Accepted Manuscript

Using zircon U–Pb ages to constrain the provenance and transport of heavy minerals within the northwestern shelf of the South China Sea

Lifeng Zhong, Gang Li, Wen Yan, Bin Xia, Yuexing Feng, Li Miao, Jianxin Zhao

PII: S1367-9120(16)30367-4 DOI: http://dx.doi.org/10.1016/j.jseaes.2016.11.019 Reference: JAES 2857

To appear in: Journal of Asian Earth Sciences

Received Date: 27 July 2016 Revised Date: 11 November 2016 Accepted Date: 20 November 2016

Please cite this article as: Zhong, L., Li, G., Yan, W., Xia, B., Feng, Y., Miao, L., Zhao, J., Using zircon U–Pb ages to constrain the provenance and transport of heavy minerals within the northwestern shelf of the South China Sea, Journal of Asian Earth Sciences (2016), doi: http://dx.doi.org/10.1016/j.jseaes.2016.11.019

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Using zircon U–Pb ages to constrain the provenance and transport of heavy

minerals within the northwestern shelf of the South China Sea

Lifeng Zhonga,b,, Gang Lib, Wen Yanb, Bin Xiaa, Yuexing Fengc, Li Miaob, Jianxin

Zhaoc

a School of Marine Sciences, Sun Yat-Sen University, Guangzhou 510006, China b Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology,

Chinese Academy of Sciences, Guangzhou 510301, China c Radiogenic Isotope Laboratory, School of Earth Sciences, The University of

Queensland, Brisbane 4072, Australia

ABSTRACT

Numerous ore-grade heavy mineral placer deposits occur in the northern South China

Sea region. Previous studies on these deposits have focused on the heavy-mineral ore resources themselves, but the provenance and transport pathways of these heavy minerals are poorly constrained. This paper presents U–Pb ages of detrital zircons from sediments within the northwestern shelf of the South China Sea, and uses this new dataset to determine the provenance and transport pathway of the sediments.

Zircons in sediments from ten areas of the northwestern shelf exhibit distinct age populations, suggesting that they have multiple provenances. Zircons in sediments from the Pearl River Mouth, Shangchuan Island, and Moyang River Mouth areas all have an obvious peak of Mesozoic ages, indicating that they have similar sediment

 Corresponding author at: 135# Xingang West Rd., Haizhu Dist., Guangzhou 510275, China. E-mail address: zhonglf9@ mail.sysu.edu.cn (L. Zhong) 1

provenances; i.e., mainly from the Pearl River, and to a lesser degree from the

Moyang River. Zircons in sediments from the areas around the Jian River Mouth and

Leizhou Bay, and off Hainan Island have an early Paleozoic age, suggesting that the sediments predominantly originate from the Yunkai massif. Zircons of the sediments from the remaining four areas, the Leiqiong Strait, Wanquan River Mouth,

Qiongdongnan, and the Outer Shelf, have Yanshannian and Indosinian age peaks in addition to an obvious early Paleozoic population, implying mixed provenances, including the Yunkai massif and Hainan Island. The sediment transport may have involved two hydrodynamic conditions in two distinct stages. First, the Guangdong

Longshore Current carried the river sediments to where they dispersed in the inner shelf; subsequently, wave-induced strong currents further transported sandy sediments southeastward to the outer shelf. In addition to explaining the provenance and transport pathways of heavy minerals within the northwestern shelf of the South

China Sea, these results provide new information relevant to exploration for heavy-mineral placer deposits.

Keywords Zircon U–Pb age; Provenance; Transport; Heavy mineral; South China Sea;

Placer deposit

1. Introduction

Marine heavy mineral placer deposits, including gold, diamond, zircon, garnet, rutile, cassiterite, xenotime, magnetite, and monazite, are distributed worldwide on beaches, in coastal waters, and on continental shelves (Baturin, 2000; Murton, 2000;

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Rona, 2003). These heavy mineral deposits are increasingly recognized as important supplements to continental endogenic ore deposits or as important sources of some disperse metals; for example, the main source of metallic zirconium from zircon placers. In addition, most heavy minerals are restricted to specific source rocks, and are thus sensitive indicators of provenance, provided that statistically distinguishable differences can be recognized between sediment source areas. Thus, heavy minerals have a wide range of applications, especially to the reconstruction of sedimentary processes, the correlation of nonfossiliferous siliciclastic strata, mining, mineral exploration, and forensic science (e.g., Mange and Wright, 2007, and references therein; Sevastjanova et al., 2012; Wong et al., 2013).

To understand the content and distribution of heavy minerals in ocean detrital sediments, and to assess reserves of economic marine heavy mineral placer deposits, it is essential to determine provenance areas and pathways of transport in coastal zones and on continental shelves (Akaram et al., 2015). Furthermore, tracing the original sources of heavy minerals can help unravel erosional and heavy mineral transport processes on the Earth’s surface. Numerous studies have examined the source areas of terrigenous debris in catchment basins (e.g., Azmon and Golik, 1995;

Barrie et al., 1988; Damiani and Giorgetti, 2008; Lahijani and Tavakoli, 2012; Liu et al., 2015; Walter et al., 2000), and traditionally, heavy mineral analyses have been used in such studies (Wong et al., 2013). However, heavy mineral associations of sedimentary deposits do not necessarily reflect their provenance, as the minerals tend to be modified during transport and by post-depositional processes (Morton and

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Hallsworth, 1999). Mineral chemistry and bulk-rock geochemistry, especially isotopic geochemistry, have also proven useful in provenance studies (e.g., Krippner et al.,

2014; Li et al., 2015; Weldeab et al., 2002; Zahid and Barbeau, 2013). In recent years, sediment provenance studies have been revolutionized by technological developments enabling single-grain radiometric dating of some components of the detrital grain population, for example U–Pb dating of zircon (Bertotti et al., 2014; Jiang et al., 2015;

Morton et al., 2008; Shao et al., 2016; Xu et al., 2016; Yao et al., 2014) and rutile

(Bracciali et al., 2013), and Ar–Ar dating of hornblende (Hemming et al., 2000), mica

(Haines et al., 2004; Van Hoang et al., 2010), and bulk sediments (Van Laningham and Mark, 2011).

In the northern South China Sea (SCS), marine resource surveys since the 1950s’ have identified marine heavy mineral placer deposits enriched in zircon, ilmenite, monazite, xenotime, and rutile primarily on modern beaches, in coastal zones, and on continental shelf (Tan and Sun, 1988; Xu and Tan, 1994; Yang et al., 2005). These discoveries have sparked renewed research interest in the deposits. However, previous studies on placer deposits have focused on exploration for, and analyses of, economic heavy mineral ore resources (Chen et al., 2006; Fu and Cen, 2008; Li, 1995; Yang,

1993; Yang and Yang, 1999; Ye, 2006a, 2006b; Zhang, 1991, 1994;). In addition, several studies on the sources of sediments in the northern SCS have been based on clay mineral assemblages (Liu et al., 2012; Liu Z.F. et al., 2010) and on bulk-rock geochemistry (Yang et al., 2008). Li et al. (2011) and Fang et al. (2014) performed heavy mineral analyses on hundreds of samples of sediments from the continental

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shelf of the northern SCS, focusing on the distribution, characteristics, and possible provenance of sediments in this area. Both studies reported that the primary factors affecting the distribution and contents of heavy minerals are hydrodynamic conditions and provenance. However, the provenance of heavy minerals in sediments on the shelf of the northern SCS has not been clearly identified. One view is that the heavy mineral sands are derived mainly from rivers, especially the Pearl River (Fang et al.,

2014); another view is that terrestrial heavy minerals originated mainly from Hainan

Island and/or the Yunkai massif (Li et al., 2011).

It has been recognized that detrital zircons are a key source of information for understanding provenances, and thus sedimentary dispersal systems (Fedo et al., 2003;

Xu et al., 2016). However, the U–Pb dating of individual detrital zircon grains in sediments from the northern shelf of the SCS has never been attempted. Thus, in this study we carried out in situ U–Pb dating of ~1600 detrital zircons from 49 sediment samples collected mainly from the northwestern shelf of the SCS, with the aim of tracing the provenance and transport pathways of heavy minerals in the sediments of this region.

2. Geological setting

The SCS (Fig. 1A), the largest marginal sea in the western Pacific region, covers an area of ~3.5 × 106 km2 over 20° of latitude. Geographically, the SCS is bordered by

South China and the Indochina Peninsula to the north and west, respectively, and to the east and south is surrounded by a chain of islands, ranging from Luzon in the

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north to Borneo in the south. Tectonically, the SCS is situated at the junction of the

Eurasian, Pacific, and Indo-Australian plates. It is bounded to the east by the Manila

Trench subduction system, where the Eurasian Plate is subducting eastward beneath the Philippine Sea Plate, and to the west by the Ailao Shan–Red River strike-slip fault system. The SCS is bounded to the north by the northern SCS rift margin, and to the northeast by the Taiwan arc–continent collision, which is actively closing the SCS basin. A conjugate rift margin lies at the southern edge of the SCS, where it has collided at Borneo and Palawan in the southern Philippines.

Oceanographically, surface circulation in the SCS is basically a basin-wide cyclonic gyre in winter, while in summer the circulation splits into a weakened cyclonic gyre north of ~12°N and a strong anti-cyclonic gyre south of ~12°N in response to seasonal changes in the monsoon wind-stress curl, with the added influence of the Kuroshio Current in its northern part (Qu, 2000; Su, 2004). The deep-water oceanography of the SCS appears to be controlled by the Bashi Strait.

Upon entering the SCS through the Bashi Strait, water of Pacific origin turns first northwestward and then southwestward along the continental margins of southeast

China and east Vietnam, suggesting that the deep-layer circulation in the SCS is predominantly cyclonic (Qu et al., 2006). Recent studies in the SCS have discovered two new oceanographic features: internal waves (Guo and Chen, 2014) and mesoscale eddies (Chen et al., 2011) that are of potential significance for studies of sedimentology.

The continental shelf is well developed on the northern and southern sides of the

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SCS (Fig. 1A). The northern continental shelf is bordered by the South China block to the north, between Taiwan Island to the east and the Indochina Peninsula to the west.

The northern continental shelf is flat and over 1600 km wide, with an average slope gradient of 1.5′. Geographically, microrelief structures such as sand waves, submarine deltas, submerged river valleys, slump structures, and rise crests occur on the bottom of the northern shelf (Cai, 2003).

Sedimentologically, the inner shelf and outer shelf exhibit contrasting characteristics (Fig. 1B). On the inner shelf, the surface sediments are dominated by modern terrigenous muddy deposits. In contrast, on the outer shelf an enormous area of sandy deposits occurs at water depths of 50–200 m. Distinct mineral assemblages occur in the different shelf areas. On the inner shelf, the diagnostic mineral assemblage is ilmenite–zircon–chlorite–mica. On the outer shelf, the characteristic mineral assemblage is zircon–ilmenite–epidote–quartz. In addition, the diagnostic mineral assemblage limonite–zircon–ilmenite–quartz is distributed off the Pearl River

Estuary, between the inner shelf and outer shelf.

The present study area is situated on the western side of the northern continental shelf of the SCS (Fig. 1B). It is estimated that in modern times the SCS has received

~700 million tons of sediments annually, of which ~80% is terrigenous material provided by surrounding rivers (Wang and Li, 2009). Around the study area, rivers with various sizes flow into the SCS, including the Pearl, Moyang, Jian, Nanduhe,

Nandu, and Wanquan rivers. Among these rivers, the Pearl River is the largest and has three main tributaries: the West River, the North River, and the East River. Other

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small rivers are distributed in western Guangdong Province and northeast Hainan

Island. The general characteristics of the drainage basins, including the length, drainage area, average gradient, annual runoff, annual sediment discharge, and bedrock outcrops, are listed in the Supporting Information (Table S1).

3. Samples and methods

A total of 48 sediment samples from the northwestern shelf of the SCS were collected using a box sampler during 2008–2013 on the cruise of the Guangdong

Province 908 Special Project on Investigation and Research of the Water Environment, and the cruises organized by a Joint Key Fund of the National Natural Science

Foundation of China (NSFC) and Guangdong Province. In addition, one sediment sample off Daya Bay was collected for comparison. More details of the sampling sites and sediment types are listed in the Supporting Information (Table S2). We divided the study region into 10 areas based on the spatial distribution of samples (Fig. 1B), designated as follows: Pearl River Mouth (Area I, from where four samples were collected), Shangchuan Island (Area II, four samples), Moyang River Mouth (Area III, five samples), Jian River Mouth (Area IV, five samples), Leizhou Bay (Area V, three samples), Leiqiong Strait (Area VI, six samples), off Hainan Island (Area VII, four samples), Wanquan River Mouth (Area VIII, three samples), Qiongdongnan (Area IX, five samples), and Outer Shelf (Area X, nine samples).

Samples of ~200 g dry weight were sieved, and zircons were extracted by conventional heavy liquid (sodium polytungstate solution; density, 2.89 g/cm3) and

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isodynamic magnetic separation techniques. All of the zircon grains were hand-picked from each sample (50–300 grains), and 50–150 of the grains were selected under a binocular microscope taking the grain sizes into consideration. The grains were mounted in epoxy disks and polished to approximately half their thickness to expose internal micro-textures. The mounts were photographed in transmitted and reflected light to ensure that the zircons selected for analysis were the least fractured in the sample and were inclusion free, after which the grains were imaged by cathodoluminescence (CL) to determine their internal micro-textures and to avoid analyses of multiple zones in a single spot, as well as to identify the genetic type of the zircon (igneous or metamorphic). The CL images were acquired using a Mono

CL3 (Gatan, USA) attached to an electron microprobe (JXA-8100) at the State Key

Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese

Academy of Sciences, Guangzhou, China.

U–Pb analyses were performed on polished grain mounts by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) at the State

Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry. The

LA–ICP–MS system consisted of an Agilent 7500a ICP–MS (Agilent Technologies,

Santa Clara, CA, USA) coupled with a Resonetic RESOlution M-50 ArF Excimer laser source ( = 193 nm) (Resonetics, Boston, MA, USA), and analyses used a laser energy of 80 mJ, a repetition rate of 10 Hz, a spot diameter of 31 m, and an ablation time of 40 s. Helium was used as the carrier gas to carry ablated aerosols to the ICP source. For calibration, we used NIST610, TEM, and 91500 as external standards, and

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29Si as an internal standard. Zircon isotope ratio calculations were performed using

ICPMS DataCal 7.0 (Liu Y.S. et al., 2008, 2010), and zircon ages were calculated using Isoplot (Version 3.23). The U–Pb ages were filtered by standard discordance tests with a 10% cutoff; discordant data (Concordance <90%) that crossed concordia within error were also included in the probability density histograms. Based on

Compston et al. (1992), 206Pb/238U ages were used for zircons younger than 1.0 Ga, while 207Pb/206Pb ages were used for grains older than 1.0 Ga.

4. Results

4.1. Zircon morphology and internal micro-textures

Representative zircon CL images are presented in Fig. 2. Zircons of the sediments from the northwestern shelf of the SCS have a range of morphologies and sizes. Most of the zircons are subangular or even rounded, indicating that they were abraded during transport over long distances. However, a small fraction of zircon grains displays perfectly elongate columnar morphologies, indicated a proximal source area. It is well established that zircons crystallize from melts at >700°C and that grains showing oscillatory textures grew during a magmatic episode in which the grains grew within a magma chamber, after the crust that originally contained them had melted and been recycled (He et al., 2013). Most of the zircons examined here show clear oscillatory zoning, indicating they crystallized from magma derived from igneous rocks. A few zircons contain inherited cores that were overgrown by sector-zoned domains, or have nebulous or taxitic internal micro-textures; these

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zircons are generally considered to be derived from amphibolite or granulite facies metamorphic rocks. Thorium/uranium ratios in combination with CL images of zircons have commonly been used to determine their origins (Harley et al., 2007; Wu and Zheng, 2004). Most zircons in this study have Th/U ratios of >0.3, suggesting their origin is igneous (Fig. 3). In contrast, zircons of metamorphic origin, which typically exhibit Th/U ratios <0.1, are rare among the zircons examined here. An igneous origin of the zircons is further supported by age concordances determined by

U–Pb dating (see below).

4.2. U–Pb ages

A total of 1687 detrital zircons from 49 samples in the different areas on the northern shelf of the SCS were selected for U–Pb isotope analysis, of which 1546 show concordant ages with concordance levels of >90%. The analytical points with concordant ages in the Phanerozoic account for 57.3% of the samples. The analytical results are shown graphically in Figs. 4–6. The following discussion is confined to the zircons with concordant ages. The entire set of raw data is available in the Supporting

Information (Table S3).

The detrital zircons of sediments from the northwestern shelf of the SCS yield ages of 10.9–3621 Ma, with the exception of one age of 4624 Ma (which shows a concordance of 99%). On the U–Pb age histogram (Fig. 4), the Phanerozoic ages occur in three distinct populations, in the Yanshannian (mainly 90–180 Ma, accounting for 13.9% of the sample points), the Indosinian–late Hercynian (200–300

Ma, 11.8%), and the Caledonian (400–510 Ma, 26.8%). The age populations in the 11

Precambrian account for 42.7% of the sample points, most of which are of Proterozoic age (36.9% of the sample points). The age distributions in the Precambrian are relative flat, with a single broad peak (600–1200 Ma) and three small peaks (1.4–1.5,

1.7–1.9, and 2.4–2.5 Ga).

In detail, the zircon ages show distinct distributions in the different areas on the northwestern shelf of the SCS (Figs. 5 and 6). Zircons in sediments from the Pearl

River Mouth (Area I), Shangchuan Island (Area II), and Moyang River Mouth (Area

III) areas show similar age populations, with four major age peaks in the Phanerozoic: late Yanshannian (90–120 Ma), early Yanshannian (130–170 Ma), late

Hercynian–early Indosinian (220–270 Ma), and Caledonian (400–500 Ma); the early

Yanshannian age population is especially strong. In the case of the Precambrian age populations, the three areas exhibit distinct features in the Archean. In addition, although the age populations in the Proterozoic are internally similar (e.g., a large population in the range of 700–1100 Ma and several smaller populations between

1200 and 2500 Ma), the differences in the strengths and ages of some of the peaks are notable and distinctive. The similarities possibly reflect a common provenance, but some small differences may express the influence of the provenance of different river drainages.

Two major zircon age populations occur in the Jian River Mouth (Area IV), at

150–170 Ma and 400–500 Ma, along with several smaller Phanerozoic populations.

Notably, the Caledonian age population in the Jian River Mouth area is relatively strong.

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The age populations of the Leizhou Bay (Area V) zircons are unique. The

Phanerozoic shows a strong well-defined population clustered around 430 ± 20 Ma, as well as three smaller peaks at 100–120, 140–170, and 220–270 Ma. In the

Precambrian, one relatively strong peak is present at 900–1100 Ma, along with several smaller peaks.

Four major Phanerozoic zircon age populations of similar strength are found for samples from the Leiqiong Strait (Area VI): late Yanshannian (100–110 Ma), early

Yanshannian (140–170 Ma), late Hercynian–early Indosinian (220–270 Ma), and

Caledonian (400–500 Ma). No strong peaks occur in the Precambrian, but a small peak is present at 700–1100 Ma and faint peaks occur at 1.4–1.5, 1.8–2.0, and 2.4–2.5

Ga.

The age populations of zircons from sediments off Hainan Island (Area VII) are similar to those of Leizhou Bay and include one strong peak in the Caledonian

(390–470 Ma) and three smaller peaks in the late Yanshannian (90–100 Ma), early

Yanshannian (140–170 Ma), and late Hercynian–early Indosinian (230–270 Ma), as well as one relatively strong peak at 700–1100 Ma and several small peaks in the

Precambrian.

Zircons of the sediments from the Wanquan River Mouth (Area VIII) and

Qiongdongnan (Area IX) areas exhibit similar Phanerozoic age populations with three major Phanerozoic age peaks in the early Yanshannian (150–170 Ma), late

Hercynian–early Indosinian (220–270 Ma), and Caledonian (400–500 Ma), and a minor age peak in the late Yanshannian (90–100 Ma). However, the Precambrian age

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populations of the two areas differ. For example, in the Qiongdongnan area, four strong age peaks are present in the Neoproterozoic and three relatively inconspicuous age peaks are present in the Mesoproterozoic and Paleoproterozoic, while in the

Wanquan River Mouth area only one small age peak is present in the Neoproterozoic, and several faint age populations are sparsely distributed in the Meso- and

Paleoproterozoic.

The age populations of zircons in sediments from the Outer Shelf (Area X) exhibit several characteristics similar to those from the Qiongdongnan area. For example, three major Phanerozoic age peaks are present, in the early Yanshannian

(150–170 Ma), late Hercynian–early Indosinian (220–270 Ma), and Caledonian

(400–500 Ma), and a minor age peak occurs in the late Yanshannian (100–110 Ma).

The Precambrian age populations are also analogous to those in the Qiongdongnan area in terms of the strengths of age peaks. However, the number of peaks in the two areas is different. In addition, a young zircon U–Pb age of 10.9 Ma (from Sample 17) was identified on the Outer Shelf, possibly indicating that heavy minerals from

Cenozoic basalts in the Leiqiong area were transported to the Outer Shelf. Strangely, the oldest zircon age of 4624 Ma also occurs on the Outer Shelf, also from Sample 17; this age is older than that of the Earth (although it has a near-perfect concordance of

99%), and is therefore not considered in the Discussion section.

One sample from Daya Bay (Sample 1) on the northeastern shelf of the SCS was selected for extraction of detrital zircons and U–Pb dating. The analytical results are presented in Figs 6 and 7. Fifty points were analyzed, among which 47 showed

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concordant ages (Fig. 7). The age populations of the detrital zircons showed two major groups in the Phanerozoic, in the Yanshannian (100–150 Ma, accounting for

51.1% of the sample points) and the Indosinian (200–250 Ma, 14.9%). In addition, two relatively strong Precambrian population peaks were present, with ages of 1.2–1.3 and 2.0–2.2 Ga. The age populations of zircons in sediments from Daya Bay were clearly different from those on the northwestern shelf, suggesting a different provenance of the sediments.

5. Discussion

5.1. Terrigenous sedimentary provenances

It is generally accepted that terrigenous sediments in the northern SCS are derived from the inputs of major rivers, including the Red River to the northwest, the

Pearl River to the north, and Taiwanese rivers to the northeast (Liu et al., 2012). The terrigenous sediments transported by the Red River are discharged and deposited mainly in the Beibu Gulf Basin, the Yinggehai Basin, and the Qiongdongnan Basin, on account of being obstructed by Hainan Island (Cai et al., 2013; Li et al., 2012; Su et al., 2014). Inputs of fine-grained detrital sediments from Taiwan rivers to the SCS accumulate mainly on the northeastern shelf and slope (Liu and Stattegger, 2014). As such, the terrigenous detrital sediments on the northwestern shelf of the SCS are derived predominantly from the inputs of rivers in the surrounding area. Six rivers drain into the region around the northwestern shelf of the SCS (Table S1), the largest being the Pearl River, which annually transports ~96 million tons of sediments to the

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sea. Other smaller rivers discharge annual sediment fluxes of ~0.3–1.9 million tons to the sea. Thus, most of the terrigenous sediments on the northwestern shelf of the SCS originate predominately from inputs of the Pearl River, as reported previously based on various research methods (Cai et al., 2013; Li et al., 2015; Liu et al., 2012, 2013).

However, the viewpoint that the sediments on the northwestern shelf of the SCS are derived mostly from the Pearl River is based mainly on analyses of lightweight and fine-grained clay minerals. Previous studies reported that distinct diagnostic mineral assemblages occur in the different shelf areas, as mentioned above, possibly implying that they are derived from multiple sources. Recent integrated offshore marine investigations have revealed through heavy mineral analyses that heavy minerals in the surface sediments of the northern shelf of the SCS are not only from the Pearl

River drainage basin (Fang et al., 2014), but also from Hainan Island and the Yunkai massif (Li et al., 2011). In addition, heavy mineral geochemistry was used by our group to decipher the provenances of surface sediments on the northwestern shelf of the SCS (Li et al., 2015). We believe that the Pearl and Jian rivers are the two main suppliers of sandy sediments to the northwestern shelf of the SCS. Furthermore, Pearl

River sediments are the major source of sandy sediments on the eastern shelf, from the Pearl River Mouth to the Moyang River Mouth. Sandy sediments on the western shelf, on the other hand, are supplied mostly by the Jian River.

In this study, detrital zircon U–Pb ages were used to trace the provenances of sediments on the northwestern shelf of the SCS. It was confirmed that the sediments on the northwestern shelf originated from South China, as samples collected from this

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region show the same detrital zircon U–Pb age distribution patterns as those of South

China, especially in showing five major magmatic events; i.e., Yanshannian (90–200

Ma), Indosinian–Hercynian (200–300 Ma), Caledonian (400–500 Ma), Jinningian

(700–1100 Ma), and Sibaonian (1100–1800 Ma) (Fig. 4). Despite the fact that five major magmatic events are widely documented in the Yangtze River drainage basin

(Yang et al., 2006), and that the Yangtze River is considered as a potential terrigenous provenance because the southward China coastal current that passes through the

Taiwan Strait may transport suspended sediments to the SCS (Liu et al., 2003; Wan et al., 2007), detrital zircon U–Pb ages of Yangtze River sands suggest that the Yangtze

River is an insignificant supplier of terrigenous sediments to the northern SCS. The zircon U–Pb age data analyzed by Safonova et al. (2010) and He et al. (2013) show a strong age population at 1.8–2.0 Ga and a Paleogene age population in the zircon

U–Pb age spectrum, which do not exist on the northwestern shelf of the SCS. In fact, modern-day Yangtze clay can reach southwards to 23°N, which is in the middle part of the Taiwan Strait (Xu et al., 2009). In addition, a conspicuous Cenozoic zircon

U–Pb age population (van Hoang et al., 2009) and muscovite Ar–Ar ages (Van Hoang et al., 2010) from the modern Red River preclude the possibility that the Red River drainage basin is the provenance area of sediments on the northwestern shelf of the

SCS.

In detail, various features of the age populations of the 10 study areas on the northwestern shelf of the SCS (Figs. 5 and 6) indicate that the different areas have contrasting sediment provenances. Zircons of the sediments from the three areas in the

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east of Shapa Town (i.e., the Pearl River Mouth, Shangchuan Island, and the Moyang

River Mouth) show very similar age populations comprising a strong Yanshannian age peak and two moderate age peaks in the Indosinian and Caledonian. These ages coincide with the ages of rocks exposed in the Pearl River and Moyang River drainage basins (Table S1), indicating that their provenance is mainly in the Pearl

River basin and, to a lesser extent, in the Moyang River basin (taking sediment loads into consideration). On the other hand, zircons of the sediments from the three areas in the west of Shapa Town (i.e., the Jian River Mouth, Leizhou Bay, and off Hainan

Island) show very strong Caledonian age populations, as well as two weak peaks in the Yanshannian and Indosinian, suggesting that their provenances are mainly from the Yunkai massif (a pre-Devonian terrane) with inputs from the Jian and Nanduhe rivers. In addition, zircons of the sediments from the remaining four areas (Leiqiong

Strait, Wanquan River Mouth, Qiongdongnan, and the Outer Shelf) show not only strong Caledonian age populations, but also two strong peaks in the Yanshannian and

Indosinian, reflecting a mixed provenance that includes the Yunkai massif and Hainan

Island, which contains widely exposed Mesozoic granitoid rocks. In particular, a

Cenozoic zircon U–Pb age of 10.9 Ma was obtained from Sample 17 from the middle of the study area, which further demonstrates that some of the sediments on the Outer

Shelf are derived from the Leiqiong area that is characterized by widely distributed

Cenozoic basalts.

In conclusion, the sandy sediments on the eastern shelf in the study area are supplied mainly by the Pearl River and to a lesser extent by the Moyang River. On the

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western shelf, the Jian River is the major source of sandy sediments, with lesser inputs from the Nanduhe River. In addition, the sediments carried by the Nandu and

Wanquan rivers have an important influence on the western shelf near Hainan Island

(Fig. 8).

5.2. Transport mechanisms

Transport and the accompanying hydraulic sorting of sediment grains are important processes in the formation of placer deposits. Despite several heavy mineral placers having been found on the northwestern shelf of the SCS (Fig. 8), surprisingly little research has focused on details of the transport mechanisms of the heavy minerals in these deposits. Previous studies on sediment transport on the northern shelf of the SCS have been based on clay mineral analyses and bulk-rock geochemistry, which have demonstrated the presence of surface and deep-water currents, including the Guangdong Longshore Current (GDLC), the SCS Warm

Current (SCSWC), the SCS Branch of the Kuroshio Current (SCSBK), and the

Branch of the North Pacific Deep Water Current (BNPDWC), which are the most important currents transporting sediments to the northern SCS from various provenances (Cai et al., 2013; Liu et al., 2011, 2012, 2013; Liu Z.F. et al., 2008, 2010).

However, the contrasting hydrodynamic properties of light and heavy mineral grains in sediments would give rise to differential transport mechanisms by flowing currents.

Differences between the characteristic mineral assemblages found on the inner and outer shelf of the northern SCS further indicate that different transport mechanisms were operating in each area. The higher densities and generally finer grain sizes of the 19

heavy minerals make them more resistant to entrainment, resulting in their transport generally as bed load, on account of their high settling velocities (Komar, 2007;

Marion and Tregnaghi, 2013; Papista et al., 2011).

In this study, detrital zircon U–Pb ages were used to probe the mechanisms of surface sediment transport on the northwestern shelf of the SCS. The ten study areas can be divided into three groups according to their age populations, indicating that at least three hydraulic conditions were involved during transport of these sandy sediments. In the inner shelf, the GDLC may be the most important hydrodynamic current transporting sandy sediments, while the cyclonic eddy derived from the

GDLC to the east of Leiqiong Strait could transport sediments from the Jian River and

Nanduhe River to waters between Leizhou and Shapa (Fig. 8). However, the GDLC does not reach the outer shelf, suggesting that other hydraulic conditions must be responsible for the transport of sediments to this region. The SCSWC, which occurs on the outer shelf as far as the shelf break of the SCS, may be responsible for this transport. However, its average current velocity (measured at Wenchang Station, Fig.

8) is only ~0.25 m/s (Wang et al., 2011), which is much lower than the minimum flow velocity of 0.55 m/s required to transport sandy sediments (Zhou, 2013), thus leading to its inability to entrain and transport sandy sediments on account of their higher settling velocities and finer grain sizes. In addition, the branch of the NPDWC seems to only influence slope sediments of the northern SCS (Shao et al., 2007). These observations possibly imply that other hydrodynamic conditions are responsible for transporting heavy minerals in sediments on the outer shelf of the northern SCS.

20

Previous studies based on in situ measurements, remote sensing images, and numerical simulations have found that internal waves, including internal solitary waves and internal tides, are widespread in the northern SCS (Guo and Chen, 2014;

Hong et al., 2015; Xu et al., 2011). Field experiments in other regions have demonstrated that near-bottom currents induced by internal waves are sufficient to re-suspend bottom sediments at depth (e.g., depths of 50 and 85 m in Massachusetts

Bay, USA; Butman et al., 2006). On the outer continental shelf of the northwestern

SCS, a sequence of internal solitary wave packets was observed to propagate roughly northwestward in September 2005 (Xu et al., 2010a). Subsequent comparisons between theoretical predictions and observational results have indicated that the northwestern SCS contains abundant highly-nonlinear internal solitary waves (Xu et al., 2010a). Moreover, during propagation the internal solitary waves would induce a strong reverse current under the pycnocline with a maximum velocity of >3.8 m/s near the bottom (Wang et al., 2011), which is sufficient to entrain and transport sandy sediments. Thus, it is reasonable to believe that strong currents induced by internal waves may provide the hydrodynamic conditions necessary to carry sandy sediments southeastward on the outer shelf of the northwestern SCS.

In this study, we obtained a zircon grain from sample 17 on the outer shelf with a

U–Pb age of 10.9 Ma. In addition, the Th/U ratio of the zircon grain is 0.33, suggesting an igneous origin (Fig. 3). Only three areas located near sample 17 could provide late Cenozoic magmatic material: basaltic magmatism on the Hainan

Island–Leizhou Peninsula, the SCS Basin, and the Indochina Block (Wang et al.,

21

2012). However, three factors make it impossible for materials to be transported from the SCS Basin and the Indochina Block to the northwestern shelf of the SCS: (1) the long distance of the site of sample 17 from the SCS Basin and the Indochina Block; (2) barriers to active bottom currents on the northern continental slope of the SCS, which extend from NE to SW (Shao et al., 2007); and (3) segmentation of the central canyon system in the Qiongdongnan Basin, which parallels the shelf break on the northern

SCS and exhibits an S-shaped geometry and a NE–NEE orientation (Su et al., 2014).

Thus, the zircon is most likely derived from the Hainan Island–Leizhou Peninsula. If so, it is possible that terrigenous detrital materials have been carried southeastward to the outer shelf, further corroborating the hypothesis of strong wave-induced currents.

In summary, the GDLC and its derived cyclonic eddy, and wave-induced strong currents are the two most likely carriers of sandy sediments on the northwestern shelf of the SCS. In addition, some sediments were probably transported by more than one process. The GDLC likely transported river sediments to the inner shelf. In detail, the

Pearl River and Moyang River sediments were transported to and dispersed across the east of the study area. Sediments from the Jian River and Nanduhe River were transported to the waters between Leizhou and Shapa by means of the cyclonic eddy derived from the GDLC to the east of Leiqiong Strait. Subsequently, strong currents induced by internal waves further transported sandy sediments southeastward to the outer shelf. The hydraulic sorting that accompanied transport resulted in the formation of the valuable heavy mineral placers on the outer shelf of the SCS (Fig. 8).

5.3. Implications for the origin of sediments in the northern shelf of the SCS

22

The sediments on the continental shelf of the SCS can be divided into two belts

(Wang et al., 1986; Fig. 1B). Sedimentation in the inner shelf occurs in a belt about 35 km wide, with sediment deposited seaward in the order of sand, silt, and clayey mud, and showing a modern sedimentary series of wave dynamics. The outer shelf deposit, which is separated from the inner shelf by a narrow transition zone, is characterized by well-sorted fine and medium sands containing more than 2% heavy minerals. As such, the outer shelf belt has good prospects for forming economically important heavy-mineral placer deposits in locations where conditions are suitable.

The origin of the deposits in the outer shelf of the northern SCS remains problematic. Shepard (1932) discovered that seaward-increasing gradation is common along the east coast of Asia. Emery (1968) proposed that the deposits in the outer shelf off East Asia between Malaya and Korea are relict sediments that were deposited during and immediately after the latest glacial stage of the Pleistocene

Epoch, and are as such unrelated to their present environments. This viewpoint had an important influence on subsequent studies of the sedimentology of the northern shelf of the SCS, with the result that some early researchers regarded the outer shelf deposits of the northern SCS as relict sediments or relict sands (Li et al., 1991; Wang et al., 1986). However, the conclusion that the relict sediment is exposed atop 70% of the surface area of continental shelves worldwide (Emery, 1968) is controversial. The deposits in certain areas of the outer shelf off East Asia have subsequently being reconsidered (Swift et al., 1971) and re-evaluated (McManus, 1975), yielding concrete evidence that some of the deposits do not consist of relict sediments.

23

Examples include the continental shelf of the East China Sea (Liu, 1997; Milliman et al., 1985) and the Taiwan Strait (Huh et al., 2011; Liao et al., 2005; Liu J.P. et al.,

2008). However, until now there has been no specialized research on the origin and sources of the sandy sediments in the outer shelf of the northern SCS, meaning their classification as relict sediments could not be challenged.

Relict sediments are classified as “sediments that were deposited long ago in equilibrium with their environments; afterward the environments changed so that the sediments are no longer in equilibrium even though they remain unburied by later sediments” (Emery, 1968). In the present study, it was found that small mountainous rivers in the western Guangdong Province transported nonnegligible sediments to the northwestern shelf of the SCS. Moreover, sediments on the inner shelf can be resuspended and transported to the outer shelf by wave-induced strong currents. As such, the sediments in the outer shelf of the northwestern SCS are modern and cannot be categorized as relict. The fact that numerous moving sand waves have been found on the seafloor of the outer continental shelf in the northern SCS (Luan et al., 2010;

Wu et al., 2006; Xia et al., 2009; Zhang et al., 2015) also argues against the viewpoint that they are relict sediments. In addition, surface sediments in the outer shelf of the northern SCS have higher concentrations of heavy metals (e.g., Cd, Ni, Cu and Zn) than background values (Gao et al., 2008), indicating the outer shelf of the northern

SCS contains sediments that have been affected by modern anthropogenic activities.

In other words, the analysis above demonstrates that modern materials have been deposited on the outer shelf of the northern SCS, so it is reasonable to conclude that

24

the sediments in this region are modern rather than relict.

6. Conclusions

We obtained in situ U–Pb ages for 1687 detrital zircons from 49 sediment samples mainly collected from the northwestern shelf of the SCS. The data show that more than half the zircons date to three epochs in the Phanerozoic: the Yanshannian, the Indosinian–late Hercynian, and the Caledonian. Moreover, most of the

Precambrian-aged zircons crystallized in the Proterozoic. Detailed analyses of age peaks of zircons in the samples show that South China is the main provenance of sediments on the northwestern shelf of the SCS. However, the sediments carried by different rivers are deposited in distinct areas. Sandy sediments in the east of the study area are supplied mainly by the Pearl River, and to a lesser extent by the Moyang

River. The Jian River and Nanduhe River catchment basins appear to be important sources of sediments in the waters between Leizhou and Shapa, and the Nandu River and Wanquan River carry sediments to the area around Hainan Island. Sediment transport likely involved two hydrodynamic conditions and two current regimes: first, the Guangdong Longshore Current carried river sediments to the inner shelf, and subsequently, wave-induced strong currents further transported the sandy sediments southeastward to the outer shelf. Our results, combined with other evidence, prove that the sandy sediments in the outer shelf of the northern SCS are modern sediments, not relict sediments.

25

Acknowledgements

We are grateful to Han Chen, Zhong Chen, and Weihai Xu for help with sample collection, and Liang Li, Nan Zhang, Xianglin Tu, and Congying Li for help with zircon U–Pb isotopic analyses. Thanks also go to Dr. Aaron Stallard of New Zealand for proofreading the manuscript. We thank Editor-in-Chief Mei-Fu Zhou and two anonymous reviewers for their suggestions that improved the manuscript. This work was supported by the NSFC–Guangdong Joint Foundation (U1133002), a research grant from the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of

Geochemistry, Chinese Academy of Sciences (SKLIG-KF-14-08), and the project of the national sea and land metallogenic map compilation and update.

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Figure captions

Fig. 1. (A) Topography of the South China Sea (SCS) and surrounding areas. (B)

Simplified surface sediment map of the northwestern shelf of the SCS, also showing the locations of samples collected in this study. Areas I–X are designated as the Pearl

River Mouth, Shangchuan Island, Moyang River Mouth, Jian River Mouth, Leizhou

Bay, Leiqiong Strait, Hainan Island, Wanquan River Mouth, Qiongdongnan, and

Outer Shelf, respectively.

Fig. 2. Cathodoluminescence (CL) images of detrital zircons of sediments from the northwestern shelf of the South China Sea.

Fig. 3. Plot of Th/U ratios versus U–Pb ages of detrital zircons from sediments on the northwestern shelf of the South China Sea. Th/U values of >0.3 indicate an igneous origin of the zircons, whereas values of <0.1 indicate a metamorphic origin.

Fig. 4. Histograms and probability density plots of detrital zircon ages in South China

(A) and the northwestern shelf of the South China Sea (B). The histogram bin width is

10 Myr for the Phanerozoic and 20 Ma for the Precambrian. Zircon U–Pb ages in

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South China are from 28 previous studies. The compiled age database is provided in the Supporting Information (Table S3).

Fig. 5. Concordia diagrams of detrital zircons of sediments from the northwestern shelf of the South China Sea. See Fig. 1 for the locations of the named study areas.

Fig. 6. Histograms and probability density plots of detrital zircon U–Pb ages of sediments from the northwestern shelf of the South China Sea. The histogram bin width is 10 Ma for the Phanerozoic and 20 Ma for the Precambrian. Raw data are presented in Table S3. See Fig. 1 for locations of the 10 study areas.

Fig. 7. Concordia diagrams for detrital zircons from sediments near Daya Bay.

Fig. 8. Schematic diagram illustrating the transport mechanisms of sandy sediments on the northwestern shelf of the South China Sea (bold purple lines). The main surface current systems are indicated by elongate arrows, according to Yang et al.

(2003) and Wang et al. (2010). Green dashed lines represent the Guangdong

Longshore Current (GDLC), orange dashed lines represent the South China Sea

Warm Current (SCSWC), and blue open arrows indicate the South China Sea Branch of the Kuroshio Current (SCSBK). The red star marks the site of Wenchang Station

(Xu et al., 2010b). The locations of heavy mineral placers (see legend in figure) were identified by Tan and Sun (1998), Zhang et al. (1992), and Yang et al. (2005). 41

42

Fig. 1.

43

Fig. 2.

44

Fig. 3.

Fig. 4.

45

Fig. 5. 46

Fig. 6. 47

Fig. 6. (Continued) 48

Fig. 6. (Continued)

49

Fig. 7

Fig. 8

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Highlights  1687 zircons from sediments on the SCS shelf were analyzed for U–Pb age  The age populations resemble those from South China, indicating their affinity  Various areas show distinct age populations, implying their different provenance  Sediment transport likely involved two hydrodynamic conditions  The sandy sediments in the outer shelf are modern sediments, not relict sediments

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