Detrital zircon ages and Hf isotopes of the early Paleozoic flysch sequence in the Chinese Altai, NW China: New constrains on depositional age, provenance and tectonic evolution

Tectonophysics 480 (2010) 213–231

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Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o

Detrital zircon ages and Hf isotopes of the early Paleozoic flysch sequence in the Chinese Altai, NW China: New constrains on depositional age, provenance and tectonic evolution Xiaoping Long a, Chao Yuan a,⁎, Min Sun b, Wenjiao Xiao c, Guochun Zhao b, Yujing Wang a, Keda Cai b, Xiaoping Xia b, Liewen Xie c a b c

Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 5 May 2009 Received in revised form 17 August 2009 Accepted 15 October 2009 Available online 27 October 2009 Keywords: Detrital zircon Hf isotope Provenance Tectonics Accretion complex Altai

a b s t r a c t Subduction–accretion complexes occur widely in the Central Asian Orogenic Belt (CAOB). Due to the scarcity of fossils, the depositional timing of the Habahe flysch sequence of the subduction–accretion complex in the Chinese Altai is poorly constrained, which gave rise to much controversy in understanding the time of the basement and the tectonic evolution of the Chinese Altai. U–Pb dating of detrital zircons from the Habahe sequence in the northwestern Chinese Altai reveals a young zircon population with a mean 206Pb/238U age around 438 Ma which, together with a mean 206Pb/238U age of 411 ± 5 Ma for the overlying rhyolite of the Dongxileke Formation, brackets the time of deposition of the sequence between early Silurian and early Devonian. The age of the Dongxileke rhyolite also indicates that the overlying Baihaba Formation possibly began to be deposited in the early Devonian, though U–Pb dating of detrital zircons from this formation gave a maximum depositional age of ∼ 438 Ma. The youngest detrital zircons and metamorphic grains of the Habahe sequence reveal different provenance to the sequence in the east. The youngest and metamorphic zircon grains, with early Paleozoic, Neoproterozoic and pre-Neoproterozoic populations, suggest a multisource for the Habahe sequence. The predominantly early Paleozoic zircons, characterized by concentric zoning, high Th/U ratios and euhedral shapes, imply that the sediments of the sequence were mostly derived from a proximal magmatic source. Based on the age patterns of the Neoproterozoic and pre-Neoproterozoic populations, the Tuva–Mongol Massif, along with adjacent island arcs and metamorphic belts, may be an alternative source region for the Habahe sequence. In view of new geochemical and chronological data for granitoids and advancement in the study of regional metamorphism in the Chinese Altai, we suggest a tectonic model of subduction beneath a huge subduction–accretion complex for the evolution of the Chinese Altai in the early Paleozoic. © 2009 Elsevier B.V. All rights reserved.

1. Introduction As one of the largest accretionary orogens in the world, the Central Asian Orogenic Belt (CAOB) or Altaids, extending from the Urals in the west to the Pacific Ocean in the east and from Siberia in the north to the Tianshan in the south, represents the most significant crustal growth in the Phanerozoic (e.g. Zonenshain et al., 1990; Sengör et al., 1993; Yin and Nie, 1996; Buslov et al., 2001; Dobretsov et al., 2004; Xiao et al., 2004; Jahn, 2004; Windley et al., 2007; Kröner et al., 2008; Xiao et al., 2009). Its early history began in the early Neoproterozoic, and the orogenic process ended with closure of the Paleo-Asian Ocean

⁎ Corresponding author. Tel.: +86 20 8529 1780; fax: +86 20 8529 0130. E-mail address: [email protected] (C. Yuan). 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.10.013

in the early Permian (Dobretsov et al., 1995; Xiao et al., 2003). The entire orogenic belt consist of various accreted terranes, such as ophiolites, seamounts, island arcs and microcontinents (e.g. Sengör and Natal'in, 1996; Jahn, 2004; Windley et al., 2002; Badarch et al., 2002; Khain et al., 2003; Xiao et al., 2004; Dobretsov et al., 2004; Kuzmichev et al., 2005; Kröner et al., 2008; Xiao et al., 2009). Subduction–accretion complexes and arc massifs are the two dominant rock associations in the CAOB (Sengör et al., 1993). Vendian and Paleozoic subduction–accretion complexes outcrop widely and show the most important features of the accretionary orogen (Sengör and Natal'in, 1996). Clastic sedimentary rocks such as flysch sequences preserve valuable geochemical and isotopic information on their provenance, which is vital to reconstruct the accretionary history of the CAOB (Coleman, 1989; Sengör et al., 1993; Chen and Hsu, 1995; Badarch et al., 2002; Khain et al., 2003; Xiao et al., 2003; Dobretsov et al., 2004; Yakubchuk, 2004; Windley et al., 2007; Kelty et al., 2008; Xiao et al., 2008, 2009).

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In the Chinese Altai, low-grade metamorphic flysch sequences are widespread and mainly consist of terigenus quartzo-feldspathic, clastic turbidites (e.g. BGMRX, 1993). The Habahe Group is the oldest sequence and, therefore, its depositional age and tectonic setting are of great importance in understanding the tectonic history of the orogen (Windley et al., 2002). However, because of the lack of systematic stratigraphic and geochronological studies, the exact depositional age and tectonic setting still remains to be constrained. For instance, due to the scarcity of fossils, the Habahe Group was variously assigned to the Middle–Upper Ordovician (GCRSX, 1981), Sinian (Wang, 1983; Peng, 1989), or Sinian–Cambrian (BGMRX, 1993). In addition, the high-grade metamorphic equivalent of this group was considered to represent a Precambrian basement (Li et al., 1996; Li and Poliyangsiji, 2001; Li et al., 2006). Therefore, several models such as an accretionary prism (Sengör and Natal'in, 1996), a passive continental margin (He et al., 1990), an active continental margin (Windley et al., 2002, 2007; Yuan et al., 2007; Long et al., 2007; Sun et al., 2008; Long et al., 2008a; Xiao et al., 2009) and a Precambrian microcontinent (Dobretsov et al., 1995; Hu et al., 2000; Buslov et al., 2001; Li et al., 2006; Wang et al., 2009), have been proposed to explain the tectonic evolution of the Chinese Altai, but no consensus has been reached. Detrital zircon, a common heavy mineral in sedimentary rocks, can survive erosion, transport, diagenesis, metamorphism, and its age pattern may provide information on the evolutionary history of the source region (McLennan et al., 2001; Fedo et al., 2003). The age of the youngest detrital zircon constrains the maximum age of deposition (e.g. Nelson, 2001; Fedo et al., 2003). This approach has been successfully

applied to sedimentary sequences which biostratigraphy cannot work effectively (e.g. Guan et al., 2002; Griffin et al., 2004; Andersen et al., 2004; Payne et al., 2006; Xia et al., 2006; Rainbird and Davis, 2007; Morton et al., 2009). Moreover, the combination of U–Pb and Hf isotopic analysis of detrital zircons has proven to be a very powerful tool to understand the nature of sedimentary provenance and tectonic setting (e.g. Nelson, 2001; Griffin et al., 2004; Andersen et al., 2004; Payne et al., 2006; Wu et al., 2007; Cawood et al., 2007a,b; Flowerdew et al., 2007; Dickinson and Gehrels, 2009). In this paper, we present a systematic zircon U–Pb and Hf isotopic studies for sandstones and meta-sandstones from the Habahe flysch sequence and overlying rhyolite in the northwestern Chinese Altai. Our results provide new constraints on their depositional age, provenance and tectonic setting, and shed some new light on the tectonic evolution of the orogen. 2. Geological setting The Chinese Altai Orogen is situated between the Sayan orogen to the north and the Junggar basin to the south, extending from East Kazakhstan, through Russia and Northern Xinjiang of China to southern Mongolia (Coleman, 1989; Xiao et al., 1990; He et al., 1990; Federovskii et al., 1995; Windley et al., 2002; Xiao et al., 2004). It consists of six NW–SE trending terranes (e.g. He et al., 1990; Windley et al., 2002; Chen and Jahn, 2002; Xiao et al., 2004; Wang et al., 2006; Yuan et al., 2007; Briggs et al., 2007; Long et al., 2007; Sun et al., 2008), and a four-domain scheme was accepted in these studies (Fig. 1). The Central domain is the largest in the Chinese Altai. The early Paleozoic

Fig. 1. Simplified geological map of the Chinese Altai (modified from He et al., 1990; Windley et al., 2002). Domains: 1, North Altai; 2, Central Altai; 3, Qiongkuer; 4, Erqis. The locations of early Paleozoic granites are marked with their U–Pb ages. Inset shows the location of the map. Microcontinents: TM, Tuva–Mongol massif; SG, South Gobi microcontinent. Subduction-accretion complexes: AM, Altai–Mongolia; GA, Gorny Altai; RA, Rudny Altai; TS, Tien Shan. Modified after Sengör and Natal'in (1996).

X. Long et al. / Tectonophysics 480 (2010) 213–231

Habahe flysch sequence mostly outcrops in this domain and largely consists of thick marine-facies terrigenous clastic rocks such as sandstone, siltstone and mudstone. The sequence is folded with steep axial planes and is separated into small units by strike-slip faults (Windley et al., 2002). In some areas, the rocks were metamorphosed to lower greenschist-facies (Wang, 1983; Peng, 1989; He et al., 1990; Windley et al., 2002; Long et al., 2008a). The flysch sequence is overlain unconformably by felsic volcanic rocks of the Dongxileke Formation and marine-facies clastic rocks of the Baihaba Formation in the northwestern Chinese Altai (Fig. 2). To the southwest, the Qiongkuer and Erqis domains are separated from the Central domain by a series of thrust faults (Fig. 1). The Devonian Kurti ophiolite was recognized within the thrust zone, and a geochemical study on pillow lavas suggested a backarc basin environment (Xu et al., 2003; Zhang et al., 2003). The Qiongkuer and Erqis domains are mainly composed of late Paleozoic clastic and volcanic rocks which are divided into the early Devonian Kangbutiebao Formation and mid-Devonian Altai Formation, respectively (He et al., 1990; BGMRX, 1993; Windley et al., 2002). Recent geochemical studies of the clastic and volcanic rocks suggested that the two formations were deposited along an active continental margin (Chen et al., 2006; Long et al., 2008b). The late Paleozoic sedimentary rocks were intruded by post-Devonian arc-related granitic and mafic plutons and Permian postorogenic granites (Wang et al., 2006; Briggs et al., 2007). Small early Paleozoic granitic plutons (450–460 Ma) occur in the Qiongkuer and Erqis domains and also show arc-related features (Yuan et al., 2007; Briggs et al., 2007).

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3. Sample description and zircon structure 3.1. Stratigraphy and sample description All samples in this study were collected from the Baihaba–Kanas profile in the northwestern part of the Central domain (Figs 1 and 2). The detailed stratigraphy and petrography are presented in Table 1. The three Habahe samples were collected from sandstone, slate and siltstone of Units 1, 3 and 5, respectively (see Fig. 2). The sandstone samples are gray and medium-grained, consisting of detrital minerals (40–55%), lithic fragments (25–40%) and cement (<15%). The detrital minerals of the sandstone are mainly composed of quartz and minor plagioclase. The slate samples mainly consist of very fine-grained quartz (45–65%), feldspars (5–15%), biotite (<5%) and cement (10– 20%). The siltstone samples have framework compositions similar to the sandstone. The Dongxileke rhyolites are porphyritic, light-gray-colored rocks with sparse phenocrysts (<15%) and massive glassy matrix (>85%). Phenocrysts are euhedral and are predominately composed of plagioclase with minor quartz and biotite. In the study area, the samples have felsic phenocrysts varying from 5 to 12% and mafic biotite phenocrysts are generally <3%. Plagioclase phenocrysts are present in the form of subhedral laths, with clear albite-Carlsbad twins. The Baihaba sample was collected from a layered slate of the formation. The slate is gray and mainly consists of very fine-grained quartz, feldspars, biotite and cement, with mineral contents similar to those of the slate samples of the Habahe Group.

Fig. 2. (a) Geological map of the northwestern Chinese Altai Orogen. (b) Geological profile in this region, its location marked as dotted line A–B in the Fig. 2a.

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Table 1 Stratigraphy, petrography and zircon characteristics of sediments in the northwestern Chinese Altai. Strata

Unit

Thickness

Petrography

Zircon sample

Features

Baihaba Formation

–

1300 m

Gray limestone, slate and calcic siltstone with lentoid limestone

L07BH08: from slate

Dongxileke Formation

–

900 m

L07BH11: from rhyolite

Habahe Group

Unit 5

1600 m

L07BH01: from siltstone

Type-I: euhedral prismatic crystals with concentric oscillatory zoning

Unit 4 Unit 3

1300 m 2100 m

Green or light-colored purple rhyolite, andesite, andesitic agglomerate, andesitic breccia and felsite with tuff breccia Green fine-grained slate with layered siltstone and mudstone Green slate with layered siltstone Gray slate

Type-I and Type-II: similar to each type of zircons of the Habahe samples, respectively Type-I: euhedral prismatic crystals with concentric oscillatory zoning

L07BH27: from slate

Type-II: subrounded to rounded crystals without significant zoning

Unit 2

1700 m

Unit 1

1000 m

Gray or green fine-grained sandstone with layered siltstone Green fine-grained to middle-grained sandstone with layered siltstone and mudstone

Th/U

Nature

>0.4

Igneous origin

0.17–1.49

Igneous origin

0.06–0.09

Metamorphic origin

L07BH16: from sandstone

3.2. Zircon structure Detrital zircons from the Habahe Group and the Baihaba Formation are generally colorless and transparent. Two types of zircon can be recognized through CL imaging (Table 1). Type-I zircons (∼95%) are characterized by a euhedral prismatic shape, concentric oscillatory zoning (Fig. 3a, b and e) and high Th/U ratios (0.17–1.49), indicating an igneous origin; Type-II zircons (∼5%) are characterized by rounded shapes, low Th/U ratios (0.06–0.09) and light luminescence without significant zoning (Fig. 3c), implying a metamorphic origin. Zircons of sample L07BH11 collected from the Dongxileke Formation are colorless, transparent and euhedral in shape. The grains are

highly luminescent with concentric oscillatory zoning and have high Th/U ratios (>0.4), suggesting an igneous origin (Fig. 3d). 4. Analytical methods 4.1. Zircon separation and CL imaging Zircons were separated using heavy liquid and magnetic techniques, and then purified by hand picking under a binocular microscope. About 100 grains were randomly selected and mounted on adhesive tape then enclosed in epoxy resin and polished to about half their thickness. After being photographed under reflected and transmitted

Fig. 3. Representative CL images for zircons from the early Paleozoic sedimentary rocks in the northwestern Chinese Altai. Analytical spots and

206

Pb/238U ages are marked.

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light, the samples were prepared for U–Pb dating and Hf isotope analysis. In order to investigate the origin and structure of zircons and to choose target sites for U–Pb and Hf isotopic analyses, cathodoluminescence (CL) imaging was carried out using a JXA-8100 Electron Probe Microanalyzer with Mono CL3 Cathodoluminescence System for high resolution imaging and spectroscopy at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. CL images of typical zircon grains are presented in Fig. 3. 4.2. U–Pb dating Zircon U–Pb dating was performed by LA-ICP-MS at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. A Geolas-193 laser-ablation system equipped with a 193 nm ArFexcimer laser was used in connection with an ELAN6100 DRC ICP-MS. Helium was used as the carrier gas to enhance the transport efficiency of the ablated material. The helium carrier gas inside the ablation cell was mixed with argon gas before entering the ICP to maintain stable and optimum excitation conditions. The analyses were conducted with a beam diameter of 63 μm with a typical ablation time of about 30 s for 200 cycles of each measurement, a 10 Hz repetition rate, and a laser power of 100 mJ/pulse (Wu et al., 2006). Zircon 91500 was used as the standard, and the standard silicate glass NIST 610 was used to optimize the instrument. The detailed analytical technique is described in Yuan et al. (2004). The common-Pb correction followed the method described by Andersen (2002). U, Th and Pb concentrations were calibrated using 29Si as an internal standard and NIST 610 as reference material. The age calculation and concordia plots were made using ISOPLOT (version 3.0) (Ludwig, 2003). Because 206Pb/238U ages are generally more precise for younger ages, whereas 206Pb/207Pb ages are more precise for older ages, we rely on 206Pb/238U ages up to 1000 Ma and 206Pb/207Pb ages if the 206Pb/238U ages are older than 1000 Ma (Gehrels et al., 2006). The results are presented in Table 2.

217

2004). Mean 176Lu/177Hf ratio of 0.0093 for the upper continental crust (Amelin et al., 1999) was used to calculate TDM2, and the following discussion is based on TDM2 ages. The Hf isotopic data are listed in Table 3.

5. Results 5.1. Samples of the Habahe Group More than fifty detrital zircons of each sample were dated (Table 2), and most U–Pb analyses are concordant or nearly concordant, except for the pre-Neoproterozoic grains (Fig. 4a–f). The three samples of this group show similar age distributions (Fig. 5a–c). U–Pb dating of Type-I zircons reveals two similar early Paleozoic zircon populations in each sample (Fig. 5a–c). The predominant population yielded similar Cambrian to Ordovician 206Pb/238U ages (454–533, 459–522, 469– 525 Ma for samples L07BH16, L07BH27, L07BH01, respectively) with age peaks at 470, 476, 491 and 500 Ma, whereas the other population exhibits the youngest 206Pb/238U ages (431–447 Ma) and forms a remarkable age peak at ∼438 Ma. The remaining Type-I zircons yielded variable ages ranging from Neoproterozoic to Archean, among which most exhibit Neoproterozoic 206Pb/238U ages (650–900 Ma) whereas only nine analyses display Neoarchen 207Pb/206Pb ages (2.5–2.9 Ga) (Fig. 5a–c). A total number of eight Type-II zircons yielded two group of metamorphic ages of ∼490 and ∼840 Ma, respectively (Table 2). The predominant early Paleozoic zircons of the three samples have similar ranges of εHf(t) values (−14 to + 14, − 9 to + 13, −15 to + 12 for samples L07BH16, L07BH27, L07BH01, respectively) (Table 3). A large number of the early Paleozoic zircons was high initial Hf isotopic ratios and positive εHf(t) values, indicating significant addition of juvenile material to the crust (Fig. 6a–c). Among the pre-Neoproterozoic zircons, most grains have negative εHf(t) values between − 1 and −15, indicating reworking of their sources (Fig. 6e).

4.3. Hf isotope analysis Zircon Lu–Hf isotopic analysis was carried out by means of LA-MCICPMS in the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, using the same Geolas-193 laser-ablation system as for U–Pb dating. Before entering the ICP-MS for U–Pb isotope analysis, the mixed carrier gas was separated into two parts. One transported the ablated sample to a Neptune multi-collector ICP-MS torch. Isobaric interference of 176Lu on 176Hf was corrected by measuring the intensity of the interference-free 175Lu isotope and using a recommended 176Lu/175Lu ratio of 0.02669 (DeBievre and Taylor, 1993) to calculate 176Lu/177Hf. Similarly, the isobaric interference of 176Yb on 176 Hf was corrected by measuring the interference-free 172Yb isotope and using a recommended 176Yb/172Yb ratio of 0.5886 (Chu et al., 2002) to calculate 176Hf/177Hf ratios. In doing so, a mean 173Yb/171Yb ratio for the analyzed spot was automatically used in the same run to calculate a mean βYb value, and then the 176Yb signal intensity was calculated from the 173Yb signal intensity and the mean βYb value (Iizuka and Hirata, 2005). The standard zircon 91500 was used in this correction to the 176Yb–176Hf interference, showing variable 176 Yb/177Hf ratios of 0.0066–0.0126 with an average of 0.0077 for 91500 (Wu et al., 2006). Detailed instrumental settings and analytical procedures are described by Xie et al. (2008). The measured 176Lu/ 177 Hf ratios and a 176Lu decay constant of 1.867 × 10− 11 yr− 1 reported by Soderlund et al. (2004) were used to calculate initial 176Hf/177Hf ratios. The chondritic values of 176Hf/177Hf = 0.0332 and 176Lu/177Hf = 0.282772 reported by Blichert-Toft and Albarede (1997) were used for the calculation of εHf(t) values. The depleted mantle evolution line is defined by present-day 176Hf/177Hf = 0.28325 and 176Lu/177Hf = 0.0384 (Griffin et al., 2004). Because zircons are generally formed in granitic magma derived from felsic crust, two-step model ages (TDM2) are more meaningful than depleted mantle model ages (Griffin et al.,

5.2. Sample of the Dongxileke Formation Thirty-one zircon spots were analyzed for U–Pb and Hf isotopic compositions (Table 2). The analyses are all concordant and mostly vary from 400 to 425 Ma, with a weighted mean 206Pb/238U age of 411 ± 5 Ma (MSWD= 0.16) (Fig. 7). The Hf isotopic analyses (Table 3) reveal high εHf(t) values varying from +4 to +10 (Fig. 6d) and resembling the Devonian granites in the Chinese Altai (Sun et al., 2008, 2009).

5.3. Sample of the Baihaba Formation Fifty-four spots were dated (Table 2). The results for Type-I zircons also reveal two early Paleozoic zircon populations (Fig. 4g and h). The older population yielded Cambrian to Ordovician 206Pb/ 238 U ages (464–524 Ma) with two age peaks at 484 and 514 Ma, respectively (Fig. 5d). The younger population exhibits similar 206 Pb/238U ages (433–442 Ma) as samples of the Habahe Group and show the same age pattern (Fig. 5). In addition, nineteen preNeoproterozoic Type-I zircons were recognized, including four Neoarchean grains (2.6–2.9 Ga). Two Type-II grains were analyzed in this sample: one yielded 206Pb/238U age of 471 Ma and the other gave 207Pb/206Pb age of 1.4 Ga. The early Paleozoic zircons have a wide range of εHf(t) values from − 9 to +16 (Table 3). Seventeen spots of the early Paleozoic population have positive εHf(t) values between +1 and +16 (Fig. 6f), whereas almost all of the Neoproterozoic grains yielded negative εHf(t) values ranging from −18 to 0. Although the pre-Neoproterozoic grains are highly variable in εHf(t) values, all four Archean grains have negative εHf(t) values between −8 and −3 which resemble those of Archean detrital zircons from the Habahe Group (Fig. 6e).

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Table 2 U–Pb data for zircons from the sedimentary and volcanic rocks in the northwestern Chinese Altai. Sample

Ratios

Spot No.

Th/U

Ages (Ma) Pb207/Pb206

Pb207/U235

1σ

Pb206/U238

1σ

Pb207/Pb206

1σ

Pb207/U235

1σ

Pb206/U238

1σ

L07BH16 (N48° 44′ 18.1″ E86° 51′ 57.9″) 1 0.53 0.05791 0.00271 2 0.65 0.05562 0.00153 3 0.76 0.05763 0.00117 4 0.82 0.05631 0.00131 5 0.09 0.05893 0.00155 6 0.52 0.05728 0.00175 7 0.67 0.05594 0.00429 8 0.52 0.05496 0.00123 9 0.43 0.05690 0.00151 10 0.67 0.05356 0.00140 11 0.48 0.05668 0.00177 12 0.31 0.12259 0.00254 13 0.55 0.05710 0.00163 14 0.43 0.05551 0.00173 15 0.47 0.06004 0.00179 16 0.38 0.05652 0.00252 17 1.49 0.05542 0.00140 18 0.08 0.07130 0.00204 19 0.64 0.05579 0.00213 20 0.60 0.05775 0.00179 21 0.45 0.06578 0.00226 22 0.64 0.05609 0.00253 23 0.49 0.06395 0.00158 24 0.59 0.05646 0.00289 25 0.95 0.05481 0.00211 26 0.71 0.05780 0.00240 27 1.27 0.05650 0.00129 28 1.02 0.05602 0.00184 29 0.39 0.05832 0.00146 30 0.39 0.05651 0.00142 31 0.69 0.05425 0.00297 32 1.00 0.05818 0.00322 33 0.69 0.16436 0.00331 34 0.45 0.05682 0.00146 35 0.53 0.05735 0.00199 36 0.41 0.05780 0.00302 37 0.90 0.05551 0.00242 38 0.32 0.06630 0.00157 39 0.64 0.11903 0.00268 40 0.89 0.06997 0.00196 41 0.36 0.06760 0.00160 42 0.82 0.06751 0.00256 43 1.11 0.05630 0.00207 44 0.60 0.05568 0.00148 45 0.35 0.05630 0.00161 46 0.62 0.05698 0.00284 47 0.82 0.05533 0.00301 48 1.01 0.05784 0.00158 49 0.06 0.05672 0.00186 50 0.60 0.16562 0.00475 51 0.41 0.05873 0.00159 52 0.40 0.05744 0.00195

0.65960 0.60531 0.61851 0.60679 0.64773 0.60059 0.55364 0.54438 0.61475 0.55770 0.60099 5.90515 0.63729 0.53831 0.67144 0.59963 0.54287 1.38106 0.57028 0.58115 1.20999 0.56881 1.03963 0.54855 0.56189 0.66493 0.57772 0.59194 0.64512 0.62670 0.59468 0.62119 9.41736 0.61596 0.55483 0.64034 0.53311 1.19588 5.74039 1.49131 1.35350 1.22438 0.57680 0.54568 0.60212 0.60141 0.53742 0.63420 0.62853 10.54787 0.69817 0.64168

0.02949 0.01592 0.01189 0.01348 0.01619 0.01743 0.04066 0.01157 0.01553 0.01391 0.01792 0.11642 0.01728 0.01599 0.01906 0.02564 0.01299 0.03756 0.02077 0.01707 0.03968 0.02463 0.02443 0.02680 0.02068 0.02640 0.01247 0.01846 0.01537 0.01495 0.03125 0.03275 0.17946 0.01507 0.01832 0.03216 0.02222 0.02678 0.12229 0.03958 0.03036 0.04425 0.02014 0.01375 0.01636 0.02860 0.02788 0.01639 0.01962 0.28853 0.01785 0.02069

0.08258 0.07890 0.07782 0.07813 0.07969 0.07602 0.07176 0.07182 0.07834 0.07549 0.07688 0.34925 0.08091 0.07031 0.08109 0.07692 0.07102 0.14043 0.07411 0.07296 0.13337 0.07352 0.11787 0.07045 0.07433 0.08340 0.07414 0.07661 0.08020 0.08041 0.07948 0.07742 0.41541 0.07859 0.07015 0.08032 0.06963 0.13078 0.34966 0.15453 0.14517 0.13149 0.07427 0.07105 0.07753 0.07652 0.07042 0.07949 0.08034 0.46174 0.08618 0.08099

0.00149 0.00100 0.00087 0.00091 0.00101 0.00103 0.00191 0.00083 0.00097 0.00093 0.00105 0.00459 0.00106 0.00096 0.00108 0.00129 0.00087 0.00198 0.00116 0.00101 0.00206 0.00125 0.00148 0.00137 0.00117 0.00137 0.00088 0.00110 0.00101 0.00100 0.00159 0.00162 0.00497 0.00100 0.00106 0.00153 0.00119 0.00164 0.00444 0.00214 0.00183 0.00220 0.00117 0.00094 0.00106 0.00148 0.00144 0.00107 0.00118 0.00759 0.00116 0.00123

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66 36 23 29 33 40 116 27 34 34 42 18 37 42 39 65 32 33 53 40 43 66 29 74 54 58 27 44 31 31 82 79 17 32 46 77 62 26 21 32 26 47 50 33 36 71 79 34 43 25 33 44

514 481 489 482 507 478 447 441 487 450 478 1962 501 437 522 477 440 881 458 465 805 457 724 444 453 518 463 472 505 494 474 491 2380 487 448 503 434 799 1937 927 869 812 462 442 479 478 437 499 495 2484 538 503

18 10 7 9 10 11 27 8 10 9 11 17 11 11 12 16 9 16 13 11 18 16 12 18 13 16 8 12 9 9 20 21 17 9 12 20 15 12 18 16 13 20 13 9 10 18 18 10 12 25 11 13

512 490 483 485 494 472 447 447 486 469 477 1931 502 438 503 478 442 847 461 454 807 457 718 439 462 516 461 476 497 499 493 481 2240 488 437 498 434 792 1933 926 874 796 462 442 481 475 439 493 498 2447 533 502

9 6 5 5 6 6 11 5 6 6 6 22 6 6 6 8 5 11 7 6 12 8 9 8 7 8 5 7 6 6 9 10 23 6 6 9 7 9 21 12 10 13 7 6 6 9 9 6 7 33 7 7

L07BH27 (N48° 42′ 05.2″ E86° 59′ 48.9″) 1 0.73 0.05657 0.00077 2 0.29 0.05895 0.00185 3 0.32 0.19795 0.00290 4 0.72 0.05676 0.00094 5 0.32 0.05595 0.00100 6 0.29 0.05439 0.00080 7 0.46 0.21168 0.00330 8 0.65 0.05759 0.00298 9 0.31 0.05705 0.00084 10 0.36 0.05595 0.00094 11 0.50 0.05574 0.00088 12 0.09 0.06740 0.00113 13 0.55 0.05723 0.00137 14 1.11 0.05790 0.00135 15 0.42 0.05520 0.00099 16 0.68 0.05622 0.00126 17 0.22 0.06381 0.00106 18 0.42 0.06712 0.00100 19 0.67 0.05730 0.00138

0.59917 0.75239 14.68070 0.59687 0.57321 0.53200 14.23604 0.65255 0.62465 0.59208 0.61738 1.29274 0.73126 0.64371 0.56176 0.60553 1.11170 1.30019 0.64294

0.00769 0.02239 0.20523 0.00937 0.00966 0.00739 0.21149 0.03223 0.00867 0.00942 0.00926 0.02056 0.01668 0.01427 0.00955 0.01290 0.01751 0.01839 0.01470

0.07683 0.09259 0.53798 0.07628 0.07433 0.07096 0.48790 0.08221 0.07943 0.07677 0.08037 0.13918 0.09271 0.08068 0.07385 0.07816 0.12642 0.14057 0.08142

0.00077 0.00132 0.00616 0.00080 0.00080 0.00072 0.00589 0.00164 0.00081 0.00081 0.00084 0.00150 0.00113 0.00097 0.00080 0.00092 0.00136 0.00146 0.00100

475 565 2809 482 450 387 2918 514 493 450 442 850 500 526 420 461 735 841 503

13 40 10 18 19 15 11 73 15 18 16 17 29 28 20 27 17 14 29

477 570 2795 475 460 433 2766 510 493 472 488 843 557 505 453 481 759 846 504

5 13 13 6 6 5 14 20 5 6 6 9 10 9 6 8 8 8 9

477 571 2775 474 462 442 2562 509 493 477 498 840 572 500 459 485 767 848 505

5 8 26 5 5 4 26 10 5 5 5 8 7 6 5 6 8 8 6

1σ

X. Long et al. / Tectonophysics 480 (2010) 213–231

219

Table 2 (continued) Sample

Ratios

Spot No.

Th/U

Ages (Ma) Pb207/Pb206

Pb207/U235

1σ

Pb206/U238

1σ

Pb207/Pb206

L07BH27 (N48° 42′ 05.2″ E86° 59′ 48.9″) 20 0.52 0.05603 0.00118 21 0.43 0.06952 0.00110 22 0.29 0.05623 0.00093 23 0.54 0.05444 0.00107 24 0.21 0.05509 0.00088 25 0.82 0.05581 0.00117 26 1.15 0.05506 0.00098 27 0.17 0.05573 0.00099 28 0.46 0.05596 0.00119 29 0.18 0.05510 0.00098 30 0.35 0.05709 0.00172 31 0.40 0.05499 0.00096 32 0.69 0.05633 0.00129 33 0.72 0.05967 0.00183 34 0.34 0.06904 0.00196 35 0.22 0.06401 0.00185 36 1.14 0.05969 0.00141 37 0.06 0.05516 0.00111 38 0.73 0.05499 0.00096 39 0.45 0.05665 0.00143 40 0.84 0.05809 0.00109 41 1.11 0.08894 0.00191 42 0.31 0.05759 0.00169 43 0.07 0.05510 0.00130 44 0.44 0.05602 0.00111 45 0.51 0.05583 0.00128 46 0.81 0.05509 0.00126 47 0.58 0.05916 0.00129 48 0.39 0.05501 0.00107 49 0.32 0.05724 0.00137 50 0.21 0.06728 0.00189 51 0.20 0.05576 0.00123 52 0.29 0.05781 0.00123 53 0.20 0.06448 0.00140 54 0.43 0.06383 0.00134 55 0.44 0.15747 0.00345 56 0.25 0.11017 0.00238 57 0.20 0.05633 0.00147 58 0.29 0.06016 0.00192 59 0.28 0.19591 0.00417 60 0.28 0.05657 0.00123 61 1.01 0.06046 0.00195 62 0.34 0.05789 0.00136 63 0.07 0.06738 0.00152

0.57531 1.44912 0.58314 0.58885 0.59095 0.57893 0.52450 0.53746 0.63312 0.58236 0.63085 0.56591 0.61274 0.68736 1.32179 1.08456 0.57168 0.61939 0.56483 0.62348 0.55804 2.25409 0.61330 0.61064 0.57860 0.54097 0.56667 0.74968 0.56067 0.55410 1.28880 0.53102 0.67145 1.11114 1.08272 6.25986 4.63782 0.57658 0.67374 12.00884 0.58660 0.68440 0.61954 1.30820

0.01153 0.02169 0.00914 0.01105 0.00891 0.01157 0.00886 0.00903 0.01283 0.00979 0.01812 0.00938 0.01335 0.02012 0.03558 0.02995 0.01285 0.01189 0.00941 0.01505 0.00993 0.04601 0.01716 0.01369 0.01091 0.01179 0.01239 0.01561 0.01039 0.01262 0.03440 0.01117 0.01367 0.02301 0.02162 0.13055 0.09556 0.01438 0.02054 0.24429 0.01221 0.02104 0.01388 0.02819

0.07452 0.15129 0.07527 0.07851 0.07786 0.07529 0.06915 0.07001 0.08212 0.07672 0.08022 0.07471 0.07897 0.08363 0.13900 0.12301 0.06954 0.08152 0.07457 0.07991 0.06975 0.18404 0.07733 0.08048 0.07500 0.07036 0.07471 0.09203 0.07402 0.07031 0.13912 0.06917 0.08436 0.12517 0.12321 0.28876 0.30579 0.07435 0.08136 0.44530 0.07532 0.08224 0.07774 0.14105

0.00086 0.00161 0.00080 0.00088 0.00082 0.00087 0.00076 0.00076 0.00096 0.00084 0.00111 0.00082 0.00095 0.00115 0.00196 0.00171 0.00086 0.00094 0.00082 0.00101 0.00079 0.00229 0.00107 0.00099 0.00087 0.00086 0.00092 0.00112 0.00086 0.00088 0.00195 0.00084 0.00102 0.00153 0.00148 0.00374 0.00383 0.00098 0.00120 0.00548 0.00093 0.00123 0.00099 0.00177

454 914 461 389 416 445 415 442 451 416 495 412 465 592 900 742 592 419 412 478 533 1403 514 416 453 446 416 573 413 501 846 443 523 757 736 2429 1802 465 609 2792 475 620 526 850

L07BH01 (N48° 40′ 29.8″ E86° 45′ 31.5″) 1 0.76 0.05724 0.00094 2 0.84 0.05916 0.00296 3 0.40 0.05800 0.00098 4 0.33 0.05967 0.00128 5 0.56 0.06337 0.00171 6 0.98 0.05674 0.00164 7 0.51 0.06206 0.00250 8 0.32 0.06896 0.00109 9 0.40 0.06501 0.00156 10 0.37 0.05972 0.00108 11 0.91 0.06175 0.00089 12 0.47 0.10072 0.00187 13 0.19 0.06907 0.00146 14 0.43 0.06265 0.00226 15 0.44 0.05661 0.00177 16 0.65 0.12468 0.00251 17 0.53 0.07210 0.00145 18 0.40 0.06464 0.00206 19 0.59 0.05801 0.00165 20 0.34 0.05932 0.00138 21 0.19 0.06904 0.00171 22 0.94 0.06535 0.00267 23 0.67 0.05743 0.00171 24 0.30 0.06259 0.00196 25 0.64 0.05774 0.00134 26 0.27 0.05934 0.00132 27 0.74 0.05709 0.00123

0.62598 0.62117 0.66976 0.66494 1.05257 0.62651 0.79266 1.29699 1.10766 0.68157 0.59685 3.31025 1.26741 0.61078 0.61149 6.11923 1.62371 0.63227 0.66539 0.78275 1.41434 1.03282 0.59770 0.60951 0.67518 0.64252 0.55390

0.00968 0.02968 0.01065 0.01351 0.02699 0.01719 0.03046 0.01923 0.02528 0.01169 0.00807 0.05800 0.02539 0.02092 0.01825 0.11727 0.03087 0.01904 0.01809 0.01735 0.03339 0.04030 0.01699 0.01811 0.01488 0.01358 0.01137

0.07931 0.07615 0.08374 0.08082 0.12046 0.08008 0.09264 0.13643 0.12358 0.08278 0.07011 0.23842 0.13311 0.07073 0.07836 0.35609 0.16340 0.07097 0.08323 0.09574 0.14866 0.11469 0.07552 0.07068 0.08485 0.07858 0.07041

0.00080 0.00143 0.00085 0.00091 0.00154 0.00104 0.00150 0.00137 0.00149 0.00087 0.00068 0.00272 0.00152 0.00108 0.00106 0.00453 0.00184 0.00101 0.00105 0.00112 0.00186 0.00194 0.00100 0.00099 0.00098 0.00090 0.00079

501 573 530 592 721 481 676 898 775 593 665 1637 901 696 476 2024 989 763 530 579 900 786 508 694 520 580 495

1σ

Pb207/U235

1σ

Pb206/U238

1σ

25 15 17 23 17 25 19 19 25 19 39 19 28 40 33 35 28 23 19 31 20 21 37 29 22 27 28 25 22 29 33 26 24 24 23 19 20 32 41 18 25 41 28 25

461 910 466 470 471 464 428 437 498 466 497 455 485 531 855 746 459 489 455 492 450 1198 486 484 464 439 456 568 452 448 841 432 522 759 745 2013 1756 462 523 2605 469 529 490 849

7 9 6 7 6 7 6 6 8 6 11 6 8 12 16 15 8 7 6 9 6 14 11 9 7 8 8 9 7 8 15 7 8 11 11 18 17 9 12 19 8 13 9 12

463 908 468 487 483 468 431 436 509 477 497 464 490 518 839 748 433 505 464 496 435 1089 480 499 466 438 464 568 460 438 840 431 522 760 749 1635 1720 462 504 2374 468 509 483 851

5 9 5 5 5 5 5 5 6 5 7 5 6 7 11 10 5 6 5 6 5 12 6 6 5 5 6 7 5 5 11 5 6 9 8 19 19 6 7 24 6 7 6 10

17 71 18 25 33 38 54 15 28 20 14 17 23 47 42 17 21 40 38 28 28 54 39 40 28 27 26

494 491 521 518 730 494 593 844 757 528 475 1483 831 484 484 1993 979 498 518 587 895 720 476 483 524 504 448

6 19 6 8 13 11 17 9 12 7 5 14 11 13 11 17 12 12 11 10 14 20 11 11 9 8 7

492 473 518 501 733 497 571 824 751 513 437 1378 806 441 486 1964 976 442 515 589 893 700 469 440 525 488 439

5 9 5 5 9 6 9 8 9 5 4 14 9 7 6 22 10 6 6 7 10 11 6 6 6 5 5

1σ

(continued on next page)

220

X. Long et al. / Tectonophysics 480 (2010) 213–231

Table 2 (continued) Sample

Ratios

Spot No.

Th/U

Ages (Ma) Pb207/Pb206

Pb207/U235

1σ

Pb206/U238

1σ

Pb207/Pb206

L07BH01 (N48° 40′ 29.8″ E86° 45′ 31.5″) 28 0.08 0.06788 0.00203 29 0.48 0.06373 0.00217 30 0.41 0.06270 0.00223 31 0.73 0.06742 0.00313 32 0.33 0.07800 0.00149 33 0.47 0.05747 0.00129 34 0.59 0.06066 0.00120 35 1.30 0.05834 0.00148 36 0.27 0.06273 0.00146 37 0.49 0.06114 0.00137 38 0.21 0.13941 0.00249 39 0.65 0.06625 0.00327 40 0.38 0.09050 0.00172 41 0.79 0.06868 0.00195 42 0.20 0.06363 0.00132 43 0.35 0.06719 0.00148 44 0.75 0.05801 0.00160 45 0.27 0.05900 0.00193 46 0.33 0.06154 0.00128 47 0.42 0.06167 0.00225 48 0.40 0.05795 0.00160 49 0.34 0.06352 0.00203 50 0.47 0.06228 0.00159 51 0.45 0.05881 0.00149 52 0.30 0.12351 0.00260 53 0.25 0.16743 0.00359 54 0.73 0.20506 0.00460 55 0.05 0.06408 0.00140 56 0.31 0.06919 0.00199 57 0.56 0.06089 0.00160 58 0.92 0.06656 0.00263 59 0.51 0.05754 0.00154 60 0.37 0.16968 0.00400 61 0.24 0.16887 0.00392 62 0.61 0.07212 0.00257

1.28708 0.61513 0.60927 1.08942 2.06420 0.64429 0.66917 0.61433 0.66532 0.72367 5.39426 0.69373 2.08864 0.72442 0.77792 1.05931 0.66992 0.74737 0.59945 0.78679 0.61809 0.79504 0.65032 0.65224 6.20525 8.87252 12.65532 0.68367 1.27265 0.65017 1.02001 0.65706 7.70062 8.15564 1.29496

0.03659 0.01990 0.02060 0.04831 0.03751 0.01377 0.01255 0.01485 0.01468 0.01542 0.09178 0.03258 0.03782 0.01958 0.01533 0.02232 0.01772 0.02332 0.01192 0.02749 0.01630 0.02427 0.01583 0.01586 0.12520 0.18235 0.27255 0.01435 0.03500 0.01632 0.03848 0.01687 0.17400 0.18163 0.04415

0.13762 0.07005 0.07053 0.11728 0.19209 0.08138 0.08008 0.07643 0.07699 0.08593 0.28089 0.07602 0.16755 0.07658 0.08876 0.11447 0.08385 0.09197 0.07073 0.09264 0.07745 0.09088 0.07583 0.08053 0.36485 0.38482 0.44817 0.07748 0.13358 0.07755 0.11128 0.08293 0.32960 0.35075 0.13041

0.00193 0.00103 0.00106 0.00220 0.00213 0.00093 0.00088 0.00093 0.00091 0.00100 0.00303 0.00148 0.00186 0.00103 0.00101 0.00135 0.00108 0.00131 0.00081 0.00143 0.00101 0.00130 0.00095 0.00100 0.00426 0.00460 0.00576 0.00091 0.00184 0.00099 0.00188 0.00107 0.00423 0.00438 0.00209

865 733 698 851 1147 510 627 543 699 644 2220 814 1436 889 729 844 530 567 658 663 528 726 684 560 2008 2532 2867 744 904 635 824 512 2554 2546 989

L07BH11 (N48° 42′ 04.1″ E86° 50′ 13.5″) 1 0.63 0.05560 0.00237 2 0.73 0.05616 0.00374 3 0.76 0.05749 0.00344 4 0.44 0.06060 0.00332 5 0.66 0.05872 0.00257 6 0.60 0.05620 0.00292 7 0.78 0.06000 0.00526 8 0.51 0.05584 0.00238 9 0.48 0.04727 0.00395 10 0.64 0.05560 0.00390 11 0.37 0.05478 0.00251 12 0.51 0.05770 0.00412 13 0.53 0.05773 0.00297 14 0.53 0.05584 0.00285 15 0.57 0.05840 0.00316 16 0.51 0.05845 0.00399 17 0.47 0.05917 0.00336 18 0.56 0.05937 0.00390 19 0.47 0.05796 0.00611 20 0.39 0.05491 0.00465 21 0.54 0.05627 0.00483 22 0.89 0.06052 0.00441 23 0.48 0.05796 0.00388 24 0.54 0.05597 0.00335 25 0.76 0.05813 0.00345 26 0.55 0.05812 0.00392 27 0.39 0.05668 0.00415 28 0.68 0.05673 0.00535 29 0.42 0.05932 0.00486 30 0.50 0.06760 0.00472 31 1.02 0.05703 0.00407

0.51309 0.50918 0.51831 0.55293 0.53314 0.51382 0.53308 0.50857 0.42569 0.52245 0.50906 0.54240 0.53331 0.50383 0.52862 0.51532 0.52656 0.52660 0.51964 0.50268 0.50301 0.48743 0.51845 0.50684 0.51793 0.52316 0.52794 0.51109 0.53722 0.66015 0.52142

0.02014 0.03104 0.02837 0.02767 0.02148 0.02446 0.04248 0.01995 0.03293 0.03372 0.02152 0.03549 0.02519 0.02375 0.02624 0.03223 0.02747 0.03194 0.05016 0.03908 0.03967 0.03254 0.03198 0.02805 0.02840 0.03256 0.03567 0.04436 0.04043 0.04242 0.03435

0.06694 0.06577 0.06540 0.06620 0.06588 0.06634 0.06446 0.06609 0.06535 0.06819 0.06744 0.06823 0.06704 0.06549 0.06570 0.06401 0.06461 0.06440 0.06509 0.06647 0.06491 0.05849 0.06496 0.06577 0.06472 0.06539 0.06767 0.06546 0.06580 0.07096 0.06643

0.00154 0.00217 0.00196 0.00188 0.00155 0.00179 0.00279 0.00154 0.00250 0.00234 0.00167 0.00244 0.00181 0.00176 0.00188 0.00223 0.00195 0.00214 0.00330 0.00276 0.00274 0.00219 0.00226 0.00211 0.00208 0.00232 0.00256 0.00304 0.00275 0.00265 0.00250

L07BH08 (N48° 42′ 06.2″ E86° 50′ 06.1″) 1 0.61 0.06161 0.00109 2 0.54 0.13209 0.00219 3 0.53 0.06117 0.00112 4 0.31 0.06101 0.00126

0.66056 7.23522 0.69651 0.69527

0.01110 0.11477 0.01213 0.01367

0.07770 0.39697 0.08253 0.08260

0.00082 0.00437 0.00088 0.00092

1σ

Pb207/U235

1σ

Pb206/U238

1σ

36 44 46 61 19 27 22 32 27 26 15 66 18 34 23 25 36 43 24 48 35 41 31 32 20 19 19 25 34 32 51 34 21 21 43

840 487 483 748 1137 505 520 486 518 553 1884 535 1145 553 584 733 521 567 477 589 489 594 509 510 2005 2325 2654 529 834 509 714 513 2197 2248 843

16 13 13 23 12 9 8 9 9 9 15 20 12 12 9 11 11 14 8 16 10 14 10 10 18 19 20 9 16 10 19 10 20 20 20

831 436 439 715 1133 504 497 475 478 531 1596 472 999 476 548 699 519 567 441 571 481 561 471 499 2005 2099 2387 481 808 481 680 514 1836 1938 790

11 6 6 13 12 6 5 6 5 6 15 9 10 6 6 8 6 8 5 8 6 8 6 6 20 21 26 5 10 6 11 6 21 21 12

436 459 510 625 557 460 604 446 63 436 403 518 520 446 545 547 573 581 528 409 463 622 528 451 535 534 479 481 579 856 493

48 78 69 61 48 59 100 48 101 84 52 83 58 59 60 78 63 76 125 102 102 81 76 68 66 76 85 112 93 74 81

421 418 424 447 434 421 434 417 360 427 418 440 434 414 431 422 430 430 425 414 414 403 424 416 424 427 430 419 437 515 426

14 21 19 18 14 16 28 13 23 22 14 23 17 16 17 22 18 21 34 26 27 22 21 19 19 22 24 30 27 26 23

418 411 408 413 411 414 403 413 408 425 421 425 418 409 410 400 404 402 407 415 405 366 406 411 404 408 422 409 411 442 415

9 13 12 11 9 11 17 9 15 14 10 15 11 11 11 14 12 13 20 17 17 13 14 13 13 14 15 18 17 16 15

661 2126 645 640

19 14 20 24

515 2141 537 536

7 14 7 8

482 2155 511 512

5 20 5 5

1σ

X. Long et al. / Tectonophysics 480 (2010) 213–231

221

Table 2 (continued) Sample

Ratios

Spot No.

Th/U

Ages (Ma) Pb207/Pb206

1σ

L07BH08 (N48° 42′ 06.2″ E86° 50′ 06.1″) 5 0.37 0.06301 0.00201 6 0.24 0.05958 0.00105 7 0.23 0.06132 0.00176 8 0.21 0.07131 0.00115 9 0.25 0.05935 0.00110 10 0.48 0.05895 0.00117 11 1.39 0.21032 0.00367 12 0.58 0.07376 0.00153 13 0.65 0.06154 0.00164 14 0.55 0.06228 0.00196 15 0.52 0.18421 0.00350 16 0.89 0.06743 0.00191 17 0.08 0.08993 0.00165 18 0.40 0.05781 0.00146 19 0.91 0.06968 0.00203 20 0.07 0.05958 0.00136 21 0.68 0.05695 0.00134 22 0.40 0.05711 0.00107 23 0.59 0.07262 0.00128 24 0.47 0.05913 0.00196 25 1.79 0.06376 0.00565 26 0.31 0.05822 0.00149 27 0.82 0.07183 0.00238 28 0.66 0.05632 0.00195 29 0.21 0.05904 0.00140 30 0.94 0.05668 0.00134 31 0.40 0.05794 0.00249 32 0.29 0.21573 0.00403 33 0.37 0.06080 0.00280 34 0.84 0.06634 0.00159 35 0.31 0.15356 0.00301 36 0.48 0.12299 0.00278 37 1.46 0.06828 0.00155 38 0.53 0.05669 0.00132 39 0.51 0.05479 0.00140 40 0.47 0.05497 0.00182 41 0.57 0.05854 0.00139 42 0.49 0.05575 0.00166 43 0.57 0.15186 0.00336 44 0.52 0.10089 0.00230 45 0.65 0.17729 0.00386 46 0.77 0.16102 0.00370 47 0.38 0.05662 0.00138 48 0.98 0.05942 0.00147 49 0.74 0.05690 0.00140 50 0.53 0.05982 0.00172 51 0.49 0.07190 0.00422 52 0.70 0.06418 0.00321 53 0.64 0.06072 0.00163 54 0.77 0.08179 0.00245

Pb207/U235

1σ

Pb206/U238

1σ

Pb207/Pb206

1σ

Pb207/U235

1σ

Pb206/U238

1σ

0.66942 0.57563 0.66129 1.39360 0.67962 0.68790 15.8837 1.26616 0.66932 0.66714 13.01778 1.24009 2.66284 0.65382 1.30534 0.62289 0.61093 0.61973 1.34768 0.68325 1.05263 0.67408 1.23678 0.58513 0.67631 0.55506 0.62709 13.23550 0.68326 1.17553 7.40709 4.13602 1.41789 0.60766 0.53428 0.53293 0.56022 0.53842 7.76009 3.08054 11.14637 7.72695 0.54171 0.61152 0.54689 0.58469 0.76498 0.62392 0.62584 0.78431

0.02037 0.00967 0.01753 0.02140 0.01193 0.01295 0.26594 0.02488 0.01705 0.02006 0.23775 0.03349 0.04643 0.01569 0.03625 0.01353 0.01364 0.01099 0.02251 0.02159 0.09021 0.01636 0.03885 0.01936 0.01522 0.01244 0.02474 0.23277 0.03001 0.02672 0.13655 0.08794 0.03046 0.01332 0.01292 0.01679 0.01253 0.01520 0.16145 0.06592 0.22797 0.16668 0.01240 0.01418 0.01263 0.01581 0.04235 0.02963 0.01576 0.02201

0.07702 0.07004 0.07820 0.14170 0.08302 0.08463 0.54770 0.12449 0.07888 0.07769 0.51259 0.13340 0.21479 0.08205 0.13590 0.07584 0.07783 0.07873 0.13465 0.08383 0.11978 0.08401 0.12493 0.07538 0.08312 0.07105 0.07850 0.44518 0.08154 0.12857 0.35000 0.24400 0.15067 0.07778 0.07075 0.07034 0.06943 0.07007 0.37074 0.22151 0.45610 0.34810 0.06940 0.07465 0.06972 0.07089 0.07717 0.07051 0.07475 0.06954

0.00106 0.00074 0.00129 0.00146 0.00089 0.00093 0.00662 0.00142 0.00098 0.00107 0.00660 0.00180 0.00238 0.00100 0.00184 0.00089 0.00092 0.00085 0.00144 0.00121 0.00345 0.00104 0.00190 0.00107 0.00100 0.00085 0.00177 0.00496 0.00147 0.00159 0.00400 0.00313 0.00182 0.00093 0.00088 0.00101 0.00084 0.00095 0.00464 0.00275 0.00555 0.00448 0.00086 0.00094 0.00087 0.00097 0.00184 0.00138 0.00099 0.00100

709 588 650 966 580 565 2908 1035 658 684 2691 851 1424 523 919 588 490 496 1003 572 734 538 981 465 569 479 527 2949 632 817 2386 2000 877 479 404 411 550 442 2367 1641 2628 2466 477 583 488 597 983 748 629 1240

41 19 30 16 20 23 13 22 34 41 15 34 17 32 35 27 29 21 18 44 133 32 39 48 29 29 48 15 63 27 17 20 25 28 32 45 28 39 19 22 19 20 29 29 29 35 74 67 32 33

520 462 515 886 527 532 2870 831 520 519 2681 819 1318 511 848 492 484 490 867 529 730 523 817 468 525 448 494 2697 529 789 2162 1661 896 482 435 434 452 437 2204 1428 2535 2200 440 485 443 467 577 492 494 588

12 6 11 9 7 8 16 11 10 12 17 15 13 10 16 8 9 7 10 13 45 10 18 12 9 8 15 17 18 12 16 17 13 8 9 11 8 10 19 16 19 19 8 9 8 10 24 19 10 13

478 436 485 854 514 524 2816 756 489 482 2668 807 1254 508 821 471 483 489 814 519 729 520 759 468 515 442 487 2374 505 780 1935 1407 905 483 441 438 433 437 2033 1290 2422 1926 433 464 434 442 479 439 465 433

6 4 8 8 5 6 28 8 6 6 28 10 13 6 10 5 6 5 8 7 20 6 11 6 6 5 11 22 9 9 19 16 10 6 5 6 5 6 22 15 25 21 5 6 5 6 11 8 6 6

6. Discussion 6.1. Depositional age The depositional age of the flysch sequence in the northwestern Chinese Altai is not well constrained. Based on microfossil studies, the Habahe Group was originally assigned to the middle to late Ordovician, whreras some geologists later suggested Sinian, Sinian to Cambrian or Cambrian to middle Ordovician ages (GCRSX, 1981; Wang, 1983; Peng, 1989; BGMRX, 1993; Cai, 1999; Windley et al., 2002; Chen and Jahn, 2002; Li et al., 2006). Recent U–Pb dating of detrital zircons from metasediments of this group in the eastern Chinese Altai revealed the youngest zircon population to have ages between 460 and 540 Ma, and some contain 389 Ma metamorphic overgrowth rims (Long et al., 2007). These data suggest a younger depositional age for the flysch sequence between early Devonian and mid-Ordovician than previously estimated.

Our U–Pb dating of detrital zircons from different lithological units of the Habahe flysch in the eastern Chinese Altai defines a new zircon population yielding a youngest age of ∼438 Ma and indicating the maximum early Silurian age of deposition. Dating of the overlying rhyolite yielded a weighted mean age of 411 ± 5 Ma, which suggests that the Habahe group must have been deposited before this age. Therefore, the Habahe flysch sequence in the northwestern Chinese Altai was probably deposited during the early Silurian to early Devonian. The Baihaba Formation, overlying the Dongxileke rhyolite, was also assigned to be late Ordovician in age (BGMRX, 1993; Cai, 1999). Detrital zircons from a slate of the group show a similar age distribution to that of the Habahe Group (Fig. 5). Although eleven grains of the youngest zircon population yielded the maximum depositional age of ∼ 438 Ma, the age of the underlying rhyolite (411 ± 5 Ma) provides a better constraint for the depositional age of the Baihaba Formation and suggests that these rocks began to be deposited not earlier than the early Devonian.

222

X. Long et al. / Tectonophysics 480 (2010) 213–231

Table 3 Lu–Hf data for zircons from the sedimentary and volcanic rocks in the northwestern Chinese Altai. Sample

176

Yb/177Hf

2σ

176

Lu/177Hf

2σ

176

Hf/177Hf

2σ

Age (Ma)

(176Hf/177Hf)i

εHf

TcDM (Ma)⁎

2σ

fLu/Hf

L07BH16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

0.009524 0.018238 0.024702 0.027398 0.009142 0.016909 0.021436 0.041966 0.030836 0.034050 0.028914 0.012024 0.026721 0.017361 0.021190 0.024356 0.067273 0.134401 0.027828 0.082618 0.023920 0.026550 0.033192 0.028785 0.034904 0.013372 0.039994 0.029721 0.034158 0.014373 0.023061 0.022015 0.005185 0.029021 0.063273 0.022156 0.022500 0.025095 0.028592 0.016595 0.059568 0.024873 0.011625 0.034154 0.031029 0.034363 0.042770 0.028404 0.009269 0.016263 0.044069 0.019310

0.000041 0.000164 0.000138 0.000141 0.000234 0.000100 0.000038 0.000888 0.000203 0.000621 0.000097 0.000148 0.000156 0.000047 0.000142 0.000269 0.002860 0.001373 0.000323 0.000912 0.000481 0.000630 0.001368 0.000164 0.000509 0.000067 0.000446 0.000348 0.002705 0.000147 0.000101 0.000503 0.000031 0.000185 0.000975 0.000091 0.000387 0.000111 0.000171 0.000073 0.001345 0.000139 0.000009 0.000311 0.000276 0.000973 0.001003 0.000507 0.000154 0.000145 0.001162 0.000058

0.000334 0.000551 0.000872 0.001063 0.000293 0.000545 0.000677 0.001334 0.001102 0.001106 0.000927 0.000385 0.000878 0.000598 0.000701 0.001002 0.002292 0.004233 0.000951 0.002652 0.000730 0.000926 0.001227 0.001037 0.001214 0.000459 0.001375 0.000949 0.001113 0.000513 0.000901 0.000731 0.000173 0.000908 0.002030 0.000786 0.000783 0.000917 0.000982 0.000601 0.001816 0.000844 0.000383 0.001097 0.000984 0.001095 0.001499 0.000890 0.000233 0.000503 0.001284 0.000629

0.000001 0.000005 0.000006 0.000006 0.000010 0.000003 0.000001 0.000028 0.000007 0.000019 0.000003 0.000005 0.000005 0.000001 0.000005 0.000011 0.000096 0.000046 0.000011 0.000034 0.000013 0.000021 0.000047 0.000007 0.000015 0.000002 0.000010 0.000011 0.000081 0.000005 0.000005 0.000016 0.000001 0.000006 0.000032 0.000003 0.000013 0.000003 0.000005 0.000002 0.000043 0.000005 0.000000 0.000012 0.000009 0.000029 0.000034 0.000015 0.000005 0.000003 0.000037 0.000001

0.282854 0.282446 0.282228 0.282773 0.282068 0.282431 0.282804 0.282338 0.282255 0.282669 0.282442 0.281135 0.282848 0.282551 0.282282 0.282835 0.282638 0.282347 0.282487 0.282693 0.282378 0.282576 0.281810 0.282876 0.282319 0.282204 0.282247 0.282407 0.282503 0.282542 0.282376 0.282172 0.281024 0.282450 0.282631 0.282875 0.282305 0.281871 0.281152 0.282193 0.282363 0.281907 0.282424 0.282812 0.282284 0.282584 0.282428 0.282498 0.282274 0.280812 0.282431 0.282321

0.000018 0.000019 0.000016 0.000017 0.000014 0.000020 0.000018 0.000016 0.000015 0.000020 0.000020 0.000015 0.000016 0.000020 0.000022 0.000015 0.000019 0.000023 0.000015 0.000019 0.000017 0.000018 0.000015 0.000015 0.000016 0.000025 0.000017 0.000018 0.000019 0.000016 0.000018 0.000018 0.000013 0.000015 0.000016 0.000017 0.000018 0.000014 0.000014 0.000017 0.000014 0.000018 0.000016 0.000014 0.000018 0.000016 0.000020 0.000014 0.000014 0.000018 0.000013 0.000014

512 490 483 485 494 472 447 447 486 469 477 1994 502 438 503 478 442 847 461 454 807 457 718 439 462 516 461 476 497 499 493 481 2501 488 437 498 434 792 1942 926 874 796 462 442 481 475 439 493 498 2514 533 502

0.282851 0.282440 0.282220 0.282764 0.282065 0.282426 0.282799 0.282327 0.282245 0.282660 0.282434 0.281121 0.282840 0.282547 0.282275 0.282826 0.282619 0.282279 0.282479 0.282671 0.282367 0.282568 0.281793 0.282867 0.282309 0.282199 0.282235 0.282398 0.282492 0.282537 0.282368 0.282166 0.281015 0.282442 0.282615 0.282868 0.282299 0.281857 0.281116 0.282183 0.282333 0.281895 0.282421 0.282803 0.282276 0.282574 0.282415 0.282490 0.282272 0.280787 0.282418 0.282315

14.08 − 0.94 − 8.89 10.40 −14.13 − 1.86 10.79 − 5.90 − 7.95 6.36 − 1.45 − 13.92 13.46 1.67 − 6.49 12.46 4.32 1.30 − 0.22 6.42 3.51 2.86 − 18.79 13.05 − 6.22 − 8.89 − 8.86 − 2.75 1.06 2.69 − 3.43 −10.86 − 6.03 − 0.92 4.06 14.37 − 7.18 −14.88 − 15.28 − 0.36 3.81 − 13.47 − 2.25 10.85 − 6.97 3.45 − 2.95 0.89 − 6.74 − 13.84 − 0.77 − 5.11

568 1324 1724 737 1998 1356 685 1542 1679 932 1339 3218 592 1148 1618 624 1015 1503 1263 916 1356 1102 2418 561 1571 1751 1704 1405 1227 1145 1454 1822 3230 1322 1024 541 1596 2282 3244 1653 1396 2214 1368 678 1625 1087 1385 1233 1627 3638 1351 1547

34 34 29 31 25 35 33 29 27 36 36 26 29 36 40 28 32 39 27 34 30 32 25 28 28 45 31 31 32 29 32 32 24 27 28 32 32 26 25 30 23 32 30 25 32 29 34 25 26 33 22 25

− 0.99 − 0.98 − 0.97 − 0.97 − 0.99 − 0.98 − 0.98 − 0.96 − 0.97 − 0.97 − 0.97 − 0.99 − 0.97 − 0.98 − 0.98 − 0.97 − 0.93 − 0.87 − 0.97 − 0.92 − 0.98 − 0.97 − 0.96 − 0.97 − 0.96 − 0.99 − 0.96 − 0.97 − 0.97 − 0.98 − 0.97 − 0.98 − 0.99 − 0.97 − 0.94 − 0.98 − 0.98 − 0.97 − 0.97 − 0.98 − 0.95 − 0.97 − 0.99 − 0.97 − 0.97 − 0.97 − 0.95 − 0.97 − 0.99 −0.98 − 0.96 − 0.98

L07BH27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

0.093031 0.057294 0.036844 0.043423 0.050566 0.032655 0.043719 0.059507 0.026359 0.026257 0.028092 0.084488 0.038067 0.049501 0.099672 0.035991 0.055777 0.036991 0.036355 0.047459 0.068220

0.003310 0.001848 0.000906 0.000240 0.000246 0.000784 0.000296 0.000536 0.000443 0.000774 0.000357 0.000594 0.000394 0.000432 0.000680 0.001020 0.000203 0.000344 0.000257 0.001278 0.000454

0.002830 0.001557 0.001224 0.001455 0.001612 0.001041 0.001345 0.001923 0.000813 0.000745 0.001026 0.002672 0.001368 0.001613 0.002960 0.001299 0.001744 0.001247 0.001248 0.001690 0.002425

0.000102 0.000043 0.000027 0.000007 0.000008 0.000022 0.000011 0.000017 0.000012 0.000023 0.000010 0.000018 0.000011 0.000015 0.000020 0.000034 0.000007 0.000011 0.000009 0.000041 0.000010

0.282453 0.282345 0.280974 0.282714 0.282325 0.282260 0.280938 0.282491 0.282324 0.282353 0.282423 0.282211 0.282260 0.282308 0.282696 0.282365 0.282104 0.282209 0.282379 0.282466 0.282152

0.000019 0.000018 0.000015 0.000016 0.000013 0.000015 0.000017 0.000020 0.000016 0.000015 0.000015 0.000015 0.000017 0.000015 0.000019 0.000016 0.000017 0.000016 0.000017 0.000014 0.000016

477 571 2809 474 462 442 2918 509 493 477 498 840 572 500 459 485 767 848 505 463 908

0.282428 0.282328 0.280908 0.282701 0.282311 0.282252 0.280863 0.282473 0.282317 0.282346 0.282413 0.282169 0.282245 0.282293 0.282671 0.282354 0.282079 0.282189 0.282368 0.282451 0.282111

− 1.67 − 3.11 − 2.72 7.95 − 6.13 − 8.68 − 1.79 0.62 − 5.25 − 4.57 − 1.73 − 2.78 − 6.04 − 5.94 6.52 − 4.12 − 7.58 − 1.89 − 3.18 − 1.14 − 3.33

1351 1502 3312 854 1566 1680 3353 1260 1547 1499 1371 1706 1652 1588 915 1483 1891 1667 1451 1312 1790

30 32 22 29 23 26 28 36 28 26 26 26 30 27 34 28 31 28 31 24 28

− 0.91 − 0.95 − 0.96 − 0.96 − 0.95 − 0.97 −0.96 − 0.94 − 0.98 − 0.98 − 0.97 − 0.92 − 0.96 − 0.95 −0.91 − 0.96 − 0.95 − 0.96 − 0.96 − 0.95 − 0.93

X. Long et al. / Tectonophysics 480 (2010) 213–231

223

Table 3 (continued) Sample

176

Yb/177Hf

2σ

176

Lu/177Hf

2σ

176

Hf/177Hf

2σ

Age (Ma)

(176Hf/177Hf)i

εHf

TcDM (Ma)⁎

2σ

fLu/Hf

L07BH27 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

0.045068 0.030582 0.032902 0.043892 0.130594 0.042151 0.027549 0.038400 0.028550 0.038161 0.029233 0.016122 0.045651 0.025099 0.058715 0.032348 0.075117 0.023796 0.097091 0.070158 0.034031 0.028903 0.047529 0.030771 0.045670 0.034146 0.068569 0.038615 0.038578 0.046464 0.052881 0.065173 0.025467 0.018184 0.015975 0.021783 0.021000 0.023655 0.089658 0.033593 0.031985 0.021979

0.000322 0.000695 0.000194 0.000596 0.003702 0.000417 0.000276 0.000452 0.000484 0.000468 0.000108 0.000187 0.000217 0.001362 0.001078 0.000506 0.000873 0.000355 0.004749 0.000423 0.000154 0.001526 0.000139 0.000196 0.001141 0.000285 0.001901 0.001393 0.001000 0.000894 0.000859 0.001284 0.001386 0.001356 0.000164 0.000089 0.000232 0.000963 0.001456 0.000834 0.000222 0.000186

0.001489 0.001050 0.001194 0.001505 0.004269 0.001366 0.000904 0.001194 0.000909 0.001259 0.001114 0.000553 0.001451 0.000860 0.001831 0.000992 0.002692 0.000783 0.003060 0.002518 0.001316 0.001031 0.001475 0.001126 0.001610 0.001127 0.002268 0.001172 0.001199 0.001586 0.001567 0.002086 0.000851 0.000638 0.000542 0.000747 0.000715 0.000813 0.002832 0.001170 0.001207 0.000734

0.000011 0.000024 0.000009 0.000019 0.000112 0.000014 0.000009 0.000018 0.000013 0.000015 0.000004 0.000006 0.000006 0.000040 0.000034 0.000016 0.000033 0.000010 0.000148 0.000015 0.000013 0.000052 0.000005 0.000006 0.000038 0.000011 0.000058 0.000043 0.000031 0.000030 0.000019 0.000039 0.000046 0.000047 0.000003 0.000003 0.000008 0.000030 0.000042 0.000029 0.000006 0.000005

0.282355 0.282409 0.282463 0.282286 0.282487 0.282267 0.282448 0.282323 0.282423 0.282537 0.282422 0.282815 0.282165 0.282098 0.282421 0.282483 0.282388 0.282303 0.282476 0.282263 0.282509 0.282510 0.282586 0.282710 0.282523 0.282180 0.282496 0.282358 0.282153 0.282344 0.282483 0.282266 0.282156 0.280963 0.281638 0.282463 0.282443 0.281019 0.282357 0.282541 0.282534 0.282158

0.000015 0.000013 0.000011 0.000014 0.000015 0.000014 0.000015 0.000013 0.000014 0.000014 0.000017 0.000015 0.000014 0.000018 0.000021 0.000015 0.000018 0.000014 0.000019 0.000017 0.000012 0.000014 0.000012 0.000015 0.000018 0.000016 0.000015 0.000017 0.000016 0.000017 0.000014 0.000017 0.000017 0.000016 0.000015 0.000012 0.000020 0.000014 0.000013 0.000015 0.000014 0.000014

468 487 483 468 431 436 509 477 497 464 490 518 839 748 433 505 464 496 435 1403 480 499 466 438 464 568 460 438 840 431 522 760 749 2429 1802 462 504 2792 468 509 483 851

0.282342 0.282399 0.282452 0.282273 0.282453 0.282256 0.282439 0.282312 0.282415 0.282526 0.282411 0.282810 0.282142 0.282086 0.282406 0.282474 0.282365 0.282295 0.282451 0.282196 0.282497 0.282501 0.282573 0.282701 0.282509 0.282168 0.282477 0.282349 0.282134 0.282332 0.282468 0.282237 0.282144 0.280933 0.281620 0.282457 0.282436 0.280976 0.282332 0.282530 0.282523 0.282146

− 4.91 − 2.46 − 0.67 − 7.37 − 1.81 − 8.65 − 0.55 − 5.78 − 1.70 1.51 − 1.97 12.76 − 3.74 − 7.77 − 3.40 0.57 − 4.20 − 5.94 − 1.79 10.82 0.86 1.39 3.23 7.12 0.92 − 8.84 − 0.32 − 5.34 − 4.03 − 6.09 0.74 − 2.16 − 5.68 − 10.61 − 0.58 − 0.97 − 0.79 − 0.71 − 5.27 2.65 1.83 − 3.34

1509 1399 1305 1634 1320 1673 1320 1561 1369 1177 1377 641 1754 1885 1403 1259 1469 1585 1323 1467 1224 1212 1091 867 1207 1791 1268 1506 1770 1539 1264 1608 1779 3405 2381 1303 1328 3195 1527 1156 1177 1743

26 23 20 25 23 24 28 23 24 25 31 27 25 31 37 27 31 24 30 31 22 23 22 26 31 28 26 30 27 29 25 28 28 21 28 23 35 20 22 27 25 24

− 0.96 − 0.97 − 0.96 − 0.95 − 0.87 − 0.96 − 0.97 − 0.96 − 0.97 − 0.96 − 0.97 − 0.98 − 0.96 − 0.97 − 0.94 − 0.97 − 0.92 − 0.98 − 0.91 − 0.92 − 0.96 − 0.97 − 0.96 − 0.97 − 0.95 − 0.97 − 0.93 − 0.96 − 0.96 − 0.95 − 0.95 − 0.94 − 0.97 − 0.98 − 0.98 − 0.98 − 0.98 − 0.98 − 0.91 − 0.96 − 0.96 − 0.98

L07BH01 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

0.065546 0.036509 0.018158 0.030921 0.048841 0.040150 0.026008 0.027593 0.043680 0.034735 0.250913 0.018169 0.057819 0.056253 0.032795 0.012333 0.022835 0.056085 0.030475 0.028654 0.019134 0.023284 0.042442 0.028986 0.029927 0.018863 0.036923 0.039545 0.036205 0.020627 0.028722

0.000504 0.000297 0.001087 0.000345 0.000728 0.000494 0.000469 0.000827 0.000101 0.000290 0.001518 0.000110 0.000259 0.000402 0.000110 0.000362 0.000325 0.000824 0.000136 0.000161 0.000491 0.000536 0.000666 0.000069 0.000353 0.000140 0.000351 0.000437 0.000426 0.000309 0.000159

0.001870 0.001212 0.000524 0.000935 0.001416 0.001227 0.000786 0.000894 0.001132 0.001021 0.006503 0.000607 0.001780 0.001581 0.000941 0.000388 0.000712 0.001913 0.001003 0.000934 0.000768 0.000815 0.001434 0.000844 0.001073 0.000576 0.001276 0.001587 0.001243 0.000665 0.000916

0.000016 0.000009 0.000030 0.000008 0.000022 0.000018 0.000013 0.000027 0.000003 0.000009 0.000023 0.000005 0.000006 0.000012 0.000004 0.000008 0.000011 0.000027 0.000004 0.000004 0.000020 0.000018 0.000020 0.000004 0.000011 0.000004 0.000010 0.000017 0.000011 0.000010 0.000004

0.282431 0.282329 0.282305 0.282376 0.282407 0.282295 0.282442 0.282191 0.282355 0.282499 0.282657 0.281143 0.282310 0.282517 0.282486 0.281134 0.282177 0.282351 0.282206 0.282445 0.282534 0.282273 0.282381 0.282424 0.282265 0.282381 0.282266 0.282558 0.282328 0.282775 0.282331

0.000017 0.000018 0.000016 0.000015 0.000022 0.000015 0.000020 0.000019 0.000018 0.000015 0.000024 0.000014 0.000015 0.000018 0.000016 0.000017 0.000016 0.000016 0.000017 0.000014 0.000016 0.000017 0.000015 0.000014 0.000017 0.000014 0.000017 0.000018 0.000018 0.000017 0.000021

492 473 518 501 733 497 571 824 751 513 437 1637 806 441 486 2024 976 442 515 589 893 700 469 440 525 488 439 831 436 439 715

0.282414 0.282318 0.282300 0.282367 0.282387 0.282284 0.282434 0.282177 0.282339 0.282489 0.282604 0.281124 0.282283 0.282503 0.282477 0.281119 0.282164 0.282335 0.282196 0.282434 0.282521 0.282262 0.282368 0.282417 0.282255 0.282376 0.282255 0.282534 0.282318 0.282770 0.282319

− 1.84 − 5.63 − 5.30 − 3.29 2.57 − 6.32 0.63 − 2.85 1.26 1.29 3.67 −21.93 0.51 0.21 0.28 −13.31 0.08 − 5.74 − 9.02 1.04 10.88 − 2.60 − 3.96 − 2.87 − 6.73 − 3.25 − 8.62 9.94 − 6.46 9.59 − 0.24

1372 1550 1570 1453 1344 1605 1310 1697 1426 1229 1044 3330 1510 1225 1259 3211 1671 1530 1757 1304 1047 1582 1461 1382 1649 1441 1674 1044 1562 741 1473

30 33 28 27 38 26 35 34 32 27 43 25 27 32 28 29 29 28 30 26 28 30 26 26 30 25 30 32 32 32 37

− 0.94 − 0.96 − 0.98 − 0.97 − 0.96 − 0.96 − 0.98 − 0.97 − 0.97 − 0.97 − 0.80 − 0.98 − 0.95 − 0.95 − 0.97 − 0.99 − 0.98 − 0.94 − 0.97 − 0.97 − 0.98 − 0.98 − 0.96 − 0.97 − 0.97 − 0.98 − 0.96 − 0.95 − 0.96 − 0.98 − 0.97

(continued on next page)

224

X. Long et al. / Tectonophysics 480 (2010) 213–231

Table 3 (continued) Sample

176

Yb/177Hf

2σ

176

Lu/177Hf

2σ

176

Hf/177Hf

2σ

Age (Ma)

(176Hf/177Hf)i

εHf

TcDM (Ma)⁎

2σ

fLu/Hf

L07BH01 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62

0.025366 0.029556 0.041739 0.054667 0.065242 0.041281 0.021687 0.025649 0.013948 0.041595 0.056068 0.077944 0.029625 0.035822 0.084160 0.029304 0.058764 0.044139 0.037812 0.028396 0.006602 0.019727 0.021587 0.056577 0.034551 0.039253 0.043808 0.023591 0.016931 0.026099 0.014703

0.000083 0.000214 0.000715 0.000336 0.001140 0.000416 0.000105 0.000601 0.000233 0.000391 0.001369 0.001850 0.000557 0.000520 0.000322 0.000459 0.001202 0.001069 0.000320 0.000106 0.000122 0.000432 0.000345 0.000298 0.001226 0.000311 0.000183 0.000148 0.000811 0.000870 0.000076

0.000754 0.000916 0.001223 0.001663 0.002046 0.001209 0.000739 0.000973 0.000491 0.001437 0.001648 0.002565 0.000952 0.001543 0.002503 0.000974 0.001740 0.001786 0.001349 0.000865 0.000218 0.000625 0.000713 0.001708 0.001056 0.001255 0.001465 0.000987 0.000550 0.000837 0.000494

0.000003 0.000006 0.000020 0.000010 0.000038 0.000011 0.000004 0.000021 0.000006 0.000012 0.000038 0.000072 0.000016 0.000022 0.000009 0.000018 0.000035 0.000039 0.000011 0.000002 0.000004 0.000014 0.000010 0.000008 0.000037 0.000009 0.000005 0.000006 0.000028 0.000029 0.000002

0.282237 0.282052 0.282444 0.282378 0.282221 0.282362 0.281102 0.282838 0.281738 0.282213 0.282471 0.281883 0.282323 0.282818 0.282395 0.282110 0.282404 0.282847 0.282379 0.282390 0.281133 0.281030 0.280889 0.282290 0.281959 0.282503 0.282544 0.282413 0.280950 0.280889 0.282373

0.000015 0.000017 0.000017 0.000018 0.000013 0.000013 0.000013 0.000013 0.000012 0.000016 0.000016 0.000017 0.000018 0.000019 0.000015 0.000018 0.000017 0.000017 0.000019 0.000016 0.000014 0.000015 0.000015 0.000015 0.000015 0.000015 0.000016 0.000015 0.000019 0.000016 0.000017

1147 504 497 475 478 531 2220 472 1436 476 548 699 519 567 441 571 481 561 471 499 2008 2532 2867 481 808 481 680 514 2554 2546 790

0.282221 0.282043 0.282433 0.282363 0.282202 0.282350 0.281070 0.282830 0.281725 0.282200 0.282454 0.281849 0.282314 0.282802 0.282374 0.282100 0.282388 0.282828 0.282367 0.282382 0.281125 0.281000 0.280850 0.282275 0.281943 0.282492 0.282526 0.282404 0.280923 0.280848 0.282366

5.93 − 14.69 − 1.04 − 4.00 − 9.63 − 3.23 − 10.55 12.45 − 5.15 − 9.76 0.82 − 17.24 − 4.79 13.57 − 4.37 −11.21 − 2.99 14.35 − 3.96 − 2.80 − 13.46 − 5.86 − 3.43 − 7.00 − 11.49 0.69 6.30 − 1.70 − 8.09 − 10.93 3.09

1510 2035 1335 1468 1757 1475 3231 620 2316 1762 1282 2324 1545 640 1459 1914 1422 594 1463 1427 3206 3247 3395 1626 2123 1233 1108 1383 3379 3517 1364

28 30 30 31 22 24 23 23 21 28 28 27 32 33 27 31 29 30 33 29 24 25 26 27 25 27 28 26 29 23 31

− 0.98 − 0.97 − 0.96 − 0.95 − 0.94 − 0.96 − 0.98 − 0.97 − 0.99 − 0.96 − 0.95 − 0.92 − 0.97 − 0.95 − 0.92 − 0.97 − 0.95 − 0.95 − 0.96 − 0.97 − 0.99 − 0.98 − 0.98 − 0.95 − 0.97 − 0.96 − 0.96 − 0.97 − 0.98 − 0.97 − 0.99

L07BH11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

0.100593 0.110358 0.072457 0.038995 0.081488 0.073940 0.097905 0.072312 0.061334 0.085972 0.054572 0.037484 0.050810 0.059212 0.074792 0.069138 0.052554 0.045834 0.045195 0.037647 0.056039 0.128576 0.053193 0.055520 0.112512 0.077811 0.037401 0.086892 0.041351 0.042484 0.073367

0.000982 0.002281 0.001724 0.000165 0.003023 0.001415 0.001565 0.000684 0.000232 0.002421 0.000567 0.000439 0.000277 0.000646 0.001322 0.000975 0.000919 0.000596 0.000365 0.000033 0.000428 0.003254 0.000345 0.000637 0.004592 0.000753 0.000080 0.000633 0.000076 0.000297 0.002726

0.003059 0.003276 0.002257 0.001222 0.002516 0.002330 0.003055 0.002222 0.001956 0.002397 0.001743 0.001225 0.001622 0.001843 0.002301 0.002160 0.001657 0.001458 0.001441 0.001218 0.001784 0.003385 0.001641 0.001752 0.003451 0.002388 0.001194 0.002374 0.001321 0.001354 0.002262

0.000031 0.000062 0.000056 0.000006 0.000092 0.000045 0.000048 0.000019 0.000006 0.000049 0.000018 0.000014 0.000009 0.000021 0.000028 0.000031 0.000027 0.000019 0.000012 0.000002 0.000014 0.000066 0.000011 0.000019 0.000139 0.000022 0.000001 0.000020 0.000002 0.000009 0.000085

0.282687 0.282665 0.282650 0.282699 0.282711 0.282705 0.282748 0.282709 0.282692 0.282807 0.282639 0.282652 0.282683 0.282695 0.282651 0.282681 0.282629 0.282696 0.282677 0.282683 0.282709 0.282766 0.282680 0.282657 0.282700 0.282652 0.282666 0.282761 0.282659 0.282679 0.282670

0.000022 0.000021 0.000020 0.000018 0.000020 0.000019 0.000030 0.000020 0.000017 0.000022 0.000019 0.000017 0.000021 0.000018 0.000019 0.000019 0.000017 0.000018 0.000016 0.000015 0.000017 0.000023 0.000014 0.000015 0.000022 0.000017 0.000015 0.000022 0.000016 0.000019 0.000017

411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411 411

0.282664 0.282639 0.282632 0.282690 0.282692 0.282687 0.282725 0.282692 0.282677 0.282789 0.282625 0.282643 0.282670 0.282681 0.282634 0.282664 0.282616 0.282685 0.282666 0.282673 0.282695 0.282740 0.282667 0.282644 0.282673 0.282634 0.282656 0.282743 0.282649 0.282669 0.282653

5.21 4.35 4.10 6.15 6.21 6.03 7.38 6.20 5.68 9.64 3.85 4.47 5.45 5.84 4.16 5.24 3.54 5.97 5.30 5.55 6.33 7.90 5.34 4.51 5.56 4.16 4.96 8.03 4.69 5.39 4.83

943 988 1000 895 892 901 831 892 919 715 1013 981 931 911 998 942 1029 905 939 926 886 805 937 979 926 998 956 798 970 934 963

39 36 34 32 34 34 53 35 31 39 33 31 38 33 33 34 31 33 29 27 31 40 24 27 36 30 28 39 30 34 28

− 0.91 − 0.90 − 0.93 − 0.96 − 0.92 − 0.93 − 0.91 − 0.93 − 0.94 − 0.93 − 0.95 − 0.96 − 0.95 − 0.94 − 0.93 − 0.93 − 0.95 − 0.96 − 0.96 − 0.96 − 0.95 − 0.90 − 0.95 − 0.95 − 0.90 − 0.93 − 0.96 − 0.93 − 0.96 − 0.96 − 0.93

L07BH08 1 2 3 4 5 6 7 8 9 10

0.036532 0.025767 0.033169 0.019626 0.051466 0.035782 0.019920 0.065419 0.031760 0.033655

0.000788 0.000118 0.000137 0.000045 0.000682 0.000570 0.000567 0.001380 0.000381 0.000883

0.001027 0.000911 0.000999 0.000565 0.001771 0.000918 0.000479 0.001775 0.000944 0.000865

0.000022 0.000004 0.000004 0.000001 0.000023 0.000015 0.000013 0.000031 0.000011 0.000012

0.282462 0.281560 0.282357 0.282421 0.282896 0.282475 0.282422 0.282146 0.282397 0.282306

0.000015 0.000018 0.000015 0.000018 0.000015 0.000018 0.000014 0.000016 0.000015 0.000018

482 2126 511 512 478 436 485 854 514 524

0.282472 0.281428 0.282454 0.282453 0.282474 0.282501 0.282470 0.282238 0.282452 0.282446

− 0.68 3.40 − 3.77 − 1.33 14.36 − 1.16 − 1.87 − 4.28 − 2.25 − 5.23

1304 2441 1486 1362 525 1291 1368 1794 1411 1571

27 32 26 32 27 32 24 27 26 33

− 0.97 − 0.97 − 0.97 − 0.98 − 0.95 − 0.97 − 0.99 −0.95 − 0.97 − 0.97

X. Long et al. / Tectonophysics 480 (2010) 213–231

225

Table 3 (continued) Sample L07BH08 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

176

Yb/177Hf

0.024787 0.030103 0.031681 0.023124 0.027529 0.026759 0.043695 0.055066 0.034690 0.029497 0.055177 0.039021 0.160740 0.021247 0.013756 0.016267 0.023530 0.067386 0.053799 0.071507 0.026063 0.046385 0.016112 0.073086 0.021629 0.013447 0.018777 0.049693 0.022726 0.015651 0.053211 0.031155 0.022739 0.025326 0.048044 0.041586 0.029330 0.158314 0.030395 0.037235 0.021557 0.017203 0.046934 0.064048

2σ

176

Lu/177Hf

0.000190 0.000126 0.000763 0.000295 0.000211 0.000194 0.001209 0.000655 0.000209 0.000224 0.001007 0.000286 0.001449 0.000199 0.000033 0.000553 0.000110 0.001608 0.001069 0.002706 0.000514 0.001676 0.000157 0.001548 0.000626 0.000196 0.000180 0.000865 0.000173 0.000104 0.000282 0.000390 0.000048 0.000261 0.000181 0.000560 0.000645 0.003894 0.000089 0.000542 0.000458 0.000078 0.001103 0.000350

0.000754 0.000903 0.001004 0.000851 0.000838 0.000932 0.001444 0.001884 0.001210 0.001040 0.001832 0.001197 0.004015 0.000800 0.000514 0.000524 0.000808 0.002136 0.001716 0.002276 0.001030 0.001668 0.000545 0.002193 0.000679 0.000447 0.000593 0.001545 0.000855 0.000597 0.001561 0.000992 0.000801 0.000813 0.001463 0.001320 0.000919 0.004453 0.001010 0.001165 0.000815 0.000545 0.001549 0.002098

⁎TcDM = t + (1/λ) × ln[1 + ((176Hf/177Hf)S, t − (176Hf/177Hf) depleted mantle, respectively.

2σ

176

0.000006 0.000003 0.000022 0.000011 0.000006 0.000006 0.000031 0.000023 0.000006 0.000007 0.000023 0.000006 0.000028 0.000007 0.000002 0.000016 0.000004 0.000047 0.000028 0.000080 0.000018 0.000056 0.000005 0.000041 0.000024 0.000007 0.000005 0.000026 0.000006 0.000003 0.000005 0.000010 0.000002 0.000009 0.000006 0.000019 0.000019 0.000104 0.000004 0.000016 0.000014 0.000002 0.000028 0.000008

0.280732 0.281919 0.282815 0.282839 0.280966 0.282184 0.282075 0.282836 0.282013 0.282811 0.282763 0.282362 0.282153 0.282904 0.281828 0.282322 0.282193 0.282879 0.282443 0.282505 0.282872 0.280894 0.282836 0.281875 0.280919 0.281152 0.282223 0.282534 0.282786 0.282678 0.282615 0.282441 0.280941 0.281857 0.280998 0.280823 0.282466 0.282901 0.282642 0.282409 0.282888 0.282519 0.282445 0.282278

DM, t)/((

176

Hf/177Hf

Lu/177Hf)

2σ

Age (Ma)

(176Hf/177Hf)i

εHf

TcDM (Ma)⁎

2σ

fLu/Hf

0.000018 0.000014 0.000014 0.000013 0.000014 0.000016 0.000016 0.000016 0.000016 0.000015 0.000016 0.000015 0.000017 0.000015 0.000021 0.000015 0.000015 0.000017 0.000018 0.000020 0.000017 0.000016 0.000013 0.000016 0.000016 0.000021 0.000016 0.000016 0.000016 0.000017 0.000017 0.000018 0.000018 0.000017 0.000016 0.000017 0.000014 0.000019 0.000013 0.000015 0.000015 0.000017 0.000020 0.000021

2908 756 489 482 2691 807 1424 508 821 471 483 489 814 519 729 520 759 468 515 442 487 2949 505 780 2386 2000 905 483 441 438 433 437 2367 1641 2628 2466 433 464 434 442 479 439 465 433

0.280920 0.282300 0.282468 0.282472 0.281061 0.282268 0.281878 0.282456 0.282259 0.282479 0.282471 0.282468 0.282264 0.282449 0.282317 0.282448 0.282298 0.282481 0.282451 0.282497 0.282469 0.280893 0.282457 0.282285 0.281260 0.281509 0.282206 0.282471 0.282498 0.282499 0.282503 0.282500 0.281272 0.281739 0.281102 0.281208 0.282503 0.282483 0.282502 0.282497 0.282474 0.282499 0.282483 0.282503

− 8.18 − 13.94 11.99 12.72 − 4.93 − 3.47 5.64 12.84 − 9.38 11.43 9.73 − 4.12 − 6.09 15.86 − 17.58 − 4.63 − 4.12 13.42 − 0.89 − 0.37 13.96 − 3.31 13.22 −15.68 − 13.22 − 13.30 0.24 1.71 9.97 6.15 3.54 − 2.36 − 13.04 3.29 − 6.34 − 15.88 − 1.54 13.40 4.68 − 3.46 14.40 0.57 − 1.81 − 8.55

3671 2205 658 614 3329 1714 1752 629 2027 671 770 1486 1853 481 2366 1537 1708 566 1342 1256 554 3456 607 2312 3502 3191 1605 1183 723 917 1048 1353 3478 2051 3350 3702 1308 564 989 1414 524 1205 1348 1666

32 25 26 23 24 28 26 29 29 28 28 28 30 28 38 26 27 29 31 34 30 17 24 27 26 37 29 29 29 31 31 33 32 30 28 26 25 32 24 26 26 31 35 37

− 0.98 − 0.97 − 0.97 − 0.97 − 0.97 − 0.97 − 0.96 − 0.94 − 0.96 − 0.97 − 0.94 − 0.96 − 0.88 − 0.98 − 0.98 − 0.98 − 0.98 − 0.94 − 0.95 − 0.93 − 0.97 − 0.95 − 0.98 − 0.93 − 0.98 − 0.99 − 0.98 − 0.95 − 0.97 − 0.98 − 0.95 − 0.97 − 0.98 − 0.98 − 0.96 − 0.96 − 0.97 − 0.87 − 0.97 − 0.96 − 0.98 − 0.98 − 0.95 − 0.94

176 Lu/177Hf)DM)], UC − (

6.2. Sedimentary provenance The Habahe flysch sequence was generally interpreted as a passive continental margin deposit (e.g. He et al., 1990; Chen and Jahn, 2002; Li et al., 2006). However, recent work in the eastern Chinese Altai revealed that detrital zircons from the group are dominated by a 460 to 540 Ma population, mostly with positive εHf(t) values (Long et al., 2007). Combining these data with the euhedral to subhedral shapes of detrital zircons and the geochemistry of sediments from this sequence, an early Paleozoic arc source, dominated by juvenile material, was suggested as the source of the clastic material (Long et al., 2007, 2008a). In this study, detrital zircons from the three units of the Habahe flysch show a similar age pattern, and their youngest zircon populations all formed at about 438 Ma, indicating that these samples share the same maximum depositional age. However, the maximum depositional age is younger than that (∼470 Ma) of sediments from the same group in the eastern Chinese Altai (Fig. 8). The difference in the maximum depositional age between the northwestern and eastern Altai may be interpreted in three different ways: (1) same source but different depositional ages; (2) different sources but similar depositional ages; and (3) different sources and different depositional ages. If the sequences in the northwestern and eastern Altai have the same source, the

where UC, S and DM are the upper continental crust, the sample and the

youngest zircon population with ages of ∼438 Ma and the metamorphic detrital zircons of ∼500 and ∼840 Ma in the northwest should also occur in the east. However, detrital zircons with such ages have not been discovered in the eastern Altai (Fig. 8; Long et al., 2007), suggesting different sources for the sediments in the two areas. The differences in the youngest zircon populations, rock assemblages and degrees of metamorphism may imply different depositional ages. Therefore, the third interpretation may be the most likely scenario for the formation of the Habahe sequence. Based on the U–Pb age data, detrital zircons of the Habahe flysch sequence in the northwestern Altai can be divided into three populations: an early Paleozoic, Neoproterozoic and pre-Neoproterozoic population (Fig. 5). In comparison with the other two, early Paleozoic detrital zircons are predominant in the Habahe sequence and generally show concentric zoning and high Th/U ratios, which are consistent with an igneous origin. Their euhedral shapes suggest that these detrital zircons experienced relatively short sedimentary transport and are probably related to proximal magmatism. Recent studies reveal that several early Paleozoic plutons are scattered in the Chinese Altai (Fig. 1; Windley et al., 2002; Shan et al., 2005; Wang et al., 2006; Yuan et al., 2007; Briggs et al., 2007; Sun et al., 2008; Sun et al., 2009). Early Silurian volcanic rocks with an age of 436 ± 4 Ma were also discovered in the

226

X. Long et al. / Tectonophysics 480 (2010) 213–231

Fig. 4. U–Pb concordia diagrams for zircon samples of the early Paleozoic sedimentary rocks.

south of the study area (Fig. 1; Zeng et al., 2007). The above subaerial volcanism, together with coeval plutons, may have provided a large proportion of the clastic material in the Habahe flysch sedimentary basin. The Neoproterozoic zircon population of the Habahe sequence shows several age peaks at 568, 752, 820 and 843 Ma (Fig. 5). An

analogous Neoproterozoic age distribution was found in the Tuva– Mongol Massif (Fig. 1), since new U–Pb dating of detrital zircons from meta-sediments in this Massif revealed Neoproterozoic age peaks at 512, 605, 572, 584 and 876 Ma (Kelty et al., 2008). Previous geochronological studies demonstrated that widespread Neoproterozoic magmatism

X. Long et al. / Tectonophysics 480 (2010) 213–231

Fig. 5. Age histogram and relative probability plots of detrital zircon samples of the early Paleozoic sedimentary rocks.

Fig. 6. Diagrams of εHf(t) values vs. crystallizing ages for zircons from the early Paleozoic sedimentary rocks.

227

228

X. Long et al. / Tectonophysics 480 (2010) 213–231

along the northern margin of the CAOB (Salnikova et al., 1998, 2001; Dobretsov et al., 2006; Gladkochub et al., 2008). U–Pb dating of metamorphic rocks in this belt yielded early Paleozoic ages at about 500 Ma (Khromykh et al., 2004; Gladkochub et al., 2008). This suggests that the metamorphic belt may be an alternate source, and some clastic material was probably transported to the northwestern Chinese Altai. Therefore, the flysch sequence in the northwestern Chinese Altai may have multiple-sources. 6.3. Nature of the Chinese Altai

Fig. 7. U–Pb concordia diagram for zircon sample of the rhyolite of the Dongxileke Formation.

occured in the Tuva–Mongol Massif and along the southern margin of the Siberian craton (Badarch et al., 2002; Tomurtogoo, 2006; Windley et al., 2007; Kelty et al., 2008, and references therein). Furthermore, the preNeoproterozoic zircon population in this study, including Archean grains, was also recognized in the Massif. Therefore, these similarities suggest that the Tuva–Mongol Massif probably was an important source for the Habahe sequence. Metamorphic detrital zircons in sedimentary rocks record regional metamorphism in their source area. Metamorphic detrital zircons of ∼ 500 and ∼840 Ma were found in the northwestern Chinese Altai. However, there are no coeval metamorphic rocks in the area, and the rounded shapes of the metamorphic zircons may imply good sorting and long distance of transportation. Recent studies reveal a wide early Paleozoic granulite-facies metamorphic belt in southern Siberia

The tectonic setting and evolution of the Chinese Altai is controversial, and several tectonic settings have been suggested, including a passive continental margin (He et al., 1990), accretionary complex (Sengör and Natal'in, 1996), Andean-type island arc (Windley et al., 2002, 2007; Sun et al., 2008; Xiao et al., 2009) and Precambrian microcontinent (e.g. Dobretsov et al., 1995; Hu et al., 2000; Buslov et al., 2001; Li et al., 2006; Wang et al., 2009). According to Sengör and Natal'in (1996), the Chinese Altai is part of the Gorny–Altay which, along with the Kolyvan–Rudny Altay, represents a large accretionary complex situated in the front of the pre-Altaid South Gobi–Tuva–Mongol Massif. In their model, the Gorny–Altay consists of an early Paleozoic accretionary wedge and magmatic arc with superimposed mid-Paleozoic magmatic arc (Sengör and Natal'in, 1996). However, a large number of Precambrian Nd whole-rock model ages for granites and gneisses led to the proposal of an Andean-type continental island arc or a Precambrian microcontinent model for the Chinese Altai. Our geochemical data for the sedimentary rocks in the Chinese Altai show an active continental margin affinity (Long et al., 2008a). Recent detrital zircon ages for high-grade metamorphic paragneisses from the central Altai indicate that their protoliths were deposited during the early Paleozoic (Sun et al., 2008). U–Pb dating of orthogneisses from the southern margin of the Chinese Altai also yielded Paleozoic ages (Wang et al., 2006; Hu et al., 2006; Yuan et al., 2007; Briggs et al., 2007). The above data show that the proposed “old” basement rocks actually formed in the early Paleozoic and not in the Precambrian. Published U–Pb and Hf isotope data of detrital zircons (Long et al., 2007; Sun et al., 2008) and data in this paper reveal a predominant zircon population with 206Pb/238U ages between 430 and 540 Ma and most grains of this population have positive εHf(t) values, suggesting derivation from juvenile melts (Long et al., 2007). Positive εHf(t) zircon values for the early Paleozoic granites and orthogneisses also suggest the Chinese Altai to be composed of predominantly juvenile material (Sun et al., 2008, 2009; and our unpublished data). The geochemistry of the Habahe sediments reveals an active continental margin setting (Long et al., 2008a). In addition, early Paleozoic dismembered ophiolite fragments with mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) chemistry were recently found in the sedimentary sequence (Wong et al., 2008). These results suggest a continuous subduction-accretion process in the Chinese Altai during the early Paleozoic. Therefore, we suggest that the Chinese Altai is a predominantly juvenile block consisting mainly of early Paleozoic subduction–accretion assemblages. This model also explains the formation of early Paleozoic granites in a subduction environment and the relation between coeval granites and sedimentary sequences in this mountain range. 6.4. Tectonic evolution of the Chinese Altai Based on recent geochemical and geochronological data for sedimentary, igneous and metamorphic rocks, the tectonic evolutionary history of the Chinese Altai can be reconstructed, and details of the subduction and accretion process are depicted as follows (Fig. 9):

Fig. 8. Contrasting relative probability plots of detrital zircons from the early Paleozoic sedimentary rocks and para-gneisses in the Chinese Altai.

Phase (1). During the Neoproterozoic, a tract of the Paleo-Asian Ocean began to be subducted northward, and a continental arc evolved along the

X. Long et al. / Tectonophysics 480 (2010) 213–231

229

Fig. 9. Tectonic model for the Chinese Altai in the early Paleozoic. More detailed history is depicted in the text.

southern margin of the Siberian Craton (Fig. 9a). Various terranes, such as ophiolites, seamounts and microcontinents, were probably accreted onto this active continental margin (Buslov et al., 2001; Zhmodik et al., 2006). With the above subduction process continuing, the Tuva–Mongol microcontinent docked with the Siberian Craton, probably during the late Neoproterozoic to early Paleozoic (Fig. 9b), as suggested by the highgrade metamorphic belt along the southern margin of the Siberian Craton (Khromykh et al., 2004; Dobretsov et al., 2006). As a natural consequence of collision, a new subduction zone was initiated along the southern margin of the Tuva–Mongol microcontinent, and subduction–accretion processes may have continued in the early Paleozoic (Fig. 9b). Phase (2). The early Paleozoic was a critical period for the accretionary complexes to develop. In the early Paleozoic, the Tuva–Mongol microcontinent had accreted onto Siberia, and subduction along its southern margin continued (Fig. 9b). Large volumes of newly-formed arc material and a small quantity of clastic sediments from the microcontinent and the metamorphic belt were incorporated into the accretionary prism. With continued growth of the accretionary prism, we speculate that rollback of the trench caused upwelling of hot asthenospheric mantle from depth and triggered partial melting of accreted juvenile material to generate widespread granitoids. Seamounts and ophiolites were also accreted onto the accretionary complex during this stage (Fig. 9b–c). Phase (3). In the early Devonian, a young arc was built on the accretionary complex, and ocean crust subduction continued (Fig. 9c). A spreading ridge may have replaced the normal oceanic crust and was subducted beneath the accretionary complex, as suggested by

Devonian high-Mg andesites (Niu et al., 1999), Nb-enriched basalts (Zhang et al., 2004), adakites (Niu et al., 2006) and high temperature– low pressure metamorphism (Wei et al., 2007). This caused a marked change in εHf(t) zircon values (Sun et al., 2009). We proposed that the contact of suboceanic mantle material with the accretionary complex and the high heat flow through a slab window was induced partial melting in the overlying accretionary complex, which was facilitated by the heat released from upwelling mantle and by the relatively low solidus of water-rich rocks in the accretionary prism (Sisson et al., 1989; Kinoshita, 2002; Bradley et al., 2003; Cole et al., 2006). This process may have resulted in the formation of large volumes of Devonian granites and felsic volcanic rocks with high εNd(t) and εHf(t) values. 7. Conclusions U–Pb dating of the Habahe flysch sequence in the northwestern Chinese Altai defines a new zircon population with the youngest ages being ∼438 Ma. These, in combination with the overlying rhyolite of the Dongxileke Formation (411 ± 5 Ma), constrain the depositional age of the Habahe Group between the early Silurian and early Devonian (411–438 Ma). The age of the Dongxileke rhyolite also indicates that the overlying Baihaba Formation probably began to be deposited in the early Devonian. The age patterns of detrital zircons in this study reveal that the early Paleozoic flysch sequences in the northwestern and eastern Chinese Altai are derived from different sources. The Tuva–Mongol Massif and adjacent island arc and metamorphic belt may be alternate source regions for the sedimentary

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sequence in the northwestern Chinese Altai. Based on new geochemical and chronological data for granitoids and metamorphic rocks, we consider the Chinese Altai as a predominantly juvenile terrane, mainly composed of accreted material added by subduction–accretion processes in the early Paleozoic. Acknowledgements This study was supported by the National Basic Research Program of China (2007CB411308), the National Natural Science Foundation of China (Grants No. 40721063, 40772130 and 40803009), the Hong Kong RGC Grants (HKU704307P and 704004P), the HKU CRCG Research Grant (200711159058), the Chinese Academy of Sciences Innovation Project (GIGCX-07-03) and the CAS/SAFEA International Partnership Program for Creative Research Teams. We are grateful to A. Kröner for his constructive comments on the manuscript. We thank Drs. Y.H. Yang, Y.D. Jiang, H.Y. Geng, J. Wong, L.M. Li, C.Q. Yin and K. Wong for their laboratory assistance. This is contribution No. IS1115 from GIGCAS. 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