Petrogenesis and tectonic significance of the plagiogranites in the Zhaheba ophiolite, Eastern Junggar orogen, Xinjiang, China

Journal of Asian Earth Sciences xxx (2014) xxx–xxx

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Petrogenesis and tectonic significance of the plagiogranites in the Zhaheba ophiolite, Eastern Junggar orogen, Xinjiang, China Lingjun Zeng a,b, Hecai Niu a,⇑, Zhiwei Bao a, Qiang Shan a, He Li a, Ningbo Li a,b, Wubin Yang a a b

Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e

i n f o

Article history: Received 2 May 2014 Received in revised form 15 September 2014 Accepted 18 September 2014 Available online xxxx Keywords: Zhaheba Ophiolite Plagiogranites Gabbros Amphibolites Anatexis

a b s t r a c t Plagiogranites (albitite and albite granite dikes/lenses) occur in the western section of the Zhaheba ophiolite, middle part of the Eastern Junggar orogen. The Zhaheba albitites (498.0 ± 5.8 Ma) and albite granites (494.6 ± 6.9 Ma) were roughly coeval and with distinct petrographic textures, i.e., cumulus texture for the albitites and granitic texture for albite granites. The Zhaheba plagiogranites are Na-enriched and LILE-depleted, resembling typical plagiogranites. Geochemical characteristics and cumulus texture of the albitites indicate that they may have been formed by early stage accumulation of albite, whereas the albite granites may have been the products of the residual magma consolidation. The eNd(t) values of the albitites and albite granites vary in the ranges of 7.2–7.7 and 6.4–7.3, respectively, whereas the eHf(t) values vary in the ranges of 11.5–17.9 and 9.8–13.9, respectively. Isotopic compositions and Zr/Hf ratios of the Zhaheba plagiogranites are similar to those of typical MORB, implying a genetic relationship with the oceanic crust. The low TiO2, Nb, Ta content, LREE enrichment and elevated (87Sr/86Sr)i values of the Zhaheba plagiogranites indicate that the rocks were likely derived from the anatexis of amphibolites, which were related to hydrothermal alteration of gabbros in intra-oceanic backarc basin. U–Pb geochronology of the Zhaheba plagiogranites indicates that it is more reasonable to connect the Zhaheba–Armantai and Karamay ophiolites to the Zaysan collision zone. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Plagiogranites are felsic plutonic rocks (including diorite, quartz diorite, tonalite, trondhjemite and albitite/anorthosite) that occur in modern oceanic crust and ophiolites (Coleman and Peterman, 1975; Coleman and Donato, 1979; Li et al., 2013). Despite their minor volume, plagiogranites provide crucial constraints on the formation ages of oceanic crust and ophiolites (Tilton et al., 1981; Jiang et al., 2008; Grimes et al., 2008, 2013) and offer a unique way to investigate oceanic basin and orogenic evolution. Three models have been proposed for the petrogenesis of plagiogranites: (1) Products of shallow differentiation of basaltic magmas in oceanic crust, representing the final stage of the oceanic crustal evolution (Coleman and Donato, 1979; Lippard et al., 1986; Jiang et al., 2008);

⇑ Corresponding author. Fax: +86 20 85290130. E-mail address: [email protected] (H. Niu).

(2) Partial melting of hydrous gabbros (or basalts) (Malpas, 1979; Gerlach et al., 1981; Koepke et al., 2004; Grimes et al., 2013) or amphibolites in the shear zones at spreading centres (Flagler and Spray, 1991); (3) Immiscibility products of a felsic melt and a Fe-enriched basaltic melt under anhydrous conditions (Dixon and Rutherford, 1979). Obviously, studies on the plagiogranite petrogenesis are very important for understanding the oceanic crustal evolution. In terms of regional tectonic significance, the Eastern Junggar orogen is located in the southwest segment of the Central Asian Orogenic Belt (CAOB), lying between the Ertix and Kalamaili faults (Fig. 1). The Dulate arc, Zhaheba–Armantai ophiolites, Yemaquan arc and Kalamaili ophiolite are distributed from north to south in the region. The widespread Late Palaeozoic volcanic rocks and minor Late Carboniferous to Permian granitoids and mafic intrusions (Li, 2004; Li et al., 2014) in the Easter Junggar orogen have preserved a complete record of the Palaeozoic magmatism– tectonism, making it one of the key areas for investigating the evolution of the CAOB’s southwest segment. The Zhaheba–Armantai ophiolites are located in the middle part of the Eastern Junggar

http://dx.doi.org/10.1016/j.jseaes.2014.09.031 1367-9120/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Zeng, L., et al. Petrogenesis and tectonic significance of the plagiogranites in the Zhaheba ophiolite, Eastern Junggar orogen, Xinjiang, China. Journal of Asian Earth Sciences (2014), http://dx.doi.org/10.1016/j.jseaes.2014.09.031

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L. Zeng et al. / Journal of Asian Earth Sciences xxx (2014) xxx–xxx 60

90° E

89° E

(a)

80

9 0 1 0 0 110120 130 140 150

Russia

Ka

Shaerbulake

T

E

50 Al

za

ta

Ju

k CA ahs y ra OB tan ng e

ng ga

F

Al

ta

rB

Kalatonke

North Xinjiang

40

ya

OB CA ina Ch NE 40

CAOB Mongolia

Ta r i m

as

rc

80

in

Xileketehalasu

NCC

90

100

110

120

(b) 130

Qinghe

Kalaxianger

Fig. 1b

Zhaheba

Du

Mesozoic-Cenozoic sediment

Laoshankou

lat

Qiakuerte

Permian volcanic-sedimentary rocks

Siberia craton

50

Fuyun

47° N

70

Urals

Qiaoxiahala

ea

rc

Camboniferous volcanic-sedimentary rocks

Kouan

Devonian volcanic-sedimentaryrocks

46° N

Ordovician-Silurian schists and genisses

Ye m

Camboniferous-Permian granitoids Silurian-Devonian granitoids

Kalamaili

Ertai

aq

Maficultramafic rocks

Dolerites

Thrust faults

Ophiolitic rocks

NormalFaults

na

rc

Armantai

K

0

89˚05′

89˚00′ (c)

ZA

ua

40km

89˚10′ Quaternary sediments

46˚35′

Carboniferous volcanic-sedimentary rocks

Middle Devonian volcanic-sedimentary rocks

Plagiogranite Lower Devonian volcanic-sedimentary rocks

Granites

Zhaheba ophiolite

Ophiolites

Plagiogranites

46˚30′

Faults

0

5km

Fig. 1. (a) Location of the study area in the Central Asian Orogenic Belt (after Shen et al., 2011) and (b) simplified geologic map of the Zhaheba area (after Niu et al., 2007). Major faults: E: Ertix thrust; F: Fuyun fault; ZA: Zhaheba–Armantai thrust; K: Kalamaili thrust.

orogen (He et al., 2001; Li, 1995, 2004; Long et al., 2012; Wang et al., 2003; Xiao et al., 2006a, 2009). The discovery of the ultrahigh-pressure (UHP) metamorphic blocks in the orogen (garnet–pyroxenite, quartz-magnesite and garnet–amphibolite) (Niu et al., 2007, 2008, 2009) suggests that the ophiolites have experienced deep subduction and the subsequent exhumation. Therefore, systematic study of the Zhaheba–Armantai ophiolites

can effectively contribute substantially to the tectonic reconstruction of the CAOB. Precious attempts on dating the Zhaheba– Armantai ophiolites (with various methods) have yielded inconsistent ages ranging from 479 Ma to 508 Ma (Liu, 1993; Jin et al., 2001; Jian et al., 2003; Huang et al., 2013; Xiao et al., 2006b, 2009). Moreover, published works on the plagiogranites in the Zhaheba–Armantai ophiolites were limited to geochronological

Please cite this article in press as: Zeng, L., et al. Petrogenesis and tectonic significance of the plagiogranites in the Zhaheba ophiolite, Eastern Junggar orogen, Xinjiang, China. Journal of Asian Earth Sciences (2014), http://dx.doi.org/10.1016/j.jseaes.2014.09.031

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L. Zeng et al. / Journal of Asian Earth Sciences xxx (2014) xxx–xxx

Ab

Ab

Qz Ab Ab Ab Ab Qz Ab 100um

a

1000um

b

250um

c

Ab Ab

Ab

Ab

Chl

Ep

Ab Ab Ep

Ab

Ab 100um

500um

d

e

100um

f

Fig. 2. Field outcrop and photomicropgraphs of the Zhaheba plagiogranites: (a) field outcrop of the plagiogranite; (b) albite granite; (c) albitite; (d) cumulus texture of albitite; (e) albitite showing plastic deformation; (f) primary epidote in albite.

investigations (Jian et al., 2003; Xiao et al., 2006b), whereas the geochemistry and petrogenesis of the rocks were largely neglected. We have identified new outcrops of plagiogranites (albitites and albite granites) in the Zhaheba region, western part of the Zhaheba–Armantai ophiolites. In this paper, we have presented new data on LA–ICP–MS zircon U–Pb geochronology, major and trace element, and Sr–Nd–Hf isotopic geochemistry for the albitites and albite granites, as well as constrained the source and petrogenesis of the Zhaheba plagiogranites.

and epidote (showing pseudomorph texture). Primary epidote (5 vol.%) also occurs in the albitites as 0.5–1 mm long euhedral columnar crystal, as well as enclosed in or interstitial among albite grains. Accessory minerals include mainly zircon and magnetite. Albite granites are light gray, showing porphyritic texture (Fig. 2). The phenocrysts are composed of subhedral to anhedral quartz of 0.1–0.2 (20 vol.%) and subhedral albite of 0.2–1.5 mm (40 vol.%). Mineral contents of the groundmass (40 vol.%) are the same as the phenocrysts. Accessory minerals include mainly zircon and magnetite.

2. Geology and petrography 3. Analytical methods The Zhaheba–Armantai ophiolites occur in a NWW-trending belt of 200 km long and 3–5 km wide, mingled in the Lower Devonian Tuoranggeku Formation (Fig. 1a). The formation is further divided into three sub-formations: tuffaceous rocks at the lower part; andesites, basalts, and with minor tuffs/tuffaceous breccias in the middle part; pyroclastic rocks with intercalated limestones and ferriferous cherts in the upper part. Recently, adakites, boninite, Nb-enriched basalts and potassic volcanic rocks were also recognized in the formation (Niu et al., 2006; Xu et al., 2001; Zhang et al., 2004), indicating a subduction-related environment. Although badly dismembered and lacking of a continuous and full ophiolite sequence, most rock types in classical ophiolites (except for sheet dike complex) occur in the Zhaheba–Armantai ophiolites, including: serpentinised harzburgite, dunite, cumulate gabbros, dolerite, pillow and massive basalts, radiolarian cherts, UHP garnet–pyroxenite, quartz–magnesite and garnet–amphibolite (Li, 1991, 1995; Niu et al., 2007). The Zhaheba ophiolite occurs in the western of the Zhaheba–Armantai ophiolites. Plagiogranites (albitites and albite granites) tectonically intruded into gabbros in the Zhaheba ophiolite as dikes or lens (Fig. 2c). Albitites are gray in colour with typical cumulus texture (Fig. 2). Albite (90 vol.%) is subhedral, 0.1–2 mm long and with weak sericite alteration. Some albite grains were plastically deformed. Biotite (5 vol.%) is interstitial, and is mostly replaced by chlorite

3.1. Zircon U–Pb dating and Hf isotope First, zircon grains were separated using conventional density and magnetic separation techniques, and then handpicked under a binocular microscope. Second, representative zircon grains were mounted in epoxy mounts, which were then polished to nearly half-section to expose internal structures. Third, all mounted zircon were studied with transmitted and reflected light micrographs as well as by cathodoluminescence (CL) imaging to reveal their internal structures. Zircon U–Pb dating was undertaken using an Agilent 7500a ICP–MS coupled with a Resonetics RESOlution M-50 193 nm laserablation system at the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS). Zircon TEM was used as the external standard for the dating, and detailed analytical procedures were outlined by e.g., Liang et al. (2009a) and Li et al. (2012). Isotopic ratios and trace element concentrations of the zircons were calculated with GLITTER 4.0 (Macquarie University), whereas the zircon ages were calculated with ISOPLOT 4.11 (Ludwig, 2008). In-situ Hf isotopic analysis on zircons with U–Pb ages using LA– ICP–MS method were carried out with a 193 nm laser attached to a Neptune multi-collector ICP–MS (LA–MC–ICP–MS) at the State Key

Please cite this article in press as: Zeng, L., et al. Petrogenesis and tectonic significance of the plagiogranites in the Zhaheba ophiolite, Eastern Junggar orogen, Xinjiang, China. Journal of Asian Earth Sciences (2014), http://dx.doi.org/10.1016/j.jseaes.2014.09.031

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L. Zeng et al. / Journal of Asian Earth Sciences xxx (2014) xxx–xxx

Table 1 Zircon U–Th–Pb isotopes of the albitites and albite granites from the Zhaheba ophiolite. Sample spots

Concentration and ratio

Isotopic ratios

U (ppm)

Th/U

207

382 73 126 155 289 211 496 155 323 387 227 267 231 258 191 22 392 367 264 179 229 212 336 242 142

0.70 0.61 0.66 0.79 1.07 0.78 1.12 0.65 0.70 0.73 0.68 0.69 0.77 0.69 0.76 0.39 0.79 0.47 0.64 0.80 0.52 0.65 0.72 0.70 0.55

Albite granite (08Zhr-2) 08Zhr-2-01 217 08Zhr-2-02 113 08Zhr-2-03 172 08Zhr-2-04 219 08Zhr-2-05 199 08Zhr-2-06 374 08Zhr-2-07 725 08Zhr-2-08 136 08Zhr-2-09 190 08Zhr-2-10 641 08Zhr-2-11 367

189 67 87 120 110 463 1047 115 151 673 257

Albite granite (08Zhr-5) 08Zhr-5-01 222 08Zhr-5-02 115 08Zhr-5-03 265 08Zhr-5-04 149 08Zhr-5-05 233 08Zhr-5-06 206 08Zhr-5-07 124 08Zhr-5-08 392 08Zhr-5-09 224 08Zhr-5-10 222 08Zhr-5-11 292 08Zhr-5-12 152 08Zhr-5-13 159 08Zhr-5-14 161 08Zhr-5-15 440 08Zhr-5-16 199 08Zhr-5-17 174 08Zhr-5-18 258 08Zhr-5-19 125 08Zhr-5-20 168 08Zhr-5-21 176 08Zhr-5-22 198 08Zhr-5-23 283

165 88 253 94 178 161 138 344 170 176 211 115 97 91 282 157 136 194 97 137 90 169 267

Albitite (11Zhr-3-3) 11Zhr-3-3-01 547 11Zhr-3-3-02 119 11Zhr-3-3-03 190 11Zhr-3-3-04 196 11Zhr-3-3-05 270 11Zhr-3-3-06 269 11Zhr-3-3-07 445 11Zhr-3-3-08 237 11Zhr-3-3-09 460 11Zhr-3-3-10 532 11Zhr-3-3-11 334 11Zhr-3-3-12 390 11Zhr-3-3-13 301 11Zhr-3-3-14 371 11Zhr-3-3-15 252 11Zhr-3-3-16 57 11Zhr-3-3-17 496 11Zhr-3-3-18 772 11Zhr-3-3-19 410 11Zhr-3-3-20 224 11Zhr-3-3-21 441 11Zhr-3-3-22 325 11Zhr-3-3-23 470 11Zhr-3-3-24 343 11Zhr-3-3-25 256

Th (ppm)

206

Apparent ages (Ma) ±r (%)

207

0.05825 0.05813 0.05454 0.05714 0.05779 0.05993 0.05733 0.05716 0.05978 0.05723 0.05593 0.05451 0.05479 0.05822 0.05464 0.05911 0.05843 0.06354 0.05548 0.06095 0.06656 0.05913 0.05954 0.05809 0.05745

0.2 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.5 0.2 0.2 0.2 0.3 0.2 0.2 0.2 0.2 0.2

0.87 0.59 0.51 0.55 0.55 1.24 1.44 0.84 0.79 1.05 0.70

0.06830 0.06459 0.06263 0.06024 0.06986 0.06734 0.06538 0.06286 0.06269 0.08443 0.05531

0.74 0.76 0.95 0.63 0.76 0.78 1.11 0.88 0.76 0.79 0.72 0.75 0.61 0.57 0.64 0.79 0.78 0.75 0.77 0.82 0.51 0.86 0.94

0.05258 0.06013 0.05815 0.06473 0.06296 0.06270 0.06915 0.06500 0.06208 0.06566 0.06745 0.05549 0.05821 0.05537 0.05564 0.05562 0.05271 0.05127 0.05458 0.06796 0.05355 0.05857 0.06577

Pb/

Pb

235

±r (%)

206

0.61904 0.64248 0.61502 0.64385 0.63942 0.6436 0.63461 0.63661 0.65911 0.62185 0.6367 0.61426 0.61861 0.64053 0.62648 0.67809 0.66145 0.68401 0.62764 0.67343 0.68946 0.64635 0.6388 0.62075 0.65164

1.8 3.6 2.6 2.7 2.3 2.3 1.8 2.5 1.9 1.7 2.0 1.8 2.2 2.1 2.3 5.7 1.8 1.7 1.9 2.6 2.1 2.4 1.9 2.1 2.3

0.8 0.6 0.5 0.5 0.6 0.5 0.4 0.6 0.6 0.7 0.4

0.78386 0.69948 0.67100 0.62542 0.74171 0.79387 0.74959 0.73744 0.70677 0.98190 0.60616

0.3 0.4 0.3 0.5 0.4 0.4 0.5 0.4 0.3 0.4 0.4 0.5 0.4 0.4 0.3 0.4 0.4 0.4 0.6 0.5 0.4 0.3 0.4

0.55163 0.66033 0.62218 0.68449 0.68789 0.71107 0.77086 0.72928 0.65689 0.70284 0.72283 0.70363 0.65501 0.62533 0.61992 0.66514 0.58059 0.55549 0.60705 0.75639 0.63455 0.64083 0.76796

Pb/

Laboratory of Isotope Geochemistry, GIGCAS. Procedures and technical parameters were described in detail by Wu et al. (2006). 3.2. Mineral geochemistry Electron microprobe analysis was carried out on plagioclase and chlorite with a JEOL JXA 8100 M electron microprobe (EMPA) at the

U

238

±r (%)

207

0.07702 0.08012 0.08176 0.08171 0.08024 0.07789 0.08031 0.08081 0.08 0.07885 0.08262 0.08183 0.08199 0.07991 0.08336 0.08341 0.08233 0.07802 0.08227 0.08007 0.07507 0.07923 0.07777 0.07746 0.08222

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

9.6 6.8 5 5.3 5.9 7.3 5.1 7.4 7.1 8.7 4.8

0.07837 0.07782 0.07871 0.07476 0.07806 0.08331 0.08209 0.08341 0.07998 0.08223 0.07828

2.8 4.9 3.5 5.2 4.5 4.1 5.3 4.3 3.3 4.8 5.1 7.1 4.8 4.9 3.5 5.2 4.1 3.9 8.0 4.6 4.4 3.3 5.2

0.07684 0.07934 0.07746 0.07747 0.07882 0.08286 0.08113 0.08099 0.07638 0.07763 0.07848 0.08725 0.07903 0.08176 0.07827 0.08250 0.07813 0.07661 0.08407 0.08122 0.08462 0.07893 0.08417

Pb/

U

Pb/235U

±r

206

Pb/238U

538.5 534.2 393.6 496.5 521.7 600.9 503.8 497.0 595.2 499.8 449.3 392.1 403.8 537.4 397.4 571.1 545.8 726.3 431.5 637.5 824.1 571.7 586.8 532.5 508.4

70.6 125.0 99.0 97.6 83.9 83.9 71.1 91.3 70.4 67.9 76.5 72.6 83.6 76.6 86.0 177.8 66.8 62.4 73.0 87.6 71.5 85.8 70.4 79.2 85.1

478.3 496.8 506.6 506.3 497.6 483.5 497.9 501.0 496.1 489.3 511.7 507.1 508.0 495.6 516.1 516.5 510.0 484.3 509.7 496.6 466.6 491.5 482.8 481.0 509.4

6.8 8.9 8.2 8.2 7.7 7.5 7.3 7.9 7.3 7.1 7.6 7.5 7.9 7.5 8.1 11.8 7.5 6.8 7.7 7.7 6.9 7.5 7.0 7.2 7.8

0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.2 0.2 0.2 0.2

587.7 538.5 521.3 493.2 563.4 593.4 568.0 560.9 542.8 694.6 481.1

54.5 40.9 30.4 32.9 34.1 41.3 29.3 43.1 42.4 44.6 30.0

486.4 483.1 488.4 464.8 484.6 515.8 508.6 516.4 496.0 509.4 485.8

10.6 12.0 10.6 10.3 10.3 13.2 8.1 10.8 10.8 14.6 11.3

0.1 0.2 0.1 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.3 0.2 0.2 0.3 0.1 0.3 0.2 0.2 0.4 0.1 0.2 0.1 0.1

446.1 514.8 491.2 529.5 531.5 545.4 580.3 556.1 512.7 540.5 552.3 541.0 511.6 493.2 489.8 517.8 464.8 448.6 481.7 571.9 498.9 502.8 578.6

18.4 30.0 22.2 31.6 26.9 24.1 30.3 25.4 20.1 28.4 30.3 42.1 29.6 30.7 22.2 31.8 26.6 25.8 50.3 26.7 27.2 20.3 30.0

477.2 492.2 481.0 481.0 489.0 513.2 502.8 502.0 474.5 482.0 487.0 539.3 490.3 506.6 485.8 511.0 485.0 475.8 520.4 503.4 523.7 489.7 520.9

8.9 11.2 8.3 9.9 8.8 9.3 8.2 8.2 6.2 9.0 15.1 14.5 10.5 19.1 7.6 14.9 10.7 9.3 22.4 7.2 10.0 8.2 6.8

±r

Department of Earth Sciences, Nanjing University. The operating conditions were as follows: accelerating voltage 15 kV, beam current 20 nA, beam diameter 1 lm. Peak and background counting times were set at 30 s. All data were corrected with standard ZAF correction procedures. Natural minerals and synthetic glasses were used as standards. Detailed procedure has been described in Wang et al. (2009).

Please cite this article in press as: Zeng, L., et al. Petrogenesis and tectonic significance of the plagiogranites in the Zhaheba ophiolite, Eastern Junggar orogen, Xinjiang, China. Journal of Asian Earth Sciences (2014), http://dx.doi.org/10.1016/j.jseaes.2014.09.031

L. Zeng et al. / Journal of Asian Earth Sciences xxx (2014) xxx–xxx

0.092 Mean=498.0±5.8Ma MSWD =3.7, n=25

0.088

Perkin–Elmer ELAN 6000 inductively coupled plasma mass spectrometer (ICP-MS) following the procedures described in detail by Li et al. (1997), with less than 5% of standard deviations for most elements.

560

(a) 11Zhr-3-3 540

520

0.084 0.080

206

Pb/

238

U

3.4. Sr–Nd–Hf isotopes 500 480

0.076 460

@07

0.072

497.9Ma

440

@14 495.6Ma

0.068 0.5

0.6

0.7 20 7

Pb /

23 5

(b) 08Zhr-2

0.8

U 600

Mean=495.0±11Ma MSWD =2.2, n=11

0.085

560

4.1. LA–ICP–MS zircon U–Pb dating

520

206

@09

480

496.0Ma

0.075 @03

440 488.4Ma

0.065 0.3

0.5

0.7 20 7

0.096

U 238

0.9

1.1

U

Mean=494.6±6.9Ma MSWD =3.1, n=23

0.088

Pb/

Pb /

23 5

580

(c) 08Zhr-5 0.092

540

0.084 500

0.080

@02

492.2Ma

0.076

@13

460

0.072 490.3Ma

0.068 0.3

0.5

0.7 20 7

Sr–Nd isotopic compositions of the samples were analysed using a Neptune Plus MC–ICPMS at the State Key Laboratory of Isotope Geochemistry, GIGCAS, following the procedures similar to those of Li et al. (2005b) and Liang et al. (2009b). The mass fractionation corrections for Nd isotopic ratios were based on the 143 Nd/144Nd ratio of 0.7219. 143Nd/144Nd ratio of the Standard Shin Etsu JNdi-1 determined during this study was 0.512115 ± 10. The Hf isotopic compositions of the samples were analysed using a Finnigan Neptune Plus MC-ICPMS at the Institute of Geology and Geophysics, CAS, following procedures similar to those of Li et al. 176 (2006). The Hf/177Hf ratios were normalised to 179 177 Hf/ Hf = 0.7325, using an exponential law for mass bias correction. 4. Results

Pb/

238

U

0.095

206

5

Pb /

0.9 23 5

U–Pb dating results for zircons from one albitite and two albite granite samples were listed in Table 1. Zircons from the albitite Sample 11Zhr-3-3 are euhedral prismatic crystals of 100–230 lm in length with aspect ratios of 2:1–5:1. They are typical magmatic zircons with clear oscillatory zoning and high Th/U ratios (0.39– 1.12) (Fig. 3a). The 206Pb/238U apparent ages of 25 analyses vary from 466.6 Ma to 516.5 Ma, with a weighted average of 498.0 ± 5.8 Ma (MSWD = 3.7). Zircons from albite granites (08Zhr-2, 08Zhr-5) are euhedral prismatic crystals of 40–200 lm in length with aspect ratios of 1:1–3:1. The zircons are magmatic with clear oscillatory zoning and high Th/U ratios (0.51–1.44) (Fig. 3b and c). The 206Pb/238U apparent ages of 11 analyses for the Sample 08Zhr-2 vary from 464.8 Ma to 516.4 Ma, with a weighted average value of 495.0 ± 11.0 Ma (MSWD = 2.2). The 206Pb/238U apparent ages of 23 analyses for the Sample 08Zhr-5 vary from 474.5 Ma to 539.3 Ma, with a weighted average value of 494.6 ± 6.9 Ma (MSWD = 3.1). The results show that the albitite and albite granites have similar U–Pb isotopic ages, which are consistent with reported zircon U–Pb isotopic ages for plagiogranites in the Zhaheba (489 ± 4 Ma) (Jian et al., 2003) and Armantai areas (503 ± 7 Ma) (Xiao et al., 2006b), within errors. Our results further ascertain that the Zhaheba–Armantai ophiolites were remnants of the Late Cambrian oceanic crust.

1.1

U

Fig. 3. Representative zircon CL images and U–Pb concordia diagrams for (a) albitite and (b and c) albite granite.

3.3. Whole-rock geochemistry Major- and trace element concentrations of representative fresh or least-altered samples were analysed at the State Key Laboratory of Isotope Geochemistry, GIGCAS. Major elements were analysed by X-ray fluorescence (XRF) following the procedures described by Li et al. (2005a), with less than 2% of standard deviations for most elements. Trace elements were determined with a

4.2. Mineral geochemistry Electron microprobe analyses of plagioclase in the Zhaheba albitites and albite granites were presented in Table 2. Plagioclase in the albitites have high contents of albite (Ab) (99.1–99.4%), minor anorthite (An) (0.4–0.7%), and negligible orthoclase (Or) (0.2–0.4%). In contrast, plagioclases in albite granites have lower Ab (91.4–95.0%), and higher An (3.5–7.2%) and Or (1.2–2.0%). 4.3. Whole-rock geochemistry The Zhaheba albite granites have higher SiO2 contents (72.4– 77.1 wt.%) than the Zhaheba albitites (52.2–59.2 wt.%), and both

Please cite this article in press as: Zeng, L., et al. Petrogenesis and tectonic significance of the plagiogranites in the Zhaheba ophiolite, Eastern Junggar orogen, Xinjiang, China. Journal of Asian Earth Sciences (2014), http://dx.doi.org/10.1016/j.jseaes.2014.09.031

6

L. Zeng et al. / Journal of Asian Earth Sciences xxx (2014) xxx–xxx

Table 2 Electron microprobe analyses of plagioclases in the albitite and albite granite from the Zhaheba plagiogranite. Sample Rock type Mineral

08Zhr-7 Al Pl

08Zhr-7 Al Pl

08Zhr-7 Al Pl

08Zhr-1 Ag Pl

08Zhr-1 Ag Pl

08Zhr-1 Ag Pl

08Zhr-1 Ag Pl

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total

69.26 – 19.68 – 0.03 – 0.13 10.82 0.03 100.54

69.34 0.01 19.77 0.03 0.01 – 0.11 10.97 0.07 102.97

68.14 – 19.14 0.02 0.03 0.01 0.08 11.38 0.04 101.71

67.53 0.00 20.93 0.03 – 0.02 1.44 10.05 0.24 100.25

68.11 – 20.34 0.03 – – 0.74 10.30 0.20 99.71

67.71 0.02 21.07 0.07 0.05 – 0.83 9.98 0.30 100.03

67.49 – 20.97 0.04 – – 0.71 10.48 0.34 100.02

Cations per 32 oxygens Si 12.046 Al 4.032 Ti – Fe – Mn 0.005 Mg – Ca 0.024 Na 3.644 K 0.007 Sum 19.758 Ab 99.2 An 0.7 Or 0.2

12.028 4.039 0.001 0.004 0.001 – 0.020 3.689 0.015 19.797 99.1 0.5 0.4

12.027 3.979 – 0.003 – 0.003 0.015 3.896 0.009 19.932 99.4 0.4 0.2

11.771 4.297 0.001 0.005 – 0.005 0.269 3.398 0.052 19.798 91.4 7.2 1.4

11.903 4.186 – 0.004 – – 0.138 3.490 0.044 19.765 95.0 3.8 1.2

11.804 4.325 0.003 0.010 0.007 – 0.156 3.374 0.067 19.746 93.8 4.3 1.9

11.787 4.313 – 0.005 – – 0.133 3.548 0.076 19.862 94.4 3.5 2.0

Rock type: Ag – Albite granite; Al – Albitite; Mineral: Pl – Plagioclase.

types of rocks are enriched in Na2O and depleted in K2O (Table 3), which are typical for plagiogranites (Coleman and Peterman, 1975). The two albitite samples (11Zhr-3-2, 11Zhr-3-3) have higher contents of K2O, MgO and Fe2OT3, which may be related to the higher biotite content. Total rare earth element (REE) contents of the albitites vary from 128 to 219 ppm, which are significantly higher than those of the albite granites (35.0–64.4 ppm). Both rock types are relatively enriched in LREE (Fig. 4), with (La/Yb)N ratios of 20.9–44.1 and 13.6–20.9 for the albitites and albite granites, respectively. The albitites show obvious positive Eu anomalies, whereas the albite granites show slightly negative Eu anomalies. The positive Eu anomalies in the albitites were probably caused by albite accumulation. Both albitites and albite granites are depleted in HFSE (Nb, Ta, Ti and P) and LILE (Rb and K) (Fig. 5), consistent with typical plagiogranites geochemistry (Coleman and Peterman, 1975; Coleman and Donato, 1979; Rollinson, 2009). 4.4. Whole rock Sr–Nd–Hf and zircon Hf isotopes The eNd(t) values of the albitites and albite granites vary in the ranges of 7.2–7.7 and 6.4–7.3 (Table 4), respectively, which are close to those of typical MORB (6.9–11.9). The initial 87Sr/86Sr ratios of the albitites and albite granites vary in the ranges of 0.705096–0.705461 and 0.704774–0.704787, respectively. They are significantly higher than typical MORB values (0.7023– 0.7033), and the former are higher than the latter (Fig. 8). In addition, whole-rock Hf isotopes of the albitite and three albite granite samples show that the rocks have similar eHf(t) values (eHf(t) = 12.6–14.5). The eHf(t) values for zircons from the albitites and albite granites vary in the ranges of 11.5–17.9 and 9.8–13.9 (Table 5), respectively. Hf isotopic compositions of the plagiogranite whole rocks and zircons are similar to those of MORB (eHf(t) = 5–25) (Salters et al., 2011), suggesting a depleted mantle source.

5. Discussion 5.1. Petrogenetic relations between the Zhaheba albitites and albite granites The albitites show typical cumulus texture (Fig. 2c), which are generally regarded to have formed by gravity settling of crystals on magma chamber floors or in-situ crystallisation on the side, roof, and bottom of magma chamber walls (e.g., Campbell, 1978, 1987; McBirney and Noyes, 1979; Morse, 1979; Sparks et al., 1984, 1985). Irvine (1980) used the residual porosity to classify cumulates as adcumulates (0–7%), mesocumulates (7–25%) and orthocumulates (>25%). The Zhaheba albitites have been identified to contain adcumulates of 5% residual porosity. Two mechanisms for the crystallisation of intercumulus melt in adcumulates have been proposed (Sparks et al., 1985): (1) Cumulates continue to grow (with similar mineral contents) when the connectivity between the intercumulus melt and the rest of the magma chamber is enough for component exchange. Continuous fractional crystallisation gradually reduces the amount of intercumulus melt and the residual porosity lower than 10%. (2) The imbalance between the hydrostatic and lithostatic pressure in the crystal mush results in deformation of the mush and the expulsion of the intercumulus melt (Meurer and Meurer, 2006). Petrographic characteristics (e.g., plastic deformation) of the Zhaheba albitites show strong evidence of compaction in the adcumulates (Fig. 2d). With the temperature decreasing, the intercumulus melt may have gradually crystallized and the residual melt became more felsic. Since the density of the residual felsic melt is lower than that of the mafic-intermediate intercumulus crystal mush, the consequent imbalance of the hydrostatic and lithostatic pressure may have led to the plastic deformation of the albite/mafic minerals and the expulsion of the residual felsic melt. The Zhaheba albite granites were formed coevally or slightly after the Zhaheba albitites. The (143Nd/144Nd)i versus SiO2 and

Please cite this article in press as: Zeng, L., et al. Petrogenesis and tectonic significance of the plagiogranites in the Zhaheba ophiolite, Eastern Junggar orogen, Xinjiang, China. Journal of Asian Earth Sciences (2014), http://dx.doi.org/10.1016/j.jseaes.2014.09.031

Al 11Zhr-3-1

Al 11Zhr-3-2

Al 11Zhr-3-3

Al 11Zhr-4

Al 08Zhr-6

Al 08Zhr-7

Ag 08Zhr-1

Ag 08Zhr-2

Ag 08Zhr-3

Ag 08Zhr-4

Ag 08Zhr-8

Ag 11Zhr-7

Ag 11Zhr-8

Ga 04Zhr-129

Ga 03Zhr-15

Ga 04Zhr-57

SiO2 TiO2 Al2O3 Fe2OT3 MnO MgO CaO Na2O K2O P2O5 LOI Total ACNK Sc Ti V Cr Mn Co Ni Cu Zn Ga Ge Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U REE LREE HREE LREE/HREE (La/Yb)N dEu

55.4 0.46 21.7 3.26 0.02 2.81 4.55 7.23 0.77 0.15 3.03 99.4 1.03 4.07 2462 37.4 43.8 216. 14.9 48.3 6.00 31.2 13.0 0.69 6.13 2154 7.77 310. 4.14 0.12 780. 53.1 99.6 11.1 39.3 4.86 1.67 3.80 0.37 1.49 0.26 0.76 0.12 0.93 0.17 7.36 0.22 2.59 8.06 0.81 218 210 7.95 26.4 40.5 1.14

54.4 0.48 23.6 4.04 0.02 3.49 1.42 6.58 2.26 0.18 3.05 99.6 1.49 3.71 3153 39.1 43.7 297. 23.2 69.4 12.5 20.4 20.0 0.52 19.4 2311 8.36 112. 4.59 0.20 954. 51.1 104. 11 38.3 4.77 1.57 3.83 0.41 1.68 0.29 0.80 0.12 0.83 0.14 2.44 0.26 3.77 9.26 0.68 219 212 8.14 26.0 44.1 1.08

55.3 0.49 23.3 3.79 0.02 3.49 1.29 6.97 1.98 0.18 2.87 99.7 1.46 3.56 2607 32.0 30.0 223. 20.0 67.4 21.5 16.7 15.6 0.48 15.2 2139 7.40 333. 3.85 0.14 749. 48.0 92.3 10.1 35.5 4.28 1.39 3.40 0.35 1.38 0.24 0.75 0.12 0.92 0.17 7.59 0.22 2.83 8.32 0.74 199 192 7.38 26.0 37.1 1.07

59.2 0.33 21.6 3.31 0.01 1.97 1 8.65 1.2 0.15 2.18 99.7 1.24 2.79 1835 21.8 32.2 183. 12.1 31.0 7.09 14.5 14.3 0.32 7.45 736. 7.30 108. 5.39 0.12 586. 30.1 56.0 6.73 24.0 3.40 1.06 2.71 0.31 1.39 0.24 0.66 0.10 0.73 0.12 2.73 0.36 3.28 6.97 0.98 128 121 6.29 19.3 29.6 1.03

56.3 0.43 21.1 2.40 0.05 2.93 6.34 6.58 0.61 0.15 2.74 99.7 0.91 3.25 2210 25.5 73.1 415. 19.2 119. 2.04 22.4 17.6 0.48 4.92 1206 5.40 222 4.29 – 685. 31.6 65.4 7.38 24.5 3.45 1.43 2.34 0.22 1.04 0.19 0.55 0.08 0.62 0.11 5.28 0.24 21.8 5.79 1.10 139 134 5.19 25.8 36.3 1.46

52.2 0.53 20.4 6.01 0.08 5.78 3.84 6.43 0.47 0.16 3.66 99.7 1.13 10.5 2641 52.7 230. 619. 36.0 167. 1.73 30.1 14.9 0.31 2.99 916. 10.8 101. 7.29 – 787. 29.1 61.4 7.84 29.4 5.10 1.73 3.69 0.43 2.18 0.39 1.06 0.14 1.00 0.14 2.55 0.40 1.69 2.47 1.11 144 135 9.07 14.9 20.9 1.16

72.4 0.19 15.4 0.51 0.01 0.80 1.03 8.85 0.11 0.07 0.31 99.8 0.92 0.99 966. 11.6 1.26 74.8 19.2 14.8 3.01 12.4 15.1 0.38 0.43 158. 5.29 114. 7.98 – 150. 15.6 29.8 3.24 10.1 1.52 0.33 1.17 0.15 0.90 0.18 0.50 0.08 0.57 0.09 3.20 0.85 3.18 5.32 0.90 64.4 60.7 3.68 16.5 19.4 0.74

75.6 0.15 13.6 0.50 0.01 0.57 0.99 7.73 0.17 0.10 0.34 99.8 0.92 1.05 789 10.4 3.63 52.6 14.2 8.43 1.51 8.48 13.3 0.55 0.68 201. 5.02 83.2 7.76 – 152. 10.2 19.5 2.34 7.78 1.31 0.33 1.08 0.15 0.84 0.16 0.49 0.07 0.53 0.08 2.58 0.83 3.14 2.99 0.83 45.0 41.6 3.44 12.1 13.6 0.82

76.6 0.18 13.3 0.38 0.00 0.49 0.89 7.41 0.13 0.06 0.26 99.8 0.95 0.51 864. 10.5 1.04 47.7 25.9 5.24 1.77 6.96 11.7 0.41 0.50 176. 4.18 99.1 6.91 – 181 13.8 25.7 2.86 8.88 1.31 0.34 1.06 0.13 0.49 0.14 0.43 0.06 0.47 0.07 2.88 0.73 2.02 4.24 0.81 55.9 53.0 2.89 18.3 20.9 0.85

77.1 0.17 13.4 0.25 0.00 0.46 0.80 7.10 0.09 0.07 0.26 99.8 1.01 0.75 886 10.5 1.12 37.8 32.2 3.3 1.89 5.45 11.0 0.41 0.34 151 5.02 105. 6.59 – 130. 14.8 29.0 3.19 9.93 1.48 0.30 1.08 0.15 0.87 0.16 0.49 0.07 0.52 0.08 2.99 0.70 2.10 4.61 0.77 62.3 58.9 3.45 17.0 20.4 0.69

75.7 0.18 13.4 0.60 0.01 1.07 0.50 7.24 0.10 0.05 0.79 99.8 1.04 1.28 854. 14.8 4.01 82.0 26.6 21.1 4.54 6.53 12.2 0.46 0.59 219. 5.87 104. 2.78 – 203. 13.5 27.0 3.12 10.1 1.69 0.36 1.37 0.18 1.02 0.20 0.59 0.09 0.62 0.10 2.87 0.29 1.93 4.48 1.12 60.2 56.0 4.22 13.3 15.5 0.71

76.5 0.17 13.2 0.88 <0.0 0.52 0.73 7.05 0.13 0.06 0.28 99.6 1.01 2.14 899. 8.63 37.5 74.3 2.2 11.9 18.6 16.8 11.8 0.57 0.77 196. 4.76 98.2 6.26 0.07 222. 13.5 24.6 2.70 8.88 1.28 0.28 1.12 0.15 0.79 0.15 0.45 0.07 0.53 0.08 2.84 0.64 3.79 4.34 0.87 54.7 51.3 3.38 15.2 18.0 0.71

74.9 0.16 13.8 1.35 <0.0 0.76 0.96 7.22 0.15 0.12 0.3 99.8 1.00 2.04 860 10.8 52.7 99.3 3.08 15.9 26.1 6.78 12.5 0.76 0.90 180. 3.91 88.5 7.88 0.14 160 8.50 15.1 1.68 5.75 0.92 0.22 0.85 0.12 0.67 0.13 0.38 0.06 0.42 0.07 2.80 0.84 3.69 2.53 0.83 35.0 32.3 2.72 11.8 14.5 0.76

49.7 1.08 14.4 11.2 0.15 8.16 7.24 3.70 1.10 0.11 2.78 99.6 0.70 52.8 6663 358 232 1207 33.1 75.3 94.1 77.9 13.9 1.19 10.4 377 16.18 56.6 5.20 – 685 7.50 16.4 2.29 10.6 2.73 0.81 3.30 0.53 3.29 0.72 2.03 0.31 1.96 0.29 1.75 0.39 0.80 1.30 0.41 52.8 40.3 12.4 3.25 2.75 0.83

46.7 1.49 17.2 12.5 0.18 6.55 4.78 3.86 1.8 0.41 4.11 99.7 1.02 35.2 8737 314 51.3 1325 38.1 42.5 93.6 98.0 18.1 1.55 42.3 780 22.5 114 9.23 – 1994 16.8 36.7 5.09 22.0 5.00 1.29 5.38 0.77 4.47 0.91 2.49 0.38 2.34 0.36 2.63 0.42 3.96 1.98 0.61 104 86.8 17.1 5.08 5.15 0.76

49.7 1.29 17.7 10.6 0.16 4.96 6.58 4.35 1.07 0.36 2.96 99.7 0.87 31.3 7899 322 9.36 1234 25.6 25.1 17.6 75.0 19.0 0.92 23.3 720 17.5 94.8 7.22 – 616 23.5 48.1 6.35 27.3 5.14 1.54 4.60 0.67 3.65 0.71 1.94 0.28 1.68 0.26 2.75 0.48 1.18 3.73 0.65 126 112 13.8 8.12 10.0 0.95

7

Rock Sample

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Table 3 Major and trace element compositions of the Zhaheba plagiogranite.

605 542 567 13.5 14.5 14.1 4.0 4.8 4.8 0.282845 0.282873 0.282862 0.000005 0.000013 0.000007 0.282884 0.282908 0.282908 0.70493 Gabbro 03Zhr-15

0.1568

0.704841 0.704833 0.704829 Albite granite 08Zhr-1 0.0065 08Zhr-4 0.0078 08Zhr-8 0.0082

0.000018

0.703838

1.3

0.1374

0.512784

0.000013

0.512343

6.56

683

0.004166 0.003807 0.004979 663 702 625 6.9 6.4 7.3 0.512351 0.512327 0.512375 0.00001 0.000011 0.000009 0.512647 0.51262 0.512703 0.0912 0.0905 0.1011 12.2 12.3 12.1

639 606 596 607 617 0.512364 0.512385 0.512391 0.512385 0.512378

(T)

18.9 19.3 16.7 21.9 21.8 0.705254 0.70528 0.705096 0.705461 0.705456 0.000005 0.000006 0.000005 0.000005 0.000014 0.705312 0.705453 0.705242 0.705669 0.705523

2r Sr/86Sr 87

Table 4 Sr–Nd–Hf isotopic results of the Zhaheba plagiogranite.

5.2. Source characteristics of the plagiogranitic magmas

(87Sr/86Sr)i

eSr

147

0.0747 0.0753 0.0729 0.0855 0.1048

Sm/144Nd

143

0.512608 0.512631 0.512629 0.512664 0.51272

Nd/144Nd

(143Nd/144Nd)i versus TiO2 plots display nearly-constant or slightly negative trends with increasing degree of fractionation crystallisation (Fig. 6). Combined with the La versus La/Sm plot (Fig. 7). These trends indicate that fractional crystallisation may have played an important role in the generation the Zhaheba plagiogranites. The petrographic and geochemical characteristics of the rocks indicate that the albitites may have been the products of earlier albite accumulation whereas the albite granites may have been formed by later consolidation of the residual magma.

Whole rock (Nd and Hf) and zircon (Hf) isotopic compositions of the albitites and albite granites are comparable to those of typical MORB, indicating a similar depleted mantle source. Moreover, all the samples plot on the mantle evolutional trend in the eHf(t) versus eNd(t) diagram (Fig. 8), indicating that the source rocks should be mantle-derived. The albitites and albite granites have relatively elevated Zr/Hf ratios of 35.6–45.9 and 31.6–35.1, respectively, which are close to that of typical MORB (33–40) (Büchl et al., 2002), but distinctly higher than those of the world average granite (25) (Münker et al., 2003), also implying that the source rocks of the Zhaheba plagiogranites were mantle-derived. The albitites and albite granites have decoupled Sr and Nd isotopic compositions and were plotted to the right of the mantle array (Fig. 9a). The initial 87Sr/86Sr ratios of the rocks vary from 0.704774 to 0.705461, whereas the eNd(t) values remain constant, showing a trend of seawater hydrothermal alteration (O’Nions et al., 1978; McCulloch and Cameron, 1983). So the decoupling feature of the Sr and Nd isotopic compositions support that seawater

0.000003 0.000004 0.000003 0.000004 0.000008

2r

Fig. 5. Rock/Primitive Mantle (PM) diagram for the Zhaheba plagiogranites. PM data are from Sun and McDonough (1989).

7.2 7.6 7.7 7.6 7.5

176

(143Nd/144Nd)i

Th K Ta Ce P Zr Sm Ti Y Lu Ba U Nb La Sr Nd Hf Eu Tb Yb

Rb/86Sr

Rb

87

0.1

eNd

1.0

(T)

TDM,2 (Ma)

10.0

Sample

Sample/PM

100.0

0.007872

Lu/177Hf

176

1000.0

0.282892

Hf/177Hf

2r

Fig. 4. REE/chondrite diagram the Zhaheba plagiogranites. Chondrite data are from Sun and McDonough (1989).

0.704785 0.704787 0.704774

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

0.000014 0.000017 0.00002

0.282818 0.000005

1

0.0082 0.0243 0.0206 0.0293 0.0094

10

12.6 4.2

eHf(0)

100

Albitite 11Zhr-3-1 11Zhr-3-2 11Zhr-3-3 11Zhr-4 08Zhr-7

eHf

TDM,2 (Ma)

albitite albite granite

(176Hf/177Hf)i

Sample/Chondrite

1000

664

L. Zeng et al. / Journal of Asian Earth Sciences xxx (2014) xxx–xxx

(T)

8

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9

L. Zeng et al. / Journal of Asian Earth Sciences xxx (2014) xxx–xxx Table 5 Hf isotopic composition of zircons from the Zhaheba plagiogranite. 176

2r

176

Albitite (11Zhr-3-3) 11Zhr-3-3-1 498 11Zhr-3-3-3 498 11Zhr-3-3-4 498 11Zhr-3-3-5 498 11Zhr-3-3-7 498 11Zhr-3-3-8 498 11Zhr-3-3-9 498 11Zhr-3-3-10 498 11Zhr-3-3-11 498 11Zhr-3-3-12 498 11Zhr-3-3-13 498 11Zhr-3-3-14 498 11Zhr-3-3-15 498 11Zhr-3-3-16 498 11Zhr-3-3-17 498 11Zhr-3-3-18 498 11Zhr-3-3-19 498 11Zhr-3-3-20 498 11Zhr-3-3-21 498 11Zhr-3-3-22 498 11Zhr-3-3-24 498 11Zhr-3-3-27 498 11Zhr-3-3-28 498 11Zhr-3-3-29 498 11Zhr-3-3-30 498

0.094614 0.091733 0.072113 0.086700 0.079343 0.095364 0.073843 0.069646 0.054088 0.070998 0.066640 0.096036 0.097614 0.120929 0.121375 0.127854 0.092215 0.069803 0.086498 0.083252 0.093385 0.099379 0.054911 0.096296 0.052152

0.001148 0.001872 0.000977 0.001135 0.001136 0.000419 0.002737 0.001012 0.001016 0.000361 0.001259 0.000312 0.000243 0.001467 0.003923 0.002114 0.000922 0.000795 0.000709 0.000819 0.000941 0.000584 0.001047 0.001192 0.000898

Albite granite (08Zhr-2) 08Zhr-2-1 495 08Zhr-2-2 495 08Zhr-2-3 495 08Zhr-2-4 495 08Zhr-2-5 495 08Zhr-2-6 495 08Zhr-2-7 495 08Zhr-2-8 495 08Zhr-2-9 495 08Zhr-2-10 495 08Zhr-2-11 495

0.052096 0.036882 0.052402 0.063814 0.070653 0.087314 0.146919 0.065880 0.065412 0.080126 0.078075

0.000196 0.000579 0.000786 0.000554 0.001249 0.002319 0.004149 0.000602 0.001808 0.000864 0.000595

Sample spot

Age (Ma)

Yb/177Hf

Lu/177Hf

2r

176

0.002504 0.002339 0.001930 0.002518 0.002322 0.002495 0.002062 0.002031 0.001586 0.002089 0.001922 0.002747 0.002583 0.003358 0.003326 0.003420 0.002444 0.001790 0.002288 0.002263 0.002599 0.002662 0.001603 0.002598 0.001447

0.000024 0.000043 0.000023 0.000027 0.000027 0.000020 0.000069 0.000028 0.000024 0.000014 0.000032 0.000010 0.000005 0.000036 0.000105 0.000042 0.000026 0.000020 0.000022 0.000021 0.000022 0.000010 0.000026 0.000026 0.000023

0.001894 0.001388 0.001773 0.002354 0.002483 0.002975 0.004998 0.002309 0.002405 0.003009 0.002487

0.000007 0.000021 0.000028 0.000020 0.000038 0.000071 0.000145 0.000017 0.000068 0.000026 0.000015

177

177

2r

176

0.282948 0.282947 0.282883 0.282927 0.282845 0.282905 0.282809 0.282826 0.282895 0.282861 0.282856 0.282925 0.282893 0.282819 0.282998 0.282930 0.282894 0.282854 0.282868 0.282924 0.282870 0.282894 0.282864 0.282937 0.282940

0.000012 0.000014 0.000009 0.000011 0.000017 0.000012 0.000010 0.000009 0.000009 0.000014 0.000009 0.000009 0.000008 0.000011 0.000013 0.000012 0.000014 0.000013 0.000012 0.000012 0.000009 0.000014 0.000009 0.000009 0.000013

0.282924 0.282925 0.282865 0.282904 0.282823 0.282882 0.282790 0.282807 0.282880 0.282842 0.282838 0.282900 0.282869 0.282788 0.282967 0.282898 0.282871 0.282837 0.282846 0.282903 0.282845 0.282869 0.282849 0.282913 0.282927

0.282874 0.282822 0.282837 0.282827 0.282763 0.282781 0.282817 0.282875 0.282786 0.282796 0.282827

0.000015 0.000014 0.000010 0.000018 0.000015 0.000020 0.000041 0.000011 0.000018 0.000021 0.000012

0.282856 0.282809 0.282820 0.282806 0.282740 0.282754 0.282770 0.282853 0.282763 0.282768 0.282804

Hf/ Hf

Hf/ Hfi

eHf (0)

eHf (T)

TDM,1 (Ma)

TDM,2 (Ma)

6.2 6.2 3.9 5.5 2.6 4.7 1.3 1.9 4.3 3.2 3.0 5.4 4.3 1.7 8.0 5.6 4.3 2.9 3.4 5.4 3.5 4.3 3.3 5.8 5.9

16.4 16.4 14.3 15.6 12.8 14.9 11.6 12.2 14.8 13.4 13.3 15.5 14.4 11.5 17.9 15.4 14.5 13.3 13.6 15.6 13.6 14.4 13.7 16.0 16.5

449 448 536 480 598 512 646 621 514 571 576 485 532 654 383 487 528 577 564 480 566 531 559 466 447

423 421 558 470 652 520 728 690 523 611 619 479 550 732 326 482 544 621 600 471 602 548 594 450 418

3.6 1.8 2.3 2.0 0.3 0.3 1.6 3.6 0.5 0.8 1.9

13.9 12.2 12.6 12.1 9.8 10.3 10.8 13.8 10.6 10.8 12.0

549 616 601 624 722 704 691 554 687 683 627

580 687 661 694 843 811 774 586 790 779 698

0.5135

0.5135

(a)

(b) 0.5130

Nd) i 14 4

FC

FC 0.5125

14 3

Nd/

0.5125

(

(

14 3

Nd/

14 4

Nd) i

0.5130

0.5120

0.5120

0.5115 50

55

60

65

70

75

80

SiO2 (wt.%)

0.5115

0

0.1

0.2

0.3

0.4

0.5

0.6

TiO2 (wt.%)

Fig. 6. Plots of (a) (143Nd/144Nd)i versus TiO2 and (b) (143Nd/144Nd)i versus SiO2 for the Zhaheba plagiogranites (Temizel et al., 2012). FC: Fractional crystallisation.

alteration has a significant impact on Sr isotopes. Compared with the Zhaheba albite granites, the Zhaheba albitites have higher degree of alteration, such as chlorite and epidote alteration (some epidotes are magmatic). Chlorite geochemistry is closely related to its formation temperature (Walshe and Solomon, 1981; Walshe, 1986; Cathelineau, 1988). The formation temperature of chlorite in the albitites varies mainly in the range of 300–350 °C (see Supplementary Table S1), indicating that the chlorites were formed by

the interactions between the albitites and the seawater. In addition to the Sample 11Zhr-4, the initial 87Sr/86Sr ratios of the albitites are positively correlated with their loss on ignition (LOI) (Fig. 9b). Given that the hydrous minerals of the albitites contain epidote and little carbonate, the LOI can to a certain extent reflect the degree of alteration. Therefore, the water/rock reaction was likely to be the main controlling factor on late stage Sr isotopic variation. Thus, we propose that, deducting the altered effect, the albitites

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L. Zeng et al. / Journal of Asian Earth Sciences xxx (2014) xxx–xxx

15

20

Gabbro

MORB Semail ophiolite

10

Troodos ophiolite

y rra

εNd ( t)

PM

5

ea ntl

10

Seawater effects

Ma

La/Sm

15

(a)

11Zhr-4

0

(b)

3

LOI

5

-5

FC

LREE-enriched components

2

1

0 0

10

20

30

40

50

60

0.704

La (ppm)

0.706 .

initial

Fig. 7. Plots of La versus La/Sm for the Zhaheba plagiogranites (Allègre and minster, 1978). PM: Partial Melting; FC: Fractional crystallisation.

0.7045

0.7050

0.7055

initial 87Sr/86Sr

-10 0.702

0.708

0.710

87Sr/86Sr

Fig. 9. (a) Initial 87Sr/86Sr versus eNd(t) diagram for the Zhaheba plagiogranites. Fields of the Semail and Troodos ophiolites (McCulloch and Cameron, 1983) are shown for comparison and (b) initial 87Sr/86Sr versus LOI diagram for the Zhaheba plagiogranites.

25 20

MORB

800

15 10

Global lower crust

Zhaheba gabbro

5

Visnes type plagiogranite

OIB

Karmoy type plagiogranite

600

0

Karmoy magenetite Gabbro Karmoy Gabbro

-5

Zr (ppm)

ε H f (t )

Zhaheba albitite Zhaheba albite granite

-10 -15

Karmoy Rayleigh fractionation trend

400

-20

Global sediments

-25 -30

-20

-15

-10

-5

0

5

200 10

15

Zhaheba Raleigh fractionation trend

ε Nd (t) 0 Fig. 8. eNd(t) versus eHf(t) diagram for the Zhaheba plagiogranites. Data taken from the Dobosi et al. (2003).

and albite granites were derived from the same source with similar Sr isotopic composition.

0

50

100

150

200

Y (ppm)

5.3. Origin of the Zhaheba plagiogranites

Fig. 10. Plot of Y versus Zr for the Zhaheba plagiogranites compared with the Karmoy ophiolite (Pedersen and Malpas, 1984). The Zhaheba Raleigh fractionation trend was calculated by the gabbro Sample 03Zhr-15. Partition coefficients for Zr and Y are from Watson and Harrison (1983) and Pearce and Norry (1979), respectively.

Three models have been proposed for the origin of plagiogranites, i.e., (1) Differentiation of basaltic magmas (Coleman and Donato, 1979; Lippard et al., 1986; Jiang et al., 2008), (2) Partial melting of metasomatised gabbros or amphibolites (Malpas, 1979; Gerlach et al., 1981; Flagler and Spray, 1991; Koepke et al., 2004; Grimes et al., 2013), (3) Liquid immiscibility in deeper crust (Dixon and Rutherford, 1979). However, the third model has largely been discarded due to the lacking of evidence for immiscible liquids in oceanic magmatic systems. The two widely accepted genetic types for plagiogranites are namely Karmoy-style (or the anatectic-type) derived from partial melting of amphibolites or amphibolite-metamorphosed gabbros and Visnes-style (or the differentiated type) related to the crystallisation differentiation of basaltic magmas (Pedersen and Malpas, 1984). The Zr and Y are incompatible elements that have great effect on the partial melting process (Hanson, 1978). So the

Visnes-style plagiogranites have higher contents of Zr, Y and Zr/Y ratios than those of the Karmoy-style plagiogranites (Fig. 10). The Zhaheba plagiogranites have low contents of Zr, Y and Zr/Y ratios, which are consistent with the Karmoy-style plagiogranites. The Zhaheba plagiogranites geochemistry shifts away from the Raleigh fractionation trend and is close to the anatectic plagiogranite region (Fig. 10), which implies that the plagiogranites may have been generated by gabbro anatexis rather than by the basaltic magma fractionation. TiO2 contents in tholeiitic melts are dependent on the oxygen fugacity (e.g., Berndt et al., 2005; Snyder and Dunning, 1993; Toplis and Carroll, 1995). A high fO2 enhances the stability of Ti oxides resulting the melts with relatively low contents of TiO2. Since MORB differentiation in nature occurs under much more reducing conditions (Bézos and Hunler, 2005; Christie et al.,

Please cite this article in press as: Zeng, L., et al. Petrogenesis and tectonic significance of the plagiogranites in the Zhaheba ophiolite, Eastern Junggar orogen, Xinjiang, China. Journal of Asian Earth Sciences (2014), http://dx.doi.org/10.1016/j.jseaes.2014.09.031

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L. Zeng et al. / Journal of Asian Earth Sciences xxx (2014) xxx–xxx

1000

1.5

Source rock(03Zhr-15)

Fractional Crystalization

1

TiO2 (wt.%)

Sample/Chondrite

Partial melting with residue of amphibolites (0.32plagioclase,0.59amphibole,0.009apatite)

Partial Melting

100

10

Zhaheba plagiogranites 30% 10% 1%

0.5 1

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

0 50

55

60

65

70

75

80

SiO2 (wt.%) Fig. 11. SiO2 versus TiO2 (Koepke et al., 2007).

1986), Koepke et al. (2007) did experiments under oxidizing conditions which were used for constructing a line delimiting the minimum amount of TiO2 expected in evolved melt generated from differentiation in tholeiitic liquids. Koepke et al. (2007) proposed that TiO2 was higher in the melts generated by MORB differentiation or liquid immiscibility and lower in the melts generated by gabbro anatexis. TiO2 contents of the Zhaheba plagiogranites are substantially lower than the melts derived from MORB differentiation or immiscibility, which further support the anatexis hypothesis (Fig. 11). In order to verify the anatexis hypothesis for the Zhaheba plagiogranites, we have selected a typical Zhaheba gabbro Sample 03Zhr-15 for geochemical modelling. The modelling was made on three assumptions, including: (1) Residue of the partial melting is amphibolite, containing 0.9% apatite, 32% plagioclase and 59% amphibole (Helz, 1976); (2) The partial melting processes follow the equation of Shaw (1970) and the mineral/melt partition coefficients follow Arth (1976); (3) The partial melting of minerals is in accordance with the modal batch partial melting with low to moderate degrees of partial melting. The results were presented in the Supplementary Table S2 and illustrated in Fig. 12. Our modelling indicates that the Zhaheba plagiogranites could be generated by 10–30% partial melting of gabbro with the amphibolite residue. Therefore, we propose that the Zhaheba plagiogranites were most likely to be derived from the gabbro anatexis. However, it is perplexing that the modelled products contain lower in LREEs than the Zhaheba plagiogranites. The paradox of LREE enrichment in the plagiogranites has drawn the attention of many geoscientists. Ludden and Thompson (1979) and Humphris (1984) proposed that under certain conditions, especially in alteration zones, seawater can selectively mobilize LREEs. Hydrothermal alterations not only change the oceanic crustal geochemistry (REE and isotopes), but also lower the melting point of the rocks. The Zhaheba albite granites have low LOI (less than 0.4 wt.%, except for the Sample 08Zhr-8), and only slightly sericite altered. Thus, alteration influence on isotopic compositions was likely to be negligible. However, comparing with the fresh gabbros ((87Sr/86Sr)i = 0.703838) in the Zhaheba ophiolite, the albite granites are enriched in 87Sr ((87Sr/86Sr)i = 0.704774–0.704787), which implying that the enrichment of 87Sr through liquid infiltration before melting (Fig. 9). The enrichment of 87Sr and LREE of the rocks indicate that the Zhaheba plagiogranites were most likely

Fig. 12. Calculated 1–30% batch modal melts of a gabbro source resembling the most mafic sample (03Zhr-15) with amphibolite residue.

to be the products of amphibolite/amphibolitised gabbro anatexis. The Zhaheba plagiogranites have pronounced negative Nb, Ta, Ti, and relatively higher LREE/HREE, which are diagnostic features of intra-oceanic subduction (Hawkesworth et al., 1993; Pearce, 2014). Some geologists (Jin et al., 2001; Liang et al., 1999; Wang et al., 2003; Xiao et al., 2009) proposed that the Armantai ophiolite derived from arc or back arc basin. The Armantai ophiolites have been dated to be 503 Ma, which are close to the Zhaheba plagiogranites. Both the Armantai and Zhaheba are in the Zhaheba– Armantai faults. So we think that the Zhaheba plagiogranites formed in intra-oceanic backarc basin. The suprasubduction-zone oceanic backarc basin ophiolites are generated by decompression melting of the asthenospheric mantle as a result of seafloor spreading (Xu et al., 2003; Dilek and Furnes, 2011) (Fig. 13a). The Zhaheba ophiolites were erupted during the spreading in the East Junggar backarc basin from the upwelling asthenospheric mantle. We propose that during the spreading, any resistance of moving the oceanic plate may manifest as low-angle shearings near the base of the crust (Fig. 13b). The addition of LREE-enriched hydrothermal solutions (the seawater from the shearing zones) made the gabbro evolve to amphibolite which lowers the melting points. The amphibolite was subsequently heated up by asthenospheric heating, probably with additional heat from shearings during plate motion, which facilitated the anatexis of amphibolite forming magma chamber, and then formed albitite and albite granite through fractional crystallisation (Temizel et al., 2012).

5.4. Regional tectonic significance The occurrence of the UHP metamorphic rock blocks indicates that the Zhaheba–Armantai ophiolites is an ultra-deep subduction zone of oceanic plate (Niu et al., 2007, 2008, 2009). The ophiolites together with coterminous the Early Paleozoic Karamay ophiolites in the West Junggar (Zhu et al., 2008), defines a west-east trend subduction zone. Many researchers believe that the Ertix fault is the tectonic suture between the Junggar–Kazakhstan and Siberian plates in northern Xinjiang province. Moreover, the Ertix fault extents westward and connects with the Zaysan collision zone as a continuous tectonic belt. Li (2004) argued against based on two factors: (1) The predecessors of the Ertix and Zaysan structural tectonic belts were significantly in terms of ages, i.e., the predecessors of the Zaysan collision zone was at least a Sinian ocean basin, whereas that of the Ertix fault was a Late Palaeozoic oceanic basin; (2) Fossils of

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L. Zeng et al. / Journal of Asian Earth Sciences xxx (2014) xxx–xxx

(a)

BAB decompression melting

arc

oceanic crust

lithosphere mantle

depleted MORB mantle

slab subduction

BAB (b)

gabbro plagiogranite

Layer1+2 Layer3 Ultramafic rock amphibolite

Fig. 13. Schematic tectonic model for the formation of the Zhaheba plagiogranites (modified after Flagler and Spray, 1991; Pearce, 2014). (a) backarc basin ophiolite and (b) Zhaheba plagiogranites formation. BAB: backarc basin; MORB: mid-ocean ridge basalt.

the Tuvaella Fauna, which were locally restricted to the edges of the Siberian plate, were found on both sides of the Ertix fault. Zircon U–Pb ages of the Zhaheba plagiogranites indicate that the Zhaheba–Armantai oceanic basin was formed in the Late Cambrian. This was close to the formation age of the ancient oceanic basin in Zaysan. The connection between the Zaysan collision zone and the ultra-deep subduction zone typifying the Zhaheba–Armantai–Karamay ophiolites seems to be more reasonable, which can better solve the question raised by Li (2004). 6. Conclusion 1. The close spatial and temporal relationships as well as similar Nd and Hf isotopic compositions of the albitites and albite granites in the Zhaheba ophiolite indicate that they were most likely comagmatic. The petrographic and geochemical characteristics of the rocks indicate that the albitites may have been formed by earlier albite accumulation, whereas the albite granites may have been the products of residual felsic magma consolidation. 2. The Zhaheba plagiogranites have Nd and Hf isotopic compositions similar to depleted mantle source, indicative of an oceanic crustal source. Low content of TiO2, Nd, Ta, significant LREE enrichment and the decoupling of Sr and Nd isotopes of the plagiogranites suggest that they may have been the products of the anatexis of amphibolites, which were generated by gabbro metasomatism in intra-oceanic backarc basin. 3. U–Pb geochronology of the Zhaheba plagiogranites indicates that it is more reasonable to connect the Zhaheba–Armantai– Karamay ophiolites to the Zaysan collision zone.

Acknowledgments We are very grateful to the handing editor, Wenjiao Xiao, and two anonymous reviewers whose polishing work and constructive comments have greatly improved the manuscript. This study was financially supported by the National Natural Science Foundation of China (U1203291, 41173040 and 41121002). We thank

Prof. Zhenhua Zhao from Guangzhou Institute of Geochemistry, CAS, for his constructive suggestions. We sincerely thank the staffs of the Key Laboratory of Isotope Geochronology and Geochemistry at GIGCAS, the Department of Earth Sciences, Nanjing University, and the Institute of Geology and Geophysics, CAS, for the analyses on whole-rock geochemistry, zircon U–Pb dating, electron microprobe mineral geochemistry and Sr–Nd–Hf isotopes. This is contribution No. IS-1966 from GIGCAS. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jseaes.2014.09. 031. References Allègre, C.J., Minster, J.F., 1978. Quantitative models of trace element behavior in magmatic processes. Earth Planet. Sci. Lett. 38, 1–25. Arth, J.G., 1976. Behavior of trace elements during magmatic processes – a summary of theoretical models and their applications. J. Res. US Geol. Surv. 4, 41–47. Berndt, J., Koepke, J., Holtz, F., 2005. An experimental investigation of the influence of water and oxygen fugacity on differentiation of MORB at 200 MPa. J. Petrol. 46, 135–167. Bézos, A., Hunler, E., 2005. The Fe3+/RFe ratios of MORB glasses and their implications for mantle melting. Geochim. Cosmochim. Acta 69, 711–725. Büchl, A., Münker, C., Mezger, K., Hofmann, A.W., 2002. High-precision Nb/Ta and Zr/Hf ratios in global MORB. Geochim. Cosmochim. Acta 66, A108. Campbell, I., 1978. Some problems with the cumulus theory. Lithos 11, 311–323. Campbell, I., 1987. Distribution of orthocumulate textures in the Jimberlana intrusion. J. Geol. 95, 35–53. Cathelineau, M., 1988. Cation site occupancy in chlorites and illites as a function of temperature. Clay Miner. 23, 471–485. Christie, D.M., Carmichael, I.S.E., Langmuir, C.H., 1986. Oxidation states of midocean ridge basalt glasses. Earth Planet. Sci. Lett. 79, 397–411. Coleman, R.G., Peterman, Z.E., 1975. Oceanic plagiogranite. J. Geophys. Res. 80, 1099–1108. Coleman, R.G., Donato, M., 1979. Oceanic Plagiogranite Revisited. Trondhjemites, Dacites, and Related Rocks. Elsevier, Amsterdam, pp. 149–167. Dixon, S., Rutherford, M., 1979. Plagiogranites as late-stage immiscible liquids in ophiolite and mid-ocean ridge suites: an experimental study. Earth Planet. Sci. Lett. 45, 45–60. Dilek, Y., Furnes, H., 2011. Ophiolite genesisi and global tectonics: geochemical and tectonic fingerpringting of ancient oceanic lithosphere. Geol. Soc. Am. Bull. 123, 387–411.

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Please cite this article in press as: Zeng, L., et al. Petrogenesis and tectonic significance of the plagiogranites in the Zhaheba ophiolite, Eastern Junggar orogen, Xinjiang, China. Journal of Asian Earth Sciences (2014), http://dx.doi.org/10.1016/j.jseaes.2014.09.031