Phase petrology of eclogites and related rocks from the Motalafjella high-pressure metamorphic complex in Spitsbergen (Arctic Ocean) and its significance

Lithos, 22 (1988) 75-97 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

75

Phase petrology of eclogites and related rocks from the Motalafjella high-pressure metamorphic complex in Spitsbergen (Arctic Ocean) and its significance TAKAO HIRAJIMA~, SHOHEI BANNO%YOSttIKUNI HIROI2 and YOSHIHIDE OHTA3 ~Department of Geology and Mineralogy, Kyoto University, Kyoto 606 (Japan) 2Department of Earth Sciences, Chiba University, Chiba 260 (Japan) 3Norsk Polarinstitutt, Postbox 158, 1330 Oslo Lufthavn (Norway)

LITHOS

Hirajima, T., Banno, S., Hiroi, Y. and Ohta, Y., 1988. Phase petrology of eclogites and related rocks from the Motalal~ella high-pressure metamorphic complex in Spitsbergen (Arctic Ocean) and its significance. Lithos, 22: 75-97. A Caledonian eclogite suite is associated with lower-grade high-pressure rocks in Motalai~ella, Oscar II Land, central-western Spitsbergen. A variety of mineral assemblages occur in the suite, depending on the bulk-rock composition; omphacite-paragonite-glaucophane-epidote-garnet in eclogites, jadeite-glaucophane-paragonite-quartz in siliceous schists, and Mg-Fe chloritoid-paragonite-epidote-garnet in pelitic schists. Petrologic and paragenetic studies are presented to characterize this unique set of mineral assemblages in relation to other eclogite suites. Mineralogical data, mainly the distribution of Fe and Mg between coexisting clinopyroxene and garnet, and the existence of impure jadeite + quartz assemblage, give approximately 580-640 °C and 18-24 kbar. New criteria for the eclogite classification are derived based on the mineral assemblages and a model petrogenetic grid. The Motalat]ella eclogites associated with the glaucophane-epidote assemblage belong to the medium-temperature portion of the eclogite facies. (Received April 28, 1987; accepted August 5, 1988)

1. Introduction

Most of the glaucophane schists and related highpressure rocks have been known from the Mesozoic and Cenozoic orogenic belts (Fig. l ). During the last decade, however, glaucophane schists and associated high-pressure rocks have been reported from several areas of the Caledonian orogenic belt of early to middle Paleozoic time: the Appalachian zone in North America (Trzcienski, 1976; Jameison, 1977; Trzcienski et al., 1984; Laird and Albee, 1981 ), and in the British and Scandinavian Caledonides (Gibbons, 1981; Krogh, 1982; Stephens and Van Roermund, 1984). The glaucophane schists and eclogites from the Caledonian rocks around Motalafjella, Oscar II Land, central-western Spitsbergen, were first described by Horsfield (1972), who re0024-4937/88/$03.50

© 1988 Elsevier Science Publishers B.V.

ported 402-621 Ma K-Ar ages for the high-pressure rocks. Ohta et al. (1983) concluded that the highpressure rocks (Vestg6tabreen Formation of Horsfield, 1972; Motalat]ella high-pressure metamorphic complex of this paper) are unconformably overlain by U p p e r Ordovician-Lower Silurian sediments (Bulltinden Formation of Horsfield, 1972; Bulltinden G r o u p of this paper). Thus, they are another example of Caledonian high-pressure rocks. In addition to their geotectonic significance, the eclogite suite of the Motalafj ella high-pressure metamorphic complex offers a unique set of mineral assemblages in a coherent sequence of high-pressure rocks, i.e. eclogites, glaucophane schists, garnetchloritoid-white mica schists and so on, which is relevant to eclogite classification. This paper describes this unique set of mineral assemblages and discusses its significance in eclogite classification.

76

' f

lithe .,,,-..,.

i

•

•, , , - , ;

olpl

;#"

...,.

.,,.

,

2

Fig. 1. Distributionof glaucophanein the world [largelybased on Miyashiro (1973); additionaldata from Dobretsov and Sobolev (1984), Thurston. (1985), Sobolevet al. (1986) and Dobretsov et al. (1988) ]. New localitiesfrom the Caledonianbelt are shownby large solid circles.

2. Geologic outline of Motalafjella

The metamorphosed supracrustals of the Svalbard Caledonides, the so-called "Hecla Hoek succession", are mainly exposed along the western and northern parts of Svalbard Archipelago (Fig. 2A). Although almost all of the Hecla Hoek rocks were metamorphosed under greenschist-amphibolite facies conditions of the lower- to intermediatepressure facies series (Ohta, 1979), eclogites are found at two areas in this succession in western Spitsbergen; one is associated with glaucophane schists at Motalal]ella (Mt. Motala), Oscar II Land, central-western part, and the other with garnet-amphibolites and blastomylonitic granite at Biskayer Peninsula, Albert I Land, in the northwestern part. This paper deals with the eclogites from Motala0ella. Roughly two-thirds of Motalaf]ella is composed of the Upper Ordovician-Lower Silurian Bulltinden Group (Armstrong et al., 1986) in the northeastern part and the Motalal~ella high-pressure metamorphic complex occupies the remainder. All planar

structures, both bedding plane and schistosity, show a monoclinical structure with westward, moderate dips (Figs. 2B and 3). Ohta et al. (1983) found a recumbent syncline in the Bulltinden Group, and the Motalat~ella high-pressure metamorphic complex occupies the core of an overturned anticline. The Motalat]ella high-pressure metamorphic complex is lithologically subdivided into two units (Hirajima et al., 1984): the Lower Unit is composed of the lower-grade schists (Fig. 3) such as lawsonite-bearing metabasites, spinel-bearing dolomites and fine-grained chloritoid-white mica schists (metapelites), whereas the Upper Unit is of -higher-grade rocks such as schistose micaceous limestones, eclogites, glaucophane schists and coarse-grained garnet-chloritoid-white mica schists. As both muscovite and paragonite are common, they are often jointly called white mica. The boundary between these units is a thrust (Fig. 3). The metabasites in the Lower Unit are mainly composed of actinolite, epidote and chlorite with a subordinate amount of lawsonite, glaucophane and pumpellyite (Hirajima et al., 1984). Thus, it is clear that both units of the Motalat]ella high-pressure metamor-

77

B

St. Jonsfjorden

A

,~E ,~E 2~

"

separating the two units is sharp, that is, rocks on both sides are not mixed up.

3. Petrography

-

,~lkm I

The main high-pressure rocks of the Upper Unit are eclogites, glaucophane schists, jadeite-bearing siliceous schists and garnet-chloritoid-white mica schists. Some descriptions of the Lower Unit rocks have been given in Hirajima et al. (1984).

' q - "

3.1. Eclogites "o I~

lee margin Bulltinden Group L _

~_

.

.

.

.

.

.

.

.

.

.

Tilloid formation

~]~ Cale-argillo-voleanicformation Undifferentiated HeclaHoek rocks m R Mota]afjella high-pressure metamorphic complex

00~ ~-I d ~

Fig. 2. A. Simplified distribution map of the Hecla Hoek succession in Spitsbergen (dotted area). The other eclogite locality (Biskayer) is shown by a star. B. Geologic map of the area from St. Jonsfjorden to Motalafjella.

phic complex underwent high-pressure metamorphism. Predominant lithologies of the Motalafjella highpressure metamorphic complex are pelitic schists. They are the chloritoid-white mica schists in the Lower Unit and the garnet-chloritoid-white mica schists in the Upper Unit, and both have very strong diaphthoritic cleavages. Hirajima et al. (1984) have shown that the association ofpelites and surrounding blocks of the Lower Unit represents a sedimentary m61ange, and this view may be extended to that in the Upper Unit, where the original structural relationship is now obscured by high-grade metamorphism. The diaphthoritic cleavages of the pelitic schists suggest that the Motalal]ella high-pressure metamorphic complex, as a whole, underwent strong shearing, tectonic mixing and retrograde mineralization during their uplift after the peak of the regional high-pressure metamorphism. The thrust

The eclogites are essentially made up of garnet porphyroblasts set in a fine-grained matrix of omphacite. The equigranular matrix accompanies subordinate amounts of glaucophane, epidote, phengite, paragonite, rutile and quartz. Glaucophane and epidote sometimes occur as idioblasts. Garnet porphyroblasts contain inclusions of omphacite, glaucophane, epidote, phengite, rutile and quartz. Edges of idiomorphic garnets penetrate into idiomorphic glaucophane in some cases [Plate I, (A) ], suggesting that the garnet crystallized simultaneously with the glaucophane. Some eclogites contain zoned amphiboles, a colorless core of winchite, and a wide bluish rim of glaucophane (see Table 1B).

3.2. Glaucophane schists Glaucophane schists show distinct schistosity and compositional banding of garnet-rich and -poor layers. The matrix consists of glaucophane, epidote, phengite and quartz with subordinate amounts of paragonite and chloritoid. The modal amount of glaucophane varies from 30% to 80%. Garnet is porphyroblastic containing abundant inclusions of glaucophane, epidote, chloritoid, rutile and quartz. Some porphyroblasts of epidote include glaucophane, phengite and quartz. Although omphacites are rarely included in garnet porphyroblasts, they are absent in the matrix. One exceptional glaucophane schist (rock No. 82092801, Tables 1 and 3) contains aegirinejadeite (Xjd = 0.5-0.7, S a c m = 0.250.40, Xdi=0.05-0.15) associated with quartz,

78

ILegendl Axis of syneline Bulltinden Group Boulder conglomerate ~V~ Alternation of shale & sandstone Fossiliferous limestone Unconformity

~

Snow & ice

~

Moraine

Motalafjella high-pressure complex Upper Unit Lower Unit ~ Diaplatlaoritic ~ Phyllites mica schists Dolomite Schistose /Serpentinite limestone ~ Limestone ~ Eclogites /Quartzite Thrust ~ Green rocks

Fig. 3. Geologic map of the vicinity of Motalat]ella.

chloritoid, garnet and paragonite in the matrix. The aegirine jadeite and accompanying glaucophane and white micas define the foliation. Chloritoid [0.13
3.3. Jadeite-bearing siliceous schists These rocks occur as thin layers, as much as 1 m thick, intercalated amongst the glaucophane schists

and eclogites, and have well-developed schistosity. They usually consist of jadeitic pyroxene, glaucophane, crossite, winchite, garnet, quartz, phengite and paragonite (Table 1D). Jadeitic pyroxenes, showing distinctive wavy extinction and anomalous interference color, usually occur as subhedral oval crystals, but sometimes as fan-shaped aggregates. In one specimen (rock No. 40-1, Tables 1 and 3 ), needles of winchite form aggregates both in the matrix and in the garnet porphyroblasts.

79

3.4. Garnet-chloritoid-white mica schists Garnet-chloritoid-white mica schists constitute the matrix of the Upper Unit and enclose lenticular

(A)

0.Smm Ill

(B)

0.2ram

PLATE I. For caption see next page.

bodies of eclogites and intercalations of glaucophane schists and siliceous schists. The main constituent minerals of this rock type are garnet, chloritoid, paragonite, quartz, phengite and car-

80 PLATE I (continued)

(C)

0.Smm III

I

A. Photomicrograph of eclogite (sample No. 5-1B ). Edge of idiomorphic garnet penetrates into idiomorphic glaucophane. B. Backscattered electron image of porphyroblastic amphiboles in garnet-chloritoid-white mica schists (sample No. 54C) (core=winchite; mantle=glaucophane; outermost rim=riebeckite). C. Backscattered electron image of zoned chloritoid in garnet-chloritoid-white mica schist (sample No. 33-2B). bonate minerals, with or without subordinate amounts of glaucophane, epidote and rutile. The garnet porphyroblasts enclose phengite, paragonite, quartz and chloritoid. Some garnets have an atoll structure; their core is usually filled with quartz. Most chloritoids show distinctive chemical zoning; an Mg-Fe chloritoid core and an Fe-rich rim [Plate I, ( C ) ] . The majority of amphiboles in this rocktype are glaucophane or crossite, and are generally homogeneous in an area of thin-section size. However, some amphiboles show distinct zoning: an inclusion-rich and pale-green core ofwinchite and an inclusion-poor and pale-blue mantle o f glaucophane, with or without a dark-blue outermost rim of magnesioriebeckite [ Plate I, (B) ].

A

Pg Ep Jd

4. Bulk chemistry

Fm C Fig. 4. Whole-rock compositions of the Motalai~ella highpressure metamorphic complex in the N(NaA102)A (A120~+ Fe2Oa)-C(CaO)-Fm (FeO + MgO + MnO) tetrahedron (open stars=eclogite; solid triangles=glaucophane schist; open triangles=glaucophane-bearing garnet-chloritoid-mica schist; solid squares=garnet-chloritoid-mica schist; solid stars = metabasite from the Lower Unit).

Whole-rock compositions of rocks of the Motala0ella high-pressure metamorphic Complex were reported by Ohta (1979) and Ohta et al. (1986).

They concluded that the garnet-white mica schists, with or without glaucophane, were derived from argillaceous sediments and that the glaucophane

81 schists were derived from mixtures of basaltic materials and pelitic (argillo-siliceous) sediments. This idea is also supported by the following observations; many eclogitic rocks have a thin rind (a few cm to 1 m) ofglaucophane schists, with mineral assemblages of intermediate composition between the eclogitic rocks and the surrounding metapelites (Fig. 4). Mixing may have taken place between the host sediments and sands derived from a basic olistolith or between the host and sands scraped off from the latter during their passage through solid media. The mineralogy, which will be discussed later, shows that they are isofacial, and mixing was finished before the major recrystallization stage.

5. Replacement textures The following replacement textures are common in the Upper Unit of the Motalafjella high-pressure metamorphic complex: (a) Most jadeitic pyroxenes are replaced to some extent by albite or phengite from the rim, and hence direct contact between jadeite and quartz is not common. In some cases, submicroscopic aegirine augite ( < 10 /tin in diameter) occurs along the boundary between albite armoring jadeitic pyroxene and quartz. Some omphacites are slightly replaced by albite along the margin. (b) The replacement of garnet by chlorite and epidote took place to various degrees, but it developed most intensely in the diaphthoritic garnetchloritoid-white mica schists, where sometimes no garnet is left at all, leaving an aggregated chlorite pseudomorph. (c) Some glaucophanes and crossites are armored by green amphiboles [actinolite, riebeckitic actinolite or hornblende, see Table 1 and Plate I, (B) ] and chlorite. Some amphiboles in the garnetchloritoid-white mica schists are decomposed to albite + white micas + chlorite + calcite aggregates.

6. Mineral chemistry Mineral analyses of the representative rocks were carried out by Hitachi-S550 with Kevex energy dispersion system at Kyoto University and JEOL 733 super-microprobe of the National Institute of Polar

Research, Japan. The mineral chemistry is shown in Table 1 for some critical assemblages.

6.1. Clinopyroxene The Fe 2+/Fe 3+ ratios of the clinopyroxenes were estimated by two methods; one following Banno (1959), that is, Xjdtjadei,e)=AlVl/(Na+Ca), Xacm(acmite ) = Fe3+/(Na+Ca) = (Na-AlVl)/ ( N a + C a ) , Xditdiopside)=Ca/(Na+Ca), and the other based on total cations as 4.00. Both methods give similar results. All the analyzed pyroxenes in the matrix of the eclogites fall in the omphacite region of the jd-acm( d i + h d + c a t ) diagram (Fig. 5), except some clinopyroxenes included in garnet. The compositions of the omiahacite grains in the matrix compare well with each other in thin-section, but some omphacite grains show oscillatory zoning with acmite-rich ( X a c m = 0.15--0.10 ) and acmite-poor parts (Xacm= 0.10-0.05). Clinopyroxenes included in garnet are homogeneous in each grain and they are generally poorer in jadeite component (Xjd= 0.15-0.35, Xacm= 0.1-0.3, Xdi = 0.4-0.6 )' than those in the matrix (Xjd=0.45--0.5, Xacm=0.05-0.2, Xdi:0.40.6). In some eclogites, Xjd of the inclusion clinopyroxenes in garnet increases from the core to the rim of garnet accompanying a decrease in KD= (Fe 2+/Mg)g . . . . l / ( Fez+/Mg)clinopy. . . . . . from 35 to 10. Most of clinopyroxenes from the siliceous schists are impure jadeite and aegirine jadeite (Fig. 5). They are homogeneous on the scale of a thin-section. Jadeitic pyroxenes are usually decomposed to albite from the rim to a various extent. When jadeitic pyroxenes are slightly richer in CaO content (Xdi>0.1), secondary aegirine augites (Xjd<0.1) are found at the outermost rim of jadeite-albite composite pool (Fig. 5 ).

6.2. Garnet Although the atoll garnets are chemically homogeneous, all the porphyroblastic garnets show very distinct chemical zonation (normal-type) with Feand Mn-enriched cores and Mg-enriched rims (Fig. 6). The Mn-compositional profile is bell-shaped, whereas the Mg zoning is bowl-shaped (Fig. 7 ). The zonation in Ca is generally weak. Some porphyroblastic garnets show slight reverse zoning at the out-

82 TABLE IA Chemical compositions o f representative minerals in eclogite (No. 24-23 ) - Mineral assemblage: Grt-Omp-Ep-Gln-Pg Mineral N.B.

Grt core

Omp

SiO_, TiO2 AL_O3 F%O~ FeO *~ MnO MgO CaO Na,O K20 Total

37.37 . 21.14 28.07 1.76 1.96 9.64 . 99.94

55.60

99.09

O=

12

6*-'

Si AI Ti Fe 3÷ Fe-"+ Mn Mg Ca Na K Total

Grt rim 37.71 .

7.02 11.65 7.75 .

Gin

Phg

Pg

37.99

38.20

57.15

46.47

21.22 26.92 0.97 3.27 9.37 -

26.59 9.36

10.57 12.33 0.09 8.59 0,24 7,27

11

99.06

99.74

97.22

96,24

6 *2

12

25

23 *3

11

11.43 5.63 6.86 11.47 7.71

0.08 23.00

.

100.04 12 2.980 2.001

2.006 0.483 .

3.040 1.978

6.019 4.937

8.006 1.745

1.780 0.065 0.385 0.794 -

1.109

0.197 1.248 0.011 1.794 0.036 1.975 15.011

.

1.815 0.074 0.392 0.759 -

0.375 0.447 0.539 4.000

Ep

48.13 0.73 25.92 5.53 1.78 0.14 0.16 10.42 94.80

.

1.993 0.462 . 0.091 0.093

Grt

.

21.48 27.46 1.10 3.33 8.96 -

6.13

8.022

55.96 .

10.94

2.983 1.989 . 1.874 0.119 0.233 0.824 -

Omp

0.042 0.127 0.367 0.440 0.536 4.000

8.020

8.007

0.010 3.883

15.958

3.290 2.249 0.038 0.316 0.181 0.010 0.021 0.907 7.012

39.13 0.39

7.46 0.78 94.23

3.004 2.981 0.021

0.935 0.064 7.005

*ETotal iron as FeO. *2Recalculated total cations as 4.0. *3Recalculated total cations in M and T sites in amphibole fis 13.0. TABLE IB Chemical compositions of representative minerals in eclogite (No. 43-3) - Mineral assemblage: Grt-Omp-Ep-Gln Ep inclu,

Wnc core

Gin rim

Hbl rim

Phg

55.01 . 8.22 8.63 0.00 7.26 13.04 6.63

38.55 . 28.23 6.43 0.16 0.00 23.74 -

53.87

58.02

45.64

7.91

8.83

98.79

97.11

7.02 0.04 15.78 7.68 3.95 0.15 96.40

9.75 11.50 0.16 9.79 0.47 6.96 11.50 96.64

51.71 0.15 24.95

19.96 0.00 8.61 10.20 2.32 0.74 96.31

3.88 0.17 0.12 10.43 94.29

6 *2

25

23 *3

23 *3

23

11

Mineral

Grt

N.B.

core

rim

matrix

inclu.

37.34 . 20.47 29.15 2.44 1.29 8.63 . 99.32

38.63 . 21.33

99.88

55.91 . 9.06 5.41 8.84 13.86 6.32 . 99.40

12

6 *2

SiO~ TiO, A1203 Fe203 FeO *~ MnO MgO CaO Na20 K~O Total O= Si AI Ti Fe 3+ Fe 2+ Mn Mg Ca Na K Total

12 3.018 1.951 . 1.971 0.167 0.155 0.747 8.007

See footnotes to Table 1A.

Omp

25.36 0.37 5.56 8.63 .

.

3.010 1.959 . 1.652 0.025 0.645 0.720 8.011

2.008 0.383 . 0.410 0.121 0.473 0.533 0.440 4.000

.

2.008 0.354 . 0.099 0.164 0.395 0.510 0.469 4.000

6.030 5.204 . 0.756 0.021 3.978 15.989

7.561 1.308

8.036 1.592

0.158 0.666 0.005 3.302 1.155 1.075 0.027 15.258

0.328 1.004 0.019 2.022 0.070 1.869 14.939

2.88

6.995 1.575

3.492 1.986 0.008

1,595

0.163

1,967 1.676 0,689 0.145 15.625

0.390 0.013 0.016 0.899 6.967

83 TABLE 1C Chemical compositions of representative minerals in glaucophane-schist (No. 46-3 ) - Mineral assemblage: Grt-Ep-Gln-Pg-Cld Gin

Ep

Mineral

Grt

N.B.

rim

core

37.11 0.07 20.66 26.48 0.27 5.43 9.5l 99.53

36.31 0.14 20.58

57.13

31.05 1.26 2.62 7.66

22.64

99.62

6.87 0.04 12.79 1.09 7.03 0.02 95.35

12

12

23 *3

SiO2 TiO~ AI.O3 Fe203 FeO *~ MnO MgO CaO Na~O K20 Total O= Si AI Ti Fe ~+ Fe :+ Mn Mg Ca Na K Total

2.939 1.928 0.004 1.754 0.018 0.641 0.807 8.091

2.937 1.962 0.009 2.100 0.086 0.316 0.664 8.074

10.38

7.883 1.688 0.399 0.454 0.005 2.631 0.161 1.881 0.004 15.045

36.63 0.06 23.43 13.03

Pg

Phg

Cld matrix

inclu.

24.14

23.80

40.16

38.66

18.51 0.13 6.38

23.06 0.15 4.27

44.92 0.07 39.43

50.37 0.28 26.11

0.63

2.28 4.06

95.93

0.06 1.44 7.11 0.14 92.92

0.47 10.55 94.12

89.32

89.94

25

11

11

12

12

0.14

5.952 4.487 0.007 1.594

-

2.946 3.047 0.004

3.410 2.084 0.014

2.006 3.934

2.013 3.855

0,035

0.129

0.006 0.032 0.904 0.012 6.986

0.410

1,286 0.009 0.790

1.631 0.011 0.538

0.061 0.911 7.018

8.025

8.048

0.019 3.941

16.001

See footnotes to Table 1A. TABLE ID Chemical compositions of representative minerals in jadeite-bearing siliceous schists - Mineral assemblage: Jd-Grt-Wnc (40-1), Jd-Grt-Gln-PgCId (82092801) Rock No.

No. 40-I

Mineral

Jd

Grt

Wnc

Jd

Gin

Grt

Pg

CId

Phg

57.10 19.18 6.92 0.20 1.09 14.44 98.93

37.97 0.02 21.88 28.35 0.58 3.48 7.47 99.74

52.21

57.11

37.64 0.10 21.90 31.42

45.79 0.15 38.44 1.11

24.65

49.70

1.69 23.33 0.69 7.74 4.58 4.83 0.08 95.14

56.13 0.12 15.57 10.22

37.39 26.54

25.34 4.50

5.67 2.05

2.69

91.73

2.93 0.18 0.33 10.24 93.62

12

23 *3

12

11

SiO: TiO. AI~O~ FeO *~ MnO MgO CaO Na,O K,O Total O=

6

82092801

98.27

9.55 11.22 0.26 10.09 0.38 7.67 0.66 96.0!

99.15

0.10 0.13 7.22 10.82 93.77

6 *2

23 *3

12

11

0.88 1.76 13.59

Si AI Ti Fe 3+ Fe -~+ Mn Mg Ca Na K

1.989 0.787 0.202 0.000 0.010 0.041 0.975 -

2.997 2.035 0.013 1.871 0.039 0.409 0.632

7.907 0.302 0.964 1.991 0.089 1.748 0.743 1.418 0.015

1.993 0.652 0.003 0.292 0.012 0.147 0.067 0.935

Total

4.004

7.984

15.177

4.000

See footnotes to Table IA.

8.001 1.577 0.244 1.091 0.017 2.108 0.057 2.083 0.055

2.983 2.045 0.006 . 2.082 . 0.670 0.174 -

15.140

7.984

. .

2.989 2.957 0.008 . 0.061 . 0.010 0.009 0.914 0.069 7.016

1.909 3.412 -

3.418 2.053 0.206

1.718

0.259

0.311 -

0.360 0.014 0.043 0.899

7.379

7.016

. .

84

Cpx

acm

• Siliceous s c h i s t • Eclogite m a t r i x 0 I n d u s i o n in e c l o g i t i c garnet A Secondary Cpx in Siliceous s c h i s t

A

&A

O0 •

di+hd÷cat

"

jd

Fig. 5. Chemical compositions of clinopyroxenes on the j d-acre- (di + hd + cat ) diagram (open circles= clinopyroxenes included in eclogitic garnets; solid circles= clinopyroxenes in eclogite matrix; triangles= clinopyroxenes in siliceous schists).

ermost rim, marked by a decrease in Mg and increases in Fe and Mn. As a rare case, there is a zoned garnet in which Mn increases without an increase in Fe, and a decrease in Mg at the outermost rim (Fig. 7B). In the latter two cases, the maximum Mg/Fe ratio is taken as the "stable rim" value at the maximum metamorphic grade. The "stable rim" composition of garnet correlates well with the coexisting minerals as will be discussed later.

curs at the core of zoned sodic amphiboles or as discrete needle crystals, and rarely as the outermost rim of glaucophane. Those constituting the core of the zoned amphiboles are accompanied by some Tschermak's substitution (Si= 7.5-7.8 for O = 23 formula basis), but Si~ 8.0 in the needle winchite (Table 1 ). The outermost rims, of such amphiboles, calcic amphiboles or magnesioriebeckite, are quite variable in composition even in one grain.

6.3. Amphibole

6.4. Epidote

Calcic (actinolite and hornblende), sodic-calcic (winchite), and sodic (glaucophane, crossite, magnesioriebeckite and riebeckite) amphiboles occur in the rocks of the Upper Unit. Glaucophane and crossite are predominant (Fig. 8) in the matrix of the eclogites and the glaucophane schists. They also occur in other rock-types in minor amounts. The sodic amphiboles are usually homogeneous over the scale of a thin-section, but, in some cases, they are distinctly zoned or rimmed by calcic amphiboles whose compositions vary from actinolite to hornblende (Table 1 ). Winchite in the strict sense, that is, intermediate between glaucophane and actinolite, oc-

Epidote commonly occurs as porphyroblasts in eclogites and glaucophane schists and sometimes exists in the garnet-chloritoid-white mica schists and the jadeite-bearing siliceous schists. It is also included in the porphyroblasts of garnet and glaucophane. Epidote is usually heterogeneous, and its YFe= (Fe3+/Fe 3+ +Al) varies from 0.1 to 0.3 even in a single thin-section. Some epidote porphyroblasts in eclogite and glaucophane schists show a zonation with a low YFecore and a high YFerim, the latter presumably being of retrograde origin. Some epidotes contain ~ 5 wt.% of rare-earth elements at the core, but the rim is free from them.

85 A) • 47-1 • 5-1b Open=Rim

( M g + F e ) < 0 . 4 , and Fe-richer dark-green rim, (Fig. 9). Core compositions of chloritoid are correlated with associated minerals; 0.30
0.05
sps ~ 4ps 0 Py~alrn Cor3 e ~

: : \1o

Rim 50

py

• -.,.V

70

60

80

B)

\

90

SpS

6.6. Other minerals

alm°sps

Zc°ra..\ gr~O~

~ m

~50 PY

Fig. 6. Compositionalvariation of garnets of representative specimens: (B) on the alm(Fe)-sps(Mn)-py(Mg) diagram; and (A) on the alm+sps(Fe+Mn)-py(Mg)-grs(Ca) diagram (open symbols showrim compositions). (A)

47-1

(B)

MgO A CaOO MnO •

5-1b

0 FeO

MnO • -34 -30

8"

-26

"34

108-

-26

620-

Rim

Core

Rim

Rim

7. Mineral assemblages

"30

4-

4

Phengite and paragonite are the micas in the rocks examined. Phengite has rather a high Si value, about 3.4 for O = 11 (Table 1 ). Rutile commonly occurs in the matrix of all kinds of rock-types and as inclusions in garnet, omphacite and sodic amphibole. Some ruffle grains are armored by retrograde sphene. Albite and chlorite are retrograde minerals. Lawsonite, which is extensively replaced by epidote and muscovite, is rarely preserved in the eclogites. Calcite, dolomite and magnesite are common in the garnet-chloritoid-white mica schists, but only calcite occurs in the eclogites, glaucophane schists and jadeite-bearing siliceous schists. Aragonite was expected, but Feigl's solution test and X-ray diffractometry of the selected specimens have failed to confirm it.

Core

Rim

Fig. 7. Chemicalzoningpattern of idioblasticgarnet. 6.5. Chloritoid Chloritoid grains often show chemical zonation; an Mg-richer pale bluish-green core, 0.2 < XMg= Mg/

The foregoing descriptions suggest that the calcic amphibole, albite and chlorite in the Upper Unit postdated the high-pressure metamorphism. Fe-rich chloritoid armoring the Mg-Fe chloritoid, and winchite armored by glaucophane are retrograde and prograde products, respectively. There is no clear evidence to suggest that needle winchite associated with jadeite (see Table 1, rock No. 40-1 ) is a majorstage mineral. Thus, the major mineral constituents of the Upper Unit are jadeite, omphacite, garnet, glaucophane, chloritoid, paragonite, phengite, epidote and quartz. Phengite and quartz occur in all the rock-types. To simplify the discussion, the sta-

86 • Porphyroblastic amphiboles in ectogite stage • Needle winchite in siliceous schist(No.40-1) ~r Retrograde amphiboles (rim of

Zoned amphiboles gin ,, /,,'~ /,m ,'~i~ /.~_: \

[] Core • Mantle * Rim

rbk '

' act

Fig. 8. Chemical composition of amphiboles on the act (Ca)-gln (Al)-rbk (Fe 3+ ) triangle [ solid circles = amphiboles mainly from eclogites and glaucophane schists; open stars= outermost rim o f zoned amphiboles; solid triangles= needle winchite from siliceous schists (rock No. 40-1)]. Zoned amphiboles (rock No. 5-4C): open squares=core; solid squares=mantle; solid

stars = outermost rim. TABLE 2

UpperUnit Core ~ ~ , , 40 30

N Riml~

II!

nn 20

1o 0

n. f l

Mg/(Mg÷Fe 2+) in chloritoid Fig. 9. Frequency distribution o f the chemical composition of chloritoid of the U p p e r Unit.

bility relations of the carbonate minerals are ignored. Dominant mineral assemblages of the Upper Unit are shown in Table 3. In describing the mineral assemblages, the abbreviations proposed by Kretz ( 1983 ) will be mostly used (Table 2). The mineral parageneses of the eclogites and associated rocks described above may be dealt with in terms of a six-component system, AI203-Fe203-FeOMgO-CaO-Na20, with excesses of SiO2 and H 2 0 or with constant H 2 0 fugacity. As the majority o f the assemblages contain garnet, which is far richer in Fe than the other ferromagnesian phases, the semiquantitative phase relations may be discussed in

Abbreviations of representative minerals and end-members

Phases: Cld = chloritoid Ep = epidote Gin = glaucophane Gri = garnet Hbl = hornblende Jd =jadeite

Ky = kyanite Lws = lawsonite Omp = omphacite Pg = paragonite Phg = phengite Wnc=winchite

End-members: acre = acmite aim = almandine cat = Ca-tschermakite component di = diopside grs = grossular

hd= hedenbergite jd =jadeite py = pyrope sps = spessartine

terms of a four-component system, A = A1203 + F e 2 0 3 - 3K20, N = NaAIO:, M = MgO and C = CaO, with almandine, quartz and H 2 0 as excess phases. The A1-Fe3+ substitution is neglected here. Figure 10 schematically shows the paragenesis : o f garnet-bearing assemblages of this system, which is a projection from the Ca-bearing almandine onto the A - C - N - M tetrahedron, assuming following XFe = Fe 2÷ / (Fe 2÷ + Mg) for the relevant phases; XFe = 2 and Xca = 1 for almandine,

87 TABLE 3 Observed mineral paragenesis of main rock-types Eclogite:

Omp-Grl-Gln-Ep-Pg (No. 24-23) Omp-Grt-Gln-Ep Omp-Grt-Gln Glaucophane schists:

Grt-Gln-Ep-Cld-Pg (No. 46-3) Grt-Gln-Ep-Pg Grt-Gln-Cld-Pg Grt-Gln-Ep Jd-Grt-Gln-Cld-Pg (No. 82092801 ) Garnet-chloritoid-white mica schists:

Grt-Ep-Cld-Pg (No. 33-2b) Grt-Cld-Pg Cld-Pg CId-Grt

lar diagram (Fig. 11 ). As we regard the system as five-component system, under arbitrarily ascribed external conditions, the maximum number of phases to coexist with garnet is 4. Thus the garnet associated with four phases has a fixed composition, or invariant in the composition figure, and those associated with three and two phases have one and two degrees of freedom, or are univariant and divariant, respectively, on the figure (Korzhinskii, 1959; Miyashiro and Shido, 1985; Hosotani and Banno, 1986). The geometry ofinvariant, univariant and divariant compositions in Fig. 11 should follow the Morey-Schreinemakers' rule, as the axes XFe and )(Ca are rerated to the chemical potential of Ca3A125i3012 (grs) and Fe3Al2Si3012( aim ) components in garnet, through respectively the formulae: Xve exp [ ( ] A a l m =

Jadeite-bearing metasiliceous rock:

Jd *SiO 2 *aH2o

Qtz fixed

Gin

M i

Ep

-

.... K y ~ /

~-~ ~

T]

)(Ca =exp [ (~grs -- G°rs)/RT]

See Table 2 for mineral abbreviations.

C

G°lm )/R

and

Jd-Grt-Ep-Wnc (No. 40-1 ) Jd-Gln-Pg

pg

--

CId

N

Fig. 10. Mineral paragenesis on the N(NaAIO2)A (A1203 + Fe203 )-C(CaO )-M(MgO ) tetrahedron projected

from quartz, muscovite and garnet (F%CaA12Si30~2).Actuallyobservedassemblagesin the Motalafjellahigh-pressure metamorphiccomplexare shownby tie-lines.

where/t and G Orefer to the chemical potential and molar free energy of the subscribed components, respectively. Thus, the triangular diagram is a modified chemical-potential diagram. The sign of the slope of univariant lines has been estimated, for instance for the Gln-Ep-Pg univariant line, as follows. The chemical equation to define the univariant line is: 5Gin (Na2A12(Fm) 3Sis O22 (OH)2) + 18Ep (Ca2A13Si3OI2(OH) )~10Pg + 5alm + 12grs + 13Qtz + 4Wtr = ( 10NaAI3 Si3Olo(OH)2 + 5 (Fm)3Al2Si3Ol2 + 12Ca3A12Si30]2

XF~= ½ for chloritoid, and XF~=0 for glaucophane and omphacite. These XF~-values are assumed to simulate the tendency of Fe-enrichment among the relevant minerals. In this projection, all the garnetbearing assemblages except Ep-Jd-Wnc(winchite) are represented.

8. Garnet compositions and mineral assemblages The relationships between paragenesis and garnet composition are shown in a Ca-Fe-Mg triangu-

+ 13SIO2 + 4H20)

( 1)

where Fm stands for Mg and Fe 2÷. This is a sliding equilibrium in relation to the Fe-Mg substitution. A high )(Ca of garnet favors the assemblage on the epidote-bearing side of the equation, and a higher Xve favors the almandine side of it. Thus the slope for this univariant line is positive in relation to the Xve and XCa axes. Figure 12 shows the "stable rim" compositions of garnet for each mineral assemblage by abbrevia-

88

aim+ s-s ,L p /\

alm.sps

~. grs'

Omp-Gln-Ep-Pg(24-23) (E) GIn÷Ep+Pg÷Cld(46-3) A Gln+Jd+Pg+Cid (82092801) • Omp+Gln_+Ep

7

\py

~

/ 8O

~ EpePg+Cld \

•

Jd*Ep+Wnc

?0 6O 5O

gr7

\py

Fig. 1 I. The relationships between paragenesis and compositions of garnet in aim + sps (Fe + Mn )-py (Mg)-grs (Ca) triangular diagram.

aim÷sps

~t" Omp.Gln,Ep.Pg(24-23) (E) GIn.Ep.Pg.Cld (46-3) A GIn+Jd+Pg+CId (82092801) • Ornp+GIn±Ep O G|n÷EpZPg • Ep. Pg÷Cld [] Pg A. Jd+Ep+Wnc

[] Pc

grs

E~Pmp~G,~

Y

I" d

Om

Ep'\GIn

Fig. 12. Relationships between paragenesis and compositions of "garnet rim" in the a l m + s p s ( F e + M n ) - p y ( M g ) - g r s ( C a ) triangular diagram. Symbols are the same as in Fig. I 1. The most magnesian compositions are selected on each sample.

89 tions (cf. Fig. l l ). The most magnesian value in each sample is regarded as the "stable rim" composition of garnet. In this figure, we have one sample each for three isophysical invariant points (assemblages); Omp-Gln-Ep-Pg (star), Gln-Ep-PgCld (circle with a dot) and Gln-Jd-Pg-Cld (open triangle). To satisfy the geometry ofunivariant lines, estimated by the above-mentioned method, the garnet composition shown by the circle with a dot should be slightly modified to be located at point 1 as shown by the broken line in Fig. 12. This treatment reduced the discrepancy between the net of univariant lines and the observed garnet compositions. Schreinemakers' analysis predicts the existence of two more invariant assemblages; one of them is Omp-Jd-Ep-Pg and the other is Omp-JdGln-Pg. They are shown in inset of Fig. 12 by the points [Gln,Cld] and [Ep,Cld], respectively. Although the last two assemblages were not found in the Motala0ella high-pressure metamorphic complex, they are compatible with the phase relations of Fig. 10. There is need of further comments. One of them is specimen No. 40-1 (solid triangle in Fig. 12, refer Table 3 ) with Jd-Ep-Wnc assemblage. Although in this model system we do not treat winchite, the garnet rim of this specimen plots on the Ca-richer side of the jadeite stable field as expected where glaucophane is unstable (Fig. 12). If winchite is an unstable phase, as we postulated earlier, this point is in the Jd-Ep field of Fig. 12 and consistent with the net. Another is specimen No. 33-2b (solid square) with Ep-Pg-Cld assemblage, which should plot above the Ep = Cld univariant line, but is set slightly apart from it. Moreover, garnets from two eclogites (solid circle) are plotted around a degenerate univariant line, Ep = Cld, beyond the stability field of the OmpEp divariant assemblage. It may be that specimen No. 33-2b and specimen No. 46-3 (circle with dot, Cld-Ep-Gln-Pg assemblage) are those eclogites affected by Fe3+-A1 substitution, which has not been taken into account in this discussion. In spite of a few inconsistencies between the assemblages of the model system and those observed, the relationships between the garnet compositions and the mineral assemblages are rather satisfactory in view of the fact that we have ignored AI-Fe 3÷ substitution. We therefore conclude that the mineral assemblages listed in Table 3 and Fig. 10 are isofacial and represent the maximum eclogite stage.

9. Equilibrium conditions of the Spitsbergen eclogites The temperature and pressure conditions of the eclogites and associated rocks in the Upper Unit of the Motalafjella high-pressure metamorphic complex may be estimated from the following observations: (a) The existence of a jadeite-quartz assemblage in the siliceous schists provides the minimum pressure, > 15 kbar at 550°C, and > 17 kbar at 600°C (Holland, 1980, 1983). (b) The common occurrence of paragonite defines the upper pressure limit, <24-26 kbar at 550°C, and <24-25.5 kbar at 600°C (Holland, 1979b). (c) The occurrence of Mg-Fe chloritoid (Xug=0.5) defines the minimum pressure, > 13 kbar at 540°C, and > 19 kbar at 640°C (Chopin, 1983). (d) KD ranges from 8 to 19 (Fig. 13) corresponding to 600 + 35 ° C at 15 kbar, 610 + 35 ° C at 20 kbar of Ellis and Green ( 1979 ). These constraints indicate that the Motalafjella eclogite was formed within the temperature range

KD=17.O

8o -~o

8

o 0

/

•

2

KD=6.5

•

/

Koo2.7

.c_

/

,,//;o u_

I0

// 0.1 I

1.0 I

I

Fe2*/Mg in pyroxene

Fig. 13. Fe2+-Mg partitioning between garnets and clinopyroxenes (open circles = clinopyroxene inclusion in garnet core; open triangles= clinopyroxene inclusion near garnet rim; solid circles = clinopyroxene in the matrix and garnet rim ).

90 of 575-645 °C at pressures of 17-24 kbar. The temperature estimated for the Motalai]ella eclogites is ~ 60°C higher than that for maximum glauc0phane stability indicated by Maresch (1977). Textural observations indicate that the majority of sodic amphiboles, except the outermost rim of the zoned amphiboles, stably coexist with omphacite, garnet, epidote and quartz. The same discrepancies with the synthetic results of Maresch (1977) have been reported from several other areas, such as the Fairbanks eclogites, Alaska, U.S.A. (600°C, 15 kbar; Brown and Forbes, 1986), Tauern Window, Austria (620 ° C, 19 kbar; Holland, 1979a) and the Adula Nappe, north Italy (Heinrich, 1986). The recent experimental studies of Koons (1982) and Carman and Gilbert (1983) show that pure glaucophane is stable beyond the upper temperature limit of Maresch ( 1977 ). As pointed out by Brown and Forbes (1986), we consider that slight chemical change of sodic amphibole strongly affects the stability field of the sodic amphibole.

10. P-T-t path of Motalafjella eclogites Dallmeyer et al. (1988) and Ohta et al. (1988) reported the 4°Ar/39Ar age of phengites from Motalafjella eclogites. They show well-defined intermediate- and high-temperature plateau ages of 460470 Ma and low-temperature shoulders ages of 360420 Ma. Based on their data and prograde and retrograde relics mentioned before, the following PT-t history is postulated. Lawsonite inclusion in core of eclogitic garnet, and KD between garnet core and inclusion clinopyroxene (Xjo = 0.2-0.3 ) indicate that the garnet core of eclogites began to crystallize at 350-400°C and 5-8 kbar, that is a very similar or slightly higher PT condition compared to that of the Lower Unit (L U in Fig. 14 ). Garnet zonation ( Fig. 6 ), increase of Xjd of included clinopyroxene from garnet core to garnet rim (Fig. 5 ), and systematic decrease of KD from garnet core to garnet rim (Fig. 13) indicate the continuous increase of prograde P-T value until the maximum stage (575-645°C at pressures of 17-24 kbar). The apparent paleogeotherm is estimated ~ 10°C/km. Newton (1986) compiled metamorphic conditions of crustal eclogites and pointed out that late Precambrian-early Paleozoic eclogites recorded a higher T / P geotherm ( 16 ° C/

Kb

~v

,,.~

/

L

D~'tI~

i

aoo

i

~

t

i

i

~o

6oo

700

i

oC

Fig. 14. Inferred P-T-t metamorphic path of Motalal]ella eclogites. UU=estimated P-T condition of the Upper Unit; LU=Lower Unit. Reaction Jd+Qtz~Ab from Holland (1980). km) than the younger (Cretaceous-Tertiary) ones ( l l ° C / k m ) . The estimated geotherm of Motalat~ella eclogite is similar to the younger-aged eclogites, and this is an obvious example of high P / T crustal-origin eclogites during late Precambrian and early Paleozoic time, against the compilation of Newton (1986). 460-470 Ma plateau ages of phengites are interpreted to date the post-metamorphic cooling age at ~315-400°C (Sisson and Onstott, 1986), and Motalat]ella eclogites had been uplifted to near the surface before the beginning of the sedimentation of the Bulltinden Group (Upper Ordovician to Lower Silurian). Since such high-pressure minerals as jadeitic pyroxenes, Mg-Fe chloritoid and glaucophane are well preserved even in metapelites of the Upper Unit, speed of uplift should be fairly fast. The Bulltinden Group and Motalat]ella highpressure metamorphic complex are folded into regional recumbent folds with development of locally penetrative axial cleavage in pelitic rocks of the Bulltinden Group. Whole-rock samples of cleaved slate display internally discordant 4°Ar/39Ar age spectra which suggest cleavage formation occurred at about 400 Ma (Dallmeyer et al., 1988). This event corresponds to the younger age of phengites from the Upper Unit. Therefore, at least, there are two possibilities for recrystallization timing of ret-

91 rograde minerals: during an uplift stage (470-460 Ma) after maximum P - T condition or a later thermal event (380-420 Ma). Because metamorphic feature of a later thermal event is not clear, we cannot exactly determine the recrystallization stage of retrograde minerals. However, as no high-pressure minerals were found during the retrograde stage, it is obvious that the retrograde P-T path runs through lower-pressure field rather than the prograde one.

11. Comparison of the phase relations with those of other areas - Petrogenetic grid

Even though eclogites are composed essentially of omphacite and garnet, recent petrographic work has revealed that they are stably associated with amphiboles (glaucophane, winchite, barroisite and hornblende), micas (muscovite, biotite and paragonite in particular), hydrous Ca-A1 silicates (lawsonite, epidote, zoisite and pumpellyite), and hydrous MgA1 silicates (magnesian staurolite and Mg-Fe chloritoid). Kyanite has also been reported from the eclogites of the glaucophanitic high-pressure terrains (Holland, 1979b; Heinrich, 1982, 1986). Therefore, it should now be possible to classify eclogites not only on the basis of partition coefficients, but also on mineral assemblages. Though the mineral assemblages of eclogitic rocks can generally be described in terms of a six-component system as we have discussed earlier, we are still far from dealing quantitatively with such a complex system. The eclogites from Motalatjella are described in terms of the paragenesis of a four-component N(NaAIO2)-CaO-A (AI203+ Fe203 )-MgO with excess of quartz, water and Fe-rich garnet. They mainly consist of the following six phases: epidote, paragonite, chloritoid, omphacite, glaucophane and jadeite. To extend the comparison to cover more eclogitic rocks, kyanite and lawsonite are added to the above six phases. Their idealized compositions are given in Table 4. High-pressure Ca-Na and Ca amphiboles may be stable in the relevant P-T field but they are not taken into account as we know little about them. In the model system, the garnet-forming reactions that we have adopted involve the decomposition of Mg-Fe chloritoid. Other garnetforming reactions are not used in the model, mainly because they have large transitional loops and are hardly approximated by model univariant reac-

tions. The following discussion is only applied to the region where jadeite + quartz is stable. As we do not have enough quantitative thermodynamic data to apply the Perkins et al. (1986) method of calculating the stable Schreinemakers' net, we chose a different way to obtain the petrogenetic grid of the eclogitic rocks in question, i.e. we calculated the possible geometry of Schreinemakers' nets of the model system giving only the slope of the univariant curves. We then chose the most plausible ones by comparing them with petrographic and synthetic evidences. The slopes of the univariant curves were calculated assuming a constant dehydration entropy of 14 cal. K - t mol-1 (Fyfe et al., 1958; Albee, 1965; Hirajima, 1983). The chemical compositions and volume of relevant phases are summarized in Table 4, in which the data sources are also described. The slopes of univariant curves calculated using the data of Helgeson et al. (1978) are slightly higher than ours, but both methods give similar topology of the nets. For our model system of four components and eight phases, we obtain eight possible nets, the invariant points of which are listed in Table 5. In calculating the nets, we used a program written by Tanabe et al. (1988) which can find out all possible nets of invariant points for systems treating univariant curves as straight lines. The geometry of the nets varies according to the assumptions of the chemistry of the phases involved. The garnet composition is variable, depending on the mineral assemblages as shown in Figs. 11 and 12. We have examined how the topology of nets changes according to the composition of excess garnet by using the compositions shown in Fig. 15. As compositions of other ferromagnesian phases are fixed to those in Fig. 10, only the reactions involving chloritoid are affected by the excess garnet. Therefore, the topology of eight nets, i.e. the combination ofinvariant points for the nets, changes with excess garnet compositions. We define the same combination for eight nets as a "set". We obtained three sets: set I corresponds to the most Fe-rich garnet (solid circle in Fig. 15 ); set II to the intermediate garnet (open circle); and set III to the garnet which is Mg-richer than the usual Group-C eclogites (triangle), respectively. The combinations of invariant points in each of the three sets are shown in Table 5, and the geometry of all the nets we ob-

92 TABLE 4 Mineral formulae and molar volumes (cm3/mol) of phases for the Schreinemakers' analysis

Na Ca Fe Mg AI Si H

Omp

Gin

Ep

CId

Pg

Jd

Ky

Lws

Grt

Qtz

W

1 l 0 1 1 4 0

2 0 0 3 2 8 2

0 2 0 0 3 3 1

0 0 0.5 0.5 2 1 2

2 0 0 0 6 6 4

l 0 0 0 1 2 0

0 0 0 0 2 1 0

0 l 0 0 2 2 4

0 0-1.2 1.2-2.4 0-1.2 2 3 0

0 0 0 0 0 1 0

0 0 0 0 0 0 2

Omp= 126.5 [1]~ Gin=258.5 [1]; Ep= 136.4 [2]; C1d=69.2 [3]; Pg=264 [4]; Jd=60.4 [1]; Ky=44.1 [•]; Lws= 101.3 [l]; Grt= 114.7-118.8 [5]:Qtz=22.7 [ l ] ; W = H 2 O a t 15 kbar, 550°C=16.73 [5] [ 1 ] = Robie and Waldbaum ( 1968); [2] =Myer ( 1966); [3] =Chopin and Moni6 ( 1984); [4] =Holland ( 1979b); [5] =Halbach and Chatterjee (1982).

tained is described elsewhere (Hirajima and Banno, 1989). Among the eight possible nets for every set, we rejected four as unlikely from consideration of the known paragenesis of eclogitic rocks. No unequivocal discrimination of the remaining four nets (Nos. 1, 3, 5 and 7) is possible, but we chose net No. 5 for every set to be the most plausible to us. We showed two of them in Fig. 16: Fig. 16A, net No. 5 of set I projected from CaFe2A12Si3Ol2 garnet and Fig. 16B, that of set II projected from Cao 6Fel8Mgo.6A12Si3Oi2garnet. As net No. 5 for set III is the same combination of invariant points as set II, it is neglected here. The difference of the characteristic net, No. 5, between sets I and II, is minor and either of the sets is consistent with the available petrographic data, but we accepted set II, because its garnet has representative composition of the study area as well as the other eclogite occurrences. We need a comment on selecting net No. 5. In other possible three nets, Nos. 1, 3 and 7, omphacite-kyanite assemblage not associated with chloritoid is stable on the higher temperature side of invariant point [24] [Gln,Cld] for which Holland (1979b) gave 650°C, 24 kbar by synthetic experiments. However, such an assemblage was described by Holland (1979a) in the Tauern Window, for which Rfiheim and Green (1974)'s calibration of the Fe-Mg partition between omphacite and garnet, which gives a lower temperature than Ellis and Green ( 1979)'s, suggests 620 + 30 ° C at 19.5 kbar. The boundary between the assemblages of Tauern and Motala0ella eclogites should be located between Rfiheim-Green's 620°C and Ellis-Green's

610 ° C of the Motala0ella eclogites. Only net No. 5 gives a reasonable temperature for the Tauern eclogites. Brown and Forbes (1986) proposed a Schreinemakers' net for the albite stability field for the same component as ours. Their net (cf. fig. 14 of Brown and Forbes, 1986) is useful for investigating mineral assemblages of lower-pressure eclogites and is joined with our net (Fig. 16) by the following two reactions: Ep+CId+Pg~-Gln+Ky

(2)

Ep+ G l n + P g ~ O m p + K y

(3)

The stability fields of eclogite from other areas are assigned on the Schreinemakers' net of Fig. 16 as follows: (1) Tauern Window and Adula Nappe. Holland (1979a) and Heinrich (1986) described the following mineral assemblages as primary in the Tauern Window (TW) and the Adula Nappe ( AN ), respectively: Kyanite-paragonite-glaucophane-omphacite (TW, A N ) Kyanite-paragonite-omphacite-epidote + magnesite ( T W )

with excess garnet and quartz. Holland (1979a) gave 620 + 30°C and 19.5 kbar for the quartz-kyanite eclogites in the Tauern Window; for the glaucophane-omphacite-kyanite-paragonite assemblage from Trescolmen, Adula Nappe, Heinrich (1986) estimated about 550-650°C and 15-22 kbar. These assemblages are incompatible with those of the Motalafjella eclogites, because of the following two reactions (Fig. 16 ):

93 Assemblages of Tauern Assemblages of vs. Window and Adula Motalafjella Nappe

aim* sps A

• Set I 0 Set II A Set Ill

/\

7

~

8o

Ep + Cld + Pg~Gln + Ky

(4)

Gin + Ep + P g ~ Omp + Ky

(5)

70

6o 50 40

TABLE 5 30

Eight independent nets of invariant points for three sets of combinations of invariant points with compositional change of excess garnet (the abbreviation number of the net and that of invariant points constituting each net are tabulated)

Eight independent nets of invariant points for Set I Net 1

[I] [Omp,Lws]; [4] [Cld,Lws]; [5] [Pg,Lws]; [7] [Ky,Lws]; [9] [GIn,Ky]; [10] [Ep,Ky]; [13] [Jd,Ky]; [20] [GIn,Pgl; [2I] [Ep,Pg]; [28] [Omp,Gln]

Net2

[1] [Omp,Lws]; [4] [Cld,Lws]; [8] [Omp,Ky]; [11] [Cld,Ky]; [18] [Pg,Jd]; [19] [Omp,Pg]; [22] [Cld,Pg]: [28] [Omp,Gln]

Net3

[2] [Gln,Lws]; [31 [Ep,Lws]; [6] [Jd,Lws]; [9] [Gln,Ky]; [10] [Ep,Ky]; [13] [Jd,Ky]; [20] [Gln,Pg]; [21] [Ep,Pg]; [28] [Omp,Gln]

Net 4

[I] [Omp,Lws]; [4] [CId,Lws]; [5] [Pg,Lws]; [8] [Omp,Ky]; [II] [Cld,Ky]; [12] [Pg,Ky]; [20] [Gtn,Pg]; [21] [Ep,Pg]; [28] [Omp,Gln]

Net5

[6] [Jd,Lws]; [13] [Jd,Ky]; [15] [Gln,Jd]; [16] [Ep,Jd]; [24] [Gln,Cld]; [25] [Ep,Cld]; [26] [Omp,Ep]

Net6

[14] [Omp,Jd]; [17] [Cld,Jd]; [18] [Pg,Jd]; [24] [Gln,CId]; [25] [Ep,Cld]; [26] [Omp,Ep]

Net 7

[3] [Ep,Lws]; [6] [Jd,Lws]; [10] [Ep,Ky]; [13] [Jd,Ky]; [15] [Gln,Jd]; [21] [Ep,Pg]; [24] [Gln,CId]; [27] [Gln,Ep]

Net 8

[4] [Cld,Lws], [11] [Cld,Ky]; [14] [Omp,Jd]; [18] [Pg,Jd]; [22] [Cld,Pg]; [23] [Omp,Cld]; [26] [Omp,Ep]

Eight independent nets of invariant points for Set H: Net 1 Net 2 Net 3 Net4 Net5 Net 6 Net 7 Net8

[1], [4]; [5]; [7]; [9]; [10]; [•3]; [•8]; [20]; [2•]; [2811 [1]; [4]; [8]; [11]; [•9]; [22]; [28] [2]; [3]; [6]; [9]; [I0]; [•3]; [•8]; [20]; [2•]; [28] [1]; [4]; [5]; [8]; [11]; [•2]; [•8]; [20]; [2•]; [28] [6]; [•3]; [•5]; [•6]; [•8]; [24]; [25]; [26] [•4]; [•7]; [24]; [25]; [26] [3]; [6]; [10]; [•3]; [•5]; [•8]; [2•]; [24]; [27] [4]; [••]; [•4]; [22]; [23]; [26]

Eight independent nets of invariant points for Set II1: Net 2 [1]; [4]; [7]; [9]; [10]; [•2]; [•3]; [•9]; [22]; [28] (All other sets are the same as those of Type II) Because of degeneracy, invariant points [23], [24] and [28] are the same.

20

grs

PY

Fig. 15. Topology change of Schreinemakers' net correlated with projecting garnet compositions. The nets with marked garnet compositions are shown in Fig. 16 as a representative for Set-I and -II topologies, respectively.

Hence, the stability field of Tauern eclogites is shown as region T in Fig. 16. (2) Sesia-Lanzo zone. Reinsch (1979) reported the following assemblages from the Sesia-Lanzo zone: (1)

(2) (3) (4) (5) (6)

Glaucophane-zoisite-paragonite Glaucophane-zoisite-omphacite Omphacite-glaucophane-paragonite Omphacite-paragonite Omphacite-zoisite-paragonite Chloritoid-zoisite-paragonite

with excess garnet, phengite and quartz. He considered that assemblages (3) and (4) were progressively formed from those of ( 1 ) and (2), but in the N-A-C-M system, all six assemblages are stable in the hatched region in Fig. 16. Based on the KD-value, Reinsch considered that the glaucophane schists [his assemblages (1) and (2)] were formed at about 450-500°C and the eclogites and omphacite felses [his assemblages (3) and (4) ] at about 550-600°C at 15 kbar. These values should indicate the P-T range of the hatched region in Fig. 16. (3) Corsica. Caron and P6quignot (1986) reported lawsonite-bearing eclogites, represented by the Omp-Alm-Gln-Lws assemblage, from Corsican (western Mediterranean) metabasalts and gave 420°C (Ellis and Green, 1979) for their crystallization temperature. This assemblage is stable in the area shown by C in Fig. 16. Therefore, it is stable on the higher-pressure and lower-temperature side

94

c

~=~ ='~ I-.

'o

<~

II

. ~ ","

~

b - -

.~,

I-0

/"

~.~ ~

uO0!

~-~

~,

< ' ~ o

o

~

-

2 o

--

w ':

~

'~

.= . ~

e~ ,.~

0

_~o~

,.-2,

.~.~ IlO~ n

uo0~

_~.~

95 of the Motalat]ella eclogites, and most probably lies in the stability field of jadeite + quartz. Even though it could be stable in the albite field, its temperature should be significantly lower than the other areas discussed in this paper. (4) Sifnos. Okrusch et al. (1978) described the following assemblages from Sifnos, Greece: Jadeite-glaucophane-garnet-epidote Omphacite-glaucophane-garnet-epidote

Acknowledgements

with excess of quartz and phengite. These assemblages are stable on the high-pressure side of the omphacite-paragonite decomposition reaction, e.g.: Gln + J d + L w s ~ O m p + Pg

tual relation of the stability fields of such high-pressure eclogites suites as Motalat]ella, the Tauern Window and Adula Nappe, Sesia-Lanzo zone and Corsica. The Motalafjella eclogite suite represents an intermediate-temperature regime and the lowest pressure among them.

(6)

which emanates from the invariant point [25] [ Ep,Cld ], in Fig. 16. The Sifnos eclogite may be one of the lowest-temperature eclogites so far known. (5) Jiaoliao Massif Donghai, east China. Enami et al. (1987) reported an eclogite mineralogy from the Precambrian gneiss and ultramafic complex of the Jiaoliao Massif, east China. This eclogite contains a unique mineral assemblage of garnet-clinopyroxene-kyanite-zoisite-corundum without albite or quartz and gives a P-T condition of 700-750°C, 1 1-25 kbar for the primary eclogite stage. Although the chemical system of the Jiaoliao Massif eclogite is slightly different from that of this paper, the former confirms one of the essential characteristics of Fig. 16, namely, that kyanite eclogite is stable only at a higher lemperature than that of the Motalal~ ella eclogite.

12. Conclusions The Motalafjella eclogite suite in Spitsbergen is marked by the assemblages omphacite-paragonite and epidote-glaucophane in eclogites, Fe-Mg chloritoid in metapelites, and jadeitic pyroxene-quartz in siliceous schists. After eliminating prograde relics (including lawsonite and winchite) and retrograde products, the formative conditions of the eclogite suite are estimated to be 575-645°C and 17-24 kbar. Schreinemakers' nets for the model system containing chloritoid, kyanite and lawsonite, with excess of almandine and others, have been constructed, and a plausible net which is consistent with the paragenesis reported from six high-pressure eclogite suites and synthetic phase relations, was chosen. The net is useful for examining the mu-

This is the fourth report from the cooperation project between the Norsk Polarinstitutt and Kyoto University, Japan. We would like to express our thanks to Professor E.H. Brown of Western Washington University for his critical discussions and reading the manuscript. Emeritus Professor D.A. Brown of Australian National University is thanked for his help in preparing this manuscript. We also express our thanks to Professor W.L. Griffin and A. Mottana for their critical comments. We gratefully acknowledge financial support for the field work of the Japanese party in 1983 from the Polar Research Committee of Kyoikusha Co. Ltd., Tokyo.

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