Provenance of the Paleoproterozoic Hutuo Group basal ...

SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

November 2012 Vol.55 No.11: 1796–1814 doi: 10.1007/s11430-012-4407-2

Provenance of the Paleoproterozoic Hutuo Group basal conglomerates and Neoarchean crustal growth in the Wutai Mountains, North China Craton: Evidence from granite and quartzite pebble zircon U-Pb ages and Hf isotopes DU LiLin1*, YANG ChongHui1, WANG Wei2, REN LiuDong1, WAN YuSheng1,3, SONG HuiXia1, GAO LinZhi1, GENG YuanSheng1 & HOU KeJun4 1 Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China; Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 3 Beijing SHRIMP Center, Beijing 100037, China; 4 Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China 2

Received May 21, 2011; accepted September 19, 2011; published online April 23, 2012

Zircon U-Pb ages (SHRIMP and LA-ICPMS) and Lu-Hf isotope data (LA-ICPMS) are presented for two granite and two quartzite pebbles from the basal conglomerates of the Sijizhuang Formation in the Hutuo Group from the Wutai Mountains area in the North China Craton. These two granite pebbles give zircon 207Pb/206Pb ages of 2513±8 Ma and 2527±8 Ma respectively, which are consistent with the emplacement ages of the Wangjiahui grey granite and Guangmingsi or Shifo granite in the Wutai Mountains. Detrital zircons from those two quartzite pebbles are mostly 2550–2490 Ma old with lesser number of 2800–2550 Ma grains, which is similar to the ages of detrital zircons from quartzites in the Gaofan Subgroup of the Neoarchean Wutai Group. Thus, the pebbles in the Hutuo Group basal conglomerates were derived locally from Wutai Mountains Neoarchean sources. Zircons from the Sijizhuang Formation conglomerate granite and quartzite pebbles mostly have positive εHf(t) values, a minority with εHf(t) values like model depleted mantle (DM) of the same age, but with most showing DM model ages 200–100 Ma. This indicates that most of the source materials were derived from the mantle within the previous 200 million years, whereas some are derived from 2550–2510 Ma juvenile crustal additions. This additional evidence suggests that in the North China Craton there was important initial polycyclic crustal formation and cratonization in the late Neoarchaean, prior to superimposed Palaeoproterozoic orogenic cycles. Hutuo Group, Sijizhuang Formation conglomerate, zircon U-Pb age, Hf isotope, sedimentary provenance, crustal growth Citation:

Du L L, Yang C H, Wang W, et al. Provenance of the Paleoproterozoic Hutuo Group basal conglomerates and Neoarchean crustal growth in the Wutai Mountains, North China Craton: Evidence from granite and quartzite pebble zircon U-Pb ages and Hf isotopes. Sci China Earth Sci, 2012, 55: 17961814, doi: 10.1007/s11430-012-4407-2

The timing of crust formation and cratonization is an important topic in understanding the early Precambrian evolution of the Earth [1]. At present, there is a dispute over the history of cratonization of North China, predominantly at *Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2012

~2500 Ma [2–10] or at ~1800 Ma [11–23]. On the basis of the regional tectonics, rock associations, geochronology, metamorphism and P-T-t paths in domains of the North China Craton (NCC), Zhao et al. [19–23] subdivided it into the Eastern Block, Western Block and the Trans-North China Orogen (TNCO). Closure of an ocean by eastward subduction led to the Eastern and Western blocks colliding earth.scichina.com

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at 1850－1800 Ma (the Lüliang Movement), with the final stabilization of the NCC. However, there are great debates over the Paleoproterozoic geological evolution in TNCO [5–7, 24–42]. The TNCO is a nearly S-N striking zone up to 1200 km long and 300–100 km wide, which is separated from the Eastern and Western Blocks by the Xinyang-Kaifeng- Shijiazhuang-Jianping and Huashan-Lishi-Datong-Duolun faults, respectively [19] (Figure 1). In the TNCO, Paleoproterozoic strata occur mainly in the Wutai, Lüliang, Zanhuang and Zhongtiao areas, where from the north to south, the Hutuo, Lüliang, Gantaohe, Jiangxian and Zhongtiao groups are exposed. They are dominated by volcano-sedimentary rocks that formed mainly after 2200 Ma [27, 28, 43–45]. The Hutuo Group in the Wutai Mountains area is the most typical Paleoproterozoic lithostratigraphic unit in the NCC (Figure 2). From base to top, the Hutuo Group is subdivided into the Doucun, Dongye and Guojiazhai Subgroups. The Doucun and Dongye Subgroups consist mainly of clastic rocks and carbonates with basic volcanic layers, whereas the Guojiazhai Subgroup is a molasse suite [47]. The Hutuo Group is mostly metamorphosed only to low-grade greenschist facies. However, near the Jingangku, it was locally metamorphosed to amphibolite facies and occurs as garnet-biotite and amphibole schists [48].

Figure 1

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Compared with the Wutai Group (Wutai Complex), Fuping Group (Fuping Complex), and Hengshan Complex, the Hutuo Group has been studied in much less detail, with debates concerning the time of deposition and its tectonic environment. In this paper, detailed field investigation of the Hutuo Group Sijizhuang basal conglomerates is combined with zircon U-Pb ages and Lu-Hf isotopic data of granite and quartzite pebbles, to constrain the time of deposition and its provenance. This can provide new insights into the chronology and tectonic setting of the Hutuo Group, and further information on the timing of crustal growth in the Wutai Mountains area of the NCC.

1 Geological background The Wutai Mountains located in the middle of the NCC contains the Wutai Group, late Archean granitoids, the Paleoproterozoic Hutuo Group, and Paleoproterozoic granites. In the eastern part of the Wutai Mountains area, the Wutai Complex is separated from the Fuping Complex by the Longquanguan Shear Zone, and in the north of the Wutai Mountains area, the Wutai Complex is isolated from the Hengshan Complex by Hutuo River (Figure 2). In the Wutai Mountains area, the Wutai Group and late Archean granitoids

Subdivision of the North China Craton (after ref. [22]). LL, Lüliang Mts.; WT, Wutai Mts.; ZH, Zanhuang; ZT, Zhongtiao Mts.

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Figure 2

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Geological sketch map of Hengshang-Wutai-Fuping in the North China Craton (after ref. [46]).

constitute a typical Neoarchean greenstone belt [48, 49]. 1.1


The Wutai Group

The Wutai Group is subdivided, from bottom to top, into the Shizui, Taihuai and Gaofan Subgroups [47, 48]. The Shizui Subgroup has undergone amphibolite facies metamorphism and mainly consists mainly of metamorphosed basic-intermediate volcanic rocks, with intermediate-acidic volcanic-sedimentary rock interlayers locally, and some clastic rocks and carbonates at the base. The Taihuai Subgroup has undergone greenschist facies metamorphism and is composed mainly of basic-intermediate-acidic volcanic rocks with some meta-sandstone and BIF assemblages in the lower part. The Gaofan Subgroup has undergone low-grade greenschist facies metamorphism, and is composed mainly of clastic rocks [48, 49]. Wilde et al. [50] noted that although all three Subgroups in the Wutai Group underwent different metamorphism, they formed in the same 2533– 2513 Ma period, and thus they suggested that the Shizui, Taihuai and Gaofan Subgroups are unrelated, but are do-

mains with different grades of metamorphism that were tectonically juxtaposed with each other. In addition, Wilde et al. [18] obtained SHRIMP zircon U-Pb ages of 2566– 2515 Ma for Neoarchean granitoids in the Wutai Mountains area, and 2117 Ma for a Paleoproterozoic pink phase of the Wangjiahui granite. Wang and Wilde [51] also reported an age of 2176 Ma for the Dawaliang granite in the Wutai Mountains area. 1.2

The Hutuo Group

The Hutuo Group is distributed mainly on the southern slope of the Wutai Mountains with a total area of 1700 km2 (Figure 3). The Hutuo Group unconformably overlies the Wutai Group [47, 49]. Ripples and cross bedding are well preserved, indicating a clear relationship between the top and bottom of the strata. The Doucun Subgroup of the Hutuo Group is composed mainly of clastic rocks, with thick conglomerates at the base. The Dongye Subgroup is composed mainly of purple sandstone/slate in the lower parts, then sandstone with stromatolitic and massive grey

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Figure 3

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Geological sketch map of Hutuo Group in the Wutai Mountains area (after ref. [47]).

carbonates intelayers in the middle to upper parts. The Guojiazhai Subgroup lies unconformably on the Dongye Subgroup and is dominated by sandstones with some conglomerates locally. Wu et al. [52] initially obtained an ID-TIMS zircon U-Pb age of 2366+103/–94 Ma from basalt of the Qingshicun Formation in the Doucun Subgroup, which suggested that the Hutuo Group formed in the Paleoproterozoic. Wilde et al. [46] reported a SHRIMP zircon U-Pb ages of 2087±9 Ma from meta-felsic tuff in the Wutai Mountains area, thereby better constraining the age of Hutuo Group as Paleoproterozoic. The Sijizhuang Formation conglomerates at the base of Hutuo Group are polymict, with abundant pebbles of quartzite (including clastic quartzite and chemical sedimentary ferroan chert), granite, BIF, chlorite schist, and volcanic rocks. The matrix between the pebbles is mainly mafic schist (Figure 4(a)) and meta-sandstone (Figure 4(b)). The pebbles are well-rounded and 30–10 cm in diameter, but have been tectonically flattened with the matrix becoming schistose (Figure 4(b)). Close to Sijizhuang (Shiji), ≥90% of the pebbles is quartzite (Figure 4(c)). Near Tanshang, 80%– 70% of pebbles is quartzite, with a few of granite, BIF and volcanic rocks (Figure 4(d)). Near Wangquanzhuang, quartzite pebbles are still dominant, but granitic ones are common (Figure 4(e)), with a few of chlorite schist (Figure 4(f)) and BIF (Figure 4(g)). The likely source of the Sijizhuang Formation conglomerates was the Wutai Group and they have the characteristics of basal conglomerates

[49]. In addition, the strongly deformed BIF pebbles contrast with less deformed matrix (Figure 4(h)) and coexistence with meta-sandstone pebble (Figure 4(b)) indicates there was deformation and metamorphism in the source regions, prior to the erosion, transport, and deposition of the Hutuo Group.

2 Sample description Two granite pebbles and two quartzite pebbles were collected from the Sijizhuang Formation conglomerates in the valley near Wangquanzhuang (Figure 3). Sample HT02-3 is a strongly deformed medium- to fine-grained biotite-granite pebble with a gneissic structure, collected from the valley 2 km north of Wangquanzhuang (GPS 38°50.075′N, 113°08.028′E all with WGS-84 datum). Sample HT11-1 is a medium- to coarse-grained chlorite bearing granite pebble with a weak gneissic structure, collected from the valley 100 m south of Wangquanzhuang (GPS 38°48.801′N, 113°07.983′E) and is medium- to coarse-grained. Both of HT02-1 and HT02-4 sample are quartzite pebbles, collected from the same outcrop as granite of HT02-3 (GPS 38°50.075′N, 113°08.028′E). They have a granoblastic texture, massive structure, and consist mainly of quartz (95%–90%), some chlorite (5%–2%), and accessory zircon and allanite.

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Figure 4 Photographs of Sijizhuang Formation conglomerates in Hutuo Group. (a) Metamorphosed mafic schist matrix in Sijizhuang Formation conglomerates; (b) sandstone matrix in Sijizhuang Formation conglomerates, with pebbles oriented due to superimposed deformation; (c) ≥90% of quartzite pebbles in conglomerates (near Sijizhuang); (d) 80%–70% quartzite pebbles in conglomerates, with granite, BIF and volcanic rock pebbles (near Tanshang); (e) granite pebbles in conglomerates (near Wangquanzhuang); (f) greenschist pebble in conglomerates (near Wangquanzhuang); (g) BIF pebble in conglomerates (near Wangquanzhuang); (h) BIF pebble that is more deformed than its matrix.

3 Analytical methods The SHRIMP zircon U-Pb analyses were performed at the Beijing SHRIMP Center, Chinese Academy of Geological Sciences. The analysis protocols follow those of Williams

[53] and Song et al. [54]. Elemental abundance of U, Th, and Pb were calibrated by using reference zircon M257 with a U concentration of 840 ppm [55] and the 206Pb/238U age was calibrated using TEMORA 1 with an age of 417 Ma [56]. Mass resolution was ≥5000 (1% peak height). The

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primary ion beam was 5–4 nA, 10 kV O2– and the spot diameter on the target was 35–30 µm. A TEMORA 1 analysis was interspersed between every four analyses of the unknowns. The software SQUID 1.02 and ISOPLOT, written by Ludwig [57, 58], were used for data processing and assessment. The common lead correction was applied using measured 204Pb abundances and model Pb compositions of Cumming and Richards [59]. LA-MC-ICP-MS zircon U-Pb dating analyses were conducted at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. Detailed operating conditions for the laser ablation system and the MCICP-MS instrument and data reduction are the same as given by Hou et al. [60]. Laser sampling was performed using a Newwave UP 213 laser ablation system. A Thermo Finnigan Neptune MC-ICP-MS instrument was used to acquire ion-signal intensities. The carrier gas was helium. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. Each analysis incorporated a background acquisition of approximately 20–30 s (gas blank) followed by 30 s data acquisition from the sample. Off-line raw data selection and integration of background and analyte signals, and time-drift correction and quantitative calibration for U-Pb dating was performed by ICPMS DataCal [61]. The zircon GJ1 was used as external standard for U-Pb dating, and was analyzed twice every 5–10 analyses. Time-dependent drifts of U-Th-Pb isotopic ratios were corrected using a linear interpolation (with time) for every 5–10 analyses according to the variations of GJ1 (i.e., 2 zircon GJ1 + 5–10 samples + 2 zircon GJ1; Liu et al. [61]). Preferred U-Th-Pb isotopic ratios used for GJ1 are from Jackson et al. [62]. Uncertainties of the preferred values for the external standard GJ1 were propagated to the ultimate results of the samples. In all analyzed zircon grains the common Pb correction was not necessary due to the low signal of common 204Pb and high 206Pb/204Pb. U, Th, and Pb concentration was calibrated by zircon M127 (with U=923 ppm; Th=439 ppm; Th/U: 0.475) [55]. Concordia diagrams and weighted mean calculations were made using ISOPLOT [58]. Zircon Lu-Hf isotope analyses were carried out in-situ using a Newwave UP213 laser-ablation microprobe, attached to a Neptune multi-collector ICP-MS at Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. The more destructive Lu-Hf analyses were performed after the U-Pb analyses by SHRIMP, with Lu-Hf and U-Pb analysis sites coinciding as closely as possible in their location. Instrumental conditions and data acquisition were comprehensively described by Hou et al. [63] and Wu et al. [64]. A stationary spot was used, with a beam diameter of 55 to 65 μm depending on the size of ablated domains. Helium was used as carrier gas to transport the ablated sample from the laser-ablation cell to the ICP-MS torch via an argon mixing chamber. In order to correct the isobaric interferences of 176Lu and 176Yb on 176Hf, 176Lu/175Lu=


1801

0.02658 and 176Yb/173Yb = 0.796218 ratios were determined [65]. For instrumental mass bias correction Yb isotope ratios were normalized to 172Yb/173Yb of 1.35274 [65] and Hf isotope ratios to 179Hf/177Hf of 0.7325 using an exponential law. The mass bias behavior of Lu was assumed to follow that of Yb. Mass bias correction protocol details were described by Wu et al. [64] and Hou et al. [63]. Zircon GJ1 was used as the reference standard, with a weighted mean 176 Hf/177Hf ratio of 0.282003 ±0.000023 (2σ, n=15) during our routine analyses. It is indistinguishable from a weighted mean 176Hf/177Hf ratio of 0.282013±19 (2σ) from in-situ analysis by Elhlou et al. [66].

4 Analytical results 4.1

Zircon U-Pb ages

4.1.1 Strongly deformed medium- to fine-grained granite pebble (HT02-3) Zircon grains in this granite pebble are ca. 200–100 μm long, with an aspect ratio 1:2 to 1:1. Most grains are euhedral, and in cathodoluminescence (CL) images they show oscillatory structure, with the character of zircons of magmatic origin (Figure 5(a), (b)). Nineteen grains were analyzed. Contents of U and Th are 464–46 ppm and 543–29 ppm respectively, with Th/U ratios of 1.21–0.46 (Table 1). Most analyses yield a closely concordant 207Pb/206Pb and 206 Pb/238U ages, with some spread of 207Pb/206Pb beyond analytical error. Accepting that this spread is due to minor variable ancient loss of radiogenic Pb (e.g., Nutman et al. [67]), then culling those with slightly younger apparent ages (1.1, 7.1, 10.1 and 19.1) yields a weighted mean 207Pb/206Pb age of 2513±8 Ma (MSWD=0.26), consistent with upper intercept age of 2520±9 Ma (Figure 6(a), (b)). This is interpreted as the emplacement age of the granite. 4.1.2 Chlorite bearing medium- to coarse-grained granite pebble (HT11-1) Zircon grains in this granite pebble are 300–200 μm long, euhedral, with an aspect ratio of 3:1 to 1:1. In CL images, most grains show oscillatory structure, typical of magmatic origin (Figure 5(c) and (d)). Twenty grains from the granite were analyzed. Contents of U and Th are 106–59 ppm and 81–40 ppm respectively, with Th/U ratios of 0.99–0.64 (Table 1). Except the strong lead loss of spot 3.1, the others yielded close to concordant ages. Excluding outlier analysis 10.1, the remaining 18 yielded a 207Pb/206Pb weighted mean age of 2527±8 Ma (MSWD=0.96; Figure 6(c)), which is interpreted as the emplacement age of granite. 4.1.3 Quartzite pebbles (HT02-1, HT02-4) Zircons are rounded, irregular or columnar with a grain size of 200–100 μm. In CL images, most zircons show oscillatory structures, some banding zones, and a few no obvious structures (Figure 5(e)–(h)). From the zircon CL images, the

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Figure 5 CL images of zircons from Sijizhuang Formation conglomerates in Hutuo Group. Blue ellipses and circles mark the sites zircon U-Pb ages by SHRIMP and LA-MC-ICPMS respectively; red circles mark the sites of LA-MC-ICPMS Lu-Hf isotope analysis.

provenance of quartzite pebble is very complicated. Rounding and truncation of internal structures indicates significant abrasion and probably prolonged transportation. Sixty grains from quartzite (HT02-1) were analyzed. Contents of U and Th are 1462–6 ppm and 246–1 ppm respectively, with Th/U ratios of 2.01–0.11 (Table 2). In a concordia diagram (Figure 7(a)), apart from a few spots with obvious lead loss, most analyses yield close to concordant ages (Table 2). The data are also portrayed in a 207 Pb/206Pb age histogram and cumulate frequency diagram (Figure 7(b)). Besides the main age group at ~2515 Ma, a few grains show older ages, with distinct minor populations at ~2788 and 2674 Ma, and a few older, Eoarchean to Mesoarchean, ones. Fifty zircon grains from the quartzite (HT02-4) were analyzed. Contents of U and Th are 447–26 ppm and 264–21 ppm respectively, with Th/U ratios of 1.34–0.2 (Table 2). The concordia diagram (Figure 7(c)) shows that eight analyses have significant loss of radiogenic Pb. Most of the rest have 207Pb/206Pb ages of 2600–2500 Ma (Table 2). The 207 Pb/206Pb histogram and frequency distribution diagram (Figure 7(d)) shows a main age peak at 2522 Ma and two older grains with ages of 3161±10 Ma and 2992±14 Ma, respectively. 4.2

Zircon Lu-Hf isotopic analyses

4.2.1 Strongly deformed medium- to fine-grained granite pebble (HT02-3) 176 Lu/177Hf and 176Hf/177Hf ratios of 19 grains are 0.000305– 0.001801 and 0.281281–0.281401 respectively, with a 176 Hf/177Hf average value of 0.281337±0.000019. All analyses have positive εHf(t) values, ranging from 1.86 to 7.14, and Hf model TDM1 ages of 2743－2559 Ma (Table 3).

4.2.2 Chlorite bearing medium- to coarse-grained granite pebble (HT11-1) 176 Lu/177Hf and 176Hf/177Hf ratios of 20 grains are 0.000354–0.001060 and 0.281307–0.281400 respectively, with a 176Hf/177Hf average of 0.281343±0.000012. All analyses have positive εHf(t) with a range from 7.71 to 3.86, and Hf model TDM1 ages of 2675–2559 Ma (Table 3). 4.2.3 Quartzite pebbles (HT02-1, HT02-4) In HT02-1 sample, the 176Lu/177Hf ratios range from 0.000288 to 0.002512, and the 176Hf/177Hf ratios are from 0.281177 to 0.281400. Eo-archean to Paleoarchean detrital zircons (3725, 3602 and 3454 Ma) have the abnormally positive εHf(t) of >20, obviously higher than that of coeval depleted mantle. A possible reason for this could be that the old components were small inherited components in ~2515 Ma zircon. Then, with the much larger sampling volume of the laser ablation analysis on zircon Lu-Hf than U-Pb analyses, the Lu-Hf analyses targeting them could actually be dominated by ~2515 Ma zircon. Another zircon grain, with an age of 2709 Ma, also has a high εHf(t) of 10.57, perhaps for the same reason. The other analyses have εHf(t) values ranging from –1.53 to 7.06, and TDM1 values are from 2914 to 2614 Ma (Table 3). In HT02-4 sample, the 176Lu/177Hf ratios are from 0.000508 to 0.002256, and the 176Hf/177Hf ratios range from 0.280857 to 0.281435. The εHf(t) values range from –4.31 to 9.93, and the corresponding TDM1 is from 3301 to 2570 Ma (Table 3).

5 Discussion 5.1 Provenance of Hutuo Group conglomerates The Sijizhuang Formation conglomerates at the base of the

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Table 1

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Zircon SHRIMP U-Pb analyses on granite pebbles of Sijizhuang Formation conglomerates a)

206 U Th Pbc Pb* 206Pb/238U Th/U (%) (ppm) (ppm) (ppm) (Ma) Strong deformed medium- to fine-grained granite pebble (HT02-3) HT02-3-1.1 0.07 123 63 0.53 49.3 2473±19 HT02-3-2.1 0.08 46 29 0.65 19.4 2552±23 HT02-3-3.1 0.00 51 30 0.61 21.1 2536±30 HT02-3-4.1 0.09 74 59 0.83 30.7 2531±18 HT02-3-5.1 0.07 97 43 0.46 40.5 2556±18 HT02-3-6.1 0.04 94 85 0.94 40.0 2598±17 HT02-3-7.1 0.09 124 86 0.72 48.8 2433±20 HT02-3-8.1 0.05 186 228 1.27 73.8 2446±12 HT02-3-9.1 0.18 81 62 0.79 31.8 2430±25 HT02-3-10.1 0.08 89 55 0.64 35.5 2452±25 HT02-3-11.1 0.05 61 59 1.01 25.2 2541±28 HT02-3-12.1 0.15 117 93 0.82 45.2 2389±22 HT02-3-13.1 0.18 94 86 0.95 31.2 2106±22 HT02-3-14.1 0.90 464 543 1.21 60.5 903.6± 8 HT02-3-15.1 0.33 267 145 0.56 66.0 1627±15 HT02-3-16.1 0.27 81 69 0.88 32.6 2465±26 HT02-3-17.1 0.09 105 101 0.99 41.7 2453±24 HT02-3-18.1 0.18 85 71 0.86 34.9 2509±26 HT02-3-20.1 0.12 80 53 0.68 33.0 2526±27 Chlorite bearing medium- to coarse-grained granite pebble (HT11-1) HT11-1-1.1 0.15 64 40 0.64 26.2 2512±27 HT11-1-2.1 0.43 59 41 0.71 24.4 2514±28 HT11-1-3.1 0.05 78 75 0.99 28.2 2256±25 HT11-1-4.1 0.22 106 81 0.79 43.4 2514±24 HT11-1-5.1 0.08 77 57 0.78 31.7 2533±27 HT11-1-6.1 0.36 62 40 0.67 25.6 2509±28 HT11-1-7.1 0.22 75 61 0.84 30.6 2503±26 HT11-1-8.1 0.27 73 58 0.82 29.4 2469±26 HT11-1-9.1 0.13 63 45 0.72 26.1 2521±28 HT11-1-10.1 -84 70 0.86 34.0 2480±25 HT11-1-10.1 0.22 63 50 0.83 26.5 2562±29 HT11-1-11.1 0.25 93 70 0.77 38.8 2545±26 HT11-1-13.1 0.13 75 49 0.68 30.4 2503±26 HT11-1-14.1 0.10 97 72 0.77 40.3 2534±25 HT11-1-15.1 0.16 67 41 0.64 28.1 2564±29 HT11-1-16.1 0.42 63 43 0.70 26.0 2528±30 HT11-1-17.1 0.19 60 47 0.80 25.0 2528±28 HT11-1-18.1 0.30 63 50 0.82 26.2 2539±28 HT11-1-19.1 0.22 80 70 0.90 32.7 2505±26 HT11-1-20.1 0.20 80 56 0.73 33.0 2529±26

Spot No.

206

207

Pb/206Pb (Ma)

Discordant (%)

207

Pb*/206Pb*

± (%)

207

Pb*/235U

± (%)

206

Pb*/238U

± (%)

2552±11 2511±18 2508±17 2519±14 2502±14 2507±12 2544±11 2509± 9 2511±16 2490±15 2517±17 2514±13 2534±15 2456±15 2497±12 2521±17 2524±13 2524±16 2550±16

3 –2 –1 0 –2 –4 4 3 3 2 –1 5 17 63 35 2 3 1 1

0.1695 0.1654 0.1651 0.1661 0.1644 0.1650 0.1686 0.16518 0.1653 0.1633 0.1659 0.1656 0.1677 0.1600 0.1640 0.1663 0.1666 0.1666 0.1692

0.66 1.1 0.98 0.85 0.84 0.73 0.68 0.55 0.95 0.87 1.0 0.77 0.91 0.90 0.69 1.0 0.79 0.93 0.97

10.92 11.07 10.97 11.02 11.03 11.29 10.66 10.51 10.43 10.42 11.05 10.24 8.93 3.320 6.491 10.68 10.64 10.93 11.19

1.1 1.5 1.7 1.2 1.2 1.1 1.2 0.8 1.6 1.5 1.7 1.4 1.5 1.3 1.2 1.6 1.4 1.5 1.6

0.4675 0.4857 0.4819 0.4809 0.4867 0.4963 0.4586 0.4615 0.4577 0.4629 0.4832 0.4487 0.3864 0.1505 0.2871 0.4658 0.4630 0.4759 0.4796

0.94 1.1 1.4 0.88 0.86 0.79 0.98 0.59 1.2 1.2 1.3 1.1 1.2 0.95 1.0 1.2 1.2 1.2 1.3

2550±18 2486±22 2537±17 2519±14 2544±16 2519±20 2510±18 2523±17 2514±18 2574±18 2534±18 2529±17 2547±16 2538±14 2538±16 2511±20 2536±19 2518±19 2542±17 2498±16

1 –1 11 0 0 0 0 2 0 4 –1 –1 2 0 –1 –1 0 –1 1 –1

0.1692 0.1629 0.1679 0.1661 0.1686 0.1661 0.1652 0.1665 0.1656 0.1717 0.1676 0.1671 0.1690 0.1680 0.1680 0.1654 0.1678 0.1660 0.1684 0.1640

1.1 1.3 1.0 0.86 0.94 1.2 1.1 1.0 1.1 1.1 1.0 0.99 0.96 0.85 0.97 1.2 1.2 1.1 0.99 0.97

11.12 10.72 9.70 10.93 11.19 10.90 10.81 10.71 10.93 11.11 11.27 11.15 11.05 11.16 11.31 10.95 11.11 11.05 11.03 10.86

1.7 1.9 1.7 1.4 1.6 1.8 1.6 1.6 1.7 1.6 1.7 1.6 1.6 1.5 1.7 1.9 1.8 1.7 1.6 1.6

0.4766 0.4770 0.4191 0.4770 0.4814 0.4758 0.4745 0.4666 0.4785 0.4691 0.4879 0.4840 0.4743 0.4816 0.4885 0.4802 0.4802 0.4827 0.4750 0.4804

1.3 1.4 1.3 1.2 1.3 1.4 1.3 1.3 1.3 1.2 1.3 1.2 1.3 1.2 1.4 1.4 1.3 1.3 1.3 1.3

a) Errors are 1, Pbc and Pb* indicate the common and radiogenic portions, respectively; error in standard calibration was 0.38%; common Pb corrected using measured 204Pb.

Hutuo Group are composed mainly of quartzite pebbles. In addition, there are some of BIF, granitoids, volcanic rocks, and chlorite schists. Rocks similar to these pebbles are widely distributed locally in Wutai Group and the Neoarchean Wutai granitoids. Based on the geochemsity and rock association, the Wutai Mountains granitoids can be divided into two stages [18, 71]. The first stage is represented by the 2566–2540 Ma Ekou, Lanzhishan and Chechang-Beitai granites, and the second stage by the 2540–2515 Ma Shifo, Guangmingsi and

Wangjiahui grey granites, which were emplaced simultaneously with formation of volcanic-sedimentary rocks of the Wutai Group [18, 71]. The age of the granite is 2513±8 Ma for pebble HT02-3 and is 2527±8 Ma for pebble HT11-1, and these are consistent with the Wangjiahui grey phase granite and medium- to coarse-grained Guangmingsi/Shifo granite in not only age, but also texture, structure, and composition. Zhang et al. [72] presented LA-ICPMS zircon U-Pb ages of 2566±15 and 2517±32 Ma from two granite pebbles from Sjizhuang Formation conglomerates in Daixian

1804

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we tend to consider that clastic quartzite pebbles in the base of Hutuo Group are from the Gaofan Subgroup. Wan et al. [45] dated detrital zircons from a Gaofan Subgroup quartzite and a quartzite pebble from the Sijizhuang Formation and found an age peak of ~2500 Ma for both, indicating their affinity. Likewise, we find age peaks of the detrital zircon grains from the two quartzite pebbles are 2522 and 2515 Ma, very similar to those found in the Gaofan Subgroup [45]. So, we conclude that quartzite pebble in Sijizhuang Formation conglomerate is mainly from the Gaofan Subgroup of the Wutai Group. Both zircon U-Pb ages of granite pebbles and quartzite pebbles and regional geology suggest that provenance of the Sijizhuang Formation at the base of the Hutuo Group should be derived mainly from the Wutai Group and late Archean granites in the Wutai Mountains. The Sijizhuang Formation conglomerate shows the characters of basal conglomerate, in accord with observations that the Hutuo Group unconformably overlies the Wutai Group and late Archean granitoids [27, 47–49]. 5.2

Figure 6 U-Pb Concordia diagrams of zircons from Hutuo Group Sijizhuang Formation conglomerate granite pebbles.

County, on the northern slopes of the Wutai Mountains, and considered the provenance of granite pebbles to be from the Ekou and Wangjiahui grey phase granite respectively. Thus we can deduce that granite pebbles in the Hutuo Group are from the late Archean granitoids in the Wutai Mountains area [47–49, 72]. In the Wutai Group, clastic quartzite is distributed mainly in the lower part of the Gaofan Subgroup, with little if any quartzite in the Shizui Subgroup [49]. In general, the Shizui Subgroup underwent amphibolite facies metamorphism [49]. So far, amphibolite and fine-grained hornblende/biotite gneiss, related with Shizui Subgroup, have not been found as pebbles in the Sijizhuang Formation conglomerates. Thus

Depositional age and environment

As the most classic Paleoproterozoic strata unit in the NCC, the age of Hutuo Group has not been well defined. Wu et al. [52] first presented an imprecise ID-TIMS zircon concordia intercept U-Pb age of 2366+103/–94 Ma from a basalt of the Qingshicun Formation in the Doucun Subgroup, which indicated the Hutuo Group formed in the Paleoproterozoic. However, basalts in Hutuo Group are continental volcanic rocks, capturing ~2500 Ma zircons when erupting [27]. Thus, this age might be composite and of no geological significance. Based on a granitic pluton with an age of 2549±22 Ma intruding in the Doucun Subgroup [73], Li and Kusky [7] suggested that Doucun Subgroup formed in the Neoarchean, and the Hutuo Group was also deposited in the Neoarchean, not in the Paleoproterozoic. However, zircon U-Pb ages of different pebbles from the Sijizhuang Formation conglomerates indicate the depositional age of the Hutuo Group should be younger than that of granite (2513 Ma). Wilde et al. [46] found that meta-felsic tuffs in the Qingshicun Formation yield SHRIMP zircon U-Pb ages of 2180±5 and 2087±9 Ma, respectively. They considered the younger one (2087±9 Ma) represented the age of volcanic rocks, indicating that the Hutuo Group formed in the Paleoproterozoic. Du et al. [27] presented a SHRIMP zircon U-Pb age of 2140±14 Ma from a basaltic andesite at the base of the Hutuo Group, and thereby confirmed that it formed in the middle Paleoproterozoic. Combining the field relationships that the Sijizhuang Formation unconformably overlies the Wutai Group and late Archean granitoids in Wutai Moutains, we consider that the Hutuo Group is younger than that of the Wutai Group and unrelated to the Wutai Group [27]. In the tectonic evolution model for the TNCO of Zhao et

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1805


Zircon La-MC-ICPMS U-Pb analyses on quartzite pebbles of Sijizhuang Formation conglomerates

Content (ppm) Pb Th U Quartzite pebble (HT02-1) HT02-1-1 100 162 200 HT02-1-2 46 55 42 HT02-1-3 29 17 37 HT02-1-4 72 57 101 HT02-1-5 78 76 96 HT02-1-6 66 63 80 HT02-1-7 221 162 1462 HT02-1-8 103 91 132 HT02-1-9 64 48 101 HT02-1-10 62 57 160 HT02-1-11 122 144 168 HT02-1-12 121 106 148 HT02-1-13 50 24 77 HT02-1-14 83 97 84 HT02-1-15 91 80 183 HT02-1-16 67 73 80 HT02-1-17 102 140 169 HT02-1-18 65 55 91 HT02-1-19 31 26 36 HT02-1-20 33 14 51 HT02-1-21 76 171 142 HT02-1-22 38 32 50 HT02-1-23 51 45 69 HT02-1-24 60 73 62 HT02-1-25 86 55 122 HT02-1-26 118 144 120 HT02-1-27 145 223 173 HT02-1-28 121 134 130 HT02-1-29 12 8 17 HT02-1-30 62 50 71 HT02-1-31 60 56 78 HT02-1-32 42 32 65 HT02-1-33 46 32 77 HT02-1-34 44 30 64 HT02-1-35 33 22 52 HT02-1-36 30 32 30 HT02-1-37 63 47 99 HT02-1-38 37 25 53 HT02-1-39 44 27 59 HT02-1-40 135 246 324 HT02-1-41 28 34 27 HT02-1-42 37 59 29 HT02-1-43 72 43 122 HT02-1-44 96 50 172 HT02-1-45 76 84 71 HT02-1-46 189 168 245 HT02-1-47 55 61 62 HT02-1-48 141 117 171 HT02-1-49 140 141 161 HT02-1-50 39 36 49 HT02-1-51 23 20 34 HT02-1-52 128 61 214 HT02-1-53 34 29 49 Spot No.

Th/U

0.81 1.31 0.46 0.57 0.78 0.78 0.11 0.69 0.48 0.35 0.86 0.71 0.31 1.15 0.44 0.91 0.83 0.61 0.72 0.27 1.21 0.64 0.65 1.17 0.45 1.20 1.29 1.03 0.46 0.71 0.72 0.49 0.41 0.46 0.43 1.05 0.48 0.47 0.45 0.76 1.26 2.01 0.35 0.29 1.18 0.69 0.97 0.69 0.88 0.73 0.60 0.29 0.59

207

Pb/206Pb

0.1648 0.1645 0.1941 0.1660 0.1655 0.1656 0.1713 0.1692 0.1671 0.1791 0.1710 0.1736 0.1820 0.1656 0.1687 0.1643 0.1658 0.1637 0.1824 0.1804 0.1820 0.1708 0.1650 0.1650 0.1830 0.1670 0.1723 0.1682 0.1651 0.1897 0.1677 0.1637 0.1640 0.1718 0.1661 0.1703 0.1657 0.1788 0.1840 0.1686 0.1667 0.3267 0.1674 0.1694 0.1828 0.1742 0.1654 0.1952 0.1738 0.1655 0.1643 0.1777 0.1653

1σ 0.0009 0.0009 0.0011 0.0009 0.0009 0.0009 0.0010 0.0009 0.0009 0.0010 0.0009 0.0009 0.0010 0.0009 0.0009 0.0009 0.0009 0.0009 0.0010 0.0010 0.0010 0.0009 0.0009 0.0009 0.0010 0.0009 0.0009 0.0010 0.0011 0.0010 0.0009 0.0009 0.0009 0.0009 0.0009 0.0010 0.0009 0.0010 0.0010 0.0009 0.0012 0.0024 0.0009 0.0009 0.0010 0.0009 0.0009 0.0010 0.0009 0.0009 0.0009 0.0009 0.0009

Isotopic ratio 1σ Pb/235U

207

8.2508 11.1513 14.5454 11.1352 11.0236 11.1315 3.2493 11.3022 10.1255 6.7361 9.7434 11.8196 12.9306 11.0060 9.2824 10.7985 8.6775 10.8434 13.0671 13.0276 9.3701 11.8185 10.9205 10.8440 12.9826 11.0460 9.3652 12.0307 11.1302 13.8094 11.4007 10.8285 10.7368 11.7685 11.1970 11.6751 10.8638 12.5676 13.6248 7.3327 11.1015 15.2027 11.1486 11.2418 12.9782 11.6642 10.9873 13.3037 11.8528 11.1715 10.7226 12.3628 10.9106

0.0987 0.1362 0.1976 0.1639 0.1325 0.1373 0.1910 0.1427 0.1196 0.0937 0.1056 0.1608 0.1524 0.1336 0.1521 0.1331 0.1957 0.1340 0.1636 0.1582 0.2762 0.1321 0.1450 0.1253 0.1657 0.1432 0.1580 0.1847 0.1415 0.1730 0.1358 0.1303 0.1205 0.1362 0.1227 0.1474 0.1248 0.1492 0.1505 0.2421 0.2184 0.4005 0.1179 0.1319 0.1503 0.1255 0.1179 0.1418 0.1580 0.1400 0.1533 0.1455 0.1208

206

Pb/238U

0.3629 0.4913 0.5435 0.4859 0.4828 0.4875 0.1388 0.4840 0.4391 0.2727 0.4129 0.4935 0.5151 0.4818 0.3984 0.4764 0.3796 0.4802 0.5190 0.5239 0.3719 0.5015 0.4801 0.4762 0.5141 0.4795 0.3949 0.5202 0.4885 0.5276 0.4930 0.4798 0.4746 0.4965 0.4893 0.4973 0.4752 0.5095 0.5367 0.3156 0.4824 0.3375 0.4828 0.4811 0.5147 0.4851 0.4816 0.4939 0.4945 0.4896 0.4729 0.5044 0.4786

1σ 0.0043 0.0059 0.0074 0.0070 0.0058 0.0061 0.0084 0.0059 0.0051 0.0038 0.0044 0.0066 0.0060 0.0057 0.0063 0.0058 0.0086 0.0058 0.0062 0.0065 0.0100 0.0055 0.0064 0.0054 0.0065 0.0061 0.0070 0.0088 0.0059 0.0066 0.0059 0.0059 0.0053 0.0057 0.0056 0.0063 0.0054 0.0060 0.0059 0.0105 0.0083 0.0088 0.0050 0.0056 0.0058 0.0048 0.0051 0.0051 0.0066 0.0061 0.0065 0.0058 0.0052

207

Pb/206Pb

1σ

2506 2502 2777 2518 2513 2514 2572 2549 2529 2656 2568 2592 2672 2513 2546 2502 2516 2494 2675 2656 2672 2566 2507 2509 2681 2527 2580 2540 2509 2740 2534 2494 2497 2576 2520 2561 2515 2643 2700 2544 2525 3602 2532 2552 2680 2598 2522 2787 2595 2513 2502 2631 2511

8 14 9 9 8 9 10 9 10 9 9 9 9 9 9 9 9 9 9 9 9 5 9 9 14 9 9 9 11 9 9 9 9 9 10 11 9 9 9 9 12 11 8 9 9 8 9 3 8 9 10 3 9

207

Age (Ma) Pb/235U 1σ 2259 2536 2786 2535 2525 2534 1469 2548 2446 2077 2411 2590 2675 2524 2366 2506 2305 2510 2685 2682 2375 2590 2516 2510 2678 2527 2374 2607 2534 2737 2556 2509 2501 2586 2540 2579 2512 2648 2724 2153 2532 2828 2536 2543 2678 2578 2522 2701 2593 2538 2499 2632 2516

11 11 13 14 11 11 46 12 11 12 10 13 11 11 15 11 21 11 12 11 27 10 12 11 12 12 15 14 12 12 11 11 10 11 10 12 11 11 10 30 18 25 10 11 11 10 10 10 12 12 13 11 10

206

Pb/238U

1σ

1996 2576 2798 2553 2539 2560 838 2545 2347 1554 2228 2586 2679 2535 2162 2512 2074 2528 2695 2716 2038 2620 2528 2511 2674 2525 2146 2700 2564 2731 2584 2526 2504 2599 2568 2602 2506 2655 2770 1768 2538 1875 2540 2532 2677 2549 2534 2587 2590 2569 2496 2633 2521

20 25 31 30 25 26 48 25 23 19 20 29 26 25 29 25 40 25 26 27 47 24 28 23 27 27 32 37 25 28 25 26 23 25 24 27 24 26 25 51 36 42 22 24 25 21 22 22 28 26 29 25 23

(To be continued on the next page)

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(Continued) Content (ppm) Spot No. Pb Th U HT02-1-54 50 168 103 HT02-1-55 72 153 110 HT02-1-56 73 72 76 HT02-1-57 75 73 248 HT02-1-58 41 31 60 HT02-1-59 6 1 6 HT02-1-60 45 35 62 Quartzite pebble (HT02-4) HT02-4-1 25 29 26 HT02-4-2 78 85 86 HT02-4-3 68 52 106 HT02-4-4 82 81 109 HT02-4-5 41 46 51 HT02-4-6 119 120 150 HT02-4-7 126 49 240 HT02-4-8 129 91 203 HT02-4-9 154 182 159 HT02-4-10 74 42 110 HT02-4-11 80 79 107 HT02-4-12 86 70 128 HT02-4-13 55 46 77 HT02-4-14 106 50 100 HT02-4-15 38 36 49 HT02-4-16 157 115 292 HT02-4-17 124 199 148 HT02-4-18 122 71 208 HT02-4-19 182 130 172 HT02-4-20 121 167 139 HT02-4-21 95 122 104 HT02-4-22 59 43 98 HT02-4-23 146 136 187 HT02-4-24 119 60 175 HT02-4-25 60 69 64 HT02-4-26 206 196 447 HT02-4-27 40 36 79 HT02-4-28 64 68 75 HT02-4-29 190 215 399 HT02-4-30 76 58 117 HT02-4-31 63 66 77 HT02-4-32 26 21 36 HT02-4-33 71 88 75 HT02-4-34 186 201 403 HT02-4-35 100 80 169 HT02-4-36 256 264 380 HT02-4-37 86 54 149 HT02-4-38 109 83 172 HT02-4-39 77 37 130 HT02-4-40 90 81 165 HT02-4-41 68 82 70 HT02-4-42 80 91 95 HT02-4-43 50 59 57 HT02-4-44 114 139 133 HT02-4-45 55 50 76 HT02-4-46 51 45 67 HT02-4-47 51 48 65 HT02-4-48 222 212 302 HT02-4-49 102 129 111 HT02-4-50 52 34 89

Th/U

207

1.63 1.39 0.95 0.30 0.51 0.17 0.57

Pb/206Pb 0.2968 0.1724 0.1804 0.1861 0.1627 0.3542 0.1734

0.0022 0.0009 0.0015 0.0012 0.0009 0.0067 0.0010

1.12 1.00 0.49 0.75 0.91 0.80 0.20 0.45 1.15 0.38 0.74 0.55 0.60 0.51 0.74 0.39 1.34 0.34 0.76 1.20 1.17 0.44 0.73 0.34 1.07 0.44 0.46 0.90 0.54 0.49 0.86 0.57 1.17 0.50 0.48 0.70 0.36 0.48 0.28 0.49 1.18 0.96 1.03 1.05 0.66 0.68 0.75 0.70 1.17 0.38

0.1650 0.1675 0.1640 0.1663 0.1649 0.1681 0.1756 0.1689 0.1684 0.1858 0.1675 0.1673 0.1720 0.2463 0.1656 0.1731 0.1685 0.1699 0.2216 0.1703 0.1673 0.1677 0.1705 0.1960 0.1665 0.1668 0.1645 0.1662 0.1722 0.1682 0.1658 0.1669 0.1652 0.1724 0.1664 0.1809 0.1680 0.1688 0.1792 0.1641 0.1671 0.1666 0.1614 0.1655 0.1614 0.1662 0.1624 0.1718 0.1640 0.1613

0.0010 0.0009 0.0009 0.0009 0.0009 0.0009 0.0010 0.0009 0.0009 0.0010 0.0009 0.0009 0.0009 0.0016 0.0010 0.0009 0.0009 0.0009 0.0012 0.0009 0.0009 0.0009 0.0009 0.0010 0.0009 0.0013 0.0009 0.0009 0.0010 0.0009 0.0009 0.0010 0.0009 0.0010 0.0010 0.0010 0.0009 0.0009 0.0010 0.0009 0.0009 0.0009 0.0009 0.0009 0.0009 0.0010 0.0010 0.0010 0.0010 0.0010

1σ

Isotopic ratio 207 1σ Pb/235U 5.4274 0.0909 9.2531 0.1095 12.6449 0.2657 5.5042 0.0864 10.8636 0.1387 16.6098 1.6004 11.7347 0.1414 10.8682 11.1628 10.7090 10.9437 10.8340 11.2332 12.0369 11.2866 11.2693 13.3577 11.1177 11.1628 11.7057 21.5228 10.7990 10.0844 11.0425 11.3605 18.0336 10.3894 11.1377 11.0924 11.4907 14.7487 11.0725 7.8617 8.4169 10.9045 6.9078 11.2515 10.9317 11.5267 10.7702 8.1172 11.1067 10.0370 11.1551 11.2265 12.6049 9.1015 11.2729 11.0628 10.7021 10.7958 11.0132 11.5378 11.0549 11.6857 10.7452 10.6818

0.1094 0.1205 0.0952 0.1155 0.1414 0.1030 0.1195 0.1216 0.1098 0.1429 0.1207 0.1123 0.1142 0.2858 0.1455 0.1028 0.1299 0.1106 0.1777 0.1201 0.1343 0.1595 0.1017 0.1739 0.1182 0.2208 0.1691 0.1092 0.1138 0.1099 0.1085 0.1899 0.1085 0.1235 0.1891 0.0981 0.1156 0.1074 0.1386 0.0854 0.1125 0.1429 0.1280 0.2110 0.1233 0.1320 0.1182 0.1326 0.1128 0.1071

206

Pb/238U 0.1331 0.3893 0.5058 0.2141 0.4841 0.3354 0.4907 0.4782 0.4831 0.4732 0.4769 0.4765 0.4844 0.4981 0.4844 0.4851 0.5213 0.4811 0.4838 0.4934 0.6326 0.4729 0.4221 0.4749 0.4844 0.5898 0.4426 0.4829 0.4799 0.4886 0.5456 0.4822 0.3360 0.3701 0.4757 0.2917 0.4850 0.4781 0.5010 0.4727 0.3407 0.4855 0.4022 0.4814 0.4824 0.5100 0.4022 0.4893 0.4811 0.4810 0.4736 0.4946 0.5033 0.4936 0.4931 0.4748 0.4801

0.0025 0.0046 0.0090 0.0030 0.0060 0.0321 0.0058

Pb/206Pb 3454 2581 2657 2709 2484 3725 2591

11 9 13 11 9 29 14

0.0049 0.0051 0.0040 0.0049 0.0062 0.0043 0.0056 0.0053 0.0047 0.0056 0.0052 0.0050 0.0049 0.0076 0.0059 0.0042 0.0055 0.0046 0.0057 0.0052 0.0060 0.0070 0.0043 0.0065 0.0052 0.0082 0.0071 0.0048 0.0052 0.0047 0.0047 0.0086 0.0047 0.0048 0.0088 0.0039 0.0050 0.0047 0.0056 0.0037 0.0049 0.0060 0.0057 0.0094 0.0055 0.0057 0.0052 0.0056 0.0048 0.0048

2507 2533 2498 2521 2506 2539 2613 2547 2542 2705 2533 2531 2577 3161 2513 2587 2542 2556 2992 2561 2531 2534 2563 2794 2524 2528 2502 2519 2579 2540 2517 2527 2510 2581 2521 2661 2538 2546 2646 2498 2529 2524 2470 2513 2472 2519 2481 2576 2498 2469

9 9 9 5 9 8 10 14 9 9 9 9 9 10 10 5 9 9 14 9 9 9 9 9 9 13 15 9 10 10 9 10 9 9 4 9 9 9 9 10 9 42 10 9 10 10 9 11 10 9

1σ

207

1σ

Age (Ma) 207 Pb/235U 1σ 1889 14 2363 11 2654 20 1901 13 2512 12 2913 93 2583 11 2512 2537 2498 2518 2509 2543 2607 2547 2546 2705 2533 2537 2581 3162 2506 2443 2527 2553 2992 2470 2535 2531 2564 2799 2529 2215 2277 2515 2100 2544 2517 2567 2503 2244 2532 2438 2536 2542 2651 2348 2546 2528 2498 2506 2524 2568 2528 2580 2501 2496

9 10 8 10 12 9 9 10 9 10 10 9 9 13 13 9 11 9 9 11 11 13 8 11 10 25 18 9 15 9 9 15 9 14 16 9 10 9 10 9 9 12 11 18 10 11 10 11 10 9

206

Pb/238U 1σ 806 14 2119 21 2639 38 1250 16 2545 26 1864 155 2574 25 2520 2541 2497 2514 2512 2546 2606 2546 2549 2705 2532 2544 2585 3160 2496 2270 2505 2547 2989 2362 2540 2527 2564 2807 2537 1868 2030 2509 1650 2549 2519 2618 2496 1890 2551 2179 2533 2538 2657 2179 2568 2532 2531 2499 2591 2628 2586 2584 2505 2528

22 22 18 21 27 19 24 23 21 24 22 22 21 30 26 19 24 20 23 23 26 30 19 27 22 40 33 21 26 20 20 37 21 23 38 18 22 20 24 17 21 26 25 41 24 24 22 24 21 21

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1807

Figure 7 U-Pb concordia and histogram with frequency distribution diagrams of zircons from the conglomerate quartzite pebbles in the Sijizhuang Formation of the Hutuo Group.

al. [21, 22, 41, 74] and Zhang et al. [72], the Hutuo Group was regarded as a back-arc assemblage of clastic rocks interlayered with volcanic rocks that formed on the margin of the Eastern Block, in a regime of Paleoproterozoic (2500– 1850 Ma) easterly-dipping subduction [74, 75]. In contrast, Kusky and Li [5], Kusky et al. [6], and Li and Kusky [7] considered that the Doucun Subgroup of the Hutuo Group and the Gaofan Subgroup of the Wutai Group as the Neoarchean deposition in a foreland basin on the Eastern Block, whereas the Dongye Subgroup represented the rift to shallow basin environment in the early to middle Paleoproterozoic (2450–2000 Ma), and the Guojiazhai Subgroup represented a late Paleoproterozoic (2000–1900 Ma) foreland basin deposits in the middle-south of the NCC. Based on the lithostratigraphy, particularly the presence of conglomerate, arkosic sandstone, pelite, and massive carbonates, the Doucun to Dongye Subgroups have been interpreted as a transgressive environment [48], with deposition in an extensional tectonic setting [26, 47]. The Guojiazhai Subgroup consists mainly of medium- to coarse-grained sandstones and conglomerates, with characteristics of molasse [47, 49]. The eruption age of 2140±14 Ma for a basaltic andesite indicates that the Hutuo Group was deposited in the middle Paleoproterozoic, not the end

of the Neoarchean [27]. Consequently, the tectonic evolution model proposed by Kusky and Li [5], Kusky et al. [6], and Li and Kusky [7] is inconsistent with the established age and lithostratigraphy of the Hutuo Group. In the basal conglomerate of the Hutuo Group, some BIF pebbles were metamorphosed to amphibolite facies [76], inconsistent with their matrix, which is only metamorphosed to low greenschist facies (Figure 4(b), (d) and (g)). Additionally, these BIF pebbles are more strongly deformed than their matrix (Figure 4(h)). This also supports that some sedimentary components of the Hutuo Group are derived from a previously metamorphosed and deformed terrane [47, 48]. Basalts of the Doucun-Dongye Subgroups are rich in Ti, with no differentiation between Nb and Ta, higher ratios of Zr/Hf. This suggests a rift-related setting rather than one related to arc volcanism [26]. This reinforces other evidence for a (2200–2100 Ma) rift setting [27]. The Guojiazhai Subgroup was deposited at the closure of the rift, unconformably overlyies the Dongye Subgroup, and formed in a different tectonic setting to the Doucun-Dongye Subgroups. As they are unrelated, we suggest that the Guojiazhai Subgroup should be separated from the Hutuo Group and renamed it as the Guojiazhai Group [28].

1808 Table 3

Du L L, et al.

Sci China Earth Sci


Zircon Lu-Hf isotopic analyses on granite pebbles and quartzite pebbles of Sijizhuang Formation conglomerates a)

176 176 Spot No. Age (Ma) 176Yb/177Hf Lu/177Hf Hf/177Hf Strong deformed medium- to fine-grained granite pebble (HT02-3) HT02-3-1 2552 0.041163 0.000808 0.281293 HT02-3-2 2511 0.019046 0.000440 0.281331 HT02-3-3 2508 0.012485 0.000305 0.281286 HT02-3-4 2519 0.019311 0.000457 0.281363 HT02-3-5 2502 0.030602 0.000925 0.281281 HT02-3-6 2507 0.018213 0.000428 0.281373 HT02-3-7 2544 0.017464 0.000401 0.281371 HT02-3-8 2509 0.044740 0.000991 0.281352 HT02-3-9 2511 0.015693 0.000500 0.281296 HT02-3-10 2490 0.031729 0.000736 0.281284 HT02-3-11 2517 0.038013 0.001288 0.281351 HT02-3-12 2514 0.035630 0.000985 0.281401 HT02-3-13 2534 0.018015 0.000492 0.281290 HT02-3-14 2456 0.061264 0.001811 0.281352 HT02-3-15 2497 0.034870 0.000945 0.281341 HT02-3-16 2521 0.022701 0.000572 0.281323 HT02-3-17 2524 0.018943 0.000577 0.281387 HT02-3-18 2524 0.015054 0.000418 0.281391 HT02-3-20 2550 0.025151 0.000720 0.281339 Chlorite bearing medium- to coarse-grained granite pebble (HT11-1) HT11-1-1 2550 0.017422 0.000409 0.281390 HT11-1-2 2486 0.019763 0.000460 0.281335 HT11-1-3 2537 0.023211 0.000539 0.281346 HT11-1-4 2519 0.048874 0.001060 0.281389 HT11-1-5 2544 0.016732 0.000413 0.281330 HT11-1-6 2519 0.021409 0.000516 0.281323 HT11-1-7 2510 0.022073 0.000540 0.281350 HT11-1-8 2523 0.018362 0.000466 0.281320 HT11-1-9 2514 0.016797 0.000465 0.281307 HT11-1-10 2574 0.022320 0.000551 0.281365 HT11-1-11 2534 0.016622 0.000427 0.281341 HT11-1-12 2529 0.021632 0.000551 0.281378 HT11-1-13 2547 0.018415 0.000495 0.281331 HT11-1-14 2538 0.024697 0.000688 0.281361 HT11-1-15 2538 0.015644 0.000437 0.281320 HT11-1-16 2511 0.013522 0.000354 0.281326 HT11-1-17 2536 0.022864 0.000603 0.281400 HT11-1-18 2518 0.018599 0.000544 0.281348 HT11-1-19 2542 0.017229 0.000482 0.281324 HT11-1-20 2498 0.023496 0.000685 0.281341 Quartzite pebble (HT02-1) HT02-1-1 2506 0.030422 0.000566 0.281303 HT02-1-2 2502 0.025967 0.000539 0.281252 HT02-1-3 2777 0.034569 0.000689 0.281200 HT02-1-4 2518 0.034356 0.000684 0.281313 HT02-1-5 2513 0.024019 0.000494 0.281328 HT02-1-6 2514 0.038488 0.000758 0.281282 HT02-1-7 2572 0.024010 0.000451 0.281303 HT02-1-8 2549 0.018203 0.000382 0.281236 HT02-1-9 2529 0.024630 0.000579 0.281230 HT02-1-10 2656 0.026788 0.000597 0.281265 HT02-1-11 2568 0.025452 0.000506 0.281307 HT02-1-12 2592 0.045376 0.001013 0.281241 HT02-1-13 2672 0.020975 0.000492 0.281214

2σ

176

Hf/177Hfi

εHf(0)

εHf(t)

2σ

TDM1

TDM2

fLu/Hf

0.000016 0.000019 0.000020 0.000018 0.000022 0.000020 0.000017 0.000021 0.000025 0.000021 0.000024 0.000023 0.000021 0.000022 0.000023 0.000022 0.000019 0.000023 0.000021

0.281253 0.281310 0.281272 0.281341 0.281237 0.281352 0.281352 0.281305 0.281272 0.281249 0.281289 0.281354 0.281266 0.281267 0.281296 0.281296 0.281360 0.281371 0.281304

–52.3 –51.0 –52.5 –49.8 –52.7 –49.5 –49.5 –50.2 –52.2 –52.6 –50.3 –48.5 –52.4 –50.2 –50.6 –51.2 –49.0 –48.8 –50.7

3.60 4.67 3.25 5.95 1.86 6.08 6.92 4.44 3.33 2.02 4.06 6.31 3.64 1.87 3.85 4.41 6.74 7.14 5.36

0.56 0.69 0.72 0.65 0.77 0.71 0.59 0.76 0.89 0.74 0.87 0.82 0.76 0.78 0.82 0.77 0.66 0.80 0.73

2719 2642 2692 2600 2743 2585 2585 2650 2693 2725 2673 2583 2700 2709 2663 2661 2575 2559 2650

2767 2681 2749 2624 2812 2609 2597 2691 2747 2794 2716 2603 2751 2774 2710 2702 2590 2570 2679

–0.98 –0.99 –0.99 –0.99 –0.97 –0.99 –0.99 –0.97 –0.98 –0.98 –0.96 –0.97 –0.99 –0.95 –0.97 –0.98 –0.98 –0.99 –0.98

0.000017 0.000015 0.000016 0.000018 0.000016 0.000016 0.000017 0.000017 0.000015 0.000016 0.000019 0.000019 0.000017 0.000018 0.000016 0.000019 0.000019 0.000024 0.000017 0.000020

0.281370 0.281313 0.281320 0.281338 0.281310 0.281298 0.281324 0.281298 0.281285 0.281338 0.281321 0.281351 0.281307 0.281328 0.281298 0.281309 0.281371 0.281321 0.281301 0.281308

–48.9 –50.8 –50.4 –48.9 –51.0 –51.3 –50.3 –51.3 –51.8 –49.8 –50.6 –49.3 –51.0 –49.9 –51.4 –51.1 48.5 –50.4 –51.2 –50.6

7.71 4.20 5.64 5.85 5.42 4.43 5.15 4.53 3.86 7.12 5.58 6.56 5.39 5.94 4.89 4.64 7.43 5.24 5.07 4.32

0.61 0.54 0.56 0.64 0.58 0.56 0.62 0.59 0.54 0.57 0.69 0.68 0.59 0.63 0.56 0.67 0.67 0.84 0.61 0.72

2560 2638 2628 2605 2641 2658 2623 2657 2675 2603 2627 2586 2646 2617 2657 2642 2559 2626 2653 2645

2564 2684 2655 2630 2671 2700 2657 2698 2723 2612 2655 2603 2675 2641 2692 2683 2566 2659 2687 2688

–0.99 –0.99 –0.98 –0.97 –0.99 –0.98 –0.98 –0.99 –0.99 –0.98 –0.99 –0.98 –0.99 –0.98 –0.99 –0.99 –0.98 –0.98 –0.99 –0.98

0.000014 0.000016 0.000016 0.000021 0.000023 0.000020 0.000019 0.000025 0.000020 0.000019 0.000020 0.000020 0.000022

0.281276 0.281226 0.281163 0.281280 0.281304 0.281245 0.281281 0.281217 0.281202 0.281234 0.281282 0.281190 0.281188

–52.0 –53.8 –55.6 –51.6 –51.1 –52.7 –52.0 –54.3 –54.5 –53.3 –51.8 –54.2 –55.1

3.34 1.48 5.62 3.78 4.51 2.44 5.05 2.25 1.23 5.33 5.00 2.30 4.09

0.48 0.57 0.57 0.75 0.80 0.70 0.66 0.88 0.70 0.69 0.72 0.71 0.79

2688 2755 2836 2682 2649 2730 2680 2765 2788 2741 2678 2804 2803

2743 2831 2853 2731 2690 2793 2713 2831 2865 2767 2711 2864 2842

–0.98 –0.98 –0.98 –0.98 –0.99 –0.98 –0.99 –0.99 –0.98 –0.98 –0.98 –0.97 –0.99


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(Continued) Spot No. Age (Ma) 176Yb/177Hf HT02-1-14 2513 0.043235 HT02-1-15 2546 0.042973 HT02-1-16 2502 0.063418 HT02-1-17 2516 0.058399 HT02-1-18 2494 0.022368 HT02-1-19 2675 0.040650 HT02-1-20 2656 0.034460 HT02-1-21 2672 0.077965 HT02-1-22 2566 0.031735 HT02-1-23 2507 0.029283 HT02-1-24 2509 0.068456 HT02-1-25 2681 0.020111 HT02-1-26 2527 0.067979 HT02-1-27 2580 0.021052 HT02-1-28 2540 0.037208 HT02-1-29 2509 0.013980 HT02-1-30 2740 0.026688 HT02-1-31 2534 0.036787 HT02-1-32 2494 0.028424 HT02-1-33 2497 0.031115 HT02-1-34 2576 0.018210 HT02-1-35 2520 0.031266 HT02-1-36 2561 0.060262 HT02-1-37 2515 0.031269 HT02-1-38 2643 0.086517 HT02-1-39 2700 0.037751 HT02-1-40 2544 0.067793 HT02-1-41 2525 0.038925 HT02-1-42 3602 0.076890 HT02-1-43 2532 0.023347 HT02-1-44 2552 0.098277 HT02-1-45 2680 0.089755 HT02-1-46 2598 0.052572 HT02-1-47 2522 0.022485 HT02-1-48 2787 0.136915 HT02-1-49 2595 0.095611 HT02-1-50 2513 0.048503 HT02-1-51 2502 0.037224 HT02-1-52 2631 0.027279 HT02-1-53 2511 0.040202 HT02-1-54 3454 0.044640 HT02-1-55 2581 0.086140 HT02-1-56 2657 0.061557 HT02-1-57 2709 0.067111 HT02-1-58 2484 0.057750 HT02-1-59 3725 0.063796 HT02-1-60 2591 0.072554 Quartzite pebble(HT02-4) HT02-4-1 2507 0.061290 HT02-4-2 2533 0.040841 HT02-4-3 2498 0.035462 HT02-4-4 2521 0.046117 HT02-4-5 2506 0.033023 HT02-4-6 2539 0.042252 HT02-4-7 2613 0.047348 HT02-4-8 2547 0.046614

Lu/177Hf 0.000816 0.001324 0.001316 0.001188 0.000461 0.000800 0.000762 0.001508 0.000629 0.000587 0.001502 0.000455 0.001369 0.000477 0.000698 0.000288 0.000575 0.000862 0.000554 0.000610 0.000386 0.000656 0.001155 0.000648 0.001877 0.000640 0.001106 0.000674 0.001653 0.000491 0.001890 0.001541 0.000916 0.000431 0.002512 0.001910 0.000976 0.000653 0.000505 0.000837 0.000856 0.001607 0.001167 0.001040 0.001017 0.001087 0.001407 0.001083 0.000848 0.000628 0.000832 0.000643 0.000748 0.000941 0.000776

Hf/177Hf 0.281263 0.281177 0.281250 0.281194 0.281254 0.281305 0.281249 0.281274 0.281259 0.281377 0.281391 0.281186 0.281398 0.281248 0.281312 0.281244 0.281244 0.281316 0.281274 0.281338 0.281226 0.281243 0.281380 0.281311 0.281287 0.281259 0.281335 0.281234 0.281191 0.281268 0.281386 0.281308 0.281273 0.281197 0.281332 0.281306 0.281293 0.281218 0.281269 0.281260 0.281250 0.281270 0.281286 0.281400 0.281362 0.281358 0.281323 0.281306 0.281240 0.281384 0.281282 0.281157 0.281271 0.281255 0.281360

εHf(0)

εHf(t)

2σ

0.000023 0.000022 0.000025 0.000025 0.000022 0.000023 0.000020 0.000022 0.000021 0.000022 0.000027 0.000020 0.000024 0.000022 0.000025 0.000026 0.000022 0.000020 0.000020 0.000019 0.000022 0.000027 0.000025 0.000024 0.000022 0.000020 0.000023 0.000032 0.000024 0.000024 0.000027 0.000026 0.000021 0.000022 0.000023 0.000026 0.000026 0.000030 0.000019 0.000028 0.000032 0.000024 0.000021 0.000021 0.000025 0.000024 0.000027

Hf/177Hfi 0.281224 0.281112 0.281187 0.281137 0.281232 0.281264 0.281211 0.281197 0.281228 0.281349 0.281319 0.281162 0.281332 0.281224 0.281278 0.281231 0.281214 0.281274 0.281248 0.281309 0.281207 0.281211 0.281324 0.281280 0.281192 0.281226 0.281281 0.281202 0.281076 0.281244 0.281294 0.281229 0.281227 0.281176 0.281197 0.281211 0.281246 0.281187 0.281244 0.281220 0.281193 0.281191 0.281227 0.281347 0.281314 0.281280 0.281253

–53.4 –56.4 –53.8 –55.8 –53.7 –51.9 –53.8 –53.0 –53.5 –49.3 –48.8 –56.1 –48.6 –53.9 –51.6 –54.0 –54.0 –51.5 –53.0 –50.7 –54.7 –54.1 –49.2 –51.7 –52.5 –53.5 –50.8 –54.4 –55.9 –53.2 –49.0 –51.8 –53.0 –55.7 –50.9 –51.8 –52.3 –55.0 –53.1 –53.5 –53.8 –53.1 –52.6 –48.5 –49.9 –50.0 –51.2

1.68 –1.53 0.10 –1.34 1.50 6.84 4.51 4.37 3.02 5.97 4.97 3.35 5.83 3.22 4.23 1.81 6.55 3.93 2.08 4.33 2.50 1.38 6.31 3.68 3.53 6.07 4.41 1.14 21.85 2.81 5.04 5.69 3.75 0.17 7.06 3.09 2.45 0.08 5.10 1.47 22.52 2.05 5.10 10.57 4.18 32.07 4.51

0.000032 0.000024 0.000022 0.000023 0.000028 0.000020 0.000025 0.000026

0.281254 0.281199 0.281354 0.281242 0.281127 0.281235 0.281208 0.281322

–51.9 –54.2 –49.1 –52.7 –57.1 –53.1 –53.6 –49.9

2.58 1.23 5.94 2.51 –1.95 2.63 3.42 5.95

2σ

176

0.81 0.78 0.88 0.90 0.78 0.81 0.73 0.78 0.76 0.78 0.97 0.73 0.85 0.80 0.89 0.91 0.80 0.70 0.70 0.69 0.78 0.97 0.87 0.86 0.79 0.70 0.83 1.15 0.87 0.86 0.97 0.91 0.75 0.80 0.82 0.91 0.94 1.08 0.66 1.01 1.12 0.85 0.74 0.76 0.88 0.86 0.96

TDM1 2759 2914 2813 2880 2747 2702 2774 2795 2752 2589 2632 2837 2614 2756 2684 2747 2768 2691 2726 2643 2779 2775 2624 2683 2804 2752 2682 2788 2921 2730 2667 2750 2753 2821 2789 2780 2730 2809 2729 2765 2780 2807 2753 2588 2639 2649 2720

TDM2 2830 3015 2898 2981 2823 2709 2808 2828 2807 2614 2665 2885 2637 2809 2727 2820 2777 2737 2795 2687 2841 2850 2641 2733 2845 2767 2721 2866 2742 2790 2696 2769 2798 2911 2790 2827 2792 2899 2758 2838 2586 2867 2780 2554 2683 2346 2755

fLu/Hf –0.98 –0.96 –0.96 –0.96 –0.99 –0.98 –0.98 –0.95 –0.98 –0.98 –0.95 –0.99 –0.96 –0.99 –0.98 –0.99 –0.98 –0.97 –0.98 –0.98 –0.99 –0.98 –0.97 –0.98 –0.94 –0.98 –0.97 –0.98 –0.95 –0.99 –0.94 –0.95 –0.97 –0.99 –0.92 –0.94 –0.97 –0.98 –0.98 –0.97 –0.97 –0.95 –0.96 –0.97 –0.97 –0.97 –0.96

1.14 0.85 0.78 0.82 0.99 0.73 0.88 0.91

2721 2793 2583 2734 2889 2744 2779 2625

2781 2868 2608 2796 3002 2804 2826 2648

–0.97 –0.97 –0.98 –0.97 –0.98 –0.98 –0.97 –0.98


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(Continued) Spot No. HT02-4-9 HT02-4-10 HT02-4-11 HT02-4-12 HT02-4-13 HT02-4-14 HT02-4-15 HT02-4-16 HT02-4-17 HT02-4-18 HT02-4-19 HT02-4-20 HT02-4-21 HT02-4-22 HT02-4-23 HT02-4-24 HT02-4-25 HT02-4-26 HT02-4-27 HT02-4-28 HT02-4-29 HT02-4-30 HT02-4-31 HT02-4-32 HT02-4-33 HT02-4-34 HT02-4-35 HT02-4-36 HT02-4-37 HT02-4-38 HT02-4-39 HT02-4-40 HT02-4-41 HT02-4-42 HT02-4-43 HT02-4-44 HT02-4-45 HT02-4-46 HT02-4-47 HT02-4-48 HT02-4-49 HT02-4-50

Age (Ma) 2542 2705 2533 2531 2577 3161 2513 2587 2542 2556 2992 2561 2531 2534 2563 2794 2524 2528 2502 2519 2579 2540 2517 2527 2510 2581 2521 2661 2538 2546 2646 2498 2529 2524 2470 2513 2472 2519 2481 2576 2498 2469

176

Yb/177Hf 0.041112 0.047994 0.038334 0.035069 0.053382 0.038513 0.045237 0.054416 0.043235 0.030881 0.120329 0.053797 0.047894 0.038103 0.064932 0.046271 0.045376 0.027734 0.043317 0.029413 0.078018 0.038886 0.066268 0.061574 0.041665 0.064641 0.098520 0.083363 0.060070 0.084618 0.085823 0.060439 0.026513 0.071618 0.041807 0.078416 0.049056 0.046818 0.053698 0.036829 0.032215 0.036174

Lu/177Hf 0.000736 0.000913 0.000748 0.000623 0.001085 0.000753 0.000800 0.000933 0.000778 0.000546 0.002256 0.000960 0.000908 0.000706 0.001133 0.000847 0.000853 0.000518 0.000863 0.000542 0.001426 0.000712 0.001314 0.001123 0.000692 0.001165 0.001791 0.001475 0.001322 0.001722 0.002084 0.001012 0.000508 0.001513 0.000781 0.001529 0.001060 0.000869 0.000955 0.000762 0.000581 0.000681

Hf/177Hf 0.281371 0.281259 0.281295 0.281304 0.281271 0.280857 0.281167 0.281356 0.281280 0.281295 0.281188 0.281307 0.281328 0.281321 0.281212 0.281246 0.281249 0.281320 0.281291 0.281244 0.281337 0.281331 0.281323 0.281291 0.281300 0.281371 0.281384 0.281435 0.281333 0.281396 0.281369 0.281365 0.281235 0.281294 0.281310 0.281287 0.281133 0.281348 0.281225 0.281260 0.281359 0.281330

176

2σ 0.000023 0.000028 0.000027 0.000020 0.000028 0.000019 0.000026 0.000024 0.000023 0.000021 0.000024 0.000022 0.000022 0.000023 0.000022 0.000022 0.000021 0.000021 0.000022 0.000020 0.000023 0.000022 0.000026 0.000028 0.000021 0.000019 0.000031 0.000025 0.000036 0.000024 0.000023 0.000021 0.000021 0.000026 0.000025 0.000026 0.000027 0.000020 0.000023 0.000019 0.000022 0.000023

Hf/177Hfi 0.281336 0.281211 0.281259 0.281274 0.281218 0.280811 0.281128 0.281310 0.281242 0.281268 0.281058 0.281260 0.281284 0.281286 0.281156 0.281201 0.281208 0.281294 0.281249 0.281218 0.281267 0.281297 0.281260 0.281237 0.281266 0.281314 0.281298 0.281360 0.281269 0.281312 0.281264 0.281316 0.281211 0.281221 0.281273 0.281214 0.281082 0.281307 0.281180 0.281223 0.281331 0.281298

εHf(0)

εHf(t)

2σ

–49.5 –53.5 –52.2 –51.9 –53.1 –67.7 –56.8 –50.1 –52.8 –52.2 –56.0 –51.8 –51.1 –51.3 –55.2 –53.9 –53.9 –51.4 –52.4 –54.0 –50.7 –51.0 –51.2 –52.4 –52.1 –49.5 –49.1 –47.3 –50.9 –48.7 –49.6 –49.8 –54.3 –52.3 –51.7 –52.5 –58.0 –50.3 –54.7 –53.5 –50.0 –51.0

6.30 5.65 3.36 3.86 2.93 2.07 –1.74 6.43 2.98 4.23 6.89 4.06 4.22 4.38 0.39 7.38 1.34 4.51 2.31 1.60 4.71 4.87 3.03 2.45 3.09 6.42 4.49 9.93 3.83 5.56 6.16 4.59 1.55 1.82 2.43 1.30 –4.31 4.75 –0.64 3.07 5.12 3.29

0.83 0.99 0.96 0.71 0.99 0.68 0.93 0.84 0.80 0.74 0.85 0.77 0.77 0.83 0.78 0.79 0.76 0.73 0.78 0.72 0.80 0.79 0.92 1.00 0.74 0.67 1.12 0.90 1.27 0.85 0.83 0.76 0.74 0.93 0.89 0.92 0.97 0.71 0.82 0.67 0.79 0.81

TDM1 2607 2772 2711 2690 2767 3301 2888 2642 2734 2698 2972 2710 2678 2674 2852 2784 2781 2662 2725 2765 2701 2660 2713 2743 2701 2637 2662 2570 2700 2641 2704 2635 2775 2767 2693 2778 2954 2647 2820 2759 2614 2659

TDM2 2626 2792 2763 2738 2821 3341 2998 2657 2790 2740 2967 2752 2720 2715 2933 2781 2856 2703 2790 2839 2734 2695 2766 2803 2758 2652 2699 2546 2745 2666 2719 2674 2849 2832 2758 2848 3090 2684 2917 2813 2648 2715

fLu/Hf –0.98 –0.97 –0.98 –0.98 –0.97 –0.98 –0.98 –0.97 –0.98 –0.98 –0.93 –0.97 –0.97 –0.98 –0.97 –0.97 –0.97 –0.98 –0.97 –0.98 –0.96 –0.98 –0.96 –0.97 –0.98 –0.96 –0.95 –0.96 –0.96 –0.95 –0.94 –0.97 –0.98 –0.95 –0.98 –0.95 –0.97 –0.97 –0.97 –0.98 –0.98 –0.98

a) Calculation parameters on zircon Lu-Hf isotopes as follows: Decay constant of 176Lu is 1.867×10-11, 176Lu/177Hf and 176Hf/177Hf ratios of CHUR are 0.0332 and 0.282772 respectively [68], 176Lu/177Hf and 176Hf/177Hf ratios of DM are 0.0384 and 0.28325 respectively [69], and fLu/Hf of felsic crust is –0.72 [70].

5.3

~2.5 Ga crustal growth in Wutai Mountains area

In the Sijizhuang Formation conglomerates, the zircons of the strongly deformed medium- to fine-grained granite pebble (HT02-3) have positive values of εHf(t), with the εHf(t) values of some spots ( 4, 6, 7, 12, 17 and 18 ) close to that of contemporaneous depleted mantle (i.e., with U-Pb ages agreeing with TDM1 model ages, Table 3, Figure 8(a)). Other zircons also have positive values of εHf(t) with narrow variation (1.86 to 5.36), but with the igneous U-Pb ages younger

than that of the Hf model ages by ≤200 million years. Thus, these zircons show uniform U-Pb ages, but a spread of Neoarchean Hf model ages (Figure 8(b)). This suggests that this granite consists of a mixture of material that was juvenile to the crust at ~2513 Ma combined with the remelting of material added to the crust ≤200 million years earlier. The zircons of medium- to coarse-grained granite pebble HT11-1 also have positive values of εHf(t) (Table 3, Figure 8(c)). A few analyzed spots, such as 1, 10, 12, and 17, have the value of ε Hf (t) close to that of contemporaneous

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Figure 8

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Pb/206Pb age vs. εHf(t) and TDM2 histogram diagrams of zircons from Hutuo Group Sijizhuang Formation conglomerate granite pebbles.

depleted mantle at ~2527 Ma, indicating juvenile crustal contributions. The other zircons also have positive εHf(t) values with narrow variation (3.86 to 5.94), with the zircon U-Pb ages younger than Hf model ages by 150 to 100 million years (Table 3, Figure 8(d)). This again shows the importance of a crust forming event at 2800–2700 Ma, with rocks formed during its being remelted at ~2527 Ma to contribute to the granite represented by sample HT11-1. The detrital quartzites (e.g., pebble HT02-1) were derived from Neoarchean granitic rocks [77]. Thus, for sample HT02-1, the zircons with U-Pb ages concentrating at 2600–2490 Ma have εHf(t) values between –1.53 and 6.31, with most being positive (Table 3, Figure 9(a)), with the two-stage TDM2 Hf model ages concentrating at 2900–2700 Ma (Table 3, Figure 9(b)). For sample HT02-4, zircons with U-Pb ages concentrating at 2620–2470 Ma have εHf(t) values of –4.31 to 6.43, with most having positive values (Table 3, Figure 9(c)), with two stage TDM2 Hf model ages concentrated at mainly 2850–2700 Ma (Table 3, Figure 9(d)). This indicates that the ~2500 Ma sources of the quartzite zircons are dominated by material separated from the depleted mantle only a short time before, indicating an important episode of Neoarchean crustal growth. Our new results are consistent with the results of Nd and zircon Hf isotopic studies in other parts of the NCC [2, 9, 78–83]. Thus, the widely distributed ~2500 Ma granitoids of the

NCC are dominated by material that was juvenile material from ~2700 to 2500 Ma.

6 Conclusions From the zircon U-Pb chronology and Hf isotopes systematics of the Sijizhuang Formation conglomerates in the Hutuo Group, integrated with previous research, we can draw the following conclusions: (1) The Hutuo Group unconformably overlies an already deformed and metamorphosed Neoarchean basement in the Wutai Mountains, and was deposited in the middle Paleoproterozoic. Two granite pebbles from the Sijizhuang Formation have zircon U-Pb ages of 2513±8 and 2527±8 Ma, with these ages and their petrologic features consistent with those of the Wangjiahui grey phase granite and the Guangmingsi/Shifo granite. Zircons from two quartzite pebbles display a most obvious peak age of ~2500 Ma, indicating likely provenance from the Neoarchean Gaofan Subgroup. Therefore, the conglomerate in the Hutuo Group is derived from the erosion of the Wutai Group and contemporaneous Wutai granitoids, and has the characteristics of basal conglomerates. (2) The Hutuo Group was related to the formation and then closure of a middle to late Paleoproterozoic rift-related

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Figure 9 207Pb/206Pb age vs. εHf(t) and TDM2 histogram diagrams of zircons from the conglomerates quartzite pebbles in the Sijizhuang Formation of the Hutuo Group.

basin. Thus, the Doucun and Dongye subgroups were deposited at the extensional environment, with clastic sediments of local provenance [28], derived from the recycling of a Neoarchean orogenic belt rather than from a coeval Paleoproterozoic volcanic arc [30]. The Doucun-Dongye Subgroups probably are related to a broader 2200–2100 Ma rifting event across the NCC [27]. On the other hand, the Guojiazhai Subgroup was deposited unconformably on the Dongye Subgroup, with the closure of the rift. Therefore, the Guojiazhai Subgroup should be separated from the Hutuo Group and renamed as the Guojiazhai Group [28]. (3) The integrated zircon U-Pb and Hf zircon data show that both the reworking of ~2700 Ma crust and crustal growth at 2550–2500 Ma are important in the Wutai mountains Neoarchean basement source materials for the Sijizhaung Formation conglomerates.

search Work from Ministry of Science and Technology of China (Grant Nos. J0721 and J0907) and National Commission on Stratigraphy of China (Grant Nos. 1212010511702-01 and 1212011120142). 1

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We thank Prof. Liu Dunyi, Zhang Yuhai, Yang Zhiqing, Yang Chun, Zhou Hui, Zhou Liqin, and Sun Huiyi for the help with mount making, cathodoluminescence images, and SHRIMP dating. We also give thanks to Prof. Song Biao and Dr. Diwu Chunrong for the explanation on the zircon U-Pb ages and Hf isotopes, to Prof. Zhai Mingguo, Prof. Liu Shuwen, Dr. Yan Zhen, and Dr. Xue Zijian for their constructive reviews and comments, to Prof. Allen P. Nutman for checking and smoothing English manuscript, and to Dr. Nasdala for providing his M257 standard zircon. This work was supported by China Geological Survey (Grant Nos. 1212010611802, 1212010711815 and 1212011120152), National Natural Science Foundation of China (Grant No. 41172171), Basic Foundation of Scientific Re-

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