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Volume 30 Issue 6
Dec.  2019
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Petrogenesis and Geodynamic Implications of the Carboniferous Granitoids in the Dananhu Belt, Eastern Tianshan Orogenic Belt

  • This paper presents new LA-ICP-MS zircon U-Pb geochronology, whole-rock major and trace element geochemistry, and Sr-Nd isotopes systematically on porphyritic granitic and K-feldspar granitic intrusions from the Dananhu belt, eastern Tianshan orogenic belt (ETOB). Zircon U-Pb dating indicates that the porphyritic granitic and K-feldspar granitic plutons were formed at 357±3 and 311±3 Ma, respectively. The porphyritic granites show geochemical and isotopic characteristics (high SiO2, low MgO and Mg#, depleted Sr-Nd isotopic values (about 0.703 4 and 6.13, respectively), with Nb/Ta (13.3-14.7) and Zr/Hf (31.0-33.9) ratios) similar to those of the crustal-derived magmas. The above characteristics suggest they were probably originated from juvenile lower crustal materials. The K-feldspar granites also have high SiO2, low MgO and Mg#, depleted Sr-Nd isotopic values (0.703 3-0.704 6 and 4.41-5.67, respectively). But some trace elements contents vary widely, with variable Nb/Ta (12.7-22.7), Zr/Hf (21.3-36.1) and Nb/La (0.38-1.07) ratios, indicating that the K-feldspar granites were formed by partial melting of juvenile lower crustal materials with old crustal materials. Combined with previous data on Carboniferous granitoids in the Dananhu belt, we infer that all the Carboniferous granitic plutons in the Dananhu belt were most likely emplaced in an island arc environment (Dananhu arc). Subsequently, a tectonic transition from oceanic subduction to post-collisional extension probably occurred in the ETOB.
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Petrogenesis and Geodynamic Implications of the Carboniferous Granitoids in the Dananhu Belt, Eastern Tianshan Orogenic Belt

    Corresponding author: Xiaoping Long, longxp@nwu.edu.cn
  • 1. Shandong Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China
  • 2. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China
  • 3. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
  • 4. Department of Earth Sciences, The University of Hong Kong, Hong Kong, China

Abstract: This paper presents new LA-ICP-MS zircon U-Pb geochronology, whole-rock major and trace element geochemistry, and Sr-Nd isotopes systematically on porphyritic granitic and K-feldspar granitic intrusions from the Dananhu belt, eastern Tianshan orogenic belt (ETOB). Zircon U-Pb dating indicates that the porphyritic granitic and K-feldspar granitic plutons were formed at 357±3 and 311±3 Ma, respectively. The porphyritic granites show geochemical and isotopic characteristics (high SiO2, low MgO and Mg#, depleted Sr-Nd isotopic values (about 0.703 4 and 6.13, respectively), with Nb/Ta (13.3-14.7) and Zr/Hf (31.0-33.9) ratios) similar to those of the crustal-derived magmas. The above characteristics suggest they were probably originated from juvenile lower crustal materials. The K-feldspar granites also have high SiO2, low MgO and Mg#, depleted Sr-Nd isotopic values (0.703 3-0.704 6 and 4.41-5.67, respectively). But some trace elements contents vary widely, with variable Nb/Ta (12.7-22.7), Zr/Hf (21.3-36.1) and Nb/La (0.38-1.07) ratios, indicating that the K-feldspar granites were formed by partial melting of juvenile lower crustal materials with old crustal materials. Combined with previous data on Carboniferous granitoids in the Dananhu belt, we infer that all the Carboniferous granitic plutons in the Dananhu belt were most likely emplaced in an island arc environment (Dananhu arc). Subsequently, a tectonic transition from oceanic subduction to post-collisional extension probably occurred in the ETOB.

0.   INTRODUCTION
  • As a representative accretionary orogenic belt, the central Asian orogenic belt (CAOB) was collaged by accretion of oceanic islands, seamounts, island arcs, accretionary wedges, ophiolites and continental blocks from the Late Proterozoic to the Mesozoic (e.g., Du et al., 2019; Xiao et al., 2015; Windley et al., 2007; Sengör et al., 1993). The eastern Tianshan orogenic belt (ETOB) is the easternmost segment of the Tianshan Mountain Range in the southern CAOB (Fig. 1; Xiao et al., 2004). Voluminous Carboniferous to Early Permian granitic intrusions and volcanic rocks are extensively exposed in ETOB and have attracted much attention (e.g., Cai et al., 2019; Wang et al., 2018; Xiao et al., 2017; Zhang et al., 2017). The ETOB is also one of the most important Cu, Ni, Au metallogenic belts in China (Qin et al., 2011), in particularly several porphyry deposits are present in the Dananhu belt, including Tuwu-Yandong, Yanxi and Chihu (Wang et al., 2018, 2015; Xiao et al., 2017; Zhang F F et al., 2016; Han et al., 2006; Zhang L C et al., 2006). Numerous studies have been conducted for the Carboniferous magmatic rocks from these deposits, which suggests that mineralization is bound up with the Carboniferous granitoids (e.g., Wang et al., 2018; Han et al., 2006). Consequently, the Carboniferous granitoids are a remarkable subject to research the metallogenesis and tectonic background of the Dananhu belt, ETOB. The Early Permian magmatic rocks were generally accompanied by extension environment and characterized by A-type granites, mafic-ultramafic intrusions, and bimodal volcanic rocks (Shi et al., 2018; Zhang et al., 2017; Mao et al., 2014; Yuan et al., 2010). Nevertheless, the tectonic setting of the Carboniferous granitoids remains controversial, with proposed settings including island arc (Wang et al., 2018; Xiao et al., 2017), rift (Xia et al., 2004; Qin et al., 2002), and post-collision settings (Gu et al., 2006).

    Figure 1.  (a) Tectonic sketch map of the central Asia orogenic belt and North Xinjiang (after Gao et al., 2011). (b) Geological map of the eastern Tianshan orogenic belt (ETOB) (after Li et al., 2016).

    In this contribution, we provide some new geochronological, whole-rock geochemical and Sr-Nd isotopic data of the Carboniferous granites in the Dananhu belt from the ETOB. Combined with available data of Carboniferous granitoids from this belt, we attempt to employ the geochemical indicators to uncover the Carboniferous evolutionary process of the Dananhu belt, ETOB.

1.   GEOLOGICAL BACKGROUND AND SAMPLE DESCRIPTIONS
  • The Chinese Tianshan orogenic belt is located between the Tarim Block and Junggar Basin and was accreted by island arc assemblages, ophiolitic mélanges, oceanic plateaus, oceanic crusts, accretionary wedges and microcontinental fragments during the Paleozoic (Fig. 1a; Kröner et al., 2007; Windley et al., 2007; Xiao et al., 2004). Generally, it is divided into the eastern and western segments by the Urumqi-Korla Road (Fig. 1b). This study focuses on the ETOB which can be subdivided into the Harlik, Dananhu, Kangguer, Yamansu and the central Tianshan belts from north to south (Fig. 1b; Xiao et al., 2013, 2004).

    The Dananhu belt, with Harlik belt, was suggested to be a united arc system formed by Kangguer ocean northward subduction from Late Ordovician to Devonian (e.g., Du et al., 2018a; Zhang et al., 2018). The strata of Harlik belt are dominated by Carboniferous and Devonian flysch sediments and magmatic rocks, with minor clastic sediments, volcaniclastics and tuffs with Ordovician and Silurian ages (Du et al., 2018a; Yuan et al., 2010). The strata in the Dananhu belt predominantly consist of pyroclastic and volcanic rocks deposited in Ordovician to Devonian, with tuffaceous siltstone, turbidite and sandstone (Zhang et al., 2018; Qin et al., 2002).

    The Kangguer belt is mainly made up of Carboniferous volcanic rocks and sedimentary, while ophiolite fragments and Devonian volcanic strata are locally deposited (Wang et al., 2014; Xiao et al., 2004). Ophiolites are comprised of meta-basalts, serpentinized peridotites and gabbros, the ages for the gabbros were suggested to be 330-494 Ma (Liu et al., 2016; Li et al., 2008). To the south, the Yamansu belt predominantly comprises Carboniferous marine strata, including mafic to felsic volcanic rocks interbedded with fine-grained clastic sediments (Zhang et al., 2016). The Central Tianshan Block is characterized by Precambrian metamorphic basement (e.g., schists, granulites, amphibolites, and migmatites). Paleozoic arc volcanic, granites and volcaniclastic rocks are distributed over it (Huang et al., 2017; Hu et al., 2000).

    Samples for this study were collected in the Dananhu belt, with porphyritic granites (42°28′15″N, 92°27′11″E) from the Devonian Dananhu Formation in Heicaotan area and K-feldspar granites (42°15′52″N, 93°19′5″E) from the Carboniferous Qieshan Formation in the northeast of Chihu (Fig. 1b). The light pink porphyritic granites show porphyroid textures (Figs. 2a, 2c). They contain 45%-55% phenocrysts mainly comprising K-feldspar (55%-60%), quartz (30%-35%), and biotite (5%-10%). The matrix minerals are composed of quartz, K-feldspar, plagioclase, biotite, and minor accessory minerals. The K-feldspar granites are pink in color and characterized by equigranular texture with coarse-grained and medium-grained (Figs. 2b, 2d). They typically contain K-feldspar (35%-40%), quartz (30%-35%), plagioclase (15%-20%), biotite (~5%) and minor accessory minerals (< 5%).

    Figure 2.  (a), (b) Field photographs of the granitic plutons in the Dananhu belt. (c), (d) Microphotographs showing the textural characteristics of the granitic rocks as seen in the thin sections under cross-polarized light. Bt. Biotite; Kfs. K-feldspar; Pl. plagioclase; Q. quartz. The porphyritic granites contain 45%-55% phenocrysts mainly comprising K-feldspar (55%-60%), quartz (30%-35%), and biotite (5%-10%). The matrix minerals are composed of quartz, K-feldspar, plagioclase, biotite, and minor accessory minerals. The K-feldspar granites are characterized by equigranular texture. They typically contain K-feldspar (35%-40%), quartz (30%-35%), plagioclase (15%-20%), biotite (~5%) and minor accessory minerals (< 5%).

2.   ANALYTICAL RESULTS
  • The analytical methods of this study can be found in ESMI. The results of this study and data from literature can be found in ESMII (Tables S1-S3).

  • Zircons extracted from the porphyritic granite (X3ET372) show euhedral and prismatic crystals with length-to-width ratios (1 : 1 to 2 : 1). The distinct oscillatory zoning of the zircons suggests they belong to igneous zircons (Fig. 3a). Twenty analyzed zircons yield concordant 206Pb/238U dates from 351 to 362 Ma with a weighted mean age (357±3 Ma; MSWD=0.37) (Fig. 3a and Table S1). This age is suggested to be the emplacement period for the porphyritic granitic pluton.

    Figure 3.  Cathodoluminescence images of representative zircon and concordia plots of zircon U-Pb dating results for the (a) porphyritic granite (X3ET372) and (b) K-feldspar granite (X3ET459) from the Dananhu belt.

    Zircon grains of the K-feldspar granite (X3ET459) exhibit shapes similar to those from the porphyritic granite. They also show universal feature for igneous zircons characterized by oscillatory zoning (Fig. 3b). Twenty-one analyses show a tight cluster and give a weighted mean 206Pb/238U age (311±3 Ma; MSWD=1.11) (Fig. 3b and Table S1), which is considered as the crystallization time for the K-feldspar granitic pluton.

  • The Early Carboniferous porphyritic granites have high SiO2 concentrations between 72.8 wt.% and 73.8 wt.% (Table S2). They show high K2O (4.18 wt.%-4.64 wt.%) and high K2O+ Na2O (8.48 wt.%-8.77 wt.%) contents (Fig. 4a and Table S2). In the TAS plot, these rocks are sub-alkaline and fall in the field of granites (Fig. 4b). They have low Fe2O3T (1.69 wt.%-1.99 wt.%), TiO2 (0.24 wt.%-0.28 wt.%), MgO (0.37 wt.%-0.51 wt.%) and Mg# (30-34) values (Table S2). The porphyritic granites have low A/CNK ratios (0.97-1.02) and are metaluminous to slightly peraluminous (Fig. 4c). They have relatively low Sr (167 ppm-211 ppm), but display high Y (20.1 ppm-24.0 ppm) contents, and thus have low Sr/Y ratios (6.93-9.86) and are plotted within the normal andesite-dacite-rhyolite field on a Sr/Y versus Y diagram (Fig. 4d). These Early Carboniferous rocks show uniform REE and trace elements patterns (Figs. 5a, 5b). On the REE pattern diagram, they display LREE-enriched patterns ((La/Yb)N=5.04-7.34) with weak negative Eu anomalies (Eu/Eu*=0.40-0.47) and HREE fractionation ((Gd/Yb)N=1.19-1.35) (Fig. 5a). On the trace elements pattern plot, they exhibit Rb, K and Pb positive anomalies, with Nb, Ta, Sr and Ti negative anomalies (Fig. 5b). They have initial 87Sr/86Sr ratios (about 0.703 4) and depleted εNd(t) values (6.13), with juvenile TDM2 ages (614 Ma) (Table S3).

    Figure 4.  (a) K2O versus SiO2 diagram (after Irvine and Baragar, 1971); (b) TAS classification diagram (after Middlemost, 1994); (c) ANK versus ACNK diagram (after Maniar and Piccoli, 1989); (d) Sr/Y versus Y diagram (after Defant and Drummond, 1990). The data of "normal granites" are from Wang et al. (2018), Xiao et al. (2017) and Zhang F F et al. (2016). The data of adakitic granites are from Wang et al.(2018, 2015), Xiao et al. (2017), Zhang F F et al. (2016), Han et al. (2006) and Zhang L C et al. (2006).

    Figure 5.  (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized trace element spider diagrams. The data of chondrite and primative mantle are from Sun and McDonough (1989).

  • The Late Carboniferous K-feldspar granites also have high SiO2 (71.1 wt.%-71.4 wt.%), K2O (2.72 wt.%-2.80 wt.%) and K2O+Na2O (6.89 wt.%-7.20 wt.%) contents, but show low MgO (0.71 wt.%-0.78 wt.%) and Mg# (33-35) values (Table S2). They are confirmed to have a high-K calc-alkaline trend and plot within the granite field (Figs. 4a, 4b). The samples are metaluminous with low A/CNK ratios (0.99-1.00) (Fig. 4c). They have variable Sr (68.1 ppm-218 ppm) and Y (6.13 ppm-20.8 ppm) contents, with low Sr/Y ratios (7.37-11.1) and are plotted outside of the adakites field (Fig. 4d). Samples from the K-feldspar granites show flexible REE and trace elements patterns (Figs. 5a, 5b). They are characterized by LREE enrichment ((La/Yb)N=2.53-5.39), with insignificant Eu anomalies (Eu/Eu*=0.71-0.77) and weak depletion in HREE ((Gd/Yb)N=0.86-1.25) (Fig. 5a). On the trace elements pattern diagram, all the rocks show negative Rb, Th, Nb and Ta anomalies, and display positive Ba and Pb anomalies, but with different Sr, Zr, Hf and Ti patterns (Fig. 5b). They have initial 87Sr/86Sr values between 0.703 3 and 0.704 6, εNd(t) values between 4.41 and 5.67, with juvenile Nd model ages (TDM2=615-720 Ma) (Table S3).

3.   DISCUSSION
  • Before the geochemical compositions of porphyritic granites and K-feldspar granites can be used to constrain the petrogenesis and source of them, the effects of alteration should be evaluated. In fact, the effects of alteration on these samples are relatively minor, given their low loss on ignition (LOI; 0.40 wt.%-1.04 wt.%) and have consistent values for mobile large ion lithosphile elements (LILEs, such as Ba, K and Pb) (Table S2).

  • Petrogeochemical characteristics of intrusive rocks record critical evidence on magma source, evolutionary process, and tectonic background (e.g., Meng et al., 2019; Xiong et al., 2019; Eby, 1992; Whalen et al., 1987; Pearce et al., 1984). Hence, it is necessary to have a clear understanding about the petrogensis for the granites in this study. The Early Carboniferous rocks have high SiO2 content (72.8 wt.%-73.8 wt.%), but display relatively low MgO (0.37 wt.%-0.51 wt.%) and Mg# values of (30-34), with relatively low compatible trace elements contents, such as Cr (3.79 ppm-5.04 ppm), Co (1.96 ppm-2.67 ppm), and Ni (1.42 ppm-2.52 ppm) (Table S2), suggesting these rocks were not the products of directly partial melting by the mantle (Du et al., 2018b; Zhang et al., 2016). Moreover, the porphyritic granites lack of mafic microgranular enclaves or inherited zircons, which does not support the mixing model by mantle- and crustal-derived components (Zhang et al., 2017; Yuan et al., 2010). On the contrary, field observations and geochemical studies suggest partial melting of the mafic or intermediate lower crust may account for the origin of porphyritic granites with high SiO2 and low MgO contents (Wang et al., 2019; Patiño Douce, 1999; Rapp and Watson, 1995). Besides, these rocks exhibit negative Sr, Ba and Eu anomalies (Fig. 5), indicating various degrees of fractional crystallization were responsible for these rocks, such as both K-feldspar and plagioclase fractionation (Wu et al., 2002).

    In general, Nb with Ta, and Zr with Hf possess similar geochemical characteristics. Crustal-derived magmas have the average Nb/Ta and Zr/Hf ratios for 11.4 and 35.7, respectively (Rudnick and Gao, 2003). The Nb/Ta and Zr/Hf ratios of the porphyritic granites (13.3-14.7 and 31.0-33.9; Table S2) are similar to those of the crustal-derived magmas, reflecting a crustal origin. Moreover, the high Y/Nb ratios for the porphyritic granites (1.97-2.42; Table S2) also suggest the crustal origin (Y/Nb > 2; Eby, 1992). In addition, the Early Carboniferous granites have depleted (87Sr/86Sr)i (about 0.703 4) and εNd(t) (+6.13) values, with juvenile TDM2 ages (614 Ma) (Fig. 6 and Table S3), implying the source of juvenile lower crust rather than Precambrian basement or highly evolved upper crust.

    Figure 6.  εNd(t) versus (87Sr/86Sr)i diagram of the Carboniferous granitoids in the Dananhu belt. The data of "Normal granites" are from Xiao et al. (2017). The data of adakitic granites are from Wang et al. (2018), Xiao et al. (2017) and Zhang et al. (2006).

    The Early Carboniferous rocks display significant HFSEs (Nb, Ta, and Ti) depleted and LILEs (Rb, K and Pb) enriched (Fig. 5 and Table S2), indicating typical subduction-related features (McCulloch and Gamble, 1991). Previous studies suggested that crustal underplating materials (e.g., Zhang et al., 2016; McKenzie, 1989) or subducted oceanic slab components (e.g., Xiao et al., 2017; Defant and Drummond, 1990) both can provide the juvenile crustal components in subduction-related environment. Nevertheless, the Early Carboniferous samples possess low Sr/Y ratios and are plotted in the normal andesite-dacite-rhyolite field instead of adakite field (Fig. 4d and Table S2), suggesting significant slab melting is precluded. Consequently, we conclude that underplated juvenile lower crustal materials are considered as the source of the Early Carboniferous porphyritic granites magma.

  • The K-feldspar granites contain SiO2 (71.1 wt.%-71.4 wt.%), MgO (0.71 wt.%-0.78 wt.%) and with Mg# values of (33-35) (Table S2), similar to those of porphyritic granites, suggesting the K-feldspar granites may be also derived from juvenile lower crustal materials. This is confirmed by the positive εNd(t) values (4.41-5.67) and Nd model ages (TDM2=615 to 720 Ma) (Table S3) for these rocks. Garnet and/or hornblende may have been major residual phases in the origin of the K-feldspar granites. If garnet is a residual phase in the source, the resultant magmas will show strong HREE depletion. Nevertheless, the K-feldspar granites display relatively low (Gd/Yb)N ratios (0.86-1.25) and flat HREE patterns (Fig. 5a and Table S2) would argue against garnet as a residual phase. The K-feldspar granites show a progressive decrease in middle and heavy REEs with increasing atomic number (Fig. 5a), suggesting breakdown of amphibole in the source.

    However, they display flexible REE and trace elements patterns, as well as (87Sr/86Sr)i values (0.703 3-0.704 6) (Fig. 5 and 6). Some trace elements contents vary widely, such as Rb (19.3 ppm-62.2 ppm), Sr (68.1 ppm-218 ppm), Nb (3.41 ppm-6.01 ppm), Ta (0.15 ppm-0.47 ppm), Zr (26.0 ppm-148 ppm) and Hf (1.15 ppm-4.10 ppm) (Fig. 5 and Table S2), hence with variable Nb/Ta (12.7-22.7) and Zr/Hf (21.3-36.1) ratios. These data indicate the Late Carboniferous samples were likely originated from multiple source materials instead of pure underplated juvenile lower crustal components (Rudnick and Gao, 2003). This is also supported by plotting in the different fields on a Sr/Y versus Y plot (Fig. 4d). Accordingly, we infer that the Late Carboniferous rocks were originated from juvenile lower crustal materials with a significant addition of other components (old crustal materials).

  • Recent investigations have revealed that the Kangguer ocean north-dipping subduction formed the Late Ordovician to Devonian Dananhu arc (Du et al., 2018a; Zhang et al., 2018). Although according to previous studies, widespread Carboniferous magmatic rocks are exposed along the Dananhua belt (Wang et al., 2018, 2015; Xiao et al., 2017; Zhang F F et al., 2016; Han et al., 2006; Zhang L C et al., 2006; Xia et al., 2004; Qin et al. 2002), these rocks were considered to be formed in different tectonic backgrounds, such as rift (Xia et al., 2004; Qin et al. 2002), island arc (e.g., Wang et al., 2018; Xiao et al., 2017), and post-collision settings (Zhou et al., 2008; Gu et al., 2006).

    The geochemical characteristics of granitoids have generally recorded critical evidence on various tectonic settings (e.g., Meng et al., 2018; Pearce et al., 1984). In this study, we not only employ the geochemical and isotopic results for the porphyritic granites and K-feldspar granites, but also incorporate the obtainable data of previous reported Carboniferous granitoids in the Dananhu belt (Table S2). Tracing the changes of the Carboniferous granitoids can give crucial clues to constrain the evolutionary process of the ETOB. The Carboniferous granitoids can be subdivided into two types by Sr/Y and Y values (Fig. 4d), the adakitic granites and "normal granites". The porphyritic granites and K-feldspar granites are plotted outside the adakites field and belong to "normal granites". Both the adakitic granites and "normal granites" show typical subduction-related feature as evidenced by depletion of HFSEs and enrichment in LILEs (Fig. 5 and Table S2; McCulloch and Gamble, 1991). Although the Carboniferous adakites in the study area were suggested to be formed dominantly by subducted oceanic crust (e.g., Wang et al., 2018; Zhang et al., 2006), but the "normal granites" were mainly formed from juvenile lower crust (e.g., Xiao et al., 2017; Zhang et al., 2016; this study). All the Carboniferous rocks show depleted bulk (87Sr/86Sr)i and εNd(t) values (Fig. 6 and Table S3), suggesting subduction zones can be considered as favorable tectonic setting for the generation of these rocks (Wang et al., 2015; Tang et al., 2010). This consideration is reinforced that all granitoids plotted in the oceanic arc field on the Rb versus (Y+Nb) and Ta versus Yb tectonic discrimination diagrams (Fig. 7; Whalen et al., 1987; Pearce et al., 1984), indicating that the Dananhu belt Carboniferous granitoids possess island arc granites characteristics that were generated in a subduction-related setting.

    Figure 7.  (a) Rb versus Y+Nb and (b) Ta versus Yb diagrams for the Carboniferous granitoids (after Whalen et al., 1987; Pearce et al., 1984). COLG. Collision granite; VAG. volcanic arc granite; ORG. ocean ridge granite; WPG. within plate granite. The data of "normal granites" are from Wang et al. (2018), Xiao et al. (2017) and Zhang F F et al. (2016). The data of adakitic granites are from Wang et al.(2018, 2015), Xiao et al. (2017), Zhang F F et al. (2016), Han et al. (2006) and Zhang L C et al. (2006).

    Nevertheless, the Nb/La values for average island arc basalts (IAB) and ocean island basalts (OIB) are 0.34 and 1.30, respectively (Rudnick, 1995; Sun and McDonough, 1989). The rocks formed in subduction settings generally have low Nb/La values (< 0.71), whereas the rocks have high Nb/La ratios (> 0.71) are therefore taken to be indicative of an intraplate component, either in a lithospheric extension or mantle plume environments (Tang et al., 2017; Condie, 1999; Pearce and Peate, 1995). In this study, we employ the Nb/La values for Dananhu belt granitoids to monitor their source component throughout the Carboniferous time. From ca. 357 to 315 Ma, both the adakites and "normal granites" show low Nb/La ratios (< 0.71; Fig. 8) and probably reflect magmas generated in subduction related environments (Tang et al., 2017; Condie, 1999; Pearce and Peate, 1995). However, ca. 311 K-feldspar granites display variable Nb/La ratios (0.38-1.07), implying they were formed by subduction related materials with a significant addition of intraplate components. This is consistent with the above-mentioned petrogenesis of the K-feldspar granites that were generated by juvenile lower crustal materials with a significant addition of other components which are probably dominated by the intraplate components, and further reflecting some sort of tectonic transition.

    Figure 8.  Nb/La versus age (Ma) diagram for the Carboniferous granitoids in the Dananhu belt. The data of Nb/La values of average ocean island basalts (OIB), island arc basalts (IAB), intraplate component and subduction related component are from Sun and McDonough (1989), Rudnick (1995), Pearce and Peate (1995) and Condie (1999), respectively. The data of "normal granites" are from Wang et al. (2018), Xiao et al. (2017) and Zhang F F et al. (2016). The data of adakitic granites are from Wang et al.(2018, 2015), Xiao et al. (2017), Zhang F F et al. (2016), Han et al. (2006) and Zhang L C et al. (2006). Symbols are the same as Fig. 7.

    In fact, most of Carboniferous felsic rocks in the ETOB belong to I-type granitoids (e.g., Wang et al., 2018; Xiao et al., 2017; Zhang et al., 2016; this study). In contrast, A-type granitoids mainly occurred in the Early Permian (e.g., Du et al., 2018c; Yuan et al., 2010). Therefore, such a tectonic transition is further supported by the widely occurrence of ca. 310-280 Ma A-type granites, mafic-ultramafic intrusions and bimodal volcanic rocks in the ETOB, implying that the ETOB probably evolved into a post-collisional tectonic environment (e.g., Du et al., 2018b, c; Zhang et al., 2017; Yuan et al., 2010).

    Moreover, the report of radiolaria in the Dananhu belt and ophiolites in Kangguer belt also proves the Carboniferous island arc model. Yang et al. (1998) reported Devonian to Carboniferous radiolaria, and Li et al. (2003) discovered possible Late Silurian to Early Carboniferous radiolaria in chert. They both suggest a Carboniferous marine sedimentary environment in Dananhu belt. The Kangguer ophiolites nearby the Dananhu belt are the youngest ophiolites (ca. 330 Ma) in ETOB (Liu et al., 2016), which probably represent remnants of the Paleo-Tianshan Ocean.

    Collectively, all the Carboniferous granitic plutons in the Dananhu belt were most likely emplaced in an island arc environment. Subsequently, a tectonic transition from oceanic subduction to post-collisional extension probably occurred in the ETOB.

    Figure 9.  A schematic diagram for the Carboniferous tectonic evolution of the ETOB. Northwards subduction of the ancient Tianshan (Kangguer) oceanic lithosphere gave rise to the Dananhu arc.

4.   CONCLUSIONS
  • (1) The Dananhu belt porphyritic granitic and K-feldspar granitic plutons were emplaced at Carboniferous (357±3 and 311±3 Ma, respectively), corresponding to Carboniferous magmatism in the Dananhu arc, ETOB.

    (2) The Early Carboniferous porphyritic granites were originated from a juvenile lower crust. The Late Carboniferous K-feldspar granites were derived from juvenile lower crustal materials with a significant addition of old crustal materials.

    (3) Coupled with the geochemical and isotopic data of previous reported Carboniferous granitoids in the Dananhu belt, all the granitoids in this belt were most likely emplaced in an island arc environment.

ACKNOWLEDGMENTS
  • We are very grateful to the editors and the two anonymous reviewers for the constructive suggestions and comments which have greatly improved the manuscript. This study was supported by the National Key Research and Development Program of China (No. 2016YFC0601003), the National Natural Science Foundation of China (Nos. 41903031, 41421002), the China Postdoctoral Science Foundation (No. 2019M652431), and the National Basic Research Program of China (No. 2014CB440801). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1256-3.

    Electronic Supplementary Materials: Supplementary materials (ESMI-Analytical Methods; ESMII-Tables S1-S3) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1256-3.

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