Journal of Earth Science  2019, Vol. 30 Issue (3): 636-646   PDF    
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Geochemistry, Zircon U-Pb Age and Hf Isotope of the Huilanshan Granitoids in the North Dabie Terrane: Implications for Syn-Collapse Magmatism of Orogen
Shengxin Liu , Haijin Xu     
School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
ABSTRACT: Syn-collapse magmatism is a critical issue for evolution of the continental orogen. The Dabie Orogen is a typical orogen which was suffered from a complete collapse. Two kinds of granitoids, namely, the coarse-grained diorite and the fine-grained granite, are recognized at the center of the Luotian extensional dome, providing an opportunity to decipher the syn-collapse magmatism in the Dabie Orogen. The diorites (125±3 Ma) are high K calc-alkaline rocks, with low SiO2 (51.9 wt.%-56.6 wt.%) and high MgO (3.5 wt.%-4.0 wt.%) contents. They are enriched in LREE and LILEs (e.g., Ba, K, Rb) and depleted in HFSEs (e.g., Ta, Nb, and Hf) with low ratio of Sr/Y (30.82-46.89). The granites (118±2 Ma) are shoshonite series rocks, with relatively high SiO2 (68.9 wt.%-72.6 wt.%) and low MgO (0.32 wt.%-0.66 wt.%) contents. They are also enriched in LREE and LILEs with weakly negative Eu anomalies (δEu=0.81-0.85), and are depleted in HFSEs with low Sr contents (338 ppm-477 ppm) and Sr/Y ratios (23.80-33.13). Therefore, the two kinds of granitoids have no geochemical characteristics of adakitic rocks, suggesting that they were generated from a normal or thinned crust level. The diorites have quite negative zircon εHf(t) values (-18.4 to -21.1), suggesting they were from partial melting of the mafic lower continental crust. The granites have relatively higher zircon εHf(t) values (-14.4 to -18.1). The granites also contains a series of old inherited zircon cores, such as two upper intercept ages of 2 628±41 and 1 840±37 Ma, and a concordant age of 807±9 Ma. All these features suggest that the granites were generated from partial melting of the felsic middle-lower continental crust. Thus, the Huilanshan Early Cretaceous granitoids coupled with the Luotian extensional dome revealed the collapsed process of the Dabie Orogen.
KEY WORDS: orogenic collapse    magmatism    partial melting    lower continental crust    Dabie Orogen    
0 INTRODUCTION

Evolution of a continental orogen is likely responsible for a process of crustal thickening by collision, subsequent delamination of the crustal root and the post-orogenic extensional collapse and thinning of the thickened crust (e.g., Xu et al., 2007; Brown, 2001; Leech, 2001). During the post-collisional collapse, it was accompanied with tectonic extension and a large amount of magmatism (e.g., Foster et al., 2001; Keay et al., 2001). The Dabie Orogen is a typical orogen which has gone through a completed orogenic cycle. It was formed by Triassic continent-continent collision between the Yangtze crust and the North China Craton, not only resulting in the ultra-high pressure (UHP) metamorphism (e.g., Xu et al., 1992; Wang et al., 1989) but also resulting in an over-thickened continental crust (e.g., Huang et al., 2008; Wang et al., 2007; Xu et al., 2007). Subsequently, it experienced extensional tectonic collapse in the Early Cretaceous (e.g., Ratschbacher et al., 2003; Hacker et al., 2000). Thus, the Dabie Orogen is an ideal place to explore the relationship between the post-collisional magmatism and orogenic collapse.

Previous studies show that the post-orogenic granitiods in the Dabie Orogen can be generally divided into two types based on the tectonic deformation, age and geochemistry (e.g., Xu et al., 2007). The early deformed granitoids (ca. 130–145 Ma) are adakitic rocks considered from partial melting of over-thickened crust or delaminated lower continental crust (e.g., Xu et al., 2012a, b, 2007; Zhang et al., 2010; Huang et al., 2008; Wang et al., 2007). On the contrary, the later un-deformed granites (ca. 110–130 Ma) are non-adakitic rocks regarded as from partial melting of thinning or normal thickness crust (e.g., Xu et al., 2007; Zhao et al., 2007; Bryant et al., 2004). These studies have made important contributions to understanding post-collisional magmatism in the continental orogen. However, the detailed process of syn-collapse magmatism is still unclear.

We focus here on Huilanshan granitoids at the center of Luotian dome in the North Dabie terrane. It's well known that the Luotian dome is an extensional dome during the orogenic collapse (e.g., Xu et al., 2002; Okay et al., 1993). In this study, two kinds of syn-collapse granitoids has been recognized: coarse-grained diorite and fine-grained granite. The main purposes are (1) to limit the formation age of the Huilanshan granitoids, (2) to reveal their petrogenesis, and (3) to discuss their implications of orogenic collapse.

1 GEOLOGICAL SETTING AND PETROLOGY

The Dabie Orogen is the west part of the Dabie-Sulu Orogen which was caused by the Triassic northward subduction of the Yangtze Craton beneath the North China Craton (e.g., Qu et al., 2018; Hacker et al., 1998). The Dabie Orogen can be divided into the following units by its petrotectonic affinity (Fig. 1b) (e.g., Zheng et al., 2005; Hacker et al., 1998; Cong and Wang, 1996) from north to south: (1) the North Huaiyang greenschist-facies metamorphic unit; (2) the North Dabie high-T granulite-facies unit also called the North Dabie terrane (NDT); (3) the central Dabie medium-T/UHP eclogite-facies metamorphic unit; (4) the South Dabie low-T/UHP eclogite-facies metamorphic unit; and (5) the Susong low-T/high-P blueschist-facies metamorphic unit. All these petrotectonic units were intruded by Early Cretaceous igneous rocks (e.g., Xu et al., 2012a, b, 2007; Wang et al., 2007; Zhang et al., 2002; Ma et al., 1998).

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Figure 1. Geological map of the Dabie Orogen (modified after Xu et al., 2012a). Fault belts: ① Jinzhai-Shucheng fault; ② Xiaotian-Mozitan fault; ③ Wuhe-Shuihou fault; ④ Mamiao-Taihu fault; ⑤ Huangpi-Guangji fault; ⑥ Tanlu fault; ⑦ Shangcheng-Macheng fault. The sizes of the intermediate-mafic dykes and mafic-ultramafic plutons are schematic.

The NDT is limited by faults as follows: the Wuhe-Shuihou fault to the south, the Shangma fault to the west, the Xiaotian- Mozitan fault to the north and the Tanlu fault to the east (Fig. 1). The NDT contains two domes, namely, the Luotian dome in the northwestern part and the Yuexi dome in the northeastern part, displaying a NW-SE oriental extensional tectonics in the Cretaceous (e.g., Xu et al., 2002; Hacker et al., 1998). The NDT contains Mid-Neoproterozoic TTG(D) orthogneisses and amphibolites (e.g., Zheng et al., 2005; Bryant et al., 2004; Hacker at al., 1998), felsic and mafic granulites (e.g., Wu et al., 2008; Hou et al., 2005), minor meta-sedimentary (e.g., quartzite, marble, calc- silicates and biotite schist), rare peridotites and eclogites (Liu et al., 2005; Xu et al., 2003). Increasing studies show that the NDT was also suffered from a Triassic metamorphism during the continental collision (details see review of Lei and Xu (2018)). Meanwhile, abundant Early Cretaceous igneous rocks are widely distributed in the Dabie Orogen (Fig. 1) (e.g., Xu et al., 2012a, b, 2007; Wang et al., 2007; Ma et al., 1998), which are closely related to its collapse.

The Huilanshan pluton, located at the center of the Luotian dome (Fig. 1), mainly consists of coarse-grained diorite and fine-grained granite. The fine-grained granite intruded into the coarse-grained diorite (Fig. 2d). A few of mafic granulites occur as lenses in the coarse-grained diorite (Fig. 2a). The wall-rocks of them are migmatites which were strongly deformed (Fig. 2e). The diorites are coarse-grained structure without deformation. They are mainly contains plagioclase (40%–50%), hornblende (20%–25%), biotite (5%–10%), quartz (5%–10%) (Fig. 2c), and K-feldspar (~5%) with minor zircon, titanite and magnetite. The granites are fine-grained structure also without deformation. They are mostly composed of plagioclase (20%), K-feldspar (25%–35%), quartz (30%–40%), biotite (5%–10%) (Fig. 2f) with minor zircon and titanite.

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Figure 2. (a) Simplified geological map of the Huilanshan of the North Dabie terrane. (b) Field photograph of the Huilanshan coarse-grained diorites showing contact relationships of the diorites with the migmatites. (c) Micrograph of the Huilanshan diorite. (d) Contact relationship of the diorites and the granites. (e) Field photograph of the Huilanshan fine-grained granites showing the contact relationships of the granites with the migmatites. (f) Micrograph of the Huilanshan granite. Amp. Amphibole; Bi. biotite; Pl. plagioclase; Kfs. K-feldspar; Qtz. quartz.
2 ANALYTICAL METHODS

Major and trace element compositions (Table S1) of whole rocks for the Huilanshan granitoids were measured at the State Key Laboratory of Geological Processes and Mineral Resources (SKLGPMR), China University of Geosciences (Wuhan), and Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, respectively. Two samples of diorite (HLS-06) and granite (HLS-02) were selected to separate zircons for internal structure study, LA-ICP-MS trace element compositions (Table S2), U-Pb dating (Table S3), and Lu-Hf isotopic (Table S4) analyses at the SKLGPMR, China University of Geosciences (Wuhan). Detailed analytical methods see the ESM attached file (ESM Ⅱ).

3 RESULTS 3.1 Zircon U-Pb Age and Trace Element Compositions

Two typical samples of granite (HLS-02) and diorite (HLS-06) were selected for LA-ICP-MS zircon U-Pb dating and trace element compositions. Data are listed in Tables S2 and S3, respectively. Parts of zircon cathodoluminescence (CL) images of the granite and diorite are shown in Fig. 3 with in-situ 206Pb/238U age and initial εHf(t) value.

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Figure 3. The CL images of representative zircon grains with 206Pb/238U age and initial Hf isotope ratio in εHf(t) from the Huilanshan granites (a)–(e) and the diorites (f)–(i) in the North Dabie terrane. Solid line circle presents the U-Pb age spot area, and broken line circle present the initial Hf isotope ratio spot area.
3.1.1 Granite (HLS-02)

Zircon grains from granite (HLS-02) are colorless and prismatic. They are generally stout prismatic and range from 150 to 300 μm in length. Base on the CL images (Figs. 3a3e), two kinds of zircons were recognized: the new growth zircons and the inherited cores. The new growth zircons display euhedral rhythmic oscillatory zoning (Figs. 3a3b). They are enriched in heavy rare earth elements (HREE) and depleted in light rare earth elements (LREE), strong positive Ce anomalies (δCe=7.3–338.8, most of them ranging from 14.6 to 77.7) and obviously negative Eu anomalies (δEu=0.05–0.41) (Table S2 and Fig. 4c). They have high ratios of Th/U (0.29–1.32) (Table S2 and Fig. 4d). All these features indicate that they are magmatic origin. As for the inherited cores, they are variably zoned with medium CL brightness (Figs. 3c3e). They are also enriched in HREE and depleted in LREE, strong positive Ce anomalies (δCe=20.3–59.1) and obviously negative Eu anomalies (δEu=0.04–0.44) (Table S2 and Fig. 4c). They also have high Th/U ratios (0.36–0.71) (Table S2 and Fig. 4d). All these features suggest that the inherited cores may represent protolith magmatic domains or multi-stage magma events.

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Figure 4. Zircon U-Pb concordia diagrams with weighted mean 206Pb/238U ages (a) and (b), chondrite-normalized REE pattern for different zircons (c), and the diagrams of zircon 206Pb/238U ages versus Th/U ratios (d) for the Huilanshan granite (sample HLS-02) from the North Dabie terrane. Values of the chondrite are from Sun and McDonough (1989).

Seventeen U-Pb spot analyses were obtained from the granitic zircons (HLS-02). These age data give two group of discordia intercept ages of 2 628±41 and 115.5±2.5 Ma, and 1 840± 37 and 111±2.5 Ma, respectively (Fig. 4a). As for the former group, two spots on inherited cores give discordant ages of 2 393.4±18.0 and 2 466±18 Ma. The later group, two spots on inherited cores give discordant ages of 1 665.5±16.8 and 1 840± 37 Ma. One spot on inherited core gives a concordant age of 807±9 Ma. Twelve young age spots on oscillatory magmatic zircon yield concordant 206Pb/238U ages ranging from 112.9±1.5 to 120.3±3.0 Ma with a weighted mean of 118±2 Ma (MSWD=1.9, n=12) (Fig. 4b), representing its formation age.

3.1.2 Diorite (HLS-06)

Zircons from the coarse-grained diorite (HLS-06) are also prismatic and transparent. Their lengths are generally 100 to 250 μm. From the CL images (Figs. 3f3i), the new growth zircons were recognized, but relict cores have not been found in its zircons. The new growth zircons display euhedral rhythmic oscillatory zoning in CL images (Figs. 3f3i). They are enriched in HREEs and depleted in LREEs, with strong positive Ce anomalies (δCe=1.6–147.2, most of them range from 10.5 to 29.8) and obviously negative Eu anomalies (δEu=0.37–0.48) (Fig. 5b). They also have high Th/U ratios (1.26–2.55) (Table S2 and Fig. 5c). All these features indicate that they are magmatic origin. Twenty-eight spots on magmatic zircon yield concordant 206Pb/238U ages ranging from 109.7±5.3 to 144.1±6.1 Ma and they yield a weighted mean of 125±3 Ma (MSWD=2.0, n=28) (Fig. 5a), representing the formation age of diorite.

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Figure 5. Zircon U-Pb concordia diagrams with weighted mean 206Pb/238U ages (a), chondrite-normalized REE pattern for zircons (b), and the diagrams of zircon 206Pb/238U ages versus Th/U ratios (c) for the Huilanshan diorite (sample HLS-06) from the North Dabie terrane. Values of the chondrite are from Sun and McDonough (1989).
3.2 Geochemistry 3.2.1 Major oxides

The granites have high SiO2 (68.9 wt.%–72.6 wt.%) and low MgO (0.32 wt.%–0.66 wt.%) contents with low Mg# values (~27.0), however, the diorites have relatively lower SiO2 (51.9 wt.%–56.6 wt.%) and higher MgO (3.5 wt.%–4.0 wt.%) contents with higher Mg# values (41.0–49.8). CaO, Al2O3, FeOT, TiO2, and P2O5 contents and Mg# values of the Huilanshan granitoids display negative correlation with SiO2 (Fig. 7). On the contrary, K2O contents of them display positive correlation with SiO2 contents, and their Na2O contents are broadly constant (Fig. 7). In the plot of alkalis versus silica (TAS diagram, Fig. 6a), the diorites are scattered around the boundary line between the alkalic and sub-alkalic field, whereas the granites fall into the sub-alkalic field. The K2O vs. SiO2 diagram (Fig. 6b) shows that the granites belong to shoshonite series rocks, and the diorites are plotted in a high-K calc-alkaline to medium-K calc-alkaline series field. The granites are weakly metaluminous to weakly peraluminous, and the diorites are metaluminous in A/NK vs. A/CNK diagram (Fig. 6c). From the major element compositions (Figs. 6 and 7), the granites are similar to the Early Cretaceous non-adakitic granites (e.g., Xu et al., 2007; Bryant et al., 2004), and the diorites are similar to the Early Cretaceous intermediate-mafic dykes (e.g., Xu et al., 2012b).

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Figure 6. Classification diagrams of the Huilanshan granite and diorite in the NDT. (a) TAS diagram, the dashed thick line shows approximate alkaline-subalkaline boundary (Middlemost, 1994). (b) K2O versus SiO2 (wt.%) diagram, series boundaries for the low-K tholeiite, medium-K calc-alkaline, high-K calc-alkaline and shoshonite are after Le Maitre et al. (1989) and Rickwood (1989). (c) Alumina saturation index shown by ANK vs. ACNK diagram (Maniar and Piccoli, 1989). Data source: Early Cretaceous non-adakitic granites in Dabie Orogen (Xu et al., 2007); Early Cretaceous low-Mg adakitic granites in Dabie Orogen, Xu et al.(2012a, b, 2007); Early Cretaceous intermediate-mafic dykes in the North Dabie, Xu et al. (2012b); and Early Cretaceous mafic-ultramafic plutons in the North Dabie (Huang et al., 2007; Zhao et al., 2005).
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Figure 7. Selected major element data for the Huilanshan granitoids plotted against SiO2 (Harker diagrams). (a) Al2O3, (b) Na2O, (c) CaO, (d) K2O, (e) FeOT, (f) Mg#, (g) P2O5, (h) TiO2. Mg#=molar [MgO/(MgO+FeOT)]×100.
3.2.2 Trace elements

The granites are enriched in LREE and depleted in HREE with weakly negative Eu anomalies (δEu=0.81–0.85) in chondrite-normalized REE pattern diagram (Fig. 8a). The granites are also enriched in LILE (e.g., Ba, K, and Rb), and depleted in HFSE (e.g., Ti, Ta, and Nb) (Fig. 8b). In addition, they have lower contents of Sr (338 ppm–477 ppm), Y (14.2 ppm–14.4 ppm), lower Sr/Y ratios (23.80–33.13) which are similar to the Early Cretaceous non-adakitic granites (e.g., Xu et al., 2007). In Sr/Y vs. Y diagram (Fig. 9), the granites are plotted into the ADR (andesite-dacite-rhyolite) field without adakitic geochemical characteristics.

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Figure 8. Diagrams of the Huilanshan granitoid from the Dabie Orogen chondrite normalized REE patterns (a) and primitive mantle normalized trace elements patterns (b). Values of the chondrite and the primitive mantle are from Sun and McDonough (1989). Data source cited are same as in Fig. 6. Average value of the data is in (b).
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Figure 9. Sr/Y versus Y diagram of Huilanshan granitoids form the Dabie Orogen (after Defant and Drummond, 1990). Symbols are same as in Fig. 6.

The diorites are also enriched in LREE with a relatively flat HREE pattern and they also display weakly negative Eu anomalies (δEu=0.65–0.86) (Fig. 8a). In primitive mantle normalized spider diagram (Fig. 8b), the diorites are enriched in LILE (e.g., Ba, K, Rb), and are depleted in HFSE (e.g., Ta, Nb, and Hf). Compared with the granites, the diorites have higher Sr (886 ppm–1 130 ppm) and Y (24.1 ppm–28.1 ppm) contents with similar Sr/Y ratios (30.82–46.89). In Sr/Y vs. Y diagram (Fig. 9), the diorites also fall into the ADR field without adakitic geochemical characteristics.

3.3 Zircon Hf Isotope

After LA-ICP-MS U-Pb dating, zircon in-situ Hf isotope analyses were carried out. The Lu-Hf isotope data for the two sample zircons are listed in Table S4.

3.3.1 Granite

Fifteen Lu-Hf analyses were carried out on the granitic zircons (Table 4). The 176Hf/177Hf and 176Lu/177Hf ratios on the Early Cretaceous age-spots are from 0.282 187 to 0.282 294 and from 0.000 58 to 0.001 458, respectively. However, the inherited cores have variable 176Hf/177Hf and 176Lu/177Hf ratios of 0.281 124–0.282 577 and 0.004 34–0.008 48, respectively. Eleven Lu-Hf analyses for the Early Cretaceous age-spots yield negative εHf (t=118 Ma) values of -14.4 to -18.1 with a weighted mean of -16.16±0.9 (Fig. 10a). Correspondingly, their single-stage "depleted mantle" Hf model ages (tDM1) range from 1 336±63 to 1 509±73 Ma (mean=1 425±37 Ma, Fig. 11a). Their two-stage "crust" Hf model ages (tDM2) range from 2 906±102 to 3 230±115 Ma (mean=3 068±82 Ma, Fig. 11b).

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Figure 10. Zircon initial Hf isotope ratio diagram of the Huilanshan granites (a), and the diorites (b) from the North Dabie terrane. Red columns represent the inherited cores εHf (t=2 628 Ma); green columns represent the inherited cores εHf (t=1 840 Ma); and blue column represents the inherited core εHf (t=807 Ma).
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Figure 11. Histograms of inherited cores zircons one-stage Hf model age (tDM1) (a), and Early Cretaceous age-spots two-stage Hf model age (tDM2) (b) for the Huilanshan granites from the North Dabie terrane. Red columns represent the (εHf (t=2 628 Ma)) inherited cores; green columns represent the (εHf (t=1 840 Ma)) inherited cores; blue column represents the (εHf (t=807 Ma)) inherited cores; and black column represent the Early Cretaceous age-spots (εHf (t=118 Ma)).

The five old inherited cores display two groups of discordia upper intercept U-Pb ages of 2 628±41 and 1 840±37 Ma and one concordant age of 807.3±9.4 Ma (Fig. 4a). The Early Paleo- Proterozoic ages (2 393.4±18.0 and 2 466.3±17.8 Ma) have mostly positive εHf (t=2 628 Ma) values (-0.8 to 4.4, Fig. 10a). Their tDM1 ages are 2 754±138 to 2 952±62 Ma (Fig. 11a). However, the Late Paleo-Proterozoic ages (1 665.5±16.8 and 1 750± 20.8 Ma) have quite negative εHf (t=1 840 Ma) values of -12.6 to -13.2 (Fig. 10a). Their tDM1 ages are 2 720±61 and 2 744±60 Ma (Fig. 11a), and tDM2 ages range from 3 860±140 to 3 912±137 Ma (Fig. 11b). The concordant Neo-Proterozoic age of 807.3±9.4 Ma shows strongly positive εHf (t=807 Ma) values of 10.6 (Fig. 10a) with tDM1 age of 949±77 Ma (Fig. 11a).

3.3.2 Diorite

Twenty-five zircon Lu-Hf analyses were obtained from the diorite (Table S4). The dioritic zircons have 176Hf/177Hf ratios of 0.282 097 to 0.282 176 and 176Lu/177Hf ratios of 0.000 392 to 0.000 909, which are a little bit lower than that of the granite. The dioritic zircons have negative εHf (t=125 Ma) values ranging from -18.4 to -21.1 with a weighted mean of -19.5±0.26 (Fig. 10b). Their tDM1 ages are 1 510±76 to 1 623±75 Ma (mean=1 550±14 Ma, n=25), and their tDM2 ages range from 3 271±121 to 3 516±121 Ma (mean=3 369±12 Ma, Fig. 12).

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Figure 12. Histograms of zircons two-stage Hf model age (tDM2) for the Huilanshan diorites from the North Dabie terrane.
4 DISCUSSION

Post-collisional granitoids are widely distributed in the Dabie Orogen. Xu et al. (2007) summarized that they were generally divided into two groups: the early deformed adakitic rocks (ca. 130–145 Ma) and the late undeformed non-adakitic rocks (ca. 110–130 Ma). Lots of works were carried out on the early adakitic rocks (e.g., Xu et al., 2012a, 2007; Wang et al., 2007), however, petrogenesis of the late non-adakitic rocks is poorly understood. From the above works on the field outcrop, petrology (Fig. 2), geochemistry (Figs. 6–9), zircon U-Pb age and Hf isotope, the Huilanshan granitoids generally belong to the late non-adakitic rocks. Their petrogenesis and implications for orogenic collapse are discussed.

4.1 Petrogenesis of the Diorite

The Huilanshan diorites have three possible formation mechanisms: mantle-derived magma, mixing source of crust and mantle, or partial melting of the mafic lower continental crust. Compared with the Early Cretaceous mafic-ultramafic rocks (MgO: 10.19 wt.%–20.09 wt.%; Ni: 101 ppm–439 ppm; Cr: 249 ppm–1 404 ppm) (e.g., Huang et al., 2007; Zhao et al., 2005; Jahn et al., 1999) in the Dabie Orogen, the Huilanshan diorites display low MgO (max. 4.0 wt.%), Ni (max. 60.8 ppm), Cr (max. 48.4 ppm) (Table S1), indicating that they have no characteristics of the mantle-derived magmas. The diorites are similar to the Early Cretaceous intermediate-mafic dykes (Xu et al., 2012b) in the geochemical characteristics, such as enrichment in the LILES (Ba, K, and Sr) and LREE and depletion in the HFSE (Zr, Hf, and Ti) (Fig. 8). Xu et al. (2012b) suggested that the Early Cretaceous intermediate-mafic dykes were derived from an enriched mantle source hybridized by foundered eclogitized lower continental crust. However, the diorites have lower Mg# values (41–49, most < 45) than that of the intermediate- mafic dykes (46–61, most > 50) (Fig. 7i), suggesting that the diorites were not from a mixing source of crust and mantle. Thus, the petrogenesis of the Huilanshan diorites might have generated from partial melting of a mafic lower continental crust, based on the evidences as follows.

First, the diorites have relatively low SiO2 contents (51.9 wt.%–56.6 wt.%) and have very negative εHf(t) values (-18.4 to -21.1) (Fig. 10), suggesting a source which has strongly mafic continental affinity. Second, the diorites have low Sr/Y ratios without adakitic geochemical characteristics (Fig. 9), suggesting they were generated from a thinned lower continental crust. Third, the formation age of the diorites is 125±3 Ma (Fig. 5a), which is consistent with the magmas from the thinned lower continental crust during the orogenic collapse (e.g., Xu et al., 2007). Fourth, the high temperature grunulite-facies metamorphism occurred at ca. 123–136 Ma (Hou et al., 2005) during the intrusion of the Huilanshan diorites. The mafic granulite as lense occurs in the Huilanshan diorite (Fig. 2). Tectonically, the Luotian dome as an extensional dome was rapidly uplifted during the Early Cretaceous collapse of the Dabie Orogen (e.g., Xu C H et al., 2002; Hacker at al., 1998). In brief, the diorites were from partial melting of the thinned mafic lower continental crust during the orogenic collapse.

4.2 Petrogenesis of the Granite

The Huilanshan fine-grained granites are undefromed and intrude into the TTG migmatites which are strongly deformed (Fig. 2). The granites have younger crystallization age of 118±2 Ma (Fig. 4b). Meanwhile, they have non-adakitic geochemical characteristics, e.g., low Sr contents (338 ppm–477 ppm) with low Sr/Y ratios (23.80–33.13) (Table S1), and flat HREE pattern with negative Sr, Ti and Eu anomlies (Fig. 8). Thus, the Huilanshan granites are similar to the Early Cretaceous non-adakitic granites in the Dabie Orogen (e.g., Xu et al., 2007; Zhao et al., 2007; Bryant et al., 2004; Zhang et al., 2002; Ma et al., 1998) (Figs. 7 and 8). It suggests that the Huilanshan granites may be formed by partial melting under the thinned lower crust (< 35 km) during the Early Cretaceous (e.g., Xu et al., 2007).

In-situ zircon Lu-Hf isotopic data show that the granites have negative εHf (t=118 Ma) values (-14.4 to -18.1), which are consistent with the late non-adakitic granites in the North Dabie terrane (εHf(t)= -9.4 to -23.4) (e.g., Xu et al., 2008; Xie et al., 2006). Correspondingly, their tDM2(Hf) ages range from 2 906± 102 to 3 230±115 Ma with a weighted means of 3 068±82 Ma (Fig. 11), suggesting they were generally from partial melting of the Archean basement. As for the five old inherited cores, zircon U-Pb dating give multi-stage ages (Fig. 4a), two groups discordia U-Pb ages: 2 393±18 and 2 466±18 Ma with an upper intercept age of 2 628±41 Ma; 1 666±17 and 1 751±21 Ma with an upper intercept age of 1 840±37 Ma, and one concordant age of 807±9 Ma. The Archean inherited cores give almost positive εHf (t= 2 628 Ma) values of -0.8 to 4.4. Their tDM1(Hf) ages are 2 754± 138 to 2 952±62 Ma which are mostly consistent with the upper intercept age of 2 628±41 Ma, suggesting the crust was formed at that time. The information recorded in the Archean inherited cores are similar to the Huangtuling felsic granulite (2 766±9 Ma) (e.g., Wu et al., 2008), which is also outcropped at the center of the Luotian dome and is very close to the Huilanshan in location. The Paleo-Proterozoic inherited cores are similar to the Huangtuling gneiss which were suffered from a Paleo-Proterozoic (1 982±14 Ma) high temperature metamorphism (Wu et al., 2008). The Paleo-Proterozoic inherited cores have negative εHf (t=1 840 Ma) value of -12.6 to -13.2 (Fig. 10), and their tDM1 ages are 2 721± 61 to 2 744±60 Ma (Fig. 11), which are similar to that of the Archean inherited cores. The zircon ages Hf isotopic data of the two group of inherited cores suggest an Archean crust went though a Paleo-Proterozoic metamorphism. The concordant Neo- Proterozoic age of 807±9 Ma is consistent with orthogneisses (ca. 700–800 Ma) which are widely distributed in the Dabie Orogen (e.g., Xu et al., 2012b; Zhao et al., 2007, 2004; Zheng et al., 2006, 2005, 2004). It has strongly positive εHf (t=807 Ma) value of 10.6 with tDM1 (Hf) age of 949±77 Ma, suggesting a juvenile crust generated from breakup of the Neoproterozoic Rodinia supercontinent (e.g., Xu et al., 2008; Zheng et al., 2005). The age and Hf isotopic data of the zircon inherited cores suggest that the source rocks of the Huilanshan granites are inhomogenous Archean crust and subsequently experienced multi-period metamorphic and magmatic events.

Considering the compositions of the Dabie Orogen (e.g., Xu and Zhang, 2018), the source rock of the Huilanshan granites has strong affinity with Yangtze Block, such as the Huangtuling felsic granulite at the center of the Luotian dome (e.g., Wu et al., 2008). Therefore, we suggest that he Huilanshan granites were formed by partial melting of the middle-lower continental crust of Yangtze Block such as felsic granulites (e.g., Wu et al., 2008; Gao et al., 1998) at the thinned crustal thickness (e.g., Xu et al., 2012a, 2007) during Early Cretaceous orogenic collapse.

4.3 Implications for Syn-Collapse Magmatism in the Dabie Orogen

The evolution of orogen generally goes through the following processes which can be defined as three stages (e.g., Brown, 2001; Leech, 2001): (1) collision, and formation of crustal thickening; (2) metamorphism of the crust root and delamination of the crustal root; and (3) extensional and thinning of the previously thickened crust. The Dabie Orogen was formed by the Triassic subduction of the Yangtze Craton and the North China Craton and resulted in the UHP metamorphism (e.g., Xu et al., 1992) and the over-thickened crust (e.g., Wang et al., 2007; Xu et al., 2007). Subsequently, it went through an extensional tectonics (e.g., Xu C H et al., 2002; Hacker et al., 1998), widespread anatexis (e.g., Xu and Zhang, 2018; Wu et al., 2008), and voluminous magmatism (e.g., Xu et al., 2012a, b, 2007; Wang et al., 2007; Zhao et al., 2005; Zhang et al., 2002; Jahn et al., 1999; Ma et al., 1998) during the Early Cretaceous orogenic collapse.

Many works suggested that the Early Cretaceous igneous rocks during the orogenic collapse are roughly divided into two groups (e.g., He et al., 2011; Wang et al., 2007; Xu et al., 2007): the early adakitic rocks (ca. 130–145 Ma) suggesting the existence of thickened continent crust (> 50 km) and the late non-adakitic rocks (ca. 110–130 Ma) suggesting a thinned continental crust (< 35 km). Most of works were focused on when the orogen began to collapse and the duration of the orogenic collapse (e.g., Xu et al., 2007; Ma et al., 2004). However, the details of the orogenic collapse are still unclear, especially for the later stage collapse process. This study suggests that the Huilanshan granitoids are composed of calc-alkaline diorite and shoshonitic granite.

They were both generated from a normal or thinned crust during the later stage (< 130 Ma) of collapse of the Dabie Orogen. As for the Huilanshan diorites (125±3 Ma, Fig. 5a), they were coeval with or closely followed by the mafic-ultramafic pluton (123–130 Ma) (e.g., Huang et al., 2007; Zhao et al., 2005; Jahn et al., 1999), the intermediate-mafic dykes (126–130 Ma) (e.g., Xu et al., 2012a; Wang et al., 2005) and the non-adakitic granites (e.g., Xu et al., 2007). The geochemical characteristics of the diorites are different from the mafic-ultramafic plutons and intermediate-mafic dykes, but similar to the non-adakitic granites. They also have very negative zircon εHf(t) values (-18.4 to -21.1) with very old zircon tMD2 age of 3 369±12 Ma. All these suggest that the diorites could have generated from partial melting of a thinned mafic lower continental crust which was similar to Huilanshan mafic granulite (e.g., Hou et al., 2005) at the center of the Luotian extensional dome. As for the Huilanshan granites (118±2 Ma, Fig. 4b), they were intruded after the diorites (Fig. 2). They are non-adakitic rocks like ones reported in previous published data (e.g., Xu et al., 2007; Bryant et al., 2004). The ages of zircon inherited cores (Fig. 4a) and zircon εHf(t) values strongly suggested that the granites were derived from partial melting of a thinned felsic middle-lower continental crust which was similar to the Huangtuling felsic granulite (e.g., Wu et al., 2008) at the center of the Luotian extensional dome. Thus, our study on the Huilanshan granitoids reveals the thinned crustal structure and the thinned process during the later stage of collapse of the Dabie Orogen.

5 CONCLUSION

Combining the studies of field work, petrology, geochemistry, zircon geochronology and Hf isotope on the Huilanshan granitoids in Dabie Orogen, we can draw the following conclusions. Two kinds of granitoids, diorite (125±3 Ma) and granite (118±2 Ma), were recognized at the center of Luotian extensional dome. The diorites have non-adakitic geochemistry and very negative εHf(t) (-18.4 to - 21.1), suggesting they were generated from partial melting of the mafic lower continental crust. The granites also have non-adakitic geochemistry and negative εHf(t) (-14.3 to -18.1), and were from partial melting of the felsic middle-lower continental crust. The two kinds of Huilanshan granitoids are syn-collapse magmatic rocks, revealing the collapsed process of the Dabie Orogen.

ACKNOWLEDGMENTS

This research was supported by the National Natural Science Foundation of China (Nos. 41572039, 41772054, and 41372076) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUGQYZX 1704). Constructive comments from the two anonymous reviewers improved the manuscript significantly. We thank Profs. Jingsui Yang and Changqian Ma for editorial handling. We thank Dr. Keqing Zong from China University of Geosciences (Wuhan) for his help in LA-ICPMS zircon U-Pb dating and Hf isotopic analysis. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-0892-y.

Electronic Supplementary Materials: Supplementary materials (ESMⅠ, Tables S1–S4; and ESM Ⅱ, Analytical Method) are available in the online version of this article at https://doi.org/10.1007/s12583-019-0892-y.


REFERENCES CITED
Brown, M., 2001. Orogeny, Migmatites and Leucogranites:A Review. Journal of Earth System Science, 110(4): 313-336. DOI:10.1007/bf02702898
Bryant, D. L., Ayers, J. C., Gao, S., et al., 2004. Geochemical, Age, and Isotopic Constraints on the Location of the Sino-Korean/Yangtze Suture and Evolution of the Northern Dabie Complex, East Central China. Geological Society of America Bulletin, 116(5): 698-717. DOI:10.1130/b25302.2
Cong, B. L., Wang, Q. C., 1996. A Review on Researches of UHPM Rocks in the Dabieshan-Sulu Region. In: Cong, B. L., ed., Ultrahigh-Pressure Metamorphic Rocks in the Dabieshan-Sulu Region of China. Kluwer Academic Publishers, Dordrecht, Boston, London. 1-7
Defant, M. J., Drummond, M. S., 1990. Derivation of some Modern Arc Magmas by Melting of Young Subducted Lithosphere. Nature, 347(6294): 662-665. DOI:10.1038/347662a0
Foster, D. A., Schafer, C., Fanning, C. M., et al., 2001. Relationships between Crustal Partial Melting, Plutonism, Orogeny, and Exhumation:Idaho-Bitterroot Batholith. Tectonophysics, 342(3/4): 313-350. DOI:10.1016/s0040-1951(01)00169-x
Gao, S., Luo, T. C., Zhang, B. R., et al., 1998. Chemical Composition of the Continental Crust as Revealed by Studies in East China. Geochimica et Cosmochimica Acta, 62(11): 1959-1975. DOI:10.1016/s0016-7037(98)00121-5
Hacker, B. R., Ratschbacher, L., Webb, L., et al., 1998. U/Pb Zircon Ages Constrain the Architecture of the Ultrahigh-Pressure Qinling-Dabie Orogen, China. Earth and Planetary Science Letters, 161(1/2/3/4): 215-230. DOI:10.1016/s0012-821x(98)00152-6
Hacker, B. R., Ratschbacher, L., Webb, L., et al., 2000. Exhumation of Ultrahigh-Pressure Continental Crust in East Central China:Late Triassic-Early Jurassic Tectonic Unroofing. Journal of Geophysical Research:Solid Earth, 105(B6): 13339-13364. DOI:10.1029/2000jb900039
He, Y. S., Li, S. G., Hoefs, J., et al., 2011. Post-Collisional Granitoids from the Dabie Orogen:New Evidence for Partial Melting of a Thickened Continental Crust. Geochimica et Cosmochimica Acta, 75(13): 3815-3838. DOI:10.1016/j.gca.2011.04.011
Hou, Z. H., 2005. Sm-Nd and Zircon SHRIMP U-Pb Dating of Huilanshan Mafic Granulite in the Dabie Mountains and Its Zircon Trace Element Geochemistry. Science in China Series D:Earth Sciences, 48(12): 2081-2091. DOI:10.1360/03yd0524
Huang, F., Li, S. G., Dong, F., et al., 2007. Recycling of Deeply Subducted Continental Crust in the Dabie Mountains, Central China. Lithos, 96(1/2): 151-169. DOI:10.1016/j.lithos.2006.09.019
Huang, F., Li, S. G., Dong, F., et al., 2008. High-Mg Adakitic Rocks in the Dabie Orogen, Central China:Implications for Foundering Mechanism of Lower Continental Crust. Chemical Geology, 255(1/2): 1-13. DOI:10.1016/j.chemgeo.2008.02.014
Jahn, B. M., Wu, F. Y., Lo, C. H., et al., 1999. Crust-Mantle Interaction Induced by Deep Subduction of the Continental Crust:Geochemical and Sr-Nd Isotopic Evidence from Post-Collisional Mafic-Ultramafic Intrusions of the Northern Dabie Complex, Central China. Chemical Geology, 157(1/2): 119-146. DOI:10.1016/s0009-2541(98)00197-1
Keay, S., Lister, G., Buick, I., 2001. The Timing of Partial Melting, Barrovian Metamorphism and Granite Intrusion in the Naxos Metamorphic Core Complex, Cyclades, Aegean Sea, Greece. Tectonophysics, 342(3/4): 275-312. DOI:10.1016/s0040-1951(01)00168-8
Le Maitre, R. W., Bateman, P., Dudek, A., et al., 1989. A Classification of Igneous Rocks and Glossary of Terms. Blackwell, Oxford
Leech, M. L., 2001. Arrested Orogenic Development:Eclogitization, Delamination, and Tectonic Collapse. Earth and Planetary Science Letters, 185(1/2): 149-159. DOI:10.1016/s0012-821x(00)00374-5
Lei, H. C., Xu, H. J., 2018. A Review of Ultrahigh Temperature Metamorphism. Journal of Earth Science, 29(5): 1167-1180. DOI:10.1007/s12583-018-0846-9
Liu, Y. C., Li, S. G., Xu, S. T., et al., 2005. Geochemistry and Geochronology of Eclogites from the Northern Dabie Mountains, Central China. Journal of Asian Earth Sciences, 25(3): 431-443. DOI:10.1016/j.jseaes.2004.04.006
Ma, C. Q., Li, Z. C., Ehlers, C., et al., 1998. A Post-Collisional Magmatic Plumbing System:Mesozoic Granitoid Plutons from the Dabieshan High-Pressure and Ultrahigh-Pressure Metamorphic Zone, East-Central China. Lithos, 45(1/2/3/4): 431-456. DOI:10.1016/s0024-4937(98)00043-7
Ma, C. Q., Yang, K. G., Ming, H. L., et al., 2004. The Timing of Tectonic Transition from Compression to Extension in Dabieshan:Evidence from Mesozoic Granites. Science in China Series D:Earth Sciences, 47(5): 453-462.
Maniar, P. D., Piccoli, P. M., 1989. Tectonic Discrimination of Granitoids. Geological Society of America Bulletin, 101(5): 635-643. DOI:10.1130/0016-7606(1989)101<0635:tdog>2.3.co;2
Middlemost, E. A. K., 1994. Naming Materials in the Magma/igneous Rock System. Earth-Science Reviews, 37(3/4): 215-224. DOI:10.1016/0012-8252(94)90029-9
Okay, A. I., Şengör, A. M. C., Satir, M., 1993. Tectonics of an Ultra-high-Pressure Metamorphic Terrane:The Dabie Shan/Tongbai Shan Orogen, China. Tectonics, 12(6): 1320-1334. DOI:10.1029/93tc01544
Qu, W., Liu, X. C., Cui, J. J., et al., 2018. 40Ar/39Ar Dating of Muscovite from the Guishan Complex in the Tongbai Orogen, Central China, and Its Geological Implications. Earth Science, 43(1): 247-258. DOI:10.3799/dqkx.2018.015
Ratschbacher, L., Hacker, B. R., Calvert, A., et al., 2003. Tectonics of the Qinling (Central China):Tectonostratigraphy, Geochronology, and De-formation History. Tectonophysics, 366(1/2): 1-53. DOI:10.1016/s0040-1951(03)00053-2
Rickwood, P. C., 1989. Boundary Lines within Petrologic Diagrams which Use Oxides of Major and Minor Elements. Lithos, 22(4): 247-263. DOI:10.1016/0024-4937(89)90028-5
Sun, S. S., McDonough, W. F., 1989. Chemical and Isotopic Systematics of Oceanic Basalts:Implications for Mantle Composition and Processes. Geological Society, London, Special Publications, 42(1): 313-345. DOI:10.1144/gsl.sp.1989.042.01.19
Wang, Q., Wyman, D. A., Xu, J. F., et al., 2007. Early Cretaceous Adakitic Granites in the Northern Dabie Complex, Central China:Implications for Partial Melting and Delamination of Thickened Lower Crust. Geochimica et Cosmochimica Acta, 71(10): 2609-2636. DOI:10.1016/j.gca.2007.03.008
Wang, X. M., Liou, J. G., Mao, H. K., 1989. Coesite-Bearing Eclogite from the Dabie Mountains in Central China. Geology, 17(12): 1085-1088. DOI:10.1130/0091-7613(1989)017<1085:cbeftd>2.3.co;2
Wang, Y. J., Fan, W. M., Peng, T. P., et al., 2005. Nature of the Mesozoic Lithospheric Mantle and Tectonic Decoupling beneath the Dabie Orogen, Central China:Evidence from 40Ar/39Ar Geochronology, Elemental and Sr-Nd-Pb Isotopic Compositions of Early Cretaceous Mafic Igneous Rocks. Chemical Geology, 220(3/4): 165-189. DOI:10.1016/j.chemgeo.2005.02.020
Wu, Y. B., Zheng, Y. F., Gao, S., et al., 2008. Zircon U-Pb Age and Trace Element Evidence for Paleoproterozoic Granulite-Facies Metamorphism and Archean Crustal Rocks in the Dabie Orogen. Lithos, 101(3/4): 308-322. DOI:10.1016/j.lithos.2007.07.008
Xie, Z., Zheng, Y. F., Zhao, Z. F., et al., 2006. Mineral Isotope Evidence for the Contemporaneous Process of Mesozoic Granite Emplacement and Gneiss Metamorphism in the Dabie Orogen. Chemical Geology, 231(3): 214-235. DOI:10.1016/j.chemgeo.2006.01.028
Xu, C. H., Zhou, Z. Y., Ma, C. Q., et al., 2002. Geochronological Constraints on 140-85 Ma Thermal Doming Extension in the Dabie Orogen, Central China. Science in China Series D:Earth Sciences, 45(9): 801-817. DOI:10.1007/bf02879515
Xu, H. J., Ma, C. Q., Ye, K., 2007. Early Cretaceous Granitoids and Their Implications for the Collapse of the Dabie Orogen, Eastern China:SHRIMP Zircon U-Pb Dating and Geochemistry. Chemical Geology, 240(3/4): 238-259. DOI:10.1016/j.chemgeo.2007.02.018
Xu, H. J., Ye, K., Ma, C. Q., 2008. Early Cretaceous Granitoids in the North Dabie and Their Tectonic Implications:Sr-Nd and Zircon Hf Isotopic Evidences. Acta Petrologica Sinica, 24(1): 87-103.
Xu, H. J., Ma, C. Q., Song, Y. R., et al., 2012a. Early Cretaceous Intermediate-Mafic Dykes in the Dabie Orogen, Eastern China:Petrogenesis and Implications for Crust-Mantle Interaction. Lithos, 154: 83-99. DOI:10.1016/j.lithos.2012.06.030
Xu, H. J., Ma, C. Q., Zhang, J. F., et al., 2012b. Early Cretaceous Low-Mg Adakitic Granites from the Dabie Orogen, Eastern China:Petrogenesis and Implications for Destruction of the Over-Thickened Lower Continental Crust. Gondwana Research, 23(1): 190-207. DOI:10.1016/j.gr.2011.12.009
Xu, H. J., Zhang, J. F., 2018. Zircon Geochronological Evidence for Partic-ipation of the North China Craton in the Protolith of Migmatite of the North Dabie Terrane. Journal of Earth Science, 29(1): 30-42. DOI:10.1007/s12583-017-0805-x
Xu, S. T., Okay, A. I., Ji, S., et al., 1992. Diamond from the Dabie Shan Metamorphic Rocks and Its Implication for Tectonic Setting. Science, 256(5053): 80-82. DOI:10.1126/science.256.5053.80
Xu, S. T., Liu, Y. C., Chen, G. B., et al., 2003. New Finding of Micro-Diamonds in Eclogites from Dabie-Sulu Region in Central-Eastern China. Chinese Science Bulletin, 48(10): 988-994. DOI:10.1007/bf03184213
Zhang, C., Ma, C. Q., Holtz, F., 2010. Origin of High-Mg Adakitic Magmatic Enclaves from the Meichuan Pluton, Southern Dabie Orogen (Central China):Implications for Delamination of the Lower Continental Crust and Melt-Mantle Interaction. Lithos, 119(3/4): 467-484. DOI:10.1016/j.lithos.2010.08.001
Zhang, H. F., Gao, S., Zhong, Z. Q., et al., 2002. Geochemical and Sr-Nd-Pb Isotopic Compositions of Cretaceous Granitoids:Constraints on Tectonic Framework and Crustal Structure of the Dabieshan Ultrahigh-Pressure Metamorphic Belt, China. Chemical Geology, 186(3/4): 281-299. DOI:10.1016/s0009-2541(02)00006-2
Zhao, Z. F., Zheng, Y. F., Wei, C. S., et al., 2004. Zircon Isotope Evidence for Recycling of Subducted Continental Crust in Post-Collisional Granitoids from the Dabie Terrane in China. Geophysical Research Letters, 31(22): 283-294. DOI:10.1029/2004gl021061
Zhao, Z. F., Zheng, Y. F., Wei, C. S., et al., 2005. Zircon U-Pb Age, Element and C-O Isotope Geochemistry of Post-Collisional Mafic-Ultramafic Rocks from the Dabie Orogen in East-Central China. Lithos, 83(1/2): 1-28. DOI:10.1016/j.lithos.2004.12.014
Zhao, Z. F., Zheng, Y. F., Wei, C. S., et al., 2007. Post-Collisional Granitoids from the Dabie Orogen in China:Zircon U-Pb Age, Element and O Isotope Evidence for Recycling of Subducted Continental Crust. Lithos, 93(3/4): 248-272. DOI:10.1016/j.lithos.2006.03.067
Zheng, Y. F., Wu, Y. B., Chen, F. K., et al., 2004. Zircon U-Pb and Oxygen Isotope Evidence for a Large-Scale 18O Depletion Event in Igneous Rocks during the Neoproterozoic. Geochimica et Cosmochimica Acta, 68(20): 4145-4165. DOI:10.1016/j.gca.2004.01.007
Zheng, Y. F., Wu, Y. B., Zhao, Z. F., et al., 2005. Metamorphic Effect on Zircon Lu-Hf and U-Pb Isotope Systems in Ultrahigh-Pressure Eclogite-Facies Metagranite and Metabasite. Earth and Planetary Science Letters, 240(2): 378-400. DOI:10.1016/j.epsl.2005.09.025
Zheng, Y. F., Zhao, Z. F., Wu, Y. B., et al., 2006. Zircon U-Pb Age, Hf and O Isotope Constraints on Protolith Origin of Ultrahigh-Pressure Eclogite and Gneiss in the Dabie Orogen. Chemical Geology, 231(1/2): 135-158. DOI:10.1016/j.chemgeo.2006.01.005