Geol. Mag. 150 (5 ), 2013, pp. 952958. c Cambridge University Press 2013 952doi:10.1017/S001675681300040X
Crustal xenoliths from Tallante (Betic Cordillera, Spain): insightsinto the crustmantle 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 EDipartimento di Fisica e Scienze della Terra, Universit di Ferrara Via Saragat 1, I44100 Ferrara, Italia
Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Universit 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)
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. Pressuretemperatureestimates for felsic xenoliths overlap (at 0.70.8 GPa)those recorded by the mantle-derived peridotite xenoliths.Therefore, we propose that an intimate association ofinterlayered crust and mantle lithologies characterizes thecrustmantle 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 crustmantle 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 crustmantle 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 crustmantle domainsthus provide suitable sources for exotic magma types such asthe Mediterranean lamproites.
Keywords: Tallante volcanism, Betic Cordillera, heterogen-eous xenolith association, crustmantle boundary.
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
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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, 1015 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 AdvantX 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 CNRIGG 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 An4058, whereas orthopyroxene is En5875. 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, Py3334, And46; 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 (PT) path that reached the solidusconditions.
3.c. PT estimates
We used the average PT method of the THERMOCALCpackage (Powell & Holland, 1994) to estimate PT val-ues for the felsic xenoliths. Calculations performed onsample TL380, which contains the quartzplagioclasegarnetcordieritespinelsillimanite assemblage, give PTconditions of 0.7 0.1 GPa and 1054 101 C. The PTquantification for the garnet-free felsic xenoliths is hamperedby the lack of a suitable mineral assemblage. Despite that,calculation of the PT position of the spinel + quartz =cordierite equilibrium for samples TL29 and TL172 yields apressure of 0.7 GPa at 1050 C. Our PT 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 0.50.8 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, Mntener & Gnther, 2001)indicate pressures of intrusion for the parental mantle-derivedliquids of 0.61.0 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).
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 PT con-ditions overlapping (at 0.70.8 GPa) those recorded bythe ultramafic xenoliths of mantle provenance (spinelplagioclase peridotites) equilibrated at 0.70.9 GPa/9401030 C (Bianchini et al. 2011). More detailed and accuratePT 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 PT conditions estimated for the crustalxenoliths. Therefore, we propose that an intimate associationof crust and mantle lithologies (interlayering at a metric tohectometric scale) characterizes the crustmantle boundary(CMB) in this area. The proposed crustmantle 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; Tuba, 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 crustmantle 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...