Elemental compositions and monazite age patterns of core sediments in the Changjiang Delta: Implications for sediment provenance and development history of the Changjiang River

Earth and Planetary Science Letters 245 (2006) 762 – 776 www.elsevier.com/locate/epsl

Elemental compositions and monazite age patterns of core sediments in the Changjiang Delta: Implications for sediment provenance and development history of the Changjiang River Shouye Yang a,⁎, Congxian Li a , Kazumi Yokoyama b a

State Key Laboratory of Marine Geology, Department of Marine Geology, Tongji University, 1239 Siping Road, Shanghai 200092, PR China b Department of Geology, National Science Museum, Tokyo, Japan Received 23 January 2006; received in revised form 20 March 2006; accepted 23 March 2006 Available online 4 May 2006 Editor: S. King

Abstract Core from a continuous borehole in the Changjiang Delta to a depth of 318.7 m dated back to the Pliocene (> 3.58 Ma) and was selected for geochemical measurements and determinations of Th–U–Pb ages of monazite, in order to investigate the changing sediment provenance and development history of the Changjiang River. Geochemical proxies including fractionation parameters of rare earth elements (cerium and europium anomalies) and elemental ratios Cr/Th, Nb/Co and Th/Co suggest that the Pliocene and Quaternary sediments have remarkably different provenances. Six peak ages of monazite grains dated at < 25, 50–200, 200–400, 400–550, 800–1000, and 1800–2000 Ma are consistent with the main tectonic and magmatic events in the Yangtze Craton. The data imply that the Pliocene sediments were mostly derived from proximal and more silicic sources whereas the Quaternary sediments were sourced from distal and more basic provenances, including the Emeishan basalt province in the upper Changjiang valley. We propose that during the Pliocene the “paleo-Changjiang” or its eastern equivalent was a locally small river draining today's lower Changjiang valley, whereas during the early Pleistocene not later than 1.18 Ma it changed its drainage pattern and developed into a large river that originated from the eastern Tibetan Plateau. This time matches well with many previous studies based upon geomorphologic, geographic and tectonic observations in the Jinshajiang valley and the Three Gorges. © 2006 Elsevier B.V. All rights reserved. Keywords: Changjiang; geochemistry; monazite; Tibetan Plateau; Quaternary

1. Introduction The major rivers draining the southeastern Tibetan Plateau have been investigated in recent years because they directly transport a large quantity of terrestrial materials eroded from the Himalayan–Tibetan Plateau ⁎ Corresponding author. Tel.: +86 21 6598 9130; fax: +86 21 6598 2208. E-mail address: [email protected] (S. Yang). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.03.042

into the marginal seas surrounding Asia and have exerted significant influence on chemical flux budget of the global ocean [1–8]. Furthermore, the basin morphology and regional fluvial patterns of these rivers are closely linked to plateau uplift and thus can reflect the Cenozoic uplift history of the Tibetan Plateau following the continental collision between India and Eurasia [6,9–13]. The Changjiang is the third longest river in the world and the fourth largest in terms of its water discharge

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[14]. The river originates from the northeastern Tibetan Plateau at elevations above 5000 m and drains more than one-fifth of the continental area of China before finally entering the East China Sea. In past the development of the Changjiang has been the focus of increasing researches [15–31]. Pioneer work on the river evolution can be traced back almost 100 yrs ago, to the work of Willis and Blackwelder in 1907 [15]. Studies of the evolution of the Changjiang and its main tributary in the upper reaches, the Jinshajiang, first flourished during the 1920–1930s [16–19]. These studies and some later attempts proposed that the Changjiang can be dated back to the Cretaceous or the Early Tertiary periods [15–21]. Since the end of 20th century the evolution of the Changjiang has been highlighted again [22–30]. Research results primarily based on geomorphologic and geographic observations suggested that the Changjiang formed in the early Quaternary (2.5–0.7 Ma) [23–27,29,30] or late Pleistocene (0.15–0.20 Ma) [10,28,31]. Therefore, the formation age and the development history of the Changjiang remain to be resolved and more substantial evidence with reliable age constraints are required. Most previous studies highlighted two regions in the Changjiang valley, i.e. the first bend located in the Jinshajiang and the Three Gorges in the upper-middle Changjiang valley (Fig. 1). In contrast, the Changjiang

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Delta forming one of the Quaternary depocenters of Changjiang sediment [32] has rarely been studied. It is evident that the Changjiang must record its evolution history in the delta area through the gradual accumulation of river sediments since its formation. In this study one continuous borehole (Core PD) with penetration to 318.7 m subsurface was taken from the Changjiang Delta. Sediments were selected for geochemical analysis and monazite dating. The main purpose of this paper is: 1) to examine downcore variations of geochemical compositions and monazite age spectra in sediments; 2) to identify the provenances of the upper Pliocene and Quaternary sediments; 3) to reconstruct the Changjiang development history during the Pliocene and Quaternary. 2. River setting and study area 2.1. Geomorphologic, geographical and geological backgrounds of the Changjiang drainage basin The Changjiang drainage basin is located between 24°27′–35°44′N and 90°33′–122°19′E and has an area of 1.8 × 106 km2 (Fig. 1). The drainage basin spans the regional structure of China with three-grade relief terraces, the highest source area with the average elevation of 3500–5000 m, the highlands in the upper-

Fig. 1. A sketch map showing the drainage basin, main distributaries, and provincial boundaries of the Changjiang. The locations of the Yangtze Craton, first bend in the Jinshajiang, the Three Gorges in the upper-middle Changjiang, and the drilling core PD in the delta area are also shown. The distributions of the Emeishan Basalt and the old (late Archean to early Proterozoic, Ar3–Pt1) metamorphic complexes are outlined. 1: Kangding Complex; 2: Kongling Complex; 3: Dabie Complex; 4: Sichuan Basin; 5: Jianghan Basin; 6: Changjiang Delta; 7: Taihu Lake.

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middle valley at elevations about 500–2000 m and the lower valley at elevations less than 500 m. The river basin drains several large catchment basins including the Sichuan, Jianghan, Dongting Lake, Poyang Lake and Taihu Lake Basins from west to east towards the river mouth (Fig. 1). Geographic boundaries of the river basin can be summarized as the upper reaches from the source area to Yichang in Hubei Province, the middle reaches between Yichang and Hukou in Jiangxi Province, and the lower reaches from Hukou to the estuary near Shanghai (Fig. 1). The four largest distributaries, the Jialingjiang, Hanjiang, Minjiang, and Yalongjiang, are all located in the upper-middle part of the Changjiang and each contains 100,000 km2 in drainage area (Fig. 1). In terms of the complicated drainage basin morphology, the Changjiang is remarkably different from the major rivers in southeast and south Asia, although they all originate within the Himalaya–Tibetan Plateau. Geologically, the Changjiang River basin is mostly situated on the Yangtze Craton, primarily framed by Mesozoic Yanshanian orogenesis and comprising structurally complicated source rocks varying from Archean

metamorphic and metasedimentary rocks to Quaternary clastic sediment [14,33,34]. The upper basin is characterized by variable rock compositions, including sporadic metamorphic rocks, Paleozoic carbonate rocks, and Mesozoic sedimentary and igneous rocks. The two major rock types include widely distributed Paleozoic carbonate rocks and upper Permian Emeishan basalt, each occupying an area about 30 × 104 km2 in southwest China (Fig. 1). The middle-lower drainage basins mostly consist of Paleozoic marine, Mesozoic terrestrial and Quaternary fluvial–lacustrine sedimentary rocks, while Mesozoic intermediate–acidic igneous rocks often occur, but over small areas. The Yanshanian granitoids are widely distributed in southeast China along the southern margin of the Yangtze Craton [33,34]. 2.2. Quaternary strata of the Changjiang Delta and the drilling core A continuously drilled core (Core PD, lat. 31°37′ 29ʺN, long. 121°23′38ʺE) was taken from the Changjiang Delta in 1999. The final drilling depth was

Fig. 2. The lithology, maganetostratigraphy and sediment grain size compositions of Core PD. C: clay; Z: silt; S: sand; MS: magnetic susceptibility (×10− 5 SI unit); Mz: mean grain size (phi); SD: standard deviation; SK: skewness; Kurt: kurtosis; Q4: Holocene strata; Q3: upper Pleistocene strata; Q2: middle Pleistocene strata; Q1: lower Pleistocene strata; N3: Pliocene strata.

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318.7 m, with the recovery averaging 81%. A prePliocene sandstone constitutes the basement at about 313.0 m subsurface. The Quaternary stratigraphy of Core PD was established by magnetostratigraphy, lithofacies, paleontology, and absolute electron spin resonance (ESR) dates. The Quaternary sediments comprise several major fining-upward and coarseningupward sedimentary cycles (Fig. 2). The stratigraphic and lithological interpretations were determined in a previous study [35]. The Pliocene strata (Unit N3, 313.0–240.0 m) contacts the overlying Quaternary strata with a distinct unconformity and primarily consists of grey hard clay with brownish iron mottles in the lower part and grey sandy sediments interbedded with clayey layers in the upper part. The lithology of the unit is suggestive of lacustrine and fluvial facies. The Quaternary strata (Q1– Q4) are remarkably different from the Pliocene, not only in sedimentary character but also in magnetic susceptibility and microfossil assemblages (Fig. 2) [35]. The Quaternary sediments are interpreted to be of fluvial origin but were influenced by several marine transgressions in the late Pleistocene and the Holocene. 3. Sampling and methodology The core was split, described and photographed in the laboratory. Subsamples were taken for grain size and geochemical analysis and microfossil examinations. In order to provide age control by magnetostratigraphy, three quartz-rich samples were selected from the core and dated by electron spin resonance (ESR) technique (ECS-106 ESR spectrometer, Bruker Com., Germany) in the Qingdao Institute of Marine Geology, China, following a standard measurement procedure [36]. Three dates are given as 106 ka at 112.8 m, 574 ka at 147.4 m, and 1186 ka at 234.6 m, with an analytic error of about 15%. It is noteworthy that the former two ESR ages are overall consistent with the paleomagnetic chronology whereas the ESR age of 1186 ka at 234.6 m ka is much younger than the paleomagnetic age. The grain size compositions of 279 sediment samples were measured by laser size analyzer (Coulter LS 230) in the State Key Laboratory of Marine Geology, Tongji University, after processing 0.2 g subsamples with 15% H2O2 and 1 N HCl to respectively remove organic matter and carbonates. A total of 95 bulk and 64 fine-grained (< 0.063 mm) sediment samples were selected for the determination of major, trace and rare earth element (REE) concentrations. The bulk samples were washed by deionized water to remove salts and dried at 50 °C in a clean oven,

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then ground in an agate mortar. The < 0.063 mm fine fraction was separated from the bulk sediments in deionized water by pippetting. An aliquot of 0.125 g powdered sample was digested with concentrated 10 ml HF, 10 ml HNO3, and 2 ml HClO4 in an airtight Teflon vessel. The solution was then eluted with 10 ml 1% HNO3. Concentrations of major and trace elements were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES, JY-38S) in State Key Laboratory of Mineral Deposits Research in Nanjing University. REEs were determined by inductively coupled plasma-mass spectrometry (ICP-MS, PQ III, Thermo Elemental) in the Korea Basic Science Institute. Analytical precision and accuracy were monitored by standards MAG-1, GSS-6 and GSD-9 and the results showed that relative deviations between measured and certified values are generally less than 5%. The application of electron microprobe analysis to monazite and zircon has been explored successfully in the last decade [37–39]. This method can determine U, Th, and Pb concentrations in domains that are ca. 2 μm in size and then used to estimate monazite age. For monazite age determination, a total of 31 fine sandy and silty sediments with each weighing above 500 g were selected from Core PD and one surface sediment sample was taken from the present-day Changjiang Estuary close to Chongming Island. The samples were washed in running tap water and then dried at room temperature. The magnetic minerals were removed using a hand magnet. Heavy minerals in the sediments were separated using methylene iodide with specific gravity around 3.3 g/cm 3 and an isodynamic separator. Monazite grains were identified and collected under a microscope, mounted on a glass slide using epoxy resin and then subjected to diamond polishing. The concentrations of U, Th and Pb in monazite grains were measured by the JEOL electron microprobe, JXL-8800, at the National Science Museum, Tokyo. The electron microprobe is equipped with four wavelength dispersive spectrometers. The synthetic minerals r-UO3 and ThO2, and natural PbCrO4 are used as standards for measuring the weight percentage of U, Th and Pb respectively. The operating conditions of the microprobe are 15 kv accelerating voltage with a 2 μm beam diameter. Probe currents for monazite are 0.2 μA. The X-ray intensities were integrated for 200 s for Th and U and 300 s for Pb analysis, and 15–20 s for other elements. The theoretical basis of monazite age dating and the detailed calculation method follow that of the chemical thorium uranium total Pb isochron method (CHIME) [37]. Monazites with 3020 Ma and 64 Ma which had been dated by SHRIMP and K–Ar methods,

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respectively, were used as internal age standards with the experiments. 4. Results and discussions 4.1. Compositions of trace elements in the core sediments Table 1 presents concentrations of trace elements and REEs in the bulk sediments and in the fine fraction. The trace elements show large variations in the bulk sediments with a coefficient of variation mostly higher than 20. In comparison, the fine fraction is characterized by smaller variations of elemental concentrations than the bulk sediments. Most elements not only in the bulk sediments but also in the fine fraction exhibit irregular variations with depth (Fig. 3). Some elements such as Ti, Co, Ni, and V yield higher concentrations in most of the fine-grained sediments than in the bulk counterparts, whereas the other elements have similar concentrations between them or are even concentrated in the bulk sediments (Table 1), suggesting a complex control of elemental concentrations in the core sediments. REE compositions of the Quaternary sediments all exhibit convex patterns with enrichments of middle REEs (MREE) relative to upper continental crust (UCC) [40], very similar to that of the modern Changjiang sediments [41]. However, weak MREE enrichment is found in the Pliocene sediments and in contrast, the heavy REEs (HREE) are relatively enriched, suggesting different REE fractionations between the Pliocene and Quaternary sediments. When normalized to East China upper continental crust (EC-UCC) [42] the REE patterns of the Quaternary sediments are more linear with weak fractionations (Fig. 4). Negative anomalies of Ce and Eu with large variations are present in the core sediments (Fig. 5). Ce exhibits a slight depletion in the bulk sediments and the fine fraction, whereas Eu shows a distinct depletion in sediments, similar to that seen in UCC and modern Changjiang sediment [41]. Notably, the Eu depletions are more significant in the Pliocene sediments than in the Quaternary sediments (Fig. 5). 4.2. Provenance discrimination based upon trace element compositions Relatively immobile elements Th and Nb are typically enriched in felsic igneous rocks whereas Sc, Co, Cr and Ti are relatively more concentrated in mafic rocks [40,43,44]. Ratios such as Th/Sc, Th/Co, Nb/Co, Cr/Th and Ti/Nb in sediments, therefore, allow a distinction to be made between felsic and basic sources.

In the present study we use element ratios Cr/Th, Nb/Co and Th/Co to identify sediment origins. Variations of these element ratios clearly show that the Pliocene sediments (N3) can be distinctly discriminated from the overlying Quaternary sediments, as shown by lower Cr/ Th and higher Nb/Co and Th/Co ratios both in the bulk and fine-grained Pliocene sediments (Figs. 6 and 7). Furthermore, the ratios of Cr/Th, Nb/Co and Th/Co show larger variations in the Pliocene compared to the Quaternary sediments. The distinct compositional differences between the Pliocene and the Quaternary sediments suggest that they originate from different provenances, i.e. there was a significant change of sediment provenance at the Pliocene/Quaternary boundary. It is evident that lower Cr/Th and higher Nb/Co and Th/Co ratios in sediments indicate more felsic source rock compositions in the provenances. Therefore, we infer that the Pliocene sediments in Core PD primarily originated from more felsic igneous sources whereas the Quaternary sediments are derived to a greater degree from more mafic sources (Fig. 7). An in-depth investigation of the background geochemical values of the aquatic environment of the Changjiang has been explored in the whole drainage basin through collecting more than 100,000 chemical data from 465 sediment sampling points distributed in the main channel and tributaries [62]. The results of this study show that the concentrations of Co and Cr in fluvial sediments yield the highest values in the Jinshajiang and the lower Yalongjiang valleys. In contrast, Th has the highest concentration of more than 15 mg/kg in the lower Changjiang valley but is generally lower than 15 mg/kg in the upper drainage basin. These observations indicate that the source sediments in the upper Changjiang valley are rich in transition metals such as Cr, Co and Ti, whereas the sources in the lower Changjiang basin have high concentrations of Th and Nb, which are highly concentrated in felsic igneous rocks. Therefore, our geochemical data imply that the Pliocene sediments from Core PD are probably mostly sourced from the lower Changjiang valley, whereas the Quaternary sediments mostly originate from upper Changjiang sources where mafic igneous rocks are more developed. The upper Permian–Triassic Emeishan basalt, occupying an area about 30 × 104 km2 in southwest China (Fig. 1), has been regarded as the only large igneous province (LIP) in China [63]. This basic LIP covers a large area in the upper Changjiang valley and thus could be a major supplier of sediments to the Changjiang. Furthermore, the basalt in the form of Triassic arcs in the Songpan Garze and Bangong Suture Zone can be the possible

Table 1 Average concentrations of some trace elements and rare earth elements in Core PD sediments (unit: ppm) n

Ti

Ba

Ni

Cr

V

Nb

Th

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Y

13.2 11.3 11.5 11.3 10.9 1.48 1.77 4.59 2.57 5.25 11 16 40 23 48 17

44.8 36.3 40.7 39.8 36.0 9.20 9.31 18.0 12.0 27.7 21 26 44 30 77 38

87.6 72.4 73.8 75.4 70.0 12.7 13.5 34.7 21.7 43.1 14 19 47 29 62 80

101 80.4 75.0 81.4 79.0 17.7 15.3 35.2 23.3 37.5 18 19 47 29 47 98

22.0 19.5 18.5 19.0 22.2 3.91 4.30 6.07 5.03 8.38 18 22 33 26 38 12

13.7 11.1 13.0 11.6 16.7 2.81 2.74 6.30 3.85 10.7 20 25 48 33 64 8.95

39.3 39.2 38.5 32.1 39.1 3.1 7.0 17.6 9.8 10.4 8 18 46 30 27 34.8

81.0 76.6 75.1 63.2 75.4 5.6 13.2 36.4 20.1 23.3 7 17 48 32 31 66.4

11.2 10.5 10.4 8.66 9.75 0.8 1.7 4.9 2.5 3.1 7 16 47 29 32

32.7 30.9 30.2 25.5 27.8 2.1 5.5 14.0 7.7 9.6 7 18 46 30 35 30.4

7.76 7.16 7.04 5.86 6.44 0.5 1.2 3.4 1.7 2.2 7 17 49 29 35 5.09

1.54 1.39 1.43 1.15 1.15 0.1 0.2 0.7 0.3 0.4 7 16 51 26 35 1.21

6.96 6.31 6.33 5.07 5.70 0.5 1.1 3.2 1.4 2.0 7 17 51 28 35

1.15 0.99 0.93 0.79 0.97 0.1 0.2 0.4 0.2 0.3 8 15 48 27 29 0.82

6.02 5.49 5.37 4.39 5.13 0.4 1.0 2.6 1.3 1.6 7 19 48 28 30

1.34 1.16 1.11 0.94 1.16 0.1 0.2 0.5 0.3 0.3 8 17 48 29 28

3.21 2.84 2.72 2.35 2.93 0.2 0.6 1.2 0.7 0.8 8 20 46 30 26

0.56 0.47 0.45 0.40 0.55 0.0 0.1 0.2 0.1 0.1 9 17 46 31 22

3.34 2.94 2.86 2.52 3.32 0.3 0.6 1.3 0.8 0.8 8 22 45 32 24 2.26

0.50 0.42 0.40 0.36 0.49 0.0 0.1 0.2 0.1 0.1 8 23 46 33 23 0.35

31.0 27.5 27.3 22.7 28.0 2.2 5.6 13.0 6.7 8.2 7 20 48 30 29 17.4

< 0.063 mm fraction Q4-Avg 13 4595 Q3-Avg 20 4312 4 4701 Q2-Avg Q1-Avg 9 4704 N3-Avg 18 4631 Q4-STD 341 708 Q3-STD Q2-STD 636 Q1-STD 778 1494 N3-STD Q4-CV 7 Q3-CV 16 Q2-CV 14 17 Q1-CV N3-CV 32

11.9 16.8 21.6 22.3 16.7 1.77 4.63 2.90 3.67 7.21 15 28 13 16 43

32.3 39.7 56.2 59.0 40.8 5.22 9.41 8.70 10.0 18.3 16 24 15 17 45

56.3 80.8 52.8 87.4 47.2 113 77.5 121 58.5 127 5.41 12.3 12.8 16.9 18.6 17.5 21.3 23.3 11.4 81.8 10 15 24 19 39 15 27 19 19 65

15.6 16.7 16.5 16.6 19.2 1.14 2.27 1.90 2.10 7.58 7 14 12 13 40

9.43 10.9 11.4 13.5 15.6 1.23 2.15 1.45 0.77 4.90 13 20 13 6 31

24.8 33.7 33.2 34.1 38.4 4.40 11.6 2.39 2.47 8.22 18 34 7 7 21

53.5 6.16 67.6 8.07 62.2 7.83 67.4 8.06 83.0 8.76 6.82 0.83 17.2 2.50 5.51 0.66 4.77 0.40 27.5 2.39 13 13 25 31 9 8 7 5 33 27

23.1 4.65 0.98 4.40 0.67 3.95 0.79 28.4 5.84 1.15 5.40 0.78 4.64 0.88 26.6 5.44 1.12 5.13 0.74 4.44 0.81 28.1 5.67 1.15 5.35 0.76 4.47 0.81 29.9 5.78 1.10 5.75 0.84 4.78 0.89 2.78 0.52 0.09 0.44 0.07 0.35 0.07 8.16 1.70 0.30 1.61 0.22 1.27 0.21 1.98 0.19 0.05 0.28 0.06 0.13 0.05 1.79 0.47 0.14 0.45 0.07 0.59 0.10 8.72 1.73 0.29 1.73 0.23 1.18 0.19 12 11 9 10 10 9 9 29 29 26 30 28 27 24 7 3 5 5 8 3 6 6 8 13 8 9 13 13 29 30 26 30 28 25 22

2.29 2.56 2.38 2.40 2.60 0.21 0.62 0.23 0.29 0.54 9 24 10 12 21

0.33 2.15 0.36 2.27 0.34 2.17 0.33 2.13 0.36 2.35 0.03 0.20 0.08 0.51 0.04 0.21 0.05 0.28 0.07 0.39 9 9 23 23 11 9 14 13 20 17

0.32 0.35 0.33 0.33 0.36 0.03 0.08 0.04 0.05 0.06 10 23 11 15 18

19.4 23.0 21.2 21.8 24.2 1.94 6.17 1.45 2.64 5.75 10 27 7 12 24

373 496 485 468 505 42.6 88.4 46.7 68.8 69 11 18 10 15 14

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Co

Bulk composition Q4-Avg 12 4681 451 Q3-Avg 30 4050 479 Q2-Avg 10 3391 509 Q1-Avg 24 3997 470 N3-Avg 19 4682 1119 359 37.4 Q4-STD Q3-STD 984 66.1 Q2-STD 1645 87.7 Q1-STD 1244 106 2492 1490 N3-STD Q4-CV 8 8 Q3-CV 24 14 Q2-CV 49 17 31 23 Q1-CV N3-CV 53 133 EC-UCC 3892 678

EC-UCC: East China upper continental crust [42]; n is sample number; Avg: average concentration; STD: standard deviation; CV: coefficient of variation. 767

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Fig. 3. Depth variations of elemental concentrations in the bulk sediment samples and the <0.063 mm fraction of Core PD.

basic sources of sediments (Peter Clift, personal communication). In addition, ultramafic and mafic rocks with different ages widely crop out in the upper Changjiang valley above Yichang [34]. Although Cenozoic basalts are widely distributed along the coastal provinces of East China, they occupy a small area in the lower Changjiang valley with sporadic distribution in Shanghai, Jiangsu and Anhui provinces [64]. In contrast,

late Mesozoic granitoids widely crop out in the lower Changjiang region, which we propose as the major source of Pliocene sediments in Core PD. REE fractionation parameters give more constraints on sediment provenance. Although REEs have been considered mobile during intense chemical weathering [45], most previous studies suggest that the single most important factor determining REE compositions in

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sands. The percentages of monazite with different crystallization ages in the core sediments are summarized in Table 2. The monazite in Core PD sediments yield ages ranging from < 25 to 2600 Ma with six main age modes occurring at < 25, 50–200, 200–400, 400– 550, 800–1000, and 1800–2000 Ma in the age histogram (Figs. 8 and 9). The results show that the vast majority of monazite grains (above 75%) have ages younger than 550 Ma although the oldest monazite grains can be dated back to the late Archean at about 2550 Ma. Fig. 8 shows that most of the Quaternary sediments bear similar monazite age patterns, with the highest peak ages present at 125–250 Ma (ca. 28–67% of total grains) and second highest at 400–550 Ma (ca. 29% of total grains). Nonetheless, the young monazite grains dated at <25 Ma are more concentrated in the upper Pleistocene and Holocene sediments relative to the lower and middle Pleistocene sediments. The Pliocene sediments, however, yield significantly different age spectrum from the Quaternary sediments, indicated by the absence of young monazite grains dated at <25 Ma and by the highest peak age occurring at 400–550 Ma with 32–69% of the total grains. The monazite grains dated at 1800–2000 Ma in the Pliocene sediment are also not as abundant as in the Quaternary Fig. 4. REE fractionation patterns of the fine-grained sediments (<0.063 mm fraction) normalized to upper continental crust (UCC, [40]) and East China upper continental crust (ECUC, [42]).

sediments is provenance [40,41,46,47]. An Eu anomaly in sediments is primarily controlled by potassium feldspar and plagioclase which bear strong Eu enrichment [40,46,47]. Core PD has different Ce and Eu anomalies between the Pliocene and Quaternary sediments, and the negative Eu anomalies are more obvious in the Pliocene sediments (Fig. 5). Both the bulk samples and the < 0.063 mm fine fraction show similar variations of Eu anomalies, suggesting that sediment grain size composition and mineral partitioning do not exert a significant control on Eu anomaly. Correspondingly, the variations of Ce and Eu anomalies in the core sediments are diagnostic of the changing sediment sources. 4.3. Monazite age spectra and tectonic–magmatic implications Monazite usually occurs as a rare accessory mineral in granitic rocks, in syenitic and granitic pegmatites and occasionally in dolomitic marble with a metasomatic origin [48]. It is resistant to weathering and is thus frequently concentrated as a detrital mineral in river

Fig. 5. Downcore variations of Ce- and Eu anomalies in the bulk sediment samples and the <0.063 mm fraction of Core PD. Eu anomaly (Eu⁎) is calculated by EuN / (SmN × GdN)1/2 and Ce anomaly (Ce⁎) by CeN / (LaN × PrN)1/2, with normalization to chondrite.

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Fig. 6. Depth variations of element ratios in Core PD. Note that the lowermost Pliocene sediments (N3) have significantly different geochemical compositions from the overlying Quaternary sediments, suggesting a distinct change of sediment provenance at the Q/N boundary.

sediments, whereas grains with ages of 800–1000 Ma are relatively more concentrated in the Pliocene sediments (Table 2, Fig. 8). A wealth of isotopic and single-mineral age data of the Yangtze Craton and its adjoining region has been achieved in the past two decades [33,49–57], which allows us to compare these monazite ages with published bedrock age data and further to constrain the possible provenances of these core sediments. The oldest monazite ages of 2425–2550 Ma in sediments are probably mostly sourced from the Kongling and/or Dabie high-grade metamorphic terrain along the northern margin of the Yangtze Craton, which is present in the middle-lower Changjiang valley (Fig. 1). The Kongling metamorphic complex is considered to be the oldest basement rocks (> 3.2 Ga) in the Yangtze Craton [51,52,54,58]. Nevertheless, due to the limited outcrop of these old metamorphic complexes in the Changjiang drainage basin monazite grains dated at > 2000 Ma are not abundant in Core PD. The monazite grains dated at 1800–2000 Ma match well with the Paleoproterozoic Luliangian Movement, which is regarded as the most important crust formation event in the Yangtze Craton [33,54]. The igneous rocks formed in this intense tectono-thermal event are not only present in the Kongling Complex, but also occur in the upper Changjiang valley such as the Kangding Complex in Sichuan and the lower basin in Zhejiang and Jiangxi Provinces [33,54]. The monazite grains dated at 1800–

2000 Ma correspondingly have higher percentages of the total than the grains at > 2000 Ma (Figs. 8 and 9). Furthermore, the Quaternary sediments contain more monazite grains of 1800–2000 Ma than the Pliocene sediments. Monazite grains dated at 800–1000 Ma are consistent with the Neoproterozoic Jinningian Movement [33,49,51,53] which resulted in wide distribution of igneous rocks in the Yangtze Craton and the Cathaysia Block [49,51,53,55–57]. Because this Neoproterozoic magmatism primarily formed bimodal basalt-dacite/ rhyolite in the Yangtze Craton and the granitoids show only sporadic outcrop in the northern margin of the Yangtze Craton [34,51,55–57], monazite grains dated at 800–1000 Ma do not yield high percentages in the core sediments (Figs. 8 and 9). Peak ages of 400–550 Ma match well with the early Paleozoic Caledonian Movement (540–360 Ma) [33] which is an intense tectonic event between the Cathaysia Block and the Yangtze Craton and consequently formed the major part of the Cathaysia Block [33,34,51]. In comparison the Yangtze Craton is relatively tectonically stable during this period. The Caledonian granitoids intruded into Sinian and Cambrian sedimentary rocks and now mainly outcrop in the middle-lower Changjiang valleys [33,34]. Therefore, we argue that monazite grains with ages of 400–550 Ma are mostly sourced from proximal provenances. These monazite grains yield the highest percentages in the Pliocene sediments

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Table 2 Percentages of monazite with different ages in different depositional units of Core PD (unit of crystallization age: 100 Ma)

Fig. 7. Discrimination plots of Cr/Th vs. Th/Co in the bulk sediment samples and the <0.063 mm fraction of Core PD. Note the Pliocene (N3) and the Quaternary sediments can be classified into two groups. The variations of element ratios suggest that the source rocks of the N3 sediments are more felsic than those of the Quaternary sediments.

(Table 2; Fig. 8), suggesting that the Pliocene sediments are dominated by erosion from a proximal provenance, whereas the Quaternary sediments have a larger provenance area. The grains dated at 200–400 Ma coincide with the Hercynian–Indosonian movement (360–195 Ma) which was strong in north and southeast China and resulted in wide distribution of Hercynian granitoids in Cathaysia and partly on the southern margin of the Yangtze Craton [33,51]. In comparison, the Triassic Indosonian movement was more intense than the Hercynian movement especially in the upper Jinshajiang and middle Changjiang basins, which resulted in widespread development of Indosonian granitoids in NE Tibet, Yunnan, Hunan and Jiangxi Provinces [34]. Higher percentages of monazite grains dated at 200–400 Ma in the Quaternary sediments further imply that during the Quaternary the Changjiang was well developed and received a large

Age (100 Ma)

Q4

0.25 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 19 19.5 20 20.5 21 21.5 22 22.5 23 23.5 24 24.5 25 25.5 26

18.3 14.4 2.8 0.8 3.4 1.5 10.0 6.5 19.0 15.2 12.1 17.9 1.1 4.2 0.0 1.5 2.8 3.4 10.0 8.0 1.4 7.2 0.9 2.7 0.6 1.5 0.2 0.0 0.2 0.0 1.1 0.4 2.8 0.4 1.1 1.1 0.8 1.5 0.2 1.1 1.1 0.4 0.0 0.0 0.2 0.0 0.2 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.2 0.4 0.0 0.0 0.5 0.4 3.2 0.8 3.5 5.3 1.7 2.7 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.4 0.2 0.4 0.0 0.0

Q3upper

Q3lower

Q2

Q1 upper

14.7 3.9 0.4 11.1 12.9 14.0 3.9 0.0 4.7 10.4 0.4 0.0 0.7 0.4 0.0 0.7 1.8 4.3 1.1 1.1 0.4 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.7 4.3 2.5 1.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

3.4 1.5 4.5 0.5 0.0 0.5 9.0 9.0 10.1 17.6 20.2 23.1 1.1 7.5 0.0 0.5 1.1 1.0 15.7 8.5 9.0 6.5 2.2 1.0 1.1 2.0 0.0 0.5 0.0 1.0 0.0 0.0 0.0 0.0 4.5 3.0 2.2 4.0 1.1 1.5 2.2 2.0 1.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.0 7.9 5.0 3.4 1.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Q1 lower

N3

6.1 2.1 2.9 8.7 15.1 20.6 5.0 0.0 1.6 10.8 5.3 1.3 0.5 0.0 0.0 0.3 1.1 5.3 2.4 0.0 1.1 0.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.3 0.0 0.0 0.0 0.0 0.3 0.0 0.0 0.8 3.4 2.9 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.3 0.0 0.5 0.0

0.0 1.0 0.5 14.4 5.2 8.9 1.3 0.5 4.2 24.7 15.7 1.8 0.3 0.0 0.0 0.0 0.8 9.2 2.6 1.0 1.3 0.5 0.0 0.0 0.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.3 2.4 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.8 0.0 0.0 0.0

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Fig. 8. Comparisons of monazite age histogram between different depositional units in Core PD. The age patterns of the Quaternary sediments are overall similar with several major age peaks. Notably, young monazite grains with ages less than 25 Ma are present in all of the Quaternary strata but absent in the Pliocene sediments.

quantity of sediments from today's upper basin, whereas in the Pliocene the sediment provenance was more restricted and proximal. The detrital grains dated at 50–200 Ma match well with the Yenshanian movement (65–195 Ma), which took place in Jurassic to Cretaceous [33,34,51]. Late Mesozoic Yenshanian magmatic activities are very intense in the upper and lower Changjiang valley and the Cathaysia Block of South China, resulting in the wide distribution of granitic rocks in southeast China and Tibet [33,34,51]. The early Yenshanian magmatic

activity (Jurassic) mainly took place in western China, including the upper Changjiang valley, while the late Yenshanian movement (Cretaceous) mostly shifted to East China and the lower Changjiang valley [33,34,49,51]. Therefore, the Yenshanian granitoids in the upper Changjiang basin generally yield older ages than those in the lower valley. The monazite grains dated at ca. 200 Ma in Core PD are much more concentrated in the Quaternary sediments than in the Pliocene sediments (Fig. 8), which again implies that the Quaternary sediments might largely come from the

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773

Fig. 9. Age spectrum of monazite in the core sediments. The main age peaks correspond to the main tectonic events in the Yangtze Craton and its adjoining area.

upper Changjiang valley, whereas the Pliocene sediments are probably mostly derived from lower valley provenances. Grains with modal ages of < 50 Ma correlate well with the Himalayan movement in relation to the India–Asia collision [6]. The timing of Tibetan Plateau uplift continues to be contentious but a widely documented interpretation is that the rapid uplift of the Plateau or major plate reorganization in Southeast Asia began around early (< 25 Ma) [59,60] or late Miocene (8 Ma) [61]. The granitoids that accompanied this intense magmatic activity only occur in the Himalayan–Tibetan Plateau and surrounding area [33,34], and therefore, monazite grains sourced from these graintic rocks can be used to link the uplift of the Tibetan Plateau. It is noteworthy that monazite grains dated at < 25 Ma are completely absent from the Pliocene sediments but are present in the Quaternary sediments albeit with variable total percentages (Fig. 8; Table 2). Therefore, we infer that during the Pliocene today's Changjiang Delta area did not receive much sediment from the eastern Tibetan area whereas during the Quaternary the northeastern Tibetan Plateau became a major supplier of sediment to the Changjiang and finally to the delta area. 4.4. Drainage pattern evolution of the Changjiang during the Pliocene and Quaternary Both elemental compositions and monazite age data suggest that the Pliocene sediments of Core PD have proximal and restricted provenances whereas the Quaternary sediments mostly originated from distal and larger provenances. Consequently, we infer that during the Pliocene the Changjiang was a smaller river draining the present-day lower Changjiang valley, whereas it developed into a large river similar to today's scale during the early Quaternary (at

> 1.18 Ma according to the ESR age at 234.6 m near the bottom of the Quaternary strata of Core PD) and its drainage basin extended to the upper valley including the Emeishan basalt LIP. As mentioned in the Introduction, previous studies have demonstrated that the Changjiang may be an old river dating back to the pre-Tertiary or to the early Pleistocene, or a very young river formed at ca. 0.15 Ma. In 1936, Barbour first proposed that the paleo-Changjiang once flowed southward to the paleo-Red River and into the South China Sea, but that the river pattern changed during the Tertiary due to abrupt river capture and reversal [19]. More recently, some studies suggested that drainage capture and a flow reversal event happened during the Miocene [12,13] or the late Pleistocene [10,28,31]. At present, we cannot give more details on the river capture and reversal of the paleo-Changjiang, i.e. whether and when the paleo-Changjiang once flowed southward into the paleo-Red River, because our existing data only indicate that during the Pliocene the “paleo-Changjiang” or its eastern tributary in East China drained a limited provenance area, whereas in the early Quaternary the drainage pattern significantly changed and a wide drainage basin similar as today's dimension was formed by at least > 1.18 Ma. The river development may be correlated with climate–tectonic coupling [6,13]. During the Neogene the southern flank of the present Changjiang Delta was continuously tectonically uplifted and experienced strong erosion, forming a wide unconformity between the Jurassic/Cretaceous baserocks and the overlying Quaternary sequence [22,32]. Stratigraphic study of Core PD shows that the bottom of the lower Quaternary strata dates back to 1.18 Ma and is underlain by Pliocene sediments, apparently indicating a hiatus located below the early Quaternary strata. Correspondingly, the Changjiang river channel progressively

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shifted southward from the north Jiangsu plain since the Pliocene with the differential subsidence of the delta plain [32,65]. The north Jiangsu plain and the delta area were dominated by lacustrine–alluvial depositional environments during the Pliocene, in which the coarse-grained sandy gravels are angular and poorly sorted with complex mineral compositions, indicating alluvial fan deposition with proximal provenances [65]. Meanwhile, Quaternary stratigraphic study of the Jianghan Basin in the middle Changjiang valley has demonstrated that the basin was also dominated by a lake environment and that the paleo-lake shrank during the late Pliocene due to tectonic uplift and an increasingly arid climate [22,25,65]. The fluvial gravels began to develop during the early Pleistocene at ca. 1.2 Ma, forming an enormous alluvial fan on the western margin of the Jianghan Basin just east of the Three Gorges [29]. The mineral compositions of rounded gravels are interpreted as being derived from distal source west of the Three Gorges including from the Emeishan Basalt LIP [25,66]. Based on neotectonic, glaciation and geomorphologic studies on the Three Gorges, Tang and Tao (1997) proposed that the Three Gorges were cut through and today's Changjiang was completed at ca. 0.7–1.0 Ma [26]. Li et al. (2001) argued that the Changjiang channelized through the Three Gorges not later than the initiation of the earlier terrace (1.16 Ma), caused by a tectonic rise accompanied by a rapid incision of the river channel [29]. Study of the fault structure developed in the Yunnan Plateau suggested that the Jinshajiang changed its flow direction from southward to eastward at about 1 Ma due to the strong tectonic uplift of the Yunnan Plateau [30]. All of these published dates concerning the Changjiang formation are consistent with our research results (at > 1.18 Ma) from the Changjiang Delta.

Pliocene sediment provenances, whereas more mafic igneous provenances account for the sources of the Quaternary sediments. The Emeishan Basalt province is widely distributed in the upper Changjiang valley and is responsible for the mafic compositions of the Quaternary sediments, whereas the acidic igneous rocks in the lower Changjiang valley account for the felsic components of the Pliocene sediments. Detrital monazite age patterns provide more constraints on sediment provenance. Six age modes occurring at <25, 50–200, 200–400, 400–550, 800– 1000, and 1800–2000 Ma match well with the late Himalayan, Yenshanian, Hercynian–Indosinian, Caledonian, Neoproterozoic Jinningian, and Luliangian Movements that took place in the Yangtze Craton and its adjoining area. Similar to the results obtained from geochemical compositions, monazite age patterns suggest that the Pliocene sediments are mostly derived from proximal provenances such as the present-day lower Changjiang valley whereas the Quaternary sediments are largely sourced from distal sources and larger provenances. Based upon these data we propose that during the Pliocene the “paleo-Changjiang” or its eastern equivalent was a locally small-scale river draining the presentday lower Changjiang valley while during the early Quaternary at > 1.18 Ma it developed into a large river with the drainage basin extending to the huge Emeishan Basalt province and the eastern Tibetan Plateau. Although this age dated by ESR method probably yields a large uncertainty, it is consistent with many previous studies based upon geomorphologic, geographic and tectonic analyses which argue that today's Changjiang channelized the Three Gorges and was completely formed at ca. 0.7–1.2 Ma. The present study sheds new light on the recognition of the development of the Changjiang during the late Cenozoic. Acknowledgements

5. Conclusions Geochemical compositions and monazite crystallization ages of the sediments from Core PD in the Changjiang Delta were presented in order to identify the sediment origins and to understand the development history of the Changjiang during the Pliocene and Quaternary. Geochemical proxies including Cr/Th, Nb/ Co, and Th/Co ratios were considered for provenance discrimination. The Pliocene sediments have distinctly lower Cr/Th and higher Nb/Co and Th/Co ratios compared to the Quaternary sediments, suggesting that more felsic source rock compositions dominated in the

This work was supported by research funds awarded by the National Natural Science Foundation of China (No. 40276018, 40206008 and 40476029) and sponsored by Shanghai Rising-Star Program (No. 04QMX1430). We are grateful to D.D. Fan, Q. Wang, B.H. Li, and B. Deng for sampling work. Thanks are extended to C.B. Lee, M.S. Choi, and T.K. Na for geochemical measurements, and Z.Y. Chen, M. Sun, G. C. Zhao and X.P. Xia for contributive discussions. Special thanks go to P. Clift, R. Cullers and the anonymous reviewers for their critical and constructive comments on the original draft.

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