Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis

Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis

Journal of Asian Earth Sciences xxx (2016) xxx–xxx Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.e...

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Journal of Asian Earth Sciences xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes

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Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis Liang Qiu a,b, Dan-Ping Yan a,⇑, Wen-Xin Yang a, Jibin Wang a, Xiangli Tang a, Shahnawaz Ariser a a b

State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 89154, USA

a r t i c l e

i n f o

Article history: Received 20 March 2016 Received in revised form 28 September 2016 Accepted 28 September 2016 Available online xxxx Keywords: Synorogenic clastic rock Indosinian orogen Geochemistry Youjiang Basin South China Block

a b s t r a c t The Indosinian orogeny marks the termination of marine deposition and the accumulation of lower Permian to Late Triassic clastic sediments in the Youjiang Basin, South China Block. Major and trace element compositions of Early to Middle Triassic sedimentary clastic rocks from Youjiang Basin were analysed to constrain their provenance and tectonic setting. Argillaceous samples have low SiO2 (average 56.95 wt.%), Al2O3 (average 15.15 wt.%), and Fe2OT3 + MgO (average 11.54 wt.%) contents, and high K2O/Na2O (average 15.61) and Al2O3/SiO2 (average 0.27) ratios, similar to mudstones from continental arc basins. Arenaceous samples have moderate SiO2 (average 76.98 wt.%), Al2O3 (average 8.41 wt.%), and Fe2OT3 + MgO (average 5.29 wt.%) contents, and moderate Al2O3/SiO2 (average 0.11) and K2O/Na2O (average 15.26) ratios, identical to those of graywackes from continental island arcs or active continental margins. Both the argillaceous and arenaceous samples have low CIA values (57–85) and relatively high ICV values (0.69–2.11), indicating that the source rocks experienced weak chemical weathering and the sedimentary detritus was derived from an immature source. Compared with late Permian to Early Triassic South China granitoids and upper crust, the samples have lower contents of high-field-strength elements (e.g., Zr, Hf, Nb, and Ta) and large ion lithophile elements (e.g., Rb, Sr, Ba, Th, U, and Pb). However, their relatively high Rb concentrations (>51 ppm), and low Rb/Sr (0.16–4.19) and Th/U (2.66–5.21) ratios, are indicative of an igneous source from a continental arc that underwent weak chemical weathering. Both the argillaceous and arenaceous samples are moderately enriched in light rare earth elements and show relatively flat chondrite-normalized heavy rare earth element patterns (LaN/YbN = 6.61–17.35; average 10.61) with strong negative Eu anomalies (Eu/Eu* = 0.54–0.89; average 0.73). In tectonic discrimination diagrams, including Th–Sc–Zr/10 and La–Th–Sc plots, the geochemical data suggest that the clastic rocks were deposited in a continental arc or margin setting. Thus, we infer that Early to Middle Triassic sediments in the Youjiang foreland basin record the transition from late Permian and Early Triassic subduction to Middle Triassic collision at the southwestern margin of the South China Block. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Sedimentary sequences record significant information on their depositional environment and tectonic setting, as well as continental growth (e.g., Najman, 2006; Lehrmann et al., 2007; Long et al., 2008; Wang et al., 2012; Verma and Armstrong-Altrin, 2016). Chemical and mineral compositions of sedimentary rocks are used to reveal their potential provenance, the tectonic settings of sedimentary basins (Zimmermann and Bahlburg, 2003; Hu et al., 2014, 2015a,b), and the evolution of orogens (Bhatia, 1983; Gu, 1994; Yang et al., 2012; Painter et al., 2014). Due to their homo-

⇑ Corresponding author.

geneity and relatively high trace element contents, clastic rocks (i.e., mudstone and sandstone) are ideal for geochemical reconstructions of provenance and tectonic setting (e.g., Long et al., 2008; Wang et al., 2016a,b). The Triassic tectonic history of the southwestern South China Block (SCB) is marked by the Indosinian orogeny (e.g., Deprat, 1914; Fromaget, 1932, 1941) that records amalgamation of the Indochina and South China blocks during the late Permian to Triassic as a result of closure of the eastern branch of the palaeo-Tethys Ocean (e.g., Lepvrier et al., 2011; Wang et al., 2013a,b; Faure et al., 2014, 2016a, 2016b; Cao et al., 2015; Halpin et al., 2016). This orogeny is characterized by a Late Triassic unconformity, large-scale granitic magmatism, fold-and-thrust belts, and depositional basins (Wang et al., 2007a, 2007b, 2013a,b; Zhang et al., 2011; Yang et al.,

E-mail address: [email protected] (D.-P. Yan). http://dx.doi.org/10.1016/j.jseaes.2016.09.020 1367-9120/Ó 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Qiu, L., et al. Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis. Journal of Asian Earth Sciences (2016), http://dx.doi.org/10.1016/j.jseaes.2016.09.020

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L. Qiu et al. / Journal of Asian Earth Sciences xxx (2016) xxx–xxx

2012; Qiu et al., 2014, 2016). However, the tectonic evolution of the Indosinian orogeny is controversial and the tectonic setting of associated sedimentary basins is poorly constrained (Metcalfe, 2002; Zhou et al., 2008; Wang et al., 2013a,b). During the Triassic, formation of the Youjiang Basin (also termed the Nanpanjiang Basin) has been attributed to various processes, such as formation as a back-arc extensional basin (Hou and Huang, 1984) or as the foreland basin of an arc at the northern Song Ma suture (Xia et al., 1993). In particular, Enos et al. (1998) suggested that the Middle–Late Triassic succession was a classic flysch to molasse sequence of a foreland basin. However, there is no consensus on the provenance of the clastic sediments in the basin, and the sediment source areas and siliciclastic flux are poorly constrained. For example, palaeocurrent data, sandstone petrology and heavy mineral data (Chaikin, 2004) of Middle Triassic flysch in the northern part of the basin (Bianyang Formation) suggest that the provenance is the Jiangnan orogen at the southeastern margin of the Yangtze Block. In contrast, detrital zircon data and the geochemistry of the Middle Triassic turbidites indicate a source from the Indosinian orogeny at the southwestern margin of the Yangtze Block (Yang et al., 2012). Furthermore, the magnetic fabric of the Middle Triassic siliciclastic rocks reveal a NE palaeoflow direction and support a southern origin from the collision of Indochina–

South China (Cai et al., 2014). Moreover, the Early Triassic clastic record is not well studied and it lacks a continuous reconstruction from the Early to Middle Triassic. This contribution uses geochemical data to further investigate the provenance and tectonic setting of these Early to Middle Triassic siliciclastic rocks. The results are compared with previously published geochronological and geochemical data, which allows us to constrain the erosional, depositional, and tectonic conditions of the Youjiang Basin during the Indosinian orogeny. The significance of our results is discussed in the framework of the evolution of palaeo-Tethys tectonism. 2. Geologic setting The SCB formed as a result of the Neoproterozoic amalgamation of the Yangtze and Cathaysia blocks (Fig. 1a; e.g., Zhou et al., 2002; Zhao et al., 2011; Wang et al., 2013a,b; Zhao, 2015), and subsequently was affected by early Palaeozoic and Mesozoic tectonic events (e.g., Lin et al., 2008; Faure et al., 2009; Charvet et al., 2010; Li et al., 2010; Wang et al., 2013a,b; Qiu et al., 2014). The SCB comprises folded and metamorphosed Proterozoic basement (e.g., Wang and Zhou, 2014; Wang et al., 2016a,b) and a folded Phanerozoic sedimentary sequence (Fig. 1; Charvet et al., 1996;

Fig. 1. Geological map of the Youjiang Basin in the South China Block. (a) Inset shows the location of the South China Block. (b) Sketch tectonic map of the Youjiang Basin and adjacent areas (modified from Yan et al., 2003; Qiu et al., 2014). (c) Geological map of the Longlin area in the northern Youjiang Basin. (d) Geological map of the Laizishan dome in the northern Youjiang Basin (BGMRGX, 1985).

Please cite this article in press as: Qiu, L., et al. Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis. Journal of Asian Earth Sciences (2016), http://dx.doi.org/10.1016/j.jseaes.2016.09.020

L. Qiu et al. / Journal of Asian Earth Sciences xxx (2016) xxx–xxx

Yan et al., 2003, 2006; Chu et al., 2012a, 2012b; Wang et al., 2013a, b). The basement consists of Neoproterozoic epimetamorphic sandy to argillaceous detrital flysch intercalated with volcanic rocks (Qiu et al., 2015a), and the cover sequence is dominated by Palaeozoic to early Mesozoic marine strata and Middle Triassic to Cretaceous clastic material (Yan et al., 2003). The Youjiang Basin lies in the southwestern part of the SCB and is bounded by the Ziyun–Luodian Fault to the northeast, the

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Shizong–Mile Fault to the northwest, and extends to the northeastern Vietnam nappes to the southwest (Fig. 1b) (Cai and Zhang, 2009; Lepvrier et al., 2011; Yang et al., 2012; Faure et al., 2014). The basin is separated from the Shiwandashan Basin by the Pingxiang–Nanning Fault to the southeast. The oldest strata in the basin are Cambrian–Ordovician shales and calcareous rocks, and these crop out on the 50-km-scale anticlines of superimposed folds (Fig. 1c). The oldest strata are overlain by Early to Middle Devonian

Fig. 2. Stratigraphic section of Early and Middle Triassic strata in the Youjiang Basin (BGMRGX, 1985).

Please cite this article in press as: Qiu, L., et al. Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis. Journal of Asian Earth Sciences (2016), http://dx.doi.org/10.1016/j.jseaes.2016.09.020

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sandstone, siltstone, and shale that are intercalated with riftaffinity basalt and dolerite (BGMRGX, 1985; Yang et al., 2012). Through the late Proterozoic to Middle Triassic, marine sediments with a thickness of 7 km accumulated within the basin (Figs. 1 and 2; Galfetti et al., 2008; Yang et al., 2012). After this protracted marine history, the basin was overthrust by Palaeozoic sedimentary rocks (Liang and Li, 2005; Lepvrier et al., 2011; Faure et al., 2014). Facies changes in the Rongdu region can be used to divide the Lower Triassic sequences into the Induan Luolou and Olenekian Ziyun formations in the west, and their equivalents in the east

are the Yelang and Anshun formations, respectively. The Luolou and Ziyun formations consist of limestone, sandstone, and claystone (Fig. 2). The Middle Triassic strata are divided into the Anisian Xinyuan and Ladinian Bianyang formations, and their equivalents are the Gejiu, Poduan, and Longtou formations in the Rongdu region. The Xinyuan Formation comprises limestone, claystone, sandstone, and siltstone in its lower part, and siltstone, silty claystone, interbedded sandstone, calcareous sandstone, and limestone in its upper part (Fig. 2). The Bianyang Formation contains thickly bedded sandstone, calcareous sandstone, and siltstone in its lower part; claystone, argillaceous sandstone, and siltstone in

Fig. 3. Field photograph and microphotograph of the clastic rocks. Abbreviations for minerals: qtz, quartz; bi, biotite; mus, muscovite.

Please cite this article in press as: Qiu, L., et al. Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis. Journal of Asian Earth Sciences (2016), http://dx.doi.org/10.1016/j.jseaes.2016.09.020

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its middle part; and calcareous sandstone, siltstone, marl, and dolomite in its upper part (Fig. 2). Both formations have rhythmical bedding and flysch sequences (Fig. 3). Previous biostratigraphic studies and detrital zircon U–Pb dating suggest that the Luolou and Ziyun formations are Early Triassic in age, whereas the Xinyuan and Bianyang formations are Middle Triassic (e.g., Lehrmann et al., 2005; Yang et al., 2012). 3. Sampling and analytical methods Samples were collected from the Luolou, Xinyuan, and Bianyang formations, from base to top, in the Longlin and Huarong areas of the Youjiang Basin (Fig. 1c and d). Twenty-seven fresh rock samples were collected for major and trace element analyses. Geochemical diagrams were made using the Corel Geological Drafting Kit (CGDK; Qiu et al., 2013). Major elements were analysed with a PANalytical Axiosadvance (Axios PW4400) X-ray fluorescence spectrometer (XRF) at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences (IGCAS), Guiyang, China. Loss on ignition (LOI) values were determined using 1 g of powder, which was heated to 1100 °C for 1 h. Major elements were measured on fused glass discs with a precision better than 2%. Trace elements were analysed using a Perkin-Elmer Sciex ELAN 6000 inductively coupled plasma–mass spectrometer (ICP–MS) at IGCAS. The powdered samples (50 mg) were dissolved in a mixture of HF + HNO3 in high-pressure Teflon bombs for 48 h at ca. 190 °C (Qi et al., 2000). Rhodium was used as an internal standard to monitor signal drift during analyses. Repeated analyses of international standard GBPG-1 were used to monitor data quality. Analyses of the international standards OU-6 and GBPG-1 are in agreement with recommended values. The analytical precision was generally better than 5% for all trace elements. 4. Analytical results The analysed samples comprise two types: argillaceous rocks with high Al2O3 and low SiO2 contents, and arenaceous rocks with low Al2O3 and high SiO2 contents (Table 1; Fig. 4). 4.1. Major elements The two rock groups are distinguished by their major elements. The argillaceous rocks have on average 15.15 wt.% (11.86– 18.85 wt.%) Al2O3, and the arenaceous rocks have on average 8.41 wt.% (5.72–10.19 wt.%) Al2O3 (Fig. 4a). The argillaceous rocks include shale, claystone, and mudstone. These samples exhibit a narrow range of SiO2, Al2O3, MgO, P2O5, MnO, and TiO2 contents, whereas they have a wide range of Na2O, K2O, and CaO contents (Table 1; Fig. 4). Their chemical compositions are similar to those of post-Archean Australian average shale (PAAS) (Taylor and McLennan, 1985). The arenaceous rocks include siltstone and sandstone. Compared with average PAAS, these samples are characterized by higher Al2O3 contents and lower SiO2 contents. Major element versus Al2O3 diagrams show no apparent correlation between the contents of most major elements and Al2O3. However, there are negative correlations between K2O, TiO2, and Al2O3, and positive correlations between SiO2, K2O, and Al2O3 on the Harker diagrams (Fig. 4). 4.2. Trace elements All of the samples show similar chondrite-normalized rare earth element (REE) patterns with moderate light REE (LREE) enrichment ((La/Yb)cn = 6.61–17.35; average 10.61), negative Eu anomalies

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(Eu/Eu⁄ = 0.54–0.89; average 0.73), and middle to heavy REE (HREE) fractionation ((Gd/Yb)cn = 1.28–2.84; average 1.87) (Table 1; Fig. 5). In primitive-mantle-normalized trace element diagrams (Fig. 5a), both rock types exhibit Pb enrichment and depletion of Ba and Sr (Fig. 5b). Argillaceous rocks have lower total REE contents and more significant REE fractionation than the arenaceous rocks. The argillaceous rocks have moderately fractionated REE patterns, with LREE/HREE ratios of 6.10–12.28 (average 9.15), (La/Yb)cn = 7.17–17.35 (average 11.26), and Eu/Eu⁄ ratios of 0.61– 0.89 (average 0.76). In contrast, the arenaceous rocks display strongly fractionated REE patterns with lower LREE/HREE ratios (6.19–8.49; average 7.59), more significant REE fractionation (La/Yb)cn = 6.61–10.02 (average 8.53), and lower Eu/Eu⁄ ratios (0.54–0.70; average 0.63) (Table 1; Fig. 5b). The argillaceous rocks show smaller Ti anomalies (3358–25,650 ppm; average 11,574 ppm) than the arenaceous rocks (1930–4911 ppm; average 3170 ppm). Both the argillaceous and arenaceous rocks have similar highfield-strength element (HFSE) contents (e.g., Nb, Ta, Zr, Hf, P, Th, and Ti), but the argillaceous rocks have a wider range of Zr concentrations than the arenaceous rocks (Fig. 6). All the rocks are enriched in large ion lithophile elements (LILE) (e.g., K, Rb, Cs, and Ba) and have similar patterns in chondrite-normalized diagrams. 5. Discussion 5.1. Provenance 5.1.1. Weathering characteristics in the sediment source area The Chemical Index of Alteration (CIA) and the Index of Compositional Variability (ICV) are effective fingerprints of chemical weathering in sedimentary source areas (Fedo et al., 1995; Cullers and Podkovyrov, 2000; Bhat and Ghosh, 2001). The CIA values can be used to evaluate the intensity of weathering of the source rocks (e.g., Fedo et al., 1995; Wang et al., 2012). High CIA values indicate the remobilization of labile cations, whereas low values imply relatively stable conditions. In the present study, the CIA values of the argillaceous rocks range from 57 to 85 with an average of 73, whereas the CIA values of the arenaceous rocks range from 65 to 81 with an average of 74. The average CIA values of the argillaceous rocks are similar to those of the arenaceous rocks, suggesting that the CIA values of both the argillaceous and arenaceous rocks are effective fingerprints but are unrelated to rock type. Moreover, the CIA values of the Early and Middle Triassic clastic rocks mainly range from 70 to 85 with an average of 73–74 (Table 1; Fig. 2), slightly higher than the values for PAAS (Table 1; Taylor and McLennan, 1985). This indicates relatively intense chemical weathering of the source that produced Al-rich minerals, indicating a hot and humid climate in the sediment source area (Bahlburg and Dobrzinski, 2011). Furthermore, the Early Triassic Luolou Formation has lower CIA values than the Middle Triassic Xinyuan and Bianyang formations, confirming a trend of increasing chemical weathering over time (Fig. 2). In general, the chemical weathering shows a tendency to increase from the Early and Middle Triassic to the Late Triassic (Fig. 7). 5.1.2. Influence of post-depositional alteration Sediments are likely to experience post-depositional processes such as interaction with hydrothermal fluid before they are preserved in the stratigraphic record (Wheatcroft and Drake, 2003; Tripathy et al., 2013). The main post-depositional process that can disturb the geochemical compositions of sediments is interaction with hydrothermal fluids (e.g., silicification and K-metasomatism) (Kendall et al., 2009; Rooney et al., 2011).

Please cite this article in press as: Qiu, L., et al. Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis. Journal of Asian Earth Sciences (2016), http://dx.doi.org/10.1016/j.jseaes.2016.09.020

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Rock type

Argillaceous rocks (ZC-)

Sample

58-0

58-1

58-2

58-3

58-5

58-6

58-7

58-8

58-9

147-1

147-5

147-6

147-8

150-2

150-5

111-1

31-1

36-1

119-1

Arenaceous rocks (ZC-) 147-4

147-7

150-4

150-6

150-7

14-1

SiO2 TiO2 Al2O3 Fe2OT3 MnO MgO CaO Na2O K2O P2O5 LOI Total K2O/Na2O SiO2/Al2O3 FeOt + MgO Al2O3/SiO2 K2O/Al2O3 ICV CIA K/Rb Li Be Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta

51.91 4.28 17.90 6.86 0.03 1.98 0.07 1.86 3.80 0.17 10.84 99.70 2.05 2.90 8.84 0.34 0.21 1.24 71.02 250.3 22.9 4.1 31.4 396 91.4 34.4 30.1 84.60 190 42.1 125.9 42.3 39.8 588 81.9 4.79 684.0 70.7 146.4 17.1 63.1 11.4 2.82 9.29 1.39 7.71 1.52 4.47 0.64 4.13 0.57 15.2 5.81

44.27 3.12 15.10 18.69 0.12 2.69 0.47 2.22 1.61 0.22 11.34 99.85 0.73 2.93 21.38 0.34 0.11 1.93 70.72 226.7 37.2 2.26 25.2 308 90.6 73.3 89.7 121.50 207 27.8 59.0 70.4 47 451 59.8 2.94 312.9 85.2 174.0 20.5 81 16.5 4.15 14.10 1.99 10.1 1.8 4.85 0.64 4 0.56 10.6 3.77

44.43 3.04 15.27 15.07 0.05 1.77 0.18 0.10 3.44 0.11 16.38 99.84 33.26 2.91 16.84 0.34 0.23 1.45 78.95 237.6 26.8 4.57 23.2 333 86.4 50.7 55.5 59.85 255 31.2 120.2 38.1 37.4 444 54.6 13 579.0 66.0 125.8 14.4 54.2 9.65 2.62 8.39 1.26 7.24 1.42 4.09 0.56 3.58 0.5 10.8 3.34

46.58 3.00 15.08 15.03 0.09 2.34 0.55 1.42 3.27 0.23 12.40 100.00 2.30 3.09 17.38 0.32 0.22 1.75 68.66 230.9 28.4 3.26 21.4 302 68.3 31.6 52.9 168.30 191 30.4 117.4 70.7 44.3 498 68.4 5.1 602.0 81.5 171.0 19.7 76.4 14.4 3.43 11.61 1.62 8.7 1.69 4.85 0.66 4.14 0.59 12.5 3.89

51.48 2.33 11.86 11.50 0.27 2.05 6.14 2.22 1.62 0.20 9.91 99.59 0.73 4.34 13.55 0.23 0.14 2.11 56.68 227.7 21.1 1.37 18.5 247 146 48.4 68 95.40 166 22.5 59.0 359.7 33.7 347 48 3.63 284.1 50.2 93.0 11 43.5 8.66 2.31 7.74 1.11 6.04 1.17 3.24 0.43 2.6 0.37 7.77 2.6

52.47 2.75 13.38 12.76 0.09 2.29 0.37 2.10 1.96 0.16 11.66 99.97 0.93 3.92 15.05 0.25 0.15 1.78 68.18 223.5 20 1.29 19.4 286 108 47.6 50.3 107.10 188 26.7 72.6 67.2 32.5 425 56.6 3.3 341.4 48.7 95.9 10.8 41.4 7.64 1.90 6.36 0.94 5.45 1.06 3.09 0.41 2.59 0.37 8.62 2.66

53.24 2.17 12.46 15.52 0.14 2.69 0.23 1.24 1.78 0.14 10.17 99.79 1.43 4.27 18.21 0.23 0.14 1.94 73.92 232.8 25.8 1.6 19.1 255 102 64.8 113 116.10 212 22.2 63.4 44.8 43 296 42.5 3.61 298.0 51.7 98.8 11.4 46.1 9.87 2.47 8.78 1.34 7.35 1.35 3.57 0.45 2.64 0.37 6.29 2.28

53.90 3.03 17.07 10.48 0.07 2.60 0.43 2.37 2.65 0.19 7.28 100.07 1.11 3.16 13.08 0.32 0.16 1.46 69.29 237.9 24.8 1.88 22.5 342 91.7 11.6 47.3 64.08 170 34.9 92.3 71.0 47.9 569 71.2 3.28 528.7 89.3 170.6 18.6 71.6 13.3 3.30 10.52 1.5 8 1.52 4.29 0.57 3.47 0.49 10.7 3.1

50.67 2.65 13.88 13.84 0.06 2.08 0.08 0.10 2.01 0.10 14.18 99.65 20.30 3.65 15.92 0.27 0.14 1.45 84.83 227.9 34.4 1.95 18.5 289 152 42.5 74.6 68.04 133 27.2 73.1 45.0 34.4 436 58.9 4.5 368.0 62.3 126.7 13.6 50.7 9.44 2.38 7.13 1.03 5.7 1.09 3.18 0.43 2.65 0.38 8.81 3.02

65.22 0.68 15.04 6.75 0.06 1.78 0.64 0.11 2.29 0.11 7.11 99.79 20.78 4.34 8.53 0.23 0.15 0.84 84.09 147.3 22.4 2.31 15.6 109 83.7 16.7 74.3 46.71 181 19 129.0 66.3 45.9 187 11.7 6.09 430.5 45.9 88.2 10.8 43.5 11.1 2.58 10.78 1.59 8.94 1.66 4.54 0.66 4.32 0.65 4.72 0.71

66.91 0.98 13.39 5.34 0.02 2.12 0.44 0.05 2.96 0.13 7.84 100.18 64.58 5.00 7.46 0.20 0.22 1.00 79.94 183.4 23.8 2.44 16.3 152 122 15.4 54.1 94.50 131 20 133.9 66.9 42.2 189 18.9 8.49 1037.4 47.3 77.9 10.5 39.9 8.03 1.68 7.73 1.16 6.83 1.33 3.77 0.53 3.29 0.49 4.71 1.32

66.28 1.13 16.66 3.33 0.02 1.56 0.12 0.06 3.70 0.07 6.85 99.79 59.83 3.98 4.89 0.25 0.22 0.71 79.80 180.4 21.9 2.75 19.5 250 120 39.9 47.4 37.17 126 23.8 170.2 40.6 41.4 214 21.5 9.59 1266.2 49.5 89.4 10.6 38.7 7.33 1.52 6.60 1.02 6.19 1.31 3.85 0.55 3.44 0.52 5.26 1.58

61.93 0.92 18.85 4.79 0.03 1.89 0.45 0.69 3.67 0.08 6.52 99.82 5.34 3.28 6.69 0.30 0.19 0.80 76.06 157.3 21 3.5 23.2 181 93.7 12.9 35.8 39.06 119 27.2 193.6 150.6 41.2 198 14.9 12.3 492.2 45.6 83.6 10.8 42.6 8.34 1.68 7.30 1.13 7.02 1.41 4.18 0.6 3.91 0.59 5.01 1.16

63.50 0.69 17.06 6.51 0.13 0.80 0.25 0.19 4.49 0.07 6.25 99.94 23.63 3.72 7.31 0.27 0.26 0.75 75.62 140.1 38.7 6.73 18 180 102 18.1 129 66.42 253 27.4 265.9 125.6 44.7 145 15.5 23.1 496.9 47.5 93.9 11.5 44.1 9.32 2.02 9.14 1.34 7.81 1.55 4.34 0.63 4.01 0.6 3.55 1.28

66.44 0.80 16.07 5.15 0.03 0.67 0.20 0.24 4.00 0.03 6.14 99.77 16.67 4.13 5.81 0.24 0.25 0.69 75.92 138.5 130 3.3 18.2 169 107 14.7 36.6 39.42 114 28.4 239.7 163.5 24.9 156 18.4 15.9 466.1 41.2 85.4 9.74 35.6 6.08 1.14 4.57 0.69 4.28 0.86 2.55 0.37 2.42 0.36 3.81 1.3

63.13 0.56 12.73 5.78 0.07 1.73 5.02 1.27 1.80 0.06 7.41 99.55 1.42 4.96 7.51 0.20 0.14 1.18 67.48 134.2 32.7 1.7 14.8 96.4 58 22.2 38.8 28.26 153 17.1 111.2 256.4 22.4 155 11.2 5.53 272.4 31.0 56.2 6.49 24.5 4.85 0.90 4.17 0.62 3.67 0.77 2.32 0.35 2.39 0.36 4.11 0.91

61.32 0.71 17.54 5.76 0.06 0.57 2.25 0.88 3.32 0.09 7.26 99.76 3.77 3.50 6.34 0.29 0.19 0.72 72.96 144.1 27.5 2.43 19.7 156 77.3 16.3 34.2 45.90 113 24 191.2 147.7 31.4 155 11.7 20 401.2 35.6 67.8 8.39 31.7 6.22 1.27 5.51 0.86 5.2 1.08 3.27 0.47 3.04 0.46 4.01 0.82

51.76 3.05 13.93 13.38 0.05 3.07 0.48 1.87 2.11 0.15 9.85 99.71 1.13 3.71 16.45 0.27 0.15 1.91 69.06 343.3 18.9 2.56 25.2 294 141 31.5 83.9 149.40 189 28 51.0 167.4 40.5 420 53.5 0.3 721.5 51.6 109.5 13.7 54.7 10.7 2.95 9.60 1.41 7.77 1.48 4.03 0.53 3.26 0.45 9.69 3.46

66.58 0.79 14.65 6.78 0.02 1.27 0.06 0.08 3.08 0.07 6.16 99.55 36.56 4.54 8.04 0.22 0.21 0.83 80.31 209.5 27.1 2.03 15.8 103 65.7 14.2 44.1 59.94 152 19.3 122.2 40.2 36.6 202 14 12.9 257.5 44.2 81.3 10.9 40 8.36 1.73 6.93 1.05 6.02 1.22 3.51 0.49 3.28 0.49 4.86 1.03

72.53 0.82 10.19 5.31 0.04 2.35 0.32 0.05 2.20 0.07 5.78 99.65 45.81 7.12 7.66 0.14 0.22 1.28 80.03 177.8 24.6 1.84 13 84.2 86.5 18.7 50.1 82.80 116 15.9 102.6 37.4 24 153 15.6 6.25 784.7 27.4 53.6 6.19 22.9 4.39 0.91 3.99 0.62 3.79 0.78 2.27 0.33 2.13 0.31 3.71 1.06

64.58 0.53 10.11 5.84 0.13 1.94 6.54 1.37 0.97 0.07 7.70 99.78 0.70 6.39 7.79 0.16 0.10 1.49 64.52 146.4 26 0.97 12.6 53.7 52.1 52.3 37.2 23.94 143 12.1 54.7 184.8 26.3 181 8.62 2.83 151.6 22.6 38.0 4.85 19.4 4.2 0.94 4.42 0.62 3.77 0.77 2.22 0.32 2.11 0.31 4.52 0.73

84.56 0.32 5.72 4.08 0.05 0.20 0.14 0.07 1.05 0.04 3.36 99.60 14.51 14.78 4.29 0.07 0.18 0.87 80.62 138.1 33.5 1.79 4.19 34.7 34 62.5 29.5 17.64 180 8.87 63.0 73.2 17.4 129 5.83 4.15 140.2 20.0 38.2 4.14 15 3.04 0.52 2.87 0.46 2.75 0.54 1.57 0.22 1.42 0.21 3.04 0.19

84.37 0.49 7.07 2.68 0.04 0.26 0.15 0.10 1.39 0.04 3.30 99.87 14.39 11.93 2.93 0.08 0.20 0.69 79.43 144.7 92 1.52 5.56 43.5 47.4 78.6 32.9 18.27 136 10.7 79.9 93.7 20.7 221 11.1 3.92 209.0 25.3 46.0 4.79 17.1 3.3 0.63 3.15 0.5 3.2 0.66 1.93 0.27 1.7 0.25 5.09 0.93

83.62 0.48 7.26 3.19 0.02 0.28 0.17 0.10 1.47 0.06 3.23 99.87 15.27 11.52 3.47 0.09 0.20 0.73 79.10 154.1 84.3 1.53 4.92 43.3 44.5 72.2 31.8 21.69 160 10.5 79.4 90.3 18.1 238 7.47 4.19 228.5 25.0 44.7 4.66 16.6 3.09 0.59 2.89 0.47 2.96 0.62 1.84 0.27 1.83 0.26 6.11 0.18

72.23 0.54 10.13 4.22 0.06 1.37 3.05 1.62 1.42 0.05 5.09 99.78 0.88 7.13 5.60 0.14 0.14 1.37 59.56 165.1 21 1.16 12.4 56.3 54.5 55.2 33.7 27.45 155 12.9 71.3 119.1 25.4 219 7.59 3.71 210.1 22.9 43.3 5.28 20.5 4.35 0.99 4.27 0.68 4.07 0.84 2.45 0.36 2.34 0.34 5.2 0.22

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Table 1 Major and trace element compositions of clastic rocks from the Youjiang Basin, South China Block (wt.% for major oxides and ppm for trace elements).

Note: dEu = Eu/Eu⁄ = EuCN/(SmCN  GdCN)1/2; dCe = Ce/Ce⁄ = CeCN/(LaCN  PrCN)1/2. N = Chondrite Normalized; the normalization value after Sun and McDonough, (1989) and Boynton (1984). LOI: loss ion ignition.

37.7 0.3 30.8 0.02 8.16 2.82 0.10 2.89 17.66 0.66 2.81 0.95 6.61 1.47 0.70 100 0.39 40.4 0.05 12.4 2.89 0.07 4.28 48.37 2.52 2.01 1.00 9.20 1.28 0.60 480 0.37 43.9 0.08 11.6 2.69 0.07 4.31 39.75 2.09 2.18 1.01 10.02 1.49 0.60 106 0.3 40.3 0.01 7.78 2.15 0.07 3.62 30.79 1.86 2.56 1.01 9.47 1.63 0.54 212 0.24 26.7 0.04 8.1 1.91 0.09 4.24 14.37 0.64 2.79 0.87 7.22 1.69 0.67 33.2 0.39 24.4 0.09 9.42 2.46 0.08 3.83 11.77 0.72 2.91 0.99 8.69 1.51 0.67 25.4 0.49 59.4 0.21 11.3 2.85 0.07 3.96 12.78 0.72 3.91 0.89 9.08 1.70 0.69 6.51 0.13 15.2 0.07 10.4 2.40 0.06 4.34 16.67 0.41 4.96 0.99 10.67 2.38 0.89 7.84 0.82 29.6 0.32 12.4 3.32 0.09 3.74 7.87 0.63 2.87 0.94 7.90 1.46 0.66 75 0.45 25.4 0.18 10.4 2.99 0.08 3.48 10.47 0.70 2.98 0.95 8.73 1.41 0.61 4.74 1.14 32.5 0.37 17.2 3.82 0.06 4.50 8.57 0.95 2.40 1.03 11.48 1.52 0.66 3.8 1.32 36.3 0.08 14.9 3.77 0.08 3.95 8.06 0.83 3.18 0.97 7.98 1.84 0.67 5.86 0.91 22.6 0.14 14.4 3.39 0.09 4.25 8.53 0.62 3.17 0.91 7.86 1.51 0.66 34.2 1.79 48.5 0.47 14.6 5.49 0.07 2.66 10.97 0.75 3.39 0.94 9.69 1.55 0.67 14.6 0.54 34.9 0.75 12.8 3.92 0.07 3.26 11.60 0.79 3.69 0.84 9.69 1.90 0.65 9.59 0.54 81.3 0.22 11.3 2.98 0.09 3.79 11.99 0.72 4.07 0.95 7.17 2.01 0.72 5.95 0.17 25.5 0.08 8.91 2.00 0.04 4.46 23.57 0.48 6.99 1.05 15.84 2.17 0.89 1.18 0.19 44.4 0.01 12.2 2.40 0.04 5.09 25.29 0.54 7.32 1.01 17.35 2.45 0.85 20.5 0.3 25.2 0.06 6.28 2.12 0.05 2.97 15.50 0.33 8.24 0.98 13.21 2.68 0.81 20.6 0.25 28.3 0.03 6.8 1.83 0.05 3.73 18.76 0.37 7.39 0.95 13.03 2.40 0.86 8.26 0.18 34.6 0.06 11.5 3.22 0.05 3.57 17.90 0.46 7.41 1.00 14.36 2.84 0.83

58-1

9.47 0.4 17.5 0.05 10.5 2.67 0.05 3.94 19.14 0.45 6.28 0.98 12.42 1.89 0.89 13.7 0.35 37 0.12 15.6 2.99 0.06 5.21 18.73 0.50 4.53 1.01 11.54 1.82 0.84 W Tl Pb Bi Th U Yb/La Th/U Zr/Sc Th/Sc La/Th Ce/Ce⁄ LaN/YbN GdN/YbN Eu/Eu⁄

10.1 0.4 28.3 0.05 14.3 3.02 0.05 4.73 23.27 0.67 5.70 1.03 13.27 2.26 0.81

16.3 0.22 23.7 0.02 7.35 1.80 0.05 4.08 21.91 0.38 6.63 1.01 12.69 1.98 0.83

150-4 147-7 147-4 58-5 58-3 58-0 Sample

58-2

Argillaceous rocks (ZC-) Rock type

Table 1 (continued)

58-6

58-7

58-8

58-9

147-1

147-5

147-6

147-8

150-2

150-5

111-1

31-1

36-1

119-1

Arenaceous rocks (ZC-)

150-6

150-7

14-1

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Silicification and K-metasomatism are interpreted to result from hydrothermal metasomatism and/or syn-depositional interaction with seawater at low temperatures (Van Kranendonk, 2006). During such processes, some major elements (e.g., Ca, Mg, and Na) are removed, and Si and K are added. Trace elements also have different mobilities under such processes (Winchester and Floyd, 1977; Polat and Hofmann, 2003; Polat and Günesß, 2009). In the present study, linear trends between major elements and Al2O3 are not obvious (Cann, 1970), and the ratios of K2O/Na2O, Al2O3/SiO2, and (Fe2OT3 + MgO)/Al2O3 increase significantly over time, indicating post-depositional silicification, K-metasomatism, and/or pyritization (Fig. 4e and f). Given their intermediate ionic potential, the REEs (except for Eu), some transition metals (e.g., Cr, Ni, and Co) and HFSE (e.g., Sc, Y, Th, Zr, Hf, Ti, Nb, and Ta) are generally regarded as immobile elements (Taylor and McLennan, 1985; Hastie et al., 2007; Polat et al., 2007). In particular, Zr and Ti are the least mobile elements (Polat and Hofmann, 2003). However, these immobile elements may be affected by a change in fluid composition and pressure– temperature conditions; e.g., the effect of H2O- to SiO2-rich partial melts or CO2-rich fluids, and/or in cases of extremely high fluid throughput (e.g., McCulloch and Gamble, 1991; Hastie et al., 2007; Qiu et al., 2015b). In Zr–Ti, Zr–Th, and Yb/La–Zr diagrams (Fig. 6a, b and d), the present samples are separated into argillaceous and arenaceous types. The argillaceous samples show high Zr contents, possibly due to zircon addition (Fig. 6), whereas data for the arenaceous samples plot in a central field, indicating that they retain their original HFSE systematics and share a similar provenance with the argillaceous samples (Long et al., 2008). In a Rb–Zr diagram (Fig. 6c), data for the samples are scattered and plot over a wide range. The argillaceous samples have higher Rb and Zr contents than the arenaceous samples, probably due to their different mineralogical compositions. Compared with the argillaceous samples, most arenaceous samples plot in narrow fields, indicating that their element contents were not significantly modified by secondary processes and potentially also share a provenance similar to the argillaceous samples. Thus, apart from some major elements (e.g., Si, K, and Fe) that were potentially modified by postdepositional alteration, most elements (e.g., REEs and HFSEs) are reliable indicators of provenance, particularly in the case of the arenaceous samples. Notably, samples with Ce/Ce⁄ (dCe) ratios between 0.9 and 1.1 are considered to be insignificantly altered (Polat and Hofmann, 2003). The Ce/Ce⁄ ratios of the argillaceous rocks are 0.84–1.05 (average 0.97) and those of the arenaceous rocks are 0.87–1.01 (average 0.97) (Table 1). These Ce/Ce⁄ ratios further imply that the major and trace elements are not significantly affected by alteration. Additionally, the samples show approximately parallel patterns in multi-element diagrams, indicating that the rocks still preserve their original REE and HFSE fingerprints (Fig. 5). Consequently, immobile elements, such as REEs and HFSEs, were used to investigate the provenance and tectonic setting of these clastic rocks, with a focus on the arenaceous samples. 5.1.3. Mineral sorting during sedimentary processes During sediment transport, sorting separates clasts into similar sizes and results in heavy mineral enrichment (e.g., zircon and monazite). The degree of sorting indicates the energy, rate, depositional duration, and transportation processes (e.g., river, debris flow, wind, or glacier) that are responsible for deposition of the sediments. Heavy minerals in clastic rocks have distinctive trace element features. For example, zircon has high Zr concentrations and monazite has high GdCN/YbCN ratios (Cullers, 1988; McLennan et al., 1990). Thus, the process of sedimentary sorting can be inferred from geochemical data. In some cases, slight enrichment of monazite and zircon in sediments causes consider-

Please cite this article in press as: Qiu, L., et al. Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis. Journal of Asian Earth Sciences (2016), http://dx.doi.org/10.1016/j.jseaes.2016.09.020

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L. Qiu et al. / Journal of Asian Earth Sciences xxx (2016) xxx–xxx

Fig. 4. Major element versus Al2O3 diagrams for clastic rocks of the Youjiang Basin. Argillaceous and arenaceous samples are shown as green and purple filled polygons, respectively. Data sources: Yang et al. (2012), Hu et al. (2014, 2015a,b); this study. Argillaceous and arenaceous samples are shown as green and purple filled polygons, respectively.

able increases in GdN/YbN ratios, which typically range from 1.0– 2.0 in sedimentary and upper crustal igneous rocks (McLennan et al., 1993). In this study, the argillaceous samples have GdN/YbN ratios of 1.41–2.84 (average 1.99), indicating a possible density sorting and enrichment of monazite. The arenaceous samples have lower GdN/YbN ratios (1.28–1.69; average 1.51) that are in the range of crustal rocks, suggesting insignificant monazite sorting and accumulation. Furthermore, in a TiO2 and Zr diagram the samples plot in two fields (Fig. 6a), with most of the argillaceous samples having higher Zr contents, indicating zircon accumulation as

also suggested by the Yb/La and Zr diagram (Fig. 6b). Th and Sc are enriched in silicic and mafic rocks, respectively, and Th/Sc ratios are unaffected by sedimentary recycling (McLennan et al., 1993; Cullers, 1994; Long et al., 2008). In contrast, the Zr/Sc ratio increases significantly during sedimentary recycling with zircon enrichment. The present samples have variable Th/Sc ratios (Table 2), with the argillaceous samples having Th/Sc of 0.33– 0.95 (average 0.59) and the arenaceous samples having values of 0.64–2.52 (average 1.41). Similarly, Zr/Sc ratios in the argillaceous samples are 7.87–25.29 (average 15.35) and in the arenaceous

Please cite this article in press as: Qiu, L., et al. Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis. Journal of Asian Earth Sciences (2016), http://dx.doi.org/10.1016/j.jseaes.2016.09.020

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Fig. 5. Chondrite-normalized REE patterns and spider diagrams of the clastic rocks. Chondrite-normalizing data are from Boynton (1984), and primitive-mantle-normalizing data are from Sun and McDonough (1989).

Fig. 6. TiO2, Th, Rb, and Yb/La versus Zr diagrams for clastic rocks of the Youjiang Basin. Argillaceous and arenaceous samples are shown as green and purple filled polygons, respectively. Data sources: Yang et al. (2012), Hu et al. (2014, 2015a,b); this study.

samples are 11.77–48.37 (average 27.12). In a Th/Sc Zr/Sc diagram, data for arenaceous samples show a positive correlation, whereas the argillaceous samples exhibit no clear correlation (Fig. 8a). This indicates that the provenance of the arenaceous and argillaceous samples was controlled mainly by source composition and sedimentary recycling with zircon enrichment, respectively.

5.1.4. Source composition Geochemical indicators and data such as the ICV value, K/Rb ratio, and REE contents can be used for the study of source compositions. Firstly, clay minerals have low ICV values because they have lower contents of K2O, Na2O, and CaO, and higher Al2O3 than non-clay minerals (Cox et al., 1995). The different ICV values of the Early Triassic Luolou Formation and Middle Triassic Bianyang

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L. Qiu et al. / Journal of Asian Earth Sciences xxx (2016) xxx–xxx

Fig. 7. (a) CIV versus ICV diagram for clastic rocks of the Youjiang Basin (after Nesbitt and Young, 1984; Cox et al., 1995; Long et al., 2008). The grey field represents the range of CIA values of Phanerozoic shale. (b) ACNK diagram for clastic rocks of the Youjiang Basin (after Fedo et al., 1995; Long et al., 2008). CIA values of 50 and 100 correspond to fresh primary igneous rocks and the most weathered rocks, respectively (Fedo et al., 1995). Data sources: tonalite (To), granodiorite (Gd), granite (Gr), and average Archean upper crust, Condie (1993); average late Permian and Early Triassic granitoids, Li et al. (2006). Arrows indicate the predicted weathering trends of To, Gd, and Gr. Argillaceous and arenaceous samples are shown as green and purple filled polygons, respectively. Data sources: Yang et al. (2012), Hu et al. (2014, 2015a,b); this study.

Table 2 Summary of the chemical compositions of clastic rocks from the Youjiang Basin and various tectonic settings. Tectonic setting

Argillaceous rocks

Arenaceous rocks

Early Triassic

Middle Triassic

OIA

Contents/Samples

Range

Average

Range

Average

Range

Average

Range

Average

Graywackesa

CA

SiO2 TiO2 Al2O3 CaO Na2O K2O Fe2OT3 + MgO Al2O3/SiO2 K2O/Na2O Al2O3/(CaO + Na2O) K2O/Al2O3 CIA Cr Ni Ba Pb Th U Zr Hf Nb La Ce Nd Sr Sc LaN/YbN Eu/Eu⁄ Zr/Sc Th/Sc Ba/Sr Th/U Zr/Hf Zr/Th

44–67 0.6–4.3 12–19 0.1–6.1 0.1–2.4 1.6–4.5 5–21 0.2–0.3 0.1–0.3 1.4–102.6 0.1–0.3 57–85 58–152 30–129 258–1266 15–81 6–17 2–5 145–588 3.6–15.2 11.2–81.9 31–89 56–174 25–81 38–359 14.8–31.4 7.2–17.4 0.6–0.9 7.9–25.3 0.3–1.0 0.8–31.2 2.7–5.2 38–53 9–58

57 1.9 15 1.0 1.0 2.8 12 0.3 0.2 27.5 0.2 74 100 61 518 34 12 3 320 7.4 38.6 55 107 49 107 20.3 11.3 0.8 15.4 0.6 7.9 4.0 43 30

65–85 0.3–0.8 6–10 0.1–6.5 0.1–1.6 1.0–2.2 3–8 0.1–0.2 0.1–0.2 1.3–29.0 0.1–0.2 60–81 34–87 30–50 140–785 24–44 8–12 2–3 129–238 3.0–6.1 5.8–15.6 20–27 38–54 15–23 37–185 4.2–13.0 6.6–10.0 0.5–0.7 11.8–48.4 0.6–2.5 0.8–21.0 2.9–4.3 39–44 16–27

77 0.5 8 1.7 0.6 1.4 5 0.1 0.2 19.2 0.2 74 53 36 287 34 10 3 190 4.6 9.4 24 44 19 100 8.8 8.5 0.6 27.1 1.4 5.0 3.9 41 20

44–54 2.2–4.3 12–18 0.1–6.1 0.1–2.4 1.6–3.8 9–21 0.2–0.3 0.7–33.3 1.4–78.4 0.1–0.2 57–85 68–152 30–113 284–684 18–44 6–16 2–3 296–588 6.3–15.2 42.5–81.9 49–89 93–174 41–81 38–360 18.5–31.4 11.2–17.4 0.8–0.9 16–25 0.3–0.7 0.8–16.2 3.0–5.2 38.7–53.2 34.8–57.8

50 2.9 15 1.0 1.5 2.5 16 0.3 7.0 19.6 0.2 71 104 65 444 29 10 3 450 10.1 60.2 67 134 59 90 22.1 13.8 0.9 21 0.5 8.1 4.2 45.1 45.1

52–85 0.3–3.0 5.7–18.9 0.1–6.5 0.1–1.9 1.0–4.5 2.9–16.5 0.1–0.3 0.7–64.6 1.3–102.6 0.1–0.3 60–84 34–141 30–129 140–1266 15–81 8–17 2–6 129–420 3.0–9.7 5.8–53.5 20.0–51.6 38.0–109.5 15–55 37–256 4.2–25.2 6.6–11.5 0.5–0.9 7.9–48.4 0.4–2.5 0.8–31.2 2.7–4.5 38–43 9.1–40.0

68 0.8 12.9 1.3 0.6 2.5 6.9 0.2 20.3 28.9 0.2 75 81 50 473 37 12 3 198 4.8 15.5 36.4 68.6 32 114 15.0 8.8 0.7 16.9 1.0 6.8 3.8 41 17.8

59 1.1 17 5.8 4.1 1.1 18 0.3 0.3 1.7 0.1 54 37 31 370 7 2 1 96 2.1 2.0 9 23 11 637 19.5 2.8 1.0 5 0.15 1.0 2.1 46 48

71 0.6 14 2.7 3.1 1.9 13 0.2 0.6 2.4 0.1 54 51 13 444 15 11 3 229 6.3 8.5 24 51 21 250 14.8 7.5 0.8 15 0.85 3.6 4.6 36 22

ACM

PM

CAB

BAB

FAB

Mudstonesb 74 0.5 13 2.5 2.8 2.9 11 0.2 1.0 2.5 0.2 51 26 10 522 24 19 4 179 6.8 10.7 33 73 25 141 8.0 8.3 0.6 22 2.59 3.8 4.8 26 10

82 0.5 8 1.9 1.1 1.7 11 0.1 1.6 2.8 0.2 59 39 8 253 16 17 3 298 10.1 7.9 34 72 29 66 6.0 10.8 0.6 50 3.06 4.7 5.6 30 19

55 0.8 15 3.0 1.8 2.1 9 0.3 1.2 3.3 0.1 64 65 32 490 14 8 3 146 4.2 11.5 23 48 25 216 16 6.3 0.7 9 0.6 2.3 2.9 35 18

54 0.7 14 3.4 2.3 2.1 9 0.3 0.9 2.5 0.2 63 81 49 526 14 7 3 116 3.4 9.8 21 45 23 210 16 5.5 0.7 7 0.5 2.5 2.4 34 16

51 0.8 12 4.5 1.6 1.5 16 0.2 1.0 2.0 0.1 50 277 330 385 3 1 1 63 1.9 4.8 9 20 11 157 23 3.4 0.9 3 0.1 1.8 1.4 34 48

Abbreviations: OIA, oceanic island arc; CA, continental arc; ACM, active continental margin; PM, passive margins; CAB, continental arc basins; BAB, back arc basins; FAB, fore arc basins. Data table is modified after Long et al. (2008). Note: dEu = Eu/Eu⁄ = EuCN/(SmCN  GdCN)1/2; dCe = Ce/Ce⁄ = CeCN/(LaCN  PrCN)1/2. N = Chondrite Normalized; the normalization value after Sun and McDonough (1989) and Boynton (1984). a Data for graywackes of various tectonic settings are from Bhatia and Crook (1986). b Data for mudstones of various tectonic settings are from McLennan et al. (1990).

Formation indicate that they were derived from immature and mature sediments, respectively (Fig. 7a), whereas there is no obvi-

ous difference between the arenaceous and argillaceous rocks. Moreover, high ICV values may identify first-cycle deposits in

Please cite this article in press as: Qiu, L., et al. Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis. Journal of Asian Earth Sciences (2016), http://dx.doi.org/10.1016/j.jseaes.2016.09.020

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Fig. 8. Geochemical diagrams illustrating weathering and source compositions for the clastic rocks of the Youjiang Basin. Template source: Th/U–Th and Zr/Sc–Th/Sc diagrams, McLennan et al. (1993); K2O–Rb diagram, Floyd (1989); La/Th–Hf diagram, Floyd and Leveridge (1987). Argillaceous and arenaceous samples are shown as green and purple filled polygons, respectively. Data sources: Yang et al. (2012), Hu et al. (2014, 2015a,b); this study.

Fig. 9. Tectonic discrimination diagrams for clastic rocks of the Youjiang Basin (after Bhatia and Crook, 1986; Roser and Korsch, 1986). Argillaceous and arenaceous samples are shown as green and purple filled polygons, respectively. Data sources: Yang et al. (2012), Hu et al. (2014, 2015a,b); this study.

active tectonic settings (Van de Kamp and Leake, 1985; Long et al., 2008). The ICV values of the argillaceous rocks in this study range from 0.69 to 2.11 with an average of 1.32, and the values of the arenaceous rocks range from 0.69 to 1.49 with an average of 1.07. The ICV values of the argillaceous and arenaceous rocks are higher than the value of PAAS, indicating that most of the clastic rocks

originated from an immature source in an active tectonic setting. Secondly, given that Rb and K are sensitive to sedimentary recycling, the K/Rb ratios of clastic rocks are ideal indicators of source composition (Floyd, 1989). The rocks in this study have K/Rb ratios of ca. 186, which is close to the value for average crust (Shaw et al., 1968). The high Rb contents (51–266 ppm, average 100 ppm; argillaceous average 125 ppm and arenaceous average 70 ppm) suggest that the sediments originated from a silicic–intermediate igneous source. In a La/Th–Hf diagram, the samples plot in the field of silicic arc rocks, apart from the Early Triassic argillaceous samples (Fig. 8c and d). Furthermore, K2O/Al2O3 ratios of the clastic rocks (Table 2; average 0.17; argillaceous average 0.18 and arenaceous average 0.16) suggest that minor K-feldspar was present in the source. Thirdly, given that REEs exhibit similar geochemical behaviour to each other and retain their original characteristics during digenesis, alteration, and metamorphism, they can provide information on source compositions and provenance. The samples in this study have negative Eu anomalies, indicating a possible source from granitoid rocks. The arenaceous samples have relatively high HREE concentrations and REE fractionations, whereas the argillaceous samples have moderate HREE concentrations and REE fractionations, which are features consistent with clastic rocks from continental arc or foreland basins (Table 2; Bhatia, 1983, 1985; Bhatia and Crook, 1986; McLennan et al., 1990). The samples plot along the tonalite–granodiorite–granite trend in an ACNK diagram (Fig. 7b), further supporting a source from granitoid rocks. Additionally, late Permian to Early Triassic granitoids from Hainan

Please cite this article in press as: Qiu, L., et al. Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis. Journal of Asian Earth Sciences (2016), http://dx.doi.org/10.1016/j.jseaes.2016.09.020

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Island and the Yunkai–Shiwandashan Massif plot around the tona lite–granodiorite–granite field, and may represent a source for the clastic rocks. Alternatively, given that these sources are now obviously eroded, the rocks exposed on Hainan Island and in the Yunkai–Shiwandashan area might just be lithologically similar to the source rocks. Thus, based on the above geochemical features we suggest that the provenance of the Early Triassic Youjiang Basin clastic rocks was dominated by materials from a continental arc. 5.2. Tectonic implications: from subduction to collision Clastic rocks in sedimentary basins occur mainly in tectonic settings such as oceanic island arcs, continental arcs, active continental margins, and passive margins. Previous studies have also suggested that clastic rocks in a basin may be deposited due to tectonic processes (e.g., Dickinson and Suczek, 1979; Bhatia and Crook, 1986; Condie, 1986). For example, SiO2 contents and K2O/ Na2O ratios increase in sandstone from oceanic island arc to passive margin settings, but Al2O3, Fe2OT3 + MgO, and TiO2 contents and Al2O3/SiO2 ratios decrease (Bhatia, 1983). The low contents of Fe2OT3 + MgO in the arenaceous rocks of the present study do not suggest an oceanic island arc origin. The major element composition of argillaceous rocks probably varies due to both different tectonic processes and even in the same tectonic regime. However, in continental arc, back-arc, and forearc basins, argillaceous rocks generally show increasing Fe2OT3 + MgO contents and decreasing SiO2 and Al2O3 contents, and decreasing K2O, Al2O3, Al2O3/SiO2, K2O/Na2O, and Al2O3/(CaO + Na2O) ratios in these tectonic settings. The argillaceous rocks in this study have low SiO2 and K2O/Na2O, and high Fe2OT3 + MgO (Table 2), indicating a continental arc basin

setting. In comparison, the sandstones have a narrow range of compositions. The best way to constrain the tectonic setting of the basin sediments is to use sandstone geochemistry. In Th–Sc– Zr/10 and La–Th–Sc diagrams (Fig. 9; Bhatia and Crook, 1986), data for most of the samples plot in or around the continental arc field. Furthermore, the HFSEs, as represented by the Hf versus La/Th diagram, indicate that the sediments originated mainly from a silicic arc source (Fig. 8d). Therefore, based on the trace element characteristics, we suggest that the clastic rocks were deposited in a foreland basin adjacent to a continental arc. Since the Indosinian orogeny was proposed over 100 years ago, late Permian to Triassic deformation, magmatism, and sedimentation have been widely documented in Vietnam and southwestern China (e.g., Wang et al., 2007a, 2007b, 2013a,b; Lepvrier et al., 2011; Faure et al., 2014, 2016a,b). However, the tectonic setting and evolution of the Palaeozoic to early Mesozoic basins, including the Youjiang, Shiwandashan, and Qinfang basins, remain unclear. Models and hypotheses proposed for the formation and evolution of these basins can be divided into two groups: (1) Pacific subduction (e.g., Li and Li, 2007), and (2) Indochina South China collision (e.g., Liang and Li, 2005; Yang et al., 2012; Hu et al., 2015a,b). Li and Li (2007) interpreted the Youjiang Basin as a retro-arc foreland basin within a 1300-km-wide intracontinental orogen that formed by flat subduction of the Pacific Plate in the Mesozoic. Yang et al. (2012) and Hu et al. (2015a,b) proposed that late Palaeozoic to Triassic detrital zircons and volcanic lithic fragments in the Youjiang Basin indicate detrital input from a subduction–collision Indosinian orogenic belt, and that it evolved into a foreland basin. Lehrmann et al. (2005) suggested that Permian carbonate strata record a long history of platform evolution and that the Triassic

Fig. 10. Schematic showing the provenance of clastic rocks, formation of the Youjiang–Shiwandashan Basin, and evolution of the Indosinian orogen (after Liang and Li, 2005; Weislogel, 2008; Faure et al., 2014; Hu et al., 2014).

Please cite this article in press as: Qiu, L., et al. Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis. Journal of Asian Earth Sciences (2016), http://dx.doi.org/10.1016/j.jseaes.2016.09.020

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clastic rocks reflect the impact of siliciclastic flux and accelerating tectonic subsidence related to tectonic convergence and foreland basin development. In addition, Hu et al. (2014, 2015a,b) proposed that: (1) the detrital and geochemical records from the late Permian to Triassic succession in the Qinfang Basin suggest derivation from recycled orogenic sources at a plate margin; and (2) plutonic and metamorphic rocks in the Yunkai Massif, Hainan Island, and northeast Vietnam were the potential source for upper Permian and Triassic siliciclastic successions in the Shiwandashan Basin, which may have been a foreland basin that developed during Indochina South China collision. Therefore, identifying the tectonic setting of these basins can provide key constraints on our understanding of the evolution of the Indosinian orogeny. In this study, the geochemical compositions of the Youjiang Early to Middle Triassic clastic rocks have been found to be similar to those of rocks from continental arc or continental margin settings. Geochemical data for Permian–Triassic clastic rocks from the Youjiang, Shiwandashan, and Qinfang basins also identify similar weathering processes, source rocks (silicic–intermediate arc compositions), and settings (continental arcs). Moreover, the discovery of late Permian arcaffinity granitoids on Hainan Island and granitic batholiths in the Yunkai Massif or Day Nui Con Voi of Vietnam suggests that these may be the source rocks (Fig. 9; Li et al., 2006; Chen et al., 2011). Based on the geochemical characteristics described above and previous studies on regional lithofacies and palaeography (e.g., Lehrmann et al., 2005), we propose a significant tectonic transition from Late Permian and Early Triassic subduction to Middle Triassic collision at the southwestern margin of the SCB (Fig. 10). Firstly, the Early Triassic argillaceous rocks and Middle Triassic clastic rocks show different source provenances, weathering conditions, and tectonic settings (Table 2). Secondly, during the Early Triassic (Anisian) most of the carbonate platforms were terminated during a deepening event and the platforms were subsequently overlain by siliciclastic turbidites due to Middle Triassic uplift and erosion of the foreland (Lehrmann et al., 2005). Thirdly, in the southwestern SCB, this reconstruction is supported by the mid-Triassic unconformity between the Ladinian and Carnian in the Qinfang Basin, and in the Norian in the Yunkai Massif (Hu et al., 2014). More importantly, in an emergent region of eastern Youjiang Basin those Late Permian–Early Triassic marine sediments are uncomfortably overlied by Late Triassic–Jurassic red clastic rocks (BGMRGX, 1985; Yang et al., 2012). All these features are consistent with the tectonic setting of the Youjiang Basin transitioning from an Early Triassic passive continental margin to a Middle Triassic synorogenic foreland of the Indosinian orogeny due to closure of the palaeo-Tethyan Ocean and Indochina–South China collision (Figs. 1b and 10; BGMRYN, 1990; Lehrmann et al., 2007; Yang et al., 2012). Moreover, the Triassic basins in the southwest SCB, including the Youjiang, Shiwandashan, and Qinfang basins, may constitute a consolidated basin that was subdivided by differential uplift during the late stages of the Indosinian orogeny. These results appear to support an Indosinian orogeny that was triggered by the closure of the palaeo-Tethys Ocean (Cai et al., 2014), rather than by intracontinental collision and orogeny driven by subduction of the palaeo-Pacific Plate. 6. Conclusions 1. The geochemistry of siliciclastic rocks from Early to Middle Triassic successions in the Youjiang Basin suggests that they experienced insignificant post-depositional alteration. CIA and ICV values indicate that the Early Triassic argillaceous rocks were derived from an immature source and experienced weak chemical weathering, whereas the Middle Triassic clastic rocks were derived mainly from a mature source with relatively strong chemical weathering.

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2. HFSE and REE geochemistry indicates that these rocks were derived from tonalite–granodiorite–granite source rocks from the southern margin of the SCB. 3. Our results support a tectonic model whereby the regional Late Triassic unconformity marks the transition from palaeoTethyan subduction to Indochina–South China collision. Subsequently, during the Middle Triassic, the Youjiang Basin evolved into a foreland basin of the Indosinian orogen.

Acknowledgements This study was supported by the National Basic Research Program of China (973 Program) Grant No. 2014CB440903, and the National Natural Science Foundation of China (NSFC) (Grant Nos. 41372212 and 41672216). We thank Liang Qi for assistance with the major and trace element analyses. This study benefited from helpful discussions with Michael Wells, Ganqing Jiang, Junting Qiu and Shuzhi Wang. We are also thankful to Lixian Tian for improving the manuscript. Editor Michel Faure and two anonymous reviewers are greatly acknowledged for their constructive comments on the manuscript.

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Please cite this article in press as: Qiu, L., et al. Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis. Journal of Asian Earth Sciences (2016), http://dx.doi.org/10.1016/j.jseaes.2016.09.020