Continental growth through accreted oceanic arc: Zircon Hf–O isotope evidence for granitoids from the Qinling orogen

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Continental growth through accreted oceanic arc: Zircon Hf–O isotope evidence for granitoids from the Qinling orogen Hao Wang a,b, Yuan-Bao Wu a,⇑, Shan Gao a, Zheng-Wei Qin a, Zhao-Chu Hu a, Jian-Ping Zheng a, Sai-Hong Yang b a

State Key Laboratory of Geological Processes and Mineral Resources, Center for Global Tectonics, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China b State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Received 31 August 2015; accepted in revised form 13 March 2016; available online 18 March 2016

Abstract The continental crust is commonly viewed as being formed in subduction zones, but there is no consensus on the relative roles of oceanic or continental arcs in the formation of the continental crust. The main difficulties of the oceanic arc model are how the oceanic arcs can be preserved from being subducted, how we can trace the former oceanic arcs through their high-Si products, and how the oceanic arcs can generate the high-Si, K-rich granitoid composition similar to the upper continental crust. The eastern Qinling orogen provides an optimal place to address these issues as it preserves the well-exposed Erlangping oceanic arc with large amounts of granitoids. In this study, we present an integrated investigation of zircon U–Pb ages and Hf–O isotopes for four representative granitoid plutons in the Erlangping unit. In situ zircon SIMS U–Pb dating indicated that the Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons formed at 472 ± 7, 458 ± 6 and 443 ± 5 Ma, respectively, all of which postdated the deep subduction of the Qinling microcontinent under the Erlangping oceanic arc. The Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons are sodic granitoid and have highly positive eHf(t) (+7.6 to +12.9) and relatively low d18O (4.7–5.0‰) values, which were suggested to result from prompt remelting of hydrothermally altered lower oceanic crust of the accreted Erlangping oceanic arc. The zircon grains from the Manziying monzogranitic pluton show similar Hf–O isotopic compositions to those of the Xizhuanghe pluton, and thus the Manziying monzogranitic pluton was likely derived from the dehydration melting of previous tonalites as exemplified by the Xizhuanghe pluton. The deep subduction of Qinling microcontinent resulted in the accretion of the Erlangping oceanic arc, which implies that arc–continent collision provides an effective way for preventing oceanic arcs from being completely subducted. The highly positive eHf(t) and relatively low d18O values of zircon grains from the granitoids in the Erlangping unit reveal that the continental crust can acquire its high-Si, K-rich nature from accreted oceanic arcs through differentiation by post-accretional magmatism, and thus highlight the significance of oceanic arcs for the generation of continental crust. Ó 2016 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

⇑ Corresponding author. Tel.: +86 2767883001; fax: +86

2767883002. E-mail address: [email protected] (Y.-B. Wu). http://dx.doi.org/10.1016/j.gca.2016.03.016 0016-7037/Ó 2016 Elsevier Ltd. All rights reserved.

Earth’s veneer of continental crust is unique among the known terrestrial planets (Rudnick, 1995; Rudnick and Gao, 2003; Hawkesworth and Kemp, 2006a), and thus ascertaining its generation and evolution holds the key to understanding the differentiation of the silicate planets

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(Taylor, 1967; Condie and Kro¨ner, 2013; Niu et al., 2013). Because the continental crust and arc magmas share similar trace-element characteristics (Rudnick and Gao, 2003; Davidson and Arculus, 2006), it is widely accepted that continental material is mainly formed in subduction zones. Given that accretionary orogens form at convergent plate margins and include many supra-subduction oceanic and/ or continental arc terranes, they are commonly viewed as the main sites of continental crust growth (Kemp et al., 2009; Cawood et al., 2013). However, it continues to attract controversy on the relative roles of oceanic or continental arcs in accretionary orogens in the formation of the continental crust (Condie and Kro¨ner, 2013). Efficient discrimination between the two types of arcs is crucial for evaluating their real contributions to the generation of the continental crust. The fundamental distinction between them is that oceanic arcs are built upon oceanic lithosphere, while continental arcs upon continental lithosphere. However, poor exposure of arc basement at the surface makes such a distinction difficult (Condie and Kro¨ner, 2013). Previous studies commonly invoked whole-rock Nd isotopic data of granitoid rocks to reveal the nature of the arc basement (Jahn et al., 2000), yet whole-rock isotopes are easily complicated by subsequent petrogenetic processes (Kemp et al., 2007). It is noteworthy that continental fragments have recently been identified in both ancient and modern oceanic arcs by U–Pb ages of zircon xenocrysts (Kro¨ner, 2010; Tapster et al., 2014). Assimilation of ancient continental material imports significantly less radiogenic Nd in granitoids derived from oceanic arcs, devaluating the whole-rock Nd isotopic compositions of granitoids in disclosing the nature of arc basement. Due to lack of pre-existing continental basement, oceanic arcs were recognized as ideal environments for the net growth of juvenile continental crust (Stern, 2002). However, the main difficulties of the oceanic arc model are how the oceanic arcs can be preserved from being subducted (Hawkesworth et al., 2009; Condie and Kro¨ner, 2013), how we can trace the former oceanic arcs through their high-Si products, and how the oceanic arcs can generate the high-Si, K-rich granitoid composition similar to the upper continental crust (Rudnick, 1995). Here, we explore the significance of accreted oceanic arc in continental crust generation with reference to the Qinling orogen and by combining U–Pb geochronological data with the distinctive archive of oxygen and hafnium isotopes in granitoid-hosted zircon. Unlike bulk rock samples, zircon is extremely retentive of the magmatic O isotope ratios, and zircon grains in equilibrium with pristine mantle-derived melts have a narrow d18O range [5.3 ± 0.3 per mil (‰)] (Valley et al., 1998). This range is insensitive to magmatic differentiation, because the fractionation, D18O (WR-Zrc), increases at nearly the same rate as d18O (WR) (Zhao and Zheng, 2003; Valley et al., 2005). Deviations outside the mantlelike range for zircon d18O value thus provide clear evidence that the O isotopic compositions of their host rocks have been changed by surficial processes or contaminated by d18O-modified material (Hoefs, 2009; Grimes et al., 2013). Mantle-like and slightly low d18O values were obtained from zircon grains in plagiogranite from modern oceanic

crust (Grimes et al., 2011) and ophiolites (Grimes et al., 2013), respectively. These are distinct from the continental granite, which commonly have higher zircon d18O values (Hoefs, 2009). The Lu–Hf system, like the U–Pb and O systems, is also extremely refractory in zircon (Cherniak and Watson, 2003). The Hf isotope compositions have been widely used to constrain when continental crust was generated (Hawkesworth and Kemp, 2006b), and to identify juvenile contributions to granitoid plutons (Kemp et al., 2007; Yang et al., 2007). Therefore, a combination of in situ U–Pb dating with Hf–O isotopic analyses on zircon would provide an effective approach to perceive petrogenetic processes and explore continental crust growth from accreted oceanic or continental arcs in accretionary orogens. In this study, zircon U–Pb geochronological data were used to deduce the timing and geodynamic processes for the generation of continental crust during the evolution of the Qinling orogen. Zircon Hf–O isotopic compositions were employed to constrain the nature of the basement. Our results also suggest that post-accretional magmatism is an instrumental way for rapid generation of granitoid continental crust in accreted oceanic arcs, and thus highlight the significance of accreted oceanic arcs for the generation of continental crust. 2. GEOLOGICAL SETTING AND SAMPLES The Qinling-Dabie-Sulu orogenic belt marks the suture zone between the North China Block (NCB) and the South China Block (SCB), forming one of the most important collisional orogens in central China (Li and Sun, 1996; Wu and Zheng, 2013). The eastern Qinling orogen represents the middle segment of this belt, and extends to the Tongbai area through the Nanyang Basin (Fig. 1a). The eastern Qinling orogen is divided into the South and North Qinling orogens by the Shangdan fault (Fig. 1b) and by the Songpa fault in the Tongbai area (Fig. 1c) (Kro¨ner et al., 1993; Ratschbacher et al., 2003; Wang et al., 2011a). The South Qinling (SQ) orogen has rock associations comparable with those of the northern margin of the SCB (Kro¨ner et al., 1993). From north to south, the North Qinling (NQ) orogen comprises the Kuanping, Erlangping, and NQ units (Kro¨ner et al., 1993; Ratschbacher et al., 2003). The Kuanping unit contains greenschist-facies metamorphic basites and sedimentary rocks. The metabasites have Neoproterozoic protolith ages of ca. 900–600 Ma, and the metasedimentary rocks show dominant Neoproterozoic age peaks of detrital zircon grains comparable with those of the NQ unit (Wu and Zheng, 2013 and references therein). Granitoids intruded the Kuanping unit mostly in the Late Mesozoic with minor in the Late Paleozoic (Su et al., 2013). The NQ unit is suggested to represent a microcontinent that consists of Proterozoic metasedimentary rocks and granitic gneisses with some metabasic blocks (Wu and Zheng, 2013 and references therein), the latter containing ultrahigh pressure (UHP) eclogite and high pressure (HP) mafic granulite (Liu and Zhou, 1995; Yang et al., 2003). The UHP eclogite has a protolith age of ca. 800 Ma and a peak metamorphic age of ca. 500–490 Ma (Wang et al., 2011b, 2013a), which were suggested to record the

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105°

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NQ: North Qinling SQ: South Qinling TB: Tongbai WDB: Western Dabie EDB: Eastern Dabie SL: Sulu YQWF: Yantai-Qingdao -Wulian Fault

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Wucheng Xiong’er unit Kuanping unit Erlangping unit North Qinling unit

Guishan complex (Shear zone)

Tongbai Complex

Late Mesozoic granitoid

Nanwan flysch

Neoproterozoic granitoid

Peridotite

Paleozoic diorite

Gabbro

Paleozoic granitoid

Sample location

Balifan tectonic melange Northern eclogite zone

Fig. 1. (a) Geological sketch map of the Qinling–Dabie–Sulu orogenic belt. (b) Simplified geological map of the eastern Qinling segment (modified after Wang et al. (2013a)). (c) Simplified geological map of Tongbai segment (modified after Liu et al. (2011)). SDS: Shangdan suture; LLF: Luonan-Luanchuan Fault.

separation of the NQ microcontinent from the SCB and its subsequent deep subduction beneath the Erlangping oceanic arc, respectively. Most parts of this unit underwent upper amphibolite-facies to granulite-facies metamorphism and migmatization, and were intruded by granitic plutons at ca. 450–400 Ma (Wang et al., 2011a, 2013b; Liu et al., 2011, 2014).

The Erlangping unit is sandwiched between the Kuanping unit in the north and the NQ unit in the south (Dong et al., 2011). It contains mafic to intermediate volcanic rocks, hypabyssal dykes, minor ultramafic rocks, fine-grained clastic rocks, and interlayered turbidite and chert with Cambrian–Silurian fossils (Ratschbacher et al., 2003; Wu and Zheng, 2013). Basaltic rocks comprise a

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major part of the volcano-sedimentary sequence, and some of them show pillow structure (Ratschbacher et al., 2003; Wu and Zheng, 2013). The mafic volcanic rocks are tholeiitic, and show different degrees of enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), while light rare earth elements (LREE) exhibit both enriched and depleted patterns (Sun et al., 1996; Dong et al., 2011). Numerous intrusions of tonalite, trondhjemite and gabbro, and rare pyroxenite are reported to intrude the volcanic sequences. The intrusives yield single-grain zircon 207Pb/206Pb ages of 470– 488 Ma (Xue et al., 1996), implying that the volcanic sequences are older than ca. 490 Ma. Ancient continental material akin to the NQ and Kuanping units has not been discovered in the Erlangping unit and most samples in this unit have positive whole rock eNd(t) values (Zhang et al., 1994; Dong et al., 2011). The combination of geological and geochemical features indicates that the Erlangping unit represents an accreted oceanic arc (Sun et al., 2002; Ratschbacher et al., 2003; Wu and Zheng, 2013). However, when the oceanic arc accreted onto the NQ unit has been a matter of debate (Sun et al., 1996; Ratschbacher et al., 2003; Dong et al., 2011). According to the precise dating of the UHP metamorphism in the NQ unit and the previous U–Pb ages of the oldest volcanic rocks in the Erlangping unit, it has recently been proposed that the Erlangping unit formed to the north of the NQ unit at >490 Ma and then accreted onto the NQ microcontinent at ca. 490 Ma by the arc–continent collision process (Wang et al., 2011b). After the accretion, the subduction zone of the Paleotethyan Ocean migrated southward (Wang et al., 2014a), and then the Erlangping oceanic arc evolved into a backarc basin during ca. 460–400 Ma (Wang et al., 2013b). The Erlangping unit was intensively intruded by early Paleozoic and late Mesozoic granitoid plutons (Wang et al., 2013b). The early Paleozoic granitoids in the Erlangping unit mainly consist of I-type tonalite, trondhjemite, granodiorite, and subordinate monzonitic granite with minor diorite and quartz diorite (Tian and Wei, 2005; Guo et al., 2010). In this study, we selected four granitoid samples from four representative plutons of the Erlangping unit for

zircon Hf–O isotope analyses and U–Pb dating. In view of the whole-rock geochemical data from the literatures (Supplementary Table S1), the Xizhuanghe pluton has low silica and high alumina contents relative to the other three plutons (SiO2: 70.89–77.44% vs. 66.84–71.57%; Al2O3: 12.09–14.17% vs. 14.11–16.78%). On the TAS chemical classification diagram (Fig. 2a), the Xizhuanghe samples plot mainly in the granodiorite field, while the samples from the other three plutons fall in the domain of sub-alkaline granite. The Taoyuan, Zhangjiadazhuang and Xizhuanghe plutons are sodic granitoids with K2O/ Na2O ratios of 0.10–0.33, 0.28–0.30 and 0.42–0.94, respectively. Based on the albite-anorthite-orthoclase diagram (Fig. 2b), the three plutons are categorized into trondhjemite, tonalite, and granodiorite, respectively. The Manziying pluton is characterized by high K2O/Na2O ratios (0.89–1.63, mostly 1.20–1.63) and thus classified as granite (Fig. 2b). In the multi-element spider diagram (Fig. 3a), the four plutons are characterized by depletion in HFSEs (Nb, Ta, P and Ti) and enrichment in LILEs (Th and U), similar to those of the continental crust (Rudnick and Gao, 2003). In addition, the four plutons show fractionated REE patterns (LaN/YbN = 1.76–32.2) with variable Eu anomalies (Eu/Eu* = 0.37–1.90) (Fig. 3b). Trondhjemite 12QL113 was taken from the Zhangjiadazhuang pluton that comprises tonalite and trondhjemite, which grade into each other with no sharp contact between them. The Zhangjiadazhuang pluton is intensively deformed with clear gneissic structure. Sample 12QL113 is mainly composed of quartz (35%), plagioclase (50%), biotite (8%), and potassium feldspar (5%), with minor accessory and secondary minerals (<2%) of zircon, epidote and chlorite. Sample 11QL126 was collected from the Manziying pluton that is predominated by monzogranite with minor syenogranite. Monzogranite 11QL126 shows massive structure and coarse-grained granitic texture. The mineral assemblage for this sample is quartz (35%), K-feldspar (35%), plagioclase (25%), and biotite (5%). The Xizhuanghe pluton occurs in the Erlangping metavolcanic suite, and mainly consists of trondhjemite, granodiorite, tonalite, and minor quartz diorite. Granodiorite

Fig. 2. Plots of Na2O + K2O vs. SiO2 (a) and An–Ab–Or (b) for the Zhangjiadazhuang, Manziying, Xizhuanghe, and Taoyuan plutons of the Qinling orogen.

H. Wang et al. / Geochimica et Cosmochimica Acta 182 (2016) 109–130 10

3

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(b) Zhangjiadazhuang Manziying Xizhuanghe Taoyuan

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Average Adakite

0

La

Ce

Pr

Nd Sm Eu

Gd

Tb

Dy

Ho

Er

Tm Yb

Lu

Fig. 3. Primitive mantle-normalized multi-element spider (a) and chondrite normalized rare earth element (b) diagrams for the Zhangjiadazhuang, Manziying, Xizhuanghe, and Taoyuan plutons of the Qinling orogen. Primitive mantle-normalized and chondritenormalized values are from McDonough and Sun (1995) and Sun and McDonough (1989), respectively, whereas the data of average upper continental crust, TTG and adakite are from Rudnick and Gao (2003), Kemp and Hawkesworth (2003) and Martin et al. (2005), respectively.

10QL161 from this pluton displays massive structure and medium-grained granitic texture, whose mineral assemblage is quartz (25%), plagioclase (50%), K-feldspar (15%), biotite (5%), and hornblende (5%). The Taoyuan pluton consists mainly of trondhjemite and biotite granite. Trondhjemite 12TB28 was sampled from this pluton. This sample exhibits massive structure and medium-grained granitic texture. It is composed of quartz (35%), plagioclase (55%), K-feldspar (<5%), and biotite (<5%) with accessory minerals (<2%) of zircon, sphene, apatite, magnetite, and ilmenite. 3. ANALYTICAL METHODS Zircon grains were separated and concentrated from 5 kg crushed rocks using standard gravimetric and magnetic techniques. The four zircon samples were cast in four respective mounts (A1407, A1752, A2214 and A2347) with zircon standards Penglai, Plesovice, Qinghu and TEM2. All the zircon grains were set within 7 mm from the center of the mount to avoid effects of sample geometry and topography (Tang et al., 2015). They were carefully imaged using transmitted and reflected light, and cathodoluminescence. Cathodoluminescence (CL) images of zircon were obtained using a LEO1450VP scanning electron microscope at the Institute of Geology and Geophysics, Chinese Academy of Science (IGGCAS) in Beijing. Zircon O isotope compositions were measured by CAMECA IMS-1280 ion microprobe at State Key Laboratory of Lithospheric Evolution in the IGGCAS, using the method described by Li et al. (2010). The Cs+ primary ion beam was accelerated at 10 kV with an intensity of ca. 2 nA, and rastered over a 10 lm area. The spot size is about 20 lm in diameter. Oxygen isotopes were measured using multi-collection mode on two off-axis Faraday cups. The mass resolution used to measure oxygen isotopes was ca. 2500. Two analyses of zircon standard Penglai were performed before and after every 5–7 sample analyses, and analyses of standard zircons Qinghu, Plesovice, or TEM2 were randomly interspersed. Each analysis consists of 20

cycles. The instrumental mass fractionation factor (IMF) is corrected according to the recommended value of d18OVSMOW = 5.31 ± 0.10‰ for zircon standard Penglai (2r; Li et al., 2010). The measured 18O/16O ratios were normalized to the VSMOW composition, then corrected for IMF as follows: IMF = d18OM–d18OStandard, and d18OSample = d18OM–IMF, where d18OM = [(18O/16O)M/ 0.0020052 1]  1000 (‰) and d18OStandard is the recommended d18O value for the zircon standard on the VSMOW scale. Measurements of zircon standards yield mountaverage d18O values of 8.32 ± 0.09‰ (2r) for TEM2, 8.20 ± 0.06‰ (2r) for Plesˇovice, and 5.41 ± 0.11‰ (2r, A1752), 5.52 ± 0.09‰ (2r, A2214), and 5.47 ± 0.16‰ (2r, A2347) for Qinghu, respectively, consistent with previously reported results (Valley, 2003; Li et al., 2013). Standard zircon oxygen isotope data are listed in the Supplementary Table S2. After the O isotope analysis, the mounts were carefully re-polished for U–Pb dating. Measurements of U–Pb isotopes were located at the same spot or domain as the O isotope analysis using the same CAMECA IMS-1280 ion microprobe at the IGGCAS. The O2 primary ion beam with an intensity of ca. 10 nA was accelerated at 13 kV. The ellipsoidal spot is 20  30 lm in size. The oxygen flooding method was used to increase the O2 pressure in the sample chamber to enhance the Pb+ sensitivity. In the secondary ion beam optics, a 60 eV energy window was used, together with a mass resolution of ca. 5400. U–Th–Pb ratios were calibrated relative to standard zircon Plesˇovice (337.13 ± 0.37 Ma) (Slama et al., 2008). Measured isotopic compositions were corrected for common Pb using non-radiogenic 204Pb. Corrections are sufficiently small to be insensitive to the choice of common Pb composition, and an average of present-day crustal composition (Stacey and Kramers, 1975) was used for the common Pb. A long-term uncertainty of 1.5% (1 RSD) for 206Pb/238U measurements of the standard zircon was propagated to the unknowns, even though the measured 206 Pb/238U error during the course of this study is around 1% (1 RSD) or less. Analyses of standard zircon Qinghu

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were interspersed, which gave a Concordia age of 159.5 ± 1.7 Ma (MSWD = 1.09). This result agrees well with the recommended values within analytical errors (Li et al., 2009). More details about the analytical procedures have been given in Li et al. (2009). The data were reduced with the ISOPLOT program of Ludwig (2003). In situ Lu–Hf isotopes of zircon were analyzed using Neptune (Plus) MC-ICPMS systems at the IGGCAS and State Key Laboratory of Geological Processes and Mineral Resources (GPMR). The zircon grains were ablated by a 193 nm ArF Laser system. Spot size of 44–60 lm was used for the analysis, with a laser repetition rate of 10 Hz at 100 mJ/pulse. Raw count rates for 172Yb, 173Yb, 175Lu, 176 (Hf + Yb + Lu), 177Hf, 178Hf, 179Hf, 180Hf and 182W were collected, and isobaric interference corrections for 176 Lu and 176Yb on 176Hf were determined using a recommended 176Lu/175Lu ratio of 0.02669 (Debievre and Taylor, 1993) and a recommended 176Yb/172Yb ratio of 0.5886 (Chu et al., 2002). The detailed analytical procedures for the two laboratories have been given by Wu et al. (2006) and Hu et al. (2012), respectively. The obtained 176Hf/177Hf ratios for zircon standards are listed in the Supplementary Table S3, which gave weighted means of 0.282503 ± 0.000007 for Mud Tank, 0.282680 ± 0.000006 for TEMORA, and 0.282011 ± 0.000010 for GJ-1. These results are in good agreement with the reported values in previous studies within analytical errors. A decay constant of 1.867  10 11/year for 176Lu (So¨derlund et al., 2004) as well as 176Lu/177Hf and 176Hf/177Hf ratios of 0.0336 and 0.282785 for the chondritic uniform reservoir (CHUR) (Bouvier et al., 2008) were adopted for calculation of the eHf(t) values. As at least 80% of all the continental crust formed at destructive plate margins (Rudnick, 1995), the composition of modern-day island arc rocks (as a proxy for arc mantle) should be used instead of MORB depleted mantle for calculation of the Hf model ages (Dhuime

(a) 12QL113@3

(b) 12QL113@5

et al., 2011). In this study, 176Hf/177Hf of 0.283158 and Lu/177Hf of 0.0378 for arc mantle (Dhuime et al., 2011) and 176Lu/177Hf of 0.015 for the average continental crust (Griffin et al., 2002) were used for calculation of the Hf model ages. 176

4. RESULTS 4.1. Trondhjemite 12QL113 Zircon grains extracted from trondhjemite 12QL113 are subhedral in shape, colorless, and transparent. The grain sizes range from 40 to 150 lm with aspect ratios of 1:1– 3:1. The zircon grains show clear CL-bright oscillatory zoning (Fig. 4a and b), indicating that these zircon grains are of magmatic origin. Thirteen SIMS data show variable Th (108–487 ppm) and U (216–821 ppm) contents and high Th/U ratios (0.42–0.71). These U–Pb isotopic analyses yield a weighted mean 206Pb/238U age of 472 ± 7 Ma (MSWD = 2.6) (Table 1 and Fig. 5a). Fourteen spots of O isotopes gave a limited d18O value range of 4.43–4.95‰ with a weighted mean of 4.7 ± 0.1‰ (MSWD = 1.6) (Table 2 and Fig. 6a). Fifteen spots of Lu–Hf isotopes obtained at the IGGCAS show 176Lu/177Hf ratios of 0.001380–0.002841 and 176Hf/177Hf ratios of 0.282803– 0.282906 (Table 3). At t = 472 Ma, the calculated eHf(t) values range from +10.5 to +14.1 with a weighted mean of +12.9 ± 0.3 (MSWD = 1.10, 1 of 15 rejected), corresponding to two-stage Hf model ages of 325–565 Ma with a weighted mean of 404 ± 22 Ma (MSWD = 1.10, 1 of 15 rejected) (Table 3 and Fig. 7a). Twelve additional Lu–Hf analyses obtained at the GPMR give similar Lu–Hf isotopic compositions to those obtained at the IGGCAS, and yield a weighted mean eHf(t) value of +12.6 ± 0.2 (MSWD = 0.35) and a weighted mean two-stage Hf model age of 424 ± 16 Ma (MSWD = 0.35) (Table 3 and Fig. 7a).

(c) 11QL126@6

(d) 11QL126@11

481.6/4.43/12.3

469.0/4.53/12.6 4.71/10.2

50µm (e) 10QL161@3

50µm (f) 10QL161@9

50µm (g) 12TB28@5 439.4/4.94/11.7

4.98/9.0

50µm

(h) 12TB28@11 444.2/5.19/12.2

454.2/4.77/9.3 466.7/4.32/8.2

50µm

50µm

50µm

50µm

Fig. 4. Typical CL images of zircon grains from trondhjemite 12QL113 (a, b), monzogranite 11QL126 (c, d), granodiorite 10QL161 (e, f), and trondhjemite 12TB28 (g, h) from the Qinling orogen. The small ellipses represent the spots of SIMS U–Pb and O isotope analyses; the large circles represent the spots of LA-MC-ICPMS Hf isotope analyses. The numbers in white, yellow and orange color fonts are the U–Pb dates (Ma), d18O (‰) and eHf(t) values, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Zircon U–Pb isotopic data obtained by SIMS for samples 10QL161, 12QL113 and 12TB28 from the Qinling orogen. Spot

[U] ppm

[Th] ppm

Th/U

10QL161 1 628 2 723 3 777 4 653 5 534 6 1123 7 668 8 456 9 839 10 637 11 638 12 638 13 758 14 1090 15 442 16 862

417 1111 876 655 405 917 545 322 623 393 514 379 556 1135 569 755

0.664 1.536 1.128 1.003 0.758 0.817 0.817 0.705 0.742 0.617 0.806 0.594 0.734 1.042 1.288 0.876

Pb Pb

±r %

207

Pb U

238

±r %

207

235

±r %

206

206

0.05 0.09 0.37 0.03 0.05 0.11 0.10 0.61 0.06 0.07 0.05 0.35 0.08 0.15 0.22 0.06

0.05831 0.05620 0.05637 0.05646 0.05560 0.05598 0.05624 0.05845 0.05590 0.05646 0.05618 0.05743 0.05603 0.05629 0.05562 0.05610

0.76 0.96 0.94 0.75 1.23 0.71 1.46 0.93 0.92 0.85 0.80 0.79 0.92 1.48 1.37 0.96

0.59719 0.56770 0.56743 0.59643 0.57029 0.55276 0.56066 0.59103 0.57863 0.56015 0.58685 0.56164 0.57648 0.52824 0.62136 0.58851

1.68 1.78 1.77 1.85 2.09 1.66 2.09 1.77 1.76 1.73 1.70 1.70 1.76 2.29 2.07 1.79

0.0743 0.0733 0.0730 0.0766 0.0744 0.0716 0.0723 0.0733 0.0751 0.0720 0.0758 0.0709 0.0746 0.0681 0.0810 0.0761

1.50 1.50 1.50 1.69 1.69 1.50 1.50 1.51 1.51 1.50 1.50 1.50 1.50 1.75 1.55 1.51

541.5 460.5 467.1 470.5 436.2 451.7 461.9 546.7 448.2 470.6 459.6 508.3 453.6 463.7 437.2 456.4

16.5 21.1 20.6 16.4 27.2 15.6 32.0 20.2 20.3 18.7 17.7 17.3 20.3 32.4 30.1 21.2

475.4 456.5 456.4 475.0 458.2 446.8 452.0 471.5 463.6 451.6 468.9 452.6 462.2 430.6 490.7 469.9

f206%

207

Pb U

Pb Pb

±r

206

207

Pb U

±r

206

Pb U

±r

6.4 6.6 6.5 7.0 7.7 6.0 7.7 6.7 6.6 6.3 6.4 6.2 6.6 8.1 8.1 6.7

461.9 455.7 454.2 475.9 462.6 445.9 450.0 456.2 466.7 447.9 470.7 441.7 463.9 424.5 502.2 472.7

6.7 6.6 6.6 7.8 7.5 6.5 6.5 6.6 6.8 6.5 6.8 6.4 6.7 7.2 7.5 6.9

235

238

12QL113 1 2 3 4 5 6 7 8 9 10 11 12 13

216 246 380 425 349 451 228 400 377 333 821 325 394

108 137 206 179 237 246 121 172 247 177 487 230 185

0.498 0.556 0.544 0.420 0.679 0.546 0.533 0.430 0.656 0.531 0.593 0.707 0.470

0.13 0.17 0.13 0.59 2.79 0.22 0.05 0.02 0.07 0.26 1.45 1.34 0.02

0.05506 0.05516 0.05564 0.05619 0.05611 0.05588 0.05685 0.05571 0.05628 0.05662 0.04699 0.05585 0.05525

1.98 2.34 1.48 1.99 2.78 1.78 1.62 1.32 1.82 2.06 2.76 3.52 1.26

0.58482 0.57005 0.59511 0.59590 0.58377 0.57011 0.60857 0.58596 0.60579 0.57852 0.47977 0.56449 0.60202

2.49 2.78 2.11 2.49 3.16 2.34 2.21 2.00 2.36 2.55 3.15 3.82 1.96

0.0770 0.0750 0.0776 0.0769 0.0755 0.0740 0.0776 0.0763 0.0781 0.0741 0.0741 0.0733 0.0790

1.50 1.50 1.50 1.50 1.50 1.52 1.50 1.50 1.50 1.50 1.51 1.50 1.50

414.6 418.6 438.0 459.9 456.5 447.8 485.7 440.7 463.3 477.0 48.6 446.4 422.4

43.7 51.4 32.7 43.5 60.4 39.1 35.4 29.2 39.8 44.9 64.7 76.3 28.0

467.6 458.0 474.1 474.6 466.9 458.1 482.7 468.3 480.9 463.5 397.9 454.4 478.5

9.4 10.3 8.0 9.5 11.9 8.7 8.5 7.5 9.1 9.5 10.4 14.1 7.5

478.4 465.9 481.6 477.7 469.0 460.1 482.0 473.9 484.6 460.8 460.5 456.0 490.3

6.9 6.7 7.0 6.9 6.8 6.7 7.0 6.9 7.0 6.7 6.7 6.6 7.1

12TB28 1 2 3 4 5 6 7 8 9 10 11 12

210 353 848 317 457 413 223 255 244 302 344 653

104 376 798 141 444 211 134 166 133 178 230 569

0.498 1.065 0.941 0.446 0.971 0.511 0.598 0.651 0.543 0.590 0.671 0.872

0.22 0.05 0.22 0.04 0.22 0.05 0.20 0.11 0.00 0.13 0.14 0.10

0.05259 0.05588 0.05447 0.05442 0.05290 0.05560 0.05349 0.05547 0.05715 0.05348 0.05269 0.05435

2.60 1.83 1.31 1.82 2.05 1.58 2.56 2.06 1.86 2.03 2.71 1.92

0.51631 0.54650 0.52329 0.54383 0.51440 0.53844 0.53108 0.53876 0.57335 0.53744 0.51824 0.53516

3.00 2.37 1.99 2.36 2.54 2.18 2.97 2.55 2.39 2.54 3.09 2.43

0.0712 0.0709 0.0697 0.0725 0.0705 0.0702 0.0720 0.0704 0.0728 0.0729 0.0713 0.0714

1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.53 1.50 1.50

311.2 447.4 390.6 388.5 324.3 436.2 349.8 431.3 497.3 349.1 315.5 385.7

58.0 40.1 29.1 40.4 46.0 34.8 56.8 45.3 40.6 45.3 60.4 42.5

422.7 442.7 427.3 440.9 421.4 437.4 432.5 437.6 460.2 436.7 424.0 435.2

10.4 8.5 7.0 8.5 8.8 7.8 10.5 9.1 8.9 9.1 10.8 8.7

443.4 441.8 434.2 451.1 439.4 437.6 448.2 438.8 452.8 453.5 444.2 444.6

6.4 6.4 6.3 6.5 6.4 6.4 6.5 6.4 6.6 6.7 6.4 6.4

4.2. Monzogranite 11QL126

(Table 2 and Fig. 6b). Fifteen Lu–Hf isotopic analyses yield Lu/177Hf ratios of 0.001220–0.002190 and 176Hf/177Hf ratios of 0.282701–0.282813. At t = 460 Ma, their corresponding eHf(t) values and two-stage Hf model ages are +6.8 to +10.8 and 536 to 793 Ma, and the calculated weighted means are +8.5 ± 0.4 (MSWD = 2.4) and 681 ± 26 Ma (MSWD = 2.4), respectively (Table 3 and Fig. 7b). 176

Zircon crystals in monzogranite 11QL126 are colorless, transparent, and subhedral. They have grain sizes ranging from 50 to 150 lm with aspect ratios of 1:1–2:1. In CL images, most zircon crystals are pronounced oscillatoryzoned without discernible cores (Fig. 4c and d). Since Guo et al. (2010) has reported a highly precise zircon U–Pb age of 459.5 ± 0.9 Ma for the Manziying pluton, we did not performed geochronological analysis on this sample. A total of 15 SIMS analyses on this sample show d18O values ranging from 4.55‰ to 5.55‰ with a weighted mean of 4.9 ± 0.1‰ (MSWD = 1.9, 1 of 15 rejected)

4.3. Granodiorite 10QL161 Zircon grains obtained from granodiorite 10QL161 are colorless, transparent, and subhedral to euhedral in shape.

116

H. Wang et al. / Geochimica et Cosmochimica Acta 182 (2016) 109–130 0.095

560

(a) 12QL113 0.085

520

U

Mean = 472 ± 7 Ma MSWD = 2.6

206

Pb/

238

480 0.075

440

510

490

0.065

400

470

450

360 430

0.055 0.4

0.5

0.6 207

0.095

0.7

Pb/ 2 3 5 U

560

(b) 10QL1 61 520 Mean = 458 ± 6 Ma 2 of 16 rejected MSWD = 2.4

Analysis #15

480

0.075

206

Pb/ 2 3 8 U

0.085

440

0.065

510

contents and high Th/U ratios (0.59–1.54). Except analyses #14 and #15 that have the youngest and oldest ages of 424.5 ± 7.2 and 502.2 ± 7.5 Ma, the other 14 analyses yield tightly clustered 206Pb/238U ages with a weighted mean of 458 ± 6 Ma (MSWD = 2.4) (Table 1 and Fig. 5b). This result coincides with the previously reported age of 461 ± 0.9 Ma within analytical errors (Guo and Chen, 2011), and is suggested to record the formation age of the Xizhuanghe pluton. Twenty-two zircon grains in this sample show a d18O value range of 4.28–5.28‰ with a weighted mean of 4.8 ± 0.1‰ (MSWD = 1.4, 3 of 22 rejected) (Table 2 and Fig. 6c). Twenty-one Lu–Hf isotopic analyses obtained at the IGGCAS give variable 176Lu/177Hf (0.001839–0.003032) but similar 176Hf/177Hf (0.282620– 0.282799) ratios. Apart from analysis #14 that gives a relatively low eHf(t) value of 3.7 and a large two-stage Hf model age of 993 Ma, the other 20 analyses give coherent eHf(t) value and two-stage Hf model age ranges of +6.0 to +9.8 and 595–841 Ma, respectively, with weighed means of +7.6 ± 0.3 (MSWD = 2.1) and 739 ± 20 Ma (MSWD = 2.1) at t = 458 Ma (Table 3 and Fig. 7c). Twenty-four additional analyses at the GPMR show similar Lu–Hf isotopic compositions, which yield a weighted mean eHf(t) value of 7.5 ± 0.2 (MSWD = 1.18), and an average two-stage Hf model age of 743 ± 12 Ma (MSWD = 1.18).

470

400

4.4. Trondhjemite 12TB28

Analysis #14 430

360 390

0.055 0.4

0.5

0.6 207

0.7

Pb/ 2 3 5 U

500 0.080

(c) 12TB28 480

0.076

460 Concordia Age = 443 ± 5 Ma MSWD = 1.9

440

206

8

Pb/ 2 3 U

0.072

0.068

Mean = 444 ± 4 Ma MSWD = 0.92

475

420

465

400

0.064

455

445

380

435

0.060 425

360 0.056 0.42

415

0.46

0.50

0.54 207

0.58

0.62

Pb/ 2 3 5 U

Fig. 5. Concordia diagrams of SIMS zircon U–Pb dating for trondhjemite 12QL113 (a), granodiorite 10QL161 (b), and trondhjemite 12TB28 (c) from the Qinling orogen.

Their lengths range from 50 to 200 lm and their aspect ratios are 1:1–2.5:1. In CL images, they exhibit clear oscillatory zoning (Fig. 4e and f), typical for magmatic zircon (Wu and Zheng, 2004). A total of 16 SIMS U–Pb isotopic data were acquired on 16 zircon grains. These analyses have variable Th (322–1135 ppm) and U (442–1123 ppm)

Zircon crystals in trondhjemite 12TB28 are colorless and transparent. They have subhedral to euhedral shapes and grain sizes of 100–200 lm with length/width ratios of 1:1–1.5:1. CL images reveal that the zircon crystals have strong luminescence and show obvious oscillatory zoning (Fig. 4g and h). Twelve SIMS U–Pb isotopic analyses were obtained on 12 zircon grains. All the analyses have moderate Th (104–798 ppm) and U (210–848 ppm) contents with high Th/U ratios (0.45–1.07). All the U–Pb isotopic analyses give coherent 206Pb/238U ages of 434.2 ± 6.3–453.5 ± 6.7 Ma, yielding consistent Concordia (443 ± 5 Ma) and weighted mean (444 ± 4 Ma) ages (Fig. 5c), which are taken as the best estimate for the formation time of the Taoyuan pluton. Fifteen SIMS data from this sample exhibit a narrow d18O value range of 4.62–5.39‰ and define a weighed mean of 5.0 ± 0.1‰ (MSWD = 2.1) (Table 2 and Fig. 6d). Twenty-two Lu–Hf isotopic analyses show variable 176Lu/177Hf ratios of 0.001533–0.003395 and relatively constant 176Hf/177Hf ratios of 0.282811–0.282919 (Table 3). At t = 443 Ma, they yield small eHf(t) value and two-stage Hf model age ranges of +10.1 to +14.0 and 310–564 Ma, and give weighted means of +12.1 ± 0.3 (MSWD = 1.3, 2 of 22 rejected) and 438 ± 19 Ma (MSWD = 1.3, 2 of 22 rejected), respectively (Fig. 7d). 5. DISCUSSION 5.1. Preservation of primary low d18O values in zircon The average d18O values of zircon grains from the four studied samples are 4.7–5.0‰, which are below the d18O

Table 2 Zircon oxygen isotopic data obtained by SIMS for samples 10QL161, 11QL126, 12QL113, and 12TB28 from the Qinling orogen. 18

O/16Om

d18O (‰)

2SE

Penglai 2@1 Penglai 2@2 10QL161@1 10QL161@2 Plesovice@4 10QL161@3 10QL161@4 10QL161@5 Penglai 2@3 Penglai 2@4 10QL161@6 10QL161@7 10QL161@8 TEM2@4 10QL161@9 Plesovice@5 10QL161@10 10QL161@11 Penglai 2@5 Penglai 2@6 10QL161@12 10QL161@13 10QL161@14 10QL161@15 10QL161@16 10QL161@17 Penglai 2@7 Penglai 2@8 10QL161@18 TEM2@5 10QL161@19 Plesovice@6 10QL161@20 10QL161@21 10QL161@22 Penglai 2@9 Penglai 2@10 Penglai1@1 Penglai1@2 11QL126@1 11QL126@2 11QL126@3 11QL126@4

A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1407 A1752 A1752 A1752 A1752 A1752 A1752

0.002015605 0.002015145 0.002014821 0.002014858 0.002021964 0.002014769 0.002015091 0.002015779 0.002015773 0.002016117 0.002015013 0.002014687 0.002015465 0.002022076 0.002013868 0.002021837 0.002013789 0.002014479 0.002015989 0.002016004 0.002014848 0.002015353 0.002014584 0.002015094 0.0020149 0.002014646 0.002015775 0.002015432 0.002014626 0.002021573 0.002014519 0.002021274 0.002014644 0.002014943 0.00201485 0.002015933 0.002015397 0.002013229 0.00201288 0.002012877 0.00201297 0.002012624 0.002012662

5.19 4.96 4.80 4.82 8.36 4.77 4.93 5.28 5.27 5.44 4.89 4.73 5.12 8.42 4.32 8.30 4.28 4.63 5.38 5.39 4.81 5.06 4.68 4.93 4.84 4.71 5.27 5.10 4.70 8.17 4.65 8.02 4.71 4.86 4.81 5.35 5.09 5.35 5.12 5.08 5.08 4.88 4.87

0.28 0.23 0.30 0.26 0.24 0.29 0.23 0.31 0.22 0.31 0.19 0.31 0.22 0.25 0.23 0.23 0.28 0.25 0.32 0.27 0.22 0.21 0.29 0.22 0.29 0.26 0.25 0.26 0.18 0.29 0.29 0.11 0.21 0.25 0.25 0.35 0.38 0.36 0.38 0.30 0.32 0.24 0.26

X position 5300 4754 4442 4342 6103 4254 4094 3850 4240 3867 3685 3475 2808 929 2936 6229 3024 2542 2944 1869 2358 1682 1634 1451 1092 958 1764 1238 733 571 316 6192 261 288 17 595 588 5762 5366 2664 2614 2524 2726

Y position 2302 2262 3029 2994 900 3217 2886 3147 2230 2235 2953 3095 3123 608 3041 731 3060 2996 2262 2255 2887 3072 3101 3046 3034 3047 2268 2308 2881 718 2848 682 2866 3043 2909 2303 2237 2340 2326 4100 4208 4334 4433

DTFA_X 35 36 45 44 40 43 41 43 38 34 42 39 40 40 40 42 42 35 36 39 40 40 39 40 40 40 37 40 34 40 38 41 39 38 39 36 39 17 20 19 20 20 22

DTFA_Y 1 4 0 2 5 3 7 5 0 0 3 12 6 5 10 6 6 8 7 0 1 5 10 4 7 5 2 0 11 8 6 6 7 9 8 7 3 2 1 16 23 25 26

DTCA_X 31 31 33 35 32 34 32 29 31 31 33 30 29 28 39 29 35 39 27 25 31 33 29 30 30 30 30 30 32 36 32 35 30 29 29 26 30 5 8 3 3 16 6

Hf isotopic analytical spot

10QL161-01 10QL161-02 10QL161-03 10QL161-04 10QL161-05

10QL161-06 10QL161-07 10QL161-10 10QL161-09 10QL161-08 10QL161-11

10QL161-12 10QL161-13 10QL161-14 10QL161-15 10QL161-16 10QL161-17

10QL161-18 10QL161-19

10QL161-20 10QL161-21

11QL126-01 11QL126-02 11QL126-03 11QL126-04 (continued on next page)

117

Mount number

H. Wang et al. / Geochimica et Cosmochimica Acta 182 (2016) 109–130

Sample

118

Table 2 (continued) Mount number

18

O/16Om

11QL126@5 11QL126@6 11QL126@7 Penglai1@3 Penglai1@4 11QL126@8 11QL126@9 Qinghu@1 11QL126@10 11QL126@11 11QL126@12 11QL126@13 11QL126@14 Penglai1@5 Penglai1@6 11QL126@15 Penglai@1 12QL113@1 12QL113@2 Qinghu@1 12QL113@3 Penglai@2 12QL113@4 12QL113@5 12QL113@6 Penglai@3 Qinghu@2 12QL113@7 12QL113@8 12QL113@9 Penglai@4 12QL113@10 Qinghu@3 12QL113@11 12QL113@12 Penglai@5 12QL113@13 12QL113@14 Qinghu@4 Penglai@6 Penglai1@5 Penglai1@6 Qinghu@2

A1752 A1752 A1752 A1752 A1752 A1752 A1752 A1752 A1752 A1752 A1752 A1752 A1752 A1752 A1752 A1752 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2347 A2214 A2214 A2214

0.002014069 0.00201241 0.002013486 0.002013833 0.002013341 0.00201239 0.002013394 0.002014272 0.002012821 0.002013127 0.002012297 0.002013293 0.002012344 0.002013573 0.002014081 0.002013241 0.002015148 0.002014013 0.002013965 0.002015422 0.002013533 0.002015041 0.002013747 0.002013734 0.002014571 0.002014871 0.002015139 0.002014019 0.00201438 0.002014461 0.002015204 0.002013787 0.002015871 0.002014575 0.00201431 0.002015517 0.00201433 0.002014203 0.002015908 0.002015298 0.002014953 0.002015775 0.002016491

d18O (‰)

2SE

5.55 4.71 5.23 5.39 5.14 4.65 5.14 5.57 4.84 4.98 4.55 5.04 4.55 5.14 5.38 4.94 5.23 4.67 4.64 5.37 4.43 5.18 4.54 4.53 4.95 5.10 5.23 4.67 4.85 4.89 5.26 4.56 5.59 4.95 4.82 5.42 4.83 4.76 5.61 5.31 4.92 5.32 5.66

0.27 0.28 0.37 0.40 0.28 0.38 0.24 0.26 0.38 0.30 0.36 0.25 0.28 0.24 0.29 0.33 0.25 0.27 0.27 0.40 0.26 0.34 0.27 0.22 0.29 0.25 0.32 0.28 0.37 0.22 0.34 0.27 0.29 0.41 0.17 0.45 0.25 0.24 0.31 0.23 0.30 0.22 0.22

X position 1843 1577 1209 4862 4182 1130 972 304 944 200 607 834 1331 3591 3016 2115 880 4123 3866 5711 3757 1620 3609 2620 1731 2171 5216 1582 1532 837 2788 33 4936 23 102 3302 1002 1799 4435 3896 2928 2426 4398

Y position 4458 4069 4257 2331 2358 4452 4309 847 4109 4441 4321 4297 4272 2308 2338 4389 2943 3688 3773 198 3858 2913 3987 3898 3884 2924 407 3942 3792 3823 2937 3729 180 3787 4051 2960 3762 4021 186 2904 2470 2556 1553

DTFA_X 24 20 20 18 12 19 19 15 17 22 21 19 21 17 14 21 24 35 34 29 34 24 35 31 29 28 28 28 28 27 29 24 27 25 23 29 22 19 25 29 3 2 24

DTFA_Y 29 20 26 2 7 29 27 15 25 29 24 27 24 3 3 21 6 2 5 5 5 6 7 8 9 2 6 10 9 7 4 9 5 8 12 3 6 8 2 3 33 35 41

DTCA_X 8 12 4 5 9 5 5 5 4 8 10 5 8 7 4 7 7 6 6 8 5 7 6 6 9 8 7 7 9 11 7 4 7 9 9 5 6 6 9 7 3 2 3

Hf isotopic analytical spot 11QL126-05 11QL126-06 11QL126-07

11QL126-09 11QL126-08 11QL126-10 11QL126-11 11QL126-12 11QL126-13 11QL126-14

11QL126-15 12QL113-01 12QL113-02 12QL113-03 12QL113-04 12QL113-05 12QL113-06

12QL113-07 12QL113-08 12QL113-09 12QL113-10 12QL113-11 12QL113-12 12QL113-13 12QL113-14

H. Wang et al. / Geochimica et Cosmochimica Acta 182 (2016) 109–130

Sample

12TB28-12 12TB28-13 12TB28-15

12TB28-09 12TB28-10 12TB28-11

12TB28-06 12TB28-07 12TB28-08

12TB28@1 12TB28@2 12TB28@3 12TB28@4 12TB28@5 Penglai1@7 Penglai1@8 12TB28@6 12TB28@7 12TB28@8 Qinghu@3 12TB28@9 12TB28@10 12TB28@11 Penglai1@9 Penglai1@10 12TB28@12 12TB28@13 12TB28@14 12TB28@15 Qinghu@4 Penglai1@11

A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214 A2214

0.002015416 0.002015086 0.002014982 0.002014419 0.002015074 0.002016485 0.002015512 0.002015264 0.002015132 0.002015035 0.002016016 0.002014933 0.002015418 0.002015554 0.002015553 0.002015555 0.002014816 0.002015582 0.002015911 0.002015447 0.002015603 0.002015938

5.12 4.95 4.90 4.62 4.94 5.64 5.16 5.03 4.97 4.92 5.41 4.87 5.12 5.19 5.19 5.20 4.84 5.22 5.39 5.17 5.05 5.22

0.45 0.37 0.26 0.29 0.38 0.16 0.34 0.31 0.29 0.29 0.20 0.21 0.25 0.21 0.22 0.26 0.30 0.34 0.23 0.39 0.30 0.30

5352 5104 5147 4857 3320 1905 1330 229 1942 3168 4054 4219 4580 5433 695 100 5550 5191 4820 4448 3904 542

306 371 542 214 339 2608 2650 422 637 782 1322 637 680 911 2628 2652 997 926 822 1022 1537 2673

20 19 18 19 12 1 2 4 2 6 21 8 8 10 7 5 13 12 10 11 23 7

40 38 39 40 39 34 35 38 40 39 40 36 41 44 29 31 36 37 40 41 41 33

3 5 5 4 3 2 1 2 2 2 3 1 2 2 2 3 0 0 1 3 3 2

12TB28-01 12TB28-02 12TB28-03 12TB28-04 12TB28-05

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119

range of 5.3 ± 0.3‰ for zircon in the normal mantle (Valley et al., 1998). Low-d18O values are commonly observed for zircon grains that crystallized from 18O-poor magma or underwent post-magmatic zircon-fluid interaction (Monani and Valley, 2001; Wei et al., 2002; Zheng et al., 2004; Gao et al., 2014). Accordingly, whether the zircon grains preserved primary magmatic O isotopic compositions should be evaluated before they are used to interpret the genesis of the granitoids. Well crystallized zircon grains have sluggish rate of uranium, thorium, lead, hafnium, and oxygen diffusion, and thus are very resistant to hydrothermal alteration, whereas metamict zircon crystals are susceptible to discordance in U–Pb age and change of d18O value during zircon-fluid interaction (Zheng and Fu, 1998; Cherniak and Watson, 2003). The degree of zircon metamictization depends on its U and Th contents and age, and high metamictization generally produces porous zircon with mottled CL texture (Booth et al., 2005). The zircon crystals in the four studied samples show perfect crystal shapes and clear oscillatory zoning (Fig. 4), and have moderate Th (mostly <800 ppm) and U (mostly <1000 ppm) contents (Table 1), and concordant to nearly concordant U–Pb ages (Fig. 5). All these characteristics indicate that the zircon grains are pristine and thus can preserve the primary magmatic O isotopic compositions. In addition, zircon crystals suffered later hydrothermal alteration would be expected to display a highly variable d18O value range and decreasing Th/U ratios, because hydrothermal fluids are typically heterogeneous in flowpath, duration of flow, and fluid/rock ratio, and are characterized by low Th/U ratios (Monani and Valley, 2001; Wei et al., 2002; Wu et al., 2009a). In contrast, zircon grains crystallized from a well-mixed magma chamber can have identical values of d18O and high Th/U ratios (commonly larger than 0.4). The zircon crystals in this study have high Th/U ratios (>0.4) and a restricted d18O value range of 4.28–5.55‰ with only 2 out of the 66 spots giving mantle-like d18O values (Valley et al., 1998) or higher. Additionally, the zircon d18O values are not correlated with their U contents or Th/U ratios (Fig. 8). Accordingly, we suggested that the lower zircon d18O values than the average mantle zircon value were the signature of the primary magma, rather than the result of later hydrothermal alteration. 5.2. Generation of the granitoids by partial melting of the accreted oceanic arc Although no precise age has been obtained for the initial formation of the Erlangping oceanic arc, it can be inferred from the following geochronological constraints. A whole rock Sm–Nd isochron age of ca. 710 Ma was acquired from basaltic rocks, and was suggested to register the formation age of the Erlangping oceanic arc (Zhang et al., 1994); Felsic volcanic rocks in the Tongbai area had zircon U–Pb ages of ca. 490 Ma, implying that the Erlangping oceanic arc had evolved into a mature island arc during the late Cambrian (Liu et al., 2013); One granitoid pluton intruded the Erlangping volcanic sequence gave a weighted mean 207Pb/206Pb age of 488 Ma, which restricted the formation age of the volcanic sequences to be older than

120

H. Wang et al. / Geochimica et Cosmochimica Acta 182 (2016) 109–130 5

4

Plagiogranite zircon range 4.9 ± 0.6 ‰

(a)

(b)

M antle zircon range 5.3± 0.3 ‰

2

Number

Trondhjemite 12QL113 Mean = 4.7 ± 0.1 ‰ MSWD = 1.6

Monzogranite 11QL126 Mean = 4.9 ± 0.1 ‰ MSWD = 1.9

2

Relative probability

3

3

Relative probability

Number

4

1 1

0

0 3.0

4.0

5.0

6.0

7.0

3.0

4.0

18

5.0

6.0

7.0

δ 1 8 O(‰)

δ O(‰) 7

5

(c)

(d)

6 4

3

2

Number

Granodiorite 10QL161 Mean = 4.8 ± 0.1 ‰ MSWD = 1.4

3

Trondhjemite12TB28 Mean = 5.0±0.1 ‰ MSWD = 2.1 2

Relative probability

4

Relative probability

Number

5

1 1

0

0 3.0

4.0

5.0

6.0

7.0

18

δ O(‰)

3.0

4.0

5.0

6.0

7.0

δ 1 8 O(‰)

Fig. 6. Histograms of zircon d18O values for samples 12QL113 (a), 11QL126 (b), 10QL161 (c), and 12TB28 (d) of the Qinling orogen. The d18O values for mantle and plagiogranite zircon are from Valley et al. (1998) and Grimes et al. (2013), respectively.

ca. 490 Ma (Xue et al., 1996). Therefore, the Erlangping oceanic arc was proposed to be formed during the late Neoproterozoic to early Cambrian (Zhang et al., 1994; Xue et al., 1996). Afterwards, it accreted onto the NQ microcontinent through arc–continent collision at ca. 500–490 Ma as revealed by the UHP metamorphism of the NQ unit (Wang et al., 2011b, 2014b). The zircon grains in the four studied granitoid plutons of the Erlangping unit show clear oscillatory zoning without old inherited cores or xenocrysts. They gave U–Pb ages of 470 Ma, 460 Ma, and 440 Ma, respectively, which are younger than the age (490 Ma) of the deep subduction of the NQ microcontinent under the Erlangping oceanic arc. A compilation of previous zircon U–Pb chronological data also revealed that the early Paleozoic magmatism in the NQ orogen mainly occurred at ca. 470–430 Ma (Wang et al., 2013b). Accordingly, we suggest a post-accretional extension setting accounted for the magmatic flare-up. Oxygen isotopic analyses for the four representative granitoid plutons revealed that they have slightly low zircon d18O values relative to the normal mantle zircon (Valley et al., 2005). Formation of low-d18O magmatic rocks are usually attributed to assimilation of high-temperature hydrothermally altered low-d18O wall rocks during magma emplacement (e.g. Bacon et al., 1989), or to remelting of high-

temperature altered source rocks (Grimes et al., 2013). The assimilation model would predict presence of crustal xenoliths and zircon xenocrysts as well as covariant Hf–O isotopic compositions (Kemp et al., 2007), which are inconsistent with the observation of the studied samples (Figs. 4 and 9). As the granitoid plutons of the Erlangping unit formed in a post-accretional setting, there are three possible source rocks for them, which are: (1) the subducted continental crust of the NQ unit; (2) the subducted oceanic crust attached with the subducted NQ continental crust; and (3) the mid-lower crust of the accreted Erlangping oceanic arc. Although continental rocks with low d18O values would have formed in continental rift settings where water–rock interaction occurs at high temperatures (above 400 °C) (e.g. Zheng et al., 2007), such low d18O values have not been documented in the NQ unit. To date, all the O isotopic analyses performed on the rocks of the NQ unit collectively gave higher zircon d18O values (6.1–9.2‰) relative to those of the normal mantle zircon (Fu et al., 2013; Qin et al., 2014, 2015). In addition, the continental rocks of the NQ unit commonly have old zircon cores and show unradiogenic Hf isotopic compositions (e.g. Wang et al., 2011b). The Hf–O isotopic observations disprove that the subducted continental crust of the NQ unit can be served as the sources of the four studied granitoid plutons. The

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Table 3 Lu–Hf isotope compositions of zircons from samples 10QL161, 11QL126, 12QL113, and 12TB28 from the Qinling orogen. Sample

Lab

176

Yb/177Hf

176

Lu/177Hf

176

Hf/177Hf

±(1r)

10QL161 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR

0.042483 0.057546 0.057965 0.050394 0.041684 0.048361 0.064341 0.067999 0.046132 0.045516 0.042929 0.055181 0.055446 0.051447 0.052116 0.056372 0.058438 0.053731 0.053604 0.043909 0.047151 0.058033 0.041767 0.050757 0.052377 0.040139 0.030908 0.049772 0.059305 0.055144 0.055772 0.084819 0.055210 0.064228 0.042589 0.044079 0.055203 0.046541 0.051946 0.046466 0.044330 0.063991 0.044816 0.058078 0.043644

0.001945 0.002565 0.002566 0.002229 0.001839 0.002160 0.002896 0.003032 0.002077 0.002008 0.001909 0.002425 0.002471 0.002307 0.002242 0.002467 0.002664 0.002367 0.002374 0.001940 0.002119 0.002532 0.001868 0.002285 0.002362 0.001779 0.001378 0.002149 0.002535 0.002429 0.002464 0.003876 0.002492 0.002805 0.001885 0.002082 0.002427 0.002089 0.002433 0.002076 0.001885 0.002796 0.001988 0.002621 0.001927

0.282720 0.282723 0.282773 0.282726 0.282722 0.282739 0.282799 0.282768 0.282738 0.282693 0.282750 0.282687 0.282697 0.282620 0.282722 0.282723 0.282779 0.282720 0.282727 0.282751 0.282751 0.282720 0.282732 0.282714 0.282752 0.282722 0.282730 0.282724 0.282728 0.282714 0.282704 0.282738 0.282706 0.282729 0.282752 0.282736 0.282723 0.282744 0.282750 0.282726 0.282724 0.282727 0.282719 0.282715 0.282743

0.000017 0.000023 0.000023 0.000018 0.000017 0.000017 0.000018 0.000022 0.000018 0.000015 0.000027 0.000017 0.000019 0.000021 0.000020 0.000018 0.000026 0.000018 0.000018 0.000029 0.000021 0.000012 0.000009 0.000014 0.000015 0.000014 0.000013 0.000013 0.000013 0.000013 0.000016 0.000019 0.000013 0.000013 0.000011 0.000014 0.000014 0.000012 0.000019 0.000012 0.000013 0.000018 0.000015 0.000012 0.000012

11QL126 1 2 3 4 5 6 7 8 9 10 11 12 13

IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS

0.037348 0.043624 0.054942 0.041117 0.044867 0.043386 0.041298 0.043145 0.044288 0.040036 0.036706 0.029582 0.045222

0.001502 0.001754 0.002190 0.001689 0.001836 0.001710 0.001671 0.001743 0.001811 0.001639 0.001502 0.001220 0.001986

0.282736 0.282732 0.282721 0.282701 0.282804 0.282789 0.282708 0.282762 0.282762 0.282813 0.282763 0.282745 0.282772

0.000022 0.000021 0.000021 0.000020 0.000024 0.000021 0.000019 0.000021 0.000023 0.000022 0.000023 0.000023 0.000020

eHf(t)

±(1r)

TNC1 (Ma)

±(1r)

TNC2 (Ma)

±(1r)

7.3 7.2 9.0 7.4 7.4 7.9 9.8 8.7 7.9 6.4 8.4 6.0 6.4 3.7 7.3 7.3 9.2 7.2 7.4 8.4 8.4 7.1 7.8 7.0 8.3 7.4 7.8 7.4 7.4 7.0 6.6 7.4 6.7 7.4 8.5 7.8 7.3 8.1 8.2 7.5 7.5 7.3 7.3 6.9 8.1

0.6 0.8 0.8 0.6 0.6 0.6 0.7 0.8 0.6 0.5 1.0 0.6 0.7 0.8 0.7 0.6 0.9 0.6 0.6 1.0 0.7 0.4 0.3 0.5 0.5 0.5 0.5 0.5 0.4 0.5 0.6 0.7 0.5 0.5 0.4 0.5 0.5 0.4 0.7 0.4 0.5 0.6 0.5 0.4 0.4

650 657 582 647 645 626 549 598 626 691 606 708 694 806 652 655 575 658 647 605 607 661 632 665 611 644 626 648 649 668 684 659 681 652 602 630 654 617 615 644 643 655 652 671 617

26 34 34 27 26 25 28 34 27 21 40 26 29 32 30 27 39 27 27 43 31 18 13 20 22 20 19 19 19 19 23 29 19 19 17 21 20 17 28 18 19 27 21 18 18

756 761 647 749 749 717 595 668 718 819 687 841 819 993 757 759 635 764 748 685 688 768 728 777 692 748 722 750 749 780 803 753 798 752 682 723 758 704 698 746 746 756 759 782 704

40 52 53 42 40 38 42 51 42 33 63 39 44 49 46 41 59 41 41 67 47 27 21 31 33 31 31 30 29 30 36 43 30 29 26 33 31 27 42 28 30 41 33 28 27

8.1 7.9 7.3 6.8 10.4 9.9 7.0 8.9 8.9 10.8 9.0 8.5 9.2

0.8 0.7 0.8 0.7 0.9 0.8 0.7 0.7 0.8 0.8 0.8 0.8 0.7

619 629 653 673 525 545 663 585 587 508 580 601 574

33 30 32 30 35 31 28 30 34 33 33 33 29

710 724 758 793 560 593 777 655 657 536 648 683 636

51 47 49 47 55 49 43 47 53 51 52 53 45

122

H. Wang et al. / Geochimica et Cosmochimica Acta 182 (2016) 109–130

Table 3 (continued) Yb/177Hf

Lu/177Hf

Hf/177Hf

eHf(t)

Sample

Lab

176

176

176

±(1r)

14 15

IGGCAS IGGCAS

0.036703 0.036549

0.001509 0.001507

0.282735 0.282734

0.000022 0.000023

12QL113 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 2 3 4 5 6 7 8 9 10 11 12

IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR GPMR

0.058452 0.042240 0.061999 0.069906 0.033277 0.048605 0.065837 0.051877 0.057658 0.064037 0.053216 0.054862 0.053805 0.046825 0.069269 0.070856 0.060637 0.064745 0.045728 0.073301 0.057567 0.059295 0.077206 0.054921 0.066443 0.037642 0.060598

0.002361 0.001716 0.002514 0.002783 0.001380 0.002024 0.002589 0.002182 0.002372 0.002681 0.002268 0.002260 0.002223 0.001962 0.002841 0.002524 0.002031 0.002180 0.001518 0.002449 0.001971 0.002022 0.002602 0.001884 0.002261 0.001330 0.002103

0.282906 0.282902 0.282859 0.282883 0.282856 0.282853 0.282887 0.282877 0.282860 0.282860 0.282892 0.282861 0.282803 0.282850 0.282891 0.282872 0.282868 0.282861 0.282854 0.282878 0.282864 0.282850 0.282867 0.282866 0.282872 0.282853 0.282854

12TB28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS IGGCAS

0.043593 0.061283 0.057877 0.046464 0.063217 0.052679 0.068328 0.065310 0.052731 0.048647 0.084914 0.069237 0.064268 0.041430 0.055113 0.072171 0.053772 0.083622 0.069305 0.092826 0.065396 0.066355

0.001742 0.002390 0.002310 0.001896 0.002505 0.002128 0.002676 0.002594 0.002090 0.001957 0.003069 0.002574 0.002363 0.001533 0.002016 0.002683 0.001970 0.003050 0.002550 0.003395 0.002414 0.002435

0.282862 0.282811 0.282843 0.282897 0.282857 0.282903 0.282911 0.282828 0.282919 0.282854 0.282876 0.282863 0.282858 0.282849 0.282857 0.282878 0.282895 0.282842 0.282865 0.282849 0.282860 0.282887

zircon crystals in the Zhangjiadazhuang and Taoyuan plutons have highly positive eHf(t) values of +12.1 to +12.9, which are identical to those of the new continental crust generated in modern island arcs worldwide (eHf(t) = + 13.2 ± 1.1, Fig. 7) (Dhuime et al., 2011). Their twostage Hf model ages (ca. 440 Ma) are very close to the times

±(1r)

TNC1 (Ma)

±(1r)

TNC2 (Ma)

±(1r)

8.0 8.0

0.8 0.8

621 622

33 34

713 714

51 54

0.000019 0.000018 0.000016 0.000016 0.000016 0.000018 0.000019 0.000018 0.000018 0.000020 0.000019 0.000019 0.000017 0.000017 0.000016 0.000019 0.000011 0.000013 0.000011 0.000012 0.000012 0.000013 0.000012 0.000011 0.000016 0.000008 0.000011

14.1 14.1 12.3 13.1 12.6 12.3 13.3 13.1 12.4 12.3 13.6 12.5 10.5 12.2 13.4 12.8 12.8 12.5 12.5 13.1 12.7 12.2 12.6 12.8 12.9 12.5 12.3

0.7 0.6 0.6 0.6 0.6 0.7 0.7 0.7 0.6 0.7 0.7 0.7 0.6 0.6 0.6 0.7 0.4 0.5 0.4 0.4 0.4 0.5 0.4 0.4 0.6 0.3 0.4

379 378 452 419 443 455 411 421 448 453 399 446 532 458 407 433 432 445 446 422 437 459 440 433 429 446 455

28 27 24 25 23 27 28 27 27 30 29 28 25 25 25 29 16 20 16 19 17 19 18 16 23 12 16

330 325 442 392 426 445 380 394 436 443 360 433 565 451 374 412 411 430 432 396 418 452 424 412 405 430 446

43 42 37 38 36 42 43 42 41 45 44 43 39 39 37 45 25 31 26 29 27 29 28 24 36 19 25

0.000021 0.000022 0.000023 0.000022 0.000023 0.000024 0.000025 0.000022 0.000022 0.000022 0.000017 0.000018 0.000017 0.000015 0.000016 0.000017 0.000015 0.000017 0.000018 0.000019 0.000015 0.000016

12.1 10.1 11.2 13.3 11.7 13.4 13.5 10.7 14.0 11.7 12.2 11.9 11.8 11.7 11.8 12.4 13.2 11.0 11.9 11.1 11.8 12.8

0.7 0.8 0.8 0.8 0.8 0.9 0.9 0.8 0.8 0.8 0.6 0.6 0.6 0.5 0.6 0.6 0.5 0.6 0.6 0.7 0.5 0.6

438 522 473 388 455 381 376 499 357 453 433 446 451 454 449 425 391 485 444 479 449 409

31 33 35 32 35 36 39 33 33 32 27 27 25 22 23 25 22 26 27 29 23 25

436 564 490 357 461 347 341 529 310 458 429 448 455 460 452 416 362 506 444 497 453 391

49 50 53 50 53 56 59 51 51 49 40 41 39 34 36 38 34 39 41 44 35 38

of the pluton formation, which implies juvenile sources. Although the Xizhuanghe and Manziying plutons show slightly low zircon eHf(t) values of +7.6 to +8.5 relative to the other two granitoid plutons, it is still comparable with modern island arcs like the Sunda and Luzon (Dhuime et al., 2011). The Neoproterozoic Hf model ages

H. Wang et al. / Geochimica et Cosmochimica Acta 182 (2016) 109–130 4

6

(b)

(a)

New continental crust range 13.2±1.1

5

3

2

1

Number

Trondhjemite 12QL113 IGGCAS Mean = 12.9±0.3 MSWD = 1.10 GPMR Mean = 12.6 ± 0.2 MSWD = 0.35

2

Relative probability

Relative probability

4

IGGCAS Mean = 8.5 ± 0.4 MSWD = 2.4

Monzogranite 11QL126

3

Number

123

1

0

0 2

7

12

2

17

7

12

17

ε H f (t)

ε H f (t) 6 12

(c)

(d) 5

10

Number

Number

4

Relative probability

6

Granodiorite 10QL161

4

Relative probability

IGGCAS Mean = 7.6 ± 0.3 MSWD = 2.1 GPMR Mean = 7.5 ± 0.2 MSWD = 1.18

8

Trondhjemite 12TB28 IGGCAS Mean = 12.1±0.3 MSWD = 1.3

3

2

1

2

0

0 2

7

12

2

17

7

12

ε H f (t)

17

ε H f (t)

Fig. 7. Histograms of zircon eHf(t) values for samples 12QL113 (a), 11QL126 (b), 10QL161 (c), and 12TB28 (d) of the Qinling orogen. The eHf(t) range for new continental crust is from Dhuime et al. (2011).

5.8

5.8

(a)

5.6

5.4

5.4

5.2

5.2

5.0

5.0

δ 1 8 O(‰)

δ 1 8 O(‰)

5.6

4.8 4.6

4.6 4.4

4.2

4.2

4.0

4.0

0

200

400

600

800

1000

U (ppm)

1200

12QL113 10QL161 12TB28

4.8

4.4

3.8

(b)

3.8 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Th/U ratio

Fig. 8. Plot of d18O versus U content (a) and Th/U ratio (b) for zircon crystals of samples 12QL113, 10QL161, and 12TB28 of the Qinling orogen.

(ca. 700 Ma) of the two plutons are coeval with the initial formation age of the Erlangping oceanic arc (Zhang et al., 1994; Xue et al., 1996). Accordingly, the Hf isotopic data indicate that these granitoid plutons were derived from the accreted oceanic crust. In typical oceanic crust, low d18O values are characteristically observed in middle-

lower crustal gabbro, with a range from 0‰ to 6‰ and typically from 4‰ to 5‰ (Stakes and Taylor, 2003). Apart from the low-d18O gabbro, subducted oceanic slab contains large portions of 18O-rich basalt and sediment (Stakes and Taylor, 2003), which are adjacent to mantle wedge and thus are prone to melting. Therefore, the zircon d18O values of

124

H. Wang et al. / Geochimica et Cosmochimica Acta 182 (2016) 109–130 10

12QL113 Zhangjiadazhuang

9

11QL126 Manziying 10QL161 Xizhuanghe

Ol

12TB28 Taoyuan

du

7

pp

er

Mantle zircon range 18 δ O = 5.3 ± 0.3 ‰

8

δ 1 O(‰)

8

6

cr

us

New continental crust range ε H f = 13.2 ± 1.1

t

Old lower crust

5

4

Plagiogranite zircon range δ O = 4.9 ± 0.6 ‰ 18

3 -5

0

5

10

15

20

ε H f (t) Fig. 9. Plot of d18O versus eHf(t) for zircon crystals of samples 10QL161, 11QL126, 12QL113, and 12TB28 of the Qinling orogen. The d18O values for mantle and plagiogranite zircon are from Valley et al. (1998) and Grimes et al. (2013), respectively, whereas the eHf(t) range for new continental crust is from Dhuime et al. (2011).

resultant slab-derived granitoids are mantle-like or higher, due to the balance of contributions of melts from 18 O-rich and 18O-poor parts of the subducted slab. This is exemplified by the modern adakites that were regarded as being derived from subducted oceanic slab (Bindeman et al., 2005). The uniformly low zircon d18O values of the studied granitoid plutons indicate that their protoliths were still preserved in lower crust level before their formation and did not involve massive supracrustal material (Fig. 9). Therefore, the oxygen isotopic evidence argues against the derivation of the Erlangping granitoids from subducted slab. Additionally, the granitoids derived from subducted oceanic slab are characterized by the geochemical features that are relatively high Sr/Y and La/Yb ratios, low heavy rare earth element (HREE) contents, and negligible Eu anomalies (namely adakitic or TTG-like signature), which are due to the presence of garnet and the absence of plagioclase in the residue (Defant and Drummond, 1990) The Zhangjiadazhuang, Manziying, and Taoyuan plutons have relatively high HREE concentrations and negative Eu anomalies (Fig. 3b), indicating that they were likely derived from a shallow crustal source buried within the stability field of plagioclase. Although the Xizhuanghe granitoids display some degrees of depletion in middle to heavy rare earth elements, and negligible Eu anomalies, they are still distinct from typical adakites by their relatively low (La/Yb)N and Sr/Y ratios as well as high Yb and Y contents (Fig. 10). As hornblende is rich in middle to heavy rare earth elements (Arth and Barker, 1976), the concave REE patterns of the Xizhuanghe granitoids may be resulted from melting of a lower crustal source at relatively high pressures with high amphibole concentration and minor plagioclase in the residue, or from

the fractional crystallization of hornblende during magma evolution. Therefore, the trace element features are also inconsistent with their origination from subducted oceanic crust. Like other oceanic profile, 18O-rich basalt and sediment and 18O-poor gabbro occupy upper and middlelower crust of oceanic arcs, respectively (Stakes and Taylor, 2003). Extensive experimental studies have been devoted to the partial melting of oceanic gabbro at middle-lower crustal levels, which show that sodic granitic melt was acquired through the reaction such as olivine + clinopyroxene + plagioclase (I) + H2O = amphibole + orthopyroxene + plagioclase (II) + hydrous sodic granitic melt (Wolff et al., 2013). Unlike subducted oceanic slab, the 18O-rich basalt and sediment in the suprasubduction oceanic arcs contribute no or little material to the hydrous sodic granitic melt, as the upper oceanic crust is not hot enough to be melted (commonly <400 °C). Therefore, the resultant sodic granitoid rocks should have relatively low d18O values (Stakes and Taylor, 2003). This has been verified by the tonalite in modern arcs (Suzuki et al., 2015) and plagiogranite in SSZ-type ophiolites (Grimes et al., 2013), which show slightly low zircon d18O values (4.85 ± 0.04‰ and 4.9 ± 0.6‰, respectively) relative to the normal mantle zircon d18O value, but comparable with our results. Although no Hf isotopic data has been directly reported for the Erlangping basaltic rocks, they have been revealed to have highly positive whole-rock eNd(t) values (2.5–7.0) (Zhang et al., 1994; Sun et al., 2002; Wang et al., 2013b), which are compatible with our Hf isotopic data taking into consideration of the positive correlation between eHf(t) and eNd(t) values for oceanic basalts (eHf(t) = 1.59  eNd(t) + 1.28) (Chauvel et al., 2008). Moreover, Paleotethyan oceanic crust in the Hong’an orogen, which

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Fig. 10. (La/Yb)N versus (Yb)N (a) and Sr/Y versus Y (b) (after Defant and Drummond (1990)) diagrams for the Erlangping granitoids in the Qinling orogen. Chondrite-normalized values are from Sun and McDonough (1989). The geochemical data of the Zhangjiadazhuang, Manziying, Xizhuanghe, and Taoyuan plutons are shown in Supplementary Table S1.

might be equivalent to the Erlangping oceanic arc, yielded eHf(t) values of +5.2 to +13.0 (Wu et al., 2009b; Zhou et al., 2015), quite similar to the results of the granitoid zircons. Simulation based on trace element data also indicates that the sodic granitoid plutons could be formed through partial melting of tholeiitic rocks from the Erlangping unit (Tian and Wei, 2005). As a result, the combination of trace elements and Hf–O isotopes suggests that the sodic granitoid plutons originated from the lower crust of the accreted Erlangping oceanic arc. The volcano-sedimentary sequence of oceanic arc terranes show conspicuous similarity with those of Archean supracrustal belt (Turner et al., 2014), both of which are intruded by sodic tonalite, trondhjemite, and granodiorite plutons, and minor potassic granite plutons. Therefore, deciphering the petrogenesis of potassic granite in oceanic arc terranes is crucial for understanding how the upper continental crust acquires its K-rich nature (Rudnick and Gao, 2003). Several origins have been proposed for the potassic granite in oceanic arc terranes, including (1) partial melting of sedimentary rocks (White and Chappell, 1983); (2) mixing of Na-rich granitoid melts with K-rich mafic magmas (Lo´pez et al., 2006); (3) extreme fractional crystallization of a mantle melt (Mushkin et al., 2003); (4) partial melting of K-rich mafic rocks (Sisson et al., 2005); and (5) dehydration melting of sodic granitoid rocks (Watkins et al., 2007). The Manziying pluton is peraluminous with high SiO2, K2O, and Al2O3 contents, and thus is classified as potassic granite (Guo et al., 2010). It could not originate by melting of sedimentary rocks, as zircon crystals from S-type granite commonly show high d18O values (>8.5‰) and negative eHf(t) values (Hoefs, 2009; Qin et al., 2014; Zhao et al., 2015), opposite to those of the Manziying pluton (d18O = 4.9 ± 0.1‰; eHf(t) = +8.5 ± 0.4). The mixing model is unfeasible for the Manziying pluton, as this scenario would predict presence of K-rich mafic rocks and covariant Hf–O isotopic compositions (Kemp et al., 2007), which are inconsistent with the geological and Hf–O isotope observation (Fig. 9). Extreme fractional

crystallization of a mantle-derived melt would generate huge intermediate intrusions accompanied with the Manziying pluton, which is not found in the Erlangping unit yet. The lack of high-K mafic rocks in the Erlangping unit also overshadows the K-rich mafic source model. The Manziying pluton consists predominantly of monogranite without accompanying intermediate-mafic components (Guo et al., 2010). It shows Hf–O isotopic compositions comparable to those of the Xizhuanghe pluton (Figs. 6 and 7), indicative of juvenile sources similar to those of the Xizhuanghe pluton. However, the Manziying pluton is much larger than the Xizhuanghe pluton in term of outcrop size (Fig. 1) and contains much more silica (SiO2 = 75.0– 77.5% vs. 66.8–71.6%) (Table S1). The geochemical compositions of the Manziying pluton do not form continuous trends with those of the Xizhuanghe pluton on Harker diagrams (Guo et al., 2010; Guo and Chen, 2011). These features imply that the Manziying pluton cannot be formed through direct differentiation of the Xizhuanghe pluton. Experimental studies have shown that dehydration melting of biotite and/or hornblende in tonalite can engender high-Si peraluminous monzogranite and the melt proportion can be more than 30% (Watkins et al., 2007). Considering that sodic granitoids are widespread in the Erlangping unit, we propose that the Manziying pluton was formed by dehydration melting of the sodic granitoid rocks that originated from the juvenile Erlangping oceanic arc crust with Hf–O isotopic compositions similar to those of the Xizhuanghe pluton. 5.3. Implications for the generation of continental crust in accretionary orogens The continental crust has bulk compositions similar to andesite (Rudnick and Gao, 2003; Davidson and Arculus, 2006). Because andesite is commonly generated in continental arc settings, the simplest way to form continental crust is by continental arc magmatism (Taylor, 1967). However, arc crust production in continental arc settings is

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mass-balanced by subduction erosion and sediment recycling, thus contributing no net mass to continental crust growth (Clift et al., 2009). In addition, the magmatic rocks in continental arcs are commonly characterized by negative eNd(t) and eHf(t) values, indicating reworking, rather than new addition, of continental crust (Kemp et al., 2009). Besides, continental arcs have remarkably higher Sr/Y (40.6–58.6) and La/Yb (24.9–28.7) ratios (Ducea et al., 2015) than those of the upper and bulk crustal averages (Sr/Y = 15.2–16.8; La/Yb = 10.5–15.5) (Rudnick and Gao, 2003) at similar silica content levels. Niu et al. (2013) proposed that ca. 65–60 Ma andesitic rocks in the Gangdese belt of southern Tibet arose from partial melting of subducted oceanic crust when the Indian continent collided with the Tibet, and thus suggested that continental collision zones were primary sites for net continental crust growth. Actually, the early Cenozoic magmatic activities can be viewed as the reworking of previous arc crust during the continental collision orogeny (Ducea et al., 2015). Syncollisional magmatism occurs very limitedly during continental subduction as exemplified by the Dabie-Sulu orogenic belt (Zheng, 2008; Zhao et al., 2012). A compilation of zircon Hf isotopic data also revealed that collisional orogen was dominated by reworking of old continental crust (Collins et al., 2011). In addition, melts arisen from subducted oceanic crust, no matter whether they occur in continental or oceanic arc settings, commonly show adakitic signatures (Defant and Drummond, 1990), which are not usually detected for the continental crust (Rudnick and Gao, 2003). Only small portions of subducted oceanic slab could return back to surface through partial melting, and thus contribute limitedly to the growth of continental crust (Clift et al., 2009). All these observations cast shadow for the significance of partial melting of subducted oceanic crust in continental crust growth. Intra-oceanic subduction zones are far removed from continents (Stern, 2002), which are considered as ideal sites for crustal growth (Draut et al., 2002; Clift et al., 2009). However, there are two main criticisms that the oceanic arc model has faced; one is that oceanic arcs should readily be subducted and have poor preservation potential, based on buoyancy and crust thickness considerations (Hawkesworth et al., 2009; Condie and Kro¨ner, 2013); the other is that the continent has high-Si, K-rich granitoid upper crust, which is commonly absent in oceanic arcs (Holbrook et al., 1999; Rudnick and Gao, 2003). Whether oceanic arcs would be subducted depends not only on buoyancy and crustal thickness, but also on subduction angle and direction (Brown and Ryan, 2011; Condie and Kro¨ner, 2013). The Erlangping unit in the eastern Qinling orogen is a paradigm for the preservation of oceanic arc in accretionary orogens (Xue et al., 1996), which accreted onto the Qinling microcontinent at ca. 500–490 Ma through arc–continent collision (Wang et al., 2011b, 2014b). Arc–continent collision is a common phenomenon in the geological record, which is exemplified by the modern Taiwan and Timor, early Cenozoic Kamchatka and Kohistan, late Paleozoic Uralides, and early Paleozoic Western Ireland (Brown and Ryan, 2011). Oceanic arc terranes were widespread in the accretionary orogens. Accordingly, we suggest that arc–continent collision is an efficient

mechanism for preventing oceanic arcs from being subducted. The preservation of the Erlangping oceanic arc provides an optimal opportunity to study how and when oceanic arcs are evolved into granitoid upper crust. Geochronological data revealed that the early Paleozoic plutons in the Erlangping unit formed at ca. 470–430 Ma (Wang et al., 2013b), postdating the accretion of the Erlangping oceanic arc. Zircon Hf–O isotopes disclosed that partial melting of the lower crust of the Erlangping oceanic arc resulted in the formation of the sodic granitoid plutons and further remelting of the sodic granitoid rocks leaded to the generation of potassic granite plutons. This indicates that the continental crust can acquire its high-Si and K-rich nature from accreted oceanic arc terranes through differentiation by multi-stage post-accretional magmatism. After the compositional differentiation, the complementary mafic restites possibly foundered into the mantle, owing to their high densities. This drives the bulk composition of the oceanic arc crust to be more continental. Numerical modeling results show the foundering can occur in accreted arc terranes, if the lower-crust/upper-mantle interface remains temperature of above 700 °C, which may account for the origin of the continental Moho (Jagoutz and Behn, 2013). Only relicts of minor ultramafic rocks were preserved in the Erlangping unit, which may be the consequence of the foundering. Overall, the Hf–O isotopic studies on granitoid-hosted zircon highlight the importance of the accreted oceanic arc for the generation of the continental crust. 6. CONCLUSIONS In situ zircon SIMS dating revealed that the Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons formed at 472 ± 7, 458 ± 6 and 443 ± 5 Ma, respectively, which postdated the accretion of the Erlangping oceanic arc. The Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons are sodic granitoid and have highly positive zircon eHf(t) (+7.6 to +12.9), and relatively low zircon d18O (4.7– 5.0‰) values. The Manziying pluton is monzogranitic and show similar Hf–O isotopic compositions to those of the Xizhuanghe pluton. It is proposed that the formation of the sodic granitoid plutons resulted from partial melting of the lower crust of the Erlangping oceanic arc shortly after its accretion and the monzogranitic pluton arose through the remelting of the sodic granitoid plutons. Accordingly, we suggest that accreted oceanic arc terranes are the main sites for net continental crust growth. Overall, arc–continent collision provides an effective way for preventing oceanic arcs from being subducted, and postaccretional magmatism can fractionate accreted oceanic arc crust into granitoid continental crust. ACKNOWLEDGEMENTS We are grateful to Qiu-Li Li, Guo-Qiang Tang, Yu Liu and Xiao-Xiao Ling for their assistance with SIMS zircon U–Pb dating and O isotope analysis as well as Jin-Hui Yang and Yue-Heng Yang for their assistance with LA-MC-ICPMS zircon Hf isotopic analysis. Critical reviews by three anonymous reviewers and edito-

H. Wang et al. / Geochimica et Cosmochimica Acta 182 (2016) 109–130 rial comments by Wei-Dong Sun substantially improved the manuscript. This study was supported by the Chinese 973 project (2015CB856106), the National Natural Science Foundation of China (41503026, 41273035 and 41173017), and the Ministry of Education of China (B07039).

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