Underplating-related adakites in Xinjiang Tianshan, China

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Lithos 102 (2008) 374 – 391 www.elsevier.com/locate/lithos

Underplating-related adakites in Xinjiang Tianshan, China Z.H. Zhao a,⁎, X.L. Xiong a , Q. Wang a , D.A. Wyman b , Z.W Bao a , Z.H. Bai a , Y.L. Qiao a b

a Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China School of Geosciences, Division of Geology and Geophysics, The University of Sydney, NSW 2006, Australia

Received 11 October 2006; accepted 15 June 2007 Available online 28 June 2007

Abstract Intermediate to acidic porphyries and dacites in the Awulale and Sanchakou area of the Xinjiang Tianshan region, China, conform to the definition of high-SiO2 adakite (HSA). These volcanic or subvolcanic rocks are characterized by high Na2O (Na2O N K2O), high Sr/Y (51–327), and strong depletions in HREE and HFSE (Nb, Ta, and Ti). Despite these characteristics, they are distinct from a suite of adakites in the same area that are inferred to be derived from partial melting of subducted slab. Differences between the two suites are evident mainly in their rock association, isotopic ages and compositions of Sr and Nd isotopes. The subducted slab-related adakites are associated with common island arc rocks, niobium-enriched basalt (NEB) and high-Mg andesite (HMA), formed in the late Carboniferous (320–334 Ma) and are characterized by relative high ɛNd(t) (+ 3.4 − + 11.6) and low (87Sr/86Sr)i ratios(0.7032–0.7063). In contrast, the Awulale and Sanchakou adakites were generated during the middle to late Permian (260–278 Ma), possess a wide range of low ɛNd(t) (+ 0.75 − + 5.69), a greater range of (87Sr/86Sr)i (0.7039– 0.7054) and have young Nd model ages (T2DM 472–699 Ma). In addition, their Mg# (35–56) and compatible element contents (MgO = 0.93–2.22 wt.%; Cr = 4.95–16.41 ppm; Ni = 2.9–25.8 ppm) are relative low compared to the subducted slab-related adakites(Mg# = 55–71; MgO = 1.22–6.78 wt.%; Cr = 24–132 ppm; Ni = 2.28–45.61 ppm). Based on these characteristics, the regional association of igneous rocks, and evidence from a global geoscience transect (GGT), the Awulale and Sanchakou adakites represent a genetically distinct suite in the Xinjiang region. We proposed that the source of these adakites was basaltic rock underplated to the base of the lower crust, that partial melted at the rutile-bearing amphiboleeclogite facies (N 650 °C and 1.5 Gpa minimum pressure (≥50 km)). © 2007 Elsevier B.V. All rights reserved. Keywords: Adakite; Underplating; Permian; Xinjiang Tianshan

1. Introduction Adakites were originally recognized in Cenozoic island arcs where they are associated with the subduction of young ≦25 Ma) oceanic lithosphere (Defant and Drummond, 1990). Compared to island arc andesites, dacites and rhyolites (ADR), adakites have high concentrations of Na and Sr, MgO b 3 wt.%, low concentra⁎ Corresponding author. Tel.: +86 20 85290024; fax: +86 20 85290130. E-mail address: [email protected] (Z.H. Zhao). 0024-4937/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2007.06.008

tions of Y and Yb, and high Sr/Y ratios (Defant and Drummond, 1990; Defant and Drummond, 1993). Atherton and Petford (1993), Muir et al. (1995) and Petford and Atherton (1996) demonstrated that the Cordillera Blanca complex in Peru and Separation Point batholith in New Zealand contained rocks that conforms to the definition of adakite, and were formed by partial melting of newly underplated basaltic crust. Partial melting of thickened lower continental crust has also put forward for the generation of Andean adakites (Kay and Kay, 1991, 1993; Kay and Mpodozis, 2001) and

Z.H. Zhao et al. / Lithos 102 (2008) 374–391

adakites in both southern Tibet (Chung et al., 2003) and eastern China (Zhang et al., 2001). Both slab-derived and crust-derived adakites were identified by Sheppard et al. (2001) in paleo-Proterozoic granites of northwest Australia. Defant and Kepezhinskas (2001) proposed that a variety of processes could produce adakites in addition to the subduction of young and hot slab. He suggested six alternative processes related to remnant slabs, oblique or fast (8–10 cm/yr) subduction, arc–arc collision, initial subduction, slab tears and flat subduction. Adakitic magma may also be generated by partial melting of slab window margins caused by subduction of oceanic ridge or plate edges as occurs in Aleution islands, Baja California, Patagonia and southern Costa Rica (Abratis and Womer, 2001; Yogodzhinski et al., 2001; Thorkeison and Breitsprecher, 2005). Defant et al. (2002) pointed out the occurrences of Cretaceous adakites in east China should encompass the melting of the lower crust. Kay and

375

Kay (2002) proposed that Andean adakites originate in three ways, which in terms of relative importance are: tectonic thickening of Andean crust, subduction-erosion of forearc crust, and subduction of young ocean crust. Kay et al. (2005) proposed that the adakitic magmas in the south-central Andes were generated by a combination of melting of the base of thickened lower crust and crust entering the mantle through subduction-erosion. The Mesozoic adakites in middle and lower reaches of the Yangtze river originated from partial melting of delaminated mafic crust in an apparently non-subduction-related setting (Xu et al., 2002). Martin et al. (2005), Martin and Moyen (2003) identified two distinct groups of adakites, according to the examination of an extensive adakite geochemical database (N 340 analyses), that may be distinguished on the basis of their silica content and other criteria. High-Si2O adakite (HSA) is considered to represent melts of subducted basaltic slab which have

Fig. 1. Geological sketch map of Permian adakites in Xinjiang Tianshan. A. Awulale adakite, to the west of the Xinjiang Tianshan region (modified after 1:200,000 geological map of Gongliu County): C2, middle Carboniferous sediments and tuff; P1, early Permian volcanic-sedimentary rocks; P2x, sandy conglomerate; P2h, tuff, dacite, basalt; P2t, siltstone, tuffaceous conglomerate; ξπ, albite porphyry; λπ, granitic porphyry; Q, Quaternary; xt105, the sample location, the others are the same. B. Sanchakou adakite, to the east of the Xinjiang Tianshan region (modified after Lang et al., 1992): 1—fault; 2—copper mine; 3—diorite; 4—conglomerate belt; 5—quartz diorite; 6—keratophyry; 7—andesite; 8—tonalite; 9—plagioclase granite porphyry; 10—bojite; 11—monzonite.

376


variably reacted with peridotite during ascent through mantle wedge. Low-Si2O adakite (LSA) was formed by melting of peridotitic mantle wedge modified by reaction with felsic slab-melts. Late Paleozoic granites and volcanic rocks are widely distributed in Xinjiang Tianshan. Adakites, Nb-enriched basalts (NEB) and high-Mg andesites (HMA) have been recognized in this region (Xiong et al., 2001a,b, 2005a,b; Wang et al., 2003, 2007; Zhao et al., 2004a,b; Zhang et al., 2006). Preliminary studies demonstrated that two suites of adakites occur in two distinct tectonic settings, based on their isotopic ages, petrology and geochemistry. A late Carboniferous suit was generated in subducted oceanic slab and is closely associated with NEB and HMA (Wang et al., 2007). The second suite of adakites has been attributed to partial melting of Permian underplated basaltic magma (Xiong et al., 2001a,b) and is the focus of this paper. Adakite petrology, major and trace elements compositions, isotopic chronology and Sr, Nd, Pb isotopic geochemistry are assessed in order to constrain the petrogenesis of the Permian adakites. 2. Outline of regional geology The Permian adakites mainly occur in the Awulale Mountains, the west Xinjiang Tianshan and they also occur in the Sanchakou area of the east Xinjiang Tianshan (Fig. 1). The west–east trending Awulale Mountains are located in the northeast part of the Yili block and are mainly composed of Permian and upper Carboniferous volcanic-sedimentary formations. The widespread Permian volcanic-sedimentary formations are divided into two parts. The lower strata (P1) include the Wulang group 1 (P1w ), which is mainly composed of sandstone, basalt, basaltic andesite, shoshonite and rhyolite, and the 2 overlying Taredetao group (P1t ), which is composed of thinly layered tuff and trachyte, shoshonite, basalt, tuffaceous siltstone and andesitic conglomerate. The upper Permian strata (P2) include Xiaoshansayi group 1 (P2x ), which is mainly composed of marlite, sandstone and tuffaceous conglomerate, and the Hamistan group 2 (P2h ) consisting of basalt, shoshonite, sodic dacite, tuff, tuffaceous conglomerate and interbeds of sandstone with conglomerate. Adakites in this area occur as small plutons of quartz albite porphyry, albite porphyry and dacite and intrude in the lower Permian volcanic-sedimentary formation. In the east part of Xinjiang Tianshan, the Sanchakou adakites occur as tonalite porphyry or plagioclase granite porphyry and intrude in the lower Carboniferous strata composed of sandstone, quartzite, shale and andesitic conglomerate.

The Carboniferous sequences dominate in the study region and the volcanic rocks in which are mainly composed of basalt, high-K calc-alkaline andesite, dacite and rhyolite. These volcanic rocks and coeval Itype granitoids exhibit the typical geochemical characteristics of arc magmatic rocks (Wang et al., 2007). In addition, there is a broad consensus that late Paleozoic sutures occur in the both northern and southern parts of the Tianshan Range. The ocean related to the study area has been named the “Carboniferous northern Tianshan ocean”, “Carboniferous Asian ocean” or “Early Caboniferous small oceanic basin” (Huang et al., 1990; Xiao et al., 1992; He et al., 1994). The age of 344.0 ± 3.4 Ma has confirmed the age of the ocean (Xu et al., 2006). 3. Analytical methods The major elements were analyzed using conventional wet chemical techniques (Li, 1997) with precision 2–5%. The trace elements, including rare earth elements, were determined by Perkin-Elmer Sciex ELAN 6000 ICP-MS at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. About 50 mg of powder sample was dissolved in high-pressure Teflon bomb using a HF + HNO3 mixture. An internal standard solution containing the element Rh was used to monitor the signal drift counting. The USGS standards BCR-1 and BHVO-1 were chosen for calibrating element concentrations in measured samples. The analytical precision for most elements was generally better than 5% (Liu et al., 1996; Li, 1997). The major chemical compositions of albite and hornblende phenocrysts in adakites were determined by electron microprobe (JEOL JXA8800) at the key laboratory for mineral deposit research in Nanjing University. Operating conditions were 15 kV at 10 nA beam current. The standards of albite, hornblende and orthoclase were used. Isotopic analyses were performed on a MAT-262 mass spectrometer at the Geology Center of Chinese Academy of Sciences. Measured 87Sr/86Sr and 143Nd/144Nd ratios were normalized to 87Sr/86Sr = 0.1194 and 143Nd/144Nd = 0.7219, respectively. The 87Sr/86Sr ratio of standard NBS987 and 143Nd/144Nd ratio of the La Jolla standard measured during this study were 0.710234 ± 7 (2σ) and 0.511838 ± 8 (2σ), respectively. Procedural blank were b100 pg for Nd and b 01 ng for Sr. The detailed analytical procedure is similar to that described by Li and McCulloch (1996). A Perkin-Elmer Sciex ElAN 6100 DRC LA-ICPMS at Northwest University was used for single zircon U–Pb dating. The system was equipped with a 193 nm ArFexcimer laser. A 30–40 μm of spot size was adopted. The


concentrations of U, Th and Pb were calibrated using 29Si as internal calibrant and NIST SRM 610 as reference material. 207Pb/206Pb and 206Pb/238U ratios were calculated using GLITTER 4.0, and then corrected using the Harvard zircon 91500 as external calibrate (Yuan et al., 2004). U–Pb ages and concordia diagram were calculated and constructed using ISOPLOT (Ver. 3) (Ludwig, 2003). 4. Petrology Quartz albite porphyry and dacite are the main adakitic rock types present and occur as small plutons each with an outcrop area of less than 2 km2 in the Awulale Mountains area. In the eastern part of Xinjiang Tianshan, the Permian adakites mainly include tonalite porphyry and plagioclase granite porphyry in complex plutons with an outcrop area of 65 km2 (Fig. 1). Albite, hornblende and lesser biotite, are the main phenocrysts in the Awulale adakites. The groundmass exhibits oriented hyalocrystalline pilotaxitic, pilotaxitic and micro-grained textures defined mainly by plagioclase with less quartz and alkaline feldspar. Plagioclases are the main phenocrysts in the Sanchakou adakites and porphyritic or blastoporphyritic textures are typical. Albite and Carlsbad twining and zonal textures are very common. Magnetite, ilmenite, apatite, zircon and sphene are the main accessory minerals. Tables 1 and 2 present electron microscope analysis of hornblende and feldspar phenocrysts in the Awulale and Sanchakou adakites. Hornblende phenocrysts (Table 1) have MgO contents greater than 13% and calculated CMg numbers in the standard crystal chemistry formula are 2.84, 2.85, 2.98 and 3.05, respectively, corresponding to more than 50% of cation C. Accordingly, the phenocrysts are classified as magnesium hornblende. Feldspar phenocrysts are typical albite with Ab values of 96.8–98.8 (Table 2). In the Sanchakou adakites, the phenocrysts of feldspar are andesines with the Ab values of 49.9–55.8 and An 43.8–49.6 (Table 2). Major element contents of the Awulale and Sanchakou adakites are listed in Table 3. For comparison, adakites attributed to the melting of subducted ocean slab- and underplated basaltic lower crust are also listed along with high-SiO2 adakite (HSA). The A/NKC ranges of the Awulale and Sanchakou adakites extend from 0.85 (meta-aluminous) to 1.22 (peraluminous). Their MgO contents are usually lower than 3 wt.% (0.93–2.22 wt.%) and are associated with wide range of Mg# (35–57, average 46). It is evident from Table 3 that the compositions of Awulale and Sanchakou adakites are similar to those of underplated lower crust-related adakites and HSA in their high SiO2 (N 60 wt.%, 62–

377

Table 1 Compositions of hornblendes in the Awulale adakite Sample no.1

xt104

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O F Cl H2O Total TSi TAl TFe3 Sum-T Cal CFe3 CTi CMg CFe2 CMn CCa Sum-C BCa BNa Sum-B AK Sum-A Sum-cat. CCl Sum-oxy.

44.09 1.84 11.96 11.77 0.148 13.35 11.82 1.62 0.41 0.075 0.023 3.0 100.08 6.29 1.72 0.00 8 0.29 1.40 0.20 2.84 0.002 0.02 0.25 5 1.55 0.45 2 0.074 0.074 15.07 0.006 23.00

xt105

xt105

xt105

40.72 2.08 12.47 15.24 0.133 13.62 11.86 2.04 0.45

43.85 1.18 10.75 14.46 0.214 14.12 11.35 1.66 0.37

44.58 0.93 9.87 12.67 0.279 14.14 11.17 1.46 0.30

0.03 1.0 100.01 5.72 2.06 0.22 8 0 1.57 0.22 2.85 0 0.02 0.34 5 1.45 0.56 2 0.08 0.08 15.08 0.007 22.63

0.04 2.0 100.01 6.2 1.79 0.01 8 0 1.70 0.13 2.98 0 0.03 0.17 5 1.55 0.46 2 0.066 0.066 15.07 0.01 22.88

0.05 5.0 100.01 6.45 1.55 0 8 0.137 1.53 0.10 3.05 0 0.03 0.14 5 1.59 0.41 2 0.055 0.055 15.06 0.012 22.99

1, the sample no. is the same as in Table 2.

71 wt.%), and Na2O (4.40–8.41 wt.%), Na2O/ K2O N 1 (1.59–9.33), and Al2O3 (N15 wt.%, 14.95–16.32 wt.%). 5. Trace element geochemistry Adakites are characterized by high LILE (particularly Sr) and low Y, relative to the island-arc rocks (Defant and Drummond, 1990). In the case of the Awulale and Sanchakou adakites, the Sr contents are 303–1633 ppm and 729–839 ppm, respectively, with most samples having Sr contents much higher than 400 ppm. Corresponding Y contents are 3.9–7.0 ppm and 12.2– 15.2 ppm, (Table 3), resulting in Sr/Y ratios between 51 and 327. From the above the differences are mainly in the high Y contents, low K contents and low Sr/Y ratios for the Sanchakou adakites. On the Sr/Y versus Y diagram, both Awulale and Sanchakou adakite samples lie in the adakite field (Fig. 2). Positive Sr and Ba anomalies can be seen in the primitive-mantle normalized spidergram, but

378


Table 2 Compositions of plagioclase in the Awulale and Sanchakou adakites (wt.%) xt104

xt105

109-9⁎

109-7⁎

S-1⁎⁎

S-2⁎⁎

Pluton

Awulale Mosizaote

Awulale Mosizaote

Awulale 109

Awulale 109

Sanchakou

Sanchakou

Rock type

Qap

Qap

Qap

Qap

Qd

Qd

SiO2 Al2O3 CaO Na2O K2O Total Si Al Ca Na K Ab An Or

68.69 20.31 0.27 10.39 0.04 99.69 2.99 1.04 0.01 0.88 0.002 98.4 1.3 0.2

69.13 20.06 0.196 11.63 0.058 101.07 2.98 1.02 0.01 0.98 0.003 98.8 0.9 0.3

68.44 19.22 0.35 11.40 0.06 99.47 3.00 0.99 0.016 0.97 0.016 96.8 1.6 1.6

68.16 19.47 0.53 11.28 0.06 99.50 2.99 1.01 0.026 0.96 0.005 96.8 2.6 0.5

58.98 26.86 8.15 5.75 0.06 99.8 2.67 1.43 0.40 0.50 0.003 55.8 43.8 0.4

58.64 27.32 9.26 5.15 0.08 100.45 2.65 1.45 0.45 0.45 0.005 49.9 49.6 0.5

Sample no.

Qap—quartz albite porphyry; Qd—quartz diorite; ⁎—Calculated from Cai et al., 1992 (unpublished). ⁎⁎—calculated from Lang et al., 1992.

the K positive anomaly of the Sanchakou adakites is relative weak in comparison with that of Awulale adakites (Fig. 3). These features are similar to those of underplated basaltic crust-related adakites(Atherton and Petford, 1993; Petford and Atherton, 1996; Xiong et al., 2003). For both Awulale and Sanchakou adakites, the concentration of high field strength elements (HFSE), such as Nb, Ta and Ti are relatively low and show strong negative anomalies on primitive-mantle normalized multielement plots (Fig. 3) with La/Nb ratios ranging between 2.9 and 8.6. The concentration of Cr (4–20 ppm) and Ni (3–26 ppm) are lower than average values for Cenozoic adakites (54 ppm Cr and 39 ppm Ni, Drummond et al., 1996, Table 3). Clear positive Eu anomalies (Eu/Eu⁎ 1.01– 1.27) and HREE depletion (La/Yb= 20.0–50.0, Yb =0.32– 0.65 ppm (b1.9 ppm)) are evident in the strongly fractionated chondrite-normalized REE patterns of the Awulale adakites. The Sanchakou adakites possess similar REE patters (Eu/Eu⁎ 1.02–1.21) but the Yb contents are high (1.32–1.67 ppm) and the La/Yb ratios are low (7.36– 7.61) in comparison with that of Awulale adakites (Fig. 4). 6. Geochronology and geochemistry of Sr, Nd and Pb isotopes 6.1. Geochronology Previous geochronological studies have employed Rb– Sr, 40Ar/39Ar and zircon U–Pb SHRIMP (sensitive high resolution ion microprobe) methods to date the Awulale and Sanchakou adakites (Table 4). In this study, zircon U–

Pb dating by LA-ICPMS was carried out on the Awulale adakite. Cathodoluminescence (CL) images of zircons are shown in Fig. 5. The zircons are euhedral, transparent and colourless. Euhedral zoning texture is clear, showing typical magmatic and no cores are observed. The compositions of zircons are listed in Table 5. The Th contents range from 27.12 to 255.63 ppm and U contents from 68.21 to 410.3 ppm with corresponding Th/U ratios of 0.37–1.07. Fig. 6 presents the U–Pb concordia plot for the LA-ICPMS data. The 206 Pb/238 U ratios agree within analytical error, yielding a weighted mean age of 259.5 ± 0.5 Ma (2σ) (MSWD = 0.72) calculated by ISOPLOT ver. 3 (Ludwig, 2003). This age is interpreted to be the crystallization age of the Awulale adakites. Zircons in Sanchakou adakites possess very similar features to those in Awulale adakite in terms of crystal shapes and Th/U ratios. The 206 Pb/ 238 U ratios for 13 analyses obtained using SHRIMP give a weighted mean age of 278 ± 4 Ma (MSWD = 0.78), representing the crystallization age of the Sanchakou adakite (Li et al., 2004). These results, together with data by other methods (Rb–Sr, 40Ar/39Ar) that show an age range of 248–268 Ma for the Awulale adakite (Li et al., 1998; Zhao, J.-M. et al., 2003) and 276–278 Ma for the Sanchakou adakite, firmly establish that these Awulale and Sanchakou adakites were formed during the middle-later Permian (Table 4). 6.2. Geochemistry of Sr, Nd and Pb isotopes Initial 87Sr/86Sr ratios of the Awulale adakites are very homogeneous with a narrow range of 0.7050–0.7054 and

Table 3 Compositions of the Awulale and Sanchakou adakites Pluton

Qunjisayi

Mosizaote

Heishantou

109

Tetiedaban

Sanchakou

Rock type

Dacite

Quartz albite porphyry

Albite porphyry


Quartz albite porphyry Tonalite porphyry

CZA UBA (N = 140) (N = 35)

xt104 xt105

xt106 xt161

xt162 xt163

xt166

xt191

xt195 xt196

xt197

S-1

S-2

65.37 66.52

68.74 62.82

62.66 62.41

67.04

71.85

70.98 70.97

71.35

62.47

66.17 65.00 63.89

TiO2

0.4

0.36

0.38

0.41

0.43

0.48

0.18

0.2

0.2

0.51

0.47

Al2O3 Fe2O3 FeO

15.55 15.56 14.95 15.97 1.71 2.09 1.96 1.56 1.45 0.86 0.72 0.88

16.32 15.98 1.78 1.64 0.84 0.87

15.34 16.21 0.84 3.66 1.47 0.75

15.52 16.32 3.99 3.2 0.75 0.73

14.17 3.93 0.25

13.88 1.08 0.44

15.2 1.08 1.01

15.54 1.15 1.11

15.7 0.72 0.62

16.27

13.03 14.63 17.40

6.52

6.06

5.79

4.21

MnO

0.04

0.01

0.12

0.08

0.06

0.04

0.02

0.05

0.08

0.04

0.14

0.08

0.07

0.05

0.03

0.13

0.11

0.22

0.08

MgO

1.58

1.67

1.75

1.32

1.37

1.18

1.02

1.44

2.06

1.89

1.89

0.69

1.13

1.12

0.93

2.22

1.83

1.91

2.47

CaO

1.79

4.07

1.72

3.87

2.14

1.41

1.19

3.42

2.38

2.75

2.15

1.44

0.6

0.84

0.83

4.90

5.80

5.55

5.23

Na2O

5.23

4.4

5.82

5.51

5.12

6.93

4.83

6.03

6.65

6.98

5.47

5.44

6.55

5.12

8.41

4.77

4.20

4.24

4.40

K2O

2.45

2.06

2.95

2.21

3.92

4.08

4.62

2.43

2.53

2.51

3

2.13

1.58

3.31

0.51

0.83

0.45

0.55

1.52

P2O5

0.15

0.21

0.22

0.15

0.17

0.13

0.12

0.24

0.23

0.2

0.21

0.10

0.10

0.10

0.09

0.25

0.21

0.22

0.19

H2O Total Na2O + K2 O Na2O/ K2 O A/NKC Mg# Cr Ni V

1.74 1.63 2.37 1.21 2.1 1.07 0.85 2.81 1.98 1.28 99.73 99.55 99.54 100.08 99.55 100.22 99.42 100.24 99.24 98.67 7.68 6.46 8.77 7.72 9.04 11.01 9.45 8.46 9.18 9.49

1.95 0.99 100.63 98.41 8.47 7.27

1.04 1.01 0.66 1.41 1.26 99.52 100.52 100.05 100.26 99.6 8.13 8.43 8.92 5.60 4.65

1.02 99.59 4.79 5.92

2.13

3.24

2.99

2.49

1.98

2.58

1.59

3.76

3.99

4.22

1.82

2.55

4.15

2.35

16.5

5.75

9.33

7.71

2.89

1.21 48 8.45 14.04 59.55

0.92 52 11.5 8.43 65.95

0.94 56 12.04 6.84 67.67

1.07 51 16.49 8.61 58.3

0.98 50 10.99 11.2 64.64

0.87 47 4.95 25.8 62.87

1.01 44 4.36 6.19 58.96

0.86 39 9.98 11.28 114.08

0.86 47 16.41 16.12 79.39

0.85 44 12.51 18.49 114.69

1.15 48 15.0 6.0 58.5

0.99 46 20.4 9.6 63.66

1.12 49 11.0 6.0 37.0

1.15 48 13.93 5.73 45.03

1.03 56 8.0 5.0 36.0

1.22 38 10.52 3.45 116.7

1.03 35 9.91 2.9 119.1

1.16 37 12.12 3.27 123.1

1.28 48 54 39 72

0.43

0.57

0.41

0.37

0.38

0.36

S-3

0.46

0.61

56.06– 72.48 0.09– 1.20 15–20 0.58– 7.30 0.02– 0.09 0.10– 2.56 1.56– 6.47 3.67– 7.24 1.28– 3.88 0.04– 0.36

64.80 0.56 16.64 4.75

0.08 2.18 4.63 4.19 1.97 0.20


Sample xt88 xt89 xt90 xt91 no. 67.59 66.56 66.39 66 SiO2

HSA (N = 267)

100 6.16 1.00– 5.57

4–133 3–50

2.13 1.25 48 41 20 95

(continued on next page)

379

380

Table 3 (continued ) Qunjisayi

Mosizaote

Heishantou

109

Tetiedaban

Rock type

Dacite


Albite porphyry


Quartz albite porphyry Tonalite porphyry

Sc Co Rb Cs Ba Sr Ta Nb Hf Zr Y

4.81 7.97 33 2.34 724 1312 0.16 2.5 2.7 98 3.9

7.52 8.43 23 0.77 519 1041 0.15 3 2.56 111 5

8.28 6.65 47 1.67 1005 1633 0.16 3 2.68 119 5

5.06 6.25 29 2.23 681 1096 0.17 2.9 2.84 103 4.4

6.16 5.6 56 1.04 579 777 0.14 2 2.17 70 4

5.89 4.64 69 1.45 585 477 0.14 1.9 2.22 68 4

16.04 4.6 83 1.49 805 334 0.14 1.8 2.2 68 4

10.19 9.54 28 1.2 708 748 0.17 2.4 2.31 79 7

10.38 12.32 30 0.89 720 620 0.17 2.5 2.28 78 6

8.82 11.98 22 0.28 388 464 0.17 2.5 2.43 86 7

6.08 7.54 32 2.75 1661 905 0.22 3.5 2.87 114 6

5.43 6.17 32 2.28 643 1120 0.20 3.2 2.92 109 5

3.0 4.17 18 0.99 389 321 0.4 5.2 2.91 97 6

6.76 4.12 48 0.58 547 303 0.39 4.9 1.87 47 6

0.99 1.333 3 0.15 157 365 0.42 5.7 2.89 99 5

390 729 0.11 1.92 2.04 77.55 12.22

5.65 5.46 9.1 10.86 10.37 13 10.8 12.02 30 1.19 284 477 485 839 771 869 0.13 0.15 0.53 2.12 2.22 8.3 2.1 2.02 3.5 70.86 68.89 117 13.95 15.17 9.5

Pb Th U Sr/Y

7.6 1.34 0.61 336

4.5 1.32 0.44 208

4.4 1.28 0.43 327

9 1.37 0.65 248

10.3 2.18 0.86 194

9.7 2.15 1.03 119

9.8 2.07 0.85 84

3 2.56 1.15 107

9.4 2.49 0.79 103

7.2 2.67 1.01 66

9.4 3.04 0.65 151

9.1 1.45 0.67 225

5 1.88 0.68 54

8.5 2.27 0.68 51

1.0 1.31 0.72 73

3.7 0.7 0.73 60

3.5 1.33 0.68 60

2.5 1.13 0.77 51

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

14.1 29.67 3.61 13.37 1.99 0.54 1.02 0.16 0.82 0.147 0.38 0.05 0.36

16.71 32.07 3.84 14.07 2.13 0.65 1.38 0.19 0.87 0.16 0.45 0.06 0.37

18.59 35.34 4.2 15.42 2.28 0.69 1.45 0.2 0.92 0.16 0.47 0.06 0.37

18.7 37.59 4.5 16.32 2.25 0.64 1.1 0.17 0.92 0.16 0.44 0.06 0.4

15.44 31.43 3.85 14.6 2.23 0.73 1.39 0.19 0.84 0.15 0.41 0.05 0.34

14.92 30.86 3.78 14.3 2.16 0.71 1.36 0.18 0.81 0.14 0.4 0.05 0.32

13.66 28.92 3.61 13.71 2.14 0.7 1.34 0.18 0.84 0.14 0.41 0.05 0.34

20.73 43.19 5.4 20.91 3.18 1.03 2.19 0.3 1.46 0.27 0.78 0.1 0.65

19.97 40.87 5.22 20.24 3.08 0.99 2.03 0.28 1.34 0.24 0.71 0.09 0.6

19.79 42.13 5.34 20.7 3.22 0.96 2.03 0.28 1.36 0.25 0.72 0.09 0.61

25.35 50.26 5.70 20.28 2.87 0.84 1.79 0.24 1.2 0.22 0.56 0.08 0.54

18.39 38.44 4.53 15.98 2.40 0.76 1.46 0.20 0.93 0.17 0.43 0.06 0.42

11.76 21.17 2.7 9.88 1.73 0.49 1.3 0.19 0.97 0.2 0.51 0.09 0.59

14.4 26.83 3.17 11.3 1.86 0.6 1.36 0.21 1.11 0.21 0.62 0.09 0.58

11.3 21.26 2.74 9.04 1.42 0.42 0.99 0.17 0.85 0.17 0.44 0.08 0.51

10.05 21.43 2.88 11.43 2.43 0.92 2.24 0.35 2.12 0.43 1.18 0.18 1.32

11.55 28.28 3.33 12.86 2.74 0.93 2.64 0.42 2.45 0.51 1.42 0.21 1.53

12.29 26.39 3.64 14.77 3.21 1.06 3.12 0.49 2.91 0.58 1.61 0.24 1.67

Lu ∑REE La/Yb

0.06 0.06 0.06 0.06 66.28 72.98 80.22 83.31 39.3 45.0 50.0 46.8

Eu/Eu⁎ 1.16 1.17 V/Sc 12.38 8.77

1.17 8.17

1.24 11.52

Sanchakou

8.40 9.99 19.15

CZA UBA (N = 140) (N = 35)

3.52 0.99 121

38.1– 617.5

108 10

55.65

18.2 3.4 0.9 2.8

0.76

0.96

0.91

0.1 0.09 51.67 62.42 20.0 25.0

0.08 49.44 22.0

0.22 57.18 7.61

0.26 0.27 0.15 69.13 72.25 7.55 7.36 19.3

1.27 1.26 10.49 10.67

1.27 3.68

1.14 9.83

1.01 1.15 12.33 6.66

1.03 36.0

1.21 13.89

1.06 1.02 1.18 21.07 22.55 7.91

1.18 11.79

2.0– 15.00

20.14 3.15 0.97 2.25 0.37 1.43

0.09 0.07 110.02 84.24 22.3 43.7

1.15 13.0

355–1512 721 565 6

19.2 37.7

0.05 0.1 0.09 0.09 66.09 100.28 95.75 97.57 40.0 32.0 33.0 32.0 1.21 7.65

52

17.53 34.65

0.05 0.05 71.71 70.03 45.4 47.0

1.19 11.2

HSA (N = 267)

1.9

0.07– 1.03

0.88 0.17

26.9– 142.9 ≥0.60

21.8 0.89

CZA-cenozoic akakites (Drummond et al., 1996); UBA-underplated basaltic rocks-related adakites (Atherton and Petford, 1993; Petford and Atherton, 1996; Muir et al., 1995); HSA-high Si2O adakites (Martin et al., 2005).


Pluton


Fig. 2. Diagram of Sr/Y against Y for the Awulale and Sanchakou adakites (after Defant and Drummond, 1993).

low positive values of ɛSr(t) (+11.5 − +17.3) (Table 6). A Sanchakou adakite sample has a low initial ratio of 87 Sr/86Sr (0.7039) and ɛSr(t) = −4.0. They are characterized by low positive ɛNd(t) values with Awulale adakite samples ranging between +0.75 − +3.26 and Sanchakou adakite sample having a value of +5.69. The Nd model ages are young for both adakite occurrences (T2DM 472– 699 Ma).The isotopic compositions of Awulale adakites are similar to those of newly underplated basaltic crustrelated adakites of Cordillera Blanca batholith in Peru (Petford and Atherton, 1996) and Separation Point batholith in New Zealand (Muir et al., 1995) but in particular distinct from those of subducted slab-related adakites in the same area. These subducted-related adakites are early Carboniferous (ca 320 Ma) and associated with Nb-enriched basalts and basaltic andesites. Some andesites and dacites possess high MgO and are similar to magnesian andesite (HMA). Geochemically, these adakites and associated volcanic rocks are characterized by high ɛNd(t) (+3.4 − +11.6), varying (87Sr/86Sr)i (0.7007–0.7063), moderately fractionated REE patterns ((La/Yb)N = 1.56–6.14), negative–positive Eu anomalies

381

(Eu/Eu⁎ = 0.76–1.34) and relative low La/Nb ratios (0.83–2.83). These characteristics of adakites were most probably related to subducted crust of the northern Tianshan ocean (Wang et al., 2007: Zhang et al., 2006). In Fig. 7, a diagram of ɛNd(t) versus (87Sr/86Sr)i illustrates that the Awulale and Sanchakou adakites define a distinct trend in the first and second quadrants that is located to the high (87Sr/86Sr)i side of the trend for slab-derived Carboniferous adakites of the same area, and of the worldwide trend of slab melt-related Cenozoic adakites. The Pb isotope compositions of the Awulale and Sanchakou adakites are listed in Table 7. In a diagram of 206 Pb/204Pb–207 Pb/204 Pb, they plot in a field where lower continental crust and MORB overlap (Fig. 8). 7. Petrology and geophysical evidence for late Paleozoic underplating in Xinjiang Tianshan 7.1. Basic granulite and basic dike swarms Underplating of mafic magma may result in the changes of the thermal and rhyological regime, such as temperature increases and viscosity decreases, and even to introduce partial melting forming hybrid lower crust (Furlong and Fountain, 1986; Liu and Furlong, 1994). Basic granulite is a typical component of lower crust and may be produced by the underplating of mantle-derived basaltic magma at the crust–mantle boundary (Drummond and Collions, 1986; Klemperer, 1989; Rudnick, 1990a,b; Gregoire et al., 1994; Rudnick and Fountain, 1995; Caress et al., 1995; Jin and Gao, 1996; Fan et al., 2005). Accordingly, the presence of basic granulite may be used as an indicator of underplating events. Basic granulite has been reported from the Tuoyun area in the southwest part of Xinjiang Tianshan (Zheng et al., 2005). It has a zircon U–Pb age of 253 ± 3 Ma, which is younger

Fig. 3. Primitive-mantle normalized trace elements spidergram (normalization after Sun and McDonough, 1989) for the Awulale and Sanchakou adakites.

382


Fig. 4. Chondrite-normalized REE patterns for the Awulale and Sanchakou adakites (normalization after Boynton, 1984).

than that of metamorphic rocks in the same area and suggests an underplating origin (Dowens, 1993). The SiO2 contents of the basic granulites are 46–52 wt.% and their REE distribution patterns show clear positive Eu anomalies (Eu/En⁎ = 1.24). The estimated equilibrium temperature and pressure are about 910 ±35 °C and 1.35 Gpa (∼ 45 km) based on mineral assemblage and compositions (Zheng et al., 2005), implying that the mafic magma was intruded into the base of continental crust. Basic dike swarms are widespread in the western Xinjiang Tianshan and provide an indicator of lithosphere extension during the underplating event. The west–east to northwest trending dikes are mainly composed of diabase, diabase porphyry and diorite and are several hundreds to thousands of meters in length and several tens of centimeters to several meters wide. A K–Ar age of 246.4 Ma has been obtained for the dikes (Wang, 1985, unpublished). Similar occurrences of basic dike swarm are observed in the western and northern margin of the Junggar basin, on the north side of Xinjiang Tianshan. In this area the basic dike swarm includes more than 40 dikes and veins of diabase, diorite and quartz diorite porphyry, intruded into the Karamay alkali-rich granite. Isotopic ages of 241– 271 Ma (K–Ar, Rb–Sr, Qi, 1993) and Sr–Nd isotopic compositions ( ɛ N d ( t) = + 6.1 − + 7.1, ( 8 7 Sr/ 8 6 Sr) i = 0.7038–0.7041, Li et al., 2004) suggest a relationship to underplated, mantle-derived magmas now preserved in the 253 Ma mafic granulites.

1990a) and 8 km in Queensland, Australia (Rudnick, 1990b). Crustal thickness in Xinjiang Tianshan is in the range of 52–62 km according to a deep seismic profile, Bouguer gravity data, and telluric electromagnetic sounding along the global geoscience transect (GGT) from Xinjiang Tianshan (Dushanzi) to west Kunlun (Quanshuigou) (Li et al., 2001; He et al., 1995). Underplating of mafic magma can result in a complex crust–mantle transitional zone (Fountain and Salisbury, 1981; Jin and Gao, 1996; Fan et al., 2005). The GGT data obtained from a transect starting from the north margin of the Tarim basin and extending across the Tianshan to the south margin of Altay revealed that the Moho discontinuity was composed of 7–8 thin layers, each of which is 2–3 km in thickness and formed a complex crust–mantle transitional zone with total thickness of about 20 km in the Tianshan mountain area (Zhao J.-M. et al., 2003). This complex zone most plausible resulted from the mafic magma underplated onto the base of continental crust. 8. Discussion 8.1. Tectonic settings Regional geology investigations combined with isotopic dating establish the adakites were emplaced into Table 4 Isotopic ages of the Awulale and Sanchakou adakites Plutone

7.2. Geophysical investigations Underplating can give rise to crustal thickening, as noted by Dowens et al. (1990) who demonstrated that the thickness of French Massif Central crust was increased by 6 km during underplating events. Underplating has also been credited with crustal thickness increases of 7 km in Ivrea, Italy (Voshage et al., 1990; Rudnick,

Rock type Age (Ma) Method

Mosizaote QAP QAP QAP QAP Qunji Dacite 109 QAP Sanchakou PGP PGP

259.5±0.5 268 ± 5 247.8 ± 5 247.0 261 ± 4 254.5 278 ± 4 276

La-ICPMS 40Ar/39Ar Rb-Sr K–Ar K–Ar K–Ar SHRIMP Rb-Sr

Reference This study Zhao Z.-H. et al. 2003 Li et al. 2004 “305” project 2000 Zhao Z.-H. et al. 2003 “305” project 2000 Li et al. 2004 Lang et al. 1992

QAP—quartz albite porphyry; PGP—Plagioclase granite porphyry.


383

Fig. 5. Cathodoluminescence (CL) images of zircons from the Awulale adakite.

volcanic-sedimentary formations in the Xinjiang Tianshan region during the middle to the late Permian (Table 4). The volcanic-sedimentary host formation is composed of basic to acid volcanic rocks, porphyries and sedimentary rocks containing plant fossils, such as Paracalamites sp., Calamites sp., Redicites Sp., Stenoocostatus and Neoggerethiopsis cf.(Institute of Geology and Mineral Resources, Xinjiang, 1991). The Permian tectonic evolution of the Xinjiang Tianshan region involved a transition from a active continental margin to a continental intraplate or postcollision settings characterized by ocean closure, crustal thickening, the generation of widespread of alkali-rich granites and basic dike swarm and lack of the association of island arc ADR (Han et al., 1999; Jahn et al., 2000; Zhao et al., 1993, 2004a,b; Li et al., 2006). All of these features are consistent with lithospheric extension driven by underplating which we also suggest was responsible for the generation of the contemporaneous adakites. 8.2. Source materials As noted above, the relatively low positive ɛNd(t) values, moderately high (87Sr/86Sr)i ratios and young Nd model ages of the Awulale and Sanchakou adakites differ

from those of subducted slab-related late Carboniferous adakites in the same area. In addition, the MgO, Cr and Ni concentrations are also lower in the Awulale and Sanchakou adakites. In addition, high-Mg andesites (HMA) and niobium-rich basalts (NEB), which are often associated with subduction-related adakites are not found associated with the Awulale and Sanchakou adakites. Such features resemble to those of adakites derived from newly underplated basaltic crust in Peru (Petford and Atherton, 1996). The Pb isotopic compositions of the Awulale and Sanchakou adakites are consistent with an origin involving a mixture of MORB and continental crust (Fig. 8). Low (87Sr/86Sr)i (0.7039) together with low K contents (3735–6889 ppm) and low La/Yb (7.36–7.61) in Sanchakou adakites should be attributed to more lower crust (K = 5063 ppm, La/ Yb = 5.3, Rudnick and Gao, 2004) entering the source region or less fractional crystallization during the petrogenesis of the Sanchakou adakites. Based on these observations, melting of subducted slab origin seems an unlikely model for the generation of the Awulale and Sanchakou adakites. However, we suggest that contemporaneous underplated basaltic magma, derived from weakly depleted mantle, may provide the source materials for the generation of the Permian adakites in the Xinjiang Tianshan region.

384

Table 5 LA-ICPMS data for zircons from the Mosizaote adakite Th

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

71.78 72.46 159.7 78.10 79.69 107.7 130.2 122.8 45.82 213.3 65.46 65.17 139.7 116.8 149.8 120.7 43.32 323.8 251.52 89.23 79.09 255.63 60.97 195.59 219.41 186.39 221.10 78.95 47.32 59.74 62.32 30.42 27.12 91.54 94.80

24.27 30.47 37.56 29.07 30.21 31.55 34.67 66.35 17.08 53.63 27.20 24.13 27.15 40.28 39.79 35.94 20.48 88.97 64.15 28.44 27.05 81.13 24.72 57.47 58.17 64.96 46.01 27.92 23.57 26.41 24.88 15.05 13.02 39.61 34.59

U

128.4 158.1 188.8 147.9 157.0 160.4 182.9 335.6 82.98 268.7 141.7 123.9 130.9 207.6 200.7 184.6 106.8 361.5 318.48 144.10 138.74 410.25 127.89 291.33 290.85 331.03 227.22 140.40 122.43 135.51 127.68 77.82 68.21 205.13 177.56

Th/ U

207

Isotope ration

0.56 0.46 0.85 0.53 0.51 0.67 0.71 0.37 0.55 0.79 0.46 0.53 1.07 0.56 0.75 0.65 0.41 0.90 0.79 0.62 0.57 0.62 0.48 0.67 0.75 0.56 0.97 0.56 0.39 0.44 0.49 0.39 0.40 0.45 0.53

0.05425 0.05708 0.0554 0.05343 0.05845 0.05418 0.05543 0.05401 0.05382 0.05288 0.05314 0.05383 0.05429 0.0518 0.05334 0.05412 0.05243 0.09012 0.05362 0.05464 0.05541 0.05093 0.05188 0.05379 0.05407 0.05251 0.07113 0.05407 0.05512 0.05518 0.05326 0.05187 0.05648 0.0537 0.05188

Pb/206Pb 1σ 170 193 14 126 213 118 129 133 245 148 115 153 131 13 143 126 122 125 82 142 122 70 132 91 93 141 202 133 142 123 121 294 190 109 112

Age (Ma) 207

Pb/235U 1σ

0.29617 0.3128 0.30826 0.30223 0.32419 0.30341 0.29672 0.30398 0.31543 0.29494 0.29844 0.30403 0.30605 0.28954 0.29892 0.30042 0.2961 0.60751 0.3012 0.30879 0.31036 0.28753 0.29259 0.30126 0.30423 0.2953 0.3790 0.30705 0.31122 0.31095 0.29896 0.27972 0.31559 0.30276 0.29019

878 103 720 686 1149 595 629 720 1402 802 583 809 678 727 743 636 629 646 373 743 615 308 691 434 0445 74 104 695 745 627 616 1559 1007 545 565

206

Pb/238U 1σ

0.03959 0.03975 0.04035 0.04103 0.04023 0.04061 0.03882 0.04082 0.04251 0.04045 0.04073 0.04096 0.04088 0.04054 0.04064 0.04026 0.04096 0.04889 0.04074 0.04099 0.04062 0.04095 0.04091 0.04062 0.04081 0.04078 0.03864 0.04119 0.04095 0.04087 0.04071 0.03912 0.04052 0.04089 0.04057

34 31 30 26 35 28 28 26 41 27 27 33 30 27 32 28 28 29 23 31 28 22 30 24 25 31 28 30 31 28 28 42 37 27 27

208

Pb/232Th 1σ

0.01318 0.01235 0.01254 0.01285 0.01246 0.01326 0.01299 0.01277 0.0133 0.01268 0.01364 0.01361 0.01297 0.01274 0.01306 0.01336 0.01431 0.01273 0.01413 0.01354 0.01386 0.01453 0.01417 0.01379 0.01354 0.01505 0.0117 0.01559 0.0154 0.01663 0.01609 0.01229 0.01487 0.01454 0.0155

207

Pb/206Pb 1σ

21 381 8 495 14 428 6 347 8 547 14 379 14 430 8 371 9 363 6 324 16 335 21 364 12 383 6 276 16 343 15 376 19 304 9 1428 9 355 18 398 16 429 9 238 19 280 11 362 11 374 20 308 0.7 961 19 374 24 417 20 420 18 340 11 279 30 471 16 358 16 280

51 76 39 55 82 32 34 57 105 65 32 45 36 61 42 35 36 12 18 40 32 15 41 22 22 43 59 38 40 33 34 133 54 29 32

207

Pb/235U 1σ

263 276 273 268 285 269 264 270 278 262 265 270 271 258 266 267 263 482 267 273 274 257 261 267 270 263 326 272 275 275 266 250 279 269 259

7 8 6 5 9 5 5 6 11 6 5 6 5 6 6 5 5 4 3 6 5 2 5 3 3 6 8 5 6 5 5 12 8 4 4

206

Pb/238U 1σ

250 251 255 259 254 257 246 258 268 256 257 259 258 256 257 254 259 308 257 259 257 259 258 257 258 258 244 260 259 258 257 247 256 258 256

2 2 2 2 2 2 2 2 3 2 2 2 2 2 2 2 2 2 1 2 2 1 2 1 2 2 2 2 2 2 2 3 2 2 2

208

Pb/232Th 1σ

265 248 252 258 250 266 261 256 267 255 274 273 260 256 262 268 287 256 284 272 278 292 284 277 272 302 235 313 309 333 323 247 298 292 311

4 2 3 1 2 3 3 2 2 1 3 4 2 1 3 3 4 2 2 4 3 2 4 2 2 4 1 4 5 4 4 2 6 3 3


Analysis Pb no. Rad


385

Fig. 6. U–Pb Concordia diagram for zircons from the Awulale adakite.

garnet (rich in Y and HREE) and rutile (rich in Ti, Nb and Ta) were residual mineral phases during the formation of the Awulale and Sanchakou adakites. In contrast, plagioclase was resolved during melting resulting in enrichment of Sr and Eu in the adakitic magma. There are many experimental works were carried out on rutile stability and HFSE partitioning. (Green and Pearson, 1986; Ryerson and Watson, 1987; Foley et al., 2000; Kelemen et al., 2002; Schimidt et al.,

8.3. Petrogenetic model In addition to the notable depletion of Nb, Ta and Ti and enrichment of Sr in primitive mantle-normalized spidergram (Fig. 3), the adakites of Awulale and Sanchakou do not show negative Eu anomalies and characterized by depletions of HREE in chondritenormalized REE patterns (Fig. 4). Experimental results suggest that these geochemical features indicate that

Table 6 Nd and Sr isotopic compositions of the Awulale and Sanchakou adakites Sample no.

Sm (ppm)

xt88 2.03 xt90 2.46 xt91 2.44 xt104 2.25 NL2-3a 0.28 NL2-13a 1.51 xt161 2.23 xt162 3.23 xt195 1.9 xt196 1.73 S2 274 TSSA (12) UBA a

Nd (ppm)

147

12.72 15.85 16.24 14.19 1.76 8.35 20.36 20.25 11.14 10.38 12.86

0.09671 0.094 0.0909 0.09592 0.09778 0.10917 0.0961 0.0953 0.1032 0.1009 0.1288

Sm/ Nd

144

143

Nd/144 Nd ± 2σ

(143Nd/144Nd)i ɛNd(t) T2DM (Ga)

Rb Sr (ppm) (ppm)

87

0.512556 ± 11 0.51257 ± 13 0.51256 ± 11 0.512547 ± 8 0.512518 ± 16 0.512542 ± 11 0.512578 ± 10 0.512567 ± 13 0.512589 ± 11 0.512642 ± 10 0.512806 ± 11

0.51239 0.51241 0.51241 0.51238 0.51236 0.51236 0.51241 0.51241 0.51241 0.51247 0.51257 0.51240– 0.5127 0.5123– 0.5126

33.58 1353 50.49 1648.1 29.80 1118.01 57.08 843.8 83.28 292.02 45.66 348.60 28.23 788.68 30.56 645.62 29.4 367.27 46.81 298.8 10.80 838.6

0.07182 0.08868 0.07715 0.1982 0.8222 0.3776 0.1036 0.137 0.2317 0.4534 0.03725

1.84 2.09 1.99 1.57 0.75 0.86 2.17 1.98 2.15 3.26 5.69 3.40– 9.11

0.598 0.578 0.585 0.620 0.699 0.685 0.568 0.588 0.571 0.472 0.618 0.349– 0.890

Rb/86Sr

87

Sr/86Sr ± 2σ ((87Sr/86Sr)i

0.705270 ± 10 0.705695 ± 12 0.705467 ± 13 0.706129 ± 16 0.70826 ± 8 0.70667 ± 2 0.705522 ± 8 0.705583 ± 6 0.706204 ± 11 0.706916 ± 20 0.704057 ± 14

0.704– 0.708

Li et al., 1998; TSSA(12): subducted slab-related adakites in Tianshan (this study; Rui et al., 2004; Wang et al., 2007); UBA: underplated basalt-related adakites (Atherton and Petford, 1993; Petford and Atherton, 1996; Muir et al., 1995).

b

0.7050 0.7054 0.7052 0.7054 0.7054 0.7054 0.7051 0.7051 0.7053 0.7053 0.7039 0.7032– 0.7042

386


Fig. 7. Diagram of eNd(t) versus (87Sr/86Sr)i for the Awulale and Sanchakou adakites. The data for Cenozoic oceanic slab-related adakites are taken from Defant et al. (1992), Kay et al. (1993), Aquillon-Robles et al. (2001). The data for the Cordillera Blanca Betholith are from Petford and Atherton (1996); Permian adakites in Xinjiang Tianshan (this study); late Carboniferous adakites in Xinjiang Tianshan (this study; Rui et al., 2004; Wang et al., 2007).

2004) For example, Xiong et al. (2005a,b, 2006) conducted an experiment using natural hydrous basalt as starting material at 1.0–2.5 Gpa and 900–1000 °C. Analyses of the quenched glasses coexisting with amphibole, rutile and garnet showed that the major element contents were very similar to those of adakite with SiO2: 59.65–68.04 wt.%, Al2O3: 17.54–19.84 wt. %, Na2O: 4.76–6.62 wt.%, Na2O/K2O N 1, A/NKC: 0.95–1.15 (i.e. weakly aluminous to peraluminous), Mg#: 37–40, and high Sr/Y ratios (115–756). Rutile has attracted considerable attention for its role in Nb and Ta budgets and Nb/Ta fractionation during subduction zone processes (Green, 1995; Rudnick et al., 2000; Foley et al., 2000; Kelemen et al., 2003). The Nb and Ta partitioning coefficients of rutile (Xiong et al., 2005b) were 51–307 and 65–417, respectively, indicating that the rutile is a Nb and Ta bearing residual mineral phase during the generation of adakites. Given that the minimum pressure of rutile stability is approximate 1.5 Gpa, the depth for adakite generation must be more than 45–50 km. Thickened crust (52–62 km) in the Xinjiang Tianshan region is consistent with this requirement. Previous experiments have also shown that adakitic rocks are generated by partial melting of metabasalt in the transition zone from amphibolite to eclogite (Rapp et al., 1991; Rapp and Watson, 1995; Sen and Dunn, 1994; Winther, 1996), corresponding to the field III (amphibole-eclogite facies) in Fig. 9 (Xiong et al., 2006). This requirement is also

consistent with the unusually thick crust of the Xinjiang area. The residue was likely composed of rutile-bearing amphibole-eclogite (Cpx + Gt + Am + Ru, temperature N650 °C, minimum pressure 1.5 Gpa). The oxygen fugacity (fO2) during partial melting of a mantle-derived rock can be monitored through the systematic change of V/Sc ratios (Lee et al., 2003, 2005; Canil, 1997, 2002; Li and Lee, 2004). Table 3 illustrates that the range of V/Sc ratios are mainly 6.6 –12.38 (average 10 34; with two outliers 3.68 and 36.0) and 13.89–22.55 (average 19.17) for the Awulale and Sanchakou adakites, respectively. These values are higher than values typical of subducted slab-related adakites in the same area (3.67–10.94, average 6.67 in Alatao; 9.2–12.8, average 10.8 in Tuwu-Yandong). In V/Sc-MgO diagram (Fig. 10), the maximum f O2 are 1 and 2 log units higher than that of FMQ buffer for the Awulale and Sanchakou adakites, respectively, but 0.5 log unit lower and higher respectively than that of FMQ

Table 7 Pb isotope compositions of the Awulale adakite Sample no.

206

Pb/204Pb

207

Pb/204Pb

208

Pb/204Pb

xt90 xt104 Ms-1⁎ Cb-1⁎

18.196 18.488 18.106 18.142

15.475 15.524 15.457 15.490

37.923 38.161 37.835 38.065


387

Fig. 8. Diagram of 206Pb/204Pb–207Pb/204Pb of the Awulale and Sanchakou adakites.

buffer for the corresponding subducted slab-related adakites in the same area. The results may suggest that the formation of Awulale and Sanchakou adakites may have occurred at relatively high fO2, which further distinguishes them from the late Carboniferous adakites of east and west Tianshan. 9. Conclusion

features typical high-SiO2 adakite (HSA), such as SiO2 N 60 wt.% (62–70 wt.%), Al2O3 13–16 wt.%, high Na2O (4.40–8.40 wt.%), low MgO (0.93– 2.22 wt.%), Ni (3–26 ppm) and Cr (4–20 ppm), Na2O N K2O, high Sr/Y ratios (51–327) and strong depletions in Nb, Ta, Ti and HREE. (2) Isotopic dating establishes ages of 260–278 Ma (middle to later Permian) for the Awulale and Sanchakou adakites. There are no HMA or NEB

(1) The Permian quartz albite porphyry, albite porphyry, tonalite porphyry and granodiorite porphyry intrusions of Awulale and Sanchakou in the Xinjiang Tianshan region have many geochemical

Fig. 9. P–T diagram showing field relevancy to the partial melting of hydrous basalt (Xiong et al., 2006). Solidus and phase boundaries of amphibole, garnet and plagioclase are from Green (1982); The phase boundary of rutile is from Xiong et al. (2005b); Field I: adakite/TTG liquids coexisting with amphibolite residue (Am + Cpx + Pl ± Gt); Field II: adakite/TTG liquids coexisting with rutile-bearing eclogite residue (Cpx + Gt + Ru ± Am); Field III: adakite/TTG liquids coexisting with rutile-bearing amphibole-eclogite residue (Cpx + Gt + Am + Ru).

Fig. 10. V/Sc versus MgO (wt.%) in Awulale and Sanchakou adakites (after Lee et al., 2005). The grey fields are for the Permian adakites of Awulale and Sanchakou, respectively; The open circle and triangle are for Alatao and Tuwu-Yandong: subducted slab-related adakites, respectively; Star: average Cenozoic adakite (Drummond et al., 1996); The three curved black lines represent (from left to right) mixing lines between lava having hypothetically high primary V/Sc with upper, average and lower continental crust.

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rock types associated with these adakites and no lithological evidence for ascent of the Permian adakitic magmas through the mantle. Chemical and isotopic evidence also distinguishes the Permian adakites from late Carboniferous adakites generated by melting of subducted slab. Relatively low but positive ɛNd(t) values (+0.75 − +5.69) and a wide range of (87Sr/86Sr)i ratios (0.7039–0.7054), together with the Pb isotopic compositions demonstrates that the Awulale and Sanchakou adakites originated from basaltic magma derived from weakly depleted mantle. Accordingly, the Awulale and Sanchakou adakites are most plausible classified as underplating-related adakite. (3) Additional support for Permian underplating in Xinjiang Tianshan comes from the thickened crust (52–62 km) and a complex crust–mantle transitional zone composed of multiple thin layers, the presence of basic granulite, and widespread of the Permian alkali-rich granites and basic dike swarm. (4) Partial melting experiments on hydrous basalt carried out at 1.0–2.5 Gpa and 900–1000 °C produced melts that are very similar to adakites in compositions. Based on these and other experimental results, the partial melting of underplated basalt beneath Xinjiang could have taken place under the temperature–pressure field of amphiboleeclogite facies prior to amphibole disappearance, leaving a residue composed of Cpx + Gt + Am + Ru. Rutile is a necessary residual mineral phase and its 1.5 Gpa minimum pressure requirement indicates that the depth of underplating-related adakite generation was at least 50 km. Acknowledgments This study was supported by grants from the National Nature Science Foundation of China (40373017), the State Key Basic Research of China (2001CB409803) and the Chinese Academy of Sciences (GIGCX-04-03). The authors are grateful to Atherton and Smithies for their thoughtful and constructive reviews. References Abratis, M., Womer, G., 2001. Ridge collision, slab-window formation, and the flux of Pacific asthenosphere into the Caribbean realm. Geology 29, 127–130. Aguillón-Robles, A., Caimus, T., Bellon, H., Maury, R.C., Cotton, J., Bourgois, J., Michaud, F., 2001. Late Miocene adakite and Nbenriched basalts from Vizcaino Peninsula, Mexico: indicators of East Pacific Rise subduction below southern Baja California. Geology 29, 531–534.

Atherton, M.P., Petford, N., 1993. Generation of sodium-rich magmas from newly underplated basaltic crust. Nature 362, 144–146. Boynton, W.V., 1984. Cosmochemistry of the rare earth elements: meteorite studies. In: Henderson (Ed.), Development in Geochemistry 2. Elsevier, Amsterdam, pp. 62–114. Canil, D., 1997. Vanadium partitioning and the oxidation state of Archean komatiite magmas. Nature 389, 842–845. Canil, D., 2002. Vanadium in peridotites, mantle redox and tectonic environments: Archean to present. Earth Planet. Sci. Lett. 195, 75–90. Caress, D.W., Mcnutt, M., Detrick, R.S., Mutter, J.C., 1995. Seismic imaging of hotspot-related crustal underplating beneath the Marquesas island. Nature 373, 600–603. Chung, S.-L., liu, D.-Y., Li, J.-Q., Chu, M.-F., Lee, H.-Y., We, D.-J., Lo, C.-H., Lee, T.-Y., Qian, Q., Zhang, Q., 2003. Adakites from continental collision zones: melting of thickened lower crust beneath southern Tibet. Geology 31, 1021–1024. Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347, 662–665. Defant, M.J., Drummond, M.S., 1993. Mount St. Helens: potential example of the partial melting of the subducted lithosphere in a volcanic arc. Geology 21, 547–550. Defant, M.J., Kepezhinskas, P., 2001. Evidence suggests slab melting in arc magmas. EOS 82, 62–69. Defant, M.J., Jackson, T.E., Drummond, M.S., De Boer, J.Z., Bellon, H., Feigenson, M.D., Maury, R.C., Stewart, R.H., 1992. The geochemistry of young volcanism throughout western Panama and southeastern Costa Rica: an overview. J. Geol. Soc. (Lond.) 149, 569–579. Defant, M.J., Xu, J.F., Kepezhinskas, P., Wang, Q., Zhang, Q., Xiao, L., 2002. Adakites: some variations on a theme. Acta Petrol. Sin. 18, 129–142. Dowens, H., 1993. The nature of lower continental crust of Europe: petrological and geochemical evidence from xenoliths. Phys. Earth Planet Inter. 79, 195–218. Dowens, H., Deputy, C., Leyreloup, A.F., 1990. Crustal evolution of the Hecynian belt of Western Europe, evidence from lower crustal granulite xenoliths. Chem. Geol. 83, 209–231. Drummond, B.J., Collins, C.D.N., 1986. Seismic evidence for underplating of the lower continental crust of Australia. Earth Planet. Sci. Lett 79, 361–372. Drummond, M.S., Defant, M.J., Kepzhinskas, P.K.S., 1996. Petrogenesis of slab-derived trondhjemite-tonalite-dacite/adakite magmas. Trans. R. Soc. Edinb. Earth Sci. 87, 205–215. Fan, Q.-C., Zhang, H.-F., Sui, J.-L., Zhai, M.-G., Sun, Q., Li, N., 2005. Magma underplating and present crust–mantle transitional zone composition: xenolith petrological and geochemical evidence. Sci. China (Series D) 48 (8), 1089–1105. Foley, S.F., Barth, M.G., Jenner, G.A., 2000. Rutile/melt partition coefficients for trace elements and assessment of the influence of rutile on the trace elements characteristics of subduction zone magmas. Geochim. Cosmochim. Acta 64, 933–938. Fountain, D.M., Salisbury, M.H., 1981. Exposed cross-section through the continental crust; implications for crustale structure, petrology and evolution. Earth Planet Sci. Lett. 56, 263–277. Furlong, K.P., Fountain, D.M., 1986. Continental crust underplating: therma considerations and seismic-petrologic consequences. J. Geophys. Res. 91, 8285–8294. Green, T.H., 1982. Anatexis of mafic crust and high pressure crystallization of andesite. In: Thorpe, R.S. (Ed.), Andesite. John Wiley, New York, pp. 465–487.

Z.H. Zhao et al. / Lithos 102 (2008) 374–391 Green, T.H., 1995. Significance of Nb/Ta as an indicator of geochemical processes in the crust–mantle system. Chem. Geol. 120, 347–359. Green, T.H., Peason, N.J., 1986. Ti-rich accessory phase saturation in hydrous mafic-felsic compositions at high PT. Chem. Geol. 54, 185–201. Gregoire, M., Mttielli, N., Nicollect, C., Cottin, J.Y., Leyrit, H., Weis, D., Shimizu, N., Giret, A., 1994. Oceanic mafic granulite xenoliths fro the Kerguelen Archipelago. Nature 367, 360–363. Han, B.-F., He, G.-Q., Wang, S.-G., 1999. Mantle-derived magmatism, underplating and the nature of basement of Junggar basin. Sci. China (Series D) 29 (1), 16–21. He, G.-Q, Li, M.-S., Liu, D.-Q., Tang, Y.-L., Zhou, R.-H., 1994. Paleozoic evolution and metallogenesis in Xinjiang, China. Xinjiang Publishing House and Hongkong, Educational and Cultural Press Ltd. pp.1–437 (in Chinese). He, G.-Q., Liu, D.-Q., Li, M.-S., Tang, Y.-L., Zhou, R.-H., 1995. The five stage model of crust evolution and metallogenic series of chief orogenic belts in Xinjiang. Xinjiang Geology 13 (2), 99–180 (in Chinese, with English abstr). Huang, J.-Q., Jiang, C.-F., Wang, Z.-X., 1990. Plate tectonics and accordion-type movement. Xinjiang Geol. Sci. 1, 3–16 (in Chinese, with English abstr). Institute of Geology and Mineralogy, No. 1 Regional geological Serving Party, Xinjiang. 1991. Paleozoic Erathem in Xinjiang. Xinjiang People's Publishing House, Urumqi, pp. 212–265, 327–328 (in Chinese, with English abst.). Jahn, B.-M., Griffin, W.L., Windley, B.F., 2000. Continental growth in the Phanerozoic: evidence from Central Asia special issue. Tectonophysics 328, 1–227. Jin, Z.-M., Gao, S., 1996. Underplating and it's geodynamical significances for the evolution of crust–mantle boundary. Geological Sciences and Technology Information 15 (2), 1–7 (in Chinese, with English abstr.). Kay, R.W., Kay, S.M., 1991. Creation and destruction of lower continental crust. Geol. Rundsch. 80, 259–278. Kay, R.W., Kay, S.M., 1993. Delamination and delamination magmatism. Tectonophysics 219, 177–189. Kay, S.M., Mpodozis, C., 2001. Central Andean ore deposits linked to evolving shallow subduction systems and thickening crust. GSA Today 11 (3), 4–9. Kay, R.W., Kay, S.M., 2002. Andean adakites: three ways to make them. Acta Petrol. Sin. 18 (3), 303–311. Kay, S.M., Ramos, V.A., Marquez, M., 1993. Evidence in Cerro Pampa volcanic rocks of slab melting prior to ridge trench collision in southern South America. J. Geol. 101, 703–714. Kay, S.M., Gody, E., Kurtz, A., 2005. Episodic arc migration, crustal thickning, subduction erosion, and magmatism in the south-central Andes. GSA Bull. 117 (1–2), 67–88. Kelemen, S., Blundy, J.D., Wood, B.J., 2002. Experimental constrins on major and trace element partitioning during partial melting of eclogite. Geochim. Cosmochim. Acta 66, 3109–3123. Kelemen, P.B., Hanghol, K., Green, A.R., 2003. One view of the geochemistry of subduction-related magmatic arcs, with an emphasis on primitive andesite and lower crust. In: Rudnick, R.L., Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry, vol. 3. Elsevier, Amsterdam, pp. 593–649. Klemperer, S.L., 1989. Deep seismic reflection profiling and the growth of the continental crust. Tectonophysics 161, 233–244. Lang, Z.-J., Shi, B., Li, T.-D., 1992. Genesis of copper deposit in Sanchakou, Hami, Xinjiang. Xinjiang Geology 10 (3), 244–252 (in Chinese, with English abst.).

389

Lee, C.-T., Brandon, A.D., Norman, M.D., 2003. Vanadium in peridotite as a proxy for paleo-f O2 during partial melting: prospect, limitations, and implications. Geochim. Cosma. Acta 67, 3045–3064. Lee, C.-T., Leeman, W., Canil, D., Li, Z., 2005. Similar V/Sc systematics in MORB and arc basalts: implications for the oxygen fugacities of their mantle source regions. J. Petrol. 46 (11), 2313–2336. Li, H.-Q., Xie, C.-F., Chang, H.-L., et al., 1998. Study on Metallogenetic Chronology of Nonferrous and Precious Metallic Ore Deposits in North Xinjiang, China. Geological Publishing House, Beijing, pp. 107–127 (in Chinese, with English abst.). Li, H.-Q., Chen, F.-W., Lu, Y.-F., Yang, H.-M., Guo, J., Mei, Y.-P., 2004. Zircon SHRIMP U–Pb age and strontium isotopes of mineralized granitoids in the Sanchakou copper polymetallic deposit, east Tianshan Mountains. Acta Geoscietica Sinica 25 (2), 191–195 (in Chinese, with English abstr.). Li, J.-Y., He, G.-Q., Xu, X., Li, H.-Q., Sun, G.-H., Yang, T.-N., Gao, I.-M., Zhu, Z.-X., 2006. Crustal tectonic framework of northern Xinjiang and adjacent regions. Acta Geol. Sin. 80 (1), 148–168. Li, X.-H., McCulloch, M.T., 1996. Secular variations in the Nd isotopic composition of Late Proterozoic sediments from the southern margin of the Yangtze Block: evidence for a Proterozoic continental collision in SE China. Precambrin Res. 76, 67–76. Li, X.-H., 1997. Geochemistry of the Longsheng ophiolite from the southern margin of Yangtze craton, SE China. Geochemical J. 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 basicintermediate dyke swarm from Xinjiang, China. Geochimica 33, 574–584 (in Chinese, with English abstr.). Li, Z.-X.A., Lee, C.-T.A., 2004. The constancy of upper mantle f O2 through time inferred from V/Sc ratios in basalts. Earth Planet. Sci. Lett. 228, 483–493. Liu, M., Furlong, K.P., 1994. Intrusion and underplating of mafic magmas: thermal-rheological effect and implications for tertiary tectonomagmatism in the North American Cordillera. Tectonophysics 237, 175–187. Liu, Y., Liu, H.-C., Li, X.-H., 1996. Simultaneous and precise determination of 40 trace elements in rock samples by ICP-MS. Geochimica 25 (6), 552–558 (in Chinese, with English abstr.). Ludwig, K.R., 2003. ISOPLOT 3. A geochronological toolkit for Microsoft excel. Berkeley Geochronology Centre Special Publication 4, 74. Martin, H., Moyen, J.-F., 2003. Secular Changes in TTG Composition: Comparison with Modern Adakites. EGS-AGU-EUG Joint Meeting, Nice, April, VGP7-IFR2O-001. Martin, H., Smithies, R.H., Rapp, R., Moyen, J.F., Champion, D., 2005. An overview of adakite, tonolite-trondhjemite -granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos 79, 1–24. Muir, R.J., Weaver, S.D., Bradshaw, J.D., Eby, G.N., Evans, J.A., 1995. The Cretaceous Separation Point batholith, New Zealand: granitoid magmas formed by melting of mafic lithosphere. J. Geolo. Sici., London, 152, 689–701. Petford, N., Atherton, M., 1996. Na-rich partial melts from newly underplated basaltic crust: the Cordillera Blanca Batholith, Peru. J. Petrology 37 (6), 1491–1521. Qi, J.-Y., 1993. Geology and genesis of vein rock group in western Zhunggar, Xinjiang. Acta Petrol. Sin. 9, 288–298. Rapp, R.P., Watson, E.B., 1995. Dehydration melting of metabasalt at 8–32 kbar implications for continental growth and crust–mantle recycling. J. Petrol. 38, 891–931.

390


Rapp, R.P., Watson, E.B., Miller, C.F., 1991. Partial melting of amphibolite/eclogite and the origin of Archean trandhjemite and tonalite. Precambrian Res, 51, 1–25. Rudnick, R.L., 1990a. Nd and Sr isotopic compositions of lower-crusta lxenoliths from north Queensland, Australia: implication for Nd model ages and crustal growth processes. Chem. Geol. 83, 195–208. Rudnick, R.L., 1990b. Continental crust: growth from below. Nature 347, 711–712. Rudnick, R.L., Fountain, D.M., 1995. Nature and composition of the continental crust: a lower crust perspective. Rew. Geophys. 33, 267–305. Rudnick, R.L., Gao, S., 2004. In: Holland, H.D., Turekian, K.K. (Eds.), Composition of the Continental Crust. Treatise on Geochemistry, vol. 3. Elsevier, Amsterdam, pp. 1–65. Rudnick, R.L., Barth, H., Horn, I., McDonough, W.F., 2000. Rutilebearing refractory eclogitees: missing link between continents and depleted mantle. Science 287, 278–281. Rui, Z.-Y., Zhang, L.-S., Chen, Z.-Y., Wang, L.-S., Liu, Y.-L., Wang, Y.T., 2004. Approach on source rock or source region of porphyry copper deposits. Acta Petrologica Sinica 20, 229–238 (in Chinese, with English abstr). Ryerson, F.J., Watson, E.B., 1987. Rutile saturation in magmas: implications for Ti-Nb-Ta depletion in island-arc basalts. Earth Planet. Sci. Lett 86, 225–239. Sen, C., Dunn, T., 1994. Dehydration melting of a basaltic composition amphibolite at 1.5 and 2.0 Gpa: implications for the origin of adakites. Contrib. Mineral. Petrol. 117, 394–409. Schimidt, M.W., Dardon, A., Chazot, G., Vannucci, R., 2004. The dependence of Nb and Ta rutile-melt portioning on melt composition and Nb/Ta fractionation during subduction processes. Earth Planet. Sci. Lett. 226, 415–432. Sheppard, S., Griffin, T.J., Tyler, I.M., 2001. High- and low-K granites and adakites at a Paleoproterozoic plate boundary in northwestrn Australia. J. Geol. Soc. Lundon. 158, 547–560. Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A.D., Norry, M.J. (Eds.), Implications for Mantle Composition and Processes, Magmatism in the Ocean Basins. Geological Society Special Publication, vol. 42, pp. 313–345. Thorkeison, D.J., Breitsprecher, K., 2005. Partial melting of slab window margins: genesis of adakitic and non-adakitic magma. Lithos 79 (1–2), 25–42. Voshage, H., Hoffman, A.W., Mazzucchelli, M., Revalenti, G., Sinigoi, S., Raczek, I., Demarchi, G., 1990. Isotopic evidence from the Ivrea zone for a hybrid lower crust formed by magmatic underplating. Nature 347, 731–736. Wang, Q., Zhao, Z.-H., Bai, Z.-H., Bao, Z.W., Xu, J.-F., Xiong, X.-L., Mei, H.-J., Wang, Y.-X., 2003. Carboniferous adakites and Nbenriched arc basaltic rocks association in the Alataw Mountains, north Xinjiang: interactions between slab melt and mantle peridotite and implications for crustal growth. Chin. Sci. Bull. 48, 2108–2115. Wang, Q., Wyman, D.A., Zhao, Z.-H., Xu, J.-F., Bai, Z.-H., Xiong, X.-L., Dai, T.-M., Li, C.-F., Zhu, Y.-C., 2007. Petrogenesis of Carboniferous adakite and Nb-enriched arc basalt in the Alataw area, northern Tianshan Range (west China): Implication for Phanerozoic crustal growth in the Central Asia Orogenic Belt. Chem. Geol. 236, 42–64. Winther, K.T., 1996. An experimentally-based model for the origin of tonalitic and trondhjemitic melts. Chem. Geol. 127, 43–59. Xiao, X.-C., Tang, Y.-Q., Feng, Y.-M., Zhu, B.-Q., Li, J.-Y., Zhao, M., 1992. Tectonic Evolution of the Northern Xinjiang and It's Adjacent Region. Geo.Publ. House, Beijing, pp. 1–180 (in Chinese).

Xiong, X.-L., Zhao, Z.-H., Bai, Z.-H., Mei, H.-J., Wang, Y.-X., Wang, Q., Xu, J.-F., Bao, Z.-W., 2001a. Adakite-type sodium-rich rocks in Awulale Mountain of west Tianshan: significance for the vertical growth of continental crust. Chin. Sci. Bull. 46 (10), 811–817. Xiong, X.-L., Zhao, Z.-H., Bai, Z.-H., Mei, H.-J., Xu, J.-F., Wang, Q., 2001b. Origin of Awulale adakitic sodium-rich rocks in west Tianshan: constrains for Nd and Sr isotopic compositions. Acta Petrol. Sin. 17 (4), 514–522 (in Chinese, with English abstr). Xiong, X.-L., Li, X.-H., Xu, J.-F., Li, W.-X., Zhao, Z.-H., 2003. Extremely high-Na adakite-like magmas derived from alkali-rich basaltic underplate: the Zhantang andesites in the Huichang Basin of SE China. Geochem. J., 37, 233–252. Xiong, X.-L., Cai, Z.-Y., Niu, H.-C., Chen, Y.-B., Wang, Q., Zhao, Z.-H., Wu, J.-H., 2005a. The late Paleozoic adakites in eastern Tianshan area and their metallogenetic significance. Acta Petrol. Sin. 21, 967–976 (in Chinese, with English abstr.). Xiong, X.-L., Adam, J., Green, T.H., 2005b. Rutile stability and rutile/ melt HFSE partitioning during partial melting of hydrous basalt: implications for TTG genesis. Chem. Geol. 218, 339–359. Xiong, X.-L., Adam, J., Green, T.H., Niu, H.-C., Wu, J.-H., Cai, Z.-Y., 2006. Trace element characteristics of partial melting produced by melting of metabasalt at high pressures: constraint on the formation condition of adakitic melts. Sci. China (Series D) 49 (9), 915–925. Xu, J.-F., Shinjio, R., Defant, M.J., Wang, Q., Rapp, R.P., 2002. Origin of Mesozoic adakitic intrusive rocks in the Ningzhen area of east China: partial melting of delaminated lower continental crust? Geology 12, 1111–1114. Xu, X.-Y., Li, X.-M., Ma, Z.-P., Xia, L.-Q., Xia, Z.-C., Peng, S.-X., 2006. LA-ICPMS zircon U–Pb dating of gabbro from the Bayingou ophiolite in the northern Tianshan Mountains. Acta Geol. Sin. 8, 1168–1175 (in Chinese with English abstract). Yogodzhinski, G.M., Lees, J.M., Churlkova, T.G., Dorondor, F., Woerner, G., Volynets, N., 2001. Geochemical evidence for the melting of subducting oceanic lithosphere at plate edges. Nature 409, 500–504. Yuan, H.-L., Gao, S., Liu, X.-M., Li, H.-M., Gunthes, D., Wu, F.-Y., 2004. Accurate U–Pb age and trace element determinations of zircon by laser ablation-inductively coupled plasma-mass spectrometry. Geostand. Geoanalytical Res. 28 (3), 353–370. Zhang, L.-C., Xiao, W.-J., Qin, K.-Z., Zhang, Q., 2006. The adakite connection of the Tuwu-Yandong copper porphyry belt, eastern Tianshan, NW China: trace element and Sr-Nd-Pb isotope geochemistry. Miner. Depos. 41, 188–200. Zhang, Q., Wang, Y., Qian, Q., Yang, J.-H., Wang, Y.-L., Zhao, T.-P., Guo, G.-J., 2001. The characteristics and tectonic-metallogenic significances of the adakites in yanshan period from eastern China. Acta Petrolol. Sin. 17 (2), 236–244 (in Chinese, with English abstr.). Zhao, J.-M., Liu, G.-D., Lu, Z.-X., Zhang, X.-K., Zhao, G.-Z., 2003. Crust-mantle transitional zone of Tianshan orogenic belt and Junggar basin and its geodynamic implication. Sci. China (Series D) 44 (9), 824–837. Zhao, Z.-H., Wang, Z.-G., Zou, T.-R., Masuda, A., 1993. The REE, isotopic composition of O, Pb, Sr and Nd and diagenetic model of granitoids in Altay region. In: Tu, G. (Ed.), New Improvement of Solid Geosciences in Northern Xinjiang. Science Press, Beijing, pp. 239–266 (in Chinese). Zhao, Z.-H., Bai, Z.-H., Xiong, X.-L., Mei, H.-J., Wang, Y.-X., 2003. 40Ar/39Ar chronological study of Late Paleozoic volcanic-hypabyssal igneous rocks in western Tianshan, Xinjiang. Geochimica 32, 317–327 (in Chinese, with English abstr).

Z.H. Zhao et al. / Lithos 102 (2008) 374–391 Zhao, Z.-H., Xiong, X.-L., Wang, Q., Bai, Z.-H., Mei, H.-J., 2004a. A case study on porphyry Cu deposit related with adakite quartz albite porphyry in Moszaote, Western Tianshan, Xinjiang, China. Acta Petrol. Sin. 20, 49–258 (in Chinese, with English abstr.). Zhao, Z.-H., Xiong, X.-L., Wang, Q., Bai, Z.-H., Xu, J.-F., Qiao, Y.-L., 2004b. The association of Late Paleozoic adakitic rocks and

391

shoshonitic volcanic rocks in Western Tianshan, China. Acta Geol. Sin. 78, 68–72. Zheng, J.-P., Lu, F.-X., Luo, Z.-H., Zhang, R.-S., Yu, C.-M., Yu, X.-L., 2005. The lower crust characteristics evidenced from the granulite xenoliths from Tuoyun, SW Tianshan, Xinjiang , Bulle. Mineral. Petrol. Geochem. 24 (suppl.), 137.