1. Introduction
The geology of the Betic area (Fig. 1) has been characterized by several orogenic cycles and extensional phases (Puga et al. Reference Puga, Fanning, Díaz de Federico, Nieto, Beccaluva, Bianchini and Díaz Puga2011) ultimately leading to widespread subduction-related and anorogenic volcanism. The last magmatic phase (Pliocene) is represented by Na-alkaline basalts erupted by the volcano of Tallante that entrained and exhumed abundant deep-seated xenoliths of both mantle and crustal provenance, attracting an intense petrological interest (Kogarko et al. Reference Kogarko, Ryabchikov, Brey, Fernández Santin and Pacheco2001; Arai, Shimizu & Gervilla, Reference Arai, Shimizu and Gervilla2003; Beccaluva et al. Reference Beccaluva, Bianchini, Bonadiman, Siena and Vaccaro2004; Rampone et al. Reference Rampone, Vissers, Poggio, Scambelluri and Zanetti2010; Bianchini et al. Reference Bianchini, Beccaluva, Nowell, Pearson and Siena2011). Unfortunately, most of these studies focused on the ultramafic xenoliths ignoring the crustal lithologies that were investigated only by Vielzeuf (Reference Vielzeuf1983). In this contribution we present new data on crustal xenoliths from Tallante that integrate the petrological information provided by the ultramafic parageneses, constraining the lithosphere stratigraphy of the area.
Simplified geological sketch map of the circum-Alboran area, reporting the xenolith sampling site (Tallante) and the location of ultramafic massifs such as Ronda and Beni Bousera.
2. Analytical methods
The investigated xenoliths, 10–15 cm in size, are extremely fresh and do not show evidence of host basalt infiltration. Rock samples were selected from unaltered chips and powdered in an agate mill. Major and trace elements (Ni, Co, Cr, V, Sc, Sr, Ba, Zr, Nb, Th) were analysed by X-ray fluorescence (XRF) on powder pellets, using a wavelength-dispersive automated ARL Advant'X spectrometer at the University of Ferrara. Accuracy and precision for major elements are estimated as better than 3% for Si, Ti, Fe, Ca and K, and 7% for Mg, Al, Mn and Na; for trace elements (above 10 ppm) they are better than 10%. Rare earth elements (REEs) were analysed by inductively coupled plasma mass spectrometry (ICP-MS) at the University of Ferrara, using an X Series Thermo-Scientific spectrometer. Accuracy and precision, based on replicated analyses of samples and standards, are estimated as better than 10% for all elements well above the detection limit. Mineral compositions were obtained at the CNR–IGG Institute of Padova using a Cameca SX 50 electron microprobe, fitted with three wavelength-dispersive spectrometers, using natural silicates and oxides as standards. Strontium isotopic analyses on mineral separates were carried out at the CNR-IGG Institute of Pisa; minerals were leached with hot 6 M HCl, digested with HF-HNO3, then Sr was separated by a conventional chromatographic technique and analyzed using a Finnigan MAT-262 multicollector mass spectrometer.
3. Petrological features of Tallante crustal xenoliths
3.a. Petrography and mineral chemistry
The investigated crustal xenoliths include both mafic and felsic parageneses. The mafic rocks consist of metagabbroids in which the pristine magmatic textures have been partially obliterated by sub-solidus processes; they are meta-norites characterized by unzoned plagioclase and orthopyroxene (Fig. 2), with accessory amounts of olivine microcrystals forming rims around orthopyroxene, and ilmenite, magnetite, chlorine-rich apatite and zircon. Microprobe investigation (online Supplementary Material at http://journals.cambridge.org/geo) indicates that plagioclase is An40–58, whereas orthopyroxene is En58–75. Similar meta-norite parageneses have been recognized in a granulite xenolith suite (entrained in Permian lamprophyres) from Central Spain (Villaseca et al. Reference Villaseca, Orejana, Paterson, Billstrom and Pérez-Soba2007).
Back-scattered scanning electron microprobe images of crustal xenoliths from Tallante; (a) and (b) refer to metagabbroid rocks TL10 and TL381, respectively; (c) and (d) refer to the felsic rock TL380. Abbreviations: Crd – cordierite; Grt – garnet; Ilm – ilmenite; Mag – magnetite; Ol – olivine; Opx – orthopyroxene; Pl – plagioclase; Qtz – quartz; Spl – spinel; Zrn – zircon.
The felsic rocks have quartz-rich parageneses containing green spinel ± garnet ± sillimanite ± feldspars. Generally, cordierite occurs between quartz and spinel, suggesting the reaction spinel + quartz = cordierite. Cordierite also forms symplectites with quartz and spinel (Fig. 2), a microstructure generally interpreted as a pseudomorph after garnet and/or Al2SiO5, which in this case was sillimanite. Accessory phases are rutile, ilmenite and magnetite. Microprobe investigation of feldspars indicates labradorite-to-bytownite composition for plagioclase and subordinate alkali feldspar (Or up to 38). Garnet is Alm60, Py33–34, And4–6; spinel is hercynite, with low ZnO contents (< 0.1 wt%). Since ZnO enlarges the spinel stability field towards lower temperatures (Nichols, Berry & Green, Reference Nichols, Berry and Green1992), the observed low content suggests that the spinel- and quartz-bearing assemblage of the Tallante felsic xenoliths equilibrated at very high-grade conditions. This is consistent with the lack of phyllosilicates in the considered parageneses that suggests a restitic character.
3.b. Bulk-rock geochemistry
The major element composition of the Tallante mafic xenoliths (online Supplementary Material at http://journals.cambridge.org/geo) shows relatively high Al2O3 (up to 23 wt%) and Na2O (up to 6 wt%) suggesting cumulus of plagioclase, as typically observed in subalkaline magma series. The trace element distribution confirms the cumulative nature of the igneous protolith highlighted by positive anomalies in strontium and europium, i.e. elements typically sequestered by plagioclase. A general enrichment in the most incompatible trace elements is observed; in particular very fractionated REE patterns characterized by enrichment in light REEs (LREEs) (LaN/YbN values up to 51) are shown in the chondrite-normalized diagram of Figure 3a. This REE distribution precludes a mid-ocean ridge basalt (MORB) fingerprint for the original magma, and suggests a calcalkaline serial affinity. Consistently, the strontium isotopic composition of plagioclase and pyroxene of metagabbro TL10 is 0.70497 (± 1) and 0.70495 (± 1), respectively; these isotopic values are higher than those expected in MORB magmas and trends towards those typical of metagabbros recorded in the Iberian Variscan belt (Villaseca et al. Reference Villaseca, Orejana, Paterson, Billstrom and Pérez-Soba2007; Andonaegui et al. Reference Andonaegui, Castiñeiras, González Cuadra, Arenas, Sánchez Martínez, Abati, Díaz García and Martínez Catalán2012).
REE distribution of crustal xenoliths from Tallante; patterns of (a) metagabbroid rocks TL10 and TL381 normalized to the chondrite values (McDonough & Sun, Reference McDonough and Sun1995) and (b) felsic rocks TL203 and TL380 normalized to the upper crust values (Taylor & McLennan, Reference Taylor and McLennan1995).
The Tallante felsic xenoliths have high SiO2 (up to 75 wt%) and Al2O3 (up to 20 wt%) in agreement with their quartz- and Al-silicate-rich modal compositions, thus suggesting derivation from sedimentary protoliths (quartz-arenite to greywacke). Unfortunately, the restitic character indicated by the mineral assemblages hampers a more precise identification of the protoliths, because processes of melt extraction variously modified the starting bulk-rock composition with depletion of low solidus components (silica and alkalis) and a relative increase of elements partitioned in residual phases characterized by low melting coefficients. The trace element distribution reveals a depletion of the most incompatible elements such as LREEs and large ion lithophile elements (LILEs). In particular the LREE depletion is emphasized in the ‘continental crust’: normalized patterns on Figure 3b, with LaN/YbN values down to 0.03. These patterns confirm the occurrence of melt extraction during a pressure and temperature (P–T) path that reached the solidus conditions.
3.c. P–T estimates
We used the average P–T method of the THERMOCALC package (Powell & Holland, Reference Powell and Holland1994) to estimate P–T values for the felsic xenoliths. Calculations performed on sample TL380, which contains the quartz–plagioclase–garnet–cordierite–spinel–sillimanite assemblage, give P–T conditions of 0.7 ± 0.1 GPa and 1054 ± 101°C. The P–T quantification for the garnet-free felsic xenoliths is hampered by the lack of a suitable mineral assemblage. Despite that, calculation of the P–T position of the spinel + quartz = cordierite equilibrium for samples TL29 and TL172 yields a pressure of 0.7 GPa at 1050°C. Our P–T estimates are therefore consistent with the available literature data (0.7 GPa/1100°C) on the Tallante felsic xenoliths (Vielzeuf, Reference Vielzeuf1983).
The almost bimineralic mineral assemblage of the mafic xenoliths is not favourable for thermobarometry. Petrography and bulk-rock composition suggests that the mafic xenoliths are cumulates of plagioclase and orthopyroxene from a calcalkaline melt. At pressures of 0.5–0.8 GPa, calcalkaline liquids are saturated with olivine + clinopyroxene + plagioclase + low-Ca pyroxene (Takahashi & Kushiro, Reference Takahashi and Kushiro1983). The lack of olivine and clinopyroxene in the main assemblage might be related to early fractionation of these phases. Olivine-bearing gabbronorites (plagioclase + orthopyroxene + clinopyroxene ± olivine ± amphibole) from pre-Alpine gabbroic intrusion in the Alps such as the Sondalo Gabbroic complex (Braga et al. Reference Braga, Giacomini, Messiga and Tribuzio2001) and the Braccia Gabbro (Hermann, Müntener & Günther, Reference Hermann, Müntener and Günther2001) indicate pressures of intrusion for the parental mantle-derived liquids of 0.6–1.0 GPa. We infer that the Tallante mafic xenoliths might have equilibrated within similar conditions, as also suggested for comparable meta-norite xenoliths from Central Spain (Villaseca et al. Reference Villaseca, Orejana, Paterson, Billstrom and Pérez-Soba2007).
4. Discussion
Several occurrences of xenoliths are known in volcanic districts of the Iberia peninsula. Some of these volcanic centres exclusively provide the exhumation of ultramafic xenoliths (Bianchini et al. Reference Bianchini, Beccaluva, Bonadiman, Nowell, Pearson, Siena and Wilson2007, Reference Bianchini, Beccaluva, Bonadiman, Nowell, Pearson, Siena, Wilson, Coltorti, Downes, Grégoire and O'Reilly2010), whereas other volcanic centres contain only felsic rocks (Ferri et al. Reference Ferri, Burlini, Cesare and Sassi2007; Villaseca et al. Reference Villaseca, Downes, Pin and Barbero1999, Reference Villaseca, Orejana, Paterson, Billstrom and Pérez-Soba2007). The volcano of Tallante is a unique volcanic centre that is characterized by an extremely heterogeneous xenolith association, including ultramafic mantle rocks and diverse crustal lithologies.
Geothermobarometry, based on equilibrium thermodynamics of balanced reactions between coexisting minerals, indicates for the crustal felsic xenoliths P–T conditions overlapping (at 0.7–0.8 GPa) those recorded by the ultramafic xenoliths of mantle provenance (spinel–plagioclase peridotites) equilibrated at 0.7–0.9 GPa/940–1030°C (Bianchini et al. Reference Bianchini, Beccaluva, Nowell, Pearson and Siena2011). More detailed and accurate P–T investigations on mantle xenoliths from Tallante were provided by Kogarko et al. (Reference Kogarko, Ryabchikov, Brey, Fernández Santin and Pacheco2001) who implemented calculations considering core/rim major element heterogeneities of pyroxene crystals in terms of diffusion profiles, suggesting that some xenoliths reflect mantle domains equilibrated at an extremely shallow level (15 km depth, or even less), conforming to the P–T conditions estimated for the crustal xenoliths. Therefore, we propose that an intimate association of crust and mantle lithologies (interlayering at a metric to hectometric scale) characterizes the crust–mantle boundary (CMB) in this area. The proposed crust–mantle association conforms to the field evidence provided by the neighbouring massifs of Ronda and Beni Bousera where the exhumed fossil CMB is characterized by mylonites (Thompson Lundeen, Reference Thompson Lundeen1978; Van der Wal & Vissers, Reference Van der Wal and Vissers1996; Tubía, Cuevas & Esteban, Reference Tubía, Cuevas and Esteban2004; Morishita et al. Reference Morishita, Arai, Ishida, Tamura and Gervilla2009 and references therein). These mylonitic domains could reflect deep trans-lithospheric shear zones (Afiri et al. Reference Afiri, Gueydan, Pitra, Essaifi and Précigout2011; Vauchez, Tommasi & Mainprice, Reference Vauchez, Tommasi and Mainprice2012) that favour inter-fingering/juxtaposition of distinct lithologies. In these interlayered CMB boundaries, partial melting due to adiabatic decompression during extensional phases preferentially involved crustal domains typically characterized by lower solidus conditions. This view is consistent with the petrological feature of the felsic xenoliths indicating a restitic character, due to melt extraction that mobilized H2O-rich fluids and incompatible elements. Anatexis was triggered by the destabilization of phyllosilicates (common minerals in meta-sedimentary rocks) that are never recorded in the studied parageneses. Partial melting possibly occurred as result of multiple episodes, and the remaining residua re-equilibrated at UHT (ultra-high temperature) conditions. Melting – although to a lesser extent – also affected the metagabbro domains in which the incipient process is revealed by a peritectic reaction due to incongruent melting of orthopyroxene (Opx → olivine + melt). The resulting crustal melts, characterized by silica-oversaturation, escaped and migrated from the source region, veined the surrounding peridotite domains and also induced an orthopyroxene-rich metasomatic aureole, as observed in some mantle xenoliths from the same locality (Arai, Shimizu & Gervilla, Reference Arai, Shimizu and Gervilla2003; Beccaluva et al. Reference Beccaluva, Bianchini, Bonadiman, Siena and Vaccaro2004; Rampone et al. Reference Rampone, Vissers, Poggio, Scambelluri and Zanetti2010; Bianchini et al. Reference Bianchini, Beccaluva, Nowell, Pearson and Siena2011).
5. Conclusion
The reported study on deep-seated xenoliths, synthesized in the cartoon of Figure 4, suggests the existence of a complex interlayered CMB beneath the Betic Cordillera. The hypothesis is supported by a series of seismic profiles that specifically investigated the Betic area, highlighting the occurrence of heterogeneous seismic velocities beneath the CMB that in the area is placed at the depth of 22–23 km (De Larouzière et al. Reference De Larouzière, Bolze, Bordet, Hernandez, Montenat and Ott d'Estevou1988), which corresponds to a pressure of c. 0.7 GPa assuming a crustal density of 3 g cm−3.
Schematic cartoons depicting (a) the depth of provenance of crustal and mantle xenoliths from Tallante; (b) details of the inferred interfingered crust–mantle boundary.
These features may be related to orogenic processes leading to the lateral juxtaposition of crustal and mantle rocks, as observed in the neighbouring massifs of Ronda and Beni Bousera. It has to be noted that interlayered crust–mantle associations are widespread throughout the peri-Mediterranean realm. For example, fossil deep crust–mantle sections such as those occurring in Ivrea-Verbano (Quick, Sinigoi & Mayer, Reference Quick, Sinigoi and Mayer1995), the Ulten Zone (Braga & Massonne, Reference Braga and Massonne2012) and central Calabria (Rizzo, Piluso & Morten, Reference Rizzo, Piluso and Morten2001) reveal analogies with the CMB beneath the Betic Cordillera. In the Ulten and Calabria zones, the field relations between contrasting lithologies show discontinuous limits delineating metric to hectometric ultramafic lenses and slices within dominant migmatitic gneisses. In the Ivrea-Verbano zone, on the other hand, prevalent (meta)gabbroic lithologies contain granulite-facies restitic metasedimentary rocks and kilometric peridotite bodies that show ductile deformation. Moreover, the indentation of crust- and mantle-derived rocks is also a current-day feature beneath the central-eastern Alps (Scarascia & Cassinis, Reference Scarascia and Cassinis1997), where granulite-facies metagabbros merge with upper mantle peridotites (Musacchio et al. Reference Musacchio, Zappone, Cassinis and Scarascia1998).
As suggested by field evidence, as well as by analogue and numerical experiments, indentation of mantle and crustal rocks can occur in supra-subduction settings where dynamics of the down-going slab and/or fluid releases trigger mantle diapirism (Brueckner, Reference Brueckner1998; Gerya & Yuen, Reference Gerya and Yuen2003; Gorczyk et al. Reference Gorczyk, Gerya, Connolly and Yuen2007; Castro & Gerya, Reference Castro and Gerya2008; Beccaluva et al. Reference Beccaluva, Bianchini, Natali and Siena2011), during continental collision and subsequent delamination of the thickened lithosphere (Tubía, Cuevas & Esteban, Reference Tubía, Cuevas and Esteban2004) or in post-collisional settings (Harris, Godin & Yakymchuk, Reference Harris, Godin and Yakymchuk2012). Slices of mantle rocks within the crust possibly occur in concomitance with trans-lithospheric shear processes (Vauchez, Tommasi & Mainprice, Reference Vauchez, Tommasi and Mainprice2012) and are also testified by recent geophysical evidence that often highlights CMB offsets in several geological frameworks (Bastow et al. Reference Bastow, Owens, Helffrich and Knapp2007; Cook et al. Reference Cook, White, Jones, Eaton, Hall and Clowes2010). A similar scenario conforms to the worldwide review of xenolith studies from non-cratonic regions provided by O'Reilly & Griffin (Reference O'Reilly and Griffin2013). These authors coherently indicate a gradational nature of the CMB, which is often characterized by peridotites and granulites that are interlayered over depths ranging from a few kilometres to tens of kilometres.
In the CMB transitional domains, silica-rich melts formed within the crustal domains segregate from their sources and interact with the surrounding mantle domains, thus forming further heterogeneities. The analogous process is observed at hand-specimen scale in some composite xenoliths from Tallante in which the peridotite matrix is cross-cut by felsic veins (e.g. Arai, Shimizu & Gervilla, Reference Arai, Shimizu and Gervilla2003; Beccaluva et al. Reference Beccaluva, Bianchini, Bonadiman, Siena and Vaccaro2004; Rampone et al. Reference Rampone, Vissers, Poggio, Scambelluri and Zanetti2010; Bianchini et al. Reference Bianchini, Beccaluva, Nowell, Pearson and Siena2011).
We conclude by emphasizing that an eteropic CMB would represent a suitable source region for exotic magma types such as lamproites. These magmas are mantle-derived ultrapotassic magmas characterized by silica-oversaturation and by trace element and Sr–Nd–Pb isotopic signatures that suggest pervasive recycling of continental crust components in their mantle sources (Conticelli et al. Reference Conticelli, Guarnieri, Farinelli, Mattei, Avanzinelli, Bianchini, Boari, Tommasini, Tiepolo, Prelević and Venturelli2009; Tommasini, Avanzinelli & Conticelli, Reference Tommasini, Avanzinelli and Conticelli2011; Prelević, Jacob & Foley, Reference Prelevic, Jacob and Foley2013). It is noteworthy that these exotic melts, also referred to as Mediterranean lamproites or Tethyan lamproites, occur within polycyclic orogenic belts, where mantle sources are affected by multiple metasomatic processes and possibly by interlayering with crustal lithologies.
Acknowledgements
The authors gratefully acknowledge Michel Grégoire and Dejan Prelević for their constructive criticisms and the Editor Phil Leat for his final comments. Moreover, the authors thank the analytical support and supervision provided by R. Tassinari (XRF and ICP-MS analyses) and R. Carampin (electron microprobe analyses).
