Zircon U–Pb geochronology, geochemistry, and Sr–Nd isotopes of the Ural–Alaskan type Tuerkubantao mafic–ultramafic intrusion in southern Altai orogen, China: Petrogenesis and tectonic implications

Journal of Asian Earth Sciences xxx (2015) xxx–xxx

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Zircon U–Pb geochronology, geochemistry, and Sr–Nd isotopes of the Ural–Alaskan type Tuerkubantao mafic–ultramafic intrusion in southern Altai orogen, China: Petrogenesis and tectonic implications Yu-Feng Deng a,b,⇑, Feng Yuan a,b, Taofa Zhou a, Noel C. White a, Dayu Zhang a,b, Xuji Guo c, Ruofei Zhang a, Bingbing Zhao a a

School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China c No. 706 Geological Party, Xinjiang Geoexploration Bureau for Nonferrous Metals, Altai 836500, China b

a r t i c l e

i n f o

Article history: Received 5 October 2014 Received in revised form 1 May 2015 Accepted 8 May 2015 Available online xxxx Keywords: Ural–Alaskan type mafic–ultramafic intrusion Tuerkubantao intrusion Parent magma Active continental margin Sulfide saturation

a b s t r a c t New U–Pb zircon ages and geochemical data including whole rock major and trace element concentrations, PGE and radiogenic isotopes are used to investigate the magma evolution processes and the sulfide saturation history of the Ural–Alaskan type Tuerkubantao mafic–ultramafic intrusion in southern Altai orogen. The Tuerkubantao intrusion consists of dunite, wehrlite, olivine pyroxenite, gabbro and diorite. Igneous zircons from a gabbro in the intrusion yielded a LA-ICP-MS U–Pb age of 370.3 ± 4.8 Ma, indicating that the intrusion was emplaced in the Late Devonian. The intrusive rocks are characterized by enrichment of large ion lithophile elements and depleted high field strength elements relative to N-MORB, which is similar to the Devonian Ural–Alaskan type intrusions in southern Altai orogen and different from Devonian volcanic rocks from ophiolites in West Junggar. The Tuerkubantao intrusive rocks have restricted (87Sr/86Sr)t ratios (0.70396–0.70453) and a large range of eNd(t) (2.84 to +3.80). The trace elements and isotope compositions are comparable with those of the volcanic rocks along the Pacific margins of the Americas. The calculated parental magma of the Tuerkubantao rocks has a high-Mg basaltic composition with 9.12 wt% MgO and 7.02 wt% FeOT. It is proposed that the primary magma was generated from partial melting of metasomatized lithospheric mantle triggered by upwelling of asthenosphere at an active continental margin. The Cu/Pd ratios in gabbros (9.26  105–32.8  105) are obviously higher than those of the wehrlites (1.18  104–1.95  104), indicating that gabbros in the intrusion have experienced sulfide segregation, whereas sulfide saturation did not occur in the wehrlites. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Alaskan/Ural-type mafic–ultramafic intrusions are thought to be formed in both the convergent plate boundary and intracontinental rift environments (Irvine, 1967; Batanova et al., 2005; Burg et al., 2009). Though these intrusions are referred to as concentrically zoned mafic–ultramafic complexes (Taylor, 1967), in most examples the zones are not well defined and are often discontinuous and asymmetric. They are restricted to late-Precambrian and Phanerozoic orogenic belts, such as those in the Cordillera of North and South America (Taylor, 1967; Irvine, 1974; Tistl et al., 1994; Pettigrew and Hattori, 2006), Urals and Koryak (Fershtater et al., 1997; Batanova et al., 2005), New South Wales (Johan, 2002), Southland of New Zealand (Spandler et al., 2000, 2003), ⇑ Corresponding author at: School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China. Tel./fax: +86 0551 62901525. E-mail address: [email protected] (Y.-F. Deng).

the Eastern Desert of Egypt (Helmy and El Mahallawi, 2003), and along the East African orogenic zone in western Ethiopia (Grenne et al., 2003). They have also been found in China in the last few decades (Chen et al., 2009; Tseng et al., 2009; Su et al., 2012; Deng et al., 2015). Although it has been proposed that these intrusions were derived from the crystallization of hydrous mafic and ultramafic magmas (Irvine, 1974; Himmelberg and Loney, 1995; Helmy and El Mahallawi, 2003; Batanova et al., 2005), the composition and crystallization history of these magmas remain controversial. Murray (1972) and Irvine (1973) regarded the Ural–Alaskan type intrusions as magma storage reservoirs for subduction zone volcanos. Irvine (1973, 1974) and Thakurta et al. (2008) argued that they resulted from fractionation of ankaramitic magma. A continuum in composition of primary magma types from picrite to ankaramite has been proposed for arc settings by Eggins (1993) and Green et al. (2004). Batanova et al. (2005) suggested a more primary melt composition (24 wt% MgO) for the Ural–Alaskan type

http://dx.doi.org/10.1016/j.jseaes.2015.05.007 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

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Y.-F. Deng et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

intrusions in Russia. Irvine (1974) explained the lithological zoning as produced by fractional crystallization and subsequent diapiric updoming of unconsolidated crystal layers with expulsion of the evolved magma. Taloy and Noble (1960) and Batanova et al. (2005) proposed the lithological zoning resulted from multiple intrusions of magmas that utilized the same conduit. The Ural– Alaskan type intrusions are known as low-sulfide systems where PGE (platinum group element) enrichment may occur mainly as platinum-iron alloy. The economic mineralization can be found in spatially associated placer deposits, including the Koryak-Kamchatka platinum belt of Russia (Tolstykh et al., 2004), the Fifield district in New South Wales, Australia (Slansky et al., 1991), Union Bay (Freeman, 2001) and Salt Chuck (Freeman, 2008) in southeastern Alaska. The studies of Cu–Ni–PGE sulfide mineralization in the Ural–Alaskan type intrusions suggested that they have a potential to host world-class sulfide-rich Cu–Ni–PGE deposits (Nixon, 1998; Helmy and Mogessie, 2001; Naldrett,

2004; Pettigrew and Hattori, 2006; Thakurta et al., 2008, 2014; Ripley, 2009). The southern part of Altai orogen is situated in the western Central Asian orogenic belt (CAOB) (Fig. 1a). Three phases of mafic–ultramafic activities (at ca. 400–370 Ma, ca. 320 Ma, and ca. 280 Ma) have been recorded (Wang et al., 2003; Niu et al., 2006; Wang et al., 2012; Zhang et al., 2012; Ye et al., 2014). Because of its Cu–Ni sulfide mineralized potential, some exploration work had been carried out at the Tuerkubantao mafic–ultramafic intrusion (Zhao, 2012; Wang et al., 2014a). The zircon U–Pb ages of the gabbro and gneissic granite are 363 and 355 Ma, respectively (Wang et al., 2012), whereas LA-ICP-MS U–Pb zircon ages indicate that the gabbro was emplaced at 394.6 ± 4.9 Ma (Guo, 2009). The intrusion is thought to be an Ural–Alaskan type intrusion above a subduction zone or part of an ophiolitic mélange in a MORB-like tectonic setting (Wang et al., 2012; Deng et al., 2015).

Fig. 1. (a) Schematic geological map of the Central Asian orogenic belt (after Jahn et al., 2000; Xiao et al., 2009); (b) simplified geological map of northern Xinjiang (after BGMX, 1993; Song et al., 2011, 2013; Deng et al., 2014); (c) simplified geological map of the Chinese Altai and adjacent areas (after Windley et al., 2002; Song and Li, 2009; Wong et al., 2010).

Please cite this article in press as: Deng, Y.-F., et al. Zircon U–Pb geochronology, geochemistry, and Sr–Nd isotopes of the Ural–Alaskan type Tuerkubantao mafic–ultramafic intrusion in southern Altai orogen, China: Petrogenesis and tectonic implications. Journal of Asian Earth Sciences (2015), http:// dx.doi.org/10.1016/j.jseaes.2015.05.007

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Deng et al. (2015) propose that the primary magma of the intrusion was derived from partial melting of previously metasomatized mantle, but there are few precise geochemical data to constrain the generation and evolution of the magma. In this paper we integrate new U–Pb zircon ages and geochemical data including whole rock major and trace element concentrations, PGE and radiogenic isotopes to investigate the magma evolution and sulfide saturation history of the Tuerkubantao intrusion.

2. Geological background The Altai orogen is located in the western CAOB, which is the world largest accretionary orogen and has been expensively studied during the last 20 years (e.g., Zonenshain et al., 1990; Buslov et al., 2001; Kröner et al., 2007, 2014; Kovalenko et al., 2004; Xiao et al., 2003, 2010; Windley et al., 2007; Polyakov et al., 2008; Izokh et al., 2010, 2011; Safonova et al., 2011; Yarmolyuk et al., 2012). It is situated between the Gorny Altai in Russia to the north and the Western Junggar to the south (Fig. 1b). It extends eastward to the Gobi Altai in southwestern Mongolia and westward to Rudny Altai in East Kazakhstan (Windley et al., 2002, 2007; Xiao et al., 2004, 2009; Buslov et al., 2004; Zhang et al., 2012), and is made up of a long-lived Paleozoic arc with minimal ancient crustal reworking, as revealed by recent studies on metamorphosed granites and sedimentary rocks (Windley et al., 2002, 2007; Chen and Jahn, 2002; Long et al., 2007; Sun et al., 2008; Wang et al., 2009). The Altai orogen consists of several fault-bounded terranes (Windley et al., 2002, 2007; Wong et al., 2010; Fig. 1c). The Altaishan Terrane is characterized by widespread Late Devonian-Early Carboniferous clastic sediments, limestones and some island arc-related volcanic rocks (Zhuang, 1993; Xiao et al., 2010). The northwest Altaishan Terrane consists of a Middle Ordovician turbidite sequence overlain by Late Ordovician volcanoclastic sediments (He et al., 1990; Long et al., 2007). The central Altaishan Terrane is the largest terrane and is made up of Cambrian-Devonian arc-related magmatic rocks. Devonian Ural–Alaskan type mafic–ultramafic intrusions have been identified in the terrane (Ye et al., 2014). The ages of Xenocrystic zircon from the rhyodacite and tonalitic gneiss and Nd crustal residence ages of amphibolites suggest that the central Altaishan terrane likely represents a Cambro-Devonian active continental margin with the Neoproterozoic basement (Hu et al., 2000; Windley et al., 2002, 2007; Wang et al., 2006; Sun et al., 2008; Xiao et al., 2009). The Qiongkuer-Abagong Terrane consists of Late Silurian-Early Devonian volcaniclastic rocks overlain by Middle Devonian turbiditic sanstone, pillow basalts and some siliceous volcanic rocks. The ca. 372 Ma Kuerti ophiolite and bimodal volcanic rocks are interpreted as remnants of an active arc-back basin system (Xu et al., 2001, 2003; Zhang et al., 2003). The geochemical studies of the volcanic rocks near the Devonian VMS deposits indicate that they formed by subduction-related processes (Wan et al., 2010, 2011). Neoproterozoic ages and subchondritic eHf of zircon xenocrysts from Cambrian-Ordovician granitoids suggest ancient crustal recycling, but positive eHf values of zircons from Devonian granites imply juvenile mantle input (Sun et al., 2008). The Carboniferous and Permian volcanic rocks and granites were not well-developed (Zhang et al., 2000; Briggs et al., 2007; Yuan et al., 2007). The Erqis Terrane, bounded to the south by the Irtysh fault, consists of Devonian to Late Carboniferous arc-related volcaniclastic rocks and intrusions metamorphosed into high-grade gneisses and contaminated by Paleoproterozoic material (Qu and Chong, 1991).

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The Shaerbulake Terrane is to the southwest of the Irtysh fault is composed of volcaniclastic rocks with Devonian-Carboniferous fossils and Permian continental volcanic rocks and sedimentary rocks (Xiao et al., 1992). Juxtaposition of boninite, adakite, high-TiO2 and low-TiO2 basalts may have been caused by a sequence of rifting and convergence in the Devonian paleo-Asian Ocean (Niu, 2005). Zircon U–Pb dating of a basalt sample gives Permian age and inherited Ordovician and Neoproterozoic zircons. This suggests that the Shaerbulake terrane belongs to an active margin built on an old micro-continent but not the young intra-oceanic subduction systems (Wong et al., 2010). The Permian Kalatongke mafic intrusions hosting the Cu–Ni sulfide deposits were located in the east of the terrane and interpreted to be formed in the post-collision setting (Song and Li, 2009; Zhang et al., 2009). The Tuerkubantao mafic–ultramafic intrusion is situated in the east of the Kekesentao mafic–ultramafic intrusion area (Fig. 1; Wang et al., 2012; Deng et al., 2015). 3. Petrography of the Tuerkubantao intrusion The Tuerkubantao mafic–ultramafic intrusion was emplaced into Middle Devonian argillaceous slate, dacite, tuff, sandstone and limestone lenses (Fig. 2a, Guo, 2009). It has a long axis of 6 km and maximum surface width of 0.5 km. Dunite, wehrlite, olivine pyroxenite, gabbro and diorite are major rock in the intrusion (Zhao, 2012). The ultramafic rocks mainly occur in the central part of the intrusion surrounded by gabbros with sharp contacts (Fig. 2b and c) and show intrusive contact with the country rocks (Fig. 2d). The dunite contains 90–95% olivine, 0–5% clinopyroxene and minor chromite (Zhao, 2012). The wehrlite contains 60–75% olivine, 5–20% clinopyroxene, 5–10% hornblende and minor biotite (Fig. 3a and b). Chromite is present as small inclusions in olivine. The olivine pyroxenite contains 45–70% pyroxene, 15–30% olivine and 5–15% hornblende (Guo, 2009). The gabbro unit is composed of plagioclase (60–65%), clinopyroxene (20–30%), hornblende (5– 10%) and minor sulfide (0–5%) (Fig. 3c). Sulfides are commonly interstitial to the clinopyroxene and plagioclase, also enclosing plagioclase crystals (Fig. 3c). The Cu–Ni sulfide mineralization is predominantly found in gabbro and only minor sulfides occur in wehrlite. Malachite and limonite can be found in the oxidation zone of the Cu–Ni sulfide mineralization (Guo, 2009; Zhao, 2012). 4. Analytical methods 4.1. Zircon U–Pb geochronology The zircons for LA-ICP-MS U–Pb dating were processed by conventional magnetic and heavy liquid separation methods, and then handpicked out using a binocular microscope to select zircon grains for analysis. Approximately 30 kg of gabbro (TE-08), collected at 47° 300 18.200 N and 86° 520 01.400 E was crushed for zircon crystal separation. Sample preparation was carried out by the laboratory of the Regional Geological Mineral Survey Institute of Hebei Province, North China and recovered 18 zircon crystals from the gabbro. The zircon grains were mounted on an epoxy resin disk and polished. Internal structures of the zircon grains were examined by cathodoluminescence (CL) before U–Pb isotopic analyses. The euhedral–subhedral zircons with striped absorption and oscillatory zoning rims in CL images were selected for U–Pb LA-ICP MS dating. LA-ICP MS analysis was performed using an Agilent 7500a quadrupole mass spectrometer and GeoLasPro laser ablation system at the School of Resource and Environmental Engineering, Hefei University of Technology. The laser ablation system used a ComPex102ArF excimer laser (wave length 193 nm) produced by

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Fig. 2. (a) Simplified geological map of the Tuerkubantao intrusion (after Deng et al., 2015); (b) outcropping contact of the wehrlite, gabbro, and country rocks; (c) field photo showing the gabbro crosscuts the wehrlite; (d) the intrusive contact relationships of the wehrlite and country rock.

Coherent Inc of the USA. The standard zircon 91500 (Wiedenbeck et al., 1995) was used to correct for inter-element fractionation, and U, Th and Pb concentrations were determined based on standard Mud Tank (732 Ma) zircons (Black and Gulson, 1978). Data processing was carried out using the Isoplot/Ex 2.49 programs of Ludwig (2001a,b). 4.2. Major and trace elements The samples analyzed in this study are from the southwestern part of the intrusion (Fig. 2a). Samples were cut with a diamond-impregnated brass blade, crushed in a steel jaw crusher that was brushed and cleaned with de-ionized water between samples and pulverized in an agate mortar in order to minimize potential contamination. Eight samples were analyzed for major element oxides. They were measured using a PANalytical Axios X-ray fluorescence spectrometer (XRF) on fused glass beads at the ALS Chemex (Guangzhou) Co. Ltd. with an analytical uncertainty less than 5%. 0.7 g powder from samples was mixed

completely with Li2B4O7–LiBO2 flux and then fused to a glass bead at 1050–1100 °C in an automatic melting instrument. Selected trace elements were determined using a Perkin–Elmer Sciex ELAN DRC-e inductively coupled plasma mass spectrometer (ICP-MS) at the Institute of Geochemistry, Chinese Academy of Sciences, with analytical uncertainty better than 10%. Samples were digested with 1 ml of HF and 0.5 ml of HNO3 in screw top PTFE-lined stainless steel bombs at 190 °C for 12 h. The analytical precision is generally better than 1% for elements with concentrations >200 ppm, and 1–3% when less than 200 ppm. The procedure for the trace elements is described in detail by Qi et al. (2000). 4.3. PGE PGE were determined by isotope dilution (ID)-ICP-MS using an improved Carius tube technique (Qi et al., 2007) in the Institute of Geochemistry, Chinese Academy of Sciences, Guiyang. Five grams of powdered sample was digested with 40 ml aqua regia in a 75 ml Carius tube placed in a sealed, custom-made,

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Sciences. Rb–Sr and Sm–Nd isotopes were determined using MC-ICP-MS: the procedural blanks were 10 pg for Sm and Nd, and 20 pg for Rb and Sr. The measured values for the LaJolla Nd standard and the SRM987 Sr standard were 143 Nd/144Nd = 0.511861 ± 10 and 87Sr/86Sr = 0.710263 ± 10 during the period of data acquisition. Lead isotopic analyses were performed on an IsoProbeT thermal ionization magnetic sectormass spectrometer (TIMS) at the Beijing Research Institute of Uranium Geology, China. The 204Pb/206Pb, 207Pb/206Pb and 208Pb/206Pb ratios of the NBS981 Pb isotopic standard determined during this study were 0.059003 ± 84 (1r), 0.91439 ± 17 (1r) and 2.16441 ± 97 (1r), respectively. The measured isotope ratios were corrected to 370.3 Ma based on U, Th and Pb contents determined by ICP-MS.

5. Analytical results 5.1. Zircon U–Pb age The results of LA-ICP-MS U–Pb zircon analyses are listed in Table 1 and the U–Pb concordia diagrams are shown in Fig. 4. Analyses of the 18 individual zircons yield a mean 206Pb/238U age of 370.3 ± 4.8 Ma with a mean square of weighted deviations (MSWD) of 0.68 (Fig. 4). The zircon U–Pb age indicates that the gabbro was formed in the Late Devonian.

5.2. Major and trace element geochemistry

Fig. 3. Photomicrographs in cross-polarized light showing typical textures of the rocks from the Tuerkubantao intrusion. (a) Olivine poikilitic texture in wehrlite; (b) biotite with third-order interference color in wehrlite; (c) plagioclase crystals enclosed in the interstitial sulfide in gabbro. Ol – olivine; Cpx – clinopyroxene; Pl – plagioclase; Hb – hornblende; Bi – biotite; Sul – sulfide. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

high-pressure, water-filled autoclave. The solution was used to collect PGE by Te-coprecipitation (Qi et al., 2004). The measured results of PGE for the reference standards TDB-1 and WGB-1 agree well with recommended values reported by Qi et al. (2004). Analytical precision and accuracy are generally better than 10%. PGEs of the blank are generally less than 1 ppb: Ir < 0.05 ppb, Ru < 0.05 ppb, Rh < 0.05 ppb, Pt < 0.05 ppb, and Pd < 0.5 ppb.

4.4. Sr–Nd–Pb isotopes Whole-rock Sr–Nd isotope analyses were carried out at Guangzhou Institute of Geochemistry, Chinese Academy of

The results of chemical analyses are presented in Table 2. In the plot and discussion that follow, all major oxide contents have been recalculated to 100% on an anhydrous basis. The wehrlites contain the highest MgO (38.8–43.0 wt%), (Fe2O3)T (8.63–11.9 wt%), and the lowest SiO2 (43.0–44.6 wt%), CaO (0.06–2.82 wt%) (Table 2, Fig. 5). The olivine pyroxenite has lower MgO (27.9–34.4 wt%) content and higher SiO2 (48.2–51.2 wt%), CaO (3.43–7.68 wt%) than the wehrlites. The gabbros have lowest MgO (5.49–11.2 wt%) content and highest SiO2 (50.5–52.2 wt%), Al2O3 (16.5–24.4 wt%) and CaO (11.6–13.4 wt%). Trace element spider diagrams normalized to normal mid-ocean ridge basalts (N-MORB) show that the Tuerkubantao intrusive rocks are enriched in large ion lithophile elements (LILE) and depleted in high field-strength elements (HFSE; Nb, Ta, Ti, Zr, and Hf) (Fig. 6a), which is similar to those of the Devonian Ural–Alaskan type intrusions in southern Altai orogen. Although the Devonian basalts and gabbros from ophiolites in West Junggar and southern Altai orogen are enriched in LILE, they contain higher HFSE contents than those of the Tuerkubantao intrusion (Fig. 6a). The wehrlite has the lowest contents of trace elements, whereas the gabbro has the highest trace elements contents (Fig. 6a). The trace element contents of the olivine pyroxenite are higher than those of wehrlite but lower than those of gabbro. The samples have variable rare-earth element (REE) contents, but most of them are enriched in LREE relative to HREE (Fig. 6b). The (La/Yb)N ratios range from 0.69 to 4.50, but most samples are between 1.36 and 3.64 (Fig. 6b), which is similar with those of the Devonian Ural–Alaskan type intrusions in southern Altai orogen. In contrast, the fractionation between LREE and HREE in the Devonian basalts and gabbros from ophiolites in West Junggar and southern Altai orogen is weaker. The REE contents of the Tuerkubantao rocks are obviously lower than those of the Devonian basalts and gabbros from ophiolites in West Junggar and southern Altai orogen (Fig. 6b). Because Pr, Nd, Sm and Eu are compatible in hornblende (Yang et al., 2000a,b), some olivine pyroxenites are enriched in those elements suggesting there was abundant hornblende in those samples.

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10.6 10.1 10.3 10.0 10.6 10.8 10.4 11.5 10.6 9.76 10.1 9.74 10.7 11.1 10.6 9.89 11.0 10.3 369 361 369 355 372 369 373 376 384 356 370 365 373 381 381 366 386 370 12.3 14.2 13.3 13.0 14.1 17.2 12.5 18.4 15.4 12.8 13.7 13.1 14.8 17.8 13.8 13.9 19.2 15.0 365 364 364 359 365 379 364 368 395 367 380 375 387 369 393 381 406 394 94.4 110 106 106 117 97.2 91.7 144 97.2 92.6 94.4 125 100.0 134 90.7 94.4 131 100.0 0.4332 0.4310 0.4310 0.4245 0.4322 0.4524 0.4315 0.4373 0.4755 0.4347 0.4534 0.4467 0.4637 0.4387 0.4730 0.4559 0.4918 0.4741

0.0174 0.0201 0.0188 0.0182 0.0200 0.0246 0.0176 0.0260 0.0223 0.0181 0.0196 0.0186 0.0214 0.0252 0.0200 0.0199 0.0282 0.0218

0.0589 0.0576 0.0589 0.0566 0.0594 0.0589 0.0596 0.0600 0.0613 0.0569 0.0591 0.0583 0.0595 0.0609 0.0609 0.0585 0.0616 0.0591

0.0017 0.0017 0.0017 0.0016 0.0017 0.0018 0.0017 0.0019 0.0018 0.0016 0.0017 0.0016 0.0018 0.0018 0.0018 0.0016 0.0018 0.0017

328 413 283 328 276 467 257 339 461 435 456 456 480 346 487 483 524 520

207

±1r Pb/206Pb 207 207

The Sr, Nd and Pb isotope data for the Tuerkubantao intrusion are calculated to an initial age of 370.3 Ma (Table 3). All rocks have relatively restricted initial (87Sr/86Sr)t (0.70396–0.70453) and large ranges of initial (143Nd/144Nd)t ratios (0.512016–0.512315) (Table 3, Fig. 7a) (Guo, 2009). The wehrlites have lower eNd(t) ratios than those of the gabbros. The eNd(t) ratios (2.84 to +3.80) of the Tuerkubantao rocks are lower than those of the Devonian volcanic rocks from ophiolites in West Junggar and plot in the field of the igneous rocks from the Pacific margins of the Americas. Compared with the Devonian Ural–Alaskan type intrusions of the southern Altai orogen, the Tuerkubantao intrusion has similar eNd(t) but more restricted (87Sr/86Sr)t ratios (Ye et al., 2014). The Tuerkubantao intrusive rocks have restricted and low (206Pb/204Pb)t (17.69–17.95), (207Pb/204Pb)t (15.47–15.52) and (208Pb/204Pb)t (37.56–37.93) values (Table 3), which plot above the northern hemisphere reference line (NHRL, Zindler and Hart, 1986) and most plot in the field of the volcanic rocks from the Pacific margins of the Americas (Fig. 7b and c). 5.4. Platinum-group elements The concentrations of PGEs in the mafic–ultramafic rocks are listed in Table 4. The content of RPGE in the Tuerkubantao mafic–ultramafic rocks ranges from 0.15 to 26.1 ppb. The wehrlite has higher PGE concentrations than the gabbro as shown by the primitive mantle-normalized diagrams (Table 4; Fig. 8a). Such features indicate that these metals are controlled by the content of chromite, olivine and pyroxene. All rocks are characterized by strong depletion in Ir, Os and Ru (IPGEs), and relative enrichment in Pt, Pd, and Cu (Fig. 8a). The Cu/Pd ratio in gabbros (9.26  105–32.8  105) is obviously higher than those of the wehrlites (1.18  104–1.95  104). On the Pd/Ir vs. Ni/Cu diagram, most samples plot in the field of high-Mg basalts and layered intrusions (Fig. 8b). 6. Discussion

0.0022 0.0026 0.0024 0.0024 0.0026 0.0031 0.0021 0.0034 0.0026 0.0023 0.0024 0.0023 0.0025 0.0031 0.0023 0.0024 0.0033 0.0026 0.0530 0.0550 0.0519 0.0530 0.0518 0.0548 0.0514 0.0532 0.0562 0.0556 0.0559 0.0558 0.0567 0.0534 0.0569 0.0568 0.0579 0.0578 0.99 0.63 0.62 0.35 0.58 0.35 0.74 0.36 0.85 0.93 0.74 0.85 0.38 0.44 1.05 0.81 0.39 0.90 653 259 417 558 310 179 595 117 408 710 601 931 259 123 889 667 155 443

Th/U U Th

647 162 257 196 180 62.3 442 42.2 346 661 443 790 98.2 54.2 932 539 61.1 399 60.7 20.33 34.1 40.2 25.2 13.27 51.3 8.69 37.3 61.5 49.2 77.5 19.04 9.20 82.7 55.1 12.05 40.5 TE-08-01 TE-08-02 TE-08-03 TE-08-04 TE-08-05 TE-08-06 TE-08-07 TE-08-08 TE-08-09 TE-08-10 TE-08-11 TE-08-12 TE-08-13 TE-08-14 TE-08-15 TE-08-16 TE-08-17 TE-08-18

Pb

Pb/206Pb

Isotopic ratios

207

Concentration (ppm)

±1r

6.1. Magma fractionation and parental magma compositions

Spot

Table 1 Results of LA-ICP-MS U–Pb dating of zircons from the gabbro in the Tuerkubantao intrusion.

Pb/235U

±1r

206

Pb/238U

±1r

Age (Ma)

Pb/235U

±1r

206

Pb/238U

±1r

5.3. Sr, Nd and Pb isotopes

The Tuerkubantao intrusion consists of multiple rocks with composition varying from ultramafic to intermediate. Sharp and lithologically reversed contacts between mafic and ultramafic rocks indicate that the intrusion was formed by multiple injections of magma. Increase of SiO2, Al2O3 and CaO and incompatible elements from wehrlite toward gabbro indicates a fractionation trend. Gradual increase of the trace element contents from olivine pyroxenite to wehrlite imply that they formed from the same parent magma. In addition, the trace element and PGE patterns of different types of rocks in the intrusion have sub-parallel trends (Figs. 6 and 8a), suggesting that the different rock types are related to each other by differentiation of the same primary magma. The wehrlites in the Tuerkubantao intrusion define a trend toward olivine (Fig. 8c), suggesting that olivine is the dominant cumulate mineral. As no chilled margins have been identified, we use the method proposed by Chai and Naldrett (1992) to calculate the composition of parental magma. The percentages of forsterite in olivine (87.6–88.5; Deng et al., 2015) indicate that Mg# (atomic Mg/(Fe + Mg)) of the silicate magma was less than 0.70, estimated by using a molar Mg–Fe distribution constant (Kd = (Fe/Mg)Ol/(Fe/Mg)magma) of 0.3 ± 0.03 (Roeder and Emslie, 1970). The intersection point extrapolated of the MgO/FeOT regression line at MgO/(MgO + FeO) = 0.7 and the trend from the olivine with the highest forsterite percentage (88.5) to average

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Fig. 4. CL images and Concordia plot of LA-ICP-MS U–Pb analyses of zircons from the gabbro in the Tuerkubantao intrusion.

Table 2 Major oxides and trace elements abundances of the Tuerkubantao intrusion. Rock type

Gabbro

Sample

TE-08

Wehrlite TE-13

TE-16

TEP1-01

TE-05

TE-10

TEP1-05

TEP1-07

Major element (wt.%) SiO2 50.0 TiO2 0.16 Al2O3 15.9 (Fe2O3)T 4.75 MnO 0.12 MgO 10.8 CaO 12.3 Na2O 1.49 K2O 0.85 P2O5 0.005 Cr2O3 0.05 LOI 3.09 Total 99.6

48.4 0.16 23.3 2.93 0.06 5.25 12.6 2.38 0.41 0.01 0.07 3.92 99.6

50.2 0.39 17.7 5.31 0.11 8.72 11.2 2.27 0.28 0.032 0.06 2.87 99.1

49.2 0.16 22.8 3.12 0.06 6.67 13.0 2.05 0.15 0.01 0.14 2.01 99.5

38.4 0.04 2.67 7.44 0.08 37.0 0.05 0.06 0.04 0.006 0.33 13.05 99.2

37.5 0.05 1.89 10.3 0.09 36.1 0.08 0.06 0 0.006 0.34 12.5 99.0

38.6 0.10 3.9 8.92 0.14 34.8 2.52 0.15 0.05 0.014 0.37 10.1 99.6

38.6 0.11 4.14 9.06 0.13 34.9 2.01 0.17 0.11 0.012 0.38 9.93 99.5

Trace element (ppm) Sc V Cr Co Ni Cu Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U Pb

16.0 72.5 432 16.2 68.1 173 12.5 367 5.23 22.5 1.27 73.2 2.51 6.18 0.69 2.81 0.68 0.36 0.71 0.15 0.79 0.19 0.55 0.08 0.57 0.07 0.54 0.11 1.07 0.35 1.49

30.0 124 377 28.7 94.7 35.3 5.98 243 10.1 25.2 1.91 43.5 3.81 9.55 1.14 4.72 1.21 0.46 1.39 0.26 1.56 0.35 0.95 0.14 0.95 0.15 0.88 0.17 1.38 0.32 1.29

18.3 75.3 816 18.9 108 8.38 3.69 258 5.13 9.9 0.53 53.1 2.32 5.27 0.59 2.51 0.66 0.39 0.79 0.12 0.87 0.15 0.42 0.08 0.51 0.07 0.28 0.07 0.3 0.11 1.26

7.44 33.9 1823 92.5 1390 26.5 1.75 6.37 1.20 3.15 0.27 10.4 0.56 1.21 0.13 0.63 0.13 0.16 0.16 0.03 0.14 0.04 0.12 0.02 0.14 0.03 0.08 0.09 0.08 0.02 0.15

7.53 33.7 1746 106 1330 67.3 0.28 8.12 1.19 3.03 0.20 4.50 0.31 0.85 0.11 0.51 0.14 0.06 0.19 0.02 0.16 0.05 0.13 0.02 0.13 0.02 0.09 0.07 0.08 0.05 0

9.95 49.1 1922 92.3 1290 56.6 1.5 16.2 2.78 7.64 0.36 1.40 0.74 1.99 0.23 1.00 0.32 0.12 0.38 0.07 0.42 0.12 0.32 0.04 0.28 0.04 0.24 0.08 0.16 0.06 0.15

9.26 48.5 1922 94.9 1330 50.9 3.49 13.1 2.92 8.38 0.39 5.49 0.74 2.00 0.26 1.32 0.44 0.16 0.43 0.1 0.45 0.12 0.34 0.04 0.31 0.05 0.16 0.07 0.15 0.04 0.25

30.8 131 295 30.6 137 61.0 30.8 200 5.50 5.06 0.36 129 1.08 3.00 0.35 1.95 0.64 0.32 0.9 0.15 0.86 0.18 0.53 0.07 0.57 0.08 0.19 0.07 0.17 0.09 0.20

Note. LOI = loss on ignition, (Fe2O3)T as total Fe.

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Fig. 5. Binary diagrams of oxides vs. MgO for the Tuerkubantao rocks. Some data of the wehrlite and gabbro are from Guo (2009) and Deng et al. (2015). The data for olivine pyroxenite are from Zhao (2012).

Fig. 6. (a) N-MORB normalized trace element spider diagrams and (b) C1 chondrite-normalized REE patterns of the Tuerkubantao intrusion. The trace elements distribution patterns of the Tuerkubantao rocks are similar to the Devonian Ural–Alaskan type intrusions in southern Altai orogen (in gray; Ye et al., 2014), and different from the Devonian gabbros and basalts from ophiolites in West Junggar (in yellow; Yang et al., 2000a,b; Lei et al., 2008) and southern Altai orogen (in blue; Xu et al., 2001). Some data of the wehrlite and gabbro are from Guo (2009) and Deng et al. (2015). The data for olivine pyroxenite are from Zhao (2012). The data for N-MORB, OIB, and C1 chondrite are from Pearce (1982) and Sun and McDonough (1989), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

composition of the wehrlite indicates that the parent magma contains 9.12 wt% MgO and 7.02 wt% FeOT (Fig. 8c and d). However, the inferred MgO content is a minimum estimation, because the olivine composition may have been enriched in Fe by exchange with trapped liquid (Barnes, 1986; Li et al., 2004). The estimated Mg# and MgO content imply that the parent magma could be high-Mg basaltic in composition, which is thought to be the parental magma of the subduction-related picrites in Southern Altai orogen (Zhang et al., 2008). This is consistent with the result inferred from the Pd/Ir vs. Ni/Cu plot (Fig. 8b).

6.2. Tectonic implications High LOI for samples (2.01–13.1 wt%) in the Tuerkubantao intrusion imply that they were probably affected by alteration.

Alteration mainly affects the abundance of mobile elements. However, HFSEs in mafic rocks are relatively immobile under alteration conditions and can be used to characterize the mafic–ultramafic rocks with respect to their original composition and tectonic environment (Pearce and Cann, 1973; Winchester and Floyd, 1977; Frey et al., 2002). The sub-parallel trace elements patterns of the Tuerkubantao rocks likely represent original trends (Fig. 6). Our new zircon LA-ICP-MS U–Pb ages (370.3 ± 4.8; Fig. 4) indicate that the Tuerkubantao intrusion was formed in the Late Devonian. However, the tectonic setting of the Devonian mafic–ultramafic intrusions in southern Altai orogen remains debated. Wang et al. (2012) proposed that the Tuerkubantao intrusion formed in a MORB-like tectonic setting on the basis of the geochemical characteristics of mafic–ultramafic intrusive rocks and

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Y.-F. Deng et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

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Table 3 Sr, Nd and Pb isotopes for the rocks of the Tuerkubantao intrusion. Rock type

Wehrlite

Gabbro

Sample

TEK-06

TEK-07

TEK-11

TEK-08

TEK-09

Rb (ppm) Sr (ppm) 87 Rb/86Sr 87 Sr/86Sr 2r (87Sr/86Sr)t Sm (ppm) Nd (ppm) 147 Sm/144Nd 143 Nd/144Nd 2r (143Nd/144Nd)t eNd(t) Th (ppm) U (ppm) Pb (ppm) 208 Pb/204Pb 2r 207 Pb/204Pb 2r 206 Pb/204Pb 2r (206Pb/204Pb)t (207Pb/204Pb)t (208Pb/204Pb)t

1.64 23.7 0.200 0.705588 0.000005 0.704534 0.26 1.01 0.153 0.512541 0.000014 0.51 0.17 0.24 0.12 8.77 37.97 0.007 15.50 0.003 17.90 0.003 17.85 15.49 37.93

5.36 77.3 0.201 0.705289 0.000007 0.704232 0.20 0.87 0.136 0.512523 0.000014 0.51 0.63

7.74 169 0.132 0.704662 0.000007 0.703964 0.27 0.98 0.164 0.512414 0.000009 0.51 2.84 0.21 0.14 4.61 37.89 0.009 15.47 0.003 17.80 0.004 17.69 15.47 37.83

9.21 204 0.131 0.705145 0.000006 0.704457 0.99 4.13 0.145 0.512537 0.000009 0.51 0.45 0.46 0.15 3.65 38.01 0.004 15.51 0.002 18.10 0.002 17.95 15.50 37.85

0.80 0.11 1.91 38.07 0.007 15.53 0.003 18.14 0.003 17.93 15.52 37.56

basalt. In contrast, Deng et al. (2015) regarded the Tuerkubantao intrusion as the Ural–Alaskan type intrusion in an arc-related setting. The Devonian picrites in the southern Altai orogen have formed in an island arc, whose primary magma was derived from depleted mantle modified by the addition of a fluid component (Zhang et al., 2008). The precursor magma of the Devonian Keketuohai mafic–ultramafic intrusion was produced by a lithospheric mantle heated by upwelling asthenosphere through a slab window during ridge subduction (Cai et al., 2012b). The samples from the Tuerkubantao intrusion are characterized by enrichment of Rb, Ba, U, and Sr, and depletion of Nb, Ta, and Ti relative to N-MORB. The trace element distribution patterns are similar to those of the Devonian Alskan-type intrusions, but distinguishable from those of the Devonian volcanic rocks from ophiolites in West Junggar and southern Altai orogen (Yang et al., 2000a,b; Xu et al., 2001; Ye et al., 2014). As shown in Fig. 7a, the eNd(t) ratios of the Tuerkubantao intrusive rocks are lower than those of the Devonian volcanic rocks from ophiolites in West Junggar, indicating that the intrusion is not a fragment of an ophiolite. In recent years, low-Ca olivine crystals (Ca < 1000 ppm) have been reported in subduction-related environments (Rohrbach et al., 2005; Kamenetsky et al., 2006; Li et al., 2012). Many workers have interpreted them as mantle xenocrysts (Ramsay et al., 1984; Schuth et al., 2004), while Kamenetsky et al. (2006) argued that those in subduction-related magmas are magmatic phenocrysts and reflect the reduced calcium and high silica content of the melt. The Ca content in olivine crystals from the Tuerkubantao intrusion is less than 1000 ppm, similar to the Ural–Alaskan type mafic–ultramafic intrusions and significantly lower than those of continental flood basalts (CFB), MORB and OIB (Fig. 9a). Furthermore, the geochemical characteristics of the Tuerkubantao rocks indicated that the primary magma resulted from partial melting of metasomatized mantle (Deng et al., 2015). Thus we proposed that the Tuerkubantao intrusion was formed in a subduction environment. Occurrences of Paleozoic ophiolites and Ural–Alaskan type intrusions along the Irtysh fault and thick Devonian and Carboniferous granites, calc-alkaline volcanic rocks and mafic–ultramafic intrusions in southern Altai orogen were thought to be evidence of

Fig. 7. Plot of (a) eNd(t) vs. (87Sr/86Sr)t, (b) (207Pb/204Pb)t vs. (206Pb/204Pb)t and (c) (208Pb/204Pb)t vs. (206Pb/204Pb)t for the Tuerkubantao rocks. The eNd(t), (87Sr/86Sr)t and the initial Pb isotope values are normalized to 370.3 Ma. Some data of the Tuerkubantao intrusion are from Guo (2009). Also shown are fields for Devonian volcanic rocks from ophiolites in West Junggar (Yang et al., 2000a,b; Lei et al., 2008). The data of the Ordovician-Devonian sedimentary rocks and granites of the Altai orogen are from Chen and Jahn (2002), Long et al. (2012), Wang et al. (2014b). NHRL is Northern Hemisphere Reference Line. The fields of MORB, UCC (upper continental crust) and LCC (lower continental crust) are after Zindler and Hart (1986), Rudnick and Gao (2003) and Yang et al. (2000a,b), respectively. The data of the Pacific margins of the Americas (including western California and southern ChileArgentina) are from James (1982), Wilson (1989), Davidson et al. (1990), Cole and Basu (1995), Stern and Kilian (1996), Miller et al. (2000), D’Orazio et al. (2001), and Gorring et al. (2003).

North-dipping subduction in the Junggar ocean during the Devonian to Carboniferous (Windley et al., 2002; Chen et al., 2006; Niu et al., 2006; Yuan et al., 2007; Zhang et al., 2008,

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Y.-F. Deng et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

Table 4 Concentrations of PGE of the Tuerkubantao intrusive rocks. Rock type

Gabbro

Wehrlite

Sample

TEK-8

TEK-9

TEK-10

TEK-11

TEK-7

TEK-13

Ir (ppb) Ru (ppb) Rh (ppb) Pt (ppb) Pd (ppb) RPGE Cu/Pd

0.005 0.035 0.004 0.062 0.047 0.15 32,87,510

0.006 0.014 0.003 0.043 0.040 0.11 9,25,848

0.23 0.46 0.16 2.11 3.98 6.94 11,769

0.30 0.88 0.24 3.32 5.88 10.61 19,507

0.59 2.56 0.55 9.01 13.41 26.12 11,476

0.30 0.76 0.22 2.52 4.31 8.11 16,862

2012;, Xiao et al., 2009; Liu et al., 2012; Deng et al., 2015; Ye et al., 2014). Because the Tuerkubantao intrusion is located at the north of the West Junggar, it was interpreted to belong to the Sawuer region of the West Junggar or the southern Altai orogen (Zhao, 2012; Guo, 2009; Wang et al., 2012). In the following discussion, we compare geochemical compositions of the Tuerkubantao intrusion and mafic–ultramafic intrusions in the West Junggar and southern Altai orogen to address this issue and to constrain the tectonic setting of the intrusion. The Tuerkubantao intrusive rocks are characterized by relatively restricted initial (87Sr/86Sr)t and large ranges of eNd(t) (2.84 to +3.8), suggesting that they had assimilated old continental crust. As there is no Precambrian basement in the Sawuer region (BGMX, 1993; Yuan et al., 2006; Zhou et al., 2006a,b,c, 2008), the crust in the Sawuer region is not thought to represent the old continental crust assimilated by the Tuerkubantao rocks. The compositional difference of the trace elements and isotopes between Devonian mafic rocks of the West Junggar and the Tuerkubantao intrusion indicate that the intrusion do not belong to the West Junggar. On the other hand, the Altai

orogen was generally regarded as an Andean-type active continental margin with a Precambrian basement (Wang et al., 2006, 2009; Niu et al., 2006; Windley et al., 2007; Xiao et al., 2008, 2010). The Tuerkubantao intrusion is located at the southern margin of the Altai orogen, and its Sr, Nd, Pb isotope and trace element compositions are similar to the Pacific active continental margin of the Americas. Thus we proposed that the intrusion was formed within an active continental margin along the southern Altai orogen (Figs. 7a–c and 9b). Because the Ordovician-Devonian sedimentary rocks and granites of the Altai orogen have highly variable (87Sr/86Sr)t ratios (Chen and Jahn, 2002; Long et al., 2012; Wang et al., 2014b), the isotopic features of the Tuerkubantao rocks suggest that they were not the interpreted source of the assimilated end-member old continental crust. On the contrary, the Sr, Nd and Pb isotopic compositions of the Tuerkubantao rocks define a trend toward the lower crust, indicating that they may have assimilated lower crust of the Altai orogen (Fig. 7). Most Tuerkubantao rocks plot relatively close to the calculated contamination curve with up to 25% contamination of depleted mantle-derived magma if lower continental crust is the contaminant (Fig. 9c). High-K calc-alkaline Devonian granitoids in the Shaerbulake Terrane were interpreted to be generated in a subduction-related setting (Shi et al., 2013). Because high-K calc-alkaline rocks cannot be formed in island-arc setting, we proposed the Devonian granitoids were formed along an active continental margin. Furthermore, the results of zircon U–Pb dating on basalt and gabbro in the Shaerbulake Terrane suggest these rocks contain the Neoproterozoic inherited zircons (Wong et al., 2010; Wang et al., 2012). The volcanic rocks contemporaneous with the Tuerkubantao intrusion consist of alkaline volcaniclastic rock, andesite, basaltic andesite and minor dacite, which are similar with those of volcanic rocks in an active continental margin

Fig. 8. (a) Primitive mantle-normalized PGE patterns of the intrusive rocks from the Tuerkubantao intrusion. (b) Diagrams of Pd/Ir vs. Ni/Cu of the Tuerkubantao intrusive rocks. (c) Plots of MgO vs. Al2O3 of the Tuerkubantao wehrlites. (d) Extrapolation of the variation diagram of FeOT vs. MgO to estimate the parental magma composition of the Tuerkubantao intrusion. The primitive mantle values are from Barnes and Maier (1999). Some data for the wehrlite and gabbro are from Guo (2009) and Deng et al. (2015). The data for Ol (olivine) and Cpx (clinopyroxene) are from Deng et al. (2015). The parental magma has 9.12 wt% MgO and 7.02 wt% FeOT.

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Y.-F. Deng et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

Fig. 9. (a) Plot of Ca vs. Fo contents in olivine (modified after Li et al., 2012). (b) Plot of the Tuerkubantao intrusive rocks in a Th–Hf–Nb discrimination diagram (Wood, 1980). (c) Modeling calculations of Sr–Nd isotopes suggesting that the Tuerkubantao intrusion experienced less than 25% contamination of LCC. The upper limit of Ca content in mantle olivine is based on Simkin and Smith (1970), Hervig and Smith (1982), O’Reilly et al. (1997), Schmidberger and Francis (1999), Jones et al. (2000). The olivine data for continental flood basalts (CFB), ocean island basalts (OIB), mid-ocean ridge basalts (MORB) and komatiites are from Sobolev et al. (2007). The olivine data for Ural–Alaskan type intrusions are from Batanova et al. (2005), Su et al. (2012) and Li et al. (2012). Olivine data for the Tuerkubantao intrusion are from Deng et al. (2015). The fields for depleted MORB mantle (DMM), UCC and LCC are after Zindler and Hart (1986), Rudnick and Gao (2003) and Windrim and McCulloch (1986), respectively. Some data of the wehrlite and gabbro are from Guo (2009) and Deng et al. (2015). The data for olivine pyroxenite are from Zhao (2012). The data for Mariana volcanics are from Pearce et al. (2005). The data for the Pacific margins of the Americas are from Cole and Basu (1995), Stern and Kilian (1996), D’Orazio et al. (2001), and Gorring et al. (2003).

(Zhang et al., 2007). As noted above, the Shaerbulake Terrane hosting the Tuerkubantao intrusion may represent a continental magmatic arc in Devonian built on a Precambrian basement. 6.3. Petrogenetic model The geochemical signature and simulated calculation for the Tuerkubantao rocks suggest the intrusion has a high-Mg basaltic parent magma. However, formation of high-Mg magma requires

11

very high mantle potential temperatures (Nisbet et al., 1993). The continental lithosphere is too cold to provide such high-Mg magmatism and most mid-ocean ridge magmatism is less magnesian (Wilson, 1989; Niu, 2005). The Devonian ophiolitic rocks, picrites, adakites, boninites, high-Ti basalts and high temperature-low pressure metamorphic rocks along the Irtysh fault at the southern margin of the Altai orogen were formed under abnormal mantle potential temperature conditions due to local upwelling of asthenosphere (Niu et al., 2006; Zheng et al., 2007; Zhang et al., 2008; Sun et al., 2009; Jiang et al., 2010; Wang et al., 2011; Shen et al., 2011, 2014; Wang et al., 2012; Cai et al., 2012a,b). The ultramafic–mafic complexes in Kekesentao area along the Erqis fault have been considered to be a late Palaeozoic ophiolite which marks the boundary between the Altai orogenic and the Junggar terrane (e.g., He et al., 1990, 1994, 2001; Xu et al., 2003; Niu et al., 2006; Xiao et al., 2008, 2009). However, the 407 Ma bimodal volcanic rocks of the Kangbutiebao Formation (Zhang et al., 2000), the 405 Ma gabbro at Keketuohai (Wang et al., 2006), the 372 Ma Kuerti mafic rocks (Zhang et al., 2003; Xu et al., 2001, 2003) and the 352 Ma Buergen ophiolitic belt (Wu et al., 2006) in the Qiongkuer-Abagong terrane and Erqis terrane (Fig. 1) reveal that these areas were in an extensional back-arc setting during the Devonian. Investigations in several geological sections and geochemical compositions of the mafic–ultramafic intrusions in Kekesentao area indicate the intrusions are mostly represented by Carboniferous arc-related intrusive rocks rather than a Devonian ophiolite complex (Zhang et al., 2012; Ni et al., 2013). The Tuerkubantao mafic–ultramafic intrusion is regarded as an Ural– Alaskan type intrusion in a subduction setting (Deng et al., 2015). Based on the detailed field observations and geochemistry of the major rock units along the Erqis belt, Zhang et al. (2012) proposed that a back-arc basin was formed along the Erqis zone during Devonian to Early Carboniferous times. On the other hand, the boninites of the eastern Saerbulake terrane were generated from sub-arc mantle fluxed by slab-derived fluid in the fore-arc area (Niu et al., 2006). The geochemical data of the granitic intrusions revealed that they were formed in an extensional forearc setting during the Devonian (Sun et al., 2006; Yuan et al., 2007; Xiao et al., 2008). Thus high-Mg magmatism in this region may be resulted from melting of metasomatized mantle underplated by ascending asthenosphere, perhaps triggered by forearc rifting or ridge subduction (Niu et al., 2006; Zhang et al., 2008; Sun et al., 2009; Shen et al., 2011; Cai et al., 2012a,b; Long et al., 2012).

6.4. Implications for Ni–Cu sulfide prospectivity When mantle-derived magma intrudes into the crust, sulfide saturation is a key process in the formation of magmatic Ni–Cu sulfide deposits (Naldrett, 2004, 2009). The sulfur content at sulfide saturation (SCSS) in mafic magma is mainly affected by the chemical composition, temperature, pressure and oxygen fugacity (f O2) of the magma (Haughton et al., 1974; Mavrogenes and O’Neill, 1999; Li and Ripley, 2005; Liu et al., 2007; Jugo et al., 2005; Jugo, 2009; Naldrett, 2009). As the partition coefficients of the Ni, Cu and PGE between liquid sulfide and liquid silicate are very high, the most mafic magmas are depleted in these metals in comparison with their concentrations in sulfides (Naldrett and Cabri, 1976; Sharpe, 1982; Peach et al., 1990; Fleet et al., 1999). Thus the Ni, Cu and PGE contents in magma can be used to identify whether or not magma is saturated with sulfur. As shown in Fig. 10, basalts in the Middle Nadezhdinsky Formation in the Noril’sk area in MORBs have extremely low Pd and Cu concentrations, which are depleted because of segregation of immiscible magmatic sulfides due to S-saturation (Bézos et al.,

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Acknowledgements We thank the management of No. 706 Geological Party, Xinjiang Geoexploration Bureau for Nonferrous Metals, for help during our field work. We benefited from discussion with professor Bin Chen. This study was financially supported by the ‘‘12th Five Year’’ National Science and Technology Support Programme (2011BAB06B01), the Program for New Century Excellent Talents in University of China (Grant no. NCET-10-0324), grants from Natural Science Foundation of China (41303031, 41172090, 41040025), the Fundamental Research Funds for the Central Universities (2013bhzx0015) and Open Funds from the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences (201102). The paper is a contribution to IGCP#592 ‘‘continental construction in Central Asia’’. Fig. 10. Scattergram of Pd vs. Cu for the Tuerkubantao intrusion, with fields for the PGE-undepleted Siberian Trap Tk + Mk + Sm basalts, the PGE-depleted Siberian Trap (Nd1–Nd3) basalts (both from Keays and Lightfoot, 2010) and mid ocean ridge basalts (MORB, from McDonough and Sun, 1995). The dashed line (from Vogel and Keays, 1997) separates fields of S-saturated magmas (below) and S-undersaturated magmas (above).

2005; Hertogen et al., 1980; Keays, 1995; Keays and Scott, 1976). The gabbros in the Tuerkubantao intrusion with high Cu/Pd ratios (9.26  105–32.8  105) fall within the S-saturated field of Vogel and Keays (1997), like the Middle Nadezhdinsky Formation of the Siberian Traps (Fig. 10). Whereas the wehrlites with lower Cu/Pd ratios (1.18  104–1.95  104) fall within the S-undersaturated field. This indicates that the gabbros have experienced sulfide segregation, but the wehrlites have not experienced this process. The relatively high Ni contents (1682–2357 ppm) in olivines of wehrlites also suggest the parent magma had not reached sulfide saturation. This is consistent with the occurrence of sulfides in the Tuerkubantao gabbros. Although the Tuerkubantao intrusion had assimilated continental rocks as discussed above, crustal contamination may not cause the magma to achieve S saturation because S-rich crustal contaminants may not have existed in the Tuerkubantao area (Keays and Lightfoot, 2010). As sulfide saturation could have been reached at 30% fractional crystallization in the Ural–Alaskan type intrusion (Thakurta et al., 2008), we propose that the sulfide saturation in gabbros was triggered by fractional crystallization. On the other hand, if magma experienced 5% olivine fractional crystallization, 50% of the Ni resource in the magma would be lost (Maier et al., 1998). Thus the magma that experienced sulfide saturation after crystallization of abundant of olivine (>5%) is not conductive to form a large magmatic Ni–Cu sulfide deposits. In conclusion, although more work needs to be done to evaluate the Cu–Ni sulfide mineralization potential of the Tuerkubantao mafic–ultramafic intrusion, mineral exploration of the intrusion should be cautious. 7. Conclusion The Ural–Alaskan type Tuerkubantao mafic–ultramafic intrusion was formed within the active continental margin of the Altai orogen during the Late Devonian (370.3 ± 4.8 Ma). Its parent magma was high-Mg basaltic magma with 9.12 wt% MgO and 7.02 wt% FeOT. The primary magma was derived by partial melting of metasomatized mantle, triggered by upwelling of asthenosphere in an extensional forearc setting along the active continental margin. The magma had assimilated lower crustal material of the Altai orogen. Gabbros in the intrusion have experienced sulfide segregation, while wehrlites have not experienced this process.

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Please cite this article in press as: Deng, Y.-F., et al. Zircon U–Pb geochronology, geochemistry, and Sr–Nd isotopes of the Ural–Alaskan type Tuerkubantao mafic–ultramafic intrusion in southern Altai orogen, China: Petrogenesis and tectonic implications. Journal of Asian Earth Sciences (2015), http:// dx.doi.org/10.1016/j.jseaes.2015.05.007