Mid-Neoproterozoic mafic rocks in the western Jiangnan orogen, South China: Intracontinental rifting or subduction?

Journal of Asian Earth Sciences 185 (2019) 104039

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Mid-Neoproterozoic mafic rocks in the western Jiangnan orogen, South China: Intracontinental rifting or subduction?

T

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Le Wana,b, , Zuoxun Zenga,b, Paul D. Asimowc, Zhihui Zenga,b, Lianhong Pengd, Daliang Xud, Yunxu Weid, Wei Liua,b,e, Chengdong Lua,b,f, Wenqi Changa a

School of Earth Science, China University of Geosciences, Wuhan 430074, China Huazhong Tectonomechanical Research Center, Wuhan 430074, China c Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA d Wuhan Center of Geological Survey, China Geological Survey, Wuhan 430205, China e Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China f Dazhou Bureau of Natural Resources and Planning, Dazhou 635000, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: South China Block Mid-Neoproterozoic Mafic rocks Western Jiangnan orogen Subduction-related magmatism Intracontinental rifting

Widely distributed Meso- to Neoproterozoic igneous rocks record the evolution of the Jiangnan orogen. We present integrated geochemical and geochronological data for basalts from the Gaoqiao area, northwestern Hunan Province. U-Pb dating of zircons indicates that the formation of Gaoqiao basalts occurred in the Neoproterozoic, no earlier than 757 ± 16 Ma. Like the contemporaneous mafic rocks in neighboring areas, the Gaoqiao basalts show trace element characteristics suggesting derivation from a source similar to ocean-island basalts (OIBs), including strong enrichment in large-ion lithophile elements (LILEs), high-field strength elements (HFSEs) and TiO2, and high Ti/Y ratios. Placed in a regional tectonic context, however, the Gaoqiao suite is most consistent with an intracontinental rift model and offers no support to the idea of ongoing subduction in middlelate Mid-Neoproterozoic time in the western Jiangnan orogen. Despite the coincidence between the age of this rift and the separation of East Antarctica-Australia from Laurentia, we argue that the rift recorded in the South China Block (SCB) was an independent and probably unrelated event during the break-up of the Rodinia supercontinent.

1. Introduction The Jiangnan orogen is traditionally considered to be a collisional belt marking the Neoproterozoic assembly of the South China Block (SCB) upon closure of an ocean between the Yangtze and Cathaysia Blocks (Guo et al., 1989; Shu and Charvet, 1996; Wang et al., 2006, 2007a,b; Zheng et al., 2008; Zhao, 2015; Yao et al., 2017). The significance of the Neoproterozoic igneous and sedimentary rocks of the western Jiangnan orogen (Zhou et al., 2009; W. Wang et al., 2012; Yao et al., 2014; Kou et al., 2018), however, depends on the position of the SCB within the Rodinia supercontinent. In reconstructions that place the SCB along the main suture between western Laurentia and Australia-East Antarctica (Li et al., 1995; Kou et al., 2018), the numerous Stype granites (825–820 Ma; Wang et al., 2006) and mafic-ultramafic rocks (835–820 Ma; Li et al., 1999; Wang et al., 2007a; Zhang et al., 2009) in the western Jiangnan orogen may correlate with coeval magmatism in Australia, Laurentia, India and South Africa, and may

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record impact of a mantle plume that triggered the break-up of Rodinia (Cui et al., 2015). On the other hand, reconstructions that place the SCB along the western margin of Rodinia as part of a giant continental margin subduction system (Zhao et al., 2017) favor a connection between the Jiangnan orogen and ongoing subduction that continued to 830 Ma, or even 750 Ma (Li et al., 1995; Ye et al., 2007; X.C. Wang et al., 2012). If there was only one rift developing at this time, in the middle of Rodinia, then the presence of OIB-affinity basalts in western Jiangnan, such as those in Hunan Province at Guzhang (Zhou et al., 2007) and Qianyang (Wang et al., 2008a) appear inconsistent with the western-margin reconstruction. However, a second, independent rift affecting the SCB could reconcile these results. Determining whether the widespread 850–740 Ma igneous rocks in the Jiangnan orogen are related to subduction, to plume-induced rifting, or to post-orogenic extension (Zheng et al., 2008; X.L. Wang et al., 2012) may therefore help resolve the question of the position of the SCB within Rodinia. In order to better understand the evolutionary history of the western

Corresponding author at: School of Earth Science, China University of Geosciences, Wuhan 430074, China. E-mail addresses: [email protected], [email protected] (L. Wan).

https://doi.org/10.1016/j.jseaes.2019.104039 Received 29 March 2019; Received in revised form 15 September 2019; Accepted 22 September 2019 Available online 23 September 2019 1367-9120/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. (a) Simplified geological maps of the studied areas in the western Jiangnan orogen. The upper-left simplified map shows the general position of the Jiangnan orogen within the South China Block. (b) The distribution of Precambrian rocks in the Jiangnan Belt and the Yangtze and Cathaysia blocks. Including three MidNeoproterozoic rift basins around the Yangtze Block (Modified after Wang and Li., 2003). (c) The Qianyang area of Hunan Province. (d) The Longsheng-SanmenjieTongdao areas on the border of northern Guangxi and western Hunan. (e) The Guzhang area of northern Hunan. (f) The Gaoqiao area in Hunan, the focus of the new sampling and analyses in this work.

2. Geological setting

Jiangnan orogen, more detailed geological, geochemical, geochronological and isotopic studies are necessary. In this paper, we provide new data from the Gaoqiao mafic suite and then place our results in context with isotopic, geochemical and geochronological data from contemporaneous mafic rocks in neighboring areas in western Hunan and northern Guangxi Province. Our aims are to unravel the petrogenesis of these mafic rocks; to determine whether they are of arc, rift, or plume origin; and thereby to constrain the Neoproterozoic tectonic and magmatic setting of the western Jiangnan orogen and the larger South China Block.

2.1. South China Block The SCB is bounded by the Qinling-Dabie-Sulu orogenic belt to the north, the Longmenshan Fault to the northwest, the Ailaoshan-Red River Fault to the southwest, and the South China Sea to the southeast (Fig. 1b). The Jiangnan orogen is a NE trending mobile belt through the middle of the SCB, separating the southeastern margin of the Yangtze Block from the northwestern margin of the Cathaysia Block (Zhang et al., 2012a). Neoproterozoic intracontinental rift basins are well developed in the SCB, including the N-S-trending Kangdian Rift, the E-W2

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Intruded the Bendong pluton Intruded the Zhaigun pluton Was intrude by the Tianpeng pluton and the mafic rocks Intruded the Sibap Group Hosted in the Danzhou Group Hosted in the Sanmenjie Group Occurred in the Wuqiangxi Formation Were uncomfortably overlain by the lower Sinian Series Intruded the Banxi Group Hosted in the Fanjinshan Group Intruded the Lengjiangxi Group Intruded the Lengjiangxi Group Occur at the bottom of the Banxi Group Host in the Jiangkou Formation of the lower Sinian Series

2.2. The Gaoqiao area Middle-late Neoproterozoic stratigraphic sequences of the lower Sinian System in the Gaoqiao area consist, from bottom to top, of the Chang'an, Fulu, Datangpo, and Nantuo Formations (Wang and Li, 2003). The Chang'an Formation (the lower part of the Jiangkou Group) dominates the southwestern Gaoqiao region and is composed of silty slate and pebbly sandstone. It is unconformably overlain by the Fulu Formation to the northeast (Wu et al., 2015). The sedimentary host rocks in the Gaoqiao area experienced chloritization and sericitization in the vicinity of NE-SW trending faults (Fig. 1e). Mafic-ultramafic rocks in the Gaoqiao area intrude exclusively into the Chang'an Formation, suggesting intrusion during the time of the lower Sinian System, previously considered to be 750–680 Ma (Wang et al., 2004). However, newly published zircon U-Pb ages show that the lower Sinian System is younger than previously thought, namely 715 Ma to a lower limit of 635 Ma (Lan et al., 2015). SHRIMP U-Pb dating of detrital zircon grains from the Jiangkou Group in the Gaoqiao area has yielded ages between 746 Ma and 752 Ma (Tang et al., 1997). A further age bracket is obtained from the 761 Ma Longsheng gabbrodiabase, hosted by the Sanmenjie Formation and never extending through the unconformity into the overlying Gongdong Formation, which is in turn overlain by the Jiangkou Formation (Ge et al., 2001). Thus, the timing of emplacement of the Gaoqiao mafic rocks into the Jiangkou Group is younger than 760 Ma. We sampled brecciated basalts from several sites in Gaoqiao. They display irregular contacts with the host sedimentary rocks (Fig. 2a). In outcrop and thin section, the sedimentary units in the region display weak deformation, indicated by stretched mineral grains, weak preferred orientation of quartz, and wavy extinction in biotite and muscovite. The basalts are grayish-green to dark grayish-green and form well-developed, corrugated massive flow structures with prominent vesicles and amygdules (Fig. 2). Breccias are locally observed in these

LA-ICP-MS LA-ICP-MS LA-ICP-MS LA-ICP-MS TIMS SHRIMP SHRIMP SHRIMP SHRIMP LA-ICP-MS LA-ICP-MS SHRIMP SHRIMP LA-ICP-MS

LA-ICP-MS SHRIMP

Li et al. (1999) Zhou et al. (2004) Wang et al. (2006) Li et al. (1999) Wang et al. (2006) Wang et al. (2006) Wang et al. (2006) Wang et al. (2006) Wang et al. (2006) Ge et al. (2001b) Zhou et al. (2007) Wang et al. (2008a) Zhou et al. (2007) wang et al. (2008a) Zhou et al. (2009) Zhang et al. (2009) Wang et al. (2007a) Wang and Li (2003) This study Hosted in the Sibao Group Hosted in the Sibao Group Layer mafic rocks hosted in the Sibao Group Intruded into the Sibao Group,underlying by the Danzhou Group and was intruded by the Sanfang pluton

trending Bikou-Hannan Rift and the NE-SW-trending Nanhua Rift along the western, northwestern and southern margins of the Yangtze Block, respectively (Fig. 1b). Early Neoproterozoic igneous rocks (825–800 Ma) are widespread in the Yangtze Block (Wang and Li, 2003; Li et al., 2008; Wang et al., 2017), mainly metamorphosed mafic-ultramafic volcanic sequences (Zhao and Cawood, 2012) interbedded with sedimentary sequences containing mostly Neoproterozoic (870–820 Ma) detrital zircons (Wang et al., 2007a, 2008b; X.L. Wang et al., 2012). These units are unconformably overlain in places by middle-late Neoproterozoic metamorphic rocks (850–720 Ma) (Zhou et al., 2004; Cui et al., 2016), conformably overlain in turn by the riftrelated 720–635 Ma lower Sinian tillite and limestone (Wang and Li, 2003; Cai et al., 2018). In the western Jiangnan orogen, abundant mafic-ultramafic intrusive rocks have been identified, which generally occur in Meso- to early Neoproterozoic sedimentary strata (Li et al., 1999; Wang et al., 2006; Zhou et al., 2007; Kou et al., 2018). Such mafic intrusive and bimodal volcanic rocks are widespread in Hunan and Guangxi Provinces (Table 1). Middle Neoproterozoic mafic-ultramafic rocks in Guangxi cover about 140 km2 in the areas of Longsheng and Sanmenjie (BGMRGX, 1985; GXRGST, 1995). Contemporaneous mafic-ultramafic rocks in Hunan are mainly found in the Qianyang, Tongdao, Guzhang and Gaoqiao areas (Fig. 1c, d, e, f), with a total outcrop area of ~ 295 km2 (Wang et al., 2004). Middle Neoproterozoic mafic dykes in Hunan commonly intrude the Wuqiangxi Formation of the Banxi Group (BGMRHP, 1988). Four mafic-ultramafic dykes have been reported in northern Guangxi with a U-Pb age of 828 ± 7 Ma (Li et al., 1999); these rocks have previously been considered to be correlated with contemporaneous igneous rocks in Australia, including the 827 Ma Gairdner Dyke Swarm and the 824 Ma Amata Dyke Swarm (Li et al., 1995; Ge et al., 2001; Ye et al., 2007; Wang et al., 2006, 2007b; X.L. Wang et al., 2012; X.C. Wang et al., 2012).

Sanfang Tianpeng Zhaigun Dongma Longsheng Sanmenjie Qianyang Guzhang Tongdao Fanjinshan Aikou Yiyang Baolinchong Gaoqiao

Northern Guanagxi Northern Guanagxi Northern Guanagxi Northern Guanagxi Northern Guanagxi Northern Guanagxi Northern Guanagxi Northern Guanagxi Northern Guanagxi Northern Guanagxi Northern Guanagxi Western Hunan Western Hunan Western Hunan Western Hunan Eastern Hunan Northeastern Hunan Northeastern Hunan Central Hunan Yangmeiao Baotan Hejiawang Bendong

Mafic–ultramafic rocks Diabase/Komatiitic basalts? Granodiorite Granodiorite Granodiorite Biotite granite Granite Granodiorite Granodiorite Gabbro–diabase Rhyo-dacite Diabase Diabase Altered ultramafic rocks Basalts Mafic–ultramafic dyke swarm High-Mg basalts Andesitic agglomerates and breccias Breccia basalts

828 ± 7 820 Ma 811 ± 4 819 ± 9 823 ± 4 804 ± 5 794 ± 8 835 ± 2 812 ± 13 761 ± 8 765 ± 14 747 ± 18 768 ± 28 756 ± 12 822 ± 15 832 ± 10 823 ± 6 814 ± 12 757 ± 16

SHRIMP

References General geology Methods Age (Ma) Rock type Locality Pluton

Table 1 Summary of ages of the Meso- to Neoproterozoic basic–felsic rocks from western Jiangnan orogen.

L. Wan, et al.

3

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Fig. 2. Field features of basalts and sedimentary rocks in Gaoqiao. (a) Field photos showing the contact relation between Gaoqiao basalts and sedimentary rocks; (c) and (e) Field photos showing fresh surface color and typical amygdaloidal structures of basalts; (b), (d) and (f) represent the microscopic characteristics of the Gaoqiao basalts.

3. Analytical Methods

basalts, containing typically 1–2 cm fragments (individually up to 7 cm), cemented by chlorite, zeolite, and epidote with minor calcite and quartz. The main phenocrysts in the basalts are elongated plagioclase, commonly altered to sericite. The matrix is dominated by chlorite (~50%), zoisite (~25%), zeolite (~10%) and iron oxides (~15%), with minor residual plagioclase. Amygdules are partly filled by iron oxides and zoisite.

3.1. Zircon U-Pb dating Zircons were separated using conventional heavy liquid and magnetic techniques. All analyzed zircon grains were purified by handpicking under a binocular microscope at the Langfang Honesty Geological Services Company, Hebei Province, China. Handpicked

Table 2 LA-LCP-MS zircon U-Pb data for Gaoqiao basalts in Hunan, western Jiangnan orogen. Spot

Th (ppm)

U (ppm)

Th/U

207

Pb/206Pb

Sample 1 2 3 4 5 6 7 8 9 10 11 12 13

GQ-2 358 975 472 245 173 413 803 587 360 1457 1262 2907 2773

621 909 576 220 201 464 772 1354 1161 913 3153 2135 2624

0.58 1.07 0.82 1.11 0.86 0.89 1.04 0.43 0.31 1.6 0.4 1.36 1.06

0.0666 0.0654 0.0663 0.0644 0.0642 0.0672 0.0643 0.0657 0.0672 0.0685 0.0667 0.0657 0.0645

± 1σ

0.0019 0.0018 0.002 0.0028 0.003 0.0022 0.0022 0.0016 0.0018 0.0022 0.0013 0.0014 0.0014

207

Pb/235U

± 1σ

206

1.2689 1.2079 1.2323 1.0733 1.0803 1.2984 1.116 1.2859 1.2927 1.0941 1.2508 1.0848 1.1245

0.0352 0.0336 0.0379 0.0443 0.0489 0.0418 0.0405 0.029 0.0332 0.0351 0.0248 0.0243 0.0264

0.1373 0.1328 0.134 0.1217 0.1233 0.1401 0.1255 0.1411 0.1383 0.1149 0.135 0.1189 0.1254

4

Pb/238U

± 1σ

207

Pb/206Pb

0.0014 0.0013 0.0014 0.0018 0.0015 0.0016 0.002 0.0014 0.0013 0.0011 0.0012 0.0013 0.0015

828 787 815 755 746 843 754 798 843 885 828 798 767

± 1σ

207

Pb/235U

± 1σ

206

Pb/238U

59 57 63 91 101 69 78 53 56 67 45 44 239

832 804 815 740 744 845 761 839 842 750 824 746 765

16 15 17 22 24 19 19 13 15 17 11 12 13

830 804 810 740 750 845 762 851 835 701 817 724 762

± 1σ

8 7 8 10 9 9 11 8 7 7 7 7 9

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most exhibit oscillatory zoning in CL images, consistent with magmatic origin (Corfu et al., 2003). Trace-element analyses were also collected on zircons from sample GQ-2. All zircon grains have similar chondritenormalized rare-earth element (REE) patterns that are strongly enriched in heavy REE (HREE) and depleted in light REE (LREE) (Fig. 3b; Supplementary table 1). Three zircon grains (Spots #10, #12, and #13) show relatively flat LREE patterns, possibly due to the presence of mineral inclusions (apatite) within the zircons. All analyses display strong negative Eu anomalies ([Eu/Eu*] = 0.14–0.48) and a range of positive Ce anomalies ([Ce/Ce*] = 1.64–62.9) (Supplementary table 1). These features further confirm their magmatic origin (Hoskin, 2003). Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jseaes.2019.104039. We obtained thirteen U-Pb analyses from this sample. Eleven of these are, within uncertainty, precisely concordant. They divide into two clusters (Fig. 3a). Two analyses (Spots #10 and #12) depart from the Concordia. This is most likely due to minor common Pb, apatite inclusions, or inaccurate measurement of 207Pb rather than any actual Pb-loss from the zircons. Measured concentrations of Th vary from 173 to 2907 ppm and of U from 201 to 3153 ppm, corresponding to a range in Th/U ratios of 0.314–1.6 (Table 2). Seven analyses (Group 1) give 206 Pb/238U ages between 804 ± 7.2 Ma and 845 ± 9 Ma with a weighted mean age of 832 ± 26 Ma (MSWD = 0.32). Group 1 spots have low Th/U (0.31–1.07) and are in the cores of grains (e.g., Spots #8 and 9; inset to Fig. 3a). The remaining six analyses (Group 2) have higher Th/U ratios (0.86–1.6) and yield younger concordant 206Pb/238U ages ranging from 701 ± 6.6 Ma to 762 ± 8.6 Ma with a weighted mean age of 757 ± 16 Ma (MSWD = 0.35). Based on their stratigraphic position within the sedimentary sequences and the distinctive appearance of the two magmatic zircon populations, we assign an age of 757 ± 16 Ma as the best estimate of the eruptive age of the Gaoqiao basalt. We therefore interpret the Group 1 spots as inherited zircon material that records some earlier magmatic event. This inference is supported by the zircon U-Pb ages (750–700 Ma) obtained by Wang et al. (2004).

zircons were documented using both optical photomicrographs and cathodoluminescence (CL) images taken on a JXA-8100 electron microprobe. Zircon U-Pb dating was completed at the State Key Laboratory of Geological Processes and Mineral Resources at the China University of Geosciences (Wuhan), using an Agilent 7700a ICP-MS with an attached 193 nm GeoLas 2005 excimer laser. The ablation rate was 10 Hz with He as the carrier gas, and the spot diameter was 32 μm. Each analysis incorporated a background acquisition of approximately 20 s (gas blank) followed by 35 s data acquisition. NIST SRM610 was used for calibrating the U and Th contents. Zircon standard 91,500 was used as the external standard for U-Pb dating and was analyzed twice between every 5 unknown analyses. The LA-ICP-MS Common Lead Correction (v3.15) method is described by Andersen (2002). Concordia diagrams and weighted mean age calculations were made using Isoplot/Ex (v3.0) (Ludwig, 2003). The analytical data are presented with 1σ error on the Concordia plots and the weighted mean ages are quoted at the 95% confidence level. The zircon U-Pb age data are listed in Table 2. 3.2. Major and trace element analyses Eight representative basalts were selected for major and trace elements analysis. All bulk samples were trimmed to remove the weathered surfaces, crushed, and subsequently powdered in an agate mill. Two basalts (GQ-1 and GQ-2) were selected for whole-rock geochemical analysis at the State Key Laboratory of Geological Processes and Mineral Resources at China University of Geosciences (Wuhan). Major element analyses were conducted by X-ray fluorescence (XRF). Rock powders were digested by HF + HNO3 in Teflon bombs for trace elements analysis in solution mode on an Agilent-7500a inductively coupled plasma mass spectrometer (ICP-MS). A set of USGS standard rocks (AGV-2, BHVO-2, BCR-2, and GSR-3) were chosen as external calibration standards. The remaining six bulk-rock samples (GQ-3, GQ-4, GQ5, GQ-6, GQ-7, GQ-8) were analyzed for major and trace elements by XRF (XRF-1800) and ICP-MS (X2) at the Hubei Provincial geological experimental testing center. Analytical uncertainties are generally better than 5%.

4.2. Major and trace elements 4. Analytical results The Gaoqiao basalts have SiO2 contents ranging from 41.3 wt% to 53.1 wt%. They display substantial heterogeneity in Al2O3 (9.7–11.45 wt%), Fe2O3T (12.3–16.6 wt%), MgO (5.6–8.7 wt%), CaO (3.2–6.6 wt%) and TiO2 (4–6.4 wt%) contents. They have a rather restricted range of relatively high Mg# values (47–58). We place their compositions in context using a compilation of existing major and trace

4.1. Zircon analysis and U-Pb ages Our zircon study focused exclusively on Gaoqiao basalt sample GQ2. Zircons from sample GQ-2 are light to dark, euhedral to subhedral grains (Fig. 3a). The grain sizes range mainly from 50 to 100 μm and

Fig. 3. (a) Representative CL images LA-ICP-MS U-Pb spots and ages for analyzed zircons of breccia basalt samples; (b) REE compositions of zircons from the sample GQ-2. 5

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Fig. 4. Rock classification diagrams for the Mid-Neoproterozoic mafic rocks from Hunan and Guangxi. (a) Nb/Y vs. 10000*(Zr/TiO2) diagram (after Winchester and Floyd, 1977). (b) SiO2 vs. FeOT/MgO diagram (after Miyashiro, 1974). Data for mafic rocks in Guzhang and Qianyang are from Zhou et al. (2007) and Wang et al. (2008a), respectively. The published Gaoqiao basalts are from Wang et al. (2004). Data for mafic rocks in Longsheng and Sanmenjiein northern Guangxi are from Ge et al. (2001) and Zhou et al. (2007), respectively.

5. Discussion

element concentration data (Supplementary table 2) from middle Neoproterozoic mafic rocks from the western Jiangnan orogen. The measured LOI values are fairly high, so for purposes of comparison we plot the element oxides recalculated to 100% total on a volatile-free basis. The dominant rock types in the Gaoqiao suite are basalts, andesitic basalts and trachybasalts (Fig. 4a). In the Nb/Y-10000*(Zr/TiO2) classification diagram, the samples cluster near the transition between the alkaline and subalkaline basalt fields, an area of the diagram not previously populated by contemporaneous mafic rocks from the region (Fig. 4a). In the SiO2-FeOT/MgO diagram, the Gaoqiao basalts mostly plot in the tholeiitic field (with two exceptions in the calc-alkaline field), consistent with other samples from Hunan (Qianyang and Guzhang), but in contrast to samples from Guangxi (Longsheng and Sanmenjie) (Fig. 4b). In a primitive-mantle normalized extended trace element diagram (spidergram), the Gaoqiao basalts show large enrichments of incompatible elements, comparable to typical ocean island basalts (OIB) (Fig. 5a). However, their patterns differ from most OIBs by displaying features typically associated with subduction-related basalts, i.e. excesses of large ion lithophile elements (LILEs: Rb, Ba, Pb and K) and minor negative anomalies in some high field strength elements (HFSEs: Nb, Ta, Hf, Ti and Y). They have remarkably deep negative Sr anomalies. The high Nb/La (0.4–0.8), Hf/Th (3.1–6), Hf/Ta (6.3–13.4), Th/Nb (0.15–0.41) and Th/Yb (1.2–2.2) ratios are all atypical among continental arc basalts (Kelemen et al., 2003). The high Nb-Ta abundance, modest Ti depletion and negligible negative Zr-Hf anomalies are also different from characteristics seen in island-arc basalts (IABs; George et al., 2003). Indeed, the Nb concentrations of the Gaoqiao rocks (15.7–23.9 ppm) are unusual in any tectonic context, being high compared to nearly all subduction-related rocks and low relative to typical OIB samples (Nb = ~67 ppm) (Sun and McDonough, 1989). The Gaoqiao samples have high total REE content and enrichment of LREEs, characterized by (La/Yb)N ratios of 9.7–25.9. They show an unusual concave-down chondrite-normalized pattern with nearly constant LREEs and a smooth decrease through the middle and heavy REEs (Fig. 5b). High (La/Yb)N ratios and low Lu/Hf ratios (0.01–0.02) are most simply interpreted to reflect melting of a source that contained residual garnet (Eduardo et al., 2013).

5.1. Geochemical interpretation The LREE-enriched yet concave-down REE patterns with low HREE concentrations are plainly different from those of both normal and enriched MORB and from subduction-related basalts. In some ways, the REE concentrations of the Gaoqiao rocks resemble typical OIBs, but again they are unique in their elevation of middle REEs and concavedown patterns. They also differ from any Neoproterozoic mafic rocks heretofore analyzed in the western Jiangnan orogen. Contemporaneous mafic rocks in Qianyang and Guzhang exhibit more typical features of OIB suites. They have general enrichment of incompatible elements punctuated by negative Rb, K and Ti anomalies and rather extreme positive Pb anomalies (Fig. 5c). They are strongly enriched in LREE relative to HREE, with a typical concave-upwards pattern (Fig. 5d). Their Zr/Nb (5.6–12) and Nb/La ratios (0.6–0.9) resemble the geochemical features of typical OIB (Zr/Nb ≈ 7.5, Nb/La ≈ 1). By contrast, the ~760 Ma Sanmenjie and Longsheng mafic rocks in northern Guangxi have trace element characteristics that are not OIB-like at all (Fig. 5e, f). The prominent negative Nb, Ta anomalies, weak depletion in Ti, positive K anomalies, and modest LREE/HREE enrichments seen in these two suites are all typical features of subduction-related basalts.

5.2. Alteration, crustal contamination and fractional crystallization Middle Neoproterozoic mafic rocks in the western Jiangnan orogen overall have high LOI values, suggesting intense alteration. Plots of Rb and Sr vs. Zr display poor correlation and substantial scatter (Fig. 6a, b), reflecting the mobile nature of Rb and Sr. By contrast, elements including Y, Hf, Ta, Nb, Nd and La are tightly correlated with Zr within each suite of samples (Fig. 6c–h), indicating that these elements were essentially immobile during alteration. In the following discussion, we put an emphasis on elements that are immobile during low-grade metasomatism and alteration, such as Ni, Cr, Y, HFSEs (Nb, Ta, Th, Zr and Hf) and REEs. Primary mantle-derived basaltic magmas generally have high Ni (> 400 ppm) and Cr (> 1000 ppm) contents (Wilson, 1989), and Mg# *(> 73) (Mahoney and Coffin, 1997). The Gaoqiao basalts, on the other hand, have relatively low Mg# (51–63), Ni (< 200 ppm) and Cr content 6

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Fig. 5. Primitive mantle (PM) normalized trace element and Chondrite-normalized REE patterns diagrams for the Mid-Neoproterozoic mafic rocks from Hunan and Guangxi province. The values of chondrite, primitive mantle, ocean island basalt (OIB), enriched mid-ocean ridge basalt (E-MORB) and normal mid-ocean ridge basalt (N-MORB) are from Sun and McDonough (1989). The average compositions of continental arc basalt (CAB) are from Tatsumi and Eggins (1995). Data sources are the same as in Fig. 4.

the Gaoqiao suite, depletion of P-Ti (Fig. 5a) and negative correlations between TiO2, P2O5 and SiO2 (Fig. 7b, c) suggest that apatite and a Tibearing phase have fractionated from most of the samples, so it is appropriate to consider the highest Ti/Y as representative of the source. The best explanation for extremely high (La/Yb)N ratios (17 ± 8) alongside low Lu/Hf ratios is substantial residual garnet during melting; Y, Yb, and Lu are strongly compatible in garnet (Jenner et al., 1993). Similar garnet signatures were noted in tholeiites from Pitanga and Paranapanemaas in the Paranà large igneous province (Piccirillo and Melfi, 1988), suggesting their derivation from a source that contained residual garnet. The absence of significant Eu anomalies argues against a role for plagioclase, whether as a residual, assimilated, or fractionated phase. Crustal contaminants, compared to mantle-derived magmas, are generally depleted in Nb and Ta but enriched in Zr and Hf (Jenner et al., 1993). The Gaoqiao basalts display prominent negative anomalies of Nb, Ta and Ti, which is consistent with significant crustal

(< 200 ppm, except for sample GQ-5), requiring that they experienced significant magmatic evolution by processes such as crustal assimilation or fractional crystallization. The simplest interpretation of the negative Nb-Ta anomalies and elevated Zr/Nb, La/Nb, Th/La, La/Sm, Th/Yb ratios in Gaoqiao basalts, since they lack other indications of subduction influence, is crustal contamination (Hawkesworth et al., 1995; Chen et al., 2014). On the other hand, the Gaoqiao, Guzhang and Qianyang suites each define rough linear arrays of increasing La and constant La/Sm (Fig. 7a), thought to be a feature of progressive fractional crystallization because La and Sm are both highly and equally incompatible in early fractionating phases in mafic magmas. The high TiO2 content (4.3–6.4 wt%) and high Ti/Y ratios (723–1271) in Gaoqiao basalts are similar to the high-Ti mafic rocks in the Emeishan large igneous province (ELIP) and the Bama region in western Guangxi (Lai et al., 2012). In general, Ti/Y, as opposed to Ti alone, reduces the effects of crystal fractionation, at least before fractionation of Ti-bearing oxides becomes significant (Xu et al., 2013). In 7

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Fig. 6. Trace elements and incompatible element ratios vs. Zr for the Mid-Neoproterozoic mafic rocks from Hunan and Guangxi Provinces. Symbols and data sources are the same as in Fig. 4.

(1.2 ppm Th, 0.2 ppm U, Nb/U = 25) (Rudnick and Gao, 2003), but a large mass fraction of lower crustal components would be required if contamination is to explain the trace element patterns. In order to quantify the mass fraction of such a component and how it varies with degree of fractionation, we may examine some trace element ratios (e.g., Zr/Hf and Nb/Ta) that are sensitive to crustal input but

contamination (Zhao and Zhou, 2009). Their contents of Th (1.8–2.9 ppm, average 2.4) and U (0.3–0.8 ppm, average 0.5) are lower than those of the upper crust (Th = 10.5 ppm, U = 2.7 ppm) (Rudnick and Gao, 2003), but their Nb/U ratios (26–70, average 44) are higher than those of the upper crust (about 9) (Zhou et al., 2004). Instead, these features seem more consistent with those of the lower crust 8

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Fig. 7. Plot of La/Sm vs. La (a) for the Mid-Neoproterozoic mafic rocks from Hunan and Guangxi Provinces; Hark variation diagrams (b, c) for the breccia basalt from Gaoqiao area to understand the fractional crystallization and crustal assimilation (AFC) process.

(Fig. 9c). In diagrams that do not explicitly include fields for continental basalts, the Hunan samples could be interpreting as trending towards OIB-like compositions, e.g., in Ce/Nb-Th/Nb, Th/Yb-Nb/Yb, and Th/Yb-Ta/Yb diagrams (Fig. 9d, e, f). In all these diagrams, the northern Guangxi suite consistently plots with continental arc suites. The distinctive positions of these two groups, as well as the unique position of the Gaoqiao samples within the overall Hunan mafic assemblage, echoes the conclusions drawn from examination of their primitive mantle-normalized trace-element patterns (Fig. 5). Although the Sanmenjie and Longsheng mafic rocks from Guangxi display arc-like features and have been assigned to an arc setting (Zhou et al., 2004, 2007), it should be mentioned that arc-like geochemical features such as depletion of Nb and Ta relative to La have also been observed in many intraplate rift and/or mantle-plume related magmas that were derived from subduction-modified lithospheric mantle sources or were contaminated by crustal materials (Hawkesworth et al., 1995). Thus, the arc interpretation of the northern Guangxi mafic rocks should be treated with caution. Zr/Sm and Ti/V ratios may help to distinguish arc basalts from intraplate basalts (Zhou et al., 2007). Experimental and natural results show that arc-related basalts have Zr/Sm ratios lower than the chondritic value (Zr/Sm = 25); Aleutian arc basalts, for example, average Zr/Sm = 20. Continental crust components have Zr/Sm ratios ranging from 24 to 42 (average 34) and intraplate basalts have high Zr/Sm ratios (> 25). With a few exceptions, the Sanmenjie and Longsheng samples from Guangxi display high Zr/Sm ratios (24–48), which are consistent with values for intraplate basalts. Moreover, the high Ti/V ratios (> 30) of these suites are distinct from those of typical arc basalts (Ti/V < 20) (Zhou et al., 2007). These observations indicate that the Sanmenjie and Longsheng gabbros are unlikely to have been emplaced in an oceanic arc setting. They are

insensitive to fractional crystallization (Weaver, 1991; Barth et al., 2000). Despite a large range in SiO2 contents, the Gaoqiao basalts have uniformly high MgO (5–8 wt%), nearly constant Zr/Hf ratios (43.9–53.2, average 46.6) and only moderately variable Nb/Ta ratios (13.4–24.3, average 17.5) (Supplementary table 2). On the other hand, strong crustal contamination would be expected to yield large ranges in Zr/Hf and Nb/Ta, approaching crust-like values towards the high-SiO2 end of the suite. The small ranges and OIB-like Zr/Hf and Nb/Ta ratios therefore argue against strong crustal contamination in the Gaoqiao suite. Other trace element ratios reinforce this conclusion: large ranges in Nb/Th and Zr/Nb ratios alongside relatively constant Th/Ta ratios (< 2.7) and Nb/La (0.4–0.8, not correlated with SiO2) are not expected in the case of significant crustal contamination (Sobolev et al., 2007; Zhang et al., 2013a) (Fig. 8). We thus infer that the Gaoqiao basalts were mainly derived from a garnet-bearing mantle source and underwent minor lower crustal contamination before or during fractional crystallization. 5.3. Tectonic setting Immobile trace elements are frequently used to discriminate the tectonic setting of mafic rocks. As shown in ternary Y-La-Nb and Ti-Zr-Y plots and the Ti-Zr diagram (Fig. 9a, b, c), ~760 Ma middle Neoproterozoic mafic rocks in the western Jiangnan orogen can be divided into two principal types. Mafic rocks in Hunan, including the new Gaoqiao data, show continental basalt affinities, whereas contemporaneous mafic rocks in northern Guangxi fall in the volcanic arc field. In some respects, the Gaoqiao samples are intermediate between the previously reported populations from Hunan and from Guangxi (Fig. 9a), whereas in other regards such as Ti-Zr contents, their position is more extreme 9

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Fig. 8. Crustal contamination diagrams of the parental magma for the Gaoqiao brecccia basalts: (a) (c) Nb/La vs. Nb/Th and Zr/Nb diagrams; (b) Ta vs. Th diagram; (d) SiO2 vs. Nb/La diagram. Symbols and data sources are the same as in Fig. 4.

Morrison, 1988). The mafic rocks in Gaoqiao, Guzhang and Qianyang overall have relatively high Nb/La ratios (0.3–1.5, average 0.8) and low La/Ta ratios (mostly 9–34, average 18, individually up to 60). Both indicators point to asthenospheric rather than SCLM sources for the Hunan mafic suites. Turning to isotopic constraints, published Sm-Nd and Rb-Sr isotope data for mafic rocks from western Hunan and northern Guangxi are listed in Supplementary table 3. Gabbros from Qianyang and mafic dykes from Guzhang both show fairly narrow ranges of εNd(t) close to zero: from +0.1 to +2.7 at Qianyang and −0.4 to +1.0 at Guzhang. Available εHf(t) data from Qianyang are consistent with the Nd isotope results, ranging from chondritic to modestly depleted (−1.9 to +8.5) (Supplementary table 3), suggesting their derivation from a modestly depleted mantle source or from a mix of enriched and depleted sources. The εNd(t) values of mafic rocks in Guangxi are between −1.7 and +5 at Longsheng and between −0.7 and +2.4 at Sanmenjie, again suggesting derivation from modestly depleted or mixed mantle reservoirs. Although the 756 Ma mafic intrusive rocks from Tongdao in southern Hunan, about 100 km southwest of Qianyang, are nearly synchronous with the mafic rocks discussed here (Wang et al., 2008a), the decoupled Nd and Hf isotopic signatures of the Tongdao suite may reflect the influence of early subduction (Polat and Münker, 2005) and metasomatism by slab-derived components (Wang et al., 2008a). The coupled NdHf isotopic compositions of the Qianyang and Guzhang suites do not point to any such subduction signature. The inference of mixing of SLCM and asthenospheric sources for the Hunan and Guangxi mafic suites (excluding Tongdao) is also supported by the correlation between La/Yb and Nb/La ratios (Fig. 10a). Furthermore, these mafic suites overall have elevated Ni contents relative to the supposed range of mantle-derived melt, plotting along the mixing line of melt-peridotite

consistent instead with a continental active margin or a previously subduction-influenced intraplate setting. Taken together, all the 768–747 Ma mafic rocks in the western Jiangnan orogen, both in Hunan (Gaoqiao, Guzhang and Qianyang) and Guangxi, may be consistently interpreted to record an intracontinental rift setting with spatially variable previous subduction influence in the source rocks. 5.4. Origin of the mafic magma The similarities in age and geochemical signatures of the Gaoqiao basalts and the mafic suite at Guzhang and Qianyang suggest that, despite their distinct origins, we should seek a consistent regional model that can explain their spatial and temporal proximity. Trace element patterns of all three suites have features typical of intraplate basalts and tend towards OIB-like compositions. Enriched sources for such intraplate basalts (Halliday et al., 1995; Lassiter et al. 2003) may be found in metasomatised subcontinental lithospheric mantle (SLCM) and as recycled components in the asthenospheric mantle (McDonough, 1989). Asthenosphere-lithosphere interaction may play a key role in producing continental basalts (Turner and Hawkesworth, 1995; Wang et al., 2009). Some incompatible trace elements ratios (e.g., Nb/La and La/Ta) have been suggested as effective indicators for distinguishing among asthenospheric mantle and SCLM source; asthenospheric mantle-derived melts generally have high Nb/La ratios, from 0.9 (NMORB) to 1.3 (OIB and E-MORB) (Sun and McDonough, 1989), while melts from ancient SCLM exhibit low Nb/La (< 0.3) ratios (Cui et al., 2015). By contrast, it is generally thought that mafic rocks with high La/Ta ratios (> 30) originated from SCLM metasomatised by subducted slab-derived fluids/melts during previous tectonic events, whereas lower La/Ta ratios characterize asthenospheric mantle (Thompson and 10

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Fig. 9. Tectonic discriminant diagrams for the Mid-Neoproterozoic mafic rocks from Hunan and Guangxi Provinces. (a) La-Nb-Y diagram (after Cabanis, 1989); (b) Ti-Zr-Y diagram (after Pearce and Cann, 1973); (c) Zr vs. Ti diagram (after Pearce, 1982); (d) Th/Nb vs. Ce/Nb (after Saunders and Tarney, 1991); (e) Nb/Yb vs. Th/ Yb (after Dilek and Furnes, 2011); (f) Ta/Yb vs. Th/Yb (after Pearce, 1983). E-MORB: enriched mid-ocean ridge basalt, N-MORB: normal mid-ocean ridge basalt, VAB: Volcanic arc basalt, WPB: Within plate basalt (N-MORB). Symbols and data sources are the same as in Fig. 4.

also plot in the lithospheric mantle field, except for a few that indicate mixing with asthenosphere-derived melts (Fig. 10a). The SCLM, because of its general depletion in incompatible elements, is easily modified by fluids or melts from subducted slabs (Elliott et al., 1997). The Guangxi suites and the Gaoqiao rocks are all characterized by high Th/ Zr and Rb/Y ratios and low Nb/Y and Nb/Zr ratios (Fig. 10c, d),

interaction (Fig. 10b), which indicate the mechanism for contamination of the asthenosphere-derived primary melts by SCLM. The Sanmenjie and Longsheng mafic rocks in Guangxi are characterized by low Nb/La (0.2–0.8, average 0.8), Nb/Ta (6–21, average 12) and Zr/Hf (31–50, average 38) ratios, all signatures associated with SCLM-derived melts. The low La/Yb and Nb/La ratios of most samples 11

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Fig. 10. Plots of elements and elemental ratios for the the Mid-Neoproterozoic mafic rocks from Hunan and Guangxi to constrain the nature of the mantle source. (a) La/Yb vs. Nb/La diagram (after Watson and Harrison, 1983); (b) Cr vs. Ni diagram (after Zhang et al., 2012a), The ranges for different rocks including Kitakami, Ochinawa, Taishan and Kiwidahi in New Zealand are from Tsuchiya et al. (2005), Shinjo et al. (1999), and Booden et al. (2010); The ~820 Yiyang and Aikou maficultramafic rocks are from Wang et al. (2007a) and Zhang et al. (2009), respectively. Plots of Nb/Zr vs. Th/Zr ratios (c) and Rb/Y vs. Nb/Y ratios (d) for the MidNeoproterozoic mafic rocks from Hunan and Guangxi showing the variations of the ratios metasomatism by subducted sediment-derived melt/fluid. Data sources are the same as in Fig. 4.

pointing to a more fluid-dominated enrichment trend compared to the melt-related enriched trend shown by the Qianyang and Guzhang suites. This is consistent with the elevated (87Sr/86Sr)i — 0.7076–0.7226 — of the Guangxi samples, compared to the Nd-Sr and Hf-Sr mantle arrays and their Nd isotope compositions. In general, such isotopic observations can be ascribed to two tectonic evolution patterns (Zhou et al., 2004): (1) a back-arc extensional setting which triggered the upwelling of depleted deep mantle components; (2) a limited amount of recycled crustal materials carried from a continental margin in an earlier subduction stage and subsequently involved in the mantle source of the mafic magma. As shown in Fig. 9a and d, some mafic rocks from Longsheng, Qianyang and Gaoqiao fall in or near the back-arc field, possibly implying their subduction affinities. We thus infer that the ~760 Ma Sanmenjie and Longsheng mafic rocks dominantly arose from a SCLM source that had been overprinted at an earlier stage by some subduction input, leading to low Th/Nb ratios (0.35–0.88, average 0.65) and negative εNd(t) values.

Neoproterozoic magmatism in Jiangnan to the effects of a mantle plume (Li et al., 1999; Wang et al., 2007a). Although the geochemical signatures of some of these rocks could be described as “OIB-like” in some ways, there is little support for plume activity in the area at this time. Compared, for example, to the voluminous (area > 2 × 105 km2) Gairdner (827 Ma) and Amata (824 Ma) Dyke Swarms in Australia (Zhou et al., 2004), associated with plume-triggered break-up of Rodinia (Zhao et al., 1994), the small-scale (< 1000 km2; X.L. Wang et al., 2012) mafic lavas in the Jiangnan orogen hardly constitute a large igneous province. Furthermore, the regional dome-like uplift expected from a plume arrival (e.g., as seen in the Emeishan large igneous province; He and Santosh, 2016) has not been reported in the Jiangnan orogen. Instead the angular unconformity between the Lengjiaxi and Banxi groups is usually considered to be related to an orogenic event (Zhou et al., 2007; X.L. Wang et al., 2012; Chen et al., 2018). In fact the bulk of early stage (830–820 Ma) mafic rocks in the western Jiangnan orogen display typical arc affinities totally unlike the contemporaneous Australian dyke swarms (Wang et al., 2004, 2008a; Zhou et al., 2009). Instead, the typical effects of the closure of an oceanic basin leading to a collisional orogeny offer more plausible mechanisms for early Neoproterozoic magmatism in the Jiangnan region (Wang et al., 2008b). Contemporaneous continental margin arc-related volcanic and plutonic rocks are widely reported along the northwestern margin of the Cathaysia Block, such as the Chencai and Longquan groups in Zhejiang, the Shenshan Group in Jiangxi and the Mayuan and Mamianshan groups in Fujian (Zhao and Cawood, 2012). Newly published

5.5. Insights for the tectonic evolution of the Jiangnan orogen The Yangtze and Cathaysia blocks are generally thought to have been amalgamated along the Jiangnan orogenic belt during the earlymiddle Neoproterozoic (1.0–0.86 Ga) (Ye et al., 2007), although stratigraphic and sedimentological data suggest that the orogenic process may have lasted until 830 Ma or even 800 Ma (Zhao, 2015). Despite this convergent tectonic setting, a number of authors have attributed early 12

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zircon U-Pb ages of these sequences extend down to 838 Ma (Li et al., 2010; Zhang et al., 2012b). The southeast-directed early Neoproterozoic continental margin arc thought to have formed the Chencai and Longquan groups in Zhejiang (Cawood et al., 2013; Yao et al., 2014) could also have been responsible for the formation of the 841 Ma Lipu diorite and the 838 Ma rhyolite-diorite in the Chencai area (Li et al., 2010). There is also evidence for a complementary northwest-directed continental margin arc along the southeastern margin of the Yangtze Block (Cawood et al., 2013), the 1032 ± 9 Ma Kunyang Group in central Yunnan (Li et al., 2013) and the 1043 ± 7 Ma Huili Group in western Sichuan (Greentree et al., 2006), which were generated in a continental arc setting (Zhang et al., 2007). Between these two complementary subduction zones, researchers have found some Neoproterozoic supra-subduction zone type ophiolites in the Jiangnan orogen, including the ~960 Ma ophiolites in northeast Jiangxi (Li et al., 1994) and the 980–820 Ma arc-related magmatic rocks and associated intrusions (Zhang et al., 2013b; Yao et al., 2014) in northeast Jiangxi. These ophiolites probably formed during the initial rifting of a back-arc basin behind the Huaiyu arc at 990 Ma (Wang et al., 2016). The early Neoproterozoic in this area was, therefore, characterized by divergent double-sided subduction and development of arc-back-arc systems (Fig. 11a). Continental margin sedimentary sequences in the Yangtze Block (Lengjiangxi Group and its equivalents) and in the Cathaysia Block (Chencai Group and its equivalents) developed in the intervening ocean basin before the collision between the Yangtze and Cathaysia blocks. They subsequently experienced intensive deformation and metamorphism during the collision (Li et al., 1995, 1999; Wang et al., 2007b, 2008a, b; Zhao and Zhou, 2011). A series of 835–800 Ma collision-related granitic rocks have also been widely recognized in both the eastern and western Jiangnan orogens (Wang et al., 2006; Zheng et al., 2008; Yao et al., 2014) (Fig. 11b). However, the exact collision time is still controversial. Geochronological studies on the high-pressure blueschists in the Jiangnan orogen have been interpreted to place the collision between the two blocks between 870 and 850 Ma (Zhao and Cawood, 1999). Other geologists argue for collision between 900 Ma and 880 Ma (Wang and Li, 2003; Wang et al., 2011) and still others suggest between 870 Ma and 830 Ma (Zhao and Cawood, 1999; Wang et al., 2007a; Zhao et al., 2011). Here we argue that the terminal collision is bracketed by the most appropriate geochronologic constraints between ~830 Ma and ~821 Ma. Firstly, middle Neoproterozoic continental margin sedimentary sequences on both the Cathaysia and Yangtze blocks predate the collision. These sequences include tuffs from the uppermost part of the Lengjiaxi and Fanjingshan groups with U-Pb ages between 840 Ma and 830 Ma (Wang et al., 2007a; Zhao et al., 2010; Gao et al., 2011) and a detrital zircon study of the Sibao Group and its equivalents yielded ages of 880–830 Ma (Wang et al., 2007a, 2008a; Zhao, 2015). Secondly, the post-collisional S-type granitoids in the Jiangnan orogen include the 829–819 Ma Sanfang, Bendong and Yuanbaoshan granitic plutons in northern Guangxi (Li et al., 1999; Wang et al., 2006) and the ~ 824 Ma Xiuning, Shexian and Xucun plutons in Anhui (Wu et al., 2006; Zheng et al., 2008) (Fig. 11b). Thirdly, the weakly metamorphosed middle Neoproterozoic strata (Banxi Group and equivalents) are generally thought to have formed shortly after the collision (Li et al., 1995; Wang et al., 2007b, 2008a,b; Zhao et al., 2011). Newly published zircon U-Pb dating for the bentonites, dacites and other volcanic rocks from the Banxi Group and equivalent strata indicate that they are most likely to have been formed between 815 Ma and 725 Ma (Wang et al., 2006; Gao et al., 2011). The post-collisional stage then account for the production of the early Mid-Neoproterozoic magmatic rocks in the western Jiangnan orogen. Bimodal magmatism typical of post-collisional orogenic evolution (Bonin, 1990) is represented by 829–819 Ma felsic intrusions orogen (e.g., Sanfang, Bendong and Yuanbaoshan) and by mafic rocks including the ~820 Ma Baotan diabases (Zhou et al., 2004), the ~823 Ma Yiyang high-Mg basalts (Wang et al., 2007b) in Hunan, and

Fig. 11. Sketch illustrating the tectonic evolution of the Jiangnan orogen between the Yangtze and Cathysia blocks in South China (See details in the text).

the ~822 Ma Fanjingshan basalts (Zhou et al., 2009) in Guizhou. The widespread 800–760 Ma rift-related volcanic rocks, e.g., the Xucun mafic rocks from the Shangshu and Puling Formations, possibly point to the occurrence of intraplate rifting during the middle Mid-Neoproterozoic (Zheng et al., 2008; X.L. Wang et al., 2012) (Fig. 11c). Deformation and magmatism occurring at this stage are probably associated with tectonic transformation to a post-collisional stage of intracontinental rifting, represented for example by the Nanhua and Kangdian rift basins (Wang and Zhou, 2012). At the end of a collisional orogeny, break-off of the subducted lithospheric slab is thought to be a natural consequence of ocean closure (Davies and Blanckenburg, 1995). Mafic rocks or K-rich lamprophyres generally accompany the break-off 13

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of a subducted lithospheric slab (Davies and Blanckenburg, 1995; Atherton and Ghani, 2002). Slab break-off or delamination of thickened subcontinental lithosphere are likely to be followed by upwelling of asthenospheric mantle. Partial melts generated by decompression may underplate and trigger melting in the remaining previously metasomatized shallow lithospheric mantle and the overlying crust (Dewey, 1988; Draut et al., 2002). Although the ~760 Ma mafic rocks in northern Guangxi (Sanmenjie and Longsheng) show some geochemical affinities with back-arc signatures, in general they should be interpreted as the products of magmatism arising out of an intracontinental rift setting. Their chemistry reflects mixing between asthenospheric mantle melts and melts generated in the previously-metasomatized lithosphere, producing both mafic magmas with “OIB-like” characteristics at Gaoqiao, Qianyang and Guzhang (Fig. 11c) and the more subduction-fluid influenced mafic rocks in northern Guangxi (Sanmenjie and Longsheng).

•

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experienced varying degrees of crustal contamination, accompanied by complex processes of assimilation and fractional crystallization. The OIB-like trace element signatures in the Gaoqiao, Guzhang and Qianyang mafic suites do not require a mantle plume component; rather the isotopic data indicate mixing between depleted mantle and slab-enriched lithospheric mantle components. The arc-like geochemical and isotopic compositions of the Sanmenjie and Longsheng gabbros in northern Guangxi probably reflect a source dominated by SCLM previously metasomatized by slab-derived fluids. The tectonic history of the Jiangnan orogen reflects a transition from early Neoproterozoic divergent double-sided subduction beneath the Yangtze and Cathaysia blocks to a Mid-Neoproterozoic collisional stage and finally late Neoproterozoic post-orogenic magmatic activity. The SCB was probably not located in the central part of Rodinia; rather, it is more likely to have been adjacent to the supercontinent margin.

5.6. Implications for supercontinent Rodinia Declaration of Competing Interest The SCB is generally thought to have been part of supercontinent Rodinia, but its position within the supercontinent and the relation between the tectonic evolution of the SCB and the breakup of Rodinia have remained unclear. Hence, for example, it is uncertain whether the lack of plume-derived geochemical signatures in Neoproterozoic magmas from the western Jiangnan orogen should be seen as inconsistent with the growing consensus that the break-up of Rodinia was triggered by the impact of a mantle plume (Li et al., 2008). One viewpoint, based on stratigraphic and tectonic analyses of synchronous sedimentary records in South China, eastern Australia and western Laurentia, places the SCB at the center of Laurentia, the “missing link” between Australia-East Antarctica and Laurentia (Li et al., 1995). In this case, the plume that disrupted Rodinia should probably have caused distinctive magmatism throughout the SCB. On the other hand, a second viewpoint—based on the unique characteristics of Grenvillian detritus in the Yangtze Block (Dehler et al., 2010; Wang et al., 2010; Wang and Zhou, 2012) and probable correlations between Neoproterozoic igneous rocks in the northwestern Yangtze Block and contemporaneous rocks in Madagascar, Seychelles and northern India (Zhou et al., 2002) —places the Yangtze Block on the periphery of Rodinia. In this case one would not expect to find magmas in the SCB related to the plume that disrupted Rodinia. In fact, Neoproterozoic mafic rocks such as the 800 Ma Huangling mafic rocks in the northern Yangtze Block (Zhao et al., 2010), the 835–812 Ma mafic rocks in northern Guangxi (Zhou et al., 2004) commonly display typical arc-like geochemical affinities. These are similar to coeval mafic rocks in westcentral Madagascar and the Seychelles Islands (Ashwal, 2002) but distinct from contemporaneous mafic rocks in Australia and North America (Zhao et al., 2010). The new geochemical data from the somewhat younger Gaoqiao mafic rocks presented here show an intracontinental rift setting consistent with a post-orogenic phase and not a mantle plume. They therefore support the hypothesis that the Yangtze Block was probably adjacent to the margin of Rodinia (Zhao and Zhou, 2009) rather than being situated between East Antarctica-Australia and Laurentia as previous thought (Li et al., 1995).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We are grateful to Prof. Y.X Wei, H.J Xu, J.X Zhang and X.Z Cui for their valuable suggestions on this paper. The authors also wish to thank the Editor-in Chief Meifu Zhou and two anonymous reviewers for their helpful comments on the manuscript. This work was financially supported by the Chinese Geological Survey (grants 1212011085340 and 12120113061700) and the National Natural Science Foundation of China (grant 41230206). References Andersen, T., 2002. Correction of common lead in U-Pb analyses that do not report 204 Pb. Chem. Geol. 192 (1–2), 59–79. Ashwal, L.D., 2002. Petrogenesis of Neoproterozoic granitoids and related rocks from the Seychelles: the case for an Andean-type Arc origin. J. Petrol. 43 (1), 45–83. Atherton, M.P., Ghani, A.A., 2002. Slab breakoff: a model for Caledonian, Late Granite syn-collisional magmatism in the orthotectonic (metamorphic) zone of Scotland and Donegal, Ireland. Lithos 62 (3), 65–85. Barth, M.G., McDonough, W.F., Rudnick, R.L., 2000. Tracking the budget of Nb and Ta in the continental crust. Chem. Geol. 165 (3–4), 197–213. BGMRGX (Bureau of Geology and Mineral Resources of Guangxi province), 1985. Regional Geology of Guangxi Autonomous Region. Geological Publishing House, Beijing, pp. 1–853 (in Chinese with English abstract). BGMRHP (Bureau of Geology and Mineral Resources of Hunan Province), 1988. Regional Geology of Hunan Province. Geological Publishing House, Beijing. Bonin, B., 1990. From orogenic to anorogenic settings: evolution of granitoid suites after a major orogenesis. Geol. J. 25 (3–4), 261–270. Booden, M.A., Smith, I.E.M., Mauk, J.L., Black, P.M., 2010. Evolving volcanism at the tip of a propagating arc: the earliest high- Mg andesites in northern New Zealand. J. Volcanol. Geoth. Res. 195, 83–96. Cabanis, B., 1989. La diagramme La/10-Y/15-Nb/8: un outil pour la discrimination des series volcaniques et la mise en evidence des processus de melange et/ou de contamination crustale. Compt. Rend. Acad. Sci Ser. ii. 309, 2023–2029. Cai, J.J., Cui, X.Z., Lan, Z.W., Wang, J., Jiang, Z.F., Deng, Q., Zhou, J.W., Chen, F.L., Jiang, X.S., 2018. Onset time and global correlation of the Cryogenian glaciations in Yangtze Block, South China. J. Paleogeogr. 20 (1), 65–86. Cawood, P.A., Wang, Y., Xu, Y., Zhao, G., 2013. Locating South China in Rodinia and Gondwana: a fragment of greater India lithosphere? Geology 41 (8), 903–906. Chen, W.T., Sun, W., Wang, W., Zhao, J., Zhou, M., 2014. “Grenvillian” intra-plate mafic magmatism in the southwestern Yangtze Block, SW China. Precambr. Res. 242, 138–153. Chen, X., Wang, X.L., Wang, D., Shu, X.J., 2018. Contrasting mantle-crust melting processes within orogenic belts: implications from two episodes of mafic magmatism in the western segment of the Neoproterozoic Jiangnan Orogen in South China. Precambr. Res. 242, 123–137. Cui, X.Z., Jiang, X.S., Wang, J., Wang, X.C., Zhou, J.W., Deng, Q., Liao, S.Y., Wu, H., Jiang, Z.F., Wei, Y.N., 2015. Mid-Neoproterozoic diabase dykes from Xide in the western Yangtze Block, South China: new evidence for continental rifting related to the breakup of Rodinia supercontinent. Precambr. Res. 268, 339–356. Cui, X.Z., Jiang, X.S., Deng, Q., Wang, J., Zhou, J.W., Ren, G.M., Cai, J.J., Wu, H., Jiang,

6. Conclusion

• We report high-Ti tholeiitic basalts from the Gaoqiao area in wes-

•

tern Hunan. They were emplaced at 757 ± 16 Ma, synchronous with several other suites of mafic rocks in the western part of the Jiangnan orogen including the ~ 747 Ma Qianyang and ~ 768 Ma Guzhang diabases in Hunan, and the ~ 760 Ma Longsheng and Sanmenjie mafic dykes in Guangxi. All these Neoproterozoic western Jiangnan mafic suites have geochemical affinity with intraplate basalts that suggest a post-orogenic intracontinental rift setting. The Gaoqiao basalts may have 14

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