Geochemistry and evolution of MORB-type eclogites from the Münchberg Massif, southern Germany

230

Earth and Planetary Science Letters, 99 (1990) 230-249

Elsevier Science Publishers B.V., Amsterdam [F~]

Geochemistry and evolution of MORB-type eclogites from the Miinchberg Massif, southern Germany H.-G. Stosch

a

and G.W. Lugmair

b

a Mineralogisch- Petrographisches Institut der Universitiit zu KiJln, Ziilpicher Str. 49, 5000 Krln 1 (F. R. G.) b UCSD, GRD A-012, Scripps Institution of Oceanography, La Jolla, CA 92093 (U.S.A.)

Received January 27, 1989; revised version accepted April 6, 1990 ABSTRACT In the Miinchberg Massif in the Variscan foldbelt of southern Germany two varieties of eclogite are known which are intercalated with amphibohte-facies meta-igneous and meta-sedimentary rocks: a dark kyanite-free and a lighter colored kyanite-bearing type. Kyanite-free eclogites, which are discussed here, have a major and trace element composition which suggests derivation from ocean-floor basalts with melt to cumulate compositions. Internal Sm-Nd isochrons (clinopyroxeneamphibole-garnet) and one Rb-Sr isochron (clinopyroxene-amphibole-mica)yield eclogitization ages in the range of 380 to 395 Ma. Thus, the age of eclogitization is only marginally higher ( < 15 Ma) than the age of amphibolite-facies metamorphism in the Mtinchberg Massif as derived from K-Ar ages of amphiboles and micas from metasediments and meta-igneous rocks. A seven point whole-rock Sm-Nd isochron for one eclogite body results in an age of 480 + 23 Ma with an initial ~Nd of 8.7 + 0.6 and is likely to record the age of igneous formation of the eclogite protoliths. Sr isotopic compositions back-calculated to that time are anomalously high and variable if compared to Nd isotopes. This can be explained by alteration with an aqueous or fluid phase with high STSr/S6Sr, most likely seawater, either during igneous formation in an oceanic rift environment or subduction-related eclogitization. In addition, some eclogites show a marked enrichment of incompatible, immobile elements and plot far below the whole-rock Sm-Nd isochron. These features are ascribed to the presence of an evolved crustal component, probably acquired during extrusion of the basaltic protoliths by mixing with country-rock gneisses.

1. Introduction T h e last = 15 years have seen a revived interest in the study of eclogites a n d r e l a t e d rocks. This interest was m a i n l y s t i m u l a t e d b y the m o u n t i n g evidence that m a n y crustal eclogites m u s t have f o r m e d u n d e r high pressure which c a n n o t be r e a c h e d in a c o n t i n e n t a l crust with n o r m a l thickness of 3 0 - 4 0 k m a n d t h a t their f o r m a t i o n m u s t s o m e h o w b e related to p l a t e collisions [1]. Petrog r a p h i c studies have shown that m o s t eclogites have a p o l y m e t a m o r p h i c history in t h a t eclogitization was followed b y one or m o r e e p i s o d e s of m e d i u m - to low-pressure m e t a m o r p h i s m w h i c h c a n eventually lead to c o m p l e t e d e c o m p o s i t i o n of the eclogite parageneses. This represents a challenge for petrological ( r e c o n s t r u c t i o n o f p r e s s u r e a n d t e m p e r a t u r e c o n d i t i o n s using a p p r o p r i a t e geotherm o b a r o m e t e r s ) as well as g e o c h r o n o l o g i c a l studies. F u r t h e r m o r e it has to b e tested on an individual basis which p o i n t o n the t e m p e r a t u r e - t i m e 0012-821x/90/$03.50

© 1990 - Elsevier Science Publishers B.V.

p a t h is b e i n g d a t e d b y which m e t h o d a n d / o r which paragenesis. U n t i l a b o u t 10 years ago eclogites were c o n s i d e r e d n o t to be suitable for geoc h r o n o l o g i c a l investigations owing to the p a u c i t y o f a p p r o p r i a t e m i n e r a l s with high K (white mica), R b / S r (white mica, a m p h i b o l e ) o r U / P b (zircon). T h e c h a r a c t e r i s t i c t w o - p h a s e a s s e m b l a g e clinop y r o x e n e + g a r n e t is, however, ideal for d a t i n g w i t h the S m - N d m e t h o d b e c a u s e g a m e t s c a n h i g h l y enrich S m over N d . W e r e p o r t here the results o f a g e o c h e m i c a l a n d r a d i o g e n i c i s o t o p e s t u d y f r o m the M i i n c h b e r g M a s s i f ( M M ) in n o r t h eastern B a v a r i a a b o u t 50 k m n o r t h of the G e r m a n d e e p - d r i l h n g site. 2. G e o l o g y of the M i i n e h b e r g Massif T h e eclogites of the M M are a m o n g the classical o c c u r r e n c e s of such rocks in central Europe. T h e y were a l r e a d y k n o w n to H a i i y (1822, cited in [2]) w h o first d e f i n e d the term "eclogite". T h e

GEOCHEMISTRY AND EVOLUTION OF MOP.B-TYPEECLOG1TES

Fig. 1. Sketch map of the Miinchberg Massif showing the eclogite localities and one meta-gabbro occurrence of which chemical a n d / o r isotopic data have been obtained (modified after [10]). Note that the eclogites occur at the boundary of two tectonic u n i t s - - t h e Liegendserie and the Hangendserie.

MM is a - - 3 0 × 1 5 km large body of mainly amphibolite-facies rocks and greenschist-facies rocks emplaced on low-grade metamorphic to unmetamorphosed sediments and volcanics of the "Saxothuringian Zone". Early geological interpretations favored a nappe origin of the MM [3,4] linking it to the Moldanubian zone in the southeast. Later, autochthonous concepts were preferred interpreting the MM as an uplifted block, e.g. [5,6]. At present, the nappe model is favored again, based on the interpretation of geologic [7] and geophysical data [8]. From the border to its center the MM (Fig. 1) consists of a sequence of: (1) greenschist-facies metavolcanics and metasediments ("Prasinit-Phyllit-Serie"), interpreted as a volcano-sedimentary series; (2) retrogressed amphibolites ("Serie der Randamphibolite"); (3) the "Liegendserie", mainly recta-sediments but also including meta-igneous augengneisses and meta-gabbroic rocks; and (4) as the highest tectonic unit a heterogeneous sequence of metasedimentary amphibolite-facies banded gneisses and meta-igneous amphibolites, the "Hangendserie", again interpreted as a volcano-sedimentary sequence (see e.g. [9]).

231

Many occurrences of eclogites and garnet amphibolites (retrogressed eclogites) have been found in the MM. A large number of them is known as isolated blocks only. Others occur as bodies intercalated with gneisses and amphibolites. The base of the "Hangendserie" is commonly assumed to be the tectonic position of all eclogites. They have been divided into two types by previous investigators [10]: (a) a dark variety consisting of clinopyroxene (cpx) + garnet (gnt) + quartz (qtz) _ amphibole (amph) + white mica (pheng[= phengite]) + rutile(rut) + zoisite(zo); and (b) and a lighter colored variety which, in addition to the mineral assemblage of group (a), is kyanite-bearing (ky). Eclogites of group (b) are more abundant in the •northeast of the MM, whereas group (a) eclogites are more frequently found in the southwestern part. In several localities both varieties occur, thus suggesting there may be a general genetic link between them. This paper deals with the dark eclogites of the Weissenstein body in the southwest of the MM. The analyzed samples are pieces of a ~- 230 m long core drilled through this body in 1969 which encountered eclogite with some intercalations of meta-sedimentary gneiss (for a detailed report see [11]). The dark variety has a chemical composition similar to that of mid-ocean ridge basalts (MORB) whereas the kyanite eclogites have been compared with high-A1 basalts and gabbros [10].

3. Relevant previous work Franz et al. [2] have studied the petrology of an eclogite sample from the Weissenstein and arrived at the conclusion that the eclogite stage was characterized by a minimum pressure of 1.3-1.7 GPa and by a temperature of ---620 + 50°C. For a retrograde symplectite stage visible in reaction rims around clinopyroxenes and phengites they estimated a pressure of ~ 0.85-1.2 GPa and essentiaUy the same temperature as for the eclogite stage. In a recent fluid inclusion study [12] an even higher pressure of eclogitization (> 2 GPa) is inferred. These high pressures of eclogitization raise the question whether the eclogites might have been tectonically mixed with the country-rocks after eclogitization [2] or whether the country-rocks experienced complete retrograde overprinting to

232

the amphibolite facies. Recently some suggestive evidence for high-pressure relics in these countryrocks has been presented [13]. Knowing the geological evolution of the MM is of obvious relevance to the German deep-drilling project. Therefore, during the pre-site investigations some effort has gone into deciphering tectonics and geochronology of the MM. Several attempts were made to date the eclogitization event; unfortunately, these have produced some controversy. Gebauer and Griinenfelder [14] have extracted zircons from two Weissenstein eclogites and from a nearby body of metagabbro (locality Steinhiigel). By assuming a common geological evolution for eclogites and metagabbro, they obtained a U - P b lower intercept between concordia and discordia age of 3 8 0 +14 - 22 Ma and an upper intercept age of 5 2 5 _ +40 31 Ma which they interpreted as the age of eclogitization and age of igneous protolith formation, respectively. The upper intercept age is controlled by the zircon fraction from the metagabbro and the lower age by those from the eclogites which individually are only slightly or not discordant. The interpretation of these authors has been criticized on the basis that the metagabbro is unlikely to be cogenetic with the eclogites [15,16]. Furthermore, metagabbro and eclogites are likely to belong to different tectonic units: The metagabbro is placed within the "Liegendserie" whereas the eclogites are considered to be part of the base of the "Hangendserie"; thus it is not clear that they must have a common geological history. Cooling ages between --400 and 360 Ma are frequently measured for various lithologies in the MM [16,17]. However, except for the eclogites, these are interpreted to date cooling after a medium-pressure metamorphic event recognized as the dominant phase of metamorphism in the MM and parts of the Moldanubian [18]. Ducrot et al. [19] have reinterpreted the data of Gebauer and Gfiinenfelder [14] and, excluding 3 one zircon fraction, calculated an age of .A.~.$. / + 116 Ma for the zircons from the eclogites alone. More recently, Mtiller-Sohnius et al. [16] argued in favor of an eclogitization age around 440 Ma also, coeval with a phase of medinm-pressure metamorphism in the "Hangendserie" as evidenced by metasediments [20]. They considered the eclogite zircons of [14] to be concordant. In addition, they presented a Rb-Sr "isochron" for bulk-rocks and clino-

H.-G. STOSCH AND G.W. L U G M A I R

pyroxenes from one dark eclogite (Weissenstein, see Fig. 1 for locality) and one kyanite eclogite from Martinsreuth. These four samples plot on a straight line corresponding to an age of 449 +_ 37 Ma which Miiller-Sohnius et al. interpreted to be the age of eclogitization. However, from Nd isotopes they also concluded that the two eclogite types are not cogenetic, thus effectively invalidating the conclusion drawn from their Rb-Sr data. Solving this controversy in geochronology is of general importance, however, because it bears on the question which events can be dated in polymetamorphic terrains with different isotope systems.

4. Samples We have investigated samples from the Weissenstein drill-core. A detailed account of the petrography and chemistry of eclogites and country-rocks has been given by Matthes et al. [11]. Sample names indicate their position in the drill-core in meters, each specimen representing between ---5 and 20 cm of the core. Major and trace element analyses of the rocks are given in Table 1. Neodymium and strontium isotope data were obtained on nine eclogite whole-rocks (including one with severe retrograde overprinting) and one meta-sedimentary gneiss from the drillcore. For comparison, a kyanite-free eclogite from the northeastern part of the MM (Fattigau occurrence) was analyzed also. In addition, from two eclogites minerals were separated and analyzed for trace element and radiogenic isotope compositions. The eclogites from the Weissenstein drill-core show highly variable degrees of preservation, ranging from only slightly altered eclogites to completely decomposed g n t - a m p h or amph-plag parageneses as a consequence of a retrograde metamorphic overprint; slightly and severely altered eclogites are mixed on the scale of meters or less [11]. The two eclogites chosen for mineral isotopic analysis are W1.60 and W6.80. Both are massive rocks which lack noticeable foliation. Sample W6.80 was selected from one of the freshest parts of the drill-core and consists (in approximately descending abundance) of garnet, clinopyroxene, quartz, rutile, amphibole and phengite. In con-

GEOCHEMISTRY AND EVOLUTION OF MOP.B-TYPE ECLOG1TES

233

TABLE 1 Bulk-rock analyses

rock-type SK)2 TIO2 AI2Os Fe203 FeO MnO

MgO

CaO Na20

E~ P~O5 l~#* La Ce Nd Sm Eu To Yb Lu Sc Cr Hf Ta "In

reck-type SIO2

"no2 AI~)S Fe2Oa FeO MnO

M~

CaO Na20

KaO P205 Mg#* Ls Ce Nd Sm Eu To Yb Ix= Sc Cr Co Hf Ta Th

W1.09 e 51.3 0.96 15.6 <0.5 8.6 0,14 8.7 11.5 4.02 0.16 0.10 101.1 0.643

W1.43 e 51.5 0.91 15.5 0.8 8.0 0.10 8.4 11.8 3.81 0=?,1 0.09 I01.I 0.632

WI.60 e 51.8 1.07 15.0 <0.5 8.4 0.15 7.9 11.3 4.00 0,43 0.II 100.2 0,6~.6

W2.30 E 49.7 1.43 14.8 0.5 9,7 0.13 8.3 12.0 2.90 0.07 0.17 99.7 0.593

W4.00 E 50.2 1.65 14.2 0.6 10.4 0.12 7.9 12.2 2.61 0.01 0.18 I00.I 0.563

W6.20 E 51.1 1.37 14.6 0.6 I0.0 0.16 8,2 11.7 3.06 0.15 0.14 101.1 0.581

W7.00 E 50.5 1,42 14.4 1.2 9.7 0.16 8.3 11,9 2.93 0.06 0.14 100.7 0.578

W9.60 e 51.2 1.39 14.7 1.0 9.5 0.09 8.1 11.7 2.94 0,21 0.16 I01.0 0.581

W15.2 E 49.5 1.56 15.1 0.9 I0.0 0.19 8.6 11.9 2.81 0.12 0.17 I00.9 0.586

W27.5 e 49.0 1.61 14.7 0.8 11.5 0.17 8.6 11.8 2.48 0.09 0.16 100.9 0.556

7.7 19.4 12.1 3.45 1.16 0.67 2.20 0.355

8.9 21.4 12.2 3.70 1.31 0.69 2.40 0.380

7.0 19.5 17.0 4.90 1.64 0.81 2.95 0.455 41.0 202 2.60

6.6 25.5 19.9 6.00 2.11 1.14 3.45 0.55 45.5 195 2.15 0.43 (0.24)

8.7 31.0 24.0 7.10 2.44 1.27 4.15 0.64

6.0 22.5 19.9 5.70 1.87 0.95 3.05 0.45

5.9 23.5 20.0 6.50 2.17 1.21 3.90 0.58

4,7 18.9 13.5 4.75 1.75 0.94 3.15 0.50 43.0 186 2.25 0.50 (0.26)

7.2 27.5 19.9 5.50 2.05 1.15 3.60 0.63 44.5 197 2.25 0.49 (0.39)

6.7 20.9 13.7 4.25 1.61 0.96 3.40 0.51 45.5 130 2.40 (0.31) (0.33)

(0.83)

W32.0 ~ 51.2 0.63 15.5 0.3 7.3 0.II 9.4 13.3 2.67 0.32 0.02 100.8 0.689

3.9 1.70 9.9 4.8 5.8 4.6 2.04 1.49 0.90 0.64 0.45 0.37 1.38 1.34 0,9,40 0.225 45.0 44.5 228 201 { 0 . 7 7 } 0.93 (0.20} (0.21) {0.44) {0.19)

W51.75 W78.98 W89.4 W96.42 W106.8 W l 1 2 W114.75W121.4 W132.6 W153.5 W205 re re/ga ga ga e e e re gn re E 48.8 48.9 57.6 46.1 48.7 49.6 50.8 53.2 51.6 73.2 51.8 1.54 1,70 1.33 1.6@ 1,88 1.90 1.56 1.39 1.88 0.64 2.08 14.9 15.9 15.4 14.2 14.0 15.0 14.6 15.1 13.7 14.1 14.4 0.4 1.0 0.4 1.6 1.3 I.I <0.5 0.2 1.0 0.5 0.3 11.0 10.9 9.1 11.7 12.6 12.3 11.6 9.9 11.7 3.80 13.9 0.19 0.16 0.23 0.17 0.21 0.23 0.18 0.19 0.22 0.06 0.24 8.9 6.6 5.0 I0.I 7.4 8.2 8.3 6.1 7,5 1.5 5.7 12.0 7.5 5.75 11.4 10.1 9.6 11,5 9.3 8.6 1.70 8.9 2.76 3.93 2.84 2,27 3.01 2.13 2.49 2.85 3.04 2,71 1.77 0.16 0.75 1.6~ 0.44 0.23 0.21 0.12 0.69 0.20 2.42 0.06 0.14 0.22 0.13 0.07 0.22 0.20 0.13 0.14 0.23 0.II 0.27 100.8 97.6 99.4 99.7 99.7 100,5 101.3 99.1 99.7 100.7 99.4 0,583 0.499 0.485 0.578 0.489 0.524 0.560 0.519 0.515 0.386 0.418 6.0 20.7 18.1 4.45 1.68 0.93 3.55 0.52 45.5 250

23.7 50.0 25.2 5.50 1.50 0.99 3.80 0.6~ 28.9 139

6.6 19.2 14.1 4.35 1.43 0.84 3.50 0.54 52.5 275

10.4 24.2 16.8 5.35 1.66 1.15 5.10 0.79 43.0 111

7.5 21.3 13.8 4.50 I.(~ 1.06 4.30 0.73 44.0 212

5,1 14.5 I0.0 3~0 1.24 0.76 3.15 0.53 45.0 149

16.4 35.5 20.5 4.95 1.47 0.91 3.80 0.59 35.1 193

11.0 25.0 16.7 4.95 1.61 1.22 5.1 0.79 41.0 126

2.55 0.39 (0.33)

4.50 0,56 5-2

3.30 0.45 0.60

3.95 0.69 0.76

3.90 0.56 (0.56)

2.70 0.43 (0.50}

4.20 0.60 3.05

3.80 0.69 0.91

22.6 50.0 18.4 4.15 0.95 0.63 2.15 0.26 10.5 50 38.4 4.60 0.86 7.3

W33.7 E 50.8 0.81 14.9 0.6 8.4 0.14 9.3 12.7 3.15 0.32 <0.02 I01.I 0,650

7.1 16.5 12.0 4.70 1.72 1.40 6.2 0.95 45.5 102 28.6 5,00 (0.33) 1.05

W36.0 ~ 53.8 1.43 15.3 0,5 I0.0 0.17 6.8 8.7 3.05 0.61 0.16 100.5 0.537 15.6 34.5 18.8 5.25 1.54 1.02 3.35 0.58 35.7 181 3.20 0.44 2.60

W43.0 W46.25 E e 48.2 49.9 1.80 1.66 15.7 15.0 0.9 1.1 12.5 10.6 0.27 0.13 8.0 8.6 11.2 11.6 1.85 2.90 0.03 0.15 0.27 0.18 I00.7 101.8 0.517 0.569 2.7 6.5 5.9 2.42 1.20 0.93 3.70 0.55 52.0 209 1.02 0.34 0.68

4.7 15.4 11.5 3.95 1.43 0.85 3.05 0.51 43.0 216 2.50 0.46 <0.2

$93

SI14

average

52.3 1.53 13,9 2.2 9.5 0.18 7.2 11.7 2.56 0.20 0.16 101.4 0.528

53.4 1.31 13,3 2,7 8.6 0.17 6.7 12.2 2.77 0.06 0.12 101.3 0.520

50.8¢1.5 1.46¢0.36 14.8±0.6

5.0 13.2 10.9 3.35 1.34 0.80 2.45 0.39 45.2 196 49.0 2.20 (0-29) (0.45)

4.3 II.I 8.6 3.05 1.07 0.62 2.10 0.35 40.0 178 45.5 1.53 (0.34)

I0.6¢2.4 7.9¢0.9 11.1±1.5 2.90¢0.58 0.20¢0.17 0.15¢-0.06

0.565±0.059

Major element concentrations [SIO~-P~5] In weight%, trace d e m e n t s ILa-Thl in wet~at p p m °Mg# = Mg/(Mg + ~.,Fe) [molecular raUo] Trace d e m e n t cone~tratlctas given In brackets Indicate values with large standard errors (2o > 5096}. Samples W1.09 - W205 a r e from the WellSenstetn body, $93 a n d $114 are dark eclogltes from Fattlgau. Cl~mtl~aUcm of rocks from the Wet8enste~n th'lll-core following Matthes et al. (1974): E = eclogtte with <5% retrograde overprint, e = ecloglte v~th 5-50% retrograde overprlnt, e = edog/tes wlth >50% retrograde overprint, re = completely retrogressed rocks probably derived from edogRes, ga = garnet amphlbol/tes, gn = metasedlmcntary g n d s s . "avorage" Is the average for all kyan/te-free edog/tes analyzed (including the 3 completely ~ samples]. This results In the following CIPW norm: quartz 0.096, o r t h o d a s e 1.2%, alI~Ite 24.596, anorth/te 26.8%, nephol/ne 0.(7)6, diops/de 12.7%0 hedenberg/te 9.6%, enstatlte 5.896, fectos/]lte 5.1%, forsterlte 5.6%, f&ya]/te 5.4%, flmentte 2.8%, apatlte 0.4%.

234

trast, Wl.60 shows significant retrograde overprint visible in reaction rims around omphacites; they consist of a fine-grained mixture of clinopyroxene II; plagioclase and subordinate amphibole II with a characteristic "fence-post" texture as described by Franz et al. [2] for a different Weissenstein eclogite. The mineralogy of W1.60 is garnet + clinopyroxene + quartz + amphibole + phengite + clinopyroxene II/plagioclase/amphibole II + zoisite.

5. Analytical procedures Eclogite whole-rocks were obtained from S. Matthes (Wiirzburg) as powders or halves of drill-core slices ~ 10-20 cm long. The core sections were cut into two or three pieces. One piece from each section was ultrasonically cleaned with deionized water, crushed using a steel press and steel mortar and finely ground in an agate ball-mill except for W1.60 which was ground in a tungsten-carbide spex-mill, a procedure which unfortunately resulted in unexpectedly high Ta contamination. A second piece from each core section was crushed using steel tools and sieved with a set of nylon sieves. This procedure was repeated until a size fraction = 0.2-0.3 mm in diameter was obtained. Concentrates of garnet, clinopyroxene, amphibole, and the non-magnetic phases were prepared from this fraction with a Frantz isodynamic separator. From these concentrates clean minerals were obtained by hand-picking under a binocular microscope. However due to its low abundance, no high-purity amphibole separate could be obtained from eclogite W6.80; the fraction labeled " a m p h " has some intergrown quartz. As a consequence of major and minor dement zoning, the garnets display a variation in color which, in the --0.2-0.3 mm diameter fraction, ranges from orange cores to slightly pink or almost colorless rims. The Mn and Ca contents of the cores are higher than those of the rims. Prior to trace element and isotope analyses, all mineral separates were ultrasonically cleaned with - - 2 N HC1, --5%HF, followed by - - 2 N HC1 and H 2 0 to remove potential grain surface contamination, and with acetone or methanol to ensure fast drying and prevent the grains from oxidizing.

H.-G. S T O S C H A N D G.W. L U G M A I R

Major element analyses of whole-rocks were performed by standard X-ray fluorescence techniques; major element compositions of minerals were obtained by electron microprobe analyses. All trace elements were determined by instrumental neutron activation analysis (INAA) at the University of Cologne. Strontium and neodymium isotope compositions were analyzed at UC, San Diego using the procedures established in that laboratory, e.g. [21].

6. Mineral chemistry Electron microprobe data for minerals from some eclogites are given in Table 2a. Garnets form the largest crystals in these rocks, mostly between -- 3-5 mm in diameter in the Weissenstein body but only = 1 mm in diameter in the Fattigau eclogites. They are almandine-rich ( - - 4 5 mole%) with major pyrope and grossular ( ~ 25% each) and a small spessartine component ( ~ 1%). They show a slight to significant element zonation with respect to Mg, Fe, Mn and Ca; Mg and Fe concentrations increase and Mn and Ca decrease from core to tim. For a detailed discussion of garnet zoning in the Miinchberg Massif eclogites see Knauer [22] who speculated that the variable Mn concentrations might be responsible for the range in color. Inclusions of quartz and rutile are very frequent, particularly in the garnet cores. In contrast to garnet, the omphacites are compositionally homogeneous. They are diopside (-5 0 - 5 5 m o l e % ) - j a d e i t e - r i c h (-- 4 0 - 4 5 % ) with hedenbergite below 10%. Unlike in the chemically rather unusual Weissenstein eclogite analyzed by Franz et al. [2], trivalent iron is not required for charge balance. Eclogite-stage amphiboles up to --2 mm in diameter occur in almost all eclogites of the Weissenstein drill-core in subordinate amounts. Typically they have low K 2 0 contents and according to the nomenclature of Leake [23] are pargasitic to edenitic hornblendes. White micas analyzed from the kyanite-free eclogites are exclusively phengites. Primary paragonite as described by Schmidt [24] was not found. The estimated modal abundance of phengite in W1.60 is -- 4% and < 1% in W6.80 (none present in the thin section used for microprobe analysis). Most phengite grains in Wl.60, particularly at the

GEOCHEMISTRY AND EVOLUTION OF MOP,B-TYPE ECLOGITES

235

TABLE 2a Electron microprobe analyses of minerals from some Weissenstein eclogites

Wl.6Om: cpx (am} 55.38 0.II 11.99 2.24 0.00 8.93 13.72 6.57

%SIO 2 %['102 %A120 3 %FeO %MnO %MgO %CaO %Na20 %K20 Site occupant3': SI 1.973 AI 0.027 +0.477 TI 0.003 Fe2+ 0.087 Mn 0.000 Mg 0.474 Ca 0.524 Na 0.454

grit (rim} 39.60 0.00 22.63 20.59 0.44 8.43 8.27 0.00

grit (core) 40.03 0.12 22.68 19,28 0.72 7.31 10.46 0.00

5.990 4.071

6.030

0.000 2.605 0.057 1.901 1,341 0.000

K

b}

%SIO 2 %TIO2 %A120 3 %FeO %Mn %MgO %CaO %Na20

W6.Sm--# gnt 39.49 0.11 0.06 10.37 22.35 3,05 21.92 0.03 0.31 9.58 7.59 14.92 8.53 5.66 0.00 ~

54.9cp:x 4

K

6.010 4.008 0.007 2.790 0.040 1.722 1.390 0.000

amph

pheng

pheng

7.0

zo

cp,xa'I

cpxII

plag

plag

45.40 0.33 16,02 9.72 0.13 12.50 9.45 4.30 0.56

45.19 0.58 17.18 9.88 0.05 12.19 9.34 4.21 0.36

50.74 0.58 27.85 1.05 0.03 3.77 0.01 0.79 10.04

50.51 0.58 28.04 1. I0 0.08 3.80 0.00 0.54 10.46

39.59 0.04 32.20 I.II 0.08 0.00 22.77 0.01

39.86 0.04 32.68 1.19 0.00 0.00 22.68 0.04

53.35 0.I0 10.57 3.55 0.03 9.72 16.33 4.75

54.26 0.08 7.22 3.03 0.08 11.86 18.71 3.46

63.97

63.73

22.24 0.I0

21.39 0.10

3.25 9.91 0.19

3.78 9.37 0.25

6.398 6.753 1.602 1.247 +1.265 +3.121 0.062 0.058 1.169 0.117 0.005 0.003 2.572 0.747 1.417 0.001 0.510 0.205 +0.646 0.066 1.705

6.722 1.278 +3.120 0.058 0.123 0.009 0.755 0.000 0.138

1.938 0.062 +0.391 0.003 0.108 0.001 0.526 0.636 0.335

1.972 0.028 +0,281 0.002 0.092 0.002 0.642 0.729 0.244

2.834 1.161

2.844 1.125

0.004

0.004

0.155 0.851

0.181 0.811

0.011

0.014

6.477 1.523 +1.170 0.014 0.036 2.431 1.160 0.092 0.016 1.642 2.652 1.689 1.444 0.(XX) 0.522 +0.668 0.I01

4.029

I (

cpx 54.89 0.13 10.49 2.68 0.03 9.91 15.09 5.40

%K20 Site occupancy: SI 1.978 AI 0.022 +0.418 TI 0.003 Fe2* 0.092 Mn 0.001 Mg 0.514 Ca 0.576 Na 0.393

amph

1.972 0.028 +0.417 0.003 0.081 0.001 0.531 0.581 0.376

W43.0m 3~.t amph 46.44 0.04 0.34 22.81 15.78 20.28 7.41 0.45 0.05 8.03 14.00 9.07 9.64 3.60 0.25 5.993 4.065 0.004 2.565 0.058 1.810 1.469

.amph 45.93 0.46 16.08 7.10 0.00 13.73 10.03 3.74 0.15

6.577 6.524 1.423 1.476 +1.210 +1.216 0.036 0.049 0.877 0.843 0.006 0.000 2.955 2.907 1.463 1.526 0.453 0.459 +0.536 +0.572 0.046 0.027

1.776

) I~-- W l l 2 m --~ cpx gnt 55.38 39.58 0.12 0.00 10.15 22.78 3,25 22.51 0.02 0.37 9.86 7.99 15.00 7.74 5.61 1.980 0.020 +0.407 0.003 0.097 0.001 0.525 0.574 0.389

5.978 4.055 0.000 2.843 0,048 1.798 1.252

Site occupancies c-~dculated on the b a s i s of 6 oxygens (cpx), 24 oxygens (gar), 23 oxygens (amph). 22 oxygens (pheng) and 8 oxygens (plagi. Where 2 values are given lor AI site occupancy the first one denotes to AI iv and the sccond one to Alvl; the first of two values listed for Na in a m p h is the site occupancy on the M4 position, the second one on the A position. Tcpx.gar (Enis & Green) 1251: W1.60: T = 661°C {p=15kbar) for gar cores and T = 688°C for gar rims W6.B0: T = 681°C (15kbar); W43.0: T~= 687°C (15kbar}; W I I 2 : T = 677°C (15kbar)

contacts with clinopyroxenes, show a reaction rim of unidentifiable composition. Rutile shows two modes of occurrence: (1) as fairly large (up to --- 1 ram) intergranular crystals,

sometimes intergrown with omphacite, and (2) as small inclusions in garnet. Quartz also occurs as tiny inclusions in garnets but also in clinopyroxene and amphibole of all eclogites. Mostly, how-

236

H.-G. STOSCH AND G.W. LUGMAIR

ever, it is found as separate grains or in dusters with grain sizes between -- 0.5 and 2 mm. Primary zoisite is rare in Wl.60 ( ~ 2-4%) where it occurs in clusters with quartz; it has not been identified in W6.80. In eclogite W1.60 all omphacites are surrounded by ~ 0.1-0.3 mm wide reaction rims of cpxix + plag + amphxx as mentioned previously and described in [2]. The jadeite component of cpxn is much lower than that of omphacite although highly variable. Plagioclase is albite-rich (> 80 mole%). The presence of these reaction rims testifies to a retrograde overprint of the eclogites at much lower pressure. If eclogitization pressures in the range of 1.5-2 GPa are assumed [2,12], equilibration temperatures can be calculated for coexisting clinopyroxene and garnet [25]. These temperatures are = 660-680 ° C for W1.60 and W6.80 without analytically significant differences between mineral cores and rims. These values are slightly ( ~ 50°C) higher than previously reported for the Weissenstein eclogite body [2,12] and higher by about the same margin than the temperature assigned to the retrograde amphibolite-facies event in the MM [18,2]. 7. Geochemistry 7.1. Bulk-rocks Whole-rock major and trace dement data are reported in Table 1. All eclogites (including the retrogressed ones) have a uniformly mafic composition and there can be no doubt that they represent metamorphosed basalts. Despite considerable variation, the overall major dement com-

2.ool

,o

o

• [--

000 0

0.60

e'.

I~ []

O 0.40 ~

D

1.00

'::~iiiiiiiiiiiiiiiiiiiiiiii~iiiiiiiii!iii~:

o D (]

I

o Eclogites • Retrogressed Eclogites 0.00

0.80

Miinchberg Massif

• •

position of the eclogites closely matches that of oceanic tholeiites [10]. The major element variation is particularly well reflected in elements like TiO 2 (-0.6-2.1%), Na20 (-- 1.8-4%) and Mg/(Mg + EFe) [= M g # ] ratios (---0.42-0.69). The range of K20 which primarily resides in phengite is very large ( = 0.010.75%). There is no systematic major element variation across the drill-core and extreme chemical heterogeneity exists even within a few meters (e.g., samples between 27.5 and 36 m depth). The variable M g # ' s are more easily assigned to igneous fractionation rather than to metamorphic features such as variations in the ratio of garnet to clinopyroxene. The fact that Ti shows a good negative correlation with M g # (Fig. 2) suggests that its variability results from the same process. On the other hand, for most samples K20 does not show a systematic dependence upon M g # (Fig. 2); the scatter may be ascribed to primary heterogeneities or to K-mobility. K could have been mobile: (1) before high-pressure metamorphism; (2) during eclogitization; or (3) during retrograde amphibolite-facies overprint which led to decomposition of phengite. Alternative (3) is least likely to be the dominant mechanism because on average partially or completely decomposed eclogites tend to have higher, not lower K-contents than well-preserved eclogites (Table 1). Some samples do show a wdldeveloped negative correlation between K and M g # (dotted field in Fig. 2) which, however, is unlikely to be ascribed to igneous differentiation alone. This will be discussed later. The major element variation of the Weissenstein eclogites is also mirrored in trace element compositions. Rare earth dement (REE) patterns

0.40

'

0.50

'

0.60

• ~

~

0.20

::~ii~))I~....

o

'

0.70 0.40

i

0.50

,D

t

i

0.60

0.70

0.00

Mg/(Mg + ,~Fe) Fig. 2. Ti vs. M g # (left) and K vs. M g # variations (fight) for dark eclogites and related rocks from the Miinchberg Massif.

GEOCHEMISTRY

AND EVOLUTION

OF MORB-TYPE

range from relative light REE-enriched (e.g. samples W1.09, Wl.43, W36.0) essentially unfractionated (e.g. W27.5, Wl14,75) to light REEdepleted (e.g. W33.7, W43) (Fig. 3). Samples from the drill core between ---2 and 15 m are characterized by somewhat peculiar REE patterns with marked La depletions relative to Ce and Nd (chondrite-normalized L a / C e -- 0.65-0.73, e.g. W4.00 in Fig. 3). Samples with the highest K content also have the highest light over heavy REE enrichment (W121.4, W36). Most eclogites and related rocks lack Eu anomalies; when present, Eu anomalies are small and mostly negative (particularly in the drill-core range between -- 100 and 135 m). Again, the samples with the highest potassium also have pronounced negative Eu anomalies. A positive Eu anomaly is observed in one or two samples only (W32.0 and, possibly, W33.7) and may be attributed to the presence of cumulate plagioclase. Heavy REE abundances range from --- 6 to 20 times chondritic (up to 30 if garnet amphibolites and retrogressed eclogites are included). Lanthanum shows more variation, between -- 5 and 50 times chondritic, and the total range of this variation is displayed in the small section of the drill-core between -- 32 and 36 m. Normalized L a / Y b ratios vary between moderate relative light REE depletion ( -- 0.5) and moderate enrichment (-- 3.1; 4.2 if retrogressed eclogites are included). Thus, REE variations are within the range measured in oceanic tholeiites, e.g. normal mid-ocean ridge basalts (N-type MORB) to incompatible element-enriched MORB (E-type MORB) (see e.g. [26]). However, patterns with maxima at Ce-Nd like those observed in the 100

i

i

W36

L

i

i

i

i

i

i

i

i

i

i

r

L

WeiJJenstein Drill Core

5o

~ 1o .r-

5

2

237

ECLOGITES

W33.7

L~& ~ fia ' Smg'. & % Dy rio E'r Tm Yb L',

Fig. 3. REE variation in five eclogites and the retrogressed eclogite W205.

o dark eclogites • retrogressed eclogitesand garnet am hibolites

Th

Hf/3 A / ~ 1 /X ~ /¢Nk~ k \

(~) N-typeMORB E-type MORB alkalinewithini (~ late l p basalts destructiveplate

Ta

Fig. 4. T a - T h - H f diagram for dark and retrogressed eclogites and garnet amphibolites from the Mtinchberg Massif.

drill-core between = 2 and 15 m are not normally found in oceanic tholeiites. Moreover, the large meter-scale variability in REE concentrations and L a / Y b ratios such as observed between 32 and 36 m of the drill core (La in W36.0 is about 9 times higher than in W33.7, and its L a / Y b = 3.6 times higher) argues against this being entirely the result of igneous fractionation if indeed oceanic-type basalts were the protoliths of the eclogites. The abundances of Sc, Cr, Hf, Ta and Th in the eclogites are within the range of the various MORB-types (e.g. [27]), but in few cases (W36.0, W121.4) low Sc and excessive Th concentrations have been measured. In the H f - T h - T a diagram [28] (Fig. 4) the great majority of Weissenstein eclogites plots into the field of E-type MORB and just barely into the field of N-type MORB thus confirming and further constraining the conclusions derived from REE patterns. It is worth noting that three samples (W36.0, W121.4, W205) plot into field 4, representative of basaltic magmas at destructive plate margins a n d / o r modified by continental crust. Most eclogites and eclogite-derived rocks have L a / T a ratios between -- 10, typical of E-type MORB, and 18.5, the average of N-type MORB, see [26] (Fig. 5). However, in this diagram more scatter is apparent than in the H f Th-Ta plot. Again, two eclogites (W36.0 and W121.4) are anomalous in having high L a / T a ratios of 35 and 27, respectively, which is due to their high La concentrations rather than low Ta, again casting doubt that their chemistry can solely be explained by crystal-liquid fractionation.

238

H.-G.

amph

0.0

0.2

0.4

0.6

0.8

0.11,--

wm Ta

Fig. 5. Ta-La diagram for dark eclogites, retrogressed eclogites and garnet amphibolites from the Miinchberg Massif.

7.2, Minerals Trace element concentrations are listed in Table 2b and chondrite-normalized REE data for

La&

RNd

STOSCH

AND

G.W.

LUGhtAIR

Eclogite WeiJLIenstein WI. 60 Sm Eu Gd Tb Dy Ho Er Tm Yb LU

Fig. 6. REE data for minerals and magnetic separates from sample W1.60. Nd data for clinopyroxene, amphibole and the garnet fractions are isotope dilution analyses; aU other data obtained by INAA. ‘*cpx junk” is a magnetic separate consisting of clinopyroxenes with inclusions of other minerals. “ > 1.5 A” is a separate non-magnetic at 1.5 A and contains quartz, rutile, zoisite, little mica and very Littlegraphite.

TABLE 2b Trace element analyses of minerals and magnetic separates


1.93

co.5 3.9

X0.35
CO.7 CO.9 4.9

w

KO.2 <0.09

CO.4 4.15

0.5-LOA l.O-l&P >1,5A=j

0.88 <0.4 19.9

cl.3 45.5

25.5

<0.22 <0.20

~0.6 4.9
2.05

CIor,

dean

“cpxjuuk”1

co.4

oraneew

2.6


4.4

pinkf!Pt

Pl=%fa 9h

0.55.7A= l-l.!&@ >1.5#@ < lOopm6’

29.9 29.1

260 300

22.6 46.0

0.74 1.44

10.7 3.69

38.3 m.0 68.0

296 200 176

68.0 45.2 67.0

0.80 0.83 (0.14)

eo.01

co.1 ~.o!?

4.05 0.162

<0.2 co.1

0.39 0.12 -31

1.13 <0.42 <0.23

0.16 <0.023 CO.04

0.69 1.68

0.28 0.37

0.08

O&Q 0.49 0.81

0.27 0.55 0.89

0.32 0.89 1.27

CO.03 (0.017)

CO.01

0.76 0.M

0.43 0.28

5.15

1.60

1.77

0.70 0.34

0.08

330

9.7 (0.12)

37,s 28.7 6.7

(1.0) 264 280 214

31.9 24.4 9.1

1.49

1.12

44.7

0.68 0.60 0.69

2.82

3.1

m.29

2.50 co.12

3.5 <0.2

7.0 ~V.18

1.03 .X0.03

78.0 4.80

241 291

57.5 11.3

1.07 1.2

1.63 1.54

2.20 0.32

3.95

co.08 7.95 CO.27

<0.013 1.19 (0.64

0.10 84.6 34.9

226 285

(0.10) 57.8 26.1

1.46 2.59

.31 1.13

0.22 3.45

0.032 0.50

6.80 48.2

159 238

4.85 53.5

0.68

<0.036 0.19 6.1

21.5

21.0

(0.092) 2.26 5.80

1.08 9.35

3.9 34.1

30.0

4.30 8.25

0.16 0.24 1.51

<0.24 0.31 3.0

CO.19 o.qs 10.9

qo.015 0.078 1.69

38.4 40.9 55.8

238 283 197

25.7 60.0

3.50 1.83 0.87

GEOCHEMISTRY AND EVOLUTION OF MORB-TYPE ECLOGITES

minerals from sample W1.60 are plotted in Fig. 6 together with the whole-eclogite data. Due to their very low concentrations of light and heavy REE, only incomplete patterns were obtained for clean clinopyroxenes and amphiboles. Clean clinopyroxene, amphibole and garnet have strongly depleted light relative to middle REE patterns. The pyroxenes show a pronounced maximum at the middle R E E with surprisingly low overall REE abundances which contrast strongly with the much higher concentrations found for cpx in other metamorphic rocks (such as granulites [29-31]); they are even unlike most clinopyroxenes from eclogite xenoliths found in kirnberlites [32]. Particularly the light REE are strongly excluded from the lattice of the clinopyroxenes from the Miinchberg Massif eclogites. Amphibole has even lower concentrations of middle REE and higher concentrations of heavy REE than clinopyroxene, again atypical of amphiboles from other metamorphic rocks or basaltic volcanics. Garnets have steep REE patterns with high heavy and middle over light REE enrichment. For both eclogites significant variation in REE concentrations between garnet cores and rims was observed with the orange cores having lower middle REE and higher heavy REE abundances than the pale pink rims. The low heavy REE abundances in clinopyroxenes and amphiboles simply reflect their equilibration with garnet into which these elements are strongly partitioned over all other common rock-forming minerals. In both eclogites clean clinopyroxene, amphibole, garnet, phengite and quartz have negligible abundances of light REE; less than 5% of the bulk-rock La is contained in these minerals. This requires one or more additional phases to be present which have not been analyzed. A fraction of rutile was analyzed from a kyanite-eclogite from the MM and was found to contain entirely negligible REE. Rutile therefore is unlikely to be a significant REE carrier in these rocks. For a kyanite eclogite from the MM, Puchelt et al. [33] have reported that in zoisite the light REE concentrations are highly enriched over the whole-rock ( = 15 times for La) and all other minerals. If the REE abundances measured by them are typical for the kyanite-free eclogites also, then -- 5-10% modal zoisite must be present in W1.60 and W6.80. However, no zoisite was found in W6.80. Sample W1.60 contains some 2-4% modal zoisite and thus

239

the major part of the light REE could be locked up in this mineral indeed. This is qualitatively confirmed by REE abundances in a fraction nonmagnetic at 1.5 A in the Frantz separator. It consists of quartz, phengite, rutile, zoisite and a few symplectite minerals. This fraction has La = 3 times higher than the total rock and is strongly light over middle and heavy REE-enriched. Secondary plag from the reaction rims around omphacites may also contain non-trivial quantities of light REE; but the small grain-size and intergrowths did not allow separation for analysis. Nevertheless, we point out that clinopyroxene and garnet from W6.80, which contains less than ~ 5% decomposition products [11], have N d concentrations -- 2.5-3 times higher than those phases from W1.60 although equilibration temperatures and pressures as well as the bulk chemistry of these two rocks are essentially identical. Consequently, the different light R E E concentrations of clinopyroxenes and garnets are likely to result from different mineralogies, i.e., equilibration of primary clinopyroxene and garnet from Wl.60 with zoisite a n d / o r secondary plagioclase + clinopyroxene. Also, note that a sample of magnetically, not hand-picked "cpx junk" which is dull and inclusion-rich with intergrowths of big rutile, has much higher light REE concentrations and a S m / N d ratio similar to the bulk (Table 2b). An additional though minor reason for the REE mass-imbalance of W1.60 may be the existence of strong concentration gradients across the drillcore; over a distance of only -- 2.5 m a La variation of up to a factor of 9 was observed [W33.7 and W36.0). A variation of a factor of --- 2 over a distance of -- 10-15 cm therefore might be feasible (--1.25 as judged from differences in the concentrations between W1.43 and W1.60) and could account for part of the difference in light REE between the rock-chip used for mineral separation and the one processed for bulk-rock analysis. For W6.80 the light REE carrier remains unidentified. The least magnetic fraction (non-magnetic at 1.5 A) containing qtz + cpx + rut + zo + pheng is light over heavy REE-enriched with a La abundance less than 20% that of the bulk; this is consistent with the near absence of zoisite in this rock. Chromium shows no systematic behavior and is partitioned about equally between clinopyroxene, garnet and amphibole. Scandium is strongly parti-

240

H.-G. STOSCH A N D G.W. L U G M A I R

TABLE 3 S m - N d - and Rb-Sr-isotopic compositions

a) whole-rocks 147S~n/144Nd l ~ q d / 1 4 4 N d

~Nd(0)

~11

Nd

87Rb/8~

87Sr/86~r

1~1

~-

0.513001 ±14 0,513001 ±14 0,512936 ±14 0.513036 ±14 0.513121 ±14 0,512536 ±14 0.513318 ±14 0.513033 ±14 0.512073 ±18 0.512931 ±14

7.1 ±0,3 7,1 ±0,3 5.8 ~.3 7.8 ±0.3 9,5 z¢.3 -2.0 ~-O,3 13.3 ~-~.3 7.7 -20.3 -11.0 =0,4 5.7 ~0.3

4-345

15.44

0.0421

0.704199

3.26

224.2

6.470

23,70

0,00131

0.079

174,5

6.518

25.00

0,0168

0,703977 ~26 0,704819 ±26

0.816

140,1

4.472

15.643

1.435

4.113

0.0338

5.342

21.07

0.1141

2.425

5.417

0.0320

3.483

11.11

0.0301

4.369

22.64

1.026

4.943

13.80

0.2332

0.1902

0.512947 =28

6.1 ~0.5

3.054

9.706

0.0186

0.1573

0.512965 ±14

6.4 ==0.3

1.949

7.488

0.0117

Wet~enstee~ Wl.60m

0.1701

W4.00m

0,1651

W15,2m

0.1576

W27.5

0.1728

W33.7m

0.2109

W36.0

0.1533

W43.0

0.2705

W114.75

0.1894

W153.5 [gneiss] W205

0.1167

FaWnaw SI14

0.2165

0.69 0,704376 +9.6 0,706052 ~26 0,704341 ~26 0.704461 +9.6 0.718042 +9.6 0.708410 ±38

1.27

108.7

7.06

179.0

0.82

73.8

1.42

136.2

76.1

214.7

3.36

41.7

0.703874 ~26

1.59

247.3

0.965

238.8

0.157

180.1

<0.011 (125pg) 1.12

185.4

b) mlnerals W1.60: "cpx Junk" cpx # 1 (25.92mg) cpx#2 (I 1.22mg) amphibole

0.4845 0.8889 1.837 3.490

(13.22mg) plnkgnt#2 (48.92mg) phenglte

0.7225

<0.00017

0.4914

0.3341

0.9303

0.4443

0.1462

<0.019

0.816~

0.1414

0.9500

0.1984

0.018

0.704070

0.00252

(9.59'mg) orauge g n t (21.74mg) plnkgnt#12

0.5791

0.703992 ~26 0.70396~ ~.26 0.703972 ±26 0.704106 ~26 0.703978

0.513787 ~20 0.514823 ~32 0.517327 ±50 0.521248

~ 2.895

0.519856 ±14

22.5 ~43.4 42.7

~.6 91.5 ± 1.0 168.0

0.4804 0.4654 1.735 2.804

0.513768 ±18 0.513724 ±16 0.516937 *9.2 0.519568 ±22

<0.0057 (125pg)

0.867

0,0059

0.932

±0.9 140.8 ±0.3

±26

22.1 ±0.4 21.2 ±0.3 63.9 z0.4 135.6 ±0.4

(290pg)

0.1776

0.704945 +9.6

51.5

839.6

0,703876 ~26 0.704007 ~26

0.0026 (54.3pg) 0.670

171.7

(6.13mg) W6.80: cpx (21.04rag) ampl~bole3 (17.93mg) orange grit (22.39mg) p i n k grit (20.02rag)

107.1

1.632

2.053

0.000044

0.8653

1.124

0.0202

1.368

0.4760

2.285

0.4926

96.07

0.0030

(68.4pg 0.0020 (39.5pg)

B l a n k corrections h a v e n o t b e e n applied to t h e s e data; typical analytical b l a n k s are: S m 10pg, Nd 20pg, Sr 60 pg, Rb 20pg. For c a l c u l a t i o n of t s o c h r o n a g e s S m - N d b l a n k corrections h a v e b e e n applied to all m i n e r a l s for w h i c h a m o u n t s dissolved a r e given. C o n c e n t r a U o n s are considered to be a c c u r a t e to ±1%o for S m a n d Nd, ±2%o for S~ a n d ±5%0 for Rb e x ~ t w h e r e b l a n k c o n t r i b u t i o n s are cons[derable. Q u o t e d errors are 2 a d either m - r u n precis[on or ± 0 . 0 0 0 0 1 4 for Nd a n d ± 0 , 0 0 0 0 2 6 for Sr reflecting t h e total r a n g e for t h e reproducibility of s t a n d a r d m e a s u r e m e n t s . All c o n c e ~ t r a U o n s in weight p p m . 1Values in p a r a n ~ a r e total a m o u n t s of m e a s u r e d Rb; 2Rb a n d Sr were accidentally c o B t ~ m i n a t e d d u r i n g Ion excharlge s e p a r a t i o n in t h e laboratory. C o n t a m t n a t i c m of S m a n d Nd c a n theTefore n o t be excluded. For S m - N d i s o c h r o n age calculations n o r m a l - s i z e b l a n k s were a s s u m e d , except w i t h a Nd isotopic c o m p o s i t i o n of t h e c o n t a m i n a n t . ~tains s o m e m t ~ g r o w t h s of q u a r t z

GEOCHEMISTRY

AND

EVOLUTION

OF

MORB-TYPE

241

ECLOGITES

tioned into garnet. The garnet rims show higher concentrations of Sc and Co than do the cores; this parallels the trends for Fe zoning. Hafnium has been found in clinopyroxene, amphibole and garnet. The concentration is highest in a magnetic separate of clmopyroxene from W1.60 in which some rutile is intergrown with clinopyroxene. The Ti-bearing mineral rutile is expected to contain some H f also and thus it is likely to be the source of the elevated H f in the "cpx junk" separate. A small amount of Cs has been measured in phengite from W6.80 which is to be expected in K-rich minerals.

7.3. Isotopes Neodymium and strontium isotope data for bulk-rocks and minerals are reported in Table 3. Not surprisingly, the eclogites have Sr concentrations within the range of E-type MORB; they vary by a factor of ~ 3. Rubidium concentrations are low though highly variable and mimic the scatter of K. As a consequence, R b / S r ratios are low and variable, too. It is immediately evident that the range in 87Sr/86Sr (=0.7040-0.7060) for the Weissenstein eclogites (excluding retrogressed sample W205) cannot be accounted for by in-situ decay of 87Rb with the measured R b / S r ratios. For example, W15.2 has the second lowest R b / S r ratio and the second highest 87Sr/86Sr. Strontium isotopic compositions as high as = 0.704 combined with relatively large Sr isotopic variations are not expected for fresh tholeiites from oceanic spreading centers, thus arguing against this being a feature of the mantle-derived melts. With respect to S m / N d ratios the eclogites encompass samples with slightly higher (W33.7) to moderately lower than chondritic values (W15.2). Sample Wl14.75 has a S m / N d ratio only slightly lower than chondritic implying that its present-day CNa-Value is almost independent of age. The value of +7.7 is in the lowest range of present-day MORB. Sample $114 from the Fattigau locality (Fig. 1) has an almost identical S m / N d ratio but an eNd(0 ) ValUe of only + 6.1 indicating some Nd isotopic heterogeneity of the dark eclogites throughout the MM. This may be ascribed to different mantle sources or point to secondary processes which modified the Nd isotopic composition after melt extraction from the mantle.

For the Weissenstein eclogites it seems that for a given end their 87Sr/86Sr ratios are quite high. 8. Discussion

8.1. Age of high-pressure metamorphism The overall chemical composition of the dark eclogite type from the M M and in particular the Weissenstein body strongly suggests that these rocks represent m e t a m o r p h o s e d ocean-floor tholeiites from mid-ocean ridges or the spreading centers of back-arc basins. Similar conclusions have also been reached for several other eclogites in central and western Europe [34-36]. Furthermore, the petrologic data indicate that eclogitization occurred at pressures in excess of -~ 1.5 GPa [2,12]. This is most easily explained by subduction-related formation for which, however, detailed geologic models for the M M have not yet been proposed. A number of K - A r data for muscovites and amphiboles is known from amphibolite-facies rocks (amphibolites, gneisses, metagranodiorite) from the " H a n g e n d - " and "Liegendserie" in the MM. They cluster around 380 Ma [17,16], see Fig. 7. Two phengites from the kyanite eclogites of Oberkotzau (Fig. 1) show even slightly higher K - A r ages of 387 and 388 Ma (the largest grainsize fractions only; smaller grain-sizes have yielded ages ~ 10 Ma younger [17]) On the other hand, there are also A r - A r data for white mica from eclogites from the same locality which are as low as = 360 Ma [37]. The reason for this discrepancy of ages is yet unknown. At least for the gneisses

•

Hangendserie. amphiboles

m,I. l:! []

Hangendserie, muscovites

[]

Liegendserie. amphiboles

•

Liegendsede. muscovites

[] )D

i 360

i

i

i

I

i

370

,

i

i

i

I

380

i

i

i

i

i

390

i

i

• [] ,

i

I 400

v

i

I

i

[

i 410Ma

Fig. 7. Compilation of published K-At ages for amphiboles and muscovites from meta-igneous (amphibolites, metagranodiorite, eclogites) and metasedimentary rocks from the MM [17,16]. Two data with ages < 360 and one with an age of 410 Ma have not been plotted. Quoted analytical errors (lo) vary between + 2.5 and 14 Ma, but are mostly < 8 Ma.

H.-G. STOSCH AND G.W. LUGMAIR

242

and amphibolites these ages are most likely to reflect cooling after the medium-pressure event of Bltimel [18]. In the eclogites this event led to formation of reaction rims around the omphacites and incipient decomposition of the phengites. Temperatures at which Ar loss becomes negligible may be around 500-550°C (amphibole) and 350 °C (white mica) [38,37]. The overlapping ages for hornblende and white mica can thus not easily be interpreted as points on a single steep cooling curve for all lithologies of the "Liegend-" and "Hangendserie", following the medium-pressure event. Rather they reflect disturbances of the K - A r isotope system and/or indicate different cooling histories for different parts of the MM [16]. A generally-held assumption is that minerals will close with respect to isotopic exchange for the Sm-Nd system and possibly also for the Rb-Sr system at significantly higher temperature than for the K-Ar system which involves a noble gas. As long as the high-pressure mineralogy is well-preserved, it might thus be expected that on a mineral to mineral scale the Sm-Nd system in the eclogites was frozen at the time of high-pressure crystallization and was not reset by the retrograde overprint at about equally high temperatures. Figure 8 shows the Rb-Sr isotope data for sample W1.60 and its minerals in an isochron diagram. Three fractions of dinopyroxene, two fractions of garnet, and one fraction each of amphibole, phengite and the whole-rock form a 0.7052 phen_~..../

Eciogite W l.60

0.7048 r~ 0.7044 ;g

t = 393+13Ma Initial = 0 . 7 0 3 9 5 2 ± 1 ~

i

bulk /

i

i

I

[

i

bun,

*q 0.7040

070401q'.~J~~

I grit 0 7038] 0.00 0.01 0.02 0.03 0.04 0.05

0.7036 0.7032 0.00

' =3~--.51~

0.05

0.'10

0.'15 87Rb/ 86Sr

0.20

Fig. 8. R b - S r isochron diagram for eclogite W1.60. The small inset shows a blowup of the low R b - S r section of the plot. A best-fit line for all samples except phengite results in essentially the same age, but with a much larger error. All errors are 2o. For h87rtb a value of 1.42 x 10 - n a -1 was used.

0.522

'

'

'

'

'

0.520"

' //~-;nk

~

Eclogite W I.6O

grit I

t=395+4Ma J p i n k

gritII

~//'~-- t = 3 ~ ± 2 ~

05

/ / " ' Iniaal~9.5~73~1o

s

~ 0.516

/

0"514" ~ P ~ ' ' f

o-513~1

1 5511;~'Jw'~,hole.rTk 0.00

°'m0

/

i

0.20

,

.

0.40

2 147Sm/144Nd

Fig. 9. S m - N d isochron diagram for eclogite Wl.60. The isochron labeled "t = 395 + 4 Ma" is calculated through "cpx junk", bulk, cpx II, amphibole and orange garnet; the line labeled " 3 8 4 + 2 Ma" is a best-fit line through "cpx junk", whole-rock, cpx II, amphibole and pink garnet II. For the calculation of this and all other S m - N d isochrons JklaTsm= 6.54 ×10 -12 a -1 was used. Data for cpx II, amphibole and the garnets were corrected for typical laboratory blanks (Sm 10pg, N d 20pg). This blank correction effectively results in a shift of the data points along the isochron towards higher S m / N d ratios and higher 143Nd/1"4 N d values and thus does not significantly change the age.

well-defined isochron of age 393 + 13 Ma which is only about 10 Ma higher than the highest K - A t ages for the "Hangendserie" amphiboles cited above [17,16]. The calculation of the best-fit line in this and all following isochron diagrams follows the procedure of Wendt [39]. Within the quoted analytical errors all data points fall on the Rb-Sr isochron. The error in the age but not the age itself is controlled by phengite. The relatively large errors are a consequence of the rather small spread in Rb/Sr ratios displayed by minerals from eclogites. Figure 9 shows the Sm-Nd data for the same eclogite. The first striking observation is the extraordinarily large spread in 147Sm//144Nd ratios (between = 0.16 and 3.5!) and, hence, Nd isotopic compositions displayed by clinopyroxene, amphibole and the garnet fractions. This demonstrates the potential power of the Sm-Nd system for the geochronology of such rocks. Secondly, all analyzed optically clean minerals have S m / N d ratios much higher than that of the whole-rock (between almost three times for cpx and = 20 times as high for pink gnt I) with Sm and Nd concentrations much lower. Only the dull, inclu-

GEOCHEMISTRY AND EVOLUTION OF MORB-TYPE ECLOGITES

sion-rich "cpx junk" has a S m / N d ratio slightly lower than the whole-rock. A straight line fit through all data points results in an age of 385 + 2 Ma (not shown in Fig. 9), i.e. within error limits identical to the Rb-Sr age. This close agreement between two independent radiogenic isotope systems which involve different minerals is very encouraging and lends credibility to the assertions that: (a) the age represents a real (metamorphic) event and not just a point on a long cooling and decompression path of the eclogites; and (b) subsequent to this date the Weissenstein eclogite body was not subjected to severe metamorphic reheating (>_ 600°C) a n d / o r high-temperature alteration again. Upon closer inspection of the Sm-Nd data for mineral separates from Wl.60, it is apparent that the three garnet fractions do not plot on a single straight line through clinopyroxenes, amphibole and the bulk. Orange garnet plots about 2.8 cN0units above and pink garnet I = 2.6 eNd-Units below a straight line which significantly exceeds the quoted analytical uncertainties. It appears justified, therefore, to fit individual straight regression lines through the different garnet fractions. The line through "cpx junk", whole-rock, clinopyroxerie, amphibole and orange garnet corresponds to an age of 395 _+4 Ma, the line through "cpx junk", whole-rock, clinopyroxene, amphibole and pink garnet II to an age of 384 _+ 2 Ma (both lines shown in Fig. 9). A line through "cpx junk", the total rock, clinopyroxene, amphibole and pink garnet I (not shown) would yield an age of 380 _+ 2 Ma. The garnet cores have recorded a slightly higher age than the garnet rims. It is not possible to decide whether pink garnet I indicates an even lower age than pink garnet II due to potential contamination of the former during chemical processing (see Table 3) and therefore less significance will be assigned to this data point. The most straightforward interpretation for the age difference revealed by garnet cores and rims is that the mineral isochrons record garnet growth during eclogitization. Cores and rims were sampled some 2-4 mm apart; the age difference would thus correspond to growth rates of --0.2-0.4 mm per Ma. On the other hand, the Sm-Nd garnet rim age is only marginally higher than the average of K - A r dates for hornblendes from meta-igneous and meta-sedimentary amphibolite-facies rocks

243

from the "Hangendserie" (380 Ma, see Fig. 7). Since the K - A r ages--except for the two eclogite micas [17]--are not related to a high-pressure event, they rather must date cooling below = 500°C after the episode of medium-pressure metamorphism. The close proximity of K-Ar, Sm-Nd and Rb-Sr ages is thus somewhat surprising. As noted before, eclogite W1.60 shows evidence of substantial retrograde overprint particularly in reaction rims around omphacite. This might be taken to argue that the Sm-Nd and Rb-Sr ages for this rock reflect the retrograde stage also and not the high-pressure event. In order to exclude this possibility we have chosen eclogite W6.80 from one of the freshest parts of the Weissenstein body for a second Sm-Nd isochron which is plotted in Fig. 10. Amphibole, clinopyroxene, garnet cores and garnet rims form an isochron of age 384 _+ 3 Ma which is identical to the garnet rim (pink garnet II) age for W1.60. All data points plot on the isochron within limits of experimental error. There is just a faint and statistically insignificant indication that also for this rock the garnet cores might have recorded a slightly higher age than the rims (amphibole, clinopyroxene, orange garnet: 385.5 __ 5.2 Ma; amphibole, clinopyroxene, pink garnet: 383.7 _+ 3.3 Ma). In the inset of Fig. 10 we have plotted the Sm-Nd mineral data for samples W1.60 and W6.80. A straight lin fitted through all data points (except orange garnet and pink garnet I from W1.60) results in an age of 0.520

./

Eclogite W6.80 0.519" 0.518"

/

orange/~ i . . . . . . .

_~ 0.51"/"

/

0.516" Z

-~ 0.515/

0.51& 0.5131

//-pink grit

t = 384+3Ma Initial = 0.512558±19 gNd(t)= 8.1i-0.4

o=t-.,0 , 7 /

/

0.516"1 I

7

0

I"

J

1

o wl.6o 2

3

0.512 1

2

3

4

147Sm/144Nd Fig. 10. S m - N d isochron diagram for eclogite W6.80. The inset shows the data for b o t h Weissenstein eclogites. The straight line is fitted t h r o u g h all data points except orange garnet and pink garnet I f r o m W1.60.

244

384 ___2 Ma. The quality of the fit indicates that at this time the Weissenstein eclogite complex was isotopically homogeneous on the scale of = 5 m and maybe more. This may, of course, be the consequence of nearly identical S m / N d ratios of the total rocks in this part of the drill-core section, inherited from the magmatic stage. The identical S m - N d mineral ages from the two eclogites strongly suggest that they indeed date eclogite metamorphism. This interpretation is supported by the chemical zoning patterns of garnet from the Weissenstein eclogites (Fe and Mg inreasing, Mn and Ca decreasing from core to rim) which are commonly interpreted as relics of prograde zoning during eclogitization [22,12]. Following Ghent et al. [40], such zonations indicate that temperatures have never exceeded = 650700 ° C because otherwise the garnets would have homogenized. On the other hand, for eclogites Wl.60 and W6.80 temperatures derived from the Ellis and Green geothermometer [25] are only ---10-20°C higher for garnet cores than for the rims which statistically is not significant. Temperature differences that small have been derived previously for the Weissenstein eclogites [22], thus suggesting the compositional zoning of garnet does not translate into a substantial difference in temperature. Additional evidence for our interpretation to be correct comes from comparison with results from other S m - N d studies. In particular, Mork and Mearns [41] have investigated a gabbro-to-eclogite transition in the western gneiss region in Norway. They found that complete chemical and textural transformation to eclogite is necessary in order to reset the S m - N d isotope clock on a mineral-tomineral scale; otherwise mixed ages between the igneous and the metamorphic phases or no ages are obtained. Cohen et al. [42] have studied granulite-facies clinopyroxene- or t h o p y r o x e n e garnet coronas which have survived in a much younger eclogite-facies shear zone in the Bergen arcs in Norway and found that they still have recorded the granulite episode. And Jagoutz [43] has reported a S m - N d mineral isochron from an eclogite xenolith which equilibrated at T > 800 ° C. All these results lend credibility to the interpretation that our S m - N d mineral isochrons record the age of eclogitization of former tholeiites in the MM, consistent with previous inferences of

H.-G. STOSCH AND G.W. LUGMAIR

Gebauer and Grianenfelder [14] from U - P b data. The small age difference of < 10 Ma between S m - N d ages for eclogites and K - A r ages for amphibolite-facies rocks, however, is somewhat surprising and presumably indicates fast decompression after eclogitization. 8.2. Protofith age and igneous evolution In systems with low fluid activities, on a whole-rock scale the S m - N d isotope clock will frequently be started at the time of igneous crystallization without being reset during subsequent metamorphic events. Since the spread in S m / N d ratios is fairly large for the Weissenstein eclogites (Table 3) we may hope that their protolith age is still recorded by the S m - N d system. Our data for the kyanite-free eclogites are plotted in Fig. 11 together with data for a country-rock gneiss from the Weissenstein drill-core. Seven out of the nine Weissenstein eclogite data points scatter closely along a straight line which corrresponds to an age of 480 + 23 Ma with an initial a43Nd/~a4Nd of 0.512464 + 29. Within error limits this age is identical to the one calculated by Gebauer and Grianenfelder in their U - P b zircon study (525 +40_3~ Ma) [14] for the upper intercept between concordia and discordia by assuming a common geological history for the Weissenstein eclogites and the metagabbro from the nearby Steinhtigel occurrence. This agreement between two independent

Kyanite-free Eclogites, Mtinchberg Massif

0.5132

w 3 3 . ~

w27.5_ ~

j

~

w152

WI.60

_o

0.5128 0.5124

z 0.5124

0.15

•

s114

w~05

J

, .

0.5120

~lt gneissW153.5 0.5120 0.10

I t = 480-~23Ma I

.

0.51161

0.10

I --"

~i~

,

|

,_

.~.~~"

"~'. J~'--~'

.4j~" . ~ , ~

0.15

0.20 1475m/144Nd

0.20

0.25 0.25

Fig. 11. S m - N d isotope evolution diagram for the kyanite-free eclogites. The small inset shows isotopic relationships backcalculated to 480 Ma. The dotted lines connecting the value for the country-rock gneiss with the 143Nd/144Nd initial of the whole-rock isochron through the data points for W36.0 and W205 represent potential mixing lines.

GEOCHEMISTRY

AND EVOLUTION

OF MORB-TYPE

245

ECLOGITES

isotope systems supports the interpretation that this is the igneous age of the eclogite protoliths, i.e. the time of extrusion and crystallization of basalts. A pre-eclogite stage of metamorphism is excluded because there is no evidence for such an event in chemical mineral zonations or eclogite textures. The high initial end of 8.7 would have been a typical value for ocean-ridge basalts at that time. Two samples, eclogite W36.0 and retrogressed eclogite W205 plot far below the isochron in Fig. 11. Both have been identified as falling into the field of mafic magmas at subduction zones or to be contaminated by continental crust in the H f T h - T a diagram (Fig. 4). In addition, W36.0 is significantly light REE-enriched, has a negative Eu-anomaly, a high L a / T a ratio (Fig. 5) and high K (Fig. 2). All these features are consistent with the interpretation that W36.0 is severely contaminated by crustal material. The N d isotopic composition of W205, its position in the H f - T h Ta diagram and the peculiar enrichment of La over Ce and Nd, but depletion of La relative to the middle R E E suggest a similar explanation for this rock. This is likely to hold true also for some other eclogites for which isotope data have not been obtained, in particular W121.4 and W78.98. Two potential ways of contamination can be envisaged: (1) in a crustal m a g m a chamber by an a s s i m i l a t i o n - f r a c t i o n a l crystallization process which seems to be important for magmas crystallizing in the deep crust (e.g. [44]); or (2) mainly by two-component mixing during extrusion of the m a g m a in an ocean basin on or into sediments derived from continental crust. At least in the case of W36.0 the latter process appears more plausible because samples a few meters up (W33.7) and down (W43.0) the drill-core lack evidence for substantial contamination. If so, uncontaminated eclogites and meta-sedimentary country-rock gneisses of the "Hangendserie" are candidates as end-members of the mixing process. In the top 74 m of the drill-core only eclogite has been recovered; country-rocks dominate the deeper parts of the core [11]. One country-rock gneiss has been analyzed (W153.5 m) and does have a Nd isotopic composition and S m / N d ratio appropriate for the crustal end-member (Fig. 11). The dotted lines connecting the N d isotopic composition of the gneiss at t = 480 Ma ago with the initial

0.5126"

Hypothetical Isotopic Relationships - 4 & O M a ago

-_

estimated

0.5124- -

~

0.5122"

~

0.5120"

~ _ _ a v e r a g e - _ _ o f

--

alteration -

•

---~

~

.~

°Fattigau

"malltle - -array"

t--

0.5118"

o.,,2ot

_

...........

0.5116" 0.702 0.704 0.766 0.708 0.710 0.712 0.5114" 0.702

0.703

0.704

0.705

87Sr ] ~ S r

Fig. 12. Hypothetical Nd-Sr isotopic relationships of dark eclogites (Weissenstein, Fattigau) ---480 Ma ago. The inset includes the country-rock gneiss and shows possible mixing curves between uncontaminated eclogite protolith and the gneiss.

143Nd//144Nd value of the whole-rock isochron through the data points for W36.0 and W205 in the inset of Fig. 11 may then be possible mixing lines. The low end value of the single measured eclogite from Fattigau ( = 2e units below the isochron) m a y be indicative of the presence of a crustal contaminant in this sample also rather than relate to different mantle sources. Figure 12 shows the N d and Sr isotopic compositions of the dark eclogites back-calculated to 480 Ma. For comparison, the average estimated " m a n t l e array" at that time is given also. All of these MORB-type eclogites--and not only those two for which crustal contamination has been i n f e r r e d - - p l o t to the right of a n d / o r above the mantle array. Since the S m - N d isotope system is usually considered to be less affected by metamorphism and alteration than the R b - S r system, we believe that 87Sr/86Sr of the eclogites, uncontaminated with respect to Nd, is anomalously high. Two reasons can be offered to account for this observation: (a) The present R b / S r ratios are much lower than the original igneous values. During eclogitization or amphibolite-facies m e t a m o r p h i s m either Rb was variably released from or radiogenic Sr introduced into the eclogites. Their Sr concentrations are well within the range of ocean-ridge basalts. However, rubidium concentrations show a large unsystematic scatter and this element might well have been mobile during metamorphism. A

246 source for the infiltration of radiogenic 87Sr/86Sr during metamorphism could have been fluids released from country-rock gneisses. In the case of closed-system evolution, gneiss W153.5 would have had 87Sr/S6Sr -- 0.7124 at 385 Ma ago. Mixing of 10-20% of such Sr with mantle Sr(STSr/86Sr < 0.703) would have been sufficient to raise the Sr isotopic composition of the eclogites to their observed values. However, such mixing or infiltration also should have severely raised Rb concentrations in the eclogites which is not seen. (b) Young MORB frequently show elevated 87Sr/86Sr as a consequence of low- and high-temperature alteration with seawater [45,46]. Presentday seawater contains 13 ppm of Sr and --450500 Ma ago had an 87Sr/86Sr composition of = 0.7085-0.7090, only slightly lower than today [47]. Neodymium concentrations, on the other hand, are extremely low in seawater (-- 3 × 10 -6 ppm [48]). Thus, the Nd isotopic composition of MORB will not be affected by alteration caused by seawater. Option (b) seems more plausible to us than a) because fluid infiltration during metamorphism should also be expected to introduce a low 143Nd/144Nd component for which there is no evidence in these seven eclogites. In addition, oxygen isotope analyses of minerals from a Weissenstein eclogite yield a calculated bulk-rock 8180 of + 3.4 [49] which is at least 2%0 lower than values expected for unaltered basalts and displaced towards values for seawater (8180 = 0). A similar displacement is observed in some sequences of oceanic crust, mostly however in the dike and gabbro complexes and not in the pillow basalt sections (for a review see [50]). The low 8t80-values displayed by many eclogites cannot be produced by closed-system equilibration of fresh ocean-ridge basalts during eclogitization, thus suggesting hydrothermally altered oceanic crust as their igneous precursors [51]. High-temperature hydrothermal alteration (but not with seawater) has also been inferred previously to have produced the low 8180 values of other eclogites [32,49,52]. We therefore suggest that the dark eclogites from the MM experienced variable degrees of alteration with water or fluids having radiogenic Sr isotopic compositions. An episode of high-temperature alteration is required to account for the low 8180 value measured by Vog.el and

H.-G. STOSCH AND G.W. LUGMAIR

Garlick [49] for a Weissenstein eclogite. The high pressures of > 1.5 GPa necessary for eclogitization [12,2] are most easily envisaged to have acted on these pieces of oceanic crust during some kind of subduction. The inset in Fig. 12 shows possible mixingcurves between the hypothetical eclogite protolith and the gneiss -~ 480 Ma ago. No attempt was made to quantitatively model the contamination process because of the large number of variables (for example Sr and Nd concentrations of endmembers, range in isotopic compositions of endmembers, extent of seawater alteration of W36.0 and W205, or Rb mobility during metamorphism). However, the mixing-line passing through W205 requires that, for simple two-component mixing, the (Sr/Nd) ratio of the uncontaminated basalt be smaller than that of the crustal end-member; the line passing through W36.0 represents the more typical case and requires (Sr/Nd)basal t > (Sr/Nd)crustalend_member. W205 has a very low Sr/Nd ratio (3.0), much lower than the gneiss, and thus it is clear that small degrees of contamination with evolved crust will affect its Sr isotopic composition much more severely than its 143Nd/144Nd. However, Sr/Nd of W36.0 is still slightly smaller than that of the gneiss. If this was also a feature of the uncontaminated protolith and if the gneiss composition matches that of the contaminant, then two-component mixing is too simple a model and magma-sediment mixing may have been accompanied by fractional crystallization. Judged from the Rb (Table 3) and Cs concentrations (W153.5 -~ 3.5 ppm, W36.0 < 0.35 ppm) simple mixing would lead to an unrealistically small crustal component in W36.0, and judged from Th concentrations (Table 1) mixing would require a crustal component higher than tolerable by the major element chemistry. Thus mixing may indeed have been accompanied by magma fractionation. The evidence for crustal contamination of some Weissenstein eclogites warrants a thorough reevaluation of possible genetic links between kyanite-free (mostly melt compositions) and kyanite-bearing eclogites (mostly cumulate compositions) as well as between eclogites and metagabbroic amphibolite-facies rocks in the MM in terms of mixing or assimilation-fractional crystallization models. This is of particular interest for

GEOCHEMISTRY AND EVOLUTION OF MORB-TYPE ECLOGITES

the Weissenstein eclogites and the meta-gabbro from the nearby Steinhiigel (see Fig. 1) which apparently have an identical igneous age ([14] and this study). 9. Summary Dark colored eclogites occur in the MM intercalated with amphibolite-facies meta-igneous and meta-sedimentary rocks. Major and trace element chemistry indicate that the eclogite protoliths were oceanic tholeiites such as E-type MORB or backarc basin tholeiites. Light REE abundances in clean clinopyroxene, amphibole and garnet are extremely low and S m / N d ratios are very high. This is ascribed to their metamorphic equilibration with minerals having high light/heavy REE ratios, probably zoisite and, maybe, plagioclase. Mineral S m - N d and Rb-Sr isotope analyses yield ages around 380-395 Ma which are interpreted to date the episode of high-pressure metamorphism. A small S m - N d age difference between garnet cores and rims is suggestive of recording garnet growth during prograde eclogitization. The mineral ages are only slightly higher and even overlap with K - A r ages of amphiboles and micas from amphibolite-facies meta-sedimentary and meta-igneous rocks of the "Liegend-" and "Hangendserie" in the Miinchberg Massif [17,16]. A whole-rock S m - N d age of 480 + 23 Ma is interpreted to reflect the age of igneous formation. The same chronology of igneous formation and high-pressure metamorphism has been inferred previously for these eclogites on the basis of U - P b analyses of zircons [14] and is very similar to that of other eclogites in the Variscan belt of central and western Europe (see, e.g. [53] and references cited therein). The end(480 Ma) value of + 8.7 confirms the interpretation that these eclogites derive from ocean-ridge basalts. 87Sr/a6Sr and 143Nd/144Nd, however, are decoupled in that the Sr isotopic ratios are too high at a given 143Nd/la4Nd. This is interpreted as the effect of alteration of oceanic crust with (sea)water or fluids with high 87~ a t / 86St. Together with the high pressures of > 1.5 GPa required for eclogitization [2,12] these features fit the model that oceanic crust like E-type MORB or back-arc basin tholeiites experienced alteration with seawater at elevated temperatures and was

247

transformed to eclogite in a subduction environment. Large variations of incompatible, immobile dements (light REE, Th) as well as Nd and Sr isotopes unsupported by S m / N d and R b / S r ratios on the scale of a few meters in the Weissenstein eclogite body indicate that portions of the eclogite have suffered from contamination with evolved continental crust, probably country-rock metasedimentary gneisses. The evidence for crustal contamination of some eclogites in the Miinchberg Massif suggests possible genetic links between kyanite-free and kyanite-bearing eclogites and meta-gabbroic amphibolite-facies rocks in the MM should be rediscussed in terms of mixing or assimulation-fractional crystallization models.

Acknowledgements S. Matthes donated the samples from the collection of the Institut ftir Mineralogie/Universittit Wiirzberg for which we are very grateful. U. Herpers kindly permitted one of us (HGS) to use the neutron activation facilities of the Abteilung Nuklearchemie der Universittit zu K/Sln. A. Jos, G. Loock and Ch. MacIsaac helped during various stages of the analytical part of this project. A previous version of this manuscript benefitted from critical comments of A. Basu, P. Horn, D. Mi~ller-Sohnius, E. Seidel and the editorial comments of F. Begemann as well as discussions on various aspects of eclogite genesis with E. Jagoutz, R. Klemd, S. Matthes and M. Okrusch. Of particular importance was a thorough and questioning review of the Ztirich group (D. Gebauer, M. Griinenfelder, A. Quadt and J.L. Paquette) which led to a major improvement of the manuscript. This work was supported by grants from DFG and NSF EAR-8618554.

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