Petrogenesis of Late Carboniferous granitoids in the Chihu area of Eastern Tianshan, Northwest China, and tectonic implications: geochronological, geochemical, and zircon Hf–O isotopic constraints

ABSTRACT This contribution presents new SIMS zircon U–Pb geochronology, major and trace element geochemistry, and zircon Hf–O isotope systematic on an example of Late Carboniferous granodiorite and porphyritic granodiorite intrusions from the Chihu area of Eastern Tianshan, Xinjiang. SIMS zircon U–Pb dating indicates that the Chihu granodiorite and porphyritic granodiorite formed at 320.2 ± 2.4 Ma and 314.5 ± 2.5 Ma, respectively. These rocks are metaluminous to weakly peraluminous with an A/CNK value of 0.92–1.58, as well as low 10000 Ga/Al, Zr + Nb + Y + Ce, and Fe2O3T/MgO values, which suggest an I-type normal island arc magmatic suite. The porphyritic granodiorite has a slightly higher Sr/Y ratio (28–37) and lower Y (6.9–11.7 ppm) and Yb (0.98–1.49 ppm) contents, suggesting mild adakite affinities. In situ Hf–O isotopic analyses using LA-ICP-MS-MC and SIMS indicate that the εHf(t) and δ18O values of granodiorite zircons vary from +11.5 to +14.9 and 4.80 to 5.85 ‰, respectively, similar to values for porphyritic granodiorite zircons, which vary from +11.9 to +17.2 and 3.78 to 4.71 ‰, respectively. The geochemical and isotopic data imply that the Chihu granodiorite and porphyritic granodiorite share a common origin, most likely derived from partial melts of the subduction-modified mantle. Based on the regional geological history, geochemistry of the Chihu intrusions, and new isotopic studies, we suggest that the Late Carboniferous magma was generated during the period of the northward subduction of the Palaeo-Tianshan ocean plate beneath the Dananhu–Tousuquan island arc.


Introduction
The Central Asian Orogenic Belt (CAOB) is one of the largest Phanerozoic accretionary orogenic belts on Earth (Şengör et al. 1993;Safonova 2009;Xiao et al. 2013;Goldfarb et al. 2014), and consists of microcontinental blocks, island arcs, oceanic crustal remnants, and continental marginal facies rocks (Coleman 1989;Jahn 2004;Windley et al. 2007;Li et al. 2013a;Pirajno 2013;Wang et al. 2015aWang et al. , 2015b. The Eastern Tianshan orogenic belt in Northwest China is located in the southern margin of the CAOB (Figure 1(A) and (B)) and contains abundant Carboniferous to Permian granitic rocks (Qin et al. 2002Zhou et al. 2010;Su et al. 2012;Shen et al. 2014;Wang et al. 2015aWang et al. , 2015b, and subordinate Devonian and Triassic granitoids (Chen et al. 2014;Han et al. 2014;Zhang et al. 2015a). This region is also one of the important Cu, Au, Fe, Mo, Ag metallogenic belts in China, and the mineralization is closely related to the granitic rocks (Han et al. 2006;Liu et al. 2007;Pirajno et al. 2011;Chen et al. 2012aChen et al. , 2012bHuang et al. 2013;Wang et al. 2015c). Consequently, Eastern Tianshan granitoids provide an excellent opportunity to study CAOB tectonic evolution and the associated metallogenesis. However, the petrogenesis and geodynamic setting of the granitoids are still under debate, with suggested settings such as rift (Qin et al. 2002), back-arc basin (Xu et al. 2003), passive continental margin , island arc (Mao et al. 2005;Zhang et al. 2008;Wang et al. 2015aWang et al. , 2015b, ridge subduction environment (Sun et al. 2010, or post-collision setting (Gu et al. 2006;Zhou et al. 2008).
Previous studies mostly focused on the Palaeozoic mineralization and magmatism in the eastern and western parts of the Eastern Tianshan orogenic belt, whereas less attention has been paid to Palaeozoic granitoids in the central part of the belt. The regional tectonic evolution of the Eastern Tianshan has not been adequately constrained until now, largely due to a lack of detailed geochronological and geochemical data. The Chihu area, located in the central Eastern Tianshan orogenic belt, is an ideal area to investigate the geodynamic processes of magmatism because of its linkage to the Dananhu-Tousuquan arc and the Kanggur-Huangshan ductile shear zone (Figure 1(C)). Similar to the nearby late Palaeozoic Tuwu-Yandong intrusions Shen et al. 2014;Wang et al. 2014Wang et al. , 2015aWang et al. , 2015bXiao et al. 2015), Chihu calc-alkaline granitoids are important intrusions in the Eastern Tianshan orogenic belt Wu et al. 2006c;Gao et al. 2015). The Tuwu-Yandong Cu deposits, located~30 km west of the Chihu deposit in the Dananhu-Tousuquan arc belt, have been studied for geochronology (Liu et al. 2003;Chen et al. 2005;Wang et al. 2014), isotopic geochemistry Shen et al. 2012;Gao et al. 2015;Xiao et al. 2015), and tectonic setting (Shen et al. 2014;Gao et al. 2015;Wang et al. 2015aWang et al. , 2015b of the intermediate-felsic intrusions. However, none of these studies have focused on the Chihu area to date, which precludes our understanding of the Chihu granitoids. Here, we present secondary ion mass spectrometry (SIMS) zircon U-Pb, whole-rock geochemistry, and Hf-O isotopic data of the Chihu granodiorite and porphyritic granodiorite in an attempt to better constrain the timing of magmatic activities, the petrogenesis of these granitoids, and the geodynamic setting of Carboniferous magmatism in the Eastern Tianshan orogenic belt.  Table 1. Previous data are from Chen et al. (1999), Zhang et al. (2002), Liu et al. (2003), Li and Chen (2004), Chen et al. (2005), Wang et al. (2005Wang et al. ( , 2015aWang et al. ( , 2015b, Wu et al. (2006aWu et al. ( , 2006c, Zhou et al. (2010), Li et al. (2011, and Shen et al. (2014).
The Jueluotage belt may be subdivided into the Xiaorequanzi-Wutongwozi and Dananhu-Tousuquan arcs in the north, the Kanggur-Huangshan ductile shear zone in the middle, and the Aqishan-Yamansu arc in the south, which are separated by the Kanggur and Yamansu faults (Figure 1(C); Xiao et al. 2004;Qin et al. 2011;Wang et al. 2016). The Xiaorequanzi-Wutongwozi and Dananhu-Tousuquan arc belts mainly consist of Lower Devonian volcanic and clastic sedimentary rocks of the Dananhu Formation, lower Carboniferous turbidites of the Gandun Formation, Carboniferous basaltic to andesitic volcanic rocks and sedimentary rocks of the Qi'eshan group, Permian calcalkaline volcanic, pyroclastic, and clastic rocks, Jurassic sandstone, and Cenozoic cover (Mao et al. 2005;Zhou et al. 2010;Shen et al. 2014;Gao et al. 2015). The Kanggur-Huangshan ductile shear belt is mainly composed of Devonian-Carboniferous volcaniclastic rocks, basalt, tuff, limestone, sandstone, and ophiolitic slices Xiao et al. 2013;Mao et al. 2014). The Aqishan-Yamansu arc belt is characterized by early Carboniferous basalt, andesite, dacite, and tuff of the Yamansu Formation and late Carboniferous rhyolite of the Tugutubulake Formation (Chen et al. 2012a;Xiao et al. 2013;Hou et al. 2014).
The main structures of Eastern Tianshan are characterized by a series of approximately E-W-trending faults, including the regional-scale Dacaotan, Kanggur, Yamansu, and Aqikuduke faults, and some small-scale faults (Figure 1(C); Mao et al. 2005Mao et al. , 2008Qin et al. 2011;Huang et al. 2013). The Eastern Tianshan intrusive rocks mainly formed in late Palaeozoic time and include diorite porphyry, plagiogranite porphyry, and granodiorite associated with Cu mineralization, and mafic-ultramafic bodies mainly associated with Cu-Ni mineralization Pirajno et al. 2011;Wang et al. 2015aWang et al. , 2015bWang et al. , 2015c.

Petrography of the Chihu granitoids
The Chihu area is located along a regional E-W-striking fault at the south margin of the Dananhu-Tousuquan arc belt (  Chihu granitoids are predominantly porphyritic granodiorite and granodiorite, which intruded the intermediate to mafic volcanic rocks of the Qi'eshan Group (Figure 2(B)).

Sample preparation
Granitoid samples for this study were collected from the Chihu area. Owing to pervasive alteration, the granitoids were more or less affected by sericitization, silicification, and chloritization. Thus, to better measure and understand the chemical composition, the least altered samples were selected for major and trace element, SIMS zircon U-Pb dating, and in situ zircon Hf-O isotopic analyses. They are composed of eight granodiorite and seven porphyritic granodiorite samples, and the sampling locations are shown in Figure 2(B).

Analytical methods
Zircons grains were separated using conventional heavy liquids, magnetic separation techniques, and handpicking under a binocular microscope at the Langfang Regional Geological Survey in Hebei Province, China. Zircon grains, together with zircon standard Penglai Plesovice (Black et al. 2004) and Qinghu (Li et al. 2009a), were mounted in epoxy, which were subsequently polished to section the crystals in half for analysis. Zircons were imaged with transmitted and reflected light micrographs as well as cathodoluminescence (CL) to reveal their internal structures, and the mount was vacuum coated with high-purity gold prior to O isotopic analyses.
Zircon oxygen isotopic analyses were conducted using the Cameca IMS-1280 SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences. The detailed analytical procedure and operating conditions are described by Li et al. (2009bLi et al. ( , 2010b. The measured oxygen isotopic data were first normalized relative to the Vienna Standard Mean Ocean Water (VSMOW) and then corrected for instrumental mass fractionation using the zircon Penglai and Qinghu standards with a δ 18 O value of 5.3 ± 0.1‰ (2σ) and 5.4 ± 0.2‰ (2σ), respectively (Li et al. 2010b(Li et al. , 2013b. The corrected δ 18 O values for the samples were reported in the standard per mil notation with 2σ errors (Li et al. 2010b).
SIMS zircon U-Pb analyses were conducted at the same domain of the O isotope analytical spots using the same Cameca IMS-1280 SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Detailed analytical and data acquisition and processing procedures are documented by Li et al. (2009a). A long-term uncertainty of 1.5% (1 RSD) for 206 Pb/ 238 U measurements of the standard zircons was propagated to the unknowns (Li et al. 2010a), despite the fact that the measured 206 Pb/ 238 U error in a specific session was generally approximately 1% (1 RSD) or less. Measured compositions were corrected for common Pb using the non-radiogenic 204 Pb method, and data processing was conducted using the ISOPLOT 3.0 program (Ludwig 2003). Corrections were sufficiently small to be insensitive to the choice of common Pb composition, and an average of present-day crustal composition (Stacey and Kramers 1975) was used for the common Pb composition, assuming that the common Pb was largely because of surface contamination introduced during sample preparation. Uncertainties on individual analyses in data tables were reported at the 1σ level; mean ages for pooled 206 Pb/ 238 U results were quoted at the 95% confidence level.
In situ Hf isotopic analyses were performed on a Newwave UP 213 laser-ablation microprobe attached to a Neptune MC-ICP-MS at the MLR Key Laboratory of Metallogeny and Assessment in Chinese Academy of Geological Sciences, Beijing. Depending on the zircon size, a stationary beam spot of approximately 60 μm diameter was employed for the analyses, and the international standard zircon sample GJ1 was used as a reference. Details on the instrumental conditions and data acquisition are given in Wu et al. (2006b). The measured values of the well-characterized zircon standard (GJ1) agreed with the recommended values to within 2σ. The weighted average of the 176 Hf/ 177 Hf ratio of the GJ1 zircon samples was 0.282000 ± 0.000011 (2σ, n = 31), consistent with the recommended values (Elhlou et al. 2006) to within 2σ. The initial 176 Lu/ 177 Hf ratios were calculated using a decay constant of 1.867 × 10 −11 year −1 for 176 Lu (Söderlund et al. 2004). The chondritic 176 Lu/ 177 Hf ratio of 0.0332 and the 176 Hf/ 177 Hf ratio of 0.282772 (Blichert-Toft and Albarède 1997) were adopted to calculate the ε Hf (t) values. The depleted mantle model age (T DM ) was measured with reference to the depleted mantle at a present-day 176 Lu/ 177 Hf ratio of 0.0384 and 176 Hf/ 177 Hf ratio of 0.28325 (Griffin et al. 2002). The crustal model age (T c DM ) was calculated using an average continental crustal 176 Lu/ 177 Hf ratio of 0.015 (Griffin et al. 2002).
Major and trace element analyses of the granitoid samples were conducted at the test centre of the Beijing Research Institute of Uranium Geology. The samples were chipped and powdered to about 200 mesh for major and trace element analyses. Major elements were determined by a Philips PW 2404 X-ray fluorescence (XRF) spectrometer with a rhodium X-ray source. Analytical precision was better than 1%, and the detailed analytical procedures were as described by Norrish and Hutton (1969). Sample powders for trace element analyses were weighed (25 mg) into Savillex Teflon beakers within a high-pressure bomb, and then were digested using HF + HNO 3 + HClO 4 acid to assure the complete dissolution of refractory minerals. Trace elements, including rare earth elements (REEs), were determined using an Element-I plasma mass spectrometer (Finnigan-MAT Ltd. German), and national geological standard reference samples GSR-3 and GSR-15 were used for analytical quality control. The analytical precision for trace elements was better than 5%, with analytical procedures being described by Qi et al. (2000).

SIMS zircon U-Pb dating
Chihu granodiorite sample (CH-9) and porphyritic granodiorite sample (CH001-12) were selected for SIMS zircon U-Pb dating, and the analytical data are listed in Supplementary Table 1 Table 1), which is consistent with a magmatic origin (Hoskin and Schaltegger 2003). Therefore, the SIMS zircon U-Pb dating results are interpreted to represent the timing of zircon crystallization and thus the age of magma emplacement. Fifteen analyses from the granodiorite sample (CH-9) yielded concordant 206 Pb/ 238 U ages varying from 311.8 to 327.5 Ma (Figure 4(B)), with a weighted mean age of 320.2 ± 2.4 Ma (MSWD = 0.98; n = 15). Except for two discordant spots (CH001-12@01 and CH001-12@15), the remaining 13 analytical spots from porphyritic granodiorite CH001-12 had 206 Pb/ 238 U ages ranging from 308.1 to 324.5 Ma (Figure 4(D)), with a weighted mean age of 314.5 ± 2.5 Ma (MSWD = 0.82, n = 13). Thus, we interpret the two weighted mean ages as emplacement ages of the granodiorite and porphyritic granodiorite, respectively.
6. Discussion    2005; Zhang et al. 2002;Wu et al. 2006aWu et al. , 2006cSun et al. 2012;Shen et al. 2014;Wang et al. 2015aWang et al. , 2015b Zhou et al. 2010). These geochronological data indicate that the Carboniferous magmatic activity was widely distributed in the middle and western parts of Eastern Tianshan (Figure 1  (C)). In addition, Chen et al. (1999) reported that the K-Ar age of the Xiaorequanzi Cu-Zn deposit is 339.1 Ma; Rui et al. (2002) obtained a Re-Os isochron age of 322.7 ± 2.3 Ma for molybdenite from the Tuwu porphyry Cu deposit; Zhang et al. (2004) reported a Re-Os isochron age of 343 Ma for veinlet-hosted and disseminated molybdenite from the Yandong area; and Zhang et al. (2010) obtained a Re-Os model age of 326.2 ± 4.5 Ma for molybdenite from the Yanxi porphyry Cu deposit. These isotopic age data suggest that Carboniferous magmatism and Cu-dominant mineral system are significant in the Eastern Tianshan orogenic belt, which are considered to be related to subduction tectonism (e.g. Mao et al. 2005;Han et al. 2006).
Precise dating of host rocks can be used to constrain the timing and duration of magmatic hydrothermal events, which is crucially important in understanding the rock-forming process and geodynamic setting (Stacey and Kramers 1975;Leng et al. 2013;Deng et al. 2014a;Zhang et al. 2015b;Wang et al. 2015d). Based on new SIMS zircon U-Pb data presented herein, the timing of granitoid intrusions in the Chihu area is well constrained. The Chihu granodiorite was emplaced at 320.2 ± 2.4 Ma (Figure 4(B)), whereas the porphyritic granodiorite was formed at 314.5 ± 2.5 Ma (Figure 4 (D)), thereby confirming that they were intruded in the Late Carboniferous. This episode is coeval with Late Palaeozoic (354-297 Ma) large-scale magmatic-metallogenesis in Eastern Tianshan (Zhang et al. 2002Wu et al. 2006aWu et al. , 2006cSun et al. 2012;Shen et al. 2014;Wang et al. 2015aWang et al. , 2015b, and this episode was also responsible for producing the Chihu Cu deposit.

Petrogenesis and source of magma
Petrogeochemical signature of intrusive rocks records important information on magma source region, magmatic process, and tectonic setting (Pearce et al. 1984;Sylvester 1998;Barbarin 1999;Sillitoe 2010); therefore, it is important to have a clear understanding of the petrogenetic history of the Chihu granitoid rocks. Aluminous minerals such as muscovite, tourmaline, and garnet, as diagnostic minerals in S-type granites (Barbarin 1999), were not identified in the Chihu granodiorite and porphyritic granodiorite. All granitoid samples in the Chihu area show positive correlation between P 2 O 5 and SiO 2 contents (Figure 10(A)), and negative correlation between Y and Rb values (Figure 10(B)), which are typical I-type granite evolution trends (Wu et al. 2003;Li et al. 2007). Furthermore, Chihu granitoids have low 10000 Ga/Al (1.26-1.87), Zr + Nb + Y + Ce (73.93-245.95 ppm), and Fe 2 O 3 T /MgO (1.69-3.44) values, falling into the non-fractionated granite field (Figure 10(C) and (D); Whalen et al. 1987). Therefore, the Chihu granitoid rocks are considered as non-fractionated I-type granites, rather than S-type or A-type granites. In addition, the Geochemically, the Chihu granodiorites are characterized by moderate HREE depletion, slightly negative Eu anomalies, and negative Nb, Ta, and Ti anomalies on the N-MORB normalized plots (Figure 7(B)), reflecting clear subduction signatures. Moreover, the adakite-like porphyritic granodiorites at Chihu have moderate LREE enrichment and weak HREE depletion, with positive or no Eu anomalies (Figure 7(A)). They also show clear depletion in high field strength elements (HFSEs), such as Nb, Ta, and Ti (Figure 7(B)), similar to those of modern subduction-related plutonic rocks (Wood et al. 1979;Briqueu et al. 1984;Shen et al. 2014), formed by the partial melting of subducted oceanic slab.
Experimental studies have shown that Mg# is a useful criterion in distinguishing melts purely derived from the crust from those involved in the mantle. Melts from the basaltic lower crust are characterized by low Mg# less than 40 regardless of the melting degree, whereas those with Mg# greater than 40 can only be obtained when a mantle component is involved (Rapp and Watson 1995;Rapp et al. 1999;Zhu et al. 2009;Guan et al. 2012 Table 2), indicating that the source for their magma was partially derived from the mantle-derived material.
These results are supported by the zircon Hf-O isotopic data (Supplementary Table 3). The Hf isotopic compositions of the granitoids from the Chihu area are characterized by positive ε Hf (t) values, which range from +11.5 to +14.9 of magmatic zircons from a granodiorite sample (CH-9), and from +11.9 to +17.2 of zircons from a porphyritic granodiorite sample (CH001-12) (Supplementary Table 3; Figure 9(A)). They also show young Hf model ages (T C DM ), which are between 383 and 594 Ma (granodiorite), and between 232 and 509 Ma (porphyritic granodiorite) (Supplementary Table 3; Figure 9(B)). In the ε Hf (t) versus U-Pb age diagram (Figure 9(C)), zircons from these samples show a spread of ε Hf (t) values close to the depleted mantle evolution line, which are higher than those of the Triassic within-plate granitoids in the Eastern Tianshan (e.g. Donggebi) (Han et al. 2014;Zhang et al. 2015a). In contrast, the Chihu ε Hf (t) values are more similar to those of the Carboniferous to Permian intrusions in Eastern Tianshan (Figure 9(D)), such as the subductionrelated granites (e.g. Tuwu and Yandong) (Shen et al. 2012;Wang et al. 2015a;b), and the mantlederived mafic-ultramafic complexes (e.g. Xiangshan,  Defant and Drummond 1990). N means normalized to chondrite (Sun and McDonough 1989). Data for the Tuwu-Yandong tonalite are from Wang et al. (2015aWang et al. ( , 2015b, and Shen et al. (2012).
Huangshan, and Tulaergen) Su et al. 2011;Tang et al. 2012), indicating the same origin by partial melting of a subduction slab or lithospheric mantle materials. Based on the geochemical affinities and the relatively inhomogeneous Hf isotopic composition (3.4ε and 5.3ε units, respectively) recorded in the Chihu granodiorites and porphyritic granodiorites, we infer that they were most likely derived from the subduction-modified mantle rocks. The interpretation is also supported by the presence of coeval intermediate to mafic volcanic rocks in the Chihu area (Wang et al. 2006b;Wu et al. 2006c).
Zircons are highly retentive of the magmatic O isotopic compositions. It is known that zircons in equilibrium with mantle-derived magmas have a very consistent δ 18 O value of 5.3 ± 0.3‰ (Valley 2003;Valley et al. 2005;Li et al. 2009b). The δ 18 O value of igneous zircons is relatively insensitive to magmatic differentiation, because the increase in the bulk rock δ 18 O value is compensated by an increase in zircon/ liquid δ 18 O fractionation, so their O isotopes can be used to trace the parental magma source (Valley et al. 2005). In this study, the zircon δ 18 O values of the granodiorites vary from 4.80 to 5.85 ‰, indicating depleted isotopic signatures close to the depleted mantle (DM) reservoir (δ 18 O = 5.3 ± 0.3‰; Li et al. 2009b). However, zircons from the porphyritic granodiorites have significantly lower δ 18 O values between 3.78 and 4.71 ‰. The lower O isotopic features recorded in the porphyritic granodiorites might be attributed to the interaction with strongly modified mantle wedge peridotite by subduction-related processes beneath the Eastern Tianshan  or reflect high-level hydrothermal activity associated with Cu mineralization.
Taking the whole data set into consideration, it is proposed that the Chihu granodiorite and porphyritic granodiorite intrusions share the same signatures as I-type granites formed in the Dananhu-Tousuquan arc, and have broadly similar formation mechanisms and magmatic sources. The primary magmas for the granodiorite (320.2 ± 2.4 Ma) and porphyritic granodiorite (314.5 ± 2.5 Ma) were most likely generated by the partial melting of subduction-modified mantle components followed by fractionation to more felsic compositions.

Geodynamic setting
The Eastern Tianshan orogenic belt occupies the middle part of the CAOB, which is considered an important polymetallic ore province in China (Goldfarb et al. 2001(Goldfarb et al. , 2014Han et al. 2006;Mao et al. 2008;Huang et al. 2013;Pirajno 2013;Deng and Wang 2015). Previous studies have revealed that the Eastern Tianshan orogenic belt experienced a long and complex geodynamic evolution, involving subduction of the Palaeo-Tianshan Ocean, collisionaccretionary, strike-slip motion, post-collisional, and intracontinental extension between the Siberian and Tarim Cratons (Pirajno et al. 2011;Santosh et al. 2011;Xiao et al. 2013;Deng et al. 2014aDeng et al. , 2014b, during which widespread late Palaeozoic volcanic rocks and granitoids were emplaced (Figure 1(C); Zhou et al. 2010;Wang et al. 2015aWang et al. , 2015b. Studies on Carboniferous andesites and granitoids in Eastern Tianshan have shown that these rocks display a subduction-related component, as evidenced by positive bulk ε Nd (t) and zircon ε Nd (t) values (Sun et al. 2008;Tang et al. 2010;Su et al. 2012). Southward and northward subduction of the Palaeo-Tianshan oceanic plate could have occurred during the Carboniferous, with ages constrained by the subduction-related Yandong-Tuwu intrusions (335−332 Ma; zircon U-Pb; Wang et al. 2015aWang et al. , 2015b and Yamansu volcanics (324.4 ± 0.9 Ma; zircon U-Pb; Hou et al. 2014). There is a broad consensus that the Palaeo-Tianshan ocean was closed in the end of Carboniferous and the Eastern Tianshan entered into a post-collisional setting since the Early Permian, as supported by the presence of the youngest ophiolite of~310 Ma and widespread bimodal volcanic rocks of~290 Ma (Qin et al. 2002;Zhang et al. 2008;Chen et al. 2011;Su et al. 2012;Huang et al. 2013). Furthermore, Mao et al. (2005) and Zhang et al. (2008) also suggested that the extensive porphyry Cu mineralization and associated magmatism that occurred in Eastern Tianshan were genetically related to the island-arc accretionary processes during the period of Carboniferous.
SIMS zircon U-Pb dating (320.2-314.5 Ma) of the granodiorite and porphyritic granodiorite obtained in this study indicates that the igneous activities that occurred in the Chihu Cu deposit of Eastern Tianshan correspond to the subduction-island arc stage as defined by Zhang et al. (2008). During subduction tectonism, the largest Tuwu and Yandong porphyry Cu deposits were formed in the Eastern Tianshan orogenic belt, with the emplacement of subduction-related granitoid intrusions, represented by the quartz diorite, diorite porphyry, and tonalite Gao et al. 2015;Wang et al. 2015aWang et al. , 2015b. These intrusions and associated mineral systems were closely related to the northward subduction of the Palaeo-Tianshan ocean plate beneath the Dananhu-Tousuquan arc (Shen et al. 2014;Wang et al. 2015aWang et al. , 2015b, further suggesting the subduction environment for Carboniferous metallogeny and magmatism. In addition, on the Ta versus Yb, and Rb versus (Y + Nb) tectonic discrimination diagrams (Figure 12(A) and (B); Pearce et al. 1984), both the granodiorite and porphyritic granodiorite samples plot within the oceanic arc field, indicating that the Chihu intrusive rocks have characteristics of island arc granites that were formed in a subduction-related setting. Combined with regional tectonic evolution, and our new isotopic age and geochemical studies, we suggest that the Palaeo-Tianshan oceanic plate subducted northward beneath the Dananhu-Tousuquan arc belt during the Carboniferous (Figure 13), and that the Chihu granodiorite and porphyritic granodiorite intrusions were derived from partial melting of the mantle components, induced by the subduction processes.

Conclusions
(1) SIMS zircon U-Pb dating indicates that the Chihu granodiorite was emplaced at ca. 320 Ma, and the porphyritic granodiorite was emplaced at ca. 314 Ma. These granitoids were precisely dated in the Chihu area, corresponding to late Carboniferous magmatism in the Eastern Tianshan orogenic belt.
(2) Geochemistry indicates that the Chihu granodiorite and porphyritic granodiorite are calc-alkaline to low-K tholeiite rocks and show moderate enrichment in LREEs with I-type granite affinity. The geochemical and zircon Hf-O isotopic data indicate that these granitoids share a broadly common origin, probably derived from partial melting of the subduction-modified mantle components followed by fractionation.
(3) Based on the regional geological history, and new geochronological and isotopic data, we suggest that the Chihu granitoid intrusions in Eastern Tianshan were generated in an arc setting, and they most likely resulted from the northward subduction of the Palaeo-Tianshan ocean plate beneath the Dananhu-Tousuquan island arc during the Carboniferous.  during the fieldwork. We also appreciate the kind help of Professor Xian-Hua Li from the Institute of Geology and Geophysics, Chinese Academy of Sciences on SIMS zircon U-Pb dating and zircon O isotope analysis. Thanks to Professor Yu-Sheng Zhai from the China University of Geosciences (Beijing) for a helpful scientific review on the original manuscript.

Disclosure statement
No potential conflict of interest was reported by the authors.