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Volume 41 Issue 4
Aug.  2020
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Yaqin Luo, Haiyan Qin, Tao Wu, Zilong Li. Petrogenesis of the Granites in the Yandangshan Area, Southeastern China: Constraints from SHRIMP U-Pb Zircon Age and Trace Elements, and Sr-Nd-Hf Isotopic Data. Journal of Earth Science, 2020, 31(4): 693-708. doi: 10.1007/s12583-020-1295-9
Citation: Yaqin Luo, Haiyan Qin, Tao Wu, Zilong Li. Petrogenesis of the Granites in the Yandangshan Area, Southeastern China: Constraints from SHRIMP U-Pb Zircon Age and Trace Elements, and Sr-Nd-Hf Isotopic Data. Journal of Earth Science, 2020, 31(4): 693-708. doi: 10.1007/s12583-020-1295-9

Petrogenesis of the Granites in the Yandangshan Area, Southeastern China: Constraints from SHRIMP U-Pb Zircon Age and Trace Elements, and Sr-Nd-Hf Isotopic Data

doi: 10.1007/s12583-020-1295-9
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  • New geochemical and geochronological data of two types of granites, which are located in Yandangshan area, southeastern Zhejiang Province, were presented to constrain their magma condition, origin and the genetic relationship between them. The SHRIMP zircon U-Pb dating of Dongshan and Hesheng granite in Yandangshan area shows that they were formed at 114±1 and 103±2 Ma, respectively. Samples from the Dongshan granite have high SiO2 (76.4 wt.%-76.9 wt.%) and total alkaline (K2O+Na2O=8.35 wt.%-8.47 wt.%) contents, but low FeOT (0.89 wt.%-1.15 wt.%), MgO (0.21 wt.%-0.22 wt.%), and CaO (0.24 wt.%-0.34 wt.%) contents and high A/CNK (~1.1) values, belonging to the peraluminous and magnesian granite. The Hesheng granite has high SiO2 (72.2 wt.%-77.5 wt.%), total alkaline (K2O+Na2O=8.05 wt.%-9.41 wt.%) and FeOT contents (1.20-2.06), and high A/CNK values (1.0-1.1), but low in MgO (0.12 wt.%-0.29 wt.%) and CaO (0.24 wt.%-0.34 wt.%) contents. Samples from the Hesheng granite also have high FeOT/MgO (6.9-10.0) and 10 000×Ga/Al (2.6-3.4) ratios similar to the ferroan/A-type granite. All the samples are enriched in LREE but have produced negative Eu anomalies (Eu/Eu*Dongshan=0.45-0.47; Eu/Eu*Hesheng=0.17-0.55), Ba, Nb, and Ta, while the REE contents of the ferroan/A-type granite (Hesheng) are higher than that of the magnesian granite (Dongshan). The (87Sr/86Sr)i value of the magnesian granite is slightly higher than that of the ferroan/A-type granite and its εNd(t) value (-6.8) is lower than the latter (-6.0--5.9). In addition, the εHf(t) value (-11.8--4.2) of magnesian granite is also lower than that of the ferroan/A-type granite (-8.3--2.0), indicating that there may be more mantle-derived components in the source area of the ferroan/A-type granite. Zircon saturation thermometer (TZr) and Ti-in-zircon thermometer (TZircon) are used to estimate the temperature of the magma source, and the results show that the magma temperature of the magnesian granite (average TZr=798℃; average TZircon=792℃) is lower than that of the ferroan/A-type granite (average TZr=862℃; average TZircon=859℃). And the oxygen fugacity of magnesian granite (ΔFMQ=1.16-3.47) are also higher than those of the ferroan/A-type granite (ΔFMQ=-0.41-1.14). Our new data indicate that both granites in this study are derived from a mixed source that consists of mantle-derived and crust-derived material. Based on the previous studies, both of the granitic plutons were formed under extension setting, and the granites transformed from magnesian to ferroan in the study area may indicate the extension was enhanced, which may be caused by the roll-back or delamination of the Paleo-Pacific oceanic slab.
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Petrogenesis of the Granites in the Yandangshan Area, Southeastern China: Constraints from SHRIMP U-Pb Zircon Age and Trace Elements, and Sr-Nd-Hf Isotopic Data

doi: 10.1007/s12583-020-1295-9

Abstract: New geochemical and geochronological data of two types of granites, which are located in Yandangshan area, southeastern Zhejiang Province, were presented to constrain their magma condition, origin and the genetic relationship between them. The SHRIMP zircon U-Pb dating of Dongshan and Hesheng granite in Yandangshan area shows that they were formed at 114±1 and 103±2 Ma, respectively. Samples from the Dongshan granite have high SiO2 (76.4 wt.%-76.9 wt.%) and total alkaline (K2O+Na2O=8.35 wt.%-8.47 wt.%) contents, but low FeOT (0.89 wt.%-1.15 wt.%), MgO (0.21 wt.%-0.22 wt.%), and CaO (0.24 wt.%-0.34 wt.%) contents and high A/CNK (~1.1) values, belonging to the peraluminous and magnesian granite. The Hesheng granite has high SiO2 (72.2 wt.%-77.5 wt.%), total alkaline (K2O+Na2O=8.05 wt.%-9.41 wt.%) and FeOT contents (1.20-2.06), and high A/CNK values (1.0-1.1), but low in MgO (0.12 wt.%-0.29 wt.%) and CaO (0.24 wt.%-0.34 wt.%) contents. Samples from the Hesheng granite also have high FeOT/MgO (6.9-10.0) and 10 000×Ga/Al (2.6-3.4) ratios similar to the ferroan/A-type granite. All the samples are enriched in LREE but have produced negative Eu anomalies (Eu/Eu*Dongshan=0.45-0.47; Eu/Eu*Hesheng=0.17-0.55), Ba, Nb, and Ta, while the REE contents of the ferroan/A-type granite (Hesheng) are higher than that of the magnesian granite (Dongshan). The (87Sr/86Sr)i value of the magnesian granite is slightly higher than that of the ferroan/A-type granite and its εNd(t) value (-6.8) is lower than the latter (-6.0--5.9). In addition, the εHf(t) value (-11.8--4.2) of magnesian granite is also lower than that of the ferroan/A-type granite (-8.3--2.0), indicating that there may be more mantle-derived components in the source area of the ferroan/A-type granite. Zircon saturation thermometer (TZr) and Ti-in-zircon thermometer (TZircon) are used to estimate the temperature of the magma source, and the results show that the magma temperature of the magnesian granite (average TZr=798℃; average TZircon=792℃) is lower than that of the ferroan/A-type granite (average TZr=862℃; average TZircon=859℃). And the oxygen fugacity of magnesian granite (ΔFMQ=1.16-3.47) are also higher than those of the ferroan/A-type granite (ΔFMQ=-0.41-1.14). Our new data indicate that both granites in this study are derived from a mixed source that consists of mantle-derived and crust-derived material. Based on the previous studies, both of the granitic plutons were formed under extension setting, and the granites transformed from magnesian to ferroan in the study area may indicate the extension was enhanced, which may be caused by the roll-back or delamination of the Paleo-Pacific oceanic slab.

Yaqin Luo, Haiyan Qin, Tao Wu, Zilong Li. Petrogenesis of the Granites in the Yandangshan Area, Southeastern China: Constraints from SHRIMP U-Pb Zircon Age and Trace Elements, and Sr-Nd-Hf Isotopic Data. Journal of Earth Science, 2020, 31(4): 693-708. doi: 10.1007/s12583-020-1295-9
Citation: Yaqin Luo, Haiyan Qin, Tao Wu, Zilong Li. Petrogenesis of the Granites in the Yandangshan Area, Southeastern China: Constraints from SHRIMP U-Pb Zircon Age and Trace Elements, and Sr-Nd-Hf Isotopic Data. Journal of Earth Science, 2020, 31(4): 693-708. doi: 10.1007/s12583-020-1295-9
  • During the Mesozoic, the South China Block (SCB) has experienced large-scale magmatism and mineralization and generated a great area of magmatic rocks over 262 920 km2 (Li et al., 2014a; Zhou et al., 2006). The widespread Mesozoic igneous rocks in southeast China are mainly distributed in the coastal areas of Zhejiang, Fujian and Guangdong provinces, and the Lower Yangtze region. More than 90% of them are granite and felsic volcanic rocks with less basalt. The Mesozoic magmatism of South China can be further divided into three stages: the Early and Late Indosinian Period (251–223 and 234–205 Ma, Triassic), the Early Yanshanian Period (180–142 Ma, Jurassic) and the Late Yanshanian Period (142–67 Ma, Cretaceous) (Mao et al., 2008; Zhou et al., 2006). Most of the Indosinian magmatic rocks in SCB are granite plutons and distributed in the inland areas with an outcrop area of about 14 300 km2. They are mainly massive, medium-grained and peraluminous S-type granites. The Early Yanshanian granite plutons are also distributed in the inland areas, most of which are parallel to the coastline except the E-W distributed Nanling granitoids, and are mostly I-type granite with a small number of A-type and S-type granites, and the outcrop area covers around 63 870 km2. The Early Yanshanian volcanic-intrusive rocks are distributed in the southern Hunan Province and the boundary among Fujian, Guangdong and Jiangxi provinces (Chen et al., 2002). In the Late Yanshanian, the outcrop area of volcanic rocks was nearly twice as large as area of plutons with a total of about 139 920 km2. Those plutons mainly distributed in the coastal region. The intrusive and volcanic rocks, mostly rhyolitic rocks, formed a NE-SW distributed volcanic-intrusive belt (Liu, 2015; Chen et al., 2008). During the past decades, lots of models are proposed for the Mesozoic igneous rocks, including the multi-block interaction mode, the Alpine-type collision model, the ridge subduction and flat slab subduction models of paleo- Pacific Plate (Gao et al., 2019; Li X H et al., 2013; Wang et al., 2013; Li Z X et al., 2012; Sun et al., 2010; Ling et al., 2009; Li and Li, 2007).

    The Late Mesozoic magmatic rocks in Zhejiang Province (Fig. 1) are mainly felsic volcanic rocks coexisting with scattered granite intrusions (Liu et al., 2014; Li et al., 2013). The origin of the granitoids was believed to be formed by the partial melting of lower-crustal materials, which consists of Neoproterozoic arc crustal rocks and ancient crustal rocks (Wu et al., 2018a). The rollback of the paleo-Pacific Plate model was generally accepted to explain the spatial and temporal evolution of the Late Mesozoic magmatism in Zhejiang Province (Gao et al., 2019; Liu L et al., 2016; Liu, 2015).

    Figure 1.  Sketch geological map of Zhejiang Province, southeastern China, and sample locations of granitoids during 80–130 Ma. Zircon geochronological and Nd isotopic data are from Qiu et al. (2004), Wong et al. (2009), He et al. (2009), Li et al. (2013), Zhao et al. (2016), Liu L et al. (2016), Pan et al. (2018) and this study. BJHJ. Baijuhuajian; DS. Dongshan; HS. Hesheng; HTY. Huangtanyang; JN. Jingning; PTS. Putuoshan; QT. Qingtian; RH. Ruhong; THD. Taohuadao; YDS. Yandangshan; YF. Yunfeng.

    Yandangshan area is located in the southeastern Zhejiang Province (Fig. 2). The igneous rocks in this area are mainly rhyolitic rocks with some porphyritic quartz syenites and granite. Previous studies proposed that these volcanic-plutonic complexes were formed by magma mixing, and derived from the mixing of mantle-derived and crust-derived melts (Yan et al., 2016, 2015; He et al., 2009; Yu et al., 2008). In this paper, we report the whole-rock major- and trace-element concentrations, Sr-Nd isotope data, SHRIMP zircon U-Pb ages, trace elements and Hf isotope concentrations of two granites in the Yandangshan area, southeastern Zhejiang Province. We aim to constrain their crystallization age, magma source and tectonic background, and explore their implications for the tectonic-magmatic evolution of SE China.

    Figure 2.  Simplified geological map of the Yandangshan region and sample locations.

  • The South China Block is bounded to the north by the North China Craton, the Tibetan Block in the west, the Indochina Block in the southwest, and the Philippine Sea Plate in the east. It is composed of the Yangtze Block in the northwest and the Cathaysia Block in the southeast, which is divided by the Jiangshan-Shaoxing suture zone (Wu et al., 2018b; Liu K et al., 2016; Li et al., 1995). In the Mesozoic, South China experienced large-scale magma and mineralization activities, which is mainly concentrated in the Cathaysia Block, including a large area of Mesozoic igneous rocks (Zhou et al., 2006). The Cretaceous magmatic rocks in Zhejiang Province are widely distributed, and the exposure covers almost 70% of the area in the province. It is mainly composed of volcanic rocks with a small number of granite intrusions. The volcanic rocks are mainly composed of rhyolite and rhyolitic volcanoclastic rock with a small amount of basalt. Most of the volcanic rocks are distributed to the southeast of the Jiangshan-Shaoxing suture zone.

    Yandangshan area is located in Yueqing, Wenzhou City, southeastern Zhejiang Province. The study area is located to the southeastern side of the Jiangshan-Shaoxing belt and adjacent to the Wenzhou-Zhenhai fault. The northeast, north-north and east-north are the main directions of faults with few northwest, north-south and east-west faults. A large volume of Cretaceous igneous rocks is exposed in and around the Yandangshan area (Fig. 2) and tuff, rhyolite, rhyolitic volcanoclastic rocks, granite and syenite are the main rock types.

    The tuff, rhyolite and rhyolitic volcanoclastic are all formed in Cretaceous and can be divided into Gaowu, Xishantou and Jiuliping formations (Fig. 2). The intrusive rocks in our study area almost all formed in the Late Yanshanian Period with the ages ranging from 67 to 142 Ma. The quartz syenite porphyry is mainly distributed in the central Yandangshan Mountain and shows intrusive relationship with the surrounding rhyolitic volcanoclastic rocks (Yan et al., 2015). Another quartz syenite pluton is located to the northwest of the Yandangshan Mountain. There are also some granite plutons, which are distributed around the Yandangshan Mountain and most of them belong to the moyite and monzonitic granite. These granites are intruded into the surrounding volcanic rocks.

    In this study, five samples were collected from two granite plutons and the sampling locations are shown in Fig. 2. The western granite pluton is located in Hesheng Town while the eastern one is in the Dongshan Village. We call them Hesheng and Dongshan granites here and after.

    The Hesheng granite (Fig. 3) is coarse-grained with 0.5–2 mm in grain sizes. Samples from it consist of plagioclase (50 –60%), quartz (20%–25%), biotite (~8%), orthoclase (~5%) and hornblende (~2%).

    Figure 3.  Field photographs and photomicrographs of the Yandangshan area granite. (a) and (b) are field photographs of YDS-43. (c) Orthoclase and quartz phenocrysts in YDS-19. (d) Plagioclase megacrysts, anhedral quartz and biotite of varying sizes are in YDS-43. Pl. Plagioclase; Or. orthoclase; Q. quartz; Bi. biotite.

    The Dongshan granite (Fig. 3) exhibits porphyritic texture with phenocryst sizes between 0.5 and 3 mm. The phenocrysts (20%–30%) are orthoclase and quartz. The matrix is composed of quartz (~50%), orthoclase (~20%) and a small number of opaque minerals.

  • Zircon grains were separated from two granitic samples by crushing, sieving, conventional heavy-liquid and magnetic separation techniques, handpicked, mounted in epoxy, and polished. Cathodoluminescence (CL) images were used to image and select analytic points after the zircons were carbon coated. The U-Pb dating of them was conducted by SHRIMP-Ⅱ at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing. Detailed analytic procedures were described by Song et al. (2002).

    The whole rock major elements analyses were analyzed by XRF at the Second Ocean Research Institute of the State Oceanic Administration. The trace elements and the zircon trace elements analyses were analyzed by Agilent 7700e ICP-MS at the Analysis and Testing Center of Wuhan Shangpu Analytical Technology. Detailed trace element analytic procedures are described the same as those in Liu et al. (2008). The relative standard deviations are lower than 5% for major elements and 10% for trace elements.

    The Sr-Nd isotope test analysis of the whole rock was completed by the State Key Laboratory of Mineralization Mechanism Research of Endogenous Metal Deposits (Nanjing University). In the mixed acid of Finnigan MAT Triton Ti, HCl, it is completely dissolved and separated by resin. The chemical process is described in the literature. The analysis result of the JNDi Nd standard sample is: 143Nd/144Nd is 0.512 105, and the test result of the NBS 987 Sr standard sample is: 87Sr/86Sr is 0.710 261.

    In situ zircon Hf isotopic analysis was carried out on a Neptune plus MC-ICP-MS in combination with a Resolution M-50-LR laser that was hosted at the State Key Laboratory of Isotope Geochemistry (Guangzhou Institute of Geochemistry). The laser parameters used in the test are 45 μm beam spot, frequency 7 Hz, energy density 80 mJ/cm2. Each point tests gas blank 28 s, laser ablation 30 s. The laser model used is the Resonetics Resolution M-50. Multi-receiving inductively coupled plasma mass spectrometry is Neptune plus. Penglai zircons were used as the reference material (Li et al., 2010) with the precision being 0.282 910±0.000 033 (2σ, N=95).

  • The major and trace element compositions for the representative samples from two granite plutons in the Yandangshan area are listed in Table 1. In the TAS diagram, the samples all fall in the granite area (Fig. 4a).

    Composition Dongshan granite Hesheng granite
    YDS-19 YDS-20 YDS-41 YDS-43 YDS-45 YDS-46
    SiO2 76.42 76.86 77.05 72.94 72.24 77.45
    TiO2 0.16 0.14 0.19 0.29 0.33 0.11
    Al2O3 12.21 11.94 12.61 13.51 13.86 12.38
    Fe2O3 1.27 0.99 1.45 2.29 2.24 1.33
    MnO 0.03 0.02 0.02 0.05 0.07 0.03
    MgO 0.22 0.21 0.15 0.27 0.29 0.12
    CaO 0.34 0.24 0.26 0.67 0.80 0.09
    Na2O 3.26 2.69 4.21 4.16 4.40 3.65
    K2O 5.10 5.78 4.55 5.00 5.01 4.40
    P2O5 0.03 0.02 0.02 0.04 0.05 0.01
    LOI 0.25 0.43 0.38 0.56 0.48 0.62
    Total 99.35 99.42 100.89 99.89 99.86 100.18
    Na2O+K2O 8.35 8.47 8.75 9.15 9.41 8.05
    Fe-index 0.84 0.81 0.90 0.89 0.87 0.91
    MALI 8.02 8.24 8.50 8.48 8.61 7.96
    ASI 1.09 1.10 1.04 1.05 1.04 1.14
    Mg# 26 30 17 19 21 15
    V 7.90 9.28 3.70 8.95 9.88 2.60
    Cr 3.78 1.92 2.81 4.93 6.93 3.10
    Co 0.93 0.74 0.54 1.15 1.12 0.52
    Ni 1.98 1.34 1.39 3.45 3.34 2.46
    Ga 12.73 12.86 17.84 18.66 19.07 22.30
    Cs 1.34 1.24 1.21 2.26 1.99 2.63
    Ba 569.80 621.06 131.59 669.41 722.86 30.05
    Rb 154.63 169.27 148.26 166.22 168.09 356.83
    Th 21.95 23.24 17.27 15.24 15.10 31.53
    U 4.18 3.56 3.14 2.95 2.35 6.08
    Nb 10.96 10.69 21.88 19.73 20.15 42.21
    Ta 1.18 1.16 1.46 1.30 1.29 2.66
    La 36.97 41.14 37.28 55.76 63.10 35.32
    Ce 66.05 76.59 74.46 107.60 120.84 49.47
    Pb 11.00 10.83 15.35 21.33 24.78 20.98
    Pr 7.10 8.38 8.54 12.47 14.47 5.77
    Nd 22.06 26.31 27.96 42.48 49.58 14.95
    Sr 145.38 137.60 38.37 92.78 106.91 28.99
    Zr 96.07 92.77 173.09 213.47 247.82 145.73
    Hf 3.32 3.35 5.99 6.27 6.91 6.85
    Sm 3.60 4.16 5.18 7.29 8.43 2.24
    Eu 0.56 0.61 0.59 1.30 1.46 0.14
    Gd 3.61 4.01 5.07 6.95 7.85 2.47
    Tb 0.60 0.66 0.90 1.09 1.20 0.51
    Dy 3.24 3.32 4.81 5.48 6.17 2.45
    Y 22.23 21.61 32.30 30.39 36.51 20.96
    Ho 0.79 0.79 1.17 1.21 1.36 0.72
    Er 2.18 2.18 3.16 3.09 3.56 2.10
    Tm 0.44 0.45 0.63 0.60 0.68 0.52
    Yb 2.75 2.65 3.62 3.11 3.71 3.14
    Lu 0.50 0.49 0.65 0.59 0.68 0.66
    Cu 4.56 3.83 4.01 8.44 6.29 4.09
    Zn 42.02 10.60 14.07 35.76 42.09 32.65
    10 000×Ga/Al 1.97 2.03 2.67 2.61 2.60 3.40
    (La/Yb)N 9.64 11.12 7.38 12.87 12.20 8.08
    Eu/Eu* 0.48 0.46 0.35 0.56 0.55 0.18
    ΣREE 150.46 171.75 174.02 249.02 283.08 120.46
    TZr (℃) 800 797 854 870 883 843
    FeOT=total iron; Fe-index=(FeOT)/(FeOT+MgO); Mg#=100×molecular Mg2+/(Mg2++Fe2+); (La/Yb)N means the ratio between chondrite-normalized La and Yb; TZr (℃) means the zircon saturation temperature.

    Table 1.  Major (wt.%) and trace element (ppm) compositions of granites around Yandangshan

    Figure 4.  Chemical classification of granites from Hesheng and Dongshan. (a) Total alkali vs. silica (TAS) diagram (after Middlemost, 1994); (b) MALI (modified alkali lime index, K2O+Na2O-CaO vs. SiO2 diagram after Frost et al., 2011); (c) ASI (aluminium saturation index, Al/(Ca–1.67P+Na+K) vs. SiO2 diagram after Frost et al., 2011); (d) Fe-index=(FeOT)/(FeOT+MgO) vs. SiO2 diagram after Frost et al. (2011).

    The Dongshan granites, which are located at the eastern part of the Yandangshan area, show high SiO2 (76.4 wt.%– 76.9 wt.%), Al2O3 (11.9 wt.%–12.2 wt.%) and K2O+Na2O (8.35 wt.%–8.47 wt.%) contents (Figs. 4b, 4c, 4d). They are high-K, calc-alkaline, peraluminous (ASI (aluminium saturation index) value=1.1). In addition, they are poor in MgO and FeOT with Mg# values are 26 and 30, respectively.

    The total REE concentrations of Dongshan granites range from 150 ppm to 172 ppm and show LREE enrichment ((La/Yb)N=9.65–11.12), exhibiting negative Eu anomalies (Eu/Eu*=0.45–0.47) on the chondrite-normalized REE plots (Fig. 5a). On the primitive mantle-normalized trace element spider diagram (Fig. 5b), they are characterized by enrichment in Rb, Th, and U and depletion in Ba, Nb, Ta and Eu.

    Figure 5.  Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element patterns (b) of Dongshan and Hesheng granite samples. Chondrite and primitive mantle-normalized values are from McDonough and Sun (1995) and Sun and McDonough (1989).

    The Hesheng granites are located in the western part of the Yandangshan area. They show high SiO2 (72.2 wt.%–77.5 wt.%), Al2O3 (12.4 wt.%–13.9 wt.%) and K2O+Na2O (8.05 wt.%–9.41 wt.%) contents (Figs. 4b, 4c, 4d). They are high-K, alkali-calcic, peraluminous rocks (ASI values=1.0–1.1). In addition, they are also poor in MgO and FeOT with Mg# values ranging from 15 to 21. The total REE concentrations of these samples range from 120 ppm to 283 ppm and also show LREE enrichment ((La/Yb)N =7.38–12.87), exhibiting negative Eu anomalies (Eu/Eu*= 0.17–0.55) on the chondrite-normalized REE plots. On the primitive mantle-normalized trace element spider diagram, they are characterized by enrichment in Rb, Th, and U and depletion in Ba, Nb, Ta and Eu.

  • The zircons from two granite samples (YDS-19 and YDS-43) were selected for SHRIMP zircon U-Pb dating. The zircons in these samples are transparent and prismatic, colorless to pale yellow. They range in size from 20 to 100 μm and have a length/width ratio of 1 : 1 to 2 : 1. Euhedral oscillatory zonings are clear in most zircon crystals. The results of SHRIMP U-Pb isotope analysis are shown in Table 2, and all of the analyses are plotted in the concordia diagrams with representative zircon CL images (Fig. 6).

    Sample Isotope content Th/U Isotope ratio Isotope age (Ma)
    Th U 207Pb/206Pb 208Pb/232Th 206Pb/238U 207Pb/206Pb 208Pb/232Th 206Pb/238U
    (ppm) Ratio 1σ Ratio 1σ Ratio 1σ Age (Ma) 1σ Age (Ma) 1σ Age (Ma) 1σ
    yds-19-01 155 222 0.72 0.056 045 4.052 052 0.005 812 3.592 944 0.050 134 3.388 675 -129 316 101 7 114 2
    yds-19-02 269 376 0.74 0.052 558 3.279 971 0.005 788 2.981 141 0.048 813 2.230 821 -104 227 106 5 115 2
    yds-19-03 100 101 1.02 0.058 727 8.287 834 0.005 855 12.439 857 0.052 377 1.613 693 557 181 118 15 113 4
    yds-19-04 281 311 0.93 0.057 196 3.458 624 0.005 817 3.019 930 0.050 798 1.033 367 324 97 113 3 113 2
    yds-19-05 316 362 0.90 0.052 733 3.447 010 0.005 296 3.763 525 0.053 946 1.008 258 -12 109 100 4 116 2
    yds-19-06 341 304 1.16 0.056 297 3.545 529 0.005 061 5.586 240 0.051 928 1.483 578 184 186 97 6 114 3
    yds-19-07 305 422 0.75 0.063 678 2.818 169 0.005 938 2.879 907 0.047 154 0.990 931 315 194 106 6 115 2
    yds-19-08 392 419 0.97 0.063 573 3.040 886 0.006 128 2.812 646 0.046 974 1.604 717 300 137 112 4 113 2
    yds-19-09 264 416 0.66 0.059 906 3.068 508 0.005 858 3.199 330 0.056 756 0.980 319 336 97 108 4 115 2
    yds-19-10 290 461 0.65 0.054 013 2.896 603 0.005 680 4.350 944 0.048 980 0.894 742 203 160 109 6 112 2
    yds-19-11 236 200 1.22 0.051 361 7.264 483 0.005 824 3.287 175 0.045 581 1.225 215 -332 593 109 7 115 3
    yds-19-12 429 652 0.68 0.059 675 2.347 003 0.005 961 2.623 513 0.056 678 1.153 297 225 148 108 5 115 2
    yds-43-01 144 105 1.41 0.090 929 4.705 492 0.005 831 3.802 847 0.047 532 1.569 100 748 653 101 12 100 3
    yds-43-02 100 52 1.99 0.085 807 9.229 725 0.005 949 4.459 693 0.044 142 2.613 675 -286 2 217 100 15 103 4
    yds-43-03 64 46 1.44 0.111 129 5.836 976 0.007 011 4.457 631 0.045 676 4.131 340 842 1 051 113 21 103 4
    yds-43-04 397 350 1.17 0.057 520 3.412 399 0.005 261 2.373 350 0.044 708 1.745 670 -20 497 98 7 101 2
    yds-43-05 92 115 0.83 0.073 166 5.942 184 0.006 049 3.727 191 0.046 960 3.133 657 474 541 104 14 106 2
    yds-43-06 229 100 2.36 0.090 236 6.908 902 0.005 623 5.995 584 0.046 731 1.692 392 914 520 105 9 105 5
    yds-43-07 174 135 1.33 0.070 143 9.912 319 0.005 683 3.174 783 0.046 728 1.135 853 474 299 106 4 101 2
    yds-43-08 169 158 1.10 0.096 291 10.125 401 0.006 654 5.876 165 0.049 582 1.997 939 1 102 502 117 15 104 3
    yds-43-09 250 147 1.75 0.071 280 9.330 733 0.005 123 2.955 667 0.048 323 1.108 690 634 302 98 4 102 2
    yds-43-10 158 108 1.51 0.086 779 15.337 130 0.005 983 3.447 224 0.047 971 1.607 066 658 615 106 7 104 3
    yds-43-11 185 151 1.26 0.125 244 10.869 659 0.007 978 3.068 647 0.044 107 3.115 663 1 164 512 124 10 105 3
    yds-43-12 203 125 1.68 0.070 492 8.281 340 0.005 641 3.180 748 0.044 938 2.083 846 943 170 114 3 101 3

    Table 2.  Results of U-Pb dating of zircons in the Hesheng and Dongshan granites

    Figure 6.  Zircon concordia diagrams and zircon CL images for the samples YDS-19 and YDS-43. Red solid circles, red dashed circles and blue dashed circles in (a) and (c) represent the points for U-Pb, Lu-Hf isotope and trace element analyses, respectively.

    Twelve analyzed spots on zircon crystals from sample YDS-19 (Dongshan granite) have 206Pb/238U ages ranging from 110.7 to 114.4 Ma, and give a weighted mean 206Pb/238U ages of 114.2±1.2 Ma (N=12, MSWD=0.3). This age is interpreted as the magma crystallization age of the Dongshan granite. The Th/U ratios range from 0.66–1.37, indicating the magmatic origin of them.

    Twelve analyzed spots on zircon crystals from sample YDS-43 (Hesheng granite) have 206Pb/238U ages ranging from 100.0 to 106.5 Ma, and give a weighted mean 206Pb/238U ages of 102.5±1.6 Ma (N=12, MSWD=0.65), representing the crystallization age of the Hesheng granite. The Th/U ratios range from 1.01–2.28, indicating a magmatic origin of them.

  • Zircon crystals, which were used for U-Pb dating, from two granite samples (YDS-19 and YDS-43) were also analyzed for their Lu/Hf ratios on the same grains (Fig. 6), and the results are listed in Table 3.

    Sample No. 176Hf/177Hf 2σ 176Lu/177Hf 2σ 176Yb/177Hf 2σ (176Hf/177Hf)i Age (Ma) εHf(t) TDM1 (Ma) TDM2 (Ma)
    yds-19-01 0.282 371 0.000 024 0.001 383 0.000 005 0.036 932 0.000 234 0.282 368 113.9 -11.8 1 258.0 1 916.0
    yds-19-02 0.282 513 0.000 020 0.001 773 0.000 006 0.046 346 0.000 203 0.282 510 114.6 -6.8 1 067.5 1 599.9
    yds-19-03 0.282 526 0.000 022 0.001 159 0.000 011 0.030 039 0.000 299 0.282 524 112.6 -6.3 1 031.9 1 569.4
    yds-19-04 0.282 540 0.000 019 0.001 592 0.000 038 0.039 796 0.000 867 0.282 536 113.1 -5.9 1 024.8 1 541.1
    yds-19-05 0.282 550 0.000 020 0.001 801 0.000 019 0.046 330 0.000 352 0.282 546 116.4 -5.4 1 015.9 1 517.3
    yds-19-06 0.282 518 0.000 023 0.001 672 0.000 025 0.044 991 0.000 778 0.282 515 113.6 -6.6 1 057.9 1 589.4
    yds-19-07 0.282 493 0.000 020 0.001 202 0.000 023 0.033 523 0.000 900 0.282 491 114.6 -7.4 1 079.9 1 642.4
    yds-19-08 0.282 516 0.000 023 0.001 719 0.000 014 0.044 898 0.000 674 0.282 513 113.1 -6.7 1 061.9 1 594.0
    yds-19-09 0.282 584 0.000 019 0.001 258 0.000 005 0.032 951 0.000 212 0.282 581 115.0 -4.2 953.1 1 439.5
    yds-19-10 0.282 570 0.000 021 0.001 000 0.000 003 0.025 658 0.000 290 0.282 568 112.4 -4.7 965.5 1 470.1
    yds-19-11 0.282 529 0.000 022 0.001 623 0.000 043 0.043 663 0.001 334 0.282 526 114.6 -6.2 1 040.5 1 563.5
    yds-19-12 0.282 526 0.000 018 0.001 435 0.000 033 0.037 485 0.001 164 0.282 523 115.1 -6.3 1 040.2 1 570.3
    yds-43-01 0.282 526 0.000 022 0.001 182 0.000 014 0.032 525 0.000 529 0.282 524 100.5 -6.6 1 032.6 1 576.6
    yds-43-02 0.282 569 0.000 024 0.002 121 0.000 093 0.061 045 0.002 330 0.282 565 103.1 -5.1 997.3 1 483.6
    yds-43-03 0.282 556 0.000 024 0.001 741 0.000 082 0.047 972 0.001 967 0.282 553 102.6 -5.5 1 005.6 1 511.2
    yds-43-04 0.282 539 0.000 024 0.001 111 0.000 009 0.030 664 0.000 275 0.282 537 100.6 -6.1 1 013.2 1 548.5
    yds-43-05 0.282 588 0.000 022 0.001 219 0.000 002 0.032 497 0.000 292 0.282 586 106.2 -4.3 946.3 1 435.2
    yds-43-06 0.282 549 0.000 027 0.002 288 0.000 125 0.066 748 0.003 707 0.282 544 104.8 -5.8 1 031.2 1 528.5
    yds-43-07 0.282 495 0.000 025 0.001 878 0.000 061 0.054 398 0.001 420 0.282 491 101.1 -7.7 1 097.3 1 649.3
    yds-43-08 0.282 552 0.000 023 0.001 201 0.000 038 0.033 653 0.001 287 0.282 549 104.3 -5.6 997.1 1 517.4
    yds-43-09 0.282 478 0.000 024 0.001 464 0.000 054 0.041 121 0.001 910 0.282 475 101.6 -8.3 1 108.8 1 684.6
    yds-43-10 0.282 602 0.000 024 0.001 676 0.000 017 0.046 640 0.000 208 0.282 598 103.7 -3.9 938.5 1 408.2
    yds-43-11 0.282 655 0.000 021 0.001 818 0.000 023 0.050 967 0.000 524 0.282 651 104.6 -2.0 865.5 1 289.0
    yds-43-12 0.282 595 0.000 026 0.002 012 0.000 048 0.056 792 0.000 939 0.282 591 100.7 -4.2 957.0 1 426.7
    εHf(t)=((176Hf/177Hf)–(176Lu/177Hf)×(eλt–1))/((176Hf/177Hf)CHUR–(176Lu/177Hf)CHUR(eλt–1))–1)×10 000; (176Lu/177Hf)CHUR =0.033 2, (176Hf/177Hf)CHU=0.282 772, λ=1.867×10-11 yr-1; TDM1 and TDM2 are calculated after Chen et al. (2014).

    Table 3.  Zircon Hf isotopic data and SHRIMP U-Pb dating data of granite around Yandangshan

    Twelve analyses for sample YDS-19 show the 176Lu/177Hf ratios range from 0.001 0 to 0.001 8, with εHf(t) values between -11.8 to -4.2 and the TDM2 model ages ranging from 1 439 to 1 916 Ma (Fig. 7). The 176Hf/177Hf ratios are 0.282 371– 0.282 584, which indicates few radiogenic ingrowths of Hf.

    Figure 7.  εHf(t) vs. age diagram for zircons from Yandangshan area granite rocks. The Hf evolution of the crustal basement in Cathaysia Block is from Xu et al. (2007), and the data of volcanic rocks in background is from Qiu et al. (2004), He et al. (2009), Liu et al. (2014), Zhao et al. (2016) and Liu L et al. (2016).

    For the sample YDS-43, twelve zircons were also analyzed. The 176Lu/177Hf ratios of these zircons are from 0.001 1 to 0.002 3, and the εHf(t) values vary from -8.3 to -2.0 with the TDM2 (two-stage depleted mantle Hf model ages) model ages ranging from 1 289 to 1 685 Ma. The 176Hf/177Hf ratios of this sample are 0.282 478–0.282 655, which is a bit higher than the zircons from YDS-19.

  • The whole-rock Sr-Nd isotopic compositions of samples from the Dongshan and Hesheng intrusions are listed in Table 4 and plotted in Fig. 8. The results of zircon U-Pb dating are used to calculate the initial 87Sr/86Sr ratios and εHf(t) values. The samples from Dongshan and Hesheng intrusions have similar (87Sr/86Sr)i ratios (0.709 4; 0.707 8 to 0.708 3, respectively) and εHf(t) values (-6.9; -6.1 to -6.0, respectively). The two-stage depleted mantle Nd model ages (TDM2) of Dongshan granite are 1 465 Ma whereas those of Hesheng granite are 1 384–1 391 Ma.

    Sample 87Rb/86Sr 87Sr/86Sr 2σ ISr 143Nd/144Nd 147Sm/144Nd 2σ TDM1 (Ga) TDM2 (Ga) εNd(t)
    Dongshan granite
    YDS-19 3.08 0.714 392 0.000 015 0.709 397 0.512 147 0.005 721 0.000 007 0.74 1.47 -6.9
    Hesheng granite
    YDS-41 11.18 0.724 119 0.000 019 0.707 833 0.512 206 0.010 073 0.000 004 0.71 1.39 -6.1
    YDS-45 4.55 0.714 944 0.000 017 0.708 318 0.512 208 0.006 481 0.000 005 0.69 1.38 -6.0
    ISr=(87Sr/86Sr)i; εNd(t)={[(143Nd/144Nd)–(147Sm/144Nd)×(eλt−1)]/[(143Nd/144Nd)CHUR−(147Sm/144Nd)CHUR×(eλt−1)−1]}×10 000; (143Nd/144Nd)CHUR=0.512 638, (147Sm/144Nd)CHUR=0.196 7, λSm=6.54×10−12 yr−1; TDM and TDM2 are single-stage and two-stage Nd model age after Wu et al. (2003), respectively.

    Table 4.  Zircon Sr-Nd isotopic data of granite around Yandangshan

    Figure 8.  εNd(t) vs. (87Sr/86Sr)i diagram of Dongshan, Hesheng granites and syenite (He et al., 2009) in Yandangshan area. Basement data are cited from Shen et al.(2009, 1999), Xue et al. (2009), Li Z et al. (2012) and Liu L et al. (2016).

  • Zircon crystals from two samples were also analyzed for their element composition on the same grains (Fig. 6), and the results are shown in Table S1. On the chondrite-normalized REE spider diagram (Fig. 9), the zircons all show strong enrichment of HREE relative to MREE and LREE, and have Ce positive anomalies and Eu negative anomalies, indicating they are unmetamorphosed magmatic zircons (Zhou et al., 2017; Hoskin and Schaltegger, 2003). In detail, zircons of Dongshan granite display smaller Eu negative anomalies (Eu/Eu*=0.13– 0.48) and smaller Ce positive anomalies (Ce/Ce*=2.04–100.65, mostly < 15) than zircons of Hesheng granite (Eu/Eu*= 0.25–0.65; Ce/Ce*=14.59–75.94). Titanium concentrations in zircons of Dongshan granite vary from 5.04 ppm to 15.66 ppm, Ce contents from 21.70 ppm to 65.67 ppm as well as Ti contents from 14.20 ppm to 45.60 ppm and Ce contents from 30.10 ppm to 91.75 ppm in zircons of Hesheng granite. However, because of the existence of the inclusions in the zircons, some elements contents may not be the true values of the zircons. Some high-error results will not be used in the next part.

    Figure 9.  Zircon chondrite-normalized REE contents diagrams of samples YDS-19 (a) and YDS-43(b), respectively. Chondrite normalized values are from Sun and MacDonough (1989).

  • Previous studies in the Yandangshan area have been focused on volcanic rocks and the syenites (Yan et al., 2016; He et al., 2009; Yu et al., 2008). In this study, our SHRIMP U-Pb dating results of the two granites in the study area yield a crystallization age of 114±1 Ma for the eastern (Dongshan) granite and 103±2 Ma for the western (Hesheng) granite.

    Granites can be commonly divided into I-, S-, and A-types (Chappell and White, 1992; Whalen et al., 1987). The A-type granite was defined as a group of alkaline, anorogenic and anhydrous granite rocks, occurring in extensional tectonic environments, anorogenic settings or post-collisional extensional settings (Wu et al., 2014; Bonin, 2007; Loiselle and Wones, 1979). A-type granite usually has higher SiO2 and Na2O+K2O (Na2O+K2O > 8.0 wt.%) contents, higher FeOT/MgO and Ga/Al (10 000×Ga/Al > 2.6) ratios, higher high field strength element (HFSE) contents (Zr+Nb+Ce+Y > 350 ppm), but lower in CaO, MgO, Al2O3, Sr, Ba, Cr, Co, Ni and V concentrations than those of I- and S-type granites (Whalen et al., 1987; Collins et al., 1982). Thus, geochemical characteristics like the Ga/Al vs. Zr+Nb+Ce+Y diagrams can be used to identify A-type granite from I-, and S-type granites. Eby (1992) further divides the A-type granite into two sub-classes, i.e., A1- and A2-types. The source region referred to by the A1-type is usually related to the mantle, whereas the A2-type granite refers to the material from the crust, which is formed in the post-collision tectonic setting.

    Frost et al.(2011, 2001) have divided the granitic rocks into ferroan and magnesian granitoids based on three values (Fe-index=(FeOT)/(FeOT+MgO); the modified alkali-lime index (MALI)=Na2O+K2O-CaO; the aluminium saturation index (ASI) =Al/(Ca–1.67P+Na+K)). The Fe-index can be used to distinguish ferroan granitoids from magnesian granitoids (Fig. 4d). Then by MALI and ASI, ferroan and magnesian granitoids can further be classified into 14 types. The enrichment of iron rather than magnesium is the most basic geochemistry characteristic of A-type granite. A-type granitoids are ferroan alkali-calcic, ferroan alkali, metaluminous and peraluminous. I-type granite mostly varies from calcic to alkali-calcic and are magnesian and metaluminous, however a small number of I-type granite which have high silica contents (> 70 wt.%) can be ferroan, alkalic and peraluminous. S-type granite are mostly magnesian, alkalic, calcic-alkalic, alkali-calcic and peraluminous, and when they have high silica contents (> 70 wt.%) can also be ferroan.

    In this study, samples from the Hesheng granite (103±2 Ma) show high K2O+Na2O contents (8.05 wt.%–9.41 wt.%), FeOT/MgO ratios (6.91–9.98), Zr+Nb+Ce+Y (302 ppm–425 ppm) and Ga/Al ratios (10 000×Ga/Al range from 2.60 to 3.40), and are all plotted into the A-type granite region (Fig. 10). They can be further divided as A2 sub-group in the Nb-Y-Ce and Nb-Y-3×Ga ternary diagrams (Fig. 11). Furthermore, samples from the Dongshan granite (114±1 Ma) have slightly lower K2O+Na2O contents (8.35 wt.%–8.47 wt.%), Fe-index values (0.81–0.84), FeOT/MgO ratios (4.3–5.2), Zr+Nb+Ce+Y (195 ppm–202 ppm) and Ga/Al ratios (10 000×Ga/Al range from 1.97 to 2.03). Therefore, we consider the granites in Dongshan to be magnesian granite.

    Figure 10.  Plots of 10 000×Ga/Al vs. K2O+Na2O (a), FeOT/MgO (b), Nd (c) and Zr (d) after Whalen et al. (1987).

    Figure 11.  Plots of the Hesheng granites in Nb-Y-Ce and Nb-Y-3×Ga diagram of Eby (1992) for subdivision of A1- and A2-type granites.

  • Watson and Harrison (1983) experimentally determined the behavior of saturated zircon in anatectic melting and derived a relationship among zircon solubility, temperature, and major element composition of melt, which was corrected into the following equation by Miller et al. (2003).

    where TZr, in kelvins, is absolute temperature that has been converted to ℃ in our study; M is a compositional factor in cation fraction and can be calculated by Eq. (2); Zrmelt is the Zr element content in melt as also Na, K, Ca, Al, and Si.

    Ti-in-zircon thermometer can also estimate the temperature of melts when zircon crystallized. The process of Ti atoms entering zircon crystals is controlled by temperature and activity of TiO2 αSiO2. Based on this, Ferry and Watson (2007) revised the thermometer calculation after study of high temperature- high pressure experiments and natural zircons, following the equation

    In the equation, Ti represents the Ti element content in zircon (ppm), and αSiO2 and αTiO2 represent the activity of SiO2 and TiO2 in the magma at that time, respectively. Therefor, it's necessary to get the estimate value of αSiO2 and αTiO2, and then we get the Tzircon, which represents the probable temperature at the time of zircon crystallization. Usually, we hold that most zircon and quartz grew at the same time in granite, and a relatively high αSiO2 is needed in primary magma evolution. Overall, we hold αSiO2 constant at 1. If rutile can be observed in zircon as inclusion, αTiO2 are saturated (αTiO2=1), but most of magma is rutile unsaturated. Same as Claiborne et al. (2010), we set the αTiO2 constant as 0.7 for a relatively high saturation level.

    The results of two methods are listed in Table S1. It shows that the TZr (797–800 ℃, 798 ℃ in average) and Tzircon of the Dongshan granite (770–827 ℃, 805 ℃ in average) are lower than those of Hesheng granite (TZr=843–883 ℃, 862 ℃ in average; Tzircon=817–959 ℃, 859 ℃ in average). Inherited or early crystalline zircons are present in both granite samples, therefore zircon was saturated in the magma source region and the zircon saturation temperatures (TZr) maybe the maximum temperature (Wu et al., 2017; Zhao, 2010; Miller et al., 2003).

    Oxygen fugacity (fO2) is also an important thermodynamic parameter of magmatic systems. Smythe and Brenan (2016) developed a method, which is based on the rocks' distinct Ce zircon-melt fractionation, to estimate fO2 from a combination of zircon-melt partitioning of Ce in zircon (Angerer et al., 2018). Whole-rock major elements and REE are seen as the melt composition in this method and then we calculated fO2 of Dongshan and Hesheng granite (Table S1), using water-free and 2.0 wt.% water contents. As shown in Fig. 9, two zircon grains from sample YDS-19 have high La contents, which may be caused by apatite inclusion, were not used in the following calculation. There are small difference between two kinds of water contents but obvious distinction of Dongshan and Hesheng granites. The Dongshan granite has high fO2 values (log fO2= -13.96– -11.87, corresponding to ΔFMQ=1.46 wt.%–2.34 wt.%; 2 wt.% H2O), whereas the Hesheng granite has lower fO2 values (log fO2= -14.39– -11.29, corresponding to ΔFMQ= -0.41 wt.%–1.14 wt.%; 2 wt.% H2O). It indicates that Dongshan granite formed in a more oxidizing environment than Hesheng granite. Based on the discussion above, the Dongshan granite has lower temperature of magma source but higher fO2 than Hesheng granite.

  • The petrogenesis of the A-type granite still remains controversial. One school thinks that A-type granites were derived from a dehydrated and melted crustal source, and characterized by high crystallization temperature and shallow emplacement (Chen et al., 2015; Wu et al., 2007). The mantle-derived magma provides a heat source for the deep-melting of the crust, and the heated crust-derived material is partially melted (Ostendorf et al., 2014). Therefore, the following mechanisms have been proposed: (1) partial melting of the lower crust (Huang et al., 2015; Frost et al., 2011); (2) mixing between mantle-derived and crustal magma (Yang et al., 2006); (3) extreme differentiation of mantle-derived tholeiitic magma or differentiation of tholeiites (Frost et al., 1999); (4) fractionation of mantle-derived mafic magma (Eby, 1992).

    The εHf(t) values of our ferroan/A-type granite (103±2 Ma) range from -8.3 to -2.0 with the TDM2 model ages of Hf range from 1 289 to 1 685 Ma and no mafic microgranular enclaves have been observed in the fields. Commonly extensive mafic rocks have spatial association with the A-type granite if the A-type granite is fractionated from mantle-derived mafic magma. However, in the study area, only several mafic dykes can be found in research area and no associated mafic rocks have been found. Therefore, the Hesheng granite cannot be fractionated from the mantle-derived mafic magma.

    We plotted εNd(t) and initial 87Sr/86Sr ratios values of our granite and volcanic rocks around Yandangshan area (He et al., 2009; Yu et al., 2008; Chen and Jahn, 1998; Lapierre et al., 1997) in Fig. 8. In the εNd(t) vs. (87Sr/86Sr)i diagram, our data are plotted in an area between mantle array and the Neoproterozoic basement of Cathaysia Block (Fig. 8b), suggesting a mixed magma source of mantle and crustal materials. In the εHf(t) vs. age diagram (Fig. 7), our samples plot above the basement but below the depleted mantle, which also supports a mixed source for these granites. Furthermore the Hesheng granites have slightly higher εHf(t) value than the Dongshan granite, which may be caused by source heterogeneity.

    For the Dongshan granite (114±1 Ma), the samples have the εHf(t) values of -11.8 and -4.2 and εNd(t) values of -6.9, with an older TDM2 model ages (1 440–1 916 Ma). Their Hf and Nd isotopes values also plot above the basement of Cathaysia Block (Figs. 7 and 8), indicating that they also cannot be simply derived from partial melting of the ancient lower crustal materials.

    In summary, we propose that both granites are derived from a mixed source consists of mantle-derived and crustal- derived materials while the crust is juvenile, the slight difference in Hf-Nd isotopes between them was most likely caused by source heterogeneity.

  • During the past decades, many models have been proposed for the tectonic and magmatic evolution of South China Block in Mesozoic. For example, Zhou and Li (2000) proposed that during 180 to 80 Ma, the slab dip angle of subduction toward SE China increased from a very low angle to a median angle. According to the change of the angle of the Pacific volcanic chain, Sun et al. (2007) proposed that at around 125 Ma, the subduction angle changed, accompanied by a series of magmatic activities. Others proposed a flat-slab subduction model to explain the change along Andean-type continental magmatic arc (280–250 Ma) and the Western Pacific-type plate margin (190–90 Ma) in South China (Zhu et al., 2016; Li Z X et al., 2012; Li and Li, 2007; Li X H et al., 2007). Nonetheless, the viewpoint that slab rollback of the Paleo-Pacific plate caused the widespread Cretaceous magmatism is generally accepted (Gao et al., 2019; Wang et al., 2016).

    Previous studies have suggested that during late Early Cretaceous (117–108 Ma), a regional-scale NW-SE transpression coming from the collision of the Philippine Block with the Asian continental margin stopped earlier crustal rifting, generated significant tectonic inversion and caused a magmatic quiescence; during Mid–Late Cretaceous (107–86 Ma), the ESE-ward subduction of paleo-Pacific Plate produced numerous A-type granites and bimodal volcanoes (Li et al., 2014b). Besides, Liu et al. (2014) indicated that in Zhejiang Province, the subduction of the paleo-Pacific Plate changed to a higher-angle subduction during 123–118 Ma and then dramatically became rapid high-angle during 110–88 Ma with the back-arc extension gradually moving from the NW to the SE Zhejiang in the meanwhile.

    Here we have also collected the geochemical, geochronological, Nd isotopic and Hf isotopic compositions of granitoids rocks during Middle and Late Cretaceous in Zhejiang Province (Table S2). In Fig. 7, the collected zircon Hf data has shown that mantle-derived materials that were injected into the felsic magma chamber, increased with decreasing time. Whole-rock Nd isotopes also show the same trend (Fig. 8). Furthermore, magma temperatures of the felsic magma were also increased with decreasing time (Fig. 12). Above all, we find that with the fast or high angle slab roll-back of the paleo-Pacific Plate, the back-arc lithospheric extension was enhanced with more A-type granitoids were formed. In this study, the Dongshan granites were formed at around 114 Ma, when the slab roll back restarted. At around 103 Ma, the back-arc lithospheric extension was enhanced and generated large numbers of A-type granitoids such as Hesheng and Taohuadao A-type granites. Our earlier study has shown that the different water contents and oxygen fugacity of the magma may be the key factors controlling the Fe-index, which was used to define the ferroan and magnesian granitoids (Wu et al., 2018a). The same maybe also happened to the Dongshan and Hesheng granites in this study.

    Figure 12.  TZr vs. age diagram for granitoids during Middle and Late Cretaceous in Zhejiang Province. The reference of zircon geochronological and zircon saturation temperatures data are in Table S2.

    Based on the previous studies, both of the granitic plutons were formed under extension setting, and the granites transformed from magnesian to ferroan in the study area may indicate the extension was enhanced, which may be caused by the roll-back or delamination of the Paleo-Pacific oceanic slab.

  • (1) The granites around Yandangshan area were formed in Early Cretaceous (114±1 and 103±2 Ma, respectively). The Dongshan granite belongs to the ferroan/A-type granites and the Hesheng granite belong to the magnesian granites.

    (2) Both granites are derived from a mixed source that consists of mantle-derived and crust-derived material. Based on the previous studies, both of the granitic plutons were formed under extension setting, and the granites transformed from magnesian to ferroan in the study area may indicate the extension was enhanced, which may be caused by the roll-back or delamination of the paleo-Pacific oceanic slab.

  • This study was funded by the National Natural Science Foundation of China (Nos. 41702047, 41541018, 41072048), Department of Science and Technology (No. 2014C33023), Geological Exploration Bureau (No. 201531) and Department of Land and Resources (No. 2015005). We are grateful for the constructive comments by two anonymous reviewers. We thank B Song of the Chinese Academy of Geological Sciences for zircon U-Pb dating. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1295-9.

    Electronic Supplementary Materials: Supplementary materials (Tables S1–S2) are available in the online version of this article at https://doi.org/10.1007/s12583-020-1295-9.

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