], in most cases it is constituted by two or three minor thrust sheets (Variegato sub-units), no more than 250 m thick altogether, that include Permo-Triassic metapelites, Triassic carbonates, and locally, at the base of the highest Variegato sub-unit, garnet–biotite-bearing graphite schists [36]. These thrust-sheets lack lateral continuity, because of frequent tectonic omissions related with Neogene extensional tectonics.

In the Sierra Cabrera, carpholite-bearing rocks are found locally, in the southeastern and in the western end of the Alpujarride outcrops (sample w.mjc.1), where remnants of at least two Variegato sub-units with carpholite assemblages are preserved at the base of the Alpujarride complex. Towards the northeast, in the Sierra de los Pinos, the fine-grained carpholite-bearing schists are found also in two much-thinned thrust sheets, of the so-called Ramonete unit [2], at the base of the Alpujarride complex, in a structural position equivalent to that of Variegato sub-units of Sierra Cabrera.

3 Petrography and thermobarometry

The studied rocks are fine-grained schists and quartzites that frequently include synfolial, lense-shaped quartz veins. The mineral assemblages present in the quartz vein samples and selected for this study are carpholite + chlorite + pyrophyllite + quartz ± white K mica (wKm) (Fig. 2). Carpholite forms centimetre to decimetre-long white to greenish fibres, clearly observable in hand specimens and included in isoclinally folded quartz veins, or relic microfibres inside the quartz veins. Chlorite shows two populations. Some chlorites form long crystals parallel to the carpholite fibres, showing equilibrium textures with carpholite. Other chlorites are found together with wKm as late alteration products of carpholite (Fig. 2B). Pyrophyllite forms small laths together with the carpholite + chlorite and the chlorite + wKm assemblages.

We used a multi-equilibrium method [14], calculating the position in the PT space of the different reactions among the aforementioned mineral assemblages. The reaction curves intersect in the proximity of one PT point when equilibrium is attained. To determine the stability fields of all the possible reactions between the present phases plus the wKm-chlorite end members, we have used the TWEEQ software of Berman [14] and activity models determined by Vidal et al. [40] for carpholite, Vidal et al. [41] for chlorite, Parra et al. [32] for wKm and Berman [13] for the remaining phases. The mineral analyses were performed with a Camebax electron microprobe at the University Paris-6. XMg in minerals is calculated as XMg=Mg/(Mg+Fe2++Mn) and XFe=Fe2+/(Mg+Fe2++Mn). Representative compositions of the metamorphic index minerals carpholite, white K mica and chlorite are given in Tables 1, 2 and 3, respectively.

Carpholite formulae (Mn,Fe,Mg)Al2Si2O6(OH)4 are calculated on the basis of a fixed number of cations (Table 1): five for the calculation of Si (considering all the elements), three for the calculation of Al, Fe, Mn and Mg (Si being ignored). This mode of calculation is used to avoid the effect of the contamination by surrounding quartz when calculating the Al, Fe, Mg and Mn content of carpholite microfibres smaller than the microprobe-beam size [25]. Fe3+ is calculated as 2-(Al + Ti). The carpholite composition varies from XMg=0.37 to XMg=0.58. MnO is always very low (<0.5 wt%). The content of F is comprised between 1 and 0 wt%.

White K mica formulae (Table 2) are calculated on the basis of 11 oxygens with total iron as FeO. The Si content varies from 3.094 to 3.196 pfu in the Variegato fine-grained schists.

Chlorite formulae (Table 3) are calculated on the basis of 14 oxygens with total iron as FeO. Their composition varies from XMg=0.80 to 0.60 in the Variegato fine-grained schists. The Si content varies from 2.61 to 3.22 pfu in the Variegato fine-grained schists.

The thermobarometric results obtained with the TWEEQ software coincide with the petrogenetic grid constraints that situate these samples in the pyrophyllite + chlorite + carpholite field, being devoid of kyanite (Fig. 3). The TWEEQ results show that the fine-grained schists have reached between 8.2±0.5 and 10.0±0.1 kbar, and between 348±28 and 408±58 °C (Fig. 3).

The fact that fresh carpholite is well preserved, being very sensitive to thermal reequilibration [40], and that no traces of andalousite or biotite have been found in these rocks, shows that these Alpujarride rocks have not suffered a later heating event, as proposed by Bakker et al. [8].

4 Discussion and conclusions

The analysed fine-grained schists of the three studied outcrops show the same mineral assemblages, which have reached equivalent PT conditions. These data, together with the fact that these Alpujarride rocks occupy the same structural position, have decided us to include them in a same Alpujarride unit, for which we have retained the name of Variegato unit, from Simon [37]. The Almagro and Almanzora units have been omitted by extensional tectonics in the two most-southern outcrops.

The Variegato unit has been considered as a single Alpujarride tectonic unit [37], although the fact that it is constituted by three thin sub-units indicates that it represents the remnants of a previous thrust pile that has been strongly thinned during Neogene extensional tectonics.

The Alpujarride complex outcropping in southeastern Betics has been described as reaching only lower-greenschist metamorphic facies [42], or lower-blueschist facies [8]. Bakker et al. [8] describe a late-heating event that affects the Alpujarride units during their exhumation path. The fact that fresh carpholite is well preserved, being very sensitive to thermal reequilibration [40], and that no andalousite or biotite has been found precludes the possibility of this re-heating event in the Variegato sub-units.

The Variegato sub-units are found lying over the Almagro unit that has only reached lower-greenschist-facies metamorphism, at approximately 4 kbar and 300 °C [35]. Moreover, the Variegato sub-units show the superposition of garnet-biotite-bearing schists on the top of wKm-chlorite bearing fine-grained schists. Then, a late to post-metamorphic thrusting event, responsible for the inverted Alpujarride metamorphic sequence, occurred prior to the brittle extensional attenuation of the Alpujarride complex during the Neogene rifting.

Acknowledgements

The French–Spanish collaboration ENS–Granada University (PICASSO programme), the CICYT Spanish projects REN2001-3868-CO3-01/MAR, REN2001-3378 and FEDER founds of the EU supported the field and laboratory research.


References

[1] F. Aldaya; V. Garcı́a-Dueñas; F. Navarro-Vilá Los Mantos Alpujárrides del tercio central de las Cordilleras Béticas. Ensayo de correlación tectónica de los Alpujárrides, Acta Geol. Hisp., Volume 14 (1982), pp. 154-166

[2] F. Alvarez Subhorizontal shear zones and their relation to nappe movements in the Cantal and Miñarros units, Eastern Betic Zone (Spain), Geol. en Mijnbouw, Volume 66 (1987), pp. 101-110

[3] J.M. Azañón; B. Goffé Ferro-magnesiocarpholite-kyanite assemblages as record of the high-pressure, low-temperature metamorphism in central Alpujarride units, Betic cordillera (SE Spain), Eur. J. Mineral., Volume 9 (1997), pp. 1035-1051

[4] J.M. Azañón; A. Crespo-Blanc Exhumation during a continental collision inferred from the tectonometamorphic evolution of the Alpujarride Complex in the central Betics (Alboran Domain, SE Spain), Tectonics, Volume 19 (2000), pp. 549-565

[5] J.M. Azañón; V. Garcı́a-Dueñas; J.M. Martı́nez-Martı́nez; A. Crespo-Blanc Alpujarride tectonic sheets in the central Betics and similar eastern allochthonous units (SE Spain), C. R. Acad. Sci. Paris, Ser. II, Volume 318 (1994), pp. 667-674

[6] J.M. Azañón; A. Crespo-Blanc; V. Garcı́a-Dueñas; M. Sánchez-Gómez Folding of metamorphic isogrades in the Adra extensional unit (Alpujarride complex, Central Betics), C. R. Acad. Sci. Paris, Ser. IIa, Volume 323 (1996), pp. 949-956

[7] J.M. Azañón; A. Crespo-Blanc; V. Garcı́a-Dueñas Continental collision, crustal thinning and nappe-forming during the Pre-Miocene evolution of the Alpujárride Complex (Alborán Domain, Betics), J. Struct. Geol., Volume 19 (1997), pp. 1055-1071

[8] H.E. Bakker; K. De Jong; H. Helmers; C. Bierman The geodynamic evolution of the Internal Zone of the Betic Cordilleras (South-East Spain): a model based on structural analysis and geothermobarometry, J. Metamorph. Geol., Volume 7 (1989), pp. 359-381

[9] J.C. Balanyá; V. Garcı́a-Dueñas Grandes fallas de contracción y extensión implicadas en el contacto entre los dominios de Alborán y Sudibérico en el Arco de Gibraltar, Geogaceta, Volume 1 (1986), pp. 19-21

[10] J.C. Balanyá; V. Garcı́a-Dueñas Les directions structurales dans le domaine d'Alborán de part et d'autre du détroit de Gibraltar, C. R. Acad. Sci. Paris, Ser. II, Volume 304 (1987), pp. 929-932

[11] J.C. Balanyá; V. Garcı́a-Dueñas; J.M. Azañón; M. Sánchez-Gómez Alternating contractional and extensional events in the Alpujarride nappes of the Alboran Domain (Betics, Gibraltar Arc), Tectonics, Volume 16 (1997), pp. 226-238

[12] J.C. Balanyá; V. Garcı́a-Dueñas; J.M. Azañón; M. Sánchez-Gómez Reply, Tectonics, Volume 17 (1998), pp. 977-981

[13] R.G. Berman Internally-consistent thermodynamic data for stoichiometric minerals in the system Na2OK2OCaOMgOFeOFe2O3Al2O3SiO2TiO2H2OCO2, J. Petrol., Volume 29 (1988), pp. 445-522

[14] R.G. Berman Thermobarometry using multi-equilibrium calculations – A new technique, with petrological applications, Can. Mineral., Volume 29 (1991), pp. 833-855

[15] M.-L. Boybaouène; B. Goffé; A. Michard High-pressure, low-temperature metamorphism in the Sebtides nappes, northern Rif, Morocco, Geogaceta, Volume 17 (1995), pp. 117-119

[16] A. Chalouan; A. Michard The Ghomarides nappes, Rif coastal range, Morocco: a variscan chip in the Alpine belt, Tectonics, Volume 9 (1990), pp. 1565-1583

[17] M.C. Comas; V. Garcı́a-Dueñas; M.J. Jurado Neogene tectonic evolution of the Alboran Sea from Mcs data, Geo-Mar. Lett., Volume 12 (1992), pp. 157-164

[18] M.C. Comas; J.P. Platt; J.I. Soto; A.B. Watts The origin and tectonic history of the Alborán Basin: insights from Leg 161 results, Proc. ODP Sci. Results, Volume 161 (1999), pp. 555-579

[19] A. Crespo-Blanc Interference pattern of extensional fault systems: a case study of the Miocene rifting of the Alboran basement (North of Sierra Nevada, Betic Chain), J. Struct. Geol., Volume 17 (1995), pp. 1559-1569

[20] F. Delgado; A. Estévez; J.M. Martı́n; A. Martı́n-Algarra Observaciones sobre la estratigrafı́a de la formación carbonatada de los Mantos Alpujárrides (Cordillera Bética), Estud. Geol., Volume 37 (1981), pp. 47-57

[21] W.P. Didon; M. Durand-Delga; J. Kornprobst Homologies géologiques entre les deux rives du détroit de Gibraltar, Bull. Soc. géol. France, Volume 7 (1973), pp. 77-105

[22] A. Garcı́a-Casco; R.L. Torres-Roldán Disequilibrium induced by fast decompression in St–Bt–Grt–Ky–Sil–And metapelites from the Betic Belt (southern Spain), J. Petrol., Volume 37 (1996), pp. 1207-1239

[23] V. Garcı́a-Dueñas; J.C. Balanyá; J.M. Martı́nez-Martı́nez Miocene extensional detachments in the outcropping basement of the Northern Alboran Basin (Betics) and their tectonic implications, Geo-Mar. Lett., Volume 12 (1992), pp. 88-95

[24] B. Goffé; A. Michard; V. Garcı́a-Dueñas; F. González-Lodeiro; P. Monié; J. Campos; J. Galindo-Zaldı́var; A. Jabaloy; J.M. Martı́nez-Martı́nez; F. Simancas First evidence of high-pressure, low-temperature metamorphism in the Alpujarride nappes, Betic Cordillera (SE Spain), Eur. J. Mineral., Volume 1 (1989), pp. 139-142

[25] B. Goffé; R. Oberhänsli Ferro- and magnesiocarpholite in the ‘Bündnerschiefer’ of the eastern Central Alps (Grisons and Engadine window), Eur. J. Mineral., Volume 4 (1992), pp. 835-838

[26] L. Lonergan; J. Platt The Malaguide–Alpujarride boundary: a major extensional contact in the Internal Zone of the eastern Betic Cordillera, SE Spain, J. Struct. Geol., Volume 17 (1995), pp. 1665-1671

[27] J.M. Martı́nez-Martı́nez; J.M. Azañón Mode of extensional tectonics in the south-eastern Betics (SE Spain). Implications for the tectonic evolution of the peri-Alborán orogenic system, Tectonics, Volume 16 (1997), pp. 205-225

[28] J.M. Martı́nez-Martı́nez, J.I. Soto, J.C. Balanyá, Orthogonal folding of extensional detachments: structure and origin of the Sierra Nevada elongated dome (Betics, SE Spain), Tectonics (in press)

[29] A. Michard; A. Chalouan; H. Feinberg; B. Goffé; R. Montigny How does the Alpine belt end between Spain and Morocco?, Bull. Soc. géol. France, Volume 173 (2002), pp. 3-15

[30] L. Morten; G.M. Bargossi; J.M. Martı́nez-Martı́nez; E. Puga; A. Dı́az de Federico Metagabbro and associated eclogites in the Lubrı́n area, Nevado-Filabride Complex, Spain, J. Metamorph. Geol., Volume 5 (1987), pp. 155-174

[31] F. Nieto; N. Velilla; D.R. Peacor; M. Ortega-Huertas Regional retrograde alteration of sub-greenschist facies chlorite to smectite, Contrib. Mineral. Petrol., Volume 115 (1994), pp. 243-252

[32] T. Parra, O. Vidal, P. Agard, A thermodynamic model for FeMg dioctahedral K-white micas using data from phase equilibrium experiments and natural pelitic assemblages, Contrib. Mineral. Petrol. (in press)

[33] J.P. Platt Comment on “Alternating contractional and extensional events in the Alpujarride nappes of the Alboran Domain (Betic, Gibraltar Arc)” by J.C. Balanyá et al., Tectonics, Volume 17 (1998), pp. 973-976

[34] H.E. Rondeel Geological investigations in the western Sierra Cabrera and adjoining areas, south-eastern Spain, PhD thesis, Amsterdam, 1965 (161 p)

[35] V.L. Sánchez-Vizcaino; M.T. Gómez-Pugnaire; J.M. Fernández-Soler Petrological features of some Alpujarride mafic igneous bodies from the Sierra de Almagro (Betic Cordilleras, Spain), Rev. Soc. Geol. Esp., Volume 4 (1991), pp. 321-335

[36] C. Sanz de Galdeano, F.J. Garcı́a-Tortosa, Almagride versus Alpujarride in the Sierra de Almagro (Betic Cordillera, Spain), C. R. Geoscience (in press)

[37] O.J. Simon Geological investigations in the Sierra de Almagro, southeastern Spain, Doctoral Thesis, Amsterdam, 1963 (164 p)

[38] O.J. Simon On the Triassic of the Betic Cordilleras (southern Spain), Cuad. Geol. Ib., Volume 11 (1987), pp. 385-402

[39] J.M. Tubı́a; J.I. Gil-Ibarguchi Eclogites of the Ojén nappe: a record of subduction in the Alpujárride complex (Betic Cordilleras, southern Spain), J. Geol. Soc. London, Volume 148 (1991), pp. 801-804

[40] O. Vidal; B. Goffé; T. Theye Experimental study of a new petrogenetic grid for the system FeOMgOAl2O3SiO2H2O, J. Metamorph. Geol., Volume 14 (1992), pp. 381-386

[41] O. Vidal; T. Parra; F. Trotet A thermodynamic model for FeMg aluminous chlorite using data from phase equilibrium experiments and natural pelitic assemblages in the 100–600 °C, 1–25 kbar PT range, Am. J. Sci., Volume 301 (2001), pp. 557-592

[42] R.L.M. Vissers; J.P. Platt; D. van der Wal Late Orogenic extension of the Betic Cordillera and the Alboran Domain: a lithospheric view, Tectonics, Volume 14 (1995), pp. 786-803

[43] R. Weijermars; T.B. Roep; B. Van den Eeckhout; G. Postma; K. Kleverlaan Uplift history of a Betic fold nappe inferred from Neogene–Quaternary sedimentation and tectonics (in the Sierra Alhamilla and Almerı́a, Sorbas and Tabernas Basins of the Betic Cordilleras, SE Spain), Geol. en Mijnbouw, Volume 64 (1985), pp. 397-411


Comments - Policy


Articles of potential interest

Alpujarride attribution of the supposed ‘Almagride Complex’ (Betic Internal Zone, Almerı́a Province, Spain)

Carlos Sanz de Galdeano; Francisco J. Garcı́a Tortosa

C. R. Géos (2002)


Pressure–temperature–time constraints on the Maghrebide mountain building: evidence from the Rif–Betic transect (Morocco, Spain), Algerian correlations, and geodynamic implications

André Michard; François Negro; Omar Saddiqi; ...

C. R. Géos (2006)


A ‘core-complex-like structure’ formed by superimposed extension, folding and high-angle normal faulting. The Santi Petri dome (western Betics, Spain)

Guillermo Booth-Rea; José Miguel Azañón; Vı́ctor Garcı́a-Dueñas; ...

C. R. Géos (2003)