Late Carboniferous high-Mg dioritic dikes in Western Junggar, NW China: Geochemical features, petrogenesis and tectonic implications

Gondwana Research 17 (2010) 145–152

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Late Carboniferous high-Mg dioritic dikes in Western Junggar, NW China: Geochemical features, petrogenesis and tectonic implications Jiyuan Yin a, Chao Yuan a,⁎, Min Sun b, Xiaoping Long a, Guochun Zhao b, Kenny Powan Wong b, Hongyan Geng b, Keda Cai b a b

Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China

a r t i c l e

i n f o

Article history: Received 19 December 2008 Received in revised form 12 May 2009 Accepted 17 May 2009 Available online 2 June 2009 Keywords: Sanukitoid Ridge subduction Junggar Cu–Au mineralization Dike

a b s t r a c t Late Paleozoic High-Mg dioritic dikes widely occur in western Junggar, NW China. Ar–Ar dating on rock chips of the dikes has yielded a plateau age of 321 ± 3 Ma, indicating an early Carboniferous age for the dikes. The dikes are enriched in SiO2 (52–57 wt.%), and characterized by high MgO (5.13–7.41 wt.%), Cr (134–204 ppm), Ni (59–141 ppm), Sr (468–724 ppm) and Ba (316–676 ppm) contents, with geochemical features analogous to those of sanukite of Setouchi volcanic belt, Japan. These dikes contain hornblende and biotite and generally have high Ba/La (27–124) and La/Nb (2.9–4.3) ratios and positive Eu anomalies, consistent with an origin from hydrous partial melting of a mantle source metasomatised by slab-derived component. The occurrence of sanukitic dikes, together with the coeval slab-related adakite in the area, implies that the western Junggar had been affected by hot, subduction-related regime, which gave rise to not only massive magmatism in the late Carboniferous, but also intensive Cu–Au mineralization in the area. © 2009 International Association for Gondwana Research. Published by ELsevier B.V. All rights reserved.

1. Introduction Accretionary orogeny plays an important role in crustal growth (e.g. Sengör, 1990; Santosh et al., 2009). The Central Asia Orogenic Belt (CAOB) covers a large area of Asia and represents the most important crustal growth in the Phanerozoic (Sengör et al., 1993; Buslov et al., 2004; Jahn, 2004; Windley et al., 2007; Zhang et al., 2009; Xiao and Kusky., 2009). The long-lasting and complex tectonic evolutionary history and plentiful mineral sources have attracted the attention of geologists worldwide (Yakubchuk, 2004; Seltmann and Porter, 2005; Shen et al., in press). As a part of the CAOB, the Junggar basin is surrounded by mountain ranges and located in the central part of the Central Asia (Fig. 1). The west Junggar is economically important, not only as an important oilfield of China, but also as a potential target for Cu–Au–Pb–Zn exploration. Of its long evolutionary history, time from Carboniferous to Permian is a critical period, during which major metal ore deposits formed in the area. There is so far no consensus on its tectonic background during the period. Based on regional geology, and results of radiolarian and isotopic dating, the western Junggar was considered by some geologists as an intra oceanic arc (Xiao et al., 2006, 2009), which was comprised of several terranes amalgamate by the end of Carboniferous (Allen et al., 1989; Buckman and Aitchison, 2004), while others argue that the western Junggar had been in a post-

⁎ Corresponding author. Tel.: +86 20 8529 1780; fax: +86 20 8529 0130. E-mail address: [email protected] (C. Yuan).

collisional environment since the early Carboniferous because the widely occurred, isotopically juvenile I- and A-type granite were interpreted to be post-tectonic (Han et al., 2006; Fan et al., 2007). Obviously, more work is needed and a correct model not only sheds light on the tectonic evolution of the area, but also helps understanding the ore-forming processes. Mafic dikes commonly occur in extensional environment, and their formation implies not only the orientation of stress field but also occurrence of important geological events (Park et al., 1995; Ernst et al., 2001; Beutel et al., 2005; Zi et al., 2008). Doleritic to dioritic dikes extensively occur in the western Junggar (Fig. 1). The dikes consist of both high-Mg and low-Mg rocks, and were considered as post-collisional association (Qi, 1993; Li et al., 2004; Han et al., 2006). However, the available 40Ar/39Ar and K–Ar ages (ca. 240–270 Ma) are mainly for the dolerite and low-Mg dioritic dikes (Zhou et al., 2008; Xu et al., 2008), and systematic age dating and geochemical studies are absent for the high-Mg dioritic dikes. In this paper, we report the Ar–Ar age and geochemistry of Mg-rich dioritic dikes in the western Junggar, which, in combination with recent studies on other rock types of this area, would provide new constrains on the tectonic regime in the critical period. 2. Geological background The western Junggar is located in the junction of Siberia, Kazakhstan and Tarim blocks and comprises terranes of various origins. Several ophiolite with ages ranging from 504 ± 60 Ma (Kwon et al., 1989) to 332 ± 14 Ma (Xu et al., 2006) have been reported in this

1342-937X/$ – see front matter © 2009 International Association for Gondwana Research. Published by ELsevier B.V. All rights reserved. doi:10.1016/j.gr.2009.05.011

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Fig. 1. Geological map of the western Junggar, NW China. Age data for Karamay and Akebastao plutons are from Han et al. (2006). Scale of the dikes is exaggerated for the purpose of recognition.

area, demonstrating a complex accretionary history starting from Cambrian (Fig. 1). Strata in the region are dominated by Devonian to Carboniferous volcano-originated sediments, e.g. tuff, tuffite, tuffaceous sandstone, siltstone, chert, intercalated with mafic to intermediate lavas. The early Carboniferous strata contain abundant fossils (e.g. radiolarian) and trace fossils, indicating a deep-sea environment (Li and Jin, 1989; Jin and Li, 1999; Guo et al., 2002). In the Late Carboniferous, the sedimentary environment changed to marineterrigenous facies that remained until the Early Permian, when red molasse emerged in the area (Jin and Li, 1999). Granitic plutons commonly occur in the west Junggar and were mostly completed in the time period of 275–340 Ma (Han et al., 2006). Two pulses of granitic plutonism have been recognized (Han et al., 2006; Zhang et al., 2006). The older pulse, at 310 Ma or earlier, was predominated by magma with composition of diorite, quartz diorite and granodiorite, some of which show characteristics of adakite rocks and have good

potential of porphyry Cu–Au mineralization (Zhang et al., 2006; Tang et al., 2009). The younger one, mostly formed after 310 Ma, is dominated by monzogranite and alkaline granite and some possess features of A-type granite or charnockite (Zhang et al., 2004; Chen and Arakawa, 2005; Han et al., 2006; Su et al., 2006; Geng et al., submitted for publication). Geochemical studies have revealed that granites in the western Junggar were mainly derived from partial melting of juvenile materials without involvement of Precambrian basement (Coleman, 1989; Feng et al., 1989; Kwon et al., 1989; Carroll et al., 1990; Chen and Jahn, 2004; Chen and Arakawa, 2005; Geng et al., submitted for publication). Dikes in the West Junggar dip steeply and occur mainly in two trends, i.e. NW–SE (280°–310°) and NE–SW (210°– 230°) (Qi, 1993; Li et al., 2004). The dikes are highly variable in size and most are less than 1 km long and less than 20 m wide, although some dikes can be a few kilometers in length. Some dikes intruded the early Carboniferous rocks while others into granitic plutons (Gao et al.,

J. Yin et al. / Gondwana Research 17 (2010) 145–152

2006; Han et al., 2006). However, the dikes rarely crosscut each other, and available geochronological data show that there is a weak relationship between the age and trending of dikes, i.e. dikes with similar ages may occur with different trends whereas those with similar trend may be generated in different pulses (Xu et al., 2008; Zhou et al., 2008). Although no high-Mg andesite has been found in the western Junggar, high-Mg dioritic dikes were reported in the area and presumably considered to be coeval with the mafic dolerite dikes in the area (Qi, 1993; Li et al., 2004; Han et al., 2006). The dikes are generally undeformed and most underwent slight alteration. Dike samples for this study were collected in the Karamay area, where the dikes intruded the Early Carboniferous rocks and are dominated by diorite or dioritic porphyry, consisting mainly of plagioclase, hornblende, clinopyroxene, biotite and opaque minerals.

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elements and between 5 and 10% for trace elements, depending on the concentration level of specific element. 4. Analytical results 4.1. Ar–Ar dating results The 40Ar/39Ar dating result, the age spectrum and isochron plots are shown in Table 1 and Fig. 2, respectively. Totally 15 steps were conducted for sample KT-02-1, and except for five each incremental-heating stages, other 10 steps form a stable plateau with an age of 310 ± 3 Ma and give an isochron age of 321 ± 3 Ma (Fig. 2a,b). The Isochron plot gives a relatively low initial 40Ar/36Ar ratio (131.4± 37), which is significantly lower than the Nier ratio (295.5)(Nier,1950), suggesting the existence of excess argon probably captured from the wall-rock during the emplacement. To eliminate the influence of the excess argon, the data were recalculated based on the initial 40Ar/36Ar ratio (131.4) and a revised plateau age (321 ± 1 Ma) was obtained (Fig. 2c). This plateau age is in agreement with the isochron age (321 ± 3 Ma), probably represents the emplacement timing of the dikes.

3. Analytical methods Hand specimens were crushed in a stainless steel mortar, and fresh rock chips of 30–60 meshes were separated by hand picking under a binocular microscope and then cleaned with deionized water for 15 min in an ultrasonic bath. The rock chips were then irradiated in the 49-2 reactor in Beijing for 54 h with a standard DRA1 sanidine of 25.26 ± 0.07 Ma. Correction factors for interfering argon isotopes derived from Ca and K are as following: (39Ar/37Ar)Ca= 8.984 × 10− 4, (36Ar/37Ar) Ca= 2.673 × 10− 4 and (40Ar/39Ar)K = 5.97 × 10− 3. A sensitivity calibration using an HD-B1 biotite standard (3.364 × 10− 10 mol/g radiogenic 40 Ar content) yielded an average value of 1.64 × 10− 15 mol 40Ar per millivolt for GV5400 mass spectrometer. The J-value uncertainty of 0.15% (1σ) was propagated into the age calculations. Mass spectrometry analysis was performed at Guangzhou Institute of geochemistry, Chinese Academy of Sciences (GIGCAS), and details of experiment have been described in Qiu and Jiang (2007). The 40Ar/39Ar dating results are calculated using the ArArCALC software package presented by Koppers (2002). Both incremental-heating and total fusion techniques were performed during the analyses. Major oxides of selected whole-rock samples were determined with a Rigaku ZSX100e X-ray fluorescence spectrometer on fused glass disks at GIGCAS. Sample powders for trace element analyses were digested with mixed HNO3 + HF acid in steel-bomb coated Teflon beakers in order to assure the complete dissolution of refractory minerals (e.g. zircon). Trace elements were analyzed using a PerkinElmer Sciex ELAN 6000 ICP-MS, following the procedures described by Li (1997). The analytical precision was better than 5% for major

4.2. Major oxides The rock samples contain intermediate SiO2 (52.3–56.9 wt.%), TiO2 (0.76–0.97 wt.%) and relatively high Al2O3 (15.9–17.5 wt.%), showing composition of typical intermediate rocks. The rocks are characterized by high MgO contents (5.13–7.41 wt.%), with Mg# generally higher than 60 (61–70). The rock is relatively sodium-rich, with high Na2O/ K2O ratios (1.8–3.0). In the SiO2 versus K2O diagram, the rock samples exhibit mediate-K calc-alkaline characteristics (Fig. 3). 4.3. Trace elements The rock samples possess relatively high Cr (134–204 ppm) and Ni (59–141 ppm) contents, consistent with their high Mg# values. They have low high field strength elements (HFSE) (Nb = 1.29–4.11 ppm; Ta = 0.09–0.29 ppm) but remarkably high Sr (468–724 ppm) and Ba (316–676 ppm), characterized by relatively high Sr/Y (27–66), Ba/Th (112–728) but low K/Rb (382–546) ratios. In comparison with the primitive mantle, the dike samples display lower Nb/Ta (8–16) but higher Zr/Hf (38–44) ratios. All the samples show LREE-enriched characteristics, with (La/Yb)N ratios between 3.2 and 5.3 (Fig. 4). Although these samples exhibit relatively low heavy rare earth

Table 1 40 Ar–39Ar dating results for the dioritic dikes of the western Junggar. Stage no.

T(°C)

36

Ar(a)

37

Ar(ca)

38

Ar(cl)

KT-02-1( J = 0.0098140 ± 0.0000147) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

300 380 480 580 680 760 840 920 1020 1140 1260 1400 1600 1800 2200

0.000209 0.000151 0.000104 0.000063 0.000033 0.000026 0.000033 0.000038 0.000029 0.000051 0.000044 0.000039 0.000056 0.000057 0.000027

Note: Tp = plateau age; Ti = isochron age.

39

Ar(k)

40

Ar(r)

Apparent age

40

(Ma), ±2 s

(%)

(%)

27.72 67.39 83.02 91.92 95.12 96.07 95.51 94.06 92.99 93.52 91.28 91.40 89.88 81.94 85.45

2.38 5.26 7.49 9.80 8.64 8.46 9.58 8.19 5.20 10.00 6.37 5.61 6.97 3.79 2.26

Ar(r)

39

Ar(k)

Tp = 321 ± 1 Ma; Ti = 321 ± 1 Ma 0.004428 0.010882 0.016846 0.026208 0.023734 0.025381 0.030542 0.030447 0.023593 0.047227 0.032593 0.023974 0.028066 0.023912 0.014893

0.000011 0.000020 0.000032 0.000044 0.000039 0.000040 0.000046 0.000041 0.000028 0.000058 0.000036 0.000031 0.000035 0.000018 0.000012

0.002712 0.005993 0.008527 0.011153 0.009835 0.009632 0.010906 0.009327 0.005918 0.011385 0.007252 0.006383 0.007937 0.004309 0.002571

0.023702 0.092084 0.151082 0.212667 0.189738 0.187255 0.211143 0.178004 0.112177 0.216678 0.137037 0.121513 0.147632 0.076755 0.046931

148.47 253.38 289.24 309.47 312.82 315.01 313.84 309.71 307.78 308.92 306.92 309.01 302.48 290.63 297.29

± 8.06 ± 2.91 ± 1.95 ± 1.81 ± 1.64 ± 1.74 ± 1.64 ± 2.04 ± 2.96 ± 2.15 ± 2.25 ± 2.77 ± 1.67 ± 3.08 ± 4.60

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Fig. 4. Chondrite normalized rare earth element pattern diagram for the high-Mg dioritic dikes. Chondrite data are from Taylor and McLennan (1985).

plagioclase. In primitive mantle normalized spider diagram, the dike samples show remarkable enrichment of LILE relative to HFSE and REE, with negative Nb–Ta and positive Pb anomalies (Fig. 5), reflecting typical characteristics of subduction-related magmas.

5. Discussion 5.1. Affinity to high-Mg andesite

Fig. 2. 40Ar/39Ar dating results for the high-Mg dioritic dikes of the western Junggar. (a) Ar– Ar age spectrum; (b) Isochron age diagram; (c) revised age plateau.

element (HREE) contents (Yb b 1.72 ppm), they exhibit only intermediately fractionated HREE ((Gd/Yb)N = 1.5–1.6). No significant negative Eu anomaly has been observed in these samples, instead, their δEu⁎ values (0.92–1.24) indicate an insignificant fractionation of

Fig. 3. Classification diagrams for the high-Mg dioritic dikes, western Junggar, K2O versus SiO2 diagram are from Gill, 1981.

The relatively high Mg# values of the dike samples make them very akin to high-Mg andesite (HMA). In the SiO2 versus MgO diagram designed for distinguishing HMA from normal andesite (McCarro and Smellie, 1998), all the dike samples fall in the HMA field (Fig. 6). HMA is a rock family consisting of several types of andesitic rocks with unique geochemical characteristics, e.g. boninite, bajaite, high-Mg adakite and sanukitoids (Rogers et al., 1985; Smithies and Champion, 2000). Boninite is usually produced in forearc environment and characterized by high SiO2 (N52 wt.%), MgO (N8 wt.%) and very low TiO2 (b0.5 wt.%), consistent with high temperature partial melting of depleted subarc mantle (Hickey and Frey, 1982; Crawford et al., 1989). Bajaite contains unusually high Sr (N1000 ppm), Ba (N1000 ppm) and have high K/Rb ratios (N1000), and is produced by disequilibrium interaction between slab-derived Si-rich melt and mantle peridotite (Rogers et al., 1985; Saunders et al., 1987). Adakites are characterized by high SiO2 (N56 wt.%), Al2O3 (N15 wt.%), with high Sr (N400 ppm) and very low HREE (Ybb 1.9 ppm), resulting from partial melting of young and/or hot subducted slab or overthickened lower crust (Defant and Drummond, 1990; Atherton and Petford, 1993). High-Mg adakite (Mg#N 50) formed from mixing of slabderived adakitic melt with mantle wedge (Shirey and Hanson, 1984; Smithies and Champion, 2000). Sanukitoids are common in Archean. They share similarities with the low-SiO2 adakites and are characterized

Fig. 5. Primitive mantle normalized spider diagram for the high-Mg dioritic dikes. Primitive mantle data are from Sun and McDonough (1989); Sanukite data of Setouchi volcanic belt are from Tatsumi et al. (2003).

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which may be ascribed to either accumulation or depressed fractionation of plagioclase. Because no accumulation texture has been observed in thin sections, the possibility of this process is unlikely. Due to dehydration of subducting slab, arc magmas may have higher H2O fugacity than delamination-related magmas. Water is an important factor in controlling crystallization sequence of minerals, which would bring an impact on fractionation of elements. As the major host mineral of Eu, crystallization of plagioclase will be depressed and postponed to later stage under high water fugacity (Müntener et al., 2001; Grove et al., 2002), giving rise to the absence of negative or presence of positive Eu anomalies. The dike samples contain hornblende and biotite, indicating high water fugacity in the magma. Therefore, the weak positive Eu anomalies of the high-Mg dikes are consistent with delayed crystallization of plagioclase, implying a fluid-assisted metasomatised mantle. In addition, remarked Nb–Ta depletion, relative enrichment of LILE and LREE, relative high Ba/La (27–124), Ba/Th (112–754) and low Th/Yb (0.73– Fig. 6. MgO versus SiO2 correlation diagram for distinguishing high-Mg andesite from normal andesite (after McCarro and Smellie, 1998).

by strong LILE and LREE enrichments, low HREE contents, high Cr and Ni contents and high Mg (≫62) (Martin et al., 2005; Tiepolo and Tribuzio, 2008), which are quite analogous to those of Cenozoic sanukite in the Setouchi Volcanic Belt, although Archean sanukitoids are more Sr-, Baand LREE-enriched (Sr N 500 ppm; Ba N 500 ppm; CeN N 100; CeN/ YbN N 10) (Defant and Drummond, 1990). Sanukite is widely believed to be generated through equilibrium reaction of a mantle peridotite with silicic liquids derived from the partial melting of the subducted sediments (Shimoda et al., 1998), and closely related to subduction of young and hot oceanic slab (Rogers et al., 1985; Kamei et al., 2004). The dikes samples contain intermediate TiO2 (N0.7 wt.%) and their MgO contents are lower than that of typical boninitic rocks, suggesting the dikes were not generate from partial melting of refractory mantle wedge. Although their relatively high Sr/Y ratios and low HREE contents are very akin to adakitic rocks (Defant and Drummond, 1990), their relatively low SiO2 (b57 wt.%), La/Yb ratios (b8) and high Mg# values preclude their origination from partial melting of subducted slab. Likewise, the relatively low Sr (468–724 ppm), Ba (316–676 ppm) contents of the dikes samples make them quite different from bajaite. Instead, as shown in Fig. 5, their trace element characteristics are quite similar to those of sanukite from Setouchi volcanic belt. 5.2. Petrogenesis The relatively high Mg# values suggest that the dikes were most likely derived from partial melting of mantle source. There are different opinions for the genesis of sanukitoids. Some workers considered sanukitoids as partial melt of a metasomatised mantle source (e.g. Stern and Hanson, 1991; Smithies and Champion, 2000; Zhao and Zhou, 2007), while others favor an interaction regime between mantle and melt from subducting slab or delaminated lower crust (Rapp et al., 1999; Smithies et al., 2004, 2007). Both of the above hypotheses require a metasomatised mantle source. In subductionrelated environment, reagent is mainly slab-derived fluid or melt, whereas in the case of an over-thickened crust, the reagent may come from delaminated lower crust. Studies of the lower Carboniferous turbidite and fossils clearly revealed a deep-sea environment in the western Junggar (Li and Jin, 1989), and the Ar–Ar dating result (321 ± 1 Ma) shows that the high-Mg dioritic dike formed prior to the Early Permian red molasse (Jin and Li, 1999). The above lines of evidence strongly indicate a normal crustal thickness in the early Carboniferous, therefore an over-thickened crust seems unlikely. The dike samples have positive Eu anomalies and high Al2O3 contents (Table 2, Fig. 4),

Table 2 Major oxide and trace element compositions of the dioritic dikes of the western Junggar. Sample

TCL-02-2

TCL-02-3

TCL-02-4

TCL-10-2

KT-01-1

KT-01-2

KT-01-3

SiO2 TiO2 Al2O3 Fe2O3T MnO MgO CaO Na2O K2O P2O5 LOI Total Mg# Na2O/K2O Cr Ni Co Sc V Rb Sr Ba Y Zr Nb Hf Ta Pb Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu δEu⁎ (La/Yb)N (Gd/Yb)N Sr/Y La/Nb Ba/Th K/Rb Th/Yb

52.3 0.79 16.8 7.44 0.13 7.41 5.53 3.71 1.39 0.12 4.34 99.9 70 2.7 204 141 32.2 21.8 177 23.8 632 534 11.7 63.2 1.34 1.59 0.11 3.87 0.85 0.36 5.12 12.0 1.73 7.82 2.09 0.86 2.17 0.36 2.06 0.45 1.18 0.18 1.14 0.17 1.24 3.2 1.6 54 3.8 628 506 0.75

54.5 0.84 17.5 7.05 0.12 5.77 4.91 4.28 1.45 0.13 3.35 99.8 66 3.0 200 133 31.9 21.4 179 21.9 717 613 12.5 65.5 1.29 1.71 0.09 19.8 0.84 0.39 5.31 12.6 1.78 8.29 2.15 0.78 2.28 0.39 2.24 0.49 1.26 0.18 1.15 0.17 1.01 3.2 1.6 57 4.1 726 546 0.73

53.0 0.81 16.9 7.41 0.12 7.17 5.26 4.01 1.39 0.13 3.50 99.7 69 2.9 154 86.0 25.5 21.2 180 23.7 627 676 13.0 70.9 1.53 1.78 0.19 3.40 0. 90 0.61 5.46 12.8 1.86 8.70 2.22 0.75 2.32 0.41 2.27 0.49 1.39 0.20 1.22 0.18 1.07 3.3 1.6 48 3.6 754 525 0.74

56.9 0.76 16.9 6.59 0.11 5.15 6.52 3.47 1.27 0.14 2.02 99.9 65 2.7 135 79.0 25 18.0 150 21.0 724 426 10.9 75.4 1.43 1.92 0.11 5.11 0.93 0.36 6.16 13.9 1.95 9.06 2.20 0.82 2.03 0.35 1.96 0.43 1.15 0.16 1.04 0.15 1.18 4.2 1.6 66 4.3 460 513 0.89

54.9 0.96 16.0 7.89 0.12 5.24 6.82 4.04 1.60 0.18 2.03 99.8 61 2.5 141 63.0 26.9 21.4 183 35.6 468 316 17.2 148 4.11 3.46 0.27 3.72 2.81 0.84 11.9 26.5 3.52 14.8 3.34 1.00 3.20 0.55 2.99 0.68 1.77 0.25 1.62 0.24 0.94 5.3 1.6 27 2.9 112 382 1.73

55.6 0.97 15.9 7.45 0.13 5.48 5.28 4.10 2.30 0.19 2.04 99.4 63 1.8 138 60.0 26.8 21.6 180 49.6 541 629 17.0 147 4.07 3.33 0.28 2.77 2.68 0.89 11.7 26.6 3.54 15.0 3.41 0.99 3.07 0.54 2.90 0.64 1.78 0.25 1.72 0.25 0.94 4.9 1.5 32 2.9 235 396 1.56

55.9 0.95 16.0 7.63 0.12 5.13 6.11 3.74 2.04 0.18 2.02 99.9 61 1.8 134 59.0 26.1 20.7 178 41.5 551 409 16.9 146 4.06 3.44 0.29 3.93 2.77 0.84 11.8 26.5 3.54 15.3 3.50 0.99 3.06 0.52 2.95 0.64 1.71 0.24 1.68 0.24 0.92 5.0 1.5 33 2.9 148 417 1.65

Mg : 100 × Mg/(Mg + total Fe).

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1.73) ratios imply that the reagent component was dominated by slabderived fluid, rather than sediment-derived melt. The above evidence, along with the relatively low HREE and relatively high water fugacity, indicates that the Mg-rich dikes were derived from hydrous partial melting of a metasomatised mantle. This is consistent with the experimental result that sanukite can be produced by partial melting of harzburgitic or lherzolitic mantle with 7–8 wt.% H2O under 1070– 1110 °C and 1.0–1.1 Gpa (Tatsumi and Ishizaka, 1982; Tatsumi, 1982, 2001). 5.3. Tectonic implications Carboniferous is a critical period for the tectonic transition and metal mineralization of the western Junggar. However, whether the western Junggar was under a subduction-dominated regime (Zhang et al., 2006; Xiao et al., 2009), or in a post-collisional environment (Chen and Arakawa, 2005; Han et al., 2006; Su et al., 2006) is still in hot debate. Some authors considered that the post-collisional phase of the western Junggar started ca. 340 Ma ago (Han et al., 1997, 2006; Chen and Jahn, 2004). However, granitic intrusions before 310 Ma are mainly I-Type (Yuan et al., 2006; Zhang et al., 2006), and zircon U–Pb dating results indicate that the A-type granite in the western Junggar generally formed around ca. 300 (296–305 Ma) (Su et al., 2006). Because formation of the granite requires a high temperature regime, the granite was believed to be resulted from partial melting of juvenile basaltic lower crust or lithospheric mantle metasomatised by slabderived fluid, induced by an upwelling mantle (Chen and Arakawa, 2005). Although many authors considered mantle upwelling as the trigger of partial melting (e.g. Chen and Jahn, 2004; Chen and Arakawa, 2005), what caused the mantle upwelling still remains unclear. Delamination of over-thickened lithosphere is usually considered to be an important mechanism for mantle upwelling, especially for post-collisional scenarios (e.g. Duggen et al., 2005), but it is not the only regime inducing mantle upwelling and partial melting of the lower crust. When oceanic ridge is subducted or there is breakoff of subducting slab, with opening of a slab window, asthenospheric mantle will upwell from deep and gives rise to decompressional melting to form mafic melt, which interacts with the lower crust to generate granitoids. In addition, edges of slab window would partially melt and/or dehydrate under high temperature to produce granodiorite and adakite (Yogodzinski et al., 2001). High-Mg andesite mainly forms by partial melting of metasomatised mantle in either subduction-related setting or post-collisional environment (Smithies and Champion, 2000; Tatsumi et al., 2003; Zhao and Zhou, 2007). The difference for the two scenarios is that high-Mg andesite in relation to subduction usually accompanies slab-derived adakite, whereas those in post-collisional environment is temporally behind the slab-derived adakite. Geological investigation for the Carboniferous strata revealed that the western Junggar was still in marine-facies environment by the end of Carboniferous (Jin and Li, 1999), thus a collision-induced crust-thickening seems unlikely in the early Carboniferous. Dikes in the western Junggar were commonly considered to be temporally behind the post-collisional granite (Han et al., 2006). However, our 40 Ar/39Ar age (321 ± 1 Ma) of the Mg-rich dioritic dikes revealed an earlier pulse of dike intrusion in the area, suggesting that the high-Mg dikes might not be generated in a post-collisional environment. A latest study has revealed wide occurrence of adakitic magmatism and porphyry copper deposit in the western Junggar during the period of 310–330 Ma (Zhang et al., 2006; Tang et al., 2009). The association of adakite and high-Mg andesite is widely ascribed to subduction of hot and young oceanic crust (e.g. Katz et al., 2004; Viruete et al., 2007; Manya et al., 2007). Considering the coeval adakitic and sanukitic magmatism in the western Junggar and their close relationship with subduction, it is logical to infer that it was the adakitic melt metasomatised the subarc mantle that subsequently partial melted to generate the high-Mg dioritic dikes. The adakite-high-Mg diorite

association therefore reflects subduction of hot and young oceanic slab and is probably responsible for the Cu–Ag mineralization of the western Junggar. 6. Conclusions (1) The dioritic dikes in the western Junggar are characterized by high Mg# values (N60), and high Ni and Cr contents, resembling the sanukite from Setouchi volcanic belt of Japan. (2) The high-Mg dioritic dikes formed from partial melting of the mantle metasomatised by slab-derived fluid/melt, and their 40 Ar/39Ar age (321 ± 1 Ma) revealed a pulse of dikes earlier than the A-type granite in the western Junggar. (3) The high-Mg dioritic dikes (sanukitoid) and coeval adakite may reflect a hot subduction regime, which gave rise to widely occurred Cu–Au mineralization in the area during the Late Carboniferous. Acknowledgements We thank Zhang Qi and Wang Qiang for their constructive suggestions. We are grateful to two anonymous reviewers whose critical reviews are very helpful in improving the manuscript. This research was jointly supported by research grants from the National Basic Research Program of China (973 Program) 2007CB411308, NSFC Projects 40721063, 40421303, 40772130, Hong Kong RGC (HKU 7043079), HKU CRCG and the CAS/SAFEA International Partnership Program for Creative Research Teams. References Allen, M.B., Zhang, C., Zhai, M., Allen, B., Saunders, A.D., Moon, C.J., 1989. Crustal accretion and mineralization in western Junggar, Xinjiang Province, northwest China. Transaction of Institute of Mineralogy and Metallogeny, Section B: Applied Earth Science B147–B149. Atherton, M.P., Petford, N., 1993. Generation of sodium-rich magmas from newly underplated basaltic crust. Nature 362, 144–146. Beutel, E.K., Nomade, S., Fronabarger, A.K., Renne, P.R., 2005. Pangea's complex breakup: a new rapidly changing stress field model. Earth and Planetary Science Letters 236, 471–485. Buckman, S., Aitchison, J.C., 2004. Tectonic evolution of Paleozoic terranes in West Junggar, Xinjiang, NW China. In: Malpas, J., Fletcher, C.J.N., Aitchison, J.C. (Eds.), Aspects of the Tectonic Evolution of China. Geological Society of London, Special Publication, vol. 226, pp. 101–129. Buslov, M.M., Fujiwara, Y., Iwata, K., Semakov, N.N., 2004. Late Paleozoic–Early Mesozoic geodynamics of Central Asia. Gondwana Research 7, 791–808. Carroll, A.R., Liang, Y., Graham, S.A., Xiao, X.C., Hendrix, M.S., Chu, J.C., McKnight, C.L., 1990. Junggar basin, northwest China: trapped Late Paleozoic ocean. Tectonophysics 186, 1–14. Chen, B., Arakawa, Y., 2005. Elemental and Nd–Sr isotopic geochemistry of granitoids from the West Junggar foldbelt (NW China), with implications for Phanerozoic continental growth. Geochimica et Cosmochimica Acta 69, 1307–1320. Chen, B., Jahn, B.M., 2004. Genesis of post-collisional granitoids and basement nature of the Junggar Terrane, NW China: Nd–Sr isotope and trace element evidence. Journal of Asian Earth Science 23, 691–703. Coleman, R.G., 1989. Continental growth of Northwest China. Tectonics 8, 621–635. Crawford, A.J., Falloon, T.J., Green, D.H., 1989. Classification Petrogenesis and Tectonic Setting of Boninites. Crawford A.J., Boninites. Academic Division of Unwin Hyaman, Ltd, London, pp. 1–49. Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arc magmas by melting of young subduction lithosphere. Nature 662–665. Duggen, S., Hoernle, K., Bogaard, P.V.D., Garbe-Schonberg, D., 2005. Post-collisional transition from subduction- to intraplate-type magmatism in the westernmost Mediterranean: evidence for continental-edge delamination of subcontinental lithosphere. Journal of Petrology 46, 1155–1201. Ernst, R.E., Grosfils, E.B., Mege, D., 2001. Giant dike swarms: Earth, Venus, and Mars. Annual Review of Earth and Planetary Sciences 29, 489–534. Fan, Y., Zhou, T.F., Yuan, F., Tan, L.G., Cooke, D., Mefre, S., Yang, W.P., He, L.X., 2007. LAICPMS zircon age of Tasite pluton in Sawuer region of west Junggar, Xinjiang. Acta Petrologica Sinica 23, 1901–1908. Feng, Y., Coleman, R.G., Tilton, G., Xiao, X., 1989. Tectonic evolution of the West Junggar region, Xinjiang, China. Tectonics 8, 729–752. Gao, S.L., He, Z.L., Zhou, Z.Y., 2006. Geochemical characteristics of the karamay granitoids and their significance in West Junggar, Xinjiang. Xinjiang Geology 24, 125–130. Geng, H.Y., Sun, M., Yuan, C., Xiao, W.J., Xian, W.S., Zhao, G.C., Zhang, L.F., Wong, K., Wu, F.Y., submitted for publication. Geochemical, Sr–Nd and zircon U–Pb–Hf isotopic studies of

J. Yin et al. / Gondwana Research 17 (2010) 145–152 Late Carboniferous magmatism in the West Junggar, Xinjiang: implications for ridge subduction. Chemical Geology (under review). Gill, T.B., 1981. Orogenic Andesite and Plate Tectonics. Springer-Verlag, Berlin, p. 390. Grove, T.L., Parman, S.W., Bowring, S.A., Price, R.C., Baker, M.B., 2002. The role of H2Orich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region, N California. Contributions to Mineralogy and Petrology 142, 375–396. Guo, H.L., Zhu, R.K., Shao, L.Y., He, D.B., Luo, Z.G., 2002. Lithofacies palaeogeography of the Carboniferous in Northwest China. Journal of Palaeogeography 4, 25–35. Han, B.F., Ji, J.Q., Song, B., Chen, L.H., Zhang, L., 2006. Late Paleozoic vertical growth of continental crust around the Junggar Basin, Xinjiang, China(Part I): Timing of postcollisional plutonism. Acta Petrologica Sinica 22, 1077–1086. Han, B.F., Wang, S.G., Jahn, B.M., Hong, D.W., Kagami, H., Sun, Y.L., 1997. Depleted-mantle source for the Unlungur River A-type granites from North Xinjiang, China: geochemistry and Nd–Sr isotopic evidence and implications for Phanerozoic crustal growth. Chemical Geology 138, 135–159. Hickey, R.L., Frey, F.A., 1982. Geochemical characteristics of boninite series volcanics: implications for their source. Geochimica et Cosmochimica Acta 46, 2099–2115. Jahn, B.M., 2004. The central Asian orogenic belt and growth of the continental crust in the phanerozoic. Geological Society, London, Special Publications 226, 73–100. Jin, H.J., Li, Y.C., 1999. Carboniferous biogenic sedimentary structures on the northwestern margin of Jungar basin. Chinese Science Bulletin 44, 368–372. Kamei, A., Owada, M., Nagao, T., Shiraki, K., 2004. High-Mg diorites derived from sanukitic HMA magmas, Kyushu Island, southwest Japan arc: evidence from clinopyroxene and whole rock compositions. Lithos 75, 359–371. Katz, O., Beyth, M., Miller, N., Stern, R., Avigad, D., Basu, A., Anbar, A., 2004. A Late Neoproterozoic (similar to 630 Ma) high-magnesium andesite suite from southern Israel: implications for the consolidation of Gondwanaland. Earth and Planetary Science Letters 218, 475–490. Koppers, A.A.P., 2002. ArArCALC—software for 40Ar/39Ar age calculations. Computers and Geosciences 28, 605–619. Kwon, S.T., Tilton, G.R., Coleman, R.G., Feng, Y., 1989. Isotopic studies bearing on the tectonics of the west Junggar region, Xinjiang, China. Tectonics 8, 719–727. Li, J.Y., Jin, H.J., 1989. The trace fossils discovery and its environment significance in Carboniferous turbidite series, the northwest border of Zhunga'er basin, Xinjiang. Scientia Geologica Sinica 63, 9–15. Li, X.H., 1997. Geochemistry of the Longsheng Ophiolite from the southern margin of Yangtze Craton, SE China. Geochemical Journal 31, 323–337. Li, X.Z., Han, B.F., Ji, J.Q., Li, Z.H., Liu, Z.Q., Yang, B., 2004. Geology, geochemistry and K–Ar ages of the Karamay basic-intermediate dike swarm from Xinjiang, China. Geochimica 33, 574–584. Manya, S., Maboko, M.A.H., Nakamura, E., 2007. The geochemistry of high-Mg andesite and associated adakitic rocks in the Musoma-Mara Greenstone Belt, northern Tanzania: possible evidence for Neoarchaean ridge subduction? Precambrian Research 159, 241–259. Martin, H., Smithies, R.H., Rapp, R., Moyen, J.F., Champion, D., 2005. An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos 79, 1–24. McCarro, J.J., Smellie, J.L., 1998. Tectonic implications of fore-arc magnesian and generation of high-magnesian andesites: Alexander island, Antarctica. Journal of the Geological Society, London 155, 269–280. Müntener, O., Kelemen, P.B., Grove, T.L., 2001. The role of H2O during crystallization of primitive arc magmas under uppermost mantle conditions and genesis of igneous pyroxenites: an experimental study. Contributions to Mineralogy and Petrology 141, 643–658. Nier, A.O., 1950. A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon and potassium. Physical Review 77, 789–793. Park, J.K., Buchan, K.L., Harlan, S.S., 1995. A proposed giant radiating dyke swarm fragmented by the separation of Laurentia and Australia based on paleomagnetism of ca. 780 Ma mafic intrusions in western North-America. Earth and Planetary Science Letters 132, 129–139. Qi, J.Y., 1993. Geology and genesis of vein rock group in Western Zhunggar, Xinjiang. Acta Petrologica Sinica 9, 288–299. Qiu, H.N., Jiang, Y.D., 2007. Sphalerite 40Ar/39Ar progressive crushing and stepwise heating techniques. Earth Planetary Science Letters 256, 224–232. Rapp, R.P., Shimizu, N., Norman, M.D., Applegate, G.S., 1999. Reaction between slabderived melts and peridotite in the mantle wedge: experimental constraints at 3.8 Gpa. Chemical Geology 160, 335–356. Rogers, G., Saunders, A.D., Terrell, D.J., Verma, S.P., Marriner, G.F., 1985. Geochemistry of Holocene volcanic rocks associated with ridge subduction in Baja California, Mexico. Nature 315, 389–392. Santosh, M., Maruyama, S., Sato, K., 2009. Anatomy of a Cambrian suture zone in southern India: Pacific type orogeny in southern India? Gondwana Research 16, 321–341. Saunders, A.D., Rogers, G., Marriner, G.F., Terrell, D.J., Verma, S.P., 1987. Geochemistry of Cenozoic volcanic rocks, Baja California, Mexico: implications for the petrogenesis of post-subduction magmas. Journal of Volcanology and Geothermal Research 32, 223–245. Seltmann, R., Porter, T.M., 2005. The porphyry Cu–Au/Mo deposits of central Eurasia I. Tectonic, geologic and metallogenic setting, and significant deposits. In: Porter, T.M. (Ed.), Super Porphyry Copper and Gold Deposits: A Global Perspective, vol. 2. PGC Publishing, Adelaide, pp. 467–512. Sengör, A.M.C., 1990. Plate-tectonics and orogenic research after 25 years—a Tethyan perspective. Earth-Science Reviews 27, 1–201. Sengör, A.M.C., Natal' in, B.A., Burtman, V.S., 1993. Evolution of the Altaid tectonic collage and Palaeozoic crustal growth in Eurasia. Nature 364, 299–306.

151

Shen, P., Shen, Y.C., Liu, T.B., Meng, L., Dai, H.W., Yang, Y.H., 2009. Geochemical signature of porphyries in the Baogutu porphyry copper belt, western Junggar, NW China. Gondwana Research 16. doi:10.1016/j.gr.2009.04.004. Shimoda, G., Tatsumi, Y., Nohda, S., Ishizaka, K., Jahn, B.M., 1998. Setouchi high-Mg andesites revisited: geochemical evidence for melting of subducted sediments. Earth and Planetary Science Letters 160, 479–492. Shirey, S.B., Harson, G.N., 1984. Mantle-derived Archean monzodiorites and trachyandesites. Nature 310, 222–224. Smithies, R.H., Champion, D.C., 2000. The Archaean high-Mg diorite suite: links to tonalite–trondhjemite–granodiorite magmatism and implications for Early Archaean crustal growth. Journal of Petrology 41, 1653–1671. Smithies, R.H., Champion, D.C., Sun, S.S., 2004. Evidence for early LREE-enriched mantle source regions: diverse magmas from the 3.0 Ga Mallina basin, Pilbara craton, NW Australia. Journal of Petrology 45, 1515–1537. Smithies, R.H., Van-Kranendonk, M.J., Champion, D.C., 2007. The Mesoarchean emergence of modern-style subduction. Gondwana Research 11, 50–68. Stern, R., Hanson, G., 1991. Archean high-Mg franodiorites: a derivative of light rare earth element enriched monzodiorite of mantle origin. Journal of Petrology 32, 201–238. Su, Y.P., Tang, H.F., Hou, G.S., Liu, C.Q., 2006. Geochemistry of aluminous A-type granites along Darabut tectonic belt in West Junggar, Xinjiang. Geochemica 35, 55–67. Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalt: implications for mantle compositions and processes. Geological Society, London, Special Publications 42, 313–345. Tang, G.J., Wang, Q., Chen, H.Q., Zhao, Z.H., Wyman, D.A., Jia, X.H., Jiang, Z.Q., 2009. Geochronology and geochemistry of the ore-bearing porphyries in the Baogutu area (western Junggar): Petrogenesis and their implications for tectonics and Cu– Au mineralization. Scientia geologica sinica 34, 56–74. Tatsumi, Y., 1982. Origin of high-magnesian andesites in the Setouchi volcanic belt, southwest Japan: II, Melting phase relations at high pressures. Earth Planetary Science Letters 60, 305–317. Tatsumi, Y., 2001. Geochemical modeling of partial melting of subducting sediments and subsequent melt–mantle interaction: generation of high-Mg andesites in the Setouehi volcanic belt, Southwest Japan. Geology 29, 323–326. Tatsumi, Y., Ishizaka, K., 1982. Origin of high-magnesian andesites in the Setouchi volcanic belt, southwest Japan: I. Petrographical and chemical characteristics. Earth Planetary Science Letters 60, 293–304. Tatsumi, Y., Shukuno, H., Sato, K., Shibata, T., Yoshikawa, M., 2003. The petrology and geochemistry of high-magnesium andesites at the western tip of the Setouchi volcanic belt, SW Japan. Journal of Petrology 44, 1561–1578. Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, pp. 1–312. Tiepolo, M., Tribuzio, R., 2008. Petrology and U–Pb zircon geochronology of amphibolerich cumulates with sanukitic affinity from Husky Ridge (Northern Victoria Land, Antarctica): insights into the role of amphibole in the petrogenesis of subductionrelated magmas. Journal of Petrology 49, 937–970. Viruete, J.E., Contreras, F., Stein, G., Urien, P., Joubert, M., Perez-Estaun, A., Friedman, R., Ullrich, T., 2007. Magmatic relationships and ages between adakites, magnesian andesites and Nb-enriched basalt–andesites from Hispaniola: record of a major change in the Caribbean island arc magma sources. Lithos 99, 151–177. Windley, B.F., Alexeiev, D., Xiao, W.J., Kröner, A., Badarch, G., 2007. Tectonic models for accretion of the Central Asian Orogenic Belt. Journal of the Geological Society 164, 31–47. Xiao, W., Kusky, T., 2009. Geodynamic processes and metallogenesis of the Central Asian and related orogenic belts: Introduction. Gondwana Research 16, 167–169. Xiao, W.J., Han, C.M., Yuan, C., Chen, H.L., Sun, M., Lin, S.F., Li, Z.L., Mao, Q.G., Zhang, J.E., Sun, S., Li, J.L., 2006. Unique Carboniferous–Permian tectonic-metallogenic framework of Northern Xinjiang (NW China): constraints for the tectonics of the southern Paleoasian Domain. Acta Petrologica Sinica 22, 1062–1076. Xiao, W.J., Windley, B.F., Yuan, C., Sun, M., Han, C.M., Lin, S.F., Chen, H.L., Yan, Q.R., Liu, D.Y., Qin, K.Z., Li, J.L., Sun, S., 2009. Paleozoic multiple subduction– accretion processes of the southern Altaids. American Journal of Science 309, 221–270. Xu, Q.Q., Ji, J.Q., Han, B.F., Zhu, M.F., Chu, Z.Y., Zhou, J., 2008. Petrology, geochemistry and geochronology of the intermediate to mafic dykes in northern Xinjiang since Late Paleozoic. Acta Petrologica Sinica 24, 977–996 (in Chinese with English abstract). Xu, X.Y., Xia, L.Q., Ma, Z.P., Xia, Z.C., Li, X.M., Wang, L.S., 2006. SHRIMP zircon U–Pb geochronology of the plagiogranites from Bayingou ophiolite in North Tianshan Mountains and the petrogenesis of the ophiolite. Acta Petrologica Sinica Acta Petrologica Sinica 22, 83–94 (in Chinese with English abstract). Yakubchuk, A., 2004. Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model. Journal of Asian Earth Science 23, 761–779. Yogodzinski, G.M., Lees, J.M., Churikova, T.G., Dorendorf, F., Woerner, G., Volynets, O.N., 2001. Geochemical evidence for the melting of subducting oceanic lithosphere at plate edges. Nature 409, 500–504. Yuan, F., Zhou, T.F., Tan, L.G., Fan, Y., Yang, W.P., He, L.X., Yue, S.C., 2006. Isotopic ages of the I type granitesin west Junggar Sawuer region. Acta Petrologica Sinica 22, 1238–1248. Zhang, L.F., Xian, W.S., Sun, M., 2004. Petrogenesis of charnockite from western Junggar, Xinjiang, China. Xinjiang Geology 22, 36–42. Zhang, L.C., Wan, B., Jiao, X.J., Zhang, R., 2006. Characteristics and geological significance of adakitic rocks in copper-bearing porphyry in Baogutu, western Junggar. Geology in China 33, 626–631. Zhang, Z., Zhou, G., Kusky, T.M., Yan, S., Chen, B., Zhao, L, 2009. Late Paleozoic volcanic record of the Eastern Junggar terrane, Xinjiang, Northwestern China: Major and

152

J. Yin et al. / Gondwana Research 17 (2010) 145–152

trace element characteristics, Sr–Nd isotopic systematics and implications for tectonic evolution. Gondwana Research 16, 201–215. Zhao, J.H., Zhou, M.F., 2007. Geochemistry of Neoproterozoic mafic intrusions in the Panzhihua district (Sichuan Province, SW China): implications for subduction-related metasomatism in the upper mantle. Precambrian Research 152, 27–47.

Zi, J.W., Fan, W.M., Wang, Y.J., Peng, T.P., Guo, F., 2008. Geochemistry and petrogenesis of the Permian mafic dykes in the Panxi region, SW China. Gondwana Research 14, 368–382. Zhou, J., Ji, J.Q., Han, B.F., Ma, F., Gong, J.F., Xu, Q.Q., Guo, Z.J., 2008. 40Ar/39Ar Geochronology of mafic dykes in north Xinjiang. Acta Petrologica Sinica 24, 997–1010 (in Chinese with English abstract).