Carboniferous magmatism and mineralization in the area of the Fuxing ...

Gondwana Research 34 (2016) 109–128

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Carboniferous magmatism and mineralization in the area of the Fuxing Cu deposit, Eastern Tianshan, China: Evidence from zircon U–Pb ages, petrogeochemistry, and Sr–Nd–Hf–O isotopic compositions Yin-Hong Wang a,⁎, Fang-Fang Zhang a, Jia-Jun Liu a, Chao-Yang Que b a b

State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, 100083, China Zijin Mining Group Northwest Corporation, Urumqi, Xinjiang, 830000, China

a r t i c l e

i n f o

Article history: Received 4 January 2016 Received in revised form 5 March 2016 Accepted 14 March 2016 Available online 14 April 2016 Handling Editor: F. Pirajno Keywords: SIMS zircon U–Pb dating Geochemistry Sr–Nd–Hf–O isotopes Fuxing Cu deposit Eastern Tianshan

a b s t r a c t The newly discovered Fuxing porphyry Cu deposit is located in the Dananhu–Tousuquan arc, adjacent to the Tuwu–Yandong Cu deposits of Eastern Tianshan, in the southern Central Asian Orogenic Belt. The Fuxing deposit is hosted by volcanic rocks (basalt and dacite) in the Early Carboniferous Qi'eshan Group and Carboniferous felsic intrusions (plagiogranite porphyry, monzogranite, and quartz diorite). New SIMS zircon U–Pb dating indicates that the plagiogranite porphyry and monzogranite emplaced at 332.1 ± 2.2 Ma and 328.4 ± 3.4 Ma, respectively. The basalts are characterized by low SiO2 contents (47.47–54.90 wt.%), a lack of Eu anomalies, strong depletion of Na, Ta, and Ti elements but positive Sr, U, and Pb anomalies, high Y (20.8–28.2 ppm) and HREE concentrations (Yb = 2.23–3.06 ppm), and relatively low (La/Yb)N (2.20–3.92) values; the dacite samples have high SiO2 contents (66.13–76.93 wt.%), clearly negative Eu anomalies, high Mg# values (36–51), and high Y (41.8–54.9 ppm) and Yb (5.76–8.98 ppm) concentrations. The basalts and dacites exhibit similar signatures as normal arc rocks, and were considered to be derived from partial melting of mantle-wedge peridotite that was previously metasomatized by slab melts. In contrast, the plagiogranite porphyry, monzogranite, and quartz diorite show the same geochemical affinity with modern adakites, which are characterized by high SiO2 contents (67.55–79.00 wt.%), minor negative to positive Eu anomalies, strong depletion of heavy rare earth elements (Yb = 0.17–1.19 ppm) and Y (1.86–10.1 ppm), positive K, Rb, Sr, and Ba but negative Nb, Ta, Th, and Ti anomalies, and high (La/Yb)N ratios and Mg# values. Moreover, these adakitic felsic intrusions display relatively high positive zircon εHf(t) values (+ 11.4 to + 18.3), low 87Sr/86Sr (0.706080–0.711239), high 143Nd/144Nd (0.512692–0.512922) ratios, and consistent zircon δ18O values (4.41‰–5.48‰), suggesting that their parental magma were most likely derived from partial melting of the subducted oceanic crust followed by mantle peridotite interaction. Based on the whole-rock geochemical and Sr–Nd–Hf–O isotopic data, as well as detailed petrographic analyses, we further suggest that the Fuxing igneous rocks and associated porphyry Cu mineralization were generated by the northward subduction of the paleo-Tianshan oceanic plate beneath the Dananhu– Tousuquan island arc during the Early Carboniferous. © 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction The Central Asian Orogenic Belt (CAOB), a world-renowned Phanerozoic orogenic collage, is sandwiched between the European and Siberian cratons in the north and the Tarim and North China cratons in the south (Şengör et al., 1993; Windley et al., 2007; Xiao et al., 2010; Pirajno, 2013; Xiao et al., 2013; W.J. Xiao et al., 2015), and formed by multiple accretion and arc–continent collision events that occurred from the Early Neoproterozoic to the Permian (Goldfarb et al., 2001; ⁎ Corresponding author at: State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, 29 Xue-Yuan Road, Haidian District, Beijing 100083, China. Tel.: +86 10 82322346 (office). E-mail address: [email protected] (Y.-H. Wang).

Liu et al., 2007; S. Li et al., 2013; Goldfarb et al., 2014; Deng et al., 2015a, 2015b). As an important component of the CAOB, the Eastern Tianshan belt is one of China's important Cu metallogenic belts (Fig. 1a,b; Qin et al., 2011; Zhai et al., 2011; Huang et al., 2013) and hosts many Cu deposits and occurrences (Pirajno et al., 1997; Charvet et al., 2007; Pirajno et al., 2011; H.Y. Chen et al. 2012; Chen et al., 2014; and references therein). In the Eastern Tianshan belt, the porphyry Cu deposits (e.g., Fuxing, Tuwu, Yandong, Linglong, Chihu, and Sanchakou) are economically important and are primarily emplaced along the Dananhu–Tousuquan arc belt (Table 1; Rui et al., 2002; Zhang et al., 2008; Wang et al., 2014, 2015a, 2015b). These porphyry Cu deposits are thought to be spatially associated with local intrusions (Chen et al., 2005; Zhang et al., 2006; B. Xiao et al., 2015), and the geneses, ages, and geodynamic settings of intrusive rocks and their

http://dx.doi.org/10.1016/j.gr.2016.03.007 1342-937X/© 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

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Y.-H. Wang et al. / Gondwana Research 34 (2016) 109–128

18 0

N

40

a

b

E

79 °

Altay

Tianshan

W Junggar

S

°E

Ce nt

200km

Fault/Suture

ra l

B

Fig. b

E Junggar

elt

Tu-Ha Basin

Orog entic

Tarim Craton

800km

c

90 °

Tianshan

W

Korla Aksu Tarim Basin

North China Craton

91°

N

44 °

Urumqi

42 ° 0

46 °

Karamay

44 ° ian

48 °

Basin

Junggar Basin As

95 °

Altay E

46 ° 0

91 °

Platform

W

East European Craton Siberian Craton

87 °

83 ° N

48 °

°E

N

120

6 0°

8 0°

92°

93°

Fig. c Beishan

94°

95°

Bogeda-Haerlike Belt

Hami

E

Hami

Tulaergen

Tu-Ha Basin

S Yuhai Sanchakou Xiaorequanzi

Dananhu-Tousuquan Arc Belt

Fuxing Yandong

1

Shiyingtan

42°

Hongshi Kanggur Hongyuntan

Chihu

Kanggur-Huangshan 3 Ductile Shear Belt

Bailingshan

2

Jueluotage B

TuwuLinglong

Da caotan K a nggur Yamansu Aqikuduke

Fault Fault Fault Fault

Xingxingxia

Yamansu

4 1 2 3 4

elt

Donggebi

t ansu Arc Bel Aqishan-Yam

Xifengshan

Baishan

Xiangshan Huangshandong Tudun Huangshan

Weiya

Tianhu 0

Central Tianshan Terrane

20km

Cenozoic-Mesozoic sediments

Late Paleozoic island arc volcanics

Late Paleozoic volcanics-sediments

Precambrian metamorphicrocks

Intrusions

Shear belt

Cu deposit

Cu-Ni sulfide deposit

Fe deposit

Mo deposit

Au deposit

Fault

Fig. 1. (a) Location of the study area in the Central Asia Orogenic Belt (modified from Jahn et al., 2000); (b) sketch map showing the geological units of the Tianshan Belt (modified from Y.J. Chen et al., 2012); (c) simplified geological map of the Eastern Tianshan Belt (modified from Huang et al., 2013).

genetic relationships with Cu mineralization remain poorly elucidated (Han et al., 2006; Y.J. Chen et al., 2012; Wang and Zhang, 2016; Wang et al., 2016c).

The newly found Fuxing porphyry Cu deposit, located in the central part of the Eastern Tianshan belt, Northwest China (Fig. 1c), is another breakthrough for porphyry Cu prospecting. The size of the deposit was

Table 1 Summary of isotopic ages for the major Carboniferous porphyry Cu deposits and related rocks in Eastern Tianshan. No.

Locations

Dating samples

Dating methods

Ages (Ma)

References

1

Fuxing

2

Yandong

3

Tuwu

4

Tuwu–Yandong

5 6

Yanxi Chihu

7

East Sanchakou

Plagiogranite porphyry Monzogranite Diorite Diorite porphyry Plagiogranite porphyry Plagiogranite porphyry Plagiogranite porphyry Plagiogranite porphyry Quartz albite porphyry Quartz porphyry Molybdenite Plagiogranite porphyry Plagiogranite porphyry Plagiogranite porphyry Molybdenite Plagiogranite porphyry Molybdenite Molybdenite Plagiogranite porphyry Granodiorite Porphyritic granodiorite Granite Granodiorite

SIMS zircon U–Pb SIMS zircon U–Pb LA–ICP–MS zircon U–Pb SIMS zircon U–Pb SHRIMP zircon U–Pb SIMS zircon U–Pb SHRIMP zircon U–Pb LA–ICP–MS zircon U–Pb LA–ICP–MS zircon U–Pb LA–ICP–MS zircon U–Pb Re–Os isochron SHRIMP zircon U–Pb SIMS zircon U–Pb SHRIMP zircon U–Pb Re–Os model SHRIMP zircon U–Pb Re–Os isochron Re–Os model SHRIMP zircon U–Pb SIMS zircon U–Pb SIMS zircon U–Pb LA–ICP–MS zircon U–Pb LA–ICP–MS zircon U–Pb

332.1 ± 2.2 328.4 ± 3.4 348.3 ± 6 340 ± 3 333 ± 4 332.2 ± 2.3 335 ± 3.7 339.3 ± 2.2 323.6 ± 2.5 324.1 ± 2.3 343 334 ± 3 332.8 ± 2.5 332.3 ± 5.9 325–321 334 ± 2 322.7 ± 2.3 326.2 ± 4.5 322 ± 10 320.2 ± 2.4 314.5 ± 2.5 323.3 ± 2.4 321.2 ± 2.5

This study This study B. Xiao et al. (2015) Shen et al. (2014a) Chen et al. (2005) Shen et al. (2014a) Wang et al. (2015b) B. Xiao et al. (2015) B. Xiao et al. (2015) B. Xiao et al. (2015) Zhang et al. (2004) Chen et al. (2005) Shen et al. (2014b) Wang et al. (2015a) Rui et al. (2002) Liu et al. (2003) Rui et al. (2002) Zhang et al. (2010) H. Wu et al. (2006) Zhang et al. (2016b) Zhang et al. (2016b) C. Wang et al. (2015) C. Wang et al. (2015)


estimated in recent years by the Zijin Mining Group Northwest Co., Ltd., and the detailed geological mapping and lab-based research undertaken during this period suggested that this was a porphyry deposit that was spatially and potentially genetically associated with plagiogranite porphyry and monzogranite. However, only minimal basic geological research has been undertaken in this area, meaning that the oreforming processes, the metallogenic setting, and the age of the oreforming related intrusions are all unclear. Here, we present new SIMS zircon U–Pb dating, whole-rock geochemical data, and in situ zircon Hf–O isotopic compositions to constrain the timing of magmatic activities and the magma source of the intrusions in the Fuxing deposit and to further explore their genetic relationship and tectonic setting. This study provides new information on the prospectivity of the wider Eastern Tianshan orogenic belt and elsewhere in the CAOB for porphyry Cu mineralization. 2. Geological setting 2.1. Regional geology The Eastern Tianshan orogenic belt, situated along the southern margin of the CAOB, is a typical Palaeozoic island arc system characterized by a complicated tectonic history and diverse styles of mineral system (Ma et al., 1993; Xiao et al., 2004; Qin et al., 2011; Pirajno,

111

2013; F.F. Zhang et al., 2015). It can be divided into the Bogeda–Haerlike belt, the Jueluotage belt and the Central Tianshan Terrane from the north to the south (Fig. 1c; Pirajno et al., 2011; Y.J. Chen et al., 2012; Xiao et al., 2013), with a series of approximately EW-trending regional-scale faults defining the boundaries, including the Dacaotan, Kanggur, Yamansu, and Aqikuduke faults, and some small-scale faults (Fig. 1c; Mao et al., 2005; H.Y. Chen et al., 2012; Huang et al., 2013). The Bogeda–Haerlike belt comprises well-developed Ordovician– Carboniferous volcaniclastic rocks, tuffs, granites and mafic–ultramafic intrusions, and contains only few porphyry Cu and Au prospects (Fig. 2a; Xiao et al., 2004; Gao et al., 2015; Tang et al., 2015). The Jueluotage belt is characterized by Devonian–Carboniferous calcalkaline andesites, basalts, rhyolites, tuffs and volcaniclastic rocks interbedded with fine-grained clastic sediments and carbonates that have undergone lower greenschist facies metamorphism, together with granitic intrusions and mafic–ultramafic complexes (Gu et al., 1999; Xiao et al., 2004; Q.G. Mao et al., 2014). The Middle Tianshan Terrane is comprised of calc-alkaline basaltic to andesitic volcanic and volcaniclastic rocks, minor I-type granites and granodiorites, and Precambrian basement rocks, and hosts some hydrothermal magnetite deposits (Shen et al., 2014a; B. Xiao et al., 2015). The Jueluotage belt may be subdivided into the Dananhu– Tousuquan arc belt, the Kanggur–Huangshan ductile shear zone, and the Aqishan–Yamansu arc belt, which are separated by the Kanggur

Fig. 2. Simplified geological map (a) and geological section (b) of the No. 28 exploration line in the Fuxing Cu deposit (modified from ZMGNC, 2015).

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and Yamansu faults (Fig. 1c). The Dananhu–Tousuquan arc belt is situated north of the Kanggur fault, and is mainly composed of Devonian volcanic and clastic sedimentary rocks of the Dananhu Formation, Carboniferous turbidites of the Gandun Formation, Carboniferous basaltic to andesitic volcanic rocks and sedimentary rocks of the Qi'eshan Group, Permian calc-alkaline volcanic, pyroclastic and clastic rocks, Jurassic sandstone, and Cenozoic cover (Zhou et al., 2010; Shen et al., 2014a, 2014b; B. Xiao et al., 2015). The extensive Carboniferous intrusions including tonalite, granodiorite, diorite porphyry, and granodiorite porphyry with adakitic affinity were emplaced in the arc belt (Shen et al., 2014b; Wang et al., 2015a; B. Xiao et al., 2015), temporally and genetically associated with several important porphyry Cu deposits of different sizes, such as the Tuwu, Yandong, Linglong, Chihu, and Fuxing deposits (Fig. 1c; Mao et al., 2005; Zhang et al., 2008; Wang et al., 2015a, 2015b). These granitic intrusions and porphyry deposits were possibly formed during an accretionary event during the Carboniferous (Xiao et al., 2013; Wang et al., 2015a, 2015b).

The middle Kanggur–Huangshan ductile shear zone lies between the Kanggur and Yamansu faults, and mainly contains the Early Carboniferous volcano-sedimentary rocks in the south, and mélanges with broken formations in the north (Xiao et al., 2013; Wang et al., 2015d). It is intruded by abundant Late Paleozoic mafic–ultramafic complexes, quartz porphyry, monzogranite, syenogranite, and granite porphyry; some of the intrusions have Late Carboniferous to Early Permian isotopic ages (Mao et al., 2005; Han et al., 2006; Wang et al., 2015c, 2016b). The Kanggur–Huangshan ductile shear zone contains a series of epithermal Au, orogenic-type Au and magmatic Cu–Ni sulfide deposits formed at ca. 290–250 Ma (Mao et al., 2005; Pirajno et al., 2011). These deposits are considered to be associated with Permian post-collision tectonism (Mao et al., 2005; Qin et al., 2011). The Aqishan–Yamansu arc belt, located between the Yamansu and Aqikuduke faults (Fig. 1c), is composed of Early Carboniferous basalt, andesite, dacite, and tuff of the Yamansu Formation and Late Carboniferous rhyolite of the Tugutubulake Formation (H.Y. Chen et al., 2012; Xiao

a

Basalt

b

Basalt

c

Dacite

d

Quartz diorite

e

Monzogranite

f

Plagiogranite porphyry

g

h

i

Aug

Pl

Pl Aug

Q

Pl 400 µ m

j

Q

1mm

k

400 µ m

l Bt

Q

Bt

Q

Pl

Q

Pl Pl

Hb

Kf Kf 400 µ m

400 µ m

400 µ m

Fig. 3. Photographs and photomicrographs showing the mineralogy and textural features of volcanic rocks and intrusions from the Fuxing Cu deposits. (a, b, g, h) basalt; (c, i) dacite; (d, j) quartz diorite; (e, k) monzogranite; (f, l) plagiogranite porphyry. Abbreviations: Pl, plagioclase; Aug., augite; Q, quartz; Hb, hornblende; Bt, biotite; Kf, K-feldspar.


et al., 2013; Hou et al., 2014). Numerous arc-related Late Paleozoic granitoids have intruded in this belt (C.Z. Wu et al., 2006; Zhou et al., 2010; W.F. Zhang et al., 2015), including the Weiquan granodiorite (297 ± 3 Ma), Bailingshan granodiorite (317.7 ± 3.7 Ma), Hongyuntan granodiorite (328.5 ± 9.3 Ma), and Xifengshan granite (349 ± 3.4 Ma). The Aqishan–Yamansu arc belt contains several Fe (−Cu) and Cu–Ag–Pb– Zn skarn deposits, including the Yamansu, Hongyuntan, Bailingshan, Chilongfeng, and Weiquan deposits. These deposits were possibly formed during the emplacement of granitic intrusions with associated hydrothermal activity (Mao et al., 2005; J.B. Wang et al., 2006; Pirajno et al., 2011). 2.2. Deposit geology The Fuxing porphyry Cu deposit is located 125 km southwest of Hami City, Xinjiang, at geographic coordinates E92°18′57″ to E92°21′21″ and N42°04′41″ to N42°05′18″(Fig. 1c). Ore bodies are mainly hosted in plagiogranite porphyry and monzogranite (Fig. 2b) that intruded intermediate to mafic volcanic rocks of the Early Carboniferous Qi'eshan Group (Liu et al., 2003; Han et al., 2006; Zhang et al., 2006). The Qi'eshan Group is composed of andesite and basalt (Fig. 3a, b, g, h) intercalated with tuff (Unit 1), andesitic or basaltic breccia (Unit 2), sandstone and siltstone with tuff and basalt interlayers (Unit 3), and gray to green conglomerate (Unit 4) from the base upwards (Han et al., 2006; Shen et al., 2014b; Gao et al., 2015). The overlying rocks of the Jurassic Xishanyao Formation consist chiefly of sandstone, siltite, mudstone, and conglomerate, and they form an angular unconformity with the strata of the Qi'eshan Group (Fig. 2a). The Carboniferous sequences in Fuxing are mainly intruded by quartz diorite (Fig. 3d, j), plagiogranite porphyry (Fig. 3f, l), and monzogranite (Fig. 3e, k). The intrusions occurred as stocks ca. 1000 m deep below the surface (Fig. 2b) and the upper part appears as dykes and apophysis. The plagiogranite porphyry and monzogranite

a

113

are associated with the porphyry-style Cu mineralization and associated hydrothermal alterations. The plagiogranite porphyry is characterized by porphyritic texture and massive structure, and mainly consists of plagioclase (45%), quartz (35%), biotite (10%), and K-feldspar (5%), with accessory minerals that include zircon and phosphates (Fig. 3f, l). Plagioclase is characterized by idiomorphic or hypidiomorphic tabular texture, showing superficial weak sericite alteration (Fig. 3l). Biotite is mainly yellow-brown in color, with a hypidiomorphic–xenomorphic flaky or fragmental texture. The monzogranite, as a relatively late event, is flesh pink and typically has a medium-coarse grain texture (Fig. 3e, k). It mainly comprises plagioclase (35%), K-feldspar (30%), quartz (20%), and biotite (5%). The accessory minerals of monzogranite are dominated by apatite, zircon, titanite, and magnetite. A series of secondary faults are present in the Fuxing area and are divided into NW–SE, NE–SW, and E–W striking groups (Fig. 2a). The poorly exposed orebodies in the Fuxing deposit have a tabular surface morphology and are elongated in the E–W direction (Fig. 2a). Using a cutoff grade of 0.5% Cu for oxidized ore and of 0.2% Cu for sulfide ore to outline the orebodies, the Fuxing main orebodies are about 555–2315 m long, 2–107 m thick, and dip toward the south at an angle of 61°–89°. In the dipping direction, the explored orebodies extend down-dip for more than 600 m (Fig. 2b), and the Cu mineralization occurs predominantly in plagiogranite porphyry and monzogranite (ZMGNC, 2015). Sulfide mineralization in the Fuxing deposit occurs as both disseminations and veinlets. Field and petrographic observation indicate that ore minerals are dominated by chalcopyrite and pyrite (Fig. 4a–c) with minor molybdenite, bornite, sphalerite (Fig. 4d), and chalcocite. Locally, pyrite grains are replaced by magnetite and hematite. Gangue minerals are composed of quartz, sericite, and chlorite with minor biotite, epidote, and calcite. The Fuxing deposit shows zonation of alteration and mineralization, as is the case for the majority of porphyry deposits. The alterations

b Py+Cp veinlet Py

Py Py veinlet

1.5cm

c

1cm

d Py Cp

Cp

Sp

Py 200 µ m

200 µ m

Fig. 4. Photographs and photomicrographs showing ore fabrics and mineral assemblages of the Fuxing Cu deposit. (a) Pyrite and chalcopyrite veinlets; (b) disseminated pyrite in the alerted rock; (c) subhedral chalcopyrite and pyrite assemblages, reflected right; (d) subhedral chalcopyrite, pyrite and sphalerite assemblages, reflected right. Abbreviations: Cp, chalcopyrite; Py, pyrite; Sp, sphalerite.

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Table 2 Whole-rock geochemical data of the volcanic rocks and intrusions from the Fuxing area (major elements: wt.%; trace elements: ppm). Sample No.

FX-4201-2

FX-4201-1

FX-4201-3

FX-4201-4

FX-4201-5

15FX-87

FX-2804-11

FX-2804-10

15FX-93

Rock type

Basalt

Basalt

Basalt

Basalt

Basalt

Basalt

Dacite

Dacite

Dacite

SiO2 TiO2 Al2O3 TFe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total Na2O + K2O Mg# Fe2O3/FeO FeO Li Be Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U YbN (La/Yb)N Eu/Eu* Sr/Y La/Sm

47.47 1.10 18.01 9.35 0.28 6.90 7.56 2.71 0.34 0.17 5.70 99.59 3.05 60 0.35 6.44 18.6 0.665 26.4 206 209 38.4 117 74.9 86.2 16.5 7.76 753 20.8 33.8 2.51 0.828 71.7 6.83 15.5 2.38 12.1 3.22 1.12 3.57 0.68 4.19 0.802 2.45 0.363 2.23 0.335 1.2 0.178 8.85 0.653 0.348 13.12 2.20 1.01 36.20 2.12

49.58 1.13 17.08 9.55 0.26 7.45 7.92 2.44 0.42 0.37 3.40 99.60 2.86 61 7.91 1.06 15.3 0.926 27.8 228 345 39.1 149 381 79 18.2 9.64 674 27.6 60.1 4.94 0.869 132 14.8 33.1 4.71 22.4 5.28 1.59 5.32 0.961 5.62 1.08 3.23 0.48 2.88 0.424 2.04 0.328 1.84 0.831 0.709 16.94 3.69 0.92 24.42 2.80

51.22 1.17 16.51 8.95 0.22 7.11 8.17 2.29 0.55 0.37 3.07 99.63 2.84 61 0.56 5.39 12.8 1.02 27.6 215 270 37.2 137 997 108 17.8 15 614 27.1 68.4 4.92 1.33 147 13.9 31.5 4.5 21.6 4.96 1.5 5.11 0.935 5.54 1.04 3.16 0.461 2.86 0.417 2.02 0.325 2.41 1.08 1.22 16.82 3.49 0.91 22.66 2.80

51.61 1.20 16.83 9.24 0.20 6.70 7.80 2.27 0.58 0.39 2.88 99.70 2.85 59 0.55 5.61 12.3 1 28.3 227 293 38.5 143 871 86.2 18.1 15.4 593 28.2 102 5.19 1.26 157 15.1 33.5 4.83 22.7 5.36 1.6 5.32 0.963 5.58 1.11 3.33 0.508 3.06 0.444 2.89 0.328 2.23 1.04 4.07 18.00 3.54 0.92 21.03 2.82

54.90 1.06 15.70 8.41 0.22 6.83 5.85 2.64 0.51 0.35 3.28 99.74 3.15 62 0.30 6.02 16 0.998 27.4 204 253 34.6 125 699 119 17 13.4 574 25.9 55.2 4.55 1.17 148 14 31.4 4.45 21.5 4.94 1.49 4.96 0.881 5.38 1.01 3.08 0.433 2.6 0.385 1.79 0.311 1.79 0.798 0.581 15.29 3.86 0.92 22.16 2.83

52.53 1.13 16.80 8.94 0.20 5.66 7.44 2.62 0.41 0.32 3.36 99.41 3.03 56 0.76 4.81 8.7 1.62 21.5 195 108 30.7 82.6 399 81.3 16.6 8.85 585 26.3 182 6.3 0.961 123 16.4 37.5 4.99 23.1 5.64 1.34 4.36 0.944 4.75 0.899 3.03 0.348 3 0.486 4.56 0.298 2.32 2.47 0.87 17.65 3.92 0.83 22.24 2.91

66.13 0.51 16.89 1.44 0.12 0.76 3.36 1.23 4.01 0.06 5.08 99.59 5.24 51 0.61 0.84 11 2.48 14.9 15.2 1.73 0.838 2.87 50 82.7 25.8 89.3 158 54.9 668 15.5 4.04 716 40.9 91.7 12.6 54.8 12.1 2.15 10.1 1.77 10.3 2.12 7.42 1.32 8.98 1.48 18.7 1.25 7.6 10.3 3.2 52.82 3.27 0.59 2.88 3.38

67.91 0.37 13.16 2.23 0.18 1.02 4.77 2.13 2.48 0.05 5.48 99.78 4.61 48 1.20 0.97 8.79 2.04 10.4 10.5 2.4 2.05 3.53 63.2 106 18.9 55.1 176 43.7 440 11.7 3.7 382 31.6 69.5 9.62 41.7 9.05 1.41 7.65 1.38 8.09 1.69 5.78 0.966 6.55 1.06 12.1 0.933 13 7.37 2.88 38.53 3.46 0.52 4.03 3.49

76.93 0.40 13.89 0.75 0.00 0.21 0.59 0.95 3.35 0.02 2.32 99.42 4.30 36 0.65 0.43 18.4 1.44 11.4 15.8 0.183 0.381 0.686 24.7 9.27 16.1 60.1 211 41.8 435 13.9 1.77 790 12.7 26.7 3.86 17.6 4.09 0.7 4.67 1.15 6.05 1.28 4.75 0.833 5.76 0.91 13.7 0.679 3.85 7.71 2.1 33.88 1.58 0.49 5.05 3.11

associated with Cu mineralization include potassic, chlorite–sericite, and phyllic alterations in plagiogranite porphyries. The lack of potassic alteration at the surface within the deposit indicates that the area has undergone minimal erosion, and that the deposit is largely preserved. On the basis of the mineralogy and micro-textures, Cu mineralization of the Fuxing deposit can be divided into four stages: Stage I is characterized by vein-like and disseminated mineralization associated with potassic alteration with an assemblage of quartz + pyrite; Stage II is represented by extensive veins containing quartz + pyrite + chalcopyrite associated with chlorite–sericite–phyllic alteration; Stage III is characterized by an assemblage of quartz + chalcopyrite + molybdenite occurring as veins or disseminations in hosting rocks; Stage IV consists

of carbonates (calcite) and minor sulfides. Stages III and IV are commonly associated with phyllic alteration (ZMGNC, 2015). 3. Samples and analytical methods The samples for this study were collected from the Fuxing Cu deposit, especially around the orebody, the sampling locations shown in Fig. 2a and b. They include plagiogranite porphyry, monzogranite, quartz diorite, dacite and basalt. The nineteen least altered samples were selected for major and rare element analysis. Two samples were chosen for SIMS zircon U–Pb isotopic dating and situ Hf–O isotopic analysis. Seven samples were selected for Sr–Nd isotopic analysis.


115

Table 2 Whole-rock geochemical data of the volcanic rocks and intrusions from the Fuxing area (major elements: wt.%; trace elements: ppm). FX-4201-24

15FX-90

15FX-91

15FX-92

15FX-94

FX-2002-2

FX-2002-3

15FX-89

FX-6

FX-7

Quartz diorite







Monzogranite

Monzogranite

Monzogranite

67.55 0.31 17.01 2.88 0.06 1.49 1.56 4.58 1.74 0.13 2.43 99.73 6.32 51 0.41 1.91 17.7 1.38 4.9 39.3 6.75 7.61 5.97 479 74 15 45.4 537 10.1 48.9 3.76 7.29 452 11 22.4 2.76 11.4 2.33 0.693 1.94 0.347 1.99 0.37 1.14 0.195 1.19 0.172 1.64 0.316 7.7 2.53 0.734 7.00 6.63 1.00 53.17 4.72

73.64 0.22 14.60 1.05 0.01 0.48 0.81 0.52 4.14 0.04 3.96 99.47 4.66 48 0.49 0.66 9.24 1.32 3.16 38 2.49 0.52 1.01 22.6 14.5 13.9 65.8 191 2.4 23.9 3.23 1.52 669 3.28 5.91 0.695 2.6 0.43 0.174 0.344 0.064 0.363 0.055 0.269 0.017 0.169 0.081 1.09 0.187 4.19 0.765 0.541 0.99 13.92 1.38 79.58 7.63

74.22 0.24 14.71 0.83 0.01 0.57 0.72 0.47 4.23 0.04 3.42 99.47 4.70 58 0.44 0.54 8.56 0.469 3.1 37.1 2.29 0.468 0.998 37.3 11.9 12.6 62.9 123 3.41 21.6 3.69 1.51 796 6.92 12.8 1.41 5.36 0.786 0.238 0.464 0.064 0.475 0.108 0.403 0.042 0.396 0.046 0.929 0.2 1.56 0.948 0.593 2.33 12.53 1.20 36.07 8.80

70.69 0.50 16.32 0.94 0.01 0.53 1.20 0.40 4.74 0.04 4.06 99.44 5.14 53 0.02 0.84 9.3 0.659 10.3 74.8 34.2 0.507 1.54 71.4 14.3 14.8 67 118 3.72 44.8 5.68 1.34 1054 7.16 14 1.67 6.91 1.22 0.29 0.899 0.134 0.594 0.116 0.412 0.068 0.736 0.095 1.52 0.286 1.96 1.1 0.939 4.33 6.98 0.85 31.72 5.87

72.39 0.24 15.17 0.91 0.01 0.52 1.58 0.38 4.37 0.07 3.76 99.40 4.75 53 1.51 0.35 9.56 0.852 3.41 50 3.36 0.465 1.35 61.1 15.3 15.3 67 257 2.55 24.4 2.62 1.79 779 8.56 15.4 1.86 7.05 1.17 0.374 0.799 0.127 0.459 0.088 0.267 0.024 0.317 0.049 0.945 0.202 5.47 1.06 0.717 1.86 19.37 1.18 100.78 7.32

70.47 0.16 12.30 1.11 0.09 0.96 5.07 4.25 1.05 0.08 4.03 99.57 5.30 63 0.78 0.59 8.62 1.23 2.92 23.1 3.28 2.95 11.5 325 41 8.98 26.2 392 7.71 21.4 1.99 2.19 159 8.02 15.5 2.03 9.07 1.72 0.696 1.55 0.252 1.33 0.241 0.747 0.106 0.671 0.108 0.929 0.237 9.79 1.38 0.708 3.95 8.57 1.30 50.84 4.66

74.89 0.16 10.26 1.86 0.10 0.55 4.88 0.38 2.36 0.08 4.11 99.64 2.74 37 2.05 0.59 16.5 1.03 1.94 16.4 2.61 4.24 5.97 2641 595 8.58 43 219 5.87 21.2 1.75 2.36 418 5.14 10.5 1.33 5.87 1.19 0.513 1.08 0.191 1.06 0.21 0.675 0.101 0.717 0.11 0.722 0.101 10.6 0.723 1.57 4.22 5.14 1.38 37.31 4.32

77.41 0.21 12.04 2.21 0.01 0.36 0.59 0.89 3.30 0.06 2.57 99.66 4.19 25 7.09 0.27 7.58 0.702 2.55 35.5 2.89 0.23 1.1 37 10 11.8 46.1 208 1.86 20.9 1.75 0.75 687 6.19 11.9 1.44 5.76 0.909 0.329 0.619 0.069 0.343 0.062 0.209 0.036 0.253 0.041 0.75 0.156 3 1.02 0.465 1.49 17.55 1.34 111.83 6.81

74.83 0.25 13.32 1.41 0.01 0.35 2.01 0.43 3.76 0.14 3.16 99.66 4.19 33 3.45 0.31 8.42 0.476 3.7 37.3 2.99 0.345 2.02 27.5 10.8 12.7 45.9 99.3 2.51 22.4 1.85 0.662 722 6.9 13.8 1.75 7.23 1.25 0.455 0.823 0.112 0.498 0.089 0.29 0.051 0.342 0.059 0.747 0.154 1.92 0.759 0.633 2.01 14.47 1.37 39.56 5.52

79.00 0.22 11.38 0.98 0.02 0.39 1.13 0.36 3.14 0.03 3.11 99.76 3.50 44 1.98 0.32 6.44 0.573 2.71 30.4 3 0.346 1.35 32.5 10 11.1 46.3 60.6 2.61 30.9 1.99 0.657 692 5.27 10.7 1.33 5.56 0.951 0.359 0.727 0.107 0.491 0.098 0.323 0.052 0.368 0.063 1.09 0.151 1.55 0.707 0.812 2.16 10.27 1.32 23.22 5.54

Zircon grains concentrates were separated via a combination of heavy liquid and magnetic separation techniques, handpicking under a binocular microscope and subsequently together with zircon standard Penglai, Pleovice (Black et al., 2004), and Qinghu (Li et al., 2009a) mounted in epoxy resin and polished to remove the upper one half of the crystals at the Langfang Regional Geological Survey in Hebei Province, China. The mount was vacuum-coated with high-purity gold prior to SIMS analyses. Before the U–Pb dating, zircon internal texture obtained based on transmitted and reflected light micrographs as well as cathodoluminescence (CL) images. Measurements of U, Th and Pb were conducted using the Cameca IMS-1280 SIMS at the Institute of Geology and Geophysics, Chinese

Academy of Sciences. Detailed operating procedures were described by Li et al. (2009a). The analyzed ellipsoidal spot size is approximately 20 × 30 μm. A long-term uncertainty of 1.5% (1 RSD) for 206Pb/238U measurements of the standard zircons were propagated to the unknowns (Li et al., 2010a), despite the measured 206Pb/238U error in a specific session being generally around 1% (1 RSD) or less. Measured compositions were corrected for common Pb using non-radiogenic 204Pb method. Corrections are sufficiently small to be insensitive to the choice of common Pb composition, and an average of present-day crustal composition (Stacey and Kramers, 1975) is used for the common Pb is largely surface contamination introduced during sample preparation. Uncertainties on individual analysis in data tables are reported at the

116


1σ level, whereas calculated weighted mean ages are within 95% confidence limit. The data was processed using the ISOPLOT 3.0 program (Ludwig, 2003). Whole-rock composition analyses were performed at the test centre of the Beijing Research Institute of Uranium Geology, China. Major elements were analyzed using a Philips PW2404 XRF with testing precision greater than 1%. Both light and heavy rare elements and trace elements were conducted using a Finnigan MAT Element I ICP–MS, with RSD (10 min) b1% and RSD (4 h) b5%. For the detailed test methods, refer to Gao et al. (2002). In situ oxygen isotope analysis was done on zircon grains that were previously dated, using the same Cameca IMS-1280 SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences. The detailed analytical procedure and operating conditions are similar to those described by Li et al. (2009b, 2010b). Measured 18O/16O values were normalized to the Vienna Standard Mean Ocean Water composition (VSMOW, 18O/16O = 0.0020052) then collected for the instrumental mass fractionation (IMF) using the zircon Penglai and Qinghu standards with a δ18O value of 5.3 ± 0.1‰ (2σ) and 5.4 ± 0.2‰ (2σ), respectively (Li et al., 2010b; X.H. Li et al., 2013). The corrected δ18O values for samples are reported in the standard per mil notation with 2σ errors (Li et al., 2010b). In situ Hf isotope analysis was performed on the zircon grains using a Neptune MC–ICP–MS and New Wave UP 213 ultraviolet LA–MC–ICP– MS at the MLR Key Laboratory of Metallogeny and Assessment in Chinese Academy of Geological Sciences, Beijing. During the analyses, helium was used as the carrier gas. Based on zircon size, a stationary beam spot size was set at either 55 or 40 μm. International standard zircon samples GJ1 were used as reference. Details of instrumental conditions and test process are given in F.Y. Wu et al. (2006) and Hou et al. (2007). The weighted average of 176Hf/177Hf of the GJ1 zircon

10 0

samples was 0.282015 ± 31 (2 SD, n = 10), which is consistent with the values reported by Elhlou et al. (2006) and Hou et al. (2007). The initial 176Lu/177Hf ratios were calculated by using a decay constant of 1.867 × 10−11 year−1 for 176Lu (Soderlund et al., 2004) and the chondritic values of 176Lu/177Hf ratio of 0.0332 and 176Hf/177Hf ratio of 0.282772 (Blichert-Toft and Albarède, 1997) were adopted to calculate the εHf(t) values. The Hf isotope depleted mantle model age (TDM) was calculated with respect to the depleted mantle with present-day 176 Lu/177Hf = 0.28325 and 176Hf/177Hf = 0.0384 (Griffin et al., 2002). The crustal model age (TcDM) was calculated with respect to the average continental crustal with a 176Lu/177Hf ratio of 0.015 (Griffin et al., 2002). Sr and Nd isotopic analyses were performed at the test centre of the Beijing Research Institute of Uranium Geology, China. The Sr–Nd isotopic measurements were made by an ISOPROBE-T multi-collector thermal ionization mass spectrometer. The 87Sr/86Sr ratio of the NBS987 standard and 143Nd/144Nd ratio of the SHINESTU standard were 0.71025 ± 7 (2σ) and 0.512118 ± 3 (2σ), respectively. The measurement accuracy of the Rb/Sr ratio was better than 0.1%. The mass fractionation of Sr isotopes was corrected using 86Sr/88Sr = 0.1194; the accuracy of the Sm/Nd ratio was better than 0.1%, and the mass fractionation of Nd isotopes was corrected using 146Nd/144Nd = 0.7219. The 87Rb/86Sr and 147Sm/144Nd ratios were calculated using the Rb, Sr, Sm, and Nd abundances measured by ICP–MS. 4. Results 4.1. Whole-rock geochemistry Whole-rock major and trace element data of the representative igneous rocks in the Fuxing deposits are presented in Table 2. The Zr/TiO2 vs. Nb/Y (Winchester and Floyd, 1976) and TAS (Middlemost,

15

a

Basalt Dacite

Rhyolite

Trachyte

N a 2O + K 2O ( % )

Z r / Ti O 2

10

-1

Trachyandesite Andesite

10 -2

Phonolite

Alkalic series

12 Rhyodacite /Dacite

b

Basalt

Foid Foidlites monzosyenite Foid monzodiorite

9 6

-3

10 -2

6

10 -1

Nb/Y

10 0

90

c

80

Sho

3

sho

1 50

55

Tuwu-Yandong plagiogranite porphyry Tuwu-Yandong volcanic rocks Chihu porphyritic granodiorite Subducted oceanic crust-derived adakites

30

s e serie alkalin

60 65 SiO 2(%)

50 40

20 10

Low K tholeiite series

0 45

Deleminated lower crust-derived adakites

80

60 s serie line alka

A

l

Calc

d

70

st a

2

60 SiO 2(%)

C ru

calc h K Hig

Granodiorite

melts

#

4

s

50

70 Mantle

s erie

Mg

K 2O ( % )

5 c niti

Granite

0 40

10 1

Sub-alkalic series

Gabbro Gabbroic diorite Diorite

Subalkaline Basalt

10

Quartz syenite

Monzonite Monzo -diorite

Foid gabbro

3

Alkali-Basalt

Syenite

Plagiogranite porphyry Monzogranite Quartz diorite

70

75

80

0 45

FC

Thickened lower crust-derived adakites

Metabasaltic and eclogite experimental melts (1-4Gpa)

55

65 SiO 2(%)

75

85

Fig. 5. (a) Zr/TiO2 vs. Nb/Y diagram for the basalt and dacite (Winchester and Floyd, 1976); (b) total alkalis vs. silica diagram for the felsic intrusions (Middlemost, 1994); (c) K2O vs. SiO2 diagram (Rollinson, 1993); (d) Mg# vs. SiO2 diagram. Data for the Tuwu, Yandong, and Chihu igneous rocks from X.M. Li et al. (2006), Han et al. (2006), Zhang et al. (2006), Shen et al. (2014a, 2014b), B. Xiao et al. (2015), Wang et al. (2015a, 2015b), and F.F. Zhang et al. (2015).


samples fall into the calc-alkaline to high-K calc-alkaline field (Fig. 5c). On the SiO2 versus Mg# diagram, these granitoids mainly plot within the subducted slab-derived adakites field (Fig. 5d). All the Fuxing basalts display consistent REE patterns, characterized by a weak LREE enrichment and a flat HREE pattern ((La/Yb)N = 2.20–3.92), without clear Eu anomalies (Fig. 7a). On the primitive mantle-normalized trace element spider diagram, they show negative Nb, Ta, and Ti anomalies and positive Sr, U, and Pb anomalies (Fig. 7b). Chondrite-normalized REE patterns of dacites are characterized by a moderate LREE enrichment and a nearly flat HREE pattern ((La/Yb)N = 1.58–3.46), with pronounced negative Eu anomalies (Eu/Eu* = 0.49–0.59; Fig. 7c). They display negative Nb, Ta, P, and Ti anomalies, and enrichment in the large ion lithophile elements (LILE), such as K, Rb, and Ba (Fig. 7d). In comparison, chondritenormalized REE patterns of the plagiogranite porphyry, monzogranite, and quartz diorite are strongly fractionated ((La/Yb)N = 5.14–19.37), with LREE enrichment and HREE depletion (Fig. 7e). They also show positive or no Eu anomalies (Eu/Eu* = 1.00–1.38), except one outlier of 0.85 (Fig. 7e). On the primitive mantle-normalized trace element patterns, they show similar patterns, characterized by clear depletion in Nb, Ta, Th, and Ti, and enrichment in K, Rb, Sr, and Ba (Fig. 7f).

1994) diagrams were chosen for lithological classification. The basalt and dacite samples correspondingly plot in the basalt to sub-alkaline basalt, and rhyodacite/dacite fields, respectively (Fig. 5a), whereas the plagiogranite porphyry, monzogranite, and quartz diorite fall in the granodiorite and granite fields (Fig. 5b), which is consistent with petrography observation. The basalt has basaltic compositions with a restricted range of SiO2 (47.47–54.90 wt.%), relatively high contents of Al2O3 (15.70–18.01 wt.%), TFe2O3 (8.41–9.55 wt.%), MgO (5.66–7.45 wt.%), and low TiO2 (1.06–1.20 wt.%), and P2O5 (0.17–0.39 wt.%) (Table 2). The dacite shows different geochemical features, having SiO2, Al2O3, TFe2O3, TiO2, and P2O5 contents of 66.13–76.93 wt.%, 13.16–16.89 wt.%, 0.75–2.23 wt.%, 0.37–0.51 wt.%, and 0.02–0.06 wt.%, respectively. The selected major oxide (Al2O3, CaO, MaO, TFe2O3, and MnO) contents of the basalt and dacite samples correlate negatively with SiO2 contents, suggesting the possible fractional crystallization process (Fig. 6). What's more, the plagiogranite porphyry, monzogranite, and quartz diorite exhibit similar geochemical features, with high concentrations of SiO2 (67.55–79.00 wt.%) and Al2 O 3 (10.26–17.01 wt.%), but low concentrations of TiO 2 (0.16–0.50 wt.%), TFe2O3 (0.83–2.88 wt.%), MnO (0.01–0.10 wt.%), and P2O5 (0.03–0.14 wt.%). On the K2O vs. Na2O diagram, most of granitoid

20

9

a

8

18

b

7 6

CaO(%)

16

A l 2O 3( % )

117

14 Basalt Dacite Plagiogranite porphyry Monzogranite Quartz diorite

12 10 8 45 8

50

55

60

5 4 3 2 1

65

70

75

0 45

80

c

0.40

7

50

55

60

65

70

75

80

50

55

60

65

70

75

80

50

55

60 65 S i O 2(%)

70

75

80

d

0.30

5

P 2O 5( % )

MgO(%)

6

4 3 2

0.20 0.10

1 0 45 10 9

50

55

60

65

70

75

0.00 45 0.30

80

e

f

0.25

8

0.20

6

MnO(%)

T F e 2O 3( % )

7 5 4

0.15

3

0.10

2

0.05

1 0 45

50

55

60 65 SiO 2( % )

70

75

80

0.00 45

Fig. 6. Harker diagrams of the Fuxing volcanic and intrusive rocks.

118


Sample/Primitive Mantle

N-MORB

E-MORB

10 0

c

10 0

10 3

e

10 2


0

10 -1

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

OIB

N-MORB

Ba U Nb La Pb Nd Zr Sm Ti Tb Y Er Yb Rb Th K Ta Ce Sr P Hf Eu Gd Dy Ho Tm Lu

d

Dacite

10 2

10 1

10 0

10 3

10 1

10

10 0

10 -1


Basalt

E-MORB

10 3

10 1

b

10 1

Dacite

10 2

10 -1

10 2

10 -1



10 3

Sample/Chondrite

OIB

10 1

10 3

Basalt

10 2

10 -1

Sample/Chondrite

a


Sample/Chondrite

10 3


f

10 2


10 1

10

0

10 -1


Fig. 7. Chondrite-normalized REE and primitive mantle-normalized spider diagrams for samples from the Fuxing deposits (normalized data are from Boynton (1984) and Sun and McDonough (1989)).

4.2. SIMS zircon U–Pb dating The representative CL images of zircon grains are shown in Fig. 8, and the SIMS zircon U–Pb analytical results are shown in Fig. 9 and Table 3. Most of the zircon grains separated from the plagiogranite porphyry (FX-1) and monzogranite (FX-3) samples are generally transparent, euhedral–subhedral, and prismatic, with an aspect ratio of 2:1 to 4:1, and exhibit clear oscillatory growth zoning in their CL images (Fig. 8). All of the analyzed zircons have variable U (45–288 ppm) and Th (14–127 ppm) contents, with Th/U ratios ranging from 0.18 to 0.51 (Table 3), indicative of a magmatic origin (Hoskin and Schaltegger, 2003). Therefore, the SIMS zircon U–Pb dating results are interpreted to represent the timing of zircon crystallization and thus the age of magma emplacement. Fifteen analyses from the plagiogranite porphyry sample yielded concordant 206Pb/238U ages varying from 325.2 to 342.2 Ma (Fig. 9a), with a weighted mean age of 332.1 ± 2.2 Ma

(MSWD = 2.7; n = 15). What's more, fifteen analytical spots from the monzogranite sample had 206Pb/238U ages ranging from 318.7 to 342.8 Ma (Fig. 9b), with a weighted mean age of 328.4 ± 3.4 Ma (MSWD = 1.4, n = 15). Thus, we interpret the two weighted mean ages as the emplacement ages of the plagiogranite porphyry and monzogranite, respectively. 4.3. Zircon Hf–O isotopes In situ Hf–O isotopic data for zircons from the two samples of FX-1 (plagiogranite porphyry) and FX-3 (monzogranite) are presented in Table 4 and shown in Figs. 10 and 11. Zircon Hf–O analyses were performed on the same grains as were used for U–Pb dating (Fig. 8). Fifteen analyses were carried out for each sample. The plagiogranite porphyry sample (332.1 ± 2.2 Ma) exhibits variable Hf isotopic compositions, with 176Hf/177Hf(t) ratios ranging from 0.282962 to 0.283082, and


a

b

FX-1 Plagiogranite porphyry 1

2

332.6±5.0

325.8 ± 4.8

5

6

334.5 ± 4.9

334.0 ± 4.9

3

330.9 ± 4.9

331.9 ± 4.9

325.2 ± 5.8

3

329.9 ± 5.0

318.7 ± 5.1

322.4 ± 4.7

6

332.5 ± 6.5

332.8 ± 4.9

342.8 ± 5.1 10

9

12

333.1 ± 4.9

15

334.2 ± 4.9

2

5

342.2 ± 5.0

11

14

1

8

332.0 ± 5.0

10

13

328.9 ± 4.8

7

9

FX-3 Monzogranite

4

328.1 ± 4.9

119

337.2 ± 5.4

332.7 ± 5.1

13

100 µ m

329.3 ± 5.0

334.6 ± 4.9

7

328.5 ± 4.8

324.5 ± 4.9 8

329.9 ± 5.9 12

11

322.2 ± 4.8 15

14

326.0 ± 5.0

4

329.9 ± 4.9 100 µ m

325.2 ± 4.8

Fig. 8. Zircon cathodoluminescence (CL) images and analysis spots from the Fuxing plagiogranite porphyry (a) and monzogranite (b).

positive εHf(t) values and TCDM ages ranging from +14.0 to +18.3 and from 169 to 445 Ma, respectively (Table 4; Fig. 10a). They yielded δ18O values ranging from 4.68‰ to 5.40‰ (Fig. 10b), with a weighted mean value of 4.99 ± 0.10‰ (2σ; n = 14). The monzogranite sample (328.4 ± 3.4) displays similar Hf isotopic compositions, with 176 Hf/177Hf ratios of 0.282894 to 0.282979, εHf(t) values of + 11.4 to +14.3 (Fig. 10c), and Hf isotopic crustal model ages of 414 to 605 Ma. They yielded δ18O values ranging from 4.41‰ to 5.48‰ (Fig. 10d), with a weighted mean value of 5.04 ± 0.18‰ (2σ; n = 15).

5. Discussion 5.1. Ages of porphyry mineralization-related magmatism in Fuxing and adjacent area Precise dating of host rocks can be used to constrain the timing and duration of magmatic hydrothermal events, which is crucially important in understanding rock-forming process and geodynamic setting (Stacey and Kramers, 1975; Leng et al., 2013; Deng et al., 2014; Li et al., 2014; Deng and Wang, 2015; Wang et al., 2015e; Zhang et al. 2016a). Based on new SIMS zircon U–Pb data presented herein, the timing of granitoid intrusions in the Fuxing area are well constrained. The Fuxing plagiogranite porphyry was emplaced at 332.1 ± 2.2 Ma (Fig. 9a), while the monzogranite was formed at 328.4 ± 3.4 Ma (Fig. 9b). The geochronology data confirms that they were intruded in the Carboniferous. Previous studies suggest that the Carboniferous porphyry mineralization-related magmatism have widely taken place in Eastern Tianshan (Table 1; Zhang et al., 2004; Chen et al., 2005; C.Z. Wu et al., 2006; Zhang et al., 2008; Shen et al., 2014a, 2014b; Wang et al., 2015a, 2015b). These magmatic rocks mainly include Yandong diorite (348.3 ± 6 Ma; B. Xiao et al., 2015), diorite porphyry (340 ± 3 Ma; Shen et al., 2014a), plagiogranite porphyry (333 ± 4 Ma; Chen et al., 2005; 332.2 ± 2.3 Ma; Shen et al., 2014a; 335 ± 3.7 Ma; Y.H. Wang et al., 2015b; 339.3 ± 2.2 Ma; B. Xiao et al., 2015), Quartz albite porphyry (323.6 ± 2.5 Ma; B. Xiao et al., 2015), and quartz porphyry (324.1 ± 2.3 Ma; B. Xiao et al., 2015), Tuwu plagiogranite porphyry

4.4. Whole-rock Sr–Nd isotopes Whole–rock Rb–Sr and Sm–Nd isotope compositions of the representative samples from the Fuxing deposit are presented in Table 5. The initial 87Sr/86Sr ratios and εNd(t) values were calculated based on the crystallization ages of ~360 Ma (Rui et al., 2002), ~340 Ma (B. Xiao et al., 2015), 332.1 Ma, and 328.4 Ma for the basalt, dacite, plagiogranite porphyry, and monzogranite. All the samples have relatively homogeneous Nd isotopic compositions with 143Nd/144Nd ratios of 0.512692 to 0.512922, corresponding to εNd(t) ranging from 3.4 to 9.8 (Table 5). Compared with the basalt and dacite, the plagiogranite porphyry and monzogranite samples have higher 87Rb/86Sr (mostly 0.64–1.64) and 87Sr/86Sr (0.706080–0.711239) values, but lower 147Sm/144Nd (0.09–0.11). However, all the samples display a relatively limited (87Sr/86Sr)i variation, which range from 0.703078 to 0.704032 (Fig. 12).

a

0.052

0.057

340

320

0.050

b

0.055

P b / 238U

P b / 238U

0.054

206

0.056

360

FX-1 Plagiogranite porphyry 15 Spots Mean = 332.1±2.2Ma MS WD = 2.7

206

0.058

FX-3 Monzogranite 15 Spots Mean = 328.4±3.4Ma MS WD = 1.4

350

0.053

330

0.051 350

344

310

336

0.049

328

330 310

320

0.048 0.33

0.35

0.37 207

Pb/ 235U

0.39

0.41

0.43

0.047 0.34

0.36

0.38 207

0.40

Pb/ 235U

Fig. 9. Concordia diagrams of zircon U–Pb ages for the Fuxing plagiogranite porphyry (a) and monzogranite (b).

0.42

120


Table 3 SIMS zircon U–Pb data of the plagiogranite porphyry and monzogranite from the Fuxing area. Spot

206

Pbc/%

U/ppm

Th/ppm

Th/U

206

Pb*/ppm

206

Isotopic ratios 238

U/206Pb*

±1σ

207

FX-1, plagiogranite porphyry, 15 spots, weighted mean age = 332.1 ± 2.2 Ma, MSWD = 2.7 FX-1@01 0.31 72 22 0.30 4.38 18.8837 1.5348 0.0524 FX-1@02 0.33 104 53 0.51 6.48 19.2883 1.5002 0.0514 FX-1@03 0.16 77 21 0.27 4.56 19.1540 1.5344 0.0529 FX-1@04 0.16 99 41 0.41 6.13 19.1021 1.5088 0.0513 FX-1@05 0.14 159 56 0.35 9.85 18.7782 1.5037 0.0525 FX-1@06 0.09 280 74 0.26 17.03 18.8076 1.5000 0.0535 FX-1@07 0.05 280 98 0.35 17.21 18.9207 1.5306 0.0536 FX-1@08 0.08 276 100 0.36 17.55 18.3425 1.5001 0.0529 FX-1@09 0.34 129 43 0.33 7.87 18.9867 1.5032 0.0525 FX-1@10 0.42 45 14 0.32 2.75 18.9277 1.5118 0.0522 FX-1@11 0.11 98 36 0.37 6.04 18.8739 1.5139 0.0535 FX-1@12 0.20 94 26 0.27 5.71 18.8593 1.5001 0.0520 FX-1@13 0.12 146 27 0.18 8.37 19.3300 1.8285 0.0529 FX-1@14 0.18 103 46 0.45 6.60 18.7910 1.5058 0.0531 FX-1@15 0.13 139 50 0.36 8.68 18.7710 1.5001 0.0523

2.5004 2.5543 2.5319 1.7819 1.5479 1.1516 0.9818 1.1475 1.4536 3.6390 2.0320 1.7003 1.4986 1.5992 1.3670

FX-3, monzogranite, 15 spots, weighted mean age = 328.4 ± 3.4 Ma, MSWD = 1.4 FX-3@01 0.22 138 63 0.46 8.65 19.0420 1.5507 FX-3@02 0.22 107 41 0.38 6.40 19.7323 1.6529 FX-3@03 0.07 175 77 0.44 10.73 19.4984 1.5017 FX-3@04 0.10 264 127 0.48 16.41 19.3674 1.5506 FX-3@05 0.11 107 37 0.34 6.57 18.8910 1.9980 FX-3@06 0.06 142 58 0.41 9.27 18.3093 1.5286 FX-3@07 0.09 133 47 0.35 8.06 19.1307 1.5067 FX-3@08 0.10 114 44 0.39 7.05 19.0454 1.8382 FX-3@09 0.20 128 59 0.46 8.16 18.6224 1.6564 FX-3@10 0.03 71 29 0.41 4.48 18.8821 1.5663 FX-3@11 0.04 130 51 0.39 7.83 19.5131 1.5156 FX-3@12 0.14 105 29 0.28 6.27 19.0461 1.5087 FX-3@13 0.07 288 92 0.32 17.37 19.0786 1.5590 FX-3@14 0.30 91 43 0.47 5.65 19.2796 1.5652 FX-3@15 0.14 94 44 0.47 5.87 19.3290 1.5199

1.2952 1.4262 1.2420 0.9218 1.6870 1.6044 1.6780 1.5805 1.6299 2.1168 1.3972 1.7242 0.9846 1.5815 1.8216

±1σ

207

Pb*/206Pb*

0.0530 0.0535 0.0537 0.0531 0.0542 0.0528 0.0530 0.0528 0.0529 0.0531 0.0534 0.0532 0.0536 0.0538 0.0526

Pb*/235U

Pb*/238U

Pb/238U

±1σ

±1σ

206

0.3825 0.3675 0.3809 0.3706 0.3853 0.3922 0.3903 0.3980 0.3812 0.3801 0.3906 0.3800 0.3771 0.3896 0.3842

2.9339 2.9623 2.9606 2.3349 2.1580 1.8911 1.8184 1.8887 2.0911 3.9406 2.5339 2.2675 2.3641 2.1966 2.0295

0.0530 0.0518 0.0522 0.0524 0.0533 0.0532 0.0529 0.0545 0.0527 0.0528 0.0530 0.0530 0.0517 0.0532 0.0533

1.5348 1.5002 1.5344 1.5088 1.5037 1.5000 1.5306 1.5001 1.5032 1.5118 1.5139 1.5001 1.8285 1.5058 1.5001

332.6 325.8 328.1 328.9 334.5 334.0 332.0 342.2 330.9 331.9 332.8 333.1 325.2 334.2 334.6

5.0 4.8 4.9 4.8 4.9 4.9 5.0 5.0 4.9 4.9 4.9 4.9 5.8 4.9 4.9

0.3837 0.3736 0.3800 0.3783 0.3958 0.3974 0.3821 0.3822 0.3914 0.3878 0.3776 0.3851 0.3874 0.3847 0.3749

2.0205 2.1831 1.9488 1.8039 2.6150 2.2160 2.2552 2.4242 2.3239 2.6333 2.0614 2.2911 1.8439 2.2251 2.3724

0.0525 0.0507 0.0513 0.0516 0.0529 0.0546 0.0523 0.0525 0.0537 0.0530 0.0512 0.0525 0.0524 0.0519 0.0517

1.5507 1.6529 1.5017 1.5506 1.9980 1.5286 1.5067 1.8382 1.6564 1.5663 1.5156 1.5087 1.5590 1.5652 1.5199

329.9 318.7 322.4 324.5 332.5 342.8 328.5 329.9 337.2 332.7 322.2 329.9 329.3 326.0 325.2

5.0 5.1 4.7 4.9 6.5 5.1 4.8 5.9 5.4 5.1 4.8 4.9 5.0 5.0 4.8

±1σ

206

Pbc (%) represents the percentage of common 206Pb in total 206Pb; * denotes radioactivity lead. Common Pb corrected using measured 204Pb.

(334 ± 3 Ma; Chen et al., 2005; 332.8 ± 2.5 Ma; Shen et al., 2014b; 332.3 ± 5.9 Ma; Wang et al., 2015a), Chihu granodiorite (320.2 ± 2.4 Ma; Zhang et al., 2016b), porphyritic granodiorite (314.5 ± 2.5 Ma; Zhang et al., 2016b) and plagiogranite porphyry (322 ± 10 Ma; H. Wu et al., 2006), and East Sanchakou granodiorite (323.2 ± 2.4 Ma; C. Wang et al., 2015) and granite (321.2 ± 2.5 Ma; C. Wang et al., 2015). Our new SIMS zircon U–Pb ages as well as these data demonstrate that the Carboniferous porphyry mineralization-related magmatic activity was well-developed in Eastern Tianshan orogenic belt (Fig. 1c). From their temporal distribution, the diorite porphyry, plagiogranite porphyry, quartz albite porphyry, granodiorite, and quartz porphyry were almost contemporaneous (Fig. 1c). However, from their spatial distribution, the Carboniferous porphyry mineralization-related magmatic rocks are linear and parallel to the Kanggur fault (Fig. 1c). The geochronological data suggest a northeasterly trend migration of the porphyry mineralizationrelated magmatism during 348–314 Ma (Table 1). In addition, Rui et al. (2002) obtained a Re–Os isochron age of 322.7 ± 2.3 Ma for molybdenite from the Tuwu–Yandong porphyry Cu deposit; Zhang et al. (2004) reported a Re–Os isochron age of 343 Ma for veinlet-hosted and disseminated molybdenite from the Yandong area; and Zhang et al. (2010) obtained a Re–Os model age of 326.2 ± 4.5 Ma for molybdenite from the Yanxi porphyry Cu deposit. Therefore, these isotopic age data suggest that the Carboniferous magmatism and porphyry Cu-dominant mineral system are significant in Eastern Tianshan orogenic belt, which are considered to be related to subduction tectonism (e.g., Mao et al., 2005, Han et al., 2006). 5.2. Petrogenesis and sources of the Fuxing magmatism 5.2.1. Basalt and dacite The basalts in the Fuxing area are characterized by high Sr (574–753 ppm), high Y (20.8–28.2 ppm) and HREE concentrations

(Yb = 2.23–3.06 ppm), and low (La/Yb)N (2.20–3.92) values (Table 2), showing geochemical signatures comparable to normal island arc rocks (Defant and Drummond, 1990). In the YbN versus (La/Yb)N diagram, the basalts plot within the typical arc field (Fig. 13a; Defant and Drummond, 1990). They have low SiO2 contents (47.47–54.90 wt.%), high Mg# values (56–62), and relatively high concentrations of compatible trace elements, such as Ni (82.6–149 ppm), Cr (108–345 ppm), and Co (30.7–39.1) (Table 2), suggesting that the mantle components were involved in their generation. They also show moderately fractionated LREEs and HREEs, enrichment in LREEs relative to HREEs, a lack of Eu anomalies (Fig. 7a), and depletion of HFSEs (Nb, Ta, and Ti) on the primitive mantle-normalized trace element spider diagram (Fig. 7b), reflecting clear subduction signatures. Moreover, the Fuxing basalts have variable La contents, but relatively narrow La/Yb ratio range, suggesting that fractional crystallization process played a prominent role in their petrogenesis (Fig. 13b; Schiano et al., 2010). This is also confirmed by the negative correlation between the selected major oxides (Al2O3, CaO, MaO, TFe2O3 and MnO) and the SiO2 contents of the volcanic rocks (Fig. 6). In addition, the basalt is characterized by positive εNd(t) value and low (87Sr/86Sr)i ratio, and is close to MORB area in the (87Sr/86Sr)i vs. εNd(t) diagram (Fig. 12). The Sr–Nd isotopic characters suggest a juvenile source for the basalts, which is consistent with their young Nd model age (TDM) of 838 Ma (Table 5). These trace element and Sr–Nd isotopic signatures mentioned above are comparable to the Tuwu–Yandong volcanic rocks in the Dananhu–Tousuquan arc belt (Fig. 12), which are interpreted to have been derived from partial melting of mantle-wedge peridotite that was previously metasomatized by slab melts (Zhang et al., 2006; Shen et al., 2014b; B. Xiao et al., 2015). Therefore, the subduction-modified mantle could have been the source for these basalts in the Fuxing area. The dacites at Fuxing also exhibit a genetic affinity with the normal arc volcanic rocks, with high Mg#(36–51), high Y (41.8–54.9 ppm)


121

Table 4 Zircon Hf and O isotopic data of the plagiogranite porphyry and monzogranite from the Fuxing area. Spot

Age/Ma

176

Yb/177Hf

176

Lu/177Hf

176

Hf/177Hf

± 2σ

176

Hf/177Hf(t)

176

Hf/177Hf CHUR(t)

εHf(0)

εHf(t)

± 2σ

TDM/Ma

TCDM/Ma

fLu/Hf

δ18O

2σ

FX-1, plagiogranite porphyry, 332.1 ± 2.2 Ma, εHf(t) = +14.0 to + 18.3 FX-1-01 332.6 0.041905 0.001785 0.283032 0.000021 FX-1-02 325.8 0.024068 0.001062 0.282977 0.000022 FX-1-03 328.1 0.029753 0.001351 0.283039 0.000020 FX-1-04 328.9 0.034108 0.001549 0.283051 0.000021 FX-1-05 334.5 0.034663 0.001514 0.282990 0.000024 FX-1-06 334.0 0.050060 0.002080 0.283088 0.000023 FX-1-07 332.0 0.041805 0.001842 0.283012 0.000021 FX-1-08 342.2 0.040418 0.001806 0.283069 0.000021 FX-1-09 330.9 0.034147 0.001522 0.283002 0.000023 FX-1-10 331.9 0.026563 0.001223 0.282969 0.000022 FX-1-11 332.8 0.030616 0.001367 0.283026 0.000023 FX-1-12 333.1 0.043425 0.001878 0.283031 0.000025 FX-1-13 325.2 0.043008 0.001920 0.283031 0.000022 FX-1-14 334.2 0.054068 0.002304 0.283096 0.000023 FX-1-15 334.6 0.042800 0.001829 0.283038 0.000022

0.283021 0.282971 0.283031 0.283041 0.282981 0.283075 0.283001 0.283057 0.282992 0.282962 0.283017 0.283019 0.283020 0.283082 0.283027

0.282565 0.282569 0.282568 0.282567 0.282564 0.282564 0.282566 0.282559 0.282566 0.282566 0.282565 0.282565 0.282570 0.282564 0.282564

9.2 7.3 9.4 9.9 7.7 11.2 8.5 10.5 8.1 7.0 9.0 9.2 9.2 11.5 9.4

16.1 14.2 16.4 16.8 14.7 18.1 15.4 17.6 15.1 14.0 16.0 16.1 15.9 18.3 16.4

0.8 0.8 0.7 0.7 0.9 0.8 0.8 0.7 0.8 0.8 0.8 0.9 0.8 0.8 0.8

318 390 304 289 376 238 348 265 359 403 323 320 320 227 309

310 428 290 266 400 186 356 221 376 445 318 313 317 169 295

−0.95 −0.97 −0.96 −0.95 −0.95 −0.94 −0.94 −0.95 −0.95 −0.96 −0.96 −0.94 −0.94 −0.93 −0.94

5.24 4.91 4.78 4.92 4.91 4.91 4.68 4.89 5.19 5.13 5.11 5.04 5.40 4.91

0.38 0.22 0.20 0.27 0.22 0.30 0.34 0.28 0.24 0.26 0.25 0.21 0.29 0.32

FX-3, monzogranite, 328.4 ± 3.4 Ma, εHf(t) = +11.4 to + 14.3 FX-3-01 329.9 0.038909 0.001684 0.282974 0.000017 FX-3-02 318.7 0.031142 0.001378 0.282987 0.000017 FX-3-03 322.4 0.065612 0.002689 0.282910 0.000019 FX-3-04 324.5 0.036169 0.001566 0.282937 0.000021 FX-3-05 332.5 0.038949 0.001694 0.282975 0.000020 FX-3-06 342.8 0.049722 0.002134 0.282930 0.000015 FX-3-07 328.5 0.035604 0.001559 0.282962 0.000018 FX-3-08 329.9 0.035211 0.001542 0.282934 0.000017 FX-3-09 337.2 0.041559 0.001819 0.282924 0.000018 FX-3-10 332.7 0.038408 0.001696 0.282966 0.000021 FX-3-11 322.2 0.033855 0.001468 0.282969 0.000019 FX-3-12 329.9 0.036031 0.001590 0.282973 0.000018 FX-3-13 329.3 0.048698 0.002112 0.282952 0.000016 FX-3-14 326.0 0.052168 0.002245 0.282929 0.000023 FX-3-15 325.2 0.035126 0.001534 0.282978 0.000019

0.282964 0.282979 0.282894 0.282928 0.282964 0.282916 0.282952 0.282924 0.282913 0.282955 0.282961 0.282963 0.282939 0.282915 0.282968

0.282567 0.282574 0.282572 0.282570 0.282565 0.282559 0.282568 0.282567 0.282562 0.282565 0.282572 0.282567 0.282567 0.282569 0.282570

7.2 7.6 4.9 5.8 7.2 5.6 6.7 5.7 5.4 6.9 7.0 7.1 6.4 5.6 7.3

14.1 14.3 11.4 12.7 14.1 12.7 13.6 12.6 12.4 13.8 13.8 14.0 13.2 12.2 14.1

0.6 0.6 0.7 0.7 0.7 0.5 0.6 0.6 0.6 0.7 0.7 0.6 0.6 0.8 0.7

400 379 508 453 400 470 417 458 475 413 405 401 438 473 394

441 414 605 526 439 541 469 531 553 459 453 442 498 554 434

−0.95 −0.96 −0.92 −0.95 −0.95 −0.94 −0.95 −0.95 −0.95 −0.95 −0.96 −0.95 −0.94 −0.93 −0.95

4.41 4.95 4.65 4.79 5.27 4.54 5.48 4.99 4.94 5.42 4.87 5.21 5.41 5.10 4.75

0.37 0.42 0.46 0.43 0.36 0.30 0.23 0.27 0.29 0.34 0.32 0.37 0.32 0.23 0.40

εHf(t) = 10,000 × ({[(176Hf/177Hf)S − (176Lu/177H f)S ×(eλt − 1)]/[(176Hf/177Hf)CHUR, 0 − (176Lu/177Hf)CHUR ×((eλt − 1)] − 1}. TDM = 1/λ × ln{1 + [(176Hf/177Hf)S − (176Hf/177Hf)DM]/[(176Hf/177Hf)S − (176Hf/177Hf)DM]. C = TDM − (TDM − t) × [( fcc − fs)/(fcc − fDM). TDM fLu/Hf = (176Lu/177Hf)S/(176Lu/177Hf)CHUR − 1. where, λ = 1.867 × 10−11/year−1 (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); ( 176 Lu/ 177 Hf) DM = 0.0384 and ( 176 Hf/ 177 Hf) DM = 0.28325 (Griffin et al., 2002); ( 176 Lu/ 177 Hf) mean

crust

[(176Lu/177Hf)mean crust/(176Lu/177Hf)CHUR] − 1; fs = fLu/Hf; fDM = [(176Lu/177Hf)DM/(176Lu/177Hf)CHUR] − 1; t = crystallization time of zircon.

7

a

7

FX-1 Plagiogranite porphyry

3 2

5

Number

Number

4

3 2

1

1 0 4.0

14

15

16 17 εHf(t)

18

19

20

21

5

c

FX-3 Monzogranite

5

4.8

d

5.2 18 δ O( )

5.6

6.0

6.4

FX-3 Monzogranite

2

Number

Relative probability


3

3 2 1

1 0 10

4.4

4

4

Number

4

0 12

13

FX-1 Plagiogranite porphyry Relative probability


5

6

b

6

6

11

12

13 14 εHf(t)

15

16

0 3.8

4.2

4.6

5.0 5.4 18 δ O( )

5.8

6.2

Fig. 10. The cumulative probability histogram of zircon εHf(t) and δ18O values for the Fuxing plagiogranite porphyry (a, b) and monzogranite (c, d).

= 0.015; f cc =

122


20

12

a

b

11

Depleted mantle

Plagiogranite porphyry Monzogranite

Supracrustal component

15 10

ε H f( t )

δ18O (‰ )

10

CHUR

0

-5 300

310

320

7

Fuxing plagiogranite porphyry Fuxing monzogranite Tuwu plagiogranite porphyry Yandong plagiogranite porphyry Chihu porphyritic granodiorite

330 340 U-Pb age (Ma)

350

8 m

/H

f c=

6 5 360

m

/H

f c=

0.7

Hf

5

Hf

9

0. 0 5

Enriched mantle

4 -15

-10

Depleted mantle

-5

0

εHf(t)

5

10

15

20

Fig. 11. (a) Correlation diagram of zircon Hf isotopes vs. U–Pb ages; (b) zircon δ18O values vs. εHf(t) diagram. The dotted lines denote the two-component mixing trends between the depleted mantle and supercrust-derived magmas. Hfm/Hfc is the Hf concentration ratio between the mantle magma (m) and crustal (c) components indicated for each curves, and the small open circles on the curves represent 10% mixing increments by assuming the mantle zircon has εHf(t) = +12 and δ18O = 5.3‰; the supracrustal zircon has εHf(t) = −12 and δ18O = 11‰ (Valley et al., 1998). The field of the depleted mantle is from Valley et al. (2005).

and Yb (5.76–8.98 ppm) concentrations, and low (La/Yb)N (1.58–3.46) values (Fig. 13a). In the La/Yb versus La diagram, the dacites display a comparable trend to the partial melting process (Fig. 13b). The isotopic signature of the dacite ((87Sr/86Sr)i = 0.703981; εNd(t) = 4.6) and the corresponding Nd model ages (720 Ma; Table 5) both support the involvement of mantle materials in the formation of the rocks. In addition, the dacite shows similar geochemical characteristics with the basalt at Fuxing, indicating that they have a relatively uniform and integrated source region. Therefore, we suggest that the dacites were also derived from the partial melting of mantle wedge peridotites that was previously modified by slab melts. 5.2.2. Plagiogranite porphyry, monzogranite, and quartz diorite The Fuxing plagiogranite porphyry are characterized by relatively high SiO 2 (70.47–74.89 wt.%), Al 2 O 3 (10.26–16.32 wt.%), Sr (118–392 ppm), Sr/Y ratios (31.72–100.78), low Y (2.40–7.71 ppm) and Yb (0.17–0.74 ppm) (Table 2), and positive Eu anomalies (Eu/Eu* = 1.18–1.38, except for one outlier of 0.85) (Fig. 7a), showing geochemical affinity with modern adakites (Defant and Drummond, 1990). In the (La/Yb) N versus YbN discrimination diagram, the Fuxing plagiogranite porphyry samples also plot well within the adakitic field (Fig. 13a), different in normal island-arc magma suggesting an adakite-like source (Defant and Drummond, 1990). There have been a variety of petrogenetic mechanisms proposed to account for the origin of adakites or adakitic rocks (Castillo, 2012), such as (a) partial melting of a subducted oceanic crust with or without contributions from a mantle wedge (Defant and Drummond, 1990; Zhu et al., 2009; Sun et al., 2010, 2011; Wang et al., 2015a, 2015b); (b) generated through crustal assimilation and fractional crystallization (AFC) processes of parental basaltic magmas (Castillo et al., 1999; Macpherson et al., 2006; Richards and Kerrich, 2007); (c) partial melting of a delaminated continental lower crust (Guo et al., 2006; Q. Wang et al., 2006; Kadioglu and Dilek, 2010); (d) partial melting of thickened lower crust (Petford and Atherton, 1996; Z.Q. Hou et al., 2013; Sui et al., 2013); (e) partial melting of subducted continental crust (Q. Wang et al., 2008); and (f) generated during magma mixing between felsic and basaltic magmas (Streck et al., 2007; Meng et al., 2014). Experimental and geochemical studies indicate that adakitic rocks derived from melting of lower crust generally have low MgO or Mg# values, and low compatible trace elements (e.g., Cr and Ni) (Yogodzinski and Kelemen, 1998; Guan et al., 2011; Zhu et al., 2011; Hou et al., 2013; Zhu et al., 2013), similar to those of experimental melts from metabasalts and eclogites (Fig. 5d; Rapp et al., 1999). However, the plagiogranite porphyries from this study have relatively high Mg# values (mostly 48–63), and moderate Cr and Ni concentrations,

which are inconsistent with adakites from lower crustal magmas, but comparable to those from the interaction of slab partial melts and mantle wedge peridotites during magma ascent (Rapp et al., 1999; Martin et al., 2005; B. Xiao et al., 2015). The Fuxing plagiogranite porphyries display high positive zircon εHf(t) values (+ 14.0 to + 18.3), low 87 Sr/86 Sr (0.707563–0.711239) and high 143 Nd/144Nd (0.512692–0.512785) ratios that are clearly different from adakitic rocks generated by partial melting of lower crust or subducted continental crust, which normally show lower εHf(t) values, higher 87Sr/86Sr and lower 143Nd/144Nd ratios (Q. Wang et al., 2006; Liu et al., 2010; Wang et al., 2015d), such as the Permian adakitic rocks from Western Tianshan in China (Zhao et al., 2008), and the adakites of Cordillera Blanca batholith in Peru (Petford and Atherton, 1996). In contrast, the adakitic plagiogranite porphyries at Fuxing are coincide with the subducted oceanic crust-derived adakites (Figs. 11a and 12), such as the Late Carboniferous Tuwu–Yandong plagiogranite porphyry (Wang et al. 2014, 2015a, 2015b; B. Xiao et al., 2015), and the Chihu porphyritic granodiorite (Zhang et al., 2016b) in Eastern Tianshan, and the Alataw adakites in Western Tianshan (Q. Wang et al., 2006). In addition, the adakitic plagiogranite porphyries are unlikely attributed to the product of AFC processes of parental basaltic magmas, because they exhibit no obvious systematic variations in geochemistry and Sr–Nd isotopic compositions associated with AFC (Castillo et al., 1999; Macpherson et al., 2006). Moreover, the zircon δ18O values of the adakitic plagiogranite porphyries vary from 4.68‰ to 5.40‰, indicating depleted isotopic signatures close to the depleted mantle (DM) reservoir (δ18O = 5.3 ± 0.3‰; Li et al., 2009b). The relatively low O isotopic features recorded in the adakitic plagiogranite porphyries might be attributed to the interaction with the overlying mantle peridotites during magma ascent. Furthermore, the adakitic plagiogranite porphyries show a positive correlation between the La and La/Yb values (Fig. 13b), implying that the partial melting process might have controlled the formation of these adakitic rocks. Therefore, we conclude that the adakitic plagiogranite porphyries at Fuxing were most probably generated by partial melting of a subducted oceanic slab, which was subsequently hybridized by mantle peridotites. The slightly positive Eu anomalies indicate plagioclase melting in the source and its retention in the magma, and the clear REE fractionation patterns are interpreted to be due to presence of garnet in the melt residue. Likewise, the negative Nb, Ta and Ti anomalies are considered to be related to the presence of rutile in their source. The Fuxing monzogranite and quartz diorite intrusions exhibit similar geochemical and Sr–Nd–Hf–O isotopic characteristics with the plagiogranite porphyry, indicating that they have a similar magma source. In the diagram of (La/Yb)N versus YbN (Fig. 13a), monzogranite

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15

Fuxing basalt Fuxing dacite Fuxing plagiogranite porphyry Fuxing monzogranite Tuwu-Yandong plagiogranite porphyry Tuwu-Yandong volcanic rocks Alataw adakites Awulale adakites

MORB

ε N d( t )

10

5 Permian adakite in Xinjiang Tianshan

0 Late Carboniferous adakite in Xinjiang Tianshan

-5

-10 0. 700

Cordillera Blanca Batholith Mantle array

0. 702

0. 704

0. 706 0. 708 87 86 ( Sr/ Sr) i

0. 710

0. 712

Fig. 12. Diagram of εNd(t) vs. (87Sr/86Sr)i for the Fuxing igneous rocks. Data source: the Tuwu–Yandong plagiogranite porphyry and volcanic rocks in Eastern Tianshan from Rui et al. (2004), Xia et al. (2004), Zhang et al. (2006), and B. Xiao et al. (2015); the Carboniferous Alataw adakites in Western Tianshan from Q. Wang et al. (2006); the Permian Awulale adakites in Western Tianshan from Zhao et al. (2008); and the field of Cordillera Blanca Batholith from Petford and Atherton (1996).

and quartz diorites all plot in the adakitic field, suggesting an adakitelike affinity (Defant and Drummond, 1990). Most of the monzogranite and quartz diorite samples exhibit high SiO2 (67.55–79.00 wt.%) and Mg# (mostly 33–51) values, which plot in the subducted oceanic crust-derived adakite field (Fig. 5d). Compared with the plagiogranite porphyry, the monzogranite shows lower zircon εHf(t) values (+11.4 to +14.3), (87Sr/86Sr)i ratios (0.703078), and similar zircon δ18O values (4.41–5.48), consistent with the interactions between slab-derived melts and mantle peridotites. Therefore, these data imply that the monzogranite and quartz diorite intrusions at Fuxing were also generated by partial melting of a subducted oceanic crust, followed by interaction with the mantle peridotites. 0.51315; (147Sm/144Nd) crust = 0.118; (87Sr/86Sr)S and (143Nd/144Nd)S are the measured values of the samples.

0.15 0.14 0.10 0.09 0.11 0.10 0.10 0.703454 0.703981 0.704032 0.703264 0.703467 0.703995 0.703078 0.000015 0.000008 0.000010 0.000009 0.000011 0.000016 0.000013 0.703679 0.707973 0.708747 0.710263 0.711239 0.707563 0.706080 0.04 0.82 1.00 1.48 1.64 0.75 0.64 360 340 332.1 332.1 332.1 332.1 328.4 Basalt Dacite Plagiogranite porphyry Plagiogranite porphyry Plagiogranite porphyry Plagiogranite porphyry Monzogranite 15FX-87 15FX-93 15FX-90 15FX-91 15FX-92 15FX-94 15FX-89

(87Sr/86Sr)i = (87Sr/86Sr)s − (87Rb/86Sr) s × (eλt − 1); 87Sr/86Sr = (Rb/Sr) × 2.8956; λRb–Sr = 1.42 × 10–11/a; (143Nd/144Nd)i = (143Nd/144Nd) s − (147Sm/144Nd) s × (eλt – 1); 147Sm/144Nd = (Sm/Nd) × 0.60456; λSm–Nd = 6.54 × 10–12/a; εNd(t) = 10,000 × [(143Nd/144Nd) i/(143Nd/144Nd)CHUR(t) − 1]; (143Nd/144Nd)CHUR(t) = (143Nd/144Nd)CHUR(0) − (147Sm/144Nd)CHUR × (eλt − 1); (143Nd/144Nd)CHUR(0) = 0.512638; (147Sm/144Nd)CHUR = 0.1967; TDM = 1/λ × ln{1 + [(143Nd/144Nd)S − (143Nd/144Nd)DM]/[(147Sm/144Nd)S − (147Sm/144Nd)DM]}; TDM2 = TDM − (TDM − t) × [(fc − fs)/(fc − fDM)]; fc = [(147Sm/144Nd) crust/(147Sm/144Nd)CHUR] − 1 = −0.4; fs = fSm/Nd = (147Sm/144Nd)S/(147Sm/144Nd) CHUR − 1; fDM = [(147Sm/144Nd)DM/(147Sm/144Nd))CHUR] − 1 = 0.08592; (147Sm/144Nd)DM = 0.21357; (143Nd/144Nd)DM =

838 720 647 632 546 655 288 1053 837 597 561 523 605 296

TDM/Ma εNd(t)

3.4 4.6 5.5 5.6 6.7 5.4 9.8 0.512349 0.512438 0.512490 0.512499 0.512553 0.512485 0.512717 0.000009 0.000006 0.000024 0.000020 0.000016 0.000009 0.000008

(143Nd/144Nd)i 2σ Nd/144Nd 143

Sm/144Nd 147

(87Sr/86Sr)i 2σ Sr/86Sr 87

Rb/86Sr 87

Age-corrected (Ma) Rock type Sample no.

Table 5 Sr–Nd isotopic compositions of samples from the Fuxing area.

0.512697 0.512751 0.512707 0.512692 0.512785 0.512703 0.512922

TDM2/Ma


5.3. Tectonic setting and mineralization implications The volcanic rocks mainly composed of basalt and dacite were found in the Fuxing area. As mentioned above, they show similar geochemical features that are characterized by weak to moderate REE fractionation patterns, negative or no Eu anomalies (Fig. 7a, c), and clear depletion in Na, Ta, and Ti (Fig. 7b, d). Most of the basalt and dacite samples fall within the subduction zone and close to E-MORB area (Pearce and Peate, 1995; Sayit and Goncuoglu, 2009) in the Th/Yb versus Nb/Yb diagram (Fig. 14a; Whattam and Hewins, 2009). Moreover, they plot in the subduction-related arc field with calc-alkaline affinities (Fig. 14b; Wood et al., 1979). Therefore, these volcanic rocks at Fuxing were most likely formed in an island arc setting associated with subduction processes. The Fuxing felsic intrusions consist of plagiogranite porphyry, monzogranite, and quartz diorite, with strong HREEs depletion, positive or no Eu anomalies (Fig. 7e), and depletion in Nb, Ta, and Ti (Fig. 7f), which are similar to those of calc-alkaline magmatic rocks formed in subduction zone environments (Rollinson, 1993; Shen et al., 2014a). What's more, the felsic intrusions fall within E-MORB and OIB area (Fig. 14a) with relatively high Th/Yb ratios ranging from 1.01 to 4.53, and plot in the volcanic arc granitoid field (Fig. 14c, d). We therefore suggest that the Fuxing intrusions (plagiogranite porphyry, monzogranite, and quartz diorite) were also formed in an island arc, namely, the Dananhu–Tousuquan arc. Although the geodynamic evolution of the Dananhu–Tousuquan arc belt has been widely discussed (Xia et al., 2004; Xiao et al., 2004; Han et al., 2006; Shen et al., 2014a; Wang et al., 2015c, 2016a, 2016b; and references therein), a consensus for the Late Paleozoic tectonic setting has not been achieved. Previous researchers proposed that the paleoTianshan ocean, which was one branch of the Paleo-Asian Ocean (Xiao et al., 2004, 2013; and references therein), may have been closed in


30

a

Basalt Dacite Plagiogranite porphyry Monzogranite Quartz diorite

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elting

124

0

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Fig. 13. Plots of (La/Yb)N vs. YbN diagram (a) and La/Yb vs. La diagram (b) for the Fuxing volcanic rocks and intrusions (modified from Defant and Drummond, 1990). N means normalized to chondrite (Sun and McDonough, 1989).

the Carboniferous and the Dananhu–Tousuquan arc belt entered a continental rift stage (Xiao et al., 1992; Xia et al., 2004; Z.M. Li et al., 2006; Xia et al., 2008). Shen et al. (2014a, 2014b) and Hou et al. (2014) argued that a south-directed subduction of the Junggar Ocean had formed the Dananhu–Tousuquan island arc and the Aqishan–Yamansu back-arc basin system. However, most researchers suggested that the Devonian–Carboniferous magmatic rocks in the Dananhu–Tousuquan arc were generated by northward subduction of the paleo-Tianshan ocean (Xiao et al., 2004; Mao et al., 2005; Han et al., 2006; Zhang et al., 2006; Wang et al., 2015a, 2015b; B. Xiao et al., 2015). Based on the presence of the youngest ophiolite of ~310 Ma and widespread bimodal volcanic rocks of ~ 290 Ma (Chen et al., 2011; Su et al., 2012) in the

10 1

Jueluotage area, it is proposed that the subduction ceased in the Early Permian as a result of collision of the Dananhu–Tousuquan arc and the Middle Tianshan massif (Xiao et al., 2004; Mao et al., 2005; Xiao et al., 2013; J.W. Mao et al., 2014). Our study about the Early Carboniferous volcanic rocks and felsic intrusions in the Fuxing area, and previous studies about the Carboniferous magmatic rocks in the Tuwu (Wang et al. 2014, 2015a), Yandong (Wang et al., 2015b), and Chihu (Zhang et al., 2016b) areas all support the subduction model in the Dananhu– Tousuquan arc, as proposed by Zhang et al. (2006) and Mao et al. (2005). In this scenario (Fig. 15), the paleo-Tianshan oceanic plate along the Kanggur fault may have been northward subducting during the Carboniferous, creating the Dananhu–Tousuquan arc in the

a

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Fig. 14. Tectonic discrimination diagrams for the Fuxing volcanic rocks and intrusions. (a) Th/Yb vs. Nb/Yb diagram (Pearce and Peate, 1995; Sayit and Goncuoglu, 2009); (b) Hf/3–Th–Ta diagram (Wood et al., 1979); (c) Rb vs. Ta + Yb diagram (Pearce et al., 1984); (d) Rb vs. Y + Nb diagram (Pearce et al., 1984). Abbreviations: IAT, island arc tholeiite; CAB, island arc calcalkaline basalt; WPT, within-plate tholeiite; WPAB, within-plate alkaline basalt; Syn-COLG, syn-collision granitoid; WPG, within-plate granitoid; VAG, volcanic arc granitoid; ORG, ocean ridge granitoid.

Y.-H. Wang et al. / Gondwana Research 34 (2016) 109–128 N

Carboniferous Fuxing Cu deposit Paleo-Tianshan ocean

Felsic intrusions

Volcanic rocks Dananhu-Tousuquan arc

Oceanic lithosphere

Continental lithosphere Mantle wedge melting

Asthenosphere

Slab melting

Oc ean ic c rus t

Fig. 15. Simplified cartoon showing the tectonic setting for the formation of the volcanic rocks and felsic intrusions in the Fuxing Cu deposit of Eastern Tianshan.

northern Kanggur fault. In this case, partial melting of mantle wedge peridotites, which were previously metasomatized by subducted slab released melts/fluids, created the basalt and dacite; melting of a subducted oceanic crust that was subsequently hybridized by mantle peridotites produced the adakitic plagiogranite porphyry, monzogranite, and quartz diorite of the Fuxing intrusions (Fig. 15). These geodynamic processes led to the formation of subductionrelated igneous rocks and associated porphyry copper deposits in Eastern Tianshan (Fig. 1c), represented by the Tuwu, Yandong, Linglong, Chihu, and the newly discovered Fuxing deposits in the central and eastern part of the Dananhu–Tousuquan arc belt (Shen et al., 2014a, 2014b; Wang et al., 2015a; B. Xiao et al., 2015). Metallogenic systems are an important component of the megaearth system, which are heterogeneously, but not randomly, distributed in time and space (Zhai et al., 2011; Santosh and Somerville, 2013; Santosh and Pirajno, 2014; Deng and Wang, 2015; Deng et al., 2015a), which are commonly controlled by the geodynamic evolution and associated magmatic activities (Chen et al., 2007; Groves and Bierlein, 2007; Chen et al., 2008; Santosh et al., 2011). In general, most porphyry Cu deposits were formed in magmatic arc settings above subduction zones of convergent plate margins (Sillitoe, 1997; Cooke et al., 2005; Sillitoe, 2010; Sun et al., 2013; Gao et al., 2015; Xue et al., 2015), and are considered to be associated with subducted oceanic crust-derived magma (Hedenquist and Lowenstern, 1994; Defant et al., 2002; Richards, 2005; Sillitoe, 2010). Various studies indicate that the oceanic crust contains more moderately incompatible elements (Cu) than the mantle and the continental crust (McDonough and Sun, 1995; Rudnick and Gao, 2003; Sun et al., 2010, 2011). The Fuxing felsic intrusions with their diagnostic adakitic chemistry were derived from oceanic slab melts, indicating that they had high contents of Cu element, which is favorable for porphyry Cu mineralization. In addition, it is well-established that the porphyry Cu deposits worldwide are closely linked to oxidized adakitic magmas (Oyarzun et al., 2001; Ballard et al., 2002; Sun et al., 2010, 2013), although several porphyry deposits formed from reduced magmas are also reported (Rowins, 2000; Smith et al., 2012; Sun et al., 2013; Cao et al., 2014). Moreover, it is suggested that the upper part of a subducted oceanic crust has a very high intrinsic oxygen fugacity (fO2) due to equilibration with seawater during hydrothermal alteration and deposition of terrigenous sediments (Mungall, 2002), which is very important for Cu mineralization in porphyry system. In the Fuxing deposit, the adakitic felsic intrusions belong to the magnetite series with relatively high Fe2O3/FeO ratios (mostly 0.41–7.09) (Table 2) and are highly oxidized (Ishihara et al., 1979). The oxidized adakitic magma led the formation of the Fuxing porphyry Cu deposit, similar to the metallogenic mechanism of the Tuwu–Yandong porphyry Cu deposits (Shen et al., 2014a, 2014b; Wang et al., 2015a, 2015b). In this contribution, we suggest that the island arc setting and associated adakitic felsic magma should be particularly suitable for the formation of the porphyry Cu deposits in the Dananhu–Tousuquan arc belt. Furthermore, the adakitic plagiogranite porphyry and related granitic rocks can be considered as an exploration target applicable to the search for new porphyry-type deposits in Eastern Tianshan and adjacent regions.

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Based on the above-mentioned tectonic setting of the Fuxing volcanic rocks and adakitic felsic intrusions, as well as the new SIMS zircon U–Pb dating and Sr–Nd–Hf–O isotopic data, an Early Carboniferous subduction zone and island arc magmatic model for the Fuxing porphyry Cu deposit can be proposed (Fig. 15). During the period of the Early Carboniferous, the northward subduction of the paleo-Tianshan oceanic crust (Wang et al., 2015a; B. Xiao et al., 2015) produced the Dananhu– Tousuquan island arc and associated volcanic rocks and felsic intrusions, of which the felsic intrusions at Fuxing, derived by partial melting of the subducting slab, were most likely responsible for the formation of the Fuxing porphyry Cu deposit (Fig. 15). 6. Conclusions (1) The Fuxing porphyry Cu deposit is located in the Dananhu– Tousuquan island arc in Eastern Tianshan. This deposit mainly occurs in the volcanic rocks (basalt and dacite) and associated felsic intrusions (plagiogranite porphyry, monzogranite, and quartz diorite). (2) The plagiogranite porphyry and monzogranite yield SIMS zircon U–Pb ages of 332.1 ± 2.2 Ma and 328.4 ± 3.4 Ma, respectively. The volcanic rocks and felsic intrusions exhibit similar signatures as arc rocks, with REE fractionation patterns and depletions of Nb, Ta, and Ti, suggesting that they were formed in a Carboniferous island arc. (3) Based on the whole-rock geochemical and Sr–Nd–Hf–O isotopic data, as well as detailed petrographic analyses, we suggest that the calc-alkaline basalt and diorite formed by partial melting of subduction-modified mantle peridotites; while the adakitic plagiogranite porphyry, monzogranite, and quartz diorite were most probably derived from partial melting of a subducted oceanic crust followed by mantle peridotites interaction. (4) The igneous rocks and associated porphyry Cu mineralization in the Fuxing area were generated by the northward subduction of the paleo-Tianshan oceanic crust beneath the Dananhu– Tousuquan island arc during the Early Carboniferous.

Acknowledgements This research was jointly funded by the Fundamental Research Funds for the National Natural Science Foundation of China (41572066 and 41030423), the Fundamental Research Funds for the Central Universities of China (2652015019 and 2652015032), the Open Research Funds for the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing) (GPMR201512), and the Geological Survey Project of China (1212011085471 and 1212011220923). We are very grateful to the Editor-in-Chief M. Santosh, Associate Editor Franco Pirajno, and two reviewers for constructive comments and improvement of the manuscript. We thank Jing Feng of the Xinjiang Bureau of Geology and Mineral Exploration for great support and assistance in our fieldwork. We also appreciate the kind help of Professor Xian-Hua Li from the Institute of Geology and Geophysics, Chinese Academy of Sciences on SIMS zircon U–Pb dating and in situ zircon O analyses. We thank Chinese Academician Yu-Sheng Zhai from the China University of Geosciences (Beijing) for a helpful scientific review of an earlier version of the manuscript. References Ballard, J.R., Palin, M.J., Campbell, I.H., 2002. Relative oxidation states of magmas inferred from Ce(IV)/Ce(III) in zircon: application to porphyry copper deposits of Northern Chile. Contributions to Mineralogy and Petrology 144, 347–364. Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mundil, R., Campbel, I.H., Korsch, R.J., Williams, I.S., Foudoulis, C., 2004. Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix effect: SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards. Chemical Geology 205, 115–140.

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