Early Mesozoic unroofing pattern of the Dabie Mountains (China): Constraints from the U-Pb detrital zircon geochronology and Si-in-white mica analysis of synorogenic sediments in the Jianghan Basin

Chemical Geology 266 (2009) 231–241

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Early Mesozoic unroofing pattern of the Dabie Mountains (China): Constraints from the U-Pb detrital zircon geochronology and Si-in-white mica analysis of synorogenic sediments in the Jianghan Basin Yuejun Wang a,⁎, Guochun Zhao b, Xiaoping Xia b, Yanhua Zhang c, Weiming Fan a, Chao Li a, Xianwu Bi d, Sanzhong Li e a

Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Department of Earth Sciences, the University of Hong Kong, Hong Kong c CSIRO Exploration and Mining, Perth, Australia d Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China e Collge of Marine Geosciences, Ocean University of China, Qingdao, 266100, China b

a r t i c l e

i n f o

Article history: Received 5 December 2008 Received in revised form 3 April 2009 Accepted 14 June 2009 Editor: D. Rickard Keywords: Zircon U-Pb geochronology Si-in-white mica Early Mesozoic synorogenic sediments Yangtze basement supply Jianghan basin Dabie HP-UHP rocks

a b s t r a c t This paper presents the results of an integrated U-Pb detrital zircon geochronology and Si-in-white mica analysis for synorogenic sediments in the Jianghan Basin to the south of the Dabie Orogen. The results provide an improved understanding of the provenance of these sediments and the unroofing pattern of the early Mesozoic Dabie Mountain. Si contents of detrital white micas range from 3.09 to 3.34 atoms pfu for the upper Triassic sandstones whereas 3.06 to 3.59 atoms pfu for the lower and middle Jurassic sandstones. The majority of detrital white micas in the lower Jurassic sandstones is phengitic and originated exclusively from the Dabie high- to ultrahigh- pressure rocks. The U-Pb dating results of the detrital zircons for seven samples suggest that these synorogenic sediments have a significant change of provenance from late Triassic to early and middle Jurassic. For the upper Triassic sandstone, the U-Pb age clusters of these zircons are characterized by ~ 420-450 Ma, ~ 750-820 Ma, ~ 1050-1200 Ma and ~ 2500 Ma with minor Luliangian (~ 1700– 2000 Ma) components. In contrast, the zircon ages of the Jurassic sandstones are dominated by the Luliangian (~ 1700–2000 Ma) ages with only minor Caledonian (~ 420-450 Ma) and Greenville (~ 10501200 Ma) ages. In combination with other available geological data, it can be concluded that the Dabie HPUHP rocks might initially be exposed to the surface at the beginning of early Jurassic (~ 190 Ma). The Jiangnan terrain (also named “Jiangnan old continental in Chinese) to the south of the Jianghan basin provided the predominant supply of upper Triassic sediments, whereas the Paleoproterozoic Yangtze crustal materials (overlying the present Dabie Complex at the time) were the important provenance of the Jurassic sediments in the Jianghan basin. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The paleogeographic patterns of a typical orogen that has undergone deep-subduction and subsequent collision and exhumation is a key issue for the understanding of its tectonic evolution. These patterns provide important information for when the subducted materials were exhumed to the surface and how long they resided in the crust during the exhumation. In the Dabie-Sulu orogenic belt, high- to ultrahigh-pressure (HP-UHP) metamorphic rocks were formed by the Triassic northward subduction of the Yangtze Block and subsequent collision with the North China Block (Fig. 1; e.g., Li ⁎ Corresponding author. Current address: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, P.O. Box 1131, Guangzhou 510640, People's Republic of China. Tel.: +86 20 85290527; fax: +86 20 85291510. E-mail address: [email protected] (Y. Wang). 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.06.012

et al., 1993, 2000; Cong, 1996; Fan et al., 2004; Wang et al., 2005). These HP-UHP rocks were exhumed to crustal levels between late Triassic and Jurassic time (e.g., Okay et al., 1993; Ames et al., 1993, 1996; Hacker et al, 1998, 2000; Li et al., 1999b, 2000; Ratschbacher et al., 2000; Wang et al., 2002). Li et al. (1999b) and Hacker et al. (1998, 2000, 2006) proposed that the initial exhumation of the HPUHP rocks might be back to ~240 Ma and their rapid exhumation firstly occurred at 226-214 Ma from mantle to middle crust levels and was followed by later rapid cooling since ~175 Ma (e.g., Liu et al., 2006). There is still controversy about the key issue of when the Dabie HP-UHP rocks were initially eroded and subsequently transported to the associated foreland basins. Two distinct hypotheses have been postulated. One advocated that the HP-UHP rocks were transported to the southern and eastern Dabie forelands during the early-middle Jurassic periods (Grimmer et al., 2003a,b; Li et al., 2005). The second hypothesis suggested a middle-late Jurassic erosion model for the

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Fig. 1. Schematic map showing the major units of the Dabie Orogen, the South and North China Blocks, and the locations of the Jianghan, Hefei, lower Yangtze foreland basins (after Hacker et al., 2000; Ratschbacher et al., 2000; Chen and Jahn, 1998; Grimmer et al., 2003a,b).

Dabie HP-UHP rocks (Li et al., 1999c, 2001;Wang et al., 2001, 2003b; Li and Wang, 2002). In addition, it is commonly considered that the Dabie Complex is dominantly composed of the orthogneiss and amphibolites and minor the metasedimentary cover, which has an affinity to the Yangtze Block (e.g., Mattauer et al., 1985; Okay et al., 1993; Hacker et al., 1998, 2000; Jahn et al., 1999; Ma et al., 2000; Oberhänsli et al. 2002; Schmid et al. 2003; Fan et al., 2004). The orthogneiss and amphibolites in the Dabie Complex (presently exposed at the surface), which exhibit highly radiogenic Pb isotopic ratios, might be representative of the Yangtze middle crust (e.g., Ma et al., 2000; Zhang et al., 2002). The key questions then are whether the present Dabie Complex were overlain by the unidentified crustal materials during the Mesozoic and what the nature of these eroded materials is. The synorogenic sediments in the foreland basins preserve the important signatures of the evolution of the associated hinterland (e.g., Bruguier et al., 1997; Gallagher et al., 1998; Grimmer et al., 2003a, b; Li et al., 2005). Detrial index minerals (e.g., Si-in-white mica, garnet and zircon) of the sediments in the foreland basins are important carriers and can provide important constraints on the unroofing history of the adjacent orogens (e.g., Thomson, 1994; Sorkhabi et al., 1996; Krol and Zeitler, 1996; Bruguier et al., 1997; Grimmer et al., 2003a,b; Li et al., 2005). It is thus reasonable to extrapolate that the sediments from the Mesozoic foreland basins related to the Dabie orogen might record the uplifting and erosion history of the Dabie Mountain. The synorogenic foreland basins related to the Dabie Orogen include the Hefei, Lower Yangtze and Jianghan basins (Fig. 1; e.g., Li et al., 2003, 2005; Liu et al., 2005; Grimmer et al., 2003a,b). The zircon U-Pb dating and white mica chemistry analyses for the Mesozoic sediments from the Hefei and Lower Yangtze foreland basins have been recently carried out (Li et al., 1999c, 2005; Grimmer et al., 2003a,b; Liu et al., 2005). However, the studies of the detrial index minerals (e.g., Si-in-white mica, and zircon) for the

Triassic–Jurassic synorogenic sandstones in the Jianghan basin are poorly documented. In this paper, we present a set of data derived from our recent mineral chemistry analysis of Si-in-white mica and LA-ICPMS U-Pb dating of detrial zircons from the upper Triassic and Jurassic synorogenic sediments in the Jianghan basin. It is also our aim for determining the timing of the initial exhumation of the Dabie HPUHP rocks and achieving a better understanding of the origin of detritus sources for sediments in these basins. 2. Geological backgrounds The Jianghan basin is a Mesozoic–Cenozoic basin developed along the southern margin of the Dabie Orogen (Fig.1, Dai et al., 2000; Liu et al., 2005). During late Triassic to Jurassic time, it was a typical synorogenic foreland basin associated with the Dabie Orogen, and was also considered a channel for transporting the crustal materials of the Dabie Mountain into the Triassic Songpan-Gangze basin (e.g., Bruguier et al., 1997; Enkelmann et al. 2007). In the basin, the synorogenic foreland sediments dominantly outcrop in the eastern part (Li et al., 2003; Liu et al., 2005), and include the upper Triassic Puqi Group, lower Jurassic Wuchang Group and middle Jurassic Huajiahu Group (Fig. 2). The basin-fill and distributions of sedimentary faces have been described by Dai et al. (2000) and Liu et al. (2005) in detail. The upper Triassic Puqi Group (300-1100 m thick) is characterized by ochre and red sandstone, siltstone, mudstone and shale, predominantly outcropped at Jinshandian (Daye) and Bishidu (Echeng). It is uncomfortably underlain by lower Triassic limestone and dolomite (e.g., Dai et al., 2000; Li et al., 2003; Liu et al., 2005). The lower Jurassic Wuchang Group is a suite of fluvial sediments mainly outcropped in the Qizhou (Qichun), Bishidu (Echeng), Jinshan (Daye) and Shenshan (Xianning) regions. This Group has an unconformable contact relationship with the upper Triassic Puqi Group by layered conglomerates at its base. These conglomerates are observed at

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Fig. 2. (a) Geological map for the Jianghan synorogenic basin, showing sample locations. (b-e) Geological map for the dashed areas in Fig. 2a. Composite stratigraphic section is adopted from the 1: 20,000 geologic map for the Puqi and Huangshi area with modifications based on our own mapping.

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Ma'anshan (Daye), Xisa (Huangshi) and Hekou (Huangshi). The lower part of the Wuchang Group is mainly composed of light gray and grayyellow thick-layered coarse-medium grained arkose sandstones, and arkose- quartzose sandstones with minor coal interbeds. Its upper part consists of gray and purple-yellow medium-fine grained sandstones with dark-gray siltstone and mudstone. The middle Jurassic Huajiahu Group unconformably overlies the Wuchang Group. Its basal part is characterized by red conglomerate and sandstone in a fluvial setting. Its middle and upper parts consists of fluvial purple-red siltstone, mudstone, and coarse-medium grained arkose-quartzose sandstones. Cretaeous and Cenozoic strata are extensively distributed in the basin, and uncomfortably overlay the pre-Cretaceous sequences. All the elevated tectonic units surrounding the Jianghan basin during the Mesozoic period are potential sources for the clastic materials accumulated in the basin. The potential clastic sources might merely be related to the contemporaneous exhumation and cooling of the Dabie orogen at the region north or/and uplifts of the Jiangnan terrain (named “Jiangnan old continental” in Chinese literature) at the region south of the Jianghan basin (Fig. 1). The Dabie Orogen was formed by the Triassic subduction of the Yangtze Block and subsequent collision with the North China Block. It can be divided into the North Huaiyang tectono-magmatic belt, North Dabie orthogneiss complex, South Dabie HP-UHP metamorphic complex and Susong greenschist/blueschist units. The North Huaiyang unit is dominantly composed of low-grade metamorphic rocks with minor amphibolite-facies rocks (e.g., Foziling and Luzhenguan Groups) intruded by Cretaceous intrusions (e.g., Okay et al., 1993; Cong, 1996). The North Dabie Complex is mainly composed of gray gneisses and subordinate migmatite, amphibolite, granulite and marble (e.g. Jahn et al., 1999; Fan et al., 2004; Wang et al., 2005). The South Dabie Complex is characterized by the occurrence of HP-UHP metamorphic rocks and consists mainly of gneiss with eclogites, garnet-bearing peridotite, jadeite quartzite, and marble (e.g., Okay et al., 1993). The Susong unit consists of muscovite–albite gneiss and two-mica gneiss with minor HP metamorphic eclogite, marble and blueschist. These HP-UHP rocks in the Dabie orogen are regarded as the representative of the deep-subducted Yangtze middle/upper crustal components, and the orthogneiss and amphibolites are generally accepted as the representative of the Yangtze middle/ lower crust (e.g., Ma et al., 2000; Zhang et al., 2002). To the south of the Jianghan basin, lies the Jiangnan terrain (representative of the Yangtze basement), which is mainly composed

of Mesoproterozoic Lengjiaxi and Neoproterozoic Banxi Groups separated by an angular unconformity. The Mesoproterozoic Lengjiaxi Group is mainly composed of sandstone, slate, spilite and volcanoclastic rocks. The Banxi Group and its equivalents are mainly a well-bedded greywacke–slate–schist succession deposited during 760–820 Ma (Wang et al., 2007). These sequences were intruded by the Jingningian, Caledonian and Yanshanian granites. There is only a limited amount of the Archaean rocks occurred in the northern Yangtze block. The available geochronological data show that the age of the Yangtze basement clusters at the peak of 2.9–3.2 Ga, 2.4-2.7 Ga, 1.9-2.1 Ga, 1.8 Ga, 0.95-1.30 Ga and 0.72-0.85 Ga (e.g., Grimmer et al., 2003a,b; Liu et al., 2006). 3. Sampling and analytical methods 3.1. Sample description Sixteen sandstones were collected from Triassic and Jurassic sediments (Fig. 2) for zircon separation and white mica microprobe analyses. Samples 03HS-3, -6, -9 and -14 are upper Triassic sandstones from Huangtuling (Qichun), Dingzhu (Echeng), Qinghu (Huangshi) and Wangjiashan (Daye), respectively. Samples 03HS-1, -7, 12, -17 are lower Jurassic sandstones from Huangtuling (Qichun), Bishidu (Echeng), Jinshandian (Huangshi) and Shenshan (Xianning), respectively. Samples 03HS-5, -8 and -13 are middle Jurassic sandstones from Huahu (Huangshi), Bishidu (Echeng) Zhangziyu (Daye), respectively. The remaining samples were collected from drill cores in the central part of the Jianghan basin (Fig. 1). These include 20JH-13 and -11 (hole Hong-7), 20JH –8 and -5 (hole Xia-3, -2292.2~-2300.0 m and -959.9~-960.5 m), and 20JH-4 (hole Paican-1, -1104.40~-1107.90 m). Based on the stratigraphic correlation, these samples are the representative of the lower (20JH-08, 20JH-13) and upper parts (20JH-4) of lower Jurassic, and lower (20JH-11) and upper (20JH-05) parts of middle Jurassic sandstones, respectively (Table 1). The mineral compositions of the samples include quartz, mica, and plagioclase with minor rutile, epidote, zircon and apatite. The sampling locations are shown in Figs. 1 and 2a-e, and their lithology and stratigraphic ages are summarized in Table 1. 3.2. Analytical methods Detrial white mica from the sixteen samples (Table 1) was randomly analyzed using EMPA-JXA-8100 electron microprobe at

Table 1 Summary of sample numbers, locations, lithology, and stratigraphic age. Samples

Location

Lithology

N Latitude

E Longitude

Stratigraphic age

Upper Triassic 03HS-3 03HS-6 03HS-9 03HS-14

Huangtuling, Qichun Dingzhu, Echeng Qinghu, Huangshi Wangjiashan, Daye

Gray, fine sandstone Ochre, fine sandstone Red, siltstone Ochre, siltstone

30°16.560′ 30°13.795′ 30°04.508′

114°56.207′ 115°00.046′ 114°54.010′

T3pq T3pq T3pq T3pq

Lower Jurassic 03HS-1 03HS-7 03HS-12 03HS-17 20JH-8 20JH-13 20JH-4

Huangtuling, Qichun Bishidu, Echeng Jinshandian, Daye Shengshan, Xianning Xia-3 drill Hong-7 drill Paican-1 drill

Green, fine sandstone Gray, greywacke Gray, coarse sandstone Gray, coarse sandstone Green, siltstone Ochre, fine sandstone Gray, coarse sandstone

30°02.119′ 30°17.333′ 30°06.803′ 29°52.488′ - 2292.2 m~- 2300.0 m

115°22.258′ 114°53.285′ 114°50.262′ 114°04.860′

J1wc J1wc J1wc J1wc J11 J11 J21

Middle Jurassic 03HS-5 03HS-8 03HS-13 20JH-5 20JH-11

Huahu, Huangshi Qiujiabian, Echeng Zhangziyu, Daye Xia-3 drill Hong-7 drill

Gray, greywacke Gray, coarse sandstone Gray, coarse sandstone Red, coarse sandstone Red, fine sandstone

30°16.990′ 30°17.835′ 30°03.493′ - 959.9 m~-960.5 m - 1769.0 m~- 1777.6 m

114°59.367′ 114°53.742′ 114°46.830′

J22 J22 J12 J22 J12

- 1104.40 m~- 1107.90 m

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the Guangzhou Institute of Geochemistry, the Chinese Academy of Sciences. Wavelength dispersive spectrometers were used with 15 kv accelerating voltage and 20 nA beam current. The analytical results of detrial mica are given in Background Dataset. Detrial zircons were taken from samples 20JH-13, 20JH-8, 20JH-11, 20JH-5, 03HS-3, 03HS-8 and 03HS-17, respectively. Zircons were separated from the crushed rocks using conventional heavy liquid and magnetic techniques and purified by handpicking under a binocular microscope. The laser ablation ICPMS (LA-ICPMS) zircon U–Pb analyses were carried out at the LA-ICPMS laboratory of Northwest University in Xi'an, China, using an Elan 6100 DRC ICP-MS from Perkin Elmer/SCIEX. The GeoLas 200 M laser ablation system (MicroLas, Göttingen, Germany) was used for laser ablation. The measurements were carried out using time-resolved analysis operating in a fast, peak-hopping sequence in the DUAL detector mode. Raw count rates for 29Si, 204Pb, 206Pb, 207Pb, 208Pb, 232Th and 238U were collected for age determination. 207Pb/206Pb and 206Pb/238U ratios were calculated using GLITTER 4.0 (Jackson et al., 2004). Detailed analytical procedures are similar to those described by Yuan et al. (2004). Analyses of the zircon standard TEMORA 1 yielded a weighted 206Pb/238U age of 415 ± 4 Ma (MSWD = 0.112, n = 24) (Yuan et al., 2004), which is in agreement with the recommended ID-TIMS age of 416.7 ± 0.2 Ma (Black et al., 2003). The analytical results are summarized in Table S1. 4. Results 4.1. microprobe analysis The compositions of 263-detrital white mica grains from upper Triassic, and lower and middle Jurassic sandstones were analyzed by electron microprobe. 4.1.1. Upper Triassic 60-detrial white mica grains from upper Triassic samples (03HS-3, -6, -9, -14) gave Si contents (pfu, calculated on 11 oxygens) of 3.093.34 (Fig. 3a), generally lower than those of the Tongbai-Dabie-Sulu HP-UHP rocks (e.g., Hacker et al., 2000; Schmid et al., 2000; Carswell et al., 2000; Grimmer et al., 2003a,b). The variations of their chemical compositions are insignificant. 4.1.2. Lower Jurassic White mica grains from lower Jurassic samples (03HS-1, -7, –12, -17 and 20JH-4, -8, -13) exhibit a wide range of Si distributions, from low Si (3.06-3.29; n =42) to intermediate (3.30-3.49, n = 63), and then to high (3.50-3.59, n = 12). Almost all of the detrital white micas from Samples 03HS-1 and -7 are phengitic (Fig. 3b). For these micas, ~54% gave Si contents of N3.3 and ~10% yielded Si contents of N3.5, similar to those of the Tongbai-Dabie-Sulu HP-UHP rocks (Fig. 3b; Hacker et al., 2000; Schmid et al., 2000; Grimmer et al., 2003a,b; Carswell et al., 2000; Schmid et al., 2000; Cong, 1996). The highest Si content of the analyzed spots is 3.59 Si atoms pfu (03HS-1-grain 4). More than 85% of the analyzed grains for samples 03HS-1 and -7 (near the Dabie-Hong'an orogen) are phengites with Si contents of N 3.30. About 50% grains for 20JH-4 and -8 (central basin locations) have Si contents of N 3.30 Si atom pfu. 4.1.3. Middle Jurassic Micas from five middle Jurassic samples (03HS-5, -8, -13, 20JH-5, -11) displays low to high Si content with unimodal and polymodal distributions. 65 out of 76 analytical spots have Si contents of 3.093.29 and the remaining spots show Si contents ranging from 3.31 to 3.56 (n= 11). All the analyzed micas for Samples 03HS-5 and -8 (near the Dabie area) have Si contents lower than 3.30, distinct from those of lower Jurassic samples (Fig. 3c). In contrast, Si contents for ~50 % grains from Sample 03HS-13 (the Daye area) are higher than 3.30. The phengitic micas also occurred in the central part of the basin (e.g., 20JH-5).

Fig. 3. Si vs. Al diagrams of the detrial white micas for the synorogenic sediments including (a) upper Triassic sandstone, (b) lower Jurassic sandstone, (c) middle Jurassic sandstone in the Jianghan basin (see Fig. 2 for sample locations).

4.2. LA-ICPMS zircon U-Pb ages Zircons from our samples show a wide range of grain sizes and morphology. Most of the analyzed detrital zircons are light brown, pink, red or colorless, and are transparent to semi-transparent. A mixture of well rounded, fragmented, poorly rounded, and euhedral zircons were observed (Fig. 4). These features might suggest distinct transport distances and material sources. Grains with well-preserved shapes most likely indicate a short distance of transportation prior to deposition (Fig. 4a-b). In contrast, rounded crystal geometries are possibly the results of a long-distance transportation or of multi-cycle erosion/deposition processes (Fig. 4c-i). The internal textures of the detrital zircons can be divided into two main types. One is represented by a core-rim texture with a homogeneous core and bright overgrown rims, which are probably related to metamorphic event (Fig. 4e, g and h). The other has a small core and strong oscillatory edge, similar to that of typical magmatic zircons (e.g., Fig. 4a-b). 282 LA-ICPMS detrital zircon U-Pb apparent ages have been obtained from one upper Triassic sandstone sample (sample 03HS-3),

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Fig. 4. Cathodoluminescence imaging of representative detrial zircon grains with a wide range in size and morphology. (a-b) grains with strong oscillatory edge and well-preserved shapes and, (c-i) the crystals with rounded shapes.

three lower Jurassic sandstones (03HS-17; 20JH-13 and 20JH-8) and three middle Jurassic sandstone samples (03HS-8; 20JH-11 and 20JH5). Most zircons gave a concordant or less disconcordant 207Pb/235U206 Pb/238U age, as illustrated in Fig. 5a-g.

4.2.1. Upper Triassic sandstone (03HS-3) Fifty-eight analyses are conducted for an upper Triassic sample (03HS-3). Most spots are plotted on or near the concordia curve (Fig. 5a). Of these analyses, spot 03HS-3.14 yielded a concordia age of 3510 Ma, and spot 03HS-3.28 gave a concordia apparent age of 2899 Ma. The results of these analyses, together with the data for spots 03HS-3.15, -3.26, -3.29, and -3.42, constitute a well-defined regression line with the upper and lower intercept ages of 2920 Ma and 804 Ma, respectively (Fig. 5a). In the histogram diagram (Fig. 6a), two dominant age clusters of ~ 750-820 Ma and ~ 2500 Ma are observed, which represent 28 % and 29 % of the analyzed grains, respectively. The ages for the remaining spots can be predominantly grouped at ~ 420-450 Ma and ~1050-1200 Ma, which represent ~12 % and ~ 21% of the total spots, respectively. The Luliangian age (~17002000 Ma) is less reflected and the Indosinian (~ 200-240 Ma) grains are not observed at all in this sample.

4.2.2. Lower Jurassic sandstones (03HS-17, 20JH-13 and 20JH–8) Ninety-eight detrial grains for three lower Jurassic samples (03HS17, and 20JH-13 and 20JH–8) gave the concordant and least disconcordant U-Pb apparent ages of 222 to 3253 Ma (Fig. 5b-d). A well-defined regression line with the upper and lower intercept ages of 1821 Ma and 1022 Ma is observed in the 20JH-8 (Fig. 5d). For these samples, only two spots (20JH-8.39 and 20JH-13.14) yielded the early Archaean concordant U-Pb apparent ages of 3219-3253 Ma. Only three Caledonian U-Pb concordant ages (452± 4 Ma, 441 ± 4 Ma and 430 ± 4 Ma) is given by 20JH-8.18, -5.04 and -5.20, respectively. For other spots, six age groups are observed (Fig. 6b). The most significant age-cluster is ~17002000 Ma with a peak of 1810-1860 Ma that is represented by 46 % of the analyzed grains. Four other age-clusters of ~220-260 Ma, ~750-820 Ma, ~2160 Ma and ~2500 Ma can also been identified, which represent ~10%, ~10 %, ~9 % and ~19 % of the analytical grains, respectively. 4.2.3. Middle Jurassic sandstone (03HS-8, 20JH-11 and -5) The U-Pb apparent ages of 126 detrial grains show a wide range from 170 Ma to 3510 Ma (Fig. 5e-g). Four grains (20JH-5.07, -5.18, -5.44 and -5.47) from sample 20JH-05 gave the age-cluster of ~ 170188 Ma (Fig. 5g), the youngest concordant apparent ages of all analytical spots. In the histogram diagram (Fig. 6c), other concordant

Fig. 5. LA-ICPMS zircon U-Pb concordia diagram (a-g) for the early Mesozoic synorogenic sediments in the Jianghan basin. (a) Upper Triassic sandstone 03HS-3. (b) Lower Jurassic sample 03HS-17. (c) Lower Jurassic sample 20JH-13. (d) Lower Jurassic sample 03JH-8. (e) Middle Jurassic sandstone 03HS-8. (f) Middle Jurassic sandstone 20JH-11. (g) Middle Jurassic sample 20JH-5.

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Fig. 6. Histograms of LA-ICPMS detrial zircon ages for (a) upper Triassic, (b) lower Jurassic and (c) middle Jurassic sandstones in the Jianghan basin with a plotting increment of 25 Ma. It is used to plot the 207Pb/206Pb apparent ages greater than 1000 Ma and the 206Pb/238U ages less than 1200 Ma.

and less disconcordant grains give four age clusters of ~ 220-260 Ma, ~ 750-820 Ma, ~ 1750-2000 Ma and ~ 2500 Ma, which are represented by ~ 13 %, ~ 14 %, ~41 % and ~14 % of the total grains, respectively. This pattern is similar to that for the lower Jurassic sandstone. In these samples, the Caledonian (~ 400-450 Ma), Jingningian (~750-850 Ma) and Greenville (~1050-1250 Ma) events were less recorded (only ~ 5 % for each event in proportion). 5. Discussions 5.1. When were the Dabie HP-UHP rocks exhumed to the surface? A major controversy surrounds the timing, at which the Dabie HPUHP rocks became exposed to the surface. Li et al. (2005) and Grimmer et al. (2003a,b) considered that these Dabie HP-UHP rocks were initially exposed to the surface in early-middle Jurassic (190–

160 Ma). However, the UHP eclogitic pebbles were only observed in the middle-upper Jurassic Fenghuangtai Formation in the Hefei basin, north of the Dabie Mountains (Wang et al., 2001, 2003a). Li et al. (1999c, 2001) thus suggested that the Dabie metamorphic complexes and their hosting HP-UHP rocks were not subject to major erosion before late Jurassic (Li and Wang, 2002). The integration of our results here with previous data provides further important constraints on the exposure time of the Dabie HPUHP rocks. In the Jianghan basin, the upper Triassic Puqi Group, lower Jurassic Wuchang Group and middle Jurassic Huajiahu Group constitute a complete synorogenic stratigraphic sequence (Figs. 1 and 2). The Si contents of the white mica from these rocks reflect large pressure variations during metamorphism (Massonne and Schreyer, 1987; Guidotti and Sassi, 1998). It is commonly considered that white micas from the HP-UHP rocks are characterized by 3.30–3.70 Si atoms pfu whereas white micas of other origins have low Si contents of b 3.3 (e.g., Hacker et al., 2000; Schmid et al., 2000; Carswell et al., 2000; Grimmer et al., 2003a,b; Li et al., 2005). As described above, there is a large contrast in chemical compositions for the detrial white micas between the upper Triassic and lower Jurassic sandstones from the Jianghan basin. High-Si detrial white micas are rarely found in the upper Triassic rocks. In contrast, the intermediate- and high-Si white micas are abundant in rocks from the lower Jurassic Wuchang Group to middle Jurassic Huajiahu Group. The majority of detrial micas for the lower Jurassic sandstones are phengites in spite of the abundant low-Si white mica observed. It is argued that the low-Si white micas may also have been derived from the Dabie HP-UHP rocks since the phengites might be strongly retrogressed. The available data however show that the Si contents of white micas from the Dabie HP-UHP rocks are greater than 3.30 Si atoms pfu during Triassic and then decrease generally from 3.48–3.30 Si atoms pfu (Triassic) to 3.35–3.10 Si atoms pfu till early-middle Jurassic (e.g., Hacker et al., 2000). This strongly suggests that low-Si micas from upper Triassic sandstones are unlikely to be the retrogressed phengites. The intermediate- and high-Si white micas from the Jurassic sandstones might have been derived exclusively from the Dabie orogen (e.g., Hacker et al., 2000; Schmid et al., 2000; Carswell et al., 2000; Grimmer et al., 2003a,b). Zircon grains with the late Triassic age have not been identified from the Jianghan Triassic sandstones. However, such zircon grains were observed in the lower and middle Jurassic sandstones. Only two possible provenance sources can be proposed: 1) the Jiangnan terrain to south of Jianghan basin; and 2) the Dabie Orogen to north of the Jianghan basin. Available data appear to argue against the occurrence of the zircon grains with late Triassic metamorphism and Indosinian granitic intrusions within the Jiangnan terrain (e.g., JXBGMR,1989). Additionally, the Indosinian granitic rocks to the south of the Jiangnan terrain (e.g., Hunan and Jiangxi Provinces) might not become a potential source for the late Triassic zircons in the Jurassic sandstones in the Jianghan basin since the Indosinian granitic rocks to the south of the Jiangnan terrain might have not been uplifted to the surface until late Jurassic, even early Cretaceous (e.g., Chen and Liu, 1995; Yue et al., 1998; Grimmer et al. 2003a,b). The absence of the Middle Triassic-Upper Jurassic Formations within the Jiangnan terrain (e.g., Mofu-Xiushui-Jiuling area) indicates the development of a Jurassic Mountain in the area. It is thus impossible for the Indosinian zircon grains sourced from the south of the Jiangnan terrain to be northwardly transported into the Jianghan basin across the early Jurassic Xiushui-Mofu-Jiuling Mountain. The remaining alternative then is that the Triassic zircons with magmatic origin in the Jianghan lower Jurassic sandstone were from the Dabie Orogen. It is noted that most of the Triassic zircons from the Jianghan lower and middle Jurassic sandstone have Th/U ratio of 0.291.81, similar to that of the most Triassic zircons for the lower Jurassic sandstones in the Hefei basin (Li et al., 2004, 2005), indicative of a magmatic origin. In contrast, the zircon grains with the Triassic U-Pb ages of ~250 to 205 Ma in the Dabie Orogen (e.g., the Suanghe, Shima, Bixiling, Wumiao, Yingshan and Maowu eclogite) exhibit the geometrical features

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of the metamorphic zircons and have Th/U ratio of less than 0.1 (e.g., Ames et al., 1993, 1996; Rowley et al., 1997; Hacker et al., 1998, 2000, 2006; Maruyama et al., 1998; Cheng et al., 2002; Ayers et al., 2002; Zheng et al., 2004, 2007; Li et al., 2005; Zhao et al., 2008 ). These features appear to suggest the occurrence of the Triassic syncollisional granitic rocks in the Dabie Orogen during early Jurassic, even though the discovery of such intrusions in the Dabie Orogen is limited (noting the presence of the Triassic plutons in the Sulu Orogen). Also taking account of the presence of the abundant phengites in the lower Jurassic sandstones, we herein infer that the Dabie Orogen might be the prominent source of the Triassic zircons for the Jianghan lower and middle Jurassic samples. In the Jianghan basin, there are few phengites and Triassic zircons in the upper Triassic sandstones. In contrast, Triassic zircon grains and voluminous intermediate- and high-Si white micas are seen in the lower and middle Jurassic sandstones. These observations suggest that the supply of the Dabie HP-UHP rocks into the Jianghan basin was minor during late Triassic but abundant during early and middle Jurassic. Gary and France-Lanord (1996) suggested that material transportation time from an erosion region to the adjacent sedimentary basins is normally less than 2 Ma. Therefore, the initial exposure of the Dabie HP-UHP rocks probably occurred at the beginning of early Jurassic (~ 190 Ma). Additional evidences supporting the consideration above include: (1) the detrial white micas with 3.58 Si atoms pfu are identified from the lower Jurassic Fanghushan sandstones in the Hefei basin (Li et al., 2005); (2) the phengites are observed in the lower Jurassic Xiangshan sandstones in the lower Yangtze River basins (Grimmer et al., 2003a,b); (3) coesite-bearing inclusions in the detrial zircons with the SHRIMP zircon U-Pb age of 234~200 Ma are observed in the lower Jurassic Fanghushan sandstones from the Hefei basin (Li et al., 2005); and (4) the provenance study on the Triassic Songpan-Ganzi basin sediments also show that the Hong'an-Dabie HP-UHP rocks were not exposed in the middle to late Triassic (Enkelmann et al., 2007). These evidences further suggest that the Dabie HP-UHP rocks served as the provenance of the Jianghan basin starting from the earliest Jurassic time. Considering that the time of the peak metamorphism and rapid exhumation time for the Dabie HPUHP rocks, it is proposed that the residing time of the exhumed Dabie HP-UHP rocks in the crust might be less than ~ 50 Ma rather than ~ 80 Ma (e.g., Webb et al., 1999; Li et al., 2000).

5.2. What materials overlay Dabie Complex during early Mesozoic? Available data show that the U/Pb zircon geochronological patterns cluster at ~3.8 Ga, ~3.3 Ga, ~3.0 Ga, ~2.5 Ga, and ~1.75-1.8 5 Ga (e.g., Zhao et al., 2000) for the North China Block and at 470–490 Ma and 390– 410 Ma for its southern margin (e.g., Zhang et al., 1989; Hacker et al., 1998, 2006; Lerch et al., 1995; Enkelmann et al., 2007). The U-Pb zircon ages for the Yangtze Block can be grouped into ~2.9-3.2 Ga, ~2.5 Ga, ~1.82.1 Ga, ~0.9-1.3 Ga, ~0.7-0.8 Ga, ~390-450 Ma and 200-250 Ma (Ames et al.,1996; Chen and Jahn,1998; Qiu et al., 2000; Grimmer et al., 2003a,b; Li et al., 2005; Liu et al., 2006). The Greenville amalgamation age- (0.9– 1.3 Ga), Jingningian rifting age- (0.7–0.8 Ga) and Indosinian age-cluster (200-250 Ma) are predominantly shown in the Yangtze Block. Our zircon U-Pb ages for the upper Triassic and Jurassic sandstones show distinctive patterns. The U-Pb zircon ages for the upper Triassic sandstone can be grouped into ~400-450 Ma, ~ 750-850 Ma, ~ 10501200 Ma and ~2500 Ma. The Luliangian (~1700-1900 Ma) component is minor and Indosinian (~ 220 Ma) grains are absent. These data indicate a significant affinity to the Yangtze Block. Taking into account the abundant low-Si white micas in the upper Triassic samples and the northwestward paleocurrent recorded in the Jianghan upper Triassic strata at the Yangxi-Huangshi-Echeng-Xianning area (e.g., Xu, 2002), the crustal materials of the Jiangnan terrain were most likely the main source for the upper Triassic fine sandstone and siltstone in the Jianghan Basin.

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However more and more data show that the Hong'an-Dabie Orogen might have supplied the upper Triassic sediments into the Songpan-Gangzi flysch basin (e.g., Bruguier et al., 1997; Weislogel et al., 2006; Enkelmann et al., 2007). This indicates that an upper Triassic transportation path through the Jianghan basin into the Songpan-Ganzi basin should be expected. Our sample locations are geographically close to the southern Dabie Orogen, and as such, coarse sandstone and greywacke should be expected theoretically. This expectation contradicts with our observation that the upper Triassic sandstone is characterized by fine sandstone. Therefore, it is inferred that the aboriginal Jianghan basin might be larger than present acreage although the exact locations of the Triassic transportation channel are unknown. The Jurassic sandstones produced the age-clusters of ~ 220260 Ma, ~ 750-820 Ma, ~1700-2000 Ma and ~ 2500 Ma with minor ~400-450 Ma and ~ 1000-1250 Ma. An age-cluster at ~ 170-188 Ma is also observed. This youngest age-cluster (Th/U ratios = 0.63-1.09) is slightly older than the deposition age of the strata (~160-175 Ma), suggesting a rapid unroofing of the adjacent hinterland during the early-middle Jurassic period (Li et al., 1999b, 2000). The other ageclusters might be interpreted as the derivation of sediments from regions north (e.g., the Dabie Orogen) or south (e.g., Jiangnan terrain) of the Jianghan basin (e.g., Grimmer et al., 2003a,b; Li et al., 2005). If the Jiangnan terrain was the dominant provenance for the Jurassic sandstone samples from the Jianghan Basin, the Neoproterozoic and Greenville ages of the zircon grains that represent the signatures of the South China Block should be expected (e.g., Hacker et al., 2006; Enkelmann et al., 2007; Liu et al., 2008 and authors’ unpublished data). However, the age populations for Jingningian, Greenville and Caledonian ages for the Jurassic samples are only ~14 %, ~5 % and ~5 % of the total grains, respectively. As documented above, the Jianghan Jurassic sandstones preserve abundant phengites. The paleocurrent data for the lower Jurassic strata in the study area (e.g., the Xianning-Huangshi area) suggest erosion regions and sediment sources from the north and northeast (e.g., Xu, 2002 and authors' unpublished data). Therefore, our dated Jurassic sandstones might be dominantly derived from the Dabie Orogen to the north rather than the Jiangnan terrain to the south of the Jianghan basin (Li et al., 1999c, 2001, 2005; Li and Wang, 2002; Grimmer et al., 2003a, b). It is noted that more than 50 % of our analyzed zircon grains for the Jurassic sandstones gave the Luliangian age (~1700–2000 Ma). Now the question arises as to the nature of the materials with the age of 1700-2000 Ma that were transported into the Jianghan basin from the Dabie Orogen during early and middle Jurassic. Two possible models can be proposed since the Paleoproterozoic zircons (1700-2000 Ma) are present in not only the North China Block but also the Yangtze Block (e.g., Gao et al., 2004; Hacker et al., 2006; Liu et al., 2008). One is that these materials were from the North China basement onto the Dabie Orogen. Zhou et al. (2008) also considered that the ShiqiaoPingshang metasediemtary rocks interbedding with the Sulu HP-UHP rocks might have an affinity to the North China Block. However, if this model were true, the expectation would have been that the North China basement rocks were thrusted southwards onto the South Dabie unit across the northern Huaiyang unit during Triassic. The abundant detrial zircons with an affinity to North China Basement should have been preserved in the Jurassic Hefei basin. However, Li et al. (2004, 2005) reported that the detrial zircon grains from the Jurassic sandstones of the Fanghushan and Fenghuangtai Formations in the Hefei basin are originally from the Yangtze basement, thus contradicting this hypothesis. An alternative model is represented by the overriding of the Paleoproterozoic Yangtze basemen over the present Dabie Complex during early-middle Jurassic period. The zircon grains with the Luliangian age-cluster in the Jianghan basin were probably from the Yangtze basement overlying the Dabie Orogen. Grimmer et al. (2003a, b) reported that a 1.9–2.1 Ga geological event might have occurred

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along the northern Yangtze Block from Qinling through Dabie into Korea. Recent data (Zheng et al., 2004, 2007; Wu et al., 2006, 2008; Zhang et al., 2006a,b,c; Zhao et al., 2008) suggested that there most likely occurred a 1.8-2.1 Ga tectonothermal event in the northern Yangtze Block and Dabie Orogen. The C-O isotopes of the marble gravel and detrial garnet from Jurassic sediments in the Hefei basins also suggest a provenance from Yangtze rather than from the North China Block (Li et al., 1999a; Wang et al., 2002). It is well known that protoliths of the Dabie Complex are predominantly characterized by the Neoproterozoic ages of ~0.7-0.8 Ga and exhibit an affinity to the Yangtze crust (e.g., Ames et al., 1996; Hacker et al., 1998; Qiu et al., 2000; Zheng et al., 2004, 2007; Zhao et al., 2008). But our analyzed Jurassic samples have abundant high-Si white mica and Luliangian zircons but less Neoproterozoic and Caledonian detrial zircons. This appears to suggest that (1) voluminous Paleoproterozoic Yangtze basement rocks might overlie the present Dabie Complex during late Triassic and early Jurassic period, and (2) the Paleoproterozoic Yangtze basement might have undergone HP-UHP metamorphism during the Triassic deep subduction (e.g., Oberhänsli et al. 2002; Schmid et al., 2003). 6. Conclusion The syntheses of our new results for the Jianghan synorogenic sediments have enabled us to reach the following conclusions for the early Mesozoic exhumation of the Dabie Mountains. (1) White micas from upper Triassic samples are characterized by low Si atom pfu. In contrast, the abundant phengitic micas with high Si atom pfu are observed in the lower and middle Jurassic sandstones, similar to those of the Tongbai-Dabie-Sulu HP-UHP rocks. (2) The U-Pb zircon ages for the upper Triassic sandstones are clustered at ~400-450 Ma, ~750-850 Ma, ~1050-1200 Ma and ~2500 Ma with few Luliangian components. For Jurassic sandstones, our analyses gave the age groups of ~220-260 Ma, ~750-820 Ma, ~1700-2000 Ma and ~2500 Ma with minor grains at the ages of ~400-450 Ma and ~1050-1200 Ma. (3) The Dabie HP-UHP rocks might initially be exhumed to the surface, and were then subject to erosion and transported into the Jianghan basin at the beginning of early Jurassic (~190 Ma). (4) The Jiangnan terrain to the south of the Jianghan basin was the major provenance of the Jianghan upper Triassic sediments. During the early and middle Jurassic period, there might be a Paleoproterozoic Yangtze basement block overlying the present Dabie Complex and these basement materials became a predominant provenance of the Jurassic sediments in the Jianghan basin Acknowledgements We would like to thank Dr. Z-Y Xu for their help during field trip and sample collection. We are grateful to Prof. D. Rickard, S. Gao and another anonymous reviewer for their critical and constructive review, which led to major improvement of the manuscript. This study was financially supported by projects from the Chinese Academy of Sciences (KZCY2yw-128, KZCX1-YW-15-1), the Ministry of Science and Technology of China (2007CB411403) and the Nature Sciences Foundation of China (40825009, 40772129) and Opening Foundation of State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, CAS (200704). This is a contribution to NO IS091104 to GIGCAS. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at doi:10.1016/j.chemgeo.2009.06.012.

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