Journal of Earth Science  2018, Vol. 29 Issue (5): 1203-1218   PDF    
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Geochronology, Geochemistry and Sr-Nd-Pb Isotopic Study of the Wulong Flower-Like Glomerophyric Diorite Porphyry (Central China): Implications for Tectonic Evolution of Eastern Qinling
Yuxiang Zhu1, Lianxun Wang1, Changqian Ma1, Chao Zhang2, Ke Wang1    
1. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;
2. Institut für Mineralogie, Leibniz Universität Hannover, Callinstr. 3, 30167 Hannover, Germany
ABSTRACT: The Wulong diorite porphyry displays a very unusual flower-like glomerophyric texture and is thus well-known as a gemstone type of "Luoyang peony stone" for non-geologists. Hereby, we present a comprehensive study in terms of petrography, zircon U-Pb geochronology, geochemistry and Sr-Nd-Pb isotope. The rock exposed as small-scale dykes in the field, intruding in the Archean migmatized biotite-plagioclase genesis at the southern margin of the North China Craton. It exhibits typical porphyrite texture with plagioclase and amphibole as dominant and subdominant phenocrysts, respectively. The diorite porphyry rocks contain 51 wt.%-59 wt.% SiO2, belonging to medium-K calc-alkaline rock series. They are characterized by enrichment of large ion lithophile elements (LILEs, e.g., Ba) and depletion of high field-strength elements (HFSEs, e.g., Nb, Ta, Zr and Hf), resembling island arc magmatic rocks. They show high initial Sr isotopic ratios (0.710 7-0.715 8), low εNd(t) values (-9.9 to -13.3) and variable initial Pb isotopic ratios (206Pb/204Pb vary from 17.3 to 19.3), similar to the coeval intermediate and mafic magmatic rocks of adjacent region. These geochemical features indicate that the Wulong diorite porphyry is likely to originate from an enriched mantle with contamination of crustal materials. Zircon U-Pb dating results reveal a magma crystallization age of 480±3 Ma, consistent with the summit of the magmatic events of the Qinling orogenic belt and reflecting the northernmost Paleozoic magmatic activity of the belt. This may indicate the influence of northward subduction of Paleotethyan oceanic slab can extend a distance of ca. 200 km to the north and reach Luoyang-Songshan region in the East Qinling Orogen.
KEY WORDS: Early Paleozoic    North China Craton    geochronology    diorite porphyry    Wulong    flower-like glomerophyric texture    

0 INTRODUCTION

The Qinling orogenic belt (QOB) marks the suture zone between the South China Block (SCB) and the North China Craton (NCC) in Central China (Fig. 1a), forming one of the major composite orogens in eastern Asia (Wu and Zheng, 2013; Dong et al., 2011a; Tseng et al., 2009). Multistage tectonic evolution between the SCB and NCC resulted in complex geological framework of the QOB which is mainly composed of the Early Paleozoic accretion-dominated North Qinling orogenic belt (NQB) and the Early Mesozoic collision-dominated South Qinling orogenic belt (SQB) (Dong and Santosh, 2016; Dong et al., 2016; Zhang et al., 2015; Shi et al., 2013; Wu and Zheng, 2013). The Early Mesozoic collision has been agreed by most geologists that mainly occurred along the Mianlue suture zone and the Dabie-Sulu ultra-high pressure orogen (Dong et al., 2013, 2011a; Wu and Zheng, 2013; Zhang et al., 2001; Meng and Zhang, 1999). However, the Early Paleozoic tectonic history of the QOB, including the precise nature of the Shangdan suture and the subduction polarity remains controversial (e.g., Zhang et al., 2015; Wu and Zheng, 2013; Dong et al., 2011a, b; Ma et al., 2007, 2006; Ratschbacher et al., 2003; Xue et al., 1996). For example, Ma et al.(2007, 2006) proposed that the southward subduction for the NCC caused the back-arc extension of the SQB and the arc magmatism of the NQB, and thus suggested a paired magmatic belt model for the Early Paleozoic tectonic history of the QOB. In contrast, other geologists proposed a northward subduction model for the SCB along the Erlangping zone, involving oceanic slab subduction and formation of arc, backarc spreading, continental deep subduction and arc-continent collision (Dong and Santosh, 2016; Dong et al., 2016, 2013, 2011a, b; Liu et al., 2016; Tang et al., 2016; Zhang H F et al., 2015; Wang H et al., 2014; Bader et al., 2013; Wang X X et al., 2013; Wu and Zheng, 2013; Ratschbacher et al., 2003; Zhang G W et al., 2001; Meng and Zhang, 2000; Xue et al., 1996).

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Figure 1. (a) Distribution of Early Paleozoic magmatic rocks in the North Qinling belt (modified after Wang X X et al., 2013). TB. Tongbai; SL. Sulu; EDB. eastern Dabie; WDB. western Dabie; YQWF. Yantai-Qingdao-Wulian fault. (b) Simplified geological map of Wulong diorite porphyry dykes (after 1/200 000 Geological Map). S-NCC. Southern margin of the North China Craton; NQB. North Qinling belt; SQB. South Qinling belt; N-SCB: northern margin of the South China Block.

Arc-related Early Paleozoic granitoids are widely distributed in the NQB (Fig. 1b; Wang X X et al., 2015, 2013; Zhang et al., 2013; Wang T et al., 2009). Accordingly, coeval intermediate and basic magmatic rocks (ca. 490–460 Ma) are also well documented along the southern part of NQB (Fig. 1b). They are represented by the Guanzizhen gabbro (Pei et al., 2007a) and Yanwan basalt in the western NQB (Chen et al., 2008b; Yan et al., 2007), the Luohansi and Sifangtai gabbros in the middle NQB (Liu et al., 2012, 2007), and the Fushui gabbro in the eastern NQB (Shi et al., 2017; Wang H et al., 2014). In contrast, Early Paleozoic magmatic rocks are rarely identified in the southern margin of the NCC (S-NCC). The Wangjiacha quartz diorite and Yanjiadian diorite are the only two Early Paleozoic intrusions found in the western S-NCC (Chen et al., 2008a; Pei et al., 2007b). While in the eastern S-NCC, no Early Paleozoic intrusions have been reported up to now.

In this work, we report a newly discovered Early Paleozoic intrusive rock, the Wulong diorite porphyry dyke. It is the first and only Early Paleozoic intrusion identified in the eastern S-NCC by current. Important to note, this rock also displays a very unusual flower-like glomerophyric texture and is famous as a gemstone of "Luoyang peony stone" for non-geologists. Despite of its popularity, the fundamental geological study of the rock is rather limited. The petrogenesis and related geological significance apart from viewing values are poorly understood. In this study, we provide an integrated investigation of in situ zircon U-Pb dating and whole-rock major and trace elements in combination with Sr-Nd-Pb isotopes for the Wulong diorite porphyry dykes, with aims to (1) precisely date the dykes; (2) constraining the origin and evolution of the dioritic magma; and (3) shed some inspiration on the Early Paleozoic tectonic evolution of East Qinling Orogen.

1 GEOLOGICAL SETTING AND SAMPLE DESCRIPATION

From north to south, the Qinling orogenic belt can be divided into four units by major sutures and faults: S-NCC, NQB, SQB and northern margin of SCB (N-SCB) (Fig. 1b; Dong et al., 2011a; Zhang et al., 2001; Meng and Zhang, 2000). The S-NCC is located between the Lounan-Luanchuan fault and Sanmenxia-Lushan fault (Fig. 1b). It mainly consists of Archean– Paleoproterozoic basements (Taihua and Dengfeng groups; Hu et al., 2014) that are unconformably overlain by Paleo– Mesoproterozoic rift-related volcanic rocks (Xiongʼer Group; Zhao and Zhou, 2009), Meso–Neoproterozoic marine facies clastic and carbonate sequences (Zhao et al., 2004) and Sinian– Ordovician carbonatites and sandstones (Zhang et al., 2001). The NQB bound by the Lounan-Luanchuan fault to the north and Shangdan Suture to the south (Fig. 1b), and comprises Kuanping Group, Erlangping Group, Qinling Group, Songshugou complex and Danfeng Group, which are unconformably covered by Carboniferous–Permian and/or Lower Triassic clastic sediments (Dong et al., 2011b). The SQB is located between the Shangdan and Mianlue suture (Fig. 1b), and consists of Precambrian metamorphosed crystalline basements and Sinian–Carboniferous sedimentary rocks (Zhang et al., 2001). The N-SCB is located in the southern part of the Mianlue suture (Fig. 1b) and comprises an Archean basement (Kongling Group) including TTG genesis and amphibolites (Zhang et al., 2001).

Magmatic rocks are widespread in both NQB and SQB but are relatively less in S-NCC and N-SCB. Numerous Early Paleozoic intrusions (Fig. 1b; Wang X X et al., 2013) exposed along the southern part of the Shangdan suture in NQB. A few Early Paleozoic alkali volcanic rocks (430 Ma; Wan et al., 2016), carbonatite-syenite complex (440 Ma; Zhu J et al., 2017) and mafic dykes (435 Ma; Dong et al., 2013) are developed in SQB. In contrast, the S-NCC and N-SCB are lacking of Paleozoic magmatism records.

The Wulong diorite porphyry is located within the S-NCC in Central China, geographically close to Luoyang City in Henan Province (Fig. 1b). It is outcropped as three small dykes (maximal width 20 m) and intruded into the Archean migmatized biotite-plagioclase gneiss behind the Wulong Village in Luoyang (Figs. 1c and 2a). The dykes are generally NWW- trending and parallel to each other in the field. Samples collected from the western two dykes are used for this study (34°31.18′N, 112°40.49′E; 34°30.98′N, 112°40.43′E).

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Figure 2. Field photographs of the Wulong diorite porphyry dykes. (a) Diorite porphyry dykes invaded into biotite-plagioclase genesis. (b) Plagioclase phenocrysts are classified into glomerocrysts and single isolated phenocrysts. (c) Flower-like glomerocrysts. (d) Irregular-shaped glomerocrysts. (e) Single tabular and stumpy phenocrysts. (f) Elongated flower-like glomerocrysts and elongated lenticular single phenocrysts.

The diorite porphyry dykes are generally grayish black (Fig. 2a), and display typically porphyritic texture with white plagioclase as dominant phenocryst. Based on topographic differences, the plagioclase phenocrysts can be divided into the glomerocrysts and single isolated phenocrysts (Fig. 2b). In most cases, the plagioclase glomerocrysts resemble blooming flowers on the outcrops and are thus defined as "flower-like glomerocrysts" (Zhu Y X et al., 2018, 2017), in which smaller crystals clustering in the center and larger crystals radially distributed in the outer part (Fig. 2c). There is also a small amount of irregular-shaped glomerocrysts with a particle size of 1 to 6 cm (Fig. 2d). For non- geologists, these glomerocrysts are referred to as the "Luoyang peony stone", since the Luoyang City is well known for peony flowers. The particle size of single phenocrysts varies from 0.5 to 4.5 cm in diameter, most of which are tabular with a few in stumpy shape (Fig. 2e). A post-magmatic deformation structure with elongated plagioclase glomerocrysts or lenticular plagioclase single phenocrysts is observed in one outcrop (Fig. 2f). These elongated phenocrysts appear to be aligned parallel and only exist in the rock fracture zone.

The matrix consists of plagioclase (~50%), amphibole (~35%), biotite (~5%) and quartz (~5%) with accessory titanite, garnet, ilmenite and zircon. Late-stage alteration and/or metamorphism occur commonly in the rocks, resulting in almost all plagioclase phenocrysts, altered to sericite and/or clinozoisite, especially in the center parts (Fig. 3a). Alteration degrees depend largely on the particle size of plagioclase crystals. Small plagioclase grains in the matrix (0.1–1 mm) are mostly fresh with typical albitic polysynthetic twins (Figs. 3b and 3c). Amphibole is anhedral and partially altered to chlorite, filling gaps in the plagioclase matrix and/or entering in the plagioclase phenocryst interior along cracks (Fig. 3c). Biotite is subhedral to anhedral with a particle size of 0.05–0.2 mm, filling gaps between the plagioclase matrix and amphibole (Fig. 3d). Very tiny and anhedral quartz crystals densely distributed around the plagioclase phenocrysts and euhedral garnet (monocrystalline or twin) occasionally inset the edge of the plagioclase phenocrysts (Fig. 3e), indicating the influence of post-magmatic metamorphism on these rocks. Euhedral titanite associated amphibole is often occur as mineral inclusion (Fig. 3f).

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Figure 3. Representing photomicrographs showing mineral assemblage and textural features of the Wulong diorite porphyry. (a) Large plagioclase phenocrysts commonly altered to sericite and/or zoisite. (b), (c) Plagioclase matrix is mostly fresh with typical albitic polysynthetic twins. (d) Amphibole fills gaps in the plagioclase matrix, in contact with biotite. (e) Anhedral quartz and euhedral garnet inset the edge of plagioclase phenocrysts. (f) Titanite occurs as mineral inclusion. Mineral abbreviations: Amp. amphibole; Bt. biotite; Chl. chlorite; Czo. clinozoisite; Grt. garnet; Pl. plagioclase; Qtz. quartz; Ser. sericite; Ttn. titanite.
2 ANALYTICAL METHODS 2.1 Whole Rock Major and Trace Elements Analyses

Bulk major elemental contents were measured for 10 fresh and representative rock samples (Table 1). The rock specimens were firstly cleaned with deionized water and crushed to powder. Major element compositions were detected by a ME-XRF26d at Australia Laboratory Services (ALS) Chemex Co. Ltd. (Guangzhou, China). Standard deviations of analysis are < 5% and detection limits are < 0.01%. Trace elements were analyzed for 5 corresponding samples (Table 2) at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences (CUG, Wuhan). Detailed sample preparation and analytical procedure can be found in Liu et al. (2008). 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 elements.

Table 1 Major element compositions (wt.%) of the Wulong diorite porphyry from the southern margin of the North China Craton
Table 2 Trace element contents (ppm) of the Wulong diorite porphyry from the southern margin of the North China Craton
2.2 Whole Rock Sr-Nd-Pb Isotope Analyses

Whole rock Sr and Nd isotopic contents for 3 rock samples were measured using a Finnigan Triton TI TIMS at the State Key Laboratory for Mineral Deposits Research (MDR), Nanjing University (NJU). Samples were adopted with resin of AG 50W X 8 and different eluent reagents. Afterwards Rb, Sr, Sm and Nd were separated and purified for the subsequent analyses of isotopes. Detailed descriptions of the analytical process were given in Pu et al. (2005). The NIST SRM-987 and JNDI-1 were used as standard yield for an 87Sr/86Sr ratio of 0.710 248±0.000 005 (2σ), and a 143Nd/144Nd ratio of 0.512 100±0.000 005 (2σ). The following reference values were used to calculate the ɛNd(t) and ISr values and the depleted mantle model ages (TDM): (143Nd/144Nd)CHUR= 0.512 638 (Goldstein et al., 1984); (147Sm/144Nd)DM=0.213 75; (143Nd/144Nd)DM=0.513 15; λSm=6.54×10-12 year-1; λRr=1.393×10-11 year-1.

Isotopic ratios of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb were performed using a mutil-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) at MDR in NJU. Rock powders (100–150 mg) were dissolved in Teflon vials with purified HF and HNO3, and were subsequently separated by the anion-exchange columns with diluted HBr as eluent. More details could be found in He (2005). NBS981 standard fields (n=5, 2σ) were analyzed with a 206Pb/204Pb ratio of 16.930 4±0.000 5, a 207Pb/204Pb ratio of 15.483 5±0.000 5, and a 208Pb/204Pb ratio of 36.674 1±0.001 3. The reference values of 206Pb/204Pb=16.941, 207Pb/204Pb=15.496 and 208Pb/204Pb=36.722 (Abouchami et al., 1999) were used to calculate initial Pb isotope ratios.

2.3 Zircon U-Pb Dating

Zircon was separated from > 10 kg pulverized rocks of the Wulong diorite porphyry dykes, picked under the microscope and put into epoxy resin to make target. Zircon grains were examined by combine transmitted and reflected light and cathodoluminescent (CL) images, which were further referred during potential zircon targets picking for trial analysis. In-situ U-Pb dating were conducted using a GeoLasPro laser ablation system (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system and an Agilent 7700e ICP-MS instrument at Shangpu Co. Ltd (Wuhan, China). The laser spot was 32 μm in diameter. An external zircon standard GJ-1 was used to normalize isotopic fractionation, and zircon standard 91500 was used as further unknown data quality certification. Data processing was conducted using software ICPMSDataCal (Liu et al., 2010, 2008). Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_version 4.1 (Lugwig, 2010).

3 RESULTS 3.1 Whole Rock Major and Trace Elements

Nine of the ten analyzed samples show uniform SiO2 (50.58 wt.%–51.68 wt.%), Fe2O3T (total iron expressed as Fe2O3; 13 wt.%–14.75 wt.%), MgO (4.00 wt.%–4.72 wt.%) and CaO (7.9 wt.%–8.7 wt.%) contents (Table 1). They are characterized by high Al2O3 (15.2 wt.%–16.38 wt.%) and Na2O (2.42 wt.%–2.93 wt.%) but low K2O (1 wt.%–1.49 wt.%), TiO2 (0.81 wt.%–0.9 wt.%), P2O5 (0.19 wt.%–0.21 wt.%) contents, with Na2O/K2O > 1, σ=1.47–2.33, Mg#=31–39, A/CNK=0.73–0.78, A/NK=2.68–2.91. As an exception, sample 15LY01 has higher SiO2 (59.81 wt.%) and lower Fe2O3T (7.37 wt.%) and CaO (4.76 wt.%) contents, probably resulted from a higher proportion of plagioclase phenocryst in the rock than others. In the total alkali-silica (TAS) diagram, all samples except sample 15LY01 are plotted in the field of gabbro within the sub-alkaline area (Fig. 4a). They belong to the middle-K calc-alkaline series in the K2O vs. SiO2 diagram (Fig. 4b).

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Figure 4. (a) Plots of Na2O+K2O contents versus SiO2 contents (after Middlemost, 1994) and (b) plots of K2O contents versus SiO2 contents (after Rollinson, 1993) of the Wulong diorite porphyry. The Fushui complex data are from Wang H et al. (2014) and Shi et al. (2017).

In the multi-element spider diagram (Fig. 5a), the Wulong diorite porphyry dykes show uniform characteristics of depletion in high field-strength elements (HFSEs) (e.g., Nb, Ta, Zr, Hf) and of enrichment in large ion lithophile elements (LILEs) (e.g., Ba). The chondrite normalized REE patterns show significant enrichment of light rare earth element (LREE) with steep slopes (LaN/YbN=4.67–7.84) and weak Eu negative anomaly (Eu/Eu*=0.74–0.97) (Fig. 5b).

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Figure 5. (a) Primitive mantle-normalized multi-element spider diagram and (b) chondrite normalized rare earth element diagram for the Wulong diorite porphyry. Primitive mantle-normalized values, chondrite-normalized values, OIB (oceanic island basalt), E-MORB (enriched mid-ocean ridge basalt) and N-MORB (normal mid-ocean ridge basalt) data are from Sun and McDonough (1989). Continental arc basalt (CAB) data are from Kelemen et al. (2003) and IAB (island arc basalt) data are from Ewart et al. (1998). The Fushui complex data are from Wang H et al. (2014) and Shi et al. (2017).
3.2 Whole Rock Sr-Nd-Pb Isotopes

Whole rock Sr-Nd isotopic data are listed in Table 3. The 87Rb/86Sr ratios vary from 0.365 to 1.669, and 87Sr/86Sr ratios from 0.713 5 to 0.727 3, 147Sm/144Nd ratios from 0.116 to 0.127 and 143Nd/144Nd ratios from 0.511 4 to 0.511 5. The isotopic ratios were recalculated using zircon U-Pb age of 480 Ma (see below) yielding (87Sr/86Sr)i values of 0.710 7 to 0.715 8, ɛNd(t) values of -9.9 to -11.3, and Nd model ages (TDM2) of 2 010–2 130 Ma. Whole rock Pb isotope compositions are variable and characterized by the higher radioactive Pb values. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios range from 17.605 to 19.639, 15.582 to 15.863 and 36.815 to 38.706, respectively (Table 3), yielding corresponding initial ratios (calculated at 480 Ma) of 17.33–19.26, 15.57–5.84 and 36.44–37.83, respectively.

Table 3 Sr, Nd and Pb isotopic ratios of the Wulong diorite porphyry from the southern margin of the North China Craton
3.3 Zircon U-Pb Ages and Trace Elements

Zircons from the Wulong diorite porphyry are colorless, transparent and subhedral-anhedral. They can be divided into two groups according to texture and composition. Group Ⅰ has a smaller size of 30–50 μm with a broader oscillatory zoning revealed by CL images (Fig. 6). These zircons display moderate Th (82 ppm–692 ppm) and U contents (372 ppm–1 460 ppm) and restricted and lower Th/U ratios from 0.25 to 0.56 (Table 4). Their 206Pb/238U ages range from 497 to 473 Ma with a weighted mean of 480±3 Ma (MSWD=0.62) (Figs. 7a and 7b). The chondrite normalized REE patterns of this group are characterized by weaker positive Ce and negative Eu (Eu/Eu*=0.43–0.69) anomalies (Fig. 7c) as well as stronger HREE enrichment (LuN=962–4 107). Group Ⅱ zircons show larger grain sizes from 40 to 55 μm. Compositional zonations in CL images are common, generally with a bright rim and a dark and rounded core (Fig. 6). The Th and U contents of the cores are intensively varying (Th=59.0 ppm–1 086 ppm, U=80.7 ppm–1 319 ppm), yielding a wide range of the Th/U ratio within 0.21–2.68 (Table 4). These zircons display inconsistent 206Pb/238U ages ranging within 992–630 Ma. They show variable positive Ce and negative Eu (Eu/Eu*=0.03–0.64) anomalies (Fig. 7c) as well as HREE enrichment (LuN=2 200–3 682).

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Figure 6. Typical CL images of zircon grains from the Wulong diorite porphyry.
Table 4 Representative zircon U-Pb isotopic data obtained by LA-ICP-MS for sample 15LY02 from the Wulong diorite porphyry, S-NCC
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Figure 7. (a), (b) Concordia diagrams of LA-ICP-MS zircon U-Pb dating for the Wulong diorite porphyry. (c) Chondrite normalized rare earth element patterns of zircons. Chondrite-normalized values are from Sun and McDongough (1989). (d) Discrimination diagram of (Sm/La)N vs. La (ppm) by zircons (after Kirkland et al., 2009).

In the La versus (Sm/La)N diagram, both group zircons show high (Sm/La) N ratios (Fig. 7d), consistent with the magmatic zircons (Kirkland et al., 2009; Zhong et al., 2006; Wu and Zheng, 2004). We suggest that Group Ⅰ zircons are formed during solidification of the Wulong glomerophyric diorite porphyry and represent the crystallization age of the rock, whereas Group Ⅱ zircons are likely inherited from the magma source or the country rocks.

4 DISCUSSION 4.1 Magma Source and Processes

Several genetic models have been proposed for intermediate igneous rocks, including partial melting of the lower crust, low degree melting of the depleted mantle, and fractional crystallization with contamination of the crustal materials (e.g., Wang L X et al., 2014; Blatter et al., 2013; Tatsumi et al., 2008; Annen et al., 2005; Tatsumi and Hanyu, 2003). The diorite porphyry investigated in this study have notably different initial 87Sr/86Sr ratios (Fig. 8a) compared to Piaochi granite and gneisses in the North Qinling (NQ) and Paleoproterozoic diorite dykes in the S-NCC, may suggest that they are not originated from such a crustal source (Wang C M et al., 2016; Wang T et al., 2009). The rocks have negative ɛNd(t) values in contrast to the NQ metabasites and oceanic arc basalts (OAB) in the Shangdan region (Fig. 8a), which contradicts with a model of partial melting of depleted mantle (Fig. 8a). Notably, these dykes are characterized by high Nb/Ta ratios (15.8–17.7), a range that is overlapping with primitive mantle (17.5±2.0; Rudnick and Gao, 2003). They show high 87Sr/86Sr ratios, low ɛNd(t) values and 208Pb/204Pb and 207Pb/204Pb ratios, similar to an enriched mantle member (EM) (Fig. 8). Their ɛNd(t) values (-9.9 to -13.3) are lower than the coeval Fushui gabbro complex (-3.97 to -5.68; Wang H et al., 2014), which also indicates a more enriched source (Fig. 8a). In the comparative diagrams, the samples of this study are plotted near the field of EM2, away from the EM1 field (Figs. 8a and 8b), suggesting that they may have an EM2-like mantle source.

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Figure 8. (a) Initial 87Sr/86Sri vs. ɛNd(t) (t=480 Ma) diagram for the Wulong diorite porphyry. The data for Fushui amphibole gabbro, Piaochi granite, OAB in the Shangdan suture, NQ (North Qinling) gneisses, NQ metabasites and Xiongʼer diorite dykes are shown in Table S2. (b), (c) Plots of 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs.206Pb/204Pb (t=480 Ma) for the Wulong diorite porphyry. Data for Early Paleozoic acid dykes from the NQ are listed in Table S2. The HIMU, MORB and enriched mantle (EM1 and EM2) fields are from Hofmann (1997). The depleted mantle (DM) data are from Hart et al. (1992).

As mentioned above, post-magmatic alteration and/or metamorphism have occurred in the Wulong diorite porphyry dykes. Nevertheless, the high field-strength elements (HFSEs, such as Nb, Ta, Zr and Hf), rare earth elements (REE) and Y are correlated with Zr (Table 1), indicating that these elements were essentially immobile during alteration and/or metamorphism and can be used to trace the magma source (Tang et al., 2012). Basaltic rocks derived from the asthenospheric mantle are characterized by enrichment of LILEs and HFSEs, which are similar to the oceanic island basalt (OIB) but quite different from the mid-ocean ridge basalt (MORB) (Zhou et al., 2016; Zhao et al., 2007). Our samples from the Wulong diorite porphyry dykes show typical arc-affinity geochemistry which are rich in LREE and LILEs and poor in HFSEs (e.g., Nb and Ta; Fig. 5a), inconsistent with an OIB-like source. They have higher Zr/Nb ratios (16–23) than that of OIB (5.8), further ruling out an asthenospheric mantle source (Zhao et al., 2010). Their geochemical features are similar to that of the coeval Fushui gabbroic complex, which has been considered to originate from an enriched lithospheric mantle (Figs. 4 and 5, Wang H et al., 2014). The high La/Ta, La/Nb, Ba/Nb, Th/Zr and Rb/Y ratios and low Nb/Zr and Nb/Y ratios of the Wulong diorite porphyry (Figs. 9a and 9b) suggest that their source have been metasomatized by subduction-related fluid (Wu et al., 2017; Mustafa et al., 2016; Kepezhinskas et al., 1997), which could be also supported by the trend of fluid enrichment in the Ba/Nb vs. Ba/La diagram (Fig. 9c). High LILEs contents and Sr isotopic ratios for the diorite porphyry indicate the presence of potassic mineral phases in the source. The relatively high Rb/Sr but low Ba/Rb ratios (Fig. 9d) indicate a phlogopite-bearing mantle source (Furman and Graham, 1999).

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Figure 9. Plots of Nb/Zr vs. Th/Zr (a), Rb/Y vs. Nb/Y (b), Ba/Nb vs. Ba/La (c), and Rb/Sr vs. Ba/Rb (d) for the Wulong diorite porphyry. Lower crust, upper crust, primitive mantle, N-MORB and OIB fields are from Hofmann et al. (1986), Sun and McDonough (1989) and Rudnick and Gao (2003). Panel is modified after Furman and Graham (1999).

As synthesized by Rudnick and Gao (2003), Ce/Pb and Nb/U ratios of mantle-derived magma (25±5 and 47±10; respectively) are remarkedly higher than those of the continental crust (average of Ce/Pb=3.9 and Nb/U=6.2), and thus have been considered as effective indicators for crustal contamination. The diorite porphyry dykes of this study have low Ce/Pb and Nb/U ratios (4.6–7.1 and 3.8–4.9, respectively), much lower than pure mantle-derived magmas and therefore suggesting the existence of crustal contamination. The inherited zircons with dark nucleus in the CL images further confirmed this view (Fig. 6), which might be captured from country rocks during magma upwelling. In the Ba/La vs. Ba/Nb diagram, most samples are plotted nearby the upper crust or lower crust field (Fig. 9c), further indicating the influence of crustal materials. The Pb isotopes of the diorite porphyry dykes are close to those of the Early Paleozoic acid rocks in the NQ, manifesting the contribution of the lower crustal materials (Figs. 8b and 8c). The slightly negative Sr anomalies and negative Eu anomalies (Eu/Eu*=0.7–0.9) (Fig. 5) for the diorite porphyry dykes could be attributed to fractional crystallization of plagioclase. In summary, the diorite porphyry dykes are derived from an enriched mantle but contaminated by crustal materials, with fractional crystallization prior to magma solidification.

The extensive replacement of plagioclase by sericite and zoisite (Fig. 3a) implies that the diorite porphyry dykes have suffered alteration after emplacement, which could be further evidenced by relatively high values of loss on ignition (0.9 wt.% and 2.3 wt.%; Table 1). The mobile elements (e.g., Rb, U and K) have relatively high abundance in the multi-element spider diagram (Fig. 5a), further indicating that the rocks have undergone low-grade alteration under advanced oxidation conditions (Deniel, 1998). The formation of sericite can be expressed by the equation (Hemley and Jones, 1964): 0.75Na2CaAl4SiO24 (andesine)+2H++ K+=KAlSi3O10(OH)2 (sericite)+1.5Na++0.75Ca2++3SiO2 (quartz). The major components changes in the rocks is the removal of Ca and Na, and some Mg, as well as the addition of a chemically equivalent amount of H. Al tends to remain constant as does K, while in some instance of sericitization K may be added to the rock. This corresponds to higher K2O contents (3.48 wt.%) of sample 15LY01-1 that has higher proportion of plagioclase phenocrysts than others (Table 1).

4.2 Northernmost Paleozoic Magmatism of the East Qinling Orogen

Our work reveals a zircon U-Pb concordant age of 480±3 Ma for the Wulong diorite porphyry, which indicates an Early Paleozoic magmatic event. Early Paleozoic magmatism are widely distributed in the NQB, forming an NNW-trend intrusive rock belt (Fig. 1a; Table S1; Dong and Santosh, 2016; Wang et al., 2015; Zhang et al., 2013; Dong et al., 2011b). In a broad view of these intrusions, most are along the northern side of Shangdan suture zone, while rare are recognized in S-NCC, north of Luonan-Luanchuan fault (Fig. 1a). The Yanjiadian diorite and Wangjiacha quartz diorite in the western part are the only two Early Paleozoic intrusions found in S-NCC (Chen et al., 2008a; Pei et al., 2007b). The Wulong diorite porphyry dyke is the first and the only Early Paleozoic intrusion recognized in the eastern part of S-NCC by current (Fig. 1a). On the other hand, it is also the northernmost Early Paleozoic intrusion in the East Qinling Orogen (Fig. 1a).

Neoproterozoic magmatism and strata are common in the South China Block and the Qinling-Dabie orogenic belt, but are absent in the NCC and the S-NCC (e.g., Gan et al., 2016; Zhang R Y et al., 2016; Dong et al., 2015, 2011b; Zhang S M et al., 2014; Lu et al., 2004; Ling et al., 2003). The inherited zircon ages of ca. 992–630 Ma in the Wulong diorite porphyry (Table 4; Fig. 6) may indicate the existence of Neoproterozoic basement in eastern of S-NCC besides the widespread Neoarchean–Paleoproterozoic Taihua and Xiongʼer Group basement.

Chemically, the Wulong diorite porphyry is enriched in LILEs and depleted in HFSEs, similar to the island arc and back- arc basaltic rocks (Fig. 5; Wang H et al., 2014). In the Ti-Sm-V diagram, the samples of this study are plotted in the island arc basalt (IAB) range (Fig. 10a; Vermeesch, 2006). High V contents and low Ti/V ratios have been documented as typical features of island arc basalts caused by high oxygen fugacity (Kelley and Cottrell, 2012; Shervais, 1982). The studied diorite porphyry displays such kind of characteristics and is rather similar to the contemporaneous Fushui amphibole gabbro and monzodiorite in the NQB (Fig. 10b) which have been proposed to have IAB features (e.g., Shi et al., 2017; Wang H et al., 2014; Zhang et al., 2013). The REE patterns and the multi-element spider diagrams of the analyzed samples are almost uniform to those of typical IAB from the Tonga- Kermadec, New Zealand (Fig. 5; Ewart et al., 1998), which is as well construable with an island-arc setting. In addition, the Wulong diorite porphyry has low Zr contents (< 130 ppm) and Zr/Y ratios (< 4), which is characterized by island arc basalt and distinct from the continental arc basalt (Xia et al., 2007).

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Figure 10. Tectonic discrimination diagrams for the Wulong diorite porphyry. (a) Ti/50-50×Sm-V diagram. (b) V-Ti/1 000 diagram (after Shervais, 1982). The Fushui complex data are from Wang H et al. (2014) and Shi et al. (2017).

Several contemporaneous gabbro intrusions and extrusive equivalents with island-arc features are distributed along the NQB, e.g., at Guanzizhen (489 Ma, Pei et al., 2007a), Yanwan and Xieyuguan basalts (483 and 472 Ma, respectively; Chen et al., 2008b; Yan et al., 2007), Luohansi (475 Ma, Liu et al., 2007); Sifangtai (460 Ma, Liu et al., 2012), and Fushui (488 Ma, Wang H et al., 2014) gabbros. A great deal of subduction-related granitoid intruded into the northern side of the Shangdan suture (Zhang et al., 2013; Wang et al., 2009). These mafic and felsic intrusive and volcanic rock series point to an island-arc terrane setting within the North Qinling belt during Early Paleozoic (Dong et al., 2007). The development of the NQB as an active continental margin with a trench-arc-basin system has been caused by a northward subduction of the Paleotethyan Ocean (Wu and Zheng, 2013; Zhang et al., 2013; Dong et al., 2011b). According to Wu and Zheng (2013), the early-stage subduction of the Paleotethyan Ocean began at > 490 Ma, along the formation of Erlangping intra-oceanic arc. Subsequently at ca. 490–480 Ma, the subduction of the NQB microcontinent beneath the Erlangping arc induces the UHP eclogite-facies metamorphism (Wang H et al., 2013, 2011), which also corresponds to the generation of intermediate and basic intrusion in the NQB and the S-NCC in an island-arc environment (Fig. 11). As one of them, the Wulong diorite porphyry is the first identified Early Paleozoic intrusion in the deformation fold belt of the eastern S-NCC, recording the northernmost Paleozoic arc magmatism in the Qinling orogenic belt, which suggests that the subduction of Paleotethyan Ocean slab can affect sub-horizontally as far as the S-NCC and reach Luoyang-Songshan area.

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Figure 11. Schematic cartoon showing the northward subduction of the Paleotethyan Ocean and the formation of the eclogites-facies metamorphism, and the arc magmatism in the S-NCC and the North Qinling units (after Wu and Zheng, 2013).
4 CONCLUSION

The Wulong diorite porphyry dykes were generated at 480 Ma, recording the northernmost Paleozoic arc magmatism in the East Qinling Orogen. It belongs to the middle-K cal-alkaline rock series, which is characterized by relatively extreme HFSEs (e.g., Nb, Ta, Zr, Hf and Y) depletion and LILE (e.g., Ba) enrichment. Combined Sr-Nd-Pb isotopic systems and geochemical data suggest that magma is derived from enriched mantle. Crustal contamination may be an important mechanism for the formation of diorite porphyry dykes. Generation of the parental magma was probably triggered by northward subduction of Paleotethyan Ocean crust in an island arc environment.

ACKNOWLEDGMENTS

Sincere gratitude is expressed to Prof. Zhendong You, who has long-termed guided and helped young geologist, in particular to the third author of this work, Dr. Changqian Ma, who is now a senior professor in mineralogy and petrology field. Financially supports are gratefully acknowledged from the National Natural Science Foundation of China (Nos. 41530211, 41502046) and the China Geological Survey (No. DD20160030). The Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUGCJ1711) is also acknowledged for partial financially support. We are grateful to Haochen Duan, Qihui Xiong, Yuchen Liu, Lian Cai and Yanqing Li for their assistance during field sampling and laboratory analysis. Special thanks to Prof. Shanrong Zhao, Drs. Chang Xu, Bin Xia, Profs. Neng-Song Chen and Paul Robinson for their assistance with sample identification and manuscript preparation. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0878-1.

Electronic Supplementary Materials: Supplementary materials (Tables S1–S2) are available in the online version of this article at https://doi.org/10.1007/s12583-018-0878-1.


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