Middle Neoproterozoic (∼845 Ma) continental arc magmatism along the northwest side of the Jiangshan–Shaoxing suture, South China: Geochronology, geochemistry, petrogenesis and tectonic implications

Precambrian Research 268 (2015) 212–226

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Middle Neoproterozoic (∼845 Ma) continental arc magmatism along the northwest side of the Jiangshan–Shaoxing suture, South China: Geochronology, geochemistry, petrogenesis and tectonic implications Zheng Liu, Yao-Hui Jiang ∗ , Guo-Chang Wang, Chun-Yu Ni, Long Qing, Qiao Zhang State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China

a r t i c l e

i n f o

Article history: Received 2 April 2015 Received in revised form 18 July 2015 Accepted 21 July 2015 Available online 30 July 2015 Keywords: Gabbro Diorite Granite Continental arc Jiangnan Orogen South China

a b s t r a c t This paper presents the first detailed zircon U–Pb chronology, major and trace element, and Nd–Hf isotope geochemistry of two Neoproterozoic plutons (Shanhou and Jiangshan) along the northwest side of the Jiangshan–Shaoxing suture, Zhejiang province, South China. The Shanhou pluton consists of gabbros, dioritic rocks and monzogranites. SHRIMP and LA-ICP-MS zircon U–Pb dating indicates that the gabbros (∼846 Ma), dioritic rocks (∼845 Ma) and monzogranites (∼847 Ma) were emplaced simultaneously. The Jiangshan pluton consists mainly of dioritic rocks that were also emplaced in the middle Neoproterozoic with a SHRIMP zircon U–Pb age of ca. 842 Ma. The Shanhou gabbros are mainly high-K calc-alkaline, and are enriched in LREE and LILE and depleted in HFSE with marked negative Ta–Nb anomalies. They have positive εNd (T) (2.0–3.5) and εHf (T) (in-situ zircon) (3.3). The Shanhou and Jiangshan dioritic rocks as well as the Shanhou granites are also high-K calc-alkaline, enriched in LREE and LILE and depleted in HFSE with marked negative Ta–Nb anomalies. They also show positive εNd (T) (0.1–4.7) and εHf (T) (insitu zircon) (0.8–5.0). Detailed elemental and isotopic data suggest that both plutons were formed in a continental arc setting with gabbros derived from partial melting of subduction-modified mantle wedge. Such gabbroic magmas underwent fractionation crystallization of clinopyroxene + amphibole, forming the less felsic dioritic rocks. Progressive fractionation crystallization of amphibole + plagioclase from the evolved melts with a little crustal assimilation produced the more felsic dioritic rocks and granites. Our new data suggest that final amalgamation between the Yangtze and Cathaysia blocks postdated 842 Ma. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The Rodinia supercontinent is generally considered to have assembled at ca. 1.3–0.9 Ga through the worldwide Grevillian orogenic events and probably rifted apart at ca. 750 Ma (e.g. Li et al., 1999, 2008a,b,c; Li, 1999; Piper, 2000; Zhou et al., 2002; Torsvik, 2003; Ernst et al., 2007; Maruyama et al., 2007). Reconstruction of the Rodinia supercontinent is a topic of international interest. It is widely accepted that the Jiangshan–Shaoxing suture and the Jiangnan Orogen resulted from the Neoproterozoic collision between the Yangtze and Cathaysia blocks. Studies on the evolution of the Jiangnan Orogen can give some significant implications for the reconstruction of the Rodinia supercontinent. The eastern segment of the Jiangnan Orogen is crucial to better understand the orogenic cycle because it contains widespread igneous plu-

∗ Corresponding author. Tel.: +86 25 89686584. E-mail address: [email protected] (Y.-H. Jiang). http://dx.doi.org/10.1016/j.precamres.2015.07.013 0301-9268/© 2015 Elsevier B.V. All rights reserved.

tons, volcanic-sedimentary sequences and two major ophiolite belts (Fig. 1a) that witness the final amalgamation between the Yangtze and Cathaysia blocks. In the earlier studies, the lower limit of the timing of the collision was constrained by the Sm–Nd mineral isochron ages (1024 ± 30 Ma, Zhou et al., 1989; 935 ± 10 Ma, Chen et al., 1991) of the gabbros within the Fuchuan ophiolites in southern Anhui province ( in Fig. 1a) and by the SHRIMP zircon U–Pb ages (∼968 Ma, Li et al., 1994) of the Xiwan plagiogranites within the NE Jiangxi ophiolites ( in Fig. 1a). Recently, Li and co-authors (Li et al., 2008a,b, 2009, 2010a,b; Ye et al., 2007) have investigated the volcanic rocks of the Shuangxiwu Group in northern Zhejiang province ( in Fig. 1a), the Taohong and Xiqiu tonalite and granodiorite stocks ( in Fig. 1a) and the Shenwu dolerites ( in Fig. 1a) that intruded into the Shuangxiwu Group, the biotite granitic lens within the NE Jiangxi ophiolite and the volcanic rocks of the Zhenzhushan Group ( in Fig. 1a) as well as the Gangbian alkaline complex ( in Fig. 1a) in Jiangxi province. On the basis of above investigation, they suggested that the southeastern margin of the Yangtze Block was an active continental margin

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during the ages of ca. 970–890 Ma coupled with the northwestward subduction of the ancient ocean between the Yangtze and Cathaysia blocks. They also concluded that the tectonic transition from plate convergence between the Yangtze and Cathaysia blocks to intracontinental rifting likely occurred during the period of 890–850 Ma, and that the final amalgamation between the Yangtze and Cathaysia blocks likely took place at or soon after ca. 880 Ma. Charvet (2013) also advocate the early Neoproterozoic amalgamation of the Yangtze Block with the Cathaysia Block. He proposed

213

that the southeastern margin of the Yangtze Block underwent the northwestward subduction of the South China Ocean between the Yangtze and Cathaysia blocks during the period of 1–0.9 Ga, with the subsequent collision between two blocks at ca. 870–860 Ma. However, other researchers proposed that the Neoproterozoic arc magmatism along the southeastern Yangtze Block was still underway after ca. 880 Ma or even lasted until ca. 800 Ma on the basis of new data concerning magmatic rocks and sedimentary strata (e.g. Zhou et al., 2004, 2009; Zheng et al., 2008,

Fig. 1. (a) Distribution of Precambrian units in the eastern segment of the Jiangnan Orogen (modified after Wang et al., 2013). ca. 827–819 Ma Fuchuan ophiolites (Zhang et al., 2012a,b, 2013a,b); NE Jiangxi ophiolites; ca. 970–890 Ma Shuangxiwu volcanic rocks (Li et al., 2009); ca. 913–905 Ma Taohong and Xiqiu tonalite and granodiorite stocks (Ye et al., 2007); ca. 849 Ma Shenwu dolerites (Li et al., 2008b); ca. 849 Ma Zhenzhushan volcanic rocks (Li et al., 2010a); ca. 848 Ma Gangbian alkaline complex (Li et al., 2010b); ca. 841 Ma Lipu diorite (Li et al., 2010c); (b) Geological map of the Shanhou pluton; (c) Geological map of the Jiangshan pluton.

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2012a; Wang et al., 2006, 2007, 2008, 2012; Li et al., 2013; Zhao et al., 2011, 2013a,b; Yao et al., 2013, 2014; Xu et al., 2014). For instance, recent new geochronological and geochemical data of the Fuchuan ophiolite suite indicate that the ophiolites formed during the ages of 827–819 Ma and exhibit typical features of suprasubduction zone (SSZ) type ophiolites (Ding et al., 2008; Zhang et al., 2012a,b, 2013a,b), which argues against the early Neoproterozoic (∼880–860 Ma) amalgamation. In addition, recent investigation on the sedimentary sequences of the Shuangqiaoshan and Xikou Groups suggest that they might have deposited in a back-arc basin at ca. 833–817 Ma (Wang et al., 2013), rendering it impossible that the final amalgamation of the Yangtze Block with the Cathaysia Block took place during the early Neoproterozoic. In this paper, we report the first detailed SHRIMP and LA-ICP-MS zircon U–Pb dating, major and trace elemental and Nd–Hf isotopic data of two gabbro-diorite-granite plutons along the northwest side of the Jiangshan–Shaoxing suture, Zhejiang province (Fig. 1). These new data allow us to explore the origin of the mafic and felsic rocks and their relationship to the evolution of the Jiangnan Orogen and to the final amalgamation of the Yangtze Block with the Cathaysia Block. 2. Geological background and geology of the plutons 2.1. Geological background The South China Block consists of the Yangtze and Cathaysia blocks, with the Jiangnan Orogen being the southeastern part of the Yangtze Block (Fig. 1a). The Yangtze Block has an Archean–Paleoproterozoic crystalline basement surrounded by late Mesoproterozoic to early Neoproterozoic folded metamorphic rocks, which are unconformably overlain by middle to late Neoproterozoic weakly metamorphosed to unmetamorphosed covers (e.g., Zhao and Cawood, 2012). In the eastern Jiangnan Orogen, the folded metamorphic rocks include the Tianli schist (Li et al., 2007), Shuangxiwu volcanic-sedimentary sequences (Ye et al., 2007; Li et al., 2009), Zhenzhushan volcanic-sedimentary sequences (Li et al., 2010a) and Shuangqiaoshan-Xikou sedimentary sequences (Gao et al., 2008; Zhao et al., 2011), which are overlain unconformably by a post-orogenic extensional basin deposits as the Nanhua Sequence (Fig. 1a). A series of Neoproterozoic granitic plutons intruded into the folded metamorphic rocks, including the Jiuling pluton in NW Jiangxi province (Li et al., 2003; Zhong et al., 2005) and the Xucun, Shexian and Xiuning plutons in southern Anhui province (Li et al., 2003; Wu et al., 2006) (Fig. 1a). Two Neoproterozoic gabbroic–granitic plutons (Shanhou and Jiangshan) have been identified in Zhejiang province along the northwest side of the Jiangshan–Shaoxing suture (Fig. 1), as we will show in this paper. The Neoproterozoic ophiolite complexes, occurring as a NE-trending belt (Fig. 1a), are in tectonic contact with the Shuangqiaoshan and Xikou groups (Bai et al., 1986). 2.2. Geology of the plutons and sampling The Shanhou pluton, located in the north of the Jinhua city (Fig. 1a), intruded into the Neoproterozoic volcanic-sedimentary sequences at ca. 845–847 Ma (see below). It was unconformably overlain by the Carboniferous-Permain or Jurassic strata in the north and is in fault contact with the Cretaceous red strata in the south (Fig. 1b). This pluton is composed of gabbros, dioritic rocks and monzogranites (Fig. 2) and was intruded by a series of doleritic dykes (Figs. 1b and 3a). The gabbros are exposed in the south part of the pluton (Fig. 1b) and have a massive structure (Fig. 3b). They consist of plagioclase, amphibole and clinopyroxene with a fine to medium-grained gabbroic texture (Fig. 3c). The dioritic rocks,

Fig. 2. Q’-ANOR (a) (Streckeisen and Le Maitre, 1979) and Zr/TiO2 vs. SiO2 (b) (Winchester and Floyd, 1977) classification diagrams for the Shanhou and Jiangshan plutons. In (b), the dashed line separates calc-alkaline and alkaline compositions.

exposed in the center of the pluton (Fig. 1b), include quartz diorite, quartz monzodiorite and quartz monzonite (Fig. 2). These rocks consist of plagioclase, K-feldspar, quartz and amphibole with a medium-grained hypidiomophic-granular texture (Fig. 3d) and a massive structure (Fig. 3a). The monzogranite is exposed in the north part of the pluton (Fig. 1b) and consists of K-feldspar, plagioclase, quartz, biotite and amphibole with a medium-grained granitic texture (Fig. 3e) and a massive structure. The Jiangshan pluton, located in the southeast of the Jiangshan county (Fig. 1a and c), intruded into the Neoproterozoic volcanic-sedimentary sequences at ca. 842 Ma (see below). It was unconformably overlain by the Carboniferous-Permain and Jurassic strata or Quaternary sediments in the north (Fig. 1c). This pluton is composed of quartz monzodiorite and quartz monzonite (Fig. 2). These rocks consist of plagioclase, K-feldspar, quartz, amphibole and biotite with a medium-grained hypidiomophic-granular texture (Fig. 3f) and a massive structure. In this study, we have collected thirteen samples from surface exposures of the Shanhou and Jiangshan plutons, including three gabbroic, eight dioritic and two granitic rocks, and sample locations are shown in Fig. 1b and c. All samples were crushed to 200-mesh using an agate mill for whole-rock geochemical analysis. Four samples, including gabbroic (SH-7-2), dioritic (SH-7-1) and granitic (SH-5-1) rocks from the Shanhou pluton and a dioritic rock (JS-1-1) from the Jiangshan pluton, were selected for SHRIMP and LA-ICP-MS zircon U–Pb dating and in-situ zircon Hf isotope

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Fig. 3. Photographs of the Shanhou dioritic rock (a) and gabbro (b) and photomicrographs (under crossed polars) of the Shanhou gabbro (c), dioritic rock (d) and monzogranite (e) and the Jiangshan dioritic rock (f). Cpx, clinopyroxene; Am, amphibole; Bt, biotite; Kfs, K-feldspar; Pl, plagioclase; Qtz, quartz.

analysis. Zircons grains were separated using magnetic and heavy liquid separation methods, and then handpicked under a binocular microscope. Zircon grains were mounted in epoxy, and then polished for subsequent analyses. 3. Analytical methods Whole-rock chemistry for major elements was determined by XRF on fused glass beads at the State Key Laboratory for Mineral Deposits Research, Nanjing University (SKLMDR, NU) with precision better than 5%. Trace elements were determined with a Finnigan Element II ICP-MS at SKLMDR, NU. The precision is generally better than 10% for all trace elements, with the majority better than 5%. Detailed analytical procedures for trace elements analysis were described by Gao et al. (2003). For Nd isotope analyses, about 100 mg of powder was dissolved in Teflon beakers with a HF + HNO3 mixture acid, and Nd was then separated and

purified by conventional cation-exchange technique. The isotopic compositions of purified Nd solutions were measured using a Finnigan Triton TI thermal ionization mass spectrometer (TIMS) at SKLMDR, NU. Detailed separation and analytical procedures are described by Pu et al. (2004, 2005). 143 Nd/144 Nd ratios are reported as measured, and normalized to 146 Nd/144 Nd of 0.7219. During the course of analysis, measurements for the La Jolla standard yielded a result of 143 Nd/144 Nd ratio of 0.511842 ± 4 (2, n = 5), which is within errors consistent with published value of 0.511857 ± 6 (Thirlwall, 1991). Total analytical blanks were 5 × 10−11 g for Sm and Nd. Zircon in-situ U–Th–Pb isotope analyses were conducted using a SHRIMP-II at the Beijing SHRIMP Center for the sample collected from the Jiangshan pluton and the granite sample collected from the Shanhou pluton. The standard TEM zircons (417 Ma) were used for inter-element fractionation, and U, Th and Pb concentrations were determined based on the standard Sri Lankan gem zircon

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SL13 (572 Ma). More detailed analytical procedures are described by Song et al. (2002). Zircon in-situ U–Th–Pb isotope analyses for the gabbro and dioritic rock collected from the Shanhou pluton were conducted using an Agilent 7500a ICP-MS equipped with a New Wave 213 nm laser sampler at SKLMDR, NU. Detailed analytical procedures are described by Xu et al. (2009). Common Pb contents were evaluated following Andersen (2002), and the 204 Pb-based method of common Pb correction was applied. In situ Hf isotope analyses were conducted on zircons that have determined U–Pb ages using a New Wave ArF 193 nm laser ablation system attached to a Neptune multi-collector ICP-MS (plus) at SKLMDR, NU. Analyses were performed predominantly with a beam size of 35 ␮m and a repetition rate of 5 Hz. Typical ablation time was about 30 s for 200 cycles. Reference zircon Mud Tank (176 Hf/177 Hf = 0.282507 ± 6; Woodhead et al., 2004) and standard zircon 91500 (Wiedenbeck et al., 1995) was used to monitor performing conditions and analytical accuracy. Detailed analytical conditions and procedures refer to Griffin et al. (2000). For the calculation of εHf values we used the chondritic values recommended by Bouvier et al. (2008).

4. Results 4.1. Zircon U–Pb dating SHRIMP and LA-ICP-MS zircon U–Pb dating results are summarized in Table 1 and shown in Fig. 4. All the zircons show regular oscillatory magmatic zoning with the size mostly between 100 and 200 ␮m (Fig. 4) and have high Th/U ratios (>0.6) (Table 1). Sixteen LA-ICP-MS U–Pb analyses for sample SH-7-2 plot in a group on the Concordia curve and yield a weighted mean 206 Pb/238 U age of 846 ± 7 Ma (MSWD = 0.42) (Fig. 4a). Twenty LA-ICP-MS U–Pb analyses for sample SH-7-1 plot in a group on the Concordia curve and yield a weighted mean 206 Pb/238 U age of 845 ± 6 Ma (MSWD = 1.06) (Fig. 4b). Nine SHRIMP U–Pb analyses for sample SH-5-1 plot in a group on the Concordia curve and yield a weighted mean 206 Pb/238 U age of 847 ± 9 Ma (MSWD = 2.4) (Fig. 4c). Ten SHRIMP U–Pb analyses for sample JS-1-1 plot in a group on the Concordia curve and yield a weighted mean 206 Pb/238 U age of 842 ± 5 Ma (MSWD = 0.74) (Fig. 4d). These ages are the best estimation of magma crystallization.

Fig. 4. LA-ICP-MS (a, b) and SHRIMP (c, d) zircon U–Pb concordant curves for the Shanhou (a–c) and Jiangshan (d) plutons. The inset shows typical cathodoluminescence (CL) images of selected zircons.

Table 1 Zircon U–Pb dating and zircon in-situ Hf isotope analysis for the Shanhou and Jiangshan plutons. Spot

U (ppm)

Th (ppm)

Th/U

207 Pb/206 Pb

207 Pb*/235 U

±%

206 Pb*/238 U

±%

206 Pb/238 U(age/Ma)

176 Lu/177 Hf

176 Hf/177 Hf

±2

εHf (T)

0.3 0.3 0.3 0.2 0.3 0.3 0.4 0.3 0.3 0.3 0.2 0.2 0.3 0.3 0.4 0.3

1.3028 1.3084 1.2988 1.3409 1.3111 1.2238 1.2427 1.2725 1.2499 1.2918 1.2552 1.3143 1.3003 1.2916 1.3029 1.2780

5.6 5.6 4.9 4.4 6.3 5.4 7.4 5.2 6.2 6.2 4.6 4.4 5.5 5.7 6.7 6.0

0.1397 0.1404 0.1412 0.1428 0.1409 0.1394 0.1407 0.1368 0.1401 0.1414 0.1372 0.1401 0.1421 0.1400 0.1398 0.1422

0.2 0.3 0.2 0.2 0.3 0.3 0.3 0.2 0.3 0.3 0.2 0.2 0.2 0.2 0.3 0.3

843.0 847.0 851.0 860.0 850.0 841.0 849.0 827.0 845.0 853.0 829.0 845.0 857.0 844.0 843.0 857.0

7.0 7.0 6.5 6.5 8.0 7.0 8.0 7.0 7.5 7.5 7.0 7.0 7.0 7.0 7.5 8.0

0.002586 0.004661 0.001409 0.002338 0.001784 0.003035 0.002613 0.002379 0.002203 0.001444 0.000939 0.001858 0.002604 0.002980 0.003226 0.002425

0.282321 0.282348 0.282347 0.282367 0.282372 0.282317 0.282450 0.282395 0.282384 0.282326 0.282393 0.282445 0.282373 0.282493 0.282376 0.282380

0.000042 0.000043 0.000029 0.000020 0.000020 0.000023 0.000027 0.000021 0.000020 0.000022 0.000022 0.000020 0.000028 0.000028 0.000036 0.000024

1.0 0.8 2.7 3.1 3.4 0.5 5.7 3.4 3.4 2.0 4.2 5.8 3.1 6.9 2.6 3.5

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.4 0.2 0.3 0.3 0.2 0.2 0.2 0.2 0.3 0.3 0.3

1.2429 1.2788 1.2439 1.2884 1.2902 1.2616 1.3090 1.2347 1.3017 1.2860 1.3006 1.2988 1.2636 1.2770 1.3053 1.3230 1.2860 1.2584 1.3283 1.3493

4.5 3.6 3.4 4.0 3.4 3.7 4.4 3.9 4.0 6.7 3.2 5.5 5.2 3.6 4.8 3.8 4.4 5.0 5.3 6.4

0.1389 0.1403 0.1381 0.1404 0.1399 0.1358 0.1435 0.1372 0.1421 0.1398 0.1396 0.1388 0.1391 0.1370 0.1417 0.1440 0.1394 0.1431 0.1440 0.1441

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.2 0.3 0.2 0.2 0.3 0.2 0.3 0.3 0.3 0.3

838.0 839.0 840.0 841.0 842.0 843.0 844.0 845.0 846.0 847.0 848.0 849.0 850.0 851.0 852.0 853.0 854.0 855.0 856.0 857.0

6.5 6.5 6.0 6.5 6.0 6.0 7.0 6.5 6.5 8.0 6.0 8.0 7.0 6.0 7.5 6.5 7.0 7.5 8.0 8.5

0.002393 0.001846 0.001516 0.001418 0.001319 0.003155 0.002838 0.002833 0.001904 0.002672 0.002295 0.002344 0.001788 0.003323 0.002603 0.001879 0.002616 0.002685 0.001949 –

0.282384 0.282441 0.282395 0.282412 0.282464 0.282456 0.282456 0.282547 0.282455 0.282444 0.282496 0.282337 0.282424 0.282454 0.282359 0.282453 0.282390 0.282419 0.282326 –

0.000026 0.000017 0.000022 0.000023 0.000022 0.000028 0.000025 0.000028 0.000022 0.000028 0.000026 0.000030 0.000025 0.000032 0.000024 0.000021 0.000024 0.000027 0.000027 –

3.2 5.5 4.1 4.8 6.7 5.4 5.6 8.9 6.2 5.4 7.4 1.8 5.2 5.4 2.5 6.3 3.6 4.6 1.8 –

1.0 0.9 0.9 1.9 1.7 0.9 0.8 1.3 1.2

1.3099 1.3233 1.3159 1.2637 1.3580 1.2563 1.2715 1.3015 1.3111

1.4 1.3 1.3 2.2 2.1 1.4 1.2 1.6 1.5

0.14166 0.14237 0.14307 0.13816 0.14214 0.13683 0.13817 0.14086 0.14045

1.0 0.9 0.9 1.0 1.2 1.2 0.9 0.9 1.0

854.0 858.1 862.0 834.2 856.8 826.7 834.3 849.5 847.2

7.7 7.4 7.5 8.0 9.6 8.9 7.1 7.3 8.0

0.000832 0.002203 0.002798 0.002431 0.002068 0.002084 0.001866 0.001248 0.001546

0.282398 0.282363 0.282346 0.282367 0.282333 0.282400 0.282361 0.282380 0.282394

0.000040 0.000045 0.000039 0.000039 0.000045 0.000045 0.000037 0.000035 0.000042

4.9 3.0 2.1 2.5 2.0 3.7 2.6 3.9 4.2

4.2 2.8 1.0 1.3 0.9 1.0 0.9 1.4 3.2 1.6

1.2967 1.3402 1.2913 1.2705 1.2969 1.2620 1.2950 1.2892 1.2880 1.2755

4.3 3.1 1.3 1.6 1.3 1.4 1.3 1.6 3.4 1.9

0.13800 0.14122 0.13970 0.13991 0.13930 0.13827 0.14007 0.13849 0.14213 0.13992

0.9 1.4 0.9 1.0 0.9 0.9 0.9 0.9 1.2 1.1

833.4 851.6 843.0 844.2 840.7 834.9 845.1 836.1 856.7 844.2

7.2 10.8 7.4 7.8 7.3 7.4 7.3 7.0 9.4 8.4

0.002860 0.000754 0.001417 0.002679 0.003576 0.002140 0.002256 0.003167 0.001520 –

0.282349 0.282315 0.282405 0.282182 0.282218 0.282385 0.282159 0.282553 0.282266 –

0.000046 0.000044 0.000046 0.000055 0.000061 0.000049 0.000054 0.000112 0.000055 –

1.6 2.0 4.6 −4.0 −3.3 3.3 −4.5 8.7 −0.1 –

±1

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Sample SH-7-2 (gabbro, Shanhou pluton), LA-ICP-MS U–Pb SH-7-2-1 263 166 1.59 0.0676 SH-7-2-2 691 321 2.15 0.0676 SH-7-2-3 513 321 1.60 0.0667 SH-7-2-5 508 347 1.46 0.0681 SH-7-2-7 359 389 0.92 0.0676 SH-7-2-8 402 219 1.84 0.0637 SH-7-2-10 216 106 2.04 0.0641 SH-7-2-11 278 165 1.69 0.0675 SH-7-2-12 400 173 2.31 0.0647 SH-7-2-13 416 305 1.36 0.0663 SH-7-2-14 318 265 1.20 0.0664 SH-7-2-16 510 399 1.28 0.0681 SH-7-2-18 201 122 1.64 0.0664 SH-7-2-20 387 189 2.05 0.0670 SH-7-2-24 220 119 1.84 0.0677 SH-7-2-25 518 243 2.13 0.0652 Sample SH-7-1 (Quartz monzonite, Shanhou pluton), LA-ICP-MS U–Pb SH-7-1-2 240 135 1.78 0.0649 SH-7-1-3 673 421 1.60 0.0661 SH-7-1-4 325 315 1.03 0.0654 SH-7-1-5 363 313 1.16 0.0665 SH-7-1-6 573 373 1.54 0.0669 SH-7-1-8 1305 384 3.40 0.0674 SH-7-1-9 527 317 1.66 0.0661 SH-7-1-10 319 287 1.11 0.0652 SH-7-1-11 343 253 1.35 0.0664 SH-7-1-12 531 234 2.27 0.0666 SH-7-1-13 905 322 2.81 0.0676 SH-7-1-14 367 219 1.68 0.0680 SH-7-1-15 141 98 1.43 0.0660 SH-7-1-18 1046 457 2.29 0.0677 SH-7-1-19 884 697 1.27 0.0670 SH-7-1-21 405 341 1.19 0.0667 SH-7-1-22 787 435 1.81 0.0670 SH-7-1-23 409 234 1.75 0.0638 SH-7-1-24 262 223 1.17 0.0669 SH-7-1-25 249 297 0.84 0.0680 Sample SH-5-1 (Monzogranite, Shanhou pluton), SHRIMP U–Pb SH-5-1-1.1 279 210 0.75 0.0671 SH-5-1-2.1 400 244 0.61 0.0674 SH-5-1-3.1 377 451 1.19 0.0667 SH-5-1-4.1 200 251 1.26 0.0663 SH-5-1-6.1 93 100 1.07 0.0693 SH-5-1-7.1 519 457 0.88 0.0666 SH-5-1-9.1 519 431 0.83 0.0667 SH-5-1-11.1 399 296 0.74 0.0670 SH-5-1-12.1 201 175 0.87 0.0677 Sample JS-1-1 (Quartz monzonite, Jiangshan pluton), SHRIMP U–Pb JS-1-1-2.1 666 1005 1.51 0.0681 JS-1-1-3.1 71 50 0.70 0.0688 JS-1-1-5.1 371 297 0.80 0.0670 JS-1-1-6.1 258 188 0.73 0.0659 JS-1-1-7.1 414 939 2.27 0.0675 JS-1-1-8.1 344 343 1.00 0.0662 JS-1-1-12.1 588 1548 2.69 0.0671 JS-1-1-13.1 643 1764 2.74 0.0675 JS-1-1-14.1 106 127 1.20 0.0657 JS-1-1-15.1 151 236 1.56 0.0661

±%

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(183–402 ppm) and V (227–386 ppm) (Fig. 6). They are all enriched in light rare earth elements (LREE) and do not show negative Eu anomalies (Fig. 7a). They are also enriched in LILE and depleted in HFSE with marked negative Ta–Nb anomalies (Fig. 7b). The Shanhou gabbros have positive εNd (T) values (2.0–3.5) (Table 2). The Shanhou dioritic rocks have SiO2 contents of 57.0–66.9 wt.% (Table 2) and are weakly peraluminous except for one sample that is metaluminous (Fig. 5a). They are also calc-alkaline to high-K calc-alkaline (Fig. 5b). These rocks are all enriched in LREE and do not show marked negative Eu anomalies (Fig. 7c). They are also enriched in LILE and depleted in HFSE with marked negative Ta–Nb anomalies (Fig. 7c). The Shanhou dioritic rocks also have positive εNd (T) values (2.3–4.7) (Table 2), similar to the Shanhou gabbros. The Shanhou granites have SiO2 contents of 66.8–67.8 wt.% (Table 2). They are metaluminous (Fig. 5a) and high-K calc-alkaline (Fig. 5b). The rocks are enriched in LREE and show moderate negative Eu anomalies (Fig. 7e). They are enriched in LILE and depleted in HFSE with marked negative Ta–Nb anomalies (Fig. 7f). The Shanhou granites also have positive εNd (T) values (0.1–1.2) (Table 2), but slightly lower than the Shanhou gabbros and dioritic rocks. From gabbros to dioritic rocks and to granites, with increasing SiO2 , there is a general decrease in Mg#, TiO2 , Fe2 O3 , MgO, CaO, V and Cr, and increase in K2 O, Y and Yb, whereas ASI, Al2 O3 , Na2 O and Sr first show an increase trend and then a decrease trend (Figs. 5 and 6). 4.2.2. The Jiangshan pluton The samples from the Jiangshan pluton generally show similar geochemical characteristics to the Shanhou dioritic rocks (Table 2; Figs. 5–7). They have SiO2 contents of 62.5–64.1 wt.%. The rocks are metaluminous (Fig. 5a) and high-K calc-alkaline (Fig. 5b). They are enriched in LREE and do not show marked negative Eu anomalies (Fig. 7g). They are also enriched in LILE and depleted in HFSE with marked negative Ta–Nb anomalies (Fig. 7h). These rocks also have positive εNd (T) values (0.5–2.8) (Table 2). 4.3. Hf isotopes in zircon

Fig. 5. SiO2 vs. ASI (a), K2 O (b) and Mg# (c) for the Shanhou and Jiangshan plutons. In (a), also shown are the trends of the fractionation crystallization of clinopyroxene (cpx) + amphibole (amp) and of amphibole (amp) + plagioclase (pl). In (c), also shown are the fields of pure crustal partial melts obtained in experimental studies by dehydration melting of low-K basaltic rocks at 8–16 kbar and 1000–1050 ◦ C (Rapp and Watson, 1995), of moderately hydrous (1.7–2.3 wt.% H2 O) medium- to high-K basaltic rocks at 7 kbar and 825–950 ◦ C (Sisson et al., 2005) and of pelitic rocks at ˇ Douce and Johnston, 1991). 7–13 kbar and 825–950 ◦ C (Patino

In-situ Hf isotope analyses have been carried out on zircons at the same spots used for the SHRIMP and LA-ICP-MS zircon U–Pb dating except for two zircon grains that were too thin for analysis. The results are given in Table 1 and illustrated in Fig. 8a. A weighted mean zircon Hf isotopic composition for each sample is presented in Table 2. The Shanhou gabbro (SH-7-2) has positive initial ␧Hf (age corrected using U–Pb age for individual grain) values, ranging from 0.5 to 6.9 with a weighted mean of 3.3. The Shanhou dioritic rock (SH-7-1) and granite (SH-5-1) also have positive initial εHf , ranging from 1.8 to 8.9 (mean 5.0) and from 2.0 to 4.9 (mean 3.2), respectively. The Jianshan pluton (JS-1-1) has relatively variable initial εHf , ranging from −4.5 to 8.7, with a weighted mean of 0.8. All the zircon εHf (T) values are coupled with their whole-rock εNd (T), and plot along the mantle array (εHf = 1.33εNd + 3.19; Vervoort et al., 1999) (Fig. 8a). 5. Discussion

4.2. Whole-rock geochemistry

5.1. The ages of the Shanhou and Jiangshan plutons

4.2.1. The Shanhou pluton The gabbro samples have SiO2 contents of 45.3–53.5 wt.% (Table 2) and are metaluminous with alumina saturation index ASI [= molar Al2 O3 /(CaO + Na2 O + K2 O)] from 0.66 to 0.82 (Fig. 5a). They are calc-alkaline to high-K calc-alkaline (Fig. 5b). The rocks have relatively high MgO (5.6–8.7 wt.%) and Mg# [= atomic Mg/(Mg + FeT )] (0.59–0.66) (Fig. 5c) as well as compatible elements such as Cr

It is generally considered that the Shanhou and Jiangshan plutons were emplaced during the Triassic (ZJBGMR, 1989). However, the ca. 847–842 Ma ages obtained in this study indicate that they were emplaced in the middle Neoproterozoic. In addition, the ages of the gabbro, dioritic rock and granite of the Shanhou pluton are consistent within uncertainties, suggesting that they were formed simultaneously.

Table 2 Major (wt.%) and trace element (ppm) and Nd-Hf isotope compositions of the Shanhou and Jiangshan plutons. Shanhou

Sample Rock type

SH-11 Gabbro

SH-7-2 Gabbro

SH-8-1 Gabbro

SH-3 QD

SH-7-1 QM

SH-9 QMD

SH-10-1 QMD

SH-4-1 MG

SH-5-1 MG

Jiangshan

SiO2 TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 LOI Total ASI Mg# V Cr Co Ni Ga Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U 147 Sm/144 Nd 143 Nd/144 Nd ±2␴ ␧Nd (T) ␧Hf (T)

51.97 0.51 17.43 7.25 0.17 7.04 7.20 3.67 1.98 0.07 2.27 99.57 0.82 0.66 227 315.6 37.7 67.6 17 41 588 10 60 3.8 465 11 22 2.72 10.5 2.37 0.75 2.18 0.28 1.91 0.41 1.15 0.17 1.01 0.15 1.65 0.27 4.36 2.01 0.44 0.1367 0.512407 8 2.0

48.01 1.02 14.29 11.60 0.19 8.69 8.55 2.67 1.52 0.33 2.39 99.25 0.66 0.60 386 402.3 42.2 81.5 21 56 661 15 73 2.9 335 19 40 5.00 20.2 4.29 1.37 3.95 0.47 3.07 0.59 1.62 0.22 1.19 0.20 1.80 0.18 4.80 2.11 0.83 0.1283 0.51236 7 2.0 3.3

53.49 0.97 15.59 7.89 0.15 5.63 6.18 5.35 1.50 0.26 2.69 99.7 0.72 0.59 235 182.9 32.7 80.3 19 50 533 15 117 5.4 325 15 32 4.09 16.0 3.30 1.06 3.19 0.42 2.89 0.59 1.63 0.23 1.41 0.23 2.74 0.33 3.93 1.62 0.45 0.1245 0.512419 13 3.5

63.05 0.63 16.58 4.42 0.07 2.17 3.32 5.28 1.46 0.17 2.88 100.02 1.02 0.50 111 38.0 12.7 16.9 20 21 714 9 161 5.7 560 13 24 3.07 11.2 2.06 0.78 1.96 0.26 1.59 0.34 0.95 0.14 0.81 0.14 3.51 0.45 4.14 2.33 0.80 0.1107 0.512404 15 4.7

66.92 0.50 15.46 3.39 0.04 1.00 2.70 5.21 3.23 0.16 1.13 99.74 0.91 0.37 59 9.7 8.2 2.6 18 54 413 17 227 12.4 862 33 60 7.18 24.4 3.84 1.03 3.44 0.47 3.06 0.67 2.00 0.28 1.87 0.32 5.14 0.84 7.05 7.48 2.07 0.0953 0.512196 17 2.3 5.0

57.90 0.72 17.92 6.68 0.15 2.49 3.71 4.94 2.37 0.36 2.49 99.72 1.03 0.43 133 5.5 12.3 22 43 769 19 173 6.5 828 23 40 6.07 23.8 4.65 1.20 4.13 0.52 3.57 0.76 2.01 0.30 1.84 0.28 3.85 0.37 4.23 2.88 0.74 0.1182 0.512399 13 3.8

57.03 0.73 18.62 7.35 0.18 2.89 2.79 6.13 1.60 0.32 2.20 99.83 1.10 0.44 134 4.0 9.6 24 33 790 17 156 5.5 561 20 35 4.82 19.8 3.86 1.23 3.58 0.44 3.13 0.66 1.90 0.27 1.56 0.26 3.53 0.33 5.20 2.26 0.74 0.1175 0.512404 8 4.0

66.79 0.44 14.45 2.79 0.07 1.01 2.80 4.64 3.88 0.15 2.17 99.19 0.85 0.42 51 6.7 3.1 18 62 196 17 183 12.9 657 37 62 6.61 21.8 3.38 0.80 3.04 0.43 2.97 0.69 1.96 0.31 1.83 0.28 4.17 0.98 11.34 7.89 2.13 0.0935 0.512125 8 1.2

67.76 63.28 0.53 0.65 14.90 16.92 3.32 4.24 0.09 0.10 1.09 1.40 2.64 3.52 5.05 4.40 3.21 3.95 0.17 0.20 1.78 1.29 100.54 99.94 0.90 0.94 0.40 0.40 60 95 19.8 5.0 7.0 8.6 1.2 19 21 60 98 291 693 22 18 239 274 13.5 12.9 607 1069 35 38 64 68 7.16 7.77 23.9 27.0 4.43 4.46 1.03 1.27 3.96 3.62 0.53 0.47 3.86 3.25 0.83 0.67 2.43 1.89 0.35 0.29 2.34 1.76 0.40 0.30 5.52 5.78 1.15 0.93 5.06 18.10 10.69 10.12 2.60 2.39 0.1118 0.1000 0.512172 0.512130 8 12 0.1 0.5 3.2 0.8

JS-1-1 QM

JS-3 QMD

JS-4 QMD

JS-5 QMD

62.49 0.78 15.22 4.90 0.14 1.66 3.72 4.48 2.79 0.20 2.91 99.30 0.89 0.40 111 11.5 11.3 2.1 19 69 373 22 217 11.1 784 28 55 5.93 22.9 3.96 1.04 3.93 0.57 3.67 0.81 2.41 0.35 2.13 0.34 4.63 0.87 11.73 10.52 1.97 0.1046 0.512192 10 1.2

63.90 0.74 15.66 4.70 0.08 2.04 3.32 4.58 2.72 0.16 2.30 100.20 0.95 0.46 112 11.2 11.8 5.1 21 62 326 16 207 7.3 703 19 33 4.46 17.1 3.35 0.87 3.03 0.40 2.79 0.61 1.81 0.27 1.69 0.27 4.60 0.65 12.04 4.65 1.19 0.1181 0.512349 15 2.8

64.08 0.75 16.36 4.70 0.08 1.95 3.11 4.75 2.64 0.18 1.59 100.19 1.00 0.45 127 17.5 12.2 6.9 22 58 402 23 250 8.6 697 23 44 5.82 21.7 4.24 1.01 4.18 0.57 4.02 0.89 2.61 0.38 2.26 0.36 5.48 0.63 9.91 5.14 1.34 0.1179 0.512344 8 2.8

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Pluton

Rock type: QM, quartz monzonite; QMD, quartz monzodiorite; QD, quartz diorite; MG, monzogranite; Total iron as Fe2 O3 ; εHf (T) denotes a weighted mean zircon Hf isotopic composition. 219

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Fig. 6. SiO2 vs. selected major element oxides (wt.%) and trace elements (ppm) for the Shanhou and Jiangshan plutons. Also shown are the trends of the fractionation crystallization of clinopyroxene (cpx) + amphibole (amp) and of amphibole (amp) + plagioclase (pl). Symbols as in Fig. 2.

5.2. Origin of the Shanhou gabbros The Shanhou gabbros have a massive rather than a layered structure (Fig. 3b) and do not show typical cumulatic texture as illustrated in Hunter (1996), suggesting that they are not cumulates. Thus we could use their whole rock chemistry to discuss the petrogenesis and tectonic setting. The Shanhou gabbros have high MgO (5.6-8.7 wt.%) and Mg# (0.59–0.66) as well as compatible elements (Cr = 183–402 ppm; V = 227–386 ppm), suggesting that they could be mantle-derived primary or near-primary melts. The gabbros show negative Ta–Nb anomalies (Fig. 7b) and have low Nb/U (3.5–11.9) and Ce/Pb (5.1–8.3) ratios [lower than MORB (∼47 and 25, respectively)], implying that the mantle source was affected by subduction processes (e.g., Thirlwall et al., 1994). They have lower εNd (T) and εHf (T) than MORB (Fig. 8), also suggesting a subduction process, in which sediment components are added to the mantle wedge

(e.g., Leata et al., 2004). Alternatively, these geochemical features could have also been caused by crustal assimilation of N-MORBlike magmas. However, the Shanhou gabbros have higher Ti/Y ratios than N-MORB, which argues against crustal assimilation, as continental crustal rocks, especially upper crustal rocks, have low Ti/Y ratios in comparison to MORB (Fig. 9). Their higher Ti/Y ratios most probably reflect lower degrees of melting than for MORB (e.g., Hergt et al., 1991; Peate et al., 1999). Furthermore, the Nb/U ratios of the Shanhou gabbros are generally lower than estimates for the continental crust (upper curst Nb/U ≈ 9), even lower than average composition of globally subducted sediment (Nb/U ≈ 5) (Fig. 9), which also argues against crustal assimilation, but suggests a subduction process. Previous studies (e.g., Ayers, 1998) indicate that the subduction-zone fluids have significantly low Nb/U ratios (≈0.22). Therefore, a N-MORB mantle affected by a slab-derived hydrous fluid could account for the low Nb/U ratios of the Shanhou gabbros. The Shanhou gabbros show some affinities

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221

Fig. 7. Chondrite-normalized (Boynton, 1984) REE patterns and primitive mantle-normalized (McDonough and Sun, 1985) trace element patterns for the Shanhou and Jiangshan plutons.

with adakites, e.g., high Sr (533–661 ppm) and low Y (10–15 ppm) and Yb (1.0–1.2 ppm) (Table 2), which also suggests their mantle source affected by a slab-derived hydrous fluid rather than a slabderived melt (Jiang et al., 2006, 2012). Partial melts derived from a subduction-modified mantle via addition of slab-melt would have very high Sr (>1100 ppm) (Martin et al., 2005; Jiang et al., 2012). In summary, we conclude that the Shanhou gabbros were most likely derived from a subduction-modified mantle wedge metasomatised via addition of slab-derived hydrous fluid.

5.3. Origin of the dioritic rocks and granites in the Shanhou and Jiangshan plutons The dioritic rocks and granites have lower Mg# than the Shanhou gabbros, but higher than partial melts under crustal P-T conditions (Fig. 5c). Coupled with similar Nd–Hf isotopic compositions to the Shanhou gabbros (Fig. 8), this suggests that they are related to the Shanhou gabbros by fractionation crystallization. Slightly lower εNd (T) of the Shanhou granites and some Jiangshan

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Fig. 8. Whole-rock εNd vs. zircon εHf (a) and MgO vs. εNd (b) for the Shanhou and Jiangshan plutons. In (a), also shown are the field of the Shuangxiwu volcanic rocks and the Taohong and Xiqiu granitoids as well as the fields for MORB, OIB and global sediments (Vervoort et al., 1999).

Fig. 10. V vs. Ti/1000 (a) (Shervais, 1982) and Th/Yb vs. Ta/Yb (b) (Pearce, 1982) for the Shanhou gabbros, and Rb vs. Y + Nb (c) (Pearce et al., 1984) for the Shanhou and Jiangshan dioritic rocks and granites. In (a), the Shuangxiwu basaltic rocks (Li et al., 2009), Gangbian mafic rocks (Li et al., 2010b) and Zhenzhushan basalts (Li et al., 2010a) are also plotted for comparison. Also shown are the fields for volcanic arc basalts (VAB), mid-ocean-ridge basalt (MORB)/back-arc basin basalts (BABB), continental flood basalts (CFB), and ocean-island basalts (OIB)/alkali basalts (AB) (Rollinson, 1993) as well as Shenwu dolerites (Li et al., 2008b). In (c), ORG = oceanic ridge granitoids, VAG = volcanic arc granitoids, syn-COLG = syn-collision granitoids, WPG = within-plate granitoids.

Fig. 9. Nb/U vs. Ti/Y for the Shanhou and Jiangshan plutons. Also shown are the average MORB, continental upper crust (Taylor and McLennan, 1985; Schmidt et al., 2004) and globally subducted sediments (GLOSS) (Plank and Langmuir, 1998).

dioritic rocks than the Shanhou gabbros (Fig. 8b) implies that they were most likely generated by fractional crystallization with a little crustal assimilation. From gabbros to dioritic rocks and to granites, with increasing SiO2 , there is a general decrease in Mg#, TiO2 , Fe2 O3 , MgO, CaO, V and Cr, and increase in K2 O, Y and Yb, whereas ASI, Al2 O3 , Na2 O and Sr first show an increase trend and then a decrease trend (Figs. 5 and 6). These geochemical characteristics suggest that the gabbroic magmas underwent fractionation crystallization of clinopyroxene + amphibole, followed by fractionation crystallization of amphibole + plagioclase (Figs. 5 and 6).

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223

Fig. 11. Cartoons showing the tectonic evolution of the eastern Jiangnan Orogen. (a) ca. 970–890 Ma, northwestward intra-oceanic subduction of the South China Ocean causing an oceanic island arc, which formed the Shuangxiwu volcanic rocks and the plagiogranites later preserved within the NE Jiangxi ophiolites. (b) ca. 880–866 Ma, northwestward flat subduction of the South China Ocean causing the obduction of Shuangxiwu arc onto the Yangtze Block, which resulted in the emplacement of the NE Jiangxi ophiolites, and formed the leucogranites within the NE Jiangxi ophiolites. (c) ca. 849–842 Ma, the southeastern margin of the Yangtze Block was a continental arc, forming the Shanhou and Jiangshan plutons, and developing a back-arc basin further inland as a consequence of slab rollback. (d) ca. 827–819 Ma, the back-arc basin evolved to a back-arc oceanic basin that was later preserved as the Fuchuan ophiolites.

In conclusion, the origin of the dioritic rocks and granites in the Shanhou and Jiangshan plutons can be summarized as below: Partial melting of subduction-modified mantle wedge generated the magmas of gabbros. Such gabbroic magmas underwent fractionation crystallization of clinopyroxene + amphibole, forming the less felsic dioritic rocks. Progressive fractionation crystallization of amphibole + plagioclase from the evolved melts with a little

crustal assimilation produced the more felsic dioritic rocks and granites. 5.4. Tectonic implications The studied gabbros, dioritic rocks and granites correspond to an association of subalkaline basalt–andesite–rhyodacite/dacite

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(Fig. 2b), typical of continental arc rock association (e.g. Condie, 1976). These rocks are mainly high-K calc-alkaline (Fig. 5b). It has long been recognized that high-K calc-alkaline magmatism could occur at active continental margins (e.g. Condie, 1976). As mentioned above, the origin of the Shanhou gabbros suggests an oceanic-slab subduction setting. They have low Ti/V ratios, which is also consistent with volcanic arc basalts (Fig. 10a). Their elevated Ta/Yb and Th/Yb ratios further suggest a continental arc rather than an oceanic island arc setting (Fig. 10b). The studied dioritic rocks and granites have relatively low Rb and Y + Nb contents with the data clearly plotting in the field of volcanic arc granite (Fig. 10c). In summary, we conclude that the Shanhou and Jiangshan plutons might have been emplaced in a continental arc setting induced by northwestward subduction of the South China Ocean. This view is reinforced by the temporal and spatial characteristics of regional magmatism and regional sedimentary records in the eastern Jiangnan Orogen. (1) Li et al. (2009) investigated the volcanic rocks of the Shuangxiwu Group ( in Fig. 1a) in Zhejiang province, which were formed at ca. 970–890 Ma, showing an association of basaltic–andesitic–rhyolitic rocks. The lowest Shuangxiwu Group (Pingshui Formation) was intruded by the Taohong and Xiqiu tonalite and granodiorite stocks at ca. 913–905 Ma (Ye et al., 2007; in Fig. 1a). These volcanic rocks and granitoids show geochemical affinities with volcanic arcs but have MORBlike Nd–Hf isotopic compositions (Fig. 8a). The basaltic rocks in the Shuangxiwu Group have low Ti/V ratios that are consistent with volcanic arc basalts (Fig. 10a). Their elevated Th/Yb relative to Ta/Yb ratios further suggest an oceanic island arc rather than a continental arc setting (Fig. 10b). Taken together, we conclude that the Shuangxiwu volcanic rocks and the Taohong and Xiqiu granitoids were most likely formed in an oceanic island arc setting induced by northwestward intra-oceanic subduction of the South China Ocean (Fig. 11a). The Xiwan plagiogranites (∼968 Ma) within the NE Jiangxi ophiolites ( in Fig. 1a) show adakitic affinities with MORB-like Nd isotopic compositions and were inferred by Li and Li (2003) to have been formed by partial melting of subducted oceanic crust. Further investigation by Gao et al. (2009) suggests that the Xiwan plagiogranites (∼970 Ma) might have been generated by differentiation of basaltic magmas derived from the mantle wedge beneath an island arc. In either case, the origin of the Xiwan plagiogranites was induced by the northwestward intra-oceanic subduction of the South China Ocean by ca. 970 Ma (Fig. 11a). (2) There exist a series of leucogranitic lenses within the NE Jiangxi ophiolites. They were emplaced at ca. 880 Ma and are peraluminous (A/CNK = 1.0–1.24) with ␧Nd (T) values of 0.8 to −3.9 (Li et al., 2008a). Li et al. (2008a) suggested that these leucogranites were likely formed by partial melting of continental margin sediments beneath a major thrust fault during the obduction of the ophiolite onto the continental crust. Thus the origin of these obduction-related granites may mark the Shuangxiwu arc obduction onto the Yangtze Block (Fig. 11b). Li et al. (2008a) interpreted such an obduction to be resulted from the continental amalgamation between the Yangtze and Cathaysia Blocks. However, considering that the southeastern margin of the Yangtze Block was still a continental arc during the ages of 847–842 Ma inferred from the origin of the Shanhou and Jiangshan plutons, we suggest that this obduction may be caused by flat subduction of the South China Ocean (Collins, 2002). The obduction most likely lasted until ca. 866 Ma, the age of the glaucophane schists at Xiwan (Shu and Charvet, 1996). The NE Jiangxi ophiolites might have been the remnants of an ancient oceanic island arc, like the Oytag ophiolite suite in the western Kunlun orogen, northwest China (Jiang et al., 2008).

(3) Li et al. (2010a) investigated the volcanic rocks of the Zhenzhushan Group in Jiangxi province ( in Fig. 1a). They erupted at ca. 849 Ma and show a basalt-dacite association. The Gangbian alkaline complex (pyroxene syenite–quartz monzonite–quartz syenite) in Jiangxi province ( in Fig. 1a) and the Shenwu dolerites in Zhejiang province ( in Fig. 1a) have also been studied that were emplaced at ca. 848 Ma (Li et al., 2010b) and 849 Ma (Li et al., 2008b), respectively. The Zhenzhushan volcanic rocks and Gangbian alkaline complex as well as Shenwu dolerites were all inferred by Li et al. (2008b, 2010a,b) to have been formed in a continental rifting setting. As mentioned above, the Shanhou and Jiangshan plutons were emplaced at ca. 847–842 Ma and their origins suggest a continental arc setting. Considering that the Zhenzhushan basalts and the Gangbian mafic rocks are consistent with back-arc basin basalts (Fig. 10a), we conclude that they were most likely formed in a back-arc extensional setting caused by slab rollback of the South China Ocean (Fig. 11c). Although the Shenwu dolerites have some affinities with within-plate basalts (Li et al., 2008b), they show clear negative Nb–Ta–Ti anomalies in the primitive mantlenormalized trace element patterns (Fig. 9 of Li et al., 2008b), typical of arc magmas. Ti–V plots with the data within the field between back-arc basin basalts and OIB (Fig. 10a) indicate that they might have been formed in a tectonic transition regime from continental arc to continental rifting, most likely in a back-arc extensional setting (Fig. 11c). More recently, Zhao (2015) suggests a divergent double subduction model for the evolution of the Jiangnan Orogen. The ∼845 Ma continental arc magmatism in the southeastern margin of the Yangtze Block, resembling the Shanhou and Jiangshan plutonism, was corresponding to the Lipu diorite (∼841 Ma, Li et al., 2010c) in the northwestern margin of the Cathaysia Block ( in Fig. 1a) that was inferred by Zhao (2015) to have been formed in a continental arc setting coupled with southeastward subduction of the South China Ocean underneath the Cathaysia Block. (4) Wang et al. (2013) have investigated the sedimentary sequences of the Shuangqiaoshan and Xikou Groups and suggest that they might have deposited in a back-arc basin at ca. 831–815 Ma and 833–817 Ma, respectively. They also indicate that the lower parts of these strata contain dominantly Neoproterozoic detritus with a pronounced age peak at 850–830 Ma, interpreted to be derived from a magmatic arc that has been mostly eroded. This is in good agreement with the ∼845 Ma continental arc magmatism in the southeastern margin of the Yangtze Block, resembling the Shanhou and Jiangshan plutonism. The back-arc basin may have been evolved to a back-arc oceanic basin by ca. 827 Ma as a consequence of progressive slab rollback, as inferred from the Fuchuan ophiolites ( in Fig. 1a) that have been dated at ca. 827–819 Ma (Zhang et al., 2012a, 2013a) (Fig. 11d).

6. Conclusions 1. The Shanhou and Jiangshan plutons are located along the northwest side of the Jiangshan–Shaoxing suture. They consist of gabbros, dioritic rocks and granites and were emplaced during the middle Neoproterozoic (∼847–842 Ma). 2. The plutons were formed in a continental arc setting induced by northwestward subduction of the South China Ocean. The gabbros were derived by partial melting of subduction-modified mantle wedge. Such gabbroic magmas underwent fractionation crystallization of clinopyroxene + amphibole, forming the less felsic dioritic rocks. Progressive fractionation crystallization of amphibole + plagioclase from the evolved melts with a little

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crustal assimilation produced the more felsic dioritic rocks and granites. 3. Integrating our new data with published data, we suggest that during ca. 970–890 Ma, northwestward intra-oceanic subduction of the South China Ocean induced the formation of an oceanic island arc, forming the Shuangxiwu volcanic rocks and the plagiogranites that were later preserved within the NE Jiangxi ophiolites. Northwestward flat subduction of the South China Ocean caused the obduction of Shuangxiwu arc onto the Yangtze Block during ca. 880–866 Ma, resulting in the emplacement of the NE Jiangxi ophiolites, and forming the glaucophane schists and leucogranites within the NE Jiangxi ophiolites. During ca. 849–842 Ma, the southeastern margin of the Yangtze Block became a continental arc resembling the Shanhou and Jiangshan plutonism. Meanwhile, a back-arc basin developed further inland as a consequence of slab rollback. Such a back-arc basin may have been evolved to a back-arc oceanic basin by ca. 827 Ma. Acknowledgments We are grateful to Guochun Zhao (Editor) and two anonymous reviewers for their thoughtful reviews and constructive comments. This work was financially supported by the National Key Basic Research Projects (No. 2012CB416706) and the National Natural Science Foundation (No. 41272083). References Andersen, T., 2002. Correction of common lead in U–Pb analyses that do not report 204 Pb. Chem. Geol. 192, 59–79. Ayers, J., 1998. Trace element modeling of aqueous fluid-peridotite interaction in the mantle wedge of subduction zone. Contrib. Mineral. Petrol. 132, 390–404. Bai, W.J., Gan, Q.G., Yang, J.S., Xing, F.M., Xu, X., 1986. Discovery of well-reserved ophiolite and its basical characters in Southeastern margin of the Jiangnan ancient continent. Acta Petrol. Mineral. 5, 289–299 (in Chinese with English abstract). Bouvier, A., Vervoort, J.D., Patchett, P.J., 2008. The Lu–Hf and Sm–Nd isotopic composition of CHUR: constraint from unequilibrated chondrites and implications for the bulk composition of terrestrial planets. Earth Planet Sci. Lett. 273, 48–57. Boynton, W.V., 1984. Cosmochemistry of the rare earth elements: meteorite studies. In: Henderson, P. (Ed.), Rare Earth Element Geochemistry. Elsevier, Amsterdam, pp. 63–114. Charvet, J., 2013. The neoproterozoic–early Paleozoic tectonic evolution of the South China Block: an overview. J. Asian Earth Sci. 74, 198–209. Chen, J.F., Foland, K.A., Xing, F.M., Xu, X., Zhou, T.X., 1991. Magmatism along the southeast margin of the Yangtze and Cathysia block of China. Geology 19, 815–818. Collins, W.J., 2002. Hot orogens, tectonic switching, and creation of continental crust. Geology 30, 535–538. Condie, K.C., 1976. Plate Tectonics and Crustal Evolution, 2nd ed. Pergamon Press, New York. Ding, B.H., Shi, R.D., Zhi, X.C., Zheng, L., Chen, L., 2008. Neoproterozoic (∼850 Ma) subduction in the Jiangnan orogen: evidence from the SHRIMP U–Pb dating of the SSZ-type ophiolite in southern Anhui Province. Acta Petrol. Mineral. 27, 375–388 (in Chinese with English abstract). Ernst, R.E., Wingate, M.T.D., Buchan, K.L., Li, Z.X., 2007. Global record of 1600–700 Ma Large Igneous Provinces (LIPs): implications for the reconstruction of the proposed Nuna (Columbia) and Rodinia supercontinents. Precambrian Res. 160, 159–178. Gao, J., Klemd, R., Long, L., Xiong, X., Qian, Q., 2009. Adakitic signature formed by fractional crystallization: an interpretation for the Neo-Proterozoic metaplagiogranites of the NE Jiangxi ophiolitic mélange belt, South China. Lithos 110, 277–293. Gao, J.F., Lu, J.J., Lai, M.Y., Lin, Y.P., Pu, W., 2003. Analysis of trace elements in rock samples using HR-ICPMS. J. Nanjing Univ. (Nat. Sci.) 39, 844–850 (in Chinese with English abstract). Gao, L.Z., Yang, M.G., Ding, X.Z., Liu, Y.X., Liu, X., Ling, L.H., Zhang, C.H., 2008. SHRIMPU–Pb zircon dating of tuff in the Shuangqiaoshan and Heshangzhen groups in South China-constraints on the evolution of the Jiangnan Neoproterozoic orogenic belt. Geol. Bull. China 27, 1744–1751 (in Chinese with English Abstract). Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., van Achterbergh, E., O’Reilly, S.Y., She, S.R., 2000. The Hf isotope composition of cratonic mantle: LAM-MCICP-MS analysis of zircon megacrysts in kimberlites. Geochim. Cosmochim. Acta 64, 133–147. Hergt, J.M., Peate, D.W., Hawkesworth, C.J., 1991. The petrogenesis of Mesozoic Gondwana low-Ti flood basalts. Earth Planet. Sci. Lett. 105, 134–148.

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