post-collisional ore-bearing adakitic porphyries from gangdese porphyry copper belt, southern tibet:...
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Lithos 126 (2011) 265–277
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Post-collisional ore-bearing adakitic porphyries from Gangdese porphyry copperbelt, southern Tibet: Melting of thickened juvenile arc lower crust
Jin-Xiang Li ⁎, Ke-Zhang Qin ⁎, Guang-Ming Li ⁎, Bo Xiao, Lei Chen, Jun-Xing ZhaoKey Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China
⁎ Corresponding author. Tel.: +86 10 82998187; fax:E-mail addresses: [email protected] (J.-X. Li), kzq
[email protected] (G.-M. Li).
0024-4937/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.lithos.2011.07.018
a b s t r a c t
a r t i c l e i n f oArticle history:Received 25 February 2011Accepted 23 July 2011Available online 30 July 2011
Keywords:AdakiticJuvenile arc lower crustHf isotopePorphyry copper depositGangdese
Porphyry Cu–Modeposits, related to theMiocene adakitic porphyries from theGangdese porphyry copper belt inthe southern Tibet, formed in a post-collisional setting. Here, we present the new zircon U–Pb ages, whole-rockchemical, and Sr–Nd and zircon Hf isotopic data for the ore-bearing adakitic porphyries fromGangdese porphyrycopper belt. LA-ICP-MS zircon dating for six samples yielded ages ranging from 19 Ma to 14Ma, indicating theyformed in the Miocene. The ore-bearing adakitic porphyries show SiO2 of 61.47–71.67%, K2O of 3.29–4.74%, andhigh Sr content (394–1106 ppm), high Sr/Y ratios (63–158), and low Y (6.12–10.3 ppm) and heavy rare earthelement contents (e.g. Yb=0.52–0.91 ppm). They show steep fractionated REE and flat HREE patterns, andstrong enrichment in large ion lithophile elements (Cs, Rb, Ba, Th, and U) and depletion of high field strengthelements (Nb), with positive Sr and negative Ti anomalies. There are no linear variations of Ba, La, Sr/Y, Dy/Yb,(87Sr/86Sr)i, and εNd(t) with increasing SiO2 content. Combinedwith the zircon positive εHf(t) values, and widerange of (87Sr/86Sr)i (0.70559 to 0.70908) and of εNd(t) (−6.8 to 0) values for all the adakitic samples, theywerelikely derived from themelting of garnet-bearing amphibolite in the juvenile arc mafic lower crust. Additionally,the adakitic porphyries with low Th/Nb ratios have lower initial 87Sr/86Sr, and higher εNd(t) and εHf(t) values,and those with high Th/Nb show higher initial 87Sr/86Sr, and lower εNd(t) and εHf(t) values. This indicates thatthe juvenile arc lower crust is derived fromthemelting of themantlewedge thatwasmetasomatizedby slabfluidor sediment melt during the Neotethyan ocean subduction. Significantly, the juvenile arc lower crust possiblyinherited the arcmagmacharacteristics of abundant inF, Cl, andCuandhighoxidation state,which contributed tothe porphyry Cu–Mo deposits in the Gangdese porphyry copper belt.
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1. Introduction
Porphyry Cu (–Mo–Au) deposits are the world's principal source ofCu and Mo (and substantial Au). Generally, most studies have shownthat they mainly occur in continental margin and island arc settings(Richards, 2003; Cooke et al., 2005; Li et al., 2008; Sillitoe, 2010; andreferences therein). However, porphyry Cu–Mo deposits also recentlyformed in post-collisional settings, such as the Gangdese porphyrycopper belt in southern Tibet. Porphyry Cu–Mo deposits in this belt areclosely related to the Miocene adakitic porphyries (Hou et al., 2004,2009; Li et al., 2007; Qin et al., 2005). Several models are proposed forthe Miocene adakitic porphyries: (i) melting of the thickened maficlower crust (Chung et al., 2003, 2005, 2009; Guo et al., 2007b; Hou et al.,2004; Xu et al., 2010; Yang et al., 2009), (ii)melting of the uppermantlesource metasomatized by the slab-derived melts (Gao et al., 2007,2010b), and (iii) melting of subducted oceanic crust (Qu et al., 2004,2007). In this study, we present new zircon U–Pb ages, whole-rockchemical, and Sr–Ndand zirconHf isotopic data forMioceneore-bearing
adakitic porphyries from Tinggong, Chongjiang porphyry Cu–Modeposits and Qaingdui porphyry-skarn Cu–Mo deposit in the Gangdeseporphyry copper belt, southern Tibet. Combined with previousgeochemical data, we propose that the ore-bearing adakitic porphyriesare likely derived fromthemeltingof thickened juvenile arcmafic lowercrust which formed during the Neotethyan ocean subduction. The arclower crust was likely derived from the melting of the mantle wedgethat was metasomatized by slab-derived fluid or melt. Like arc magma(McInnes and Cameron, 1994; Mungall, 2002; Sillitoe, 1997), the arclower crust shouldbeenriched in F, Cl, H2O, andCuandofhighoxidationstate, contributing to the formation of porphyry Cu–Mo deposits insouthern Tibet.
2. Geology of the Gangdese porphyry copper belt
The Gangdese porphyry copper belt in southern Tibet (Fig. 1a)mainly occurs in the Lhasa Terrane bounded by the Indus-Yarlungsuture (IYS) and Bangong-Nujiang suture (BNS). Due to thenorthward subduction of the Neotethyan ocean (IYS), the arcgranitoids (Fig. 1b) and volcanic rocks formed in the southern LhasaTerrane during the Early Jurassic–Late Cretaceous (Chu et al., 2006; Jiet al., 2009;Wen et al., 2008, Zhang et al., 2007, 2010; Zhu et al., 2008,
ChongjiangChongjiang
Porphyry Cu-Mo deposit
Porphyry-skarn Cu deposit
Xigaze
LhasaLhasa
JiamaJiama
MozhugongkaMozhugongka
Lhasa Terrane
Arc granitoids Oligocene granites Nornal fault
NuriNuri
Collisional granitoids Miocene porphyry intrusion
THS 00 20km20km
Thrust fault
TTS
GCT
THS
TTS
GCT
XFB
Tethyan Himalayan sequence
Triassic-Tertiary volcanic-sedimentary sequenece
Gangdese central thrust
Xigaze forearc basin
XietongmenXietongmen
IYS
QulongQulongQiangduiQiangdui
NanmuNanmu
QuxuQuxuNimuNimu
TinggongTinggong
BairongBairongNanmulinNanmulin
XFB
0 200kmIndia
Lhasa terrane
Qiangtang terrane
Songpan-Ganze Terrane
Xigaze
Coqen
Zhongba
Gerze
GangarChangdu
Lhasa
BNS
JSS
IYS
AKSIndus-Yarlung suture
Bangong-Nujiang suture
Jinshajiang suture
Ayimaquin-Kunlun sutute
34°
30°
26°
98°94°90°86°82°78°
Thust belt
Detachment fault
Suture
Stike-slipe fault
IYS
BNS
JSS
AKS
a
88° 89° 90° 91° 92°
30°
29°
30°
29°
88° 89° 90° 91° 92°
N
Gangdese porphyry copper belt
b
Fig. 1. Sketch tectonic and location map (a), and generalized geologic map (b) of the Gangdese porphyry copper belt in the southern Tibet (after Hou et al., 2004).
266 J.-X. Li et al. / Lithos 126 (2011) 265–277
2009). Subsequently, the voluminous Gangdese collisional granitoidbatholiths (Fig. 1b), were emplaced and Paleocene–Eocene potassicand ultrapotassic volcanism and Lizizong volcanic rocks were eruptedin the southern Lhasa Terrane during the continental collision (~65 to50 Ma) between the Indian and Eurasian plates (Dong et al., 2005;Gao et al., 2010a; Ji et al., 2009; Mo et al., 2007, 2008). Eocene–Oligocene adakitic granites and porphyries (Chung et al., 2009) wereemplaced, and formed many porphyry-skarn copper deposits in thesouthern Lhasa Terrane (Nuri, etc., Li et al., 2006). Importantly, theMiocene ore-bearing adakitic porphyries (Fig. 1b), mainly monzo-granite and granodiorite, intruded the Gangdese granitoid batholithand surrounding Triassic-Tertiary sedimentary sequence (TTS) asstocks or dikes, and formed a series of porphyry Cu–Mo deposits(Qulong, Jiama, Bairong, Tinggong, Chongjiang, etc.). Most age results,obtained mainly by Ar–Ar and zircon U–Pb dating methods (Chunget al., 2003; Hou et al., 2004; Li et al., 2007; Qu et al., 2009; Xu et al.,2010), indicate that these adakitic porphyries and the associatedCu–Mo mineralization formed between ca. 20 and 10 Ma. Addition-ally, most studies suggested that the emplacement of the Mioceneadakitic porphyries were locally controlled by the NS-striking normalfault systems (Fig. 1b) that occurred between ca. 18 and 13 Ma(Blisniuk et al., 2001; Coleman and Hodges, 1995; Williams et al.,
2001). Simultaneously, the Miocene potassic and ultra-potassicvolcanic rocks erupted in the southern Lhasa Terrane. These magmasformed from the low degree of melting of lithospheric mantle thatwasmetasomatized by sediment-derivedmelts (Ding et al., 2003; Gaoet al., 2009b; Miller et al., 1999; Nomade et al., 2004; Turner et al.,1996; Williams et al., 2001, 2004; Zhao et al., 2009).
3. Sampling, analyses and results
3.1. Sampling and petrography
Samples were collected from the ore-bearing porphyries ofBairong, Tinggong porphyry Cu–Mo deposits and the Qiangduiporphyry-skarn Cu–Mo deposit in the Gangdese porphyry belt,southern Tibet (Fig. 1b; Table 1). All these samples are granodioritesand monzogranites and porphyritic textures with 30–45% pheno-crysts of plagioclase, K-feldspar, quartz, amphibole and biotite. Thegranodiorite porphyries are composed of quartz (20–25%), plagioclase(40–45%), K-feldspar (10–15%), amphibole (6–8%) and biotite (3–5%)and accessory minerals (b2%) including zircon, apatite, titanite, andFe–Ti oxides. The monzogranite porphyries consist of quartz (20–25%), plagioclase (25–35%), K-feldspar (25–30%), amphibole (4–5%)
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and biotite (3–5%) and accessory minerals (b1%) including zircon,apatite, titanite, and Fe–Ti oxides.
3.2. Analytical methods
3.2.1. Major, trace elements and Sr–Nd isotopes analysesThe samples were powdered in an agate mortar. Major elements
were analyzed on fused glass disks with an X-ray fluorescencespectrometer (Shimadzu XRF-1500) at the Institute of Geology andGeophysics, Chinese Academy of Sciences (IGGCAS), Beijing, China.The loss-on-ignition (LOI) was measured as the weight loss of thesamples after 1 h baking at a constant temperature at 1000 °C. Thepowders (1.2 g) were fused with lithium tetraborate (Li2B4O7, 6 g) at1050 °C for 20 min. The precision for major elements is better than 2%relative. The accuracy and reproducibility were monitored by Chinesenational standard sample GSR1 (granite, Table 1), with relativestandard deviation of better than 1%. The more detailed analyticalprocedure was reported by Guo et al. (2007b). Major element data arepresented in Table 1.
Trace elements were determined using an ELEMENT inductivelycoupled plasma mass spectrometer (ICP-MS) at IGGCAS. The powders(40 mg) were dissolved in distilled 1 ml HF and 0.5 ml HNO3 in aTeflon screw-cap capsules. The solutions were heated at 170 °C for10 days, dried and redissolvedwith 2 ml HNO3 in the capsules. Finally,the solutions were diluted in 1% HNO3 to 50 ml before analysis.The precision for trace elements is better than 5% relative. Thestandard sample GSR1 (Table 1) was used to monitor the analyticalaccuracy and reproducibility, with relative standard deviation ofbetter than 3%. Trace elements data are listed in Table 1. The detailedanalytical procedures follow those of Guo et al. (2007b) and Jin andZhu (2000).
Sr–Nd isotopeanalyseswereperformedona FinniganMAT262massspectrometer at IGGCAS. Sample powders were spiked with mixedisotope tracers, then dissolved with a mixed acid (HF: HClO4=3:1) inTeflon capsules for 7 days. Rb, Sr, Sm and Ndwere separated in solutionusing AG50W×8 (H+) cationic ion-exchange resin columns. The massfractionation corrections for Sr and Nd isotopic ratios were based on86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219, respectively. The inter-national standard NBS987 gave 87Sr/86Sr=0.710245±10 (n=15, 2sigma) and standard NBS607 gave 87Sr/86Sr=1.20030±25 (n=15).The BCR-1 standard yielded 143Nd/144Nd=0.512625±8 (n=12).Detailed analytical procedures for the Sr–Nd isotope measurementsare reported byGuoet al. (2007b). The Sr–Nd isotopedata are presentedin Table 1, and analytical errors are given as 2σ.
3.2.2. Zircon U–Pb geochronology and in-situ Hf isotopes analysisZircon crystals were obtained from crushed rock using a combined
method of heavy liquid and magnetic separation techniques.Individual crystals were handpicked under a binocular microscope.They were mounted in epoxy and polished to expose the cores of thegrains. The sites for zircon U–Pb age and Hf isotope analysis wereselected on the basis of the cathodoluminescence (CL) imaging, whichwas carried out using a scanning electron microscope at IGGCAS.
The zircon U–Pb age and Hf isotope analysis were conducted on aNeptune MC-ICP-MS equipped with a 193-nm laser at IGGCAS. Beforeanalysis, the sample surface was cleaned with ethanol to eliminatepossible contamination. During the analyses, a laser repetition rate of 6–8 Hz at 100 mJ was used. The spot sizes are 40–60 μm. Every 5 sampleanalyses was followed by analysis of a suite of zircon standards, i.e.,Harvard zircon 91500 (Wiedenbeck et al., 1995), Australian NationalUniversity standard zircon TEMORA 1 (Black et al., 2003) and NIST SRM610. Each spot analysis consisted approximately of 30 s backgroundacquisition and 40 s sample data acquisition. The detailed analyticaltechnique has been described byWu et al. (2006) and Xie et al. (2008).For zircon U–Pb age analysis, 207Pb/206Pb, 206Pb/238U, 207Pb/235U(235U=238U/137.88), and 208Pb/232Th ratios are corrected using the
Harvard zircon 91500 (Wiedenbeck et al., 1995) as the externalcalibrant. Common Pb contents were evaluated using the methoddescribed by Andersen (2002). The age calculations and concordiadiagrams were generated using ISOPLOT (ver 3.0) (Ludwig, 2003). Theuncertainties for individual analyses (ratios and ages) are quoted at the1σ level, whereas the errors on concordia and weighted mean ages arequoted at the 2σ level.
During zircon Hf isotopes analyses, TEMORA 1 analyzed as anunknown sample yielded a weighted 176Hf/177Hf ratio of 0.282675±0.000006 (2σ, MSWD=3.0, n=60), which is in good agreement withthe recommended Hf isotopic ratio (0.282686±0.000008, Black et al.,2003; Wu et al., 2006). During data acquisition for Hf isotopes,176Hf/177Hf isotopic ratio of 0.282300 is recommended as the standardvalue for 91500 (Wu et al., 2006), which is corrected the Hf isotopicmeasurements. Initial 176Hf/177Hf is calculated according to thecorresponding spot age, and the value of εHf(t) is calculated relativeto the chondritic reservoir with a 176Hf/177Hf ratio of 0.282772 and176Lu/177Hf of 0.0332 (Blichert-Toft and Albarede, 1997). Single-stageHf model ages (TDM1) are calculated relative to the depleted mantlewhich is assumed to have a linear isotopic growth from 176Hf/177Hf=0.279718 at 4.55 Ga to 0.283250 at present, with a 176Lu/177Hf ratio of0.0384 (Griffin et al., 2000; Vervoort and Blichert-Toft, 1999). Two-stage Hf model ages (TDM2) are calculated by assuming a mean176Lu/177Hf value of 0.022 for the average lower crust (Amelin et al.,1999).
3.3. Results
3.3.1. Zircon U–Pb agesThe zircons of the studied samples are mostly euhedral, and reveal
long to short prismatic forms with mean crystal sizes of 100–350 μm(Fig. 2). Most zircons show obvious oscillatory zoning (Fig. 2) andTh/U ratios of 0.4–3.1, higher than 0.1, in accordance with those ofigneous zircons (Fernando et al., 2003; Samuel andMark, 2003). Thus,the zircon U–Pb ages can be interpreted as representing the emplace-ment age of the host rocks.
Samples BR-17 and BR-18, from the Bairong porphyry Cu–Modeposit, yielded 206Pb/238U ages of 14.2±0.9 Ma (2σ, MSWD=0.6)and 14.8±0.5 Ma (2σ, MSWD=0.5), respectively (Fig. 3a, b). Onexenocrystic zircon from BR-17, the monzogranite porphyry, shows anolder 206Pb/238U age of 54±7 Ma (Figs. 2 and 3a), identical to the ageof collisional Gangdese granitoid batholith (Dong et al., 2005; Ji et al.,2009). Sample TG-39, from the Tinggong, yielded a 206Pb/238U age of16.0±0.8 Ma (2σ, MSWD=0.4; Fig. 3c). Samples QD-1, QDZK001-116 and QDZK001-216, from the Qiangdui porphyry-skarn deposit,yielded 206Pb/238U age of 17.1±0.8 Ma (2σ, MSWD=0.4), 19.1±0.9 Ma (2σ, MSWD=4.0) and 19.0±1.0 Ma (2σ, MSWD=3.0),respectively (Fig. 3d, e, f). In the sample QDZK001-216, the ages ofthree zircon grains are discordant, indicating they possibly have somecommon Pb (Fig. 3f). Moreover, the three samples roughly show fourgroups of inherited zircon ages: ~30 Ma, 50–65 Ma, ~80 Ma, and~180 Ma (Figs. 2 and 3d, e, f), corresponding to the post-collisionalOligocene (Chung et al., 2009; Li et al., 2006), main collisionalmagmatism (Ji et al., 2009; Mo et al., 2007, 2008), and Late Cretaceous(Wen et al., 2008) and Middle Jurassic magmatism related to theNeotethyan ocean subduction (Chu et al., 2006; Ji et al., 2009; Zhuet al., 2008), respectively.
3.3.2. Major and trace elementsAll of the studied samples mainly lie within the high-K calc-alkaline
field in the diagram of K2O versus SiO2 (Fig. 4a). They display SiO2 of61.47–71.67%, TiO2 of 0.26–0.54%, Al2O3 of 13.28–17.85%, MgO of 0.78–2.14%, and K2O of 3.29–4.74%, with K2O/Na2O of 0.85–1.29. Thesecharacteristics, together with the high Sr content (394–1106 ppm),Sr/Y ratios (63–158, Fig. 4b), and lowY (6.12–10.3 ppm) andheavy rareearth element (HREE) contents (e.g. Yb=0.52–0.91 ppm), indicate that
Table 1Geochemical data for the ore-bearing adakitic porphyries from the Gangdese porphyry copper belt, Southern Tibet.
Sample GSR1 DBQ-1 BR05-2 BR05-3 BR05-4 BR-15 BR-17 BR-18 TG-39 TZK 1101-32 QDZK 001-116 QDZK 001-216 QD-1
Locality Standard Bairong Tinggong Qiangdui
(wt.%)SiO2 72.66 68.43 71.67 65.58 67.08 67.04 66.18 66.23 65.11 63.77 66.20 61.47 62.87TiO2 0.30 0.44 0.26 0.44 0.46 0.48 0.52 0.51 0.54 0.49 0.41 0.47 0.45Al2O3 13.38 15.39 13.28 15.58 15.75 15.61 16.32 16.09 15.34 14.56 15.66 17.85 16.24Fe2O3T 2.16 2.74 1.56 2.67 3.08 3.19 3.04 3.36 3.15 3.19 2.02 1.59 2.79MnO 0.06 0.04 0.02 0.03 0.03 0.04 0.03 0.03 0.10 0.08 0.03 0.06 0.04MgO 0.45 1.29 0.78 1.36 1.46 1.48 1.63 1.55 2.04 1.72 1.22 1.45 2.14CaO 1.59 2.76 1.78 2.89 2.88 3.22 2.11 2.26 3.38 2.99 4.45 6.24 5.70Na2O 3.13 4.23 3.41 4.47 4.14 4.48 4.65 5.86 3.95 3.92 4.03 3.65 3.74K2O 5.01 3.59 4.39 3.35 3.65 3.54 3.61 3.29 3.82 4.74 4.13 3.96 3.61P2O5 0.10 0.17 0.15 0.16 0.19 0.21 0.22 0.21 0.23 0.33 0.18 0.20 0.20LOI 0.62 0.43 2.23 3.03 0.80 0.57 1.15 0.63 2.28 3.70 1.60 2.52 1.67Total 99.45 99.50 99.53 99.56 99.52 99.86 99.46 100.02 99.94 99.49 99.95 99.44 99.44FeO 1.58 1.00 1.75 1.59 1.03 1.05 1.12 1.90 1.92 1.35 0.80 1.43
(ppm)Li 130 14.0 20.6 36.3 13.4 9.4 19.1 12.2 28.9 25.2 6.20 5.31 18.3Be 12.5 2.78 3.93 2.45 3.20 3.05 3.22 2.39 3.35 5.57 1.67 1.90 1.39Sc 6.12 4.77 2.85 4.76 5.27 5.59 4.97 4.74 6.22 5.38 5.89 6.15 6.51V 23.3 53.7 28.8 55.0 59.1 58.7 56.7 61.9 64.7 53.5 74.9 84.2 80.7Cr 4.48 18.5 15.5 23.8 24.4 27.3 39.5 19.5 62.2 70.0 114 356 151Co 3.26 7.05 4.11 7.41 7.57 6.75 9.88 4.77 8.49 8.31 7.62 10.29 8.20Ni 2.46 7.38 7.80 12.4 9.72 12.2 27.3 15.7 22.3 44.21 9.57 180 8.54Cu 1.61 23.1 9.80 9.69 19.9 50.5 136 3.40 146 66.2 928 5118 820Zn 29.5 28.9 28.2 32.9 30.7 39.4 51.8 35.4 80.2 94.1 37.7 180 20.1Ga 20.0 18.3 17.8 18.5 19.0 18.2 20.1 19.6 18.7 18.6 19.0 20.6 17.0Rb 477 136 245 107 135 126 122 126 152 259 66.3 74.3 47.7Sr 110 716 394 833 782 734 766 676 636 657 999 1106 807Y 61.2 7.86 6.12 7.00 8.00 8.89 8.21 6.65 8.20 10.4 6.56 7.01 8.33Zr 175 171 145 148 142 160 153 127 170 292 104 109 102Nb 40.3 7.98 8.87 5.82 7.89 7.91 6.41 5.86 8.34 14.1 3.72 4.35 4.18Cs 39.0 8.98 6.99 11.5 10.6 6.89 4.04 7.83 9.57 14.0 5.91 1.12 4.54
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Table 1 (continued)
Sample GSR1 DBQ-1 BR05-2 BR05-3 BR05-4 BR-15 BR-17 BR-18 TG-39 TZK 1101-32 QDZK 001-116 QDZK 001-216 QD-1
Locality Standard Bairong Tinggong Qiangdui
Ba 334 667 673 874 852 675 999 777 790 1196 824 821 704La 56.4 31.8 37.9 28.2 31.6 34.6 34.4 25.6 33.1 48.1 18.9 21.3 20.2Ce 111 62.5 69.2 54.7 62.0 66.5 65.5 50.4 65.4 101 37.1 41.5 39.9Pr 13.0 7.30 7.78 6.28 7.22 8.14 8.25 6.30 8.44 12.88 4.87 5.46 5.27Nd 47.4 26.7 26.2 22.1 26.2 30.4 32.2 24.3 34.0 53.7 18.1 19.8 20.3Sm 9.75 4.63 4.09 3.82 4.53 4.90 5.06 3.91 5.80 8.93 3.18 3.37 3.45Eu 0.84 0.97 0.77 0.92 1.05 1.05 1.16 1.05 1.22 1.78 0.81 0.76 0.91Gd 9.09 2.95 2.55 2.39 3.00 3.18 3.16 2.80 3.57 5.54 2.17 2.48 2.57Tb 1.62 0.34 0.27 0.29 0.34 0.40 0.39 0.33 0.41 0.62 0.25 0.28 0.31Dy 10.3 1.49 1.06 1.23 1.52 1.70 1.61 1.35 1.66 2.39 1.22 1.35 1.57Ho 2.18 0.26 0.18 0.22 0.27 0.30 0.26 0.23 0.29 0.39 0.23 0.24 0.29Er 6.43 0.72 0.50 0.61 0.71 0.84 0.74 0.62 0.79 1.09 0.57 0.61 0.75Tm 1.06 0.11 0.08 0.10 0.11 0.13 0.11 0.09 0.11 0.15 0.08 0.09 0.11Yb 7.06 0.74 0.59 0.62 0.72 0.79 0.69 0.59 0.73 0.91 0.52 0.54 0.69Lu 1.09 0.11 0.09 0.09 0.10 0.12 0.10 0.09 0.11 0.14 0.08 0.08 0.11Hf 6.47 4.70 4.84 4.30 4.24 5.02 4.69 3.68 4.88 8.56 3.08 3.25 3.01Ta 7.70 0.57 0.64 0.36 0.54 0.68 0.49 0.44 0.60 0.98 0.25 0.30 0.31Tl 1.93 1.23 2.21 1.08 1.56 1.10 1.45 1.23 2.27 3.21 0.35 0.45 0.30Pb 31.5 33.8 64.5 33.8 33.0 26.9 20.2 12.5 42.1 95.5 15.7 12.5 8.5Bi 0.52 0.19 0.49 0.29 0.53 0.20 0.51 0.53 0.18 1.16 0.08 1.24 0.06Th 53.5 28.0 53.4 19.6 29.4 30.0 21.5 14.9 30.3 66.1 6.55 7.01 7.49U 19.4 5.59 11.78 4.49 4.91 3.53 3.04 2.86 5.08 10.4 1.80 2.42 2.0587Rb/86Sr 0.5631 1.7072 0.3841 0.4877 0.4874 0.4689 0.5782 0.6618 0.6810 0.1853 0.1965 0.449087Sr/86Sr 0.706052 0.709443 0.705832 0.706271 0.706244 0.706173 0.705722 0.706393 0.707500 0.705633 0.705927 0.7065602σ 12 10 14 13 11 12 14 15 13 12 11 13(87Sr/86Sr)i 0.705932 0.709079 0.705750 0.706167 0.706140 0.706073 0.705599 0.706252 0.707355 0.705588 0.705880 0.706451147Sm/144Nd 0.0988 0.0886 0.0966 0.0996 0.0959 0.1139 0.0998 0.1070 0.1087 0.1043 0.1025 0.1045143Nd/144Nd 0.512427 0.512318 0.512504 0.512430 0.512495 0.512411 0.512551 0.512313 0.512278 0.512611 0.512612 0.5126292σ 15 12 10 13 15 14 11 13 12 15 12 10(143Nd/144Nd)i 0.512418 0.512309 0.512495 0.512420 0.512485 0.512400 0.512541 0.512303 0.512268 0.512599 0.512601 0.512617t(Ma) 15 15 15 15 15 15 15 16 16 19 19 17εNd(t) −3.9 −6.0 −2.4 −3.9 −2.6 −4.3 −1.5 −6.2 −6.8 −0.3 −0.3 0.0
Notes: Initial Sr and Nd isotope ratios were obtained by using their formation age, as shown in Table 1.
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BR-17 BR-18
TG-39 QD-1
17±1Ma(7.9±0.6)
61±10Ma(11.9±0.6)
182 ±13Ma(14.0±0.7)
15±2Ma(3.7±0.6)
17±2Ma(3.8±0.7)
36±5Ma(10.7±0.8)
15±1Ma(2.1±0.6)
16±1Ma(3.3±0.5)
15±2Ma(7.8±0.5)
13±2Ma(2.7±0.5)
54±7Ma(4.4±0.7)
15±1Ma(5.1±0.6)
0303
0606
1212
03030808
0505
0202
1010 09091212
1616
QZK001-1 16 QZK001-216
1919
0404
21±4Ma(8.4±0.5)
61±5Ma(11.1±0.5)
1313
79±1Ma(11.7±0.5)
1111
0101
20±1Ma(9.6±0.4)
0404
33±2Ma(10.6±0.5)
59±3Ma(10.9±0.4)
1010
Fig. 2. Representative cathodoluminescence (CL) images of zircons from the Gangdese ore-bearing adakitic porphyries with analytical numbers, U–Pb ages and εHf(t) values (scalebar=100 μm).
270 J.-X. Li et al. / Lithos 126 (2011) 265–277
the samples can be identified as adakites (Defant andDrummond, 1990;Martin, 1999; Xu et al., 2002). Moreover, the studied samples showsteep fractionated REE and flat HREE patterns (Fig. 5a), with(La/Yb)N=21.1–46.1 and Eu/Eu*=0.73–0.93. In the trace elementspider diagram (Fig. 5b), they show strong enrichment in large ionlithophile elements (LILE, e.g., Cs, Rb, Ba, Th, and U) and the depletion ofhigh field strength elements (HFSE, e.g., Nb), with positive Sr andnegative Ti anomalies.
3.3.3. Sr and Nd isotopesThe studied ore-bearing adakitic porphyries have the (87Sr/86Sr)i
values ranging from 0.70559 to 0.70908 and εNd(t) values rangingfrom −6.8 to 0 (Fig. 6, Table 1), identical to the previously publishedSr–Nd isotopic data for the Miocene adakitic porphyries (Gao et al.,2007, 2010b; Guo et al., 2007b; Hou et al., 2004; Xu et al., 2010). TheSr–Nd isotope range of the Miocene adakitic porphyries partiallyoverlaps with those (Sr–Nd isotopic values recalculated for 20 Ma) ofthe Jurassic Yeba basalt (Zhu et al., 2008) and Lizizong volcanic rocks(Mo et al., 2007, 2008).
3.3.4. Zircon Hf isotopesZircons from the ore-bearing adakitic porphyries in the Gangdese
porphyry copper belt show mostly low 176Lu/177Hf ratios (b0.002,Fig. 7a). The two adakitic porphyries (BR-17 and BR-18) from theBairong deposit show zircon 176Hf/177Hf ratios ranging from 0.282813to 0.283023, corresponding to εHf(t) values of 1.8 to 9.2 (mean=4.5;Fig. 7b, Table S2). One xenocrystic zircon shows a 176Hf/177Hf ratio of0.282864 and corresponding εHf(54 Ma) values of 4.4 (Fig. 7b). Onesample (TG-39) from the Tinggong deposit has zircon 176Hf/177Hf ratiosbetween 0.282799 and 0.282901 and corresponding εHf(t) values
ranging from 1.4 to 5.0 (mean=3.3; Fig. 7b). Zircons from the adakiticporphyries at Qiangdui (QD-1, QDZK001-116 and QDZK001-216) have176Hf/177Hf ratios of 0.282869–0.283043, corresponding to εHf(t) valuesof 3.9 to 10.0 (mean=8.2; Fig. 7b). The four groups of inherited zircons,~30 Ma, 50–65Ma, ~80 Ma, and ~180 Ma, from the Qiangdui sampleshave 176Hf/177Hf ratios of 0.282917–0.283054 (εHf(t) of 5.7–10.7,mean=9.1), 0.283019–0.283072 (εHf(t) of 10.0–11.9, mean=10.8),0.283015–0.283058 (εHf(t) of 10.1–11.7, mean=10.8), and 0.283061(εHf(t) of 14.0), respectively (Figs. 3d, e, f and 7b). The zirconHf isotopicvalues decrease with decreasing age (Fig. 7b).
4. Discussion
4.1. Petrogenetic model for Miocene ore-bearing adakitic porphyries inthe Gangdese copper belt
Most studies indicate that adakitic rocks can be formed by differentprocesses: high-pressure (HPFC: involving garnet, Macpherson et al.,2006) and low-pressure fractionation (LPFC: involving olivine+clinopyroxene+plagioclase+amphibole+titanomagnetite, Castilloet al., 1999; Gao et al., 2009a) of hydrous basaltic magma, partialmelting of a subducted slab (Defant and Drummond, 1990; Martin,1999; Qu et al., 2004, 2007), magma mixing between basaltic andrhyolitic magma (Guo et al., 2007a; Qin et al., 2010), low degree partialmelting of metasomatized mantle (Gao et al., 2007, 2010b; Jiang et al.,2006), and partial melting of a thickened mafic lower crust (Chiaradia,2009; Chunget al., 2003;Hou et al., 2004; Karsli et al., 2010; Shafiei et al.,2009; Topuz et al., 2005, 2011; Xu et al., 2002). Thus, there is stillconsiderable debate on the origin of adakitic rocks.
60
20
20
10
40
20
10
20
6080
60
80
40
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.00 0.02 0.04 0.06 0.08 0.10 0.12
206 Pb
/238 U
206 Pb
/238 U
207Pb/235U 207Pb/235U
206 Pb
/238 U
206Pb/238U weighted age:
206Pb/238U weighted age:
206Pb/238U weighted age:
206Pb/238U weighted age:
206Pb/238U weighted age:
206Pb/238U weighted age:
(a) BR-17(Bairong)Monzograniteporphyry
14.2 ± 0.9 Ma(n=23, MSWD=0.6)
54 ± 7 Ma
14.8 ± 0.5 Ma(n=23, MSWD=0.5)
16.0 ± 0.8 Ma(n=26, MSWD=0.4)
(b) BR-18(Bairong)Monzograniteporphyry
17.1 ± 0.8 Ma(n=17, MSWD=0.4)
34.5 ± 3.2 Ma(n=3, MSWD=0.1)
63.9 ± 4.7 Ma(n=3, MSWD=0.2)
182 ± 13 Ma
19.1 ± 0.9 Ma(n=12, MSWD=4.0)
54.9 ± 4.5 Ma(n=3, MSWD=1)
76.1 ± 3.8 Ma(n=8, MSWD=4.1)
19.0 ± 1.0 Ma(n=16, MSWD=3)
59.9 ± 3.2 Ma(n=5, MSWD=2)
50
40
30
0.000
0.002
0.004
0.006
0.008
0.00 0.01 0.02 0.03 0.04 0.05 0.06
50
40
30
0.000
0.002
0.004
0.006
0.008
0.00 0.01 0.02 0.03 0.04 0.05 0.06
(c) TG-39 (Tinggong)Monzograniteporphyry
(d) QD-1(Qiangdui)Monzograniteporphyry
(e) QDZK001-116(Qiangdui)Granodioriteporphyry
(f) QDZK001-216(Qiangdui)Granodioriteporphyry
240
200
160
120
80
0.00
0.01
0.02
0.03
0.04
0.0 0.1 0.2 0.3 0.4
100
40
0.000
0.004
0.008
0.012
0.016
0.020
0.00 0.04 0.08 0.12 0.16 0.20
100
80
40
0.000
0.004
0.008
0.012
0.016
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
206Pb/238U age:
206Pb/238U age:
Fig. 3. Zircon U–Pb concordia diagrams and weighted ages of the ore-bearing adakitic porphyries from the Gangdese porphyry copper belt.
271J.-X. Li et al. / Lithos 126 (2011) 265–277
Except for Tinggong, the Bairong and Qiangdui ore-bearingadakitic porphyries mainly show a partial melting trend rather thana fractional crystallization trend in the La–La/Sm diagram (Fig. 8),consistent with published data for the ore-forming adakitic porphy-ries from the Gangdese porphyry copper belt (Gao et al., 2007). Lacontent deceases, and Sr/Y and Dy/Yb ratios rapidly increase withincreasing SiO2 content during HPFC (Macpherson et al., 2006). Ba andLa contents decrease and Dy/Yb ratios increase with increasing SiO2
content during LPFC (Castillo et al., 1999; Gao et al., 2009a; Richardsand Kerrich, 2007). The ore-bearing adakitic porphyries from theGangdese copper belt display neither of these trends (Fig. 9a, b, c, d)thus indicating that fractional crystallization is not responsible for
generating these rocks. Moreover, the simultaneous Miocene potassicand ultra-potassic basaltic magmas could not be precursors for theadakitic porphyries, because they have higher (87Sr/86Sr)i and lowerεNd(t) values than them (Fig. 6). Also, the Sr–Nd isotopic character-istics of the Miocene potassic and ultra-potassic volcanic rocks suggestthat they were derived from the low degree melting of the lithosphericmantle metasomatized by sediment-derived melts (Ding et al., 2003;Gao et al., 2009b; Miller et al., 1999; Nomade et al., 2004; Turner et al.,1996; Williams et al., 2001, 2004; Zhao et al., 2009). This suggests themechanism of the melting of the upper mantle source metasomatizedby the slab-derived melts (Gao et al., 2007, 2010b) for these adakiticporphyries should be ruled out. Additionally, there are no obvious
Tholeiite Series
Calc-alkalineSeries
Shoshonite Series
45 50 55 60 65 70 75
01
32
45
67
K2O
High-Kcalc-alkaline
Series
Published Miocene adakite
Bairong
Tinggong
Qiangdui
0
100
200
300
400
0 10 20 30 40
Y(ppm)
Sr/Y
Adakite
Normal arc magmas
SiO2
(b)
(a)
Fig. 4. Plots of (a) K2O versus SiO2 (Peccerillo and Taylor, 1976) and (b) Sr/Y versus Y(Defant and Drummond, 1990), indicating that the Gangdese ore-bearing porphyriesbelong to high-K calc-alkaline series and adakitic rocks, respectively.
Sam
ple/
cho
ndri
te
1
10
100
1000
Sam
ple/
prim
itive
man
tle
0.1
10000
1
10
100
1000
(a)
(b)
Cs Rb Ba Th U Nb K La Ce Pb Pr Sr P Nd ZrSm Eu Ti Dy Y Yb Lu
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Published Miocene adakite
Published Miocene adakite
Fig. 5. Chondrite-normalized REE patterns (a) and primitive mantle-normalizedelement spider diagram (b) for the Gangdese ore-bearing adakitic porphyries. Thenormalizing values for REE and trace elements are from Boynton (1984) and Sun andMcDonough (1989), respectively.
0.702 0.704 0.706 0.708 0.710
-8
-4
12
8
4
0
-12
-16
-20
Post-collisionadakites
0.712 0.714 0.716
Ultra-potassic rocks
Yarlung MORB
JurassicYeba basalts
Lizizong volcanic rocks
87Sr/86Sri
εNd(
t)
Fig. 6. Sr and Nd isotopic compositions of the Gangdese ore-bearing adakitic porphyriesin southern Tibet. Data for Jurassic Yeba basalt and Paleocene–Eocene Lizizong volcanicrocks are from Zhu et al. (2008) and Mo et al. (2007). Data for Miocene potassic andultra-potassic volcanic rocks, and Yarlung MORB are from Ding et al. (2003), Gao et al.(2009b), Mahoney et al. (1998), Miller et al. (1999), Nomade et al. (2004), Turner et al.(1996), Williams et al. (2004) and Zhao et al. (2009). Data for the published Mioceneadakitic porphyries are from Gao et al. (2010b), Guo et al. (2007b), Hou et al. (2004)andXu et al. (2010).
272 J.-X. Li et al. / Lithos 126 (2011) 265–277
trends of crustal contamination and magma mixing presented in the(87Sr/86Sr)i, εNd(t) versus SiO2 diagram (Fig. 9e, f). Moreover, zircon Hfisotopes in each sample mainly display limited variations in theiroverall εHf(t) values (Fig. 7). These evidences suggest that magmamixing for the origin of the adakitic porphyries in the Gangdese copperbelt should be ruled out. However, the presence of some old zircons(Fig. 7) suggests that little crust contamination may have played a rolein the generation of the ore-bearing adakitic porphyries in theGangdese belt.
In addition, the ore-bearing adakitic porphyries have high K2Ocontent, and high (87Sr/86Sr)i and low εNd(t) (Fig. 6), which are incon-sistent with slab-melting adakitic magma (Defant and Drummond,1990;Martin, 1999).Moreover, theNeotethyanoceanic slab subductionceased after the India–Eurasian continental collision (65–50 Ma), andthe subducted slab possibly sunk into the deep mantle in the Eocene(Kohnand Parkinson, 2002;Wen et al., 2008). Therefore, partialmeltingof the subductedNeotethyan slab (Qu et al., 2004, 2007) is not a suitableexplanation for the origin of the adakitic porphyries. Importantly, theMiocene ore-bearing porphyries are mostly identical to adakitic meltsderived from the melting of thickened mafic lower crust (Fig. 10a). Theadakitic porphyries show high Sr/Y and La/Yb, low Y and HREE, andrelative enrichment of Sr (Fig. 5b), suggesting the presence of residualgarnet and the absence of plagioclase in the source (Rapp et al., 2002;
Rapp and Watson, 1995; Xiong et al., 2005). However, they also showflat HREE patterns, indicating that amphibole played a more importantrole than garnet in the generation of the adakitic melt (Huang and He,2010;Moyen, 2009).Moreover, the adakitic porphyries have lowNb/Taratios, which are consistent with adakitic melts derived from theamphibolite melting (Fig. 10b; Foley et al., 2002; Xiong, 2006).Therefore, garnet-bearing amphibolite melting in the mafic lower
0 0.001 0.002 0.003 0.0040.2826
0.2827
0.2828
0.2829
0.2830
0.2831
0.2832
0 20 40 60 80 100 120 140 160 180 200
20
10
0
-10
-20
DM
CHUR
(a)
(b)
176 H
f/17
7 Hf
176Hf/177Hf
Age(Ma)
Miocene adakites
Inherited zircon
~50-60Ma Gangdse granitesand Lizizong volcanic rocks
εNd(
t)
Fig. 7. Variation of zircon initial 176Hf/177Hf versus 176Lu/177Hf (a), and εHf(t) versusAge (Ma) diagram (b) for the Gangdese ore-bearing adakitic porphyries.
273J.-X. Li et al. / Lithos 126 (2011) 265–277
crust is a plausiblemechanism for the generation of ore-bearing adakiticporphyries from the Gangdese porphyry copper belt.
4.2. Source features
The Gangdese ore-bearing adakitic porphyries are derived frompartial melting (Figs. 8, 9) and, therefore, their geochemical
0 10 20 30 40 50 60
0
2
4
6
8
10
12
La
La/
Sm
Parti
al m
eltin
g
Fractional crystallization
Fig. 8. La/Sm–La diagram for the Gangdese ore-bearing adakitic porphyries, indicatingtheir evolution is mainly controlled by partial melting.
characteristics can be used to characterize the source. Trace elementratios, such as La/Yb, Th/Nb, Ba/La and La/Nb, are widely used toidentify the metasomatic agents and estimate the flux from thesubducted slab (Hawkesworth et al., 1993; Pearce and Peate, 1995).Sediment melts have high Th/Yb and low Ba/La ratios (Plank andLangmuir, 1998; Kessel et al., 2005; Hanyu et al., 2006) while slabfluids have high Ba/La and low Th/Yb ratios (Hanyu et al., 2006;Hawkesworth et al., 1993; Kessel et al., 2005; Pearce and Peate, 1995).The Gangdese adakitic porphyries show the trends of lower Th/Nb andelevated Ba/La, and elevated Th/Nb and lower Ba/La (Fig. 11a). Thisindicates that the source rocks (mafic lower crust) likely were derivedfrom themelting of themantle wedgemetasomatized by slab fluids orsediment melts. The adakitic rocks with lower Th/Nb ratios havelower initial 87Sr/86Sr and higher εNd(t) values, and those with higherTh/Nb ratios show higher initial 87Sr/86Sr and lower εNd(t) values(Fig. 11b, c). Recent studies indicate that Hf is insoluble in slab-derived fluids, but it can be more soluble in slab melts, which shouldoverprint the enriched εHf signature of the subducted sediments inthe mantle wedge (Barry et al., 2006; Hanyu et al., 2006; Woodheadet al., 2001). The Gangdese ore-bearing adakitic porphyries with lowTh/Nb ratios show high εHf(t) values, but those with high Th/Nbratios display low εHf(t) values (Fig. 11d). This suggests that themaficlower crust source was possibly derived from the melting of themantle wedge metasomatized by slab-fluid or sediment melt.
The Sr–Nd isotope range (Fig. 6) of the Miocene adakiticporphyries partially overlaps with those of the Jurassic Yeba basalt(Zhu et al., 2008) and Lizizong volcanic rocks (Mo et al., 2007, 2008).Combined with the zircon positive εHf(t) of all the adakitic samples,the source could be juvenile arc lower crust derived from the meltingof the mantle wedge metasomatized by slab fluid or sediment meltduring the Neotethyan ocean subduction. The juvenile arc lower crustpossibly inherited the arcmagma characteristics of abundant F, Cl, andCu and high oxidation state (McInnes and Cameron, 1994; Mungall,2002; Sillitoe, 1997), which contributed to the porphyry Cu–Modeposit in the Gangdese porphyry copper belt.
4.3. A geodynamic scenario for the ore-forming adakitic prophyries
Several models for the ore-bearing adakitic porphyries from theGangdese belt have been proposed (Chung et al., 2003; Guo et al.,2007b; Hou et al., 2004). Slab-breakoff after collision along IYS wasproposed as a cause of Miocene adakitic magmatism (Hou et al.,2004). In the case of a slab-breakoff, the Miocene adakitic magmatismshould be confined to a restricted region. However, the Mioceneadakitic magmatism occurs in the Gangdese belt with a length of~1000 km. We therefore regard slab-breakoff as an unlikely cause fortheMiocene adakiticmagmatism. A delaminationmodel seems to be aplausible mechanism for the generation of adakitic porphyries fromthe Gangdese porphyry copper belt (Chung et al., 2003; Guo et al.,2007b). Importantly, this study stresses partial melting of the juvenilearc lower crust as contributing to the porphyry Cu–Mo deposits in theGangdese porphyry copper belt.
Before ~65 Ma, a series of arc magmas formed (Fig. 12a) by theNeotethyan ocean subduction in the southern Lhasa Terrane. Arcgranitoids (Chu et al., 2006; Ji et al., 2009;Wen et al., 2008, Zhang et al.,2007, 2010; Zhu et al., 2009) and volcanic rocks (Zhu et al., 2008)formed in the Early Jurassic–Late Cretaceous. Importantly, the basalticmelts derived from the mantle wedge metasomatized by the slab fluidor sediment melt (Zhu et al., 2008), are underplated forming thejuvenile arcmafic lower crust, and somemelts continued to rise formingbasalts at surface (Fig. 12a). The juvenile arc lower crust inherited thearc magma characteristics of F, Cl, and Cu and high oxidation state(McInnes and Cameron, 1994; Mungall, 2002; Sillitoe, 1997). In theMiocene, following the Indian–Eurasian continental collision, thejuvenile arc mafic lower crust underwent tectonic thickening and wasmetamorphosed into garnet-bearing amphibolite (Fig. 12b). The crust
LPFC
LPFC
HPFC
HPFC
LPFC
HPFC
Crustal contamination
or magm amixingCru
stal c
ontam
inatio
n
or m
agma m
ixing
Ba
400
600
1000
1200
800 La
20
30
40
50
Sr/Y
50
100
150
200
Dy/
Yb
2.0
2.5
3.0
0.705
0.706
0.707
0.708
0.709
-10
-6
-2
4
6
50 55 60 65 70 7550 55 60 65 70 7550 55 60 65 70 75
(87Sr
/86Sr
) i
(a) (b)
(d)
(c)
(e) (f)
-4
-8
0
2
SiO2 SiO2 SiO2
Fig. 9. Ba, La, Sr/Y, Dy/Yb, (87Sr/86Sr)i, and εNd(t) versus SiO2 for the Gangdese ore-bearing adakitic porphyries. HPFC: high-pressure fractionation (involving garnet, Macphersonet al., 2006) and LPFC: low-pressure fractionation (involving olivine+clinopyroxene+plagioclase+amphibole+titanomagnetite, Castillo et al., 1999).
Subducted oceanicslab-derived adakites
Delaminated lowercrust-derived adakites
Adakite by melting ofthickened lower crust
50 55 60 65 70 75
8
6
4
2
0
MgO
Adakitic melt derived fromrutile eclogite melting
Adakitic melt derivedf romrutile-fre ee clogite melting
Adakitic melt derived fromamphibolite melting
Nb/
Ta
1 10 100 10000
5
10
15
20
25
30
Amphibolitefractional
1%
5%
10%
15%
(a)
(b)
Zr/Sm
SiO2
Fig. 10. Plots of (a) MgO versus SiO2 (after Karsli et al., 2010) and (b) Nb/Ta versus Zr/Sm (after Foley et al., 2002) for the Gangdese ore-bearing adakitic porphyries.
274 J.-X. Li et al. / Lithos 126 (2011) 265–277
was thickened to ≥55 km (Chung et al., 2009) in the Eocene, and theeclogitizatized crust root with thickened lithospheric mantle becameunstable and began downwelling and thinning (Fig. 12b). Thelithospheric mantle thinning led to extensional tectonics representedby normal faults and mafic dikes (18 Ma–13Ma). Resultant upwardcounterflow of the hotter asthenosphere greatly raised the geothermand caused partial melting of the remaining juvenile arc mafic lowercrust to form the ore-bearing adakitic melts. The adakitic melts,enriched in F, Cl, and Cu and of high oxidation state, ascended to theshallow crust to form the ore-bearing adakitic porphyries in theGangdese porphyry copper belt, Southern Tibet.
5. Conclusions
Porphyry Cu–Mo deposits in the Gangdese porphyry copper belt insouthern Tibet formed in a post-collisional setting, different fromthe typical subduction-related porphyry Cu–Mo–Au deposits. TheGangdese porphyry Cu–Mo deposits are closely related to Miocene(20–10 Ma) adakitic porphyries. This study, mainly focusing on the ore-bearing adakitic porphyries, indicates that they were likely derivedfrom the melting of garnet-bearing amphibolite in juvenile arc maficlower crust, which was derived from the melting of the mantle wedgemetasomatized by slab fluids or sediment melts during the Neotethyanocean subduction. Significantly, the juvenile arc lower crust possiblyinherited the arc magma characteristics of abundant F, Cl, Cu elementsand high oxidation state, which contributed to the porphyry Cu–Modeposit in the Gangdese porphyry copper belt, southern Tibet. Thismodel provides a possible mechanism for the origin of porphyry Cudeposits in a post-collisional setting.
Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.lithos.2011.07.018.
Acknowledgments
We thank Jin-Biao Chen and Xiao-Chun Wang for their assistancewith the field works and sample collection, Yue-Heng Yang and Jin-Feng Sun for their help with the LA-ICP-MS analyses, and Prof. He Li
0 1 3 4 5
0
2
4
6
8(c)
Sediment m
elt
Slab fluid
Ba/
La
10
20
30
40
50
(d)
Sediment melt
Slab
flu
id
0 1 32 4 5
(b)
Slab fluid
Sediment melt
-10
-6
-2
4
6
-4
-8
0
2
0.705
0.706
0.707
0.708
0.709(a)
Sla
b fl
uid
Sediment melt
2
Th/Nb Th/Nb
(87Sr
/86Sr
) i
εNf(
t)
εNd(
t)
Fig. 11. Diagrams of Ba/La, (87Sr/86Sr)i, εNd(t), and εHf(t) versus Th/Nb for the Gangdese ore-bearing adakitic porphyries.
Asthenosphere
Lhasa terrane
Lithospheric mantle
Neo-TethysN
Mantle wedgemelting
Slab dehydration
(a) > 65Ma
Slab melting
Asthenosphere mantlecounterflow
Juvenile lower crustJuvenile lower crust
(b) 20-10Ma
India
Lithospheric mantle
Juvenile lowercrust melting
Mioceneadakite
Garnet-bearing amphibolite
CuFCl
FCl
Cu
FCl
Cu
F ClCu
F ClCu
F
Cl
Cu
Lhas aterrane
Fig. 12. Cartoon showing: (a) before ~65 Ma a series of arc magmas and the juvenile arcmafic lower crust formedbybasalticmelts derived from themetasomatizedmantlewedgeduring the Neotethyan ocean subduction; (b) at ca. 20–10 Ma melting of garnet-bearingamphibolite of thickened juvenile arc mafic lower crust, heated by the hotterasthenosphere upward counterflow, formed the ore-bearing adakitic melts. The adakiticmelts, enriched in F, Cl, and Cu and of highoxidation state, ascended to the shallowcrust toform the ore-bearing adakitic porphyries in the Gangdese belt, southern Tibet.
275J.-X. Li et al. / Lithos 126 (2011) 265–277
and Xin-Di Jin concerning the analysis of major and trace elements,and Assistant Prof. Chao-Feng Li referring to the Sr–Nd isotopeanalysis. The English text was edited with assistance from G. NelsonEby. Constructive comments by Gültekin Topuz and an anonymousreviewer. This study was funded by National Natural ScienceFoundation Project (40902027, 40772066 and 41072059).
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