pb and o isotope systematics in granulite facies xenoliths, french massif central: implications for...

342 Earth and Planetary Science Letters, 102 (1991) 342-357 Elsevier Science Publishers B.V., Amsterdam

[CH]

Pb and O isotope systematics in granulite facies xenoliths, French Massif Central: implications for crustal processes

H. Downes a, P.D. K e m p t o n b, D. Briot b,c, R.S. H a r m o n b and A.F. Ley re loup a

Department of Geology, Birkbeck College, Malet Street, London WC1E 7HX, UK h NERC Isotope Geoseiences Laboratory, Keyworth NG12 5GG, UK

c Laboratoire de G~ochimie, Universitk Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France d Laboratoire de P&rologie, G.P.F.A. UniuersitO des Sciences et Techniques de Languedoc, Place E Bataillon, 34060 Montpellier, France

Received May 11, 1990; revised version accepted September 13, 1990

ABSTRACT

Pb and O isotope data are represented for a suite of granulite facies xenoliths found within Tertiary alkaline volcanic rocks of the Massif Central, France. The suite consists of ultramafic and mafic cumulates, metagabbros which are considered to represent basic liquids, felsic meta-igneous lithologies (charnockites) and metasediments. Ranges of 8180 values are +6.9 to + 9.8%~ for mafic xenoliths, + 9.3 to + 10.2%o for felsic recta-igneous samples and + 6.1 to + 11.8%o for the metasediments. By comparison, d180 values for mantle peridotites from the same region range from + 5.1 to + 6.9%0, whilst local Hercynian granitoids vary from +8.6 to +12.0%o. The 2°6pb/2°4pb ratios of the granulite xenoliths are between 17.77 and 19.19, 2°7pb//z°4pb ratios vary from 15.51 to 15.69, and 2°8pb/Z°4pb ratios range from 38.07 to 40.07. In general, metasedimentary granulites have the more radiogenic Pb isotope compositions, whereas mafic meta-igneous samples are less radiogenic. These isotopic characteristics can be explained as the result of the interaction of mafic magmas with the metasedimentary crust into which they intruded. The release of heat also provoked melting of the more fusible parts of the lower crust and led to the formation of late-orogenic Hercynian granitoids. However, an additional component which provides less radiogenic Pb is also needed in the source of the granitoids; this may be the felsic meta-igneous xenoliths or midd le /uppe r crustal gneisses.

1. The lower crust

The lower continental crust is a major geochemical reservoir within the Earth's lithosphere, but it is poorly characterised in terms of its petrology, mineralogy and chemical composition because of its inaccessibility. Granulite facies xenoliths which are brought to the surface in alkaline volcanic rocks offer one method of sampling and analysing the deeper regions of the crust [1-4]. Such xenoliths represent an important source of information about the deep crust, including its mineralogy, major element, trace element and REE geochemistry, and isotopic character. A variety of processes are probably involved in the formation and modification of the lower crust, but major periods of basalt underplating and intracrustal melting are considered to be among the most important [5-7].

We present Pb and O isotope analyses for a

0012-821X/91/$03.50 © 1991 - Elsevier Science Publishers B.V.

well-characterised suite of lower crustal xenoliths from the French Massif Central. These samples have previously been studied using trace elements combined with Sr and Nd isotopes [7]. The mafic granulite xenoliths are considered to be a product of basalt underplating during Hercynian orogenic activity, with the older crustal material being represented by granulitic metasediments. During intrusion, partial melting of fusible portions of this crust yielded the abundant Hercynian granitoids of the region [8]. The new data presented here can be used to constrain the Pb and O isotopic ratios of the lower crust, and to test these hypotheses.

It has long been considered that the lower crust has a low /~ (238U/2°4pb) value as a result of depletion in U during granulite facies metamorphism because exposed granulitic terrains generally display low/~ values [9]. This would lead the lower crust to have unradiogenic Pb isotopic compositions, and thus be the major reservoir of unra-

Pb A N D O I S O T O P E S Y S T E M A T I C S IN G R A N U L I T E F A C I E S X E N O L I T H S , F R A N C E 343

diogenic Pb required to balance the radiogenic Pb isotopic character of the upper crust and upper mantle. We will show that, in common with several other xenolith suites [9], the Massif Central granulites are characterised by radiogenic Pb-isotope ratios.

The O isotopic nature of the lower crust is also ~8oorly constrained. It might be expected to have

O/160 ratios that vary systematically with lith- ology, but generally fall between mantle values (ca. 6%0) and upper crustal values (> ca. 8%0, which are higher because material formed at low temperatures at, or near, the Earth's surface are strongly enriched in 180. In the Massif Central granulite suite, mafic meta-igneous samples have higher 8180 values than those of spinel peridotite xenoliths from the same region, whereas 8180 values for metasedimentary granulites and Her- cynian granitoids also overlap extensively.

2. Granulitic xenoliths of the Massif Central

A suite of granulitic xenoliths found in the late Tertiary alkaline volcanoes of Bournac and Roche Pointue in the French Massif Central are well characterised in terms of their mineralogy, metamorphic history and nature of their protoliths [10-12] and a high-quality geochemical database (including REE, and Sr and Nd isotope ratios) for both meta-igneous and metasedimentary xenoliths

has been produced [7]. This work has added considerably to the worldwide database for lower crustal rocks because the Massif Central granulite suite includes numerous metasedimentary granulites, which are less common in other xenolith suites, in addition to abundant mafic granulites.

The mafic meta-igneous xenoliths of the Massif Central are considered to represent both magmatic liquids emplaced into the lower crust and cumulates from those liquids. The metasediments are mostly "shale-like" in bulk composition, but some immature sedimentary protoliths such as greywackes have also been identified [7]. The felsic meta-igneous xenoliths exhibit more composi- tional variety and may represent either anatectic material formed from the melting of the metasediments or ancient orthogneisses, but their origins are more ambiguous than the mafic granulites.

The age of the lower crustal xenoliths from the Massif Central is poorly constrained. The deposi- tional or stratigraphic age of the metasediments is probably. Late Proterozoic and some zircons separated from them yield poorly constrained Late Proterozoic ages. Both felsic meta-igneous and metasedimentary granulites contain zircons which form discordia yielding lower-intercept U-Pb ages clustering around 300-280 Ma, i.e., of late Hercynian age [13,14]. Figure 1 shows data for monazite and zircons separated from several Bournac xenoliths [14]. No isochron relationships

O. I 0 soo Granulite Xenoliths

0.09 Bournac (Massif Central) soo

OO8oo7 +o 0.06

0 . 0 5 300

£O . . . . . o 0.04 2 3

0.03 J " ? zircons '1/ BOU , 0.02 l o o / • monazites J 0.01 , , ~ Z~ zircons BOU 8

0 I I I I I I I I I

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

207pb/235 u

Fig. 1. Concordia diagram showing zircons and monazite from granulite xenoliths of the Massif Central. Data from [14].

344

are observed in the Sm-Nd or Rb-Sr systematics [7]. Instead, correlations of 143Nd/144Nd-Sm/Nd and 87Sr/86Sr-Rb/Sr for the Massif Central xenoliths are considered to be mixing lines between mantle-derived marie magmas of late Her- cynian age, represented by meta-igneous xenoliths, and pre-existing crust, represented by the metasediments [7]. The age of the crust into which the marie magmas were intruded is probably Early Proterozoic (2.1-1.9 Ga) as inliers of this age are found elsewhere in the western part of the Hercynian orogen [15]; ion-microprobe analyses of zircons from metasediments in the upper crust of the southern Massif Central yield similar ages [161.

It has been proposed [7] that the pre-Hercynian crust of the Massif Central consisted predomi- nantly of metasedimentary and felsic meta-igneous lithologies, including 540-500 Ma orthogneisses formed by melting of the felsic crust at the end of the Pan-African orogeny [8]. This crustal structure would be similar to that of the present- day crust of the Hegau /Urach district of the Moldanubian zone of southern Germany [17]. Par- tial melting of the siliceous lower crust, induced by underplating of abundant mafic magmas during the Hercynian orogeny, is thought to have given rise to the abundant peraluminous granitoids that were intruded into the upper crust between 360 and 280 Ma [8]. The presence of mantle-derived mafic magmas in the Hercynian orogen is also indicated by isotopic analyses of lamprophyres from Western Europe [18].

3. Analytical techniques

We have determined Pb and O isotope ratios (Tables 1 and 2) in a wide range of xenolith lithologies that includes ultramafic cumulates, plagioclase cumulates and felsic meta-igneous xenoliths, as well as samples identified as former basic magmatic liquids and metasediments. De- tails of the mineralogy of the samples analysed are given in Appendix I. The O isotopic character of the present-day upper mantle of the region is constrained by 6aSO values for whole-rock spinel peridotite xenoliths from the Massif Central (Ta- ble 3), and by the 6 1 8 0 values of their host basalts [19,20]. In Table 4 we also present 6180 values for a variety of orthogneisses and Hercynian granitoids exposed in the region.

H. DOWNES ET AL.

TABLE 1

O isotope data for Massif Central granulite xenoliths

Sample Number 8180(%o SMOW)

Mafic liquids BAL51 (3854) +7 .2+0 .0 17B1 (3591) +8.8 RPI0 (3210) +8 .9+0 .2 RP12 (3209) + 8.1 + 0.0 BALI]20 (3180) + 9.8

Mean + 8.6 ± 0.9

Mafic and ultramafic cumulates BAL84 (3857) +8.2±0.1 BAL210 (3200) + 6.9 BOU13 (5927) + 8.9

Mean + 8.0 ± 0.8 Mean of 8 marie xenoliths + 8.3 ± 0.9

,4 cid and intermediate meta-igneous BALl204 (3222) + 10.2 BALl046 (3192) + 9.7 BAL904X (3198) + 9.3

Mean + 9.7 + 0.4 Mean for 11 meta-igneous + 8.7 + 1.0 xenoliths

Khondalito-kinzigitic metasediments 8330 (8433) +11.8+0.0 RP41 (2841) +10.2+0.1

Mean + 11.0 + 0.8

Metasedimentary granulites without Al eSiO 5 8320 (8432) +6.3+0.11 RP50 (3207) + 11.0 RP6 (3206) + 9.4

Mean + 8.3 + 2.0 Mean for 5 metasedimentary + 9.7 + 2.0 xenoliths

O isotope ratios (as 8a80 values relative to the SMOW standard) were determined mass spectro- metrically, using a VG SIRA 10 mass spectrometer, on CO 2 produced from oxygen liberated by overnight reaction of whole-rock powders with BrF 5 at 550°C, converted into CO 2 by reaction with hot graphite. Details are given in [21]. Ana- lytical precision is _+0.2 to 0.3%0. International reference standard NBS28 gives a 6180 value of +9.6%~ in the N IG L laboratory. The O isotope analyses of the Massif Central granitoids (Table 4) were obtained at SURRC, using similar proce- dures which yielded equivalent precision.

Pb AND O ISOTOPE SYSTEMATICS IN GRANULITE FACIES XENOLITHS, FRANCE 345

TABLE 2

Pb isotope data for Massif Central granulite xenoliths

Sample 206 Pb/204 Pb 207 Pb/204 Pb 20spb / 2o4 Pb

Mafic hquids BL51 18.141 15.619 38.177

17B1 18.114 15.640 38.562

RP10 18.681 15.683 39.315

RP12 18.011 15.623 38.067

B A L l l 2 0 18.227 15.628 38.313

5212 a 18.390 15.580 38.16 5213 a 18.268 15.581 38.13

5214 a 18.057 15.611 38.16 5215 a 18.005 15.577 38.03

5216 a 18.026 15.620 38.09

5217 a 18.054 15.555 37.93 5218 ~ 18.038 15.615 38.25 168 b,c 18.266 15.670 38.50

Mean 18.164±0.201 15.609±0.035 38.265±0.351

Mafic and ultramafic cumulates BAL84 18.347 15.636 38.587

BAL210 18.244 15.588 38.161

BOU13 18.253 15.653 38.420

Mean 18.276+0.038 15.637___0.031 38.417+0.159

Felsic meta-igneous BALl204 17.773 15.518 38.134

BALl046 18.074 15.552 38.305

BAL904X 18.640 15.670 38.660 2520 b,d 18.530 15.645 38.73

802 bx 18.535 15.676 39.10

TABLE 3"

O isotope characteristics of whole rock mantle peridotites from

the Massif Central (calculated from mineral analyses using

modes from [34])

Sample 81s0(%~ SMOW)

Ta l3 5.58

Bo83-74 5.84

Bo83-73 5.67

Btl 5.53

Bt2 5.25

Bt39 6.03 BR12 5.58

RP83-68 5.17

MC19 a 6.8

MC20 a 6.5 MC32 a 5.9

Mean 5.80 +_ 0.49

a Data from [20].

for Massif Central granulite xenoliths [23-25], but these were obtained on samples for which the bulk chemistry and mineralogy were not reported. The xenoliths reported in [23] were described as being 'metabasic', and brief descriptions of those xenoliths analysed by Michard-Vitrac et al. [25] can be found in Appendix I. We have, therefore, reported

Mean 18.310+0.332 15.600_+0.058 38.586+0.339

Metasediments 8330 18.860 15.695 40.069

RP41 18.261 15.620 38.618 8320 19.188 15.623 39.205 RP50 18.244 15.657 38.372

RP6 18.560 15.680 39.436

803 18.190 15.663 38.49 509 b,f 18.737 15.687 38.75

199 b 18.637 15.676 38.58

BALl b 18.697 15.727 39.35

Mean 18.597+0.308 15.661_0.034 38.511+0.358

a Data from ref. [23]. b Data from ref. [25].

c MCFA (Sr data from ref. [7]). d BALl42 (Sr and Nd data from ref. [26]).

e BAL802 (Sr and Nd data from ref. [7]).

f BAL509 (Sr and Nd data from ref. [7]).

Pb isotope data on lower crustal granulite xenoliths exist for suites from the southwestern U.S.A. [4,22], the Eifel [9], and eastern Australia [9]. A small number of Pb isotopic analyses exist

TABLE 4

Oxygen isotope data for granitoids from Massif Central (Sr and Nd isotope analyses reported in [8])

Sample Locality 8180(%0 SMOW)

Undeformed Hercynian granitoids VELAY Velay + 10.9 _+ 0.3

101 Cantal + 10.5 201 Margeride + 10.8

202 Issoire + 8.9 _+ 0.2

203 La Goutelle + 8.6

204 Volvic + 10.5

5312 Chateauponsac + 10.6 5740 Piegut + 11.3

6042 Cognac + 12.0

7189 Gu&et + 9.8

GLC1 Cantal + 9.9

Mean + 10.3 ± 0.9

Pre-Hercynian orthogneisses 2328 Mulatet + 7.0 5581 Thaurion + 10.4 8903 Monts du Lyonnais + 8.1

Mean + 8.5 + 1.4

346 H. D O W N E S E T AL.

these previously published analyses in Table 2 for comparison. In some cases, the samples analysed by Michard-Vitrac et al. [25] are identical to those for which the Sr and Nd isotope compositions are reported by Downes et al. [7] and Ben-Othman et al. [26], and the reported Sr and Nd values have been used in the construction of some of the inter-isotope variation diagrams. Pb isotope data for four other Massif Central granulite xenoliths [27], have also been included in Table 2. Pb isotope analyses of lamprophyres and K-feldspars from Hercynian granitoids have also been reported [18,25,28].

Pb isotope analyses were obtained on a Finne- gan MAT 261 multicollector mass spectrometer at the Open University. 100-200 mg of unleached powder were dissolved using HF-HNO3; residues were converted to nitrate and finally chloride. 1 ml of 1M HBr was then added to the residue. Pb was separated using columns prepared from PVP disposable pipette tips fitted with a 2 mm diame- ter polypropylene plug and 3 -4 drops of Dowex 1 × 8 200-400 mesh anion exchange resin; com- pletely new columns were prepared for each sample in order to minimise the Pb blank. Pb con- centrations range from 2.76 ppm in meta-igneous samples to 11 p p m in metasediments [27]. Blanks were less than 0.6 ng. Ratios were normalised to accepted values for NBS981: 2°6pb/2°4pb = 16.937, 2°Tpb/2°npb = 15.541, 2°8pb/2°4pb =

36.721. Mass fractionation was -0 .14% per amu.

4. Results

The 6180 values of the Massif Central granulitic xenoliths are highly variable (Table 1) ranging from +6.3 to +11.8%o. In general, mafic meta-igneous lithologies have low 8180 values (mean = +8.3 _+ 0.9%o), whereas felsic meta-igneous samples are slightly more 180-rich (mean = +9.7 + 0.4%0). Cumulate samples have a slightly lower average 8180 value than that of samples thought to represent magmatic liquids (+8.0%o, cf. + 8.6%o), consistent with a small 180 enrichment in differentiated melts during fractional crystallisation. Metasedimentary xenoliths exhibit the greatest O isotope range (+6 .3 to +11.8%o) with four of the five samples having 8180 values > + 9.0; thus metasediments are, on average, significantly more 1SO-rich than the meta-igneous xenoliths.

When compared with lower crustal xenoliths from other areas [21], the Massif Central xenoliths exhibit 8180 ranges for particular lithological types which are similar to those from the Nor th Hessian Depression (Germany) [29]. For mafic xenoliths, the 8180 range is slightly greater than that for xenoliths from provinces such as Chudleigh (Australia), the Eifel (Germany), the Bearpaw Mountains and the Basin and Range (USA), where 8180 values generally fall between +6.2 and +8.8%0. The Massif Central metasedimentary xenoliths are generally similar in O isotope composition to those from Kilbourne Hole (USA) [30] and North Hessia (Germany) [29] which typically have 8180 values in excess of + 9%0.

The 8180 values of the mafic meta-igneous xenoliths are higher than those for mantle xenoliths from the Massif Central (Table 3). Most of these peridotites have very low 8a80 values between +5.1 and +6.0%0, although a value of +6.8%o has been reported [20]. The mean 6180 value for Massif Central mantle peridotites is +5.80 + 0.49%0, which is identical to the mean value ( + 5.85%o) for Tertiary and Quaternary al- kali basalts from the area [19].

Hercynian granitoids from the area (Table 4) have 8180 values in the range +8.6 to 12.0%o. These exposed plutonic rocks are somewhat less enriched in 180 than Hercynian granitoids from the Northern Schwarzwald (6a80 = +11.5 to + 13.5%o [31]), and they do not exhibit any indica- tion of meteoric-hydrothermal depletion in 180 seen in Hercynian granitoids of the southern Schwarzwald (8180 = +2.3 to +11.5%o [31]). Three pre-Hercynian orthogneisses from the Mas- sif Central show a large spread in 8180 values from +7.0 to + 10.4%o (mean = + 8.5 + 1.4%o). This range overlaps the range in 8 |80 values for Schwarzwald ortho- and paragneiss (+1 .6 to + 10.4%o [31]), although it is clear that some of these rocks have also had their 180/160 ratios lowered by meteoric-hydrothermal alteration associated with granitoid intrusion.

When 8180 values are plotted against SiO 2 for mantle peridotites, lower crustal granulites and Hercynian granitoids from the Massif Central (Fig. 2), a general positive correlation is observed in the order: peridotites, mafic granulites, metasediments, felsic granulites and granitoids. The position of the mafic granulites is consistent with

Pb AND O ISOTOPE SYSTEMATICS IN G R A N U L I T E FACIES XENOLITHS, FRANCE 347

mixing between mantle-derived melts with 8180 values of ca. 5.8%0 and a80-enriched silicic crustal material. However, the felsic meta-igneous granu- rites do not plot on an extension of this trend, as a SiO z gap is present; this feature may indicate that there is no direct relationship between the mafic and felsic samples. The metasediments have high and variable 6180 values similar to those of the granitoids, but their SiO 2 contents are much lower. This may be due to their restitic nature as residues after the partial melting event which formed the granitoids; their original 180/a60 ratios may have been much higher and closer to observed values for clastic sediments. The granitoids themselves have high 6180 values partly as a result of being formed by partial melting of the metasediments, and partly due to mineralogical controls (i.e., their high quartz content).

In Fig. 3, Pb isotope compositions are plotted against 8180 values. The mafic and metasedimentary suites form a broad, positively correlated trend, in which the mafic granulites have low 6180 values coupled with relatively low 2°7pb/2°4pb and 2°spb/2°4pb ratios, and the metasediments in general have high 8180 values and slightly higher 2°7pb/2°4pb and 2°8pb/2°apb ratios. Metasedi- mentary xenolith 8320 has an anomalously low 6180 value and an unusually high 2°6pb/2°apb ratio; thus, it is clearly quite different from the other metasediments. One felsic sample plots in

the overlap between the mafic and metasedimentary granulites, but the other two felsic granulites extend away from the centre of this field towards lower Pb isotope ratios and slightly higher 6t80 values.

The 2°Tpb/2°4pb vs. 2°6pb/2°4pb and 2°8pb/ 2°4pb vs. 2°6pb/2°4pb diagrams for the Massif Central granulite xenoliths (Fig. 4) illustrate the complexity of Pb isotope relationships. There is significant overlap among the different lithological groups, but the mafic meta-igneous samples tend to have the least radiogenic Pb isotope values and the metasediments the most radiogenic compositions. Mafic granulites fall in scattered fields with broad positive trends. The metasediment array somewhat overlaps that for the mafic xenoliths. The felsic meta-igneous xenoliths, however, exhibit a wider range of compositions than mafic meta-igneous xenoliths. Two of the felsic meta-igneous samples have lower 2°Tpb/2°4Pb compositions and fall below the main group, but the other three plot in the overlap between the mafic and metasedimentary xenolith fields. This confirms the diverse nature of the felsic granulites and indicates that rocks of different origins may be present in the group. Felsic granulites with low 2°7pb/2°4pb and 2°6pb/2°4pb ratios may have a Proterozoic component in their source.

The Massif Central data exhibit a positive lin- ear correlation in plots of Z°7pb/2°4pb and

+12.0

+11.0

+10.0 o

0o +9.0 0 0

6" v +8.0 o

oo 7.~ +7.0

+6.0

+5.0

M

40

Mass'if Central nitoids

£q Metasediment

45 50 55 60 65 70 75 80

SiO 2 Fig. 2. ~ l S o VS. SiO 2 for mafic, felsic and metasexiimentary granulites, with 5 additional data points for marie granulites from K. Mengel (unpubl. data). Field of mantle xenoliths from Massif Central shown by capital M. Field of granitoids also shown (SiO 2

values from [8]).

2°8pb/2°4pb vs. 2 ° 6 p b / 2 ° 4 p b ; they fall to the right of the geochron and above the Northern Hemi- sphere Reference line (Fig. 4). In this respect, they are similar to the majority of granulite xenoliths worldwide but differ markedly, however, from granulite terrains and from some granulite xeno-

liths whose host erupted through Archaean continental crust [9]. The mafic granulites also lie at the end of the Stacey and Kramers [32] growth curve for Pb in the continental crust (Fig. 4).

5. Nature and origin of the lower crust beneath the Massif Central

40.5-

40.0

38.0-

37.5 +6.0

. .Q

39.5- 0

~ 39.0-

o c,4 38.5-

15.7

nJ~ 8 3 2 0

o ~ c

15.6

15.5/

• Mafic ] X Felsic I ~ ~

8 3 2 0 []

A I I J ~ I I

+7.0 +8.0 +9.0 +10.0 +11.0 +12.0 5180 (0/00 SMOW)

348 H . D O W N E S E T A L .

+6.0 +7.0 +8.0 +9.0 +10.0 +11.0 +12.0 5180 (0/00 SMOW)

20.0 l

g 19°[ °F 13_

° i O c~ 18.0

i

17.0 +6.0

C i q h I I

+7.0 +8.0 +9.0 +10.0 +11.0 +12.0 ,5180 (0/00 SMOW)

Fig. 3. 2°spb/Z°4pb-8180 (A), 2°7pb/2°4pb 8180 (B), and 2°6pb/2°4pb-8180 (C), relationships for Massif Central

granulite xenoliths.

Major differences exist between the lower crustal granulite xenoliths and the granulitic material presently found in the upper part of the continental crust of the Massif Central [33]. The mafic xenoliths are characterised by medium pressure (4-9 kbar) parageneses and show a simple cooling and decompression history [12]. Those which have higher pressure relict parageneses (10- 14 kbar), show adiabatic decompression before cooling. In contrast, granulites from the present- day upper crust (associated w i t h the Leptyno- Amphibolite Group) have higher pressure parageneses (13-15 kbar) and are associated with eclogites. The chemistry of the two groups is also different, with the exposed mafic granulites being of MORB or low-Ti tholeiite affinity, whereas the mafic xenoliths are compositionally more akin to continental tholeiites. The exposed granulites are found above the major thrust plane which forms the suture zone of the Hercynian orogen and are probably part of a meta-ophiolite complex. There- fore, it is considered that the granulite xenoliths represent a part of the crust which is different from that represented by the granulites presently exposed in the upper crust.

All metasedimentary xenoliths appear to have undergone metamorphism at low pressures (4-7 kbar). These results are interpreted as indicating that the crust beneath the Massif Central has a broadly layered structure, with mafic meta-igneous lithologies generally underlying, but also intruding, the metasedimentary and felsic meta-igneous material. Some contacts between mafic and metasedimentary lithologies have been observed in the xenolith suite. Thus the geobarometric data give support to the hypothesis that mafic magmas were intruded into the base of the crust in a major underplating event and that the pre-existing crust was composed of the metasedimentary and felsic recta-igneous rocks.

When 8180 values for the granulite xenoliths are plotted against 87Sr/86Sr and ]43Nd/]44Nd

Pb AND O ISOTOPE SYSTEMAT1CS IN GRANULITE FACIES XENOL1THS, FRANCE 349

41,0

40.0

..o

_~ 39.0 13-

0o c~

38.0

37.0 17.0

O Mafic )~ Felsic [ ] Metasediment

18.0 19.0 20.0

206 pb/204pb

.~ 16.0 13_

o

IX. c~

15.5

16.5 l

[]

15.0 q i 17. 18.0 19,0

206 p b / 2 0 4 p b

NHRL [ ]

B I

20.0

Fig. 4. 2°spb/2°4pb vs. 2°6pb/2°4pb (A), and 2°7pb/2°4pb vs. 2°rpb/2°4pb (B), isotopic variations for granulite facies xenoliths of the Massif Central. Shown for reference are the geochron and Northern Hemisphere Reference Line (NHRL) [37] and Stacey and

Kramers [32] growth curve, with ticks showing 0.25 Ga intervals.

ratios, corrected to the probable age of intrusion of the mafic magmas (300 Ma), well-defined co- variations are observed (Fig. 5). The 180 /160 ratios increase with increasing (87Srj86Sr)300 ratios from low values in the mafic liquids and cumulates to intermediate values of both isotope ratios in the felsic meta-igneous xenoliths. The metasediments fall on an extension of this trend to higher 87Sr/86Sr and higher 8 1 8 0 , The converse relationship is observed between (143Nd/144Nd)300 and 8180 such that the highest (143Nd/144Nd)300 and lowest 180/~60 ratios characterise the mafic meta- igneous samples, and lower (143Nd/144Nd)300 and

higher 8180 values are seen in the felsic and metasedimentary lithologies. The felsic granuhtes fall between the mafic granulites and metasediments in Fig. 5.

Together these observations may indicate that the magmas which formed the mafic xenoliths were contaminated by crustal material similar to the metasedimentary granulites. The contaminant may have been a melt from the metasedimentary material with high 87Sr/86Sr and high 8a80. How- ever, such a process cannot be precisely modelled since the metasediments are restitic; hence both their 8180 values and their R b / S r ratios may have

350 H. D O W N E S ET AL.

0 o3 o o

v

+13.0

+12,0

+11.0

+10.0

+9.0

+8.0

+7.0

+6.0 0,700

\ edns tral

I I

0.710 0,720 87Sr/86Sr (at 300 Ma)

A I 0.730

+13.0 Massif Central

+12.0 / granit0ids

~) +10.0 / [~k. x ~ ~ i i / [ x Felsic o ~ i m e n t

+9.0

+8.0

+7.0

B +6.0 I I I t 0.5117 0.5119 0.5121 0.5123 0.5125

143Nd/144Nd (at 300 Ma)

Fig. 5. (SVSr/86Sr)300 vs. ~]80 (A) and (14~Nd/144Nd)300 vs. 6180 (B) for granulite xenoliths from the Massif Central. Also shown fields for Hercynian granitoids from Massif Central (ref. [8] and this paper).

been strongly modified during melting. Therefore, the crustal endmember is unconstrained. Because there is no evidence within the xenoliths for underplating of high 6t80 altered oceanic lithosphere, the mantle-derived endmember of such a mixing process would probably have a ~tSo value of ca. + 5.8%0, similar to that of the mantle xenoliths, but its Sr and Nd isotopic characteristics are

poorly constrained. The trends in Fig. 5 of increasing 3180 with increasing (SVSr/86Sr)300 and decreasing (143Nd/]44Nd)30o, and the adjacent positions of the mafic and metasedimentary xenolith fields could be interpreted as representing an increasing degree of crustal contamination of the mafic liquids, with the contaminant being the metasediments or crust of similar isotopic corn-

P b A N D O I S O T O P E S Y S T E M A T I C S I N G R A N U L I T E F A C I E S X E N O L I T H S , F R A N C E 351

positions. From the point of view of these isotope data, the felsic meta-igneous xenoliths could be formed either from an extreme degree of crustal contamination of the basic liquids (linked to fractional crystallisation), or to melting of the metasediments and mixing between the felsic melt and more mafic magmas. The felsic xenoliths may, in part, represent the product of the fusion of the metasediments and mixing of mafic magma into the partial melt.

In a diagram of Pb isotopic composition against present-day 87Sr/86Sr and 143Nd/]44Nd ratios (Fig. 6), broad trends are also observed: mafic granulites have the lower Sr and Pb isotope ratios

and higher 143Nd/144Nd ratios whereas metasediments tend to have higher Sr and Pb isotope ratios and lower 143Nd/144Nd ratios. The felsic meta-igneous samples plot either in the overlap between the two fields or fall away from the rest of the data to lower 2°7pb/z°4pb ratios. One felsic xenolith, BALl42, has an extreme 87Sr/a6Sr value, and thus its Sr and Pb isotope ratios are identical to some of the metasediments. The metasediments and mafic meta-igneous xenoliths show some overlap, but of the five felsic meta-igneous xenoliths analysed, only two fall consistently in the region of overlap. As noted above, it is likely that the felsic meta-igneous samples are a complex group

15.7

JO 4- 0 Oa .~ 15.6 13-

0 CM

A 15.5 I

0.5118 0.5130

I • Mafic / - - / ~ ' ~ >K~,..~ ~ I ~ Felsic .

I 1 I I I

0.5120 0.5122 0.5124 0.5126 0.5128

143 Nd/144Nd

15.7-

JE~ ........ ~" ' - .

~0 4L ,,;

n 15.6-

0 CM

B 15.5 I I I n t

0.700 0,705 0.710 0.715 0.720 0.725 0,730

875r/ 86Sr

Fig. 6.2°TPb/2~Pb- 143Nd/l~Nd (A) and 2°Tpb/2°4Pb- 87Sr/86Sr (B), and relationships for Massif Central granulite xenoliths.

352 H. D O W N E S E T AL.

of rocks which include both ancient crustal material and lithologies which were produced by anatexis of the metasediments and mixing of basic components during basaltic underplating. Pre- Hercynian diatexites and orthogneisses from the Schwarzwald [28] also have unradiogenic Pb isotope ratios similar to those of the unradiogenic felsic xenoliths; hence these felsic lithologies may be derived from pre-Hercynian migmatites in the middle crust. This may indicate the presence of Proterozoic crustal material similar in age to the 1.9-2.1 Ga inliers elsewhere in the Hercynian orogen [15].

The Sr-Pb and Nd-Pb systematics of the mafic granulites are similar to those observed for mafic granulite xenoliths from the Eifel [9], although the range in 2°Tpb/z°npb variation is much larger for the Massif Central. However, both Massif Central and Eifel granulite suites show markedly contrast- ing behaviour to granulite xenoliths from the Geronimo Volcanic Field (USA) [4], where trends to decreasing 207pb/204Pb with increasing 87Sr/86Sr and decreasing ]43Nd/144Nd are ob-

served. This demonstrates clearly that the isotopic signature produced by contamination of mafic

melts is dependent upon the nature of the crust into which they were intruded.

Figure 7 shows the (87Sr/86Sr)300-(la3Nd/ 144Nd)300 variation of Massif Central mantle xenoliths [34], lower crustal xenoliths [7] and exposed Hercynian lamprophyres and granitoids [8,18,35]. Of note are both the within-group variations and between-group overlap which defines a continuous convex-downward trend. As the data array contain lithologies derived from three dis- tinct domains (the mantle, the lower crust and the present-day surface), it can be considered representative of a section through the lithosphere from sub-continental lithospheric mantle to the upper crust. The variation in 875r/86Sr ratios reflects the increase in R b / S r value from the lower to upper crust. As these different rock types have very similar S m / N d ratios, there is very little change in 143Nd/ln4Nd from felsic meta-igneous and metasedimentary xenoliths to granitoids.

Corresponding 3180 ranges through the continental lithosphere from the sub-continental mantle to the upper crust are also shown in Fig. 7. The gradual 180 increase in the lithosphere is comparable to the observed increase in 87Sr/86Sr and de-

0.5135 Mantle Xenoliths ~ ( 5 1 8 0 = +5.2 to +6.0)

- - R o c k Type 8180 (°/oo SMOW 0.5130 ~ i • M----~c-- ~gt - - -o +9.--.-.-.-.-.-.-.-.8----

o o )K Felsic +9.3 to +10.2 0")

~ ~ [~ Metasediment +9.4 to +11.8

"~ 0.5125 Z ~1- ~ / V " Lamprophyres ~ ' ~ , L '~ j ' (8180 = +72 t° + 9 8 ) Granitoids Z

co ~ .6 to +12.0) 0.5120

"":::::i!i!i!i!i!i!i!i:i::::: '

).70 0.71 0.72 0.73 0.74 0.75

87Sr/86Sr (at 300 Ma) Fig. 7. 87Sr/86Sr300-143Nd/]44Nd30o variation in spinel peridotite xenoliths [34], granulite facies lower crustal xenoliths [7], Hercynian granitoids of the French Massif Central [8,35] and Hercynian lamprophyres from Western Europe [18]. 8180 ranges for

peridotites, granulites and granitoids also shown. Only data which define the field of Sr and Nd isotopic variation have been used.

Pb AND O ISOTOPE SYSTEMATICS IN G R A N U L I T E FACIES XENOLITHS, FRANCE 3 5 3

crease in J43Nd/I44Nd. The mantle-derived spinel peridotites have low 8t80 values typical of mantle material, whereas the mafic meta-igneous xenoliths are all displaced to higher 6180 values ( + 6.9 to + 9.8%0). This indicates that the magmas from which these rocks were formed were either not

derived from normal low-tSo mantle, or that they were strongly contaminated by the continental crust during their emplacement. The compositions of the basic liquids have been variously described as tholeiitic or calc-alkaline and strong crustal contamination of an originally tholeiitic magma

40.5

39.5

38.5

37.5

36.5 17.(

15.8

o

"~ 15.6 EL O (Xl

• Mafic )~ Felsic [ ] [ ] Metasediment

Massif Central granitoids E ] ~ (K-feldspar) \ ~( ~ [ ]

Schwarzwald granitoids (K-feldspar) Massif Central granitoids

~ m £ : '~ " (whole rock)

tamorphic rocks ~ / / / / / / / / / / (K-feldspar)

W. Europe ~

.amprophyres I I A I 18.0 19.0 20.0

206 pb/204pb

Massif Central granitoids (K-feldspar)

Massif Central granitoids ~ [ ~ ~ / (whole rock)

Schwarzwald granitoids j ~ ~ ~ "

(K-feldspar)

' ~ W. Europe Lamprophyres

Schwarzwald metamorphic rocks (K-feldspar)

D 15.4 I I ~ l

17.0 18.0 19.0 20.0

206 pb/204pb

Fig. 8. Comparison of 2°spb/2°4pb vs. 2°6pb/2°4pb (A) and 2°7pb/2°4pb vs. 2°6pb/2°4pb (B) for K-feldspars from Hercynian granitoids of the Massif Central [25] and the Schwarzwald [28], whole rock Hercynian granitoids of the Massif Central corrected to 300 Ma (A. Shaw, pets. commun., 1990), lamprophyres of Western Europe corrected to 300 Ma [18], and K-feldspars from

pre-Hercynian gneisses of the Schwarzwald [28], with those for Massif Central granulite xenoliths.

354 H. D O W N E S ET AL.

might produce such ambiguous geochemical characteristics. The high 8180 values observed for the metasediments might be appropriate for that of the contaminant of the mafic magmas, but it is more likely that the metasediments are restitic and their original 8180 values were higher and more similar to those of present-day clastic sediments.

6. The lower crust and the origin of Hercynian granitoids

The Massif Central contains abundant Her- cynian granitoids, which range in age from 360 to 280 Ma [36]. The origin of these peraluminous granitoids has been attributed to melting of lower crustal meta-igneous and metasedimentary rocks, represented by the granulite xenoliths [8], or to melting of upper crustal meta-volcanics and metasediments [35]. The fields of the Sr and Nd isotope initial ratios [8] and 8180 values of Massif Central Hercynian granitoids are shown on Fig. 5. The Hercynian granitoid field overlaps most of the felsic meta-igneous and metasedimentary data, supporting the hypothesis [8] that the Massif Central granitoids are the result of melting of the more felsic parts of the lower crust. Granitoid 8180 values are very similar to those of the metasedimentary granulites, indicating that these more fusible part of the lower crust probably played a major role in their origin. However, since the three orthogneisses from the Massif Central show a large range in 8180 values (+7 .0 to + 10.4%~), and ortho- and paragneisses from the Schwarzwald vary between + 1.6 to + 10.4%o [31], it is clear that upper crustal material could also be involved in the source of the granitoids.

When the Pb isotope data for K-feldspars from Massif Central granitoids [25] are compared with that for the granulite xenoliths (Fig. 8), the granitoid K-feldspar data form a field with rather lower 2°6pb/2°apb ratios than the metasediments. The data are not directly comparable, due to the lack of age correction on the granulite data, but they would appear to contradict the hypothesis of formation of the granitoids from melting of dominantly metasedimentary lower crustal lithologies. There are also abundant Pb isotope data on whole rocks and K-feldspars from Hercynian granites of the Schwarzwald [28]. The Schwarzwald granitoids do not significantly overlap the field of Massif

Central metasedimentary xenoliths, but are displaced to less radiogenic compositions and, therefore, tend to fall in the field of mafic xenoliths.

The low Pb isotope ratios in some of the Hercynian granitoids could be due to the involve- ment of some middle or upper crustal material in their source, as the upper crustal metamorphic rocks [28], and some of the lower crustal felsic xenoliths discussed here, have less radiogenic compositions than the mafic meta-igneous xenoliths. If these lithologies were involved in the generation of the granitoids, the effect would be seen mainly in the Pb isotope ratios and would not be detectable in the Sr or Nd isotope ratios.

Alternatively, unradiogenic mantle-derived material, similar to the magmas which formed the mafic granulites, could also be a component in the source of the granitoids. A major mantle component has been identified in the early 360 Ma quartz diorites of the Massif Central [A. Shaw, pers. commun., 1990 and in the late (295 Ma) lamprophyres of the Hercynian belt [18]. Although their present-day Pb isotope compositions are elevated compared with the mafic xenoliths, the lamprophyres have unradiogenic initial Pb isotope compositions and (143Nd/144Nd)300 values [18] which are similar to those of the mafic granulites (Fig. 8). Their (87Sr/86Sr)300 compositions are, however, slightly higher than those of the mafic granulites for a given (143Nd/14~Nd)300 value. These lamprophyres do not significantly overlap the field of initial Sr and Nd isotope ratios of the Massif Central granitoids (Fig. 7), but they indicate that mantle-derived magma was sometimes permitted to rise to high levels in the Hercynian crust.

The intrusion of granitic magma into the upper crust of the region occurred near the end of the Hercynian orogeny, postdating the main metamorphic events. A similar event occurred in the same region at the close of the Cadomian or Pan-African orogeny (600-500 Ma), when granitoids representing the oldest rocks yet dated in the Massif Central (e.g., the Thaurion and Palan- ges orthogneisses) were intruded. Most of these orthogneisses have initial Sr and Nd isotopic signatures similar to those of the metasedimentary xenoliths at that time [8]. It is possible that near the end of each orogenic cycle, after metamorphism and crustal thickening has occurred, a

Pb AND O ISOTOPE SYSTEMATICS IN GRANULITE FACIES XENOLITHS, FRANCE 355

period of crustal relaxation and thinning takes place during which basic magmas are intruded into the lower crust. The transfer of heat from the mantle provokes partial melting of the pre-existing crust and concomitant late-orogenic granulite- facies metamorphism.

7. Conclusions

The Pb and O isotope data for the three main types of granulite xenoliths from the Massif Central vary in a consistent manner. In general, mafic granulites have the least radiogenic Pb and Sr isotopes, the most radiogenic 143Nd/144Nd, and the lowest 8180 values. In contrast, the metasedimentary granulite xenoliths have a more radiogenic Pb and Sr isotopic character, low 143Nd/144Nd ratios and high 180/160 ratios. Felsic meta-igneous samples either fall into the overlap between the two main groups, or plot to low 2°7pb/2°4pb and 2°6pb/2°4pb ratios. This may indicate that there are several origins for the felsic xenoliths: one group may be ancient meta- granitoids, while a second group are probably melts of the metasediments or extreme contami- nants of the basic magmas.

An origin for the mafic xenoliths involving mixing between mafic magmas and pre-existing crust is indicated by both the Pb and O isotopic data. Thus, the results are consistent with the hypothesis of basaltic underplating having added considerably to the crust during its history. Like the Eifel granulite xenoliths [9], the Massif Central xenoliths plot to the right of the geochron (Fig. 4). Therefore, as shown by Rudnick and Goldstein [9] for lower crustal xenolith suites from elsewhere in the world, the Massif Central xenoliths are too radiogenic in Pb isotopic character to be the mis- sing unradiogenic component which could solve the Pb isotope paradox. Our data indicate that in order to obtain the Pb isotopic characteristics of the Hercynian granitoids, non-radiogenic Pb has to be added to radiogenic Pb in lower crustal melts. The Pb isotope ratios of K-feldspars from Hercynian granitoids from the Massif Central and Schwarzwald appear to require a component which

is less radiogenic than the metasediments, and some of the felsic meta-igneous lithologies. In our model, this non-radiogenic Pb is obtained from exposed metamorphic rocks or some lower crustal felsic granulites, which may be representative of middle to upper crustal lithologies, or from the mantle.

The O isotope data demonstrate clearly that the mafic xenoliths are contaminated by high 180 continental crust, which could be represented by the metasediments. These data also support the hypothesis of infracrustal melting of metasedimentary and felsic meta-igneous rocks to form the Hercynian granitoids of the Massif Central, since the 8180 values of the granitoids are similar to the lower crustal lithologies, and there is extensive overlap between the fields on plots of 8 1 8 0 - 87Sr/S6Sr and 8180 - 143Nd/a44Nd at 300 Ma (Fig.

5). Finally, the evidence presented here indicates

that periods of crustal thinning at the end of major orogenic events (e.g., the Hercynian orogeny, the Cadomian orogeny) are linked to the intrusion of basic magma into the lower part of the crust. Such underplating gives rise to granulite facies metamorphism and crustal anatexis, due to the transfer of heat from the mantle to the crust. Granitoids which are intruded into the upper part of the crust during this time will have dominantly crustal isotopic signatures, since the material transferred from the mantle appears rarely to be able to penetrate into the upper crust.

Acknowledgements

O isotope analyses on Hercynian granitoids were provided by A.E. Fallick of the Scottish Universities Research and Reactor Centre. M. Fowler and P. Greenwood are thanked for laboratory assistance with O isotope analyses at NIGL. Thanks are due to K. Mengel for the use of his unpublished data, and to K. Mengel, R. Rudnick and an anonymous reviewer for thorough reviews of the manuscript. N E R C Isotope Geosciences Laboratory Publication Series Number 25.

356 H. DOWNES ET AL

APPENDIX I

Basic petrographic characteristics of Massif Central granulite xenoliths

Sample Analysis Petrographic type number

Mafic liquids BAL51 3854 Pyriclasite 17B1 3591 Pyriclasite RP10 3210 Q-fsp Pyriclasite RP12 3209 Pyriclasite BALl 120 3180 Pyriclasite

Mafic and ultramafic cumulates BAL84 3857 Pyriclasite BAL210 3200 Pyriclasite BOU13 5927 Quartz pyriclasite

Acid and intermediate meta-igneous BALl204 3222 Charno-enderbitic gneiss BALl046 3192 Charno-enderbitic gneiss BAL904X 3198 Charnockitic gneiss

Metasediments 8330 8433 Khondalite RP41 2841 Khondalito-kinzigite 8320 8432 AI 2 SiOs-free granulite RP50 3207 A12SiOs-free granulite RP6 3206 AI 2SiOs-free granulite

Samples reported by Michard-Vitrac et al. [25] BALl42 2520 Charnockitic gneiss BAL803 2850 AI 2SiOs-free granulite 509 2151 Khondalito-kinzigite 802 2849 Charno-enderbitic gneiss 199 2481 Charnockitic gneiss 168 2146 Pyriclasite BALl 2845 Charnockitic gneiss

Petrographic description

plag, opx, cpx, ilm, ap, rt plag, opx, cpx, ilm, ap, hb (pargasite), sp plag, gt, qtz, opx, rut, ksp, ilm, blot, zir plag, cpx, opx, ilm, ap plag, cpx, gt, opx, biot, hb, ilm, ap

opx, cpx, plag, biot, ilm plag, opx, cpx, sp, hb (pargasite) qtz, plag, opx, gt, biot, ilm, ap, zir

qtz, plag, ksp, opx, gt, rut, ilm, ap, zir qtz, plag, opx, gt, biot, rut, ilm, ksp, ap, zir qtz, ksp, plag, opx, rut, ilm

qtz, plag, ksp, gt, sill, biot, ilm, rut, zir qtz, plag, ksp, gt, sill, rut, ilm, zir, gph, ap qtz, plag, ksp, gt, ilm, rut, ap qtz, plag, ksp, gt, rut, blot, ap, zir qtz, plag, gt, ksp, biot, rut, ilm, ap, zir

qtz, plag, ksp, opx, rut qtz, plag, ksp, gt, rut, ap, zir, mo, gph qtz, plag, ksp, gt, sill, rut, ap, zir, gph qtz, plag, ksp, opx, gt, rt, ilm, he, mg qtz, plag, ksp, opx, gt, rut, ilm, ap, zir plag, opx, gt, biot, ilm, ap, rut qtz, plag, ksp, opx, biot, gt, rut, ilm

Plag = plagioclase feldspar; qtz = quartz; opx = orthopyroxene; cpx = clinopyroxene; gt = garnet; ilm = ilmenite; ap = apatite; rt = rutile; hb = hornblende; sp = spinel; biot = biotite; zir = zircon; sill = sillimanite; ksp = K-feldspar; gph = graphite; mo = monazite; mg = magnetite; he = hematite.

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