rutile, titanite, apatite, zircon, opaque, and calcic amphibole + oligoclase (± diopside)-symplectite after pyroxene. The calc-silicate intercalated with marble forms ...
Eur. J. Mineral. 1994,6,995-1000 SHORT NOTE
A coesite inclusion in dolomite in Dabie Shan, China: Petrological and rheological significance HANS-PETER SCHERTL and ARAL I. OKAY*
Research Group on High Pressure Metamorphism, Institut für Mineralogie, Ruhr-Universität Bochum, D-44780 Bochum, Germany
Abstract: Coesite partially inverted to quartz occurs as an inclusion within dolomite in a calc-silicate rock in Dabie Shan, China. The calc-silicate rock consists mainly of dolomite, calcite, quartz, and phengite that is partially replaced by phlogopite, plagioclase, and secondary phengite. There are also minor amounts of epidote, coesite, rutile, titanite, apatite, zircon, opaque, and calcic amphibole + oligoclase (± diopside)-symplectite after pyroxene. The calc-silicate intercalated with marble forms an over 8-m-thick band in felsic gneiss. It comprises calcium-rich eclogite blocks with coesite and diamond inclusions in both, pyroxene and garnet. Radial cracks are absent in dolomite surrounding the coesite/quartz inclusion suggesting that the volume increase associated with the coesite-quartz inversion was largely accommodated by movement along the cleavage planes. The presence of coesite within dolomite in calc-silicate rocks confirms that sedimentary rocks deposited on continental crust can be subducted to depths of over 100 km, and that even a carbonate mineral like dolomite is able to preserve coesite during the process of eduction. Key-words: coesite, dolomite, calc-silicate-rock, ultrahigh-pressure-metamorphism, Dabie Shan.
Introduction Since its initial discovery in continental crustal rocks in the Western Alps (Chopin, 1984), coesite, the high-pressure polymorph of SiO2, has been described from several regional metamorphic terrains as inclusions within garnet, omphacite, kyanite, jadeite, tourmaline, wagnerite, and zircon (e.g. Smith, 1984; Okay et ai, 1989; Wang et al., 1989; Hirajima et ai, 1990; Sobolev & Shatsky, 1990; Reinecke, 1991; Schertl, 1992; Chopin & Sobolev, 1994). Although there is general agreement that coesite, found as inclu
sions in these metamorphic minerals, has formed under nearly static pressure conditions inside its stability field, the areal extent of ultrahigh-pressure- (UHP) metamorphism is much more con troversial. Coesite is generally found in eclogites which make up only a very small but ubiquitous fraction of the metamorphic terrain dominated by petrologically uninformative gneisses. Even in the case of the UHP-metamorphic pyrope-quartzite rocks of the Dora-Maira massif there is up to now no unequivocal petrological evidence that the external country rock gneiss has also ex perienced the UHP-metamorphism.
*Present address: ITÜ Maden Fakültesi, Jeoloji Bölümü, Ayazaga, Istanbul, Turkey. 0935-1221/94/0006-0995 $ 1.50 © 1994 E. Schweizerbarfsche Verlagsbuchhandlung. D-70176 Stuttgart
996
H.-P. Schertl, A. I. Okay
Marble, consisting dominantly of calcite (± dolomite), forms a minor constituent in several UHP terrains. Here, we report for the first time coesite as an inclusion in dolomite from a calcsilicate rock from Dabie Shan in China, which provides evidence that the calc-silicate rock and a s s o c i a t e d marble have u n d e r g o n e UHP metamorphism. It also shows that a common car bonate mineral like dolomite can preserve coesite inclusions and thus can act as a "high-pressure container".
Geological background Dabie Shan occurs in the eastern part of the Quinling orogen, which was formed during the Triassic collision of the Sino-Korean and Yangtze blocks (e.g. Mattauer et al., 1985; Hsü et al., 1987). It is dominantly a gneiss-granite terrain and comprises several tectonic slices, one of which is the eclogite zone with the UHProcks. The eclogite zone is sandwiched between two amphibolite-facies tectonic slices (Okay et al., 1993) and comprises medium-grained, felsic gneiss with eclogite and impure marble bands and boudins. The marble layers that range in thickness from 10 cm to 10 m contain eclogite blocks that may represent pre-metamorphic marly layers in limestone. Such eclogites are characterised by grossular-rich garnets and diopside-rich pyroxenes (Okay, 1993; Wang & Liou, 1993). Coesite and diamond are best preserved in these calcium-rich eclogites as inclusions within garnet and pyroxene (Xu et al., 1992). Similar coesite-bearing eclogites were also recently de scribed as boudins in marble from Mali in Africa (Caby, 1994).
Petrography and mineral chemistry The outcrop with coesite in dolomite is a small road-side marble quarry 4.6 km southeast of Wumiao (sample 230 on Fig. 2 of Okay, 1993). The 8-m-high quarry wall exposes 0.3-1 m thick marble and calc-silicate bands with 3-60 cm large eclogite blocks. The eclogite blocks from this outcrop contain coesite and diamond inclusions in both, pyroxene and garnet (Okay, 1993). The sample with coesite in dolomite (230C)
comes from a calc-silicate band. The rock is me dium-grained with a grain-size range of 0.2 to 2 mm and shows a granoblastic texture. It consists mainly of calcite (~ 25 modal percent), dolomite (21 %), quartz (16 %), phlogopite (13 %), plagioclase (9 %), phengite (7 %), epidote with allanite cores (3 %), and calcic amphibole (3 %). Coesite, diopside, apatite, titanite, rutile, zircon, mag netite, pyrite, and chalcopyrite occur individually in amounts of less than one modal percent. The inferred peak UHP-mineral assemblage in the rock was dolomite + aragonite + coesite + phen gite + epidote/allanite + clinopyroxene + rutile + opaque. All aragonite must have reacted to cal cite which generally contains appreciable magne sium and iron and has an average composition of (Ca89-9iMg7-8Fe2-3; Table 1). A few dolomite crystals contain calcite inclusions which are con siderably poorer in magnesium and iron (Caçç) than the calcite in the matrix (Table 1). These calcite inclusions may represent aragonite that were converted to calcite later in the metamorphic history of the rock in contrast to the matrix aragonites that have equilibrated earlier with the ferromagnesian minerals. Dolomite has an aver age composition of Ca47-52Mg38-45Fe7-9∙ Phengite with a maximum Si-content of 3.60 per formula unit (p.f.u.) is largely pseudomorphed by green and brown phlogopite, plagioclase (albites5-75), and secondary phengite with minimum Si-con tents of 3.31 p.f.u.. The brown phlogopite is char acterised by higher amounts of TiO2 (up to 2.7 wt.%) in comparison with the green variety (TiO2 ~ 0.2 wt.%), whereas the Mg/(Mg + Fe) ratio in both types is similar and ranges between 0.72 and 0.77. Fine-grained symplectite of calcic amphi bole with variable composition (Table 1) and plagioclase (albite -so) represent former clinopy roxene. A fine-grained secondary diopside occurs in the outermost zone of one of these symplectites. Epidote grains with Fe 3+ /(Fe 3+ + Al) ratios of about 0.15 contain allanite cores with Ce and minor La as shown by the energy-dispersive sys tem of the electron microprobe. The allanite cores also contain ~ 1.3 wt.% MgO (Table 1). Titanite with up to 3.7 wt.% AI2O3 and rutile occur as individual crystals with no reaction re lationships. Electron microprobe analysis of coesite indicates pure SiO2. Coesite/quartz textures Coesite, partially inverted to quartz, occurs as a 0.2 mm large inclusion within a 1.1 mm large
Table 1. Representative microprobe analyses of minerals from the coesite-bearing calc-silicate (sample 230C; n.d. = not determined).
dolomite
calcite
pheng i t e
(inclusion in
dolomite)
amphibole
biotite
(coarse
(fine
grained)
grained)
(green)
(brown)
pyroxene
titanite
epidote
plagioclase
coesite
rutile
sio2
0.07
0.01
0.02
53.46
50.06
39.60
36.91
46.44
50.73
52.37
63.60
37.38
29.41
100.13
0.02
TiO2
0.01
0.00
0.06
0.31
0.41
0.11
2.10
0.21
0.16
0.01
0.04
27.58
35.47
0.00
98.89
A 1
0.01
0.02
0.01
23.09
25.83
15.78
16.06
12.18
5.00
2.13
22.83
0.26
2.69
0.00
0.06
-
-
-
-
5.37
2.46
2.43
0.09
8.00
0,33
-
-
2°3
Fe2O3
-
-
-
FβO
6.07
1.55
0.17
1.44
1.85
8.94
10.59
6.25
6.36
3.63
-
-
-
0.00
0.49
MnO
0.12
0.19
0.18
0.00
0.01
0.02
0.00
0.10
0.07
0.07
0.02
0.09
0.05
0.01
0.00
MgO
23.00
3.44
0.22
4.90
4.08
20.24
16.91
13.00
16.85
13.46
0.01
0.12
0.02
0.00
0.00
CaO
33.11
54.34
58.77
0.00
0.00
0.13
0.17
9.31
11.97
23.81
3.23
23.70
29.72
0.03
0.00
Nâ2O
0.00
0.00
0.00
0.17
0.20
0.04
0.09
2.70
0.73
1.38
10.05
0.00
0.05
0.00
0.00
κ2o
0.00
0.02
0.01
10.12
10.76
9.97
10.01
0.70
0.35
0.00
0.14
0.02
0.00
0.01
0.00
F
n.d.
n.d.
n.d.
0.00
0.00
n.d.
0.25
0.03
0.01
0.00
0.00
0.00
0.57
n.d.
0.00
Σ
62.4 0
59.57
59.42
93.49
93.20
94.84
93.09
96.29
94.69
99.05
100.00
96.39
98,31
100.18
99.46
⅜at.
1
1
1
o*
11
11
11
11
0*
23
23
0*
6
8
0*
0*
12.5
⅜at
3
⅞a t .
1
1
Ca
0.473
0.898
0.990
Si
3.601
3.419
2.876
2.780
Si
6.732
7.403
Si
1.938
Si
2.811
Si
2.946
Si
0.964
Si
1.000
0.000
Mg
0.457
0.079
0.005
Al
0,399
0.581
1.124
1.220
Al
1.268
0.597
Al
0.062
Al
1.189
Al
0.054
Al
0.104
Al
0.000
0.001
Fe
0.068
0.020
0.002
Ti
0.874
Ti
0.000
0.993
Mn
0.001
0.003
0.002
Fe3 +
0.008
Fe
0.000
0.006 0.000
Fe3
Σ
4.000
4.000
4.000
4.000
Σ
8.000
8.000
Σ
2.000
Si
0.001
0.000
0.000
Ti
0.000
0,000
0.001
Al
1.434
1.498
0.228
0.205
Al
0.813
0.263
Al
0.032
Al
0.000
0.000
0.000
Ti
0.016
0.021
0.006
0.119
Ti
0.023
0.017
Ti
0.000
Na
0.000
0.000
0.000
Fe
0.081
0.106
0.543
0.667
Fe3+
0,586
0.270
Fe3 +
0.068
K
0.000
0.000
0.000
Mn
0.000
0.001
0.001
0.000
Fe2+
0.758
0.777
Fe2
Mg
0.492
0.416
2.191
1.898
Mn
0.012
0.008
Mg
2.809
3.664
Σ
2.023
2.042
2.969
2.889
Ca
0.000
0.000
0.010
0,014
Na
0.022
0.027
0.006
0.013
Ca
1.466
1.883
R
0.870
0.937
0.924
0.962
Na
0.770
0.208
K
0.130
0.065
Σ
2.366
2.156
Σ
1.000
1.000
1.000
Σ
Σ
0.892
0.964
0.940
5.001
4.999
0.989
F
0.000
0.000
0.059
OH
2,000
2.000
1.941
+
0.003
Mn
0.001
Mg
0.001
Σ
4.005
0.112
Ca
0.153
Mn
0.002
Na
0.861
Mg
0.743
K
0.008
Ca
0.944
Na
0.099
K
0.000
Σ
2.000
+
Σ
1.022
Σ
3.000
Mn
0.001
Mn
0.000
Al
2.507
Mg
0.001
Mg
0.000
0.000
Fe3+
0.475
Ca
1.044
Ca
0.000
0.000
Ti
0.015
Na
0.003
Na
0.000
0.000
K
0.000
K
0.000
0.000
Σ
2.999
Σ
1.000
1.000
F
0.059
Σ
2.997
Mn
0.006
Mg
0.014
Ca
2.001
Na
0.001
K
0.002
Σ
2.024
1
The analyses were performed on a CAMEBAX electron microprobe (acceleration voltage 15 kV, beam current 15 nA). Most analyses were obtained using a defocussed beam (~ 5 µm). Standards used were pyrope (Si, Al, Mg), andradite-glass (Ca, Fe), rutile (Ti), spessartine (Mn), K-glass (K), jadeite (Na), and topaz (F). O* : anhydrous oxygen basis.
998
H.-P. Schertl, A. I. Okay
Fig. l. Photomicrograph (plane polarised light) showing the coesite inclusion in dolomite (dol = dolomite, cc = calcite, coe = coesite, qz = quartz, ap = apatite); notice the cleavage {1011} and the prominent twinning plane {0112}.
dolomite crystal (Fig. 1 and 2). In thin section coesite is easily recognisable from its higher re fractive index against the surrounding quartz. Under crossed polars the inverted quartz shows a feathery texture (Fig. 2b); these palisade-like, often extremely fine-grained quartz crystals are orientated radially around the relic coesite. These textural characteristics of partly paramorphosed coe sites are well known in UHP-metamorphic rocks from other localities (e.g. Chopin, 1984, Fig. 3; Gillet et al., 1984, Fig. 1). Another re markable feature of quartz and coesite is revealed under the cathodoluminescence-microscope: the relic coesite exhibits a bluish luminescence colour, which is identical to that of coesite crys tals of the Dora-Maira massif (Schertl, 1992), while the surrounding quartz is characterised by a reddish-violet luminescence colour. In UHP-metamorphic rocks, the host silicates that surround the coesite show radial cracks around these inclusions formed as a result of stress caused by the volume increase during the coesite-quartz inversion (e.g. Chopin, 1984, Fig. 3). In contrast, the coesite investigated here is enclosed in a dolomite that shows none of these prominent radial cracks. The fine "cracks" around the coesite in dolomite, some of which terminate after some distance from the inclusion are parallel to the {1011} cleavage planes in
dolomite (Fig. 1 and 2a). The volume expansion during the coesite-quartz inversion was probably largely accomodated by movement along these cleavage planes. The dolomite host also shows prominent twin lamellae on {0112} (cf. Fig 2b) which represent a common dolomite deforma tion-twinning (Barber, 1977). The {0112} twin ning is not confined to the dolomite with coesite but occurs in nearly all the other dolomite crys tals in the section indicating that it is not related to the coesite-quartz inversion.
Conclusions The presence of coesite inclusion in dolomite indicates that, in contrast to calcite, the assem blage dolomite + coesite is stable at pressures over 28 kbar and at temperatures of 800 ± 50°C, which is the estimated temperature during the UHP-metamorphism in Dabie Shan (Okay, 1993). This is in agreement with the experimen tal work on the high-pressure stability of dolomite (e.g. Kraft et al., 1991; Ross & Reeder, 1992). The dolomite + coesite + clinopyroxene paragenesis buffers the CO2-activity through the reaction: 1 dolomite + 2 coesite = 1 diopside + 2 CO2.
999
Coesite in dolomite
Fig. 2a. Detailed view of the coesite inclusion in dolomite (photomicrograph in plane polarised light). The coesite (coe, higher relief) is partly inverted to quartz (qz, lower relief). The cleavage-planes {1011} are indicated by dashed lines.
Fig. 2b. Same photomicrograph in crossed polars showing feathery, palisade-like replacement texture of quartz after coesite. The twin-planes {0112} are marked.
This reaction, calculated using the hostdolomite composition and an omphacite with a diopside activity of 0.5 suggests a CO2-activity of about 0.2 at the PT-conditions of the UHPmetamorphism in Dabie Shan, using the thermodynamic data set of Berman (1988) and the GeO-Calc programme of Brown et al. (1988). The volume increase during the coesite-quartz inversion was largely accommodated by move ment along the cleavage planes in dolomite which may explain the lack of radial fractures around the inclusion. The presence of coesite in dolomite in calc-silicates also confirms that, during orogeny, sedimentary rocks deposited on the continental crust can be subducted to depths of over 100 km and provides positive evidence that marble layers as well as the eclogites in
Dabie Shan have undergone the UHP metamorphism. Acknowledgements: We thank Werner Schreyer for helpful comments on the manuscript. The authors are also grateful to Olaf Medenbach for the universal-stage measurements. The second author thanks the Alexander von Humboldt Foundation for a supporting grant. Critical re views and suggestions by J. Ingrin and N. Mancktelow have improved the manuscript.
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H.-P. Schertl, A. I. Okay
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Received 5 April 1994 Accepted 8 July 1994
