Introduction

During the Late Mesozoic period, extensive magmatism took place in southeast China with which economically significant W, Sn, Mo, Bi, U, Cu, Pb, Zn, Nb, Ta, REE, and Sb mineralization is genetically associated. This igneous activitiy is divided into two main age groups, the Early Yanshanian Jurassic (180 to 140 Ma; J2–J3) and Late Yanshanian Cretaceous (140 to 97 Ma; K1) (Li 2000; Zhou and Li 2000). The geodynamic setting and tectonic regime of Yanshanian magmatism is a hot topic of international interest and remains controversial. Many contrasting models are proposed: (1) Alpine-type collision between the Yangtze and Cathaysia Blocks (Hsü et al. 1988; 1990); (2) continental rifting and basin formation (Gilder et al. 1991; Li 2000); (3) subduction of the paleo-Pacific plate at an active continental margin; for the latter model mechanisms have been suggested by different researchers, including a normal continental arc (Jahn et al. 1990; Charvet et al. 1994; Martin et al. 1994; Lan et al. 1996; Lapierre et al. 1997), shallow subduction and roll-back (Zhou and Li 2000; Zhou et al. 2006), and foundering of a flat-subducting slab followed by slab roll-back (Li et al. 2007; Li and Li 2007).

Precise estimates of the onset and duration of magmatism are important not only for constraining the age relations with mineralization, but also for better understanding processes of mantle dynamics and melt generation within thinned continental crust during extension. The Xiangshan volcanic-intrusive complex, containing the largest volcanic-hosted uranium deposits in China, is located in Jiangxi Province, Southeast China. The timing and duration of the magmatism forming this complex has been controversial. Previous studies of the regionally extensive volcanic-intrusive units at Xiangshan gave a large age variation (e.g. Chen et al. 1999; Yu 2001; Fan et al. 2005; Zhang and Li 2007) and it is difficult to link the age data with information on the magma evolution of individual systems (Yang et al. 2010). Previously, the volcanic activity at Xiangshan was considered to take place mainly in the Late Jurassic and continued to the Early Cretaceous (158 to 135 Ma) (Jiang et al. 2005), and two magmatic cycles were distinguished (e.g. Xia et al. 1992; Wu 1999). Recently, ages of 135 ± 1 Ma and 135 ± 1 Ma were obtained for a rhyodacite and a rhyodacitic porphyry, respectively (Yang et al. 2010), indicating that the large-scale Xiangshan volcanic-intrusive activity took place in the Early Cretaceous rather than in the Late Jurassic. New data presented here indicate that the Xiangshan volcanic-intrusive activity was rather short-lived and that some of the ages obtained by previous researchers are inaccurate and need to be reinvestigated. In this study, we present precise zircon U–Pb ages, together with major and trace element and Sr–Nd–Hf isotopic data for the whole Xiangshan complex, in an attempt to better constrain the ages of the various rock types and the petrogenetic processes involved.

Geological setting

The Xiangshan intrusive-volcanic complex in Jiangxi Province, SE China, is located in the Ganhang tectonic belt (Gilder et al. 1996), a tectonic suture zone between the Yangtze and Cathaysia blocks (Fig. 1). The complex, comprising a resurgent caldera (collapsed caldera and resurgent dome association), is located at the northwest end of a zone of volcanic-intrusive complexes in SE China, and has ellipsoidal shape. The Xiangshan basin is approximately 26.5 km long and 15 km wide, covering an area of about 309 km2. It is a large-scale volcanic collapse basin (Fig. 2), with the basement of the complex consisting of greenschist to amphibolite facies metamorphic rocks (schists, amphibolites) of Early- to Meso-Proterozoic age and Neoproterozoic rocks of the Sinian period (phyllites, slates, sandstones). Since the late 1950s, a number of giant uranium deposits have been explored and mined in this area. A series of Cretaceous-Tertiary red-bed basins were deposited in the Gan-Hang Belt (Fig. 1). The NE-trending basins are infilled with red clastic sedimentary rocks along with marls and evaporites (gypsum), which are locally interlayered with the volcanic rocks. The basins are mainly located to the NW of the volcanic-intrusive complex belt, and are thought to have been deposited in a back-arc extensional environment (Zhou and Li 2000).

Fig. 1
figure 1

Geological sketch map showing the Gan–Hang Belt in Southeast China. (Modified after Yu et al. 2006)

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Fig. 2
figure 2

Geological sketch map showing the volcanic-intrusive complex in the Xiangshan district (Modified after Fang et al. 1982; Fan et al. 2001a). 1: glutenite; 2: crystal tuffs and porphyritic lava; 3: siltite and rhyodacite; 4: gritstone and glutenite; 5: gritstone; 6: metamorphic rocks; 7: granite porphyry; 8: granite; 9: lamprophyre dyke; 10: lava neck (inferred); 11: faults; 12: sampling locations

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The volcanic rocks from the Xiangshan include rhyolitic crystal tuffs, welded tuffs, rhyodacites (including rhyodacitic porphyry), acidic porphyritic lava, associated subvolcanic rocks such as monzogranite-porphyry and granite-porphyry, and late dykes such as quartz monzonitic porphyry and lamprophyre. The first volcanic cycle of the two previously distinguished magmatic cycles represents a fissure eruption and is composed mainly of rhyodacite that yielded a single-grain zircon U–Pb age of 158.1 ± 0.2 Ma (Yu 2001) and rhyolitic welded tuff. Based on the geologic occurrence and petrologic characteristics, two types of rhyodacite were identified in the Xiangshan uranium ore field (Wu 1999). It was recognized that rhyodacite, which contains hematite bands, is of volcanic origin, but rhyodacitic porphyry without hematite bands belongs to a hypabyssal intrusive phase (Wu 1999). Field relations show that the rhyodacitic porphyry is intrusive and penetrates within rhyodacite, or as layers overlap the rhyodacite, and occurs as tongue-like bodies that interpenetrates clastoporphyritic extrusives (Wu 1999; Wu et al. 2003; Fan et al. 2005). The second volcanic cycle was a central vent eruption and was composed mainly of an intrusive facies and an extrusive facies of felsic porphyroclastics that constitute the main part of the Xiangshan volcanic-intrusive complex; that yielded a single-grain zircon U–Pb age of 140.3 Ma (Chen et al. 1999). After the effusion, the volcano collapsed, creating ring fractures. Finally, hypabyssal rocks, such as monzogranite-porphyry and granite-porphyry (with a single-grain zircon U–Pb age of 135.4 Ma, Chen et al. 1999), were intruded along the ring fractures, forming ring dykes (Fig. 2). Mafic magmatism also occurs in the Xiangshan area, including lamprophyre dykes (with a single-grain zircon U–Pb age of 125.1 ± 3.1 Ma, Fan et al. 2005) and quartz monzonitic porphyry dykes (with a singled-grain zircon U–Pb age of 129.5 ± 2.0 Ma, Fan et al. 2005). These subvolcanic rocks contain mafic microgranular enclaves (Fan et al. 2001b). More details on the petrography, mineralogy, lithogeochemistry, and isotope geochemistry of the Xiangshan volcanic-intrusive complex are provided in Jiang et al. (2005) and Yang et al. (2010), and references therein.

Petrology

The mineralogical and petrographic features of various types of rocks in the Xiangshan intrusive-volcanic complex are summarized in Table 1. In brief, the rhyodacite contains hematite bands and is purple in hand specimens with slight flow structures, whereas the rhyodacitic porphyry does not contain hematite bands and is gray-green in hand specimens. The porphyritic lavas make up the bulk of the Xiangshan volcanic-intrusive complex. The subvolcanic rocks are composed mainly of the monzogranite-porphyry and granite-porphyry. Quartz monzonitic porphyry crops out in the northern part of the volcanic-intrusive complex as small dykes cutting the basement metamorphic rocks.

Table 1 Lithology and mineralogy of the studied magmatic rock types from the Xiangshan volcanic-intrusive complex
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Mafic microgranular enclaves (defined as “quenched enclaves” by Fan et al. 2001b and Jiang et al. 2005), are hosted by the subvolcanic rocks, which show ovoid bodies with the long axis ranging from several centimetres to 60 cm. Large enclaves contain back-veins and quenched margins. Within the enclaves, or at the boundary between the enclaves and the host rock, K-feldspar xenocrysts are rounded and show sieve-like resorption textures, filled by the enclave material. They are compositionally identical to K-feldspar in the host rocks (Fan et al. 2001b).

Sampling and analytical methods

Samples for this study have been collected from surface exposures, underground mines and open pits. All the samples were collected away from areas of uranium mineralization, and are thus largely unaltered. The sample locations are shown in Fig. 2.

CL imaging of zircon

The samples were processed through crushing, conventional magnetic and heavy liquid separation methods to extract zircons for U–Pb dating, and then zircon grains were handpicked under a binocular microscope for analysis. The zircon grains were mounted in epoxy, and then polished to section the crystals in half for analysis. Zircons were documented with transmitted and reflected light micrographs as well as cathodoluminescence (CL) images to reveal their internal structures. The CL images for LA–ICP–MS zircon U–Pb dating were taken using a MonoCL3+ detector (manufactured by Gatan, U.S.A.) attached to a scanning electron microscope at the State Key Laboratory of Continental Dynamics, Northwest University in Xi’an. While the CL images for SHRIMP zircon U–Pb dating were performed at the Scanning Electron Microscope Group of Beijing SHRIMP Center, Chinese Academy of Geological Sciences.

Zircon U–Pb dating

The LA–ICP–MS zircon U–Pb analyses for samples XS-05 and XS-59 were carried out at the State Key Laboratory for Mineral Deposits Research using an Agilent 7500a ICP-MS equipped with a New Wave Research 213 nm laser ablation system at Nanjing University. The ablated material is transported in a He carrier gas through 3 mm i.d. PVC tubing and then combined with Ar in a 30 cm3 mixing chamber prior to entering the ICP-MS for isotopic measurement. Mass discrimination of the mass spectrometer and residual elemental fractionation were corrected by calibration against a homogeneous zircon standard, GEMOC/GJ-1 (608 Ma). Samples are analyzed in ‘runs’ of ca. 15 analyses, which include ten to 12 unknowns, bracketed by two to four analyses of the standard. The unknowns include one analysis of a well-characterized zircon standard, Mud Tank (735 Ma), as an independent control on reproducibility and instrument stability, and a weighted mean age of 735 ± 13 Ma (2σ, n = 16) was obtained for Mud Tank zircon during our routine analyses. Analyses were carried out with a beam diameter of 30–40 μm, 5 Hz repetition rate, and energy of 10 to 20 J/cm2. Data acquisition for each analysis took 100 s (40 s on background, 60 s on signal). Raw c ount rates for 206Pb, 207Pb, 208Pb, 232Th, and 238U were collected for age determination. Detailed analytical procedures are similar to those described by Jackson et al. (2004). The raw ICP-MS data were exported in ASCII format and processed using GLITTER (Van Achterbergh et al. 2001). Common Pb contents were evaluated using the method described by Andersen (2002). The age calculations and plotting of Concordia diagrams were made using Isoplot v. 3.23 (Ludwig 2003).

The SHRIMP zircon U–Pb analyses for samples XS-29-1, XS-12, XS-30-2 and XS-63 were performed using the SHRIMP II ion microprobe at Curtin University of Technology, Australia, via a remote control system operated in the State Key Laboratory for Mineral Deposits Research, Nanjing University. A primary ion beam of 4.5 nA, 10 kV O−2 and 25 to 30 μm spot diameter were used. Mass was analyzed at a mass resolution of 5000 (1% peak height). The standard TEM zircons (417 Ma) of the Geological Survey of Australia were used to correct for inter-element fractionation. After every three unknown zircon measurements, the zircon standard (TEM) was measured to control the reproducibility and instrument stability, and a weighted mean age of 417 ± 3 Ma (2σ, n = 13) was obtained for TEM zircon during our routine analyses. The 204Pb based method of common Pb correction was applied. Details of the analytical processes, principles and parameters of the equipment have been previously published (Compston et al. 1984; Williams and Claesson 1987; Compston et al. 1992; Williams et al. 1996; Williams 1998; Song et al. 2002).

Zircon Lu–Hf isotopes

Zircon Hf isotope analyses were carried out using a Newwave UP213 laser-ablation microprobe, attached to a Neptune multi-collector ICP-MS at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. Instrumental conditions and data acquisition were comprehensively described by Wu et al. (2006) and Hou et al. (2007). A stationary spot was used for the present analyses, with a beam diameter of either 40 μm or 55 μm depending on the size of ablated zircon domains. Helium was used as the carrier gas to transport the ablated sample from the laser-ablation cell to the ICP-MS torch via an Ar gas mixing chamber. In order to correct the isobaric interferences of 176Lu and 176Yb on 176Hf, 176Lu/175Lu and 176Yb/173Yb ratios (normalizing ratios of 0.02658 and 0.796218, respectively; Chu et al. 2002) were determined. The mass 176 isobaric interference correction functions can be expressed as:

$$\begin{array}{*{20}l} {{^{{{\text{176}}}} {\text{Hf}} = {\text{176}}_{{\text{m}}} - [^{{175}} {\text{Lu}} \times (^{{176}} {\text{Lu/}}^{{175}} {\text{Lu}})_{{\text{t}}} ({\text{M}}_{{176}} /{\text{M}}_{{175}} )^{{\beta ({\text{Lu}})}} + ^{{173}} {\text{Yb}} \times (^{{176}} {\text{Yb/}}^{{173}} {\text{Yb}})_{{\text{t}}} ({\text{M}}_{{176}} /{\text{M}}_{{173}} )^{{\beta ({\text{Yb}})}} ]} \hfill} \\ {{\beta = \ln ({\text{R}}_{{\text{m}}} /{\text{R}}_{{\text{t}}} )/\ln ({\text{M}}_{{\text{A}}} /{\text{M}}_{{\text{B}}} ),} \hfill} \\ \end{array} $$

Where β is the mass bias coefficient, Rm is the measured ratio of the two isotopes and Rt is the accepted ratio of the two isotopes. Zircon GJ1 was used as the reference standard, with a weighted mean 176Hf/177Hf ratio of 0.282013 ± 27 (2σ) during our routine analyses. It is not distinguishable from a weighted mean 176Hf/177Hf ratio of 0.282013 ± 19 (2σ) (Elhlou et al. 2006) from in-situ analysis.

For the calculation of εHf(t) values, we have adopted a decay constant for 176Lu of 1.867 × 10−11 year−1 (Soderlund et al. 2004) and chondritic present-day values of 176Lu/177Hf (0.0336) and 176Hf/177Hf (0.282785) derived from Bouvier et al. (2008). Depleted mantle Hf model ages (TDM) were calculated using the measured 176Lu/177Hf ratios of zircon, assuming that the depleted mantle reservoir has a 176Hf/177Hf = 0.283250 at present day, with a 176Lu/177Hf value of 0.0384 (Griffin et al. 2000). The mantle extraction model age (T CDM ) for the source rocks of the magmas was calculated by projecting initial 176Hf/177Hf ratios of the zircon to the depleted mantle model growth line using a mean 176Lu/177Hf value (0.015) for average continental crust (Griffin et al. 2002).

Major and trace elements and Sr–Nd isotopes of whole-rocks

The samples for whole-rock chemical analysis were crushed to 200-mesh using an agate mill. Whole-rock major and trace elements, and Sr and Nd isotopic compositions were determined at the State Key Laboratory for Mineral Deposits Research, Nanjing University. The whole-rock major element compositions were determined by a Jobin Yvon 38S ICP-AES, whereas trace and rare earth elements (REEs) were measured by a Finnigan Element II HR-ICP-MS. About 50 mg of powered sample was dissolved in high-pressure Teflon bombs using a HF + HNO3 mixture. Rh was used as an internal standard to monitor signal drift during counting. The analytical precision is better than 10% for most trace and rare earth elements. Detailed analytical procedures for trace and rare earth elements are described by Gao et al. (2003).

Sr and Nd isotopic compositions were measured using a Finnigan Triton TI TIMS following the methods of Pu et al. (2004, 2005). About 50 mg samples were dissolved in the same way as for trace element analyses. Complete separation of Sr was achieved by a combination of cation-exchange chromatography in H+ form and pyridinium form with the DCTA complex. Nd was then separated from the REE fractions by cation-exchange resin using HIBA as eluent. After purification, the separated Sr was dissolved in 1 μL of 1N HCl and then loaded with TaF5 solution onto W filaments for TIMS analysis. The separated Nd was dissolved in 1 μL of 1N HCl and then loaded with H3PO4 solution onto Re double-filaments. 87Sr/86Sr and 143Nd/144Nd ratios are reported normalized to 86Sr/88Sr ratio of 0.1194 and 146Nd/ 144Nd ratio of 0.7219, respectively. During the period of laboratory analysis, measurements of NIST SRM-987 Sr standard yielded 87Sr/86Sr ratio of 0.710252 ± 16 (2σ, n = 65), and the JNdi-1 Nd standard yielded 143Nd/144Nd ratio of 0.512121 ± 16 (2σ, n = 67). Total analytical blanks were 5 × 10−11 g for Sm and Nd and (2∼5) × 10−10 g for Rb and Sr. For the calculation of ISr, εNd(t) and Nd model ages, the following parameters were used: λRb = 1.42 × 10−11 year−1 (Steiger and Jäger 1977); λSm = 6.54 × 10−12 year−1 (Lugmair and Marti 1978); (147Sm/144Nd)CHUR = 0.1967 (Jacobsen and Wasserburg 1980); (143Nd/144Nd)CHUR = 0.512638 (Goldstein et al. 1984); (143Nd/144Nd)DM = 0.513151, (147Sm/144Nd)DM = 0.2136 (Liew and Hofmann 1988).

Results

Zircon U–Pb chronology

Porphyritic lava (samples XS-05, XS-29-1, XS-59)

Zircons in porphyritic lava are colorless or buff to transparent, euhedral to subhedral, elongate to stubby grains. Concentric zoning and a typical magmatic oscillatory zonation is common and no inherited cores were observed (see insets in Fig. 3a–c). The results are reported in Tables 2 and 3. All of the Th/U ratios of zircons for porphyritic lava vary between 0.25 and 1.83, mostly clustering around 0.4–0.6, consistent with a magmatic origin (Hoskin and Black 2000; Belousova et al. 2002). Thus, the zircons can represent the formation age of the rocks. The U–Pb concordia diagrams for porphyritic lavas are shown in Fig. 3a–c. The weighted mean 206Pb/238U age for samples XS-05 (Yunji), XS-29-1 (Jurong’an) and XS-59 (Youfangcun) are 136 ± 1 Ma (2σ, MSWD = 1.3), 132 ± 1 Ma (2σ, MSWD = 0.58) and 135 ± 1 Ma (2σ, MSWD = 1.5), respectively.

Fig. 3
figure 3

Zircon U–Pb concordia diagram for porphyritic lava (a, b, c), granite porphyry (d, e) and quartz monzonitic porphyry (f) from Xiangshan volcanic-intrusive complex. The insets show typical CL images of zircons with ages and εHf(t) values. Solid line circles are the location of U–Pb analyses and dotted line circles are the location of Hf analysis spots in a, c. For b, d, e, f, solid line circles are the location of U–Pb analyses and Hf analysis spots

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Table 2 LA-ICPMS zircon U–Pb dating of porphyritic lava from Xiangshan volcanic-intrusive complex in SE China
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Table 3 SHRIMP zircon U–Pb dating of porphyritic lava, granite porphyry and quartz monzonitic porphyry from Xiangshan volcanic-intrusive complex in SE China
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Granite porphyry (samples XS-12, XS-30-2)

Zircons are euhedral, up to 200 μm long, having length/width ratios of about 2:1. Most of them are relatively transparent and colorless. Euhedral concentric zoning is common in most crystals. No inherited zircon cores were observed. Th/U ratios mostly vary between 0.28 and 0.78. The results are listed in Table 3. The best estimate of the crystallization ages of samples XS-12 (Shazhou) and XS-30-2 (Jurong’an), based on the mean 206Pb/238U ratio, are 137 ± 1 Ma (2σ, MSWD = 0.81) and 137 ± 1 Ma (2σ, MSWD = 1.8) (Fig. 3d, e).

Quartz monzonitic porphyry (beschtauite, sample XS-63)

Zircons were mostly clear and euhedral with concentric zoning and length/width ratios of about 2:1. Rounded zircon cores are occasionally observed within a few idiomorphic grains. The results are listed in Table 3. One analysis of a zircon core (spot XS-63-5.2) is discordant, yielding a 207Pb/206Pb age of 1679 Ma. For 12 of 17 analyses, 206Pb/238U ratios are indistinguishable within analytical uncertainties, corresponding to a single age population with a weighted mean 206Pb/238U age of 136 ± 1 Ma (2σ, MSWD = 1.3) (Fig. 3f). This age is interpreted as the crystallization age of the quartz monzonitic porphyry (sample XS-63 from Youfangcun).

Geochemical and isotopic data

Major and trace elements

The Xiangshan volcanic-intrusive complex is peraluminous, with values of the alumina saturation index A/CNK >1.00 except for some subvolcanic rocks and a quartz monzonitic porphyry, which are metaluminous (A/CNK <1.00) (Fig. 4). The Xiangshan volcanic-intrusive complex has a wide range of chemical compositions, with SiO2 of 62.52% to 76.94%, Al2O3 of 11.55% to 15.66%, MgO of 0.17% to 2.45%, Fetot of 1.10% to 5.63%, CaO of 0.56% to 4.00% (Table 4). The samples are relatively high in total alkali contents, with total K2O + Na2O ranging from 5.58% to 8.51%. The samples also have high K2O contents, with all data plotting in the high-K calc-alkaline and shoshonite fields in a K2O vs SiO2 classification diagram, except for quartz monzonitic porphyry that is medium-K calc-alkaline (Fig. 5g). On binary major and trace element vs SiO2 variation diagrams (Fig. 5a–f), samples from the volcanic-intrusive complex show a general decrease in Al2O3, TiO2, Fetot (FeO + Fe2O3), MgO, CaO, and P2O5, whereas K2O (Fig. 5g) and Na2O (not shown) do not correlate with SiO2.

Fig. 4
figure 4

A/NK vs A/CNK diagram (Maniar and Piccoli 1989) showing the peraluminous character of volcanic-intrusive complex and the metaluminous character of mafic microgranular enclaves from Xiangshan. A = Al2O3, C = CaO, N = Na2O, K = K2O (all in molar proportion)

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Table 4 Major (wt.%) and trace element (ppm) compositions of the Xiangshan volcanic-intrusive complex and mafic microgranular enclave in SE China
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Fig. 5
figure 5

Chemical variation diagrams for the Xiangshan volcanic-intrusive complex and mafic microgranular enclaves. Symboles are as in Fig. 4

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Chondrite-normalized REE patterns of the Xiangshan volcanic-intrusive complex invariably show light REE (LREE) enrichment and negative Eu anomalies (Fig. 6a). LREE are enriched relative to HREE in all rocks (LREE/HREE = 3.98 to 17.85). Differences between samples are most pronounced in the LREE abundances (LaN = 58 to 308), REE pattern slopes (LaN/YbN = 3.47 to 29.33) and degree of the Eu anomaly (Eu/Eu* = 0.18 to 0.82) (Table 4). In the primitive mantle-normalized variation diagrams, all the Xiangshan volcanic-intrusive rocks show enrichment in Rb and Th (U) and characteristic depletion in Ba, Nb (Ta), Sr, P and Ti (Table 4) (Fig. 6b). The samples are rich in high field strength elements and Ga with high Ga/Al ratios. Samples from enclave centers which show the least chemical modification and therefore can be considered as representative compositions of the mafic microgranular enclave magma show the following geochemical characteristics (Jiang et al. 2005): intermediate SiO2 contents, high MgO and low TiO2 contents, high Mg-numbers and high concentrations of Sc, Ni, Co and V; enrichment in alkalis, high K2O contents with high K2O/Na2O ratios, high light REE and large ion lithophile element contents, low initial εNd(T) values (−4.2) and high initial 87Sr/86Sr ratios (0.708147).

Fig. 6
figure 6

a Chondrite-normalized REE patterns (normalization values are from Boynton 1984) and b primitive-mantle-normalized multi-element spidergrams (normalization values are from McDonough and Sun 1995) for Xiangshan volcanic-intrusive complex and mafic microgranular enclaves

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Sr–Nd isotopic compositions

Eighteen samples from the Xiangshan volcanic-intrusive complex show a wide range of calculated 87Rb/86Sr (1.26 to 20.23) and measured 87Sr/86Sr (0.712489 to 0.750231) ratios due to significant fractionation and variable chemical compositions, while their age-corrected initial 87Sr/86Sr (ISr) ratios cluster between 0.708516 and 0.715057 (Table 5). The Xiangshan volcanic-intrusive complex also displays variable calculated 147Sm/144Nd ratios ranging from 0.09 to 0.17, and measured 143Nd/144Nd ratios between 0.512098 and 0.512268. Calculated initial εNd(T) values range from −6.9 to −8.7 and the T CDM model ages are mostly between 1.49 and 1.64 Ga (Table 5), except for the quartz monzonitic porphyry sample which has a higher εNd(T) value of −5.7 and a younger T CDM model age of 1.40 Ga. The mafic microgranular enclaves center has a relatively high ε Nd(T) value (−4.2) with the T CDM model age of 1.27 Ga, and a low initial 87Sr/86Sr value (0.708058).

Table 5 Nd and Sr isotopic systematics of the volcanic-intrusive complex and mafic microgranular enclaves from Xiangshan, SE China
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Zircon Hf isotopic compositions

Twenty-one spot analyses of Hf isotopes were obtained for rhyodacite (Sample XS-30-1, Yang et al. 2010), yielding εHf(T) values between −5.7 and −8.5 (Fig. 7a), corresponding to T CDM model ages between 1,550 Ma and 1,720 Ma (Fig. 7f). Both the εHf(T) values and T CDM model ages show nearly unimodal distributions, with an average of εHf(T) = −7.0 and T CDM  = 1,630 Ma.

Fig. 7
figure 7

Histogram of εNd(T) values and Hf model ages for the Xianghsan volcanic-intrusive complex

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Eighteen spot analyses were obtained for a rhyodacitic porphyry (Sample XS-30-3, Yang et al. 2010), yielding εHf(T) values between −6.9 and −10.1 (Fig. 7b), corresponding to T CDM Hf model ages between 1,621 Ma and 1,823 Ma (Fig. 7g). Both the εHf(T) values and T CDM Hf model ages show nearly unimodal distributions, with an average of εHf(T) of −8.5 and T CDM Hf model age of 1,721 Ma.

Sixty-one spots of in situ Hf isotope analysis have been determined on zircon from porphyritic lava (sample XS-05, XS-29-1 and XS-59) (Table 6). Zircons in porphyritic lava are characterized by clearly negative initial εHf values, ranging from −6.3 to −10.3 with a weighted mean of −8.3 (Fig. 7c), and corresponding to T CDM Hf model ages between 1,585 Ma and 1,832 Ma with an average of T CDM Hf model age of 1,706 Ma (Fig. 7h).

Table 6 LA-MC-ICP-MS in situ analysis of zircon Lu-Hf isotopic composition of volcanic-intrusive complex from Xiangshan, SE China
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Thirty-four spots were analysed on zircon from subvolcanic-rocks (sample XS-12 and XS-30-2) (Table 6). All domains show negative εHf(T) values between −6.1 and −9.9 with a average of −8.6 (Fig. 7d). Correspondingly, their Hf model ages are 1571 to 1813 Ma, with an average of 1731 Ma (Fig. 7i).

Twenty-one spots were analysed on zircon from quartz monzonitic porphyry (Sample XS-63) (Table 6). Uniformly negative εHf(T) values scatter between −5.2 and −10.2, with an average of −7.7 (Fig. 7e). Correspondingly, their Hf model ages are 1514 to 1832 Ma with an average of 1672 Ma (Fig. 7j).

A summary of Hf isotopic data for all zircons from the Xiangshan volcanic-intrusive complex is presented in Table 7. Overall, Hf isotopic data of the zircons are all negative and show nearly unimodal distributions, and T CDM of late Paleo-Mesoproterozoic ages.

Table 7 Lu–Hf isotopic character for zircons from the Xiangshan volcanic-intrusive complex
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Discussion

Timing of the Xiangshan volcanic-intrusive complex

Previously published geochronological data suggest that the formation of the Xiangshan volcanic-intrusive complex corresponds to two cycles of volcanic-intrusive activity mainly in the Late Jurassic (158–135 Ma) (Jiang et al. 2005), which lasted for more than 20 million years.

In this study, the high spatial resolution zircon U-Pb dating results demonstrate that emplacement of the various igneous units at Xiangshan took place within a rather short time span (ca. 2 Ma). Our new data indicate that the Xiangshan volcanic-intrusive complex was formed during a Cretaceous magmatic event in SE China that was generated in response to the peak of an extensional tectonic regime (Li 2000). The Xiangshan complex comprises a series of felsic volcanic and intrusive rocks that overlie the Mesoproterozoic basement. The stratigraphically oldest rhyodacite yielded a U–Pb zircon age of 135 ± 1 Ma. The overlying rhyodacitic porphyry was dated at 135 ± 1 Ma (Yang et al. 2010). Our new ages for three porphyritic lava samples from Yunji (Sample XS-05), Jurong’an (Sample XS-29-1), Youfangcun (Sample XS-59) area are 136 ± 1 Ma, 132 ± 1 Ma and 135 ± 1 Ma, respectively. The felsic magmas at Xiangshan is composed of sub-volcanic rocks (granite porphyry). This unit yielded U–Pb zircon ages of 137 ± 1 (Shazhou, Sample XS-12) and 137 ± 1 Ma (Jurong’an, Sample XS-30-2), respectively. The final stage of magmatism at Xiangshan involved emplacement of quartz monzonitic porphyry dykes and lamprophyric dykes. The quartz monzonitic porphyry yielded a U–Pb zircon age of 136 ± 1 Ma (Youfangcun, Sample XS-63). All these ages are identical within analytical errors.

The precise zircon U–Pb dating of magmatic events at Xiangshan demonstrates that the volcanic-intrusive activity is ephemeral. Field relations indicate that the intrusive rocks postdated the Xiangshan volcanism, but it is clear from present geochronologic data that the age gap might be rather small. Emplacement of the volcanic-intrusive complex took place within a short time period of 135∼137 Ma, which is indistinguishable from the age uncertainties of the individual samples.

Petrogenesis of Xiangshan volcanic-intrusive complex

Genetic type

Most igneous rocks of Xiangshan volcanic-intrusive complex are peraluminous (Fig. 4). The samples show a SiO2 variation between 65.35% and 76.94% and they all have low TiO2 contents (mostly <0.5 wt%). They are enriched in REE (especially LREE), HFSE (except Nb and Ta, which are relatively depleted) and Ga contents, but are depleted in Ba, Sr, Nd, Ti, P and transition metals. The Ga/Al ratios are high. These geochemical characteristics are comparable to shoshonites (Morrison 1980). In general, all the samples from the volcanic-intrusive complex have an A-type affinity, as indicated by high Ga/Al ratios. They plot in the A-type fields in the 10,000 × Ga/Al vs Zr and Nb classification diagrams (not shown, Whalen et al. 1987, 1996; Jiang et al. 2005).

Geothermometry using mineral equilibria (Jiang et al. 2005) and homogenization temperatures of magmatic inclusions (Xia et al. 1992) indicate a high crystallization temperature (>850°C) for the Xiangshan magmas. Trace element geochemistry and Sr–Nd–O isotope systematics imply that the Xiangshan magmas were probably derived from partial melting of Meso-Proterozoic metamorphosed lower-crustal rocks that had been dehydrated during an earlier thermal event (Jiang et al. 2005). Jiang et al. (2005) also suggested a phlogopite-bearing spinel harzburgitic lithospheric mantle source for the mafic microgranular enclave magmas. A back-arc extensional setting related to subduction of the Palaeo-Pacific plate has been favored to explain the petrogenesis of the Xiangshan volcanic-intrusive complex and its mafic microgranular enclaves (Jiang et al. 2005).

Origin of the volcanic-intrusive complex

The Xiangshan volcanic-intrusive complex shows a narrow range of εNd(T) values (−7.4 to −8.8), whereas their initial 87Sr/86Sr ratios vary from 0.7087 to 0.7152 (Fig. 8). Previous researchers considered that the Xiangshan volcanic-intrusive complex was derived mainly from partial melting of crustal rocks (e.g. Wang et al. 1991, 1993; Liu et al. 1992; Fan et al. 2001a, b) and derived from a common magmatic source due to their similar Sr–Nd isotopic characteristics (e.g. Shen et al. 1992; Xia et al. 1992; Fan et al. 2001a; Jiang et al. 2005).

Fig. 8
figure 8

Initial 87Sr/86Sr vs εNd(t) diagram for the volcanic-intrusive complex and mafic microgranular enclaves from the Xiangshan. The Sr and Nd isotope data for orthometamorphic and parametamorphic rocks are from Yuan et al. (1991) and Hu et al. (1999), ISr and εNd(T) were calculated at 135 Ma for these metamorphic rocks. Symboles are as in Fig. 4

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Because of the high closure temperature of the zircon Hf isotopic system (higher than the closure temperature of zircon U–Pb isotopic system; Patchett 1983; Cherniak et al. 1997; Cherniak and Watson 2003), zircon can record the characteristics of different types of source rocks. Therefore, the zircon Hf isotope system has become an important tool to constrain the origin of magmas, especially to decipher processes of crustal evolution and mantle-crust interaction (Griffin et al. 2000, 2002). In all volcanic-intrusive rocks, the approximately 135 Ma old zircons are characterized by negative initial εHf(t) values, ranging from −5.2 to −10.3, with most values concentrated in the range of −7 to −9 (Figs. 7a–e, 9). The negative and unimodally destributed εHf(t) values indicate the same magmatic source for all rocks of the complex and predominantly crust-derived material. Based on the Sr–Nd isotopic composition, Jiang et al. (2005) suggested that the volcanic rocks were probably derived from partial melting of Mesoproterozoic metamorphic rocks including both orthometamorphic and parametamorphic rocks at depth (Fig. 8). A SHRIMP U–Pb zircon age of 1766 ± 19 Ma has been reported for the amphibolites exposed in NW Fujian-SW Zhejiang in the Cathaysia Block (Li et al. 1998). This age is similar to the Nd model ages (1.49 to 1.64 Ga) and Hf model ages (1.55 to 1.83 Ga) (Fig. 9). Therefore, we contend that the Xiangshan volcanic-intrusive complex may have been derived from the remelting of Meso-Palaeoproterozoic metamorphic rocks from Xiangshan basement.

Fig. 9
figure 9

206Pb/238U ages vs εHf(t) of zircons from the Xiangshan volcanic-intrusive complex

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However, it was still unclear whether there is any contribution from mantle-derived magmas. One of the aims of this study was to understand the possible mixing between mantle-derived magmas and crustal melts by applying the Sr–Nd–Hf isotopic systems. For the rhyodacite, rhyodacitic porphyry, porphyritic lava and granite porphyry, similar Nd model ages (1.49 to 1.64 Ga) and Hf model ages (1.55 to 1.83 Ga) have been calculated, indicating a major Paleo-Mesoproterozoic crustal source. In contrast, the quartz monzonitic porphyry has inconsistent Nd and Hf model ages. The quartz monzonitic porphyry has a comparable zircon Hf model age of 1.51 to 1.83 Ga, but a younger whole-rock Nd model age of 1.40 Ga. It appears that the zircon Lu–Hf and whole-rock Sm–Nd isotope systems in quartz monzonitic porphyry were decoupled and evolved in different ways, suggesting the involvement of a major Paleo-Mesoproterozoic crustal source and a subordinate younger mantle component. This occurrence should be basically related to the fact that zircon retains its Hf isotope signature acquired during crystallization from the initial magma, whereas the whole-rock Sm–Nd system was readily equilibrated with the new melt (mafic microgranular enclave magma) and hence gave the lower TDM values. The higher εNd(T) values of −5.7 for quartz monzonitic porphyry also indicates the involvement of mantle-derived magma, although we also cannot rule out the possibility of remelting of larger amounts of older basaltic underplated material.

Fractionation processes

On binary major element vs SiO2 variation diagrams, compositional gaps are common features of the complex, whereas major oxides (except K2O and Na2O) show linear trends. The REE of the complex are characterized by significant enrichment of LREE relative to HREE, and the REE patterns for complex are actually inhomogeneous. The granite porphyry has the highest LREE content and lowest HREE content (14.81 > LREE/HREE > 17.85), its distribution curves showing significantly steeper trends than other rocks (Fig. 6a). Porphyritic lava shows the lowest LREE content and has the largest negative Eu anomalies (0.42 > Eu/Eu* > 0.18) (Fig. 6a). The primitive mantle-normalized multi-element diagrams (McDonough and Sun 1995) (Fig. 6b) show significant negative Ba, Sr, P and Ti anomalies in the porphyritic lava, though the complex has relatively similar patterns to the primitive mantle-normalized element spidergrams.

Taking into account that magmatic activity in the Xiangshan volcanic-intrusive complex is contemporaneous based on the precise zircon U–Pb ages of this study, and they have similar whole rock Sr–Nd and zircon Hf isotopic compositions, fractional crystallization, rather than magma mixing or partial melting of heterogeneous sources during the formation of these rocks, is favored to explain the diversity of the element variations. This is supported by depletions in Ba, Sr, Nb, P, Ti and Eu shown in the spidergrams (Fig. 6). Strong Eu depletion requires extensive fractionation of plagioclase and/or K-feldspar. Fractionation of plagioclase would result in negative Sr and Eu anomalies, and that of K-feldspar would produce negative Eu and Ba anomalies.

In log–log diagrams of Ba vs Sr, Ba vs Rb, Ba/Sr vs Sr and Rb/Sr vs Sr (Fig. 10), it appears that the granite porphyry is less influenced by crystal fractionation, and could be regarded as the initial composition of the melt. Ba changes little in the crystallization of rhyodacite, rhyodacitic porphyry and granite porphyry, but decreases rapidly in the crystallization of porphyritic lava. This is explained by the separation of plagioclase in the crystallization stage of rhyodacite, rhyodacitic porphyry, and by fractionation of K-feldspar in the crystallization stage of porphyritic lava. In addition to major phases, accessory minerals seem to have controlled much of the REE variation. The decrease of LREE with increasing SiO2 suggests a separation of minerals with high LREE, such as apatite, zircon and allanite, which are important accessory minerals in these rocks. According to the diagram of (La/Yb)N vs La (Fig. 11), the variation of REE contents seems to be controlled by fractionation of allanite during magmatic evolution.

Fig. 10
figure 10

a Ba vs Sr, b Ba vs Rb, c Ba/Sr vs Sr and d Rb/Sr vs Sr diagrams showing that fractionation of plagioclase plays an important role in the rhyodacite (and rhyodacitic porphyry) stage of differentiation; whereas in the prophyritic lava stage, separation of K-feldspar appears to have controlled the variation of these elements. Partition coefficients of Rb, Sr and Ba are from Philpotts and Schnetzler (1970). Symboles are as in Fig. 4

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Fig. 11
figure 11

(La/Yb)N vs La diagram showing the change of REE patterns by separation of accessory minerals, especially allanite and apatite. Partition coefficients are from Fujimaki (1986) for apatite, Mahood and Hildreth (1983) for zircon and allanite, and Yurimoto et al. (1990) for monazite. Symboles are as in Fig. 4

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A new integrated model for the origin of the Xiangshan volcanic-intrusive complex

A model for the origin of the Xiangshan volcanic-intrusive complex has been previously proposed by Jiang et al. (2005). However, based on the geochronology and petrological data presented in this paper, this model needs significant modification. The volcanic activity of Xiangshan occurred during the Early Cretaceous (about 135 Ma), rather than Late Jurassic to Early Cretaceous (158 to 135 Ma), which was cited in the Jiang et al. (2005)’s model.

Moreover, there are lamprophyre dykes in the volcanic-intrusive complex, showing that the mafic magmas were emplaced at the felsic magma emplacement levels. In the Jiang et al. (2005) model, the mafic microgranular enclaves were generated in the lower crust where the felsic magma chamber formed. Furthermore, although the enclaves are darker and finer-grained than their host granitoids, most are intermediate to felsic in composition. The relatively fine-grained microstructures indicate relatively rapid crystallization. The enclave magma crystallized more rapidly than the felsic magma (Zeck 1970), once thermal equilibrium has been established. The felsic and mafic magmas could remain separate (Yoder 1973), particularly if small volumes of more mafic magma are enclosed in large volumes of felsic magma and if mixing is minimal (Kouchi and Sunagawa 1983), because chilling increases the viscosity of the more mafic globules, as well as promoting viscosity during rapid crystallization. Mafic microgranular enclaves typically show little or no evidence of disintegration after their injection into felsic magma, and the surrounding granitoid is generally not obviously modified. Both the compositional gap and discontinuous compositional trend between the mafic microgranular enclaves and their host volcanic rocks at Xiangshan suggest minimal scale mixing between mafic microgranular enclave magma and their host granitoid magma (Fig. 5). Besides, disintegration of enclaves should disseminate finer-grained material (Vernon 1984) than is typical of granite porphyry at Xiangshan. Furthermore, the report of lamprophyric dykes (Fan et al. 2005), resembling enclaves in adjacent granitoids suggests that enclave magmas can exist either independently of, or as a separate layer in, their host granitoid magma bodies. So the compositional variety of the enclave magmas was produced before their injection and away from the exposed site of final mingling. Consequently, we propose a two-magma-chamber model for the development of the Xiangshan volcanic-intrusive complex and mafic microgranular enclaves. The formation of the Xiangshan mafic microgranular enclaves can be explained by the injection of the mafic magma from a deep seated magma chamber toward the hypabyssal felsic magma chamber.

Based on the petrological and geochemical data presented in this paper, a new integrated model for the origin of the Xiangshan volcanic complex is proposed, as shown in Fig. 12a–d. During the Cretaceous, it is suggested that an extensional environment predominated in SE China (Li 2000), and Zhou and Li (2000) have suggested a model associated with the Palaeo-Pacific plate subduction and underplating of mafic magmas for the origin of Late Mesozoic igneous rocks in SE China. At Early Cretaceous (about 135 Ma) time, the dip angle of the subducted slab increased, resulting in oceanward migration of the active magmatic zone (Fig. 12a) (Zhou and Li 2000; Jiang et al. 2005). At the same time, back-arc extension, and upwelling of the astenthenosphere, as a consequence of slab roll-back, resulted in partial melting of the lithospheric mantle, generating the high-Mg potassic magmas in the lower crust (Jiang et al. 2005). Underplating of these anomalously high-temperature (>1,200°C) (Jiang et al. 2005) melts into the crustal source region induced partial melting. Such crustal melts diapirs rose and assembled to form hypabyssal felsic magma chambers, which then erupted to form the Xiangshan volcanic rocks (such as rhyodacite, and followed by the fractionation crystallization of plagioclase and allanite, Fig. 12b) and the Xiangshan intrusive rock (porphyritic lava, and followed by the fractional crystallization of K-feldspar and allanite, Fig. 12c). The plutonic high-Mg potassic magma chamber and hypabyssal felsic magma chamber were connected throughout the extensional tectonics. High-Mg potassic magmas from the plutonic magma chamber injected into the hypabyssal felsic magma chamber through the extensional channel or faulted structures. The composition of the high-Mg potassic magma may have been changed by concurrent hybridization and assimilation of partial melts over the large depth range during the protracted upward percolation in the plexus of crustal conduits, and mafic microgranular enclaves appear to crystallize at granitoid host magma emplacement levels (Fig. 12d).

Fig. 12
figure 12

Schematic diagram showing magma generation processes for the Xianghsan volcanic-intrusive complex and mafic microgranular enclaves. The volcanic-intrusive complex was formed in a continental extensional setting as a consequence of slab roll-back (a modified after Jiang et al. 2005). The complex has undergone advanced fractional crystallization with separation of plagioclase, K-feldspar and accessory minerals such as allanite in the crystallization of rhyodacite, rhyodacitic porphyry and porphyroclastic extrusives (b, c). The formation of the Xiangshan mafic microgranular enclaves can be explained by the injection of the basaltic magma from deep seated magma chamber toward hypabyssal acidic magma chamber at the acidic magma emplacement levels (d). See text for more discussion

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Conclusions

On the basis of the integrated geochronological, geochemical and Sr–Nd–Hf isotopic analyses of the Xiangshan volcanic-intrusive complex, SE China, the following scenario can be outlined:

  1. (1)

    U–Pb zircon dating of the volcanic-intrusive complex from Xiangshan provides insights into the extrusive and intrusive activity at Xiangshan, which took place within a short time span (135–137 Ma), and overlaps with the peak episode of an extensional tectonic regime during the Cretaceous in SE China.

  2. (2)

    Geochemical data indicate that all the samples from the Xiangshan volcanic-intrusive complex have A-type affinities. Their similar whole rock Sr–Nd and zircon Hf isotopic data suggest that igneous rocks in the Xiangshan volcanic-intrusive complex have the same magmatic source, and were derived mainly from Paleo-Mesoproterozoic crustal rocks, without significant addition of mantle-derived magma (except quartz monzonitic porphyry, which has higher εNd(T) values of −5.8, and may indicate the involvement of a subordinate younger mantle-derived magma for its magmatic origin). Geochemical studies indicate that rocks from the complex have variable REE patterns and they show negative anomalies of Ba, Nb, Sr, P, Eu and Ti in spidergrams, suggesting that these rocks have undergone advanced fractional crystallization with separation of plagioclase, K-feldspar and accessory minerals such as allanite.

  3. (3)

    Detailed petrographic, chemical, and isotope studies of the Xiangshan volcanic complex and its mafic microgranular enclaves provide information on their genesis and evolution. Our model interprets this Cretaceous complex as forming in an extensional and back-arc environment. The formation of the Xiangshan mafic microgranular enclaves can be explained with the injection of mafic magma from a deep seated magma chamber into a hypabyssal felsic magma chamber at a higher emplacement level in the crust.