Pergamon Journal of Southeast Asian Earth Sciences, Vol. 9, No. d, pp. 335-344, 1994

Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All fights reserved

0743-9547/94 $7.00 + 0.00

Ophiolites of the Hengduan Mountains, China: characteristics and tectonic settings

ZHANG QI,* ZHOU DEJIN,* ZHAO DASHENG,* HUANG ZHONGXIANG,~ HAN SONG,~" JIA XIUQIN'~ and DONG JINQUAN t

*Institute of Geology, Chinese Academy of Science, Beijing 100029, People's Republic of China and ?Institute of High Energy Nuclear Physics, Chinese Academy of Science, Beijing 100083, People's Republic of China

AIMract--The Hengdnan Mountains contain Neotethyan and Palaeotethyan ophiolites. Dengqen ophiolite is representative of Neotethyan ophiolite. Its extrusive series are typical fore-arc boninite and are considered to have formed in a fore-arc basin setting. It was emplaced at the end of the Juras~c. The Palaeotethyan ophiolite is widely distributed over the Hensduan Mountains. It is composed mainly of metamorphic peridotite (mainly lher/.ofite), gabbro and basalt. Cumulative nltramafic rocks and sheeted dike complex are rarely exposed. Baudts in most of Palaeotethyan ophiofites have geochemical features similar to MOREs. Based on their geochemical and associated sedimentological evidence, most of the Palaeotethyan ophiolites (except Changning--Menglian ophiolite in big ocean basins) were formed in small ocean basins.

INTRODUCTION

THE TtrrHY~ tectonic zone, from Spain to Sumatra, is the largest east-west tectonic zone on the Earth. It trends north-south in its eastern part and extends to Southeast Asia. The Hengduan Mountains are situated at a change in trend of this zone (Fig. 1). The Hengduan Mountains are adjacent to the Qinghai-Xizang Plateau. Detailed studies have been made on ophiolites in the Qinghai- Xizang Plateau at Yarlung-Zangbo and Bangcongco- Dongqiao areas. However, these ophiolites display similarities to ophiolites in the eastern Mediterranean (Wu and Deng 1980; Pearce and Deng 1988). The Neotethyan ophiolites in the Hengduan Mountains, however, are not well known.

The Palaeotethyan ophiolites are also well developed in the Hengduan Mountains. In contrast, only one Neotethyan ophiolite is exposed in the Hengduan Mountains, the Dengqen ophiolite, which is connected with the Bangongco--Dongqiao ophiolite belt.

This paper deals with Neotethyan and Palaeotethyan ophiolites in the Hengduan Mountains and discusses their origin and tectonic environments. The relation- ship between the Yunnan Hengduan Mountains ophio- lites and Southeastern Asian ophiolites is intriguing, (Hutchison 1975, 1982; Huang et al. 1984). We consider that the Ailaoshan ophiolite belt in its southern part may be divided into two sub-belts; one connected with the Song Ma ophiolite belt in Viet Nam, and the other with the Nan River ophiolite belt in Thailand.

GEOLOGICAL BACKGROUND

1. Neotethyan ophiolites

The Dengqen ophiolite of Xizang, situated in the northwestern of Hengduan Mountains, and in the

eastern part of Bangongco-Dongqiao ophiolite belt is an example of Neotethyan ophiolite (Fig. 2). The Dengqen ophiolite is a north-inchned rock slab with a basement of harzburglte. From bottom to top, it is composed of cumulus orthopyroxenite, cumulus quartz norite, homo- geneous quartz norite and dike complex (Fig. 3). The dike complex mainly consists of quartz diabase from several centimetres to 2 metres in width with symmetrical chilled edges. Plaglogranite veinlets, several centimetres in width, penetrates the dike complex.

In the northern part of the ophiolite, flysch sandstone and shale (Upper Jurassic (Js)) are associated with blocks of radiolarian siliceous rock, pillow lava, gabbro and sandstone. Generally, these blocks are about ten to several tens of metres in width. The sandy shale matrix is strongly sheared. Flexural folds are common, and these are intercalated with sandstone lens; thus, constituting a typical ophiolite melange. Radiolarian fossils are of Early Jurassic (Jm) age (Li Hongshang 1988). In the southern part of the ophiolite belt, red detrital rocks of Cretaceous to Tertiary age, unconformably overlie the ophiolite. It is inferred that the ophiolites were emplaced before the Cretaceous.

2. Palaeotethyan ophiolite

Several Paiaeotethyan ophiolites have been described previously. These include: (1) Litang ophiolite of Sichuan (Wang Liangchen et al. 1988), (2) Deqing (Wang Peishen 1983), (3) Tongchangjie (Tang Jiawen 1982) and (4) Shuanggou (Tang Jiarei 1986, Zhang Qi et al. 1988). Characteristics of the Tongchangjie and Shuanggou ophiolites are mainly discussed.

Tongchang]ie ophiolite melange in Yunxian County, Yunnan Province

The Tongchangjie ophiolite is situated in the Changning-Menglian ophiolite belt (Fig. 2). The matrix

335

336 ZHANG QI et ai

.~ ~ :__~, 0 BEI J ING

TETHYAN ZONE \ ~.~'~...Ik=m...~ f ~ ]~ ~. /

~ ~"~'-~TETHYAN ZONE / \ J ~ _ / ,

Fig. 1. Sketch map of Tethyan tectonic zone. The rectangular area in the map shows the location of Hengduan Mounta ins in the Tethyan tectonic zone.

of the ophiolite melange is composed of two kinds of rocks; strongly deformed and sheared sericite-schist, sericite quartz schist, dimicaceous schist, etc. and green schists (chlorite-schist, actinolite-schist, and epidote- actinolite--schist, etc.), derived from metamorphosed basalt. The blocks of ophiolite melange consists of

~~ ~?7~~n ~ YANGTZE ~ ' ~ 4 /l MASSIF

'.. • / o

' /

: v n t a g Slatumggou

Fig. 2. Sketch map of ophiolites in Hengduan Mountains and adjacent area.

1. Ultramafic rocks; 2. Oranitoids; 3. C-P rift basalts; 4. C-P island arc voleanics; 5. Palaeotethyan ophiolites; 6. Neotethyan ophiolites; ALO = Ailaoshan Ophiolites belt; BDO = Bangongco-Dongqiao Ophiolite belt; C M O = Changning-Mengl ian Ophiolite belt; JRO = Jinshajiang Ophiolite belt; N R O = N a n River Ophiolite belt; SMO = Song Ma Ophiolite belt; YLO = Yushu-Li tang Ophiolite belt;

YNO = Yarlung Z angbo -Naga Hill Ophiolite belt.

basalt, gabbro, websterite, serpentinite, radiolarian siliceous rocks, and limestone. The large basalt block covers an area of 1 km 2. It is composed of high-Ti basalt intercalated with tuff and volcanic detrital rocks, with weak deformation. The limestone block (Middle Carboniferon-Permian) ranges from several metres to more than 200 m in width. In the western part of Tongchangjie, the limestone block occurs as a nappe and overlies the ophiolite melange. Thin-bedded radiolarian siliceous rocks are purple, grey, black or green. Their age is of Late Permian (P2) (Wu and Li 1989). Gabbro amphibole's K-Ar age is 383 Ma; greenschist actinolite K-Ar age is 172Ma; the basalt's Nd model age (TcHuR) is 251-293Ma (its e N d = - 1 . 9 to +2.5) (Zhang et al., in press). Therefore, we consider that the Changning-Menglian oceanic basin might have opened in the early Devonian and closed at the end of Permian. The age of 172 Ma might represent an intra-continental orogenic event.

Shuanggou ophiolite in Xinping County, Yunnan Province

The Shuanggou ophiolite is situated in the south of Ailaoshan ophiolite belt (Fig. 2). It is composed of three kinds of rocks (Fig. 4). From the base upwards, the rock sequence is: metamorphic peridotite (mainly plagioclase-lherzolite, some spinel lherzolite and harzburgite, rarely dunite), gabbro and basalt. Gabbro, including fine-grained gabbro, and pegmatitic gabbro, is relatively complex. Moreover, veinlets of tonalite and diorite occur. Veins less than 20 cm wide of rodingite parallel the foliation of metaperidotite.

The Shuanggou ophiolite occurs as rock slabs thrust westward over red detrital rocks of the Yiwanshui Formation of Late Triassic age. In the south of Shuanggou area, gravels of ultramafic rocks are found in the basal conglomerate of the Yiwanshui Formation. There are different opinions on the age of the Shuanggou ophiolite. Some consider it to be Proterozoic (Fan 1986; Yang 1986). However, we have an age for the gabbro and diorite (Fig. 5). The Ar-Ar age spectrum of clinopyroxene in gabbro gives its plateau age of 339.2 Ma + 13.4 Ma, which represents the crystallization

Ophiolites of the Hengduan Mountains, China 337

RELONG I I I I N-

4000 t-

O , l k , 2 " t Kz -E t

Fig. 3. Section of the opliiolite in I ~ g q ~ , Xi~ng. I. Red detrital rocks from Upper Cretaceous to Tertiary; 2, Harzburgite; 3. Dunite; 4. Cumulative orthopyroxenite; 5.

Cumulate quartz norite; 6. Quartz norite; 7. Diabase dike; 8. Limestone (Upper Jurassic).

MAOHE

Fig. 4. Geological map of Shuanggou ophiolite in Xinping County, Yon n~_n Province.

1. Red fragmental rock of Yiwan~ul Formation of Upper Triassic; 2. Carboniferotm-Petmain dightly metamorphic fragmental rock; 3. Basalt; 4. C_mbbto-diabate; 5. Metamorphic petidotite; 6. Quartz porphyry; 7. Radiolarian fouil~ 8. Foliation; 9. Thrust fault; 10. Slip

fault; il. Schistoaity; 12. Slip thrust fault.

age of clinopyroxene. The apparent age in the low- temperature zone is very high, and might be due to nuclear impact and not have any geological significance.

"The apparent age in the high-temperature zone is 458 Ma which might be due to the excess argon in clinopyroxene, and its geological significance is not apparent. The U-Pb (low-point) age of zircon in diorite is 256 Ma (Fig. 6). Accordingly, the Shuanggou ophiolite was probably formed during Carboniferous to Permain time, and cmplaced before the Late Triassic.

PETROGRAPHY

1. Mantle peridotite

Most of the harzburgites and dunites are of fine- medium grained mosaic and medium-coarse grained platy texture. Some olivines have kink bands and de- formed twins. The average olivine compositions are: Dengqen Fo = 91.8, Shuanggou Fo = 90.2. Generally, orthopyroxene is of medium-coarse grained porphyro- clast with undulatory extinction and kink bands (Dengqen average orthopyroxene Mg](Mg+Fe2+), 92.8; Shuanggou, 90.3). A small amount of grossularite and hydrogrossular exist also in the Shuanggou lherzo- lite. It is inferred that grossularites were altered from plagioclase. Originally, the Shuanggou lherzolite was a plagioclase-lherzolite. Melting textures are developed in lherzolite based on field survey and microscope obser- vation (Zhang et al. 1988, 1991) and exhibit three types: (a) clustered aggregates of clinopyroxene; (b) dark trapped melt pocket (mainly composed of grossularite and hydrogarnet), and (c) light-colour trapped pocket (composed of chlorite, a small amount of serpentines and glass). Except for the minerals mentioned above, a small amount of orthopyroxene, spinel and epidote also occur in the melt aggregate. The phenomena of this initial melting in ophiolites has been reported elsewhere (Dick 1977, Quick 1981, Bloomer and Hawkins 1983, Hawkins and Evans 1983, Nicolas and Dupuy 1984). It also occurs in ultramafic nodules of basaltic rocks (Maalee and Printzlau 1979). The melt aggregates of Cpx, Gt, Chl and Sp have been interpreted as solidified

338 ZHANG Ql et ai.

1200

~ 80C

t 0I

0

mm The platean age = 339Ma-q-13.4Ma

' ' io ' 6'0 ' 8 ' 0

Released n A r ( ~ )

Fig. 5. 4°Ar/39Ar age spectra for elinopyroxene in Shuanggou gabbro.

100

pockets of in situ partial melts or liquids that infiltrated a harzburgite host. These aggregates and their host have been referred to as "impregnated peridotites" and represent evidence of initial partial melting or metasomatism of the mantle•

2. Cumulative uitramafic rock

Olivine has not been found in the Dengqen cumulus pyroxenite and gabbro. Cumulus minerals are mainly composed of bronzite, diopside, and plagioclase (calci- clase-labradorite) in pyroxenite. A small amount of quartz occurs in the cumulus interstices of orthopyroxe- nes. This shows that primary magma is strongly SiO2 supersaturated.

3. Gabbro

The Dengqen quartz norite has gabbroic textures• Its dark minerals are mainly euhedral-semieuhedral bronzite and diopside. Plagioclase occurs as euhedral short, platy crystals (labradorite--calciclase). Quartz,

• 500

~ ' •026t.329 4./- °000338 6 - .050331 4-/- .000687 T - • 25586 ÷ / - 0 Sa (::OR. COEF. = . 9999115 / /

l aO0 /

• 4.00~ . 0 f t | I I i t

o 2.0 4.0 6.0 e.o a°7pb / 23~LJ

Fig. 6. 2°~pb/238U vs 2°TPb/235U isochron diagram of zircon for Shuanggou diorite.

being fine-grained, occurs in the interstices between pyroxene and plagioclase. The contents of quartz in quartz-norite range from 7 to 15%. This mineral assemblage indicates that the primary magma was rich in Si, Mg and Ca. It could be the product of fractional crystallization under high pressure•

4. Diabase and basalt

The Dengqen quartz diabase has quenching and fine- grained diabase textures. Except for the filling of grained pyroxene, a small amount of fine-grained quartz occurs between the framework of acicular and striped plagio- clase. There are two types of basalts in the ophiolites of this area: one is aphanophyric (phaneric) basalt, the other is porphyritic basalt• According to dominant porphyroblastic minerals, the latter includes olivine basalt, plagioclase--basalt and pyroxene-basalt.

GEOCHEMISTRY

1. Dengqen ophiolite

The Dengqen metaperidotite is rich in refractory elements, such as Mg, Cr, Ni and Co, poor in Ca, AI, Ti, V, Sc and REE (Table 1), with a LREE-depleted pattern (Fig. 7). Boudier and Nicolas (1985) divided ophiolites into two subtypes: strongly depleted harzbur- gite and weakly depleted lherzolite. Dengqen ophiolite belongs to the first subtype. The Dengqen quartz norite is rich in SiO2; its SiO2 content is between 51 and 59% (average 55.6%). Its composition is between that of gabbro--diorite and diorite• The rock has a gabbroic texture, and plagioclase is rich in Ca (An is 72 to 92). It is worth noting that Dengqen norite is poor in Ti, Sc, V and HREE, but rich in Si, Mg, Cr, Ni and Co. There are two types of REE patterns: LREE-enriched and U-shape patterns (Fig. 7). In summary, it appears that there are some similarities between Dengqen quartz norite and boninite (Hickey and Frey 1982)• Compared

Ophiolites of the Hengduan Mountains, China 339

i

0 . 1 ~

I I I I La C e Nd Sm E . I I

Tb Yb L,,

Fig. 7. Chondr/te normalized REE patterns from Dengqen ophiolite in Xizang.

1-2. Harzburite; 34 . Orthopyroxenite; 5-7. Cumulate quartz norite; 8. Quartz norite; 9-11. Diabase; 12. Plagiogranite.

SO

20 t-

~lO

2

R~b I( LJa Sr H'f Z'r Sm Ti Y Y'b

Fig. 8. Geochemical patterns of boninites in Dengqen. MORB, Cape Vogel (Papua New Guinea), and chondrite normalized values are after Hickey and Frey (1982); Dengqen (average of

diabases).

with MORB, Dengqen quartz diabase is poor in high field-strength elements (Ti, Zr, Y, P and HREE), but rich in Si, refractory elements (Mg, Cr, Ni) and large ion lithophile elements (Rb, K, Ba). These are similar to that of boninite in Cape Vogel (Hickey and Frey 1982) (Fig. 8). It can be seen that the REE distribution of the Dengqen plagiogranite is similar to that of quartz

Table 1. Chemical composition of Denqen ophiolite

No. 1 2 3 4 5 6 7 8 9 10 11 12 Rocks Hz Hz Op Op CQN CQN CQN QN D D D Pt-Gr Sample 314 326 309 307 280 294 294-I 293 284 282 283 283-1

Major elements (wt%) SiO2 39.62 38.75 53.26 57.14 51.25 55.7 55.44 55.1 51.94 59.79 58.87 72.49 TiO 2 0.01 0.02 0.09 0.14 0.1 0.13 0.14 0.19 0.12 0.22 0.25 0.17 AI203 0.18 0.5 5.18 3.81 14.11 11.41 5.78 11.81 14.65 12.92 15.05 11.36 Fe203 5.72 3.74 1.86 0.85 1.27 0.54 1.09 0.73 1.01 0.97 1.49 0.22 FeO 2.09 2.91 10.1 4.64 6.59 6.95 9.21 4.89 7.16 8.51 7.12 2.85 MnO 0.08 0.11 0.23 0.14 0.1 0.13 0.17 0.11 0.13 0.1 0.09 0.03 MgO 38.91 40.31 20.8 20.47 11.14 12.81 17.88 9.52 9.25 5.59 4.83 2.42 CaO 0.02 0.2 7.05 9.98 10.69 8.01 7.21 11.54 9.64 7.81 7.93 4.69 Na20 0.34 0.96 1.75 1.3 0.54 4.09 2.86 3.23 3.76 4.05 gaO 0.01 0.01 0.07 0.11 0.29 0.5 0.24 0.13 0.3 0.13 0.16 0.31 P2Os 0.02 0.02 0.01 0.01 0.01 0.02 0.03 0.02 0.04 0.02 0.19 H20 + 11.71 12.6 0.82 0.72 3.02 1.75 1.44 1.87 2.68 1.28 1.07 1.43 H20- 0.53 0.69 0.02 0.24 0.36 0.3 0.12 0.16 0.33 0.16 0.18 0.15 Loss 0.41 0.54 0.2 0.38 0.04 0.09 0.11 0.02 Total 99.31 100.4 100.02 1 0 0 . 5 9 1 0 0 . 6 8 99.58 99.37 1 0 0 . 2 8 1 0 0 . 0 9 1 0 0 . 7 7 1 0 0 . 8 2 100.36

Trace elements (ppm) La 0.0225 0.0575 0.309 0.52 0.7 1.41 1.49 2.09 1.495 2.64 3.3 3.82 Ce 0.769 0.538 0.947 1.73 1.57 3.26 3.33 4.27 2.665 4.86 5.19 6.13 Nd 2.95 Sm 0.179 0.253 0.216 0.365 0.425 0.558 0.338 0.71 0.644 0.529 Eu 0.0295 0.0254 0.077 0.100 0.127 0.236 0.087 0.228 0.133 0.262 0.241 0.22 Tb 0.0425 0.045 0.052 0.098 0.087 0.101 0.104 0.114 0.111 0.262 0.193 Yb 0.064 0.241 0.246 0.272 0.262 0.264 0.326 0.467 0.47 0.526 0.273 Lu 0.011 0.0123 0.083 0.052 0.059 0.091 0.088 0.095 0.079 0.131 0.125 0.096 Sc 3 4 38 21 26 18 36 16 17 17 29 21 V 25 32 155 120 130 140 170 175 162 140 200 96 Cr 3800 2360 1570 2000 670 920 1460 730 280 80 74 7 Co 140 98 69 50 62 53 63 26 40 46 44 11 Ni 3800 2130 320 310 170 262 1800 120 89 62 62 36 Zr 7 24 30 23 I 1 24 30 48 26 56 26 30 Y <5 <2 <2 <2 <5 <2 <5 <2 <2 <5 <5 2 Rb 1 2 2 3 3 3 3 2 4 2 4 Sr 9 12 22 25 42 22 25 63 51 62 77

Analynis methods: major elements, XRF; trace ©lements, ICP; Ree, INAA. Both XRF and ICP are analysis in Institute of Geology, Chinese Academy of Science; INAA is Institute of High Energy Nuclear Physics, Chinese Academy of Science.

Hz, Harzburgite; Op, Orthopyroxenite; CQN, Cumulative quartz norite; QN, Quartz norite; D, Diabase; Pl-Gr, Plagiogranite. SEAES 9/4---D

340 ZHANG QI et at.

Fable 2. Chemical composit ion of Shuanggou ophiolite

No. l 2 3 4 5 f~ 8 9 10 ~! !2 Rock Hz Hz Hz Hz Ga D D D Ba Ba QI) QD Sample YT-342 YT-368 YT-37t H-15 TY-344 ZH1-39 YT-284 SH-29 M-10 YT-346 H--2O ZHI.-20

Major elements (wt%) SiO 2 40.34 39.68 38.7 39.99 47.01 46.6 48.10 46.68 49.23 44.82 65.39 63.77 TiO 2 0.08 0.1 0.03 0.03 i05 1,74 0.88 1.02 1.39 2.05 0.51 0 i5 A1203 24 2,21 1.84 1.27 I2.02 16.93 15.17 14.70 15.68 20.79 18.78 [61 Fe203 5.49 4.81 2.5 5.77 0 34 FeO 7.59 7.46 7.24 1.05 2.49 6.3 997 10.19 2.3 7.84 12i 23 MnO 0.15 0.13 0.13 0.1 0,13 0.14 0.16 0.16 0.14 0.1 0,04 0.03 MgO 35.33 36.02 36.6 37.4 12.73 7.03 8.50 8.79 9.08 8.22 0.77 2.64 CaO 1.83 0.43 0.15 0.17 13.61 9.02 11.33 11.75 10.26 7.04 1.68 2.8 Na:O 0.12 0.1 0.09 0.03 1.42 3,72 1.83 1.77 1.67 0.73 t0.65 9 I [ K20 0.02 0.07 0.05 0.01 0.94 0.44 0.16 0.23 0.08 2.24 005 0 14 P205 0.01 0.03 0.02 0.11 0.25 0.09 0.10 0.12 0 I H20 + 10.7 12.2 12.78 12.64 2.56 4,81 1~48 H 2 0 - 0.76 0.52 1.07 1.41 0.18 0.40 012

LOSS 0.73 0.76 1.23 0,39 4.87 3.12 2.33 3.12

Total 100.06 99.71 99.94 99.59 99.45 99.54 99.31 99.72 100.81 93.95 100.62 100.27

Trace dements (ppm) La 0.595 0.492 1.27 0..423 0.733 12 1.62 2.24 7.86 13.2 25 36.8 Ce 1.25 1.06 2.83 2.75 25.4 2.85 17.1 27.5 62.8 95.3 Nd 0.616 0,577 1.62 17.6 6.11 7.61 14.2 41.7 60 Sm 0.129 0.152 0.437 0,128 0.392 5.01 2.27 2.51 4.17 4.81 13.3 17.8 Eu 0.0323 0.0429 0.111 0.077 0.138 1.75 0,721 0.852 1.69 1.69 2.44 2.07 Tb 0.0291 0.042 0.0893 0.101 0.079 1.06 0.561 0,575 0.931 1.06 3.0.1 4.08 Yb 0.15 0.233 0.313 0.325 0,315 2.62 1.51 2.53 2.8 2.95 11.1 21.3 Lu 0.0255 0.0404 0.0476 0.051 0.042 0.408 0.264 0.365 0.33 0.404 1.4 3.22 Sc 13 11 7 9 52 37 31 52 35 34.6 10 4 Co 92 101 97 94 58 32 52 56 28 26.6 4 6 Ta 0.171 0.109 0.039 0.052 0.133 0.766 0.157 0.062 0.434 0.93 0.402 1.12 Th 0.675 0.619 0.139 0.144 0,174 1.07 0.156 0.27 0.889 1.41 2.4 4.22 Hf 0.418 0.306 0.213 4.24 1.67 1.84 4 4.63 28.5 16 U 0.5 0.53 0.923 0.84 0.84 0.831 1.04 1.1 Cs 1.2 1.04 0.536 0.255 I 1.8 2.24 0.29 0.862 3.69 0.594 0.402 V 122 77 88 53 186 210 232 250 149 180 65 14 Cr 1744 2397 2488 2750 1460 219 370 440 347 200 17 23 Ni 1802 3071 3360 2060 145 143 91 106 209 48 15 14 Zr 18.6 20 51 174 66 66 147 140 814 238 Y 4 2 4 5 27 30 30 27 21 122 15l Nb 0.8 2,1 12 7 6.4 18 Rb 10.6 95 12 Ba 16 52 186 38 99 74 393 190 237 26 Sr 14 15 1 4 65 250 155 110 379 185 86 81

Hz, Harzburgite; Ga, Gabbro; D, Diabase; Ba, Basalt; QD, Quartz diorite.

diabase. It seems that there is a genetic relationship between the Dengqen plagiogranite and boninite.

2. Shuanggou ophiolite

The composition of the Shuanggou metaperidotite (Table 2) differs from that of the Dengqen harzburgite. The former is rich in Si, A1, Ca, Ti, Na, K, V, Sc, REE, with LREE-enriched pattern (Fig. 9), and

corresponds to the lherzolite subtype of Boudier and Nicolas (1985).

The Shuanggou diabase and basalt include low-Al and high-A1 types, all the diabases and some basalts belonging to the low-Al type, while plagioclase-basalts belong to the high-A1 type. These two types of rock have similar relations between MgO vs other main elements, and similar Mg' values, but with different A1203 contents. This suggests that they were derived

Table 3. Nd-Sr isotopic data of Shuanggou ophiolite

Sample S m ( p p m ) Nd(ppm) t47Sm/ l"Nd 143Nd/l~N(2~) TCMuR(Ma) TDM(Ma) ~ Nd(0) e Nd(t)

H-26 6.329 46.728 0.0819 0.513057 + 77 0 109 8. I 13. I YG-41 0,739 2.67 0.1672 0.513032 _+ 25 - 2053 400 7.6 8.6 YT-266 3.45 9.449 0.2208 0.513167 + 17 3307 340 10.3 9.5 YT-324 2.37 10.80 0.1327 0.513191 "t- 66 0 0 10.7 12.8

Sample Rb(ppm) Sr(ppm) STRb/~Sr ~Sr/at'Sr(2a) (STSr/~Sr)i e Sr(0) e Sr(t)

H-26 0.497 96,05 0.0149 0.704 142 + 49 0.70409 - 5.1 - 1.8 YG~I1 0.425 87.20 0.0112 0.703788 + 50 0.70375 - 9.9 - 6.8 YT-266 6.518 211.66 0.0889 0.705456 -t- 19 0,70513 13.6 12.9 YT-324 7.533 195.00 0.1115 0.704540 _+ I 1 0.70413 0.6 - 1.3

Analysis methods: SR and Nd isotopes using VG-354 in Institute of C~ology, Chin~¢ Academy of Sciences by Qiao G ~ g . Calculate parameters: IND(0)= 0.51264, (147Sm/144Nd) CHUR =0.1967, ISr(0)= 0.7047, (87Sr/86Sr) UR : 0.0847, t = 256Ma, H-26 and YG-41, Quartz diorites; YT-266, Diabase; YT-324, Gabbro.

Ophiolites of the Hengduan Mountains, China

Table 4. Oxygen isotopic data of Shuanggou ophiolite

341

Sample HI6 Y T - 3 4 0 YT-343 YT-324 YT-266 YT-267 YT-327 M-10 H-26 ZHI-20 Rock Hz Hz Hz Ga D D Ba Ba QD QD

6 I~0(%e) 5.43 6.20 5.30 5.92 7.36 9.42 10.13 1 I. 18 11.45 10.05

Analysis method: 0 isotopes by MAT-251EM, using BrFs methods in Institute of Geology, Chinese Academy of Sciences by Huo Weiguo. Precision is +0.2Y~.

from different magma sources. They have obvious differ- ences in trace elements and their ratios. The low-Al type is higher in Sc, Y, V, and lower in Sr, K, Ti (Table 2), thus their Ti/V, Ti/Sc, Ti/Y, Ti/Cr ratios are about half of high-Al type. In Fig. 10, the evolutionary trends of these two types are separated, the average Ti/V ratio of low-A1 type is 28, while that of high-Al type is 51, and is in the alkalic basalt area. It is known that Ti and V are coherent in the same kind of rocks or in different homogenetic rocks, and the difference of Ti/V ratio could reveal the difference of their source (Shervais 1982). In Fig. 10, both picrite and plagioclase-basalt plot in the same area, while most of the gabbros plot near the diabase area. This suggests that gabbro has a close genetic relation with diabase, which is also demonstrated by their close association in outcrops.

The high-Al type and low-Al types differ also in their REE characteristics. The former is slightly LREE- enriched, while the latter is LREE-depleted. The low-A1 type (LREE-depleted) correspond to N-MORB, while the high-Al type to E-MORB. They could be derived from different sources, the low-Al type being derived from depleted mantle, while the high-Al type might be derived from depleted mantle, but with the addition of LILE-enriched material.

The Shuanggou gabbro has an LREE-depleted pattern. Diorite and tonalite have plain or slightly LREE-depleted or an enriched patterns with obviously negative Eu-anomaly, the REE contents are high, up to 100 times of chondrite (Fig. 9).

Nd isotopic ratios of the Shuanggou gabbro, diabase, basalt, diorite and tonalite are high, their ~43Nd/144Nd ratios are 0.5130-0.1532, 8 Nd( t )= 7.9-12.8 (Table 3),

i

I i I i I I L I (~ Nd Sm Eu Tb Yb Lu

Fig. 9. Chondrite normalized REE patterns from Shuanggou ophiolite in Xinping County. Open circles represent metamorphic peridotite; Slanting crosses indicate diabue; Triangles indicate basalt; Dots indicate diorite and tonalite;

Shading indicate the field of 8abbro.

342 ZHANG QI et aL

which reveals that they were derived from a depleted mantle source. Their 875r/S6Sr ratios are 0.7037-0.7054, S r = - 9 . 9 - + 13.6, showing a wide range. In Fig. 11, most of the plots deviate from mantle array which might be due to the high e Sr caused by sea water's alternation. 6~sO also has a wide range, from 5.92-11.45%o. (Table 4), within the range of Oman and Xigaze's 6 J80 (Oman, 6~80=3.6-12.6%o, after Pearson et al. 1991; Xigaze, 6180 = 4.8-16%o, after Agrinier et al. 1988). The 6t80 of basalt and diorite are high, which might be affected by sea water's alternation in low temperature. The 6~80 of metamorphic fluid formed by dehydration of hydrothermal alternation is +4 to + 12%. (Barrett and Friedrichsen 1989).

3. Tongchangfie ophiolite

The Tongchangjie metaperidotite composition is similar to that of Shuanggou, although the former has a wider range (Zhang et al. 1985). The Tong- changjie meta-basalt's REE pattern is LREE- depleted (Zhang et al. 1985), its average Ti/V ratio is 23, Ti/Sc ratio is 214, Ti/Y ratio is 208, and 143Nd/l'~Nd ratio is 0.51308-0.51323 (Zhang et al. in press). These data infer that the Tongchangjie ophiolite is similar to N-MORB type ophiolite, and was derived from strongly depleted asthenosphere mantle.

DISCUSSION

1. Depleted degree of mantle

The Dengqen residual metaperidotite is mainly composed of harzburgite (norm Di content is only 0.79%). It has considerable quantities of dunite and

Vp0m

/

IAB ~ / ~ / / / ORT

400

/ o o

• 4

0 I I I 0 5000 10000 15000

T ip0m

Fig. 10. Ti-V diagram of Shuanggou Ophiolite. 1. Gabbro; 2. Diabase; 3, Picrite; 4. Basalt.

¢ 0. 5130 c..

I

O. 5125

MORB

\ \ \ \

, \ , \ 0. 703 0. 705

87Sr/"Sr Fig. 11. Sr-Nd isotopic diagram. 1. Gabbro; 2. Diabase; 3. Diorite.

× 1

A 2

• 3

o. d

podiform chromite. Compared with undepteted mantle, it is rich in Mg', poor in AI, Ca, V, Sc, Ti and REE. Olivine and orthopyroxene are also rich in Mg. These characteristics show that the Dengqen residual peridotite belongs to the harzburgite subtype (Boudier and Nicolas 1985) and represents strongly depleted mantle.

Compared with the Dengqen ophiolite, the meta- peridotite is mainly lherzolite, and dunite occurs rarely in the Shuanggou ophiolite. Both olivine and orthopyroxene are relatively poor in Mg. The Cr/(Cr + A1) ratio of spinel is low (0.45-0.54) (Zhang et al. 1988). The chemical composition of the rock is rich in AI, Ca, V, Ti, Sc, and REE, belonging to lherzolite subtype, so it might be derived from a slightly depleted mantle.

2. Tectonic environments of ophiolites

Neotethyan ophiolite. As discussed above, the Dengqen harzburgite was strongly depleted. Bonatti and Michael (1989) show that degree of depletion in metape- ridotite is related to the environments in which peridotite was formed. Dengqen is approximately equivalent to the active marginal environment which is related to mature ocean or subduction. The Dengqen cumulate belongs to the orthopyroxene type, following the classification of Ishiwatari (1985). Plagioclase in the cumulate is rich in Ca (An = 72-94), showing that Dengqen cumulate was crystallized under high-P.2 o conditions. Both quartz norite and quartz diabase belong to the boninite series. Sun and Nesbitt (1978) indicated that boninite and low-Ti basalt could be formed by remelting of a severely depleted mantle source in the water-bearing condition. They raised a wet melting model of a depleted mantle wedge over a subduction zone. The Dengqen ophiolite quartz diabases are similar to those of boninite in Bonin Island and Mariana Arc of the Western Pacific Ocean (Hickey and Frey 1982). They were probably formed in the environment of a fore-arc basin of intraoceanic island arc, which was related to tension in the fore-arc

Ophiolites of the Hengduan Mountains, China 343

basin above the dehydrated subducted oceanic litho- sphere (Zhang and Yang 1986).

Palaeotethyan ophiolite. The basement rocks in the Palaeotethyan ophiolites are mainly composed of slightly depleted lherzolite. The geochemical character- istics of gabbro, diabas¢ and basalt here are similar to the corresponding rocks retrieved from the modern ocean floor. For example, average TiO2 contents of basalts for different individual ophiolite is from 0.88 to 1.60 wt%, their Tip/, Ti/Sc and Ti/Y ratios are similar to that of MORB (Ti/V=23-35, Ti/Sc=214--340, Ti/Y -- 208-370). So most of the basalts plot in the field of the MORB (Zhang et al. 1989).

The REE patterns of the Shuanggou diabase and basalt include both LREE-depleted and LREE-enriched. It is suggested that the LREE-enriched high-A1 type of rock is formed from LILE-enriched mantle source and at rift environment of the early extension stage of small ocean basin, which represent the products from slow spreading ocean ridge. The LREE-depleted low-Al type might be formed by rapid spreading of a small ocean basin at its mature stage.

Based on the discussion above, Tongchangjie ophio- lite belongs to N-MORB type and the ocean basin existed for a long period (late Devonian-late Permian). Pelagic abyssal deposit had been found (Liu 1991), which implies that the Changning-Menglian ocean basin was very big.

3. The relationship between Yunnan and Southeast Asian ophiolites

Many authors had discussed the southward extension of Yunnan Palaeotethyan ophiolites and the relationship between Yunnan ophiolites and southeastern Asian ophiolites (Hutchison 1975, 1982, Huang et al. 1984, Ban" and Macdonald 1987). In Fig. 2, it can be seen that the Ailaoshan ophiolite could be separated into two sub-belts: one stretches into the Song Ma Line of Viet Nam, while the other turns to the southwest and con- nects with the Nan River ophiolite of Thailand. Huang et al. (1984) suggested that the Nan River suture zone should be connected with the Changning--Shuangjiang belt (we name it the Changning-Mengiian belt). The Changning-Mengiian belt should extend into the eastern part of Burma. The peridotite-diorite complex in the south of Jinghong (Zhao et al. 1991) is similar to the pyroxenite-diorite complex in the west of Chiang Mai, Thailand (Barr et al. 1990). The Lincang granite corre- sponds to the granite belt of western Thailand. The Chiang Mai volcanic belt which is of within-plate volcanic rocks and of Carboniferous-Permian age (Barr et al. 1990), corresponds to Wusu within-plate basalt in Lineang block (Zhou et al. 1992). The Chiang Rai island arc volcanic rock (C-P) (Ban" et al. 1990) corresponds to Yanxuanqiao, Simao and Jinggu island-arc volcanic rocks (C3-P2) (Zhou et al. 1992). Besides, both Ailaoshan and Nan River ophiolites are westward sub- ducted, and both of them were emplaced before late Triassic. The Ailaoshan, Nan River and Song Ma suture

zones limit the boundary of Yangtze plate, Indosinian plate, Lincang-Chiang Mai block. Shan-Thai-Ma block should be located at the west of Changning-Menglian suture zone (Fig. 2). More detailed work is needed before identifying the southward extension of Changning- Mengiian suture zone.

REFERENCES Agrmier, P., Javoy, M. and Girardcau, J. 1988; Hydrothermal activity

in a peculiar oceanic ridge: Oxygen and hydrogen isotopic evidence in the Xigaze ophiolite (Tibet, China). Chem. Geol. 71, 313--335.

Barr, S. M. and Macdonald, A. S. 1987. Nan River suture zone, northern Thailand. Geology IS, 907-910.

Bart, S. M., Tanthmkrit, C., Yaowanoiyothin, W. and Macdonald, A. S. 1990. Petrology and tectonic implications of Upper Palaeozoic volcanic rocks of the Chiang Mai belt, northern Thailand. J. SE Asian Earth Sci. 4, 37-47.

Barrett, T. J. and Friedrichsen, H. 1989. Stable isotopic composition of atypical ophiofite rocks from east Liguria, Italy. Chem. GeoL tO, 71-84.

Bloomer, S. H. and Hawkins, J. W. 1983. Gabbroic and uitramafic rocks from the Mariana trench: an island arc ophiolite. In: The Tectonic and Geological Evolution of Southeast Asian Seas and Islands (Edited by Hayes, D. E.), Part 2, pp. 294-317. Am. Geophys. Union, Washington, DC.

Bonatti, E. and Michael, P. J. 1989. Mantle peridotites from continen- tal rifts to ocean basins to subduction zones. Earth Planet. Sci. Lett. 91, 297-311.

Boudier, F. and Nicholas A. 1985. Harzburgite and Iherzolite subtypes in ophiolitic and oceanic environments. Earth Planet. ScL Left. 76, 84-92.

Dick, H. J. B. 1977. Partial melting in the Josephine peridotite--l: the effect on mineral composition and its consequence for geobarometry and geothermometry. Am. J. Sci. 277, 801--832.

Dobretsov N. L. 1989. Tectonic problems and ophiolite belts of South Siberia, IGCP Project 224, Report No. 4, pp. 89-100. Osaka, Japan.

Fan Chengiun 1986. The tectonic-metamorphic belt of Ailao Mt. in Yunnan Province. Yunnan Geology 5, 281-291 (in Chinese with English abstract).

Hawkins, J. W. and Evans, C. A. 1983. Geology of the Zambales Ranges, Luzon, Philippine Islands: Ophiolite derived from an island arc-back arc basin pair. In: The Tectonic and Geological Evolution of Southeast Asian Seas and Islands (Ecfited by Hayes, D. E.), Part 2 Am. Geophys. Union, Washington, DC pp. 95-123.

Hickey, R. L. and Frey, F. A. 1982. Geochemical characteristics of boninite series volcanics: Implications for their source. Geochim. Cosmochim. Acta 46, 2099-2115.

Huang Jiqing, Chen Guomin and Chen Binwei 1984. Preliminary analysis of the Tethys-Hirnalayan tectonic domain. Acta GeoL Sinica 58, 1-17 (in Chinese with English abstract).

Hutchison, C. S. 1975. Ophiolite in Southeast Asia. Bull. Geol. Soc. Am. 86, 797-806.

Hutchison, C. S. 1982. Southeast Asia. In: The Ocean Basins and Margins (Edited by Nairn, A. E. M. and Stehli, F. G.), Vol. 6, pp. 451-512. Plenum Press, New York.

Ishiwatari, A. 1985. Igneous petrogenesis of the Yakuno ophiolite (Japan) in the context of the diversity of ophiolites. Contrib. Miner. Petrol. 89, 155-167.

Li Hongsheng 1988. Early Jurassic (Late Pliensbachina) radiolaria from the Dengqen area, Xizang (Tibet). Acta MicropalaentoL Sinica, 5, 323-330 (in Chinese with English abstract).

Liu Benpei, Feng Qinglei and Fang Nianqiao 1991. Tectonic evolution of the Paleo-Tethys in Changning-Menglian and Lancangilang belts, Western Yunnan, IGCP Project 321, Abstract pp. 189-192, Kunming, China.

Maalze, S. and Printzlau, I. 1979. Natural partial melting of spinel lherzolite. J. PetroL 20, 727-741.

Nicolas, A. and Dupuy, C. 1984. Origin of ophiolitie and oceanic Iherzolites. Tectonophys. 110, 177-187.

Pearce, J. A. and Den8 Wanmin 1989. The ophiolite of the Tibet C-eograverses, Lhasa to Golmud (1985) and Lhasa to Kathmandu (1986). In: The Geological Evolution of Tibet: Report of the 1985 Royal Society-Academia Sinica geograverse of the Qinghal-Xizang Plateau. Phil. Trans. Roy. Soc. Lond. A32'7, 215-238.

344 ZHANG Qi et al,

Pearson, D. G., Davies, G. R., Nixon, P. H., Greenwood, P. B. and Mattey, D. P. 1991. Oxygen isotope evidence for the origin of pyroxenites in the Beni Bousera peridotite massif, North Morocco: derivation from subducted oceanic lithosphere. Earth Planet. Sci. Lett. 102, 289-301.

Quick, J. E. 1981. Petrology and petrogenesis of the Trinity peridotite, an upper mantle diapir in the eastern Klamath Mountains, Northern California. J. Geophy. Res. 1386, 11837-11863.

Shervais. J. W. 1982. Ti-V plots and the petrogenesis of modern and ophiolitic lavas. Earth Planet. Sci. Lett. 59, 101-118.

Sun, S. S. and Nesbitt, R. W. 1978. Geochemical regularities and genetic significance of ophiolitic basalts. Geology 6, 689~93.

Wang Liancheng, Li Dazhou, Zhang Qi and Zhang Kuiwu 1985. Ophiolite melange in Litang, Sichuan Province. Acta Petrol. Sinica !, 17-27 (in Chinese with English abstract).

Wu Haoruo and Deng Wanmin 1980. Basic geological features of the Yarlung Zangbo ophiolite belt, Xizang, China. In: Ophiolites, Proc. Intern. Ophi. Syrup Cyprus 1979 (Edited by Panayiotou, A.), Cyprus Geol. Sur. Dep., 462472,

Wu Haoruo and Li Hongsheng 1989. Carboniferous and Permian radiolaria from Menglian area, western Yunnan. Acta Micropalaeontol. Sinica, 6, 337-343 (in Chinese with English abstract).

Yang Jiarui 1986. The features of ophiolite sequence and their geological significance in Shuanggou area, the central segment of Ailao Mountain, Yunnan. Yunnan Geol. 5, 292-302 (in Chinese with English abstract).

Yang Jiawen 1982. Discussion on the ophiolite at fongchanglJe, Yunxian County. Yunnan Geol. I, 59-7t (in Chinese with English abstract).

Zhang Qi and Li Dazhou 1989. A survey of suture zone in Hengduan Mountains, southwestern China. In: Advances in Geoscienee (Edited by Wang, S.), No. 1, pp. 231-238. China Ocean Press.

Zhang Qi, Li Dazhou and Zhang Kuiwu 1985. Preliminary study on Tongchangjie ophiolite melange from Yun County, Yunnan Province. Aeta Petrol. Siniea, 1, (3), 1-14 (in Chinese with English abstract).

Zhang Qi, Li Dazhou and Zhang Kuiwu 1989. Preliminary study for Palaeotethyan ophiolites in Hengduan Mountain Region (HMR), China. J. SE Asian Earth Sci. 3, 249-254.

Zhang Qi, Zhang Kuiwu, Li Dazhou and Wu Haiwei 1988. A preliminary study of Shuanggou ophiolite from Xinping County, Yunnan Province. Acta Petrol. Sinica, 4, 37-48 (in Chinese with English abstract).

Zhang Qi and Yang Ruiyin 1986. The boninite-like pluton in ophiolite from Dingqing, Xizang, and its geological significance. Chinese Sei. Bull. 31, 405--408.

Zhang Qi, Zhao Dasheng and Li Dazhou 1991. Initial melting of metaperidotite in Shuanggou ophiolite from Xinping County, Yunnan Province. Acta Petrol. Sinica 1, 1-15.

Zhao Dasheng, Zhang Qi, Wang Mingxin, Huang Zhongxiang 1991. The Geochemical features and tectonic setting of Paleng peridotite-diorite complex in Jinaghong County, Yunnan, Chinese Sei. Bull., 36, 1974-1978.