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J. metamorphic Geol., 1996, 14, 549563

Pressuretemperature conditions and retrograde paths of eclogites,garnetglaucophane rocks and schists from South Sulawesi, Indonesia

K . M I YA Z A K I ,1 I . Z U L K AR N A I N ,2 J . S O P A H E L U WA K A N 2 A N D K . WA K I TA 1

1Geological Survey of Japan, 11-3 Higashi, Tsukuba, Ibaraki 305, Japan2Research and Development Centre for Geotechnology, Jl. Cisitu, 21/154D, Bandung, 40135 Indonesia

A B S T R A C T High-pressure metamorphic rocks exposed in the Bantimala area, c. 40 km north-east of Ujung Pandang,were formed as a Cretaceous subduction complex with fault-bounded slices of melange, chert, basalt,turbidite, shallow-marine sedimentary rocks and ultrabasic rocks. Eclogites, garnetglaucophane rocksand schists of the Bantimala complex have estimated peak temperatures of T=580630 C at 18 kbarand T=590640 C at 24 kbar, using the garnetclinopyroxene geothermometer. The garnetomphacite

phengite equilibrium is used to estimate pressures. The distribution coefficient KD1=[(Xpyr)3

(Xgrs)6/

( Xdi

)6]/[(Al/Mg)M2,wm

(Al/Si)T2,wm

]3 among omphacite, garnet and phengite is a good index for metamor-phic pressures. The K

D1values of the Bantimala eclogites were compared with those of eclogites with

reliable PTestimates. This comparison suggests that peak pressures of the Bantimala eclogites were P=1824 kbar at T=580640 C. These results are consistent with the PTrange calculated using garnetrutileepidotequartz and lawsoniteomphaciteglaucophaneepidote equilibria.

The estimated PTconditions indicate that these metamorphic rocks were subducted to c. 6585 kmdepth, and that the overall geothermal gradient was c. 8 C km1. This low geothermal gradient can beexplained with a high subduction rate of a cold oceanic plate. The retrograde paths of eclogite andgarnetglaucophane rocks suggest that these units were refrigerated during exhumation, consistent withdecoupling of the high-P rocks and ascent due to buoyancy force during continued underflow of the coldoceanic plate.

Key words: eclogite; high-pressure metamorphism; Indonesia; PTconditions; retrograde metamorphism.

plate subducted toward the West KalimantanI N T R O D U C T I O N

Continent.A Cretaceous subduction complex, the BantimalaComplex, is exposed in the Bantimala area, east of

G E O L O G I C A L S E T T I N GPankajene, South Sulawesi (Figs 1 and 2). It is madeup of fault-bounded slices of Cretaceous accretionary Cretaceous subduction complexes of Indonesia are

distributed in West and Central Java, Southsediments, ultrabasic rocks and Cretaceous high-pressure metamorphic rocks (Sukamto, 1986). Wakita Kalimantan, and South Sulawesi (Fig. 1). Before the

opening of the Makassar Strait, the Bantimalaet al. (1994, 1996) presented the following scenario ofthe evolution of the Bantimala Complex. The high- Complex constituted a single subduction complex

with the subduction complexes in Java and Southpressure metamorphic rocks were formed in the LateJurassic or earliest Cretaceous by subduction of an Kalimantan (Hamilton, 1979). Cretaceous plutons

occur in West Kalimantan and the basement of theoceanic plate toward the West Kalimantan Continent.Subduction ceased in the Albian, and the high- western Java Sea (Hamilton, 1979). The eastern and

southern arms of the Sulawesi subduction complexpressure metamorphic rocks were exhumed beforeand during the deposition of middle Cretaceous are underlain by a Tertiary complex consisting mainly

of high-pressure metamorphic rocks and ophiolitesradiolarian chert.This paper describes the occurrence, mineral assem- (Parkinson, 1991). These rocks are structurally

overlain by the BanggaiSula continental fragmentsblages, mineral chemistry, peak pressure and tempera-ture conditions, and retrograde metamorphism of (Hartono, 1990), as a result of eastward-directed

subduction.eclogites, garnetglaucophane rocks and schists of theBantimala Complex. These results contribute to anunderstanding of the evolution of the palaeo-oceanic

Correspondence: Kazuhiro Miyazaki (email: g8707@gsj.go.jp)

549

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550 K . M I YA Z A K I E T A L .

Fig. 1. Tectonic map of the Indonesian region (modified from Wakita et al., 1994).

The lawsonite-bearing and hematite-bearing glauco-O U T L I N E O F T H E G E O L O G Y O F T H E

phane schists are repectively interlayered with lawson-B A N T I M A L A C O M P L E X

ite-bearing chloritemica schists or albiteactinolitechlorite schists. The garnetglaucophane schists areThe Bantimala area is located about 40 km north-east

of Ujung Pandang, South Sulawesi (Fig. 2). The interlayered with garnetchloritoidglaucophanequartz schists or garnetglaucophanequartz schistsdetailed geology of this area was investigated by

Sukamto (1975, 1978, 1982, 1986). The Bantimala (Fig. 3). All three types of glaucophane schists are infault contact with each other. Eclogite and garnetComplex is about 10 km wide in the Bantimala area;

it is surrounded by Tertiary and Quaternary sedimen- glaucophane rock occur as tectonic blocks withinsheared serpentinite (Figs 4 and 5). KAr ages oftary and volcanic rocks, and unconformably covered

by Late Cretaceous to Palaeocene sedimentary rocks. phengite from these rocks (Wakita et al., 1994, 1996)

are as follows: garnetglaucophane rocks (1327,The complex is intruded by Palaeogene diorite.The Bantimala Complex is composed of tectonic 1136 Ma); mica-rich part intercalated with garnet

glaucophane rock (1246 Ma); and micaquartzslices of high-pressure metamorphic rocks, sedimentaryrocks and ultrabasic rocks (Fig. 2). The boundary schists intercalated with hematite-bearing glaucophane

schists (1146, 1156 Ma).faults were active before the Palaeocene, and some ofthem were partly reactivated in Cenozoic time. The The sedimentary rocks are identified as melange,

turbidite and shallow-marine clastic rocks. Melangesmetamorphic rocks in the Bantimala Complex consistof glaucophane schist, albiteactinolitechlorite include clasts and blocks of sandstone, siliceous shale,

chert, basalt and schist in a sheared shale matrix. Aschist, chloritemica schist, garnetglaucophanequartz schist, garnetchloritoidglaucophanequartz radiolarian assemblage from chert is assigned a middle

Cretaceous (late Albianearly Cenomanian) age, andschist, serpentinite, garnetglaucophane rock and eclo-gite. Predominant lithologies are glaucophane schists the chert unconformably overlies the high-pressure

metamorphic rocks (Wakita et al., 1996).that are divided into three types: very fine-grainedlawsonite-bearing glaucophane schist; hematite-bearing The ultrabasic rocks are mostly serpentinized peri-

dotite, locally including chromite lenses.glaucophane schist; and garnetglaucophane schist.

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P- T C O N D I T I O NS , S U L A WE S I 551

Fig. 2. Simplified geological map of theBantimala Complex, South Sulawesi(modified from Sukamto, 1986).

epidote, phengite, rutile, quartz and, in very rare cases,chloritoid. The matrix contains subordinate amountsof epidote, phengite, rutile and quartz. Idioblasticglaucophane occurs rarely in the matrix. Magnesio-hornblende occurs as a matrix mineral in one sample(P-04).

The garnetglaucophane rocks are characterized bymodally abundant glaucophane. Garnet porphyrob-lasts (up to 5 mm) are set in a matrix of glaucophane(0.20.75 mm), containing subordinate amounts ofepidote, omphacite, phengite, rutile and quartz. In veryrare cases, the matrix contains no omphacite.

The mineral paragenesis of the eclogites and garnet

glaucophane rocks are as follows (abbreviations afterKretz, 1983): eclogites, Omp+Grt+Ep+Phengite+

Fig. 3. Outcrop of garnetglaucophane schist (dark coloured) Qtz+Rt; Omp+Grt+Gln+Ep+Phengite+Qtz+Rt;intercalated with garnetchloritoidglaucophanequartz schist Omp+Grt+Gln+Hbl+Ep+Phengite+Rt; and( light coloured). This outcrop occurs along the Cempaga

Omp+Grt+Ep+Phengite+Rt; and garnetglauco-River.phane rocks, Gln+Grt+Omp+Ep+Phengite+Qtz+Rt; Gln+Grt+Ep+Phengite+Qtz+Rt.

P ET R OG R AP H YGarnetglaucophane schists and their associated rocks

Eclogites and garnetglaucophane rocksGarnetglaucophane schists show distinct schistosityand compositional banding of garnet-rich and garnet-The eclogites are made up essentially of garnet

porphyroblasts (up to 1 cm) set in a matrix of a fine- poor layers. The euhedral garnet ranges from 0.1to 1 mm. The matrix consists of glaucophanegrained omphacite (0.010.05 mm). Garnet porphyro-

blasts have inclusions of omphacite, glaucophane, (0.10.75 mm), epidote, phengite and quartz with

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552 K . M I YA Z A K I E T A L .

Fig. 4. Geological map along the CempagaRiver. This figure shows occurrence ofeclogites, garnetglaucophane rocks andschists.

R E T R O G R A D E M I N E R A L P A R A G E N E S I S

Some of the eclogites, garnetglaucophane rocks andschists underwent variable degrees of retrograde meta-morphism. In general, the garnetglaucophane schistssuffered more extensive retrograde metamorphism thanthe other rock types.

Chlorite and lawsonite are found in some eclogitesand garnetglaucophane rocks. In sample P-04 (eclo-gite), these phases occur in particular domains showingwell-developed chlorite aggregates and coarse-grained lawsonite patches (1 2 mm). Outside thesedomains the mineral assemblage is garnet+epidote+omphacite+hornblende+glaucophane+rutile(Fig. 6a). Lawsonite has inclusions of omphacite,garnet, epidote, glaucophane, hornblende, titanite andFig. 5. Outcrop along the Pateteyang River of garnet

glaucophane rock associated with sheared serpentinite. rutile rimmed by titanite. This shows that the retro-grade domains had the same mineral assemblagebefore lawsonite crystallized. In these retrograde por-

tions (Fig. 6b), fractures in garnet are filled by chloritesmall amounts of rutile. Omphacite is rare in theseand lawsonite, and garnet rims are completely replacedrocks. Garnetchloritoidglaucophanequartz schistsby chlorite. Prismatic glaucophane (0.70.2 mm) isare intercalated among the garnetglaucophane schists,rimmed by strong blue- and lavender-coloured crossite.and show compositional banding of mica-rich andOmphacite ( 1.00.5 mm) is rimmed by pale-green-mica-poor layers. Euhedral garnets are set in a matrixcoloured chloromelanite. Titanite occurs instead ofof quartz, glaucophane (0.10.75 mm), epidote andrutile. Therefore, the retrograde mineral assemblage isphengite with a small amount of chloritoid. In veryinterpreted to be Chloromelanite+Crossite +Lws+rare cases, chloritoid is absent (garnetglaucophaneChl+Ttn.quartz schist). The mineral assemblages of these rocks

A negligible amount of albite occurs with chloriteare as follows: garnetglaucophane schists, Gln+Grt+and partly replaces chloromelanite and crossite rims.Ep+Phengite+Qtz+Rt; and Gln+Grt+Omp+Therefore, it is interpreted as a later retrograde mineralEp+Phengite+Qtz+Rt; garnetchloritoidglauco-crystallized after the chloromelanitelawsonite assem-phanequartz schist, Grt+Gln+Cld+Ep+Phengite+blage. Fine-grained phengite is also found partly alongQtz; and garnetglaucophanequartz schist, Grt+interfaces between lawsonite and chloromelanite.Gln+Ep+Phengite+Qtz.

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P- T C O N D I T I O NS , S U L A WE S I 553

Fig. 7. Photomicrograph of garnetglaucophane schist (sampleFig. 6. Photomicrograph of eclogite (sample P-04). (a)

Mg-51). (a) Albite porphyroblast in garnetglaucophane schist.Omphacitegarnetepidotehornblende in major part. (b)Garnet rim is replaced by chlorite and epidote within albite

LawsonitegarnetglaucophaneNa-pyroxene in retrogradeporphyroblast. (b) Actinolite and chlorite within albiteportion.porphyroblast.

Albite porphyroblasts (up to 1.0 mm) in sampleM I N E R A L C H E M I S T R YMg-51 (garnetglaucophane schist) occur in particular

layers in which chlorite is more abundant than in the Mineral analyses were carried out using either a JEOLother parts (Fig. 7). The major part of this sample 8800 or a JEOL 733 EPMA at the Geological Surveyconsists of glaucophane, epidote, garnet, quartz, phen- of Japan. Accelerating voltage, specimen current and

gite and rutile. Omphacite, garnet, epidote, zoned beam diameter were kept at 15 kV, 12 nA on Faradayamphibole, actinolite, chlorite, titanite and hematite cup and 2 mm, respectively. The mineral assemblagesare enclosed in albite porphyroblasts. The omphacite of analysed samples are shown in Table 1 and mineralhas irregular interfaces against albite, and the garnet chemistries are listed in Table 2. The Fe

2O

3content of

rim is replaced by chlorite. The amphibole is zoned sodic pyroxene was estimated on the assumption offrom a glaucophane core, through a crossite inner Al+Fe3+=Na. The Fe3+/Fe2+ value of amphibolemantle and winchite outer mantle to an actinolite was calculated as total cations=13 exclusive of K, Narim. Hematite occurs only in albite porphyroblasts. and Ca (O=23).Therefore, the following retrograde mineral assem-blages are inferred: Ab+Chl+Crossite+Ep+Ttn+

ClinopyroxeneHem+Qtz; Ab+Chl+Winchite+Ep+Ttn+Hem+

Assuming the pyroxene components are jadeiteQtz; and Ab+Chl+Act+Ep+Ttn+Hem+Qtz.( jd), acmite (acm), diopside (di) and hedenbergite(hd ), the end-member mole fractions are calculatedas X

jd=Al/(Na+Ca), X

acm=Fe3+/(Na+Ca),

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554 K . M I YA Z A K I E T A L .

Table 1. Mineral assemblages of analysedsamples.S am pl e n o. T yp e G rt P yr oxe ne G ln A ct H bl E p L ws Cl d A b C hl Ph en gi te Q tz O the r

Mg-47a Ecl. + + + + i + + Rt

Mg2 18a Ecl. + + + + Rt

P-04 Ecl. + + i,r + + r r r + Rt, Ttn*

Mg-49a Grt-G ln + + + + + + Rt

Mg-51 Grt-Gln + i + r + r r + + Rt, Ttn*, Hem*

Mg-50 Grt-Qtz + + + + + +

+: prograde mineral, i: inclusion in garnet or albite, r: retrograde mineral, *: retrograde mineral. Ecl. : eclogites, Grt-Gln:

garnetglaucophane schists, Grt-Qtz: garnetchloritoidglaucophanequartz schist.

Table 2. Representative analyses of pyroxene (O=6) .

Eclogites Garnet-glaucophane schists

Rock no. Mg-47a Mg-47a Mg-47a Mg2-18a P-04 P-04 P-04 Mg49a Mg-51

N.B. core rim f-matrix inc. in Grt major part r-portion

SiO2

55.21 55.54 55.64 55.68 53.77 55.31 53.93 55.06 55.48

TiO2

0.00 0.02 0.01 0.05 0.05 0.08 0.04 0.04 0.1

Al2O3 7.02 10.02 10.01 10.63 8.53 7.82 5.08 8.21 8.44Cr

2O

30.00 0.04 0.00 0.04 0.01 0.05 0.01 0.07 0.05

FeO* 5.85 5.15 5.3 6.37 11.27 7.17 12.44 7.4 9.08

MnO 0.01 0.05 0.01 0.02 0.14 0.19 0.23 0.07 0.01

MgO 10.25 8.42 8.29 7.16 5.36 8.95 7.13 8.36 7.5

CaO 15.7 13.5 13.22 11.85 10.42 14.18 12.86 13.96 12.68

Na2

O 5.5 6.92 6.88 7.67 8.63 6.79 7.39 6.29 7.39

K2

O 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.02 0.00

Total 99.53 99.66 99.35 99.46 98.19 100.56 99.10 99.46 100.73

Si 1.992 1.987 1.995 1.994 1.978 1.977 1.983 1.993 1.984

Al 0.298 0.422 0.423 0.448 0.370 0.329 0.220 0.350 0.356

Ti 0.000 0.001 0.000 0.001 0.001 0.002 0.001 0.001 0.003

Cr 0.000 0.001 0.000 0.001 0.000 0.001 0.000 0.002 0.001

Fe3+ 0.086 0.057 0.055 0.084 0.246 0.141 0.306 0.091 0.156

Fe2+ 0.090 0.097 0.104 0.107 0.101 0.073 0.076 0.133 0.115

Mn 0.000 0.001 0.000 0.001 0.004 0.006 0.007 0.002 0.000

Mg 0.551 0.449 0.443 0.382 0.294 0.476 0.391 0.450 0.400

Ca 0.606 0.517 0.508 0.454 0.411 0.543 0.506 0.541 0.486

Na 0.384 0.479 0.478 0.532 0.615 0.471 0.526 0.441 0.512K 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000

Total 4.008 4.012 4.005 4.004 4.020 4.020 4.016 4.005 4.013

jd(%) 30.11 42.37 42.92 45.45 36.02 32.5 21.31 35.65 35.64

acm (%) 8.69 5.74 5.56 8.48 23.96 13.93 29.65 9.25 15.68

di(%) 52.59 42.68 41.72 35.99 29.8 46.44 41.03 42.53 37.79

hd (%) 8.61 9.22 9.8 10.08 10.22 7.14 8.01 12.58 10.89

* Total Fe as FeO. Calculated values. f-matrix: fine-grained matrix; r-portion: retrograde portion; inc. in Grt: inclusion in garnet.

Xdi

=[Ca/(Na+Ca)][Mg/(Mg+Fe2+)] and Xhd

= a very distinct chemical zonation (normal-type) withFe- and Mn-enriched cores and Mg-enriched rims. The[Ca/(Na+Ca)][Fe2+/(Mg+Fe2+)]. Most analysed

pyroxene in the eclogites and garnetglaucophane zonation in Ca is generally weak, although garnet insample P-04 (eclogite) is depleted in the grossularschists falls in the omphacite region of the

jdacm(di+hd) diagram (Fig. 8 ). The jadeite content component at the rim. The composition of garnet rims

in the eclogites and garnetglaucophane schists isat rims of omphacite ranges from Xjd=0.35 to 0.45,and usually the rim is more jadeite-rich than the core. similar (Xprp

=0.200.30 and Xgrs

=0.200.25). Thegrossular content of garnet in garnetchloritoidRetrograde zonation is observed in the retrograde

domains in sample P-04 (eclogite). Omphacite is glaucophanequartz schists is slightly lower (Xgrs

=

0.150.20).rimmed by more acmite-rich sodic pyroxene (chlorome-lanite: X

jd=0.21 and X

acm=c. 0.30).

AmphiboleGarnet

Glaucophane in eclogites, garnetglaucophane schistsand garnetchloritoidglaucophanequartz schists isThe mole fractions of garnet end-members are

calculated as follows: Xpyr

=Mg/(Fe+Mn+Mg+ usually homogenous over the scale of a thin-section,but, in some cases, it is distinctly zoned (sample P-04)Ca), X

alm=Fe2+/(Fe2++Mn+Mg+Ca), X

sps=

Mn/(Fe2++Mn+Mg+Ca) and Xgrs

=Ca/(Fe2++ or rimmed by winchite and actinolite (sample Mg-51).Winchite occurs also in the core of zoned sodicMn+Mg+Ca). Garnet compositions are shown in

Fig. 9. Garnet in the garnetglaucophane schists shows amphiboles in the garnetglaucophane schist (sample

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Table 2. (continued) Representative analyses of garnet (O=12).

Eclogites Garnet-glaucophane schists Grt-Qtz

Rock no. Mg-47a Mg-47a Mg2-18a P-04 P-04 P-04 Mg-49a Mg-49a Mg-50 Mg-50

N.B. core rim core medium rim core rim core rim

SiO2

37.84 38.176 38.06 37.72 37.33 37.85 37.92 38.78 38.76 39.36

TiO2

0.07 0.094 0.07 0.13 0.05 0.02 0.22 0.03 0.11 0.14

Al2

O3

20.64 21.396 21.64 21.07 21.57 22.48 20.60 21.29 21.33 21.55

Cr2O

30.00 0.013 0.00 0.05 0.10 0.01 0.00 0.03 0.00 0.00

FeO* 28.33 25.104 25.41 28.01 26.39 23.99 26.03 26.83 25.60 28.7

MnO 1.19 0.687 0.98 1.66 1.22 0.61 3.97 0.5 6.44 0.69

MgO 2.58 4.92 4.65 2.90 4.26 7.22 1.65 4.15 3.45 5.19

CaO 9.07 8.807 8.41 8.84 8.27 7.38 9.36 8.83 6.34 6.23

Na2

O 0.01 0.02 0.02 0.05 0.02 0.10 0.02 0.00 0.08 0.00

K2

O 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0 0.01 0.00

Total 99.73 99.217 99.23 100.45 99.21 99.67 99.77 100.45 102.12 101.86

Si 3.017 3.001 2.995 2.986 2.96 2.934 3.029 3.026 3.015 3.028

Al 1.939 1.982 2.006 1.965 2.016 2.053 1.939 1.958 1.955 1.953

Ti 0.004 0.006 0.004 0.008 0.003 0.001 0.013 0.002 0.006 0.008

Cr 0.000 0.001 0.000 0.001 0.003 0.00 0.000 0.001 0.000 0.000

Fe2+ 1.888 1.650 1.672 1.853 1.750 1.555 1.739 1.751 1.665 1.846

Mn 0.081 0.046 0.065 0.111 0.082 0.04 0.269 0.033 0.424 0.045Mg 0.306 0.576 0.545 0.342 0.503 0.834 0.196 0.483 0.400 0.594

Ca 0.774 0.741 0.709 0.749 0.703 0.612 0.801 0.738 0.528 0.514

Na 0.001 0.003 0.002 0.008 0.003 0.015 0.003 0.000 0.012 0.000

K 0.000 0.000 0.001 0.001 0.000 0.000 0.000 0.000 0.001 0.000

Total 8.010 8.004 7.999 8.026 8.024 8.045 7.990 7.991 8.007 7.987

pyr (%) 10.04 19.12 18.22 11.2 16.57 27.41 6.54 16.07 13.25 19.82

alm (%) 61.93 54.75 55.91 60.64 57.59 51.14 57.87 58.27 55.19 61.56

sps (%) 2.64 1.52 2.18 3.65 2.71 1.31 8.94 1.09 14.06 1.49

grs (%) 25.39 24.61 23.7 24.51 23.13 20.14 26.65 24.57 17.5 17.13

* Total Fe as FeO. GrtQtz: garnetchloritoidglaucophanequartz schist.

Table 2. (continued) Representative analyses of amphibole (O=23).

Eclogites Garnet-glaucophane schists

Grt-QtzRock no. P-04 P-04 P-04 P-04 Mg-49 Mg-49 Mg-51 Mg-51 Mg-51 Mg-51

N.B. Mg-47a inc. in Grt major part r-portion, core r-portion, rim core rim core mantle-1 mantle-2 rim Mg-50

SiO2

54.85 56.44 49.84 56.83 56.65 53.92 58.37 57.25 55.33 57.41 53.69 58.77

TiO2

0.01 0.01 0.18 0.07 0.00 0.00 0.08 0.01 0.08 0.02 0.01 0.02

Al2

O3

8.51 9.95 8.39 10.15 7.49 5.84 9.97 10.33 6.68 5.92 1.20 9.43

Cr2O

30.04 0.09 0.09 0.04 0.01 0.07 0.00 0.01 0.00 0.00 0.00 0.00

FeO* 19.86 13.00 8.16 8.55 14.65 10.22 8.49 11.20 18.27 12.96 13.74 9.73

MnO 0.04 0.09 0.24 0.24 0.23 0.10 0.01 0.00 0.17 0.36 0.46 0.00

MgO 5.61 9.25 15.29 12.23 9.98 14.61 12.70 10.57 8.80 10.79 15.02 12.35

CaO 0.08 0.72 10.08 1.63 0.79 7.08 1.63 0.69 1.38 7.5 11.08 1.29

Na2

O 6.75 7.30 3.07 6.60 7.32 3.61 6.30 7.36 6.79 3.52 1.12 6.88

K2

O 0.01 0.02 0.32 0.02 0.02 0.10 0.03 0.02 0.00 0.07 0.05 0.02

Total 95.76 96.85 95.65 96.37 97.13 95.55 97.58 97.43 96.36 98.49 96.34 98.69

Si 7.931 7.892 7.221 7.822 7.937 7.682 7.888 7.873 7.836 8.050 7.811 7.923

Al (IV ) 0.069 0.108 0.779 0.178 0.063 0.318 0.112 0.127 0.164 0.000 0.189 0.077

Al ( VI) 1.380 1.531 0.653 1.469 1.174 0.662 1.475 1.547 0.952 0.978 0.016 1.421

Ti 0.002 0.001 0.019 0.007 0.000 0.000 0.008 0.001 0.009 0.002 0.001 0.002

Cr 0.004 0.010 0.010 0.004 0.001 0.008 0.000 0.001 0.000 0.000 0.000 0.000

Fe3+ 0.763 0.367 0.029 0.445 0.658 0.474 0.495 0.411 0.911 0.000 0.394 0.479

Fe2+ 1.638 1.153 0.960 0.539 1.057 0.743 0.464 0.877 1.252 1.519 1.277 0.618

Mn 0.005 0.010 0.029 0.028 0.027 0.012 0.002 0.000 0.021 0.043 0.056 0.000

Mg 1.209 1.928 3.300 2.508 2.082 3.101 2.556 2.164 1.856 2.253 3.255 2.481

Ca 0.012 0.108 1.564 0.241 0.119 1.081 0.235 0.101 0.209 1.127 1.726 0.187

Na 1.892 1.977 0.862 1.761 1.987 0.996 1.650 1.961 1.864 0.957 0.316 1.798

K 0.001 0.003 0.058 0.004 0.004 0.018 0.005 0.004 0.001 0.012 0.009 0.003

Total 14.904 15.083 15.480 15.004 15.109 15.090 14.890 15.065 15.051 14.987 15.045 15.542

Na in M4 1.99 1.89 0.44 1.76 1.88 0.92 1.76 1.9 1.79 0.87 0.27 1.81

YFe3+

35.61 19.32 4.23 23.24 35.93 41.72 25.14 20.97 48.9 0.00 96.01 25.19

(%)

XFe2+

57.53 37.42 22.54 17.7 33.67 19.33 15.37 28.83 40.29 40.27 28.18 19.94

(%)

agln

0.03 0.14 0.02 0.25 0.11 0.04 0.26 0.2 0.04 0.04 0.00 0.24

* Total Fe as FeO. Calculated values. f-matrix: fine-grained matrix; r-portion: retrograde portion; YFe3+

: Fe3+/(Fe3++Al(VI)); inc. in Grt: inclusion in garnet; agln

: activity of glaucophane

(see Table 3); XFe2+

: Fe2+/(Fe2++Mg); GrtQtz: garnetchloritoidglaucophanequartz schist.

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556 K . M I YA Z A K I E T A L .

Table 2. (continued) Representative analyses of epidote Table 2. (continued) Representativeanalyses of lawsonite (O=8) .(O=12.5).

Garnet-glaucophane Rock no. N.B. Eclogites P-04 r-portion

Eclogites schists

Grt-Qtz SiO2

38.17

Rock no. Mg-51 TiO2

0.23

Al2

O3

31.66N.B. Mg-47a Mg2-18a P-04 Mg-49a core Mg-50

Cr2O

30.10

Fe2O

3* 1.47SiO

238.40 37.81 38.15 38.95 37.65 39.05

TiO2

0.10 0.09 0.08 0.02 0.01 0.05 MnO 0.00

MgO 0.01Al2

O3

25.44 26.71 25.51 24.71 24.66 22.00

Cr2O

30.12 0.00 0.00 0.01 0.04 0.00 CaO 17.34

Na2

O 0.07Fe2O

3* 10.83 8.81 11.22 12.04 11.06 13.52

MnO 0.07 0.02 0.32 0.06 0.03 0.42 K2

O 0.01

Total 89.05MgO 0.14 0.08 0.05 0.05 0.04 0.02

CaO 22.89 23.31 23.31 23.16 23.64 22.68Si 1.995

Na2

O 0.01 0.02 0.04 0.00 0.00 0.02Al 1.950

K2

O 0.01 0.00 0.00 0.00 0.02 0.00Ti 0.009

Total 98.00 96.84 98.68 99.00 97.14 97.76Cr 0.002

Fe3+ 0.058Si 3.018 2.991 2.989 3.040 3.000 3.109

Al 2.356 2.490 2.356 2.273 2.316 2.064 Mn 0.000

Mg 0.001Ti 0.006 0.005 0.005 0.001 0.000 0.003

Cr 0.008 0.000 0.000 0.001 0.003 0.000 Ca 0.970Na 0.007Fe3+ 0.64 0.524 0.661 0.707 0.663 0.810

Mn 0.005 0.001 0.021 0.004 0.002 0.028 K 0.000

Total 4.992Mg 0.016 0.010 0.006 0.006 0.004 0.002

Ca 1.926 1.974 1.956 1.936 2.018 1.934

Na 0.001 0.003 0.006 0.000 0.000 0.004 * Total iron as Fe3+. r-portion: retrograde portion.

K 0.001 0.000 0.000 0.000 0.002 0.000

Total 7.976 7.999 8.000 7.968 8.009 7.953

YFe3+

(%) 21.37 17.39 21.92 23.72 22.25 28.18Table 2. (continued) Representative analyses of phengite

aczo

0.36 0.48 0.35 0.28 0.32 0.07(O=22).

* Total iron as Fe3+. aczo

: activity of clinozoisite (see Table 3); YFe3+

: Fe3+/(Fe3++Al(VI));Garnet-glaucophaneGrtQtz: garnetchloritoidglaucophanequartz schist.

Eclogites schists Grt-Qtz

Rock no. Mg-47a Mg218a P-04 Mg-49a Mg-51 Mg-50Table 2. (continued) Representativeanalyses of chloritoid (O=12).

SiO2

53.51 50.87 50.47 53.38 51.60 49.77

TiO2 0.12 0.20 0.15 0.04 0.10 0.11Eclogite Grt-Qtz Al2O

324.05 26.67 25.81 26.66 25.96 25.52

Cr2O

30.00 0.02 0.09 0.00 0.00 0.00Rock no. N.B. Mg-47a inc. in Grt Mg-50

FeO* 2.90 2.25 3.02 2.67 3.62 3.85

MnO 0.00 0.00 0.00 0.02 0.00 0.11SiO2

25.34 26.01MgO 4.38 4.07 4.09 3.83 4.16 2.84TiO

20.00 0.00

CaO 0.05 0.00 0.03 0.00 0.00 0.00Al2O

340.83 40.65

Na2O 0.16 0.36 0.22 0.33 0.24 0.87Cr

2O

30.00 0.00

K2O 10.90 10.85 10.61 9.64 10.44 9.85FeO* 19.86 20.17

Total 96.08 95.27 94.48 96.59 96.13 92.92MnO 0.00 0.19

MgO 6.38 5.63Si 7.099 6.807 6.832 6.974 6.865 6.866

CaO 0.00 0.01Al 3.760 4.205 4.117 4.105 4.070 4.148

Na2O 0.00 0.02

Ti 0.011 0.020 0.015 0.004 0.010 0.012K

2O 0.00 0.00

Cr 0.000 0.001 0.005 0.000 0.000 0.000Total 92.40 92.69

Fe2+ 0.322 0.252 0.342 0.292 0.403 0.444

Mn 0.000 0.000 0.000 0.002 0.000 0.013Si 2.042 2.091Mg 0.866 0.811 0.824 0.746 0.824 0.584Al 3.876 3.849Ca 0.008 0.000 0.004 0.000 0.000 0.000Ti 0.000 0.000Na 0.042 0.094 0.057 0.085 0.061 0.232

Cr 0.000 0.000 K 1.844 1.851 1.832 1.606 1.771 1.733Fe2+ 1.337 1.355Total 13.952 14.040 14.029 13.814 14.005 14.031Mn 0.000 0.013

Mg 0.765 0.674

* Total Fe as FeO. GrtQtz: garnetchloritoidglaucophanequartz schist.Ca 0.000 0.001

Na 0.000 0.004

K 0.000 0.000

Total 8.020 7.987Other minerals

XMg

(%) 36.40 33.20

Epidote in eclogites, garnetglaucophane schists and* Total Fe as FeO. X

Mg: Mg/(Mg+Fe2+); inc. in Grt:

siliceous schist is only weakly zoned, with variation ininclusion in garnet; GrtQtz: garnetchloritoidglauco-phanequartz schist. Y

Fe3+[=Fe3+/(Fe3++Al)] being within0.01. The

YFe3+

of epidote in the garnetglaucophane schist(sample Mg-51) varies from 0.22 (core) to 0.28 (rim).Mg-49a). Magnesiohornblende occurs as inclusions

within garnet and as a matrix mineral in the eclogite Chloritoid occurs in the garnetchloritoidglauco-phanequartz schist and, rarely, chloritoid is enclosed(sample P-04). The magnesiohornblende is rimmed by

glaucophane in the retrograde portion of sample P04. in garnet porphyroblasts of the eclogites. The XMg

of

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P- T C O N D I T I O NS , S U L A WE S I 557

Table 2. (continued) RepresentativeP E A K PT E S T I M A T E S F O R E C L O G I T E S A N Danalyses of chlorite (O=28).G A R N E T GL A U C O P H A N E S C H I S T S

Eclogite Grt-Gln

In this section, we estimate metamorphic temperaturesRock no. N.B. P-04 r-portion Mg-51 and pressures of eclogites and garnetglaucophaneSiO

227.92 25.03 schists. K

D=(Fe2+/Mg)

garnet/(Fe2+/Mg)

clinopyroxenebe-

TiO2

0.01 0.00 tween garnet and clinopyroxene rims ranges from 11Al

2O

319.02 19.49

to 13. These correspond to 580630 C at 18 kbar andCr2

O3

0.06 0.00

FeO* 17.66 26.88 590640 C at 20 kbar using the calibration of PowellMnO 0.24 0.70 (1985). Jadeite content in omphacite coexisting withMgO 21.20 13.01

quartz, but not with albite, ranges from 35 to 45%.CaO 0.04 0.00Na

2O 0.07 0.00 We calculated a minimum pressure of equilibration

K2O 0.02 0.03

using the one-site model of Banno (1986) for theTotal 86.22 85.14

activity of the jadeite component in C2/c (disordered)Si 5.740 5.520

pyroxene and excess enthalpy of 0.9 kcal (Table 3); theAl 4.609 5.064Ti 0.001 0.000 excess enthalpy was estimated by Banno (1986),Cr 0.005 0.000

assuming symmetric simple solution on the single-siteFe2+ 3.034 4.956

Mn 0.041 0.131 model with experimental results of Holland (1983).Mg 6.493 4.273 The results show that metamorphic pressure isCa 0.009 0.000

>1213 kbar at T=600 C.Na 0.026 0.000K 0.004 0.008

Total 19.961 19.952

Peak PTestimates using garnetomphacitephengite* Total Fe as FeO. r-portion: retrograde portion; GrtGln: equilibriumgarnetglaucophane schist.

Okay (1993 ) showed that the garnetclinopyroxenephengite assemblage is a good geobarometer as it is

Table 2. (continued) Representative not H2O-dependent, and isopleths of Si in phengite

analyses of albite (O=8). have low dP/dT. This geobarometer is based onfollowing reaction:E cl og it e G rt G ln sc hi st

pyrope+2 grossular+3 celadonite=6 diopsideRock no. N.B . P -04 r-porti on Mg-51

SiO2

65.92 68.40 +3 muscovite , (1)TiO

20.03 0.00

Al2O

320.06 19.81 Mg

3Al

2Si

3O

12+2Ca

3Al

2Si

3O

12Cr2

O3

0.06 0.00

FeO* 0.31 0.06 +3K(MgAl)Si2Si

2O

10(OH)

2=6CaMgSi

2O

6MnO 0.00 0.00MgO 0.28 0.03 +3KAl

2(SiAl)Si

2O

10(OH)

2. (2)

CaO 0.23 0.03

Na2O 11.62 12.11 The DG of this reaction expressed in terms of the

K2O 0.03 0.00

chemical potential of each component (mi), is as follows:

Total 98.53 100.44

Si 2.936 2.979 DG=mpyr

+2mgrs

+3mcel

(6mdi

+3mms

)=0 , (3)Al 1.053 1.017

Ti 0.001 0.000 m0pyr

+2m0grs

+3m0cel

(6m0di

+3m0ms

)Cr 0.001 0.000

Fe2+ 0.012 0.002 =RTln{[(apyr

) (agrs

)2/(adi

)6]/[(acel

)3/(ams

)3 ] } . ( 4 )Mn 0.000 0.000

Mg 0.019 0.002

where m0i is chemical potential of pure phase i at givenCa 0.011 0.001 PT, a

iis activity of each component and R is the gasNa 1.003 1.022

K 0.002 0.000 constant. For representation in a more convenientTotal 5.037 5.023 form, DG0 is as follows:

* Total Fe as FeO. r-portion: retrograde portion; Grt-Gln DG0=m0pyr

+2m0grs

+3m0cel

(6m0di

+3m0ms

) .schist: garnetglaucophane schist.

We define equilibrium coefficient Keq

as follows:

Keq

=[(apyr

) (agrs

)2/(adi

)6 ]/[(acel

)3/(ams

)3] .chloritoid ranges from 0.33 to 0.36. Lawsonite occursin the retrograde domain of eclogite (P-04 ) and has a Equation (4) becomesslightly higher Fe

2O

3(=1.47 wt%). Phengite is a

Keq

=exp[DG0/(RT)] .common mineral in all types of metamorphic rocks inthis district. It has rather high Si value, about 7.00 for Given appropriate thermodynamic data and activity

models, the equilibrium coefficient Keq

can be directlyO=22, in eclogites and garnetglaucophane schists(Table 2 ). related to pressure and temperature. The following

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558 K . M I YA Z A K I E T A L .

Fig. 8. Chemical compositions ofclinopyroxene in the jdacm(di+hd)diagram.

Fig. 9. Chemical compositions of garnet inthe (alm+sps)pyrgrs triangular diagram.

activity models for garnet, clinopyroxene and white adi

=(cdi

Xdi

) .mica are used.

White mica: independent mixing-on-sites model (referGarnet: ionic solution model for garnet

to Holland & Powell, 1990),(Mg,Fe,Mn,Ca)

3Al

2Si

3O

12,

ams

=4cms

(XK,A

) (XV,M1

) (XAl,M2

)2(XAl,T2

)(XSi,T2

) ,a

pyr=(c

pyrX

pyr)3 ,

acel

=4ccel

(XK,A

) (XV,M1

) (XMg,M2

) (XAl,M2

) (XSi,T2

)2 ,a

grs=(c

grsX

grs)3 .

Clinopyroxene: single-site model, where Xi

and ci

are the mole fraction of component i

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P- T C O N D I T I O NS , S U L A WE S I 559

Table 3. Solid-solution models used in calculations of equilibria (7) and (8).

aczo

=XM3Al

czo

gln agln

=(XM4Na

)2[AlVI/(AlVI+Fe3+)]2[Mg/(Mg+Fe2+)]3 Evans (1990)

jd ajd

=(cjd

Xjd

) assuming symmetric simple solution on one-site with GE=Wjd-di

Xjd

Xdi

+Wjd-hd

Xjd

Xhd

+Wjd-acm

Xjd

Xacm

+Wdi-hd

Xdi

Xhd

+Wdi-acm

Xdi

Xacm+Whd-acm Xhd Xacm, Wjd-di=Wjd-hd=3.766 kJ (=0.9 kcal ) and Wjd-acm=Wdi-hd=Wdi-acm=Whd-acm=0.RTln c

jd=3.766 (1Xt

jd) ( 1X

jdX

acm).

di adi

=(cdi

Xdi

)

RTln cdi

=3.766 (Xacm

+Xjd

) Xjd

grs agrs

=(ggrs

Xgrs

)3 Berman ( 1990)

3RTln cgrs

were given by Berman ( 1990) as a function of T, P, Xgrs

, Xalm

, Xpyr

and Xsps

.

and the activity coefficient of component i, respectively. Spitsbergen eclogite was estimated as P=1824 kbarat 580640 C using jadeite+quartz and paragoniteX

i,jrepresents the mole fraction of i iron in j site of

white mica. Then the distribution coefficient (KD1

) and stabilities (Hirajima et al., 1988). Their estimation oflower pressure limit is given by stability of ratio of activity coefficient (Kc) as follows:jadeite+quartz, but a jadeite+quartz assemblage was

KD1

=[(Xpyr

)3 (Xgrs

)6/(Xdi

)6 ]/not found in the Bantimala metamorphic rocks.However, Hirajima et al. (1988) showed that mineral[(Al/Mg)

M2,wm(Al/Si)

T2,wm]3,

assemblages systematically change with the composi-Kc=(c3pyr

c6grs

/c6di

)/[(cms

/ccel

)3] ,tion of coexisting garnet at the same pressure andtemperature. Comparing the compositions of thewhere (Al/Mg)

M2,wmand (Al/Si)

T2,wmare the Al/Mg

ratio in M2 site and the Al/Si ratio in T2 site of white garnets from this work with those of Hirajima et al.(1988), as shown in Fig. 9, it can be seen that the bulkmica. We assume that (Al/Mg)

M2,wm=(Al+Si8)/(Mg)

and (Al/Si)T2,wm

=(8Si)/(Si4) for O=22. Keq

is composition of the metamorphic rocks in this regionis not favourable to form the jadeite+quartz assem-

Keq

=KD1

Kc ,blage. We conclude that the metamorphic pressure ofequilibration of eclogites and garnetglaucophaneandschists of the Bantimala Complex was 1824 kbar.

KD1

=(1/Kc)EXP[(DG0/(RT)] . (6 )

The distribution coefficient KD1

is obtained directly Estimate of peak PTusing mineral paragenesis involvingfrom compositions of coexisting minerals. However, Ti-minerals and lawsonitethe term Kc (T, P, X

i

) is necessary for estimatingConstraints on pressure, temperature and the activitypressure and temperature. Qualitative relations of K

D1,

of H2O can be set by comparison of the mineral

pressure, temperature and composition of minerals inassemblages with computed phase equilibria. The

natural metamorphic rocks are evaluated in this paper.mineral assemblages of eclogites and garnet glauco-

Figure 11 shows a plot of (Xpyr

)3(Xgrs

)6/(Xdi

)6 vs.phane schists have a high variance. We compare the

[(Al/Mg)M2,wm

(Al/Si)T2,wm

]3 of ultrahigh-pressureobserved mineral assemblages to phase equilibria to

(UHP) metamorphic rocks from China (Hirajimaset broad limits on the PT a

H2Oconditions under

et al., 1990; Okay, 1993; Wang & Liou, 1993) , high-which the phases equilibrated.

pressure metamorphic rocks from SpitsbergenFor minerals with solid-solutions, it is necessary to

(Hirajima et al., 1988) and high-pressure metamorphicmake estimates of the displacements of the equilibria.

rocks from Sanbagawa (Enami et al., 1994). The KD1 For garnet, the solution model of Berman (1990) was

values of Dabie Shan 1 are scattered, with the twoused; for epidote and glaucophane, the solution models

lower values being obtained from eclogite lenses andof Evans (1990) were used. For the activities of jadeite

bands in acidic gneiss. Okay (1993) suggested thatand diopside components in C2/c pyroxene, an

these lower values of KD1 were due to re-equilibration extended single-site model of Banno (1986) was used,at lower pressures during uplift of these metamorphici.e. the symmetric simple solution model for a four-

rocks. Except for two lower values of KD1

of Dabiecomponent ( jadeiteacmitediopsidehedenbergite)

Shan 1, the data in Fig. 11 suggest that KD1

decreasessystem with excess enthalpies of W

jddi=W

jdhd=0.9

systematically from UHP metamorphic rocks to high-kcal and W

jdacm=W

acmdi=W

acmhd=W

dihd=0. The

pressure metamorphic rocks (Sanbagawa metamorphicsolid solution models used in this paper are listed in

rocks). Therefore, it seems that the value of KD1

is aTable 3. The database of Holland & Powell (1990) was

good index of metamorphic pressure.used to calculate phase equilibria.

The KD1

values of Indonesian eclogite are close toThe equilibrium

that of the Spitsbergen eclogite. Mineral assemblage,mineral compositions and metamorphic temperature 3Grs+5Rt+2Qtz+H

2O=2Czo+5Ttn , (7 )

of the Indonesian eclogites are similar to those ofSpitsbergen eclogite. The similarity of the K

D1value 3Ca

3Al

2Si

3O

12+5TiO

2+2SiO

2+H

2O

in both regions suggests that metamorphic pressuresare also similar. The metamorphic pressure of the =2Ca

2Al

3Si

3O

12(OH)+5CaTiSiO

5,

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560 K . M I YA Z A K I E T A L .

Fig. 10. Chemical compositions of sodicamphibole.

in epidote. For garnet (Xgrs

=0.25 and Xpyr

=0.16) andepidote (XM3

Al=0.27) in sample Mg-49a ( garnetglauco-

phane schist), the equilibrium lies near 17 kbar at580 C and 16 kbar at 620 C for a

H2O=1 (Fig. 12).

For garnet (Xgrs

=0.24 and Xpyr

=0.18) and epidote(XM3

Al=0.49) in sample Mg218a (eclogite), the equilib-

rium lies near 12 kbar at 560 C and 10 kbar at 610 Cfor a

H2O=1 (Fig. 12). Assuming eclogites, garnet

glaucophane rocks and garnetglaucophane schistswere formed under the same PT a

H2Oconditions, and

using the temperature range given by the garnetclinopyroxene geothermometer, the minimum pressureranges from 17 kbar at 580 C to 16 kbar at 620 C.

The assemblage glaucophaneepidotequartzomphacite is common in the eclogites, garnetglaucophane rocks and schists. The equilibrium

Gln+6Czo+2Qtz+14H2O=2Jd+9Lws+3Di,

(8 )

Na2Al2Mg3Si8O22(OH)2+6Ca2Al3Si3O12 (OH)

+2SiO2

+14H2O=2NaAlSi

2O

6

+9CaAl2

Si2O

7(OH)

2(H

2O)+3CaMgSi

2O

6,

Fig. 11. Plot of the distribution coefficient KD1

of reaction (1)in the (X

pyr)3 (X

grs)6/(X

di)6 vs. [(Al/Mg)

M2,wm(Al/Si)

T2,wm]3 can be used to set maximum pressure on the assem-

diagram. Bantimala (this study), Dhoghai (Hirajima et al.,blage. It shifts to higher pressure with decreasing1990), Dabie Shan 1 (Okay, 1993), Dabie Shan 2 (Wang &glaucophane component (Mg end-member) inLiou, 1993), Spitsbergen (Hirajima et al., 1988), Sanbagawa

(Enami et al., 1994). Na-amphibole. For clinopyroxene (Xjd

=0.36, Xacm

=

0.16, Xdi

=0.38 and Xhd

=0.11), glaucophane (agln

=

0.20) and epidote (XM3Al

=0.32) in sample Mg-51can be used to set PT limits on the assemblagegarnetrutilequartzepidote, which is present in the (garnetglaucophane schist), the equilibrium lies near

24 kbar at 580 C and 27 kbar at 650 C for aH2O

=1eclogite and garnetglaucophane schists. The equilib-rium gives a minimum pressure that shifts to the lower (Fig. 12). For clinopyroxene (X

jd=0.42, X

acm=0.06,

Xdi

=0.43 and Xhd

=0.09), glaucophane (agln

=0.03)pressure side with increasing clinozoisite component

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P- T C O N D I T I O NS , S U L A WE S I 561

Fig. 12. Calculated PTdiagram forequilibria (7 ) and (8 ), and inferred PTpathof the Bantimala eclogite and garnetglaucophane schist. Activities of phase orphase-components according to Table 3.Equilibria (7a), (7b) and (7c) represent

Grossular+

Rutile+

Quartz+

Water=

Clinozoisite +Titanite equilibrium (7) forthe sample Mg-49a (garnetglaucophaneschist), Mg2-18a (eclogite) and P-04(eclogite), respectively. Equilibria (8a), (8b),(8c) and (8d) representGlaucophane+Clinozoisite+Quartz+Water=Omphacite+Lawsonite equilibrium (8) forMg-51 (garnetglaucophane schist), Mg-47a(eclogite), P-04 (eclogite, major part) andP-04 (eclogite, retrograde portion),respectively. GrtCpx (max) and GrtCpx(min) represent metamorphic temperaturesestimated with the garnet (rim) clinopyroxene geothermometer of Powell(1985). Reaction Jd+Qtz=Ab fromHolland (1980). Inferred amphibole stability

in hematite-bearing basic schist (Otsuki &Banno, 1990). Na-amp: Na-amphibole(magnesioribeckitecrossite), Win: winchite,Bar: barroisite, Hbl: hornblende.

and epidote (XM3Al

=0.28) in sample Mg-47a (eclogite), glaucophane (agln

=0.25) and epidote (XM3Al

=0.35) inthe major part of sample P-04 (eclogite), the equilibriumthe equilibrium lies near 26 kbar at 600 C and 29 kbar

at 660 C for aH2O

=1 (Fig. 12). Assuming the eclogites, lies near 350 C at 10 kbar and 525 C at 20 kbar fora

H2O=1 (Fig. 12). For clinopyroxene (X

jd=0.21, X

di=garnetglaucophane rocks and schists were formed

under the same PT aH2O

conditions, and using the 0.41, Xacm

=0.30 and Xhd

=0.08) and glaucophane(a

gln=0.03) in the retrograde domain, in combinationtemperature range given by the garnetclinopyroxene

geothermometer, the maximum pressure ranges from with epidote (XM3Al

=0.35) in the major part of sampleP-04, the equilibrium is essentially the same (Fig. 12).24 kbar at 580 C to 27 kbar at 650 C.

The stability region of the assemblages garnet The progress of Reaction (8 ) from the left to the right

side requires water, and thus depends strongly on therutilequartzepidote and glaucophaneepidotequartzomphacite with a

H2O=1 are consistent with the addition of water. The inferred PT trajectory must

cross this equilibrium (Fig. 12). In the retrogradepeak PT condition estimated with thegarnet+omphacite+phengite equilibrium. domain of sample P-04, Ti minerals included in

lawsonite patches are always titanite or rutile rimmedby titanite. This shows that rutile was unstable before

R E T R O G R A D E P A T H O F E C L O G I T E S , G A R N E T lawsonite was produced. Therefore, the retrograde PT

G L A U C O P H A N E R O C K S A N D S C H I S T Strajectory must cross Equilibrium (7) before crossingEquilibrium (8) (Fig. 12). These data show that theseThe retrograde mineral assemblage of eclogite (sample

P-04: chloromelanite+crossite+Lws+Chl+Ttn in rock units were refrigerated during upward motion.Sodic amphiboles in the garnetglaucophane schistthe retrograde portion) suggests that the retrograde

PT trajectory must be located in the stability region (sample Mg-51) were finally rimmed by actinolite,and the assemblage hematite+actinolite+albite isof chloromelanite+lawsonite. This stability region can

be calculated using Equilibrium (8 ). For clinopyroxene observed. This suggests that the later PT trajectoryof this rock was located on the lower pressure side of(X

jd=0.33, X

di=0.46, X

acm=0.14 and X

hd=0.07),

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562 K . M I YA Z A K I E T A L .

the crossite stability region ( Brown, 1974; Otsuki & in the simple jadeitediopside system. It is possible toevaluate the effect of the ordering on Equilibrium (8)Banno, 1990). Brown (1977) showed that variation in

Fe2+/Mg had little effect on the stability of crossite. using hypothetical compositions of minerals, clinopyrox-

ene (X

jd

=Xdi

=0.50), glaucophane (

agln

=0.25) and epi-Otsuki & Banno (1990) showed semiquantitative phaserelations of actinolitewinchitemagnesinoriebeckite dote (XM3

Al=0.35 ) with Hollands (1990) results. The

equilibrium with P2/n omphacite shifts to 0.5 kbar at(or crossite) associated with albite, chlorite, hematiteand quartz. The composition of zoned amphibole 600 C and to 1 kbar at 400 C, below the equilibrium

with C2/c omphacite. Therefore, it seems that thecoexisting with hematite, quartz and chlorite withinan albite porphyroblast in sample Mg-51 varies ordering effect does not affect strongly Equilibrium (8).through glaucophane, crossite, winchite and actinolite.Therefore, the inferred PT trajectory lies near 5 kbar

Tectonic implicationsat 350 C (Fig. 12).

The retrograde paths of eclogite and garnetglauco- The peak PT conditions of the eclogites, garnetglaucophane rocks and schists were estimated as T=phane schist show that these deeply subducted meta-

morphic rocks were cooled during upward motion. A 580640 C and P=1824 kbar. This means that theserocks were subducted to 6585 km depth (assumingsimilar path was reported from Franciscan metamor-

phic rocks (Ernst, 1988). density=2850 kgm3), under an overall geothermal

gradient of c. 8 C km1. Calculations of the thermalstructure of subduction zones suggest that such low

D I S C U S S I O Ngeothermal gradients occur where shear stress and basalheat flux are low, and subduction angle and thermal

PT estimates and solid-solution modelsconductivity are high (Peacock, 1992). Very high fluidpressures along the plate contact are probably the wayFor the PT estimation using the omphacitegarnet

phengite equilibrium, we used only the relations shear stresses are reduced (Dumitru, 1991). In suchcases, the effect of shear heating becomes low, and thebetween PT and compositions of minerals. The

equilibrium coefficient KD1

among omphacite, garnet low geothermal gradient can be explained simply by ahigh rate of subduction of a cold oceanic plate.and phengite is a function of PTand the composition

of the minerals, and the same value of KD1

will give Some deeply subducted metamorphic rocks sufferedretrograde metamorphism. The retrograde path of thethe same equilibrium pressure when temperature and

the compositions of the minerals are the same. In the eclogite in Fig. 12 suggests that the Bantimala eclogiteevidently was refrigerated during upward motion.absence of experimental data, observed K

D1

to KD1were linked in natural samples for which pressures Ernst (1988) reviewed retrograde blueschist PTpaths,

in which some of the PT paths, such as of tectonicwere estimated with independent methods.Pressures and temperatures were estimated with blocks in the Franciscan Complex, are similar to the

retrograde path of eclogite in this study. He suggestedgarnetrutilequartzepidote and lawsoniteomphaciteglaucophaneepidote equilibria, but these are dependent that the upward motion took place as tectonically

imbricated slices (e.g. Ernst, 1971), as laminar returnon the solid-solution models chosen for each mineral.The symmetric simple solution on single-site model was flow in a melange zone (Cloos, 1982; Shreve & Cloos,

1986), and perhaps partly as lateral spreading/exten-used for disordered omphacite. Holland (1983 ) proposedan asymmetric simple solution on two-site model, but sion of an underplated accretionary prism (Platt, 1986,

1993). Platt (1993) pointed out that the laminar returnBanno (1986) pointed out that jadeitediopside solid-solution may be explained with semi-ideal solution flow may provide a mechanism for some occurrences

of high-grade tectonic blocks in mud-matrix melanges.(symmetric simple solution with small excess enthalpy)on a single-site model by introducing strong interaction However, such mud-matrix melanges do not occur in

this region. The eclogites are enclosed in shearedof cations between M1 site and M2 site. Therefore, thesolid-solution behaves like a semi-ideal solution on a serpentinite. The serpentinite matrix+eclogite blocks

are less dense than the mantle material they displace.single-site model, although the solid-solution for ompha-cite must be treated as random mixing on a two-site When underflow is maintained, leading to refrigeration

of the accretionary section, decoupled high-pressuremodel. He also suggested that the difference betweensingle- and two-site models is not large at rocks (the serpentinite matrix+eclogite blocks) may

ascend due to buoyancy force (Ernst, 1988).300 C

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P- T C O N D I T I O NS , S U L A WE S I 563

petrology of eclogites and related rocks from the Montalafjellaschist represents the later segment of the upward path.high-pressure metamorphic complex in Spitsbergen (ArcticThe later segment suggests nearly isothermal decompo-Ocean) and its significance. L ithos, 22, 7597.

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2ONa

2OCaOMgOMnO

Development Centre for Geotechnology, and his staff FeOFe2O

3Al

2O

3TiO

2SiO

2CH

2O

2. Journal of

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Hirajima, T., Banno, S., Hiroi, Y. & Ohota, Y., 1988. Phase Received 8 September 1995; revision accepted 11 March 1996.