Tectonic evolution of the Qiangtang Block, northern Tibet during the Late Cisuralian (Late Early Permian): Evidence from fusuline fossil records

Palaeogeography, Palaeoclimatology, Palaeoecology 350–352 (2012) 139–148

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Tectonic evolution of the Qiangtang Block, northern Tibet during the Late Cisuralian (Late Early Permian): Evidence from fusuline fossil records Yi-chun Zhang a, b,⁎, Shu-zhong Shen b, G.R. Shi a, Yue Wang b, Dong-xun Yuan b, Yu-jie Zhang c a b c

School of Life and Environmental Sciences, Deakin University, Melbourne Burwood Campus, 221 Burwood Highway, Burwood, Victoria 3125, Australia State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, 39 East Beijing Road, Nanjing 210008, China Chengdu Center, China Geological Survey, 2 Renming Road North, Chengdu 610081, China

a r t i c l e

i n f o

Article history: Received 14 January 2012 Received in revised form 22 June 2012 Accepted 28 June 2012 Available online 6 July 2012 Keywords: Tibet Qiangtang Block Permian Tectonic evolution Fusuline faunas

a b s t r a c t The tectonic evolution of the Qiangtang Block in the Qinghai–Tibetan Plateau has been a controversial subject for a long time. In this paper, the discovery of new stratigraphic and fusuline fossil evidence from the Permian sequences (Qudi and Lugu formations) of the Qiangtang Block is reported and the palaeogeographical position and tectonic history of this block during the Late Cisuralian (Late Early Permian) are discussed. The Qudi Formation is typified by thick turbidite deposits and contains Artinskian fusulines such as Pseudofusulina and Chalaroschwagerina. The fusulines were deposited as grains involved in debris flow deposits, suggesting a synchronicity with the depositional time of the turbidites. The subsequent Lugu Formation is dominated by seamount-type carbonates with an irregular basalt base. Fusulines Cancellina, Pseudodoliolina and Parafusulina in the base of the carbonates confirm its age as middle Kungurian. The transition from the turbidite Qudi Formation to the seamount Lugu Formation is here interpreted to be a continuous depositional process recording the Qiangtang Block's separation from the Indian Plate. This separation signaled the opening of the Neotethys Ocean between the Qiangtang Block and the Indian Plate. Palaeogeographically, the Qiangtang Block's separation is comparable with the Baoshan Block's separation in the east and Central Pamir's separation in the west. By contrast, the ultimate opening of the Neotethys Ocean by the separation of India–Pakistan and northern Oman is apparently much later than this event recorded in the Qiangtang Block. Consequently, it is interpreted that the opening of the Neotethys Ocean in the whole northern Gondwanan margin is a diachronous series of events. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The Late Palaeozoic is an important time interval marked by drastic climatic change and dramatic tectonic evolution for many ancient tectonic plates and blocks (Sengör et al., 1988; Stampfli and Borel, 2002; Shi and Waterhouse, 2010). In the Mediterranean area, for example, the Late Palaeozoic tectonic events are interpreted to have involved the separation of a slice of continents along the northern margin of Gondwana, resulting in the formation of the Neotethys Ocean at the expense of the shrinking Palaeotethys Ocean in the north (Stampfli and Borel, 2002). However, the process and accurate timing of this event has not yet been fully revealed (e.g. Robertson and Searle, 1990; Gaetani and Garzanti, 1991; Stampfli et al., 1991; Angiolini et al., 2003a,b; Chauvet et al., 2008; Zhu et al., 2010). The same is also true for the tectonic evolution of the blocks from the Qinghai–Tibet Plateau where high altitude, severe field conditions and complex tectonic

⁎ Corresponding author at: School of Life and Environmental Sciences, Deakin University, Burwood Campus, 221 Burwood Highway, Burwood, Victoria 3125, Australia. Tel./fax: +61 03 92517304. E-mail address: [email protected] (Y. Zhang). 0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2012.06.025

structures have hampered the full recognition and understanding of this opening process in the Qinghai–Tibet Plateau. Tectonically, the Qinghai–Tibet Plateau comprises a complex assembly of allochthonous blocks or terranes divided by major fault zones or sutures that may represent palaeo-oceans (Yin, 1997; Yin and Harrison, 2000). These sutures are, from south to north, respectively the Yarlung–Zangbo suture zone, the Bangong–Nujiang suture zone and the Longmu Co–Shuanghu suture zone (Fig. 1). The tectonic history of these blocks/terranes has been a subject of ongoing debate since the 1970s. For instance, the Qiangtang Block (=West Qiangtang Block of Sengör et al., 1988), bounded by the Longmu Co–Shuanghu suture zone to the north and the Bangong– Nujiang suture zone to the south, has been generally included in the Cimmerian Continent of Sengör (1979). It was believed to have drifted away from Gondwana during the Late Cisuralian time (Sengör, 1979; Metcalfe, 2002; Ueno, 2003). However, the accurate timing of the rifting event and the subsequent opening of the Neotethys Ocean has never been constrained due to limited work previously undertaken in the Qiangtang Block. Similarly, the relationship between the Qiangtang Block and the Lhasa Block is another point of contention recently (Metcalfe, 2002; Zhu et al., 2009; Zhang et al., 2010). Some scholars have suggested

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Fig. 1. Map showing the location of the studied area and the distribution of Artinskian turbidites of the Qudi Formation. Data from Liang et al., 1983; Hu, 1984; Chengdu Institute of Geology and Resources, 2005; Guangxi Institute of Geological Survey, 2005 and our own observations.

that the Lhasa Block, together with the Qiangtang Block, may have been separated from Gondwana in the Late Cisuralian (Shi et al., 1995; Shi and Archbold, 1998; Ueno, 2003). However, another opinion is that the Lhasa Block did not drift away until the Late Triassic (Metcalfe, 2002, 2009). The stratigraphical and palaeontological records are thus vital in helping to explain and constrain these discrepancies and, ultimately, to resolve the timing of the rifting of the Qiangtang Block. New and updated stratigraphical and palaeontological data also add important information to better understand the spatial and temporal palaeobiogeographical relationships between the Qiangtang and Lhasa Blocks with respect to their respective changing positioning relative to Gondwana during the Permian. This paper presents the results of our recent field discoveries and subsequent laboratory studies of new Permian depositional sequences and fusuline faunas in Rongma Town in the central west Nyima County and Cuozheqiangma Town in the north of Nyima County and west of Shuanghu Special District in northern Tibet (Fig. 1). These newly discovered sequences and faunas provide the first solid age constraints for the tectonic evolution of the Qiangtang Block during the Late Cisuralian. 2. Stratigraphy The Lower Cisuralian strata in the Qiangtang Block are composed of glacimarine diamictites and volcanic rocks, which were together named the Horpatso Group by Norin (1946). Subsequently, this group was divided into three formations, respectively the Cameng, Zhanjin and Qudi Formations in ascending order (Liang et al., 1983). The Cameng and Zhanjin Formations in the studied area are composed of metamorphosed rocks such as slate, schist and quartzite intercalated with several layers of basalt and mafic dyke swarms (Li et al., 2004a; Wang et al., 2009; Zhai et al., 2009). One spectacular feature in these formations is the presence of glacimarine deposits such as slate pebble dropstones, which have been considered to be related to

the Late Palaeozoic Ice Age (Liang et al., 1983; Li et al., 1995; Zhang et al., 2009a). Fossils are scarce in the Zhanjin Formation except for the sporadically-preserved bivalve Eurydesma. By contrast, the Zhanjin Formation in Duoma County in the western part of the Qiangtang Block is more fossiliferous. In Duoma County it contains the coral Amplexocarinia–Cyathaxonia assemblage, the brachiopod Ambikella– Anidanthus fusiformis assemblage and the bivalve Eurydesma–Mourlonia assemblage, which were all assigned to a Late Asselian to Early Sakmarian Age (Liang et al., 1983). It is also worthy to note here that the metamorphosed Cameng and Zhanjin Formations do not represent the crystalline basement rocks because much older strata such as Ordovican cephalopod-containing rocks have been revealed in the Qiangtang Block (Li et al., 2004b). The metamorphism of the Cameng and Zhanjin Formations probably resulted from local contact metamorphism. The Qudi Formation conformably overlies the Zhanjin Formation. It consists of mudstone, shale, sandstone and volcanic rocks with a thickness of 2000 to 3000 m (Fig. 2). This formation is widely distributed from Shuanghu Special District westward to Duoma County (Fig. 1). It has been widely regarded as turbidite deposits both in terms of facies and deep water trace fossils (Liang et al., 1983; Hu, 1984). The Qudi Formation at Jiaomuri Hill (Jiaomuri section, N: 33°16′ 39.2″; E: 87°1′27.84″) is composed of mudstones, sandstones and volcanic rocks showing features of turbidite deposition (Fig. 3A). These deposits have been depicted as showing several Bouma sequences in sandstones (Institute of Geological Survey and Jilin University, 2005). Additionally, abundant grazing trace fossils have been found on the surface of the mudstones, pointing to a deep water depositional environment (Hu, 1984). However, in the Gangtangcuo area (Gangtangcuo section, N: 33°5′21″; E: 86°39′43″), north of Rongma Town, this sequence is replaced by metamorphosed shale and siliceous rocks (Fig. 3B). Discoveries of slumped limestone bodies preserved in the volcanic breccias and the debris flow deposits in the turbidites of the Qudi Formation are of great interest (Fig. 3C–D).

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Fig. 2. Stratigraphic columns of the Permian strata at the studied area, also showing the stratigraphic occurrences of fusulines and corals. The coordinations of these sections are, respectively: Jiaomuri section, N: 33°16′39.2″; E: 87°1′27.84″; Gangtangcuo section, N: 33°5′21″; E: 86°39′43″; Mari section, N: 33°15′23.11″; E: 87°30′31.54″; and Duoma section, N: 33°16′59.52″; E: 87°27′24.12″.

These two types of deposition (i.e., limestones and deep water turbidites) are recorded at both Jiaomuri Hill and the Gangtangcuo area. As shown in Fig. 3D, the debris flow deposits in the Qudi Formation contain numerous fusulines as well as muddy debrites. Fig. 3E, a photomicrograph shows that the fusuline tests are well preserved in the debris flows, which are surrounded by basalt breccias. This observation suggests that the fusuline tests were deposited as grains in debris flows. Additionally, the image in Fig. 3F shows that one fusuline test was compressed laterally, making the fusuline test deformed but not fractured. This phenomenon implies that the fusuline was subjected to pressure as it flowed with the other debris prior to lithification. Both phenomena discussed above indicate that numerous fusulines accumulated on the surface of the carbonate sea bed and were not consolidated (lithified) in the sediment prior to the deposition of the turbidites. Therefore, it is concluded that the depositional age of the turbidites is synchronous with the age of the fusulines discovered in the debris flow deposits. In the south of Cuozheqiangma Town, large scale basalt flows can be observed, capped by massive limestones (Fig. 4A, B). The combined basalt and limestone units form the Lugu Formation. The Lugu Formation covers a large area, as does the Qudi Formation. Locally, the basalt is also found interbedded with the limestone. The limestone in the Lugu Formation is

considered to be seamount-type carbonates for three reasons. First, the limestone contains many bioclastic grains, but it is devoid of detrital siliciclastics. Second, the limestone varies greatly in facies and depositional environments. The limestone outcrops near Cuozheqiangma Town (N: 33°12′5.68″; E: 87°51′43.34″) are reefal in character with massive sponges and corals (Fig. 4C), whereas the equivalent limestones in the west are characterised by shallow water fusulines and compound corals (Fig. 4D). Such a great change in depositional environments is probably due to the rugged basalt basal topography as opposed to a stable continental shelf. Third, the geochemistry of the basalt in the basal part of the Lugu Formation shows that it was island arc type basalt that formed near a mid-oceanic ridge (Zhai et al., 2006). Thus, the limestone above the basalt can provide age constraints for the formation of the sea-mount type carbonates. 3. Fusuline assemblages and their ages The fusulines in the Qudi Formation are composed mainly of Pseudofusulina and Chalaroschwagerina. However, the species are not evenly distributed from the Rongma area in the west to the Cuozheqiangma area in the east. For example, the fusulines from the Jiaomuri section (at Jiaomuri Hill) are composed of

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Fig. 3. Artinskian turbidite-type deposition of the Qudi Formation (F: fusulines; L: limestones; V: volcanic rocks). A. Turbidites in the Jiaomuri section; B. slightly-metamorphosed shale in the Gangtangcuo section, north of Rongma Town. C. Limestone blocks (L) slumped into volcanic rocks (V). D. Fusuline deposited as grains (F) in debris flow deposits. E. Microscopic picture showing the preservational status of fusuline grains in the debris deposits. F. Microscopic picture showing a ductilly deformed fusuline test.

Chalaroschwagerina vulgaris (Schellwien), C. globosa (Schellwien), Pseudofusulina insignis Leven, P. pamirensis Leven and Praeskinerella pavlovi (Leven), whereas the fusulines from the Gangtangcuo section consist mainly of Chalaroschwagerina solita Skinner and Wilde, Pseudofusulina atetsensis Nogami, P. norikurensis krafftiformis Leven and P. tumidiscula Leven, even though both areas share many genera (Fig. 2, 5). Most Pseudofusulina species in the fauna have been widely reported from the Sakmarian strata in West and Middle Asia. For example, Pseudofusulina insignis, P. pamirensis, and P. tumidiscula occur in Central Pamir (Leven, 1967, 1993, 2009), P. insignis and P. pamirensis occur in Central Iran (Leven and Gorgij, 2007), and P. norikurensis krafftiformis occurs in North Pamir (Leven, 1967). Unlike the fusuline assemblages from Central Pamir (Leven, 1993), the fusuline fauna in the Qudi Formation lacks pseudoschwagerinids such as Zellia and Sphaeroschwagerina and instead contains Chalaroschwagerina and Praeskinnerella. Praeskinnerella pavlovi has also been reported in the Yakhtashian (Artinskian) seamount-type

carbonates in the Changning–Menglian suture zone in western Yunnan (Ueno et al., 2003) and in the Artinskian Buqinshan Formation in East Kunlun, Qinghai Province, China (Pospelov et al., 2005). According to Leven (2004), the first occurrences of the genera Chalaroschwagerina and Praeskinnerella are not older than Yakhtashian. Also, some species of Pamirina, the representative genus of the Yakhtashian stage, have been reported from the Qudi Formation in the Duoma area (Nie and Song, 1983). Thus, the Pseudofusulina–Chalaroschwagerina assemblage in the Qudi Formation of the Jiaomuri and Gangtangcuo areas is here considered to be Artinskian in age. The Lugu Formation, like the Qudi Formation, also exhibits a variety of microfacies and contains different fusuline associations in different sections. In the Duoma section (N: 33°16′59.52″; E: 87°27′24.12″), for example, the fusulines are dominated by Cancellina primigena (Hayden), Nankinella orbicularia Lee and Sphaerulina hunanica Lin, whereas the nearby Mari section (N: 33°15′23.11″; E: 87°30′31.54″) contains Chusenella douvillei (Colani), Pseudodoliolina ozawai Yabe and Hanzawa,

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Fig. 4. Lugu Formation of seamount-type carbonates on a basalt base. A. Seamount-type carbonates in the Cuozheqiangma area. B. Seamount-type carbonates in the Jiaomuri area. C. Reefal limestone of seamount-type carbonates. D. Compound corals found in the basal part of the carbonates.

Parafusulina multiseptata (Schellwien), P. lata Reichel, Schwagerina hupehensis Chen, Neofusulinella giraudi Deprat and Monodiexodina kattaensis (Schwager) (Fig. 2, 6). As has been pointed out above, the different microfacies and fusuline associations in different sections are

interpreted here to have resulted from the rugged and irregular basalt base. Of those species, Cancellina primigena (Hayden) is the most important species to provide a solid age constraint. It has been widely

Fig. 5. Selected fusulines from the Qudi Formation. 1–3. Chalaroschwagerina vulgaris (Schellwien, 1909), JMR—107, 22, 36; 4. Chalaroschwagerina solita Skinner and Wilde, 1966, GTC—82/1; 5–6. Pseudofusulina insignis Leven, 1993, JMR—72, 105; 7–8. Chalaroschwagerina globosa (Schellwien, 1909), JMR—76, 70; 9–10. Pseudofusulina norikurensis krafftiformis Leven, 1967, GTC—31, 20; 11. Pseudofusulina atetsensis Nogami, 1961, GTC—5; 12–13. Pseudofusulina pamirensis Leven, 1993, JMR—45, 64; 14. Praeskinnerella pavlovi (Leven, 1967), JMR—79; 15–16. Pseudofusulina tumidiscula Leven, 1993, GTC—53, 28.

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Fig. 6. Selected fusulines from the Lugu Formation. 1. Nankinella orbicularia Lee, 1934, DMC—0.5 m–10; 2–4. Cancellina primigena (Hayden, 1909), DMC—0.5 m–23, 12, 26/5; 5. Chusenella douvillei (Colani, 1924), MR 12.5 m–12/2; 6. Pseudodoliolina ozawai Yabe and Hanzawa,1932, MR 12.5 m–7; 7. Parafusulina multiseptata (Schellwien, 1898), MR 38 m–6; 8. Parafusulina lata Reichel, 1940, MR 18 m–4; 9. Schwagerina hupehensis Chen, 1956, MR 12.5 m–28; 10–11. Neofusulinella giraudi Deprat, 1915 MR 25 m–25/2, 10 m–4/2; 12. Sphaerulina hunanica Lin, 1977, DMA 8.2 m–6/2; 13. Monodiexodina kattaensis (Schwager, 1887), MR 18 m–12.

recorded from the uppermost part of the Chihsia Formation and the lowest part of the Maokou Formation in South China (Sheng, 1963; Yang, 1985; Zhou and Xie, 1997). It has also been reported from Afghanistan (Leven, 1997), Japan (Huzimoto, 1936; Ueno et al., 2006), Transcaucasia (Leven, 1998), South Pamir (Leven, 1967) and Turkey (Leven and Okay, 1996). It is significant to note that in the section dominated by Cancellina, there are no species of Misellina or Neoschwagerina, indicating that the fauna is equivalent to Cancellina liuzhiensis Zone in South China (Yang, 1985; Xiao et al., 1986). Moreover, the Cancellina fauna has been ascribed to the lower part of the Xiangboan Stage of South China (Jin et al., 1999) and the Kubergandian Stage of the Tethyan scale (Leven, 2004). The other component of the Lugu fusuline fauna (Mari section) is characterised by abundant Parafusulina, Chusenella and Pseudodoliolina specimens. They can be correlated with the Parafusulina multiseptata Zone in the uppermost Chihsia Formation in South China (Lee, 1931; Sheng, 1962; Zhou and Zhang, 1984). This Parafusulina-dominated fusuline fauna has been regarded as synchronous with the Cancellina fauna of a restricted shelf origin (Sheng and Jin, 1994). Regardless of their different species compositions, both the Cancellina-dominated and Parafusulina-dominated fusuline fauna of the lower part of the Lugu Formation indicate a Kubergandian age. However, a precise correlation between the fusuline-defined Kubergandian Stage and the conodont-defined Kungurian–Roadian stages is currently not well resolved (Henderson and Mei, 2003; Leven, 2004; Wang et al., 2011; Shen et al., in press). The Kubergandian is conventionally correlated to Roadian (Jin et al., 1997) because the type section of the Kubergandian Stage in Southeast Pamir bears Roadian ammonoids in its middle part (Chediya et al., 1986). However, recently discovered material from the Luodian section in South China has confirmed that the index Murgabian Neoschwagerina simplex assemblage can range downward into Kungurian (Mei et al., 2002; Henderson and Mei, 2003). Additionally, materials from an exotic limestone block at Hatahoko in Japan also support the scenario in the Luodian section that the Murgabian fusulines coexist with middle to upper Kungurian conodonts (Shen et al., in press). Consequently, the Kubergandian Stage, lying below the Murgabian Stage, is here considered to be broadly correlative with the middle part of

the Kungurian Stage on the international Permian timescale (Wang et al., 2011). 4. Tectonic implications and correlations 4.1. Tectonic implications As early as 1979, Sengör proposed the concept of a Cimmerian Continent, which included tectonic blocks geographically ranging from West Asia through Afghanistan, Tibet and the western Yunnan province of China, Myanmar, western Thailand, western Peninsular Malaysia, and northwest Sumatra (Sengör, 1979; Sengör et al., 1988; Metcalfe, 2009). These land masses all record a history of rifting from Gondwana in the Late Palaeozoic and drifting northward rapidly, resulting in the subsequent opening of the Neotethys Ocean and the closure of the Palaeotethys Ocean. The Qiangtang Block was often included in the Cimmerian Continent (e.g., Metcalfe, 2002, 2009; Ueno, 2003). However, its actual tectonic history during the Permian time has been controversial due to limited biostratigraphical data. Thus, the deposits and fossils in the Permian of the Qiangtang Block, presented above, are of great significance in timing the tectonic separation and subsequent drift history of this block. As shown above, the Qudi Formation characteristically contains thick turbidite deposits. The fusulines found in the turbidite deposits were transported but not stratigraphically reworked. Fusuline biostratigraphy suggests that the turbidites were formed in the Artinskian time. The Lugu Formation, characterised by seamount-type carbonates, was subsequently deposited on a basalt surface. The Cancellina, Pseudodoliolina and Parafusulina fusuline fauna in the basal part of the Lugu Formation confirms that the age of these seamount-type carbonates is middle Kungurian. When plotting the occurrences of the turbidite deposition on a map, it is clear that the turbidites are distributed from Shuanghu Special District in the east to Duoma County in the west (Liang et al., 1983; Hu, 1984; Chengdu Institute of Geology and Resources, 2005; Guangxi Institute of Geological Survey, 2005). In addition, the turbidites are

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relatively close to the Bangong–Nujiang suture zone in the west and even closer to the Longmu Co–Shuanghu suture in the east (Fig. 1). In particular, the area between the occurrences of the turbidites and the Bangong–Nujiang suture zone is occupied largely by Mesozoic marine deposits. These findings suggest that the presence of Qudi-type turbidites close to the Bangong–Nujiang suture zone cannot be excluded. It is thus proposed here that the formation of the turbidites in the Qudi Formation, that have a substantial regional distribution covering much of the Qiangtang Block, was most likely related to, and triggered by, the initial rifting and tectonic separation of the Qiangtang Block from Gondwana in the Artinskian time. This led to an extensive eruption of basalt in the newly-formed Neotethys Ocean, on which was formed the sea-mount type carbonates of the Lugu Formation in the Middle Kungurian time (Fig. 7). The inferred rifting of the Qiangtang Block is also supported by widely reported mafic dyke swarms in the Zhanjin Formation with strong geochemical affinity to mafic magma bodies of intra-plate extensional origin (Deng et al., 1996; Yin and Harrison, 2000; Li et al., 2004a; Wang et al., 2006; Wang et al., 2009; Zhai et al., 2009). It is thus reasonable to assume that the Qiangtang Block began to break up from Gondwana in as early as the Sakmarian time, an event that would have been preceded by emplacement of mafic dyke swarms as seen in the Zhanjin Formation, and followed by rifting in the Artinskian time as recorded by the deposition of turbidites of the Qudi Formation. The formation of sea-mount type carbonates in the Lugu Formation may be interpreted as evidence for drifting of the Qiangtang Block from Gondwana. Moreover, the inferred rifting and drifting process is also in good agreement with the palaeomagnetic data from the Qiangtang Block which suggests a quick northward movement starting from the Cisuralian (Li et al., 2004). Drifting of the Qiangtang Block does not mean that it was being separated from the Lhasa Block in the Artinskian time even though these two blocks are currently juxtaposed in the central and northern Tibet. The Permian basalt in the Lhasa Block has been interpreted to have formed in a subduction system (Zhu et al., 2010). In addition, the Permian ecologite and granite in the central Lhasa Block indicate

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a collision setting (Yang et al., 2009; Zhu et al., 2009). In other words, unlike the Permian basalt of the Qiangtang Block, none of the Permian magmatic rocks known so far from the Lhasa Block shows affinity to a rift tectonic setting. Furthermore, the Lhasa Block is very different from the Qiangtang Block with respect to its Permian depositional sequences and palaeontological contents. Firstly, rift-related turbidite deposits of the Qudi Formation are widespread in the Qiangtang Block, but similar deposits are not yet known in the Lhasa Block. The chronostratigraphic equivalent of the Qudi Formation in the Lhasa Block is the Angie Formation, a succession of siliciclastic rocks that suggests a tectonically stable depositional environment and transitions from clastic to limestone facies (Xia, 1983). Secondly, the oldest fusulines in the Qiangtang Block are as early as the Artinskian in age. Kungurian compound corals and fusulines have been found in the base of the Lugu Formation (Figs. 4, 6), which indicate a relatively warmer-water environment, whereas the oldest warm-water fusulines and compound corals in the Lhasa Block are Late Guadalupian (Middle Permian) (Zhang et al., 2010). The warm-water faunas in the Qiangtang Block are much older than those in the Lhasa Block, indicating that these blocks differed in their rifting time from Gondwana and/or palaeogeographic position relative to Gondwana within the Neotethys Ocean (Zhang et al., 2010). The Lhasa Block was unlikely to have been separated from the Himalaya Tethys Zone of the Indian Plate as late as the Late Triassic as suggested by Metcalfe (2002). The main Himalaya Tethys Zone in southern Tibet is dominated by cold-water fauna and cool-temperate flora in the Lopingian (Late Permian), whereas the Lhasa Block contains warm-water fauna dated as old as the Late Guadalupian. This supports the argument that the Lopingian of the Lhasa Block differs from that of the Himalaya in both biota and depositional sequence (Shen et al., 2000, 2001; Zhang and Shen, 2007). Our further study confirms that the Permian sequences in the Lhasa Block resemble those from the Tengchong Block and the Sibumasu Block in western Yunnan, China. Further, all these three blocks are united by their collective close relationships, in both stratigraphy and biotas, with northwest Australia, rather than with India and the Himalaya Tethys Zone in southern Tibet.

Fig. 7. Schematic illustrations showing the tectonic evolutionary process of the Qiangtang Block through the Cisuralian.

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Thus, it is not plausible to place the Lhasa Block between the Qiangtang Block in the north and the Himalaya Tethys Zone of the Indian Plate in the south for at least, the Permian Period. Instead, it is suggested that the Qiangtang Block may have been located directly adjacent to the Indian Plate before the Artinskian time, especially when considering that the rift-related basalt occurs in both the Qiangtang Block and the Himalaya Tethys Zone in southern Tibet but is missing in the intervening Lhasa Block (Zhu et al., 2002; Zhu et al., 2010). Consequently, the Mesotethys Ocean, an ocean basin envisaged by Metcalfe (Metcalfe, 1996, 2002, 2009) to have formed when the Qiangtang Block separated from the Lhasa Block in the Late Cisuralian time, might never have existed. Sea-mount type carbonates of the Lugu Formation were deposited on basalt in the Neotethys Ocean after the Qiangtang Block had drifted away from the Indian Plate. Interestingly, sea-mount type carbonates have also been reported from Ladakh, northwest Himalaya (Mathur and Pal, 1979; Lys et al., 1980; Robertson, 1998) and Oman (Vachard et al., 2002; Weidlich, 2007). Within the Yarlung–Zangbo suture zone, similar carbonate rocks of comparable age are also known and have long been described as “exotic limestone blocks” (Shen et al., 2003a, 2003b, 2003c; Shi et al., 2003; Zhang et al., 2009b; Wang et al., 2010). Although a critical study is clearly required about the depositional and tectonic origins of these so-called “exotic limestone blocks”, it is possible that they were also sea-mount type carbonates formed in the Neotethys Ocean after the Qiangtang Block became separated from the Indian Plate. 4.2. Correlations Judging from its Permian stratigraphic and biogeographical affinities, the Qiangtang Block was likely in geographic proximity to a number of other blocks. The spatial and temporal correlations of the inferred Sakmarian–Artinskian rifting event are vital in reconstructing the nature of this event along the northern Gondwanan margin. The Qiangtang Block is similar to the Baoshan Block in western Yunnan Province of China in terms of Permian stratigraphy and palaeontology. The massive basalts of the Woniusi Formation were widely regarded as an indication of the Baoshan Block's separation from Gondwana (Wopfner, 1996). The age of the Woniusi Formation has been estimated to be Middle to Late Artinskian (Wang and Sugiyama, 2002) because the topmost part of the underlying Dingjiazhai Formation was dated as Middle Artinskian based on the presence of the conodonts Sweetognathus bucaramangus, S. whitei and Mesogondolella bisselli (Mei et al., 2002; Ueno et al., 2002). The western extension of the Qiangtang Block cannot be directly traced because it is truncated by the Karakorum fault (Gaetani et al., 1990; Gaetani and Garzanti, 1991). However, the Permian sequences have the potential to provide a valuable clue to find the counterparts of the Qiangtang Block in the Pamir Plateau. The Qiangtang Block is bounded by the Qamdo Block with typical Cathaysian fauna in the north. The counterpart of the Qiangtang Block in the west is Central Pamir, north of which is the Darvaz–North Pamir zone (Leven, 1967; Pashkov and Sholv'man, 1979). Another similarity between the Qiangtang Block and Central Pamir lies in the Sakmarian–Artinskian fusulines. The fusulines from the Dangikalon Formation in Central Pamir bear strong similarities to those in the Qudi Formation in the Qiangtang Block. Both units contain elongated Pseudofusulina elements (Nie and Song, 1983; Leven, 1993). More importantly, the Rushan–Pshart suture, between Central Pamir in the north and Southeast Pamir in the south, contains Late Cisuralian rift-related basalts (Leven, 1995), suggesting that a rifting basin was being formed between Central Pamir and Southeast Pamir during the Late Cisuralian. Evidence of this rifting basin has also been documented in Karakorum where a deeper depositional environment trend toward the northeast from Artinskian onward has been recognised (Gaetani et al., 1995; Gaetani, 1997). Additionally, basalts

with overlying Kubergandian carbonates have also been found in the northern part of Southeast Pamir, a position close to the Rushan– Pshart suture (Leven, 1967). These lines of evidence strongly suggest that the Cisuralian depositional sequences and inferred tectonic events in the Qiangtang Block are correlative with those from Central Pamir and the Rushan–Pshart suture. Thus, it seems clear that the Neotethys Ocean was formed south of the Baoshan, Qiangtang and Central Pamir blocks during the Artinskian. The following brief comments on the Permian stratigraphy of northern India and the Arabia Plate give a broader, continent-wide perspective on the opening process and timing of the Neotethys Ocean across the northern peri-Gondwanan margin. The opening of the Neotethys Ocean in these areas has been linked to a possible triple-junction formed between the Cimmerian Continent, Arabia and India, possibly in the form of an aulacogen similar to the proposed Arabia–India–Madagascar rift model (Hauser et al., 2002; Chauvet et al., 2009). The drift of the Cimmerian Continent from the Arabia Plate has been dated to the Guadalupian on evidence from rift-related magmas in northern Oman (Besse et al., 1998; Chauvet et al., 2009), even though the initial opening of the Neotethys Ocean here may have occurred as early as the Sakmarian according to some scholars (e.g., Angiolini et al., 2003a, b). A comparable Guadalupian Age has also been favored for the ultimate opening of the Neotethys Ocean in northern India (Gaetani et al., 1990; Chauvet et al., 2008), although there, as in Oman, the initial break-up may have been as early as the Late Sakmarian (Garzanti et al., 1994, 1996, 1999). Hence, it seems apparent that the rifting and drifting events in the Qiangtang Block, the Baoshan Block and Central Pamir were not, as previously thought, synchronous with those in the India–Pakistan and Oman. The opening of the Neotethys Ocean north of the Indian Plate was significant biogeographically. For example, the rifting and drifting of the Qiangtang Block from the Indian Plate may have provided a corridor for the warm-water oceanic current to flow easterly along the newly-formed Neotethys Ocean. If so, this would explain why there exists sporadic warm-water fauna in the Himalaya Tethys Zone (Shi et al., 2003). Additionally, the drifting of the Qiangtang Block northward from the Indian Plate during the Artinskian, coupled with background climatic amelioration after the Late Palaeozoic Ice Age (Montañez et al., 2007; Fielding et al., 2008; Shi and Waterhouse, 2010), may have triggered the formation of the transitional Cimmerian biogeographical province during the Guadalupian (Shi et al., 1995; Shi and Archbold, 1998). The subsequent movement of this block into the palaeoequatorial zone resulted in its entry into, and final integration with, the warm-water palaeoequatorial Cathaysian province after the Late Guadalupian (Shi et al., 1995; Shen and Shi, 2004; Shen et al., 2009). 5. Conclusions Newly discovered Permian stratigraphic sections and fusuline faunas provide important age constraints for the timing of the tectonic evolution of the Qiangtang Block during the Cisuralian. Based on fusuline biostratigraphy, the age of turbidites in the Qudi Formation is constrained to be Artinskian and that of the seamount-type carbonates of the overlying Lugu Formation is dated as the Middle Kungurian. The Cisuralian depositional sequences of the Qiangtang Block, are characterised by a succession of mafic dykes embedded within massive glacial diamictites, turbidites and seamount-type carbonates that overlie a basalt base. This succession is interpreted to indicate a continuous and sustained tectonic process beginning with break-up in the Sakmarian, followed by active rifting in the Artinskian, and finally, separation and northward drifting by the Middle Kungurian. Additionally, it is argued that the Lhasa Block, currently situated between the Qiangtang Block in the north and the Himalaya Tethys Zone and India in the south, was unlikely to have attained this same or a similar spatial configuration in the Permian relative to the Qiangtang Block and India. Instead, we suggest that the Qiangtang Block was more likely to have

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