Age, petrogenesis and tectonic implications of Early Devonian bimodal volcanic rocks in the South Altyn, NW China

Journal of Asian Earth Sciences 111 (2015) 733–750

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Age, petrogenesis and tectonic implications of Early Devonian bimodal volcanic rocks in the South Altyn, NW China Lei Kang ⇑, Pei-Xi Xiao, Xiao-Feng Gao, Ren-Gang Xi, Zai-Chao Yang Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, MLR, Xi’an Center of Geological Survey, CGS, Xi’an 710054, China

a r t i c l e

i n f o

Article history: Received 21 January 2015 Received in revised form 3 April 2015 Accepted 22 June 2015 Available online 22 June 2015 Keywords: Early Devonian Bimodal volcanic rocks Petrogenesis Tectonic magmatic evolution South Altyn

a b s t r a c t In this paper, we report zircon U–Pb dating, Hf isotopes, geochemical and Sr–Nd isotopic data, with the aim the petrogenesis and regional tectonic evolution of Early Devonian bimodal volcanic rock in the South Altyn, NW China. New LA-ICPMS zircon U–Pb isotopic data constrained them at ca. 406 Ma. The mafic samples are characterized by high Fe, Cr and Ni contents, low Ti and Mg contents, slightly enriched LREE patterns, and low (La/Yb)N, La/Nb and La/Ta ratios, and positive eNd(t) values (+3.3 to +3.4), indicating that they were likely derived from strong batch-melting of the asthenosphere in the spinel facies field. The felsic rocks show an A-type affinity, with high alkalis, Fe, Ga, Zr, Nb, Ce and Y contents, low Mg, Sr content, high Rb/Sr and Ga/Al ratios, enrichment in LILE (e.g., Rb, K, Th, U and LREE) and depletion in Ba, Sr, Nb, Ta, P and Ti, and fractionated REE patterns with very strong negative Eu anomalies. These features, along with distinct eNd(t) values ( 0.5 to +2.3) and mostly positive eHf(t) ( 0.29 to +5.18), indicate that the felsic rocks were mainly generated by partial melting of the crust in low pressure and high temperature conditions, and simultaneously underwent slight magma mixing of such melts with mantle magma. According to the petrogenetic schemes and geological background of the Early Devonian bimodal volcanic rocks (tholeiite and A-type dacite–rhyolite), they should have formed in a post-collisional extensional setting. Moreover, on the basis of spatial and temporal distribution, and formation mechanism, the tectonic magmatic evolution of the early Paleozoic South Altyn Tagh could be divided into three stages: r 505–472 Ma (continental collision), the magmatite formed under high-pressure conditions due to the deep subduction and initial tearing of continental slab; s 467–450 Ma (continental slab break-off), the magmatite formed at high temperature and low pressure in virtue of felsic upper crust uplifting and mantle magma underplating; and t 432–385 Ma (post-collisional extension), the magmatite consists of A-type granites and bimodal volcanic rocks, which are the products of the interaction between mantle and crust, and asthenosphere upwelling. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Bimodal volcanism typically characterizes an extensional environment, which can occur within various tectonic settings, including continental rifts (Duncan et al., 1984; Garland et al., 1995; Pin and Marini, 1993), within-plate extensional settings (Coulon et al., 1986; Hildreth et al., 1991; Turner et al., 1992; Frost and Frost, 1997; Frost et al., 1999; Bonin, 2004), intra-oceanic islands (Geist et al., 1995; Jolly et al., 2008), oceanic island arcs (Frey et al., 1984; Pin and Paquette, 1997), incipient backarc depressions (Hochstaedter et al., 1990), mature islands/active continental margins (Frey et al., 1984; Hochstaedter et al., 1990; Pin and Paquette, ⇑ Corresponding author at: Xi’an Center of Geological Survey, No. 438, East Youyi Street, Xi’an 710054, China. E-mail address: [email protected] (L. Kang). http://dx.doi.org/10.1016/j.jseaes.2015.06.004 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

1997) and back-arcs (Hochstaedter et al., 1990; Pearce et al., 1995, 1999; Elliott et al., 1997; Ewart et al., 1998; Shinjo and Kato, 2000). A-type rhyolites with tholeiitic basalts seem to be the most enigmatic out of all these various associations of mafic and felsic rocks which are associated with post-collision to within-plate regimes (Hildreth et al., 1991; Turner et al., 1992; Frost and Frost, 1997; Frost et al., 1999, 2001). In the last decade, the South Altyn Tagh was recognized as significant high-pressure/ultrahigh-pressure (HP/UHP) terrain (Liu et al., 1997, 2002, 2005, 2007, 2009, 2012, 2013; Wang et al., 2011; Zhang et al., 2001a,b, 2004, 2005; Zhang and Meng, 2005; Cao et al., 2009). Previous studies indicate that the metamorphic ages which represent deep subduction of continental crust range from 480 to 504 Ma (Cao et al., 2009; Liu et al., 2007, 2009, 2010, 2012; Wang et al., 2011; Zhang et al., 1999, 2004, 2005) and the retrograde metamorphic ages which represent continental slab

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break-off range from 450 to 455 Ma (Liu et al., 2012, 2013). However, the dynamic mechanism of the post-collision after the deep subduction of continental crust and continental slab breakoff, are still not clear. A massive magmatite which is closely related to the collisional orogeny, outcrops in the South Altyn Tagh from 502 to 385 Ma (Fig. 1A) (e.g., Wang et al., 2008, 2014; Cao et al., 2010; Sun et al., 2012; Yang et al., 2012; Kang et al., 2013, 2014, in press), and initially can be subdivided into three stages: (1) 500 Ma (continent–continent collision and crustal thickening); (2) 462–451 Ma (extensional tectonic setting and break-off of deep subducted continental slab; and (3) 426–385 Ma (post-collisional extension and crustal thinning) (Yang et al., 2012; Liu et al., 2013; Wang et al., 2014). At present, the petrogenesis and melting mechanisms tectonic setting of the first two stages of magmatism are almost clear due to their ages being identical to the metamorphic and retrograde metamorphic ages for HP/UHP metamorphic rocks (e.g., Ma et al., 2009, 2011; Cao et al., 2010; Sun et al., 2012; Yang et al., 2012; Kang et al., 2013, 2014, in press), but the third stage of magmatism is still not clear because of few studies (e.g., Wu et al., 2007; Wang et al., 2008, 2014). Moreover, the post-collisional (either late or postorogenic) setting are normally accompanied by voluminous and granitoid magmatism (e.g., Bonin, 2004; Liégeois, 1998; Turner et al., 1992, 1999), but recently all magmatic studies about the post-collision (the third stage) of the South Altyn Tagh focused on granites (e.g., Wu et al., 2007; Wang et al., 2008, 2014). Recently, typical bimodal volcanic rocks (tholeiites and A-type felsic volcanic rocks) in the third stage have been discovered in the eastern section of the South Altyn Tagh tectonic melange belt, which are helpful to study the magmatic events, mantle–crust interaction and orogenic events during the post-collisional stage. In this paper, we present geochronological and geochemical data for the bimodal volcanic rocks to examine their precise age, petrogenesis and tectonic implications. Works about diverse magmatites at different stages provide key opportunities to study the tectonic evolution for the early Paleozoic in South Altyn Tagh, we therefore will discuss the early Paleozoic collision orogenic process.

2. Geological background The Altyn Tagh marks the northern margin of the Qinghai-Tibet Plateau, lying between the Tarim block to the north and the Qaidam block, the Qilian orogen and the Kunlun belt to the south (Fig. 1B). From north to south, the Altyn Tagh can be divided into four units (Xu et al., 1999; Liu et al., 2009, 2012): (1) the north Altyn Archean complex (NAAC); (2) the north Altyn oceanic-type subduction complex (NAOSC); (3) the Milanhe-Jinyanshan block (MJB); and (4) the South Altyn Tagh subduction-collision complex (SATSC) (Fig. 1C). South Altyn Tagh subduction-collision complex can be divided into the South Altyn Tagh HP/UHP terrain and the South Altyn Tagh tectonic melange belt (Yang et al., 2012). The South Altyn Tagh tectonic melange belt extends E–NE for 700 km along the Altyn fault from the east of the Mangnai to west of the Apar, with 71 mafic–ultramafic rocks outcropping in the region. In detail, there are mafic volcanic rock, pillow lava and silicolite in the Apar area, abundant ultramafic rocks and mafic volcanic rock, minor gabbro, andesite and silicolite with an absence of dyke swarm and pillow lava in the Mangnai area (Wang et al., 1999). As a portion of the South Altyn Tagh tectonic melange belt, the ultramafic rocks in the Mangnai consist of dunite and harzburgite with strong serpentinization which are abounding in amosite. Previously, these rocks were considered an ophiolite formation which was known as the Apar-Mangnai ophiolitic melange belt (Dong et al., 1995; Liu et al., 1998; Wang et al., 1999; Li et al.,

2009; Zhang and zhou, 2001). Recently, the mafic–ultramafic rocks in the South Altyn Tagh tectonic melange belt were divided into two types: some with characteristics of an ophiolitic formation with an age P500 Ma (e.g., Liu et al., 1998; Wang et al., 1999; Li et al., 2009); the others appeared to have formed within a plate extensional settings during 453–467 Ma (e.g., Ma et al., 2009, 2011; Wang et al., 2014). 3. Petrology and sampling In the recent years, numerous volcanic-sedimentary rocks in the tectonic lens or massif were found within the Mangnai area in the eastern section of the South Altyn Tagh tectonic melange belt, according to the regional geological survey of the Mangnai area at 1:250,000 scale (XACGS, 2012) (Fig. 1D). These volcanic-sedimentary rocks consists mostly of volcanic rocks that are predominantly amaranth rhyolites–dacites with minor greyish-green basalts which contact in conformable relationship (Figs. 2 and 3a), a typical bimodal sequence. The sedimentary successions consist of dominant amaranth/greyish-green pelitic siltstones, sandstones and subordinate weakly schistose mudstones and griotte with clear stratification (Fig. 2). In addition, massive rhyolite–dacite rock masses are exposed in the northern area of the South Altyn Tagh tectonic melange belt (Fig. 1D). The basalts (11A13) are greyish-green1 in color and commonly a microscopic porphyritic texture with phenocrysts of plagioclase up to 0.5–2 mm in size. The groundmass includes fine-grained plagioclase, pyroxene, amphibole and magnetite, and is characterized by the fasciculate variolitic structure of fibroid pyroxene and plagioclase (Fig. 3b and c). The rhyolites (11A10) are amaranth (reddish-rose) in color with partial vesicular structure, and commonly have a porphyritic erosion texture with quartz and plagioclase up to 0.2–1.3 mm in diameter. The groundmass is composed mainly of quartz, plagioclase and glass with fluxion/flow texture (Fig. 3c and d). The dacites (11A08) are brownish red in color and commonly show porphyritic erosion texture with phenocrysts of anorthoclase, plagioclase and quartz up to 0.2–2.8 mm in size. The groundmass is composed mainly of tiny quartz crystals and plagioclase with microscopic poikilitic structure (Fig. 3e and f). 4. Analytical methods 4.1. Zircon U–Pb isotopic dating Zircons were extracted at the Langfang Regional Geological Survey, Hebei Province, China. Based on CL images, zircon LA-ICP-MS trace element and U–Th–Pb isotopic analyses were performed on these zircons at the State Key Laboratory of Continental Dynamics, Northwest University, China. CL investigation was performed on a scanning electron microscope (Quanta 400 FEG of FEI Company, USA) equipped with Mono CL3+ cathodoluminescence detector, Gatan Company, Britain, and the LA-ICP-MS for in situ zircon U–Pb dating consistent with an ICP-MS (Agilent 7500a) and an excimer laser ablation system (193 nm, Geolas 200M, Lambda Physic). Trace element and U–Th–Pb isotopic data were acquired simultaneously on the same spot (30 lm diameter). The isotopic ratios and element concentration of zircons were calculated using GLITTER (ver. 4.0, Macquarie University). The weighted mean calculations and concordia diagrams were made by using ISOPLOT (version 3.0) (Ludwig, 2003). The detailed analytical technique is described in Yuan et al. (2004). 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.

L. Kang et al. / Journal of Asian Earth Sciences 111 (2015) 733–750

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Fig. 1. Geological map of the Mangnai area in the South Altyn Tagh, northwestern China (figures (A)–(C) are modified after Wang et al. (2014); figure (D) is modified after XACGS (2012)).

Fig. 2. Representative geological sections nearby Shimiankuang area showing the basalt, rhyolite and dacite, and the relation of them and their country rocks in the South Altyn Tagh tectonic melange belt.

4.2. Major and trace element determination Major elements were analyzed on fused glass disks by X-ray fluorescence spectrometry. Trace elements (including REE) were determined by a Agilent 7500a ICP-MS at the State Key Laboratory of Continental Dynamics in Northwest University,

Xi’an, China. Indium was used as an internal standard to correct for matrix effects and instrument drift. The Chinese national standards GSR-1 (granite) and GSR-3 (basalt) were used to monitor analyses. Errors for major element analysis are within 1%, except for P2O5 (5%), and for most trace elements (including REE) are within 10%.

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Fig. 3. Representative outcrop photos and micrographs of bimodal volcanic rocks from the South Altyn Tagh tectonic melange belt. (a) Basalts and rhyolites in conformable contact relationship; (b and c) photomicrographs of basalt; (d and e) photomicrographs of rhyolite; (f and g) photomicrographs of dacite. Mineral abbreviations in photomicrographs: Pl – plagioclase; Px – pyroxene; Fsp – Feldspar; Q – quartz; Atc – anorthoclase.

4.3. Sr–Nd isotopic analyses

4.4. In-situ Lu–Hf isotope analyses

Sr–Nd isotope ratios were determined on a Finnigan Trition thermal ionization mass spectrometer at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. Sample powders were digested in Te on bombs with mixed agents of double distilled HNO3 and HF acids at 190 °C for 48 h. The measured 143 Nd/144Nd and 87Sr/86Sr ratios were normalized to 146 Nd/144Nd = 0.7219 and 86Sr/88Sr = 0.1194, respectively. During the period of analysis, NBS987 standard yielded an average 87 Sr/86Sr value of 0.710215 ± 10 (2s) and La Jolla standard gave an average 143Nd/144Hd value of 0.511837 ± 1 (2r), which agree well with recommended values within analytical errors (Ling et al., 2003). Total procedural Sr and Nd blanks are <4 ng and <1 ng, respectively.

In situ zircon Lu–Hf isotopic measurements were done using the Nu Plasma HR Multi-Collector (MC)-ICPMS, equipped with a 193 nm ArF Laser, with a spot size of 44 pm, a 10 Hz repetition rate, and a laser power of 100 mJ/pulse at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an. The analytical protocols were similar to that outlined in Yuan et al. (2008). The interference of 176Lu on 176Hf was corrected by measuring the intensity of the interference-free 175Lu, using the recommended 176 Lu/175Lu ratio of 0.02669 (DeBievre and Taylor, 1993) to calculate 176Lu/177Hf. Similarly, the isobaric interference of 176Yb on 176 Hf was corrected by using a recommended 176Yb/177Yb ratio of 0.5886 (Chu et al., 2002). Zircon 91500 was used as the reference standard. A decay constant value of 1.865  10 11 a 1 for 176Lu (Scherer et al., 2001), the present day chondritic ratios of

L. Kang et al. / Journal of Asian Earth Sciences 111 (2015) 733–750 176

Hf/177Hf = 0.282772 and 176Lu/177Hf = 0.0332 (Blichert-Toft and Albarede, 1997) were adopted to calculate eHf values. Single-stage Hf model ages (TDM1) are calculated relative to the depleted mantle with a present day 176Hf/177Hf ratio of 0.28325 and 176Lu/177Hf of 0.0384 (Vervoort and Blichert-Toft, 1999), and two-stage Hf model ages (TDM2) are calculated by assuming a mean 176 Lu/177Hf value of 0.015 for the average continental crust (Griffin et al., 2002). 5. Analytical results 5.1. In situ zircon U–Pb and Hf isotope The in situ zircon LA-ICPMS U–Pb and Lu–Hf isotopic data are presented in Tables 1 and 2, respectively. The zircons selected for analysis were euhedral–subhedral and displayed striped absorption or fine-scale oscillatory growth zoning in the CL images (Fig. 5). Zircons in the dacites (11A08.1) are 50–170 lm long and 30– 110 lm wide, and are dominantly euhedral and prismatic with typical oscillatory growth zoning (Fig. 4a). 206Pb/238U ages determined from 22 analytical spots ranged from 401.4 Ma to 408.8 Ma with a weighted mean 206Pb/238U age of 405.8 ± 1.2 Ma (n = 22, MSWD = 0.78) (Fig. 5a). This age is taken to the extrusive age of the dacites. 17 Hf isotopic spot analyses were conducted on zircons from the dacites with radio genic 176Hf/177Hf ratios ranging from 0.282515 to 0.282679, and eHf(t) = 0.29 to 5.18. Two-stage model ages were calculated based on a felsic upper crust 176Lu/177Hf ratio of 0.008 (Taylor and McLennan, 1985), and TDM2 values of these zircons vary from 963 to 1219 Ma. Zircons extracted from the rhyolites (11A10.4) for analysis were 60–310 lm long and 40–140 lm wide, and were dominantly euhedral and prismatic, and with typical oscillatory growth zoning (Fig. 4b). Their 206Pb/238U ages (from 16 spots) varied between 403.1 Ma and 410 Ma, yielding a weighted mean age of 406.1 ± 1.2 Ma (MSWD = 0.61) (Fig. 5b). These can be interpreted as the extrusive age of the rhyolites. As noted above, the dacites from the rhyolites–dacites rock masses and the rhyolites from the volcanic-sedimentary rocks are contemporary volcanic rocks which belong to Early Devonian. 5.2. Major and trace element geochemistry The major and trace element data of the Early Devonian bimodal volcanic rocks (basalt, rhyolite and dacites samples) from the South Altyn Tagh tectonic melange belt are listed in Table 3. Petrographic investigations have revealed that most of the sample shave been subjected to alteration, which is also evident from their high LOI contents, ranging from 1.89 wt.% to 4.81 wt.%. Such alterations could have modified the contents of mobile elements, such as Na, K, Ca, Cs, Rb, Ba, and Sr. Therefore, we use only immobile elements (e.g., the high field strength elements (HFSEs), the rare earth elements (REEs), transitional elements, and Nd isotopes) for classifying the rocks and discussing their tectonic setting and petrogenesis. The SiO2 contents of the magmatic rocks of the Early Devonian volcanic rocks varied from 47.16 to 72.89 wt.% and had a distinct compositional gap between SiO2 = 51 wt.% and 67 wt.% (Table 3), displaying typical bimodality. The samples were classified as basalt, dacite and rhyolite on the Zr/TiO2 vs. Nb/Y plot (Winchester and Floyd, 1977) (Fig. 6a). The mafic rocks are sub-alkaline and tholeiite in terms of the Nb/Y vs. Zr/TiO2 and Co vs. Th binary diagrams (Fig. 6a and b). They are characterized by relatively low TiO2 (0.98–1.08%), MgO (4.03–5.56%) and P2O5 (0.14–0.16%) contents, high content of

737

T

Fe2O3 (8.31–11.9%), and moderate concentrations of Al2O3 (15.99–17.99%) and CaO (7.68–11.23%) (Table 3). On the chondrite-normalized REE diagrams, they exhibit slight LREE enrichment (LaN/YbN = 1.67–1.81) without negative Eu anomalies (Eu/Eu⁄ = 0.96–1.15) (Fig. 7a). Their primitive mantle-normalized diagrams are characterized by moderate content in most trace elements, such as Th, U, Nb, Ta, La, Ce, Hf, Zr and Y. Such trace element patterns without significant Nb–Ta depletion relative to La are typical of E-MORBs (Fig. 7b) (Sun and Mc Donongh, 1989). In contrast, the felsic rocks (dacite and rhyolite) belong to the high-K calc-alkaline and Shoshonitic group (Fig. 6b). They have wide ranges of TiO2 (0.23–0.61%), high alkalis (K2O + Na2O = 7.07–10.31%) and TFe2O3 (2.43–3.16%), and low MgO (0.42–0.71%). On the chondrite-normalized REE pattern diagram (Fig. 7c), these measurements indicate LREE-enriched patterns (LaN/YbN = 5.43–7.89) with strong negative Eu anomalies (Eu/Eu⁄ = 0.45–0.61). On the primitive mantle-normalized diagram (Fig. 7d), they are enriched in LILE (e.g., Rb, K, Th, U and LREE) and depleted in Ba, Sr, Nb, Ta, P and Ti. 5.3. Sr–Nd isotopic compositions Whole rock Rb–Sr and Sm–Nd data for five basalt, dacite and rhyolite samples from the Early Devonian bimodal suites are given in Table 4. The initial 87Sr/86Sr ratios (ISr), initial 143Nd/144Nd ratios (INd) and eNd(t) values were calculated for the new U–Pb zircon ages (ca. 406 Ma) that was determined in this study. Due to most of the sample had been subjected to alteration in their history, the primary Sr compositions of the samples analyzed may have been changed (Xu and Castillo, 2004). Therefore, we will also not use the Sr isotopic results in the interpretation below. The mafic rocks (basalts) have similar isotopic compositions with positive eNd(t) values (+3.3 to +3.4) and Nd model ages (TDM) from 1440 Ma to 1459 Ma, while the felsic rocks (dacite and rhyolite) have a wide range of eNd(t) values ( 0.5 to +2.3) and Nd model ages of TDM = 1023 and 1181 Ma. 6. Discussion 6.1. Petrogenesis of bimodal volcanic rocks 6.1.1. Petrogenesis of the mafic rocks The low silica contents (SiO2 = 47.16–50.39%), relatively high concentrations of TFe2O3 (8.31–11.9%), and high Cr and Ni contents (175–295 ppm and 152–260 ppm, respectively) in the mafic rocks suggest that they were derived from a mantle source. Furthermore, ratios of their incompatible elements such as Nb/Ta (15.68–16.41), Nb/U (25.61–39.48), Zr/Hf (43.99–45.13), (La/Nb)N (0.89–0.99) and (Th/Nb)N (0.99–1.02) are comparable with primitive mantle (for primitive mantle, Nb/Ta = 17.5 ± 0.5, Nb/U  30, Zr/Hf = 36.27, (La/Nb)N = 0.98–1.00, (Th/Nb)N  1; Sun and Mc Donongh, 1989). Nevertheless, their low Mg# (0.42–0.50) indicate that they not represent primary magmas, and SiO2 concentrations correlate negatively with MgO concentrations (Fig. 8a), indicating the mafic magmas may have experienced some crystal fractionation. The trends of increasing concentrations of Ni and Cr with increasing MgO (Fig. 8b and c) suggest olivine fractionation, the increasing contents of TFe2O3 and TiO2 within creasing MgO (Fig. 8d and e) indicate titanic magnetite fractionation. P2O5 concentrations positive correlation with MgO concentrations (Fig. 8f) reflect apatite fractionation. In contrast, Al2O3 concentrations correlate negatively with MgO concentrations (Fig. 8g), and a lack of negative Eu anomalies on the chondrite-normalized REE diagrams indicate an absence of plagioclase fractionation during the process of magmatic evolution.

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Table 1 LA–ICP–MS zircon U–Pb dating data for Early Devonian dacite (11A08.1) and rhyolite (11A10.4) from the South Altyn Tagh tectonic melange belt. Sample no.

Isotopic ratios 207

Pb/206Pb

Ages (Ma) 207

Pb/235U

206

Pb/238U

208

Pb/232Th

207

Pb/206Pb

207

Pb/235U

206

Pb/238U

208

Pb

Th

U

Th/U

Pb/232Th

1r

Ratio

1r

Ratio

1r

Ratio

1r

Age

1r

Age

1r

Age

1r

Age

1r

0.05449 0.05788 0.05621 0.05503 0.05614 0.05726 0.05562 0.05489 0.05890 0.05667 0.05562 0.05399 0.05421 0.05591 0.05655 0.05528 0.05648 0.05992 0.05477 0.05740 0.05525

0.00120 0.00095 0.00134 0.00234 0.00109 0.00115 0.00089 0.00182 0.00318 0.00131 0.00095 0.00120 0.00080 0.00116 0.00108 0.00107 0.00094 0.00176 0.00089 0.00176 0.00153

0.05449 0.05788 0.05621 0.05503 0.05614 0.05726 0.05562 0.05489 0.05890 0.05667 0.05562 0.05399 0.05421 0.05591 0.05655 0.05528 0.05648 0.05992 0.05477 0.05740 0.05525

0.00120 0.00095 0.00134 0.00234 0.00109 0.00115 0.00089 0.00182 0.00318 0.00131 0.00095 0.00120 0.00080 0.00116 0.00108 0.00107 0.00094 0.00176 0.00089 0.00176 0.00153

0.06546 0.06504 0.06472 0.06507 0.06463 0.06521 0.06467 0.06539 0.06408 0.06455 0.06494 0.06532 0.06562 0.06425 0.06474 0.06483 0.06498 0.06438 0.06544 0.06527 0.06534

0.00046 0.00039 0.00049 0.00075 0.00043 0.00044 0.00039 0.00061 0.00095 0.00048 0.0004 0.00046 0.00038 0.00044 0.00043 0.00043 0.00040 0.00057 0.00039 0.00058 0.00054

0.02013 0.02131 0.02102 0.02030 0.02082 0.02140 0.02144 0.02029 0.02270 0.02064 0.02157 0.01975 0.02137 0.02126 0.02191 0.02145 0.02236 0.02146 0.02201 0.02049 0.02129

0.00025 0.00020 0.00024 0.00040 0.00021 0.00022 0.00020 0.00027 0.00072 0.00022 0.00022 0.00026 0.00018 0.00029 0.00024 0.00025 0.00022 0.00022 0.00020 0.00031 0.00025

391.2 524.9 459.8 413.5 457.6 501.0 437.0 407.5 563.4 477.9 437.0 370.5 379.7 448.5 473.1 423.3 470.4 600.7 403.0 506.5 422.1

48.52 35.85 52.55 91.94 42.47 44.29 34.75 71.74 113.5 50.80 37.06 49.27 32.88 45.23 42.23 42.09 36.73 62.41 35.80 66.56 60.18

406.1 424.5 412.8 407.5 411.9 421.7 408.9 408.2 425.4 414.7 410.4 402.4 405.2 408.6 414.9 407.7 415.7 433.1 407.8 422.9 410.2

6.68 4.75 7.42 13.7 5.80 6.17 4.45 10.5 18.2 7.19 4.87 6.68 3.92 6.24 5.71 5.72 4.77 9.71 4.57 10.0 8.72

408.8 406.2 404.3 406.4 403.7 407.2 403.9 408.3 400.4 403.3 405.6 407.9 409.7 401.4 404.4 404.9 405.8 402.2 408.6 407.6 408.0

2.79 2.39 2.94 4.53 2.59 2.68 2.34 3.72 5.76 2.88 2.42 2.80 2.28 2.68 2.58 2.59 2.4 3.46 2.38 3.53 3.26

426.2 420.5 406.1 416.5 427.9 428.9 405.9 453.8 413.0 431.3 395.2 427.4 425.2 438.0 428.9 446.9 429.2 439.9 410.0 425.9 426.2

4.00 4.77 7.99 4.16 4.38 3.97 5.28 14.3 4.27 4.45 5.22 3.65 5.71 4.80 5.01 4.43 4.37 4.02 6.08 4.94 4.00

70.100 170.64 100.11 36.940 184.84 137.40 176.11 43.490 160.52 96.700 167.87 81.600 201.32 186.89 163.11 172.00 256.37 99.960 142.25 35.770 54.850

121.82 248.36 242.74 95.740 377.30 285.23 241.66 153.56 265.36 277.01 216.11 127.15 252.17 233.07 252.49 238.74 331.25 475.71 206.83 87.460 169.68

220.88 550.77 310.62 111.97 593.42 433.11 580.77 125.50 508.30 298.60 556.44 264.56 663.09 627.39 534.41 568.70 846.86 278.69 464.40 108.79 164.19

0.55 0.45 0.78 0.86 0.64 0.66 0.42 1.22 0.52 0.93 0.39 0.48 0.38 0.37 0.47 0.42 0.39 1.71 0.45 0.80 1.03

11A10.4-01 11A10.4-02 11A10.4-03 11A10.4-04 11A10.4-05 11A10.4-06 11A10.4-07 11A10.4-08 11A10.4-09 11A10.4-10 11A10.4-11 11A10.4-12 11A10.4-13 11A10.4-14 11A10.4-15 11A10.4-16

0.05542 0.05180 0.05270 0.05520 0.05962 0.05760 0.05399 0.05521 0.05371 0.05301 0.05475 0.05588 0.05270 0.05344 0.05399 0.05526

0.00088 0.00080 0.00084 0.00082 0.00077 0.00118 0.00101 0.00086 0.00078 0.00085 0.00086 0.00085 0.00082 0.00081 0.00082 0.00074

0.49421 0.46296 0.46874 0.49468 0.53998 0.51842 0.48149 0.49650 0.48170 0.47518 0.49553 0.50055 0.47291 0.47854 0.48302 0.49575

0.00658 0.00593 0.00634 0.00607 0.00540 0.00956 0.00796 0.00653 0.00576 0.00649 0.00661 0.00644 0.00624 0.00613 0.00615 0.00526

0.06469 0.06483 0.06453 0.06499 0.06567 0.06526 0.06467 0.06521 0.06504 0.06500 0.06563 0.06495 0.06507 0.06493 0.06487 0.06505

0.00039 0.00039 0.00039 0.00039 0.00038 0.00047 0.00043 0.00040 0.00039 0.00040 0.00041 0.00040 0.00040 0.00040 0.00040 0.00038

0.02043 0.01908 0.01942 0.01957 0.02217 0.02130 0.01949 0.02085 0.01917 0.01784 0.01976 0.01969 0.01673 0.01728 0.01859 0.01912

0.00019 0.00019 0.00019 0.00016 0.00016 0.00026 0.00022 0.00022 0.00016 0.00017 0.00018 0.00017 0.00017 0.00015 0.00017 0.00015

429.1 276.8 315.8 420.3 589.9 514.3 370.4 420.4 358.6 329.2 402.0 447.2 315.8 347.4 370.6 422.8

34.38 34.79 35.88 32.66 27.81 44.60 41.52 34.24 32.52 35.79 34.50 33.24 34.91 33.95 33.87 29.28

407.8 386.3 390.3 408.1 438.4 424.1 399.1 409.3 399.2 394.8 408.7 412.1 393.2 397.1 400.1 408.8

4.47 4.12 4.38 4.13 3.56 6.39 5.45 4.43 3.95 4.46 4.49 4.36 4.30 4.21 4.21 3.57

404.1 404.9 403.1 405.9 410.0 407.6 404.0 407.3 406.2 405.9 409.8 405.7 406.4 405.6 405.2 406.3

2.39 2.35 2.38 2.36 2.28 2.82 2.62 2.43 2.35 2.45 2.47 2.42 2.43 2.41 2.41 2.31

408.8 382.1 388.8 391.8 443.1 426.0 390.1 417.1 383.8 357.4 395.4 394.1 335.4 346.2 372.3 382.8

3.76 3.73 3.71 3.20 3.19 5.20 4.35 4.29 3.14 3.40 3.48 3.34 3.32 2.97 3.28 2.97

175.50 133.63 134.57 182.17 259.14 239.09 135.95 161.21 195.91 215.13 222.53 145.86 141.80 156.59 169.54 159.99

209.22 131.51 156.22 253.11 1163.9 257.28 152.29 197.09 159.74 295.50 255.01 188.05 167.89 183.42 240.48 200.48

555.28 424.81 424.81 613.63 1109.8 721.15 397.33 509.62 571.99 700.15 674.19 455.70 428.11 555.90 546.03 510.81

0.38 0.31 0.37 0.41 1.05 0.36 0.38 0.39 0.28 0.42 0.38 0.41 0.39 0.33 0.44 0.39

L. Kang et al. / Journal of Asian Earth Sciences 111 (2015) 733–750

Ratio 11A08.1-01 11A08.1-02 11A08.1-03 11A08.1-04 11A08.1-05 11A08.1-06 11A08.1-07 11A08.1-08 11A08.1-09 11A08.1-10 11A08.1-11 11A08.1-12 11A08.1-13 11A08.1-14 11A08.1-15 11A08.1-16 11A08.1-17 11A08.1-18 11A08.1-19 11A08.1-20 11A08.1-21

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L. Kang et al. / Journal of Asian Earth Sciences 111 (2015) 733–750 Table 2 Lu–Hf isotopic data for Early Devonian dacite (11A08.1) from the South Altyn Tagh tectonic melange belt. Sample no.

t (Ma)

176

Yb/177Hf

176

Lu/177Hf

Hf-01 Hf-02 Hf-03 Hf-04 Hf-05 Hf-06 Hf-07 Hf-08 Hf-09 Hf-10 Hf-11 Hf-12 Hf-13 Hf-14 Hf-15 Hf-16 Hf-17

408.8 406.2 404.3 403.7 406.4 403.9 408.3 400.4 403.3 405.6 407.9 409.7 401.4 404.9 404.4 405.8 408.0

0.022407 0.028942 0.034754 0.030319 0.029434 0.022807 0.039751 0.021207 0.040251 0.020980 0.017211 0.025267 0.038098 0.028292 0.032449 0.023486 0.033913

0.000826 0.001093 0.001327 0.001158 0.001135 0.000864 0.001560 0.000842 0.001540 0.000806 0.000660 0.000989 0.001423 0.001073 0.001254 0.000890 0.001344

176

Hf/177Hf

2r

0.282550 0.282628 0.282626 0.282573 0.282656 0.282628 0.282664 0.282639 0.282679 0.282609 0.282515 0.282634 0.282617 0.282620 0.282612 0.282631 0.282602

0.000008 0.000007 0.000013 0.000014 0.000012 0.000009 0.000015 0.000010 0.000019 0.000010 0.000010 0.000008 0.000011 0.000009 0.000010 0.000007 0.000009

eHf(0) 7.9 5.1 5.2 7.0 4.1 5.1 3.8 4.7 3.3 5.8 9.1 4.9 5.5 5.4 5.7 5.0 6.0

eHf(t) 0.91 3.53 3.36 1.55 4.54 3.54 4.72 3.89 5.18 2.93 0.29 3.86 2.95 3.26 2.89 3.69 2.61

2r

TDM1(Hf)(Ma)

TDM2(Hf)(Ma)

0.30 0.25 0.44 0.51 0.43 0.33 0.52 0.37 0.67 0.35 0.37 0.28 0.37 0.32 0.36 0.24 0.33

990 887 895 965 848 882 847 865 824 907 1034 876 911 897 913 878 929

1158 1022 1029 1122 971.0 1020 963.0 999.0 936.0 1052 1219 1008 1048 1035 1054 1014 1071

fLu/Hf 0.98 0.97 0.96 0.97 0.97 0.97 0.95 0.97 0.95 0.98 0.98 0.97 0.96 0.97 0.96 0.97 0.96

Fig. 4. Cathodoluminescence (CL) images of zircons for Early Devonian dacite (11A08.1) and rhyolite (11A10.4), with analytical identification number. Dotted line circles represent locations of Lu–Hf analysis spots, solid line circles indicate spots of LA-ICP-MS U–Pb Analysis spots, labeled with their 206Pb/238U ages.

In general, high (Th/Nb)N (1; Saunders et al., 1992), low Nb/La (P1; Kieffer et al., 2004) and (Th/Ta)N (<1; Neal et al., 2002), and Nb–Ta depletion are reliable trace elements characteristics of crustal contamination. Mild enrichments in Nb–Ta relative to La (i.e., Nb/La P 1; 1.04–1.16), (Th/Nb)N  1 (0.99–1.02) and (Th/Ta)N > 1 (1.89–1.94) of the Early Devonian basalt indicate that the magma

did not suffer appreciable crustal contamination (Xia et al., 2013; Xia, 2014). Low (La/Yb)N ratios usually reflect a melting regime dominated by relatively large melt fraction and/or spinel as the predominant residual phase, whereas high (La/Yb)N ratios are indicative of smaller melt fractions and/or garnet control (Yang et al., 2007; Zhang

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L. Kang et al. / Journal of Asian Earth Sciences 111 (2015) 733–750

Fig. 5. U–Pb concordia diagrams for zircons from Early Devonian dacite (11A08.1) and rhyolite (11A10.4). Data-point error crosses are 2r.

et al., 2008). Nearly-flat to chondrite normalized HREE patterns (Fig. 7c), relatively high HREE and Y contents (Yb: 2.60–2.88 ppm, Lu: 0.38–0.41 ppm, Y: 26.0–29.4 ppm) for mafic volcanic rocks suggest that there was no residual garnet in the source (e.g., Wilson, 1989; Luais and Hawkesworth, 1994; Maheo et al., 2009; Genç and Tüysüz, 2010). The relatively low (La/Sm)N (1.04–1.54) and (Tb/Yb)N (1.00–1.67) ratios (Fig. 10a), in combination with relatively flat MREE to HREE patterns (Fig. 7c) of these mafic rocks, suggest that they may have been formed by a relatively high degree of partial melting of a mantle source in the spinel facies field and thus at depths shallower than 60–70 km (Zhang et al., 2008). This is consistent with their moderate TiO2 and total Fe2O3 concentrations that are similar to those of glasses produced by partial melting experiments on natural anhydrous fertile spinel peridotite (HK-66; Hirose and Kushiro, 1993) (Fig. 10b). Moreover, these mafic rocks plot along the batch-melting curve of spinel peridotite in La/Sm vs. Sm/Yb diagram (Fig. 10c), suggesting that the mafic rocks were derived from strong batch-melting of spinel peridotite. It is noted that mafic magmas derived from asthenosphere typically have La/Nb ratios of <1.5 and La/Ta ratios <22, whereas those from lithosphere have La/Nb ratios >1.5 and La/Ta ratios >22 (Fitton et al., 1988; Saunders et al., 1992; Li et al., 2013a). Low La/Nb ratios (0.86–0.96), low La/Ta ratios (13.91–15.12) and positive eNd(t) values (+3.3 to +3.4) indicate that the Early Devonian basalt should have originated from the asthenosphere. 6.1.2. Petrogenesis of the felsic rocks The Early Devonian felsic volcanic rocks (rhyolite and dacite) are all characterized by high SiO2, Na2O + K2O and TFe2O3 contents, low MgO, strong negative Eu anomalies, low Ba (385–864 ppm) and Sr (65.79–147.5 ppm) contents, relatively high Rb/Sr (0.98–1.68, av. 1.28) and Ga/Al ratios (2.43–2.68), and high concentrations in Ga (15.3–20.5 ppm), Zr (343–469 ppm), Nb (17.0–31.4 ppm), Ce (86.5–140 ppm) and Y (44.8–61.7 ppm). Overall, these are typical of major and trace element geochemical features of A-type granites (e.g., Whalen et al., 1987), and they all fall into the A-type granite field in Ga/Al vs. Zr (Fig. 11a) plots, are comparable to the A-type granites in the South Altyn Tagh tectonic melange belt (e.g., Wang et al., 2008) (Fig. 7c and d). Moreover, the felsic volcanic rocks have low Sr (65.8–144 ppm) and LaN/YbN (5.43–7.89), high Y (44.8–61.7 ppm) and Yb (4.51–5.74 ppm), and strong negative Eu anomalies. These features indicate that they originated from a source that lacked garnet but contained plagioclase in the residue which indicating partial melting in low pressure (e.g., Petford and Atherton, 1996; Zhang et al., 2006). The calculated zircon saturation temperatures (TZr) of the felsic volcanic samples yield saturation temperatures of

859–880 °C (Table 3). Therefore, these are suggested that Early Devonian felsic volcanic rocks are typical A-type magmatite which formed in low pressure and high temperature conditions. The petrogenesis of A-type felsic rocks as a part of bimodal suites has been a highly debated topic. Several main categories of petrogenetic schemes have been proposed for their origin, including: (1) melting of felsic crust (Clemens et al., 1986; Creaser et al., 1991; Patiño Douce, 1997; King et al., 1997; Brewer et al., 2004); (2) fractionation products of mantle-derived tholeiitic and alkaline magmas (Turner et al., 1992; Mushkin et al., 2003; Peccerillo et al., 2003; Tian et al., 2010; Genç and Tüysüz, 2010); and (3) combination of crustally-derived felsic magma with mantle-derived mafic magma (Foland and Allen, 1991; Frost and Frost, 1997; Frost et al., 1999; Mingram et al., 2000; Yang et al., 2006; Zhang et al., 2008). Although the Early Devonian A-type felsic volcanic rocks could have been produced by extreme differentiation of coeval basalt parent magmas, there are against this possibility. First, the volume of A-type felsic volcanic rocks in the South Altyn Tagh tectonic melange belt is much larger than the mafic ones. Second, there is a ‘‘gap’’ between mafic and felsic rocks, or a lack of intermediate compositions. In contrast, these felsic volcanic samples display an overall enriched pattern except for depletion in P, Eu and Ti due to fractional crystallization (Fig. 7c and d), low Mg# (0.23– 0.33) and strong Nb–Ta negative anomalies, Hf model age TDM2 values vary from 963.0 to 1219 Ma and Nd model ages of TDM = 1023 and 1181 Ma, indicating that these felsic magmas should have been generated from a crustal source. This is further supported by the Yb/Ta vs. Y/Nb diagram (Fig. 11d), in which the felsic volcanic rocks exhibit similarities to the crust between OIB and IAB. A pure crustal origin is untenable for these Early Devonian A-type felsic volcanic rocks, though the felsic volcanic rocks are closely related to the crust. Currently, the combined interaction between crustal and mantle sources for the origin of A-type magmas has been widely recognized (Foland and Allen, 1991; Frost et al., 1999; Liu et al., 2005a,b; Yang et al., 2006), mean while the Early Devonian A-type felsic volcanic rocks should be suitable to this model because: (a) The positive or slightly negative Nd isotopic compositions (eNd(t) = +2.3 and 0.5) of the felsic volcanic rocks (Fig. 9a), and almost positive Hf isotope (eHf(t) = 0.29 to +5.18, av. +3.12) (Fig. 9b) of the dacite preclude their derivation by anatexis of the highly evolved continental crust. (b) Melting experiments of Patiño Douce (1997) demonstrated that dehydration melting of hornblende-bearing granitoids in the crust (P < 4 kbar, suitable for depths of 15 km or less)

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L. Kang et al. / Journal of Asian Earth Sciences 111 (2015) 733–750 Table 3 Major and trace element compositions of the Early Devonian bimodal volcanic rock in the South Altyn Tagh tectonic melange belt. Rock type

Basalt

Rhyolite

Dacite

Sample

11A13.1

11A13.2

11A13.3

11A13.4

11A13.5

11A13.6

11A10.1

11A10.2

11A10.3

11A10.4

11A08.1

11A08.2

11A08.3

SiO2 TiO2 Al2O3 T Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total Mg# La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y P REE (La/Yb)N Eu/Eu⁄ Ce/Ce⁄ V Cr Co Ni Ga Rb Ba Th U Nb Ta Sr Nd Zr Hf Sm Eu Yb M TZr

47.27 0.98 16.04 9.89 0.15 4.76 11.23 3.99 0.54 0.14 4.81 99.8 0.49 7.01 15.3 2.05 9.49 2.62 1.11 3.27 0.60 4.13 0.92 2.68 0.40 2.61 0.38 26.9 52.6 1.81 1.15 0.94 141 373 45.4 223 19.6 10.9 202 0.48 5.95 7.31 0.46 583 9.49 77.4 1.74 2.62 1.11 2.61

47.16 0.98 15.99 9.86 0.15 4.74 11.20 3.97 0.55 0.14 4.78 99.52 0.49 6.98 15.2 2.07 9.50 2.70 1.12 3.27 0.61 4.19 0.93 2.70 0.40 2.68 0.39 26.8 52.7 1.76 1.15 0.93 142 375 45.3 220 16.1 10.7 203 0.39 5.89 7.30 0.47 589 9.50 76.5 1.72 2.70 1.12 2.68

48.63 0.94 17.99 11.17 0.14 4.16 8.07 4.57 0.58 0.15 3.05 99.45 0.42 7.13 15.0 2.24 10.4 2.91 1.07 3.61 0.67 4.52 0.99 2.92 0.43 2.88 0.41 29.4 55.2 1.67 1.01 0.88 164 400 47.3 181 16.2 10.5 186 0.36 6.16 8.01 0.51 479 10.4 84.5 1.92 2.91 1.07 2.88

50.39 0.99 16.97 8.31 0.13 4.29 8.13 5.34 0.75 0.15 4.32 99.77 0.50 6.88 14.6 1.99 9.17 2.54 0.99 3.13 0.58 3.94 0.87 2.57 0.38 2.60 0.39 26.0 50.6 1.78 1.07 0.92 165 378 43.0 152 16.8 9.12 303 0.47 9.07 7.26 0.46 397 9.17 76.2 1.71 2.54 0.99 2.60

49.83 0.88 17.53 10.29 0.14 4.03 8.43 4.66 0.72 0.14 3.44 100.10 0.44 6.66 14.6 2.00 9.34 2.59 1.03 3.15 0.58 4.05 0.90 2.67 0.39 2.66 0.39 26.5 51.0 1.69 1.10 0.93 232 361 44.9 178 15.5 9.96 240 0.32 7.70 7.62 0.48 450 9.34 80.2 1.80 2.59 1.03 2.66

48.12 1.08 16.61 11.90 0.15 5.56 7.68 3.95 0.77 0.16 3.57 99.55 0.48 6.82 14.7 2.18 10.2 2.85 0.99 3.46 0.64 4.33 0.95 2.75 0.40 2.72 0.39 27.6 53.4 1.69 0.96 0.89 145 411 54.9 260 17.3 19.3 210 0.34 8.47 7.92 0.48 450 10.2 83.8 1.86 2.85 0.99 2.72

72.29 0.25 13.35 2.64 0.06 0.42 1.68 3.35 3.38 0.06 2.87 100.40 0.24 43.2 89.5 10.6 40.5 8.24 1.63 7.89 1.33 8.46 1.77 5.27 0.80 5.38 0.79 53.5 225 5.43 0.61 0.96 4.94 5.28 3.70 2.99 15.3 123 385 17.0 0.28 17.6 1.54 103 40.5 354 8.44 8.24 1.63 5.38 1.31 868

72.58 0.24 13.28 2.43 0.05 0.42 1.36 3.27 3.40 0.06 2.46 99.55 0.25 44.2 91.5 10.9 41.8 8.66 1.66 8.28 1.39 8.64 1.79 5.21 0.79 5.25 0.76 54.5 231 5.68 0.59 0.96 4.95 4.52 3.30 2.35 19.3 125 506 16.9 0.28 17.0 1.48 86.9 41.8 343 8.25 8.66 1.66 5.25 1.24 870

72.89 0.25 13.19 3.16 0.03 0.47 0.84 3.34 3.57 0.06 1.96 99.76 0.23 42.5 88.5 10.4 39.3 7.76 1.46 7.14 1.16 7.15 1.49 4.39 0.67 4.51 0.65 44.8 217 6.37 0.59 0.96 5.01 5.81 3.90 4.23 19.3 110 600 17.4 0.20 18.2 1.52 65.8 39.3 358 8.47 7.76 1.46 4.51 1.18 880

70.89 0.23 13.30 2.67 0.05 0.45 2.08 3.24 3.42 0.06 3.12 99.51 0.25 41.9 86.5 10.2 39.3 7.98 1.58 7.78 1.31 8.37 1.72 5.06 0.78 5.12 0.75 52.5 218 5.54 0.61 0.96 4.88 3.93 3.27 2.45 17.3 129 404 16.3 0.19 16.9 1.46 77.8 39.3 351 8.30 7.98 1.58 5.12 1.40 859

68.43 0.57 14.42 2.85 0.08 0.70 1.72 3.99 4.89 0.12 2.09 99.86 0.33 65.5 132 15.4 56.8 10.8 1.57 9.54 1.51 9.10 1.85 5.44 0.82 5.61 0.84 57.0 316 7.89 0.46 0.94 2.61 7.78 2.47 4.41 19.1 146 864 25.2 0.20 29.8 2.03 148 56.8 431 10.6 10.8 1.57 5.61 1.58 864

68.95 0.61 14.68 2.94 0.08 0.66 1.43 4.77 4.18 0.13 1.89 100.30 0.31 69.0 140 16.5 61.2 11.8 1.74 10.6 1.66 10.01 1.99 5.87 0.87 5.95 0.89 61.7 338 7.84 0.47 0.95 2.75 9.21 3.56 5.51 19.7 126 706 25.4 0.20 31.4 2.09 128 61.2 466 11.3 11.8 1.74 5.95 1.54 875

67.76 0.57 14.23 2.92 0.08 0.71 2.09 4.19 4.80 0.12 2.37 99.84 0.32 67.0 135 15.8 58.5 11.1 1.58 9.90 1.57 9.53 1.92 5.67 0.86 5.74 0.86 59.5 325 7.88 0.45 0.94 2.62 3.89 3.31 2.04 20.5 151 792 25.4 6.57 31.2 2.09 144 58.5 469 11.3 11.1 1.58 5.74 1.71 862

Note: Major element concentrations in wt.%, trace element data as ppm. LOI = loss on ignition; Mg# = Mg2+/(Mg2+ + TFe2+); Eu/Eu⁄ = (Eu)cn/[(Gd)cn + (Sm)cn]/2; Ce/Ce⁄ = (Ce)cn/ [(La)cn + (Pr)cn]/2; TZr, zircon saturation thermometer (Watson and Harrison, 1983).

is a likely origin for high-silica, A-type granites, but it is certain that the high magmatic temperature for the Early Devonian A-type felsic volcanic rocks (Table 3) in the upper crust requires heat and/or mass transfer from mantle-derived, hot basaltic magmas in the origin of A-type rocks, thus chemical interactions between hot basaltic magmas and crustal melts will have had typically taken place. These were strongly proven by the wide range of 87 Sr/86Sr isotopic ratios (0.70753–0.71246) and distinct eNd(t) values ( 0.5 to +2.6) for the Early Devonian felsic volcanic rocks, which should show contamination by the mantle (analogous to Early Devonian mafic rocks) and upper crust (Fig. 9a). (c) In the South Altyn Tagh tectonic melange belt, A-type felsic volcanic rocks have similar geochemical characteristics to

massive coeval A-type granites (Fig. 7c and d), which exhibit evidence for magma mixing between mantle melts and crustal magmas (Wang et al., 2008). In the light of the discussion above, we conclude that the Early Devonian felsic volcanic rocks (rhyolite and dacite) were mainly produced by partial melting of crust in low pressure and high temperature conditions, simultaneously existing mantle magmatic upwelling and slightly magma mixing in an extensional setting. 6.2. Tectonic implications Bimodal magmatic suites from different settings commonly display distinct geochemical characteristics and diagnostic relationships between mafic and felsic end-members (e.g., Pin and

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L. Kang et al. / Journal of Asian Earth Sciences 111 (2015) 733–750

Fig. 6. Geochemical classification diagrams for the Early Devonian bimodal volcanic rock in the South Altyn Tagh tectonic melange belt. (a) Zr/TiO2 vs. Nb/Y diagram from Winchester and Floyd (1977) and (b) Th vs. Co (Hastie et al., 2007).

Fig. 7. (a and c) Chondrite-normalized REE pattern and (b and d) primitive mantle-normalized trace element pattern for the Early Devonian mafic and felsic volcanic rocks, respectively. Chondrite-normalized and primitive mantle-normalized values are from Boynton (1984) and Sun and Mc Donongh (1989), respectively. Upper crust, Middle crust and Lower crust values are Rudnick and Gao (2003). Data for A-type granite that formed in 424 Ma in the South Altyn Tagh tectonic melange belt are from Wang et al. (2008). Table 4 Rb–Sr and Sm–Nd isotopic compositions for the Early Devonian bimodal volcanic rock in the South Altyn Tagh tectonic melange belt. Sample

Rock type

Age (Ma)

87

Rb/86Sr

87

Sr/86Sr

11A10.2

Rhyolite

406

2.9252

0.72936

±4

0.71246

0.11655

0.51240

±4

0.5

1181

0.45

11A08.1

Dacite

406

4.0102

0.73070

±4

0.70753

0.12762

0.51257

±8

2.3

1023

0.40

11A13.1 11A13.2 11A13.4

Basalt

406 406 406

0.0522 0.0611 0.0618

0.70801 0.70809 0.70809

±6 ±3 ±5

0.70773 0.70775 0.70774

0.16963 0.17176 0.17013

0.51273 0.51275 0.51274

±6 ±6 ±6

3.3 3.4 3.3

1440 1459 1450

0.21 0.20 0.20

Error (2r)

87

Sr/86Sr(i)

147

Sm/144Nd

143

Nd/144Nd

Error (2r)

eNd(t)

TDM (Ma)

fSm/Nd

Note: eNd(t) is calculated with 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1967 for present-day CHUR (Jacobsen and Wasserburg, 1980); TDM is calculated with Nd/144Nd = 0.51315 and 147Sm/144Nd = 0.2137 for present-day depleted mantle (Goldstein et al., 1984; Peucat et al., 1988).

143

Paquette, 1997; Zhang et al., 2008; Meng et al., 2011). For example, those from oceanic islandarcs generally contain voluminous mafic members and subordinate felsic members, yet these members share similar geochemical characteristics such as low-K and strong depletion in incompatible elements (e.g., Ikeda and Yuasa, 1989). The mafic and felsic rocks in island arc and back-arc settings are

characterized by an enrichment in LILEs and LREEs, and a depletion in HFSEs (Frey et al., 1984; Hochstaedter et al., 1990; Pin and Paquette, 1997; Genç and Tüysüz, 2010; Xie et al., 2011). Continental rift bimodal volcanics typically consist of abundant felsic and minor mafic rocks with similar geochemical characteristics, such as high-K and a clear enrichment in LILEs and HFSEs

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Fig. 8. MgO vs. SiO2 (a), Cr (b), Ni (c), TFe2O3 (d), TiO2 (e), P2O5 (f), Al2O3 (g), and K2O (h) variation diagrams for the Early Devonian bimodal volcanic rock in the South Altyn Tagh tectonic melange belt. Yusupuleke granite is data from Wang et al. (2008, 2014), Tula granite is data from Wu et al. (2007).

(e.g., Duncan et al., 1984; Pin and Marini, 1993; Garland et al., 1995; Li et al., 2013a). In contrast, the mafic rocks from the post-collisional to within-plate extensional settings are typically tholeiitic gabbros and/or basalts, whereas the coeval felsic rocks generally display the geochemical characteristics of A-type magmatic rocks (Turner et al., 1992; Frost and Frost, 1997; Frost et al., 1999; Bonin, 2004; Zhang et al., 2008; Li et al., 2014). As mentioned above, the Early Devonian bimodal volcanic rocks in the South Altyn Tagh tectonic melange belt are composed mainly of dacite, rhyolite, and minor basalt which belong to A-type felsic magmatite and tholeiite group respectively (Fig. 6a and b), hence should formed in the post-collisional to within-plate extensional settings.

The mafic rocks show high degrees of partial melting of the asthenosphere at shallow depths, and the tectonic setting generally advocating for such a successive mafic magmatism in a given area is a post-collisional extensional regime (e.g., Turner et al., 1999; Zhang et al., 2008). Due to Th, Ta and Hf being strongly incompatible elements, their ratios of each other would vary weakly during mantle melting and crystallization differentiation, and therefore are suitable for tectonic setting discrimination (Wang et al., 2001). In Th/Hf vs. Ta/Hf diagram, the Early Devonian basaltic rocks fall into continental rift tholeiitic (Fig. 12). Especially, in the South Altyn Tagh tectonic melange belt, these mafic rocks are an analogue for the Changshagou mafic rocks (ca. 467 Ma) (Ma et al., 2009) and Mangnai mafic rocks

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Fig. 9. (a) eNd(t) vs. 87Sr/86Sr(i) (Zindler and Hart, 1986) for the Early Devonian bimodal volcanic rock and (b) eHf(t) vs. U–Pb ages of zircons from the dacite in the South Altyn Tagh tectonic melange belt. EMI and EMII – enriched mantle I and II sources; HIMU – high-l mantle source; DM – depleted mantle source; PM – primitive mantle. Upper crust values are from Jahn et al. (1999).

Fig. 10. (a) (La/Sm)N vs. (Tb/Yb)N, (b) TiO2 vs. Fe2O3 and (c) La/Sm vs. Sm/Yb diagrams for the mafic volcanic rocks in the South Altyn Tagh tectonic melange belt. (a) The fields for tholeiitic, transitional and alkali basalts are after Zhi et al. (1990). The horizontal dashed line separates expected fields for melting garnet- and spinel-bearing peridotite as determined for the Cenozoic basalts in the Basin and Range province (Wang et al., 2002). (b) The fields for peridotitic melts are after Fallon et al. (1988) and Hirose and Kushiro (1993). (c) Modal mineralogy for spinel- and garnet-peridotites are taken from Wilson (1989).

Fig. 11. Chemical classification diagrams for the felsic magmatite in the South Altyn Tagh tectonic melange belt. (a) Ga/Al vs. Zr diagram (Whalen et al., 1987) where all data fall into the A-type granite and (b) the Yb/Ta vs. Y/Nb diagram (Eby, 1992) exhibiting similarities to average crustal ratios between OIB and IAB (island-arc basalt); Yusupuleke granite is data from Wang et al. (2008, 2014), Tula granite is data from Wu et al. (2007).

(ca. 453 Ma) (Wang et al., 2014) which formed in the initial post-collisional stage of South Altyn Tagh orogeny (Fig. 13), and differing from E-MORBs (ca. P500 Ma) (Wang et al., 1999) and MORBs (ca. P500 Ma) (Li et al., 2009) as the part of Apar-Mangnai ophiolite. Conversely, the A-type felsic volcanic rocks, together with massive coeval A-type granites in the South Altyn Tagh tectonic

melange belt (e.g., Wu et al., 2007; Wang et al., 2008), are the felsic end-members of the Early Devonian bimodal magmatic suites. Such successive magmatism is generally developed in an extensional environment during the closing to post-orogenic worldwide (Hildreth et al., 1991; Turner et al., 1992; Frost and Frost, 1997; Frost et al., 2001; Mushkin et al., 2003). Furthermore, Nb–Y–Ce and Nb–Y–Ga ternary diagrams of Eby (1992) (Fig. 14a and b) also

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break-off during the early Paleozoic, which necessarily led to multiple magmatic activities such as upwelling of mantle magma, remelting of crust and mixing of mantle and crustal magma (Pitcher, 1983; Miller et al., 1999). Diverse magmatite ranges in age from Middle Cambrian (502 Ma) to Middle Devonian (385 Ma), consists of ultramafic, mafic and felsic igneous rocks (Fig. 1A and Table 5) with tholeiitic, calc-alkaline and alkali series, and are the recorders of the entire orogenic process from continental collision to post-collision (e.g., Wu et al., 2007; Wang et al., 2008; Cao et al., 2010; Sun et al., 2012; Yang et al., 2012; Kang et al., 2013, 2014, in press). According to the nature and mechanism of magmatic activity, we come to the conclusion that tectonic magmatic evolution of the early Paleozoic South Altyn Tagh should be divided into three stages: (1) 502–472 Ma; (2) 467–450 Ma; and (3) 432–385 Ma (Fig. 15). Fig. 12. Ta/Hf vs. Th/Hf tectonic discriminant diagram (Wang et al., 2001) for the mafic volcanic rocks in the South Altyn Tagh tectonic melange belt, I-Destructive plate margins N-MORB; II-Convergent plate margin (II1. Oceanic islands basalt; II2 Continental margins basalt); III-Intra-oceanic islands basalt and seamount basalt, TMORB and E-MORB; IV-Within-continental plate (IV1. Continental rifts/Continental margins rifts tholeiite; IV2. Within-plate alkali basalt; IV3. Continental extensional zone/initial rift basalt); V-Mantle plume basalt.

indicate that Early Devonian felsic volcanic rocks belong to the A2-type granites which indicate a post-orogenic extensional tectonic setting (Liégeois, 1998). The South Altyn Tagh HP/UHP metamorphic belts mark the convergent lithospheric plate boundaries due to the paleo oceanic subduction and continental collision (Liu et al., 2009, 2012, 2013). The Early Devonian bimodal volcanic rocks (ca. 405.8 Ma and 406.1 Ma) and massive coeval A2-type granites (ca. 385.2 Ma and 424 Ma) outcropping along the South Altyn HP/UHP belts are obviously younger than the continental deep subduction (ca. 486–504 Ma) and slab break off (ca. 455 Ma), so the region during the Early Devonian should be in the stage of post-collision. In addition, the studies have shown that post-collisional extension can be attributed to slab detachment or break-off (Davies and von Blanckenbry, 1995) which can cause upwelling of the asthenosphere, subsequently perturbing the original thermal gradient, and resulting in crustal partial melting (Bonin, 2004), even leading to mixing of mantle melts and crustal magmas. This diagenetic model conforms to the petrogenetic schemes and geological background of the Early Devonian bimodal volcanic rocks, Consequently, they should formed in post-collisional extensional setting. 6.3. Tectonic magmatic evolution of the early Paleozoic South Altyn Tagh Currently, studies of HP/UHP metamorphic rocks in the South Altyn Tagh have indicated continental deep subduction and slab

6.3.1. 505–472 Ma: deep subduction of continental crust during the stage of continental collision (Fig. 16a) Only a small quantity of granites (e.g., Kang et al., 2014, in press; Sun et al., 2012) outcropped in the South Altyn Tagh (Fig. 1A and Table 5), all which formed during the HP/UHP metamorphism (486–504 Ma) during the stage of continental crust deep subduction. Studies have shown that these granites should have formed in high-pressure conditions of crustal thickening in the collision orogenic stage, as according to their adakitic chemical characteristics of high (La/Yb)N, high Sr and low Y contents (Sun et al., 2012; Kang et al., 2014, in press). In particular, the interaction between mantle magma and the subducted slab was indicated by abundant mafic magmatic enclaves (MME) in the Yumuquan Magma Mixing Granite (Sun et al., 2012), and led to the subducted continental crust being weakened and initially tearing due to this thermal weakening (Toussaint et al., 2004a,b; Li et al., 2013b; Li, 2014; Burov et al., 2014). Moreover, these granites all outcropped in the southern region of Altyn Tagh fault zone (Fig. 15), this distribution characteristic is remarkably consistent with the molten rock lying to the left of the suture zone (the side of the subducted continental crust) according to the numerical geodynamic modeling of one-sided steep continental subduction (Li et al., 2008, 2013b; Li, 2014). 6.3.2. 467–450 Ma: exhumation during the stage of continental slab break-off (Fig. 16b) Massive intrusive rocks (Ma et al., 2011; Wang et al., 2014; Cao et al., 2010; Kang et al., 2013, in press; Yang et al., 2012) formed during this stage (Fig. 1A and Table 5), and are composed of diverse rock types (e.g., augite peridotite, gabbro, diorite, monzonitic granite and alkali granite), which apparently coeval with the retrograde metamorphic ages of the South Altyn Tagh HP/UHP terrain

Fig. 13. Comparisons of the Early Devonian basaltic rocks with P500 Ma MORB (the most basic basalt sample, 07b-42, Mg# = 75.39) (Li et al., 2009) and E-MORB (the most basic basalt sample, 95A37, Mg# = 63) (Wang et al., 1999) belong to Apar-Mangnai ophiolite, 467 Ma continental rift mafic rocks (the most basic gabbro sample, 07b-147, Mg# = 70.7) (Ma et al., 2009) and 453 Ma post-collisional mafic rocks (the most basic diabase sample, 10A-06/7, Mg# = 65) (Wang et al., 2014) in the South Altyn Tagh tectonic melange belt.

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Fig. 14. Chemical classification diagrams of A-type granite (after Eby (1992)). (a) Nb–Y–Ce and (b) Nb–Y–Ga ternary diagrams for the subdivision of A1- and A2-type granites (Eby, 1992) indicating belong to the A2-type granites. Yusupuleke granite is data from Wang et al. (2008, 2014), Tula granite is data from Wu et al. (2007).

Table 5 The main features of early Paleozoic magmatite for the South Altyn Tagh. No.

Magmatite

Age (Ma)

Rock

Rock series

Geochemical characteristics

Sources

Temperature (°C) and pressure (GPa)

Reference

1

Changshagou granite

503.1 ± 1.7

Quartz diorite

Metaluminous calc-alkaline

High (La/Yb)N, high Sr and low Y, adakite

Lower crust

2

Yumuquan

497 ± 2

Granodiorite, monzonitic granite

High (La/Yb)N, high Sr and low Y, adakite

Lower crust

Kang et al. (2014) Sun et al. (2012)

3

Mangnai granite

472 ± 1

Monzonitic granite

Metaluminous– Peraluminous calc-alkaline I-type granite Weakly peraluminous Shoshonitic S-type granite

High-temperature (900) and highpressure (1.2–1.5) High-pressure

High (La/Yb)N, moderate Sr and low Y, high Sr/Y ratios

Upper crust (sandstone and mudstone)

Low-temperature (<800) and highpressure (>1.8)

Kang et al. (in press)

4

Changshagou

467 ± 1

Tholeiitic

LREE enrichment relative to HREE

E-MORB

High-temperature and low-pressure

5

Tatelekebulake

462 ± 2

Augite, peridotite, gabbro, quartz diorite, granodiorite Monzonitic granite, syenogranite

Low Sr and high Y, low Sr/Y ratios

Middle-lower crust

6

Mangnai diorite

458 ± 6

Quartz diorite

High Sr and high Y, low Sr/Y ratios

Upper crust (metabasalt)

High-temperature (800) and lowpressure (<0.8) High-temperature (>800) and lowpressure (<1)

7

Mangnai diabase

453 ± 5

Diabase

Peraluminous calc-alkaline S-type granite Metaluminous– peraluminous calc-alkaline I-type granite Tholeiitic

Ma et al. (2009, 2011) Cao et al. (2010)

E-MORB

Low-pressure

8

Dimunalike

453 ± 3

Syenogranite

Peraluminous calc-alkaline S-type granite

Slightly LREE enrichment relative to HREE Low Sr and high Y, low Sr/Y ratios

High-temperature (800) and lowpressure (1)

9

East of Tatelekebulake

451 ± 2

Monzonitic granite, syenogranite, quartz diorite

Peraluminous S-type granite

High (La/Yb)N, low Sr and low Y, high Sr/Y ratios

Upper crust (sandstone), TcDM = 1200– 1465 Ma Upper crust (sandstone and mudstone)

Wang et al. (2014) Yang et al. (2012)

High-temperature (>800) and highpressure

Kang et al. (2013)

Yusupuleke

432 ± 4, 424

Monzogranites

Metaluminous calc-alkaline A-type granite

Low Sr and high Y, low Sr/Y ratios

Middle to lower crustal

High-temperature (796–831) and lowpressure

Bimodal volcanic rocks

406 ± 1

Dacite, rhyolite, and basalt

Tholeiitic and A-type volcanic rock

Asthenosphere and felsic crust

High-temperature and low-pressure

Tula

385 ± 8

Syenogranite

Metaluminous calc-alkaline A-type granite

Slightly LREE enrichment relative to HREE for basalt, low Sr and high Y, low Sr/Y ratios for dacite and rhyolite Low Sr and high Y, low Sr/Y ratios

Wang et al. (2008, 2014) This study

Crust

High-temperature and low-pressure

Wu et al. (2007)

10

11, 12

13

(450–455 Ma) (Liu et al., 2012, 2013). The continental slab broke off as a result of the subducted continental crust being weakened and torned, and the broken-off slab mainly consisted of mafic lower crust, lithosphere and residual oceanic crust with the absence of felsic upper crust because of the low density of felsic

Kang et al. (in press)

upper crust (Burov and Yamato, 2008; Burov et al., 2014). Slab break-off primarily gave rise to partial or complete loss of the slab pull force (Duretz et al., 2012) and buoyant flow (Royden et al., 2008; Li et al., 2013a,b, 2014). The structural model analyses have proven that the broken-off slab can only provide felsic upper crust

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Furthermore, slab break-off could have triggered the inflow of hot asthenosphere to where the slab ruptured (Duretz et al., 2012; Li et al., 2013a,b), the Changshagou mafic–ultramafic intrusion and Mangnai diabase dikes should have recorded the mantle magma underplating, which provided heat for the upper crust melting. This magmatic formation mechanism can better explain the formation of the mafic–ultramafic intrusion, and is suitable for the characteristics of the medium-acidic magmatic activities at high temperature and low pressure (e.g., Cao et al., 2010; Yang et al., 2012; Kang et al., in press) in this stage of the South Altyn Tagh.

Fig. 15. Probability of zircon U–Pb age of the early Paleozoic magmatite, showing three stages of tectonic magmatic evolution. Data references see Table 5.

6.3.3. 432–385 Ma: extension during the stage of post-collision (Fig. 16c) Abundant intrusive rocks and a small quantity of volcanic rocks in this stage were distributed in the southern region of the Altyn Tagh fault (Fig. 1A and Table 5). The intrusive rocks consist of the Yusupuleke (ca. 424 Ma; Wang et al., 2008) and Tula intrusions (ca. 385 Ma; Wu et al., 2007), which belong to A-type granites (Fig. 11), and with elemental compositions of low Sr, high Y and low Sr/Y ratios. The volcanic rocks are Early Devonian bimodal volcanic rocks (ca. 406 Ma), which are comprised of tholeiite and A-type dacite–rhyolite (this study). Moreover, these A-type granites and A-type felsic volcanics should have the same petrogenesis, based upon the fact that they have the same major element evolution trend (Fig. 8) and similar trace element compositions (Fig. 7), so these felsic volcanic should be the extrusive facies of the coeval A-type granites. This characteristic magmatic association should have formed in the post-collisional extensional setting (e.g., Wu et al., 2007; Wang et al., 2008, 2014 and this study). Moreover, the tholeiite which originated in the asthenosphere, the dacite– rhyolite relate to the mixing of mantle melts and crustal magmas, and abundant mantle-derived enclaves (MME) in Yusupuleke intrusion all indicate existing interaction between mantle and crust, and even asthenosphere upwelling in deep crust (Wang et al., 2008, 2014). Particularly, the ages of these magmatites (424–385 Ma) are 26–65 Ma younger than the continental slab break-off stage (467–450 Ma). After the continental slab break-off, the relaxation of the sustained regional stress caused the tectonic setting to transfer from slab break-off (vertical uplifting) to post-collisional (horizontal extension).

7. Conclusions The following conclusions can be drawn from this study:

Fig. 16. Early Paleozoic tectonic magmatic evolution pattern of South Altyn Tagh. (a) 505–472 Ma Deep subduction of continental crust during the stage of continental collision; (b) 467–450 Ma Exhumation during the stage of continental slab break-off; and (c) 432–385 Ma Extension during the stage of post-collisional. Figures (a) and (b) are modified after Davies and von Blanckenbry (1995), Li et al. (2013b), Li (2014) and Burov et al. (2014).

uplift with no mafic lower crust (Zhang et al., 2001a,b; Li, 2004; Li et al., 2005), thus the medium-acidic magmatic activities of this stage can be characterized by remelting of upper crust in reduced stress conditions (e.g., Yang et al., 2012; Kang et al., 2013, in press).

(1) New LA-ICPMS zircon U–Pb dating indicates that the new discovered bimodal volcanic rocks in the South Altyn formed at ca. 406 Ma. (2) Geochemical and isotopic tracing suggests that the mafic volcanic rocks were likely derived from strong batch-melting of the asthenosphere. The rhyolite and dacite were generated by partial melting of the crust, and suffered slight contamination from the mantle. (3) According to the petrogenetic schemes and geological background, the Early Devonian bimodal volcanic rocks (tholeiite and A-type dacite–rhyolite) should formed in a post-collisional extensional setting. (4) On the basis of spatial and temporal distribution, associations and formation mechanism of magmatite, the tectonic magmatic evolution of the early Paleozoic South Altyn Tagh could be divided into three stages: (a) 505–472 Ma, the stage of continental collision, magmatic activities were characterized by high-pressure conditions due to the deep subduction and initial tearing of continental crust;

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(b) 467–450 Ma, the stage of continental slab break-off, these magmatites formed during felsic upper crust uplifting and mantle magma underplating; (c) 432–385 Ma, the stage of post-collisional extension, the magmatite consists of A-type granites and bimodal volcanic rocks, and are the products of the interaction between mantle and crust, and asthenosphere upwelling. Acknowledgements The author is very grateful to the helpful discussions with many colleagues, including Drs. Liang Liu, Chao Wang, Wenqiang Yang and Ran Wang, thanks Bor-ming Jahn, Irene Yao, Baodi Wang and the other anonymous reviewer, and editors for their valuable comments and suggestions. This work is supported by the National Natural Science Foundation of China (Grant No. 41202044), China Geological Survey (Grant Nos. 1212011085034 and 12120113044500) and the Natural Science Foundation of Shaanxi Province, China (Grant No. 2012JM5004). 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.2015.06. 004. These data include Google maps of the most important areas described in this article. References Blichert-Toft, J., Albarede, F., 1997. The Lu–Hf geochemistry of chondrites and the evolution of the mantle-crust system. Earth Planet. Sci. Lett. 148, 243–258. Bonin, B., 2004. Do coeval mafic and felsic magmas in post-collisional to withinplate regimes necessarily imply two contrasting, mantle and crustal, sources? A review. Lithos 78, 1–24. Boynton, W.V., 1984. Cosmochemistry of the rare earth elements: meteorite studies. In: Henderson, P. (Ed.), Rare Earth Element Geochemistry. Elsevier, Amsterdam, pp. 63–114. Brewer, T.S., Ahall, K.I., Menuge, J.F., Storey, C.D., Parrish, R.R., 2004. Mesoproterozoic bimodal volcanism in SW Norway, evidence for recurring pre-Sveconorwegian continental margin tectonism. Precambr. Res. 134, 249– 273. Burov, E., Yamato, Ph., 2008. Continental plate collision, P–T–t–z conditions and unstable vs. stable plate dynamics: insights from thermo-mechanical modelling. Lithos 103, 178–204. Burov, E., Francois, T., Yamato, P., Wolf, S., 2014. Mechanisms of continental subduction and exhumation of HP and UHP rocks. Gondwana Res. 25, 464–493. Cao, Y.T., Liu, L., Wang, C., Chen, D.L., Zhang, A.D., 2009. P–T path of Early Paleozoic pelitic high-pressure granulite from Danshuiquan area in Altyn Tagh. Acta Petrol. Sinica 25, 2260–2270 (in Chinese with English abstract). Cao, Y.T., Liu, L., Wang, C., Yang, W.Q., Zhu, X.H., 2010. Geochemical zircon U–Pb dating and Hf isotope compositions studies for Tatelekebulake granite in South Altyn Tagh. Acta Petrol. Sinica 26 (11), 3259–3271 (in Chinese with English abstract). Chu, N.C., Taylor, R.N., Chavagnac, V., Nesbitt, R.W., Boella, R.M., Milton, J.A., Germain, C.R., Bayon, G., Burton, K., 2002. Hf isotope ratio analysis using multicollector inductively coupled plasma mass spectrometry: an evaluation of isobaric interference corrections. J. Anal. At. Spectrom. 17, 1567–1574. Clemens, J.D., Holloway, J.R., White, A.J.R., 1986. Origin of an A-type granites: experimental constraints. Am. Mineral. 71, 317–324. Coulon, C., Maluski, H., Bollinger, C., Wang, S., 1986. Mesozoic and Cenozoic volcanic rocks from central and southern Tibet: 39Ar/40Ar dating, petrological characteristics and geodynamic significance. Earth Planet. Sci. Lett. 79, 281– 302. Creaser, R.A., Price, R.C., Wormald, R.J., 1991. A-type granites revisited: assessment of a residual-source model. Geology 19, 163–166. Davies, J.H., von Blanckenbry, R.G.F., 1995. Slab break off-A model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens. Earth Planet. Sci. Lett. 129 (1–4), 85–102. DeBievre, P., Taylor, P.D.P., 1993. Table of the isotopic composition of the elements. Int. J. Mass Spectrom. Ion Processes 123, 1–149. Dong, X.Y., Li, X., Ye, L.H., 1995. China Ultramafic Rocks. Geological Publishing House, Beijing, pp. 38–50 (in Chinese). Duncan, A.R., Erlank, A.J., Marsh, J., 1984. Regional geochemistry of the Karoo igneous province. Spec. Publ. Geol. Soc. Afr. 13, 355–388. Duretz, T., Schmalholz, S.M., Gerya, T.V., 2012. Dynamics of slab detachment. Geochem. Geophys. Geosyst. 13, Q03020.

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