Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis

Journal of Asian Earth Sciences xxx (2017) xxx–xxx

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Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis Xiangsong Wang a,b,c, Keda Cai a,b,⇑, Min Sun d, Wenjiao Xiao a,b,e, Xiaoping Xia c,f, Bo Wan c,e, Zihe Bao a,b,c, Yannan Wang a,b,c a

Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China Xinjiang Key Laboratory of Mineral Resources and Digital Geology, Urumqi 830011, China c University of Chinese Academy of Sciences, Beijing 100029, China d Department of Earth Sciences, The University of Hong Kong, Pokflam Road, Hong Kong, China e State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China f State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510460, China b

a r t i c l e

i n f o

Article history: Received 7 September 2016 Received in revised form 1 March 2017 Accepted 16 March 2017 Available online xxxx Keywords: Tianshan Central Asian Orogenic Belt Terrane assembly Slab break-off Accretionary orogenesis

a b s t r a c t Late Carboniferous to Early Permian is a critical period for the final amalgamation of the Central Asian Orogenic Belt (CAOB). However, as most of the accreted terranes of the CAOB are unclear in tectonic nature and origin, the timing and processes of their mutual amalgamation have been poorly constrained. To understand assembly of the West Junggar Terrane with the Yili Block, a suite of the late Paleozoic magmatic rocks, including ignimbrite, rhyolite and granite, in northwestern Chinese Tianshan Belt were studied for their petrogenesis and tectonic implications. Our new results of secondary ion mass spectrometry (SIMS) zircon U-Pb dating reveal two separate magmatic episodes, ca. 300 Ma volcanism (ignimbrite and rhyolite) and ca. 288 Ma plutonsim (biotite granite). Geochemically, for the ca. 300 Ma volcanism, the ignimbrites have low SiO2 (65.8–71.5 wt.%) and Mg# (6–13) values, and exhibit arc affinity with significantly enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE) such as Nb, Ta and Ti. The whole-rock eNd(t) and zircon eHf(t) values range from +6.9 to +7.0 and +9.9 to +14.1 respectively, indicating a juvenile basaltic lower crustal origin. Rhyolites have slightly high SiO2 (72.7–74.0 wt.%) and K2O (3.86–4.53 wt.%) contents, high zircon d18O (11.67–13.23‰) values, and low whole-rock eNd(t) (+2.9 to +3.8) and zircon eHf(t) (+2.8 to +10.0) values, which may suggest sediment involvements during magma generation. In contrast, for the ca. 288 Ma plutonism, the biotite granites have obviously higher SiO2 (74.7–75.5 wt.%) contents and whole-rock eNd(t) (+7.7 to +8.8), zircon eHf(t) (+9.8 to +12.7), and lower zircon d18O (5.99–6.84‰) values, than those of the ca. 300 Ma volcanic rocks, which are consistent with signatures of juvenile magma source. According to our estimates of zircon saturation temperatures, together with their contrasting genesis, we attribute the formation of ca. 300 Ma high temperature (815–938 °C) volcanism to oceanic slab break-off during assembly of the West Junggar Terrane with the Yili Block, and relate the generation of ca. 288 Ma low temperature (723–735 °C) plutonism to subsequent strike-slipping of North Tianshan Fault that facilitated introduction of water-fluxes triggering hydrous partial melting of juvenile lower crust. The sequential magmatic episodes in the northwestern Chinese Tianshan Belt may provide a crucial clue to a tectonic transition from plate convergence to intra-plate adjustment during the formation of the Kazakhstan Orocline in the late Paleozoic. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding author at: Xinjiang Institute of Ecology and Geography, Chinese, Academy of Sciences, 818 South Beijing Road, Urumqi, Xinjiang, China. E-mail address: [email protected] (K. Cai).

The Central Asian Orogenic Belt (CAOB) covers an immense area of the Asian territory and represents one of the largest and

http://dx.doi.org/10.1016/j.jseaes.2017.03.013 1367-9120/Ó 2017 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Wang, X., et al. Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis. Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.03.013

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X. Wang et al. / Journal of Asian Earth Sciences xxx (2017) xxx–xxx

longest-lived accretionary orogenic collages on the Earth (Cai et al., 2011; Jahn, 2004; N.B. Li et al., 2015; P.F. Li et al., 2015; Sengör et al., 1993; Sengör and Natal’In, 1996; Wan et al., 2017; Windley et al., 2007; Xiao et al., 2015). In contrast to other collision-type and/or subduction-type orogenic belts, the CAOB shows a distinctive occurrence of a gigantic tectonic collage after complicated evolutionary processes involving multi-stage accretion and collision of diverse tectonic terranes, such as island arcs, ophiolites, seamounts, accretionary complexes and microcontinents (Cai et al., 2014; Chen et al., 2014; He et al., 2015; Jahn et al., 2000; Windley et al., 2007; Xiao et al., 2009). The presentday tectonic framework of the CAOB is generally regarded as a result of the final amalgamation of the Kazakhstan and TuvaMongol collage systems along the Irtysh Shear Zone as well as the convergence of the Tarim and North China cratons along the

South Tianshan-Solonker suture zone (Sengör et al., 1993; Sengör and Natal’In, 1996; Windley et al., 2007; Xiao et al., 2015). The Kazakhstan collage system occupies a critical tectonic position in the CAOB, and is characterized by a horseshoe-shaped structure, known as the Kazakhstan Orocline. The Tianshan orogenic belt occurs as a backbone along the southern limb of the collage system and separates the Tarim craton to the south from the Junggar terranes to the north (Fig. 1). The tectonic evolution of the Tianshan belt is believed to have a genetic link with the bending of the Kazakhstan Orocline and subduction of ancient oceanic plates, including Junggar Ocean and South Tianshan Ocean (Xiao et al., 2015 and references therein). The Tianshan orogenic belt, which extends for more than 2500 km from Uzbekistan to SW Mongolia, consists mainly of granitoids, metamorphic rocks, sedimentary sequences, and voluminous volcanic rocks (An et al.,

Fig. 1. (a) Geological map of the Kazakhstan collage system in the Western CAOB. Modified after Windley et al. (2007) and Xiao et al. (2015). (b) The Kazakhstan Orocline before and after bending. Modified after Bazhenov et al. (2012).

Please cite this article in press as: Wang, X., et al. Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis. Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.03.013

X. Wang et al. / Journal of Asian Earth Sciences xxx (2017) xxx–xxx

2013; B. Wang et al., 2007; Zhu et al., 2006). These rocks are important geologic archives for understanding the geodynamic evolution of the CAOB. The northwestern Chinese Tianshan Belt is a part of the Tianshan orogenic belt and is situated inboard in the southern limb of Kazakhstan collage system, and thus retains a record of the bending processes of the Kazakhstan Orocline and the associated evolution of the Junggar Ocean in the late Paleozoic (Windley et al., 2007; Xiao et al., 2013, 2015). The late Paleozoic igneous rocks are widespread in the region, however, due to inadequate high-precision geochronological and geochemical data, their petrogenesis and tectonic implications have been poorly understood (Han et al., 2009; Li et al., 2014; Tang et al., 2010, 2013; Wang et al., 2009; Q. Wang et al., 2007; Xia et al., 2012; Yin et al.,

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2016; Zhang et al., 2012; Zhang et al., 2016), which impedes our understanding of the regional tectonic evolution. The objectives of this study are to reveal the petrogenesis of the igneous rocks from the northwestern Chinese Tianshan Belt, and delineate their tectonic settings through systematic analyses of zircon U-Pb dating, Hf-O isotopes, and whole-rock element as well as Sr-Nd isotopic compositions. 2. Geological background The Kazakhstan collage system consists mainly of the Chingiz arc, the Kokchetav microcontinent, and the North Tianshan-Yili arc (Fig. 1). As the southern limb of the Kazakhstan collage system,

Fig. 2. Geological map of the Western Chinese Tianshan and Alataw area with the sample locations (modified from Xinjiang Bureau of Geology and Mineral Resources XBGMR, 1993). (a) The Western Chinese Tianshan Belt. (b) The Alataw area in the northwestern Chinese Tianshan Belt.

Please cite this article in press as: Wang, X., et al. Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis. Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.03.013

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the North Tianshan-Yili arc underwent a long-lived evolutionary history, including Paleozoic accretion and collision, Mesozoic thermal subsidence, and Cenozoic thrusting and uplift (Allen et al., 1991; Wang et al., 2011; Windley et al., 1990). The Chinese western Tianshan Orogenic Belt is an important portion of the North Tianshan-Yili arc. It can be subdivided into four tectonic units from south to north, including the South Tianshan Belt (STB), the Central Tianshan Block (CTB), the Yili Block (YB) and the Chinese North Tianshan Belt (NTB) (Fig. 2a). These units are separated by three fault systems of the South Nalati-Kawablak Fault, the North Nalati Fault and the North Tianshan Fault. The STB is commonly considered as a Paleozoic accretionary complex and consists of Paleozoic volcano-sedimentary rocks (Kröner et al., 2013; Ma et al., 2014; Shu et al., 2002). It is dominated by Lower Cambrian black shales and phosphoric silicates, as well as Cambrian-Carboniferous marine/non-marine carbonates, clastic rocks, cherts and inter-layered volcanic materials, which are unconformably overlain by Permian fluvial deposits and volcanic rocks (Allen et al., 1993; Carroll et al., 1995; XBGMR, 1993). A Paleozoic HP/UHP metamorphic belt, several ophiolitic mélanges and numerous granitic intrusions are exposed along the northern margin of the STB (Gao et al., 2011; Han et al., 2011; Long et al., 2008). The CTB is comprised of a Precambrian metamorphic basement that is covered by early to late Paleozoic volcanic and sedimentary rocks (Gao et al., 1998, 2009; He et al., 2014; Hu et al., 2000; Huang et al., 2015; Shu et al., 2004). The basement rocks comprise amphibolite- and granulite-facies metamorphic rocks (Xiao et al., 1992; XBGMR, 1993). The Late Silurian to Carboniferous islandarc volcanic and volcanoclastic rocks were thrust over the Precambrian metamorphic rocks (Gao et al., 1998; Zhu et al., 2005, 2009). The NTB is a Paleozoic accretionary complex that is primarily composed of Devonian-Carboniferous volcanic rocks, turbidites and ophiolitic mélanges, and they form a series of imbricate structures (Gao et al., 1998; Li et al., 2014; Wang et al., 2008; Xu et al., 2005, 2006a; Zhu et al., 2013). Permian terrestrial sedimentary and volcanic rocks unconformably overlie these earlier strata (Q. Wang et al., 2007; Zhu et al., 2013). The Bayingou ophiolites that are exposed within this complex have ages of 344–325 Ma, possibly representing slices of the Carboniferous Junggar oceanic lithosphere along the NTB (Xu et al., 2005, 2006a, 2006b). The Sikeshu granitic pluton intruded the ophiolitic mélange, and was considered as a stitching pluton with an age of 316 Ma (Han et al., 2009). During the Permian, large-scale dextral ductile shearing movement occurred along the North Tianshan Fault (NTF) (Allen and Vincent, 1997). The timing of the ductile deformation was constrained at 290–245 Ma (De Jong et al., 2008; Laurent-Charvet et al., 2002, 2003; B. Wang et al., 2006; Yin and Nie, 1996; Zhou et al., 2001). The YB may be formed by the amalgamation of the Chu-Yili and Aktau-Junggar microcontinents in the early Paleozoic (Wilhem et al., 2012; Windley et al., 2007). The Precambrian basement rocks include granitic gneiss and amphibolite, migmatite and schist, which are mainly exposed in the Wenquan region of the northwestern Chinese Tianshan Belt (Chen et al., 2000; Hu et al., 2000). Cambrian-Ordovician rocks crop out in the southeast of the Sayram Lake (Fig. 2a), consisting of cherts, siltstones and carbonates. The Silurian sequences including flysch, limestone, volcanic and volcano-sedimentary rocks occurring in the Borohoro area (Fig. 2a) (B. Wang et al., 2006; XBGMR, 1993). The Devonian rocks are distributed in the Alataw and Borohoro areas and include conglomerate, sandstone, siltstone, basalt and andesitic porphyry. Carboniferous strata are widespread in the YB and mainly consist of limestone, sandstone, shale and volcanic rocks (Q. Wang et al., 2007; Wang et al., 2009; XBGMR, 1993). The Permian strata occur sporadically in the Alataw and Awulale areas, and they are dominated by intermediate-acid volcanic and sedimentary rocks

(Fig. 2a) (XBGMR, 1993). The Late Devonian to Early Permian igneous rocks crop out widely in the YB (N.B. Li et al., 2015; Tang et al., 2010, 2013). Numerous porphyry-type copper deposits are associated with the Devonian to Late Carboniferous granitoid intrusions in the Lailisigaoer and Lamasu-Dabate areas (Zhang et al., 2010; Zhu et al., 2012). In addition, many epithermal gold deposits are also considered to have a close relation to the Carboniferous volcanic rocks in the Tulasu Basin (Fig. 2a) (An et al., 2013; Zhai et al., 2009). The Alataw area in the northwestern Chinese Tianshan Belt was a late Paleozoic convergent margin, occupies the north part of the YB, and is adjacent to the south of the West Junggar Terrane (WJT) (Fig. 2b). It is mainly composed of Devonian-Permian strata. The Devonian strata consist of shallow marine sedimentary rocks, while the Carboniferous strata are composed of sandstone, limestone, basalt, andesite, dacite and rhyolite. The Permian strata of the Wulang Formation mainly consist of terrestrial volcanic rocks, which unconformably overlie the older strata (XBGMR, 1993). The granitoid intrusions are widely exposed along the Alataw area and formed at the time interval of 267–306 Ma (Chen et al., 1994, 2000; Chen et al., 2007; Liu et al., 2005; Yin et al., 2016). The NTF is distributed on the east of Alataw area, and some granites occur as spindle shape along the NTF (Fig. 2a; XBGMR, 1993). The WJT is situated on the northeast of the NTF, and is generally considered as a complicated tectonic collage of intra-oceanic arc and accretionary systems, represented by diverse volcanic rocks, granitoids and accretionary complexes (Xiao et al., 2008). 3. Sample description Felsic volcanic rocks and granites were sampled from the Alataw area in the northwestern Chinese Tianshan Belt (Fig. 2b). The volcanic rocks are considered as dominant components of the Wulang Formation, including ignimbrites and rhyolites. The ignimbrites are dark gray, showing depositional lamination and contain possibly volcanic clasts (1–2 cm). These angular to rounded felsic fragments may represent transported debris from a pyroclastic eruption. The phenocrysts are rare and the groundmass shows cryptocrystalline texture (Fig. 3a and b). The rhyolites exhibit porphyritic texture with phenocrysts of K-feldspar (15–20 vol.%), quartz (10–15 vol.%), and biotite (1–2 vol.%). K-feldspar phenocrysts are subhedral to anhedral. Quartz phenocrysts commonly show anhedral and embayed texture. Their groundmass is mainly composed of K-feldspar (45–50 vol.%) and quartz (20–25 vol.%) (Fig. 3c and d). The biotite granite samples were collected from a granitic intrusion that occurs between YB and WJT, and it has a spindle shape extending parallel to the NTF (Fig. 2b). It is a gray medium-fine grained biotite granite, consisting of euhedral plagioclase (35–40 vol.%) with clear zoning, anhedral quartz (25–30 vol. %) and K-feldspar (15–20 vol.%), strip-shaped biotite (5 vol.%) and minor accessory minerals (<5 vol.%), such as zircon, apatite and magnetite (Fig. 3e and f). 4. Analytical methods 4.1. Zircon U-Pb dating After sample crushing, zircon grains were separated by conventional heavy liquid and magnetic separation techniques. Zircon grains from the >25 lm non-magnetic fraction were hand-picked and mounted on adhesive tape, then enclosed in epoxy resin and polished to about half of their thickness. In order to investigate internal structures of potential analyzed zircons and choose potential target sites for U-Pb and Hf-O analyses, Cathodoluminescence

Please cite this article in press as: Wang, X., et al. Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis. Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.03.013

X. Wang et al. / Journal of Asian Earth Sciences xxx (2017) xxx–xxx

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Fig. 3. Field photos and photomicrographs under cross-polarized light of the magmatic rocks from the Alataw area in the northwestern Chinese Tianshan Belt. (a and b) The ignimbrite. (c and d) The rhyolite. (e and f) The biotite granite. Mineral abbreviation: Qtz, quartz; Pl, plagioclase; Bi, biotite; Kfs, K-feldspar.

(CL) images were obtained for zircons prior to analysis, using a JXA-8100 Electron Probe Micro-analyzer with a Mono CL3 Cathodoluminescence System for high resolution imaging and spectroscopy at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS). Secondary ion mass spectrometry (SIMS) zircon U-Pb analyses were conducted using a CAMECA IMS1280-HR system at the State Key Laboratory of Isotope Geochemistry, GIG CAS. Analytical procedure is similar to that described by Li et al. (2009). The O2 primary ion beam with an intensity of 10 nA was accelerated at 13 kV. The ellipsoidal spot is about 20 lm  30 lm in size. The aperture illumination mode (Kohler illumination) was used with a 200 lm primary beam mass filter (PBMF) aperture to produce even sputtering over the entire analyzed area. Oxygen flooding was used to increase the O2 pressure to 5  10 6 Torr in the sample chamber, enhancing Pb+ sensitivity to a value of 25 cps/nA/ppm for zircon. This great enhancement of Pb+ sensitivity is crucial to improve precision of 207Pb/206Pb zircon measurement. Positive secondary ions were extracted with a 10 kV potential. In the secondary ion beam optics, a 60 eV energy window was used, together with a mass resolution of 5400. Rectangular lenses were activated in the secondary ion optics to increase the transmission at high mass resolution. A single electron multiplier was used in ion-counting mode to measure secondary ion beam intensities by the peak jumping sequence: 196 (90Zr216O, matrix reference), 200 (92Zr216O), 200.5 (background), 203.81 (94Zr216O, for mass

calibration), 203.97 (Pb), 206 (Pb), 207 (Pb), 208 (Pb), 209 (177Hf16O2), 238 (U), 248 (232Th16O), 270 (238U16O2), and 270.1 (reference mass). The integration time for these mass are1.04, 0.56, 4.16, 0.56, 6.24, 4.16, 6.24, 2.08, 1.04, 2.08, 2.08, 2.08, and 0.24 s, respectively. Each measurement consisted of seven cycles, and the total analytical time per measurement was 12 min. Calibration of Pb/U ratios is relative to the standard zircon Plešovice (337.13 Ma) (Sláma et al., 2008), which was analyzed once every four unknowns, based on an observed linear relationship between ln (206Pb/238U) and ln (238U16O2/238U) (Whitehouse et al., 1997). A long-term uncertainty of 1.5% (1 RSD) for 206Pb/238U measurements of the standard zircons was propagated to the unknowns (Q.L. Li et al., 2010), despite that the measured 206Pb/238U error in a specific session is generally around 1% (1 RSD) or less. U and Th concentrations of unknowns were also calibrated relative to the standard zircon Plešovice, with Th and U concentrations of 78 and 755 ppm, respectively (Sláma et al., 2008). Measured compositions were corrected for common Pb using non-radiogenic 204 Pb. Common Pb is very low, and is largely derived from laboratory contamination introduced during sample preparation (Ireland and Williams, 2003). An average of present-day crustal composition (Stacey and Kramers, 1975) is used for the common Pb. A secondary standard zircon Qinghu (Li et al., 2013) was analyzed as unknown to monitor the reliability of the whole procedure. Five analytical spots conducted during the course of this study yield a concordant age of 158.5 ± 3.3 Ma, identical to its recommended

Please cite this article in press as: Wang, X., et al. Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis. Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.03.013

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Table 1 SIMS zircon U-Pb isotopic analyses for the magmatic rocks from the Alataw area, North Yili Block. Analysis

Content (ppm)

Th/U

Isotopic ratios 207

Th

U

Ignimbrite C14BL21-1 C14BL21-2 C14BL21-3 C14BL21-4 C14BL21-5 C14BL21-6 C14BL21-7 C14BL21-8 C14BL21-9 C14BL21-10 C14BL21-11 C14BL21-12 C14BL21-13 C14BL21-14 C14BL21-15 C14BL21-16 C14BL21-17 C14BL21-18 C14BL21-19

11 15 22 8 55 21 9 20 18 13 14 42 8 39 26 16 45 17 40

37 48 48 27 89 42 29 41 41 27 53 91 25 71 52 51 76 36 73

0.28 0.31 0.47 0.31 0.62 0.49 0.30 0.48 0.45 0.46 0.27 0.46 0.32 0.55 0.50 0.30 0.59 0.47 0.55

Rhyolite C14BL01-1 C14BL01-2 C14BL01-3 C14BL01-4 C14BL01-5 C14BL01-6 C14BL01-7 C14BL01-8 C14BL01-9 C14BL01-10 C14BL01-11 C14BL01-12 C14BL01-13 C14BL01-14 C14BL01-15 C14BL01-16 C14BL01-17 C14BL01-18 C14BL01-19 C14BL01-20 C14BL16-1 C14BL16-2 C14BL16-3 C14BL16-4 C14BL16-5 C14BL16-6 C14BL16-7 C14BL16-8 C14BL16-9 C14BL16-10 C14BL16-11 C14BL16-12 C14BL16-13 C14BL16-14 C14BL16-15 C14BL16-16 C14BL16-17 C14BL27-1 C14BL27-2 C14BL27-3 C14BL27-4 C14BL27-5 C14BL27-6 C14BL27-7 C14BL27-8 C14BL27-9 C14BL27-10 C14BL27-12 C14BL27-13 C14BL27-14 C14BL27-15

107 25 50 62 61 134 65 131 102 121 84 42 74 58 43 90 51 44 69 70 37 116 162 282 78 139 68 67 46 98 65 89 54 81 46 61 47 77 559 86 116 167 210 75 68 95 160 181 56 93 80

234 57 120 137 134 222 122 208 170 194 186 101 162 122 101 152 126 103 160 152 95 246 333 394 165 231 108 147 104 191 131 158 122 175 99 138 108 188 672 243 190 258 330 146 116 196 259 294 140 195 179

0.46 0.44 0.41 0.45 0.45 0.60 0.53 0.63 0.60 0.62 0.45 0.41 0.46 0.47 0.42 0.59 0.41 0.43 0.43 0.46 0.40 0.47 0.49 0.72 0.47 0.60 0.62 0.45 0.44 0.51 0.50 0.56 0.45 0.46 0.46 0.44 0.43 0.41 0.83 0.35 0.61 0.65 0.64 0.52 0.59 0.49 0.62 0.62 0.40 0.47 0.44

206

Isotopic ages (Ma) 1r

207

0.04196 0.05652 0.05247 0.03346 0.04557 0.05790 0.03638 0.05100 0.04717 0.05296 0.05831 0.04814 0.05053 0.05947 0.06373 0.05145 0.05294 0.05074 0.05475

22.34 13.17 18.07 43.92 6.24 12.74 37.54 32.91 21.66 17.15 16.61 9.04 29.00 12.92 7.36 12.70 11.03 19.03 7.13

0.05094 0.05161 0.05251 0.05270 0.05511 0.05014 0.05260 0.04844 0.04834 0.04961 0.05474 0.04580 0.05188 0.05093 0.05233 0.04847 0.05298 0.04893 0.05286 0.04959 0.05420 0.05017 0.04625 0.05146 0.05164 0.05347 0.05254 0.04926 0.04919 0.05193 0.05360 0.04635 0.05141 0.04921 0.05128 0.05201 0.05117 0.04841 0.05316 0.05287 0.05232 0.05155 0.05147 0.05231 0.05673 0.04926 0.05469 0.05304 0.04915 0.05099 0.05137

2.29 11.00 3.08 4.11 2.65 4.02 5.41 14.24 4.02 5.10 2.77 13.94 4.99 26.18 5.14 6.38 5.15 9.99 3.10 4.67 3.55 5.20 7.62 1.86 3.12 2.20 5.65 5.72 6.28 3.53 5.49 7.58 4.77 3.43 6.41 3.76 5.36 4.71 1.42 2.94 4.66 3.91 3.40 5.35 6.56 3.91 2.84 6.44 8.41 4.18 4.72

Pb/

Pb

1r

206

0.27475 0.37989 0.33133 0.21907 0.30278 0.37012 0.24387 0.33405 0.31488 0.36259 0.38338 0.31196 0.31907 0.39065 0.42346 0.34308 0.35108 0.34800 0.35731

22.39 13.24 18.12 43.94 6.40 12.81 37.57 32.97 21.70 17.21 16.66 9.14 29.04 13.01 7.48 12.79 11.11 19.12 7.26

0.33069 0.34497 0.35168 0.34438 0.37013 0.32425 0.34320 0.32640 0.32064 0.32541 0.36292 0.29986 0.33800 0.33766 0.35079 0.31869 0.34623 0.32079 0.34080 0.32815 0.35810 0.32395 0.30361 0.33996 0.34121 0.35226 0.34298 0.31902 0.32069 0.34235 0.34582 0.30377 0.33646 0.32102 0.33166 0.34185 0.33570 0.31909 0.35780 0.34812 0.34884 0.34270 0.34929 0.33912 0.37355 0.32150 0.35740 0.34967 0.32252 0.34539 0.34430

2.65 11.09 3.37 4.33 2.97 4.23 5.57 14.31 4.27 5.28 3.08 14.00 5.17 26.23 5.31 6.52 5.34 10.08 3.38 4.86 3.80 5.37 7.74 2.29 3.40 2.58 5.90 5.87 6.47 3.78 5.65 7.71 4.96 3.68 6.55 3.99 5.53 4.85 1.81 3.15 4.80 4.07 3.58 5.47 6.66 4.07 3.05 6.54 8.49 4.34 4.85

Pb/

235

U

238

1r

207

0.0475 0.0487 0.0458 0.0475 0.0482 0.0464 0.0486 0.0475 0.0484 0.0497 0.0477 0.0470 0.0458 0.0476 0.0482 0.0484 0.0481 0.0497 0.0473

1.36 1.36 1.36 1.55 1.40 1.36 1.34 1.90 1.40 1.42 1.37 1.40 1.38 1.50 1.36 1.44 1.34 1.81 1.38

0.0471 0.0485 0.0486 0.0474 0.0487 0.0469 0.0473 0.0489 0.0481 0.0476 0.0481 0.0475 0.0473 0.0481 0.0486 0.0477 0.0474 0.0475 0.0468 0.0480 0.0479 0.0468 0.0476 0.0479 0.0479 0.0478 0.0473 0.0470 0.0473 0.0478 0.0468 0.0475 0.0475 0.0473 0.0469 0.0477 0.0476 0.0478 0.0488 0.0478 0.0484 0.0482 0.0492 0.0470 0.0478 0.0473 0.0474 0.0478 0.0476 0.0491 0.0486

1.34 1.44 1.36 1.37 1.34 1.34 1.34 1.34 1.45 1.35 1.34 1.35 1.35 1.57 1.35 1.34 1.40 1.35 1.34 1.35 1.34 1.34 1.34 1.34 1.35 1.34 1.69 1.35 1.58 1.34 1.34 1.38 1.36 1.35 1.34 1.35 1.35 1.13 1.12 1.13 1.16 1.11 1.12 1.12 1.13 1.12 1.13 1.12 1.12 1.18 1.15

Pb/

U

Pb/206Pb

1r

207

229 473 306 835 25 526 603 241 58 327 541 106 219 584 733 261 326 229 402

485 268 366 942 145 257 799 621 449 348 327 201 561 258 149 268 233 389 152

238 268 308 316 417 202 312 121 116 177 402 13 280 238 300 122 328 144 323 176 379 203 0 262 269 349 309 160 157 283 354 0 259 158 253 286 248 120 335 323 299 265 262 299 481 160 400 330 155 241 258

52 234 69 91 58 91 119 305 92 115 61 306 110 513 113 144 113 219 69 105 78 117 184 42 70 49 124 129 141 79 119 189 106 78 141 84 119 107 32 65 103 87 76 118 139 89 62 140 186 94 105

Pb/235U

1r

206

Pb/238U

246 327 291 201 269 320 222 293 278 314 330 276 281 335 359 300 306 303 310

50 38 47 84 15 36 78 87 54 48 48 22 74 38 23 34 30 51 20

299 307 289 299 303 292 306 299 305 312 300 296 289 300 303 304 303 313 298

4 4 4 5 4 4 4 6 4 4 4 4 4 4 4 4 4 6 4

290 301 306 300 320 285 300 287 282 286 314 266 296 295 305 281 302 283 298 288 311 285 269 297 298 306 299 281 282 299 302 269 294 283 291 299 294 281 311 303 304 299 304 297 322 283 310 304 284 301 300

7 29 9 11 8 11 15 36 11 13 8 33 13 70 14 16 14 25 9 12 10 13 18 6 9 7 15 15 16 10 15 18 13 9 17 10 14 12 5 8 13 11 9 14 19 10 8 17 21 11 13

297 305 306 299 307 295 298 308 303 300 303 299 298 303 306 300 299 299 295 302 302 295 300 302 302 301 298 296 298 301 295 299 299 298 296 300 300 301 307 301 304 304 310 296 301 298 299 301 300 309 306

4 4 4 4 4 4 4 4 4 4 4 4 4 5 4 4 4 4 4 4 4 4 4 4 4 4 5 4 5 4 4 4 4 4 4 4 4 3 3 3 3 3 3 3 3 3 3 3 3 4 3

1r

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7

X. Wang et al. / Journal of Asian Earth Sciences xxx (2017) xxx–xxx Table 1 (continued) Analysis

Content (ppm)

Th/U

Isotopic ratios 207

Th

U

C14BL27-16 C14BL27-17 C14BL27-18 C14BL27-19 C14BL27-20

97 202 74 110 107

193 306 171 230 177

0.51 0.66 0.43 0.48 0.60

Biotite granite C14TL55-1 C14TL55-2 C14TL55-3 C14TL55-4 C14TL55-7 C14TL55-8 C14TL55-9 C14TL55-10 C14TL55-11 C14TL55-12 C14TL55-15 C14TL55-16 C14TL55-17 C14TL55-19

185 342 167 162 438 271 224 222 291 413 459 442 386 74

738 660 438 387 537 531 416 436 1298 956 756 998 582 203

0.25 0.52 0.38 0.42 0.82 0.51 0.54 0.51 0.22 0.43 0.61 0.44 0.66 0.36

Pb/206Pb

Isotopic ages (Ma) 1r

207

0.05098 0.05244 0.05304 0.05282 0.05422

4.80 2.67 5.78 4.18 4.86

0.05142 0.05057 0.04889 0.04926 0.04942 0.05343 0.05175 0.05058 0.05218 0.05158 0.05167 0.05280 0.04955 0.04535

8.54 5.00 6.15 8.27 3.75 4.85 15.48 8.27 3.56 3.35 1.84 1.38 5.55 17.30

Pb/235U

1r

206

0.33096 0.35114 0.36154 0.34891 0.35928

4.93 2.90 5.89 4.33 4.99

0.32627 0.32229 0.30491 0.30955 0.31350 0.33686 0.32424 0.31901 0.32115 0.32928 0.32105 0.33047 0.30886 0.28905

8.68 5.23 6.34 8.42 4.06 5.09 15.55 8.41 3.88 3.69 2.40 2.06 5.76 17.37

value. Uncertainties on single analyses are reported at the 1r level; mean ages for pooled U-Pb analyses are quoted with a 95% confidence interval. Data reduction was carried out using the Isoplot/ Ex 3 software (Ludwig, 2003). SIMS zircon U-Pb isotopic data are presented in Appendix Table 1. 4.2. Zircon oxygen isotopes Zircon oxygen isotopes were measured using the same CAMECA IMS1280-HR SIMS at the State Key Laboratory of Isotope Geochemistry, GIGCAS. The detailed analytical procedures were similar to those described by X.H. Li et al. (2010a). The 133Cs+ primary ion beam with an intensity of 2 nA was accelerated at 10 kV and focused to an area of ɸ 10 lm on the sample surface and the size of analytical spots is about 20 lm in diameter (10 lm beam diameter +10 lm raster). Oxygen isotopes were measured in multicollector mode by using two off-axis Faraday cups. Total analytical time per spot was about 3.5 min, including 30 s of pre-sputtering, 120 s of automatic tuning of the secondary beam, and 64 s of analysis. The measured oxygen isotopic data were corrected for instrumental mass fractionation (IMF) using the Penglai zircon standard (d18OVSMOW = 5.3‰, X.H. Li et al., 2010b), which was analyzed once every four unknowns, using sample-standard bracketing (SSB) method. The internal precision of a single analysis generally was better than 0.1‰ (1r) for the 18O/16O ratio. As discussed by Kita et al. (2009) and Valley and Kita (2009), internal precision for a single spot (commonly < 0.1‰, 1r) is not a good index of analytical quality for stable isotope ratios measured by SIMS. Therefore, the external precision, measured by the spot-to-spot reproducibility of repeated analyses of the Penglai standard, 0.30‰ (2r, n = 24) is adopted for data evaluation. Seven measurements of the Qinghu zircon standard during the course of this study yielded a weighted mean of d18O = 5.54 ± 0.18‰ (2r), which is consistent with the reported value of 5.4 ± 0.2‰ within analytical errors (Li et al., 2013). The Zircon oxygen isotopic compositions are presented in Appendix Table 2. 4.3. Zircon Hf isotope analyses Zircon Hf isotope analyses were carried out using an ArF excimer laser ablation system, attached to a Neptune Plasma multicollector ICP-MS, at GIGCAS. A spot size of 40 lm was used for most analyses and the ablation spot for Hf analysis was sited at the same and/or near domain for the U-Pb dating. The setting

Pb/238U

1r

207

0.0471 0.0486 0.0494 0.0479 0.0481

1.11 1.13 1.12 1.12 1.12

0.0460 0.0462 0.0452 0.0456 0.0460 0.0457 0.0454 0.0457 0.0446 0.0463 0.0451 0.0454 0.0452 0.0462

1.55 1.53 1.54 1.53 1.54 1.54 1.54 1.54 1.53 1.53 1.53 1.54 1.53 1.54

Pb/206Pb

1r

207

240 305 331 321 380

107 60 126 92 106

260 221 142 160 168 347 275 222 293 267 271 320 174 37

185 112 138 183 85 106 320 181 79 75 42 31 125 374

Pb/235U

1r

206

Pb/238U

290 306 313 304 312

13 8 16 11 13

297 306 311 302 303

3 3 3 3 3

287 284 270 274 277 295 285 281 283 289 283 290 273 258

22 13 15 20 10 13 39 21 10 9 6 5 14 40

290 291 285 287 290 288 286 288 282 292 284 286 285 291

4 4 4 4 4 4 4 4 4 4 4 4 4 4

1r

yielded a signal intensity of 10 V at 180Hf for the standard zircon 91500. Typical ablation time was 26 s, resulting in pits 20–30 lm deep. Masses 172, 173, 175–180 and 182 were simultaneously measured in static-collection mode. Data were normalized to 176 Hf/177Hf = 0.7325, using exponential correction for mass bias. The isobaric interference of 176Lu on 176Hf is negligible, on account of the extremely low 176Lu/177Hf in zircon (normally < 0.002) (Iizuka and Hirata, 2005). The mean bYb value was applied for the isobaric interference correction of 176Yb on 176Hf in the same spot. The ratio of 176Yb/172Yb (0.5887) was applied for the Yb correction. Detailed instrumental settings and analytical procedures were described by (Wu et al., 2006). The measured 176Lu/177Hf ratios and the 176Lu decay constant of 1.865  10 11 yr 1 were used to calculate initial 176Hf/177Hf ratios (Scherer et al., 2001). The chondritic values of 176Hf/177Hf = 0.0332 and 176Lu/177Hf = 0.282772 were used for the calculation of eHf values (Blichert-Toft and Albarède, 1997). The present-day 176Hf/177Hf = 0.28325 and 176 Lu/177Hf = 0.0384 values were used to calculate the depleted mantle model age (TDM) (Griffin et al., 2004). The Lu-Hf composition of the analyzed spots is presented in Appendix Table 2.

4.4. Major and trace element analyses Major elements were obtained by X-ray fluorescence spectrometry (XRF) at the ALS Laboratory Group, Guangzhou, using fused lithium tetraborate glass pellets. Loss on ignition (LOI) values was obtained using 1 g of powder heated to 1100 °C for 1 h. The analytical uncertainties are estimated at 1–5%, which is determined on the Chinese National standard GSR-3. Trace elements, including REEs, HFSEs and LILEs, were determined with a Bruker M90 inductively coupled plasma mass spectrometer (ICP-MS) at the Guizhou Tuopu Resource and Environment Analysis Center, using the method of Qi and Grégoire (2000). About 0.05 g of powdered sample was placed in a PTFE bomb, and 1 ml of HF and 1 ml of HNO3 were added, and then the sealed bombs were placed in an electric oven and heated to 190 °C for about 36 h. After cooling, the bombs were heated on a hot plate to evaporate to dryness. 500 ng of Rh was added as an internal standard, and then 2 ml of HNO3 and 4 ml of water were added. The bomb was again sealed and placed in an electric oven at 140 °C for about 5 h to dissolve the residue. After cooling, the samples were diluted 3000 times for ICP-MS measurement. By using the normal sensitivity mode, the sensitivity of the

Please cite this article in press as: Wang, X., et al. Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis. Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.03.013

8

X. Wang et al. / Journal of Asian Earth Sciences xxx (2017) xxx–xxx

Table 2 Zircon Lu-Hf and O isotopic compositions for the magmatic rocks from the Alataw area, North Yili Block. ±2r

T (Ma)

(176Hf/177Hf)i

eHf (t)

TDM (Ma)

±2r

TCDM (Ma)

0.282974 0.282963 0.282873 0.282961 0.282972 0.282990 0.282959 0.282913 0.282926 0.282908 0.282928 0.282942 0.282944 0.282941 0.282925

0.000014 0.000014 0.000016 0.000014 0.000015 0.000013 0.000013 0.000015 0.000014 0.000015 0.000012 0.000015 0.000014 0.000014 0.000014

300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4 300.4

0.282965 0.282954 0.282864 0.282954 0.282960 0.282983 0.282951 0.282905 0.282921 0.282902 0.282919 0.282935 0.282934 0.282928 0.282917

13.4 13.0 9.9 13.0 13.2 14.1 12.9 11.3 11.9 11.2 11.8 12.4 12.3 12.1 11.7

401 418 546 414 410 373 421 487 460 488 468 443 447 457 470

20.09 19.82 22.88 20.64 22.68 18.85 18.31 21.92 19.34 20.88 18.03 21.00 19.92 21.06 19.80

462 487 691 486 473 419 494 599 561 604 567 530 533 546 572

0.95 0.96 0.96 0.97 0.94 0.97 0.96 0.96 0.98 0.97 0.96 0.97 0.95 0.94 0.96

0.000914 0.000687 0.000856 0.001025 0.001330 0.001219 0.001110 0.001032 0.001287 0.001439 0.000814 0.001158 0.001094 0.000825 0.001357

0.282809 0.282811 0.282838 0.282821 0.282862 0.282820 0.282814 0.282771 0.282858 0.282803 0.282823 0.282777 0.282815 0.282797 0.282777

0.000011 0.000013 0.000012 0.000011 0.000013 0.000012 0.000010 0.000013 0.000013 0.000012 0.000010 0.000012 0.000011 0.000011 0.000012

300.7 300.7 300.7 300.7 300.7 300.7 300.7 300.7 300.7 300.7 300.7 300.7 300.7 300.7 300.7

0.282804 0.282807 0.282833 0.282815 0.282855 0.282813 0.282808 0.282766 0.282850 0.282795 0.282819 0.282770 0.282808 0.282792 0.282770

7.7 7.9 8.8 8.1 9.5 8.1 7.9 6.4 9.4 7.4 8.3 6.6 7.9 7.3 6.5

627 620 585 612 558 617 623 682 564 644 605 677 622 643 680

16.13 17.59 17.03 16.11 18.23 17.71 14.80 18.98 18.54 17.69 14.15 16.48 16.31 15.41 16.99

828 820 761 802 712 807 818 914 722 847 794 904 817 855 905

0.98 0.98 0.98 0.97 0.97 0.97 0.97 0.97 0.97 0.96 0.98 0.97 0.97 0.98 0.96

Rhyolite 0.033494 0.045796 0.043721 0.067196 0.045204 0.048534 0.044281 0.051073 0.062566 0.053591 0.037208 0.036969 0.024304 0.055851 0.034145

0.000804 0.001119 0.001151 0.001554 0.001072 0.001291 0.001042 0.001216 0.001495 0.001402 0.000889 0.000889 0.000599 0.001334 0.000834

0.282800 0.282732 0.282746 0.282738 0.282874 0.282740 0.282763 0.282770 0.282779 0.282673 0.282776 0.282846 0.282826 0.282824 0.282819

0.000012 0.000013 0.000011 0.000012 0.000012 0.000014 0.000011 0.000011 0.000011 0.000014 0.000011 0.000011 0.000010 0.000011 0.000012

298.6 298.6 298.6 298.6 298.6 298.6 298.6 298.6 298.6 298.6 298.6 298.6 298.6 298.6 298.6

0.282795 0.282726 0.282740 0.282729 0.282868 0.282732 0.282757 0.282763 0.282770 0.282665 0.282771 0.282841 0.282822 0.282817 0.282814

7.4 4.9 5.4 5.1 10.0 5.2 6.0 6.3 6.5 2.8 6.5 9.0 8.4 8.2 8.1

638 740 720 740 537 732 695 687 681 830 673 574 598 612 612

16.68 17.89 15.64 16.57 17.45 19.37 15.12 15.16 16.06 19.91 16.07 15.50 14.49 15.76 16.27

848 1006 974 998 683 991 936 921 905 1143 903 745 787 799 805

0.98 0.97 0.97 0.96 0.97 0.97 0.97 0.97 0.96 0.96 0.98 0.98 0.98 0.97 0.98

C14BL27 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Rhyolite 0.153292 0.045376 0.043520 0.060862 0.058715 0.027054 0.027465 0.066056 0.039441 0.038588 0.033442 0.027493 0.061126 0.034575

0.003602 0.001103 0.001034 0.001422 0.001413 0.000661 0.000670 0.001545 0.000970 0.000933 0.000811 0.000670 0.001518 0.000838

0.282730 0.282867 0.282821 0.282818 0.282853 0.282835 0.282825 0.282753 0.282835 0.282820 0.282814 0.282816 0.282781 0.282774

0.000016 0.000012 0.000011 0.000012 0.000011 0.000011 0.000010 0.000012 0.000011 0.000010 0.000011 0.000011 0.000012 0.000010

303 303 303 303 303 303 303 303 303 303 303 303 303 303

0.282709 0.282860 0.282815 0.282810 0.282845 0.282832 0.282821 0.282745 0.282829 0.282815 0.282810 0.282813 0.282773 0.282769

4.4 9.8 8.2 8.0 9.3 8.8 8.4 5.7 8.7 8.2 8.0 8.1 6.7 6.6

796 548 612 622 572 586 601 718 592 612 618 613 677 675

24.35 16.97 15.79 16.49 15.56 15.78 13.58 17.91 15.30 14.70 16.15 15.65 16.79 13.67

1040 698 801 812 732 763 787 961 769 802 813 807 897 905

0.91 0.97 0.97 0.96 0.96 0.98 0.98 0.96 0.97 0.98 0.98 0.98 0.96 0.98

C14TL55 1 2 3 4 5 6 7 8

Biotite granite 0.041192 0.063621 0.067643 0.066791 0.058720 0.090755 0.076223 0.096910

0.001024 0.001631 0.001692 0.001634 0.001414 0.002405 0.001795 0.002314

0.282907 0.282962 0.282880 0.282922 0.282961 0.282899 0.282962 0.282885

0.000011 0.000013 0.000009 0.000011 0.000010 0.000015 0.000012 0.000012

287.5 287.5 287.5 287.5 287.5 287.5 287.5 287.5

0.282901 0.282953 0.282871 0.282913 0.282953 0.282887 0.282952 0.282872

10.9 12.7 9.8 11.3 12.7 10.4 12.7 9.9

490 418 538 476 418 520 420 540

15.52 18.17 13.60 15.93 14.85 21.68 16.73 17.03

615 496 684 588 497 649 499 681

0.97 0.96 0.96 0.96 0.96 0.94 0.95 0.94

Grains

176

C14BL21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Yb/177Hf

176

Lu/177Hf

176

Ignimbrite 0.067201 0.068563 0.061793 0.040452 0.087659 0.042716 0.060432 0.059471 0.027621 0.036931 0.067682 0.050545 0.074746 0.093348 0.062569

0.001729 0.001700 0.001566 0.001102 0.002164 0.001133 0.001543 0.001501 0.000724 0.001003 0.001705 0.001295 0.001880 0.002251 0.001555

C14BL01 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Rhyolite 0.038181 0.027599 0.033894 0.042870 0.057150 0.051967 0.047011 0.042517 0.056559 0.064577 0.033725 0.047836 0.044477 0.033956 0.055785

C14BL16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Hf/177Hf

f Lu/Hf

d18O

±2r

12.04 12.16 12.30 11.69 11.77 11.93 12.87 12.73 12.79 12.25 13.23 12.26 12.50 12.55 12.13

0.21 0.20 0.22 0.24 0.37 0.26 0.22 0.20 0.23 0.16 0.20 0.17 0.21 0.19 0.25

6.84 6.18 6.59 6.59 6.39 6.36 6.22 5.99

0.18 0.18 0.20 0.13 0.18 0.15 0.19 0.14

Please cite this article in press as: Wang, X., et al. Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis. Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.03.013

9

X. Wang et al. / Journal of Asian Earth Sciences xxx (2017) xxx–xxx Table 2 (continued) Grains

176

Yb/177Hf

9 10 11 12 13 14 15 16

0.081725 0.061755 0.083350 0.072624 0.078099 0.070346 0.060405 0.060018

176

Lu/177Hf

176

Hf/177Hf

0.001948 0.001437 0.002013 0.001831 0.001941 0.001789 0.001506 0.001621

0.282955 0.282889 0.282905 0.282935 0.282943 0.282933 0.282941 0.282903

±2r

T (Ma)

(176Hf/177Hf)i

eHf (t)

TDM (Ma)

±2r

TCDM (Ma)

0.000011 0.000013 0.000010 0.000012 0.000013 0.000012 0.000010 0.000010

287.5 287.5 287.5 287.5 287.5 287.5 287.5 287.5

0.282945 0.282882 0.282894 0.282925 0.282933 0.282923 0.282933 0.282894

12.4 10.2 10.6 11.7 12.0 11.6 12.0 10.6

432 521 507 460 450 463 448 503

16.38 18.37 14.74 17.31 18.84 17.38 13.93 14.79

516 660 632 561 544 566 544 631

f Lu/Hf

d18O

±2r

0.95 0.96 0.95 0.95 0.95 0.95 0.96 0.96

6.24 6.52 6.48 6.84 6.29 6.31 6.43 6.51

0.16 0.22 0.23 0.21 0.21 0.15 0.13 0.17

The 176Hf/177Hf and 176Lu/177Hf ratios of chondrite and depleted mantle at the present day are 0.282772 and 0.0332, and 0.28325 and 0.0384, respectively, Blichert-Toft and Albarède (1997), Griffin et al. (2000). k = 1.867  10 11 a 1, Söderlund et al. (2004). (176Lu/177Hf)c = 0.015, T = crystallization age of zircon.

instrument was adjusted about 5  105 cps (counts per second) for 1 ng ml 1 of 115In and 2  105 cps for 1 ng ml 1 of 232Th. The AMH-1 (andesite) and OU-6 (slate) as reference materials were used as pure elemental standards for external calibration. Analytical precision is generally better than ±5–10%. The major and trace element results are presented in Appendix Table 3. 4.5. Whole-rock Sr-Nd isotope analyses The whole-rock Sr-Nd isotopic compositions were measured at the Guizhou Tuopu Resource and Environmental Analysis Center using the method of Qi and Grégoire (2000). Chemical separation was undertaken by conventional ion-exchange techniques. Approximately 50 mg of powdered sample was placed in a PTFE bomb, after that 1 ml of HF and 1 ml of HNO3 were added. The sealed bombs were placed in an electric oven and heated to 185 °C for 36 h. After cooling, the bombs were heated on a hot plate to evaporate to dryness. 0.5 ml HCl was added and evaporated to dryness. And then 4 ml of 1.5 mol L 1 of HCl was added. In order to dissolve the residue, the bomb was again sealed and placed in an electric oven at 125 °C for about 5 h. The solution was centrifuged at 4  103 rpm for 5 min, and then the supernatant was loaded onto preconditioned Dowex 50 W  8 cation exchange resin columns (8  102 mm) for separation of sample matrix and Sr from Rb using 1.5 mol L 1 of HCl. LREE was eluted with 6 ml of 6 mol L 1 of HCl. The solution was evaporated to dryness and dissolved with 1 ml of 0.25 mol L 1 of HCl. The resulting solution was loaded onto the pre-conditioned Ln resin columns for separation of Nd from La, Ce, Pr, and Sm. A Neptune MC-ICP-MS was used to measure the 87Sr/86Sr and 143Nd/144Nd isotope ratios. NIST SRM987 and JMC-Nd were used as certified reference standard solutions for 87Sr/86Sr and 143Nd/144Nd isotopes ratios, respectively. Analyses of NIST SRM-987 gave 0.710210 ± 0.000037 (2SD, n = 14) while the JMC Nd standard gave 0.511106 ± 0.000002 (2SD, n = 8). BCR-1 was used as the reference material. The Nd-Sr isotope compositions are presented in Appendix Table 4.

5. Analytical results 5.1. Zircon U-Pb age 5.1.1. Ignimbrite The ignimbrite (sample C14BL21) was collected from the Wulang Formation for zircon U-Pb isotopic dating (Table 1; Fig. 2b). Zircon grains of sample C14BL21 are prismatic, transparent, and colorless with lengths ranging from 110 to 150 lm and widths between 60 and 100 lm. In CL images, they display wellpreserved concentric oscillatory zoning. Their Th/U ratios are from 0.27 to 0.62 (Table 1; Fig. 4a), which indicates a magmatic origin (Belousova et al., 2002). Nineteen analyses give relatively coherent apparent 206Pb/238U ages of 289 to 313 Ma, yielding a weighted

mean 206Pb/238U age of 300 ± 3 Ma (Table 1; Fig. 5a), which is considered to represent the eruption age of the ignimbrites. 5.1.2. Rhyolite Three rhyolite samples were collected from the Wulang Formation (Fig. 2b). The zircon grains are light-yellow and transparent to semi-transparent, euhedral stubby prismatic crystals with lengths of 100–300 lm and length/width ratios of 1–3. The clear oscillatory zoning in CL images (Fig. 4b–d), and high Th/U ratios (0.35–0.83), imply a magmatic origin. Twenty zircon grains from sample C14BL01 were analyzed for U-Pb isotopic compositions, generating a weighted mean 206 Pb/238U age of 301 ± 5 Ma (Table 1; Fig. 5b), which are similar to the age of the ignimbrite (C14BL21). Seventeen zircon grain U-Pb isotopic analysis of sample C14BL16 give 206Pb/238U ages varying from 295 to 302 Ma, with a mean age of 299 ± 2 Ma (Table 1; Fig. 5c). Nineteen zircon U-Pb isotopic analysis for sample C14BL27 yield relatively coherent apparent 206Pb/238U ages varying from 296 to 309 Ma and a weighted mean 206Pb/238U age of 303 ± 1 Ma (Table 1; Fig. 5d). 5.1.3. Biotite granite The biotite granite samples were collected from the intrusion along the NTF in the northwestern Chinese Tianshan Belt (Fig. 2b). Zircon grains from sample C14TL55 are euhedral prismatic crystals 80–150 lm long with the length to width ratios of 2–6 and light-yellow in color (Fig. 4e). The CL images show that zircon grains have clear oscillatory zonation (Fig. 4e), and the moderate U (203–1298 ppm) and Th (74–459 ppm) contents and high Th/U ratios (0.25–0.82) (Table 1) imply a magmatic origin. Fourteen zircon grains were analyzed and showed a weighted mean 206Pb/238U age of 288 ± 1 Ma (Table 1; Fig. 5e). 5.2. Zircon Hf-O isotopic compositions 5.2.1. Ignimbrite Sample C14BL21 has high initial zircon 176Hf/177Hf ratios (0.282864–0.282983), positive eHf(t) (+9.9 to +14.1) values and young TCDM (419–691 Ma) ages (Table 2). 5.2.2. Rhyolite Sample C14BL01 is characterized by relatively high zircon initial 176Hf/177Hf ratios (0.282766–0.282850), positive eHf(t) (+6.6 to +9.5) values, with relatively young TCDM ages ranging from 712 to 914 Ma, and high zircon d18O values between +11.67‰ and +13.23‰ (Table 2). Zircon grains from sample C14BL16 gave high initial 176Hf/177Hf ratios (0.282665–0.282868), positive eHf(t) (+2.8 to +10.0) values, and TCDM ages in range of 683–1143 Ma (Table 2). Zircon Hf isotopic analysis for sample C14BL27 shows high initial 176Hf/177Hf ratios (0.282709–0.282860) and positive eHf(t) values from +4.4 to +9.8, with TCDM ages ranging from 698 to 1040 Ma (Table 2).

Please cite this article in press as: Wang, X., et al. Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis. Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.03.013

10.62 3.39 13.24 2.69 9.63 0.778 1.48 4.61 82.4 26.1 77.1 78 63.6 468 8.67 5.19 359 50 104 13 55.5 10.8 1.64 10.67 1.82 10.4 2.13 6.48 0.96 6.31 0.92 10.4 0.31 1.03 0.37 4.53 0.06 8.44 2.41 5.68 1.40 0.46 274.63 39.69

15.75 6.01 21.66 1.84 7.63 1.37 1.55 19.12 190.4 28.3 115 60.7 61.1 745 9.72 6.67 331 35.4 75.6 9.74 44.1 9.57 1.83 9.45 1.67 9.84 2.12 6.21 0.93 6.33 1.01 14.6 0.47 1.39 0.47 7.61 0.13 6.84 3.09 4.01 1.23 0.58 213.80 37.56

22.41 3.59 9.1 13.9 11.1 2.46 3.14 12.08 64.56 18.1 130 68 39.8 224 7.51 4.75 537 27 57 7.17 28.7 5.94 0.61 6.19 1.03 6.36 1.33 4.06 0.61 3.7 0.56 5.93 0.51 0.954 0.64 19 0.12 11.4 2.53 5.23 1.38 0.31 150.25 23.83

23.58 3.24 9.96 13.6 5.75 2.29 1.21 6.96 66.88 18.5 122 61 41.7 233 7.39 4.33 559 29.6 61 7.68 30.2 6.4 0.64 6.21 1.11 6.79 1.42 4.24 0.62 3.97 0.60 6.46 0.51 0.895 0.60 18.1 0.07 12.3 2.62 5.35 1.29 0.31 160.48 24.96

73.2 0.26 12.85 2 0.03 0.25 1.26 3.63 3.99 0.03 1.61 99.1 1.10 1.02 23 825 21.51 3.15 9.3 13.6 29.7 2.24 5.51 7.38 68.4 18.4 129 69.4 41 231 7.31 4.59 569 28.1 57.1 7.33 28.6 6.19 0.61 5.86 1.08 6.49 1.37 4.03 0.61 3.87 0.58 5.93 0.45 1.27 0.59 17.6 0.06 11.3 2.57 5.21 1.25 0.30 151.82 23.89

73.1 0.26 12.9 2.02 0.03 0.24 1.4 3.37 4.26 0.03 1.8 99.4 1.26 1.02 22 824 24.84 2.82 8.97 13.6 7.15 2.43 2.63 7.68 69.92 18 124 59.2 40.4 227 6.8 3.88 539 28.3 58.8 7.38 29.7 6.01 0.56 6.14 1.1 6.3 1.31 4.07 0.59 3.75 0.60 5.96 0.42 0.806 0.55 17.6 0.04 11.7 2.42 5.41 1.35 0.28 154.61 23.86

73.5 0.25 12.8 1.92 0.03 0.24 1.22 3.68 3.96 0.03 1.6 99.2 1.08 1.02 23 822 18.9 3.17 9.35 14.1 7.63 2.84 2.21 7.82 65.44 18.2 141 54.7 40.8 229 7.13 4.59 545 27.8 58.7 7.48 29.1 6.2 0.61 6.19 1.1 6.47 1.34 3.99 0.59 3.73 0.57 5.81 0.46 0.932 0.63 17.6 0.05 11.8 2.69 5.35 1.37 0.30 153.88 23.99

73 0.26 12.9 1.99 0.03 0.24 1.28 3.15 4.37 0.04 1.74 99 1.39 1.05 22 823 25.83 2.39 9.88 14.2 6.77 2.57 1.81 7.04 68.48 19.1 137 64.4 43.6 239 7.23 3.76 530 30 63.1 7.69 31.2 6.42 0.66 6.62 1.11 6.71 1.45 4.29 0.60 3.9 0.60 6.09 0.46 1.06 0.55 16.2 0.06 11.7 2.47 5.52 1.40 0.31 164.35 25.28

72.9 0.27 12.9 2.08 0.03 0.26 1.42 3.59 3.86 0.04 1.87 99.2 1.08 1.02 23 821 12.42 2.12 7.41 12.7 7.47 2.11 2.58 7.43 102.4 16.9 124 50.9 41.1 237 7.3 4.26 584 30.3 63.7 8.29 31.9 6.79 0.67 6.84 1.22 7.36 1.55 4.67 0.69 4.34 0.67 7.18 0.62 1.23 0.66 18.4 0.08 13 2.5 5.01 1.30 0.30 168.99 27.34

72.9 0.27 12.9 2.07 0.03 0.26 1.34 3.36 4.2 0.04 1.87 99.2 1.25 1.03 23 821 16.38 3.57 7.78 13 6.8 2.09 1.42 7.11 55.68 17.5 135 62.1 39.6 223 6.92 4.88 558 28.6 59.1 7.32 29.6 6.19 0.63 6.09 1.09 6.64 1.39 4.29 0.61 3.73 0.59 6.27 0.48 0.933 0.65 17 0.09 11.6 2.49 5.50 1.35 0.31 155.86 24.42

72.9 0.25 12.9 1.88 0.02 0.22 1.31 3.19 4.4 0.03 1.84 98.9 1.38 1.04 21 815 15.39 3.49 7.97 12.6 7.55 2.17 1.85 6.40 65.44 17.5 129 55.2 41.2 223 7.52 4.42 533 28.4 58.9 7.56 28.8 6.2 0.61 6.02 1.1 6.66 1.46 4.23 0.62 3.94 0.59 6.18 0.5 1.21 0.62 16.1 0.06 12.3 2.78 5.17 1.26 0.30 155.09 24.62

73.2 0.26 13 1.98 0.03 0.24 1.21 3.36 4.2 0.04 1.77 99.3 1.25 1.06 22 815 17.64 2.58 8.38 13.4 6.79 2.44 1.45 7.65 65.2 18.3 129 63 41.9 234 6.61 4.35 558 29.5 59.7 7.85 31.2 6.47 0.6 6.57 1.13 7.03 1.39 4.28 0.62 4.08 0.61 6.38 0.33 0.691 0.63 17.7 0.03 12 2.66 5.19 1.33 0.28 161.02 25.70

72.7 0.26 12.95 2.11 0.03 0.26 1.42 3.38 4.25 0.04 1.95 99.4 1.26 1.02 22 820 25.11 3.32 8.29 13.2 5.63 2.1 0.88 9.36 67.28 17.6 136 56.6 40.8 238 7.08 4.5 584 28.5 58.5 7.47 29.1 6.1 0.6 6.32 1.08 6.59 1.39 4.14 0.59 3.81 0.56 6.34 0.47 1.56 0.67 18.7 0.06 11.3 2.39 5.37 1.37 0.29 154.75 24.48

73.1 0.27 13 2.02 0.03 0.26 1.08 3.26 4.53 0.04 1.6 99.2 1.39 1.06 23 821 16.2 3.81 8.37 13.1 8.32 2.55 1.94 7.66 65.12 18.3 131 53.8 41.9 233 7.05 4.37 538 28.8 60.4 7.42 29.7 6.26 0.58 6.18 1.13 6.79 1.42 4.18 0.62 3.93 0.59 6.02 0.46 1.54 0.63 19.7 0.09 12 2.61 5.26 1.30 0.28 158.00 24.84

72.7 0.26 12.9 2.01 0.03 0.26 1.38 3.27 4.22 0.03 2.02 99.1 1.29 1.04 23 819 19.26 2.35 8.79 14.8 10.3 2.28 2.76 8.00 75.84 18.7 127 58.9 42.9 256 7.3 4.07 508 29.1 60.2 7.64 29.2 6.38 0.65 6.36 1.15 6.74 1.43 4.26 0.65 3.97 0.61 6.58 0.46 0.953 0.60 16.6 0.06 12 2.51 5.26 1.33 0.31 158.34 25.17

72.7 0.27 12.9 2.01 0.03 0.24 1.44 3.53 3.92 0.04 2.04 99.1 1.11 1.02 22 828 23.49 2.81 9.61 14.9 13.5 2.51 2.43 8.08 77.68 18.9 130 82.7 42.4 247 7.38 4.69 631 29.2 60.5 7.52 30.1 6.5 0.62 6.24 1.1 6.69 1.4 4.16 0.61 3.9 0.58 6.61 0.47 1.22 0.63 19.2 0.07 12.1 2.69 5.37 1.32 0.29 159.11 24.67

73.5 0.26 12.9 2.1 0.03 0.27 1.09 3.36 4.05 0.03 1.31 98.9 1.21 1.08 23 825 17.1 3.01 9.16 14.3 13.5 1.89 3.04 7.13 68.64 18.4 134 65.7 40.9 242 7.16 5.41 568 27.6 57.1 7.14 28.3 6.03 0.59 5.91 1.1 6.45 1.37 4.11 0.61 3.79 0.60 6.26 0.45 0.918 0.59 18.1 0.07 11.9 2.56 5.22 1.29 0.30 150.70 23.94

74 0.27 12.85 1.63 0.03 0.2 1.4 3.13 4.36 0.04 1.82 99.7 1.39 1.04 22 823

Fe2OT3 is total iron as Fe2O3. Mg# = molar 100  Mg/(Mg + Fe). Subscript N means Chondrite-normalized; dEu = 2 ⁄ EuN/(SmN + GdN).

7.596 5.06 20.77 2.3 6.4 1.19 0.9 5.90 132 25.5 98.5 109 60.4 691 9 10.7 398 34.3 72.7 9.26 41.3 8.91 1.76 8.92 1.63 9.46 1.92 5.99 0.90 6.07 0.93 13.7 0.42 1.31 0.45 10.1 0.15 6.59 1.9 4.05 1.22 0.60 204.05 35.82

10.98 4.78 21.74 2.34 9.51 1.3 2.05 12.40 234.4 26.8 99.8 92.1 59.9 718 9.27 8.81 264 35.1 75.6 9.75 43 9.16 1.78 8.71 1.55 9.37 1.99 5.91 0.84 5.92 0.93 13.8 0.45 1.37 0.45 6.91 0.26 6.73 3.09 4.25 1.22 0.60 209.61 35.22

6.687 5.63 19.88 3.16 10.3 1.29 2.97 11.20 129.6 25.6 105 107 60.4 642 6.43 7.54 325 35.5 74.2 9.56 42.3 9.06 1.8 9.1 1.65 9.77 2.02 6.01 0.91 6.29 0.92 13.2 0.19 0.358 0.41 6.77 0.03 6.79 2.13 4.05 1.20 0.60 209.09 36.67

67.5 0.35 14.85 6.16 0.04 0.38 1 3.02 3.35 0.08 2.69 99.4 1.11 1.43 13 938

Li Be Sc V Cr 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 W Tl Pb Bi Th U (La/Yb)N (Gd/Yb)N dEu RREE HREE

71.5 0.19 13.7 4.23 0.02 0.17 0.72 4.22 2.55 0.04 1.72 99.1 0.60 1.24 9 888

Rhyolite

67 0.32 14 5.51 0.09 0.27 2.11 3.91 2.75 0.08 3.31 99.4 0.70 1.06 10 934

73.2 0.28 13 2.05 0.03 0.24 1.14 3.33 4.36 0.04 1.58 99.3 1.31 1.06 21 821

65.8 0.31 14.15 6.66 0.12 0.18 1.77 4.2 3.05 0.08 2.75 99.1 0.73 1.05 6 930

Ignimbrite

65.8 0.28 13.85 5.87 0.11 0.18 2.61 3.43 3.08 0.07 3.67 99 0.90 1.01 7 922 26.46 2.51 9.29 13.8 14.2 2.28 4.17 8.32 67.28 17.8 132 66.9 40.6 237 7.02 4.68 563 26.9 54.3 6.89 28 6.16 0.56 5.9 1.1 6.35 1.34 3.77 0.57 3.69 0.54 5.91 0.48 1.02 0.59 18.7 0.09 11.4 3.03 5.23 1.32 0.28 146.07 23.26

72.7 0.25 12.65 1.92 0.03 0.25 1.63 3.17 4.23 0.03 2.13 99 1.33 0.99 23 821 21.69 1.98 9.71 13.6 13.7 2.34 2.96 7.28 77.84 18.9 131 60.2 42 242 7.6 3.91 515 28.5 58.7 7.33 29.5 6.4 0.62 6.25 1.12 6.73 1.33 3.93 0.60 3.8 0.58 6.2 0.52 1.2 0.63 18.6 0.05 11.7 2.47 5.38 1.36 0.30 155.38 24.33

73 0.26 12.75 2.07 0.03 0.25 1.16 3.51 4.21 0.03 1.68 99 1.20 1.02 22 823 28.53 3.81 9.7 14.3 11.9 2.47 3.47 7.82 73.2 19 140 46.7 45.4 243 7.45 4.64 497 29.3 63 7.61 30.9 6.51 0.67 6.33 1.19 6.95 1.43 4.11 0.63 4.05 0.58 5.99 0.52 1.14 0.58 18.5 0.04 12.2 2.21 5.19 1.29 0.31 163.26 25.27

72.9 0.26 12.75 2.07 0.03 0.25 1.4 3.33 4.14 0.03 1.78 98.9 1.24 1.02 22 823 13.41 4.19 10.6 17.6 16.5 2.82 3.46 8.16 64.16 19.7 123 112 31.8 267 7.19 3.18 580 26.1 54.9 6.72 26.9 5.79 0.72 5.2 0.951 5.46 1.05 3.18 0.44 3 0.44 6.88 0.36 0.405 0.49 12.9 0.02 11.1 1.5 6.24 1.43 0.39 140.85 19.72

73 0.3 13.3 2.31 0.02 0.3 1.58 3.52 4.17 0.04 0.46 99 1.18 1.01 23 832 11.52 4.47 10.56 18.3 12.4 2.95 3.39 10.40 86.4 19.8 116 99.8 39.8 265 6.66 4.11 576 26.8 55.7 6.73 27.9 5.88 0.68 5.85 1.04 6.03 1.26 3.74 0.55 3.63 0.51 6.44 0.27 0.308 0.50 17.6 0.05 11.1 2.19 5.30 1.33 0.35 146.30 22.61

73.1 0.31 13.3 2.39 0.03 0.33 1.52 3.67 3.95 0.04 0.4 99 1.08 1.02 24 831 12.96 3.91 8.94 16.5 10.5 2.62 8.79 11.76 65.28 18 131 101 41.7 240 7.92 3.18 618 28.8 60.6 7.59 31.5 6.89 0.68 6.65 1.22 7.38 1.5 4.36 0.65 4.33 0.60 6.57 0.52 1.85 0.65 17 0.20 12.8 2.86 4.77 1.27 0.30 162.76 26.70

73.1 0.3 13.2 2.38 0.02 0.34 1.24 3.43 4.49 0.04 0.52 99.1 1.31 1.03 25 822 8.856 3.11 8.76 15.4 9.09 2.37 2.81 16.56 288.8 17.6 111 92 29.1 246 7.51 6.24 612 27.3 59.3 7.33 30 6.16 0.79 5.79 1.03 5.75 1.1 3.21 0.49 3.37 0.51 6.78 0.44 2.96 0.59 23 0.08 12.1 1.76 5.81 1.42 0.40 152.13 21.25

73.7 0.29 13.35 2.26 0.03 0.28 1.5 3.66 4.02 0.04 0.33 99.5 1.10 1.02 22 824 84.06 5.73 7.3 24.8 7.12 1.93 2.82 9.04 73.84 15.8 94.2 203 13.4 74.8 8.43 8.33 709 17.9 32.6 3.42 12.5 2.42 0.46 2.2 0.351 1.85 0.362 1.14 0.16 1.08 0.17 2.01 0.64 1.49 0.38 14.4 0.09 8.47 0.964 11.89 1.69 0.60 76.62 7.32

75.5 0.13 13.4 1.25 0.04 0.34 1.54 3.66 3.27 0.04 0.46 99.6 0.89 1.08 39 723 75.51 3 7.13 16.1 7.75 1.87 1.84 9.52 66.4 14.9 88.7 202 11.7 73.5 6.93 6.46 734 15.3 28.4 2.95 11.1 1.96 0.39 1.77 0.32 1.7 0.351 0.981 0.16 1.06 0.14 2.05 0.68 0.432 0.35 15.2 0.07 7.6 0.855 10.35 1.38 0.63 66.58 6.48

74.9 0.11 13.55 1.11 0.04 0.31 1.54 3.79 3.27 0.04 0.46 99.1 0.86 1.08 39 722

Biotite granite

86.58 3.31 6.84 17 7.1 1.87 1.76 9.68 70.64 15.7 88.1 208 12.1 78.2 7.59 7.51 725 18.8 34.2 3.75 13.1 2.39 0.42 1.97 0.338 1.88 0.335 1.12 0.16 0.99 0.16 2.21 0.6 0.333 0.37 15.1 0.11 8.55 0.957 13.62 1.65 0.57 79.61 6.95

75.1 0.13 13.65 1.28 0.05 0.35 1.63 3.85 3.09 0.04 0.42 99.6 0.80 1.08 39 727 90.9 3.05 6.7 16.8 6.94 1.93 2.2 8.96 56.32 15.3 93.8 200 13.1 79.8 8.18 8.05 745 17.2 31.7 3.47 12.3 2.38 0.46 2.07 0.373 1.87 0.374 1.16 0.17 1.17 0.17 2.33 0.73 0.355 0.37 15.4 0.11 8.35 0.959 10.54 1.46 0.62 74.86 7.35

75.3 0.14 13.65 1.36 0.05 0.38 1.56 3.72 3.25 0.04 0.43 99.9 0.87 1.09 39 728

89.28 3.61 6.96 18.4 13.1 1.99 5.65 9.68 67.04 15.5 90.9 208 13.5 86.8 8.01 9.13 731 19.4 35 3.85 13.9 2.28 0.46 2.29 0.369 1.94 0.398 1.11 0.17 1.13 0.18 2.37 0.64 0.336 0.36 14.9 0.07 9.1 1.08 12.31 1.68 0.61 82.48 7.59

74.7 0.13 13.55 1.31 0.05 0.35 1.63 3.79 3.04 0.05 0.42 99 0.80 1.08 38 735

77.76 1.43 6.33 16.9 6.1 1.49 1.05 7.55 52.72 14.4 86.3 206 9.59 74.4 7.06 7.68 765 18.3 34.3 3.8 13.4 2.4 0.45 1.93 0.301 1.52 0.289 0.794 0.12 0.785 0.12 2.06 0.79 0.444 0.36 14.6 0.10 9.13 0.787 16.72 2.03 0.62 78.50 5.85

75.2 0.11 13.45 1.13 0.04 0.3 1.58 3.74 3.18 0.04 0.33 99.1 0.85 1.08 38 723

100.8 3.86 6.55 16.7 5.43 1.73 1.18 9.84 56.56 14.8 104 192 13.5 84.1 7.94 19.4 695 16.6 30.5 3.25 11.8 2.14 0.43 2.11 0.369 1.99 0.398 1.21 0.17 1.12 0.17 2.46 0.76 0.377 0.48 15.1 1.65 7.97 1.01 10.63 1.56 0.61 72.26 7.54

75.5 0.13 13.35 1.28 0.05 0.34 1.58 3.76 3.12 0.03 0.38 99.5 0.83 1.07 38 732

C14BL22 C14BL23 C14BL24 C14BL25 C14BL26 C14BL02 C14BL03 C14BL04 C14BL05 C14BL06 C14BL07 C14BL08 C14BL09 C14BL10 C14BL11 C14BL12 C14BL13 C14BL14 C14BL15 C14BL17 C14BL18 C14BL19 C14BL20 C14BL28 C14BL29 C14BL30 C14BL31 C14TL56 C14TL57 C14TL58 C14TL59 C14TL60 C14TL61 C14TL62

SiO2 TiO2 Al2O3 Fe2OT3 MnO MgO CaO Na2O K2O P2O5 L.O.I Total K2O/Na2O A/CNK Mg# T/°C

Sample

Table 3 Major (wt.%) and trace element (ppm) data for the magmatic rocks from the Alataw area, North Yili Block.

10 X. Wang et al. / Journal of Asian Earth Sciences xxx (2017) xxx–xxx

Please cite this article in press as: Wang, X., et al. Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis. Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.03.013

Nd)C

0.40 338

1)]/[( (ekt ( Nd)S Sm/

338

Nd)C Sm/

144

8.8 0.7043

147 144

(1) TDM = (1/k)ln{1 + [( Nd/ Nd)DM ( Nd/ Nd)S (( Sm/ Nd)DM ( Sm/ Nd)S]}. (2) T2DM = (1/k)ln{1 + [( Nd/ Nd)S ( Nd/ Nd)DM (147Sm/144Nd)DM]} and (3) k = 6.54  10 6, (147Sm/144Nd)c = 0.118, (147Sm/144Nd)DM = 0.21357, (143Nd/144Nd)DM = 0.513151, (147Sm/144Nd)CHUR = 0.1967.

144 143

0.512939 12.3 2.38 0.000006 0.709849

143 144 147

1.36 200

144

11

5.2.3. Biotite granite Sample C14TL55 has high zircon initial 176Hf/177Hf ratios (0.282871–0.282953), relatively high eHf(t) (+9.8 to +12.7) values, with young TCDM (496–684 Ma) ages, and low d18O (+5.99‰ to +6.84‰) values (Table 2). 5.3. Major and trace element geochemistry 5.3.1. Ignimbrites The ignimbrites have low SiO2 (65.8–71.5 wt.%), high K2O (2.55–3.35 wt.%) and total alkali (K2O + Na2O = 6.37–7.25 wt.%) (Table 3). These rocks are medium to high-K calc-alkaline and peraluminous (Fig. 7a, c, and d), with K2O/Na2O ratios ranging from 0.6 to 1.11, and ASI values of 1.01–1.43 (Table 3). The coherent REE patterns are characterized by moderate enrichment of LREE and nearly flat HREE ((La/Yb)N = 4.01–5.68; (Ga/Yb)N = 1.20–1.40), with negative Eu anomalies (Eu/Eu⁄ = 0.46–0.60) (Table 2; Fig. 8a). It is noted that negative Ba, Nb, Ta, and Ti anomalies are apparent on the primitive mantle normalized spider diagrams (Fig. 8b). 5.3.2. Rhyolite The samples of rhyolites have high SiO2 (72.7–74.0 wt.%) and K2O (3.86–4.53 wt.%) contents, with K2O/Na2O ratios ranging from 1.08 to 1.39 (Table 3). In the TAS diagram, all samples plot in subalkaline and rhyolite field (Fig. 7a). These rocks have low Fe2OT3 (1.63–2.39 wt.%), MgO (0.2–0.34 wt.%) and TiO2 (0.25–0.31 wt.%) with low Mg# (21–25) values, which are of typical high-K calcalkaline and metaluminous (ASI = 0.99–1.08) (Table 3; Fig. 7c and d). The rhyolite samples are also characterized by LREE enrichment ((La/Yb)N = 4.77–6.24), with flat HREE patterns ((Ga/Yb)N = 1.25– 1.43) and moderate negative Eu anomalies (Eu/Eu⁄ = 0.28–0.40) (Table 3; Fig. 8c), similar to those of the ignimbrites. The primitive mantle-normalized trace element spider diagrams of the samples show enrichments of Rb, Th, U, K and depletions of Ba, Nb, Ta, and Ti (Fig. 8d). 5.3.3. Biotite granite Samples of the biotite granites have higher SiO2 (74.7–75.5 wt.%) and Na2O (3.66–3.85 wt.%), and slightly lower K2O (3.04–3.27 wt. %) with K2O/Na2O (0.80–0.89) ratios (Table 3; Fig. 7c), than those of the ignimbrites and rhyolites. The rocks are sub-alkaline and weakly peraluminous (ASI = 1.07–1.09), characterized by low Fe2OT3 (1.11–1.36 wt.%), MgO (0.3–0.38 wt.%) and TiO2 (0.11– 0.14 wt.%), with low Mg# (38–39) values (Table 3). They are significantly enriched in LREE ((La/Yb)N = 10.35–16.72) and less fractionated HREE ((Ga/Yb)N = 1.38–2.03), with weak Eu anomalies (Table 3; Fig. 8e). The primitive mantle-normalized diagrams show obvious enrichment in Rb, Th, U, K and depletion in Nb, Ta, and Ti (Fig. 8f).

144

93.8 287.5 C14TL59

Biotite Granite

Rhyolite

Rhyolite

Rhyolite

Ignimbrite

C14BL22 C14BL25 C14BL06 C14BL09 C14BL17 C14BL20 C14BL29 C14BL31 C14TL56

143

300.4 300.4 300.7 300.7 298.6 298.6 303 303 287.5

105 77.1 141 135 134 140 116 111 94.2

143

2.84 2.86 7.46 6.29 5.91 8.68 3.37 3.49 1.34 107 78 54.7 62.1 65.7 46.7 99.8 92 203

144

147

0.1178

144

0.512861 0.512840 0.512690 0.512698 0.512701 0.512683 0.512654 0.512694 0.512883 0.1304 0.1184 0.1297 0.1273 0.1297 0.1282 0.1283 0.1250 0.1178 42.3 55.5 29.1 29.6 28.3 30.9 27.9 30 12.5 9.06 10.8 6.2 6.19 6.03 6.51 5.88 6.16 2.42 0.000008 0.000006 0.000007 0.000007 0.000006 0.000011 0.000007 0.000007 0.000006 0.719545 0.718293 0.739553 0.735251 0.732664 0.743920 0.721236 0.722823 0.709696

Sm (ppm) (2r) Sr/86Sr 87

Rb/86Sr 87

Sr (ppm) Rb (ppm) Age Rock name Sample

Table 4 Whole Sr-Nd isotopic compositions of the magmatic rocks from the Alataw area, North Yili Block.

Nd (ppm)

147

Sm/144Nd

143

Nd/144Nd

((

0.512718 0.000004

147

531 498 838 800 818 835 888 786 428 6.9 7.0 3.6 3.8 3.8 3.5 2.9 3.8 7.7 0.512605 0.512608 0.512435 0.512448 0.512446 0.512432 0.512402 0.512449 0.512661 0.000003 0.000003 0.000003 0.000003 0.000004 0.000003 0.000003 0.000003 0.000004

0.7074 0.7061 0.7077 0.7084 0.7075 0.7069 0.7069 0.7079 0.7042

(t)

INd (t) (2r)

Isr (t)

eNd

TDM (Ma)

147

Sm/

0.34 0.40 0.34 0.35 0.34 0.35 0.35 0.36 0.40 501 497 773 752 755 778 825 751 428

144

fSm/Nd T2DM (Ma)

X. Wang et al. / Journal of Asian Earth Sciences xxx (2017) xxx–xxx

5.4. Whole-rock Nd-Sr isotopic compositions Whole-rock Nd-Sr isotopic compositions of the ignimbrites are rather homogeneous, with (87Sr/86Sr)i ratios of 0.7061–0.7074, eNd(t) values of +6.9 to +7.0, and young Nd model ages of 497–501 Ma (Table 4). The rhyolite samples show low eNd(t) (+2.9 to +3.8) values and initial Sr isotopic compositions ((87Sr/86Sr)i = 0.7069–0.7084), with Nd model ages varying from 751 to 825 Ma (Table 4). In contrast to the ignimbrites and rhyolites, the biotite granites have the highest eNd(t) (+7.7 to +8.8) values, and lowest initial Sr isotopic compositions ((87Sr/86Sr)i = 0.7042–0.7043), with Nd model ages varying from 338 to 428 Ma (Table 4).

Please cite this article in press as: Wang, X., et al. Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis. Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.03.013

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Fig. 4. Cathodoluminescence images of representative magmatic zircon grains analyzed for U-Pb ages, Lu-Hf and O isotopes. The red and blue circles indicate the SIMS analytical spots for U-Pb and O isotopes, respectively. The yellow circles denote the LA-ICPMS analytical spots for Lu-Hf isotopes. Numbers near the analytical spots are the U-Pb ages (within parentheses), eHf(t) values [within square brackets] and d18O values .

Fig. 5. Zircon U-Pb concordant plots for the magmatic rocks from the northwestern Chinese Tianshan Belt.

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Fig. 6. (a) Initial 87Sr/86Sr versus eNd(t) diagram; (b) eHf(t) values versus d18O values, the eHf(t) range for new continental crust is from Dhuime et al. (2011). (c and d) eHf(t) values versus crystallization ages for the magmatic zircon grains from the northwestern Chinese Tianshan Belt.

Fig. 7. Major element plots for the magmatic rocks from the northwestern Chinese Tianshan Belt. (a and b) TAS classification diagram, the alkaline and sub-alkaline division is after Irvine and Baragar (1971). (c) SiO2 versus K2O diagram (after Maitre, 1989). (d) A/NK versus A/CNK diagram (Maniar and Piccoli, 1989). Data for the magmatic rocks are from Q. Wang et al. (2007) and Liu et al. (2005) and this study.

6. Discussion 6.1. Timing of the Wulang formation Volcanic rocks in the Alataw area of northwestern Chinese Tianshan Belt were collectively assigned to the Wulang Formation, which was mainly distributed in the east of the Alataw area

(Fig. 2b) (XBGMR, 1993). Although previous isotopic dating study reported a Rb-Sr isochron age of 307 ± 15 Ma for these volcanic rocks, the Wulang Formation was envisaged as Early Permian volcanic stratum because it unconformably overlies on the assumed Middle Carboniferous Dongtujinhe Formation (XBGMR, 1993). However, recent study revealed that the Wulang volcanic Formation includes Late Carboniferous adakitic rocks (Q. Wang et al.,

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X. Wang et al. / Journal of Asian Earth Sciences xxx (2017) xxx–xxx

Fig. 8. Chondrite-normalized REE patterns and primitive mantle-normalized spider diagrams for the magmatic rocks from the northwestern Chinese Tianshan Belt. (a and b) The ignimbrite. (c and d) The rhyolite. (e and f) The biotite granite. Normalizing values are from Sun and McDonough (1989).

2007), arguing against the formation age of Early Permian. Our four high precise SIMS zircon U-Pb ages (ca. 303–299 Ma) strongly favor a Late Carboniferous deposition age of the Wulang Formation in the northwestern Chinese Tianshan Belt. 6.2. Petrogenesis 6.2.1. Ignimbrites and rhyolite The ignimbrites and rhyolites have coeval crystallization ages (ca. 300 Ma), suggesting that they may represent a same volcanic episode and have a genetic link. The ignimbrites are felsic in composition (SiO2 = 65.8–71.5 wt.%), and peraluminous (ASI = 1.01–1.43). Generally, the felsic volcanic rocks might have been produced by either differentiation of mantle-derived mafic melt through assimilation and fractional crystallization (AFC) processes, or partial melting of metabasaltic rocks (Bonin, 2007; Defant and Drummond, 1990; Eby, 1992; Rapp and Watson, 1995). The homogeneous Sr-Nd and Lu-Hf isotopic compositions preclude the possibility of notable wall-rock contamination and/or assimilation. Few feldspar and quartz phenocrysts in the ignimbrites indicate that the parental magma ascent and erupted rapidly (Fig. 3b). There is no contemporary intrusion with comparable isotopic compositions in the area. Thus, their precursor magma may be not significantly affected by fractional crystallization. In addition, the relationship between Mg# values or MgO and SiO2 contents argues against olivine and pyroxene fractionation during magma evolution (Fig. 9d). The ignimbrites display much lower Cr, Co, Ni contents than those of the intermediateacidic volcanic rocks generated by melting of mantle wedge (Fig. 9a–c) (Q. Wang et al., 2007). Moreover, the relatively constant Al2O3 (13.7–14.85 wt.%) contents, nearly uniform REE and trace element abundances, collectively support that fractional crystal-

lization could not be an important magmatic process. Therefore, these geochemical fingerprints of ignimbrites likely record the primitive chemical composition of parental magma, which may result from partial melting of the metabasaltic rocks. Experimental studies suggest that partial melts from basaltic lower crust rocks typically show low Mg# (<40) regardless of melting degrees (Fig. 10) (Rapp and Watson, 1995; Schmidt et al., 2004). The ignimbrites have relatively low Mg# (6–13) values and compatible elements contents (e.g., Cr, Co and Ni), consistent with the melting of lower basaltic crust. The negative Eu, Ba and Sr anomalies of the ignimbrites imply that they may be derived from a stability field of plagioclase (Fig. 7c and d). The positive eNd(t) (+6.9 to +7.0) values and relatively young TCDM (497–501 Ma) ages of the ignimbrites are strikingly different from those of the Neoproterozoic basement of the YB which has strongly negative eNd(t) values (eNd(t) = 8.5 to 11.7 (Hu et al., 2000); eNd(t) = 5.0 to 8.2 (Chen et al., 2000)) (Fig. 6a), indicating that the Neoproterozoic basement rocks cannot be as the dominant source. In contrast, the strongly positive zircon eHf(t) (+9.9 to +14.1) values and relatively juvenile TCDM model ages (419–691 Ma) support that the ignimbrites were derived from partial melting of juvenile lower crust. Since the rhyolites have higher SiO2 contents than those of the coeval ignimbrites, they seem to constitute a well-defined linear correlation between major oxides and SiO2 contents. The similarities in age and composition suggest that the rhyolites and ignimbrites may be comagmatic products. However, the remarkably low zircon eHf(t) (+2.8 to +10.0) and whole-rock eNd(t) (+2.9 to +3.8) values of the rhyolites, indicating that the magmatic source of the rhyolites should be different from that of the ignimbrites. The rhyolites also have relatively high Yb contents and flat HREE patterns, resembling partial melts from sources above the

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Fig. 9. Harker diagrams showing the trace element variations of the magmatic rocks from the northwestern Chinese Tianshan Belt. Data sources for the magmatic rocks in the Alataw area are from Q. Wang et al. (2007) and this study.

Fig. 10. SiO2 versus Mg# diagram. The field of metabasaltic and eclogite experimental melts (1–4.0 GPa) is from Rapp et al. (1999) and reference therein.

garnet stability field (<10 kb) (Patiño Douce, 1996; Singh and Johannes, 1996). Experimental studies showed that dehydration melting of metabasaltic or amphibolitic rocks can produce melts characterized by high SiO2 (66–75 wt.%) and low MgO (0.24– 1.70 wt.%) contents at middle crustal level (Beard and Lofgren, 1991; Patiño Douce, 1999; Rapp and Watson, 1995; Wolf and Wyllie, 1994). The rhyolites have high SiO2 (72.7–74 wt.%) and relatively low MgO (0.2–0.34 wt.%), resembling these experimental melts. Sisson et al. (2004) obtained K-rich melts, with K2O/ Na2O > 1 at SiO2 > 65%, by using medium-to-high K basaltic compositions as starting materials. The rhyolites have much higher K2O, TiO2, Rb, Cs content, higher K2O/Na2O (>1.0) and Rb/Sr (mostly > 1.0) ratios and lower Al2O3/TiO2 (<200) ratios than those

of ignimbrites, which requests a source dominated by K-enriched minerals. The oxygen isotopes of igneous zircon grains that crystallize directly from the parental melts are inherited from the source rocks with little influence from closed-system magmatic processes (Wang et al., 2013). Sedimentary rocks generally have high d18O values (>10‰; Valley et al., 2005) due to their interaction with surface water at low temperatures (Hoefs, 2009). Thus granitic melts derived from partial melting of such rocks must have higher d18O values than those originating from partial melting of igneous rocks. The rhyolites have higher zircon d18O (+11.67 to +13.23‰) (Fig. 6b) than mantle-derived melts (+5.3 ± 0.3‰) (Valley et al., 1998), supporting a derivation from a metasedimentary source or involvement of sedimentary materials. The rhyolites have lower wholerock eNd(t) (+2.9 to +3.8) and variable zircon eHf(t) (+2.8 to +10.0) values relative to the ignimbrites, consistent with significant involvement of sedimentary materials. If the Nb-enriched basalts from the Alataw area are taken as mafic end-members, the sedimentary rocks from the basement of the YB are applied as the other, it can be estimated that roughly up to 50% of the sediments were involved. Given that the mafic end-member of the Nb-enriched basalts has the highest eNd(t) (eNd(t) = +11.4), the actual proportion of basement materials may be higher than the estimated value. Based on the above discussion, we propose that the rhyolites may be partial melts from a juvenile basaltic lower crust with involvement of the Neoproterozoic basement sedimentary materials. 6.2.2. Biotite granite The lack of mafic microgranular enclaves (MME) in the biotite granites (Fig. 3e), and their homogeneous Nd-Hf isotopic compositions make magma mixing or mingling unlikely. The biotite granites have low A/CNK ratios (1.07–1.08) and K2O/Na2O ratios (0.80–0.89) (Fig. 7d), inconsistent with melts from greywacke or pelitic source, since their partial melts generally give rise to peraluminous melts (Patiño Douce, 1999). Moreover, the biotite

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X. Wang et al. / Journal of Asian Earth Sciences xxx (2017) xxx–xxx

granites are characterized by relatively low HREE and Yb contents, with high (La/Yb)N (10.35–16.72) values (Fig. 8f), possibly indicating residual garnet and/or amphibole in their source. If garnet is the primary residual mineral in the source, the partial melts will exhibit a strong HREEs depletion, and have a progressive decrease in HREEs with increasing atomic number as a result of the partition coefficients for these elements. Whereas, if amphibole is dominant in the source, the intermediate and felsic magmas will display concave-upward patterns between the middle and heavy REEs (Rollinson, 1993). Given that amphibole is a major K-bearing mineral and has much higher K2O contents than garnet, the presence of amphibole in the residue phase can depress the potassium content of partial melts and induce low K2O/Na2O ratios (Rapp and Watson, 1995; Sen and Dunn, 1994). Our biotite granite samples show nearly flat HREE patterns with (Gd/Yb)N = 1.38–2.03, and have low K2O/Na2O ratios, indicating that residual amphibole might have been present in the magma source. Experimental studies have shown that the presence of amphibole is largely affected by addition of external aqueous fluids during partial melting (Gardien et al., 2000). Thus the biotite granites are likely products of H2O-saturated partial melting (Beard and Lofgren, 1991; Patiño Douce and Beard, 1995). The biotite granites possess lower d18O (+5.99 to +6.84‰) values than the rhyolites. The zircon crystals that have low d18O values commonly crystallized from 18O-poor magma or underwent post-magmatic zircon-fluid interaction (Gao et al., 2014; Monani and Valley, 2001; Wei et al., 2002; Zheng et al., 2004). The zircon grains of the biotite granites exhibit intact crystal shapes and clear oscillatory zoning (Fig. 4), together with low Th (<500 ppm) and U (mostly < 1000 ppm), as well as concordant U-Pb ages (Fig. 5e), suggesting that the zircon crystals may have preserved their primary magmatic O isotopic compositions. Furthermore, the d18O values of the biotite granites are analogous to those of the I-type granite, as melts of igneous rocks. The biotite granites have relatively radiogenic whole-rock Sr isotopic compositions that diverge from the mantle array, but these strikingly homogeneous wholerock Nd and zircon Hf-O isotopic compositions preclude significant assimilation of ancient crustal materials. Collectively, the biotite granites have much lower initial 87Sr/86Sr ratios (0.7045–0.7056), relatively positive whole-rock eNd(t) (+7.7 to +8.8) values with younger Nd (338–428 Ma) model ages, and zircon eHf(t) (+9.8 to +12.7) values with Hf (496–684 Ma) model ages, indicating that they were likely derived primarily from a quite juvenile crustal source that resembles the West Junggar Terrane affinity (Fig. 6a). 6.3. Magmatic response to a tectonic transition of accretionary orogenesis In the Carboniferous, the North Yili Block was suggested to be located on a convergent margin as a result of the subduction of Junggar oceanic plate, which is revealed by several lines of evidence as follows. (1) the Bayingou ophiolites distributed in the NTB have zircon U-Pb ages of 344 ± 3.4 Ma and 325 ± 7 Ma (Fig. 2a) (Xu et al., 2006a, 2006b), (2) the Carboniferous volcanic rocks show typical arc-like geochemical features (Li et al., 2014; Long et al., 2008; Tang et al., 2010, 2012; Q. Wang et al., 2007; Zhang et al., 2010, 2012; Zhu et al., 2009), and (3) the Late Carboniferous adakitic rock association includes adakite, high-Mg andesite and Nb-enriched basalt in the northwestern Chinese Tianshan Belt (Q. Wang et al., 2006, 2007). In the Early Permian, terrestrial sedimentary and volcanic rocks unconformably covered the Late Carboniferous epicontinental marine strata, and the red beds were well developed in the NTB (B. Wang et al., 2007, 2009). Additionally, the Carboniferous igneous rocks are mainly calc-alkaline, while the Permian ones are dominated by calc-alkaline, alkaline and transitional series,

suggesting that the latter were probably formed in a postsubduction setting (Wang et al., 2009). Furthermore, numerous Early Permian A-type granites have been identified in the North YB (Tang et al., 2010). Collectively, these lines of geological evidence support that the final amalgamation between the YB and West Junggar Terrane likely occurred in the latest Carboniferous. The magmatic rocks are dated at the time interval of 303– 288 Ma in this study, which fall in the period of tectonic transition from oceanic subduction to post-subduction regimes. Arc-continent collision/amalgamation has been an important process during accretionary orogenesis, leading to significant continental growth throughout the Earth history (Brown et al., 2006; Rudnick and Fountain, 1995), such as the cases in the modern Pacific margin (e.g., Taiwan, Japan and Caribbean) (Huang et al., 2006; Harris, 2006; Iturralde-Vinent et al., 2008). Slab break-off commonly occurs in the early stage of continent-continent or arc-continent collision, which may be attributed to strongly positive buoyancy of arc/continental lithosphere resisting subduction (Gerya et al., 2004; Huw Davies and von Blanckenburg, 1995; Wong A Ton and Wortel, 1997). As a result, once the oceanic slab detaches, hot asthenospheric mantle will rise up and cause partial melting of overlying mantle and crust (Coulon et al., 2002; Ferrari, 2004; Huw Davies and von Blanckenburg, 1995), which generally results in high heat fluxes to generate high temperature magmatism. The zircon saturation thermometry is usually employed to estimate the crystallization temperatures of granitic magmas (Hanchar and Watson, 2003). In the present study, the ignimbrites and rhyolites from the Alataw area have temperature of 888–938 °C and 815–832 °C, respectively, clearly higher than 723–735 °C of the biotite granites in the NTF (Table 3). Absence of inherited zircon, which may imply zirconium undersaturated melt compositions, can be regarded as a key evidence to estimate the minimum initial magma temperature at the source (Miller et al., 2003; Watson and Harrison, 1983). The volcanic rocks were as a result of a high temperature magmatic event, which caused the flare-up magmatism with high eHf(t) values at ca. 300 Ma (Fig. 11). We interpret this magmatic episode having a link with arc-continent collision/amalgamation of the WJT and YB, which intrigued the asthenospheric upwelling through slab break-off (Fig. 12b). The tectonic event of oceanic slab break-off can match many geologic facts as follows. (1) After the Late Carboniferous, the volcanic magmatism began waning in the NTB and North YB, and the previously assumed Permian Wulang Formation was actually formed in the Late Carboniferous. (2) The Early Permian molasse occurred along the NTB and North YB. (3) The absence of North Tianshan accretionary complexes between the Alataw area and WJT, possibly imply tectonic erosion during the arccontinent collision or later strike-slip movement (Shyu et al., 2011). (4) The majority of granitic intrusions along the NTF are synkinematic granites with high temperature fabrics and strong deformation that may have been caused by Early Permian large strike-slip shearing (Wang et al., 2009). We also note that slab break-off is an ubiquitous tectonic scenario in accretionary orogenic belts, like in Taiwan and Japan. After arc-continent collision/amalgamation, the large-scale dextral ductile shearing movement occurred along the NTF and was dated at 290–245 Ma (Allen and Vincent, 1997; De Jong et al., 2008; Laurent-Charvet et al., 2002, 2003; B. Wang et al., 2006; Yin and Nie, 1996; Zhou et al., 2001). Biotite granite was emplaced during this period, and its generation is inferred to have relation to water-fluxes partial melting, which may be as the leading reason for its low zircon saturation temperature (Becker et al., 1998; Ebadi and Johannes, 1991). The shear zones may have acted as a channel for facilitating regional metamorphic fluids and then for extracting melts, which promoted the generation, extraction and

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Fig. 11. Zircon saturation temperature and isotopic parameters as a function of age for the Late Carboniferous and Early Permian North YB and NTB magmatic rocks: a. Rock zircon saturation temperature versus age. Temperatures are calculated using the equations of Watson and Harrison (1983). b. Rock eHf(t) versus age (data source: Chen et al., 2000; Li et al., 2014; Liu et al., 2005; Tang et al., 2010; Q. Wang et al., 2007; Zhang et al., 2012; Zhang et al., 2016).

emplacement of these magmas by water-fluxed melting of lower crust (Fig. 12c) (Weinberg and Hasalová, 2015). 6.4. Implication for the formation of the Kazakhstan Orocline It is generally accepted that the Kazakhstan collage system comprises several orogenic components that were amalgamated to generate a long, single composite continent by the Silurian (Kröner et al., 2007; Safonova and Santosh, 2014; Safonova et al., 2009; Windley et al., 2007). The palaeomagnetic data suggested that the orogenic systems around the Kazakhstan Orocline were nearly straight in the Middle Devonian (Bazhenov et al., 2003; Collins et al., 2003; Levashova et al., 2007; Van der Voo et al., 2006), and subsequently underwent 180° bending in the late Paleozoic (Abrajevitch et al., 2008; Sengör and Natal’In, 1996; Xiao et al., 2010). The evolution of the Kazakhstan Orocline may involve bending during subduction and large-scale strike-slip after subduction termination (Van der Voo et al., 2006; Xiao et al., 2010, 2015). The internal margin of the Kazakhstan Orocline was an active margin from the Early Devonian to the Late Carboniferous

Fig. 12. Schematic diagrams for the Carboniferous to Early Permian tectonic evolution of the North Yili Block. (a) The Junggar oceanic plate subducted beneath the Yili Block during the Carboniferous. (b) The arc-continent collision/amalgamation between the West Junggar Terrane and Yili Block caused the slab break off of the Junggar oceanic plate, the oceanic lithospheric plate continued to sink due to the negative buoyancy and tore off the oceanic lithosphere, which resulted in upwelling of deep asthenospheric mantle through the gap, and the break-off of the oceanic lithosphere induced an extensional regime and caused mantle upwelling and underplating of mantle-derived magmas as well as melting of the lower crust in the Alataw area. (c) Early Permian large-scale dextral strike-slip movement induced magmatism with diverse origins.

due to the Junggar oceanic plate subduction (Alexeiev et al., 2011; Bykadorov et al., 2003; Filippova et al., 2001; Windley et al., 2007). Apparently, the Junggar oceanic plate subduction has a close relationship with the Kazakhstan Orocline. The onset of the bending may result from asymmetric trench retreating in the Late Devonian to Carboniferous (Li et al., 2016). However, the ending of the oroclinal bending still remains enigmatic. Growing

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evidence indicates subduction termination of the Junggar oceanic plate may be in the latest Carboniferous to the Early Permian (Briggs et al., 2007; Buslov et al., 2004; Charvet et al., 2007; Filippova et al., 2001; Sengör and Natal’In, 1996). Our ca. 300 Ma high temperature volcanism further suggests that oceanic slab break-off tectonic scenario, likely as a result of the arc-continent collision/amalgamation, took place in the latest Carboniferous, which places a temporal constraint on the assembly of the West Junggar Terrane with the Yili Block. On the other hand, the subsequent ca. 288 Ma low temperature plutonism may be facilitated by strike-slipping of North Tianshan Fault, caused by intra-plate adjustment after terrane assembly during the formation of the Kazakhstan Orocline. 7. Conclusions (1) Two magmatic episodes at ca. 300 Ma and 288 Ma were revealed in northwestern Chinese Tianshan Belt. The ca. 300 Ma volcanic episode generated ignimbrites and rhyolites, and the ca. 288 Ma plutonic episode produced biotite granites. (2) Both ignimbrites and rhyolites were dominantly derived from partial melting of juvenile basaltic lower crust, although the latter requests more involvement of basement sedimentary rocks. The biotite granites were generated by partial melting of the juvenile lower crust in the presence of external water-fluxes. (3) A slab break-off tectonic scenario is proposed to account for the high-temperature volcanic magmatism at ca. 300 Ma, and the subsequent large-scale strike-slip may be a leading reason for the generation of the 288 Ma biotite granite. The two contrasting late Paleozoic magmatic episodes were likely as responses to tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis.

Acknowledgments The present study was financially supported by the Major Basic Research Project of the Ministry of Science and Technology of China (Grants: 2014CB448000), Hong Kong Research Grant Council (704313P), and National Science Foundation of China (41622205). This work is a contribution to the Talent Awards to KDC from the China Government under the 1000 Talent Plan and the Hundred Talent Program of Xinjiang Government. This manuscript is benefited significantly from polishing work by Drs. Zhiwei Bao, Pengfei Li, Samuel B.D. Crace and Ratheesh Kumar R. T. Two anonymous reviewers and editors are gratefully appreciated for their constructive suggestions and comments that improved the manuscript considerably. References Abrajevitch, A., Van der Voo, R., Bazhenov, M.L., Levashova, N.M., McCausland, P.J.A., 2008. The role of the Kazakhstan orocline in the late Paleozoic amalgamation of Eurasia. Tectonophysics 455, 61–76. Alexeiev, D.V., Ryazantsev, A.V., Kröner, A., Tretyakov, A.A., Xia, X., Liu, D.Y., 2011. Geochemical data and zircon ages for rocks in a high-pressure belt of Chu-Yili Mountains, southern Kazakhstan: implications for the earliest stages of accretion in Kazakhstan and the Tianshan. J. Asian Earth Sci. 42, 805–820. Allen, M.B., Vincent, S.J., 1997. Fault reactivation in the Junggar region, northwest China: the role of basement structures during Mesozoic-Cenozoic compression. J. Geol. Soc. 154, 151–155. Allen, M.B., Windley, B.F., Zhang, C., 1993. Palaeozoic collisional tectonics and magmatism of the Chinese Tien Shan, central Asia. Tectonophysics 220, 89–115. Allen, M.B., Windley, B.F., Zhang, C., Zhao, Z.Y., Wang, G.R., 1991. Basin evolution within and adjacent to the Tien Shan Range, NW China. J. Geol. Soc. 148, 369– 378.

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Please cite this article in press as: Wang, X., et al. Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis. Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.03.013