Porphyry and skarn Au–Cu deposits in the Shizishan orefield, Tongling, East China: U–Pb dating and in-situ Hf isotope analysis of zircons and petrogenesis of associated granitoids

Ore Geology Reviews 43 (2011) 182–193

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Porphyry and skarn Au–Cu deposits in the Shizishan orefield, Tongling, East China: U–Pb dating and in-situ Hf isotope analysis of zircons and petrogenesis of associated granitoids Xiao-Nan Yang a, Zhao-Wen Xu a,⁎, Xian-Cai Lu a, Shao-Yong Jiang a, Hong-Fei Ling a, Liang-Gen Liu b, Da-Yuan Chen b a b

State Key Laboratory for Mineral Deposit Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, PR China No. 321 Geological Party of Anhui Bureau of Geology and Explorations, Tongling 244033, PR China

a r t i c l e

i n f o

Article history: Received 27 April 2010 Received in revised form 9 August 2010 Accepted 10 September 2010 Available online 14 October 2010 Keywords: Zircon U–Pb dating Sr, Nd, Pb and Hf isotopic geochemistry Thermobarometry Early Cretaceous Tongling

a b s t r a c t The magnetite-series (I-type) calc-alkaline granitoid suit, ranging from pyroxene monzodiorite to granodiorite, is associated with the porphyry and skarn gold–copper deposits at the Shizishan orefield in Tongling district, Anhui Province. In-situ U–Pb dating and Hf isotope analysis of magmatic and inherited zircons are combined with whole rock Sr–Nd–Pb isotopic data and mineral thermobarometry to interpret the petrogenesis. The magmatic zircons from the quartz monzodiorites yield weighted average 206Pb/238U ages of ca. 139 Ma and mean εHf(t) value of −19.8 ± 3.9 (1σ), while those from the pyroxene monzodiorite show a similar mean age but notably higher mean εHf(t) value (−8.5 ± 1.4). The inherited zircons from the quartz monzodiorite yield ages of 0.8, 2.0 and 2.4 Ga with mean εHf(t) value of −2.9 ± 1.4, while those from the pyroxene monzodiorite show younger ages (165 to 245 Ma) but similar mean εHf(t) value (−5.6 ± 4.5). Whole rock Sr–Nd–Pb isotope data indicate that crustal material significantly contributed to the magma. Mineral thermobarometry results reveal that the depths of the discrete magma chambers were about 23 km, and 10 to 2 km deep. The data above combined with previous studies suggest that: 1) The magma emplacement and crystallization (typically for zircons) mainly occurred at about 139 Ma, consistent with the age of mineralization; 2) The primary pyroxene monzodioritic magma might have mixed with the magma produced by partial melting of the Yangtze lower crust, and accumulated in the magma chamber at ca. 23 km deep in the lower crust level; 3) AFC and magma mixing were the dominate processes for the magmatic evolutions at shallow level (2 to 10 km), where the circumstances were favorable for mineralization. © 2010 Elsevier B.V. All rights reserved.

1. Introduction More than 200 ore deposits occur along the middle-lower reaches of the Yangtze River from eastern Hubei Province in the west to southern Jiangsu Province in the east. These deposits constitute the Yangtze Metallogenic Belt (Chang et al., 1991; Zhai et al., 1992; Fig. 1). Individual deposits consist mainly of porphyry-style, skarn and stratabound massive sulphide orebodies (Pan and Dong, 1999). The total gold reserves in this belt have been estimated at more than 600 t (Zhao et al., 1999). Early geological investigations into this belt accompanied the great progress in exploration for large ore deposits in China between the 1970's and 1990's. Using a fundamental geochemical approach, as well as S–Sr–Nd–Pb isotopic methods (Zhang, 1986; Chang et al., 1991; Chen, 1991; Zhai et al., 1992; Xing

⁎ Corresponding author. Tel./fax: + 86 25 8359 28 04. E-mail address: [email protected] (Z.-W. Xu). 0169-1368/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.oregeorev.2010.09.003

and Xu, 1995; Tang et al., 1998), researchers have established the spatial and temporal relationships between polymetallic mineralization, tectonics, stratigraphy and the Late Jurassic to Early Cretaceous (Yanshanian) magmatism (Pan and Dong, 1999; Zhao et al., 1999). These pioneering studies were regional in scope to assist reconstruction of the regional tectonic history from Late Paleozoic continental rifting, via continent–continent collision to Early Cretaceous intracontinental tectono-magmatic event (Chang et al., 1991; Pirajno et al., 2009). However, precise dating results for the Yanshanian plutons were not available at that time. More work is still needed to investigate the heterogeneity within each pluton for petrogenetic modelling. Petrologists and economic geologists extended their research into these two directions at the Shizishan orefield in the Tongling mining district, where preliminary geology and petrochemistry has been well documented (Tang et al., 1998; Pan and Dong, 1999; Wang et al., 2003, 2004b, 2007; Huang et al., 2004). Zircons of different age groups have been found in these plutons during geochronological studies

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Fig. 1. Regional geological sketch map showing the distribution of major mining districts along the Yangtze River, East China, and the location of the Shizishan orefield (modified from Pan and Dong, 1999).

(Wu et al., 2008a,b; Yang et al., 2008; Xie et al., 2009). In-situ Hf isotopic data reported here are essential for interpreting the origin of these zircons, and will have further implications for the sources of metals and for the geodynamic evolution of the region. Sr–Nd–Pb isotopes and geothermobarometry are also powerful tools for petrogenetic research. Moreover, published Sr–Nd–Pb isotope data are incomplete for the quartz monzodiorites, and previous thermobarometric estimates are contradictory. In this study, petrography, zircon U–Pb dating and Hf isotope analysis, whole rock Sr–Nd–Pb isotopic geochemistry and mineral thermobarometry have been employed and combined to place tight constraints on: (1) the magmatic ages and material sources for the mineralized plutons; (2) the major magmatic processes and the equilibrated physicochemical conditions for the magmas, and (3) the circumstances favouring Au–Cu mineralization. 2. Geological setting The Tongling district, Anhui Province, in the central part of the Yangtze metallogenic belt, is one of the seven major mining districts in the region (Fig. 1). It is situated at the northern margin of the Yangtze Craton and lies to the south of the Qinling–Dabie orogenic belt. There are 45 deposits and 76 plutons within the district (Pan and Dong, 1999). The five major orefields of the Tongling mining district are named Shizishan, Tongguanshan, Tianmashan, Fenghuangshan and Xinqiao (Chang et al., 1991). The Shizishan orefield is located about 7 km east of Tongling city, in the central part of the Tongling district (Chang et al., 1991; Zhai et al., 1992). The orefield is located on the southeastern limb of the NEtrending Qingshan anticline formed during the Indosinian orogenic event (ca. 230 to 220 Ma; Gu et al., 2007). There are near N–S, E–W, NE and NW trending faults of Late Jurassic to Early Cretaceous age in this orefield (Fig. 2). Outcropping strata consist mainly of Triassic thinly-layered limestone, which overlies Permian and Carboniferous limestone and Upper Devonian and Silurian strata (mainly sandstone). Orefield reserves total about 1.5 Mt Cu; the largest single deposit (Dongguashan) contains about 0.94 Mt Cu metal and 22 t Au (Zaw et al., 2007). There are two types of skarn: the predominant type is that which replaces the carbonate layer in the carboniferousTriassic strata; the other appears at the contact zones between the intrusive and the limestone country rocks (Pan and Dong, 1999). Ore minerals in the main orebodies are usually chalcopyrite, pyrrhotite and pyrite with minor amount of sphalerite, galena, molybdenite and

native gold. Skarn zonation is usually from altered pluton to garnet skarn, diopside skarn, wollastonite/tremolite marble and marble for the carbonate country rocks (Pan and Dong, 1999). The orebodies of the Au–Cu skarn deposits are hosted at different vertical depths from around −875 m to −90 m, and the porphyry style mineralization occurs in the altered Qingshanjiao pluton below the skarns. The country rocks and the associated plutons for these deposits are summarized in Table 1. Re–Os isochron ages of ore minerals from the Datuanshan Cu deposit and Chaoshan Au deposit are 139.1 ± 2.7 Ma (Mao et al., 2006) and 141.7 ± 9.9 Ma (Wang et al., 2008b), respectively, while the Rb–Sr isochron age of fluid inclusions in orebearing quartz veins in the Dongguashan deposit is 134 ± 11 Ma (Xu et al., 2005). Some Late Paleozoic SEDEX-style deposition found in the host strata prior to the Mesozoic mineralization was considered making minor contribution to the metal resource in this region (Gu and Xu, 1986; Gu et al., 2007). Geological fluid mapping was carried out to investigate the SEDEX fluid systems in detail (Hou et al., 2007). However, we will focus our study on the mineralized plutons, because major Au–Cu mineralization is associated with them (Lu et al., 2008). The plutons mainly consist of pyroxene monzodiorite, quartz monzodiorite and granodiorite, and are typically emplaced into Silurian to Triassic sedimentary strata (Tang et al., 1998). Detailed petrochemistry, a few Sr–Nd–Pb isotope data and experimental simulation of petrogenesis indicate that they constitute a magmatic rock suite which originated from mixing between juvenile mantle and ancient crustal materials (Xing and Xu, 1995, 1996; Xing et al., 1997; Wang et al., 2003, 2007; Huang et al., 2004; Gao et al., 2006; Yan et al., 2008). Specifically, these plutons were considered similar to adakite in terms of geochemical composition, even though they formed under intra-continental environment rather than in an island-arc like ‘normal’ adakite (Zhang et al., 2001; Wang et al., 2003, 2004b). Zircon U–Pb (Wang et al., 2004a; Yang et al., 2007; Wu et al., 2008a; Xie et al., 2009) and amphibole Ar–Ar ages (Wang et al., 2008a) of some plutons roughly constrained the magmatic event to between 143 and 135 Ma. 3. Petrography and whole rock chemistry Samples of the Baimangshan pyroxene monzodiorite were collected along underground adits between −90 m and −120 m (below sea level) at the Chaoshan gold mine, while those of the Datuanshan quartz monzodiorites were collected between −520 m and −560 m in the Datuanshan copper mine. Samples of the Qingshanjiao quartz monzodiorites were collected either along

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Fig. 2. Geological sketch map of the Shizishan orefield at Tongling, East China, showing the distribution of outcrops and the location of the studied plutons.

underground adits between −730 m and −875 m in the Dongguashan copper mine or from drill cores at equivalent depths. Petrographic observations of each sample were carried out to ensure that only fresh samples were analyzed. A summary of the petrographic features for the plutons is given in Table 2. Samples are usually intermediate- to coarse-grained with heterogranular or porphyritic (including amphibole and plagioclase phenocrysts) texture. Variable proportion of plagioclase, hornblende, K-feldspar, quartz, biotite, and clinopyroxene are the major minerals, while apatite, magnetite, titanite, and zircon are accessory phases. Subhedral to euhedral plagioclase shows a zonal texture, while subhedral to anhedral K-feldspar develops Carlsbad-albite compound twins or Carlsbad twin. Hornblende and clinopyroxene are euhedral to subhedral; quartz and biotite are anhedral. Zircon is usually colorless, columnar and euhedral. Particularly, in the pyroxene monzodiorite and granodiorite, there are pyroxene and hornblende megacrysts (Du et al., 2004) as well as gabbroic xenoliths with either porphyritic or gabbro texture (Du and Lee, 2004). The Baimangshan, Qingshanjiao and Datuanshan plutons belong to metaluminous magnetite-series (I-type) granitoids with SiO2 content ranging from 56% to 65% (Huang et al., 2004; Wang et al., 2007; see Supplementary Table S2). They are classified as high-potassium calcalkaline granitoids for the high alkali content (Na2O + K2O = 7.0 ± 0.5%, Huang et al., 2004; Wang et al., 2007). They show enrichment of Rb, Ba, Sr, Th, U and LREE with (La/Yb)N ratios of 21.07 to 24.28, relative

depletion of Nb, Ta and Ti, and absence of Eu anomaly with Eu/Eu* values of 0.92 to 1.03 (Huang et al., 2004; Wang et al., 2007). These common geochemical features suggest that the plutons may have originated from a similar source. In addition, the pyroxene monzodiorite, quartz monzodiorite, and granodiorite exhibit systematic variations in major and trace element contents with SiO2 content, for instance, FeO, MgO, and CaO contents are negatively correlated to SiO2 content, while REE fractionation is proportional to that (Huang et al., 2004; Wang et al., 2007). These consecutive variations imply that these plutons were formed under a series of magmatic processes during their evolution.

4. Analytical methods 4.1. Zircon U–Pb dating and Hf isotope analysis Fresh whole rock samples were crushed to less than 300 μm, and zircons were separated by using conventional heavy liquid and magnetic techniques. For each pluton, 60 to 80 zircon grains in diverse crystal shapes were picked out under microscope and mounded in rows at the surface of an epoxy resin cylinder. Then the cylinder was grounded and polished until the interior of the zircon grains was well exposed. Zircon morphologies and internal texture were further examined with Back-Scattered Electron (BSE) imaging on JEOL JXA8100 electron microprobe.

Table 1 Summary of mineralized plutons and associated ore deposits in the Shizishan orefield. Pluton

Lithology

Ore deposit(s)

Ore deposit type(s)

Host strata

Commodities & reserves Status

Baimangshan Qingshanjiao Datuanshan Shizishan

Pyroxene monzodiorite Quartz monzodiorite Quartz monzodiorite–granodiorite (Quartz) monzodiorite

Skarn deposit Porphyry–Skarn deposit Skarn deposit Skarn deposit

Middle Triassic Carboniferous Lower Triassic Lower Triassic

~ 5.2 t Au (Fe) ~ 0.94 Mt Cu; (~22 t Au) ~ 0.23 Mt Cu; (Mo, Au) ~ 0.13 Mt Cu; (Au)

Jiguanshan

Pyroxene–quartz monzodiorite

Chaoshan Dongguashan Datuanshan Shizishan East, Shizishan West Jiguanshan

Skarn deposit

Middle Triassic ~ 1.4 Mt Fe; (Ag)

Metals in parentheses are less abundant.

In production In production, ongoing exploration In production Past producer In production

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Table 2 Summary of petrography of the igneous rock samples. Pluton

Rock type

Main minerals (vol.%)

Accessory minerals

Texture and structure

Baimangshan

Pyroxene monzodiorite

mt, ap, tit, (zir)

Qingshanjiao

Quartz monzodiorite

Datuanshan

Quartz monzodiorite– granodiorite

Pl (50 to 60, An = 63 to 46), kf (10 to 12), hb (15 to 18), cpx (5 to 8) Pl (55 to 65, An = 54 to 30), kf (10 to 12), hb (10 to 12), qz (8 to 12), bio (1 to 3) Pl (55 to 65, An = 50 to 17), kf (10 to 12), hb (8 to 10), qz (10 to 18), bio (1 to 3)

Porphyric or subhedral heterogranular texture; massive structure Subhedral heterogranular or porphyric texture; massive structure Subhedral heterogranular or porphyric texture; massive structure

cpx (b 2%), mt, ap, tit, zir mt, ap, tit, zir

ap: apatite; bio: biotite; cpx: clinopyroxene; hb: hornblende; kf: K-feldspar; mt: magnetite; pl: plagioclase; qz: quartz; tit: titanite; zir: zircon.

Zircon U–Th–Pb isotopes were analyzed using an Agilent 7500 ICPMS, coupled with a New Wave Research 213 nm wave-length laser microprobe. We followed the analytical procedure introduced by Jackson et al. (2004). The repetition rate used for all analyses was 5 Hz, the aperture beam diameter/iris ratio was 15%, beam expander was 0 and the incident pulse energy was adjusted to between 10 and 20 J/cm2. The spot size of the laser was about 40 μm. The raw count rates for 206Pb, 207Pb, 208 Pb, 232Th and 238U were collected during age determination. Mass discrimination of the mass spectrometer and residual elemental fractionation were corrected by calibrations against a homogeneous standard zircon, GEMOC/GJ-1 (609 Ma). The analyses were carried out in repeated runs. In each run, two to four measurements of the standard were done at the beginning and end; twelve measurements of unknown zircons were done in between. The unknowns also included another near-concordant standard zircon, Mud Tank (735 Ma; Black and Gulson, 1978) as an independent control of reproducibility and instrument stability. The precision and accuracy of LA-ICP-MS measurements on those standards have been documented (Jackson et al., 2004). The mean concentrations of U and Th in the GJ-1 standard are 230 ppm and 15 ppm, respectively, whereby concentrations of U and Th at each analytical spot could be derived by comparing the background-corrected count rates of the unknowns to those of the GJ-1 standard. Raw data were evaluated and corrected for common Pb (Anderson, 2002) before age calculations (see Supplementary Table S1). Weighted average ages and U–Pb concordia diagram were calculated and plotted with the Isoplot plugin for Excel (Ludwig, 2000). In this study, a weighted mean 206Pb/238U age of 602 ± 4 Ma (1σ, n = 18) was obtained for the GJ-1 standard, which agrees with the previous value of 599.8 ± 2.4 Ma (1σ, n = 8) acquired by TIMS analyses (Jackson et al., 2004). For zircon grains younger than 1.0 Ga, the 206Pb/238U ages are used, while otherwise the 207Pb/206Pb ages are referred to (Griffin et al., 2004). Zircon Hf isotopes were in-situ measured on a Neptune multi-collector ICP-MS (with a 193 nm laser ablation microprobe attached) at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. Spot sizes of ca. 60 μm and a laser repetition rate of 10 Hz at 100 mJ were used. Raw count rates for 172Yb, 173Yb, 175Lu, 176(Hf+Yb+Lu), 177Hf, 178 Hf, 179Hf, 180Hf and 182 W were recorded simultaneously, and isobaric interference corrections for 176Lu and 176Yb on 176Hf were made based on precise determinations. 176Lu was calibrated using the 175Lu value, while 176 Yb/172Yb ratio of 0.5887 and mean βYb value obtained on the same spot were employed for the interference correction of 176Yb on 176Hf (Iizuka and Hirata, 2005). Details of the analytical technique are published by Wu et al. (2006). During analyses, the 176Hf/177Hf and 176Lu/177Hf ratios of the standard zircon 91500 were 0.282299±0.000035 (2σ, n=29) and 0.00030, respectively. The measured average 176Hf/177Hf ratio agrees with the ratios acquired by solution method (0.282302±0.000008 (2σ), Goolaerts et al., 2004; 0.282306±0.000008 (2σ), Woodhead et al., 2004). 4.2. Whole rock Sr–Nd–Pb isotope analyses Fresh rock samples were powdered to b74 μm in an agate mortar, and then about 50 mg powder of each sample was completely digested with a mixed HNO3 + HF solution. After adding appropriate spikes, Sr

and Nd were separated by using the standard ion exchange techniques. Their isotopic ratios were determined using a VG354 thermal ionization mass spectrometer at the Modern Analysis Center, Nanjing University, following the procedure described in detail by Wang et al. (1988). 87 Sr/86Sr and 143Nd/144Nd were normalized to 86Sr/88Sr= 0.1194 and 146 Nd/144Nd= 0.7219, respectively, for correcting isotopic fractionation effects while measuring. The blanks of the whole procedure were b1 ng for Sr and b60 pg for Nd. Reproducibility and accuracy of the Sr and Nd isotopic analyses have been checked by running the Sr standard SRM987 and Nd standard La Jolla, with measured mean 87Sr/86Sr value of 0.710224 ± 0.000020 (2σ, n = 15) and mean 143Nd/144Nd value of 0.511864 ± 0.000008 (2σ, n = 10), respectively, during the experiment. Analyses of whole rock Pb isotopes were carried out in our laboratory according to the following procedure: about 50 mg powder of each sample was completely dissolved in concentrated HNO3 + HF. After dried, the residue was redissolved in HBr + HNO3 and loaded into a 100 μL column with AG 1-X8 anion-exchange resin. The extracted Pb was then purified in a second column. Approximately 100 ng Pb was loaded onto each single rhenium filament using the silica-gel technique (Gerstenberger and Haase, 1997). Pb isotope analyses were performed on a Finnigan MAT Triton TI thermal ionizing mass spectrometer (TIMS). Analytical reproducibility of 0.01% (2σ) for 206Pb/204Pb, 0.01% for 207Pb/204Pb and 0.02% for 208 Pb/204Pb was attained in this study. Mass fractionation corrections were made based on the value of NBS-981 standard (Todt et al., 1996). 4.3. Mineral composition analysis for thermobarometry Chemical compositions of amphibole, plagioclase and K-feldspar of each pluton were analyzed using a JEOL JXA-8100 electron microprobe in our laboratory in order to make mineral thermobarometry calculations. An electron beam size of b1 μm, an accelerating potential voltage of 15 kV, and a probe current of ca. 15 nA were applied during element (Si, Ti, Al, FeTotal, Mn, Mg, Ca, Na, K) determinations. The standards were natural minerals, including hornblende (for Si, Na, Mg, Al, Ca, Ti), K-feldspar (for K), and biotite (for K, Ti, Fe). Matrix effects were corrected using the ZAF software provided by JEOL. The accuracy of the reported values during the analyses is 1% to 5%, depending on the absolute element concentrations. 5. Results 5.1. Zircon U–Pb dating and Hf isotopes Zircons from the Qingshanjiao and Datuanshan quartz monzodiorites share similar appearance, and could be divided into two groups by morphology. The dominant group consists of elongate transparent crystals (about 180 to 220 μm long) which contain oscillatory-zones under BSE imaging (Figs. 3a, b and 4a–c). The weighted average 206Pb/238U age of this group is 138.8±1.6 Ma (MSWD=2.2, Fig. 3c, d) for Qingshanjiao, and 138.7±1.3 Ma (MSWD=0.87, Fig. 4e, f) for Datuanshan. The average ages represent the time of magmatic crystallization. The εHf (t) values of the magmatic zircons are −13 to −25 for Qingshanjiao and −17 to −28 for Datuanshan. The average two-stage Hf model age (TcDM) of this group

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Fig. 3. (a and b) Back-scattered electron (BSE) images showing representative zircon morphologies from the Qingshanjiao quartz monzodiorite. The smaller circle indicates the spot for U–Pb dating, while the other one indicates that for Hf isotope analysis. 206Pb/238U ages were used for zircon grains younger than 1.0 Ga, otherwise 207Pb/206Pb ages were used. Initial Hf isotopic ratios were calculated with the in-situ U–Pb age of each zircon grain and converted to ε unit. (c) weighted average diagram with a weighted mean 206Pb/238U age of 138.8 ± 1.6 Ma (n = 17), MSWD = 2.2; and (d) U–Pb concordia plot for the magmatic zircon grains.

from Qingshanjiao (2.4±0.2 Ga) and Datuanshan (2.5±0.3 Ga) are similar. The minor group of zircons is squat to equant in crystal shape with irregular or skeletal outlines (Figs. 3a–b and 4a, d), and yields variable ages for both plutons, including 0.8 Ga, 2.0 Ga and 2.4 Ga. The εHf(t) values for this group of zircons range from −1.6 to −15, which is generally higher than the previous group. The average TcDM value of Hf isotopes for the 2.0 to 2.4 Ga zircons from both quartz monzodiorites is 3.2±0.2 Ga, while that for the 0.8 Ga zircons is 2.0±0.2 Ga. Age and Hf isotope data indicate that this group consists of inherited zircons. Zircon grains in the Baimangshan pyroxene monzodiorite can also be classified into two groups. The minor group of zircon grains which typically have acute-angled pyramids (Fig. 5a, c, d) yields a weighted average age of 136.3 ± 5.2 Ma (MSWD = 2.6, Fig. 5e, f), which agrees with the published dating results for the Baimangshan pluton (Table 3, Wang et al., 2004a, 2008a; Wu et al., 2008a). Compared to magmatic zircons in the quartz monzodiorites, this group shows higher εHf(t) values (from −6.5 to −10) but younger average TcDM (1.4 ± 0.1 Ga). Zircon grains of the dominant group usually have squat to equant shape with irregular or skeletal outlines (Fig. 5a–d). Their 206Pb/238U ages are older (from 165 to 245 Ma) than the magmatic age, and crowd around 166 Ma, 200 Ma and 235 Ma. The εHf(t) values for these zircons range from −0.7 to −9.7, which approximate to the values of the inherited zircons in the quartz monzodiorites. The average TcDM value of Hf isotopes for this group of zircons is 1.7 ± 0.1 Ga. The complex results of ages and Hf isotopes of the different zircon groups, which have implications for the material source and a dynamic geological evolution, are discussed below.

5.2. Rb–Sr isochron and initial strontium isotopic compositions The Rb–Sr isochron age for the Baimangshan pyroxene monzodiorite is 138.8 ± 6.9 Ma (MSWD = 1.3, N = 6) with initial 87Sr/86Sr ratio (ISr) of 0.7073 (Gao et al., 2006), that for the Qingshanjiao quartz monzodiorite is 135.6 ± 1.4 Ma (MSWD = 1.3, N = 7) with ISr of 0.7075 (Xu et al., 2005), and that for the Datuanshan quartz monzodiorite is about 135.2 ± 9.2 (MSWD = 3.1, N = 6) with ISr of 0.7084 (Fig. 6). The isochron age of each pluton, although containing larger uncertainty, approximates to its weighted average zircon 206 Pb/238U age. Meanwhile, the increase of ISr from pyroxene monzodiorite to granodiorite may suggest an increase in upper crustal material assimilated by the magmas. 5.3. Mineral thermobarometry Aluminium concentrations in amphibole can be used to estimate crystallization pressure for granitic batholiths (Anderson and Smith, 1995). As a barometer, it has been calibrated using experimental data at ~760 °C (Johnson and Rutherford, 1989) and ~ 675 °C (Schmidt, 1992). The amphiboles we analyzed are all calcic amphiboles (composition data in Supplementary Table S4), and can be further classified into the subgroups of magnesiohastingsite and magnesiohornblende (Leake et al., 1997). Their Fe/(Fe + Mg) ratios vary from 0.46 to 0.68, which never exceed the typical range of the amphiboles used in most experimental and empirical calibrations. After normalizing the amphibole compositions on assumption that Σcations – Ca– K–Na = 13, total Al were used to roughly estimate the equilibrated

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Fig. 4. (a–d) BSE images showing representative zircon morphologies from the Datuanshan quartz monzodiorite–granodiorite. The smaller circle indicates the spot for U–Pb dating, while the other one indicates that for Hf isotope analysis. 206Pb/238U ages were used for zircon grains younger than 1.0 Ga, otherwise 207Pb/206Pb ages were used. Initial Hf isotopic ratios were calculated with the in-situ U–Pb age of each zircon grain and converted to ε unit. (e) U–Pb concordia plot for the magmatic zircon grains; (f) weighted average diagram with a weighted mean 206Pb/238U age of 138.7 ± 1.3 Ma (n = 10), MSWD = 0.87.

pressure with the equation proposed by Johnson and Rutherford (1989). For the quartz monzodiorite, the magnesiohastingsite from the Qingshanjiao pluton yields higher pressure around 6.2 kbar, while the others (magnesiohornblende) yield pressures around 0.5 kbar for Qingshanjiao and 1.2 kbar for Datuanshan. The magnesiohastingsite in the enclaves from Baimangshan was equilibrated at about 6.1 to 6.4 kbar (Wu et al., 1997; Du and Lee, 2004; Lei et al., 2010), while the magnesiohornblende from the host rock was about 0.5 kbar. These pressure data will be referred to when applying the amphiboleplagioclase thermometer. The amphibole-plagioclase thermometer is applied using the Hbpl program of Holland and Blundy (1994). Equilibrated temperatures were solved with the pressures defined using amphibole-barometry above (Table 4). The Temperature A that requires silica saturation was adopted for the quartz monzodiorite samples, while otherwise Temperature B was used (discussions on the application of the two temperature equations could be found in Holland and Blundy, 1994). The temperatures for the enclaves in the Baimangshan pyroxene monzodiorite were estimated to be over 850 °C (Wu et al., 1997; Du and Lee, 2004; Lei et al., 2010), while our data suggest that the host rock is equilibrated at about 700 °C. The quartz monzodiorite to granodiorite are equilibrated at about 780 °C to 730 °C (Table 4). We examined our previous barometry results by comparing the temperature estimates to the temperature range of the calibration experiments by Johnson and Rutherford (1989). The former matches the latter well, except for the ~700 °C sample of Baimangshan, for which the pressure was recalculated base on the equation of Schmidt (1992) and updated in Table 4.

To further evaluate our temperature–pressure estimations, temperature effects on Al-in-amphibole barometer (Anderson and Smith, 1995) were considered here. With a new equation from Anderson and Smith (1995), coupled pressure and temperature (P2 and T2 in Table 4) were solved by iteration on a computer. The latest results generally agree with the values above, if the uncertainties are taken into account, except for the ca. 890 °C enclaves from Baimangshan. However, applying this correction to samples much higher than 800 °C should be treated with caution (Anderson and Smith, 1995), we doubt the revised pressure estimates for the enclaves in Baimangshan is true. The final estimated temperatures for the enclaves and host rock of the Baimangshan pyroxene monzodiorite are about 900 °C (Wu et al., 1997; Du and Lee, 2004; Lei et al., 2010) and 697 °C, respectively, while the pressures are from ca. 6 kbar (Wu et al., 1997; Du and Lee, 2004; Lei et al., 2010) and 0.4 kbar, respectively, which are almost equivalent to lithostatic pressures of 23 and 1.4 km, respectively. The temperature and pressure for the Jiguanshan pyroxene monzodiorite are about 850 °C and 2.8 kbar (ca. 10 km, lithostatic; Lei et al., 2010). The temperatures for the Qingshanjiao quartz monzodiorite are between 730 and 760 °C, which are almost equivalent to lithostatic pressures of 23 km and 4 km, respectively. The temperature and pressure for Datuanshan are ca. 790 °C and 0.4 kbar (ca. 1.6 km, lithostatic), respectively. Applying two-feldspar thermometry (Putirka, 2008) to the Qingshanjiao quartz monzodiorite yields temperatures ranging from 902 °C to 428 °C (see Supplementary Table S5), but for the test of equilibrium (Putirka, 2008), activity difference between plagioclase and alkalifeldspar reached minimum at about 621 °C. This temperature is

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Fig. 5. (a–d) BSE images showing representative zircon morphologies from the Baimangshan pyroxene monzodiorite. The smaller circle indicates the spot for U–Pb dating, while the other one indicates that for Hf isotope analysis. 206Pb/238U ages were used for zircon grains younger than 1.0 Ga, otherwise 207Pb/206Pb ages were used. Initial Hf isotopic ratios were calculated with the in-situ U–Pb age of each zircon grain and converted to ε unit. (e) weighted average diagram with a weighted mean 206Pb/238U age of 136.3 ± 5.2 Ma, MSWD = 2.6; and (f) U–Pb concordia plot for all zircon grains.

probably the equilibration temperature of feldspar crystallization for the quartz monzodiorite (Table 4). Additionally, we performed calculations for the Baimangshan pyroxene monzodiorite (using data from Du and Lee, 2004), and obtained an equilibration temperature of approximately 702 °C (Table 4). 5.4. Initial neodymium and lead isotopic compositions Whole rock Nd and Pb isotope data are listed in Supplementary Table S3. Initial Nd and Pb isotopic ratios at the crystallization age constrained by the zircon dating results in this study are calculated (Supplementary Table S3). U, Th, and Pb concentrations of whole rocks measured by ICP-MS (Huang et al., 2004; Wang et al, 2007; see Supplementary Table S2) were also used for calculation of initial Pb isotopic ratios. Initial Nd isotopic composition, εNd(t), of the Qingshanjiao and Datuanshan quartz monzodiorites are about −14 and −9, respectively. These values are very close to those for the Shizishan and Dongguashan quartz monzodiorites in the orefield (between −10 and −12, Wang et al., 2003, 2004b). Moreover, the εNd(t) values for the quartz monzodiorites are generally lower than those for the Baimangshan pyroxene monzodiorite (between −6 and −8, Wang et al., 2008a; Yan

et al., 2008), which suggests that the more felsic magmas contained more crustal material than the pyroxene monzodioritic one. The average two-stage Nd model ages (TDM2) for the Qingshanjiao and Datuanshan quartz monzodiorite are about 2.1 and 1.7 Ga, respectively, which are higher than that for the Baimangshan pyroxene monzodiorite (about 1.4 Ga; Wang et al., 2008a; Yan et al., 2008). In the (208Pb/204Pb)i−(206Pb/204Pb)i diagram (Fig. 7a), the initial Pb isotopic compositions vary from EM I (Beier et al., 2007)-like compositions (data-points on the upper-right side) to those of the Yangtze Lower Crust (on the left side). Furthermore, the initial Pb isotopic ratios are negatively correlated to 1/Pb (Fig. 7b), indicating the mixing between initial pyroxene monzodioritic magma and material from the Yangtze Lower Crust. 6. Discussion 6.1. Age, source material and the dynamic magmatic evolution The zircon U–Pb dating results for magmatic zircons from the plutons of this study agree well with published hornblende Ar–Ar and zircon U–Pb ages for the Baimangshan, Datuanshan and other granitoids in the Shizishan orefield (Table 3; Wang et al., 2004a,

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Table 3 Geochronological data for the mineralized plutons at the Shizishan orefield. Plutons

Lithology

Method

Age (Ma)

Data source

Baimangshan

Pyroxene monzodiorite

Qingshanjiao

Quartz monzodiorite

Datuanshan

Quartz monzodiorite– Granodiorite

Jiguanshan Shizishan

Quartz monzodiorite Quartz monzodiorite

Zircon 206Pb/238U Zircon 206Pb/238U Zircon 206Pb/238U Amphibole Ar–Ar Amphibole Ar–Ar Amphibole Ar–Ar Whole-rock Rb–Sr isochron Zircon 206Pb/238U Whole-rock Rb–Sr isochron Zircon 206Pb/238U Zircon 206Pb/238U Whole-rock Rb–Sr isochron Zircon 206Pb/238U Zircon 206Pb/238U

136.3 ± 5.2 142.9 ± 1.1 138.2 ± 0.8 139.7 ± 0.3 138.6 ± 0.4 138.3 ± 0.6 138.8 ± 6.9 138.8 ± 1.6 135.6 ± 1.4 139.3 ± 1.2 138.7 ± 1.3 135.2 ± 9.2 139.9 ± 1.1 135.1 ± 3.3

This study Wang et al. (2004a) Wu et al. (2008a) Wang et al. (2008a) Wang et al. (2008a) Wang et al. (2008a) Gao et al. (2006) This study Xu et al. (2005) Wu et al. (2008a) This study This study Wu et al. (2008a) Xie et al. (2009)

2008a; Wu et al., 2008a; Xie et al., 2009). The Rb–Sr isochron age for Datuanshan, although not as accurate as zircon dating, is still comparable to these ages (Table 3). All age data indicate that the magmas started to be emplaced at about 143 Ma, and achieved their peak activity at about 139 Ma. This magmatic event could continue at least until about 135 Ma, as implied by the Rb–Sr isochron ages and zircon U–Pb ages of the Shizishan and Baimangshan plutons (Xu et al., 2005; Gao et al., 2006; Xie et al., 2009; this study). Based on the combined U–Pb age and Hf isotopic composition of each zircon, the source of inherited zircons could be evidently distinguished from that of the magmatic ones. The inherited zircons in the quartz monzodiorites have ages of 0.8, 2.0 and 2.4 Ga and slightly negative εHf(t) values (−5.6 ± 4.5, error in 1σ), while those in the Baimangshan pyroxene monzodiorite yield ages of about 165 to 245 Ma and εHf(t) values (−2.9 ± 1.4) in a similar range. The magmatic zircons from all three plutons have a comparable weighted average 206Pb/238U age, but the average εHf(t) values of those in the Baimangshan pyroxene monzodiorite and in the quartz monzodiorites are lower (−8.5 ± 1.4) and the lowest (−19.8 ± 3.9), respectively. The decrease in εHf(t) values of the magmatic zircons from quartz monzodiorites compared to those in pyroxene monzodiorite reflects increased assimilation of crustal material in the more felsic magma. In Fig. 8, the inherited zircons from quartz monzodiorites scatter at the intercepts of the Archean and Paleoproterozoic crustal derivatives on the chondrite line, while the magmatic zircons lie near the two crustal evolution curves. The zircons from pyroxene monzodiorite are in the intermediate zone connecting the chondrite line to the crustal curve. The distribution of zircon samples represents the dynamic geological

evolution of heavy Hf isotope enrichment, and suggests that the old metamorphic-sedimentary rocks (1.9 to 1.2 Ga) in the upper crust may have been increasingly assimilated into the magma. This point is also supported by the increase of ISr and TDM2 of Nd isotopes from pyroxene monzodiorite (0.7073, 1.4 Ga) to quartz monzodiorite– granodiorite (0.7075 to 0.7084, 1.7 to 2.1 Ga, respectively). Thus primary pyroxene monzodioritic magma generation in the Shizishan orefield was significantly affected by mixing or assimilation processes which introduced large amount of upper crustal material into the evolved magmas. The U–Pb ages and Hf isotopes of the inherited zircons record the time of different magmatic events and their sources in Tongling (maybe even valid for the Yangtze block). Besides the inherited zircons from Qingshanjiao with ages of ~ 2.0 Ga and ~2.4 Ga (Yang et al., 2008), another group of ages of ~800 Ma has been found in this study. Unsurprisingly, inherited zircons of these three age groups also exist in Datuanshan, while those of ca. 800 Ma and 2.0 Ga were previously reported for the Dongguashan pluton (Wu et al., 2008a). Moreover, for the plutons in the orefields adjacent to Shizishan, such as the Fenghuangshan, Shatanjiao and Xinqiao plutons, inherited zircons of ca. 750 Ma, 2.2 Ga and 2.5 Ga were found (Wu et al., 2008b). These age groups may be characteristic for the potential input rocks into the Early Cretaceous magmas in Tongling. The age and Hf isotope pair will be more powerful. For instance, the metamorphic zircons of 2.0 Ga occurred in the Kongling terrane in the northern part of the Yangtze Block, which is the common basement strata and the only Archean microcontinent in southern China. These also have a εHf (t) value of~−6.5 (Zhang et al., 2006), or −6.7 to −2.8 and −2.1 to 2.0 (Wu et al., 2009), and could be a possible source for the ~ 2.0 Ga zircons at the Shizishan orefield.

6.2. Pressure, temperature and petrogenesis

Fig. 6. Rb–Sr isochron ages and initial Sr isotopic compositions for the Datuanshan quartz monzodiorite–granodiorite.

In previous studies, the Al–Ti-in-amphibole thermobarometer was applied to the xenoliths and host rocks in both the Shizishan (e.g., Wu et al., 1997; Du and Lee, 2004) and Tongguanshan orefields (e.g., Du et al., 2007; Cao et al., 2009). However, different barometry equations were employed by these authors, which makes it difficult to compare their results. Furthermore, in a few previous studies, the temperature estimates have exceeded the temperature range at which the barometry equation was calibrated, and applying such equation to those temperatures would tend to over-estimate the equilibrated pressure. We thus collected the published composition data, and did all the calculations together with our data. The barometric results for both the pyroxene monzodiorites and the quartz monzodiorites are consistent and yield two discrete pressure ranges. The lower magmareservoir may exist at a depth of about 23 km, and the magmatic process(es) below and within this magma chamber can be implied

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Table 4 Thermobarometric estimates for the plutons (error in 1σ). Pluton

TZr

Baimangshan

683 ± 20 °C 682 ± 11 °C 697 ± 12 °C

PAmp/kbar

TAmp-pl/°C

P2/kbar

T2/°C

0.46

697

0.37

697

6.10 6.40 ± 0.29 2.8

890 920 ± 19 852

2.02 1.86 ± 0.66

885 895 ± 16

6.21 ± 0.36 0.50 ± 0.40

757 ± 20 728 ± 22

6.28 ± 0.50 0.98 ± 0.65

757 ± 19 726 ± 19

1.22 ± 0.63

784 ± 21

0.42 ± 0.25

797 ± 33

Tfeldspar/°C

702 (enclaves) (enclaves) Jiguanshan Qingshanjiao

Datuanshan

774 ± 11 °C

763 ± 9 °C

621 ± 12

Data source Huang et al. (2004) Wang et al. (2003) Wang et al. (2008a) This study Du and Lee (2004) Du and Lee (2004) Wu et al. (1997)Lei et al. (2010) Lei et al. (2010) Huang et al. (2004) This study Wang et al. (2007) This study

Zircon saturation temperature (TZr) in the 2nd column were calculated from TZr = 12900/[2.95+ 0.85 M + ln(496000/Zrmelt)] (Watson and Harrison, 1983) and converted to centigrade; Pressure estimation based on Al-in-amphibole barometer (PAmp) in the 3rd column was yielded from a suitable equation: P (±0.6 kbar)= −3.01+ 4.76Altot (Schmidt, 1992), if 655 °Cb TAmp-pl b 700 °C or P (±0.5 kbar)= −3.46+ 4.23Altot (Johnson and Rutherford, 1989), if TAmp-pl N 720 °C; Amphibole-plagioclase temperature (TAmp-pl) in the 4th column were generated with the hb-pl program (Holland and Blundy, 1994). Coupled temperature (T2) and pressure (P2) in the 5th and 6th column were solved by iteration as temperature effects on the Al-in-hornblende barometer were considered, and the equation is P (±0.6 kbar)= −3.01+ 4.76Altot − {[T(°C) − 675]/85} × {0.53 Altot + 0.005294[T(°C) − 675]} (Anderson and Smith, 1995). Two-feldspar temperatures (Tfeldspar) in the 7th column were calculated after Putirka (2008).

Fig. 7. (a) 208Pb/204Pb vs. 206Pb/204Pb diagram for the plutons in Shizishan orefield showing the isotopic variation from EM I (Beier et al., 2007)-like compositions (upperright) to the Yangtze Lower Crust (left side). (b) 208Pb/204Pb vs. 1/Pb diagram illustrating the mixing line between initial pyroxene monzodioritic magma and the Yangtze Lower Crust. YLC = Yangtze Lower Crust.

from other evidence. Partial melting process may have happened since the Ta/Sm ratio values correlate with Ta concentrations (Fig. 9, Allègre and Minster, 1978). Moreover, partial melting with residue of amphibole in the source region often produces melts enriched in LILE and depleted in Nb, Ta and Ti (Rapp et al., 2003), which may account for the petrochemistry of the plutons in the Shizishan orefield (Wang et al., 2003; Huang et al., 2004). A portion of the residue (general estimate of about 5–25%, Miller et al., 2003) could be carried in the melt as restite (Chappell et al., 2000). The enclaves in the Baimangshan pyroxene monzodiorite are probably such restite. At the same time, the Pb isotope data suggest possible source contributions from the Yangtze Lower Crust to the initial magma. The primary magma may have caused the partial melting of the lower crust and mixed with the crustal magma. The mixed magma is more likely to accumulate in the lower magma chamber of a depth of about 23 km than in the shallow one. Regarding regional tectonics, the late stage of the collision between the Sino-Korean Craton and the Yangtze Craton was estimated to have taken place at 209 ± 2 Ma (Ames et al., 1993), when ultra-high pressure metamorphism and lithospheric thickening resulted from this continental collision. The environment of crustal thickening usually produces granitoids with zircon inheritance (Miller et al., 2003), the same as those at Shizishan orefield. Later, movement of the Pacific Plate began to control regional tectonics (Zhou and Li, 2000). The shift from a convergent to an extensional environment in the South China Block occurred at about 160 Ma (Zhou, 2006), which triggered magma intrusion into the upper crust from the lower chambers. New magma chambers could be formed at depths from 2 to 10 km in the Shizishan orefield as implied by the amphibole barometer. Some lines of evidence may explain the magmatic processes within the shallow magma chamber. The liner increase of (Ce/Yb) against Ce concentration from pyroxene monzodiorite to granodiorite (Yang et al., 2007) would be the result of combined wallrock assimilation and fractional crystallization (AFC) processes (Hart and Allège, 1980). We performed simulation calculations on Sr–Nd isotope compositions with classic equations (DePaolo, 1981). Based on petrographic observations and previous studies (e.g., Huang et al., 2004; Gao et al., 2006), we suppose that plagioclase is the major mineral component that fractionates during magmatic evolution from pyroxene monzodiorite to granodiorite; the partition coefficients for Sr and Nd are thus assumed to be DSr = 1.8 and DNd = 0.08, respectively (Arth, 1976). These partition coefficients have been assigned in previous Sr–Nd isotope studies as indicators of crustal contamination (e.g., James, 1981). Fig. 10 shows that the distribution

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Fig. 8. Plot of in-situ U–Pb age vs. εHf (t) value. 206Pb/238U ages were used for zircon grains younger than 1.0 Ga, otherwise 207Pb/206Pb ages were taken; Hf isotopic ratios were corrected against the in-situ U–Pb age of each zircon grain and converted to ε unit. The dashed lines were drawn assuming that crust growth occurred respectively at about 2.9 and 1.9 Ga, which were the statistic peak ages of zircons from the juvenile continental crust (Condie, 1998). The inherited zircons from quartz monzodiorites scatter at the intercepts of the crustal evolution curves on the chondrite line, while the magmatic zircons lie between the two curves of old crust (2.9 and 1.9 Ga). The zircons from pyroxene monzodiorite are in the intermediate zone connecting the chondrite line to the crustal curve. The distribution of the samples reflect a dynamic geological evolution, and suggests that old crustal material (1.9–1.2 Ga) could have been assimilated into the magma.

of initial Sr–Nd isotopes that can be predicted by two AFC simulation curves with the assumption that the mass of assimilated material approximate 60% of the crystallized material. The simulation is compatible with the mixing signature of zircon Hf isotopes and the petrographic mixing textures (Di et al., 2005). Hence, the AFC process is very likely to have occurred in the shallow level magma chambers. Additionally, with zircon inheritance in the plutons, Zr saturation temperature (TZr) provides the maximal estimate of initial magma temperature (Miller et al., 2003). TZr and the temperature of the host rock estimated with amphibole-plagioclase thermometer are almost identical for each pluton, which indicates that the magmas were very likely to have achieved Zr saturation, and the inherited zircons from the metamorphic basement rocks and/or Mesozoic sedimentary strata in the upper crust were able to be preserved in the plutons. This argument is supported by the occurrence of peak Zr concentration in Zr–SiO2 diagram (Yang et al., 2008), and is also compatible with the

Fig. 9. Ta/Sm–Ta diagram shows a partial melting trend. Data plotted with open symbols are from Huang et al. (2004) and Wang et al. (2007), while those with filled symbols are from Zhai et al. (1992), Wang et al. (2003) and Wang et al. (2004b).

191

Fig. 10. Plot of initial Sr isotopic compositions vs. εNd(t) values showing that the magmatic evolution from pyroxene monzodiorite to granodiorite was controlled by combined wallrock Assimilation and Fractional Crystallization (AFC) process. The most depleted pyroxene monzodiorite sample was assumed to represent the initial magma composition, and the dashed curves are simulating the variations of initial Sr–Nd isotopic ratios under AFC (after DePaolo, 1981) toward two boundary isotopic compositions at the wallrock side (the Yangtze Upper Crust; Chen, 1991), while keeping other parameters constant. Sr and Nd concentrations of the Yangtze Upper Crust are 273 × 10−6 (Gao et al., 1998) and 20 × 10−6 (Ling et al., 2009), respectively, while Sr and Nd isotopic compositions of the Baimangshan pyroxene monzodiorite (Wang et al., 2008a; Yan et al., 2008), Shizishan monzodiorite (Wang et al., 2003, 2004b), Dongguashan monzodiorite (Wang et al., 2003) were collected from literature. The plagioclase is assumed to be the major mineral component that is fractionating, and the partition coefficients for Sr and Nd are DSr = 1.8 and DNd = 0.08 (Arth, 1976), respectively. Ma = mass of assimilated material; Mc = mass of crystallized material.

contrasting Hf isotopic compositions between inherited and magmatic zircons (Fig. 8).

6.3. Consequences of magmatic processes for the mineralizations The new zircon ages, Sr–Nd–Pb–Hf isotopic data and thermobarometry results are combined for further discussion on the potential links between magmatic activity and mineralization. The weighted average zircon 206Pb/238U ages of the Datuanshan quartz monzodiorite and Baimangshan pyroxene monzodiorite are comparable to the Re–Os isochron ages of ore minerals from the Datuanshan Cu deposit (Mao et al., 2006) and the Chaoshan Au deposit (Wang et al., 2008b), respectively. Meanwhile, both the mean zircon U– Pb age and the Rb–Sr isochron age (Xu et al., 2005) for the Qingshanjiao quartz monzodiorite are also in agreement with the Rb–Sr isochron age of the ore-forming fluid for the Dongguashan Cu deposit (Xu et al., 2005), although some uncertainties exist for the Rb–Sr ages. There were several favorable conditions for generation of giant magmatic-hydrothermal mineralization coincident with magmatic activity. The lower pressure estimates in this study are equivalent to lithostatic depths of about 1.4 to 10 km, which are the favorable depths for the formation of magmatic-hydrothermal deposits. There are extensive Carboniferous to Triassic limestone strata in the Tongling area, whereby skarns develop in the contact zones between the intrusives and limestone. The complex interactions between the magma and the wallrock may result from the heterogeneity of the intrusive rock and the magmatic zircons during crystallization, as is revealed by Sr–Nd–Hf isotopic compositions. Material exchange may further increase the potential for mineralization. In addition, the quartz monzodiorites that were emplaced at 700 to 800 °C have induced medium temperature calcic (diopside- and grandite-bearing) skarns and significant mineralizations within the sedimentary strata and at the carbonate-intrusion contacts (i.e., the Dongguashan Cu deposit; Gu et al., 2007). In the central and deeper part of the orefield where quartz monzodiorites are abundant, porphyry and skarn copper mineralization

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predominates, whereas in the northern and upper parts where pyroxene monzodiorite was emplaced, skarn gold mineralization appears (Tang et al., 1998). Several factors may account for this difference. These include: (1) magma composition and source material. Copper deposits tend to be associated with the quartz monzodiorite, while the gold deposit is associated with the pyroxene monzodiorite (Tang et al., 1998). (2) The nature of host rocks. In contrast to the porphyry-style mineralization occurring in the plutons, the skarns developed either in the sedimentary strata or in the contact zone with the limestone (Pan and Dong, 1999). (3) Thermodynamic conditions of the intrusive rock. Temperature variation of the pyroxene monzodioritic magma is larger than that of the more felsic magmas as suggested by the different mineral thermobarometers. Further studies are still needed for greater quantitative understanding of the relationships between magmatic processes and metallogenesis. 7. Conclusions The following conclusions can be drawn from our combined studies on U–Pb dating and in-situ Hf isotopes of zircons and Sr–Nd– Pb isotopes and mineral thermobarometry for the petrogenesis of the mineralized plutons at the Shizishan orefield in the Tongling mining district, East China. 1) The magma emplacement and crystallization (typically for zircons) mainly occurred at about 139 Ma, which is close to the mineralization time constrained by published Re–Os dating results. 2) The primary pyroxene monzodioritic magma may have mixed with the magma produced by partial melting of the Yangtze lower crust, and accumulated at the magma chamber of ca. 23 km deep in the lower crust level. 3) AFC and magma mixing were the dominant processes for the magmatic evolutions at shallow level (2 to 10 km), where the circumstances were favorable for mineralization.

Supplementary materials related to this article can be found online at doi:10.1016/j.oregeorev.2010.09.003. Acknowledgments This work was supported by the Chinese National Science Foundation (No. 49873016), the Ph.D. foundation program of the Ministry of Education of China (20070284011), and the Scientific Research Foundation of Graduate School of Nanjing University (2008CL010). We express our gratitude to Chen Bangguo, Liu Jinghua, and Wang Bin (Shizishan Mine Company) for their assistance during fieldwork. We are also appreciative of the efforts made by Chief Editor Nigel J. Cook and two anonymous reviewers to help us improve this manuscript. References Allègre, C.J., Minster, J.F., 1978. Quantitative models of trace element behavior in magmatic processes. Earth and Planetary Science Letters 38, 1–25. Ames, L., Tilton, G.R., Zhou, G.Z., 1993. Timing of collision of the Sino-Korean and Yangtse cratons: U–Pb zircon dating of coesite-bearing eclogites. Geology 21, 339–342. Anderson, T., 2002. Correction of common Pb in U–Pb analyses that do not report 204Pb. Chemical Geology 192, 59–79. Anderson, J.L., Smith, D.R., 1995. The effects of temperature and fO2 on the Al-inhornblende barometer. American Mineralogist 80, 549–559. Arth, J.G., 1976. Behavior of trace elements during magmatic processes – a summary of theoretical models and their applications. Journal of Research of the U.S. Geological Survey 4, 41–47. Beier, C., Stracke, A., Haase, K.M., 2007. The peculiar geochemical signatures of São Miguel (Azores) lavas: metasomatised or recycled mantle sources? Earth and Planetary Science Letters 259, 186–199.

Black, L.P., Gulson, B.L., 1978. The age of the Mud Tank carbonatite, Strangways Range, Northen Territory. BMR Journal of Australian Geology and Geophysics 3, 227–232. Cao, Y., Du, Y.S., Pang, Z.S., Li, S.T., Zhang, J., Zhang, Z.C., 2009. Underplating and assimilation- fractional crystallization of Mesozoic intrusions in the Tongling area, Anhui province, East China: evidence from xenoliths and host plutons. International Geology Review 51, 542–555. Chang, Y.F., Liu, X.P., Wu, Y.C., 1991. The copper–iron belt along the middle and lower reaches of the Changjiang River. Geological Publishing House, Beijing. 379 pp. (in Chinese with English abstract). Chappell, B.W., White, A.J.R., Williams, I.S., Wyborn, D., Wyborn, L.A.I., 2000. Lachlan fold belt granites revisited: high- and low-temperature granites and their implications. Australian Journal of Earth Sciences 47, 123–138. Chen, J.F., 1991. Simple mixing, AFC mixing and isotope data explanation of granitoids. Anhui Geology 1, 19–27 (in Chinese with English abstract). Condie, K.C., 1998. Episodic continental growth and supercontinents: a mantle avalanche connection? Earth and Planetary Science Letters 163, 97–108. DePaolo, D.J., 1981. Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth and Planetary Science Letters 53, 189–202. Di, Y.J., Zhao, H.L., Wu, G.G., Zhang, D., Zang, W.S., Liu, Q.H., 2005. Genesis of the intrusive rocks from the Tongling area during the Yanshanian and mixing of threeend-member magma. Geological Review 51, 528–538 (in Chinese with English abstract). Du, Y.S., Lee, H.K., 2004. Discovery of gabbroic xenoliths from Early Cretaceous intrusions in Tongling, Anhui Province, China and its geological significance. Geological Journal of China Universities 10, 332–342 (in Chinese with English abstract). Du, Y.S., Qin, X.L., Tian, S.H., 2004. Mesozoic magmatic to hydrothermal process in the Tongguanshan ore field, Tongling, Anhui province, China: evidence from xenoliths and their hosts. Acta Petrologica Sinica 20, 339–350 (in Chinese with English abstract). Du, Y.S., Li, S.T., Cao, Y., Qin, X.L., Lou, Y.E., 2007. UAFC-related origin of the Late Jurassic to Early Cretaceous intrusions in the Tongguanshan ore field, Tongling, Anhui Province, East China. Geoscience 21, 71–77 (in Chinese with English abstract). Gao, S., Luo, T.C., Zhang, B.R., Zhang, H.F., Han, Y.W., Zhao, Z.D., Hu, Y.K., 1998. Chemical composition of the continental crust as revealed by studies in East China. Geochimica et Cosmochimica Acta 62, 1959–1975. Gao, G., Xu, Z.W., Yang, X.N., Wang, Y.J., Zhang, J., Jiang, S.Y., Ling, H.F., 2006. Petrogenesis of the Baimangshan pyroxene diorite intrusion in Tongling Area, Anhui Province: constraints from Sr–Nd–Pb–O isotopes. Journal of Nanjing University (Natural Sciences) 42, 269–279 (in Chinese with English abstract). Gerstenberger, H., Haase, G., 1997. A highly effective emitter substance for mass spectrometric Pb isotope ratio determinations. Chemical Geology 136, 309–312. Goolaerts, A., Mattielli, N., de Jong, J., Weis, D., Scoates, J.S., 2004. Hf and Lu isotopic reference values for the zircon standard 91500 by MC-ICP-MS. Chemical Geology 206, 1–9. Griffin, W.L., Belousova, E.A., Shee, S.R., Pearson, N.J., O'Reilly, S.Y., 2004. Archean crustal evolution in the northern Yilgarn Craton: U–Pb and Hf-isotope evidence from detrital zircons. Precambrian Research 131, 231–282. Gu, L.X., Xu, K.Q., 1986. On the Carboniferous submarine massive sulphide deposits in the lower reaches of the Changjiang (Yangtze) River. Acta Geologica Sinica 2, 176–188 (in Chinese with English abstract). Gu, L.X., Zaw, K., Hu, W.X., Zhang, K.J., Ni, P., He, J.X., Xu, Y.T., Lu, J.J., Lin, C.M., 2007. Distinctive features of Late Palaeozoic massive sulphide deposits in South China. Ore Geology Reviews 31, 107–138. Hart, S.R., Allège, C.J., 1980. Trace-element constrains on magma genesis. In: Hargraves, R.B. (Ed.), Physics of Magmatic Processes. Princeton University Press, Princeton, pp. 121–159. Holland, T., Blundy, J., 1994. Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. Contributions to Mineralogy and Petrology 116, 433–447. Hou, Z.Q., Yang, Z.S., Meng, Y.F., Zeng, P.S., Li, H.Y., Xu, W.Y., 2007. Geological fluid mapping in the Tongling area: implications for the Paleozoic submarine hydrothermal system in the middle-lower Yangtze metallogenic belt, East China. Acta Geologica Sinca (English Edition) 81, 833–860. Huang, S.S., Xu, Z.W., Gu, L.X., Hua, M., Lu, X.C., Lu, J.J., Nie, G.P., Zhu, S.P., 2004. A discussion on geochemical characteristics and genesis of magma in Shizishan Orefield, Tongling area, Anhui province. Geological Journal of China Universities 10, 217–225 (in Chinese with English abstract). Iizuka, T., Hirata, T., 2005. Improvements of precision and accuracy in in situ Hf isotope microanalysis of zircon using the laser ablation-MC-ICPMS technique. Chemical Geology 220, 121–137. Jackson, S.E., Pearson, N.J., Griffin, W.L., Belousova, E.A., 2004. The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U–Pb zircon geochronology. Chemical Geology 211, 47–69. James, D.E., 1981. The combined use of oxygen and radiogenic isotopes as indicators of crustal contamination. Annual Reviews of Earth and Planetary Science 9, 311–344. Johnson, M.C., Rutherford, M.J., 1989. Experimental calibration of the aluminium-inhornblende geobarometer with application to Long Valley caldera (California) volcanic rocks. Geology 17, 837–841. Leake, B.E., Woolley, A.R., Arps, C.E.S., Birch, W.D., Gilbert, M.C., Grice, J.D., Hawthorne, F.C., Kato, A., Kisch, H.J., Krivovichev, V.G., Linthout, K., Laird, J., Mandarino, J., Maresch, W.V., Nickel, E.H., Rock, N.M.S., Schumacher, J.C., Smith, D.C., Stephenson, N.C.N., Ungaretti, L., Whittaker, E.J.W., Youzhi, G., 1997. Nomenclature of amphiboles. European Journal of Mineralogy 9, 623–651. Lei, M., Wu, C.L., Gao, Q.M., Gui, H.P., Liu, L.G., Guo, X.Y., Gao, Y.H., Chen, Q.L., Qin, H.P., 2010. Petrogenesis of intermediate-acid intrusive rocks and enclaves in Tongling

X.-N. Yang et al. / Ore Geology Reviews 43 (2011) 182–193 area and the application of mineral thermobarometry. Acta Petrologica et Mineralogica 29, 271–288 (in Chinese with English abstract). Ling, M.X., Wang, F.Y., Ding, X., Hu, Y.H., Zhou, J.B., Zartman, R.E., Yang, X.Y., Sun, W.D., 2009. Cretaceous ridge subduction along the lower Yangtze River belt, Eastern China. Economic Geology 104, 303–321. Lu, J.J., Guo, W.M., Chen, W.F., Jiang, S.Y., Li, J., Yan, X.R., Xu, Z.W., 2008. A metallogenic model for the Dongguashan Cu–Au deposit of Tongling, Anhui province. Acta Petrologica Sinica 24, 1857–1864 (in Chinese with English abstract). Ludwig, K.R., 2000. Isoplot–a Geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication No 1a. Mao, J.W., Wang, Y.T., Lehmann, B., Yu, J.J., Du, A.D., Mei, Y.X., Li, Y.F., Zang, W.S., Stein, H.J., Zhou, T.F., 2006. Molybdenite Re–Os and albite 40Ar/39Ar dating of Cu–Au–Mo and magnetite systems in the Yangtze River valley and metallogenic implications. Ore Geology Reviews 29, 307–324. Miller, C.F., McDowell, S.M., Mapes, R.W., 2003. Hot and cold granites? Implications of zircon saturation temperatures and preservation of inheritance. Geology 31, 529–532. Pan, Y.M., Dong, P., 1999. The lower Changjiang (Yangzi/Yangtze River) metallogenic belt, East Central China: intrusion- and wall rock-hosted Cu-Fe-Au, Mo, Zn, Pb, Ag deposits. Ore Geology Reviews 15, 177–242. Pirajno, F., Ernst, R.E., Borisenko, A.S., Fedoseev, G., Naumov, E.A., 2009. Intraplate magmatism in Central Asia and China and associated metallogeny. Ore Geology Reviews 35, 114–136. Putirka, K., 2008. Thermometers and barometers for volcanic systems. In: Putirka, K., Tepley, F. (Eds.), Minerals, Inclusions and Volcanic Processes: Reviews in Mineralogy and Geochemistry, 69, pp. 61–120. Rapp, R.P., Shimizu, N., Norman, M.D., 2003. Growth of early continental crust by partial melting of eclogite. Nature 425, 605–609. Schmidt, M.W., 1992. Amphibole composition in tonalite as a function of pressure: an experimental calibration of the Al-in-hornblende barometer. Contributions to Mineralogy and Petrology 110, 304–310. Tang, Y.C., Wu, Y.C., Chu, G.Z., Xing, F.M., Wang, Y.M., Cao, F.Y., Chang, Y.F., 1998. Geology of copper–gold polymetallic deposits in the along-Changjiang area of Anhui province. Geological Publishing House, Beijing. 351 pp. (in Chinese with English abstract). Todt, W.C., Cliff, R.A., Hanser, A., 1996. Evaluation of a 202Pb–205Pb double spike for high-precision lead isotope analysis. Geophysical Monograph 95, 429–437. Wang, Y.X., Yang, J.D., Tao, X.C., Li, H.M., 1988. A study of the Sm–Nd method for fossil, mineral, rocks and its application. Journal of Nanjing University (Natural Sciences) 24, 297–308 (in Chinese with English abstract). Wang, Q., Xu, J.F., Zhao, Z.H., Xiong, X.L., Bao, Z.W., 2003. Petrogenesis of the Mesozoic intrusive rocks in the Tongling area, Anhui Province, China and their constraint on geodynamic process. Science in China (Series D) 46, 80–805. Wang, Y.B., Liu, D.Y., Zeng, P.S., Yang, Z.S., Tian, S.H., 2004a. SHRIMP U–Pb geochronology of pyroxene diorite in the Chaoshan gold deposit and its Geological significance. Acta Geoscientica Sinica 25, 423–427 (in Chinese with English abstract). Wang, Y.L., Wang, Y., Zhang, Q., Jia, X.Q., Han, S., 2004b. The geochemical characteristics of Mesozoic intermediate-acid intrusive of the Tongling area and its metallogenesis-geodynamic implications. Acta Petrologica Sinica 20, 325–338 (in Chinese with English abstract). Wang, Y.J., Liu, J.H., Xu, Z.W., Fang, C.Q., Jiang, S.Y., Yang, X.N., Zhang, J., Li, H.Y., 2007. Petrochemical characteristics and discussion on the genesis of the Datuanshan quartz diorite in Tongling area, Anhui Province. Contributions to Geology and Mineral Resources Research 22, 264–269 (in Chinese with English abstract). Wang, J.Z., Li, J.W., Zhao, X.F., Qian, Z.Z., Ma, C.Q., 2008a. Genesis of the Chaoshan gold deposit and its host intrusion, Tongling area: Constraints from 40Ar/39Ar ages and elemental and Sr–Nd–O–C–S isotope geochemistry. Acta Petrologica Sinica 24, 1875–1888 (in Chinese with English abstract). Wang, J.Z., Li, J.W., Zhao, X.F., Qian, Z.Z., Ma, C.Q., Qu, W.J., Du, A.D., 2008b. Re–Os dating of pyrrhotite from the Chaoshan gold skarn, Eastern Yangtze Craton, Eastern China. International Geology Review 50, 392–406. Watson, B.E., Harrison, M.T., 1983. Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth and Planetary Science Letters 64, 295–304. Woodhead, J., Hergt, J., Shelley, M., Eggins, S., Kemp, R., 2004. Zircon Hf-isotope analysis with an excimer laser, depth profiling, ablation of complex geometries, and concomitant age estimation. Chemical Geology 209, 121–135. Wu, C.L., Zhou, X.R., Huang, X.C., Zhang, C.H., Xu, S., Gui, H.P., Chen, S.Y., 1997. Enclave petrology of intermediate-acid intrusive rocks in Tongling district, Anhui. Acta Geoscientia Sinica 18 (2), 182–191 (in Chinese with English abstract).

193

Wu, F.Y., Yang, Y.H., Xie, L.W., Yang, J.H., Xu, P., 2006. Hf isotopic compositions of the standard zircons and baddeleyites used in U–Pb geochronology. Chemical Geology 234, 105–126. Wu, C.L., Dong, S.W., Gui, H.P., Guo, X.Y., Gao, Q.M., Liu, L.G., Chen, Q.L., Lei, M., Wooden, J.L., Mazadab, F.K., Mattinson, C., 2008a. Zircon SHRIMP U–Pb dating of intermediate-acid intrusive rocks from Shizishan, Tongling and the deep processes of magmatism. Acta Petrologica Sinica 24, 1801–1812 (in Chinese with English abstract). Wu, G.G., Zhang, D., Di, Y.J., Zang, W.S., Zhang, X.X., Song, B., Zhang, Z.Y., 2008b. SHRIMP zircon U–Pb dating of the intrusive in the Tongling metallogenic cluster and its dynamic setting. Science in China (Series D) 51, 911–928. Wu, Y.B., Gao, S., Gong, H.J., Xiang, H., Jiao, W.F., Yang, S.H., Liu, Y.S., Yuan, H.L., 2009. Zircon U–Pb age, trace element and Hf isotope composition of Kongling terrane in the Yangtze Craton: refining the timeing of Palaeoproterozoic high-grade metamorphism. Journal of Metamorphic Geology 27, 461–477. Xie, J.C., Yang, X.Y., Sun, W.D., Du, J.G., Xu, W., Wu, L.B., Wang, K.Y., Du, X.W., 2009. Geochronological and geochemical constraints on for mation of the Tongling metal deposits, middle Yangtze metallogenic belt, east-central China. International Geology Review 51, 388–421. Xing, F.M., Xu, X., 1995. The essential features of magmatic rocks along the Yangtze River in Anhui province. Acta Petrologica Sinica 11, 409–422 (in Chinese with English abstract). Xing, F.M., Xu, X., 1996. AFC mixing model and origin of intrusive rocks from Tongling area. Acta Petrologica et Mineralogica 15, 10–15 (in Chinese with English abstract). Xing, F.M., Zhao, B., Xu, X., Zhu, C.M., Zhao, J.S., Cai, E.Z., 1997. Experimental study on the genesis of intrusive rocks in the Tongling area, Anhui. Regional Geology of China 16, 267–274 (in Chinese with English abstract). Xu, Z.W., Lu, X.C., Ling, H.F., Lu, J.J., Jiang, S.Y., Nie, G.P., Huang, S.S., Hua, M., 2005. Metallogenetic mechanism and timing of late superimposing fluid mineralization in the Dongguashan diplogenetic stratified copper deposit, Anhui Province. Acta Geologica Sinica (English edition) 79, 405–413. Yan, J., Chen, J.F., Xu, X.S., 2008. Geochemistry of cretaceous mafic rocks from the Lower Yangtze region, eastern China: characteristics and evolution of the lithospheric mantle. Journal of Asian Earth Sciences 33, 177–193. Yang, X.N., Xu, Z.-W., Zhang, J., Wang, Y.J., Xu, X.S., Jiang, S.Y., Ling, H.F., Liu, L.G., Chen, D.Y., 2007. Geochronology and origin of Nanhongchong pluton in Shizishan orefield, Anhui province. Acta Petrologica Sinica 23, 1543–1551 (in Chinese with English Abstract). Yang, X.N., Xu, Z.W., Xu, X.S., Ling, H.F., Liu, S.M., Zhang, J., Li, H.Y., 2008. Zircon U–Pb geochronology and its implication for the temperature of Yanshanian magma in Tongling, Anhui province. Acta Geologica Sinica 82, 510–516 (in Chinese with English Abstract). Zaw, K., Peters, S.G., Cromie, P., Burrett, C., Hou, Z.Q., 2007. Nature, diversity of deposit types and metallogenic relations of South China. Ore Geology Reviews 31, 3–47. Zhai, Y.S., Yao, S.Z., Lin, X.D., Zhou, X.R., Wan, T.F., Jin, F.Q., Zhou, Z.G., 1992. Fe–Cu (Au) metallogeny of the middle-lower Changjiang region. Geological Publishing House, Beijing. 234 pp. (in Chinese). Zhang, R.H., 1986. Sulfur isotopes and pyrite-anhydrite equilibria in a volcanic basin hydrothermal system of the Middle to Lower Yangtze Valley. Economic Geology 81, 32–45. Zhang, Q., Wang, Y., Qian, Q., Yang, J.H., Wang, Y.L., Zhao, T.P., Guo, G.J., 2001. The characteristics and tectonic-metallogenic significances of the adakites in Yanshanian period from eastern China. Acta Geologica Sinica 17, 236–244 (in Chinese with English abstract). Zhang, S.B., Zheng, Y.F., Wu, Y.B., Zhao, Z.F., Gao, S., Wu, F.Y., 2006. Zircon U–Pb age and Hf–O isotope evidence for Paleoproterozoic metamorphic event in South China. Precambrian Research 151, 265–288. Zhao, Y.M., Zhang, Y.N., Bi, C.S., 1999. Geology of gold-bearing skarn deposits in the middle and lower Yangtze River Valley and adjacent regions. Ore Geology Reviews 14, 227–249. Zhou, X.H., 2006. Major transformation of subcontinental lithosphere beneath eastern China in the Cenozoic–Mesozoic: review and prospect. Earth Science Frontiers 13, 50–64 (in Chinese with English abstract). Zhou, X.M., Li, W.X., 2000. Origin of late Mesozoic igneous rocks in southeastern China: implications for lithosphere subduction and underplating of mafic magma. Tectonophysics 326, 269–278.