SmNd isotopic systematics of a gabbro-eclogite transition

Lithos, 19 (1986) 255 267

255

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Sm-Nd isotopic systematics of a gabbro-eclogite transition MAI BRITT E. MORK* and EUAN W. MEARNS Mineralogisk-Geologisk Museum, Sars Gate 1, 0562 Oslo 5 {Norway)

LITHOS

M~brk, M.B.E. and Mearns, E.W., 1986. Sm-Nd isotopic systematics of a gabbro-eclogite transition. In: W.L. Griffin (Editor), Second International Eclogite Conference. Lithos, 19:255 267. Sm-Nd isotopic systems have been studied in: (1) a sequence of coronitic olivine gabbros showing different degrees of transition to eclogite; (2) coronitic leuco-gabbro norite; and (3) country-rock eclogite, all from the Nord~byane-Brattvgg area, western Norway. Isochrons defined by combinations of wholerock samples and relict igneous phases give ages of 1198 _+56 and 1289 + 48 Ma (olivine gabbro) and 926 -+ 70 Ma (leuco-gabbro norite) and have been interpreted in terms of two different episodes of igneous intrusion. In gabbro with advanced corona formation, relict augite tends to retain its original Nd isotopic composition, while plagioclase may show selective disturbance related to recrystallization and partial replacement by garnet. In completely eclogitized samples, there is a correlation between the degree of isotopic equilibrium and microstructural equilibrium. Whole-rock eclogites occurring within gabbro are characterized by pseudomorphs after igneous phases, and preserve original igneous Nd isotopic composition. Results for eclogite minerals suggest Caledonian ages, but these phases have not achieved complete isotopic equilibrium. In contrast, Nd isotopic systems in texturally equilibrated and strongly deformed eclogite within adjacent gneisses have been disturbed on a whole-rock scale. Minerals from an external eclogite define a Sm-Nd isochron with an age of 400 _+ 16 Ma which is interpreted to date synkinematic eclogite equilibration at high-P and -T conditions. (Received November 15, 1985; accepted May 12, 1986)

1. Introduction The Western Gneiss Region (WGR) of Norway has been subjected to a high-P and -T (HPT) metamorphic regime (Krogh, 1977, 1980; Griffin et al., 1985). HPT-mineral assemblages have commonly been preserved in basic rocks, both as thoroughly equilibrated eclogites and as coronitic gabbros and dolerites with incomplete reactions (Gjelsvik, 1952; Griffin and R~heim, 1973; Bryhni et al., 1977; Griffin et al., 1985; M~brk, 1985). Field relations and petrology of coronitic gabbros and eclogites in the vicinity of Flems~y (Brattv~gNord~byane district; Fig. l a and d) demonstrate a series o f reaction stages between gabbro and eclogite

(M~brk, 1985, 1986). Recent dating of gabbros and dolerites from the WGR (T6rudbakken, 1982; Mearns, 1984) indicate Mid-Proterozoic intrusion ages, while Sm-Nd mineral ages and U-Pb zircon ages for eclogites suggest Caledonian eclogite metamorphism (Griffin and Brueckner, 1980, 1985; Gebauer et al., 1985). S m - N d dating of igneous minerals has proven to be a powerful tool for dating basic and ultrabasic rocks (Jacobsen and Wasserburg, 1979; Edwards and Wasserburg, 1985; Mearns, 1984, 1986b). The relationships preserved on Flems~by therefore provide an opportunity to study: (1) the effects of HPT-metamorphism on isotopic systems in the relict igneous assemblages overprinted by a much later metamorphism; and (2) the rela-

*Present address: Institute of Biology and Geology, University of Troms~b, 9000 Troms4, Norway. 0024-4937/86/$03.50

© 1986 Elsevier Science Publishers B.V.

256

tionship between mineralogical, chemical and isotopic equilibrium in rocks showing different degrees of eclogite transition. In this article we address the problems of: (1) The ages of the various gabbro intrusions in the vicinity of FlemsCy. (2) The age of the HPT-metamorphism which led to eclogitization of the gabbros. (3) The mineralogical and chemical reactions leading to isotopic equilibrium or disequilibrium.

1.1. Geological :;etting The WGR comprises multiply deformed and complexly interrelated ortho- and paragneisses. The gneisses have protolith ages in the range 900-1760 Ma (Pidgeon and R~heim, 1973; Brueckner, 1979; Lappin et al., 1979; Harvey, 1983; Mearns, 1984;

N

+

M~brk, in prep.). The area of the present study is in the northwestern part of the WGR and falls in the highest temperature part of a regional gradient, defined by isotherms based on eclogite geothermometry (Krogh, 1977; Griffin et al., 1985). The geology consists of granitic gneisses with variable degrees of migmatitization, amphibolites/mafic gneisses and strongly deformed paragneisses (Fig. 1a). Gjelsvik (1952) distinguished between two types of gabbro (dolerite) intrusions: olivine-bearing and olivine-free. The gabbros occur as intrusions in the granitic gneisses, and have been particularly well protected from deformation and pervasive metamorphism in the axial zone of an antiform along the south side of the Brattv~g peninsula. Eclogites are most frequent in the migmatic gneisses on the Nord6yane islands. Olivine gabbro on FlemsCy (lot. GE, Fig. ld)

LEGEND

.GE

Metasediments, metabasites

Har

. \ ;~__~=~

EMSQY

Granitic gneisses Granitic augengneisses Amphibolite Gabbro/dolerite Hbl- migm atite, amPhi bol itic gneisses Eclogite occurrence Brattvag

r Loc.GE

7

Fig. 1. a. Geological map of FlemsCy, adjacent islands and tile Irattvlg peninsula with sampling locations. FlemsC~y: GE = gabbroeclogite body (see (d)) and EH = layered country-rock eclogite. HaramsCy (S~eberget quarry): SAE = leucogabbro norite body (samples SAE1 and HI) cut by a fine-grained basic dyke (SD). Gr~btshornet (Brattv~g peninsula): GR = olivine-gabbro body. b. and c. Location of the map area of (a). d. Map showing the distribution of corona gabbro (hatched area), transitional gabbro-eclogite (dotted area) and eclogite (black) within the gabbro-eclogite complex on the island of FlemsCy (loc. GEL Sample numbers for corona gabbro (GEl5), eclogites (GE4a and GE7) and pegmatitic gabbro (GE3b) which is situated within the transitional gabbro-eclogite zone.

257

shows all stages of transformation to eclogite. It can be demonstrated that some of the associated country-rock eclogites (loc. EH) have been derived from the same gabbros (M~brk, 1985; M~brk and Brunfelt, in prep.). The eclogite-forming metamorphism reached pressure and temperature conditions in the range 15-20 kbar and >700°C. Eclogite formation locally occurred under both relatively static conditions by replacement reactions within the gabbro, but was mostly accompanied by pervasive deformation, The HPT-metamorphism was followed by cooling through amphibolite facies conditions, associated with boudinage and two phases of folding.

within gabbro vs. country-rock eclogite. Sampling locations are shown in Fig. l a. Different gabbro types are represented by leucogabbro-norite from Haram (samples SAE1 and H1) and olivine gabbro from Gr6tshornet (GR2) and Flems~by (GEl5 and GE3b). The Gr~btshornet occurrence represents the gabbro that has been least affected by metamorphism, while GEl5 is the least reacted sample within the gabbro-eclogite body on Flems~by (Fig. l d). Eclogitized volumes within the Flems~by gabbro are represented by GE7 and GFAa (Fig. ld), Countryrock eclogites are represented by two samples, EH5 and EH13, from a layered eclogite 600 m west of the GE location on Flerns~by (M~brk, 1985, fig. 1 detailed map, and Fig. la).

1.2. SampliJzg strategy

1.3. Description of samples

Sampling was designed to cover: (a) different types of gabbro: (b) different degrees of HPT-metamorphic transition within gabbro; and (c) eclogite

1.3.1. GR2 (Grbtshornet) This is a small body of medium-grained olivine gabbro consisting of cumulus olivine and plagioclase with intercumulus augite, ilmenite and minor pleonaste. All the relict

I:ig. 2. Photomicrograph (crossed polars) of coronitic gabbros. a. Gr~tshornet olivine-gabbro (GR2) showing igneous plagioclase laths (P1), olivine (O1) and augite (Cpx). Metamorphism is restricted to coronas between olivine and plagioclase. b. Haram, coarse-grained leucogabbro-norite (SAE 1) showing a part of a relict clinopyroxene crystal. Note thin amphibole corona around the pyroxene. The original plagioclase has been transformed to a pseudomorphic plagioclase aggregate. Garnet (black) is concentrated in marginal parts of the plagioclase domains.

258

.513£ 143Ndf

igneous minerals are fairly well preserved (Fig. 2a). Reaction within augite is restricted to fine-grained exsolution of opaque phases mainly along the rims. A weak colouring of plagioclase is also observable in distinct zones, but the original polysynthetic albite twins are well preserved (Fig. 2a). Metamorphic reaction is restricted to tiny coronas (total width 0.1-0.2 ram) consisting of orthopyroxene, clinopyroxene ± amphibole and garnet, which have formed between olivine and plagioclase.

fro~ p ~ x

=Nd

Cpx

~ 5~'

.512E

® /.-'"Cpx

GEIS~ ~

pl~//,,,~/ / Opx .512(

//Y

.511~

/// //"

,"

/

.a485

/,// /

.

.bs . . . .

.oo . . . .

.;o .

.is

. . .

UmNd/~'4Nd

.~,o'

'

'";'Sm]"4Nd

/

.5126

@

,5124

. . . .

q .3484 ] .3483

/ 5

GE3b~H

SD2~%1289 J reference line

:H13

GEl5 O" GE4a

.5122

~O S~EI

/

.5120

•3484

........

***+* .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

+ . . . . . .

.3483 i

i

t

/

~

i

=

i

I

t

i

t

h

i

i

L

.08 .O9 .10 ,11 .12 .13 .14 .15 .16 .17 .18 .19 .20 .21 . 2 2 . 2 3 147Sm/144Nd

12

©

8 4

-Stadtlandet -16 i

0

i

I

i

i

500

i

i

i

i

i

1000

i

[

]

1

I

1500

Fig. 3. Sm-Nd isochron diagram for corona-gabbros. a. Squares: whole rock, plagioclase and clinopyroxene from Gr6tshornet (GR2). Open circles: whole-rock samples of corona-gabbro (GE3b and GEl5) and GEl5 plagio-

1.3.2. GEl5, GE3b, GE7, GE4a (Flems~y) (Fig. ld) This occurrence of olivine gabbro showing different degrees of transition to eclogite has been described in detail elsewhere (M~rk, 1985, 1986). GEI5 is an olivine gabbro with mineralogy and igneous phase relations similar to those described for GR2. Corona reactions are, however, much more advar]ced. Metamorphic phases (Opx, Grt, Omp, Spl) constitute more than 40% of the mode. Clinopyroxene has preserved its relict igneous composition (low-Ti augite; MCrk, 1985), but has, to a greater extent than in the previous case, been coloured by fine-grained opaque phases. Plagioclase has been affected to a greater extent than in GR2 by pervasive growth of garnet coronas, and by reaction to produce an intergrowth of more sodic plagioclase and tiny spinel needles in central parts (M~brk, 1986, figs. lb and 2c). GE3b represents a late igneous pegmatitic vein within the gabbro, consisting of plagioclase, augite and ilmenite, with coronas of garnet and amphibole. The plagioclase has recrystallized to a fine-grained aggregate within its relict igneous domains. GE7 is an eclogite consisting of garnet, omphacite, phlogopite, ilmenite and apatite, representing a completely reacted part of the gabbro. However, the metamorphic phases perfectly pseudomorph the igneous textures (Fig. 3b; M~rk, 1985). Garnet has replaced the igneous plagioclase laths, while omphacite and phlogopite occur in finegrained, polygonal aggregates which mimic the intercumulus domains. clase and clinopyroxene from tile Flems~y gabbro. Filled circles: whole-rock samples (GE7 and GE4a) of completely eclogitized parts of the Flems6y gabbro for comparison. Diamonds: whole rock, plagioclase, orthopyroxene and clinopyroxene from the sample SAE1 and whole rock from Ill, Haram leucogabbro-norite. b. Comparisons of all the whole-rock samples for coronagabbros and eclogites within gabbro (symbols as in (a), Haram dyke (SD2) and country-rock eclogites EH5 and EH13 (filled triangles). c. e-evolution diagram to illustrate the effect of crustal contamination in producing a model age for the Haram gabbro (MCrk and Mearns, 1985) older than the 930 Ma isochron age (SAE1). In contrast, olivine gabbro (GEl5) falls on a depleted mantle evolution curve for southwest Scandinavia (SWD). Note that the dyke composition (SD) is substantially less contaminated than SAE1, H1. e-values have been calculated using the chondrite values derived by Jacobsen and Wasserburg (1984) recalculated to ~46Nd/ 144Nd normalization.

259

GE4a is chemically identical to G E l 5 , b u t has been completely transformed to eclogite of similar mineralogy as GE7. It is also closer to m e t a m o r p h i c equilibrium than GE7 microstructurally, but garnet m a y still mimic the laths of igneous plagioclase (Fig. l d ; M~brk, 1986).

1.3.3. EH13, EH5 {layered country-rock eclogite) This eclogite is strongly foliated and compositionally layered. Some of the layers are chemically and mineralogically identical to the eclogitized gabbro described above (M~brk, 1985; M~brk and Brunfelt, in prep.), b u t relict igneous textures are obliterated completely. The sample EH13 is from a foliated layer from the central part of the body and consists of omphacite, garnet, amphibole and futile, apparently in textural equilibrium. EH5 represents the central part of an adjacent smaller lens. This sample shows symplectitization of omphacite.

1.3.4. SAE1, H1, SD2 (Haram, quarry at Saeberget) SAE1 is a coarse-grained ( 0 . 5 - 2 cm grain size) pegmatitic leucogabbro-norite consisting of c u m u l u s plagioclase and i n t e r c u m u l u s o r t h o p y r o x e n e , clinopyroxene and ilmenite. Subsolidus reactions involve exsolution of elinop y r o x e n e in o r t h o p y r o x e n e , and limited exsolution of an

opaque phase in clinopyroxene. Some pyroxene grains have been subjected to kinking and fracturing. Plagioclase has been transformed to fine-grained plagioclase aggregates with minor spinel _+ epidote, p s e u d o m o r p h i n g the original plagioclase domains (Fig. 2b). Corona structures (total width up to 0.35 m m ) have developed between the original domains of plagioclase and mafic phases (clinopyroxene, orthopyroxene, ilmenite) and involve various combinations of amphibole, garnet, biotite, and m e t a m o r p h i c clinopyroxene and plagioclase. Garnet has preferentially f o r m e d in marginal parts of the plagioclase d o m a i n s (Fig. 2b). H1 has original mineralogy similar to SAE1 and has been less affected by m e t a m o r p h i s m , b u t SAE1 was preferred for mineral analyses because of its larger grain-size. SD2 is a fine-grained dyke within the coarse-grained gabbro. In spite of the apparently well-preserved igneous field appearance of the dyke (Gjelsvik, 1952, fig. g) it consists of a m e t a m o r p h i c assemblage of plagioclase, diopside, hornblende, biotite, garnet and opaque phases.

1.4. Analytical procedures T h e size o f e a c h s a m p l e is a r o u n d eral s e p a r a t e s ( c a . 5 0 - 2 0 0

mg) have been

TABLE1 Sm-Nd isotope data Sample No. GR2 GR2 GR2 GR2

w.r. w.r. Cpx PI

Sm (ppm)

Nd (ppm)

4.18 4.15 6.21 1.46

17.3 17.2 17.7 7.62

laTSm/144Nd

~a~Nd/~44Nd

+- 2o

~4SNd/~44Nd

-+ 20

0.14667 0.14703 0.21242 0.11657

0.512348 0.512344 0.512867 0.512109

0.000005 0.000005 0.000025 0.000004

0.348415 0.348415 0.348400 0.348416

0.000004 0.000004 0.000025 0.000005

GE3b w.r.

7.77

32.9

0.14338

0.512302

0.000013

0.348413

0.000015

GEl 5 w.r. G E l 5 Cpx G E l 5 PI

3.96 8.25 1.06

17.4 27.6 7.93

0.13775 0.18125 0.08125

0.512263 0.512631 0.511910

0.000004 0.000005 0.000007

0.348416 0.348414 0.348411

0.000005 0.000005 0.000011

GE4a w.r. GE4aw.r. GE4aGrt GE4aOmp GE4a Phi GE4a Apa

3.33 3.28 1.31 4.00 0.15 233

14.4 14.2 3.09 15.2 0.80 1475

0.14024 0.14050 0.25713 0.15990 0.11396 0.09586

0.512290 0.512284 0.512577 0.512357 0.512255 0.512145

0.000005 0.000005 0.000007 0.000006 0.000009 0.000009

0.348408 0.348409 0.348413 0.348400 0.348400 0.348421

0.000005 0.000004 0.000007 0.000005 0.000015 0.000012

(;E7 w.r. GE7 Grt GET Omp

2.84 1.12 2.55

12.9 1.99 9.41

0.13372 0.34248 0.16482

0.512226 0.512782 0.512399

0.000011 0.000024 0.000039

0.348436 0.348410 0.348383

0.000012 0.000027 0.000032

EH5 w.r.

3.02

12.0

0.15334

0.512352

0.000006

0.348417

0.000006

E H I 3 w.r. EHI3 Grt EH13Omp

1.75 0.91 2.72

0.22133 0.77813 0.18041

0.512461 0.513949 0.512348

0.000009 0.000064 0.000012

0.348426 0.348366 0.348437

0.000009 0.000086 0.000011

SD2 w.r.

2.95

0.16878

0.512555

0.000006

0.348414

0.000006

4.82 0.71 9.16 10.6

HI w.r.

5.48

24.3

0.13701

0.512118

0.000004

0.348412

0.000005

SAEI w.r. SAE1 Cpx SAE10px SAICI Pl

4.91 3.99 1.01 1.25

22.11 13.7 3.86 8.99

0.13472 0.17691 0.15919 0.08422

0.512100 0.512362 0.512245 0.511891

0.000004 0.000023 0.000016 0.000006

0.348414 0.348420 0.348404 0.348419

0.000005 0.000011 0.000018 0.000005

D

duplicate analyses.

103 c m 3. M i n -

D

1)

cleaned

260 by hand-picking under a binocular microscope to exclude heterogeneous grains. The analytical procedure follows that described by Mearns (1986 in this special issue; Oslo Laboratory). Isotopic ratios have been measured using a fully automated Vacuum Generators 354 five collector mass spectrometer. In the period December 1984 to August 1985 replicate analyses of the laboratory standard JM (Johnson & Matthey Nd203 No. 5819093A) give a 143Nd/ ~44Nd ratio 0.511125 + 0.000008 (2o). Isotopic ratios have been normalized to 146Nd/l~Nd = 0.7219. Duplicate analyses (chemistry and mass spectrometry) of whole-rock samples GR2 and GEl5 are reproducible within two standard errors (Table 1).

2. Corona gabbros

2.1. Concentralion data Analytical results for Sm and Nd are presented in Table 1. Concentration data in terms of partition coefficients may be used to test if the minerals have preserved igneous Sm and Nd distributions. Clinopyroxene/whole-rock partition coefficients for Sm and Nd for SAE1 are within the range reported for natural basalts (D = 0.2-1; Schnetzler and Philpotts, 1968, 1970; Green and Pearson, 1985; B. Sundvoll, unpublished data, 1985), while slightly higher values for GR2 and GEl5 are compatible with the cumulus nature of olivine (M~brk, 1985). Sm/Nd distribution coefficients between clinopyroxene and plagioclase (i.e. Sm/Nd(cpx)/Sm/ Nd(pl)) in the coronites GR2, GEl5 and SAE1 show a limited range (1.8 2.2) and are shnilar to igneous values reported elsewhere (Edwards and Wasserburg, 1985). These relations show that the minerals may have preserved igneous Sm and Nd distributions,

2.2. Age of gabbro intrusions Sm-Nd results for pyroxene, plagioclase and whole rock for corona gabbro from Gr6tshornet, Flems~by and ttaram are given in Table 1 and plotted in Fig. 3a. Clinopyroxene + plagioclase + wholerock data from Gr~btshornet are collinear and define an age of 1198 -+ 56 Ma (MSWD = 0.06) and 143Nd/ 144Ndi = 0.511192 + 0.000048 [e(t) = +1.6]. The good fit and high initial ratio of the isochron, corn-

bined with the fact that the igneous mineral phases are so well preserved, suggest that this is the age of igneous intrusion. Whole-rock samples of both coronites (GEl5 and GE3b) and eclogitized samples (GE7 and GE4a) within the Flems~by gabbro as well as relict augite (GEl5) fall close to the Gr6tshornet line, but define a separate isochron with an age 1289 -+ 48 Ma (MSWD = 0.20) and 14aNd/l~Ndl = 0.511100 -+ 0.000018 [fit) = +2.0] (Fig. 3a). Collectively all these results fall onto a Sm-Nd pyroxene + plagioclase + whole-rock isochron from a gabbro from Kr~keness (Mearns, 1984; 1258 -+ 56 Ma, 1 = 0.51113 + 0.00006) ca. 100 km south of Nord~byane. This fact is perhaps the best evidence that the three isochrons record the age of the gabbro/ dolerite intrusions which appear to have crystallized from melts derived from uniformly depleted mantle. In contrast to GR2, plagioclase from the advanced coronite GEl5 has been selectively disturbed and falls above the Flems~by gabbro isochron (to be discussed below). Mineral and whole-rock data from the Haram leucogabbro-norite fall below the line defined by the olivine gabbros (Fig. 3a and b). Clinopyroxene + orthopyroxene + whole-rock from the coarsegrained sample SAE1 and whole rock from H1 define an isochron with a younger date of 926 -+ 70 Ma (MSWD = 0.78) and intercept ratio of 0.511284 -+ 0.000032 [e(t) = - 3 . 5 ] (Fig. 3a). As in the case for GEl5, plagioclase from SAE1 falls above the isochron. The fine-grained dyke SD2 from Haram does not fall on the isochron (Fig. 3b). Negative e(t) values could mean either contamination of the magma with crustal material, or isotopic resetting of the mineral system. The Haram isochron can therefore be interpreted in terms of two models: (1) That it dates the intrusion of a gabbroic magma that has assimilated crustal Nd. (2) That it records metamorphic resetting. Model 1 implies that plagioclase has been selectively disturbed isotopically as suggested for GEl5, while orthopyroxene and clinopyroxene have retained their original distributions. This model is preferred on the basis of: (a) comparisons with the relations in the olivine gabbros; (b)mineral microstructures and diffusion data (to be discussed below); and (c) the coarse grain size of the minerals used for analyses. However, supplementary radiometric dating of this gabbro-type by other methods is desirable to resolve this problem.

261 Initial ~43Nd/144Nd ratios for gabbros GEl5 and GR2 fall on the Southwest Scandinavian depleted mantle curve established by Mearns (1984, 1986b, fig. 3c). In contrast, accepting the isochron from Haram as an intrusion age, this leucogabbro-norite appears to be contaminated by older crustal Nd and has an older model age than the olivine gabbros (Fig. 3c). The dyke (SD) intruding the Haram leucogabbro-norite also appears to be crustally contaminated although not to 'the same extent as the coarsegrained Haram rocks (SAE1 and H1). This is consistent with Gjelsvik's (1952) model based on major element chemistry, of crustal contamination by assimilation at depth, and also with his observation that the Haram dyke is compositionally "closer to an ordinary basalt magma" than the coarse-grained Haram rocks.

2.3. Effect of metamorphism Coronitic gabbros which show different degrees of eclogitization allow examination of isotopic behaviour during metamorphic transformation. The isochron relations suggest that plagioclase has been disturbed isotopically in GEl5 and SAE1, but not in the least reacted rock GR2, while relict pyroxenes have retained their original composition in all samples. The isotopic retention within clinopyroxene is consistent with experimental studies which show extremely low volume-diffusion coefficients for Sm (and St) in diopside (ca. 10-X4cm2s -1 (Freer et al., 1982; Sneeringer et al., 1984) even at temperatures far above the metamorphic conditions in the samples studied. Diffusion rates in plagioclase are generally several orders of magnitude greater than in clinopyroxene (see summary by Yund, 1983; includes Sr but not REE). In feldspars the colnpensation temperature is near the "closure temperature" around 460°C (Hart, 1981). Plagioclase may therefore have lower closure temperatures for diffusion of Sm and Nd and be easier to disturb isotopically than clinopyroxene. In contrast, Morse (1984) calculated much lower maximum diffusion coefficients (ca. 10 -20 cm2s -1) for the Na,Si-Ca,A1 exchange in igneous plagioclase on the basis of the chemical zoning. Diffusion rates would, however, be increased by high defect concentrations, channeling by deformation or exsolution and the operation of intergranular diffusion (cf. deformationinduced diffusion and recrystallization of oligoclase; White, 1975).

The low closure temperature for plagioclase is not compatible with the preservation of the isotopic systems within plagioclase in GR2. Rather, the excellent preservation of the igneous phases and the pyroxene + whole-rock isochron relationship in this sample may be used to indicate that plagioclase has a higher closure temperature (T > 700°(2) for diffusion of Sm and Nd in the absence of deformation and recrystallization processes; the isotopic disturbance of plagioclase in the more advanced coronites GEl5 and SAE1 is thus possibly due to the mineralogical and microstructural development. The metamorphic effects on ptagioclase in GEl5 involve adjustments towards more Na-rich composition and local replacement by garnet. These transformations would be accompanied by a strong Sm/Nd fractionation from the plagioclase host into the product garnet since garnet has higher Sm/Nd ratios than plagioclase (see Section 3). The position of plagioclase (GEl5) above the isochron (Fig. 3a) can thus be attributed to the reduced Sm/Nd in relict plagioclase resulting t'rom local formation of garnet within plagioclase. Partition coefficient data show that olivine has very low concentrations of Sm and Nd (Zielenski, 1975; McKay, 1986). Thus, while Fe and Mg has diffused from olivine into plagioclase to form garnet (M~brk, 1985), it is unlikely that olivine could contribute significant amounts of REE into the plagioclase domains. Corona formation may therefore result in selective disturbance of the relict igneous phases depending on local reaction mechanisms, without disturbance of the whole-rock system. In SAE1, isotopic disturbance of plagioclase could also have been enhanced by the thorough recrystallization of plagioclase, and the appearance of amphibole in the corona products. Since pyroxenes participate in the corona reactions in the leucogabbro-norite there is also a chance that pyroxene has been locally disturbed, ttowever, the coarse grain size of the pyroxenes that were used and careful elimination of mixed grains suggest that the analyses represent the igneous isotopic distributions for pyroxenes in SAE1.

3. Eclogites Eclogites GE4a and GE7 are parts of the same metagabbro as the coronites GEl5 and GE3b and represent more advanced reaction stages. The eclog-

262 ites EH5 and EH13 are from an adjacent countryrock ecloglte occurrence. Earlier studies of eclogitized gabbro (GE) and of the adjacent country-rock eclogite (EH) suggest a common origin with respect to: (1) protolith composition; and (2) metamorphic evolution (MCrk, 1985; MCrk and Brunfelt, in prep.). However, the layered and foliated country-rock eclogite body covers a larger compositional range. Trace element patterns in the sample used for mineral dating (EH13) have been altered during metamorphism (MCrk and Brunfelt, in prep.). EH13 also differs from the eclogites GE7, GE4a within the gabbro in having a simpler mineralogy (see Section 1.3) and a closer approach to metamorphic equilibrium both microstructurally and in terms of mineral chemistry. In contrast, GE7 and GE4a are both chaiacterized by garnet pseudomorphs after igneous plagioclase.

!)43Nd/l'~Nd

.5128[ ,

.512{~ ,5124

Grt

I

5122

w,r.

14SNd/l'~Nd

slx~

.5118

' .3485 1,3484 .3483

t

.

.

k

.10

--

l

a

J5

!

L_Z

.20

•

E

X

L &

J

L

&

l

--

--X

.25 147Sm/12~N d

.35

"4SNd/'~4Nd .512~[

@

Grl

5124

omp Phi

.5122

w.r

Ap

145Nd/"44Nd !

5120

.3484

.5118

3. l. Concentration data

@

+

,

.,0

-

+

.15

•

.3484

.20 ,47Sm/,,~Nd25 r

Whole-rock Sm/Nd ratios for eclogites in gabbro are similar to those in corona-gabbros. In contrast, the country-rock eclogite EH13 has significantly lower Sm and Nd contents, and a much higher Sm/Nd-ratio. The low Sm and Nd contents in EH13 is compatible with significantly lower contents of normative apatite than in the other samples. Apatite, for example in GE4a, is enriched in Sm and Nd by a factor of 100 relative to the other minerals (Table 1). Comparisons with the coronite data show that garnet has a higher Sm/Nd ratio than the mineral it has replaced (plagioclase), and that this ratio has reached the highest value for garnet in the equilibrated eclogite EH13. Sm/Nd distribution coefficients for Grt/Omp are in the range 1.6-4.3, also with the highest value for the country-rock eclogite (EH13) comparable with data for HPT equilibrated country-rock eclogites elsewhere (Ulsteinvik and Selje eclogites; Griffin and Brueckner, 1985).

3.2. Iso topes 3.2.1. Eclogites within gabbro GE7: On an isochron diagram the garnet, omphacite and w.r. do not define a line (Fig. 4a). This indicates isotopic disequilibrium, and the relative position of the mineral and w.r. values suggests the

Fig. 4. Sm-Nd isotope plot for eclogites within gabbro (Flems~by): a. Garnet (Grt), omphacite (Omp) and whole-rock (w.r.) disequilibrium in GE7, an eclogite with pseudomorphic igneous microstructures. b. Garnet (Grt), omphacite (Omp), phlogopite (Phl), apatite (Ap) and whole-rock (w.r.) disequilibrium in eclogite GE4a. presence of an additional phase with low Sm/Nd ratio (see below). GE4a: The whole-rock composition lies within a polygon defined by garnet, omphacite, phlogopite and apatite (Fig. 4b). Thus, different mineral pairs or mineral-w.r, combinations give rise to different apparent ages: Grt + w.r. = 407 + 39 Ma and 379 -+ 23 Ma, Grt + Omp = 329 + 79 Ma and 346 -+ 30 Ma (= P h l + Omp) for GE7 and GE4a, respectively, all of which are probably meaningless.

3.2.2. Country-rock eclogite; Ett13 Omp + w.r. + Grt in EH13 define an isochron with an age of 400 -+ 16 Ma (MSWD = 0.10) (Fig. 5). Whole-rock EH13 is also characterized by a much higher 147Sm/144Nd ratio than the eclogites within gabbro (Fig. 3b). The closer approach to isotopic equilibrium in this sample than in GE7 and GE4a suggests a correlation between the degree of isotopic and metamorphic equilibration, and that the apparent equilibrium in EH13 may be related to recrystallization and penetrative deformation.

263

l 14;Nd/144Nd

143Nd/144Nd .5141

Ate400 Ma

.512~l

,

.

,

~

i

wc ~ f .5124

k,,~...

.51~81

,

i

--~*

.76 .77 .78 .79

O/mmp/--~

_

145Nd/144Nd

.5122

]

~1~

~ .,4,,, I

i .20

.25

_

_

w~s_

__*o_~p .

.

.

.

.

.

.

.

7 .............................................. G rt '4/Sm/'44Nd

t .34a~ ~

147Sm/144Nd

Fig. 5. Sm-Nd isochron diagram for foliated country-rock eclogite (EH13) with a close approach to equilibrium, both microstructurally and isotopically, between omphacite (Omp), garnet (Grt) and who/e-rock (w.r.). 3.3. Discussion

The isotopic disequilibrium between metamorphic phases in eclogitized gabbros is of particular interest since several two-point Sm-Nd ages (Grt + Cpx) have been reported in the literature (Griffin and Brueckner, 1981, 1985). Since it is also clear that the eclogites within gabbro have been subjected to a similar metamorphic history as the more thoroughly equilibrated country-rock eclogite (EH13), the variable degree of isotopic equilibration must be related to factors other than P and T alone, Isotopic disequilibrium between the mineral phases can be explained by: (a) diffusion parameters and blocking temperatures; and (b) fluid availability during reaction and cooling and selective contamination. Two alternative models are discussed below: (1) total isotopic HPT-equilibrium was never achieved for GET and GE4a, or (2) the disequilibrium was induced during cooling. 3.3.1. Model 1 - Metamorphic disequilibrium A model of metamorphic disequilibrium is preferred. Such a model is based on the correlation between petrographic and isotopic disequilibrium which suggests that isotopic equilibrium was never achieved in the eclogites with pseudomorphs. The pseudomorphic textures have formed via a transitional reaction stage with corona structures (M~brk, 1985). These microstructures have been interpreted in terms of restricted diffusion between felsic and mafic domains. In such a situation plagioclase is the main source of Sm and Nd for garnet (which re-

Fig. 6. Schematical sketch illustrating the Sm-Nd isotopic development within pseudomorphic domains after igneous augite (Aug) and plagioclase (P1) during the transition from gabbro to eclogite explaining the isotopical disequilibrium in GE4a and GE7. Crystallization of metamorphic phases is accompanied by fractionation of Sm/Nd and equilibration of ~43Nd/~44Nd towards the mean whole-rock value. However, under static conditions, complete equilibrium is not reached and the metamorphic phases garnet (Grt) and omphacite (Omp) retain memory from the igneous phases they replace. places it completely) while omphacite would to a large extent be controlled by the original augite composition. This may give some insight to the processes which govern isotopic equilibration durilag metamorphic crystallization (Fig. 6). For example the garnet + whole-rock date for eclogite GE4a is younger than expected; this could be explained if the garnet in part inherited the low 143Nd/144Nd ratio from the plagioclase that it replaces. Likewise clinopyroxene + whole-rock dates are too old which may reflect inheritance in the omphacite of the high X43Ndf14aNd ratio of the igneous augite. If this explanation is valid, then it would appear that recrystallization and deformation are necessary to erase domainal disequilibrium inherited from the igneous parent; the equilibrated "end-member" is exemplified by EH13. 3.3.1.1. The role o f apatite. The very high Nd concentration and the low Sm/Nd ratio of apatite (Table 1; GE4a) means that apatite is an important sink for Nd with extremely low 143Nd/l'~Nd compositions that is tied up in localized micro-domains. Thus, apatite may play an important role in controlling mineral + whole-rock equilibria during metamorphic reactions. This is demonstrated in sample GE4a where apatite completes the mineral polygon which surrounds the whole-rock point (Fig. 4b). Furthermore, examination of P2Os concentrations

264

~43Nd/ 144Nd I

element mobility and isotopic equilibration for EH13 than for the eclogites within the gabbro.

e~ e .~~GO~

At~400Ma ,~O~ w~~-m~, tlasOmatised __ . . . . . -J eCh '47Sm//'44Nd Fig. 7. Schematical sketch illustrating the possible role that apatite plays in controlling fractionation of the Sm/Nd system in whole-rock eclogite during m e t a m o r p h i s m . Apatite m a y have a composition somewhere on the line between the filled and open circle in the diagram, depending on the degree to which it has equilibrated during m e t a m o r p h i s m . Metasomatic dissolution and removal of apatite during m e t a m o r p h i s m would lead to a change in the whole-rock Sm-Nd isotopic composition indicated by the arrow. This m e c h a n i s m can explain the high ~47Sm/~44Nd ratio of EH13 compared to the other eclogites.

in the eclogites reveals that the eclogites with the highest Sm/Nd ratios, which fall off the igneous isochron (Fig. 3b), also have lowest P2Os concentrations. This could be explained in terms of selective dissolution of apatite during metamorphism, accompanied by partial removal of P and REE. For example, using GE7 as a reference rock, and subtracting EH13's Sm and Nd concentrations from it leaves a "residue" with a Sm/Nd ratio not very far from the value expected in apatite (0.14 vs. 0.16 in apatite from GE4a). The effect of metasomatic removal of apatite during metamorphism at 400 Ma leading to an increase in the w.r. 147Sm/144Nd ratio is illustrated schematically in Fig. 7. Thus, dissolution of apatite and removal of P, Sm and Nd (and presumably other trace elements; M6rk and Brunfelt, in prep.) could possibly explain the increase of 147Sm/la4Nd in EH13 relative to the other eclogites. This process could be simultaneous with the eclogite formation and would also require operation of a fluid phase during eclogitization of EH13. All these parameters are compatible with greater

3.3.2. Model 2 Blocking temperatures An alternative way of interpreting isotopic disequilibrium between minerals is in terms of the blocking temperature concept which has been applied to other isotopic systems (cf. J~iger and Hunziker, 1979). This model involves isotopic disequilibrium evolving during cooling, caused by minerals having different blocking temperatures for REE diffusion. The apparent isochron relations between phlogopite, omphacite and garnet in eclogite GE4a (Fig. 4b) is not compatible with such an interpretation since phlogopite probably has several orders of magnitude higher diffusion rates than garnet and would have a much lower blocking temperature for Nd diffusion. Another possibility is that some of the minerals have been selectively disturbed by late fluid interaction. The earlier reported irregularities of Fe3+/ Fe 2+ for omphacite in eclogitized gabbros (M~brk, 1985) combined with the close microstructural association of phlogopite and omphacite in these rocks could possibly suggest that the young apparent age defined by these minerals (ca. 340 Ma; GE4a) records a late disturbance. However, this date is not considered geologically meaningful since Rb-Sr cooling age for phlogopite in GE4a is significantly older (378 -+ 8 Ma; w.r. + Phl, Table 2). The earlier interpretation (model 1) of incomplete Sm-Nd isotopic equilibrium during the metamorphism is thus preferred.

4. Summary 4.1. Sm-Nd isotopic behaviour during the gabbroeclogite transition GR2, GEl5, GE7, GE4a and EH13 represent a sequence of samples showing increasing degree of

TABLE2 Rb-Sr isotope data for sample GE4a Sample No.

Rb (ppm)

Sr (ppm)

~TRb/86Sr

87Sr/S6Sr

-+ 20

GE4a w.r. GE4a Phi

19.4 186

311 39

0.18055 14.0022

0.705434 0.780050

0.000010 0.000020

379 +- 8 Ma

265 transformation from corona gabbro to eclogite. The samples represent a development from advanced coronite (GEl5) to eclogites with different degree of preservation of pseudomorphs after the igneous phases (GE7 and GE4a) to completely recrystallized and foliated eclogite (EH13). Taking the less extensively reacted corona gabbro (GR2) to represent the original olivine-gabbro system, the following relations between metamorphic transitions and Sm-Nd isotope development are suggested: (1) On a whole-rock scale (ca. 1 1) original Sm-Nd isotopic distribution has been preserved independently of the degree of HPT-reaction. Exceptions are eclogites which have been isolated as smaller bodies within the gneisses where the transitions have been accompanied by penetrative deformation and fluidenhanced diffusion (EH13). (2) Within corona gabbros with advanced transformations, augite, in spite of the exsolution of opaque phases, retains its original igneous Sm-Nd isotopic composition through P and T conditions of 15-20 kbar and > 700°(7. The retention of Sm-Nd isotopic relations within plagioclase from Gr95tshornet (GR2) similarly suggests a high blocking temperature for diffusion of Nd in plagioclase. However, in the more advanced coronitic gabbros (GEl5, SAE1), plagioclase has been disturbed isotopically. This disturbance is due to: (a) continuous chemical reactions within plagioclase (GEl5, SAE1), (b) Sm and Nd fractionation into the garnet which invades plagioclase during metamorphism (GEl5, SAE1); and (c) recrystallization of plagioclase (SAE1). (3) Metamorphic minerals in eclogites with pseudomorphic textures are not in equilibrium with respect to 143Nd/144Nd. The disequilibrium is attributed to incomplete isotopic exchange during reaction ("memory effect within pyroxene"). The amount of apatite in the system, and the degree to which it can dissolve, may also be a critical factor. (4) The relatively higher degree of microstructural equilibrium for GE4a than for GE7 is accompanied by an approach to an isochron for the polygon defined by *43Nd/144Nd-14VSm/144Nd for garnet + omphacite + phlogopite + apatite. (5) Combinations of garnet + omphacite and garnet + whole rock from GE4a and GE7 give apparent ages that are significantly younger (Grt + Omp) or similar/slightly younger (Grt + w.r.) than

the 400 Ma age suggested from Grt + w.r. + Omp in the equilibrated rock EH13. (6) The isotopic developments in the sequence of rocks illustrate that volume diffusion, even at fairly extreme P-T conditions, is not sufficient for resetting of 143Nd/144Nd isotopes in medium-grained gabbroic systems. Sm-Nd mobility is related to metamorphic reactions. The isotopic data, combined with the structural and petrographical relations (cf. M6rk, 1985), may indicate that processes such as penetrative recrystallization, deformation and dissolution into an intergranular fluid (as inferred for EH13) are required to produce Sm-Nd isotopic equilibrium between the product phases.

4.2. Age relations The protolith age for olivine gabbro and associated eclogite is 1200-1300 Ma. The Haram leucogabbronorite may be a later Precambrian intrusion with an age of 926 -+ 70 Ma. The 400 -+ 16 Ma age on the equilibrated eclogite is interpreted to date the metamorphism, or more precisely, the main deformation that accompanied the eclogite formation. The 400 Ma age is close to, but slightly younger than Sm-Nd garnet + omphacite ages reported elsewhere in the WGR (Griffin and Brueckner, 1980, 1985; 407-447 Ma). In the present case, garnet + omphacite pairs yield ages that are similar to or lower than the EH13 isochron. It is apparent that two-point ages may be insufficient for dating metamorphism unless total mineralogy, reaction microstructures and the state of metamorphic equilibrium are also discussed. Rb-Sr biotite and K-feldspar cooling ages from adjacent gneisses in the area (HargSy; M~Srk, in prep.) are 20 30 Ma younger, suggesting that extensive uplift and cooling occurred in the time interval 400-370 Ma following the HPT-metamorphism (see also Lux, 1986).

Acknowledgements This paper is contribution No. 5 in the Norwegian Lithosphere Project (ILP), supported by the Norwegian Research Council for Science and the Humanities (NAVF) (Subproject No. D.40.37.063; Petrogenetic processes in the continental lithosphere).

266

References

Brueckner, H.K., 1979. Precambrian ages from the GeirangerTafjord-Grotli area of the Basal Gneiss Region, west Norway. Nor. Geol. Tidsskr., 59: 141-153. Bryhni, I., Krogh, E. and Griffin. W.L., 1977. Crustal derivation of Norwegian eclogites: a review. Neues Jahrb., Mineral. Abh., 130: 49-68. Edwards, R.L. and Wasserburg, G.J., 1985. The age and emplacement of obducted oceanic crust in the Urals from Sm-Nd and Rb-Sr systematics. Earth Planet. Sci. Lett., 72: 389- 404. Freer. R., Carpenter, M.A., Long, J.V.P. and Reed, S.J.B., 1982. "Null result" diffusion experiments with diopside: implications for pyroxene equilibria. Earth Planet. Sci. Lett., 58: 285 292. Gebauer, D., Lappin, M.A., Grfinenfelder, M. and Wyttenbach, A., 1985. The age and origin of some Norwegian eclogites: A U-Pb zircon and REE study. In: D.C. Smith and Ph. Vidal (Guest-Editors), Isotope Geochemistry and Geochronology of Eclogites. Chem. Geol. (Isot. Geosci. Sect.), 5 2 : 2 2 7 - 2 4 8 (special issue). Gjelsvik, T.. 1952. Metamorphosed dolerites in the Gneiss Area of SunnmC~re on the west coast of southern Norway. Nor. Geol. Tidsskr., 30: 33-134. Green, T.H. and Pearson, N.J., 1985. Rare earth element partitioning between clinopyroxene and silicate liquid at moderate to high pressure. Contrib. Mineral. Petrol., 91:24 36. Griffin, W.L. and BrucckneL H.K., 1980. Caledonian Sm-Nd ages and a crustal origin Norwegian eclogites. Nature (London), 285:319 321. Griffin, W.L. and Brueckner, H.K., 1985. REE, Rb-Sr and Sm-Nd studies of Norwegian eclogites. In: D.C. Smith and Ph. Vidal (Guest-Editors), Isotope Geochemistry and Geochronology of Eclogites. Chem. Geol. (Isot. Geosci. Sect.), 52:249 271 (special issue). Griffin. W.L. and R3heim, A., 1973. Convergent metamorphism of eclogites and dolerites, Kristiansund area, Norway. Lithos, 6:21 40. Griffin, W.L., Austrheim, H., Brastad, K., Bryhni, I., Krill, A., Krogh, E.5., M~brk, M.B.E., Qvale, H. and T6rudbakken, B., 1985. High pressure metamorphism in the Scandinavian Caledonides. In: D. Gee and B. Sturt (Editors), The Caledonide Orogen - Scandinavia and Related Areas. Wiley, Chichester, pp. 783-802. Hart, S.R., 1981. Diffusion compensation in natural silicates. Geochim. Cosmochim. Acta, 45:2?9 291. Harvey, M.A., 1983. A geochemical and Rb Sr study of the Proterozoic augen orthogneisses of the Molde peninsula, west Norway. Lithos, 16: 325-338. Jacobsen, S.B. and Wasserburg, G.J., 1979. Nd and Sr isotopic study of the Bay of Islands ophiolite complex and the evolution of the source of mid-ocean ridge basalts. J. Geophys. Res.,84:7429 7445. Jacobsen, S.B. and Wasserburg, G.]., 1984. Sm-Nd isotopic evolution of chondrites and achondrites, II. Earth Planet. Sci. Lctt., 67: I37-150.

J/iger, E. and Hunziker, J.C. (Editors), 1979. Lectures in Isotope Geology. Springer, Berlin, 329 pp. Krogh, E.J., 1977. Evidence of a Precambfian continentcontinent collision in western Norway. Nature (London), 267(5606): 17-19. Krogh, E.J., 1980. Compatible P-T conditions for eclogites and surrounding gneisses in the Kristiansund area, western Norway. Contrib. Mineral. Petrol., 75: 387-393. Lappin, M.A., Pidgeon, R.T. and van Breemen, O., 1979. Geochronology of basal gneisses and mangerite syenites of Stadtlandet, west Norway. Nor. Geol. Tidsskr., 59: 161-181. Lux, D.R., 1985. K/Ar ages from the basal gneiss region, Stadtlandet area, western Norway. Nor. Geol. Tidsskr., 65: 277-286. McKay, G.A., 1986. Crystal/liquid partitioning in basaltic systems: Extreme fractionation of REE in olivine. Geochim. Cosmochim. Acta, 50: 69-79. Mearns, E.W., 1984. Isotopic studies of crustal evolution in western Norway. Ph.D. Thesis, University of Aberdeen, Aberdeen (unpublished). Mearns, E.W., 1986a. Sm-Nd ages for Norwegian garnet peridotite. In: W.L. Griffin (Editor), Second International Eclogite Conference. Lithos, 1 9 : 2 6 9 - 2 7 8 (this special issue). Mearns, E.W., 1986b. Magmatic and metamorphic ages from Western Norway and isotopic evolution in depleted South West Scandinavian mantle. (In preparation.) Morse, S.A., 1984. Cation diffusion in plagioclase feldspar. Science, 225(4661): 504-505. M~brk, M.B.E., 1985. A gabbro to eclogite transition on Flems~y, Sunnm~bre, western Norway. In: D.C. Smith, G. Eranz and D. Gebauer (Guest-Editors), Chemistry and Petrology of Eclogites. Chem. Geol., 50: 283310 (special issue). M~brk, M.B.E., 1986. Coronite and eclogite formation in olivine gabbro (Western Norway): Reaction paths and garnet zoning. Mineral. Mag., 50:417 426. M~brk, M.B.E. and Brunfelt, A.O., 1986. ChemicaI investigation of a "common origin" hypothesis for country rock eclogites and a gabbro (Flems~by, West Norway). (In preparation.) Pidgeon, R.T. and R~heim, A., 1972. Geochronological investigation of the gneisses and minor intrusive rocks from Kristiansund, west Norway. Nor. Geol. Tidsskr., 52: 241-256. Schnetzler, C.C. and Philpotts, J.A., 1968. Partition coefficients of rare earth elements and barium between igneous matrix material and rock-forming-mineral phenocrysts - I. In: L.H. Ahrens (Editor), Origin and Distribution of tile Elements. Pergamon, Oxford, pp. 9 2 9 938. Schnetzler, C.C. and Philpotts, J.A., 1970. Partition coefficients of rare earth elements between igneous matrix material and rock forming mineral phenocrysts 1I. Geochim. Cosmochim. Acta, 3 4 : 3 3 1 - 3 4 0 . Sneeringer, M., Hart, S.R. and Shimizu, N., 1984. Strontium and samarium diffusion in diopside. Geochim. Cosmochim. Acta, 48: 1589-1608.

267

T~brudbakken, B.O,, 1982. En geologisk unders~kelse av nordre Trollheimen red Surnadal reed hovedvekt p5 sanimenhengen mellom strukturer, metamorfose og aldersforhold. Cand. Real. Thesis, University of Oslo, Oslo (unpublished). White, S., 1975. Tectonic deformation and recrystallization of oligoclasc. Contrib. Mineral. Petrol., 50: 287-304.

Yund, R,A., 1983. Difflision in feldspars. In: P.H. Ribbe (Editor), Feldspar mineralogy. Mineral. Soc. Am., Roy. Mineral. (2nd ed.), 2: 362. Zielinski, R.A., 1975. Trace element evaluation of a suite of rocks from Reunion Island. Indian Ocean. Geochim. C()smochim. Acta, 3 9 : 7 1 3 - 7 3 4 .