Shaoxing-Pingxiang Fault and Late Early Paleozoic Juxtaposition of the Yangtze Block and the West Cathaysia Terrane, South China

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Origin and age of the Shenshan tectonic mélange in the Jiangshan- Shaoxing-Pingxiang Fault and late Early Paleozoic juxtaposition of the Yangtze Block and the West Cathaysia terrane, South China

Lijun Wang1,2, Kexin Zhang1,3, Shoufa Lin2,†, Weihong He1, and Leiming Yin4 1State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China 2Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada 3Institute of Geological Survey, China University of Geosciences, Wuhan 430074, China 4Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China

ABSTRACT mélange is not an ophiolitic mélange, but extensional tectonics, passive margin evolution, rather a tectonic mélange that formed as a subduction zones, collisional tectonics, strike- When and how the Yangtze Block (Yang- result of movement along the JSP Fault in the slip tectonics, and intracontinental deformations tze) and the West Cathaysia terrane (West early Paleozoic. We suggest that Yangtze and (Naylor, 1982; Harris et al., 1998; Huang et al., Cathaysia) in South China were amalgam- West Cathaysia were two separate microcon- 2008; Festa et al., 2010; Osozawa et al., 2011; ated are critical to a better understanding of tinents, were accreted to two different parts Wakabayashi and Dilek., 2011). Tectonic mé- the Neoproterozoic to early Paleozoic tectonic of the northern margin of Gondwana in the langes at plate boundaries are commonly sub- evolution of South China and remain highly early Early Paleozoic, and juxtaposed in the ordinate to broken formations and can provide debatable. A key to this debate is the tec- late Early Paleozoic through strike-slip move- much useful information through analyses of tonic significance of the Jiangshan-Shaoxing- ment along the JSP Fault. We further suggest their contact relationships, matrix characteris- Pingxiang (JSP) Fault, the boundary between that the ca. 820 Ma collision in the Jiangnan tics and detailed studies of the blocks (e.g., Fes- Yangtze and West Cathaysia. The Shenshan Orogen took place between Yangtze and a ta et al., 2010, Robertson and Ustaömer, 2011; mélange along the JSP Fault has the typical (micro)continent that is now partly preserved Zhang et al., 2014; Raymond, 2019). Mélanges block-in-matrix structure and is composed as the Huaiyu terrane and was not related to at plate boundaries are not only of the subduc- of numerous shear zone-bounded slivers/ West Cathaysia. We compare our model for tion-accretion-related types but also other types lenses of rocks of different types and ages that South China with the accretion of terranes in that are not directly associated with the closure formed in different tectonic environments, the North American Cordillera and propose of oceans (e.g., strike-slip mélanges). Under- including middle to late Tonian volcanic and a similar model for the relationship between standing the mélange-forming processes and volcanogenic sedimentary rocks (turbidite) the Avalon and Meguma terranes in the Ca- related units in the geological record is of first- of arc/back-arc affinity, a series of middle nadian Appalachians, i.e., the two terranes order significance in understanding the tectonic Tonian ultramafic to mafic plutonic rocks of were accreted to two different parts of the evolution of mountain belts and the amalgama- oceanic island basalt affinity, a carbonaceous Laurentian margin and were later juxtaposed tion of terranes. This requires documentation of shale that was deposited in a deep marine through margin-parallel strike slip faulting. the spatial and temporal relationships between environment, and a red mudstone. U-Pb zir- the mélange-forming processes and tectonics. con ages and acritarch assemblages (Leios- INTRODUCTION The Jiangshan-Shaoxing-Pingxiang Fault phaeridia-Brocholaminaria association) found (JSP Fault; Fig. 1B) is one of the most important in the turbidite confirm its Tonian age, and Mélanges are mappable units or bodies of fault zones in South China and is traditionally fossils from the carbonaceous shale (Asterid- rocks characterized by a lack of internal conti- considered as the boundary between the Yang- ium-Comasphaeridium and Skiagia-Celtibe- nuity of contacts or strata and by the inclusion tze Block (Yangtze) and the Cathaysia Block rium-Leiofusa) constrains its age to the Early of blocks of different ages and origins in a fine- (Cathaysia) (Ren et al., 1990, 1998; Guo et al., to Middle Cambrian. Field relationships and grained matrix (typically shale/slate, sandstone, 1989; Wang and Mo, 1995; Xu et al., 1992; available age data leave no doubt that the ul- or serpentinite and less commonly carbonate, Li, 1993; Zhang et al., 2015b). Some studies tramafic-mafic rocks are exotic blocks (rather evaporate, or volcanic rocks) (block-in-matrix suggest that it is a Neoproterozoic suture zone than intrusions) in the younger metasedimen- fabric) (Greenly, 1919; Hsü, 1968; Silver and resulting from the amalgamation between the tary rocks. We conclude that the Shenshan Beutner, 1980; Raymond, 1984). Mélanges are two blocks (Shui 1986; Shu et al., 2014; Yao not unique to subduction/accretion zones, as they et al., 2019) and was reactivated in the early Shoufa Lin https://orcid.org/0000-0003-1172- also occur in other tectonic or sedimentary envi- Paleozoic to accommodate intraplate deforma- 922X ronments (Raymond, 1984; Festa et al., 2010; tion between them (Sun et al., 2018), where- †Corresponding author: [email protected]. Zhang et al., 2014). They can be associated with as others argue that it is an early Paleozoic

GSA Bulletin; Month/Month 2021; 0; p. 1–17; https://doi.org/10.1130/B35963.1; 14 figures; 1 supplemental file. published online 7 April 2021

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A B

Figure 1. (A) Map showing the tectonic-stratigraphic divisions of China (modified from Zhang et al., 2015b). (B) Map showing the tectonic framework of South China (modified from Zhao and Cawood, 2012; Lin et al., 2018a).

subduction-accretionary zone (He et al., 2015; the mélange, its relationship with the JSP Fault terrane and Southwest terrane (Yao et al., 2019). Qin et al., 2015; Zhang et al., 2015b; Dong, and implications of our results for the tectonic The Northeast Jiangxi Fault is the suture zone 2016; Pan et al., 2016; Liu et al., 2018). Ex- significance of the JSP Fault, the Neoprotero- between the Huaiyu and Jiuling terranes (Zhang posed in the middle segment of the JSP Fault zoic to Paleozoic evolution of South China, and et al., 2015a; Lin et al., 2018a). Following the as- is the Shenshan mélange (Fig. 1B) which was the position of South China in the Gondwana sembly between the two terranes at ca. 820 Ma, previously considered to be an early Paleozoic supercontinent. In particular, we emphasize the the Jiangnan Orogen underwent regional exten- subduction-accretionary mélange or ophiolitic importance of strike-slip faulting in the tectonic sion, resulting in the development of the Nanhua mélange (Pan et al., 2016; Zhang et al., 2016). evolution of South China. Rift Basin (Wang and Li, 2003; Li et al., 2005; However, the presence of the mélange has Shu et al., 2011; Wang et al., 2012; Wang et al., been disputed; Yang et al. (2012) and Wang GEOLOGICAL SETTING 2014). This was followed by the deposition of et al. (2019) suggest that the lithological units Cryogenian to early Paleozoic successions in involved are all of Neoproterozoic ages and the South China was previously considered to this basin, characterized by earlier clastic rocks volcano-sedimentary rocks form part of a con- have formed by the amalgamation of Yangtze to sourced from Yangtze, tillite, dolostone cap, and tinuous volcano-sedimentary sequence, intrud- the northwest and Cathaysia to the southeast, re- later (passive margin) platform and basin and ed by mafic and ultramafic rocks, all formed in sulting in the intervening Jiangnan Orogen (Guo slope facies (Fig. 2). The rift system features the a middle to late Neoproterozoic rift basin (the et al., 1989; Charvet et al., 1996; Li et al., 2002; Tonian to Cambrian sedimentary sequences in Nanhua Rift Basin). Thus, it is important for a Zhao and Cawood, 2012; Zhang et al., 2013). the Jiangnan Orogen (Fig. 2B). better understanding of the tectonic evolution The most recent studies suggest that the Jiang- Across the JSP Fault, Cathaysia is divided into of South China to investigate if the Shenshan nan Orogen and Cathaysia formed by accretion/ the West Cathaysia terrane (West Cathaysia) and mélange is truly a mélange and, if so, what collision of multiple terranes at different times, the East Cathaysia terrane (East Cathaysia) by type of mélange (ophiolitic mélange or not) involving arc-continent accretion, continent- the Northwest Fujian Fault (Fig. 1B; Lin et al., and of what age (Paleozoic or Neoproterozoic). continent collision, and orogen-parallel large- 2018a). West Cathaysia is featured by two suites In this paper, we report results of a detailed scale strike-slip motion (Fig. 1B; Yin et al., 2013; of Neoproterozoic arc rocks (Fig. 2B). The ear- field-based investigation of the Shenshan mé- Xia et al., 2018; Lin et al., 2018a, 2018b; Yao lier suite includes ca. 1.0–0.9 Ga meta-rhyolite lange. After a brief introduction of the geologi- et al., 2019; Wang et al., 2020b). and meta-basalt that sporadically occur in the cal setting, we describe the various lithologies in The Jiangnan Orogen occurs to the north of Yunkai and Wuyi domains (Wang et al., 2013b; the mélange and present U-Pb geochronological the JSP Fault (Fig. 1B) and includes Proterozoic Xia et al., 2018). The later suite is mainly ex- and biochronological evidence for the ages of arc/back-arc basin assemblages. It consists of the posed in the Wuyi domain, consisting of thick the (volcano-)sedimentary rocks in the mélange. Mesoproterozoic to Neoproterozoic composite turbidite sequences, volcanogenic sedimentary We conclude with a discussion on the origin of Huaiyu terrane and the Neoproterozoic Jiuling rocks, granite, rhyolite and rhyolitic tuff with

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Figure 2. (A) The Early Cambrian tectonic lithofacies and palaeogeography map of South China showing the contrasting sedimentary sequences across the Jiangshan-Shaoxing-Pingxiang Fault (based on Wang, 1995; Liu and Xu, 1994; Zhang et al., 2020). (B) Summary of the main characteristics of the pre-Devonian principal lithological units and tectonothermal events for the Jiangnan Orogen and the West Cathaysia terrane (modified from Wang et al., 2020b). HT/HP—high temperature/high pressure.

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ages of 850–700 Ma (Wan et al., 2007; Xia characterized by several Cryogenian volcano- et al., 2013a; Shu et al., 2014; Chen et al., 2015; et al., 2015; Yu et al., 2018; Jiang et al., 2019; genic and hydrothermal iron formations and Lin et al., 2018a). Qi et al., 2019). In contrast to the Jiangnan Oro- early Paleozoic siliceous rock layers (Fig. 2B). The contrast in pre-Devonian geological his- gen, the late Neoproterozoic to early Paleozoic These pre-Devonian rocks are highly deformed tory between the Jiangnan Orogen and West sedimentary sequences in West Cathaysia are and metamorphosed (up to granulite facies) Cathaysia across the JSP Fault is summarized dominated by thick sequences of clastic rocks between 460 and 420 Ma (the early Palaeozoic in Figure 2B. The pre-Devonian rocks on both derived from Pan-African-aged sources, and Wuyi-Yunkai orogeny) (Li et al., 2009; Wang sides of the fault are unconformably overlain

Figure 3. (A) Simplified geological map of the Shenshan area, Jiangxi Province, China showing structural relationships and lithological units in the Shenshan tectonic mélange (based on the geological map of Yichun City [scale 1:250,000], Geological Survey Institute of Jiangxi Province, 2013, and additional data collected during this study). Also shown are acritarch and zircon sample locations. Inset is an equal-

area lower-hemisphere projection of poles to tectonic foliation (S1) and lineation (L1) in the map area. (B) A schematic cross-section (location shown in A) showing the structural relationships among the different lithological units in the Shenshan tectonic mélange.

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by the Middle Devonian strata (an overlapping subduction-accretion mélange, which they rather than sedimentary, as is evidenced from sequence) and the two sides have shared a com- termed the “Shenshan ophiolite mélange,” that the strong deformation of the matrix (Figs. 5 mon geological history since. formed as a result of the closure of the proposed and 6). In the northern part of the area, toward East Cathaysia consists of Paleoproterozoic early Paleozoic Huanan Ocean between Yang- the JSP Fault, the rocks are more intensely de- (1.9–1.8 Ga) metasedimentary and meta-plu- tze and Cathaysia. Wang et al. (2020b) shows formed than those in the south and mylonite, tonic rocks (Yu et al., 2009; Lin et al., 2018a, that the dominant turbidites in the study area tectonic foliation, and lineation are well devel- and references therein). These rocks experienced were derived from local contemporaneous late oped (Fig. 5). Due to later folding (Fig. 5D), intense Mesozoic (ca. 250–230 Ma) amphibolite Neoproterozoic arc igneous rocks. the foliation and lineation have variable orien- to granulite (and locally eclogite) facies meta- tations (Fig. 3A). Poles to the foliation define a morphism (Zhao et al., 2017) but not the early GEOLOGY OF SHENSHAN MÉLANGE great-circle girdle, indicating a shallowly west- Paleozoic metamorphism, and are suggested to northwest-plunging fold axis (Fig. 3A, inset). have originated from an Indosinian-aged (early The Shenshan mélange was first reported by Quartz-carbonate veins are common. They are Mesozoic) orogen to the south through strike- Pan et al. (2016). It is located to the south of subparallel to the foliation and are variably de- slip motion along the Northwest Fujian Fault Xinyu City, to the north of the town of Liangshan formed (Fig. 5A). Evidence for strong shearing (Lin et al., 2018a, 2018b). in Jiangxi Province (Fig. 3A). In the north, the is widespread at both the outcrop and micro- Available structural and geochronological mélange is unconformably overlain by Creta- scopic scales (Fig. 5). data indicate that the JSP Fault has a compli- ceous sedimentary rocks (Fig. 3A). In the south, The Shenshan mélange includes four litho- cated structural history. It records a major sinis- it is in tectonic contact with Cryogenian to Edia- logical units: turbidite, carbonaceous shale, red tral strike-slip movement in the early Paleozoic caran Cathaysia-type sedimentary rocks that mudstone, and ultramafic to mafic rocks. They (Wang et al., 2015; Zhang et al., 2018; Sun host the Xinyu banded iron formation. During are described below. For simplicity, we use their et al., 2018). This deformation took place under the early Paleozoic Wuyi-Yunkai orogeny, these protolith names in our description and the prefix greenschist-facies conditions, with a tempera- sedimentary strata experienced greenschist-fa- “meta-” is implied. ture between 425 °C and 475 °C (Zhu and Zhou, cies metamorphism and were intruded by S-type 1994; Sun et al., 2018). A mylonite in the fault granite. The metasedimentary rocks and granite Turbidite zone yielded a biotite 40Ar/39Ar plateau age of are unconformably overlain by the Middle De- 431 ± 4 Ma, interpreted as a cooling age (Wang vonian sedimentary rocks (Fig. 2B). The turbidite is the dominant rocks in the et al., 2015). The fault was reactivated during Detail field work in this study shows that the Shenshan mélange. It comprises tuffaceous the Mesozoic (Sun et al., 2018), although Meso- Shenshan mélange consists of various fault- sandstones, siltstones, and mudstones, locally zoic movement is minor compared to the early bounded slivers (Fig. 3). The mélange has a containing felsic tuff layers. It experienced Paleozoic one. There is no evidence for Neo- block-in-matrix structure at the outcrop scale greenschist-facies metamorphism with well- proterozoic deformation along the fault (Zhang with blocks of ultramafic to mafic plutonic developed cleavage in siltstones and mudstones et al., 2018). The fault has an overall northeast rocks embedded in a turbidite (Fig. 4A) or car- (or slate) (Figs. 6A and 6B). strike, but changes to an east-northeast direction bonaceous shale matrix (Fig. 4B), or blocks of In the southern part of the study area, the de- in its central segment, including in the study area carbonaceous shale and red mudstone embed- formation is weaker and Bouma sequences with (Fig. 1). It is not clear if this change in strike is a ded in the turbidite (Fig. 3B). Such a block-in- parallel laminations are locally well preserved primary feature and was present during the early matrix feature is also evident at the map scale in low-strain zones (Figs. 6A). In the northern Paleozoic movement, or is due to later deforma- (Fig. 3). Most of the blocks range from tens of part of the area, the turbidite is intensely de- tion. Shu et al. (1995) reported evidence for centimeters to tens of meters in size, but some formed and highly foliated and lineated. Here, early Paleozoic sinistral strike-slip movement are much larger (Fig. 3). The enormous sizes of it represents the main mélange matrix with along an east-northeast–trending shear zone many of the blocks (up to a few kilometers in blocks of ultramafic-mafic rocks embedded (the Nanchang-Wanzai shear zone) to the north length; Fig. 3) make it unlikely that the mélange in it or is interleaved with carbonaceous shale of the study area. Similar age, orientation, and is a deformed olistostrome. by fault. kinematics indicate that this shear zone and the The primary cause of fragmentation and Recent zircon U-Pb dating and whole rock JSP Fault were potentially kinematically related. mixing of the various lithologies is tectonic geochemical studies by Wang et al. (2020b) The study area is located in Jiangxi Province, China, in the northwestern margin of West Ca- thaysia and in the JSP Fault zone (Figs. 1B and A B 3A). It is dominated by Precambrian rocks that consist of low- to medium-grade metamorphosed turbidite (DGMRJP, 1997; Yang et al., 2012; Wang et al., 2020b). Some researchers suggested that these meta-sedimentary rocks were formed in a continental-rift setting, triggered by extensional tectonics during the late Neoproterozoic, similar to the sedimentary rocks in the Nanhua Rift Basin on the north side of the JSP Fault (Figs. 2A and 2B; Yang et al., 2012; Wang et al., 2014; Shi et al., 2015). More Figure 4. Field photographs showing the block-in-matrix structure in the Shenshan tectonic recently, Pan et al. (2016) and Zhang et al. mélange of South China. (A) A gabbro block within a turbidite matrix. (B) A gabbro block (2016) proposed that these rocks represent a within a carbonaceous shale matrix. The gabbro blocks are outlined by the dashed lines.

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A B slices and are in fault contact with the turbidite or the ultramafic-mafic rocks (Fig. 3). Most of them are highly deformed and have a strong foliation (Fig. 6C). During this study, organic- walled microfossils are found in the carbonaceous shale, as well as in the turbidite. They are described in detail below to constrain the ages and the depositional environment of these rocks.

Red Mudstone

C D In the southern part of the study area, the mélange contains a hemipelagic red mudstone (shale) of several meters to hundreds of meters thick, mostly finely laminated (Fig. 6D). The age of this lithology is unknown. These rocks are tectonically mixed with tuffaceous turbidite, and are highly deformed (Fig. 6D), resulting in strong foliation. They also contain abundant foliation-parallel, variably deformed carbonate and quartz veins.

Figure 5. Field photographs and photomicrograph of highly deformed rocks (mylonite) in Ultramafic to Mafic Rocks the Shenshan tectonic mélange of South China. (A) Deformed siltstone with quartz veins in the turbidite. (B) Deformed carbonaceous shale with well-developed tectonic foliation and Ultramafic and mafic bodies mostly occur in stretching lineations. (C) Mylonitic tuffaceous siltstone. (D) Folds in foliation in tuffaceous the northern part of the Shenshan mélange. They siltstone. Qtz—quartz. have elongate and lozenge shapes and occur at various sizes. Larger bodies are a few hundred indicate that these tuffaceous turbidites were Carbonaceous Shale meters to a few kilometers long and are in fault deposited at ca. 790–740 Ma and were sourced contact with the turbidite and the carbonaceous mostly from locally distributed coeval arc igne- The carbonaceous shales (slate) are mainly shale (Fig. 3). Smaller ones are commonly sur- ous rocks. Additional zircon U-Pb data are pre- exposed in the northern part of the area. They oc- rounded by slickensides and occur as centimeter- sented below. cur in discontinuous, meter- to km-scale tectonic to meter-scale blocks in the metasedimentary rocks (Fig. 4). AB The ultramafic and mafic rocks consist of highly altered cumulate pyroxenite, gabbro, and diabase. Spinel-bearing serpentinized cumulate pyroxenite are exposed in the west of the Shenshan area, mostly in a 2-km-long body (Fig. 3A). It is massive, dark grayish-green in color (Fig. 7A) and has cumulate medium- grained textures (Fig. 7C). Primary minerals are mainly pyroxene, olivine, hornblende, spinel, and magnetite. Altered olivine crystals are enclosed in hornblende or pyroxene in spinel- CD bearing serpentinized pyroxenite (Fig. 7C). Serpentine commonly forms as complete pseu- domorphs of olivine and pyroxene. Chrome spinel is observed as disseminated euhedral to subhedral grains and vermicular textured clus- ters of grains (Fig. 7C). The gabbro is of grayish-green color alter- nating with yellowish-brown color and has a medium-grained texture (Fig. 7B). It has under- gone some degree of alteration and its pre-alteration modes are estimated at pyroxene (60%), plagioclase (25%), and minor Fe-Ti oxides. Pri- Figure 6. Photographs of representative lithologies and sedimentary structures of sedimentary mary pyroxene grains are replaced by tremolite. rocks in the Shenshan tectonic mélange of South China. (A) Tuffaceous siltstone with bedding The plagioclase is partly to completely replaced (S0) and foliation (S1). (B) Tuff layer. (C) Carbonaceous shale. (D) Highly deformed red mudstone. by fine-grained zoisites Fig. 7D( ).

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A B ing was carried out using the SQUID and Isoplot programs (Ludwig, 2003). The zircon U-Th-Pb isotopic data are given in Table S1 in the Supple- mental Material1. The uncertainties for the isotopic ratios of individual analyses are quoted at 1σ, whereas uncertainties for weighted mean ages are given at 95% confidence level.

Sample Description and Results The sample (S202) is a gray to light green fine- grained felsic tuff (collected at 27°42′32.8″N, C D 114°52′40.4″E; Fig. 3). It is foliated and metamorphosed to greenschist facies (Fig. 6B). The sample yielded abundant zircon, generally small in size with lengths of 30–100 µm (mostly <50 µm) and aspect ratios from 1:1–2:1. They can be divided into three groups based on mor- phology and internal textures shown under CL images (Fig. 8). Zircon grains from Group 1 are smaller in size and mostly equant. They include euhedral to subhedral grains or crystal fragments and display oscillatory zoning, indicating a mag- Figure 7. Field photographs and photomicrographs of ultramafic to mafic rocks in the Shen- matic (most likely volcanic) origin. Grains from shan tectonic mélange of South China. (A) Foliated cumulate olivine hornblende pyroxenite. Group 2 are subhedral to subrounded and some (B) Gabbro. (C) Olivine hornblende pyroxenite. (D) Gabbro. Pl—plagioclase; Ol—olivine; of them have a narrow bright rim in their CL Cpx—clinopyroxene; Hb—hornblende; Spl—spinel. images. They also exhibit oscillatory zoning, consistent with a magmatic origin. These two The gabbro has yielded a zircon U-Pb age of resolution ion microprobe (SHRIMP) method. groups dominate the zircon populations from 791 ± 9 Ma (Wang et al., 2019). Geochemical In addition, statistical analyses were conducted our sample. Group 3 zircon are subrounded to investigations indicate that these mafic rocks are on the laser ablation–inductively coupled plas- rounded and are obviously detrital zircon. Some generally similar to oceanic island basalt (OIB) ma–mass spectrometry (LA-ICP-MS) zircon of them have core and rim structures. and high-Nb and Nb-enriched mafic rocks. data of Wang et al. (2020b). A total of 31 analyses were conducted on Combining with their εNd(t), εHf(t), and δ18O 31 grains. Three of the analyses were rejected values, these ultramafic-mafic rocks were most SHRIMP U-Pb Zircon Geochronology due to high common lead. 16 of the analyses likely derived from an enriched OIB-like mantle were on the Group 1 zircon. Their U contents source (Wang et al., 2019). Analytical Techniques and Th/U ratios are 48–354 ppm and 0.57– Different bodies of the ultramafic to mafic Zircon grains were separated from a sample 1.84, respectively. They form a tight cluster rocks may have different compositions and grain of the felsic tuff using the conventional min- and define a weighted mean 206Pb/238U age of sizes but individual bodies are generally inter- eral separation method, involving crushing, and 812.5 ± 7.5 Ma (mean square weighted devia- nally homogenous in composition and grain size. gravity and magnetic separation procedures. tion [MSWD] = 0.88) (Fig. 9B). This is inter- Notably, the grain size does not correspond to the Grains were mounted in epoxy and ground to preted as the age of volcanism and sedimenta- size of a body, and some very small bodies have expose their cores. Cathodoluminescence (CL) tion of the felsic tuff. Six of the analyses were coarser grains. There is no evidence for chilled images were taken to reveal the internal structure on the Group 2 zircon. Their U contents and margins or contact metamorphism. Instead, the of the zircons which helped in selecting areas Th/U ratios are 83–199 ppm and 0.57–1.41, contacts are invariably sheared and commonly for analyses. respectively. Five of them form a tight cluster altered. It is evident that these ultramafic-mafic The zircon was analyzed for U-Pb isotopes and define a weighted mean 206Pb/238U age of rocks are not intrusions into the supracrustal by SHRIMP II at the Beijing SHRIMP Center, 853.0 ± 15 Ma (MSWD = 1.13) (Fig. 9C). It is rocks as previously suggested (e.g., Yang et al., Institute of Geology, Chinese Academy of Geo- interpreted as the age of inheritance or detrital 2012; Wang et al., 2019). Instead, they are more logical Sciences, Beijing, China. The analytical zircon. Five analyses on the Group 3 detrital likely exotic blocks embedded in the carbona- procedures were similar to those described by zircon grains or their cores yield 207Pb/206Pb ceous shale and turbidite. This interpretation is Williams (1998). The intensity of the primary ages from ca. 2458 to 2523 Ma (Fig. 9A). One − 207 206 reinforced by evidence that the ultramafic-mafic O2 ion beam was ∼2.7 nA (at an energy of analysis has a Pb/ Pb age of ca. 1737 Ma, rocks are older than the supracrustal host rocks 10 kev), and the spot size was around 18 µm. Ref- (see below). erence standard zircons M257 (U = 840 ppm; 206Pb/238U age = 562 Ma) (Nasdala et al., 2008) 1Supplemental Material. Photos of acritarchs, U-Pb ZIRCON GEOCHRONOLOGY and TEMORA 1 (206Pb/238U age = 417 Ma) table of U-Pb zircon data and results of mixture (Black et al., 2003) were used for calibration of modeling of U-Pb zircon data from the Shenshan 206 238 tectonic mélange of South China. Please visit https:// To better constrain the ages of the supracrustal elemental abundance and Pb/ U, respective- doi.org/10.1130/GSAB.S.14058050 to access 204 rocks in the Shenshan mélange, zircon from a ly. The measured Pb abundances were applied the supplemental material, and contact editing@ felsic tuff layer was dated by the sensitive high- for the common lead correction. Data process- geosociety.org with any questions.

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with M, respectively, are well above the thresh- old of 0.05 that gives a 95% confidence that the populations compared are identical (or more precisely, the difference between them are statistically insignificant). In contrast, theP values for comparing samples B and H with samples L and M are below 0.05, indicating that the two sets of samples have statistically different zircon populations. The Neoproterozoic ages appear to vary con- tinuously from ca. 730 to 900 Ma. As there is no evidence for almost continuous igneous ac- tivities lasting for over 150 m.y. in the middle to late Tonian in this part of South China, or South China in general, it is likely that the data is a Figure 8. Representative cathodoluminescence images of three groups of zircons from tuff result of mixing of multiple age components. sample S202 from the Shenshan tectonic mélange of South China. Circles indicate sensitive Mixture modeling (Sambridge and Compston, high-resolution ion microprobe analytical spots with numbers as in Table S1 (see footnote 1994) was performed using Isoplot to help iden- 1). The ages are indicated for each analysis. tify distinct age components (or peaks) in each sample. Following the advice from Sambridge and Compston (1994, p. 381), we “attempt to also interpreted as from a detrital zircon zoic to Archean ones. The middle to late Tonian estimate the minimum number of components (Fig. 9A). ages of the four samples are shown in a cumula- necessary to explain the distribution of age mea- tive probability plot (Gehrels, 2011) in Figure 10 surements,” so that “the temptation to over-inter- Insights from Mixture Modeling and in histograms in Figure 11. pret the data is limited.” Through trial and error, of LA-ICP-MS Zircon Data Figure 10 also shows the results of a Kol- we choose three age components for samples B mogorov-Smirnov (K-S) test, a nonparametric and H and four components for samples L and Wang et al. (2020b) report LA-ICP-MS U-Pb test that compares the cumulative distributions of M. This choice is the most reasonable one as zircon ages from four samples of the tuffaceous two data sets, using an Excel macro by George decreasing the numbers of components would metasedimentary rocks in the study area. The lo- Gehrels. The probabilities (P values) obtained significantly increase the values of relative mis- cations of the samples (B, H, L, M) are shown in from the K-S test were used to compare poten- fit for each sample, and increasing them would Figure 3A. The data reported are between 95% tial similarities in zircon populations of the four not significantly decrease the misfit, but would and 105% concordant. The ages are dominantly samples. The P values of 0.965 and 0.333, for significantly reduce the consistency of the age middle to late Tonian, with minor Paleoprotero- comparing samples B with H and samples L components identified among the samples (see

A BC

Figure 9. Sensitive high-resolution ion microprobe U-Pb zircon data for tuff sample S202 from the Shenshan tectonic mélange of South China. (A) Concordia diagram showing all data. (B and C) Weighted mean age diagrams (above) and concordia diagrams (below) of Group 1 and Group 2 zircons, respectively (MSWD—mean square of weighted deviation; “n” indicates number of analyses; the dashed ellipse is datum not included in the calculation). (D) 206Pb/238U age frequency-probability histograms of Group 1 and Group 2 zircons. Arrows indicate the age components (peaks) identified by mixture modeling. Inset table shows results of mixture modeling.

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significant younger component at ca. 742 Ma Figure 10. Cumulative prob- (∼18%–25%). These ages correspond well with ability plot of Neoproterozoic known igneous ages in West Cathaysia (e.g., zircon ages from tuffaceous ca. 818 Ma for a felsic volcanic rock [Li et al., turbidite samples B, H, L, and 2005]; ca. 756 and 774 Ma for K-bentonites M from the Shenshan tectonic [Zhou et al., 2018]; ca. 770 Ma for a rhyolitic mélange of South China from tuff [Qi et al., 2019]; ca. 735 Ma for a rhyolite Wang et al. (2020b). Inset gives [Jiang et al., 2019]). Notably, the two older com- the results of a Kolmogorov- ponents (ca. 812–822 Ma and ca. 859–864 Ma) Smirnov (K-S) test (Sircombe are identical within error with the SHRIMP ages and Hazelton, 2004). The prob- presented above from sample S202. These indi- abilities (P values) compare cate that the age components identified by mix- each permutation of pairs ture modeling are geologically significant. amongst the four samples. Considering that the three sets of samples (B&H, L&M, and S202) are all tuffaceous and have distinct youngest age components (ca. 742 Ma, ca. 771–779 Ma, and ca. 812 Ma, Table S2; see footnote 1). Application of mixture The age components identified from mixture respectively), it is unlikely that they have the modeling to the SHRIMP data presented above modeling for the four samples of Wang et al. same depositional age. The easiest and most for sample S202 yields two age components at (2020b) are given in Figures 11A–11D. They likely interpretation is that they have differ- 811 ± 8 (84% of the data) and 858 ± 26 Ma are remarkably consistent among the samples. ent ages and the youngest age component for (14%) (Fig. 9D). These are identical within er- All the samples contain two major components each sample is the age of syn-depositional ror with the ages of the Group 1 and Group 2 at ca. 771–779 Ma (∼38%–47%) and ca. 812– volcanism and thus the age of sedimentation zircons presented above (812.5 ± 7.5 Ma and 822 Ma (∼26%–43%), and a minor component of the unit concerned; this is supported by the 853 ± 15 Ma, respectively), supporting the va- at ca. 859–886 Ma (mostly ca. 859–864 Ma; observation that the zircon grains in the young- lidity of the modeling method. ∼5%–19%). Samples L and M also contain a est age group in each sample are all euhedral

A C

B D

Figure 11. 206Pb/238U age frequency-probability histograms of Neoproterozoic zircons from tuffaceous turbidite samples B, H, L, and M from the Shenshan tectonic mélange of South China from Wang et al. (2020b). Arrows indicate the age components (peaks) identified by mixture modeling. Inset tables are results of mixture modeling on all data from each sample. “n” indicates number of analyses.

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to subhedral. In any case, the available age A B C D data indicate that the turbidite sequence in the Shenshan mélange includes a mixture of rocks of different ages, although they cannot be separated into different map units due to similar lithology, strong deformation (and mixing), and poor exposure.

ACRITARCH PALEONTOLOGY E F G

Acritarchs (organic-walled microfossils) from early Paleozoic rocks have great potential for dating sedimentary rocks. In several cases, paleopalynology has proven to be a powerful tool in dating the matrix of tectonic mélanges (Calon et al., 2002; Beccaletto et al., 2005) or low-grade metasedimentary clastic sequences in tectonic boundaries (Montenari and Ser- H I J vais, 2000). Acritarchs have been reported from the carboniferous shale in the Shenshan mélange (Wang et al., 2020a). During this study, acritarchs were also found in the turbidite. Here, we provide a detailed and systematic description of the acritarchs, to enrich the biostratigraphy and constrain the depositional ages of the sedimentary rocks in the Shenshan mélange. Figure 12. Acritarchs from the carbonaceous shale and the tuffaceous turbidite in Paleopalynological techniques the Shenshan tectonic mélange of South China. From the carbonaceous shale: (A) Celtiberium?papillatum Moczydłowska, 1998; (B) Asteridium pallitum (Volkova, 1968) Acritarch specimens were extracted from Moczydłowska, 1991; (C) Asteridium lanatum (Volkova, 1969) Moczydłowska, 1991; (D) Co- both the carbonaceous shale and the turbi- masphaeridium agglutinatum Moczydłowska, 1988; (E and F) Skiagia ciliosa (Volkova, 1968) dite in the study area. Because the rocks have Downie, 1982; (G) Skiagia cf. insigne (Fridrichsone, 1971) Downie, 1982; (H) Leiofusa sp. experienced low-grade metamorphism, spe- From the turbidite: (I) Leiosphaeridia sp. (J) Palaeopleurocapsa reniforma Ogurtsova and cial care was taken to extract the poorly pre- Sergeev, 1987. All scale bars are 10 μm. Additional photos are available in the Data Reposi- served organic-walled microfossils. Samples tory (Figs. S1, S2, and S3; see footnote 1). of ∼50 g were collected and treated with the palynological preparation techniques as out- Results from the Carbonaceous Shale cies is featured by larger vesicles and numer- lined in Wang et al. (2020a). The material Six out of the 40 processed rock samples ous simple longer and slender processes. The investigated and discussed herein is perma- from the carbonaceous shale unit yielded a small vesicles are circular to oval in outline. The pro- nently housed in the collections of the Insti- amount of acritarchs (Figs. 12A–12H; Figs. S1 cesses are densely distributed in the proximal tute of Geological Survey, China University and S2) accompanied by organic fragments. part, and some of them are agglutinated to each of Geosciences, Wuhan. Some species have been reported by Wang et al. other. The tips of the processes are acuminate (2020a), such as Asteridium spinosum, Aste- and flexible. Results ridium lanatum, and Comasphaeridium sp. Ad- Celtiberium and Skiagia: The identified ditional species are described below. Celtiberium?papillatum Moczydłowska, 1998 Acritarchs in the study area are metamor- Asteridium: This species is characterized by is featured by circular to subcircular vesicles phosed and carbonized, as indicated by Raman small vesicles and solid processes. It is the domi- with a number of short processes distributed on spectroscopy (Wang et al., 2020a). As a result, nant acritarch in our samples (Fig. 12B; Figs. them, and the diameter of the vesicle is usually they are fuscous-black in color and nearly opaque S1 and S2). The newly identified Asteridium 15–25 μm. The processes have various shapes; under transmitted-light microscopes, and not all pallitum (Volkova, 1968) Moczydłowska, 1991 most processes have regularly conical shape and distinctive characteristics are available. The ac- (Fig. 12B) has an oval vesicle covered by a few some are papilliform with rounded bases and ritarchs from the turbidite are dominated with irregularly distributed processes. The diameter tapering points. Most processes are hollow and spheromorphic form (Leiosphaeridia) (Fig. 12I; of the central body is ∼10–15 μm and the length some are connected with the inner cavity of the Fig. S3; see footnote 1), and some fragments of the processes is usually 2 μm. The bases of vesicle (Fig. 12E). of benthic algae (Fig. 12J). The acritarch taxa the processes are slightly widened and conical Skiagia ciliosa (Volkova, 1969) Downie, from the carbonaceous shale are characterized and the tips are blunt. Excystment is not usually 1982: This species is characterized by larger with acanthomorphic and netromorphic forms observed in this species. oval vesicles with longer hollow processes. The (Figs. 12A–12H; Figs. S1 and S2), which are Comasphaeridium agglutinatum diameters of the central body are 20–50 μm, and distinct from those from the turbidite. Moczydłowska, 1988 (Fig. 12C): This spe- the length of the processes are usually 3–10 μm.

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The processes are usually thin and slender. Some ornate-Fimbriaglemerella membranacea, He- Results from the Turbidite morphotypes have longer and wider processes liosphaeridium dissimilare-Skiagia ciliosa, and The acritarch assemblages from the siltstone (Fig. 12F). Volkovia dentifera-Liepainaplana assemblage in the turbidite are dominated with spheromor- Skiagia cf. insigne (Fridrichsone, 1971) zones (Moczydłowska, 1991; Fig. 13). As re- phic form (Leiosphaeridia) and some fragments Downie, 1982: This species is rarely found in viewed by Moczydłowska and Yin (2012), these of benthic algae which were identified asBro - our samples (Fig. 12G). It is characterized by four Lower Cambrian microfossil zones occur cholaminaria Germinosphaera sp. (see Yin and spherical or slightly ellipsoidal vesicles covered between 541 and 510 Ma. Li, 1978) and Palaeopleurocapsa reniforma by numerous thick, robust, cylindrical processes. The acritarches found in the carbonaceous Ogurtsova and Sergeev, 1987. The process bases are wide, shaped as truncated shale in this study are typical Cambrian taxa. Leiosphaeridia sp. (Fig. 12I): This species cones. The terminations of the processes are The Asteridium lanatum and Comasphaeridium is the most abundantly preserved microfossil shaped as wide, flat funnels, closed distally and agglutinatum, with the previously reported As- in our turbidite samples. Its vesicles are mor- divided into several parts. teridium spinosum and Comasphaeridium sp., phologically simple and circular to subcircular Vesicles of Leiofusa sp. are fusiform in out- can be correlated with the Asteridium tornatum- in outline. Vesicle diameters range in size from line (Fig. 12H). Its surface construction cannot Comasphaeridium velvetum Zone, which occurs 15 to 150 μm, mostly from 30 to 80 μm. The be observed due to carbonization. It is 120 µm in the basal Cambrian (Wang et al., 2020a). The surface of the vesicle is smooth to shagrinate. long and 30 µm wide. Its vesicles are smooth. Asteridium pallitum also occurs in the Lower The vesicle walls vary in thickness, opening by Cambrian and appears in the Skiagia-Fimbria- median split or partial rupture of vesicle wall. Age and Depositional Environment of the glomerella Zone, whereas the Skiagia ciliosa Germinosphaera sp. (Fig. S3): This species Carbonaceous Shale and Skiagia cf. insigne always occur in the high- has a spherical to subspherical vesicle with Many Cambrian palynofloras and acritarch er Volkovia dentifera-Liepainaplana Zone. The thicker wall; the surface of the vesicle is psilate. zonations have been studied and defined world- Celtiberium?papillatum were previously report- A long conic-cylindrical process is featured in wide. They have contributed to comprehensive ed in the Upper Cambrian Sosnowiec Formation this species. The vesicle is 35–60 μm in diameter biostratigraphical correlations and paleoeco- in the Silesia area of Poland (Moczydłowska, and the process is 10–15 μm long. logical interpretations (Shergold and Cooper, 1998). The Leiofusa sp. is rare and is confined to Palaeopleurocapsa reniforma Ogurtsova 2004; Babcock et al., 2005; Yin et al., 2010; the Middle Cambrian to Late Ordovician strata and Sergeev, 1987 (Fig. 12J): This species is Babcock et al., 2014). The Lower Cambrian (Ghelli Formation; Fig. 13). The acanthomor- recognized by its irregularly shaped colonies acritarch zones are established in the East Eu- phic acritarch assemblages we obtained from and a number of small cells. The rounded to ropean Platform which has the most complete the carbonaceous shale were usually deposited subrounded cells are tightly packed in subpar- and continuous thick Lower Cambrian succes- in a relatively deep basin environment below allel rows which comprise the variously shaped sion (Moczydłowska, 1991). They are also in- storm base. In addition, the lithological features colonies. The diameter of the cells ranges from creasingly identified in other regions, such as and sedimentary structures indicate that the car- 8 to 15 μm, and the wall is 0.5–1.5 μm thick. Iberia, South China, South Australia, Laurentia, bonaceous shale was deposited in an open shelf and Siberia (Moczydłowska, 1991, 1998; Mon- basin, below wave base and with low energy. Age of the Turbidite tenari and Servais, 2000; Moczydłowska and Therefore, the acritarch assemblages obtained Simple spherical vesicles (Leiospherids) Yin, 2012). The four zones defined in the Lower from the carbonaceous shale constrain the Early are morphological forms commonly found in Cambrian are, in ascending order: the Asteridium to Middle Cambrian age and the sedimentary Neoproterozoic sediments. Leiosphaeridia tornatum-Comasphaeridium velvetum, Skiagia environment of this unit. are known from ca. 1.8 Ga throughout the

Figure 13. Stratigraphic and geographical occurrences of acritarch assemblages from carbonaceous shales in the Cambrian (data from Baltica and Poland modified from Moczydłowska, 1991, 1998; data from Tarim, Yangtze, and Lesser Himalaya from Yao et al., 2005). Guzh.—Gu- zhangian; Drum.—Drumian; Fortun.—Fortunian.

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Proterozoic–Phanerozoic and thus have no rocks are not part of a coherent stratigraphic se- ern part of the mélange, toward the center the stratigraphic value. However, Germinosphaera quences deposited in a Neoproterozoic rift basin JSP Fault (Fig. 3). In addition, the slivers in the sp. and Palaeopleurocapsa reniforma typically as suggested by Yang et al. (2012); and the maf- mélange have elongate and lozenge shapes and occur in Neoproterozoic sequences. Germinos- ic-ultramafic rocks are not intrusions in the “rift are all parallel to the JSP Fault (Fig. 3A). Most phaera sp. is found in the Neoproterozoic sedi- sequence” as suggested by Wang et al. (2019), sedimentary rocks in the matrix are mylonitic mentary sequence in the eastern Officer Basin but are blocks enclosed in the metasedimentary with well-developed foliation and lineation. of South Australia and Palaeopleurocapsa re- matrix, an interpretation strongly reinforced by They contain foliation-parallel quartz veins that niforma occurs in the Neoproterozoic (ca. 800– the fact that the mafic-ultramafic rocks are sig- formed during deformation (Fig. 5), presumably 750 Ma) Chichkan Formation of South Kazakh- nificantly older than most of the hosting metased- resulting from metamorphic fluids generated stan (Ogurtsova and Sergeev, 1987). imentary rocks. This study supports the idea of during shearing along the JSP Fault. The mafic Some acritarch taxa have previously been Pan et al. (2016) and Zhang et al. (2016) that the rocks are variably altered, particularly along reported in the turbidite in the study area, such rock units described here in the Shenshan area their contacts with the matrix. as Baculimorpha brevis, Conusmorpha brevis, constitute a mélange. However, the OIB affinities It is well established that the Jiangnan Oro- Laminaritesz sp., Leiominusula minunta, Leiop- of the Neoproterozoic ultramafic-mafic rocks, gen (and Yangtze) and West Cathaysia on the sophosphaera densa, and Taeniatum crassa sp. and the significant age gap between the Neopro- two sides of the JSP Fault have contrasting pre- (Ling et al., 2000). These taxa are also restricted terozoic turbidite and ultramafic-mafic rocks and Devonian geological history (Fig. 2B). This in- to Neoproterozoic strata. Therefore, the U-Pb the Cambrian carbonaceous shale, suggest that dicates a significant movement along the fault in zircon and the acritarch data consistently indi- this mélange is unlikely to be an early Paleozoic the early Paleozoic, coincident with the age of cate that the turbidite in the Shenshan mélange subduction-related ophiolitic mélange developed the JSP Fault, which is an early Paleozoic sinis- is middle to late Tonian in age. in an accretionary wedge. Instead, overall evi- tral strike-slip fault (Shu et al., 1995; Wang et al., dence suggests that it is a tectonic mélange that 2015; Sun et al., 2018; Zhang et al., 2018). DISCUSSION comprises multiple tectono-stratigraphic and Based on the close spatial and temporal rela- tectono-magmatic units in the middle segment of tionship between the Shenshan mélange and the Nature of the Shenshan Mélange the JSP Fault zone at the northern margin of West JSP Fault, we infer that deformation localization Cathaysia. and the resultant strike-slip movements along the As defined by Raymond (1975) and Festa JSP Fault were the triggering mechanisms for et al. (2010), a mélange, as a body of mixed Age of the Shenshan Mélange the formation of the Shenshan tectonic mélange. rocks, contains blocks, both exotic and native, We propose that the movement and deformation that are derived from different stratigraphic units As described above, the Shenshan mélange led to extensive fragmentation, dismember- or sequences, different tectonic settings, various contains slivers and blocks of very different ages. ment, and mixing of the rocks that had formed paleogeographic domains, and/or dissimilar The ultramafic-mafic rocks and the turbidite are in different geological environments at different metamorphic zones. Neoproterozoic and the carbonaceous shale is times. We suspect that there exists a continuum Our results described above show that the Early to Middle Cambrian. The latter is signifi- from tectonic mélanges to broken formations, Shenshan mélange consists primarily of turbi- cant, as it indicates that the mélange must be Mid- corresponding to intensity of deformation and dites structurally intercalated with carbonaceous dle Cambrian or younger. Considering that there structural level of their formation. shale, ultramafic-mafic rocks, and minor red is no evidence for a major tectonic event in the It should be noted that early Paleozoic mé- mudstone (Fig. 3). It shows the typical block- region after the Early Devonian (marked by the langes have also been reported in other areas in-matrix structure, at both the map and out- sub-Middle Devonian regional unconformity), along the JSP Fault, such as the Chencai and crop scales (Figs. 3 and 4). The different units we conclude that the mélange must have formed Yingyangguan mélanges (Fig. 1B; Qin et al., have different ages and origins. The gabbro in during the early Paleozoic, as initially proposed 2015; Dong, 2016; Zhao et al., 2018). They have the ultramafic-mafic rock unit has an age of by Pan et al. (2016) and Zhang et al. (2016), even all been interpreted as the result of tectonic mix- 791 ± 9 Ma (Wang et al., 2019). This unit shows though most major lithological components in ing of Neoproterozoic and early Paleozoic rock OIB-like high-Nb/Nb-enriched basalt geochemi- the mélange are Neoproterozoic in age. units. We suggest that the formation of these cal signatures and was derived from an OIB-like tectonic mélanges was also related to the early mantle source (Wang et al., 2019). The domi- Relationship of the Shenshan Tectonic Paleozoic movement along the JSP Fault. nant middle to late Tonian tuffaceous turbidite Mélange with the Jiangshan-Shaoxing- unit is a mixture of rocks of various ages (ca. Pingxiang (JSP) Fault Implications for the Tectonic Framework of 742 Ma for samples L&M, ca. 771–779 Ma for South China B&H, and ca. 812 Ma for S202) that formed in The Shenshan mélange incorporates lithologi- arc/back-arc settings (Wang et al., 2020b). The cal components of different ages and provenanc- Tectonic mélanges are not unique to subduc- carbonaceous shale unit is Early to Middle Cam- es. This implies that the various sedimentary and tion or collision zones; they also occur in other brian in age and was deposited in a deep marine magmatic rocks enclosed in the mélange were tectonic boundary settings such as strike-slip sedimentary environment; rocks of this type and assembled at an active tectonic boundary. As faults and transform boundaries (Karig, 1980; age are widely distributed in West Cathaysia, mentioned above, available data do not support Raymond, 1984; Festa et al., 2010). In addition, such as in the lowest part of the Early Cambrian the interpretation that the mélange is a subduc- the amalgamation of blocks and/or terranes is Niujiaohe Formation (DGMRJP, 1997). tion-related mélange. not necessarily related to accretion or collision The rocks in the different lithological units are The Shenshan tectonic mélange is closely as- and the contrasting geological histories between characterized by strong deformation, mixing, and sociated, spatially, with the JSP Fault. There is two adjacent blocks (terranes) do not necessar- markedly different ages. Hence the relationships an increase in disruption of strata and intensity ily indicate closure of an ocean between them. among them cannot be primary. The sedimentary of deformation from the southern to the north- Strike-slip motion between terranes has played

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an important, and even primary role in disrupt- not an ophiolitic or subduction-related mélange the JSP Fault has led to an interpretation that ing, transporting, and amalgamating terranes in but a tectonic mélange related to the early Pa- they were not amalgamated until the early Paleo- many mountain belts, as shown by the mid-Cre- leozoic strike-slip movement of the JSP Fault. zoic, through closure of the proposed “Huanan taceous paleogeographic reconstruction of dif- Ocean” (He et al., 2015; Liu et al., 2018; Zhao ferent terranes in the Northern Cordillera (e.g., A model for the Relationship between the et al., 2018). Here we propose a model to rec- Haggart et al., 2006; Wyld et al., 2006), alloch- Yangtze Block and the West Cathaysia Terrane oncile evidence for the above two very different thonous arc terranes in the Philippines (Karig Evidence for a ca. 820 Ma collision in the Ji- interpretations (Fig. 14). The strike-slip motion et al., 1986), and the Appalachian-Caledonian angnan Orogen (the Jinning /Sibao orogeny) has along the JSP Fault in the early Paleozoic plays Orogen (e.g., Hutton, 1987; Lin et al., 2007, led many to suggest that Yangtze and Cathaysia an important role in this model. 2013). The importance of strike-slip motion in were amalgamated in the Neoproterozoic (e.g., It is generally accepted that the Jiangnan Oro- terrane amalgamation has also been proposed for Li et al., 2002; Zhao and Cawood, 2012; Shu gen formed due to a collision at ca. 820 Ma, fol- South China (Lin et al., 2018a). In this study, we et al., 2014; Yao et al., 2019), whereas the con- lowing the closure of a ca. 850–825 Ma arc-back conclude that the Shenshan tectonic mélange is trasting pre-Devonian geological history across arc system (Fig. 14A, see a1 and a2; Yin et al.,

A B

Figure 14. (A and C) Schematic diagrams showing the proposed model for the middle to late Tonian and early Paleozoic tectonic evolution of, and the relationships among, the southeastern Yangtze Block, the Huaiyu terrane, and the West Cathaysia terrane. (B) Simplified Ordo- vician paleogeographic reconstructions showing relative paleopositions of the Yangtze Block and the West Cathaysia terrane in Gondwana (modified from Cocks and Torsvik, 2013; Harper and Servais, 2014; Yao et al., 2014). See text for further explanation. (D) Simplified map of the Northern Cordillera showing the distribution of Palaeozoic to early Mesozoic allochthonous terranes and major faults (simplified from Colpron and Nelson, 2011). JSP—Jiangshan-Shaoxing-Pingxiang.

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2013; Zhang et al., 2013; Zhao, 2015; Lin et al., herent terrane before its accretion to Gondwana ping strike-slip faults, including transform faults. 2018a; Yao et al., 2019). The corresponding su- in the early Paleozoic. The recent interpretation Some (parts) of these strike-slip faults spatially ture zone is the Northeast Jiangxi Fault (not the that the North and South Wuyi domains in West coincide with the original suture zones, whereas JSP Fault) (Figs. 1 and 14A). We propose here Cathaysia were two separate “blocks” and were others cut across and dismembered the original that the collision took place between Yangtze not amalgamated until ca. 700 Ma (Jiang et al., terranes and juxtaposed terranes that might or and a continent/microcontinent of which the 2019) is thus consistent with our model. It is also might not have been in contact initially (e.g., Huaiyu terrane was a part. We here name this consistent with the suggestion that West Cathay- Price, 1994; Monger, 2008). Cumulative amount continent/microcontinent the Great Huaiyu ter- sia was located at the periphery of Rodinia in the of dextral strike-slip motion along the faults to the rane (GHT) (Fig. 14A). middle to late Tonian (Wang et al., 2020b). east of the Queen Charlotte(-Fairweather) Fault Following the collision, the area underwent There exists considerable debate concerning (Fig. 14D) is estimated to be at least ∼800 km extension from ca. 815 Ma (Fig. 14A, see a3; the position of South China in the Gondwana and up to ∼2300 km (Monger and Gibson, Wang and Li, 2003; Wang et al., 2012; Yao et al., supercontinent (Yang et al., 2004; Li et al., 2019, and references therein). In addition, there 2019). The resulting Nanhua Rift Basin overlies 2008; Cocks and Torsvik, 2013; Cawood et al., is a dextral motion of over 1000 km along the the Jiangnan Orogen and the surrounding area 2013; Xu et al., 2013; Yao et al., 2014; Zhang Queen Charlotte(-Fairweather) Fault in the last (Fig. 14A). At this time, West Cathaysia was part et al., 2015c; Wang et al., 2020c; Yang et al., 50 m.y., based on the northward motion of the of a magmatic arc system (Zhou et al., 2018; Ji- 2020). All the reconstruction models to date Yakutat Block (terrane) (Fig. 14D; McCrory and ang et al., 2019; Qi et al., 2019), possibly located have assumed that Yangtze and Cathaysia were Wilson, 2013). along the western margin of the Rodinia super- parts of a single microcontinent before their The “Arctic realm terranes” in the North continent, as proposed by Wang et al. (2020b) amalgamation with Gondwana, and individual American Cordillera (Fig. 14D) have affinities (Fig. 14A, see b1), although its location relative models were based on data from either Yangtze with the northern Caledonides, the Timanide to Yangtze is uncertain. or West Cathaysia. Models based on data from Orogen, and the Urals in Europe. They probably In our model, Yangtze and West Cathaysia Yangtze (including palaeomagnetic, paleonto- occupied a position to the northeast of Lauren- were two separate microcontinents, as indicat- logical, stratigraphic, and detrital zircon data) tia in the early Palaeozoic and are interpreted to ed by their contrasting pre-Devonian geological suggest that South China collided with Indochi- have traveled thousands of kilometers westward history (Fig. 2B), and were accreted to two dif- na and Iran in the Early Cambrian (Torsvik and through transform faulting along the northern ferent parts of the northern margin of Gondwana Cocks, 2009; Zhang et al., 2015c; Yang et al., margin of Laurentia in the mid-Paleozoic, before in the early Paleozoic (Fig. 14B). West Cathay- 2020) and migrated to a low latitude position being accreted to the western margin of Lauren- sia was the lower plate during the associated close to northwestern Australia during the Or- tia (Colpron and Nelson, 2011). collision and underwent high-pressure–high- dovician and Silurian (Yang et al., 2004; Cocks Another potential analogue is in the Canadian temperature metamorphism (the Wuyi-Yunkai and Torsvik, 2013; Harper and Servais, 2014; Appalachians. Here, the Avalon and Meguma orogeny) (Lin et al., 2018a, 2018b). Late dur- Han et al., 2015; Fang et al., 2019). In contrast, terranes (both originated from the northern mar- ing or following the collision, the JSP Fault was models based on data from West Cathaysia (in- gin of Gondwana) have contrasting geological initiated subparallel to the continental margin cluding stratigraphic and detrital zircon data) histories before deposition of a Late Devonian– (possibly due to oblique collision or trans- suggest that South China had a close link with Early Carboniferous overlapping sequence (the form faulting), and sinistral strike-slip motion either northern India or the junction of eastern Horton Group) and are now separated by a major along the fault juxtaposed West Cathaysia with India, western Australia, and East Antarctica in strike-slip fault (the Cobequid-Chedabucto Fault Yangtze (Fig. 14C). A major part of the GHT the early Paleozoic (Duan et al., 2012; Xu et al., or the Mina Fault zone). The pre-Late Devonian moved away from Yangtze and the Jiangnan 2013; Yao et al., 2014; Xue et al., 2020). In our relationship between the two terranes is contro- Orogen, either as a result of movement along model, Yangtze and West Cathaysia were two versial (e.g., Murphy et al., 2004; van Staal et al., the JSP Fault in the early Paleozoic (as shown in separate microcontinents and were accreted to 2009). Although many geologists believe that the Fig. 14C), or earlier, during Rodinia breakup in two different parts of the northern margin of two terranes represent two separate microconti- the Tonian (following the 815–720 Ma rifting; Gondwana (Fig. 14B). It thus offers a solution nents, no direct evidence for closure of an ocean Fig. 14A, see a3); the current Huaiyu terrane to the long-standing controversy concerning in between (either an ophiolite or an arc) has is what remains of the GHT (Fig. 14C). Part of the position of South China in Gondwana that been identified. It is likely that the terranes were the Nanhua Rift Basin and potentially part of the is consistent with the available data from both accreted to two different parts of the Laurentian Jiangnan Orogen may also have moved away. Yangtze and West Cathaysia. margin and were later juxtaposed through margin-parallel strike-slip faulting (possibly induced Implications for Rodinia and Gondwana Comparison with the North American by oblique collision), similar to what is proposed Reconstruction Cordillera and the Northern Appalachians here for South China. A very important aspect of the above model is Our proposal that Yangtze and West Cathaysia that the collision in the Jiangnan Orogen and the were two separate microcontinents and were jux- CONCLUSIONS amalgamation between Yangtze and West Cathay- taposed through strike-slip faulting after their ac- sia were two separated events. The former took cretion to the Gondwana margin is similar to what Available field, geochronological, paleon- place at ca. 820 Ma, possibly at the late stage of has been proposed for the North American Cor- tological, and geochemical data show that the Rodinia assembly, and did not involve West Ca- dillera (Fig. 14D). In the Canadian and Alaskan Shenshan tectonic mélange comprises a mixture thaysia. The latter occurred in the late Early Pa- Cordillera, some major outboard terranes were of middle Tonian OIB-affinity ultramafic to maf- leozoic, possibly related to Gondwana assembly. accreted to the western margin of North America ic rocks, middle to late Tonian arc-related tuffa- An important implication of this model is that by the Middle Jurassic, but the current terrane ceous turbidites, an Early to Middle Cambrian West Cathaysia did not have to be a coherent ter- boundaries are, in many places, Cretaceous or carbonaceous shale, and a red mudstone of un- rane at ca. 820 Ma, but it had to have been a co- younger continental margin-parallel steeply dip- known age. These rocks occur in fault-bounded

14 Geological Society of America Bulletin, v. 130, no. XX/XX

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