Geochronology and petrogenesis of Miocene granitic intrusions related to the Zhibula Cu skarn deposit in the Gangdese belt, southern Tibet

Journal of Asian Earth Sciences 120 (2016) 100–116

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Geochronology and petrogenesis of Miocene granitic intrusions related to the Zhibula Cu skarn deposit in the Gangdese belt, southern Tibet Jing Xu a, You-ye Zheng a,b,⇑, Xiang Sun b,⇑, Ya-hui Shen b a b

State Key Laboratory of Geological Processes and Mineral Resources, and Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China

a r t i c l e

i n f o

Article history: Received 13 July 2015 Received in revised form 27 January 2016 Accepted 28 January 2016 Available online 28 January 2016 Keywords: Cu skarn deposit Porphyry–skarn Zircon U–Pb age Zircon Hf–O isotope Zhibula Tibet

a b s t r a c t The Zhibula Cu skarn deposit (19.5 Mt ore @ 1.64%), near the Qulong porphyry Cu–Mo deposit, is located in the Gangdese porphyry copper belt in southern Tibet. The deposit is a typical metasomatic skarn that is related to the interaction of magmatic-hydrothermal fluids and calcareous host rocks. Stratiform skarn orebodies are mainly distributed in the contact between tuff and marble in the lower part of the Jurassic Yeba Formation. Endoskarn zonations for an outward trend are observed in the granodiorite, which grade from a fresh granodiorite to a weakly chlorite-altered granodiorite, a green diopsidebearing granodiorite, and a dark red-brown garnet-bearing granodiorite. The Zhibula granodiorite and monzogranite have similar secondary ion mass spectrometry (SIMS) zircon U–Pb ages of 16.9 ± 0.3 Ma and 17.0 ± 0.2 Ma, respectively. They exhibit different fractional crystallization from granodiorite (SiO2 = 64.8–69.3 wt.%) to monzogranite (SiO2 = 72.3–76.8 wt.%). Both the granodiorite and monzogranite are characterized by high Al2O3 (12.6–16.7 wt.%) and K2O (1.5–5.5 wt.%) contents, high Sr/Y (35–151) and La/Yb (19–48) ratios, and variable MgO (0.16–3.91) and Mg# (31–61) values. They display features of enrichment in large ion lithophile elements (LILEs, e.g., Rb, Ba, Sr, and K), depletion in high field strength elements (HFSEs, e.g., Nb, Ta, Ti, and P), and moderate negative Eu anomalies (dEu = 0.58–0.98). They show restricted in situ zircon Hf isotopic compositions (+6.7 to +8.8; only one sample is +4.5) and consistent d18O values (+6.0‰ to +6.6‰). The geochemical data indicated that the Miocene Zhibula granitic intrusions formed by the magma that were characterized by high Sr/Y ratios and were derived from the partial melting of the thickened juvenile lower crust, which may have been metasomatized by the slab melts during subduction of the Neo-Tethyan oceanic crust and were induced by the convective removal of the thickened lithosphere. In addition, the Zhibula Cu skarn deposit and the Qulong Cu–Mo deposit form a significant porphyry–skarn ore system. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The Indian–Asian collision and continuous northward movement of the Indian continent created the Himalayan–Tibetan plateau, which has the thickest crust on Earth (Dewey and Burke, 1973; Yin and Harrison, 2000). The geophysical data reveal that the Tibetan plateau has been thickened to approximately twice the thickness of normal continental crust (ca. 70–80 km; Molnar, 1990; Owens and Zandt, 1997). The Gangdese porphyry copper belt that is located in southern Tibet is one of the most important Cu belts in China (Rui et al., 2003, 2004; Zheng et al., 2004, 2007, ⇑ Corresponding authors at: State Key Laboratory of Geological Processes and Mineral Resources, and Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China (Y.-y. Zheng). E-mail addresses: [email protected] (Y.-y. Zheng), [email protected] (X. Sun). http://dx.doi.org/10.1016/j.jseaes.2016.01.026 1367-9120/Ó 2016 Elsevier Ltd. All rights reserved.

2015; Hou et al., 2011; Wu et al., 2016). Most porphyry Cu (–Mo) deposits related to high-Sr/Y granitic rocks have been recognized in collisional settings in the Gangdese belt. These high-Sr/Y magmas occur in an arcuate EW-trending belt parallel to the Yarlung– Tsangpo suture zone in southern Tibet (Hou et al., 2004, 2011; Zheng et al., 2004, 2013; Wu et al., 2016). Several petrogenetic models have been proposed for high Sr/Y granitic rocks, including (1) melting of a subducted oceanic slab, followed by interaction with the overlying mantle peridotite (Defant and Drummond, 1990; Martin, 1999; Kilian and Stern, 2002; Qu et al., 2004; Zhu et al., 2009); (2) melting of a mantle wedge source, metasomatized by slab-derived melts (Grove et al., 2003, 2005; Samaniego et al., 2005; Gao et al., 2007, 2010); (3) melting of a thickened basaltic lower crust or thickened (juvenile) mafic lower crust (Atherton and Petford, 1993; Xu et al., 2002; Chung et al., 2003; Hou et al., 2004; Hou et al., 2011; Mo et al., 2007; Li et al., 2011; Guan et al.,

J. Xu et al. / Journal of Asian Earth Sciences 120 (2016) 100–116

2012); (4) melting of a subducted mafic continental lower-crust (Wang et al., 2008; Xu et al., 2010); (5) assimilation and fractional crystallization (AFC) of mantle-derived magmas during their transit through the continental crust (Castillo et al., 1999; Macpherson et al., 2006); and (6) magma mixing between siliceous crustal melts and basaltic magma from metasomatized mantle (Streck et al., 2007; Chen et al., 2013). Ore deposits hosted in and around porphyry intrusions emplaced into carbonate rocks represent a major source of Cu, Pb, Zn, Ag, and Au worldwide (Baker et al., 2004). Copper skarn ore systems are particularly common in orogenic zones related to subduction, both in oceanic and continental subduction geotectonic settings (Einaudi et al., 1981; Meinert et al., 2005). The oreforming intrusions associated with Cu skarn deposits are typical I-type granitoids and consist of a relatively large range of rocks, such as granodiorite and granodiorite porphyry (Meinert et al., 2005; Hezarkhani, 2006; Zhang et al., 2014). These intrusions also have high-K, Na-rich, and high-Mg contents, with Fe2O3/FeO ratios > 0.4 (Kim et al., 2012), and exhibit high oxygen fugacity and water-rich conditions (e.g., enrichment of biotite, sphene, hornblende, and magnetite; Foley and Wheller, 1990). The porphyry–skarn metallogenic system is undoubtedly significant and has been extensively studied in recent decades. Some researchers have constrained the porphyry–skarn mineralization systems by geochronology, igneous geochemistry, and ore-forming fluid, such as the Lamasu Cu deposit (Zhu et al., 2012), Hongniu–Hongshan Cu deposit (Peng et al., 2014), Huangshaping W–Sn deposit (Li et al., 2014), and Luming–Xulaojiugou Mo–Pb–Zn deposit (Hu et al., 2014). However, further studies on the processes of the formation and evolution of porphyry–skarn deposits are necessary to determine their genetic links. The Zhibula Cu skarn deposit has been researched via metallogenic epoch (16.9 ± 0.64 Ma, Li et al., 2005) and skarn characteristics (Xu et al., 2014, 2016). However, systematic studies on the intrusive rocks related to skarn mineralization, particularly in comparison with the Qulong magmatic rocks, have not yet been performed extensively. Here, determined by systematical investigation of the alteration and mineralization of the local geology and combined with the SIMS zircon U–Pb geochronology, zircon Hf–O isotope, and detailed geochemistry of the Zhibula granodiorite and monzogranite, we provide robust constraints on magmatic and metallogenic epochs, petrogenesis, and the relationship between Zhibula and Qulong deposits. 2. Regional geology The Himalayan–Tibetan plateau primarily contains four tectonic domains that include, from south to north, the Himalaya, Lhasa, Qiangtang, and Songpan-Ganzi terranes, which are separated by the Indus-Yarlung Zangbo suture zone (IYZSZ), Jinsha suture zone (JSSZ), and Bangong-Nujiang suture zone (BNSZ) (Fig. 1a and b). The Lhasa block is divided into the northern Gangdese (NG), central Gangdese (CG), Gangdese back-arc fault uplift belt (GBAFUB), and southern Gangdese (SG), which are separated by the Shiquan River-Nam Tso Mélange Zone (SNMZ), Gar-LunggarZhari Nam Tso-Comai Fault (GLZCF) and Luobadui-Milashan Fault (LMF) (Fig. 1c). The Gangdese porphyry copper belt discussed in the paper is located to the east of the 1000 km-long Gangdese tectonic-magmatic belt in SG and has experienced a complex tectonic history from the Early Jurassic subduction of the NeoTethyan ocean to the Cenozoic Indian–Asian continental collision and post-collision (Yin and Harrison, 2000; Chung et al., 2003; Hou et al., 2011; Zheng et al., 2014a; Sun et al., 2013, 2016, in press). The Gangdese porphyry copper belt is the largest copper belt in China that has been found thus far and where more than 10 large and super-large ore deposits have been discovered,

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including the Qulong porphyry Cu–Mo deposit (10 Mt of Cu and 0.44 Mt of Mo metal; Zheng et al., 2015), the Zhunuo porphyry Cu deposit (2.3 Mt of Cu metal; Zheng et al., 2007), and the Jiama porphyry–skarn Cu–Mo–Au–Ag–Pb–Zn deposit (5 Mt of Cu, 0.55 Mt of Mo, 105 t of Au, 7000 t of Ag, and 0.56 Mt of Pb + Zn metal; Tang et al., 2011) (Fig. 1). The Zhibula Cu skarn deposit is located approximately 2 km south of the Qulong porphyry Cu–Mo deposit (Fig. 2a). The magmatism can be generally divided into four phases in this metallogenic belt, including the Lower Jurassic arc volcanic rocks and arc granites of the Yeba Formation, granites related to subduction of the Late Cretaceous, volcanic rocks of the Linzizong Group and the coeval collision-type granites of the Paleocene to Eocene and the porphyries and granites associated with largescale porphyry Cu deposits of the Oligocene and Miocene (Geng et al., 2005; Zheng et al., 2014a). The structure distribution is nearly E–W trending, whereas the main ore-controlling structures are compound N–E fracture zones with multiphase and inherited characteristics (Zheng et al., 2004). 3. Geology of the ore deposit The Lower Jurassic Yeba Formation, which predominantly comprises tuff that has partly metamorphosed to (pyroxene–clinozoisite) hornfels and mixed with a small amount of marble and meta-quartz sandstone, is the major lithostratigraphic unit (Li et al., 2005). The formation strikes west-northwest with a dip of 70–90° and is consistent with the regional structures (Fig. 2b and c). Many thrust faults lie in the mine and control the distribution of the orebodies, which developed in breccias, cataclastic rocks, and fault gouges with skarn and mineralization such as garnet, magnetite, and minor chalcopyrite. Large intrusions were not observed and no outcrops were evident in this area, except for some granitoid dikes or apophyses that were composed of granodiorite and monzogranite in the drill cores. Endoskarn was locally observed in granodiorite proximal to contact areas and consists of an assemblage of garnet ± diopside ± chlorite. Endoskarn is generally zoned from a dark red-brown garnet at the skarnintrusive contact to a green diopside-bearing granodiorite, a weakly chlorite altered granodiorite, and a fresh granodiorite (Fig. 3, at depths of 420.5–432 m in the drill hole ZK2014). Similarly to the endoskarn in the granodiorite, the exoskarn also reflects a certain order of zonation, beginning with a fresh tuff, changing to a grayish chlorite hornfels, then to a yellowish-green epidote skarn, and finally to a dark brownish-red garnet skarn. Skarns, particularly the dark red-brown garnet skarn, are closely spatially associated with mineralization and contain ore minerals, such as chalcopyrite, bornite, and magnetite. Only trace chalcopyrite and bornite were observed in the granodiorite that is associated with typical garnet and diopside endoskarn (Fig. 3). Four orebodies are primarily hosted in the interbeds between tuff and marble in the Yeba Formation; these layers are less lenticular and parts of the orebodies are controlled by fractures in the tuff (Fig. 2b and c). The orebodies contain 0.32 Mt of Cu metal (average grade 1.64 wt.%, Li et al., un published). A wall rock alteration that is dominated by skarnization is extensively developed and is closely spatially associated with the copper mineralization. Skarns are mainly hosted in the contacts between tuff and marble and in the fractures of the tuff. Parts of them are distributed in the contacts between marble and granodiorite. The ore minerals mostly consist of chalcopyrite and bornite, followed by magnetite, molybdenite, pyrite, galena, sphalerite, and pyrrhotite, along with minor chalcocite, hematite, and arsenopyrite. The gangue minerals are commonly composed of grandite and diopside, followed by epidote, wollastonite, calcite, and quartz and small amounts of tremolite, actinolite, chlorite, and sericite. The Zhibula skarn mineralization is similar to those of typical metasomatic skarn deposits

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70

90

80

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120

Tarim

36

40 Central Asian Orogenic Belt

40

Qiangtang

30 H im

a la y

an O

Fig b 20

0

ro g e

32

JSSZ

Fig c South China Craton

90

500 km

0

100

Late Triassic

Ophiolitic melange zone

Chengdu

Early Jurassic

Porphyry Cu-Mo(Au) deposit

Kunming

Early Cretaceous

Skarn Cu(Pb-Zn)deposit

zi

120

82 32

pan Gan

Late Cretaceous

Himalayas

20

1000 km

Granitoids

Song

BNSZ Gangdese Lhasa IYZSZ

30 n

Lanzhou

Kunlun

Tarim Craton Beijing Nor th C r a C h in a to n

(b)

100

90

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(a)

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Xungba

(c)

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Gerze

86 88 SNMZ

IYZSZ

Nyainrong

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BNSZ

Naqu 31

31

CG

Sangba SNMZ

GLZCF

Damxung Mamba

30

GBAFUB Namling Zhunuo

Tinggong

Jiru Chongjiang Xiongcun Pusangguo

100 km

Nimu

29

IYZSZ

Saga 84

86

Lhasa Dabu

Jiama

Zhibula

Chuibaizi

Bayi

Fig.2a

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Xigaze 88

30

Gongbo gyamda

Qulong

SG

82 50

NG

Coqen

Hor LMF

0

94

Nyima

90

92

94

Fig. 1. (a) Simplified structural map of China. (b) Tectonic framework of the Himalayan–Tibetan plateau (modified after Zhu et al., 2009). (c) Simplified geologic map of the Lhasa block showing major tectonic subdivisions, distribution of main deposits, and position of the study area (modified after Zheng et al., 2014a). Abbreviations: JSSZ = Jinsha Suture Zone, BNSZ = Bangong-Nujiang Suture Zone, SNMZ = Shiquan River-Nam Tso Mélange Zone, GLZCF = Gar-Lunggar-ZhariNam Tso-Comai Fault, LMF = LuobaduiMilashan Fault, IYZSZ = Indus-Yarlung Zangbo Suture Zone, SG = Southern Gangdese, CG = Central Gangdese, GBAFUB = Gangdese Back-Arc Fault Uplift Belt, and NG = Northern Gangdese.

that are related to magmatic-hydrothermal fluids, which develop though a prograde stage, early retrograde stage, and late retrograde stage (Meinert et al., 2005). 4. Samples and analytical techniques 4.1. Petrography The fresh granodiorite and monzogranite in this study were collected from the drill cores ZK2014 and ZK2407 (the drillhole locations are shown in Fig. 2b). The Zhibula granodiorite is light gray, coarse- to medium-grained and principally composed of plagioclase (40–45 vol.%), quartz (20–25 vol.%), and K-feldspar (15–20 vol.%), followed by biotite (10–15 vol.%) and hornblende (5–10 vol.%). Plagioclase is a tabular crystal (1.5–2.5 mm) and commonly exhibits well-developed carlsbad–albite twins and minor oscillatory zoning. K-feldspars (2–3 mm) show Carlsbad twinning. Biotite (0.3–1 mm) exhibits distinct pleochroism in planepolarized light and changes from brown to light brown. Hornblende is euhedral fine to medium grain. The accessory minerals, including sphene, zircon, magnetite, and apatite, occur as inclusions within the essential minerals (Fig. 4a and b). The Zhibula monzogranite is light off-white and medium-grained and primarily consists of plagioclase (25–30 vol.%), quartz (30–35 vol.%), and K-feldspar (30–35 vol.%) along with minor biotite (3–5 vol.%) and hornblende (
3 vol.%). Plagioclase is a tabular crystal (1–2 mm) and is characterized by slight minor oscillatory zoning and evident polysynthetic twins. Quartz exhibits irregular grain (0.5–1.5 mm) texture, and biotite is subhedral tabular. The accessory minerals include zircon and apatite (Fig. 4c and d). 4.2. Zircon U–Pb dating The zircons from the Zhibula granodiorite and monzogranite were separated by conventional techniques at the Institute of

Regional Geology and Mineral Resource Survey of Hebei Province, China. Zircon grains were separated by conventional magnetic and density techniques to concentrate nonmagnetic and heavy fractions. After sample preparation, zircon grains, which were free of visible inclusions and major fractures, were handpicked and embedded in epoxy resin and then polished to expose the grain centers. Cathodoluminescence images were obtained using a JXA-8100 electron microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) in Beijing, to characterize the internal structure of the zircons. Then the zircons showing obvious oscillatory zoning were selected for U–Pb dating and Hf–O isotopic analyses. Zircon grains from the granodiorite are mostly between 100 and 250 lm in size, with length/width ratios ranging from 2:1 to 3:1. Cathodoluminescence images reveal obvious oscillatory zoning of these zircons. Zircon grains from the monzogranite are mainly between 150 and 300 lm in size, with length/ width ratios of approximately 2:1–3:1 and are also show oscillatory zoning on cathodoluminescence images. Zircon U–Pb isotopic analyses were conducted by secondary ion mass spectrometry (SIMS) on a Cameca IMS-1280 at IGGCAS. U–Th–Pb isotopic ratios and absolute abundances were determined relative to the standard zircons TEMORA and 91500 (Black et al., 2003; Wiedenbeck et al., 1995). The detailed analytical procedures have been described by Li et al. (2009). The mass resolution used to measure the U–Pb isotope was 5400 during the analytical session. The analytical spot size is approximately 10–15 lm. Corrections were sufficiently small to be insensitive to the choice of common Pb composition, and an average of present-day crustal composition (Stacey and Kramers, 1975) was used, assuming that the common Pb is largely surface contamination or from the gold coating introduced during sample preparation. Uncertainties on individual analyses in data tables are reported at the 1r level; mean ages for pooled U/Pb analyses are quoted at the 95% confidence interval. Data reduction was performed using the Isoplot 3.0 (Ludwig, 2003).

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200 m

N

A’

F1

N29°35'12?

E91°37'01?

(b) 0

F9

ZK2407 F2 ZK2014

E91°33'54?

(a)

400 m

N

N29°38'47?

E91°38'19? 0

Fig.c

A

F5

F3

F7

F6

Biote garanodrite

Granite porphyry

Rhyolite porphyry

Tuff

Porhyry orebody

Skarn orebody

F4 F8

Fig.b

N29°35'04?

N29°36'12?

Monzoganite Biote porphyry monzoganite

Tuff

Skarn

F1

Orebody Quartz vein

E91°35'24?

Fault

Drill hole

A’ 5358.86 (m)

(c )

A

Meta-quartz Marble sandstone

14°

0

50 m 5300

5200 Tuff

Meta-quartz sandstone

Marble

Skarn

Orebody

Quartz vein

Fault

Fig. 2. (a) Regional geological map of the Qulong ore district including the Zhibula ore deposit (modified after Zheng et al., 2004). (b) Geological map of the Zhibula Cu skarn deposit. (c) Geological cross section A–A0 (No. 16 exploration line).

4.3. Zircon Hf–O isotopic analyses In situ zircon oxygen isotopic measurements were also conducted using the Cameca IMS-1280 at IGGCAS. For oxygen isotopic analyses, the intensity of the Cs+ primary ion beam was 2–3 nA, and it was accelerated at 10 kV. The spot size is 10–15 lm. Oxygen isotopes were measured in multi-collector mode using two off-axis Faraday cups. Uncertainties of individual analyses are reported at the 1r level. The internal precision of a single analysis was generally better than 0.2‰ for the 18O/16O ratio. Values of d18O were standardized to VSMOW (Vienna Standard Mean Ocean Water, 18 O/16O = 0.0020052) and reported in standard per mil notation. The instrumental mass fractionation factor (IMF) was corrected using the 91500 zircon standard with (d18O) VSMOW = 9.9‰ (Wiedenbeck et al., 2004). The measured 18O/16O ratio was normalized using VSMOW compositions and then corrected for the IMF. The Penglai zircon standard analyzed during the course of this study yielded a weighted mean of d18O = 5.28 ± 0.06‰ (95%

confidence level, n = 54), which is consistent within error with the reported value of 5.31 ± 0.10‰ (Li et al., 2010). The detailed working conditions and the analytical procedures have been described by Li et al. (2010). In situ zircon Hf isotopic analyses were subsequently done on the same spots or the same age domains for age determination of the concordant grains. Zircons were performed using a Neptune plus MC–ICP–MS, coupled to a Geolas 2005 excimer ArF laser ablation system with a beam diameter of 44 lm at the State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences (Wuhan). The 91500, GJ-1, and NIST610 standards were analyzed during the analyses. Details of instrumental conditions and data acquisition are given in Hu et al. (2012) and Deng et al. (2010). The eHf(t) values (parts in 104 deviation of initial Hf isotopic ratios between the zircon sample and chondritic reservoir) and TCDM (zircon Hf isotope crustal model ages based on a depleted mantle source and an assumption that the protolith of the zircon’s host magma has an average continental crustal

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Depth (m) Lithostragraphic column

10.0

Zonaon

Gangue minerals

Ore minerals

Greyish chlorizaon hornfels

Chlorite (major), biote and epidote (minor)

Pyrite (major), chalcopyrite (minor)

Dark red-brown garnet skarn

Garnet (94%), fissure epidote (minor)

Magnete (mostly), chalcopyrite (minor)

Yellow-green epidote skarn

Epidote (80%), chlorite and quartz (major), caleite (minor)

Chalcopyrite (major), pyrite (minor)

1

1

2014-50m

52.0

2

53.2

2014-88m

3

55.0 Greyish chlorizaon hornfels

Chlorite (major), biote Pyrite (major), and epidote (minor) chalcopyrite (minor)

Grey tuff mixed chlorizaon hornfels

Quartz or quartz vein (major), chlorite and garnet (minor)

Pyrite (minor), chalcopyrite (minor)

Yellow-green epidote skarn

Epidote (80%), chlorite and quartz (major), caleite (minor)

Chalcopyrite and pyrite (major), magnete (minor)

80.0 2

2014-337m

4

316.0

339.0

2014-368m

3

5 4

Dark red-brown garnet skarn

Garnet (95%), quartz, caleite, and chlorite (minor)

Locally chalcopyrite and pyrite

Grey tuff mixed dark red-brown garnet skarn

Garnet (45%)

Pyrite (minor), chalcopyrite (minor)

Dark red-brown garnet granodiorite

Garnet (40-45%), diopside (5-10%), chlorite (5%)

Chalcopyrite (trace) Bornite (trace)

Green diopside granodiorite

Diopside (15-20%), chlorite (5-10%)

Chalcopyrite (trace) Bornite (trace)

Weak chlorizaon to fresh granodiorite

Chlorite (minor)

None

Grey tuff

Quartz vein (minor)

None

409.0 2014-421m

6

420.5

421.0

5

6

2014-423m

7

425.0 7

432.0

2014-427m

466.0

Fig. 3. Lithostratigraphic column and zonation of skarn in the drill hole ZK2014. The last column is the typical hand specimen photographs of wall rock, granodiorite, skarn, and hornfels at different drill hole depths. The scale bars in all photographs are one centimeter in length. 176

Lu/177Hf ratio of 0.015) were calculated following Griffin et al. (2002), using the 176Lu decay constant given in Blichert-Toft and Albarède (1997). 4.4. Major and trace element analyses Whole-rock geochemical analyses were determined at the Institute of Regional Geology and Mineral Resource Survey of Hebei Province, China. All samples were ground in an agate mortar to 200 mesh. The analytical uncertainty of XRF analyses for major elements was within 5%, and the uncertainty of the elements examined here was also less than 5% for the ICP–MS analyses. The Chinese national rock standards of GB/T114506.28-1993 for major

elements, GB/T14506.2-1993 for H2O+ and H2O, LY/T1253-1999 for loss-on-ignition (LOI), and DZ/T0223-2001 and JY/T016-1996 for trace elements were prepared using the same procedure to monitor the analytical reproducibility. The detailed analytical procedures follow Liang and Grégoire (2000). 5. Analytical results 5.1. Zircon U–Pb ages The SIMS zircon U–Pb data are summarized in Table 1 and shown in Fig. 5. Analyses of 17 spots of zircons from the Zhibula granodiorite exhibit a broad range of U (157–662 ppm) and Th

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(b)

(a)

Q

Hbl

Pl

Kfs

Bt Kfs Hbl

Spn Qtz

Pl

Mt

Pl

Bt

Qtz

1 cm

200μm

(d)

(c)

Kfs Bt

Qtz

Qtz Bt

Pl Pl Kfs

1 cm

200μm

(f)

(e)

Grt

Pl

Di

Grt Di

Grt

Pl 1 cm

200μm

(h)

(g)

Pl

Di Di

Qtz

Qtz

Grt

Spn

Di

Pl 1 cm

200μm

Fig. 4. Representative hand specimen photographs (a, c, e, and g) and photomicrographs (b, d, f, and h) of the Zhibula drill core. (a and b) Photograph of granodiorite. Plagioclases have twins and minor oscillatory zoning. (c and d) Photograph of monzogranite with medium-grained texture and lesser dark-colored minerals. (e and f) Dark red-brown garnet skarn in endoskarn with minor diopsides. (g and h) Green diopside skarn in endoskarn without garnets. Mineral symbols: Qtz, quartz; Kfs, K-feldspar; Pl, plagioclase; Hbl, hornblende; Bt, biotite; Spn, sphene; Mt, magnetite; Di, diopside; Grt, garnet.

(57–329 ppm) contents. The high Th/U ratios (0.35–0.70) combine with the euhedral shape and the internal growth zoning of the dated zircons are indicative of the typical magmatic origin

(Hoskin and Black, 2000). The 207Pb corrected ages are relatively restricted on 15.9–18.0 Ma and the weighted mean age is 16.9 ± 0.3 Ma (MSWD = 3.0) (Fig. 5a), which represents the

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Table 1 SIMS zircon U–Pb data of Miocene Zhibula granitic intrusions. Spot

U (ppm)

Th (ppm)

Th/ U

f206%a

Uncorrected for common Pb 206

Pb/238U

±1s (%)

207

Pb/206Pb 206

Zhibula monzogranite (Z2407-6), Wghtd. Avg of common Pb corrected 1 233 89 0.38 0.0027 2 0.0516 2 293 103 0.35 0.0026 2 0.0519 3 222 82 0.37 0.0026 3 0.0538 4 291 82 0.28 0.0027 3 0.0514 5 128 40 0.31 0.0027 3 0.0454 6 374 231 0.62 0.0027 2 0.0568 7 279 174 0.62 0.0026 2 0.0479 8 247 202 0.82 0.0026 2 0.0409 9 183 79 0.43 0.0027 2 0.0528 10 252 89 0.35 0.0026 2 0.0576 11 222 95 0.43 0.0026 2 0.0505 12 207 92 0.44 0.0026 2 0.0501 13 145 66 0.46 0.0027 3 0.0499 14 110 44 0.41 0.0027 3 0.0514 15 1744 1157 0.66 0.0026 2 0.0481 16 577 254 0.44 0.0027 2 0.0529 17 208 117 0.56 0.0026 3 0.0556 18 518 166 0.32 0.0028 2 0.0481 19 151 72 0.48 0.0025 3 0.0585 20 228 129 0.57 0.0027 2 0.0522 21 177 50 0.28 0.0027 2 0.0507 22 210 104 0.49 0.0027 2 0.0497 a

±1s

238

Pb/238U age – 16.9 ± 0.3 Ma 13 17.1 0.3 8 15.5 0.4 11 15.7 0.4 16 16.0 0.5 7 16.3 0.3 8 16.7 0.4 6 16.6 0.4 5 16.7 0.4 6 16.9 0.4 8 17.5 0.4 10 15.0 0.6 8 16.6 0.5 10 17.7 0.4 7 16.0 0.3 9 17.3 0.4 11 15.8 0.3 9 17.2 0.4

206

t207/

±1s

Comm. Pb corrb

Common Pb correctedc 206

Pb/238U

206

(MSWD = 3.0, N = 17) 223 319 5.2 604 165 3.7 451 244 5.6 273 373 3.0 23 162 2.3 9 184 2.0 17 152 1.9 406 120 1.9 76 133 1.4 173 191 1.4 350 218 7.9 122 185 5.6 150 243 2.0 88 154 1.5 52 228 1.5 46 270 1.4 188 219 2.7

Pb/238U age – 17.0 ± 0.2 Ma 8 16.7 0.4 9 16.5 0.4 13 16.4 0.5 7 17.0 0.6 14 16.7 0.5 6 17.1 0.4 9 16.5 0.4 9 15.7 0.4 10 16.8 0.5 8 16.2 0.3 10 15.6 0.4 9 16.1 0.4 13 16.2 0.7 15 16.3 0.6 3 16.7 0.3 6 17.2 0.3 13 15.9 0.6 6 18.0 0.3 10 15.6 0.5 11 16.6 0.4 9 16.1 0.5 9 16.3 0.6

(MSWD = 1.2, N = 22) 268 182 4.2 279 197 2.5 363 284 3.0 260 165 2.9 35 341 2.5 483 133 1.9 94 211 2.1 297 234 7.3 322 235 3.1 516 167 1.8 220 221 6.6 200 204 4.2 192 310 7.9 260 347 5.2 105 71 0.6 326 140 1.0 437 284 6.7 104 131 0.3 547 209 4.3 293 247 3.6 228 214 6.1 180 209 5.3

±1s (%)

207

Pb/235U

±1s (%)

207

Pb/206Pb

±1s (%)

238

t206/

±1s

Yes Yes No Yes Yes Yes Yes Yes Yes Yes No No Yes Yes Yes Yes No

0.0027 0.0024 0.0024 0.0025 0.0025 0.0026 0.0026 0.0026 0.0026 0.0027 0.0023 0.0026 0.0027 0.0025 0.0027 0.0025 0.0027

2 2 3 3 2 2 2 2 2 2 4 3 2 2 2 2 2

0.0154 0.0102 – 0.0094 0.0095 0.0108 0.0110 0.0143 0.0131 0.0143 – – 0.0125 0.0123 0.0167 0.0120 –

13 41 – 56 29 30 21 20 15 19 – – 24 19 10 24 –

0.0421 0.0306 – 0.0273 0.0270 0.0300 0.0311 0.0399 0.0362 0.0381 – – 0.0330 0.0359 0.0451 0.0356 –

13 41 – 56 29 30 21 20 15 19 – – 24 19 9 24 –

17.2 15.8 16.5 16.4 16.7 17.1 16.9 16.9 17.1 17.7 16.2 17.6 18.0 16.2 17.3 16.0 17.8

0.4 0.4 0.4 0.5 0.3 0.4 0.4 0.4 0.4 0.4 0.5 0.4 0.4 0.3 0.4 0.3 0.4

No Yes Yes Yes No Yes Yes No Yes Yes No No No No Yes Yes No Yes No No No No

0.0026 0.0026 0.0025 0.0026 0.0026 0.0027 0.0026 0.0024 0.0026 0.0025 0.0024 0.0025 0.0025 0.0025 0.0026 0.0027 0.0025 0.0028 0.0024 0.0026 0.0025 0.0025

3 2 3 3 3 2 2 3 3 2 3 3 4 4 2 2 4 2 3 3 3 4

– 0.0114 0.0105 0.0102 – 0.0153 0.0109 – 0.0102 0.0150 – – – – 0.0156 0.0166 – 0.0175 – – – –

– 29 45 41 – 17 29 – 65 24 – – – – 5 11 – 7 – – – –

– 0.0321 0.0300 0.0280 – 0.0417 0.0309 – 0.0282 0.0433 – – – – 0.0435 0.0452 – 0.0456 – – – –

– 29 44 41 – 17 29 – 65 24 – – – – 5 11 – 7 – – – –

17.3 16.8 16.7 17.4 17.1 17.2 16.8 17.1 17.2 16.3 16.6 16.7 17.5 17.1 16.8 17.2 16.8 18.0 16.1 17.1 17.0 17.1

0.4 0.4 0.5 0.6 0.5 0.4 0.4 0.4 0.4 0.3 0.4 0.4 0.5 0.5 0.3 0.3 0.5 0.3 0.4 0.4 0.4 0.4

Percentage of 206Pb that is common Pb. Pb comm. Pb correction as per Yang et al. (2014). Due to very low level of Pb in zircon and high comm. Pb, correction only reliable for Comm. Pb corrected data. Note absence of corrected 207Pb/235U and 207Pb/206Pb data for some zircon.

b 207 c

t206/

206

Pb/238U ratio in some zircon.

J. Xu et al. / Journal of Asian Earth Sciences 120 (2016) 100–116

Zhibula granodiorite (Z2014-4), Wghtd. Avg of common Pb corrected 1 248 96 0.39 0.0027 2 0.0421 2 241 124 0.52 0.0025 2 0.0600 3 157 61 0.39 0.0026 2 0.0560 4 162 57 0.35 0.0026 3 0.0517 5 391 187 0.48 0.0026 2 0.0456 6 291 142 0.49 0.0027 2 0.0459 7 467 293 0.63 0.0026 2 0.0464 8 622 274 0.44 0.0027 2 0.0549 9 527 329 0.62 0.0027 2 0.0475 10 307 149 0.49 0.0028 2 0.0495 11 168 73 0.43 0.0025 3 0.0535 12 250 139 0.56 0.0027 2 0.0485 13 469 327 0.70 0.0028 2 0.0490 14 436 260 0.60 0.0025 2 0.0478 15 300 123 0.41 0.0027 2 0.0451 16 235 140 0.60 0.0025 2 0.0469 17 249 158 0.63 0.0028 2 0.0427

±1s (%)

107

J. Xu et al. / Journal of Asian Earth Sciences 120 (2016) 100–116

17 16

0.07

15

207

Pb-correction age

18

207

0.07

Mean = 16.9 0.3 MSWD = 3.0,N=17 (a) Granodiorite

19

Pb-correction age

0.08

0.08

14

17 16 15

Pb 206

Pb

0.06

Pb /

0.06

207

206

18

14

207

Pb /

Mean = 17.0 0.2 MSWD = 1.2,N=22 (b) M onzogranite

19

0.05

0.05

20

0.04 320

19

18

340

17

360

16

380 238

U/

400 206

20

15

420

440

0.04 310

19

330

18

350

17

370 238

Pb

16

390

U/

206

15

410

430

Pb

Fig. 5. U–Pb Tera-Wasserburg diagrams for zircons from the (a) Zhibula granodiorite and (b) monzogranite.

crystallization age of the granodiorite. 22 analyses of zircon grains from the monzogranite exhibit an extensive variation on U (110–1744 ppm) and Th (40–1157 ppm) contents. The Th/U ratios (0.28–0.82) and the petrography of zircons also suggest their magmatic origin. The zircon 207Pb corrected ages range from 16.1 to 18.0 Ma and exhibit a weighted mean age of 17.0 ± 0.2 Ma (MSWD = 1.2) (Fig. 5b). 5.2. Zircon Hf–O isotopes Details of zircon Hf and O isotopic results are shown in Table 2. The zircons of the granodiorite display 176Yb/177Hf and 176Lu/177Hf ratios ranging from 0.0127 to 0.0253 and 0.0004 to 0.0009, respectively (eHf(t) = +7.4 to +8.8, only one sample is +4.5, average +8.0, n = 17). The zircon 176Yb/177Hf ratios of the monzogranite are between 0.0096 and 0.0205, with the 176Lu/177Hf ratios of 0.0003–0.0007 (eHf(t) = +6.7 to +8.6, average +7.5, n = 22) and the d18O values of the monzogranite range from +6.02‰ to +6.61‰ (n = 22, average 6.30‰).

Chondrite-normalized REE patterns and primitive mantlenormalized spider diagrams are shown in Fig. 7. The entire Zhibula granodiorite and monzogranite samples exhibit light rare earth P element (LREE)-enriched patterns with REE (74.66– 157.36 ppm), potentially indicating their similar magmatic origin. The high fractionated REE patterns (LaN/YbN = 13.9–34.7, SmN/YbN = 2.6–6.8, GdN/YbN = 1.7–3.8) indicate that the garnets with minor amphiboles were major residual phases of the magmatic source in the process of the partial melting. The moderate negative Eu anomalies (dEu = 0.58–0.98) probably reflect the fractional crystallization of plagioclase in the genesis of the Zhibula intrusions. The primitive mantle-normalized spider diagrams also show that the Zhibula intrusions with the characteristics of enrichment in large ion lithophile elements (LILEs, e.g., Rb, Ba, Sr, and K) and depletion in high field strength elements (HFSEs, e.g., Nb, Ta, Ti, and P) are similar to those of the Qulong intrusions. The negative Nb, Ta, and Ti anomalies are commonly considered to be produced by ilmenite and/or sphene fractionation. The negative P anomalies may be related to apatite separation.

5.3. Major and trace elements 6. Discussion The results of whole-rock geochemical analyses are listed in Table 3. The Zhibula monzogranites are characterized by higher SiO2 contents (72.3–76.8 wt.%) than granodiorites (64.8–69.3 wt. %). In contrast, the monzogranites have lower MgO contents (0.2– 0.6 wt.%) and Mg# values (31.2–40.1) than the granodiorites do (MgO = 1.1–3.9 wt.%, Mg# = 45.1–61.1). The K2O/Na2O ratios of the monzogranites and granodiorites range from 1.0 to 2.1 and 0.4 to 0.7, respectively. The Al2O3 contents are 15.9–16.7 wt.% and 12.6–14.8 wt.%, respectively. In the Q–A–P diagrams (Bowden et al., 1984; Fig. 6a), most samples fall within the granodiorite and monzogranite fields, which are consistent with classifications performed by observing specimens by hand and microscopy. The SiO2 versus K2O plot (Peccerillo and Taylor, 1976; Fig. 6b) shows that they belong to calc-alkaline and high-K calc-alkaline rocks and that the monzogranites contain more high-K than the granodiorites do. Both them are characterized from metaluminous to peraluminous on the A/CNK–A/NK diagram (Maniar and Piccoli, 1989) and are similar to I-type granitoids (Chappell and White, 1974; Fig. 6c). The thorium contents increase with the increasing Rb contents (Fig. 6d), which is consistent with the evolution trends of the I-type magmas (Chappell and White, 1992).

6.1. Petrogenesis of the Zhibula granitic intrusions Based on petrographic observation, the samples used for wholerock geochemical analyses were fresh and had undergone almost no hydrothermal alteration (Fig. 4a–d). The loss-on-ignition is low between 0.47 and 1.79 wt.% (Table 3). Additionally, all of the samples display similar and subparallel patterns of REE and trace element concentrations (Fig. 7). We therefore suggest that alteration had no effect on the distribution on major and trace elements and that they can be used to document the petrogenesis of Miocene Zhibula granitic intrusions in this study. Comparing the Zhibula granodiorite and monzogranite with the Qulong intrusions associated with mineralization reveals similarities in their geochemical characteristics, that is, high Sr/Y (34.7–151.4) and La/Yb (19.4–48.3) ratios and low Y and Yb contents (5.4–9.8 ppm and 0.5–1.0 ppm, respectively); most scholars refer to rocks with these characteristics as adakite (Fig. 8a and b). However, the term adakite originally referred to a combination of volcanic and intrusive rocks produced by subduction during island arc setting and is associated with the partial melting of subducting hot and young

108

J. Xu et al. / Journal of Asian Earth Sciences 120 (2016) 100–116

Table 2 Zircon Hf–O isotopic data of Miocene Zhibula granitic intrusions. Spot

Age (Ma)

176

Yb/177Hf

176

Lu/177Hf

176

Hf/177Hf

1r

eHf (0)

eHf(t)

TDM (Ma)

TCDM (Ma)

fLu/Hf

d18O (‰)

Zhibula granodiorite (Z2014-4) 1 17.2 0.025258 2 15.8 0.018840 3 16.5 0.016847 4 16.4 0.017409 5 16.7 0.015634 6 17.1 0.015548 7 16.9 0.012657 8 16.9 0.017251 9 17.1 0.020039 10 17.7 0.015951 11 16.2 0.020128 12 17.6 0.017670 13 18.0 0.017757 14 16.2 0.018266 15 17.3 0.017051 16 16.0 0.018474 17 17.8 0.016812

0.000880 0.000644 0.000615 0.000612 0.000521 0.000538 0.000422 0.000705 0.000688 0.000552 0.000703 0.000622 0.000605 0.000628 0.000669 0.000638 0.000593

0.282889 0.282982 0.282993 0.282977 0.282978 0.282988 0.282991 0.282985 0.283002 0.282984 0.283012 0.282990 0.282981 0.282993 0.282999 0.282971 0.282976

0.000013 0.000010 0.000010 0.000010 0.000010 0.000010 0.000011 0.000011 0.000011 0.000012 0.000012 0.000013 0.000013 0.000011 0.000012 0.000011 0.000012

4.1 7.4 7.8 7.2 7.3 7.7 7.8 7.5 8.1 7.5 8.5 7.7 7.4 7.8 8.0 7.0 7.2

4.5 7.8 8.2 7.6 7.7 8.0 8.1 7.9 8.5 7.9 8.8 8.1 7.8 8.2 8.4 7.4 7.6

513 379 363 386 383 369 364 375 352 375 337 367 380 363 354 395 386

712 527 505 537 534 514 508 521 488 521 467 511 528 505 492 550 538

0.97 0.98 0.98 0.98 0.98 0.98 0.99 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98

n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.

Zhibula monzogranite (Z2407-6) 1 17.3 0.013229 2 16.8 0.011203 3 16.7 0.016922 4 17.4 0.016973 5 17.1 0.011361 6 17.2 0.016200 7 16.8 0.020547 8 17.1 0.017197 9 17.2 0.014309 10 16.3 0.011775 11 16.6 0.014483 12 16.7 0.018180 13 17.5 0.015629 14 17.1 0.016137 15 16.8 0.018550 16 17.2 0.013103 17 16.8 0.009615 18 18.0 0.016510 19 16.1 0.009616 20 17.1 0.015019 21 17.0 0.009752 22 17.1 0.015503

0.000549 0.000404 0.000682 0.000639 0.000460 0.000658 0.000692 0.000600 0.000532 0.000405 0.000560 0.000637 0.000547 0.000582 0.000728 0.000553 0.000362 0.000646 0.000335 0.000520 0.000410 0.000529

0.282991 0.282971 0.282969 0.282966 0.282951 0.282968 0.282975 0.282962 0.282956 0.282963 0.282973 0.282975 0.282979 0.282963 0.282962 0.282977 0.282985 0.283005 0.282984 0.282987 0.282985 0.282983

0.000011 0.000010 0.000010 0.000013 0.000010 0.000012 0.000011 0.000012 0.000011 0.000011 0.000011 0.000011 0.000009 0.000010 0.000012 0.000011 0.000011 0.000011 0.000012 0.000011 0.000012 0.000010

7.8 7.0 7.0 6.9 6.3 6.9 7.2 6.7 6.5 6.8 7.1 7.2 7.3 6.8 6.7 7.2 7.5 8.3 7.5 7.6 7.5 7.5

8.1 7.4 7.3 7.2 6.7 7.3 7.6 7.1 6.9 7.1 7.5 7.5 7.7 7.1 7.1 7.6 7.9 8.6 7.9 8.0 7.9 7.8

365 392 398 401 420 399 389 407 414 403 391 389 382 405 408 385 372 346 373 370 372 376

508 549 553 558 587 555 540 567 578 565 545 541 532 564 567 537 520 480 523 516 520 524

0.98 0.99 0.98 0.98 0.99 0.98 0.98 0.98 0.98 0.99 0.98 0.98 0.98 0.98 0.98 0.98 0.99 0.98 0.99 0.98 0.99 0.98

6.53 6.11 6.22 6.14 6.25 6.41 6.19 6.27 6.15 6.43 6.56 6.33 6.53 6.61 6.02 6.33 6.25 6.16 6.12 6.03 6.52 6.41

±2r

0.44 0.41 0.41 0.26 0.25 0.25 0.32 0.38 0.45 0.40 0.35 0.33 0.27 0.38 0.36 0.49 0.29 0.27 0.31 0.49 0.30 0.39

Notes: eHf(t) = 10,000  {[(176Hf/177Hf)S  (176Lu/177Hf)S  (ekt  1)]/[(176Hf/177Hf)CHUR,0  (176Lu/177Hf)CHUR  (ekt  1)]  1}. TDM = 1/k  ln{1 + [(176Hf/177Hf)S  (176Hf/177Hf)DM]/[(176Lu/177Hf)S  (176Lu/177Hf)DM]}. TCDM = TDM  (TDM  t)  [(fcc  fLu/Hf)/(fcc  fDM)]. fLu/Hf = (176Lu/177Hf)S/(176Lu/177Hf)CHUR  1. where, k = 1.867  1011 year1 (Soderlund et al., 2004); (176Lu/177Hf)S and (176Hf/177Hf)S are the measured values of the samples; (176Lu/177Hf)CHUR = 0.0332 and (176Hf/177Hf)CHUR,0 = 0.282772 (Blichert-Toft and Albarède, 1997); (176Lu/177Hf)DM = 0.0384 and (176Hf/177Hf)DM = 0.28325 (Griffin et al., 2000); (176Lu/177Hf)mean crust = 0.015; fcc = [(176Lu/177Hf)mean crust/ (176Lu/177Hf)CHUR]  1; fDM = [(176Lu/177Hf)DM/(176Lu/177Hf)CHUR]  1; t = crystallization time of zircon; n.a., not analyzed.

(625 Ma) oceanic slabs (Defant and Drummond, 1990; Defant and Kepezhinskas, 2001; Richards and Kerrich, 2007). Moyen (2009) mentioned that rocks with an ‘‘adakitic signature” could be achieved through different processes (e.g., melting of a high Sr/Y or La/Yb source, fractional crystallization or AFC, and interactions of felsic melts and the mantle). Some researchers also argue that not all rocks characterized by calc-alkaline to high-K calcalkaline granitoids in porphyry deposits are adakite (Richards and Kerrich, 2007). Therefore, to avoid confusion, we use descriptive terms such as ‘high Sr/Y granitic rocks’ rather than ‘adakitic rocks’. Several different petrogenetic models have been proposed to interpret the magma generation of Miocene high-Sr/Y granitic rocks in southern Tibet in recent years, including the partial melting of the subducted Neo-Tethyan oceanic crust, the upper mantle, the subducted Indian lower crust, and the thickened (juvenile) mafic lower crust (Qu et al., 2004; Gao et al., 2007; Xu et al., 2010; Chung et al., 2003; Hou et al., 2004). The Zhibula granodiorite and monzogranite have high K2O (1.5–5.5 wt.%) contents, low MgO (0.2–3.91 wt.%) contents, variable Mg# (31.2–61.1) values, and depletion in compatible elements such as Cr (6.9–14.3 ppm) and Ni (2.7–12.8 ppm); these characteristics are distinct from the calc-alkaline magma formed

by the partial melting of the subducted oceanic slab (e.g., the Early Cretaceous intrusion associated with subduction in southern margin of the Lhasa block, Zhu et al., 2009). Subsequently, the melts would have interacted with the overlying mantle wedge, resulting in high Mg# numbers during magma ascent (Stern and Kilian, 1996; Kilian and Stern, 2002). Such rocks are characterized by relatively low SiO2 (mainly <65 wt.%) contents, high MgO contents, and high Mg# (up to 70) values and are rich in compatible elements (e.g., Cr and Ni; cf. Garrison and Davidson, 2003), which are not the cases in the Zhibula magma (Fig. 9a and b). In addition, the Zhibula intrusions are rich in K2O, possessing high K2O/Na2O ratios ranging from 0.4 to 2.1, which is inconsistent with slabmelting adakitic magma with K2O/Na2O < 0.5 (Defant and Drummond, 1990; Martin, 1999). Furthermore, previous studies have indicated that the Neo-Tethyan oceanic slab probably broke off and sunk into the deep mantle at approximately 50 Ma (Kohn and Parkinson, 2002; Chung et al., 2009; Lee et al., 2009), whereas, the emplacement ages of the Zhibula intrusions were approximately 17 Ma, which suggest that the Zhibula intrusions occurred within a postcollisional setting rather than a subduction setting. Hence, partial melting of previously subducted oceanic slab cannot explain the formation of the Zhibula magma.

109

J. Xu et al. / Journal of Asian Earth Sciences 120 (2016) 100–116 Table 3 Major (wt.%) and trace element (ppm) compositions of Miocene Zhibula granitic intrusions. Sample#

SiO2 Al2O3 TiO2 Fe2O3 FeO CaO MgO K2O Na2O MnO P2O5 H2O+ H2O LOI Total La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y W Mo Cu Pb Zn Bi Co Ni Cd Nb Ta Zr Hf Th U Li Cs Be Ti V Cr Ga Rb Sr Ba Sc Ag Sn B Au As Sb Mg# A/CNK Na2O + K2O K2O/Na2O TFeO RREE La/Yb dEu Sr/Y

Zhibula granodiorite

Zhibula monzogranite

Z2014-1

Z2014-2

Z2014-3

Z4705-1

Z4705-2

Z4705-3

Z2407-1

Z2407-2

Z2407-3

Z2407-4

Z2407-5

69.33 15.88 0.38 0.87 1.60 3.33 1.12 2.04 4.58 0.03 0.15 0.31 0.12 0.50 99.81 18.39 37.03 4.54 17.17 2.90 0.77 2.17 0.29 1.25 0.25 0.67 0.10 0.62 0.08 6.41 0.66 0.37 20.3 17.2 35.4 0.03 8.9 9.9 0.02 4.18 0.30 79.3 5.07 6.93 1.51 10.91 1.74 1.86 2284 75.5 14.3 24.35 62.2 936.0 739.5 6.91 0.04 0.7 2.5 0.47 0.5 0.07 45.61 1.01 6.62 0.45 2.38 86.22 29.9 0.90 145.9

68.72 16.07 0.41 0.97 1.44 3.56 1.20 2.02 4.68 0.03 0.16 0.28 0.13 0.55 99.81 20.80 46.82 5.96 22.48 3.81 1.01 2.85 0.37 1.72 0.32 0.90 0.16 0.94 0.12 8.65 0.59 0.65 15.9 17.0 31.5 0.03 8.6 9.5 0.01 4.99 0.51 95.1 6.79 8.88 1.90 7.68 1.49 1.85 2476 70.5 13.3 22.36 46.3 849.3 736.4 5.93 0.05 0.9 5.7 0.57 0.6 0.07 48.17 0.98 6.70 0.43 2.30 108.25 22.1 0.89 98.2

65.70 16.71 0.53 0.56 1.34 4.89 1.62 3.23 4.51 0.04 0.21 0.23 0.14 0.47 99.79 25.13 50.43 6.14 23.42 3.88 1.09 2.86 0.35 1.52 0.28 0.76 0.11 0.72 0.13 7.31 0.40 2.71 23.3 17.0 34.4 0.03 7.8 12.8 0.03 4.82 0.40 91.3 5.53 7.62 2.49 3.94 1.22 1.74 3166 85.4 12.9 22.92 61.9 1106.0 799.7 7.02 0.03 1.4 3.5 0.79 0.7 0.07 61.13 0.84 7.73 0.72 1.84 116.81 35.0 0.95 151.4

65.94 16.31 0.49 1.63 1.70 3.15 3.91 1.46 2.85 4.49 0.04 0.18 0.34 0.70 99.70 20.31 42.79 5.45 21.08 3.52 1.01 2.60 0.32 1.35 0.25 0.66 0.11 0.60 0.10 6.64 0.39 0.63 16.4 16.5 39.1 0.07 7.6 10.4 0.02 3.54 0.35 98.8 6.82 5.72 1.65 6.83 3.33 1.41 2944 79.9 13.0 19.85 65.8 918.0 644.0 5.69 0.04 0.9 16.7 0.73 0.8 0.06 45.18 0.93 7.34 0.63 3.15 106.79 24.40 0.98 138.35

64.82 16.07 0.52 1.18 2.40 3.44 3.42 1.60 2.77 4.24 0.05 0.20 0.96 1.77 99.02 22.75 47.12 5.95 22.91 3.85 1.09 2.80 0.35 1.51 0.27 0.74 0.11 0.68 0.11 7.33 6.17 3.03 0.35(%) 42.3 72.7 0.89 11.4 12.3 0.27 3.91 0.39 105.0 7.40 8.39 2.28 12.89 6.71 1.54 3124 91.9 14.3 22.66 109.1 971.0 658.0 6.51 2.90 1.4 8.9 12.28 4.8 0.23 45.31 0.99 7.01 0.65 3.44 106.79 23.89 0.96 132.57

66.10 16.24 0.49 1.51 1.82 3.16 3.83 1.46 3.05 4.42 0.04 0.18 0.24 0.62 99.76 21.94 46.64 5.94 23.04 3.85 1.10 2.81 0.35 1.51 0.27 0.73 0.11 0.67 0.11 7.17 0.55 0.64 28.2 16.7 36.4 0.06 8.1 10.9 0.01 3.42 0.27 100.3 7.60 6.17 1.89 7.02 3.52 1.58 2956 80.6 14.1 20.41 76.1 963.0 722.0 5.84 0.04 1.3 5.2 0.82 0.9 0.07 45.10 0.93 7.47 0.69 3.16 106.79 23.52 0.98 134.22

76.81 12.58 0.12 0.05 0.41 0.96 0.16 5.53 2.64 0.01 0.04 0.16 0.14 0.58 99.88 17.86 33.12 3.73 12.84 2.08 0.57 1.74 0.23 1.05 0.20 0.54 0.09 0.54 0.07 5.40 0.27 1.08 11.2 39.0 17.7 0.01 2.4 2.7 0.02 2.50 0.30 57.7 3.02 12.51 1.85 8.02 2.55 0.73 696 24.9 6.9 13.55 117.5 282.8 879.0 5.92 0.02 0.5 7.0 0.88 1.0 0.14 38.24 1.04 8.17 2.09 0.45 74.66 32.9 0.90 52.4

73.31 13.56 0.21 0.44 1.08 2.28 0.40 3.48 3.25 0.02 0.08 0.70 0.23 1.74 99.84 19.63 36.02 4.05 14.15 2.30 0.61 2.01 0.28 1.39 0.27 0.72 0.12 0.70 0.09 7.25 0.24 2.79 15.3 30.3 21.2 0.03 6.4 5.8 0.03 3.47 0.34 66.2 3.86 13.98 1.74 13.78 8.90 1.13 1271 57.0 8.0 18.31 103.3 458.0 1008.0 6.23 0.04 0.7 17.9 0.56 1.0 0.07 32.62 1.02 6.73 1.07 1.47 82.33 28.1 0.85 63.2

72.31 14.75 0.19 0.33 0.84 2.38 0.40 3.68 3.60 0.02 0.07 0.63 0.21 1.29 99.84 19.53 36.29 4.07 14.12 2.29 0.65 1.91 0.27 1.28 0.25 0.66 0.11 0.62 0.08 6.68 0.31 1.92 15.2 28.5 22.6 0.03 5.1 5.2 0.02 3.12 0.31 60.8 2.41 12.73 1.10 12.06 6.91 1.41 1151 45.8 7.8 18.40 87.7 584.0 983.0 5.51 0.05 0.8 14.5 0.42 0.8 0.11 38.42 1.04 7.27 1.02 1.13 82.13 31.6 0.93 87.4

73.77 13.53 0.18 0.39 0.67 1.81 0.26 4.61 2.83 0.01 0.04 0.69 0.27 1.79 99.88 19.64 35.94 3.96 13.69 2.31 0.55 2.04 0.31 1.67 0.35 0.99 0.17 1.02 0.13 9.75 0.23 2.88 12.8 35.2 18.8 0.02 4.1 3.8 0.04 3.97 0.49 61.7 1.96 13.95 2.15 14.86 8.42 0.94 1067 41.9 8.2 16.52 124.8 338.4 732.7 7.66 0.03 1.0 18.4 0.79 0.9 0.06 31.17 1.05 7.43 1.63 1.02 82.76 19.3 0.76 34.7

73.83 12.88 0.29 0.41 1.32 2.07 0.63 3.85 2.73 0.03 0.09 0.82 0.21 1.59 99.72 37.10 71.06 8.03 28.19 4.27 0.76 3.45 0.43 1.85 0.34 0.89 0.13 0.77 0.10 9.03 0.76 2.54 373.5 30.0 33.0 0.04 8.5 6.8 0.04 3.80 0.41 75.2 3.21 18.58 1.44 11.71 8.12 0.86 1751 63.2 9.8 15.30 90.9 349.9 1301.0 9.83 0.17 0.8 18.3 1.39 1.2 0.09 40.11 1.04 6.58 1.41 1.68 157.36 48.3 0.58 38.8

Notes: Mg# = 100Mg2+/(Mg2+ + total Fe2+)] (Molar); total FeO = FeO + 0.89  Fe2O3; A/CNK = Al2O3/(CaO + Na2O + K2O) (Molar); dEu = 2  EuN/(SmN + GdN); (La/Yb)N is chondrite-normalized ratio (Sun and McDonough, 1989).

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Q

Monzogranite Granodrite

(a)

7

(b)

rite

K 2 O(W t.%)

6 5

Shoshonite

4

Gra

nod

3 2

Hi

gh

-K

Monzogranite

lc-

lin

e

Calc-alkaline 1

Quartz monzonite

Quartz monzodiorite Monzodiorite

ca

ka al

Monzonite

0

P

A

Tholelitic 40

45

50

55

60

65

70

75

80

SiO 2 (Wt.%)

3

16

(c) Metaluminous

2.5

Peraluminous I-type

14

S-type

2

12 Th (ppm)

A/NK

(d)

1.5

10 I-t

8

1

y

pe

S-t 6

0.5

tre

nd

ype

tren

d

Parlkaline 4

0 0.5

1.5

1

20

2

60

A/CNK

100 Rb (ppm)

140

Fig. 6. Discrimination diagrams for the Zhibula granodiorite and monzogranite. (a) Quartz–alkali-feldspar–plagioclase diagram (Q–A–P) (Bowden et al., 1984). (b) K2O versus SiO2 diagram (Peccerillo and Taylor, 1976). (c) A/NK versus A/CNK diagram. {A/NK = molar ratio of [Al2O3/(Na2O + K2O)]; A/CNK = molar ratio of [Al2O3/(CaO + Na2O + K2O)]} (Maniar and Piccoli, 1989; Chappell and White, 1974). (d) Rb versus Th diagram.

1000

1000

(b)

Monzogranite Granodrite Sample/Primitive mantle

Sample/Chondrite

(a)

100

Qulong porphyry

Qulong granodrite

10

Qulong porphyry

100

10

1

Qulong granodrite 0.1

1 La

Ce

Pr

Nd

Sm Eu

Gd

Tb

Dy

Ho

Er

Tm Yb

Lu

Rb Ba Th U

K Ta Nb La Ce Sr Nd P Zr Hf Sm Ti Y Yb Lu

Fig. 7. (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized trace element patterns of the Zhibula granodiorite and monzogranite. Values for the chondrite and primitive mantle are from Sun and McDonough (1989). Data of the Qulong porphyry and Qulong granodiorite are from Yang (2008) and Zheng et al. (2013).

The magmatic zircons of the Zhibula granodiorite and monzogranite show positive eHf(t) values (+6.7 to +8.8) with a young TCDM age of 463–711 Ma, which conform closely with those of the

Qulong granodiorite (+6.4 to +10.6, Yang, 2008; +5.6 to +9.3, Hu et al., 2015; Fig. 10). Moreover, these values are similar to the zircon eHf(t) values of the Eocene Gangdese batholith (+6.0 to +13.4,

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200

Zhibula monzogranite Zhibula granodrite Qulong porphyry Qulong granodrite

70 60

Adakites

150 50 La/Yb

Sr/Y

Adakites 100

40 30

Normal arc magmas

50

Normal arc magmas

20 10

(b)

(a) 0

0 0

5

10

15 Y(ppm)

20

25

0

0 5

1 Yb(ppm)

1 5

2

Fig. 8. Discrimination diagrams of (a) Sr/Y ratios versus Y contents and (b) La/Yb ratios versus Yb contents for the Zhibula granodiorite and monzogranite (Defant and Drummond, 1990; Moyen, 2009).

175

(a)

Subduction-related adakite

1000

150

Subduction-related adakite 100 Ni(ppm)

125

Ni(ppm)

(b)

100

10

75 50

Lower crust-derived adakite

1

Lower crust-derived adakite

25 0 25

35

45

55

65

75

Mg#

0 1 0 1

1

10 Cr(ppm)

100

1000

Fig. 9. Discrimination diagrams of (a) Ni contents versus Mg# values and (b) Ni contents versus Cr contents for the Zhibula granodiorite and monzogranite. Data of slab melting and lower-crustal melting are from Guan et al. (2012). Data of the Qulong porphyry and Qulong granodiorite and symbols are the same as in Fig. 8.

Ji et al., 2009), the Cretaceous Gangdese batholith (+6.2 to +16.5, Chu et al., 2011; Zhu et al., 2009), and the Linzizong volcanics (+0.5 to +8.5, which was restricted from +5.2 to +8.5; Lee et al., 2007; Fig. 11). However, they are markedly different from the magma derived from the melting of an upper mantle, namely the enriched subcontinental lithospheric mantle, for which eHf(t) values usually range from 12 to 36 (Griffin et al., 2000). Additionally, metasomatized mantle peridotite is composed mainly of pyroxenite. Partial melting of pyroxenite produces basalts, rather than adakites. This is proved by the lack of large volumes of coeval mafic magmatic rocks in the Lhasa terrane (Chung et al., 2009; Zheng et al., 2012; Hu et al., 2015). We therefore preclude the possibility that the Zhibula magma was derived from the melting of an upper mantle metasomatized by slab-derived melts. Similarly, melts produced by a subducted lower crust might have subsequently interacted with the overlying lithospheric mantle and formed adakites characterized by high Cr, Ni, and Mg# values (Wang et al., 2008). Even if the melts did not fully interact with the overlying lithospheric mantle, their isotopic signatures should

be similar to those of the Eocene high-Sr/Y granitic rocks of the northern Himalayas (Zeng et al., 2011). However, the zircon eHf(t) values of the Zhibula magmatic rocks are not in agreement with those of the Eocene high-Sr/Y granitic rocks of the northern Himalayas (<6, Zeng et al., 2011). We therefore conclude that the melting of the subducted Indian continental lower crust proposed by Xu et al. (2010), cannot explain the petrogenesis of Miocene Zhibula magma. Rapp et al. (1991) proposed that the lower crustal source composed of eclogites and/or garnet-bearing amphibolites at a depth of at least 40–50 km can produce the high Sr/Y and La/Yb magma. Although Nomade et al. (2004) suggested that there was a much thinner crust of ca. 35 km in southern Tibet during the Miocene, many researches have demonstrated that the thickness of the Tibetan crust has been >40–55 km in Miocene (Mo et al., 2007; Chung et al., 2009; Guan et al., 2012). This is because melting of eclogites and garnet amphibolites were observed; Chung et al. (2003) and Hou et al. (2004) reported that high-Sr/Y granitic magma from continental collision zones in Tibet was derived from the partial

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20

Zhibula monzogranite Zhibula granodrite Qulong granodrite Linzizong volcanics Eocene Gangdese batholith Cretaceous Gangdese batholith

ε Hf(t)

15

Depleted mantle

0. 5G a

10

(176 Lu /

177

Hf)= 0.01 5

5

1.0Ga

0

0

20

60

40

80 100 Zircon U - Pb age(Ma)

140

120

160

Fig. 10. Plot of zircon eHf(t) versus U–Pb ages for the Zhibula ore deposit. Data of the Qulong granodiorite come from Yang (2008) and Hu et al. (2015), the Linzizong volcanics are from Lee et al. (2007), the Eocene Gangdese batholith are from Ji et al. (2009), and the Cretaceous Gangdese batholith are from Chu et al. (2011).

10

Hf

DM

0.

/Hf

C

7

=1.

5

30

0.2

8

50

0.0

δ 18O

Zhibula monzogranite (17Ma) Qulong granic rock (17Ma) Linzhi two-mica granite(26Ma) Tinggong K-feldspar granite (50Ma) Langxian two-mica granite(76-80Ma)

Ancient lower Crust

5

70

6

90

Enriched Mantle

4 -15

Depleted Mantle

Mantle Zircon

-10

-5

0

5

10

15

Hf(t) Fig. 11. Plot of eHf(t) versus d18O isotopes of zircons from the Zhibula monzogranite. The solid lines denote the two component mixing trends between the mantle- and lower crust-derived magmas. HfDM/HfC is the ratio of the Hf concentration in the depleted mantle (DM) over that of the crustal (C) melt indicated for each curve, and ticks on the curves represent 10% mixing increments by assuming the mantle zircon has eHf(t) = 12 and d18O = 5.3‰ (Valley et al., 1998) and that the ancient lower crust zircon has eHf(t) = 5.2 and d18O = 9.4‰ (cf., Zheng et al., 2012). The ratio of Hf concentrations in the DM and C end members (HfDM/HfC) is indicated for each. Data of the Tinggong K-feldspar granite (50 Ma) are from Du (2013), Langxian two-mica granite (76–80 Ma) are from Zheng et al. (2014b), Linzhi two-mica granite (26 Ma) are from Zheng et al. (2012), and Qulong granitic rock (17 Ma) are from Hu et al. (2015).

melting of the ancient Lhasa thickened lower crust. However, the eHf(t) values of the Zhibula intrusions are clearly distinct from those of the Early Carboniferous granitoids (6.8 to 4.9) of the Jiacha and Langxian areas in southern Lhasa Terrane, which were generated by the partial melting of the ancient Lhasa continental lower crust (Ji et al., 2012). As previously mentioned, the Hf isotope compositions are similar to those of high-Sr/Y granitic rocks from the Qulong deposit (+6.4 to +10.6, Yang, 2008) and Linzizong volcanics (+5.2 to +8.5, Lee et al., 2007) that were derived from the thickened juvenile lower crust (Mo et al., 2007; Yang, 2008; Hou et al., 2011). Therefore, the thickened juvenile lower crust plausibly explains the source of the Zhibula intrusions. Hou et al. (2015) investigated that Miocene Cu–Mo deposits-related porphyries are spatially confined to the Jurassic arc and concluded

that remelting of the lower crustal sulfide-bearing Cu-rich Jurassic cumulates took place in a thickened crust (>50–55 km) within the amphibole and garnet stability field (Chiaradia, 2013), which is consistent with Cenozoic collision-induced crustal thickening in southern Tibet. We thus infer that materials of cumulates possibly contributed to the thickening of the juvenile lower crust. This conclusion is also compatible with our new in situ zircon oxygen isotope data. Zircon is extremely retentive of its magmatic oxygen isotope ratio (Valley et al., 1998). Monzogranite and granodiorite from the Zhibula ore deposit exhibit consistent ages and geochemistry, indicating that they originate from the same magmatic source. Therefore, we analyzed zircons of monzogranite to examine oxygen isotopes. As listed in Table 2, the Zhibula monzogranite zircon d18O values vary across a centralized area between

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Marble Tuff Skarn orebody Qulong porphyry

Zhibula skarn Cu deposit

Cu-Mo deposit

Indian continental crust Lhasa continental crust

Indian lithospheric mantle Partial melting of thickened juvenile Lhasa lithospheric mantle lower crust

Upwelling hot asthenosphere mantle

Convective removal Fig. 12. Schematic image showing the petrogenetic model for Miocene Zhibula intrusions (modified from Sillitoe, 2010).

+6.0‰ and +6.6‰ and are similar to the values of mantle zircon (5.3 ± 0.3‰, Valley et al., 1998). Zheng et al. (2012) obtained ancient inherited zircon with an age of 1660 Ma, d18O value of 9.4‰ and eHf(t) of 5.2 from the Linzhi granitoid and presented that this zircon represents the isotopic composition of ancient lower crust (Zheng et al., 2014b). Thus, to simulate the magmatic source combination, we use this zircon as one endmember of the ancient lower crust and the depleted mantle as the other. As visible in the plot of eHf(t) versus d18O zircon isotopes (Fig. 11), the Hf–O isotope compositions of the Zhibula monzogranite are similar to those of the Qulong high Sr/Y granitic rocks (17 Ma, Hu et al., 2015), the Tinggong K-feldspar granite (approximately 50 Ma, Du, 2013), and the Langxian two-mica granite (76–80 Ma, Zheng et al., 2014b), and they clearly differ from the Linzhi two-mica granite (approx. 26 Ma, Zheng et al., 2012). On the basis of simulation results, most zircons in the monzogranite precipitated from melts that contained approximately 80% depleted mantle-derived melt (HfDM/HfC = 0.7), which is similar to the compositions of the MORB (eHf(t) = +13.9, d18O = +5.8‰, Hoefs, 2009). In conclusion, we infer that Miocene Zhibula magma was derived from the partial melting of the thickened juvenile lower crust may have been metasomatized by the slab melts during subduction of the Neo-Tethyan oceanic crust. 6.2. Implication for geodynamic scenarios Precise in situ SIMS zircon U–Pb data reveal that the ages of the Zhibula granodiorite (16.9 ± 0.3 Ma) and monzogranite (17.0 ± 0.2 Ma) are similar to those of the Qulong porphyry (17.6 ± 0.7 Ma, Rui et al., 2003; 17.7 ± 0.3 Ma, Yang, 2008) and granodiorite (16.7 ± 0.3 Ma, Zheng et al., 2013). Both of them are consistent with large-scale porphyry and skarn mineralization as well as the magmatism of the Gangdese metallogenic belt during the Miocene (e.g., Jiama granite porphyry, 14.2 ± 0.2 Ma, Tang et al., 2011; Bangpu monzogranite porphyry, 16.2 ± 0.2 Ma, Wang et al., 2011; Tinggong monzogranite porphyry, 17.0 ± 0.6 Ma, Rui et al., 2004; Zhunuo monzogranite porphyry, 15.6 ± 0.6 Ma, Zheng et al., 2007) (the deposit locations are shown in Fig. 1c). Several geodynamic processes have been proposed for the Miocene

Gangdese magmatism, including subducted Neo-Tethyan oceanic slab breakoff (Kohn and Parkinson, 2002; Hou et al., 2004), delamination of the thickened lithosphere (Chung et al., 2003; Li et al., 2011), and convective removal of the thickened lithosphere (Ji et al., 2009; Zheng et al., 2014a). In the case of slab breakoff, subduction terminated in the Eocene (Lee et al., 2009; Chung et al., 2009; Ji et al., 2009), and the Miocene magmatism was confined to a restricted region rather than the Gangdese belt with an approximate length of 1000 km (Li et al., 2011). We therefore consider that slab breakoff is unlikely to be associated with the Zhibula intrusions. This is also in agreement with the conclusions regarding Miocene Qulong intrusions that were proposed by Yang (2008) and Hu et al. (2015). Delamination of the lower crust would cause upwelling of the asthenosphere and generate magma with high MgO, Cr, and Ni contents, as well as Mg# numbers, such as Mesozoic Ningzhen adakitic rocks with the contents of MgO (1.52–3.99 wt.%), Cr (19.6–113 ppm), and Ni (12.0–64.6 ppm) (cf. Xu et al., 2002) and Xinglonggou adakites from northeast China with the contents of MgO (average of 3.7 wt.%, but up to 5.7 wt. %), Cr (127–402 ppm), and Ni (82–311 ppm) (cf. Gao et al., 2004). Therefore, the Zhibula magma was unlikely derived from the delaminated lower crust. During the Miocene (18–13 Ma), following the Indian–Asian continental collision, the regional tectonic stress change from compression to extension (Yin and Harrison, 2000; Hou et al., 2004; Zheng et al., 2014a; Wu et al., 2016) was conducive to asthenosphere upwelling. The convective removal of the thickened lithosphere in southern Tibet directly resulted in upwelling of the asthenosphere and triggered the partial melting of the thickened juvenile lower crust (Fig. 12). 6.3. Porphyry–skarn system Granodiorite and monzogranite are the only two types of intrusions in the Zhibula mine. Because of the relatively low degree of exploration in this area, the outcrop of magmatic rocks and the relationship between intrusions and wall rocks, except for small amounts of granodiorite and monzogranite observed in drill cores, could not be observed. Precise in situ SIMS zircon U–Pb ages reveal that the Zhibula granodiorite and monzogranite formed at

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approximately 17 Ma, which is consistent with the isochron age of molybdenite Re–Os (16.9 ± 0.64 Ma, Li et al., 2005). The granodiorite and monzogranite exhibit similar geochemical features with Itype granitoids and display consistent petrogenesis and magmatic source, both of which are seemingly related to Cu skarn mineralization. Copper concentrations cannot be used directly as a geochemical parameter to prove or disapprove the link between magma and Cu deposits, whereas the crystallization processes appear to be more important than bulk magma chemistry in determining the intrusion-associated Cu mineralization. This is because Cu concentrations may drop quickly when magmas evolve to higher SiO2 contents (
58 wt.%) because of the oxygen fugacity fluctuation induced by crystallization of Fe–Ti oxides and subsequent sulfate reduction (S2) that scavenges Cu into magmatic fluids (Foley and Wheller, 1990; Mungall, 2002; Liang et al., 2009; Sun et al., 2011). Candela and Holland (1984) concluded that the concentration of Cu increases with the elevated concentration of chloride and oxygen fugacity, as the clathrate (e.g., (CuCl)0, (CuCl3)2, and (CuCl3)3) (Bertelli et al., 2009). However, Cl concentrations decrease and F concentrations increase, evolving to higher crystal fractionation. Thus, high fractionation of magma makes no contribution to the release of more Cu for mineralization. The Zhibula monzogranites exhibit higher SiO2 contents (72.3–76.8 wt.%) than granodiorites (65.7–69.3 wt.%) and show higher fractional crystallization. Moreover, the monzogranites have lower fO2 (Fe2O3/ FeO = 0.13–0.58) than that of granodiorites (Fe2O3/FeO = 0.41– 0.68) (Kim et al., 2012), and the granodiorites are also water-rich (e.g., enrichment of biotite, sphene, hornblende, and magnetite; Foley and Wheller, 1990). Therefore, it plausible that the granodiorite is more likely intrusion-associated Cu skarn mineralization in the Zhibula deposit. In addition, Meinert (1987) observed that skarn is zoned away from contact in the sequence, namely from garnet to pyroxene, to pyroxenoid and to marble in the northeast zone of the Groundhog Zn–Pb–Cu–Ag skarn deposit. Meinert et al. (1997) investigated the Big Gossan Cu–Au skarn and proposed that, from the contact to distal marble, the skarn zonations change from garnet-rich skarn to pyroxene-rich skarn, and the colors of garnet change from dark red-brown, to brown and finally to pale green. Similar phenomena were also observed by Atkinson and Einaudi (1978) in the Carr Fork Cu district, Zhang et al. (2014) in the Tongshan Cu deposit, and Peng et al. (2015) in the Hongniu–Hongshan Cu deposit. Skarn was produced by metasomatism related to magmatichydrothermal fluid and the alteration in the contact was the most intensive (Einaudi et al., 1981; Meinert et al., 2005). In the Zhibula ore deposit, the granodiorites exhibit typical endoskarn zonations, from the contact to fresh granodiorite, correspondingly varying from a dark brownish-red garnet-bearing granodiorite, to a green diopside-bearing granodiorite, to a weakly chlorite-altered granodiorite and to a fresh granodiorite (Fig. 3, at the depth of 420.5– 432 m in drillhole ZK2014; Fig. 4e–h). Moreover, skarn Cu mineralization generally occurs in proximal rather than distal intrusions, such as Pb and Zn mineralization (Meinert et al., 2005; Sillitoe, 2010). These are the principal lines of geological evidence indicating that the intrusion is closely associated with skarnization and mineralization. Summarizing, we infer that the granodiorite is related to the Zhibula Cu skarn deposit. In recent years, the relationships between skarn mineralization and other types of deposits have received increased attention, although the porphyry–skarn metallogenic system is undoubtedly the most significant and most studied. Generally, this appears in different types of orebodies within one deposit or in different ore deposits in the same ore field or ore district (e.g., Lamasu Cu, Zhu et al., 2012; Hongniu–Hongshan Cu, Peng et al., 2014; Huangshaping W–Sn, Li et al., 2014; and Luming–Xulaojiugou Mo–Pb–Zn, Hu

et al., 2014). Although the ore-forming intrusion of the Qulong porphyry Cu–Mo deposit is monzogranite porphyry (Zheng et al., 2004; Yang, 2008), its orebodies are distributed in both monzogranite and granodiorite (Yang et al., 2009; Fig. 2a), both of which were observed in the drill cores from the Zhibula ore deposit. As discussed previously, the Zhibula Cu skarn deposit is located approximately 2 km south of the Qulong porphyry Cu–Mo deposit (Fig. 2a). Their similar zircon U–Pb ages (16.7–17.7 Ma, Rui et al., 2003; Yang, 2008; Zheng et al., 2013), molybdenite Re–Os isochron ages (16.4–16.9 Ma, Meng et al., 2003; Li et al., 2005), geochemistry, and zircon Hf–O isotopic components (Yang, 2008; Hu et al., 2015) indicate that they have a common magmatic source and similar evolutionary processes. We therefore deduce that the intrusions in the Zhibula and Qulong mine were probably deeply interconnected (Fig. 2a; Li et al., 2005; Xu et al., 2014). It is similar to the model proposed by Sillitoe (2010) that the porphyry–skarn metallogenic systems always follow zonings that are center to the distal of intrusions are porphyry Cu ± Au ± Mo deposits, Cu– Au skarns, and less commonly Zn–Pb and/or Au skarns. On the basis of their similar geology, petrogenesis, and geochronology, the Zhibula Cu skarn deposit and the Qulong Cu–Mo deposit form a significant porphyry–skarn ore system.

7. Conclusions (1) The Zhibula granodiorite associated with Cu skarn mineralization exhibits a similar crystallization age of approximately 17 Ma to monzogranite. They exhibit different fractional crystallization from granodiorite to monzogranite. (2) The Zhibula granodiorite and monzogranite formed form the magma with high Sr/Y ratio signature, which was derived from the partial melting of the thickened juvenile lower crust and may have been metasomatized by the slab melts during subduction of the Neo-Tethyan oceanic crust. The convective removal of the thickened lithosphere of southern Tibet is the primary geodynamic process. (3) The Zhibula Cu skarn deposit constitutes a typical porphyry– skarn metallogenic system with the Qulong porphyry Cu– Mo deposit.

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