LA-ICPMS U–Pb zircon age constraints on the provenance of Cretaceous sediments in the Yichang area of the Jianghan Basin, central China

Cretaceous Research 34 (2012) 172e183

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LA-ICPMS UePb zircon age constraints on the provenance of Cretaceous sediments in the Yichang area of the Jianghan Basin, central China Chuanbo Shen a, b, *, Lianfu Mei a, Lei Peng c, Youzhi Chen c, Zhao Yang d, Guangfu Hong c a

Key Laboratory of Tectonics and Petroleum Resources, China University of Geosciences, Ministry of Education, Wuhan 430074, China State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China c Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China d Department of Geology, Northwest University, Xi’an 710069, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 November 2010 Accepted in revised form 29 October 2011 Available online 10 November 2011

Detrital zircon UePb ages have been shown to be a powerful tool for provenance analysis and determining the exhumation of sediment source areas. This paper presents the results of detrital zircon LAICPMS UePb ages for Cretaceous sediments from the Yichang area of the Jianghan Basin, central China. The results provide new information on the provenance of these sediments and the detailed exhumation process of the Huangling Dome. Zircons with different age populations have been derived from the strata of the Huangling Dome. The Liantuo, Gucheng and Nantuo formations and the Kongling complex were exhumed, leading to deposition of the early Cretaceous Wulong Formation, which provides the sources of zircons with age peaks at 3.1e3.0, 2.5 and 1.8 Ga. Exhumation of the Huangling granitoid and contemporary volcanics provided the source of the late Cretaceous Luojingtan Formation, which contains zircons with age peaks at 1.1e0.95 and 0.83e0.74 Ga. The Qinling-Dabie orogeny supplied zircons with an age cluster of 0.27 to 0.18 Ga. These results indicate the timing of initial exhumation for the Huangling granite. They also show how overlying strata was first uplifted and eroded, followed by exposure of underlying strata at the surface during continued exhumation. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: UePb zircon ages Provenance Cretaceous Exhumation Jianghan Basin

1. Introduction Provenance studies of clastic sedimentary rocks are not only an important part of basin analysis, but also a bridge between the basin and adjacent orogens. This is useful for characterizing sediment source areas and tracking any changes in source area over time, which by extension permits palaeogeographic reconstructions (e.g., Sircombe, 1999; Weltje and Eynatten, 2004) and sheds light on the exhumation history of the source areas (e.g., Bernet and Spiegel, 2004; Najman, 2006). Of the various techniques utilized in sediment provenance studies, single-grain age dating of detrital zircon, which is unaffected by sedimentary fractionation processes and directly reflects the age distribution of the zircon-bearing rocks of the source terrain, has been widely used to obtain information regarding provenance, source area exhumation, and landscape evolution (Dickinson and Gehrels, 2003; Bernet and Spiegel, 2004; Andersen, 2005). This has been successfully applied to diverse

* Corresponding author. Key Laboratory of Tectonics and Petroleum Resources, China University of Geosciences, Ministry of Education, No. 388 Lumo Road, Wuhan 430074, China. E-mail address: [email protected] (C. Shen). 0195-6671/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2011.10.016

regions, including the Alps (e.g., Spiegel et al., 2000), the Appalachians (e.g., Park et al., 2010), the Colorado Plateau (e.g., Dickinson and Gehrels, 2003), the Himalayas (e.g., Amidon et al., 2005; Najman, 2006), the Tianshan (e.g., Li and Peng, 2010), and the Qinling-Dabie (e.g., Wang et al., 2009). The field area is at Yichang, southeastern Three Gorges. This belongs to the sub-basin of the northwestern part of the Jianghan Basin (Fig. 1). Previous provenance work has focused on TriassiceJurassic sandstones, associated with initial exhumation of the Dabie HP-UHP rocks (Liu et al., 2003, 2005; Li et al., 2005; Wang et al., 2009). There has been less research into the sediment source for the Cretaceous strata in the basin. However, the Cretaceous Period was an important time in tectonic history, and saw climate change, petroleum accumulation, and mineralization in China (Chen, 2000; Zhang, 2000; Yu et al., 2006; Sha, 2007; Chen et al., 2009; Cao and Wang, 2009; Li et al., 2010). The transition from the palaeo-Tethyan tectonic system to the circum-Pacific tectonic system occurred during the Cretaceous, resulting in the formation of an NeE extending volcanic rock (Xie et al., 2006) and a switch from compression to extension in eastern China (Davis et al., 2002; Li and Li, 2007; Hsieh et al., 2008). The Cretaceous sandstones have petroleum exploration potential in the Jianghan Basin (Chen et al., 2009). Therefore, it is of great scientific significance to study the

C. Shen et al. / Cretaceous Research 34 (2012) 172e183

A

173

B

C

Fig. 1. Geological setting of the Yichang area of the Jianghan Basin. A, sketch map showing the location of the study area and highlighting possible source areas. B, simplified geological map for the Yichang area in the Three Gorges and sample location, modified from Liu et al. (2008). C, palaeocurrent analysis of the gravel flat surface and cross-bedding, showing a southeast orientation, suggesting the sediment may be derived from the northwest. LYB, Lower Yangtze Basin; HL, Huangling Dome; JHB, Jianghan Basin.

chronology and provenance of the Cretaceous strata in the basin. In this paper, we present zircon UePb ages determined by laser ablation inductively coupled plasma mass spectrometry (LAICPMS), for the Cretaceous clastic sedimentary rocks in the Jianghan Basin. The zircon ages enable a comprehensive understanding of the provenance of these sediments and the exhumation of their source areas. 2. Geological setting The Yichang area is located in the southeast of the Three Gorges in western Hubei province, a northwestern part of the Jianghan Basin and to the northwest of the Huangling Dome in central China (Fig. 1A, B). Elevation in this area decreases from west to east, from mountains (about 1700 m) in the northwest to the Jianghan Plain (<50 m) in the southeast (Xiang et al., 2007). The Jianghan Basin,

covering nearly 28,000 km2, is a large MesozoiceCenozoic petroliferous basin, bounded by NNE-trending normal faults, and contains sediments spanning the Cretaceous to Quaternary. All of these basin-filling deposits are similar in that they initiate with relatively coarse-grained alluvial fan and/or deltaic deposits that generally fine upward into lacustrine mudstones (Dai et al., 2000). The basin is surrounded by the Qinling-Dabie orogeny (NE), the Jiangnan orogen (S), and the Huangling Dome (NW), the potential source areas for the Cretaceous sediments (Fig. 1A). The Qinling-Dabie Orogen stretches for about 2000 km in eastcentral China and was formed by Triassic subduction of the Yangtze Block beneath the North China Block (Hacker et al., 1995, 2000; Meng and Zhang, 2000; Webb et al., 1999; Ratschbacher et al., 2003). After its formation in the Triassic, initial exhumation that exposed the Dabie HP-UHP rocks occurred during the early Jurassic (w190 Ma), which provided the predominant supply of the Jurassic

Fig. 2. Photographs of selected samples in the field. Stars denote the sample locations. The hammer shaft is ca. 35 cm long. A, sample 5-98. B, sample 5-112.

174

C. Shen et al. / Cretaceous Research 34 (2012) 172e183

sediments in the Jianghan Basin (Wang et al., 2009). The QinlingDabie Orogen was reactivated during the Cretaceous Period by Pacific back-arc extension, resulting in regional exhumation (Hu et al., 2006; Shen et al., 2009a, b). The Jiangnan Orogen, composed mostly of the Mesoproterozoic Lengjiaxi and Neoproterozoic Banxi sedimentary groups, separated the Yangtze and Cathaysia blocks (Wang et al., 2007, 2009). These sequences were intruded by the Jingningian, Caledonian and Yanshanian granites (Wang et al., 2009). Detrital zircon UePb ages of the sedimentary sequences in the Jiangnan Orogen reveal three age clusters: 2.5e2.4, 1.8e1.6 and 1.0e0.86 Ga, most likely derived from the Yangtze and/or Cathaysia blocks (Wang et al., 2007). The Huangling Dome is a NEE-striking anticline with high basement uplift. It has exposed the high-grade metamorphic complex of the ArchaeanePalaeoproterozoic Kongling Group and Huangling intrusive granite, surrounded by marine strata from the Sinian to the Triassic (Fig. 1B). The age of the Kongling Group has been dated at 2.90e2.95 Ga, and detrital zircon UePb ages from the Archaean metapelites are 2.87e3.28 Ga (Gao et al., 1999; Qiu et al., 2000; Zhang et al., 2006a, b; Liu et al., 2008). The Huangling Batholith is unconformably overlain by the Liantuo Formation (748
12 Ma; Ma et al., 1984). Emplacement of the Huangling granite occurred between 740 and 850 Ma, which is commonly thought to be associated with rifting of the Rodinia supercontinent (Li XH et al., 2003; Li ZX et al., 2003; Zhang et al., 2006a). Cretaceous sediments comprise, from base (Lower Cretaceous) to top (Upper Cretaceous), the Shimen (K1s), Wulong (K1w), Luojingtang (K2l), Honghuatao (K2h) and Paomagang (K2p) formations. The Shimen Formation varies from 12 to 185 m in thickness and is composed of grey, yellow and red conglomerates with interbedded siltstones. Palaeozoic limestone underlies this formation in a sharp unconformity, which can be observed at the entrance of the Xiling Gorge and at Nanjingguan (Mao and Wang, 1999). The Wulong Formation is 714e1696 m thick and consists of brownegrey fine- to medium-grained sandstones, siltstone in the lower unit and sandy conglomerate in the upper unit. The Lower Cretaceous Luojingtan Formation is 270e812 m thick and consists of redewhite massive conglomerate with thin-bedded sandstone and siltstone. The Honghuatao Formation is 269e1420 m thick and underlies the Paomagang Formation (263e805 m thick), both of which are composed of massive red fine-grained sandstone, siltstone and mudstone.

mounted in epoxy and polished to expose the grain centres. Before in-situ UePb isotope analyses, the morphology and internal structure of the zircons were studied by binocular microscope and cathodoluminescence (CL) imaging, which were used as guides to find appropriate spots for the isotope analyses (Fig. 3). CL images

3. Samples and analytical methods 3.1. Sample descriptions Samples 5-98 (E11116.6890 , N30 40.5960 , 59 m asl) and 5-112 (E11127.4800 , N30 40.027, 123 m asl) were collected from the upper part of the Lower Cretaceous Wulong Formation (K1w) and the lower part of the Upper Cretaceous Luojingtang Formation (K2l) respectively (Figs. 1B and 2). Sample 5-98 is located in Dianjun town within Yichang City and the lithology is grey-red siltstones and medium-grained sandstones with interbedded sandy conglomerate and conglomerate (Fig. 2A). The sample 5-112 for UePb dating was collected from the sandstones (Fig. 2B). Palaeocurrent analysis of the gravel flat surface and oblique bedding of the Wulong and Luojingtan formations all show a southeast orientation, indicating that the sediment source may come from the northwest (Fig. 1C). 3.2. Analytical methods Rock samples were crushed in a steel jaw crusher. A shaking bed and heavy liquid techniques were used to concentrate heavy minerals. Zircon crystals were separated from other heavy minerals by hand picking under a binocular microscope. They were then

Fig. 3. Cathodoluminescence imaging of representative zircon grains with a wide range of size and morphology. A, sample 5-98. B, sample 5-112.

Table 1 UePb isotopic ratios and ages of detrital zircons from the Early Cretaceous Wulong Formation in the Yichang area of the Jianghan Basin, central China. Grain No.

Th (ppm)

U (ppm)

Th/U

Ratios (common-Pb corrected) 207

287 213 151 89.6 95.0 6.28 49.4 337 51.1 236 136 53.4 49.7 201 356 311 97.9 87.1 164 162 355 37.4 58.5 52.2 177 93.8 123 110 17.1 49.5 51.7 467 36.7 161 166 68.0 64.6 120 45.9 136 252 83.7 98.6 77.6 33.3 48.2 140 170 203 274 324 214

422 303 203 106 179 820 120 517 357 555 460 233 90.5 410 604 644 234 124 247 454 387 77.8 84.0 623 442 218 185 126 18.2 244 353 645 87.4 235 194 261 239 98.5 43.5 255 238 102 608 70.9 28.1 36.7 318 146 539 454 308 296

0.69 0.71 0.75 0.85 0.53 0.01 0.41 0.66 0.14 0.43 0.30 0.23 0.55 0.49 0.59 0.49 0.42 0.71 0.67 0.36 0.92 0.48 0.70 0.08 0.40 0.43 0.67 0.88 0.95 0.20 0.15 0.73 0.42 0.69 0.86 0.26 0.27 1.23 1.06 0.54 1.07 0.83 0.16 1.10 1.20 1.32 0.44 1.17 0.38 0.61 1.06 0.73

0.05321 0.17017 0.16873 0.05632 0.17438 0.11559 0.18317 0.05106 0.12396 0.1121 0.05293 0.11394 0.05224 0.15765 0.07283 0.05259 0.1629 0.15887 0.11528 0.15661 0.05374 0.16847 0.16904 0.11464 0.05117 0.14155 0.12402 0.07198 0.16667 0.11517 0.11515 0.05036 0.16718 0.17144 0.12316 0.11549 0.06428 0.05306 0.05768 0.05357 0.07345 0.04736 0.05155 0.11822 0.09332 0.16889 0.05216 0.05241 0.11851 0.05556 0.11476 0.05198


1s

207

0.00164 0.00241 0.00117 0.00173 0.00168 0.00072 0.00242 0.00089 0.00137 0.00073 0.00096 0.00094 0.00255 0.00096 0.00134 0.00122 0.00116 0.00175 0.00129 0.00123 0.00148 0.00161 0.00234 0.00072 0.00111 0.00147 0.00124 0.00142 0.00271 0.00097 0.0009 0.00105 0.00182 0.0011 0.00094 0.00082 0.00152 0.00192 0.00293 0.00142 0.00091 0.00258 0.0009 0.00171 0.002 0.00182 0.00129 0.00262 0.00077 0.00174 0.00153 0.0021

0.17751 10.69069 11.19639 0.39804 11.68002 5.31239 12.25579 0.3112 5.75189 5.20504 0.31752 5.46549 0.20503 9.81909 1.62351 0.30472 10.3278 8.93111 4.52362 9.70125 0.26467 11.17586 10.17083 5.18243 0.27567 7.63069 6.34272 1.57757 10.29733 4.93891 5.12687 0.20247 10.6332 11.49211 6.27124 5.29751 0.87118 0.3339 0.37088 0.32358 1.50897 0.16313 0.41699 5.09545 3.17402 10.72037 0.29472 0.24179 5.40613 0.31759 4.80831 0.24598

Pb/235U

Ages (common-Pb corrected, Ma)
1s

206

0.00561 0.13412 0.14331 0.01246 0.12565 0.04116 0.12495 0.00567 0.08247 0.03747 0.00592 0.05119 0.00952 0.09401 0.03827 0.00876 0.18359 0.20126 0.09048 0.16173 0.00762 0.14565 0.108 0.03868 0.00585 0.11624 0.0872 0.03064 0.16926 0.04468 0.05485 0.00423 0.15664 0.15788 0.10885 0.1039 0.0176 0.01241 0.01749 0.00897 0.02107 0.00872 0.0074 0.07448 0.06838 0.12183 0.00808 0.01058 0.04186 0.01062 0.05636 0.01076

0.0243 0.45564 0.47971 0.0514 0.48555 0.33225 0.48527 0.04415 0.33547 0.33597 0.04346 0.34776 0.02913 0.45082 0.1609 0.04185 0.45916 0.40698 0.284 0.44906 0.03598 0.48098 0.43638 0.327 0.03909 0.38964 0.36962 0.15936 0.45062 0.31076 0.32253 0.02919 0.46049 0.48507 0.36847 0.33168 0.0983 0.04608 0.04854 0.04384 0.14878 0.02516 0.05859 0.31239 0.24722 0.45918 0.04068 0.03378 0.32907 0.04138 0.30387 0.03423

Pb/238U


1s

208

0.00026 0.00297 0.00399 0.0005 0.00422 0.00234 0.00409 0.00038 0.00355 0.00224 0.00035 0.00306 0.00036 0.00304 0.00202 0.00041 0.00362 0.00373 0.00225 0.00424 0.00035 0.00404 0.0039 0.00206 0.00032 0.00426 0.00364 0.00202 0.00531 0.00245 0.00303 0.00022 0.00412 0.00316 0.00282 0.00221 0.0012 0.00046 0.00078 0.00037 0.00107 0.0003 0.00056 0.00289 0.00271 0.00367 0.00042 0.0007 0.00214 0.00058 0.00195 0.00046

0.00746 0.1262 0.13336 0.01617 0.13173 0.09881 0.13346 0.01397 0.15091 0.09497 0.01485 0.09962 0.00931 0.12501 0.04746 0.01311 0.12828 0.11653 0.08568 0.12545 0.01108 0.13254 0.12094 0.09461 0.01266 0.11083 0.11182 0.04767 0.13223 0.08931 0.11267 0.0089 0.1292 0.1321 0.10532 0.09341 0.03012 0.01397 0.01499 0.01308 0.04482 0.00758 0.01783 0.08807 0.07127 0.12726 0.01308 0.01069 0.09376 0.01324 0.08749 0.01142

Pb/232Th


1s

207

0.00012 0.00088 0.00117 0.00028 0.00104 0.00276 0.00121 0.00019 0.0037 0.00067 0.00023 0.00112 0.00027 0.00089 0.00078 0.00022 0.0011 0.0013 0.00102 0.00107 0.00015 0.00162 0.00117 0.00101 0.00021 0.00147 0.00161 0.00065 0.00214 0.00094 0.01885 0.0001 0.00159 0.00094 0.00103 0.00083 0.0004 0.00021 0.00033 0.00022 0.00041 0.00016 0.00037 0.00109 0.00099 0.00121 0.00021 0.00022 0.00069 0.00029 0.0006 0.00031

338 2559 2545 465 2600 1889 2682 243 2014 1834 326 1863 296 2431 1009 311 2486 2444 1884 2419 360 2542 2548 1874 248 2246 2015 985 2524 1883 1882 212 2530 2572 2002 1887 751 331 518 353 1027 68 266 1930 1494 2547 292 303 1934 435 1876 285

Pb/206Pb


1s

207

52 24 11 52 8 6 22 26 12 6 28 8 83 8 28 48 19 26 25 16 47 11 24 6 34 13 12 20 13 7 9 35 13 14 20 26 51 66 75 47 17 95 24 14 24 9 44 62 6 49 25 76

166 2497 2540 340 2579 1871 2624 275 1939 1853 280 1895 189 2418 979 270 2465 2331 1735 2407 238 2538 2450 1850 247 2188 2024 961 2462 1809 1841 187 2492 2564 2014 1868 636 293 320 285 934 153 354 1835 1451 2499 262 220 1886 280 1786 223

Pb/235U


1s

206

5 12 12 9 10 7 10 4 12 6 5 8 8 9 15 7 16 21 17 15 6 12 10 6 5 14 12 12 15 8 9 4 14 13 15 17 10 9 13 7 9 8 5 12 17 11 6 9 7 8 10 9

155 2420 2526 323 2551 1849 2550 278 1865 1867 274 1924 185 2399 962 264 2436 2201 1611 2391 228 2532 2334 1824 247 2121 2028 953 2398 1744 1802 185 2442 2549 2022 1847 604 290 306 277 894 160 367 1752 1424 2436 257 214 1834 261 1710 217

Pb/238U


1s

208

2 13 17 3 18 11 18 2 17 11 2 15 2 14 11 3 16 17 11 19 2 18 17 10 2 20 17 11 24 12 15 1 18 14 13 11 7 3 5 2 6 2 3 14 14 16 3 4 10 4 10 3

150 2402 2530 324 2501 1905 2532 280 2841 1834 298 1919 187 2381 937 263 2440 2228 1662 2389 223 2516 2308 1827 254 2125 2142 941 2510 1729 2158 179 2456 2508 2024 1805 600 280 301 263 886 153 357 1706 1392 2421 263 215 1811 266 1695 230

Pb/232Th


1s 2 16 21 6 19 51 22 4 65 12 5 21 5 16 15 4 20 24 19 19 3 29 21 19 4 27 29 12 38 17 342 2 28 17 19 15 8 4 7 4 8 3 7 20 19 22 4 4 13 6 11 6

C. Shen et al. / Cretaceous Research 34 (2012) 172e183

5-98_01 5-98_02 5-98_03 5-98_04 5-98_05 5-98_06 5-98_07 5-98_08 5-98_09 5-98_10 5-98_11 5-98_12 5-98_13 5-98_14 5-98_15 5-98_16 5-98_17 5-98_18 5-98_19 5-98_20 5-98_21 5-98_22 5-98_23 5-98_24 5-98_25 5-98_26 5-98_27 5-98_28 5-98_29 5-98_30 5-98_31 5-98_32 5-98_33 5-98_34 5-98_35 5-98_36 5-98_37 5-98_38 5-98_39 5-98_40 5-98_41 5-98_42 5-98_43 5-98_44 5-98_45 5-98_46 5-98_47 5-98_48 5-98_49 5-98_50 5-98_51 5-98_52

Pb/206Pb

(continued on next page) 175

1799 1645 2022 222 250 2661 773 2452 13 13 48 4 3 23 8 18 1806 1661 1704 230 256 2718 776 2486 16 18 48 10 6 20 13 21 1834 1727 1936 236 256 2947 771 2499 21 27 54 82 37 18 32 26 1863 1802 2189 306 259 3108 759 2505 0.0011 0.00145 0.00471 0.00019 0.00016 0.00157 0.00155 0.00124

Pb/232Th 208


1s Pb/238U 206


1s Pb/235U 207


1s Pb/206Pb 207

0.09311 0.08479 0.10522 0.01104 0.01246 0.1407 0.03901 0.12897 0.00266 0.00258 0.00978 0.00059 0.00049 0.0055 0.00146 0.00405 0.3233 0.29395 0.30249 0.03626 0.04058 0.52439 0.128 0.47047 0.00098 0.00107 0.00549 0.0023 0.00099 0.0055 0.00096 0.00137

0.09322 0.09816 0.32052 0.01276 0.00742 0.35414 0.02731 0.23811 5.08745 4.47669 5.7323 0.26108 0.28682 17.21926 1.13757 10.71477


1s

Pb/

53.3 175 137 176 247 233 6.01 78.4 5-98_53 5-98_54 5-98_55 5-98_56 5-98_57 5-98_58 5-98_59 5-98_60

242 409 113 98.0 391 232 213 121

0.22 0.43 1.23 1.81 0.64 1.01 0.03 0.65

0.11391 0.11013 0.13697 0.05247 0.05139 0.23816 0.06451 0.16472

Pb Pb/

206 207

Th (ppm) Grain No.

Table 1 (continued )

U (ppm)

Th/U

Ratios (common-Pb corrected)

235 207

U


1s

206

Pb/

238

U


1s

208

Pb/

232

Th


1s

Ages (common-Pb corrected, Ma)

20 27 86 4 3 28 30 22

C. Shen et al. / Cretaceous Research 34 (2012) 172e183


1s

176

were taken using a Quanta 400FEG environmental scanning electron microscope (ESEM) equipped with an Oxford spectrometer and a Gatan CL3þ detector at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. The operating conditions for the CL imaging were at 15 kV and 20 nA. UePb dating and trace element analysis of zircon were simultaneously performed using LA-ICPMS analyses at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China. A Geolas 2005 system equipped with a 193 nm Lambda Physic ArF-excimer laser with 70 mJ energy at a repetition ratio of 8 Hz coupled to an Agilent 7500 quadrupole ICPMS was used for ablation. The spot diameter was 32 mm for all analyses. Detailed analytical procedures are similar to those described by Yuan et al. (2004). The time-resolved spectra were processed off-line using Glitter software (ver4.0, Macquarie University, Sydney, Australia) to calculate the isotopic ratios using zircon 91500 and GJ-1 as external standards which were analyzed twice for every five analyses. The age of the zircon 91500 and GJ-1 standards used for calculation were 1062.4
0.8 (2s) Ma for 206Pb/238U (Wiedenbeck et al., 1995) and 608.5
1.5 (2s) Ma for 207Pb/206Pb (Jackson et al., 2004). Concentrations of U, Th, Pb and trace elements were calibrated using 29Si as an internal standard and NIST SRM610 as an external reference standard. Common Pb was corrected by ComPbCorr#3_151 using the method of Andersen (2002), with errors quoted at 1s level for all individual analyses. The reported ages (Tables 1 and 2) were calculated and Concordia and probability diagrams (Figs. 4 and 5) were made using ISOPLOT program ver3.25 (Ludwig, 2003). Ages of grains older than 1200 Ma were calculated from their 207Pb/206Pb ratio, whereas ages of younger grains were based on their 206 Pb/238U ratio (Cawood and Nemchin, 2000). To identify the different age clusters, 50e100 grains per sample should be dated (Dodson et al., 1988; Bernet and Spiegel, 2004). In this study, 60e71 grains per sample were fully randomly analyzed, which can provide a 95% probability of finding different populations (Andersen, 2005). 4. Results The LA-ICPMS UePb ages and analytical data of the zircons are summarized in Tables 1 and 2 respectively and graphically illustrated in the concordia diagrams (Fig. 4) and probability diagram (Fig. 5). The Th/U ratios are plotted against the UePb ages in Fig. 6. Errors on individual age analyses are cited as 1s. 4.1. Wulong Formation (K1w) Zircons from the Wulong Formation (sample 5-98) show various grain sizes and morphologies. The grains are predominantly prismatic, poorly rounded, fragmented and euhedral, suggesting short transport distances. Most of the analyzed zircons are light brown, pink, pale red or colourless and generally clear or weakly cloudy, with crystals ranging from 25 to 230 mm in length, and their length/ width ratios are mostly 1.5:1 to 3:1. The CL images show variable brightness and structure and can be divided into three main types (Fig. 3A). Some zircons display distinct core-rim textures with homogeneous cores and bright overgrown rims, which are probably related to metamorphic events (e.g., grains 28, 31, 49; Fig. 3A). These zircons have relatively low Th/U ratios ranging from 0.03 to 0.20, consistent with metamorphic origin (Rubatto, 2002; Bingen et al., 2004; Zhang et al., 2006b). The others have small cores and strong oscillatory zoning, indicating a single growth history, similar to that of typical magmatic zircons (e.g., grains 21, 25, 43; Fig. 3A). A few zircons developed a very narrow rim with strong CL brightness (e.g., grains 13, 19, 38, 42; Fig. 3A), indicating later growth (Zhang et al., 2006a). Concentrations of U range from 18.2 to 820 ppm,

Table 2 UePb isotopic ratios and ages of detrital zircons from the Late Cretaceous Luojingtan Formation in the Yichang area of the Jianghan Basin, central China. Grain No.

Th (ppm)

U (ppm)

Th/U

Ratios (common-Pb corrected)
1s

207

0.11502 0.16691 0.16428 0.11343 0.06938 0.18221 0.11327 0.11249 0.15381 0.06719 0.1103 0.11276 0.11239 0.11283 0.14802 0.10677 0.17403 0.11365 0.05739 0.06755 0.16421 0.05121 0.11369 0.10397 0.07639 0.10905 0.11298 0.07829 0.05267 0.11752 0.12154 0.11456 0.06831 0.16382 0.06841 0.07652 0.05197 0.10145 0.1029 0.05045 0.05191 0.22127 0.11343 0.06995 0.05218 0.11609 0.06437 0.06882 0.0663 0.11388 0.11442 0.05042

0.00082 0.00276 0.00123 0.00077 0.00169 0.00595 0.0009 0.00079 0.00143 0.00095 0.00073 0.0008 0.0007 0.00094 0.00677 0.00148 0.00158 0.0008 0.00073 0.00136 0.00214 0.00097 0.00081 0.00243 0.00089 0.00149 0.00084 0.00056 0.00202 0.00134 0.00128 0.00186 0.0008 0.00127 0.00124 0.00107 0.0029 0.00311 0.00143 0.00206 0.00238 0.00302 0.00078 0.00175 0.00205 0.00097 0.00067 0.00301 0.00256 0.00084 0.00076 0.00129

5.47399 11.3958 10.6137 5.00751 1.4052 11.9661 5.20461 5.30297 8.79716 1.26122 5.00159 4.73139 4.71079 4.89086 7.66922 4.3155 12.0749 4.99656 0.68636 1.30429 10.941 0.30022 5.27565 4.19216 1.81536 4.8183 5.2744 2.09992 0.30124 5.12043 6.04827 5.06636 1.32034 10.1171 1.29101 1.98685 0.22983 3.71873 3.93311 0.18586 0.23657 16.6651 5.42496 1.33019 0.2005 5.5139 1.1639 1.37542 1.26707 5.32045 4.98004 0.29234

Pb/206Pb

5-112_01 5-112_02 5-112_03 5-112_04 5-112_05 5-112_06 5-112_07 5-112_08 5-112_09 5-112_10 5-112_11 5-112_12 5-112_13 5-112_14 5-112_15 5-112_16 5-112_17 5-112_18 5-112_19 5-112_20 5-112_21 5-112_22 5-112_23 5-112_24 5-112_25 5-112_26 5-112_27 5-112_28 5-112_29 5-112_30 5-112_31 5-112_32 5-112_33 5-112_34 5-112_35 5-112_36 5-112_37 5-112_38 5-112_39 5-112_40 5-112_41 5-112_42 5-112_43 5-112_44 5-112_45 5-112_46 5-112_47 5-112_48 5-112_49 5-112_50 5-112_51 5-112_52

49.6 5.45 45.2 142 39.0 14.1 102 35.3 116 155 157 121 69.8 183 135 23.4 112 78.0 312 40.0 44.2 192 79.2 8.22 74.4 19.8 153 553 336 45.3 261 104 149 101 90.9 104 118 53.7 97.7 54.3 215 30.9 93.5 31.2 66.8 209 93.0 7.37 66.3 35.2 74.1 98.3

272 13.8 119 412 45.6 11.1 232 319 84.8 145 415 254 445 140 144 36.2 98.1 313 586 70.1 24.9 288 211 13.5 211 36.9 174 583 832 300 122 252 253 341 91.0 97.3 180 89.0 88.5 108 184 959 283 43.1 118 137 367 36.7 170 228 365 198

0.18 0.40 0.38 0.35 0.86 1.28 0.44 0.11 1.38 1.08 0.38 0.48 0.16 1.32 0.95 0.65 1.15 0.25 0.54 0.57 1.79 0.67 0.38 0.62 0.35 0.54 0.89 0.95 0.41 0.15 2.14 0.41 0.59 0.30 1.01 1.07 0.66 0.61 1.11 0.51 1.18 0.03 0.33 0.73 0.57 1.54 0.26 0.20 0.39 0.16 0.20 0.50

Pb/235U

Ages (common-Pb corrected, Ma)
1s

206

0.08258 0.22056 0.12187 0.04929 0.03481 0.42356 0.04318 0.04043 0.08403 0.01721 0.0368 0.0547 0.0368 0.05352 0.42176 0.07608 0.26438 0.09525 0.01327 0.03146 0.17591 0.00594 0.04839 0.0999 0.02141 0.06616 0.03998 0.01558 0.01205 0.05428 0.06538 0.07012 0.01858 0.16029 0.02803 0.04243 0.013 0.10816 0.07247 0.00737 0.01051 0.26597 0.04594 0.03411 0.00753 0.04884 0.0118 0.06627 0.04677 0.05661 0.06355 0.00846

0.34466 0.49727 0.46789 0.31956 0.14738 0.47669 0.33258 0.34106 0.41484 0.13568 0.32813 0.30355 0.30332 0.31396 0.37431 0.29356 0.50038 0.31828 0.08671 0.13983 0.484 0.0427 0.33583 0.29343 0.17212 0.32007 0.33784 0.194 0.0413 0.31596 0.36034 0.32076 0.14104 0.44593 0.1373 0.18835 0.03216 0.26585 0.27694 0.02701 0.03317 0.54618 0.3464 0.13831 0.02811 0.34334 0.13081 0.14353 0.1386 0.33798 0.31464 0.04218

Pb/238U


1s

208


1s

207

0.00239 0.00469 0.00301 0.00209 0.0013 0.00998 0.00215 0.00206 0.00327 0.001 0.0021 0.0032 0.00202 0.00239 0.00875 0.0023 0.0058 0.00231 0.00068 0.00101 0.00388 0.00046 0.00214 0.00291 0.00138 0.00235 0.00217 0.0012 0.00038 0.00241 0.00285 0.00274 0.00216 0.00374 0.00108 0.00134 0.00046 0.0026 0.00299 0.00027 0.00037 0.00636 0.0025 0.00172 0.0003 0.00221 0.00089 0.00224 0.00154 0.00218 0.00195 0.00038

0.09649 0.13275 0.12778 0.09222 0.04395 0.13883 0.09267 0.09263 0.11329 0.04052 0.09376 0.08805 0.08769 0.09039 0.0962 0.08439 0.13079 0.09149 0.02634 0.04172 0.13452 0.01343 0.0957 0.08923 0.05059 0.093 0.09568 0.05691 0.01253 0.09274 0.10002 0.09237 0.04176 0.12277 0.04163 0.05578 0.01063 0.07752 0.08046 0.00852 0.01011 0.12863 0.10102 0.0424 0.00922 0.09862 0.0416 0.05108 0.04231 0.10005 0.09226 0.01382

0.00102 0.00291 0.00112 0.00068 0.00072 0.00319 0.00072 0.00092 0.00088 0.00037 0.00071 0.00091 0.00082 0.00068 0.00398 0.00111 0.00148 0.00088 0.00028 0.00068 0.00125 0.00016 0.0009 0.00191 0.00068 0.00131 0.00068 0.00057 0.00035 0.0017 0.0007 0.00075 0.00035 0.0011 0.00045 0.00052 0.00025 0.00064 0.00091 0.00021 0.0002 0.00722 0.00105 0.00078 0.00024 0.00064 0.00043 0.0023 0.0004 0.00138 0.00164 0.00023

1880 2527 2500 1855 910 2673 1853 1840 2389 844 1804 1844 1838 1846 2323 1745 2597 1859 506 855 2499 250 1859 1696 1105 1784 1848 1154 315 1919 1979 1873 878 2495 881 1109 284 1651 1677 216 281 2990 1855 927 293 1897 754 893 816 1862 1871 214

Pb/232Th

Pb/206Pb


1s

207


1s

206

17 20 11 9 36 32 7 6 7 16 6 9 6 10 62 21 21 24 29 38 16 26 8 29 12 15 6 7 75 9 9 30 14 16 32 31 103 58 19 73 81 12 7 32 66 8 11 74 83 10 14 50

1897 2556 2490 1821 891 2602 1853 1869 2317 828 1820 1773 1769 1801 2193 1696 2610 1819 531 848 2518 267 1865 1672 1051 1788 1865 1149 267 1840 1983 1830 855 2446 842 1111 210 1575 1621 173 216 2916 1889 859 186 1903 784 878 831 1872 1816 260

13 18 11 8 15 33 7 7 9 8 6 10 7 9 49 15 21 16 8 14 15 5 8 20 8 12 6 5 9 9 9 12 8 15 12 14 11 23 15 6 9 15 7 15 6 8 6 28 21 9 11 7

1909 2602 2474 1788 886 2513 1851 1892 2237 820 1829 1709 1708 1760 2050 1659 2615 1781 536 844 2545 270 1867 1659 1024 1790 1876 1143 261 1770 1984 1793 851 2377 829 1112 204 1520 1576 172 210 2809 1917 835 179 1903 792 865 837 1877 1764 266

Pb/235U

Pb/238U


1s

208

11 20 13 10 7 44 10 10 15 6 10 16 10 12 41 11 25 11 4 6 17 3 10 15 8 11 10 6 2 12 14 13 12 17 6 7 3 13 15 2 2 27 12 10 2 11 5 13 9 10 10 2

1862 2519 2431 1783 869 2628 1791 1791 2169 803 1811 1706 1699 1749 1857 1637 2484 1769 525 826 2551 270 1847 1728 997 1797 1847 1119 252 1793 1927 1786 827 2341 824 1097 214 1509 1564 172 203 2446 1945 839 186 1901 824 1007 838 1927 1784 277

Pb/232Th


1s 19 52 20 13 14 57 13 17 16 7 13 17 15 13 73 21 27 16 6 13 22 3 17 35 13 24 13 11 7 32 13 14 7 20 9 10 5 12 17 4 4 129 19 15 5 12 8 44 8 25 30 5

C. Shen et al. / Cretaceous Research 34 (2012) 172e183

207

(continued on next page) 177

415 232 836 829 2494 809 851 2213 806 742 1800 2331 937 2817 2867 817 1637 891 1008 0.00025 0.00023 0.00061 0.00046 0.00147 0.00043 0.00052 0.02212 0.00031 0.00053 0.00073 0.00105 0.00037 0.00108 0.00107 0.00035 0.00072 0.00059 0.00039

408 328 865 832 3022 806 882 1822 840 779 1851 2587 989 2945 2939 799 1763 894 965

43 61 37 26 33 17 23 30 73 27 7 5 9 5 7 16 14 33 25

420 250 833 829 2825 799 856 1666 816 743 1849 2489 970 2926 2929 826 1700 878 989

9 8 15 11 16 8 10 23 18 10 7 7 6 7 9 8 11 14 12

422 241 823 826 2557 794 844 1546 807 729 1842 2365 959 2890 2906 834 1647 872 997

3 2 7 6 24 6 7 21 7 6 12 12 6 16 17 6 10 7 7

Pb/232Th 208


1s Pb/238U 206


1s Pb/235U 207


1s Pb/206Pb 207

0.02073 0.01154 0.04221 0.04187 0.1313 0.04082 0.04301 0.11569 0.04068 0.03737 0.09314 0.12225 0.04745 0.14954 0.15241 0.04123 0.08438 0.04506 0.05111 0.00055 0.00036 0.00131 0.001 0.00553 0.00103 0.00118 0.0041 0.00129 0.00102 0.00257 0.00274 0.00111 0.00389 0.00421 0.001 0.00206 0.00126 0.00128 0.06757 0.03815 0.13612 0.13669 0.48691 0.1311 0.13985 0.27102 0.13343 0.11978 0.3307 0.44314 0.16039 0.56573 0.56947 0.1382 0.29106 0.14476 0.16732 0.01322 0.00965 0.03275 0.0234 0.25143 0.01729 0.02346 0.11831 0.04031 0.02099 0.04464 0.07509 0.01481 0.12653 0.16305 0.01695 0.05801 0.03169 0.03 0.51241 0.27911 1.27117 1.26138 15.1532 1.1971 1.32274 4.16094 1.23393 1.07806 5.17707 10.6036 1.59845 16.836 16.8944 1.25655 4.33554 1.37529 1.64738 0.00106 0.00184 0.00158 0.00106 0.00454 0.0008 0.00113 0.00314 0.00228 0.0013 0.00077 0.00126 0.00065 0.00132 0.00136 0.0007 0.00092 0.00125 0.00069

Pb/

1.13 0.51 1.09 0.86 0.43 0.75 0.68 0.07 0.86 0.64 0.31 0.32 0.97 0.52 0.54 0.80 0.43 1.12 0.89 294 338 61.3 117 119 127 98.1 50.9 198 97.4 123 132 301 143 169 275 85.1 84.8 269 5-112_53 5-112_54 5-112_55 5-112_56 5-112_57 5-112_58 5-112_59 5-112_60 5-112_61 5-112_62 5-112_63 5-112_64 5-112_65 5-112_66 5-112_67 5-112_68 5-112_69 5-112_70 5-112_71

261 667 56.7 137 275 171 145 726 232 154 403 410 311 276 314 345 200 76.2 304

Pb/

0.05489 0.05297 0.06788 0.06681 0.22571 0.06599 0.06844 0.11136 0.06707 0.06514 0.11316 0.17296 0.0721 0.21517 0.21437 0.06578 0.1078 0.06883 0.07128

Pb


1s

207 206 207

Th (ppm) Grain No.

Table 2 (continued )

U (ppm)

Th/U

Ratios (common-Pb corrected)

235

U


1s

206

Pb/

238

U


1s

208

Pb/

232

Th


1s

Ages (common-Pb corrected, Ma)

5 5 12 9 26 8 10 401 6 10 14 19 7 19 19 7 13 11 8

C. Shen et al. / Cretaceous Research 34 (2012) 172e183


1s

178

and Th from 6.01 to 356 ppm with Th/U ratios varying between 0.01 and 1.81 (Table 1), but most are above 0.1 (Fig. 6A). The apparent ages of 60 grains show a wide range from 155 to 3108 Ma (Fig. 4A) and contain groups at 3108, 2682e2246, 2198e1494, 962e894, 776e604 and 367e155 Ma on the concordia diagrams (Fig. 4A), with age peaks at ca. 3.1, 2.5, 1.8, 0.95, 0.74 and 0.27e0.19 Ga (Fig. 5A). One grain yielded an age older than 3.0 Ga at 3108
28 Ma and the four youngest grains have concordant ages of 185
2, 185
1, 160
2 and 155
2 Ma (Table 1). 4.2. Luojingtang Formation (K2l) Zircons from the Luojingtan Formation (sample 5-112) have similar crystal features to those from sample 5-98. Most zircons are euhedral, transparent or translucent, and 35e200 mm in length, with aspect ratios between 1.5:1 and 2:1. CL images of some zircons reveal oscillatory zoning (e.g., grains 41, 47, 48, 49; Fig. 3B), indicative of magmatic origin. Others exhibit inherited cores (e.g. grains 06, 10, 15, 50; Fig. 3B). U and Th concentrations vary from 13.5 to 959 ppm and 5.45e553 ppm respectively. Th/U ratios vary between 0.03 and 1.79 (Table 2), but most are upper 0.1 (Fig. 6B). All 71 spot analyses yielded zircon UePb apparent ages that range from 172 to 3022 Ma (Table 2). The age populations are grouped into seven major ranges, 3022e2990, 2673e2323, 1979e1651, 1154e959, 886e729, 536e422 and 270e172 Ma on the concordia diagrams (Fig. 4B), with age peaks at ca. 3.0, 2.5, 1.8, 1.1, 0.83, 0.5e0.4 and 0.27e0.18 Ga (Fig. 5B). The four oldest zircons are 3022, 2945, 2939 and 2990 Ma, and the two youngest have concordant ages of 179
2 and 172
2 Ma (Table 2). 5. Discussions and conclusions 5.1. Provenance of detrital zircons Combining samples 5-98 and 5-112, 131 detrital zircon UePb ages from the Cretaceous sediments give seven age clusters with peaks at ca. 3.1e3.0, 2.5, 1.8, 1.1e0.95, 0.83e0.74, 0.5e0.4 and 0.27e0.18 Ga (Fig. 5C), which are represented by 3.8% (5 grains), 18.3% (24 grains), 33.6% (44 grains), 6.1% (8 grains), 15.3% (20 grains), 1.5% (2 grains) and 21.4% (28 grains) of the total analyzed grains (Tables 1 and 2) respectively. These different age groups might indicate different sources of detrital zircons. Palaeocurrent indicators reveal that the sediment sources came from the northwest, mostly from the Huangling Dome. Although many studies indicate that Archaean rocks and inherited zircons existed widely in the Yangtze Craton (Gao et al., 1999; Qiu et al., 2000; Yuan et al., 2004; Yuan and Hu, 2006; Zhang et al., 2006a, b; Wang et al., 2009), Archaean rocks are only exposed in the Kongling metamorphic complex of the Huangling Dome. The Kongling complex consists of dioritic, tonalitic, trondhjemitic and granitic gneisses (DTTG) in the lower unit and metasedimentary rocks in the upper unit, with some amphibolites and locally preserved mafic granulite intruded as lenses, boudins and layers (Gao et al., 1999). SHRIMP and LA-ICPMS UePb zircon analyses revealed DTTG granitoid magmatism at 2.90e2.95 Ga (Qiu et al., 2000; Zhang et al., 2006b). Gao et al. (1999) also obtained wholerock Nd model ages of 2.9e3.3 Ga from gneisses. Detrital zircon UePb ages from the Archaean metapelites are 2.87e3.28 Ga, and the rocks have SmeNd depleted mantle model ages of 3.07e3.21 Ga (Gao et al., 1999; Qiu et al., 2000). Abundant ages of around 2.95 Ga were obtained from the clastic sedimentary Liantuo Formation in the Huangling Dome (Zhang et al., 2006a). Liu et al. (2008) also acquired ca. 3.0 Ga detrital zircons through systematic studies of UePb dating and Hf isotopes of 711 detrital zircons from eight clastic sedimentary rocks from Liantuo, Gucheng and Nantuo Formations in the Huangling Dome and deemed they are from the Kongling complex. Thus,

C. Shen et al. / Cretaceous Research 34 (2012) 172e183

179

Fig. 4. Concordia plots of LA-ICPMS zircon UePb analytical results. A, B, early Cretaceous sample 5-98. C, D, late Cretaceous sample 5-112.

our analytical five detrital zircons with ages peaking at 3.1e3.0 Ga most likely come from the Kongling complex (Fig. 5C). Age populations with peaks at 2.5 Ga and 1.8 Ga are evident in both detrital samples (Fig. 6), with the 1.8 Ga peak accounting for 36.6% and 30.0% of all analyzed grains from samples 5-98 (Fig. 6A) and 5-112 (Fig. 6B) respectively. However, there are some controversies about the sources of these two prominent age peaks and the key issue is whether these zircons are from the North China Block or the Yangtze Block. Liu et al. (2008) found that these age peaks also occur in the two blocks, but that the 2.5 Ga peak is predominant in the North China block and the 1.8 Ga peak is predominant in the Yangtze Block. Zircon age groups of 2.55e2.40 and 2.05e1.80 Ga can be obtained from the Liantuo, Gucheng and Nantuo formations of the Huangling Dome (Zhang et al., 2006a; Liu et al., 2008), which are coincident with the UePb ages of the trondhjemitic gneiss and metasediment (Qiu et al., 2000), the whole-rock SmeNd isochron ages of the amphibolite and paragneiss (Ling et al., 2001) and the zircon ages of the migmatite (Zhang et al., 2006b) in the Kongling area of Huangling Dome. Similar detrital zircon age populations of 2.5e2.4 and 1.8e1.6 Ga also occur in the basement sedimentary sequences of the Jiangnan orogen and have been suggested to come from the recycled sedimentary materials of the Yangtze Block (Wang et al., 2007). To the west of the Kongling metapelites, the Quanyishang K-feldspar granite of ca. 1.85 Ga intruded the Kongling terrane (Yuan et al., 1991), which probably provided the source of zircons with age peaks at 1.8 Ga. Therefore, we consider the Huangling Dome as one of the major sources for the zircons with age clusters of ca. 2.5 and 1.8 Ga, integrating the results of the palaeocurrent analysis. We found minor late Mesoproterozoic to early Neoproterozoic age components with a peak at 1.1e0.95 Ga. These zircons are absent from the North China Block, whereas a significant peak at

985 Ma occurs in the Huangling complexes and coeval igneous rocks (Liu et al., 2008). It is commonly thought these ages are equivalent to the time of the Grenville Orogeny, which have been reported in the Yangtze Block and the Jiangnan Orogeny (Wang et al., 2007, 2009). We believe these zircons are most likely to have been derived from the Huangling Dome, at least from the recycled sedimentary sources. Middle Neoproterozoic magmatism was widespread in South China and is commonly related to the breakup of the Rodinia Supercontinent (Li XH et al., 2003; Zheng et al., 2004; Greentree et al., 2006; Zhang et al., 2006b) by oceanic subduction and arc accretion (Liu et al., 2008). The Huangling granitoid underlies the Liantuo Formation and intrudes the Kongling complex, consisting of four plutons which have zircon UePb ages from 850 to 740 Ma (Ma et al., 1984; Li XH et al., 2003; Li ZX et al., 2003; Zheng et al., 2004; Zhang et al., 2006b). Hence, zircons with a UePb age peak at 0.83e0.74 Ma from the Cretaceous sandstones are most likely derived from the Huangling granitoid and contemporary volcanics. The 0.5e0.4 Ga age range is poorly represented in the detrital zircons of this study, with only two grains within this range from sample 5-112. This age corresponds to a global tectonic-magmatic event termed the “Caledonian Movement” in Western Europe (Ireland et al., 1998) and the “Tacantins Movement” in North America (Rino et al., 2004), which is expressed in the Qinling-Dabie Orogeny (Ratschbacher et al., 2000, 2003; Enkelmann et al., 2007), the late Triassicemiddle Jurassic sandstones of the Jianghan Basin (Wang et al., 2009) and the Cathaysia Block (Xiang and Shu, 2010). However, the age has not been documented in the Precambrian clastic sediments of the Huangling Dome, which might suggest that the Caledonian-age tectonothermal event was not significant in this area. The zircons with an age peak of 0.27 to 0.18 Ga represent

180

C. Shen et al. / Cretaceous Research 34 (2012) 172e183

an important and abundant group, accounting for 21.4% of the total analyzed grains in this study. This time period is commonly thought to cover the formation and exhumation of the Dabie HP-UHP rocks (Hacker et al., 2000, 2006; Grimmer et al., 2003; Ratschbacher et al., 2003; Enkelmann et al., 2007; Wang et al., 2009).

A

5.2. Implication for the exhumation of Huangling Dome

B

C

Fig. 5. Relative probability plots of UePb ages for concordant detrital zircons. A, Wulong Formation sample 5-98. B, Luojingtang Formation sample 5-112. C, total grains.

A

Synorogenic sedimentary rocks are the remnants of rock exposed at Earth’s surface in the past that have since been eroded (Bernet and Spiegel, 2004). Poorly rounded conglomerates are widespread in the Wulong and Luojingtan formations, indicating rapid accumulation from a proximal source that is related to rapid exhumation of the Huangling Dome during the Cretaceous Period. Distinct patterns and possible changes of zircon UePb ages with time, e.g., Early or Late Cretaceous, can provide some new information on the exhumation of the sedimentary source areas. As stated above and shown in Fig. 6, zircons with different age populations from the Cretaceous clastic rocks have been derived from the strata of the Huangling Dome, which may record the Cretaceous exhumation process and landscape evolution of the Huangling Dome. Zircons with age peaks at ca. 2.5, 1.8 and 0.27e0.19 Ga comprise the most abundant age groups for the Early Cretaceous sample 598, which are represented by 26.7%, 30.0% and 33.3% of the total grains respectively (Fig. 6A). However, one prominent age population with a peak at 0.83 Ga is observed in the Late Cretaceous sample 5-112 (Fig. 6B), which accounts for 25.4% of the analyzed grains. Moreover, the proportion of age clusters with peaks at 3.0, 1.8 and 1.1e0.95 Ga increased, whereas the age group with a peak at 0.27e0.18 Ga decreased. According to the discussion above, a relationship between provenance and exhumation details for the Huangling Dome and Cretaceous sediments can be established (Fig. 7). At the Wulong stage of the Early Cretaceous, the Huangling granitoid was not exposed at the surface. The Liantuo, Gucheng and Nantuo formations and the Kongling complex overlying the Huangling granitoid were subjected to rapid uplift and exhumation, leading to deposition of the Cretaceous Wulong Formation (Fig. 7A). Because of the mass of recycled sedimentary materials from the Liantuo, Gucheng and Nantuo sandstones, the Wulong Formation comprises mainly fine- to medium-grained sandstones and conglomerate with quartz sandstone and minor granite gneiss. New apatite fission track dating for the Kongling granite gneiss indicates that it experienced rapid exhumation at 109 Ma (Shen, unpublished data), which is consistent with this study. At the Luojingtan stage of the Late Cretaceous, the Huangling granitoid was exposed at the surface because of the continuous exhumation of the overlying strata and began to supply the sediment of the Luojingtan Formation (Fig. 7B), resulting in the

B

Fig. 6. Plots of Th/U ratios versus UePb ages. A, Wulong Formation sample 5-98. B, Luojingtang Formation sample 5-112.

C. Shen et al. / Cretaceous Research 34 (2012) 172e183

181

A

B

Fig. 7. Simplified model for the relationship between the exhumation process of the Huangling dome and deposition of the Cretaceous sediments. A, at the Wulong stage of the Early Cretaceous: The Huangling granitoid was not exposed at the surface, the Liantuo, Gucheng and Nantuo formations and the Kongling complex experienced rapid uplift and exhumation, leading to deposition of the Cretaceous Wulong formation. B, during the Luojingtan stage of the late Cretaceous the Huangling granitoid was exposed at the surface, supplying sediment to the Luojingtan Formation.

abrupt increase of detrital zircons with a 0.83 Ga age peak and abundant conglomerate deposition. At the same time, the older zircons (age peaks at 3.0 and 1.8 Ga) are more abundant in sample 5-112 than in 5-98, which may point to a shallow exhumation depth in the Wulong stage of the Early Cretaceous. The Huangling Dome, however, was subjected to further uplift and exhumation owing to late intensification of exhumation in the Luojingtan stage of the Late Cretaceous, resulting in the exposure of older rocks and granite. These results were the first to report the initial time of exposure of the Huangling granite. Recently, increasing evidence from apatite and zircon fission track and apatite (UeTh)/He dating and thermal history modelling revealed that the Huangling Dome underwent a relatively long cooling following its formation, and that rapid cooling and exhumation occurred at 160e98 Ma, ages that fall mostly around 106e91 Ma (Hu et al., 2006; Liu et al., 2009; Shen et al., 2009b), also support our model. In addition, the apatite fission track age distribution from strata of the Huangling Dome also provides evidence for this exhumation model. Apatite fission track dating indicates that the 145e115 and 107 Ma ages derive from Silurian and Cambrian strata respectively (Liu et al., 2009) in the eastern Huangling Dome (Fig. 1). Apatite fission track ages change from 129 to 91 Ma from the eastern to central Huangling granite (Shen et al., 2009b). New apatite fission track dating for the Kongling granite gneiss and the Huangling granite also indicates that rapid exhumation of the Kongling complex occurred at 109 Ma, but that exhumation of the Huangling granite occurred at 92 Ma (Shen, unpublished data). These lines of evidence all suggest that the overlying strata were first uplifted and eroded and then underlying strata were exposed

owing to exhumation of the Huangling Dome. Our results provide new and more detailed evidence for the rapid exhumation of the Huangling granite; but further research is required to determine if the onset of exhumation was during the Late Jurassic (Liu et al., 2005; Hu et al., 2006) or the Late Triassic (Dai, 2002; Xu et al., 2005; Yan and Guo, 2006) or the Cretaceous (Liu et al., 2009; Qu et al., 2009). Acknowledgements We thank Zhaochu Hu, Hujun Gong, Zhenbing She and Changgui Gao for their expert assistance in UePb dating; David J. Batten and anonymous reviewers for comments and suggestions; Shaofeng Liu and Sanzhong Li for their kind help in discussions; Adam Szulc and Tom Wittenschlaeger polishing the English. This work was financially supported by the National Natural Science Foundation of China (No. 40902038), the PetroChina Innovation Foundation (No. 2009D-5006-01-08), the Natural Science Foundation of Hubei Province in China (No. 2009CDB217) and the Special Fund for Basic Scientific Research of Central Colleges, China University of Geosciences (Wuhan, No. CUGL100411). References Amidon, W.H., Burbank, D.W., Gehrels, G.E., 2005. UePb zircon ages as a sediment mixing tracer in the Nepal Himalaya. Earth and Planetary Science Letters 235, 244e260. Andersen, T., 2002. Correction of common lead in UePb analyses that do not report 204 Pb. Chemical Geology 192, 59e79.

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