Zheng Duan, Guang-fu Xing, Sheng-bing Liao, Ping-li Chu, Wen-cheng Huang, Yan-hui Zhu, Xu-jie Shu,Chang-bo Li

Nanjing Center of China Geological Survey, Nanjing 210016, China

ABSTRACT

Numerous dark enclaves with different shapes are found in Jiuling Neoproterozoic granitoids. Precise LAICP-MS U-Pb dating was conducted on zircons extracted from two microcrystalline enclave samples,yielding crystallization ages of 822.6±5.8 Ma and 822.2±6.2 Ma, respectively. The consistent ages within analytical errors with the host granitoids suggested that they were the products of the same magmatism.The microcrystalline-dioritic enclaves commonly show plastic forms and contain similar plagioclase megacrysts to the host rocks, and both of the enclaves and host granitoids showed a complex composition and structural imbalance in plagioclases. Furthermore, the apatites with a euhedral acicular shape occurred widely in the microcrystalline-dioritic enclaves. All of these petrographic features above imply magma mixing is involved in their diagenesis. The enclaves and host granitoids show a marked zircon trace element difference and Hf isotopic signatures without correlation in zircon trace element pairs but form their own system between enclaves and host granitoids. Additionally, most of the zircons show extremely high εHf (t) with εHf (t) =3.54-11.94 from the southern samples, and εHf (t) =1.0-9.09 from the central region. Some zircons with the higher εHf (t) are similar to the zircons from the juvenile island arc in the eastern segment of Jiangnan Orogen. Integrated geological and Hf isotopic characteristics suggest microcrystalline-dioritic enclaves were derived from the partial melting process of the Mesoproterozoic crust which enriched juvenile island arc materials and mixed with the granitic magma that remelted from the Mesoproterozoic continental crust which relatively enriched ancient sediments and mixed with the host granitoid in diagenesis.

Keywords:

Zircon U-Pb age

Zircon trace element compositions

Hf isotopic compositions

Magma mixing

Jiuling Pluton

1. Introduction

“Jiangnan Orogen ”, which formed through the amalgamation of the Yangtze Block with the Cathaysia Block, is a Precambrian geological mass near the NE trend with a length of 1500 km and a width of 200 km, mainly composed of Neoproterozoic low metamorphic sedimentary series,Neoproterozoic granitoids and a few mafic igneous. The Jiuling Pluton is the largest Neproterozoic intrusion in the South China Block (＞ 2500 km2), however it is exposed in the eastern segment of Jiangnan Orogen (Li WX et al., 2003)(Fig. 1). There is still much controversy focussed on the origin of these Neoproterozoic granitoids. There are mainly three kinds of views as follows: (1) remelted from the lower crust induced by mantle plume activity (Li WX et al., 2003,2008); (2) derived from the post-collisional and post-orogenic magmatic activities, caused by the oceanic crust subducted into the Yangtze Block (Wang XL et al., 2006, 2014); (3)formed in the rift setting after subduction and collision and related to the collapse of the arc-continent collisional orogenesis (Wu RX et al., 2006; Zheng YF et al., 2008). In recent years, Jiuling Pluton has been the subject of substantial research, and the accepted geological scheme can be summarized as following two aspects: (1) Jiuling Pluton invaded at 820–825 Ma; (2) pluton containing garnets, cordierites and other aluminum-rich accessory minerals, are typical of S-type granitoids characteristics, and are mainly formed by continental crust remelting with no appreciable addition of mantle components (Li XH et al., 2003; Xue HM et al., 2010; Zhao KD et al., 2013).

Fig. 1.A geological sketch map of the studied area. a-Simplified geological map of South China (modified from Yao JL et al., 2014); b-Simplified geological map of the Xiushui-Wuning area in NW Jiangxi (modified from 1:50 000 the Luoxi sheet, the Huangshaqiao sheet, and the Shimenlou sheet geological map).

The dark enclaves in the granitoids play a huge part in revealing the deep geodynamic processes, origin, emplacement and petrogenesis of magma, which cause this to be an important window for understanding deep geological processes (Barbarin B, 2005). Most of the dark enclaves in Jiuling Neoproterozoic granitoids are microcrystalline dioritic enclaves. However, detailed research is rarely conducted.Recently, studies such as Sun KK et al. (2017) argued that the dark enclaves in Jiuling granodiorites were formed from the interaction of mafic magma with a potassium-rich crust, and the enclaves have relatively uniform and higher εNd(t) values than that of the host granite. While the εHf(t) of the enclaves showed a wide range, and are obviously distinctly different from their εNd(t) (Sun KK et al., 2017), this feature may not simply be caused by the interaction of basalt magma with the potassium-rich crust. On the other hand, the zircons can preserve the initial Hf compositions well, so the εHf(t) of zircons are more suitabe to imply the information of the source regions than the εNd(t) of the entire rock (Andersen T et al., 2002; Griffin RF et al., 2004). Thus, the enclaves with a wide range of εHf(t) may imply their original materials are complex, even maybe of a mix in the origin of the region.Furthermore, Didier J and Barbarin B (1991) argued that the microcrystalline dioritic enclaves derived from magma mixing/mingling, and are commonly found in calc-alkaline granitic rocks. Research into mingling/mixing has been a hot topic in granite research of late. Thus, we provide research on these kinds of enclaves, that may contribute to a further understanding of the source area material composition of Jiuling Pluton. However, magma mixing will lead to obvious element migration, which may cover some aspects of the magma source, but the information in the magma source can be well maintained by the Lu-Hf isotope system during the process of magmatic evolution. Therefore, it is appropriate to identify the trail of the magmatic source and the specific magmatism process (Janoušek V et al., 2004). Thus, we use MC-LA-ICPMS to analyze the Lu-Hf isotope of zircons from microcrystalline dioritic enclaves in the studied area, which is expected to identify the different original source area material composition of Jiuling Pluton, and it can provide more details about the diagenetic evolutionary process of Jiuling Pluton.

2. The geological background and sample description

The "Jiangnan Orogen" is a collision belt between the Yangtze Block and the Cathaysia Block. It starts from the northern Guangxi in the west and passes through southwestern Guizhou, northeastern Guizhou, western Hunan,northwestern Jiangxi, northeastern Jiangxi, southern Anhui,western Zhejiang, and northern Zhejiang to the East. The granitoids of the Neoproterozoic are widely exposed in this belt, and all these rocks intrude into the metasedimentary strata. Wang XL et al. (2014) divided the Jiangnan Orogen into two segments: the eastern segment and the western segment of Jiangnan Orogen, with the boundary extending from the north to the middle of Hunan. The Neoproterozoic granitoides in the eastern segment of Jiangnan Orogen are mainly distributed in southern Anhui (such as Xiuning Plutons, Xucun Plutons and Yixian Plutons) and northwestern Jiangxi (such as Jiuling Pluton).

Jiuling Pluton is located in the eastern segment of Jiangnan Orogen (Fig. 1a) and contains numerous dark enclaves inside. Jiuling Pluton located in northwestern Jiangxi, is a granitic complex body, which is mainly composed of biotite-granodiorite, tonalite and biotitemonzogranite. The biotite-granodiorites consist mainly of plagioclase (30%-35%), K-feldspar (10%-15%), quartz(30%-40%), biotite (10%-15%) and indigo blue cordierites in euhedral hexagonal columnar, but the cordierites are mostly sericitized; the minerals in tonalite are plagioclase(45%-55%), K-feldspar (＜5%), quartz (25%-35%) and biotite (15%), most of the plagioclase is andesine with obvious zonal structures; the biotite-monzogranite is mainly composed of plagioclase (20%-30%), K-feldspar (mainly is perthite) (25%-35%), quartz (30%-40%) and biotite(10%-15%) (Duan Z et al., 2017). The SHRIMP zircon U-Pb dating of northern Jiuling granodiorite is 819±9 Ma measured by Li XH et al. (2003). Zhong YF et al. (2005) argues that the main body of Jiuling Pluton (all samples are biotitegranodiorite) emplaced at 828±8 Ma by SHRIMP zircon UPb. Ma TQ et al. (2009) measured SHRIMP zircon U-Pb ages of 816±5 Ma in Zhangbangyuan Pluton, Northeastern Hunan.Zhang FF et al. (2011) measured that the age of Jiuling granitoid (biotite-monzogranite) is 813±4 Ma in the eastern segment of Jiuling Pluton and 823±3 Ma in the southern segment of Jiuling Pluton, while Xiyuankeng Pluton (biotitemonzogranite) yields206Pb/238U age of 805±3 Ma in the southwestern segment of Jiuling Pluton. Wang XL et al.(2014) measured the LA-ICPMS zircon U-Pb age of Xiyuankeng and Jiuling granodiorite are 804±3 Ma and 820±4 Ma, respectively. Recently, the authors also obtained the rock forming age of biotite-granodiorite, tonilite and biotite-monzogranite, which are 821.6–824.0 Ma and 819.5–823.6 Ma and 820.4–824.5 Ma respectively used LAICPMS zircon U-Pb dating (Duan Z et al. 2018 , in press).

In general, the abundant dark enclaves in Jiuling Pluton can be classified into host rock xenoliths and the microcrystalline-dioritic enclaves. The host rock xenoliths are dark gray with hornfelsic obviously. The protolith is attributed to metasedimentary rocks of the Shuangqiaoshan Group,mostly angular and a few are oval with a major axis of 8–25 cm and there are mostly sharp and clear distinct boundaries within the host rock. While the microcrystalline-dioritic enclaves are dark gray, mostly rounded, and oval, the major axis is 5–20 cm, a few are 1–2 cm (Fig. 2). The major axisminor axis ratio is generally between 1.5 and 3.0. In addition,some of them are elongated to varying degrees but without solid deformation, and the boundary between the enclaves and the host rock is distinct but partially presents a narrow transitional zone. Furthermore, the narrow dark chilled margin and the light felsic halos grow around the enclaves (Fig. 2),which imply that there is an obvious assimilation and contamination between the enclaves and the host rocks. The mineral assemblages of the microcrystalline-dioritic enclaves are consistent with the host rock, but apparently more enriched in biotites. The enclaves display a porphyaceous texture, while the phenocrysts mainly consist of plagioclase in short columna, occasionally imbedded with crystal quartz, the matrix is mainly comprised of phenocrystalline biotite, that is significantly different from the host granitoids with a granitic texture. The main mineral assemblages of microcrystallinedioritic enclaves are plagioclase (30%-40%), biotite(35%-45%), quartz (10%-15%) and a small amount of K-feldspar. Some of the plagioclase phenocrysts show irregular form, which are inlayed in the quartz (Fig. 3b, c). The accessory minerals include ilmenite, monazite, garnet, zircon,apatite, etc. Importantly, the apatites are widely dispersed in the enclaves in tiny acicular crystal with the length of 0.1–0.2 mm, and their length-width ratio is about 10:1–20:1. A few microcrystalline-dioritic enclaves also contain the other metamorphic minerals like garnet, cordierite, andalusite, etc.Additionally, the geochemistry of the enclaves showed that the microcrystalline-dioritic enclaves are more enriched in CaO, Fe2O3T, MgO, Cr, Ni, Co, V, but depleted in SiO2when compared with the host rocks. The enclaves are also rich in LILE like Rb, Th, U, Pb, but have a depletion of HFSE like Nb, Ta, Ti, Ba, and present right-inclined type REE distribution patterns (Sun KK et al., 2017).

3. Analytical methods

Oxides (%) of major elements in biotites and plagioclases were measured with the JEOL JXA-8230 electron microprobe at the Shandong Analysis Center of the China Metallurgical Geology Bureau and the JEOL JXA-8200 microprobe at Bayerisches Geoinstitute. Silicate minerals were used as the standards for Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K and P. The following conditions were applied in deriving the measurements. The beam diameters of EMP are 10 μm. The accelerating voltage was 20 kV at a current of 10 nA for all the mineral elements. The peak counting time was 10 s for Na and K, and 20 s for the other elements. Analytical errors ＜2%for SiO2, Al2O3, CaO; and that ＜3% for FeO, MgO, TiO2; and that ＜5% for MnO, Na2O, K2O. The calculation of Fe2+and Fe3+came after Zheng QR et al. (1983), and the crystal chemical formula is calculated with the principle of electricity price equilibrium, and the results are listed in Table 1.

The zircons were separated from the microcrystallinedioritic enclaves using a conventional method for heavy mineral concentrations, and were then handpicked under a binocular microscope, mounted 150 grains in epoxy resin randomly and polished to about half thickness.Cathodoluminescence (CL) images were made of the zircons prior to U-Th-Pb analyses in order to reveal the internal textures and to select the sites for LA-ICPMS and MC-LAICPMS analyses (Fig. 4).

Fig. 2.Field morphological characteristics of the microcrystalline-dioritic enclaves in Jiuling Pluton within the studied area. a-the microcrystalline-dioritic enclaves with porphyaceous texture, the phenocrysts are mainly plagioclase in short columna and the matrix is mainly composed of phenocrystalline biotite; b-the light felsic halos grow around the enclaves, where the mingling area is between the enclaves and the host rocks, and the obvious “large” plagioclase in the enclaves are the xenocrysts from the host rock; c-the narrow dark chilled margin grows around the enclaves with sharp and clear distinct boundaries between the enclaves and the host rock; d-banded enclaves in the granodiorite, revealing the injecting process of microcrystalline-dioritic magma.

Fig. 3.Microscopical characteristics of the microcrystalline-dioritic enclaves in Jiuling Pluton within the studied area. Ap-apatite; Pl-plagioclase; Kf-K-feldspar; Ads-adesine; Qz-quartz; Bt-biotite; Ser-sericite. The micrographs are perpendicular polarized light.a-the main mineral assemblages of microcrystalline-dioritic enclaves are plagioclase, biotite, quartz and a small amount of K-feldspar with abundant needle-like apatites; b-the plagioclase phenocrysts in enclaves display an irregular form, and are inlayed in the quartz, implying a mixing/mingling has developed between the enclaves and host rock; c-the plagioclase phenocrysts with an irregular form inlay the quartz and grow abundant needlelike apatites; d-plagioclase phenocryst in the host granitoids usually develop a relatively sodium-rich core (An41) with a corrosion borde, and are enclosed within a calcium-rich plagioclase (An52-38) with idiomorphic form.

Zircon trace elements and U-Pb dating were carried out using a Thermo X2 inductively coupled plasma mass spectrometer (ICP-MS, Thermo Corporation, USA). The ablation system was Germany Coherent Geolas Pro. That was hosted at the Testing Center of Shandong Bureau of China Metallurgical Geology Bureau. Analyses were carried out with a beam diameter of 32 μm, 8 Hz repetition rate. The ablated aerosol was carried by helium. Every seventh sample analysis was followed by two standard zircon 91500, two NIST SRM610 and one PL measurement. The zircon U-Pb isotopic ratio was corrected by using zircon 91500 as an external standard, while the trace element of zircons analysis used NIST SRM610 as an external standard,29Si as an internal standard element. Detailed laboratory test conditions,standard U-Th-Pb isotope analysis ratio and analysis process.A detailed description of the analytical technique was reported Li FC et al. (2016). Fractionation correction and results were calculated by ICPMS dataCal (Liu YS et al.,2008; 2010). Common Pb was corrected using the method described by Andersen (2002). The results were calculated using the Isoplot/Ex_ver3 (Ludwig KR, 2003).

Zircon Hf isotopic experiments were conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific,Germany) in combination with a Geolas 2005 excimer ArF laser ablation system (Lambda Physik, Göttingen, Germany)that was hosted at the state Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan. Zircon Hf isotopic fractionation was corrected using standard zircon 91500. Isobaric interference of176Yb on176Hf was corrected by a laboratory-established formula: βYb=0.912×βHf. A detailed description of the zircon Hf isotope analytical technique can be found in Hu ZC et al.(2012). The single zircon U-Pb age was used in the calculation of εHf(t) values adopting the chondrule with176Lu/177Hf=0.0336,176Hf/177Hf=0.282785 (Bouvier A et al.,2008). The single-stage Hf model ages (TDM1) was calculated relative to the depleted mantle with176Hf/177Hf=0.28325 and176Lu/177Hf=0.0384 (Vervoort JD et al., 1999), and the twostage Hf model ages (TDM2) were calculated with a mean176Lu/177Hf=0.015 of the average continental crust (Griffin WL et al., 2002).

The analytical results of zircon U-Th-Pb-Hf isotopic compositions are summarized in Table 2 and Table 3.

4. Analytical results

4.1. EMP mineral major elements

The authors have obtained the major composition of plagioclases and biotites in host granitoids and enclaves by EMP. The chemical composition of biotites are shown in Table 1, and the classification is shown in Fig. 5a. The biotites of biotite-granodiorite, tonalite, biotite monzogranite and microcrystalline-dioritic enclaves are all enriched in Fe,Al, but poor in Mg, and all samples are dropped into the annite area in the diagram of Mg-(Fe2++Mn)-(AlⅥ+Fe3++Ti)(Foster MD, 1960) (Fig. 5a). Zhou ZX (1988) used the diagram of MgO-FeOT/(FeOT+ MgO) to distinguish biotites in three different geological sources (Fig. 5b), and Fig. 5b indicates that all the biotites of biotite granodiorite, tonalite,biotite monzogranite and microcrystalline-dioritic enclaves are characterized by crust-derived granites. Thus, the major chemical composition of dioritic enclaves and the host granitoids show similar content without significant difference.

Table 1.The electron-microprobe composition of biotite and plagioclase from microcrystalline-dioritic enclaves and host granitoids.

Table 2.LA-ICP-MS zircon U-Pb dating data of the microcrystalline-dioritic enclaves in the studied area.

Table 3.MC-LA-ICP MS zircon Lu-Hf isotopes of the microcrystalline-dioritic enclaves in the studied area.

However, the plagioclase phenocrysts in the dioritic enclaves and host granitoids show complex compositions and structural imbalances (Fig. 3b, c), which are also supported by the EMP data of the plagioclase with zonal structure. Fig. 3d shows plagioclase phenocryst in the host granitoids with a melted gulf-like shape but uniform composition (An41) with lighter gray in the core. The core is surrounded by a darker gray plagioclase that is enriched in Ca with a relatively regular form (An52-38), which in turn is wrapped in a Na-rich plagioclase (An21), with a clear line of demarcation between the different crystal zones. Besides, the plagioclase number from the edge (An21) is similar to the plagioclase (An24) that is inserted into the plagioclase with a gulf-like shape.

Fig. 4.Zircons CL images and the site of analyzed points from microcrystalline-dioritic enclaves within the studied area.

4.2. LA-ICP-MS zircon trace elements and U-Pb ages

The characteristics of zircons trace elements of Neoproterozoic biotite-granodiorite, tonalite, biotite monzogranite, Mesozoic granitoids and microcrystallinedioritic enclaves in the studied area have been researched comprehensively in this study (Fig. 1b). The average ΣREE of biotite granodiorite is 1670.5×10-6, the average ΣREE of tonalite is about 2015.8×10-6, that of biotite monzogranite is about 4415.0×10-6, and that of the captured zircons (about 820 Ma) in Mesozoic granite is about 1926.1×10-6. All of the zircons show depleted LREEs, enriched in HREEs, strongly positive Ce anomalies, and strongly negative Eu anomalies on the REE distribution patterns of zircons.

Two samples of microcrystalline-dioritic enclaves were measured by LA-ICP-MS zircon U-Pb dating and zircon traceelement analysis. The sampling locations are shown in Fig.1b. Samples of 5539BT were collected from the central region of the study area, and a total of 15 zircons were tested. They are all concordant or nearly concordant. Additionally, the mostly zircon CL images are dark, indicating that Th and U contents of zircons are relatively high. One zircon (-05) shows light CL images, and its length-width ratio is about 3:1 with a corrosion border. In addition, clear oscillatory zonations are well-developed and Th/U ratios are high, indicating its magmatic origin. The measured Th and U contents of zircons are 144×10-6and 579×10-6, respectively, with a Th/U of 0.25,indicating obvious characteristics of magmatic zircon (Wu YB et al., 2004). Due to errors in counting statistics during the analysis,207Pb/206Pb ages are more precise for older (＞1.0 Ga)zircons, while206Pb/238U ages are more precise for younger zircons (Griffin WL et al., 2004). Therefore, we use the207Pb/206Pb ages for older zircons, and206Pb/238U ages for younger zircons in the following discussion. The207Pb/206Pb ages of this zircon with a light colored CL image is 1.7Ga, it may be the captured zircon from the magma source, and it contains lower total rare earth element content (∑REE) is 1677.3×10-6; For the other 14 zircons, the Th, U content range from 45.8×10-6to 834×10-6and 214×10-6to 1187×10-6,respectively, with Th/U between 0.14 to 0.77, attribution to magmatic zircon with a weighted mean206Pb/238U age of 822.2±6.2 Ma (MSWD = 0.08) (Fig. 6a). The trace element content of these 14 zircons is higher, with an average of 3219.3×10-6. In Fig. 7, all the zircons exhibited the characteristics of depleted light rare earth elements (LREE),gradually enriched in heavy rare earth elements (HREE),intense positive Ce anomalies, and intense negative Eu anomalies.

Fig. 5.Classification diagram for biotite (a) (after Foster MD, 1960) and FeOT/(FeOT+MgO)-MgO diagram on material sources of biotites (b)(after Zhou ZX, 1988) in biotite-granodiorite, tonalite, biotite monzogranite and microcrystalline dioritic enclaves from the studied area.

Fig. 6.U-Pb Concordia diagram of the zircons from the microcrystalline-dioritic enclaves within the studied area.

Sample 6106BT was collected from the southern segment of the studied area and was close to the Shuangqiaoshan Group of surrounding rocks. A total of 15 zircons were tested.All dating points are concordant or nearly concordant, and the overall zircon CL images are dark, indicating which contain high Th, U content. The U-Pb age can be divided into three groups (Fig. 6b). Among them, two zircons show a long columnar shape with a 2:1 length-width ratio and a dark CL image, and develop typical magmatic oscillatory zonations.The measured Th and U contents of zircons are 1127×10-6,1144×10-6and 1240×10-6, 1579×10-6, respectively, and the Th/U is 0.98, indicating obvious characteristics of magmatic zircon (Wu YB et al., 2004), yielding206Pb/238U ages of 906 Ma and 869 Ma, which may be the captured zircons from the surrounding rocks in the process of Jiuling Pluton intruding into the Shuangqiaoshan Group, and they have relatively low REE contents of 2649.6×10-6and 2930.8×10-6(∑REE),respectively. The other two zircon CL images are relatively light, also develop the magmatic oscillatory zonations with Th, U content are 56.4×10-6, 135×10-6and 467×10-6, 655×10-6,respectively. Th/U was 0.41 and 0.71, yielding207Pb/206Pb age of 1.6 Ga and indicating they may directly belong to the source materials. Additionally, they show very low content of the rare earth elements of 986.4×10-6and 445.9×10-6(∑REE),respectively. On the other hand, Th and U content of the other 11 zircons range from 50.9×10-6to 2157×10-6, 135×10-6to 1082×10-6, Th/U is 0.17 –2.99 with a weighted mean206Pb/238U age of 822.6±5.8 Ma (MSWD = 0.11). The trace element content of these 11 zircons is higher with an average of 2809.9×10-6. In Fig. 7, zircons all showed the feature of depleted light rare earth elements, enriched in heavy rare earth elements, strong positive Ce anomalies, and strongly negative Eu anomalies.

4.3. Zircon Hf isotopic compositions

The zircon Hf analyses were performed on the same domain of the same grain as those used for U-Pb dating.

From the microcrystalline-dioritic enclave (Sample 5539BT), fifteen analyses were obtained for zircon Hf isotopic compositions. Fourteen newly formed magmatic zircons (weighted mean206Pb/238U age of 822.2±6.2 Ma)yield different initial176Hf/177Hf ratios ranging significantly from 0.281571 –0.282520 with significantly different εHf(t)values. The εHf(t) of measurement point TW61006B14 (822 Ma) is extremely low with -24.36 and TDM2is 3.26 Ga,indicating it originated from the Palaeoarchaean crust.Similarly, the εHf(t) of measurement point -01 is also very low at -8.64 with TDM2at 2.27 Ga, implying it may also be derived from the Palaeoarchaean crust. However, the εHf(t) of the other 12 zircons are all greater than 0 with εHf(t) values from 1.49 to 9.09 (the average is about 4.86) (Fig. 8a) and the TDM2model is 1.14–1.64 Ga, indicating they originated from the Mesoproterozoic crust. Additionally, one zircon xenocryst from the source (207Pb/206U age is 1.7 Ga) yield initial176Hf/177Hf ratios of 0.282667, with a εHf(t) value at -0.26 and TDM2at 2.46, implying they originated from Paleoproterozoic crust.

Fig. 7.Chondrite-normalized REE patterns of zircons from the microcrystalline-dioritic enclaves (chondrite values after Boynton WV et al.,1984).

Fifteen zircon grains from the microcrystalline-dioritic enclave (Sample 6106BT) were chosen for Hf isotopic determination. Eleven magmatic zircons from them (with a weighted average age of 822.6±5.8 Ma) also yield quite different initial176Hf/177Hf ratios ranging significantly from 0.282359 to 0.282596 with εHf(t) values from 3.54 to 11.90(the average is about 7.23), corresponding to TDM2ranging from 0.95 Ga to 1.50 Ga, implying that they are similarly derived from the partial melting of the Mesoproterozoic continental basement. Two zircon xenocrysts (906 Ma, 869 Ma) captured from Neoproterozoic surrounding rocks yield approximate initial176Hf/177Hf ratios of 0.282522 and 0.282540, with a εHf(t) value of 10.35 and 11.32 and the TDM1is 1.02 Ga and 0.99 Ga, respectively. However, two zircon xenocrysts (about 1.6 Ga) from the source regions yield quiet different initial176Hf/177Hf ratios of 0.281543 and 0.282018,respectively, corresponding to the εHf(t) value of -6.11 and 8.18 (Fig. 8b) with TDM2and TDM1at 2.34 Ga and 2.78 Ga,1.70 Ga and 1.78 Ga, respectively. Thus, we infer that the source materials of the two zircon xenocrysts may be derived from the Archean crust (about 2.78 Ga) and the Paleoproterozoic crust (about 1.70 Ga), respectively.

5. Discussion

5.1. The petrogenesis of the microcrystalline-dioritic enclaves

The recently published O, Hf, and Nd isotopic data suggest that the Neoproterozoic granitoids from the Eastern Segment of Jiangnan Orogen showed characteristics that range from I-type to S-type granites (Wang XL et al., 2013,2014). Wang XL et al. (2014) divided the Jiangnan Orogen into eastern and western segments largely along a nearly N-S boundary stretching across northern and central Hunan Province and the oxygen isotopes of the granitoids from the western segment varied from 8.5‰ to 10.5‰ (δ18O), while the granitoids from eastern segment ranged from 6.0‰ to 8.5‰ (Wang XL et al., 2013), which indicate a large proportion of mature continental crust was involved in the source region of the granitic magma from the western segment, while younger materials were involved in the magmatic rocks in the eastern segment, which is consistent with their Hf and Nd isotopic characteristics. In addition, the δ18O of the zircons gradually changed from 3‰ of the core to 6‰ of the sides from the peraluminous granitoids in the eastern segment of the Jiangnan Orogen, thus indicating mature continental compositions significantly increased in the late stage of magmatic evolution (Wang XL et al., 2013). This suggested the original granitic magma maybe I-type granitic magma in the eastern segment of Jiangnan Orogen, whereas in the late stage of magmatic evolution, the magma gradually transitioned from I-type granitoids to S-type granitoids. So,what caused the transition from I-type granitoids to S-type granitoids for Jiuling Pluton?

Enclaves in igneous rocks provide important information on the origin and evolution of magma. Furthermore,microcrystalline-dioritic enclaves that formed earlier, research helps us to understand the evolution of granitic magma in the early stage wells. In addition, granite with only microcrystalline-dioritic enclaves (also called surmicaceous enclave) as crust petrogenesis (also called C-type granite) and granites with microcrystalline-balsatic enclaves as crust mixing with mantle petrogenesis or mantle petrogenesis (also called M-type granite), respectively. The dark enclaves in the studied area mainly consist of microcrystalline-dioritic enclaves without microcrystalline-basaltic enclaves,indicating the host granite attribution to C-type granite.

The origin of dark enclaves in intermediate-acidic igneous has always been under debate. The origin of these enclaves mainly includes, (1) Fractional crystallization of comagmatic evolution (Chappell BW et al., 1987); (2) Residues from lower Crust (Chappell BW et al., 1992); (3) The mixing of mafic magma and acidic magma (Perugini D et al., 2003;Barbarin B et al., 2005; Feeley TC et al., 2008). Increasingly studies show that most of the dark enclaves are the result of magma mixing. However, issues such as the mechanism of magma mixing are still under debate.

As discussed above, the microcrystalline-dioritic enclaves may represent the external magma of the injection, and mid with the host magma. The host granitoids were plastic when the external magma was injected into the host magma, due to the enclaves elongated varying degrees but without solid deformation in the studied area, and the boundary between the enclaves and the host rock is distinct but partially present within a narrow transitional zone, which implies there are material exchanges between the enclaves and the host granitoids in the studied area. On the other hand, experimental petrology showed that when the magma is rapidly cooled, the crystalline apatite is elongated along the C axis with the length-width ratio up to 20:1 (Wyllie PJ et al., 1962), so that the microcrystalline-dioritic enclaves that contain the acicular apatites are generally regard as a sign of magmatic mixing and the result of a mixing/mingling between hot, basaltic magma and cool, felsic magma (Piccoli PM et al. 2002).While, the microcrystalline-dioritic enclaves in the studied area contain a lot of acicular apatites (Fig. 3b, c), which indicate that they have received intense magmatic mixing with the host granitoids. In addition, the crystals in these enclaves are obviously more granular than the host rocks and show typical magmatic structures. The petrographic features also show that the phenocrysts of plagioclase are significantly larger than the matrix in the enclaves, which may be captured from the host granitoids. All of these characteristics imply the mixture in the dark enclaves represents the external mafic magma which is injected into the granitic magma, and that the host granitoids were still flowing at the time of injection for the plastic shape and chilled margin of the enclaves in the studied area.

Moreover, as shown in Fig. 3d, plagioclase phenocryst in the host granitoids usually developed a relatively sodium-rich core (An41) with a corrosion border, which was enclosed in a calcium-rich plagioclase (An52-38) with idiomorphic form. We infer these characteristics were caused by a small volume of basaltic magma that formed the enclaves, and this was injected into the acidic magma chamber that formed the host granitoids. The plagioclase crystalized from the acidic magma and eroded into an irregular shape by the magma of enclaves with higher temperatures, so that, the plagioclases became a new growth point wrapped around calcium-rich plagioclase.With the decline in temperature and the continuous crystallization of the magma, the calcium-rich plagioclase was again wrapped by relatively sodium-rich plagioclase that continued to crystallize from the host magma. This imbalance in composition and structure is attributed to the result of rapid crystallization and incomplete mixing (Janoušek V et al.,2004).

Similar structural imbalances have also been found in dioritic enclaves, as shown in Fig. 3b, c within the yellow dotted line area, where plagioclase phenocrysts often have an irregular form. Therefore, the plagioclase, previously crystallized in the host granitoids, may have migrated into the enclaves and be eroded by the enclave magma with high temperatures. Thus, the phenomena above fully indicate that there was an active bidirectional material exchange between the enclaves and host granitoids, which also indicate that the two magmas had coexisted long enough that the crystal could migrate across the boundaries.

Based on comprehensive research correlations of zircon trace elements in the biotite-granodiorite, tonalite, biotitemonzogranite and microcrystalline-dioritic enclaves in the studied area, all show that the four kinds of zircons from different igneous groups have similar REE distribution patterns (Fig. 7). They all display light depletion rare earth elements, gradually enriched by heavy rare earth elements,strongly positive Ce anomalies and negative Eu anomalies,and have the typical characteristics of magmatic zircons.Positive Ce anomaly is due to Ce3+being oxidized to Ce4+,and its ionic radius similar to Zr and Hf, which make Ce3+easier to occupy zircon lattice than light rare earth elements.Additionally, the negative Eu anomaly is related to the crystallization of feldspar. Besides this, from granodiorite,tonalite to monzogranite, the zircon REE content increases gradually, an indication that the three kinds of magma components above are different, which implies the source materials of the three granitoids above may be different. In addition, the contents of rare earth elements in the same rock also present few distinctions, such as zircons with the same age and lithology display a relatively large variation range of REE (Fig. 7). Furthermore, there is also a significant difference in the composition of zircon rare earth elements between the newly crystallized zircons and the zircon xenocrysts from the Neoproterozoic granitoids. For instance,the zircon xenocrysts with an age of about 1.5Ga–1.7Ga contain the lowest total rare earth elements (∑REE) (Fig. 7),while the fluid metasomatism zircons with an age of about 720–750 Ma show the highest content of total rare earth elements (Fig. 7), but the content of the zircon rare earth elements with the age of 820–950 Ma are close to each other.However, the total rare earth elements of the zircon xenocrysts with the age of 850–950 Ma is slightly lower than that of newly crystallized zircons with the age of 820–330 Ma. Therefore, on the one hand, these features imply that the source components of the granitoids are heterogeneous; On the other hand, they are also influenced by the early crystallization of the accessory minerals, that transformed the content of the residual melt. Additionally, the former implies that the Neoproterozoic granitoids in the studied area were formed by partial melting of sedimentary materials with different zircon U-Pb ages, in which the characteristics of rare earth elements in zircon xenocrysts were still reserving part of the information from the original magma; While the latter showed that the late magma with enrichment in rare earth elements have a significant impact on the composition of the zircons that crystallize in the latter stage of magma.

It is worthwhile pointing out that there are marked differences of zircon trace element distribution patterns without correlation in zircon trace element pairs but forming their own system between enclaves and the three granitoids(host rocks), according to the diagrams of zircon trace element pairs (Fig. 9). Such as the blue and the purple dotted line area shown in Fig. 9. Besides this, from biotitegranodiorite, tonalite to biotite-monzogranite, the content of rare earth elements gradually increases with δEugradually decreases (within the area of blue dotted line, Fig. 9b) but increases in the dioritic enclaves with δEugradually decreasing (within the area of the purple dotted line, Fig. 9b).In addition, in these diagrams, the trace element composition of zircons from the tonalite is between that of biotitegranodiorite and biotite-monzogranite (Fig. 7), indicating the three magmatic components are obviously different, but there is also some correlation on petrogenesis of the three types of granitoids. Additionally, the geochemical characteristics of these three types of granites indicate that the source materials of tonalite are a mixture from the source of biotitegranodiorite and the source of biotite-monzogranite (Duan Z et al., 2017). Furthermore, we also note that there are other regularities in the trace element content of zircons from the same rock, and some element pairs also present correlations.Such as, there is a positive correlation between Y-Th, U-Th and Y-ΣREE (Fig. 9), which is probably due to the fact that Y, REE can displace Zr and Si in zircon lattice during the crystallization of zircon. In addition, the extraction of the melt and the crystallization of the accessory minerals can also change the composition of the magma, and then the magmatic components are continuously changed by external factors(such as magmatic mixing). Combined with the REE and trace element composition of zircons, we infer that the evolution of magma itself (melt extraction, mineral crystallization and heterogeneity of the original components)and magma mixing affect both magmatic component changes.Additionally, trace elements in the microcrystalline dioritic enclaves have significant differences with that of the trace elements in the Neoproterozoic host granitoids, but forming their own system. Furthermore, the range of zircon trace elements in the enclaves entirely crosses the variable boundary of that in the three Neoproterozoic granitoids in the diagram of zircon trace elements (Fig. 9). Therefore, the linear relationship between the trace elements in the microcrystalline-dioritic enclaves and the zircons of the Neoproterozoic granitoids is significantly different and cannot be explained by separation and crystallization. Moreover, the distinct Hf isotopic composition of Neoproterozoic granitoids and microcrystalline-dioritic enclaves (Duan Z et al., 2017 and this article) also largely preclude the possibility of separation and crystallization.

Fig. 9.Diagrams of zircon trace element correlations from the microcrystalline-dioritic enclaves.

On the other hand, in general, it is difficult to tell whether these enclaves are crystallized from magma with different properties by using Sr-Nd isotopes since the Sr-Nd isotopic compositions between dark enclaves and host rocks easily tend toward homogenization. However, the Hf isotopic composition of zircons can record isotopic compositional changes during magma mixing and differentiation, thus effectively identifying the mixing effect of different magmas(Griffin WL et al., 2002; Li XH et al., 2007). In this study, the Hf isotope composition of the Neoproterozoic granitoids and microcrystalline dioritic enclaves in Jiuling Pluton are significantly different, but the age of crystallization is the same. The εHf(t) of zircons from the Jiuling Neoproterozoic granitoids ranged from -6.76 to 9.22 (Duan Z et al., 2017),whereas the εHf(t) of zircons from the microcrystalline dioritic enclaves range from 1.49 to 11.90. Which indicate that both of the enclaves and the host granites attribution to different types of magma, may originate from different source regions. Additionally, the enclaves are generally higher than that of the host rocks with the same crystallization age. The microgranular and igneous-texture of the enclaves imply the enclaves are not residual of the remelting sedimentary rocks or the cumulate fractional crystallization in the early stage.Therefore, the origin of the microcrystalline-dioritic enclaves cannot be explained by the cumulative fractional crystallization of homologous magma or refractory residuals of that remelted from the sedimentary rocks, but may be caused by magmatic mixing between the mantle-derived basaltic and crust-derived granitic magmas. Thus, the characteristics of the external mafic magma source zone represented by the microcrystalline-dioritic enclaves should be an important issue worth exploring.

The study of dark enclaves is of great importance for the identification of mantle-derived magmatic components in granite (Didier J et al., 1991; Barbarin B et al., 2005).Because the information relating to magmatic geochemistry and lithofacies is easily covered up where the Sr-Nd isotopic composition tends to be uniform, it is difficult to identify the exact source of the mantle source component through the Sr-Nd isotope. Since Hf isotopes of zircons can record changes in isotopic composition during magma mixing and differentiation (Griffin WL et al., 2002; Li XH et al., 2007),and distinguish the distinction of Hf isotopic compositions between the host granites and the enclaves (Yang JH et al.,2006). Therefore, the study of Hf isotopes in granites is becoming an important method for studying the interaction of crust-mantle magma (Wu FY et al., 2007).

Previous studies have shown that zircon Hf isotopes can provide evidence for identifying magmatic source areas and specific magmatic processes (Wu FY et al., 2007). However,the Lu-Hf isotopic characteristics of microcrystalline-dioritic enclaves show significant differences in (176Hf/177Hf)iand εHf(t) with host rocks (Fig. 8a). Therefore, we can rule out the possibility that the microcrystalline-dioritic enclaves are the schlieren derived from the early stage of magmatism.Although the age of the enclaves is consistent with that of the host rocks, the enclaves developed a typical igneous texture,which indicates enclaves are also not surrounding rock xenoliths. Besides which based on their geochemical characteristics, these microcrystalline-dioritic enclaves have a pronounced Eu anomaly (Sun KK et al., 2017). While in the process of crystallization differentiation, the melt will present obvious Eu negative anomalies since the fractionation of plagioclase, while the residual melt should show the positive anomalies of Eu and Sr. Therefore, the enclaves are not the residue of the host rocks. Thus, microcrystalline-dioritic enclaves may represent another source region. Furthermore,studies have shown that the zircons have consistent forming ages but with widely varying εHf(t) values, generally associated with mixing magmas originating from different source regions (Griffin WL et al., 2002).

However, regional geological data shows the Shuangxiwu Group in northern Zhejiang and the Zhangshudun ophiolite in northeastern Jiangxi exposed SSZ-type ophiolite sets, arc volcanic rocks and subduction-related granites and adakites dating from about 900 to 970 Ma (Li XH et al., 2008, 2009,2003; Wu RX et al., 2006; Ye MF et al., 2007). Related plagiogranite, andesite, hornblendhite-bearing diorite and granodiorite dating from 900-930 Ma, which are of intra-arc igneous with positive εNd(t) and εHf(t) values (Ye MF et al.,2007; Chen ZH et al., 2009a, b; Li XH et al., 2009). For instance, Li XH et al. (2009) argued that the Shuangxiwu volcanic rock as a typical Neoproterozoic island arc magma product, and obtained the Shuangxiwu volcanic rock age of 890–970 Ma with the zircon εHf(t) value ranged from 11.0 to 15.3, and TDM1range from 0.72 Ga to 1.40 Ga. Wu XL et al.(2006) studied the Neoproterozoic granodiorites in the southeastern margin of Yangtze block and concluded that the zircons with an age of 882 Ma from granodiorite inherited from arc source igneous rocks. The εHf(t) value is between 2.89–11.08, the range of TDM1is 0.99–1.32 Ga. Chen ZH et al. (2009a) systematically reported the geochemical characteristics, zircon geochronology of high-Mg diorite, Nbevriched basaltic porphyrite and plagiogranite in the Pingshui area of the Jiangshan-Shaoxing geosuture zone. Additionally,the εHf(t) values of zircon from high-Mg diorite and plagiogranite range from 8.6 to 13.2 and 11.0 to 16.2,respectively. While the TDM1values vary from 0.94 to 1.14 Ga and 0.81 to 1.01 Ga (Chen ZH et al., 2009a). Besides, Chen ZH et al. (2009b) studied the keratophyre of Pingshui Group,Zhejiang province and obtained its age ranged from 904 Ma to 906 Ma with εHf(t) ranged from 8.6 to 15.4, and the TDM1values ranged from 0.84Ga to 1.11 Ga. It is suggested that there may be juvenile island arc crustal growth events in south China during early Neoproterozoic (about 1000–900 Ma).

Most zircons of the microcrystalline-dioritic enclaves in the study area have very high εHf(t) values, and TDM1also concentrates at about 1.0 Ga (Table 2). The εHf(t) of southern sample 6106BT (822.6±5.8 Ma) is 3.540–11.94, while that of the TDM1mode is 0.92–1.25 Ga. The εHf(t) of central sample 6106BT range from 1.0–9.09, the TDM1is 1.03–1.38 Ga.Which are approximate to that of the high-Mg diorite, Nbenriched basaltic rocks, plagiogranite and keratophyre described above in Shuangxiwu Group and the Jiangshan-Shaoxing geosuture zone in northern Zhejiang. Which indicate that the source materials of the enclaves are related to the eastern arc-like igneous rocks and may be mainly composed of clastics derived from the Meso-Neoproterozoic(1000–900 Ma) juvenile island arc materials. In addition, the εHf(t) of two zircon xenocrysts from 6106BT (906 Ma, 869 Ma) is 10.35 and 11.32, with a TDM1of 1.02 Ga and 0.99 Ga,respectively. Which are similar to the arc-like volcanics of Shuangxiwu Group and eastern segment of Jiangshan-Shaoxing geosuture, which indicate that the two zircon xenocrysts may be directly derived from the juvenile island arc igneous in eastern segment of Jiangshan-Shaoxing suture zone less affected by the host granitoids.

It is noteworthy that the εHf(t) values of the southern sample (6106BT) are higher than that of the central sample(5539BT) implies that the southern region accepted more detrital material from the juvenile island arc igneous rocks,while the central region accepted less, but more Mesoproterozoic ancient basement materials similar to the Neoproterozoic Shuangqiaoshan group.

In summary, the microcrystalline-dioritic enclaves may represent another source region, which partial melting from the Mesoproterozoic crust that enriched juvenile island arc materials, and mixed with the granitic magma that remelted from the Mesoproterozoic continental crust that enriched ancient arenite. In the process of magma mixing, although both exchanged a large amount of minerals and elements, they did not reach a complete balance between the components and structures, especially the Hf isotopic composition, and some of the zircon xenocrysts even remained an intact Hf isotope composition of the source. However, the Lu-Hf isotopic composition of newly crystalized zircons (about 820 Ma) in the microcrystalline-dioritic enclaves have been influenced to some extent, but still higher than the εHf(t) of the host granitoids.

5.2. The inspiration of regional tectonic magmatism

The Neoproterozoic magmatic activity recorded in the Jiangnan Orogen indicates the intra-oceanic arc igneous assemblage of 1.0–0.9 Ga was developed in the Shuangxiwu area in Northeastern Zhejiang Province and the Zhangshudun area in the Northeastern Jiangxi Province, and presented the εHf(t) and εNd(t) isotopic characteristics of the depleted mantle, indicating the existence of early Neoproterozoic ocean-ocean subduction (Li XH et al., 2007; Xia Y et al.,2018). In addition, the glaucophane schist of about 860 Ma and obduction-type granite of about 880Ma in the Xiwan area(Shu LS et al., 1994; Li XH et al., 2008; Charvet J et al.,1996) recorded the anatexis of sedimentary rocks in the backarc basin beneath the ophiolite which obducted upon the island arc crust during the collision between the Shuangxiwu arc and the Zhangshudun arc (Cox J et al., 1999).

Basalts developed in the early stage of the back-arc basin showing the features of the arc and gradually transited to NMORB as the basin continued to expand (Gribble WL et al.,1998), while the Northwestern margin of Cathaysia block exposed the mafic rocks of about 860-840 Ma showing the features of arc-like and N-MORB together (Xia Y et al.,2018), suggesting that there developed a mature back-arc basin during this period in the Northwestern margin of the Cathaysia block (the eastern segment of Jiangnan orogen).

The active continental margin of Yangtze block is characterized by widely exposed peraluminous granitoids.The peraluminous S-type granites were formed in the orogenic belt and are the result of juvenile crustal reworkings and/or experienced partial melting of the mid-lower crust. The juvenile clastics from the erosional arc-continent gradually decreases toward the west, resulting in the gradual decrease of εHf(t) and εNd(t) from the eastern segment to the western segment (Wang XL et al., 2014). The age of arc-like magmatic rocks on the margin of the Yangtze block are concentrated at about 850–825Ma, and the peak of peraluminous granitic magmatism is about 825Ma (Charvet J,1996; Wang XL et al., 2014; Xia Y et al., 2018). Recently, a large amount of geochronology data on the metamorphic basement of Jiangnan orogen has been published and the final collisional time of the intra-oceanic arc and Yangtze block has been limited to about 825 Ma along the Jingdezhen-Yifeng-Wanzai fault (Wang XL et al., 2007, 2014; Yao JL et al.,2013; Xia Y et al., 2017). Moreover, the arc-continent collision led to the shortening and thickening of the crust. The process of crustal shortening and thickening is followed by erosion, retrogradation and orogeny, which is the result of the crust-mantle isotope rebalancing (Leech ML, 2001). Such as,some sedimentary formations (eg, Luojiamen, Baizhu and Hetong Formations) are typically gravel, indicating rapid uplift, erosion, short-distance handling and deposition processes, corresponding to the collapsing and stretching with orogenic belts (Wang XL et al., 2003). Following the collision between intra-oceanic arc and Yangtze Block, the region of the southeastern margin of Yangtze block developed a wide range of extension.

Based on the research of the Neoproterozoic magmatic rocks in south China and the related spatiotemporal distribution characteristics of the rocks, the suture zone between the Yangtze and Cathaysia block can be divided into three zones: intra-oceanic arc zone, the active continental margin of Cathaysia block, and the active continental margin of the Yangtze Block (Xia Y et al., 2018). Neoproterozoic granitoids exposed in Jiangnan orogen are spatially distributed in zonal distribution, which is difficult to explain using the mantle plume model. In addition, there are no typical products of the mantle plume as continental flood basalts nor a large number of radial dykes in southern China. Additionally, in recent years, some scholars have also argued that the high-Mg andesite (Yiyang Komatitic basalt) derived from the subduction (Zhao JH, 2009; Zheng YF et al., 2008), which was once believed to be evidence of mantle plume. In fact,there is no reliable report of continental flood basalts (CFB)and ocean island basalts (OIB) derived from mantle plumes at the margin of the Yangtze Block. Its worthwhile noting that in recent years, the information of Neoproterozoic arc magma activities in the margin of the Yangtze Block gradually became clear. For example, SSZ-type ophiolites of about 830 Ma and basalts of 850–825 Ma related to arc volcanism in the southern Anhui province (Li XH et al., 2003, Yao JL et al.,2013; Zhang SB et al., 2012a; Zhang CL et al., 2013), and high-Mg diorites of about 830 Ma developed in the Sibao Group (Chen et al., 2014), northern Jiangxi, southestern Hunan and northern Guangxi (Li XH et al., 2003, Yao JL et al., 2014; Zhou JH et al., 2009). Zhang YZ et al. (2012a) also obtained zircons from gabbro in the Fuchuan ophiolite with the U-Pb age of 824±3 Ma and shows the geochemical characteristics of the back-arc environment.The contemporaneous pillow basalt and spilite (Zhang SB et al.,2012b) developed in the Zhangyuan area in northeastern Jiangxi and in the Leigong area in northwestern Jiangxi province are also characterized by the geochemical properties of the back-arc basin. These characteristics indicate that along the southern margin of Yangtze block developed back-arc basin of 824–860 Ma, with the closure time of the basin likely to be later than 830 Ma. This also shows that it indeed developed the active continent marginal arc magmatism of Neoproterozoic and develope the back-arc basin along the southern margin of the Yangtze block. The source region materials of Jiuling Pluton may have originated from the back-arc basin. In addition, it accepted the juvenile igneous clastics drived from the intra-oceanic arc and the ancient sedimentary clastics drived from the Yangtze block.Meanwhile, it also indicates that the formation of the Neoproterozoic granitoids in Jiangnan orogen is closely related to the subduction and subsequent post-collisional magmatism at 850–805 Ma (Wang XL et al., 2006, 2013,2014; Zhou JC et al., 2009).

Based on the regional geological data of stratigraphy,magmatism and metamorphism in the studied area, we infer that Jiuling pluton formed in the post-collision stage of the arc-continental collision orogen.In addition, the Neoproterozoic granitoids in the eastern segment of Jiangnan orogen are enriched in biotite and cordierite, which is attributed to the CPGs (cordierite granites) (Wang XL et al.,2013). Furthermore, these granitoids all originated in the arccontinent collision and post-collision tectonic setting (Duan Z et al., 2017). While, the arc from the arc-continent collision zone should belong to the intra-oceanic arc (Xia Y et al.,2018). Therefore, the uplift and erosion of the arc-continent collision zone provide a distinct source area for collisional S-type granites, while the Neoproterozoic back-arc basins developed along the southeastern margin of Yangtze block accepted the juvenile igneous clastics originated from the intra-oceanic arc (Shuangxiwu arc or Zhangshudun arc) in the eastern segment of Jiangnan Orogen, and the ancient continental crust materials in the southeastern margin of the Yangtze Block simultaneously. Since then, sedimentary materials partially melted due to the extension in postcollision of the Yangtze and the Cathaysia Block and formed Neoproterozoic granitoids that enriched ancient continental crust materials and microcrystalline-dioritic enclaves which enrichment in juvenile intra-oceanic arc clastics in the study area. Furthermore, both magmas mingled with each other at about 822 Ma.

6. Conclusions

Based on the geology, petrology, geochronology, zircon trace elements and Hf isotopic composition of the microcrystalline-dioritic enclaves from the Jiuling Neoproterozoic granitoids, we draw the following main conclusions.

(i) The microcrystalline-dioritic enclaves from Jiuling Neoproterozoic granitoids formed at about 822 Ma which is consistent with the host granitoids within the error range and belongs to the same magmatic products. The field and petrographic features also indicate that there is magma mixing between microcrystalline-dioritic enclaves and host granioids.

(ii) The Lu-Hf isotope indicates that the microcrystallinedioritic enclaves may be derived from the crustal partial remelting process that enriched juvenile intra-oceanic arc clastics and mixed with the host granitic magma formed by remelting of Mesoproterozoic crust which enriched ancient continental crust materials.

(iii) There are more juvenile intra-oceanic arc clastics in the southern part of the source region of microcrystallinedioritic enclaves, while the content of ancient continental crust materials in the central part is higher, which reveals the amalgamation between the Yangtze and Cathaysia blocks are juvenile intra-oceanic arcs from the south and ancient continental crust from the north.

Acknowledgment

This study was financially supported by geological survey projects 12120113064800, 121201008000160902, 12121008-000150004 of the China Geological Survey, and the National Key Research and Development Program of China(2016YFC0600203). We appreciate the thoughtful reviews provided by two anonymous reviewers and the Executive Editor-in-Chief Dr. Yang Yan. This research was supported and guided by Senior Engineer Shen Jialin from Nanjing Center of China Geological Survey, Professor Liu Yongsheng from Chinese University of Geosciences (Wuhan), Engineer Lin Peijun from the Testing Center of the Shandong Bureau of China Metallurgical Geology Bureau.