Jiangnan Orogen in South China: Developing from divergent double subduction

    Jiangnan Orogen in South China: Developing from divergent double subduction Guochun Zhao PII: DOI: Reference:

S1342-937X(14)00276-7 doi: 10.1016/j.gr.2014.09.004 GR 1321

To appear in:

Gondwana Research

Received date: Revised date: Accepted date:

17 June 2014 10 September 2014 12 September 2014

Please cite this article as: Zhao, Guochun, Jiangnan Orogen in South China: Developing from divergent double subduction, Gondwana Research (2014), doi: 10.1016/j.gr.2014.09.004

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Jiangnan Orogen in South China: developing from divergent

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Guochun Zhao

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double subduction

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Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong

Contacts:

Department of Earth Sciences

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James Lee Science Building The University of Hong Kong Pokfulam Road, Hong Kong

Tel:

852-28578203

Fax:

852-25176912

E-mail:

[email protected]

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ACCEPTED MANUSCRIPT Abstract The Jiangnan Orogen is considered as a continent-continent collisional belt resulting from the closure of a Meso-Neoproterozoic ocean separating the southeastern

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margin of the Yangtze Block from the northwestern margin of the Cathaysia Block. Recent data indicate the existence of early Neoproterozoic (1000-825 Ma) volcanic arc

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assemblages on both sides of the orogen, suggesting that the ocean lithosphere between the Yangtze and Cathaysia blocks must have undergone divergent double-sided subduction during the period of 1000-825 Ma. The divergent double subduction

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eventually resulted in the closure of the ocean basin at ~825 Ma, leading to the soft collision of the Yangtze and Cathaysia blocks to form the Jiangnan Orogen, without

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involvement of continental deep subduction, high-grade metamorphism of continental crust and uplift/exhumation of high-grade metamorphic rocks. Shortly after the

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collision, the initial detachment of the ocean lithosphere from the overlying crust and

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sedimentary sections induced underplating of mantle magmas, triggering partial melting of accretionary-wedge strata to form some peraluminous (S-type) granites in

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the period 825-815 Ma. Finally, the sinking of the oceanic slab pulled down the overlying strata to form some basins in which the Banxi Group and its equivalent strata

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including bimodal volcanic rocks were formed in the period 815-750 Ma.

Keywords: Jiangnan Orogen; divergent double subduction; Yangtze Block; Cathaysia Block; collision; Neoproterozoic.

1. Introduction Development of most classical Phanerozoic continent-continent collisional belts (e.g. the Alpine, Himalayan and Dabie-Sulu belts) resulted from a single-sided subduction where an ocean lithosphere moved under a continent on one side but on the other side it was directly connected with a passive margin of another continent (Yin and Harrison, 2000; Hacker et al., 2000; Compagnoni, 2003; Zheng et al., 2003; Schmid et 2

ACCEPTED MANUSCRIPT al., 2004). When the ocean lithosphere was completely consumed by subduction, the subducting oceanic slab would pull down the passive margin of the continent beneath another continent along the subduction zone, resulting in at least doubly thickening of

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the crust and high-grade metamorphism of the subducted continental crust at depths (Hacker et al., 2000; Zheng et al., 2003). Later, the subducted continental slab may have

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undergone uplift/exhumation due to rapid erosion, slab break-off or detachment of the oceanic slab when it was transformed to eclogite in the deep mantle, leading to

Compagnoni, 2003; Searle et al., 2003).

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large-scale folding and thrusting and exposing of high-grade rocks (Hacker et al., 2000;

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In some cases, an ocean lithosphere would be subducted on both sides, forming divergent double-sided subduction zones. Soesoo et al. (1997) are the first to present a series of scenarios on the divergent double subduction model and applied it to explain

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the development of the Lachlan Belt in eastern Australia. According to Soesoo et al.

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(1997), the initial stage of divergent double subduction evolution involves the development of two ―normal‖ subduction zones, deposition of accretionary-wedge

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sediments on the margins of the overriding plates, and arc magmatism in the two continental margin arcs. Further subduction on either side of the oceanic slab eventually leads to the closure of the ocean basin and then the two continental blocks

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are welded together, without obvious continental subduction. Later, the oceanic lithosphere is detached from the overlying sedimentary section and crust, and sinks into the deep mantle, leading to the development of extensional basins and emplacement of plutons and volcanism. Although the application of this model to the Lachlan Belt invited hot debates (O’Halloran and Bryan, 1998; Cayley and Taylor, 1998; Soesoo et al., 1998), divergent double-sided subduction models have been successfully applied to some other orogenic belts. For example, Xiao et al. (2003) applied a divergent double subduction model to interpret the final closure of the eastern segment of the Paleo-Asian Ocean. A modern example for divergent double subduction is the Molucca ocean plate that is subducting in two directions under the Eurasian Plate to the west and the Philippine Sea plate to the east, forming the Sangihe arc in the west and Halmahara 3

ACCEPTED MANUSCRIPT arc in the east, respectively (Hall, 2000). In this contribution, we attempt to apply a divergent double subduciton model to the Jiangnan Orogen in South China, which is characterized by some special features that cannot be well explained by single-sided

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subduction models.

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2. Special features of the Jiangnan Orogen

The Jiangnan Orogen has long been considered as a continent-continent collisional belt along which the Yangtze Block in the northwest and the Cathaysia Block in the

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southeast amalgamated to form the South China continent (Fig. 1; Zheng et al., 2013), though the timing of the collision is still controversial, with the predominant views

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arguing for the Neoproterozoic collision (Shu et al., 1994, 2014; Li, 1999; Zhao and Cawood, 1999; Li et al., 2002, 2007, 2008b, 2009; Wang et al., 2004, 2006a, 2007,

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2008a,b; Zhou et al., 2009; Zhao et al., 2011; Shu, 2012; Cawood et al., 2013). In the

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last two decades, a large number of tectonic models have been proposed for the formation and evolution of this orogen (e.g. Li et al., 1999, 2003, 2010; Shu et al., 1999,

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2008a,b, 2011; Zhou et al., 2002a,b, 2006a,b; Wang et al., 2003a, 2004, 2006a, 2007, 2010, 2012a, 2013a; Shu, 2006, 2012; Zheng et al., 2006, 2007, 2008a,b; Zhao and Zhou, 2008; Zhao et al., 2008), but none of these models can well explain all of the

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following special features of the orogen: (1) There is no much evidence for continental deep subduction operative in the Jiangnan Orogen following the closure of the ocean, which typifies many other typical continent-continent collisional belts in the world (e.g. the Alpine, Himalayan and Dabie-Sulu belts). Nearly all lithologies in the Jiangnan Orogen were only metamorphosed in sub-greenschist to greenschist facies (Shu et al., 1995; Shu and Charvet, 1996; Zhou et al, 2006, 2009), unlike other typical continent-continent

collisional belts that contain large amounts of high-grade metamorphic rocks forming at the subduction stage of continental crust following the closure of an ancient ocean. (2) Most researchers believe that the finally collision between the Yangtze and Cathaysia blocks occurred in the early Neoproterozoic, with ages ranging from 4

ACCEPTED MANUSCRIPT 900-880 Ma (Li et al., 1995, 2007, 2008a,b, 2009) to 860-820 Ma (Shu et al., 1994, 1995, 1999, 2008, 2011; Zhao and Cawood, 1999; Wang et al., 2004, 2006a, 2007; Shu, 2006; Zhou et al., 2006, 2009; Zhao et al., 2011; Cawood et al., 2013), but few

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such-aged metamorphic ages have been obtained for lithologies in the Jiangnan Orogen, except a glaucophane Ar/Ar age of 866±14 Ma produced from blueschist (Shu et al.,

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1994) and mica Ar/Ar ages of 1042-942 Ma from the Tianli schist (Li et al., 2007). This is different from other typical continent-continent collisional belts where numerous reliable metamorphic ages were produced, especially from metamorphic zircons and

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monazites. Mineral Ar/Ar ages are often open to different interpretations and cannot be

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used to precisely constrain the timing of collisional events. (3) It still remains unknown or controversial regarding the polarity of the subduction between the Yangtze and Cathaysia blocks, with some people arguing for a

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southeastward subduction (Hsü et al., 1988; Cawood et al., 2013), whereas others believe

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a northward subduction (Shui, 1987; Shu et al., 1995, 1999; Li et al., 2007, 2009; Wang

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et al., 2004, 2006a, 2007, 2008a,b; Zhao et al., 2011). (4) Controversy has also surrounded the tectonic nature of the middle Neoproterozoic (815-750 Ma) weakly metamorphosed strata (Banxi Group and its

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equivalents), which unconformably overlie the early Neoproterozoic metamorphosed strata (Sibao Group and its equivalents), but are unconformably overlain by late Neoproterozoic (<780 Ma) unmetamorphosed cover (called the Sinian System in the Chinese literature). (5) It is still unclear about the origin of the middle Neoproterozoic (825-815 Ma) peraluminous

(S-type)

granites,

which

intrude

the

early

Neoproterozoic

metamorphosed strata (Sibao Group and its equivalents), and are in tectonic contact with the 815-750 Ma Banxi Group and its equivalents. Although these special features characterized by the Jiangnan Orogen cannot be well explained by any single-sided subduction models, they are consistent with divergent double-sided subduction on both sides of the orogen. Recent geological data 5

ACCEPTED MANUSCRIPT support the existence of early Neoproterozoic magmatic arcs on the southeastern margin of the Yangtze Block and northwestern margin of the Cathaysia Block (Wang

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et al., 2004, 2007; Zhou et al., 2006, 2009; Li et al., 2009; Zhang et al., 2012b, 2013).

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3. Neoproterozoic arc on the southeastern margin of the Yangtze Block

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The Yangtze Block is considered to have Archean-Paleoproterozoic crystalline basement (e.g. the Kongling, Huangtuling and Houhe complexes) surrounded by late Mesoproterozoic to early Neoproterozoic folded belts, which are unconformably

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overlain by Neoproterozoic unmetamorphosed Sinian cover (Wang and Mo, 1995; Zhao and Cawood, 2012). The late Mesoproterozoic to early Neoproterozoic folded

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belts can be further subdivided into the Jiangnan Orogen in the southeast and the Panxi-Hannan Belt in the western and northern margins of the block (Fig. 1; Zhao and

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Cawood, 2012).

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In the northeastern segment of the Jiangnan Orogen, the Jiang-Shao Fault is conventionally considered as a suture zone separating the southeastern margin of the

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Yangtze Block from the northwestern margin of the Cathaysia Block (Fig. 2). The basement rocks on the southeastern margin of the Yangtze Block are early Neoproterozoic (970-825 Ma) volcanic-sedimentary strata metamorphosed in

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greenschist facies, which are intruded by middle Neoproterozoic (825-815 Ma) peraluminous (S-type) granites and unconformably overlain by middle Neoproterozoic (815-750 Ma) weakly metamorphosed strata and late Neoproterozoic (<750 Ma) unmetamorphosed Sinian cover (Wang and Mo, 1995; Li et al., 2003; Wang and Li, 2003; Zhong et al., 2005; Wang et al., 2011; Zhao and Cawood, 2012; Yao et al., 2014; Fig. 2). The early Neoproterozoic (970-825 Ma) metamorphosed volcanic-sedimentary strata are considered as pre-collision assemblages, represented by the Sibao Group in north Guangxi, Fanjingshan Group in northeast Guizhou, Lengjiaxi Group in central Hunan, Shuangqiaoshan Group in northwest Jiangxi, Shangxi Group in south Anhui, and Shuangxiwu Group in west Zhejiang (Shu et al., 1994, 1995, 1999; Wang et al., 2004, 2007; Zhao and Cawood, 2012; Fig. 2). The middle Neoproterozoic (825-815 Ma) 6

ACCEPTED MANUSCRIPT peraluminous (S-type) granites include the Sanfang, Bendong, Tianpeng, Motianling and Yuanbaosha granites that intrude the Sibao Group (Li, 1999; Li et al., 2003, Wang et al., 2006a), the Gangbian granite intruding the Fanjingshan Group (Wang et al.,

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2006b, Li et al., 2010a), the Jiuling granodiorite intruding the Shuangqiaoshan Group (Li et al., 2003), and the Xucun, Shexian and Xiuning granitoids intruding the Shangxi

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Group (Li et al., 2003; Wu et al., 2006; Zheng et al., 2008a,b). These granites were not deformed and metamorphosed, with a massive structure, interpreted as post-orogenic granites (Zheng et al., 2007, 2008a). The middle Neoproterozoic (815-750 Ma) weakly

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metamorphosed strata are represented by the Danzhou, Xiajiang, Banxi, Luokedong and Likou groups, which unconformably overlie the Sibao, Fanjingshan, Lengjiaxi,

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Quangqiaoshan and Shangxi groups, respectively (Wang and Mo, 1995). It still remains controversial about the tectonic setting of the middle Neoproterozoic strata, with some

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researchers favoring a post-orogenic environment (Wang et al., 2004, 2012a, 2013;

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Zheng et al., 2007, 2008; Zhao and Cawood, 2012), whereas others argued that they formed under a rifting system, named the Nanhua Rift System (Li et al., 2003; Wang

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and Li, 2003; Zhang et al., 2013b). Some of volcanic assemblages in the early Neoproterozoic metamorphosed volcanic-sedimentary strata are considered to have developed under magmatic arc

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environments (Wang et al., 2004, 2007; Li et al., 2009; Zhang et al., 2012b, 2013). The oldest arc assemblages at the southeastern margin of the Yangtze Block are represented by volcanic rocks from the Shuangxiwu Group in west Zhejiang, which consists predominantly of volcanic and pyroclastic rocks interlayered with felsic tuffs, tuffaceous sandstones and siltstones that were deformed and metamorphosed in greenschist-facies (Li et al., 2009). These volcanic rocks are dominated by andesite, dacite and rhyolite as well as volcaniclastic rocks, which yielded SHRIMP U-Pb zircon ages around 970–890 Ma (Li et al., 2009). Geochemically, volcanic rocks from the Shuangxiwu Group possess highly positive ԑNd(T) (+5.4 to +8.7) and ԑHf(T) (+11.0 to +15.3) values and are characterized by enrichments in Th and LREE but depletions in Nb, Ta, Zr, Hf and Ti, similar to volcanic rocks in modern magmatic arcs (Li et al., 7

ACCEPTED MANUSCRIPT 2009). Other early Neoproterozoic metamorphosed volcanic-sedimentary strata,

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represented by the Sibao Group and its equivalents at the southeastern margin of the

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Yangtze Block, were mainly formed in the period between 860-825 Ma (see later

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discussion). These groups consist predominantly of greenschist-faces metamorphosed sandstone, siltstone, shale, volcanic rock and tuff, of which the volcanic rocks range from komatiitic basalts, through tholeiites and andesites, to dacites and rhyolites (Wang

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et al., 2004); some of the andesite and dacite have Mg-numbers ranging from 71 to 25, classified as high-Mg andesite and dacite (Zhang et al., 2012b, 2013). The komatiitic

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basalts are high in Al2O3/TiO2, MgO, Ni and Cr, and are depleted in Nb and Ti, exhibiting the geochemical characteristics of the plume–arc interaction (Wang et al., 2004), whereas the other volcanic rocks are strongly depleted in Nb and Ti but enriched

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in LILEs, typical of volcanic arc origin (Wang et al., 2004; Zhang et al., 2012b, 2013),

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which led Wang et al. (2004, 2007), Zhou et al. (2009) and Zhang et al. (2012b, 2013) to propose that these volcanic rocks developed in a subduction zone along the

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southeastern margin of the Yangtze Block, though some others argue that these volcanic rocks, together with those 850–750Ma igneous rocks in the Panxi-Hannan Belt along the western and northern margin of the Yangtze Block, were products of

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anorogenic magmatism in intracontinental rift basins related to mantle plume activities during the breakup of Rodinia (Li et al., 2002, 2003, 2008a,b; Wang and Li, 2003; Wang et al., 2011, 2012b). We believe that although some bimodal volcanic rocks from early Neoproterozoic metamorphosed volcanic-sedimentary strata may have developed in back-arc basins or rift zones, the 860-825 Ma andesite-dominant volcanic assemblages (e.g. the Baolinchong volcanics) along the southeastern margin of the Yangtze Block were most likely formed under magmatic arc environments. In addition, some Neoproterozoic ophiolitic assemblages have been recognized in the southeastern margin of the Yangtze Block, represented by the 980-900 Ma Gandongbei ophiolite in NE Jiangxi Province and the ~820 Ma Fuchuan ophiolite in southern Anhui Province. These ophiolites exhibit typical features of SSZ type ophiolites (Zhang et al., 2012, 8

ACCEPTED MANUSCRIPT 2013a), which also support the existence of a magmatic arc at the southeastern margin of the Yangtze Block.

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4. Neoproterozoic arc on the northwestern margin of the Cathaysia Block Precambrian basement rocks in the Cathaysia Block are sparsely exposed in the Badu,

Wuyishan,

Nanling,

Yunkai

and

Hainan

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Chencai,

areas

along

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northeast-southwest-trending belt bounded by the Chenzhou-Linwu-Jiang-Shao Fault in the northwest and the Zhenghe-Dapu Fault in the southeast (Fig. 1; Zhao and

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Cawood, 2012), and the rest areas of the block are covered by Phanerozoic igneous rocks (especially Mesozoic granitoids) and sedimentary rocks. Unlike the Yangtze

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Block that contains minor Archean rocks, the Cathaysia Block has no Archean rocks exposed on the surface, but the presence of numerous Archean detrital zircons and

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inherited/xenocrystic zircons implies the existence of Archean crust hidden

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underground in the block or adjacent blocks (Zheng and Zhang, 2007 and references wherein). The oldest Precambrian basement in the Cathaysia Block is represented by

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Paleoproterozoic granitoids and supracrustal rocks, named the Badu Complex (Xu et al., 2007; Yu et al., 2009, 2010, 2012), which are only exposed in the northeastern sector of the block (Fig. 1).

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More than 95% of Precambrian basement in the Cathaysia block is composed of Meso- to Neoproterozoic volcanic-sedimentary successions metamorphosed mainly from greenschist to lower amphibolite facies, represented by the Chencai Group in central Zhejiang, the Longquan Group in southwest Zhejiang, the Mayuan and Mamianshan groups in northwest Fujian, the Shenshan Group in south Jiangxi, the Yunkai Group in central-west Guangdong, and the Baoshan and Shilu groups in the Hainan island (Zhao and Cawood, 2012). Traditionally, these groups were considered as Paleo-Mesoproterozoic basement rocks, but recent SHRIMP and LA-ICP-MS data have revealed that except the ~1.43 Ga Baoshan and Shilu groups in the Hainan island (Li et al., 2002, 2008b), nearly all other groups were formed during Neoproterozoic time (Li et al., 2010b; Shu et al., 2011; Yao et al., 2011; Yao et al., 2013). The volcanic 9

ACCEPTED MANUSCRIPT rocks from these Neoproterozoic metamorphic strata are composed predominantly of basalts and rhyolites, with minor andesites and dacites, some of which display arc affinities in geochemical composition. The oldest Neoproterozoic meta-volcanic rocks

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reported in the Cathaysia Block are the 997-978 Ma amphibolites (metabasites) from the Yunkai Complex (Fig. 1), which have Nb/La = 0.33–0.54 and Nb = 1.71–2.85 ppm,

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and exhibit geochemical signatures similar to those of the Saunders island-arc (Zhang et al., 2012a; Wang et al., 2013). Northeast of the Yunkai area is the Nanling region where the Neoproterozoic rocks are represented by the 970±8 Ma Jingnan volcanic

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rocks dominated by rhyolite and rhyolitic greywackes (Shu et al., 2008). Geochemically, the Jingnan volcanic rocks are enriched in ΣREE, Rb, Th and Ce, and

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depleted in Ba, Sr, Eu, Ti, P and Nb-Ta, with moderate negative Eu and Sr anomalies, which are similar to those geochemical features of acidic volcanic rocks on continental

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arcs (Shu et al., 2008). In the Wuyishan, Badu and Chencai areas northeast of the

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Nanling region, an adequate amount of mafic to felsic volcanic rocks metamorphosed up to amphibolite facies has been recognized from the Neoproterozoic Mayuan,

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Mamianshan, Longquan and Chencai groups (Kong et al., 1995; Wang et al., 2013; Yao et al., 2013). Available geochemical data show that amphibolites from the Chencai and Longquan groups can be geochemically divided into two types, with one similar to

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N-MORB and the other similar to those basalts forming on magmatic arcs (Kong et al., 1995; Ye et al., 1995; Chen et al., 1999; Yao et al., 2013). Most recently, Yao et al. (2013) obtained a LA-ICP-MS U-Pb zircon age of 879 ± 10 Ma for the Chencai hornblende gneiss, interpreted as the protolithic age of the hornblende gneiss that is considered to be the product of subduction magmatism. Wang et al. (2013) obtained SHRIMP U-Pb zircon ages of 970 ± 10 Ma and 969 ± 13 Ma for two amphibolite samples collected from the Longquan Group in the Wuyishan area, confirming their early Neoproterozoic origin. Intruding the metavolcanic rocks of the Chencai and Longquan groups are some diorites of which the Lipu diorite in the Chencai area yielded a SHRIMP

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Pb/238U age of 841 ± 6 Ma (Li et al., 2010b), which is nearly

identical to a SHRIMP 206Pb/238U age of 838 ± 5 Ma obtained for a rhyolite from the same group (Li et al., 2010b). Kong et al. (1995) proposed that the mafic to felsic 10

ACCEPTED MANUSCRIPT metavolcanic rocks and associated diorites in the Wuyishan and Chencai areas represented an early Neoproterozoic continental margin arc, which is most recently advocated by Cawood et al. (2013) and Wang et al. (2013). Taken together, we conclude

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that like the southeastern margin of the Yangtze Block, the northwestern margin of the

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Cathaysia Block also developed in a magmatic arc during early Neoproterozoic time. 5. Timing of final collision between the Yangtze and Cathaysia blocks When the Yangtze Block collided with the Cathaysia Block to form the intervening

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Jiangnan Orogen is still a controversial issue. Although some researchers argue that the collision between the two blocks may not have occurred until the early Paleozoic (Chen

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and Rong, 1999; Gu et al., 2002), and Hsü et al. (1988) even speculated that the Yangtze and Cathaysia blocks amalgamated in the early Mesozoic, most other people believe

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that the collision of the two blocks happened in the early Neoproterozoic (Shu et al.,

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1994, 1995; Li et al., 1995; 2002, 2007, 2009; Zhao and Cawood, 1999; Li, 1999; Wang et al., 2004, 2007, 2008a,b; Zhao et al., 2011). However, the exact time of the

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Neoproterozoic collision between the two blocks has invited a hot debate, with one school of thought proposing that the collision occurred at some time between 900-880 Ma (Li et al., 2003; Wang and Li, 2003; Li et al., 2007, 2009; Wang et al., 2011),

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whereas others argue that the collision may have occurred at some time between 870-830 Ma (Shu et al., 1994, 1995; Zhao and Cawood, 1999; Wang et al., 2004, 2007, 2008a,b; Shu et al., 2011; Zhao et al., 2011). However, recent zircon U-Pb age data obtained for the Sibao, Fanjingshan, Lengjiaxi, Shuangqiaoshan and Shangxi groups in the Jiangnan Orogen are supportive of neither the 900-880 Ma nor 870-830 Ma collisional models (Zhao and Cawood, 2012 and references wherein). There is a broad agreement that the sedimentary-volcanic protoliths of these groups developed before the collision, and were deformed and metamorphosed during collision between the Yangtze and Cathaysia blocks (Shu et al., 1994, 1995; Li et al., 1995; 2002, 2007, 2009; Li, 1999; Wang et al., 2007, 2008a,b; Zhao et al., 2011). Thus, the collision between the Yangtze and Cathaysia blocks must have occurred at some time after the formation of these groups. Recent zircon U-Pb age data reveal that all of these groups were formed 11

ACCEPTED MANUSCRIPT in the period 850-825 Ma (Wang et al., 2007, 2008a,b, 2011d, 2012c,d; Gao et al., 2008, 2010a,b, 2011; Zhou et al., 2009; Zhang et al., 2012b, 2013), which indicate that the collision must have happened at some time after 825 Ma. On the other hand, these

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groups are unconformably overlain by middle Neoproterozoic weakly metamorphosed Banxi Group and equivalent strata, which are considered to have formed shortly after

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collision between the Yangtze and Cathaysia blocks (Shu et al., 1994, 1995; Li et al., 1995; 2002, 2007, 2009; Zhao and Cawood, 1999; Li, 1999; Wang et al., 2007, 2008a,b; Gao et al., 2008, 2010a,b, 2011; Zhao et al., 2011). The bentonites, dacites and other

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volcanic rocks from the Banxi Group and its equivalent strata yielded SHRIMP U-Pb zircon ages of ~815 Ma (Wang et al., 2003b, 2006b; Gao et al., 2011). Thus, the timing

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of collision between the Yangtze and Cathaysia blocks can be restricted to a short period between 825-815 Ma. It deserves mentioning here that minor blueschists have

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been found in the Jiangnan Orogen (e.g. Shu and Zhou, 1988; Zhou, 1989; Zhou and

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Shu, 1989; Zou et al., 1995; Zhou and Zou, 1996), and Shu et al. (1994) obtained a mineral Ar/Ar age of 866 ± 14 Ma for a blueschist sample collected from NE Jiangxi

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Province. However, this age is open to different interpretations as it may represent the timing of a subduction event, not the timing of collision between the Yangtze and Cathaysia blocks.

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6. A divergent double-sided subduction model for the Jiangnan Orogen The presence of volcanic arc assemblages on the southeastern margin of the Yangtze Block and the northwestern margin of the Cathaysia Block and their age data allow us to propose the following divergent double-sided subduction model for formation and evolution of the Jiangnan Orogen: (A) In the period 1000-825 Ma, the Yangtze and Cathaysia blocks were separated by an old ocean whose lithosphere was undergoing divergent double-sided subduction beneath the southeastern margin of the Yangtze Block and the northwestern margin of the Cathaysia Block (Fig. 2A). The initial stage (1000-890 Ma) of divergent double subduction evolution involved the development of volcanic arcs on both the margins, represented by the 970–890 Ma Shuangxiwu arc volcanic rocks at the southeastern 12

ACCEPTED MANUSCRIPT margin of the Yangtze Block and the 997-970 Ma Yunkai, Nanling and Wuyishan arc-relate volcanic rocks on the northwestern margin of the Cathaysia Block. This was followed by development of accretionary-wedge basins on both the margins in the

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period 890-825 Ma, forming a series of volcanic-sedimentary associations, represented by the Sibao, Fanjingshan, Lengjiaxi, Shuangqiaoshan and Shangxi groups on the side

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of the Yangtze Block, and the Chencai, Longquan, Mayuan, Mamianshan, Wanquan, Louziba, Shenshan and Yunkai groups on the side of the Cathaysia Block. Associated with arc volcanism at this stage was the emplacement of minor diorites, represented by

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the ~840 Ma Lipu diorite pluton in the Cathaysia Block (Fig. 2A).

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(B) At ~825 Ma, the divergent double subduction on both sides of the oceanic slab eventually led to the closure of the ocean basin and then the Yangtze and Cathaysia blocks were welded together (soft collision) to form the Jiangnan Orogen, without

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involvement of continental deep subduction, high-grade metamorphism of continental

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crust and uplift/exhumation of high-grade metamorphic rocks (Fig. 2B).

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(C) Shortly after the soft collision, the oceanic lithosphere started to be detached from the overlying crust and sedimentary sections, which induced underplating of mantle magmas, triggering partial melting of accretionary-wedge strata to form some

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peraluminous (S-type) granites in the period 825-815 Ma (Fig. 2C). (D) Finally, the sinking of the oceanic slab pulled down the overlying strata to form some basins in which the Banxi Group and its equivalent strata were formed in the period 815-750 Ma (Fig. 2D), in association with bimodal magmatism, represented by 800–760 Ma volcanic rocks and mafic dykes in the Jiangnan Orogen (Wang et al., 2012). Thus, the proposed divergent double subduciton model can reasonably explain all special features that characterize the Jiangnan Orogen but cannot be well explained by a single-sided subduction model.

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ACCEPTED MANUSCRIPT Acknowledgements We would like to thank Yongei Zheng, Liangshu Shu, Xianhua Li, Xiaolei Wang,

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Yuejun Wang and Jinhai Yu for their thoughtful discussions. The work was partly

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supported by a NSFC project (41190075) and Hong Kong RGC GRF (7063/13P and

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7060/12P).

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References

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Fig.1. Schematic map of South China showing the Jiangnan Orogen intervening between the Yangtze Block in the northwest and the Cathaysia Block in the southeast.

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Fig. 2. Geological map showing the distribution of Precambrian rocks in the Jiangnan Belt and the Yangtze and Cathaysia Blocks.

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Fig.3. A series of schematic sections showing the tectonic evolution of a divergent double subduction system in the Jiangnan Orogen along which the Yangtze and Cathaysia Blocks amalgamated to form South China.

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► Volcanic rocks with arc affinities have been recognized on both sides of the Jiangnan Orogen. ► A divergent double subduction model is applied to the Jiangnan Orogen. ► Such a model can explain the absence of continental deep subduction in the Jiangnan Orogen.

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