Petrology and P–T history of the Wutai amphibolites: implications for tectonic evolution of the Wutai Complex, China

Precambrian Research 93 (1999) 181–199

Petrology and P–T history of the Wutai amphibolites: implications for tectonic evolution of the Wutai Complex, China Guochun Zhao a,b,*, Peter Cawood a, Liangzhao Lu b a Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO U1987, 6001, Perth, W.A., Australia b Department of Geology, Changchun University of Science and Technology, Changchun, 130061, China Received 30 January 1998; accepted 14 August 1998

Abstract The Wutai Complex represents the best preserved granite-greenstone terrane in the North China Craton. The complex comprises a sequence of metamorphosed ultramafic to felsic volcanic rocks, variably deformed granitoid rocks, along with lesser amounts of siliciclastic and carbonate rocks and banded iron formations. Petrological evidence from the Wutai amphibolites indicates four metamorphic evolutionary stages. The M assemblage is composed of 1 plagioclase+quartz+actinolite+chlorite+epidote+biotite+rutile, preserved as mineral inclusions in garnet porphyroblasts. The metamorphic conditions for this assemblage cannot be quantitatively estimated. The M stage is 2 represented by garnet porphyroblasts in a matrix of quartz, plagioclase, amphibole, biotite, rutile and ilmenite. P–T conditions for this assemblage have been estimated using the program T at 10–12 kbar and 600–650°C. The M assemblage is shown by amphibole+plagioclase±ilmenite symplectic coronas around embayed garnets and yields 3 P–T conditions of 6.0–7.0 kbar and 600–650°C. M is represented by chlorite and epidote rimming garnet, chlorite 4 rimming amphibole and epidote replacing plagioclase under greenschist-facies conditions of 400–500°C and relatively lower pressures. Taken together, the qualitative P–T estimates from M and M and the quantitative P–T estimates 1 4 from M and M define a clockwise P–T path for the Wutai amphibolites. 2 3 The estimated P–T path from the four stages suggests that the Wutai Complex underwent initial burial and crustal thickening (M +M ), subsequent isothermal exhumation (M ), and finally cooling and retrogression (M ). This 1 2 3 4 tectonothermal path, along with those of the Fuping and Hengshan complexes, which bound the southeast and northwest margins, respectively, of the Wutai Complex, is considered to record the early Paleoproterozoic collision between the eastern and western segments of the North China craton. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Amphibolite facies; Continental collision; Decompression reactions; Deformation; Greenschist facies; Metamorphism

1. Introduction Amphibolite is a common lithological unit within the internal, higher grade zones of most * Corresponding author. Tel.: +61 8 9266 3421; fax: +61 8 9266 3153; e.mail: [email protected]

orogenic belts. Determination of the conditions of metamorphism within these zones, especially P–T–t paths, can provide valuable information about the tectonic processes which accompanied metamorphism and orogenesis. Most metamorphic reactions observed in amphibolites are continuous; the mineral assemblages change little over a wide

0301-9268/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0 3 0 1 -9 2 6 8 ( 9 8 ) 0 0 09 0 - 4

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P–T range ( Thompson et al., 1982), and until recently, there were no adequate solution models for amphiboles, making it relatively difficult to quantitatively deduce P–T conditions using conventional thermobarometry, compared with metapelites and mafic granulites. Recently, however, several new thermobarometric techniques based on internally consistent thermodynamic databases (e.g. Refs. Powell and Holland, 1988; Berman, 1991; Gordon, 1992; Gordon et al., 1994; Gottschalk, 1997; Holland and Powell, 1998) and new solution models for amphiboles (Ma¨der and Berman, 1992; Ma¨der et al., 1994) have been developed and these can be applied for estimating P–T conditions of amphibolites and other rocks. In this study, we examine metamorphic textures and mineral assemblages in order to establish the metamorphic evolution and we use the T technique (Berman, 1991) to obtain P–T information from the amphibolitic rocks of the Wutai Greenstone Belt of the North China craton and outline the implications for the tectonothermal processes within this segment of the craton.

2. Regional setting The North China craton consists of an Archean to Paleoproterozoic assemblage of orthogneisses, paragneisses, granitoid rocks, ultramafic to felsic volcanic and intrusive rocks and minor siliciclastic and carbonate sedimentary rock units (Bai, 1986; Li et al., 1990; Tian, 1991; Bai et al., 1992; Wang et al., 1996). The Wutai Complex is located some 300 km SW of Beijing ( Fig. 1). The complex consists of supracrustal rocks and granitoid rocks. The supracrustal rocks comprise a lower sequence of amphibolites, paragneisses, banded iron formations, calc-silicate rocks and marbles, metamorphosed to amphibolite facies, overlain by a middle sequence of greenschist and an upper sequence of clastic sedimentary rocks metamorphosed to lower to upper greenschist facies (Fig. 2). The granitoid rocks include tonalites, trondhjemites, granodiorites and granites. The Wutai Complex defines a NE–SW trending belt and the metamorphic grade increases towards the contacts with the adjacent Fuping and Hengshan blocks to the SE and NW,

respectively (Fig. 2). The Hengshan and Fuping blocks consist of a variety of grey granitoid gneisses, amphibolites, mafic granulites and metasedimentary rocks in granulite facies. The Wutai Complex is considered to unconformably overlie the Fuping and Hengshan blocks (Bai, 1986; Tian, 1991). The regional structure of the Wutai Complex is characterised by a large composite ‘synform’ ( Fig. 3) which developed during four main deformational episodes (Bai, 1986; Ma et al., 1987; Tian, 1991). Structural features recognised for D are small rootless intrafolial folds ( F ) and an 1 1 associated early foliation (S ). D is the dominant 1 2 deformation phase in the Wutai Complex and is represented by a series of ubiquitous isoclinal, recumbent folds ( F ) of varying scale accompanied 2 by a penetrative foliation (S ). The composite 2 ‘synform’ of the Wutai Complex is a regionalscale F fold, superimposed by a series of F folds 2 3 ( Fig. 3). D is characterized by asymmetric upright 3 folds ( F ) and associated cleavage (S ). D is 3 3 4 represented by open and symmetric folds ( F ), the 4 amplitude of which ranges from 5.0 cm up to 10 m, and an axial-planar fracture cleavage (S ). 4 Controversy has surrounded the age of the Wutai Complex. Some workers believe the Wutai Complex marks the base of the Proterozoic in China (Ma et al., 1987; Tian, 1991), whereas others consider it to be Archean in age (Bai, 1986; Wang and Bai, 1986; Xu et al., 1991,Sun et al., 1992). Recently, Wilde et al. (1997) have obtained S U–Pb ages of between 2524±8 and 2553±8 Ma from the igneous zircons in the felsic volcanic rocks from the middle sequence of Wutai Complex and associated granitoid rocks, confirming an Archean age for the lower and middle parts of the Wutai Complex.

3. Wutai amphibolites The Wutai amphibolites are exposed towards the inferred base of the Wutai Complex (see Figs. 2 and 3) and are associated with ultramafic rocks, garnet pyroxenites (plagioclase-free), felsic paragneisses, mica-schists, banded iron formations (BIF ), calc-silicate rocks and marbles.

G. Zhao et al. / Precambrian Research 93 (1999) 181–199

183

Fig. 1. Distribution of Early Precambrian rocks in the Sino–Korean Craton (revised after Jahn, 1990).

Based on field and petrographic evidence, the following types of amphibolite have been recognised: (1) schistose amphibolites, with a hornblende+plagioclase+quartz±garnet±biotite ±epidote assemblage; (2) massive amphibolites, with a hornblende+plagioclase+quartz±garnet assemblage; (3) hornblende-plagioclase gneisses, with a hornblende+plagioclase+quartz±biotite ±garnet assemblage; and (4) amphibole-mica schists, with a hornblende+plagioclase+muscovite+biotite+quartz±garnet±epidote assemblage. Types 1 and 2 are the dominant amphibolites in the Wutai Complex, commonly intercalated with felsic paragneisses or calc-silicate rocks, whereas types 3 and 4 occur only as thin sheets within felsic paragneisses. On the basis of geochemical characteristics, the protoliths of the amphibolites have previously been assigned to igneous (type 1 and 2) and pyroclastic (type 3 and 4) origins (Li et al., 1986). A recent geochemical study, however, suggested that all four types of amphibolites are derived from tholeiitic basalt which formed in an island arc environment (Dang and Zhao, 1993). The Wutai amphibolites can also be divided into garnet-bearing and garnet-free assemblages. The spatial distribution of garnet-bearing and garnet-

free amphibolites is irregular and in some cases the two types occur in the same outcrop. This implies that the presence of garnet is controlled not only by P–T conditions, but also by bulk chemical compositions. The Wutai amphibolites locally preserve various primary igneous fabrics including ophitic and porphyritic textures, suggesting a volcanic or subvolcanic origin for the precursor of the Wutai amphibolites.

4. Metamorphic stages On the basis of microstructural relations, four separate metamorphic stages have been recognised from the Wutai amphibolites. The detailed chronological relationships between these stages, the metamorphic crystallization and the different deformational episodes are shown in Fig. 4. The M assemblage is represented by mineral 1 inclusions in garnet porphyroblasts. In most cases, the M minerals are randomly oriented, but in a 1 few cases, they define the S foliation (S =S ), i i 1 outlined by oriented chlorite, biotite and deformed quartz. S is oblique to the matrix mineral foliation i

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Fig. 2. Geological sketch map of the Wutai Complex (revised after Bai et al., 1992).

S . M is the oldest metamorphic episode preserved 2 1 in the Wutai amphibolites, and predates the growth of porphyroblasts and the D phase of deforma2 tion. Thus, we ascribe the M metamorphic fabric 1 to the D episode. According to the S preserved 1 i in the innermost domains of the large garnet porphyroblasts, the most complete mineral assemblage that can be identified for the M stage 1 includes plagioclase+quartz+actinolite+chlorite +epidote+biotite±rutile±opaque minerals. The mineral inclusions are typically 0.05–0.1 mm in diameter whereas the host garnet is 1–3 mm. Plagioclase and quartz are the dominant mineral

inclusions in garnets. Chlorite occurs both as single flakes and in composite inclusions. Epidote appears either as single inclusions or locally associated with chlorite and quartz. Rare rutile only occurs in the core region of garnet porphyroblasts. The M stage represents the growth of coarse 2 garnet porphyroblasts and matrix minerals of amphibole+plagioclase+quartz+biotite±clinopyroxene±rutile±ilmenite. Their development is simultaneous with the deformation D . Amphibole 2 and biotite in the M assemblage define the 2 regional schistosity (S ). Amphibole occurs as sub2 hedral grains (0.5–1.5 mm), and ranges modally

G. Zhao et al. / Precambrian Research 93 (1999) 181–199

185

Fig. 3. Schematic cross-section of the Wutai Complex (revised after Tian, 1991). The location of the cross-section is shown in Fig. 2.

to An . Biotite commonly appears in minor 20 amounts. The M assemblage is composed of 3 amphibole+plagioclase [Fig. 5(a)], ilmenite+ plagioclase [Fig. 5(b)] and ilmenite+ plagioclase+amphibole symplectic coronas [Fig. 5(c)] around the embayed garnets. Symplectic amphibole occurs as fine subhedral grains, intergrown with symplectic plagioclase. In some cases, amphibole+plagioclase and ilmenite+plagioclase symplectites completely replace garnet grains. These assemblages suggest that the following generalized reactions have taken place: garnet (core)+plagioclase (matrix) Fig. 4. Mineral crystallization-deformation diagram for the Wutai amphibolites. Symbols are: i, inclusions; m, matrix; m-c, matrix (rim) p-c, porphyroblasts (core); p-r, porphyroblasts (rim); r, rim; re, retrogressive minerals; s, symplectites. Mineral symbols are after Kretz (1983).

+quartz (matrix)+H O=garnet (rim) 2 +plagioclase (symplectite) +amphibole (symplectite),

(1)

garnet+plagioclase (matrix)+rutile (matrix) from 30 to 70%. The amphibole varies from yellowish green to blue in color, but is commonly bluish green. Plagioclase and quartz occur primarily as xenoblastic grains (0.5–1.0 mm). Garnet occurs as subhedral to subrounded porphyroblasts (up to 3.0 mm) containing M mineral inclusions. All 1 M plagioclases are Na-rich and range from An 2 11

=plagioclase (symplectite) +ilmenite (symplectite),

(2)

where in both cases symplectic plagioclase is more An-rich than matrix plagioclase. For example, An content is between 37 and 46 for symplectic plagioclase, and between 10 and 21 for matrix plagio-

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Fig. 5. Photomicrographs showing representative metamorphic reaction textures of the rocks studied. Scale bar: 1.0 mm. (a) Plagioclase+amphibole symplectite (M ) around embayed garnet grain, plane light. (b) Plagioclase+ilmenite symplectite (M ) around 3 3 garnet grain, plane light. (c) Plagioclase+ilmenite+amphibole symplectite (M ) around garnet grains, plane light. (d) Chlorite 3 (M ) rimming garnet, plane light. Mineral symbols are after Kretz (1983). 4

clase. Similar reactions were reported from other amphibolites and mafic granulites (Mengel and Rivers, 1991). The M assemblage is shown by (1) chlorite 4 replacing garnet grains [Fig. 5(d )]; (2) chlorite replacing matrix amphibole and biotite grains; (3) actinolite replacing hornblende; and (4) epidote replacing plagioclase grains. These retrogressive minerals may be produced by the following generalized hydration reactions ( Yardley, 1989):

amphibole+H O=chlorite+actinolite 2 +quartz+epidote.

amphibole+plagioclase+H O=chlorite 2 +epidote+quartz,

Eleven samples of amphibolites were chosen as the most appropriate for characterizing the different stages of the tectonothermal evolution; their location is given in Fig. 2. Samples 0902, 0910, 1007, 1010, 1102 and 1308 are schistose amphibolites; samples 0909, 1011, 1103 and 1303 are massive amphibolites; sample 1004 is a mica-

(3)

plagioclase+garnet+H O=epidote+quartz, 2 (4) garnet+H O=chlorite+quartz, 2

(5)

(6)

The M minerals do not show any deformational 4 features, suggesting they formed probably after the deformation D . 4

5. Mineral chemistry

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amphibole schist and sample 1312 is a amphiboleplagioclase gneiss. Mineral compositions required for P–T calculations were determined by analysing carbon-coated polished thin sections on an EPM-81Q electron microprobe at Beijing University. Analyses were performed with ~15 kV accelerating voltage, 20 nA beam current and ~1 mm beam diameter. Natural minerals were used as standards for major elements; synthetic oxides and silicate minerals were used for some minor elements. Data reduction was performed according to the procedure of Bence and Albee (1968) with the alpha factors of Albee and Ray

(1970). Fe3+ concentrations in ferromagnesian silicates were estimated by using the scheme of Droop (1987). A representative selection of the minerals used for P–T calculations in the Wutai amphibolites is included in Tables 1–5. 5.1. Garnet Garnet is grossular-rich, relatively pyrope- and spessartine-poor almandine, with minor amount of andradite contents ( Table 1). No pronounced compositional zoning is present. The average composition for the core is Alm Sps 0.576 0.104

Table 1 Garnet compositions used in T analysis Sample 0910 no. Core

1004 Core

1010 Core

1011 Core

1102 Core

1103 Core

1308 Core

1312 Core

0902 Rim

0909 Rim

1002 Rim

1007 Rim

1110 Rim

1303 Rim

SiO 2 TiO 2 Al O 2 3 Fe O 2 3 FeO MnO MgO CaO Na O 2 KO 2 Totals

37.06 0.07 21.02 1.79 25.83 2.62 1.01 10.77 0.05 0.02 100.24

37.68 0.10 21.04 0.47 25.18 6.07 0.91 9.06 0.12 0.01 100.64

37.29 0.20 20.77 0.23 26.25 3.88 0.81 9.92 0.08 0.02 99.45

36.93 0.16 20.46 1.53 24.07 6.78 0.91 8.93 0.06 0.01 99.85

37.71 0.05 21.26 0.99 26.32 2.70 1.13 10.38 0.12 0.07 100.73

37.27 0.20 20.92 0.48 25.12 5.45 0.93 9.32 0.11 0.00 99.80

37.15 0.11 20.80 0.40 24.98 5.12 0.74 10.04 0.01 0.04 99.39

37.05 0.14 21.05 0.61 25.62 3.19 1.08 10.51 0.02 0.03 99.30

37.90 0.21 20.67 0.64 26.13 4.90 1.12 9.25 0.11 0.02 100.94

37.59 0.09 20.96 0.04 26.32 3.40 1.01 9.80 0.09 0.16 99.46

37.17 0.11 20.97 2.16 25.46 2.71 1.06 10.05 0.28 0.09 100.06

37.76 0.01 21.20 0.89 26.46 2.60 1.15 10.30 0.15 0.04 100.56

37.43 0.17 20.62 0.00 25.21 5.31 1.48 8.79 0.03 0.06 99.10

37.36 0.09 20.90 0.32 26.54 3.54 0.84 9.90 0.05 0.07 99.61

Structural formula Si 3.002 Ti 0.005 Al 1.980 Fe3+ 0.019 Fe2+ 1.784 Mn 0.241 Mg 0.101 Ca 0.852 Na 0.008 K 0.007 Sum 8.000 Alm Sps Prp Grs Adr

0.599 0.081 0.034 0.276 0.010

(12 oxygens) 2.958 3.002 0.004 0.006 1.978 1.976 0.107 0.028 1.724 1.678 0.177 0.410 0.120 0.108 0.921 0.773 0.008 0.019 0.002 0.001 8.000 8.000 0.586 0.060 0.041 0.258 0.053

0.565 0.138 0.036 0.246 0.014

3.002 0.012 1.972 0.014 1.768 0.265 0.097 0.856 0.012 0.002 8.000

2.977 0.010 1.944 0.093 1.623 0.463 0.109 0.771 0.009 0.001 8.000

2.987 0.003 1.986 0.059 1.744 0.181 0.133 0.881 0.018 0.007 8.000

2.992 0.012 1.980 0.029 1.686 0.371 0.111 0.802 0.017 0.000 8.000

2.996 0.007 1.977 0.024 1.685 0.350 0.089 0.867 0.002 0.004 8.000

2.979 0.008 1.995 0.037 1.723 0.217 0.129 0.905 0.003 0.003 8.000

3.010 0.013 1.935 0.038 1.735 0.330 0.133 0.787 0.017 0.002 8.000

3.017 0.005 1.983 0.002 1.767 0.231 0.121 0.843 0.014 0.016 8.000

2.968 0.007 1.974 0.130 1.700 0.183 0.126 0.860 0.043 0.009 8.000

2.995 0.001 1.983 0.053 1.755 0.175 0.136 0.875 0.023 0.004 8.000

3.017 0.010 1.960 0.000 1.700 0.363 0.178 0.759 0.005 0.006 7.998

0.592 0.089 0.032 0.280 0.007

0.549 0.156 0.037 0.213 0.047

0.593 0.062 0.045 0.269 0.030

0.568 0.125 0.037 0.255 0.015

0.563 0.117 0.030 0.278 0.012

0.579 0.073 0.043 0.286 0.019

0.581 0.111 0.445 0.245 0.019

0.579 0.078 0.041 0.283 0.001

0.593 0.064 0.044 0.232 0.065

0.597 0.060 0.046 0.270 0.027

0.567 0.121 0.059 0.257 0.000

Adr=Fe3+/2; Grs=(Ca-3Adr)/(Fe2++Mg+Mn+Ca); Alm=Fe2+/( Fe2++Mg+Mn+Ca). Fe3+ is derived following the scheme of Droop (1987).

Prp=Mg/(Fe2++Mg+Mn+Ca);

Sps=Mn/(Fe2++Mg+Mn+Ca);

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Table 2 Amphibole compositions used in T analysis Sample 0910 1004 1010 1011 1102 1103 1308 no. Matrix Matrix Matrix Matrix Matrix Matrix Matrix

1312 0902 Matrix Symp.

0909 Symp.

1002 Symp.

1007 Symp.

1110 Symp.

1303 Symp.

SiO 2 TiO 2 Al O 2 3 Fe O 2 3 FeO MnO MgO CaO Na O 2 KO 2 Totals

39.97 0.75 14.46 6.08 18.13 0.17 5.21 10.97 1.30 1.14 98.18

39.46 0.75 13.52 7.91 18.53 0.37 4.44 10.85 1.05 1.16 98.04

39.95 0.67 13.75 7.56 18.92 0.25 4.49 10.92 1.06 1.17 98.75

39.90 0.62 13.02 7.92 18.23 0.38 4.72 10.83 1.07 1.02 97.70

41.42 0.49 15.42 4.14 16.95 0.52 6.32 11.20 1.55 1.03 99.06

39.28 0.62 14.49 3.29 19.79 0.42 4.96 11.20 2.00 1.17 97.22

39.39 0.82 13.08 7.77 19.53 0.38 4.76 10.22 1.98 1.27 99.20

39.99 0.76 14.19 4.12 20.93 0.27 4.35 10.72 1.95 1.26 98.53

Structural formula (23 oxygens) Si 6.112 6.066 6.162 Ti 0.073 0.095 0.088 Al 2.658 2.375 2.578 Fe3+ 0.386 0.900 0.477 Fe2+ 2.575 2.516 2.697 Mn 0.055 0.050 0.035 Mg 1.150 1.093 0.999 Ca 1.867 1.687 1.770 Na 0.603 0.591 0.583 K 0.232 0.250 0.248 Sum 15.712 15.622 15.637

37.88 0.31 13.74 8.38 19.43 0.31 3.15 11.96 1.95 1.15 98.26

39.76 0.65 12.76 6.19 20.51 0.24 4.85 11.13 2.13 1.06 99.28

39.34 0.70 13.29 7.55 19.27 0.21 5.00 10.30 2.03 1.26 98.96

39.09 0.64 13.09 4.38 22.53 0.27 3.49 11.20 1.99 1.05 97.73

39.27 0.45 13.33 7.42 21.03 0.10 3.93 10.84 1.91 1.15 99.44

39.86 0.68 13.98 8.09 17.42 0.50 5.05 10.95 1.12 1.08 98.73

5.942 0.037 2.541 0.989 2.549 0.041 0.736 2.010 0.593 0.230 15.669

6.125 0.075 2.317 0.717 2.643 0.031 1.113 1.837 0.636 0.209 15.705

6.060 0.081 2.413 0.875 2.483 0.027 1.148 1.700 0.606 0.248 15.642

6.153 0.076 2.429 0.519 2.966 0.036 0.819 1.889 0.607 0.211 15.706

6.066 0.052 2.427 0.863 2.717 0.013 0.905 1.794 0.572 0.227 15.636

6.087 6.116 6.103 0.078 0.086 0.087 2.517 2.608 2.465 0.929 0.700 0.921 2.225 2.321 2.397 0.065 0.022 0.048 1.149 1.188 1.023 1.792 1.799 1.798 0.332 0.386 0.315 0.211 0.223 0.229 15.383 15.448 15.388

6.127 6.176 0.077 0.072 2.486 2.376 0.873 0.922 2.427 2.359 0.032 0.050 1.026 1.089 1.795 1.796 0.315 0.321 0.229 0.202 15.388 15.364

6.197 0.055 2.720 0.466 2.121 0.066 1.409 1.799 0.450 0.197 15.478

Symp.=symplectite. Fe3+ is derived following the scheme of Droop (1987).

Pyp Grs And , with that of the rim being 0.036 0.259 0.024 Alm Sps Pyp Grs And . 0.583 0.085 0.046 0.262 0.022 Spessartine ranges from 0.060–0.156 in the core regions to 0.060–0.078 in the rim regions and is accompanied by a slight increase in pyrope contents from core to rim. It would be expected that the rim compositions of garnet should be lower in Ca than the core regions since symplectic plagioclases around embayed garnets are relatively Ca-rich, however, no such compositional variation was detected. This suggests that the garnets were re-equilibrated during later metamorphic stages of M and M . 3 4 5.2. Amphibole Table 2 lists the compositions of matrix and symplectic amphiboles. There are no systematic compositional variations from core to rim within

single amphibole grains, but a pronounced compositional variation exists between matrix and symplectic amphiboles. The matrix amphibole is higher in Na+ and Fe2+ than symplectic amphibole ( Table 2). The compositions of the matrix amphibole vary mainly from ferroan-paragasite and ferro-paragasite to hastingsite, whereas symplectic amphiboles are mainly ferro-tschermakite, following the nomenclature of Leake (1997). The compositional difference between the matrix and symplectic amphiboles may be attributed to different metamorphic reactions: the matrix amphiboles and plagioclases and associated garnet porphyroblasts have been produced from the M mineral assemblage of actinolite+chlorite 1 +albite+epidote+quartz, whereas symplectic amphiboles and plagioclases have been produced from garnet porphyroblasts and matrix plagioclases through Eq. (1).

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G. Zhao et al. / Precambrian Research 93 (1999) 181–199 Table 3 Plagioclase compositions used in T analysis Sample 0910 no. Matrix

1004 Matrix

1010 Matrix

1011 1102 1103 Matrix Matrix Matrix

1308 Matrix

1312 Matrix

0902 Symp.

0909 1002 1007 1110 1303 Symp. Symp. Symp. Symp. Symp.

SiO 2 TiO 2 Al O 2 3 FeO* MnO MgO CaO Na O 2 KO 2 Totals

65.44 0.08 21.76 0.03 0.07 0.02 2.80 10.18 0.11 100.49

65.21 0.12 21.34 0.17 0.10 0.04 2.32 11.02 0.05 100.37

63.84 0.07 22.36 0.10 0.03 0.06 3.23 10.10 0.10 99.89

63.80 0.03 23.06 0.03 0.03 0.10 3.75 10.09 0.13 101.02

62.12 0.00 23.98 0.32 0.00 0.03 4.80 9.58 0.16 100.99

62.80 0.13 22.88 0.13 0.06 0.20 7.25 6.73 0.03 100.21

62.97 0.04 22.41 0.02 0.13 0.04 7.33 6.06 0.28 99.28

64.42 0.05 21.99 0.13 0.10 0.06 2.88 10.41 0.07 100.11

Structural Si Ti Al Fe Mn Mg Ca Na K Sum

formula (8 oxygens) 2.842 2.868 2.868 0.002 0.003 0.004 1.144 1.124 1.107 0.004 0.001 0.006 0.004 0.003 0.004 0.004 0.001 0.003 0.136 0.131 0.109 0.890 0.865 0.940 0.004 0.006 0.003 5.030 5.002 5.043

An Ab Or

13.2 86.4 0.4

13.2 86.3 0.6

10.4 89.4 0.2

2.824 0.002 1.166 0.003 0.001 0.004 0.153 0.866 0.006 5.025 14.9 84.5 0.6

64.60 0.06 21.36 0.18 0.02 0.02 2.40 11.14 0.18 99.96

2.858 0.002 1.114 0.006 0.001 0.001 0.114 0.956 0.010 5.062 10.6 88.5 0.9

65.57 0.11 21.89 0.09 0.09 0.06 2.40 10.95 0.13 101.29

2.858 0.004 1.125 0.003 0.003 0.004 0.112 0.925 0.007 5.041 10.7 88.6 0.7

2.796 0.001 1.192 0.001 0.001 0.007 0.176 0.858 0.007 5.039 16.9 82.4 0.7

2.737 0.000 1.245 0.011 0.000 0.002 0.227 0.818 0.009 5.049 21.5 77.6 0.9

2.775 0.004 1.192 0.004 0.002 0.013 0.343 0.577 0.002 4.912 37.2 62.6 0.2

2.803 0.001 1.176 0.001 0.005 0.003 0.350 0.523 0.016 4.877 39.4 58.8 1.8

62.00 0.03 23.52 0.09 0.01 0.09 7.59 6.54 0.05 99.92

2.750 0.001 1.230 0.003 0.000 0.006 0.361 0.562 0.003 4.916 39.0 60.7 0.3

63.36 0.01 21.93 0.33 0.06 0.09 8.13 5.30 0.09 99.30

2.816 0.000 1.149 0.011 0.002 0.006 0.387 0.457 0.005 4.834 45.6 53.8 0.6

63.48 0.02 22.23 0.12 0.08 0.01 7.24 6.07 0.12 99.37

2.818 0.001 1.163 0.004 0.003 0.001 0.344 0.522 0.007 4.863 39.4 59.8 0.8

62.77 0.02 21.16 0.17 0.03 0.05 9.17 5.65 0.32 99.34

2.809 0.001 1.116 0.006 0.001 0.003 0.440 0.490 0.018 4.884 46.4 51.7 0.9

Symp.=symplectite; FeO* = total iron; An=Ca/(Ca+Na+K ); Ab=Na/(Ca+Na+K ); Or=K/(Ca+Na+K ).

5.3. Plagioclase A clear compositional variation exists between the matrix and symplectic plagioclases in the Wutai amphibolites ( Table 3). The matrix plagioclases are oligoclase, with An , whereas the sym10.4–21.5 plectic plagioclases are andesine, with An . 37–46 This difference is consistent with the compositional variation between the matrix and symplectic amphiboles, and thus should result from different reactions producing the matrix and symplectic minerals. Some symplectic plagioclases with lower An contents were detected in domains far away from resorbed garnet grains. This suggests that the original compositions of symplectic plagioclase have been changed by diffusion between the matrix plagioclase and symplectic plagioclase, and/or by the re-equilibrium during M or M . Thus, the key 3 4

to the determination of the M P–T conditions is 3 the recognition of the symplectic plagioclase with a more anorthitic composition than the matrix plagioclase. 5.4. Biotite Biotite has variable composition, especially in X [X =Mg/(Mg+Fe2+)], depending on the Mg Mg proximity to other ferromagnesian minerals. For example, biotites in contact with garnet grains show higher X . The matrix biotite compositions Mg listed in Table 4 were analysed from biotite grains surrounded by plagioclase or quartz. These matrix biotite grains show little compositional variation from core to rim, with relatively low Al O 2 3 (<17 wt.%) and MgO (<8 wt.%) contents and high FeO* (total Fe as FeO) (>25 wt.%) and

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Table 4 Biotite compositions used in T analysis Sample no.

0910

1004

1010

1011

1102

1103

1308

1312

SiO 2 TiO 2 Al O 2 3 Fe O 2 3 FeO MnO MgO CaO Na O 2 KO 2 Totals

33.51 2.63 16.84 1.30 26.30 0.19 6.11 0.07 0.22 9.17 96.35

34.02 1.12 14.08 3.78 23.31 0.01 7.94 0.01 0.49 9.12 93.89

34.71 3.66 15.22 0.00 26.39 0.07 6.43 0.09 0.18 9.17 95.93

35.29 1.80 15.61 0.00 25.88 0.17 6.36 0.15 0.80 9.47 95.54

35.33 3.81 15.16 0.00 26.34 0.15 7.03 0.00 0.37 9.33 97.53

34.56 3.00 14.81 1.96 24.87 0.04 7.77 0.02 0.36 8.99 96.40

34.39 2.73 15.21 0.00 27.21 0.32 6.54 0.08 0.64 9.40 96.53

34.05 1.59 15.89 1.37 26.62 0.33 6.01 0.24 0.32 8.95 95.38

Structural formula (11 oxygens) Si 2.640 Ti 0.156 Al 1.564 Fe3+ 0.077 Fe2+ 1.733 Mn 0.013 Mg 0.717 Ca 0.006 Na 0.034 K 0.923 Sum 7.862 X Mg AlIV AlVI

0.293 1.360 0.204

2.741 0.068 1.337 0.229 1.570 0.001 0.953 0.001 0.077 0.938 7.915

2.733 0.217 1.413 0.000 1.738 0.005 0.755 0.008 0.027 0.922 7.818

2.790 0.107 1.455 0.000 1.711 0.011 0.749 0.013 0.123 0.956 7.915

2.734 0.222 1.383 0.000 1.705 0.010 0.811 0.000 0.056 0.922 7.842

2.704 0.177 1.366 0.116 1.628 0.003 0.906 0.002 0.055 0.898 7.854

2.716 0.162 1.416 0.000 1.797 0.021 0.770 0.007 0.098 0.948 7.936

2.716 0.095 1.494 0.082 1.776 0.022 0.714 0.021 0.049 0.912 7.881

0.378 1.259 0.078

0.303 1.267 0.146

0.304 1.210 0.245

0.322 1.266 0.117

0.358 1.296 0.070

0.300 1.284 0.132

0.286 1.284 0.210

X =Mg/(Mg+Fe2+). Mg Fe3+ is derived following the scheme of Droop (1987).

TiO contents ( Table 4). The X ranges from 2 Mg 0.286 to 0.378, Ti from 0.095 to 0.217 and AlVI from 0.078 to 0.245. There is no systematic correlation between X and the TiO content in the Mg 2 analyzed samples.

change in P–T condition between the M and M 2 3 metamorphic stages.

5.5. Ilmenite

Of the four metamorphic stages into which the tectonothermal evolution of the Wutai amphibolites can be divided, only the M and M stages 2 3 have appropriate mineral assemblages for P–T calculations. Moreover, they represent the main stages in the metamorphic evolution of the Wutai amphibolites and seem to control the key part of the geometry of the P–T path. The prograde M 1 and retrograde M stages are also very important 4 for the characterisation of the whole geometry of

Ilmenite is present both in matrix and symplectic assemblages. Analyses of matrix and symplectic ilmenites are shown in the Table 5. The main compositional differences between matrix and symplectic ilmenites are in the molar content of hematite (Fe O ). The symplectic ilmenite has higher 2 3 hematite component than the matrix ilmenite. This compositional variation may be attributed to the

6. Metamorphic conditions

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G. Zhao et al. / Precambrian Research 93 (1999) 181–199 Table 5 Ilmenite compositions in the Wutai amphibolites Sample 0910 1004 1010 1102 1103 1011 1308 1312 0902 no. Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Symp.

0909 Symp.

1002 Symp.

1007 Symp.

1110 Symp.

1303 Symp.

SiO 2 TiO 2 Al O 2 3 Fe O 2 3 FeO MnO MgO CaO Na O 2 KO 2 Totals

0.24 50.12 0.03 5.68 42.46 1.23 0.05 0.10 0.27 0.06 100.24

0.06 47.47 0.07 10.26 42.26 0.15 0.00 0.07 0.05 0.01 100.40

0.05 48.51 0.04 9.03 42.92 0.23 0.05 0.03 0.08 0.01 100.96

0.20 47.88 0.05 9.51 42.84 0.21 0.01 0.02 0.03 0.02 100.77

0.22 48.35 0.05 9.05 41.93 0.13 0.04 0.16 0.29 0.02 100.24

0.22 50.35 0.05 4.84 42.71 1.13 0.04 0.16 0.29 0.02 99.82

0.01 50.42 0.08 6.27 41.98 1.53 0.09 0.02 0.34 0.02 100.76

Structural formula Si 0.006 Ti 0.955 Al 0.001 Cr 0.000 Fe3+ 0.092 Fe2+ 0.901 Mn 0.024 Mg 0.002 Ca 0.004 Na 0.014 K 0.001 Sum 2.000

0.16 51.47 0.07 2.41 43.53 1.65 0.00 0.07 0.25 0.01 99.61

(3 oxygens) 0.000 0.004 0.948 0.978 0.002 0.002 0.000 0.000 0.118 0.046 0.878 0.920 0.032 0.035 0.003 0.000 0.001 0.002 0.016 0.012 0.001 0.000 2.000 2.000

0.06 51.51 0.04 3.59 43.01 1.73 0.15 0.03 0.28 0.01 100.41

0.002 0.971 0.001 0.000 0.068 0.901 0.037 0.006 0.001 0.014 0.000 2.000

0.02 50.13 0.09 6.02 42.29 1.44 0.11 0.04 0.20 0.06 100.39

0.001 0.947 0.003 0.000 0.114 0.888 0.031 0.004 0.001 0.010 0.002 2.000

0.02 51.73 0.07 2.90 43.34 1.91 0.05 0.09 0.21 0.03 100.35

0.20 50.88 0.05 3.65 42.41 2.71 0.01 0.12 0.13 0.02 100.19

0.001 0.977 0.002 0.000 0.055 0.910 0.041 0.002 0.002 0.010 0.001 2.000

0.005 0.963 0.001 0.000 0.069 0.893 0.058 0.000 0.003 0.006 0.001 2.000

0.04 50.66 0.01 5.54 42.27 1.97 0.10 0.03 0.19 0.08 100.88

0.001 0.953 0.000 0.000 0.104 0.884 0.042 0.004 0.001 0.009 0.003 2.000

0.10 47.12 0.03 12.12 40.61 0.23 0.05 0.10 0.27 0.06 100.68

0.003 0.890 0.001 0.000 0.229 0.853 0.005 0.002 0.003 0.013 0.002 2.000

0.006 0.947 0.001 0.000 0.107 0.892 0.026 0.002 0.003 0.013 0.002 2.000

0.002 0.901 0.002 0.000 0.195 0.892 0.003 0.000 0.002 0.002 0.000 2.000

0.001 0.915 0.001 0.000 0.171 0.900 0.005 0.002 0.001 0.004 0.000 2.000

0.005 0.905 0.001 0.000 0.180 0.901 0.004 0.000 0.001 0.001 0.001 2.000

0.006 0.915 0.001 0.000 0.171 0.883 0.003 0.002 0.004 0.014 0.001 2.000

Symp.=symplectite. Fe3+ is derived following the scheme of Droop (1987).

the path, but the ‘real’ M mineral assemblage is 1 not completely known and no well-calibrated geothermobarometers are suitable for estimating the P–T conditions of their mineral assemblages. Therefore, P–T conditions of the M and M 1 4 stages can only be qualitatively estimated. Metamorphic P–T conditions of the M and 2 M stages were estimated using the program 3 T (thermobarometry with estimation of equilibrium state, Berman, 1991) ( Version 1.02). In this multi-equilibrium approach, P and T are determined from the intersection of three or more independent reactions in P–T space using the thermodynamic data of Berman (1988) for selected end-member phases. One of the advantages of the T technique is that the equilibration state of a sample may be assessed by comparing the intersection positions of all equilibria for a given

assemblage. A good convergence is consistent with the assumption of equilibrium whereas divergence would suggest that one or more phases were erroneously included in the stable assemblage. Activitycomposition relations for garnet, biotite, plagioclase, and amphibole were computed using the models of Berman (1990), McMullin et al. (1991), Fuhrman and Lindsley (1988) and Ma¨der et al. (1994), respectively. Ideality was assumed for ilmenite. In estimating average P–T conditions using the I program (Berman, 1991), we only accepted those P–T estimates from the equilibria intersections with a standard deviation of less than ±0.3 kbar for pressure and ±20°C for temperature after no more than one iteration of the exclusion analysis, which re-calculates the position and standard deviation of the intersection after removing intersections lying more than 1.5 s

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from the mean. Overall uncertainties for multiequilibrium thermodynamic calculations are difficult to assess quantitatively since they also include errors in electron microprobe analysis, solution models and thermodynamic properties. We adopt a maximum ‘aggregate uncertainty’ for our T results of ±1.0 kbar and ±50°C, based on an earlier suggestions by Essene (1989) for single thermobarometers. 6.1. M stage 1 The representative mineral assemblage of the prograde metamorphic stage (M ) in the Wutai 1 amphibolite is largely a greenschist facies assemblage. Generally, the temperature of this assemblage ranges from 400 to 500°C ( Yardley, 1989), but the pressure of the greenschist facies assemblage cannot be determined. Considering the presence of rutile which has the smallest molecular

Table 6 Equilibria used in the T analysis (e.g. Berman, 1991). Numbers correspond to those on the intersection diagram in Fig. 4 1. 2. 3. 4. 5. 6. 7. 8.

Alm+Phl=Py+Ann 2Ann+2Py+8Rt+FeTr=FeTs+8Qz+2Phl+8Ilm FePa+4Rt+Py+Ann=Ab+4Ilm+Phl+FeTs FeTr+8Rt+2Alm=8Ilm+8Qz+FeTs Alm+4Rt+FePa=FeTs+4Ilm+Ab 8Qz+2FePa=FeTs+FeTr+2Ab Ab+Ann+Py+4Rt+FeTr=FePa+8Qz+Phl+4Ilm Ab+Alm+4Rt+FeTr=FePa+8Qz+4Ilm

Abbreviations: Ab, albite; Alm, almandine; Py, pyrope; Qz, quartz; FeTr, ferro-tremolite; FeTs, ferro-tschermakite; FePa, ferro-paragasite; Ilm, ilmenite; Rt, rutile; Ann, annite; Phl, phlogopite.

volume of the TiO polymorphs and tends to occur 2 in relatively high-pressure assemblage (Deer et al., 1992), the pressure of this stage cannot be lower than 3–4 kbar.

Fig. 6. T plot for the M stage. Numbers on reaction lines refer to mineral equilibria listed in Table 2. 2

G. Zhao et al. / Precambrian Research 93 (1999) 181–199

6.2. M stage 2 To reduce the effect of M resetting, the estimates 3 of P–T conditions during the M stage are based 2 on the core compositions of garnet, matrix amphibole, plagioclase, ilmenite and biotite from those samples which are devoid of symplectites around embayed garnets. The end-member phases used in calculations for the M stage were albite, anorthite, 2 pyrope, almandine, beta-quartz, Fe-tremolite, Fe-tschermakite, Fe-pargasite, phlogopite, annite, ilmenite and rutile. Grossular was omitted in calculations because grossular-bearing equilibria invariably fell far outside the intersections defined by other equilibria. This may be related to less reliable thermodynamic data for grossular (Berman, 1991), or that the selected plagioclases were not in equilibrium with the grossular-rich garnet cores. The lack of good intersections for the grossularbearing equilibria may also arise if the equilibria have little feedback effect from Fe–Mg exchange and hence do not slide down-pressure as the activity of almandine or pyrope changes (Harley, 1989). The choice of the end-member Fe-tremolite, Fe-tschermakite and Fe-pargasite was based on the fact that the main compositions of the matrix amphibole (M ) vary mainly from ferroan-pargas2 ite to ferro-pargasite, and to hastingsite. Table 6 lists eight possible equilibria that can be written for the selected end-member phases, of which only three are linearly independent. T results for eight amphibolite samples presented graphically in Fig. 6 show excellent intrasample convergence of the eight possible equilibria (three independent) generated for garnet–amphibole–plagioclase– quartz–biotite–ilmenite–rutile assemblage of the M stage. The average P–T conditions for the M 2 2 stage are 600–650°C and 10–12 kbar. Average values of P–T display good intersample agreement, suggesting that the M equilibrium assemblages 2 were preserved, despite later overprints (M –M ). 3 4 6.3. M stage 3 The rim compositions of garnet and the core compositions of symplectic amphibole and plagioclase from six samples were selected for determining the P–T conditions of the M stage. The end3

193

member phases used in the calculations were albite, anorthite, pyrope, almandine, grossular, betaquartz, tremolite, tschermakite, Fe-tschermakite and Fe-pargasite. Ion-exchange equilibria among the end-members of the symplectic amphibole invariably fell far outside the intersections defined by other equilibria. Fe-tremolite and pargasite were therefore excluded in the T analysis. The 17 possible equilibria (three independent) for these end-members are listed in Table 7 and the T analysis results of six samples are shown in Fig. 7. The results show excellent intrasample convergence for the garnet–amphibole–plagioclase–quartz assemblage of the M stage. Most 3 samples have standard deviations of less than 0.3 kbar and 5°C. The P–T data display good intersample compatibility and yield an average temperature of 600–650°C and pressure of 6.0–7.0 kbar for the M stage. 3 6.4. M stage 4 No quantitative geothermobarometric calculations are possible for estimating the temperature and pressure of the retrograde stage (M ). 4 Table 7 Equilibria used in the T analysis (e.g. Berman, 1991). Numbers correspond to those on the intersection diagram in Fig. 5 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

4Gr+2Py+12Qz+3Tsc=3Tr+12Ann 5Tsc+24Qz+4Gr+2Alm=12An+3Tr+2FeTs 15Tsc+24Qz+16Gr+8Alm+6Ab=48An+Tr+6FePa Tsc+Alm=Py+FeTs 4Gr+5Py+12Qz+3FeTs=3Tr+12An+3Alm 9Tsc+8Gr+8Alm+6Ab=24An+4Py+3Tr+6FePa 3Ab+4Alm+2Gr+3Tsc=3FePa+6Qz+3Py+6An 6FePa+24Qz+8Py=6Ab+8Alm+3Tr+3Tsc 2Gr+5Py+18Qz+3FePa=3Tr+6An+4Alm+3Ab 4FeTs+5Tsc+8Gr+4Alm+6Ab=24An+3Tr+6FePa 3FeTs+2Gr+Alm+3Ab=6An+6Qz+3FePa 6FePa+5Tsc+24Qz=6Ab+3Tr+8FeTs 6Ab+8Gr+5Py+9FeTs=6FePa+3Tr+24An+Alm 6Ab+8Gr+4Py+Tsc+8FeTs=6FePa+3Tr+24An 4FeTs+Py+2Gr+Ab=6An+6Qz+Tsc+3FePa 5Py+24Qz+6FePa=3FeTs+3Tr+5Alm+6Ab 9Ab+10Gr+5Py+12FeTs=9FePa+3Tr+6Qz+30An

Abbreviations: Ab, albite; An, anorthite; Alm, almandine; Gr, grossular; Py, pyrope; Qz, quartz; Tr, tremolite; Tsc, tschermakite; FeTs, ferro-tschermakite; FePa, ferro-paragasite.

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Fig. 7. T plot for the M stage. Numbers on reaction lines refer to mineral equilibria listed in Table 3. 3

The retrograde mineral assemblage chlorite+ epidote+actinolite±plagioclase±quartz represents greenschist facies conditions, with probable temperatures of 400–500°C ( Yardley, 1989).

7. P–T path of the Wutai amphibolites The combination of textural characteristics of reaction relationships, mineral chemistry and multi-equilibria thermobarometric calculations of the Wutai amphibolites defines a clockwise P–T evolutionary path as summarised in Fig. 8. Some uncertainties for the early prograde and late retrograde metamorphic stages exist, but the large and key part of the path is well constrained by quantita-

tive P–T estimates for the M and M metamor2 3 phic stages. Early greenschist-facies metamorphism (M ) 1 and S planar fabric represented by the mineral 1 inclusions within garnet porphyroblasts suggest that the Wutai amphibolite unit was buried and deformed under a low-T gradient during the first deformational episode recognized in the rocks. The temperature–pressure estimates of 600–650°C and 10–12 kbar from the core compositions of the matrix and porpyroblast minerals indicate relatively high-pressure amphibolite facies metamorphism during the M stage. The high pressures are 2 supported by the presence of kyanite-bearing schist associated with the amphibolite in the Wutai Complex. Wang et al. (1997) obtained P–T condi-

G. Zhao et al. / Precambrian Research 93 (1999) 181–199

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thermal decompression occurred between the M 2 and M stages in the Wutai amphibolites. This is 3 further supported by sillimanite or andalusite overgrowing kyanite ( Tian, 1991) and the presence of cordierite+sillimanite symplectite around those garnets which contain kyanite inclusions in pelitic gneiss in the Wutai Complex (Liu, 1996a,b). Isothermal decompression paths require that exhumation of deep-seated metamorphic rocks is rapid relative to the rate of thermal relaxation and cooling ( England and Thompson, 1984; Thompson and England, 1984). Decompression was followed by greenschist facies retrogression. Evidence for this regional retrogression is indicated by a number of hydration reactions, mainly involving the replacement of garnet by chlorite, Fe-paragasitic or Fe-hastingsitic hornblende by actinolite and plagioclase by epidote. Considering the relationships between the metamorphic and deformational episodes ( Table 1), the M stage most likely occurred when 4 the metamorphic terrane was being exhumed and hence, the pressures were decreasing.

8. Tectonic implications Fig. 8. P—T path of the Wutai amphibolites. The approximate limits of the metamorphic facies are those provided by Yardley (1989). Al SiO transitions are from Holdaway and 2 5 Murkhopadhyay (1993). Mineral symbols are after Kretz (1983).

tions of 12.5–13.5 kbar and 545–585°C from a kyanite-gedrite schist. Further support is given by the presence of kyanite+Mg-rich chlorite+talc (?) in magnesian pelites associated with the Wutai amphibolite (Liu, 1996a,b). In the model system MgO–Al O –SiO –H O, this assemblage is stable 2 3 2 2 only at high pressures (Schreyer, 1988). The pressure of 10–12 kbar implies that the Wutai amphibolites were buried to a depth of more than 35 km during the M stage and D . 2 2 On the other hand, plagioclase+ amphibole±ilmenite symplectites around embayed garnet grains record medium-pressure amphibolite-facies metamorphic conditions of 6.0–7.0 kbar and 600–650°C for the M stage. Therefore, iso3

Two opposing views on the tectonic evolution of the Wutai Complex have been proposed. One considers the Wutai Complex to represent an island arc, deformed and metamorphosed during collision of the Fuping and Hengshan complexes (Bai, 1986; Bai et al., 1992; Li et al., 1990; Wang et al., 1996), whereas the other considers the belt to be the result of closure of an ensialic rift system ( Tian, 1991; Yuan and Zhang, 1993; Li and Qian, 1995). These tectonic models are based primarily on regional studies and on limited geochronological data, and only a few metamorphic studies have been undertaken on the major lithotectonic units within the complex. The P–T path for the Wutai amphibolites, in combination with data on the age range of the complex and the timing of metamorphism ( Wilde et al., 1998), places important constraints on the setting of the complex and on models of its relationship to the adjacent Fuping and Hengshan complexes. Lithological and geochronological data

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for the Hengshan, Fuping and Wutai complexes indicate that they are all characterized by the emplacement of major silicic igneous bodies in the Neoarchean at around 2550–2520 Ma and a metamorphic and deformational event at around 1800 Ma (Cawood et al., 1998; Wilde et al., 1997). Geochemical data from the Wutai Complex indicate that these bodies formed in a supra-subduction zone setting ( Wilde et al., 1997). These data demonstrate that the high grade Hengshan and Fuping complexes do not represent an older cratonic basement to the low- to medium-grade Wutai Complex as suggested in the models of Tian (1991) and Wang et al. (1996). Rather it suggests that the three complexes all represent elements of a single magmatic arc system that was tectonically disrupted and juxtaposed at around 1800 Ma. The estimated P–T evolutionary path established for the amphibolites in the Wutai Complex requires this phase of disruption and juxtaposition to occur during a phase of mountain building which involved an initial phase of crustal thickening (M1+M2), followed by near-isothermal exhumation (M3) and final cooling and retrogression (M4). This tectonothermal process is very similar to those of the Fuping and Hengshan complexes.

The Fuping and Hengshan mafic granulites also underwent decompression and cooling retrogression stages after the peak metamorphic stage (Liu, 1996a,b; Zhao et al., in review). Thus rather than metamorphism and deformation of the Wutai Complex being related to localized interaction with the Fuping and Hengshan complexes, either through closure of an intracratonic rift ( Tian, 1991; Yuan and Zhang, 1993; Li and Qian, 1995) or a large oceanic tract (Bai, 1986; Bai et al., 1992; Li et al., 1990; Wang et al., 1996), we consider the Wutai Complex, along with the Fuping and Hengshan complexes to be part of a single system and it was the accretion of this entire system to the North China Craton at around 1800 Ma that accounts for the tectonothermal evolution of the complexes. Recent research on lithological, structural, metamorphic and geochronological data from across the North China Craton suggests that the craton can be divided into eastern, central and western zones, separated from each other by the Xinyang–Kaifeng–Jianping Fault and Huanshan–Lishi–Datong–Duolun Fault (Fig. 9) ( Zhao et al., 1998; Zhao et al., in review). The eastern zone is dominated by Palaeoarchean to

Fig. 9. Schematic extent of the eastern and western continental blocks and the central arc zone in the North China Craton.

G. Zhao et al. / Precambrian Research 93 (1999) 181–199

Mesoarchean TTG plutons and Neoarchean anatectic granitoid rocks and minor amounts of supracrustal rocks. The structural style of this zone is dominated by oval domes and the metamorphic evolution is characterized by the isobaric cooling (IBC )-type anticlockwise P–T–t paths (Cui et al., 1991; Liu and Lin, 1991; Li, 1993; Sun et al., 1993; Ge et al., 1994; Zhao et al., 1998). This zone probably represents the oldest continental block within the North China Craton, termed the eastern continental block ( Zhao et al., 1998). The western zone is characterized by large volumes of Archean TTG and mafic granulites and Palaeoproterozoic khondalite series. The metamorphic evolution of the zone is characterized by anticlockwise P–T–t paths for the TTG gneisses and mafic granulites, and clockwise P–T–t paths for the khondalite series (Jin et al., 1991; Liu et al., 1993; Lu and Jin, 1993). The Archean TTG gneisses and mafic granulites with anticlockwise P–T–t paths are part of the western continental block ( Zhao et al., in review), whereas the Palaeoproterozoic khondalite series represents passive continental margin deposits on this continental block (Jin et al., 1991; Liu et al., 1993; Zhao et al., in review). The Wutai, Hengshan and Fuping complexes, along with the Lu¨liang, Zhongtiao, Huaian and Northern Hebei complexes lie in the central zone. They represent Neoarchean continental magmatic arc regions in the eastern continental block (Bai, 1986; Li et al., 1990; Wang et al., 1996; Zhao et al., in review) and were metamorphosed and deformed, along with the Paleoproterozoic khondalite series on the western continental block, during amalgamation of the eastern and western continental blocks in the late Palaeoproterozoic. The mineral reaction textural relations and P–T path of the amphibolites from the Wutai Complex record the tectonothermal history of collision and mountain building between the eastern and western continental blocks which resulted in the final assembly of the North China Craton.

Acknowledgments We thank E. Ghent and D. Whitney for their comments on an earlier version of this paper. G.C.

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Zhao is particularly grateful to E. Ghent for encouraging the application of T multiequilibrium analysis to the Wutai amphibolites. The paper benefited substantially from detailed reviews by S. Harley and F. Mengel. The work was supported by a NSFC Grant No. 49772144) to L.Z. Lu and G.C. Zhao and an ARC Large Grant to S.A. Wilde and P. Cawood. This is Tectonics Special Research Centre publication No. 34.

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