Ordovician meta-sedimentary rocks from the northern Altai-Mongolian

Journal of Asian Earth Sciences 96 (2014) 69–83

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Geochemical study of the Cambrian–Ordovician meta-sedimentary rocks from the northern Altai-Mongolian terrane, northwestern Central Asian Orogenic Belt: Implications on the provenance and tectonic setting Ming Chen a,b, Min Sun a,b,⇑, Keda Cai c, Mikhail M. Buslov d,e, Guochun Zhao a, Elena S. Rubanova d,e a

Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China HKU Shenzhen Institute of Research and Innovation, Shenzhen, China Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China d Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk 630090, Russia e Novosibirsk State University, Novosibirsk 630090, Russia b c

a r t i c l e

i n f o

Article history: Received 18 May 2014 Received in revised form 19 August 2014 Accepted 25 August 2014 Available online 2 September 2014 Keywords: Altai-Mongolian terrane Meta-sedimentary rocks Provenance Accretionary prism

a b s t r a c t The Altai-Mongolian terrane (AM) is a key component of the Central Asian Orogenic Belt (CAOB), but its tectonic nature has been poorly constrained. This paper reports geochemical compositions of Cambrian– Ordovician meta-sedimentary rocks from the northern AM to trace their source nature and depositional setting, which in turn place constraints on the geodynamic evolution of the AM. The Cambrian–Ordovician meta-sedimentary rocks from the northern AM show variable major-element compositions, with negative correlation between SiO2 and TiO2, Al2O3, Fe2OT3, MgO and K2O. Their high ICV values (1.18–2.53) and relatively low CIA values (37.9–76.3) indicate that the sediments were immature and probably underwent mild to moderate chemical weathering. The low-SiO2 samples are characterized by relatively restricted SiO2/Al2O3 (mostly 2.60–6.07) and low Rb/Sr ratios (0.02–1.89), implying their proximal deposition without obvious sedimentary sorting and recycling. In contrast, the high-SiO2 samples show much higher SiO2/Al2O3 ratios (15.4–19.9) possibly due to sedimentary sorting and/or silicification. All these samples yield relatively high Al2O3/TiO2 ratios (15.6–22.8), strong LREEs/ HREEs differentiation ((La/Yb)N = 4.86–10.7) and obvious negative Eu anomalies (dEu = 0.61–0.83). Combined with their Th/Sc, Zr/Sc, La/Th and Co/Th ratios comparable with intermediate-acidic magmatic rocks, we infer that these kinds of magmatic rocks served as a major source for the investigated meta-sedimentary rocks. The TiO2, Al2O3 and Fe2OT3 + MgO concentrations are mostly higher than typical sediments from passive margin, and the Th/U, La/Sc, Th/Sc, Eu/Eu⁄, Zr/Hf, Zr/Th and La/Th ratios are quite similar to sediments from continental arcs. These data suggest that the Cambrian–Ordovician metasedimentary rocks from the northern AM were most likely deposited in an environment related to a continental arc setting rather than a passive regime. These rocks show strong similarities to their counterparts in the Chinese Altai (CA, southern AM) and Tseel terrane (southeastern extension of the CA in western Mongolia) in terms of geochemical compositions and depositional setting. With combination of recent isotopic studies for detrital zircons, our data suggest that the AM probably represented a coherent accretionary prism along a continental arc in the early Paleozoic. Ó 2014 Published by Elsevier Ltd.

1. Introduction The Central Asian Orogenic Belt (CAOB) is a type accretionary orogen that covers an immense area among the East European, Siberian, Tarim and North China continents (Fig. 1 inset; e.g., Zonenshain et al., 1990; Jahn et al., 2000; Windley et al., 2007). ⇑ Corresponding author at: Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China. E-mail address: [email protected] (M. Sun). http://dx.doi.org/10.1016/j.jseaes.2014.08.028 1367-9120/Ó 2014 Published by Elsevier Ltd.

Numerous magmatic arcs and accretionary prisms were generated and amalgamated by long-lived subduction–accretion processes from the latest Mesoproterozoic to late Paleozoic or possibly to early Mesozoic (e.g., Khain et al., 2002, 2003; Xiao et al., 2003, 2009, 2010; Charvet et al., 2007; Wang et al., 2007), making it the most important site of Phanerozoic crustal growth on the Earth (e.g., Jahn et al., 2000; Jahn, 2004; Kovalenko et al., 2004; Yuan et al., 2007; Sun et al., 2008). Minor Precambrian continental blocks, such as the Tuva-Mongolian terrane, also played an important role in the formation of this huge accretionary collage

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M. Chen et al. / Journal of Asian Earth Sciences 96 (2014) 69–83

Fig. 1. Simplified geological map showing the tectonic framework of the northwestern Central Asian Orogenic Belt (CAOB, modified after Buslov and Safonova (2010)). Fig. 1 inset shows the location of the CAOB. Abbreviations: TC, Tarim continent; NCC, North China continent; TM, Tuva-Mongolian terrane; B, Barguzin; CTUS, Charysh-TerektaUlagan-Sayan.

(e.g., Badarch et al., 2002). Revealing the tectonic nature and evolution of different units is essential to reconstruct the accretionary processes of the CAOB, however, most of which are still not well understood (e.g., Sengor et al., 1993; Mossakovsky et al., 1993; Buslov et al., 2001; Windley et al., 2007). For instance, the Altai-Mongolian terrane (AM) situated in the northwestern CAOB (Fig. 1) has been variously viewed as a micro-continent (e.g., Dobretsov et al., 1995; Buslov et al., 2001; Windley et al., 2002; Li et al., 2006; Yang et al., 2011) or an accretionary prism (e.g., Mossakovsky et al., 1993; Sengor et al., 1993; Sengör and Natal’in, 1996; Long et al., 2007, 2008, 2010, 2012; Yuan et al., 2007; Sun et al., 2008; Xiao et al., 2009). The former view considered that a Precambrian crystalline basement probably exists beneath the AM mainly based on the discovery of Proterozoic xenocrystic zircons in some volcanic rocks (e.g., Windley et al., 2002), Proterozoic whole-rock Nd model ages of igneous and meta-sedimentary rocks (e.g., He et al., 1990; Hu et al., 2000; Li et al., 2006; Wang et al., 2006, 2009), as well as high-grade metamorphism (e.g., Bureau of Geology and Mineral Resources of Xinjiang (BGMRX), 1993). Accordingly, the thick early Paleozoic flysch-like sedimentary sequences throughout the AM were suggested to be deposited in a passive continental margin (Zonenshain, 1972; Zonenshain et al., 1990; He et al., 1990). However, no Precambrian basement rocks have been recognized so far. Some other scientists argued that these early Paleozoic sedimentary sequences were possibly deposited in a fore-arc/back-arc basin or island arc setting based on the identification of volcaniclastic fragments in the sedimentary rocks (Mossakovsky and Dergunov, 1985; Watanabi et al., 1994; Byamba and Dejidmaa, 1999). Recent studies in the Chinese Altai (CA, the southern AM in northwestern China) and Tseel terrane (the southeastern extension of the CA in western Mongolia; Jiang et al., 2012) show that the early Paleozoic

flysch-like sedimentary sequences and high-grade paragneisses contain detrital zircons predominantly of ca. 550–430 Ma old (Long et al., 2007, 2010; Sun et al., 2008; Jiang et al., 2010, 2012; Yang et al., 2011). These rocks show strong geochemical affinities with those in an active continental margin or continental arc (Long et al., 2008, 2012; Jiang et al., 2012). In addition, the widespread Paleozoic granitic rocks in the CA are characterized by positive zircon eHf(t) values and young two-stage Hf model ages (e.g., Sun et al., 2008, 2009; Cai et al., 2011a,b), indicating their derivation from juvenile sources. Accordingly, the accretionary prism model was proposed to interpret the evolution of the CA in the early Paleozoic (Long et al., 2007, 2008, 2010, 2012; Yuan et al., 2007; Sun et al., 2008). However, such kind of studies has not been extended to the northern AM in Russia. The provenance and tectonic setting of the early Paleozoic sedimentary sequences in this area is crucial to understand the tectonic nature of the whole terrane, but little is known. This study is focused on the most widely distributed Cambrian– Ordovician meta-sedimentary sequences (Fig. 2) and a thick assumed late Proterozoic turbidite sequence (Fig. 3) in the northern AM. These sequences probably represent the oldest deposition in the northern AM, and thus recorded the early evolution of this terrane. Systematic whole-rock major- and trace-element analyses were conducted to trace their source nature and deposition setting, which in turn place constraints on the tectonic nature of the AM and accretionary orogenic processes in the northwestern CAOB. 2. Geological setting The AM in the northwestern CAOB is separated from the Gorny Altai terrane (GA) to the north by the Charysh-Terekta-UlaganSayan (CTUS) suture-shear zone (Fig. 1), from the Rudy Altai


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Fig. 2. The geological units of the northern Altai-Mongolian terrane to the south of the Gorny Altai terrane (modified after Daukeev et al., 2008).

Fig. 3. The geological units of the northern Altai-Mongolian terrane to the southeastern of the Gorny Altai terrane (modified after Buslov and Safonova, 2010).

terrane (RA) to the west by the North-Eastern fault, and from the Junggar terrane to the south by the Irtysh fault. It covers an area about 1000 km long and up to 250 km wide that extends from Russia to northwestern China to western Mongolia. In general, this terrane is dominated by thick terrigenous sedimentary sequences that were intruded by extensive early–middle Paleozoic plutonic rocks (Fig. 4; e.g. Yuan et al., 2007; Daukeev et al., 2008; Sun et al., 2008, 2009; Glorie et al., 2011; Cai et al., 2011a,b). In the CA, the early Paleozoic sedimentary sequences (Fig. 4) were grouped into the Habahe Group (Long et al., 2007 and references therein). Some of them underwent greenschist-facies metamorphism. Detrital zircons from the sedimentary rocks of this group are predominantly 550–430 Ma old, with minor Precambrian grains (Long et al., 2007, 2010). High-grade paragneisses cropping out in this area have been proved to be the metamorphic

counterparts of the Habahe Group with similar provenance (Long et al., 2007, 2008, 2010, 2012; Sun et al., 2008; Jiang et al., 2010; Yang et al., 2011). Whole-rock geochemical studies show that the Habahe Group was deposited in an active continental margin or continental arc setting in the late Ordovician to Silurian with sources mainly from intermediate-felsic igneous rocks of a nearby magmatic arc (Long et al., 2007, 2008, 2010, 2012). The Habahe Group was intruded mainly by metaluminous to peraluminous granitoids with early–middle Paleozoic ages (e.g., Windley et al., 2002; Wang et al., 2006; Yuan et al., 2007; Sun et al., 2008; Cai et al., 2011a), and to a lesser extent A-type granitic plutons with late Paleozoic to Mesozoic ages (Cai et al., 2011b). Compared with the CA, the AM in Russia and Mongolia receives less attention. Our study area is located in the northern AM in Russia (Figs. 2 and 3). To the north is the GA (Fig. 1), which was

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Fig. 4. Simplified stratigraphic columns comparing the northern Altai-Mongolian terrane (AM) in the Russia (based on the geological maps of Daukeev et al. (2008) and Buslov and Safonova (2010)), the Chinese Altai in northwestern China (after Long et al., 2012) and Mongolian Altai in western Mongolia (after Badarch et al., 2002).

regarded as an accretionary prism built on the late Neoproterozoic to early Paleozoic Kuznetsk-Altai intra-oceanic island arc (e.g., Buslov et al., 2002; Ota et al., 2007; Kruk et al., 2010, 2011; Glorie et al., 2011). The terrigenous sedimentary sequences of the AM in our study area consist of a ca. 6 km thick middle Cambrian to early Ordovician flysch-like deposition, and transgressively overlying middle Ordovician to early Silurian grey marine sediments and Devonian volcaniclastic sedimentary rocks (Fig. 2; Buslov et al., 2001 and references therein). The flysch-like deposition includes greenschist-facies meta-sandstones and meta-siltstones, which have been isoclinally folded (Fig. 5a). These units were strongly disturbed due to strike-slip faulting in the late Paleozoic (Buslov et al., 2004a, 2004b; Buslov, 2011; Buslov et al., 2013). To the east of the GA, a thick turbidite sequence that was assumed to be late Proterozoic outcrops consists mainly of quartz-feldspar/polimictic sandstones and siliceous shales (Fig. 3; Buslov and Safonova, 2010). This strongly deformed turbidite sequence was intruded by middle–late Paleozoic granitic plutons with formation of zonal metamorphic rocks and partially covered by Devonian sedimentary sequences (Fig. 3; Buslov and Safonova, 2010). The igneous rocks in the northern AM are dominated by Devonian to early Carboniferous granitoids (Figs. 2 and 3; Vladimirov et al., 1997; Daukeev et al., 2008; Glorie et al., 2011; Cai et al., 2014) with arc-like geochemical compositions (Kruk et al., 2011). A few ca. 470–450 Ma granitic plutons intruded the middle Cambrian to early Ordovician meta-sedimentary sequences along the CTUS suture-shear zone, possibly recording the middle–late Ordovician magmatism in the northern AM (Glorie et al., 2011).

3. Samples description Samples HKAM002, 003, 004 and 005 (GPS: N 49°470 49.200 , E 88°240 39.800 ) are fine- to medium-grained meta-sandstones that were collected from the middle Cambrian to early Ordovician meta-sedimentary sequences to the south of the GA (Figs. 2 and 5a). These rocks mainly consist of quartz (60–70%), chlorite (15–20%), Fe–Ti oxides (5–10%), with minor plagioclase (1–2%), muscovite (1%) and lithic fragments (1%) (Fig. 5b). Most of the minerals are angular, with poor sorting. The medium-grained quartz grains are supported by the fine-grained

matrix. The minerals in these samples are weakly orientated due to the greenschist-facies metamorphism. Samples HKAM011, 012, 013, 014 and 015 (GPS: N 49°450 14.400 , E 88°220 29.800 ) were also collected from the middle Cambrian to early Ordovician meta-sedimentary sequences to the south of the GA (Fig. 2). They can be divided into two groups, i.e. fine- to medium-grained meta-sandstones and fine-grained meta-siltstones. Samples HKAM011, 012 belong to the former group, and are mainly composed of quartz, chlorite, Fe–Ti oxides and lithic fragments that are similar to those of samples HKAM002, 003, 004, 005. Samples HKAM013, 014, 015 belong to the other group, and are dominated by the mineral assemblages of quartz (60%), chlorite (35–40%) and Fe–Ti oxides (5%). The preferentially orientated chlorites show clear foliation, which was further deformed as tiny S-shape folds (Fig. 5c). Samples HKAM030, 031, 032 and 033 (GPS: N 50°050 17.600 , E 89°020 43.100 ) were coarse-grained meta-sandstones collected from a turbidite sequence to the southeast of the GA (Fig. 3). They have been highly deformed with strong foliation and are predominantly composed of quartz (>85%), with minor plagioclase and Fe–Ti oxides. The quartz grains are mostly polycrystalline and well orientated (Fig. 5d). Detrital zircons from the meta-sedimentary sequences represented by samples HKAM002 to HKAM005 and HKAM011 to HKAM015 show similar age spectra with those counterparts in the CA and MA, but totally different spectra with those in the GA (Chen et al., 2014a). This implies that these sequences probably represent the northernmost part of the AM, although they were situated along the CTUS suture-shear zone (Figs. 1 and 2; Chen et al., 2014a). The youngest detrital zircons are late Cambrian to early Ordovician old (Chen et al., 2014a), broadly consistent with the assigned age of middle Cambrian to early Ordovician for these sequences. The turbidite sequence represented by samples HKAM030, 031, 032, 033 was previously considered to be deposited in the late Proterozoic (Buslov and Safonova, 2010). However, detrital zircons from this sequence have a nearly identical age spectrum (unpublished data of Keda Cai) with those from samples HKAM002 to HKAM005 and HKAM011 to HKAM015. The youngest grains are ca. 480 Ma old (unpublished data of Keda Cai), thus implying a maximum deposition age of late Cambrian to early Ordovician. The absence of ca. 470–450 Ma detrital zircons, which could be provided by the coeval magmatism in the northern AM (Glorie et al., 2011), may suggest that the turbidite sequence had


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Fig. 5. Field occurrence (a) and petrographic observation (c–d) of representative meta-sedimentary rocks in the northern Altai-Mongolian terrane. (a) Middle Cambrian to early Ordovician greenschist-facies meta-sandstone to the south of the Gorny Altai terrane; (b) fine-medium grained meta-sandstone (HKAM004) with a mineral assemblage of quartz + chlorite + Fe–Ti oxides (minor) + plagioclase (minor); (c) fine-grained meta-siltstone (HKAM015) that is dominated by quartz and chlorite; (d) coarse-grained metamorphosed quartz sandstone (HKAM033) that is dominated by quartz (>85%). The symbols ‘‘()’’ and ‘‘(+)’’ in the up right angles of each diagram represents planepolarized light and perpendicular polarized light, respectively.

a deposition age prior to ca. 470–450 Ma. Therefore, samples from the above-mentioned two different units are put together to constrain the source nature and tectonic setting of the northern AM in the Cambrian to Ordovician. 4. Analytical methods Samples were crushed and powdered to 200 meshes in an agate mill. Whole-rock major-element concentrations were determined by X-ray fluorescence (XRF) on fused glass beads in the Department of Earth Sciences, the University of Hong Kong. The analytical accuracies are about ±1% for SiO2, ±2% for other oxides with concentrations greater than 0.5 wt.% and ±5% for minor oxides with concentrations greater than 0.1% (Chen et al., 2014b). Traceelement concentrations were analyzed on a Perkin-Elmer Sciex ELAN 6000 inductively coupled plasma mass spectrometer (ICP-MS) for nebulized sample solutions at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. Analytical procedures are similar to those described by Liu et al. (1996) and Li (1997). About 50 mg sample powders for each sample were dissolved in high-pressure Teflon bombs using a HF + HNO3 mixture. An internal standard solution containing single element Rh was used to monitor signal drift during counting. A set of international and Chinese national rock standards, including BHVO-2, G-2 and GSP-2, were chosen for calibrating element concentrations. Analytical errors are generally less than 15%. The major- and trace-element concentrations are presented in Tables 1 and 2, respectively. 5. Results 5.1. Major elements Samples from the northern AM show variable major-element compositions (Table 1; Fig. 6) and can be subdivided into two

groups based on their SiO2 concentrations. The first group (lowSiO2 group), including samples HKAM002-005 and HKAM011015, is characterized by fine- to medium-grained textures and relatively low SiO2 concentrations (50.7–71.8 wt.%). Samples 95-1/3 and 95-4/21 reported by Kruk et al. (2010) are also classified into this group based on their low SiO2 concentration (58.0–69.0 wt.%). Their TiO2, Al2O3, Fe2OT3 and MgO abundances are 0.60–0.93 wt.%, 11.8–19.8 wt.%, 5.28–9.97 wt.% and 2.49– 6.65 wt.%, respectively. Their SiO2/Al2O3 ratios vary between 2.60 and 6.07, and Na2O/K2O ratios range from 0.28 to 2.98, thus these samples fall within the greywacke field in the diagram of Pettijohn et al. (1987, see Fig. 7). The other group (high-SiO2 group) includes samples HKAM030, 031, 032 and 033. They are coarse-grained and have high SiO2 concentrations of 83.8–88.8 wt.%, which are consistent with their high contents of quartz (Fig. 5d). These four samples show relatively restricted TiO2, Al2O3, Fe2OT3 and MgO abundances of 0.19–0.31 wt.%, 4.20–5.50 wt.%, 1.94–2.63 wt.% and 1.09– 1.60 wt.%, respectively (Fig. 6). The high-SiO2 samples have SiO2/ Al2O3 and Na2O/K2O ratios of 15.4–19.9 and 0.98–8.62, respectively, and are plotted in the litharenite and subarkose fields (Fig. 7). In the Harker diagrams, SiO2 shows negative correlation with TiO2, Al2O3, Fe2OT3, MgO and K2O (Fig. 6). The poor correlation between the SiO2 and Na2O is possibly due to the mobility of sodium during diagenesis and metamorphism. The CIA (Chemical Index of Alteration, Al2O3/(Al2O3 + CaO + Na2O + K2O) 100, molar ratio; Nesbitt and Young, 1982) and PIA (Plagioclase Index of Alteration, (Al2O3 K2O)/(Al2O3 + CaO + Na2O K2O) 100, molar ratio; Fedo et al., 1995) values of the low-SiO2 group are 63.5–76.3 and 65.0–82.0, respectively, while those of the high-SiO2 group are 37.9–65.0 and 37.6–68.3, respectively (Table 1; Fig. 8). ICV (Index of Compositional Variability, (Fe2O3 + K2O + Na2O + CaO + MgO + TiO2)/Al2O3, molar ratio; Cox et al., 1995) values of these two groups of rocks are 1.19– 1.50 and 1.44–2.53, respectively (Table 1). Additionally, both the

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Table 1 Major-element compositions of meta-sedimentary rocks from the northern Altai-Mongolian terrane. Sample

Low-SiO2 group HKAM002 HKAM003 HKAM004 HKAM005 HKAM011 HKAM012 HKAM013 HKAM014 HKAM015 95-1/3 68.45 0.68 13.31 5.99 0.04 3.18 0.52 2.97 1.57 0.18 2.43 99.31 0.53 5.14 19.56 68.39 1.26 71.64 2.47 2.16

71.80 0.69 11.84 5.28 0.05 2.49 0.72 2.81 1.49 0.18 2.39 99.73 0.53 6.07 17.23 65.82 1.27 68.72 2.32 1.93

65.24 0.78 14.09 7.03 0.06 3.76 0.68 3.64 1.32 0.20 2.66 99.47 0.36 4.63 17.98 66.68 1.38 68.89 2.33 2.00

65.02 0.84 15.33 6.02 0.06 3.76 0.53 2.80 2.12 0.21 2.92 99.61 0.76 4.24 18.19 70.39 1.19 74.92 2.69 2.38

67.27 0.71 14.08 5.98 0.06 3.52 1.36 3.19 1.41 0.19 2.77 100.54 0.44 4.78 19.75 65.28 1.33 67.40 2.54 1.88

64.43 0.76 15.10 6.20 0.07 4.04 0.73 3.29 1.69 0.20 3.16 99.67 0.51 4.27 19.74 68.34 1.28 71.40 2.54 2.16

CIA = Al2O3/(Al2O3 + CaO + Na2O + K2O) 100, molar ratio (Nesbitt and Young, 1982). ICV = (Fe2OT3 + K2O + Na2O + CaO + MgO + TiO2)/Al2O3, molar ratio (Cox et al., 1995). PIA = 100 (Al2O3 K2O)/(Al2O3 + CaO + Na2O K2O), molar ratio (Fedo et al., 1995). A/NK = Al2O3/(Na2O + K2O), molar ratio. A/CNK = Al2O3/(CaO + Na2O + K2O), molar ratio. a Data are from Kruk et al. (2010). b Data are from McLennan et al. (1993). c Data are from Rudnick and Gao (2003).

52.25 0.93 19.78 9.04 0.10 5.88 1.05 2.79 2.75 0.24 4.45 99.27 0.99 2.64 21.23 71.78 1.29 76.78 3.15 2.54

55.95 0.75 17.22 8.48 0.10 5.60 1.61 2.10 2.42 0.19 4.80 99.22 1.15 3.25 22.83 70.11 1.40 74.64 3.42 2.35

50.67 0.87 19.52 9.97 0.10 6.65 1.57 2.87 2.22 0.21 5.29 99.95 0.77 2.60 22.46 70.60 1.43 74.23 3.32 2.40

69.02 0.60 12.21 5.44 0.12 3.79 1.36 3.10 1.04 0.13 3.66 100.47 0.34 5.65 20.35 63.46 1.50 64.97 2.40 1.74

a

95-4/21

Average HKAM030 HKAM031 HKAM032 HKAM033 Average

58.04 0.81 18.05 9.97 0.21 4.67 1.03 1.52 2.25 0.12 3.44 100.12 1.48 3.22 22.28 76.34 1.20 82.04 4.39 3.23

62.56 0.77 15.50 7.22 0.09 4.30 1.02 2.83 1.84 0.19 3.45 0.71 4.23 20.15 68.84 1.32 72.33 2.87 2.25

88.79 0.22 4.52 2.45 0.04 1.22 0.64 0.66 0.68 0.07 1.26 100.54 1.02 19.66 20.62 64.98 1.44 68.32 2.99 1.86

86.10 0.20 5.48 1.94 0.03 1.09 1.53 1.41 0.69 0.05 1.68 100.20 0.49 15.71 28.01 53.76 1.50 54.31 2.18 1.16

84.59 0.31 5.50 2.63 0.05 1.60 1.65 1.70 0.29 0.07 2.10 100.49 0.17 15.38 17.49 52.91 1.80 53.07 2.18 1.12

83.84 0.19 4.20 2.04 0.05 1.20 3.31 1.49 0.17 0.06 3.15 99.70 0.12 19.94 22.05 37.92 2.53 37.56 1.97 0.61

85.83 0.23 4.92 2.27 0.04 1.27 1.78 1.32 0.46 0.06 2.05 100.23 0.45 17.67 22.04 52.39 1.82 53.31 2.33 1.19

62.80 1.00 18.90 7.10 0.11 2.20 1.30 1.20 3.70 0.16

66.60 0.64 15.40 5.50 0.10 2.48 3.59 3.27 2.80 0.15

3.08 3.32 18.90 73.22 0.89 81.72 3.75 2.73

0.86 4.32 24.06 56.00 1.36 57.41 2.22 1.27


SiO2 TiO2 Al2O3 Fe2OT3 MnO MgO CaO Na2O K2O P2O5 LOI Total K2O/Na2O SiO2/Al2O3 Al2O3/TiO2 CIA ICV PIA A/NK A/CNK

PAASb UCCc

High-SiO2 group a

Table 2 Trace-element compositions of meta-sedimentary rocks from the northern Altai-Mongolian terrane. Sample

Low-SiO2 group

PAASb UCCc

High-SiO2 group

HKAM002 HKAM003 HKAM004 HKAM005 HKAM011 HKAM012 HKAM013 HKAM014 HKAM015 95-1/3a 95-4/21a Average HKAM030 HKAM031 HKAM032 HKAM033 Average

a

c

12.4 4288 116 317 361 16.0 97.4 2.91 21.5 12.7 65.2 61.7 18.9 209 7.71 3.24 307 27.3 59.8 7.61 28.5 5.62 1.06 4.64 0.616 3.55 0.718 1.86 0.291 1.83 0.278 5.42 0.774 3.16 12.9 1.70 7.58 1.04 16.83 2.11 2.20 1.24 24.52 20.52 1.06 38.58 16.19 144 0.62 1.00 10.71 3.14 2.10

18.3 4847 131 279 463 21.9 85.8 3.96 57.9 15.3 59.2 119 26.0 177 9.20 3.06 313 33.8 70.8 8.67 32.3 6.44 1.41 6.02 0.867 5.04 0.959 2.52 0.390 2.55 0.371 4.67 0.869 7.19 10.4 1.93 5.40 0.57 9.69 3.25 1.85 2.10 26.87 27.37 0.50 37.91 17.03 172 0.68 0.99 9.52 3.39 1.95

Data are from Kruk et al. (2010). Data are from McLennan et al. (1993). Data are from Rudnick and Gao (2003).

20.7 5591 158 285 478 21.5 94.3 3.96 53.9 18.3 95.5 82.6 29.8 181 10.3 5.14 451 34.5 77.5 8.85 33.3 6.77 1.49 6.37 0.926 5.61 1.08 2.80 0.429 2.65 0.387 4.91 1.01 5.56 12.0 2.11 5.69 0.58 8.74 2.88 1.66 1.79 23.74 30.84 1.16 36.90 15.11 183 0.68 1.06 9.34 3.29 1.99

17.4 4602 121 259 490 19.0 86.0 24.0 60.2 14.7 57.1 227 23.5 164 7.21 3.50 352 27.0 55.6 6.92 25.4 5.19 1.26 5.16 0.758 4.45 0.911 2.35 0.361 2.28 0.338 4.41 0.664 11.3 8.21 1.49 5.51 0.47 9.43 3.29 1.55 2.31 31.59 28.06 0.25 37.18 19.97 138 0.74 0.97 8.48 3.36 1.87

18.1 4849 126 228 522 20.0 92.1 37.5 66.8 16.3 68.0 99.1 22.8 148 7.90 4.37 386 23.0 50.2 6.28 22.9 4.71 1.04 4.54 0.713 4.31 0.87 2.29 0.330 2.13 0.326 4.05 0.716 6.11 7.92 1.57 5.05 0.44 8.19 2.91 1.27 2.52 28.78 32.76 0.69 36.56 18.68 124 0.68 1.00 7.77 3.15 1.76

25.0 6083 183 220 734 29.7 133 49.3 104 23.9 111 100 32.0 177 10.7 6.15 599 32.4 70.9 8.57 32.6 6.75 1.73 6.72 1.02 6.16 1.21 3.21 0.496 3.18 0.473 4.91 0.969 3.32 10.6 2.51 4.21 0.42 7.07 3.06 1.30 2.80 20.75 34.42 1.11 35.99 16.69 175 0.78 1.02 7.31 3.10 1.75

21.5 4757 150 192 730 27.6 124 41.2 98.0 20.8 103 76.4 26.3 126 8.94 5.91 527 24.3 52.6 6.56 24.5 5.05 1.19 5.28 0.812 4.90 0.973 2.72 0.420 2.72 0.401 3.61 0.797 2.64 8.45 1.75 4.82 0.39 5.84 2.87 1.13 3.26 22.68 37.90 1.35 34.81 14.86 132 0.70 1.00 6.40 3.10 1.61

25.1 5342 172 188 792 33.8 143 5.23 102 23.5 89.4 94.9 28.9 154 9.69 5.24 492 28.5 61.3 7.45 28.1 5.70 1.29 5.72 0.880 5.27 1.09 2.96 0.475 3.07 0.482 4.25 0.881 2.18 9.88 2.49 3.96 0.39 6.12 2.89 1.14 3.42 18.98 34.73 0.94 36.18 15.57 152 0.69 1.01 6.67 3.23 1.54

3596

4855

32.0 82.0 19.8 124 8.50

76.0 199 26.0 118 8.50

140 22.2 37.4

390 21.8 39.5

17.6 3.96 0.880 3.89 0.640

20.0 4.78 1.10 4.50 0.780

1.70 0.240 0.500 3.40

2.46 0.360 0.500 3.10

6.60 2.00 3.30

5.30 1.10 4.82

3.36

4.11

29.0 0.39 248 18.79

41.1 0.38 236 22.26

0.68

0.71

9.37 3.62 1.89

6.36 2.94 1.51

19.4 4928 139 251 540 23.0 103 19.5 66.5 17.8 75.3 110 25.1 158 8.85 4.52 394 27.5 57.5 7.55 26.4 5.45 1.23 5.20 0.793 4.81 0.960 2.55 0.393 2.44 0.363 3.75 1.26 5.01 9.14 1.83 5.10 0.54 9.15 3.10 1.54 2.40 25.88 30.99 0.81 74.42 17.73 151 0.70 1.00 8.24 3.27 1.78

5.00 1262 42.0 239 297 7.19 21.6 1.58 12.2 5.31 20.4 42.5 9.94 40.7 2.03 0.672 160 7.50 16.0 1.99 7.54 1.60 0.423 1.78 0.295 1.84 0.400 1.01 0.152 0.969 0.140 1.08 0.153 1.82 1.74 0.549 3.17 0.35 8.13 4.31 1.50 4.13 137.67 31.03 0.48 37.53 23.39 41.6 0.76 0.99 5.55 3.03 1.52

4.36 1139 34.7 272 223 5.66 16.0 1.08 9.53 5.31 22.0 112 8.43 59.8 2.07 0.883 174 8.48 16.8 2.12 7.63 1.51 0.396 1.56 0.245 1.51 0.301 0.810 0.141 0.920 0.133 1.57 0.192 2.29 2.49 0.613 4.05 0.57 13.71 3.41 1.94 2.28 109.34 19.05 0.20 38.05 24.07 42.5 0.78 0.94 6.61 3.62 1.41

4.97 1818 51.6 251 349 7.95 23.0 1.38 17.4 5.69 10.1 84.5 8.85 93.9 2.90 0.480 105 5.12 10.4 1.33 4.99 1.17 0.327 1.37 0.236 1.60 0.354 1.02 0.163 1.09 0.156 2.39 0.257 3.25 3.26 0.780 4.18 0.66 18.88 1.57 1.03 2.44 76.80 19.37 0.12 39.32 28.76 29.3 0.79 0.95 3.37 2.83 1.04

3.92 1074 37.7 245 374 6.37 18.2 1.46 12.9 4.07 5.47 72.7 6.17 46.5 1.86 0.328 71.0 4.82 9.89 1.26 4.77 1.02 0.288 1.15 0.175 1.12 0.240 0.681 0.112 0.714 0.106 1.30 0.169 2.48 1.81 0.481 3.76 0.46 11.86 2.67 1.23 3.52 135.67 23.09 0.08 35.88 25.74 26.3 0.81 0.96 4.84 3.05 1.33

4.56 1324 41.5 252 310 6.79 19.7 1.37 13.0 5.10 14.5 78.0 8.35 60.2 2.22 0.591 128 6.48 13.3 1.68 6.23 1.33 0.359 1.47 0.238 1.52 0.324 0.880 0.142 0.923 0.134 1.59 0.193 2.46 2.32 0.606 3.79 0.51 13.15 2.99 1.43 3.09 114.87 23.13 0.22 37.69 25.49 35.0 0.79 0.96 5.09 3.13 1.32

16 5993 150 110

14 3836 97 92

55

160 200 27 210 19 15 650 38.2 79.6 8.83 33.9 5.55 1.08 4.66 0.774 4.68 0.991 2.85 0.405 2.82 0.443 5 1.28 20 14.6 3.1 4.71 0.91 13.13 2.62 2.39

47 28 67 17.5 82 320 21 193 12 4.9 628 31 63 7.1 27 4.7 1 4 0.7 3.9 0.83 2.3 0.3 2 0.31 5.3 0.9 17 10.5 2.7 3.89 0.75 13.79 2.95 2.21

7.53 28.54 0.80 42.0 14.4 185 0.63 1.02 9.72 4.44 1.37

8.76 19.87 0.26 36.4 18.4 148 0.69 1.00 11.12 4.26 1.65

75

b

15.8 3991 92.5 291 294 17.5 75.0 7.69 34.2 14.4 72.2 64.0 21.6 165 8.65 4.07 378 27.7 57.0 7.01 24.7 4.93 1.10 4.43 0.708 4.02 0.832 2.23 0.347 2.27 0.333 4.07 0.726 3.57 8.32 1.45 5.73 0.53 10.44 3.32 1.75 2.10 35.02 24.17 1.13 40.55 19.84 138 0.71 0.98 8.73 3.62 1.61


Sc Ti V Cr Mn Co Ni Cu Zn Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U Th/U Th/Sc Zr/Sc La/Th La/Sc Co/Th Cr/Th Ti/Zr Rb/Sr Zr/Hf Zr/Th P REE dEu dCe (La/Yb)N (La/Sm)N (Gd/Yb)N

76


Fig. 6. Binary plots showing the geochemical variation of the meta-sedimentary rocks in northern Altai-Mongolian terrane. Data of the upper continental crust (UCC) and post-Archean Australian average shale (PAAS) are from Taylor and McLennan (1985) and Rudnick and Gao (2003), respectively.

two groups of rocks show similar K2O/Na2O and Al2O3/TiO2 ratios but distinct SiO2/Al2O3 ratios (Table 1). The A/NK (Al2O3/(Na2O + K2O), molar ratios) and A/CNK (Al2O3/(CaO + Na2O + K2O), molar ratios) values of the low-SiO2 group samples are 2.32–4.39 and 1.74–3.23, and those of the high-SiO2 group samples are 1.97–2.99 and 0.61–1.86, respectively (Table 1). 5.2. Trace elements The low-SiO2 samples yield sub-parallel and highly fractionated chondrite-normalized rare earth elements (REEs) patterns (Fig. 9a). They have (La/Yb)N ratios (where subscript N refers to chondritenormalized values) ranging from 6.36 to 10.7 (average = 8.24) and negative Eu anomalies of 0.62–0.78 (average = 0.70; Table 2). The REEs concentrations of these samples vary from 124 to

183 ppm (average = 151 ppm, Table 2), which are comparable with those of the upper continental crust (UCC) and Post Archean Australian Shale (PAAS) (Fig. 9a). In contrast, the high-SiO2 samples are characterized by extremely low REEs concentrations, which range from 26.3 to 42.5 ppm with an average value of 35.0 ppm. Although they show sub-parallel REEs patterns similar to those of the low-SiO2 samples, the high-SiO2 samples are diagnostic by relatively weak REEs fractionation ((La/Yb)N = 3.37–6.61, average = 5.09) and negative Eu anomalies (dEu = 0.76–0.81, average = 0.79). All the samples in the northern AM are nearly free of Ce anomalies (dCe = 0.94–1.06) and show relatively flat heavy rare earth elements (HREEs) patterns ((Gd/Yb)N = 1.04–2.10; Table 2). The low-SiO2 samples possess large ion lithosphere elements (LILEs, e.g., Rb, Ba, K, Pb and Sr) and high field strength elements (HFSEs, e.g., Nb, Ta, Zr, Hf, Th and U) mostly lower than those of


77

totally distinct patterns. All the LILEs and HFSEs are extremely depleted in the UCC-normalized spider diagram (Fig. 9b). Additionally, Ni, Sc, V and Co contents of the low-SiO2 samples are 75.0–143 ppm, 12.4–25.1 ppm, 92.5–183 ppm and 16.0–33.8 ppm, respectively, much higher than those of the high-SiO2 samples (Ni = 16.0–23.0 ppm, Sc = 3.92–5.00 ppm, V = 34.7–51.6 ppm and Co = 5.66–7.95 ppm; Table 2). However, samples of both groups have similar Cr concentrations, which are 188–317 ppm and 239–272 ppm, respectively. 6. Discussion

Fig. 7. Classification of the early Paleozoic meta-sedimentary rocks in the northern Altai-Mongolian terrane (after Pettijohn et al., 1987). Symbols as in Fig. 6.

Chemical compositions of sedimentary rocks are controlled by several key factors, including (1) the chemical weathering of source rocks (e.g., Nesbitt and Young, 1982), (2) sedimentary sorting during transport and sedimentation (e.g. Mclennan et al., 1993; Cullers, 1994a,b; Jian et al., 2013), (3) burial diagenesis and metamorphism (Fedo et al., 1995, 1996) and (4) the provenance and depositional setting (e.g., Bhatia and Crook, 1986; Roser and Korsch, 1986). Evaluating the effect of the former three factors is thus a prerequisite before we use the compositions of sedimentary rocks to trace the source nature and tectonic setting that they deposited. 6.1. Source-area weathering

Fig. 8. CIA vs. ICV diagram for the meta-sedimentary rocks in the northern AltaiMongolian terrane (after Nesbitt and Young, 1984; Cox et al., 1995). Symbols as in Fig. 6.

the UCC and PAAS (Table 2). In particular, Pb and Sr are variable but strongly depleted in the UCC-normalized spider diagrams (Fig. 9b). Other trace elements, such as Yb, Lu and Y, are comparable with those of the UCC and PAAS. In contrast, the high-SiO2 samples yield

Mobile elements, such as Na, K and LILEs, will be depleted while the relatively immobile elements (e.g., Al2O3, TiO2, REEs and HFSEs) will be enriched during weathering processes (Nesbitt and Young, 1982, 1984; Harnois, 1988; McLennan et al., 1993). The CIA values have been widely applied to quantify the degree of weathering (e.g., Nesbitt and Young, 1982; Long et al., 2008). With increasing chemical weathering in the source areas, the feldspar would be preferentially decomposed, resulting in the enrichment of Al2O3 and depletion of K2O and Na2O. In general, fresh igneous rocks have CIA values lower than 50, while the residual clays have such values close to 100 (McLennan et al., 1993). The investigated samples show highly varied CIA values ranging from 37.9 to 76.3 (Table 1; Fig. 8), mostly higher than those of fresh upper continental crust (UCC = 48; Rudnick and Gao, 2003) but lower than those of PAAS (70–75, McLennan et al., 1993), implying mild to moderate weathering of the source rocks. The Plagioclase Index of Alteration (PIA) is another parameter to quantify the degree of chemical weathering that precludes the influence of K-metasomatism (Fedo et al., 1995). Samples in this study show

Fig. 9. Chondrite-normalized REEs patterns and upper continental crust-normalized spider diagrams of meta-sedimentary rocks in the northern Altai-Mongolian terrane. Data of the chondrite are from Sun and McDonough (1989) and those of the upper continental crust are from Rudnick and Gao (2003). Symbols as in Fig. 6.

78


such values ranging from 37.6 to 82.0, which are mostly lower than those of PAAS (PIA = 82; Table 1; McLennan et al., 1993) and partially overlap with those of the UCC (PIA = 57; Table 1; Rudnick and Gao, 2003), further indicating the relatively weak but varied weathering. This conclusion is also in accordance with the relatively restricted Rb/Sr ratios (0.02–1.89; Table 2), since a substantial increase of Rb/Sr would occur during weathering of the source rocks (McLennan et al., 1993). 6.2. Sedimentary sorting Sedimentary sorting during transport and sedimentation controls the mineralogical and chemical compositions of sediments and can be evaluated by textural maturity with respect to grain sizes and shapes as well as mineralogical and geochemical compositions (McLennan et al., 1993). The low-SiO2 samples are all fine- to medium-grained, with mineral compositions dominated by quartz (angular to subrounded) and chlorite (Fig. 5b and c). The observations indicate that these meta-sedimentary rocks underwent insignificant sedimentary sorting, which can be further testified by their low SiO2/ Al2O3 ratios and high REEs contents comparable with those of the PAAS and UCC (Table 2; Fig. 9a) (Taylor and McLennan, 1985; Rudnick and Gao, 2003). In contrast, the high-SiO2 samples contain more than 85% quartz and have much higher SiO2/Al2O3 ratios (15.38–19.94). The coarse-grained texture and high proportion of quartz are possibly due to sedimentary sorting to some extent. Accordingly, the REEs, LILEs and HFSEs would be diluted (Fig. 9a and b) as a result of the extremely low abundances of such elements in quartz. All the investigated samples yield obvious negative Eu anomaly (Table 2), which is in line with the fact that they are mainly composed of quartz and chlorite, without significant content of feldspars that are always enriched in Eu (McLennan et al., 1993). Heavy minerals (e.g., zircon, monazite and allanite) would be possibly accumulated by sedimentary sorting with enrichment of Zr, Hf and REEs, as well as high (Gd/Yb)N ratios (McLennan et al., 1993 and references therein). Most fine- to medium-grained meta-sandstones from the northern AM have Zr abundances between 118 and 209 ppm, implying insignificant enrichment of zircons. As a result of the dilution of quartz, the high-SiO2 samples from the northern AM yield much lower Zr contents (40.7– 93.9 ppm; Table 2). Additionally, all samples yield (Gd/Yb)N ratios varying from 1.04 to 2.10 (only the sample HKAM003 has this ratio more than 2.0; Table 2, Fig. 6h), which are comparable with those of post-Archean sedimentary and upper crustal igneous rocks (1.0– 2.0, McLennan et al., 1993). This indicates insignificant concentration of monazite because it is significantly enriched in REEs and has a very steep chondrite-normalized HREEs pattern (McLennan et al., 1993). 6.3. Diagenesis and sedimentary recycling Apart from sedimentary sorting and chemical weathering, diagenesis after sedimentation can also affect the geochemical compositions of sedimentary rocks. Usually silicification and K-metasomatism are regarded as the two most important geological processes during diagenesis (Cullers et al., 1993; Fedo et al., 1995), when Ca, Mg and Na, as well as minor Fe and Sr would be removed while Si and K would possibly be added (Cullers et al., 1993; Long et al., 2008). The low-SiO2 samples from the northern AM show relatively restrict SiO2/Al2O3 (2.60–6.07; Table 1), and K2O/Na2O ratios (0.34–1.48; Table 1) and negative correlation between SiO2 and K2O (Fig. 6f). These results imply insignificant silicification and K-metasomatism during the diagenesis. The coarse-grained

Fig. 10. A–CN–K plot for the (meta-)sedimentary rocks in the northern AltaiMongolian terrane (after Fedo et al., 1995). Data of the tonalite (To), granodiorite (Gd), granite (Gr) and average Archean upper crust are from Condie (1993). Arrows represent the predicted weathering trends of To, Gd and Gr. Symbols are same as in Fig. 6. Ka: kaolinite; Chl: chlorite; Gi: gibbsite; Sm: smectite; IL: illite; Mu: muscovite; Pl: plagioclase; Kfs: K-feldspar.

meta-sandstones from the northern AM yield extremely high SiO2/Al2O3 ratios (15.97–19.94), which were possibly attributed to silicification to some extent in addition to sedimentary sorting (see Section 6.1). In the A–CN–K diagram (Nesbitt and Young, 1984, 1989), the studied samples are plotted sub-parallel to the weathering trends (Fig. 10), and thus underwent negligible K-metasomatism. Same as sedimentary sorting, the recycling of sediments would also cause the concentration of heavy minerals (e.g. zircon and monazite). However, the samples are characterized by small ranges of Zr and REEs abundances (see discussion in Section 6.1), indicating insignificant sedimentary recycling. Th/U and Rb/Sr ratios can also be applied to constrain the degree of sedimentary recycling. All the meta-sedimentary samples have Th/U ratios from 1.4 to 7.6, which follow the weathering trend and close to that of the UCC (Fig. 11b). This indicates that these samples possibly underwent simple recycling, which would always take place at an active tectonic regime (Long et al., 2008). The restricted range of Rb/Sr ratios (0.02–1.89) further confirms this conclusion (McLennan et al., 1993). 6.4. Source nature 6.4.1. Implications from major elements SiO2/Al2O3 ratios and the Index of Compositional Variability (ICV) are two important parameters to quantify the maturity of sediments (Cox et al., 1995; Roser et al., 1996). With increasing weathering and sedimentary recycling, the modal proportion of quartz would increase, whereas feldspar, mafic minerals and lithic fragments would decrease, leading to progressively elevated SiO2/ Al2O3 ratios. Since the removal of soluble elements, such as Na, K and Ca, the ICV values would increase. The fine- to mediumgrained samples (low-SiO2 group) from the northern AM show SiO2/Al2O3 ratios mostly between those of felsic and mafic igneous rocks (around 5 and 3, respectively; Roser et al., 1996), suggesting that the primary sediments of these meta-sedimentary rocks were immature and possibly derived from the eroded igneous rocks rather than recycled sedimentary rocks. This can be also validated by their high ICV values (1.19–2.04; Table 1), and the preservation of plagioclase (Fig. 5b and c). The high-SiO2 samples have ICV values (1.44–2.53) comparable with those of the low-SiO2 ones, which


79

Fig. 11. Geochemical diagrams showing the source nature of the meta-sedimentary rocks in the northern Altai-Mongolian terrane. (a) Rb-K2O diagram (after Floyd et al., 1989); (b) Th–Th/U diagram and (c) Zr/Sc–Th/Sc diagram (after McLennan et al., 1993); (d) Hf–La/Th diagram (after Floyd and Leveridge, 1987); (e) La/Sc–Co/Th diagram (after Gu et al., 2002). Data of samples from Chinese Altai are from Li et al. (2006) and Long et al. (2008), and those of samples from Mongolian Altai are from Jiang et al. (2012).

may also imply immature sources. The much higher SiO2/Al2O3 ratios of the high-SiO2 samples can be explained by strong sedimentary sorting, with significant enrichment of quartz (Fig. 5d). As demonstrated by Hayashi et al. (1997), sandstones and mudstones commonly retain the original Al2O3/TiO2 values of their parental rocks and thus they can be an effective indicator to the provenance of sedimentary rocks. Samples from the northern AM show Al2O3/TiO2 values mostly higher than that of primitive continental arc basalt (average = 16.02; Kelemen et al., 2003), but close to the primitive continental arc andesite (average = 20.20; Kemp and Hawkesworth, 2003), precluding significant mafic to ultra-mafic igneous rocks in the sediment sources. In the A–CN–K diagram (Nesbitt and Young, 1984, 1989), almost all samples are plotted in the area between the weathering trends of tonalite and granodiorite (Fig. 10), indicating the source rocks were possibly a mixture of intermediate to felsic magmatic rocks, which is consistent with the conclusion based on the Al2O3/TiO2 values.

6.4.2. Implications from trace elements REEs are a group of elements that behave uniformly and have extremely low water–rock partition coefficients. Therefore, most REEs are transferred from the source rocks into clasts (Gao and Wedepohl, 1995). It is generally accepted that REEs are immobile during most of weathering, transport, diagenesis and low to medium grade metamorphism and thus a reliable indicator to the provenance (Chaudhuri and Cullers, 1979). Mafic igneous rocks are always characterized by low REEs abundances and insignificant or without negative Eu anomaly, whereas felsic ones are featured by higher REEs contents and obvious negative Eu anomaly (Cullers et al., 1997 and references therein). The low-SiO2 samples from the northern AM show moderate to high REEs abundances (124–183 ppm; Table 2), indicating that the parental rocks were possibly intermediate to felsic igneous rocks. In contrast, the high-SiO2 samples show extremely low REE concentrations but similar REE patterns with those of low-SiO2 samples (Fig. 9a), which was possibly due to the dilution of quartz. Samples of both groups show obvious negative Eu anomalies, implying that the

80


parental rocks of the sediments underwent feldspar fractionation. Additionally, they are characterized by intense fractionation between the LREEs and HREEs (Fig. 9a) and thus preclude significant mafic igneous rocks in the sources. The transition elements (e.g., Cr, Sc, Ni, Co and V) are preferentially concentrated in mafic minerals such as pyroxene and olivine. Therefore, sedimentary rocks that were derived predominantly from mafic igneous rocks could be featured by elevated concentrations of these elements (Cullers et al., 1997). In contrast, high field strength elements (e.g., Zr, Hf, Th and U) are more easily incorporated into melts during magma differentiation, resulting in high contents of these elements in felsic igneous rocks (Feng and Kerrich, 1990). All these elements are quite immobile and cannot be significantly affected by diagenesis and metamorphism, and consequently are of significant importance in discriminating the source nature of sedimentary rocks (Armstrong-Altrin et al., 2004). The Cr, Ni, Co, V contents of low-SiO2 samples from the northern AM are almost all lower than those of MORBs (mid-ocean ridge basalts) and arc basalts, but mostly in the range between granites and andesites (Kelemen et al., 2003; Kemp and Hawkesworth, 2003), implying that the source rocks were probably dominated by intermediate to felsic rocks. The low Ni, Co, V and Sc contents of the high-SiO2 samples possibly resulted from dilution of quartz, and thus cannot truly reflect the source nature. The source nature can be also addressed by ratios of some trace elements, such as K2O/Rb, (La/Yb)N, La/Sc, Th/Sc, Zr/Sc, Th/Co and Cr/Th (Table 3; McLennan et al., 1993; Armstrong-Altrin et al., 2004 and references therein). Although K and Rb are considered mobile during the transport, diagenesis and metamorphism, their ratios have proven to be relatively uniform and effective in distinguishing the source rocks (Shaw, 1968; Long et al., 2008; El-Bialy, 2013). As shown in Fig. 11a, meta-sedimentary rocks from the northern AM have K2O/Rb ratios mostly ca. 150–230 and plot close and along the magmatic trend (K2O/Rb = 230, Shaw, 1968). Most of these samples have high K2O and Rb concentrations, which are indicative of felsic to intermediate igneous rocks in the source areas (Floyd and Leveridge, 1987). In addition, Th and Zr are incompatible during most igneous activities and will be enriched in the felsic igneous rocks, while Sc is compatible and will preferentially enter olivine, pyroxene and other minerals that crystallize in the early magmatic process (McLennan and Taylor, 1991). Therefore, ratios such as Th/Sc, Zr/Sc are very sensitive to the source rocks and can be ideal parameters in discriminating the source natures of sedimentary rocks (Table 3). The samples from the northern AM are mostly plotted along the igneous differentiation line and in the area between the average compositions of andesite and granite (Fig. 11c; McLennan et al., 1993). This indicates that the sediments were dominated by the erosion of felsic to intermediate igneous rocks in the source areas. A similar conclusion can be further illustrated by the Th/U–Th, La/Th–Hf and La/Sc–Co/Th binary plots (Fig. 11b, d and e).

6.5. Depositional setting A close relationship exists between the geochemical compositions of sedimentary rocks and the tectonic settings (e.g. Bhatia, 1983; Roser and Korsch, 1986; Bhatia and Crook, 1986; McLennan et al., 1990; McLennan and Taylor, 1991; Long et al., 2008). Generally, TiO2, Al2O3, Fe2OT3 + MgO concentrations and Al2O3/SiO2 ratios of sandstones would decrease while SiO2 concentrations, K2O/Na2O and Al2O3/(CaO + Na2O) ratios would increase with the change from oceanic island arc to continental island arc to active continental margin to passive margin (Bhatia, 1983). Most of our samples show relatively large ranges of these values possibly due to their chemical mobility during the weathering and diagenesis (Table 1). However, the high concentrations of TiO2, Al2O3 and Fe2OT3 + MgO preclude their formation in a passive regime, which is also manifested by the SiO2–K2O/Na2O tectonic discrimination diagram (Roser and Korsch, 1986; Fig. 12a). Moreover, relative immobile trace elements such as REEs and HFSEs are also effective in discriminating the tectonic setting of sedimentary rocks. Systematic increases of La, Ce, HFSEs, and ratios of Th/U, La/Sc and Th/Sc have been reported in graywackes from oceanic island arc to passive continental margin, together with the decreases in Eu/Eu⁄, Zr/Hf, Zr/Th, La/Th and Ti/Zr ratios (e.g. Bhatia, 1985; Bhatia and Crook, 1986). Samples from the northern AM yield average values of these trace elements or ratios similar to the continental arc setting, suggesting that the protoliths of these rocks were most likely deposited in a continental arc setting. In the tectonic discrimination diagrams such as La/Sc–Ti/Zr, La– Th–Sc and Th–Sc–Zr/10 (Bhatia and Crook, 1986), nearly all these samples are plotted in or around the field of continental arc setting (Fig. 12b–d), suggesting the above conclusion. 6.6. Tectonic implications The AM was previously suggested as a micro-continent with a Precambrian basement of Gondwana affinity (Dobretsov et al., 1995; Hu et al., 2000; Buslov et al., 2001; Windley et al., 2002; Li et al., 2006; Wang et al., 2006, 2009; Yang et al., 2011). The widely distributed early Paleozoic meta-sedimentary sequences were considered to be deposited in a passive continental margin (e.g., He et al., 1990; Zonenshain et al., 1990). However, the extensive early–middle Paleozoic granitic rocks in the CA have zircons with positive eHf(t) values and young two-stage Hf model ages, indicating that they were mainly derived from partial melting of juvenile crustal material without significant contribution of ancient crustal rocks (Yuan et al., 2007; Sun et al., 2008; Cai et al., 2011a,b). Moreover, detrital zircons from the early Paleozoic meta-sedimentary rocks in the CA and Tseel terrane (southeastern extension of the CA in western Mongolia) are dominated by the ca. 540–430 Ma population, only with minor Precambrian grains (Long et al., 2007, 2010; Sun et al., 2008, 2009; Jiang et al., 2010, 2011, 2012). Similar results have also been recently obtained for detrital zircons

Table 3 Elemental ratios of meta-sedimentary rocks from the northern Altai-Mongolian terrane and sediments of other sources. Northern Altai-Mongolian terrane

La/Sc Th/Sc Cr/Th Eu/Eu⁄ a b c d

Low-SiO2 group

High-SiO2 group

1.13–2.20a (1.54b) 0.39–1.04 (0.54) 18.98–35.02 (25.88) 0.62–0.74 (0.70)

1.03–1.94 (1.43) 0.35–0.66 (0.51) 76.80–137.67 (114.87) 0.76–0.81 (0.79)

Represents the range of the elemental ratios. Represents the average ratios. Data are after Amstrong-Altrin et al. (2004). Data are from Rudnick and Gao (2003).

Sediments from felsic sourcesc

Sediments from mafic sourcesc

Upper continental crustd

2.50–16.3 0.84–20.5 4.00–15.0 0.40–0.94

0.43–0.86 0.05–0.22 25.0–500 0.71–0.95

2.21 0.75 8.76 0.72


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Fig. 12. Tectonic setting discrimination diagrams for meta-sedimentary rocks in the northern Altai-Mongolian terrane. (a) SiO2–K2O/Na2O diagram (after Roser and Korsch, 1986). (b) La/Sc–Ti/Zr, (c) La–Th–Sc and (d) Th–Sc–Zr/10 diagrams are after Bhatia and Crook (1986). Symbols and data sources are same as in Fig. 11. OIA: oceanic island arc; ACM: active continental margin; CIA: continental island arc; PM: passive margin.

from the Cambrian–Ordovician meta-sedimentary sequences in the northern AM (Chen et al., 2014a; unpublished data of Keda Cai). These data do not support the existence of a Precambrian basement for the AM. In addition, the geochemistry of the Cambrian–Ordovician meta-sedimentary rocks from the northern AM suggests that they were mainly sourced from felsic to intermediate igneous rocks in a continental arc or active continental margin (Figs. 10 and 11). These geochemical characteristics are consistent with those reported for the early Paleozoic meta-sedimentary rocks from the CA and Tseel terrane (Figs. 11 and 12; Long et al., 2008, 2012; Sun et al., 2008; Jiang et al., 2012). The consistency of the meta-sedimentary rocks from the northern AM, CA (southern AM) and Tseel terrane (southeastern extension of the CA) in terms of both the geochemical composition and tectonic setting confirms that they were deposited on a coherent continental arcrelated tectonic setting with similar sediment sources in the early Paleozoic. Studies have shown that in the Neoproterozoic to Ordovician, two distinct island arcs, namely the Dunzhugur and Lake island arcs, formed and subsequently accreted to the western margin of the Precambrian Tuva-Mongolian terrane (e.g., Kuzmichev et al., 2001; Badarch et al., 2002; Rudnev et al., 2009, 2012; Yarmolyuk et al., 2011). These subduction–accretion related magmatic rocks possibly provided substantial sediments in formation of the early Paleozoic meta-sedimentary rocks in the AM (Long et al., 2008, 2012; Jiang et al., 2011, 2012; Chen et al., 2014a). Therefore, we consider that the AM possibly represented an accretionary prism related to the magmatic arcs in the western Mongolia in the early Paleozoic, rather than a Precambrian continent block with passive marginal deposition.

7. Conclusions (1) The studied meta-sedimentary rocks from the northern Altai-Mongolian terrane (AM) underwent differential sedimentary sorting. Most of them were dominated by immature sources with mild to moderate chemical weathering. (2) The meta-sedimentary rocks from the northern AM were mainly derived from intermediate to felsic igneous rocks and were deposited in a continental arc-related setting. (3) The meta-sedimentary rocks from the northern AM are quite similar to their counterparts in the Chinese Altai (CA, southern AM) and Tseel terrane (southeastern extension of the CA in western Mongolia) in terms of both geochemical compositions and depositional setting, suggesting that the entire AM probably represents a coherent accretionary prism along a continental arc in the early Paleozoic. Our data favor an arc accretion mechanism in formation of the AM instead of a passive marginal deposition surrounding a Precambrian continental block. Acknowledgements This study was financially supported by the Major Basic Research Project of the Ministry of Science and Technology of China (Grant: 2014CB44801), Hong Kong Research Grant Council (HKU705311P and HKU704712P), National Science Foundation of China (41273048, 41190075) and a HKU CRCG grant. The work is a contribution to the IGCP592 by the Joint Laboratory of Chemical Geodynamics between HKU and CAS (Guangzhou Institute of Geochemistry) and Germany/Hong Kong and PROCORE France/

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