Detrital zircon U-Pb-Hf isotopes and provenance of ...

Gondwana Research 51 (2017) 193–208

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Detrital zircon U-Pb-Hf isotopes and provenance of Late Neoproterozoic and Early Paleozoic sediments of the Simao and Baoshan blocks, SW China: Implications for Proto-Tethys and Paleo-Tethys evolution and Gondwana reconstruction Tianyu Zhao a,b, Qinglai Feng a,b,⁎, Ian Metcalfe c, Luke A. Milan c, Guichun Liu b, Zhibin Zhang b a b c

State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China School of Earth Sciences, China University of Geosciences, Wuhan 430074, China Earth Sciences, Earth Studies Building C02, School of Environment and Rural Science, University of New England, Armidale, NSW 2351, Australia

a r t i c l e

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Article history: Received 11 November 2016 Received in revised form 28 June 2017 Accepted 30 July 2017 Available online 02 August 2017 Handling Editor: S. Kwon Keywords: Gondwana Proto-Tethys Baoshan and Simao blocks Detrital zircon ages Paleogeography

a b s t r a c t Early Paleozoic evolution of the northern Gondwana margin is interpreted from integrated in situ U-Pb and Hfisotope analyses on detrital zircons that constrain depositional ages and provenance of the Lancang Group, previously assigned to the Simao Block, and the Mengtong and Mengdingjie groups of the Baoshan Block. A metafelsic volcanic rock from the Mengtong Group yields a weighted mean 206Pb/238U age of 462 ± 2 Ma. The depositional age for the previously inferred Neoproterozoic Lancang and Mengtong groups is re-interpreted as Early Paleozoic based on youngest detrital zircons and meta-volcanic age. Detrital U-Pb zircon analyses from the Baoshan Block define three distinctive age peaks at older Grenvillian (1200–1060 Ma), younger Grenvillian (~960 Ma) and Pan-African (650–500 Ma), with εHf(t) values for each group similar to coeval detrital zircons from western Australia and northern India. This suggests that the Baoshan Block was situated in the transitional zone between northeast Greater India and northwest Australia on the Gondwana margin and received detritus from both these cratons. The Lancang Group yields a very similar detrital zircon age spectrum to that of the Baoshan Block but contrasts with that for the Simao Block. This suggests that the Lancang Group is underlain by a separate Lancang Block. Similar detrital zircon age spectra suggest that the Baoshan Block and the Lancang Block share common sources and that they were situated close to one another along the northern margin of East Gondwana during the Early Paleozoic. The new detrital zircon data in combination with previously published data for East Gondwana margin blocks suggests the Early Paleozoic Proto-Tethys represents a narrow ocean basin separating an “Asian Hun superterrane” (North China, South China, Tarim, Indochina and North Qiangtang blocks) from the northern margin of Gondwana during the Late Neoproterozoic-Early Paleozoic. The Proto-Tethys closed in the Silurian at ca. 440–420 Ma when this “Asian Hun superterrane” collided with the northern Gondwana margin. Subsequently, the Lancang Block is interpreted to have separated from the Baoshan Block during the Early Devonian when the Paleo-Tethys opened as a back-arc basin. © 2017 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

1. Introduction Assembly of the Gondwana supercontinent took place in the Late Precambrian-Early Paleozoic (Cawood and Buchan, 2007). Present-day East and mainland Southeast Asia comprises a collage of tectonic blocks, namely North and South China, Tarim, Indochina, North Qiangtang, Sibumasu, South Qiangtang and Lhasa. These blocks are interpreted to have successively rifted from Gondwana and accreted to Eurasia during the Paleozoic and Early Mesozoic. This process involved the opening and closure of three paleo-oceans, the Paleo-Tethys, Meso-Tethys and ⁎ Corresponding author. E-mail address: [email protected] (Q. Feng).

Ceno-Tethys (Metcalfe, 2013). Recent studies of ophiolitic components within suture zones of the south-west region of China (Longmu CoShuanghu Suture Zone in Tibet and Changning-Menglian Suture Zone in the San-Jiang region) have revealed evidence of a far older paleoocean in the Early Paleozoic (Zhai et al., 2010, 2016; B. Wang et al., 2013; Z. M. Peng et al., 2014). This older ocean basin was named Proto-Tethys as an interpreted predecessor of Paleo-Tethys (Zhai et al., 2010, 2016; Deng et al., 2014; Hu et al., 2014b). However, different models have been proposed for the nature and tectonic evolution of the Gondwana Proto-Tethys margin and its relationship to Paleo-Tethys based on different interpretations of the nature of the Gondwana margin (passive or active) and on different interpretations of the age-duration and closure ages for the Proto-Tethys and Paleo-Tethys. Some

http://dx.doi.org/10.1016/j.gr.2017.07.012 1342-937X/© 2017 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

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T. Zhao et al. / Gondwana Research 51 (2017) 193–208

authors have interpreted that the Gondwana Proto-Tethys margin was a passive continental margin based on Early Paleozoic (ca. 530–460 Ma) granitoids in Lhasa, Himalaya, South Qiangtang, and Sibumasu being interpreted as having been emplaced in a post-collisional, extensional setting (Miller et al., 2001; Song et al., 2007; Liu et al., 2016). Alternatively, others argue that these granitoids are subduction related and that the Gondwana Proto-Tethys margin was an active continental margin during the Early Paleozoic based on the presence of a

contemporaneous (ca. 530–460 Ma) angular unconformity formed by Andean-type orogenesis during subduction of the Proto-Tethys Ocean lithosphere beneath the Indian and Australian margin of Gondwana (Cawood et al., 2007; Zhu et al., 2012; Y. Wang et al., 2013; Ding et al., 2015). The age-duration and closure ages for the Proto-Tethys and Paleo-Tethys are also disputed. Some researchers propose that the combined Proto-Tethys and Paleo-Tethys oceans are in fact a long-lived single ocean basin throughout the Paleozoic that underwent only one

Fig. 1. (a) Tectonic outline of Southeast Asia.1 = Qinling-Dabie, 2 = Jinshanjiang, 3 = Ailaoshan, 4 = Song Ma,5 = Longmu Co-Shuanghu, 6 = Changning-Menglian, 7 = Chiang MaiInthanon, 8 = Jinghong, 9 = Luang Prabang, 10 = Nan-Uttaradit, 11 = Indus-YalungZangbo, 12 = Shan Boundary, 13 = Banggong; (b) geological map of the Baoshan showing regional tectonic relationships, Late Neoproterozoic-Paleozoic sediments, Paleozoic and Cenozoic magmatic rocks and the sample locations of this study. The ages for magmatic rock are shown by yellow circles (Duan et al., 2006; Mao et al., 2012; M. Dong et al., 2013; Y. Wang et al., 2013; Nie et al., 2016; Xing et al., 2016; Liu et al., 2017). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


orogenic cycle and closed in the Triassic (Hu et al., 2014b; Zhai et al., 2016). Others have proposed that the Proto-Tethys closed at ca. 490– 470 Ma (Wang et al., 2012b; Hu et al., 2015; Li et al., 2016) or ~430 Ma (Zhang et al., 2014; C. Zhang et al., 2015) before the Paleo-Tethys opened in the Devonian (Jian et al., 2009; Metcalfe, 2013). In order to address the uncertain nature of the Proto-Tethys and varied tectonic models for this ocean basin we undertook investigations of two tectonic blocks, the Baoshan and Simao blocks, which are bounded by sutures/relics of both Paleo-Tethys and Proto-Tethys. Recently, newly discovered NS-trending Early Paleozoic ophiolites (interpreted as Proto-Tethys) were reported between the Lancang Group and Lincang batholith to the east of the Paleo-Tethys Changning-Menglian Suture Zone (Fig. 1b, Liu et al., 2017). The Lancang Group was previously regarded as representing the western margin of the Simao Block but in view of Proto-Tethys ophiolites forming its eastern boundary it may represent a separate continental block (Lancang Block). The paleo-positions of the Baoshan and Simao blocks remain equivocal, hindering our understanding of the relationship between the Proto-Tethys and Paleo-Tethys. The Baoshan Block (as a western component of the Sibumasu Terrane) is interpreted to originate from Gondwana (Metcalfe, 2013). However, whether it originated on the NW Australian margin (Agematsu and Sashida, 2009; Dopieralska et al., 2012; Cai et al., 2015; Metcalfe and Aung, 2014) or the Indian margin of Gondwana (D. Li et al., 2015) is debated. The Simao Block was previously interpreted as representing a northern extension to the Indochina Block (Metcalfe, 2013), however, recent work, particularly in Laos now indicates that the Simao Block was separated from the Indochina Block up until the Triassic (Qian et al., 2016; Rossignol et al., 2016). Uncertainty related to the original locations of both Baoshan and Simao on the Gondwana margin, and disparate models for the evolution and relationships between the Proto-Tethys and Paleo-Tethys require further constraining data. In this study, we present new detrital zircon in situ U-Pb and Lu-Hf isotope data for Late Neoproterozoic and Early Paleozoic sedimentary rocks of the Baoshan and Simao blocks. These data provide constraints on depositional ages and detrital provenance of the sedimentary rocks. Using this new information, the palaeogeographic positions of the Lancang and Baoshan blocks during Gondwana assembly are inferred and a new tectonic model for Early Paleozoic tectonic evolution of the northern margin of Indo-Australian Gondwana is presented. 2. Geological background and sampling The study area comprises the following tectonic units from east to west: the Simao Block, the Changning-Menglian Suture Zone and the Baoshan Block (Fig. 1a). A new NS-trending Early Paleozoic zone of ophiolites between the Lancang Group and Lincang batholith in the Simao Block is also reported (Fig. 1b, Liu et al., 2017) and leads us to propose a Lancang Block underlying the Lancang Group. 2.1. The Simao Block The Simao Block is separated from the South China Block by the Ailaoshan Suture Zone and bounded by the Changning-Menglian Suture Zone to the west (Fig. 1). The precise boundary of the eastern margin of the Simao Block is an open question and the Simao Block could either represent a northern segment of the Indochina Block (Metcalfe, 2013) or be interpreted as being separated from the Indochina Block up until the Triassic by the Luang Prabang belt (Qian et al., 2016; Rossignol et al., 2016). The metamorphosed basement of the Simao Block consists of Precambrian meta-volcanic and metasedimentary rocks. The Neoproterozoic Lancang Group comprises low-grade metamorphic volcanic-sedimentary rocks that are unconformably overlain by the Lower Carboniferous Nanduan Formation (Nie et al., 2015). The Lancang Group outcrops are adjacent to the Triassic Lincang granite batholith to the east and are in fault contact with the Chongshan Group to the north. The

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protoliths of the Chongshan group are mainly sandstones, calcareous rocks, and basic volcanic rocks and the Group is dated as MiddleUpper Proterozoic (Wang et al., 2000). The Lancang metamorphic group is a sequence of low-grade metamorphosed volcanic-sedimentary rocks and can be subdivided into the Huimin and Manlai Formations in the study area. The Huimin Formation is dominated by metamorphosed basalt-andesite lavas with meta-sandstone, slate and schist interlayers. The Manlai Formation is dominated by low-grade sandstone, shale and mudstone (Fig. 2). Calc-alkaline volcanic rocks to the east of the Lincang batholith with uppermost Silurian zircon U-Pb ages of ~ 420 Ma are present in the Dazhonghe area (Mao et al., 2012). This sequence of dacite lavas and associated massive sulfide mineralization from the Dapingzhang area yield a Silurian (Wenlock) zircon U-Pb age of 429 Ma and Re-Os isotope data on Mo-rich bulk ore samples define an isochron of 429 ± 10 Ma (Lehmann et al., 2013). The Lincang batholith intruded the Lancang Group and was overlain by terrestrial red-beds of Middle Jurassic age to the east. The Lincang batholith formed during the continental collision between the Baoshan Block (Sibumasu Terrane) and the Simao Block (G. Dong et al., 2013). Recent geochronological studies indicate that the Lincang batholith is a complex consisting of at least two intrusive pulses (late Permian and Middle to Late Triassic) (Deng et al., 2014). 2.2. The Baoshan Block The Baoshan Block is bordered by the NS-trending ChangningMenglian Suture Zone to the east, and by both the NW-trending Nujiang fault and Luxi Suture Zone to the west (Fig. 1). It is regarded as a disrupted northern component of the Sibumasu Block with an affinity to Gondwana based on stratigraphic, paleontological, and paleomagnetic data and was rifted from the supercontinent during the late Early Permian and collided with the Simao Block in the Middle to Late Triassic (Wang et al., 2001; Metcalfe, 2013; Ali et al., 2013; Deng et al., 2014). The Gongyanghe Group located in the western part of the Baoshan Block consists of weakly metamorphosed turbiditic and bathyal-facies sandstone, shale and carbonate rocks with rare fossils and volcanicrock intercalations and is considered to be the metamorphosed basement to the block (Fig. 1, Yang et al., 2012). Cao and Lu (1991) reported Early Cambrian trace fossils at the base of the Gongyanghe Group. Luo (1985) reported Early-Middle Cambrian acritarchs at the top of the Gongyanghe Group. Intraplate metabasalts from the Gongyanghe Group yield a Late Cambrian zircon U-Pb age of ~ 499 Ma (Yang et al., 2012). The Neoproterozoic Mengtong Group and Ordovician-Silurian Mengdingjie Group represent slope-basin deposits on the eastern margin of the Baoshan Block. The Mengtong Group comprises low-grade metamorphosed sandstone, slate and schist, with some felsic and mafic volcanic rocks. The Mengdingjie Group is composed mainly of low-grade metamorphosed sandstone, mudstone and interbedded basic volcanic rocks and siliceous limestone, which commonly underwent low-grade greenschist-amphibolite facies metamorphism. These two Groups are in fault contact. Upper Paleozoic marine successions include diamictites and pebbly mudstones which are interpreted to be of glacial origin (Wopfner, 1996). Early Paleozoic, Permian and Late Cretaceous to Paleocene granitoids are the main magmatic record in the Baoshan Block. The Pinghe monzogranites and Pingdajie granite give Ordovician U-Pb ages of 486–455 Ma (M. Dong et al., 2013; Y. Wang et al., 2013). A Middle Permian A-type granitic pluton yields a zircon U-Pb age of 266 Ma (Lin et al., 2010). The Late Mesozoic to Early Cenozoic magmatic Huataolin granite was dated at 100–60 Ma (Chen et al., 2007). 2.3. The suture zones The Changning-Menglian Suture represents the Paleo-Tethys in the study area, outcropping as dismembered ophiolites and associated

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Fig. 2. Stratigraphic columns for the Lancang, Mengtong and Mengdingjie groups (revised from 1:250, 000 geological map of Jinghong and Fengqing, Yunnan). The star symbol denotes the locations of the collected samples. The arrows symbol show the reported meta-igneous rocks age (Nie et al., 2015; Xing et al., 2016). The stratigraphic columns show wavy patterns reflecting a metamorphic past.

oceanic sedimentary rocks, opened in the Devonian and closed in the Late Triassic (Metcalfe, 2013). It is contiguous with the Longmu CoShuanghu Suture in Central Qiangtang and the Chiang Mai-Chiang Rai Suture in northern Thailand (Deng et al., 2014; Metcalfe, 2013; Metcalfe et al., 2017). The deep-water sedimentary rocks include late Middle Devonian to Middle Triassic pelagic radiolarian-rich cherts (Liu et al., 1991; Feng and Ye, 1996; Metcalfe, 2013) and seamount carbonates dated by fusulinids as Early Carboniferous to Late Permian in age (Ueno, 2003; Wu et al., 1995). NMORB-like basalts and gabbros from the Ganlongtang-Dongba ophiolite complex slightly to the northeast of the Gengma area give Lower Carboniferous U-Pb zircon ages of 349–331 Ma (Duan et al., 2006). OIB-like basalt from the Laochang area gives Upper Carboniferous U-Pb zircon ages of 323–307 Ma (Deng et al., 2014). Supra Subduction Zone (SSZ)-type meta-gabbro has been dated with zircon U-Pb at 270–264 Ma (Jian et al., 2009). Recently, another North-South Early Paleozoic ophiolitic mélange belt, which is located between the Lancang Group and Lincang batholith, was reported by Liu et al. (2017) (Fig. 1). It consists of phengite-bearing quartz schist, green-schist and amphibolite schist, tonalite and anorthosite, laminated amphibolite and cumulate gabbros. They are respectively equivalent to pelagic sediment, sea-floor basalt, plagiogranite, mafic cumulate rock and HP-UHP pyrolite in ophiolite suite. U-Pb zircons from cumulate anorthosite is dated as late Early Ordovician (470 Ma) from the Mannahe section and metagabbro from the Dananmei section gives a Late Ordovician age of 440 Ma (Liu et al., 2017) (Fig.1b). Samples for detrital zircon U-Pb-Hf analyses were collected from both the Baoshan and Simao blocks (Fig. 1b). Two undeformed sandstones (15ND10 and 15ND12) from the Mengdingjie Group of the Baoshan Block are well sorted and consist mainly of sub-angular to

rounded quartz grains (95%), lithic grains (4%) and minor feldspar and mica (1%) (Fig. 3a). A weakly foliated quartz schist sample, 15NDG-21, from the Mengtong Group of the Baoshan Block is texturally mature, and is dominated by quartz fragments and minor clay matrix (Fig. 3c). These groups were mostly metamorphosed under high pressure and low temperature conditions, which records continental collision during the closure of the Proto-Tethys or Paleo-Tethys ocean between the Simao and Baoshan blocks (Wei et al., 1984). The meta felsic volcanic sample Mh6 is grayish white in color in hand specimen and is recrystallized and exhibits a foliate texture in thin section. The K-feldspar and quartz phenocrysts with minor mica and accessory minerals are distributed in a sericitized groundmass. Two metamorphosed sandstones (15NDG1 and 15NDG2) comprising primarily quartz and elongate biotite laths were collected from the Lancang Group (Fig. 3b) of the Simao Block. 3. Analytical methods Conventional heavy liquid and magnetic techniques were employed to separate zircon grains, which were then picked by hand under a binocular microscope. More than 200 grains were randomly selected, mounted in an epoxy resin and polished to expose the cores. Cathodoluminescence (CL) images for revealing internal structures were obtained using a JEOL JXA-8100 electron microscope. Zircon UPb dating was performed at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences (Wuhan). U-Pb isotope analysis was conducted with a 7500a ICP-MS coupled with a GeoLas 2005 system. Detailed analytical procedure followed Liu et al. (2010). In our analysis, the spot size was 32 μm


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Fig. 3. Photomicrographs of samples from the (meta) sandstones in Mengdingjie, Mengtong and Lancang Groups (a, b, c) and meta-felsic volcanic rock from the Mengtong Group (d) (All images are under cross-polarized light). (a) 15ND10; (b) 15NDG1; (c) 15NDG 21; (d) Mh6. Abbreviations for minerals: Q, quartz; Pl, plagioclase; M, mica; R, rock fragments.

with each analysis including a background gathering of around 20 s and 50 s of data acquisition from the samples. The standard 91500 and GJ-1 were used as external standard for U-Pb dating and to determine the elemental fractionation. Zircon 91500 was analyzed twice every 5 analyses. Off-line selection and integration of background and analytical signals, time-drift correction and quantitative calibration for U-Pb dating were performed using ICPMSDataCal (Liu et al., 2008). Concordia diagrams and weighted mean calculations were made using Isoplot/ Ex_ver3 (Ludwig, 2003). Hafnium isotope analyses on dated zircon grains were performed on a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) equipped with a Geolas 2005 laser ablation system (Lambda Physik, Göttingen, Germany) at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. Laser ablation system conditions and the MC-ICP-MS instrument as well as analytical procedure are described in detail by Hu et al. (2012). Analyses were carried out with a laser beam at 44 μm in diameter and 20 s background signal followed by 50 s of ablation signal acquisition. The standard 91500 was analyzed twice every 10 analyses. Interference of 176Yb on 176Hf was corrected by 176Yb/173Yb = 0.7876 (McCulloch et al., 1977). The minor interference of 176Lu on 176Hf was corrected by 176Lu/175Lu = 0.02656 (Blichert-Toft et al., 1997). The values of initial Hf isotope and 176Hf/177Hf ratios were calculated with the 176Lu decay constant of 1.865 × 10− 11 a−1 (Scherer, 2001) and the measured 176Lu/177Hf ratios. Two-stage Hf model ages (TCDM) were calculated by assuming a 176Lu/177Hf ratio of 0.015 for the average continental crust (Griffin et al., 2000). 4. Results A total of 6 samples (15NDG1, 15NDG2, 15NDG21, 15ND10, 15ND12 and Mh6) were selected for zircon U-Pb dating. Two samples (15NDG1 and 15NDG 21) were analyzed for Lu-Hf isotopes. Two samples (15NDG1, 15NDG2) are from the Simao Block, while the rest are from Baoshan. All U-Pb ages and Lu-Hf isotope data analyzed in this paper are listed in Supplementary Tables S1 and S2. Uncertainties of individual

analyses are at 1-sigma level. All U-Pb analyses for each sample are plotted on concordia diagrams and on distribution of age frequency diagrams (Fig. 4). Only analyses with N 90% concordance were used in frequency diagrams (Discordance = 1 − (100 ∗ (1 − abs(68Age − 75Age) / ((68Age + 75Age) / 2))). The 206Pb/238U ratio was calculated for zircon grains b1000 Ma, whereas 207Pb/206Pb ratio was used for older zircon grains because N1 Ga 206Pb/207Pb ages have an uncertainty of 0.5–2% (at 1–sigma level), whereas b 1.0 Ga 206Pb/207Pb ages have considerably greater uncertainty (Gehrels et al., 2008). Zircon crystals from the six sedimentary rock samples are generally 50 to 250 μm in size, with aspect ratios of 1:1 to 4:1. Most of the analyzed zircons are transparent to semitransparent and subeuhedral to rounded, implying long distance transport or multi-cycled sedimentary processing. Most grains show oscillatory zoning under CL images with high Th/U ratios (N 0.1), characteristic of a magmatic origin (Hoskin and Ireland, 2000). Some grains display bright structure-less characters and low Th/U ratios (b0.1), which is interpreted to be of metamorphic origin. Zircon grains from samples Mh6 are transparent in color, euhedral and prismatic. Their crystal lengths range from 150 to 250 μm, with length/width ratios of 2:1 to 5:1. Most of them show oscillatory zoning in CL images, suggesting their magmatic origin. A few zircon grains have residual cores and show core-rim structures. 4.1. Detrital zircon U-Pb ages 4.1.1. Simao Block 4.1.1.1. Lancang Group (15NDG1 and 15NDG2). Sixty-nine detrital zircon grains from sample 15NDG1 have been analyzed and yielded 64 concordant (discordance b 10%) ages. Ages range from 3208 to 494 Ma, with distinct peaks at ca. 1179, 968 and 566 Ma and an age group between 2600 and 2400 Ma. 63 detrital zircons were analyzed for sample 15NDG2, which yielded 60 concordant ages (ca. 2809–455 Ma) with three major peaks at ca. 1140, 946 and 552 Ma. Age spectra for the two samples from the Lancang Group exhibit a similar range of ages, which fall into four groups: 0.6 to 0.5 Ga, 1 to 0.9 Ga, 1.2 to 1.1 Ga and

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Fig. 4. U-Pb dating concordia diagrams of samples and relative probability density diagrams of U-Pb detrital zircon ages. 15ND10 and 15ND12 from the Mengdingjie Group, 15NDG1 and 15NDG2 from the Lancang Group, 15NDG-21 and Mh6 from the Mengtong Group. Mh 6 is a meta-felsic volcanic rock from the Mengtong Group. Data were filtered in green. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2.6 to 2.4 Ga (Fig. 5c). The youngest age from the two samples is 455 Ma, which provides the maximum depositional age of the Lancang Group. 4.1.2. Baoshan Block 4.1.2.1. Mengtong Group (15NDG 21 and Mh6). A total of 83 analyses were conducted and yielded 74 interpretable ages (with 90% concordance) for sample 15NDG20 from the Mengtong Group. They range in age from 3744 to 487 Ma, with age peaks at ca. 2388, 1152, 972 and 562 Ma (Fig. 5b). The youngest age of 487 Ma provides the maximum depositional age of the Mengtong Group. The zircon grains from sample Mh6 are mostly euhedral and show magmatic zoning on CL images (Fig. 4l). Sixteen analyses gave a weighted mean 206Pb/238U age of 462 ± 2 Ma (Fig. 4l), which is interpreted as the eruption age of sample Mh6. Three analyses gave older 206Pb/238U apparent ages of 1676 Ma, 749 Ma and 787 Ma, indicating ages of inherited zircons. The occurrence of old zircons indicates their derivation from partial melting of a mixed sedimentary source that was composed of old crustal rocks or basement material.

Group. The U–Pb age vs. εHf(t) diagram is shown in Fig. 6. The εHf(t) values show a wide range of εHf(t) and TCDM model ages, suggesting complex sources for the host magmas of these zircons. Overall, the εHf(t) values in the 2.8–2.3 Ga age population yielded a range of εHf(t) values from −23 to +28 with model ages (TDM2) of 4.0–1.5 Ga, suggesting reworking of older crustal components and involvement of juvenile material during magma generation. Similarly, the prominent zircon age population of 1.2–0.9 Ga, shows a broad range of εHf(t) values from −14.0 to +16.1, and Hf model ages (TDM2) of 2.5– 0.9 Ga. Some detrital zircons in this population give positive values, indicating involvement of juvenile materials during Late MesoproterozoicEarly Neoproterozoic times, while the majority yielded negative values, indicating reworking of older crustal components. Most zircons with crystallization ages of 0.65–0.45 Ga show strong negative εHf(t) values (− 26 to − 1, except for two zircons with εHf(t) values of +8.0 and +1.9) and yield Hf model ages (TDM2) ranging from 2.7 Ga–1.9 Ga, suggesting their derivation mainly from the reworking of Neoarchean to Middle Paleoproterozoic rocks with small amounts of juvenile crust. 5. Discussion

4.1.2.2. Mengdingjie Group (15ND10 and 15ND12). Sixty-two detrital zircons from sample 15ND10 were analyzed for U-Pb ratio, yielding 55 concordant ages (ca. 3058 to 482 Ma) with four peaks ca. 1600, 1169, 963 and 552 Ma. Fifty-nine concordant ages of 64 dated-zircons were obtained from sample 15ND12, which vary from 2780 to 528 Ma. There are four major age peaks at ca. 2439, 1164, 954 and 561 Ma, respectively. Ages from the two samples lie in the ranges of 0.65 to 0.55 Ga, 1 to 0.9 Ga, 1.2 to 1.1 and 2.5 to 2.3 Ga (Fig. 5a). 4.2. Zircon Hf isotope compositions Ninety-three zircon Hf isotope compositions were analyzed on the same domain as the U–Pb analyses from Lancang Group and Mengtong

5.1. Constraints on depositional ages of the sedimentary sequences 5.1.1. Lancang Group The Lancang Group consists of two formations, the Huimin and Manlai Formations. Traditionally, they have been interpreted to be Neoproterozoic (BGMRY), but the depositional ages of the Lancang group have not been well defined. According to the microfossil flora of the Huimin Formation, the Lancang Group was regarded as Late Proterozoic (Lei, 1982) or Precambrian to Cambrian (Liu, 1985). Zhai et al. (1990) obtained a Sm-Nd isochron age of 1287.4 ± 90.4 Ma and Rb-Sr isochron age of 1064 ± 84 Ma from blueschist, but the Rb-Sr isochron age of greenschist is 533 ± 12 Ma. The depositional age of the Lancang


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LA-ICP-MS U-Pb zircon dating of two samples from the Lancang Group is used to constrain the maximum depositional age of the sediments. The youngest zircon from sample 15NDG1 yields an age of ca. 494 Ma and that from sample 15NDG2 yields an age of about 455 Ma. Given that the sampling sites are close to each other and received similar parent material, the sampled strata possibly represent a synchronous sedimentary sequence. Thus, youngest single grain age from the Lancang Group is 455 Ma, which means the Lancang Group was deposited younger than 455 Ma. Recently, Dickinson and Gehrels (2009) proposed that the mean age of the youngest cluster of U-Pb grain ages provides the most conservative measure of youngest age. The youngest weighted mean ages of the Lancang Group is 521 ± 4 Ma. However, the youngest zircon age should provide a good estimate of the maximum age unless there are significant lead loss issues or the zircon is a metamorphic zircon. The 455 Ma zircon shows no significant Pb loss or metamorphic structure. Moreover, intra-formational metavolcanic rocks from the Huimin Formation in the Lancang Group give zircon U-Pb ages of ca. 460 Ma (Fig. 4l) (Nie et al., 2015; Xing et al., 2016). The coincidence of youngest detrital zircon (~455 Ma) with the volcanic zircon U-Pb age provide a more robust constraint on the depositional age of the Lancang Group. In addition, chlorite from greenschist and crossite from blueschist give Ar/Ar cooling ages of 410 ± 15 Ma and 409.8 ± 23.6 respectively from the upper part of the Lancang Group (Cong et al., 1993), which should be younger than the formation of the Lancang Group. Moreover, the Lancang Group is overlain by the Devonian-Carboniferous Nanduan Formation with an angular unconformity (Feng et al., 1996; Nie et al., 2015). Therefore, deposition of the Lancang Group probably started ~455 Ma (Late Ordovician) and terminated before 410 Ma (Early Devonian), much younger than the previously inferred Neoproterozoic.

Fig. 5. Relative probability pattern of U-Pb detrital zircon age distributions of the Mengdingjie, Mengtong and Lancang groups for comparison to the Simao Block. The detrital age of the Simao Block data from (Xia et al., 2014). Important age peaks are shown in color bands. N = total number of samples, n = total number of analyses. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Group remains equivocal and no reliable zircon U–Pb ages have previously been determined. We here present robust evidence the depositional age of the Lancang Group.

5.1.2. Mengtong Group According to limited micropaleontologic evidence, metamorphic grade and regional stratigraphic correlation, previous researchers suggest that the Mengtong Group was deposited in the Neoproterozoic. However, no precise depositional ages determined by zircon U-Pb geochronological data for the Mengtong Group have been reported. The youngest weighted mean ages of the Mengtong Group are 558 ± 15 Ma. The youngest single zircon of the sandstone from the Mengtong Group gives a concordant age of ca. 487 Ma, indicating the maximum depositional sedimentary age being Early Ordovician. In addition, ca. 460 Ma meta-felsic volcanics have been found in the northern part of the Mengtong Group (Fig. 1b). These data argue convincingly that the Mengtong Group was probably deposited during the Early Paleozoic rather than previously estimated Neoproterozoic, which is similar to our interpretation for the Lancang Group. 5.1.3. Mengdingjie Group The Mengdingjie Group was traditionally considered to be Ordovician-Silurian based on paleontological data in the 1:250, 000 Geological Maps of Fengqing (BGYP). The youngest weighted mean ages of the Mengdingjie Group are 552 ± 5 Ma, which is older than its biostratigraphical age. The youngest U-Pb zircon age (ca. 482 Ma) constrains the maximum depositional age as Early Ordovician, which is also compatible with the previously assigned Ordovician-Silurian age. Consequently, a maximum Ordovician depositional age for the Lancang, Mengtong and Mengdingjie stratigraphic units is therefore inferred. 5.2. Provenance of the sedimentary sequences

Fig. 6. Plot of epsilon Hf vs. U-Pb ages of detrital zircons from the Mengtong and Lancang groups. The previously reported detrital zircons from the Western Australia and Tethyan Himalaya were also plotted for comparison (Veevers et al., 2005; Zhu et al., 2011).

Our detrital zircons from the three Early Paleozoic stratigraphic units together define three obvious age populations: ~ 1.1, ~ 0.95 and 0.65– 0.5 Ga, followed by minor age distributions of ~ 2.7, ~ 2.4, 1.7–1.4 Ga (Fig. 7a). Precambrian magmatism of these ages has not been reported in the Baoshan/Sibumasu Block or the Simao Block and the sub-rounded

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or anhedral shapes of these zircons suggest a long-distance transport or multi-cycled sedimentary processing before deposition. The oldestknown igneous rocks reported in the Baoshan Block with in-situ zircon U-Pb age were obtained from the Gongyanghe Group (499 Ma meta-basalt) and Pinghe granite (502–476 Ma; Chen et al., 2007; Liu et al., 2009; Yang et al., 2012; M. Dong et al., 2013; Y. Wang et al., 2013). Farther to the south, the Khuu Tao orthogneiss and the Khao Dat Fa granite yield concordant ages of 501 Ma and 477 Ma respectively in northern Peninsular Thailand (Lin et al., 2013; Kawakami et al., 2014). The oldest exposed basement of metamorphic rocks in the Baoshan Block, including the Mengdingjie, Mengtong and Lancang groups, are here reinterpreted to be Early Paleozoic rather than Precambrian in age. The Middle Cambrian-Early Ordovician Machinchang and Jerai Formations are the oldest sedimentary strata in Malaysia (Lee, 2009). Thus, there are no unequivocal exposures of Precambrian basement rocks reported in the Baoshan/Sibumasu Block. In addition, information on the basement of the Simao Block is largely lacking. The Paleoproterozoic Ailaoshan and Dahongshan groups in the south-western Yangtze Block used to be interpreted as the basement of the Simao Block and Yangtze Block. However, the earliest reported magmatic rocks in the Simao Block are Silurian (Mao et al., 2012; Lehmann et al., 2013; Zhao et al., 2016). Detrital zircon grains from the Simao Block also record a Late Ordovician age peak at ca. 447 Ma (Fig. 5d; Xia et al., 2014). Hence, we suggest that the b 0.5 Ga zircons in our samples may come from local igneous rocks, but the ones N0.5 Ga have been derived from sources outside of the blocks. The presence of Pan-African (600– 500 Ma) and Grenville (1330–900 Ma) detrital zircons in Early

Paleozoic strata is a characteristic feature of a Gondwana-derived source (Myrow et al., 2010; Cawood et al., 2013; Xu et al., 2013). The large PanAfrican (600–500 Ma) age population is possibly related to the formation of the Gondwana supercontinent. The assembly of Gondwana formed the Prydz-Darling Orogen (600–500 Ma) between Antarctic and India and southwest Australia (Fig.9 Fitzsimons, 2000; Cawood et al., 2007). The ~530 Ma peak metamorphism recorded the final collision of East and West Gondwana (Shabeer et al., 2005). Other possible sources of these detrital zircons in our samples are the accretionary orogenic events on the margins of East Gondwana, such as the 480–530 Ma Ross-Delamerian orogeny (Cawood, 2005; Cawood et al., 2007; Cawood and Buchan, 2007). It is noticeable that the identified Pan-African age of detrital zircons (600–500 Ma) with large εHf(t) variation (− 25.9 to +8.0) and Hf model age (2.8–0.95 Ga), indicate the occurrence of juvenile crustal materials and reworking of Paleoproterozoic-late Archean crustal material. Therefore, the Pan-African orogens (600–500 Ma) related to subduction-collisional events provided an important provenance supplement during the Early Paleozoic (Fig. 9). The Grenvillian age (1330–900 Ma) of detrital zircons in our samples yielding large εHf(t) variation (−14 to +16) and Hf model age (2.5–0.98 Ga) are indicative of complex source region(s). Our samples include both younger and older Grenvillian ages (990–900 Ma and 1330–1130 Ma), suggesting possible sources both from the Albany-Fraser orogen (1330– 1130 Ma) in SW Australia and Rayner-Eastern Ghats orogen (960– 990 Ma) in Antarctica and India (Boger et al., 2000; Fitzsimons, 2000; Halpin et al., 2012; Cawood et al., 2013). A small percentage of Grenvillian age detrital zircons show low Th/U values, which indicate

Fig. 7. Comparison of the age distribution pattern of detrital zircon age from Baoshan Block and different blocks of Gondwana (Cawood and Nemchin, 2000; Veevers et al., 2005; Pullen et al., 2008, 2011; Myrow et al., 2009, 2010; Dong et al., 2010, 2011; Zhu et al., 2011; Mcquarrie et al., 2013; Wang et al., 2014; Xia et al., 2014; D. Li et al., 2015; C. Wang et al., 2016; J. Zhang et al., 2015). Important age peaks are shown in color bands. Data exhibited are all detrital age spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


metamorphic rock sources. The paragneisses in the Mesoproterozoic Northampton Complex in Western Australia were metamorphosed and deformed at 1090–1020 Ma (Ksienzyk et al., 2012) and ∼ 950– 940 Ma metamorphic zircons were reported in the Mangalwar Complex of India, which is similar to some metamorphic grains in our samples. The Neoarchean age population (ca. 2400–2700 Ma) coincides with ages from the supercontinent Kenorland (Belousova et al., 2010). Late Archean zircons in our samples give εHf(t) values of − 13.1 to + 28.3 with model ages of 3.9–1.5 Ga, suggesting reworking of Archean crust in the source region as well as additional juvenile crust during this period. Archean-Early Paleoproterozoic crustal rocks are also common in Australia, Antarctica and India (Boger et al., 2000; Fitzsimons, 2000; Griffin et al., 2004; Reid et al., 2014). Therefore, we consider that the provenance of our samples likely reflect sources from different orogenic events of East Gondwana blocks (Fig. 9). 5.3. Tectonic implications 5.3.1. An independent Lancang Block with affinity to the Baoshan Block As discussed in the previous section, the detrital zircon age spectra of the Lancang Group of the Lancang Block closely resembles those from the Mengtong and Mengdingjie groups of the Baoshan Block. They share a similar zircon age distribution pattern characterized by Neoarchean (~2500 Ma), older Grenvillian (1200–1060 Ma), younger Grenvillian (~960 Ma) and Pan-African (650–500 Ma) age peaks (Figs. 5 and 7). In contrast, the detrital zircons from sedimentary rocks of the Simao Block have major age groups at ~2.5, 1.7–1.4, ~0.95, 0.6 and 0.47 Ga, which is slightly different from the pattern for the Lancang Group of the Lancang Block (Figs. 5 and 7). The peak ages of ~1160 Ma and ~560 Ma from the Lancang Group are less significant in the Simao Block (Fig. 5). These data indicate that the Lancang Group received significantly more abundant older Grenvillian (1330–1130 Ma) and Pan-African (650–500 Ma) age material than the Simao Block and is similar in terms of provenance to the Baoshan Block. Moreover, the Lancang Group contains metavolcanic rocks dated at ca. 460 Ma (Middle Ordovician) which are of similar age to widespread metarhyolite (Mengtong Group, this study) and granites (Nie et al., 2015; Xing et al., 2016) in the Baoshan Block. In contrast, Ordovician magmatism appears to be absent in the Simao and Indochina blocks, which are characterized by Silurian igneous rocks (Mao et al., 2012; Lehmann et al., 2013). Based on zircon age spectra, the provenance sources of the Lancang Group of the Lancang Block are closely comparable to those of the Baoshan Block but significantly different to those of the Simao Block. In addition, Early Paleozoic ophiolitic rocks discovered between the Lancang Group and Lincang granites indicate the presence of a Proto-Tethys suture zone in SW Yunnan which is to the east of the Changning-Menglian Suture Zone (the Paleo-Tethys) and separates the Lancang Block and the Simao Block (Fig. 1b; Liu et al., 2017). The Proto-Tethys ophiolitic mélange along the eastern boundary of the Lancang Block extends from the Yunxian to Shuangjiang areas (Fig. 1b). The Lancang Block cannot be a western sector of the Simao Block during the Early Paleozoic, but appears to show a tectonic affinity with the Baoshan Block at that time. The Baoshan Block sedimentary rocks and the Lancang Group of the Lancang Block were deposited in linked basins that received similar sedimentary material from Gondwana during the Early Paleozoic. 5.3.2. Positions of the Baoshan and Lancang blocks in Gondwana The Baoshan Block, that formed the western tip of the Sibumasu Block, was an integral component of East Gondwana until the opening of the Meso-Tethys Ocean in the late Early Permian (Metcalfe, 2013). However, the exact paleo-position of Baoshan on the margin of Gondwana is still debated. Multidisciplinary data, including similar Paleozoic stratigraphy, biotic assemblages, Late Carboniferous-Early Permian glacio-marine deposits, paleoenvironmental indicators, detrital zircon age distribution patterns and old crustal basement suggests a NW

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Australia origin for the Baoshan/Sibumasu Block (Metcalfe, 1988, 1996, 2011, 2013, 2017; Wopfner, 1996; Jin, 2002; Agematsu and Sashida, 2009; Dopieralska et al., 2012; Cai et al., 2015; Metcalfe and Aung, 2014; G. Li et al., 2015). In contrast, D. Li et al. (2015) argue that the Baoshan-Sibumasu Block was along the Indian Block margin in the Early Paleozoic based on detrital zircon provenance analyses. Recently, Ali et al. (2013), Y.C. Xu et al. (2014) and Y.J. Xu et al. (2014) have suggested that the Baoshan Block was situated near the junction of northern India and northwestern Australia based on paleomagnetic data. Zhu et al. (2011) used detrital zircon age spectra to discriminate whether some Peri-Gondwana blocks were derived from the Indian or the Australian margins of Gondwana based on the presence/absence of younger (~ 950 Ma) and older (~ 1170 Ma) Grenvillian detrital zircons. The older (~1170 Ma) Grenvillian detrital zircons were most likely sourced from the Wilkes-Albany-Fraser belt in southwest Australia, whereas the younger (~ 950 Ma) detrital zircons were interpreted to have been derived from the Rayner-Eastern Ghats belt between India and Antarctica (Fig. 9). The Wilkes-Albany-Fraser belt supplied detritus to the Permian Collie, Perth and Lhasa Basins to the north, showing an age distribution yield of 1130–1330 Ma (Cawood and Nemchin, 2000; Veevers et al., 2005; Zhu et al., 2011) while the Rayner-Eastern Ghats belt with magmatic events dated 960–900 Ma in India and Antarctica supplied sedimentary detritus to the northern margin of the Indian Craton, the Himalaya, and the South Qiangtang Block (DeCelles, 2000; Tobgay et al., 2010; Dong et al., 2011; Mckenzie et al., 2011b; Z. Zhao et al., 2014; M. Wang et al., 2016). The Lhasa Block shows an older Grenvillian peak age of ~1170 Ma which corresponds to the Wilkes-Albany-Fraser belt but lacks the younger (~950 Ma) Grenvillian detrital zircons, suggesting that the Lhasa Block was originally part of Australia (Zhu et al., 2011). Therefore, different detrital zircon patterns from the different blocks can be a useful method to constrain paleogeographic reconstructions. The presence of both late Mesoproterozoic (~ 1150 Ma) and Early Neoproterozoic (~ 960 Ma) detritus in our samples suggests both the Wilkes-Albany-Fraser belt in southwest Australia and the Rayner-Eastern Ghats belt in India as the material sources for the Baoshan Block. Moreover, Neoarchean-Early Paleoproterozoic granitic gneisses, metavolcanics and granites from the Yilgarn Craton and Pilbara Craton Western Australia formed at 3.05 Ga–2.62 Ga (Griffin et al., 2004; Li et al., 2016), whereas thermal events in India took place younger at ca. 2.6– 2.4 Ga (Mondal et al., 2002). Thermal events around 1600 Ma are found in both the Wilkes-Albany-Fraser belt and Rayner-Eastern Ghats belt (Dobmeier et al., 2006; Cawood and Korsch, 2008; Upadhyay et al., 2009; Kirkland et al., 2011). The Late Neoarchean-Early Paleoproterozoic and ca. 1600 Ma detrital zircons from the Baoshan Block match well with source regions in India, Western Australia and their adjacent cratons (Fig. 9). Therefore, we propose that in the Early Paleozoic the Baoshan Block was located in the transitional zone between northeast Greater India and northwest Australia to receive detritus from both cratons (Fig. 9). This is also consistent with available paleomagnetic data, which positions the Baoshan Block within the junction of India and Australia before separating from Gondwana (Ali et al., 2013; Y.C. Xu et al., 2014; Liao et al., 2015). The age spectra for the Lancang Group are similar to the Baoshan Block rather than the Simao Block. This indicates that the Lancang Block with the Lancang Group received zircons from similar sources to the Baoshan Block and formed part of the Gondwana margin close to or perhaps even part of the Baoshan Block. 5.3.3. Comparison with other major Gondwana derived blocks and Gondwana reconstruction Although the component continental blocks of present-day East and Southeast Asia were located adjacent to Indian-Australian Gondwana, the precise paleogeography of this region during the Early Paleozoic remains unclear (Cawood et al., 2013; Metcalfe, 2013; Torsvik and Cocks, 2013; Burrett et al., 2014). Some researchers argue that an ‘Asian Hun superterrane’ was located at the northern margin of East Gondwana

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during the Early Paleozoic without the presence of a Proto-Tethys (Xu et al., 2013; Y.C. Xu et al., 2014). Detrital zircon ages from the Early Paleozoic Baoshan samples are compared with detrital age spectra of potential neighboring blocks in Fig. 7, including South China, Tarim, Indochina, South Qiangtang, Lhasa and Himalaya, to enhance our understanding of the paleogeography of present-day East and Southeast Asia during the Early Paleozoic. Pan-African (600–500 Ma) and Grenvillian (1300–900 Ma) detrital zircons in Paleozoic strata are characteristic features of Gondwana provenance, and suggests that Indochina and South China (an amalgamation of Yangtze and Cathaysia in the Early Neoproterozoic) were part of Gondwana during the Early Paleozoic (Duan et al., 2011; Cawood et al., 2013; Xu et al., 2013; Usuki et al., 2013; C. Wang et al., 2016; Q. Chen et al., 2016). The positions of the Tarim and North China blocks have been interpreted near South China along the northern Gondwana margin based on similar paleomagnetic data, faunas and detrital zircons (McKenzie et al., 2011a; Metcalfe, 2013; P. Zhao et al., 2014; Z. Li et al., 2015; Lee et al., 2016). However, compared with the Himalaya, Sibumasu, South Qiangtang and Lhasa blocks, the existence of Pan-African (0.6–0.5 Ga) detrital zircons in the “Asian Hun superterrane” are restricted to a relatively small number (Fig. 7). The “Asian Hun superterrane” probably comprised the Tarim, Indochina. North China, South China and North Qiangtang blocks (Fig. 9). Recently, abundant Ediacaran-Cambrian tuff layers and some meta-volcanic rocks with an age of ca. 528 Ma have been identified in South China (Table 1, Compston et al., 1992, 2008; Jenkins et al., 2002; Zhou et al., 2004; Condon et al., 2005; Zhang et al., 2005, 2008; Chen et al., 2009; Ding et al., 2002; Wang et al., 2012a; Okada et al., 2014; D. Chen et al., 2015; Liu et al., 2015). These tuffs and meta-volcanic rocks were probably the source for the Neoproterozoic-Cambrian detrital zircons in the Early Paleozoic sedimentary sequence in South China. Furthermore, exposures of Grenvillian basement rocks were reported in South China and other parts of the “Asian Hun superterrane” (Xu et al., 2004; Tan et al., 2006; Shu et al., 2008; Song et al., 2012). Therefore, the presence of Pan-African (600–500 Ma) and Grenville (1300–900 Ma) detritus in Early Paleozoic sedimentary rocks is not a solid evidence to suggest ties between the blocks and Gondwana. Moreover, existing studies indicate that the geological records of tectonic-thermal/magmatic events are different between the “Asian Hun superterrane” and the northern Gondwana margin blocks (Fig. 8). The “Asian Hun superterrane” experienced ca. 460–400 Ma magmatic events, which were much younger than those events (550–460 Ma) of the northern Gondwana margin (Fig. 8). Thus, magmatic activities of the “Asian Hun superterrane” were different to those of the northern Gondwana margin blocks during the Early Paleozoic. Moreover, Early Paleozoic ophiolites discovered in the Longmu Co-

Shuanghu and Changning-Mengliang Suture Zone (Zhai et al., 2010; B. Wang et al., 2013; Hu et al., 2014b; Z.M. Peng et al., 2014), represent fragments of oceanic lithosphere that once separated the “Asian Hun superterrane” from the main part of the eastern Gondwana margin. However, Proto-Tethys must have been a narrow ocean, because close faunal affinities and paleomagnetic data suggest close proximity of North China, South China, Tarim, Indochina and the NE Gondwana Himalayanwest Australian region during the Early Paleozoic (Thanh et al., 1996; McKenzie et al., 2011a; Metcalfe, 2013). The Longmu Co-Shuanghu and Changning-Mengliang zones record the remnants of both Proto-Tethys and Paleo-Tethys oceans and provide opportunities to enhance our understanding of the evolution of the Proto-Tethys and its relationship to Paleo-Tethys. 5.3.4. Evolution of the Proto-Tethys and its relationship to Paleo-Tethys In order to present our new tectonic evolution model, it is necessary to first discuss previous studies in the context of our new data. Some workers argue that Early Paleozoic granites in the Himalayas, Lhasa, Qiangtang and Sibumasu were emplaced in an extensional/post-collisional environment in response to the final Gondwana assembly (Miller et al., 2001; Song et al., 2007; Yang et al., 2012; Liu et al., 2016). Others, however, suggested that these Early Paleozoic granites resulted from subduction of the Proto-Tethys plate beneath the Gondwana margin (Cawood et al., 2007; Y. Wang et al., 2013; Ding et al., 2015; Li et al., 2016). Further support for subduction of the Proto-Tethys Ocean beneath Gondwana comes from coeval mafic rocks that are sensitive to geodynamic processes (Zhu et al., 2012). Moreover, recent new discoveries of Early Paleozoic subduction-related arc volcanic rocks, including basaltic andesite, andesites and dacite, were reported in the Lancang Group, which reveals that the subduction of the Proto-Tethys Ocean beneath the Lancang Block occurred in the Ordovician (Nie et al., 2015; Xing et al., 2016). Our new detrital zircon data suggest that the Lancang Group of the Lancang Block had a tectonic affinity with the Baoshan Block and the northern Gondwana margin during the Early Paleozoic. The Baoshan Block is tectonically separated from the Simao Block by the recently discovered Early Paleozoic ophiolitic mélange between them. Based on published data and our new data here presented, we suggest that the Proto-Tethys subducted beneath the northern margin of Gondwana during the Early Paleozoic. Cawood and Buchan (2007) argue that the termination of subduction zones within an assembling supercontinent is likely to cause synchronous subduction initiation along the supercontinent's margin because of global plate compensation. From this geodynamic perspective, Early Paleozoic magmatism and coeval metamorphism along the Gondwana

Table 1 Compilation of Neo-Proterozoic tuff U-Pb geochronological data from South China. Age (Ma)

Location

Formation

Method

Reference

542.6 ± 3.7 542.1 ± 5 524.2 ± 5.1 522.3 ± 3.7 535.2 ± 1.7 536.3 ± 5.5 635.4 ± 1.3 632.5 ± 0.48 550.5 ± 0.7 526.4 ± 5.4 523.9 ± 6.7 526 ± 1.1 522.7 ± 4.9 636.3 ± 4.9 635.2 ± 0.6 621 ± 7 555.2 ± 6.1 654.2 ± 2.7 654 ± 3.8 662.9 ± 4.3

Bahuang Ganziping Panmen Bahuang Meishucun Ganziping Wuhe-Gaojiaxi Jijiawan Jijiawan Chengjiang Gorges Meishucun Taoying Maopingdong Maopingdong Jiuqunao Jiuqunao Hunan Hunan Zhailanggou

Mid-upper Liuchapo Formation Basal Liuchapo Formation Basal Niutitang Formation Basal Niutitang Formation Fifth ash of Yuhucun Formation Upper Liuchapo Formation Doushantuo Formation Basal Doushantuo Formation Uppermost Doushantuo Formation Shuijingtuo Formation Shuijingtuo Formation Shiyantou Formation Niutitang Formation Nantuo Formation Doushantuo Formation Doushantuo Formation Doushantuo Formation Middle Datangpo Formation Top Datangpo Formation Basal Datangpo Formation

SIMS U-Pb zircon SIMS U-Pb zircon SIMS U-Pb zircon SIMS U-Pb zircon SIMS U-Pb zircon SHRIMP U-Pb zircon Single zircons Single zircons Single zircons LA-ICP-MS LA-ICP-MS SHRIMP SHRIMP SHRIMP SHRIMP SIMS zircon U-Pb SIMS zircon U-Pb SIMS U-Pb zircon SIMS U-Pb zircon IDTIMS U-Pb

(D. Chen et al., 2015) (D. Chen et al., 2015) (D. Chen et al., 2015) (D. Chen et al., 2015) (Zhu et al., 2009) (Chen et al., 2009) (Condon et al., 2005) (Condon et al., 2005) (Condon et al., 2005) (Okada et al., 2014) (Okada et al., 2014) (Compston et al., 2008) (Wang et al., 2012a) (Zhang et al., 2008) (Zhang et al., 2008) (Zhang et al., 2005) (Zhang et al., 2005) (Liu et al., 2015) (Liu et al., 2015) (Zhou et al., 2004)


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Fig. 8. The age-data of granitic rocks during the Early Paleozoic orogeny from the blocks along the northern margin of East Gondwana (Miller et al., 2001; Gehrels et al., 2003a, 2003b, 2006; Chen et al., 2007; Cawood et al., 2007; Wang et al., 2007, 2011; Lee and Whitehouse, 2007; Quigley et al., 2008; Usuki et al., 2009; Pullen et al., 2011; Mitchell et al., 2012; Zhu et al., 2012; Wang et al., 2012b; Chen et al., 2012; M. Dong et al., 2013; Y. Wang et al., 2013; Huang et al., 2013; Lin et al., 2013; Lehmann et al., 2013; Guo et al., 2013; S. Zhao et al., 2014; Kawakami et al., 2014; Hu et al., 2014a; Tran et al., 2014; Z. Peng et al., 2014; Ge et al., 2014; Wang et al., 2015; Hu et al., 2015; Ding et al., 2015; Shi et al., 2015; Li et al., 2016; Liu et al., 2016; Zhao et al., 2016; Xu et al., 2016 and its references).

margin is thought to be related to subduction of the Proto-Tethys Ocean (Cawood et al., 2007; Zhu et al., 2012). High-pressure granulites from central Qiangtang record a peak HP metamorphism at ca. 427–422 Ma (Zhang et al., 2014). Contemporaneous ultrahigh-pressure metamorphic events are also recorded in Tarim (ca. 430 Ma, Zong et al., 2012). Silurian (440–430 Ma) garnet amphibolite (retrograded from eclogite) of the Longyou area, South China was thought to have formed during the collisional orogenic event between the South China Block and eastern Gondwana (X. Chen et al., 2015). Recently, C. Zhang et al. (2015) reported ca. 420 Ma post-collision gabbroic intrusions from the Cathaysia Block, which indicate the transition from subduction to post collision with Gondwana. Therefore, the main outboard micro-continental fragments collided with the northern margin of Gondwana in the Silurian. This event led to the rapid exhumation of HP basic granulites (ca. 392–389 Ma) and the opening of the Paleo-Tethys ocean basin (Zhang et al., 2014). Studies of ocean plate stratigraphy, paleomagnetic data, sea mount rock associations, ophiolites, accretionary complex rocks, cover sequences, and stitching plutons indicate that the Paleo-Tethys ocean basin opened in the Early Devonian and closed on the Triassic (Metcalfe, 2013; Stampfli et al., 2013). The opening of the Paleo-Tethys Ocean is recorded by rifting of the “Asian Hun superterrane” from northern Gondwana (C. Chen et al., 2016; Han et al., 2016). The present Lancang Block is separated from the Baoshan Block by the Changning-Mengliang Suture, which represents the Paleo-Tethys that opened in the Devonian. Therefore, we interpret that the Lancang Block was likely separated from the Baoshan Block during the opening of the Paleo-Tethys Ocean in the Devonian. We here offer a new tectonic evolution model for Proto-Tethys and Paleo-Tethys during the Early to Middle Paleozoic (Fig. 9). The Proto-Tethys represented a narrow ocean separating the “Asian Hun superterrane” from the northern margin of Gondwana during the Late Neoproterozoic-Early Paleozoic. During the period 500–460 Ma, the Proto-Tethys Ocean was subducted beneath the northern margin of Gondwana generating voluminous subduction-related igneous rocks. At ca. 440–420 Ma, the “Asian Hun superterrane” collided with northern Gondwana causing regional metamorphism observed in Central Qiantang, South China and Tarim. Then during the Early Devonian, the Paleo-Tethys Ocean opened initially as a back-arc basin.

6. Conclusions 1. Detrital zircon ages of the Lancang and Mengtong Groups in SW Yunnan show that the deposition of the groups began in the Ordovician rather than previously inferred Neoproterozoic. 2. In situ U-Pb ages for detrital zircons from sedimentary rocks in the Baoshan Block and the Lancang Block define three main age populations, including late Mesoproterozoic (~ 1150 Ma), Early Neoproterozoic (~ 960 Ma) and Pan-African age (600–500 Ma) along with a few Early Paleoproterozoic ages. 3. The distribution pattern of the U-Pb ages and Hf isotopic data of zircons suggests that the Lancang Group (Lancang Block) was paleogeographically close to the Baoshan Block on the northern margin of East Gondwana during the Early Paleozoic. Detritus from the Wilkes-Albany-Fraser belt in southwest Australia and from the Rayner-Eastern Ghats belt in India were the main sources for Paleozoic sediments of the Baoshan Block and the Lancang Block. 4. Both the Baoshan and Lancang blocks were in the transition zone between India and Australia along the Gondwana margin in the Early Paleozoic. 5. The Early Paleozoic Proto-Tethys was a narrow ocean between the “Asian Hun superterrane” and NE Gondwana margin. The Gondwana Proto-Tethys margin was an active continental margin during the Early Paleozoic. The Proto-Tethys closed along the northern margin of Gondwana during the Silurian. 6. Following the Silurian closure of the Proto-Tethys, the Paleo-Tethys Ocean opened initially as a back-arc basin in the Early Devonian. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.gr.2017.07.012. Acknowledgements We thank the Editor (Sanghoon Kwon) and three reviewers for their constructive comments that have greatly improved the manuscript. Discussions with Dr. Guozhen Xu and Dr. Hu Zhang helped to improve this paper. This study was financially supported by projects from the Natural Science Foundation of China (41672222, 41190073 and 41172202), the China

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Fig. 9. Schematic tectonic cartoons showing the Early Paleozoic tectonic evolution of the northern margin of Indo-Australian Gondwana. SQ = South Qiangtang, NQ = North Qiangtang, IC = Indochina Block, SC = South China, NC = North China, SWB = South West Borneo, EJ-WS = East Java-West Sulawesi. Partly after Zhu et al. (2011), Metcalfe (2013), Ali et al. (2013) and Han et al. (2016).

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