Petrogenetic evolution of orogenic lherzolite massifs in the central and western Pyrenees

TECTONOPHYSICS ELSEVIER

Tectonophysics 292 (1998) 145-167

Petrogenetic evolution of orogenic lherzolite massifs in the central and western Pyrenees J. Fabribs

a,*, J.-E Lorand a, J.-L. Bodinier b

"Laboratoire de Mingralogie, URA-CNRS 736, Mus#um National d'Histoire Naturelle, 61 rue Buffon, 75005 Paris, France b UMR 5569 - - G#ofluides-Bassins-Eau, ISTEEM, C.P. 057, Universitg de Montpellier 2, Place E. Bataillon, 34095 Monrpellier Cedex 05, France

Received 30 June 1997; accepted 12 January 1998

Abstract

A petrological and geothermobarometric study of several tens of representative spinel peridotite and co-existing pyroxenite samples indicates that orogenic lherzolite massifs of the central-western Pyrenees (CWP) display significant differences from those of the eastern Pyrenees (EP). They are characterized by a predominance of clinopyroxene-rich spinel lherzolites, an abundance of coarse-granular textures, the absence of thick harzburgitic bands, lower proportions of pyroxenites, the absence of high-pressure amphibole-pyroxenite dykes, and a greater development of high-stress deformation textures which define 100m-scale mylonitic shear zones. The highly fertile compositions of the CWP peridotites can be accounted for by accretion of asthenospheric protoliths to the lithosphere via passive cooling or very limited degrees of adiabatic melting. This accretion event was followed by a thermal relaxation, resulting in the steady-state equilibrium stage (1050°C and 15-18 kbar) recorded by the garnet pyroxenites and granular lherzolites. Compared to the single decompression and cooling step identified in the EP lherzolite massifs, the CWP massifs record a two-step, non-adiabatic uplift. The earliest event is a nearly isothermal (1050-950°C) decompression from 60 km up to 25 km depth caused by a lithospheric thinning event, possibly related to the late Hercynian extension. The further uplift step from 25 km to 15 km depth was accompanied by cooling down to 600°C and high-stress mylonitic deformation, resulting in tectonic denudation and emplacement of the western Pyrenean massifs onto the floor of small Albo-Aptian pull-apart basins via large-scale shear zones. Geospeedometric considerations yield estimates around 5-10 Ma for the duration of cooling and mylonitic deformation, coeval with the mid-Cretaceous anticlockwise motion of the Iberian and European plates. So, the western Pyrenean peridotite massifs left the mantle between 109 and 117 Ma and were uplifted to shallower levels (15 km) than the EP massifs. © 1998 Elsevier Science B.V. All rights reserved. Keywords: upper mantle; orogenic lherzolites; Pyrenees; continental lithosphere; geochemistry; geospeedometry

1. I n t r o d u c t i o n

The lithospheric upper mantle beneath continents is sampled by undersaturated lavas and oro* Corresponding author. Tel.: +33 (1) 4079-3522; Fax: +33 (1) 4079-3524; E-mail: [email protected]

genic peridotites. Whereas xenoliths are accidentally collected in areas of continental rifting, orogenic peridotites have been emplaced in tectonically active regions which show evidence of superimposed tectono-magmatic events (Menzies and Bodinier, 1993). Orogenic peridotite massifs so far studied have recorded various stages of magmatism and

0040-1951/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PI1 S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 0 5 5 - 9

146

J. Fabriks et al./Tectonophysics 292 (1998) 145-167

lithospheric thinning (e.g. Bodinier et al., 1987a; Downes et al., 1991; Rampone et al., 1995; Van der Wal and Bodinier, 1996) which can be unravelled because structural relationships between constituent rock types are preserved. Feedback constraints on the structural development of orogenic zones can also be drawn from the study of orogenic peridotites. In particular, their deformation textures and the compositional zoning of their constituent minerals may provide geospeedometric estimates for the activity of large-scale lithospheric shear zones during continental extension and breakup (Vissers et al., 1995). Unlike the other orogenic lherzolite massifs known in the world which are large but isolated bodies, the Pyrenean lherzolite massifs occur as about 40 small lenses disseminated over 200 km along the Pyrenean Range. Thus, they provide a large-scale probe of the sub-continental lithospheric mantle in a continental suture zone, the north Pyrenean metamorphic zone separating the Iberian Peninsula from the western European plate. Most previous studies have focussed on eastern Pyrenean massifs which are located in the Arirge department (see Fabrirs et al., 1991; Downes et al., 1991; Mukasa et al., 1991; and references therein). In contrast, very little is known on the seventeen massifs, some with large dimensions (2 × 3 km), which crop out farther to the west, extending to the Atlantic coast. Previous works are mostly unpublished (e.g. Rio, 1967; Monchoux, 1970; Gaudichet, 1974) or have focussed on specific aspects (e.g. garnet pyroxenites; Kornprobst and Conqurrr, 1972). A preliminary survey of these massifs revealed differences in lithology and mineralogy (Fabrirs et al., 1991) compared to those of the eastern Pyrenean massifs. These are discussed in more detail in the present paper, which is based

on a study of several tens of representative spinel peridotite samples, newly collected in the largest massifs from the central and western Pyrenees, and reference samples of co-existing pyroxenites. A detailed geothermobarometric study of both peridotites and pyroxenites has been done to refine the models previously proposed for the evolution of orogenic peridotite massifs throughout the Pyrenean Range. Major and trace element contents of peridotites provide additional constraints on the nature of the protolith of Pyrenean peridotite massifs. 2. Geological setting

The 40 orogenic peridotite massifs recognized in the Pyrenees are distributed into 7 groups (Fig. 1). To the east, the Salvezines, Prades-Bestiac, VicdessosLherz, and Castillon groups are the eastern Pyrenean (EP) massifs. The central and western Pyrenean peridotite massifs (CWP), located to the west of the Castillon north Pyrenean massif, are seventeen lherzolite bodies, ranging in dimension from a few metres to 3 km. They are usually divided into three main groups (Lacroix, 1894; Monchoux, 1971), i.e. Tuc Desse-Arguenos, Avezac-Moncaut, and Turon de Trcou6re-Col d'Urdach. The Tuc Desse-Arguenos group referred to as the central Pyrenean peridotite massifs comprises six massifs, of relatively large dimensions (from 500 m to 3 km) (Fig. 1A), which crop out in a geological setting very similar to those of the Ari~ge massifs. They are located to the south of granitoids and gneiss forming the 'north Pyrenean Hercynian massifs', the Milhas massif in that case. They are all embedded within the north Pyrenean metamorphic zone (NPMZ), a narrow (0-5 km wide), steeply

Fig. 1. Structural sketch map of the Pyrenees showing the location of the main groups of orogenic lherzolite massifs. Dotted area: Hercynian basement (north Pyrenean massifs and Palaeozoic axial zone). Oblique ruling: Mesozoic sediments of the north Pyrenean zone and equivalents. Black: north Pyrenean metamorphic zone. SPU = South Pyrenean units; PAZ = Palaeozoic axial zone; NPF = north Pyrenean fault; NPFT = north Pyrenean frontal thrust. (A) Structural location of the Tuc Desse-Arguenos group of orogenic lherzolite massifs (after Can6rot and Debroas, 1988). Legend: 1 = Upper Cretaceous flysch (upper Cenomanian to lower Senonian); 2 = black flysch (upper Albian to lower Cenomanian); 3 = Jurassic and Lower Cretaceous (carbonated rocks); 4 = Cretaceous metamorphic zone; 5 ----orogenic lherzolite massif; 6 = Palaeozoic units. B.F. = Bouigane fault. Orogenic lherzolite massifs: 12 = Arguenos-Moncaup; 13 = Terres N~res; 14 = Col d'Aillos; 15 = Tuc Desse; 16 = Tuc de Haurades; 17 = Porter d'Aspet. (B) Structural sketch map of the western part of the north Pyrenean zone (after Canrrot, 1988). Legend: 1 = diapiric Triassic sediments; 2 = Cretaceous metamorphic band; 3 = lherzolite massif. Orogenic lherzolite massifs: 3 = Col d"Urdach; 4 :- Prdaing; 5 = Saraill6; 6 = Turon de Trcourre; 7 = Moncaut; 8 = Montaut.

147

J. FabriCs et al./Tectonophysics 292 (1998) 145-167

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J. FabrWs et al./Tectonophysics 292 (1998) 145-167

east-west-dipping, band of Mesozoic metasediments affected by a low pressure-high temperature metamorphic peak (T = 550-650°C, P < 3-4 kbar), dated at 95 Ma (e.g. Montigny et al., 1986; Golberg and Leyreloup, 1990). The central Pyrenean peridotite massifs lie along the northern edge of a major E - W fault set (the 'Bouigane fault'), which continues to the east across the Ballongue Cretaceous sedimentary basin to the Coumes peridotite massif of the Castillon group (Monchoux, 1968) (Fig. IA). This fault marks there the southern boundary of the NPMZ. Granulite facies rocks, representing lower crust and lenses of mantle-derived rocks were emplaced into the NPMZ (Vielzeuf, 1984). Moreover, friable granulated agglomerates containing sapphirine, kornerupine, gedrite, corundum, cordierite and phlogopite have been found at the contact of the Tuc Desse and Arguenos-Moncaup massifs (Monchoux, 1972). They are interpreted as silica-deficient Mg-rich crustal rocks deriving from the middle-upper continental crust and metamorphosed at 800-900°C and total pressures ranging between 5 and 9 kbar by contact with the lherzolite (Vielzeuf and Kornprobst, 1984). The geological setting of the westernmost peridotite massifs (Avezac-Moncaut, Turon de Trcoubre-Col d'Urdach) differs significantly because of the absence of north Pyrenean Hercynian massifs. Granulite facies rocks (Vielzeuf, 1984) and sapphirine-bearing metamorphic rocks are lacking. The Palaeozoic basement is restricted to some outcrops (gneiss, schists, quartzites, granites) in the vicinity of the lherzolite bodies. Peridotite massifs are isolated in the southern faulted limbs, or in the periclinal ends of major anticlines, which affect essentially Jurassic-Cretaceous sediments of the north Pyrenean zone. Some massifs (e.g. Turon de Trcou~reUrdach group and the western part of the AvezacMoncaut group) are located along the diapiric zone of Moncaut-Sarrance-Roquiague (Fig. 1B), which resuited from the Lower Cretaceous halokinesis of Triassic evaporites followed by a strong N-S extension with crustal thinning during the Cretaceous (Can6rot, 1988). This structure affects a narrow belt of metamorphosed post-Palaeozoic formations which, there, marks the NPMZ. Near the NW contact of the Col d'Urdach massif, of large dimensions (1500 x 1000 m), decimetre-sized pebbles of altered lherzolite are

enclosed in lower Cenomanian flyschs, indicating a pre-Cenomanian age for the emplacement of the lherzolite into the upper crust, at least in this part of the Pyrenees (Schoeffler et al., 1964; Gaudichet, 1974). 3. Main lithological features of the CWP massifs This study is concerned with seven of the largest CWP massifs (Portet d'Aspet, Tuc Desse, ArguenosMoncaup, Montaut, Moncaut, Turon de Trcourre, Col d'Urdach). These are mainly composed of foliated lherzolites associated with spinel websterite microlayers (less than 1% of the total volume of the lherzolite massifs). Metamorphic foliation can be very strong (e.g. in the Turon de T6cou~re lherzolite massif) and has affected a hectometre-scale tectonite shear zone bounded by granular lherzolites. This foliation affects both lherzolites and websterites. Where mylonitization is very strong (e.g. Turon de Trcou~re), spinel websterites are strongly boudinaged. Thick harzburgitic bands are entirely absent. In addition to the diffuse websterite layering, thicker spinel websterite bands (4-12 cm thick) are heterogeneously distributed throughout the peridotite masses. Clinopyroxene-poor lherzolites (<1 m in thickness) have locally been observed on either side of isolated single spinel websterite layers. Starting with the EP massifs, a westward variation exists in the nature of isolated pyroxenite layers throughout the Pyrenean Range. Plagioclase replacing spinel is common in the CWP massifs, especially at Saraill6 (one of the westernmost massifs), whereas this mineral is very rare in spinel websterites from the EP massifs. Conversely, garnet pyroxenites occur in the Tuc DesseArguenos group as well as in several EP massifs (Lacroix, 1894, 1917; Monchoux, 1970; Komprobst and Conqurrr, 1972). Although their field relations with surrounding peridotites are not clearly visible, garnet pyroxenites seem to occur as solitary sills, 550 cm thick, devoid of compositional zoning and intensely boudinaged. Garnet pyroxenite was reported by Lacroix (1894) from the Moncaut massif but none in the massifs located farther to the west, i.e. in the Turon de Trcourre-Col d'Urdach group. In the latter massif, a unique, Ti-rich spinel websterite has been collected in addition to normal spinel websterites. CWP massifs have undergone hydrothermal alteration, the degree of which varies from one massif

J. FabriCset al./Tectonophysics 292 (1998) 145-167

to another. For instance, the Moncaut peridotites are 40 to 60% serpentinized, those from Col d'Urdach up to 80% and the small outcrop of Portet d'Aspet is almost totally serpentinized. In contrast, Turon de Trcou~re peridotites are less than 10% serpentinized. In all CWP massifs, serpentinization is generally pervasive and developed independently from the tectonic contacts. Relic cores of olivine crystals are isolated within a mesh-textured serpentine ('pseudomorphic lizardite') and orthopyroxene is partly replaced by a 'bastite' pseudomorph. Moreover, the Avezac-Moncaut group display a pervasive, locally intense, development of fine-meshed carbonate. Actinolitic amphibole and talc may also replace pyroxenes, especially in the Tuc Desse, Arguenos-Moncaup and Col d'Urdach massifs (Monchoux, 1970; Gaudichet, 1974). Spinel is transformed into diaspore in some websterites of the Saraill6 and Col d'Urdach massifs and the spinelplagioclase pyroxenites in the Saraill6 massif underwent a partial rodingitic alteration giving rise to grossular + diopside + chlorite + vesuvianite assemblages (Gaudichet, 1974).

4. Analytical methods Major- and trace-element contents of 58 peridotite, 3 garnet pyroxenite and 1 spinel websterite samples have been determined by wet chemical methods, AAS and 1NAA at the University of Montpellier. The REE were determined by INAA (for more details, see Savoyant et al., 1984). Electron microprobe analyses of the main silicates and spinel were done in 38 samples of peridotite and 7 pyroxenites (including 3 garnet pyroxenites), using the automated CAMECACAMEBAX electron microprobe of the Musrum National d'Histoire Naturelle (MNHN), Paris. Representative bulk-rock analyses are given in Table 1, while the ranges of mineral compositions are listed in Tables 2 and 3. The complete set of data is available from the authors on request. 5. Peridotites 5.1. Modal composition

Modal compositions of the peridotites have been calculated by a least-square routine from bulk-

149

rock chemistries and averaged mineral compositions (MNHN; unpubl.). Among the 58 samples studied, 46 are spinel lherzolites; their average clinopyroxene modal content is 14 -4- 2 wt.%. This great abundance of lherzolite cannot be attributed to sampling bias, as the same relative proportion of lherzolite was found in a systematic sampling made along a NE-SW traverse, perpendicular to the foliation, in the Turon de Trcou~re massif. The refractory wall rocks of pyroxenites are clinopyroxene-poor lherzolites (5-10 wt.% Cpx). Only one sample (TUR15) has a harzburgitic modal composition (4 wt.% Cpx), but its clinopyroxene content is probably underestimated due to the degree of serpentinization (L.O.I. --- 7.95). Accessory minerals (normally < 1%), like Ti-pargasite, Cu-Fe-Ni sulphides (see Lorand, 1991) and plagioclase, are disseminated throughout the host rock. Disseminated pargasite is ubiquitous and relatively abundant (1-2 wt.%) in some samples from Tuc Desse, Arguenos-Moncaup and Turon de Trcourre. This mineral may be intimately intergrown with vermicular spinel or can occur as coarser crystals at the triple junctions between olivine and pyroxene crystals, or sometimes as inclusions in orthopyroxene. There is no sign of disequilibrium between the four anhydrous phases and the disseminated Ti-pargasite. Plagioclase replaces spinel porphyroclasts in the most mylonitized lherzolites from Turon de Trcou~re. 5.2. Texture

The peridotites exhibit a wide range of textures, sometimes within a single massif. The oldest recognizable type is coarse granular according to the nomenclature of Harte (1977). This texture is well preserved at Tuc Desse, Arguenos-Moncaup and Moncaut (Fig. 2a). The grain size is the largest found in the Pyrenean peridotite massifs. Many olivines and orthopyroxenes are centimetre-sized with some crystals reaching up to 4 cm in diameter. They may be free of sub-grain and kink-band boundaries and their grain boundaries are irregular and curved. Clinopyroxene crystals generally are smaller. Both pyroxenes generally have few exsolution lamellae. Relic associations of lobed and/or vermicular spinel grains intergrown with pyroxenes suggest a preexisting protogranular texture (Mercier and Nicolas, 1975). Large neoblasts (0.15-0.5 mm diameter) with

J. Fabriks et al./Tectonophysics 292 (1998) 145-167

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Table 2 Range of mineral chemistry of the major phases in CWP spinel peridotites, grouped according to lithology Olivine

Orthopyroxene

Clinopyroxene

Spinel

41.58-40.37 10.49-9.18 50.22-48.57 0.49-0.15 -

57.14-54.38 5.78-2.14 0.48-0.09 7.01-5.35 34.71-31.46 0.95-0.22 -

52.70-50.61 1.05-0.44 8.10-4.81 1.30--0.22 2.83-2.22 16.98-13.71

61.46-54.91 12.77-6.35 13.38-9.54 20.74--18.26

Cpx-poor spinel lherzolites SiO2 40.92-40.20 TiO2 A1203 Cr203 FeO 9.50-8.31 MgO 51.18-49.22 NiO 0.38-0.20 CaO Na20 -

55.55-54.22 4.61-1.52 0.52-0.06 6.52-5.47 33.61-33.22 0.43-0.21 -

Spinel harzburgites SiO2 41.0(040.35 TiO2 A1203 Cr203 FeO 10.07-9.13 MgO 49.86--48.81 NiO 0.38-0.08 CaO Na20 -

56.34-54.76 4.69-1.66 0.51q). 19 6.34-5.78 34.53-33.20 0.37-0.27 -

Spinel lherzolites SiO2 TiOz A1203 Cr203 FeO MgO NiO CaO Na20

120° triple junctions may represent 5 to 15% of some coarse-granular peridotites. This, and the occurrence of subgrain boundaries in some olivines mark incipient recrystallization into a porphyroclastic texture. A transitional texture between coarse-granular and porphyroclastic predominates in the Arguenos-Moncaup, Tuc Desse and Moncaut massifs although it can be observed in other C W P massifs. It is characterized by an increasing development of a metamorphic foliation defined by elongation of olivine and pyroxenes and alignment of spinel grains. Porphyroclasts show tight substructures and kink bands. Locally, olivine porphyroclast boundaries are underlined by trails of small-size neoblasts (0.05-0.1 m m diameter), which indicate the imprint of a c o n t i n u i n g / i n c r e a s i n g highstress deformation (Fig. 2b). These secondary-foliated porphyroclastic textures are characterized by a

21.93-18.60 2.30-0.97 52.34--51.55 0.58--0.40 6.88-4.73 1.49-0.44 2.40-2.12 15.44--14.14

54.42-32.70 35.66-14.43 15.20-11.48 19.60-14.51

21.87-20.28 2.27-1.67

52.69-46.91 17.77-12.65 19.79-16.55 18.11-15.59

very heterogeneous distribution of the recrystallized matrix on the scale of a thin section. The porphyroclastic texture grades into mylonitic textures in the 100-m-thick shear zone that affects Turon de T6cou6re. Similar, but much thinner, mylonitic zones ( < 1 0 cm thick) are also found in the Tuc Desse massif. In mylonitic lherzolites, deformed and rounded porphyroclasts ( 1 - 3 m m diameter) of olivine and clinopyroxene, and extremely stretched and thinned porphyroclasts of orthopyroxene with undulose extinction (Fig. 2c) are enclosed in a mosaic-textured matrix which is surrounded by anastomosing thin bands of ultrafine-grained ( < 1 0 - 2 0 Ixm) material. The latter cannot be resolved by optical microscopy and it imports a fluidal aspect to the texture (Fig. 2d). The fine-grained matrix may also isolate lenticular domains, a few rnm thick, made up

J. Fabriks et al./Tectonophysics 292 (1998) 145-167

153

Table 3 Range of mineral compositions of the major phases in CWP pyroxenites, grouped according to lithology Orthopyroxene

Clinopyroxene

Spinel

55.44-53.86 0.29-0.05 5.17-2.84 0.39-0.13 6.84-6.06 34.38-32.75 0.62-0.19 -

51.91-49.33 0.98-0.39 7.69-5.46 1.01-0.07 2.92-1.96 15.36-13.41 22.02-20.07 2.17-0.87

66.86-57.63 9.38-0.90 12.31-10.12 21.37-19.42 -

50.59-49.37 1.47-1.31 8.84-7.70 0.17-0.00 2.87-2.34 14.43-13.21 21.98-21.30 1.69-1.46

68.87-67.34 0.25-0.04 12.15-10.97 21.01-19.88 -

51.53-50.17 1.29-0.71 9.18-6.67 0.504).04 3.06-2.43 14.76-13.30 21.04-19.90 2.13-1.20

63.59-60.44 3.91-3.51 13.96-13.17 19.09-17.85 -

Garnet

Spinel websterites SiO2 TiO2 A1203 Cr203 FeO

MgO CaO Na20

Coarse-grained spinel websterites SiO2 TiO2 A1203 Cr203 FeO

MgO CaO Na20

53.91-52,15 0.254).05 6.38-4.99 8.27-7.77 31.69-30.57 0.97-0.23 -

Garnet pyroxenites SiOz TiO2 A1203 Cr203 FeO

MgO CaO Na20

54.83-52.96 0.28-0.05 6.63--4.07 0.29-0.01 9.04-6.68 33.39-30.19 0.47-0.22 -

of a recrystallized olivine mosaic with larger grain size (20-150 t~m). The transformation of spinel into plagioclase (An37_48) that characterizes the highly mylonitized peridotites in the Turon de T6cou~re massif involves several successive microstructural changes. In the first stage, spinel porphyroclasts are surrounded by a continuous corona of plagioclase, which progressively recrystallized as polygonal neoblasts (50 Ixm), with 120° triple junctions (see fig. 4f in FabriCs et al., 1991). In the second stage, spinel porphyroclasts are themselves recrystallized, forming strongly flattened aggregates of polygonal neoblasts. In the final stage, plagioclase and spinel neoblasts are dispersed in the fine-grained matrix, along with Ti-pargasite grains. Plagioclase-producing reactions do not involve olivine or pyroxenes. On the other hand, a colourless metamorphic amphibole (Ti-free pargasite) is often observed in the mylonitic bands that contain plagioclase.

41.89--40.77 24.84-23.37 0.62-0.06 13.34-9.34 19.82-14.74 6.85-4.44

5.3. Whole-rock geochemistry The range of major-element contents of peridotites encompasses the one defined by the EP peridotites with, however, more fertile lherzolite compositions (Fig. 3). Except for a moderate LREE depletion, samples DES7, PH1, TUR7 have major and trace element abundances very similar to primitive upper mantle (PUM) estimates (4.2 wt.% A1203; 3.2 wt.% CaO; 0.18 wt.% TiO2; Mg/Ca = 10; Lu = 68 ppb; HREE abundances 2 × C 1 - chondrite; see McDonough and Sun, 1995). The relatively undepleted nature of the mantle sampled by the CWP massifs is shown by the large number of samples containing more than 3.6 wt.% A1203 (Fig. 3). The abundances of CaO, Na20, TiO2 and HREE (e.g. Yb) correlate positively with A1203 whereas the Mgnumber correlates negatively. Such elemental covariations in mantle rocks have been widely attributed

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to variable degrees of partial melting and melt extraction (Frey et al., 1985; Bodinier, 1988a). Nevertheless, some scatter is observed for CaO, Na20 and TiO2, suggesting that these elements were somewhat remobilized after the melting event. The highly serpentinized samples from Portet d'Aspet, Urdach and to a lesser extent Moncaut are markedly lower in CaO and Na20, two elements that are known to be mobile in hydrothermal fluids and are often depleted relative to A1203 in serpentinized peridotites. Several lherzolites, the Turon de T6cou~re harzburgite (TUR15), and two clinopyroxene-poor lherzolites from Arguenos-Moncaup (MON1, MON3) have higher TiO2 and Na20 contents, which correlate with a larger amount of amphibole in these samples. Their

Ti/V ratios are also higher compared to those of the other refractory peridotites. The lherzolites have relatively homogeneous REE patterns (Fig. 4) characterized by a moderate LREEdepletion (La/YbN = 0.2-0.3). Chondrite-normalized REE patterns of the most fertile samples (DES7, PHI) display an almost fiat segment between the middle and heavy REE, and about twice the chondritic abundances of Tb (0.104 ppm) and Yb (0.44 ppm). Apart from these fertile samples, all of the CWP lherzolites (except MON4) show HREE fractionation and Tb/YbN ratios < 1. The Tb/Yb ratios decrease in parallel with decreasing Yb, suggesting that garnet was present in the source (Bodinier et al., 1988). However, this ratio varies significantly

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1

L)

-'-

PH1

¢-

=

I

0.5

ej

o i

i

i

i

0.1!5

i --~[

i

T

i

T~I

i

i

r

Turon de T6cou~)re

~

TUR16

5.4. Mineral chemistry of major phases (Table 2)

0.5

0.1

,

2

i

i

i

i

i

i

p

i

i

i

i

i

Urdach

-i

0.5

01

between the massifs, regardless of the degree of fertility. The Arguenos-Moncaup and Urdach massifs tend to be relatively unfractionated, with ratios near 1 (YbN around 1) even in rather refractory peridotites. Conversely, the Moncaut and Turon de T6cou6re massifs show more fractionated patterns, with Tb/YbN < 1 in fertile lherzolites (YbN around 2). As observed in various suites of mantle rocks (e.g. McDonough and Frey, 1989; and references therein), refractory peridotites have higher LREE/HREE ratio than the coexisting lherzolites, a feature which is not consistent with partial melting. Moreover, the clinopyroxene-poor lherzolites from Tuc Desse and those from Arguenos-Moncaup have markedly distinct REE patterns. The two clinopyroxene-poor lherzolite wall rocks from Tuc Desse (DES5 and DES6) are depleted in middle and heavy REE, but their patterns display marked LREE enrichments. Such REE patterns have been frequently reported for harzburgites in orogenic lherzolite massifs and basalt-borne xenoliths (Bodinier et al., 1988; McDonough and Frey, 1989; Downes et al., 1991; Van der Wal and Bodinier, 1996). The clinopyroxene-poor lherzolites MON1 and MON3 from Arguenos-Moncaup display convex REE patterns with the apex at Sm, which closely resemble those of the peridotites adjacent to the amphibole pyroxenite veins from the Lherz massif, in the EP massifs (Bodinier et al., 1988, fig. 6A; Bodinier et al., 1990).

LaCe

. . . .

SmEu

.

.

o

.

.

.

YbLu

Fig. 4. Chondrite-normalized REE abundances for spinel peridotites from two central (Tuc Desse, Arguenos-Moncaup) and three western (Moncaut, Turon de T6cou6re, Col d'Urdach) Pyrenean massifs. Normalizing values are taken from McDonough and Frey (1989).

Olivine shows very little compositional variation. Fo contents range between 89.0 in the most fertile lherzolites to 91.3 in the clinopyroxene-poor lherzolites. Orthopyroxene exhibits a narrow range of Mgnumber (0.895-0.915) but a large range of A1 and Cr contents, which vary according to both bulk-rock chemistry and orthopyroxene grain size. The A1203 contents of cores of large, cm-sized megacrysts from coarse-granular lherzolites and porphyroclasts from deformed lherzolites tend to correlate positively with bulk-rock A1203 contents (Fig. 5). Up to 5.8 wt.% A1203 is found in some orthopyroxene cores, in agreement with the fertile bulk-rock composition of the CWP peridotites. The AlzO3 contents of porphyroclasts less than 1.5 mm in diameter decrease

J. Fabrids et al./Tectonophysics 292 (1998) 145-167 CORE COMPOSITIONS

157

AI

6~20C

OPX

e~ <

O0 5

o

s " "t; 4 I

1

I 3

°O

I 4

°

°

°

•

100 8

°

O

rock AlzO3 %

I 2

o00

"

g.

3

°

oO

-

°

c,x

O

° °

°

°

7-

°

t9

•

~,

;

a' °

0 0

6

5

I 1

I 2

r o c k AlzO3 % i i 3 4

Fig. 5. Variation of A1203 (wt.%) content of, respectively,orthopyroxenesand clinopyroxenes(core compositions)vs. A1203 (wt.%) whole-rockcontent in the CWP peridotites.

in parallel with decreasing grain size, especially in the Turon de Trcourre mylonitic lherzolites (Fig. 6). Core-to-rim decrease of the contents of A1 and Cr are also observed in the large orthopyroxene megacrysts and porphyroclasts. The neoblasts are significantly poorer in A1 and Cr than the porphyroclast rims. Clinopyroxenes are chromian diopsides ( C a 4 2 _ 5 0 Fe4-sMg45_53; Cr203 = 0.7-1.5 wt.%). As observed for orthopyroxene, there is a positive correlation between the A1203 contents in the cores of large clinopyroxene crystals and the bulk-rock A1203 contents (Fig. 5). The cores of the large crystals from the fertile lherzolites (e.g. TUR7, PHI, URD1) contain up to 8.1 wt.% A1203 and are also the richest in Na20 (up to 2.3 wt.%) and TiO2 (up to 1.0 wt.%). Na and Ti correlate negatively with Cr. All of the clinopyroxene porphyroclasts are characterized by a core-to-rim decrease of A1, Cr, Na and Ti. The clinopyroxene neoblasts are significantly poorer in A1, Na, Ti and Cr than the porphyroclasts. The A1 and Na contents are particularly low in the neoblasts from the Turon de

r 0.1

__L

grain siz~ 1

lOmm

Fig. 6. Variation of AI content (x 1000) in core orthopyroxene (atoms per 6 oxygens) vs. grain size from Turon de Trcou~re peridotites. Trcou~re mylonitized peridotites, probably because spinel is replaced by plagioclase (An40-45). Spinel from lherzolites is very poor in chromium, in agreement with the fertile nature of bulk-rock compositions. Like the pyroxenes, its compositional variations are related to both bulk-rock chemistry and grain size. The spinel grains larger than ~0.2 mm show relatively constant Mg-(0.72-0.80) and Cr-number (Cr-number, defined as Cr/(Cr + A1 + Fe3+); 0.025-0.15) (Fig. 7). Spinels richer in chromium (0.2 < Cr# < 0.41) occur in the two clinopyroxene-poor lherzolites from Arguenos-Moncaup. In spinel grains smaller than 0.1-0.2 mm, Mg# and Cr#, respectively, decrease and increase in parallel with decreasing grain size. 6. Pyroxenites

6.1. Petrography The spinel-plagioclase websterites typical of the westernmost massifs are characterized by a coarse-

158

J. Fabriks et al./Tectonophysics 292 (1998) 145-167

Mg/(Mg+Fe 2+)

0.8 •

•

O•

°gO O ql,~

o

•

0.7

~• 0.6

i

I

i

L

Cr/(Cr+AI+Fe3+) • 0.2

.'..•

.

":

...

!

•

0.1

•

•

••

•% 15

L

I

I

0.1

0.3

1.0

I

10.0 g r a i n size (rnm)

Fig. 7. Variation of Mg/(Mg + Fe 2+) and Cr/(Cr + A1 + Fe3+), respectively (core compositions) vs. grain size for spinels in Turon de T6cou~re peridotites.

granular texture (grain size up to 1 cm), with clinopyroxene generally more abundant than orthopyroxene. Orthopyroxene/clinopyroxene modal ratios decrease from the core to the rim of the thickest layers in the Urdach massif. The garnet pyroxenites of the Arguenos-Moncaup massif occur as three types, i.e. spinel-garnet websterite, garnet websterite, and garnet clinopyroxenite (Kornprobst and Conqu6r6, 1972). Like the peridotites, these garnet pyroxenites have coarse-granular to porphyroclastic textures. Relic megacrysts of orthopyroxene and clinopyroxene, up to 5 cm in length, are frequent. They coexist either with garnet and large green spinel crystals (spinel-garnet websterites), or only with garnet (garnet websterites and clinopyroxenites). Clinopyroxene megacrysts exsolved abundant orthopyroxene, spinel and/or gar-

net, sometimes visible on hand sample scale. Orthopyroxene is rich in thinner clinopyroxene, spinel and/or garnet exsolutions. Small (1-3 mm) garnet, more or less replaced by kelyphitic assemblages (spinel + orthopyroxene + plagioclase), also occurs along the grain boundaries of the pyroxenes. These textural criteria, coupled with modal compositions, indicate that each of the three types of garnet pyroxenites can be accounted for by unmixing and secondary recrystallization of primary assemblages composed of Al-rich clinopyroxene + orthopyroxene + spinel (spinel-garnet websterites), or Al-rich clinopyroxene ÷ garnet (garnet websterites and garnet clinopyroxenites). In the latter two types of garnet pyroxenites, some garnets are considered belonging to the primary assemblage because they form much larger, cm-sized grains containing rutile exsolutions. Spinel-garnet websterites are also present in the Moncaut massif (sample H43; Lacroix, 1917) and one garnet clinopyroxenite has been sampled from Tuc Desse (sample H41; Lacroix, 1917). Almost all pyroxenite samples contain minor amounts of interstitial brown amphibole, which is especially abundant in the Moncaut spinel-garnet websterite. Kaersutite replacing clinopyroxene is present in a coarse-grained spinel websterite from Col d'Urdach (sample III33b).

6.2. Mineral chemistry (Table 3) The thinnest spinel websterite layers show mineral compositions identical to those of peridotites. In contrast, different compositions are observed in the coarse-grained spinel websterite from Col d'Urdach (sample lII33b) and the garnet pyroxenites. In both rock types, orthopyroxene is AlzO3-rich and clinopyroxene TiOe-rich (about 1 wt.%) and CrzO3-poor. The highest TiO2 contents (1.3-1.5 wt.%) correspond to sample II133b which otherwise contains large kaersutite crystals. As observed in the peridotites, A1203 contents decrease significantly towards the rims of pyroxene porphyroclasts. Primary pale green spinel in spinel-garnet websterites is very poor in chromium (3.53.9 wt.% Cr203). Primary garnets have core compositions Pyry0Alm17Gr12Spl, whereas the smaller subsolidus garnets are less magnesian (Pyr63-61AIm22_26Gr12Spl-3). Plagioclases in spinel-

Z Fabriks et al,/Tectonophysics 292 (1998) 145-167

plagioclase websterites from Saraill6 have a restricted compositional range (An39-46), whereas in the garnet websterite H43 from Moncaut, the plagioclase replacing spinel is a labradorite (An51-53). Disseminated brown amphibole is a kaersutite (Mg# = 0.820-0.86; TiO2 = 4.7-6.0 wt.%).

159

900 _T "6 1 0-2 10 -3 •

//

7. Geothermobarometry

•

eQ

" 10_4 i0-5

/

7.1. Peridotites

The strong A1-Cr zonation patterns recorded by the pyroxenes and spinel and the close relationship between these patterns and the grain size of minerals from the fine-grained matrix clearly indicate both partial re-equilibration stages at decreasing temperature and increasing stress and strain rate. In order to quantify these stages, geothermometry was performed in terms of crystal habit, grain size and core/rim distinctions, as outlined by Conqutr6 and FabriCs (1984). Temperatures were obtained by applying the two-pyroxene thermometer (Wells, 1977), the olivine-spinel thermometer (FabrEs, 1979) and the spinel-orthopyroxene-olivine thermometer (Sachtleben and Seck, 1981). The temperatures computed by both pyroxene thermometers from core compositions of large pyroxene porphyroclasts cluster around 950-1000°C: for instance, the sp-opx-ol thermometer yields an average temperature of 1000 ± 32°C for peridotites from Turon de T6cou~re. Rim analyses of large orthopyroxene crystals, as well as the neoblast compositions, yield systematically lower temperatures, about 700-750°C for the Tuc Desse massif, and down to 630 ± 35°C for the Turon de Ttcou~re massif. This variation reflects the significant decrease of A1 content that parallels the decreasing grain size of orthopyroxene (Fig. 6), and accordingly the increasing intensity of shear deformation. As is usual for orogenic lherzolites, the olivinespinel geothermometer yields lower temperatures than do the pyroxene geothermometers (FabriCs, 1979). For ten samples from the Turon de T6coutre massif, the temperatures calculated on core analyses of 54 mineral pairs range between 890 ° and 595°C and decrease as the grain size of spinel decreases (Fig. 8). Such a variation clearly indicates a continuous cooling stage from --~900°C down to 600°C.

grain size ,

I

0.1

I

I

1.0mm

Fig. 8. Relations between calculated temperature from core composition of olivine-spinel pairs and grain size of spinel from Turon de Ttcou~re peridotites. The cooling rate curves (°C year -1) are from Ozawa (1984).

Equilibrium pressures of spinel peridotites are difficult to determine owing to the lack of a reliable geobarometer. The absence of plagioclase and garnet in the coarse-granular- and porphyroclastic-textured peridotites places very wide constraints on the equilibrium pressure between 8 and 17 kbar in the relevant temperature range around 950°C. However, the development of plagioclase rims around spinel in the most deformed, mylonitized lherzolites, especially at Turon de Trcourre, indicates pressures lower than 8 kbar for temperatures of 600-650°C (Fig. 9). In these rocks, the mineral assemblage olivine + plagioclase + orthopyroxene + clinopyroxene + spinel may provide more accurate information on pressure conditions through the two following independent mineral reactions: CaAlzSizO8Plag + Mg2SiO4ol = MgAl2SiO6opx + CaMgSizOrcp x

(1)

2Mg2SiO4ol + CaA12Si2Osplag = Mg2SizO6op× + CaMgSi206cp× + MgAlzO4sp (2) The equilibrium conditions of these reactions were calculated for samples TUR1 and TUR16 using the Thermo-Calc program (Holland and Powell, 1990). The curves representative of the two equilibrium reactions (1) and (2), intersect at 4.8-5.0 (+1.5) kbar

160

J. FabriCset aL /Tectonophysics 292 (1998) 145-167 TEMPERATURE (°C) 600

BOO

1000

1200

1400

2O

'° i 2c

60

30

Fig. 9. P-T path inferred for pyroxenitesand mylonitic lherzolites from the CWP massifs. The phase diagram for the ultramafic system is constructed after data from Takahashi (1986) for melting curve of dry peridotite KLB-1 and from Ito and Kennedy (1968) for B3P1 composition. Dashed lines represent steady-state conductivegeothermsof 90, 120, and 150 mW m-2 (Pollack and Chapman, 1977). The enclosed felds refer to P-T conditions for: (A) primarycrystallizationof garnet pyroxenites; (B) peak metamorphismof north Pyrenean granulites (Vielzeuf, 1984); (C) Cretaceous metamorphism of the north Pyrenean zone (Golberg and Leyreloup, 1990). The arrow indicates the latest stage of subsolidus re-equilibration recorded by eastern Pyreneanperidofitemassifs (see fig. 7 in FabriCset al., 1991). and 600-620 (4-50)°C. These latest equilibration conditions recorded by the mosaic-textured matrix in mylonitized peridotites from the western Pyrenean massifs are very close to the transition between the spinel and plagioclase lherzolite fields. So, the high-stress deformation episode that produced the mylonitized peridotites was accompanied by a simultaneous decrease of pressure (down to about 5 kbar) and temperature (down to 600-650°C). Because of its lack of textural relationships with olivine and pyroxenes and its relatively sodic composition, plagioclase likely formed by reaction between spinel and Na-bearing metamorphic fluids which also produced the colourless metamorphic amphibole. It is the mylonitic deformation which favoured plagioclase nucleation.

tion with the two-pyroxene thermometer (Wells, 1977) and the orthopyroxene-olivine-spinel thermometer (Sachtleben and Seck, 198l). All reveal the same equilibration temperature range (9001000°C) as that recorded by the pyroxene cores in the surrounding peridotites (Table 4). Concerning the garnet clinopyroxenite sample H41 (Tuc Desse) in which clinopyroxene is extensively replaced by pargasite, the temperatures estimated from the garnet/clinopyroxene-Fe/Mg geothermometer (Ellis and Green, 1979) are consistent with those obtained using the Holland and Blundy (1994) thermometer based on the AI(IV) content of amphibole (930°C vs. 920 + 26°C at 6 kbar, respectively). Pressure was estimated by using the Harley (1984b) barometer which is based on the transfer reaction involving the solubility of A1 in orthopyroxene coexisting with garnet. This barometer coupled with the Harley (1984a) thermometer allows simultaneous estimation of equilibrium temperature and pressure for each garnet-orthopyroxene pair. The core compositions of garnet-coarse orthopyroxene pairs yield 1066 4- 50°C, 16.4 4- 2.6 kbar for sample MON19, and 1037 i 50°C, 15.8 + 2.5 kbar for sample H43. These pressure estimates are more or less reproduced by applying a recent geobarometer (Nimis, 1995) based on crystal structure modelling of clinopyroxene from basaltic systems (Table 4). There is, however, a discrepancy in sample H41. In this particular case, one may suspect that the clinopyroxene barometer is probably less reliable, because of the extensive replacement of this mineral by secondary (Ti-poor) pargasite. When applied to subsolidus garnet coexisting with orthopyroxene neoblasts or porphyroclast rims, the Harley barometer and thermometer yield, respectively, 833 ± 35°C, 7.6 -4-2.0 kbar and 740 4- 50°C, 6.0 4- 2.5 kbar, values consistent with the occurrence of plagioclase in the kelyphitic corona surrounding garnet. 8. Discussion

7.2. Pyroxenites

Pyroxenites allow independent estimates of equilibrium temperatures and pressure to be made. The garnet-orthopyroxene thermometer (Harley, 1984a) and the garnet-clinopyroxene thermometer (Ellis and Green, 1979) were used in conjunc-

8.1. P - T - t conditions for the emplacement of the CWP lherzolite massifs into the Albo-Cenomanian pull-apart sedimentary basins

The highest P - T conditions (950-1000°C and 15-18 kbar), recorded by the mineral core corn-

J. FabriOs et al./Tectonophysics 292 (1998) 145-167

161

Table 4 Mean apparentequilibriumconditions(±1 S.D.) for the pyroxenitesin central-westernPyrenees Sample

Twell~

Zss81

TEG79

TGa/Opx

937 ± 18

987 ± 15

884 ± 30

976 ± 28 831 ± 28 930 ± 40

1037 ± 740 ± 1066 ± 833 ±

PGa/Opx

PCpx

15.8 ± 6.0 ± 16.4 ± 7.6 ±

12.8 ± 1.6

Garnet pyroxenites H43 cg H43 sg MON19 cg MONI9 sg H41

50 50 50 35

2.5 2.5 2.6 2.0

15.4 ± 1.0 6.1 ± 1.5

Spinel pyroxenites TURI7 C23 III33b A2b core A2b rim

931 ± 26 814 i 25

1021 ± 3 0 1011 ± 2 8

972 ± 10 823 ± 27

11.1 ± 12.6 ± 12.3 ± 10.5 ± 6.5 ±

1.1 2.0 2.0 2.0 2,0

Temperatures are given in °C and pressures in kbar. TWells, two-pyroxene thermometer of Wells (1977); Tss81, spinel-orthopyroxene-olivine thermometer of Sachtleben and Seck (1981); TEG79: Fe-Mg/garnet--clinopyroxene thermometer of Ellis and Green (1979); TGa/Opx: Fe-Mg/garnet-orthopyroxene thermometer of Harley (1984a); PGa/Ovx, garnet-orthopyroxene barometer of Harley (1984b); Pcpx, clinopyroxene barometer of Nimis (1995). cg, coarse grain of orthopyroxene; sg, small grain of orthopyroxene. Massifs: H43. Moncaut; MONI9, Arguenos-Moncaup; H41, Tuc Desse; TUR17, Turon de T6cou~re; C23 and III33b, Urdach; A2b, Saraill&

positions in the garnet pyroxenites and the coarsegranular peridotites define the earliest steady-state equilibrium stage that can be identified in the CWP massifs. A similar stage has already been identified and referred to as D1-R1 in the eastern Pyrenean massifs (Fabri6s et al., 1991). Starting from this D1-R1 stage, the sub-solidus evolution of lherzolitic massifs strongly diverged between the EP and CWP massifs. While a single decompression and cooling step has been identified in the eastern Pyrenees, the CWP garnet pyroxenites record a two-step, non-adiabatic uplift. The earlier step is a nearly isothermal (1050-950°C) decompression from 50-60 km up to about 25 km. In contrast, the further uplift step from 25 to 15 km depth was accompanied by a more accentuated cooling down to 600°C and a highstress mylonitic deformation at increasing stress values, as indicated by the progressive reduction in neoblast grain size. The P - T conditions recorded by neoblast and porphyroclast-rim compositions of the mylonitized samples cluster at the transition between the spinel and plagioclase lherzolite stability fields, which is consistent with the fact that pyroxenites and mylonitized peridotites contain both spinel and plagioclase. These conditions define a steep steadystate conductive geotherm of about 120-150 mW m -2 (Pollack and Chapman, 1977) which fits the geotherm (about 140 mW m -2) developed in the

Mesozoic sediments of the north Pyrenean zone during the Cretaceous high-temperature-low-pressure metamorphism. So, there is little doubt that the mylonitic deformation is related to the emplacement of the CWP massifs at a depth of 10-15 km into the mid-Cretaceous pull-apart sedimentary basins. As shown by mineral chemistry, the mineral assemblages only partially re-equilibrated during the latest uplift stage into the mid-Cretaceous sedimentary basins. So, the temperature data along the 150120 mW m -2 geotherm in fact represent blocking temperatures. The fact that the lowest blocking temperatures correspond to the highest degrees of mylonitization shows that the reduced grain size allowed diffusive exchange to continue to low temperatures in the mylonites. Such a relationship between blocking temperature and grain size can be used to estimate cooling rates from the olivinespinel geospeedometer of Ozawa (1984). The result ( a b o u t 10-4°C year-l; Fig. 8) combined with the lowest blocking temperature (600°C) recorded by the olivine-spinel geothermometer suggest a duration of about 3 Ma for the latest uplift step. Values of the same order of magnitude are obtained by considering the parameters governing the mylonitic deformation. The geopiezometer of Ross et al. (1980), applied to the grain sizes of syntectonic olivine neoblasts, indicates a deviatoric stress of about 1 ± 0.5 kbar,

162

J. Fabriks et al./Tectonophysics 292 (1998) 145-167

which transposes into a shear strain of about 10 -13 s -1 at 800°C using the Karato et al. (1986) equation. A shear velocity of about 1.0 4- 0.6 mm year -1 is deduced from the shear strain, taking into account a mean thickness of 100 to 500 m for mylonitized bands (cf. Turon de Trcourre). This shear velocity brackets the duration of the last uplift step between 5 and 13 Ma. Whether estimated from the Ozawa's getspeedometer or from the conditions of the mylonitic deformation, the time span of the last uplift stage of the CWP massifs agrees well with the maximum duration postulated for the unusually strong geothermal anomaly that characterized the NPMZ during the mid-Cretaceous (3 to 6 Ma; Albarrde and Michard-Vitrac, 1978). Moreover, such shear velocity is supported by independent stratigraphic and structural constraints from the Albo-Cenomanian pull-apart sedimentary basins. For each individual basin, the extent of the left displacement along the E - W segment of the north Pyrenean fault did not exceed 20 km (Debroas, 1987). As the pull-apart basins developed between 114 Ma and 95 Ma during the sinistral rifting of the Iberian and European plates, the mean relative velocity of the elementary motion along the NPMZ was about 1 mm year -1. These independent constraints provide additional supports to our conclusion that the latest stage of emplacement of the CWP massifs and the related mylonitic deformation of the peridotites were coeval with the mid-Cretaceous anticlockwise motion of the Iberian and European plates along the NPMZ. It has been postulated that the CWP massifs were emplaced into the crust before the onset of the Cretaceous alkaline magmatism because they are totally devoid of the late generation of high-pressure amphibole-rich dykes known in some EP lherzolite massifs (Fabrirs et al., 1991). This assumption is confirmed by the presence of acid, plumasite-like, dykes recently dated at about 104 Ma in the Avezac and Col d'Urdach massifs (B. Azambre, pers. commun., 1997). Taking conservative values (5-13 Ma) for the uplift from 25 to 15 km and a minimum crustal emplacement age of 104 Ma, one may deduce that the mantle slices left the mantle between 117 and 109 Ma in the westernmost Pyrenees. This age is, within error, consistent with the beginning of the anticlockwise rotation of Spain relative to Europe (114 Ma).

8.2. The 100 mW m -2 steady-state geotherm and the Late Hercynian continental lithospheric thinning The nearly isothermal branch of the P - T path from depths of 60 km (DI-R1 stage) to 25 km (Fig. 9) imposes to consider an important lithosphere thinning event predating the middle Cretaceous emplacement of the CWP massifs into the Albo-Cenomanian pullapart basins. It must be noted that the P - T conditions of the last equilibration of some spinel websterites from the Turon de Trcoubre-Col d'Urdach group (10-13 kbar; 1000 -4- 50°C) correspond to a geothermal gradient of 100-110 mW m-2). This gradient fits the conditions of equilibration of Hercynian granulites coexisting with the lherzolite massifs in the NPMZ (Vielzeuf, 1984). One may therefore infer that the continental lithosphere thinning event recorded by the westernmost Pyrenean lherzolite massifs is in part related to the generalized extension that affected the Variscan Belt of western Europe since the Stephanian (Burg et al., 1994), especially in the Pyrenees (Vissers, 1992). From shear-wave splitting measured in the Pyrenees, Vauchez and Barruol (1996) suggested that the mantle seismic anisotropy recorded in this orogen may be inherited from a lithospheric structure formed mostly during the Hercynian orogeny. The intersection between the 100 mW m -2 geothermal gradient and the dry solidus of the lherzolites (Fig. 9) constrains the thickness of the lithosphere at about 50-60 km at this period. Thus, the extension of the nearly isothermal branch to low-pressure (8 kbar) would be younger and related to the middle Cretaceous lithospheric thinning. 8.3. Garnet pyroxenites and the D1-R1 equilibration stage The oldest equilibration stage recorded by garnet pyroxenites (D1-R1) and coarse-granular peridotites corresponds to a model 80-90 mW m -2 continental geotherm (Pollack and Chapman, 1977). This latter intersects the dry lherzolite solidus at about 80 4- 10 km (Fig. 9). If our conclusion relating the nearly isothermal decompression path shown by the CWP massifs to the late Hercynian lithosphere thinning were correct, then this thickness should characterize the continental lithosphere beneath the Pyrenees before the late Hercynian extension. Much older model

J. Fabriks et al. / Tectonophysics 292 (1998) 145-167

ages, up to the Mesoproterozoic, have been suggested for garnet pyroxenites by preliminary Re/Os isotope systematics (Reisberg and Lorand, 1995b). However, these ages need to be refined before they can be used in a quantitative discussion. Petrographic criteria indicate that present-day mineralogy and texture of the garnet pyroxenites result from unmixing and subsolidus recrystallization of primary assemblages composed of various proportions of Al-rich orthopyroxene and clinopyroxene megacrysts with minor spinel and gamet. Wholerock compositions allow a reconstruction of these primary assemblages by using binary plots such as Ca vs. Mg (Fig. 10), Ca vs. Na, and Si vs. A1. The data points of spinel websterites lie very close to the clinopyroxene-orthopyroxene tie line (Fig. 10). However, the pyroxenes alone cannot explain the high AlzO3 contents of some samples, which testify to the presence of small amounts of spinel in their primary parageneses. Conversely, the bulk-rock compositions of garnet clinopyroxenite (M12, MONll) and garnet websterite (M17) effectively plot away from the clinopyroxene-orthopyroxene tie-line towards the composition of primary garnet, which supports the assumption that garnet was present in the primary mineral assemblage.

1614-

~ 0 MONH•

12-

aOo (Wt.%)

8

C)

\

I• 3 I

H41 11141

\\

~

I~ ! I

t.Z.._2_l .... M20• 11144

] [ MONI0•• • 116 / MONI2• • • H43 A2b

4 15

20

25

30

M g O (wt.~) Fig. 10. Bulk-rock wt.% contents of CaO vs. MgO in pyroxenites from CWP lherzolite massifs. Symbols: 1 = garnet pyroxenite; 2 = spinel websterite from Col d'Urdach; 3 = Saraill6; 4 = Turon de T6cou~re; 5 = primary composition of clinopyroxene from Arguenos-Moncaup; 6 = primary composition of garnet from Arguenns-Moncaup (Kornprobst and Conqu6r6, 1972). Enclosed fields are the compositions of high P - T clinopyroxenes (CPX) and orthopyroxenes (OPX) after Bender et al. (1978) and Green et al. (1979).

163

Mineral assemblages similar to those reconstructed for the CWP pyroxenites have been produced experimentally at high-pressure (16-18 kbar) from olivine tholeiites. The olivine-rich tholeiite B3P1 (Mg-number = 78) used by Ito and Kennedy (1968) crystallized the same assemblage (cpx + opx + sp) as the garnetspinel websterite (M20, MON 10, MON 12), while the primary assemblages (cpx + ga zk (opx)) inferred for the garnet websterite (M 17) and garnet c linopyroxenite (M12, MON11) have been obtained by Green and Ringwood (1967) from an olivine tholeiite with lower Mg-number (72). The bulk compositions of pyroxene megacrysts recalculated by incorporating the various exsolved minerals (Kornprobst and Conqu6r6, 1972) compare well with the pyroxenes crystallized at 15 kbar near the liquidus (at about 1350°C) of the tholeiitic basalt 527-1-1 (Bender et al., 1978) and at 15-20 kbar for the basalt DSDP 3-18 + 9 wt.% olivine composition (Green et al., 1979). These striking analogies make the interpretation already proposed by Conqu6r6 (1977) and Bodinier et al. (1987b) for the eastern Pyrenean garnet pyroxenites also valid for the CWP massifs, i.e. the thick spinel- or garnet-bearing pyroxenite layers are high-pressure crystalline segregates separated from tholeiitic melts circulating within open cracks. The clinopyroxene grid of Gasparik (1984) indicates that the primary assemblages started to crystallize in the magmatic conduits at 1300-1350°C, 1617.5 kbar ('A' in Fig. 9). Thus, the primary magmatic pyroxenes unmixed and recrystallized down to the D1-R1 equilibration stage at nearly constant pressure. As shown by Sautter and Fabribs (1990), such an isobaric cooling is characteristic of narrow magmatic conduits solidifying in a colder lithospheric mantle. It can be deduced that the CWP lherzolite massifs were still parts of a 80-km-thick lithosphere when they were cross-cut by the tholeiitic melts. At a later stage after this magmatic event, the mantle section represented by CWP massifs was percolated by small melt fractions which precipitated interstitial disseminated Tipargasite both in peridotites and layered pyroxenites (FabriCs et al,, 1991). 8.4. Nature of the mantle protolith of the CWP massifs

The close similarity of some lherzolite major element compositions with Primitive Upper Mantle

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(PUM) estimates has been outlined above. Likewise Os isotopic ratios recently determined by Reisberg and Lorand (1995a) in samples TUR7 and DES7 (0.1284) suggest a derivation from the convecting mantle source more radiogenic than the Modern Depleted Morb-source Mantle (0.1247; Meisel et al., 1996). These values fit the range of Os isotopic compositions postulated for the hypothetical Primitive Upper Mantle (0.1290 4- 0.009; Meisel et al., 1996). Assuming that this reservoir contained about 4.2 wt.% A1203 (cf. TUR7), the inversion method of Bodinier (1988b) indicates that the CWP lherzolites experienced between 0 and 7% of magmatic removal. The low-degree melting determined above is symptomatic of a limited adiabatic decompression path which likely started at the spinel-garnet boundary. This is constrained in Fig. 9 by the depth at which lherzolites equilibrated after partial melting (50 -4- 10 km) and the location of the dry solidus of four phase peridotites. The variation of Tb/Yb vs. Yb indicates that garnet was present in the source of some massifs and lobed and/or vermicular spinel intergrown with pyroxenes may be decomposition products of this garnet. Because of the small degree of melting determined above, it is highly unlikely that the few clinopyroxene-poor lherzolites identified in the CWP massifs represent residues of a higher degree of partial melting. Their systematic occurrence as wall rocks of thin spinel websterite layers indicates that they are products of melt-rock reactions (e.g. Kelemen, 1990; Bodinier et al., 1991). Only melt-rock reactions, implying precipitation of olivine and dissolution of pyroxene, can account for the geochemical characteristics of these refractory peridotites that are not consistent with partial melting, such as the relatively low Mg-number, the elevated Ti contents, and the LREE/HREE ratios higher than in coexisting lherzolites. Both the highly fertile nature of the lherzolites and the scarcity of refractory rocks can be explained by a geodynamic situation in which the newly created lithosphere markedly thickens by conductive cooling and downward capture of the uppermost convecting mantle (Wilson et al., 1994; Carlson, 1995). Such a situation is known in transform zones of modern ocean floors (Seyler and Bonatti, 1997).

9. Conclusion The present study puts forward significant differences as regards lithology, whole-rock chemistry and phase chemistry between the EP and CWP lherzolite massifs, which suggest that these two groups of massifs occupied distinct tectonic environments during the primordial accretion event to the sub-continental lithospheric mantle. The highly fertile compositions of CWP peridotites can be accounted for by accretion of asthenospheric protoliths to the lithosphere via passive cooling or very limited degrees of adiabatic melting, perhaps in a transform zone setting. A series of events clearly happened in the westernmost massifs before the middle Cretaceous uplift along mantle shear zones, which have not been recognized elsewhere in the Pyrenees. The isothermal decompression from 60 to 30 km recorded by the pyroxenites is likely to be related to the lithospheric thinning event that started during the late Hercynian extension and ended during the Middle Cretaceous. It is assumed that the westernmost massifs left the mantle between 117 and 109 Ma and were uplifted to shallower levels (15 km) compared to the EP massifs. Thus they escaped to metasomatic reactions with the alkali magmatism erupted throughout the north Pyrenean zone and the Spanish Basque Country during the middle Cretaceous (113-85 Ma) counterclockwise rotation of Iberia with respect to Europe. Tectonic denudation at relatively high temperature and emplacement by shear zones directly onto the floor of the small Albo-Cenomanian pull-apart basins would also explain the extensive hydrothermal alteration of the westernmost Pyrenean massifs (e.g. carbonation at Avezac-Moncaut and serpentinization/rodingitic alteration). The time span proposed for the sequence of emplacement of the CWP massifs is closely similar to that deduced from radiometric ages (3.4 4- 3.1 Ma) for tectonic denudation and cooling of the mantle beneath the west Galicia margin during the continental break-up between Iberia and Newfoundland (Scharer et al., 1995).

Acknowledgements This paper has benefited from constructive discussions with B. Azambre. We are grateful to L. Reis-

J. Fabriks et al./Tectonophysics 292 (1998) 145-167

berg, H. Downes, M.A. Menzies and two anonymous referees for their critical comments and helpful suggestions on the various versions of the manuscript.

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