Radiolarian age constraints on Mesotethyan ocean evolution, and their implications for development of the Bangong Nujiang suture, Tibet more

Journal of the Geological Society Radiolarian age constraints on Mesotethyan ocean evolution, and their implications for development of the BangongNujiang suture, Tibet Alan T. Baxter, Jonathan C. Aitchison and Sergey V. Zyabrev Journal of the Geological Society 2009; v. 166; p. 689-694 doi:10.1144/0016-76492008-128 Email alerting service Permission request Subscribe click here to receive free email alerts when new articles cite this article click here to seek permission to re-use all or part of this article click here to subscribe to Journal of the Geological Society or the Lyell Collection Notes Downloaded by University of Hong Kong Libraries on 21 June 2009 © 2009 Geological Society of London Journal of the Geological Society, London, Vol. 166, 2009, pp. 689–694. doi: 10.1144/0016-76492008-128. Radiolarian age constraints on Mesotethyan ocean evolution, and their implications for development of the Bangong–Nujiang suture, Tibet A L A N T. BA X T E R 1 , J O NAT H A N C . A I T C H I S O N 1 * & S E R G E Y V. Z YA B R E V 2 1 Tibet Research Group, Department of Earth Sciences, University of Hong Kong, Hong Kong SAR, China 2 Institute for Tectonics & Geophysics, 65 Kim Yu Chen Street, Khabarovsk, 680000 Russia *Corresponding author (e-mail: jona@hku.hk) Abstract: Radiolarian discoveries indicate that deep-marine conditions prevailed in central Tibet from the early Middle Jurassic until well into the Early Cretaceous (late Hauterivian–early Aptian; 131–121 Ma) and help to constrain the temporal extent of oceanic conditions along the Bangong–Nujiang suture. These new fossils occur in chert or siliceous mudstone blocks associated with the Lagkor Tso ophiolitic melange. Basin ´ inversion associated with closure of Mesotethys between the Qiangtang and Lhasa terranes was accompanied by melange formation and predated regionally widespread deposition of overlying shallow-marine late ´ Aptian–Albian orbitolinid limestones. The c. 1700 km long Bangong–Nujiang suture zone is marked by scattered occurrences of ophiolitic rocks extending from Bangong Tso (tso ¼ lake) in the west (808E), through Lagkor Tso (848E), Dong Tso (858E) and Dongqiao–Xainxa (908E) to east of Nujiang (978E) (Pan & Zheng 1983) (Fig. 1). The zone is somewhat diffuse and varies greatly in width to .200 km near Dongqiao. It separates the Qiangtang and Lhasa terranes in central Tibet and has been interpreted to mark the trace of a former Mesotethyan ocean that is thought to have opened in the Permian, with closure in the Late Jurassic (Dewey et al. 1988; Metcalfe 2002). Two broadly subparallel zones of ophiolitic rocks are locally discriminated with differing hypotheses as to their origins. In western Tibet, the ophiolitic melange around Rutog is subparallel ´ to outcrops further south near Shiquanhe. Some workers regard these as discrete suture zones (Matte et al. 1996). Others regard them as klippe of a main Bangong–Nujiang suture rooted to the north (Girardeau et al. 1984, 1985; Kapp et al. 2003). A similar pattern is seen in the Gertse district of central Tibet with northern occurrences near Dong Tso and more southerly outcrops around Lagkor Tso. Further east, ophiolitic outcrops have a more extensive north–south distribution from Dongqiao to Xianxa. Ophiolitic melanges structurally overlie sedimentary rocks of the ´ Lhasa terrane and are regarded as remnants of a series of southdirected nappes. The ophiolites are themselves overlain by midCretaceous shallow-marine limestones (Girardeau et al. 1984, 1985). Various radiometric ages have been reported for the ophiolitic rocks. Notably, most are Jurassic with no older ages. Pearce & Deng (1988) inferred a Jurassic age from late magmatic amphiboles. Gabbros from the Dong Tso (Shemalagou) ophiolite in central Tibet yield an age of 191 Æ 22 Ma (Qiu et al. 2004). Middle Jurassic sensitive high-resolution ion microprobe (SHRIMP) U–Pb ages (177.1 Æ 1.4 to 162.5 Æ 8.6 Ma) have recently been reported from Bangong Tso (Shi 2007) and Lagkor Tso (166 Æ 2.5 Ma) (Zhang et al. 2007). In many areas, the ophiolitic rocks have a suprasubduction-zone signature (Girardeau et al. 1986; Pearce & Deng 1988; Liu et al. 2002; Shi et al. 2004) and where accompanied by other arc-related rocks these too have similar ages (Guynn et al. 2006). The youngest reported 689 magmatic age for rocks of the suture zone is a U/Pb (single zircon fusion method) age of 128 Ma for ophiolitic gabbro in Nagqu County (Chen et al. 2006). Closure of a Mesotethyan ocean along the Bangong–Nujiang suture is widely regarded as an event that occurred some time within the range of Late Jurassic to Early Cretaceous (Dewey et al. 1988; Yin & Harrison 2000). Although this event is significant for understanding the tectonic evolution of Tibet, the age estimate is remarkably imprecise and lies within an interval when numerous other events were taking place in the region. We present new fossil data that place important temporal constraints on the age of the Bangong seaway and its closure. Local geology Our investigations have concentrated on little-studied portions of the Bangong–Nujiang suture in Gertse County of Ngari prefecture in western Tibet. The presence of complete ophiolite successions (e.g. Dong Tso 50 km east of Gertse township) confirms the generation of oceanic lithosphere, with geochemical data indicating that this occurred in a suprasubduction-zone environment (Wang et al. 2008). A narrow east–west-trending (.75 km2 ) zone of serpentinite-matrix ophiolitic melange crops ´ out north of Lagkor Tso c. 25 km south of the town of Gertse. Although the existence of ophiolitic rocks in this area has been known for some time they have not attracted detailed investigation. Several researchers (Kidd et al. 1988; Pearce & Deng 1988) have mentioned Lagkor Tso as part of a larger sublinear east– west-trending belt of Mesotethyan ophiolites. More recently, Zhang et al. (2007) and Wang et al. (2008) have reported aspects of the Lagkor Tso area. Three main units are recognized in the area (Cheng & Xu 1986): (1) Permian limestone crops out in the south and lies beneath ophiolitic melange in the footwall of a steeply NE´ dipping thrust fault; poorly preserved fossils including ammonoids and trilobites are locally abundant in the limestones; (2) ophiolitic rocks at Lagkor Tso occur as blocks within serpentinite-matrix melange thrust southwards over the Permian lime´ stone; (3) Aptian–Albian orbitolinid limestone unconformably overlies the melange. ´ 690 A . T. BA X T E R E T A L . Fig. 1. Map showing the location of the Bangong–Nujiang suture ophiolite occurrences together with an inset map of the Lagkor Tso area indicating the distribution of ophiolitic melange and radiolarian sample locations. JS, Jinsha suture; BNS, Bangong–Nujiang suture; KF, Karakoram Fault; SS, Shyok ´ suture; IS, Indus suture; YTSZ, Yarlung Tsangpo suture zone. Ultramafic lithologies (harzburgites and pyroxenites) are the dominant blocks in the melange, and are up to 1 km in size. ´ They are accompanied by other subordinate lithologies that include gabbro, pillow basalt, chert, limestone, volcaniclastic sandstone, granodiorite, tonalite, felsic volcanic rocks and amphibolite, all of which are enveloped within a sheared serpentinite-matrix. Rare pegmatitic gabbros are present, as are felsic (tonalitic) dykes. Extensive knockers of granodiorite and diorite are a distinctive feature distinguishing this melange from nearby ´ ophiolite occurrences at Dong Tso. Pillow basalts are an important but minor component of the melange. They are locally ´ associated with purple and green pillow breccia or red ribbonbedded chert. Where primary stratigraphic contacts are preserved, siliceous, radiolarian-bearing, mudstones are intercalated with, and overlie, the basalts. Amphibolites are locally extensive in the east (Wang et al. 2008). Radiolarians are commonly used as key indicators of the timing of deep-marine sedimentation within suture zones and often prove critical to constraining the timing of important events. Although widely used in stratigraphic investigations along other sutures in the region such as the Yarlung Tsangpo suture zone (Ziabrev et al. 2003, 2004; Aitchison & Davis 2004) and its lateral correlative, the Indus suture zone in Ladakh (Danelian & Robertson 1997; Kojima et al. 2001; Zyabrev et al. 2008), few detailed investigations of radiolarians have been reported from the Bangong–Nujiang suture. In central Tibet, Mesozoic radiolarians have been reported from marine clastic sediments of the Rila Formation, a unit regarded as Late Jurassic–Early Cretaceous (Qu et al. 2003) but no identifications or illustrations of the fauna were given. Late Jurassic (late Kimmeridgian–early Tithonian) radiolarians have been reported from near Bangong Lake (Li 1986; Yang & Wang 1990). Although the samples were reported as coming from the Mugagangri Group descriptions of their field occurrence suggest that these cherts were collected from a disrupted (melange) unit. ´ Radiolarian age assignment We report radiolarians from chert or siliceous mudstone blocks in the Lagkor Tso melange. Thirty potentially radiolarian-bearing ´ samples, of which three yielded moderately preserved radiolarians (Fig. 2), were collected from grey–green siliceous mudstones and red or green cherts. One sample, LTO 703, was collected from siliceous mudstone intercalated with pillow basalts. Correlation with detailed Jurassic (Baumgartner et al. 1995; Gorican et al. 2006) and Cretaceous (Jud 1994; O’Dogherty 1994; O’Dogherty & Guex 2002) radiolarian studies from further west in Tethys were used to determine the stratigraphic position of the fossiliferous sediments with Unitary Association (UA) zones correlated to the geological time scale of Gradstein et al. (2004) (Fig. 3). Sample 04052403 yielded well-preserved radiolarians indicative of the early Aalenian to middle Bajocian interval (UA Zones 1–4; 176–168 Ma). A further sample R A D I O L A R I A N AG E , T I B E T 691 Fig. 2. Composite plate of radiolarians from the three Lagkor Tso sample locations. All scale bars represent 100 ìm. (a–c) from sample 05052412; (d–f) from sample 04052403; (g–p) from sample LTO 703. (a) Archaeotritabs hattori Dumitica; (b) Parashuum sp. cf. P izense (Pessagno & Whalen); . (c) Paronaella sp. cf. P. skowkonaensis Carter; (d) Bernoullius sp. cf. B. rectispinus s.l. Kito, De Wever, Danelian & Cordey; (e) Parashuum izense (Pessagno & Whalen); (f) Parashuum sp. cf. P. officerense (Pessagno & Whalen); (g) Acaeniotyle sp. cf. A. umbilicata (Rust); (h) Crolanium puga ¨ (Schaaf); (i) Dictyomitra communis (Squinabol); (j) Sethocapsa orca (Foreman); (k) Tethysetta usotanensis (Tumanda); (l) Pseudodictyomitra carpatica (Lozyniak); (m) Pseudoeucyrtis apochrypha O’Dogherty; (n) Thanarla brouweri (Tan); (o) Thanarla pacifica (Tan); (p) Xitus clava (Parona). 692 A . T. BA X T E R E T A L . Fig. 3. Age ranges of radiolarians from the three Lagkor Tso sample locations. For Jurassic taxa the Unitary Association Zones (UAZ) of Baumgartner et al. (1995) are applied. Ranges of Cretaceous taxa utilize Jud (1994), O’Dogherty (1994) and O’Dogherty & Guex (2002). (05052412) from several kilometres along strike also yielded abundant well-preserved Aalenian (UA Zones 1–2; 176– 172 Ma) taxa. Sample LTO 703, however, yielded abundant wellpreserved radiolarians unlike any in the previous two samples. Taxa present indicate assignment to the latest Hauterivian to early Aptian UA 29–35 (131–121 Ma) of O’Dogherty & Guex (2002), making this the youngest radiolarian-bearing deep-marine sediment reported from the Bangong–Nujiang suture. Discussion The new data presented herein allow considerable refinement of the estimated timing of both opening and closure of an ocean basin between the Qiangtang and Lhasa terranes. The Bangong– Nujiang suture is traditionally viewed as the trace of a Mesotethyan ocean, which opened between the Lhasa and Qiangtang terranes during the Permian. However, Permian and Triassic strata in this region are continental margin sediments and there is little evidence of open ocean conditions at that time. The ages of amphibolites associated with ophiolitic rocks in suture zone settings are commonly used to infer the timing of oceanic closure and their emplacement onto a continental margin. Latest Early Jurassic Ar–Ar amphibolite ages have thus been cited as evidence of ocean closure in the Dongqiao area (Zhou et al. 1997). However, radiolarian faunas younger (Late Jurassic) than this have been reported from elsewhere along the suture (Yang & Wang 1990; Wang & Yang 1991; Qu et al. 2003) indicating continuing marine conditions. The radiolarians documented herein clearly indicate the presence of an oceanic basin until well into the Early Cretaceous, as does the presence of Early Cretaceous ophiolitic rocks at Nagqu (Chen et al. 2006). This is paradoxical and we suggest that, although local emplacement events cannot be ruled out, older metamorphic ages might best be regarded as recording local thermal events (Wang et al. 2008). Radiolarians extracted from cherts as well as radiometric ages for ophiolitic rocks with which these cherts are associated in the central and western parts of the Bangong–Nujiang suture are confined to between 170 and 130 Ma. The Lagkor Tso and other disrupted ophiolites associated with the Bangong–Nujiang suture are typically blanketed by Aptian–Albian shallow-marine limestone units (Marcoux et al. 1987; Zhang 2000; Zhang et al. 2002), suggesting that a major basin inversion event must have occurred shortly after late Barremian–early Aptian radiolarite deposition. Available data therefore suggest that the Bangong ocean basin was not as ancient as previously suggested. It both opened and closed later than previously thought. If the Mesotethyan ocean was growing throughout a 40 Ma interval it should have become substantial and a record of its subduction might reasonably be expected. For this to have happened, even at modest subduction rates, it must have begun well in advance of final ocean closure. Despite numerous investigations along the suture (e.g. at Bangong Tso (Matte et al. 1996; Shi et al. 2004); Lagkor Tso (Zhang et al. 2007); Dong Tso (Qiu et al. 2004); Dongqiao (Girardeau et al. 1984, 1986; Pearce & Deng 1988; Zhou et al. 1997); Dingqing (Liu et al. 2002); and Nagqu (Chen et al. 2006)) there are few detailed descriptions of many of the classic features that might be expected where a major ocean has closed. Subduction-related volcanism (Dewey et al. 1988), collision-related deformation, crustal thickening, mountain building and related molasse formation are notably lacking (Schneider et al. 2003) and there is little geophysical manifestation of the suture (Haines et al. 2003). Structural data do not indicate a high degree of obliquity in convergence, and any regional compression appears to have been orthogonal to the trend of the suture. Apart from c. 170 Ma arcrelated rocks associated with the Bangong–Nujiang suture itself, there is little evidence of subduction-related volcanism in this region prior to 130 Ma in the form of either arc volcanic rocks or magmatic zircons in the regional detrital record north and south of the Bangong–Nujiang suture (Kapp et al. 2007). Thus, we contend that it is unlikely that the Bangong–Nujiang suture segment of the Mesotethyan ocean was ever substantial. The absence of conventionally expected elements of a suture zone along the Bangong–Nujiang suture may explain the proliferation of contrasting models proposed to interpret its development. It is variously regarded as a collisional suture (Coulon et al. 1986), a continent–arc or continent–forearc collision zone overprinted by a continent–continent collision (Pearce & Deng 1988), a rift (Taner & Meyerhoff 1990), a strike-slip dominated rift (for the Dongqiao–Nagqu basin) (Mattern et al. 1998), a strike-slip basin (Schneider et al. 2003), or a back-arc basin (Haines et al. 2003). More recently, Guynn et al. (2006) proposed accretion of a microcontinent to the Qiangtang terrane in the Early–Mid-Jurassic (Amdo basement). Fundamental issues such as the direction of any associated subduction remain contentious. Allegre et al. (1984) inferred south-directed subduction as a result of the paucity of contemporaneous plutonic and/or volcanic bodies north of the suture, whereas various other workers have invoked north-directed R A D I O L A R I A N AG E , T I B E T 693 subduction (Zhou et al. 1997; Kapp et al. 2003; Guynn et al. 2006), with Yin & Harrison (2000) proposing a doubly vergent subduction zone between the Qiangtang and Lhasa terranes. The clusters of age data around both 170 and 125 Ma are probably also significant. The geochemical signature of many rocks along the suture indicates development above a subducting slab. In this context, the back-arc basin model originally argued on the basis of geophysical observations (Haines et al. 2003) is appealing. However, we suggest that any Bangong–Nujiang suture back-arc is unlikely to have been associated with subduction under the northern margin of the Qiangtang terrane. As an alternative we propose that early subduction of Neotethyan oceanic lithosphere under the southern margin of the Lhasa terrane to form an Andean-style continental convergent margin was coeval with, and probably implicated in, development of the Bangong–Nujiang suture basin. Subsequent shortening of the Lhasa terrane notwithstanding, magmatism along the Bangong– Nujiang zone probably occurred in a back-arc region associated with the Neotethyan subduction system. We also note that the Neogene Karakoram Fault truncates the Bangong–Nujiang suture and westernmost Lhasa terrane, the northern edge of which ophiolitic rocks have over-thrust. As pointed out by workers in NW India (Rolland et al. 2000; Robertson & Collins 2002) the logical correlation of the Lhasa terrane across this structure is with the Ladakh or Dras arcs. Thus the Bangong–Nujiang suture most probably correlates with the Shyok suture (Phillips et al. 2004). The model proposed by Rolland et al. (2000) fits well with observations along the Bangong–Nujiang suture and a back-arc model scenario. We postulate that the Bangong–Nujiang suture–Shyok suture zones represent the trace of a former zone of back-arc basin–marginal sea. Its development was associated with southward rifting of the Lhasa terrane off Eurasia after initiation of Neotethyan subduction along its southern margin. The present-day margin of eastern Asia, where similar basins have developed behind a chain of continental and intra-oceanic island arcs (Kamchatka, Kurile Islands, Japan and Ryukyu Islands), provides a modern analogue. Opening and closure of the Bangong marginal sea was most probably linked to changes in plate geometry and/or variations in the dip of the subducting Neotethyan slab that were related to positive bathymetric features on its surface. 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