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Geol. Mag. 150 (5 ), 2013, pp. 952–958. c Cambridge University Press 2013 952 doi:10.1017/S001675681300040X RAPID COMMUNICATION Crustal xenoliths from Tallante (Betic Cordillera, Spain): insights into the crust–mantle boundary GIANLUCA BIANCHINI , ROBERTO BRAGA & ANTONIO LANGONE Dipartimento di Fisica e Scienze della Terra, Università di Ferrara – Via Saragat 1, I44100 Ferrara, Italia Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Bologna – Piazza di Porta S. Donato 1, I40126 Bologna, Italia Istituto CNR di Geoscienze e Georisorse (IGG), Via Ferrata 1, I27100 Pavia, Italia (Received 11 February 2013; accepted 26 April 2013; first published online 6 June 2013) Abstract The volcano of Tallante (Pliocene) in the Betic Cordillera (Spain) exhumed a heterogeneous xenolith association, including ultramafic mantle rocks and diverse crustal lithologies. The latter include metagabbroids and felsic rocks characterized by quartz-rich parageneses containing spinel ± garnet ± sillimanite ± feldspars. Pressure–temperature estimates for felsic xenoliths overlap (at 0.7–0.8 GPa) those recorded by the mantle-derived peridotite xenoliths. Therefore, we propose that an intimate association of interlayered crust and mantle lithologies characterizes the crust–mantle boundary in this area. This scenario conforms to evidence provided by the neighbouring massifs of Ronda and Beni Bousera (and by other peri-Mediterranean deep crust/mantle sections) where exhumation of fossil crust– mantle boundary reveals that this boundary is not sharp. The results are discussed on the basis of recent geophysical and petrological studies emphasizing that in non-cratonic regions the crust–mantle boundary is often characterized by a gradational nature showing inter-fingering of heterogeneous lithologies. Silica-rich melts formed within the crustal domains intruded the surrounding mantle and induced metasomatism. The resulting hybrid crust–mantle domains thus provide suitable sources for exotic magma types such as the Mediterranean lamproites. Keywords: Tallante volcanism, Betic Cordillera, heterogen- eous xenolith association, crust–mantle boundary. 1. Introduction The geology of the Betic area (Fig. 1) has been characterized by several orogenic cycles and extensional phases (Puga et al. 2011) 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. 2001; Arai, Shimizu & Gervilla, 2003; Beccaluva et al. 2004; Rampone et al. 2010; Bianchini et al. 2011). Unfortunately, most of these studies focused on the ultramafic xenoliths ignoring the crustal lithologies that were investigated only by Vielzeuf (1983). In this contribution we present new Author for correspondence: [email protected] data on crustal xenoliths from Tallante that integrate the petrological information provided by the ultramafic parageneses, constraining the lithosphere stratigraphy of the area. 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-HNO 3 , 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 metagab- broids 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

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Page 1: Crustal xenoliths from Tallante (Betic Cordillera, Spain) insights into the crust–mantle boundary 13_Bianchini

Geol. Mag. 150 (5 ), 2013, pp. 952–958. c© Cambridge University Press 2013 952doi:10.1017/S001675681300040X

R A P I D C O M M U N I C AT I O N

Crustal xenoliths from Tallante (Betic Cordillera, Spain): insightsinto the crust–mantle boundary

G I A N L U C A B I A N C H I N I∗†, RO B E RTO B R AG A‡ & A N TO N I O L A N G O N E¶∗Dipartimento di Fisica e Scienze della Terra, Università di Ferrara – Via Saragat 1, I44100 Ferrara, Italia

‡Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Bologna – Piazza di Porta S. Donato 1,I40126 Bologna, Italia

¶Istituto CNR di Geoscienze e Georisorse (IGG), Via Ferrata 1, I27100 Pavia, Italia

(Received 11 February 2013; accepted 26 April 2013; first published online 6 June 2013)

Abstract

The volcano of Tallante (Pliocene) in the Betic Cordillera(Spain) exhumed a heterogeneous xenolith association,including ultramafic mantle rocks and diverse crustallithologies. The latter include metagabbroids and felsic rockscharacterized by quartz-rich parageneses containing spinel± garnet ± sillimanite ± feldspars. Pressure–temperatureestimates for felsic xenoliths overlap (at 0.7–0.8 GPa)those recorded by the mantle-derived peridotite xenoliths.Therefore, we propose that an intimate association ofinterlayered crust and mantle lithologies characterizes thecrust–mantle boundary in this area. This scenario conformsto evidence provided by the neighbouring massifs of Rondaand Beni Bousera (and by other peri-Mediterranean deepcrust/mantle sections) where exhumation of fossil crust–mantle boundary reveals that this boundary is not sharp.The results are discussed on the basis of recent geophysicaland petrological studies emphasizing that in non-cratonicregions the crust–mantle boundary is often characterized by agradational nature showing inter-fingering of heterogeneouslithologies. Silica-rich melts formed within the crustaldomains intruded the surrounding mantle and inducedmetasomatism. The resulting hybrid crust–mantle domainsthus provide suitable sources for exotic magma types such asthe Mediterranean lamproites.

Keywords: Tallante volcanism, Betic Cordillera, heterogen-eous xenolith association, crust–mantle boundary.

1. Introduction

The geology of the Betic area (Fig. 1) has been characterizedby several orogenic cycles and extensional phases (Pugaet al. 2011) ultimately leading to widespread subduction-related and anorogenic volcanism. The last magmatic phase(Pliocene) is represented by Na-alkaline basalts erupted bythe volcano of Tallante that entrained and exhumed abundantdeep-seated xenoliths of both mantle and crustal provenance,attracting an intense petrological interest (Kogarko et al.2001; Arai, Shimizu & Gervilla, 2003; Beccaluva et al. 2004;Rampone et al. 2010; Bianchini et al. 2011). Unfortunately,most of these studies focused on the ultramafic xenolithsignoring the crustal lithologies that were investigated onlyby Vielzeuf (1983). In this contribution we present new

†Author for correspondence: [email protected]

data on crustal xenoliths from Tallante that integratethe petrological information provided by the ultramaficparageneses, constraining the lithosphere stratigraphy of thearea.

2. Analytical methods

The investigated xenoliths, 10–15 cm in size, are extremelyfresh and do not show evidence of host basalt infiltration.Rock samples were selected from unaltered chips andpowdered in an agate mill. Major and trace elements (Ni,Co, Cr, V, Sc, Sr, Ba, Zr, Nb, Th) were analysed by X-rayfluorescence (XRF) on powder pellets, using a wavelength-dispersive automated ARL Advant’X spectrometer at theUniversity of Ferrara. Accuracy and precision for majorelements are estimated as better than 3 % for Si, Ti, Fe,Ca and K, and 7 % for Mg, Al, Mn and Na; for traceelements (above 10 ppm) they are better than 10 %. Rareearth elements (REEs) were analysed by inductively coupledplasma mass spectrometry (ICP-MS) at the University ofFerrara, using an X Series Thermo-Scientific spectrometer.Accuracy and precision, based on replicated analyses ofsamples and standards, are estimated as better than 10 %for all elements well above the detection limit. Mineralcompositions were obtained at the CNR–IGG Instituteof Padova using a Cameca SX 50 electron microprobe,fitted with three wavelength-dispersive spectrometers, usingnatural silicates and oxides as standards. Strontium isotopicanalyses on mineral separates were carried out at the CNR-IGG Institute of Pisa; minerals were leached with hot 6 MHCl, digested with HF-HNO3, then Sr was separated by aconventional chromatographic technique and analyzed usinga 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 andfelsic parageneses. The mafic rocks consist of metagab-broids in which the pristine magmatic textures havebeen partially obliterated by sub-solidus processes; theyare meta-norites characterized by unzoned plagioclaseand orthopyroxene (Fig. 2), with accessory amounts ofolivine microcrystals forming rims around orthopyroxene,and ilmenite, magnetite, chlorine-rich apatite and zircon.Microprobe investigation (online Supplementary Material at

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Figure 1. Simplified geological sketch map of the circum-Alboran area, reporting the xenolith sampling site (Tallante) and the locationof ultramafic massifs such as Ronda and Beni Bousera.

http://journals.cambridge.org/geo) indicates that plagioclaseis An40–58, whereas orthopyroxene is En58–75. Similar meta-norite parageneses have been recognized in a granulitexenolith suite (entrained in Permian lamprophyres) fromCentral Spain (Villaseca et al. 2007).

The felsic rocks have quartz-rich parageneses containinggreen spinel ± garnet ± sillimanite ± feldspars. Generally,cordierite occurs between quartz and spinel, suggesting thereaction spinel + quartz = cordierite. Cordierite also formssymplectites with quartz and spinel (Fig. 2), a microstructuregenerally interpreted as a pseudomorph after garnet and/orAl2SiO5, which in this case was sillimanite. Accessory phasesare rutile, ilmenite and magnetite. Microprobe investigationof feldspars indicates labradorite-to-bytownite compositionfor plagioclase and subordinate alkali feldspar (Or up to 38).Garnet is Alm60, Py33–34, And4–6; spinel is hercynite, with lowZnO contents (< 0.1 wt %). Since ZnO enlarges the spinelstability field towards lower temperatures (Nichols, Berry& Green, 1992), the observed low content suggests that thespinel- and quartz-bearing assemblage of the Tallante felsicxenoliths equilibrated at very high-grade conditions. This isconsistent with the lack of phyllosilicates in the consideredparageneses that suggests a restitic character.

3.b. Bulk-rock geochemistry

The major element composition of the Tallante maficxenoliths (online Supplementary Material at http://journals.cambridge.org/geo) shows relatively high Al2O3

(up to 23 wt %) and Na2O (up to 6 wt %) suggestingcumulus of plagioclase, as typically observed in subalkalinemagma series. The trace element distribution confirms thecumulative nature of the igneous protolith highlighted bypositive anomalies in strontium and europium, i.e. elementstypically sequestered by plagioclase. A general enrichmentin the most incompatible trace elements is observed; inparticular very fractionated REE patterns characterized byenrichment in light REEs (LREEs) (LaN/YbN values upto 51) are shown in the chondrite-normalized diagram ofFigure 3a. This REE distribution precludes a mid-oceanridge basalt (MORB) fingerprint for the original magma,and suggests a calcalkaline serial affinity. Consistently, thestrontium isotopic composition of plagioclase and pyroxeneof metagabbro TL10 is 0.70497 (± 1) and 0.70495 (± 1),respectively; these isotopic values are higher than thoseexpected in MORB magmas and trends towards thosetypical of metagabbros recorded in the Iberian Variscan belt(Villaseca et al. 2007; Andonaegui et al. 2012).

The Tallante felsic xenoliths have high SiO2 (up to75 wt %) and Al2O3 (up to 20 wt %) in agreement withtheir quartz- and Al-silicate-rich modal compositions, thussuggesting derivation from sedimentary protoliths (quartz-arenite to greywacke). Unfortunately, the restitic characterindicated by the mineral assemblages hampers a moreprecise identification of the protoliths, because processesof melt extraction variously modified the starting bulk-rockcomposition with depletion of low solidus components (silicaand alkalis) and a relative increase of elements partitioned

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Figure 2. Back-scattered scanning electron microprobe images of crustal xenoliths from Tallante; (a) and (b) refer to metagabbroidrocks 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.

in residual phases characterized by low melting coefficients.The trace element distribution reveals a depletion of the mostincompatible elements such as LREEs and large ion litho-phile elements (LILEs). In particular the LREE depletion isemphasized in the ‘continental crust’: normalized patternson Figure 3b, with LaN/YbN values down to 0.03. Thesepatterns confirm the occurrence of melt extraction during apressure and temperature (P–T) path that reached the solidusconditions.

3.c. P–T estimates

We used the average P–T method of the THERMOCALCpackage (Powell & Holland, 1994) to estimate P–T val-ues for the felsic xenoliths. Calculations performed onsample TL380, which contains the quartz–plagioclase–garnet–cordierite–spinel–sillimanite assemblage, give P–Tconditions of 0.7 ± 0.1 GPa and 1054 ± 101 ◦C. The P–Tquantification for the garnet-free felsic xenoliths is hamperedby 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 apressure of 0.7 GPa at 1050 ◦C. Our P–T estimates are there-fore consistent with the available literature data (0.7 GPa /1100 ◦C) on the Tallante felsic xenoliths (Vielzeuf, 1983).

The almost bimineralic mineral assemblage of the maficxenoliths is not favourable for thermobarometry. Petrographyand bulk-rock composition suggests that the mafic xenoliths

are cumulates of plagioclase and orthopyroxene from acalcalkaline melt. At pressures of 0.5–0.8 GPa, calcalkalineliquids are saturated with olivine + clinopyroxene +plagioclase + low-Ca pyroxene (Takahashi & Kushiro,1983). The lack of olivine and clinopyroxene in the mainassemblage might be related to early fractionation ofthese phases. Olivine-bearing gabbronorites (plagioclase +orthopyroxene + clinopyroxene ± olivine ± amphibole)from pre-Alpine gabbroic intrusion in the Alps such asthe Sondalo Gabbroic complex (Braga et al. 2001) andthe Braccia Gabbro (Hermann, Müntener & Günther, 2001)indicate pressures of intrusion for the parental mantle-derivedliquids of 0.6–1.0 GPa. We infer that the Tallante maficxenoliths might have equilibrated within similar conditions,as also suggested for comparable meta-norite xenoliths fromCentral Spain (Villaseca et al. 2007).

4. Discussion

Several occurrences of xenoliths are known in volcanicdistricts of the Iberia peninsula. Some of these volcaniccentres exclusively provide the exhumation of ultramaficxenoliths (Bianchini et al. 2007, 2010), whereas othervolcanic centres contain only felsic rocks (Ferri et al. 2007;Villaseca et al. 1999, 2007). The volcano of Tallante is aunique volcanic centre that is characterized by an extremelyheterogeneous xenolith association, including ultramaficmantle rocks and diverse crustal lithologies.

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Figure 3. REE distribution of crustal xenoliths from Tallante;patterns of (a) metagabbroid rocks TL10 and TL381 normalizedto the chondrite values (McDonough & Sun, 1995) and (b) felsicrocks TL203 and TL380 normalized to the upper crust values(Taylor & McLennan, 1995).

Geothermobarometry, based on equilibrium thermody-namics of balanced reactions between coexisting miner-als, indicates for the crustal felsic xenoliths P–T con-ditions overlapping (at 0.7–0.8 GPa) those recorded bythe ultramafic xenoliths of mantle provenance (spinel–plagioclase peridotites) equilibrated at 0.7–0.9 GPa/940–1030 ◦C (Bianchini et al. 2011). More detailed and accurateP–T investigations on mantle xenoliths from Tallante wereprovided by Kogarko et al. (2001) who implemented calcula-tions considering core/rim major element heterogeneities ofpyroxene crystals in terms of diffusion profiles, suggestingthat some xenoliths reflect mantle domains equilibrated atan extremely shallow level (15 km depth, or even less),conforming to the P–T conditions estimated for the crustalxenoliths. Therefore, we propose that an intimate associationof crust and mantle lithologies (interlayering at a metric tohectometric scale) characterizes the crust–mantle boundary(CMB) in this area. The proposed crust–mantle associationconforms to the field evidence provided by the neighbouringmassifs of Ronda and Beni Bousera where the exhumed fossilCMB is characterized by mylonites (Thompson Lundeen,1978; Van der Wal & Vissers, 1996; Tubía, Cuevas & Esteban,2004; Morishita et al. 2009 and references therein). Thesemylonitic domains could reflect deep trans-lithospheric shearzones (Afiri et al. 2011; Vauchez, Tommasi & Mainprice,2012) that favour inter-fingering/juxtaposition of distinctlithologies. In these interlayered CMB boundaries, partial

Figure 4. Schematic cartoons depicting (a) the depth ofprovenance of crustal and mantle xenoliths from Tallante; (b)details of the inferred interfingered crust–mantle boundary.

melting due to adiabatic decompression during extensionalphases preferentially involved crustal domains typically char-acterized by lower solidus conditions. This view is consistentwith the petrological feature of the felsic xenoliths indicatinga restitic character, due to melt extraction that mobilizedH2O-rich fluids and incompatible elements. Anatexis wastriggered by the destabilization of phyllosilicates (commonminerals in meta-sedimentary rocks) that are never recordedin the studied parageneses. Partial melting possibly occurredas result of multiple episodes, and the remaining residuare-equilibrated at UHT (ultra-high temperature) conditions.Melting – although to a lesser extent – also affected themetagabbro domains in which the incipient process isrevealed by a peritectic reaction due to incongruent meltingof orthopyroxene (Opx → olivine + melt). The resultingcrustal melts, characterized by silica-oversaturation, escapedand migrated from the source region, veined the surroundingperidotite domains and also induced an orthopyroxene-richmetasomatic aureole, as observed in some mantle xenolithsfrom the same locality (Arai, Shimizu & Gervilla, 2003;Beccaluva et al. 2004; Rampone et al. 2010; Bianchini et al.2011).

5. Conclusion

The reported study on deep-seated xenoliths, synthesizedin the cartoon of Figure 4, suggests the existence of a

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956 R A P I D C O M M U N I C AT I O N

complex interlayered CMB beneath the Betic Cordillera.The hypothesis is supported by a series of seismic profilesthat specifically investigated the Betic area, highlightingthe occurrence of heterogeneous seismic velocities beneaththe CMB that in the area is placed at the depth of 22–23 km (De Larouzière et al. 1988), which corresponds toa pressure of c. 0.7 GPa assuming a crustal density of3 g cm−3.

These features may be related to orogenic processesleading to the lateral juxtaposition of crustal and mantlerocks, as observed in the neighbouring massifs of Rondaand Beni Bousera. It has to be noted that interlayeredcrust–mantle associations are widespread throughout theperi-Mediterranean realm. For example, fossil deep crust–mantle sections such as those occurring in Ivrea-Verbano(Quick, Sinigoi & Mayer, 1995), the Ulten Zone (Braga& Massonne, 2012) and central Calabria (Rizzo, Piluso &Morten, 2001) reveal analogies with the CMB beneath theBetic Cordillera. In the Ulten and Calabria zones, the fieldrelations between contrasting lithologies show discontinuouslimits delineating metric to hectometric ultramafic lenses andslices within dominant migmatitic gneisses. In the Ivrea-Verbano zone, on the other hand, prevalent (meta)gabbroiclithologies contain granulite-facies restitic metasedimentaryrocks and kilometric peridotite bodies that show ductiledeformation. Moreover, the indentation of crust- and mantle-derived rocks is also a current-day feature beneath the central-eastern Alps (Scarascia & Cassinis, 1997), where granulite-facies metagabbros merge with upper mantle peridotites(Musacchio et al. 1998).

As suggested by field evidence, as well as by analogueand numerical experiments, indentation of mantle and crustalrocks can occur in supra-subduction settings where dynamicsof the down-going slab and/or fluid releases trigger mantlediapirism (Brueckner, 1998; Gerya & Yuen, 2003; Gorczyket al. 2007; Castro & Gerya, 2008; Beccaluva et al. 2011),during continental collision and subsequent delamination ofthe thickened lithosphere (Tubía, Cuevas & Esteban, 2004)or in post-collisional settings (Harris, Godin & Yakymchuk,2012). Slices of mantle rocks within the crust possibly occurin concomitance with trans-lithospheric shear processes(Vauchez, Tommasi & Mainprice, 2012) and are also testifiedby recent geophysical evidence that often highlights CMBoffsets in several geological frameworks (Bastow et al.2007; Cook et al. 2010). A similar scenario conforms tothe worldwide review of xenolith studies from non-cratonicregions provided by O’Reilly & Griffin (2013). These authorscoherently indicate a gradational nature of the CMB, whichis often characterized by peridotites and granulites that areinterlayered over depths ranging from a few kilometres totens of kilometres.

In the CMB transitional domains, silica-rich melts formedwithin the crustal domains segregate from their sources andinteract with the surrounding mantle domains, thus formingfurther heterogeneities. The analogous process is observedat hand-specimen scale in some composite xenoliths fromTallante in which the peridotite matrix is cross-cut by felsicveins (e.g. Arai, Shimizu & Gervilla, 2003; Beccaluva et al.2004; Rampone et al. 2010; Bianchini et al. 2011).

We conclude by emphasizing that an eteropic CMBwould represent a suitable source region for exotic magmatypes such as lamproites. These magmas are mantle-derivedultrapotassic magmas characterized by silica-oversaturationand by trace element and Sr–Nd–Pb isotopic signatures thatsuggest pervasive recycling of continental crust componentsin their mantle sources (Conticelli et al. 2009; Tommasini,Avanzinelli & Conticelli, 2011; Prelevic, Jacob & Foley,2013). It is noteworthy that these exotic melts, also referred

to as Mediterranean lamproites or Tethyan lamproites, occurwithin polycyclic orogenic belts, where mantle sources areaffected by multiple metasomatic processes and possibly byinterlayering with crustal lithologies.

Acknowledgements. The authors gratefully acknowledgeMichel Grégoire and Dejan Prelevic for their constructivecriticisms and the Editor Phil Leat for his final comments.Moreover, the authors thank the analytical support andsupervision provided by R. Tassinari (XRF and ICP-MS analyses) and R. Carampin (electron microprobeanalyses).

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