U–Pb detrital zircon geochronology and its implications: The early Late Triassic Yanchang Formation, south Ordos Basin, China

Journal of Asian Earth Sciences 64 (2013) 86–98

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U–Pb detrital zircon geochronology and its implications: The early Late Triassic Yanchang Formation, south Ordos Basin, China Xiangyang Xie a,⇑, Paul L. Heller b a b

Department of Geosciences, University of Arkansas, Fayetteville, AR 72701, United States Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, United States

a r t i c l e

i n f o

Article history: Received 16 October 2012 Received in revised form 14 November 2012 Accepted 22 November 2012 Available online 13 December 2012 Keywords: Late Triassic LA-ICP-MS U–Pb detrital zircon Yanchang Formation Ordos Basin

a b s t r a c t U–Pb detrital zircon geochronology has been used to identify provenance and document sediment delivery systems during the deposition of the early Late Triassic Yanchang Formation in the south Ordos Basin. Two outcrop samples of the Yanchang Formation were collected from the southern and southwestern basin margin respectively. U–Pb detrital zircon geochronology of 158 single grains (out of 258 analyzed grains) shows that there are six distinct age populations, 250–300 Ma, 320–380 Ma, 380–420 Ma, 420– 500 Ma, 1.7–2.1 Ga, and 2.3–2.6 Ga. The majority of grains with the two oldest age populations are interpreted as recycled from previous sediments. Multiple sources match the Paleozoic age populations of 380–420 and 420–500 Ma, including the Qilian–Qaidam terranes and the North Qilian orogenic belt to the west, and the Qinling orogenic belt to the south. However, the fact that both samples do not have the Neoproterozoic age populations, which are ubiquitous in these above source areas, suggests that the Late Triassic Yanchang Formation in the south Ordos Basin was not derived from the Qilian–Qaidam terranes, the North Qilian orogenic belt, and the Qinling orogenic belt. Very similar age distribution between the Proterozoic to Paleozoic sedimentary rocks and the early Late Triassic Yanchang Formation in the south Ordos Basin suggests that it was most likely recycled from previous sedimentary rocks from the North China block instead of sediments directly from two basin marginal deformation belts. Published by Elsevier Ltd.

1. Introduction

2. Geology setting and tectonic history

The Ordos Basin is a typical intracontinental basin situated in north central China with an area of over 250,000 km2. The basin developed on the Archean granulites and lower Proterozoic greenschists of the North China block (Yang et al., 2005) and is filled with Paleozoic to Cenozoic age sedimentary rocks with thicknesses that exceed 8 km in the southwestern part. Geographically, the Ordos Basin is surrounded by the Lang and Daqing Mountains to the north, the Lüliang Mountains to the east, the Qinling Mountain ranges to the south and the Liupan and Helan mountain ranges to the west (Fig. 1). A regional seismic survey shows that, except where locally affected by normal faults active in Late Proterozoic times, the basin fill gently dips (<2°) to the west (Yang, 2002). In this study, U–Pb detrital zircon geochronology was utilized to study the provenance of the early Late Triassic Yanchang Formation in the south Ordos Basin. Results will provide insight into provenance, sediment dispersal patterns, and basin margin tectonics and its impact on sedimentation during the deposition of the early Late Triassic Yanchang Formation in the south Ordos Basin.

The Mesozoic to Cenozoic geologic history of central China was strongly influenced by continental accretion along the southern margin of the Eurasian continent (Darby and Ritts, 2000; Liu and Yang, 2000; Darby and Ritts, 2002; Ritts et al., 2004, 2006; Xia et al., 2006b; Yin et al., 2009; Li et al., 2010). This can be simplified as the amalgamation of microplates within China under the background of complex interplay between plates of the Circum-Pacific zone and the Tethys-Himalaya zone (Watson et al., 1987; Yin and Harrison, 2000). Micro- and macro-continental blocks and islandarc fragments were welded together by a series of suturing and collisional events during the Late Paleozoic and Mesozoic time (Zhang et al., 1984; Watson et al., 1987; Yin and Nie, 1996) (Fig. 2). Prior to the Permian, the history of this region evolved as part of the North China block. From Permian to Triassic time, the Triassic collision between the North and South China blocks and the Early Paleozoic suture zone between the Qaidam block and the Eurasian plate took place along basin margins (e.g., Watson et al., 1987; Yin and Harrison, 2000; Song et al., 2012a,b). Beginning in the Early to Late Triassic time the Qinling orogenic belt to the south and the Liupanshan thrust belt to the west formed the southern and western basin boundaries, respectively. The basin evolved into a foreland basin with its asymmetric profile dipping toward the

⇑ Corresponding author. Tel.: +1 479 575 4748; fax: +1 479 575 3469. E-mail address: [email protected] (X. Xie). 1367-9120/$ - see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jseaes.2012.11.045

X. Xie, P.L. Heller / Journal of Asian Earth Sciences 64 (2013) 86–98

87

Fig. 1. Simplified regional geologic map of the Ordos Basin showing the study area, sample locations, major fault systems, and deformation belts around the Ordos Basin, north-central China (modified from Darby and Ritts (2002) and Ritts et al., (2004)). Stars: locations of detrital zircon samples. TC: Sample-TC8; AK: Sample-AK8. Light and dark grey represents the major deformation belts around the Ordos Basin. White represents the young graben systems around basin margins.

A A B B Tarim

Ordos Basin

E

C D

North China Block

G F H I

South South ChinaBlock China block K J

Fig. 2. Sketch map of major tectonic belts of China (modified from Zhang et al. (1984)). A series of microcontinents were amalgamated by collision and suturing events. Grey areas show the Precambrian cratons, including the North China block, the South China block, and the Tarim block. (A) Junggar–Xingan fold belt; (B) Tianshan–Tumen fold belt; (C) North Qilian fold belt; (D) Central and south Qilian fold belt; (E) Qaidam–Kunlun–Qinling fold belt; (F) Songpan Ganzi terrane; (G) Qiangtang terrane; (H) Lhasa block; (I) Indian craton; (J) Youjiang backarc basin; (K) south China fold belt.

X. Xie, P.L. Heller / Journal of Asian Earth Sciences 64 (2013) 86–98

south-west margin and a fluvial-deltaic-lacustrine depositional environment (Liu, 1998; Liao et al., 2007). Starting in the Jurassic time, localized rifting and uplifting has been documented around the basin margins (Yang, 2001; Yang et al., 2005). In Cenozoic time, large scale strike-slip movement and intraplate deformation created the Hetao graben system to the north and the Fenwei graben system to the south and east (Ma et al., 1982; Lu and Ding, 1985; Zhang et al., 1995; Mercier et al., 1996; Zhang et al., 1998). 3. U–Pb detrital zircon geochronology The ability to fingerprint source areas with different zircon age populations makes the U–Pb detrital zircon geochronology very useful in provenance studies, especially when distinctive U–Pb age signatures in source rocks are well defined. 3.1. Sampling and method Two outcrop samples were collected from the first sandstone body right below the regional marker horizon – the Zhangjiatan oil shale, which can be easily identified along the outcrops and traced using geophysical wireline logs in the subsurface based on

its high resistivity signatures (Yang et al., 1990). This marker also serves as a datum and provides an opportunity to document and compare the spatial change and difference of sediment sources in the south Ordos Basin during the early Late Triassic of the Yanchang Formation. Sample AK8 was collected from the southwestern section, the Ruishuihe outcrop, Ankou area; and sample TC8 was collected from the Jinsuoguan outcrop, Tongchuan area (Fig. 1 and 3). All detrital zircons were removed from fine- to medium-grained sandstones of fluvial origin, collected from Jinsuoguan, Tongchuan in the southern Ordos Basin and Ruishuihe, Ankou in the southwestern Ordos Basin (see Fig. 1 for sample locations). Standard mineral separation techniques were performed in the following order: disc mill, Wilfley table, Frantz-LB-1 magnetic separator and heavy liquids. The magnetic separator was operated at 0.5a, 1.0a, and 1.4a mps set at a side slope of 25° and a down slope of 15° in an effort to retain all zircon grains. Heavy liquids used include Bromoform (q P 2.85 g/cm3) and Methylene Iodide (q > 3.3 g/ cm3) done in two steps. Processed grains were separated in alcohol under a binocular microscope using optical and physical characteristics (e.g. color, shape, degree of rounding, transparency, and grain size). Representatives of each population were selected for continued analysis. The final picking of grains was done using both

Southwest

100

Lake 200 300 500 600

Delta plain Lake

700

Braided to meandering river

Lake

400 500 600

Delta plain Delta front Lake

800

Delta front

G: Gravel VCS: Verg coarse grain sandstone CS: Coarse grain sandstone MS: Medium grain sandstone FS: Fine grain sandstone M: Mudstone and shale

1000

900

Delta plain

TC8

G VCS CS MS FiS M

1100

Sample locations

1200

Braided river

Delta Delta plain front

200

AK8

300

Delta plain Delta front

100

Datum: Regional marker bed: the Zhangjiatian Shale

Delta front Lake

South

400

88

(m)

Shale

Plane bedding sandstone

Mudstone

Tabular bedding sandstone

Coal seam

Cross bedding sandstone

G VCS CS MS FS M

Fig. 3. Measured stratigraphic section of the Lower Late Triassic Yanchang Formation showing the detrital zircon sample locations in the southern and southwestern part of the Ordos Basin. The southern stratigraphic section is located at Jinsuoguan, Tongchuan (Latitude: 35°130 0.2500 N; Longitude: 109°10 58.7900 E). The southwestern stratigraphic section is located at the Ruishuihe, Ankou (latitude: 35°70 19.2700 N; longitude: 106°460 42.8000 E). Two samples (AK8 and TC8) were collected right below the regional strata correlation marker horizon—the Zhangjiatan Shale.

X. Xie, P.L. Heller / Journal of Asian Earth Sciences 64 (2013) 86–98

random and non-random picking methods (e.g. Gehrels et al., 2000a) in order to get the maximum mixture of different grain types and a representative age population. The ratio of grains picked was evenly split between random and non-random grains. Splits of all samples were mounted in epoxy, then polished and imaged with reflected light by an optical microscope. Backscatter and cathodoluminescence images were used to characterize internal features of zircon grains. All images served as a reference map for locating laser targets. Separated zircon grains from south and southwest Ordos Basin exhibited a wide range in size and morphology, including light pink to colorless, translucent prismatic euhedral crystals, and dark brown to yellow, rounded crystals with different zoning under CL imaging (Fig. 4). All LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) measurements were conducted at the GeoAnalytical Laboratory at Washington State University. Analyses were done using a New Wave UP-213 laser-ablation system in conjunction with a ThermoFinnigan Element2 single-collector double-focusing magnetic sector ICP-MS. Fixed 30 lm diameter spots were used with a laser frequency of either 10 or 5 Hz, depending on estimated age and Pb abundance. Each analysis consisted of a 6-s warm-up period, with an 8-s delay in recording data to enable the samples to reach the plasma. At this time 300 rapid scans measured atomic masses 202Hg, 204(Hg+Pb), 206Pb, 207Pb, 208Pb, 232Th, 235U, and 238U. These were done over a 35-second period. The Element2 was tuned before each analytical session using the NIST 612 glass standard to maximize signal intensity and stability (Chang et al., 2006). For these measurements, both counting and analog modes were used, depending upon signal ranges. The counting mode was used for signals up to about 4  106 cps, generally for 202Hg, 204(Pb+Hg), 206 Pb, 207Pb, 208Pb, and 235U, and the analog mode was used for a signal range of 105–1010 cps, such as 232Th and 238U. All operating parameters, including ICP-MS and laser settings, were held constant for standards and unknowns to minimize potential bias between samples and standards. At the beginning of each analytical run, a standard was analyzed until fractionation was stable and the variance in the measured 206 Pb/238U and 207Pb/206Pb ratios was at or near ±2% (1 ± standard deviation). Laser-induced time-dependent fractionation was corrected by normalizing measured ratios in both standards and samples to the beginning of the analysis using the intercept method. In this study, all Pb/U ratio and ages were calibrated with reference to known zircon standards: Peixe. The Peixe standard is from the Rio de Peixe area of Brazil with 206Pb/238U 564 ± 4 Ma TIMS age (Gehrels et al., 2000b; Chang et al., 2006). During the sample runs the standard was analyzed two or three times for every 10 unknown analyses.

0

89

In this study, all grain analyses were evaluated carefully using both conventional and Tera–Wasserburg concordia diagrams. Grains yielding ages with more than 10% discordance were not included in the final interpretation. All analytical uncertainties were listed as 2r and uncertainties in age were given with a 95% confidence level. In general, the final reported ages were based on 207 Pb/206Pb ages for those grains older than 1000 Ma and 206 Pb/238U ages for those grains with ages younger than 500 Ma. There are no ages between 500 and 1000 Ma in this study. All diagrams were produced using the Isoplot V3.0 program of Ludwig (2003). 3.2. Results 3.2.1. Sample-TC8 This sample was collected from the lower Yanchang Formation in the southern part of the south Ordos Basin, and is a grey to green, thick-bedded fluvial fine-grained sandstone and siltstone overlain by a dark grey, black mudstone and shale. It was collected from a green, fine-grained sandstone bed located about 35 m above the base of the unit (Fig. 1 and 3). In total, 114 single detrital zircon grains were analyzed from this sample, and 68 grains are selected for the final interpretation (e.g., <±10% discordance) (Table 1 and Fig. 5). Sample-TC8 yielded ages between 260.8 ± 5.1 Ma and 2652.2 ± 26 Ma. Within this distribution, the age spectra shows several peaks at 283, 332, 369, 449, 1845, 1885, 1934, 1995, 2247, 2345, 2510 and 2665 Ma (Fig. 5). About 9% of those are in the age group 250–300 Ma, 4.5% fall within 320–380 Ma, and 4% belong to 410–480 Ma. The rest were all within two older age groups, 1.7–2.1 and 2.3–2.6 Ga. There is no grains with ages between 500 Ma and 1.5 Ga (Fig. 7). 3.2.2. Sample-AK8 This sample was collected from the southwestern part of the Ordos Basin, and is composed of thick-bedded yellow to green fluvial sandstone interbedded with shale and some thin coal seams towards the top of the section. It was collected from a medium to coarse-grained fluvial sandstone at 30 m above the base of the unit (Fig. 1 and 3). Altogether, 144 single detrital zircon grains were analyzed and 90 grains were chosen for the final interpretation (e.g., <±10% discordance) (Table 2 and Fig. 6). These zircon grains range in age from 252.6 ± 8.7 to 2623.4 ± 27.6 Ma with peaks at 277, 295, 415, 476, 1678, 1778, 1860, 1965, 2158, 2277, 2354 and 2511 Ma (Fig. 6). About 10% fall within age group 250– 300 Ma, 3% fall within 410–480 Ma, and the rest belong to the two older age groups, 1.7–2.1 and 2.3–2.6 Ga. No grains fall within ages between 500 Ma and 1.5 Ga (Fig. 7).

100 µm

Fig. 4. Typical Cathodoluminescence images of detrital zircon grains from Sample TC-8 and Sample AK-8 showing grain internal structures. The majority grains are light pink to colorless, translucent prismatic euhedral crystals with different zoning, cores, fractures, and sometime with thin metamorphic/igneous overgrowths. CL imaging has been used to target coherent domains within grains for laser spots. Circles showing laser analyzing spots.

90

X. Xie, P.L. Heller / Journal of Asian Earth Sciences 64 (2013) 86–98

Table 1 Sample-TC8 U–Pb detrital zircon LA-ICP-MS analysis results. Sample location: Latitude: 35°130 0.2500 N; Longitude: 109°10 58.7900 E; Italic values: analyses with discordance larger than ±10%. Sample

Isotopic ratio 207

TC8d_120a TC8d_119a TC8d_118a TC8d_116a TC8d_115a TC8d_114a TC8d_113a TC8d_110a TC8d_109a TC8d_108a TC8d_105a TC8d_104a TC8d_103a TC8d_100a TC8d_99a TC8d_97a TC8d_95a TC8d_94a TC8d_93a TC8d_92a TC8d_89a TC8d_88a TC8d_87a TC8d_83a TC8d_81a TC8d_77a TC8d_76a TC8d_75a TC8d_73a TC8d_72a TC8d_70a TC8d_69a TC8d_68a TC8d_67a TC8d_66a TC8d_65a TC8d_62a TC8d_60a TC8d_59a TC8d_58a TC8d_57a TC8d_56a TC8d_53a TC8d_51a TC8d_48a TC8d_47a TC8d_46a TC8d_44a TC8d_43a TC8d_42a TC8d_41a TC8d_37a TC8d_36a TC8d_35a TC8d_34a TC8d_32a TC8d_29a TC8d_28a TC8d_24a TC8d_23a TC8d_20a TC8d_16a TC8d_12a TC8d_11a TC8d_7a TC8d_3a TC8d_2a TC8d_1a

Pb/235U

5.4145 6.1623 0.2949 0.5609 10.3179 0.3145 7.9703 0.3070 10.8717 10.9225 10.7103 11.1251 9.2138 10.2532 5.5905 11.5771 5.9059 6.3094 10.5918 6.1465 10.6569 11.5246 10.7420 10.5437 8.0238 7.3529 8.8144 6.1116 5.0008 10.0668 0.3317 9.1236 5.8561 5.1263 7.8968 5.6803 10.6510 0.5843 0.3228 6.4166 11.5803 5.0612 10.1597 5.5501 0.4442 10.4586 11.3956 10.1580 11.3781 11.3908 8.5572 0.5581 0.3194 5.4906 8.3706 8.5934 5.1271 11.2225 4.9583 5.3828 6.0783 9.4596 0.4187 10.2107 10.5835 0.3881 6.5162 10.6719

Discordance large than 10% TC8d_117a 4.0445 TC8d_112a 5.4992 TC8d_111a 3.4618

Apparent ages ±1r

206

0.1621 0.1830 0.0090 0.0173 0.3051 0.0098 0.2389 0.0084 0.2809 0.2817 0.2773 0.2879 0.2386 0.2474 0.1402 0.2724 0.1421 0.1504 0.2660 0.1599 0.2503 0.2860 0.2610 0.3926 0.3033 0.2727 0.3222 0.2257 0.1865 0.3750 0.0139 0.3686 0.2314 0.2070 0.3123 0.2262 0.4392 0.0239 0.0133 0.2524 0.4556 0.2055 0.2441 0.1377 0.0107 0.2547 0.2912 0.2708 0.2666 0.2723 0.2042 0.0097 0.0070 0.1037 0.1408 0.1454 0.1502 0.3145 0.1373 0.1510 0.1894 0.3017 0.0131 0.3137 0.2545 0.0098 0.1571 0.3504 0.1214 0.1695 0.1045

Pb/238U

±1r

Error Corr.

206

0.3382 0.3656 0.0413 0.0728 0.4682 0.0443 0.4109 0.0427 0.4795 0.4780 0.4670 0.4732 0.4466 0.4563 0.3450 0.5027 0.3624 0.3723 0.4710 0.3653 0.4912 0.5051 0.4550 0.4738 0.4173 0.3948 0.4306 0.3696 0.3262 0.4681 0.0459 0.4533 0.3629 0.3393 0.3889 0.3422 0.4717 0.0753 0.0454 0.3805 0.5097 0.3320 0.4668 0.3441 0.0590 0.4604 0.4594 0.4516 0.4927 0.4937 0.4109 0.0719 0.0448 0.3482 0.4066 0.4299 0.3344 0.4946 0.3205 0.3503 0.3683 0.4570 0.0569 0.4559 0.4881 0.0528 0.3838 0.4841

0.0067 0.0071 0.0008 0.0015 0.0091 0.0009 0.0081 0.0009 0.0097 0.0097 0.0095 0.0097 0.0091 0.0088 0.0071 0.0094 0.0070 0.0071 0.0098 0.0079 0.0092 0.0102 0.0089 0.0128 0.0116 0.0106 0.0113 0.0099 0.0089 0.0127 0.0011 0.0111 0.0084 0.0083 0.0090 0.0081 0.0121 0.0018 0.0011 0.0087 0.0116 0.0081 0.0060 0.0048 0.0007 0.0062 0.0071 0.0074 0.0057 0.0062 0.0051 0.0008 0.0006 0.0048 0.0046 0.0048 0.0065 0.0089 0.0056 0.0063 0.0073 0.0096 0.0011 0.0088 0.0090 0.0010 0.0071 0.0063

0.8534 0.8518 0.8425 0.8318 0.8499 0.8047 0.8483 0.8343 0.8869 0.8844 0.8889 0.8903 0.8842 0.8676 0.8763 0.8689 0.8709 0.8639 0.8868 0.8840 0.8654 0.8788 0.8735 0.8652 0.8677 0.8617 0.8601 0.8635 0.8649 0.8649 0.7569 0.8063 0.8015 0.8052 0.8018 0.8028 0.8069 0.7858 0.7733 0.7983 0.7987 0.7988 0.8008 0.7953 0.7887 0.8011 0.8123 0.7959 0.7963 0.7957 0.7961 0.7500 0.6498 0.8339 0.8118 0.7961 0.8077 0.8072 0.8075 0.8115 0.8049 0.8153 0.7979 0.8067 0.8677 0.7995 0.8681 0.7837

0.2526 0.3035 0.2106

0.0051 0.0061 0.0042

0.8552 0.8310 0.8496

Pb/238U

±1r (Ma)

206

Pb/207Pb

±1r (Ma)

Best age

±1r (Ma)

1877.8 2008.5 260.8 453.2 2475.8 279.4 2219.2 269.6 2525.1 2518.5 2470.3 2497.6 2380.2 2423.1 1910.5 2625.6 1993.4 2040.2 2488.0 2007.1 2575.8 2635.6 2417.3 2500.0 2248.0 2145.1 2308.5 2027.5 1820.0 2475.3 289.3 2409.9 1996.0 1883.1 2117.8 1897.2 2491.1 468.1 286.2 2078.5 2655.6 1848.3 2469.4 1906.2 369.3 2441.3 2436.8 2402.4 2582.5 2586.9 2219.2 447.6 282.7 1926.2 2199.5 2305.4 1859.9 2590.3 1792.1 1936.0 2021.5 2426.5 356.7 2421.5 2562.6 331.8 2094.1 2545.0

32.3 33.6 5.1 8.8 39.6 5.3 37.0 5.4 42.3 42.0 41.8 42.2 40.3 38.9 33.7 40.3 33.0 33.1 42.6 37.1 39.5 43.7 39.4 55.9 52.5 48.8 50.6 46.3 42.9 55.5 6.8 49.1 39.6 39.8 41.8 38.7 52.6 10.9 6.8 40.4 49.5 39.2 26.3 22.8 4.6 27.1 31.1 32.6 24.7 26.5 23.3 4.7 3.7 23.1 21.1 21.7 31.2 38.1 27.4 30.1 34.1 42.1 6.8 38.8 38.7 5.9 32.9 27.2

1897.3 1989.3 276.1 446.4 2453.6 263.2 2235.3 291.7 2501.7 2514.9 2521.0 2562.5 2341.5 2487.0 1919.3 2528.1 1929.4 1999.1 2488.2 1986.5 2427.6 2512.6 2570.0 2470.2 2220.3 2164.6 2327.9 1954.9 1818.5 2412.1 303.9 2299.5 1911.5 1792.8 2314.6 1962.3 2495.0 463.2 267.0 1990.5 2505.3 1808.7 2433.0 1910.9 398.0 2505.2 2652.2 2488.5 2532.7 2531.1 2357.7 464.1 271.2 1869.7 2337.9 2287.3 1818.8 2503.2 1835.3 1823.1 1951.3 2346.9 344.8 2480.9 2426.1 341.0 2001.9 2454.4

30 29 39 39 28 44 29 34 20 21 20 20 21 20 22 20 21 21 20 22 20 20 20 32 33 34 33 34 35 33 63 43 44 45 42 44 42 58 61 44 42 46 26 29 36 26 26 28 26 26 27 26 38 19 17 18 32 29 31 31 34 32 44 32 21 34 22 40

1897.3 1989.3 260.8 453.2 2453.6 279.4 2235.3 269.6 2501.7 2514.9 2521.0 2562.5 2341.5 2487.0 1919.3 2528.1 1929.4 1999.1 2488.2 1986.5 2427.6 2512.6 2570.0 2470.2 2220.3 2164.6 2327.9 1954.9 1818.5 2412.1 289.3 2299.5 1911.5 1792.8 2314.6 1962.3 2495.0 468.1 286.2 1990.5 2505.3 1808.7 2433.0 1910.9 369.3 2505.2 2652.2 2488.5 2532.7 2531.1 2357.7 447.6 282.7 1869.7 2337.9 2287.3 1818.8 2503.2 1835.3 1823.1 1951.3 2346.9 356.7 2480.9 2426.1 331.8 2001.9 2454.4

30 29 5.1 8.8 28 5.3 29 5.4 20 21 20 20 21 20 22 20 21 21 20 22 20 20 20 32 33 34 33 34 35 33 6.8 43 44 45 42 44 42 10.9 6.8 44 42 46 26 29 4.6 26 26 28 26 26 27 4.7 3.7 19 17 18 32 29 31 31 34 32 6.8 32 21 5.9 22 40

1452.1 1708.6 1231.8

26.0 30.2 22.4

1897.1 2116.9 1944.8

29 31 30

91

X. Xie, P.L. Heller / Journal of Asian Earth Sciences 64 (2013) 86–98 Table 1 (continued) Sample

Isotopic ratio 207

TC8d_107a TC8d_102a TC8d_101a TC8d_98a TC8d_96a TC8d_91a TC8d_90a TC8d_86a TC8d_85a TC8d_84a TC8d_82a TC8d_80a TC8d_79a TC8d_78a TC8d_74a TC8d_71a TC8d_64a TC8d_63a TC8d_61a TC8d_55a TC8d_54a TC8d_50a TC8d_49a TC8d_45a TC8d_40a TC8d_39a TC8d_38a TC8d_33a TC8d_31a TC8d_30a TC8d_25a TC8d_22a TC8d_19a TC8d_17a TC8d_15a TC8d_14a TC8d_13a TC8d_10a TC8d_9a TC8d_8a TC8d_6a TC8d_5a TC8d_4a

Pb/235U

3.9120 7.7316 0.2731 0.3814 0.3621 4.0392 0.2730 0.2830 0.3728 0.2867 0.2775 0.3412 0.3494 8.1883 0.3461 10.4807 0.5427 0.3428 4.3105 0.4280 6.6638 0.3425 0.4796 0.3120 0.3940 0.3967 0.3161 0.4940 0.3069 4.2238 0.4439 0.3082 3.0039 0.3957 0.3104 3.2987 7.8468 0.3711 0.3486 4.7218 0.3833 0.4317 0.3865

Apparent ages ±1r

206

Pb/238U

0.1047 0.2100 0.0075 0.0094 0.0101 0.0967 0.0092 0.0067 0.0143 0.0109 0.0103 0.0132 0.0131 0.3081 0.0133 0.4024 0.0219 0.0136 0.1718 0.0120 0.1615 0.0085 0.0138 0.0082 0.0084 0.0085 0.0057 0.0090 0.0059 0.1150 0.0154 0.0149 0.0987 0.0167 0.0099 0.1088 0.2580 0.0094 0.0110 0.1206 0.0129 0.0126 0.0095

0.2405 0.3795 0.0377 0.0438 0.0481 0.2250 0.0393 0.0408 0.0484 0.0399 0.0411 0.0480 0.0468 0.3792 0.0468 0.4293 0.0569 0.0438 0.2168 0.0467 0.3143 0.0453 0.0424 0.0425 0.0488 0.0426 0.0420 0.0552 0.0415 0.2252 0.0536 0.0379 0.1420 0.0460 0.0444 0.2279 0.3309 0.0518 0.0453 0.2692 0.0456 0.0489 0.0503

±1r

Error Corr.

206

Pb/238U

0.0052 0.0083 0.0008 0.0008 0.0010 0.0043 0.0009 0.0007 0.0013 0.0011 0.0011 0.0013 0.0012 0.0104 0.0013 0.0120 0.0013 0.0010 0.0051 0.0007 0.0041 0.0006 0.0008 0.0006 0.0007 0.0006 0.0005 0.0006 0.0005 0.0038 0.0011 0.0016 0.0032 0.0015 0.0008 0.0051 0.0074 0.0010 0.0011 0.0054 0.0011 0.0009 0.0009

0.8938 0.8955 0.8584 0.8409 0.7548 0.8679 0.6206 0.8246 0.8397 0.8404 0.8396 0.8259 0.8415 0.8655 0.8281 0.8557 0.7758 0.7849 0.8035 0.6807 0.8006 0.7611 0.7904 0.7505 0.7088 0.7490 0.7573 0.7110 0.7520 0.8039 0.6489 0.9175 0.8260 0.8590 0.7563 0.8189 0.8220 0.8326 0.8246 0.8742 0.7573 0.6730 0.8416

1389.5 2074.0 238.7 276.2 302.6 1308.3 248.7 257.9 304.8 252.0 259.6 302.1 294.6 2072.4 294.9 2302.5 356.8 276.6 1264.9 293.9 1761.7 285.8 267.8 268.0 307.1 269.1 265.0 346.3 262.0 1309.5 336.5 239.6 856.1 289.7 279.8 1323.2 1842.6 325.6 285.3 1536.9 287.5 307.5 316.6

±1r (Ma)

206

Pb/207Pb

±1r (Ma)

26.7 38.7 5.0 5.2 6.1 22.6 5.5 4.6 8.1 6.7 6.6 8.0 7.7 48.6 7.8 54.0 8.0 6.2 27.1 4.2 20.3 3.6 4.8 3.7 4.2 3.9 3.1 3.8 3.3 20.0 6.6 9.8 17.9 9.4 5.2 26.6 35.5 6.0 6.9 27.3 7.0 5.7 5.7

1925.3 2320.0 307.1 714.5 398.1 2100.8 211.1 208.2 445.9 292.2 146.1 266.4 378.8 2419.2 355.1 2625.3 904.0 480.8 2278.5 823.3 2388.6 403.5 1245.8 341.7 550.8 853.0 397.4 771.8 356.8 2176.8 606.0 568.2 2383.9 688.4 229.2 1714.0 2576.9 283.1 446.6 2059.4 637.4 744.3 439.9

22 21 32 28 41 21 60 31 47 48 48 50 46 33 49 34 54 57 43 44 27 38 35 40 33 29 27 27 29 29 56 42 32 46 49 35 32 32 39 22 47 45 30

Best age

±1r (Ma)

0.6

0.4 0.28

206

Pb/

238

U

2500

0.24 0.20

2800

206

Pb

1500

Pb/

0.16

207

0.2

2000

0.12 0.08

1200 400

0.04

500

0.00

0

4

8

12

16

238

U/

0.0 0

2

4

6

8 207

Pb/

10

20

24

28

206

Pb

12

14

235

U

Fig. 5. Pb/U concordia diagram of single detrital zircon ages for Sample-TC8 with insert Pb/U Terra-Wasserburg Concordia diagram. Only analyses with the discordance less than 10% are plotted. Uncertainties in age are given with a 95% confidence level. Error ellipses are at the 2r level. Number: 68 out of 114.

92

X. Xie, P.L. Heller / Journal of Asian Earth Sciences 64 (2013) 86–98

Table 2 Sample-AK8 U–Pb detrital zircon LA-ICP-MS analysis results. Sample location: Latitude: 35°70 19.2700 N; and Longitude: 106°460 42.8000 E; Italic values: analyses with discordance larger than ±10%. Sample

Isotopic ratio 207

AC8d_140a AC8d_139a AC8d_138a AC8d_137a AC8d_136a AC8d_133a AC8d_132a AC8d_131a AC8d_130a AC8d_129a AC8d_128a AC8d_127a AC8d_126a AC8d_125a AC8d_123a AC8d_122a AC8d_121a AC8d_118a AC8d_117a AC8d_116a AC8d_115a AC8d_112a AC8d_109a AC8d_107a AC8d_106a AC8d_105a AC8d_104a AC8d_103a AC8d_101a AC8d_100a AC8d_99a AC8d_97a AC8d_94a AC8d_92a AC8d_91a AC8d_89a AC8d_88a AC8d_85a AC8d_84a AC8d_83a AC8d_82a AC8d_81a AC8d_80a AC8d_79a AC8d_78a AC8d_77a AC8d_76a AC8d_75a AC8d_74a AC8d_71a AC8d_70 AC8d_68a AC8d_66a AC8d_65a AC8d_64a AC8d_61a AC8d_59a AC8d_58a AC8d_56a AC8d_55a AC8d_54a AC8d_53a AC8d_51a AC8d_49a AC8d_47a AC8d_42a AC8d_40a AC8d_38a AC8d_36a AC8d_35a AC8d_33a AC8d_31a

Pb/235U

5.3509 11.6273 4.9566 11.1770 5.6575 10.7749 11.3789 5.7025 11.1471 0.3122 11.3552 13.2239 5.4151 11.2957 5.4205 8.6752 9.5949 11.4370 0.3544 11.6199 6.0292 10.5541 7.3562 4.9956 0.2964 0.2981 6.1919 10.6960 10.0602 0.3057 4.2743 9.8000 4.7888 10.3393 6.4578 11.2959 10.6548 11.0419 4.8254 4.5753 4.4904 5.6327 9.1658 10.9340 10.8952 0.5831 0.3320 0.2825 5.1735 5.2180 0.5068 5.3738 5.4335 0.5089 11.7126 5.0238 11.2022 9.9332 5.0823 10.4569 10.3641 8.7947 5.0863 5.0869 5.0025 4.8080 5.4614 5.6762 0.3354 11.1661 9.9180 10.9769

Apparent ages ±1r

206

Pb/238U

0.1091 0.2285 0.1014 0.2240 0.1093 0.2176 0.2321 0.1142 0.2777 0.0077 0.2749 0.3329 0.1295 0.2721 0.1328 0.2070 0.2342 0.4046 0.0113 0.3792 0.1822 0.3347 0.2510 0.1493 0.0104 0.0099 0.1840 0.2660 0.2672 0.0074 0.0988 0.2210 0.1412 0.3565 0.2173 0.3664 0.5123 0.3560 0.2499 0.1939 0.1740 0.2256 0.4037 0.4331 0.4387 0.0238 0.0142 0.0110 0.2003 0.1579 0.0156 0.1856 0.1965 0.0187 0.4225 0.1828 0.3799 0.3367 0.2288 0.4703 0.4556 0.3832 0.2226 0.2770 0.2724 0.1379 0.1403 0.1698 0.0114 0.5785 0.4469 0.4945

0.3466 0.5061 0.3363 0.4936 0.3599 0.4736 0.4992 0.3534 0.4903 0.0439 0.5014 0.5424 0.3560 0.5008 0.3485 0.4400 0.4548 0.4881 0.0487 0.4956 0.3657 0.4649 0.3988 0.3183 0.0415 0.0415 0.3799 0.4698 0.4633 0.0432 0.3019 0.4433 0.3086 0.4692 0.3781 0.4913 0.4853 0.5056 0.3154 0.3065 0.3132 0.3538 0.4177 0.4817 0.4980 0.0761 0.0460 0.0398 0.3440 0.3390 0.0658 0.3435 0.3410 0.0675 0.4899 0.3194 0.4778 0.4614 0.3244 0.4550 0.4503 0.4086 0.3331 0.3300 0.3140 0.3257 0.3548 0.3420 0.0467 0.4988 0.4373 0.4910

±1r

Error corr.

206

Pb/238U

0.0052 0.0071 0.0051 0.0072 0.0050 0.0070 0.0075 0.0051 0.0063 0.0005 0.0058 0.0071 0.0039 0.0057 0.0042 0.0048 0.0054 0.0144 0.0012 0.0133 0.0087 0.0119 0.0124 0.0085 0.0013 0.0012 0.0102 0.0107 0.0114 0.0009 0.0064 0.0091 0.0085 0.0105 0.0079 0.0094 0.0190 0.0096 0.0138 0.0087 0.0072 0.0090 0.0126 0.0118 0.0126 0.0019 0.0013 0.0009 0.0081 0.0074 0.0014 0.0108 0.0113 0.0022 0.0160 0.0108 0.0149 0.0143 0.0133 0.0185 0.0180 0.0160 0.0132 0.0164 0.0156 0.0079 0.0077 0.0091 0.0013 0.0243 0.0175 0.0197

0.8439 0.8406 0.8456 0.8429 0.8406 0.8460 0.8437 0.8368 0.7981 0.7742 0.8013 0.7949 0.8039 0.8023 0.7991 0.8035 0.7985 0.9087 0.8801 0.9138 0.8978 0.9017 0.9394 0.9376 0.9401 0.8828 0.9416 0.9501 0.9528 0.9155 0.9508 0.9485 0.9541 0.8851 0.8771 0.8780 0.9235 0.8796 0.9410 0.8926 0.8876 0.8932 0.8911 0.8903 0.8871 0.8722 0.8804 0.8782 0.8931 0.8972 0.8816 0.9398 0.9406 0.9329 0.9342 0.9550 0.9487 0.9440 0.9574 0.9533 0.9578 0.9504 0.9550 0.9663 0.9653 0.9140 0.9240 0.9424 0.8793 0.9647 0.9486 0.9501

1918.1 2640.0 1868.9 2586.4 1981.6 2499.4 2610.5 1950.9 2571.9 276.9 2619.9 2793.5 1963.3 2617.2 1927.4 2350.5 2416.5 2562.3 306.5 2594.8 2008.9 2461.2 2163.3 1781.4 261.9 262.3 2075.6 2482.7 2454.2 272.4 1700.9 2365.3 1733.9 2480.0 2067.5 2576.5 2550.2 2637.9 1767.1 1723.7 1756.3 1952.7 2250.2 2534.8 2605.2 472.6 290.0 251.9 1905.7 1881.7 410.6 1903.6 1891.5 421.1 2570.0 1786.9 2517.6 2445.5 1811.2 2417.6 2396.8 2208.7 1853.2 1838.6 1760.4 1817.3 1957.4 1896.2 294.1 2608.5 2338.4 2574.9

±1r (Ma)

206

Pb/207Pb

±1r (Ma)

Best age

±1r (Ma)

24.8 30.5 24.4 31.0 23.4 30.6 32.0 24.2 27.1 3.1 25.0 29.7 18.7 24.5 20.1 21.5 24.0 61.9 7.4 57.2 41.1 52.0 56.7 41.6 8.2 7.2 47.4 46.9 49.9 5.7 31.6 40.6 41.5 46.0 36.8 40.5 82.0 40.8 67.5 42.8 35.3 42.7 57.3 51.3 53.9 11.3 7.9 5.6 38.6 35.6 8.7 51.5 53.9 13.5 68.9 52.5 64.5 62.6 64.3 81.5 79.4 72.9 63.5 78.9 75.9 38.5 36.7 43.4 8.0 103.5 78.0 84.6

1831.9 2524.0 1747.0 2499.5 1864.4 2507.6 2510.7 1911.3 2506.5 267.0 2499.9 2623.4 1804.6 2493.1 1845.2 2264.0 2380.0 2557.1 318.8 2558.1 1949.9 2503.8 2148.5 1861.6 278.6 288.3 1929.6 2508.8 2428.7 257.4 1672.9 2459.2 1840.8 2454.0 2012.8 2525.3 2447.8 2438.7 1815.5 1770.2 1696.6 1887.3 2446.5 2503.7 2441.6 436.2 300.2 259.5 1784.3 1826.2 447.3 1855.5 1888.7 398.6 2591.0 1865.4 2558.2 2414.5 1858.0 2524.2 2526.7 2413.5 1811.7 1829.3 1889.0 1750.3 1826.5 1962.0 290.7 2480.6 2502.8 2478.6

20.1 18.2 20.3 18.5 19.3 18.4 18.7 20.0 27.6 39.5 27.4 27.6 29.5 27.4 29.6 28.1 27.9 24.9 34.7 22.5 24.3 23.4 20.4 18.8 27.2 35.2 17.9 13.1 13.7 22.4 13.3 12.1 15.9 30.1 32.2 30.1 32.1 30.2 32.7 38.4 38.5 36.9 36.6 34.9 35.7 49.7 50.4 49.7 37.2 25.8 34.1 21.3 22.0 29.4 21.4 19.4 17.9 18.9 23.6 22.9 21.4 23.1 23.7 25.7 25.9 21.4 18.0 18.0 36.8 22.9 24.2 24.0

1831.9 2524.0 1747.0 2499.5 1864.4 2507.6 2510.7 1911.3 2506.5 276.9 2499.9 2623.4 1804.6 2493.1 1845.2 2264.0 2380.0 2557.1 306.5 2558.1 1949.9 2503.8 2148.5 1861.6 261.9 262.3 1929.6 2508.8 2428.7 272.4 1672.9 2459.2 1840.8 2454.0 2012.8 2525.3 2447.8 2438.7 1815.5 1770.2 1696.6 1887.3 2446.5 2503.7 2441.6 472.6 290.0 251.9 1784.3 1826.2 410.6 1855.5 1888.7 421.1 2591.0 1865.4 2558.2 2414.5 1858.0 2524.2 2526.7 2413.5 1811.7 1829.3 1889.0 1750.3 1826.5 1962.0 294.1 2480.6 2502.8 2478.6

20.1 18.2 20.3 18.5 19.3 18.4 18.7 20.0 27.6 3.1 27.4 27.6 29.5 27.4 29.6 28.1 27.9 24.9 7.4 22.5 24.3 23.4 20.4 18.8 8.2 7.2 17.9 13.1 13.7 5.7 13.3 12.1 15.9 30.1 32.2 30.1 32.1 30.2 32.7 38.4 38.5 36.9 36.6 34.9 35.7 11.3 7.9 5.6 37.2 25.8 8.7 21.3 22.0 13.5 21.4 19.4 17.9 18.9 23.6 22.9 21.4 23.1 23.7 25.7 25.9 21.4 18.0 18.0 8.0 22.9 24.2 24.0

93

X. Xie, P.L. Heller / Journal of Asian Earth Sciences 64 (2013) 86–98 Table 2 (continued) Sample

Isotopic ratio 207

AC8d_30a AC8d_28a AC8d_26a AC8d_24a AC8d_22a AC8d_21a AC8d_20a AC8d_18a AC8d_15a AC8d_14a AC8d_10a AC8d_9a AC8d_8a AC8d_7a AC8d_4b AC8d_4a AC8d_3a AC8d_1a

Pb/235U

10.9042 10.0614 4.6031 5.2181 4.3966 4.9630 5.6097 9.7760 5.7079 4.9199 10.5692 10.4152 0.3113 10.9858 5.7743 5.7110 9.6020 4.5632

Discordance larger than 10% AC8d_13a 8.1175 AC8d_135a 6.0809 AC8d_134a 0.4171 AC8d_124a 5.7581 AC8d_120a 0.3196 AC8d_119a 0.3428 AC8d_113a 0.3012 AC8d_111a 0.2973 AC8d_110a 0.3462 AC8d_108a 5.2246 AC8d_102a 0.3317 AC8d_98a 5.5100 AC8d_96a 3.1237 AC8d_95a 0.3344 AC8d_90a 0.5665 AC8d_87a 0.4393 AC8d_86a 0.3737 AC8d_73a 11.6710 AC8d_69a 2.7013 AC8d_67a 3.6237 AC8d_57a 6.2726 AC8d_52a 11.8428 AC8d_48b 4.3271 AC8d_46a 0.3454 AC8d_45b 6.3367 AC8d_44a 0.3448 AC8d_43a 0.4631 AC8d_41a 4.5616 AC8d_39a 0.3481 AC8d_37a 3.9350 AC8d_34a 6.8762 AC8d_32a 4.8522 AC8d_29a 0.2915 AC8d_27a 4.4943 AC8d_25c 0.3913 AC8d_25b 0.3198 AC8d_25a 0.2797 AC8d_23a 2.7621 AC8d_19a 4.6290 AC8d_17a 0.3090 AC8d_16a 0.3067 AC8d_11a 2.6406 AC8d_6a 0.2873 AC8d_5a 0.5979 AC8d_2a 3.6598

Apparent ages ±1r

206

0.5521 0.4403 0.2009 0.1537 0.1293 0.1982 0.1500 0.2751 0.2278 0.1618 0.3234 0.4579 0.0101 0.3747 0.2465 0.1658 0.3357 0.1373 1.5705 0.1163 0.0092 0.1373 0.0106 0.0103 0.0097 0.0106 0.0146 0.1541 0.0107 0.1208 0.0775 0.0086 0.0188 0.0152 0.0143 0.3615 0.0798 0.1425 0.2287 0.5064 0.1900 0.0233 0.2727 0.0189 0.0138 0.2213 0.0146 0.1527 0.1775 0.2190 0.0151 0.2094 0.0471 0.0137 0.0107 0.1112 0.1555 0.0099 0.0093 0.0865 0.0093 0.0835 0.1018

Pb/238U

±1r

Error corr.

206

0.4768 0.4446 0.2987 0.3297 0.2908 0.3130 0.3468 0.4448 0.3408 0.3080 0.4725 0.4418 0.0435 0.4774 0.3492 0.3433 0.4656 0.3073

0.0219 0.0172 0.0115 0.0083 0.0073 0.0114 0.0076 0.0104 0.0119 0.0083 0.0136 0.0185 0.0012 0.0153 0.0134 0.0092 0.0153 0.0087

0.9556 0.9500 0.9472 0.9450 0.9398 0.9626 0.9317 0.9311 0.9560 0.9325 0.9637 0.9677 0.9036 0.9612 0.9515 0.9514 0.9619 0.9588

2513.4 2371.2 1684.7 1836.7 1645.3 1755.3 1919.4 2371.9 1890.3 1731.0 2494.5 2358.7 274.6 2515.9 1930.8 1902.3 2464.3 1727.5

0.3627 0.3855 0.0531 0.3707 0.0432 0.0488 0.0418 0.0409 0.0406 0.3514 0.0450 0.2814 0.2246 0.0454 0.0750 0.0466 0.0494 0.5365 0.1970 0.2282 0.2757 0.4165 0.2113 0.0400 0.3326 0.0434 0.0592 0.2905 0.0472 0.2390 0.4123 0.2993 0.0403 0.2871 0.0486 0.0453 0.0383 0.1654 0.2945 0.0406 0.0382 0.1734 0.0399 0.0257 0.2056

0.0684 0.0051 0.0008 0.0041 0.0010 0.0011 0.0011 0.0011 0.0013 0.0094 0.0013 0.0055 0.0051 0.0010 0.0014 0.0010 0.0011 0.0118 0.0040 0.0084 0.0093 0.0160 0.0084 0.0022 0.0129 0.0021 0.0015 0.0134 0.0014 0.0086 0.0091 0.0120 0.0018 0.0120 0.0058 0.0017 0.0013 0.0061 0.0087 0.0010 0.0009 0.0047 0.0012 0.0035 0.0049

0.9977 0.8298 0.7463 0.8046 0.7723 0.8507 0.8785 0.8604 0.8099 0.9441 0.8861 0.9320 0.9441 0.9184 0.8516 0.8617 0.8045 0.8836 0.8754 0.9589 0.9523 0.9512 0.9547 0.8966 0.9532 0.9557 0.8900 0.9746 0.7501 0.9621 0.9295 0.9478 0.9299 0.9520 0.9903 0.9435 0.9273 0.9654 0.9504 0.8546 0.8905 0.9440 0.9266 0.9849 0.8978

1994.8 2101.8 333.7 2032.8 272.3 307.4 263.7 258.5 256.8 1941.2 283.5 1598.4 1306.1 286.1 466.5 293.6 310.7 2768.9 1159.0 1325.1 1569.6 2244.8 1235.7 253.1 1851.2 274.1 370.7 1643.8 297.4 1381.7 2225.3 1687.8 254.6 1627.0 306.1 285.8 242.6 986.6 1664.1 256.8 241.9 1030.9 252.1 163.4 1205.1

In short, the overall age distributions of the Sample-TC8 and Sample-AK8 are very similar. The detrital zircon grains from the Sample-TC8 have a narrow peak at 266 Ma and several smaller peaks at 370, 450, 1937, 1998, 2346, and 2514 Ma (Fig. 7). The Sample-AK8 has three very high and narrow peaks at 277, 1863, and 2511 Ma. In general, the detrital zircon age distribution is characterized by five major age population groups: 250–300 Ma, 320–380 Ma, 380–420 Ma, 420–500 Ma, 1.7–2.1 Ga, and 2.3–

Pb/238U

206

±1r (Ma)

Best age

±1r (Ma)

95.0 76.5 56.9 40.1 36.1 55.5 36.3 46.1 57.2 40.8 59.4 82.2 7.6 66.6 63.7 44.2 67.1 42.6

2516.6 2499.1 1829.0 1876.7 1794.0 1880.0 1915.6 2449.5 1978.0 1892.9 2479.1 2567.2 279.6 2526.8 1954.8 1966.1 2340.9 1760.7

25.2 23.3 25.7 17.9 18.8 19.7 18.1 17.9 21.4 22.2 13.8 18.5 31.5 15.7 23.6 15.9 16.3 15.6

2516.6 2499.1 1829.0 1876.7 1794.0 1880.0 1915.6 2449.5 1978.0 1892.9 2479.1 2567.2 274.6 2526.8 1954.8 1966.1 2340.9 1760.7

25.2 23.3 25.7 17.9 18.8 19.7 18.1 17.9 21.4 22.2 13.8 18.5 7.6 15.7 23.6 15.9 16.3 15.6

315.8 23.9 4.8 19.1 5.9 6.6 6.5 7.0 8.3 44.5 7.7 27.7 26.7 6.3 8.4 6.1 6.8 49.5 21.3 44.0 46.9 72.4 44.5 13.9 62.1 13.2 8.9 66.4 8.4 44.5 41.4 59.2 11.3 59.8 35.3 10.4 7.9 33.5 43.1 5.9 5.6 26.0 7.2 22.0 26.4

2480.0 1870.6 489.3 1842.7 359.0 236.2 298.5 315.8 666.3 1763.3 350.7 2251.8 1640.1 347.8 402.1 879.9 407.7 2431.6 1613.9 1882.6 2507.8 2875.7 2328.8 694.6 2204.0 514.5 481.7 1862.7 349.7 1947.3 1970.6 1920.2 307.3 1857.2 543.8 248.8 325.0 1973.0 1863.9 417.6 536.4 1806.4 296.1 2546.5 2086.1

23.3 19.7 32.2 29.4 47.7 36.8 35.4 41.2 52.1 17.8 33.6 13.7 15.2 22.8 44.1 40.0 53.6 25.9 28.4 20.0 18.7 21.6 22.5 63.1 22.7 35.7 30.1 19.6 61.7 19.0 17.2 26.0 43.2 26.0 36.1 33.0 32.9 18.9 19.2 37.7 30.7 20.5 27.5 40.1 21.4

±1r (Ma)

Pb/207Pb

2.6 Ga. Both samples show a age gap between 500 Ma and 1.5 Ga (Fig. 7).

4. Potential source areas Previous geochronology studies in central China provide a relatively well-constrained framework for provenance interpretation

94

X. Xie, P.L. Heller / Journal of Asian Earth Sciences 64 (2013) 86–98

4.1. Qinling orogenic belt

and correlation. Potential source areas for the southern and western Ordos Basin consist of three areas: the Qinling orogenic belt to the south; the Qilian–Qaidam terranes and the North Qilian orogenic belt which are exposed in the Liupanshan thrust belt to the west (Tang and Guo, 1992; Zhou et al., 1994); and the North China block to the north and northeast.

Previous U–Pb geochronology studies of those basement rocks show that the Qinling orogen belt is characterized by rocks of the Early Paleozoic age (e.g., 400–520 Ma) with peaks at 404, 415, 455, and 485 Ma (Mattauer et al., 1985; Kröner et al., 1988;

0.6

0.4

Pb/

238

U

2500

206

0.24 2800

Pb/

206

Pb

1500

207

0.2

0.16 2000 0.08

1200 400

500

0.00

0

8

16 238

U/

0.0 0

4

8

12

207

Pb/

24

206

Pb

16

235

U

Fig. 6. Pb/U concordia diagram of single detrital zircon ages for Sample-AK8 with insert Pb/U Terra-Wasserburg concordia diagram. Only analyses with the discordance less than 10% are plotted. Uncertainties in age are given with a 95% confidence level. Error ellipses are at the 2r level. Number: 90 out of 144.

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X. Xie, P.L. Heller / Journal of Asian Earth Sciences 64 (2013) 86–98

Reischmann et al., 1990; Zhang et al., 1991; Chen et al., 1992; Lerch et al., 1995; Sun et al., 1996; Xue et al., 1996). Besides, there are some Mesozoic ages (e.g., 200–250 Ma) with age peaks at 210 and 240 Ma and minor older Neoproterozoic age peaks at 770, 900, and 950 Ma (Zhang et al., 1997; Sun et al., 2002; Ling et al., 2003; Yan et al., 2003; Yang et al., 2003; Jin et al., 2005) (Fig. 7). 4.2. Qaidam-Qilian terranes and the North Qilian orogenic belt As the western boundary of the Ordos Basin, the Qaidam-Qilian terranes and the North Qilian orogenic belt are the second potential source areas (Fig. 7), which are exposed in the Liupanshan thrust belt to the west. The Qilian terrane and the North Qilian orogenic belt are dominated by Early Paleozoic ages, 400–430 and 450–490 Ma ages, with some as old as Neoproterozoic ages, e.g., 760–800 to 1000 Ma (Mao et al., 2000; Yang et al., 2002; Cowgill et al., 2003; Gehrels et al., 2003a,b; Liu et al., 2003; Zhang et al., 2003; Song et al., 2004, 2005; Wu et al., 2005; Liu et al., 2006; Song et al., 2006; Tseng et al., 2006; Song et al., 2007; Zhang et al., 2007; Song et al., 2010; Song et al., 2012a). The Qiadam terrane has age clusters at 260–280, 350–365, 380–410, 420–460, and 485– 500 Ma (Zhang et al., 2000; Yang et al., 2002; Gehrels et al., 2003b; Song et al., 2004; Xiao et al., 2004; Wu et al., 2005; Song et al., 2006; Wu et al., 2006). 4.3. North China block The North China block has a complex tectonothermal history with some basement rock ages as old as 3.8 Ga (Li, 1980; Jahn and Zhang, 1984; Yang et al., 1986). Typically U–Pb zircon ages from those basement rocks cluster around 1.8–2.0 and 2.4–2.8 Ga (Kröner et al., 1988; Sun et al., 1991; Tang and Guo, 1992; Kröner et al., 1993; Song et al., 1996; Wilde et al., 1997; Kröner et al., 1998; Zhao et al., 2000; Guan et al., 2002; Li et al., 2004; Luo et al., 2004; Wilde et al., 2004; Xu et al., 2004a,b; Zhao et al., 2004; Zheng et al., 2004a,b; Guo et al., 2005; Kröner et al., 2005; Li et al., 2005; Peng et al., 2005; Wilde et al., 2005; Gao et al., 2006; Hou et al., 2006; Kröner et al., 2006; Lu et al., 2006; Wan et al., 2006; Xia et al., 2006a,b) (Fig. 7). Another major potential sediment source for the south Ordos Basin is pre-Mesozoic siliciclastic sedimentary rocks. Previous detrital zircon studies along the northern margin of the North China block and the northwestern Ordos Basin provide a good reference for those recycled pre-Mesozoic sedimentary rock sources (Cope et al., 2005; Darby and Gehrels, 2006). Darby and Gehrels (2006) show that detrital zircon ages of the upper Proterozoic to Ordovician strata from the northwestern Ordos Basin have two major age clusters, 1.8–2.1 and 2.5–2.8 Ga, with several small age peaks at 2.2, 2.3, and 2.4 Ga. Cope et al. (2005) reported that Carboniferous to Permian sandstone strata in north margin of the North China block have detrital ages clusters with peaks at 295, 310, 320, 385, and 400 Ma and some minor peaks at 265 and 365 Ma (Fig. 7). Considering those two locations are the northwest and north margins of the Ordos Basin, presumably all the preMesozoic strata within the Ordos Basin will have similar age populations. 5. Discussion and conclusions Detrital zircon age spectra from Sample-TC8 and Sample-AK8 yielded a very similar age distribution, which suggests that these two areas share similar provenance. They both have two populations of Precambrian grains that range from 1.8 to 2.0 Ga, and 2.2 to 2.8 Ga and a Paleozoic detrital age peak at 240–490 Ma, but

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peaks within the Paleozoic population do not exactly match each other. The broad range of detrital zircon ages suggests that multiple sources were involved at the south Ordos Basin during the Triassic deposition of the Yanchang Formation. U–Pb detrital zircon geochronology of 158 single grains (out of 258 analyses) can be subdivided into six distinct age groups, 250– 300 Ma, 320–380 Ma, 380–420 Ma, 420–500 Ma, 1.7–2.1 Ga, and 2.3–2.6 Ga. Sample TC-8 does not have the age population of 380–420 Ma while sample AK-8 does not have 320–380 Ma age population. The two oldest populations represent the typical basement rock ages of the North China block (Li and Cong, 1980; Sun and Cui, 1980; Jahn and Zhang, 1984; Yang et al., 1986; Tang and Guo, 1992; Yang, 2001; Gehrels et al., 2003b). Darby and Gehrels (2006) report that detrital zircon ages of the upper Proterozoic to Ordovician strata from the northwestern Ordos Basin have two major age clusters, 1.8–2.1 and 2.5–2.8 Ga, with several small age peaks at 2.2, 2.3, and 2.4 Ga. (Fig. 7). Sediments derived from those sources will inherit their age distribution signatures and characteristics. Considering the factors that all these detrital grains were a dark color and rounded, and the fact that the basement of the North China is not exposed in this local area except in the core part of the Qinling orogenic belt, it is most likely that the two old age populations were recycled from previous sedimentary rocks. Sample-TC8 has a peak at 266 Ma and several smaller peaks at 370, 450, 1937, 1998, 2346, and 2514 Ma. Sample-AK8 has three peaks at 277, 1863, and 2511 Ma. Among young age populations, there are multiple sources of matching age groups of 380-420 and 420–500 Ma, including the Qilian–Qaidam terranes, the North Qilian orogenic belt, the north and the west Qinling orogenic belt, and the recycled Proterozoic to Paleozoic strata. It is unlikely that these two sources were the direct sources for the south Ordos Basin, even though Early Paleozoic ages (400–560 Ma) have been reported both from the Qilian–Qaidam terranes (Zhang et al., 2000; Yang et al., 2002; Gehrels et al., 2003b; Song et al., 2004; Xiao et al., 2004; Wu et al., 2005; Song et al., 2006; Wu et al., 2006) and the Qinling orogenic belt, due to the fact that both samples do not have the Neoproterozoic age populations (800–1000 Ma), which are very common for both the Qilian–Qaidam terranes and Qinling orogenic belt (Mattauer et al., 1985; Kröner et al., 1988; Reischmann et al., 1990; Zhang et al., 1991; Chen et al., 1992; Lerch et al., 1995; Sun et al., 1996; Xue et al., 1996; Mao et al., 2000; Yang et al., 2002; Cowgill et al., 2003; Gehrels et al., 2003a,b; Liu et al., 2003; Zhang et al., 2003; Song et al., 2004, 2005; Wu et al., 2005; Liu et al., 2006; Song et al., 2006; Tseng et al., 2006; Song et al., 2007; Zhang et al., 2007; Song et al., 2010, 2012a). Also implied is that the Qinling orogenic belt to the south and the Liupanshan thrust belt to the west were not active sediment sources during the deposition of the early Late Triassic Yanchang Formation. Similarity between the early Late Triassic Yanchang Formation and the Proterozoic to Paleozoic sedimentary rocks of the North China block suggests that previous sedimentary rocks within the North China block were the primary sediment sources for the early Late Triassic Yanchang Formation. Additionally, both samples show there is a 250–300 Ma age population, which is abundant in the recycled Paleozoic strata source from north (Cope et al., 2005). Sample TC-8 has two minor age peaks within age group 320–380 Ma. There are no known sources of the 250–300 Ma age population in the surrounding areas matching this age population. Further studies are required to fully understand and correlate this source. The main conclusions from this study are: 1. Detrital zircon geochronology of two Yanchang Formation samples shows similar age distributions. These samples are characterized by six major detrital zircon grain age populations—250–

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5.

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300 Ma, 320–380 Ma, 380–420 Ma, 420–500 Ma, 1.8–2.0 Ga, and 2.2–2.8 Ga. The broad range of detrital zircon ages suggests that source terranes for the south Ordos Basin during the Triassic deposition of the Yanchang Formation was either spatially large or from more complex terranes which have source rocks with different origins. The two oldest age groups, 1.8–2.0 and 2.2–2.8 Ga, match ages of underlying North China block basement rocks. Most likely those two old age groups are recycled from previous sedimentary rocks. Multiple sources match age groups of 380–420 and 420– 500 Ma, including the Qilian–Qaidam terrane, the north and the west Qinling orogeny belts. However, both samples do not have the Neoproterozoic age populations (800–1000 Ma) suggesting that the Qilian–Qaidam terranes and the Qinling orogenic belt were not the direct source for the south Ordos Basin. Similar age distribution between the early Late Triassic Yanchang Formation and the Proterozoic and Paleozoic sedimentary rocks from the North China block implies that those grains were directly derived from the North China block and inherited similar ages from previous strata. Both sample TC-8 and sample AK-8 show similar age distribution with sedimentary rocks from the North China block. Detrital zircon grains with those ages are likely recycled. Sample TC-8 has two minor age peaks at age group 320– 380 Ma. There are no known sources in surrounding areas. Partially overlapping age with the Proterozoic to Paleozoic strata suggests that some could be inherited from this recycled source.

Acknowledgements The authors thank Amy Weislogel and an anonymous reviewer for their constructive reviews of the manuscript. The authors also thank Professor Y.Y., Yang at Xian Shiyou University in China for providing help in the field and sample collection. Thanks to Dr. Kevin Chamberlain at the Department of Geology and Geophysics, University of Wyoming and Dr. Jeff Vervoort at the School of Earth and Environmental Sciences, Washington State University for discussions regarding sample preparation, analysis, and data processing. This research has been supported in part by an IAS postgraduate Grant, a GSA student research grant, an AAPG Grant-in-Aid program, and the summer independent research award from the College of Art and Science, University of Wyoming.

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