A jadeite-quartz-glaucophane rock from Karangsambung, central Java, Indonesia
The Island Arc (1998) 7, 223±230
Thematic Article
A jadeite±quartz±glaucophane rock from Karangsambung,
central Java, Indonesia
K. MIYAZAKI1, J. SOPAHELUWAKAN2, I. ZULKARNAIN2
1
AND
K. WAKITA1
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, conglomerate 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±phengite±quartz equilibrium, peak pressure and temperature of the jadeite±quartz±glaucophane rock are P 22 2 kbar and T 530 40 °C. The estimated P±T conditions
indicate that the rock was subducted to ca 80 km depth, and that the overall geothermal
gradient was 7.0 °C/km. This rock type is interpreted to have been generated by the
metamorphism of cold oceanic lithosphere subducted to upper mantle depths. The exhumation 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 metamorphic 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 metamorphic rocks. Before the opening of the Makassar Strait, these complexes may have
constituted a single subduction complex (Hamilton 1979).
The Luk-Ulo Complex (Asikin 1974) of the
Karangsambung area of Central Java is composed of a chaotic mixture of various kinds of
sedimentary, igneous and metamorphic rocks.
Kenter et al. (1976) reported the late Early
Accepted for publication April 1997
Cretaceous foraminifera Orbitolina from limestone 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 pelitic schist was obtained by Kenter et al. (1976).
The ages of sedimentary and metamorphic rocks
are very similar to those of the Bantimala Complex (Wakita et al. 1996).
This paper describes a jadeite±quartz±glaucophane 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 Complex.
224 K. Miyazaki et al.
Fig. 1 Tectonic map
of Central Indonesia
(modi®ed from Wakita et
al. 1994a).
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 ultrabasic rocks, limestone, rhyolite, conglomerate
and metamorphic rocks. Sandstone usually alternates with shale, while chert is often interbedded 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 ophiolite (Suparka 1988) are distributed in the central
part of the complex (Fig. 2). The Tertiary sequence 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 (Natori et al. 1978).
Fig. 2 Geologic map of the Karangsambung area (modi®ed from Wakita
et al. 1984b).
Jadeite±quartz±glaucophane rock, Karangsambung 225
METAMORPHIC ROCKS
JADEITE±QUARTZ±GLAUCOPHANE ROCK
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 ®negrained. Others contain, in addition to the above
minerals, biotite and/or hornblende, and are
slightly more coarse grained. Epidote amphibolite 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 association: 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 samples 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±glaucophane 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 glaucophane, 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 retrograde mineral.
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 sedimentary 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 included within the serpentinite which is distributed between the pelitic schists and the nonmetamorphosed 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), glaucophane-rich domain, and quartz-rich domains
(Fig. 3b). Small quartz inclusions are included in
the jadeite patches (Fig. 4). Areal fraction ratio
( 0.23) of jadeite and quartz in the patches is
close to that ( 0.27) of albite decomposition
reaction; albite jadeite + quartz. Very small
amounts of glaucophane (
Thematic Article
A jadeite±quartz±glaucophane rock from Karangsambung,
central Java, Indonesia
K. MIYAZAKI1, J. SOPAHELUWAKAN2, I. ZULKARNAIN2
1
AND
K. WAKITA1
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, conglomerate 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±phengite±quartz equilibrium, peak pressure and temperature of the jadeite±quartz±glaucophane rock are P 22 2 kbar and T 530 40 °C. The estimated P±T conditions
indicate that the rock was subducted to ca 80 km depth, and that the overall geothermal
gradient was 7.0 °C/km. This rock type is interpreted to have been generated by the
metamorphism of cold oceanic lithosphere subducted to upper mantle depths. The exhumation 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 metamorphic 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 metamorphic rocks. Before the opening of the Makassar Strait, these complexes may have
constituted a single subduction complex (Hamilton 1979).
The Luk-Ulo Complex (Asikin 1974) of the
Karangsambung area of Central Java is composed of a chaotic mixture of various kinds of
sedimentary, igneous and metamorphic rocks.
Kenter et al. (1976) reported the late Early
Accepted for publication April 1997
Cretaceous foraminifera Orbitolina from limestone 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 pelitic schist was obtained by Kenter et al. (1976).
The ages of sedimentary and metamorphic rocks
are very similar to those of the Bantimala Complex (Wakita et al. 1996).
This paper describes a jadeite±quartz±glaucophane 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 Complex.
224 K. Miyazaki et al.
Fig. 1 Tectonic map
of Central Indonesia
(modi®ed from Wakita et
al. 1994a).
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 ultrabasic rocks, limestone, rhyolite, conglomerate
and metamorphic rocks. Sandstone usually alternates with shale, while chert is often interbedded 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 ophiolite (Suparka 1988) are distributed in the central
part of the complex (Fig. 2). The Tertiary sequence 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 (Natori et al. 1978).
Fig. 2 Geologic map of the Karangsambung area (modi®ed from Wakita
et al. 1984b).
Jadeite±quartz±glaucophane rock, Karangsambung 225
METAMORPHIC ROCKS
JADEITE±QUARTZ±GLAUCOPHANE ROCK
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 ®negrained. Others contain, in addition to the above
minerals, biotite and/or hornblende, and are
slightly more coarse grained. Epidote amphibolite 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 association: 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 samples 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±glaucophane 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 glaucophane, 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 retrograde mineral.
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 sedimentary 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 included within the serpentinite which is distributed between the pelitic schists and the nonmetamorphosed 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), glaucophane-rich domain, and quartz-rich domains
(Fig. 3b). Small quartz inclusions are included in
the jadeite patches (Fig. 4). Areal fraction ratio
( 0.23) of jadeite and quartz in the patches is
close to that ( 0.27) of albite decomposition
reaction; albite jadeite + quartz. Very small
amounts of glaucophane (