A jadeite-quartz-glaucophane rock from Karangsambung, central Java, Indonesia

Thematic Article

A jadeite±quartz±glaucophane rock from Karangsambung,

central Java, Indonesia

K. MIYAZAKIIYAZAKI1, J. SOPAHELUWAKANOPAHELUWAKAN2, I. ZULKARNAINULKARNAIN2ANDAND K. WAKITAAKITA1

1_{Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, Ibaraki 305, Japan}2_{Research and Development Center for}

Geotechnology, Jl. Cisitu, 21/154D, Bandung, 40135 Indonesia

Abstract High-pressure metamorphic rocks are exposed in Karangsambung area of central Java, Indonesia. They form part of a Cretaceous subduction complex (Luk±Ulo Complex) with fault-bounded slices of shale, sandstone, chert, basalt, limestone, con-glomerate and ultrabasic rocks. The most abundant metamorphic rock type are pelitic schists, which have yielded late Early Cretaceous K±Ar ages. Small amounts of eclogite, glaucophane rock, garnet±amphibolite and jadeite±quartz±glaucophane rock occur as tectonic blocks in sheared serpentinite. Using the jadeite±garnet±glaucophane±phen-gite±quartz equilibrium, peak pressure and temperature of the jadeite±quartz±glau-cophane rock areP ˆ 22 ‹ 2 kbar andT ˆ 530 ‹ 40 °C. The estimatedP±Tconditions indicate that the rock was subducted to ca 80 km depth, and that the overall geothermal gradient was7.0°C/km. This rock type is interpreted to have been generated by the metamorphism of cold oceanic lithosphere subducted to upper mantle depths. The ex-humation from the upper mantle to lower or middle crustal depths can be explained by buoyancy forces. The tectonic block is interpreted to be combined with the quartz±mica schists at lower or middle crustal depths.

Key words: Cretaceous subduction complex, geothermal gradient, high-pressure meta-morphic rocks, Indonesia, Karangsambung, P±T conditions, tectonics.

INTRODUCTION

The Cretaceous subduction complexes at the southeastern margin of Sundaland in Indonesia are distributed in West and Central Java, South Kalimantan, and Central and South Sulawesi (Fig. 1). These complexes are characterized by the chaotic occurrence of sandstone, shale, chert, basalt, ultrabasic rocks and high-pressure meta-morphic rocks. Before the opening of the Ma-kassar Strait, these complexes may have constituted a single subduction complex (Hamil-ton 1979).

The Luk-Ulo Complex (Asikin 1974) of the Karangsambung area of Central Java is com-posed of a chaotic mixture of various kinds of sedimentary, igneous and metamorphic rocks. Kenter et al. (1976) reported the late Early

Cretaceous foraminifera Orbitolina from lime-stone in the Luk±Ulo Complex. Wakita et al. (1994b) reported an Early to Late Cretaceous radiolarian assemblage from shale and chert. These rocks occur as blocks and slices in the complex. A late Early Cretaceous K±Ar age (117 ‹ 1.1 Ma) for mica in a coarse-grained peli-tic schist was obtained by Kenter et al. (1976). The ages of sedimentary and metamorphic rocks are very similar to those of the Bantimala Com-plex (Wakitaet al. 1996).

This paper describes a jadeite±quartz±glau-cophane rock, which occurs as tectonic blocks in the Luk±Ulo Complex. Because the jadeite± quartz assemblage is diagnostic of subduction zone metamorphism, these results contribute to an understanding of the tectonic evolution of the Luk±Ulo Complex, and the relationship between the Bantimala Complex and the Luk±Ulo Com-plex.

The Island Arc(1998)7_{, 223±230}

GEOLOGIC SETTING

The Karangsambung area is not only underlain by the Luk±Ulo Complex but also by Eocene to Miocene volcanic and clastic rocks. The complex consists of shale, sandstone, chert, basic to ultra-basic rocks, limestone, rhyolite, conglomerate and metamorphic rocks. Sandstone usually al-ternates with shale, while chert is often inter-bedded with limestone. These constituents of the complex occur as tectonic blocks and slabs. The long axes of these blocks and slabs trend

east-northeast±west-southwest, parallel to the strike of sedimentary rocks occurring as slabs. Large tectonic slabs consisting of dismembered ophio-lite (Suparka 1988) are distributed in the central part of the complex (Fig. 2). The Tertiary se-quence is gently folded with an east±west trending vertical axial plane, and is divided into the Karangsambung, Totogan, and Waturanda Formations in ascending order (Fig. 2). The Luk±Ulo Complex is unconformably overlain by the Karangsambung Formation, which yields Eocene foraminifera (Natoriet al. 1978).

Fig. 1 Tectonic map of Central Indonesia (modi®ed from Wakitaet al._1994a).

METAMORPHIC ROCKS

Most of the metamorphic rocks in the Luk±Ulo Complex are pelitic schists in which albite, quartz and muscovite are abundant. The next most abundant minerals are chlorite, garnet and clinozoisite. Small amounts of sphene and graphite are also present. Some of the pelitic schists do not contain garnet, and are very ®ne-grained. Others contain, in addition to the above minerals, biotite and/or hornblende, and are slightly more coarse grained. Epidote amphibo-lite in which barroisite, garnet, epidote, albite, biotite and phengite are present, is intercalated with garnet-bearing pelitic schists.

Two samples of pelitic schists from the Luk± Ulo Complex were collected for K±Ar age dating. The localities of the samples are shown in Fig. 2. The mineral associations are as follows: Samples: KS18 and KS23: Albite porphyroblast-bearing garnet±quartz±muscovite schist. Mineral associ-ation: garnet, quartz, albite, muscovite, chlorite, carbonate, graphite, tourmaline, sphene, apatite and opaque. K±Ar age data of muscovite are shown in Table 1. The average data of the sam-ples range from 110 ‹ 6 to 115 ‹ 6 Ma and are consistent with the K±Ar age of Kenter et al. (1976).

Small amounts of garnet±amphibolite, eclogite, glaucophane rock and jadeite±quartz±glaucop-hane rock occur as tectonic blocks in sheared serpentinite. The garnet±amphibolite contains hornblende, garnet, plagioclase, zoisite and quartz, and has suffered mylonitization. The eclogite consists of garnet, omphacite, barroisite, epidote, paragonite, rutile and rutile rimmed by sphene. The glaucophane rock contains glaucop-hane, acmite, chlorite, epidote and phengite. The jadeite±quartz±glaucophane rock consists of jadeite, glaucophane, garnet, phengite, quartz, albite and rutile, where albite is the only retro-grade mineral.

JADEITE±QUARTZ±GLAUCOPHANE ROCK

The jadeite±quartz±glaucophane rock (sample no.: KS10) was collected from a boulder in the Muntjar River near to the boundary between the pelitic schist and non-metamorphosed sedimen-tary rocks (Fig. 2). There are many boulders of eclogite, glaucophane rocks and serpentine at the same location. Therefore, it is inferred that these blocks are tectonic blocks which were once in-cluded within the serpentinite which is distrib-uted between the pelitic schists and the non-metamorphosed sedimentary rocks.

The distribution of minerals in the jadeite± quartz±glaucophane rock is heterogeneous (Fig. 3a). Three domains are recognized: dusty jadeite patches (2±5 mm in length), glauco-phane-rich domain, and quartz-rich domains (Fig. 3b). Small quartz inclusions are included in the jadeite patches (Fig. 4). Areal fraction ratio (ˆ 0.23) of jadeite and quartz in the patches is close to that (ˆ 0.27) of albite decomposition reaction; albite ˆ jadeite + quartz. Very small amounts of glaucophane (<5%), phengite (<1%), garnet (<1%) and rutile (<1%) occur as inclu-sions. In many cases, rims of the jadeite patches were replaced by albite, in addition to direct contact with quartz which is also observed (Fig. 3b). Albite veins are developed in the patches. The glaucophane-rich domain consists of ®ne-grained needles of glaucophane. Albite (0.3± 2.0 mm) occurs in the domains. In very rare cases, small jadeite patches (2 mm) also occur, and the rim is replaced by albite. The quartz-rich domains consist of equigranular quartz and small amounts of glaucophane, phengite, rutile, albite and sphene. Euhedral garnet (<0.3 mm) occurs in each domain and also between the domains.

MINERAL CHEMISTRY

Mineral analyses were carried out using the JEOL 8800 at the Geological Survey of Japan. Acceler-ating voltage, induced beam current and beam diameter were kept at 15 kV, 12 nA on Faraday cup and 2 lm, respectively. Representative

min-eral chemistries are shown in Table 2. The Fe2O3

content of sodic pyroxene was estimated on the assumption of Al + Fe3+ ˆ Na. The Fe3+/Fe2+ value of amphibole was calculated as total cat-ions ˆ 13 exclusive of K, Na and Ca (O ˆ 23).

Assuming the pyroxene components are jade-ite (jd), acmjade-ite (acm), diopside (di) and he-Table 1 K±Ar data of muscovite from quartz-mica schist of

the Luk-Ulo MeÂlange Complex

Sample

Separation of muscovite and measuring of age data were done by Teledyne Isotopes.

denbergite (hd), the end-member content is cal-culated as Xjd ˆ Al/(Na + Ca), Xacm ˆ Fe3+/ (Na + Ca), Xdi ˆ [Ca/(Na + Ca)] ´[Mg/(Mg +

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

Fe2+)]. The analyzed jadeite has some heteroge-neity (Xjd ˆ 0.89±0.95). We observed jadeite

(Xjd ˆ 0.95) in direct contact with

quartz-inclu-sions in the jadeite patches.

The contents of the garnet end-members are calculated as follows:

Xpyr ˆMg=…Mg‡Fe‡Mn‡Ca†;

XalmˆFe=…Mg‡Fe‡Mn‡Ca†;

XspsˆMn=…Mg‡Fe‡Mn‡Ca†and

XgrsˆCa=…Mg‡Fe‡Mn‡Ca†:

All of the garnets show very distinct chemical zoning (normal type) with Mn-rich cores and Fe-rich rims. The core-composition is; Xalm ˆ 0.48 and Xsps ˆ 0.26, and the rim-composition is; Xalm ˆ 0.73 and Xsps ˆ 0.05. The grossular

content decreases from core (Xgrs ˆ 0.23) to rim (Xgrs ˆ 0.15). The pyrope content is very low (Xpyr ˆ 0.03±0.07).

XFe2+ ˆ Fe2+/(Mg + Fe2+) for glaucophane

ranges from 0.47±0.49, and YFe3+ ˆ Fe3+/(Al (VI) + Fe3+) for glaucophane decreases from core to rim. In some cases, glaucophane is rim-med by magnesioriebeckite at the contact with albite. The Si content in phengite is 6.6±6.7 (O ˆ 22). Na content in phengite is 0.2 (O ˆ 22).

P±T ESTIMATION

Garnet, phengite, and glaucophane are included in the jadeite patches. These minerals also occur outside the jadeite patches, therefore, it is in-ferred that the jadeite±quartz±glaucophane± garnet±phengite is an equilibrium assemblage.

The areal fraction ratio of jadeite and quartz in the dusty jadeite patches is close to that of the albite decomposition reaction;

AlbiteˆJadeite‡Quartz

NaAlSi3O8ˆNaAlSi2O6‡SiO2 …1†

Therefore, it is envisaged that original albite crystals were changed isochemically to the dusty jadeite patches, and that the metamorphic pres-Fig. 4 Photomicrograph of a dusty jadeite patch. Quartz (Qtz inc.)

as inclusion in jadeite (Jd). Crossed polars.

sure was higher than the equilibrium (1). Using the experimental data of Holland (1980) and as-suming a pure jadeite composition, the minimum pressure of jadeite±quartz equilibrium is 11 kbar at 400°C and 16 kbar at 600°C.

Metamorphic temperature is estimated by the garnet±phengite geothermometer (Krogh & RaÈ-heim (1978)), and result is 480°C at 15 kbar and 600°C at 23 kbar using the garnet-rim compo-sition. Krogh & RaÈheim (1978) suggested that the garnet±phengite geothermometer gives a high temperature when phengite contains signi-®cant amounts of Fe3+. Therefore, the estimated temperature is a maximum temperature.

Constraints on pressure, temperature and the activity of H2O can be set by comparison of the

mineral assemblages with computed phase equi-libria. The mineral assemblages of the jadeite± quartz rock have a high variance as regards the phase rule. We compare the observed mineral assemblage to phase equilibria to set broad limits on the P-T-aH2O conditions under which the

phases reached equilibrium. For minerals with

solid-solutions, it is necessary to estimate the displacements of the equilibria. For garnet solid solutions, the solution model of Berman (1990) was used. For glaucophane, we used the solution model by Evans (1990). For jadeite, an extended one-site model of Banno (1986) was used, that is the symmetric solution model for a four-com-ponent (jadeite±acmite±diopside±hedenbergite) system with excess enthalpies of Wjd-di ˆ Wjd-hd ˆ 0.9 kcal and Wjd-acm ˆ Wacm-di ˆ Wacm-hd ˆ Wdi-hd ˆ 0. The solid solution models used in

this paper are listed in Table 3. The database of Holland and Powell (1990) was used to calculate the phase equilibria.

can be used to set the minimum pressure limits on the garnet±jadeite±quartz assemblage. It shifts to higher pressures with decreasing fer-Table 2 _{Representative analysis of jadeite (Jd). garnet (Grt). galucophane (Gln), magnesioriebeckite (Mrb) and phengite (Phe)}

in sample KS10

SiO2 58.38 59.34 58.99 36.64 37.27 37.17 56.30 55.90 53.97 49.55 49.48

TiO2 0.22 0.04 0.07 0.15 0.12 0.11 0.04 0.12 0.02 0.35 0.33

Al2O3 22.12 24.35 24.04 20.90 21.43 21.38 12.48 12.53 1.37 29.33 28.63

Cr2O3 0.00 0.03 0.00 0.00 0.00 0.01 0.03 0.01 0.00 0.01 0.04

FeO 2.43 1.58 1.81 21.87 21.63 31.93 13.02 12.98 23.24 2.75 2.86

MnO 0.14 0.03 0.04 12.02 11.96 2.28 0.10 0.00 0.54 0.04 0.00

MgO 1.24 0.34 0.40 0.81 0.80 1.72 7.61 7.66 8.40 2.77 2.74

CaO 2.39 0.53 0.68 8.41 7.90 5.10 0.29 0.44 1.39 0.01 0.00

Na2O 13.85 15.25 15.18 0.07 0.07 0.00 7.54 7.32 6.66 0.78 0.76

K2O 0.01 0.00 0.03 0.01 0.00 0.00 0.03 0.04 0.05 9.95 9.58

Total 100.78 101.50 101.24 100.88 101.17 99.69 97.42 96.98 95.64 95.53 94.41

Oˆ 6 6 6 12 12 12 23 23 23 22 22

Si 1.984 1.984 1.981 2.947 2.971 2.995 7.839 7.812 7.973 6.605 6.662

Al 0.886 0.959 0.951 1.981 2.013 2.030 2.048 2.063 0.239 4.607 4.542

Ti 0.006 0.001 0.002 0.009 0.007 0.006 0.004 0.013 0.002 0.035 0.033

Cr 0.000 0.001 0.000 0.000 0.000 0.001 0.003 0.001 0.000 0.001 0.004

Fe3+,  _{0.026 0.029 0.037 0.000 0.000 0.000 0.138 0.167 1.455 0.000 0.000}

Fe2+ 0.043 0.016 0.014 1.471 1.442 2.151 1.378 1.349 1.415 0.307 0.322

Mn 0.004 0.001 0.001 0.819 0.807 0.155 0.012 0.000 0.068 0.004 0.000

Mg 0.063 0.017 0.020 0.097 0.095 0.206 1.578 1.596 1.848 0.549 0.548

Ca 0.087 0.019 0.025 0.725 0.675 0.440 0.043 0.066 0.221 0.002 0.000

Na 0.912 0.988 0.987 0.011 0.011 0.000 2.033 1.982 1.906 0.201 0.189

K 0.000 0.000 0.001 0.011 0.000 0.000 0.006 0.006 0.009 1.691 1.645

Total 4.010 4.014 4.018 8.060 8.020 7.984 15.082 15.054 15.135 14.002 13.953

jd 0.887 0.953 0.940 pyr 0.031 0.032 0.070 YFe3+ _{0.068 0.082 0.873}

acm 0.026 0.028 0.036 alm 0.473 0.478 0.729 XFe2+ _{0.466 0.458 0.434}

di 0.052 0.010 0.014 sps 0.263 0.267 0.053

hd 0.035 0.009 0.010 grs 0.233 0.223 0.149

*table Fe as FeO; calculated value (see text); YFe3+_{= Fe}3+_/(Fe3+_{+ Al(VI)); XFe}2+_{= Fe}2+_/(Fe2+_{+ Mg).}

roglaucophane component. For pure ferroglau-cophane (aFe-gln ˆ 1), paragonite (XNa ˆ 0.95)

and the measured compositions of garnet and jadeite in sample KS10, the equilibrium lies near 17 kbar at 437°C and 21 kbar at 337°C. For glaucophane (XM4Na ˆ 0.98, XFe2+ ˆ 0.466 and

YFe3+ ˆ 0.068; aFe-gln ˆ 0.084) in sample KS10,

the equilibrium lies near 20 kbar at 550°C and 21 kbar at 500°C. The observed Fe±Mg distri-bution coef®cient between garnet and glaucop-hane is 11.93. Fe±Mg distribution coef®cient calculated with the database of Holland and Powell (1990) is 11.99 at P ˆ 20 kbar and

T ˆ 550°C. Therefore, the observed distribu-tion coef®cient is consistent with the calculated distribution coef®cient.

can be used to set the minimum temperature on the assemblage. It shifts to the high temperature side with decreasing chloritoid component. For chloritoid (XFe ˆ 0.9) and the measured com-positions of garnet, jadeite and glaucophane in sample KS10, the equilibrium lies near 421°C at 25 kbar and 497°C at 17 kbar. For chloritoid (XFe ˆ 0.5), the equilibrium lies near 480°C at 25 kbar and 548°C at 21 kbar. We do not have any chloritoid in our samples. XFeof chloritoid in

metamorphic rocks for which the metamorphic conditions are 20 ‹ 2 kbar and 430 ‹ 30°C (Okay & Kelley 1994) is0.9. XFeof chloritoid in

metamorphic rocks for which metamorphic con-ditions are 21 ‹ 3 kbar and 610 ‹ 30°C is ca 0.7 (Mizayaki et al. 1996). The Fe±Mg distribution coef®cients between glaucophane and chloritoid in the literature range from 0.06 to 0.138. Using

these distribution coef®cients, a possible value of

XFeof chloritoid coexisting with the glaucophane

(XFe2+ ˆ 0.466) becomes 0.86±0.93. Therefore,

the assumed value (XFe ˆ 0.9) of chloritoid is consistent with the observed values in meta-morphic rocks at high pressures and low to moderate temperatures.

The equilibrium

QuartzˆCoesite …4†

gives the maximum pressure, because of the absence of coesite and its pseudomorph. The stability region of the assemblage of jadeite± quartz±garnet±glaucophane with aH₂O ˆ 1 are

presented in Fig. 5. The results show that the pressures and temperatures of the jadeite± quartz±glaucophane rock are P ˆ 22 ‹ 2 kbar and T ˆ 530 ‹ 40°C.

TECTONIC IMPLICATIONS

The peak P-T conditions of the jadeite±quartz± glaucophane rock was estimated as P ˆ 22 ‹ 2 kbar and T ˆ 530 ‹ 40°C. This means that the rock was subducted to 80 km depth, and that the overall geothermal gradient was7°C/ km. Calculations of the thermal structure of subduction zones suggest that such low geo-thermal gradients occur where shear stress and basal heat ¯ux are low, and the subduction angle and thermal conductivity are high (Peacock 1992). In the peak stage of very high-pressure metamorphism of the jadeite±quartz±glaucop-hane rock, penetrative deformation was absent. Therefore, the effect of shear heating is low, and the low geothermal gradient can be explained simply by a high rate of subduction of a cold oceanic plate.

Table 3 _{Activity models}

Fe-glaucophane Na2Fe2Al2Si8O22(OH)2 (XM4Na)2(1)YFe3+)2X_Fe2+)3 0.084 (data from Table 2)

Fe-chloritoid FeAl2SiO5(OH)2 XFe 0.5, 0.7, 0.9 (assumed)

Jadeite NaAlSi2O6 cXjd, RT in

c= 3.766(1)Xjd)(1)XjdXacm)

Xjd= 0.953,Xacm= 0.028,Xdi= 0.010,

Xhd= 0.009 (data from Table 2) assuming symmetric simple

solution on one-site model of Banno (1986)

Quartz SiO2 pure phase 1.0

The jadeite±quartz±glaucophane rock occurs as a tectonic block in sheared serpentinite. This tec-tonic block + sheared serpentinite is in faulted contact with the pelitic schists. Metamorphic pressure of the more abundant pelitic schists is signi®cantly lower than that of the tectonic block, as albite is stable in the pelitic schists. The tectonic blocks + serpentinite matrix are interpreted to have decoupled at upper mantle depths from the subducting oceanic plate, and have ascended to lower or middle crust depths due to buoyancy forces. The tectonic block was combined with the pelitic schists at lower or middle crust depths.

COMPARISON WITH METAMORPHIC ROCKS IN THE BANTIMALA COMPLEX

Both the Luk±Ulo (LU) and Bantimala (BA) Complexes are characterized by the chaotic oc-currence of sandstone, shale, chert, basalt, ultra-basic rocks and high-pressure metamorphic rocks. These complexes may have been part of a single subduction complex (Hamilton 1979). The ages of the chert in the Lu and BA Complexes were Early to Late Cretaceous and Middle Cretaceous, res-pectively (Wakitaet al. 1994a, 1994b, 1996). The lithologies of the high-pressure metamorphic

rocks in each region are different. The most abundant rock type in the LU Complex is a pelitic schist, whereas that in the BA Complex consists of lawsonite-bearing or hematite-bearing glaucop-hane schists. Small amounts of epidote amphibo-lite, which contains barroisite, garnet, epidote, albite, biotite and phengite, are intercalated with the pelitic schists in the LU Complex. Small amounts of the pelitic schists are intercalated with the lawsonite-bearing or hematite-bearing glau-cophane schists in the BA Complex. However, K± Ar ages of the most abundant rocks in each region are similar, K±Ar ages for mica in the pelitic schists from the LU Complex were 117 ‹ 1.1 Ma (Kenter et al. 1976), 110 ‹ 6 and 115 ‹ 6 Ma (in this study). K±Ar ages for mica in the pelitic schists intercalated with hematite-bearing glau-cophane schists in the BA Complex were 114 ‹ 6 and 115 ‹ 6 Ma (Wakita et al. 1996). These ages are interpreted to be exhumation ages.

Small amounts of metamorphic rocks subduc-ted to upper mantle depths occur as tectonic blocks within sheared serpentinite in both re-gions: jadeite±quartz±glaucophane rock in the LU Complex, and eclogite, garnet±glaucophane rocks and schists in the BA Complex. Miyazaki

et al. (1996) showed that eclogites, garnet±glau-cophane schists and rocks in the BA complex were subducted to 65±85 km depth, and the overall geothermal gradient was 8°C km)1_.

They proposed that the tectonic blocks included in the serpentinite were decoupled at upper mantle depths from the subducting oceanic plate, and ascended to lower or middle crustal depths due to buoyancy forces. The tectonic blocks are interpreted to have combined with the lawsonite-bearing or hematite-lawsonite-bearing glaucophane schists, which are more abundant than the tectonic blocks in the BA Complex, at lower or middle crustal depths. Similarities in the very low overall geothermal gradient of the tectonic blocks and the ascent of the tectonic blocks from upper mantle depths in both regions are consis-tent with the interpretation that the LU and BA Complexes formed part of a single subduction complex.

CONCLUSION

The Luk±Ulo (LU) Complex consists of shale, sandstone, chert, basic to ultrabasic rocks, lime-stone, rhyolite, conglomerate and metamorphic rocks. The ages of the sedimentary rocks are Fig. 5 Calculated P-T diagram for equilibria (2)

parago-nite + glaucophane ˆ jadeite + garnet + quartz + water and (3) chloritoid + glaucophane ˆ jadeita + garnet + quartz + water. Grt-Phengite represents metamorphic temperature estimated with garnet (rim)-phengite geothermometer of Krogh and RaÈheim (1978). Reaction jadeite + quartz ˆ albite from Holland (1980). Reaction quartz ˆ coesite was calculated using thermodynamic data of Holland and Powell (1990).

Early to Late Cretaceous. K±Ar ages for the most abundant quartz±mica schist is late Early Creta-ceous. We found the tectonic block of jadeite± quartz±glaucophane rock within the LU Complex. The peak pressure and temperature were calcu-lated at P ˆ 22 ‹ 2 kbar and T ˆ 530 ‹ 40°C. These values mean that the overall geothermal gradient was 7°C/km and burial depth was

80 km. This rock type is interpreted to have been generated by metamorphism of cold oceanic lithosphere subducted to upper mantle depths. The exhumation from upper mantle depths to the lower or middle crust can be explained by buoy-ancy forces. The tectonic block is interpreted to have been combined with the pelitic schists at the lower or middle crust. The metamorphism and exhumation of the tectonic block are similar to those of the tectonic blocks in the Bantimala (BA) Complex, and are consistent with the interpr-etation that the LU and BA Complexes formed part of a single subduction complex.

ACKNOWLEDGEMENTS

Dr Ir. S. Suparka, Director of the Research and Development Center for Geotechnology, and his staff are thanked for their assistance and for many useful discussions. We thank Dr C. D. Parkinson for critically reading the manuscript. K. Miyazaki would like to express thanks to Professor S. Banno and two anonymous review-ers for their critical reviews and suggestions for improvement.

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