2. Department of Geology, Modibbo Adama University of Technology, Yola P. M. B. 2076, Nigeria;
3. Institute of Mineralogy and Petrology, Faculty of Sciences, University of Zagreb, Zagreb HR-10000, Croatia;
4. Department of Geology, Faculty of Earth and Environmental Sciences, Bayero University, Kano, Nigeria;
5. Department of Mineral and Petroleum Resources Engineering, Kaduna Polytechnic, Kaduna, Nigeria
Several contrasting petrogenetic processes in an extension setting (within plate or post-collisional setting) give rise to a geochemically and minerallogically distinct group of granitoids generally dubbed as A-type (Grebennikov, 2014; Vilalva and Vlach, 2014; Peng et al., 2012; Nardi and de Fatima DallʼAgnol and de Bitencourt, 2009; Oliveira, 2007; Katzir et al., 2007; Litvinovsky et al., 2002; King et al., 1997; Eby, 1992). Minerallogically, these granitoids compose of iron-rich mafic silicate mineral such as hedenbergite, ferrohastingsite and annite as well as sodic pyroxene (e.g., aegirine) and sodic amphiboles (e.g., arfvedsonite and/or riebeckite). Chemically, these diverse groups of rock are generally characterized by remarkable enrichment in high field strength elements (HFSE) (e.g., Nb, Ta, and Zr), F, REE (Wang et al., 2018; Eby, 1992). Additionally, they have high alkali content, high FeOT/MgO and Al/Ga, but low CaO content compared to other granitoids (Whalen et al., 1987). Typically, they cover wide spectrum in composition ranging from strictly alkaline/peralkaline, metaluminous and occasionally, peraluminous (Bonin, 2007; Martin, 2006). These petrologically distinct granitoids (also referred to as A-type) are widespread both in space and time (e.g., Litvinovsky et al., 2002; Whalen et al., 1987). Largely due to their association with significant Nb, Ta, Sn, U and REE mineralization, A-type granites continue to attract attention from several researchers globally (e.g., Jiang et al., 2018; Li et al., 2018, 2014; Dostal and Shellnut, 2015; Shellnutt et al., 2009). However, in spite of significant advances recorded recently in applying experimental petrology, trace element and isotopic systematics in constraining the petrogenesis and geodynamic setting of igneous rocks, the genesis of A-type granitoids still remain highly controversial (e.g., Litvinovsky et al., 2015). The major controversy lies in the determination of the most suitable magma source for these granitoids (Bonin, 2007; Martin, 2006; Eby, 1992). Several petrogenetic models involving crustal, mantle or the direct mixing of these two distinct end members sources have been proposed for genesis of A-type granites (Litvinovsky et al., 2015, 2011; Dostal et al., 2014; Jahn et al., 2009; Martin, 2006; Wu et al., 2002; King et al., 1997; Patiño Douce, 1997; Turner et al., 1992). These A-type granites are more compositionally diverse than those were initially recognized by Loiselle and Wones (1979), who introduced the term "A-type" to denote granitoids that are mildly "alkaline" and "anorogenic"; the term "anhydrous" was later introduced by Bowden (1985), to highlight their low oxygen fugacity and water content. Importantly, the significantly high content of both large-ion lithophile elements (LILE) and HFSE in these rocks suggests that they are derived either from enriched OIB (oceanic island basalts)-like mantle source or from continental crust (Eby, 1992). However, isotopic composition of two enriched OIB mantle reservoirs: EMI and EMII typically overlap with those of continental crust (Zindler and Hart, 1986). Hence, some OIB mantle derived A-type granitoids could be misinterpreted as crustally derived (e.g., Litvinovsky et al., 2015). In this regard, more composite data and geological input are therefore required in constraining the origin of A-type granites (Wang et al., 2018; Litvinovsky et al., 2015; Dostal et al., 2014; Jahn et al., 2009; Shellnutt et al., 2009).
In this contribution, we present major and trace elements, mineral chemistry, Sr-Nd isotope as well as zircon U/Pb data of some alkaline granites and syenites from Suizhou-Zaoyang region within the Tongbai-Hongʼan-Dabie orogenic belt in Central China. The Huangyangshan and Guanzishan alkaline granitoids are arguably the least known in South Qinling orogenic belt. Our aim is to constrain the petrogenesis of the granitoids and infer their tectonic implication within the framework of geodynamic evolution of the Tongbai-Hongʼan-Dabie orogenic belt in Central China.1 GEOLOGICAL SETTING AND SAMPLE DESCRIPTION 1.1 Geological Setting
The Tongbai-Hongʼan-Dabie Orogen which is part of the E-W trending Qinling orogenic belt is outcropped between the North China Block (NCB) and Yangtze Craton (Dong and Santosh, 2016), linking Kunlun-Qilian Orogen to the west and Dabie-Sulu Orogen towards the east (Fig. 1a). Shangdan suture has subdivided the Qinling Orogon into south and north (Fig. 1a), with North Qinling believed to be part of the NCB, while the South Qinling as part of the Yangtze Craton before the Mianlue Ocean opened up in the Devonian (Wang et al., 2017; Zhang et al., 2001). During the Early Mesozoic, the NCB and the South China Block (SCB) merged after the closing of the Mianlue Ocean Basin (Dong and Santosh, 2016; Dong et al., 2011; Zhang et al., 2001). The Tongbai-Hongʼan-Dabie orogenic belt in the South Qinling is further divided into six tectonic units of different rock assemblages, separated by sutures and major fault systems such as Xiaotian-Mozitan suture, Xixian suture, Tanlu fault, Tongbai-Mozitan fault, Xiangfan-Guangji fault, Dawu fault and Shangma fault. These six tectonic units include the Beihuaiyang tectonic region, which comprised predominantly of Meso–Neoproterozoic Luzhengguan orthogneisses and Paleozoic metaclastic rocks with greenschist facies metamorphic characteristics. The South Dabie unit which is divided by Xiaotian-Mozitan suture has its components ranging from high-pressure (HP) amphibolites, eclogites and schist facies to ultra-high-pressure (UHP) eclogite facies (Zhu et al., 2017) rocks. Other tectonic units include Nanwan, South Hongʼan, North Tongbai and South Tongbai units (Fig. 1a). A major feature in South Tongbai tectonic unit is the Meso–Neoproterozoic Wudang uplift, which includes low-grade metamorphosed sedimentary-volcanic rocks of the Wudangshan and Yaolinghe groups (Wang et al., 2016). In South Qinling, many giant granitic and dioritic intrusions of Neoproterozoic Age, such as the Fenghuangshan and Douling plutons (Dong and Santosh, 2016) are overlain by the thick Sinian–Cambrian sedimentary sequences, Cambrian–Ordovician carbonate rocks, Silurian shale, Devonian–Carboniferous clastic sediments intercalated with limestone and limited Permian–Triassic sandstone (Zhang et al., 2001).
South Qinling orogenic belt has experienced widespread Early Paleozoic magmatism extending to the east from North Dabashan Mountains to Suizhou-Zaoyang regions. Mafic and intermediate outcrops constitute the dominant rock units in these areas including limited occurrences of carbonitite and syenitic complexes (Wang et al., 2017; Cao L et al., 2015; Cao Q et al., 2015; Xu et al., 2008; Zhang et al., 2007; Dong et al., 1998; Yu, 1992; Li, 1991). The Guanzishan and Huangyangshan alkaline granitoids occur in the southern margin of Tongbai Orogen outcropping within the South Tongbai tectonic zone and they form part of the intermediate to felsic rocks found in this region.1.2 Sample Description
The sampling locations are shown in Figs. 1b and 1c. The Huangyangshan pluton is covered in the southern part by Cretaceous meta-sedimentary rocks, comprising matrix supported conglomeration of different rocks (sandstone, limestone, marble, etc.). Huangyangshan pluton consists mainly of quartz syenites and very little occurrence of alkaline granite (Fig. 1b), whereas in Guanzishan area, nepheline syenite, quartz syenite and minor occurrence of alkali granites occur (Fig. 1c). The plutons from both areas are composed of fine-medium-coarse grained and porphyritic textures (Fig. 2). They range in color from dark to grayish brown containing amphiboles, quartz and K-feldspars under visual observation in hand specimen. The rocks in Huangyangshan are composed of K-feldspar (~45%), amphiboles (~25%), biotite (~20%) and plagioclase (~10%). The feldspars have undergone incipient alteration (Figs. 2a–2d). They range from subhedral to euhedral and include orthoclase feldspars and sanidine.
A small massive stock of Guanzishan nepheline syenites outcropped in Yulong Village. The rocks are medium-coarse grained nepheline-bearing syenites, with grayish-dark green color and massive structure. They compose of feldspars ~50%, biotite ~20%, amphibole ~20% and nepheline ~10% (Figs. 2e and 2f). Rocks in this area are altered and characterized by subhedral texture. A total number of eight samples have been used in this study. Six samples are from Huangyangshan Massif, while two samples are from Guanzishan. The distribution of samples from Huangyangshan and Guanzishan are shown as red stars in Figs. 1b and 1c, respectively.2 ANALYTICAL METHODS 2.1 Mineral Chemistry
The composition of amphiboles from Huangyangshan and Guanzishan plutons were determined at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, with a JEOL JXA-8100 electron probe micro analyzer equipped with four wavelength-dispersive spectrometers (WDS). Initially, the samples were coated with a thin conductive carbon film prior to analysis. The precautions suggested by Zhang and Yang (2016) were used to minimize the difference of carbon film thickness between samples and obtained a ca. 20 nm uniform coating. During the analysis, an accelerating voltage of 15 kV, a beam current of 20 nA and a 10 μm spot size were used to analyze the minerals. The ZAF procedure was employed in order to correct for the atomic number, absorption and fluorescence effects. Amphibole stoichiometry and nomenclature were determined based on recommendation by Hawthorne et al. (2012) using an Excel spreadsheet programmed for amphibole classification (Version 1.9, Locock, 2012) (Table 1).
Eight samples from Huangyangshan and Guanzishan alkaline granitoids were first crushed using steel crusher and subsequently pulverized to < 200 mesh size using an agate mill. Analysis of major element compositions was carried out at the ALS Laboratory, Guangzhou, using X-ray fluorescence (XRF) techniques. Analytical precision for major element varies from 2% to 5%. Trace elements were analyzed using Agilent 7700e ICP-MS at the Wuhan Sample Solution Analytical Technology Co. Ltd., Wuhan, China following the procedures outlined by Liu et al. (1996). International standard materials (e.g., AGV-2, BHVO-2, RGM-2 and RGM-2) were measured to monitor data quality during analysis, which show a correlative standard deviation of ±5%–10% for most of the trace elements.2.3 Zircon U-Pb Dating
Zircons from Huangyangshan (quartz syenite) and Guanzishan (nepheline syenite) were separated using conventional density and magnetic separation techniques and handpicked under a binocular microscope. The grains were subsequently mounted in epoxy resin, polished to half their thickness and they were later photographed in transmitted and reflected light. The morphology and internal structures of zircons were examined using cathodoluminescence (CL) imaging prior to U-Pb isotopic analysis. Zircon CL images were obtained at the Wuhan Sample Solution Analytical Technology Co. Ltd., Wuhan, China, using an analytical scanning electron microscope (JSM-IT100) connected to a GATAN MINICL system. Zircon U-Pb dating was conducted by the LA-ICP-MS method at the same laboratory, using an Agilent 7500a ICP-MS equipped with a 193 nm ComPex102-ArF laser-ablation system (Coherent Inc, USA). Helium was used as the carrier gas, and a spot size of 32 μm with a repetition rate of 6 Hz was applied to all analyses with a 10 J/cm2 energy density. Zircon 91500 was used as an external calibration standard for age calculation, while NIST SRM 610 and Plesovice were also used for quality control. All analyzed 207Pb/206Pb and 206Pb/238U ratios were calculated using ICPMSDataCal (Liu et al., 2010). The age calculations and concordia plots were made using ISOPLOT (Ludwig, 2003).2.4 Whole Rock Sr-Nd Isotopes
Whole rock Sr and Nd isotopic compositions of quartz syenite, alkaline granite and nepheline syenite from Huangyangshan and Guanzishan were determined using a micromass isoprobe multi-collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS) at the Qingdao Speed Analysis and Testing Company Limited, Shandong, China. Analytical procedures for Sr and Nd isotopes are described in detail by Li et al. (2004) and Wei et al. (2002). Chemical separation of Sr and Nd is similar to the methods described by Li and McCulloch (1998). Sample powder (∼50–100 mg) were digested with distilled HF-HNO3 in screw-top PFA beakers at 120 ℃ for 15 d. Sr and REEs were then separated using cation columns, followed by separation of Nd from the REE fraction using HDEHP columns. All isotopic data were analyzed by MC-ICP-MS. The 87Sr/86Sr value of the NBS987 standard and 143Nd/144Nd value of the JNdi-1 standard were 0.710 288±0.000 028 (2σ) and 0.512 109±0.000 012 (2σ), respectively; all measured 143Nd/144Nd and 86Sr/88Sr values were fractionated and corrected to 146Nd/144Nd=0.721 9.3 RESULTS 3.1 Amphibole Composition
The composition of amphiboles from the Guanzishan and Huangyangshan alkaline granitoids are presented in Table 1, Fig. 3. Owing to alteration in Guanzishan samples, only very few fresh points were analyzed. The amphibole-group minerals have a double silicate chain structure and a generic chemical formula of AB2CⅥ5TⅥ8O22(OH) (Leake et al., 1997). According to Leake et al. (1997) classification, calcic amphiboles generally have Ca+Na > 1 (apfu) in the B-site with Na being less than 0.5 (apfu) while sodic-calcic amphiboles typically have Ca+Na > 1 apfu in the B-site with Na ranging between 0.5 and 1.5 (apfu). High contents of Na in the B-site (> 1.5 apfu) and alkalis in the A-site (> 0.5 apfu) are indicative of sodic amphiboles. The amphiboles in the study area are subdivided into riebeckite, potassic arfvedsonite, ferro-eckermannite, ferro-ferri-winchite, ferro-katophorite and ferro-ferri-katophorite (Locock, 2012, Table 1). Majority of the amphiboles are sodic and only few of them fall within the range of sodic-calcic affinity (Fig. 4).3.2 Whole Rock Major and Trace Elements Composition
Major and trace elements data are presented in Table 2. Silica (SiO2) content in the rocks ranges from 59.35 wt.% to 72.20 wt.%. The granitoids are typically alkaline as shown on (Al2O3+CaO)/(FeOT+Na2O+K2O) vs. 100(MgO+FeOT+TiO2)/SiO2 discrimination diagram (Fig. 5a) and peralkaline in composition (Fig. 5b). The alkaline suites range in composition from quartz syenites to alkaline granites sensu-stricto; though one sample from Guanzishan area plots within the field of nepheline syenite as shown on the discrimination diagram proposed by De la Roche et al. (1980) (Fig. 5c). Similarly, on the 10 000×Al/Ga vs. Y and Na2O+K2O discrimination diagram proposed by Whalen et al. (1987), all the granitoids plot within the field of A-type granite (Figs. 6a and 6b). The potassium content in the granitoids is high (K2O=3.65 wt.%–6.78 wt.%) and in Na2O+ K2O-CaO vs. SiO2 diagram (Fig. 6c), they plot within the field of alkaline granites. According to the classification scheme proposed by Frost et al. (2001), the granites are ferroan and alkalic in composition (Fig. 6d) similar to the granites in Gardar Province, South Greenland (Frost et al., 2001).
In the primitive mantle multi-element normalized diagram, the granitoids are characterized by enrichment of both LILE such as Rb, Th, La, Ce, and Nd and HFSE such as Nb, Ta and Zr (Figs. 7a and 7b) which is typical of rocks derived from OIB-like sources (Eby, 1992). Additionally, in the Yb/Ta vs. Y/Nb and Ce/Nb vs. Y/Nb binary plots, the syenite and peralkaline granites from both Guanzishan and Huangyangshan plot within OIB field (Figs. 7c and 7d). Similarly, some coeval mafic rocks (e.g., Ziyang-Zhenba gabbro and diabase) and some rock units with similar chemical composition such as Mogou and Sandaogou syenite from South-Central China also plot within or near a field typical of OIB derived rocks (Figs. 7c and 7d). Negative Ba and Sr in Guanzishan and Huangyangshan syenites and granites (Fig. 7a) suggest feldspar fractionation. Furthermore, the syenites and the granites also show enrichment in LREE but depletion in HREE with high (La/Yb)N values (5.97 to 16.6) and a pronounced negative Eu anomaly (Fig. 7b).3.3 Zircon U-Pb Ages
The results of LA-ICP-MS U-Pb dating are presented in Table 3. Thirty zircon grains were analysed from Guanzishan nepheline syenite (16SZ06-01) and another twenty-three zircon grains from Huangyangshan quartz syenite (16SZ03-05). The zircons are generally pale yellow and transparent and are subhedral to euhedral in shapes. However, some few zircons are colorless and prismatic in shape. The zircons range in sizes from 125 to 290 μm (Figs. 8a and 8b). They generally exhibit oscillatory zoning in CL images and have Th/U ratios ranging from 0.3 to 0.9 typical of magmatic zircon (Belousova et al., 2002). Sample 16SZ06-01 is nepheline syenite from Guanzishan and a total number of thirty zircon spots were analyzed from this sample. They plot on a concordia diagram with a weighted mean 206Pb/238U age of 450±3 Ma (MSWD=2.5) (Figs. 8c and 8d). Sample 16SZ03-05 is a medium to coarse-grained quartz syenite from Huangyangshan. Results of analyses of eight zircon spots with low degree of discordance (less than 10%) from this sample yielded 206Pb/238U ages varying from 430±6 to 456±9 Ma, and plot on a concordia diagram, with a weighted mean age of 441±7 Ma (MSWD=1.5) (Figs. 8e and 8f).
Whole rock Sr and Nd data are presented in Table 4. The 87Rb/86Sr ratio is rather high in all the rocks analyzed. The Guanzishan nepheline syenites and alkaline granites have 87Rb/86Sr values of 3.74 and 19.77, respectively, while the Huangyangshan alkaline granites have 87Rb/86Sr values ranging from 48.28 to 140.84, respectively. Such high 87Rb/86Sr ratios in the rocks often create some uncertainties in calculating ISr values (e.g., Feng et al., 2014; Jahn et al., 2009, 2004, 2000; Wu et al., 2002). Hence, discussion regarding source constrains of the granitoids is centered mainly on whole rock Nd isotope as well as major and trace elements compositions. The 143Nd/144Nd in Guanzishan nepheline syenite and alkaline granite are 0.512 544 and 0.512 521, respectively, while in Huangyangshan pluton the 143Nd/144Nd ranges from 0.512 544 to 0.512 66, respectively. The values of ɛNd(t) in Guanzishan nepheline syenite and alkaline granite are +1.81 and +2.26, respectively and the calculated two-stage model age for these rocks are 1 040 and 1 003 Ma, respectively (Table 4). This ɛNd(t) is close to the values recorded from Wudang mafic dikes in South Qinling (Table 4). On the other hand, the Huangyangshan alkaline granite has ɛNd(t) values ranging from +2.61 to +3.46 and a relatively younger two-stage Nd model age values ranging from 906 to 975 Ma, respectively.
The mineralogical composition of the syenite-granites association from Suizhou-Zaoyang region, comprising of alkaline amphiboles such as riebeckite, coupled with their significant enrichment in high field strength elements such as Nb, Zr, Y, as well as their high REE content (except Eu), when compared to S-type and I-type granitoids suggest that the granitoids are typically syenite and A-type (e.g., Eby, 1992). Similarly, the nepheline syenite associated with Suizhou-Zaoyang region A-type suites is also characterized by high content of Zr, Y and REE (except Eu). According to Whalen et al. (1987) it is generally very difficult to distinguish A-type granite from highly fractionated I-type granitoids owing to the fact that the mineralogical and chemical composition of these two distinct granitoids could overlap. However, the characteristically high content of Ga/Al ratio, high Na2O+K2O and FeOT(FeOT+MgO) (Figs. 6a–6d) further suggest that the granitoids under investigation are A-type (e.g., Wu et al., 2002) and nepheline syenites. Additionally, the granitoids are depleted in P2O5, Sr, Ba and Ti when compared to typical S-type granitoids, which are always peraluminous. The Guanzishan and Huangyangshan nepheline syenites and granites association were emplaced between 450±3 and 441±7 Ma, respectively, which correspond to Late Ordovician to Early Silurian Period (Fig. 8). The LA-ICP-MS zircon U-Pb age of Huang- yangshan determined in this study is quite similar to the SHRIMP zircon age (439±6 Ma) determined by Ma et al. (2005). This age is however, significantly higher than the Rb/Sr isochron age of 215 Ma reported by Qiu (1993).4.2 Magma Source
Peralkaline A-type granites and syenites generally form at anomalously higher temperature than other granitoids, which imply that mantle derived magmas generally play significant role in their genesis (e.g., Wu et al., 2002; King et al., 1997; Turner et al., 1992). In this respect, fractional crystallization of mantle derived magmas has been considered as important mechanisms for the genesis of peralkaline A-type granites and syenites (Litvinovsky et al., 2015; Shellnutt et al., 2009; Bonin, 2007; Shellnutt and Zhou, 2007; King et al., 1997; Turner et al., 1992). However, peralkaline A-type granitoids are generally characterized by anomalous enrichment of HFSE as well as REE, which is the hallmark of rock derived from enriched/metasomatised mantle (Shellnutt et al., 2011; Bonin, 2007; Upton et al., 2003; Eby, 1992, 1990; Sutcliffe et al., 1990). Nonetheless, models involving partial melting of sialic crustal materials and/or fusion of crustal materials modified by enrichment of LILE and volatiles at higher pressure have also been proposed for the origin of peralkaline A-type granitoids (Martin, 2006; Lubala et al., 1994). For instance, Collins et al. (1982) argued that A-type granite could be derived through partial melting of chemically depleted (restitic) lower crustal sources. However, A-type granites derived through the above process are generally peraluminous rather than peralkaline in composition (e.g., Martin, 2006). Alternatively, Martin (2006) proposed a model involving melting of "fenitized" lower crustal source for the formation of A-type granite. But experiment carried out by Litvinovsky et al. (2000) has revealed that syenitic magmas cannot form through melting of sialic crustal material even at pressures as high as 15 to 25 kbar. According to Litvinovsky et al. (2015), peralkaline granites and syenites characterized by high K2O and Na2O are generally derived from K-rich basaltic or andesitic magmas with negligible crustal input.
Elements Nb, Ta and Ti have useful application in differentiating crustal derived rocks from rocks that formed from mantle magmas as well as in tracing contamination of mantle derived rocks via crustal assimilation (e.g., Wang et al., 2017; Niu and OʼHara, 2009). This stem from the fact that rocks that formed from melting of crustal material generally showed significant depletion in Nb, Ta, and Ti when compared to rocks that formed from mantle derived magmas (e.g., Niu and OʼHara, 2009; Rudnick and Gao, 2003; Taylor and McLennan, 1985; Taylor, 1977). This suggests a genetic link between the formation of continental crust and subduction related (island arc) magmatism (Taylor and McLennan, 1985; Taylor, 1977). The absence of negative Nb and Ta anomalies in peralkaline granites and syenites from both Guanzishan and Huangyangshan is therefore inconsistent with rocks derived from melting of crustal sources. Negative Ti anomalies in these rocks point to magmatic evolution involving the fractionation of hornblende and Fe-Ti oxide. The geochemical characteristics of the syenites and granites such as their high K2O and Na2O and low Y/Nb and Y/Ta ratio < 2 are typical of A-type suites that are commonly considered as derivatives of enriched OIB mantle sources (Bonin, 2007; Eby, 1992, 1990). Similarly, ratios of some LILE and HFSE such as Th/Ta, Th/Nb, Rb/Nb, Ba/Nb generally reflect the source materials from which igneous rock were derived (e.g., Shellnutt et al., 2009). This is because these trace elements remain largely immobile throughout the course of magmatic differentiation (Wang et al., 2017; Shellnutt et al., 2009; Shellnutt and Zhou, 2007). Binary plots of these trace element ratios revealed that the peralkaline granites of both Guanzishan and Huangyangshan plutons were largely derived from an enriched mantle sources (Figs. 9a–9e). Typically, mantle derived rocks generally have lower Th/Ta ratio≈2 compared to upper crust (Th/Ta≈6.9) or lower crust (Th/Ta≈7.9) (Shellnutt et al., 2009; Rudnick and Gao, 2003). The average Th/Ta values obtained from the samples of peralkaline granitods are approximately 2 (Fig. 9c), which further suggests that the peralkaline granitoids were most likely derived from the mantle. Furthermore, the peralkaline Guanzishan pluton yielded the ɛNd(t) values ranging from +1.81 to +2.26 (Table 4). These slightly positive ɛNd(t) values suggest an origin from enriched mantle source (e.g., Feng et al., 2014; Jahn et al., 2009; Winter, 2001). Importantly, the peralkaline nature of the rocks coupled with their OIB-like geochemical characteristic (Figs. 7 and 9) are also compatible with rocks that were derived from enriched mantle sources (Shellnutt et al., 2009; Bonin, 2007; King et al., 1997; Eby, 1992). Additionally, the high K2O content in the granitoids is consistent with rocks derived from EM mantle sources (Jackson and Daskupta, 2008). In the plots of ɛNd(t), Nb/U vs. ɛNd(t) and ɛNd(t) vs. age (Ma), peralkaline granitoids from Guanzishan and Huangyangshan both plot in the field of EM2 mantle derived rocks (Figs. 10a, 10b and 10c).4.3 Petrogenesis
Fractional crystallization of alkaline transitional or theolitic basalt with minimal or no crustal assimilation have been proposed as the main petrogenetic processes that give rise to peralkaline alkali-calcic to alkali granite and syenite (Frost and Frost, 2011; Eby, 1992, 1990; Loiselle and Wones, 1979). The positive ɛNd(t) values in nepheline syenites and alkaline granites from Guanzishan and Huangyangshan plutons indicate that they probably formed from fractional crystallization of basic magmas. In this respect, significant depletion in Ba, Sr, P, Ti and Eu indicate fractionation of feldspars, biotite, apatite, amphiboles and Ti-rich augite. According to Barker (1987), fractionation of olivine, Ti-rich augite and plagioclase from an alkaline basic magma can give rise to silica-undersaturated nepheline normative or silica oversaturated quartz-normative evolved magmas.
Amphibole group minerals play an important role in ensuring transition from SiO2 undersaturated to SiO2 saturated melt (Martin, 2007). Amphiboles in the syenites and peralkaline granites investigated show variation from sodic-calcic to sodic in composition (Fig. 4a). The absence of SiO2 poor calcic amphiboles implies that they were removed from depth to form a cumulate (Papoutsa and Pe-Piper, 2014; Martin, 2007; Giret et al., 1980). The presence of nepheline normative syenite in Guanzishan pluton suggests that this rock probably formed from fractional crystallization of mafic magmas. Similarly, the variation in amphibole composition from riebeckite to arfvedsonite in Huangyangshan quartz syenite and granite which is marked by decreasing CaO, MgO and Mg# contents and increasing SiO2 (Table 2), is typical of magma evolution through fractional crystallization (Pe-Piper, 2007; Giret et al., 1980). Plot of Fe2+/(Fe2++Mg2+) vs. Al show that the rocks were formed under the condition of low oxygen fugacity (Fig. 4b), which is typical of amphiboles found in A-type granites (Papoutsa and Pe-Piper, 2014).
Wang et al. (2017) established a genetic link between mafic, ultramafic and nepheline synenite in Guanzhishan via a liquid line of decent involving fractional crystallization and/or assimilation fractional crystallization (AFC) of OIB mantle derived magmas. By contrast, we obtained an average U/Pb age of 450 Ma from Guanzishan nepheline syenite which is slightly older than the age of mafic rocks (Gaotan diabase 440 Ma and Banjiuguan gabbro 439 Ma) as reported by Wang et al. (2017) and a little younger than the Wudang mafic dikes 460 Ma (Nie et al., 2016). Apart from that, the Guanzhishan nepheline syenites are geochemically more enriched in K2O than the Gaotan diabase (Fig. 7a). This further negates the hypothesis involving the derivation of geochemically more evolved nepheline syenite magmas from fractional crystallization of the original mafic magma from which the Gaotan diabase was formed. This is because fractional crystallization that resulted in strong Ba depletion (Fig. 7a) involving either K-feldspar or biotite fractionation is expected to have resulted in K2O depletions in Guanzhishan nepheline syenites relative to Gaotan diabase. On the contrary, K2O is even more enriched in nepheline syenites and granites than in Gaotan diabase. More so, the possibility of crustal assimilation that could lead to increase in K2O in Guanzhishan nepheline syenites and peralkaline granitoids from Huangyangshan pluton has also been ruled out based on both trace element and ɛNd(t) data. The mean U/Pb age obtained from Huangyangshan (441 Ma) is consistent with the 440 to 439 Ma peak of alkaline magmatism in Gaotan diabase and Banjiuguan gabbro, respectively (Wang et al., 2017). We therefore infer that the Guanzhishan nepheline syenites and granites were formed from fractional crystallization of OIB-like basic magmas derived from upwelling of metasomatized lithospheric mantle during the Late Ordovician extension. The emplacement of such granitoids was probably controlled by some deep seated fault and/or shear zones. The Huangyangshan quartz syenite and granite on the other hand probably formed from upwelling and subsequent assimilation fractional crystallization of enriched aesthenospheric mantle derived magmas. The formation of these granitoids therefore marked the main period of rifting in the Early Silurian, which facilitated significant mantle upwelling (e.g., Ma et al., 2005). This period also marked the peak of magmatism in South Qinling (Wang et al., 2017).4.4 Tectonic Implication
A-type granitoids are generally emplaced in extensional environment: post-collisional or anorogenic (Bonin, 2007; Wu et al., 2002; Eby, 1992, 1990; Whalen et al., 1987). In a broad sense, Eby (1992, 1990) subdivided A-type granite into two major groups: namely, A1 and A2 based on their peculiar trace element compositions especially their Nb/Y ratio and Nb-Y-Ce contents. According to the authors, the A1 subgroups are products of fractional crystallization of basaltic magmas derived from OIB related sources and are generally emplaced in anorogenic setting such as continental rift or intraplate settings (e.g., Eby, 1992). The A2 groups on the other hand are associated with post-collisional or post-orogenic settings and are generally derived from subcontinental lithosphere or lower crust. Without exception, the peralkaline syenites and alkaline granites from Suizhou-Zaoyang region plot within the A1 field (Figs. 11a and 11b). Typically, all the granitoids from Suizhou-Zaoyang region plot within the same field in Pearce et al. (1984) tectonic discrimination diagram (Figs. 11c and 11d) which implies that they are within plate anorogenic granites associated with crustal extension and/or rifting. Importantly, the granitoids have Y/Nb and Y/Ta ratio < 2 (Fig. 7c) which further suggest that they were mainly derived from O1B related mantle sources (Eby, 1992). Additionally, the ferroan and alkalic features of the granitoids (Figs. 6c and 6d) are also typical of granitoids of the A1 group (e.g., Dall'Agnoll et al., 2012), which are notably different from A2 granite that are generally alkali-calcic. Similarly, coeval mafic rocks and some younger perakaline suites with similar chemical composition from Mogou and Sandaogou also plot within the field typical of OIB derived rock. We therefore infer that peralkaline granitiods from Suizhou-Zaoyang region and some plutons with comparable composition such as Mogou and Sandaogou were mainly derived from OIB magmas with low contribution from ancient crustal sources. These rocks are linked directly with the coeval mafic rocks in the area (e.g., Wang et al., 2017). Previous studies (e.g., Wang et al., 2017; Ma et al., 2006, 2005, 2004) have revealed that the Dabie orogenic belt has experienced complex tectonic history especially during the Late Ordovician to Late Cretaceous periods. During the Early Paleozoic, while the subduction of the ancient ocean was taking place towards the south (Fig. 12), concurrently, the northern margin of Yangtze Craton began to break (Xu et al., 2008; Ma et al., 2005), resulting into back arc extension in the southern fringe of Qinling-Dabie (Ma et al., 2006). The products of magmatisms that resulted from the subduction of this ancient Qinling Plate in the north and the subsequent back arc extension/rifting from the south (Fig. 12) are considered as typical example of "paired magmatic belts" (Ma et al., 2006). This short period of extension and subsequent rifting prompted the emplacement of A-type granites (Wang et al., 2017; Ma et al., 2005). Extension during the Late Ordovician triggered the upwelling of small volume of metasomatized lithospheric OIB-like melt (e.g., Wang et al., 2017). The Guanzishan nepheline syenite and A-type granite formed from the evolved fraction of these melts (Fig. 12). The emplacement of these rocks was probably controlled by some deep-seated fault/shear zones. Extension and subsequent rifting continued into Early Silurian Period (Ma et al., 2006, 2004). This period facilitated the upwelling of metasomatized asthenosphere-derived OIB sources and the emplacement of mafic rocks such as the Gaotan diabase (Fig. 7). The Huangyangshan peralkaline quartz syenites and alkaline granites probably formed from the same mafic magma through assimilation fractional crystallization (Fig. 12).5 CONCLUSION
Peralkaline A-type granitoids and nepheline syenites were episodically emplaced in Suizhou-Zaoyang region between 441±7 and 450±3 Ma, which corresponds to Early Silurian and Late Ordovician periods, respectively. All the A-type suites and nepheline syenites showed affinity for within plate setting hence, they are anorogenic senso-stricto. The emplacement of the granitoids marked important periods of extensions and/or rifting from Late Ordovician to Early Silurian periods. Geochemical and isotopic evidences suggest that these peralkaline syenites and granites originated from fractional crystallization and/or assimilation fractional crystallization of enriched OIB-like mantle sources. We therefore conclude that the peralkaline syenites and granites in Suizhou-Zaoyang region and by extension those within the Qinling orogenic belt and NCC, with parallel geochemical characteristics e.g., the Sandaogou and Mogou syenites were largely derived from OIB magmas which originated from enriched aesthenospheric and lithospheric mantle with minor contribution from older continental crust material. These A-type suites and nepheline syenites are co-genetic with coeval mafic rocks in the area. Episodic extension and/or rifting triggered significant asthenospheric mantle upwelling and subsequent mixing with lithospheric mantle sources. The peralkaline syenites and alkaline granites from Suizhou-Zaoyang region exhibit A1-type granite affinity with all the granitoids from the studied region plotting in within plate granite in the tectonic discrimination diagram, which imply that they are within plate anorogenic granites associated with the Ordovician crustal extension and Silurian rifting.ACKNOWLEDGMENTS
Professor Changqian Ma has received long-term guidance and mentorship from Prof. Zhendong You, which is highly appreciated. We also acknowledge the support from the National Natural Science Foundation of China (No. 41502046), partial financial support by the China Geological Survey (No. DD20160030), and the Fundamental Research Funds for the Central Universities, China University of Geosciences, Wuhan (No. CUGCJ1711) are also acknowledged. We also thank Prof. Bernard Bonin and two anonymous reviewers whose painstaking reviews have significantly improved this work. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0877-2.
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