Spatiotemporal Reconstruction of Late Mesozoic Silicic Large Igneous Province and Related Epithermal Mineralization in South

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Journal of Geophysical Research: Solid Earth

RESEARCH ARTICLE Spatiotemporal reconstruction of Late Mesozoic silicic 10.1002/2016JB013060 large igneous province and related epithermal Key Points: mineralization in South China: Insights from • A Late Mesozoic northwestward and subsequent eastward migration of the the Zhilingtou volcanic-intrusive complex South China SLIP emplacement sequence is established Guo-Guang Wang1, Pei Ni1, Chao Zhao1, Xiao-Lei Wang1, Pengfei Li2, Hui Chen1, An-Dong Zhu1, and • The SLIP formed in an arc and 1 subsequent back-arc setting related Li Li

to a northwestward subduction and 1 following slab retreat State Key Laboratory for Mineral Deposits Research, Institute of Geo-Fluids, School of Earth Sciences and Engineering, 2 • Epithermal mineralization formed by Nanjing University, Nanjing, China, Department of Earth Sciences, University of Hong Kong, Hong Kong, China melting of metal-rich juvenile crust in response to tectonic transition from arc to back-arc regimes Abstract Silicic large igneous provinces (SLIPs) generally reflect large-scale melting of lower crustal materials and represent significant metal reservoirs. The South China Block-Coastal Region (SCB-CR) SLIP Supporting Information: hosts several large epithermal deposits. To better understand these deposits, we document the • Supporting Information S1 spatiotemporal framework of the host SLIP across the SCB-CR. Using zircon U-Pb dating and geochemical and isotopic analysis, we identify four stages of emplacement. Stage 1 felsophyre (circa 149 Ma) shows a chemical Correspondence to: fi P. Ni, af nity to highly fractionated I-type granites. Stages 2 and 3 of low-Mg felsic volcanics (circa 128 to 111 Ma) [email protected] and stage 4 felsite (circa 100 Ma) have higher εHf(t) and εNd(t) values than stage 1 felsophyre, suggesting a significant contribution of newly underplated juvenile crust to the magma sources. Stage 4 diabase (circa ˗ Citation: 101 Ma) was likely produced by melting of subduction metasomatized asthenospheric mantle. Together with Wang, G.-G., P. Ni, C. Zhao, X.-L. Wang, reliable published data, we build a new spatiotemporal framework of volcanics and infer that the majority of P. Li, H. Chen, A.-D. Zhu, and L. Li (2016), the SCB-CR SLIP was related to the gradual northwestward subduction of the Izanagi plate beneath South Spatiotemporal reconstruction of Late Mesozoic silicic large igneous province China in a continental arc setting during circa 170 to 110 Ma, and minor contribution was from the eastward and related epithermal mineralization in retreat of the subducting slab in a back-arc setting during circa 110 to 90 Ma. We conclude that the South China: Insights from the large-scale epithermal mineralization was generated by melting of the metal-rich, thin (30–40 km), newly Zhilingtou volcanic-intrusive complex, J. Geophys. Res. Solid Earth, 121, underplated hydrous juvenile crust during the tectonic transition from arc to back-arc settings. 7903–7928, doi:10.1002/2016JB013060.

Received 6 APR 2016 Accepted 18 OCT 2016 1. Introduction Accepted article online 19 OCT 2016 Mafic large igneous provinces (LIPs) are characterized by short-lived (<5 Ma), large volume (>100,000 km3) Published online 14 NOV 2016 mafic-ultramafic rocks in intraplate settings [Bryan and Ernst, 2008; Coffin and Eldholm, 1994; Ernst and Buchan, 2003]. In contrast to classic mafic LIPs, silicic large igneous provinces (SLIPs) are defined by (1) extru- sive volumes of > 10,000 km3 dacitic to rhyolitic ignimbrites; (2) emplaced over relatively long time intervals (generally > 20 Ma); and (3) spatially/temporally associated with volcanic rifted margins or failed continental rifts [Bryan, 2007; Bryan et al., 2002]. Examples include the Jurassic Chon Aike province and related rocks in the rift zone of West Antarctica [Pankhurst et al., 1998], the Early Cretaceous Whitsunday igneous province in the volcanic rifted margin of eastern Australia [Bryan et al., 1997], and the Oligocene to Early Miocene Sierra Madre Occidental province in the Basin and Range extensional regime of western Mexico [Camprubi et al., 2003; Ferrari and Rosas-Elguera, 2002]. These well-known SLIPs are reported to have formed in back-arc extensional regimes associated with accreted continental margins or intraplate rifts [Bryan, 2007; Bryan et al., 1997; Ferrari and Rosas-Elguera, 2002]. In contrast, SLIPs generated in continental arc settings are not well studied. The South China Block (SCB) is characterized by the widespread development of Late Mesozoic granitoids, diorites, gabbros, and their volcanic counterparts (Figure 1a). Additionally, over 90% of these magmatic rocks are granitoids and equivalent volcanic rocks with minor basalts [Zhou et al., 2006]. The SCB is also famous for the Qing-Hang Cu-Au-Pb-Zn-Ag porphyry-skarn metallogenic belt, the Nanling W-Sn-Nb-Ta quartz vein and granite-related metallogenic belt, and the Coastal Region epithermal Au-Ag-Cu-Pb-Zn metallogenic belt [Ni et al., 2015; Pirajno and Bagas, 2002; So et al., 1998; G. G.Wang et al., 2015a; Zhao et al., 2013]. Considering the great geological and economic significance of this region, the tectonic set-

©2016. American Geophysical Union. ting of the SCB during Late Mesozoic has attracted considerable attention but remains highly debated All Rights Reserved. [Gilder et al., 1996; Hsü et al., 1988; Li and Li, 2007; Sun et al., 2007; G. G. Wang et al., 2015a; Xie et al.,

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Figure 1. (a) Sketch map showing the Late Mesozoic volcanic-intrusive complex belt in South China [after G. G. Wang et al., 2012], ZJ = Zhejiang, FJ = Fujian, GD = Guangdong, JX = Jiangxi, HN = Hunan, HB = Hubei, GX = Guangxi, GZ = Guizhou, and AH = Anhui; (b) simpliﬁed geological map of the Zhilingtou igneous rocks and associated mineralization.

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1996; Xie et al., 2001; Zhou et al., 2006]. At least three competing tectonic models have been proposed, including a collision model [Hsü et al., 1988], a mantle plume model [Xie et al., 1996; Xie et al., 2001], and a paleo-Pacific (or Izanagi) plate subduction model [Li and Li, 2007; Sun et al., 2007; Zhou et al., 2006]. Hsü et al. [1988] reported the collision model and stressed the Late Mesozoic assemblage between the Yangtze and Cathaysia blocks, but an increasing number of observations have revealed that the amal- gamation between these two blocks occurred during Neoproterozoic instead of Late Mesozoic [Zhao et al., 2013; Zheng et al., 2008]. Xie et al. [1996] and Xie et al. [2001] proposed the mantle plume model to explain the Late Mesozoic granitic magmatism in South China. However, mantle plumes generally result in the rapid formation of flood basalts [Stein and Hofmann, 1994; Tarduno et al., 1991] that are absent in the SCB [Q. Liu et al., 2012b; Zhou et al., 2006]. Given obvious drawbacks of the above men- tioned two models, most researchers tend to agree with the paleo-Pacific plate subduction model [Jiang et al., 2015; Li and Li, 2007; Sun et al., 2007; G.G. Wang et al., 2015a; Zhou et al., 2006]. The paleo-Pacific subduction model remains hotly debated regarding the initiation time and drifting directions of the paleo-Pacific plate. The subduction initiation time of the paleo-Pacific plate remains controversial, involving three different views: Li and Li [2007] proposed that the subduction started in the Late Permian and an intraplate setting developed in Yanshanian period; Jiang et al. [2015] argued that the subduction started in the Early Jurassic and then repeated advances and retreats of the slab occurred beneath the South China Block; Zhou et al. [2006] and Sun et al. [2007] proposed that the subduction of the paleo- Pacific plate beneath the SCB initiated in the Middle Jurassic. Finally, two different drifting directions of the paleo-Pacific plate were proposed. Sun et al. [2007] argued that the pacific plate drifted southwestward before circa 125 Ma instead of northwestward as proposed by Zhou et al. [2006], Li and Li [2007], and Jiang et al. [2015]. Volcanic rocks are widely distributed in Zhejiang, Fujian, and Guangdong provinces in the South China Block- Coastal Region (SCB-CR) and cover an area of 100,000 km2. Given that more than 95% magmatic rocks are felsic in composition, they may constitute the SCB-CR SLIP [Wang and Zhou, 2005]. Recently, numerous robust zircon U-Pb dating results have suggested that the volcanics in Fujian and Guangdong provinces were erupted between the Late Jurassic and the Late Cretaceous (circa 168 to 95 Ma) [Guo et al., 2012; Liu et al., 2015]. In addition, some reliable zircon U-Pb dating data were obtained from volcanics in northern and southeastern Zhejiang province and yielded ages of circa 140 to 88 Ma [L. Liu et al., 2012a]. Voluminous felsic volcanics in southwestern Zhejiang province crop out and constitute an important part of the SCB-CR SLIP, but no robust ages have been obtained from the volcanics in this region to date. It is crucial to clarify whether the felsic volcanics in southwestern Zhejiang province formed between the Middle and the Late Jurassic, as proposed by the Bureau of Geology and Mineral Resources of Zhejiang Province [1989] or the Early Cretaceous similar to the felsic volcanics in other regions of Zhejiang province [L. Liu et al., 2012a]. Moreover, the combination of recently obtained reliable dating results from volcanics can provide a clear spatiotemporal framework of the SCB-CR SLIP and deeper insights into its tectonic evolution. Epithermal deposits, first proposed by Lindgren [1933], are important sources of gold, silver, and base metals and form at shallow crustal levels (<1.5 km) and at low temperatures (<300°C) [Robb, 2005; Simmons et al., 2005]. Epithermal deposits are generally hosted by coeval or slightly older volcanic rocks. Orebodies occur in veins with steep dips that formed through dilation and extension [Simmons et al., 2005]. Such hydrother- mal systems commonly develop in continental arc, intra-arc, back-arc, and intraplate rift settings [Sillitoe and Hedenquist, 2003]. SLIPs have great economic significances and are closely linked with large-scale epithermal mineralization. The Sierra Madre Occidental SLIP in western Mexico was reported to host more than 800 Early Miocene (circa 24 to 20 Ma) epithermal Au-Ag deposits or occurrences [Camprubi et al., 2003]. The SCB-CR SLIP is also characterized by the widespread occurrence of epithermal Au-Ag deposits such as the Zijinshan, Wuziqilong, Yueyang, Dongyang, Qiucun, Taihuashan, Zhilingtou, Babaoshan, Wubu, and Dalingkou deposits [Pirajno and Bagas, 2002; Zhong et al., 2014], but the links between SLIP and large-scale epithermal mineralization remain unclear. Studies on the Zhilingtou volcanic and intrusive complex are of great significance because this complex belongs to a suite of voluminous felsic volcanics in southwestern Zhejiang province. This complex lacks robust dating constraints and hosts the largest Au-Ag deposit in Zhejiang province [The No. 7 Team of the Zhejiang Bureau of Geology and Mineral Resources, 2006]. In this contribution, we first conducted systematic LA-ICP-MS zircon U-Pb geochronology as well as elemental and Sr-Nd-Hf isotopic analyses to investigate

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Figure 2. Typical ﬁled and photomicrographs of four-stage Zhilingtou igneous rocks, including (a and b) stage 1 felsophyre, (c and d) stage 2 rhyolite, (e and f) stage 3 dacite, (g and h) stage 4 diabase, and (i and j) stage 4 felsite. Abbreviations: Pl, plagioclase; Kfs, alkaline feldspar; Aug, augite; Qz, quartz.

WANG ET AL. SLIP AND EPITHERMAL MINERALIZATION 7906 AGE L LPADEIHRA IEAIAIN7907 MINERALIZATION EPITHERMAL AND SLIP AL. ET WANG ora fGohsclRsac:SldEarth Solid Research: Geophysical of Journal Table 1. A Summary of the Fundamental Results of the Zhilingtou Volcanic-Intrusive Complex Stage Rock Type Field Occurrence Ages (Ma) Elements Major (wt %) Minor (ppm) Sr-Nd Isotopes Zircon Hf Isotopes Sample Location 87 86 1 Felsophyre Intruding Badu 149 SiO2 77.09–77.34; K2O 5.60–6.02; initial Sr/ Sr = 0.703; εHf(t)=À18.6 ~ À14.0; ZLT055 (longitude: 119° basement strata P2O5 0.01–0.02; TiO2 0.12–0.13; εNd (t)=À5.2; TDM2 = 1.36 Ga TDM2 = 2.08–2.36 Ga 25′41.5″, latitude: 28° Rb 226–302; Zr, 183–212; Ba 46–74; 36′42.5″); ZLT054-057 Sr 17–31; Eu 0.05–0.13 near-ZLT055 87 86 2 Rhyolites The lower part of 128 SiO2 67.92–76.11; K2O 4.04–6.22; initial Sr/ Sr = 0.712–0.714; εHf(t)=À13.5 ~ À1.8; ZLT199 (longitude: 119° the Moshishan Rb 111–331; MgO 0.20–1.43; εNd (t)=À6.6;TDM2 = 1.46 Ga TDM2 = 1.30–2.02 Ga 25′36.4″, latitude: 28° Group Al2O3 mostly < 12.71; Sr 11–140; 37′14.8″); ZLT201-212 Mg# 25–37; Sr/Y 0.47–12.27 near to ZLT199; ZLT259 (longitude: 119°25′48.1″, latitude: 28°37′23.3″); ZLT237- 241 near to ZLT259 87 86 3 Dacites The upper part of 111 SiO2 64.46–70.20; Al2O3 13.92–16.28; initial Sr/ Sr = 0.711–0.714; εHf(t)=À11.5 ~ À3.6; ZLT065 (longitude: 119° the Moshishan Sr 237–473; K2O 3.00–5.78; MgO 0.42–1.05; εNd (t)=À7.6 ~ À5.8; TDM2 = 1.39–1.89 Ga 25′48.9″, latitude: 28° Group Mg# 28–35; Sr/Y 9.84–18.91 TDM2 = 1.38–1.53 Ga 37′24.2″); ZLT172-176 from 38 exploration line underground tunnel at 420 m level; ZLT276-280 from 3841 underground tunnel at 380 m level 87 86 4 Felsite Cutting volcanics 100 SiO2 74.98–76.87; K2O 3.96 5.68; initial Sr/ Sr = 0.709–0.712; εHf(t)=À8.1 ~ À2.2; ZLT006-010 from of the CaO 0.24–1.20; MgO 0.09–0.26; P2O5 < 0.01; εNd (t)=À0.6 ~ +0.2; TDM2 = 1.30–1.67 Ga underground tunnel Moshishan Nb 11.3–13.8; Ti 527–686; Sr 82–157; TDM2 = 0.89–0.95 Ga along V-1 ore body at Group Ba 434–605; Mg 16–36 180 m level; ZLT137- 138 from underground tunnel along V-2 ore body at 180 m level 87 86 Diabase 101 SiO2 44.91–47.11; K2O 2.63–3.12; initial Sr/ Sr = 0.707–0.708; ZLT001-004 from 10.1002/2016JB013060 Na2O 3.14–3.43; MgO 4.37 5.59; εNd (t)=À5.0 ~ À2.2; εHf(t)=À5.0; TDM2 = 1.48 Ga underground tunnel Ba 477–2182; Sr 586–2476; Mg# 44–55; TDM2 = 1.08–1.31 Ga along V-1 orebody at Nb 13.5–15.5; Ta 0.75–0.89; Ti 816–9077 180 m level Journal of Geophysical Research: Solid Earth 10.1002/2016JB013060

the petrogenesis of the Zhilingtou igneous rocks. Together with recently published work on the SCB-CR SLIP [Guo et al., 2012; Li et al., 2014; Liu et al., 2015; L. Liu et al., 2012a], we aim to reconstruct the spatiotemporal framework of the SCB-CR SLIP and its corresponding tectonic setting. Furthermore, the links between the SCB-CR SLIP and large-scale epithermal mineralization are discussed in details.

2. Geological Background and Rock Characteristics The SCB comprises the Yangtze and Cathaysia blocks, and the two blocks are separated by the Jiang-Shao Fault (JSF). The two blocks were amalgamated together in the Neoproterozoic Jiangnan Orogeny along the JSF fault [Wang et al., 2012; Zheng et al., 2008]. The JSF fault, as a suture zone, later reactivated as a precursor to the development of the Nanhua rift basin [Wang and Li, 2003]. The Early Paleozoic Kwangsian (Wuyi-Yunkai) Orogeny is characterized by the angular unconformity between pre-Devonian deformed rocks and Devonian strata resulting from the northward underthrusting of the Cathaysia Block beneath the Yangtze Block [Charvet et al., 2010; Ting, 1929]. The SCB experienced the Triassic Indochina Orogeny in response to the collision between the Indochina and South China blocks [Carter et al., 2001] and between the South China and North China blocks [Li et al., 2001; Ratschbacher et al., 2000]. Most researchers agree that the subduction of the paleo-Paciﬁc plate beneath the South China Block occurred during the Yanshanian period (200 to 65 Ma), although disagreement exists on in terms of the details of the subduction process [Gilder et al., 1996; Jiang et al., 2015; Li and Li, 2007; Sun et al., 2007; G. G. Wang et al., 2015a; Zhou et al., 2006]. The study area is located in the western region of Zhejiang province in the Cathaysia Block. The regional basement includes the Badu Group (or Badu Complex), which consists of sillimanite-mica schists, amphibolites, and gneisses, and represents the oldest rocks known in the Cathaysia Block [Yu et al., 2012]. Zircon U-Pb dating and Lu-Hf isotopic analysis of the Badu complex, coeval granitoids, and intraplate basaltic rocks suggest that two major episodes of crustal growth occurred in this region at 1.8–1.9 Ga and 2.7–2.9 Ga [Li, 1997; Yu et al., 2012; Zhao et al., 2014]. The cover lithologies are Yanshanian (200 to 65 Ma) felsic volcanic rocks, which can be further divided into lower and upper volcanic series [Gu, 2005]. The lower volcanic series, i.e., the Moshishan Group, are subdivided into the Dashuang, Gaowu, Xishantou, Chawan, and Jiuliping formations. The lower volcanic series are widespread with a total outcrop area of more than 40,000 km2 and exhibit close spatial and genetic relationships with Au-Ag polymetallic deposits such as the Zhilingtou, Babaoshan, Wubu, and Dalingkou deposits [Pirajno et al., 1997].The upper volcanic series, i.e., Yongkang Group, are scattered in dozens of basins and can be subdivided into the Guantou, Chaochuan, and Fangyan formations [The Bureau of Geology and Mineral Resources of Zhejiang Province, 1989]. The studied rock samples were collected from the Zhilingtou Au-Ag polymetallic deposit, which has 24.5 metric ton of Au at an average grade of 12.94 g/t and 527.7 t of Ag at an average grade of 278 g/t. The rock units in the Zhilingtou deposit include basement rocks of the Badu Group and cover rocks of the Moshishan Group (the lower volcanic series) (Figure 1b). The lower part of the Moshishan Group contains medium-coarse grained quartz sandstones and tuffaceous sandstones with rhyolitic tuff interlayers at the base and rhyolitic tuff breccia at the top. The upper part of the Moshishan Group simply contains a dacitic volcanic succession. In addition to volcanics, at least two stages of Late Mesozoic intrusive rocks occur in the Zhilingtou deposit. A felsophyre occurs sporadically in the basement units of the Badu Group, and the crosscutting relationships between volcanics and felsophyre are unclear. Bimodal intrusive rocks including felsite and diabase dykes intruded felsic volcanics of the Moshishan Group. The felsophyres are composed of K-feldspar, quartz, plagioclase, and biotite (Figures 2a and 2b). The rhyolites exhibit porphyritic textures and consist mainly of quartz, alkaline feldspar, plagioclase, and biotite (Figures 2c and 2d). The dacites display a porphyritic texture and contain plagioclase, quartz, alkaline feldspar, biotite, and hornblende (Figures 2e and 2f). The felsic member of the bimodal rocks is mainly composed of quartz, alkaline feldspar, and plagioclase with minor amounts of biotite, whereas the maﬁc member is composed of biotite, plagioclase, hornblende, K-feldspar, and quartz (Figures 2g–2j).

3. Sampling and Analytical Methods Zircons for cathodoluminescence (CL) imaging, U-Pb dating, and Lu-Hf isotopic studies were collected from eight samples of the Zhilingtou volcanic and intrusive rocks. Thirty-six samples were chosen for

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the analysis of the whole-rock major and trace elements, and nine samples were selected for the analysis of the Rb-Sr and Sm-Nd isotopes. A summary of the Zhilingtou igneous rocks is presented in Table 1. The supporting information provides data tables listing the zircon U-Pb isotope (Table S1), major and trace element (Table S2), Sr and Nd isotope (Table S3), and zircon Hf isotope results (Table S4).

The zircon grains for LA-ICP-MS U-Pb dating were separated from the selected igneous rocks using con- ventional heavy liquid and magnetic separation methods. Representative zircon grains were handpicked under a binocular microscope, mounted on an epoxy resin disk and polished down to nearly half the sec- tion to expose internal structures for LA-ICP-MS analysis. Zircons were documented with transmitted and reﬂected light photomicrographs and CL images to reveal their internal structures. The mounts were vacuum coated with high-purity gold prior to examination to avoid charging. The CL images were taken with the scanning electron microscope at the State Key Laboratory of Continental Dynamics at Northwest University, and in situ LA-ICP-MS zircon U-Pb dating was performed using an Agilent 7500 ICP-MS, coupled with a 213 nm New Wave laser microprobe at the State Key Laboratory for Mineral Deposit Research at Nanjing University. Analytical procedures are described by Jackson et al. [2004]. Common Pb was corrected according to the method proposed by Andersen [2002]. The analyses presented here have been corrected assuming recent lead loss. Data processing was carried out using Isoplot programs of Ludwig [2001].

Samples for elemental and Sr-Nd isotopic analysis were initially examined by optical microscopy to select unaltered or the least altered samples. Major element contents of the samples were determined at the State Key Laboratory for Mineral Deposit Research at Nanjing University using an X-ray ﬂuorescence spectrometer, with a precision of better than 5%. Trace elements, including rare earth elements (REEs), were analyzed using a Finnigan Element II ICP-MS at the State Key Laboratory for Mineral Deposit Research at Nanjing University following procedures described by Gao et al., [2003]. The analytic precision for trace and rare earth elements is better than 10%.

Sr and Nd isotopic compositions were measured using a Triton Thermal Ionization Mass Spectrometer at the State Key Laboratory for Mineral Deposit Research at Nanjing University, following the methods of by Pu et al. [2005]. 87Sr/86Sr and 143Nd/144Nd ratios are normalized to a 86Sr/88Sr value of 0.1194 for Sr and to a 146Nd/ 144Nd value of 0.7219 for Nd. During the analysis, measurements resulted in a 143Nd/144Nd ratio of 0.511842 Æ 4(2σ, n = 5) for the La Jolla standard (recommended value = 0.511850) and 87Sr/86Sr ratio of 0.710260 Æ 10 (2σ, n = 30) for the NBS-987 Sr standard (recommended value = 0.710245). Total analytical À À blanks were 5 × 10 11 g for Sm and Nd and 2 to 5 × 10 10 g for Rb and Sr.

The zircon Hf isotopic analysis was conducted in situ using a New Wave UP213 laser ablation microprobe, attached to a Neptune multicollector ICP-MS at the Institute of Mineral Resources at Chinese Academy of Geological Sciences in Beijing. Instrumental conditions and data acquisition were comprehensively described by Hou et al., [2007] and Wu et al., [2006]. A stationary spot was used for the present analysis, with a beam diameter of 40 or 55 μm depending on the size of ablated domains. The zircon GJ1 was used as a reference standard during our routine analysis, with a weighted mean 176Hf/177Hf ratio of 0.281998 Æ 0.000024 (2σ, n = 14). This value is undistinguishable from a weighted mean 176Hf/177Hf ratios of 0.282000 Æ 0.000005 176 177 (2σ) using the solution analysis method described by Morel et al. [2008]. Initial Hf/ Hf ratios εHf(t) were calculated with the reference to the Chondritic Uniform Reservoir (CHUR) at the time of zircon growth from À À the magma. The adopted decay constant for 176Lu is 1.865 × 10 11 yr 1 [Scherer et al., 2001], and the chondritic176Hf/177Hf ratio of 0.282772 and 176Lu/177Hf ratio of 0.0332 are from Blichert-Toft and Albarède, 176 177 [1997]. Two-stage model ages (TDM2) were calculated assuming Lu/ Hf = 0.015 for the average continental crust [Grifﬁnetal., 2002].

4. Results 4.1. Zircon Petrography and U-Pb Dating 206 238 CL images of the representative zircon grains with Pb/ U ages and εHf(t) values are shown in Figure 3. Zircons are generally euhedral, prismatic (ranging from short to long), colorless, and transparent. The crystal grains range from 60 to 240 μm in length, with axial (length/width) ratios varying from 1:1 to 3:1. Most of the

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Figure 3. Typical CL images and LA-ICP-MS U-Pb concordant curves for zircons from the Zhilingtou igneous rocks.

zircons show oscillatory or planar zoning in the CL images (Figure 3), similar to zircons observed in typically magmatic rocks [Hoskin and Schaltegger, 2003]. LA-ICP-MS zircon U-Pb results of igneous rock samples are summarized in Tables 1, S1, and Figure 3. We identify four stages of magmatism at Zhilingtou. Thirty-two results from stage 1 felsophyre dikes are close to the concordia curve and yield a weighted mean 206Pb/238U age of 149 Æ 1Ma(n = 32, mean square [sum of] weighted deviation (MSWD) = 0.63). Twenty-eight analytical spots from stage 2 rhyolites have a weighted mean 206Pb/238U age of 128 Æ 1 Ma (n = 28, MSWD = 1.01). Twenty-ﬁve spots from stage 3 dacite were concentrated on a concordant curve and show a weighted mean 206Pb/238U age of 111 Æ 1Ma (n = 25, MSWD = 0.26). Bimodal rocks formed in stage 4 have a weighted mean 206Pb/238U age of 101 Æ 1Ma(n = 8, MSWD = 0.06) and a weighted mean 206Pb/238U age of 100 Æ 2Ma(n = 13, MSWD = 2.7) for diabase and felsite, respectively.

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Figure 4. (a) TAS diagram [after Middlemost, 1994], (b) SiO2 versus Zr/TiO2 classiﬁcation diagram after [Winchester and Floyd, 1977], (c) K2O versus SiO2 [after Le Maitre et al., 1989], (d) Mg# versus SiO2, data sources: G. G. Wang et al. [2014] and the references therein, (e) Sr/Y versus Y [after Defant et al., 2002], (f) P2O5 versus SiO2, and (g and h) Nb versus 10,000*Ga/Al and Rb/Ba versus Zr + Ce + Y [after Whalen et al., 1987] for the igneous rocks at Zhilingtou.

The presented zircon U-Pb dating results suggest that four stages of volcanic and intrusive rocks in the Zhilingtou area formed during the Late Jurassic to Early Cretaceous (circa 149 to 100 Ma). Volcanic rocks formed in stages 2 and 3 and erupted at circa 128 Ma and 111 Ma, respectively. Intrusive rocks formed in stages 1 and 4 and intruded at circa 149 and 100 Ma, respectively.

4.2. Whole-Rock Elements The whole-rock major and trace element data are presented in Table S2 and Figure 4, with 4 samples from stage 1 felsophyre, 11 samples from stage 2 rhyolite, 9 samples from stage 3 dacite, and 5 and 7 samples from stage 4 diabase and felsite, respectively.

Samples of stage 1 felsophyre have very high concentrations of SiO2 (77.09 to 77.34 wt %), K2O (5.60 to 6.02 wt %), Rb (226 to 302 ppm), and high ﬁeld strength elements (HFSEs, e.g., Zr, with values of 183 to

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Figure 5. Normalized REE patterns and trace element spidergrams for the volcanic-intrusive rocks in the Zhilingtou deposit. Chondrite and primitive mantle data are from Boynton [1984] and Sun and McDonough [1989], respectively. Average OIB, OAB, and CAB data are from Sun and McDonough [1989] and Kelemen et al. [2003]. Abbreviations: OIB oceanic island basalt, OAB oceanic arc basalt, and CAB continental arc basalt.

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212 ppm), but low contents of TiO2 (0.12 to 0.13 wt %), P2O5 (0.01 to 0.02 wt %), Ba (46 to 74 ppm), Sr (17 to 31 ppm), and Eu (0.05 to 0.13 ppm) (Tables 1 and S2). These samples plot in the region of granites on the TAS diagram (Figure 4a). They display obvious enrichment in light rare earth elements (LREEs) and marked Eu depletions in the REE patterns (Figure 5a). In spidergrams, they show negative Ba, Nb, Sr, Eu, and Ti anomalies and positive Pb anomalies (Figure 5b). Elemental data suggest that stage 1 felsophyre can be classiﬁed as a highly fractionated I-type granites [Wu et al., 2003] (Figures 4e and 4f).

Samples of stage 2 rhyolite have high SiO2 (67.92 to 76.11 wt %), K2O (4.04 to 6.22 wt %), and Rb (111 to 331 ppm) contents but low MgO (0.20 to 1.43 wt %) and Al2O3 (mostly less than 12.71 wt %), Sr (11 to 140 ppm) contents, low Mg# (25 to 37) values, and low Sr/Y (0.47 to 12.27) ratios (Tables 1 and S2). They plot

in the domains of rhyolite and rhyodacite in the SiO2 versus Zr/TiO2*0.0001 diagram and belong to the high-K calc-alkaline series (Figures 4b and 4c). All the samples fall into the low Mg# region (Figure 4d). REE patterns exhibit a certain degree of LREE enrichment and negative Eu anomalies (Figure 5c). The spidergrams show that they have typical arc magma geochemical afﬁnities, such as clear enrichments in large ion lithophile elements (LILEs) and Pb, but relative depletions of Nb, Ta, and Ti (Figure 5d) [Rudnick and Gao, 2003].

Sample of stage 3 dacite shows intermediate contents of SiO2 (64.46 to 70.20 wt %), Al2O3 (13.92 to 16.28 wt %), and Sr (237 to 473 ppm), high contents of K2O (3.00 to 5.78 wt %), but low contents of MgO (0.42 to 1.05 wt %), low Mg# (28 to 35) values, and low Sr/Y (9.84 to 18.91) ratios (Tables 1 and S2). They fall into

the dacite domain in the SiO2 versus Zr/TiO2*0.0001 diagram and belong to the high-K calc-alkaline to shoshonitic series (Figures 4b and 4c). REE patterns exhibit a certain degree of LREE enrichment with no obvious Eu anomalies (Figure 5e). Similar to stage 2 volcanics, stage 3 volcanic rocks also show low Mg# values (Figure 4d), and an arc magma geochemical signature (e.g., Nb, Ta, and Ti depletions but Pb enrich-

ment) (Figure 5f). However, unlike stage 2 volcanics, stage 3 volcanic rocks have higher P2O5, Sr, and Ba contents but lower SiO2 and Rb contents (Table S2 and Figure 4e).

Samples from stage 4 diabase have low SiO2 abundances of 44.91 to 47.11 wt % and plot in the basalt region in the SiO2 versus Zr/TiO2*0.0001 diagram and in the monzodiorite domain in the TAS diagram (Figures 4a and 4b). They have high contents of K2O (2.63 to 3.12 wt %), Na2O (3.14 to 3.43 wt %), MgO (4.37 to 5.59 wt %), Ba (477 to 2182 ppm), and Sr (586 to 2476 ppm) and high Mg# (44 to 55) values, but low contents of Nb (13.5 to 15.5 ppm), Ta (0.75 to 0.89 ppm), and Ti (8163 to 9077 ppm) (Figures 4c and 4e and Tables 1 and S2). REE and trace element diagrams show that these samples are enriched in LREEs, LILEs (e.g., Ba and Sr), and Pb but depleted in Nb, Ta, and Ti (Figures 5g and 5h), suggesting a subduction-related metasomatized magma source [Sun and McDonough, 1989].

Samples from stage 4 felsite have high abundances of SiO2 (74.98 to 76.87 wt %) and K2O (3.96 5.68 wt %) but low abundances of CaO (0.24 to 1.20 wt %), MgO (0.09 to 0.26 wt %), P2O5 (<0.01 wt %), Nb (11.3 to 13.8 ppm), Ti (527 to 686 ppm), Sr (82 to 157 ppm), and Ba (434 to 605 ppm) as well as Mg values (16 to 36) (Tables 1 and S2). They plot in the granite domain in the TAS diagram and belong to the high-K calc-alkaline series (Figures 4a and 4c). They show enrichment in LREEs and a certain degree of negative Eu anomalies (Figure 5i). Compared with stage 1 felsophyre, stage 4 felsite lacks strong Eu, Sr, and Ba depletions, indicating a relatively low degree of magma fractionation (Figures 4e and 4f). These samples show negative anomalies in Ba, Nb, Sr, and Ti and positive anomalies in Pb in spidergrams (Figure 5j), suggesting a subduction-related metasomatized magma source.

4.3. Whole-Rock Sr-Nd Isotopes Whole-rock Rb-Sr and Sm-Nd isotopic compositions were determined for nine samples. These measured data and calculated initial 87Sr/86Sr and 144Nd/143Nd ratios are presented in Tables 1 and S3 and Figure 6. The sample from stage 1 felsophyre has low initial 87Sr/86Sr ratios of 0.703 in good agreement with its extremely high

Rb/Sr ratio of 13.5. Stage 1 felsophyre has a εNd(t = 149 Ma) value of À5.2 and a two-stage Nd model age (TDM2) of 1.36 Ga (Table S3). Rhyolite and dacite samples from stages 2 and 3 have similar Sr-Nd isotopic com- 87 86 positions, with initial Sr/ Sr ratios of 0.711 to 0.714, negative εNd(t = 128–111 Ma) values of À7.6 to À5.8, and two-stage Nd model ages (TDM2) of 1.38 to 1.53 Ga (Table S3 and Figure 6). Compared with stages 2 and 3 rocks, the bimodal rock samples from stage 4 have similar initial 87Sr/86Sr ratios of 0.707 to 0.712 but

higher εNd(t = 111–100 Ma) values of À5.0 to +0.2 and lower two-stage Nd model ages (TDM2) of 0.89 to 1.31 Ga (Table S3 and Figure 6).

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4.4. Zircon Hf Isotopes Eight samples that were dated using the zircon U-Pb method were also analyzed by LA-MC-ICP MS to measure the Lu-Hf isotopes in the same domains or similar structures. Initial 176Hf/177Hf ratios

denoted by εHf(t) values and two-stage Hf model ages for the analyzed zircons were calculated using weighted mean age. The zircon Hf isotopic data are listed in Tables 1 and S4 and Figure 7. Eighteen analyses for stage 1 felsophyre

yield very negative εHf(t) values of À18.6 to À14.0 (Figure 7a). They correspond to 87 86 Figure 6. εNd(t) versus ( Sr/ Sr)i diagram showing the isotopes for two-stage Hf model ages (TDM2) ranging the volcanic-intrusive rocks in the Zhilingtou deposit. Data for the Dexing juvenile crust-derived felsic rocks are taken from G. G. Wang from 2.08 to 2.36 Ga (Table S4). Zircon et al. [2015a]. Data for MORB and the approximate ﬁelds of mantle grains from stage 2 rhyolite exhibit rela- reservoirs are taken from Zindler and Hart [1986] and references tively higher negative εHf(t) values of therein. Data for the Darongshan S-type granite are taken from Qi À13.5 to À1.8 (Figure 7b). et al. [2007]. Correspondingly, their two-stage Hf model

ages (TDM2) range from 1.30 to 2.02 Ga (Table S4). Twenty-three analyses for stage 3 dacite show Hf isotopic compositions similar to the analytical data for stage 2 rhyolite. Stage 3 dacite has

negative εHf(t) values of À11.5 to À3.6 and two-stage Hf model ages (TDM2) ranging from 1.39 to 1.89 Ga (Figure 7c and Table S4). Zircon grains from stage 4 bimodal rocks have slightly higher εHf(t) values, with εHf(t) values of À8.1 to À2.2 (Figure 7d). The corresponding two-stage Hf model ages (TDM2) range from 1.30 to 1.67 Ga (Table S4).

Figure 7. Histograms of in situ zircon εHf(t) values of the volcanic-intrusive rocks in the Zhilingtou deposit.

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5. Discussion 5.1. Petrogenesis of the Zhilingtou Volcanic-Intrusive Complex 5.1.1. Hf and Nd Isotopes as Indicators of Crustal Residence Time and Their Decoupling The variations in Hf and Nd isotopic compositions have been well developed as an unambiguous petrogenetic indica- tor to decipher the continental crustal growth and evolution because of radio- active decay of 147Sm and 176Lu [DePaolo et al., 1991; Harrison et al.,

2005; Vervoort et al., 1996]. εHf(t) and εNd(t) values have generally been used to indicate the deviation of the Figure 8. Diagram of εHf(t) versus U-Pb ages for zircons from the Zhilingtou volcanic and intrusive rocks. The full line of crustal extraction 176Hf/177Hf and 143Nd/144Nd values of 176 177 is calculated by using Lu/ Hf ratio of 0.015 for the average conti- the sample from that of the CHUR in nental crust. The data for the basement in East Cathaysia are from Xisheng units of parts in 104, respectively Xu et al. [2007]. Symbols are the same as those in Figure 4. [DePaolo, 1988; Vervoort et al., 2000]. Generally speaking, the Hf and Nd isotopes of igneous rocks derived from recycled ancient continental crustal rocks with low Sm/Nd and

Lu/Hf ratios have negative εHf(t) and εNd(t) values, while those of igneous rocks derived from the depleted mantle with high Sm/Nd and Lu/Hf ratios have positive εHf(t) and εNd(t) values [Flierdt et al., 2002; Harrison et al., 2005]. The depleted mantle model age (TDM) has been used to quantitatively constrain the crustal residence time of magma source of depleted mantle-derived rocks that generally have positive εHf(t) and εNd(t) values [DePaolo et al., 1991]. In contrast, the two-stage model age (TDM2) assumes that the parental magma was produced from ancient continental crust that commonly has negative εHf(t) and εNd(t) values [Vervoort and Blichert-Toft, 1999]. Given that the Zhilingtou igneous rocks have negative εHf(t) and εNd(t) values (Tables 1, S3 and S4), we use the two-stage model age (TDM2) for zircon Hf and whole-rock Nd isotopes to express the crustal residence time. As summarized in Table 1, the two-stage whole-rock Nd model ages of the Zhilingtou volcanic-intrusive com-

plex are clearly lower than the two-stage zircon Hf model ages (TDM2). Therefore, it is necessary to determine which isotope system is reliable before using them to discuss the magma source signature. The Nd isotope disequilibrium is common between crustal protoliths and melts formed by crustal anatexis, because the REE budget (e.g., Nd) can be controlled by accessory phases (e.g., garnet, apatite, and monazite) [Ayres and Harris, 1997; Davies and Tommasini, 2000]. For example, the batch melting modeling implied that the proto- lith with a real model age of 1.7 Ga yielded melts with an unrealistic estimated Nd model ages ranging from 0.66 to 2.24 Ga due to the Nd isotope disequilibrium [Davies and Tommasini, 2000]. In contrast, zircon Hf isotopes are much more reliable because (1) zircon has high Hf concentrations and low Lu/Hf ratios; (2) the crystallization age can be directly dated by zircon U-Pb techniques; (3) zircon is extremely resistant to Lu/Hf disturbance; and (4) it is easy to monitor whether the zircon Lu-Hf isotopic system is closed via the combined use of the U-Pb system [Amelin et al., 1999; Kemp et al., 2006; Vervoort et al., 1996]. As a consequence, in this

study, we used zircon two-stage Hf model age (TDM2) in order to trace the magma source and origin of igneous rocks. 5.1.2. Petrogenesis of Stage 1 Felsophyre

Stage 1 felsophyre has very negative εHf(t) values of À18.6 to À14.0 and relatively old two-stage Hf model ages (TDM2) from 2.08 to 2.36 Ga, suggesting that it is primarily derived from ancient rather than juvenile crust [G. G. Wang et al., 2014; Xiong et al., 2014]. Detrital zircons from the Oujiang River in the East Cathaysia Block indicate that the basement in Zhejiang province is dominantly Paleoproterozoic in age (1. 85 to 2.40 Ga) with minor Archean components [Xu et al., 2007]. The zircon Hf isotopes from stage 1 felsophyre plot in the area of crustal evolution of the East Cathaysia Block (Figure 8), indicating that stage 1 felsophyre was likely generated by melting of Cathaysia Block continental crust. Together with

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the low MgO contents and Mg# values (Table 1) of the Zhilingtou stage 1 felsophyre, few mantle materials were involved in parental magmatic melt.

Stage 1 felsophyre has very high contents of SiO2, Rb, and HFSEs (e.g., Zr, Nb, Ce, and Y) but extremely low

contents of P2O5, Eu, Ba, and Sr, indicating that it can be classified as a highly fractionated I-type granite (Table S1 and Figures 4 and 5). Essential minerals in stage 1 felsophyre are quartz, perthite, and plagioclase with minor biotite, implying that magmas of stage 1 felsophyre experienced significant fractionation. Strong fractional crystallization is demonstrated by significant depletions in Ba, La, Ce, Sr, Eu, and Ti in the spidergrams (Figures 5a and 5b). The obvious Eu, Sr, and Ba depletions likely result from extensive fractionation of plagioclase and K-feldspar, the strong Nb and Ti negative anomalies require Ti-bearing phase separation (e.g., ilmenite and titanite), and the clear P depletion is related to apatite separation [G. G. Wang et al., 2014; Wu et al., 2003]. As shown by the Sr-Ba diagram (Figure 9a and Table S5), the stage 1 felsophyre data plot in the region among the plagioclase, K-feldspar, and biotite evolutional lines. Because K-feldspar increases during magma differentiation, K-feldspar is likely not the component responsible for the depletion of Sr and Ba [Wu et al., 2003]. The reasonable explanation is that the fractionation of plagioclase occurred during the early stage and the fractionation of plagioclase and biotite developed during the late stage. Additionally, the accessory minerals appear to affect much of the

REE variation. In the diagram of (La/Yb)N versus La, the variation in REE contents seems to be consistent with the separation of monazite and allanite, apatite, and sphene but is not inﬂuenced by zircon separation (Figure 9b and Table S6). 5.1.3. Petrogenesis of Stages 2 and 3 Felsic Volcanics Maﬁc microgranular enclaves (MMEs) occur in some Late Mesozoic igneous rocks in the SCB-CR [Li et al., 2014; Xu et al., 1999], and these igneous rocks com-

monly have higher zircon εHf(t) and Figure 9. Modeling fractionation process during the generation of whole-rock εNd(t) values than basement stage 1 felsophyre in the Zhilingtou deposit. (a) Ba versus Sr diagram rocks in the Cathaysia Block [Guo et al., showing the separation of plagioclase and biotite. Partition coefficients 2012; Li et al., 2014; Liu et al., 2015], indi- of Sr and Ba are from Hanson [1978]. (b) (La/Yb)N versus La diagram showing separation of accessory minerals, especially allanite, monazite, cating that a magma mixing process and apatite. Partition coefficients are from Arth [1976] for apatite, Green occurred. However, the lack of MMEs in and Pearson [1986] for titanite, Mahood and Hildreth [1983] for zircon, the Zhilingtou felsic volcanics strongly Green et al. [1989] for allanite, Yurimoto et al. [1990] for monazite, and argues against magma mixing. In Wu et al. [2003] for hornblende. Abbreviations: Cpx = clinopyroxene, Opx = orthopyroxene, Hbl = hornblende, Bi = biotite, Pl = plagioclase, addition, the Zhilingtou felsic volcanics Kf = K-feldspar; Ap = apatite, Allan = allanite, Sph = sphene, have a low-Mg geochemical signature Mon = monazite, Zr = zircon. The blue filled diamond symbols are the (Table 2 and Figure 4d), precluding the fi same as those in Figure 4, and the red lled yellow square are possibility of a significant input of samples from the selected candidate representing parent magma mantle-derived mafic melts. Thus, magma [G. G. Wang et al., 2014]. Data for modeling fractional crystallization of stage 1 felsophyre are listed in Tables S5 and S6. Modeling procedure mixing is unlikely to have generated the for fractional crystallization is described in Text S1. Zhilingtou stages 2 and 3 felsic volcanics.

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The low-Mg geochemical signature strongly argues against the direct contribution of mantle materials into

these felsic volcanics, whereas juvenile continental crust is a good candidate to explain the higher εHf(t) values [Vervoort and Patchett, 1996; G. G. Wang et al., 2015a; Zheng et al., 2008]. Growth of the juvenile crust likely occurred during the Early Cretaceous in the SCB-CR. First, most of Hf isotope data from stages 2 and 3

felsic volcanics display systematically higher zircon εHf(t) values (À13.5 to À1.8) than those from stage 1 felsophyre (Table S2) and basement rocks in the Cathaysia Block [Xu et al., 2007]. As shown in the t-εHf(t) diagram (Figure 8), stages 2 and 3 felsic volcanic samples plot above the evolutionary trend of the basement rocks in the Cathaysia Block. Second, the Early Cretaceous igneous rocks in the SCB-CR have the youngest Hf-Nd model ages and lowest initial Sr isotopes among those in South China, indicating that the input of the asthenospheric mantle materials resulted in significant crustal growth [Gilder et al., 1996]. The newly underplated juvenile crustal materials during the Early Cretaceous beneath the SCB-CR could be gabbroic sills in compositions near the crust-mantle transition zone similar to those described in the Oman ophiolites [Kelemen et al., 1997]. This phenomenon is supported by the fact that mantle-derived mafic magmas are denser than most crustal rocks and therefore tend to pool near the base of the crust to form an underplated layer [Richards, 2003]. Besides as part of magma source materials, the underplated basaltic magma likely also provides a significant amount of heat for crustal anatexis and the eruption of large-scale felsic volcanism [Wilkinson, 2013]. Therefore, partial melting of the mixture of newly underplated juvenile and ancient continental crust materials likely explains the genesis of stages 2 and 3 low-Mg felsic volcanics. Stages 2 and 3 volcanics show various major and trace element concentrations, which can be explained by

source differences or magmatic processes. They share similar εHf(t) values and two-stage Hf models ages, suggesting that they have similar magma sources and probably have constant proportions of juvenile and ancient crustal materials. Thus, magmatic processes play a crucial role in the changes in the geochemical compositions of these felsic volcanics. The observed variations in the magma compositions can be generated by different degrees of magma fractionation or partial melting. Figure 10 displays data from stage 2 and 3 on Zr/Nb versus Zr and La/Sm versus La diagrams. In both diagrams, the data show increasing Zr/Nb and La/Sm ratios with increasing Zr and La content, respectively, revealing geochemical trends that are consistent with partial melting rather than fractional crystallization. Hence, we conclude that partial melting is the predominant process responsible for the formation of the Zhilingtou felsic volcanics. 5.1.4. Petrogenesis of Stage 4 Bimodal Rocks

Compared to the Zhilingtou intermediate to acid rocks, stage 4 diabase has lower concentrations of SiO2 (44.91 to 47.11 wt %) and higher MgO (4.37 to 5.59 wt %), indicating that it is the most maﬁc rocks in the Zhilingtou region. Stage 4 diabase has trace element patterns similar to those of OIBs, except for the deple-

tions in Nb, Ta, and Ti. Additionally, the εNd(t) and εHf(t) values of the diabase are higher than those of the enriched mantle end-members (EMI or EMII, Figure 6). The high incompatible element contents and εNd(t) and εHf(t) values reveal that a signiﬁcant contribution of the depleted asthenospheric mantle. In addition, stage 4 diabase has much lower εNd(t) and εHf(t) values than typical the depleted asthenospheric mantle, indicating strong metasomatism likely occurred in the mantle source region or crustal contamination took place the ascent of mantle-derived magma to the shallow crustal environments. Stage 4 diabase has very high Sr

Figure 10. Partial melting discriminative diagrams for the Zhilingtou felsic volcanics. (a) Zr/Nb versus Zr diagram; (b) La/Sm versus La diagram. Abbreviations: PM = partial melting; FC = fractionation crystallization. Symbols are the same as those in Figure 4.

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contents and lacks a negative Ta anomaly, which is a contrast to the coeval crust-derived felsite, suggesting that it experienced little crustal contamination. The enrichments in LILEs (Ba, Th, Pb, La, Ce, etc.) and depletions in the HFSEs (Nb, Ta, Ti, etc.) of stage 4 diabase are similar to average compositions of arc basalts (Figures 5g and 5h), suggesting that the mantle source region was metasomatized in an arc or back-arc setting. An arc or back-arc setting for stage 4 diabase is consistent with by the NW directed subduction of the paleo-Paciﬁc plate beneath the South China along its southeastern margin during Late Mesozoic [Sun et al., 2007; G. G. Wang et al., 2015a; Zhou et al., 2006]. The absence of a negative Eu anomaly in the chondrite-normalized REE patterns and the presence of a positive Sr anomaly in the spidergrams preclude signiﬁcant fractional crystallization of plagioclase, indicating that little magma fractionation occurred during the generation of stage 4 diabase.

Similar to stages 2 and 3 felsic volcanics, stage 4 felsite has higher εHf(t) values than basement rocks in the Cathaysia Block (Figures 7 and 8), implying involvement of newly underplated juvenile crust or mantle materials as magma sources. The extremely low Mg content of stage 4 felsite (Table 1) strongly argues against a mantle source but supports a mixed source involving newly underplated juvenile crust and ancient continental crust. Partial melting of the juvenile continental crust likely resulted from upwelling of coeval hot basaltic

melts of metasomatized mantle. The high SiO2 contents of the stage 4 felsite commonly indicate a high degree of fractionation. However, the weak Eu and Sr negative anomalies in the normalized REE patterns and spidergrams (Figures 5i and 5j) suggest no strong feldspar fractionation occurred. Thus, partial melting and slight fractionation are likely responsible for the formation of stage 4 felsite.

5.2. Spatiotemporal Distribution of the SCB-CR SLIP The formation ages of the SCB-CR SLIP have drawn great attention over the past few decades because of their tectonic signiﬁcance and their relation to large-scale epithermal mineralization. Geologists originally proposed that abundant volcanic successions in Zhejiang, Fujian, and Guangdong provinces mainly formed during the Middle to Late Jurassic, with minor episodes during the Cretaceous [The Bureau of Geology and Mineral Resources of Fujian Province, 1985; The Bureau of Geology and Mineral Resources of Zhejiang Province, 1989], based on various methods such as K-Ar, 40Ar-39Ar, Rb-Sr, and U-Pb methods on single minerals or whole rocks [Li et al., 1989; L Liu et al., 2012; Yang et al., 2008; Yu and Xu, 1999]. However, the relatively low closure temperatures do not preclude resetting of both K-Ar and Rb-Sr isotope systems associated with later thermal disturbances [Dodson, 1973; Li et al., 1999]. In contrast, the U-Pb isotopic analysis on individual zircon is regarded as a reliable method due to higher closure temperatures [Cliff, 1985]. Recently, some Jurassic to Cretaceous ages have been reported based on SHRIMP or LA-ICP-MS single grain zircon U-Pb analysis of felsic volcanics from Guangdong and Fujian provinces [Guo et al., 2012; Liu et al., 2015]. In addition, felsic volcanics from northern and southeastern Zhejiang province are dated to yield a Cretaceous formation age (circa 140 to 88 Ma) [L. Liu et al., 2012a]. The dating results of the Zhilingtou rhyolites and dacites suggest that voluminous felsic volcanics of the Shimaoshan Group in southwestern Zhejiang province formed during Early Cretaceous (circa 128 to 110 Ma). Therefore, our results provide a robust age constraint on voluminous felsic volcanics throughout Zhejiang province and suggest that the formation ages are Early Cretaceous to early Late Cretaceous instead of the previously proposed Middle to Late Jurassic formation ages. The data on volcanics in southwestern Zhejiang (e.g., the Zhilingtou volcanics), together with recently obtained data on volcanics from northern and southeastern Zhejiang, Guangdong, and Fujian provinces [Guo et al., 2012; L. Liu et al., 2012a, 2015], provide a great opportunity to reconstruct the spatiotemporal framework of the entire SCB-CR SLIP. In the following we provide a summary of the three episodes of volcanism that occurred in the SCB-CR. The ﬁrst volcanic sequence formed during the Middle to Late Jurassic and is characterized by minor occurrences of felsic volcanic rocks in northeastern Guangdong and southern Fujian provinces (168–145 Ma) [Guo et al., 2012] as well as minor dacite and felsophyre in southeastern Zhejiang province (177 Ma) [L. Liu et al., 2012a; this study]. The second Early Cretaceous felsic volcanic succession is predominant and constitutes the main part of the SCB-CR SLIP in Zhejiang and northern Fujian provinces (143–110 Ma) [Guo et al., 2012; L. Liu et al., 2012a, 2015; this study]. The third episode of Late Cretaceous bimodal volcanic-intrusive rocks occur along the easternmost coastline in the SCB-CR, and our dating results suggest that they formed during 110 to 90 Ma [Liu et al., 2015; this study]. The new spatiotemporal framework is characterized by minor Middle to Late Jurassic felsic volcanic rocks in northeastern Guangdong and southern Fujian provinces, voluminous Early Cretaceous volcanics in Zhejiang and northern

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Fujian provinces, and minor Late Cretaceous bimodal volcanic-intrusive rocks along the easternmost coastline of SCB. The newly established spatiotemporal framework of the SCB-CR SLIP demonstrates a northwestward migration of the volcanic activity during 170 to 110 Ma followed by an eastward migration during 110 to 90 Ma.

5.3. Tectonic Evolution of the SCB-CR SLIP Although most current studies support a subduction model of the paleo-Pacific plate along the margin of the South China Block during Late Mesozoic [Jiang et al., 2015; Li and Li, 2007; Q. Liu et al., 2012b; Sun et al., 2007; Zhou et al., 2006], two key issues including the subduction initiation time and the principal drifting direction of the subducting plate remain controversial. The subduction of the paleo-Pacific plate has been suggested by some authors to have started in the Late Permian (circa 250 Ma) [Li and Li, 2007] or Early Jurassic (circa 197 Ma) [Jiang et al., 2015]. However, the South China Block is characterized by ~ E-W tectonic elements in the Triassic and Early Jurassic [Zhang et al., 2009] and this orientation is inconsistent with the coeval westward subduction of the paleo-Pacific plate. Additionally, the voluminous felsic arc volcanics in the SCB-CR SLIP were produced during the Middle Jurassic to Early Cretaceous (Figure 11) rather than the Late Permian or Early Jurassic. Furthermore, the absence of the contemporaneous accretionary wedge along the margin of the South China Block strongly argues against the Late Permian or Early Jurassic initial subduction. The initiation of the paleo-Pacific plate subducting beneath the South China Block most likely occurred in the Middle Jurassic [Dong et al., 2008; Isozaki, 1997; G. G. Wang et al., 2015a; Y. Wang et al., 2015b; Zhou et al., 2006]. This interpretation is supported by the following observations: (1) the initial rapid formation of the Pacific plate at

Figure 11. Distribution of three episodes of volcanic successions in the SCB-CR SLIP. ZJ = Zhejiang, FJ = Fujian, GD = Guangdong, JX = Jiangxi, and HN = Hunan. The age data are from Guo et al. [2012], L. Liu et al. [2012, 2015], Wang et al., [2012], and this study.

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Figure 12. Tectonic evolution history of the SCB-CR SLIP in South China. (a and b) Plate reconstruction at 160 Ma and narrow arc magmatism in the SCB-CR between circa 170 and 140 Ma; (c and d) plate reconstruction at 120 Ma and wide arc magmatism in the SCB-CR 140 to 110 Ma; (e and f) plate reconstruction at 100 Ma and bark-arc magmatism in the SCB-CR 110 to 90 Ma. Plate reconstruction at 160 Ma, 120 Ma, and 100 Ma is after Wessel and Müller [2015]. La = Laurasia; SA = South America; NA = North America, SCB = South China Block, MT = Meso-Tethys, WG = Western Gondwanaland; EG = Eastern Gondwanaland, Ch = China, Pac = Paciﬁc, Phx = Phoenix, Far = Farallon, Iza = Izanagi.

circa 175 Ma in response to the rapid breakup of the Pangean supercontinent (Figure 12a) [Storey, 1995; Veevers, 2004]; (2) the widespread development of Jurassic accretionary complexes along the East Asian continental margin such as the Mino-Tanba and Chichibu belts in Southwest Japan, the Helongjiang Complex in NE China, the Tananao Complex in Taiwan, and the North Palawan Block in the west Philippines [Isozaki, 1997; Fu-Yuan Wu et al., 2007; Yui et al., 2012]; (3) the change from nearly E-W trending folding during the Triassic to Early Jurassic to NNE trending faulting and folding during the Middle Jurassic in South China and nearby region [Y. Wang et al., 2015b; Zhang et al., 2009]; and (4) the marine regression in the Yongmei Basin in Fujian province during approximately the late Early Jurassic and the initiation of felsic continental volcanism during the Middle Jurassic [Xing et al., 2002]. The main drifting direction of the subducting paleo-Paciﬁc plate is another crucial issue, and at least two different viewpoints have been proposed [Goldfarb et al., 2007; Li and Li, 2007; Q. Liu et al., 2012b; Sun et al., 2007; Wang et al., 2011]. The Paciﬁc plate drifted southwestward during 180–125 Ma according to Wang et al. [2011]. This model plausibly explains systematical distribution of the Late Mesozoic large-scale igneous events and associated mineralization in South China from NE to SW [Wang et al., 2011; Zhang et al., 2013]. However, the spatiotemporal framework of the SCB-CR SLIP indicates that the felsic arc volcanism experienced an obvious northwestward shift during the Jurassic to Early Cretaceous (Figure 11), which strongly argues against the existence of coeval southwestward subduction. In addition, porphyry-skarn Cu-(Au) deposits including the Dexing, Yinshan, Yongping, Dongxiang, Jiande, Baoshan, Qibaoshan, and

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Yuanzhuding deposits are primarily distributed along the NE trending Jiang-Shao Fault and have similar metallogenic ages ranging from Middle to Late Jurassic [Li et al., 2015; Mao et al., 2011; Wang et al., 2013]. More importantly, the Pacific plate first formed at circa 175 Ma and was merely a small triangular plate far from South China until 140 Ma (Figure 12a) [Bartolini and Larson, 2001; Müller et al., 2008]. As a consequence, the southwestward drifting history of the Pacific plate before circa 125 Ma [Sun et al., 2007] should not be simply used to discuss the Late Mesozoic setting of South China. The subducting plate beneath the South China Block from the Middle Jurassic to Early Cretaceous is most likely the Izanagi plate [Engebretson et al., 1985; Müller et al., 2008; Whittaker et al., 2007], which moved NWward or NNWward (Figures 12a, 12c, and 12e) [Goldfarb et al., 2007; Seton et al., 2012] and controlled the SCB-CR SLIP. Because previous tectonic models cannot interpret well the spatiotemporal framework of the SCB-CR SLIP, the subduction initiation time, and the principal drifting direction of the subducting plate, it is necessary to propose a new three-episode tectonic model. Episode 1 (circa 170 to 140 Ma): minor arc volcanics formed locally along the Zhenghe-Dapu Fault in Fujian and Guangdong provinces, suggesting that the initial northwestward subduction of the Izanagi plate beneath the South China Block [Gilder et al., 1996] likely occurred in the Middle Jurassic and that a narrow volcanic arc formed (Figures 12a and 12b). Episode 2 (circa 140 to 110 Ma): the widespread felsic volcanic succession in Zhejiang and northern Fujian provinces suggests that northwestward gradual subduction occurred and that a broad subduction zone formed during the Early Cretaceous. In general, the width of the subduction zone is related to the slab dip [Gutscher et al., 2000; Tatsumi, 2005], and thus, the broad subduction zone in South China likely resulted from the shallow slab subduction of the Izanagi plate at that time (Figures 12c and 12d). Episode 3 (circa 110 to 90 Ma): the Late Cretaceous magmatism was characterized by prevailing bimodal volcanic-intrusive rocks along the easternmost part of the SCB-CR, suggesting a back-arc setting. We propose that the opening of the Shi-Hang back- arc basin system [Gilder et al., 1996] resulted from the eastward retreat of the trench system due to roll back of the subducting Izanagi plate (Figures 12e and 12f).

5.4. Implication for Epithermal Mineralization in the SCB-CR SLIP The SCB-CR SLIP is characterized by large-scale epithermal mineralization, including the Zhilingtou (24.5 metric ton Au and 528 metric ton Ag), Wubu (645 metric ton Ag), Dalingkou (870 metric ton Ag), Dongyang (10 metric ton Au), Zijinshan (313 metric ton Au), and Yueyang (1843 metric ton Ag) deposits [The No. 7 Team of the Zhejiang Bureau of Geology and Mineral Resources, 2006; Pan et al., 2015; Zhang et al., 2012]. With regards to the large-scale epithermal mineralization in subduction zone (e.g., Zhilingtou), it remains a conundrum whether the metal is derived from the mantle or the overriding continental crust [Chiaradia, 2014; Lee et al., 2012; Mungall, 2002; Wilkinson, 2013]. One view is that melting of the mantle wedge destabilizes sulfides and releases the metals into the melt, forming the metal-rich magma [Ling et al., 2009; Mungall, 2002]. This hypothesis requires that the ore-related magmas show significant contribution of mantle materials; thus, they are characterized by high MgO contents and Mg values [Stern and Kilian, 1996]. However, the low Mg, Cr, and Ni geochemical characteristics of the Zhilingtou arc volcanics argue against the involvement of mantle-derived magmas (Table 1 and Figure 13a). An alternative view is that melting of the continental curst can form ore deposits with- out the contribution from the mantle [Chiaradia, 2014; Lee et al., 2012]. Our results suggest the Zhilingtou ore- related volcanics, probably most of felsic volcanics in the SCB-CR, formed by remelting of the newly underplated juvenile basic continental crust, indicating the significant contribution of the continental crust rather than the mantle. The newly underplated juvenile crustal materials could have formed beneath the SCB-CR during the Early Cretaceous resulting from mantle-derived basaltic melts stalling near the crust-mantle transition zone due to the relatively high density of basaltic magma [Kelemen et al., 1997]. Continental crust is generally thought to have lower metal concentrations than primary basaltic magmas in arc environments [Mungall, 2002; Wang et al., 2006]. However, juvenile continental crust may not adhere to this pattern. In fact, the metal released from the subduction-modified mantle can be transported to the base of the crust by mantle-derived mafic magma, resulting in an initial enrichment. Such an initial enrichment has been observed in natural environment, for example, high concentrations of Au are present in subarc mantle xenoliths near Lihir Island, Papua New Guinea [McInnes et al., 1999]. Additionally, the importance of this type of initial metal enrichment in juvenile crust for ore deposits has also been advocated by many authors, based on the Pb isotope constraints, the precise dating of ancient mineralization, and the geochemistry of ore- related porphyries [Core et al., 2006; Lee et al., 2012; Q. Liu et al., 2012b; Pettke et al., 2010; Richards, 2013;

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Figure 13. (a) MgO versus SiO2 diagram for the igneous rocks in the Zhilingtou deposit: Data are sourced from G. G. Wang et al.[2015a] and reference therein; (b) LaN/YbN versus YbN for the igneous rocks in the Zhilingtou deposit [after Drummond and Defant, 1990]; (c) epithermal deposits in the SCB-CR SLIP (e.g., Zhilingtou) formed by remelting of metal-rich nearly underplated juvenile crust in an arc setting; (d) porphyry deposits in the SCB (e.g., Dexing) formed by melting of delaminated subcontinental lithospheric mantle and thickened continental crust.

Sillitoe et al., 2014; G.G. Wang et al., 2015a]. Thus, remelting of the metal-enriched juvenile lower continental crust may be a necessary step for the formation of a large number of epithermal deposits, particularly for low- Mg felsic volcanics in subduction zones. Crustal thickness likely plays an important role in controlling the occurrence of the large-scale mineralization in the subduction zone [Haschke et al., 2006; Kay et al., 1999]. Rare earth elements (REE) fractionation (e.g.,

LaN/YbN ratios) provides useful information to predict the crustal thicknesses via pressure-sensitive residual minerals [Kay and Mahlburg-Kay, 1991]. The LaN/YbN ratios reﬂect pressure-dependent changes from clinopyroxene to amphibole to garnet in the residual mineralogy in the magma source region. Low ratios of

LaN/YbN (≤20) generally indicate clinopyroxene-dominated mineral residues (30–35 km depth), intermediate ratios (LaN/YbN > 20–30) indicate amphibole-dominated mineral residues (40 km depth), and high ratios (LaN/YbN ≥ 30) reﬂect garnet-bearing mineral residues [Kay and Mahlburg-Kay, 1991; Rapp and Watson, 1995]. In Figure 13b, the Zhilingtou felsic volcanics have low LaN/YbN ratios (9.93 to 19.45) suggesting a thin continental crust (30–35 km depth). In contrast, the Middle Jurassic Dexing porphyry deposits have higher

LaN/YbN ratios (up to 33.16), indicating a thickened crust (≥40 km depth). Similarly, the central Andean porphyry Cu mineralization (e.g., EI Teniente and EI Indio)-related porphyries also have high LaN/YbN ratios (up to 36.1), which are attributed to crustal thickening induced by the repeated Chilean type ﬂat-slab subduction [Kay et al., 1999]. The comparison between epithermal and porphyry mineralization-related igneous

rocks indicates low LaN/YbN ratios and thin continental crust favored the formation of SLIPs and related large-scale epithermal mineralization such as the Zhilingtou deposit (Figure 13c). In contrast, the porphyry mineralization should be generated in a thickened crust setting such as the Dexing ore ﬁeld (Figure 13d). The Pb element is mobile in the subduction zone environment, and its contents increase signiﬁcantly when slab subduction begins. The low Pb contents at circa 148 Ma (Figure 14a) suggests that the Zhilingtou region was an intraplate setting at that time and the presence of a narrow arc in the SCB-CR during the Jurassic. The Pb concentrations are higher in the Zhilingtou igneous rocks formed after circa 128 (Figure 14a), indicating that a broad arc occurred during Early Cretaceous. The Pb concentrations are higher in the Zhilingtou igneous rocks formed after circa 110 Ma (Figure 14a), suggesting that a slab rollback-related back-arc setting occurred during the Late Cretaceous (after circa 110 Ma). Such changes are also supported by the variations in the

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mean zircon εHf(t) values (Figure 14 b) and the variations in the distance between trench and arc volcanics (Figure 14c). The Zhilingtou Au-Ag deposit developed at circa 110 Ma (this study) and the giant Zijinshan epithermal Au-Cu ore deposit formed at circa 111 Ma [Jiang et al., 2013]. Ages of host rocks at the Zhilingtou and Zijinshan deposits suggest that the large-scale epithermal mineralization was probably produced in a specific environment in response to the transition from a continental arc setting to a back-arc setting (Figures 14c and 14d). The continental arc setting (circa 140 to 110 Ma) commonly represents a compressional environment, but the back-arc setting indicates an extensional regime (after circa 110 Ma). During the transitional from arc to back-arc settings, the relaxa- tion of stress commonly reactivate preexisting faults, which provide Figure 14. Geochemical variations versus ages of the igneous rocks in the channels favorable to ore fluids ε Zhilingtou deposit. (a and b) Pb contents and Hf(t) values of the Zhilingtou ascent, resulting in precipitation igneous rocks changes with the time, respectively; (b and c) distance (km) between trench and volcanic arc; (d) epithermal mineralization occurred in and ore deposit formation in the response to tectonic transition from arc to back-arc regimes. shallow crust. The coupling between changes in crustal stress regime and the generation of epithermal deposits was also identified in Bajo de la Alumbrera in Argentina, Bingham in USA, and Ok Tedi and Ertsberg in Papua New Guinea [Sillitoe, 2008; Solomon, 1990]. Similarly, we conclude that the tectonic transition from arc to back-arc settings in the SCB-CR provides a favorable environment that explains the development of the large-scale epithermal mineralization associated with the SLIP emplacement.

In summary, the large-scale epithermal mineralization in the SCB-CR SLIP is associated with melting of the metal-rich juvenile continental crust rather than the mantle. In addition, the Early Cretaceous epithermal mineralization in the SCB-CR SLIP is related to a thin arc resulting from the tectonic transition from arc to back-arc regimes. In contrast, the Jurassic porphyry mineralization in the South China Block correlates to the thickened continental crust in an intraplate environment. Thus, we concluded that the large-scale epithermal mineralization was generated by melting of the metal-rich, newly underplated, thin (30–40 km) hydrous juvenile crust during a tectonic transition from arc to back-arc settings.

6. Conclusions We have established that four stages of magmatism characterizing the SCB-CR SLIP between 149–100 Ma. Stage 1 consists of highly fractionated granite that emplaced at circa 149 Ma, stages 2 and 3 felsic volcanics erupted at circa 128–110 Ma, and stage 4 bimodal intrusive rocks emplaced at circa 100 Ma. The progressively

higher εHf(t) and εNd(t) values from early to late stages suggest that the contribution of juvenile components increased over time. The low Mg values of the Zhilingtou felsic volcanics suggest that juvenile continental crust was primarily involved in the generation of the parent magma. Stage 4 maﬁc dikes have relatively high

εHf(t) and εNd(t) values and Nd, Ta, and Ti depletions, suggesting a metasomatized asthenospheric mantle source.

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Based on the results in this paper and previously published data, we suggest that the formation of the SCB-CR SLIP involved in three episodes of magmatism: episode 1 consists of a localized felsic arc magmatism that occurred along the Zhenghe-Dapu Fault in Guangdong and Fujian provinces at 170 to 140 Ma, episode 2 includes widespread felsic arc magmatism that developed in Zhejiang and Fujian provinces during circa 140 to 110 Ma, and episode 3 consists of bimodal igneous rocks that crop out mainly along the easternmost margin of the SCB during circa 110 to 90 Ma. The spatiotemporal distribution in the SCB-CR SLIP reveals that the initial subduction of the Izanagi plate beneath Guangdong and Fujian provinces in the SCB began during the Middle to Late Jurassic. Subduction then shifted northwestward beneath Zhejiang and Fujian provinces during the Early Cretaceous and followed by an eastward subduction retreat during the late Early Cretaceous to Late Cretaceous. Remelting of the metal-enriched juvenile lower continental crust is critical to the epithermal mineralization-

related low-Mg felsic volcanics in the SCB-CR SLIP. The low LaN/YbN ratios of the felsic volcanics reﬂect the thin continental crust conditions, which favored the formation of the SLIP and the related large-scale epithermal mineralization. We conclude that the change from an arc setting to a back-arc setting in the SCB-CR SLIP induced the generation of the large-scale epithermal deposits.

Acknowledgments References We sincerely thank Jie-Xiong Hua from ’ the Suichang gold mine and Jia-Run Liu, Amelin, Y., D.-C. Lee, A. N. Halliday, and R. T. Pidgeon (1999), Nature of the Earth s earliest crust from hafnium isotopes in single detrital – Guo-Ai Xie, and Chang-Zhi Wu from zircons, Nature, 399(6733), 252 255. – 204 – Nanjing University for assistance during Andersen, T. (2002), Correction of common Pb in U Pb analyses that do not report Pb, Chem. Geol., 192,59 79. field work. We are grateful to Bing Wu Arth, J. G. (1976), Behaviour of trace elements during magmatic processes: A summary of theoretical models and their applications, J. Res. – for assistance with the LA-ICP-MS zircon U. S. Geol. Surv., 4(1), 41 47. U-Pb dating, to Wei Pu for her assistance Ayres, M., and N. Harris (1997), REE fractionation and Nd-isotope disequilibrium during crustal anatexis: Constraints from Himalayan leu- – with the whole-rock Sr and Nd isotope cogranites, Chem. 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