RESEARCH New Biostratigraphic Evidence of Late Permian to Late

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RESEARCH

New biostratigraphic evidence of Late Permian to Late Triassic deposits from Central Tibet and their paleogeographic implications

Gui-chun Wu1,*, Zhan-sheng Ji2, Wei-hua Liao3, and Jian-xin Yao1 1KEY LABORATORY OF STRATIGRAPHY AND PALAEONTOLOGY, MINISTRY OF LAND AND RESOURCES, INSTITUTE OF GEOLOGY, CHINESE ACADEMY OF GEOLOGICAL SCIENCES, BEIJING 100037, CHINA 2CHINESE ACADEMY OF GEOLOGICAL SCIENCES, BEIJING 100037, CHINA 3NANJING INSTITUTE OF GEOLOGY AND PALAEONTOLOGY, CHINESE ACADEMY OF SCIENCES, NANJING, 210008, CHINA

ABSTRACT

Triassic deposits in the Bangong-Nujiang Suture Zone are important for understanding its tectonic nature and evolutionary history, but have not been systematically studied due to a lack of biostratigraphic data. For a long time, the Upper Triassic Quehala Group featuring clasolite has been regarded as the only rocky unit. In recent years, the silicite-dominated Gajia Formation that bears radiolarian fossils was suggested to represent Ladinian to Carnian deposits. The Upper Permian and Lower Triassic rocks have never been excavated and thus are considered to be absent. This research, however, reveals that fossils aged from the Late Permian to Anisian of the Middle Trias- sic and Norian of the Late Triassic have been preserved in the central Bangong-Nujiang Suture Zone, which provides evidence of Upper Permian to early Middle Triassic deposits and provides new insights on the Upper Triassic strata as well. A new Triassic strata succession is thus proposed for the Bangong-Nujiang Suture Zone, and it demonstrates great similarities with those from Lhasa to the south and Qiangtang to the north. Therefore, we deduce that the Bangong-Nujiang Suture Zone was under a similar depositional setting as its two adjacent terranes, and it was likely a carbonate platform background because limestones were predominant across the Triassic. The newly acquired biostratigraphic data indicate that Lhasa and Qiangtang could not have been located on two separate continents with disparate sedimentary settings; therefore, the Bangong-Nujiang Suture Zone likely did not represent a large ocean between them. This conclusion is supported by lithostratigraphic and paleomagnetic research, which revealed that Lhasa and Qiangtang were positioned at low to middle latitudes during the Early Triassic. Combining this conclusion with fossil evidence, we suggest that the three main Tibetan terranes were in the same palaeobiogeographic division with South China, at least during the Latest Permian to Early Triassic. The Early Triassic conodont species Pachycladina obliqua is probably a fossil sign of middle to low latitudes in palaeogeography.

LITHOSPHERE; v. 11; no. 5; p. 683–696 | Published online 27 June 2019 https://doi.org/10.1130/L1046.1

INTRODUCTION 2012). This view generally considers the Lhasa (Ji et al., 2018a), separated only by a wide sea- terrane to have remained on the north margin way rather than a large ocean (Yang et al., 1984; The Bangong-Nujiang Suture Zone (BNSZ), of Gondwanaland, while the Qiangtang terrane Chen et al., 2001). which is bounded by the Lhasa terrane to the had drifted northward as one component of the These disputes primarily resulted from rarely south and the South Qiangtang terrane to the Cimmerian Continent during the Late Permian documented stratigraphic data. The ocean opin- north, is an E–W trending tectonic unit in central to Late Triassic (Fig. 2) (Wakita and Metcalfe, ion was based on siliceous rock producing radio- Tibet (Pan et al., 2013; Zhu et al., 2016) (Fig. 1). 2005; Muttoni et al., 2009; Metcalfe, 2013; Zhu larian fossils of the Late Triassic Carnian in the In research into dispersion, amalgamation, and et al., 2013). The other view considers Lhasa Dingqing area (Wang et al., 2002) or on the convergence between Eurasia and Gondwana- and Qiangtang to be closely connected, both radiolarian-bearing siliceous Gajia Formation in land, the tectonic background of Bangong-Nuji- having had a common migration history dur- the Nagqu area of central Tibet (Pan et al., 2012; ang during the Triassic has always been a tan- ing the Permian to Triassic (e.g., Scotese and Zhu et al., 2013). The viewpoint placing Lhasa talizing subject (Scotese and McKerrow, 1990; McKerrow, 1990; Wang et al., 2003). In this and Qiangtang in the same plate depended on Pan et al., 2012; Metcalfe, 2013). At present, opinion, disputes still exist about their deposi- different stratigraphic information. For example, disagreements abound, as represented by two tional setting. Some explained them as a land the land and continental rift opinion was based opposite opinions. The mainstream view is that since the Late Permian that began rupturing as a on the knowledge that Lower to Middle Trias- it was one part of the Tethyan Ocean between continental rift along the BNSZ at the end of the sic deposits were absent; the only affirmative Eurasia and Gondwanaland (e.g., Pan et al., Late Triassic (Zhao et al., 2001) (Fig. 3). Oth- lithological unit is the clasolite-characterized ers pointed out that Lhasa and Qiangtang were Quehala Group of the Upper Triassic (Fig. 1C). *Corresponding author: [email protected] in a similar marine setting during the Triassic In contrast, the sea perspective was due to the

82° 84° 86° 88° 90° 92° Zhapu B Lugu Dinggu Shuanghu NQ LSSZ 33° Gumu Geji RiganpeiCo Rima SQ Oma Ejiu Gaize 32 Dongco BNSZ ° DaruCo Nierong 0 60 120(km) Sailipu SelinCo Jiaqun Nima Fig.C LS Nagqu Beila Longgar Coqen Ban’ge SQ LSSZ 31° NQ A DangreyongCo Xainza BNSZ Dingqing NamuCo Fig.B Jiangrang

LS ★ Lhasa Beijing IYSZ LSSZ: Longmu Tso-Shuanghu Suture Zone; BNSZ: Bangong-Nujiang Suture Zone; IYZSZ: Indus-Yarlung Fig.A Himalaya Terrane Zangbo Suture Zone; NQ: Northern Qiangtang; SQ: Southern Qiangtang; LS: Lhasa terrane

90°36′ 90°42′

0 ′ K Qusehai T3Q K1q D1d D2-3c P1x 1 ° 4 J2-3Jn 3 Da’erdong Fm.Chaguoluoma Fm. Xiala Fm. D1d Q K Devonian Devonian B Permian K1q o Q ' Jiaqun Daruc B K q 1 T3Q J1-2X J2-3Jn

K1q Quehala Gr. Xihu Gr. Jienu Gr. Q Q K Triassic Jurassic Jurassic

K1q K1q P1x K E K1q 5 ′ Laqingduo J2-3Jn P1x J2-3Jn T3Q

J2-3Jn J2-3Jn Granites Granites Qushenla Fm.

1 ° 3 J Jn A 2-3

3 K Cretaceous ' E Tertiary Cretaceous A K1q J2-3Jn D2-3c 1 D d n Shemari J Q 3 T Q - 3 2 J J2-3Jn Quatenary Fault Unconformity T3Q D1d Q T Q D2-3c 3 D1d Q B B' D1d A A' D2-3c T3Q D d 2X 1 D c J 1- 2-3 Shemari sections West Jiaqun 90°36′ 90°42′ section C

Figure 1. Sketch map showing (A and B) the main tectonic terranes of Tibet (simplified after Pan et al., 2013), and (C) the geological map for the study area (modified after Chen et al., 2015).

close marine Triassic deposits and faunas found a complete Triassic succession has been estab- comprehensive biostratigraphic investigation to in both Lhasa and Qiangtang (Yang et al., 1984; lished in accordance with the sections found in the north of Ban’ge area in the central BNSZ. As Chen et al., 2001; Ji et al., 2018a). the Coqen and Wenbudangsang areas (Ji et al., a result, deposits for the Upper Permian to early Stratigraphic evidence has invariably been 2007b; Wu et al., 2007, 2014), implying that the Middle Triassic and the Norian of the Upper Tri- a main contributor to interpretations of tec- west Lhasa terrane was in a carbonate platform assic were first discovered in the study area. The tonic nature. However, Triassic sedimentary setting during the Triassic (Ji et al., 2018a; Wu et details are given in the following sections. data remain rare at present, especially in the al., 2018). Similar deposits were found in South BNSZ, where a Lower Triassic deposit has Qiangtang as well (Guizhou Institute of Geologi- GEOLOGICAL SETTING never been reported and is thus believed to be cal Survey, 2005; Zhang et al., 2005; Li et al., missing, which contrasts greatly with adjacent 2018). The common sedimentary evidence dis- The investigated area is located south and areas where Triassic marine sediments were discovered on both sides suggests that they should southwest of Daruco Lake (Fig. 1). Tectonically, closed to be well developed. For example, in be affiliated with a similar carbonate platform it belongs to the Dongkaco substratigraphic divi- the Lhasa terrane to the south, Lower Triassic setting. As a tectonic zone between these two ter- sion in central BNSZ. According to a 1:250000 deposits are widespread in the western portion ranes, biostratigraphic research in the BNSZ will Geological survey on the Ban’ge Sheet (Chen (Ji et al., 2006, 2007a, 2007b, 2007c; Wu et al., be helpful in providing crucial evidence to sup- et al., 2015), the rocky units in this area consist 2007, 2014, 2017, 2018; Zheng et al., 2007), and port this hypothesis. Accordingly, we launched a primarily of the Lower Devonian Da’erdong

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Under this stratigraphic background, the Palaeozoic rocks are commonly treated as bro- Siberia ken blocks inserted within the Mesozoic strata, Jungar Greenland and the Upper Permian to the Triassic strata Tarim Panthalassa below the Quehala Group clasolite was thought Kazakhstan Qaidam Ocean 30ºN to be absent (Fig. 1). Because previous stud- Europe Mongolia ies have not provided reliable age information Panthalassa RTF North North for the related strata, we sought to strengthen hys trench BNO: Bangong-Nujiang Ocean America Paleo-Tet China A (the present BNSZ) the biostratigraphic investigations of the origi- Iran KK QT South China QT: Qiangtang Terrane 0º nally described Jienu Group near Shemari Vil- LS: Lhasa Terrane Adria Indochina lage and the Da’erdong Formation in western Intra-Pangea dextral shear BNO A : Central Afghanistan KK : Karakoram Jiaqun County (Fig. 1). As a result, we acquired South Sibumasu trench America Neo-T RTF : Ridge-trench-fault Late Permian corals and Early to Middle Trias- Arabia 30ºS ethys sic conodonts from the Jurassic Jienu Group Africa LS and Late Triassic Norian corals from the Devo-

Greater India nian Da’erdong Formation. The new fossil data Australia reveal that the prior stratigraphic assignment is Antarctica incorrect. More importantly, these data allow for a revision of the ages for the related strata and their contacting relationship as well. Figure 2. Representative view of the Bangong-Nujiang Suture Zone (BNSZ) during the Triassic as the Bangong-Nujiang Ocean (BNO), which belongs to a part of the Tethys Ocean. Under this view, the Lhasa Terrane (LS) was in the northern margin of Gondwanaland while the Qiangtang Terrane NEWLY OBTAINED BIOSTRATIGRAPHIC had drifted across the equator during the Triassic (modified after Muttoni et al., 2009). DATA

Late Permian to Middle Triassic Biostratigraphy in the Shemari Section Indian Neo- North Tibet Bayankala Block Block Tethyan Old Land The Shemari section, located north of She- LS BNSZ QT mari Village, starts at point A (N31°34′31.3″, E90°39′33.8″, H4875 m) and ends at point 3 T 3 A′ (N31°34′41.2″, E90°39′24.8″, H4889 m) (Fig. 1). The rocks are characterized by dolomite interbedded with limestone, occasionally filled with Cretaceous basalts in some beds (Fig. 4), Indian Neo- North Tibet Bayankala and have been described as the Jurassic Jienu Block Tethyan Old Land Block Group but with no fossil record (Chen et al., LS BNSZ QT 2015). Through our detailed biostratigraphic T1-T2 investigation, we obtained fossils ranging from the Late Permian to the Middle Triassic, indicating that the strata here have been erroneously correlated and need to be revised. Figure 3. Sedimentary model demonstrating that the Bangong-Nujiang Suture Zone (BNSZ) was in Altogether, there are 26 beds. According to the same old land together with the Lhasa terrane (LS) and the Qiangtang terrane (QT) during the 3 the fossils and lithology, beds 10–13 are a repeti- Early to Middle Triassic (T1-T2) that was rifted as a continental rift at the end of the Late Triassic (T3 ) (modified after Zhao et al., 2001). tion of beds 1–9, with a fault developed between beds 9 and 10. To clearly demonstrate the fossil distribution in the column figure, the beds below

Formation (D1d), the Middle to Upper Devonian faulted fragments that thrust into Mesozoic rocky the fault are omitted, but some typical fossil spe-

Chaguoluoma Formation (D2–3c), the Middle units, such as Jurassic rocks. The Quehala Group cies there are marked at an estimated position

Permian Xiala Formation (P2x), the Upper Tri- is a set of sandstone interbedded with slate, and level (Fig. 4). The rock above bed 26 cropped

assic Quehala Group (T3Q), the Early to Mid- it may contact the Devonian Da’erdong Forma- out poorly, and sampling for this part is diffi-

dle Jurassic Xihu Group (J1–2X), the Middle tion in angular unconformity or with the Jurassic cult. Nevertheless, we acquired a Late Permian

to Upper Jurassic Jienu Group (J2–3Jn), and Jienu Group in parallel unconformity. The Xihu coral fauna (Fig. 5), two early Triassic conodont the Lower Cretaceous Qushenla Formation Group is also a sandstone set that unconformably faunas, and a Middle Triassic conodont fauna

(K1q) (Fig. 1). contacts with the Upper Triassic Quehala Group from this section (Fig. 6), which are described The Da’erdong Formation is characterized by and is conformably overlain by the Jurassic Jienu in detail below. limestone, and it is usually regarded as faulted Group. The Jienu Group is in turn unconform- fragments within Mesozoic rocks. The Xiala ably overlain by the Cretaceous Qushenla For- Late Permian Coral Fauna Formation features dolomite bearing abundant mation and features slate and sandstone inter- Late Permian coral fauna was obtained from fossils, such as corals, brachiopods, fusulinids, bedded with andesite, limestone, and dolomitic beds 1, 8, and 10 (Fig. 4). It primarily consists of and bivalves, and it is also believed to represent limestone. species belonging to the genus Waagengophyllum

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Si l a g 5 e 2 c e r i l s r. d s

d -40 . g a i i x 4 r 2 . e T M

-39 a l l . i a a a e t p t t n Figure 4. Compound biostrati- l u a 3 c a o i n c 2 a s d r r m r l graphic column map showing t l n r u o s 2 f e e o n i l

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-33 l o p a o 1 i p e s p h s s h o t s o o e C a o i e e p h N N s N C o e N ) c s 7 i m i ( 1 s s 0 s n 0 1 ' e a i n A r a a m h s T u A u n u . q a t r v i a n p c a l e r i u t 6 o b a s i p 0 1 . a s w s p t 5 l e s x p t a l o s a o c u s i i e i n r e o L s a s d i n h s S n s o d o a t a . p i i e a i i f m r d n l t d 5 n 0 r c o n a -32 o s c 4 u y i h s f m c u h i c h 0 m a u l 1 2 d c n o l l r r l o a e o s a E o y i

-31 e P L N s P h -24 d n p n . i

-30, -23 e o F o m i 3 p n 0 c H z i e 5 -29 u s a g

1 1 -21 i e h j a d n a u i a o W l M l . . g 2 e -20 f p n 1 d r a a m c m s i b u u l l b l l m i 1 y y r -28 1 H h h e p -10 p o o n n e r P e g g

e -27 a mark of the float sampling a a p a 0 a

1 Note: the prefix of the samplings is 091020 W p W -26, -1

U The green color marks samplings from bed 1 to bed 9, the locations of which are estimated according to the fossils and lithology.

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1 2 3

6 7 4 29mm 5

14 11

8 9 10 12 13 15 Figure 5. Coral fossils from the Shemari section (1–7) and Jiaqun section (8–15). Sample numbers: 1–4: 091020-20; 5–7: 091020-26; 8–10: 091021-13; 11: 100527-1; 12: 100527-7; 13: 100527-5; 14: 100527-13; 15: 100527-16. 1–5: Waagengophyllum crassiseptatum Wu

(P3); 6: Waagenophyllum cf. ganhaiziense Fan (P3); 7: Waagenophyllum? sp.; 8: Distichophyllia tenuisa Deng et Zhang; 9–11: Pseudo- retiophyllia nazhacunensis Deng et Zhang; 12: Distichophyllia cf. tenuisa Deng et Zhang; 13: Procylolites? sp.; 14–15: Retiophyllia sp.

(Fig. 5). Among them, the species Waagengo in the western Tethyan area (Perri and Andra- classification needs to be scrutinized in the phyllum crassiseptatum Wu and Waagenophyl ghetti,1987; Aljinovic et al., 2006; Kolar- future. The age of this genus remains unclear lum cf. ganhaiziense Fan indicate a Late Perm- Jurkovsek et al., 2017), the USA (Solien, 1979), due to the rarity of acquired materials. In South ian age (Wang et al., 2003). We also obtained South China (Wang and Cao, 1981; Jiang et al., China, the genus was aged to the Early Induan rare ramiform conodont specimens from sample 2000), and Tibet (Xia and Zhang, 2005; Zheng due to its association with Hindeodus post 091020-12 in bed 1, such as Hibbardelloides sp. et al., 2007). parvus (Yang et al., 2014), whereas in Slovenian, These specimens could not be used to accurately In contrast, Parafurnishius has been rarely it was aged to the Early Olenekian, although determine the age of the related rocks because documented. It was found in Induan strata in other accompanying species were not observed they have a relatively long geologic time span, Sichuan Province, South China, by Yang et al. (Chen et al., 2016). The current research shows ranging from the Permian to Triassic; thus, in this (2014), who described Parafurnishius as a new that the specimens of this genus could co-occur study, the Late Permian age is mainly determined genus, with Parafurnishius xuanhanensis being either with Hindeodus cf. postparvus in sam- by the coral fossils. its type species. Similar specimens were discov- ple 091020-21 or with Pachycladina obliqua ered in the Idrija-Ziri area of Slovenia, although in samples 091020-23 and 091020-30 (Fig. 4). Induan to Early Olenekian Conodont they were designated as another genus Platyvil The latter species spread from Late Induan to Fauna of the Early Triassic losus, with two species Platyvillosus corniger Early Olenikian. Thus, we deduce that this taxon Induan to Early Olenekian Conodont fauna and Platyvillosus regularis discriminated (Chen should range from the Early Induan to the Early of the Early Triassic was found in beds 9 and et al., 2016). The specimens from South China Olenekian of the Early Triassic. 13. The conodont specimens are grouped into and Slovenia share common characteristics with Hindeodus, Pachycladina, and Parafurnishius, those of Platyvillosus because of their broad Late Olenekian Conodont Fauna of the which are typical Early Triassic genera. Among platform, although they are obviously different Early Triassic them, specimens of Pachycladina and Parafur from the latter because of the highly developed The Late Olenekian conodont fauna of nishius dominate over those of Hindeodus in tall denticles on the oral side. Thus, we think it the Early Triassic were discovered in sample abundance. is more reasonable to assign them to the genus 091020-37 of bed 21 and float sample 091020- Pachycladina, a shallow water facies index Parafurnishius. The present specimens from the 34 collected from bed 20. The specimens were fossil of the Late Induan to the Early Olenekian, BNSZ are obviously similar to this taxon. Here, abundant and well preserved, and they were is frequently found in dolomitic rocks. Speci- we tentatively put them under the name Parafur assigned to Chiosella, Triasspathodus, and mens of this genus have been widely reported nishius xuanhanensis. The detailed systematic Magnigondolella.

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Specimens of Triasspathodus and Magni gondolella dominate in terms of yield. For the genus Triasspathodus, its typical species of the Late Spathian, including Triasspathodus homeri and Triasspathodus triangularis, are recognized. Regarding the genus Magnigondolella, most 4 5 specimens could be assigned to Magnigon dolella ex. gr. regale because of their high den- 3 ticles, which are arranged uniformly, a typical 1 2 characteristic of the species Magnigondolella regale (Mosher, 1970). This species ranges from the latest Early Triassic to the earliest Anisian (Orchard, 2007). In contrast, specimens of the genus Chio 8a sella are rare in this fauna, although the key 7a species Chiosella timorensis is identified. The first appearance datum (FAD) of Chiosella timo rensis is believed to be the base of the Anisian (Xia and Zhang, 2005), although Goudemand 7b 8b et al. (2012) noted that the FAD of this species 6 is more likely the Late Spathian because it was found to co-occur with the ammonoids Neopo panoceras haugi (Hyatt and Smith) and Keyser lingites inyoense (Smith), two diagnostic species of the Late Spathian Haugi zone (Goudemand et 10a al. 2012). Based on this analysis, the assemblage 11 of conodont fauna in bed 19 belongs to the Late 9 Olenekian of the Early Triassic. 20 0 µm 10b Late Anisian Conodont Fauna of the Middle Triassic The Late Anisian conodont fauna of the Mid- dle Triassic is acquired from sample 091020- 39 in bed 24. It is noteworthy that the sample 18 was collected as a float form but is characterized by limestone, which is in accordance with the rocks in bed 24. Therefore, we deduce that it should have been sourced from bed 24 or upward along the slope. This conjecture requires further investigation for validation. Nevertheless, the fauna and its age are important for providing reliable information on the deposits during the 12 Middle Triassic. 13 14 15 16 17 19 The specimens in this fauna are abundant and well preserved, and most of them belong to Figure 6. Conodonts from the Shemari section. The scale bar for 8a and 8b is 300µm and for the others is 200µm. 1 and 3 are from Sample 091020-30; 2 is from Sample 091020-23; 4–6 are from gondolellid elements. They could be assigned Sample 091020-21; 7–11 are from Sample 091020-37; 12–13 are from Sample 091020-34; 14–20 are as Magnigondolella bifurcata, Magnigon from Sample 091020-39. 1–3: Parafurnishius xuanhanensis, Pa; 4: Hindeodus cf. postparvus, Pa; 5: dolella constricta, and Paragondolella lieber Hadrodontina aequabilis, Sc; 6: Pachycladina obliqua, Sb; 7a, 7b: Triasspathodus homeri, Pa. Lat- mani, which are index fossils of the Late Anisian eral view (7a) and aboral view (7b) for the same specimen. 8a, 8b: Triasspathodus triangularis Pa; (Sweet et al., 1970; Kovács, 1994). Therefore, Lateral view (8a) and aboral view (8b) for the same specimen. 9: Chiosella gondolellides, Pa; 10a, we believe that the present fauna represents the 10b: Triasspathodus homeri, Pa. Lateral view (10a) and aboral view (10b) for the same specimen. 11: Chiosella timorensis, Pa; 12–13: Magnigondolella regale; 14: Paragondolella constricta, Pa; 15–16: Late Anisian of the Middle Triassic. Paragondolella leibermani, Pa; 17–19: Paragondolella bifurcata, Pa.

Late Triassic Norian Fossils from the Jiaqun Section subdivided (Fig. 4). Previous research correlated designated as Pseudoretiophyllia nazhacunen this set of limestone to the Lower Devonian sis and Distichophyllia tenuisa. Both of these

Section BB′ is located west of Jiaqun Town, Da’erdong Formation (D1d), but there is no fossil are diagnostic species of a Late Triassic Norian with a starting coordinate point B N31°39′01.8″, evidence (Fig. 4). Through our biostratigraphic age (Liao and Deng, 2013); thus, we revise the E90°34′07.2″, and H4803. The rocks here investigation, however, we acquired coral fos- limestone as Late Triassic Norian strata from are characterized by limestone, with 7 beds sils (Fig. 5) from bed 3 (Fig. 4) that could be the Devonian Da’erdong Formation.

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CONODONT CORRELATIONS Zheng et al., 2007; Wu et al., 2007, 2014, 2017, Pachycladina fauna is younger than Triasspath 2018). A complete conodont succession rang- odus triangularis. The available data regarding Studies investigating Triassic conodont bio- ing from the Late Permian to the Triassic has this fauna assemblage are generally from below stratigraphy in Tibet were initially performed thus been established, based on the limestone- the Triasspathodus triangularis zone (Orchard, in southern Tibet in the mid-1970s (Wang and dominated Dibuco section (Ji et al., 2006; Wu et 2007). Here, we insist that the Pachycladina Wang, 1976). To date, marine Triassic sections al., 2007) and the Wenbudangsang section (Wu fauna represents a Late Induan to Early Ole- have been shown to cover the Lhasa terrane (Ji et al., 2014). The dolomite-type succession has nekian age, which is older than the Neospatho et al., 2006, 2007a, 2010; Wu et al., 2007, 2014, also been recognized in the Zozo section (Ji et dus triangularis zone. 2017, 2018; Zheng et al., 2007) and the Qiang- al., 2007a; Zheng et al., 2007) and the Namutso The above mentioned correlations among tang terrane (Xia and Zhang, 2005; Guizhou section (Wu et al., 2017) (Fig. 7). faunas from different areas in Tibet suggest that Institute of Geological Survey, 2005; Zhang et al., Late Permian conodonts are rare in terms the newly recognized Late Permian to Middle 2005; Li et al., 2018). The recognized conodont of yield in the Shemari section, which is very Triassic conodont faunas in the BNSZ are well biozones from different parts of Tibet could similar to observations in the Wenbudangsang correlated with those in southern Tibet, the Lhasa be well correlated with the international zones section (Wu et al., 2014) and the Zozo section (Ji terrane, and the Qiangtang terrane. established by Sweet et al. (1970) (Fig. 7). To et al., 2007a). The present Induan to Early Ole- determine the differences between conodont fau- nekian conodonts are characterized by elements DISCUSSION nas from the study area and from the other areas of Pachycladina, which clearly differ from those in Tibet, we carried out a thorough comparison. in the limestone-dominated sections but are simi- Succession for Upper Permian to Triassic lar to those in the dolomite-developed sections in Strata in the BNSZ Correlation with Southern Tibet Lhasa. The Late Olenekian fauna in the present section is similar to that found in the limestone- Based on new biostratigraphic data and Southern Tibet, also known as the Himalaya type section, sharing common specimens of Chi previously reported rocky units, we propose terrane, has attracted extensive interest from osella, Magnigondolella, and Thiassospathodus. a stratigraphic scheme for Upper Permian to biostratigraphic researchers (Wang and Wang, For the Middle Triassic, conodont fauna in the Upper Triassic strata in the BNSZ that is obvi- 1976; Orchard et al., 1994; Shen et al., 2006; Shemari section are older than those found in ously different from that of previous research Yuan et al., 2018). To date, excluding the prob- the limestone-dominated sections of Lhasa. The (Fig. 8). The details are documented in the fol- lematic Late Permian conodonts, a relatively Shemari section is characterized by Anisian spe- lowing subsections. complete Triassic conodont succession has been cies, including Neogondolella bifurcata, Neogon established (Fig. 7). dolella constricta, and Paragondolella lieber Mujiuco Formation for Upper Permian to For the Induan to Early Olenekian faunas, mani, whereas the limestone-dominated section Lower Triassic Strata both Southern Tibet and the Shemari section of Lhasa bears specimens of Quadralella polyg The Mujiuco Formation, defined in the have specimens of Hindeodus, although South- nathiformis and Paragondolella inclinata rang- Xainza area of the Lhasa terrane, is a set of dolo- ern Tibet has a relatively high productivity and ing from Late Ladinian to Early Carnian (Fig. 7). mite ranging from the Late Permian to Early diversity of the specimen. Nonetheless, the Triassic according to the coral fauna Waagen difference between the two areas seems to be Correlation with the Qiangtang Terrane ophyllum-Liangshanophyllum-Lobatophyllum more conspicuous: the Shemari section is char- (Cheng et al., 2002) and conodont specimens of acterized by specimens of Pachycladina, Para To date, Triassic conodonts in Qiangtang Pachycladina (Wu et al., 2017), which spreads furnishius, whereas the southern Tibet section have been sporadically documented. The con- extensively in the Lhasa terrane (Wu et al., 2018). features specimens of Neospathodus. This differ- odont sequence there is assembled from a few The Upper Permian and Lower Triassic strata ence is probably caused by a diverse depositional sections. Nevertheless, Early Triassic and Late discovered in the Shemari section coincide well background, with the study area likely being in Triassic biozones have been recorded as covering with the diagnosis of the Mujiuco Formation in much shallower water than southern Tibet. The nearly the whole area. For example, the Early both lithology and fossils. Meanwhile, Late Late Olenekian faunas are similar in both areas. Triassic Olenekian Triasspathodus homeri-Tri Permian coral fossils and Early Triassic con- For example, both faunas have Triasspathodus, asspathodus triangularis assemblage zone was odonts occurred simultaneously in this section, Magnigondolella, and Chiosella species. The found to have spread in both South Qiangtang indicating that the Upper Permian and Lower Middle Triassic Anisian faunas, such as Neogon (Guizhou Institute of Geological Survey, 2005; Triassic were likely continuously deposited. In dolella bifurcata and Neogondolella constricta, Zhang et al., 2005) and North Qiangtang (Xia addition, the obtained Early Triassic conodont are also similar in the two areas. and Zhang, 2005). In addition, the Late Triassic faunas include not only Late Induan to Early Norian Epigondolella zone and the Early Trias- Olenekian Pachycladina fauna but also Late Correlation with the Lhasa Terrane sic Pachycladina-Hadrodontina-Parachirogna Olenekian species, such as Chiosella timoren thus were recognized from north Qiangtang (Xia sis, Magnigondolella ex. gr. regale, and Thias Triassic conodont studies in the Lhasa ter- and Zhang, 2005) (Fig. 7). sospathodus homeri, ensuring that the Mujiuco rane began in the 1980s, focusing on regions The Late Olenekian Triasspathodus homeri- Formation could extend up to the end of the near Lhasa City, where only the Early Triassic Triasspathodus triangularis fauna occur in both Lower Triassic. Triasspathodus homeri zone and Norian Epigon Qiangtang and the BNSZ. The Pachycladina- dolella fauna of the Late Triassic were reported Hadrodontina-Parachirognathus fauna in North Gajia Formation for the Middle Triassic (Sun et al., 1981; Ji et al., 2003). Dramatic prog- Qiangtang is also similar to the present Pachy Ladinian to the Late Triassic Carnian Strata ress was achieved in the 21st century, with suc- cladina fauna, both characterized by produc- The Gajia Formation, discovered in the cessive discoveries of more marine Triassic sec- ing index species of shallower water facies. We Nagqu area and characterized by siliceous rocks tions (Ji et al., 2006, 2007a, 2007b, 2007c, 2010; disagree with Xia and Zhang (2005) that the bearing Ladinian radiolarian fossils (Nimaciren

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International Lhasa (or Gangdese) conodont zones Southern Tibet Qiangtang Bangong-Nujiang s e

e Limestone-type Dolomite-type g i r a t e Ji et al., 2007 S S Ji et al., 2006, 2007 Xia et al., 2005 Wang et al., 1976 Zheng et al., 2007 This paper Sweet et al.,1971 Zhang et al., 2005 Tian, 1982 Wu et al.,2014 Wu et al., 2017 . . n a m m i t h . F . F a h D h c Q . R i s . m . s n Ep.bidentata

a Ep.postera- m a m i i o F r

r Ep.aff.abneptis- c Ep.multidentata Ep.sp. l F T o . F i Ep.cf.primitia J r e M N Ep.abneptis Ep.abneptis e p p n . p n a a m g U

i Qd.polygnathiformis Qd.polygnathiformis Qd.polygnathiformis i n R . F r -Pg.inclinata h a Z

C Ns.newpassensis Ep.diebeli c i s s n a a c i i i Ep.mungoensis r s g T s n Pg.excelsa a i e i l d r d a Pg.mombergensis T d L i e l M d

n Pg.bifurcata d a i Pg.constricta

i -Pg.constricta s M i -Pg.leibermani n p A

u Mg.regale o Ch.timorensis r Ch.timorensis Ch.timorensis Ch.timorensis -Mg.regale Ng.jubata Ns.gondolellides Ch.gondolelloides . g G . n -Ng.jubata Ts.triangularis m Ts.triangularis -Ts.triangularis o . F m l -Ts.homeri -Ts.homeri -Ts.homeri n u m n Platyvillosus

Ns.collinsoni a T a e F u i Ng.milleri l q a Ng.milleri o F k i i c n u n

Ns.conservativus n g h e e n c l s . r i

Parachirognathus- a g a s O m Nvs.waageni T s Nvs.waageni n Furnishius / G

. Pf.xuanhanensis- a i Yi o F r

Ns.pakistanensis m Pach.-Hd. / c Pach. obliqua .

T Pach.obliqua

-Parach. u r

Ns.cristagalli m Ns.cristagalli i o F e j c

Ns.dieneri Ns.dieneri u

w Ns.dieneri u g F i o j M Sws.kummeli n Pf.xuanhanensis- L u Sws.kummeli a n

Sws.kummeli s a Ng.prediscreta H.postparvus M a u

H.postparvus h d s i n

I Ng.carinata I.isarcica Z Ns.krystyni I.staeschei H.typicalis H.parvus H.parvus

Ng.taylorae-

. H.praeparvus Ng.changxingensis Rare Ramiform n m elements a . .

i Ng.meishanensis m g m u F n o -H.julfensis n i . g a F a g F x l i g m n g

Ng.yini-Ng.zhangi a n a i m n o s r a l o F X e g

h Ng.changxingensis e c i n u C a i

r P -Ng.deflect L j d e u u p -Ng.postwangi b p M n U e n a W i

g gondolellids n

i elements p a

i (pending detailed h

c future study) u W

Ep.-Epigondolella; Pg.-Paragondolella; Ng.-Neogondolella; Mg.-Magnigondolella; Qd.-Quadralella; Ch.-Chiosella; H.-Hindeodella; I.-Isarcicella;Ns.-Neospathodus;Ts.-Triasspathodus; Nvs.-Novispathodus; Sws.-Sweetospathodus Pach.-Pachycladina;Parach.-Parachirognathus; Hd.-Hadrodontina;Pf.-Parafurnishius;Gl.-Gladigondolella; Zh.-Zhulong; J.-Jiangrang; D.-Dibuco; Ml.-Mailonggang; Qh.-Quehala; ?-undetermined; Fm.-Formation

Figure 7. Conodont zones of late Permian to Triassic among the main divisions of Tibet.

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Rao et al., 1987 Chen et al., 2015 Pan et al., 2004 Nimaciren et al., 2005 Zhu et al., 2013 This paper m s e e t i Bangongco-Nujiang SR

s Gangdese- r e y Bangongco-Nujiang SR Bangong-Nujiang Nianqingtanggula SR Northern Lhasa Bangongco-Nujiang SSR S S Dongqiao-Jiangco SSR (central part) Nagqu-Luolong SSR Dongkaco Dongkaco SSR r e p Sandy Shale, p u Jienu Group

e Siltstone, l

d Jienu Group c d i i s

s Volcanic tuff m a r r u Mugagangri Group J1 J e Xihu Group w o l Quehala Formation r e

p Quehala Group p Quehala Group Mailonggang Formation u Quehala Group e c l i d s d s Gajia Formation Gajia Formation i a i m r Xihu Group T r

e ? w o l ?

r Mujiuco Formation e p p u e n l a i d

d ? m i r e m P

r Xiala Formation e

w Xiala Formation o l

SR SSR ? Stratigraphic Angular Substratigraphic Absent Unknown Parallel Conformity Region Region strata strata unconformity unconformity

Figure 8. Main schemes for the Permian to Jurassic strata division in the Bangong-Nujiang Suture Zone (BNSZ).

and Xie, 2005), has been treated as a Ladinian to Upper Triassic was also found in the Lhasa ter- Palaeozoic Permian limestones there thrust up Carnian rocky unit in the central BNSZ (Pan et rane, where the limestone known as the Mailong- into the Jurassic strata (Chen et al., 2014). How- al., 2012). The present investigation does not pro- gang Formation has been proven to be Norian ever, our biostratigraphic studies revealed that vide sufficient lithological information regarding and the clasolite has been proven to be Rhatian to the so-called Permian rocky unit is in fact of the Middle Triassic strata. From typical Anisian Early Jurassic through biostratigraphic investiga- a Jurassic age because abundant Jurassic coral conodont fauna, we learn that the Middle Trias- tions (Wang et al., 1983; Lin et al., 1989; Ji et al., fossils were recognized within it (Ji et al., 2011). sic sediments were definitely deposited in this 2007c). Thereby, we correlate the limestone in Thus, the mélange phenomenon in the Oma area area and the Anisian strata feature limestone. It the BNSZ to the Mailonggang Formation. The is incredible. is noteworthy that the siliceous component in overlying clasolite is assigned as the Quehala The study area is another typical site featur- the rocks increased at the end of our measured Formation, to refer to the clasolite above the ing a tectonic mélange phenomenon (Chen et al., section, making it plausible to consider the Gajia Norian limestone. 2015; Lai et al., 2017). Similarly, through bio- Formation as a Late Middle Triassic rocky unit. stratigraphic work, the “Jurassic blocks” that had Knowledge of the Middle Triassic is obviously The Tectonic Mélange Phenomenon in been described as inserted by the Permian Xiala insufficient at present. It is a problem demanding the BNSZ Formation (see Fig. 1, section AA′) are revised further investigation in the future. to Upper Permian to Middle Triassic strata, and Traditionally, the BNSZ was believed to be the “Devonian rocks” contacting with the Trias- Upper Triassic Strata characterized by developing tectonic mélange. sic Quehala Group in unconformity (see Fig. 1, The Upper Triassic rocky unit in the BNSZ The main characteristic in the stratigraphy is section BB′) are correlated with the Upper Trias- has been believed to be a set of clasolite that the Palaeozoic rocks usually thrust into the sic Mailonggang Formation. defined as the Quehala Group and contacting Mesozoic strata by fault block forms (Chen et The above two examples from the Oma area with the Palaeozoic limestones in unconformity al., 2014, 2015). and the present Ban’ge area indicate that it is (BGMRX, 1993; Chen et al., 2015). This work, This tectonic mélange interpretation collides problematic to explain the BNSZ as a tectonic however, demonstrates that the limestone below with the biostratigraphic research. For exam- mélange zone. At least, the biostratigraphic evi- the Upper Triassic clasolite was, in essence, ple, the Oma area in the western part of the dence at present does not support that. To verify Late Triassic Norian rather than Palaeozoic. An BNSZ was thought to be a representative region this hypothesis, more investigative work needs identical limestone and clasolite pattern for the with developed tectonic mélanges in which the to be done in the future.

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Strata Correlations in Lhasa, the BNSZ, mud shale with radiolarian-bearing silicite and future to provide reliable age information. In and Qiangtang basalt interbedded; the Upper Triassic Norian addition, the depositions through the Triassic Riganpeico Formation characterized by lime- are dominated by limestones, so we conjecture In Lhasa, the Upper Permian to the Trias- stone again; and the Rhatian Samenxiong For- that the three areas were likely affiliated with sic succession has been well established based mation made up of a set of clasolite rocks (Li a similar carbonate platform system from the on biostratigraphic research in the Coqen and et al., 2018). Meanwhile, Lower Triassic dolo- Late Permian to the Late Triassic, albeit with Wenbudangsang areas (Ji et al., 2007b, 2007c, mite is dispersed in both Lhasa and Qiang- a somewhat changed depositional setting. The 2018b; Wu et al., 2014). In ascending order, it tang (Fig. 9). extensively distributed Lower Triassic sections includes the Upper Permian Wenbudangsang A comparison showed that the strata succes- in Lhasa and Qiangtang (Fig. 10) could pro- Formation dominated by limestone bearing rich sion established in the BNSZ has many similari- vide evidence for this deduction. According to chert-nodules; the Lower to Middle Triassic ties to that in Lhasa and Qiangtang (Fig. 9). For biostratigraphic and lithostratigraphic analyses, Garenco Formation characterized by limestone; example, the Lower Triassic was characterized the Lower Triassic deposits in Lhasa consisted the Ladinian to the Carnian Zhulong Formation by limestone; the Ladinian to Carnian had an of limestone-type and dolomite-type deposits featuring abundant siliceous rocks interlayered increased siliceous ingredient; the Norian strata (Ji et al., 2018b). Regarding the Early Triassic with thin-layered limestone; the Norian Jian- were limestone again; and the Rhatian featured palaeolithofacial map, the dolomitic evapora- grang Formation characterized by limestone clasolite. Depositions better reflect the sedimen- tion facies alternates with the carbonate plat- again; and the Rhatian to Early Jurassic Dibuco tary setting. The Triassic succession commonly form depression facies (Wu et al., 2018). The Formation (or Jianzhanong Formation) marked found in these three areas indicated that exten- same model should be applicable in Qiangtang, by a set of clasolite (Fig. 9). sive marine transgression took place since the where the dolomite was also reported from the The Triassic strata in Qiangtang were less Early Triassic, which reached a climax during Qiangduo area crossing the BNSZ (Guo et al., documented, and most work was focused in the Ladinian to Carnian and was then followed 1991), whereas the dominated bioclastic lime- the Upper Triassic strata (e.g., Bo et al., 2017). by recession beginning in the Norian. Regarding stone or oolitic limestone was documented from Recent research launched in the Zoqingco sec- the lithology, the increasing silicite component, the eastern part (Fig. 10). tion of Southern Qiangtang, however, prelimi- which was usually interbedded with limestone, New lower Triassic evidence from the Ban’ge narily disclosed that the stratigraphic succes- indicates that the seawater deepened (Fig. 9). area showed that the central BNSZ was likewise sion there is extremely similar to Lhasa, which From the column variation, the BNSZ and dominated by carbonate (mainly dolomite) includes the Lower Triassic Zishisang Forma- its two adjacent areas were assumed to be syn- facies, affirming that Lhasa, Qiangtang, and the tion dominated by limestone; the Middle Trias- chronized (Fig. 9), although biostratigraphic BNSZ belonged to the same depositional system sic Zoqingco Formation featuring developing investigations should be strengthened in the during the Early Triassic (Fig. 10).

Lhasa Terrane Bangong-Nujiang South Qiangtang Series Carbonate Platform Carbonate Platform (central part) Depression Facies Facies

Rhatian Dibuco Fm Quehala Fm Quehala Fm Samenxiong Fm r e

p Jiangrang Fm Mailonggang Fm Mailonggang Fm Riganpeico Fm p Norian U Si

c Carnian Si i

s Zhulong Fm Si Si Si s Si a e Si i undetermined Si l

r Ladinian Si Zoqingco Fm d Si T d

i Si

M Anisian Si r

e Olenekian Garenco Fm Zishisang Fm

w Oula Fm o

L Induan Mujiuco Fm Mujiuco Fm

Upper Permian Wenbudangsang Fm

Clasolite Dolomite Limestone Limestone bearing Limestone Silicite-Limestone Shale Basalt Silicite Flint-nodule (presumed) (presumed)

Figure 9. Sketch column map showing the Upper Permian to Upper Triassic strata succession among the Bangong-Nujiang Suture Zone (BNSZ), Lhasa, and Qiangtang. The succession of Lhasa is based on research by Ji et al. (2007b, 2018b); the succession of Qiangtang is after work by Guo et al. (1991) and Li et al. (2018); and the succession of the BNSZ is from BGMRX (1993), Wang et al. (2002), Nimaciren et al. (2005), and this research. Fm—formation.

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82° 84° 86° 88° 90° 92° Qiangduo 11 Lugu Dinggu Shuanghu 33° 10 NQ 33° Gumu LSSZ 9

6 Geji Rima 8 SQ Oma Gaize 32° Nierong Dongco BNSZ 32° DaruCo SelinCo Nima Jiaqun 2 Sailipu 1 5 31° NLS Nagqu Longgar Coqen Ban’ge 31° 7 DangreyongCo 4 Xainza 3 Dolomite Oolitic Clastic Dolomite Jiangrang NamuCo facies Limestone Limestone 0 60 120(km) 1 Note : Red lines mark the rough geographic boundary for the main Tibetan terranes. BNSZ: Bangong-Nujiang Suture Zone; Limestone Bioclastic Limestone Section nubers facies Limestone and locations LSSZ: Longmu Tso-Shuanghu Suture Zone; NQ: Northern Qiangtang; SQ: Southern Qiangtang; NLS: Lhasa terrane

Figure 10. Sketch map showing the Early Triassic palaeolithofacies covering from North Lhasa, the Bangong-Nujiang Suture Zone, and Qiangtang, based on stratigraphic data. Section names and sources: 1: Shemari section, from this paper; 2: Gajia section, from Nimaciren et al. (2005); 3: Namutso section, from Wu et al. (2017); 4: Mujiuco section, from Cheng et al. (2002); 5: Rendo section, from Wu et al. (2018); 6: Zozo section, from Ji et al. (2007a) and Zheng et al. (2007); 7: Dibuco section, from Ji et al. (2007b); 8: Wenbudangsang section, from Wu et al. (2017); 9: Zishijiabori section, from Zhang et al. (2005); 10: Zoqingco section, from Li et al. (2018); 11: Talikeganlishan section, from the Guizhou Institute of Geological Survey (2005).

Palaeogeographic Implications come from biostratigraphic, lithostratigraphic al., 2013), whereas Lhasa was in the southern and paleomagnetic evidence. For example, the hemisphere between 16.5° ± 3.9° S and 18.4° In research on the palaeogeographic evo- Permian paleogeographic models for the major ± 10.7° S (Zhou et al., 2016). Thus, the paleo- lution of Tibetan terranes, the paleo positions terranes in the Qinghai-Tibet Plateau proposed magnetic data together with the lithostratigraphic of Lhasa and Qiangtang have been a debated by Zhang et al. (2013) were dependent on bio- analysis both insist that Lhasa and Qiangtang topic, although the latest Permian to Early Tri- stratigraphic and lithostratigraphic data. For the were located at low to middle latitudes during assic period is usually neglected (e. g., Met- same reason, the usual avoidance of the Latest the Early Triassic rather than distantly separated calfe, 2013). Two apparently different views Permian to the Early Triassic during palaeogeo- in two continents (Fig. 11). The conodont spe- exist. One view asserted that the two terranes graphical discussions is also caused by scarce cies Pachycladina obliqua may be a fossil sign were located on two sides of the Tethyan Ocean. biostratigraphic and lithostratigraphic data. Thus, marking a low to middle latitude. It is basically Under this view, Lhasa stayed in Gondwana- our newly acquired biostratigraphic evidence produced from shallower water facies such as land, whereas Qiangtang drifted northward as from the BNSZ together with the collected infor- dolomite (sometimes in oolitic limestone). And, a component of the Cimmerian Continent. The mation from Lhasa and Qiangtang is helpful for according to the collected documents, this distance between them enlarged in the Late pushing this research forward. species spread only along middle to low lati- Triassic when Qiangtang rapidly moved to the Biostratigraphically, the Lower Triassic tudes, such as in North America, the western north (Belov et al., 1986; Muttoni et al., 2009; deposits in the three regions are common not Tethys, and South China (Fig. 11). Nevertheless, Metcalfe, 2013; Wakita and Metcalfe, 2005; only in developing carbonate rocks but also in the accuracy of the positions still depends on Zhu et al., 2013; Yan et al., 2016; Lucas, 2017). producing the shallow water index conodont spe- acquisition of more reliable biostratigraphic and The other view posited that Lhasa and Qiang- cies Pachycladina obliqua. In addition, the Late paleomagnetic data in the future. tang both departed from Gondwanaland as a Permian coral genus Waagengophyllum in this part of the Cimmerian Continent (e.g., Scotese study was recognized in the other two regions CONCLUSIONS and McKerrow, 1990; Wang et al., 2003). This (Wang et al., 2003). Similar depositions and fos- viewpoint tended to assert that the two terranes sils indicate that Lhasa, Qiangtang, and BNSZ (1) Biostratigraphic progress has been made were in the same plate; however, the knowledge were not far separated from each other. At least in the central BNSZ. By performing a biostrati- about their depositional setting was incongru- it was impossible that they existed in two conti- graphic study in the Ban’ge area of the central ous. Some researchers described them as a land nents with quite different depositional settings. BNSZ, we discovered fossils of Late Perm- with rare marine sediments primarily deposited This opinion is supported by lithostratigraphic ian to Late Triassic Norian for the first time. during the Triassic (Zhao et al., 2001), whereas and paleomagnetic research. First, the exten- Four faunas are recognized altogether, includ- others considered them to be in a common sea sively distributed carbonate (especially dolomite) ing the Late Permian coral fauna Waagengo background as suggest by sporadic biostrati- from the Neoproterozoic to the Middle Triassic phyllum crassiseptatum–Waagengophyllum cf. graphic and lithostratigraphic data (Yang et al., was a sign of a warm, shallower water environ- ganhaiziense; the Early Triassic (the Induan to 1984; Chen et al., 2001; Boucot et al., 2013; Ji ment that developed only along the low to middle the early Olenekian) conodont fauna Pachy et al., 2018a). latitudes (Chen et al., 2001). Second, the mag- cladina obliqua–Parafurnishius xuanhanensis– It is well known that in the palaeogeograph- netic data indicated that Qiangtang was approxi- Hindeodus cf. postparvus; the Early Triassic ical analysis, the determined factors mostly mately equatorial (Huang et al., 1992; Boucot et (the Late Olenekian) conodont fauna Chiosella

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the Early Triassic were in low to middle latitude positions and were not far away from each other. 60ºN Combined with biostratigraphic research, we Siberia suggest that the BNSZ and two other adjacent

Tarim terranes were in the same carbonate platform, Kazakhstan Mongolia which was located in middle to low latitudes. 30ºN Greenland Eurasia The conodont species Pachycladina obliqua mountains NC N-America may be a fossil indicator to signal the palaeo- landmass Palaeotethys Distributions of Pachycladina geographic locations. shallow sea and slope QT º LS: Lhasa terrane 0 lran BN SC QT: Qiangtang terrane ACKNOWLEDGMENTS LS BN: Bangong-Nujiang This study was financially supported by the National Natural Africa NC: North China Science Foundation of China (41472030, 41972034), the Min- SC: South China istry of Science and Technology of China (2015FY310100), and S-America Neotethys º 30 S Arabia the China Geological Survey (DD20160120-02, DD20160120-04, DD20160126, DD20190008). We are indebted to Yukio Isozaki, Gondwanaland Shuan-hong Zhang, Peng-ju Liu, Hai-yan Wang, and Su-ping Li for their useful suggestions and skillful assistance during India different stages in preparing this work. We thank Ya-dong º 60 S Australia Sun, Kurt Stuewe, and the other anonymous reviewers for their constructive comments concerning this manuscript. We Antarctica would also like to acknowledge the professional manuscript services of American Journal Experts. Figure 11. Map showing the relative positions of Lhasa, Qiangtang, and Bangong-Nujiang during the Early Triassic based on paleomagnetic, lithostratigraphic, and biostratigraphic data (modified REFERENCES CITED after Lucas, 2017). Aljinovic, D., Kolar-Jurkovsek, T., and Jurkovsek, B., 2006, The lower Triassic shallow marine succession in Gorski Kotar region (external Dinarides, Croatia): Lithofacies and Conodont dating: Rivista Italiana di Paleontologia e Stratigrafia, v. 112, no. 1, p. 35–53. timorensis–Magnigondolella regale–Triasspath geological investigations are needed to finally Belov, A.A., Gatinsky, Yu.G., and Mossakovsky, A.A., 1986, A odus homeri–Triasspathodus triangularis; the accept or deny BNSZ as a tectonic mélange. precis on pre-Alpine tectonic history of Tethyan paleo- Middle Triassic Anisian conodont fauna Para (3) The stratigraphic succession established oceans: Tectoninophysics, v. 127, p. 197–211, https://doi .org/10.1016/0040-1951(86)90061-2. gondolella bifurcata–Paragondolella constricta– in the BNSZ could be well correlated with that BGMRX (Bureau of Geology and Mineral Resources of Xizang Paragondolella leibermani; and the Late Tri- in both Lhasa and Qiangtang. In BNSZ, Lhasa, Autonomous Region), 1993, Regional geology of Xizang assic Norian coral fauna Pseudoretiophyllia and Qiangtang, the Triassic strata successions (Tibet) Autonomous Region: Beijing, Geological Pub- lishing House, 450 p. (in Chinese with English Abstract). nazhacunensis–Distichophyllia tenuisa. Com- have many similarities, including limestone or Bo, J.F., Wang, X.L., Gao, J.H., Yao, J.X., Wang, G.H., and bined with regional biostratigraphic informa- dolomite for the Lower Triassic; the siliceous Hou, E.G., 2017, Upper Triassic reef coral fauna in the tion from previous research, a strata succession ingredient increasing to climax in the Ladin- Renacuo area, northern Tibet, and its implications for palaeobiogeography: Journal of Asian Earth Sciences, v. 146, scheme for the BNSZ is proposed as follows: ian to Carnian; limestone for Norian strata; and p. 114–133, https://doi.org/10.1016/j.jseaes.2017.05.006. the Mujiuco Formation for the Upper Permian clasolite for the Rhatian rocks. These similar Boucot, A.J., Chen, X., and Scotese, C.R., 2013, Phanerozoic paleoclimate: An atlas of lithologic indicators of climate: to the Lower Triassic; the Gajia Formation for depositions suggest that they were formed in Society for Sedimentary Geology, SEPM Concepts in the Middle Triassic Ladinian to the Upper Trias- the same sedimentary setting, probably in the Sedimentology and Paleontology, v. 11, 478 p. sic Carnian; the Mailonggang Formation for the same carbonate platform because limestones Chen, X., Yuan, Y.P., and Boucot, A.J., 2001, Paleozoic climate evolution of China: Beijing, Science Press, 325 p. (in Upper Triassic Norian; and the Quehala Forma- dominate across the Triassic. The extensively Chinese). tion probably for the Upper Triassic Rhaetian to distributed Lower Triassic sections in Lhasa and Chen, Y.L., Zhang, K.Z., Gou, Y.D., and Wen, J.H., 2014, Chi- the Lower Jurassic. Qiangtang enforce this deduction. nese Regional Geological Survey Report (1:250000 Oma Sheet): Beijing, China University of Geosciences Press, (2) Our biostratigraphic research further (4) This research provides new biostrati- 163 p. (in Chinese). challenges the opinion that the BNSZ is a tec- graphic evidence for the paleogeographic recon- Chen, Y.L., Chen, G.R., and Zhang, K.Z., 2015, Chinese Re- tonic mélange zone. The investigated area had struction of Lhasa and Qiangtang. Due to a lack gional Geological Survey Report (1:250000 Ban’ge Sheet): Beijing, China University of Geosciences Press, been thought to be a representative site for of biostratigraphic data, the Late Permian and 253 p. (in Chinese). developing tectonic mélange, one main sign of Early Triassic period has always been avoided Chen, Y.L., Kolar-Jurkovsek, T., Jurkovsek, B., Aljinovic, D., and Richoz, S., 2016, Early Triassic conodonts and car- which is that Palaeozoic sediments were thrust in most palaeogeographic research on Lhasa and bonate carbon isotope record of the Idrija-Ziri area, Slo- into Mesozoic strata as fault block forms. How- Qiangtang. Therefore, the newly discovered bio- venia: Palaeogeography, Palaeoclimatology, Palaeoecol- ever, our biostratigraphic findings oppose this stratigraphic data are of great palaeogeographi- ogy, v. 444, p. 84–100, https://doi.org/10.1016/j.palaeo .2015.12.013. view. The “Jurassic strata,” which were previ- cal significance. Similar depositions and fossils Cheng, L.R., Wang, T.W., Li, C., and Wu, S.Z., 2002, Establish- ously thought to be inserted by the Permian in the BNSZ, Lhasa, and Qiangtang indicate that ment of the Upper Permian Mujiu Co Formation and ru- strata are in fact Upper Permian to Middle Tri- they should belong to the same palaeobiogeo- gose coral assemblage in the Xainza area, northern Tibet: Gelological Bulletin of China, v. 21, no. 3, p. 140–143 (in assic rocks; similarly, the originally described graphic division. Therefore, they were unlikely Chinese with English abstract). “Devonian rocks” contacting the Upper Triassic to have been located in two different continents Goudemand, N., Orchard, J.M., Bucher, H., and Jenks, J., 2012, clasolite in unconformity are actually Upper Tri- with completely different depositional settings, The elusive origin of Chiosella timorensis (Conodont Tri- assic): Geobios, v. 45, p. 199–207, https://doi.org/10.1016 assic Norian strata. Both examples show that the and the BNSZ at that time could not have been /j.geobios.2011.06.001. sedimentary mélange is basically misunderstood a large ocean. This opinion is supported by Guizhou Institute of Geological Survey, 2005, 1:250000 Dinggo and Gyamco Sheets in Xizang: Sedimentary because of the erroneous strata assignment. lithostratigraphic and paleomagnetic research, Geology and Tethyan Geology, v. 25, no. 1–2, p. 45–50, More stratigraphic evidence and other related which showed that Lhasa and Qiangtang during (in Chinese with English abstract).

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basin evolution: Beijing, Science Press, 439 p. (in Chinese Lhasa Terrane, Tibet, and its paleogeographic implica- D.Z., and Lü, J., 2013, Qinghai-Tibet Plateau and its ad- with English abstract). tions: Journal of Asian Earth Sciences, v. 121, p. 108–119, jacent areas explanation book of 1:3000000 Mesozoic Zheng, Y.Y., Xu, R.K., Wang, C.Y., Ma, G.T., Lai, X.L., Ye, D.J., https://doi.org/10.1016/j.jseaes.2016.02.006. tectonic-lithofacial geological map: Beijing, Geological Cao, L., and Liang, J.W., 2007, Discovery of Early Triassic Zhu, D.C., Li, S.M., Cawood, P.A., Wang, Q., Zhao, Z.D., Liu, Publishing House, 309 p. (in Chinese). conodonts in western Gangdisê and the establishment S.A., and Wang, L.Q., 2016, Assembly of the Lhasa and of the Tangnale Formation: Science in China Series D: Qiangtang terranes in central Tibet by divergent double Earth Science, v. 50, no. 12, p. 1767–1772. subduction: Lithos, v. 245, p. 7–17, https://doi.org /10 .1016 Zhou, Y.N., Cheng, X., Yu, L., Yang, X.F., Xu, H.L., Peng, X.M., /j.lithos.2015.06.023. MANUSCRIPT RECEIVED 19 OCTOBER 2018 Xue, Y.K., Li, Y.Y., Ye, Y.K., Zhang, J., Li, Y.Y., and Wu, H.M., Zhu, T.X., Jiang, X.S., Feng, X.T., Zhang, Y.J., Mao, X.D., Zhu, REVISED MANUSCRIPT RECEIVED 4 MAY 2019 2016, Paleomagnetic study on the Triassic rocks from the L.D., Zhou, M.K., Wang, X.F., Xiong, G.Q., Wu, H., Liu, MANUSCRIPT ACCEPTED 4 JUNE 2019

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