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Submarine reworking of exhumed subcontinental mantle rocks:field evidence from the Lherz peridotites, French Pyrenees

Yves Lagabrielle and Jean-Louis BodinierGeosciences Montpellier, Universite de Montpellier 2 and CNRS Cc 60, Place Eugene Bataillon, 34095 Montpellier, France

Introduction

The Pyrenean peridotites consist ofabout 40 fragments of generally well-preserved subcontinental mantle, afew hundred meters to 3 km across(Fabries et al., 1991, 1998). Most ofthem occur within a narrow belt ofMesozoic sediments running over� 400 km, parallel to the NorthPyrenean Fault (NPF) that marksthe limit between the Iberia andEurasia plates (Fig. 1). The signifi-cance of these small mantle frag-ments scattered along a majortectonic boundary has long been amatter of debate. Various scenarioshave been proposed for theiremplacement, ranging from purelytectonic mechanisms, such as solidintrusion of hot or cold mantle rocksinto sediments (Minnigh et al., 1980;Vielzeuf and Kornprobst, 1984), totectono-sedimentary processes involv-ing disintegration and transport ofmantle rocks previously exhumedon the seafloor (Choukroune, 1973;

Fortane et al., 1986). However, mostattempts to solve the question ofPyrenean peridotite emplacementwere published more than 20 yearsago. Since that time, considerableprogress has been made in ourunderstanding of mantle exhumationat passive margins and mid-ocean-ridges. In particular, an importantresult of recent studies of ocean-continent transitions (OCT) is thatsubcontinental lithospheric mantle isexposed on the ocean floor at thefoot of most non-volcanic passivemargins. Mantle rocks exhumed insuch settings experienced reworkingand transport leading to local accu-mulation of typical ultramafic-rich clastic sedimentary sequences(Whitmarsh et al., 2001; Manatschal,2004).Here, we re-examine the geological

setting of the Pyrenean peridotites inthe area of Etang de Lherz, the type-locality of lherzolite, and we focus onthe relationships between the perido-tite bodies and the surrounding sedi-ments. The aim of this study was toidentify further the emplacementmechanisms of the Pyrenean perido-tites in the light of the scenariosrecently suggested for mantle exhu-mation in extensional and oceanicenvironments.

Emplacement of the Pyreneanlherzolites: geological constraintsand former models

Five constraints based on significantgeological, petrological and geochem-ical data must be considered whenaddressing the mode of emplacementof the Pyrenean peridotites.1 Fresh to moderately serpentinized

lherzolite is the predominant rocktype in all peridotite bodies. At Etangde Lherz, the peridotites show petro-logical and geochemical character-istics typical of sub-continentallithosphere, such as low equilibrationtemperatures (800–900 �C), a widerange of isotopic compositions, oldosmium ages and vein-related, meta-somatic features comparable to thoseobserved in mantle xenoliths (Bodi-nier et al., 1988, 1990, 2004; Downeset al.,1991; Fabries et al., 1991; Reis-berg and Lorand, 1995). The lherzo-lites were recently interpreted asformed at the expense of old,�2.5 Ga (Reisberg and Lorand,1995), harzburgitic lithosphere byigneous refertilization (Le Roux et al.,2007). This process probably occurredduring the post-Variscan, post-colli-sional thermal event responsible forgranulitic metamorphism in the wes-tern Pyrenees (Pin and Vielzeuf, 1983).

ABSTRACT

In the Pyrenees, the lherzolites nowhere occur as continuousunits. Rather, they always outcrop as restricted bodies, nevermore than 3 km wide, scattered across Mesozoic sedimentaryunits along the North Pyrenean Fault. We report the results of adetailed analysis of the geological setting of the Lherz massif(central Pyrenees), the type-locality of lherzolites and one of themost studied occurrences of mantle rocks worldwide. The Lherzbody is only 1.5 km long and belongs to a series of ultramaficbodies of restricted size (a few metres to some hundreds ofmetres), occurring within sedimentary formations composedmostly of carbonate breccias originating from the reworking ofMesozoic platform limestones and dolomites. The clasticformations also include numerous layers of polymictic brecciasreworking lherzolitic clasts. These layers are found far from anylherzolitic body, implying that lherzolitic clasts cannot derive

from the in situ fragmentation of an ultramafic body alone, butmight also have been transported far away from their sourcesby sedimentary processes. A detailed analysis of the contactsbetween the Lherz ultramafic body and the surroundinglimestones confirms that there is no fault contact and thatsediments composed of ultramafic material have beenemplaced into fissures within the brecciated carapace of theperidotites. These observations bear important constraints forthe mode of emplacement of the lherzolite bodies. We inferthat mantle exhumation may have occurred during Albianstrike-slip deformation linked to the rotation of Iberia along theproto-North Pyrenean Fault.

Terra Nova, 20, 11–21, 2008

Correspondence: Y. Lagabrielle, Geosci-

ences Montpellier, Universite de Montpel-

lier 2 and CNRS Cc 60, Place Eugene

Bataillon, 34095 Montpellier, France. Tel.:

04 67 14 35 85; e-mail: yves.lagabrielle@

gm.univ-montp2.fr

� 2008 Blackwell Publishing Ltd 11

doi: 10.1111/j.1365-3121.2007.00781.x

After this event, the peridotites under-went cooling in the mantle lithospherebefore being cross-cut by a late gen-eration of amphibole-pyroxenitedykes (Conquere, 1971; Bodinieret al., 1987; Vetil et al., 1988). Thesedykes are considered to represent meltconduits for mid-Cretaceous (Albian)alkaline magmatism in the Pyrenees(Golberg et al., 1986; Montigny et al.,1986; Bodinier et al., 1987).2 The lherzolites never constitute

continuous units, but always appearas restricted bodies never more than3 km long. They are never associatedwith larger gabbro or pillow-lavaformations of MORB affinity. There-fore, they cannot be considered asparts of an ophiolitic suite represent-ing typical oceanic lithosphere. Theirmode of emplacement necessarily dif-fers from the obduction of coherentthrust sheets as typical in Tethyanophiolites.3 The lherzolite bodies are always

closely associated with sedimentaryformations representing invertedMesozoic basins. They occur withina geological environment often char-acterized by voluminous detrital for-mations (flyschs) including turbiditesand debris flows linked to tectonicinstabilities and sedimentary transportand deposition in basins of Cretaceousage, generally Albian to Cenomanian.4 The eastern Pyrenean lherzolites

are located in a belt of HT-LP Creta-ceous metamorphism linked to theNPF that has been dated between110 and 85 Ma (Albarede andMichard-Vitrac, 1978; Montigny

et al., 1986; Golberg and Maluski,1988). Mesozoic limestones now occuras highly deformed marbles bearingtypical Pyrenean mineralogical assem-blages including scapolite, phlogopite,diopside, etc. This led many authorsto suggest a causal relationshipbetween ultramafic intrusions andheat transfer from lower lithophericlevels (see Golberg et al., 1986; Gol-berg and Maluski, 1988; for refer-ences). The involvement of hot fluidsas mechanism of heat transfer leadingto dynamothermal metamorphismwas inferred from the heterogeneityin the distribution of the isograds. Thethermal anomaly was linked to impor-tant crustal thinning and fluid circu-lation along the NPF area during thesinistral drift of Iberia (Dauteuil andRicou, 1989).5 At Etang de Lherz, the ultramafic

rocks are surrounded by a variety ofvoluminous breccia formations com-posed of clasts of ultramafic andcarbonate rocks of various sizes (fromless than one mm to a few dm), mixedin different proportions.Finally, because of the wide range

of observations listed above, the pro-cesses of emplacement of the Pyreneanlherzolite bodies have long been amatter of debate and led to highlydiffering interpretations with two con-trasting end-members, namely:1 The lherzolites have been

emplaced tectonically within theMesozoic sedimentary sequences.Brecciation is regarded as the resultof the intrusion of solid mantle rocksinto upper crustal levels during rifting

processes preceding the Pyreneanorogeny (Vielzeuf and Kornprobst,1984; and references therein). Minnighet al. (1980) proposed that the brec-cias derive from in-situ disaggregationof carbonate sediments as the result ofquenching caused by endothermicdecomposition reactions of the car-bonates. This would imply heat trans-fer, fluid circulation and brecciationby hydraulic fracturing of both theperidotites and the sedimentary host-rocks at a very large scale. Thisinterpretation is in conflict with theresults of later investigations showingthat the Pyrenean metamorphism isnot related to direct contact betweenhot mantle rocks and sedimentaryformations (Golberg, 1987; Dauteuiland Ricou, 1989). Gravity modellingalso confirms that the size of the Lherzbody was not sufficient to allow heattransfer leading to the HT-LP meta-morphism of the entire Lherz region(Anderson, 1984).2 The lherzolites are not intrusive

bodies but are clasts within sedimen-tary formations and have beenexhumed as portions of the Mesozoicbasement of the Pyrenean basinsbefore or during the Cretaceous.Indeed, some authors report primarysedimentary contacts between thelherzolites and surrounding sedimen-tary formations (Choukroune, 1974;Duee et al., 1984; Fortane et al.,1986).Choukroune (1973, 1980) proposed asedimentary origin for the brecciasaround Etang de Lherz, arguing that:(a) lherzolite clasts and monogeniclherzolitic breccias layers are abun-

Fig. 1 Location map of the lherzolitic bodies in the eastern Pyrenean belt (modified from Choukroune, 1974).

Cretaceous exhumation of pyrenean mantle • Y. Lagabrielle and J.-L. Bodinier Terra Nova, Vol 20, No. 1, 11–21

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12 � 2008 Blackwell Publishing Ltd

dant in a wide area, some km awayfrom the main body of Lherz, (b) thebreccias are associated with fine-grained ultramafic clastic rocks show-ing graded bedding and flute-caststypical of sediment transport in ansubaqueous environment and, (c) thebreccias systematically contain peb-bles of deformed marbles exhibitingPyrenean metamorphic mineralogicalassemblages in a matrix that experi-enced weaker metamorphic overprint.

Breccias in the Etang de Lherzregion and contact betweenlherzolite and limestones

The lherzolites of the Etang de Lherzregion are exposed within the Aulusbasin, a Mesozoic basin inverted dur-ing the Pyrenean orogeny. The formerbasin now corresponds to a narrowbelt of Jurassic and Lower Cretaceouslimestones, marbles and dolomites,which have been deformed and

brought into a vertical position be-tween the Paleozoic units of the TroisSeigneurs to the North and of theAxial Zone to the South. The meta-sediments are characterized by a widevariety of breccias, as shown on theBRGM geological map (Colchenet al., 1997; Ternet et al., 1997)(Fig. 2). The lherzolites appear asnumerous small bodies, ranging insize from a few meters to severalhundreds of meters, some of themexhibiting a brecciated fabric. Massiveultramafic bodies frequently appear inclose association with layers of brec-cias having a mixed ultramafic-carbonate composition. Ultramaficbreccias may also form mappablelenses, up to 300 m long, alignedwithin the limestones. The largestperidotite exposure, with a rectangu-lar shape and 1.5 km long (Fig. 3),forms the Lherz body. Besides thelherzolitic bodies, minor ophitic andgabbroic bodies are observed (Fig. 2).

We studied breccias located at var-ious distances from the lherzolitebody, as well as within the massifitself.Detailed observations along the

trail from Port de Lherz to Pic deGirantes (ravin de Paumeres, north ofthe Fontete-Rouge summit), revealthat the entire peridotite-bearing sec-tion consists of clastic rocks with anoverall vertical bedding (Fig. 2).There is no massive carbonate forma-tion exposed here. Therefore, theJurassic age of the host-marbles ofthe peridotite bodies cannot be pro-ven. The largest volume of clasticrocks consists of breccias and minorconglomerates mainly composed offragments of white to pink marbleand dolomite, with additional ultra-mafic fragments (Fig. 4). Carbonateclasts are angular to subangular,rarely rounded, with sizes rangingfrom one mm to a few dm. Thebreccias are generally poorly sorted,

Fig. 2 Simplified geological map of the Aulus-Lherz region showing the distribution of the lherzolite bodies and associatedlithologies (redrawn after the BRGM Aulus les Bains 1 : 50 000 geological map by Colchen et al. (1997), after Vielzeuf andKornprobst (1984), and according to our own observations). Note that small lherzolite bodies from 1 dm to 100 m across arescattered around the body of Lherz. Thick dotted line delineates the region where polymictic debris-flow deposits have beenrecognized during our study. Hatched area represents the region north of the main ultramafic bodies, where strongly deformedMesozoic sediments are exposed, and where rare sedimentary clastic deposits have been observed.

Terra Nova, Vol 20, No. 1, 11–21 Y. Lagabrielle and J.-L. Bodinier • Cretaceous exhumation of pyrenean mantle

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but locally, ultramafic sandstonessequences show graded and cross bed-ding, and slumps. Some large marbleclasts are foliated and exhibitmm-sized retromorphosed scapolites,confirming post-metamorphic brecci-ation, as earlier reported by Choukro-une (1973, 1980). Dark-grey clasts ofmetapelitic rocks (Liassic, Aptian?)are abundant in some layers, andin such cases, graded bedding isobserved. Locally, the ultramaficbreccias form m-thick, vertical, mo-nomictic layers and lenses interbeddedwithin the carbonate clastic forma-tions. The breccias contain variousproportions of matrix, and in matrix-supported breccias, the compositionof the matrix varies from pure car-bonate to pure ultramafic material.Isolated blocks of breccias made up ofmarble clasts floating within a mono-mictic graded, ultramafic sandstonematrix are abundant close to the Portde Lherz.Various breccia-types are observed

close to, and within the main body ofLherz.Type 1 is found along the southern

and eastern borders of the ultramaficoutcrops, where it forms a 50 to 200 mthick, continuous carapace of mono-mictic breccias (Le Roux et al., 2007)(Fig. 3). These breccias never includecarbonate clasts and often appear aslayers sandwiched between panels ofmassive peridotite. The continuity ofmantle structures is generally pre-served throughout these breccias, atleast at the outcrop scale. Websteritelayers and hornblendite (‘‘lherzite’’)

dikelets, for instance, can be followedover several meters within the brec-cias, being only slightly offset alongbrittle faults, which indicate little dis-placement between clasts. This type ofin situ formed breccias bear charac-teristics of cataclastic breccias.Type 2 breccias are observed close

to the contact with the carbonatesediments (Fig. 3, Site LHZ 6). Theiraspect resembles that of the cataclasticbreccias but they typically includeisolated clasts of limestones and lookmore heterogenous. The transitionbetween type 1 and 2 is not sharp,suggesting that the source of theultramafic debris can be found withininternal cataclatic fault zones cross-cutting the lherzolitic body.Type 3 breccias are observed in

restricted places within the massif (forinstance at Sites LHZ 4 and LHZ 5,Fig. 3) where they appear to fill largefissures opened within the peridotites.They are composed of thin layers ofgraded ultramafic sandstones (Fig. 5)with cross-lamination, includingcm-sized blocks of fresh lherzoliteand isolated serpentine clasts. Thesedeposits closely resemble the gradedultramafic sandstones overlying andinserted within the ultramafic base-ment of some Apenninic and Alpineophiolites (Decandia and Elter, 1972;Cortesogno et al., 1981; Bernoulliand Weissert, 1985). Fragments ofultramafic minerals such as pyroxene,olivine and spinels, less than 1 mmacross form most of the fine-grainedmatrix and are associated with grainsof serpentinite and millimetric frag-

ments of deformed marbles (Fig. 4).Type 3 further differs from the othertypes of breccias in Lherz in showingan extreme variety of ultramafic clastsin term of mineralogical composition(peridotites, websterites, honblendites,etc.), mantle textures (coarse-granularto mylonitic) and degree of serpenti-nization (fully serpentinized clasts andfresh lherzolite debris). The majorityof the peridotite clasts are composedof fresh, pyroxene-rich, coarse-granu-lar lherzolites different from the Lherzperidotites. These rocks are morereminiscent of peridotites from otherbodies, such as Fontete Rouge (Fab-ries et al., 1991). This points to amixed and relatively far provenance ofthe ultramafic clasts in Type 3 breccia.Besides the ultramafic clasts, the brec-cias generally contain debris of car-bonate rocks. These include clasts ofdeformed marbles showing here alsometamorphic minerals typical of thePyrenean metamorphism. Clasts offormer monomictic carbonate brec-cias, or of polymictic ultramafic-car-bonate breccias are often observed.Type 4 breccias are clastic rocks

closely resembling the so-called ophi-calcites of the Apenninic and Alpineophiolites (Lemoine et al., 1987) thatdeveloped on seafloor exposures ofexhumed mantle (Figs 3 and 5). Theytypically include dominant, poorlysorted, angular clasts of serpentiniteand minor poorly serpentinized lherz-olites. As frequently observed inAlpine ophicalcites (e.g. Fruh-Greenet al., 1990), late carbonate veinscross-cut the breccias, a consequenceof circulation of fluids after deposi-tion. Such ophicalcite breccias infillfissures opened within the massiveperidotites as observed along the roadat Site LHZ 8 (Fig. 3).We closely studied the contacts

between carbonates and ultramafics.These are well exposed along the roadat Sites LHZ 2a and LHZ2b (Figs. 3and 6) and show a highly contortedoutline. Two main observations haveto be emphasized. (1) Where observed,the carbonates are always welded ontothe irregular surface of the ultrama-fics. (2) The contact is not rectilinearat a meter scale; rather, the surface ofthe lherzolites is fractured and fissuresare filled with carbonate brecciasmixed with ultramafic debris as illus-trated in Fig. 6c. These observationspreclude a fault contact at this loca-

Fig. 3 Detailed map of the Lherz body and location of sample sites and of main typesof breccia formations as discussed in the text. Black stars refer to ultramafic breccialocations, white star represents light-coloured sandstone site. Map after Le Rouxet al. (2007) and Conquere and Fabries (1984).


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tion. Late carbonate veins cross cutboth the lherzolites and the marblebreccias suggesting post-depositionalmetamorphic evolution and fluid cir-culation. Detailed field observationand thin-section analysis show thatthe sedimentary layer in direct contactwith the lherzolites is a carbonate

micro-breccia reworking mm-sizedlherzolitic clasts and isolated mineralgrains originating from the disaggre-gation of the lherzolite (pyroxene,olivine, serpentinite and amphibolesfrom lherzolite dikelets). This is wellobserved at Site LHZ 2b (Fig. 6). AtSite LHZ 2a, the lherzolites show a

mylonitic fabric, cut and offset bylater small faults and fissures, which isclearly cut by the contact with thesediments. Two meters to the south ofthe carbonate-ultramafic contact atSite LHZ 2a, the carbonate brecciasform vertical strata made up of frag-ments of pink to white-grey marbles

Fig. 4 The polymictic breccias. Numerous examples of sedimentary breccias are well exposed in the region between the Lherz andFontete Rouge bodies, an area delineated by a thick dotted line in the map of Fig. 2. Evidence of sedimentary origin andsubaqueous transport of clasts includes graded bedding, frequent erosional channels and cross-bedding, mixing from differentsources, round shape of some clasts, etc. (a) Graded polymictic carbonate-lherzolite breccias with interlayered polymicticsandstone beds. (b) Lens of bedded, pure ultramafic sandstone (white arrows) within polymictic carbonate-ultramafic breccia. (c)Matrix-supported breccia. Angular clasts are deformed marbles, the orange-coloured matrix is made up of lherzolitic sandstone.(d) Monomictic ultramafic breccia. (e) Thin section of a fine-grained ultramafic sandstone (grains of serpentinite and millimetricfragments of deformed marbles), breccia of Type 3, location Fig. 3.


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and dolomites. Some of these beds, upto 1 m thick, include up to 30–40% oflherzolitic clasts outlining the verticalbedding. Such clear bedding of thebreccias precludes an origin throughquenching and hydraulic fracturingbecause of hot rock intrusion.We observed new exposures of mid-

dle- to high-grade metamorphic rocksbetween peridotites and carbonatesalong the northern boundary of theEtang de Lherz area (Site LHZ 7,Figs. 2, 7). Lacroix (1895) and Monc-houx (1970, 1972) have describedthree predominant types within simi-lar rocks: (1) biotite- and scapolite-rich rocks, (2) spinel amphibolites, (3)anthophyllite- and phlogopite-rich

rocks, also containing variable pro-portions of magnesian hornblende,sapphirine, kornerupine, Al-spinel,altered cordierite, scapolite, calcite,tourmaline, and subordinate amountsof rutile and apatite. Monchoux(1972) interpreted the sapphirine-bearing assemblages as mafic crustalrocks modified by the combinedeffects of metasomatism and meta-morphism (800–900 �C and 0.6–0.9GPa) in contact aureoles of peridotitesduring the early stages of their exhu-mation. The deep-seated metamorphicrocks would have been draggedtowards the surface during later stagesof the exhumation of the peridotites.Our observations indicate the

existence of two distinct rock facieswithin this suite. One (Type a) is madeof generally broken, idiomorphic min-erals (except for phlogopite) that havelost all of their primary texturalrelationships. This highly friable rocktype is carbonate-free and probablyderives from a metamorphic protoliththrough intense cataclasis (Monc-houx, 1972). The other rock facies(Type b) is distinguished by the pres-ence of a carbonate matrix and thepresence of small clasts (<1 cm) ofultramafic rocks (Fig. 7). It is identicalto Type a in terms of mineralogicalcomposition, except that the mineralsare broken into smaller pieces. In thefield, the Type b facies contains clastsof Type a rocks, of ultramafic and ofadditional undetermined rocks (workin progress). It occurs in direct contactwith the Lherz peridotite; it is associ-ated with ultramafic sediments, andobviously results from sedimentaryreworking of the Type a.

Interpretation: sedimentary originof some of the Lherz breccias

The observations reported above dem-onstrate that the lherzolites and someassociated deep-seated rocks havebeen submitted to sedimentaryreworking. However, apart from Type1 breccias regarded as cataclasticbreccias, tectonic fragmentation alonecannot explain a number of highlydistinctive features such as: (1) theoccurrence of numerous polymicticultramafic–carbonate breccia layersinterbedded within the clastic sedi-mentary sequence of the Aulus basin,some km away from the main Lherzbody; (2) the presence of gradedultramafic sandstones within fissuresof the Lherz body, and (3) the mixingof serpentinized clasts with unweath-ered mantle debris in ophicalcite-typebreccias. These features imply: (1)reworking and transport of a two-component clastic material by gravi-tational processes, including rockfalls, debris flows and grain flows, (2)reworking of ultramafic fine-grainedmaterial and deposition in a sub-aqueous environment within fissuresopened within an ultramafic basementand (3) mixing of debris from varioussources including regions of highlyserpentinized mantle. Since serpenti-nization of mantle rocks is thought todevelop generally at temperatures

Fig. 5 (a) Ophicarbonate and (b) lherzolitic pebbly sandstones containing angularclasts of coarse-grained lherzolite, (c, enlargement of b). (a) is a Type 4 breccia, (b)belongs to the Type 3 breccias (see text for descriptions and Fig. 3 for location ofsamples).


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lower than 300–350 �C (Andreaniet al., 2007), the association ofpoorly serpentinized lherzolites andserpentinites suggests that brecciationprocesses did not occur undertemperature conditions calculated forthe Pyrenean metamorphism. Theseobservations demonstrate that ultra-mafic rocks have been exhumed andexposed on the seafloor together withMesozoic sediments, and have beenincorporated as exotic debris within aclastic sequence as synthesized inFig. 8. Because of the progressivecontact between Type 1 cataclasticbreccias and Type 2 sedimentary brec-

cias, we may propose that tectonicbrecciation at depth preceded sedi-mentary reworking. Brecciationoccurred possibly during shearingleading to exhumation, but furtherinvestigations are requested to betterconstrain the P,T conditions and thekinematic of this cataclastic event.Finally, taking into account the

clastic origin of the ultramafic mate-rial around the Etang de Lherz, wehave to address the question of thesignificance of the Lherz body itself.The clastic formations are almostcontinuous around the ultramaficbody, with a remaining doubt along

its northwest side where exposures arescarce. Under such conditions, theLherz body might represent either asmall remnant of the ultramafic base-ment of the sedimentary basin, atectonic slice along a major detach-ment fault, or a large olistolith embed-ded within a clastic sedimentarysequence. These interpretations are inline with gravity modelling showingthat the Lherz body is of small sizeand is not rooted within the Pyreneanbasement (Anderson, 1984). In thefirst two cases, the Lherz body wouldexhibit a tectonic contact on its north-western side against the tectonized

Fig. 6 Detailed views of the contact between the main body of Lherz and the carbonates of the Aulus basin at Sites LHZ 2a andLHZ 2b, along the southern edge of the massif (see Fig. 3 for location). Here, there is no tectonic contact between the mantle rocksand the sediments. Rather, as frequently observed in Alpine-Apenninic ophiolites, the sediments infill open fissures within theperidotites. Note that the limestones are carbonate micro-breccias, most probably emplaced as debris flows above the exhumedmantle basement. Photographs (a), (b) and (c) are taken from Site LHZ 2b (Fig. 3) exposing the contact on the eastern side of theroad. Photograph (d) is from a block recently fallen down from the exposure at Site LHZ 2a, on the western side of the road.


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corridor along the NFP. In the thirdcase, the polymictic clastic formationreworking the mantle debris would becompletely disconnected from its ori-ginal basement of unknown composi-tion.

Geodynamic implications andconclusions

Our new field observations in theregion of Etang de Lherz bear impor-tant constraints for the mode ofemplacement of the Pyrenean lherzo-lites. Following Choukroune (1973,1974, 1980), we confirm that thelherzolite bodies around Etang deLherz are enclosed within a sedimen-

tary clastic sequence resulting fromthe accumulation of debris reworkedfrom exposures of platform carbon-ates and ultramafic rocks. This inter-pretation is consistent with field datain the Bearn region where lherzoliticbodies also form restricted exposuresregarded as olistholiths emplacedwithin flysch formations of Creta-ceous age (Duee et al., 1984; Fortaneet al., 1986). In that sense, the sedi-mentary sequence of the Aulus basincan be compared to the Cretaceous-Eocene successions of the NorthernApennines where gravity depositsincluding ophiolitic debris flows(olistostromes) and olistoliths cropout extensively (Abbate et al., 1970;

Marroni and Pandolfi, 2001). Thelargest ophiolitic olistoliths of theApennines are 1–2 km long and 200–300 m thick, a size similar to that ofthe Lherz body. Gravity emplacedultramafic-rich bodies showing char-acteristics similar to the Lherz poly-mictic clastic sequences have been alsoreported from various orogenic beltssuch as the Californian Coast Ranges(e.g. Lockwood, 1971), or the internalAlps (Deville et al., 1992).From these observations, it is clear

that subcontinental mantle has beenexposed on the floor and ⁄or along theflanks of a deep, tectonically activebasin that now forms the Aulusregion. At present, exhumation ofmantle rocks is known to occur atthe foot of non-volcanic continentalmargins such as the Galicia-IberiaAtlantic margin (Boillot et al., 1980,1985; Abe, 2001; Whitmarsh et al.,2001; Manatschal, 2004), along theaxial reliefs of slow-spreading ridges(Lagabrielle et al., 1998; Karsonet al., 2006), within small oceanicbasins (Seyler and Bonatti, 1988), oralong the walls of oceanic fracturezones (Bonatti et al., 1971; Auzendeet al., 1989). In these different geody-namic settings, mantle exhumation isalways accompanied by sedimentsyielding large volumes of debris ofdominantly ultramafic composition,ranging from fine-grained turbiditesto debris-flows and rock slides, as firstreported on the flanks of the GorringeBank (Lagabrielle and Auzende,1982). In the Pyrenean case, mantleexhumation cannot be viewed as aprocess linked to the opening of awide ocean, as no relicts of oceaniclithosphere are present within themountain belt. In contrast to theTethyan ophiolites, the low degree ofpervasive serpentinization of most ofthe Pyrenean lherzolites suggests veryrapid exhumation followed by instan-taneous sedimentary reworking andburial within detrital sequences. Thisis consistent with transtensive condi-tions at the foot of a rapidly stretchedcontinental crust. This scenario isillustrated in Fig. 9. It may haveoccurred during the opening of aseries of pull-apart basins along theIberia ⁄Europe plate boundary due tooceanic spreading in the Bay of Bis-caye during the Albian (Le Pichonet al., 1970; Choukroune and Mat-tauer, 1978; Olivet, 1996).

Fig. 7 Antophyliite- and vermiculite-rich sandstones, polymictic and ultramaficbreccias exposed along the northern limit of the Lherz body at Site LHZ 7 (seeFig. 3 for location).

Fig. 8 Cartoon depicting possible geological setting and mode of emplacement of theultramafic bodies and associated clastic rocks.


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Thinning of the continental litho-sphere in the region where lherzolitesare now exposed is confirmed byadditional evidence, as follows:

1 Radiometric ages (Ar-Ar and Sm-Nd) obtained on amphiboles fromLherz and Caussou. These agesindicate crustal emplacement ofthe ultramafic rocks around 110–105 Ma (Henry et al., 1998).

2 Pyrenean HT metamorphism. Thisinvolves heat transfer throughfluid circulation and occurred inresponse to continental thinningduring the Late Cretaceous.

3 Alkaline magmatism of Cretaceousage (105 Ma), that indicates partialmelting of upwelling mantle.

4 A major mylonitic event dated at110–100 Ma, which occurred along

normal faults cross-cutting Paleo-zoic basement close to the NPF(Costa and Maluski, 1988; de SaintBlanquat, 1993). One of these faultstestifies to fluid circulation and Mgenrichment linked to continuousshearing between 112 and 97 Ma(Scharer et al., 1999). The occur-rence of several basement unitsexhibiting granulitic metamorphicassemblages located along theNPF (Fig. 1) indicates that thelower continental crust has beenalso exhumed, possibly along thesefaults, a fact already noticed byVissers et al. (1997).

An important point must be addedto this discussion. Numerous clasts ofcarbonate included in the polymicticbreccias exhibit HT-LP Pyrenean

metamorphic parageneses and inter-nal deformation. This would imply apost-Albian age for the clastic sedi-mentation. On the other hand, theclastic sequence itself has experienceda metamorphic evolution, whichwould in turn indicate pre-LateAlbian sedimentation. Although thispoint needs further investigations,most of the geological data suggestthat mantle exhumation occurred dur-ing the Albian (Fig. 9). The ultramaficrocks may have been emplaced atshallow lithospheric levels earlier thanthe Cretaceous, during Variscan post-orogenic crustal thinning or duringTriassic or Liassic rifting episodes.According to some kinematic recon-structions (Sibuet et al., 2004), a realmof thinned crust and ⁄or oceanic crustwas present before the Cretaceous

Fig. 9 Geodynamic model of mantle exhumation in the particular context of the Pyrenean belt. (a) Timing of rotation of Iberia(after Olivet, 1996), thick green lines indicate zones of active mantle exhumation. (b) Scenario of mantle exhumation in a pull-apartbasin. (c) Evolution of the zone of exhumed mantle during basin inversion due to the Pyrenean orogeny.


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between the Iberian and Europeanplates.

Acknowledgements

This is a contribution of the French‘‘GDR Marges’’ Program (INSU-CNRS, Total, IFP, BRGM, Ifremer).We thank J.M. Dautria, M. Seranne,M. Lopez, B. Peybernes, M. de SaintBlanquat for stimulating discussions inthe field and help in the analysis of thinsections and interpretation of somesedimentary features. M. Daignieres,R. Caby and F. Boudier, and studentsof Masters from Montpellier are alsothanked for fruitful discussions. Dori-ane Delmas and Christophe Nevadoprovided high quality thin sections.Weare grateful to Daniel Bernoulli and ananonymous reviewer for careful andconstructive reviews.

References

Abbate, E., Bortolotti, V. and Passerini, P.,1970. Olistostromes and olistoliths. Sed.Geol., 4, 521–557.

Abe, N., 2001. Petrochemistry of serpenti-nized peridotite from the Iberia AbyssalPlain (ODP Leg 173): its character inter-mediate between sub-oceanic to sub-con-tinental upper mantle. In: Non-VolcanicRifting of Continental Margins: EvidenceFrom Land and Sea (R.C.L. Wilson, R.B.Whitmarsh, H.P.J. Taylor and N. Fro-itzheim, eds), Geol. Soc. London Spec.Publ., 187, 143-159.

Albarede, F. and Michard-Vitrac, A., 1978.Age and significance of the North Pyre-nean metamorphism. Earth Planet. Sci.Lett., 40, 327–332.

Anderson, H.J., 1984. Gravity modelling ofthe lherzolite body at Lers (French Pyr-ennes); some regional implications. Geol.Mag., 122, 51–56.

Andreani, M., Mevel., C., Boullier, A.-M.and Escartin, J., 2007. Dynamic controlon serpentine crystallization in veins:constraints on hydration processes inoceanic peridotites. Geochem. Geophys.Geosystems, 8, 2. Q02012 ⁄2006GC001373.

Auzende, J.M., Bideau, D., Bonatti, E.,Cannat, M., Honnorez, J., Lagabrielle,Y., Malavieille, J., Mamaloukas-Fran-goulis, V. and Mevel, C., 1989. Directobservation of a section through slow-spreading oceanic crust. Nature, 337,6209.

Bernoulli, D. and Weissert, H., 1985. Sed-imentary fabrics in Alpine ophicalcites,South Pennine Arosa zone, Switzerland.Geology, 13, 755–758.

Bodinier, J.-L., Fabries, J., Lorand, J.-P.,Dostal, J. and Dupuy, C., 1987. Geo-

chemistry of amphibole pyroxeniteveins from the Lherz and Frey-chinede ultramafic bodies (Ariege,French Pyrenees). Bull. Miner., 110,345–358.

Bodinier, J.-L., Dupuy, C. and Dostal, J.,1988. Geochemistry and petrogenesis ofEastern Pyrenean peridotites. Geochim.Cosmochim. Acta, 52, 2893–2907.

Bodinier, J.-L., Vasseur, G., Vernieres, J.,Dupuy, C. and Fabries, J., 1990. Mech-anisms of mantle metasomatism: geo-chemical evidence from the LherzOrogenic peridotite. J. Petrol., 31, 597–628.

Bodinier, J.-L.,Menzies,M.A., Shimizu,N.,Frey, F.A. and McPherson, E., 2004. Sil-icate, hydrous and carbonate metasoma-tism at Lherz, France: contemporaneousderivatives of silicate melt-harzburgitereaction. J. Petrol., 45, 299–320.

Boillot, G., Grimaud, S., Mauffret, A.,Mougenot, D., Kornprobst, J., Mergoil-Daniel, J. and Torrent, G., 1980. Ocean-continent boundary of the Iberianmargin: a serpentinite diapir west of theGalicia Bank. Earth Planet. Sci. Lett.,48, 23–34.

Boillot, G., Winterer, EL., Meyer, A.W.,Applegate, J., Baltuck, M., Bergen, J.A.,Comas, M.C., Davies, T.A., Dunham,K., Evans, C.A., Girardeau, J., Gold-berg, D., Haggerty, J., Jansa, L.F.,Johnson, J.A., Kasahara, J., Loreau,J.P., Sierra, E.L., Moullade, M., Ogg, J.,Sarti, M., Thurow, J., Williamson,M.W., 1985. Resultats preliminaires dela campagne 103 du Joides Resolution(Ocean Drilling Program) au large de laGalice (Espagne): sedimentation et dis-tension pendant le ‘‘rifting’’ d�une margestable: hypothese d�une denudation tec-tonique du manteau superieur. C. R.Acad. Sci. Paris, 301, 627–632.

Bonatti, E., Honnorez, J. and Ferrara, G.,1971. Peridotite-gabbro-basalt complexfrom the equatorial Mid-Atlantic Ridge.Philos. Trans. Roy Soc. Lond., 268, 385–402.

Choukroune, P., 1973. La breche de Lherzdite ‘‘d�explosion liee a la mise en placedes lherzolites’’ est une breche sedimen-taire d�age cenozoıque. C. R. Acad. Sci.Paris, 277, 2621–2624.

Choukroune, P., 1974. Structure et evolu-tion tectonique de la zone Nord-Pyrene-enne. Analyse de la deformation dans uneportion de chaıne a schistosite subverti-cale. These d�Etat, USTL, Montpellier.

Choukroune, P., 1980. Comment on‘‘Quenching: an additional model foremplacement of the lherzolite atLers (French Pyrenees)’’ Geology, 8,514–515.

Choukroune, P. and Mattauer, M., 1978.Tectonique des plaques et Pyrenees: surle fonctionnement de la faille transfor-mante nord pyreneenne; comparaison

avec des modeles actuels. Bull. Soc. Geol.Fr., 20, 689–700.

Colchen, M., Ternet, Y., Debroas, E.J.et al., 1997. Carte geologique de laFrance au 1 ⁄50000, feuille 1086, Aulus-les Bains, 1997. editions BRGM,Orleans, France.

Conquere, F., 1971. Les pyroxenolites aamphibole et les amphibolites associeesaux lherzolites du gisement de Lherz(Ariege, France): un exemple du role del�eau au cours de la cristallisation frac-tionnee des liquides issus de la fusionpartielle de lherzolites. Contrib. Mineral.Petrol., 33, 32–61.

Conquere, F. and Fabries, J., 1984.Chemical disequilibrium and its ther-mal significance in spinel- peridotitesfrom the Lherz and Freychinedeultramafic bodies (Ariege; FrenchPyrenees). In: Kimberlites II: TheMantle and Crust-Mantle Relationships(J. Kornprobst, ed), pp. 319–332.Elsevier, Amsterdam.

Cortesogno, L., Galbiati, B. and Principi,G., 1981. Descrizione dettagliata dialcuni caratteritici affioramenti di brecceserpentiniche de la Liguria orientale edinterpretazioni in chiave geodinamica.Ofioliti, 6, 47–76.

Costa, S. and Maluski, H., 1988. Use of the40Ar-39Ar stepwise heating method fordating mylonitic zones: an example fromthe St. Barthelemy Massif (NorthernPyrenees, France). Chem. Geol., 72, 127–144.

Dauteuil, O. and Ricou, L.E., 1989. Unecirculation de fluides de haute tempera-ture a l�origine du metamorphisme cre-tace nord-pyreneen. Geodinamica Acta,3, 237–250.

Decandia, F.A. and Elter, P., 1972. Lazona ofiolitifera del Bracco nel settorecompreso fra Levanto e la Val Graveglia(Appennino Ligure). Mem. Soc. Geol.Ital., 11, 503–530.

Deville, E., Fudral, S., Lagabrielle, Y.,Marthaler, M. and Sartori, M., 1992.From oceanic closure to continentalcollision: a synthetic view from the HP-LT belt of the Western Alps. Geol. Soc.Am. Bull., 104, 127–139.

Downes, H., Bodinier, J.-L., Thirlwall,M.F., Lorand, J.-P. and Fabries, J.,1991. REE and Sr-Nd isotopic geo-chemistry of the Eastern Pyrenean peri-dotite massifs: sub-continentallithospheric mantle modified by conti-nental magmatism. Orogenic Lherzolites

and Mantle Processes, (M.A. Menzieset al, eds) J. Petrol., 97–115.

Duee, G., Lagabrielle, Y., Coutelle, A. andFortane, A., 1984. Les lherzolites asso-ciees aux chaınons bearnais (PyreneesOccidentales): mise a l�affleurement ante-dogger et resedimentation albo-ceno-manienne. C. R. Acad. Sci. Paris, 17,1205–1209.


.............................................................................................................................................................


Fabries, J., Lorand, J.-P., Bodinier, J.-L.and Dupuy, C., 1991. Evolution of theupper mantle beneath the Pyrenees: evi-dence from orogenic spinel lherzolitemassifs. Orogenic Lherzolites and Mantle

Processes, (M.A. Menzies et al, eds)J. Petrol., 55–76.

Fabries, J., Lorand, J.-P. and Bodinier,J.-L., 1998. Petrogenetic evolution oforogenic lherzolite massifs in the centraland western Pyrenees. Tectonophysics,292, 145–167.

Fortane, A., Duee, G., Lagabrielle, Y. andet Coutelle, A., 1986. Lherzolites and theWestern ‘‘Chaınons Bearnais’’ (FrenchPyrenees): structural and paleogeo-graphical pattern. Tectonophysics, 129,81–98.

Fruh-Green, G.L., Weissert, H. and Ber-noulli, D., 1990. A multiple fluid historyrecorded in Alpine ophiolites. J. Geol.Soc. London, 147, 959–970.

Golberg, J.-M., 1987. Le metamorphismemesozoıque dans la partie orientale desPyrenees: relation avec l�evolution de lachaıne au Cretace. Doctorat thesis,Montpellier II.

Golberg, J.-M. and Maluski, H., 1988.Donnees nouvelles et mise au point surl�age du metamorphisme pyreneen. C. R.Acad. Sci. Paris, 306, 429–435.

Golberg, J.-M., Maluski, H. and Leyre-loup, A.F., 1986. Petrological and agerelationship between emplacement ofmagmatic breccia, alkaline magmatism,and static metamorphism in the NorthPyrenean Zone. Tectonophysics, 129,275–290.

Henry, P., Azambre, B., Montigny, R.,Rossy, M. and Stevenson, R.K., 1998.Late mantle evolution of the Pyreneansub-continental lithospheric mantle inthe light of new 40Ar-39Ar and Sm-Ndages on pyroxenites and peridotites(Pyrenees, France). Tectonophysics, 296,103–123.

Karson, J.A., Fruh-Green, G.L., Kelley,D.S., Williams, E.A., Yoerger, D.R.and Jakuba, M. 2006. Detachmentshear zone of the Atlantis Massifcore complex, Mid-Atlantic Ridge,30�N. Geochem. Geophys. Geosystems,7, Q06016, doi: 10.1029/20O5GC001109.

Lacroix, A., 1895. Les phenomenes decontact de la lherzolite et de quelquesophites des Pyrenees. Bull. Carte Geol.Fr., 6, 181–186.

Lagabrielle, Y. and Auzende, J.M., 1982.Active in situ disaggregation of oceaniccrust and mantle on Gorringe Bank:analogy with ophiolitic massives.Nature,297, 490–493.

Lagabrielle, Y., Bideau, D., Cannat, M.,Karson, J.A. and Mevel, C. 1998 Ultra-mafic-mafic plutonic rock suites exposedalong the Mid-Atlantic Ridge (10�N–30�N): symmetrical–asymmetrical distri-bution and implications for seafloorspreading processes. In: Faulting andMagmatism at Mid-Ocean Ridges (W.R.Buck, P.T. Delaney, J.A. Karson and Y.Lagabrielle, eds), pp. 153–176. AmericanGeophysical Union, Washington, DC.

Le Pichon, X., Bonnin, J. and Sibuet, J.C.,1970. La faille nord-pyreneenne: failletransformante liee a l�ouverture du Golfede Gascogne. C. R. Acad. Sci. Paris, 271,1941–1944.

Le Roux, V., Bodinier, J.-L., Tommasi, A.,Alard, O., Dautria, J.-M., Vauchez, A.and Riches, A.J.V., 2007. The Lherzspinel lherzolite: refertilized rather thanpristine mantle. Earth Planet. Sci. Lett.,259, 599–612.

Lemoine, M., Boillot, G. and Tricart, P.,1987. Ultramafic and gabbroic oceanfloor of the Ligurian Tethys (Alps,Corsica, Apennines): in search of agenetic model. Geology, 15, 622–625.

Lockwood, J.P., 1971. Sedimentary andgravity slide emplacement of serpenti-nites. Geol. Soc. Am. Bull., 82, 919–936.

Manatschal, G., 2004. New models forevolution of magma-poor rifted marginsbased on a review of data and conceptsfrom West Iberia and the Alps. Int. J.Earth Sci., 93, 432–466.

Marroni, M. and Pandolfi, L., 2001. Debrisflow and slide deposits at the top of theInternal Liguride ophiolitic sequence,Northern Appennines, Italy: a record offrontal tectonic erosion in a fossilaccretionary wedge. Island Arc, 10,9–21.

Minnigh, L.D., Van Calsteren, P.W.C. andden Tex, E., 1980. Quenching: an addi-tional model for emplacement of thelherzolite at Lers (French Pyrenees).Geology, 8, 18–21.

Monchoux, P., 1970. Les lherzolites pyr-eneennes: contribution a l�etude de leurmineralogie, de leur genese et de leurstransformations. These d�Etat thesis,Toulouse.

Monchoux, P., 1972. Roches a sapphirineau contact des lherzolites pyreneennes.Contrib. Mineral. Petrol., 37, 47–64.

Montigny, R., Azambre, B., Rossy, M. andThuizart, R., 1986. K-Ar study of Cre-taceous magmatism and metamorphismfrom the Pyrenees: age and length ofrotation of the Iberian Peninsula.Tectonophysics, 129, 257–273.

Olivet, J.L., 1996. La cinematique de laplaque Iberie. Bulletin des Centres de

Recherche Exploration-Production Elf-Aquitaine, 20, 131–195.

Pin, C. and Vielzeuf, D., 1983. Granulitesand related rocks in Variscan medianEurope; a dualistic interpretation.Tectonophysics, 93, 47–74.

Reisberg, L. and Lorand, J.-P., 1995.Longevity of sub-continental mantlelithosphere from osmium isotopesystematics in orogenic peridotitemassifs. Nature, 376, 159–162.

de Saint Blanquat, M, 1993. La faillenormale ductile du massif du SaintBathelemy. Evolution hercynienne desmassifs nord-pyreneens catazonaux con-sideres du point de vue de leur histoirethermique. Geodin. Acta, 6, 1, 59–77.

Scharer, U., de Parseval, P., Polve, M. andde Saint Blanquat, M., 1999. Formationof the Trimouns talc-chlorite deposit(Pyrenees) from persistent hydrothermalactivity between 112 and 97 Ma. TerraNova, 11, 30–37.

Seyler, M. and Bonatti, E., 1988. Petrologyof a gneiss-amphibole lower crust unitfrom Zabargad island, Red Sea. Tec-tonophysics, 150, 177–208.

Sibuet, J.C., Srivastava, S.P. and Spakman,W., 2004. Pyrenean orogeny and platekinematics. J. Geophys. Res., 109, doi:10.1029/2003JB002514.

Ternet, Y., Colchen, M., Debroas, E.J.,Azambre, B., Debon, F., Bouchez, J.L.,Gleizes, G., Leblanc, D., Bakalowicz,M.,Jauzion, G., Mangin, A. and Soule, J.C.,1997. Notice explicative, Carte geol.France (1 ⁄50 000), feuille Aulus les Bains(1086), Orleans: BRGM, 146 p.

Vetil, J.Y., Lorand, J.-P. and Fabries, J.,1988. Conditions de mise en place desfilons des pyroxenites a amphibole dumassif ultramafique de Lherz (Ariege,France). C. R. Acad. Sci. Paris, 307,587–593.

Vielzeuf, D. and Kornprobst, J., 1984.Crustal splitting and the emplacement ofPyrenean lherzolites and granulites.Earth Planet. Sci. Lett., 67, 87–96.

Vissers, R.L.M., Drury, M.R., Newman, J.and Fliervoet, T.F., 1997. Myloniticdeformation in upper mantle peridotitesof the North Pyrenean Zone (France):implications for strength and strainlocalization in the lithosphere. Tectono-physics, 279, 303–325.

Whitmarsh, R.B., Manatschal, G. andMinshull, T.A., 2001. Evolution ofmagma-poor continental margins fromrifting to seafloor spreading. Nature,413, 150–154.

Received 11 June 2007; revised versionaccepted 26 September 2007


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