Full text of DOI:10.1007/BF02901171

NOTES
logite zone was considered as high-pressure “cold” ec-
logite zone according to Okay[8] because there was not
coesite to have been found. However, a gneissic sample
collected in this zone has a plenty of coesite inclusions in
zircons. Thus, we think that the glaucophane eclogites
zone is also UHP zone with lower metamorphic tempera-
ture conditions[10], although more works will still be
needed. The rocks in the UHP zone consist of granitic to
tonalitic orthogneisses, calc-silicate gneiss, marble,
metapelite, jadeite quartzite, eclogite and ultramafic rocks.
We collected granitic to tonalitic orthogneisses as well as
mica schist interlayers among these gneisses. The sample
localities are marked in fig. 1, and the rock types are
shown in table 1.
Coesite inclusions in zircon
from gneisses identified by la-
ser Raman microspectrometer
in ultra-high pressure zone of
Dabie Mountains, China
LIU Jingbo1, YE Kai1, CONG Bolin1,
Maruyama Shegnori2 & FAN Hongrui1
1. Institute of Geology and Geophysics, Chinese Academy of Sciences,
Beijing 100029, China;
2. Department of Earth and Planetary Sciences, Tokyo Institute of Tech-
nology, 2-12-1 Ookayama, Meguro Tokyo, 152, Japan
2
Sample preparation and analytical technique
Zircons were separated from samples of 0.5 to sev-
Abstract
The mineral inclusions in zircon from gneisses
eral kilograms. 40ü200 zircon grains were mounted in
the petroepoxy, then polished to about half of grains. The
inclusion minerals in zircon are identified with a Jasco
NRS Laser Raman microspectrometer at the Department
of Earth and Planetary Sciences of Tokyo Institute of
Technology. The spectrometer has an Ar-ion laser with
514.5 mm line at 20 mW. Spectra were measured with 2ü
10 s counting time and 0.5 wave number steps, and the
spot size of analysis is about 1.5 ȝm.
in ultra-high pressure (UHP) zone of the Dabie Mountains
were identified by using a laser Raman microspectrometer.
Coesite occurs as inclusions in zircons from all types of
gneiss. Other important minerals, such as jadeite, omphacite,
aragonite, barite, and anhydrite were also found as inclusion
minerals. These discoveries indicate that (ν) gneissic coun-
try rocks had metamorphosed at the same time as the en-
closed eclogites; and (ξ)
ꢀ2 -bearing fluids were present
SO4
in the UHP metamorphic process, which is manifested by
occurrence of barite and anhydrite coexisting with coesite.
Zircon is a unique mineral on the research of UHP
metamorphism. It is well known that zircon, as a me-
chanically strong mineral, is an excellent container for
coesite preservation[1,2]. The identification of coesite in
zircon[3,4] has made the debate on metamorphic relation-
ship between the UHP eclogites and their country gneisses
ü
7]
in the Dabie Mountains and the Sulu region[5
to the
light. However, it is necessary for a systematic research on
all type of gneisses in these regions. We collected gneissic
samples from so-called “cold” eclogite to “hot” eclogite
zones along approximately south-north profile in the
Dabie Mountains[8,9]. The inclusion minerals in zircons of
samples are carefully identified by using the laser Raman
technique. Our results will provide basic data for inter-
pretation on the relationship between the UHP eclogites
and their country gneisses.
1
Geologic setting and sampling
The UHP zone of the Dabie Mountains is a part of
orogen formed in the Triassic by the subduction of Yang-
tze craton beneath the North China craton. The orogen
comprises a succession of metamorphic zones from north
to south characterized by increasing metamorphic degree
from transitional blueschist-greenschist, through epidote
amphibolite, glaucophane eclogite to coesite eclogite.
Glaucophane eclogite-bearing zone south of coesite ec-
Fig. 1. Geological map of Dabie Mountains (simplified after Okay,
1993). 1, Paleozoic-recent unmetamorphosed sediments; 2, granitoid
plutons; 3, northern Dabie Complex; 4, hot eclogite terrain; 5, cold ec-
logite terrain; 6, Susong Complex; 7, coesite-bearing samples; 8, coe-
site-free samples.
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Chinese Science Bulletin Vol. 46 No. 22 November 2001
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NOTES
Fig. 2. Microscopic photos showing coesite inclusions in zircon. Zircon sizes in photos (a)ü(e) are about 145, 260, 250, 120 and 250
ȝm along the long axis, respectively. anh, Anhydrite; ana: anatase; ap, apatite; coe, coesite; phen, phengite; qz, quartz.
bands of coesite and quartz in the spectra of same inclu-
3
Results
sion (fig. 3). The sample 99db-22 was collected from
“cold” eclogite zone defined by Okay (1993) (fig. 1). The
occurrence of coesite inclusions in zircon implies that
glaucophane eclogite zone had also suffered the UHP
metamorphism (fig. 2).
The identified inclusion minerals in zircon are shown
in table 1. Some typical microscopic photos and spectra of
inclusion minerals are shown in figs. 2 and 3. The inclu-
sion minerals include coesite, quartz, clinopyroxene, rutile,
anatase, apatite, sphene, aragonite, calcite, barite, anhy-
drite, white mica (phengite, muscovite and paragonite),
feldspar and garnet.
Clinopyroxene has been identified in zircon of 8
samples. Jadeitic pyroxene is present in samples 99db-187
and 99db-22 (fig. 3), and omphacite inclusions occur in
zircons of other samples. Apatite is ubiquitous in zircon.
Rutile, white mica and sphene are also common in zircon.
Anatase was identified in three samples (99db-19, 99db-
146 and 99db-98). Aragonite and calcite are present in
zircon of 99db-112 (figs. 2 and 3). Feldspar inclusions
occur in zircons of most of the samples, and it is impossi-
ble to discriminate them as Na-rich plagioclase or K-
feldspar on the basis of Raman spectrum. Garnet is very
rare in the zircons that we studied (99db-130). The inter-
esting and important discovery is to identify barite and
anhydrite in some samples (99db-95, 99db-130, 99db-187,
99db-161, 99db-17) (figs. 2 and 3, and table 1). The pres-
ence of two minerals indicates that there existed the sulfu-
ric acid-bearing fluids in the metamorphic process, even
in the UHP stage.
Coesite inclusions in zircon were found in seven of
fifteen samples. Zircons that contains coesite inclusions
are usually multi-faceted, short prismatic or equate, and
larger than 200 ȝm in size (table 1). Zircons in some sam-
ples (99db-112, 99db-130) are fine-grained, and coesite
inclusions are present but very rare. Coesite has not been
found as inclusions in zircons of other 8 samples, and zir-
cons in those samples are usually fine-grained, long pris-
matic or equate and evidently smaller than coesite-bearing
zircons (usually ˘120 ȝm for equate zircon and ˘200
ȝm for long prismatic zircon along long axis (table 1).
Coesite is a common inclusion mineral in zircon in some
samples (Samples: 99db-19, 99db-22, 99db-95, 99db-161
and 99db-187), and 10%ü30% zircon grains checked
have coesite inclusions. Coesite was often partly trans-
formed towards quartz as manifested by coexistence of
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Chinese Science Bulletin Vol. 46 No. 22 November 2001

Fig. 3. Representative laser Raman spectra of inclusion coesite (Coe), aragonite (A), jadeite (Jd), barite (B) and anhydrite (Anh). Zr: Zircon.
coesite inclusions of zircon display different degrees of
4
Discussion
Zircon has played a key role in solving the debate on
transformation to quartz, the preservation of coesite in
zircon is an important factor whether or not coesite can be
identified. The grain size of zircon seems to play a role in
preservation of coesite according to the present observa-
tion. It is a common phenomenon that transformation
from coesite to quartz ruptured the host zircon, whereas
the size of zircon probably is a parameter that controls the
fracturing of zircon. Fine-grained zircon is easily frac-
tured by the internal pressure of inclusion yielded from
transformation from coesite to quartz, but coarse-grained
zircon is more difficult. As the fracture is produced, coe-
site will hardly be preserved. On the other hand, the ca-
thodoluminescence images of zircons display that zircons
the relationship between the UHP eclogites and their
country gneisses in the Dabie and Sulu regions[3, 4]. We
have found that all types of gneisses have coesite-bearing
zircons, which means that eclogites and their country
gneisses had suffered the same coeval metamorphic his-
tory. However, coesite is not present as inclusions in zir-
con of each sample. We do not think this case as evidence
supporting “foreign” model[5, 6] since samples as the same
type of gneiss either have coesite inclusions in zircon or
not. This implies that other factors are responsible for the
presence of coesite inclusions of zircon. Since most of
Chinese Science Bulletin Vol. 46 No. 22 November 2001
1915

NOTES
als, Daklady Akademii Nauk, USSR, 1994, 334: 488.
2. Chopin, C., Sobolev, N. V., Principal mineralogic indicators of
UHP in crustal rocks (eds. Coleman, R. G., Wang, X. M.), Ultra-
high Pressure Metamorphism, New York: Cambridge University
Press, 1995, 96ü131.
3. Tabata, H., Yamauchi, K., Maruyama, S. et al., Tracing the extent
of a UHP metamorphic terrane: Mineral-inclusion study of zircons
in gneisses from the Dabie Shan (eds. Hacker, B. R., Liou, J. G),
When Continent Collide: Geodynamics and Geochemistry of Ul-
tra-high-pressure Rocks, Dordrecht: Kluwer Academic Publishers,
1998, 261ü274.
had the complicated history of recrystallization and
growth, and there are distinct differences in different sam-
ples. Zircons of some samples show previously igneous
zircon cores with oscillatory zones and less metamorphic
overgrowth rims, whereas zircons of other samples had
multi-growth metamorphic zones with or without previous
igneous zircon cores. Thus, it is possible that zircons of
some samples either formed after the UHP stage or did not
grow during the UHP stage. If so, coesite will be impossi-
bly found in zircons of these samples.
4. Ye, K., Yao, Y., Katayama, I. et al., Areal extent of ultra-high
pressure metamorphism in the Sulu terrane of east China: evi-
dence from coesite inclusions in zircon from country rock granitic
gneiss, Lithos., 2000, 52: 157.
5
Conclusions
5. Cong, B. L., Zhai, M. G., Carswell, D. A. et al., Petrogenesis of
ultrahigh-pressure rocks and their country rocks at Shuanghe in
Dabieshan, central China, European Journal of Mineralogy, 1995,
7: 119.
6. Zhao, Z. Y., Wang, Q. C., Cong, B. L., Coesite-bearing ultrahigh
pressure metamorphic rocks from Donghai, northern Jiangsu
Province, eastern China: “Foreign” or “In Situ”? Scientia Ge-
ologica Sinica, 1992, 1(1): 43.
7. Wang, X. M., Liou, J. G., Regional ultrahigh-pressure coe-
site-bearing eclogitic terrane in central china: evidence from
country rocks, gneiss, marble and metapelite, Geology, 1991, 19:
933.
8. Okay, A. I., Petrology of a diamond and coesite-bearing meta-
morphic terrain: Dabie Shan, China; European Journal of Miner-
alogy, 1993, 5: 1.
9. Okay, A. I., Sengor, A. M. C., Satir, M., Tectonics of an ultra-
high-pressure metamorphic terrane: the Dabie Shan/TongBai shan
Orogen, China, Tectonics, 1993, 12(6): 1320.
10. Wang, X. M., Liou, J. G., Maruyama, S., Coesite-bearing eclogites
from the Dabie Mountains, central China: Petrogenesis, P-T paths,
and implications for regional tectonics, Journal of Geology, 1992,
100: 231.
Zircon from all type of gneisses of UHP zone in the
Dabie Mountains is a good witness of UHP metamorphic
event. Coesite, jadeite, omphacite, aragonite, etc. were
identified as inclusions in zircons. Barite and anhydrite
that formed in fluid-related circumstance were identified
-2
in zircon, which indicates that SO4 -bearing fluids were
present. The presence of the “cold” eclogite zone in the
Dabie Mountains was suspected due to the discovery of
coesite inclusion in zircon of gneiss from “cold” eclogite
zone.
Acknowledgements We are grateful to Katayama Ikou for his help in
the laser Raman analysis and sample preparation. This work was sup-
ported by the National Natural Science Foundation of China (Grant No.
49972029) and the Ministry of Science and Technology of China (Grant
No. G1999075501).
References
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container of inclusions of diamond, coesite and coexisting miner-
(Received March 26, 2001)
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