Geochemistry and petrogenesis of Eastern Pyrenean peridotites

Gcwhimicu o ~‘~mwchihimicu Anu Vol. 52. pp. 2893-2907 Copyright 0 1988 Pergsmon Press pk. Printed in U.S.A.

00167037/88/$3.00 + .OO

Geochemistry and petrogenesis of Eastern Pyrenean peridotites

J. L. BODINIER', C. DUPUY’ and J. DOSTAL~

I Centre G&@ique et Gkopbysique, Universiti des Sciences et Techniques du Languedoc, 34060 Montpellier, France z Department of Geology, Saint Mary’s University, Halifax, Nova Scotia, Canada, B3H 3C3

(Received September 14, 1981; accepted in revisedform September 23, 1988)

Abatnct-The high-temperature peridotite bodies of the Eastern Fyrkkes (France), which are composed of spine1 peridotites containing bands of pyroxenites and veins of amphibole-beating uhrabasic rocks, have gone through a multi-stage evolution. The petidotites underwent partial melting in the stability field of garnet resulting in major variations of Mg, Al, Ca, Na, Ti, Sc, V, Ni and HREE. Then the peridotite residue was invaded by basaltic melts. The pyroxenite bands in the peridotites are high-pmssun crystal segregates from these melts. Subsequently, after cooling in subcontinental lithospheric conditions, the peridotites interacted with alkali magma which was probably associated with the Ctetaceous alkali magmatism of the Pyn%es. In addition to the crystallixation of amphibole-rich ultrabasic reeks in vein-conduits and the re-equilibration of the wall-rock peridotites leading to LREE, Ti and Fe enrichments, this event was accompanied by extensive metasomatic proasses The metasomatism locally affected lhetzolitea, producing an inctease of the modal proportions of clinopyroxene (k amphibole) (Caussou massif). The metasomatism was more widespread in the harxburgites where it produced an enrichment of LREE relative to HREE without a significant change in the modal composition.

INTRODUCTION

THE HIGH-TEMPERATURE peridotite massifs of the Eastern Pyr&n&s (France) represent pieces of the upper mantle tec- tonically emplaced into the crust (AVE LALLEMANT, 1967; CONQUIS, 1977.1978). They arc composed mainly of spine1 peridotites with bands of pyroxenites and veins of amphibole rich ultmbasi~ rocks (CONQUBRE, 1977, 1978). Their size, reaching up to 1 km in length, overcomes the sampling lim- itations usually encountered in the investigation of ultramafic xenoliths, and thus, the massifs provide a good opportunity to study upper mantle composition and processes and the relationship among the various rock-types.

The purpose of this study is to present major and tmce element data on peridot& from nine ultrama6c bodies of the Eastern Pyr&&-s, to evaluate the chemical interaction between the peridotites and the pyroxenite bands and veins and to provide insights into the diverse processes which affected upper mantle composition. Previously we showed that anhydrous pyroxenite bands were formed by precipitation from basaltic magmas similar to Mesozoic tholeiite (B~DINIER

ef al., 1987a) while amphibole pyroxenite veins were derived from alkali basaltic magma (E~ODINIER ef al., 1987b).

GEOLOGICAL AND PETROGRAPHIC NOTES

Numerous peridotite bodies ranging in size from a few m to 1 km crop out within a belt up to 1 km wide called the Internal Metamorphic Zone (RAWER, 1959), which stretches along the North Pymnean Fault (Fig. 1). The belt has tectonic contacts and was affected by high-T low-P metamorphism during the Albo-Ccnomanian (ALBAREDE and MICHARD Vrnuc, 1978; GOLBERG et al., 1986; MONTIGW et al., 1986).

Nine massifs of the Eastern Pyr&&s were studied. They range in sixe from about 5 X 5 m (Pit Couder) to 700 X 1600 m (Lhetz). Apart from brecciated and serpentinixcd zones up to several m wide, the mass& are composed of relatively fresh ultrama6c rock containing usually < 15% serpentine. A brief description of the individual bodies and the

2893

analyzed samples is given in the Appendix. The bodies consist of spine1 lherzolites which are intercalated with bands of harzburgites and clinopyroxene-poor lherzolites up to several tensof meters thick (e.g. J_ACROIX, 1901, 1917; bVIER, 1959; AVE LALLEMANT, 1967; CONQU~RE, 197 1,1977,1978). The layering also is paralleled by bands of spine1 websterites (3- 20 cm thick), and in the Lhetz, Freychinide and Elestiac bodies, by bands of rare garnet pyroxenites (0.3- 1 m thick). The garnet pyroxenites are subpamllel to the layering, but in places they cut the layering and foliation at a high angle. Locally, these pyroxenites (anhydrous pyroxenites) predominate vol- umetrically over peridotites. The ultramatic sequences of the Lherz, FmychinZde and Bestiac mass& are crosscut by veins of phlogopite-bearing homblendites and amphibole-bearing pyroxenites up to 30 cm thick (hydrous pyroxenites).

The peridotites display a significant variation ‘in modal compositions, particularly in the proportions of clinopyroxene, both within the individual massifs and among the massifs. Spine1 lherzolites that contain 10 to 15% clinopyroxene and 2 to 3% spine1 predominate. Harxburgites (~5% clinopyroxene) and clinopyroxene-poor lherzolites (5 to 10% clinopyroxene) form bands 5 to 60 m thick which constitute up to about 20% of the complexes (e.g. Lherz). Clinopyrox- ene-rich lhet-zolites which contain 20 to 30% clinopyroxene and 4 to 8% spine1 occur in zones adjacent to the pyroxenite bands. At the Caussou massif, clinopyroxene-rich ihetzolites unrelated to the pyroxenite bands comprise about half of the massif. Amphibole (Table 1) is present in small amounts (< 1%) in peridot& of some massifs (e.g. Lherx); its abundance tends to increase close to the contact with the amphibole pyroxenite dikes. In addition, irregular patches of peridotite up to I m in sixe with as much as 20% of amphibole (Ti-pargasite) occur locally in the Caussou massif (CON- QUER& 1971, 1978).

The mass& underwent a complex precmplacement evolution including at least two distinct stages of deformation and recrystallization (CONQU~RE and FABRIEES, 1984). The first stage took place at temperatures of about 900” to 1000°C

2894 J. L. Bodinjer, C. Dupuy and J. Dostal

+ FONTCTE AOUGE _+

+ -+ i- + t i + ._ _+.

FIG. 1. A: Simplified geological map of the Pyr6n6e-s (modified after CHOUKROUNE and S~GURET, 1973). I) Cenozoic and Mesozoic sediments; 2) Internal Metamorphic Zone: Mesozoic metasedimentary rocks; 3) Pre-Mesozoic basement. B and C: Simplified geological maps of the Internal Metamorphic Zone in the Lherz area (modihed after MONCHOUX, 1970) showing locations of the peridotite bodies (black). The other symbols are the same as in Fig. IA.

and a depth of 50 to 70 km. The second episode of deformation (700”-750°C) was accompanied by synkinematic decompression at a depth of about 25 km. The variation of texture of the peridotites is related to the two cooling stages. A coarse-grained protogranular texture well-developed in the Fontete rouge massif was probably formed at the end of the first stage. The second episode of deformation was responsible for a widespread porphyroclastic to equigmnular texture with a distinct foliation (CONQUI?R& 1978; CONQU!?R~ and FA- BR&S, 1984).

SAMPLING AND ANALYTICAL METHODS

The studied peridotites from Lherz. Font& rouge, FreychinZde, Best& and Caussou are from the suite described by CONQUW (1978) and FABRICS and CONQU~R~Z (1983). The samples from the other massifs were newly collected for this project. Sixty-seven samples were selected for the chemical analyses. This set includes 45 samples of “massive peridotites” that were collected at a d/stance > 1 m from the pyroxenite bands or dikes. The rest of the samples are “wall-rock peridotit& collected at a distance of I to 5 cm from the pyroxenites, with the exception of kelyphite lherrolites from the Be&c massif. some of which are from a distance ofabout 10 cm. The compositions of the representative peridotites are given in Table I and the trace

element abundances of separated mineral phases are given in Table 2. The major element compositions of the analyzed minerals were reported by CONQUERS ( 1978).

The minerals were separated by an electromagnetic technique accompanied by hand-picking under a binocular microscope. The es- timated purity of the mineral separates is better than 99%. The purified separates were washed in hot distilled water.

The major elements, Sr, Cr. Co, V, Ni and Zn, were determined by atomic absorption. Rare earth elements (REE) were analyzed by the radiochemical neutron activation technique (SAVOYANT ei al.. 1984), whereas Th, Ta, Hf and Sc were determined by instrumental neutron activation. The precision and accumcy ofthe analytical data have been reparted by BODINIER et al. (1987a) and can be aho ju&d from the results on the replicate analyses of the standard rock UBN given in Table I.

GEOCHEMISTRY

Major elements

The analyzed peridotites display variations in their major element composition (Table 1) with the Lherz maaaif having the largest span of values. In Lherz, the (Mgl vaiues (Mg/Mg + Fe2+) range from 0.91 in massive harzburgites to 0.80 in the wall-rock Ihenolites.

Pyrenean peridotites 2895

LNERZ

13-104 71-321 71-322 71-107 ,I-,24 70-321 71-367 71-,26

” ” L- 1.. I. I. L L

SIO) (2) 41.58

*‘z”3 0.62 Fe*O,(T) 8.,0 H”” 0.12

w 44.07 Cd “.,R

&?a 0.04

K?O 0.005 71”) D.05 L.0.l. ,.9,

IHRI .9”9

: 99.08

h (Pd 0.05‘ ce 0.062 M sm 0.030 E” 0.01‘ Th 0.010 Yb 0.065 ,.,I 0.011 SC Nf SC 5.6 ” 34 CT ,725 CO 132 L” 14

42.95 0.6‘ 8.24 0. I2

43.68 “.‘” 0.05 0.006 0.“2 2.59

.910

98.70

“.“2, 0.08,

0.018 0.007

0.005 O.026

0.001 4

U.“, 6.5

32 127”

130 65

44.91 1.40

R.B7 0. I,

41.3, 0.98

0.12 0.01, 0.05 1.00

.901

98.80

0.06, 0.17

0.071 0.029 0.020

0.09, 0.015 ‘

Cl.“, 8.3

‘8 ,665

127 112

‘3.95 1.61 8.20 0.1,

‘I.65 l.‘O

“.I, 0.008 U.“, 1.15

.908

98.,0

0.28 0.51

0.09 0.29

0.018 0.1,

0.024

5

II.0 55

,010

120 72

‘2.9, 2.8”

“.bO 0.13

,a.24

2.60 0.22 0.006

0.1, 2.92

.895

98.60

0.15 0.‘5

0.47 0.,9

0.075 0.054

O.,Z 0.052 5

0.1, 14.5

,7 2‘20

II6 91

‘3.1‘ 2.97

8.25 0.1,

37.68

,.“O 0.2”

0.007 0.12-

2.21

.897

97.8‘

6.17 0.68

0.27

0.11

0.07

0.29 0.1143 9

O.IIl L5.L

85

I490 117

79

44.1, 3.40

8.21 0.13

37.20

,.:I 0. ,b

0.02,

U.1, 0.88

.a97

97.74

U.23 0.91

“. 2”

“.I, 0.075

0.17 0.060

10 “.I5

16.2

88 )095

115 69

41.16 ,.6”

8.97 0. I‘

%.LR

1.1” ;i.z,

Il.cc’,

0. Lb

?.“”

.887

9”.“9

0.18 0.69

0. 2 5

11.10

0.08 C.‘O

0.002 ”

0.: 5 I5.h

RI 2325

II, 76

I FOHTETE ROUGE FREYCHIVEOE

FM-1‘ 72-4,2 ?2-228 FON-27 :,-151 7s’r25 71.3,5 II-,,6 7,-,,9 ” H L L L I. I. L I.

so2 (2) 44. I‘

*’ PI 0.67 ~“a%‘” 8.61

0.1,

HF.0 “.95 00 0.62 &P 0.0, 3,” 0.00,

L.Oil. 0.0‘ 0.97

‘,.I* 0.7, 8.96 0.12

‘4.57 0.89 0.0‘ 0.012

0.05 ,.a2

411.22 44.31 3. I5 1.27 11.52 8. I‘ O.!, 0.13

3R.7, 37.51 3.13 2.*0 0.18 0.27 0.110 I “.“,>I 0. : I 1C.11 0.69 2.09

lhcl .911 .907 .906 .9”” .a92 .89, .899 .90” .894

r 10”. I‘ 99.95 99.‘5 99.1, 99.RO 98.0, 99.19 98.90 99.09

La (PPd 0.14 0.35 “.I, 0.12 U.14 ce 0.22 0.68 0.47 0.55 0.60 Nd 0. lb 0.55 0.,9 sm U.067 0.2, “.I7 0.25 “.,I E” 0.028 0.10 0.070 0.10 “.,I Tb 0.1119 0.080 U.0‘8 “.“72 “. 080 Yb 0. I5 0. ,R O.,, 0.30 0.39 I.” 0.028 0.060 u.057 “.OSL~ “.OtJ7 Sr ‘ 42 I7 9 7 II R B “I 0.0, 0. 1 I O.OR 0. I I 0. to SC 4.2 I,.‘ 13.3 I5.u 15.0 ” ,I ,I 50 90 90 78 12 82 “5 CT 2935 27,s 1285 2235 2500 2490 22011 2,20 10,O co 121 119 II5 102 107 115 I16 115 Li‘ 7.n 110 110 59 157 100 58 55 II 60

LO.,. -

L- Loss on igntttoo; FezO,(T) - Total Fe ils Pep,; iHRl - tag& + 7%“: H - “arrhurplrcs;

- CPX-ooer soincl IhermlItc”: I. - Solnel Iherzolllea. nl IS belO” derecrlon Iimits I” a,, IhF

In the massive peridot&s, [Mg] and MgO correlate neg- atively with A&Or, (20, NarO and TiOr (Fig. 2). Their compositional range is equivalent to that observed in the ultramalic xenoliths (HUTCHINSON ef al., 1975; DUPUY ef al., 1986; STOSCH et al., 1986) and also in other high-tempemture peridotite bodies such as Ronda (FREY ef al., 1985). The exceptions to the variation trends displayed on Fig. 2 are the kelyphite lherzolites from Bestiac, which are depleted in Ca relative to the massive spine1 lherzolites, and the clinopyroxene/amphibole-rich peridotites of Caussou, which are enriched in Ca, Ti, Na and K.

but slightly depleted in Ca. Similar compositional differences were noted in the Ronda massif (FREY et ai.. 1985) and in ultramafic xenoliths of alkali basalts (WILSHIRE and JACKSON, 1975; IRVING, 1980; KEMPTON, 1985).

Rare earth elements

The chondrite-normalized REE abundances in the separated minerals are shown in Fig. 3. All the analyzed clinopyroxenes have REE contents higher than the chondrlte abundances; they display three distinct shapes of patterns corresponding to different host rocks.

The wall-rock peridotites have highly variable compositions. In comparison with the massive peridotites, the rocks

1. Clinopyroxenes of the massive lherzolites (samples 7 l-

adjacent to the anhydrous pyroxenite bands are enriched in 32 1 and 70-367) are depleted in light REE (LREE) and have

Al, Ti, Mn, Ca and Na, and those which occur next to the an unfmctionated distribution of heavy REE (HREE) with

La, amphibole pyroxenite veins are enriched in Fe, Ti, and Mn

- 2 and h - 9 (N-chondrite-normalized). The patterns resemble those ofchnopyroxene from unmetasomatized

2896 J. L. Bodinier, C. Dupuy and J. Dostal

0.1” 0.10 0.1” 0.“” II. I2 11.13 0.13 0.2" 0.45 0.3" 0.14 11.01, 0.02 0.04

.897 0.899 .a97 .9"1 ."98 ,899 .895

98.42 98.70 99.06 99.39 99.03 99.15 99.46

0.19 0.30 0.42 0.089 O.il2 0.24 cl.39 0.53 0.64 0.89 0.37 0.40 0.71 0.87

98.81

0.52 1.02 0.98

La (mm) C.2 Nd 0.4 0.42 0.49 0.30 0.26 0.58 0.86

0.17 0.20 0.17 0.15 0.146 0.22 0.28 0.075 0.076 0.065 0.065 0.061 0.086 0.101 0.048 0.056 0.046 0.050 0.040 0.055 0.065

0.33 0.115 0.073 0.121

0.29 0.11 0.075 0.11 0.36 0.058

0.15 14.4 76

2360 LO"

47

0.39 O.L5 0.099 O.IS

Srn E” Tb HO Yb L”

0.081 0.103 0.075 0.081 0.086 O.OY7 O.L"7 0.30 0.35 0.33 0.30 0.34 0.41 0.42 0.42

0.072 14

0.40 0.063

18 0.38

14.6 74

224” 106

75

0.052 0.059 0.056 0.05” 0.060 0.070 0.075 II 17 24 4 4 9 24 Il.*, 0.09 0.08 0.08 cl.1 I ".I* 0.19 13.5 11.5 14.0 LO.9 14.0 IS.1 14.3

sr Hf -0.16

13.0 75 78 85 78 83 9" 88

2880 249" 2855 2i9O 2440 2375 2325 90

245" LO6 112 110 112 111 11” 107 11”

2110 2035 2000 2095 2045 196" 2000 ,900 85 100 92 210 84 151 78 71

Cr

LHERZ FKEYCHINEDE BESTIAC 69-356 72-443b 7*-44% 72-282 73-114 72-357 72-364 72-268 72-452 72-267 BES-5 72-451

L I‘+ I,+ I.+ L+ ‘ I.+ KL KL KL+ L+ L+

45.67 45.48 45.53 44.60 42.50 44.25 63.54 3.59

13.19 0.18

36.28 2.45 0.19 0.008 II.22 0.06

43.15 3.94 8.97 0.14

36.05 3.55 0.34 0.006 0.16 2.01

43.73 4.22 (1.73 0.13

36.92 3.76 0.32 0.004 0.16 1.26

4.80 5.88 2.90 9.00 11.82 11.47 0.14 0.1.8 0.17

4.15 3.9, 4.08 10.50 10.35 9.23

3‘. 2” 29.32 37.51 3.76 5.68 2.04 0.39 0.38 0.19 0.013 0.003 0.005 0.19 0.29 0.14 0.90 0.05 0.32

46.02 44.22 5.18 6.27 9.65 8.26 0.14 0.14

32.76 33.30 4.48 5.75 0.54 0.60 0.013 0.009 0.42 0.25 0.13 0.66

iM81 .845 .885 0.892

99.71 98.32 99.23

0.15 0.12 0.13 34.17 39.82 38.73 3.40 2.20 2.98 0.29 0.21 0.42 0.002 0.02 0.05 0.21 u.14 0.19 1.61 1.08 0.57

.863 .883 .893

99.08 100.43 LOO.63

0.870 .887

99.33 99.46

0.10 0.20 0.63 0.82 0.83 0.77 0.33 0.32 O.L2 0.12 0.084 0.08

0.20 0.66

.88* .R30 .867

99.06 99.08 100.22

0.03 0.14 0.23 ".i6 U.72 0.44

0.08 0.20 0.29 0.41 0.64 0.77

0.28 0.22 0.52 “. 14 0.1 I 0.10 0.20 0.063 0.09 O.08 0.16 0.055

0.51 0.27 1.4” 0.90 L.94 0.95 0.70 0.50

0.51 0.086

12.4 80

0.47 0.076 9 0.20

II.1 *II*

2785 114

103 lb*5 114

0.49 0.075 9 0.15

18.0

0.61 0.78 0.32 0.12

II”

0.12

156

0.048 22

/4

7 6 0.13 0.19 0.04

2570

53.2

**al

25.3

1315

12.1

108 III 125

0.6, 0.20 0.21 0.32

102

0.094

65

0.091

83

0.11 0.073 0.077 u.09

0.13 0.12

,680

0.51

,610

0.34

2140

0.16 0.086 0.046 0.022 6

0.13

125

O.,19

114

11.2

109

9.6

44.90 4.86 9.29 0.14 33.10 5.50 0.64

126

O.lw5 0.34 0.72

.875

99.50

0.33 L.20 1.78

2960

0.74 0.28 0.23

0.84 0.13

v.40 22.7

85

0.25 0.21 0.14 0.13 0.20 0.62 0.69 0.1, o.,,

0.28 21.4

119 108 17,” 1870 IO6 92 ,680

70 43

Si”,(%)

A1203 fnnd”W

&” C.9” Nap KO do L.OfI.

P

La (Ptm) ce Nd 8” E” Tb Ho Yb L” ST Hf SC Y

peridotite xenoliths (MENZIES, 1983) and from other high- temperature peridotite mar& (e.g., OTTONELLO et al., 1984).

2. The REE pattern of clinopyroxene from the clinopyroxenepoor lhenolite 7 I- 107 is characterized by LREE enrichment; its abundances of HREE are lower than those of the previous type. The protile resembles those of clinopyroxene from some metasomatized peridotite xenoliths (MFNZIES,

1983).

3. The third type of clinopyroxene patterns is from clinopyroxene/amphibolerich peridotites ot’Caues~u (CAU-2) and Frey&i&de wall-rock peridot&s adjacent to amphibole pyroxenite veins (73- 1). The pattern is convex with the apex at Nd and Sm. and ruembks the clinopyroxene me@crysB of some alkali m (IRVING aad FREY, 1984) arid clino-

pyroxenes from amphibole pyroxenite dikes that aoas-cut the pcridotites in the Lhen body (B~DINIER ef al., 1987b).

Fynnean peridotites 2897

TMJLE *-a: M"R A"" TRACE U.EnENT AWNOMKIS OF YAl.L-ROCK PERICOTITES AOJACEIT TO MPHIBOLE- PYR"XE"IlE VEINS AN" AMPHIBOLE PERIWTlfBS FRW CAUSS""

0.17 “.“27

, .I0 0.47 “.iS 0.075 0. I1 0.3, 0.0‘9

0.“‘ 0.25 II.3 92

1975 I“ 100

0.32 "."SO

O.“,

70-122 *I.-

7”-II8 I.+

0.55 1.16 1.2" 11.1) 0.12 0.065

0.23 0.012

I, IO 0.02 0.12 9.5

62 2525 129 80

CI"SS"" 70-192 AL'

0.58 0.62 1.69 I.tl2 I.,, 1.12 0.4, 0.41 0. Ib 0.14 “.“I, 0.077 0.06 0.09 0.18 0.22 0.025 0.0,"

19 ‘

"."I 0.06 0.I‘ 0.15 9.8 10.1

6, 69 2‘5" 2185 12s 120 92 Inn

7"-195 70-i AL' A+L

0.48 0.5” 1.42 1.72 I.07 1.45 'J.-l6 0.47 0.12 0.16 0.064 ".""I 0.1: 0.1‘4 0.27 0.02, 0."‘"

I‘ 18 2 1" 0.02 0.0, 0.15 0.21 8.0 10.1 58 85

2310 ,970 i,l I28 9" YY

"BLi CAL'-3 A?. (1) (2)

a& 1 0.12 xg" ‘2.Lb Ca" I.20

?6" U.15

do 0.0‘

L."il. ".I7 0.32

I%!1 .898

I 98.5"

Ia (Pp.) 0.53 Ce 1.2 Nd I., sm 0.36 E” 0.12 Tb 0.07, HO 0.095 Yb 0.16 L" 0.029 sr 1" IS 1" *a "."4 "1 0.70 SC 6.7 " 1" Cr 2060 co IIh zn 7s

4,. 1” 12.66 I2.M 1”. I” ‘,.52 ‘4.48

3.“” 3.36 3.1" ,. 3” 3.18 3.82 8.45 8.55 9.35 8.95 9.“” n.79 “.I, 0.i‘ 0.)‘ 0. I I 0.17 0.1,

39.2" NT.48 37.0‘ 17.,5 37.04 >1.,7 3.22 ,.91 4.,5 5.81 1.90 4.34 ".‘2 0.h I 0.6" U.PL 0.68 0.82 0.02 O."? 0.14 0.22 0.19 0.22 0.19 0.21 ".)I 0.7" 0.76 0.9, 0.0" ".bR 0.72 "."2 0.99 0.64

.9"0 ."9R .&36 .R9"

98.3, 98.12 99.0, 98.19

0.84 1.0 3." 0.9, 0.29 ".I‘

0.39 0.06

27 1"

0.88 3.4 L." 1.21 0.41 0.21 0.25 0.45 "."69

,I It,

0.51 1.0‘ I,.‘ II.1 68 70

72," 2380 100 IO" b6 14

1.2" I.4 4.) 5.0 I., 6.1 I.90 2.1" 0.59 (i.66 0.12 0.35 0.27 0.,2 0.5D C.60 0.079 0.079 ,Y 81 22 10 0.38 0.5, 2.79 2.8"

1P.2 L3.9 76 82

215" 22"" 105 106 36 16

.BY2

LOO.03

1.5 5.) 7.1 2.,5 0.74 O.,b 0.12 0.6, 0.08‘

82 31

2.84 14.2 85

228" 101 33

.I386

99.52

2.15 9.3 9.9 2.88 U.88 3.01

U.74 u.12

100

,.24 17.3 9,

2705 1"‘ 32

39.7 (0.9) 2.9 (0.1) 8.1 (0.2) 0.126 CO."",,

35.2 to.27 1.17 ("."6, 0.14 (0.03) 0.02, ~0.0087 0.11 CO.OI7 12.0 CO.17

9Y.67 99.H7

0.13 ~0.01) 0.88 ~0.l"~ 0.64 CD.05) 0.211 (0.012) 0.079 ("."05) 0.06" ("."04) 0.087 ~0.005) 0.302 ("."I,) 0.048 ~0.0"‘) 7 (I,

," (2) 0.01, ~0.008, 0.13 (0.",4) IZ.8 to.17 6‘ (10,

2,lS (55, 106 (1) 94 (II)

0.6 ,.I 0.9 0.2‘ 0.08 0.06

".,I 0.0‘ I" 30 0.0, 0.13 I,.5 75

?,"0 11" 92

Yi.9 2.9 8.4 0.12

35.7 1.22 0.1" 0.02 0.1 I I,.‘

The amphiboles from the Caussou massif have patterns The orthopyroxenes have REE contents close to the chon- very similar to those of clinopyroxene and also of amphibole drite abundances for HREE and lower for the other REE megacrysts in alkali basalts as reported by LIOTARD et al. (Fig. 3B). Two orthopyroxenes from the massive lherzolites (1983) and also amphibole from amphibole-bearing pyrox- possess patterns with positive slopes, increasing from LREE enites (BODINIER ei al., 1987b). The calculated amphibole/ to HREE. Such profiles of otthopyroxene have been described clinopyroxene partition coefficients of REE (Fig. 4) decrease from spine1 lherzolite xenoliths (OTTONELLO, 1980; STOSCH, regularly from LREE to HREE and closely resemble the val- 1982) and other high-temperature peridotite massifs (OT- ues reported both from megacrysts in basalts (IRVING and TONELLO et al.. 1984). The orthopyroxene from a wall-rock FREY, 1984) and from Pyrenean amphibole pyroxenite veins peridotite adjacent to an amphibole pyroxenite vein has a (BODINIER et al.. 1987b). pattern enriched in LREE relative to the other otthopyrox-

2898 J. L Bodinier. C. Dupuy and J. Dostal

-II 17 3 4 5 6 I 0 12 3 4 5 6 7

r4 .5 96 r7 v 8

FIG. 2. Variations of whole-rock compositions: A1203 vs. [Mg] ratio, CaO, Ti02, SC, Yb and Ce. 1) harzburgites and clinopyroxencpoor spine1 lhmolites; 2) massive spinel lherzolites; 3) kelypbite berzoi& from Be&c; 4) clinopyroxene/ amphibole hanburghs from Caussou; 5) amphibole spine1 lherzolites from Caussou; 6) clinopyroxene/ampbibole- rich lherzolites from Caussou; 7) wall-rock peridotites adjacent to amphibole pyroxenite veins; 8) wall-rock peridotites adjacent to anhydrous pyroxenite bands.

enes, as does the coexisting clinopyroxene. The chnopyroxene/orthopyroxene partition coefficients (Fig. 4) decrease from LREE to HREE, showing a shape similar to the mineral pairs from spine1 peridotite xenohths (oTTONELL 1980;

STOSCH, 1982). Olivine has very low REE abundances which display a V-shaped pattern (Fig. 3C) comparable to those reported from dunites (FREY, 1984).

The massive peridotite compositions display two types of REE patterns (Fig. 5) which resemble those of the constituent clinopyroxenes (Fig. 3). The first type is characterized by a LREEdepleted profile. The samples from various massifs differ by their HREE distribution, which ranges from the lhetzolites of the Best&, including the kelyphite lherzolite (sample 72-268) with a flat HREE segment (Fig. 5C) to those with a fractionated HREE distribution (Fig. SB). The kelyphite-bearing lherzolite 72452 is strongly depleted in Yb and Lu (Fig. 5C). Such LREEdepleted patterns have been reported from spine1 lherzolite xenoliths (FREY and PRINZ,

1978; JACKHJTZ ef al., 1979; STOSCH and SECK, 1980; IRVING,

1980) and from high-temperature peridotite bodies ( L~UBET et al., 1975; LOUBET and ALLURE, 1982; FREY, 1984).

The second type of REE patterns in the massive peridotites, similar to those of clinopyroxene from sample 7 l-107 (Fig. 3A), is found in refractory rocks such as clinopyroxenepoor lherzolite 71-107 from Lhen (Fig. 5A) and PGER-2 from Pit de G&al (Fig. 5B). Their REE abundances are lower than those of chondrites, but the pattern of these samples displays selective LREE enrichment; such U-shaped patterns have been frequently reported from xenoliths (e.g., STOSCH and SECK, 1980; DLJPUY et al., 1986) and ophiolitic harzburgites

(ALL~GRE et al., 1973; NOIRET et al., 1981; FREY, 1984; PRINZHOFER and ALLEGRE, 1985).

The chnopyroxene/amphibole peridotites from Caussou (Fig. 5D) have a convex REE profile with the apex at Nd and Sm and have higher REE contents than amphibolepoor peridotites. The REE patterns of the Caussou peridotites closely resemble those of the amphibole pyroxenite veins from the Lherz and Freychintie massifs (BODINIER et al., 1987b) and mica or amphibole-bearing type II xenoliths of FREY and PRINZ (1978) and IRVING (1980). The wall-rock peridotites (Fig. 6) have REE abundances and patterns intermediate between the massive lhelzolites and the adjacent pyroxenite bands and veins (BODINIER ef al., 1987a,b)

As in several peridotite xenoliths and high-temperature peridotite bodies (FREY and GREEN, 1974; FREY, 1984), HREE in the massive peridotites correlate with major elements. In particular, Yb increases with the increase of Al203 (Fig. 2). The correlation disappears in the chnopyroxene/ amphibole-rich peridotites from Caussou and the wall-rock peridotites surrounding the amphibole pyroxenite veins. Fig- ure2suggeststhatLREEinthemassiveperidotitesalsodis play a weak positive correiation with A&03.

Incompatible elements (3, Ba, Hf and Ta)

The concentrations of Sr and Hf in the puidotites vary widely with the highest values in the amphibok-rich peridotites of Caussou. Moat analyzed samples display a positive correlation between Sr and LRBE with Sr/Ce ratioa around ten. However, a few samples of various rock-types, particu-

Pyrewan peridot& 2899

I I I I I I I I I-

Amphibole -

P

< ’ ’ I I I I I I I

; 2- @ OPX

5 z 1 i

.5

-1 .1 -,

I I I I I I IS 0 Ohvine

.Ol I La Ce Nd !3m Eu Tb Ho Yb Lu

FIG. 3. Chondrite-normalized REE abundances of separated minerals. Dashed lines: amphiboles; dssheddottal lines: pymxenes from wall-rock peridot&s adjacent to amphibole pymxenite veins; solid lines: pymxcnes and olivinc from massive peridotites. Shaded field: amphiboles from amphibole pyroxenite veins (BODINIER TV al.. 1987b). Symbols are the same as in Fig. 2. Normalizing values after NAKAMURA (1974).

larly from the Pit Couder body, have higher Sr/Ce ratios (up to 27) due to Sr enrichment.

Compared to the massive peridotites of the Lherz and Freychinede bodies, the wall-rock peridotites from contacts with the anhydrous pyroxenite bands have higher Hf/LREE and lower Hf/HREE ratios, whereas the wall-rock lhetzolites from close to the amphibole pyroxenite dikes have lower Hf/ LREE and higher Hf/HREE ratios. In agreement with the partition coefficient values, the relative variation of Hf in the peridotite wall-rocks of the anhydrous pyroxenite probably is caused by clinopyroxene crystallization, whereas the low Hf/LREE ratios at the contact with the amphibole pyroxenites reflect the relative enrichment of LREE without significant modification of the modal proportions relative to massive peridotites. Ra and Ta are below the detection limit (respectively <5 and co.01 ppm) in most samples except for the

peridotites of Caussou and the wall-rock peridot&es adjacent to amphibole-bearing pyroxenites in Lherz and FreychWde.

Transition elements

The abundance of Sc and V displays a distribution similar to that of Yb, as shown on Fig. 2. The increase of each of these elements with the increase of A1203 is indicative of their moderately incompatible characters. The Ti/V ratio decreases with the increase of the Yb content from massive lherzolites to massive harzburgites (from 13 to 3) in agreement with the more incompatible character of Ti (Fig 7). Zn remains rather constant in massive peridotites but is enriched relative to A120, in the peridotites next to the amphibole pyroxenite veins and in the kelyphite lhetzolites from Bestiac. On the other hand, the abundances of Cr are highly variable. The Al-poor refractory peridotites have higher Cr concentrations (2,500-2,900 ppm for Al20, < 1.8%) than the Al- rich peridotites ( i,700-2,400 ppm for A1203 > 4%). The wide range of Cr concentrations in high temperature peridotites has been attributed by FREY et al. (1985) to the heterogeneous distribution of spinel.

The abundances of transition elements in the mineral phases are given in Table 2, whereas the values of the partition coefficients (KD) for the mineral pairs are reported on Table 3. The K. for orthopyroxene-olivine and clinopyroxene-orthopyroxene are closely comparable to the data reported by STOSCH (198 1) from spine1 peridotite xenoliths. In contrast, the clinopyroxene-orthopyroxene KD for SC, Ti, V and Cr are higher than those obtained from the anhydrous pyroxenite bands (BODIN~ER et al., 1987a). The amphiboleclinopyrox- ene KD for SC (0.7) and Co (- 1.3) are within the range of reported values for megacrysts in alkali basalts (IRVING and FREY, 1984) and the amphibole pyroxenite veins (BODINIER

et al., 1987a).

5 I I I I I I I I I II, La Ce Nd Sm Eu Gd lb Oy Er Yb Lu

FIG. 4. Clinopyroxene/orthopyroxene and amphihole/clinopyroxene partition coefficients for REE. Cpx/Opx: Ce. Nd and Eu contents in sample 71-321 and Ce, Nd and Tb in sampie 70-367 have &en extrapolated. The shaded field delineates the field of spine1 lherzolite xenoliths (after OTTONELLO, 1980, and STOKH, 1982). Amph/Cpx: the dashed line represents the megacrysts (IRVING and FREY, 1984) and the dotted line the values obtained from the amphibole pyroxenite veins (BODINIER et a!.. 1987b).

2900 J. L. Bodinier. C Duouy and J. Dostal

I-

t- 3

1

.5

.1

.O!i

3

I

.5 v) W I-

a

is

?

2’

u B 1

.5

4cl

10

5

1

.5 E 1

_-- ____ ----- _____---- ,,

v- ,’ __*-

-p- .*’ _a-- ___.*----a-

__-+<-

e -- ,” _. .*”

1.1 , .* *---” ,. .- fl-:‘:i I

‘a ,& *- *... +11* _

71.

d I I I I I I I I l-

I I I I I I I I I

1 I I I I I I I I .a Ce Nd !%I Eu Tb Ho Yb Lu

FIG. 5. Chondrite-nonnahzed REE abundances of the massive peridotites, the kelyphitc peridotites and the Caussou amphibole peridot&s. The symbols are the same as in Fig. 2.

A: Lhcrz (solid lines) Font&e rouge (line with crosses) and Frey- chin&e (dashed lines).

B: Pie Couder (short dashed lines). Porteteny (solid lines), Sem (long dashed lines), and Pit de G&al (dotted lines).

C: Bestiac. D: Causaou. The shadad tiekk amphibole pyroxenite veins (BoD.

INIER et al., 1987b).

I I I I I I

I I I I I I La Ce Nd Sm Eu Tb Yb Lu

RG. 6. Chrondrite-normalized REE abundances in the wail-rock peridotites. The symbols are the same as in Fig. 2.

A: peridotites adjacent to the amphibole pyroxenite veins-Lherz (solid lines) and FreychinMe (dashed lines). The shaded field-amphibole pyroxenite veins (BODINIER er al.. 1987b).

8: peridotites adjacent to the anhydrous pyroxenite bands-L-hen. (solid lines), Freychin&e (short dashed lines) and Bmtiac (long dashed lines). The shaded field-anhydrous (layered) pyroxenite bands (BODIKIER et al.. 1987a; our unpublished data).

15-

TI/V

/- ,r 1

,’ . ._ -- ., 10 _.:_ /.’ l * :., / - , /

* fl’ ‘.. :+ /‘Randa l ., .

/ . 5: * 9 /* i’ l

. .

-* .‘* ,*/

- ,‘* /

,/ ,

_ (_/” Yb “pm

0 I I I I 0 1 2 3 4 5

RG. 7. Variations of Ti/V ratio vs. Yb in the massive pcridotites. The symbols are the same as in Fig. 2. The dashed curve delineates the field of Ronda peridotites after FREY ef al. (1985).

2901

Ol.l”lNE ORTHOPYROXENE Cl.INOPYROXEWE MPHIBOLe

11-321 70-367 71-321 70-367 73-1 71-107 71-321 70-367 73-1 CA"-2 CA"-, CA"-2

I.4 (PP") 0.022 0.020 0.050 0.040 0.24 3.42 0.77 0.75 5.1 2.4 1.4 5.2 0.090 0.037 0.45 5.62 3.3 2.8 15.8 9.6 17.1 II., cc

NC4 2.61 4.5 3.2 14.1 14.0 21 24 sm 0.005 0.006 0.064 O.II 0.92 1.73 1.40 1.23 ‘.RO 7.6 8.1 E" 0.002 0.029 0.053 0.33 0.72 0.59 1.46 1.40 2.4 2.4 Tb 0.035 0.030 0.21 0.48 0.4‘ 0.79 0.81 I.21 1.16 HO 0.38 0.6‘ 1.14 1.20 Yb 0.020 0.010 0.21 0.19 0.18 0.17 1.79 2.01 1.76 1.53 ,.I35 1.92 Lu 0.045 0.047 0.040 0.20 0.26 0.33 0.28 0.21 0.24 0.25

333 3‘8

II 120 " 6 cr 163 co 134 h‘i 3040 2" 25

0.03 1.2

I20 ‘

35

136 2880

26

0.04 0.04 11.9 14.6

720 780 78 83

,810 1360 II 50

800 587 655 22 23 4,

0.09 10.5

L 140 70

1210 55

2100 5

6850

0.10 1.01

68.2 3480 276

6230 22

361 13

0.06 1.14

87.2 4140

304 ‘720

19 298 I5

90 110

0.25 0.10 0.22 0.29

2.39 0.0 5.2 5.2

67.0 49.9 39.2 36.9 28440 26940

321 307

4460 4060 ,460 1350

20 30 39 38

630 615 25 16

DI!XUSION

As already suggested by FREY ef al. (1985), the presence of small amounts of serpentine in the peridotites has only a minor effect on the element distribution. Furthermore, many analyzed samples are devoid of this mineral phase. A few samples from Lhetz and Freychidde which contain 15 to 30% serpentine (e.g., 7 l-325,7 l-324 and 72-425) have major and trace element compositions comparable to the freshest samples such as FOR-l, POR-2 and SEM-1. However, the variations of Sr and LREE and the increase of the SrfCe ratio in the small peridotite body of Pit Cbuder may be related to the presence of carbonate veinlets crosscutting the rocks and more specifically to fluid interaction with the surrounding marbles. The abundances of these elements increase with increasing proportions of veinlets. This is also shown in Fig. SB where the vein&poor sample (FCOU-2) has a REE pattern with a negative slope from Lu to La whereas the veinlet- rich peridotite (PCOU3) displays an enrichment in La and Ce with (La/Nd)N > 1. The concentrations of the other elements do not appear to be affected. Thus the distribution of major and trace elements in the large majority of the samples probably reflects mantle processes.

Two aspects of the petrogenesis of the peridotite massifs will be discussed in turn:

TABLE 3: UINENAL PARTlTION COEPPIClENTS FOR TRANSITlON ELENENTS

OPXlOL I I (I) (1)

CPXIOPX

(2) (3)

SC I2 5.7 - 6.4 3.4 - 5.0 2.0 - 8.0 Ti 4.8 - 5.3 2.0 - 2.5 " 13 - 21 3.5 - 3.7 2.9 Cr IL - 39 3.4 - 3.7 1.3 - 2.9 1.4 - 3.0 co 0.36 - 0.40 0.36 - 0.60 0.3, - 0.43 0.36 - 0.49 N1 0.20 - 0.26 0.45 - 0.51 0.60 0.39 - 0.62 Z" 0.9 0.59 - 0.65

1. compositional variations of the massive peridotites as an indicator of partial melting processes;

2. element variations related to metasomatic processes in the peridotites.

Compositional variations in the massive peridotites

The variation of elements such a.~ Al, Ca, Ti and HREE encountered in the massive peridotites of the Eastern Pyrknks (Table 1, Fig. 2) have also been reported from ultramafic xenoliths in alkali basal@ and from other high temperature peridotites and were interpreted to represent residue after partial melting (FREY and GREEN, 1974; RINGWOOD, 1975; OTTONELLO et al., 1984; FREY et al., 1985). In the Pyrenean peridotites, this hypothesis is further confirmed by the decrease of the element ratios involving the more incompatible element in the numerator (e.g., Ti/V, Yb/Sc, Hf/Yb) from the Al-rich lhexzolites to the harzburgites.

However, the Tb/Yb ratio does not show a regular trend when plotted against Yb (Fig. 8A), and the HREE fractionation in several samples (Fig. 5) with (Tb/Yb)N < I may suggest that garnet was a residual phase. In order to evaluate this assumption, the expect@ behaviour of the Tb/Yb ratio during partial melting is shown on Fig. 8B. In the garnet-free lherzolitic source, the ratio remains nearly constant with the increasing degree of batch melting (F) when the residual proportion of clinopyroxene is higher than 5% (F < 15%). When the residue becomes a hanburgite (F = 15-30%), the ratio decreases significantly. In the garnet-bearing lherzolitic residue, the Tb/Yb ratio is related to the modal proportion of garnet in the source. Alternatively, a fractional and continuous melting process (LANGMUIR ef al., 1977) can produce a distinct decrease of the Tb/Yb ratio by a low degree of melting (F c 1 5%, Fig. 8B) from a source with only a mod- erate proportion of garnet (< 10%). However, the model of continuous melting does not fit the data; in particular, it fails to explain the high Tb/Yb ratio in the more refractory rocks. On the other hand, the value of this ratio in the refractory rocks is consistent with a batch melting model with or without residual garnet in the source. The presence of residual garnet

3902 J. L. Bodinier. c’. Dupuy and J. Dostal

.3

2

.l

.3

.2

.l

0

Tb;’ Yb --

t

Tb/Yb

F. 2 Yb wm

I I -I 0 1 2 3 4

FIG. 8. A: Variations of Tb/Yb ratio vs. Yb in the massive peri- domes- I) Lhen and FreychinMe: spine1 Iherzolites, 2) Lherz, Frey- chin&de and Font&e rouge: harzburgites and clinopyroxene-poor lhetzolites, 3) Pit Cot&r, Porteteny and Sem, 4) Pit de G&al. Empty stars: Ronda Iheczolites(FREY et al., 1985), solid stars: Lanzo lherzolites (BODINIER, 1988), empty circles: har&ugites and duuites from New Caledonia ophiolites (PRINZHOFER and ALL~GRE, 1985). The symbols are the same as in Fig 2. The error bars represent analytical uncertainties calculated according to ALBAR~DE and PROVOST ( 1977).

B: Variations ofthe Tb/Yb ratio vs. Yb in the residue after melting of Iherzolites with 2X chondritic abundances of Tb (0. I04 ppm) and Yb (0.44 ppm). Solid lines-equilibrium batch melting of the source with the following modal composition:

01 opx cpx Grt

A 0.50 0.30 0.20 0

B 0.55 0.25 0.15 0.05

C 0.55 0.25 0.10 0.10

D 0.55 0.20 0.10 0.15

The melting proportions of the phases were calculated according to equations of PRESNALL m al. (1979) for the stability field of spine1 (A) and those of MYSEN (1970) for the garnet field (B, C and D). The equations were modified to reach complete melting of garnet at 15% melting, clinopyroxene at 25% and orthopyroxene at 40%. The dotted lines represent the boundaries of the different mineral assem- blages in the residue. The mineral/liquid partition coefficients were taken from FREY ef al. (I 978). The long dashed line represents the composition of the residue aher continuous melting (LANGMUIR el al., 1977) of a source with 10% garnet (type C), and with 2% melting increments and 2% of retained liquid in the residue. The short dashed line shows a residue after fractional mehing (SHAW, 1970) of the D type source.

in the source may be expected for the massive peridotite where the variation of Tb/Yb exceeds analytical error and for the peridotite of the Pit de Geral massif (field 4, Fig. 8A) where

the decrease of Yb is accompanied by an increase of Tb/Yb.

The degree of partial melting may be roughly approximated at 5 to 10% for the massive lherzolite and at I5 to 25% for the clinopyroxene-poor lherzolites and harzburgites.

In order to evaluate the chemical characteristics of magmas produced during melting, the REE compositions of liquids in equilibrium with the massive lherzolites have been calculated and are shown in Fig. 9. The liquids display marked enrichments of LREE relative to HREE with the (Ce/Yb), ratio ranging between two and five. They resemble the patterns of the continental tholeiites related to the opening of the Atlantic Ocean (BER.I.RANU er al., 1982). The Mesozoic tholeiites are abundant in the Pyre&es (e.g.. AI IBERT, 1985)

and have been considered to be the parental magma for the anhydrous pyroxenites of the Lherz and Freychinede massifs (BODINIER 1~ ul.. 1987a).

Metasomatic processes itI peridolites

Wall-rock peridotites. Compared to massive peridotites. the composition of the wall-rock peridotites changes (Figs. 2 and 4) and the character of these variations depends upon whether the peridotites are in contact with anhydrous or amphibole-bearing pyroxenite. This may suggest an interaction between the pyroxenite and the wall-rock peridotite. Similar interactions have been reported from ultramafic xenoliths (e.g.. WILSHIRE and SHERVAIS, 1975; IRVING, 1980; KEMP-

TOM, 1985) and high temperature peridotite massifs (POLVI?

and ALL!&RE, 1980; FREY et al.. 1985). The peridotite adjacent to the amphibole-bearing veins

which were derived from alkali basalt magmas (BODINIER et al., 1987b) is enriched in LREE, Hf, Sr, and Ti relative to the massive peridotite. Also, the wall-rock peridotites have higher Ce/Yb (Fig. IO). Such enrichment appears in several ultramafic xenoliths and is interpreted to be the result of interaction with silicate melt and/or fluid (HARTE, 1983; MENZIES ef al., 1987). The observed increase of several incompatible elements such as LREE and Hf in the wall-rock peridotites cannot. however, be the result of a simple mixing with an alkali basalt magma. Otherwise, the content of Th

FIG. 9. Calculated chondritcnormalixed REE abundances of liquids in equilibrium with the massive Ihetzolites. The partition coefficients used are the same as in Fig. 8B. The modal proportions of garnet were taken as 2% for samples with Yb - 0.3 ppm (PCGU-2, 7 l-324 and 71-335) and 4% for samples with Yb - 0.4 ppm (SEM-I and PDR-3). The other minerals were taken in the proponions: 01 = 0.60, Opx = 0.25. Cpx (+ Grt) = 0.15. The dashed lines delineate the field of Mesozoic continental tholeiites related to the opening of the At- lantic from the Pyre&s and Morccco (BERTRAND PI al., 1982; AL- IBERT, 1985).

Eyrenean peridotites 2903

50: I I I I1111~ I I I I I I I I / \ I ‘4 I -

Ce/ Yb “,A” \ -

,.\..i

‘fb PP~ .l I I l111111 I I I Illll I I I

.Ol .05 .l .5 1 5

FIG. 10. Variations of Ce/Yb VS. Yb in the ultmmalic rocks of the Eastern Pyrenean bodies. The symbols of the rock-types are the same as in Fig. 2. R-Ronda peridotites (FREY ef al., 1985), L-Lanzo lhetzoiites (BODINIER, 1988) AP-anhydrous pyroxenite. bands (BODINIER ef al.. 1987a), HP-amphibole pyroxenite homblendite veins (BODINIER et al., 1987b), AB-alkali basalt% The solid line represents the calculated composition of the peridotites with dunitic to lhmolitic compositions equilibrated with alkali basalts containing Ce = 100 ppm and Yb = 2 ppm. The partition coefficients are the same as in Fig 8B. The heavy dashed line delineates the mixing field between lherzolites (S-20% clinopyroxene) equilibrated with alkali basalt magma and the amphibole pyroxenite veins.

should be above the detection limit (-0. I ppm). Similarly, the interaction with fluid alone cannot explain the increase of elements such as Ti. Nevertheless, the REE pattern and content of clinopyroxene separated from the wall-rock peridotites (Fii. 3), which are very similar to those of clinopyroxene from amphibole clinopyroxenite veins, favoum an interaction with a LREEcnriched basaltic liquid. Such an enrichment process may result from equilibration with an alkali basaltic magma, as possibly suggested by the calculations IV- ported on Fig. 10. In the case of r-e-equilibration of peridotite with an alkali basalt melt, these calculations indicate a drastic increase of the Ce/Yb ratio accompanied by a slight decrease of Yb and reflect fairly well the chemical change from massive spine1 lherzolites to wall-rock peridotites, which corresponds to the trend from solid circles to solid triangles on Fig. 10.

The peridot&s that occur next to the anhydrous pyroxenite band have higher Al, Ti, Ca, SC, Hf and HREE abundances than the massive peridotites. The wall-rock peridotites have geochemical features intermediate between the massive peridotites and anhydrous pyroxenite bands (E%ODINIER et al., 1987a), as shown on Fig. 10, where they plot within the field of mixing of the two rock-types. These geochemical chatac- teristics are consistent with solid state mixing during deformation and recrystalhzation and/or interaction with a basaltic melt From which clinopyroxene crystals were also precipitated. Clinopyroxene is the dominant phase of the anhydrous py-

roxenites (CONQU~RE, 1977). and its observed high proportion in the wall-rock peridot&s adjacent to the anhydrous pyroxenites can account for the high content of SC, Ti, Hf and HREE. In a few samples which are characterized by HREE fractionation (Tb/yb)N < 1, very low contents of LREE (e.g., samples 72-282 and 72-364, Fig 68) and high Sc contents (>SO ppm), the precipitation of garnet may have accompanied clinopyroxene crystallization. However, the garnet subsequently disappeared during subsolidus recrystallization.

The wall-rock peridotites of both amphibole pyroxenite veins and anhydrous pyroxenite bands display distinct Fe enrichment resulting in low [Mg] values (down to 0.80-0.83). In several samples, the Fe enrichment (up to I S- 17% Fe203) is even higher than the one reported from composite ultramafic xenoliths which are characterized by [Mg] values lower than 0.89 (WILSHIRE and SHERVAIS, 1975; WILSHIRE et al.. 1980; IRVING, 1980; GURNEY and HARTE, 1980; HARTE,

1983). In xenoliths, the Fe enrichment has been usually interpreted to be the result of interaction between peridotites and Fe-rich silicate melts (Fe or Fe-Ti metasomatism of HARTE, 1983, and MENZIES et al., 1987).

However, the clinopyroxene/amphibole-rich peridotites from Caussou, which are unrelated to pyroxenite bands or dikes, have “normal” [Mg] values similar to the massive spine1 Iherzolites, although their interaction with a postulated alkali basalt melt apparently led to complex re-equilibration, as evidenced by the REE distribution (see below, and Fig. 10). The [Mg] and REE characteristics of the various metasomatized peridotites from Pyrenean bodies are illustrated in Fig. 11. The difference between the “wall-rock metasomatism” and the “pervasive metasomatism” of Caussou suggests

-75, _ _ \ 1

Ce/Yb -./

0 5 10 15

FIG. 11. Variations of the (Mg] vs. Ce/Yb ratio. Symbols are the same as in Fig. 2. AP-anhydrous pyroxenite bands (BODINIER et al.. 1987a. and our unpublished data for the Restiac massif); HP- amphibole pyroxenite veins (~ODINIER a al., 1987b).

2904 1. L. Bodinier. C. Dupuy and J. Dostal

that interaction between silicate melt and peridotite alone cannot explain the enrichment of Fe and that a complemen- tary process has occurred in peridotites adjacent to the pyroxenite segregates.

In the wall-rocks, the [Mg] ratio regularly decreases toward the pyroxenite contact, and this decrease is accompanied by an increase of the ratio in the pyroxenite bands and dikes from the centre to the margins (CONQUI?R& 1978; BODINIER

et al., 1987b). The lowest [Mg] values (0.80-0.85) are observed in the thin peridotite bands (~3 cm) intercalated between thick pyroxenite layers and veins (samples 69-356, 73- 1 14, 70-249, 73-2, 73-3 and 73-l-3). Conversely, the highest [Mg] ratios in pyroxenites (0.89-0.91) are observed in the thin bands and veins in the peridotites (BODINIER et al.,

1987b). BODINIER et al. (1987b) have attributed these characteristics of the wall-rocks to a re-equilibration of the Mg- rich minerals of the peridotites with the Fe-rich minerals of the pyroxenites. The postulated process would correspond to a Fe-Mg interdiffusional cation exchange, which may have started when the first magmatic minerals were plated onto the conduit-walls. Thereafter, this process was probably fa- cilitated by grain-boundary diffusion through the network of interstitial liquid infiltrated into the wall-rocks. The chemical exchange between segregates and peridotites, responsible for Fe enrichment, and the interaction between liquids and peridotites. responsible for incompatible trace element enrichment, were probably broadly contemporaneous. Thus, the postulated mechanism of Fe enrichment is part of the metasomatic processes which all&ted the peridotites adjacent to the magma-conduits. However, the peridotites from Caussou, which underwent “pervasive” metasomatism, were not affected by this process. Mn and Co display a positive correlation with Fe and a distinct enrichment in the wall-rock peridotites relative to the massive Ihetzolites, including the strongly metasomatized peridotites from Caussou. This suggests that these elements were probably also affected by the diffusional re-equilibration.

Clinopyroxene/amphibole-rich peridotites from Caussou. Except for their higher content of several incompatible elements, including Na, K, Sr, Ba, Ta, Hf, Ti and REE, these rocks resemble the wall-rock peridotites adjacent to the amphibole pyroxenite dikes. However, unlike the wall-rocks, the Caussou peridotites have abundances of Fe, Cr, Sc and HREE similar to those of the massive peridotites. On Fig. IO, the clinopyroxene/amphibole-rich peridotites of Caussou plot between massive lherzolites and alkali basal& suggesting that they may be the result of the mixing of these two com- ponents. However the Th content below the detection limit and the low La/Sm ratio (Fig. 5D) negate a simple mixing of a residual peridotite with an alkali magma. More plausible is a petrogenetic model involving m-equilibration of residual minerals with migrating alkali basaltic magma and precipitation of clinopyroxene and amphibole. This process is consistent with the REE composition of the clinopyroxene and amphibole (Fig. 3A) which resembles that of megacrysts Tom alkali basaltic lavas (IRVING and FREY, 1984). It is ah con-

sistent with the high clinopyroxene and amphibole abundances of the Caussou peridotite, and the calculations reported on Fig. 10 would fit the data of Caussou, provided they take into account this increase ofthe clinopyroxene and amphibole proportions.

Harzburgite bands. Although a partial melting model can explain the variation trends of the lhelzolites and also the variations of major and some trace elements in the more

refractory rocks, it cannot readily account for the high LREE/

HREE ratios in the clinopyroxene-poor lherzolites and harzburgites. These rocks commonly are enriched in LREE with (La/Sm)h > I, more so than Ihetzolites from the same body (e.g.. sample 71-107; Fig. SA). In fact, it appears that the most refractory rocks have the highest Ce/Yb ratio, whereas during equilibrium melting, the Ce/Yb ratio decreases with

increasing degree of melting. The LREE enrichment is not related to secondary or alteration processes since the high- temperature mineral phases of these rocks (e.g., clinopyroxene of sample 71-107; Fig. 3) have high Ce/Yb ratios.

Patterns with an enrichment of LREE are common in ultramafic xenoliths of alkali basalts (FREY, 1984) and have been attributed to metasomatism of these mantle rocks by LREE-enriched melts or fluids (MENZIES ef al.. 1987). How- ever, the REE pattern of the clinopyroxene from harzburgite 7 I - 107 (Fig. 3A) marked by enrichment of La and Ce, differs from that ofclinopyroxenes from the lherzolites which equilibrated with an alkali magma (samples 73-l and CAU-2) enriched in Nd and Sm. NAVON and STOLPER (1987) have recently argued that such REE patterns may also reflect a chromatographic effect produced by porous flow percolation of a LREE-enriched melt in the peridotites. This hypothesis is consistent with the experimental work of TORAMARU and FUJI1 (1986) which suggests preferential infiltration of magma through adjacent olivine-poor rocks. Thus, the different types of LREEcnrichment observed in lherzolites and harzburgites, respectively, might be related to the same alkali metasomatic event but reflect two distinct mechanisms of magma circu- lation in the two rock-types: vein-conduits in the Iherzolites and porous flow percolation in the harzburgites.

SUMMARY AND IMPLICATIONS

The Eastern Pyrenean peridotites have undergone a complex multi-stage evolution, including various processes of mineral segregation and metasomatism from migrating magmas. The metasomatism played an important role during the evolution of several rock-types of the Pyrenean peridotite ma&f& particularly the amphibole pyroxenite veins, Fe-rich wall-rock peridotites, CPX-AMPH-rich peridotites and LREE-enriched harzburgites. The me&somatic processes of

the Pyrenean peridotite show close similarities to those ob-

sexved in the ultramafic xenoliths in alkali basals (e.g., WtL- SHIRE and SHERVAIS, 1975; IRVING, 1980; MENZIES et al., 1987).

The systematic compositional variations in the massive petidotites indicate that they are residue after different degrees of melting. The lherzolites arc residual after SmalIer amounts of melting than the harzburgites; for example, according to our model, 15-256 melt extraction is required for ban- burgites and clinopyroxenepoor lhetzolites and less than 155% for lhetzolites. HREE fractionation may suggest that garnet was a residual phase after melting, as inferred for the Ronda massif(Fk~~ et al.. 1985). However, numerous samples from the Eastern Pyr&&s have lower Tb/Yb than the Ronda peridotites, suggesting that they contained a higher proportion

pyrenean peridotites 2905

of garnet. The melts in equilibrium with the massive lherzolites of the Pyr@es are enriched in LREE, in contrast to lherzolites related to an oceanic setting, which were equilibrated with liquids similar to the N-type MORB (e.g., Lanzo, BODINIER, 1988). We may infer that magmas formed by partial melting of the peridotites and also parenta! magmas of the anhydrous pyroxenites of the Eastern Pyrcnean peridotite bodies (BODINIER et al., 1987a) have geochemical characteristics similar to the Mesozoic continental tholeiites of the PyrGes, which are related to the early stages of the opening of the Atlantic (ALIBERT, 1985). However, the isotope data (Porvk, and ALLEGRE, 1980; ALIBERT, 1985; and, HAMELIN and ALLEGRE, 1985) indicate that the lherzolites are not the residue after the extraction of these tholeiites. The Pyrenean continental tholeiites were probably generated in deeper parts of the upper mantle, and, during their ascent to the surface, the anhydrous pyroxenites were segregated in the vein conduits. Interaction of the tholeiitic magma with the wall-rock peridotites adjacent to the conduits might be responsible for the enrichment of HREE relative to LREE associated with an increase of modal proportions of clinopyroxene.

Aher this event, the peridotites cooled in the spine! stability field (-900°-1000°C at depth of SO-70 km) (CONQUERI? and FABRI& 1984). This is probably related to the accretion of the diapir into the subcontinental lithosphere. Subse- quently, the peridotites underwent loca! changes of their pri- mary residual composition due to infiltration of alkali basalt liquids. With the exception of the Caussou massif, migration of the alkali basalt magma in the lherzolites through vein- conduits led to the flow segregation of amphibole-rich pyroxenites and to the interaction with the wail-rock peridotites. Minor modal metasomatism of the wall-rock peridotites (amphibole of: phlogopite f ilmenite) suggests that the magma infiltrated up to a few cm from the contact. Changes in the chemical composition of the peridotites are especially marked by LREE enrichments, implying that this process led to the re-equilibration of the peridotite minerals with the alkali basalts. However, we suggest that the distinct enrichment of Fe (and also Mn and Co) in the wall-rock peridotites is probably due to a diffusional exchange between the pyroxenites and peridotites which occurred mainly during the crystallization of the veins. The peridotites of the Caussou massif also were modified by metasomatism, which produced an enrichment in clinopyroxene and amphibole at the expense of orthopyroxene and spinel. All these minerals appear to have been

re-equilibrated with an alkali basaltic liquid. The distinct metasomatic features of the Caussou peridotites may be a result of a pervasive percolation of the magma through the lherzolite. However, in the other massifs, the porous flow

percolation mi&t have been more effective in the harzburgites than in the lherzolites. Such a mechanism can also explain the enrichment of LREE relative to middle and heavy REE in the harzburgites. The similar ages obtained for amphibole from the Caussou peridotites (103 M.y., ALBAREDE and MI-

CHARDVITRAC, 1978) and from Lherz homblendites (101 M.Y., GOLBERG et al., 1986) suggest that the alkali metaso-

matic processes were contemporaneous and possibly related to the middle-Cretaceous alkali magmatism of the Pyr&&s (MONTIGNY et al., 1986). These metasomatic processes occurred towards the end of the second stage of the synkinematic recrystallization of the peridotites under decreasing T-P con-

ditions (650“-7OO“C and 25-30 km, FABR& and CON- QU!~R!& 1983, and CONQU~~R~ and FABRIC%, 1984).

Acknowledgements-We thank Drs. J. Fabrics and J. P. Lot-and for many useful discussions and for providing samples. We are grateful to Drs. F. A. Frev. M. Roden. P. D. Kempton and J. Nielson for their helpful comments on an earlier version of the manuscript. The research was supported by the Centre Giologique et Geophysique, C.N.R.S.U.S.T.L., Montpellier, France, and the Natural Sciences and Engineering Research Council of Canada (operating grant A3472).

Editorial handling: F. A. Frey

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Pyrenean peridot&s 2907

APPENDIX

LHERZ (I ,600 X 700 m in size, very good exposure) consists of harzburgites, spinel lhetzolites, anhydrous pyroxenite bands and amphibok (AMPH) pyroxenite veins. The ultramatic rocks am breceiated along the margins of tbe massif, particularly along the southern and eastern contacts. Limestone breccias rarely occur along faults within the massif. The analyzed peridotites contain 5 to 15% serpentine with the exception of the harzburgites (samples 73-104 and 7 l-325) and the spine1 (SP) lherzolite 7 I-324 which contain 20 to 30% serpentine. All the samples are foliated and display porphyroclastic to equigranular textures. Ohvine (OL) and orthopyroxene (OPX) porphyroclasts are about 2 to 3 mm in size, whereas clinopyroxene (CPX) is generally smaller (- 1 mm). The massive peridotites comprise three rock-types:

Harzburgites (OL 75-85% OPX 15-25% CPX I-5%, SP -I%, AMPH traces-samples 73-104 and 71-325), CPX-poor spine1 Iher- zolites (OL 65-75%. OPX 20-30%, CPX 5-8%, SP I-2t-samples 71-322 and 71-107) and soinel Iherzolites (OL 50-60%. OPX 25- 3046, CPX IO-I 5%: SP 2-j%, AMPH traces-samples 7 i-324, 7 I- 321, 70-367 and 71-326).

The peridotites adjacent to the anhydrous pyroxenite bands have either modal compositions similar to the massive peridotites (spine1 lherrolite sample 69-356) or are enriched in CPX (OL 35-50%, OPX 25-35%. CPX 15-25%, SP 3-6%-samples 72443a, 72-443b, 72- 282 and 73-114). The amount of interstitial amphibole in these wall- rocks is usually higher than in the massive peridotites (e.g. - 1% in sample 73-I 14 and I-2% in sample 69-356). The peridotites adjacent to the amphibole pyroxenite veins have modal composition comparable to the CPX-poor spine1 lherzolites (sample 70-249) or the spine1 lhetzolites (samples 73-2 and 73-3) except for additional 3 to 5% AMPH and traces of ilmenite.

FONTf TE ROUGE (300 X I50 m in size, good exposure) is composed mainly of peridot& and rare anhydrous pyroxenite bands. The analyzed peridot& am relatively fresh (18% alteration). They are not foliated and usually have a coarse-grained protogranular texture with OL and CPX up to I cm in size. Porphyroclastic to fine- grained (I-2 mm) equigranular textures are rare. Most peridotites lack interstitial amphibole. The peridotites include harzbutgites (samples FON-I and 72-4320). CPX-poor spine1 lherzolites (sample 72-228) and spine1 lherzolites (samples FON-2 and 73-15 I). The modal composition of the individual rock-types are comparable to those from Lherz.

FREYCHINfDE (1,000 x 300 m in size, poor exposure) contains the same rock-types, even with a similar degree of alteration (5-15% serpentine), as the Lherz massif with the exception of missing harz- but&es. The peridotites also are well-foliated with porphyroclastic to equigranular textures and always comprise trace amounts of interstitial amphibole. The analyzed samples include massive spinei lherzolites (samples 72-425, 71-335, 71-336 and 71-339). the pet-i- dotites adjacent to the anhydrous pyroxenite bands (spine1 lherzolite 72-357 and CPX-rich spine1 lherzolite 72-364) and the amphibole spine1 Ihmolites adjacent to the amphibole pyroxenite veins (72. 108b, 73-la, 73-I-1, 73-l-2 and 73-l-3).

PIC COUDER complex includes two small peridotite bodies (< 10 m in size, MONCHOUX, 1970). The body which has been sampled is a rounded block (5 X 5 m large) composed of spine1 lherzolites with ~3% serpentine. The peridotites, particularly near the contacts. contain carbonate veinlets. The veinlets are relatively abundant in sample PCOU3, rare in samole PCOU-I and absent in samole PCGU-2. The lhenolites have p&phyroclastic texture. ~_

PGRTETENY massif ( I50 X 80 m in size, poorly exposed) consists ofspinel lherzolites with only trace amounts of serpentine. The rocks resemble the protogranular peridotites from Fontete rouge. They

contain coarse porphyroclasts of OL, OPX and CPX set in finger- grained equigranular matrix. Sample POR-I lacks intetstitial amphibole while PGR-2 and PGR-3 contain tracz amounts of thii mineral.

SEM (250 X 100 m in size, poor exposure). Two analyzed samples (SEM-1 and SEM-2) are coarse-grained porphyroclastic spine1 lher- zotites virtually without serpentine. They have a higher proportion of interstitial amphibole (-1%) than the equivalent massive peridotites ofthe Lhet-z massif

BESTIAC complex includes several ultramafic bodies up to 300 x 100 m in size (MONCHOUX, 1970). Some larger bodies are highly serpentinized and contain numerous limestone inclusions. The sampled body ( I40 X 60 m in size) is made up of relatively fresh peridotites and anhydrous pyroxenite bands. In addition to typical massive spinet Iherzolites, the Bestiac complex contains a kelyphite-bearing layered sequence (FABRICS and CONQUER& 1983) composed of lhenolites intercalated with bands of garnet websterites 3 to 20 cm thick and olivine Webster&es Q 3 cm thick. Three lherzolite samples were collected from this sequence. The CPX-rich lhenolite 72-267 which contains minor amounts of kelyphite and spine1 (~5%) was adjacent to a thin olivine websterite band. Kelyphite lherzolite 72-268 is from the centre of a 20 cm thick peridotite band sandwiched between garnet websterite layers. Lherzolite 72-452 which contains I to 2% kelyphite was picked up IO cm from a garnet webstetite band. The other two samples (7245 1 and BE!+5) were collected away from the keiyphite sequence. There are CPX-rich spine1 Iherzolites adjacent to thin spine1 websterite bands (a3 cm thick). Compared to CPX- rich lherzolites from Lherz and FreychinMe. these rocks from Bestiac have a higher proportion of CPX relative to OPX with a modal CPX/ OPX ratio close to I (FABRICS and CONQUI%&, 1983). All the Bestiac peridotites contain trace amounts of amphibole.

CAUSSOU (100 X 60 m, good exposure) is composed of CPX- rich spine1 Iherzolites with subordinate spinal lherzolites and rare spine1 websterite bands. In addition, AMPH-rich peridotites form irregular patches, up to I m large. The massif is intersect4 near the contacts by carbonate veinlets. Although the massif contains numerous 1 to 2 cm thick veins of serpentine, the peridot&s are relatively fresh; the analyzed samples have 58% serpentine. The texture of the peridotites is porphyroclastic to equigmnular with amphibole occur- ring both as porphyroclasts and neoblasts (CONQUERS, 1978). CPX- rich spine1 lherzolites (samples 70-I 18, 70-189,70-I92 and 70-195) contain I5 to 25% CPX, which usually forms crystals larger than porphyroclasts of OL and OPX. Compared to the CPX-rich wall- rock peridotites, they have higher proportions of OL (60-6596 as opposed to 35-50%) and lower abundances of OPX (I O-20% vs. 25- 35% in the wall-rocks) and SP (2-3% vs 4-69). The proportion of CPX/OPX is typically &I. Samples 70- I92 and 70- 195 contain 5 to 10% AMPH, whereas samples 70-I 18 and 70-189 have only trace amounts of AMPH. The modal composition of spine1 lherzolites (samples 70- 122 and 70- 120) is closely comparable to similar rocks from the other massifs apart from the higher proportions of AMPH in Caussou. The contents of this mineral is about 3% in sample 70- 122 and 2% in sample 70-120. The AMPH-rich lherzolites (samples 70-5 and CAU-3) have the highest amounts of amphibole among the studied peridotites (15-20%) (OPX and CPX both 8-15% and SP < 0.5%). The analyzed peridot&s contain ~8% serpentine and display porphyroclastic to equigranular texture.

PtC DE GERAL (500 X 200 m, good exposure) is made up of predominantly CPX-poor spine1 Iherrolites. The analyzed samples, which contain 5 to 15% serpentine, have porphyroclastic to equi-

granular texture. They are rather similar to the rocks from Lherz and Freychinede. Their abundance of interstitial amphibole is very small (-0.2%). Lherzolite 7 l-264 has a minor amount of phlogopite.

Geochemistry and petrogenesis of Eastern Pyrenean peridotites

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