Geochemical constraints on the nature of magma

Lithos 284–285 (2017) 30–49

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Geochemical constraints on the nature of magma sources for Triassic granitoids from South Qinling in central China Ying-Hui Lu, Zi-Fu Zhao ⁎, Yong-Fei Zheng CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China

a r t i c l e

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Article history: Received 14 November 2016 Accepted 29 March 2017 Available online 07 April 2017 Keywords: Orogenic granitoids Source nature Geochemical differentiation Petrogenetic mechanism Collisional orogen

a b s t r a c t A combined study of zircon U-Pb ages and Lu-Hf isotopes, whole-rock major-trace elements and Sr-Nd isotopes as well as mineral chemistry and O isotopes was carried out for Triassic granitoids from the South Qinling orogen in central China. Model calculations were also performed to examine the trace element fractionation during partial melting of crustal rocks. The results provide insights into the nature of magma sources for these granitoids. LA-ICPMS zircon U-Pb dating yields concordant ages of 208 ± 2 to 216 ± 3 Ma for these granitoids from the Shahewan (SHW), Caoping (CP) and Zhashui (ZS) plutons, and no relict zircon cores are identified by the CL imaging and U-Pb dating. The SHW and CP granitoids contain hornblende and are metaluminous with A/CNK ratios of 0.84 to 0.93. They exhibit relatively low SiO2 contents (62.88–69.04 wt.%) but high contents of FeOT, MgO and TiO2, and slightly to negligibly negative Eu anomalies (δEu = 0.79–0.89). Zircons from them show mantle-like δ18O values of 4.71 to 5.72‰. In contrast, the ZS granites contain no hornblende and are metaluminous to weakly peraluminous with A/CNK ratios of 0.99 to 1.03. They show relatively high SiO2 contents (69.32–75.94 wt.%) but low FeOT, MgO and TiO2 contents, and moderate negative Eu anomalies (δEu = 0.63–0.81). They have slightly low zircon δ18O values of 4.60 to 4.83‰. All of these granitoids show arc-like trace element distribution patterns with enrichment in LREE and LILE (e.g., Rb, K and Pb) but depletion in HFSE (e.g., Nb, Ta and Ti). Geochemical comparison and modeling indicate that these granitoids are different from adakitic rocks originating from the thickened lower continental crust. Compared with the composition of felsic melts produced by petrological experiments of various lithologies, it appears that these granitoids are derived from dehydration melting of metabasaltic sources at normal lower crustal depths, and experienced varying degrees of fractional crystallization. The SHW and CP granitoids were crystallized under the conditions of variably low pressures of 0.7–2.3 kbar, temperatures of 628–766 °C and higher fO2 than the ZS granites. On the other hand, these granitoids show moderate whole-rock initial 87Sr/86Sr ratios of 0.7045 to 0.7055 and slightly negative εNd(t) values of −4.0 to −1.5 as well as slightly negative to positive zircon εHf(t) values of −1.3 to 3.2. They have two-stage Nd model ages of 1.05–1.38 Ga and Hf model ages of 1.04–1.39 Ga. These whole-rock εNd(t) and zircon εHf(t) values are comparable with those for Neoproterozoic Yaolinghe metabasalts and mafic-ultramafic intrusions in South Qinling but inconsistent with those for Neoproterozoic Bikou metavolcanics and mafic-ultramafic intrusions in the northern margin of the Yangtze craton. Together with their non-adakitic features, it is suggested that they were derived from partial melting of the mafic lower crust in South Qinling rather than the subducted Yangtze continental crust beneath South Qinling. The continental collision between the North China Block and South China Block in the Late Triassic would have brought about reworking of the mafic lower crust for the granitoid magmatism in the Qinling orogen. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Granites have been subjected to numerous intensive studies because they are the most common type of magmatic rocks in the continental crust and occur in a variety of tectonic settings, providing significant information on the growth and reworking of continental crust, crust-mantle interaction and mineralization (e.g., Gill, 2010; Hawkesworth and Kemp, ⁎ Corresponding author. E-mail address: [email protected] (Z.-F. Zhao).

http://dx.doi.org/10.1016/j.lithos.2017.03.028 0024-4937/© 2017 Elsevier B.V. All rights reserved.

2006; Kemp et al., 2007; Rudnick, 1995). However, granitic rocks show great diversity with different mineralogical assemblages and geochemical compositions (e.g., Brown, 2013; Clemens and Stevens, 2012; Pitcher, 1997). Thus, the type classification, source nature, differentiation mechanism and tectonic setting of their production are still the most debated subjects in granite petrogenesis. So far, as summarized by Clemens and Stevens (2012), various models have been proposed to account for geochemical variations within granitic rock suites, including mixing of mantle- and crustal-derived magmas, fractional crystallization of basaltic magmas, peritectic assemblages entrainment, restite unmixing and

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melting of mixed materials, among which mixing between felsic and mafic magmas were often suggested (e.g., Collins, 1996; Barbarin, 2005; Kemp et al., 2007; Yang et al., 2007). Because of these complexities, granite petrogenesis has been enigmatic in petrology and geochemistry. Although differentiation of mantle-derived magmas could be involved in the genesis of some granites, reworking of ancient and juvenile crustal rocks are still thought to be the dominant way to produce continental granites (Zheng et al., 2015). Clemens et al. (2011) argued that most I-type granitic magmas are purely crustal in origin, whereas their source rocks are commonly formed by the mixing of mantle- and crust-derived materials such as arc volcanic rocks, which could have the mixed isotopic signatures between mantle and crustal components. On the other hand, I-type granites can be produced by partial melting of ancient and juvenile igneous rocks (e.g., Zhao et al., 2007, 2008), resulting in the mixed isotopic signatures that look like mixing between the crustal and mantle sources. Furthermore, the geochemical diversity of I-type granites is primarily caused by a series of differences in partial melting conditions (e.g., temperature, pressure and water fugacity) and the extent of fractional crystallization (e.g., Chappell and White, 1992; Chappell et al., 2004; Clemens et al., 2010 Miller et al., 2003; Patiňo Douce and McCarthy, 1998). Since normal geothermal gradients and crustal thickness cannot provide sufficient heat for partial melting of crustal rocks, mantle-derived magmas are usually invoked to play a significant role in providing the heat necessary for crustal melting for granitic magmas (e.g., Annen and Sparks, 2002; Brown, 2013; Clemens, 2003; Huppert and Sparks, 1988; Petford and Gallagher, 2001).

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Therefore, it is essential to identify the source nature and petrogenesis of granites in order to understand the crustal architecture and the tectonic evolution of orogenic belts. It is common that intensive and extensive magmatism are developed in orogenic belts where granitoids are dominated (e.g., Barbarin, 1999; Pearce et al., 1984; Pitcher, 1983). The Qinling orogen constitutes the most dominated part of the Central Orogenic Belt of China (Fig. 1), which was formed by the continental collision between the North China Block (NCB) and South China Block (SCB) in the Triassic (Dong et al., 2011; Ratschbacher et al., 2003; Wu and Zheng, 2013). Widespread Triassic I- and S-type granitoids are developed in the Qinling orogen, mainly in the South Qinling zone (SQZ) (e.g., Li et al., 2015; Lu et al., 2016; Wang et al., 2013b), but contemporaneous mafic rocks are scarce. These granitoids were mainly emplaced in the Late Triassic (e.g., Li et al., 2015; Lu et al., 2016, and references therein). However, their petrogenesis and tectonic settings are still hotly debated. Based on their zircon Hf isotopes and the conspicuous occurrences of mafic microgranular enclaves (MMEs) in some granitic plutons, especially in the northern margin of the SQZ, many studies proposed that the Itype granitoids there were produced by mixing of magmas derived from partial melting of the lower continental crust and enriched lithospheric mantle or depleted asthenospheric mantle (e.g., Gong et al., 2009a, 2009b; Liu et al., 2011; Wang et al., 2011; Zhang et al., 2008). Other studies suggested that some granitoids were produced by partial melting of the subducted Yangtze continental crust or subducted sediments and underwent interaction with the overlying mantle wedge

Fig. 1. Simplified geological map showing (a) the location of the Qinling orogen in central China, (b) its tectonic division and (c) the distribution of Triassic granitoids (modified from Dong et al. 2011). The studied area is marked by a blue solid rectangle. Abbreviations: SDS, Shangdan suture; MLS, Mianlue suture; LLWF, Lingbao-Lushan-Wuyang fault; MBXF, Mianlue-Bashan-Xiangguang fault; LLF, Luonan-Luanchuan fault; LDF, Longmenshan-Dabashan fault; EPG, Erlangping Group; KPG, Kuanping Group; QC, Qinling Complex.

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peridotite (e.g., Jiang et al., 2010; Qin et al., 2010, 2013). However, the geochemical evidence for a direct contribution of mantle materials to the origin of granitoids is circumstantial. On the other hand, the geodynamic mechanism for the generation of these granitoids has been also controversial. Different hypotheses have been proposed: (1) the granitoid magmatism resulted from the northward subduction of the Paleo-Tethys oceanic crust and thus has a tectonic setting of volcanic arc (e.g., Li et al., 2015; Zhang et al., 2001); (2) these granitoids were formed in a syn-collisional setting (e.g., Lu et al., 2016; Sun et al., 2002; Zhou et al., 2008); (3) these granitoids were formed in a post-collisional setting (e.g., Qin et al., 2010, 2013; Zhang et al., 2008), especially for rapakivi-textured granitoids that are considered to represent the product of extensional tectonism after the continental collision between the NCB and the SCB along the Mianlue suture (e.g., Lu et al., 1999; Wang et al., 2008, 2011); (4) these granitoids were formed under episodic tectonism from subduction-related to syncollisional to post-collisional settings (e.g., Dong et al., 2011, 2012; Jiang et al., 2010; Liu et al., 2011). These disputes have led to various interpretations about the timing of the closure of the Mianlue ocean and the continental collision between the SCB and the NCB. In addition, some granitoids are characterized by adakitic signatures in trace element composition, such as high La/Yb and Sr/Y ratios but low Y and Yb contents, which are normally considered to be resulted from partial melting of the subduction-thickened continental crust (e.g., Qin et al., 2010, 2013; Zhang et al., 2008). Furthermore, some granites were previously regarded as I-type but later studies are identified as S-type, which were produced by partial melting of the metasedimentary rocks in response to the collision orogeny in the SQZ (Lu et al., 2016, and references therein). Therefore, reasonable identifications of the type, source nature and petrogenetic mechanism of these granitoids would have great significance for understanding the crustal architecture and geodynamic evolution of the Qinling orogen. In terms of the existing results and still hotly debated issues, we have collected granitoid samples from three granitoid plutons (i.e. the Shahewan, Caoping and Zhashui plutons) in the Dongjiangkou area close to the Shangdan suture in the SQZ. In this paper, we present a combined study of petrography, LA-ICPMS zircon U-Pb ages, whole-rock major-trace elements and Sr-Nd isotopes, zircon Lu-Hf isotopes as well as mineral O isotopes and mineral chemistry for these granitoids from South Qinling. The results provide new insights into the nature of source rocks and the effect of anatectic processes on the geochemical composition of these granitoids as well as the geodynamic evolution of the SQZ. 2. Geological setting The Chinese mainland is characterized by the assembly of several ancient cratonic blocks along orogenic belts (Zheng et al., 2013). Among which, the Qinling orogen is composed of several accretionary to collisional orogens between the NCB and the SCB, being one of the major orogenic belts in China (e.g., Mattauer et al., 1985; Ratschbacher et al., 2003; Wu and Zheng, 2013; Zhang et al., 2001). It extends westly toward the Kunlun and Qilian orogens, and eastly toward the Tongbai, Hong'an, Dabie and Sulu orogens (Fig. 1a). The Dabie-Sulu orogenic belt was produced by the continental collision between the NCB and the SCB in the Triassic, with the well-known occurrence of ultrahigh pressure (UHP) metamorphic rocks (Zheng, 2008). In contrast, the Qinling orogen was built by a series of tectonic processes from the subduction of continental crust in the Cambrian through the subduction of Paleotethyan oceanic crust in the Carboniferous to the subduction of continental crust in the Triassic, which led to the final amalgamation between the NCB and the SCB (e.g., Dong et al., 2011; Meng and Zhang, 2000; Wu and Zheng, 2013; Zhang et al., 2001). The geological framework of the Qinling orogen was built up by two major suturing events between three blocks and several major faults (Fig. 1b). From north to south, these include the southern margin of the North China Block (S-NCB), the North Qinling zone (NQZ), the

South Qinling zone (SQZ) and the northern margin of the South China Block (N-SCB), which are respectively separated by the Shangdan suture in the north and the Mianlue suture in the south (Dong et al., 2011; Meng and Zhang, 2000; Zhang et al., 2001). It is widely accepted that the Shangdan suture resulted from Middle Paleozoic closure of the Shangdan Ocean and collision between the NQZ and the SQZ (e.g., Mattauer et al., 1985; Zhang et al., 2001), whereas the Mianlue suture was formed by the Late Triassic closure of the Mianlue Ocean and collision between the SQZ and the SCB (e.g., Dong et al., 2011; Li et al., 1996, 2007), and then overprinted by the south-directed overthrust of the Mianlue-Bashan-Xiangguang Fault (MBXF) in the Late Jurassic to Early Cretaceous in the eastern part (Dong et al., 2011). The NQZ is bounded by the Luonan-Luanchuan fault (LLF) in the north and the Shangdan suture in the south (Fig. 1b); it is predominately composed of Precambrian basement, Neoproterozoic and Early Paleozoic ophiolites, and volcanic-sedimentary assemblages, which are unconformably overlain by middle Carboniferous to Permian sedimentary rocks (Dong et al., 2011). The main lithological units in the NQZ, from north to south, are the Kuanping Group, Erlangping Group, Qinling Group and Danfeng Group, which are separated from each other by thrusts or ductile shear zones (Dong et al., 2011; Zhang et al., 2001). The Kuanping Group is chiefly composed of greenschists, mica schists, quartzites, marbles and amphibolites, whose protoliths are tholeiitic basalts, wackes, and carbonate rocks, respectively (Wu and Zheng, 2013). It was intruded by the Late Mesozoic and Late Paleozoic granitoids (Wang et al., 2013b; Zhang et al., 2013). Recent studies suggest that the Kuanping Group was formed during the Late Neoproterozoic to Early Paleozoic (e.g., Shi et al., 2013). The Early Paleozoic Erlangping Group is composed of ophiolite, metavolcanic and metasedimentary complexes (Wu and Zheng, 2013), which were intensively intruded by Early Paleozoic and Late Mesozoic granitic plutons (Wang et al., 2013b; Zhang et al., 2013). The Qinling Group, the main tectonic unit of the NQZ, contains marbles, amphibolites and gneisses, whose protoliths are limestones, interlayer of continental tholeiitic basalts and clastic rocks, respectively (Dong et al., 2011). Previous studies suggested that the Qinling Group was formed in the Paleoproterozoic (Zhang et al., 1994b), and underwent early Neoproterozoic amphibolite-facies metamorphism and late Early Paleozoic greenschist-facies metamorphic overprint (Dong et al., 2011). However, new zircon U-Pb dating results indicate that the Qinling Group was developed in the Early Neoproterozoic or Late Mesoproterozoic, and metamorphosed in the Early Paleozoic (Shi et al., 2013, and references therein). The Danfeng Group is composed of ophiolite and subduction-related volcanic and metasedimentary rocks, which are associated with the Early Paleozoic closure of backarc basin and arc-continent collision (e.g., Dong et al., 2011; Wu and Zheng, 2013), and represents the main tectonic boundary between the SQZ and the NQZ. The SQZ, located between the Shangdan suture in the north and the Mianlue suture in the south (Fig. 1b), is characterized by a south-vergent imbricated thrust-fold system (Zhang et al., 2001). It is mainly composed of Meso-Neoproterozoic metasedimentary and metavolcanic rocks and Early Paleozoic to Early Mesozoic sedimentary rocks (Dong et al., 2011; Zhang et al., 2001). The Meso-Neoproterozoic metasedimentary and metavolcanic rocks are mainly rift-type volcanic-sedimentary assemblages, which were metamorphosed under greenschist- to lower amphibolite-facies conditions, yielding the Foping metamorphic complex, the Douling Group, the Wudang Group and the Yaolinghe Group (e.g., Dong et al., 2011; Ling et al., 2008; Zhang et al., 2001; Zhu et al., 2014). All these Precambrian basements are covered by sedimentary strata, including the uppermost Neoproterozoic clastic and carbonate rocks, Cambrian to Ordovician limestones, Silurian shales, and Devonian to Carboniferous clastic sediments with intercalated limestones (Dong et al., 2011). Permian to Lower Triassic sandstones only occur in the northern part of the SQZ (Zhang et al., 2001). Additionally, the SQZ was intensively intruded by the Early Mesozoic granitoids, which are associated with the subduction and collision between the SQZ and the SCB along the Mianlue suture (Dong et al., 2011; Lu et al., 2016).

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To the south of the SQZ, the Mianlue suture is a large-scale, southward curved fold-thrust system, which is mainly composed of discrete ophiolitic mélange as exposed to various thrust slices bound by brittle and/or ductile faults with different scales of deformation, formation ages, tectonic settings and origins (Li et al., 2007). South to this zone is the N-SCB (Fig. 1c), which includes the Neoarchean basement, Middle to Late Proterozoic volcanic-sedimentary assemblages and Neoproterozoic intrusions, such as the Yudongzi Group, Hannan complex and Bikou Group. All these units are unconformably overlain by Sinian carbonates and clastic rocks, Cambrian to Ordovician continuous marine limestones, Silurian shales and siltite, Permian to Middle Triassic limestones, and then are unconformably covered by Late Triassic to Cretaceous continental facies clastic rocks (Dong et al., 2011). Mesozoic granitoids widely occur in the SQZ, roughly parallel to the Mianlue suture, with an outcrop area of ~6000 km2 (Fig. 1c). The granitoids of the SQZ have been grouped into three suites: the Guangtoushan suite, the Wulong suite and the Dongjiangkou suite from southwest to northeast (Zhang et al., 1994a). The reliably geochronological data indicate that these granitoids were mainly emplaced during ca. 205– 225 Ma (Li et al., 2015; Lu et al., 2016, and references therein). The Dongjiangkou suite, close to the Shangdan suture in the SQZ, is composed of the Dongjiangkou, Zhashui, Caoping and Shahewan plutons. These granitoids show high Na2O contents, low K2O/Na2O and (87Sr/86Sr)i ratios, and variable δ18O values of 4.1 to 9.0‰, and thus they were suggested to belong to I-type (Zhang et al., 1994a). However, many recent studies proposed obviously different models of petrogenesis for these granitoids (e.g., Gong et al., 2009a, 2009b; Hu et al., 2016; Jiang et al., 2010; Lu et al., 1999; Liu et al., 2011; Qin et al., 2010; Wang et al., 2008, 2011; Zhou et al., 2008). In this regard, it remains hotly debated with respect to both the nature of magma sources and geodynamic setting.

3. Samples and petrography The present study focuses on the Dongjiangkou suite in the SQZ, which is close to the Shangdan suture. The samples were collected from the Shahewan (SHW), Caoping (CP) and Zhashui (ZS) plutons (Fig. 1c). The SHW pluton is dominated by medium- to coarse-grained biotite-hornblende porphyritic monzogranite and granodiorite with an outcrop area of ~104 km2. Its country rocks are the Neoproterozoic Qinling complex in the north and the Paleozoic Liuling Group in the south (Gong et al., 2009a). It shows sharp intrusive contact and distinct contact aureoles with country rocks. The SHW pluton is prominently characterized by the presence of MMEs and alkali feldspar megacrysts in local areas, their amounts generally decrease from the margin to the center of the pluton (Wang et al., 2011). The MMEs have variable shapes and sizes, mainly being elliptical or rounded, with sizes ranging from several centimeters to decimeters. They also show clear quenched margins with the host granitoids, without obvious foliations or preferred orientations of their component crystals (Fig. 2a). Alkali feldspar megacrysts are usually monocrystal or ovoid, with various sizes ranging from ca. 2 to 5 cm, and exceptionally up to 14 cm (Fig. 2a). Petrographic observations for the SHW granitoids found that some alkali feldspar megacrysts are surrounded by plagioclase (Fig. 2a), which were regarded as rapakivi-textured granitoids (Lu et al., 1999; Wang et al., 2008, 2011). Alkali feldspar megacrysts can be also found in MMEs occasionally, but do not display such texture (Fig. 2a). The SHW granitoids are composed of quartz (20–25%), K-feldspar (15–25%), plagioclase (25–30%), biotite (5–10%) and hornblende (5–8%) with accessory zircon, titanite, apatite and magnetite (Fig. 2b). K-feldspar phenocrysts are euhedral to subhedral, some of which have large corroded cores; hornblende phenocrysts are small and euhedral (Fig. 2b). Biotite occurs not only in matrix but also as inclusion in plagioclase and quartz, some biotite rims have been altered to chlorite (Fig. 2b).

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The CP pluton is located in the west of the SHW pluton about 10 km with an outcrop area of ~184 km2, and composed mainly of porphyritic and medium- to coarse-grained granodiorite and monzogranite with MMEs of various scales and shapes in local areas. Its country rocks are migmatitic gneiss of the Qinling Group in the north and sandy slate in the south (Gong et al., 2009a). The CP porphyritic granitoids are composed of alkali feldspar megacrysts, with sizes of ca. 1 to 3 cm, in a medium- to coarse-grained matrix, and hardly show the rapakivitexture as the SHW granitoids (Hu et al., 2016; this study). The CP granodiorites mainly show massive structures and subhedral medium-grained textures without alkali feldspar megacrysts (Fig. 2c). The CP pluton also contains some MMEs in local areas, which display variable shapes and sizes, mainly being elliptical, rounded or ductile shapes, with sizes ranging from several centimeters to decimeters (Fig. 2c). Most MMEs show clear contact with the host granitoids and without orientation or elongation of mineral grains as those in the SHW pluton (Fig. 2c). The mineral assemblages are quartz (20–30%), K-feldspar (10–20%), plagioclase (30–35%), biotite (5–8%) and hornblende (5–10%) with accessory zircon, titanite, apatite and magnetite (Fig. 2d). Most plagioclase crystals occur as euhedral laths with well-developed polysynthetic twinning, whereas oscillatory zoning is rarely observed (Fig. 2d). Hornblende occurs as large euhedral to subhedral crystals, which are euhedral when contacting with K-feldspar and quartz (Fig. 2d). The hornblende crystals are usually homogeneous and rarely display disequilibrium textures. Biotite varies from pale reddish brown to yellowish brown and greenish brown in color, and some biotite rims were partly altered to chlorite (Fig. 2d). The ZS pluton with an outcrop area of ~264 km2, located in the west of the SHW pluton about 50 km, intruded into the Devonian Liuling Group in the southwest, the Carboniferous Eryuhe Group in the east and the Danfeng Group in the north, showing sharp intrusive contacts with the country rocks (Gong et al., 2009a). It is composed of medium-grained biotite monzogranite and granite with massive structure (Fig. 2e). The granites are mainly composed of quartz (20–30%), Kfeldspar (15–25%), plagioclase (20–35%) and biotite (3–8%) with accessory zircon, titanite, apatite and magnetite (Fig. 2f). In contrast to the SHW and CP plutons, no hornblende was observed in all samples from the ZS pluton. Some plagioclase crystals occur as euhedral laths with well-developed polysynthetic twinning and oscillatory zoning, and the others occur as subhedral ragged flakes (Fig. 2f). The MMEs are rare in the ZS pluton compared to the CP and SHW plutons (Fig. 2e). The ZS granites do not display rapakivi-texture as the SHW granitoids. In summary, there are some differences among the three plutons in the field occurrence and petrographic features. The SHW and CP plutons have similar mineral assemblages and contain relatively abundant MMEs in the marginal facies, whereas the ZS pluton is characterized by the absence of hornblende, alkali feldspar megacryst and MMEs. In addition, the SHW granitoids show ovoid alkali megacrysts with plagioclase rims which are regarded as rapakivi-texture, whereas the CP and ZS granitoids have no such unusual texture. 4. Analytical methods 4.1. Whole-rock major-trace elements and Sr-Nd isotopes Fresh samples were crushed to powders of ~200 meshes in the agate mortar for whole-rock major-trace element and Sr-Nd isotope analyses. Whole-rock major and trace elements were analyzed at the ALS Chemex Company in Guangzhou, China. Major elements were measured using an X-ray fluorescence spectrometer (XRF) with the analytical precision better than ±2–5%. Trace elements were analyzed by using an inductively coupled plasma-mass spectrometry (ICP-MS) with the analytical precision better than ± 5% for most trace elements. Chinese standard GBW07103 (granite GSR-1) was used to analyze and monitor the analytical quality. Whole-rock Sr-Nd isotope analyses were carried out on a Thermo Scientific Neptune MC-ICP-MS at the State Key Laboratory of

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Fig. 2. Field photographs and microphotographs show typical textures for the Shahewan (SHW), Caoping (CP) and Zhashui (ZS) granitoids in the South Qinling zone. (a) Porphyritic monzogranite with MME from the SHW pluton; (b) biotite-hornblende monzogranite of the SHW pluton; (c) coarse to medium-grained granodiorite with MMEs in the CP pluton; (d) medium-grained granodiorite of the CP pluton; (e) medium-grained granite without MMEs in the ZS pluton; (f) medium-grained granite of the ZS pluton. Mineral abbreviations: Qz—quartz; Pl—plagiocalse; Kfs—K-feldspar; Bt—biotite; Hbl—hornblende; Mag—magnetite; Ttn—titanite.

Lithospheric Evolution in the Institute of Geology and Geophysics, Chinese Academy of Sciences (CAS), Beijing. Chemical separation was undertaken by conventional ion-exchange techniques. The detailed procedures have been described in Yang et al. (2010, 2012). During the analyses, the mass fractionation corrections were based on 86 Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219 for Sr and Nd isotopic ratios, respectively. The measured values for the NBS987 Sr standard and the JNdi-1 Nd standard were 143Nd/144Nd = 0.512119 ± 0.000012 (2σ, n = 3), and 87Sr/86Sr = 0.710233 ± 0.000024 (2σ, n = 4) during the period of data acquisition, respectively. Furthermore, USGS reference materials GSP-2 and BCR-2 were also used to analyze for Sr-Nd isotopes, yielding ratios of 0.765107 ± 22 and 0.704999 ± 16 for 87Sr/86Sr, and 0.512635 ± 9 and 0.512951 ± 9 for 143Nd/144Nd, respectively, consistent with the recommended values well within analytical errors (Weis et al., 2006). Single-stage Nd model ages (TDM1) were calculated relative to the depleted mantle with a present-day 143Nd/144Nd ratio of 0.51315 and 147Sm/144Nd ratio of 0.2137 (Depaolo, 1988). Two-stage

Nd model ages (TDM2) were calculated relative to the average continental crust with a 147Sm/144Nd ratio of 0.118 (Jahn and Condie, 1995). 4.2. Zircon U-Pb ages and Lu-Hf isotopes Zircon grains were separated from 3 to 5 kg samples using conventional magnetic and heavy liquid separation techniques, and handpicked under a binocular microscope. Zircon grains were mounted in epoxy resin, and then polished to expose the cores of zircons and coated with carbon. Cathodoluminescence (CL) images were taken for zircon grains to reveal their external morphology and internal structure, which were further used as a guide to the selection of spots for U-Pb and Lu-Hf isotope analyses. In-situ zircon U-Pb isotope analysis was carried out using an Agilent 7500a ICP-MS equipped with a 193 nm ComPex102-ArF laser-ablation system (Coherent Inc., USA) at the School of Resources and Environmental Engineering in Hefei University of Technology, Hefei. Detailed

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operating conditions for the laser ablation system and the ICP-MS instrument and data reduction can be found in Liu et al. (2008, 2010). Off-line selection and integration of background and analyte signals, time-drift correction, and quantitative calibration for U-Th-Pb dating were performed by ICPMSDataCal (Liu et al., 2010). Common Pb contents were carried out using the EXCEL program of ComPbCorr#151 (Andersen, 2002). Age calculations and plotting of concordia diagrams were made using ISOPLOT (version 3.0) (Ludwig, 2003). After the U-Pb isotope analysis, in-situ zircon Lu-Hf isotope analysis was conducted by LA-MC-ICPMS in combination with a GeoLas 193 nm excimer laser ablation system at the State Key Laboratory of Lithospheric Evolution in Institute of Geology and Geophysics, CAS, Beijing. Analyses of zircons were conducted with a beam diameter of 44 μm or 60 μm and a repetition rate of 8 Hz. The detailed analytical procedures and data acquisition were described by Wu et al. (2006). Zircon Mud and GJ-1 were used as the reference standards, with weighted mean 176Hf/177Hf ratios of 0.282503 ± 0.000021 (2σ, n = 11) and 0.282019 ± 0.000024 (2σ, n = 11) during our analyses, respectively, consistent with the recommended 176Hf/177Hf ratios of 0.282507 ± 0.000006 for Mud (Woodhead and Hergt, 2005) and 0.282015 ± 0.000019 for GJ-1 (Elhlou et al., 2006) within analytical errors. A decay constant of 1.867 × 10−11 year−1 for 176Lu (Söderlund et al., 2004) as well as 176Hf/177Hf and 176Lu/177Hf ratios of 0.282772 and 0.0332 for the CHUR (Blichert-Toft and Albarède, 1997) were adopted for calculation of the εHf(t) values. Single-stage Hf model ages (TDM1) were calculated relative to the depleted mantle with the present-day 176Hf/177Hf ratio of 0.28325 and 176Lu/177Hf ratio of 0.0384 (Griffin et al., 2000). Two-stage Hf model ages (TDM2) were calculated relative to the average continental crust assuming a 176Lu/177Hf value of 0.015 (Griffin et al., 2002). 4.3. Mineral chemistry and O isotopes Mineral compositions were determined using an EPMA 1600 (Shimadzu) electron microprobe and a JEOL JXA-8230 electron microprobe at the University of Science and Technology of China (USTC) and the Hefei University of Technology, Hefei, respectively. The quantitative analyses were made under the conditions of 15 kV accelerating voltage, a beam current of 20 nA, and a beam diameter of 1 or 5 μm. The accuracy of the reported values for the analyses is ±1–5% depending on the abundance of the element. Oxygen isotope analysis of selected minerals was conducted by the laser fluorination technique using a Finnigan Delta XP mass spectrometer equipped with a CO2 laser at the CAS Key Laboratory of Crust-Mantle Materials and Environments in USTC, Hefei. Oxygen isotope data were reported by the δ18O notation, which has part per thousand differences (‰) relative to the VSMOW (Vienna standard mean ocean water) standard. Repeat measurements gave analytical errors better than ±0.1‰ (1σ) for δ18O (Zheng et al., 2002). Three reference minerals were used: δ18O = 10.0 ± 0.1‰ for zircon 91500 (Zheng et al., 2004), δ18O = 3.7 ± 0.1‰ for garnet 04BXL07 (Gong et al., 2007), and δ18O = 11.11‰ for quartz GBW04409 (Zheng et al., 1998). 5. Results 5.1. Zircon U-Pb ages Four granitoid samples from the SHW pluton (14SHW03, 14SHW06), the CP pluton (14CP05) and the ZS pluton (13QL104) were selected for the zircon U-Pb dating. Zircon grains extracted from the three SQZ plutons are generally enhedral in shape, prismatic, colorless, and transparent. Most zircon grains show clear oscillatory zoning as shown in CL images (Fig. 3), which are typical of magmatic origin. The zircon grain lengths range from 60 to 300 μm with aspect ratios of 1:1 to 4:1. The zircon U-Pb isotope data are listed in supplementary Table S1 and illustrated in Fig. 3.

35

As shown in Fig. 3, most of the U-Pb isotopic data are concordant within the analytical errors. Twenty-one analyses display Th/U ratios of 0.42–1.15 and yield a weighted mean 206Pb/238U age of 214 ± 3 Ma with MSWD = 0.56 (Fig. 3a; Table S1) for the SHW sample 14SHW03, and the other three analyses are discordant. For the SHW sample 14SHW06, except for one analysis plotting away the concordia curve, the remaining twenty-four analyses show Th/U ratios of 0.5–1.21 and yield a weighted mean 206Pb/238U age of 210 ± 2 Ma with MSWD = 0.83 (Fig. 3b; Table S1). Twenty-three analyses give Th/U ratios of 0.42–1.23 and yield a weighted mean 206Pb/238U age of 216 ± 3 Ma with MSWD = 0.68 (Fig. 3c; Table S1) for the CP sample 14CP05, the remaining one analysis is discordant. A total of twenty-five analyses were performed for the ZS sample 13QL104; they show Th/U ratios of 0.25–1.9 and yield a weighted mean 206Pb/238U age of 208 ± 2 Ma with MSWD = 1.02 (Fig. 3d; Table S1). In summary, the zircon U-Pb dating of the three studied plutons yields the relatively uniform ages of 208 ± 2 to 216 ± 3 Ma. These UPb ages are interpreted as the magmatic crystallization ages for the SHW, CP and ZS granitoids, which are consistent with previously reported results (Li et al., 2015; Wang et al., 2013b, and references therein). 5.2. Whole-rock major and trace elements Major and trace element data are listed in supplementary Table S2 and plotted in Figs. 4 to 7 together with the literature data from Gong et al. (2009a). All the samples fall in the sub-alkali field on the total alkali-silica (TAS) diagram (Fig. 4a), with rock types of quartz monzonite and granodiorite for the SHW and CP plutons but granite for the ZS pluton. On the normative CIPW albite-anorthite-orthoclase diagram (Fig. 4b), the SHW and CP plutons are categorized into granodiorite, and the ZS pluton is classified as granite with a few straddling the granodiorite. The SHW and CP samples are metaluminous with A/CNK values of 0.84 to 0.93 (Fig. 5a), and have moderate SiO2 contents (62.88–69.04 wt.%) and high K2O contents (3.55–4.40 wt.%). In contrast, the ZS samples are metaluminous to weakly peraluminous with A/CNK values of 0.99 to 1.04 (Fig. 5a), and have relatively high SiO2 contents (69.32–75.94 wt.%) and K2O contents (3.7–4.90 wt.%). All of these samples fall in the domain of high-K calcalkaline series (Fig. 5b). On the Harker diagrams (Fig. 6), the SHW and CP samples display higher contents of TiO2, P2O5, MgO, CaO and FeOT than those of the ZS samples, and the ZS samples plot along an extension of the SHW and CP samples trend. Except for K2O and Na2O, all the samples show clear negative correlations between major elements and SiO2 contents. The SHW and CP samples exhibit a small range in K2O/Na2O ratios from 0.9 to 1.16 (close to 1.0), whereas the ZS samples show a large range in K2O/Na2O ratios from 0.87 to 1.33 (Table S2). On the chondrite-normalized REE diagram (Fig. 7a, c and e), the SHW and CP granitoids exhibit similar REE distribution patterns with enrichment in LREE relative to HREE and (La/Yb)N = 11.6–21.5, relative flat HREE patterns with (Dy/Yb)N = 1.25–1.52, high total REE contents of 113.7–165.9 ppm, and slightly to negligibly negative Eu anomalies with δEu = 0.79–0.89 (Table S2). In contrast, the ZS granites exhibit more fractionated REE patterns with (La/Yb)N ratios of 14.4 to 32.5, low total REE contents of 69.5 to 155.6 ppm and moderate negative Eu anomalies with δEu = 0.63–0.81 (Table S2). In addition, the ZS granites show concave-up REE distribution patterns with (Dy/Yb)N = 0.4–1.34, especially for sample 13QL118. On the primitive mantle-normalized diagram (Fig. 7b, d and f), the SHW and CP granitoids show similar trace element distribution patterns, with enrichment in large ion lithophile elements (LILE) such as Rb, Sr, K and Pb, but depletion in high field strength elements (HFSE) such as Nb, Ta and Ti. In comparison to the SHW and CP granitoids, the ZS granites exhibit relatively higher Rb but lower Ba, Sr and Pb as well as more depletion in Nb, Ta, P and Ti (especially for sample 13QL118). Nevertheless, all samples from the SHW, CP and ZS plutons exhibit arc-like trace element distribution patterns, typical of the

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Fig. 3. Concordia diagrams of zircon U-Pb isotope data analyzed by the LA-ICPMS for the SHW, CP and ZS granitoids in the South Qinling zone. Inserted figures are representative zircon CL images. The circle denotes the domain of in-situ zircon U-Pb isotope analysis with beam diameters of 32 μm. Apparent 238U/206Pb ages are also shown.

continental crust (e.g., Rudnick, 1995; Rudnick and Gao, 2003). However, it is noteworthy that there are significant differences in some majorand trace-element compositions between these granitoids. Based on the calibration of Watson and Harrison (1983), whole-rock Zr saturation temperatures (TZr) are calculated by using available data for major elements and Zr contents in these granitoids (Table S2). The results show TZr values of 774 to 801 °C (average of 783 °C) for the SHW pluton, 759 to 781 °C (average of 767 °C) for the CP pluton and 722 to 795 °C (average of 770 °C) for the ZS pluton. As illustrated by the zircon CL imaging and U-Pb dating (Fig. 3) as well as the previous studies (Gong et al., 2009a; Hu et al., 2016; Wang et al., 2011), there are no or few relict zircons as cores or individual grains in these granitoids. As suggested by Miller et al. (2003), for inheritance-rich

(N10% grains with premagmatic cores) granitoids, TZr should place an upper limit on magma crystallization temperature; for inheritancepoor (b10% grains with premagmatic cores) granitoids, TZr would approach to the temperature of melt segregation and thus provide a minimum initial crystallization temperature at the magma source. Therefore, the calculated TZr values for the SHW, CP and ZS granitoids are approximate to or slightly lower than their magma crystallization temperatures. 5.3. Whole-rock Sr-Nd isotopes Whole-rock Sr-Nd isotope data for the SHW, CP and ZS granitoids are listed in Table 1 and plotted in Fig. 8 together with the literature

Fig. 4. Total alkali vs. SiO2 (TAS) (a) and An-Ab-Or (b) diagrams for the SHW, CP and ZS granitoids in the South Qinling zone. Data for black solid triangles, purple solid squares and green solid circles represent the SHW, CP and ZS granitoids, respectively, from this study. Black open triangles, purple open squares and green open circles represent the literature data for the SHW, CP and ZS granitoids, respectively, from Gong et al. (2009a). Abbreviations: GD—gabbroic diorite, MD—monzodiorite, Mz—monzonite, QM—quartz monzonite.

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Fig. 5. Diagrams of A/NK vs. A/CNK (a) and K2O vs. SiO2 (b) (the classification is after Rollinson, 1993) for the SHW, CP and ZS granitoids in the South Qinling zone. Data sources and symbols are the same as those in Fig. 4.

data from Zhang et al. (2006b), Jiang et al. (2010) and Wang et al. (2011). The SHW granitoids show relatively homogeneous (87Sr/86Sr)i ratios of 0.7051 to 0.7059 and εNd(t) values of − 3.5 to − 0.5 with two-stage Nd model ages (TDM2) of 1.05–1.27 Ga. The CP granitoids have (87Sr/86Sr)i ratios of 0.7044 to 0.7057 and εNd(t) values of − 3.0 to − 0.6 with TDM2 of 1.06–1.25 Ga, similar to those of the SHW granitoids. The ZS granites exhibit similar (87Sr/86Sr)i ratios of 0.7040 to 0.7057, but slightly enriched Nd isotope compositions with εNd(t) values of −4.6 to −3.6, and TDM2 of 1.30–1.38 Ga (Table 1). In addition, the (87Sr/86Sr)i ratios and εNd(t) values for these granitoids show no obvious positive or negative correlations with SiO2 contents for an individual pluton or as a whole (Fig. 9). 5.4. Zircon Lu-Hf isotopes Three of the four samples that were previously performed for zircon U-Pb dating were chosen for in-situ Lu-Hf isotope analysis on or adjacent to the same domains. The zircon Lu-Hf isotope data are listed in

supplementary Table S3 and illustrated in Fig. 10. Initial 176Hf/177Hf ratios are denoted as εHf(t) values, which were calculated at the crystallization ages of each pluton. All the zircons have 176Lu/177Hf ratios much lower than 0.002 (Table S3), indicating only minor accumulation of radiogenic Hf after the formation of granitoids. Therefore, it can be inferred that the present 176Hf/177Hf ratios can nearly represent the initial 176 Hf/177Hf ratios when the granitoids were emplaced (Wu et al., 2007). Twenty-three analyses were performed on the SHW sample 14SHW06, yielding εHf(t) values of − 1.3 to 3.2 (Fig. 10a) and twostage Hf model ages of 1039 to 1323 Ma (Fig. 10b). Twenty-four analyses for the CP sample 14CP05 give zircon εHf(t) values of − 0.1 to 3.1 (Fig. 10c), corresponding to two-stage Hf model ages of 1048 to 1256 Ma (Fig. 10d). Twenty-three analyses for the ZS sample 13QL104 yield εHf(t) values of − 0.7 to 2.3 (Fig. 10e), corresponding to twostage Hf model ages of 1098 to 1394 Ma (Fig. 10f). As a whole, the zircon Hf isotope compositions are not only relatively homogeneous but also broadly similar to each other. 5.5. Mineral chemistry

Fig. 6. Hacker plots of major elements for the SHW, CP and ZS granitoids in the South Qinling zone. Data sources and symbols are the same as those in Fig. 4.

Representative microprobe analyses of major mineral phase from the SHW, CP and ZS granitoids are presented in supplementary Table S4 and plotted in Fig. 11. Plagioclase crystals of these samples mostly show normal compositional zoning. The compositions of plagioclase are mainly oligoclase with less andesine for the SHW and CP granitoids; their An contents are from 17 to 38 (Fig. 11a; Table S4). Plagioclases of the ZS granites show slightly low calcium contents (An = 13–28), falling in the domain of oligoclase (Fig. 11a; Table S4). Hornblendes from the SHW and CP granitoids show relatively uniform chemical compositions (Table S4). These hornblendes show low (Na + K)A values (b0.5), high CaB values (N 1.5) and Mg/(Mg + Fe2+) ratios ranging from 0.63 to 0.88. Based on the nomenclature scheme proposed by Leake et al. (1997), most of them are classified into magnesiohornblende; only a few hornblende rims probably suffered later alteration and fall into the actinolite field (Fig. 11b). Biotites from the SHW, CP and ZS granitoids have relatively homogeneous chemical compositions (Table S4). The SHW and CP biotites are significantly enriched in Mg but depleted in Fe, with relatively low Fe/(Fe + Mg) ratios of 0.36–0.44, whereas the ZS biotites show relatively high Fe/(Fe + Mg) ratios of 0.45–0.48 (Table S4); all of them fall into the field of eastonite (Fig. 11c; Foster 1960). Biotites of the SHW, CP and ZS granitoids are characterized by enrichment in Al2O3 (13.21–14.15) and MgO (11.84–15.38) but depletion in FeOT (15.00–19.83), falling in the C-type region (Fig. 11d), which is typical for orogenic calc-alkaline granitoids (Abdel-Rahman 1994). Furthermore, biotites from the three plutons also have some differences in major elements (e.g., FeOT/MgO, A/CNK and Mg#; Table S4), which are consistent with whole-rock compositions (Table S2).

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Fig. 7. Chondrite-normalized REE patterns and primitive mantle-normalized trace element distribution patterns for granitoids in the South Qinling zone. (a) and (b) for the SHW granitoids; (c) and (d) for the CP granitoids; (e) and (f) for the ZS granites. The chondrite and primitive mantle data are from McDonough and Sun (1995). Red lines denote the lower crust compositions of Rudnick and Gao (2003), which are shown for comparison.

5.6. Mineral O isotopes The O isotope compositions of mineral separates for the granitoids from the SHW, CP and ZS plutons are listed in Table 2 and plotted in Fig. 12. The SHW and CP granitoids show limited variations in mineral O isotope composition, with δ18O values of 4.71 to 5.72‰ for zircon, 8.23 to 9.31‰ for quartz and 6.55 to 7.49‰ for plagioclase. The ZS

granites exhibit relatively low δ18O values of 4.60 to 4.83‰ for zircon, 7.93 to 8.62‰ for quartz and 4.96 to 6.30‰ for plagioclase. All the samples display nearly equilibrium O isotope fractionations between quartz and zircon (Fig. 12a), yielding apparent O isotope temperatures of 606–805 °C for the SHW and CP granitoids and 612–723 °C for the ZS granites (Table 2). However, plagioclase is in significant O isotope disequilibrium when paired with quartz for some samples

Table 1 Whole-rock Sr-Nd isotope compositions for Triassic granitoids in the South Qinling zone. Pluton

Shahewan

Caoping

Zhashui

Sample No.

14SHW01

14SHW03

14SHW06

14CP02

14CP03

14CP05

13QL95

13QL104

13QL106

Rb (ppm) Sr (ppm) 87 Rb/86Sr 87 Sr/86Sr 2σ (87Sr/86Sr)i Sm (ppm) Nd (ppm) 147 Sm/144Nd 143 Nd/144Nd 2σ εNd (t) TDM1 (Ma) TDM2 (Ma) fSm/Nd

99.8 841 0.3433 0.706369 0.000020 0.7053 5.76 28.6 0.1218 0.512387 0.000013 −2.8 1263 1219 −0.38

111 781 0.4112 0.706550 0.000019 0.7053 4.42 22.9 0.1167 0.512381 0.000007 −2.8 1207 1222 −0.41

128.5 772 0.4816 0.707021 0.000026 0.7055 3.77 18.5 0.1232 0.512355 0.000008 −3.5 1337 1272 −0.37

84.9 681 0.3606 0.706133 0.000018 0.7050 4.05 19.1 0.1282 0.512411 0.000008 −2.6 1316 1190 −0.35

100 680 0.4254 0.706359 0.000020 0.7051 4.73 24.2 0.1182 0.512451 0.000007 −1.5 1114 1113 −0.40

92.2 707 0.3773 0.706249 0.000025 0.7051 5.47 25.2 0.1312 0.512454 0.000009 −1.8 1286 1126 −0.33

145.5 472 0.8919 0.707353 0.000015 0.7046 3.46 20.2 0.1035 0.512322 0.000007 −3.6 1146 1300 −0.47

156.5 470 0.9634 0.707794 0.000019 0.7048 4.84 24.8 0.1180 0.512322 0.000008 −4.0 1317 1317 −0.40

184.5 418 1.2771 0.708451 0.000027 0.7045 2.79 16 0.1054 0.512323 0.000010 −3.6 1164 1300 −0.46

Notes: 87Rb/86Sr and 147Sm/144Nd ratios were calculated using Rb, Sr, Sm and Nd contents, measured by ICP-MS. Single-stage Nd model ages (TDM1) were calculated relative to the depleted mantle, and two-stage Nd model ages (TDM2) were calculated relative to the average continental crust at the time of magma crystallization (t = 215 Ma).

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(Fig. 12b), especially for the ZS granitoids, indicating that they suffered different degrees of retrograde resetting by post-magmatic alteration. 6. Discussion 6.1. Estimates of temperature, pressure and oxygen fugacity for magma emplacement

Fig. 8. Whole-rock Sr-Nd isotope diagram for the SHW, CP and ZS granitoids in the South Qinling zone. Symbols are the same as those in Fig. 4. Data sources: the SHW, CP and ZS granitoids (this study; Jiang et al., 2010; Wang et al., 2011; Zhang et al., 2006b); the other Triassic granitoids from the South Qinling zone (SQZ; Jiang et al., 2010; Liu et al., 2011; Qin et al., 2010, 2013; Zhang et al., 2006b); the Triassic granitoids from the North Qinling zone (NQZ; Jiang et al., 2010; Wang et al., 2011; Zhang et al., 2006b); the Triassic granitoids from the southern margin of the North China Block (S-NCB; Ding et al., 2011); the Triassic granitoids from the West Qinling zone (WQZ; Zhang et al., 2007); the Triassic granitoids from the Songpan-Garze fold zone (Zhang et al., 2006a); postcollisional granitoids from the Dabie-Sulu orogenic belt (Yang et al., 2005; Wang et al., 2007a); the Neoproterozoic Bikou Group and mafic-ultramafic intrusions in the northern margin of the South China Block (N-SCB; Xia et al., 2007; Zhao and Zhou, 2009); the Neoproterozoic Yanglinghe Group and mafic-ultramafic intrusions in the SQZ (Wang et al., 2013a; Xia et al., 2008; Zhu et al., 2014); the Cenozoic subducted oceanic crust-derived adakites (Sajona et al., 2000).

The total Al contents of hornblende are linearly correlated with the pressure of magma crystallization, and thus can provide a basis for estimation of crystallization pressures and emplacement depths for granitic batholiths (Anderson and Smith, 1995; Schmidt, 1992). Holland and Blundy (1994) proposed the amphibole-plagioclase thermobarometry to estimate P-T conditions for silica-saturated and silica-undersaturated magmas. Furthermore, Anderson and Smith (1995) proposed a revised expression for the Al-in-hornblende barometer by incorporating the effect of temperature, which would be more useful under the conditions of near-solidus with T b 800 °C and Fetot/(Fetot + Mg) b 0.65. As reviewed by Anderson et al. (2008), the amphibole-plagioclase thermometer of Holland and Blundy (1994) in combination with the Al-in-hornblende barometer of Anderson and Smith (1995) can be used to estimate the emplacement P-T conditions of granitic rocks. In order to obtain precise emplacement P-T data, it is essential to select the outermost unaltered rims of hornblende and plagioclase for thermobarometry, or touching pairs of crystals (Blundy and Cashman, 2008). The calculated P-T results are 650–766 °C and 1.4–2.3 kbar for the SHW pluton, 628–724 °C and 0.7–1.7 kbar for the CP pluton (Table 3). Therefore, it is estimated that the emplacement depths are ca. 4.7–7.5 km and 2.4–5.7 km (1 kbar ≈ 3.3 km within the crust; Table 3) for the SHW and CP pluton, respectively. The P-T conditions of magma crystallization for the ZS pluton cannot be obtained due to its lack of hornblende. Oxygen fugacity (f O2) is the principal factor controlling the Fe/Mg compositions of mafic minerals (such as biotite and hornblende), which are independent of whole-rock Fe/Mg compositions (e.g., Anderson and Smith, 1995; Anderson et al., 2008; Wones, 1981). With increasing fO2, biotite and hornblende would become more Mg-rich with markedly increasing Fe3+/(Fe3+ + Fe2+) and decreasing Fe/(Fe + Mg) ratios at the expense of components released by K-feldspar and magnetite (Wones, 1981; Anderson et al., 2008). Based on the normalized results of hornblende and biotite, the hornblendes from the SHW and CP granitoids have similar Fe/(Fe + Mg) ratios of 0.23–0.4 (Table S4), suggesting that they crystallized at high fO2 environment (Anderson and Smith, 1995; Wones, 1981). As such, the biotites are characterized by relatively low Fe/(Fe + Mg) ratios of 0.36–0.44 at 2 to 3 log units above the QFM oxygen buffer for the SHW and CP granitoids, but slightly high Fe/(Fe + Mg) ratios of 0.45–0.48 at 1.5 to 2 log units above the QFM oxygen buffer for the ZS granites (Fig. 11c; Table S4; Anderson et al., 2008). This indicates that the SHW and CP plutons crystallized under much more oxidation conditions than the ZS pluton (Anderson et al., 2008; Wones, 1981). 6.2. Origin of low δ18O granitoids

Fig. 9. Relationships between whole-rock Sr-Nd isotope compositions and SiO2 contents for the SHW, CP and ZS granitoids in the South Qinling zone. (a) (87Sr/86Sr)i vs. SiO2; (b) εNd(t) vs. SiO2. Symbols are the same as those in Fig. 4. Data are from this study, Jiang et al. (2010) and Wang et al. (2011).

As shown in Table 2, the SHW and CP granitoids have zircon δ18O values of 4.71‰ to 5.72‰ (Fig. 12a), which are similar to normal mantle zircon values of 5.3 ± 0.3‰ (Valley et al., 1998). In contrast, the ZS granites have relatively low zircon δ18O values of 4.60 to 4.83‰ (Fig. 12a). On the other hand, some samples experienced varying extents of postmagmatic alteration as indicated by O isotope disequilibria between quartz and plagioclase (Fig. 12b). This is consistent with petrographic observations that some plagioclase and biotite crystals have altered rims (Fig. 2b, d and f). Thus, some minerals would not preserve their magmatic O isotope compositions (Wei et al., 2002; Zhao et al., 2007). In this regard, it is important to evaluate these low zircon δ18O values which may crystallize from low δ18O magmas or result from postmagmatic hydrothermal alteration by low δ18O fluids (e.g., Monani and Valley, 2001; Wei et al., 2002; Zheng et al., 2004).

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Fig. 10. Zircon εHf(t) values and two-stage Hf model ages (TDM2) for the Triassic SHW, CP and ZS granitoids in the South Qinling zone.

Fig. 11. Mineral compositions for the SHW, CP and ZS granitoids in the South Qinling zone. (a) Plagioclase compositions; (b) classification diagram of hornblendes are after Leake et al. (1997); (c) and (d) classification diagrams of biotites are after Foster (1960) and Abdel-Rahman (1994), respectively. Symbols are the same as those in Fig. 4.

Y.-H. Lu et al. / Lithos 284–285 (2017) 30–49 Table 2 Mineral and whole-rock O isotope compositions for Triassic granitoids in the South Qinling zone. Sample No.

δ18O (‰)

Δ18OQz-Zr (‰)

TQz-Zr (°C)c

TZr (°C)

Qz

Pl

Zr

WRa

WRb

Shahewan 14SHW01 14SHW02 14SHW03 14SHW04 14SHW05

9.11 8.90 8.50 8.23 8.57

7.00 6.67 6.87 6.55 6.86

5.12 5.44 5.72 5.06 4.94

6.61 6.93 7.27 6.60 6.51

7.42 7.74 8.03 7.36 7.18

3.99 3.47 2.78 3.17 3.63

606 681 805 730 656

781 779 778 779 794

Caoping 14CP01 14CP02 14CP03 14CP04 14CP05

8.68 9.14 9.21 8.83 9.31

6.88 7.62 7.23 7.17 7.49

4.71 5.19 5.55 5.33 5.56

6.36 6.85 6.89 6.75 6.93

7.09 7.54 7.91 7.70 7.93

3.97 3.95 3.67 3.50 3.75

609 611 651 676 638

761 765 764 762 762

Zhashui 13QL95 13QL102 13QL104 13QL105 13QL106 13QL118

7.93 8.32 8.51 8.59 8.31 8.62

5.28 5.49 5.23 5.44 4.96 6.30

4.72 4.60 4.74 4.65 4.83

6.48 6.36 6.54 6.39 6.67

7.04 6.89 6.98 6.97 7.15

3.21 3.72 3.77 3.94 3.48

723 643 636 612 679

774 782 795 774 776 722

Abbreviations: Qz-quartz, Pl-plagioclase, Zr-zircon, WR-whole-rock. a Respresents the calculated whole-rock δ18O values based on Valley et al. (1994). b Calculated whole-rock δ18O values based on measured zircon δ18O values, isotope fractionation factors between zircon and magmatic rocks (Zhao and Zheng, 2003), and wholerock Zr saturation temperatures (TZr, based on Watson and Harrison, 1983). c Denotes the O isotope temperatures for Qz-Zr pairs that are theoretically calibrated following the calibration of Zheng (1993).

Zircon has extremely high stability and relatively slow rate of oxygen diffusion (Watson and Cherniak, 1997; Zheng and Fu, 1998), and thus can preserve its primary O isotope compositions despite later geological processes such as subsolidus high-T water-rock interaction and dry granulite-facies metamorphism (e.g., Valley, 2003; Zheng et al., 2004). In the present case, for the zircon grains that show wellpreserved magmatic zoning on CL images and are lacking in cracks or fractures, the vast majority of their analyses give concordant U-Pb ages (Fig. 3) and relatively uniform zircon δ18O values (Table 2). All these characteristics argue against the possibility that the zircons from the SHW, CP and ZS granitoids are metamict and that the slightly low δ18O values could result from recrystallization or diffusion at subsolidus temperatures (Cherniak and Watson, 2003; Valley, 2003; Wei et al., 2002). Instead, the low zircon δ18O values would represent the primary magmatic O isotope compositions rather than the secondary O isotope signature due to the hydrothermal alteration in the post-magmatic stage. Therefore, the host granitoids were produced by partial melting of pre-existing 18O–depleted crustal rocks.

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Table 3 Estimates of P-T conditions for crystallization of Triassic granitoids in the South Qinling zone. Pluton

Hornblende

Plagioclase

T (°C)

P (kbar)

D (km)

Shahewan

14SHW01-Hbl1-1 14SHW01-Hbl1-4 14SHW01-Hbl2-1 14SHW01-Hbl2-6 14SHW06-Hbl1-1 14SHW06-Hbl1-4 14CP02-Hbl2-5 14CP03-Hbl1-1 14CP05-Hbl1-1 14CP05-Hbl1-4 14CP05-Hbl2-5 14CP05-Hbl3-1 14CP05-Hbl3-5

14SHW01-Pl1-1 14SHW01-Pl1-4 14SHW01-Pl2-1 14SHW01-Pl2-4 14SHW06-Pl2-1 14SHW06-Pl2-4 14CP02-Pl2-5 14CP03-Pl1-1 14CP05-Pl3-1 14CP05-Pl3-4 14CP05-Pl2-6 14CP05-Pl1-1 14CP05-Pl1-5

765 766 678 650 705 710 715 628 699 688 711 673 724

1.6 1.4 1.8 2.3 1.9 1.5 0.8 0.7 1.7 1.7 1.6 0.8 1.4

5.4 4.7 6.1 7.5 6.2 5.1 2.7 2.4 5.7 5.5 5.2 2.6 4.6

Caoping

On the other hand, in terms of the original definition by Chappell and White (1974), S-type granites are produced by partial melting of metasedimentary rocks which have experienced low-T supracrustal chemical weathering, whereas I-type granites are derived from partial melting of metaigneous rocks which did not experience the sedimentary process. As such, granitic melts originated from the low δ18O metaigneous rocks can have relatively low δ18O values (b7.5‰ for zircon, and b8.5‰ for whole-rock) compared to those originating from the high δ18O metasedimentary rocks (Hoefs, 2009). Further studies indicate that the majority of I-type granites have whole-rock δ18O values of 6.5 to 8.5‰ whereas the majority of S-type granites have δ18O values of 9.0 to 12.0‰ (Lu et al., 2016, and references therein). Although I- and S-type granites have notably different O isotope compositions, their whole-rock δ18O values may change a little bit due to post-magmatic hydrothermal alteration. In fact, the post-magmatic alteration is evident for some granitoids in this study as discussed above, and thus the measured whole-rock δ18O values cannot completely reflect the O isotope compositions of their primary magmas. In this regard, two approaches are used to estimate the primary whole-rock δ18O values based on measured zircon δ18O values and other parameters (Valley et al., 1994; Zhao and Zheng, 2003). The calculated results give whole-rock δ18O values of 6.36 to 7.27‰ for the SHW and CP granitoids, and 6.36 to 6.67‰ for the ZS granites based on the approach of Valley et al. (1994) but 7.18 to 8.03‰ for the former and 6.89 to 7.15‰ for the later based on the approach of Zhao and Zheng (2003) (Table 2). In either case, the estimated whole-rock δ18O values are slightly lower than those for common I-type granites (e.g., Harris et al., 1997; Kemp et al., 2007; O'Neil and Chappell, 1977; Zhao et al., 2007). Therefore, such low zircon and whole-rock δ18O values suggest that the SHW, CP and ZS granitoids were derived from partial melting of pre-existing 18O−depleted metaigneous rocks.

Fig. 12. Mineral-pair O isotope plots for the SHW, CP and ZS granitoids in the South Qinling zone. Isothermal lines are calculated using the O isotope fractionation equations of Zheng (1993). Symbols are the same as those in Fig. 4.

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6.3. The source nature of granitoids The SHW, CP and ZS granitoids in the Dongjiangkou suite exhibit low A/CNK ratios of 0.84–1.03, relatively high Na2O contents (normally N3.2 wt.%), and low zircon and whole-rock δ18O values (b 6‰ for zircon, and mostly b 8‰ for whole-rock). In addition, the SHW and CP plutons contain hornblende. All these features indicate that they are derived from partial melting of the metaigneous rocks and thus belong to Itype granites (e.g., Chappell and White, 2001; Harris et al., 1997; Kemp et al., 2007; O'Neil and Chappell, 1977; Zhao et al., 2007). The same conclusion was also reached by previous studies (e.g., Zhang et al., 1994a; Gong et al., 2009a). However, there have been different propositions for the source nature of the SHW, CP and ZS plutons and their adjacent plutons in the Dongjiangkou suite. Some authors proposed that these Triassic granitoids were produced by mixing of magmas derived from partial melting of the mafic lower crust of the SQZ or the subducted Yangtze slab and metasomatized lithospheric or depleted mantle (e.g., Gong et al., 2009a, 2009b; Liu et al., 2011; Wang et al., 2011; Zhang et al., 2008). Others hypothesized that some granitoids originated from interaction between the subducted Yangtze continental crust and the overlying mantle wedge (Qin et al., 2010, 2013). In addition, Jiang et al. (2010) suggested that the Caoping and Dongjiangkou granitoids were derived from partial melting of the subducted sediments due to dehydration of the underlying igneous oceanic crust, with subsequent interaction with the overlying mantle wedge. All the SHW, CP and ZS granitoids are characterized by enrichment in LREE, LILE and Pb but depletion in HFSE such as Nb, Ta and Ti (Fig. 7; Table S2), typical of the continental crust (e.g., Rudnick, 1995; Rudnick and Gao, 2003). These granitoids show consistently low whole-rock (87Sr/86Sr)i ratios of 0.7040 to 0.7059 and slightly negative εNd(t) values of −4.6 to −0.5 (Fig. 8 and Table 1), with two-stage Nd model ages (TDM2) of 1.05 to 1.38 Ga. Such isotope compositions are consistent with those for the Neoproterozoic Yaolinghe Group metabasalts and contemporaneous mafic-ultramafic intrusions in the SQZ (e.g., Wang et al., 2013a; Xia et al., 2008; Zhu et al., 2014), but different from those for the uncontaminated metavolcanic rocks of the Neoproterozoic Bikou Group and mafic-ultramafic intrusions in the NSCB (e.g., Xia et al., 2007; Zhao and Zhou, 2009) as well as the Cenozoic subducted oceanic crust-derived adakites (Sajona et al., 2000). Furthermore, the granitoids from the Dongjiangkou suite also exhibit obviously different Sr-Nd isotope compositions from contemporaneous granitoids in the western Qinling zone (Zhang et al., 2007), the Songpan-Garze orogen (Zhang et al., 2006a), the S-NCB (Ding et al., 2011; Zhang et al., 2006b) and the Dabie-Sulu orogenic belt (Wang et al., 2007a; Yang et al., 2005). In addition, their Sr-Nd isotope compositions are less enriched than those Triassic S-type granites at Guangtoushan and Huayang in the southern margin of the SQZ (Lu et al., 2016). On the other hand, zircon Hf isotopes provide a further constraint on the source nature of granitoids from the SHW, CP and ZS plutons. Although the crust-mantle differentiation always produces the juvenile crust with positive εHf(t) values, the juvenile crust is aged with time and becomes the ancient crust with negative εHf(t) values. Thus, zircon εHf(t) values can be used as petrogenetic discriminators between ancient and juvenile crustal materials (e.g., Zheng et al., 2006, 2007). As depicted in Fig. 10, zircons from the three plutons show similarly narrow variation in εHf(t) values from − 1.3 to 3.2, with two-stage Hf model ages (TDM2) of 1039 to 1394 Ma (Table S3). Generally, wholerock Nd and zircon Hf model ages can approximately reflect the average residence time of the juvenile crust since the crustal-mantle differentiation (e.g., Goldstein et al., 1997; Zheng et al., 2006, 2007) and thus be used as indicators of provenance ages. It is observed that the wholerock two-stage Nd model ages of 1.05–1.38 Ga are consistent with the zircon two-stage Hf model ages of 1.04–1.39 Ga (Tables 1 and S3) for these granitoids in the SQZ. Thus, both Nd and Hf model ages provide a reliable constraint on the crustal residence time of source rocks,

suggesting that the SHW, CP and ZS granitoids would be derived from reworking of the crustal rocks with the Mesoproterozoic growth ages. Such ages of crustal growth are common in the northern edge of the South China Block (Zheng et al., 2006, 2009). In collisional orogens, both the subducted and overlying continental crust can serve as magma source of granitoids (e.g., Jiang et al., 2012; Zhang et al., 2010). In this regard, the Neoproterozoic Bikou metavolcanics and mafic-ultramafic intrusions in the N-SCB may serve as components of the subducted crust, whereas the Neoproterozoic Yaolinghe metabasalts and mafic-ultramafic intrusions in the SQZ may serve as components of the overlying crust; both of them can be approximately regarded as source rocks in terms of isotopic compositions for the Triassic granitoids in the SQZ. As depicted in Fig. 13, there are large variations in both the whole-rock Nd isotope compositions of the Neoproterozoic Bikou metavolcanics (Xia et al., 2007) and the zircon Hf isotope compositions of the Neoproterozoic mafic-ultramafic intrusions in the N-SCB (Zhao and Zhou, 2009), with young to old Nd-Hf model ages. In contrast, there are relatively limited variations in the wholerock Nd isotope compositions of the Neoproterozoic Yaolinghe metabasalts (Ling et al., 2002; Xia et al., 2008) and the zircon Hf isotope compositions of the Neoproterozoic mafic-ultramafic intrusions in the SQZ (Wang et al., 2013a). Furthermore, the whole-rock εNd(t) and zircon εHf(t) values for the Triassic granitoids in the SQZ fall within the ranges of average continental crust evolution, which are bracketed by those of the Neoproterozoic Yaolinghe metabasalts and the Neoproterozoic maficultramafic intrusions in the SQZ (Fig. 13), respectively. Thus, these data, together with the O isotopic data, suggest that the magma source of the Triassic granitoids would be the mafic lower continental crust, which has similar isotopic compositions to the Neoproterozoic Yaolinghe metabasalts and mafic-ultramafic intrusions in the SQZ that was produced by rift magmatism in response to the breakup of the Rodinia supercontinent in the Middle Neoproterozoic (e.g., Zheng et al., 2007). Therefore, the Triassic granitoids would originate from the overlying crust rather than the subducted continental crust. Although they also have the original source from the subducted Yangtze continental crust, they contain less amounts of the ancient terrigenous sediment than the latter in view of their Nd-Hf isotope compositions. 7. Constraints on the petrogenesis of granitoids As discussed above, all the geochemical features of the SHW, CP and ZS granitoids from the Dongjiangkou suite indicate that they are derived from partial melting of the metaigneous rocks. On the other hand, there are significant variations in both major and trace element compositions among the three plutons (Figs. 4 to 7; Table S2). In general, the compositional variations of granitoids mainly depend on some factors such as the compositions of source rocks, partial melting conditions (including temperature, pressure, water fugacity and so on), and magma evolution processes such as magma mixing, fractional crystallization and assimilation of country rocks (e.g., Clemens and Stevens, 2012; Collins, 1996; Depaolo, 1981; Gao et al., 2014; Kemp et al., 2007; Lu et al., 2016; Zhao et al., 2015). Combined with previously published data, it is clear that the SHW, CP and ZS granitoids display limited variations in whole-rock Sr-Nd isotope compositions, zircon δ18O values and εHf(t) values within each pluton (Figs. 8 to 10 and 12; Tables 1, 2 and S3). Due to the scarce of contemporary mafic igneous rocks in the SQZ, it excludes the possibility that they were formed by extensive fractional crystallization from basaltic magmas. In addition, their SiO2 contents are not correlated with (87Sr/86Sr)i ratios and εNd(t) values (Fig. 9). Xenocryst zircons and wall-rock xenoliths are also absent in these granitoids (this study; Gong et al., 2009a; Wang et al., 2011), which are the diagnostic features of granitic systems that underwent the crustal assimilation (e.g., DíazAlvarado et al., 2011). These results suggest that the crustal assimilation did not play a considerable role in their petrogenesis. Therefore, it is unlikely that assimilation and fractional crystallization (AFC) processes of

Y.-H. Lu et al. / Lithos 284–285 (2017) 30–49

43

Fig. 13. Plots of zircon U-Pb ages vs. whole-rock εNd(t) values (a) and zircon εHf(t) values (b) for the SHW, CP and ZS granitoids in the South Qinling zone. Data sources and symbols are the same as those in Fig. 9. Also shown for comparison are whole-rock Nd isotope compositions of the Neoproterozoic Yaolinghe Group (Ling et al., 2002; Xia et al., 2008) and Bikou Group (Xia et al., 2007) metavolcanic rocks, and zircon Hf isotope compositions of the Neoproterozoic mafic-ultramafic intrusions in the South Qinling zone (SQZ, Wang et al., 2013a) and the southern margin of the North China Block (N-SCB, Zhao and Zhou, 2009).

the mantle-derived basaltic magma could be responsible for the petrogenesis of the SHW, CP and ZS granitoids in the SQZ. On the other hand, in terms of high Mg#, Cr and Ni contents for some samples as well as the occurrence of MMEs in local areas, it is suggested that mantle-derived magmas play a substantial role in producing the Triassic granitoids in the SQZ (e.g., Gong et al., 2009a, 2009b; Hu et al., 2016; Qin et al., 2010; Wang et al., 2008, 2011; Zhang et al., 2008). Recent studies do indicate that the injection of hot mafic magmas into the cold felsic host rock would form quenched magma globules, in which the loss of H2O from mafic to felsic domains drives an increase of melt fraction in the outer zone through local lowering of the solidus temperature, and provides positive feedback in the form of chemical quenching of the enclave core through increasing its solidus temperature (Pistone et al., 2016). Furthermore, as suggested by Clemens et al. (2016), the occurrences of MMEs would be a sign of mingling and local hybridisation, and some MMEs show coarser margins, which may be related to the diffusion and migration of H2O from the thermally quenched mafic magma globules toward the felsic host magma. Moreover, large-scale mafic-felsic magma mixing would be limited by the formation of biotite-rich rinds on MMEs, which might be formed by late-stage chemical reactions between the solidified enclaves and melts or fluids after progressive crystallization and cooling of the host magma body (Farner et al., 2014). The MMEs in the SHW and CP plutons show fine grain size, acicular apatite crystals and lath-like plagioclase, together with the absence of any flow foliation, indicating that quenching crystallization of the magmas mostly occurred in the shallow environment. As such, the following lines of evidence argue against a considerable contribution from the mantle-derived magma to the petrogenesis of granitoids: (1) the MMEs only occur in local areas, which decrease from the margin to the center in the SHW and CP plutons (this study; Wang et al., 2011). They only represent 1–2% of the volume of these granitoid plutons; by simple calculations of mass balance, it is the enclave magmas that may become hybridised, leaving the volumetrically dominant host granitic magmas relatively unaffected; (2) the contemporaneous ZS pluton almost has no MMEs, and exhibits low MgO contents with Mg# b45 (Table S2), and low Cr and Ni contents (Gong et al., 2009a), indicating no involvement of the mantle-derived mafic magma in the origin of the ZS granites; (3) the coeval mantle-derived mafic igneous rocks are extremely scarce in the target region; (4) within an individual pluton, there are no systematic changes in initial isotopic ratios with chemical compositions such as MgO and SiO2 (Fig. 9; Tables 1 and S2). Therefore, the chemical variations within a single pluton are unlikely to be mainly caused by magma mixing. This is consistent with the assumption that large-scale mixing of felsic and mafic

magmas is unlikely because of their contrast in chemical compositions and physical parameters such as viscosity, density and temperature (e.g., Grasset and Albarade, 1994; Sato and Sato, 2009; Sparks and Marshall, 1986). As suggested by Roberts and Clemens (1993), most high-K calc-alkaline I-type granitoids can be solely derived from partial melting of common crustal rocks; direct chemical input from the mantle at the time of granitic magmatism is unnecessary. On the other hand, it is generally assumed that underplating and emplacement of significant volumes of hot basaltic magmas into the lower continental crust or at the crust-mantle interface may supply the heat to initiate and promote large-scale crustal anatexis in collisional orogens (Bergantz, 1989). Numerical simulations suggest that periodic influx of basaltic magmas can induce partial melting of a mafic (amphibolitic) lower crust (Petford and Gallagher, 2001). Available experimental data indicate that dehydration melting reactions of amphibolites within the lower crust can produce significant volumes of granodioritic to trondhjemitic melts, particularly in the region of high heat flow (e.g., Wyllie and Wolf, 1993; Rapp and Watson, 1995). Because there was no significant contribution from mantle-derived materials to the origin of granitoids in the SQZ, the mantle may have primarily provided heat for their generation. The limited variations in the Sr-Nd-Hf isotope compositions of granitoids from the SQZ suggest their origination from relatively homogeneous crustal sources (Figs. 8 to 10). While the isotopic signatures of granitoids are primarily inherited from their source rocks, their elemental contents can be affected by a number of factors such as fractional crystallization and partial melting conditions (P, T or fH2O). As illustrated in Fig. 14, the SHW and CP granitoids show a narrow range of SiO2 contents (62.88–69.04 wt.%) and lack notable negative Eu anomalies with δEu = 0.75–0.89 (Table S2). Their SiO2 contents are not correlated with Eu/Eu⁎, Rb/Sr, (La/Yb)N and (Dy/Yb)N ratios (Fig. 14), suggesting that no extensive fractionation of feldspar, hornblende and garnet for the SHW and CP granitoids. By comparison, the ZS granites exhibit relatively high SiO2 contents (69.32–75.94 wt.%), moderate negative Eu anomalies with δEu = 0.63–0.92, and their SiO2 contents are negatively correlated with Eu/Eu⁎ ratios but positively correlated with Rb/Sr ratios (Fig. 14a, b), indicating varying degrees of plagioclase fractionation because plagioclase has excessively high Eu and Sr contents. Particularly, the ZS sample 13QL118 is highest in its SiO2 content of 75.94 wt.%, FeOT/MgO ratio of 4.98 and (K2O + Na2O)/CaO ratio of 11.59 (Table S2), suggesting that it is a highly fractionated granite (HFG; Whalen et al., 1987). In addition, SiO2 contents are not correlated with (La/Yb)N ratios (Fig. 14c) but negatively correlated with (Dy/Yb)N ratios (Fig. 14d), suggesting fractionation or residue of hornblende to some extent during magmatism. This is also supported by concave-upward

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Y.-H. Lu et al. / Lithos 284–285 (2017) 30–49

Fig. 14. Plots of whole-rock SiO2 vs. Eu/Eu⁎ ratios (a), Rb/Sr ratios (b), (La/Yb)N ratios (c) and (Dy/Yb)N ratios (d) for the SHW, CP and ZS granitoids in the South Qinling zone. Data sources and symbols are the same as those in Fig. 4.

distribution patterns for the ZS granites (Fig. 7e), because hornblende has high partition coefficients for middle REE in felsic melts (Rollinson, 1993). As argued above, all geochemical features of the SHW, CP and ZS granitoids in the SQZ indicate that they were derived from partial melting of the metaigneous rocks with no involvement of supracrustal rocks in their magma sources. In addition, all the samples have relatively low Al2O3/(FeOT + MgO + TiO2) and (Na2O + K2O)/CaO ratios (Fig. 15) and generally fall into the field of amphibolite-derived melts (Patiňo Douce, 1999; Lu et al., 2016), further indicating that the SHW, CP and ZS granitoids were produced by dehydration melting of mainly basaltic metaigneous rocks. Furthermore, low Rb/Sr (b 0.4), Rb/Ba (b 0.2) and high Zr/Hf (N30) ratios (Table S2) also indicate that their parental magmas were produced by partial melting of metabasaltic sources. On the other hand, previous studies have argued that some of the Triassic granitoids in the SQZ such as the Dongjiangkou and Wulong plutons (e.g., Qin et al., 2010, 2013; Zhang et al., 2008) have geochemical characteristics similar to modern adakites (Defant and Drummond, 1990; Martin, 1999), but proposed that these granitoids may be produced by the mixing of mantle-derived mafic magma with ancient crustderived felsic magma produced in the stability field of garnet. Although some SHW, CP and ZS samples display relatively high Sr/Y and (La/Yb)N ratios (Table S2), the majority of them fall into the field of continental arc andesite, dacite and rhyolite (ADR) or transition from adakite to ADR (supplementary Fig. S1). Moreover, the SHW, CP and ZS granitoids exhibit limited variations in Sr/Y, (La/Yb)N and (Dy/Yb)N ratios, which are clearly different from high Sr/Y granitoids (HSG) produced by partial

melting of the thickened lower continental crust in the Dabie orogen (He et al., 2011), but similar to low Sr/Y granitoids (NG) derived from partial melting of normal lower continental crust (supplementary Fig. S2). Therefore, the SHW, CP and ZS granitoids are not adakitic rocks. In order to further discriminate granitoids from adakites, we have made a trace element modeling in this study. The modeling utilizes bulk =CLCC + F(1− Dbulk )], in which the batch melting equation: Cmelt i i /[Di i LCC melt denote the concentrations of element i in the lower conCi and Ci tinental crust and melt, respectively, and F is the melt fraction. The partition coefficients used in the calculations are from Johnson (1998) and Qian and Hermann (2013). As discussed above, the geochemical compositions demonstrate that the SHW, CP and ZS granitoids were derived from the dehydration melting of metabasaltic sources, which may be the lower continental crust (LCC) in the SQZ. Therefore, the assumed source is the average LCC (Rudnick and Gao, 2003). Based on the experimental results by Wyllie and Wolf (1993) and Qian and Hermann (2013), we consider the effects of minerals with various proportions (e.g., Cpx, Opx, Pl, Amp and Gt) in the residuum. The modeling results suggest that the trace element compositions of these granitoids can be appropriately obtained by ~ 20% to 30% batch melting of the LCC, with the residual mineral assemblage Cpx + Opx + Amp + Pl + Gt and at a proportion of 40:20:30:8:2 (supplementary Fig. S3). Thus, our modeling indicates that the SHW, CP and ZS granitoids can be produced by partial melting of the mafic protoliths in the LCC, leaving behind a granulite residue. They cannot be produced by partial melting of the thickened lower continental crust with the eclogite residue. Beside the protolith nature and fractional crystallization, the melting temperature is also an important factor to affect granitoid compositions (e.g., Gao et al., 2014; Lu et al., 2016; Zhao et al., 2015). The zircon saturation thermometer (TZr) can be used to estimate the temperatures of partial melting for granitoids (Hanchar and Watson, 2003; Miller et al., 2003). As discussed above, the calculated TZr values for the SHW, CP and ZS granitoids are approximate to or slightly lower than their magma crystallization temperatures. As illustrated in Fig. 16, the ZS granites show higher Al2O3/TiO2, Rb/Sr ratios and lower P2O5 and Zr + Ce + Nb + Y contents than the SHW and CP granitoids at similar TZr values. This can be ascribed to the significant influence of source nature and fractional crystallization. On the other hand, the SHW granitoids exhibit relatively higher TZr values, P2O5 and Zr + Ce + Nb + Y contents than the CP granitoids (Fig. 16b, d), indicating that melting temperatures would have played an important role in dictating their geochemical compositions. In addition, the SHW granitoids exhibit a typical rapakivi texture with some alkali feldspar megacrysts surrounded by plagioclase (Fig. 2a; Lu et al., 1999; Wang et al., 2008, 2011), which was considered to be formed by the isothermal decompression dissolution of the MMEs after magma mixing in a postcollisional setting (e.g., Lu et al., 1999; Wang et al., 2008, 2011; Zhang et al., 2008). However, Zhou et al. (2008) argued that the SHW

Fig. 15. Plots of major element compositions for granitic melts experimentally produced by dehydration melting of various lithologies, compared to those for the SHW, CP and ZS granitoids in the South Qinling zone. Data for experimental melts are from Patiňo Douce (1999) and Lu et al. (2016), and references therein. Data sources and symbols are the same as those in Fig. 4.

Y.-H. Lu et al. / Lithos 284–285 (2017) 30–49

Fig. 16. Plots of zircon saturation temperature (TZr) vs. Al2O3/TiO2 ratios (a), P2O5 contents (b), Rb/Sr ratios (c) and Zr + Ce + Nb + Y contents (d) for the SHW, CP and ZS granitoids in the South Qinling zone. Data sources and symbols are the same as those in Fig. 4 (not including sample 13QL118).

granitoids have geochemical characteristics similar to the nearby CP and other syn-collisional granitoids in the SQZ, but in contrast to typical rapakivi-textured granitoids, and thus formed in a syn-collisional setting. Therefore, the magma mixing and decompression process cannot completely explain the rapakivi texture of the SHW pluton. On the other hand, melting temperatures could also have played an important role in the formation of rapakivi textures (e.g., Müller et al., 2008; Hu et al., 2016). In this regard, it needs to be further investigated with respect to the nature and formation mechanism of the rapakivi texture in the SQZ granitoids. In summary, the SHW, CP and ZS granitoid magmas would be produced by the partial melting of metabasaltic sources at the normal lower crustal depth, and subsequently experience varying degrees of fractional crystallization. The melting temperature and thus the extent of partial melting would have played an important role in producing these granitoids. On the other hand, local small-scale hybridisation may have occurred at the contact zone between the MMEs and the felsic host magma, resulting in the local formation of some SHW and CP granitoids with relatively high MgO (Mg#), Cr and Ni contents (e.g., Gong et al., 2009a; Wang et al., 2011). Furthermore, the mantle could provide significant heat energy for the generation of these granitoids. 8. Implications for the tectonic setting of granitoid magmatism The Triassic continental collision between the South and North China Blocks would have ultimately built the Qinling-Tongbai-Hong'an-DabieSulu orogenic belt (Dong et al., 2011; Mattauer et al., 1985; Meng and Zhang, 2000; Wu and Zheng, 2013; Zhang et al., 2001). The occurrences of Triassic UHP metamorphic rocks and syn-exhumation igneous rocks are prominent in the Dabie-Sulu orogenic belt (e.g., Zheng, 2008, and references therein; Zhao et al., 2012), providing a natural laboratory to investigate the tectonic processes of continental subduction and exhumation in the eastern segment. In contrast, there are no Triassic UHP metamorphic rocks in the western segment (the Qinling orogen). However, the Triassic granitoids are widespread in the Qinling orogen (Fig. 1c). Reasonable understanding of their petrogenesis can place important constraints on the formation and evolution of the Qinling orogen during the Triassic continental collision between the South and North China Blocks. Although many studies have been devoted to the tectonic setting of Triassic granitoids in the Qinling orogen, there is still great controversy on this topic, with mainly two kinds of viewpoint. Some researchers hypothesized that the granitoid magmatism resulted from the northward subduction of the Paleo-Tethys oceanic crust and thus has a tectonic setting of volcanic arc (e.g., Li et al., 2015; Zhang et al., 2001). In contrast, the other researchers advocated that the granitoids were

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produced in a postorogenic setting and thus postdates the final amalgamation between the South and North China Blocks. In either case, the magmatism may be related either to delamination of the thickened orogenic crust (e.g., Zhang et al., 2008), or to breakoff of the subducted Mianlue oceanic slab (e.g., Sun et al., 2002; Qin et al., 2010, 2013). On the other hand, the Triassic granitoids in the Qinling orogen were divided into different stages based on their emplacement ages, in correspondence to syn-subduction, syn-collision and post-collision stages, respectively (e.g., Dong et al., 2011, 2012; Jiang et al., 2010; Liu et al., 2011; Wang et al., 2013b). In doing so, some confusion has been caused by defining the time of magmatism relative to a given collisional orogeny. Zircon U-Pb dating indicates that the SHW, CP and ZS granitoids in the SQZ were emplaced at 208–216 Ma, in accordance with ca. 205–225 Ma emplacement ages for the vast majority of granitoids in the Qinling orogen (Lu et al., 2016, and references therein). Such ages postdate the UHP metamorphic ages of 225–245 Ma in the Dabie-Sulu orogenic belt (e.g., Li et al., 1993; Zheng, 2008; Zheng et al., 2009). This suggests that the crystallization ages of Triassic granitoids in the SQZ may be later ca. 10–30 Myr than the UHP metamorphic episode in the Dabie-Sulu orogenic belt. Considering the absence of contemporaneous basaltic to andesitic igneous rocks in the SQZ, on the other hand, it is unlikely that these granitoids would be the direct product of continental arc magmatism during the Triassic subduction of the Paleo-Tethys oceanic crust beneath the SQZ (Lu et al., 2016). There are also metamorphic ages of 221–242 Ma for the Heigouxia metavolcanic rocks in the Mianlue ophiolite suture, which were generally accepted to mark the timing for the closure of the Paleo-Tethys ocean (Li et al., 1996). Furthermore, previous studies have revealed that the initial collision between the SCB and the SQZ occurred at ca. 223 ± 2 Ma in view of muscovite Ar/Ar dating for a ductile shear zone at the Mianlue suture (Chen et al., 2010). Based on paleomagnetic studies and the indentation model (e.g., Zhao and Coe, 1987; Yin and Nie, 1993), it is suggested that the collision between the SCB and the NCB would have started from east to west, with a scissor-like closure between them. The initial contact in the easternmost part would take place in the Early Triassic or the Late Permian, and then continental collision occurred in the Late Triassic by a clockwise rotation to the westernmost part. The two blocks were assembled together in the western part (the Qinling orogen) during the Late Triassic, while it would be rotated clockwise to cause tectonic extension in the eastern part (the Dabie-Sulu orogenic belt) where syn-exhumation magmatism was produced by decompressional melting of the UHP metamorphic rocks (e.g., Zhao et al., 2012; Zheng et al., 2015). In this regard, the SHW, CP and ZS granitoids would be produced in a syn-collisional setting in association with the tectonic transition from compression to extension in the Late Triassic. Based on the structural and paleogeographic evolution of the foreland basin belt along the northern margin of the SCB, it is generally accepted that the SCB would have subducted northward beneath the southernmost margin of the newly accreted NCB in the Qinling orogen and thus collided with it from the late Middle Triassic to Late Triassic (Liu et al., 2005; Wu and Zheng, 2013). Substantially, the subduction of continental crust in the Qinling orogen is contemporaneous with the exhumation of UHP metamorphic slices in the Sulu orogen (Liu et al., 2015; Zheng et al., 2015). In addition, many granulites have been found within the Mianlue suture in the SQZ (e.g., Liang et al., 2013), showing ca. 200–214 Ma ages for amphibolite-facies retrogression. Such ages are consistent with the emplacement time of the SHW, CP and ZS granitoids. Nevertheless, the Late Triassic SHW, CP and ZS granitoid plutons, being similar to the other contemporary granitoid plutons in the SQZ, exhibit no or weak deformation and discordantly cut across regional structures. For example, the SHW and CP granitoid plutons intruded into the Shangdan suture and the Guangtoushan granitic pluton intruded into the Mianlue suture (Fig. 1c). This indicates that these granitoids were produced in the tectonic setting of orogenic extension within the framework of continental collision in the SQZ. Such a tectonic interpretation is

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further reinforced by the stratigraphic successions of the SQZ, where the metamorphic Neoproterozoic basement underlies the non-metamorphic Neoproterozoic to Triassic sedimentary sequences, which are unconformably covered by the Jurassic sequence, indicating the Middle to Late Triassic collisional and compressional setting in the SQZ (Dong et al., 2011). Taken together, all the above observations indicate that the SHW, CP and ZS granitoids in the SQZ were produced in a syncollisional setting in association with the tectonic transition from compression to extension in the Late Triassic. Based on the regional tectonic evolution, some previous studies considered that the subducting slab breakoff model (Davies and von Blanckenburg, 1995) could be used to explain the geodynamic mechanism of the Late Triassic granitoid magmatism in the SQZ, and argued that the slab breakoff took place at a very shallow depth, which can greatly disturb the asthenosphere beneath the thickened continental lithosphere of the Qinling orogen and thus lead to the granitoid magmatism (e.g., Sun et al., 2002; Qin et al., 2010, 2013). This model predicted a linear thermal pulse along a collisional orogen to generate a linear zone of high-K calc-alkaline magmatic rocks with rapid mantle upwelling and sedimentary basin development (Davies and von Blanckenburg, 1995). However, contemporaneous mantle-derived mafic igneous rocks are extremely scarce in the target region. As discussed above, the granitoids of this study were not produced by partial melting of the tectonically thickened lower continental crust, which is inconsistent with the slab breakoff model. In addition, if the slab breakoff did take place, decompressional melting of the asthenospheric mantle would happen to produce mid-ocean ridge basalts (MORB)-like igneous rocks along a linear zone (Zheng et al., 2015). However, there are only small amounts of contemporaneous mafic dikes and lamprophyres in the SQZ (Wang et al., 2007b), and no MORB-like igneous rocks have been found so far in the target region. The Triassic I- and S-type granitoids are widespread in the Qinling orogen, especially in the SQZ (e.g., Li et al., 2015; Lu et al., 2016; Wang et al., 2013b; Zhang et al., 1994a). In terms of their outcrop areas and spatial distributions (Fig. 1c), the I-type granitoids are exposed over areas of ~3200 km2 (~60%) and are mainly located in the northern margin of the SQZ along the Shangdan suture, such as the Dongjiangkou pluton, Shahewan pluton, Caoping pluton, Zhashui pluton, Wulong and Laocheng plutons, etc. (e.g., Gong et al., 2009a, b; Jiang et al., 2010; Qin et al., 2010, 2013; this study). The S-type granitoids occupy areas up to ~2200 km2 (~40%) and are mainly located in the southern margin of the SQZ along the Mianlue suture, such as the Guangtoushan pluton, Huayang pluton, Yanzhiba pluton, Lanbandeng pluton, Huoshaodian and Jiangjiaping plutons, etc. (e.g., Jiang et al., 2010; Lu et al., 2016, and references therein). These I-type and S-type granitoids have almost the same emplacement ages (~215 Ma), and thus formed in a similar tectonic setting. Therefore, they are petrogenetically paired and belted in their spatial distributions, which are roughly parallel to the collisional suture zone. Their spatial distribution feature is quite similar to that of Cenozoic granitoids in the Himalayan orogen, where a discontinuous chain of S-type lecucogranite occurs in the southern High Himalayan belt whereas numerous discrete plutons of I-type granodiorite and granite occur in the northern Trans-Himalayan belt (e.g., Chung et al., 2005; Gill, 2010). The paired granite belts would be produced by anataxis of the subducted metasediment for S-type granites but reworking of the marginal arc lower crust for I-type granites. In this regard, the Late Triassic granitoids in the SQZ may be produced in a tectonic setting similar to the Cenozoic granitoids in the Himalayan orogen, where partial melting of the crustal sources for both I- and Stype granites is associated with the continental collision, recording the tectonic transition from compression to extension. It is widely accepted that the SCB was deeply subducted to mantle depths (N120 km) in the Triassic and experienced UHP eclogite-facies metamorphism and subsequently underwent rapid exhumation in the Dabie-Sulu orogenic belt (Zheng, 2008, and references therein). However, no UHP metamorphic rocks of Triassic age have been found in the

Qinling orogen (e.g., Dong et al., 2011; Wu and Zheng, 2013). The exposure of HP to UHP metamorphism in the Qinling-Tongbai-Hong'anDabie-Sulu orogenic belt is similar to that in the Himalayan orogen, where UHP metamorphic rocks are only found in its westernmost part, whereas the majority of it only contains HP eclogite-facies metamorphic rocks (Ding et al., 2016, and references therein). As argued before, the Late Triassic granitoids in the SQZ were derived from dehydration melting of the mafic lower continental crust beneath the SQZ. Except for muscovite-rich metapelitic rocks at pressures of less than ~ 1.0 GPa, the dehydration melting of common crustal rocks requires temperatures in excess of ~ 850 °C. It is generally thought to be impossible that crustal rocks can be heated to such high temperatures under the conditions of normal geothermal gradients and even crustal thickening in collisional orogens. For this reason, the mantle-derived basaltic magmas are usually considered the key heat source for partial melting of the mafic lower continental crust (e.g., Clemens, 2003; Patiňo Douce and McCarthy, 1998). The contemporaneous MMEs and few lamprophyre dykes occur within these plutons in the SQZ (e.g., Gong et al., 2009a, 2009b; Qin et al., 2010; Wang et al., 2007b, 2011), indicating that mantle-derived magmas could have served as the heat source for the dehydration melting of the lower continental crust. The high heat flux is also required by the large volume of contemporaneous granitoids in the SQZ (Dong et al., 2011; Li et al., 2015; Wang et al., 2013b). This is possibly realized by either underplating of the mantle-derived mafic magmas (Clemens, 2003) or the asthenospheric mantle along thinned orogens (Zheng and Chen, 2016). In particular, high heat flow can be provided by the asthenospheric mantle when the thickened orogenic lithosphere is thinned by active continental rifting (Zheng and Chen, 2017). As such, partial melting of crustal rocks in collisional orogens is spatially and temporally associated with the asthenospheric upwelling in response to lithospheric extension. In summary, the following scenario is proposed as the tectonic mechanism for the generation of a paired granite belt in the Late Triassic in the SQZ. (1) The subduction of the ancient oceanic crust beneath the northern margin of the Yangtze craton generates the juvenile crust of Late Mesoproterozoic age in the SQZ. (2) The Mianlue ocean was opened in the Late Paleozoic but became closed prior to the continental collision between the SCB and the NCB in the Late Triassic. (3) While the subducted metasediment underwent partial melting to produce the S-type granites in the southern margin of the SQZ (Lu et al., 2016), the mafic lower crust underwent reworking to produce the I-type granitoids in the northern margin of the SQZ (this study). The occurrence of the paired granite belt may provide a petrological record of the tectonic development from the oceanic subduction to continental collision. The difference in the compositions of source rocks is the key to the differentiation between contemporaneous S- and I-type granitoids in the collisional orogen. 9. Conclusions The SHW, CP and ZS granitoids from the Dongjiangkou suite in the SQZ were emplaced at 208 ± 2 to 216 ± 3 Ma, consistent with the subduction of continental crust along the Mianlue suture in the Qinling orogen. These granitoids were derived from partial melting of the metabasaltic rocks under the conditions of normal lower crustal depths and thus are of I-type and non-adakitic affinity. These granitoids have low and homogeneous whole-rock (87Sr/86Sr)i ratios, slightly negative whole-rock εNd(t) and positive zircon εHf(t) values with Late Mesoproterozoic two-stage Nd and Hf model ages. Furthermore, the source nature, fractional crystallization and melting temperature have played important roles in dictating their compositional variations. In addition, these granitoids show relatively limited variations in whole-rock εNd(t) and zircon εHf(t) values, and are genetically linked to the Neoproterozoic Yaolinghe metabasalts and mafic-ultramafic intrusions in the SQZ. This suggests their origination from the lower continental crust of the SQZ, which would have similar isotopic compositions to the Yaolinghe Group, rather than the subducted Yangtze continental

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crust. There are also contemporaneous S-type granites in the southern part of the SQZ, forming a paired granite belt in the Qinling orogen. The difference in the composition of source rocks is substantial to the differentiation between the I- and S-type granites. The continental collision between the NCB and the SCB in the Late Triassic would have brought about not only the partial melting of metasedimentary rocks for the S-type granites but also the reworking of mafic lower crust for the I-type granitoids in the same orogen. Acknowledgments This study was supported by funds from the Chinese Ministry of Science and Technology (2016YFC0600103, 2015CB856102), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB18000000) and the Natural Science Foundation of China (41473034). We appreciate the assistance of Fuqiang Dai, Yuwei Tang and Fei Zheng with the field sampling and Sr-Nd isotope analyses, Quanzhong Li with the LA-ICPMS zircon U-Pb dating, Xiangping Zha with O isotope analyses, Yonghong Shi and Qiongxia Xia with the electron microprobe analyses as well as Yueheng Yang and Jinhui Yang with wholerock Sr-Nd and zircon Lu-Hf isotope analyses. We are grateful to Dr. Zuochen Li and one anonymous reviewer for their comments that greatly helped the improvement of the presentation. We thank Dr. Nelson Eby for his editorial handling.

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