Petrogenesis and tectonic implications of the Early

Tectonophysics 712–713 (2017) 270–288

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Petrogenesis and tectonic implications of the Early Paleozoic intermediate and mafic intrusions in the South Qinling Belt, Central China: Constraints from geochemistry, zircon U–Pb geochronology and Hf isotopes Ruirui Wang a,b,⁎, Zhiqin Xu c, M. Santosh d,e, Fenghua Liang f, Xuehai Fu a,b a

School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China Key Laboratory of Coalbed Methane Resources & Reservoir Formation Process, China Ministry of Education, Xuzhou 221116, China c State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China d Centre for Tectonics, Resources and Exploration, University of Adelaide, SA 5005, Australia e School of Earth Sciences and Resources, China University of Geosciences Beijing, Beijing 100083, China f Innovative Research Center of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China b

a r t i c l e

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Article history: Received 18 July 2016 Received in revised form 9 May 2017 Accepted 26 May 2017 Available online 31 May 2017 Keywords: Intermediate and mafic magmatic rocks South Qinling Belt Early Paleozoic Zircon geochronology and Lu-Hf isotopes Geochemistry and tectonic setting

a b s t r a c t The characteristics and tectonic implications of the Early Paleozoic alkaline magmatic belt in the South Qinling Belt, which was originally part of the northern Yangtze Block prior the Devonian, have remained elusive. Whether this magmatic belt is related to rifting of the passive continental margin, to back-arc extension in the active continental margin, or to mantle plume activity is debated. Understanding the origin and geodynamic significance of this magmatic belt can provide new constraints on the Early Paleozoic tectonic evolution of the northern Yangtze Block. Here we present zircon U-Pb data from a suite of nepheline syenite, quartz syenite, diabase, and gabbro from the northern margin of the Yangtze Block which show an age range of ca. 435–440 Ma. The εHf(t) values of the intermediate rocks up to 16.59 suggest magma generation from depleted mantle sources and new crustal growth. Geochemically, the syenites showing high total alkali contents and are enriched in LREE, LILE (Rb, Ba, and K), and HFSE (Th, U, Nb, Ta, Zr, and Hf), with depletion in Sr, P, and Ti. The intermediate and mafic magmatic rocks were generated through magmas sourced from the subcontinental lithospheric mantle metasomatized by asthenospheric mantle and underwent fractional crystallization without significant crustal contamination. The magmatic suite represents a significant phase of crustal extension in the northern margin of the Yangtze Block. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The assembly of the supercontinent Pangea marks one of the major global tectonic events in the Phanerozoic (Domeier et al., 2012; Stampfli et al., 2013; Domeier and Torsvik, 2014; Li et al., 2017a, 2017b). However, the configuration of this supercontinent, particularly the timing and geodynamics of the assembly of East Asian continents remain debated. Of particular importance in this context is the amalgamation between the North China Block (NCB) and South China Block (SCB) (Jahn and Chen, 2007; Dong et al., 2013; Li et al., 2017a), the popular models on which relate the timing to Triassic based on several lines of evidence including the formation of ultrahigh-pressure metamorphic rocks and paleomagnetic data (e.g. Sengör, 1985; Zhao and Coe, 1987; Yin and Nie, ⁎ Corresponding author at: School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China. E-mail address: [email protected] (R. Wang).

http://dx.doi.org/10.1016/j.tecto.2017.05.021 0040-1951/© 2017 Elsevier B.V. All rights reserved.

1996; Meng and Zhang, 1999; Zhang et al., 2001; Torsvik et al., 2008). However, some workers have argued for an Early or mid-late Paleozoic collision between the North and South China Blocks based on Paleozoic collision-related granitoids, deformation, benthic faunas, data from sedimentary units and Pb isotopic compositions (e.g. Mattauer et al., 1985; Zhang et al., 1997; Dong et al., 2013). The Qinling Orogen, made up of North Qinling Belt (NQB) and South Qinling Belt (SQB), is a collisional orogen between the NCB and SCB (Zhang et al., 2001; Dong et al., 2011; Wu and Zheng, 2013) (Fig. 1a and b). The Early Paleozoic magmatic suites in the Qinling Orogen provide important clues to evaluate whether the NCB and SCB collided in the Early or mid-late Paleozoic. These magmatic rocks in the NQB are suggested to be related to subduction, collision and post-collision (Wang et al., 2015b). However, the petrogenesis and tectonic setting of similar aged rock suites in the SQB remain controversial. Some workers proposed that the Early Paleozoic magmatism in the SQB occurred in the passive continental margin as a result of the opening of

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Fig. 1. (a) Topography of China and adjacent regions showing the tectonic units and location of the South Qinling belt. (b) Geological map of the South Qinling.

Mianlue ocean which is a branch of Paleo-Tethyan ocean (Dong et al., 2011; Wu and Zheng, 2013; Chen et al., 2014; Liu et al., 2016). However, others correlated the magmatism to a back-arc extensional setting (Ma et al., 2006; Wang et al., 2009b, 2015a; Zhang, 2010). A third group invoked mantle plume to explain this magmatic event (Yan, 2005; Xu et al., 2008; Long, 2016).

The previous work focused mainly on the mafic rocks in the SQB, with less attention to the intermediate magmatic rocks which include nepheline syenite, quartz syenite, and trachyte, among other rock types. The syenites occurring usually as small intrusions and enriched in alkali elements, are resistant to later alteration and modification of their trace element budget (Tchameni et al., 2001). Thus, detailed

Fig. 2. Geological maps showing sampling locations.

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investigations on the Early Paleozoic intermediate and mafic magmatic rocks in the SQB can provide insights into the tectono-magmatic history of the region.

With a view to precisely date the Early Paleozoic intermediate and mafic suites in the SQB and to evaluate their geochemical and petrogenetic aspects, we carried out zircon U-Pb-Lu-Hf isotopic and whole-rock

Fig. 3. (a) and (b) Field photos and photomicrographs of the Guanzishan nepheline syenite; (c) and (d) Field photos and photomicrographs of the Haoping quartz syenite; (e) and (f) Field photos and photomicrographs of the Gaotan diabase; (g) and (h) Field photos and photomicrographs of the Banjiuguan gabbro. Mineral abbreviations are as follows: Bt = biotite, Chl = chlorite, Cpx = clinopyroxene, Ne = nepheline, Pl = plagioclase, Prt = perthite, Q = quartz, Ser = sericite.

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geochemical analyses of representative samples. We use the data to reconstruct the tectonic setting and geodynamic processes in this region during the Early Paleozoic. 2. Geologic background and samples descriptions 2.1. Geologic background The E-W trending Qinling Orogen connects the Kunlun–Qilian Orogen to the west and Dabie–Sulu Orogen to the East (Fig. 1a and b) (Dong and Santosh, 2016). The orogen has been subdivided into South Qinling and North Qinling by the Shangdan Suture (Fig. 1a and b). The North Qinling originally belonged to the NCB, whereas the South Qinling was part of the northern margin of the Yangtze Block (YB) prior to the opening of the Mianlue Ocean in the Devonian (Zhang et al., 2001). The initiation of the Mianlue Ocean spreading is represented by bimodal

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volcanic rocks (Dong et al., 2011). The Early Mesozoic amalgamation of the NCB and SCB resulted in the closure of the Mianlue Ocean which separates the SQB from the SCB since the Mid-Devonian to Mid-Triassic (Zhang et al., 2001; Dong et al., 2011; Dong and Santosh, 2016). The South Qinling has a Precambrian basement overlain by Sinian to Mesozoic sedimentary rocks (Zhang et al., 2001). The Early-Middle Neoproterozoic Douling Complex is composed of biotite gneiss and schist with subordinate hornblende gneiss, marble and basalt (Liu et al., 2016). The most widely exposed basement strata occur in the Wudang Uplift in South Qinling and include low-grade metamorphic rocks of the Wudangshan and Yaolinghe Groups (Wang et al., 2016) (Fig. 1c). Several Neoproterozoic plutons occur in South Qinling, among which two large intrusions are the Fenghuangshan pluton (also called Tiewadian pluton) and Douling pluton, which are mainly composed of granitic and dioritic intrusions (Dong and Santosh, 2016). These are covered by Sinian (Ediacaran) clastic and carbonate

Fig. 4. (a) Mg-(AlVI + Fe3++Ti)-(Fe2++Mn) triangular diagram (after Foster, 1960); (b) Ternary classification diagram of feldspars (after Deer et al., 1992); (c) Discrimination diagram of the series for pyroxene (Morimoto et al., 1988); (d) Discrimination diagram of Na pyroxenes (Morimoto et al., 1988).

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sediments, Cambrian–Ordovician limestone, Silurian shale, Devonian– Carboniferous clastic sediment intercalated with limestone and limited Permian–Triassic sandstone (Zhang et al., 2001). The cover rocks have undergone low grade metamorphism. The diverse physical properties of the cover rocks have led to the development of sub-décollements, especially in the Silurian strata (Xu, 1988; Wang et al., 2013b). In the Xunyang area in north of the Ankang Fault, the structural deformation in the cover comprise listric structures which change downwards from upright folds through isoclinal folds to recumbent folds (Xu, 1988). The Chengkou-Fangxian Fault also shows listric shape, and connects the secondary fault in the deep décollement. The Early Paleozoic magmatic belt in the SQB extends eastward from North Dabashan Mountain to the Suizhou and Zaoyang area. The belt is composed mainly of intermediate and mafic magmatic rocks and a few carbonatite-syenite complexes (Li, 1991; Yu, 1992; Dong et al., 1998; Zhang et al., 2007; Xu et al., 2008; Cao et al., 2015a, 2015b). The mafic rocks include gabbro, gabbro-diabase, diabase, and diabase-porphyrite whereas the intermediate rocks are composed of quartz syenite, syenite, nepheline syenite, trachyte, and diorite.

2.2. Samples description The sampling locations are shown in Fig. 1b and Fig. 2. The Guanzishan pluton in the Suizhou-Zaoyang area shows intrusive contact with Sinian schist and Neoproterozoic mafic rocks (Fig. 2a). The intermediate rocks in the North Daba Mountain are controlled by the NWSE trending fault and intrude along the bedding planes of the Silurian strata (Fig. 2b). The Early Paleozoic mafic rocks mainly exposed in the North Daba Mountain which is a branch of the SQB. The NW-SE trending mafic rocks are tens to hundreds of meters in width, and hundreds to few thousands of meters in length (Fig. 2c and d). They show intrusive contact with the surrounding Cambrian, Ordovician and Silurian strata, and occur mostly as sills.

The Guanzishan nepheline syenite is light grey green, with a medium-grained and porphyritic and subhedral texture and massive structure. The rock is composed of perthitic alkali feldspar (~ 65 vol%) as phenocrysts, Na pyroxene (jadeite and aegirine; ~ 10 vol% in total), nepheline (~10 vol%), sericite (~5 vol%) and epidote (~5 vol%) as matrix minerals (Fig. 3a and b; Fig. 4; Appendix A.1). The accessory minerals include sphene, magnetite, zircon and apatite. The Haoping biotite quartz syenite is ash black, with a coarsegrained and porphyritic texture and massive structure. The rock is composed of perthitic alkali feldspar (~ 75 vol%) as phenocrysts together with perthitic alkali feldspar, biotite, quartz, calcite and epidote (~ 20 vol% in total) as matrix minerals (Fig. 3c and d; Fig. 4a and b). The accessory minerals include sphene, magnetite, zircon, and apatite. The biotite is magnesian (Fig. 4a). The Gaotan diabase is celadon, with an ophitic texture and massive structure. The rock is predominantly composed of plagioclase (~70 vol%), clinopyroxene (~15 vol%), and minor amounts of chlorite, Fe-Ti oxides (~10 vol% in total) (Fig. 3e and f; Fig. 4 b and c). The Banjiuguan gabbro is celadon, with a gabbroic texture and massive structure. The rock is composed of plagioclase (~ 55 vol%), clinopyroxene (~35 vol%), and minor amounts of chlorite, olivine, FeTi oxides (~10 vol% in total) (Fig. 3g and h; Fig. 4 b and c). 3. Analytical methods 3.1. Zircon U-Pb dating using LA–ICP–MS Zircon grains for U–Pb dating were extracted by standard density and magnetic methods, followed by hand-picking under a stereoscopic microscope. The selected zircons grains were mounted in epoxy and polished down to half-section. Transmitted and reflected light micrographs and cathodoluminescence (CL) images were utilized to guide the U–Th–Pb isotope analysis. Zircon U–Pb dating was conducted with a laser-ablation inductively coupled plasma mass spectrometer (LA–

Fig. 5. Cathodoluminescence (CL) images of representative zircon grains with 206Pb/238U ages and εHf(t) values. The small circles are the spots for U-Pb dating and the large and dashed circles are the spots for Hf analyses.

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ICP–MS) at the Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS). U–Pb dating was performed on a Thermo Finnigan Neptune MC– ICP–MS instrument equipped with Newwave UP 213 laser ablation system. All of the spot analyses were performed with a beam diameter of 25 μm, a 10 Hz repetition rate, and an energy of 2.5 J/cm2. Standards GJ1 and SRM610 were served as the standard sample. Raw signal data were processed using the software ICPMSDataCal8.0 (Liu et al., 2010) and the results were plotted using Isoplot 3.0 (Ludwig, 2003).

3.2. Zircon Lu-Hf isotope Zircon Hf isotope analyses were carried out in situ using a Neptune multi–collector ICP–MS attached by a Newwave UP213 laser–ablation microprobe at the Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS) (Beijing, China). Detailed instrumental conditions and data acquisition procedures were comprehensively described by Wu et al. (2006) and Hou et al. (2007).

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The analytical spot has a diameter of 55 μm. Helium was used as carrier gas to transport the ablated sample from the laser-ablation cell to the ICP–MS torch via a mixing chamber mixed with Argon. Detailed methods and parameters for the correction of isobaric interference and instrumental mass bias can be found in Chu et al. (2002) and Wu et al. (2006). Correction for the isobaric interference of 176Lu and 176Yb with 176 Hf utilize 176Lu/175Lu = 0.02658 and 176Yb/173Yb = 0.796218 (Chu et al., 2002). For the instrumental mass bias correction, the Yb isotope ratios were normalized to 172Yb/173Yb = 1.35274 (Chu et al., 2002) and the Hf isotope ratios were normalized to 179Hf/177Hf = 0.7325 using an exponential law. The mass bias behavior of Lu was assumed to follow that of Yb, and the mass bias correction protocol details were identical to those described by Wu et al. (2006) and Hou et al. (2007). The initial 176Hf/177Hf ratios were calculated with reference to the chondritic uniform reservoir (CHUR) at the time of zircon growth from magmas. The εHf(t) values were calculated based on the zircon U–Pb ages, the chondritic rations of 176Lu/177Hf (0.0332) and 176 Hf/177Hf (0.282772) (Blichert-Toft and Albarède, 1997). The singlestage model ages (TDM) were calculated based on a depleted mantle source with present day 176Hf/177Hf at 0.28325, using the 176Lu decay

Fig. 6. U-Pb concordia plots and weighted mean ages for zircons.

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constant of 1.865 × 10−11 year−1 (Scherer et al., 2001). The two-stage model age (TDMC) is calculated under the assumption of a mean 176 Hf/177Hf of 0.015 for the average continental crust (Griffin et al., 2000). Zircon GJ-1 was used as the reference standards, with a weighted mean 176Hf/177Hf ratio of 0.282015 ± 19 (2σ), according to an in situ analysis by Elhlou et al. (2006). 3.3. Whole-rock geochemistry Samples were crushed and powdered in an agate mortar to a grain size b 200 mesh. Major elements were analyzed by a ARL9800XP + X–ray fluorescence spectrometer (XRF) at the National Research Center of Geoanalysis. The analytical precision is better than 2%. Detailed analytical procedures were described in Franzini et al. (1972). Trace element contents were determined using a Finnigan Element II inductively coupled plasma mass spectrometer (ICP–MS) at the National Research Center of Geoanalysis (Beijing, China). 3.4. Electron microprobe Mineral chemical compositions of the studied rocks were analyzed using a JEOL JXA–8900 electron microprobe (EPM) at the Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, CAGS. The operating conditions are 15 kV accelerating voltage, 20 nA beam

current, and 5 μm defocused electron beam diameter. Natural or synthetic standards were utilized for calibration. 4. Results 4.1. Zircon U-Pb and Lu-Hf isotopes 4.1.1. Guanzishan nepheline syenite The zircon grains of sample QL11-2-3 are prismatic, colorless and euhedral (Fig. 5a). Their length ranges from 150 to 250 μm in size, with ratios of length to width from 1:1 to 3:1. Zircon grains with weak banded zoning and high Th/U values (0.68–1.28) imply their magmatic origin. Thirty one analyses are on the Concordia curve, and yield a weighted mean 206Pb/238U age of 434.8 ± 1.2 Ma (MSWD = 3.2) (Fig. 6a; Appendix A.2). Twenty Hf isotopic spot analyses were obtained from twenty grains. The zircons show varying initial 176Hf/177Hf ratio (0.282619 to 0.282906) and εHf(t) value (3.81 to 13.37) with a weighted mean of 7.19 ± 0.71 (Fig. 7a and c; Appendix A.3). The single-stage model ages (TDM) range from 523 Ma to 896 Ma, and the two-stage model ages (TCDM) from 567 Ma to 1178 Ma (Appendix A.3). 4.1.2. Haoping quartz syenite The zircon crystals of sample QLhp are prismatic, colorless and euhedral-subhedral (Fig. 5b). Their length ranges from 50 to 120 μm

Fig. 7. (a) Zircon εHf(t) versus age diagram; (b) Th versus U diagram; (c) Histogram of initial Hf isotope ratios for zircons of Guanzishan nepheline syenite; (d) Histogram of initial Hf isotope ratios for zircons of Haoping quartz syenite. DM, Depleted Mantle; CHUR, Chondritic Uniform Reservoir.

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in size, with ratios of length to width from 2:1 to 3:1. Most Zircon grains display clear oscillatory zoning in CL images with Th/U values of 0.50 to 2.74, which indicate their magmatic origin. Twenty grains are analyzed. Fifteen analyses are on the Concordia curve, and yield a weighted mean 206 Pb/238U age of 435.1 ± 1.2 Ma (MSWD = 2.7) (Fig. 6b; Appendix A.2). The age is interpreted as the crystallization age of sample QLhp and as the best estimate of intrusion age of the pluton. Five zircon grains have 206Pb/238U ages between 637 Ma and 750 Ma. Fourteen Hf isotopic spot analyses were obtained from fourteen grains for sample QLhp. The most zircons show varying initial 176 Hf/177Hf ratio (0.28153 to 0.28294) and εHf(t) value (− 34.41 to 16.59) with a weighted mean of 12.1 ± 2.4 (Fig. 7a and d; Appendix A.3). The single-stage model ages (TDM) are between 433 Ma and 2974 Ma, and the two-stage model ages (TCDM) between 431 and 3554 Ma (Appendix A.3).

4.1.3. Datan diabase Sample QL15-3-1 was collected from diabase sills in Dabashan Mountain. Zircon grains from this sample are prismatic-acicular, colorless and euhedral (Fig. 5c). Their length ranges from 50 to 150 μm in size, with ratios of length to width from 1:1 to 6:1. Zircon grains with weak banded zoning and high Th/U values (1.44 to 2.79) imply their magmatic origin. Twenty three analyses are on the Concordia curve, and yield a weighted mean 206Pb/238U age of 440.0 ± 0.5 Ma (MSWD = 0.8) (Fig. 6c; Appendix A.2).

4.1.4. Banjiuguan gabbro Sample QL4-9-1 was collected near Banjiuguan. Zircon grains from this sample are prismatic, colorless and euhedral (Fig. 5d). Their length ranges from 50 to 150 μm in size, with ratios of length to width from 1:1 to 1:4. Zircon grains without any zoning and high Th/U values (1.25 to 3.45) imply their magmatic origin. Nineteen analyses are on the Concordia curve, and yield a weighted mean 206Pb/238U age of 439.9 ± 0.5 Ma (MSWD = 0.12) which is considered the crystallized age of the pluton (Fig. 6d; Appendix A.3).

4.2. Major and trace elements The Early Paleozoic magmatic rocks have experienced variable degrees of alteration as evidenced by the existence of secondary minerals and variable loss on ignition (LOI) values (0.59–1.87 for the intermediate rocks; 2.90–3.56 for the mafic rocks) (Appendix A.4). The major elements are re-calculated on volatile-free basis. Some alteration indices can be used to test for mobility as proposed by Spitz and Darling (1978). In the Al2O3/Na2O versus Na2O diagram (Spitz and Darling, 1978), the samples of the mafic rocks show slight Na-loss or are fresh (Fig. 8). In contrast, the samples of the nepheline syenite show Na-alteration, whereas the quartz syenite only weak Na-alteration (Fig. 8). The samples of nepheline syenite have underwent NaMetasomatism, and secondary minerals, such as Na pyroxene (jadeite and aegirine), albite, sericite, epidote and so on, were formed. Zirconium is one of the most immobile elements during alteration, and could be an alteration-independent index to test the mobility of the other trace elements (Yao et al., 2012). The bivariate plots of Zr against selected trace elements can reflect the mobility of these trace elements during alteration (figures not shown). The contents of high field strength elements (HFSEs, e.g. Nb, Ta, Zr, Hf, Th, U, and Y), and rare earth elements (REEs) show positive correlation with the contents of Zr, indicating that such elements were immobile during alteration. In contrast, the large ion lithophile elements (LILEs, e.g. Cs, Sr, and Ba) do not correlate well with Zr, suggesting the mobility of these elements during alteration. Therefore, the immobile elements will be used in the following discussion.

Fig. 8. Al2O3/Na2O versus Na2O diagram (after Spitz and Darling, 1978). Data source of the Early Paleozoic intermediate and felsic rocks in the South Qinling Belt: Huang et al., 1992; Zhang et al., 2002; Ma et al., 2004; Xu et al., 2008; Wang et al., 2009a, 2015a; Yan, 2005; Li et al., 2009; Wang, 2014; Cao et al., 2015b; Chen, 2016; Long, 2016; Wan et al., 2016. Data source of the Early Paleozoic ultramafic and mafic rocks in the South Qinling Belt: Huang et al., 1992; Dong et al., 1998; Xu et al., 2001; Zhang et al., 2002; Yan, 2005; Li et al., 2009; Zhang, 2010; Chen et al., 2010, 2014; Xiang et al., 2010; Wang, 2014; Xie et al., 2014; Cao et al., 2015a; Wang et al., 2012, Wang et al., 2015a; Chen, 2016; Long, 2016; Wan et al., 2016.

4.2.1. Guanzishan nepheline syenite and Haoping quartz syenite The samples of Guanzishan nepheline syenite have medium SiO2 (56.54–57.10 wt%), low TiO2 (0.19–0.23 wt%), MnO (0.19–0.21 wt%), MgO (0.06–0.12 wt%), CaO (0.91–1.05 wt%), high K2O (5.76–6.09 wt%), Na2O (8.49–8.50 wt%), Al2O3 (19.48–19.78 wt%), and FeOT (5.34– 5.60 wt%) (Fig. 9a; Appendix A.4). The rocks show varying ACNK (0.88–1.06) and ANK (0.95–1.10) (Fig. 9b). The nepheline syenite is alkaline-peralkaline according to the Na2O + K2O-CaO versus SiO2 diagram and SiO2 versus A.R diagram (Fig. 9c, d). In the chondrite-normalized REE distribution patterns (Fig. 10a), the nepheline syenite exhibits enrichment of LREE (La/Yb)N = 7.93–15.93 and flat HREE with strongly negative Eu anomalies (Eu/Eu* = 0.07–0.17). Primitive mantle normalized trace element spider diagram shows depletion of Sr, P and Ti, and enrichment in LILE (Rb, and K) and HFSE (Th, U, Nb, Ta, Zr, and Hf) (Fig. 10b). The Sr, P, and Ti negative anomalies may be related to the fractional crystallization of apatite, titanium and iron oxides, and feldspar. The samples of Haoping quartz syenite have high SiO2 (63.64– 64.78 wt%), Al2O3(15.38–17.29 wt%), FeOT (1.97–3.20 wt%), Na2O (4.66–5.33 wt%) and K2O (4.93–5.82 wt%), and low TiO2 (0.97– 1.08 wt%), MnO (0.06–0.22 wt%), MgO (0.76–1.62 wt%) and CaO (0.96–1.54 wt%) (Fig. 9a; Appendix A.4). The analyzed samples are metaluminous to peraluminous according to the A/CNK (A/CNK = Al2O3/(CaO + Na2O + K2O)mol.) and A/NK (= Al2O3/(Na2O + K2O)mol.) (Fig. 9b). The quartz syenite is alkaline according to the Na2O + K2O-CaO versus SiO2 diagram and SiO2 versus A.R diagram (Fig. 9c, d). Their REE patterns normalized by Chondrite (Boynton, 1984) show that all the samples are enriched in light rare earth elements (LREE), and have flat heavy REE (HREE) patterns (Fig. 10c). Primitive mantle normalized trace element spider diagram show depletion of Pb, Sr, P and Ti, and enriched in K and HFSE (Nb, Ta, Zr, and Hf) (Fig. 10d).

4.2.2. Gaotan diabase The samples of Datan diabase have low SiO2 (41.67–48.52 wt%), K2O (0.87–1.24 wt%), K2O + Na2O (2.21–5.00 wt%) and MnO(0.15–

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Fig. 9. (a) Total alkali versus silica (TAS) classification diagram (after Middlemost, 1994), the boundary between alkaline and subalkaline area is from Irvine and Baragar (1971); (b) A/CNK versus A/NK diagram (after Shand, 1943); (c) Na2O + K2O-CaO versus SiO2 diagram (after Frost et al., 2001); (d) SiO2 versus A.R (Alkalinity ratio) [Al2O3 + CaO+(Na2O + K2O)]/[Al2O3 + CaO-(Na2O + K2O)] diagram (Wright, 1969). The published data sources of the magmatic rocks in the South Qinling Belt are same to Fig. 8.

0.21 wt%), high TiO2 (2.63–3.02 wt%), MgO (3.64–12.76 wt%), P2O5 (0.39–0.53 wt%), Al2O3 (12.07–16.94 wt%), FeOT (11.01–13.39 wt%), and CaO (7.55–10.5 wt%) (Fig. 9a; Appendix A.4). The samples are metaluminous according to their A/CNK (0.62–0.71) and A/NK (2.05– 3.90) (Fig. 9b). In the chondrite-normalized REE distribution patterns (Fig. 8c, d), the samples of diabase are enriched in light rare earth elements (LREE) (Fig. 10e). Primitive mantle normalized trace element spider diagram show depletion of Pb, Ti, and Yb, and enriched in LILE (Rb, and K) and HFSE (Nb, and Ta) (Fig. 10f). All the normalized trace element and REE show OIB-like (oceanic-island-basalt-like) patterns (Fig. 10e and f). 5. Discussion 5.1. Timing of the Early Paleozoic magmatism in the SQB The emplacement timing of the mafic rocks in the North Daba Mountains have been investigated in previous works. The clinopyroxene K-Ar age of the diabase is reported as 471.4 ± 23.2 Ma (Teng and Teng and Li, 1990). The 39Ar-40Ar analysis of phlogopite

from the kimberlite in association with mafic rocks yielded an age of 431.9 Ma (Huang et al., 1992). The whole-rock Rb-Sr isochron age of the mafic rocks in Zhenping is 447.9 ± 10.6 Ma (He et al., 1999). Zircon U-Pb ages reported in recent studies from the mafic rocks range from 399 Ma to 451 Ma (Zhang et al., 2007; Wang et al., 2009a; Zhang, 2010; Zou et al., 2011; Xie et al., 2014; Chen et al., 2014; Wang, 2014; Cao et al., 2015a; Xiang et al., 2016). Previous K-Ar, U-Pb and Rb-Sr isotopic dating of the intermediate rock suite yielded a wide range of ages from 306 Ma to 215 Ma for the alkaline complexes in Huashanzhai, Guanzishan, Shaxiongdong, and Miaoya (Li, 1991). However, the riebeckite quartz syenite in Suizhou area shows zircon U-Pb age of 439 ± 6 Ma (Ma et al., 2004), and the syenite porphyry veins in the Qiaomaichong gold deposit in Suizhou area have U-Pb ages of 415 ± 7 Ma and 477 ± 6 Ma (Cao et al., 2015b). The pyroxene diorites in Gaoqiao and Xuhe in North Daba Mountain were dated at 438.4 ± 3.4 Ma and 438.4 ± 3.1 Ma (Wang, 2014). The available age data constrain the peak magmatism at ca. 440 Ma (Fig. 11). In this contribution, the samples of the nepheline syenite, quartz syenite, diabase and gabbro are dated between 435 and 440 Ma confirming the peak magmatic stage.

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Fig. 10. (a) Chondrite-normalized REE patterns of Guanzishan nepheline syenite; (b) Primitive mantle-normalized trace element spider diagrams of Guanzishan nepheline syenite; (C) Chondrite-normalized REE patterns of Haoping quartz syenite; (d) Primitive mantle-normalized trace element spider diagrams of Haoping quartz syenite; (e) Chondrite-normalized REE patterns of Gaotan diabase; (f) Primitive mantle-normalized trace element spider diagrams of Gaotan diabase; The chondrite values are from Boynton (1984), and the primitive mantle values are from Sun and McDonough (1989). The data for OIB, E-MORB and N-MORB are from Sun and McDonough (1989). The published data sources of the magmatic rocks in the South Qinling Belt are same to Fig. 8.

5.2. Petrogenesis 5.2.1. Magma sources Several genetic models have been proposed for the origin of the syenites, including: (1) partial melting of crustal rocks (Whalen et al., 1987; Tchameni et al., 2001); (2) mixture of mantle-derived basaltic magmas and crustal-derived granitic melt (Zhao et al., 1995; Mingram et al., 2000); and (3) fractional crystallization of mantle-derived

magmas, combined with varying degrees of crustal contamination (Fitton, 1987; Litvinovsky et al., 2002; Kumar et al., 2007). For the Guanzishan nepheline syenite and Haoping quartz syenite we exclude the second model because of the absence of any mafic microgranular enclaves (MMEs) in these plutons. Based on the zircon saturation temperature (Watson and Harrison, 1983), the initial magma temperatures of the Guanzishan nepheline syenite and Haoping quartz syenite are estimated as 905–1006 °C, 846–

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Fig. 11. Probability of zircon U-Pb ages of the intermediate and mafic rocks in the South Qinling Belt. Age source of the intermediate rocks: Ma et al., 2004; Xu et al., 2008; Wang, 2014; Wang et al., 2015a; Cao et al., 2015b; This research. Age source of the mafic rocks: Zhang et al., 2007; Wang et al., 2009a; Zhang, 2010; Zou et al., 2011; Xie et al., 2014; Chen et al., 2014; Wang, 2014; Cao et al., 2015a; Xiang et al., 2016; This research.

Fig. 12. (a) (Na2O + K2O)/CaO versus Zr + Nb + Y + Ce diagram (after Whalen et al., 1987); (b) 10000Ga/Al versus Y diagram (after Whalen et al., 1987); (c) Nb-Y-Ce triangular diagram (after Eby, 1992); (d) Nb-Y-3Ga triangular diagram (after Eby, 1992).

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989 °C, respectively. The lack of inherited zircon grains suggests that the primary magma was zircon-undersaturated, and thus the temperatures provide only the minimum estimates (Miller et al., 2003). This is comparable to the high temperature feature of A-type granites with minimum magma temperature of 830 °C (Clemens et al., 1986). According to the classification diagrams proposed by Whalen et al. (1987), Eby (1992) and Frost et al. (2001), the samples of intermediate rocks from Guanzishan and Haoping show A-type granite affinity (Fig. 12a, b; Fig. 9c). Furthermore, the intermediate rocks are geochemically similar to A1-type granites which are derived from sources such as oceanic island basalts (OIB) in continental rift or intraplate settings (Eby, 1992). Based on plots in the Yb/Ta versus Y/Nb diagram (Fig. 13a), Th/Yb versus Nb/ Yb diagram (Fig. 13b), Zr/Nb versus Nb/Th diagram (Fig. 13c), and εNd(t) versus (87Sr/86Sr)i diagram (Fig. 13d), most of the magmatic rocks in our study show OIB affinity. This inference is also supported by the OIB–like trace–element patterns (Fig. 10).

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The EM I(enriched mantle component I), EMII(enriched mantle component II) and HIMU (high U/Pb mantle component) are the three end-members to be considered for the magma sources of the Early Paleozoic magmatic rocks in the SQB (Fig. 13c and d). However, the OIBlike rocks are usually associated with low-degree partial melting of asthenosphere mantle or related to mantle plume (Thompson, 1984; Eby, 1992; Scarrow et al., 1998). The majority of εHf(t) values of the syenites plot above the evolutionary array of the zircons of the basement rocks in the Yangtze Block (Fig. 14). Moreover, the high εHf(t) values of some zircon grains are even fall on the depleted mantle line, suggesting the input of depleted mantle materials. Two types of mantle metasomatism have been recognized: (1) subduction-related metasomatized mantle source enriched in LILE and LREE, but depleted in HFS elements with very low HFS/LREE ratios (Nb/La b 0.3); and (2) asthenosphere-derived melt-metasomatized mantle enriched in LILE, LREE and HFS elements with high HFS/LREE

Fig. 13. (a) Yb/Ta versus Y/Nb diagram (after Eby, 1992); (b) Th/Yb versus Nb/Yb diagram (after Pearce, 2008); (c) Zr/Nb versus Nb/Th diagram (after Condie, 2005); (d) εNd(t) versus initial 87Sr/86Sr diagram (after Qi and Zhou, 2008). DM, depleted mantle; MORB, middle oceanic ridge basalt; HIMU, high U/Pb mantle component; EMI, enriched mantle componentI; EMII, enriched mantle component II; DEP, deep depleted mantle; En, enriched component; REC, recycled component. Data source: The DM and mantle array, Zindler and Hart (1986); MORB, HIMU, EMI and EMII, Hart et al. (1992); OIB, Wilson (1989); Yangtze upper/middle and lower crust, Gao et al. (1999), Ma et al. (2000) and Chen and Jahn (1998). The published data sources of the magmatic rocks in the South Qinling Belt are same to Fig. 8.

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Fig. 14. Hf isotopes of zircon grains in the Early Paleozoic intermediate magmatic rocks. The Hf isotopic data of the syenite in Qiaomaichong gold deposit, Zhouan mafic-ultramafic intrusion, magmatic rocks in South Qinling Belt, sedimentary rocks in South Qinling Belt, magmatic rocks in northwestern and western Yangtze Block, Kongling complex is according to Cao et al. (2015b), Wang et al. (2016) and references therein.

ratios (Nb/La N 1) (Li et al., 2003). The intermediate and mafic rocks are enriched in LILE, LREE and HFSE and have high HFSE/LREE ratios with the majority N 1. Thus, a subduction-related metasomatized mantle source seems unlikely as the magma source for the intermediate and mafic rocks. In contrast, the parental magma of the intermediate and mafic rocks might have originated from lithospheric mantle metasomatized by asthenosphere-derived melt. 5.2.2. Assimilation and fractional crystallization (AFC) The Nb, Ta, and Pb are sensitive to crustal contamination which will deplete the magma in Nb, and Ta, and enrich in Pb (Rudnick and Gao, 2003). However, the intermediate and mafic rocks are not depleted in Nb and Ta, and are not enriched in Pb, which excludes any significant role of crustal contamination in the petrogenesis of these rocks. The ratios of immobile trace elements such as high La/Sm (N 4.5) and high (Th/ Nb)N (≥1) are important indicators of crustal contamination (Saunders et al., 1992; Lassiter and DePaolo, 1997). The low La/Sm (3.45 to 4.01), low (Th/Nb)N (0.70 to 0.87) also rule out a significant role of crustal contamination in the petrogenesis. The La/Yb versus La diagram and Zr/Nd versus Zr diagram reveal that fractional crystallization is the dominant factor during the magma evolution. The intermediate rocks display close spatial and temporal relations to mafic and ultramafic rocks of the South Qinling Belt. It seems plausible that the latter was produced by assimilation and/or fractional crystallization (AFC) of the former because of their same magma sources as stated previously and the similar evolutional trends on the Harker diagrams (Fig. 15). The variable Mg# (39.51 to 65.37), Cr (8.24 to 57.1 ppm), and Ni (4.56 to 224 ppm) of the mafic rocks indicate significant fractional crystallization (Appendix A.4). From ultramafic-mafic rocks to intermediate-felsic rocks, there is a general increase in K2O (Fig. 15d), and

decrease in CaO, Fe2OT3, TiO2, V, Cr, and Sr with increasing SiO2 (Fig. 15e, f, g, h, i and j). Meanwhile, Al2O3, MgO and Na2O show an initial increasing trend and then a decrease with increase in SiO2 (Fig. 15a, b and c). These geochemical features might suggest that the ultramafic-mafic magmas experienced fractional crystallization of clinopyroxene and amphibole, followed by fractional crystallization of amphibole and plagioclase. Pronounced depletions in Ba, Sr, P, Ti and Eu demonstrate an advanced fractional crystallization during the formation of the nepheline syenite (Fig. 10b). Strong Eu, Ba and Sr depletion of the Guanzishan nepheline syenite requires extensive fractionation of plagioclase and/ or K-feldspar. On the Sr versus Ba diagram, the trend of separation of K-feldspar for Guanzishan nepheline syenite and separation of plagioclase for Haoping quartz syenite can be observed (Fig. 16c). However, separation of K-feldspar is not plausible because K2O increases with increasing SiO2 for the Guanzishan nepheline syenite (Fig. 16d). This is interpreted as separation of biotite and plagioclase at the late stage, rather than K-feldspar, which is further supported by the strong depletion of Eu in the normalized REE diagram. The fractional crystallization of plagioclase can also be inferred from Rb/Sr versus Sr diagram (Fig. 16d). The P and Ti negative anomalies may be related to the fractional crystallization of apatite, titanium, and iron oxides. 5.3. Tectonic implications 5.3.1. Early Paleozoic tectonic setting in the SQB The South China Block (SCB), formed by amalgamation between the Yangtze Block (YB) in the northwest and Cathaysia Block in the southeast during the Neoproterozoic (Wang et al., 2007; Zhao et al., 2011; Li et al., 2009), is considered to have been along the northern margin of supercontinent Gondwana in the Early Paleozoic (Wang et al., 2010,

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Fig. 15. SiO2 versus selected major element oxides (wt%) and trace elements (ppm) for the Early Paleozoic magmatic rocks in South Qinling Belt. Also shown are the trends of the fractional crystallization of clinopyroxene (cpx) + amphibole (amp) and amphibole (amp) + plagioclase (pl). The published data sources of the magmatic rocks in the South Qinling Belt are same to Fig. 8.

Wang et al., 2013a). During this period, the southeastern SCB underwent intra-continental orogenesis accompanied by the widespread ca. 435–440 Ma magmatism (Zhang et al., 2011; Yao et al., 2012; Wang et al., 2013a). Meanwhile, the northern SCB also experienced extensive magmatism and deformation (Yu, 1992; Huang et al., 1992; He et al., 1999; Wang et al., 2015a). The tectonic discrimination diagrams show that the Early Paleozoic alkaline magmatic rocks in the northern SCB are related to a within-plate setting (Fig. 17a, b, c and

d). The different sedimentary successions in the two sides of the Chengkou-Fangxian Fault which was primarily a north-dipping normal fault in the Early Paleozoic indicate the extension to the north of the fault (Meng et al., 1996). To the south of the fault, shelf sediments occur, whereas deep-water carbonates, turbidites and bedded cherts dominate the northern domain of the fault (Meng et al., 1996). The sedimentary sequence changes northward into thick platform carbonates in the Zhenan-Xichuan area (Mei et al., 1995).

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Fig. 16. (a) La/Yb versus La diagram, the inset is a schematic CH vs. CH/CM diagram (CH and CM refer to highly in compatible element concentration and moderately compatible element concentrations, respectively); (b) Zr/Nb versus Zr diagram (after Geng et al., 2009); (c) Ba versus Sr diagram (after Li et al., 2007); (d) Rb/Sr versus Sr diagram (after Zhang and Zou, 2013).The published data sources of the intermediate and felsic rocks in the South Qinling Belt are same to Fig. 8.

The Early Paleozoic extension in the SQB has been variably interpreted as rifting in the passive continental margin (Dong et al., 2011; Wu and Zheng, 2013; Chen et al., 2014), mantle plume activity (Zhang et al., 2002; Yan, 2005; Xu et al., 2008), or back-arc extension in a subduction setting (Ma et al., 2006; Wang et al., 2009b, 2015a). The South Qinling region experienced magmatic episodes at ca. 800– 630 Ma, ca. 451–399 Ma, and ca. 248–190 Ma magmatism (Wang et al., 2015b, 2016). The Early Paleozoic magmatism was long-term and discontinuous as compared to those in the other two periods. Thus, correlating the Early Paleozoic tectonics of the SQB to back-arc extension in a subduction setting is not reasonable. Moreover, there is no evidence for subduction-related sequences, and therefore we exclude back-arc extension in a subduction setting as the mechanism for the magmatism. Our geochemical data on the intermediate and mafic rocks suggest OIB–like sources. Such magma sources are generally attributed to the rise of mantle plumes or asthenosphere upwelling (Eby, 1992). The vertical uplift in the Early Paleozoic may be the result of the rise of mantle plumes or asthenosphere upwelling (Meng et al., 1996; Mei et al., 1999). However, the mantle plume model is not plausible due to the lack of radial dike swarms or any massive magma eruption (Choudhuri and Nemčok, 2017). Therefore, asthenospheric upwelling is the preferred mechanism to interpret the petrogenesis of the Early Paleozoic magmatic rocks.

5.3.2. Geodynamic model Extension of the continental lithosphere has been considered to develop in response to regional stress field (passive rifting) or thermal upwelling of the asthenosphere (active rifting) (Corti et al., 2003). However, passive rifting can change into active rifting, because the extension of the continental lithosphere may result in a wider updoming of the asthenosphere and the lithosphere above (Frisch et al., 2011). Continental rifting can lead to large-scale symmetric patterns by pure shear deformation or asymmetric patterns by simple shear deformation (Wernicke, 1981; Corti, 2012). The asymmetric rifts developed in the passive continental margins are characterized by a gently dipping master fault (detachment fault) cutting at low angles through the crust to the base of the lithosphere (Frisch et al., 2011). In the SQB, the strata and faults are mostly northeast-dipping, and the Chengkou-Fangxian Fault acted as the master fault. During the asymmetric rifting, the basement rocks underneath the master fault usually uplift in a dome-like fashion (Lister and Davis, 1989). Such phenomenon can also be observed in the SQB where the vertical uplift was due to thermodynamic process (Meng et al., 1996; Mei et al., 1999). Hence, we consider that asymmetric rifting may have occurred during the Early Paleozoic in the SQB. Combining our new results with those from previous studies, we present a model of asymmetric rifting for the Early Paleozoic tectono– magmatic evolution of the South Qinling belt (Fig. 18). We speculate

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Fig. 17. (a) Rb versus Y + Nb diagram (after Pearce et al., 1984); (b) Nb versus Y diagram (after Pearce et al., 1984); (c) Zr/Y versus Zr diagram (after Pearce, 1982); (d) Hf/3-Th-Ta triangular diagram (after Wood, 1980). WPG = within plate granites; ORG = ocean ridge granites; VAG = volcanic arc granites; syn-COLG = syn-collision granites; E-MORB = enriched middle oceanic ridge basalt; N-MORB = normal middle oceanic ridge basalt; OIB = oceanic island basalt; IAB = island arc basalt. The published data sources of the magmatic rocks in the South Qinling Belt are same to Fig. 8.

that extension in the northern margin of the SCB (passive continent margin) triggered the upwelling of asthenosphere–derived OIB–like sources. The mafic rocks were likely derived from the lithospheric

mantle metasomatized by asthenosphere-derived OIB-like melt. The intermediate rocks were probably generated from the same basaltic magma by AFC process.

Fig. 18. Geodynamic model for the Early Paleozoic magmatism in the South Qinling Belt. See text for discussion.

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6. Conclusion Based on the results presented in this study and existing data, we draw the following conclusions regarding the Early Paleozoic tectonic evolution of South Qinling: (1) The intermediate and mafic magmatic rocks in the South Qinling Belt were formed at ca. 435 to 440 Ma. (2) The intermediate and mafic magmatic rocks were sourced from subcontinental lithospheric mantle metasomatized by asthenosphere-derived OIB-like melt and have experienced fractional crystallization without significant crustal contamination. (3) The magmatic rocks were formed due to asthenospheric upwelling and lithospheric extension in the passive continental margin.

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