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Volume 27 Issue 3
Jun.  2016
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Zuomin Zhou, Changqian Ma, Caifu Xie, Lianxun Wang, Yuanyuan Liu, Wei Liu. Genesis of highly fractionated Ⅰ-type granites from Fengshun complex: Implications to tectonic evolutions of South China. Journal of Earth Science, 2016, 27(3): 444-460. doi: 10.1007/s12583-016-0677-3
Citation: Zuomin Zhou, Changqian Ma, Caifu Xie, Lianxun Wang, Yuanyuan Liu, Wei Liu. Genesis of highly fractionated Ⅰ-type granites from Fengshun complex: Implications to tectonic evolutions of South China. Journal of Earth Science, 2016, 27(3): 444-460. doi: 10.1007/s12583-016-0677-3

Genesis of highly fractionated Ⅰ-type granites from Fengshun complex: Implications to tectonic evolutions of South China

doi: 10.1007/s12583-016-0677-3
More Information
  • Corresponding author: Changqian Ma, cqma@cug.edu.cn
  • Electronic Supplementary Material: Supplementary materials (Tables S1–S6) are available in the online version of this article at http://dx.doi.org/10.1007/s12583-016-0677-3.
  • Received Date: 2015-11-10
  • Accepted Date: 2016-03-01
  • Publish Date: 2016-06-10
  • The South China Block is characterized by the large-scale emplacement of felsic magmas and giant ore deposits during the Yanshanian. We present zircon Hf isotopic compositions, whole-rock major and trace element compositions of the Fengshun complex, located in eastern Guangdong Province, South China. The Fengshun complex is a multi-stage magmatic intrusion. It is composed of two main units, i.e., the Mantoushan (MTS) syeno-monzogranites, alkali feldspar granites and the Hulutian (HLT) alkali feldspar granites. LA-ICPMS zircon dating shows that the complex emplaced in 166-161 and 139±2 Ma, respectively. Geochemically, the MTS granites show relatively various geochemical compositions with low REE contents (87.76×10-6-249.71×10-6), Rb/Sr ratios (1.19-58.93), pronounced Eu negative anomaly (0.01-0.37) and low Nb/Ta ratios (2.40-6.82). In contrast, the HLT granites exhibit relatively stable geochemical characteristics with high REE contents (147.35×10-6- 282.17×10-6), Rb/Sr ratios (2.05-10.30) and relatively high Nb/Ta ratios (4.45-13.00). The isotopic data of the MTS granites display relatively enriched values, with ISr varying from 0.708 2 to 0.709 7, εNd(t) from -7.8 to -6.9 and εHf(t) from -7.4 to -3.2, in comparison with those of the HLT which are ISr=0.703 05-0.704 77, εNd(t)=-5- -3.4 and εHf(t)=-0.7-1.8). The two-stage model ages of the MTS granites (T2DM(Nd)=1.51-1.59 Ga and T2DM(Hf)=1.26-1.48 Ga) are also higher than those of the HLT granites (T2DM(Nd)=1.21-1.34 Ga and T2DM(Hf)=0.96-1.10 Ga). Thus the MTS and HLT granites might originate from different sources. The former is more likely derived from partial melting of Meso-Proterozoic basement triggered by upwelling of asthenosphere and/or underplate of the basaltic magma and then extensive fractional crystallisation, similar to the genesis of Early Yanshanian granitoids of the EW-trending tectono-magmatism belt in the Nanling range. In comparison, the latter might have involved with asthenosphere component, similar to the Early Cretaceous granitoids of NE-NNE-trending granitoid-volcanic belt in coastal region, southeastern China. We propose that the MTS granites were mainly formed in Paleo-Tethyan post-orogenic extensional tectonic setting whereas the HLT granites were formed in the back-arc extensional tectonic setting. The period at 139 Ma represents the initial time of roll-back of the paleo-Pacific Plate in SE-trending.
  • Electronic Supplementary Material: Supplementary materials (Tables S1–S6) are available in the online version of this article at http://dx.doi.org/10.1007/s12583-016-0677-3.
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Genesis of highly fractionated Ⅰ-type granites from Fengshun complex: Implications to tectonic evolutions of South China

doi: 10.1007/s12583-016-0677-3

Abstract: The South China Block is characterized by the large-scale emplacement of felsic magmas and giant ore deposits during the Yanshanian. We present zircon Hf isotopic compositions, whole-rock major and trace element compositions of the Fengshun complex, located in eastern Guangdong Province, South China. The Fengshun complex is a multi-stage magmatic intrusion. It is composed of two main units, i.e., the Mantoushan (MTS) syeno-monzogranites, alkali feldspar granites and the Hulutian (HLT) alkali feldspar granites. LA-ICPMS zircon dating shows that the complex emplaced in 166-161 and 139±2 Ma, respectively. Geochemically, the MTS granites show relatively various geochemical compositions with low REE contents (87.76×10-6-249.71×10-6), Rb/Sr ratios (1.19-58.93), pronounced Eu negative anomaly (0.01-0.37) and low Nb/Ta ratios (2.40-6.82). In contrast, the HLT granites exhibit relatively stable geochemical characteristics with high REE contents (147.35×10-6- 282.17×10-6), Rb/Sr ratios (2.05-10.30) and relatively high Nb/Ta ratios (4.45-13.00). The isotopic data of the MTS granites display relatively enriched values, with ISr varying from 0.708 2 to 0.709 7, εNd(t) from -7.8 to -6.9 and εHf(t) from -7.4 to -3.2, in comparison with those of the HLT which are ISr=0.703 05-0.704 77, εNd(t)=-5- -3.4 and εHf(t)=-0.7-1.8). The two-stage model ages of the MTS granites (T2DM(Nd)=1.51-1.59 Ga and T2DM(Hf)=1.26-1.48 Ga) are also higher than those of the HLT granites (T2DM(Nd)=1.21-1.34 Ga and T2DM(Hf)=0.96-1.10 Ga). Thus the MTS and HLT granites might originate from different sources. The former is more likely derived from partial melting of Meso-Proterozoic basement triggered by upwelling of asthenosphere and/or underplate of the basaltic magma and then extensive fractional crystallisation, similar to the genesis of Early Yanshanian granitoids of the EW-trending tectono-magmatism belt in the Nanling range. In comparison, the latter might have involved with asthenosphere component, similar to the Early Cretaceous granitoids of NE-NNE-trending granitoid-volcanic belt in coastal region, southeastern China. We propose that the MTS granites were mainly formed in Paleo-Tethyan post-orogenic extensional tectonic setting whereas the HLT granites were formed in the back-arc extensional tectonic setting. The period at 139 Ma represents the initial time of roll-back of the paleo-Pacific Plate in SE-trending.

Electronic Supplementary Material: Supplementary materials (Tables S1–S6) are available in the online version of this article at http://dx.doi.org/10.1007/s12583-016-0677-3.
Zuomin Zhou, Changqian Ma, Caifu Xie, Lianxun Wang, Yuanyuan Liu, Wei Liu. Genesis of highly fractionated Ⅰ-type granites from Fengshun complex: Implications to tectonic evolutions of South China. Journal of Earth Science, 2016, 27(3): 444-460. doi: 10.1007/s12583-016-0677-3
Citation: Zuomin Zhou, Changqian Ma, Caifu Xie, Lianxun Wang, Yuanyuan Liu, Wei Liu. Genesis of highly fractionated Ⅰ-type granites from Fengshun complex: Implications to tectonic evolutions of South China. Journal of Earth Science, 2016, 27(3): 444-460. doi: 10.1007/s12583-016-0677-3
  • The Mesozoic geology of South China Block (SCB) is characterized by widespread granitic intrusive-volcanic rocks, which are closely related to ore deposits such as Mo, W, Sn, U, REE, Nb-Ta, Cu, Pb and Zn (Luo et al., 2015; Wang et al., 2015; Li et al., 2010; Zhou et al., 2006). Some ore deposits are world-class such as major granite-related W-Sn deposits. The ages of the intrusive-vocanic rocks and the ore deposits can be divided into two periods: Early Mesozoic period (Triassic) and Late Mesozoic period (Jurassic to Cretaceous). The Triassic rocks are obviously less than Jurassic and Cretaceous rocks, and lack of coeval volcanic rocks (Li and Li, 2007; Zhou et al., 2006). The Late Mesozoic (Yanshanian in Chinese literature granitoids are characterized by predominant Ⅰ-type granitoids, fewer amounts of A- and S-type granitoids, accompanied by coeval volcanic rocks. The Jurassic granitoids are dominantly distributed in the interior of the SCB. However, the Mesozoic tectonic regime of the magmatism and mineralization remain intensely controversial, particularly for the Yanshanian tectonic evolution. Different models have been proposed, such as intracontinental orogeny (Cui et al., 2013a, b; Tang et al., 2013; Dong et al., 2007), post-orogenic extension relative to Tethyan tectonic regime (Indosinian orogeny) (Bai et al., 2007; Chen et al., 2002), back-arc extension due to slab roll-back (Jiang et al., 2009), lithospheric extension in the intraplate setting casused by delamination of the subducted slab (Wang et al., 2014; Li and Li, 2007), basaltic underplating in response to subduction (Wang et al., 2000; Zhou and Li, 2000), reactivating (Deng et al., 2011; Xie et al., 2006; Wang et al., 2005; Xu and Xie, 2005) and the breakup of the subduction slab (Mao et al., 2008). It is now widely accepted by many geologists that the Late Mesozoic magmatism is dominated by the flat-slab subduction of the paleo-Pacific Plate beneath the SCB (Li and Li, 2007; Zhou et al., 2006). However, the Early Yanshanian (Jurassic) magmatism is not consistent with the model (Liu et al., 2012; Wang et al., 2011; Chen et al., 2008). Although the Cretaceous granitoids are NE-NNE-trending, which are mainly distributed along the southeastern coast and associated with voluminous felsic volcanic rocks, the Jurassic magmatism is mainly concentrated in the interior Nanling range of the SCB in EW-trending distribution, with minor coeval volcanic rocks, especially rare arc-related volcanic rocks. It was proposed that the Mesozoic magmatism experienced a coastward migration in SE-trending, particularly in the Yanshanian (Li and Li, 2007; Zhou et al., 2006). However, Wang et al. (2011) suggested that the 180–125 Ma magmatic rocks were gradually younger northeastward. In addition, the timescale during ca. 139–85 Ma is a significant period of magmatic activity and the Cretaceous granitoids have been attributed to the subduction of the paleo-Pacific Plate by most researchers (Li and Li, 2007; Zhou and Li, 2000), but it is still debated for the initial time of the roll-back of paleo-Pacific Plate (Li et al., 2014; Yang et al., 2012; Wang et al., 2011; Li and Li, 2007). On the other hand, the magmatism during ca. 150–140 Ma is greatly weak in the SE China interior as "magmatic quiescence" (Liu et al., 2012; Li et al., 2010). Little attention is paid to it. We consider that this period is very important to constraining the initial time of the roll-back. Mesozoic calc-alkaline Ⅰ-type granites are widely distributed in the Fengshun area, eastern Guangdong Province, SE China, which is geographically located at a joint between NNE-NE-trending tectono-magmatic belt of the coast and the EW-trending tectono-magmatism- metallogenic belt of the Nanling range. In this study, we conduct an intergrated geochronological, geochemical and Sr-Nd-Hf isotopic investigation on these granites. We aim to (1) explain the Middle to Late Jurassic magmatism distributed in Nanling range. (2) Shed new light on the startup time of the roll-back of the subducted slab.

  • The South China Block (SCB) is surrounded by the North China Craton in the north, the Philippine Sea Plate in the southeast and the Tibetan Plateau in the west. It consists of the Cathaysia Block in the southeast and the Yangtze Craton in the northwest (Fig. 1a). There are two main fault systems in SCB, namely EW-stricking and NE-stricking fault. The EW-stricking fault mainly lies in the interior Nanling range whereas the NE-stricking one is distributed in the whole SCB, such as southeastern coastal region. The EW-striking Nanling range is located in the Cathaysia Block, which is a greatly important tectonic-magmatic-metallogenic belt (Liu et al., 2014). The Fengshun complex is located in the southeastern margin of the Cathysia Block, whereas a joint is developed between the NE-striking coastal region and the EW-striking Nanling range. The complex intruded into the Early Jurassic strata (muddy siltstone and siltstone mudstone) and the Late Jurassic rhyolite porphyry in the north and intruded into the Late Triassic strata (feldspar quartz sandstone) in the southwest (Fig. 1b).

    Figure 1.  Distribution of Mesozoic granites and volcanic rocks in South China (a) (modified after Sun, 2006) and geological sketch map of the Fengshun complex (b).

    The complex is mainly composed of two individual units. One is Mantoushan (MTS) granite, with an outcrop area of ~400 km2. According to petrography and field appearance, two different varieties of the unit can be distinguished: (1) MTS1 is medium-coarse grained gigantic phenocryst biotite syeno- monzogranite. It is widespread, with relatively less alkali feldspar but more plagioclase. (2) MTS2 is fine-grained alkali feldspar granite. It is relatively rare, occurring as small stock with an area of ~25 km2, rich in alkali feldspar. The other unit is Hulutian (HLT) medium-coarse-grained alkali feldspar granite, with an area of ~250 km2. The complex contains rare enclaves except for Hulutian granites. However, previous researchers took the complex as a single unit of "Jiexi granite" and zircon U-Pb age dating indicated that the "unit" formed during the Cretaceous (ca. 135 Ma, Zhao et al., 2012). No significant petrological and geochemical differences were observed. Previous Rb-Sr ages on whole rocks for the MTS and HLT granites yielded two episodes, at ca. 145, 126 Ma, respectively (Xu and Yue, 1999). We consider that the Rb-Sr ages may not represent their real intrusive age. This study shows that Fengshun complex is a Yanshanian multi-stage complex instead of Late Yanshanian.

    It can be seen that the MTS1 granite intruded into the MTS2 granite in field geological investigation. MTS1 is porphyaceous texture and the phenocryst is mainly perthite (10%–15%) of 1×2–2×3 cm in diameter. The matrix is mainly composed of alkali feldspar (30%–45%), plagioclase (An=8–25, 15%–25%), quartz (25%–35%) and biotite (5%–10%). The alkali feldspars are mainly microcline and perthite, subordinate orthoclase (Figs. 2a, 2b). The plagioclases are mainly andesines with zoning textures. MTS2 consist of alkali feldspar (55%–65%), plagioclase (5%–15%), quartz (20%–30%) and biotite (±3%) with grain size of about 1 mm. The alkali feldspar is microcline and perthite. It developed several graphic textures (Fig. 2c). The plagioclases develop polysynthetic twinning, zoning textures and subordinate myrmekitic texture (Fig. 2d).

    Figure 2.  Microscope photographs of the granitic rocks from Fengshun complex. (a)–(b). MTS perthite, graphic texture and brown biotite in medium-coarse grained huge-phenocryst syenogranite; (c) MTS-perthite and graphic texture in the fine-grain alkali feldspar granite; (d) MTS myrmekitic texture in the fine-grain alkali feldspar granite. Pth. Perthite; Pl. plagioclase; Q. quartz; Bt. biotite; Mc. microcline; Kf. K-feldspar.

    HLT consists of alkali feldspar (55%–65%), plagioclase (5%–10%), quartz (25%–33%) and biotite (5%–7%) with grain size of 3–5 mm. The alkali feldspar is perthite, accessory orthoclase.

  • Zircon grains were separated by magnetic and density techniques to concentrate non-magnetic, heavy fractions. The representative zircons were hand-picked under a binocular microscope, and then mounted them on the adhesive tape, enclosed in epoxy resin, polished down to expose the grain centers. The transmitted and refleted light images as well as cathodoluminescence images were documented before analysis. Zircon U-Pb isotopes were analyzed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan), using an Agilent 7500a ICP-MS attached to a GeoLas 2005 laser ablation system (DUV 193 nm ArF-excimer laser). A spot size of 32 μm was applied to all analyses. Standard zircon 91500 was taken as the external standard. Detailed analytical procedures can be found in Liu et al. (2010). ISOPLOT software was used to calculate weighted zircon ages and to construct the concordia plots (Liu et al., 2010).

  • Whole-rock samples for trace elements analysis were digested by HF+HNO3 in teflon bombs and analyzed with an Agilent 7500a ICP-MS at the GPMR (State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences). The detailed sample preparation and analytical procedures for ICP-MS analyses are described by Liu et al. (2008). Reported analyses on reference materials of AGV-1 and GSR-3 show a relative standard deviation of < 5% for REE and 5%–12% for other trace elements.

  • Sr and Nd isotopic compositions were measured by Finnigan Triton TI thermal ionization mass spectrometry (TIMS) at the State Key Laboratory for Mineral Deposits Research, Nanjing University. The detailed analytical procedures are presented in Pu et al. (2005). About 50 mg samples were dissolved as for trace element analyses. Complete separation of Sr was achieved by a combination of cation-exchange chromatography in H+ form and pyridinium form with the DCTA complex. Then Nd was separated from the REE fractions by cation-exchange resin using HIBA as eluent. After purification, the separated Sr was dissolved in 1 μL of 1 N HCl, and then loaded with TaF5 solution onto W filaments for Sr isotopic analysis on TIMS. The separated Nd was dissolved in 1 μL of 1 N HCl and then loaded with H3PO4 solution onto Re double-filaments for Nd isotopic analysis on TIMS. The 143Nd/144Nd and 87Sr/86Sr ratios are normalized to the natural 146Nd/144Nd ratio of 0.721 9 and 86Sr/88Sr ratio of 0.119 4, respectively. During the period of analysis, measurements of NIST SRM987 Sr standard and JNdi-1 Nd standard yielded 87Sr/86Sr=0.710 252±0.000 016 (2σ) and 143Nd/144Nd= 0.512 121±0.000 016 (2σ), respectively. Total analytical blanks were 5×10-11 g for Sm and Nd, (2–5)×10-10 g for Rb and Sr.

  • Zircon Lu-Hf isotope in situ analyses were carried out using a neptune plus MC-ICP-MS (Thermo Fisher Scientific, Germany) by a combination of a Geolas 2005 excimer ArF laser ablation system (Lambda Physik, Göttingen, Germany) at the GPMR. The detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument and analytical method were comprehensively described by Hu et al. (2008). Zircon standard 91500 yielded a weighted average 176Hf/177Hf ratio of 0.282 308±0.000 013 (1σ, n=16), GJ-1 and TEM yielded weighted average 176Hf/177Hf ratios of 0.282 017± 0.000 011 (1σ, n=6) and 0.282 695±0.000 011 (1σ, n=6), respectively. A decay constant of 176Lu of 1.865×10-11 yr-1 (Scherer et al., 2001) was adopted, and εHf(t) values were calculated using the present-day chondrite values of 176Hf/177Hf= 0.282 772 and 176Lu/177Hf=0.033 200. Single-stage of depleted mantle model ages (TDM) were calculated using ratios of 176Hf/177Hf=0.283 25 and 176Lu/177Hf=0.038 400 at present day (Griffin et al., 2000). Two-stage model Hf ages (T2DM) were calculated with a mean 176Hf/177Hf ratio of 0.015 000 for average continental crust (Griffin et al., 2000).

  • Four representative granites are selected for LA-ICPMS zircon U-Pb dating. Samples 13GD04-1 and 13GD05-1 were selected from MTS1 and sample 14GD06-1 was selected from MTS2 of Mantoushan granites. Sample 13GD12 was selected from Hulutian granites. Zircon grains are transparent, light brown in color, euhedral to subhedral and columnar with lengths of 50–280 μm and length-to-width ratios of 1.5 : 1–5 : 1. Zircon mostly exhibit typical oscillatory, linear zoning, and minority of them show core-rim structure. Zircons from four granites mostly have ratios of Th/U > 0.4. The analytical results for zircon U-Pb ages are listed in Table S1 and Fig. 3.

    Figure 3.  Representative cathodoluminescence (CL) images of zircon grains and zircon U-Pb concordia diagrams.

    A total of 20 zircons and 22 spots for sample 13GD04-1 were analyzed, 20 spots defined a weighted mean 206Pb/238U ages of 162±3 Ma (MSWD=1.9), which represent the intrusive age of the granite. The other two spots (8, 11) have low concordia and were excluded from the weighted average age calculations. Twenty-two zircons and 22 spots for sample 13GD05-1 were analyzed. Twenty-one spots were plotted on the concordant curve in the concordia diagram and defined a weighted mean 206Pb/238U ages of 161±2 Ma (MSWD=2.1), which could be interpreted as the crystallisation age. The No. 18 spot is an inherited zircon, showing a low concordia and was omitted from the weighted average age calculations. A total of 11 zircons and 14 spots for sample 14GD06-1 were analyzed. Eleven spots were plotted on the concordant curve in the 207Pb/235U-206Pb/238U concordia diagram, indicating that the U-Pb isotopic systems were basically closed after formation. Eleven spots defined a weighted mean 206Pb/238U ages of 166±2 Ma (MSWD=1.1), which represents the intrusive age of the granite. The other two spots (6, 7) obtained around 178 Ma, probably indicating zircon xenocrysts or inherited zircons (low concordia of No. 7 spot). The 206Pb/238U ages of No. 11 spot is 156±2 Ma, probably a juvenile zircon in the late stage. Three zircons were omitted from the weighted average age calculations. All analyzed spots of 19 zircons and 19 spots for sample 13GD12 were plotted on the concordant curve. All spots yielded a weighted mean 206Pb/238U ages of 139±2 Ma (MSWD=1.7), representing the intrusive age.

  • The results of major and trace-element compositions are given in Table S2. Major element compositions have been recaluculated as LOI (loss on ignition) free for plotting on the Q'-ANOR diagram (Fig. 4). They are metaluminous (Fig. 5). The MTS granites reveal more variations of TiO2, TFe2O3, CaO, P2O5 contents than those of the HLT granites. In the MTS granites, MTS2 exhibit some differences. MTS2 are relatively SiO2-rich and low Fe, Ti, Mg, Ca, and the differentiation index DI are the highest (mean value of 96.6).

    Figure 4.  Q'-ANOR normative composition diagram (Streckeisen and Le Maitre, 1979) for classification, where Q'=100×Q/(Q+Or+Ab+An) and ANOR=100×An/(Or+An).

    Figure 5.  A/CNK-ANK diagram.

    Chondrite-normalized REE pattern for these granites are shown in Fig. 6a. The MTS granites show larger variations of trace elements than those of the HLT granites. The MTS granites have relatively lower REE contents and more pronounced Eu negative anomaly (0.01–0.37) and lower Nb/Ta ratios (2.40–6.82). The MTS granites reveal REE tetrad effect (1.01–1.15). In contrast, REE concentrations of the HLT granites exhibit relatively higher values of 147×10-6– 282×10-6 (mean value of 200×10-6), relatively higher Nb/Ta ratios (4.45–13.00). The HLT granites have no REE tetrad effect (mean TE1, 3=0.99) except one sample. In the MTS granites, MTS2 have lowest REE concentrations (88×10-6–149×10-6), mean value of 115×10-6, and LREE/ HREE ratios range of 1.06–1.89, showing that relatively flat REE pattern and REE tetrad effect (mean TE1, 3=1.08). All granites have the similar patterns in primitive-mantle- normalised multi-element diagram (Fig. 6b), with pronounced negative Ba, Sr, Nb, P and Ti anomalies and evidently positive Rb, Th, U, La and Ce anomalies.

    Figure 6.  (a) Chondrite-normalized REE patterns and (b) primitive mantle- normalized trace element patterns (normalized values are from Sun and McDonough, 1989).

  • The Sr-Nd isotopic data for whole-rocks are given in Table S2 and Fig. 7. Zircon U-Pb concordia ages in this study are used to calculate the initial Sr-Nd isotopic compositions. The Nd isotopic model ages were calculated using the formulation proposed by Li and McCulloch (1996). The MTS granties have evolved radiogenic isotopic compositions with ISr ratios ranging from 0.708 24 to 0.709 68 and corresponding εNd(t) values of -7.8 to -6.9. Their two-stage depleted-mantle model ages (T2DM) range from 1.51 to 1.59 Ga. The HLT granites have relatively lower ISr values (0.703 05 to 0.704 77) and higher εNd(t) values of -5.0 to -3.4. The two-stage depleted-mantle model ages (T2DM) (1.21–1.34 Ga) are younger than those of the MTS granites.

    Figure 7.  Diagram of εNd(t) versus crystallization ages. The base map is after Shen et al. (1993). Symbols as in Fig. 5. Data of gray boxes in Table S3.

  • Zircon Hf isotopic data were listed in Table S4 and Figs. 89. For the MTS granites, two samples (13GD04-1 and 13GD05-1) were analysed. 176Hf/177Hf ratios of 13 analyses of sample 13GD04-1 are consistent, ranging from 0.282 528 to 0.282 582, εHf(t=162) values of -5.1 to -3.3 and corresponding to T2DM ranging from 1.26 to 1.36 Ga. For sample 13GD05-1 also from Mantoushan granites, 176Hf/177Hf ratios of 12 analyses vary from 0.282 469 to 0.282 584, εHf (t=161) values of -7.4 to -3.2 and corresponding to T2DM values from 1.26 to 1.48 Ga. Conversely, 14 analyses of Sample 13GD12 from the HLT granites have higher 176Hf/177Hf ratios of 0.282 665 to 0.282 739 and higher εHf (t=139) values of -0.7 to +1.8, corresponding to younger T2DM values from 0.96 to 1.10 Ga.

    Figure 8.  Histograms of zircon εHf(t) values and two-stage Hf model ages.

    Figure 9.  Diagram of zircon εHf(t) versus crystallization ages. The DM and crustal evolution reference lines are after Griffin et al. (2000). The Hf isotope evolutionary area for the East Cathaysia metamorphic basement is from Xu et al. (2007).

  • The Fengshun complex is characterized by high SiO2 and alkali contents (Na2O+K2O=7.27%–9.72%), K(K2O/Na2O= 1.09%–2.14%) and NKA=0.72–0.99 with mean values of 0.91, and belonged to high-K calc-alkaline series. The differentiation index DI range from 86 to 98 (mean value of 94), implying a highly fractionated granite affinity. It is metaluminous and largely different from typical peraluminous S-type granties. Wolf and London (1994) suggested that the apatite reaches saturation in metaluminous and weakly peraluminous (ACNK < 1.1) magma. Thus, P2O5 contents in the magma such as I- and A-type granites decrease with increasing SiO2 contents, resulting in very low P2O5 contents in highly fractionated granitoid. In contrast, the apatite is under-saturated in strongly peraluminous magma due to its high solubility. Consequently, P2O5 contents in the strongly peraluminous magma such as S-type granites are either roughly constant or slightly increase with increasing SiO2 contents. As shown in Fig. 10, P2O5 contents for all of samples are below 0.1% and decrease with increasing SiO2 content, different from typical S-type granties (Chappell, 1999). The Zr, Y, Zn contents are similar to the LFB Bega Ⅰ-type granites in Australia and Y-Rb, Th-Rb diagrams illustrate the characteristics of Ⅰ-type granite (Fig. 10). In the geochemical discrimination diagrams they are mostly plotted in the Ⅰ-type or fractionated Ⅰ-type granite fields (Fig. 11). The isotopic compositions for lower ISr, T2DM(Nd) and T2DM(Hf) as well as higher εNd(t) and εHf(t) values are also distinct from the typical S-type granite in SCB (Qi et al., 2007; Chen et al., 1999).

    Figure 10.  Chemical variation diagrams. Data of LFB Bega Ⅰ-type granites are after Collins et al. (1982). Symbols as in Fig. 5

    Figure 11.  Diagrams of the geochemical discrimination. The base maps (a) are from Nakada and Takahashi (1979) and (b) from Whalen et al. (1987) and (c) from Sylvester (1989).

    In addition, a negative correlation between Zr content with SiO2 content and existence of inherited zircons demonstrate that zircons are saturated. The calculated zircon saturation temperatures (TZr) vary from 705 to 751 ºC (with a mean of 728 ºC, Table S2), all below those of the typical A-type granite (800–900 ºC, Liu et al., 2003). The Zr+Nb+Ce+Y contents are also below those of A-type granite (350×10-6, Whalen et al., 1987) except sample 13GD15-1 (Table S2). The measured FeOt/MgO ratios of 2.32 to 8.72, with a mean of 4.86, are obviously below the average value of A-type granites (10, Whalen et al., 1987; or 13.4, Turner et al., 1992). Consequently, the Fengshun granites are highly fractionated high-K calc-alkali Ⅰ-type granites.

  • Calc-alkaline Ⅰ-type granites could be generated either by assimilation and fractional crystallization (AFC) of mantle- derived basaltic sources or by mixing of mafic magma with crust-derived felsic magma. Granitoids generated by such processes are normally characterized by a wide range of chemical and isotopic compositions and abundant mafic enclaves. It is unlikely that the granitoids generated directly from mantle because Mesozoic acidic magmatic rocks are widely distributed in SE China lacking of corresponding large-scale mafic igneous rocks, and the SiO2 contents values of our samples are very high (> 73%).

    The MTS granites were probably originated from the Meso-Proterozoic basement crust. It is supported by the petrography and isotopic compositions. Mafic enclaves are rarely observed in the MTS granites. Those granites have relatively lower εNd(t), εHf(t) and higher two-stage Nd and Hf isotopic model ages (T2DM) than the HLT granites. The εNd(t) and εHf(t) values vary from -7.8 to -6.9, -3.2 to -7.4, respectively. T2DM(Nd) values and T2DM(Hf) values range from 1.51–1.59, 1.26–1.48 Ga, respectively. The isotopic compositions are consistent with those of granitic rocks in Nanling range (Fig. 7). The period during 1.4–1.5 Ga is an extremely important period of crust growth in southeastern China and the Early Yanshanian granites were originated from the Meso-Proterozoic metamorphosed sedimentary rocks (Wang and Shen, 2003).

    In contrast, the HLT granites are more likely generated from a mixed parental magma between mantle-derived magma and Meso-Proterozoic basement crust, several reasons are listed as follows: (1) mafic enclaves commonly occur in the HLT granites. (2) The HLT granites have relatively higher εNd(t), εHf(t) and lower two-stage Nd and Hf isotopic model ages (T2DM) than those of the MTS granites. The εNd(t) and εHf(t) values vary from -5.0 to -3.4, -0.6 to +1.8, respectively. T2DM(Nd) values and T2DM(Hf) values range from 1.21 to 1.34, and 0.96 to 1.10 Ga, respectively. These model ages are clearly below the Nd isotopic model ages of the metamorphic basement in Caythasia Block (Chen et al., 1999) and the Hf isotopic model ages of Darongshan-Shiwandashan S-type granites (Qi et al., 2007). (3) The Sr-Nd isotopic compositions are similar to those of granitic rocks along the coast region in southeastern China, which are considered as mixing between ancient basement and mantle-derived materials, which implies that the HLT granites may be generated from a mixed crustal protolith with both juvenile basaltic crust and the Meso- Proterozoic basement crust.

  • The MTS granites show large variations of chemical compositions. Rb/Sr ratios range from 1.19 to 58.93, far higher than the average ratio of upper crust (Taylor and Mclennan, 1985). K/Rb ratios range from 89.90 to 179.34, far lower than the average ratio of upper crust (Rudnick and Gao, 2003). The MTS granites have lower Nb/Ta ratios than the HLT granites. Unlike the HLT granites, the MTS granites show REE tetrad effect, especially the MTS2 granites. In the MTS granites, Rb/Sr ratios of the MTS2 granites are the highest and the K/Rb ratios are the lowest. Siderophile elements such as V, Cr and Ni contents are lower than those of MTS1. MTS2 have lowest REE concentrations (88×10-6–149×10-6), with mean value of 115×10-6. LREE/HREE ratios of MTS2 granites range from 1.06–1.89, showing relatively flat REE pattern and REE tetrad effect (mean TE1, 3=1.08), probably in response to strongly crystallization and fractionation and interaction by F-Cl-rich fluid (Zhao et al., 1992).

    All these characteristics illustrate that the MTS granites have experienced highly fractionated evolution. Strong Eu depletion requires extensive fractionation of the plagioclase and/or K-feldspar, because the fractionation of plagioclases can result in depletion of Sr-Eu elements and the fractionation of K-feldspars can promote depletion of Sr-Eu elements. It can be seen on simulation log-log diagrams of Sr versus Ba and Rb (Figs. 12a, 12b). Both Sr and Ba contents decrease with the differentionated evolution, demonstrating fractional crystallization of plagioclase and K-feldspar during the formation of these granites. The P depletions are mainly relative to fractionation of apatites.

    Figure 12.  Sr versus Ba and Sr versus Rb plots (the data represent the percentage of the remnant melt. Bt. Biotite; Pl. plagioclase; Kf. K-feldspar; Hbl. hornblende; Cpx. clinopyroxene; Opx. orthopyroxene.

    The HLT granites show relatively more stable chemical compositions than the MTS granites. Nb/Ta ratios are higher than those of MTS, but lower than that of the continent crust (11, Taylor and Mclennan, 1985). Rb/Sr ratios range from 2.05 to 10.30 and K/Rb ratios range from 133.46 to 186.80, similar to the MTS granites. In Fig. 12, both Sr and Ba contents decrease with the magmatic evolution, consistent with strong Eu depletion. Highly evolved magmas are in general rich in H2O, B, F, and/or Cl, and often show non-CHARAC ratios and REE tetrad effects. Different from MTS granites, no REE tetrad effect occurs in HLT granites. The Zr/Hf and Nb/Ta ratios of the HLT granites are higher than those of the MTS granites, in particular of sample MTS2. As a result, MTS granites were more likely to have undergone obvious magmatic differentiation, during which intense interaction of the residual melts with aqueous hydrothermal fluids leading to the above highly fractionated signatures has taken palce.

  • The Yanshanian magmatisms in South China have undergone two major tectono-magmatic episodes since the Indosinian orogeny, namely Early Yanshanian (Jurassic) and Late Yanshanian (Cretaceous). During Indosinian post-orogenic stages, N-S-trending extensional stress resulted in the formation of EW-trending fault systems and the emplacement of granitic bodies, which formed dominantly at ca. 160 Ma. The Late Yanshanian magmatism is related to the NWW to NW-ward subduction of the paleo-Pacific Plate during the Cretaceous which formed dominantly 135–90 Ma. The distinct episodes of the MTS and HLT granites are probably related to the two fracture systems. The MTS granites at ca. 160 Ma were a part of intraplate magmatism in EW-trending Nanling range, whereas the HLT granites were formed in the paleo-Pacific subduction regime.

    Xu and Yue (1996) suggested that the 165–160 Ma magmatism in eastern Guangdong Province underwent transformation from the intraplate environment to active continental margin environment. Some other researchers suggested that it was formed in compressive orogenic setting (Cui et al., 2013a, b; Dong et al., 2007). Many geologists insisted that the Jurassic magmatic rocks in Nanling and adjacent regions formed in intraplate extension including post-orogenic and rifting regime (Li et al., 2007b; Ma et al., 2006; Hua et al., 2005).

    However, there are several different viewpoints on the geodynamic mechanism resulted from lithospheric extension in ca. 160 Ma as follows: Tectonic relaxation and extension followed by Indosinian collisional orogen, namely post-orogenic setting (Bai et al., 2007; Wang et al., 2007; Chen et al., 2002), back-arc extension owing to slab roll-back (Jiang et al., 2009), intraplate lithospereic extension due to delamination of the paleo-pacific flat-subducted slab initiated since Indosinian (Li and Li, 2007), reactivation of EW-striking faults by the subduction of the paleo-Pacific Plate (Deng et al., 2011; Xie et al., 2006; Wang et al., 2005; Xu and Xie, 2005) and a large window developed after subducted plate was torn up (Mao et al., 2008). The following evidences in this study suggest that the magmatism at ca. 160 Ma such as the MTS granites were likely formed in post- orogenic setting: (1) The granites are located in eastern Nanling range. No direct evidence can confirm Indosinian subduction occurred (Wang et al., 2011). Up to now, there are no Indosinian vocanic rocks or adakitic rocks in the whole Cathysia Block. Therefore, the flat subduction model for paleo-Pacific Plate since Indosinian is not convincing. (2) The magmatism in Nanling range is mainly E-W-striking, inconsistent with NE-striking generated by back-arc extension theoretically as a result of the paleo-Pacific subduction. (3) These granites belong to high-K calc-alkaline series, similar to large-scale Early Yanshanian granitoids in Nanling region (Zhang et al., 2003). They have Nd-Hf isotopic affinity with those of granitoids in Nanling region (Figs. 7 and 9). It is a typical post-orogenic rock assemblage with the coeval A-type granites, alkaline rocks and intraplate basalts. Some syenites and bimodal volcanic rocks outcropped, which indicates the rift setting, the amount of these rocks is too small and likely formed in local extensional condition. Therefore, it needs further investigations on the viewpoints of fault reactivitation and opened window. (4) Neither MTS nor HLT granites have any metamorphic and deformational characteristics. There are no any orientation textures and compressive shear structure for the granites in western Nanling region (Bai et al., 2007). They mostly occurred in the extensional fault belts illustrating the passive emplacement mechanisms. In fact, most of the granitoids in Nanling region, formed dominantly at ca. 160 Ma, do not exhibit metamorphic and deformational characteristics.

    On the contrary, the magmatism at ca. 139 Ma such as the HLT granites is associated with the Cretaceous granitoids along the southeastern coastal region in SE China (Figs. 7 and 9). We suggest that the HLT granites were formed in the back-arc extensional tectonic setting. The period at 139 Ma represents the initial time of roll-back of the paleo-Pacific Plate in SE-trending. The following evidences can support this viewpoint. (1) In Fig. 8, three stages have obviously different Nd isotopes. The stage during 156–166 Ma have relatively low εNd(t) values and large variation, probably reflecting break-up of thicken crust. In Fig. 10, the lower εHf(t) values between 147–140 Ma correspond to an evident compressive event in SE China interior, and the relatively higher εHf(t) values between 130–140 and 155–170 Ma indicate two extensional events, respectively. According to Sr-Nd-Hf isotopic compositions, the source of late-stage HLT granites could be involved with asthenosphere material. (2) The coastal magmatism mainly formed at 139–85 Ma. This magmatism is characterized by predominant volcanic rocks, which is different from Jurassic magmatism. The total outcrop area of the Cretaceous magmatic rocks is ~1 393 920 km2 and exhibits an NE-NNE-trending magmatic belt (Sun, 2006; Zhou et al., 2006). The distribution of granitoids is consistent with that of the coeval volcanic rocks (Fig. 1). Generally, the tectonic settings of the southeast coast granitoids have been considered as paleo-Pacific Plate subduction (Chen et al., 2008). (3) Wong et al. (2009) proposed a model of irregular roll-back of the subducted paleo-Pacific Plate to interpret the genesis of the A-type granites (125–100 Ma) in the coastal area. The Early Cretaceous A-type granitic rocks along the Gan-Hang belt have duration from 137 to 122 Ma, indicating the extension (back-arc extension or intra-arc rift) is likely to start as early as ca. 137 Ma (Yang et al., 2012). Zhu et al. (2014) documented that strong extension might have been driven by the roll-back of paleo-Pacific Plate since Early Cretaceous strata (ca. 140 Ma), which is much earlier. (4) In Fig. 13, the Cretaceous granitoids with intrusion ages between 139 and 85 Ma in coastal region were progressively younger towards ocean. (5) As shown in Fig. 14, the intensive magmatism including the abundant A-type granites emerged since ca. 139 Ma. It was a consequence of the rollback of the subduction slab in response to the sharp increasing of εNd(t) and εHf(t) values after a timescale of 150–140 Ma.

    Figure 13.  Distribution and ages of the Cretaceous granitoids in SE China. Data are listed in Table S5.

    Figure 14.  Diagram showing tectonics, magmatism, sedimentary and compression-extensional tectonic cycles from 250–80 Ma in South China.

    It needs to be noticed that the episode of the magmatism (140–150 Ma) was taken as magmatic quiescence (Li et al., 2010). Li (2013) pointed that the magamtism (145–137 Ma) have weak magmatic activities and they are adakitic rocks and gneiss. It is ubiquitous for unconformity between Jurassic and Cretaceous strata (Li, 2013; Zhang et al., 2013) as well as the sedimentary gap during ca. 150–140 Ma (Li, 2013; Dong et al., 2007) in South China Block. Guo et al. (2012) suggested that the Nanyuan Formation (143–130 Ma) was uncomfortably overlying Douling Formation (168–145 Ma) in South China Block. Xing et al. (2008) proposed that the previous "Nanyuan Formation" should be divided into the Shekou Formation (163–150 Ma) and the uncomfortably overlying Nanyuan Formation (142–130 Ma) in South China Block. Li (2000) and Holloway (1982) argued that there was a significant magmatic gap and widespread regional unconformity approximately at the Jurassic-Cretaceous boundary in South China. We propose that the period during ca. 150–140 Ma is a magmatic quiescence in response to the compressive tectonic setting.

    Consequently, the magmatism at ca. 160 Ma such as the MTS granites emplaced in post-orogenic extensional stage, accompanied with the basalt underplating and partial melting of the Meso-Proterozoic basement. It should be noted that the subduction of the paleo-Pacific Plate at this period has not been denied in this study, but it is not the primary tectonic mechanism. Until ca. 150–140 Ma, the period was likely a supreme timescale of subdution of paleo-Pacific Plate. Since 139 Ma, the roll-back of the slab started and then led to the lithospheric extension and thinning. It further induced asthenosphere upwelling, which triggered partial melting of the Meso- Proterozoic basement involved with the asthenosphere material, and subsequently generated from the extensive magmatic differentiation of the parental magma.

  • Although the SCB underwent intra-continental orogeny, the gradual and intermittent thinning of lithosphere mantle resulted in upwelling of asthenosphere material since ca. 165 Ma (Hua et al., 2005). In Fig. 14, it shows that the SCB has undergone four compressive-extensional tectonic cycles since Triassic. Although two compressive collision events are relatively short during ca. 150–140 and 174–170 Ma, lasting for 4–10 Ma, the two periods greatly influence the whole South China (Fig. 14) in respond to pervasive absence of strata (Mao et al., 2009) and magmatic gap or distinctly weakening magmatic activity. The South China Block has undergone lithosphere extensional and thinning during 170–150 and 140–120 Ma.

    As can be seen in Fig. 15, the age distribution of Mesozoic mineralization and corresponding tectonic setting in South China are consistent with the tectonic cycles proposed in the context. The mineralizations are products by intense tectono- magmatism. It is a critically important characteristic for the Mesozoic mineralization in South China Block that the formations of most ore deposits are closely relative to the granitic magmatism. South China has the world-class W-Sn deposits, with porphyry Cu-Mo, vein-type Ag-Pb-Zn, and epithermal Au-Ag-(Cu) deposits. South China also is the world-class granitic province, formed predominantly at Mesozoic, especially ca. 160 Ma in Nanling range. The Nangling region is the largest W-SN metallogenic belt in the world. Li et al. (2008) argued that the higher fractionation occur from Cu (Mo) deposit to Sn (W) deposit. Many of the Mesozoic granitoids in South China are highly evolved (Li et al., 2007b), especially in Nanling range such as the MTS granites. Zhao et al. (2010) pointed that the magmatic rocks bearing REE tetrad effect are generally highly evolved and closely associated with W, Sn and rare-metal mineralization. The complexation of REE and rare-metal with the halogens, released from the highly evolved magma, might have played a significant role in the generation of the REE tetrad effect and mineralization. The onset of the mineralization commonly is at the tectonic transition from collision to extension and the large-scale extension is accompanied by the large-scale mineralization (Figs. 1415). It is more likely that the ore-forming fluids released from the asthenosphere upwelling and revolution process of granitic magma contributed to the Mesozoic mineralization in South China. Overall, we suggest that the South China Block has undergone four tectono-magmatism- metallogenic cycles in Mesozoic, accompanied by three large-scale magmatism and mineralization events in Yanshanian.

    Figure 15.  Age histogram of the Mesozoic ore deposits in South China (the age data in the plots are listed in Table S6).

  • (1) Zircon LA-ICPMS U-Pb dating results indicate that the Fengshun complex consists of Yanshanian multi-stage complex plutons. Mantoushan granites are formed at 161–166 Ma, whereas Hulutian granites are dated at 139±2 Ma.

    (2) The granites belong to high-K calc-alkaline Ⅰ-type granites, mainly underwent fractionation and crystallization of plagioclases, K-feldspars and apatites.

    (3) The MTS granites formed in post-orogenic extensional tectonic setting similar to granitoids in the Nanling range. The lithospheric extension and thinning induced asthenosphere upwelling, accompanied by the basalt underplating and partial melting of the Meso-Proterozoic basement. The subduction of the paleo-Pacific Plate at ca. 160 Ma was not the primary tectonic mechanism. Until 150–140 Ma, the period was likely a supreme timescale of subdution of paleo-Pacific Plate. Since 139 Ma representative of the HLT granites, the roll-back of the subducted slab was started and then led to the lithospheric extension and thinning. It further induced asthenosphere upwelling, which triggered partial melting of the juvenile crust and Meso-Proterozoic basement, and subsequently generated from the extensive magmatic differentiation of the parental magma. The South China Block has undergone four compressive-extensional tectono-magmatism-metallogenic cycles, accompanied by 3-large-scale magmatism in Yanshanian.

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