Stratigraphic and sedimentological constraints on the age and tectonic evolution of the Neotethyan ophiolites along the Yarlung

Tsangpo suture zone, Tibet

JONATHAN C. AITCHISON l, AILEEN M. DAVIS l

, ALEXANDRA V. ABRAJEVITCH 1

, JASON R. ALI l, BADENGZHU2, JIANBING LIU1,3, HUI

LU0 1,4, ISABELLA R. C. MCDERMID l & SERGEY V. ZIABREV 1

1 Tibet Research Group} Department of Earth Sciences} University of Hong Kong} Polifulam Road} Hong Kong SAR} P.R. China (e-mail: jona@hku.hk)

2Geological Team No.2} Tibet Geological Survey} Lhasa} Tibet} P.R. China 3 Present address: Institute of Geology and Mineral Resources} Bureau of Geology and Mineral Resources of Xinjiang} 16 Youhao Beilu Road} Urumqi} Xinjiang} P.R. China

4Present address: Nanjing Institute of Geology and Palaeontology} Laboratory of Palaeobiology and Stratigraphy} Academia Sinica} Nanjing 210008} P.R. China

Abstract: Ophiolitic rocks distributed along the Yarlung Tsangpo suture zone in southern Tibet are the few remaining fragmentary remnants of many thousands of kilometres of the ocean space that formerly existed between India and Eurasia. Portions of mid-Jurassic and mid­Cretaceous intra-oceanic island arcs can be recognized amongst those rocks that have been studied in detail. Complete suprasubduction zone ophiolite successions are preserved in the Dazhuqu terrane, which crops out both east and west of Xigaze. Radiolarians in inter-pillow cherts and immediately overlying sedimentary rocks indicate a Barremian ophiolite generation event. Palaeomagnetic data show that this ophiolite formed at equatorial latitudes south of the Lhasa terrane before its south-directed emplacement onto the northern margin of India. Highly refractory ultramafic rocks in the Luobusa ophiolite appear to be of Mid-Jurassic age and are potentially related to intra-oceanic island arc remnants in the nearby Zedong terrane. Ophiolitic massifs along the suture in western Tibet are thrust southwards onto northern India and record Late Jurassic ocean-floor development. Miocene north-directed back-thrusting associated with India-Asia collision has further complicated interpretation of regional geology. The ophiolitic rocks of the Yarlung Tsangpo suture zone provide evidence for the former existence of multiple oceanic island arc segments within Neotethys and suggest that consumption of the oceanic space between India and Asia was more complicated than has been predicted by existing models.

The vast Neotethyan Ocean that Once separated Eurasia and India finally closed during the Cen­ozoic along the Yarlung-Tsangpo suture ZOne (YTSZ) in Tibet. The suture traverses southern Tibet meridionally, at c. 29'N, and is marked by a discontinuous belt of ophiolitic bodies (Fig. 1). It extends westwards beyond Tibet into northern India and experiences a major change in strike near Namche Barwa (7782 m 29°40'N, 095°10'E) in eastern Tibet, where it heads southward towards Burma. Although most of what Once lay within Neotethys was subducted, and otherwise destroyed during continental collision, a few remnants of Tethyan affinity are preserved along, and marginal to, the YTSZ. The suture is characterized by the presence of a series of outcrops of rocks of

ophiolitic affinity, the nature and distribution of which are reviewed in this paper.

The ophiolitic rocks along the suture (Table 1) have locally been mapped in detail by geologists of the Bureau of Geology and Mineral Resources of the Xizang Autonomous Region during the search for Cr resources in this region (Badengzhu 1979, 1981; Zhang & Fu 1982; Wei & Peng 1984). The first detailed studies of the ophiolites were carried out as part of investigations during the 1980s into the nature of the India-Asia collision zone. In particular, this work concen­trated on the Xigaze region, where a major zone of outcrops, up to 25 krn wide, includes several ophiolitic massifs. These rocks are near continu­ously exposed for over 175 krn along strike from

From: DILEK, Y. & ROBINSON P. T. (eds) 2003. Ophiolites in Earth History. Geological Society, London, Special Publications, 218, 147-164. 0305-8719/03/$15 © The Geological Society of London 2003.

148 1. C. AITCHISON ET AL.

EURASIA 81 ° 90°

l:l. Gangrinboche

Lhasa terrane Namche Barwa

Lhasa Ngamring Xigaze .Qu~u

e Renbung Liuqu eBainang Zedong

Indian terrane

Fig. 1. Geological sketch map of the Yarlung Tsangpo suture zone across southern Tibet together with settlements and places mentioned in the text. YTSZ, Yarlung Tsangpo suture zone; BNS, Bangong-Nujiang suture; MCT, Main Central thrust of the Himalaya; MBT, Main Boundary thrust of the Himalaya. The approximate position of the Yarlung Tsangpo is shown as a continuous line and the suture is shown as a subparallel grey zone.

near Renbung, 250 Ian SW of Lhasa, to Ngamring Tso. Numerous descriptions of the petrography, geochemistry, structure, metamorphism, palaeo­magnetism and tectonic evolution of these rocks emanated from investigations by Chinese, French, British and German researchers (Nicolas et al. 1981; Shackleton 1981; Tapponnier et aZ. 1981; Burg 1983; Allegre et aZ. 1984; Girardeau et aZ. 1984b, 1984c, 1985a, 1985b, 1985c; Wang et al. 1987; Girardeau & Mercier 1988; Einsele et aZ. 1994; Durr 1996). Although tectonically disrupted and heavily attenuated, sections locally display a complete ophiolite sequence from fresh Cr diop­side-rich harzburgites to marine sedimentary cover developed on top of pillowed mafic volcanic rocks (Nicolas et aZ. 1981; Girardeau et aZ. 1984b, 1985a, 1985c). Interpretations of these rocks were much influenced by models available at that time for understanding ophiolite generation. The ophio­lite was generally regarded to have been generated at a mid-ocean ridge and was considered to represent the basement to forearc turbidites of the Xi gaze Group.

More recent investigations include studies of the Luobusa ophiolite east of Zedong (Huang et aZ. 1981; Bai et aZ. 1993,2000; Zhou & Robinson 1994; Zhou 1995; Zhou et aZ. 1996,2002; Griselin et aZ. 1999; Hu 1999; Hebert et al. 2000, 2001) and re-examinations of the geochemistry and mineralogy of ophiolitic occurrences along the suture zone (Wang et aZ. 1999; Hebert et aZ. 2000, 2001). The suprasubduction zone affinity of this ophiolite has been widely recognized in these contemporary studies.

The first information regarding the age of the ophiolite became available in reports of Sino­French expeditions to this area in the early 1980s (Marcoux et aZ. 1982). The cherts as well as fine­grained clastic deposits that depositionally overlie

the ophiolite were accorded a late Albian to possibly early Cenomanian age based on radiolar­ian faunas. Other radiolarian fossils described later from the same deposits were interpreted as upper Albian-lower Cenomanian (Li & Wu 1985) or lower Cenomanian (Wu 1986) based on correla­tion with the Archaeospongoprunum techamaensis Zone of California.

Over the past six summers, members of the Tibet Research Group of the Department of Earth Sciences at the University of Hong Kong have made detailed field investigations of stratigraphic relationships amongst rocks exposed along the Yarlung Tsangpo suture zone (Aitchison et al. 2002a). Research began with the aim of testing existing models and the hope of being able to develop a new and improved understanding of the tectonic evolution of this region. In particular, when compared with areas such as the modern western Pacific, the existing model involving con­sumption of the Tethys at a single long-lived north-dipping subduction zone developed along the southern margin of the Lhasa block appeared likely to be oversimplified. Detailed regional investigations have resulted in the better discrimi­nation of several discrete terranes together with other important units such as syncollisional conglomerates along the suture (Fig. 2). Micro­palaeontological studies have provided high-preci­sion age constraints for ophiolitic and subduction complex rocks along the suture. Together these results can be used to place constraints on the timing of interactions between the terranes present and the development of the collision zone between India and Asia. This work has resulted in the recognition of the possibility that remnants of more than one subduction system that may have existed within Neotethys are preserved within the suture zone (Aitchison et aZ. 2000).

Table 1. Summary of key characteristics of various ophiolitic rocks along the Yarlung Tsangpo suture zone

Area Lithologies Longitude Lithological-tectonic Age constraints Affinity unit

Jungbwa Ultramafic (harz and 81°E Jungbwa ophiolite Late Jurassic 152 ± 33 Ma MOR cpx-poor 1hz) (Ar/Ar, Miller et al. 2003)

Dangxiong Mostly ultramafic, 83°E Dangxiong ultramafic Undetermined Unknown minor basalt

Xigaze Complete ophiolite 85°E-90oE Dazhuqu terrane Mid-Cretaceous, Barremian SSZ (radiolarians, Ziabrev 2001)

Zedong Mostly harzburgite 92°E Luobusa ophiolite Mid-Jurassic 177 ± 31 Ma SSZ (Sm-Nd, Zhou et al. 2002)

Primary contacts

Nappe thrust southwards over northern edge of Indian terrane Thrust southwards over northern edge of Indian terrane Thrust southwards over Bainang terrane accretionary complex and Indian terrane; later (Miocene) north-directed back­thrusting over Gangrinboche facies conglomerates Originally southwards onto Indian terrane; later (Miocene) north-directed back­thrusting over Gangrinboche facies conglomerates onto Lhasa terrane; close (faulted) spatial relationship with late Mid­Jurassic Zedong terrane intra-oceanic arc rocks

~ ~ r' c:: Z Cl ...., en >­Z Cl ." o en c:: ...., c:: ~ tn N o Z tn o ."

~ o r' ....... ...., tn en

~ '-0

D Quaternary

~ Gangrinboche conglomerate

8 Liuqu Formation

Lhasa terrane

D intrusives

~ Sangn Group

D Xigaze terrane

o Zedong terrane

~ Dazhuqu terrane

• Gyantze

~ Bainang terrane

~ Indian terrane

/ depositional contact

/ fault

/ thrust fault

Fig. 2. (a) Geological map of the Xigaze-Renbung area (modified after Wang et ai. 1987) indicating the disposition of major tectonic entities in this region. GCT, Great Counter thrust. The distribution of Yamdrok melange is not shown but it mostly crops out within the northern Indian terrane within the 20 km south of the suture zone. (b) Geological map of the Zedong-Luobusa area indicating the disposition of the main tectonic entities in this region (modified after Badengzhu 1979, 1981). RZT, Renbu-Zedong thrust.

VI o

:--n

>­~ n ::c: ...... r:/)

o z ~ '-'l ~ t---<

YARLUNG TSANGPO SUTURE ZONE OPHIOLITES 151

Regional tectonic framework

Several terranes and tectonically significant units that developed before, and during, the India­Eurasia collision are recognized within and bound­ing the suture (Aitchison et al. 2000, 2002a; Fig.

South ...

3). As some knowledge of these rocks is useful in the context of evolution of the suture zone a brief description is provided below.

To the north of the suture lies the Lhasa terrane, a microcontinental block that had detached from the northern periphery of Gondwana and docked

North

Regional E-W extension .. Great Counter Thrust

SOUth-directed ~ oPhiolite emplacement

100Ma

150Ma

200Ma

'" :J

I o

Indian Bainang

amphibolite facies metamorphic event

affeciing blocks later incorporated into melange

~ I'-///,J ophiolite

generation

: Ar/Aron ~Jungbwa 1-

Dazhuqu Zedong Xigaze Lhasa

Fig. 3. Summary time-space plot for terranes and tectonically significant units recognized within, and bounding, the Yarlung Tsangpo suture that developed before, and during, the India-Eurasia collision.

152 J. C. AITCHISON ET AL.

with Asia by the Late Jurassic (Allegre et al. 1984; Yin & Harrison 2000). It is reported to contain a Middle Proterozoic to Lower Cambrian metamorphic basement overlain by Palaeozoic to middle Cretaceous shallow-marine and terrestrial deposits. The southern portion of the Lhasa terrane, immediately north of the suture, is marked by Upper Jurassic-Lower Cretaceous metasedi­mentary and metavolcanic rocks (Sangri Group), which are intruded by the regionally extensive calc-alkaline Gangdese (Trans Himalayan) bath­olith. These igneous rocks record extensive mag­matism that resulted from northward subduction of Neotethyan oceanic lithosphere beneath the Lhasa terrane. Andesites within the Sangri Group are intercalated with fossiliferous Upper Jurassic­Lower Cretaceous clastic and carbonate deposits (Badengzhu 1979; Bureau of Geology and Mineral Resources of Xizang Autonomous Region Geolo­gical Team No. 2 1979; Badengzhu 1981; Pearce & Deng 1988) and represent the oldest subduc­tion-related volcanic rocks in the southern Lhasa terrane. Radiometric ages reported from Gangdese plutons range from 153 ± 6 Ma (Murphy et al. 1997) to 30.4 ± 0.4 Ma (Harrison et ai. 2000). Extrusive units within remnants of the volcanic carapace of the batholith occur in the Lingzizong Formation and range from 60 to 48 Ma (Maluski et ai. 1982; Xu et al. 1985). These volcanic rocks overlie the Upper Cretaceous Takena Formation with an angular unconformity indicating a latest Cretaceous deformational episode in the Lhasa terrane. Cretaceous units in the southern Lhasa terrane are also reported to contain andesitic volcanic rocks (Yin et al. 1988) and volcanogenic detritus (Coulon et al. 1986). Radiometric and biostratigraphic data thus both appear to indicate that subduction-related magmatism, associated with the consumption of Neotethyan oceanic litho­sphere along the southern margin of the Lhasa terrane, commenced in the Late Jurassic and lasted until mid-Oligocene.

The southern margin of the Lhasa terrane is marked by a regionally extensive sequence of conglomerates; the 'Gangrinboche' facies (Aitchison et al. 2002b). These conglomerate units, known by a variety of local geographical names (Luobusa, Dazhuqu, Qiuwu and Kailas formations) crop out along the strike of the suture for at least 1500 krn (Aitchison et al. 2002b). They record the latest Oligocene-earliest Miocene deposition of coarse clastic sediments. Each unit lies unconformably upon a basement of Lhasa terrane rocks that initially was the sole source of sediment. Up-section detritus from sources south of the suture then becomes increasingly dominant. Development of these conglomerates was a direct result of the India-Asia collision and our under-

standing of these siliciclastic rocks has significant implications for regional tectonic models (Aitchison & Davis 2001; Aitchison et al. 2002b, 2003). The southern extent of outcrop of the Gangrinboche facies is typically marked by a north-directed thrust fault known variously as the Great Counter thrust or Renbu Zedong thrust system (Gansser 1964; Yin et al. 1994). Numerous strands of this fault exist. It carries rocks of both suture zone and Indian terrane affinity in its hanging wall and places them over a footwall of southern Lhasa terrane rocks. In some areas, northward displacement along this Miocene fault system has totally obscured or removed remnants of the YTSZ, which is marked only by a zone containing schistose serpentinite-matrix melange.

To the south of the northernmost strand of this fault system, a succession of several kilometres thickness composed of volcaniclastic turbidites (Xigaze terrane) abuts the Lhasa terrane and Gangrinboche conglomerates. These rocks extend westwards along the northern flank of the suture zone from around 90°E. The Xigaze terrane is thrust northwards over upper Oligocene-lower Miocene Gangrinboche facies conglomerates (Aitchison et al. 2002b). The original geometry of the India-Asia collision suggests that the Xigaze terrane should have been thrust southwards over the suture zone ophiolites and northern Indian terrane. However, at its southern boundary the terrane at present lies in the footwall of a younger (late Micoene) north-directed thrust with ophioli­tic rocks of the Dazhuqu terrane or Paleogene Liuqu Conglomerate (Davis et al. 2002) in the hanging wall. Sparse fossils indicate that the turbidites have an upper Albian to Coniacian stratigraphic range (Bassoullet et al. 1984; Wiedmann & Durr 1995; Wan et al. 1998). As the oldest fossils reported are not from the base of the section, it is possible that sedimentation may have commenced before the late Albian. The top of the Xigaze turbidite sequence is truncated by erosion. These rocks are interpreted as a forearc succession that developed in association with north-directed subduction of Neotethyan oceanic lithosphere be­neath the Lhasa terrane (Shackleton 1981; Burg & Chen 1984; Girardeau et al. 1984b; Einsele et al. 1994; Durr 1996; Wang et al. 1999). Development of the terrane is likely to have been intimately related to evolution of the magmatic arc along the southern edge of the Lhasa terrane (Einsele et al. 1994; Durr 1996). Although the Xigaze terrane is conventionally regarded as being floored by the Dazhuqu ophiolite (e.g. Burg & Chen 1984; Girardeau et al. 1984a; Einsele et al. 1994; Durr 1996), these two units are ubiquitously in tectonic contact and are best regarded as separate terranes (Aitchison et al. 2000).

YARLUNG TSANGPO SUTURE ZONE OPHIOLITES 153

A tectonic sliver of Middle Jurassic intra­oceanic island arc rocks (Zedong terrane) crops out at the northern edge of the suture between Zedong and Luobusa. This terrane is entirely bounded by north-directed thrusts related to the Miocene Renbu-Zedong thrust system (Yin et al. 1994). The base of the section is faulted and begins with a thin (several metres) succession of arc tholeiitic lavas overlain by a c. 15 m thick sequence of red ribbon-bedded chert then 1000+ m of volcaniclastic breccias of shoshonitic affinity. The succession is cut by numerous shoshonitic dykes with minor intrusions of diorite and leucogranite (McDermid et al. 2001 b). Both radiometric and biostratigraphic data indicate the onset of magmatism in the late Mid-Jurassic. Radiometric ages of between 152 and 161 Ma (McDermid 2002; McDermid et al. 2002) are in accord with Bajocian-early Callovian radiolarian faunas recovered from the underlying chert. The terrane is interpreted as remnants of an intra­oceanic magmatic arc (Aitchison et al. 2000; McDermid et al. 2001a; McDermid 2002) poten­tially similar to other terranes known from else­where along the suture in NW India and Pakistan.

In most areas, the main trace of the suture is marked by ophiolitic rocks such as those asso­ciated with the Dazhuqu terrane. These rocks are discussed in more detail elsewhere in the text.

Accretionary prism rocks are common on the southern side of the suture zone, and include those of the Bainang terrane (Aitchison et al. 2000). This terrane lies between ophiolitic rocks of the Dazhuqu terrane to its north and the Indian terrane to the south. Early contacts between these terranes are south-directed thrust faults (Burg 1983; Burg & Chen 1984; Ratschbacher et al. 1994). It contains units previously referred to as infra­ophiolitic thrust sheets of radiolarites (Burg & Chen 1984) or Upper Jurassic-Lower Cretaceous red radiolarites (Girardeau et al. 1984a). Good exposures exist near Donglha, Xialu and Bainang. Radiolarians reported from siliceous rocks near Xialu have ages ranging from the Mid-Jurassic to Cretaceous (Aptian) (Wu 1993; Matsuoka et al. 2001a, 2001b). Detailed geological mapping and investigations of radiolarian biostratigraphy eluci­date the structure, stratigraphy and evolution of the terrane (Ziabrev et al. 2000, 2003a; Ziabrev 2001). It contains numerous north-facing and chiefly south-verging tectonic slices that experi­enced structural imbrication during the Cretaceous as the Bainang terrane accretionary prism devel­oped. These slices incorporate various lithologies: chert, siliceous and tuffaceous mudstones, lime­stone, siliceous and calcareous shales. The most complete and well-exposed piece of the terrane near Bainang has been mapped and sampled for

radiolarian biostratigraphic study (Ziabrev 2001). Five lithotectonic units can be discriminated based on their characteristic lithologies and structural styles. Radiolarians allow reconstruction of a relict stratigraphy that records a long history of sedi­mentation in different portions of Tethys since the Late Triassic (Ziabrev 2001). This stratigraphy records the northward travel of an oceanic plate and its mid-Cretaceous approach towards a sub­duction zone in a remote intra-oceanic setting (Ziabrev et al. 2003a).

Passive margin rocks of the Indian terrane or Tethyan (Tibetan) Himalaya lie south of the suture. They include thick Permian to Lower Cenozoic continental rise deposits (Liu 1992), which merge southward into a continuous Ordovi­cian to Eocene shelf sedimentary succession of marine carbonates, sandstone, siltstone and shale (Bureau of Geology and Mineral Resources of Xizang Autonomous Region 1993; Jadoul et al. 1998). The development of the passive continental margin facing the Tethyan domain was punctuated by a series of rifting episodes related to Gondwana disintegration and associated with intra-plate vol­canism (Gaetani & Garzanti 1991). In the western Himalaya the Zanskar shelf merges northward with Mesozoic slope-rise deep-sea deposits of the Lamayuru complex and its distal equivalent, the Karamba complex (Danelian & Robertson 1997; Robertson 1998). Fragments of carbonate-topped Permian seamounts in the Ladakh Himalaya (Robertson & Sharp 1998) and south Tibet (Aitchison et al. 2000), which locally fringe the northern Indian passive margin, may have devel­oped on the oldest Tethyan oceanic crust. Passive margin was terminated by the Cenozoic India­Asia collision. Disruption of northern Indian margin rocks into widespread regional melange zones accompanied Paleogene collision events (Liu 2001; Liu et al. 2002). However, continuing deposition of marine carbonate sediments in the Indian terrane until at least the Eocene-Oligocene boundary (Wang et al. 2002) suggests that con­tinent-continent collision and final closure of Tethys was a later event.

Extensive zones of disrupted rocks occur as the Yamdrok melange to the south of, and parallel to, the Yarlung Tsangpo ophiolites. They extend several hundreds of kilometres with a width of several to tens of kilometres (Liu & Einsele 1996). Field investigations near Gyangze indicate that these mud-matrix melanges are sandwiched within zones of Mesozoic rocks of India passive margin affinity (Liu 2001). Contacts are irregular and transgress bedding, indicating a likely tec­tonic and/or diapiric origin for the melange (Pini 1999). The ages of cherty blocks in the melange range from Late Jurassic to Late Cretaceous.

154 J. C. AITCHISON ET AL.

Siliceous siltstones and mudstones within the melange matrix have yielded lower Aptian radi­olarians. A new uppermost Paleocene radiolarian fauna from the matrix of the melange near Gyantze places important maximum age con­straints on the timing of formation of this unit (Liu & Aitchison 2002).

Several conglomerate units are exposed along the YTSZ and each one possibly documents discrete tectonic events. Rapidly deposited coarse clastic rocks of the Liuqu Conglomerate record a Paleogene phase of sedimentary basin develop­ment along the suture (Davis et al. 1999, 2001, 2002). Sediment is dominated by conglomerates, and deposition occurred in both terrestrial and sub-aqueous settings. Facies changes are com­monly abrupt, with rapid changes in clast types, grain size and depositional styles. Proximal depos­its are locally offset relative to their original source terranes. These coarse clastic sedimentary rocks are interpreted as having been deposited in oblique-slip basins that developed as an intra­oceanic island arc collided with the leading (northern) edge of the Indian margin (Davis et al. 1999, 2001, 2002). Sediments were derived from both areas and the conglomerates record aspects of the history of collision between these terranes. The absence of clasts derived from terranes to the north of the Yarlung-Tsangpo suture suggests that basins in which the coarse clastic units accumu­lated developed before the main collision between India and Asia.

The original disposition of terranes within the suture zone has been greatly disrupted and former relations between terranes are not well con­strained. Reconstruction of the tectonic evolution of the area is therefore difficult. Most early models (Allegre et al. 1984; Searle et al. 1987) suggested that a single Andean-type convergent plate margin along the northern side of Tethys was responsible for destruction of the Neotethys, although the possibility of additional subduction zones was considered by some workers (Proust et al. 1984). The co-occurrence and north-south distribution of the Zedong (magmatic arc), Dazhu­qu (suprasubduction zone ophiolite) and Bainang (subduction complex) terranes led to their inter­pretation as evidence of the former existence of a south-facing intra-oceanic subduction system that lay within the Tethys (Aitchison et al. 2000) and the possible existence of more than one conver­gent margin. Analogy with the modem western Pacific and SE Asia suggests that reality may have been even more complex. As more details and constraints on the evolution of terranes within the YTSZ become available, the complexity and sophistication of models for this zone is likely to further increase.

Ophiolitic rocks along the Yarlung Tsangpo suture zone

Dazhuqu terrane: Xigaze

Ophiolitic rocks in the Xigaze region are the best known occurrences along the suture because of their proximity to one of the few highways through Tibet (Wang et al. 1987). Outcrop is near continuous from Ngarnring-Tso eastwards to Re­nbung. The ophiolite in this area was the subject of detailed investigations by Chinese and French geologists particularly in the early 1980s. Early investigations recorded details of the petrology and geochemistry of lithologies present, the meta­morphic and structural history of the ophiolitic massifs (Nicolas et al. 1981; Shackleton 1981; Tapponnier et al. 1981; Burg 1983; Allegre et al. 1984; Girardeau et al. 1984b, 1984c, 1985a, 1985b, 1985c; Wang et al. 1987; Girardeau & Mercier 1988; Einsele et al. 1994; Durr 1996).

Numerous massifs of mostly ultramafic rocks are accompanied by all other elements of an ophiolite suite and together these rocks are as­signed to the Dazhuqu terrane (Aitchison et al. 2000). Ultramafic massifs dominate the zone of outcrop with major occurrences at Dazhuqu (Fig. 4e), Qunrang, Xigaze, Jiding, Liuqu and Ngarnr­ing. Other elements of an ophiolite suite are also present, although lower-crustal sections are not well preserved and have locally been eliminated by tectonic attenuation of the section. Sheeted dykes are well developed both SE of Qunrang (Fig. 4b) and SW of Sagui. An important observa­tion of Girardeau et al. (1985c) was that diabase dykes occur at all levels throughout the entire ophiolite section, indicating the possibility of more than one phase in the magmatic evolution of this terrane.

Most massifs preserve fragments of a basaltic section with a thin sequence of overlying sedimen­tary strata. Marine siliceous and fine-clastic de­posits that cover the ophiolite mostly crop out along the northern margin of the Dazhuqu terrane. These deposits are referred to as the Chongdu Formation (R. L. Cao 1981, cited by Bureau of Geology and Mineral Resources of Xizang Auton­omous Region 1993). Sections of pillow basalt overlain by a few metres of chert then volcaniclas­tic sediments are best exposed in the Qunrang district. Other sections are known from south of Jiding at Donglha (two sections; Fig. 4c), Sagui, Beimarang, Bainang, Dazhuqhu and Polio. In most cases, the pseudo-stratigraphies of ophiolitic sec­tions are north-facing although south- and west­facing sections occur at Donglha and Bainang, respectively. Early investigations of the Chongdu Formation presented preliminary age data

YARLUNG TSANGPO SUTURE ZONE OPHIOLITES 155

Fig.4. (a) Ophiolitic rocks exposed at Dangxiong near the headwaters of the Yarlung Tsangpo, western Tibet. Photograph taken from 300 20'N, 082°55'E looking eastwards. Ultramafic rocks crop out in the low hills on the far side of the river fiats. (b) Steeply east-dipping dykes exposed in upper-crustal levels of the Dazhuqu ophiolite SE of

" Qunrang. Photograph taken from 29°09'N, 089°01'E looking eastwards. (c) An overturned sequence of pillow basalts overlain by a thin «20 m) sequence of uppermost Barremian to upper Aptian ribbon-bedded radiolarian cherts exposed at Donglha (29°08'N, 088°25'E) along a canal located SSE of Jiding. (d) Rocks of the Dazhuqu ophiolite observed looking westwards from a garnet amphibolite knocker locality (29°09'N, 089°20'E) in serpentinite-matrix melange at the base (south) of the ophiolite. Serpentinite-matrix melange lies in the foreground; rocks on the ridge on the left of the photo are dominated by cherts of the Bainang terrane. The centre right of the photo is dominated by serpentinized harzburgites and the mountain over the river valley in the distance is mostly a north-facing section of diabase and gabbro. (e) Serpentinized harzburgites of the Dazhuqu terrane (viewed looking west), which form the high (5000+ m) table-topped mountain, have been thrust southwards over the northern margin of the Indian terrane near Renbung. Thick beds within the Dazhuqu conglomerate crop out on the hill slopes on the sides of the Yarlung Tsangpo in the foreground. (f) View looking southwards towards the Purang valley from between Mapham Tso (Lake Manasarovar) and Lhanak Tso (Lake Raksas) south of Gangrinboche (Mt Kailas, 6714 m). The Jungbwa ophiolite crops out on the hills to the SW of Lake Raksas. (g) View from the Luobusa Chromite mine looking westwards up the Yarlung Tsangpo. Outcrops of the Lhasa terrane and Gangdese batholith lie in the distance beyond the ophiolite. (b) Northward-dipping tectonic slices of basalt, pillow breccia and basaltic hyaloclastite overlain by red ribbon­bedded chert crop out on the southeastern side of ultramafic massifs near Dangxiong (300 09'N, 083°14'E).

156 J. C. AITCHISON ET AL.

(Marcoux et al. 1982) and suggested that the ophiolite had formed in the Cenomanian.

On the basis that the Xigaze ophiolite sections are thinner than those reported from other classic ophiolites, such as those of Cyprus or Oman, the Sino-French group considered the ophiolite to be somewhat different (Nicolas et al. 1981). They suggested that it had formed at a slow-spreading ocean ridge located proximal to the margin of the southern Lhasa block (Pozzi et al. 1984). The presence of diabase dykes throughout the section suggests primary structural attenuation of the ophiolite in response to tectonic extension at the ridge system where it was generated. Further section removal probably occurred during tectonic emplacement onto India or later India-Asia collision.

More recent workers have re-examined the geochemistry of some of these rocks (Hebert et al. 2000, 2001) and undertaken detailed investigations of the radiolarian biostratigraphy (Zyabrev et al. 1999; Ziabrev 2001; Ziabrev et al. 2003b) and palaeomagnetism (Abrajevitch et al. 2001, 2002, 2003) of sediments immediately overlying the ophiolite.

The northern margin of the ophiolite lies in the hanging wall of a steep north-directed Mio­cene 'back' -thrust. A tectonic contact with a footwall of fore-arc basin turbidites of the Xi­gaze terrane is ubiquitous (Aitchison et al. 2003). Although the ophiolite is typically north­facing with its uppermost levels exposed near the contact, no depositional continuity with the Xigaze terrane can be demonstrated. Earlier suggestions of original depositional continuity between Dazhuqu and Xigaze terranes can be dismissed. The Dazhuqu terrane consists of numerous discrete and seemingly unrelated mas­sifs. Individual ophiolitic massifs preserve dis­crete structural levels in different proportions. Palaeomagnetic data indicate that massifs had origins at different locations and have experi­enced varying degrees of tectonic rotation (Abrajevitch et al. 2001, 2002, 2003). The purportedly overlying Xigaze terrane is, on the other hand, structurally coherent and contains bedding packages, which can be traced along strike for several tens of kilometres.

The southern margin of the ophiolite is typi­cally dominated by ultramafic rocks with develop­ment of a wide (up to several hundreds of metres) zone of serpentinite-matrix melange. Locally this melange contains rare knockers, which have ex­perienced amphibolite-facies metamorphism (Fig. 4d). In the Bainang district, an S-shaped sigmoidal bend in the YTSZ preserves earlier contacts with the Bainang terrane that predate later north­directed back-thrusting. Ophiolitic rocks and ser-

pentinite-matrix melange lie in the hanging wall of a south-directed thrust that is locally truncated by strike-slip faults (Girardeau et ai. 1985b; Ratschbacher et ai. 1994). Together with rocks of the Bainang terrane the ophiolite was emplaced southwards onto the northern edge of India before the India-Asia collision.

Sedimentary rocks immediately overlying the ophiolite have been the subject of recent investiga­tions by a research team from the University of Hong Kong, which show that they differ even between nearby localities (Ziabrev et al. 2003b). Overall, the sedimentary sections exhibit a coar­sening-upward trend from chert and siliceous mudstones intercalated with, and immediately above, pillow basalts. Up-section sedimentary rocks become predominantly volcaniclastic and include basaltic agglomerates, felsic tuffs, and volcano lithic turbidites indicating an island-arc depositional environment. Detailed investigations of well-preserved, abundant radiolarians (Fig. 5) provide high-precision biostratigraphic age con­straints (Fig. 6) on the timing of the eruption of ophiolitic basalts in this region (Zyabrev et ai. 1999; Ziabrev 2001; Ziabrev et al. 2003b). These results collectively show that ophiolitic rocks in the Xi gaze district were generated in an intra­oceanic suprasubduction zone setting within a relatively short (c. 6 Ma) interval between the late Barremian (c. 123 Ma) and mid-Aptian (c. 117 Ma). Accumulation of sediments upon the newly generated ophiolite initially occurred in a series of discrete rift-controlled sub-basins asso­ciated with various spreading centres. An increas­ing flux of arc-derived volcaniclastic sediment up-section indicates activity in a nearby volcanic arc. The Dazhuqu terrane appears to have devel­oped in an intra-oceanic setting within Tethys where it was isolated from any continental influ­ence.

Application of high-precision radiolarian bios­tratigraphy in constraining the ages of basalts and overlying sediments in the Dazhuqu terrane has made it possible to reinvestigate the palaeomag­netism of these rocks so as to better constrain where they might have formed. Results show that the ophiolite formed at equatorial to very low northern latitudes (Abrajevitch et al. 2001, 2002, 2003). Some fragments of the ophiolite have clearly experienced a counter-clockwise rotation whereas sedimentary successions within the Xi­gaze terrane, previously reported to conformably overlie the ophiolite, are coherent over many kilometres of their strike length. All sections from which conformable contacts with the Xigaze terrane had been reported have been re-examined and north-directed thrusting is ubiquitous between these two terranes.

YARLUNG TSANGPO SUTURE ZONE OPHIOLITES 157

Fig. 5. Some representative mid-Cretaceous (lowermost upper Aptian VA8) radiolarians from siliceous mudstones intercalated with volcaniclastic sediments, which overlie pillow basalts of the Dazhuqu terrane SE of Qunrang (29°08'N, 089°02'E) (scale bars represent 100 1Jll1). (a) Acaeniotyle umbilicata(Riist); (b) A. diaphorogona Foreman; (c) Crucella hispana O'Dogherty; (d) Hiscocapsa grutterinki (Tan); (e) Pseudodictyomitra hornatissima (Squinabol); (t) Pseudodictyomitra pentacolaensis Pessagno; (g) Stichomitra communis Squinabol; (h) Thanarla brouweri (Tan); (i) Turbocapsula costata (Wu); (j) Xitus clava (Parona).

Dazhuqu terrane: Ngamring Tso-Saga­Zongba-Jungbwa

Ophiolitic rocks crop out more or less continu­ously in a westerly direction from Ngamring Tso to Saga. The general distribution of these rocks has been mapped (Zhang & Fu 1982; Burg 1983) but they have not been the subject of any detailed investigation. Outcrop occurs mostly as ophiolitic melange although a large ultramafic massif is present a few kilometres south of the river crossing at Sangsang (29°24'N, 086°43'E). From Ngamring west, outcrop lies north of the Yarlung Tsangpo but to the south of the main road leading to Saga (29°20'N, 085°25'E). Just east of the turnoff for the northern road to Tsochen and western Tibet the main road crosses to the south of the ophiolite. Outcrop is mostly obscured by Quaternary allu­vium in a swampy area. West of the turnoff, ophiolitic rocks, mostly diabases and basalts, are exposed on the uppennost southern slopes of a large peak (6000+ m) and outcrop continues further westwards on high peaks. From Saga

westwards to beyond Zongba (29°45'N, 083°55'E), limited occurrences of ophiolitic rocks occur well to the north of the main (only) road and these rocks have not been studied. West of Zongba, a settlement in the large graben leading northwards from Mustang in Nepal, a significant occurrence of ophiolitic rocks is encountered near Dangxiong (300 09'N, 083°14'E). The ophiolite here is domi­nated by ultramafic massifs (Fig. 4a), which include Cr-bearing dunite bodies. This mineraliza­tion has been explored in detail by geologists of the Bureau of Geology and Mineral Resources of the Xizang Autonomous Region. The ophiolitic rocks have been emplaced southwards onto rocks of Indian affinity and lie several kilometres south of the suture zone. Numerous, south-vergent, thin (decametre-scale) tectonic slices of northward-dip­ping basalt, pillow breccia and basaltic hyaloclas­tite overlain by red ribbon-bedded chert crop out on the SE side of the ultramafic massifs (Fig. 4h). Although we have examined these rocks for radi­olarian faunas, no identifiable material has been recovered so far from acid residues.

158 J. C. AITCHISON ET AL .

..... Age (Ma) C, 110 ..... ("II t: ("II

cis cis ca C, '-..t: .&:. t: t: m m ::l ca a '-t: t: t:

0 0 ::l ::l C C a c..

I ca

~ C)

115 ca N

en • :::l 0 W u ::l « cr f-

I ::l w .2 ..t:

0:: "0 N 120 u ca

D- C 0::

I w

~ I ~ 0

125

Fig. 6. Radiolarian-based ages of supra-ophiolitic sedimentary sections associated with the Dazhuqu terrane ophiolite. Unitary associations (UA) after Baumgartner et al. (1995) and O'Dogherty (1994). Chronological scale after Gradstein et al. (1994).

Further westwards, ophiolitic rocks are again encountered when Mapham Tso (Lake Manasaro­var) is reached south of Gangrinboche (Mt Kailas, 6714 m, 31°04'N, 081°19'E). The ophiolitic rocks here are referred to as the Jungbwa (Yungbwa) ophiolite (Gansser 1964; Searle et al. 1987; Murphy et al. 2002; Miller et al. 2003; Murphy & Yin 2003). Outcrop occurs mostly to the south and west of Lhanak Tso (Lake Raksas, Fig. 4f), where it is dominated by a large (>3500 km2 )

ultramafic massif. Harzburgite and cpx-poor lher­zolite are the dominant lithologies and isotopic data for these rocks (Miller et al. 2003) suggest that peridotite melting occurred during the Early Jurassic. Ultramafic rocks in this region are locally cut by pegmatitic gabbronorite and rare normal mid-ocean ridge basalt (N-MORB) type tholeiitic basalt dykes for which Miller et al. (2003) reported a Late Jurassic 40 Arj39 Ar age of 152 ± 33 Ma. Additional outcrops of ultramafic rocks occur further NW in the Kiogar region, where no reports of the upper levels of any ophiolitic succession exist and the most detailed descriptions of these rocks are from recon­naissance studies only (Gansser 1964). All the ophiolitic rocks lie in the hanging wall of south­directed thrusts, which transported them over rocks of Indian affinity (Gansser 1964). The Jungbwa ophiolite has been thrust 30-40 km south of the YTSZ (sensu stricto) and lies atop passive margin sediments of the Indian terrane (Murphy & Yin, 2003). The Kiogar ophiolite has

been thrust a similar distance southwards over the Indian terrane and development of extensive mud­matrix melange (Gansser 1964) may have accom­panied its emplacement. The northern side of the YTSZ (sensu stricto) in the Kailas region is marked by the north-directed South Kailas thrust along which zones of sheared serpentinite and ophiolitic melange occur (Yin et al. 1999).

Dazhuqu terrane: Renbung-Quxu

Outcrops of ophiolitic rocks, which occur as blocks in a serpentinite-matrix melange, can be traced eastwards intermittently from Renbung to west of Lhasa airport. The largest zone of outcrop is located on the SW side of the Yarlung Tsangpo to the south of a 6126 m peak on which Miocene Gangrinboche facies conglomerates lie uncon­formably upon Lhasa terrane rocks (Aitchison et al. 2002b). South of Quxu, serpentinite-matrix melange with large blocks of ultramafic rocks crops out together with diabase dykes and basalts where the YTSZ passes through a narrow col (Jiangdanyako, 29°18'N, 0900 43'E) between Lhasa terrane granites and the Indian terrane. The ophio­litic rocks lie within shear zones associated with the north-directed Renbu-Zedong thrust system.

Ophiolitic rocks: Zedong-Luobusa

A further major occurrence of ophiolitic rocks along the YTSZ crops out to the SE of Lhasa near

YARLUNG TSANGPO SUTURE ZONE OPHIOLITES 159

Zedong and Luobusa. The largest chromite deposit (Fig. 4g) in China is currently being worked at Luobusa and rocks there have been the subject of numerous investigations (Badengzhu 1979, 1981; Huang et al. 1981; Bai et al. 1993, 2000; Zhou & Robinson 1994; Zhou 1995; Zhou et ai. 1996, 2002; Griselin et ai. 1999; Hu 1999; Hebert et al. 2000, 2001). The Luobusa ophiolite is dominated by a harzburgitic mantle section that has experi­enced Late Miocene India-Asia collision-related north-directed thrusting over a dunite transition zone and ophiolitic melange (Zhou et ai. 1996). Lenses and pods of dunite are abundant within the ultramafic section and these rocks contain the chromite mineralization. Restricted occurrences of gabbros are also known but no diabase dykes have been reported from Luobusa. Together, the entire package has been thrust northwards over Lower Miocene Luobusa conglomerates (Gangrinboche facies) on the southern margin of the Lhasa terrane (Aitchison et ai. 2002b) and is itself overthrust by Triassic Indian terrane rocks. The original structural relationship between the ophio­litic rocks and the Indian terrane is not preserved.

The geochemistry and petrology of the Luobusa ophiolite are distinctive. Spinels from the Luobusa massifs have Cr numbers as high as 0.69-0.94 and olivines are more forsteritic than elsewhere along the YTSZ (Wang et al. 1999; Hebert et al. 2000, 2001). Such values are similar to those for rocks produced from boninitic melts and are con­siderably more refractory than for rocks from elsewhere along the suture. One of the most unusual features of the Luobusa ophiolite is the reported occurrence of small amounts of micro­diamonds, graphite, SiC and other rare minerals found in association with chromitites (Bai et ai. 1993, 2000; Hu 1999).

Ophiolitic rocks at Luobusa and Zedong are intimately associated with island arc tholeiitic rocks of the Zedong terrane. At Luobusa, this terrane is limited to a small zone of overturned variolitic pillow basalts, which are stratigraphi­cally overlain by red mudstones. Outcrop is much more extensive near Zedong, where the terrane consists of a sequence in which island arc tholeii­tic pillow basalts are overlain by a thin succession of red radiolarian cherts, which are themselves overlain by an up to 1 km thick pile of shoshonitic autoclastic breccias (McDermid 2002). The exact nature of the original relationship between these two terranes remains indeterminate, as all contacts are faulted. Nevertheless, limited age data from the Luobusa ophiolite (Sm-Nd age of 177 ± 31 Ma on gabbro dykes given by Zhou et al. 2002) are similar to U-Pb, Ar-Ar and fossil­based ages for rocks of the Zedong terrane (McDermid 2002; McDermid et al. 2002). The

relationship of ophiolitic rocks in the Zedong­Luobusa district to other ophiolitic rocks along the suture remains ambiguous. Although ophiolitic rocks in the Zedong to Luobusa area lie in an identical structural position along the YTSZ, both the compositions (Wang et al. 1999; Hebert et al. 2000, 2001) and ages of these rocks (Ziabrev 2001; McDermid 2002; McDermid et al. 2002; Ziabrev et al. 2003b) differ from those of other ophiolitic rocks of the YTSZ. Further work is clearly required to resolve this enigma. We note reports of similar ages for the oldest ophiolitic rocks of the Spontang ophiolite in Ladakh, NW India (Pedersen et al. 2001), and suggest that it is entirely possible that remnants of more than one intra-oceanic island arc are preserved between India and Asia.

Emplacement

Kinematic indicators along the southern margin of the Dazhuqu terrane ophiolites near Bainang, SE of Xigaze, indicate its southward emplacement onto the feather-edge of the northern Indian continent (Girardeau et ai. 1985c; Ratschbacher et ai. 1994). Ophiolites further west along the suture have similarly been emplaced southwards onto the northern Indian passive margin. Precise constraints on the timing of this emplacement event have proved difficult to obtain with seemingly contra­dictory datasets. Traditionally the ages of amphi­bolite-facies metamorphic rocks within melange zones at the base of ophiolitic successions have been interpreted as indicative of emplacement events. In this case, Late Cretaceous ages for amphibolites from near Bainang (Wang et al. 1987) would seemingly indicate obduction at that time. A tectonic event of this nature, however, seems inconsistent with any events recorded in regional sedimentary successions, which should reflect such an event. An alternative interpretation of the Late Cretaceous metamorphic ages known from various localities along the suture within Tibet (Zhou cited by Aitchison et ai. 2000) and further afield in NW India and Pakistan (Searle et al. 1999) is that they represent an ophiolite­specific thermal event such as the subduction of a spreading centre (Shervais 2001). The timing of emplacement of ophiolitic rocks onto the northern margin of India is perhaps better constrained from the sedimentary record within, and south of, the suture. Distal Indian passive margin sediments in the northern Tethyan Himalayan zone, north of Qomolangma (Mt. Everest) record a significant tectonic disturbance and the development of a major disconformity around the Cretaceous­Paleogene boundary (Wan et ai. 2002) before resumption of normal carbonate accumulation in a

160 J. C. AITCHISON ET AL.

quiescent environment until the time of India­Asia collision. Along the suture itself, ophiolitic rocks as well as strata of the Indian and Bainang terranes are unconformably overlain by Paleogene Liuqu conglomerates. These coarse clastic sedi­ments are interpreted to have accumulated during the collision of the ophiolitic terrane and the Indian continental margin, which lay to its south (Davis et al. 2002). Other Lower Miocene con­glomerates along the suture contain clasts from all nearby terranes, including those north of the suture, and constrain the timing of collision be­tween India and Asia (Aitchison et al. 2002b). Regional development of mud-matrix melange was extensive along the northern Indian margin in the Paleogene (Gansser 1964; Shackleton 1981; Liu & Einsele 1996; Liu 2001). These melanges possibly developed in response to ophiolite obduc­tion and are analogous to similar rocks in con­temporary arc-continent collision zones such as Timor (Barber et al. 1986) and Taiwan (Chang et al.2000).

Discussion

As a result of detailed investigations along the YTSZ over the past few years, it is now possible to provide a better understanding of ophiolites in the region. Improved precision for Upper Meso­zoic radiolarian biostratigraphy (O'Dogherty 1994; Baumgartner et al. 1995) allows more precise age constraints to be placed on any interpretations of the development of this region. Radiolarian ages from sediments immediately overlying the ophiolite constrain the timing and duration of the ophiolite generation event. Biostra­tigraphic investigations of numerous sections in­dicate that the Dazhuqu terrane ophiolite formed in the Barremian (mid-Cretaceous) with accumu­lation of volcaniclastic sediments continuing into the Aptian (Zyabrev et al. 1999; Ziabrev 2001; Ziabrev et al. 2003b). Results of palaeomagnetic investigations indicate generation of the Dazhuqu terrane ophiolite at equatorial to low northern latitudes at least 1000-1500 km south of Asia's margin (Abrajevitch et al. 2001,2003; Abrajevitch jevitch 2002). Some fragments of the ophiolite have experienced a counter-clockwise rotation and individual ophiolitic massifs appear to have differ­ent travel histories. Although many workers had previously inferred depositional continuity be­tween ophiolitic rocks of the Dazhuqu terrane and volcaniclastic turbidites of the Xi gaze terrane, detailed investigations of all known localities where this relationship has been inferred unequi­vocally reveal that this contact is tectonic. Sedi­mentary rocks overlying the ophiolite are volcaniclastic, as are those in the Xi gaze terrane,

but the sources of the two suites of rocks were different. Furthermore, the structural evolution of both terranes was remarkably different and there is no evidence that these two entities shared a common history before India-Asia collision.

Present interpretation of the Dazhuqu terrane ophiolite as having an origin within a suprasub­duction zone setting is supported by detailed mineralogical and geochemical studies in the Xi gaze area (Wang et ai. 1987; Hebert et al. 2000, 2001). The close spatial association of these rocks with the subduction complex assemblage of the Bainang terrane suggests the existence of a south­facing intra-oceanic island arc at near equatorial latitude within Neotethys during the mid-Cretac­eous (Aitchison et al. 2000). As more data become available from along the YTSZ, interpre­tation of this zone becomes increasingly compli­cated. Ages for ophiolitic assemblages in essentially the same structural positions at Xigaze, Jungwa and Zedong are distinctly different. It now seems that the YTSZ contains remnants of at least one, and possibly two, intra-oceanic island arc systems. Given the complexity of analogous mod­em settings such as the western Pacific, it is clear that early interpretations were oversimplified. Mul­tiple arcs may have coexisted in a wide Tethys with no single simple convergent margin. It is also probable that plate boundaries were not necessa­rily parallel to one another. The suture extends from one side of Tibet to the other and beyond. Despite a few detailed studies of isolated occur­rences, the Yarlung Tsangpo ophiolites have re­ceived remarkably little attention relative to their significance in interpretation of the evolution of Tethys and the India-Asia collision.

Continuing Tibet research at the University of Hong Kong is supported by grants from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project Nos. HKU7102/98p, 7299/99P and 7069/01 P). We thank colleagues in the Geological Society of Tibet for their assistance in arranging the logistics related to this research. The authors gratefully acknowledge the constructive reviews of Y. Dilek and P. Robinson, which helped to improve the manuscript.

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