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

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.

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).

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].

4 Discussion et conclusions

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 P–T 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 P–T 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 P–T 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.

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.

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 P–T space of the different reactions among the aforementioned mineral assemblages. The reaction curves intersect in the proximity of one P–T 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. X_Mg in minerals is calculated as X_Mg=Mg/(Mg+Fe²⁺+Mn) and X_Fe=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 X_Mg=0.37 to X_Mg=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 X_Mg=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].

The analysed fine-grained schists of the three studied outcrops show the same mineral assemblages, which have reached equivalent P–T 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.