1bNd isotope geochemistry of the alkaline magmatism from the Cretaceous North Pyrenean Rift Zone (France-Spain)

Chemical Geology, 97 ( 1992 ) 3 3 - 4 6 Elsevier Science Publishers B.V., A m s t e r d a m

33

[31

REE and Sr-Nd isotope geochemistry of the alkaline magmatism from the Cretaceous North Pyrenean Rift Zone (France-Spain) Michel Rossy ~, Bernard A z a m b r e b and Francis Albarbde c aLaboratoire de POtrologie, Universitk de Franche-Comtk, Route de Gray, F-25030 Besanfon Cedex, France bDkpartement de Pktrologie, Universitk Pierre et Marie Curie, 4 place Jussieu, F- 75252 Paris Cedex 05, France cCentre de Recherches POtrographiques et Gkochimiques-Ecole Nationale Sup&rieurede GOologie, F-54501 Vandoeuvre-lbs-Nancy Cedex, France (Received January 8, 1991 ; revised and accepted September 18, 199 l )

ABSTRACT Rossy, M., Azambre, B. and Albarbde, F., 1992. REE and Sr-Nd isotope geochemistry of the alkaline magmatism from the Cretaceous North Pyrenean Rift Zone (France-Spain). Chem. Geol., 97: 33-46. Cretaceous alkaline magmatism extends E-W within the north-Pyrenean Rift Zone from the Mediterranean Sea to the Spanish coast of the Bay of Biscay and consists of both effusive and intrusive rocks displaying a wide petrographic diversity: basalts, trachytes, lamprophyres, gabbros, teschenites, ultramafic cumulates and nepheline syenites. Sixty samples representing the main rock-types were analyzed for major and trace elements including the REE. From these, thirteen samples, mainly from mafic rocks, were selected for determination of Sr and Nd isotopic compositions. 87Sr/86Sr ratios range from 0.703 to 0.7064 and tn3Nd/144Nd ratios from 0.5127 to 0.5129. eya(t)-values of Pyrenean samples ( + 2 to + 6) are those commonly found for continental volcanism, particularly from extension and rift settings. High 87Sr/86Sr ratios in some samples coupled with high 6~80-values suggest they suffered contamination by a Sr-rich, Nd-free fluid such as seawater. Regularities of REE distribution in basalts and associated teschenites substantiate that they formed by variable degrees of melting from LREE-enriched mantle sources which had similar (La/Yb)N close to 5 and probably werc situated beneath the lithosphere. LREE enrichment took place 600-1000 Ma ago and is thus related neither to Mesozoic events in the Pyr6n6es nor to the breakup of Pangea. Finally, the isotopic data are used to discuss the genesis of the continental basalts which is interpreted in term of a three-component mixing: the absence ofbasalts with Nd as radiogenic as in MORB and the scarcity of mixed enriched types are shown to reflect unique S r / N d systematics among the components present in the mantle source of continental basalts.

1. Introduction Tholeiitic and alkaline magmas are commonly found together both in continental rifts and in intra-plate oceanic islands, although alkaline or even peralkaline magmas are also known outside of rift structures. The genesis of continental alkaline series therefore poses several important general questions: (a) Are these series typical features of particular geodynamic events like mantle hot spots or rift formation?

(b) Is their source similar to that of oceanic island basalts (OIB)? (c) In a given area, are tholeiitic and alkaline series genetically related? Mesozoic magmatic activity around the North Atlantic Ocean generally consists of both tholeiitic and alkaline episodes, the TriassicLiassic and Cretaceous, respectively. In the Pyr6n6es, the Mesozoic magmatism is represented on one hand by Triassic-Liassic tholeiitic dolerites (Montigny et al., 1982; Azambre et al., 1987), on the other by Middle Creta-

0 0 0 9 - 2 5 4 1 / 9 2 / $ 0 5 . 0 0 © 1992 Elsevier Science Publishers B.V. All rights reserved.

34

M. ROSSY ET A L ,

ceous alkaline rocks (Montigny et al., 1986). Alkaline magmatism of similar age is known on the west European margin, particularly in Portugal (Rock, 1982), and on the east American margin (see McHone and Butler, 1984, for a review) in both continental (White Mountains Magmas Series and New England-Qu6bec Province) and oceanic environments (New England and Newfoundland Seamounts). One of the notable features of many alkaline provinces is their linearity (especially in the Pyr6n6es, the Monteregian Hills of Qu6bec and the New England Seamounts). On the American margin, linear features have been interpreted as hot spot traces (Crough, 1981; Duncan, 1984; Foland et al., 1986) or old lineaments reactivated during the regional extension related to the opening of the North Atlantic Ocean (McHone and Butler, 1984; B6dard, 1985). In the Pyr6n6es, Cretaceous magmatic rocks are located in a narrow, eastwest-trending zone, the North Pyrenean Rift Zone (Fig. 1 ) which separates a Paleozoic do-

main to the south (Axial Zone) from the Aquitaine Basin to the north. The North Pyrenean Zone may correspond to a Cretaceous lithospheric scale transform fault zone between Iberian and European plates (Le Pichon et al., 1971; Choukroune, 1976; Mattauer, 1985). Crustal extension and thinning in this setting may account for the alkaline magmatism (this study), the tectonic emplacement of spinel lherzolite bodies (Vielzeuf and Kornprobst, 1984) and the short duration (Middle Cretaceous) of a high-T--low-P regional metamorphism (Ravier, 1959: Bernus-Maury, 1984; Golberg, 1987 ). This work is a geochemical study of effusive and intrusive alkaline and peralkaline rocks associated with the North Pyrenean Cretaceous continental rift. Trace-element and isotopic characteristics which have not been modified by postmagmatic alteration, particularly rare-earth element (REE) and Nd isotopic compositions, will be used to discuss the genesis of alkaline magmas from the Pyrdn6es and their potential relationships with the

'I N

0

50Km

'91SC'4 Y~,,o eTOULOUSE

;ANTANDER

/S

SAN

PAU BILBAO •

/

eTARBES

..~.................:.

~.. ~

RO0- D \

..:.:.....

" ' : ' : " '"- . .".' . ' .: . : : " " : " " ' " ". . . . : ' " . ' . '.". . . "" -". .". . : "" . . : .•' : . . . ' . .'."

e ZARAGOZA

. : ....

] •

IIN i

LERIOA I

Fig. 1. Sketch m a p o f the Pyr6n6es showing the location of analyzed samples within the E - W - t r e n d i n g N o r t h Pyrenean Zone. Sample numbers refer to Tables 1-3. Dotted area represents the Paleozoic basement.

ALKALINE MAGMATISM FROM THE CRETACEOUS NORTH PYRENEAN RIFT ZONE

Triassic tholeiites. Aspects of petrography, mineralogy and major- and trace-element compositions relevant to fractionation processes will be found in Azambre et al. (1992). Finally, the relevance of the present results to the understanding of the continental basalt genesis will be discussed.

2. Geological setting Cretaceous magmatic rocks from the Pyr6n6es are known from numerous exposures and several boreholes within the North Pyrenean Rift Zone, from the Mediterranean Sea in the east to the Spanish coast of the Bay of Biscay (Bilbao area) to the west. Their emplacement is well dated by both field relationships (Late Albian to Turonian) and K-Ar ages (85-113 Ma according to Montigny et al., 1986). Two geographical provinces of very different size have been distinguished, the Corbi6res to the extreme east and the Central and Western Pyr6n6es. In the Corbibres province, between Narbonne and Perpignan, the magmatism consists of an exclusively intrusive association: stocks of nepheline syenite smaller than 0.5 km 2, and sills and dykes of monchiquites from 10 cm to several metres wide, intrude Mesozoic sediments of the "Nappe des Corbibres" (Lacroix, 1920; Barrab6, 1952; Azambre, 1967). In the Central and Western Pyr6n6es, alkaline rocks cluster mainly in three zones; St Gaudens, Bagn6res-Oloron and the Spanish Basque Country. Around St Gaudens, holes drilled during petroleum exploration intersected up to 1000 m of volcaniclastic rocks and subordinated lava flows and intrusions interbedded with Cretaceous sediments (Dubois and Seguin, 1978 ). The volcanic rocks consist of alkali basalts and some intermediate lavas. Intrusive rocks are represented by a few dykes and/or sills of alkali dolerites and lamprophyres. In the Bagn6res-Lourdes-Oloron area, subaqueous basaltic lava flows and many intrusives are exposed. Intrusions consist essen-

35

tially of alkali dolerite sills (up to several tens of metres thick), in places strongly fractionated as picrites, teschenites, analcite syenites (Lacroix, 1920; Viennot, 1927; Azambre, 1967 ). Southeast of Bilbao in the Spanish Basque Country, the Biscay syncline contains a thick subaqueous sequence (in places > 1000 m) of differentiated basalts and trachytes and numerous sills of alkali dolerites (teschenites s.1.) (San Miguel de la Camara, 1952; Rossy, 1988). Teschenitic sills, in close association with the lavas, are generally intrusive into sedimentary rocks located below the volcanic pile, but may crosscut the volcanics. In the Corbi~res and some areas in the Western Pyr6n6es, intrusive rocks retained their magmatic mineralogy while in other areas (i.e. Bigorre), they suffered retromorphic alteration with or without preservation of relict magmatic phases. In most volcanics (particularly in the Western Pyr6n6es and Spanish Basque Country) low-temperature alteration is extensive and the only relics are clinopyroxenes and occasional amphiboles.

3. Results Detailed description of main rock-types (lamprophyres from the Corbibres, basalts from the Central Pyr6n6es, teschenites, dolerites and alkali basalts from the French and Spanish Western Pyr6n6es), mineral chemistry and exhaustive major- and trace-element data (sixty samples) can be found in Azambre et al. (1992) and only a brief summary is presented here. In spite of the large variability of mineral assemblages, mafic rocks have fairly similar major-element compositions through the whole province. The basaltic rocks of the Pyr6n6es and associated teschenites are typically alkaline as shown by: (1) their mineral chemistry, particularly their clinopyroxene trends from Cr-diopside to AI- and Ti-rich salires; (2) their high TiO2 contents; and ( 3 ) their REE patterns displaying the high (La/Yb)N values ( 10-20 ) and the upward concavity typ-

36

M. ROSSYET AL.

TABLE 1 Location (from west to east, see Fig. 1 ) and brief petrographic description of samples analysed for Sr and Nd isotopes Sample

Location

Rock-type

Description

B3168

SBC*

basalt

MB3149

SBC*

basalt

B2100

SBC*

basalt

4049

SBC*

teschenite

MIX-691

Western teschenite Pyren6es

MIE-691

Oloron

basalt

HAB-69

Oloron

picrite

BLU-A

Oloron

teschenite

SA-1

St Gaudens

basalt

A 101B

St Gaudens

lamprophyre

ROQ-D

Corbi~res

monchiquite

FEU-A

Corbibres

monchiquite

FI-A

Corbi+res

nephelinesyenite

pillow flow; rare microphenocrysts of chloritized olivine and Cr-diopside set in a groundmass displaying an intersertal texture with laths of plagioclase and clinopyroxene; interstices filled with chlorite, opaque minerals, sphene and some carbonates 1-m-thick dyke; porphyritic lava with phenocrysts of olivine (now chloritized ) and zoned clinopyroxene; microlitic groundmass with zoned plagioclase laths, clinopyroxene with kaersutite overgrowths, Ti-magnetite and some chlorite patches massive flow or sill ( > 10 m thick) with pillowed bottom; porphyritic lava with phenocrysts of olivine (rare and chloritized), clinopyroxene, plagioclase and opaque minerals; fine-grained groudmass with doleritic to intersertal texture made of plagioclase laths with minute inclusions of pumpellyite, clinopyroxene, sphene, chlorite and some carbonate sill; fine-grained texture with olivine (sparse and replaced by a chlorite-carbonate mixture), clinopyroxene, Fe-Ti-oxides, kaersutite, plagioclase, interstitial analcite and some secondary carbonate dyke; fine-grained intersertal texture; phenocrysts of olivine (replaced by chlorite, serpentine or carbonate), plagioclase, clinopyroxene mantled by amphibole, FeTi-oxides. apatite and some biotite pillow flow; intersertal texture with albitized plagioclase laths, quench clinopyroxene, chloritized glass, carbonate and some pumpellyite sill; poikilitic texture with cumulus olivine and plagioclase both enclosed in clinopyroxene crystals; intercumulus amphibole, phlogopite and plagioclase dyke; doleritic to intersetal texture with olivine phenocrysts (now altered to carbonate ), clinopyroxene, amphibole, zoned plagioclase, sparse analcite and biotite borehole sample (2658-m depth ); porphyritic lava with phenocrysts of olivine and zoned clinopyroxene, microphenocrysts of plagioclase and Fe-Ti-oxides set in a microcrystalline mesostasis with plagioclase microlites borehole sample (4003-m depth ); phenocrysts of olivine (rare and altered to carbonate), clinopyroxene, Fe-Ti-oxides, amphibole and biotite; microgranular groundmass made of clinopyroxene, abundant amphibole, apatite and rare alkali feldspar, plagioclase and analcite dyke; phenocrysts of olivine, clinopyroxene, amphibole and microcrysts of clinopyroxene, biotite, amphibole; glassy groundmass with mm-sized ocelli filled with analcite and calcite; this dyke contains cm-sized xenoliths of spinel lherzolites dyke; phenocrysts of olivine and clinopyroxene and micropbenocrysts of clinopyroxene and amphibole; analcitic and glassy groundmass containing ocelli filled with calcite, glass or analcite stock; medium-grained intersertal texture with perthite, nepheline, analcite, aegyrine. hastingsite and biotite; accessories: zircon, wohlerite, allanite, garnet

*SBC = Spanish Basque Country.

ical of alkaline rocks. Monchiquites from the Corbi6res, which moreover include lherzolite xenoliths, present the same characteristics with even higher (La/Yb)N-Values. Thirteen samples, selected as typical of the regional magmatism, were analysed for Sr-Nd

isotopes. They include five basalts, three lamprophyres, three teschenites, one picrite and one nepheline syenite. Locations and petrographic descriptions are summarized in Table 1. REE, Th and U (Table 2) were measured by

24.8 54.7 69.6 27.2 47.4 29.4 30.3 6.37 42.6 76.7 80.4 76.9 72.0

55.5 106 146 52.3 94.5 56.0 58.8 12.1 88.3 134 143 132 121

26.03 44.38 63.64 24.31 38.99 23.11 22.62 5.58 45.34 65.76 56.83 51.42 21.88

5.17 8.11 10.84 4.70 7.26 4.24 4.48 1.30 8.25 10.50 9.35 8.56 2.66

1.71 2.82 4.4 1.86 2.57 1.47 1.75 0.54 2.64 2.93 3.02 2.76 1.01

4.78 6.66 9.7 4.8 6.53 3.62 4.76 1.53 6.68 7.83 7.88 7.22 3.2

Gd

3.92 5.57 6.83 4.05 4.78 3.2 4.11 1.41 4.26 4.82 5.56 5.06 2.87

Dy

1.92 2.34 2.88 2.02 2.04 1.55 1.99 0.75 1.81 2.07 2.6 2.3 2.1

Er

mg-number [ = M g / ( M g + 0.9Fetot) ] from major-element data in Azambre et al. ( 1992 ) (see text ).

0.673 0.634 0.600 0.647 0.693 0.637 0.608 0.804 0.622 0.525 0.662 0.630 0.097

Eu

1.72 2.2 2.33 1.9 1.67 1.58 1.85 0.71 1.36 1.76 2.26 1.99 2.79

Yb

0.27 0.27 0.35 0.27 0.26 0.23 0.28 0.11 0.17 0.26 0.32 0.25 0.43

Lu

22.0 30.7 35 23.5 25.7 20.6 23.9 8.8 21.8 26.4 30.3 28.2 22.1

Y

U

Th

Rb (ID)

Sr (ID) 1.23 2.38 1.36 0.91 4.94 0.99 0.14 0.02 1.88 39.8 3.78 2.9 11.1

3.55 7.12 5.59 4.74 7.92 4.15 1.97 0.59 5.66 17.6 11.8 9.9 39.4

8.77 49.9 29.7 70.2 23.7 36.5 10.5 28.9 4.99 81.0 30.4 33.1 214

382 887 1,359 1,522 1,255 548 1,029 272 946 1,324 1,472 1,345 615

t~

©

N

>. Z

t'~

.<

basalt basalt basalt basalt teschenite teschenite teschenite picrite basalt lamprophyre monchiquite monchiquite ne-syenite

Sm (ID)

B3168 MB3149 B2100 MIE-6 91 4049 MIX-691 BLU-A HAB-69 SA- 1 AIOIB ROQ-D FEU-A FI-A

Nd (ID)

Z ©

Ce

Rock-type

Sample

mgLa number

©

>

Trace-element concentrations (ppm) determined by ICP emission spectroscopy except for those labelled ID (isotope dilution)

TABLE 2

m ¢3 7~

,q

© ~r

,..]

K

~r >

38

M. ROSSYET AL

TABLE 3 Isotope composition data Sample

8VSr/E6Sr*2a

87Sr/86Sr ( t = 9 0 Ma)

143Nd/144Nd'2o

~NU(90 Ma )

B3168 MB3149 B2100 MIE-691 4049 MIX-691 BLU-A HAB-69 SA-1 AIOIB ROQ-D FEU-A FI-A

0.705423 ±0.000032 0.703763±0.000031 0.704225±0.000036 0.706238 ±0.000035 0.704501 ±0.000033 0.706386 ± 0.000034 0.705648 ± 0.000039 0.705353 ± 0.000032 0.703272 ±0.000033 0.703873 ±0.000035 0.706054 ±0.000034 0.704315 ±0.000030 0.704681 ±0.000034

0.705338 0.703555 0.704144 0.706067 0.704431 0.706139 0.705610 0.704960 0.703252 0.703646 0.705977 0.704224 0.703394

0.512719 ±0.000023 0.512858 ±0.000020 0.512785± 0.000024 0.512769 ± 0.000031 0.512812 ±0.000033 0.512739 ± 0.000032 0.512905 ± 0.000021 0.512748 ± 0.000021 0.512763 ±0.000017 0.512712±0.000015 0.512796±0.000027 0.512771 +0.000022 0.512711 ± 0.000023

2.45 5.27 3.87 3.39 4.35 2.94 6.08 2.77 3.42 2.58 4.19 3.69 2.83

0.5134

n

i

MORB

i

•

0.5132 I L

~

Basalts

i

•

Monchiquites

© Teschenites [] Ne-Syenite

~ l ~ t ~ Geronim°

..RioGrande ssif Central

~"

zo5128 0.5126 0.5124 --

"~ / A ~ / . , / ~)~\ seawater Kenya~k• ~ ;~x " ~ ~z / / ~~////~o","lm~ ///.'~_ contamination ~I - - °,~ (

~ - ~ , O s l o //////~Rift U///

, __ , 0.703 0.704

0.765

0.706

0.767

0.708

87 Sr / 86 Sr Fig. 2. 143Nd/144Nd vs. 87Sr/86Sr diagram. M O R B (midocean ridge basalts) a n d OIB (oceanic island basalts) fields from Zindler a n d H a r t ( 1 9 8 6 ) ; see text for other references. The horizontal arrow emphasizes the shift in isotope c o m p o s i t i o n s due to interaction with seawater.

inductively coupled plasma (ICP) emission spectroscopy at the Centre de Recherches P& trographiques et G6ochimiques with a precision of ~ + 10% and are reported in Azambre et al. (1992). Nepheline syenites have scattered La/Th ratios (0.8-4.3). The remaining samples, except a few with low Th concentrations, for which the data are presumably much less precise, have relatively constant La/Th ratios (5-10). For the Sr-Nd isotope determinations (Ta-

ble 3), analytical techniques, blanks and standard data have been previously reported by A1±bert et al. (1983). Leaching experiments aimed at identifying potential contamination upon emplacement were carried out in cold 2.5 N HC1. The scatter of the 87Sr/86Sr ratios (0.703-0.7064) exceeds that expected for the 143Nd/144Nd ratio (0.5127-0.5129) if Pyr6nean magmas followed the common trends of mantle-derived volcanics. In the Sr-Nd isotopic diagram (Fig. 2), representative analyses from the Pyr6n6es delineate a horizontal field which is highly oblique to the mantle array (oceanic ridges and islands). This feature occurs in other continental volcanic series (W6rner et al., 1986; Zindler and Hart, 1986). In the Pyr6n6es, the range of eN~(t)-values ( + 2 to + 6) is relatively small and corresponds to the values commonly found for continental volcanism, particularly from extension and rift settings. The variation of eyd(t) ( - 2 to + 7) observed in alkaline basaltic suites with low or moderate K content from East Africa (Norry et al., 1980; Vollmer and Norry, 1983), the French Massif Central (Chauvel and Jahn, 1984; Downes, 1984), western Germany (AI±bert et al., 1983; W6rner et al., 1986), Colorado Plateau and Rio Grande Rift, U.S.A. (A1±bert et al., 1986; Perry et al., 1987) and Oslo

39

ALKALINE MAGMATISM FROM THE CRETACEOUS NORTH PYRENEAN RIFT ZONE

Rift, Norway ( N e u m a n n et al., 1988) remains distinctly lower than values of depleted mantle from mid-oceanic ridges ( + 9 to + 11 ).

07065 I

4. Discussion

0 7056 [

I

~---T

87 S r /

~"

"~

66Sr

"T

-~

•m ROQ-D

0.7060

MIE 691

0

87Sr/86Srratios

do not seem to be correlated with other chemical (Na, K, Rb, etc.) parameters characteristic of water-rock interaction. Monchiquite ROQ-D from the Corbi6res is a hypabyssal rock with an extremely fresh mineralogy and a very high Sr content, but its 87Sr/ 86Sr is relatively high (0.706) whereas the altered basalt 3149 has Sr isotopic composition close to mantle values (0.7035). Acid leaching of ROQ-D failed to reveal the presence of a crustal labile component whereas its &'80 (M. Javoy, pers. commun., 1990) reaches the high value of + 11.1%o. Partial replacement o f m e sostasis, which forms ~ 50% of the rock, by analcite indicates interaction with low-temperature fluids. In the same area, monchiquite FEU-A has similar characteristics [emplacement, mineralogy, occurrence of ultramafic xenoliths, major- and trace-element contents, a high j~80 of + 10.7°/o0 (M. Javoy, pers. commun., 1990) and positive ~Nd(t)_value ( + 3 . 7 ) ] but its 875r/86Sr ratio is distinctly lower (0.7042). Although the possibility that Sr isotopic heterogeneity reflects that of the parent mantle cannot be absolutely excluded, it seems more likely that Sr isotope variation reflects contamination by a Sr-rich, Nd-free fluid with high 87Sr/S6Sr values such as seawater, or water which had percolated through surrounding carbonated sediments. The vertical array of mafic samples in a 87Sr/86Sr vs. mg number plot (Fig. 3) is indeed consistent with their significant contamination by a Mgpoor component which may be assigned to supracrustal rocks. For the rest of the discussion, only the samples with the lowest 87Sr/86Sr ratios ( < 0.705 ), chosen as an arbitrary cut-off, will be taken as representative of the unaltered magma. The Nd isotopic compositions of alkaline

]

©

MIXe81

BLUA •

• Basalts

i t

O Teschenites • Monchiquiles

0 7050

B3T68

HAB 59

[] Ne-Syenite 0.7045

O 4049

0.7040

•

•B2100 FEU.A

O AIOf B

0.7035-

• B3149 [~ FI-A

•

07030 i 0.00

I

'

J.

0.10

0.20

030

__~

0.40

~:

_.

050

060

SA I

mg [

070

_ t

0 80

0 90

Fig. 3. 87Sr/86Srvs. mg-number [Mg/(Mg+0.9 Fetot)] for the Pyrenean samples. Isotopic heterogeneityrequires more than one source of Sr. The vertical array suggests that one component is Mg-poor and has radiogenic Sr, which are features typical of the upper continental crust. rocks from the Pyr6n6es, as well as of other alkaline series cited above, are close to those of OIB and distinct from mid-ocean ridge basalts (MORB) (Fig. 2 ). The similarity between the Nd and Sr isotopic properties of OIB and continental alkali basalts (CAB) is generally taken as an evidence that their c o m m o n source is situated below the lithosphere (All6gre et al., 1981 ). The Nd isotope composition of ultramafic rocks from Lherz, Caussou and Freychin6de (Downes et al., 1991 ) is significantly more radiogenic than that of alkaline rocks from the Pyr6n6es: a genetic link between these ultramafic rocks, which can be taken as samples of the local lithosphere, and the alkaline rocks is therefore unlikely (Fig. 4). CAB and OIB also share a relatively high Ti content ( 3% TiO2 for mg number ~ 0.60) which make them distinct from continental tholeiites and MORB. In the more mafic samples, absence of plagioclase at the liquidus (Azambre et al., 1992) makes A1 behave as an incompatible element. The relatively homogeneous Nd isotopic compositions and nearly constant L a / T h ratio

M. ROSY

40

n amphibole peridotites (Caussou)

l spine1 lherzolites (Lherz) 1 0 homblendites (Lherz)

i

m Z

:.

I

Pirenean Alkaline Magmas

0.5126

0.5124

.’ :~~,,:~~_..:::.: ,;: .,

-“i .I‘.. .‘% ,.,, >..;;,, ‘, ..,.,’ i,. .,‘.:..;‘:.:.,, _;.. ,;,_:‘) .,,’....“i.. : :,, _:,::_ ;;,;.; ...,,,_.y..

0.5126

c:

j

~

_I 0.703

1

_A 0.704

0.705

~-_0.706

0.707

0.708

87Sr /86Sr Fig. 4. Sr and Nd isotope compositions of alkaline rocks and uhramafic rocks from the Pyretrees. Peridotite data from Downes et al. ( 199 1). Caussou amphibole-lherzolites may represent depleted spine1 peridotite metasomatized by alkaline magmas (Fabrib et al., 1989). Conversely, Lherz homblendites (lherzites) may represent small batches of these alkaline magmas which interacted with the surrounding Iherzolite.

along the whole belt, as well as, to a lesser extent, the Ba/Th ratio ( 100) (Azambre et al., 1992) suggest that the Pyrenean Cretaceous malic and differentiated magmas were extracted from sources displaying little differences in their geochemical characteristics. As shown by Azambre et al. ( 1992), the REE contents of basalts and teschenites from the Western PyrCnCes (Spanish Basque Country) form a coherent set of data, while trachytes and feldspathoidal syenites may result from the fractionation of alkali basalts and lamprophyres. Lamprophyres from the Corbieres have also higher La contents than basalts but exhibit larger light REE (LREE) enrichments (Fig. 5 ). From the Nd isotopic data and Sm/Nd ratios, it can be deduced that the source have suffered the superimposition of: ( 1) an old fractionation event ( > 2 Ga) which led to relative depletion in LREE and Rb; and (2) a recent fractionation in the opposite sense. The latter event is particularly evident in the inferred source of basalts in the Spanish Basque Country. With the exception of five samples of clearly differentiated or cumulative character

ET AL.

( 703, 1166, 1183, 3053, 3059) these basalts and associated teschenites define a good alignment in the (La/Yb ), vs. La, plot with an intercept distinct from zero (Fig. 5 ), which suggests that they formed by variable degrees of melting from mantle sources with similar (La/ Yb)N. The small number of immobile trace elements does not permit the degree of melting and source mineralogy to be found by inversion. However, if the melting is neither cotectic nor peritectic (Albarede, 1983 ), then it is possible to use the conventional notation of Shaw ( 1970) and Treuil and Joron ( 1975) for partial melting with the assumption that La behaves as an almost perfectly incompatible element. In chondrite-normalized concentrations, it can be written:

where the subscript 0 refers to the source values; P yb is the distribution coefficient for the virtual mineral composition of the melt: and Dyb the bulk solid/liquid distribution coefficient for a degree of melting close to zero. Since Pyb is necessarily positive, the intercept is less than (La/Yb)O. The value of the intercept, calculated by linear regression, has been found to be 5.32 1.5 (at the 95% confidence level). As (La/Yb), is less than the minimum (La/Yb), ratio of the basalts (lo), its value in the mantle source is necessarily in the range of N 5- 10 times the chondritic ratio, the lower limit being the most probable. Strictly speaking, this conclusion is only valid for a melting in which the proportions of melted minerals do not change with the degree of melting (Shaw, 1970; Albarede, 1983) and such conditions probably are never strictly fulfilled in basaltic systems (e.g., Jaques and Green, 1980; Morse, 1982 ). One of the conditions for the hypothesis of an enriched source to be erroneous would be a large variation of 1 - PYb related to garnet rapidly melting out. However, in such a case, major-element variations would be larger than

ALKALINE

MAGMATISM

FROM

THE

CRETACEOUS

NORTH

PYRENEAN

RIFT

ZONE

41

30

A101B FEU-A •

25

.oo-~

SA4.0" ÷ . ~ 20

I [ /

15

10

~

B2100 ~3

,~'1~B3149 • lID

FI-A

4049

• Q

t/ ~ . , / M I E•- 6 9 1

0

~

-:

B3168

I

~HAB-69

"~ ~ • •

• Monchiquites

Basalts

[~ Ne-Syenite

Q Teschenites O

I

0

_ I

1 O0

i

i

t

200

i

300

i

J

400

i

(La)N L

500

i

600

Fig. 5. (La/Yb)N vs. LaN plot for Pyrenean alkaline magmas. Large symbols represent the samples analyzed in this work for isotope compositions, minor symbols are from Azambre et al. (1992). Barred symbols represent basalts from the St Gaudens boreholes. The regression line has been calculated from all the basalts but the St Gaudens samples.

is actually observed (Azambre et al., 1992). The succession of events which have affected REE could thus be reduced to the following sequence: (1) A long-term evolution in a LREE-depleted environment, presumably the normal sub-lithospheric mantle of the Pyrenean region. (2) Recent preferential extraction of LREE by a basaltic liquid or a metasomatic fluid. The trace-element imprint of the liquid is subsequently found in the mantle, fertile although REE-poor, through which this liquid rises. We identify this surrounding mantle with the Pyrenean sub-lithospheric mantle. The boundary layers of the convecting mantle, inclusive of any detached part of the continental lithosphere (McKenzie and O'Nions, 1983), are places where isotopic heterogeneities may survive mantle convection. (3) Melting of this enriched peridotite during the Cretaceous to produce the alkaline Pyrenean magmas. Due to unusual regularities in La and Yb distributions, this sequence may be distin-

guished from a one-stage model where a LREEdepleted source undergoes very low degrees of melting (Alibert et al., 1983). The apparent age of the last enrichment event [stage (2) ] may be roughly estimated from the model age relative to the ordinary depleted mantle, i.e. from the hypothetical time the source of the Pyrenean alkali basalts and the depleted mantle had identical 143Nd/laaNd ratios. A lower bound of 0.12 for the 1475m/ laaNd ratio in the source is the value ofbasalts and corresponds to the extreme case where melting would not produce S m / N d fractionation, whereas an upper bound of 0.15 may be estimated by interpolating the S m / N d ratios as a log-linear function of atomic number between La and Yb assuming a m i n i m u m (La/ Yb)N ratio of 5. The enrichment event is thereby constrained to have happened in the time interval 1000-600 Ma: if this event is unique, it is related neither to the breakup of the Pangea (Triassic to Cretaceous) nor to the emplacement of the Triassic continental tholeiites [which are in any case too mildly en-

42 riched in incompatible elements to represent a suitable metasomating agent (Alibert, 1985 ) ], nor with any Alpine geodynamic event. Amphibole is an ubiquitous mineral of the North Pyrenean peridotites and is found in lherzolite inclusions in the Corbibres monchiquites. It becomes locally abundant (5-10%) at Caussou (Conqu6r6, 1971 ), in hydrous pyroxenites cross-cutting the foliation, and in lherzites. Some of these amphiboles have been consistently dated at ~ 100 Ma, i.e. they are nearly contemporaneous with the Cretaceous alkaline magmatism (Verschure et al., 1967; A1bar6de and Vitrac-Michard, 1978; Goldberg et al., 1986). Amphibole crystallization in lithospheric peridotites beneath the Pyr6n6es therefore does not represent a precursor event of the alkaline magmatism. The opposite view, that these amphiboles could reflect a reaction between the Cretaceous alkaline magmas and the sub-Pyrenean lithosphere, has recently been advocated by Fabribs et al. (1989). This view is consistent with the findings of Downes et al. ( 1991 ) that amphibole-bearing peridotites from Caussou and lherzites have Nd isotope compositions intermediate between that of alkaline rocks and that of the depleted spinel lherzolites of Lherz (Fig. 4 ). Small volumes of alkaline magmas may have interacted with the surrounding depleted lithosphere in much the same way as small batches of alkaline basalts react with the oceanic lithosphere under the Hawaiian islands (Chen and Frey, 1983 ). It is likely that the sequence of discrete events suggested above is an oversimplification. A model in which the enrichment is more or less a continuous process is also acceptable: the 600-1000-Ma range would therefore correspond to a time constant impossible to relate to a unique enrichment event. Whatever metasomatic m e d i u m - - magma or fluid - - is considered, both the single episode and the continuous enrichment models imply that the size of the geochemical heterogeneities must be significantly smaller than the volume of locally melted mantle source. Moreover, whether this

M. ROSSY ET AL.

enrichment of the asthenospheric mantle is due to subduction zone processes (dehydration of the slab, migration of magmas) or to intra-plate volcanism cannot be deduced from the present data. If the isotopic properties of the mantle source result from the combined signatures of the depleted asthenosphere and enriched components, Sr and Nd isotope compositions of continental basalts would be expected to span the whole range between the two end-members. This point has been addressed by WOrner et al. (1986) and Zindler and Hart (1986) who point out that no continental basalt is known with eNd(t) in excess of +7, whereas the MORB source has values of + 8 to + 11. They also observe that, in the Nd-Sr isotopic plot, continental basalt series define trends consistently converging on a "node" associated with an isotopic composition that they consider to represent a true mantle component (PREMA, for prevalent mantle). Trends of continental basalts fanning out from a point plotting below the composition of the depleted mantle endmember may be explained by assuming that Nd and Sr in these basalts represent a mixture of three components: (1) a depleted-mantle component labelled "DM"; (2) an "enriched" component with low 87Sr/g6sr and 143Nd/J44Nd ratios corresponding to the Ti metasomatism of Menzies ( 1983 ), or to the EM I component of Zindler and Hart ( 1986); (3) an "enriched" component with high 87Sr/86Sr and low 143Nd/144Nd ratios reminiscent of the continental crust signature (Hawkesworth and Vollmer, 1979), and corresponding to the potassic metasomatism of Menzies ( 1983 ) or to the E M I I component of Zindler and Hart ( 1986 ). The specific isotopic values of these components are largely immaterial to the rest of the discussion. Such a breakdown of Sr and Nd isotopic compositions into three end-members ob-

ALKALINEMAGMATISMFROMTHE CRETACEOUSNORTH PYRENEANRIFTZONE

viously does not apply to those Pyrenean samples which have interacted with a Sr-rich fluid during magma emplacement. Absence of strong relationships between the isotopic and geochemicalproperties, particularly the major elements, shows that magma mixing is not the process responsible for the mixture properties. All components must therefore coexist in the source of magmas. Again, to account for the regional isotopic homogeneity of the Pyrenean alkaline magmas, heterogeneities must occur on a scale smaller than the volume of mantle melted by each magmatic event. For a given range of S r / N d ratios, the mixing hyperbolae D M - E M I and D M - E M II intersect near the node where all the continental magma series appear to converge and which behaves as a fictitious end-member. Endmembers could be chosen with quite different isotopic properties, in particular for EM I and EM II, but mixing would nonetheless result in the same pattern. Without further geochemical evidence, it may prove difficult to show that a suggested component (e.g., PREMA) is not a node generated by a more distant end-member and a particular distribution of S r / N d ratios. Fig. 6 shows a possible hyperbolic mixing triangle with ( S r / N d ) DM= 0.25, ( S r / N d ) EM1= 2, and (Sr/Nd)EM H=0.1 and a node at STSr/ 86Sr=0.7035 and 143Nd/144Nd=0.5129. Because the hyperbolae have strong curvatures of opposite sign, the probability for a point to fall close to this apparent end-member is stronger than anywhere else. The rate of variation of an isotopic ratio Rm in the mixture per increment of mass fraction between two end-members labelled 1 ( D M ) and 2 (EM I or E M I I ) varies inversely with the square off2, the mass fraction added to DM, according to: dR.,/(R2 -R~ ) dA

C~/C2 2

where C is the concentration of either Sr or Nd; and R its isotopic composition. Similar formulae hold for f3 which results in contour lines

43

0.5134

1

i

- -

i

~

-

-

-

-

]

r

MORB 0.5132 "O Z

"0 Z

0.5130

~Geronimo

0.5128

0.5126

~,~ c~on ~ ' . . . / 0.5124 _ _

0.51:

~ ~

~-~

0.703 0.704 0.705 0.706 0.707

DM (0.5, 5)

-0 Z "O

EM-II (o. ~, 8)

0.51;

Z

0.511

EM-I (1, 0.2)

0.702

0.704

0.706

0.708

0.710

87 Sr / 86 Sr Fig. 6. Mixing ternary diagrams for three components with the following parameters: DM (depleted mantle): 87Sr/ 86Sr=0.7023, 143Nd/144Nd=0.51325, Sr=0.5, N d = 5 ; EM-I (enriched mantle I): 87Sr/S6Sr=0.7032, 143Nd/ ~44Nd=0.5110, S t = l , Nd=0.2; and EM-H (enriched mantle II): 87Sr/86Sr=0.720, 143Nd/~44Nd=0.5124, Sr=0.1, N d = 8. Only relative concentrations are important in defining mixing relationships. The upper diagram (contour lines at 10% weight fraction of each end-member) is an enlarged area of the lower diagram (20%). Samples plotting between EM-I and EM-H are rare, which is consistent with the existence of a nodal point in the mixing ternary diagram.

closely spaced about the node in Fig. 6. It is therefore expected that more points gather in the vicinity of the node than in other parts of the mixing triangle. It has been repeatedly emphasized that, away from the depleted side, continental basalt data split in two branches whereas isotopic compositions intermediate between EM I and E M I I

44

are rarely observed (Menzies, 1983; Zindler and Hart, 1986 ). The contrasting curvature of the D M - E M I and D M - E M II branches implies a systematic variation of the S r / N d ratio of the end-members such as: (Sr/Nd)EM iI < (Sr/Nd)DM < (Sr/Nd)EMt resulting in an apparent shift of the EM IE M I I mixing hyperbola towards the center of the mixing triangle. Hence the existence of a node in the isotopic data and the scarcity of magmas intermediate between EM I and EM II are probably reflecting the same Sr/Nd systematics among the end-members present in the source of continental basalts.

M. ROSSY ET AL.

The Service d'Analyse of the C.R.P.G. and M.G. Marchal are thanked for the data produced during this work. M. Javoy kindly provided oxygen isotope data of monchiquites. P. Dubois and J.P. S6guin (SNEAP) gave borehole samples from St Gaudens. Reviews by C. Chauvel, B. Dupr6, J. Fabribs and R. Varne were appreciated. New steps in understanding Pyrenean metasomatic fluids owe a great deal to the life-long efforts of Mrs. J. Mign6 from Jurangon. This work was supported by untagged funds from the Universit6 Pierre et Marie Curie, Universit6 de Franche-Comt6 and Centre de Recherches P6trographiques et G6ochimiques.

5. Conclusions The 878r/86Sr ratios of the North Pyrenean alkalic magmas are scattered as a result of interaction with seawater a n d / o r surrounding rocks, whereas the end (t)-values fall in a rather narrow interval of + 2 to +6, which makes them similar to ocean island basalts. The mantle source, similar to the source of ocean island basalts, is problably located beneath the lithosphere and has a (La/Yb)N ratio close to 5. The LREE enrichment took place 600-1000 Ma ago, probably as a result of multiple events. This enrichment is not related to the breakup of the Pangea, nor to the Triassic tholeiitic volcanism contemporaneous with the breakup. Alkaline continental magmas are interpreted in terms of a three-component mixing between the normal depleted mantle and two enriched mantle components. The S r / N d systematics of these three components may account for the convergence of the continental basalt series on a unique isotopic "'node", as well as, for the scarcity of lavas with compositions intermediate between the two enriched components.

Acknowledgements We are particularly indebted to Dani6le Dautel for her skillful analytical contribution.

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ALKALINE MAGMATISM FROM THE CRETACEOUS NORTH PYRENEAN RIFF ZONE

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45

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46

daires des Pyr6n6es. M6m. Soc. G6ol. Fr. No. 86, 250 pp. Rock, N.M.S., 1982. The Late Cretaceous igneous province in the Iberian Peninsula and its tectonic significance. Lithos, 15:111-131. Rossy, M., 1988. Contribution ~ l'6tude du magmatisme m6sozo'ique du domaine pyr6n6en, I. Le Trias dans l'ensemble du domaine; II. Le Cr6tac6 dans les Provinces Basques d'Espagne. Thesis, University of Besan9on, Besan~on (unpublished). San Miguel de la Camara, M., 1952. Las erupciones y las rocas volcanicas de las Vascongadas. Munibe, 2-3:115130. Shaw, D.M., 1970. Trace element fractionation during anatexis. Geochim. Cosmochim. Acta, 34: 237-243. Treuil, M. and Joron, J.L., 1975. Utilisation des 616ments hygromagmatophiles pour la simplification de la mod61isation quantitative des processus magmatiques - Exemples de l'Afar et de la dorsale m6dio-atlantique. Soc. ltal. Mineral. Petrol., 31: 125-174.

M. ROSSY ET AL.

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