Chemical Geology, 89 ( 1991 ) 243-262 243 Elsevier Science Publishers B.V., Amsterdam

[21

Geodynamic implications of geochemical data for the Pyrenean ophites (Spain-France)

D. B6ziat a, J.L. Joron b, P. Monchoux a, M. Treuil b and F. Walgenwitz c °Laboratoire de Min&alogie et Cristallographie, L.A. No. 67 du CN.R.S., Universit~ Paul Sabatier, 39 all~es J. Guesde,

F-31400 Toulouse, France bLaboratoire de G~ochimie Compar~e et Systkmatique et Institut de Physique du Globe, L.A. No. 196 du C.N.R.S., Universit~

Pierre et Marie Curie (Paris VI), 4 Place Jussieu, F- 75230 Paris Cedex 05, France, and Groupe des Sciences de la Terre, Laboratoire Pierre Siie, C.N.R.S., Centre d'l~tudes Nucl~aires Saclay, F-91191 Gif-sur- Yvette Cedex, France

cSoci~t~ Nationale Elf-Aquitaine, C.S.T.C.S., avenue Larribeau, F-64018 Pau Cedex, France

(Received May 22, 1990; accepted for publication August 16, 1990)

ABSTRACT

B6ziat, D., Joron, J.L., Monchoux, P., Treuil, M. and Walgenwitz, F., 1991. Geodynamic implications of geochemical data for the Pyrenean ophites (Spain-France). Chem. Geol., 89: 243-262.

Field observations and radiometric data (K/Ar) of Pyrenean tholeiites issued from numerous large sills in Mesozoic basins indicate a Liassic intrusion. Variations in the chemical composition of these tholeiitic rocks show that the magmatic liquids are slightly differentiated via fractional crystallization and/or in situ density fractionation. Data on the highly hygromagmaphile (HYG) elements exhibit a noteworthy homogeneity of compositions and display, relative to chondrite- normalized abundances, a distinctly negative Ta anomaly and a slight enrichment of highly HYG elements. These geo- chemical features are significantly different from ocean-type tholeiites due to the geochemical and mineralogical properties of their mantle source. The peculiar composition of the Pyrenean mantle source can be interpreted as the result of the Hercynian convergence process in which the dehydration of the subducted slab induced hydrothermal transformation and high-P-T hydrous pyroxenite segregation. The opening of the North Atlantic Ocean could have produced the ophites by inducing melting in this subcontinental mantle; with the enlargement of the new ocean basin, such a magma source would become progressively less available.

I. Introduction

Ophites are basaltic dolerites. They are a widespread rock type associated with the Keu- per continental sediments in the Pyrenean Belt (Fig. 1 ).

The petrology of these Keuper magmatic rocks has received considerable attention (Bossi~re, 1968; Fonteilles and Muffat, 1970; Thiebault, 1973; Walgenwitz, 1976; Lago, 1980; Azambre and Rossy, 1981; Azambre et al., 1981, 1987); however, trace-element data were available (Castellarin et al., 1978; Meschede, 1987 ) for only a few ophites from northern Spain.

The purpose of this paper is to present re- suits of a trace-element geochemical study concerning a large number of ophite samples and a few alkali basalt samples from the Cor- bi~res area, France (East Pyr6n6es) taken for comparison, and to emphasize characteristics of ophitic magmas.

The results focus on the discussion on whether there has been crustal contamination or if the composition of these rocks reflects that of their mantle source. We propose a model which takes into account the tectonic features and the geochemical characteristics of peri-At- lantic magmatism, relating them to an oro- genic phase preceding the beginning of crustal

0009-2541/91/$03.50 © 1991 - - Elsevier Science Publishers B.V.

244 D. BI~ZIAT ET AL.

thinning and rifting. This model is based on the particular nature of the sub-continental man- tle to explain the high degree of homogeneity of composition within a large geographic province.

2. Geological and geodynamic setting, age

The ophites are exclusively associated with the Keuper evaporitic sequence of Triassic basins. Generally, the ophitic bodies present fault contacts with their surrounding sedi- ments. However, in some outcrops as Estopi- nan, Spain (Lago et Pocovi, 1982) they show evidence of a sill structure. At its contacts, the ophitic body presents flow structures such as flow wrinkles and load casts similar to those described between two sediments of different density. Furthermore, the Keuper sediments are transformed (discoloured and compacted) over ~ 1 m; we note also the lack of assimila- tion at contacts with the country rock.

The alkali rocks from Corbi~res, which have been extensively studied by Azambre and Rossy ( 1981 ), are also associated with Keuper evaporitic sediments corresponding to epicon- tinental deposits of a stable continental mar- gin (Curnelle et al., 1980). They often exhibit an effusive character and are generally trans- formed into spilitic assemblages. The chemis- try and the mineralogy of the primary igneous phases show the alkali affinity of these rocks. Furthermore, Azambre and Rossy ( 1981 ) in- dicated for the S te Eugtnie outcrop the pres- ence of peridotite xenoliths (lherzolites to spi- nel harzburgites) which could be the solid source for the magmas of the Eastern Pyrtntes.

K-Ar data (Walgenwitz, 1976; Btziat, 1983; Montigny et al., 1983 ) yield ages of 190 Ma for the ophites, and 190-210 Ma for the alkali ba- salts. So, ophites and alkali basalts seem to be penecontemporaneous, in agreement with geo- logical observations.

During Permo-Triassic times, the initial

phase of the North Atlantic opening was marked by crustal thinning along the axis of the future ocean. As the Atlantic Ocean opened, the newly formed continental margins were frag- mented into small basins. An accumulation of 3000-5000 m of clastic sediments and evapo- ritic deposits followed with the concomitant extrusion of the eastern North American tho- leiites (240-160 Ma) and the Moroccan tho- leiites ( 190-180 Ma) (Manspeizer et al., 1978; McBride et al., 1989 ). Similarly, ophitic mag- mas seem to result from early extensive events resulting from the opening of the North Atlan- tic Ocean between the African and North American plates (Pitman and Talwani, 1972) beginning 180 Ma ago.

3. Petrographic notes

Mineral phases together with their ranges of composition are the following (Btziat, 1983; Azambre et al., 1987):

( 1 ) zoned plagioclase (An2o_8o) poikiliti- cally enclosed by augite (Wo23_asEn35_5oFsio_ 42); these two minerals together make up al- most 90% (by volume) of the rocks;

(2) pigeonite (Wo l oEnso-75Fs~ 5-40 ) , usually confined to the rims of larger augite crystals, and rarely orthopyroxene;

(3) olivine (Fo55_84), generally replaced by green-brown secondary material (chloritoser- pentine) as inclusions in pyroxenes;

(4) Fe-Ti-oxides consisting of titanomag- netites with 0 .23<Usp<0.58 and ilmenites with 0.023 < Hem< 0.110;

(5) interstitial granophyric groundmass (quartz, biotite, actinolite, chlorite, zircon, al- lanite and micropegmatite ).

Textural variations such as chilled margins or pegmatitic facies occur on the outcrop scale. However, local variations are limited to facies enriched in olivine at Louslitges or Arbas (Azambre et al., 1981; Btziat and Walgenwitz, 1983).

GEODYNAMIC IMPLICATIONS FOR THE PYRENEAN OPHITES 245

- - ! I ! I

Gulf of Gascogne / Dax

~0o,onc,~r "-/, . ~ : ~ " ~ . . - " ~ ' ~ , , C " - - ~ / ' ~ ~ ...~.. - - ' a~il~, ~ ~ stJ~oo ~."~2~F " 1 ~ . .

2/Q CORDILLERA C AN:::::C A A z peit i x ~ ~ l @° ~)~ ~ " ' ~

° " ~,~ ~ ~ ' ~ '" '" "') DO Vitoria ' -'~'~ ,,I -

Oligocene and post- 00phite Oligocene terrains • Alkali basalt

i I Mesozoic and Eocene 0 10 20 30 40 50 Km i i i i i i

Hercynian terrains

Pyrenean metamorphic domain | I a I

Fig. 1. Location of studied ophite and alkali basalt samples from the Pyr6n6es s.l.

uo!-I

DP!J?-I f

D an~- ~ ......'." ~-~ ..... ~ ...... .... ' "" "' "" :" i~

~.o - :'.7:-'"

UDuS!dj~ a "-~.

~.ln9 ×!o3

J~-OJJDADN

J

a~,S ~ a!ua6n 3 ..'-

"':'-~..~ aUUOSSD :)JD~)

.[, ,b. ,','.;f';,~ r'°°':% '~.' -/' ~"; "e ; '" /?"

-~

"y~.*~, .'7.i¢~, ..'4:: ... >~-:~.: ~~ S

...... ,.,. ~\";~ /..:" e-e' • ? ~"

"~:, r" "" ~ L'..~hI.~ s ,," ~s S°" .... I" "~" ' "'U'"v': ~J el~ n 0-10 •

Io • '" seq JD.L "~"~ ' L.

ss I"ID d

o asnolnoj_

eOu~nv ?..LO

8~-g~'dd

GEODYNAMIC IMPLICATIONS FOR THE PYRENEAN OPHITES

4. Analytical methods

Analytical methods used for samples crushed by an agate mill and selected after a mineral- ogical study are:

( 1 ) atomic absorption spectrometry for ma- jor elements (precision: __ 1%);

(2) instrumental neutron activation (Chayla et al., 1973 ) with irradiation followed by chemical separation for lanthanides (Joron and Ottonello, 1985). The accuracy of the method is ~ __ 5% for each trace element ana- lysed (Jaffrezic et al., 1980). Because the chilled margins are rare and often altered, the samples selected generally have the common ophitic to sub-ophitic texture. 180 samples from 105 outcrops have been studied by B6ziat (1983). 30 representative samples from dif- ferent localities in the Pyr6n6es (Fig. 1) are used in this summary.

5. Geochemistry

The ophites display both major- and trace- element similarities with normal-type mid- ocean ridge basalts (N-MORB): SiO2 (49.3%), A1203 (14.2%), FeO* (10.3%), MgO (7.9%), CaO (11.4%), TiO2 (0.91%) and Na20 (2%); K20 (0.5%) is somewhat higher. The contents in transition elements, clearly heterogeneous, are on the average similar to N- MORB with Cr 285 ppm and Ni=90 ppm, whereas the contents in hygromagmaphile ele- ments (HYGE) show limited variation* and are clearly higher than in N-MORB with Rb (11 ppm), Ba (100 ppm), Zr (75 ppm), Ta (0.33 ppm), Th (1.15 ppm) and U (0.30 ppm) (Wood et al., 1979a, b) (Table I).

*A few samples exhibit anomalous enrichments in alkalis (K, Rb, Cs) and U: these anomalies can be explained by the great mobility of these elements during hydrothermal alteration (sericitization of plagioclase) or supergene al- teration (Bougault et al., 1979 ).

249

5.1. Differentiation

The identification of processes that have led to the observed geochemical variations follows models proposed by several authors (Treuil, 1973; Joron et al., 1980a, b). Simple binary diagrams C=f(C*), in which the concentra- tion C of an element is plotted against that C* of most HYGE, have been used.

Plotting the data of HYGE (Fig. 2 ), we note that most ophite samples plot very closely to straight lines passing through the origin. If transition elements are plotted vs. Th on a log- arithmic scale, samples fall into a very narrow domain corresponding to linear depletion with strong negative slope (Fig. 3). The trace-ele- ment behavior illustrated in these diagrams is consistent with fractional crystallization as the major differentiation process in the suite, in accordance with models deduced from major- element data.

At Arbas, significant chemical fractionation occurs between the base and the top of the sin- gle sill (B6ziat and Walgenwitz, 1983). The basal sample (653, Table I ) is enriched in fer- romagnesian minerals (Ol+Cpx, 54%; Plag, 39%) whereas the top sample (654, Table I ) is enriched in plagioclase (O1+ Cpx, 36%; Plag, 49%). Crystal settling of the denser Fe-Mg phenocrysts has apparently occurred here after emplacement of the sill.

The range of compositions displayed by samples issued from the whole Pyrenean area is larger than that observed in a single locality. No clear regional trend is observed.

The Th concentration of primary liquids, C°h, may be estimated by a simple graphical technique (Treuil, 1973; All~gre et al., 1977) using a log CNi vS. log Cash diagram. C°h is equal to the Th concentration corresponding to COTh = 300 ppm found in unevolved magmas originating from partial melting of peridotite. It has been shown (Bougault, 1977; Bougault et al., 1979) that the concentration of Ni does not vary significantly either in the residue or

2 5 0 D. BI~ZIAT ET AL.

TABLE I

Major-, trace- and rare-earth element analyses

Location 1 2 3 4 5 6 7 8 9 10 11 12

Sample 884 885 709 712 653 654 880 881 875 872 877 871 692 120

SiO2 (wt.%) 49.0 49.0 50.0 49.0 48.0 48.0 49.5 50.0 48.5 49.0 T i Q 0.77 0.73 0.94 1.00 0.77 1.10 0.77 0.97 0.87 0.90 A1203 12.5 14.0 13.2 14.2 13.0 15.5 11.7 14.2 14.4 13.6 Fe203 3.09 2.99 3.35 3.34 5.35 4.99 3.44 2.82 3.89 3.09 FeO 7.1 6.1 7.5 6.6 5.5 6.1 8.6 6.8 6.1 7.1

MnO 1650 1390 1680 1870 1390 1750 1810 1550 1600 1590 MgO 11.6 10.l 8.7 7.5 11.2 6.4 10.3 8.3 8.3 7.8 CaO 12.2 13.3 11.3 12.5 11.1 12.0 11.3 11.5 12.0 11.9 Na20 1.5 1.6 1.8 2.0 1.4 2.l 1.5 2.0 1.7 1.9 K20 0.80 0.36 0.50 0.38 0.60 0.80 0.41 0.67 0.34 0.38

P (ppm) 490 490 360 350 380 380 505 700 695 730 U 0.15 0.18 0.25 0.34 0.52 0.31 0.20 0.42 0.25 0.39 Th 0.94 0.93 1.22 1.22 1.01 1.29 0.61 1.12 1.04 1.21 Zr 51 53 73 63 61 94 74 55 96 77 Hf 1.67 1.39 1.96 2.11 1.45 2.23 1.14 2.03 1.94 2.18 Ta 0.28 0.25 0.33 0.36 0.25 0.40 0.12 0.32 0.33 0.35 Ba 109 71 77 93 75 181 70 126 108 108 Cs 0.61 0.47 1.88 1.10 0.77 0.67 0.42 0.40 0.39 0.43 Rb 12.9 10.0 14.3 8.2 14.6 16.2 9.2 14.8 9.9 11.0 Sb 0.05 0.11 0.10 0.32 0.21 0.10 0.09 0.33 0.04 0.12 Cr 454 456 299 210 631 111 523 378 269 211 Co 53.8 49.6 48.7 37.1 54.1 41.6 66.4 50.1 44.5 45.3

Ni 178 147 115 99 229 62 183 140 112 95 Sc 35.4 38.0 36.9 37.0 35.5 37.2 41.5 37.1 34.9 36.9 La 6.0 5.2 6.4 7.4 5.2 8.3 4.2 6.4 6.6 7.5 Ce 14.9 11.4 16.6 16.0 12.6 19.1 10.0 16.7 14.3 16.5

Sm - - - 1.58 - - Eu 0.81 0.76 0.97 0.95 0.72 1.06 0.69 0.93 0.92 0.99 Tb 0.40 0.39 0.47 0.50 0.39 0.57 0.32 0.47 0.46 0.51 Yb - - - 1.26 - - Lu . . . . 0.18 -

f ( % ) 74 75 57 57 - - 62" 58

[Mg] 0.56 0.56 0.47 - - 0.48 0.46

49.5 49.1 49.7 50.0 0.90 0.94 0.90 0.89

13.0 14.2 14.6 15.7

3.28 3.92 1.94 2.02 7.2 6.8 7.6 7.8

1570 1860 1690 1730 8.3 7.8 8.7 6.1

11.2 11.5 12.6 11.3 1.8 1.9 1.8 2.0 0.41 0.58 0.40 0.38

680 690 325 490 0.23 0.35 0.18 - 1.06 1.28 0.77 -

50 85 46 - 1.27 2.00 1.57 - 0.33 0.35 0.24 -

102 152 85 116 0.40 0.84 0.82 -

t2.6 16.5 9.3 10.1 0.08 - 0.5 -

296 179 345 113 49.6 44.5 49.4 40.0

124 88 119 77 36.2 36.3 37.9 -

6.9 7.3 4.7 - 15.0 16.5 10.8 - 2.42 2.51 - - 0.90 1.I0 0.84 - 0.48 0.53 0.39 - 1.80 2.27 - 0.27 0.30 - -

66 - 91 -

0.47 - 0.46

See Fig. I for locations, f = calculated values of the differentiation index; [ Mg ] = Mg/( Mg + Fe 2 + ); - = not determined.

in the liquid as a result of varying degrees of partial melting. The corresponding value of COTh is 0.7--+0.2 ppm (Fig. 3). This C°h-Value can be used to compute a differentiation (crys- tal fractionation) index f = C°h / Ca-h (Treuil, 1973 ). f-values for our samples vary between 0.6 and 0.9, indicating that ophites result from moderate degrees of crystal fractionation. They display a sharp correlation with Mg-number (Table I ).

When we plot the least evolved samples in the same discrimination diagram, i.e. in an

AFM diagram, ophite samples clearly fail in the tholeiitic field (Fig. 4 ).

5.2. Hygromagmaphile element (HYGE) data." Ta anomaly

Sample data normalized to chondrites are plotted on a multi-element diagram (Sun et al., 1979; Wood et al., 1979a, b), using normali- zation coefficients and classification of Bou- gault (1980). We do not include alkalis and al- kaline earths because of their mobility (see

GEODYNAMIC IMPLICATIONS FOR THE PYRENEAN OPHITES 251

13 14 15 16 17 18 19

716 743 745 697 718 732 734 728 740

20 21 22 23 24 Alkal i basal ts

96 109 104 889 867 S te Eug6nie D u r b a n

50.5 48.0 48.5 49.5 47.4 49.2 50.0 49.6 0.97 0.90 0.87 0.97 0.77 0.80 0.80 0.87

14.4 14.2 12.7 15.1 12.5 12.5 12.3 13.4 2.71 2.51 2.81 3.06 2.46 2.24 1.83 2.29 7.9 7.9 8.9 7.4 7.4 8.6 8.7 8.1

1750 1660 1780 1780 1340 1910 1750 1790 7.1 8.6 10.0 6.6 12.5 10.4 10.2 9.8

11.4 12.5 11.9 11.9 11.8 12.3 12.1 11.8 1.8 1.7 1.5 1.9 1.4 1.3 1.3 1.6 0.46 0.36 0.30 0.55 0.62 0.45 0.38 0.43

370 365 270 450 325 230 240 325 0.28 0.20 0.16 0.30 0.21 0.22 0.33 0.26 1.14 0.97 0.80 1.46 0.88 1.01 0.90 0.90

69 64 48 91 63 97 119 73 2.21 1.97 1.53 2.14 1.51 1.43 1.45 1.49 0.37 0.30 0.25 0.39 0.26 0.27 0.27 0.27

73 53 63 144 47 103 85 95 0.67 0.97 0.89 0.46 1.18 0.76 0.73 0.81

14.2 11.1 7.8 12.5 12.1 12.5 12.2 10.5 0.12 0.04 0.02 0.04 0.13 0.04 0.11 0.04

163 278 415 108 649 501 438 452 42.8 47.1 59.5 41.1 55.9 53.2 56.8 52.6 75 122 158 63 245 157 161 149 37.7 39.8 39.4 35.5 35.6 38.2 36.1 37.7

7.2 6.4 5.2 7.7 5.0 5.8 6.1 5.1 16.5 13.5 11.0 17.0 10.2 14.9 14.9 14.5

2.29 1.88 2.68 1.69 - 0.98 0.90 0.64 1.13 0.68 0.84 0.88 0.84 0.52 0.47 0.38 0.56 0.37 0.43 0.42 0.42

1.80 1.55 2.35 1.65 - 0.28 0.24 0.30 0.25 -

87 - 81 69 78 78

0.47 - 0.57 0.48 0.48 0.48

50.0 49.0 47.6 48.0 48.2 48.8 0.97 0.77 0.67 0.92 0.87 0.97

14.2 13.4 14.0 14.9 13.6 14.9 2.35 2.73 3.15 2.55 3.59 3.04 7.5 7.7 6.5 6.5 7.1 7.6

1780 1790 1510 2010 1570 1590 7.6 9.5 9.0 8.5 8.8 5.8

11.5 11.7 12.4 9.2 11.5 9.8 2.3 1.6 1.5 2.7 2.0 2.5 0.22 0.46 0.31 2.00 0.70 1.20

310 340 280 340 550 725 2.32 4.22 0.28 0.21 0.30 0.34 1.25 1.29 1.67 1.66 1.09 1.26 1.08 1.40 4.36 4.70

77 66 79 104 66 94 255 225 1.89 2.19 1.88 2.04 2.02 2.27 5.40 4.80 0.36 0.39 0.33 0.34 0.33 0.39 3.84 3.80

46 94 39 389 100 126 318 390 0.63 0.87 0.63 3.28 0.83 0.80 2.00 3.90 6.0 4.6 9.4 61.5 17.4 36.2 23.5 16.2 0.52 0.49 0.94 0.14 0.13 0.10 0.12 0.18

219 273 337 228 330 75 234 308 46.7 46.2 42.9 38.2 47.3 41.9 39.6 53.8 97 116 147 97 119 52 158 252 39.0 34.7 37.2 37.8 38.6 35.4 23.0 20.4

7.7 8.9 6.9 7.3 6.6 7.6 37.3 32.6 15.3 18.4 15.2 16.7 16.0 15.2 76.0 67.5

- 2.68 2.23 - 2.43 5.0 4.4 0.98 1.01 0.96 0.98 0.96 1.03 2.70 2.30 0.52 0.49 0.47 0.51 0.46 0.54 0.95 0.81 - 1.88 1.80 - 2.28 1.99 1.75 - 0.29 0.27 - 0.34 -

42 - 64 56 65 - -

0.44 - 0.52 0.5 0.49 -

Section 5 ). The family of sub-parallel patterns (Fig. 5a) exhibits low enrichment over chon- drite for the highly HYG elements, with a dis- tinctly Ta negative anomaly. As shown in Fig. 6, Ta increases with Th very closely to a straight line passing through the origin, and has to be considered as a HYGE. This Ta anomaly is not due to fractional crystallization (E1-Azzouzi et al., 1982; Briqueu et al., 1984). These patterns show a positive Eu anomaly more or less pro- nounced which may be ascribed to density fractionation leading to variable plagioclase

accumulation (e.g., top sample 654 from Ar- bas with Eu= 1.06 ppm). The slight differ- ences of HYGE enrichment noted among the various samples could be the result of frac- tional crystallization.

Data on contemporaneous least fraction- ated tholeiites from eastern North America (high-TiO2 quartz-normative tholeiites; Rag- land et al., 1971; Smith et al., 1975) and Mo- rocco (Anti-Atlas and Haut-Atlas; Bertrand et al., 1982 ) exhibit similar HYGE patterns (Fig. 5b).

252 D. BI~ZIAT ET AL.

"l-a

o

**~*

/f" /0 , . . .

, Th

Fig. 2. Variation of highly HYG elements in ophites (F. C. = fractional crystallization model, P.M. = partial melting model).

300

1OO

Ni

\ \

, \ *

~" *

• \ ~ , \ *

\ \

10 i i I ; i I i

o., c~ h . ~ o . 7 1 Th

Fig. 3. Th vs. Ni plotted on a logarithmic scale (C°h = initial Th concentration: see text for explanation ).

G E O D Y N A M I C IMP LICATI ONS FOR THE PYRENEAN OPHITES 253

F

A M

Fig. 4. A - F - M diagram (A=Na20+K20; F=FeO+ 0.9Fe203; M= MgO, in wt.%).

Data on the oceanic tholeiites (Hofmann, 1988 ) exhibit no Ta negative anomaly. Walvis Ridge basalts, South Atlantic Ocean, have both Ta negative anomaly and an enrichment in highly HYG elements (Bougault, 1980) (Fig. 5b).

Data on alkali rocks from Corbi6res, simi- larly for the two samples, display a clearly en- riched spectrum with respect to slightly HYG elements, typical of alkali basalts from oceanic islands (e.g., Emperor Seamounts, Leg 55, west Pacific; Cambon et al., 1980) and from conti- nental provinces (e.g., Chalne des Puys, Mas- sif Central, France; Villemant et al., 1979) (Fig. 5a).

The source of the magmas can also be iden- tified by ratios of HYGE. Treuil and Joron (1975, 1976), and Joron and Treuil (1977) noted that for rocks crystallized from rela- tively unfractionated liquids (high degrees of partial melting and low degrees of fractional crystallization), the ratios of the HYGE are only slightly modified and are approximately equal to their ratios in the solid source that underwent partial melting. This result can be applied to ophites since, after Green ( 1971 ),

o LJ

" (a)

to

[

I t I , I IL I, , I , , ThLG Ce ZrSm Eu Tb Yb Lu

Tel Hf Ti Y

100

W R - -

(b)

IO

I I I, I, h i I I Th LciCe ZrSm Eu Tb Yb Lu V

To Hf Tt Y

Fig. 5. Chondrite-normalized multi-element diagram. a. Individual patterns for ophites and alkali basalt (0). b. Patterns for high-TiO2 quartz-normative tholeiites from eastern North America ( I ) , Haut-Atlas tholeiites from Morocco ( • ) ; W.R. =Walvis Ridge basalts (Bougault, 1980); and average N-type MORB (Hofmann, 1988) compared with ophites ( stippled field).

254 D. BI~ZIAT ET AL.

0.3

0.2

0.1

Ta

0

Fig. 6. Th vs. Ta diagram.

Ta

0.4

,~** • • &* *

Th I I !

2

Th

Hf/3

Fig. 7. Th-Hf/3-Ta diagram (A = field for N-type MORB; B=field for E-type MORB; C= field for within-plate ba- salt; D=field for magma series at destructive plate mar- gins; • = ophites; • = alkali basalts; [] = low-TiO2 quartz- normative tholeiites and II=high-TiO2 quartz-norma- tive tholeiites, from eastern North America; O = Anti-At- las tholeiites and O=Haut-Atlas tholeiites, from Morocco).

quartz or olivine tholeiitic magmas are pro- duced by high degrees of partial melting ( 10- 20%).

The comparison of different HYGE ratios for the least fractionated ophites with tholei- ites from North America (Pennsylvania, U.S.A. ) and Morocco allows us to identify the original tectonic environment. Worldwide, the Th/Ta, Th/La and T h / H f ophitic ratios range from low values typical of alkali basalts and tholeiites from diverging plates (e.g., Th/Ta staying very close to 1 and varying in the re- stricted area going from 0.5 to 1.5 ) to very high values typical of calc-alkali basalts from con- verging plates (e.g., Th/Ta generally > 10) (Joron and Treuil, 1977; Briqueu et al., 1984) (Table II ). Such data are observed in the least differentiated tholeiites from eastern North America and Morocco. The values of alkali ba- salts from Corbirres are extremely close to those measured in Neogene magmatic series from the Chaine des Puys and in ocean island basalts (Villemant et al., 1979 ).

Discrimination diagrams of Meschede (1986) and Wood et al. ( 1979a, b) distin- guish between different types of MORB and continental tholeiites. As we do not measure

GEODYNAMIC IMPLICATIONS FOR THE PYRENEAN OPHITES 2 5 5

TABLE II

Hygromagmaphile element (HYGE) ratios in Pyrenean ophites and alkali basalts from Corbi~res, and average values from peri-Atlantic tholeiites

Sample Th/Ta Th/La Th/Hf La/Ta No.

Ophites, Pyren6es, Spain-France

692 3.2 0.16 0.49 19.6 709 3.7 0.19 0.62 19.4 712 3.4 0.15 0.58 22.8 718 3.4 0.17 0.58 19.6 728 3.3 0.18 0.60 18.9 732 3.7 0.17 0.71 21.5 734 3.3 0.15 0.62 22.6 736 3.3 0.16 0.51 20.8 740 4.6 0.22 0.88 21.4 745 3.2 0.18 0.52 18.0 104 3.7 0.17 0.62 21.5 109 3.3 0.17 0.58 20.0 653 4.0 0.19 0.70 20.8 654 3.2 0.16 0.58 20.7 872 3.5 0.16 0.56 21.4 877 3.2 0.16 0.57 20.0 877 3.5 0.17 0.55 20.0 884 3.4 0.16 0.56 21.4 885 3.7 0.18 0.68 20.8 889 3.3 0.16 0.53 20.0

Average 3.5 0.17 0.59 20.6

Standard deviation 0.3 0.02 0.06 1. I

Morocco Anti-Atlas 4.2 0.19 0.69 21.3 Haut-Atlas I 4.0 0.21 0.95 18.5 Haut-Atlas II 4.0 0.17 0.65 22.2 Haut-Atlas Ill 5.2 0.24 0.75 26.3

Pennsylvania, U.S.A.

high-TiO~ quartz- tholeiite 4.6 0.26 1.0 17.7

low-TiO2 quartz- tholeiite 9.4 0.38 1.4 25.0

Alkali basalts, Corbi6res, France

Durban 1.14 0.12 0.81 9.7 S te Eug6nie 1.02 0.14 1.02 8.6

Nb and Y contents, we use the Th-Hf/3-Ta diagram (Wood et al., 1979a, b) (Fig. 7). Continental tholeiites plot in the field of magma series at destructive plate margins, quite separately from the field of within-plate basalts (WPB). A further distinction between continental tholeiites and calc-alkali basalts seems necessary for this diagram; the Th/Ta

ratio may be such a discriminant which gives the differences between the two groups. It should be noted that the Corbi~res alkali ba- salts plot in the WPB field.

So, the specific features ofophites are: ( 1 ) a quite good similarity of compositions

with a slight enrichment in the highly HYG elements relative to chondrites;

(2) a distinct Ta negative anomaly similar to that of peri-Atlantic tholeiites and Walvis Ridge basalts, with Th/Ta values intermediate between those of diverging plate boundaries and those of converging plate boundaries.

6. Discussion

It is a generally accepted hypothesis that the continental upper mantle is geochemically dif- ferent from the oceanic upper mantle because of the continental crust-oceanic crust-mantle differentiation processes (Hofmann, 1988). Various models have been proposed for the sub-continentalmantle and processes of con- tinental genesis.

6.1. A crustal contamination process

A crustal contamination process, proposed in the past by several authors (Compston et al., 1968; Faure et al., 1974; Carter et al., 1978; A1- l~gre et al., 1982; Dostal et al., 1983; Dupuy and Dostal, 1984; Alibert, 1985), could theo- retically produce the specific features of ophites. Moreover, the continental crust ma- terial (Taylor and McLennan, 1985; Loubet et al., 1988) and oceanic sedimentary material (Kay and Kay Mahlburg, 1988 ) are character- ized by Ta depletion. Evidence of fractional crystallization has been shown in Section 5.1. Hence, the development of crustal contami- nation accompanied by fractional crystalliza- tion could lead to the AFC (assimilation-frac- tional crystallization) model (All~gre and Minster, 1978; DePaolo, 1981 ) if contamina- tion takes place in the magmatic chamber, or to the AEC (assimilation-equilibrium crystal-

2 5 6 D . B t ~ Z I A T E T A L .

lization) model (Devey and Cox, 1987) if contamination takes place during ascent. Both models produce geochemical relationships (but in the opposite sense) between the differentia- tion index and the amount of contamination. Ophite data show no clear correlation of Th/ Ta ratios o r 875r /86Sr isotope ratios (after Ali- bert, 1985) with the degree of fractionation such as the differentiation index fo r Mg-num- ber (Fig. 8).

Moreover, AEC processes should yield a marked heterogeneity in tholeiitic magma by

'~I/TII (a)

~ ~ ~ fo/ go

I i I 1 /

o 90 80 70 60 5

Mg-no :f: :~

(b)

~Sr I I I

15 2O 25 3O

Fig. 8. Crustal contamination-differentiation relation- ships. a. Th/Ta v s . f b. Sr vs. Mg-number (after Alibert, 1985).

variable addition of crustal material within the large peri-Atlantic province. However, ophites and peri-Atlantic tholeiites exhibit low U (0.3 ppm) and Th ( 1.15 ppm) contents, low Rb/ Sr (0.06) and high K/Rb ( 380 ) (Dupuy et al., 1988) relative to those measured in upper continental crust and high homogeneity of HYGE ratios.

These arguments argue against significant crustal contamination. The composition of the least differentiated ophites could reflect the primary geochemical characteristics of the mantle source.

6.2. A mantle source differing from the MORB mantle by its chemical composition or mineralogical composition

Several authors (O'Nions and Clarke, 1972; Brooks et al., 1976; All6gre et al., 1982; Ber- trand et al., 1982 ) noted that continental flood basalts (CFB) are richer in some HYGE (Th, U, Ba, K, Sr) than N-MORB, and suggested that these characteristics reflect the existence of a less depleted or enriched upper part of the continental lithosphere which is the assumed source of the CFB. This reservoir could have been affected by recycled crustal material and by ancient metasomatic events (Mahoney et al., 1982; Carlson, 1984; Weaver and Tarney, 1983; Nelson et al., 1986). The Nd-Sr isotope data of the ophites are consistent with this hy- pothesis (Alibert and Montigny, 1983; Ali- bert, 1985). Such a hypothesis justifies the compositional differences between CFB and N- MORB. However, such a hypothesis does not explain whether or not the Ta negative anom- aly is "primary", i.e. whether it exists in man- tle material, or is "secondary", i.e. created either by high-pressure crystallization or by melting processes.

The Pyrenean peridotite bodies, which are pieces of upper mantle tectonicaUy emplaced into the continental crust, occur within the Cretaceous metamorphic zone along the North Pyrenean fault. They consist mainly of layered

GEODYNAMIC IMPLICATIONS FOR THE PYRENEAN OPHITES 257

spinel lherzolites intercalated with two types of cumulates formed by high-P-T mineral segre- gation: amphibole pyroxenite or hornblendite veins and spinel or garnet pyroxenite veins (Monchoux, 1970; Conqu6r6, 1978). Bodi- nier et al., ( 1987a, b) have calculated the bulk partition coefficients (D) for anhydrous and hydrous pyroxenites (Table IV in Bodinier et al., 1987b). The DTa-Values (0.15-0.50) are higher than DTh-Values (0.03) for amphibole pyroxenites, whereas DTa (0.03) is identical to DTh (0.02) in the anhydrous pyroxenites. From the calculated rare-earth element (REE) com- positions of the liquid in equilibrium with the constituent minerals, Bodinier et al. relate an- hydrous pyroxenites to tholeiitic magmas (ophites), and the amphibole pyroxenites to Cretaceous alkali basalts exposed in the Pyr6- n6es. However, the isotope data (Polv6 and All6gre, 1980; Hamelin and All6gre, 1985) in- dicate that the lherzolites are not phases resid- ual to melting of these ophites which were probably generated in deeper parts of the up- per mantle. Such an anhydrous pyroxenite seg- regation justifies the light REE (LREE) en- richment of ophite, but not its Ta negative anomaly. Only a hydrous pyroxenite segrega- tion predating Triassic time could lead to Ta negative anomaly in residual magma. Given the anhydrous mineralogy of the ophites, it is very unlikely that they were ever in equilib- rium with a hydrous pyroxenite segregation.

As well, Treuil et al. (1979) demonstrated that Ta may be fractionated from Th during melting. Their work demonstrated that Ta, Hf and REE have higher partition coefficients in biotite or in amphibole than in orthopyroxene or olivine. However, Th and U have the same partition coefficients in all these minerals. The above authors suggested that the high Th/Ta ratios and the HYGE fractionation observed in converging plate margin basalts were the re- sults of the partial melting of a hydrous mantle source. From the same rocks, Briqueu et al. ( 1984 ) invoked

"the presence of a phase (titanate?) in low propor- tions, stable under the P - T and H20 conditions in- duced by the geodynamic context".

However, Green and Pearson (1986), and Ryerson and Watson (1988) showed that Ta depletion cannot be attributed to residual ti- tanate minerals; also, during a partial melting process, biotite or amphibole would quickly enter the melt and would not thus generate the observed negative Ta anomaly.

Instead, the cause of Ta anomalies is more likely to be "primary". The mantle source of ophites may have been affected by a conver- gence process during which the dehydration of the subducted slab induced hydrothermal transformation (Treuil et al., 1979; Joron et al., 1983), similar to what has been proposed by Lorentz and Nicholls (1976) and Bard et al. (1980) for the Hercynian belt in western Eu- rope. The subducted slab could produce a fluid that contains itself Ta depletion and which may ascent through the overlying peridotite man- tle, changing its chemical composition (Arcu- lus and Powell, 1986; Hofmann, 1988; Ryer- son and Watson, 1988). If such a mantle was stabilized beneath the continental crust (Jor- dan, 1978 ), it could produce during the early extensive events, at Permo-Triassic time, tho- leiitic magmas with their characteristic Ta anomalies. Nevertheless, the absence of"typi- cal"subduction-zone basalts associated with the ophites along with intermediate Th/Ta ra- tios suggests that ophitic magma could be gen- erated in a post-orogenic zone.

Treuil et al. (1979), Joron et al. ( 1980a, b) and All6gre et al. ( 1981 ) emphasized that al- kali basalts from continental or oceanic areas have similar isotopic or chemical features and that they originate from a similar mantle source. In our case, two chemical analyses are not sufficient to discuss this hypothesis. Nevertheless, it should be noted that the Cor- bi6res alkali basalts show all the characteristics of within-plate or diverging plate basalts (e.g., Th/Ta ratio close to l, and no Ta negative anomaly). Like the Pyrenean tholeiitic mag-

2 5 8 D. BI~ZIAT ET AL.

matism, the alkali basalts could have been gen- erated at the time of the opening of the North Atlantic. We believe that, as during the Per- mian period, the Triassic basins evolved in op- posed tectonic contexts on both sides of a line roughly located on the Cerdagne fault (Bixel and Lucas, 1983). This explains the sharp sep- aration between the two magmatic provinces.

7. Conclusions

The present geochemical study comple- ments earlier petrographic and geologic stud- ies (Brziat, 1983). Ophites and peri-Atlantic tholeiites show a noteworthy similarity of compositions, with a slight enrichment of the most highly HYG elements relative to chon- drites and distinct negative anomalies of Ta are less pronounced than that of magmas associ- ated with convergence zones. These geochem- ical features could be characteristic of conti- nental tholeiites generated in post-orogenic settings by continental breakup and the open- ing of an ocean basin. Such magmas may arise relatively uncontaminated through the crust and their peculiar features could reflect sub- continental mantle characteristics produced by the effects of metamorphism or metasomatic processes in the mantle related to plate subduction.

These results are in agreement with the ex- istence of a large extensive tectonic field in the Pyrrnres, induced by the opening of the North Atlantic ocean at the Triassic-Liassic limit, occurring after the Hercynian orogenic phase.

Acknowledgements

We would like to thank two anonymous re- viewers for their suggestions and comments which have contributed to the final version of the text.

References

Alibert, C., 1985. A Sr-Nd isotopic and REE study of late Triassic dolerites from the Pyrrnres (France) and the

Messejana Dyke (Spain and Portugal). Earth Planet. Sci. Lett., 73: 81-90.

Alibert, C. and Montigny, B., 1983. Nd and Sr isotopic composition and REE geochemistry of Triassic basalts from the Pyrrnres. Terra Cognita, 3:133 (abstract).

Allbgre, C.J. and Minster, J.F., 1978. Quantitative models of trace element behavior in magmatic processes. Earth Planet. Sci. Lett., 38: 1-25.

Allrgre, C.J., Treuil, M., Minster, J.F., Minster, B. and Albarrde, F., 1977. Systematic use of trace elements in igneous processes, Part I. Fractional crystallization processes in volcanic suites. Contrib. Mineral. Petrol., 60: 57-75.

All~gre, C.J., Duprr, B., Lambert, B. and Richard, P., 1981. The subcontinental versus suboceanic debate, I. Lead-neodymium-strontium isotopes in primary al- kali basalts from a shield area: the Ahaggar volcanic suite. Earth Planet. Sci. Lett., 52: 85-92.

Allrgre, C.J., Duprr, B., Richard, P., Rousseau, D. and Brooks, C., 1982. Subcontinental versus suboceanic mantle, II. Nd-Sr-Pb isotopic comparison of conti- nental tholeiites with mid-ocean ridge tholeiites and the structure of the continental lithosphere. Earth Planet. Sci. Lett., 57: 25-34.

Arculus, R.J. and Powell, R., 1986. Source component mixing in the regions of arc magma generation. J. Geo- phys. Res., 91" 5913-5926.

Azambre, B. and Rossy, M., 1981. Caract~re alcalin du magmatisme triasique des Corbi~res Orientales. Bull. Soc. Grol. Fr., 23: 253-262.

Azambre, B., Rossy, M. and Elloy, R., 1981. Les dolrrites triasiques (ophites) des Pyrrnres: donnres nouvelles fournies par les sondages prtroliers en Aquitaine. Bull. Soc. Grol. Fr., 23: 263-269.

Azambre, B., Rossy, M. and Lago, M., 1987. Caractrris- tiques prtrologiques des dolrrites tholriitiques d'hge triasique (ophites) du domaine pyrrnren. Bull. Mi- nrral., 110: 379-396.

Bard, J.P., Burg, J.P., Matte, Ph., Ribeiro, A., 1980. La chaine hercynienne d'Europe occidentale en termes de tectonique des plaques. In: Colloq. C6, 26th Int. Geol. Congr., Paris, pp. 233-246.

Bertrand, H., Dostal, J. and Dupuy, C., 1982. Geochem- istry of Early Mesozoic tholeiite from Morocco. Earth Planet. Sci. Lett., 58: 225-239.

B6ziat, D., 1983. l~tude p~trologique et g6ochimique des ophites des Pyr6n6es - Implications g6odynamiques. Thesis (3~me cycle), University of Toulouse, Tou- louse, 60 pp.

B6ziat, D. and Walgenwitz, F., 1983. Mise en 6vidence d'un processus d'accumulation dans le gisement de dol6rite (ophite) d'Arbas (Haute-Garonne). C.R. Acad. Sci. Paris, 296: 1435-1440.

Bixel, F. and Lucas, C., 1983. Magmatisme, tectonique et s6dimentation dans les foss6s st6phano-permiens des Pyr6n6es occidentales. Rev. G6ol. Dyn. G6ogr. Phys., 24(4): 329-342.

GEODYNAMIC IMPLICATIONS FOR THE PYRENEAN OPHITES 259

Bodinier, J.L., Guiraud, M., FabriCs, J., Dostal, J. and Dupuy, C., 1987a. Petrogenesis of layered pyroxenites from the Lherz, Freychin6de and Prades ultramafic bodies (Ari+ge, French Pyrrnres). Geochim. Cosmo- chim. Acta, 51: 279-290.

Bodinier, J.L., FabriCs, J., Lorand, J.P., Dostal, J. and Dupuy, C., 1987b. Bull. Minrral., 110: 345-358.

Bossi~re, G., 1968. l~tude p6trographique de la r6gion de Bedous. Thesis (36me cycle), University of Paris VI, Paris, 129 pp.

Bougault, H., 1977. l~vidence de la cristallisation frac- tionn6e au niveau d'une ride m6diooc6anique: Co, Ni, Cr. FAMOUS, Leg 37 du DSDP. Bull. Soc. G6ol. Fr., 7, 19: 1207-1212.

Bougault, H., 1980. Contribution des 616ments de transi- tion/l la comprrhension de la grn~se des basaltes ocra- niques - Analyse des 616ments traces dans les roches par spectromrtrie de fluorescence X. Thesis (Docto- rat), University of Paris VII, Paris, 221 pp.

Bougault, H., Joron, J.L. and Treuil, M., 1979. Altera- tion, fractional crystallisation, partial melting, mantle properties from trace elements in basalts recovered in the North Atlantic. In: M. Talwani, C.G. Harrison and D.E. Hayes (Editors), M. Ewing Serie 2, Deep Drill- ing Project in the Atlantic Ocean: Ocean Crust. Am. Geol. Union, Washington, D.C., Geodyn. Proj., Sci. Rep., 48: 352-368.

Bougault, H., Cambon, P., Corrr, O., Joron, J.L. and Treuil, M., 1980. Evidence for variability ofmagmatic processes and upper mantle heterogeneity in the axial region of the Mid-Atlantic Ridge near 22 °N and 36 ° N. Tectonophysics, 55:11-34.

Briqueu, L., 1984. Grochimie isotopique du strontium et du nrodyme des ophites pyrrnrennes. In: I~tude du magmatisme associ6 aux zones de subduction ~ l'aide de traceurs grochimiques multiples: 616ments traces et rapports isotopiques 875r/a6Sr, 143Nd/144Nd. Thesis (Sciences), Universit6 des Sciences et Techniques du Languedoc, Montpellier.

Briqueu, L., Bougault, H. and Joron, J.L., 1984. Quanti- fication of Nb, Ta, Ti and V anomalies in magmas as- sociated with subduction zones: petrogenetic implica- tions. Earth Planet. Sci. Lett., 68: 297-308.

Brooks, C., James, D.E. and Hart, S.R., 1976. Ancient lithosphere, its role in young continental volcanism. Science, 193: 1086-1094.

Cambon, P., Joron, J.L., Bougault, H. and Treuil, M., 1980. Leg 55 Emperor seamounts: trace elements in transitional tholeiites, alkali basalts and hawaiites, mantle heterogeneity or homogeneity and magmatic processes. In: E.D. Jackson and I. Koizumi (Editors), Init. Rep. Deep Sea Drill. Proj., Vol. 55. U.S. Gov. Print. Off., Washington, D.C., pp. 585-597.

Carlson, R.W., 1984. Isotopic constraints on Columbia River flood basalt genesis and the nature of the sub- continental mantle. Geochim. Cosmochim. Acta, 48: 2357-2372.

Carter, S.W., Evenson, N.M., Hamilton, P.J. and O'Nions, R.K., 1978. Nd and Sr isotope evidence for crustal contamination of continental volcanics. Science, 202: 743-747.

Castellarin, A., Lucchini, F., Rosell, J., Rossi, P., Sartori, R. and Savelli, C., 1978. Preliminary data on Meso- zoic ophites from the southern Pyr6nres. Mineral. Pe- trogr. Acta, 22: 85-93.

Chayla, B., Jaffrezic, H. and Joron, J.L., 1973. Analyse par activation dans les neutrons ~pithermiques. C.R. Acad. Sci. Paris, 277: 273-275.

Compston, W., McDougall, I. and Heier, K.S., 1968. Geochemical comparison of the Mesozoic basaltic rocks of Antarctica, South Africa and Tasmania. Geo- chim. Cosmochim. Acta, 32: 129-149.

Conqurrr, F., 1978. Prtrologie des complexes ultrama- fiques de lherzolites ~ spinelle de l'Ari~ge (France). Th~se (Doctorat), University of Paris VI, Paris, 333 PP.

Curnelle, R., Dubois, P. and Seguin, J.C., 1980. Bassin d'Aquitaine. Substratum antr-tertiaire et bordures mrsozoiques en France: introduction ~ la grologie du Sud-Ouest. 26th Int. Geol. Congr., Paris, pp. 47-58.

DePaolo, D.J., 1981. Trace elements and isotopic effects of combined wall-rock assimilation and fractional crystallization. Earth Planet. Sci. Lett., 53: 189-202.

Devey, C.W. and Cox, K.G., 1987. Relationships be- tween crustal contamination and crystallization in continental flood basalt magmas with special refer- ence to Deccan Traps of the Western Ghats, India. Earth Planet. Sci. Lett., 84: 59-68.

Dostal, J., Dupuy, C. and Baragar, W.R.A., 1983. Geo- chemistry and petrogenesis of basaltic rocks from Coppermine River area, Northwest Territories. Can. J. Earth Sci., 20: 684-698.

Dupuy, C. and Dostal, J., 1984. Trace element geochem- istry of some continental tholeiites. Earth Planet. Sci. Lett., 67: 61-69.

Dupuy, C., Marsh, J., Dostal, J., Michard, A. and Testa, S., 1988. Asthenospheric and lithospheric sources for Mesozoic dolerites from Liberia (Africa): trace ele- ment and isotopic evidence. Earth Planet. Sci. Lett., 87:100-110.

E1-Azzouzi, M., Bougault, H., Maury, R. and Villemant, B., 1982. Application du diagramme de Coryell et Ma- suda "61argi" h l'rtude du fractionnement du titane et du vanadium dans la s6rie alcaline de la chaine des Puys. C.R. Acad. Sci. Paris, 295(I1): 1117-1120.

Faure, G., Bowman, J.R., Elliot, D.H. and Jones, L.N., 1974. Strontium isotopic composition and petrogene- sis of the Kirkpatrick basalt, Queen Alexandra Range, Antarctica. Contrib. Mineral. Petrol., 48:155-169.

Fonteilles, M. and Muffat, S, 1970. l~tude prtrographique de deux dolrrites (ophites) h pigeonite et olivine des Pyrrnres occidentales. Bull. Soc. Fr. Minrral. Cristal- Iogr., 93: 555-570.

Green, D.H., 1971. Composition of basaltic magmas as

2 6 0 D. BI~ZIAT ET AL.

indicators of conditions of origins: application to oceanic volcanism. Philos. Trans. R. Soc. London, 268: 707-725.

Green, D.H. and Pearson, N.J., 1986. Ti-rich accessory phase saturation in hydrous mafic-felsic compositions at high P, T. Chem. Geol., 54: 185-201.

Hamelin, B. and All~gre, C.J., 1985. Lead isotopic com- position of high temperature peridotites from Lherz, Lanzo, Beni-Bousera and the genesis of isotopic het- erogeneities in the Earth's mantle. Eos (Trans. Am. Geophys. Union ), 66: I 14 (abstract).

Hofmann, A.W., 1988. Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust. Earth Planet. Sci. Lett., 90: 297-314.

Jaffrezic, H., Joron, J.L., Treuil, M. and Wood, D.A., 1980. A study of the precision attained by neutron activation analysis using international standard rocks GS-N and BCR-I as examples - A discussion of a geochemical model accounting for the estimated errors. J. Ra- dioanal. Chem., 55 (2): 417-425.

Jordan, T.H., 1978. Composition and development of the continental tectosphere. Nature (London), 274: 544- 548.

Joron, J.L. and Ottonello, G., 1985. Radiochemical neu- tron activation analysis of rare earth elements in peri- dotitic rocks. J. Radioanal. Nucl. Chem., 88 (2): 259- 272.

Joron, J.L. and Treuil, M., 1977. Utilisation des pro- pri6t6s des 616ments fortement hygromagmaphiles pour l'6tude de la composition chimique et de l'h6t6rog6- n6it6 du manteau. Bull. Soc. G6ol. Fr., 7, 20(4): 521- 531.

Joron, J.L., Bougault, H., Wood, D.A. and Treuil, M., 1978. Applications de la g6ochimie des 616ments en traces ~ l'6tude des propri6t6s et des processus de g6n6se de la crofite oc6anique et du manteau sup6rieur. Bull. Soc. G6ol. Fr., 20(4): 521-531.

Joron, J.L., Treuil, M., Jaffrezic, H. and Villemant, B., 1980a. l~tude g6ochimique des 616ments en traces dans les s6ries volcaniques du rift d'Assal - Identification et analyse des processus d'accr6tion. Bull. Soc. G6ol. Fr., 7, 22(6): 851-862.

Joron, J.L., Treuil, M., Jaffrezic, H., Villemant, B. and Richard, P., 1980b. G6ochimie des 616ments en traces du magmatisme de l'Afar et de la m6gastructure Mer Rouge-Afar-Golfe d'Aden - Implications p6trog6n6- tiques et g6odynamiques. Bull. Soc. G6ol. Fr., 7, 22(6 ): 945-957.

Joron, J.L., Cabanis, B. and Treuil, M., 1983. M6thodes d'identification des s6ries volcaniques anciennes ba- s6es sur la g6ochimie des 616ments en traces - Compa- raison avec les s6ries r6centes: exemples d'application. Bull. Cent. Rech. Explor. Prod. Elf-Aquitaine, 7: 273- 284.

Kay, R.W. and Kay Mahlburg, S., 1988. Crustal recycling

and the Aleutian arc. Geochim. Cosmochim. Acta, 52: 1351-1359.

Lago, M., 1980. Estudio geol6gico, petr616gico, geochi- mico y de aprovechamiento industrial de rocas ofiticas en el Norte de Espafia. Thesis (Doctorat), University of Zaragosa, Zaragosa, 444 pp.

Lago, M. and Pocovi, A., 1982. Nota preliminar sobre la presencia de estructuras fluidales en las ofitas del area de Estopinan (Huesca). Acta Geol. Hispan., 17: 227- 233.

Lorentz, V. and Nicholls, I.A., 1976. The Permo-Carbon- iferous basis and range province of Europe - An appli- cation of plate tectonics. In: H. Falke (Editor), The Continental Permian in Central, West and South Eu- rope. pp. 313-342.

Loubet, M., Sassi, R. and Di Donato, G., 1988. Mantle heterogeneities: a combined isotope and trace element approach and evidence for recycled continental crust materials in some OIB sources. Earth Planet. Sci. Lett., 89:299-315.

Mahoney, J.J., McDougall, J.D., Lugmair, G.W., Murali, A.V., Sankar Das, M. and Gopalan, K., 1982. Origin of the Deccan Traps flows at Mahabaleshwar inferred from Nd and Sr isotopic and chemical evidence. Earth Planet. Sci. Lett., 60: 47-60.

Manspeizer, W., Puffer, J.H. and Cousminer, H., 1978. Separation of Morocco and Eastern North America: a Triassic-Liassic stratigraphy record. Geol. Soc. Am. Bull., 89: 901-920.

McBride, J.H., Nelson, K.D. and Brown, L.D., 1989. Evi- dence and implications of an extensive early Mesozoic rift basin and basait/diabase sequence beneath the southeast Coastal Plain. Geol. Soc. Am. Bull., 101: 512- 520.

Meschede, M., 1986. A method of discriminating be- tween different types of mid-ocean ridge basalts and continental tholeiites with the Nb-Zr -Y diagram. Chem. Geol., 56:207-218.

Meschede, M., 1987. The continental geochemistry of Triassic ophites of northern Spain. Neues Jahrb. Geol. Pal~iontol., Monatsh., 19: 287-296.

Monchoux, P., 1970. Les lherzolites pyr6n6ennes. Thesis (Doctorat), University of Toulouse, Toulouse, 240 pp.

Montigny, B., Azambre, B., Rossy, M. and Thuizat, R., 1983. l~tude K/Ar du magmatisme basique 1i6 au Trias sup6rieur des Pyr6n6es - Cons6quences m6thodolo- giques et pal6og6ographiques. Bull. Min6ral., 105: 673- 680.

Nelson, D.R., McCulloch, M.T. and Sun, S.S., 1986. The origins of ultrapotassic rocks as inferred from Sr, Nd and Pb isotopes. Geochim. Cosmochim. Acta, 50:231- 245.

O'Nions, R.K. and Clarke, D.B., 1972. Comparative trace element geochemistry of Tertiary basalts from Baffin Bay. Earth Planet. Sci. Lett., 15: 436-446.

Pitman, W.C. and Talwani, M., 1972. Sea floor spreading

GEODYNAMIC IMPLICATIONS FOR THE PYRENEAN OPHITES 261

in the North Atlantic. Geol. Soc. Am. Bull., 83: 619- 646.

Polv6, M. and All6gre, C.J., 1980. Orogenic lherzolite complexes studied by 87Rb-87Sr: a clue to understand- ing the mantle convection processes? Earth. Planet. Sci. Lett., 51: 71-93.

Ragland, P.C., Brunfelt, A.O. and Weigand, P.W., 1971. Rare earth abundances in Mesozoic dikes from the United States. In: A.O. Brunfelt and E. Skinner (Edi- tors), Activation Analyses in Geochemistry and Cos- mochemistry. Oslo-Bergen-Troms6 University For- lage, pp. 227-235.

Ryerson, F.J. and Watson, E.B., 1988. Rutile saturation in magma: implications for T i -Nb-Ta depletion in is- land-arc magmas. Earth Planet. Sci. Lett., 86: 225-239.

Smith, R.C., Rose, A.W. and Lanning, R.M., 1975. Geol- ogy and geochemistry of Triassic diabase in Pennsyl- vania. Geol. Soc. Am. Bull., 86: 943-955.

Sun, S.S., Nesbitt, R.B. and Sharaskin, A.Y., 1979. Geo- chemical characteristics of mid-ocean ridge basalts. Earth Planet. Sci. Lett., 44:119-138.

Taylor, S.R. and McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Ox- ford, 312 pp.

Thiebaut, J., 1973. Au sujet des ophites pyr6n6ennes (le point des travaux actuels). Ann. Sci. Univ. Besangon, 3(20): 5-13.

Treuil, M., 1973. Crit6res p6trologiques, g6ochimiques et structuraux de la gen6se et de la diff6renciation des magmas basaltiques - Exemple de l'Afar. Thesis (Doctorat), University of Orl6ans, Orl6ans.

Treuil, M. and Joron, J.L., 1975. Utilisation des 616ments hygromagmaphiles pour la simplification de la mod6- lisation quantitative des processus magmatiques. Rend. Soc. Ital. Mineral. Petrol., 31: 125-174.

Treuil, M. and Joron, J.L., 1976. l~tude g6ochimique des 616ments en traces dans le magmatisme de l'Afar - Im-

plications p6trog6n6tiques et comparaison avec le magmatisme de l'Islande et de la dorsale m6dio-atlan- tique - Afar between continental and oceanic rifting. E. Schweizerbart, Stuttgart, pp. 26-79.

Treuil, M., Joron, J.L., Jaffrezic, H., Villemant, B. and Calas, G., 1979. G6ochimie des 616merits hygromag- maphiles, coefficients de partage min6raux/liquide et propri6t6s structurales de ces 616ments dans les liq- uides magmatiques. Bull. Soc. Fr. Min6ral. Cristal- logr., 102: 402-409.

Villemant, B., Jaffrezic, H., Joron, J.L. and Treuil, M., 1979. Distribution coefficients of major and trace ele- ments: fractional crystallization in the alkali basalt se- ries of Chaine des Puys (Massif Central, France). Geochim. Cosmochim. Acta, 45:1997-2016.

Walgenwitz, F., 1976. l~tude p6trologique des roches in- trusives triasiques, des 6cailles du socle profond et des gites de chlorite de la r6gion d'Elizondo (Navarre es- pagnole). Thesis (36me cycle), Besangon, University of Besangon, 172 pp.

Weaver, B.U and Tarney, J., 1983. Empirical approach to estimating the composition of the continental crust. Nature (London), 310: 575-577.

White, W.M. and Schilling, J.G., 1978. The nature and origin of geochemical variation in Mid-Atlantic ridge basalts from the Central North Atlantic. Geochim. Cosmochim. Acta, 42:1501-1516.

Wood, D.A., Joron, J.L. and Treuil, M., 1979a. A reap- praisal of the use of trace elements to classify and dis- criminate between magma series erupted in different tectonic settings. Earth Planet. Sci. Lett., 45: 326-336.

Wood, D.A., Joron, J.L., Treuil, M., Norry, M. and Tar- ney, J., 1979b. Elemental and Sr isotope variations in basic lavas from Iceland and the surrounding oceanic floor: the nature of mantle source heterogeneity. Con- trib. Mineral. Petrol., 70: 319-339.