Journal of Southeast Asian Earth Sciences, Vol. 3, Nos 1--4, pp. 249-254, 1989 0743-9547/89 $3.00 + 0.00 Printed in Great Britain Maxwell Pergamon Macmillan pie

Preliminary study of Palaeo-Tethyan ophiolites in Hengduan Mountain Region (HMR), China

ZHANG QI, L~ DAZHOU and ZHANG KuIwu

Institute of Geology, Academia Sinica, Beijing, People's Republic of China

Abstract--Four Palaeo-Tethyan ophiolite zones have been distinguished in the Hengduan Mountain Region (HMR), China. These ophiolites are usually associated with rifting terrains, which were formed earlier than the ocean crust, so that the HMR was set in a tensile stress setting, transforming from continental crust into transitional crust during the late Paleozoic, especially in the Permian. The geochemical composition of basalts from the Palaeo-Tethyan ophiolites studied here have only slight variation and are roughly similar to those of the N-MORB and T-MORB, apart from the ocean island. According to field observations and geochemical data the Palaeo-Tethyan ophiolites here were formed at small and narrow ocean basins, with no apparent relationship to subduction and island arc volcanic activity. It indicates that the origin and development of basaltic magma from the Palaeo-Tethyan ophiolites have distinctive characteristics that differ from the Neo-Tethyan ophiolites.

THE HENGDUAN Mountain Region (HMR) covers southern Qinghai, eastern Xizang, western Sichuan and Yunnan Province. Up to now, four Palaeo-Tethyan ophiolite zones have been distinguished in this area. They are: the Jinsha River ophiolite zone, the Yushu- Litang ophiolite zone, the Ailao Mountain ophiolite zone and the Changning-Menglian ophiolite zone (Fig. 1).

The ophiolitic melange is well developed in the HMR, in which the Litang ophiolitic melange especially dis- plays characteristics distinct from the others. The matri- ces of the Litang ophiolite melange are composed of basalts, basaltic breccias, greenschists, etc., which have all undergone intense shear strains; blocks composed of Upper Triassic limestones which are less deformed, pillow lavas, gabbros, etc. It is possible that the Litang ophiolite was originally formed in a transform fault setting where, soon afterwards, it underwent structure mixing during the subducting process. The REE patterns of the basaltic rocks also support the view that the basalts were formed in a transform fault setting.

The Shuanggou ophiolite is a typical example within the Ailao Mountain ophiolite zone. Its basal part, composed of lherzolites, is less depleted. Above this there are gabbros and diabases and the top is occupied by basalts. It is generally overlapped by slightly meta- morphosed detrital rocks intercalated with basaltic rocks and a few radiolarian-bearing siliceous rocks. The au- thors suggest that the features of the Shuanggou ophi- olite are similar to that of the Trinity ophiolite in the western U.S.A. (Quick 1981). The foliation within the lherzolites strikes approximately north-south and dips steeply. The lineation of oriented orthopyroxenes in the lherzolites is almost vertical to the earth's surface. The Moho discontinuity, marked by a boundary fault be- tween the metamorphic periodotites and gabbro units, dips to the east or to the west at low or moderate angles. Thus, the foliations are at a high angle to the Moho discontinuity (Fig. 2), which indicates that the Shuang-

gou ophiolite is quite close to the spreading centre within an ocean basin.

It is interesting that the Palaeo-Tethyan ophiolites are often associated with rifting terrains which were formed earlier than the ocean crust itself, so that the HMR was set in a tensile stress setting which transformed from continental crust into transitional crust during the late Palaeozoic and especially in the Permian. Therefore, the tectonic settings of the ophiolite here are characterized by small ocean basins derived from the attenuation of continental crust along passive margins. It is possible that the ocean crust occurred in the position of strong tensile stresses during the early Triassic.

The Changning-Menglian ophiolite zone is different from other zones described above. The opening time of the ocean basin apparently began in the late Devonian or early Carboniferous and extended through the late Permian. It is possible that the Changning-Menglian ocean basin was once larger. There are no rifting terrains in the vicinity of the Changning-Menglian ophiolite zone. The subduction in the belt was very strong during the Permian, the most developed period of the con- tinental extension on the western margin of the Yangtze Plate. Therefore, the Jinsha River, the Yushu-Litang and the Ailao Mountain ocean basins all belong to the back-arc basin settings, although they are situated at the back of sialic crust that is distinctively different from that of the west Pacific Ocean at present. Therefore there are no IAT-type basalts in the three ophiolite zones. It is suggested that the subduction only disturbed the hot state of the deep mantle beneath the continental crust in the east of the Changning-Menglian areas.

There are three features in the petrological association of the Palaeo-Tethyan ophiolites in the HMR. First, the bottom of the ophiolites is characterized by lherzolite and harzburgite which are less depleted, only a few dunite occurrences have been found here, but there is plagioclase--lherzolite comparing to primary mantle in the Shuanggou area (Table 1). Therefore, the degree of

249

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Fig. 1. Distribution of the Palaeo-Tethyan ophiolites in the HMR. (I) Palace-suture zone, dashed lines show assumed boundary and top of triangle indicates the direction of subduction; (2) meta-peridotites in ophiolite; (3) rift-type basalts (Permian); (4) volcanic rocks of islands arc and active continental margin settings (Mid-Upper Triassic); (5) granitoids (~ 5); (6) faults. JO, Jinsha River ophiolite zone; YLO, Yushu-Litang ophiolite zone; AO, Ailao Mt. ophiol/te zone; CMO,

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partial melting is lower ( < 2 0 % ) here. Second, the cumulates are less developed or almost lacking, such as in the Shuanggou ophiolite, which indicates that the magma reservoir is small beneath the ocean floor and that the spreading velocity of the ocean basin is very slow. Third, the lack of sheeted dyke swarms in the

H M R implies a relatively stable, nontensional stress regime different from that prevailing in normal mid- ocean spreading centres. However, it is surmized that such an environment may exist in a spreading centre of back-arc or marginal basin where spreading is more diffusely distributed (Evarts 1977). We consider that it is

Palaeo-Tethyan ophiolites in H M R , China 251

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Palaeo-Tethyan ophiolites in HMR, China 253

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Fig. 4. Chondrite normalized REE patterns of basalts in the HMR (a) LREE-depleted type (N-MORB); (b) LREE-slightly enriched type (T-MORB). (1) Area of Shuanggou basalts; (2) Area of Tongchangjie meta-basalts; (3) Gongka pillow lava from Deqin County (after Peng Xinjie, personal communication); (4) Shuanggou basalt (Sh-35); (5) Litang basalts and

diabases (after Liu Baotian, personal communication); (6) Area of west Alpine basalts (Beccaluva et al. 1984).

not favourable for the formation of sheeted dyke, if the spreading velocity of ocean basin is very slow at the early opening stage.

The geochemical composition of the Palaeo-Tethyan ophiolites here are surprisingly similar. For example, the basalts in the ophiolite zones can be divided into two types by TiO: contents: type A is characterized by average TiO2 contents (ranges of average values for individual ophiolite from 0.88 to 1.60wt%) and the TiO~ contents of type B are higher than 2%. Cor- relatively, their Ti/V, Ti/Sc and Ti/Y ratios are also different (Table 2), the former is similar to that of MORB (Ti/V=23-35, Ti/Scffi214-340, Ti /Y= 208-370), and the latter corresponds with oceanic island basalts (Ti/V = 51-56, Ti/Sc ffi 553-822, Ti/Y = 39%843). All of the basalts with average TiO~ con- tents plot in the field of the MORB in Fig. 3. The Baimaxueshan pillow lavas from Deqin County, Yun- nan Province within the Jinsha River ophiolite zone have lower abundances in Ni and Cr (average 44 and 73 ppm, respectively). Some rocks project in the field of island arc basalts, but their TiO: (av. 1.35%), Y abundance (23 ppm) and Ti/V ratios (28) are higher and Sr abun- dance (107ppm) is lower than that of IAT (TiO~ contents usually less than 0.8% Ti/V<20, average Y = 12 ppm, Sr = 200 ppm, after Ashley et al. 1979, Sun

1980, Shervais 1982). It is possible that crystal differentiation of basalts causes low abundance in Ni and Cr. The low MgO content (av. 5.2%) and MgO/(MgO + FeO) ratio (0.47) of Baimaxueshan pillow lavas show that they belong to the evolved rocks.

The REE distributions of MORB-type basalts from the HMR can also be divided into two types: type A is LREE-slightly depleted, their REE abundances are 8-20 times as high as that of chondrite; the REE patterns are slightly convex up and similar to N-MORB (Fig. 4). Type B is LREE-enriched with LaN/YbN = 1.2-2.0, their REE abundances are 8-40 times higher than chon- drite and similar to those of the T-MORB and west alpine ophiolites (Beccaluva et ai. 1984). The pillow lavas from Litang ophiolite, Sichuan Province are LREE enriched; those from Jinsha River and Changning-Menglian ophiolite zones are LREE de- pleted. However, both types appear in the Shuanggou basalts. It is known that the T-MORB is situated in highlands of oceanic floor or transform fault environ- ments. Field observations show that the Litang ophi- olite melange was probably formed in a transform fault environment (Wang et al. 1985), and this observation is consistent with the REE data obtained.

The current petrologic studies have shown an appar- ent scarcity of MORB within the lavas of ophiolites from

254 ZHANG QI et al.

Phanerozoic orogenic belts (Coleman 1984), but they are abundant in the HMR. However, the basalts of the Palaeo-Tethyan ophiolite from the HMR are more or less similar to those of N-MORB or T-MORB (transi- tional type MORB), apart from the ocean island basalts (Table 2). The evidence provided by the geochemical characteristics of basalts indicate that the ocean crust of the Palaeo-Tethyan ocean were formed at spreading centres, with no apparent relationship to subduction and island arc volcanic activity. But it does not imply that there were wide oceanic basins like the present Pacific or Atlantic Ocean at that time. It only indicates that the origin and the developing process of basaltic magma studied here are roughly similar to that of MORB, but distinctly different from that of the Neo-Tethyan ophiolites. There is little doubt that the Neo-Tethyan ophiolites also have the MORB-type basalts, but they are characterized by IAT and boninites, which implies an extremely depleted character of the source mantle material. It is likely that they represent the productions of island-arc or back-arc tectonic environment related to subduction in intra-ocean basin (Sun and Nesbitt 1978, Serri 1981, Cameron 1985).

The following conclusions are drawn from field obser- vations and geochemical data studied. The Palaeo- Tethyan ophiolites were formed at small and narrow ocean basins. However, it is surprising that the volcanic activities related to island-arc or active continental mar- gin settings are well developed here, which is composed mainly of tholeiitic and calc-alkali basaltic and rhyolitic rocks, a small amount of andesites, showing some features of bimodal volcanism. It is probable that the volcanism occurred in the inter-arc basin settings, indi- cating that the convection of mantle accompanied by the asthenosphere ascended beneath the island-arc areas within continental crust was active. On the other hand the Neo-Tethyan ophiolites, with IAT and boninites, are characteristic of intra-ocean subduction. (Sun and Nesbitt 1978, Coleman 1984). Thus the closing influence of ocean basin was greatly offsetted by subduction of intra-ocean. It is striking that there is no strong island- arc volcanism related to large scale subduction in the Neo-Tethys during the closing of this ocean, for exam- ple, Greece, Cyprus, Turkey and Syrian from the eastern Mediterranean, and the Banggong Co-Nujiang ophiolite zone from Xizang. The exception is that of the Yarlung Zangbo ophiolite (Zhang and Yang 1986).

In commenting on the diversity of ophiolites,

Coleman (1984) made a brilliant summation for Tethyan-type ophiolite. He emphasizes that Tethyan ophiolites were formed in small spreading ocean basins along passive continental margins and were character- ized by IAT and boninites. The present authors note that Coleman's Tethys generally correspond to the Neo- Tethys, including the eastern Mediterranean ophiolites. The research about the Neo-Tethys has been carried out for nearly 100 yr, but for the Palaeo-Tethys it has just begun. Up to now, the accummulated data for the Palaeo-Tethys are scarce but the Palaeo-Tethyan ophi- olites from the HMR have displayed some features dis- tinct from the Neo-Tethys by our study. The authors think that if we can make a detailed analysis for these features and give a right interpretation, we would be able to get further knowledge about the plate tectonic model and the development related to the continental orogenic belt.

REFERENCES

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