Geochronology and Geochemistry of the Karadaban Bimodal Volcanic Rocks in the Altyn Area, Xinjiang: Implications for the Tectonic Evolution of the Altyn Ocean

Hindawi Geoﬂuids Volume 2019, Article ID 6256398, 25 pages https://doi.org/10.1155/2019/6256398

Research Article Geochronology and Geochemistry of the Karadaban Bimodal Volcanic Rocks in the Altyn Area, Xinjiang: Implications for the Tectonic Evolution of the Altyn Ocean

Wen-Bin Jia ,1,2 Guang-Sheng Yan ,3 Xiao-Fei Yu,2 Yong-Sheng Li,2 Sandro Conticelli ,4 and Ze-Zhong Du2

1College of Earth Science, Jilin University, Changchun 130061, China 2Development and Research Centre of China Geological Survey, Beijing 100037, China 3China Geological Survey, Beijing 100037, China 4Dipartimento di Scienze della Terra, University of Florence, Florence 50121, Italy

Correspondence should be addressed to Guang-Sheng Yan; [email protected]

Received 19 October 2018; Revised 30 March 2019; Accepted 13 May 2019; Published 19 August 2019

Academic Editor: Ling-Li Zhou

Copyright © 2019 Wen-Bin Jia et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Detailed geochronological, geochemical, and Sr-Nd-Hf isotopic data are presented for early Paleozoic volcanic rocks in the Karadaban area from the northern Altyn region, NW China, with the aim to constrain their petrogenesis and tectonic implications. The Karadaban volcanic rocks show a bimodal distribution in composition, with rhyolite and basalt. The LA- ICP-MS zircon U-Pb age indicates that the volcanic rocks were erupted at 512 Ma. The mafic rocks are calc-alkaline, enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE; Ba and U) and depleted in high-field strength elements (HFSE; Nb and Ta). These features together with their depleted isotopic signature (initial 87Sr/86Sr = ε 0 70413 – 0 70817, Nd t =27 to 3.7) suggest that they were likely derived from a depleted mantle source but mixed with crustal components while upwelling. The felsic rocks show an A-type affinity, with high alkalis and Rb/Sr and Ga/Al ratios; enriched in LILE (e.g., Rb, K, Th, U, and REE) and depleted in Ba, Sr, Nb, P, and Ti; and with fractionated REE patterns with strong negative Eu anomalies. The combination of the decoupling of εNd t values (−2.5 to −6.3) and εHf t values (+5.5 to +14.7) in the setting of subduction indicates that the felsic rocks were generated by partial melting of the juvenile crustal as a result of magma upwelling. The geochemical and Sr-Nd-Hf isotopic characteristics, coupled with regional geology, indicate that the formation of the Karadaban bimodal volcanic rocks involves an extensional regime associated with a subduction-related environment. The rifting of the back arc in response to the retreat of the subducting northern Altyn oceanic lithosphere may account for the Karadaban bimodal volcanic rocks.

1. Introduction ore deposits [12, 13]. Moreover, of all these various associations of mafic and felsic rocks, it is perhaps the A-type Bimodal volcanism typically characterizes an extensional rhyolites with basalts that are the most enigmatic but may environment, which can occur within various tectonic set- also prove to be the most instructive. Therefore, the recogni- tings, including continental rifts [1, 2], within-plate exten- tion of such distinctive bimodal igneous suites in ancient sional settings [3–5], intraoceanic islands [6], ocean island orogenic belts can not only provide diagnostic geodynamic arcs [7, 8], incipient back-arc depressions [9], mature islan- tracers for constraining the tectonic evolution of these belts d/active continental margins [7–9], and back arcs [9–12]. In but also document important information on crustal growth each of these modern tectonic environments, the volcanic by magmatic underplating in continental interiors. activity may give rise to specific features, such as lithological The Altyn Tagh Belt (ATB) marks the northern margin assemblage, geochemical signature, and type of associated of the Qinghai-Tibet Plateau [14], lying between the Tarim 2 Geofluids block to the north, the Qaidam block, the Qilian orogen, and indicator of the palaeosubduction zone [52]. Moreover, the Kunlun orogenic belt to the south [15, 16]. As one of the there are many mafic-ultramafic plutons that are devel- most important tectonic zones in northwestern China, the oped in the NASC, in which the ophiolite belt is dated Altyn Tagh carved and dominated several tectonic units to at 524–437 Ma, with the features of the ocean island basalt, match the eastern extrusion of the Tibetan Plateau [17–21]. indicating that the oceanic crust may have developed In the last decade, the North Altyn Tagh region was probably in the Early Cambrian [14, 53, 54]. Researches recognized as an ophiolite [14, 22–24], with HP/UHP meta- about the age of felsic rocks in NASC show that they morphic rocks [25–27] and igneous rocks [28–32]. Previous mainly ranged from 500 Ma to 420 Ma, which is associated studies show that three stages can be divided about the with the setting of subduction to collision to postcollision collisional orogeny in the Altyn area: the oceanic-continent [55–57]. The NASC is about 20 km wide, which extends collision stage [31, 32], the oceanic-continent subduction from Hongliugou eastward to Lapeiquan for 150 km, and is stage [33–35], and the postcollisional extension stage [36, 37]. controlled by the NEE-Altyn faults. In the Altyn region, the At present, the mechanisms of the tectonic setting of the last Paleozoic formation of the Zhuo’abulake and Simierbulake two stages of magmatism are almost clear due to the zircon volcanic sedimentary rock series is located at the center of U-Pb ages from igneous rocks and metamorphic ages from the Karadaban fault, which is fault-contacted to the Cenozoic HP/UHP metamorphic rocks [14, 31, 34, 35, 37, 38]. How- and Archean strata. In addition, there is a suite of Karadaban ever, the oceanic-continental collision stage of magmatism bimodal volcanic rocks that developed in the north of NASC, in the north Altyn region is still unclear, which is subject which occurred in the Zhuo’abulake Formation (Figure 2). to few systematic studies. In addition, the oceanic- Magmatism peaked at the early Paleozoic and stretched continental subduction setting is always accompanied by along the NE faults, which produced a series of granodioritic the mixture of crustal and mantle magmatism, but most of and granitic rocks. Chen et al. reported that the age of the studies are focused on granite only. Recently, there was Kaladawan felsic volcanic rocks (zircon SHRIMP U-Pb a set of bimodal volcanic rocks discovered in the NE of the method) was 485–477 Ma, and the characteristic of geochem- North Altyn Tagh, which is helpful for studying the istry indicated a setting of active continental margin [34]. Cui magmatic events and determining the timing of oceanic- et al. (2010) verified the geochemistry of basic volcanic rocks continental subduction. In this paper, we present U-Pb in the Karadaban region. These rocks showed the features of zircon dating and geochemical and Sr-Nd-Hf isotopic tholeiitic basalt and conjectured an ocean island arc setting, composition for Karadaban bimodal volcanic rocks from which developed at the hanging wall of subduction in the the north Altyn region. Our aims are (1) to constrain petro- early Paleozoic [58]. However, the timing of the subduction genesis and magma sources of bimodal volcanic rocks in the was still disputable. subduction setting and (2) to discuss the early Paleozoic evolutionoftheAltynTaghregion. 3. Petrology and Sampling 2. Geological Background In this study, 12 samples within the Karadaban bimodal volcanic rocks were collected for geochemical analysis, 8 of The Altyn Mountain is located at the juncture of the them were used for Sr-Nd isotopic analysis, and analyses of Tarim Basin, the Qaidam Basin, and the Qilian orogen Hf isotopes were made on zircon grains from samples in Western China [19, 39–43], which plays a significant KB-26 and KB-81 that were previously dated by laser role in accommodating convergence between India and ablation multiple collector inductively coupled plasma mass Eurasia [20, 44, 45]. The Altyn Tagh fault system is the spectrometry (LA-ICP-MS). Lithology of these samples are first-order tectonic unit that marks the northern margin of basalt and rhyolite (Figure 3). the Qinghai-Tibet Plateau and influences the geological The rhyolite samples are gray to light yellow in color, evolution of surrounding areas. Pin and Paquette proposed massive, and porphyritic and rhyolitic in texture, which are that the sedimentation in the southern Tarim Basin was asso- interbedded with the basalt (Figure 3(a)). The felsic samples ciated with this large-scale (>1600 km long) intracontinental contain 10–15% plagioclase and quartz phenocrysts in a fault system [8] (Figure 1(a)). groundmass consisting of glassy cryptocrystalline material The tectonic units of the ATB have been divided into and feldspar and quartz (crystal sizes < 0 1mm). The quartz four different schemes (Figure 1(b)) [19, 47–49]: the North is subhedral-anhedral, some of which has been rounded by Altyn Tagh Archean complex (NAAC), the North Altyn dissolution (Figures 3(b) and 3(c)). The plagioclase is subhe- Tagh subduction-collision complex (NASC), the Milanhe- dral and lath-shaped (up to 0.5 mm) and hypidiomorphic in Jinyanshan block (MJB), and the South Altyn Tagh texture. The groundmass is typically a fine-grained assem- subduction-collision complex (SATSC) (Figures 1(b) and blage of plagioclase, quartz, and muscovite. Muscovite is 1(c)) [19, 25, 50]. In the NAAC region, the HP argilla- present as microscopic flakes (generally <0.1 mm), prefer- ceous rocks, glaucophane schist, and eclogite occurred entially oriented aggregates, and disseminated single crystals belonging the HP-LT metamorphic belt. Previous studies with a preferred orientation (Figure 3(d)). The dark greyish- showed that the phengite in the eclogite and the parago- green basalts occur as massive layers with a porphyritic nite in the glaucophane schist have a yield of 39Ar–40Ar texture. The main phenocrysts are plagioclase and pyroxene isochron ages of 513 ± 5 Ma and 497 ± 10 Ma, respectively (Figures 3(e) and 3(f)). The plagioclase is euhedral, tabular, [51]. These HP-LT metamorphic rocks are an important and up to 0.3–0.4 mm, and the pyroxene occurs as thin Geofluids 3

(a) Alashan (b) Tarim Milan I I b Hongliugou c II N39° Tibet Ruoqiang III

India 500 km Qaidam Basin Mangya Huatugou Tarim Basin IV N38° I: North Altyn Tagh Archean complex (NAAC) II: North Altyn Tagh subduction-collision complex (NASC) III: Milanhe-Jinyanshan block (MJB) 0 100 km IV: South Altyn Tagh subduction-collision complex (SATSC) Tula E86° E88° E90° E92°

(c) 92° Tarim Basin 478 ± 3.3 Ma N 488~477 Ma Zhang et al., 2012 518 ± 4.1 Ma Chen et al., 2016 Gai et al., 2015 512 ± 2.8 Ma 506 ± 2.3 Ma 494 ± 5.5 Ma Tis paper 40° 512 ± 1.5 Ma Han et al., 2012 Meng et al., 2015 Gao et al., 2011 North Altyn Tagh Annanba Altyn Tagh

Hongliugou

Lapeiquan

488 ± 2 Ma Ni et al., 2017 Bashikaogong 39°

500 ± 1.2 Ma 482 ± 1.56 Ma Qaidam Basin 484 ± 4.9 Ma Kang et al., 2011 Wu et al., 2006 Wu et al., 2017 0 40 km 90° Suoerkuli Basin Fig 2 92° 94°

Strata

Cenozoic Early Palaeozoic granite

Early Palaeozoic Ophiolitic melange

Meso-Neoproterozoic Regional fault

Archeozoic- Strike-slip Paleoproterozoic fault

Figure 1: (a) Tectonic outline of Western China; (b) sketch map of the Altyn region; (c) tectonic map of the northern Altyn Tagh region (after [46]). tabular crystals up to 0.5–0.6 mm in size. The groundmass 4.1. Major and Trace Elements. Major and trace elements includes fine-grained plagioclase, pyroxene, and magnetite, were analyzed in the Analytical Laboratory of Research which is characterized by the fasciculate variolitic structure Institute of Uranium Geology, Beijing (BRIUG). Major ele- of fibroid pyroxene and plagioclase (Figure 3(g)). ment compositions were measured by wavelength dispersive X-ray fluorescence spectrometry (XRF) on fused glass beads 4. Analytical Methods using a Philips PW2404 spectrometer; analytical uncertainties are less than ±1% for major elements. FeO content The Karadaban igneous samples were collected from adits and was determined by conventional wet chemical titration outcrops (Figure 2), which were selected for petrographic technique. Both trace element and REE abundance were studies under thin sections. Some of the igneous samples were analyzed using ICP-MS (Finnigan MAT Ltd.); uncertainties crushed to 200 meshes for major element, trace element, and are less than ±5% for trace elements ≥ 20 ppm, ±3% for La Sr-Nd isotopic analyses; other samples were chosen for zircon and Lu, and ±2% for other REEs. The detailed analytical U-Pb dating and Hf isotopic measurements. procedures are given in Wang et al. [59]. 4 Geofluids

91°30″ 91°45″ 92°00″

4747 6 O zhzh 6

″ 1 zhzh ″ O1

10 KB-23- 10

39 KB-58- 4747 39 zh4 Q O1 45 zhzh5 KB-19- 1 KKB-138B-138 KB-26KB-26 5125 ± 2.8 MMa TisT paperpp KKB-81B-81 1616 O zh4 KB-82KB-82 1 O zhzh4 Q 1 KB-46KB-46 zhzh4 O1 KB-132KB-132 KKB-76B-76 KB-70

505 ± 1.8 Ma 18 Unpublished data

″ 4 zhzh ″ O1

° 3 2 zhzh KB-33KB-33 39 O1 1188 0 220000 m zh 39 O1

Ordovician 4th Quaternary zh4 MarbleMarble FaultFault PyritizationPyritization Q O1 Zhuo’abulake Fm. Ordovician 6th Ordovician 3rd zh6 zh3 Quartz porphyry Alteration zone Pb-Zn orebody O1 Zhuo’abulake Fm. O1 Zhuo’abulake Fm. Ordovician 5th Ordovician 2nd zh5 zh2 Diabase Sample position O1 Zhuo’abulake Fm. O1 Zhuo’abulake Fm. sericitization

Figure 2: Geological map of the Karadaban area (after [34]).

4.2. Zircon U-Pb Dating. Cathodoluminescence (CL) images 4.4. Sr-Nd Isotopic Analyses. Samples for isotopic analysis were obtained by using a scanning electron microscope were dissolved in Teflon bombs after being spiked with 84 87 150 147 (SEM, Leo 1450VP, Germany) at the Chinese Academy of Sr, Rb, Nd, and Sm tracers prior to HF+HNO3 (with Geological Sciences before in situ U-Pb and Hf isotopes a ratio of 2 : 1) dissolution. Rubidium, Sr, Sm, and Nd were analyses. Zircon LA-ICP-MS U-Pb dating was performed separated using conventional ion exchange procedures using a laser ablation inductively coupled plasma mass and measured using multicollector mass spectrometer spectrometer (LA-ICP-MS) at the MLR Key Laboratory of (IsoProbe-T) at the Analytical Laboratory of BRIUG [66]. Metallogeny and Mineral Assessment, Institute of Mineral Procedural blanks were <100 pg for Sm and Nd and Resources, Chinese Academy of Geological Sciences (CAGS), <500 pg for Rb and Sr. 143Nd/144Nd were corrected for Beijing. The laser spot diameter and frequency were 40 μm mass fractionation by normalization to 146Nd/144Nd = and 10 Hz, respectively. The Harvard zircon 91500 was used 0 7219, and87Sr/86Sr ratios were normalized to 86Sr/88Sr = as an external standard for zircon U-Th-Pb analyses and 0 1194. Typical within-run precision (2σ) for Sr and Nd NIST610 as an external standard to calculate the contents was estimated to be ±0.000015. The measured values for of U, Th, and Pb. The 207Pb/206Pb and 206Pb/238U ratios were the La Jolla and BCR-1 Nd standards and the NBS-607 calculated using the GLITTER program [60], and common Sr standard were 143Nd/144Nd = 0 511853 ± 7 (2σ) and Pb was corrected using the method of Andersen [61]. Age 0 512604 ± 7 (2σ), respectively, and the measured value calculations and concordia plots were done using Isoplot during the period of data acquisition is 87Sr/86Sr = (ver. 3.0) [62]. The details of the analytical techniques were 1 20042 ± 2 (2σ). documented by Hou et al. [63]. 5. Results 4.3. Lu-Hf Isotope Analyses. In situ zircon Hf isotopic analyses were carried out at the Guangzhou Institute of Geochemistry, 5.1. Major and Trace Elements. Twelve whole-rock major Chinese Academy of Sciences, using a Neptune MC-ICP-MS and trace element data are presented in Table 1. The loss with an ArF excimer laser ablation system. During analyses, on ignition (LOI) for the rocks was in the generally range the spot sizes of 32 and 63 μm and a laser repetition rate of from 0.74 to 3.68 wt.% (one sample at 6.87 wt.%), which 10 Hz with 100 mJ were used. During analyses, the 176Hf/177Hf probably reflects the presence of alteration-related hydra- and 176Lu/177Hf ratios of standard zircon (91500) were tion. Considering the potential modification from the 0 282294 ± 15 (2σn, n =20) and 0.00031, similar to the concentrations of mobile components [67, 68], the effects commonly accepted 176Hf/177Hf ratio of 0 282284 ± 3 (1σ) of alteration and the mobility of the major and trace measured using the solution method [64, 65]. elements were evaluated prior to the geochemical and Geofluids 5

Basalt Rhyolite

(a)

Rhyolite Ms

(b) (c) (d)

90°

Glass

Basalt Pl

(e) (f) (g)

Figure 3: Photographs of outcrops and photomicrographs showing the geological and mineralogical characteristics of the Kaladaban bimodal volcanic rocks: (a) outcrop of bimodal volcanic rocks; (b, c) outcrop of and hand specimen of rhyolite; (d) photographs of rhyolite; (e, f) outcrop of and hand specimen of basalt; (g) photographs of basalt. Pl: plagioclase; Px: pyroxene; Q: quartz; Ms: muscovite.

petrological investigation. Only immobile elements were The SiO2 content of the volcanic rocks lie within a range used to classify rock samples and discuss their tectonic from 44.74 to 76.14 wt.% and with a compositional gap at – settings and petrogenesis. 46 68 wt.% SiO2, which shows the bimodal feature (Table 1). 6 Table 1: Major (%) and trace element (ppm) data for the volcanic rocks in the northern Altyn Tagh region.

Rock type Basalt Rhyolite Sample KB-19 KB-23 KB-70 KB-58 KB-26 KB-33 KB-46 KB-76 KB-81 KB-82 KB-132 KB-138

SiO2 45.43 46.65 44.80 44.74 76.14 68.06 75.64 71.25 72.60 68.78 68.70 71.41 TiO2 1.79 3.75 1.57 2.98 0.19 0.41 0.26 0.20 0.32 0.65 0.52 0.23 Al2O3 17.42 14.46 15.48 16.93 12.79 11.59 10.31 9.60 14.60 13.79 12.73 14.57 T Fe2O3 12.19 17.12 11.49 13.90 2.07 8.30 4.91 6.86 2.64 4.54 7.50 3.06 MnO 0.48 0.50 0.66 0.64 0.06 0.17 0.05 0.28 0.05 0.17 0.22 0.01 MgO 8.74 4.23 8.63 7.67 0.57 5.21 2.76 6.12 0.86 1.33 2.05 1.33 CaO 6.61 7.85 10.59 1.29 0.30 0.13 0.27 0.11 0.17 1.08 0.54 0.35

Na2O 3.10 2.80 1.77 3.11 5.79 1.28 0.19 0.05 2.46 4.27 0.17 3.99 K2O 0.80 0.18 1.44 0.67 1.27 1.30 2.11 1.22 3.80 4.08 4.56 2.81 P2O5 0.24 0.53 0.21 0.65 0.03 0.07 0.03 0.04 0.04 0.10 0.05 0.03 LOI 3.11 1.92 3.35 6.87 0.74 3.42 3.41 3.68 1.87 1.20 1.47 1.68 Total 96.80 98.07 96.64 92.57 99.21 96.52 96.54 95.72 97.54 98.79 97.03 97.79 A/CNK 0.97 0.76 0.65 2.07 1.12 3.09 3.33 6.03 1.72 1.03 2.06 1.42 A/NK 2.92 3.01 3.46 2.90 1.17 3.30 3.96 6.85 1.79 1.21 2.44 1.52

Na2O/K2O 3.89 15.56 1.23 4.68 4.56 0.98 0.09 0.04 0.65 1.05 0.04 1.42 Rb 31.00 4.95 60.50 46.70 38.70 71.40 101.00 23.80 143.00 117.00 115.00 92.70 Ba 1012.00 194.00 981.00 888.00 618.00 214.00 396.00 458.00 935.00 862.00 14744.00 635.00 Th 4.87 5.08 4.88 11.90 24.90 15.80 13.50 11.80 28.50 30.40 22.70 24.70 U 1.26 1.94 1.39 4.41 8.24 4.59 3.65 3.12 7.60 6.88 6.84 6.40 Nb 11.50 11.70 10.90 13.00 19.40 18.50 18.30 22.60 26.50 16.10 25.30 24.70 Ta 0.64 0.70 0.56 0.82 1.65 1.43 1.34 1.53 1.91 1.54 1.92 2.02 K 6607.48 1494.15 11953.23 5520.07 10542.08 10791.11 17514.80 10127.04 31543.24 33867.48 37851.89 23325.40 Cr 188.00 174.70 197.00 215.30 4.13 18.00 14.00 2.46 4.26 10.00 4.04 3.40 Ni 98.80 112.00 93.60 118.46 2.04 9.56 6.74 1.07 1.93 4.81 2.17 2.51 V 297.00 264.00 275.00 315.00 5.31 25.10 20.30 8.51 6.51 75.40 3.08 4.97 Co 50.20 35.30 53.90 67.90 0.43 2.57 1.28 0.61 1.24 9.66 0.85 1.70 La 24.70 25.60 27.60 43.70 61.20 51.00 45.20 47.20 56.90 46.00 59.70 66.80 Ce 50.10 61.50 56.70 83.60 107.00 110.00 85.10 79.80 93.10 86.20 102.00 131.00 Pb 36.10 44.80 44.70 283.00 8.48 4.06 5.99 18.40 7.30 94.50 91.00 6.18 Pr 6.22 10.00 7.93 12.90 15.30 13.20 10.30 9.02 13.80 10.20 15.60 15.50 Sr 370.00 394.00 460.00 144.00 107.00 32.10 26.60 22.60 75.80 80.70 322.00 104.00 Nd 26.70 48.50 36.50 57.00 58.10 52.40 39.00 33.00 50.40 39.70 66.50 61.40 P 1038.70 2308.71 929.59 2841.15 109.11 314.23 139.66 157.11 174.57 440.79 213.85 139.66 Geo

Sm 5.92 12.40 8.14 13.70 11.80 10.50 7.91 7.17 10.50 8.16 14.80 12.30 ﬂ uids Zr 116.00 295.00 107.00 289.00 372.00 363.00 261.00 439.00 442.00 225.00 527.00 334.00 Geo ﬂ uids

Table 1: Continued.

Rock type Basalt Rhyolite Sample KB-19 KB-23 KB-70 KB-58 KB-26 KB-33 KB-46 KB-76 KB-81 KB-82 KB-132 KB-138 Hf 3.23 8.12 3.03 7.40 11.50 10.50 7.65 11.20 12.60 7.87 15.70 10.30 Eu 1.67 3.50 2.29 4.03 1.25 1.28 0.91 1.05 1.31 1.09 2.22 1.38 Ti 10728.35 22475.58 9409.78 17860.60 1144.76 2427.36 1582.28 1192.70 1905.93 3883.78 3086.65 1402.48 Gd 5.87 11.90 8.10 13.00 10.50 9.77 8.08 7.60 10.00 8.37 14.70 11.70 Tb 1.08 2.29 1.44 2.42 1.93 1.97 1.77 1.63 1.93 1.61 2.78 2.22 Dy 6.09 13.10 8.23 14.00 10.90 11.70 11.20 10.40 12.00 9.25 16.20 13.00 Y 33.20 69.70 39.00 74.50 53.90 60.40 62.30 61.10 73.70 56.40 85.00 69.40 Ho 1.21 2.62 1.50 2.65 2.20 2.38 2.29 2.17 2.51 1.88 3.22 2.56 Er 3.58 7.76 4.15 7.37 6.91 6.62 6.54 6.69 7.87 5.94 10.00 7.76 Tm 0.52 1.18 0.59 1.10 1.11 1.02 0.96 1.06 1.27 0.92 1.55 1.18 Yb 3.23 7.31 3.51 7.13 7.57 6.13 6.12 6.91 8.17 5.83 9.95 7.51 Lu 0.49 1.13 0.54 1.03 1.13 0.88 0.84 1.08 1.17 0.87 1.52 1.14 ΣREE 137.37 208.79 167.22 263.63 296.90 278.85 226.22 214.78 270.93 226.02 320.74 335.45 LREE 115.31 161.50 139.16 214.93 254.65 238.38 188.42 177.24 226.01 191.35 260.82 288.38 HREE 22.06 47.29 28.06 48.70 42.25 40.47 37.80 37.54 44.92 34.67 59.92 47.07 LREE/HREE 5.23 3.42 4.96 4.41 6.03 5.89 4.99 4.72 5.03 5.52 4.35 6.13 La/Nb 2.15 2.19 2.53 3.36 3.15 2.76 2.47 2.09 2.15 2.86 2.36 2.70 La/Ta 38.53 36.57 49.37 53.49 37.09 35.66 33.73 30.85 29.79 29.87 31.09 33.07

LaN/YbN 5.49 2.51 5.64 4.40 5.80 5.97 5.30 4.90 5.00 5.66 4.30 6.38 δEu 0.86 0.87 0.85 0.91 0.34 0.38 0.35 0.43 0.39 0.40 0.45 0.35 δCe 0.96 0.94 0.93 0.85 0.83 1.02 0.93 0.89 0.79 0.93 0.80 0.96 Note: major elements were obtained by X-ray fluorescence spectrometer (XRF) at the Analytical Laboratory of BRIUG, Beijing, using fused lithium-tetraborate glass pellets. The analytical precision as determined on the Chinese national standard GSR-3 was generally around 1–5%. Trace elements were analyzed using a PerkinElmer ELAN-DRC-e ICP-MS at the Analytical Laboratory of BRIUG, Beijing. The powdered samples fl ~ ° (50 mg) were dissolved in high-pressure Te on bombs using a HF+HNO3 mixture for 48 h, at 195 C. Rh was used as an internal standard to monitor signal drift during counting. The international standards GBPG-1 and OU-6 and the Chinese national standard GSR-3 were used for analytical quality control, with the precision generally better than 5%. 7 8 Geofluids

1 Comendite pantellerite Phonolite

2 Rhyolite 0.1 Tr achyt e Dacite

Zr/TiO Trachyandesite Andesite 0.01 Basanite Andesite/basalt

Subalkaline basalt Alkaline basalt

0.001 0.01 0.1 110 Nb/Y

Karadaban basalt Karadaban rhyolite Kaladawan rhyolite (a) 4

3 Alkalic

(%)

2 2

TiO Toleiitic

0 0.05 0.1 0.15 ⁎ Zr/P2O5 10000

Karadaban basalt (b) Figure ﬁ 4: Nb/Y vs. Zr/TiO2 (a) and TiO2 vs. Zr/P2O5 (b) diagrams (after [69]). (b) Diagram for the classi cation of the Karadaban bimodal volcanic rocks in the northern Altyn Tagh region. Data of the Kaladawan rhyolite from [34].

Combining with the Zr/TiO2-Nb/Y plot, the samples were e.g., Rb, U, and Ba) with moderate to negative anomalies identified as basalt and rhyolite (Figure 4(a)). Mafic rocks, in high-field strength elements (HFSEs; e.g., Nb and Ta) on the TiO2-Zr/P2O5 diagram (Figure 4(b)), show alkalic (Figure 5(b)). – T features, with relatively low K2O contents (0.18 1.44 wt.%), The rhyolite exhibits a wide range of Fe2O3 content T – – – high concentrations of Fe2O3 (11.49 17.12 wt.%), MgO (2.07 8.30 wt.%), with low TiO2 (0.19 0.65 wt.%) and – – – – (4.23 8.74 wt.%), and TiO2 (1.79 3.75 wt.%). On a chondrite- P2O5 (0.03 0.10 wt.%) content, and high Al2O3 (9.60 normalized REE diagram, the samples exhibit slightly LREE 14.57 wt.%). The A/NK and A/CNK values are 1.17–6.85 enrichment (LaN/YbN =251 – 5 64), with weak negative Eu and 1.03–6.03, respectively, which classify the felsic samples anomalies (σEu = 0 85 – 0 91) (Figure 5(a)). The primitive- as peraluminous. The felsic rock samples show La/Yb N mantle-normalized trace-element spider diagram shows values of 4.30–6.38 and are characterize by LREE enrichment slight enrichment in large-ion lithophile elements (LILEs; with moderate HREE and LREE fractionation and show Geofluids 9

1000 1000

100

100 10

itive mantle) itive

1 Rock (chondrite) 10

Rock (prim Rock

0.1

1 0.01 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Rb T Nb K Ce Pr Nd Sm Hf Ti Tb Y Er Yb Ba U Ta La Pb Sr P Zr Eu Gd Dy Ho Tm Lu Basalt Basalt (a) (b) 1000 1000

100

100 10

ORB)

Rock (E-M Rock (chondrite) 10

0.1

1 0.01 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Rb T Nb K Ce Pr Nd Sm Hf Ti Tb Y Er Yb Ba U Ta La Pb Sr P Zr Eu Gd Dy Ho Tm Lu A-type granite Karadaban rhyolite Hongliugou granite Kaladawan rhyolite A-type granite Karadaban rhyolite Lapeiquan granite Highland 4337 granodiorite Hongliugou granite Kaladawan rhyolite Lapeiquan granite Highland 4337 granodiorite (c) (d)

Figure 5: Chondrite-normalized REE element pattern and primitive-mantle normalized trace element spider grams for the bimodal volcanic rocks. The normalized values of chondrite are from [70], and primitive mantle values are from [71]. Data of the A-type granite are from [72], Lapeiquan granite are from [24], Hongliugou granite are from [57], Kaladawan rhyolite are from [34], and Highland 4337 granodiorite are from [32]. strong negative Eu anomalies (δEu = 0 35 – 0 45). They are (sample KB-26) rhyolites are mostly display euhedral crystals also enriched in LILE (e.g., Th, U, and K) and are relatively with clear oscillatory zoning, euhedral, and up to 50 μmin depleted in HFSE and LREE, with remarkably negative Nd, length, with length to width ratios of nearly 2 : 1. Their CL P, and Ti anomalies (Figure 5(d)). images commonly show oscillatory zoning (Figure 6), which indicated a typical feature of magmatic zircons [73]. 5.2. Zircon U-Pb Geochronology. Two samples were collected For sample KB-26, 206Pb/238U ages determined from 24 for LA-ICP-MS U-Pb zircon dating. The results are listed in analytical spots ranged from 502 to 523 Ma with a weighted Table 2. Zircons selected from the medium- to coarse- mean age of 512 ± 3 Ma (n =23, MSWD = 1 16). For sample grained (sample KB-81) and medium- to ﬁne-grained KB-81, 206Pb/238U ages determined from 26 analytical spots 10 Table 2: LA-ICP-MS zircon U-Pb dating data for Karadaban rhyolite (KB-26, KB-81) from the northern Altyn Tagh region.

Isotopic ratios Ages (Ma) Spot no. 207 Pb/206 Pb 207 Pb/235 U 206 Pb/238 U 207 Pb/206 Pb 207 Pb/235 U 206 Pb/238 U Pb (ppm) Th (ppm) U (ppm) Th/U Ratios 1σ Ratios 1σ Ratios 1σ Age 1σ Age 1σ Age 1σ KB-26-1 0.05943 0.00173 0.66147 0.01805 0.08121 0.00092 583.4 69.43 515.5 11.03 503.4 5.50 179.22 370.82 399.64 0.93 KB-26-2 0.05904 0.00197 0.66388 0.02115 0.08200 0.00102 568.6 72.21 517.0 12.91 508.0 6.07 123.32 251.45 301.98 0.83 KB-26-3 0.06160 0.00230 0.69672 0.02517 0.08246 0.00106 661.1 74.99 536.8 15.06 510.8 6.29 99.43 236.65 248.95 0.95 KB-26-4 0.06051 0.00196 0.67705 0.02272 0.08114 0.00115 620.4 74.99 525.0 13.76 502.9 6.83 154.70 317.22 392.62 0.81 KB-26-5 0.06212 0.00171 0.71062 0.02002 0.08284 0.00080 679.6 63.88 545.1 11.88 513.0 4.77 166.46 348.84 435.81 0.80 KB-26-6 0.05778 0.00277 0.64371 0.02960 0.08131 0.00153 520.4 110.17 504.6 18.29 503.9 9.13 119.30 233.79 362.94 0.64 KB-26-7 0.06759 0.00998 0.74943 0.09941 0.08220 0.00236 857.4 313.87 567.9 57.76 509.3 14.03 72.24 126.11 209.43 0.60 KB-26-8 0.06709 0.00357 0.75325 0.04424 0.08096 0.00171 840.4 111.88 570.1 25.63 501.8 10.17 152.07 321.51 385.92 0.83 KB-26-9 0.06001 0.00196 0.68555 0.02180 0.08313 0.00096 605.6 70.36 530.1 13.13 514.8 5.72 138.37 270.60 366.74 0.74 KB-26-10 0.06166 0.00188 0.71332 0.02235 0.08387 0.00096 661.1 66.66 546.7 13.24 519.2 5.69 155.19 331.54 381.97 0.87 KB-26-11 0.06045 0.00200 0.69004 0.02203 0.08306 0.00076 620.4 76.84 532.8 13.24 514.4 4.53 185.01 370.24 446.68 0.83 KB-26-12 0.06368 0.00253 0.71492 0.02687 0.08180 0.00084 731.5 78.70 547.7 15.91 506.9 5.01 138.00 285.23 355.98 0.80 KB-26-13 0.05910 0.00198 0.68358 0.02316 0.08365 0.00076 572.3 74.06 528.9 13.97 517.8 4.54 158.96 331.23 379.46 0.87 KB-26-14 0.05821 0.00232 0.66206 0.02606 0.08279 0.00095 538.9 88.88 515.9 15.92 512.8 5.68 113.33 243.40 306.63 0.79 KB-26-16 0.05920 0.00201 0.70101 0.02393 0.08602 0.00125 576.0 78.69 539.4 14.28 531.9 7.40 190.04 361.89 432.30 0.84 KB-26-17 0.05913 0.00270 0.67400 0.03558 0.08217 0.00146 572.3 127.76 523.1 21.58 509.1 8.70 165.05 378.47 221.56 1.71 KB-26-18 0.05868 0.00262 0.67151 0.03046 0.08362 0.00180 553.7 98.13 521.6 18.50 517.7 10.72 198.49 415.08 424.27 0.98 KB-26-19 0.05990 0.00199 0.68925 0.02301 0.08357 0.00121 611.1 72.21 532.4 13.83 517.4 7.17 173.72 337.42 377.19 0.89 KB-26-20 0.05808 0.00174 0.64937 0.02015 0.08131 0.00126 531.5 66.66 508.1 12.40 504.0 7.51 318.15 645.51 558.34 1.16 KB-26-21 0.05859 0.00189 0.68120 0.02204 0.08464 0.00116 553.7 70.36 527.5 13.31 523.8 6.92 195.70 338.88 417.41 0.81 KB-26-22 0.05928 0.00358 0.68206 0.03900 0.08349 0.00326 576.0 132.23 528.0 23.55 516.9 19.38 159.41 200.48 402.64 0.50 KB-26-23 0.06142 0.00196 0.68436 0.01974 0.08144 0.00093 653.7 68.51 529.4 11.90 504.7 5.52 193.52 293.47 459.68 0.64 KB-26-24 0.06116 0.00484 0.67772 0.04259 0.08179 0.00295 655.6 170.35 525.4 25.78 506.8 17.56 165.11 212.71 383.31 0.55 KB-81-1 0.06004 0.00213 0.66399 0.02230 0.08091 0.00107 605.6 75.91 517.1 13.61 501.6 6.39 26.49 187.64 330.97 0.57 KB-81-2 0.05821 0.00203 0.65346 0.02105 0.08228 0.00114 538.9 77.77 510.6 12.93 509.7 6.77 40.88 352.60 490.77 0.72 KB-81-3 0.06149 0.00381 0.68379 0.03845 0.08167 0.00213 657.4 133.32 529.1 23.19 506.1 12.70 27.98 190.92 345.59 0.55 KB-81-4 0.06088 0.00233 0.68519 0.02497 0.08238 0.00118 635.2 83.32 529.9 15.05 510.3 7.03 35.97 246.22 435.34 0.57 KB-81-5 0.05833 0.00143 0.66421 0.01685 0.08261 0.00082 542.6 53.70 517.2 10.28 511.7 4.88 48.21 329.28 585.66 0.56 KB-81-6 0.06265 0.00597 0.71734 0.07026 0.08308 0.00219 696.0 203.68 549.1 41.56 514.5 13.03 36.85 394.07 406.53 0.97 KB-81-7 0.06094 0.00240 0.69022 0.02642 0.08248 0.00106 636.7 85.17 532.9 15.87 510.9 6.34 27.12 218.68 320.83 0.68 KB-81-8 0.05847 0.00244 0.65658 0.02750 0.08163 0.00098 546.3 90.73 512.5 16.86 505.8 5.85 21.62 130.54 270.59 0.48

KB-81-9 0.05860 0.00201 0.66130 0.02126 0.08232 0.00093 553.7 78.69 515.4 12.99 509.9 5.51 47.42 382.00 561.75 0.68 Geo

KB-81-10 0.05723 0.00204 0.65293 0.02387 0.08296 0.00129 501.9 77.77 510.3 14.67 513.7 7.66 41.15 295.92 494.37 0.60 ﬂ KB-81-11 0.05881 0.00225 0.65222 0.02397 0.08123 0.00140 561.1 83.32 509.8 14.73 503.4 8.37 39.12 298.38 478.59 0.62 uids Geo ﬂ uids

Table 2: Continued.

Isotopic ratios Ages (Ma) Spot no. 207 Pb/206 Pb 207 Pb/235 U 206 Pb/238 U 207 Pb/206 Pb 207 Pb/235 U 206 Pb/238 U Pb (ppm) Th (ppm) U (ppm) Th/U Ratios 1σ Ratios 1σ Ratios 1σ Age 1σ Age 1σ Age 1σ KB-81-12 0.06282 0.00263 0.70654 0.02906 0.08236 0.00136 701.9 89.65 542.7 17.29 510.2 8.12 30.47 185.74 375.81 0.49 KB-81-13 0.05858 0.00304 0.66136 0.03487 0.08226 0.00154 550.0 112.95 515.4 21.32 509.6 9.14 34.35 204.96 421.38 0.49 KB-81-14 0.05774 0.00222 0.65190 0.02464 0.08241 0.00126 520.4 87.95 509.6 15.15 510.5 7.53 43.03 280.42 531.47 0.53 KB-81-15 0.06107 0.00232 0.68192 0.02667 0.08146 0.00133 642.6 76.84 527.9 16.10 504.8 7.91 39.98 273.58 493.02 0.55 KB-81-16 0.06055 0.00176 0.69754 0.02070 0.08384 0.00119 633.4 61.10 537.3 12.38 519.0 7.07 84.00 430.41 1047.75 0.41 KB-81-17 0.05988 0.00341 0.67154 0.03465 0.08291 0.00142 598.2 122.21 521.6 21.05 513.5 8.43 18.15 95.22 225.51 0.42 KB-81-18 0.06111 0.00288 0.70427 0.03351 0.08431 0.00127 642.6 101.84 541.3 19.97 521.8 7.56 27.92 147.84 342.84 0.43 KB-81-19 0.05864 0.00216 0.67410 0.02493 0.08355 0.00124 553.7 79.62 523.2 15.12 517.3 7.38 77.50 604.90 919.21 0.66 KB-81-20 0.05860 0.00236 0.67375 0.03269 0.08375 0.00275 553.7 88.88 523.0 19.84 518.5 16.36 74.02 694.08 839.68 0.83 KB-81-21 0.06190 0.00196 0.73603 0.02451 0.08623 0.00126 672.2 62.80 560.1 14.34 533.2 7.48 49.82 345.96 562.44 0.62 KB-81-22 0.06049 0.00282 0.70626 0.03306 0.08539 0.00125 620.4 101.84 542.5 19.67 528.2 7.44 23.17 103.25 282.14 0.37 KB-81-23 0.05967 0.00188 0.67760 0.01948 0.08299 0.00103 590.8 68.51 525.3 11.79 514.0 6.11 54.79 360.50 643.54 0.56 KB-81-24 0.05925 0.00185 0.68230 0.02168 0.08385 0.00130 576.0 68.51 528.2 13.09 519.1 7.72 48.62 343.90 562.80 0.61 KB-81-25 0.05836 0.00160 0.65881 0.01870 0.08181 0.00099 542.6 63.88 513.9 11.45 506.9 5.90 55.56 436.41 655.44 0.67 KB-81-26 0.05794 0.00136 0.65695 0.01507 0.08249 0.00107 527.8 56.47 512.7 9.24 511.0 6.37 98.72 850.43 1118.25 0.76 11 12 Geoﬂuids

510 Ma 509 Ma 514 Ma 510 Ma 508 Ma 512 Ma (8.6) (5.6) (10.3) (5.5) (4.5) (11.9)

50 m

KB-26 rhyolite Mean = 512.2 ± 3.0 Ma 0.092 MSWD = 1.16; n = 24 560

0.088

U 0.084 520

238

Pb/

206 0.080 540 480 0.076 500

0.072 0.45 0.65 0.85 1.05 207Pb/235U (a)

510 Ma 506 Ma 514 Ma 510 Ma 511 Ma 511 Ma (5.5) (11.0) (10.3) (8.8) (14.7) (10.2)

50 m

KB-81 rhyolite Mean = 512.2 ± 2.8 Ma 0.090 MSWD = 0.97; n = 26

540

0.086

238

Pb/ 0.082 206 500 545

0.078

505

0.074 0.5 0.6 0.7 0.8 0.9 207Pb/235U (b)

Figure 6: U-Pb concordia diagrams for zircons from rhyolite (KB-26, KB-81). Data-point error crosses are 2σ; yellow circles represent locations of Lu-Hf analysis spots; red circles indicate spots of LA-ICP-MS U-Pb analysis. Geoﬂuids 13

Table 3: Zircon Lu-Hf isotopic data for rhyolite (KB-26, KB-81) from the northern Altyn Tagh region.

t 176 177 176 177 176 177 σ 176Hf/177Hf ε ε T T Sample no. (Ma) Yb/ Hf Lu/ Hf Hf/ Hf 2 i Hf(0) Hf(t) DM1 (Ma) DM2 (Ma) fLu/Hf KB-26-1 503 0.251434 0.003659 0.282705 0.000027 0.282670 -2.4 7.5 834 995 -0.89 KB-26-2 508 0.169238 0.002433 0.282607 0.000026 0.282584 -5.8 4.5 949 1186 -0.93 KB-26-3 510 0.217950 0.003014 0.282726 0.000032 0.282697 -1.6 8.6 788 930 -0.91 KB-26-4 502 0.204649 0.003001 0.282644 0.000032 0.282616 -4.5 5.5 909 1117 -0.91 KB-26-5 513 0.169703 0.002366 0.282637 0.000032 0.282614 -4.8 5.7 904 1115 -0.93 KB-26-6 503 0.172736 0.002428 0.282691 0.000031 0.282668 -2.9 7.4 825 999 -0.93 KB-26-7 509 0.173820 0.002490 0.282637 0.000033 0.282613 -4.8 5.6 907 1119 -0.92 KB-26-8 501 0.234971 0.003456 0.282859 0.000034 0.282827 3.1 13.0 596 642 -0.90 KB-26-9 514 0.263981 0.003888 0.282780 0.000035 0.282743 0.3 10.3 724 824 -0.88 KB-26-10 519 0.161216 0.002365 0.282724 0.000034 0.282701 -1.7 8.9 776 915 -0.93 KB-26-11 514 0.193726 0.002916 0.282776 0.000041 0.282748 0.2 10.5 710 811 -0.91 KB-26-12 506 0.065240 0.000959 0.282643 0.000039 0.282634 -4.6 6.3 861 1074 -0.97 KB-26-13 517 0.215409 0.003017 0.282837 0.000038 0.282808 2.3 12.7 622 675 -0.91 KB-26-14 512 0.207593 0.003169 0.282819 0.000039 0.282789 1.7 11.9 651 722 -0.90 KB-81-01 501 0.193927 0.002814 0.282829 0.000043 0.282803 2.0 12.1 630 697 -0.92 KB-81-02 510 0.165903 0.002392 0.282632 0.000030 0.282609 -5.0 5.5 912 1129 -0.93 KB-81-03 506 0.286633 0.004118 0.282808 0.000031 0.282769 1.3 11.0 687 771 -0.88 KB-81-04 510 0.241194 0.003556 0.282895 0.000033 0.282861 4.4 14.4 543 559 -0.89 KB-81-05 511 0.281597 0.004062 0.282903 0.000033 0.282864 4.6 14.5 538 551 -0.88 KB-81-06 514 0.171756 0.002528 0.282724 0.000028 0.282699 -1.7 8.8 780 922 -0.92 KB-81-07 511 0.301216 0.004433 0.282912 0.000035 0.282870 5.0 14.7 530 538 -0.87 KB-81-08 506 0.184141 0.002788 0.282874 0.000032 0.282847 3.6 13.8 563 593 -0.92 KB-81-09 510 0.213336 0.003195 0.282675 0.000033 0.282645 -3.4 6.7 867 1047 -0.90 KB-81-10 514 0.224146 0.003340 0.282708 0.000031 0.282676 -2.3 7.9 822 975 -0.90 KB-81-11 503 0.215673 0.003251 0.282701 0.000035 0.282670 -2.5 7.5 830 995 -0.90 KB-81-13 510 0.192282 0.002833 0.282762 0.000037 0.282735 -0.4 9.9 730 844 -0.91 KB-81-14 511 0.160352 0.002419 0.282766 0.000028 0.282743 -0.2 10.2 716 826 -0.93 KB-81-15 505 0.167696 0.002422 0.282716 0.000032 0.282693 -2.0 8.3 789 942 -0.93 ranged from 502 to 533 Ma with a weighted mean age of 5.4. Sr-Nd Isotopic Compositions. The results of Sr-Nd 512 4±28Ma (n =26, MSWD = 0 97). The age of 512 Ma analysis for the rhyolite samples are presented in Table 4. 87 86 represents the crystallization time of zircon in the rhyolite. The whole-rock initial Sr/ Sr ratios and εNd t values have According to the relationship between rhyolite and basalt in been calculated at t = 512 Ma on the basis of the zircon U-Pb the outcrop, the two rocks are the contemporary volcanic ages of this study (Figure 6). The initial 87Sr/86Sr ratios for the rocks, which belong to the early Paleozoic. rhyolite vary from 0.71140 to 0.73287 with 143Nd/144Nd ratios ranging between 0.51207 and 0.51231; they have 5.3. In Situ Zircon Hf Isotopes. Zircon Lu-Hf isotopic data are 87 86 Sr/ Sr i values of 0.70895–0.71307, εNd t values of −6.3 listed in Table 3. Their initial εHf t values were calculated 87 86 to −2.5, and TDM1 of 1.94–1.63 Ga. The initial Sr/ Sr ratios using their U-Pb zircon ages. for basalt vary from 0.70467 to 0.70864 with 143Nd/144Nd Fourteen Hf isotopic spot analyses were conducted on 176 177 ratios ranging between 0.51269 and 0.51277; they have zircons from the rhyolite with radiogenic Hf/ Hf(t) 87 86 Sr/ Sr values of 0.70413–0.70817, εNd t values of 2.7 ratios varying from 0.282584 to 0.282827, with an average i to 3.7, and TDM1 of 1.79–1.59 Ga. of 0.282694. Their εHf t and Hf single-stage model ages (TDM1 Hf ) range from 4.5 to 13.0 (with an average of 8.5) and 596–949 Ma (with an average of 790 Ma), respectively. 6. Discussion Fourteen in a total of ﬁfteen Hf isotopic spot analyses were conducted on zircons from the rhyolite with radiogenic 6.1. Geochronological Framework. The North Altyn Tagh 176Hf/177Hf(t) ratios varying from 0.282432 to 0.282870, subduction-collision complex is one of the most important with an average of 0.282728. Their εHf t and Hf single- tectonic units in Western China. The study on Karadaban stage model ages (TDM1 Hf ) range from 5.5 to 14.7 (with bimodal volcanic rocks provides new evidences for the early an average of 10.4) and 538–912 Ma (with an average of Paleozoic tectonic evolution. In this study, we obtained the 710 Ma), respectively. zircon LA-ICP-MS U-Pb age conducted on the rhyolite in 14

Table 4: Rb-Sr and Sm-Nd isotopic compositions for the Early Cambrian bimodal volcanic rock in the North Altyn Tagh tectonic belt.

87Rb/86Sr 87Sr/86Sr 87Sr/86Sr σ 147Sm/144Nd 143Nd/144Nd 143Nd/144Nd σεNd t T Sample Rock Age (Ma) m m i 2 m m i 2 DM (Ga) KB-26 Rhyolite 512 1.072 0.71677 0.70895 3 0.1225 0.51207 0.51166 2 -6.3 1.81 KB-33 Rhyolite 512 0.1004 0.71380 0.71307 3 0.1260 0.51217 0.51175 2 -4.5 1.70 KB-46 Rhyolite 512 0.9701 0.71785 0.71077 3 0.1362 0.51231 0.51185 2 -2.5 1.66 KB-76 Rhyolite 512 3.2086 0.73287 0.70946 3 0.1276 0.51223 0.51180 1 -3.5 1.63 KB-132 Rhyolite 512 0.0103 0.71140 0.71133 2 0.1440 0.51226 0.51178 1 -4.0 1.95 KB-19 Basalt 512 0.0717 0.70656 0.70604 2 0.1806 0.51277 0.51217 4 3.6 1.75 KB-23 Basalt 512 0.0532 0.70811 0.70772 6 0.1698 0.51269 0.51212 6 2.8 1.59 KB-70 Basalt 512 0.0645 0.70864 0.70817 3 0.1764 0.51271 0.51212 3 2.7 1.79 Geo ﬂ uids Geoﬂuids 15

the northern Altyn region, and the age of volcanic rocks was mobile elements such as K2O, Rb, Ba, and Cs. However, the 512 4±28Ma and 512 2±3Ma, which belongs to early Fe, Al, Ca, Mg, REE, and HFSE elements are considered to Cambrian. It is an important stage for researchers to study be relatively immobile during the low-temperature alteration the evolution of the northern Altyn region. [65, 85–87]. Accordingly, the following discussion of the Previous studies show that the magmatic activity in the magma geochemical composition will focus on the immobile Altyn area generated a series of magma evolution from basic element and their ratios, as well as isotopic compositions. to felsic rocks, which mainly occurred from 520~480 Ma [29, 30, 56, 76, 77]. The early Paleozoic tectonic belt of 6.2.1. Petrogenesis of the Mafic Rocks. The low silica content Hongliugou-Lapeiquan was recognized, and the results (SiO2 =4474 – 46 65 wt %) and relatively high concentra- T – – showed that the basic volcanic rocks are mainly with the tions of Fe2O3 (11.49 17.12 wt.%), Cr (175 215 ppm), and features of MORB and OIB [78]. In this belt, the LA-ICP- Ni (94–118 ppm) content of the mafic rocks suggest that a MS zircon U-Pb age of the eutectic gabbro is 513 ± 3 Ma, mantle component played a prominent role in their genesis which can be classified as a subduction zone ophiolite, and [88]. The slight lack of negative Eu anomalies on the mainly developed in the oceanic subduction belt [79]. Liu chondrite-normalized REE diagrams indicates an absence et al. (1998) reported that the whole-rock isochron age of the of plagioclase fractionation during the process of magmatic pillow basalt in Hongliuquan area is 524 ± 44 Ma [80], which evolution (Figures 5(a) and 5(b)). Meanwhile, the basalt probably represented the forming age of ocean island basalt. displays a calc-alkaline nature and is enriched in LREEs Gao et al. (2011) obtained a LA-ICP-MS zircon U-Pb age of and is also depleted in HFSEs (e.g., Nb and Ta). These fea- 513 ± 2 Ma from the plagiogranite, which is intrusive in the tures are similar to the subduction-related magmas [89]. ophiolite mélange. Chen et al. (2016) obtained the SHRIMP However, the basaltic samples show MgO, Cr, Ni, V, and ~ zircon U-Pb age of 482 477 Ma from the felsic volcanic rocks Co are negatively correlated with SiO2, indicating that the in the Kaladawan area, which indicated that the characteristic basaltic magma is not primitive and possibly experienced of magmatism continued from 520 Ma to 477 Ma. In addi- some fractional crystallization of pyroxene. This is consistent tion, there are high pressure-ultrahigh pressure (HP/LP) with the presence of pyroxene as the dominant phenocryst metamorphic zones in the tectonic belt, and the 39Ar–40Ar in the basalt. The Karadaban basalt is enriched in Th isochron age of 513 ± 5 Ma from muscovite in the blueschist (4.08–11.90 ppm) and positively correlated the plots of showed that the HP/LP was effected by the oceanic subduc- Nb/La and Nb/Th (Figure 7(a)). So we conjectured the tion. In this paper, we verified that the age of felsic volcanic involvement of Th-enriched component in its petrogenesis, rocks in Kaladaban is 512 Ma, corresponding with the setting which is due to the crustal contamination [37]. The continen- of the north Altyn region. Combining with previous studies, tal crust is typically depleted in Nb and Ta [90], and the we inferred that there was an ocean basin that existed at upper continental crust is enriched in La and Th, while the the north Altyn region in the late Neoproterozoic and lower continental crust is not always enriched in Th [91]. the subduction activities probably continued until the late Combining with the (Th/Ta)PM vs. (La/Ta)PM plot, the basalt Ordovician. Since the oceanic crust subduction and colli- samples define an UCC trend (Figure 7(b); [92]), implying sion, a series of subduction-related volcanic rocks were that the contaminator is the highly evolved upper continental developed in the north Altyn region [34]. The magmatic crust. It is noted that mafic magmas derived from the activity in the northern Altyn region is multistage and prob- asthenosphere typically have La/Nb ratios of <1.5 and La/Ta ably successive from Neo-Proterozoic to Late Paleozoic. ratios of <22, whereas those rocks from lithosphere have La/Nb ratios > 1 5 and La/Ta ratios > 22 [93, 94]. High 6.2. Petrogenesis of Bimodal Volcanic Rocks. Bimodal La/Nb ratios (2.15–3.36), La/Ta ratios (36.57–53.49), and volcanic rocks occur within a variety of tectonic settings, such positive εNd t values (+2.7 to +3.7) indicate that the as continental rifts, oceanic islands, zones of continental basalt was derived from a depleted mantle source but break-up, back-arc basins, orogenic belts, intraoceanic island mixed with crustal components while upwelling. Mantle- arcs, and mature island arcs/active margins [9, 81, 82]. derived magmas generally assimilate crustal materials during Recent studies have proposed two models of the genesis of their ascent or their storage in a crustal magma chamber [95]. bimodal volcanic rocks, depending on the cognate of the The Nb/U values (2.16–9.13) are lower than those of the rhyolites and basalts (e.g., [83]). Since the mafic and felsic source mantle (34), MORB (47 ± 11), or OIB (52 ± 15), magmas originated from different sources, the trace element further indicating that the basalt was affected by the crustal contents and the Sr and Nd isotopic compositions should contamination [96]. Moreover, the mafic rocks from the differ between the basalts and the rhyolites [84, 85], thereby Karadaban area are Nb-Ta depleted but enriched in Zr, Hf, providing a reliable indicator for the genesis of bimodal and Ti (Figure 5(b)), yielding patterns similar to those volcanic rocks. expected for crustal material which suggests that these Samples from the northern Altyn region have experi- magmas assimilated crustal material during ascent [97]. enced less alteration. The loss of ignition (LOI) values for Excluding the garnet residue in the source, the Nb/Yb the mafic and felsic volcanic rocks in the study area are and Ta/Yb ratios are effective in constraining the relative 1.92–6.87 and 0.74–3.68 wt.%, respectively. This alteration depletion of the mantle source when the degree of partial is a result of low-temperature metamorphism related to melting is even lower [70, 98]. Meanwhile, the Th/Yb ratio intense deformation and fluid infiltration, whose effects are is a sensitive indicator of the inclusion of subduction zone principally evident in the various types of abundance of the materials in magma [87, 91]. The Th/Yb vs. Nb/Yb diagram 16 Geofluids

2.8

7.0

2.2 6.0

PM 1.6

Nb/T 5.0

(T/Ta)

Crustal contamination

1.0 4.0 Upper continental crust Lower continental crust 0.4 3.0 0.1 0.2 0.3 0.4 0.5 0.6 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Nb/La (La/Ta)PM (a) (b) Figure 7: Plots of Nb/La vs. Nb/Th (a) and (Th/Ta)PM vs. (La/Ta)PM (b) for the basalt of Kaladaban bimodal volcanic rocks (after [37]).

10 Active continental margins e

OIB sourc 1 Enriched Oceanic arc system

Th/Yb E-MORB

0.1 rray a e

sourc MORB MORB-OIB

Depleted

0.01 0.1 1 10 100 Nb/Yb

Figure 8: Nb/Yb vs. Th/Yb diagram for the basalt of the North Altyn Tagh bimodal volcanic rocks. The ﬁeld of MORB-OIB is from [99].

(Figure 8) indicates that the basalt samples in the present low Al2O3, CaO, and MgO content; depletion in HFSEs (e.g., study developed in an active continental margin, mainly Nb, Ta, P, Ti, and Hf); and strong negative Eu anomalies. close to the boundary between the E-MORB and OIB arrays. These compositional features suggest that the samples in In summary, the geochemical and Sr-Nd isotope data the present study originated from a source that lacked garnet demonstrate that the basalt of the Karadaban bimodal volca- but contained plagioclase in the residue, which indicates nic rocks was derived from the depleted mantle source and partial melting under low-pressure conditions [100, 101]. experienced fractional crystallization, which was dominated The absence of Ba, Nb, Ta, Sr, P, and Ti, as shown in the by the fractionation of pyroxene and crustal contamination primitive-mantle-normalized trace-element spider diagram possibly by upper crustal materials. (Figure 5(d)), indicates similar trace element composition of A-type granites [101], and the felsic rocks generally display 6.2.2. Petrogenesis of the Felsic Rocks. Felsic volcanic rocks are similar patterns to the basalts. However, the Nb/Ta and T characterized by high SiO2,K2O+Na2O, and Fe2O3 content; Th/Ta ratios of the felsic rocks are low, suggesting that the Geoﬂuids 17

5000 10 IAB 2000

1000

Average crust

)

Yb/Ta OIB Zr (ppm Zr A-type granite 100

I-, S-, & M-type granites

10 0.1 1 4 6 8 10 0.1 1 10 ⁎ 10000 Ga/Al Y/Nb

Karadaban rhyolite Highland 4337 granodiorite Karadaban rhyolite Highland 4337 granodiorite Hongliugou granite South Kaladawan granite Hongliugou granite South Kaladawan granite (a) (b)

Figure 9: Chemical classiﬁcation diagrams for the felsic volcanic rocks in the North Altyn Tagh region. (a) Ga/Al vs. Zr diagram (after [110]) where all data fall into the A-type granite and (b) the Yb/Ta vs. Y/Nb diagram (after [102]) exhibiting similarities to average crustal ratios between OIB (ocean island basalt) and IAB (island-arc basalt). Data of the Hongliugou granite are from [57], Highland 4337 granodiorite are from [32], and south Kaladawan granite are from [46].

Nb Nb

A1-type granite A1-type granite

A2-type granite A2-type granite

Y Y3Ga Ce Karadaban rhyolite Highland 4337 granodiorite Hongliugou granite South Kaladawan granite

Figure 10: Plots of the volcanic rocks from the North Altyn Tagh region in Nb-Y-3Ga and Nb-Y-Ce (after [102]). A1: anorogenic granite; A2: postorogenic granite. Data of the Hongliugou granite are from [57], Highland 4337 granodiorite are from [32], and south Kaladawan granite are from [46]. rhyolite is contaminated by upwelling basaltic magma. Fur- felsic rocks are A-type granites and exhibit similarities to thermore, the Karadaban felsic samples fall into the A-type the crust between OIB and IAB. Based on the subdivision granite area on a Ga/Al-Zr plot (Figure 9(a)), and the Yb/Ta of A-type granites [102], the Karadaban felsic samples are and Y/Nb ratios of the felsic samples are higher than those of type A2 (Figure 10); however, the other granites (e.g., High- OIB and close to IAB (Figure 9(b)). This indicated that the land 4337 and Hongliugou) show I-type features. It coincides 18 Geoﬂuids

Table 5: Summary of zircon U-Pb ages for igneous rocks in the Altyn Tagh region.

No. Locality Age (Ma) Method Lithology Tectonic setting Reference 1 Hongliutan 518 5±41 U-Pb LA-ICP-MS Plagiogranite Subduction zone [16] 2 Lapeiquan 512 1±15 U-Pb LA-ICP-MS Plagiogranite Subduction zone [24] 3 North Highland 4337 506 2±23 Zircon SHRIMP Granodiorite Island arc [74] 4 Hongliugou 500 3±12 U-Pb LA-ICP-MS Granite Subduction zone [57] 5 Highland 4337 494 4±55 Zircon SHRIMP Granodiorite Active continental margin [32] 6 Kaladawan 488~477 Zircon SHRIMP Dacite Subduction zone [34] 7 North Altyn region 488 ± 2 U-Pb LA-ICP-MS Rhyolite Island arc [75] 8 South Kaladawan 484 2±49 Zircon SHRIMP Monzonite granite Active continental margin [46] 9 North Bashikaogong 481 6±56 U-Pb LA-ICP-MS Quartz diorite Subduction zone [30]

18 DM 12 MORB

6 PM

t) HIMU OIB 0

Nd(

–6 E-MORB EMII –12 EMI

–18 0.700 0.704 0.708 0.712 0.716 87 86 ( Sr/ Sr)i Karadaban rhyolite Karadaban basalt

87 86 Figure 11: Sr/ Sr i vs. εNd t diagram for the Karadaban bimodal volcanic rocks. EMI and EMII-enriched mantle I and II sources, HIMU-high-l mantle source, DM-depleted mantle source, and PM-primitive mantle. Upper crust values are from [111]. with the amount of island arc granitoid developed in the the solubility of Nd was higher than that of Hf and led to the north of Karadaban volcanic rocks [32, 57]. Hence, we higher Nd/Hf ratios in the fluid/melt derived from the suggest the Karadaban felsic rocks probably generated in an subducted plate [107, 108]. Coincidentally, from Cambrian extensional environment. to Ordovician, the whole north Altyn region was in a subduc- Two types of process have been proposed for the origin of tion setting, and a great deal of crustal growth occurred the silicic end-member in the bimodal magmatic suite, through arc-related magmatism (Table 5). For the Nd-Hf including (1) extensive fractional crystallization from a isotopic decoupling, the Hf isotope composition of zircon is common mantle-derived magma parental to the mafic end- more realistic than Nd. The positive zircon εHf t (4.5 to member, coupled with crustal contamination [103], and 14.7) and TDM2 ages of 551 to 1186 Ma suggest a juvenile (2) crustal anatexis caused by mantle-derived mafic magma source rather than an old basement rocks. In addition, with distinct isotopic compositions [104–106]. In the case the wide range of εHf t indicated the mixture of the of the felsic rocks, the distinct difference documented by end-member during the magma evolution [109]. When the whole-rock εNd t and zircon εHf t values between the the subduction metasomatized mantle-derived felsic crustal basalt and felsic rocks indicated that the basalt could not have materials were rejuvenated, the newly formed magmas been produced by fractional crystallization of the coeval would inherit the decoupled Nd-Hf characteristics such ε t − − basalt. However, the negative Nd ( 6.3 to 2.5) and posi- as Karadaban rhyolite in accordance with A2-type granites tive εHf t (4.5 to 14.7) of Karadaban felsic rocks showed representing magmas derived from the continental crust Nd-Hf isotopic decoupling. The decoupling was probably which has been through a cycle of island-arc magmatism effected by fluid/melt metasomatism in the subduction zone; (Figures 11 and 12; [102]). As presented above, the continued Geofluids 19

Depleted mantle 20 440 Ma 970 Ma 1.4 Ga 10 2.7 Ga 177 Hf = 0.015 176 Lu/ CHUR 0

Ηf(t)

–10

177 Hf = 0.015 –20 176 Lu/

–30 0 500 1000 1500 2000 2500 3000 t(Ma) KB-26 rhyolite KB-81 rhyolite

Figure 12: Plot of εHf t values versus U-Pb ages of zircons from the bimodal volcanic rocks in the North Altyn Tagh region. extension of the crust and repeated underplating of subse- diagrams. Based on the Hf-Th-Nb and Zr-Ti-Y ternary quent mafic magma partial melting of the juvenile crust to diagrams, the Karadaban rocks are classified as volcanic arc generate felsic melts will ascend and erupt on the surface calc-alkaline basalts (Figure 13), which support the hypothe- forming the Karadaban rhyolite. sis that magmatic traces existed in the study area [99]. The Based on the discussion above, we conclude that the felsic Karadaban basalts exhibit both arc-like and within-plate volcanic rocks produced mainly by partial melting of the basalt affinities, implying that they are probably formed juvenile crustal and the origin melt source itself likely within a back-arc tectonic setting. Generally, the Kaladaban resulted from mantle-derived magmatic upwelling in an bimodal volcanic suite in the north Altyn region is likely an extensional setting, probably near the subduction-related analogue of the Dzungaria Ocean in the southern Central active continental margin. Asian Orogenic Belt during the late Paleozoic [70]. The Karadaban bimodal volcanic rocks resemble those of the 6.3. Tectonic Implications. The north Altyn region is a part back-arc side of the Dzungaria arc, which are believed to of the Proto-Tethys tectonic domain. The Cambrian- result from the initiation opening of the Dzungaria Ocean Ordovician basic rocks and calc-alkaline granite in this [70, 112]. On the other hand, regional geological data suggest region are interpreted to be island arc volcanic rocks that the final closure of the Altyn Ocean did not occur until [34, 35]. SHRIMP zircon ages of 506 ± 2 Ma for a grano- the Early Ordovician and even until the later Ordovician in diorite and 481 ± 5 Ma for arc granite indicate that the the north Altyn region [24, 35]. This demonstrates that the Cambrian to early Ordovician age is related to arc magma- Kaladaban bimodal volcanism was produced in a back-arc tism [30, 31]. In addition, bimodal volcanic rocks indicate environment (Lapeiquan back-arc basin), rather than in the an extensional setting in which they form, and several types postorogenic or intracontinental rifting setting. of the extensional setting can account for the generation The Altyn region is of great significance to realize the of bimodal volcanic suite, such as within-plate (continental evolution of the Tibetan Plateau and the tectonic pattern rifting), passive continental marginal rifting, and incipient of Western China. It developed as a unified back-arc back-arc or continental active margins [9]. Hence, we pro- basin system between the northern Altyn Complex and pose that the Karadaban bimodal volcanism could be MJB before the formation of the Altyn Tagh, which is regarded as a significant indication of arc-related exten- the northern boundary of the Proto-Tethys in China sional tectonics. [113]. Three stages can be identified by previous studies: On the one hand, according to the feature of trace north Altyn Oceanic rifting (~750 Ma) [47, 114], north Altyn element, the Karadaban volcanic suite displays some geo- oceanic subduction (520~460 Ma) [34, 76, 77, 115], and chemical features of subduction-related melts, such as oceanic consumption followed by continental collision enrichment of LILEs but depletion in HFSEs and Sr-Nd-Hf (440~420 Ma) [29, 109, 116, 117]. This paper mainly isotope compositions of magmas. This suggests that the focuses on the study of the bimodal volcanic rocks developed mixing of juvenile crust and mantle-derived magma, as well during the subduction of the northern Altyn Ocean. Com- as the Karadaban bimodal volcanic suite, was formed in a bined with previous studies, the subduction is presumed to subduction-related environment. The conclusion is further have occurred around 520~460 Ma (Table 5) [19, 77, 117] supported by a series of tectonomagmatic discrimination from the plentiful research of regional magmatic sequences 20 Geofluids

Hf/3 Ti/100

IAT N-MORB

E-MORB IAB WPB ICA

WPB CAB CAB IAB

⁎ T Nb/16 Zr Y 3 Karadaban basalt IAT: island arc tholeiite Lapeiquan diabase ICA: island arc calc-alkaline basalt CAB: calc-alkaline basalt WPB: within-plate basalt

Figure 13: Hf-Th-Nb and Zr-Ti-Y tectonic discriminant diagram of Karadaban basalt. Data for the Lapeiquan diabase are from [16]. to the mafic to intermediate to felsic rocks [31, 46, 109]. probably generated by partial melting of the juvenile Based on our new data and previous studies, we propose a crustal as a result of basic magma upwelling model for the tectonic evolutions of the north Altyn region during the Early Cambrian. This model emphasizes a south- (3) The generation of the Karadaban bimodal volcanic ward subduction system of the oceanic lithosphere (Altyn rocks was not only related to an active continen- Ocean) between the NASC and MJB. Beneath the MJB, its tal margin setting but also associated with the consolidated NASC margin during the early Paleozoic period subduction-related back-arc extension environment and an arc and back-arc system developed, resembling the present western Pacific continental margin, appearing during Data Availability the early Cambrian [24, 35, 46]. The Karadaban bimodal vol- The data used to support the findings of this study are canic rocks formed in the incipient stage of the Cambrian available from the corresponding author upon request. back-arc basin/extension. The system might have lasted into the Late Ordovician, during which the back-arc basin Conflicts of Interest gradually became mature and finally closed during the Early Silurian. In conclusion, the north Altyn oceanic crust The authors declare that there are no conflicts of interest subducted towards the MJB and transmitted the latter into regarding the publication of this paper. the active continental margin. The magma, which derived from the upwelling and mixing caused by subduction, Acknowledgments eventually emplaced into the crust as Kaladaban bimodal volcanic rocks. This study was financially supported by the Development and Research Centre of the China Geological Survey, Beijing (Grant No. 12120113090000). 7. Conclusions Supplementary Materials The following conclusions can be drawn from this study. Supplementary Table 1: major (%) and trace element (ppm) (1) New LA-ICP-MS zircon U-Pb dating indicates that data for the volcanic rocks in the northern Altyn Tagh region. the newly discovered bimodal volcanic rocks in the Supplementary Table 2: LA-ICP-MS zircon U-Pb dating data northern Altyn Tagh area formed at ca. 512 Ma for Karadaban rhyolite (KB-26, KB-81) from the northern Altyn Tagh region. Supplementary Table 3: zircon Lu-Hf (2) Whole-rock geochemical and Sr-Nd-Hf isotopic isotopic data for rhyolite (KB-26, KB-81) from the northern compositions suggest that the mafic rocks were Altyn Tagh region. Supplementary Table 4: Rb-Sr and derived mainly from a depleted mantle source but Sm-Nd isotopic compositions for the volcanic rocks in mixed with crustal components. The felsic rocks were the northern Altyn Tagh region. (Supplementary Materials) Geofluids 21

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