A strontium, neodymium and oxygen isotope study of hydrothermal metamorphism and crustal anatexis in...

Contrib Mineral Petrol (1988) 100:399-417 Contributions to Mineralogy and Petrology �9 Springer-Verlag 1988

A strontium, neodymium and oxygen isotope study of hydrothermal metamorphism and crustal anatexis in the Trois Seigneurs Massif, Pyrenees, France* M.J. Bickle i, S .M. Wickham 2 * ~ H.J. Chapman I, and H.P. Taylor, Jr. 2 1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, United Kingdom 2 California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125, USA

Abstract. Nd, Sr, and O isotope analyses have been made on metamorphic and igneous rocks and minerals from a 310 340 Ma Hercynian-age metamorphic terrane in the Pyrenees, France. Lower Paleozoic shales and phyllites have STSr/S6Sr values of 0.707-0.717 at 310 Ma, but model values at 310 Ma of 0.709-0.736 (based on assumed depositional age of 450 Ma and an initial STSr/S6Sr=0.707). On a regional scale, 87Sr/86Sr was homogenized to about 0.713 to 0.717 in the higher-grade pelitic schists during metamorphism. Much of this STSr/S6Sr exchange occurred at very low grades (below the biotite isograd), but significant changes also accompanied the 6180 lowering of the phyllites (+ 13 to + 16) during their transformation to andalusite- and sillimanite-grade schists (6180= +11 to +12); all of these effects are attributed to pervasive interactions with hydrothermal fluids (Wickham and Taylor 1985). The data also show that a syn-metamorphic plutonic complex, dominated by a biotite granite body, was derived by mixing of a relatively mafic magmatic end-member (SVSr/S6Sr~ 0.7025~).7050 and 61so~ +7.5 to +8.0) with two metasedimentary sources, both having 87Sr/86Sr~0.715 and 61so ~ + 10.0 to + 12.0, but with one being more homogeneous than the other. The more homogeneous component and the (mantle-derived?) magmatic end-member dominate at low structural levels within the complex. The less homogeneous end-member that dominates at high levels is clearly derived from the local Paleozoic pelitic schists. A Rb-Sr age of 330+20 Ma was obtained on hornblende from a deep level within the complex, which fixes this age for the regional metamorphism, as well. Although a post-metamorphic granodiorite magma body at Trois Seigneurs also dis- plays heterogeneities in 6180 and S7sr/a6Sr (and thus does not give a clear-cut Rb-Sr isochron), the data are consistent with an emplacement age between 260 and 310 Ma, similar to ages of other late granodiorites in the Pyrenees. 143Nd/ l~4Nd is very uniform within the Hercynian crust, both at Trois Seigneurs (eNd = --3 to --7) and elsewhere in the Pyrenees; almost all igneous lithologies have depleted-mantle, mid-Proterozoic model ages, consistent with efficient recycling of crustal material following original crustal accre-

* Contribution No. 4544, Division of Geological and Planetary Sciences, California Institute of Technology ** Present address: Department of the Geophysical Sciences, Uni- versity of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA Offprint requests to: M.J. Bickle

tion in this area at about 1600 Ma or earlier. Rb-Sr mineral ages exhibit a complex cooling history reflecting late Her- cynian and Mesozoic thermal events. Our results show that profound homogenization of the S7Sr/S6Sr and 180/160 ratios of large volumes of the crust can occur during regional metamorphism and crustal anatexis, particularly in regions undergoing extensional tectonics. Such processes can significantly modify the isotopic compositions of the protoliths of granitic magmas; this may explain why many peraluminous Hercynian granitoids of Western Europe have anomalously low (87Sr/86Sr) initial values compared to their probable sedimentary parent rocks.

Introduction

During the Late Carboniferous, the Paleozoic rocks of the Eastern Pyrenees were metamorphosed in high thermal gradient environments associated with granitoid plutonism (e.g. Zwart 1979). A number of different tectonic interpreta- tions have been proposed for this Hercynian orogenic episode, including various models that invoke regional com- pression (see Matte 1986), such as the Andean-type conver- gent plate model of Nicolas (1972) and the continental collision models of Burrett (1972) and Dewey and Burke (1973). More recently, Wickham and Oxburgh (1985, 1986, 1987) suggested that the high T-low P metamorphic sequences in the Pyrenees were generated by localized crustal extension and associated intrusion of mantle-derived magmas.

Discrimination among the different Hercynian tectonic models requires detailed documentation of the timing of metamorphic, deformational, and plutonic events, as well as a better understanding of the sources of the granitic magmas; these represent the main focus of the present study, which presents Rb-Sr, Sm-Nd and O isotope whole- rock data, and some Rb-Sr mineral data, on metamorphic and plutonic rocks from the Trois Seigneurs Massif in the Pyrenees. This terrane (Fig. 1) exposes a typical Hercynian metamorphic sequence and a range of plutonic rock types, and it has previously been the subject of other detailed field, petrological, and geochemical studies (Zwart 1959; Allaart 1959; Wickham 1984, 1987a; Wickham and Taylor 1985; 1987a).

Our new data are intended to constrain more closely the timing of metamorphism and plutonism, the sources

400

France

St. Girons

TROIS SEIGNEURS MASSIF

AGLY _ z" MASSIF

Perpignan

+++++

Spain

ANDORRA- MONT LOUIS

I �9 �9 ' ' " " " " Figueras

o 50 KM l J

I [ Mesozoic end Tertiary sediments ~ Orthogneisses

Low-grade Paleozoic I Basel gneisses

High-grade mica schists, migmofites, [ ~ Late gronodiorites anatectic granites

Fig. l . Hercynian basement outcrop in the Pyrenees in southern France and northern Spain, showing the location of the Trois Seigneurs Massif and other localities referred to in the text (based on maps in various literature sources listed in the references). The boundary between the low-grade Paleozoic and the high-grade mica schists, migmatites and anatectic granites corresponds approximately to the 'biotite-in' isograd. Late granodiorite names are shown in boxes

of the igneous rocks, and whether or not extensive Sr isotopic homogeniza t ion occurred within the metasedimentary rocks. The lat ter quest ion is especially interesting because the high-grade pelites, psammites, and carbonates appear to have been pervasively infi l trated by aqueous fluids during Hercynian metamorphism, as evidenced by the overall lowering and homogeniza t ion of the 180/160 rat ios in the metamorph ic and plutonic rocks (Wickham and Taylor 1985, 1987a).

Geological setting

Hercynian of the Pyrenees

In the Pyrenees, segments of Hercynian basement were uplifted during Tertiary Alpine deformation (Fig. 1). In the North Pyrenean Zone, there are a number of such basement slices, made up of Paleozoic (and possibly Late Proterozoic) metasedimentary rocks intruded by various types of granite. Recent summaries of the Hercynian geology of the Pyrenees are given by Wickham and Oxburgh (1986, 1987), Zwart (1979), and several papers in Banda and Wickham (1986).

The grade and intensity of Hercynian metamorphism generally increase toward deeper structural (and stratigraphic) levels, characteristically reaching upper amphibolite facies (with development of abundant pelitic migmatites) within the Paleozoic sedimentary sequences. In the Trois Seigneurs Massif (and in several other areas as well) the migmatites grade downward into a biotite-rich peralu- ruinous granitoid several km in size (Wickham 1984, 1987a), In addition, numerous peraluminous leucogranite bodies up to 1 km

in size are associated with the migmatites and micaschists. On the basis of chemical and isotopic compositions, mineralogy, and field relationships, both of these magma-types appear to have been largely derived by partial melting of the local Lower Paleozoic pelitic lithologies (Wickham 1984, 1987 a, b; Wickbam and Taylor 1985; Bickle et al. 1985).

In addition to the above bodies, a distinctive suite of much larger biotite-hornblende granodiorite plutons was emplaced throughout the Pyrenees (Zwart 1979). Most workers have concluded that these bodies (which are typically several hundred km 2 in outcrop area, see Fig. 1) were intruded after the Hercynian metamorphic peak, and they are here termed "late granodiorites" (e.g. Zwart 1979; Wickham 1984, 1987a; Wickham and Taylor 1985). These granodiorites are characteristically emplaced into weakly metamorphosed Upper Paleozoic rocks at fairly shallow upper crustal levels, but at Trois Seigneurs a small pluton of this type was intruded at deeper levels, and this body cross-cuts the regional metamorphic isograds (see Fig. 2).

Along the southern margin of the Axial Zone of the Pyrenees (Fig. 1), metamorphic rocks of the upper part of the Hercynian Paleozoic sequence are locally unconformably overlain by Upper Carboniferous to Permian sedimentary and volcanic rocks that post-date the Hercynian-age metamorphism and penetrative deformation (see Fig. 2 in Wickham and Oxburgh 1986). It is important to note that very little of the Paleozoic sedimentary sequence is missing at this unconformity, where Westphalian D or Stephanian rocks typically rest on Lower Carboniferous or Devonian strata, never on high-grade basement. The youngest deformed rocks yet identified below the unconformity are of Namurian or Lower West- phalian age (Zwart 1979, 1986), implying deposition between 326 and 310 Ma (Hess and Lippolt 1986). The oldest rocks above the

401

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Sei(

+•+++++++*+++++§ + + + + + + + . + + + + § 2 4 7 + + + + + + + §

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des Trois

11

~Etang. :2~ : i i : : i ! : ! i i ! i i i : : ! i :~

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Silurian shales

Cambro- Ordovician phyl[ite

[ [ Andalusite schists

Sillimanite schists (+ andalusite )

Sillimanite gneiss (migmatite)

Biotite granite - quartz diorite

Biotite granodiorite

Muscovite granite 1 i

Mesozoic sediments 0 km B

Fig. 2. Simplified geological map of the Trois Seigneurs Massif showing the locations of samples analyzed in this study (modified after Wickham 1987a). The numbers on the map correspond to the localities listed in Tables 2 through 6

unconformity are Westphalian D in age (~ 310-306 Ma, Hess and Lippolt 1986). Thus, the unconformity places stringent age brackets on the main Hercynian deformational and metamorphic events (about 326 Ma-310 Ma). The occurrence of Stephanian (296-286 Ma, Hess and Lippolt 1986) and Lower Permian volcanic rocks attests to the continuation of igneous activity until at least 270 Ma.

Geology and stable isotope geochemistry of the Trois Seigneurs Massif

Within the Trois Seigneurs Massif (Fig. 2), there is a continuous change from chlorite-grade shales and slates, through andahisite and sillimanite-grade mica-schists, to a relatively narrow migmatite zone, over a horizontal distance of only 5-7 kin. The migmatites are gradational into, and structurally underlain by, a biotite-cordierite granite body rich in metasedimentary xenoliths (the "deep" biotite granite).

Wickham and Taylor (1985, 1987a) showed that the schists at grades higher than the 'andalusite in' isograd were shifted to lower 6180 values (+ 11 to + 13) than their low-grade protoliths, which have 6180> +14 to +15. Carbonate horizons (10-30m thick) were even more profoundly depleted in 180, producing homogeneous whole-rock fi180 values essentially identical to those of the surrounding pelitic schists and psammites. Also, the 6180 values of the leucogranite bodies and of the deep biotite granite (but not the late granodiorite) were found to be similar to the (5180 values of the schists.

Wickham and Taylor (1985) estimated that material-balance water-rock ratios of at least 0.2 and probably more than 0.5 were necessary to produce the observed depletion in 18 0 in the metasediments. They also concluded that the aqueous fluid circulation mostly took place during prograde Hercynian metamorphism, and had largely ceased by the time that peak metamorphic temperatures

were reached. The D/H ratios of muscovites are abnormally high from a variety of lithologies throughout the metamorphic and anatectic sequences in the Pyrenees (OD = - 2 6 to -31), implying that the metamorphic-hydrothermal fluids were derived from a very uniform, high-D source, most probably sedimentary formation waters or exchanged seawater (Wickham and Taylor 1985, 1987 b).

Previous geochronological studies in the Pyrenees

The disruption of the Pyrenees by Alpine-age thrusting precludes accurate correlation between the Hercynian metamorphic struc- tures and the Upper Carboniferous sedimentary record. Any such correlations are thus necessarily based on a comparison of the radiometric ages of metamorphic and intrusive events with the fossil stratigraphic record.

Previously published ages from the Hercynian basement in the Pyrenees are listed in Table 1 (recalculated to 2Rb=1.42 • 10 -11 a-1). The main features of these data are: (1) Rb-Sr whole-rock ages are generally 260-280 Ma for the late granodiorites. (2) A whole-rock Rb-Sr age of 328+15 Ma was obtained on the syn- metamorphic(?) "granite profond" in the Canigou Massif (see Fig. 1), consistent with U-Pb zircon ages on the same suite (Vitrac- Michard and All+gre 1975a, b). (3) A U-Pb zircon age of 310_+ 10 Ma was obtained on granulite-facies gneisses at Port de Saleix (Postaire 1982). (4) U-Pb zircon ages of 309_+5, 315_+5 and 315 + 5 Ma were reported for charnockites from the Agly Mas- sif (Postaire 1982), compatible with the U-Pb zircon age of 314___ 7 Ma (Respaut and Lancelot 1983). (5) A Rb-Sr whole-rock study of the Aston Massif (Majoor and Priem 1987) indicates an almost complete homogenization of the Sr isotopic compositions at two distinct localities within the Aston orthogneiss at about 315 Ma; in this same study, ages of 301 _+ 15 Ma were determined for the Ax granite and 292_+13 Ma for the Soulcem muscovite-

402

Table 1. Published radiometric ages from Hercynian plutonic lithologies in the Eastern Pyrenees

Locality Unit Method Age + / - 2 Refer- Ma sigma ences

1 Syn-metamorphic ages

Canigou "Grani te Profond" Rb-Sr whole-rock 328 15 (1) Port de Saleix Granulite facies U-Pb zircon 310 10 (2)

quartzite upper intercept Agly Ansignan charnockite U-Pb zircon 309 5 (2)

lower intercept Agly Ansignan charnockite U-Pb zircon 315 5 (2)

(basic facies) upper intercept Agly Ansignan charnockite U-Pb zircon 315 5 (2)

(Leucocratic facies) upper intercept Agly Ansignan charnockite U-Pb zircon 314 7 (3)

upper intercept

2 Late granodiorites"

Querigut (0.7095) Querigut (0.70911 + 16) Querigut-Millas-Mont Louis composite

isochron (0.710-+ 1) Costabonne (0.711 _+ 1) Maladeta (0,7117 _ 2) Millas Bassies b (0.7114 -+ 26)

3 Muscovite ages

Rb-Sr whole-rock 280 - (4) Rb-Sr whole-rock 304 5 (5)

Rb-Sr whole-rock 275 12 (1) Rb-Sr whole-rock 262 10 (1) Rb-Sr whole-rock 277 7 (6) Rb-Sr whole-rock 269 12 (7) Rb-Sr whole-rock 284 81 (9)

Aston-Hospitalet Rb-Sr muscovite 264 to 285 11 (8) to whole-rock 11 (7 ages)

Canigou Rb-Sr muscovite 269 - (1) to whole-rock

References: (1) Vitrac-Michard and Allegre 1975a; (2) Postaire 1982; (3) Respaut and Lancelot 1983; (4) Ben Othman et al. 1984; (5) age recalculated from data in Ben Othman et al. 1984, but excluding aplite data and sample Qt15 gives age as shown and initial ratio of 0.70911 _+16, MSWD=4.2 ; (6) Vitrac-Michard et al. 1980; (7) Vitrac-Michard 1975; (8) Jaeger and Zwart 1968; (9) Majoor and Priem 1987 a The calculated initial 87Sr/86Sr values at each locality are listed in parentheses b Strongly reset along Alpine shear zones

granite swarm (the latter are geologically analogous to the Trois Seigneurs leucogranites, see above). (6) Rb-Sr whole-rock and muscovite ages between 285 and 269 Ma were found for various lithologies in the Aston-Hospitalet and Canigou Massifs. (7) Other Rb-Sr and K-Ar mineral ages are younger, mostly between 100 and 250 Ma, reflecting later (e.g. Alpine) events on minerals with lower blocking temperatures (Vitrac-Michard and All~gre 1975 a; Albar- ede and Vitrac-Michard 1978; Montigny et al. 1986). (8) Some recent U-Pb age determinations have been made using the SHRIMP ion-probe. Zircon from a syn-metamorphic leucogranite in the Trois Seigneurs Massif gave an age of 338• Ma, and similar analyses of zircon from the Trois Seigneurs late granodiorite suggest an age close to 280 Ma (Williams et al., in prep.).

These earlier studies imply that Hercynian metamorphism in the Pyrenees may have occurred as early as 328__15Ma to 338_+10 Ma (from the Canigou and Trois Seigneurs ages) or as late as 292_+ 13 to 315 _+ 5 Ma, based on several U-Pb zircon ages on the Agly and Port de Saleix granulites and on the Rb-Sr ages from the Aston Massif. This range of radiogenic age determinations for the metamorphism is significantly wider than the 306-326 Ma stratigraphic age of the Upper Carboniferous unconformity in the Pyrenees. Ages on the late granodiorites are up to 50 Ma younger.

The whole-rock and muscovite Rb-Sr ages of about 285 to 270 Ma (Table 1) probably date the time when the rocks cooled through the ~ 500 ~ C muscovite Rb-Sr blocking temperature. Thus the existing data-set is consistent with the hypothesis that the oro-

genic events occurred over a 50 Ma period. The Stephanian to lower Permian volcanic rocks (306-270 Ma, Harland et al. 1982) may be petrogenetically related to the late granodiorites, because they span a similar time range, and they are compositionally distinct from the syn-metamorphic peraluminous granites.

Analytical methods

Sr was separated using standard cation techniques. Blanks of < 1 ng are negligible compared to the sample size. Isotope analyses were made on a V-G Isomass 54 E mass spectrometer. Analysis of NBS987 on this machine over a three-year period gave a 87Sr/ S6Sr ratio of 0.710230_+30 (2a). Rb/Sr ratios on whole-rock powders were determined by XRF spectrometry to a precision of • 1% (2a on the ratio) using a Phillips PW1400 spectrometer (Har- vey and Atkin 1981) calibrated against Rb/Sr ratios on USGS standards (de Laeter and Abercrombie 1970). Mineral separates were dissolved in HF, HNO3 and HC1 in bombs at 150~ or 200 ~ C as necessary, and aliquoted for separate isotope ratio and isotope dilution Rb and Sr analysis. USGS standard BCR-1 analyzed in this way gave an Rb/Sr ratio of 0.1405 and a 87Sr/86Sr ratio of 0.705097 • 50 (2a). Decay and isotope constants are those of Steiger and Jaeger (1977) and all Rb-Sr ages quoted in the text have been recalculated to a decay constant of 1.42 • 10 -11 a - 1. Sample dissolution for Sm-Nd analyses (HF, HNO3 and HC1 stages) was in teflon bombs at 200 ~ C. Sm and Nd were separated by standard column and reverse phase techniques (Cohen et al.

0730 f ......... ......... 1 Late granodiorite

Age 310 -+ 6 Ma 87Sr/86Sri 0~ -+ 17

MSWD 1,5

0-725

0-720

~ [ d e d from regressior 0-715 ~ J I I I I I I I J I I I I I I I I I

1-0 2"0 3-0

5 7 R b / 8 6 S r

Fig. 3. Rb-Sr isochron diagram for the Trois Seigneurs late granodiorite. If three of the twelve samples are omitted from the regression (open circles), the remaining data (solid circles) define an isochron representing an age of 310_+6 Ma. The 6~80 values of the granodiorite samples fall into two distinct groups, as shown; the highJgO samples typically have low 87Rb/s6sr and are all from locality 2 near Etang d'Arbu (see Figs. 2 and 4). These data indicate that the granodiorite magma was not isotopically homogeneous at the time of intrusion

403

07301 ......... ......... 1 Late granodiorite

Age 310 • 6 Ma 87Sr/S6Sri 0.71059 • 17

MSWD 1.5

0-725

j j

Col de Rose j . . . ~ ' ~

~ Reference Isochlon

0-720 ~ - Pic de J j ~ P e y ~ ,

~ ~ Etong d'Arbu

~ ' ~ 0 sam, pie exc luded from regression 0.715 , / ~ K i I 1 I I I I I I i i i i i i i i

1"0 2-0 3.0

0 7 R b / 8 6 S r

Fig. 4. Rb-Sr isochron diagram for the late granodiorite (same data as shown in Fig. 3), showing fields for the three different sample localities (see also Fig. 2). Each of the three localities preserves a tighter range of 8:Rb/86Sr and 87Sr/86Sr than the whole data set taken together, and two of the groups define younger (shallower) slopes than the dashed line, which is the 310 Ma isochron from Fig. 3. A 280 Ma reference isochron (heavy solid line) is also shown, representing an age that is more typical of other late granodiorites in the Pyrenees. The isotopic heterogeneity, both for oxygen (Fig. 3) and strontium, and the shallower slopes of the Pic de Peyroutet and Etang d'Arbu data, both suggest that the 310 Ma age is too old, and that the data-points may in part represent a mixing array, thereby obscuring the true age of intrusion

1988). 14aNd/144Nd ratios were normalised to a 146Nd/144Nd ratio of 0.7219. An analysis of USGS standard BCR-t gave X43Nd/ 144Nd = 0.512655 + 18 and Sm/Nd = 0.2279 _+ 2. CHUR model ages and end values were calculated using the chondritic values of Was- serburg et al. (1981). Depleted mantle model ages (tDM) were calculated for a source evolving to a present-day 143Nd/144Nd ratio of 0.513150. Error estimates are quoted at 95% confidence limits and isochron regressions are after York (1969). 180/160 analyses are reported relative to the SMOW scale, and were done in the same manner as described by Wickham and Taylor (1985, 1987a). NBS-28 has a 6280 = + 9.60 in our laboratory.

Results

Trois Seigneurs late granodiorite

Regression of nine whole-rock Rb-Sr analyses of the late granodior i te gives an isochron corresponding to an age of 3 1 0 + 6 Ma, initial 87Sr /86Sr=0.7106+2 and M S W D = 1.5 (Fig. 3, Table 2a); however, this regression excludes 3 data- points that do not fit the isochron line, and because this Rb-Sr age is older than a U-Pb ion probe zircon age of about 280 M a on this same pluton (Williams et al., in prep.), we suggest that the 310 M a value may be too old. The U-Pb age is p robab ly to be preferred given the insensiti- vity o f U-Pb zircon systematics to such problems as petro- genetic heterogeneity and post -emplacement al terat ion, which are known to affect Rb-Sr geochronology. Indeed, the granodior i te samples exhibit considerable 180/160 heterogeneity, and these whole-rock &180 values show some correlat ion with Rb/Sr rat io (Fig. 3). Fur thermore , i f the samples are subdivided into suites by sample localities, ex-

clusion of the three anomalous data-points does not appear to be justified (Fig. 4). Unfor tunately , regressions of da ta from even the best defined individual localities (Pic de Peyr- outet and Etang d 'Arbu) are insufficiently precise to distinguish between a 3]0 M a age and a 280 M a age (Figs. 3 and 4).

We therefore interpret the 310_+6 M a Rb-Sr age to be an art ifact of both sample selection and incomplete homogenization of 87Sr/86Sr rat ios within a magma formed from two or more sources. The spread in 180/160 rat ios confirms such heterogeneity, and if the more Rb/Sr rich source also had a slightly higher 878r/86Sr ratio, the apparen t Rb/Sr isochron age would indeed be too old. Isotopic inhomoge- neities on this scale are not uncommon in crustal granitic melts (e.g. Hill et al. 1985), and much more dramat ic effects of this type are observed in the deep bioti te granite at Trois Seigneurs, as discussed below.

A 280 M a age for the late granodior i te is within the range of Rb-Sr whole-rock ages exhibited by other " l a t e " granodiori tes in the Pyrenees (Table 1), with the exception of the age recalculated for the Querigut p luton by excluding the aplitic samples (see Table 1). The initial 87Sr/86Sr ratio of 0 .7106+2 at 310___6 M a (or 0.7114 at 280 Ma) is also within the range (0.709 to 0.711) exhibited by the other late granodiori tes (Table 1).

Biotite granite

Rb-Sr isotopic composi t ions of 7 h igher-Rb/Sr samples o f the deep bioti te granite from Trois Seigneurs define a relatively imprecise " i s o c h r o n " of 309__. 34 M a (initial 87Sr/

Table 2. Rb-Sr and O isotope data for sample suites (a) from the late granodiorite, (b) the deep biotite granite, (c) a regional suite of pelitic schists, and (d) a detailed sample traverse at a single outcro p locality. The sample localities are shown on Fig. 2

Sample 2or Locality number a Rb b Sr b Rb/Sr 87Rb/86Sr 8VSr/86Sr error (87Sr/86Sr)31o e 6180

a Late granodiorite

5 TS2783 131.8 185.9 0.709 2.053 0.719707 48 0.710650 + 9.64 5 TS2883 120.8 188.9 0.639 1.852 0.718810 42 0.710640 + 9.54 5 TS2983 126.2 179.3 0.704 2.039 0.719262 54 0.710267 + 9.87 5 TS3083 77.1 208.3 0.370 1.071 0.716072 50 0.711347 -

6 TS3183 141.0 180.0 0.783 2.269 0.720563 42 0.710553 + 9.86 6 TS3283 137.5 181.7 0.757 2.192 0.720175 50 0.710505 + 9.57 6 TS3383 138.6 166.8 0.831 2.408 0.721193 48 0.710570 + 9.88 6 TS3483 141.8 179.6 0.789 2.287 0.720631 38 0.710541 + 9.61

2 TS3583 120.7 211.9 0.570 1.650 0.717929 42 0.710650 + 10.60 2 TS3683 85.3 214.5 0.397 1.151 0.715637 50 0.710559 + 9.97 2 TS3783 1!3.9 218.6 0.521 1.509 0.717184 70 0.710527 + 10.04 2 TS3883 135.1 188.7 0.716 2.073 0.719264 48 0.710119 + 9.35

b Biotite granite

7 TS4083 147.7 161.3 0.916 2.654 0.726719 62 0.715010 + 11.28 7 TS4183 130.1 167.3 0.778 2.254 0.725030 70 0.715086 + 11.18 7 TS4283 156.8 173.2 0.905 2.624 0.725640 56 0.714064 + 10.26 7 TS4383 147.0 158.1 0.930 2.695 0.726644 54 0.714754 + 11.42

4 TS4483 111.4 180.0 0.619 1.793 0.723033 44 0.715123 + 11.31 4 TS44283~ 67.1 193.7 0.347 1.004 0.718951 14 0.714522 - 4 TS4583 77.5 224.3 0.346 1.000 0.714770 46 0.710358 + 9.63 4 TS4683 99.8 231.9 0.430 1.247 0.719283 28 0.713782 + 10.13

3 TS4883 58.9 276.1 0.213 0.617 0.709355 28 0.706633 + 8.84 3 TS4983 52.4 266.7 0.196 0.569 0.709254 44 0.706744 + 8.41

10 TS9283 236.2 146.1 1.617 4.691 0.736100 56 0.715405 + 10.10 10 TS9383 138.0 119.1 1.159 3.360 0.729419 44 0.714596 + 10.67

c Regional suite ofpelitic schists

11 TS4382a 245.8 83.0 2.960 8.604 0.754132 60 0.716174 + 11.47 12 TS8681 158.5 58.7 2.702 7.838 0.733883 76 0.699304 + 11.68 13 TS10581 121.3 71.2 1.704 4.942 0.734739 112 0.712936 + 11.21 14 TSl1882 167.9 152.8 1.099 3.186 0.727424 50 0.713368 + 12.27 15 TS12481 128.9 152.9 0.843 2.443 0.727064 58 0.716286 + 12.62 16 TS15582 137.8 98.5 1.399 4.058 0.735741 96 0.717838 + 12.29 17 TS24182 160.5 72.0 2.229 6.472 0.742776 140 0.714223 + 11.24

1 TP9 d 18.6 3188 0.006 0.017 0.708653 14 0.708578 -

d Detailed sample traverse at pelitic schist locality 224~ (6180 = + 12.07 ~)

22 TP184 88.7 136.1 0.651 1.887 0.724247 22 0.715922 22 TP185 73.0 134.5 0.543 1.573 0.723527 11 0.716588 22 TP186 106.4 102.4 1.038 3.011 0.730502 16 0.717217 22 ~ TP187al 115.8 56.6 2.045 5.935 0.739558 14 0.713376 22 ~ TP187a2 107.9 56.4 1.913 5.550 0.738075 18 0.713589 22 ~ TP187bl 104.5 49.4 2.116 6.142 0.739229 12 0.712134 22 ~ TP187b2 103.3 42.9 2.409 6.994 0.742491 11 0.711635 22 r TP187b3 108.9 45.5 2.896 6.956 0.743683 16 0.712996 22 r TPI87b5 114.3 48.2 2.871 6.884 0.742715 20 0.712343 22 ~ TP187b5 113.9 46.9 2.430 7.056 0.743182 15 0.712052 22 TP188 109.7 90.6 1.211 3.510 0.729574 15 0.714088

a Sample numbers as given in Wickharn (1987a) and Wickham and Taylor (1985) b Rb and Sr concentrations given in ppm ~ We have throughout this paper calculated initial 87Sr/86Sr and 143Nd/144Nd ratios at 310 Ma for all igneous and metamorphic lithologies where we have been concerned with modeling metamorphic or magrnatic mixing processes. This was done in spite of the fact that our best estimate of the age of regional metamorphism at Trois Seigneurs is about 330 Ma, based on the Rb-Sr age of hornblende from the biotite granite (330_+ 20 Ma) and a leucogranite zircon age of 338+_ 10 M a (Williams et al., in prep.). In any case, recalculating initial isotope ratios to 330 M a instead of 310 Ma would have a trivial influence on the calculated initial isotopic values, and would not significantly alter any of our conclusions

Metaearbonate sample ~180 value of sample SI35A from this same locality (Wickham and Taylor 1985) Seven small slices (cm-sized). These give an ' i soehron ' with an age of 261 ___ 52 M a and 'initial 8VSr/86Sr' of 0.7171 +48 , MSWD = 23 Regression of these 5 samples (using the average of the small slices) gives an age of 270___ 60 and an ' initial STSr/86Sr' of 0.7174 + 21,

MSWD 393 Marie xenolith in the biotite granite

0 . 7 4

0"73

0 -72

0.71

I I I I

Biotite Granite e /

Age 309 • 34M8

87Sr/a6Sri 0 -7149 • 13

MSWD 49 / e

I

69

0 sample exc luded f rom regression

o.7~ I I I I 1 2 3 4

97Rb/sesr

Fig. 5. Rb-Sr isochron diagram for the deep biotite granite showing the crude 309 • 34 Ma isochron defined by the seven high Rb/Sr samples (solid circles). Four low Rb/Sr data points fall well below the isochron (open circles) and have been excluded from the regression. These samples come from outcrops at Lapege and Illier (Fig. 2) at the deepest structural levels exposed in the biotite granite complex

86Sr=0.7149• M S W D = 4 9 , filled circles on Fig. 5, Table 2 b). Four low-Rb/Sr data-points (and a mafic xenolith, not plotted) lie below the " i sochron" and were excluded from this regression. The strontium isotopic compositions o f all these samples recalculated to 310 Ma are plotted against 87Rb/86Sr on Fig. 6, which also shows their whole-rock 5180 values. The 7 higher-Rb/Sr samples define a horizontal line on Fig. 6 indicating a degree of SVSr/S6Sr homogeneity, whereas the low Rb/Sr samples define an approximately linear trajectory with a positive slope, suggesting mixing between a low-SVSr/S6Sr end-member and a high-STSr/86Sr end-member.

Let us assume that the high-87Sr end-member has 5 1 s o = + 1 0 . 5 and lies on the well-defined, horizontal 0.7149 line, and that the low-SVSr end-member has 5~80= +7.5 to +8 .0 and S7Sr/86Sr=0.7025 to 0,7050, and lies on the extrapolated regression line drawn through the 4 low-Rb/Sr data points. A simple mixing calculation then produces a close match with the measured isotopic compositions, as shown on Fig. 6. The lowJSO, low-87Sr end- member is constrained by the mixing relationships to have 5tsO in the range +7.5 to +8 .0 because its (878r/86Sr)31o ratio is unlikely to be lower than 0.7025-0.705.

Another way of looking at this same data-set (Fig. 7) shows that the whole-rock 51SO values exhibit a positive correlation with the 87Sr/86Sr values at 310 Ma. This kind of correlation is very commonly observed in granitic magmas containing both a crustal and a mantle component (e.g. see Taylor and Silver 1978; Taylor 1980; Fleck and Criss 1985; Taylor and Sheppard 1986). The samples with highest S7Sr/86Sr and 51sO on Fig. 6 were mostly collected from relatively high structural levels within the pluton (see Fig. 2). The isotopic compositions within this part o f the pluton overlap those o f the adjacent Lower Paleozoic metapelitic rocks (Fig. 7), and in the field these pelites can be

405

0 . 7 4

0 . 7 3

87Sr 86St

0.72

0.71, +8.4 (~ / t

0 . 7 " ' I

I I I I

+10.{

+,1.3:3/./ I

"

+8.8 za +9.4 calc

I I I 2 3 4 5

87Rb/86Sr

Fig. 6. Rb-Sr isochron diagram showing the same biotite granite data shown in Fig. 5 (open circles), but also plotting the calculated isotopic compositions that these samples would have had at 310 Ma (filled circles). Measured 6180 values are shown above each of the measured data points. The 310 Ma values of the seven samples used in the regression on Fig. 5 plot close to the horizontal line at 87Sr/SrSr=0.7149, indicating a fairly homogeneous Sr isotopic composition throughout this part of the complex at the time of emplacement. The other four samples define a linear (mixing?) trajectory, apparently involving a low-gVSr, low-St80 end- member and a high-SVSr, highJ80 end-member. The 5t80 values of mixtures also vary approximately linearly along this line, and by extrapolating to the ordinate (S7Rb/86Sr=0) and to the 87Sr/ S6Sr=0.7149 line, we can place tight constraints on the isotopic compositions of the end-members generating these mixing trends. Using end-member 6180 values of + 10.5 and either + 7.5 or + 8.0, the calculated 5180 of each of the four samples (shown in italics) can be seen to closely match the measured 61sO value. The lowJ80 end-member certainly cannot have a 5180 any lower than + 7.0, and it probably had a value somewhere between + 7.5 and + 8.0, as illustrated by the shaded field on the diagram

observed to grade into the biotite granite through a zone of pelitic migmatites.

The lowJSO, low-STSr/86Sr samples on Fig. 7 are the same 4 samples with low Rb/Sr ratios from Figs. 5 and 6. These four samples include: (1) two samples of hornblende diorite that crop out near Lapege (Fig. 2; also see Wickham and Taylor 1985); and (2) two out of three samples from a nearby outcrop of biotite-cordierite granodiorite at Illier, which is rich in mafic xenoliths (see Wickham 1987a, Fig. 6b). Both of these outcrops are among the deepest structural levels exposed in the biotite granite complex, which also makes them almost the deepest samples available from the entire Trois Seigneurs Massif. The correlations among Rb/Sr, 87Sr/S6Sr, 180/160, and structural level are a result o f a systematic compositional heterogeneity within the biotite granite body; these effects are readily explained by incomplete homogenization (see Wickham 1987a; 1987b) between a mantle- or lower crust-derived, Hercynian-age (diofitic?) magma and the local metasedimentary rocks that form the source for the main part o f this pluton (Wickham and Taylor 1985).

However, if we examine Figs. 6 and 7 in more detail , it is seen that at least two kinds of upper crustal melts

406

0 7 4 5

0.740

0Y55

OY50

o ~, 0.725

o'-:, I.o %

0.715

OY05

I I k

�9 BIOTITE GRANITE included in isochron fit

o B-G excluded from isochron fit

�9 PELITIC SCHISTS x MODEL PEL[TIC

SCHISTS

/ -

I

1 Ix [ I I I

T I

S TROIS SEIGNEURS I i%DEL PELITES

T0't 510 M4 l

I

I

[

REGIONAL SUITE

1 I I I I I I I I 0"7007 + 8 + 9 +10 +11 +12 +15 +14 +15 +16

~tSO Fig. 7. Plot of 87Sr/S6Sr vs. 6180, showing the biotite granite (B-G) data from Fig. 6 together with data for the regional suite of Trois Seigneurs (T-S) pelitic schists and the regional suite of shales and phyllites (Tables 2 and 5). The vertical and horizontal brackets within the shale field indicate the range of isotopic compositions at the five outcrop localities that were studied, with the centers of the brackets indicating the average value at each locality. Also shown are: (a) a calculated field (crosses enclosed by the dashed envelope) of model pelitic schists representing the 87Sr/86Sr values that these samples would have attained at 310 Ma if they had evolved from sediments deposited at 560 Ma with an initial 87Sr/ 865r ratio of 0.707, assuming that they behaved as closed systems with respect to Rb and Sr (note that if they had also behaved as closed systems with respect to oxygen, the 6180 values of these model schists would also have been much higher prior to metamorphism than is shown in the figure, namely about +15); and (b) the calculated range (vertical dashed lines) of model 87Sr/S6Sr ratios that the regional suite of shales and phyllites would have attained at 310 Ma if they had evolved from sediments deposited at 450 Ma with an initial S7Sr/86Sr = 0.707, again assuming closed-system be- havior. The schist data-point with the vertical brackets represents the group of intermediate-grade samples from locality 22 that was studied in some detail (Table 2). The measured T-S pelitic schists define a fairly tight cluster of points, with much more homogeneous 87Sr/86Sr values at 310 Ma than the range of values shown by the two groups of model pelites. This suggests that, like the 6180 values, the SVSr/StSr ratios were changed and partially homogenized in the metasedirnents during metamorphism, The mixing line for the biotite granite suite from Fig. 6 is also shown (it transforms to the curved triangular stippled region on this diagram). The homogenized Trois Seigneurs pelites clearly constitute a plausible

18 87 high- O, high- Sr end-member source material for the main body of the biotite granite

may be required to explain the well-defined biotite-granite mixing trend (albeit of only 4 points). Mixing among the following three distinct magmatic components is the sim- plest way to explain these trends: (1) A dioritic(?) magma with an initial 87Sr/S6Sr of about 0.703-0.705 and 6180 of +7.5 to + 8.0. (2) An anatectic, peraluminous granitic magma derived from the Lower Paleozoic pelitic l i tho lo-

gies; this magma-type appears to have variable S7Sr/SGSr 0.714-0.716 and 6180 ~ + 10 to + 12. (3) A more homogeneous granitic/granodioritic magma with S7Sr/S6Sr~0.715 and 6180 ~ + 10.5. Because of its high STSr/S6Sr ratio and peraluminous chemical composition, this third magma type must also be derived from a pelitic crustal source, but one with a slightly lower 6180 value and lower Rb/Sr ratio (and possibly other compositional differences as well) compared to the Trois Seigneurs pelitic schists. This could represent more homogeneous material from a somewhat deeper structural level, or it could represent metasediments that were even more depleted in 1 s O by metamorphic-hydrothermal fluids than the presently exposed pelites at Trois Seig- neurs. Convective overturn and mixing within the biotite- granite magma chamber might also have played a role in the greater degree of isotopic homogeneity of this component.

Magma type (2), which makes up the main part of the biotite granite pluton, has some other interesting features in common with the pelitic schists. The samples of schist and granite with high STRb/86Sr all have systematically lower 6180 and lower ppm Sr than the low-SVRb/86Sr samples of schist and granite (Figs. 6 and 8, Table 2). The spread in 6180 amounts to only a little more than one per mil, but the correlations are clear-cut. This suggests that, at least for the inhomogeneous end-member, such sub- fie isotopic signatures in the protolith may survive the melting episode, in spite of an overall depletion of 180 in the biotite granite relative to the pelite precursor (618 0 = + 10.1 to +11.4 in the granite compared to a range of +10.6 to + 12.7 in 15 outcrop localities of pelitic schist; see Ta- ble 3).

On the basis o f our present sampling of the biotite granite, it is difficult to constrain the relative proportions o f the three magma-types described above. Evidence for the existence of (1) and mixing between (1) and (3) is so far only documented in the outcrops at Illier and Lapege at the very deepest structural levels that are exposed. Our present Sr and O data, coupled with petrological and compositional similarities between the upper and central part o f the biotite granite and the overlying metasediments (including the striking similarity of refractory xenolith types), imply that this main part of the pluton is dominated by component (2) (Wickham 1987 a). Component (3) may represent the extra magmatic component suggested by Wickham (1987a) to be responsible for small discrepancies in C a t and A12Oa content between some biotite granite samples and the local Paleozoic pelitic metasediments. Detailed geochemical mapping of the biotite granite complex may in the future provide valuable insights into the kinematics of these complex mixing processes between the distinct batches of magma.

Pelitic schists

Six out of seven samples of metapelitic rocks from metamorphic grades above the 'andalusite in ' isograd define a poor Rb-Sr " i sochron" equivalent to an age of 314+80 Ma, initial 87Sr/86Sr=0.7151+45, M S W D = 5 0 0 (Fig. 8, Table 2). The other sample was not included in the regression because it gives an absurd Paleozoic STSr/ StSr ratio lower than 0.700 (Table 2). The estimated 87Sr/ StSr ratio in these other six metasediments is essentially identical to the initial 87Sr/S6Sr value derived for the main

Trois Seigneurs Regional Peli/es

(87Sr/86Sr)i = 0.71514 -+ 79 (510 Ma)

0 ~ 6

87Sr

86Sr

0.75

0 .74

0.73

I I J [ I I

"Age" 514 -+ 80 Ma (87Sr/86Sr)i = 0.71506 + 446

MSWD = 499 ~ _

J j x +11.2

+12.3 J _ +11. 7 • + ,, 2 * 2 "

~ 1 ~ x +12.3

0 . 7 2 I I I I I I 5 4 5 6 7 8 9

87Rb/86Sr

Fig. 8. Rb-Sr isochron diagram for pelites collected from various localities within the Trois Seigneurs metamorphic sequence (see also Fig. 2 and Table 2c). Six of the pelites define a crude linear array representing an age of 314_+80 Ma. A seventh data point (TS8681) is suspect and has been rejected because it gives an absurdly low initial 87Sr/86Sr ratio. The data indicate regional homogenization of 87Sr/S6Sr ratios in the pelites during the Hercynian metamorphism. 6180 values for each sample are shown in italics, indicating that the more Rb-rich samples tend to be slightly lower in 180; this might possibly be attributed to these samples being more micaceous and less quartz-rich, as quartz-rich assemblages ought to be richer in 180, either because of equilibration, or because they are less susceptible to hydrothcrmal exchange. This pat- tern is also seen in the biotite granite (see Fig. 6), adding further support to the hypothesis that the pelites are a major source component of the biotite granite

407

body of the deep bioti te granite (Fig. 5). This similarity is nicely i l lustrated in Fig. 7, which shows that the pelites provide an obvious h ighJSO, high-87Sr/S6Sr source material for the bioti te granite.

One other locali ty of pelitic schist me tamorphosed at a somewhat lower grade was studied in detail ; this outcrop locality lies about 100 m from the ' anda lus i te - in ' isograd, in the vicinity of t h e ' b iot i te- in ' isograd (locality 22, Fig. 2). Five separate samples were analyzed for S7Sr/S6Sr, and one of these samples was cut into cm-sized slices, giving 7 micro- samples that were analyzed individually (Table 2). This composite suite of samples independent ly defines a crude 270 M a linear a r ray on a Rb-Sr isochron d iagram (Table 2), and the 180/160 and 878r/S6Sr ratios in this outcrop are very similar to the values in the higher-grade andalusite- and sil l imanite-grade schists (Fig. 7; Table 3).

The uniformity of S7Sr/S6Sr in all these pelitic schists at about 310 M a is interpreted as indicating that substantial Sr isotope homogenizat ion took place within these metasediments during the various stages of the Hercynian orogeny. The t ransformat ion of the schists into the bioti te granite was responsible for still further homogenizat ion of SVSr/86Sr ratios, as well as a further slight deplet ion in aso, as described above.

The pelitic and psammit ic sediments are p robab ly o f Cambr ian or Ordovician age (Zwart 1979). Thus, major enrichments in radiogenic S7Sr would have been generated in the 15~300 M a time interval between sedimentation- diagenesis and Hercynian metamorphism, and this would be expected to produce a very heterogeneous set of S7Sr/ S6Sr ratios in the sedimentary rocks just pr ior to metamorphism. F o r example, if the initial STSr/86Sr rat io of the metasedimentary rocks was 0.707 at a plausible depositional age of 560 Ma, the present range of Rb/Sr compositions exhibited by the regional suite of pelites would have resulted in S7Sr/S6Sr rat ios between 0.716 and 0.738 (Fig. 7). The small-slice psammite samples (see below)

Table 3. Oxygen and strontium isotope data (average values) at various outcrop localities of pelitic schist, phyllite, and shale in the Trois Seigneurs massif, arranged into four groups as a function of grade of metamorphism"

Parameter studied b Original Low grade Intermediate grade High grade sedimentary rock (shales and phyllites) (vicinity of the (andalusite and (model values)d biotite isograd) sillimanite zones)

Mean 51sO value ? +14.54-0.7 (8) +12.1 4-0.9 (5) + 11.5_+0.6 (16) Range of SXsO ? +13.3 to +16.0 +10.5 to +13.3 +10.6 c to +12.7

Mean 87Sr/86Sr value 0.7202_+ 0.0053 (5) 0.7130-t- 0.0022 (5) 0.7153 (1) 0.7151 4-0.0016 (6) e Range of 87Sr/86Sr 0.7133 to 0.7326 0.7089 to 0.7166 - 0.7129 to 0.7178 Average Sr content 89 ppm (5) 89 ppm (5) 102 ppm (1) 98 ppm (7) Average Rb content 138 ppm (5) 138 ppm (5) 98 ppm (1) 160 ppm (7) Average Rb/Sr ratio 1.55 1.55 0.96 1.62

a Data are from the present study and from Wickham and Taylor (1985). The tabulated 87Sr/S6Sr, Sr, and Rb data represent the mean values for the various localities, using a single average value for each of these outcrop localities where multiple samples were studied b The 4- indicates average deviation from the mean value, with the number of analyzed localities given in parentheses. All 878r/86Sr values are calculated at 310 Ma directly from the analyses, except for the sedimentary rock (model) values, which are calculated assuming original deposition at 450 Ma ~ One sample with an anomalously low 5180 value of + 8.8 is not included (Wickham and Taylor 1985) a The 5180 values are unknown, but the low-grade model strontium isotope values of the sedimentary rocks at 310 Ma can be calculated, as discussed in the text, by assuming an initial STSr/86Sr =0.707 at 450 Ma and the analyzed Rb/Sr ratio ~ Excludes locality 12, which exhibits an absurdly low (87Sr/86Sr)31o value (see Table 2)

408

would have even lower values, an average s 7Sr/S6Sr of 0.710 at 310 Ma for TP2 and 0.716 for TP3 (values not plotted 0.73o on Fig. 7). eTsr

The original sedimentary compositional layering in the SSSr mica-schists, psammites, and carbonates at Trois Seigneurs is very well preserved (albeit substantially deformed). Inas- 0.725 much as the concentrations of trace elements such as Rb and Sr are mainly controlled by the original protoliths and by the mineral assemblages, we would in general expect that these element concentrations would not be drastically 0.720 changed during regional metamorphism. For example, original illite-rich and carbonate-poor layers would all be expected to exhibit relatively high Rb/Sr ratios even at higher metamorphic grades. Although it is probable that the Rb/Sr o.715 ratios in the metasedimentary rocks were somewhat modified during the several stages of Hercynian metamorphic- hydrothermal events, it is inconceivable that these ratios would have all been changed by exactly the right amounts to accurately mimic the effects of a 310 Ma isotopic homogenization event. Also, note that the average Sr and Rb contents of the low-grade shales and phyllites are very similar to those of the high-grade pelitic schists (Table 3); their average Rb/Sr ratios are 1.55 and 1.62, respectively, suggesting little or no change in Rb/Sr, in spite of the fact that the average f i lso was lowered from +14.5 to +11.5 (Table 3).

Considerable homogenization of 87Sr/86Sr has clearly occurred in the metasediments, as illustrated in Fig. 7. This 0.730 figure shows the aforementioned calculated values for the Trois Seigneurs model pelitic schists at 310 Ma, assuming eTsr a Cambrian depositional age of 560 Ma, an initial 878r/86Sr 86Sr at 560 Ma of 0.707, and that the 87Rb/S6Sr ratios in these

0.725 samples remained essentially unchanged since deposition. All but one of these samples should have attained much higher 87Sr/S6Sr ratios at 310 Ma than are in fact observed, and three samples have S7Sr/86Sr ratios at 310 Ma that are drastically lower than the calculated "mode l " values 0.720 at 310 Ma. This implies that not only were the SVSr/S6Sr ratios in the metasediments partially homogenized during Hercynian metamorphism (generating the crude metasediment isochron of Fig. 8 and the fairly tight cluster of sample 0.715 points of Fig. 7), but some of these ratios were also substantially lowered as a result of mixing with relatively unradiogenic strontium (87Sr/86Sr < 0.715). Thus, all of the pelites attained fairly uniform S7Sr/S6Sr ratios at 310 Ma, and 0.71c some of these ratios were much less radiogenic than they should have been, given the initial 87Rb/S6Sr of each sample.

A possible low-87Sr/S6Sr source might be limestones in the Lower Paleozoic sequences, which would typically have STSr/86Sr ratios of ~ 0.708 (e.g. sample TP9, Table 2c). Al- ternatively the less radiogenic strontium could have been introduced by the fluids themselves; the marine formation waters (brines) that may have circulated through these rocks typically contain several hundred ppm strontium (Carpenter et al. 1974; Land 1987) and they probably have 87Sr/S6Sr lower than 0.715. In this respect, note that the large quantity of fluid that flushed the metasedimentary sequences during prograde metamorphism, and which was responsible for homogenizing 180/160 and STSr/S6Sr ratios in the metasediments, is thought on the basis of D/H measurements to have been ultimately of marine origin (Wick- ham and Taylor 1985).

P s a m m i t e s l i ce d a t a - T P 3 and T P I O i I i

TP3 �9

Age = 268 + 4 2 Ma [R = 0.712166+1396

. .d Age = 284 -+ 7 Me e , ~ " IR = 0.711343+135 MSWD = 3.58

j / , ,

/J- 0.710 I I I

1 2 3 e 7 R b / e 6 S r

Fig. 9. Rb-Sr isochron diagram for small slice samples TP3 and TPI0. The data again indicate Hercynian-age metamorphic homogenization of 875r/86Sr ratios in the metasediments. In this case the initial ratios are lower than in the regional pelite samples from Fig. 8, because of the influence of an adjacent low-STSr carbonate layer

Combined p s a m m i t e s l i ce da ta i

Age = 3 0 3 + 14Ma

IR =.710813+-231

MSWD = 9 5 . 2 5

1 2 e 7 R b / e e s r

TP2 0 TP3 �9 TP8 �9 TPIO TP11 �9 TP12 Z~

I 3

Fig. 10. Rb-Sr isochron diagram for all the small slice samples. The data are consistent with homogenization of 87Sr/86Sr over distances of several meters during the Hercynian metamorphism. As in Fig. 9, the initial 87Sr/S6Sr ratio is anomalously low because of the influence of the adjacent carbonate layer. Away from the carbonates, typical initial 87Sr/86Sr values in the metapelites are about 0.715, as illustrated in Fig. 8

Psammitic schists

Six samples of psammite have been studied in more detail (see Tables 4a and b). The samples were collected between 2.5 and 6.5 m above (i.e. on the lower grade side of) a 17 meter-thick metacarbonate horizon (locality No. 1 on Fig. 2). Centimeter slices were cut from each specimen, and Rb-Sr data from these small-slice samples are listed in Tab-

409

Table 4. Rb-Sr and O isotope data for the small-slice psammite samples (a), together with regressions calculated for each group of samples (b)

2o- Sample" Rb Sr Rb/Sr S7Rb/86Sr 87Sr/86Sr error (87Sr/86Sr)31o ~180

a Psammite small slice samples

TP2a 99.9 446.3 0.224 0.648 0.713188 68 0.710329 - TP2b 116.0 485.9 0.239 0.691 0.713547 152 0.710499 - TP2c 124.1 508.9 0.244 0.706 0.713366 66 0.710251 + 12.37 TP2d 116.2 434.5 0.267 0.774 0.713761 76 0.710346 - TP2e 112.4 438.6 0.256 0.742 0.713766 80 0.710493 - TP2f 86.8 484.6 0.179 0.518 0.712977 58 0.710692 -

TP3a2 129.6 137.5 0.943 2.731 0.722637 48 0.710589 - TP3a3 132.6 134.4 0.987 2.859 0.723237 60 0.710624 - TP3a4 125.6 222.9 0.563 1.632 0.718611 52 0.711411 + 12.24 TP3a5 132.4 177.4 0.746 2.162 0.720502 66 0.710964 - TP3a6 133.2 126.3 1.055 3.056 0.723409 72 0.709927 - TP3b2 130.4 144.9 0.900 2.607 0.722698 68 0.711197 - TP3b3 130.6 173.9 0.751 2.175 0.720881 84 0.711286 - TP3b4 130.3 179.9 0.724 2.098 0.719746 72 0.710490 - TP3b5 131.5 146.5 0.898 2.601 0.721939 52 0.710464 - TP3b6 131.5 168.6 0.780 2.258 0.720149 64 0.710187 -

TP8a 93.8 304.8 0.308 0.891 0.714715 28 0.710784 - TP8av 84.8 325.9 0.260 0.753 0.714151 24 0.710829 + 12.39 TP8b 71.1 339.5 0.209 0.606 0.713716 36 0.711043 + 12.56 TP8c 55.4 329.6 0.168 0.487 0.713046 40 0.710898 + 12.70

TP10a 90.9 346.9 0.262 0.759 0.714417 30 0.711069 +12.68 TPI0b 128.5 172.3 0.746 2.161 0.720080 18 0.710546 +12.77 TP10c 136.8 214.0 0.639 1.851 0.718637 134 0.710471 + 12.71 TP10d 130.5 138.4 0.944 2.734 0.722461 68 0.710399 + 12.80

TPl la 133.2 295.3 0.451 1.306 0.715696 18 0.709934 + 12.85 TPl lb 129.5 160.0 0.810 2.346 0.720194 28 0.709844 + 13.18

TPI2a 121.0 450.5 0.269 0.778 0.713954 50 0.710522 + 12.82 TP12b 107.2 453.8 0.236 0.684 0.713628 40 0.710610 + 12.59

Sample Number of slices Age (Ma) Initial 87Sr/86Sr MSWD b

b Regressions on small slice samples

TP2 6 217___82 0.7113___ 8 12 TP3 10 274 • 43 0.7119 + 14 57 TP10 4 284_+ 8 0.7113_+ 2 4 TP8 4 278_+47 0.7112___ 5 22

(all data including TPll , TP12) 28 303 • 14 0.7108_+ 2 95

a All samples are from locality 1 b M S W D = FU/(N-2) where FU is the weighted sum of the squares of the residuals and N is the number of data points (see McIntyre et al. 1966)

le 4a. The groups of slices from each sample give regressions corresponding to ages between 2 1 7 _ 8 2 M a and 284-t-8 Ma. Two of these (from TP3 and TP10) are illustrated on an isochron diagram in Fig. 9. Combined, the Rb-Sr data on 28 individual cm-sized slices from all six specimens give a regression equivalent to an age of 3 0 3 + 1 4 M a , initial 87Sr/86Sr=0.7108_2, M S W D = 9 5 (Fig. 10). These Rb-Sr systematics reflect post-Hercynian perturbations on a cm-scale but nevertheless imply that Sr isotopes were substantially homogenized over distances of several meters during the Hercynian metamorphism.

The inferred SVSr/S6Sr ratios in the slice samples at 310 Ma are close to 0.710-0.712 (Table 4; Figs. 9 and 10); these are even lower than the values of about 0.715 given by the regional suite of pelitic schists (Figs. 7 and 8). Such

effects are most readily explained by the influence of the immediately adjacent carbonate layer. The carbonates (with very low Rb/Sr ratios) have 87Sr/86Sr of 0.708 to 0.709, much lower than in the Rb-rich pelites, and this value has changed very little since deposition. This is illustrated by sample TP9 (Table 2c) from the metacarbonate layer adjacent to the high-grade side of the meter-spaced psammite suite TP2-TP12. Pelites and psammites out to at least 25 meters from the upper margin of this metacarbonate layer show markedly reduced 87Sr/86Sr ratios at 310 Ma, compared with typical values of about 0.715 elsewhere in the metapelites. Strontium isotope profiles across and ad ja - cent to these and other carbonate units in the area will be discussed in detail in a future publication (Bickle et al., in prep.) and are reported by Chapman et al. (1987).

410

Table 5. Rb-Sr and O isotope data for the samples of low-grade shale and phyllite from 5 localities (a, b, c, d, e), together with regressions calculated for each group of small-slice samples (/0 and for each outcrop locality (g). Sample localities are indicated on Fig. 2

2~r Model ~ Sample Rb Sr Rb/Sr 87Rb/86Sr 87Sr/86Sr error (87Sr/86Sr)31o (87Sr/a6Sr)31o

a Locality 18 (6180 whole-rock= +14.92+0.08, +14.20, and +14.34)

TP108al" 118,7 38.8 3.055 8.878 0.751337 18 0.712169 0.724746 TP108a2" 120.1 39.9 3.011 8.749 0.750911 26 0.712311 0.724488 TP108bl a 148.4 58.6 2.531 7.352 0.746516 13 0.714080 0.721695 TP108b2 a 148.5 58.6 2.527 7.340 0.746707 16 0.714327 0.721670 TP108b3" 144,7 56.7 2.550 7.407 0.747005 11 0.714328 0.721805 TPl10 168,5 59.0 2.771 6.049 0.747232 22 0.711722 0.723088 TPl13 197,4 47.2 4.179 12.159 0.765348 15 0.711708 0.731302 TP114 183,9 59.4 3.093 8.990 0.755581 12 0.715919 0.724969 T P l l 5 146,5 42.8 3.425 9.951 0.751301 19 0.707398 0.726890 TPI16 b 88.5 274.4 0.322 0.934 0.716249 13 0.712129 0.708866 TPl17 b 96.5 228.7 0.422 1.221 0.717221 20 0.711833 0.709441

b Locality 19 (6180 whole-rock= + 14.40+_0.16 and + 16.03)

TPl19 195.9 63.1 3.105 9.023 0.752427 18 0.712619 0.725035 TP120 165.3 45.7 3.614 10.507 0.757809 14 0.711456 0.728001 TP121 150.6 75.5 1.993 5.784 0.739606 19 0.714089 0.718561 TP 122al ~ 193.4 162.4 1.191 3.453 0.727707 9 0.712472 0.713902 TP122a2" 195.7 164.5 1.189 3.448 0.727822 17 0.712612 0.713891 TP122bl a 187.1 169.8 1.102 3.194 0.726812 30 0.712719 0.713385 TP122b2a 183.3 175.8 1.043 3.023 0.726020 11 0.712684 0.713042 TP122cl a 193.1 167.8 1.150 3.335 0.727290 14 0.712578 0.713665 TP122c2 a 191.3 160.8 1.190 3.449 0.727806 15 0.712591 0.713893 TP123 73.3 100.8 0.727 2.105 0.720641 11 0.711353 0.711208

c Locality 20 (6180 whole-rock= + 13.53++_0.05)

TP126al a 142.6 113.1 1.260 3.656 0.732844 16 0.716716 0.714307 TP126a2 a 140.0 111.5 1.256 3.643 0.732901 14 0.716829 0.714281 TP126bl a 139.1 114.2 1.219 3.534 0.732445 12 0.716852 0.714064 TP126b2 a 139.6 114.6 1.218 3.532 0.732573 13 0.716990 0.714060 TP126cl a 139.5 111.6 1.250 3.625 0.732803 60 0.716813 0.714244 TP126c2" 139.1 112.2 1.240 3.596 0.732884 82 0.717018 0.714188 TP127 147.7 114.8 1.286 3.730 0.733098 12 0.716641 0.714456 TP128 137.1 104.7 1.310 3.800 0.733353 20 0.716588 0.714596 TP129 134.2 90.7 1.479 4.291 0.735254 21 0.716325 0.715576

d Locality 21 (6180 whole-rock= + 13.26)

TP 130 138.8 29.0 4.793 13.950 0.768684 17 0.707139 0.734883 TP131 125.0 24.9 5.024 14.627 0.772592 24 0.708061 0.736236 TP133 128.7 38.2 3.371 9.800 0.754827 20 0.711594 0.726587

e Locality 23 (6180 whole-rock= + 15.02)

TP190 101.4 104.5 0.971 2.815 0.727547 12 0.715129 0.712626 TP191 140.9 115.7 1.218 3.532 0.729974 11 0.714394 0.714059

Locality Number of Age (Ma) Initial 8VSr/86Sr MSWD samples

f Regressions on small slice samples

18 (TP108) 5 214_+ 14 0.7243_+ 15 1.5 19 (TP122) 6 287_ 22 0.7137 +_ 10 0.8 20 (TP126) 6 269 _+ 123 0.7190 _+ 63 2.2

g Regressions at each locality, using the average for the small-sliced samples

18 7 313_+ 22 0.7120_+ 7 187 19 5 329_ 30 0.7111+16 110 20 4 260+_ 24 0.7193_+13 1.3 18, 19, 20, 21, 23 21 d 336_+ 27 0.7120_+12 594

Small slices (cm-sized) b Carbonate-rich samples c Model 8VSr/86Sr values calculated at 310 Ma, assuming closed system with respect to Rb and Sr, and original sediment deposition at 450 M a with 87sr/a6Sr ini t ial=0.7070 d Includes the 3 data points excluded on Fig. 11

4 1 1

~ 8 0 0

~o75o

, ~ ~ i I ' 1 l 1 L

Low grode sheles end phy l l i tes

Age 280 _+ 2 4 M s 8-"Sr/SeSri 0.7'I67 + 18

MSWD 289 / . / . ' j o

/ e

x overoge for cm-sized slices

o sample excluded from regression 0 . 7 0 0 L ~ J ~ I ~ ~ J ~ I ~ ~ ~

0 5 10 15

87Rb / /86Sr

Fig. 11. Rb-Sr isochron diagram for the regional suite of Lower Paleozoic shales and phyllites (18 samples). The average values of the cm-sized slices from three specimens are indicated by the crosses (see Table 5). Three datapoints were excluded from this regression (open circles)

Shales' and phytlites

Five localities at low metamorphic grade were investigated as a control on strontium isotope systematics in the possible precursors to the higher grade schists (18, 19, 20, 21, and 23, see Fig. 2). Some of these shale and phyllite samples are from the same outcrops studied by Wickham and Tay- lor (1985). Several samples were collected from each outcrop locality, and at three of the localities, selected specimens were cut into cm-scale slices (TP 108, TP 122, and TP 126). The analytical data are given in Table 5.

The Rb-Sr isotopic data from the shales and phyllites define linear arrays on Rb-Sr isochron diagrams, on a range of scales from the cm-slice samples to the entire suite of low-grade samples. I f these arrays are interpreted as isochrons, they imply an age of homogenization between 250 Ma and 350 Ma, which is within error identical to the age of the main Hercynian metamorphic event (Table 5, Fig. 11). As was the case with the higher grade schists, the scatter on these regressions implies that strontium isotopic homogenization was by no means complete, except on the cm-slice scale. If we exclude three samples with very low Rb/Sr ratios, two of which are very carbonate-rich, and if the slice data from each of the large samples is averaged, an ' isochron' of 2 8 0 + 2 4 M a is obtained (Fig. 11). This gives a SVSr/S6Sr at the time of homogenization of 0.7167 4-18, similar to the range of values displayed by the higher-grade pelitic schists (Fig. 7). I f the three data-points are not excluded, we obtain a much poorer fit for the samples with high STRb/arSr ratios, but we still get a Hercynian age of 336 4-27 Ma, together with a somewhat lower 'initial' STSr/S6Sr of 0.7120 + 12 (Table 5).

The overall range of (S7Sr/S6Sr)31o and 6180 for each locality is indicated on Fig. 7, which shows that the shales and phyllites extend to somewhat lower S7Sr/86Sr ratios than the regional suite of pelitic schists, as well as having distinctly higher 3180 values than the schists (Table 3). We have also calculated model 87Sr/86Sr values for these samples at 310 Ma, assuming a depositional age of 450 Ma

rather than the somewhat more realistic 560 Ma sedimentary model-age utilized above for the regional suite of pelitic schists. In these latter calculations we wished to be as con- servative as possible in our estimates of the original 87Sr/ S6Sr ratios of these sediments; because Upper Ordovician fossils (Thiebaut 1956) occur at locality 18 (Fig. 2), which is close to the highest stratigraphic level in the area, the age of the sedimentary pile in the Trois Seigneurs massif is constrained to be older than 420 to 450 Ma.

It is unlikely that the initial S7Sr/S6Sr of any of these sediments would have been lower than 0.707, because Ordo- vician sea water itself was about 0.708 to 0.709 (Burke et al. 1982). Therefore, combining this value with the very conser- vative 450 Ma age, we may calculate minimum initial STSr/ S6Sr ratios for these sediments (Table 5, Fig. 7), assuming that they have retained their original depositional Rb/Sr ratios. At three localities the model values of individual samples extend up as high as 0.728-0.736, well above the measured (87Sr/SrSr)31o values for these samples, while at two of the outcrop localities the model 310 Ma values are slightly lower than the measured (SVSr/S6Sr)31o values (Fig. 7). Clearly, there have been major SVSr/S6Sr changes between the time of deposition of these sediments and the final stages of Hercynian metamorphism.

We interpret the isotopic data from the shales and phyllites as indicating that a profound homogenization of SVSr/ 868r commenced in these rocks at the very lowest grades of prograde metamorphism during the Hercynian orogeny. The extent of isotopic homogenization is well illustrated by comparing the wide range of model (s VSr/86Sr)31 o values with the narrow range of 310 Ma values calculated from the measured SVSr/S6Sr values (Table 5). Subsequent to the postulated low-grade homogenization event, further homogenization is recorded in the rocks of the Trois Seigneurs Massif during prograde metamorphism at grades at and above the biotite isograd. This additional degree of 878r/ S6Sr homogenization at the higher grades was accompanied by appreciable lowering of 3180 in these rocks (Fig. 13; Table 3). It is therefore logical to infer that the 87Sr/S6Sr effects are also the result of fluid-rock interaction during metamorphism (Wickham and Taylor 1985). However, even at the highest grades, isotopic homogenization was not perfectly completed, as evidenced by the fairly steep STSr/S6Sr gradients preserved at the psammite/carbonate contacts.

An important unanswered question is whether the Her- cynian event merely homogenized strontium isotopic compositions within the pile of metasediments or whether external input of strontium caused bulk shifts in strontium isotopic composition. Weighted for Sr contents, the mean model (STSr/SrSr)31o of 0.7159-1-I0 compares with a post- homogenization mean measured (S7Sr/S6Sr)31o of 0.7133_+ 13 (21 whole-rock samples). In addition, the model 87Sr/S6Sr estimate varies rapidly with the estimated deposition age; for example, the model estimates above would be reduced from 0.7159 to 0.7130 if the deposition age were reduced by 30 Ma to 420 Ma. I f we could be certain that the true mean depositional age of the sedimentary section was somewhat older (e.g. 500-580 Ma, see Zwart 1979), then there would have to have been some external input of strontium into the pelites. However, even in this case the necessary Sr probably could all be derived from limestones elsewhere in the sedimentary section. Thus, with the available data, and particularly with the uncertainty over

412

Table 6. Rb-Sr ages of whole-rock samples and mineral separates

Locality Sample number Rb a Sr ~ Rb/Sr 87Rb/a6Sr 8VSr/S6Sr + / - Age +/-- (Ma)

Late Granodiorite

2 TS3583 whole-rock 120.7 211.9 0.570 1.650 0.717929 42 2 TS3583 hornblende 7.3 62.1 0,117 0.339 0.715152 76 149 5.0

Biotite Granite

3 TS4883 whole-rock 58.9 276.1 0.213 0.617 0.709355 28 3 TS4883 plagioclase 4.3 626.0 0.007 0.020 0.707881 76 174 10 3 TS4883 K-feldspar 97.3 47.3 2.058 5.96 0.710384 68 14 1.0 3 TS4883 biotite 344.6 7.1 48,6 143.8 0.945518 104 116 1.2 3 TS4883 hornblende 1.1 3.2 0.333 0.964 0.710984 82 330 20

3 TS4983 whole-rock 52.4 266.7 0.196 0.569 0.709254 44 3 TS4983 plagioclase 7.3 651.0 0.011 0.033 0.707710 52 203 9.1 3 TS4983 biotite 248.3 6.3 39.22 115.6 0.897794 " 332 115 1.2 3 TS4983 K-feldspar 3.7 94.9 0.039 0.112 0.710226 40 (150) 8 TS2283 whole-rock 139.3 157.9 0.882 2.557 0.725633 56 8 TS2283 muscovite 652.7 6.6 9 8 . 5 8 0 320.6 1.977233 116 277 2.8 8 TS2283 biotite 603.9 6.48 93.2 297.3 1.756042 124 246 2.4

Leucogranite Pod ( ~ 30 m thick)

9 TS7782 whole-rock 189.0 19.6 9.643 28.20 0.816233 58 9 TS7782 muscovite 251.6 48.8 5.16 14.99 0.766344 76 266 6.3

a Concentrations in ppm

the calculation of the model values, it is not yet possible to determine if there was any external input of less radiogenic strontium into the metasedimentary pile.

Whereas the strontium isotope systematics permit us to conclude that the individual shale and phyllite samples have undergone major strontium isotope changes since they were originally deposited, we cannot utilize the oxygen isotope systematics to draw such a conclusion. There is probably no way to accurately determine the original 6180 of these sediments at the time o f deposition. Therefore, although it is conceivable that significant 180/160 changes also might have accompanied the changes of strontium isotopic composition at very low grade, we can only prove the 3%0 depletions in 6x80 that occurred at higher grades as the shales and phyllites were transformed to andalusite- and sillimanite-schists (Table 3).

Mineral ages

Mineral whole-rock Rb-Sr ages from three samples of the biotite granite and two leucogranites with retrogressive muscovite are listed in Table 6. As expected from previous K - A r and Rb-Sr mineral age studies in the Pyrenees (Jaeger and Zwart 1968; Vitrac-Michard and All+gre 1975; Albar- ede et al. 1978), the minerals with lower blocking temperatures exhibit a range of ages. The oldest mineral age listed in Table 6 is 330___20 Ma for an amphibole from the outcrop of quartz diorite at Lapege. In the present study, this probably provides the best available constraint on the age of the Hercynian metamorphism at Trois Seigneurs. Musco- vite and whole-rock ages of 277___ 3 and 266_+ 6 Ma on a biotite granite sample and a leucogranite from the metamorphic sequence lie within the 285-264 Ma range of muscovite ages from the Aston Massif (Table 1), which lies in the Axial Zone to the south of the Trois Seigneurs Massif (Fig. 1). Biotite ages for the Trois Seigneurs Massif range from 246_+2 Ma on a biotite granite sample to 116_+1.2

and 115_+ 1.2 Ma in the Lapege quartz diorite body. The older of these age limits is similar to a "disturbed" 39Ar/ 4~ age of 237 Ma determined by Costa and Maluski (1988) on an undeformed sample of "Trois Seigneurs gneiss" (the exact location of this sample was not reported).

The muscovite and biotite ages quoted above imply that the Hercynian rocks cooled through the Rb-Sr blocking temperature for muscovite (,-~ 500 ~ C) and biotite (~ 350~ by 277 Ma and 246 Ma, respectively. At Trois Seigneurs, the observed peak metamorphic conditions correspond to a temperature of about 700 ~ C at 12 km depth (Wickham 1987a). The crust at this level might be expected to cool by conduction to ~500 ~ C in 5 to 10 Ma and to ~350 ~ C in ~25 Ma to 100 Ma depending on the steady state thermal gradient (Carslaw and Jaeger 1959). However, the muscovite age (representing a temperature of about 500 ~ C) is 30-60 Ma younger than the 305-340 Ma age range estab- lished for the Hercynian metamorphism and melting events; this discrepancy could be due to continued intrusion of magmas into the upper levels of the Hercynian crust (as represented by the late granodiorites and the Permian volcanic rocks).

The 115-116 Ma biotite ages from near the base of the biotite granite probably reflect the effects of the North Pyr- enean metamorphism (Montigny et al. 1986), which at least locally reached upper greenschist facies in the adjacent Me- sozoic carbonates along the North Pyrenean Fault Zone. These ages are readily analogous to Late Mesozoic biotite ages and whole-rock Rb-Sr ages on altered samples in the Aston-Hospitalet Massif in the Axial Zone to the south (Jaeger and Zwart 1968; Majoor and Priem 1987).

Sm-Nd isotopic data

Three samples of the biotite granite, including a sample with a low initial 87Sr/86Sr ratio from the Illier outcrop (TS4583), have indistinguishable Nd isotopic compositions

Table 7. Sm-Nd isotopic compositions

Locality Sample Sm" Nda Sm/Nd 147Sm/ 143Nd/ _ TC~UR _ TDM _ eNdb (ppm) (ppm) 144Nd 144 Nd (Ma) (Ma) (310 Ma)

413

Late Granodiori~

5 TS2883 9.14 50.01 0.1828 0.11048 0.512140 16 882 28 1491 24 --6.3

Biotite Granite

7 TS4083 7.68 39.36 0.1952 0.11800 0.512153 22 966 42 1608 36 --6.6 7 TS4183 6.36 32.33 0.1968 0.11896 0.512140 24 978 24 1624 20 -6 .6 4 TS4583 5.80 28.01 0 .2071 0.12522 0.512172 22 995 48 1684 38 -6 .3 3 TS4883 3.08 15.67 0.1963 0.11867 0.512334 24 595 48 1310 38 -2 .8

Xenolith in Biotite Granite

4 TS44283 5.23 27.45 0.1905 0.11514 0.511830 10 1509 20 2037 18 -12.5

Psammitic Metasediment

t TP10a 6.35 30.71 0.2067 0.12497 0.512269 20 786 42 1514 36 - 4 . 4 1 TP10c 7.17 36.90 0.1944 0.11752 0.512161 22 920 42 1567 36 -6 .2

a Sm and Nd concentrations + / - 5%, ratios to + / - 0.2% b 2 sigma error on epsilon Nd ca + / - 0.4

at 310 M a (Table 7), with TCHUR model ages of 966-995 M a and Toga model ages o f 1608-1684 Ma. A mafic xenolith from the Illier outcrop gives an older model age (Tcnug = 1509 Ma, TDM=2037 Ma). Older crust must have predo- minated in the sources of all these samples. The hornblende diori te sample (TS4883) gives a substantial ly younger model age (TcHug 595 + 48 Ma, ToM = 1310 + 38 Ma). It is possible that this represents a mixture between a Hercynian mantle- derived magma and an older crustal component (see also Wickham and Taylor 1985, Fig. 7), a l though the model ages could equally well be explained by a heterogeneous crustal source.

Thus, the Sm-Nd isotopic composi t ions of the bioti te granite body confirm its derivat ion from crustal sources, but do not differentiate between two plausible models: a mixture of Hercynian-age mantle-derived magma and an older crustal detri tal component in the Paleozoic metasediments; or heterogeneity in the source material . Indeed, we know that limited heterogeneity is present, because a mafic xenolith within this body has a significantly older model age than the other samples o f bioti te granite. The absence of a more readily dist inguishable Hercynian-age mantle magmat ic component suggests that the mantle-derived magma bodies o f this age that provided the principal heat source for the metamorphism may have ponded towards the base o f the crust (perhaps confined by the thick anatectic zone which formed above them, see Fig. 12 in Wick- ham and Taylor 1987a).

The Sm-Nd isotopic da ta from a late granodior i te sample (TS2883; Table 7) give a TcnvR model age of 882 Ma, a depleted mantle age of 1491 Ma, and a end value of --6.3 4-0.4 at 310 Ma. This composi t ion and model age lie within the range exhibited by the Trois Seigneurs bioti te granite and by the Querigut plutonic complex (Fig. 12); thus these da ta also indicate a p redominant ly crustal source. However, these similarities appear to be more a reflection of uniformity of 143Nd/lr in the Hercynian crust rather than signifying derivat ion from the same li thological unit. This uniformity is also reflected in the Sm-Nd isotopic composi t ions of two psammite samples from Trois Seigneurs (TP10a and TP10c), even though these samples have anom-

0 . 5 1 3 5 0 MOR8

0 .5150C - o

o 3 ~la 0 .5125s

3rda

0 ,5120s z

. ~ ERI

0.5115s L

7 i i - - l - l i i

• BIOTITE GRANITE �9 TROIS SEIGNEURS 3 present ~ B-G e.xd~ded from GRANODIORITE

isochron flf o PSAMMITE �9 MAFIC XENOLITR

CHUR o BASAL GNDSSES 4- presen!

TROIS SEIGNEURS BIOTITE GRANITE SUITE

AGLY ASTON

- - ~BASAL GNEISSES

0.5110C# ~ ~ ~ 1 ~ [ I 0 . 7 0 0 0 .705 02 '10 0.715 0 . 7 2 0 0 . 7 2 5 0 . 7 5 0 0 2 ' 5 5 0 . 7 4 0

(87Sr /86Sr )o f 310 Mo

Fig. 12. Nd-Sr isotope correlation diagram for rocks from Trois Seigneurs, also showing available data from other localities in the Pyrenees (Ben Othman et al. 1984), as well as the average values of the "depleted" (MORB) and bulk Earth (CHUR) reservoirs. Where shown, the tie l&es connect the present day 143Nd/144Nd and 87Sr/86Sr values (solid dots) with the value that each sample would have had at 310 Ma. 143Nd/144Nd ratios are extremely uniform in most lithologies, whereas STSr/86Sr varies widely between the very 87Sr-rich basal gneiss samples and the less evolved, more primitive compositions shown by some of the plutonic lithologies. The Trois Seigneurs biotite granite suite and the late granodiorite from Querigut define similar trends, suggesting that both types of magmas formed by a broadly similar mixing process that in- volved a primitive, mantle-derived end-member and a Paleozoic metasediment or basal gneiss end-member. A mafic xenolith from deep within the biotite granite has an anomalously low t43Nd/ t44Nd ratio and is the only sample so far analyzed from the Pyren- ees that falls below the trend defined by the rest of the data

alously low S7Sr/86Sr rat ios (ascribed to their proximity to a metacarbonate layer).

In contras t to the (much earlier) biot i te granite body, we know that the source o f the late granodior i te must be deep seated and presently unexposed in the Pyrenees, because most of the Hercynian basement rocks representative of the upper and middle crust in the Pyrenees at about 3 0 0 M a have 5180 values of + 1 0 to + 1 6 and 878r/S6Sr values of 0.710-0.740, whereas the late granodior i tes were

414

clearly derived from a very large-sized, geochemically uniform lithological reservoir with a lower ~180 between +9 and + 10 and a lower 878r/86Sr of about 0.709-0.711 (Ta- bles 1 and 2). It is thus clear that the Sm-Nd data cannot be used to distinguish between a lower crustal source material and an upper crustal (Paleozoic metasedimentary rock) source. The psammitic schists, the biotite granite, and the late granodiorite have virtually indistinguishable 143Nd/ 144Nd and Sm-Nd ratios, and these values are all within the range exhibited by analogous samples elsewhere in the Pyrenees (Fig. 12).

Discussion

The ubiquitous mixing phenomena involving two (or more) end-members in the granitic plutons studied in this work preclude any determination of exact emplacement ages for these bodies using whole-rock Rb-Sr analyses. However, we are able to assign approximate ages to these bodies, and the new isotopic data presented in conjunction with previously published data provide important constraints on the origin and evolution of the Hercynian magmas of the Pyrenees, as well as upon the geochemistry and metamorphic history of their country rocks.

Sr isotopic compositions in the higher grade pelitic schists of the Trois Seigneurs Massif were homogenized on a scale of tens of meters at approximately 310 340 Ma. However, the local influence of intercalated metacarbonate layers can be observed (Figs. 9 and 10). Note that the existence of a relatively good regional isochron (Fig. 8) does not necessarily imply regional homogenization, only that homogenization distances are of the same order as the scale of the heterogeneities (Roddick and Compston 1977). This homogenization event appears to involve mixing between a reservoir of relatively unradiogenic strontium and another component with much more radiogenic strontium; the low- s 7Sr end-member must have been derived either from Paleo- zoic carbonate or from the metamorphic-hydrothermal fluids, whereas the high-87Sr component clearly represents Rb-rich detrital material in the original Lower Paleozoic shales.

The strontium isotopic compositions of samples from the main body of the deep biotite granite at Trois Seigneurs are practically identical to those of the adjacent high-grade pelitic metasediments of Paleozoic age. Although the biotite granite has slightly lower c~180 values than the pelitic schists, the initial 87Sr/86Sr ratio of seven isochron samples of the granite is 0.7149_ 13, which is within error exactly the same as that of the regional suite of metapsammitic and metapelitic samples at 310 Ma. This is consistent with the hypothesis that this body was mostly derived at about this time by in situ melting of this same metasedimentary source material (Wickham 1987 a, b).

The most striking thing about the Nd and Sr isotope data from the Pyrenees (including the Trois Seigneurs Mas- sif, the Querigut complex, and the Aston, Agly and Canigou Massifs) is the relative uniformity of a43Nd/144Nd isotope ratios in the Hereynian crust in a variety of different metamorphic and igneous lithologies, from various structural levels and with various ages, compared to the wide range in 87Sr/86Sr shown by the same samples (Fig. 12). The field of data-points from the Trois Seigneurs biotite granite ex- tends from the low 87Sr/86Sr end of the basal gneiss field back to the most primitive sample (the Lapege diorite) and

overlaps the field of data from Querigut; both complexes can be interpreted in terms of mixing between a more primitive mantle-like end-member and crustal material with a uniform end of about --6 and 87Sr/86Sr of about 0.715 at 310-340 Ma. However, the biotite granite represents a relatively small, cogenetic plutonic complex, whereas the Querigut complex includes several compositionally distinct plutons, some of which may span a range of emplacement ages and may be petrogenetically unrelated (Ben Othman et al. 1984).

Based only on the neodymium isotope systematics, the basal gneisses would represent a viable source material for many of the granitic bodies in the Pyrenees (including those at Trois Seigneurs). However, the wide variation in both S7Sr/86Sr (Fig. 12) and 6a80 (Wickham and Taylor 1987a) in these lithologies rules out this possibility, unless these gneisses became drastically transformed and geochemially homogenized by some event that we do not yet observe in the exposed basement sections in the Pyrenees (e.g. Agly).

Figures 7 and 13 illustrate the similar, but much more homogeneous Sr isotopic compositions of the main part of the Trois Seigneurs biotite granite compared with the high-grade pelitic schists. This is consistent with the biotite granite being derived from the pelitic schists, but also being further homogenized by convective mixing in the magma chamber. However, notwithstanding the greater spread of STSr/86Sr in the pelitic schists, the most important feature of Figs. 7, 12, and 13 is the similarity and uniformity of the 6180 values, t43Nd/144Nd ratios, and 87Sr/86Sr ratios in the upper parts of the biotite granite body and in the pelitic schists, as well as in the leucogranite pods that were formed by partial melting and local melt segregation within the schists (these pods have 6180= +11.0 to +13.4 and initial 87Sr/86Sr = 0.7171 + 18 ; Wickham and Taylor 1985; Majoor and Priem 1987).

The Trois Seigneurs pelitic schists occupy a much more homogeneous (8VSr/86Sr)31o field (0.713-0.718) than the calculated field they would be expected to occupy if they had evolved as closed systems with their present 87Rb/86Sr ratios from Cambro-Ordovician sediments. This field of (875r/86Sr)31o values is remarkably homogeneous compared with the fields occupied by typical basal gneiss lithologies from the Agly Massif (Fig. 13), or even from the large orthogneiss massifs at Aston and Canigou, which also appear to represent partially homogenized (pre-Paleozoic?) basement (Vitrac-Michard and Alltgre 1975) or Lower Pa- leozoic intrusive bodies (Zwart 1968; Majoor and Priem 1987). Lithologies similar to those exposed in these regions probably formed a major erosional source material for the detrital Paleozoic sediments. We know that the 6180 values of the Trois Seigneurs pelitic schists have been shifted from the values of + 14 to + 16 typical of their low-grade protoliths to values of about +11 to +13 as a result of the Hercynian metamorphism (Wickham and Taylor 1985), and it seems likely that 87Sr/86Sr ratios reached their final, relatively homogeneous values of 0.713-0.717 at this time by the same metamorphic-hydrothermal processes.

The Trois Seigneurs data are compared with data from the Maladeta complex, farther to the west, in Fig. 13. The Trois Seigneurs late granodiorite plots close to the Mala- deta granodiorite, which is a much larger pluton, though with a similar chemical composition. The two plutons were probably derived from the same type of geochemically uniform source, which must be mainly crustal based on the

415

0:725

0,720

0Z15

-g

%

0Z10

0.705

-}-6 +7 +8 +9 +10 +11 +12 +15 +14 +IU + lb

8180

Fig. 13. Plot of 873r/S6Sr at 310 Ma vs 6180, comparing the Trois Seigneurs (T-S) granite and pelite data with data from the Mala- deta plutonic complex in the central Pyrenees (M1, M2, and M3; Vitrac-Michard et al. 1980). Also shown are fields for the measured values of Trois Seigneurs shales and phyllites at 310 Ma (Fig. 7), together with the calculated range of model values at 310 Ma for these same rocks (see text). The range of values shown by the Agly basal gneisses and the Aston and Canigou orthogneisses are also indicated (Jaeger and Zwart 1968; Vitrac-Michard and All6gre 1975; Ben Othman et al. 1984; Majoor and Priem 1987; Wickham and Taylor 1985, 1987; unpublished data), because these lithologies could have formed a major erosional source component for the Lower Paleozoic detrital sediments. Note that only a single 6180 analysis is available for Canigou (+10.44), so the 1 per mil-wide field is simply centered on that value. For Aston, the two available 5~80 values are + 11.38 and +12.38, and these define the width of that field. The large range of 87Sr/S6Sr values at Aston may be somewhat misleading in the light of a recent study by Majoor and Priem (1987); they analyzed 19 samples of orthogneiss from two areas within this massif (Gorge de Merens and Laparan), and demonstrated that profound 87Sr/86Sr homogenization occurred in the two areas during Hercynian metamorphism (0.7241-0.7278 and 0.7193-0.7205, respectively). The measured range of (SVSr/ 86Sr)31o values in these two areas is much narrower than the variation in the Jaeger and Zwart (1968) study. The Trois Seigneurs pelitic schists are an obvious high-180, high-SVSr end-member for the biotite granite suite, and they have much more homogeneous 87Sr/86Sr ratios than either the model shales or the basal gneisses and orthogneisses. The large arrows indicate schematically how the 5180 and SVSr/S6Sr values of the shales and phyllites have been changed during prograde Hercynian metamorphism. Fields for the upper mantle, for the main part of the Peninsular Ranges batholith (PRB) from Taylor and Silver (1978), and for the Hercyn- tan granites from Brittany and southwest England (Sheppard 1986) are also shown. Despite their having major metasedimentary components in their sources, these other Hercynian granitic rocks also have relatively low initial S7Sr/S6Sr ratios, suggesting that the source rocks for these magmas may also have been affected by the same sort of STSr/86Sr homogenization process that appears to have affected the Trois Seigneurs pelites

Nd isotope systematics. This source is presumed to be deep seated and presently unexposed in the Pyrenees because the Hercynian basement rocks representative of the upper and middle crust in the Pyrenees have 6180 values greater than + 10 and S7Sr/S6Sr values of 0.710-0.740, whereas the

late granodiorites were clearly derived from a very large- sized reservoir with a lower 6180 between + 9 and +1 0 and a lower 87Sr/86Sr o f about 0.709-0.711 (Tables 1 and 2). The Maladeta complex includes a gabbro-norite body and a two-mica granite in addition to the voluminous granodiorite. Although Wickham and Taylor (1985) suggested that the Maladeta gabbro-norite might be related to the Lapege diorite, our new data show that the latter has a distinctly lower 87Sr/86Sr. The Maladeta 2-mica granite lies close to the high-lSO end of the biotite granite composition trend, and it also was probably derived from similar pelitic source material. Thus, the granodiorite, the two- mica granite, and the norite at Maladeta are probably not cogenetic, confirming the interpretation of Vitrac-Michard et al. (1980) that these represent separate batches of magma derived from different crustal source regions, with little or no mixing between them. This also partly explains the differences between the Maladeta rocks and the definitely cogenetic biotite granite suite at Trois Seigneurs, where mixing synchronous with emplacement did occur.

The present study has demonstrated that large volumes of the Hercynian crust were profoundly isotopically modified during metamorphism, and that this occurred before incorporation of this mainly metasedimentary material into crustally derived granitic magma bodies such as the deep biotite granite at Trois Seigneurs. In this respect it would be extremely misleading to take the 61sO value and the calculated model 878r/86Sr value of a low-grade Cambro- Ordovician shale from the Pyrenees and use these values directly as a representative end-member for any of the granitic lithologies, despite the fact that modified material of this type undoubtedly formed a major protolith component of some of the granites. The shales and phyllites underwent such profound isotopic modification by hydrothermal processes during the extended period o f diagenesis and prograde metamorphism that, in any magma mixing process, this end-member would have very different oxygen and Sr isotopic compositions than if the original sediments had remained perfectly closed systems.

This point is well demonstrated in a recent paper dis- cussing the petrogenesis of ~340 Ma Hercynian S-type granites o f the Armorican Massif, northwest France (Peu- cat et al. 1988). On the basis of strontium isotope data, these authors discounted local detrital sedimentary material as a major source for these S-type, peraluminous granites because they believed that the SVSr/S6Sr ratio of such material at 340 Ma should have been much higher (>0.715) than their measured granite values (0.706-0.715). This "s t ront ium paradox" is perfectly resolved if the protoliths to the granites (Lower Paleozoic and Late Precambrian pelitic sediments) evolved in the same way as did the Lower Paleozoic pelites in the Pyrenees, namely by having their 87Sr/86Sr ratios homogenized and lowered during the extended diagenetic and prograde metamorphic episode(s) that preceded anatexis. Peucat et al. (1988) concluded that the Armorican peraluminous granites must have been derived by melting of "young unradiogenic source rocks that have now generally disappeared." We believe that a more plausible alternative is that the STSr/86Sr ratios of the metasedimentary protoliths were homogenized and lowered by aqueous fluids during prograde metamorphism, so that the isotopic compositions of the high-grade metasediments that were finally melted were markedly different than the age- corrected values of the original sediments.

416

Figure 13 shows da ta from some other Hercynian granites, including Bri t tany and southwest England, all of which are thought to have major metasedimentary components in their sources (Sheppard 1986). I t is notable that the high 3180, sedimentary end-members in these granites also have relatively low initial 87Sr/86Sr rat ios - again lower than the typical values that Rb-r ich pelitic sedimentary mater ia l would be expected to evolve towards in as little as 100 M a of aging since deposit ion. Al though the source materials for these plutons have not been identified accurately enough to make precise estimates of the likely 87Sr/86Sr that they would have a t ta ined at the time of anatexis, it is plausible that major reductions in S7Sr/S6Sr and 3180 also occurred in these regions during the metamorphic-anatec t ic event that generated the granites. In many cases, 8~Sr/86Sr rat ios appear to be shifted downwards , which suggests mixing with a major low-87Sr/S6Sr reservoir. Obvious candidates for this reservoir are carbonate sediments or the Sr-rich marine format ion waters that might be impor tan t components of, or available to flow through, the metasedimentary sections. M a j o r changes in impor tan t geochemical pa rame- ters thus may occur th roughout huge volumes of the crust, as a result of isotopic exchange with metamorphic -hydrothermal fluids. In using isotope systematics to identify the sources of granit ic plutons, it is par t icular ly impor tan t to be aware o f these potent ia l changes which may mask the identi ty o f some impor tan t source mater ia ls (e.g. shales).

Acknowledgments. Financial support for this work was provided by NERC grant no. GR3/6457 and NSF grant no. EAR-8313106. SMW acknowledges a research fellowship at Trinity Hall, Cam- bridge, a NERC postdoctoral fellowship, and a Visiting Research Associateship at the California Institute of Technology. The XRF Rb-Sr determinations were carried out at the University of Nott- ingham under the direction of Peter Harvey and Brian Atkin. We are grateful to Samuel Epstein, Jack Coulson, Joop Goris, Joe Ruth, and Maurice Haslop for laboratory assistance, and also to Linda Marnoch for her patience with the preparation of the manuscript. Discussions with Ron Oxburgh and Steve Sparks have been very helpful, and we thank Christian France-Lanord and Simon Sheppard for their reviews of the manuscript.

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Received November 11, 1987 / Accepted August 1, 1988 Editorial responsibility: J. Hoefs

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