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Volume 30 Issue 6
Dec.  2019
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Zircon U-Pb Ages and Hf Isotopes of Neoproterozoic Me-ta-Igneous Rocks in the Liansandao Area, Northern Sulu Orogen, Eastern China, and the Tectonic Implications

  • The Sulu Orogen preserves the Neoproterozoic tectonic-magmatic events, corresponding to the breaking up of the Rodinia supercontinent. The ages and petrogenesis of meta-igneous rocks in the Liansandao area in the northern Sulu Orogen are not well-constrained. This study reports zircon U-Pb ages and Hf isotopes of these rocks from the Liansandao area. Three meta-igneous rock samples give similar weighted mean 206Pb/238U ages of 744±11, 767±12, and 762±15 Ma, respectively, indicating the Neoproterozoic crystallization ages. These rocks formed coevally with the Wulian and Yangkou intrusions that located along the Yantai-Qingdao-Wulian fault zone. The Neoproterozoic ages indicate that the meta-igneous rocks from the Liansandao area have affinity to the Yangtze Block. The three samples have εHf(t) values of -7.2- -10.5, -6.0- -17.5, and -6.8- -12.0, respectively. These negative εHf(t) values indicate a primarily crustal source. However, the widely various εHf(t) values that are higher than the continental crust, suggesting magma mixing between mantle-derived materials and the continental crust or source heterogeneity. Combined with the Hf model ages and geochemical characteristics, the monzodiorite (sample LSD-2) is most likely to be mantle-derived magma then interacted with ancient continental crust, and the granitic protolith (samples LSD-1 and LSD-3) in the Liansandao area might derive from the re-melting of a Paleoproterozoic continental crust at ~750 Ma, resulting from the upwelling and underplating of mantle-derived magma formed in an extensional setting due to the break- up of the Rodinia supercontinent.
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Zircon U-Pb Ages and Hf Isotopes of Neoproterozoic Me-ta-Igneous Rocks in the Liansandao Area, Northern Sulu Orogen, Eastern China, and the Tectonic Implications

    Corresponding author: Fanxue Meng, mfx1117@163.com
  • 1. Shandong Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China
  • 2. Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China
  • 3. Instituto Universitario de Geología (Universidad de A Coruña), Campus Universitario de Elviña, 15071 Coruña, Spain

Abstract: The Sulu Orogen preserves the Neoproterozoic tectonic-magmatic events, corresponding to the breaking up of the Rodinia supercontinent. The ages and petrogenesis of meta-igneous rocks in the Liansandao area in the northern Sulu Orogen are not well-constrained. This study reports zircon U-Pb ages and Hf isotopes of these rocks from the Liansandao area. Three meta-igneous rock samples give similar weighted mean 206Pb/238U ages of 744±11, 767±12, and 762±15 Ma, respectively, indicating the Neoproterozoic crystallization ages. These rocks formed coevally with the Wulian and Yangkou intrusions that located along the Yantai-Qingdao-Wulian fault zone. The Neoproterozoic ages indicate that the meta-igneous rocks from the Liansandao area have affinity to the Yangtze Block. The three samples have εHf(t) values of -7.2- -10.5, -6.0- -17.5, and -6.8- -12.0, respectively. These negative εHf(t) values indicate a primarily crustal source. However, the widely various εHf(t) values that are higher than the continental crust, suggesting magma mixing between mantle-derived materials and the continental crust or source heterogeneity. Combined with the Hf model ages and geochemical characteristics, the monzodiorite (sample LSD-2) is most likely to be mantle-derived magma then interacted with ancient continental crust, and the granitic protolith (samples LSD-1 and LSD-3) in the Liansandao area might derive from the re-melting of a Paleoproterozoic continental crust at ~750 Ma, resulting from the upwelling and underplating of mantle-derived magma formed in an extensional setting due to the break- up of the Rodinia supercontinent.

0.   INTRODUCTION
  • In the Neoproterozoic period, tectonic and magmatic activities were intense and widespread on the Earth, as response to the assembly and break-up of the Rodinia supercontinent. Contemporaneous, global events include the occurrence of low-latitude glacier (i.e., "snowball" Earth), the reproduction of the Ediacaran biota, and the Cambrian Explosion (Zheng, 2003).

    The "Rodinia supercontinent" assembled through the Greenville-age orogen centered on the Laurentian continent (McMenamin and McMenamin, 1992). The break-up of the Rodinia supercontinent has an obvious temporal and spatial heterogeneity, and the timing and mechanism of the break-up event remains controversial. Earlier studies suggested that the breaking occurred in the Early Cambrian (Dalxiel, 1992) or Neoproterozoic (~720 Ma; Jiang et al., 2018; Lu et al., 2004). However, some researchers have proposed that the break-up event lasted for a longer period of time (~820-570 Ma) and probably resulted from multi-stage mantle-plume activities, leading to a non-synchronous break-up of different masses (Li et al., 2008). The widespread Neoproterozoic magmatism can provide important information for the understanding of geodynamic processes of the break-up of the Rodinia supercontinent.

    In China, some continental blocks, especially the Tarim and Yangtze blocks, preserve relevant Neoproterozoic magmatic records. These records comprise widespread Neoproterozoic bi-modal volcanic rocks, basic dike swamps, intra-plate granitic and gabbroic intrusions, and A-type granites, indicating an extensional environment of crustal breaking and thinning in this period.

    The Yangtze Block contains widespread Neoproterozoic magmatic rock suites, recording the transformation of the block from an orogenic to a non-orogenic environment (Jiang et al., 2018; Yin et al., 2018; Xu Y et al., 2016; Yang and van Loon, 2016). The 850-820 Ma magmatic-volcanic suites are mainly distributed in the Panxi-Bikou-Hannan belt in the western margin (Chen et al., 2015; Du et al., 2014; Dong et al., 2012; Wang Q H et al., 2012), and the Jiangnan orogenic belt in the eastern margin (Wang Y J et al., 2016, 2013; Zhang and Wang, 2016; Zhang et al., 2015, 2013; Zhao and Asimow, 2014; Charvet, 2013), and rare in the northeastern margin of the Yangtze Block (Bader et al., 2013). It is believed that the magmatism and sedimentation in the northeastern margin of the block mainly occurred during Mid-to Late Neoproterozoic time (He et al., 2016; Li and Zhao, 2016; Liao et al., 2016; Zhu D C et al., 2016; Zhu X Y et al., 2015; Chen et al., 2013; Liu and Zhang, 2013), although the formation time and tectonic setting is unclear due to the lacking of systematic chronological and geochemical research. Previous studies suggested that the Neoproterozoic metamorphic rocks in the Sulu orogenic belt represent the metamorphic basement of the Yangtze Block, corresponding to the large-scale magmatism produced by the breaking of the Rodinia supercontinent, and the ultrahigh-pressure (UHP) metamorphism which occurred in the Late Triassic. The study area, i.e., the Sulu Orogen, as the eastern extension of the Qinling-Dabie Orogen, is an ideal place to understand the petrological and geochemical changes during the deep subduction of the continent and the evolution of the Rodinia supercontinent. Although the Sulu Orogen has drawn tremendous attention on the processes of Triassic subduction, collision, and tectonic exhumation, the studies on Precambrian tectono-magmatic evolution in this region remain insufficient, with rare available Neoproterozoic ages (He et al., 2016; Hacker et al., 2006; Zheng et al., 2004). To clarify the Precambrian tectonic evolution of the Sulu orogeny, this study carries out zircon LA-ICP-MS U-Pb dating and Hf isotopic analyses on the Precambrian meta-igneous rocks in the Liansandao area in the central-northern section of the Sulu Orogen. In combination with regional geology, we aim to reveal the petrogenesis of the Precambrian magmatism in the study area, and discuss their relationship with the break-up of the Rodinia supercontinent.

1.   GEOLOGICAL SETTING AND PETROGRAPHY
  • The Sulu Orogen is an SW-NE-oriented belt in eastern China, characterized by well-developed high-pressure to ultrahigh-pressure metamorphic rock masses, and it has been regarded as the eastern extension of the Qinling-Dabie Orogen (Wang et al., 2019; Chen et al., 2018; Cui et al., 2018; Liang et al., 2018; Meng Y K et al., 2018; Yang T et al., 2018; Li J et al., 2017; Yang R C et al., 2017, 2016; Kong et al., 2015; Li X-P et al., 2013, 2011a). The Sulu Orogen is bounded by the Tanlu fault zone to the west, the Jiashan-Xiangshui fault zone to the south, and the Yantai-Qingdao-Wulian fault zone to the north. The orogenic region is composed of the northern Wulian Complex belt dominated by greenschist-facies metamorphic rocks; the middle UHP metamorphic belt lies between the Wulian-Weihai fault zone and the Shuyang-Lianyungang fault zone, which is ~160 km in width and mainly consists of granitic gneiss-eclogite-marble; the southern high-pressure metamorphic belt between the Shuyang-Lianyungang fault zone and the Jiashan-Xiangshui fault zone, characterized by quartz and/or glaucophane schists. The area from the Jiashan-Xiangshui fault to the northern margin of the Yangtze Block mainly consists of low-grade and non-metamorphic rocks. The UHP metamorphic rocks in Dabie-Sulu Orogen are mainly composed of granitic gneiss, with minor other types of rocks occurred as enclosed lenses or slices (Xia et al., 2016; Li X-P et al., 2014, 2011a, b; Ni et al., 2013) (Fig. 1a).

    Figure 1.  (a) Sketch geological map of the Dabie-Sulu Orogen (modified from Chen et al., 2016). (b) Sketch geological map of the study area and sample locations. LT, MT and HT denote low, medium and high temperature, respectively; LP, HP and UHP denote low, high and ultrahigh pressure, respectively; the elongated gray area within LT/HP blueschist zone denotes the mafic body.

    The study area in the central part of the Sulu Orogen is located in the Liansandao area of Qingdao City, Shandong Province. The rock outcrops in this region can be subdivided into three tectonic zones as follows: a northern NE-trending regular schistose belt, a central NE-trending ductile shear zone, and a southern inclined vertical fold belt (Fig. 1b).

    The northern NE-trending regular schistose belt is dominated by granulite-facies rocks, which consists of garnet-bearing biotite granulite, mica quartz schist, and dolomitic albitite granulite. The rocks display obvious NE-trending schistosity with nearly vertical inclination, constituting a regional hypo-metamorphic schistose belt.

    The central ductile shear zone is mainly composed of severe deformed albitite epidote schist, which enwraps small albitite epidote lenses and large biotite granulite lenses. The ductile shear zone is a strike-slip zone with an NE-trending and a steep inclination. In the interior and exterior of the shear zone, the folding of schistose and the deformation of lens are regular, and the shear zone is characterized by a combination of left-lateral strike-slip and oblique thrust. This zone contains some dykes of medium to coarse-grained feldspar granite porphyry, trending to ~180° with widths of 6-8 m and lengths of ~50 m.

    The southern plunging vertical fold belt has a nearly upright hinge, and contains different types of metamorphic rocks with the schistose and blasto-bedding as the index plane. This belt constitutes a continuous complex of synform and antiform structures. The folds in this belt similarly show obvious solid rheology of deep structural layers with thinner limbs and thicker hinge.

    In this study, we collected three rock samples (LSD-1, LSD-2, and LSD-3) from the central and southern regions of the study area. Sample LSD-1 (35°54'09.82"N, 120°11'29.12"E) is gneissic granodiorite in the central section of the study area. The fresh surface is gray with a lepido granoblastic texture and gneiss structure (Fig. 2c). The major mineral assemblage consists of quartz (40%-45%), feldspar (30%-35%), biotite (15%-20%). The biotite is subhedral flaky-scaly, with a continuous parallel arrangement. The xenomorphic-granular quartz and feldspar granules are evenly distributed between the mica. There are also small amounts of epidote and metal opaque minerals. Other accessories include zircon, apatite, magnetite, etc. (Fig. 2d). A clear contact exists between the intrusions and the main body of the shear zone (Fig. 2a).

    Figure 2.  Photographs (a)-(c), (e), (g) and microphotographs (d), (f), (h) showing the meta-igneous rocks from Liansandao. Field photographs showing the contact relationships of granite porphyry and gneissic granodiorite (a), and gneissic monzonite diorite and garnet-bearing granitic gneiss (b); photographs and microphotographs showing the hand specimans and microstructures of samples LSD-1 (c), (d), LSD-2 (e), (f), and LSD-3 (g), (h); Qtz. quartz; Pl. plagioclase; Bi. biotite; Kfs. K-feldspar.

    Sample LSD-2 (35°54'09.52"N, 120°11'30.38"E) is a gneissic monzonite diorite collected from the southern inclined vertical fold belt. The fresh rock is gray/green-gray in color, with a lepido granoblastic texture and gneissic structure (Fig. 2e). The mineral assemblage includes potassium feldspar (30%-35%), biotite (25%-30%), quartz (25%-30%), hornblende (5%). Most of the potassium feldspars are subhedral short columnar with quartz inclusions. The biotites occur discontinuously in subhedral flaks or banded shapes, and some of them are weakly chloritized. The quartz is translucent xenomorphic-granular. There are a small number of metamorphic pyroxenes; accessories are zircon, apatite, and magnetite (Fig. 2f).

    Sample LSD-3 (35°54'08.27"N, 120°11'29.01"E) is a garnet-bearing granitic gneiss collected from the lens of the southern inclined vertical fold belt. The lenses were deformed with the fold belt, very likely the garnet-bearing granitic gneiss intruded roughly parallel into the foliation of the metamorphic rock before folding, and was then deformed by the contemporaneous metamorphism occurred during the formation of the inclined vertical fold (Fig. 2b). The fresh rock is grayish-white-gray, with a flakey granular crystalloblastic texture and gneissic structure (Fig. 2g). The minerals consist of translucent xenomorphic-granular quartz (35%-40%), xenomorphic-granular feldspar (30%-35%), banded subhedral biotite (15%-20%), and garnet residue (5%). The accessories include zircon, apatite, and magnetite, etc. (Fig. 2h).

2.   ANALYTICAL METHODS
  • Zircon grains were separated from rock samples using conventional heavy liquid and magnetic techniques, followed by hand-picking under a binocular microscope. Purified zircon grains were then mounted in epoxy and polished to about half thickness to expose the cores of the grains.

    The cathodoluminescence (CL) images are used to show the internal texture of zircons and to select optimum spots for U-Pb dating. Zircon CL images were obtained at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China, using an analytical scanning electron microscope (JSM-IT100) connected to a GATAN MINICL system. The imaging condition is 10-13 kV voltage of electric field and 80-85 µA current of tungsten filament. U-Pb dating and trace element analyses of zircon are simultaneously conducted by LA-ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are described by Zong et al. (2017).

    Laser sampling was performed using a GeolasPro laser ablation system that consists of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. An Agilent 7700e ICP-MS instrument was used to acquire ion-signal intensities. The spot size and frequency of the laser was set to 32 µm and 5 Hz, respectively. Zircon 91500 and glass NIST610 are used as external standard for U-Pb dating and trace element calibration, respectively. Repeated analyses of 91500 give a weighted mean age of 1 061±10 Ma (MSWD=0.72, n=21). An Excel-based software ICPMSDataCal was used to perform off-line selection and integration of background and analyzed signals, time-drift correction and quantitative calibration for trace element analysis and U-Pb dating (Liu Y S et al., 2010, 2008). Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3 (Ludwig, 2003).

    In-situ Hf isotopic analyses of the zircons were carried out using a new wave UP213 laser-ablation microprobe attached to a Neptune multi-collector ICP-MS at the MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China. Helium was used as a carrier gas, with a beam diameter of ~44 μm, and laser-ablation time of 26 s during analyses. Zircon GJ-1 was used as the reference standard, and the results of the standard are indistinguishable from the recommended weighted mean 176Hf/177Hf ratio of 0.282 015±0.000 008 (2σ, n=10). The standard reference BHVO-2 gives 176Hf/177Hf=0.283 093±0.000 003, identical to the reference value within analytical error. The instrumental conditions and the data acquisition methods have been described comprehensively by Hou et al. (2007).

3.   RESULTS
  • Most of the zircons analyzed here show metamorphic overgrowth rims. However, they are too narrow to be analyzed. Thus, we focus on the magmatic zircons (core) and the analytical results are shown in Table S1.

  • Thirteen zircon grains from the gneissic granodiorite (LSD-1) were analyzed using LA-ICP-MS. The zircons are typically short to long prismatic with the lengths of 150 to 250 μm (individual grains can reach up to 250 μm), and length to width ratios of 2 : 1-2.7 : 1. The zircon grains are characterized by oscillatory zoning and high Th/U ratios (> 0.4). The jagged edge of some grains suggested that they might have undergone magmatic corrosion (Fig. 3a). The 13 analyzed data fall near the concordia line and give the apparent 206Pb/238U ages of 716±5 to 771±8 Ma, yielding a weighted mean age of 744±11 Ma (MSWD=5.7) (Fig. 4a).

    Figure 3.  Zircon CL images of meta-igneous rocks from the Liansandao area with 206Pb/238U age and εHf(t) value for (a) LSD-1, (b) LSD-2 and (c) LSD-3. Yellow and red dashed circles represent the spots for U-Pb dating and the spots for Hf analyses.

    Figure 4.  U-Pb diagrams of concordia and weighted mean ages of zircons for (a) LSD-1, (b) LSD-2 and (c) LSD-3.

  • Nineteen spots were analyzed on the zircons from the gneissic monzonite diorite (LSD-2). The zircons are mainly long prismatic; a minor amount of zircons are ellipsoidal. They have length-width ratios of 2 : 1-2.7 : 1. Zircon grains also show clear oscillatory zoning, and some of them have inherited core. In addition, they have Th/U ratios of > 0.4. They also show the existence of magmatic corrosion edge (Fig. 3b). All the analyzed spots plot on or nearby the concordant line, giving apparent 206Pb/238U ages from 721±9 to 805±8 Ma, with a weighted mean of 767±12 Ma (MSWD=5.4) (Fig. 4b).

  • Fourteen spots were analyzed on the zircons from the garnet-bearing granitic gneiss (LSD-3). They are similar to the magmatic zircons from LSD-2 described above (Fig. 3c). The obtained 206Pb/238U ages range from 704±13 to 797±12 Ma, and give a weighted mean age of 762±15 Ma (MSWD=7.8) (Fig. 4c), which is consistent with that of LSD-2 within error.

  • Thirty-one representative concordant grains out of the dated zircons were analyzed for the Hf isotope, and the results are shown in Table S2 and Fig. 5. The εHf(t) value and two-stage model ages were calculated using the measured 206Pb/238U ages in this study.

    Figure 5.  εHf(t) vs. U-Pb age diagram for the meta-igneous rocks from Liansandao.

    The Hf isotopic compositions of zircon grains from sample LSD-1 are relatively uniform. They have 176Hf/177Hf ratios from 0.282 038 to 0.282 120, and εHf(t)= -7.2 to -10.5, indicating a continental crust origin. Combined with the TDM2 ages of 2 105 to 2 286 Ma, it is likely that the protolith derived from re-melting of Paleoproterozoic ancient continental crust.

    The 176Hf/177Hf ratios of zircon grains from the LSD-2 range widely from 0.281 804 to 0.282 184, with εHf(t) values range from -5.9 to -17.6. The higher εHf(t) values than that of the continental crust and the trend toward the depleted mantle indicate the contribution of mantle-derived material. However, the negative εHf(t) values indicate the involvement of continental crust. The TDM2 ages range from 2 004 to 2 774 Ma, and most of them are Paleoproterozoic except for five grains are 2 506 to 2 774 Ma.

    The zircon grains from the sample LSD-3 have 176Hf/177Hf= 0.281 987-0.282 120, and εHf(t)= -6.8- -12.0, which are similar to those of LSD-1 and LSD-2. The TDM2 ages also range from 2 104 to 2 399 Ma. In a word, the Hf isotopic compositions of zircons from the three samples are comparable, suggesting a common source.

4.   DISCUSSION
  • Magmatism in the Sulu Orogen occurred primarily in the eras of the Meso-Neoproterozoic, and Mesozoic. Field observations and contact relationships in this study suggest that the meta-igneous rocks from Liansandao are intrusive suites metamorphosed and deformed in the Meso-Neoproterozoic. However, there are still no accurate radiometric ages on these meta-igneous rocks. As stated above, the zircons of the three meta-igneous rocks from the Liansandao area show oscillatory zoning and have high Th/U ratios, indicating a magmatic origin, and the zircon ages represent the formation ages of the protolith. The zircons from the three samples yield the concordant U-Pb ages of 747±12, 767±12 and 765±21 Ma, respectively, which are consistent with each other within error. Thus, the protoliths of the meta-igneous rocks in the Liansandao area formed contemporaneously in the Neoproterozoic. Previous studies have confirmed that the 750 to 850 Ma magma activities were common in the Yangtze Block (He et al., 2017; Yang Y N et al., 2016; Bader et al., 2013; Charvet, 2013). However, magmatism in this period was rare in the North China Craton and only occurred to the east of the Tanlu fault zone, which is generally considered to have a Yangtze Block affinity (Fig. 6). The reported Neoproterozoic magmatisms along the Sulu Orogen mainly occurred in the areas of Rongcheng, Weihai, Yangkou, Wulian, and Donghai (Liu et al., 2018; He et al., 2016; Chen M et al., 2013; Chen R X et al., 2010; Hacker et al., 2006; Zheng et al., 2004). Therefore, we suggest that the protoliths of the meta-igneous rocks in the Liansandao area represent synchronous magmatic events in the Sulu Orogen and the Yangtze Block. It is worth noted that the magmatic suites in the Yangkou, Wulian, and Liansandao areas are distributed along and possibly controlled by the NNE-oriented Wulian-Yantai fault.

    Figure 6.  Distribution of the Neoproterozoic magmatism in China (modified from Zhao and Cawood, 2012; map of China after GS (2016) 1594).

  • The Hf isotopic compositions of zircon can not only be used to identify the source characteristics, but also are useful tools to study the continental crust growth (Zhang Y et al., 2016; Wu F Y et al., 2007; Griffin et al., 2002).

    As previously mentioned, the negative εHf(t) values of the zircons in the three rock samples from the Liansandao area in this study seem to indicate a crustal genesis. The two-stage model ages of the samples concentrated around 2.0-2.3 Ga imply that they might result from the re-melting of Paleoproterozoic continental crust materials. Furthermore, the trace-elemental data also have "arc or continental-crust-like" characteristics such as depletion in HFSEs and enrichment in LILEs (our unpublished data). However, it should be noted that the Hf isotopic compositions of all the samples in this study are inhomogeneous. In particular, the Hf isotopes of sample LSD-2 vary widely, which indicate mixing of at least two components. Although the εHf(t) values are negative, they spread toward positive value, which might indicate a contribution of mantle-derived materials (Fig. 5). Sample LSD-2 has lower SiO2 (50.7 wt.%-52.1 wt.%) and higher MgO contents (4.26 wt.%-5.03 wt.%) than those of samples LSD-1and LSD-3 (our unpublished data) and melts of continental crust, also indicating a mantle origin or involvement of mantle-derived material (Meng F X et al., 2018). We thus infer that the sample LSD-2 is most likely formed from mantle-derived magma mixed with continental crustal material to a different extent. The ancient continental crust materials might be Paleoproterozoic to Archean. The Archean basement is generally considered as an important feature of the North China Craton (e.g., Liu et al., 1992). However, recent studies have shown that the Archean basement rocks are also sporadically exposed in the Yangtze Block (Wang et al., 2013a, b), e.g., several Mid-Archean 207Pb/206Pb ages (Liu et al., 2011) and some Neo-Archean inherited zircon ages (Liu and Liou, 2011) have been found in the Sulu Orogen. Previous studies suggested that the subduction and orogeny ceased (Fig. 7a) and transformed into an extensional environment at ~800 Ma or earlier in the northern margin of the Yangtze Block (e.g., Duan et al., 2018; Deng et al., 2016). Then, the mantle-derived magma might upwell and emplace in the continental crust and have interacted with ancient continental crustal material, responding to the extensional setting. In contrast, the relatively uniform Hf isotopic compositions and Hf model age of samples LSD-1 and LSD-3, combined the "continental-crust like" trace-elemental composition, suggest that they were derived from re-melting of Paleoprotorozoic continental crust. It is not surprisingly considered that the protoliths of samples LSD-1 and LSD-3 are granitic. In extensional environment, the mantle-derived magma underplated beneath the continental crust might provide heat and materials to the production of granitic magma. Previous studies on Nd isotope of granitic gneiss in the Jiaonan area, central Sulu Orogen showed that all the samples have negative εNd(t) values, with model ages mainly at 2.0 Ga. These results suggested that the magma was derived from Paleoproterozoic crust (Xue et al., 2007). In addition, granitic gneisses from the Rongcheng UHP terranes in the northern Sulu Orogen showed εHf(t) values ranging widely from positive to negative (Li et al., 2007). Meanwhile, the two-stage model ages of the zircons with the negative εHf(t) are concentrated at 1.9-2.3 Ga, with some around 2.6-2.8 Ga. These data imply that both ancient crust and mantle-derived components had contributed to the magma formation and the rocks document the crust-mantle mixing characteristics. The similarities of these rocks to Liansandao meta-igneous rocks studied here lead us to believe that they might be cogenetic. In summary, the protolith of the samples LSD-1 and LSD-3 in the Liansandao area is most likely a result from the re-melting of the Paleoproterozoic continental crust beneath Yangtze Block caused by underplating of mantle-derived magma in Neoproterozoic period (750-770 Ma). However, the protolith of sample LSD-2 might be a result of mantle-derived magma formed in extensional setting that then mixed with ancient continental material during upwelling. The age characteristics suggest that the magmatic thermal event was the product of the break-up of the Rodinia supercontinent (Li et al., 2008). It means that the protolith of the meta-igneous rocks in the Liansandao area have recorded the magma responses of the extensional environment during the break-up of the Rodinia supercontinent. The decompression and melting of the mantle during the break-up of Rodinia supercontinent led to the formation of basic magma (e.g., LSD-2). Furthermore, the basic magma upwelled and underplated onto the lower part of the continental crust and resulted in the partial melting of the continental crust, producing the granitic rocks. The break-up event caused the multiple generations of ultrabasic-basic rocks in various regions at the northeastern margin of the Yangtze Block with the age of 800-750 Ma, which is believed to represent an important episode of continental crustal growth in the Yangtze Block (Zeng et al., 2016; Xue et al., 2011; Zhao and Zhou, 2009).

    Figure 7.  Geodynamic evolutionary models for the northern margin of Yangtze Block during the Neoproterozoic (modified from Wang et al., 2017).

  • Magmatic activities during Neoproterozoic period are common in the Yangtze Block (see above), and this period is also contemporaneous with the Rodinia supercontinent break-up. Although the mechanism of the Rodinia supercontinent break-up remains controversial, in essence the following scenarios are discussed: (1) super mantle plume effects (Li X H et al., 2010; Wang et al., 2008a; Li Z X et al., 2003); (2) island arc models (Zhao et al., 2011; Wang et al., 2008b; Zhou et al., 2002); and (3) plate-rift models (Zheng et al., 2008; Wu Y B et al., 2007; Wu R X et al., 2006), and all of them suggest an extensional environment (Li S Z et al., 2015).

    There is no evidence to support the existence of the mantle plume. Furthermore, the geochemical studies on the ~770-780 Ma gneissic granites in the eastern margin of the Dabie ultra-high pressure metamorphic zone, suggested that the northeastern margin of the Yangtze Block was attributed to a passive rift setting during Mid-Neoproterozoic period, instead to an active rift environment caused by mantle plume upwelling (Hu et al., 2010). In addition, there is no evidence for the presence of continental flood basalts and oceanic island basalts indicating mantle plume activity. The ~800 Ma gneissic granites in the Daleishan area at the northern margin of the Yangtze Block possess typical post-orogenic A-type granite characteristics, indicating a back-arc extension environment during this period (Cao et al., 2017). The chronological and Hf isotopic analysis of zircons from the granite in the Lujiazhai area in the southern part of Dabieshan also shows that it was the ~816 Ma collapsed product formed in gravity/extension environment. These findings indicate that the northeastern margin of the Yangtze Block has transformed from a compressional to an extensional regime at ~820 Ma. It was proposed that the northern and western margins of the Yangtze Block collided at 1 000-900 Ma, while the southeastern margin is an active continental margin. At ~820 Ma, the Rodinia supercontinent broke up, producing numerous A-type granites and basic rocks in extensional settings with the ages of 800-780 Ma. With further extension, alkaline and basic rocks occurred at 760-750 Ma (Deng et al., 2016). The paleogeography data provided further supports. For example, an inland sea existed between the Australian Block and the Siberian Block during the break-up of the Rodinia supercontinent. Then, the sea constantly expanded, subducted, and collided with the South China Craton (Li Z X et al., 2013). Based on the integrated paleo-geographic reconstruction data of the Rodinia supercontinent, Hu et al. (2007) speculated that the northeastern margin of the Yangtze Block belonged to an active continental margin environment with oceanic subduction during the earlier Neoproterozoic period. The formation ages of the protoliths of the three meta-igneous rocks in the Liansandao area are ~770-750 Ma as suggested in this study, thus, it was deduced to form in the rifting extensional background after subduction and collision events.

    The subduction ceased (Fig. 7a) and transformed into an extensional environment at ~820 Ma or earlier. At this time, the Rodinia continent started to break up, leading to decompression melting of mantle material, which then underplated to the base of the lower continental crust. These actions provided sufficient heat for ancient crustal melting to produce the felsic magma (Fig. 7b). The mafic pluton at the northern margin of the Yangtze Block roughly experienced the latest regional magmatic activities during the break-up of the Rodinia supercontinent, and the average 206Pb/238U age of the zircon was 637±4 Ma (Wang M X et al., 2012). In conclusion, the widespread magmatic activities at the northern margin of the Yangtze Block were the response to the Rodinia supercontinent break-up. The results of this study and the existing data on granitic rocks indicated the participation of mantle-derived material with mantle isotope characteristics. Therefore, during this period, important events related to continental crust growth in the Yangtze Block had occurred, and the granitic magma recorded the melt extraction events associated with the continental crust growth event (Zheng, 2003).

5.   CONCLUSIONS
  • In this study, we firstly report zircon U-Pb dating and Hf isotopic results of meta-igneous rocks from the Liansandao area and reach the following conclusions.

    (1) The zircon U-Pb ages of three meta-igneous rocks samples from the Liansandao area are determined to be 744±11, 767±12, and 762±15 Ma, respectively. The results indicate that the protolith formed in the Neoproterozoic period, coeval with the Yangkou and Wulian Neoproterozoic magmatisms.

    (2) The Hf isotopes of zircons show a relatively large variation. Combined with the existing researches and the Hf model ages in this study, the granitic protoliths (samples LSD-1, LSD-3) in the Liansandao area are most likely to be derived from the partial melting of the Paleoproterozoic Yangtze continental crust caused by unpderplating of basic magma, and sample LSD-2 origined from mantle-derived magma responding to extensional setting that interacted with ancient continental crust materials during upwelling and emplacement into the continental crust.

    (3) The northeastern margin of the Yangtze Block changed to an extensional setting at ~820 Ma or earlier, and the Rodinia supercontinent started to break-up, then the mantle materials underwent decompression melting to produce basic magma that underplated to the base of the lower continental crust to produce the granitic magma. Furthermore, the addition of basic magma at ~800-750 Ma suggests an important continental crust growth event in the Yangtze Block.

ACKNOWLEDGMENTS
  • This work was financially supported by the Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology (No. MGQNLM201902), the National Natural Science Foundation of China (Nos. 41472155, 41876037), the Scientific and Technological Innovation Project of the China Ocean Mineral Resources R & D Association (No. DY135-N2-1-04), the Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (No. 2016RCJJ008), and the SDUST Research Fund (No. 2015TDJH101). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1252-7.

    Electronic Supplementary Materials: Supplementary materials (Tables S1-S2) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1252-7.

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