Journal of Earth Science  2019, Vol. 30 Issue (2): 323-334   PDF    
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Geochronology and Petrogenesis of Mafic-Intermediate Intrusions on the Northern Margin of the Central Tianshan (NW China):Implications for Tectonic Evolution
Cai Hongming 1,2, Yang He 3, Gong Xiangkuan 1     
1. School of Geology and Mining Engineering, Xinjiang University, Urumqi 830047, China;
2. Geography Postdoctoral Research Station, Xinjiang University, Urumqi 830047, China;
3. Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China
ABSTRACT: Late Paleozoic mafic-intermediate intrusions are widespread in the central Tianshan belt, but their tectonic settings remain controversial. Petrogenesis of these intrusions can provide insights into the tectonic evolution of the belt and its adjacent areas. This study presents new whole-rock geochemical and zircon U-Pb geochronology and Hf isotopic data for the Yaxi diorite and Qianzishan gabbro on the northern margin of the central Tianshan (NW China). Zircon U-Pb dating yielded the magma crystallization ages of 313±4 and 295±4 Ma for the Yaxi diorite and Qianzishan gabbro, respectively. They have lower Mg# values, Ni and Cr contents than typical mantle-derived primary melt, with negative correlations between MgO, TFeO and SiO2 contents, indicating clinopyroxene and olivine fractionation during magma evolution. They are characterized by enrichment of large ion lithophile elements (e.g., Rb, Ba and Sr) and depletion of high field strength elements (e.g., Nb, Ta and Ti) with high Ba/Th and Rb/Y, suggesting that their mantle sources had been metasomatized by slab-derived fluids. In addition, the Qianzishan gabbro has high Al2O3 contents (19.54 wt.%-20.88 wt.%) and positive Eu anomalies (Eu/Eu*=1.09-1.42), which can be attributed to accumulation of plagioclase. Geochemical and zircon Hf isotopic compositions reveal that both the Yaxi diorite and Qianzishan gabbro were derived from depleted lithospheric mantle in the spinel stability field with insignificant crustal contamination. In association with previous investigations, we suggest that the Yaxi and Qianzishan intrusions were emplaced in a subduction-related environment, which means that the subduction of the Junggar Ocean lasted at least to the earliest Permian.
KEY WORDS: mafic-intermediate intrusions    geochemistry    petrogenesis    tectonic implication    central Tianshan    
0 INTRODUCTION

The Chinese Tianshan orogenic belt, situated between the Tarim Block to the south and the Junggar terrane to the north, occupies a crusial position in the southwestern central Asian orogenic belt (CAOB) (Zhang et al., 2016a). As the most essential part of the Chinese Tianshan orogen, the central Tianshan is a linkage between the Tarim Block and Junggar terrane and played a pivotal role in tectonic evolution of the orogen (Lei et al., 2011), which in turn is important in deciphering evolutionary history of the CAOB. Although numerous studies have been carried out on the Late Paleozoic tectonic evolution of the central Tianshan belt, the Late Paleozoic tectonic setting of the region is still debated. Several conflicting views have been proposed, including (a) passive continental margin related to the northward subduction of the Tianshan Ocean (Xiao et al., 2014), (b) continental arc setting resulted from the southward subduction of the Junggar Ocean (Zhang et al., 2016b; Zhang et al., 2015a; Tang et al., 2012), (c) related to the northward subduction of the south Tianshan Ocean (Sun et al., 2006), and (d) post collisional/orogenic setting (Zhang et al., 2016b; Ma et al., 2015; Dong et al., 2011; Chai et al., 2008). The uncertainty of the tectonic setting has limited our understanding on the tectonic evolution of the Chinese Tianshan belt and the CAOB.

Late Carboniferous to Early Permian mafic-intermediate intrusions are widely distributed in the central Tianshan belt, and could provide important constraints on tectonic regime of the region. Recently, many mafic-intermediate intrusions have been recognized on the northern margin of the central Tianshan belt (southwest of the Yamansu area, NW China). Previous field geological observations suggested that they might be the products of Triassic magmatism (Bureau of Geology and Mineral Resources of Xinjiang Uygur Autonomous Region, BGMRXUAR, 2013). However, this assertion lacks evidence from accurate isotopic chronology, which limits our understanding of their petrogenesis and tectonic settings. Therefore, two mafic-intermediate plutons (the Yaxi and Qianzishan) have been chosen for detailed zircon U-Pb isotopic study. Our results show that they formed in the Late Carboniferous to the earliest Permian, rather than the Triassic, suggesting that the formation ages of these assumed Triassic intrusions should be reassessed. Furthermore, we also present whole-rock geochemical and zircon Lu-Hf isotopic data for the Yaxi and Qianzishan plutons. The combined results are used to reveal the mantle source and petrogenesis, providing new constrains on Late Carboniferous to the earliest Permian tectonic setting of the central Tianshan belt, which therefore shed light on the tectonic evolution of the Chinese Tianshan orogenic belt.

1 GEOLOGICAL SETTING AND PETROGRAPHY

The Chinese Tianshan orogenic belt is traditionally subdivided from west to east into "western Tianshan" and "eastern Tianshan" roughly along longitude ~86°E (Zhang et al., 2016b), and they have different evolution history during Paleozoic time (Han and Zhao, 2018; Xiao et al., 2004). The focus of this study is related to the eastern Tianshan, which can be subdivided into the north (NTS), central (CTS), and south (STS) Tianshan belts, separated from each other by Weiya fault and Kumishi-Xingxingxia fault (Xiao et al., 2004) (Fig. 1). The NTS is mainly composed of Late Paleozoic magmatic arc (e.g., Harlik, Bogda, Dananhu) related to southward subduction of the Junggar Ocean beneath the eastern Tianshan. Closure time of the ocean has been a matter of debate in suggestions including Early Paleozoic (Dong et al., 2011; Wang et al., 2011), Middle Devonian–Early Carboniferous (Ma et al., 2014), Late Carboniferous–Early Permian (Zhang et al., 2016b; Zhang et al., 2015a; Chen et al., 2011) and Permian–Triassic (Xiao et al., 2009). The STS resulted from the collision between the Tarim block and CTS caused by closure of the south Tianshan Ocean (Han and Zhao, 2018). Although the closure time of the south Tianshan Ocean remains controversial (Han and Zhao, 2018; Zhang et al., 2017; Zhang et al., 2016b; Ma et al., 2014), recent researches suggested that its eastern part closed before Carboniferous (Zhang et al., 2016b; Su et al., 2011a, b ).

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Figure 1. Simplified geological map of eastern Tianshan showing the location of the study area (modified after Zhang et al., 2016b).

The central Tianshan is separated from the northern Tianshan by the Weiya fault and from the Southern Tianshan by the Kumishi-Xingxingxia fault, respectively (Fig. 1) (Zhang et al., 2016b). It is characterized by occurrence of Proterozoic metamorphic basement and overlying Cambrian-Carboniferous volcanic-sedimentrary successions (Zhang et al., 2016b; Lei et al., 2011). The basement is divided into the Xingxingxia, Kawabulag and Tianhu groups, which underwent a regional greenschist- to amphibolite-facies metamorphism (Xia et al., 2004; Hu et al., 1998). The volcanic-sedimentary strata are dominated by greenschists, slates, limestones and volcanic-siliciclastic rocks (Han and Zhao, 2017). Early Paleozoic to Early Mesozoic ultramafic to felsic intrusions are widely distributed in the region, with formation ages mainly during ca. 500 to ~250 Ma for the granitoid (Han and Zhao, 2018) and Late Carboniferous to Middle Triassic for the ultramafic to mafic intrusions (Su et al., 2012; Tang et al., 2011; Wang et al., 2008; Mao et al., 2006).

For the purpose of this study, samples were collected from the Yaxi and Qianzishan intrusions on the northern margin of the central Tianshan belt (Fig. 2).

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Figure 2. Geological map showing the occurrence of the Yaxi and Qianzishan intrusions (modified after BGMRXUAR, 2013).

The Yaxi intrusion occurs along the Weiya fault that extends for hundreds of kilometers (Fig. 2), with an outcrop area of ca. 3.8 km2. It intruded the Precambrian Xingxingxia Group and consists of Fe-V mineralized gabbro and unmineralized diorite. The contacts among the different rock units are transitional. The diorite is medium- to fine-grained in texture and gray in colour (Fig. 3a), mainly composed of plagioclase (45%–50%) and hornblende (40%–45%), with less clinopyroxene (5%–10%) and quartz (< 3%) as well as accessory amounts of zircon, apatite and magnetite (Fig. 3b). Clinopyroxene and plagioclase grains generally occur as euhedral to subhedral, and quartz is occasionally present as anhedral crystals.

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Figure 3. Representative field photos and photomicrographs of the Yaxi diorite (a, b) and the Qianzishan gabbro (c, d). Ol. olivine; Cpx. clinopyroxene; Amp. amphibole; Pl. plagioclase.

The Qianzishan intrusion is exposed ca. 7.8 km south of the Weiya fault, with an outcrop area of ca. 3.2 km2. It also intruded the Precambrian Xingxingxia Group, and mainly consists of medium- to fine-grained gabbro (Fig. 3c), which consists of clinopyroxene (40%–55%), plagioclase (40%–45%), hornblende (2%–5%), with traces of apatite, zircon, and Fe-Ti oxides (Fig. 3d). Plagioclase shows twinning, occasionally altered to clay minerals. Clinopyroxene is occasionally altered to chlorite.

2 ANALYTICAL METHODS

Whole-rock major and trace element analyses were carried out at ALS Chemex (Guangzhou) Co. Ltd.. For major element analyses, the prepared samples were fused with lithium metaborate-lithimu tetraborate flux, and then poured into a platinum mould. The resultant disk was in turn analysed by X-ray fluorescence spectroscopy. Analytical precision is better than 2%. For trace element analyses, two subsamples were prepared, one of which was digested with perchloric, nitric and hydrofluoric acids, and the residue is leached with dilute hydrochloric acid and diluted to volume. The solution is then analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). The other subsample was added to lithium metaborate flux, mixed well and fused in a furnace at 1 025 ℃. The resulting melt is then cooled and dissolved in an acid mixture containing nitric, hydrochloric and hydrofluoric acids. This solution is then analyzed by ICP-MS. According to the actual situation of the sample and the digestion effect, the comprehensive value is the final test results, with analytical uncertainties generally less than 10%.

Zircons were separated from samples YS16H01 and YS16H20, using conventional heavy liquid and magnetic separation techniques at Geological Service Ltd., Langfang. The zircon crystals were mounted in an epoxy mount and then polished to expose their centres. Pre-analytical cathodoluminescene (CL) images of all zircons were obtained to investigate their internal structures and to choose target sites for U-Pb and Hf isotopic analyses.

Zircon U-Pb isotopic analyses were performed using laser ablation multicollector ICP-MS at Institute of Mineral Resources, Chinese Academy of Geological Sciences (IMR-CAGS), Beijing. A spot size of 20 μm with a repetition rate of 8 Hz was used for all analyses. U-Th-Pb ratios were determined relative to the Plesovice standard zircon, and the absolute abundances of U, Th, Pb were determined using the NIST 610 standard glass. Detailed analytical procedures are reported in Hou et al. (2009). Isotopic ratios were calculated using ICPMSDataCal (Liu et al., 2010) with concordia diagram plotting and weighted mean age calculations using Isoplot (v.3.0) (Ludwig, 2003).

In-situ zircon Lu-Hf isotope measurements were also performed using LA-MC-ICP-MS at IMR-CAGS. Analytical spots were located close to the pits generated during U-Pb analyses or in the same growth domains as inferred from CL images. The analytical protocol used was the same as outlined in Hou et al. (2007). The analyses were undertaken using spot size of 40 μm, an 20 Hz repetition rate and a laser beam energy density of 8 J/cm2. Zircon 91500 and GJ-1 were used as the reference standard. The decay constant of 1.865×10-11 yr-1 for 176Lu was adopted (Scherer et al., 2001). Initial 176Hf/177Hf ratio, denoted as εHf(t), was calculated relative to the chondritic reservoir with a 176Hf/177Hf ratio of 0.282 772 and 176Lu/177Hf ratio of 0.033 2 (Blichert-Toft and Albarède, 1997). The single-stage Hf model age (TDM) was calculated relative to the depleted mantle with a present-day 176Hf/177Hf ratio of 0.283 25 and 176Lu/177Hf ratio of 0.038 4 (Griffin et al., 2000).

3 RESULTS 3.1 Zircon U-Pb Data

LA-MC-ICPMS zircon U-Pb dating results are listed in Supplementary Table S1 and illustrated in Fig. 5, inserted with representative zircon CL images.

Zircons from sample YS16H01 (Yaxi diorite) are mostly euhedral to subhedral prismatic crystals with a size range of 40 to 180 μm in length and length/width ratios of 1 : 1–2 : 1. These zircons show oscillatory zoning (Fig. 4a), indicating an igneous origin (Rubatto and Gebauer, 2000). Fifteen zircons were analyzed, showing Th of 63×10-6–2013×10-6, U of 160×10-6–2 299×10-6, with Th/U ratios of 0.17–1.96 (Table S1). Among them, two zircons with 206Pb/238U ages of 833 and 245 Ma were obtained. The older zircon is likely inherited grain, and the younger zircon is possibly related to the effects of later tectonothermal events. The remaining concordant zircons form a tight cluster on a Concordia and yield a weighted mean 206Pb/238U age of 313±4 Ma (MSWD=0.41, n=13; Fig. 4a), which is interpreted as the magma crystallization age of the Yaxi diorite.

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Figure 4. Zircon U-Pb Concordia diagrams inserted with representative computed laminography (CL) images of Yaxi diorite (YS16H01) (a) and Qianzishan gabbro (YS16H20) (b). U-Pb dating pits are depicted by smaller solid circles, and Lu-Hf analysis pits by larger broken circles.

Zircons from sample YS16H20 (Qianzishan gabbro) occurred as subhedral stubby prismatic crystals with a size range of 25–170 μm in length and aspect ratios of 1 : 1 to 2 : 1. In Computed Laminography (CL) images, most of the zircon grains show concentric zoning (Fig. 4b). For geochronology, nineteen analyses were obtained from 19 grains. They have variable Th (127×10-6 to 16 279×10-6) and U (233×10-6 to 9 416×10-6) concentrations with Th/U ratios ranging from 0.10 to 1.73 (Table S1). All the analyses are concordant, four of which yielded two 206Pb/238U age populations of 820 and 405–410 Ma and are interpreted as xenocrysts. The remaining concordant zircons form a consistent group with a weighted mean 206Pb/238U age of 295±4 Ma (MSWD=0.33, n=15; Fig. 4b), considered as the crystallization age of the Qianzishan gabbro.

3.2 Major and Trace Elements

Whole rock major and trace element data for the Yaxi and Qianzishan intrusions are given in Table 1 and plotted in Figs. 5, 6. All analyzed samples show sub-alkaline and calc-alkaline affinities.

Table 1 Major (%) and trace element (×10-6) compositions of the Yaxi diorite and Qianzishan gabbro
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Figure 5. (a) TAS (le Bas et al., 1986) and (b) AFM of the Yaxi and Qianzishan intrusions diagrams. Modified after Irvine and Baragar (1971), major oxides are in wt.% (water-free).
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Figure 6. (a) Primitive-mantle normalized trace element spider diagrams and (b) chondrite-normalized REE patterns of the Yaxi and Qianzishan intrusions. Primitive-mantle, Chondrite, OIB, N-MORB and E-MORB values from Sun and McDonough (1989).

The Yaxi diorite samples have a narrow range of chemical compositions, with SiO2 (55.31 wt.%–58.23 wt.%), TiO2 (0.54 wt.%–0.70 wt.%) and Al2O3 (15.58 wt.%–16.22 wt.%), typical of intermediate rocks. They are characterized by high Mg# values (~62), MgO (6.04 wt.%–7.52 wt.%), CaO (6.05 wt.%–6.66 wt.%) and Na2O (4.37 wt.%–5.04 wt.%), but low K2O (0.17 wt.%–0.91 wt.%) contents. In a trace element spider diagram, they show enrichment of Rb, Ba, Th and light REE values, with negative Nb, Ta, Ti anomalies (Fig. 6a). They have total REE contents of 46×10-6–52×10-6, displaying weakly right-inclined REE patterns ((La/Yb)N=3.61–4.24) and insignificant Eu anomalies (Eu/Eu*=0.89–1.18) (Fig. 6b).

The Qianzishan gabbro samples are mafic in nature, with lower SiO2 (45.93 wt.%–48.69 wt.%), Na2O (1.21 wt.%–1.83 wt.%) and K2O (0.15 wt.%–0.37 wt.%), but higher Mg# values (63–75), MgO (8.10 wt.%–9.29 wt.%), CaO (12.21 wt.%–15.96 wt.%) and Al2O3 (19.54 wt.%–20.88 wt.%) contents than the Yaxi diorite. Furthermore, compared to the Yaxi diorite, the Qianzishan samples have lower trace element and REE contents (Fig. 6). They also have negative Nb, Ta, Zr, Hf and Ti anomalies in a trace element spider diagram (Fig. 6a) and gently sloping REE patterns ((La/Yb)N=1.91–3.14, Fig. 6b), with positive Eu anomalies (Eu/Eu*=1.09–1.42).

3.3 Zircon Hf Isotope

In-situ zircon Lu-Hf isotope data are summarized in Table 2 and exhibited in Fig. 7. For all samples, the initial εHf(t) and TDM values are calculated at their magma crystallization ages. Ten spot analyses were obtained for the zircons from sample YS16H01, yielding 176Lu/177Hf ratios of 0.001 243 to 0.002 058, εHf(t) values of -0.8 to +5.1 and TDM values of 437 to 757 Ma. Six analyses from sample YS16H20 exhibit 176Lu/177Hf ratios of 0.000 524 to 0.002 548, εHf(t) values of +3.6 to +14.4 and TDM of 358 to 792 Ma. In Fig. 7, all analyses in this study plot below the depleted mantle evolution curve.

Table 2 LA-MC-ICPMS zircon Lu-Hf isotope data of the Yaxi diorite and Qianzishan gabbro
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Figure 7. Zircon εHf(t) versus age diagrams for Yaxi diorite and Qianzishan gabbro.
4 DISCUSSION 4.1 Fractional Crystallization and Crustal Assimilation

The Yaxi diorite samples have MgO of 6.04 wt.% to 7.52 wt.%, with Mg# values of 62–63, Ni of 113×10-6–127×10-6 and Cr of 407×10-6–462×10-6, indicating that their magma was dominantly mantle-derived. But their Mg#, Ni and Cr are still lower than that of typical mantle-derived primary melts (Mg#=73–81, Ni > 400×10-6 and Cr > 1 000×10-6, Wilson, 1989), suggesting that they must have experienced crystal fractionation (Zhang et al., 2016b). The effects of crystal fractionation can also be identified from the small variability of La/Sm ratios (2.46 to 2.95) (Sun et al., 2008). Fractionation of clinopyroxene and olivine is supported by the negative correlations between MgO, TFeO and SiO2 contents (unpublished data).

Similarly, the Qianzishan gabbro samples have Mg# values (63–75), Ni (18×10-6–130×10-6) and Cr (158×10-6–533×10-6) contents, slightly lower than those of typical mantle-derived primary melts (Wilson, 1989), indicating an evolved magma probably underwent fractionation of clinopyroxene and olivine (Zhang et al., 2016b), as evidenced by the negative correlations between MgO, TFeO and SiO2 as well (not shown). Noteworthy in the major element composition is their high Al2O3 contents (19.54 wt.%–20.88 wt.%), similar to high alumina basalts defined by (Sisson and Grove, 1993). Such high Al2O3 contents could be caused by preferential fractionation of olivine and pyroxene relative to plagioclase under effect of H2O (Sisson and Grove, 1993). Alternatively, the high Al2O3 contents can be explained using plagioclase accumulation (Wagner et al., 1995), which is evidenced by positive correlation between Eu/Eu* ratios and the Sr contents (not shown) and positive Eu anomalies (Eu/Eu*=1.09–1.42, Fig. 6b). Thus, we propose that the Qianzishan intrusion was not directly solidified from a primary mantle-derived melts, and underwent fractional crystallization of olivine and pyroxene and accumulation of plagioclase.

Crustal assimilation appears to be inevitable when mantle derived magmas ascend through continental crust (Watson, 1982) and modify element and isotope compositions (DePaolo, 1981). Both the Yaxi diorite and Qianzishan gabbro have low concentrations and narrow ranges of K2O (Table 1), suggesting minimal crustal contamination for these samples (Zhao and Zhou, 2007). Additionally, crustal materials have high SiO2, La/Sm, Th/Nb, and low Sm/Nd and Nb/Ta values. Thus, crustal contamination would result in positive correlations between SiO2 and La/Sm, SiO2 and Th/Nb, but negative correlation of SiO2 and Sm/Nd, SiO2 and Nb/Ta (Cui et al., 2017). However, this is not the case in the Yaxi and Qianzishan intrusions (Fig. 8), suggesting limited effect of the crustal contamination on element compositions. Furthermore, the studied samples have relatively high Ti/Zr (49.6 to 270.6) and Ti/Y (281.1 to 588.2) ratios, much higher than those of the typical crustal rocks (Ti/Zr < 30, Ti/Y < 200, (Wagner et al., 1995)), also precluding significant crustal assimilation (Zhang et al., 2016b).

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Figure 8. Plots of selected trace element ratios versus SiO2 for the Yaxi and Qianzishan intrusions. (a) La/Sm, (b) Th/Nb, (c) Sm/Nd, and (d) Nb/Ta. Symbols are the same as those in Fig. 5.

The narrow range of εHf(t) values (-0.8 to +5.1) for the Yaxi diorite further suggest the absence of crustal contamination (Fig. 7). On the other hand, The Qianzishan gabbro has variable εHf(t) values (+3.6 to +14.4), that can be attributed to effect of crustal contamination (Cui et al., 2017; Ma et al., 2017) or involvement of an asthenospheric mantle source (Zhang et al., 2016b). Considering their εHf(t) values are higher than those of the Yaxi diorite, and partly overlaps with those of the 290–280 Ma asthenosphere-derived gabbros from the CTS, such as Tianyu gabbro (εHf(t) of +3.6 to +7.6; Tang et al. (2011)) and gabbroic dykes from Weiya and Xingxingxia areas (εHf(t) of +3.0 to +9.8; Zhang et al. (2016b)), we interpret the scattered εHf(t) values as being due to involvement of asthenosphere mantle. However, the crustal contamination could not be excluded base on the zircon Hf isotope compositions alone.

Conclusively, the element compositions of the samples from Yaxi and Qianzishan intrusions have not been significantly affected by crustal contamination and thus can be used to fingerprint the nature of their mantle sources.

4.2 Mantle Sources

The Yaxi diorite and Qianzishan gabbro display εHf(t) values of -0.8 to +5.1 and +3.6 to +14.4, respectively. Their dominant positive εHf(t) values suggest that the intrusions were derived from Hf isotope depleted mantle sources. Such depleted mantle sources could be produced by previous melt extraction from the mantle wedge. Meanwhile, their trace element compositions are characterized by enrichment in large ion lithophile elements (LILEs: e.g., Cs, Rb, Ba and Sr) and LREE, and depleted in Nb, Ta and Ti (Fig. 6). Given the unsignificant effect of crustal assimilation on element compositions, the enrichment feature could be explained by subduction modification in the mantle sources (Su et al., 2011; Sun et al., 2008). The distinct types of modification could be discriminated by trace element ratios. Previous studies suggested that LILEs (e.g., Rb, Ba and Sr) and other mobile trace elemenets (e.g., U, Pb) are effectively transported by subduction-related aqueous fluid (Sun et al., 2008; Class et al., 2000; Elliott et al., 1997; Pearce and Peate, 1995).

In contrast, Th is transferred efficiently from the slab only when sediment melts are involved (Labanieh et al., 2012). Therefore, the magma sourced from the mantle metasomatised by slab-derived fluid should have high Ba/Th and Sr/Th ratios, whereas those derived from the sources modified by the subducted sediments should have an elevated Th content. For the Yaxi and Qianzishan plutons, all samples have high Ba/Th and Sr/Th ratios and relatively low Th contents (Figs. 9a, 9b), consistent with fluid-induced enrichment. The metasomatism features of their mantle sources may be further examined through a plot of Rb/Y versus Nb/Y (Zhao and Zhou, 2007) and Sr/Nd versus Th/Yb (Woodhead et al., 1998). As shown in Figs. 10c, 10d, the spots of the Yaxi diorite and Qianzishan gabbro lie parallel to the Rb/Y axis and the Sr/Nd axis, demonstrating that their mantle sources were metasomatized by aqueous fluids.

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Figure 9. Plots of (a) Ba/Th vs. Th, (b)Sr/Th vs. Th, (c) Rb/Y vs. Nb/Y and (d) Sr/Nd vs. Th/Yb for the Yaxi and Qianzishan intrusions. Symbols are the same as those in Fig. 5.
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Figure 10. Non-modal batch melting model for Gd/Yb vs. Dy/Yb (Tang et al., 2014). Ticks along the curves indicate the degree of melting in percent. Symbols are the same as those in Fig. 5.

In addition, The Yaxi diorite and Qianzishan gabbro are characterized by flat Chondrite-norimalized HREE patterns (Fig. 6), suggesting their generation at relatively shallower mantle level (Tang et al., 2014). This interpretation is further supported by the Gd/Yb and Dy/Yb ratios, which can effectively discriminate the fingerprint of spinel from garnet in the mantle source (Tang et al., 2014). All the samples have low Dy/Yb and Gd/Yb ratios (Fig. 10), which plot on the melting curve of spinel peridotite rather than garnet peridotite, indicating their derivations from small degrees (ca. 5%) of melting of upper mantle sources within the spinel stability field.

In summary, the Late Carboniferous to Early Permian mafic-intermediate intrusions were probably derived from the partial melting of subduction-metasomatized lithospheric mantle in the spinel stability field, which still preserves isotopically depleted feature.

4.3 Tectonic Implications

Late Carboniferous to Early Permian magmatism in the Tianshan orogenic belt have been interpreted as the products of (1) a mantle plume (Qin et al., 2011; Su et al., 2011a, b ; Xia et al., 2008; Zhou et al., 2004); (2) subduction-related magmatism (Zhang et al., 2016b; Zhang et al., 2015a; Su et al., 2012; Tang et al., 2012; Sun et al., 2006); and (3) post collisional/orogenic tectonothermal events (Zhang et al., 2016b; Ma et al., 2015; Zhang et al., 2013; Dong et al., 2011; Chai et al., 2008). The Yaxi and Qianzishan intrusions emplaced as small-volume stock and do not have OIB signatures (Fig. 6), ruling out a plume-related origin (Zhang et al., 2013; Zhao and Zhou, 2007). In fact, the mantle plume model is considered unlikely due to lacking of basic geological evidence, such as radiating mafic dike swarms and circular crustal uplift patterns (Zhang et al., 2016b; Ma et al., 2015; Wang et al., 2014; Tang et al., 2012; Shu et al., 2011b).

The Late Carboniferous Yaxi diorite (313 Ma) and the earliest Permian Qianzishan gabbro (295 Ma) presented here are calc-alkaline, enriched in LILEs and depleted in HFSEs, with significant negative Nb, Ta and Ti anomalies, suggesting their formation in an arc setting. This consideration is corroborated by their trace element compositions, mostly plotting into the arc-related fields in the Hf/3-Th-Ta diagram (Fig. 11a). Such a Late Carboniferous (313 Ma) arc system is consistent with the presences of (1) ca. 326 Ma Bayingou ophiolite in NTS (Xu et al., 2006), which means that the Junggar Ocean was not closed until the Late Carboniferous (Zhang et al., 2015b); (2) ~311 Ma Luotuogou Nb-enriched basalt derived by partial melting of the metasomatized mantle wedge, which was triggered by slab melts during subduction of Junggar oceanic crust (Wang et al., 2006); (3) arc-related intrusions distributed in CTS, such as ca. 324 Ma granitic gneisses (Zhang et al., 2015a), 312–310 Ma gabbros and diorites (Zhang et al., 2016b; Tang et al., 2012), and Late Carboniferous Alaskan-type mafic-ultramafic complex (Su et al., 2012), which were also interpreted as being due to southward subduction of the Junggar oceanic plate. However, this magmatic scenario is different from Sun et al. (2006), proposing subduction of the south Tianshan Ocean responsible for formation of the Late Carboniferous granitoid at the south side of the Weiya fault. Recently, Zhang et al. (2016b) and references therein suggested the closure of the eastern segment of the south Tianshan Ocean occurred in the Devonian. Combined with the absence of Late Devonian to Late Carboniferous subduction-related magmatic rocks in the southern central Tianshan (Xingxingxia area) and northern Beishan (Zhang et al., 2015a), we attribute the Late Carboniferous arc setting on the central Tianshan to subduction of the Junggar Ocean.

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Figure 11. (a) Hf/3-Th-Ta (Wood, 1980) and (b) 2Nb-Zr/4-Y (Meschede, 1986) diagrams for the Yaxi and Qianzishan intrusions. Data for 290–280 Ma central Tianshan (CTS) basaltic magmas are shown for comparison (Zhang et al., 2016b; Tang et al., 2011; Chai et al., 2008). Symbols are the same as those in Fig. 5. WPA. within plate alkaline basalts; WPT. within plate tholeiite basalts; P-type MORB. mid-ocean ridge basalts from plume influenced regions; N-type MORB. normal mid-ocean ridge basalts; VAB. valcanic arc basalts.

As for the earliest Permian Qianzishan gabbro (295 Ma), its arc environment is further supported by the following facts: (1) In basalt tectonic discrimination diagram (Fig. 11b), all the Permian samples plot in the field of N-type mid-ocean ridge basalts and volcanic arc basalts, distinct from the 290–280 Ma basaltic magmas also from the CTS (Zhang et al., 2016b; Tang et al., 2011; Chai et al., 2008), which mainly fall into the within-plate basalt fields and were interpreted as products of post collisional/orogenic events. Considering that their trace element compositons different from those of N-MORB (Fig. 6), an arc environment for them is reasonable; (2) The Qianzishan gabbro is characterized by higher Al2O3 (19.54 wt.%–20.88 wt.%) and relatively low Na2O (1.21 wt.%–1.82 wt.%) contents, similar to high alumina basalts erupted from the volcanoes of Aso and Towada in Japan (Hawkesworth et al., 1995). Such elevated Al2O3 contents are an inherent features of subduction related magmas, and high alumina basalts were considered as products of fractional crystallization in the presence of 2 wt.%–4 wt.% H2O released from the subducted oceanic slab (Hawkesworth et al., 1995). Additionally, the earliest Permian arc system is not in conflict with the strike-slip shearing of the nearby ductile shear zone, such as the Weiya fault (265 Ma; Shu et al. (2002)) and the Kangguer-Huangshan zone (260–247 Ma), which postdated the formation of the Qianzishan rocks and were considered to be response of collisonal or obique-collisional tectonics (Shu et al., 2011). However, the arc setting for the Qianzishan gabbro seems inconsistent with the presence of 295–280 Ma bimodal volcanic rocks from the Bogda area (Chen et al., 2011) and ~298 Ma high-K calc-alkaline granites from Harlik belt (Wang et al., 2009), which were considered to mark the initiation of the post-collision extension. This problem can be conciliated by different time of collision/accretion of the island arcs in the Junggar Ocean (Xiao et al., 2004). In this scenario, accretion of the Dananhu-Yamansu arc to the CTS (probably after the earliest Permian) could postdate accretion of the Harlik-Bogda belt to the Angara (Late Carboniferous).

Based on the above, and considering their emplacement into the CTS Precambrian basement, we propose that both the Late Carboniferous Yaxi diorite and the earliest Permian Qianzishan gabbro formed in a continental arc setting, implying that the arc setting on northern margin of the CTS persisted until ca. 295 Ma and subduction of the Junggar Ocean lasted at least to the earliest Permian.

5 CONCLUSIONS

Whole-rock geochemical and zircon U-Pb-Hf isotopic studies of the Yaxi diorite and Qianzishan gabbro from the northern margin of the central Tianshan belt lead to the following conclusions:

(1) The Yaxi and Qianzishan intrusions formed at 313 and 295 Ma, respectively.

(2) Both the Yaxi diorite and Qianzishan gabbro were derived from the partial melting of lithospheric mantle sources in the spinel stability field, which were modified by fluids from the subducted slab.

(3) The discoveries of the Late Carboniferous and the earliest Permian arc-related intrusions in this study, together with other arc-related magmatism in the central Tianshan belt, demonstrate that the arc setting on northern margin of the CTS persisted until ca. 295 Ma and subduction of the Junggar Ocean lasted at least to the earliest Permian.

ACKNOWLEDGMENTS

This research was supported by the Key Laboratory of Xinjiang Uygur Autonomous Region (No. 2016D03002), the National Natural Science Foundation of China (No. 41562010) and the China Postdoctoral Science Foundation (No. 2017M613257) and Doctoral Scientific Research Foundation of Xinjiang University (No. BS100127). Constructive comments from two anonymous reviewers and guidance from the editors are greatly appreciated. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-1205-6.

Electronic Supplementary Material

Supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-018-1205-6.


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