Permian granites in the Pyrenees: the Aya pluton (Basque Country) Yoann Dene ` le, 1,3 Jean-Louis Paquette, 2 Philippe Olivier 3 and Pierre Barbey 4 1 Ge ´osciences Montpellier, UMR CNRS 5243, Universite ´ Montpellier II, Place Bataillon, F-34095 Montpellier Cedex 5, France; 2 LMV, Universite ´ Blaise Pascal, CNRS-IRD, 5, rue Kessler, F-63038 Clermont-Ferrand, France; 3 GET-Universite ´ de Toulouse-CNRS-IRD-OMP, 14 Avenue E. Belin, F-31400 Toulouse, France; 4 CRPG-CNRS, Nancy-Universite´, BP 20, F-54501 Vandœuvre-le `s-Nancy, France Introduction Late Stephanian and Permian times in Western Europe correspond to a tran- sitional period between the final stages of the Variscan orogeny and the Alpine cycle that started with the dislocation of the Pangea. An exten- sive rift system developed in the northern foreland of the Variscan belt (e.g. Neumann et al., 2004; Timmer- man, 2004) coevally with a major dextral translation between North Africa and Europe (Ziegler and Clo- etingh, 2003) and in association with widespread magmatism. The Variscan segment of the Pyrenees (Fig. 1) is considered to belong to the southern foreland of the Variscan belt (Barn- olas and Chiron, 1996), although its palaeogeographical position remains largely unknown. Magmatic rocks are represented by calcalkaline and pera- luminous plutons emplaced all along the Pyrenean chain. Their age and context of emplacement were consid- ered, before the 1990s, to be emplaced during the Permian (whole-rock Rb-Sr isochron ages at 270–290 Ma; Vitrac-Michard and Alle` gre, 1975; Michard-Vitrac et al., 1980; Alibert et al., 1988; Bickle et al., 1988; Debon and Zimmermann, 1988; Majoor, 1988; Gibson, 1989) in a distensive tectonic regime (Wickham and Tay- lor, 1985; Wickham and Oxburgh, 1985; van den Eeckhout and Zwart, 1988; Gibson, 1991; Vissers, 1992), whereas they are now interpreted as Westphalian to Stephanian (zircon U-Pb dating at 300–312 Ma) syntec- tonic bodies (Romer and Soler, 1995; Paquette et al., 1997; Roberts et al., 2000; Maurel et al., 2004; Olivier et al., 2004, 2008; Ternet et al., 2004; Gleizes et al., 2006). A widespread Lower to Upper Permian magmatism is recorded in the Pyrenees (Fig. 1), such as the volcanic formations of the Sierra de Cadi, Ossau, Anayet, Panticosa, Cin- co Villas and Rhune massifs (Gisbert, 1981; Marti, 1986; Bixel, 1988; Cabanis and Le Fur-Balouet, 1989; Valero, 1991; Briqueu and Innocent, 1993; Gilbert et al., 1994; Innocent et al., 1994; Barnolas and Chiron, 1996; Debon et al., 1996; Lasheras Adot et al., 1999; Lago et al., 2004; Perini et al., 2004). Nevertheless, no Permian granite pluton coeval with this volcanism was known so far in the Pyrenees. On the occasion of a geo- chronological study of Variscan gran- ites, we dated the Aya massif, the westernmost pluton of the Pyrenees (Fig. 1) that was considered to be chemically distinct from the other Pyrenean plutons (Debon et al., 1996), but not dated. The aim of this article is to reassess from new geo- chronological, structural and geo- chemical data, the characteristics of the Aya pluton, highlight its Saxonian age and replace it in the post-colli- sional geodynamic evolution of the southern foreland of the Variscan belt. Geological setting Two Stephanian–Permian tectono- magmatic cycles showing a transition from calcalkaline to alkaline chemis- try, are identified in the Pyrenees (Bixel, 1988). The first cycle corre- sponds to (i) a bimodal episode with peraluminous dacites ⁄ rhyolites and low-K calcalkaline andesites, well- represented in the Sierra de Cadi massif and in the Midi dÕOssau caul- dron complex, as well, where they were dated at 278 ± 5 and 272 ± 3 Ma (zircon U-Pb; Briqueu and Innocent, 1993); (ii) high-K calcalka- line andesites and dacites probably Autunian in age (Bixel, 1988); and (iii) peraluminous potassic rhyolites, prob- ably Autunian and restricted to the eastern Pyrenees. The second cycle, intrusive in Saxonian red beds, corre- sponds first to transitional-alkaline andesites observed only in the central Pyrenees, and then to alkali basalts occurring as sills and dykes mainly in the Cinco Villas massif. The Aya pluton (c. 13 · 4.5 km) was emplaced in low-grade Devo- nian–Westphalian metasediments of the Cinco Villas massif wherein it developed a 100- to 1000-m-thick contact aureole (Fig. 1). According to the studies of Pesquera (1985), Pesquera and Pons (1990) and Olivier et al. (1999), the Aya pluton (Fig. 2a) ABSTRACT We show that the Permian volcanism in the Pyrenees was accompanied with emplacement of granite plutons with tran- sitional chemistry. Reassessment of age, structure and petro- logical characteristics of the Aya pluton, intrusive in the Cinco Villas massif (Basque Country), indicates that it was emplaced at 267.1 ± 1.1 Ma in a dextral transtensive context and that it marks a transition between the high-K calcalkaline and the alkaline emissions of the first and second Permian volcanic cycles, respectively. This age is close to that of c. 267 Ma reported for lamprophyres dykes of the Pyrenean Axial Zone. These data characterize a period of transition between the Variscan orogeny and the subsequent Alpine cycle in the Pyrenees, with an earlier extensive regime that became widespread in the Pyrenees during the Mesozoic times. Terra Nova, 24, 105–113, 2012 Correspondence: Pierre Barbey, CRPG- CNRS, Nancy-Universite´, BP 20, F-54501 Vandœuvre-le`s-Nancy, France. Tel.: +33 3 83 59 42 34; fax: +33 3 83 51 17 98; email: [email protected]Ó 2011 Blackwell Publishing Ltd 105 doi: 10.1111/j.1365-3121.2011.01043.x
Transcript
Permian granites in the Pyrenees: the Aya pluton (BasqueCountry)
Yoann Denele,1,3 Jean-Louis Paquette,2 Philippe Olivier3 and Pierre Barbey4
1Geosciences Montpellier, UMR CNRS 5243, Universite Montpellier II, Place Bataillon, F-34095 Montpellier Cedex 5, France; 2LMV,
Universite Blaise Pascal, CNRS-IRD, 5, rue Kessler, F-63038 Clermont-Ferrand, France; 3GET-Universite de Toulouse-CNRS-IRD-OMP,
14 Avenue E. Belin, F-31400 Toulouse, France; 4CRPG-CNRS, Nancy-Universite, BP 20, F-54501 Vandœuvre-les-Nancy, France
Introduction
Late Stephanian and Permian times inWestern Europe correspond to a tran-sitional period between the final stagesof the Variscan orogeny and theAlpine cycle that started with thedislocation of the Pangea. An exten-sive rift system developed in thenorthern foreland of the Variscan belt(e.g. Neumann et al., 2004; Timmer-man, 2004) coevally with a majordextral translation between NorthAfrica and Europe (Ziegler and Clo-etingh, 2003) and in association withwidespread magmatism. The Variscansegment of the Pyrenees (Fig. 1) isconsidered to belong to the southernforeland of the Variscan belt (Barn-olas and Chiron, 1996), although itspalaeogeographical position remainslargely unknown. Magmatic rocks arerepresented by calcalkaline and pera-luminous plutons emplaced all alongthe Pyrenean chain. Their age andcontext of emplacement were consid-ered, before the 1990s, to be emplacedduring the Permian (whole-rockRb-Sr isochron ages at 270–290 Ma;Vitrac-Michard and Allegre, 1975;Michard-Vitrac et al., 1980; Alibertet al., 1988; Bickle et al., 1988; Debonand Zimmermann, 1988; Majoor,1988; Gibson, 1989) in a distensive
tectonic regime (Wickham and Tay-lor, 1985; Wickham and Oxburgh,1985; van den Eeckhout and Zwart,1988; Gibson, 1991; Vissers, 1992),whereas they are now interpreted asWestphalian to Stephanian (zirconU-Pb dating at 300–312 Ma) syntec-tonic bodies (Romer and Soler, 1995;Paquette et al., 1997; Roberts et al.,2000; Maurel et al., 2004; Olivier etal., 2004, 2008; Ternet et al., 2004;Gleizes et al., 2006).A widespread Lower to Upper
Permian magmatism is recorded inthe Pyrenees (Fig. 1), such as thevolcanic formations of the Sierra deCadi, Ossau, Anayet, Panticosa, Cin-co Villas and Rhune massifs (Gisbert,1981; Marti, 1986; Bixel, 1988;Cabanis and Le Fur-Balouet, 1989;Valero, 1991; Briqueu and Innocent,1993; Gilbert et al., 1994; Innocentet al., 1994; Barnolas and Chiron,1996; Debon et al., 1996; LasherasAdot et al., 1999; Lago et al., 2004;Perini et al., 2004). Nevertheless, noPermian granite pluton coeval withthis volcanism was known so far in thePyrenees. On the occasion of a geo-chronological study of Variscan gran-ites, we dated the Aya massif, thewesternmost pluton of the Pyrenees(Fig. 1) that was considered to bechemically distinct from the otherPyrenean plutons (Debon et al.,1996), but not dated. The aim of thisarticle is to reassess from new geo-chronological, structural and geo-chemical data, the characteristics ofthe Aya pluton, highlight its Saxonian
age and replace it in the post-colli-sional geodynamic evolution of thesouthern foreland of the Variscan belt.
Geological setting
Two Stephanian–Permian tectono-magmatic cycles showing a transitionfrom calcalkaline to alkaline chemis-try, are identified in the Pyrenees(Bixel, 1988). The first cycle corre-sponds to (i) a bimodal episode withperaluminous dacites ⁄ rhyolites andlow-K calcalkaline andesites, well-represented in the Sierra de Cadimassif and in the Midi d�Ossau caul-dron complex, as well, where theywere dated at 278 ± 5 and 272 ±3 Ma (zircon U-Pb; Briqueu andInnocent, 1993); (ii) high-K calcalka-line andesites and dacites probablyAutunian in age (Bixel, 1988); and (iii)peraluminous potassic rhyolites, prob-ably Autunian and restricted to theeastern Pyrenees. The second cycle,intrusive in Saxonian red beds, corre-sponds first to transitional-alkalineandesites observed only in the centralPyrenees, and then to alkali basaltsoccurring as sills and dykes mainly inthe Cinco Villas massif.The Aya pluton (c. 13 · 4.5 km)
was emplaced in low-grade Devo-nian–Westphalian metasediments ofthe Cinco Villas massif wherein itdeveloped a 100- to 1000-m-thickcontact aureole (Fig. 1). Accordingto the studies of Pesquera (1985),Pesquera and Pons (1990) and Olivieret al. (1999), the Aya pluton (Fig. 2a)
ABSTRACT
We show that the Permian volcanism in the Pyrenees wasaccompanied with emplacement of granite plutons with tran-sitional chemistry. Reassessment of age, structure and petro-logical characteristics of the Aya pluton, intrusive in the CincoVillas massif (Basque Country), indicates that it was emplacedat 267.1 ± 1.1 Ma in a dextral transtensive context and that itmarks a transition between the high-K calcalkaline and thealkaline emissions of the first and second Permian volcanic
cycles, respectively. This age is close to that of c. 267 Mareported for lamprophyres dykes of the Pyrenean Axial Zone.These data characterize a period of transition between theVariscan orogeny and the subsequent Alpine cycle in thePyrenees, with an earlier extensive regime that becamewidespread in the Pyrenees during the Mesozoic times.
consists of gabbro, diorite, granodio-rite, monzogranite and leucogranitedistributed in two main lithologicalunits:
1 A peripheral unit occurring mainlyin the northern part of the pluton,and consisting of leucocratic, fine-to coarse-grained, locally porphy-ritic biotite–ilmenite syenogranitesand monzogranites, which containlocally mafic enclaves.
2 An inner unit corresponding to amafic–silicic complex and compris-ing fine- to medium-grained gabbro,diorites (augite, hornblende, biotite,ilmenite, with interstitial K-feld-spar), fine- to medium-grained,equigranular and porphyritic gran-odiorites (hornblende, biotite,ilmenite) and granites (biotite,ilmenite); this unit is considered toresult from mingling between tho-leiitic mafic magmas and crust-derived silicic ones.
Structural data
Westphalian metasediments of theCinco Villas massif show a well-defined Variscan Sv schistosity asso-ciated to multi-scale isoclinal folds.Its trajectories show a whole N70�Edirection in the eastern part of themassif (Fig. 2a), that becomesinflected in the western contact aure-ole and to the south of the pluton,with the predominance of N20�Eorientation and the presence ofa well-developed triple point in thesouth-western border of the pluton.
In the contact aureole, anintense folding is associated to asecond generation of schistosity(SP slaty-cleavage) well-developedin its north-western part. Thefold axes, slaty-cleavage and contactzone between the pluton and itscountry rocks present very closeorientations.Gravimetric study and modelling of
the pluton (Olivier et al., 1999) dem-onstrate its relative flat floor and thepresence of a root zone below themost mafic rocks. As a consequencethe Aya plutonic rocks form a slightlyasymmetric laccolith-shaped intru-sion, with a c. N35�E long axis (Fig. 2c), an outlying root zone to the SSWand a major body developed to theNNE. From anisotropy of magneticsusceptibility studies and image anal-ysis, Olivier et al. (1999) showed theexistence of a good correlation be-tween magnetic lineations and folia-tions and sub-magmatic mineral ones.Magnetic foliations are generallyhomogeneously oriented betweenN120�E and N90�E except above theroot zone where NE–SW directionsprevail (Fig. 2b). Dip of foliationplanes is steep to moderate and con-verges to the root zone. In the intru-sion core, lineations are moderatelyplunging and present a mean directionaround N55�E, being slightly obliqueto the elongation direction of theintrusion. By contrast, in the periph-ery of the laccolith, lineations (slightlyto moderately plunging) are perpen-dicular to the contact with the countryrocks.
The total anisotropy parameter(Pp%), which characterizes the inten-sity of the finite strain in paramagneticgranites (e.g. Gleizes et al., 2001;Denele et al., 2008), ranges from 0.4to 3.9, with a mean value at 1.2(Fig. 2b). These values are very lowcompared to those of the large syntec-tonic plutons, as for example the Ca-uterets pluton showing a mean Pp% at3.4 (Gleizes et al., 1998). Such lowvalues imply a low intensity of defor-mation during emplacement of thepluton. Nevertheless, mapping ofPp% shows an overall evolution fromvery low deformation in its core to low-moderate deformation on it borders.
Chemical compositions
Whole-rock major and trace elementcompositions are given in Table 1 andtaken from Pesquera (1985). Rocksamples plot close to the limit betweenhigh-K calcalkaline and shoshoniticseries in a K2O-SiO2 plot (Fig. 3a)and are distinguishable from the otherplutonic bodies of the Variscan seg-ment of the Pyrenees by lower CaOcontent and MgO ⁄MgO + FeOtot.
ratio, but higher TiO2 and K2O con-tents. According to the classificationof Frost and Frost (2011), the granitesamples are defined as ferroan, pera-luminous and alkali-calcic. In Y +Nbvs. Rb or Nb plots (not shown;Whalen et al., 1987), Aya granitesand granodiorites fall in the WPGfield, whereas leucogranites fall in theCOLG field. Compositions are chem-ically close to A-type granites on the
Aya pluton
CV
50 kmCambrian to upper Carboniferous metamorphic rocks
Stephano-Permian basins and volcanic rocks (numbers represent the tectono-magmatic cycles)
Variscan plutons
Aya pluton
N P F
N P F
AtlanticOcean
Mediterranean
Sea
N o r t h - P y r e n e a n T h r u s t
A X I A LZ O N E
Rhune basin
Aragon-Béarn basin
Anayet basin
Sierra de Cadi basin
Ossau volcaniccomplex
France
1
1
1
2
2
Fig. 1 Geological map of the Pyrenees with location of the Aya pluton and of the main Stephano-Permian basins and volcanicrocks (numbers indicate the tectono-magmatic cycle). Redrawn from Barnolas and Chiron (1996).
Permian granites in the Pyrenees • Y. Denele et al. Terra Nova, Vol 24, No. 2, 105–113
Devonian-Westphalianmetasediments with metamorphicisogrades
Cinco-Villas massif
RB
i da so a
Krausse et al. (1972)
Alpine faults
S2
SPOur study
50
37
75
2–4
1–2< 1
Pp%
Magnetic foliationtrajectoriesMagnetic lineation pattern
Schistosity
AAY-3Y-3AY-3 AY-2Y-2AY-2
AY-1Y-1AY-1
10 km5
5 km
10 km
N
4794
4790
596 600 604
4786
Pesquera and Pons (1990)
3 km
Very
low
sub-m
agmatic
deformation
Very
low
sub-m
agmatic
deformation
(UTM kilometric coordinate system)
Fig. 2 (a) Petrographic and structural map of the Aya pluton and its country rocks. Inset: map of the total magnetic anisotropyparameter (Pp%) modified from Olivier et al. (1999), and of the foliation trajectories in the country rocks. (b) Magnetic fabrics inthe Aya pluton (modified from Olivier et al., 1999). (c) Block diagram showing the geometry of the Aya laccolith and relationshipsbetween internal submagmatic fabrics and the regional schistosity in country rocks.
Terra Nova, Vol 24, No. 2, 105–113 Y. Denele et al. • Permian granites in the Pyrenees
basis of 104 Ga ⁄Al ratios > 3,Zr + Nb + Ce + Y > 360 andNb + Y > 47. However, Aya gran-
ites are not strictly A-type, but morelikely transitional, considering theMg-rich chemistry of their ferromag-
nesian minerals (Mg ⁄Mg + Fe ra-tios > 0.7 for augite and >0.5 foramphibole) (Eby, 1990). Multi-ele-ment mantle-normalized plot (Fig. 3f)shows clear Nb-Ta negative anoma-lies. This and the Th ⁄Yb vs. Ta ⁄Ybratios in the gabbros (c. 2.6 vs. 0.36)suggest that the parent magma in-volved a crustal component. The pat-terns of the Aya gabbros also showsimilarities with those of La Rhunebasalts, with the exception of large ionlithophile elements.
87Sr ⁄ 86Sr initial ratios of the gabbroand monzogranite (Table 2) suggestcontamination by a crustal compo-nent, whereas that of the leucogranitesample is anomalously low (0.685) andimprecise due to very high 87Rb ⁄ 86Sr(possible resetting of the Rb-Sr sys-tem). Nd isotopic compositions()3.3 £ eNd £ )8.3) and TDM (1.0–1.5 Ga) are in the same range as theOssau intermediate and silicic lavas(Innocent et al., 1994). These valuesare also reminiscent of those given bythe 280-Ma plutonic rocks from Cor-sica (Paquette et al., 2003) and suggestthe contribution of lower crustal me-tasediments. Innocent et al. (1994)showed that La Rhune basalts haveNd isotopic compositions ()2.1 £eNd £ 0.8; average Sm ⁄Nd = 0.19)characteristic of sub-continental litho-spheric mantle. Their Nd model agesnormalized to CHUR (173–462 Ma)are consistent with their emplacementage (c. 269 Ma). Owing to the simi-larities in age and composition be-tween the Aya gabbros and La Rhunebasalts, it is suggested that the part ofcrustal recycling in the genesis of theAya magmas (TCHUR = 570 Ma forthe gabbro) remained moderate, inagreement with the absence of inher-ited zircons (see below).
Geochronological data
Three samples from the main rock typeswere selected for dating (Fig. 2a). U-Pbisotopic compositions were determinedby ID-TIMS on zircon fractions (Pa-quette and Pin, 2001) for one gabbrosample, and by in situ LA-ICPMS(Laser ablation: Resonetics, Nashua,NH, USA; ICPMS: Agilent, Tokyo,Japan) (Tiepolo, 2003; Paquette andTiepolo, 2007) for two granites samples.U-Pb data (Fig. 4) are available inSupporting Information. Zircon grainsare yellow-coloured, poorly translucent,
Table 1 Major (wt%) and trace element (p.p.m.) compositions of leucogranite,
monzogranite and gabbro samples from the Aya pluton.
Rock type Gab Gab Grd Mzg Lgr
Sample ref. AYA-D2 AYA-1b AY4ter AYA-7 AYA-1a
SiO2 55.68 56.71 57.3 73.2 75.78
Al2O3 14.74 15.58 17.65 13.06 12.04
FeOtot. 8.82 7.81 6.94 2 0.9
MnO 0.14 0.12 0.06 0.03 0.01
MgO 2.94 2.28 2.23 0.36 0.09
CaO 5.33 4.85 0.45 0.49 0.68
Na2O 3.38 3.67 2.68 2.65 3.29
K2O 3.5 3.54 4.86 5.6 4.89
TiO2 2.27 1.88 1.99 0.32 0.11
P2O5 0.8 0.72 0.39 0.08 –
LOI 1.54 1.51 5.18 1.45 0.84
Total 99.16 98.68 99.73 99.23 98.61
Ba 1125 1198 540 273 74
Be 2.6 3.2 3.6 4.5 3.6
Co 18.8 16.1 – 2.9 0.5
Cr 42.6 – – 8.9 42.1
Cs 3.3 3.4 2.8 3 2.7
Cu 36.4 22.9 – – –
Ga 25 25.6 23.7 21.1 24.1
Ge 1.9 1.8 1.6 1.6 2.1
Hf 9.7 9.7 6.1 7.1 5.2
Mo 2.4 1.2 0.8 0.3 2.3
Nb 30.7 29.2 16.7 12.8 18.2
Ni 27 7 – – 20
Pb 114 25 44 28 16
Rb 109 108 219 235 348
Sn 7 7.1 – 3.7 7.7
Sr 482 504 187 70 33
Ta 2.2 2.1 1.4 1.5 3
Th 16.2 14.9 15.1 62.4 51
U 2.7 2.7 16 7 10.3
V 123 94 – 12 2
W 1.7 1.4 – 0.8 7.1
Y 63 63 25 34 113
Zn 200 118 – 32 –
Zr 419 455 262 243 123
La 92.07 90.62 20.24 43.63 56.09
Ce 189 188.6 40.21 102.9 112.8
Pr 23.69 21.35 5.594 10.07 14.52
Nd 89.73 85.61 20.77 34.46 50.49
Sm 16.54 15.77 4.17 6.96 12.53
Eu 2.76 2.78 0.66 0.5 0.01
Gd 13.4 12.86 3.77 5.82 12.54
Tb 2.04 1.95 0.62 0.1 2.52
Dy 11.69 11.29 3.88 6.05 16.92
Ho 2.22 2.12 0.82 1.18 3.53
Er 6.04 6 2.55 3.42 10.67
Tm 0.9 0.897 0.4 0.54 1.69
Yb 5.92 5.81 2.91 3.73 11.32
Lu 0.87 0.87 0.49 0.54 1.63
Whole-rock major and trace elements were analysed by ICP-AES and ICP-MS (CRPG-CNRS, Nancy),
respectively. Sample preparation, analytical conditions and limits of detection are detailed in Carignan et al.
(2001). Gab, gabbro; Grd, granodiorite; Mzg, monzogranite; Lgr, leucogranite. –Below detection limit or not
determined.
Permian granites in the Pyrenees • Y. Denele et al. Terra Nova, Vol 24, No. 2, 105–113
with numerous cracks and inclusions.They are euhedral and moderately elon-gated with a maximum length–widthratio of 3 : 4. In cathodoluminescenceimages, they show a strong oscillatoryzoning with dark domains illustratinghigh trace-element content (i.e. [U] oftenabove 1000 p.p.m.). These high concen-trations occur in the three facies and arerelated to the magma source.
Twenty-nine spots were analysed byLA-ICPMS in the Aya1 leucogranitesample. All spots plotted in a Tera–Wasserburg diagram define a lowerintercept age of 268.3 ± 1.7 Ma,which is statistically equivalent to themean 206Pb ⁄ 238U age of 267.2 ±1.2 Ma (Fig. 4a). No older inheritedcore was identified. Twenty-one spotswere analysed by LA-ICPMS in the
Aya2 monzogranite sample. The re-sults are also concordant to sub-con-cordant and yield a lower interceptcomparable to the mean 206Pb ⁄ 238Uage of 269.3 ± 1.3 Ma (Fig. 4b).Despite repeated analyses in the centreof the grains, no older age was mea-sured. Five multigrain zircon fractionsfrom the gabbro sample were analysedby ID-TIMS. All fractions are con-
2.0
2
1
TiO2
FeOtot.
4.0 6.0 8.0 10.0 12.0
other Pyrenean plutons
Aya pluton
granite
granodiorite
gabbro-diorite
La Rhune basalts
1
2
3
4
5
6
7 K2O
SiO2
50 55 60 65 70 75
Arc tholeiite seriesCalc-alkaline series
High-K calc-alkaline seriesShoshonitic series
(a) (d)
(b)
(c)
1
3
5
7
9 MgO
2.0 4.0 6.0 8.0 10.0 12.0
FeOtot.
1000
100
101104*Ga/Al
Zr
I-type
and
S-type
LFB
WM
(e)
LuLa Tb Dy Ho Er Tm YbCe Pr Nd Sm Eu Gd1
10
100
Aya graniteAya gabbro-diorite
Bordères pluton
La Rhune basalts
Sam
ple/
chon
drite
(f)1
10
100
Aya gabbro-diorite
La Rhune basalts
Sam
ple/
prim
itive
man
tle
YTbTiSmZrRbCs Ba Th U K Ta Nb La Ce Sr Nd P Hf
Fig. 3 Plots of (a) K2O vs. SiO2, (b and c) MgO and TiO2 vs. FeOtot. (wt%), (d) Zr vs. 104 Ga ⁄Al (p.p.m.), (e) REE patternsnormalized to chondrite (Evensen et al., 1978), and (f) multi-element patterns normalized to the primitive mantle (Wood et al.,1979). Data from the Aya pluton (Pesquera, 1985 and this work) are compared to other plutonic bodies of the Variscan segment ofthe Pyrenees (average data from Debon et al., 1996), to La Rhune Permian basalts (Lasheras Adot et al., 1999) and to the fields ofWhite Mountains (WM) and Lachland Fold Belt (LFB) A-type granites (Eby, 1990).
Terra Nova, Vol 24, No. 2, 105–113 Y. Denele et al. • Permian granites in the Pyrenees
cordant and yield a concordant age of266.8 ± 0.4 Ma (Fig. 4c). No inher-ited memory was detected.Owing to the different dating tech-
niques used in this study, it is difficultto establish a relative chronologybetween the three samples. They allthree display similar ages within errorlimits, consequently the crystallizationof the zircon crystals is interpreted asthe emplacement time of the Ayamassif at a mean value of 267.1 ±1.1 Ma. The absence of inherited zir-con crystal or core indicates either thelack of recycled component into themagma source and thus the prevailingparticipation of a juvenile end-mem-ber or, more likely, the fast chemicaldisequilibrium and dissolution ofinherited zircon crystals.
Table 2 Sr-Nd isotopic compositions of gabbro, monzogranite and leucogranite
Isotopic compositions were determined at the CRPG-CNRS (Nancy, France) by MC-ICP-MS (Nd) and TIMS
(Sr). CHUR composition used in the calculations: 143Nd ⁄ 144Nd CHUR = 0.512638, 147Sm ⁄ 144Nd = 0.1967
(decay constant = 6.54 · 10)12). TDM model ages are calculated according to the depleted mantle model of
Ben Othman et al. (1984).
Intercept at 266.8 + 0.4 MaMSWD = 0.022, n = 6
Probability of concordance: 0.86
238U/206Pb23.4 23.6 23.8 24.0 24.2
0.05163
0.05160
0.05157
0.05154
AYA plutondiorite
268
269
266266
207 P
b/20
6 Pb
300 290 280250 240
207 P
b/20
6 Pb
238U/206Pb
2321 250.049
0.053
0.057
0.061
AYA plutonleucogranite
Lower intercept at 268.3 + 1.7 Ma
MSWD = 0.62, n = 29
Mean 206Pb/238U age: 267.2 + 1.2 Ma
20 µm
207 P
b/20
6 Pb
238U/206Pb21 22 23 24 25 26
0.050
0.052
0.054
0.056
0.058
AYA plutonmonzogranite
300290 280
250 240
50 µm
Lower intercept at 269.4 + 2.3 Ma
MSWD = 0.98, n = 19
Mean 206Pb/238U age: 269.3 + 1.3 Ma
(a) (b)
(c)
Fig. 4 Tera–Wasserburg (1972) diagrams for the three main rock types of the Aya pluton: (a) leucocratic, coarse-grained granite;(b) medium-grained porphyritic granodiorite; (c) fine-grained gabbroic enclave. Data point error ellipses are 2r. In (a) and (b) areshown cathodoluminescence images of zircon showing their strong oscillatory zoning.
Permian granites in the Pyrenees • Y. Denele et al. Terra Nova, Vol 24, No. 2, 105–113
U-Pb ages, with a mean at 267.1 ±1.1 Ma, show that the Aya plutonemplaced in Saxonian times duringinfilling of Permian basins by conti-nental detrital and volcanic deposits.This invalidates the previous modelsof syn-collisional emplacement of theAya pluton during the late D2 Vari-scan transpressive stage (Olivier et al.,1999), coeval with the emplacement ofthe large calcalkaline plutons (300–312 Ma). Moreover, the Aya pluton isdevoid of N110�E-trending shear-bands observed in all the largeVariscan calcalkaline plutons of thePyrenees, and showing high-tempera-ture solid-state deformation micro-structures with dextral movementsand a thrust component.Sub-magmatic stretching in the core
of the pluton (c. N55�E), elongationof the intrusion at map scale in a closeN40�E direction and inflection of themain Variscan schistosity trajectoriesof the Cinco Villas massif in thewestern contact aureole, with a triplepoint in the south-western border,suggest an NE–SW extensional stresscomponent and a N90�E wrench com-ponent during emplacement. A modelin transtensive regime for the coevalformation of the Saxonian basin andalkaline dike swarm of the La Rhunearea in the north-eastern contact aure-ole of the Aya pluton was also pro-posed by Lago et al. (2004). Suchasymmetric laccolith with stretchinglineations parallel to the major axiswas evidenced in various extensionaldomains (Baldwin et al., 1993; Deneleet al., 2011). The very low deforma-tion recorded in the Aya plutonimplies very slow extensional velocityduring emplacement. Increase ofdeformation from the centre to theborders of the pluton, and the pres-ence of lineations perpendicular to themain axis exclusively in the borderssuggest a lateral push of the magmasduring emplacement that may explainthe intense folding and developmentof a slaty-cleavage in the contactaureole in a close direction to thecontact. Inflexion of the regionalschistosity in a large area to the southof the Aya pluton and regional gravi-metric anomaly (Barnolas andChiron, 1996) suggest the presence ofa second pluton with the same elon-gation to the south.
The Aya pluton is likely to repre-sent the deep expression of the UpperPermian volcanism, and it highlightsthe intensity of this magmatism in thesouthern foreland of the Variscan belt.Our data corroborate an importantmagmatic stage at 267 Ma, in closeagreement with the K-Ar age of267 ± 11 Ma reported for lampro-phyres dykes of the Pyrenean AxialZone showing chemical affinity withthe second Permian volcanic cycle(Debon and Zimmermann, 1993;Perini et al., 2004).Evolution of the Stephanian–
Permian basins seems to be contempo-raneous with a transition fromlate-orogenic transpressive to anoro-genic transtensive regimes, with a pro-gressive increase of extensivecomponent. The initial trans-tensionalstage (c. 275 Ma) corresponds to cal-calkaline volcanism and coarse detritalsedimentation infilling strongly subsid-ing and isolated sub-basins. The lasttrans-tensional stage (c. 267 Ma) ischaracterized by transitional-alkalinemafic and silicic plutonism and volca-nism, coeval with fine-grained sedi-ments that extensively overlapprevious units, connecting the sub-basins. This Permian stage in the Pyre-nees represents the earlierst evidence ofextensive regime in the southern part ofthe Variscan chain, which led later tothe formation of the Bay of Biscay rift.
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Received 15 June 2011; revised versionaccepted 2 November 2011
Supporting Information
Additional Supporting Informationmay be found in the online versionof this article:
Table S1. Zircon U-Pb data ob-tained by in situ Laser ablation ICP-MS for the Aya leucogranite sample.Table S2. Zircon U-Pb data ob-
tained by in situ Laser ablation ICP-MS for the Aya monzogranite sample.Table S3. U-Pb data obtained by
ID-TIMS on zircon fractions from theAya gabbrodiorite sample.Please note: Wiley-Blackwell are
not responsible for the content orfunctionality of any supporting mate-rials supplied by the authors. Anyqueries (other than missing material)should be directed to the correspond-ing author of the article.
Terra Nova, Vol 24, No. 2, 105–113 Y. Denele et al. • Permian granites in the Pyrenees
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