Early Cretaceous radiolarians from the Spongtang massif, Ladakh, NW India: implications for Neo-Tethyan evolution more

Journal of the Geological Society Early Cretaceous radiolarians from the Spongtang massif, Ladakh, NW India: implications for Neo-Tethyan evolution Alan T. Baxter, Jonathan C. Aitchison, Jason R. Ali and Sergey V. Zyabrev Journal of the Geological Society 2010; v. 167; p. 511-517 doi:10.1144/0016-76492009-099 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 12 July 2010 © 2010 Geological Society of London Journal of the Geological Society, London, Vol. 167, 2010, pp. 511–517. doi: 10.1144/0016-76492009-099. Early Cretaceous radiolarians from the Spongtang massif, Ladakh, NW India: implications for Neo-Tethyan evolution 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, JA S O N R . A L I 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: alantbaxter@gmail.com) Abstract: The discovery of two Early Cretaceous (mid-Valanginian–mid-Aptian range) radiolarian faunal assemblages from ribbon-bedded cherts collected near Photoskar in northern Ladakh, NW India, provides the first robust biostratigraphic age constraints associated with the Spongtang massif. This klippe of relict Neotethyan suprasubduction-zone ophiolitic rocks and related arc volcanic rocks crops out 30 km south of the Indus suture in Ladakh. The radiolarian assemblages, the age assignment of which lies between published radiometric ages, provide new constraints on the evolution of this intra-oceanic island arc system. Critically, from them it can be inferred that the system was appreciably long-lived (Jurassic–Cretaceous) and more continuous than is commonly considered. The Spongtang ophiolite (Reuber et al. 1992) is one of numerous fragments of suprasubduction-zone ophiolitic rocks and associated arc volcanic rocks that formerly made up parts of an intraoceanic arc system that lay within Neotethys (Aitchison et al. 2000; Aitchison & Davis 2004). As India migrated northwards during the Late Mesozoic, Neotethyan oceanic lithosphere was consumed beneath this intra-oceanic arc system. When the northern margin of Greater India collided with the arc, rocks that had formed in the suprasubduction zone were emplaced southwards onto it in a series of nappes. This occurred prior to final closure of the Neotethyan Ocean, portions of which lay north of this converging intra-oceanic arc system, and ultimately, collision between India and Asia that occurred c. 20 Ma later (Aitchison et al. 2002, 2007a). In the Ladakh region of NW India, elements of this system include the Spongtang ophiolite and Spong arc (Reuber et al. 1992; Corfield et al. 2001; Pedersen et al. 2001) together with the Nidar ophiolite (Maheo et al. 2004; Ahmad et ´ al. 2008; Zyabrev et al. 2008). Correlatives of these rocks can be traced east across the Karakoram Fault into Tibet where they form parts of the Dazhuqu terrane and probably also the Zedong terrane (Aitchison et al. 2000). Major occurrences of correlative suprasubduction-zone ophiolitic rocks in Tibet crop out at Jungbwa (Miller et al. 2003; Chan 2008), Saga and Sangsang (Bedard et al. 2010), Xigaze (Nicolas et al. 1981; Girardeau et ´ al. 1984, 1985a; Wang et al. 1987; Dubois-Cote et al. 2005; Dupuis et al. 2005), Dazhuqu (Girardeau et al. 1985b; Xia et al. 2003), Zedong (McDermid et al. 2002; Aitchison et al. 2007b) and Luobusa (Zhou et al. 1996) (Fig. 1). To the west they can be correlated with the Bela, Muslim Bagh and Waziristan ophiolites in Pakistan (Moores et al. 1980; Jan et al. 1985; Sarwar 1992; Beck et al. 1995; Zaigham & Mallick 2000). We note that this is not the same correlation as that suggested by Khan et al. (2009), who conflated the ophiolites that have been overthrust southwards onto the Indian margin together with the Kohistan–Ladakh arc. The latter, however, represents an entirely different intra-oceanic island arc system lying to the north of the Indus suture, which collided with Eurasia’s southern margin during the Middle to Late Cretaceous (Petterson & Windley 1985; Treloar et al. 1989, 1996, 2003; Treloar 1997). 511 Accounting for Miocene–Recent displacement across the rightlateral Karakoram Fault clearly shows that this sequence of rocks is more appropriately correlated with the Lhasa terrane (Robinson 2009), with which it was formerly laterally continuous. The Spong arc and related Spongtang ophiolite are located in the Ladakh region of NW India. They have been thrust southward over Permian to Eocene sedimentary rocks of the northern Indian passive margin, which have been described in detail elsewhere (Fuchs 1982; Garzanti et al. 1987; Gaetani & Garzanti 1991) and is located c. 30 km south of its root in the Indus suture zone. Previous work on this massif has been limited because of access issues, but Reuber (1986) described crustal units of the Spongtang ophiolite and discussed possible oceanic thrusting and obduction processes. An open-ocean, mid-ocean ridge basalt (MORB) origin was inferred because ‘transform faults and intra-oceanic thrusts are not known in BAB [back-arc basalts]’ (Reuber 1986; Garzanti et al. 1987). More recent studies have shown that the Spongtang massif can be differentiated into two distinct suites of related affinities. The oldest is the Spongtang ophiolite, which contains lower and upper mantle representatives as well as crustal rocks. These rocks are gradationally overlain by a younger, c. 500 m thick, volcano-sedimentary package that developed in an island-arc environment (Corfield et al. 2001). Much of this intra-oceanic island arc appears to have foundered during collision with India, but tuffs erupted from it are present in numerous localities (Badengzhu 1979). Pedersen et al. (2001) reported a U–Pb zircon age for a diorite intruding high-level gabbros in the Spongtang ophiolite at 177 Æ 1 Ma and suggested that this represents a minimum age for generation of oceanic material. They also dated an andesitic sample from the overlying Spong arc at 88 Æ 5 Ma, providing one of the few Late Cretaceous ages for these suprasubductionzone rocks. Although there is general agreement that this assemblage of intra-oceanic subduction-related rocks records the development of a northward-dipping (Corfield et al. 2001) intra-oceanic subduction zone, and its eventual collision with the northern margin of India, the timing of both the development of this 512 A . T. BA X T E R E T A L . intra-oceanic island arc system and its eventual collision with northern India remains controversial. Many early workers contended that the ophiolite was emplaced onto the northern Indian margin in the Early Eocene as it is thrust over Palaeocene– Eocene limestones (Fuchs 1982). However, Searle (1986) argued that this thrusting was linked to a later event, the India–Asia collision, with ophiolite obduction having occurred earlier during Cretaceous–Tertiary times. This was disputed by Garzanti et al. (2005), who noted ‘not a single piece of evidence from the stratigraphic record of the Zanskar Range indicates or even hints that the Spongtang ophiolite was emplaced onto the Indian margin during the Late Cretaceous’. We concur and suggest that the first evidence of its emplacement appears to be recorded with the appearance of ophiolitic detritus in Palaeocene sedimentary units: the Chogdo Formation in the Zanskar valley (Searle et al. 1990) and in Dibling (Garzanti et al. 1987). In this paper we report the first detailed description of radiolarian faunal assemblages from the Spong arc succession. They provide new constraints on the timing of development of this intra-oceanic island arc system. Our results indicate that its development was perhaps more continuous that previously thought. We also discuss evidence that constrains the timing of ophiolite emplacement. between India and Asia (Molnar & Tapponnier 1977; Tapponnier et al. 1981; Thakur & Sharma 1983; Allegre et al. 1984; Burg & ` Chen 1984; Mercier & Li 1984; Searle et al. 1987). These continental masses were separated by an expansive Neotethyan ocean with fragments of associated oceanic lithosphere preserved along the sutures (Sengor 1987; Sengor et al. 1988). Advances in ¨ ¨ our understanding of ophiolites and the associated realization that, rather than being fragments of the subducting slab, where preserved, such bodies are more likely to be fragments of oceanic lithosphere that formed above a subducting slab in a suprasubduction-zone setting (Bloomer et al. 1995; Shervais 2001; Dilek & Newcomb 2003; Hawkins 2003; Dilek & Robinson 2004) cast new light on our understanding of suture zone development. By the beginning of this millennium it had been recognized that ophiolites and associated rocks along the Indus and Yarlung Tsangpo suture zones represented fragments of intra-oceanic island arc systems that had developed within the Neotethyan Ocean (Aitchison et al. 2000; Corfield et al. 2001; Maheo et al. ´ 2004). By carefully studying such rocks the history of the development of the former Neotethyan Ocean can be elucidated. Radiolarians Radiolarian microfossils provide age constraints on sediments associated with the suprasubduction-zone ophiolites as well as material off-scraped from the downgoing slab during subduction. Representatives of the sedimentary cover of the Neotethyan ocean floor were accreted together with distal northern Indian Neotethyan intra-oceanic island arc system By the 1980s it was widely recognized that the Indus and Yarlung Tsangpo suture zones represented the locus of collision Fig. 1. Map of the Himalayan region showing the location of Indus and Yarlung–Tsangpo suture zones. Also highlighted are eight regions from which Tethyan radiolarians have been reported. 1, Kahi melange (Beck et al. 1995); 2, Shergol melange (Kojima et al. 2001); 3, Spongtang ophiolite (Reuber ´ ´ 1992; Corfield et al. 2001); 4, Nidar ophiolite (Kojima et al. 2001; Zyabrev et al. 2008); 5, Sangdanlin (Chan, 2006); 6, Xigaze ophiolite (Ziabrev et al. 2003); 7, Bainang terrane (Ziabrev et al. 2004); 8, Zedong terrane (Aitchison et al. 2007b). IS, Indus suture; YTSZ, Yarlung–Tsangpo suture zone; BNS, Bangong–Nujiang suture; JS, Jinsha suture; STDS, South Tibet detachment surface; MCT, Main Central thrust; MBT, Main Boundary thrust. S P O N G TA N G R A D I O L A R I A N S 513 margin sediments and combined into a subduction complex that developed in association with the Neotethyan intra-oceanic arc system. In NW India these rocks occur in the Karamba Formation, which contains Middle Jurassic to Late Cretaceous radiolarians (Danelian & Robertson 1997). In central southern Tibet they are found in the Bainang terrane, which contains Late Triassic to Middle Cretaceous radiolarians (Ziabrev et al. 2004). The successions in both areas are remarkably similar. Elsewhere, radiolarians extracted from inter-pillow cherts constrain the timing of formation of oceanic crust of the suprasubduction-zone assemblage. Middle Cretaceous radiolarians faunas are the most common and have been reported in detail from ophiolites in the Xigaze region (Ziabrev et al. 2003) of southern Tibet and the Nidar region in Ladakh (Kojima et al. 2001; Zyabrev et al. 2008). Middle Jurassic radiolarians have also been reported from cherts overlying island arc tholeiitic pillow basalts at Zedong (Aitchison et al. 2007b) (Fig. 1). Albian radiolarians including Pseudodictyomitra pentacolaensis have been reported from along the Indus suture in a chert block from the Shergol ophiolitic melange (Kojima et al. 2001). In a study ´ of the correlative Waziristan ophiolite in NW Pakistan, Late Cretaceous radiolarians (Cryptamphorella conara and Amphipyndax pseudoconulus) were reported from kilometre-scale blocks of radiolarite in the Kahi melange (Beck et al. 1995). Thus the ´ age range indicated by radiolarians for the Neotethyan intra- oceanic island arc assemblage is broadly similar to that indicated by radiometric age determinations for the associated igneous lithologies. Spongtang ophiolite Previous biostratigraphic work related to the Spongtang area is limited. Reuber et al. (1992) reported the occurrence of radiolarians from a chert sample collected on the east flank of Photong Kangri that indicated Late Jurassic (terminal Callovian to Tithonian) affinity. However, one of the two forms they reported as Minifusus parvisingula is a nomen dubium; the misspelt generic name is presumably meant to be Mirifusus whereas the specific epithet is the name of another related genus Parvicingula. Nevertheless, both genera belong to the subfamily Parvicingulinae (Pessagno 1977) and the presence of either would suggest assignment to the Late Jurassic to Early Cretaceous. Another Late Jurassic to Early Cretaceous taxon, Pantanelium [sic] lanceola, was reported by Reuber et al. (1992). In a subsequent report a rotaformid test was identified in a float sample from the Photong river valley by Corfield et al. (2001). Although they claimed this to be a biostratigraphically important Cretaceous radiolarian, it sheds little further light on the age of the rocks concerned. All samples were from float of indeterminate source and none of the radiolarians were illustrated. Thus, the best age Fig. 2. Geological map (adapted from Corfield et al. 2001) of the Spongtang massif showing distribution of rocks associated with the Spongtang ophiolite and Spong arc and their structural relationships with surrounding rocks. The sample sites from which the red ribbon-bedded radiolarian cherts were collected for use in this study are also shown. 514 A . T. BA X T E R E T A L . Fig. 3. Composite plate of biostratigraphically important radiolarians from the two Spong arc sample locations. (All scale bars represent 100 ìm.) Sample 07092504: (a) Svinitzium pseudopuga (Dumitrica); (b) Dictyomitra ?communis (Squinabol); (c) Hiscocapsa sp.; (d) Hiscocapsa uterculus (Parona); (e) ?Pseudodictyomitra carpatica (Lozyniak); (f) Pseudodictyomitra nuda (Schaaf); (g) Thanarla brouweri (Tan); (h) Mictyoditra lacrimula (Foreman); (i) Xitus clava (Parona). Sample 07092506: (j) Acaeniotyle umbilicata (Rust); (k) Pseudocrolanium puga (Schaaf); (l) Crucella bossoensis Jud; (m) Dictyomitra communis (Squinabol); (n) Siphocampium davidi Schaaf; (o) Sethocapsa sp. cf. S. simplex Taketani; (p) Pantanellium lanceola (Parona); (q) Tethysetta boesii (Parona); (r) Pseudodictyomitra carpatica (Lozyniak); (s) ?Pseudoeucyrtis apochrypha O’Dogherty; (t) Pseudoeucyrtis hanni (Tan); (u) Thanarla brouweri (Tan); (v) Thanarla lacrimula (Foreman); (w) Suna hybum (Foreman); (x) Xitus cf. ex. gr. carpatica-hornitissima. S P O N G TA N G R A D I O L A R I A N S 515 constraints on the suprasubduction-zone rocks at Spongtang are provided by the radiometric ages given by Pedersen et al. (2001), who reported a U–Pb age of 177 Æ 1 Ma from a diorite in the high-level gabbros in the Spongtang ophiolite and 88 Æ 5 Ma for an andesite from the overlying Spong arc succession. For this study, six red ribbon-bedded chert samples, of which two were productive, were collected for radiolarian analysis during November 2007 on a reconnaissance fieldtrip to the Zanskar–Photoksar region. All samples were collected from west of Photoksar on the northern flank of Photang Kangri, where the rocks have been mapped by Corfield et al. (2001) as part of the Spong arc volcano-sedimentary succession (Fig. 2). Those workers suggested that a gradational contact exists between the uppermost crustal rocks of the Spongtang ophiolite and pillow lava volcaniclastic rocks and cherts of the Spong arc from which the radiolarian samples were collected. Because of heavy snowfall at the time of collecting our field party was unable to verify the nature of this contact. The samples were processed in the HKU biostratigraphy laboratory and prepared using standard radiolarian extraction techniques, as described by Pessagno & Newport (1972). Each sample was broken into c. 3 cm3 pieces, suspended in plastic netting and immersed in a 5% HF solution for 12–24 h. This was repeated, up to 10 times, for some samples, to maximize the capture of radiolarian tests. Acid residues were sieved, with fossils concentrated in the 63 ìm fraction. The sieves were thoroughly cleaned of all materials between each sample. Radiolarians were picked and mounted onto an SEM stub, coated in a Ti–Au alloy and photographed using an SEM. These photographs were compared with type specimens in the literature and an age range was compiled. Samples 07092504 and 07092506 both yielded moderately to well-preserved tests (Fig. 3). scale of Gradstein et al. (2004). Based on established age ranges for radiolarian taxa present within samples 07092504 and 07092506 they are both representative of the latest Valanginian to early to mid-Aptian (Fig. 4). Discussion The Photang radiolarian cherts form part of a .500 m thick basalt–andesite volcanic and volcano-sedimentary Spong arc assemblage described by Corfield et al. (2001), who interpreted these rocks to represent an intra-oceanic island arc suite distinct from that of the Dras arc to the north. They are a component of the supra-ophiolite sediments and commonly contain radiolarians (McDermid 2002; Ziabrev et al. 2003; Aitchison et al. 2004; Zyabrev et al. 2008). Similar rocks lie in the same structural position elsewhere along the suture zone (e.g. Nidar, Xigaze, Zedong) and the biostratigraphic ages reported herein correlate extremely well with those studies. Furthermore, they fill a gap between the radiometric ages reported from this area by Pedersen et al. (2001). The combined 165–85 Ma range of radiometric and radiolarian ages indicates that the intra-Tethyan oceanic island arc was long-lived. The proposition of an extensive, long-lived intra-oceanic arc in Neotethys is further strengthened by the seismic tomography results of Van der Voo et al. (1999), who interpreted a highvelocity mantle anomaly (slab III), stretching from the Mediterranean to southern India, as evidence for intra-Neotethys subduction. Furthermore, Abrajevitch et al. (2005) linked this mantle anomaly with subduction beneath the Dazhuqu ophiolite in southern Tibet, which is interpreted to be of island arc affinity. Recent workers (Bedard et al. 2010) have reported that ´ the Saga and Sangsang ophiolites are derived from a complex arc–back-arc setting in Neotethys. Given that the first unequivocal indication in the sedimentary record of collision between this intra-oceanic island arc and the northern margin of India is the abrupt appearance of serpentinite and other ophiolite-derived detrital clasts in the Palaeocene Chogdo Formation (Searle 1986) in NW India and at Sandinglan in central southern Tibet (Ding et al. 2005; Aitchison et al. 2007a), this suggests that intra-oceanic subduction should have continued until c. 55 Ma. Age assignment Identification of taxa and age assignment for Middle Jurassic to Middle Cretaceous radiolarian assemblages are based on recent taxonomic studies and biostratigraphic zonations of Tethyan radiolarians (Jud 1994; O’Dogherty 1994; Baumgartner et al. 1995). Biostratigraphic data are correlated to the geological time Fig. 4. Chart of stratigraphic ranges of radiolarians present in samples 07092504 and 07092506. Age assignments for Middle Jurassic to Middle Cretaceous radiolarian assemblages are based on recent taxonomic studies and biostratigraphic zonations of Tethyan radiolarians (Jud 1994; O’Dogherty 1994; Baumgartner et al. 1995). Published radiometric dates related to the Spongtang ophiolite are also indicated. 516 A . T. BA X T E R E T A L . Dilek, Y. & Newcomb, S. (eds) 2003. Ophiolite Concept and the Evolution of Geological Thought. Geological Society of America, Special Papers, 373. Dilek, Y. & Robinson, P.T. (eds) 2004. Ophiolites in Earth History. Geological Society, London, Special Publications, 218. Ding, L., Kapp, P. & Wan, X. 2005. Paleocene–Eocene record of ophiolite obduction and initial India–Asia collision, south central Tibet. Tectonics, 24, 1029, doi:10.1029/2004TC001729. Dubois-Cote, V., Hebert, R., Dupuis, C., Wang, C.S., Li, Y.L. & Dostal, J. 2005. Petrological and geochemical evidence for the origin of the Yarlung Zangbo ophiolites, southern Tibet. 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