1 Introduction

In the Cretaceous, South China was surrounded by several continental terranes and oceanic plates, such as the North China Block to the north, the Songpan-Ganzi Block to the west, the Indochina Block to the southwest, the Pacific Plate to the east, and the Neo-Tethys Plate to the south. South China is famous for large-scale magmatism and related mineralization, especially in the Mesozoic. A variety of tectonic models have been proposed to explain these characteristics specifically and the Mesozoic tectonic evolution of South China more generally (Gilder et al. 1991, 1996; Li 2000; Zhou and Li 2000; Zhou et al. 2006; Li et al. 2007, 2012; Sun et al. 2007a, 2012b; Ling et al. 2009; Mao et al. 2011; Wang et al. 2011). The models can be classified into three groups: active continental margin related to subduction of the Pacific Plate; continental rifting and extension; and mantle plume event. More recently, it has been proposed that the Neo-Tethys Plate subducted beneath the South China Block in the Cretaceous (Sun et al. 2016b; Sun 2016).

In addition to the giant W, Sn, Sb, Nb, Ta, U, and rare-earth element (REE) deposit belts, a number of copper deposits occur in South China as well. Porphyry or skarn copper deposits are the most important deposit types, accounting for ~70%–80% of the world’s total copper reserves (Sillitoe 2010; Sun et al. 2015a), and mainly form along convergent belts; for example: the Andean region in South America. Copper deposits in the northeast part of the South China Block have been well-studied. In contrast, the genesis and tectonic setting of copper deposits in the southern part of the South China Block remain obscure.

Shilu is a porphyry–skarn Cu–Mo deposit in the Yangchun Basin, in the south of the South China Block. Previous studies on this deposit have considered geologic features as well as geochronology, geochemistry, and mineral trace element composition. A range of ages have been obtained through different techniques, including a zircon U–Pb evaporation age of 125 Ma, a mineral Rb–Sr age of 122 Ma, an 40Ar–39Ar age of ~100 Ma, and ore-bearing and ore-barren granodiorite Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) zircon U–Pb ages of ~107 and ~104–107 Ma, respectively (Zhao et al. 1985; Yu et al. 1998; Duan et al. 2013; Zheng et al. 2015). Therefore, the formation age of the Shilu intrusion remains controversial. For the ore-forming age, a molybdenite Re–Os isochron age of 104.1 ± 1.3 Ma has been obtained (Zhao et al. 2012). The genesis and tectonic setting of the deposit are also ambiguous. It is located close to the transition zone between the eastern Tethys and the Pacific tectonic realms. The effect of the two major tectonic realms on this region remains unclear. Some previous work has proposed that the Shilu intrusion formed in an extensional environment related to subduction of the Pacific Plate (Duan et al. 2013; Zheng et al. 2015). In addition, the variation of magma compositions has been used to argue that the region belonged to an intraplate setting and experienced upwelling of the asthenosphere mantle and extension of overlying lithosphere during the Early Jurassic to the Early Cretaceous (Li et al. 2001).

In this study, we present LA-ICP-MS zircon U–Pb ages, molybdenite Re–Os ages, zircon Hf isotopes, and whole-rock major and trace elements. These data provide firmer constraints on the timing and geochemical characteristics of the Shilu intrusion and deposit, with implications for the tectonic setting of Late Cretaceous magmas in this region.

2 Geological setting

2.1 Regional geology

The Shilu Cu–Mo deposit is located in the Yangchun Basin, South China (Fig. 1). The Yangchun Basin is a NE–SW-trending fault-bounded basin formed from an Indosinian synclinorium, southeast of the Wuchuan-Sihui deep fault (Li et al. 2000; Zheng et al. 2015). The area has complex geological structure and has experienced intensive magmatic activity. NE–SW-trending faults define the tectonic framework. E–W-trending faults can also be found, e.g., the Taishan-Yangchun tectonic belt (Zhang 2008).

Fig. 1
figure 1

a Sketch geologic map of South China and surrounding plates. b Geological map of the Yangchun basin, South China. c Geological map of the Shilu Cu–Mo deposit. The black pentacles are sample sites

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The outcropping strata in this basin include Cambrian, Devonian, Carboniferous, Jurassic, and Quaternary strata (Fig. 1; Luo 1988; Zheng et al. 2013). The basement is composed of Cambrian low-grade metamorphic flysch and sandy shales distributed on both sides of the basin, which are covered by Upper Paleozoic carbonate rocks, neritic clastic rocks, and sandy shales. Jurassic terrestrial clastic rocks unconformably overlie the Paleozoic strata (Zheng et al. 2013).

Abundant granitic intrusions of different rock types with varying mineralization have been recognized. These intrusions vary from intermediate to acidic in composition, including monzonite, monzonitic granite, granodiorite, and granite (Fig. 1). The ages of these rocks range from Caledonian to Yanshanian, with most magmatic activity having occurred in the Yanshanian. Several polymetallic deposits closely related to granitic intrusions have been found, such as the Shilu porphyry–skarn Cu–Mo deposit, the Bengkeng-Shiwu Cu–Pb–Zn deposit, the Xishan Sn–W deposit, and the Yingwuling W–Sn deposit (Chen 1988; Wu and Zeng 2011; Duan et al. 2013; Mei et al. 2013; Zheng et al. 2015).

2.2 Ore deposit geology

Shilu is a porphyry–skarn Cu–Mo deposit with average grades of 0.9% for Cu and 0.21% for Mo (Zheng et al. 2015). NE-trending faults are the dominant tectonic framework in this mine, in keeping with the regional fault structure. Ore bodies can be divided into two ore types: oxidized ore and primary ore. The oxidized ore bodies are mainly distributed in weathered skarn and Quaternary strata, and were an important ore type in the past. The primary ore bodies are mainly hosted in the skarn belts between intrusions and carbonate country rocks (Fig. 1; Zhang 2008; Zhang and Zhang 2012).

The strata cropping out in the deposit include the Cambrian Bacun Group (εbc); the Upper Devonian Maozifeng Formation (D3m) and Tianziling Formation (D3t); the Lower Carboniferous Menggongao Formation (C1ym) sandstone and shale; the Shidengzi Segment (C1ds) limestone; the Ceshui Segment (C1dc) sandstone, shale, and hornstone; the Zimenqiao Segment (C1dz) dolomite and limestone; the Middle Carboniferous Huanglongqun Group (C2hn) dolomite and limestone; and Quaternary sediments (Fig. 1).

Different petrographic features were recognized for the Shilu intrusion based on field observation and thin section studies under optical microscope. Granodiorite and quartz diorite are the main rock types (Fig. 2a, b). The granodiorite samples were composed of plagioclase (~60%), K-feldspar (~15%), quartz (~20%), biotite (~8%), and minor amphibole (<5%) (Fig. 2e). More dark minerals and sulfides were observed in the quartz diorite as compared to granodiorite (Fig. 2e, f). The quartz diorite samples primarily consisted of plagioclase (~55%), K-feldspar (~12%), quartz (~18%), biotite (~8%), and amphibole (~8%) (Fig. 2f). Both granodiorite and quartz diorite samples contained accessory minerals of magnetite, zircon, apatite, and titanite.

Fig. 2
figure 2

Specimen photographs and micrographs of the Shilu intrusion. a Granodiorite; b Quartz diorite; c Molybdenite, chalcopyrite and pyrite in quartz vein; d Skarn; e Micrograph of granodiorite, cross-polarized light; f Micrograph of quartz diorite, cross-polarized light. Am amphibole, Bt biotite, Mt magnetite, Pl plagioclase, Qz quartz

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3 Samples and analytical methods

3.1 Whole-rock major and trace elements

Nine fresh intrusive rock samples, derived from outcrops and drill holes, were collected from different positions in the deposit (Fig. 1). They were broken to small fragments, washed and dried, and then crushed to 200 mesh. Whole-rock analyses for major and trace elements were conducted at the ALS Laboratory Group, an Australian analytical laboratory in China. The major elements were analyzed using X-ray fluorescence spectrometry (XRF), specifically a PANalytical Axios. First, the loss on ignition (LOI) values were determined. Samples were weighed and then heated in a muffle furnace and then weighed after ignition to calculate the LOI. Then, the baked whole-rock powders were mixed with lithium borate and fused at over 1000 °C in a smelting furnace. The whole-rock major elements were then measured using XRF. To analyze trace elements, fused samples were progressively dissolved in nitric acid, hydrochloric acid, and hydrofluoric acid, and then analyzed by an Agilent 7700× ICP-MS. Whole-rock F content was analyzed by ion electrodes using an American Accumet AR50. Whole-rock Cl content was determined by ion chromatography on a Switzerland Metrohm IC930.

3.2 Zircon U–Pb dating

Three samples used for zircon U–Pb dating were collected from surface and drill-hole localities (Fig. 1c). Zircon grains were separated from the samples using standard density and magnetic separation techniques. The separated grains were fixed in epoxy and then polished down to near half section to expose internal structures. Cathodoluminescence and optical microscopy images were taken of zircon grains, and were used for choosing appropriate spots for laser ablation. Zircon U–Pb ages and trace element compositions were analyzed using LA-ICP-MS at the Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The LA-ICP-MS system is comprised of a Resonetics RESOlution S-155 ArF-Excimer laser source (λ = 193 nm) and an Agilent 7900 ICP-MS. The ablation conditions were 4 J·cm−2 laser energy, ablation frequency of 8 Hz, and a spot diameter of 29 μm. The SRM 610 glass and the TEMORA zircon were used as external calibration standards and the 91Zr was selected as the internal standard. Detailed information for analytical procedures and conditions are described by Li et al. (2012). Data processing was performed using ICPMSDataCal (Liu et al. 2008; Lin et al. 2016). Concordia diagrams and weighted mean age calculations were processed using Isoplot (Ludwig 2003).

3.3 Molybdenite Re–Os dating

Four molybdenite samples were selected for Re–Os isochron dating. Samples SL27–1 and SL33 were derived from quartz veins with metal mineralization, and samples ZK4503-662 and ZK4503-686 were from molybdenite veins in skarn. Samples were purified using the alcohol floating method and then molybdenite grains were carefully chosen under binocular microscope, before being crushed to 200 mesh in an agate mortar. Re–Os isotope analyses were conducted at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, using Thermo Scientific X Series 2 ICP-MS. Detailed information for analytical conditions and procedures can be obtained from earlier publications (Sun et al. 2010b, 2015b).

3.4 Zircon Hf isotope analyses

In-situ zircon Hf isotope analyses were performed on a Resonetics M-50 laser ablation system coupled with a Neptune MC-ICP-MS at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. A spot diameter of 45 μm and an ablation frequency of 8 Hz were used for analyzing the spots where U–Pb had been analyzed. The zircon Penglai was used as a reference standard, with a recommended 176Hf/177Hf ratio of 0.282906 ± 0.000001 (2s) (Li et al. 2010). Detailed analytical methods have been described by Wu et al. (2006).

4 Results

4.1 Whole-rock major and trace elements

Major and trace element data are presented in Table 1. Our data are plotted in Fig. 3, along with previous results from other studies. Most of the samples have LOI values less than 2 wt%. The major element compositions vary slightly, with SiO2 contents of 64.5 wt% –67.4 wt%, Al2O3 contents of 15.4 wt%–16.6 wt%, MgO contents of 1.66 wt% –2.52 wt%, Na2O contents of 2.43 wt%–2.99 wt%, and K2O contents of 2.68 wt%–3.56 wt%. In the SiO2 against K2O diagram, the Shilu intrusion belongs to the high-K calc-alkaline series (Fig. 3a). The diagram of A/CNK versus A/NK shows that the intrusive rocks are metaluminous (A/CNK < 1.1) (Fig. 3b). Volatile contents show low fluorine (520–590 ppm) and high chlorine (110–220 ppm) characteristics.

Table 1 The compositions of major oxides (wt%) and trace elements (ppm) for the Shilu intrusion
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Fig. 3
figure 3

a The diagram of SiO2 versus K2O and b the diagram of A/CNK versus A/NK for the Shilu intrusion

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All the samples, including previously published data, show similar chondrite-normalized REE patterns (Fig. 4a), with enrichment of LREE [LREE/HREE = 7.69–11.0, (La/Yb)N = 7.78–13.4], and slightly negative Eu anomalies (δEu = 0.75–0.82). In the primitive mantle normalized spidergram (Fig. 4b), all samples display characteristics of arc magmatic rocks, with positive anomalies of U, K, and Pb and negative anomalies of Nb, Ta, P, and Ti for high field-strength elements. In addition, the Shilu pluton has high Sr and low Y and Yb contents, indicating the absence of plagioclase (both residual and crystallized) and residual garnet in the magma source. However, some previous data (Zheng et al. 2015) have shown strong Zr and Hf negative anomalies, which have resulted from incomplete dissolution of the zircon samples during analysis.

Fig. 4
figure 4

a Chondrite-normalized rare earth elements pattern and b primitive mantle-normalized incompatible elements spider diagram for the Shilu intrusion. Chondrite and primitive mantle data are from Sun and McDonough (1989)

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4.2 Zircon U–Pb dating and trace elements

Three samples from the Shilu intrusion, including granodiorite and quartz diorite, were selected for zircon U–Pb dating. The results of isotopic compositions and ages are listed in Table 2. We obtained 23, 17, and 39 valid data points for samples SL01, SL30, and ZK8502-594, respectively, according to the following three criteria: (1) no cracks, defects, or inclusions; (2) more than 30 s of laser ablation time; and (3) a highly concordant age. Most zircon grains exhibited a euhedral prismatic shape with lengths of 100–300 μm. As in the CL image presented, zircon grains showed typical oscillatory zones (Fig. 5). The analytical results show that all zircon grains had high Th/U ratios (average value 0.43, 0.55, and 0.54, respectively), typical of magmatic zircon (Sun et al. 2002; Wu and Zheng 2004). The U–Pb concordia diagrams for the three samples are shown in Fig. 5. The weighted mean 206Pb/238U ages for SL01, SL30, and ZK8502-594 were 106.6 ± 1.3, 104.4 ± 1.0, and 103.9 ± 0.5 Ma, respectively (Fig. 5). Different zircon ages between granodiorite and quartz diorite indicate multiple magmatic pulses.

Table 2 LA-ICP-MS zircon U–Pb dating results for the Shilu intrusion
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Fig. 5
figure 5

Zircon U–Pb concordia and weighted mean age diagrams for the Shilu intrusion

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Zircon trace element data were obtained along with zircon U–Pb analyses. These are listed in Supplementary material Table S1 together with zircon Ce4+/Ce3+ ratios, and EuN/EuN* ratios. As shown in the chondrite-normalized REE patterns (Fig. 6), all zircon grains presented typical patterns, i.e. enriched in HREE and depleted in LREE. They exhibited slightly negative Eu and strongly positive Ce anomalies.

Fig. 6
figure 6

Chondrite-normalized REE pattern diagram for zircons. Chondrite data are from Sun and McDonough (1989)

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4.3 Molybdenite Re–Os dating

Molybdenite is a frequently-used mineral for determining ore-forming age, because it is a common mineral in a variety of deposits and its osmium content is mainly radiogenic due to low common osmium and high rhenium concentrations. Re–Os isotopic compositions of four molybdenite samples are listed in Table 3. Four samples had variable concentrations of 187Re and 187Os, ranging from 2.43 to 201.24 ppm and 4.11 to 343.90 ppb, respectively. Molybdenite Re–Os model ages were calculated using the formula: t = (1/λ)*ln(1 + 187Os/187Re), where λ is the decay constant of 187Re (1.666 × 10−11 year−1) (Smoliar et al. 1996). The model ages for samples SL27-1, SL33, ZK4503-662, and ZK4503-686 were 101.3 ± 0.4 Ma (2σ), 102.5 ± 0.4 Ma (2σ), 101.5 ± 0.9 Ma (2σ), and 101.7 ± 0.6 Ma (2σ), respectively. All the samples had identical model ages within error, yielding a weighted mean age of 101.9 ± 1.0 Ma (MSWD = 6.6). The 187Re–187Os isochron age was 102.2 ± 2.9 Ma (MSWD = 9.4) (Fig. 7), which is consistent with the weighted mean of the model ages within error. The large MSWD is likely due to variation of initial Os content or formation age.

Table 3 Re and Os isotope data for the molybdenite samples from the Shilu Cu–Mo deposit
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Fig. 7
figure 7

Re–Os isochron and weighted mean age diagrams for the molybdenite samples from the Shilu deposit

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4.4 Zircon Hf isotopes

Zircon Hf isotope compositions are listed in Supplementary material Table S2. Twenty-two zircon grains were analyzed from SL01 and twenty grains from ZK8502-594. Most 176Lu/177Hf ratios were less than 0.002, indicating there has been little radiogenic 176Hf accumulation since zircon crystallization. The initial 176Hf/177Hf ratios were calculated using the measured 176Lu/177Hf ratios and the 176Lu decay constant of 1.865 × 10−11 year−1 (Scherer et al. 2001). The zircon εHf(t) values ranged from −10.7 to −3.3, and were calculated using the corresponding zircon age, the chondritic 176Hf/177Hf ratio of 0.282772, and 176Lu/177Hf ratio of 0.0332 (BlichertToft and Albarede 1997).

5 Discussion

5.1 Oxygen fugacity of magma

Oxygen fugacity is an important factor for porphyry copper deposits (Mungall 2002; Sillitoe 2010; Sun et al. 2013b, 2015a). Zircon is a good recorder of magmatic oxygen fugacity (Ballard et al. 2002). Ce and Eu have Ce3+, Ce4+ and Eu2+, Eu3+ valence states, respectively, which have different partition coefficients in zircon. Ce4+ and Eu3+ are more easily accommodated in zircon than Ce3+ and Eu2+, respectively. Therefore, Ce4+/Ce3+ and EuN/EuN* ratios in zircon can reflect relative oxidation states of magmas (Ballard et al. 2002; Liang et al. 2006). Ballard et al. (2002) proposed that porphyry copper mineralization is closely associated with zircon Ce4+/Ce3+ > 300 and EuN/EuN* > 0.4, which are useful indices for evaluating the mineralization potential. In this study, the zircon Ce4+/Ce3+ ratios of Shilu ranged from 138 to 1114, with an average value of ~619; and the EuN/EuN* ratios ranged from 0.43 to 0.59, with an average value of ~0.53. Both indicators are significantly higher than the reference values. In the EuN/EuN* versus Ce4+/Ce3+ diagram (Fig. 8), most of the Shilu samples plotted in the overlap fields of ore-bearing porphyry in Chuquicamata–El Abra and Dexing porphyry copper deposits (Zhang et al. 2013), indicating that the Shilu intrusion crystallized from highly oxidized magma. The chemical composition of biotite in the granodiorite also displayed a high oxygen fugacity between nickel–nickel oxide and the hematite-magnetite buffer (Zheng et al. 2015). Magmas of porphyry or skarn copper deposits generally have high oxygen fugacity, usually more than two orders of magnitude higher than the fayalite–magnetite–quartz (ΔFMQ + 2) oxygen buffer (Mungall 2002), but lower than the hematite-magnetite oxygen buffer (Sun et al. 2013b, 2015a). Oxygen fugacity strongly affects the sulfur species in magmas and their solubility. Sulfur in magmas is mainly present as sulfate when the oxygen fugacity is higher than ΔFMQ + 2, and the sulfur solubility in magmas increases from ~1000 ppm to >1 wt%, correspondingly (Jugo et al. 2005; Jugo 2009). Therefore, high oxygen fugacity can transform more sulfide to sulfate, which induces sulfur-undersaturated magmas and consequently allows assimilation of more sulfide. Copper is incompatible in sulfur-undersaturated magmas; when initial concentration of copper is high in oxidized magmas (Sun et al. 2013b, 2015a), conditions are favorable for copper mineralization. The oxygen fugacity in convergent-margin magmas is usually higher than in other geologic settings (e.g., intraplate setting), which is related to plate subduction and release of oxidized fluids (Brandon and Draper 1996; Parkinson and Arculus 1999; Sun et al. 2007a, b, 2010a). Therefore, the genesis of the high oxygen fugacity of the Shilu intrusion may be related to slab subduction.

Fig. 8
figure 8

Zircon EuN/EuN* versus Ce4+/Ce3+ diagram. The fields of ore-bearing and ore-barren porphyry in Chile and ore-bearing porphyry in Dexing are modified from Zhang et al. (2013). The red rhombuses represent adakitic rocks from Shilu

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5.2 Adakitic rock

The term “adakite” was first proposed to describe magmas derived from partial melting of young, subducted oceanic slab (Defant and Drummond 1990). It was defined simply by its chemical composition. Typical adakite has SiO2 ≥ 56 wt%, Al2O3 ≥ 15 wt%, MgO < 3 wt%, high Sr (usually ≥400 ppm), and low Y and Yb (generally ≤18 and 1.9 ppm, respectively). Thus, adakite usually has high Sr/Y and La/Yb ratios. Shilu intrusive rocks measured here were consistent with the parameters of typical adakite, except that a few samples had slightly higher Y and Yb contents (Tables 1, 4), which may have resulted from crustal contamination.

Table 4 Geochemical comparison of typical adakite and the Shilu intrusion
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Because of the close genetic association between adakite and porphyry copper deposits, the rock has received great attention in recent years (Moyen 2009; Sun et al. 2010a, 2011, 2012a, 2013b; Hu et al. 2015). Given that adakite is defined by chemical characteristics, therefore, it may also be produced by partial melting of thickened lower continental crust (Chung et al. 2003; Wang et al. 2006). However, the average Cu concentration of lower continental crust is ~26 ppm (Rudnick and Gao 2003), and the mantle wedge may have copper contents lower than primitive mantle (<30 ppm) (McDonough and Sun 1995; Sun et al. 2011); both have Cu concentrations much lower than that of oceanic crust (~100 ppm) (Sun et al. 2003, 2011). Therefore, adakitic magmas derived from oceanic crust should contain much higher Cu contents than those from the lower continental crust and are therefore favorable for Cu mineralization (Sun et al. 2011). The low oxygen fugacity of lower continental crust is another unfavorable factor for scavenging Cu from magma sources.

Different magma sources can also be effectively constrained using a multi-element diagram of Sr/Y versus (La/Yb)N (Ling et al. 2011; Liu et al. 2010; Sun et al. 2011). All our samples together with previous data plotted in the field of partial melting of subducted oceanic crust in Fig. 9. Combined with the above discussion, the most plausible mechanism for Shilu adakitic rocks and Cu–Mo mineralization is the partial melting of subducted oceanic crust.

Fig. 9
figure 9

Sr/Y versus (La/Yb)N diagram for the Shilu intrusion. The base map is modified after Ling et al. (2013). SOC subduction oceanic crust, LCC lower continental crust. (La/Yb)N ratios are chondrite-normalized values, and chondrite date are from Sun and McDonough (1989)

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5.3 Magma mixing and crustal contamination

Previous researchers have shown that Shilu adakitic rocks have high radiogenic Sr and low εNd(t) (Li et al. 2001; Zheng et al. 2015). It is generally assumed that normal adakitic magmas derived from the partial melting of oceanic slab should have positive εNd(t) and low radiogenic Sr (close to MORB in isotope composition), e.g., Cook adakite (Stern et al. 1984). However, because of assimilation of enriched mantle components as well as continental crust, the isotope composition is readily changed during magma ascent. In the two-component (87Sr/86Sr)i versus εNd(t) mixing diagram, most of the Shilu samples fell near the field defined by the Lower Yangtze River Belt (LYRB) adakites and one sample in the field of the Dexing adakites (Fig. 10). Previous work has determined that the LYRB adakites were caused by ridge subduction, and these adakitic magmas mixed with type II enriched mantle (EMII) and were contaminated by crustal materials (Ling et al. 2009). The Dexing adakites were also derived from the partial melting of subducted oceanic crust and assimilated oceanic sediments or crustal materials (Zhang et al. 2013). In addition, the Nd isotope composition of the Shilu intrusion is notably higher than the Dabie adakites derived from the partial melting of lower continental crust (Ling et al. 2011). The South China Block is characterized by EMII (Li and Yang 2003) and the Shilu adakitic rocks are located on a mixing line between adakitic magmas and an EMII source, close to the mixing line between adakitic magmas and average continental crust. Therefore, Shilu adakitic rocks may be explained by the mixing of adakitic magmas with an EMII component contaminated by crustal materials or oceanic sediments. Zircon (176Hf/177Hf)i isotope compositions of Shilu adakitic rocks are also higher than an oceanic crust source, indicating the contamination of crustal materials or oceanic sediments. The compositions of hornblende and biotite indicate that the Shilu intrusion is the product of crust-mantle mixed-source magma as well (Zheng et al. 2015). Crustal contamination is probably also responsible for the slightly higher Y and Yb contents than typical adakites in some samples. Magmatic contamination commonly occurs during magma ascent through assimilation. To some extent, assimilation is similar to partial melting, in that the wall rock is partially melted by intruding magma and the melt mixes into the magma. Both Y and Yb are moderately incompatible elements in the absence of garnet. The Y (20–21 ppm) and Yb (2.0–2.2 ppm) concentrations of the mid- and upper- continental crust (Rudnick and Gao 2003) are marginally higher than the discrimination line of adakite. Partial melting, however, produces magmas with Y and Yb ~ 3 times higher than the source at 10% of partial melting. Therefore, crustal contamination elevates the Y and Yb contents.

Fig. 10
figure 10

Sr–Nd isotope mixing model for adakitic rocks in the Shilu deposit. Mixing simulation curves are modified after Ling et al. (2009). The Cook adakite is considered from the partial melting of oceanic plate and has a similar composition of Sr–Nd isotope with N-MORB (Kay et al. 1993; Stern et al. 1984); The compositions of EMI, EMII and average continental crust are from references (Guo et al. 2003; Hofmann 1997; Li and Yang 2003; Rudnick and Gao 2003). The mixing lines with circle symbols are for Sr = 400 ppm, and the mixing lines with cross symbols are for Sr = 1000 ppm. The fields of Sr–Nd isotope compositions for Dexing adakites, LYRB adakites, and Dabie adakites are modified from Zhang et al. (2013), where LYRB Lower Yangtze River Belt

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Rhenium concentrations vary dramatically in molybdenites of different origin (Mao et al. 1999). The Re/Mo ratio of the continental crust (2.5 × 10−3–2.5 × 10−4) (Rudnick and Gao 2003; Sun et al. 2003), however, corresponds to hundreds to thousands of ppm Re in molybdenite. Major fractionation between Re and Mo occurs during reduction. Organic-enriched sediments have low Re/Mo. Therefore, molybdenite formed in climax-type deposits has low Re, whereas that associated with subduction-related porphyry Cu deposits from South America have high Re (Sun et al. 2016a). In this study, the Re concentrations of molybdenite reached n × 10−4 levels, similar to porphyry Cu deposits from South America (Sun et al. 2016a), with the exception of sample ZK4503-662 (n × 10−6). The high Re concentration in molybdenite is consistent with the slab melting model, whereas the low Re in sample ZK4503-662 is consistent with contamination of crustal materials with high organic concentrations.

Based on trace element analysis, we propose that the Shilu adakitic magmas were caused by the partial melting of subducted oceanic slab, which may have interacted with EMII and crustal materials during magma ascent.

5.4 Geochronology

Accurate isotopic age is crucial for understanding the relationships among magmatic rocks, mineralization, and tectonic events. Previous studies have obtained several ages for the Shilu intrusion. A zircon U–Pb evaporation age of 125 Ma was obtained by the isotope geological age data assembly group in 1983; Zhao et al. (1985) determined an isochron age of 122 ± 1 Ma using mineral Rb–Sr dating; and Yu et al. (1998) obtained an 40Ar–39Ar plateau age of 99–101 Ma through K-feldspar analysis. In recent years, LA-ICP-MS zircon U–Pb ages of 107 ± 0.7, 107.2 ± 2.0, 106.7 ± 1.4, and 104.1 ± 2.0 Ma have been obtained for granodiorites (Duan et al. 2013; Zheng et al. 2015). An ore-forming age of 104.1 ± 1.3 Ma was obtained using molybdenite Re–Os dating (Zhao et al. 2012). The zircon evaporation age is much older than new results, likely because it is determined by 207Pb/206Pb and even a small amount of inherited Pb can perturb the analysis. It is also difficult to obtain precise mineral Rb–Sr isochron ages at such young ages.

In this study, new zircon U–Pb ages suggest that the Shilu intrusion formed between 106.6 ± 1.3 and 103.9 ± 0.5 Ma. The oldest age is consistent with previously determined LA-ICP-MS zircon U–Pb ages (Duan et al. 2013; Zheng et al. 2015). The youngest zircon U–Pb age is younger than previously published ages. These results indicate multiple magmatic pulses. Consistently, previously published K-feldspar mineral 40Ar–39Ar plateau ages (99–101 Ma) are also slightly younger, which probably represent a thermal resetting event. Moreover, magmatic activity has been documented at around 100 Ma in western Guangdong (Geng et al. 2006). For ore-forming age, molybdenite Re–Os dating defined an age of 102.2 ± 2.9 Ma in this study, which is consistent with previous findings (Zhao et al. 2012) and identical to the youngest zircon U–Pb age within error, indicating a genetic relationship with Cu–Mo mineralization.

5.5 Tectonic setting

As discussed above, Shilu adakitic rocks derived from the partial melting of subducted oceanic crust. The question then is: How could subducted oceanic crust reach Shilu? The South China Block was surrounded by the Pacific and the Neo-Tethys Oceans in the Cretaceous. Therefore, the most likely candidate is either the Pacific Plate or the Neo-Tethys Plate. In recent years, most researchers have argued that the subduction of the Pacific Plate may account for the formation of Cretaceous magmas and mineralization in South China.

Based on island chains, the Pacific Plate subducted southwestward beneath South China before 125 Ma (Sun et al. 2007a). It reached as far south as the Nanling Range at ~160 Ma and was immediately followed by slab rollback (Wang et al. 2011; Li et al. 2012; Sun et al. 2012b). Asthenosphere upwelling induced by slab rollback and phengite decomposition due to the elevated temperature released abundant fluorine and formed large-scale highly fractionated granites and W–Sn mineralization in the Jurassic (Wang et al. 2011; Li et al. 2012). Therefore, the Pacific Plate before 125 Ma was not responsible for adakitic rocks in the Yangchun district, south of the Nanling Range, and thus the partial melting of a Pacific slab cannot account for the Shilu adakitic rocks (Fig. 11). Furthermore, the Pacific Plate experienced a clockwise rotation of about 80° at ~125 Ma, changing the drift direction from southwest to northwest (Sun et al. 2007a, 2013a). As presented in previous studies, the ridge between the Pacific and Izanagi Plates subducted along the Lower Yangtze River belt at ~140 ± 10 Ma, forming extensive adakites and an important metallogenic belt (Ling et al. 2009; Sun et al. 2010a). The ridge then reached the Xuhuai arc at ~130 Ma, forming Cretaceous adakites (Fig. 11) (Ling et al. 2013). The distance between the two adakite belts is about 500 km, corresponding to a northward migration rate of ~5 cm·year−1. The ridge migrated northward faster after 125 Ma due to the reorientation and the northwestward subduction of the Pacific Plate. So, the ridge is likely located in the north of the Korean Peninsula. More than 2000 km separated the ridge and the position of the Shilu copper deposit at ~104 Ma (Fig. 11). Therefore, this Pacific ridge was not responsible for the Shilu adakitic rocks either.

Fig. 11
figure 11

Sketch map of magmatic belts in eastern China and different locations of the Pacific ridge at different times. The location of magmatic belts and the Pacific ridge are modified from Ling et al. (2013)

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The influence of the Neo-Tethys on the South China Block has received much less attention (Sun 2016) because its current trench is located to the south of the South China Sea, which is far away from the present position of the South China Block (Fig. 12). However, the position of the trench in the Cretaceous needs to be reconstructed because dramatic tectonic movements have occurred in Southeast Asia since that time. Considering the extrusion of the Indochina Block and the opening of the South China Sea (Tapponnier et al. 1982) and Borneo away from the South China Block (Ben-Avraham and Uyeda 1973), the trench should have been much closer to the South China Block at that time (Fig. 12).

Fig. 12
figure 12

Sketch map of present plates (Royden et al. 2008) and plates reconstruction at ~125 Ma (based on Ben-Avraham and Uyeda 1973; Royden et al. 2008; Seton et al. 2012). Grey areas show the present location of blocks. Red lines are estimated positions of subduction zones. Blue lines show the boundary lines between the Pacific and the Tethys

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The Neo-Tethys Plate started to subduct northward at ~125 Ma (Sun 2016). With normal plate subduction of the Neo-Tethys, however, it is difficult to not only reach Shilu but to get partial melting as well because it was still several hundred kilometers away from Shilu and would have been cold. However, if the Neo-Tethys ridge subducted at ~104 Ma, that would be the best mechanism for Shilu adakitic rocks and copper mineralization (Fig. 13).

Fig. 13
figure 13

Cartoon map illustrating the evolution history of the Neo-Tethys plate. a Neo-Tethys plate started to subduct northward at ca.125 Ma. b The Neo-Tethys ridge subducted beneath South China Block at 104 Ma. The ridge was parallel with subduction zone, which formed a magmatic line from Tibet Plateau to Canton in China

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Ridge subduction is common along convergent margins, especially in South America. Many ridge subductions are associated with large-sized porphyry copper deposits, because of slab melting and high oxygen fugacities (Sun et al. 2010a). Oceanic crust closer to a spreading ridge is younger, hotter, and lighter, usually leading to low-angle subduction. Ridge subduction can extend far away from subduction zones through flat subduction, e.g., over 1000 km (Coney and Reynolds 1977; Livaccari et al. 1981; Li and Li 2007; Espurt et al. 2008). The low fluorine and high chlorine of the Shilu intrusion also indicate shallow subduction, because chlorine in recycled oceanic crust has high mobility relative to fluorine in the early-stage of subduction (Lassiter et al. 2002). The Neo-Tethys ridge was parallel to the subduction zone, similar to the subduction of the northeast Pacific ridge in western North America (Cole and Basu 1995). Subduction of the Neo-Tethys ridge formed a magmatic line along the subduction zone from the Tibet Plateau to Canton in southern China (Fig. 13). A signature of Late Cretaceous (~100 Ma) Neo-Tethys ridge subduction has been found in the Gangdese belt (Guan et al. 2010; Ye et al. 2015). In addition, Li et al. (2014) determined that South China tectonic evolution is characterized by the alternate appearance of compression and extension based on Cretaceous basin magmatism and paleo-tectonic stress field. There was a compression environment at around 110 Ma. To sum up, we propose that the eastern Neo-Tethys ridge was parallel to the subduction zone and subducted beneath the South China Block, and the partial melting of subducted oceanic crust formed the Shilu adakitic rocks associated with Cu–Mo deposits.

6 Conclusions

  1. 1.

    The Shilu intrusion has geochemical characteristics of adakitic rocks with high Al2O3 and Sr, as well as high Sr/Y and La/Yb ratios, and low Y and Yb contents. It also has high oxygen fugacity as indicated by high zircon Ce4+/Ce3+ and EuN/EuN* ratios.

  2. 2.

    Shilu adakitic magmas may have assimilated EMII and crustal materials during ascent and emplacement.

  3. 3.

    The Shilu intrusion formed between 106.6 ± 1.3 and 103.9 ± 0.5 Ma, with multiple magmatic pulses. Our analysis returned a molybdenite Re–Os isochron age of 102.2 ± 2.9 Ma, which is identical to the youngest zircon U–Pb age within error.

  4. 4.

    Shilu adakitic rocks formed by the partial melting of subducted oceanic crust at ~107–103 Ma. The subduction of the Neo-Tethys ridge is the most likely source.