ARTICLES · 2009-09-09 · ARTICLES 952 Chinese Science Bulletin Vol. 51 No. 8 April 2006 Chinese Science Bulletin 2006 Vol. 51 No. 8 952—962 DOI: 10.1007/s11434-008-0952-7 SHRIMP

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952 Chinese Science Bulletin Vol. 51 No. 8 April 2006

Chinese Science Bulletin 2006 Vol. 51 No. 8 952—962 DOI: 10.1007/s11434-008-0952-7

SHRIMP zircon U-Pb dating for subduction-related gran-itic rocks in the northern part of east Junggar, Xinjiang ZHANG Zhaochong1,2, YAN Shenghao3, CHEN Bailin4, ZHOU Gang5, HE Yongkang5, CHAI Fengmei1, HE Lixin5 & WAN Yusheng2

1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;

2. Institute of Geology, Chinese Academy of Geological Sciences, Bei-jing 100037, China;

3. Institute of Mineral Resources, Chinese Academy of Geological Sci-ences, Beijing 100037, China;

4. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100037, China;

5. No. 4 Geological Party, Xinjiang Bureau of Geology and Mineral Resources, Altay 836500, China

Correspondence should be addressed to Zhang Zhaochong (email: [email protected])

Abstract SHRIMP U-Pb zircon dating on the Xileketehalasu granodiorite porphyry and Kalasayi monodiorite porphyry that intrude middle Devonian Beitashan Formation at the north part of east Junggar region shows that they were formed at 381±6 Ma and 376±10 Ma respectively. They are interpreted as subduction-related granitic rocks, which is the first report that the isotopic ages for the granitic rocks range from 350 to 390 Ma. Another determined age for the Kalasayi monodiorite porphyry is 408±9 Ma, representing the age of underlain Lower Devonian volcanic rocks. Thus, the U-Pb dates suggest that the northeastward subduction of Junggar ocean from southwest occurred at 408 to 376 Ma (the real inter-val may be larger). Because the ore-bearing porphyry intruded following the formation of the volcanic rocks of middle Devonian Beitashan Formation, their tec-tonic setting is similar to the Andes Mountains that hosts world-class porphyry copper deposits, and the researched area could be regarded as a potential area for prospecting large porphyry copper deposits.

Keywords: SHRIMP U-Pb zircon age, subduction, granitic porphyry, east Junggar.

A great deal of studies have recently devoted to the Central Asian orogenic belt (CAOB). Some of the studies have proposed that CAOB is a tectonic frame of

complex mosaic fragments, link of multiple suture zone and mountain-basin coupling, and has undergone some important geologic processes, such as bidirectional subduction and intracontinental orogeny during Meso-zoic and Cenozoic[1－3]. Moreover, the continental crust growth is assumed to occur by Paleozoic subduction and underplating of basic magmas[4] during the postcollisional stage and partial melting of juvenile crust, which is represented by a large scale of post-collisional granites at 290 to 300 Ma (average 294 Ma)[5―7]. How-ever, no geochronological study has been made for the ages of subducted granites in the northern part of the east Junggar region that is regarded as an important part of CAOB.

The northern part of east Junggar region is a Phan-erozoic accretion orogenic belt[8], tectonically located at the integrated region between Siberian and Kazak-stan-Junggar plates. Before early Paleozoic, the re-searched area belonged to a part of ancient Central Asian plate, and underwent the processes of stable con-tinental margin. The previous studies have shown that at least from Early Ordovician, there was a Junggar ocean (connecting with Sangzai ocean in the west and south Mongolian ocean in the east) separating Junggar plate from Altay plate, thereafter underwent the proc-esses of subduction, collision and accretion[2,7,9,10]. Af-ter late Paleozoic, the Junggar ocean gradually closed subsequent to southward draft of Siberian plate and northward draft of Siberian and Tarim plate, and com-pletely closed in the Middle Carboniferous[11]. However, ages for the beginning and the end of the subduction of Junggar ocean have not been constrained. Recently, we determined the ages of the Xileketehalasu and Kalasayi granitic porphyries by SHRIMP method when we stud-ied the porphyry copper deposits in the researched area. The results provide further confirmation of occurrence of the subducted granites in the area, and thus place constraints on timing of the onset and termination of the subduction of Junggar ocean. In this paper, we re-port the U-Pb dating results, and discuss their implica-tions for geodynamics and prospecting for porphyry copper deposits.

1 Geologic setting and petrology The researched area is located in the south side of

the Irtylish-Mayinebo fault, which marks the boundary between Junggar orogenic belt to south and Altay oro-genic belt to the north (Fig. 1). The regional strata in the north part of the east Junggar region consists of

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Fig. 1 Sketch regional geologic map of the south margin of Altay orogenic belt (a), geological map of Kalasayi (b) and Xileketehalasu regions (c). Regional tectonic location is shown at the right upper corner[12], and the square marks the location of regional geologic map. (a) 1. Quarternary; 2. Permian continental volcanic rocks; 3. Lower Carboniferous volcanic-sedimentary rocks; 4. Middle Devonian volcanic rocks; 5. Cambrian-Ordovician sandstone and limestone; 6. middle-upper Proterozoic gneiss and schist; 7. Devonian-Carboniferous granite; 8. Permian granite; 9. mafic intrusion; 10. ultramafic intrusion; 11. fault and its number (⑤represents Irtylish fault). (b) 1. Basalt and andesite; 2. diorite porphyry; 3. quartz diorite porphyry; 4. monodiorite porphyry; 5. diorite dyke; 6. syneite porphyry; 7. andesitic tuff; 8. Quaternary. (c) Q, Quaternary; β, basalt; TSs, basaltic tuff; β+α, ba-salt+andesite; δ, diorite; ξγ, syenite porphyry; δµ, diorite porphyry; ξπ, quartz porphyry; γ0, granite porphyry; λπ, granodiorite and quartz diorite porphyry; λϕ, fracture alteration zone; Bt, biotite alteration. Stars represent the locations of the sampling. Kalasayi and Xileketehalasu regions are shown in (a).

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Lower Devonian Tuoranggekuduke Formation of ma-rine pyroclastic rocks and sedimentary rocks, Middle Devonian Beitashan Formation of intermediate-basic volcanic rocks and pyroclastic rocks and carbonate rocks, Middle Devonian Yundukala Formation of vol-canic rocks, Carboniferous volcanic-sedimentary vol-canic rocks and Permian continental coal-bearing sedimentary rocks. The Xileketehalasu and Kalasayi porphyries intruded Middle Devonian strata, which consist chiefly of basic rocks with minor intermediate rocks and sedimentary rocks in the upper part. In addi-tion, minor picrites occurred in the lower part. The ge-ology and petrology of these two porphyries are de-scribed as follows:

The geology of the Xileketehalasu porphyry was de-scribed previously[13]. The central coordinate is 46°34′37″N and 90°03′24″E. The porphyry consists of granodiorite and quartz diorite porphyry, which change gradually. It strikes 325°, dipping southwest, consistent with regional structure line. It is 640 m long, dozens of meters wide, up to 170 m, with an area of 0.04 km2. It underwent very slightly deformation, appearing ellipse shape (Fig. 1). The rock display granite texture, con-sisting of biotite (1%―4%), orthoclase (15%―25%), oligoclase (30%―40%) and quartz (5%―25%). The rock underwent strong alteration, and the alteration types mainly contain K-feldspar, quartz, biotite and sericite alteration. The rock was completely mineral-ized (peacock alteration) on the surface. The coordinate of the sample for SHRIMP dating is 46°34′0.2″N and 90°03′32.1″E.

The Kalasayi porphyry is situated at the southeast of the Xileketehalasu porphyry (Fig. 1). Its coordinate is 46°20′13″N and 90°16′28″E. The rock consists of dio-rite porphyry, quartz diorite porphyry and monodiorite porphyry. It occurs in ellipse shape, and has not been deformed. Its long axis strikes northwest, the same as the regional tectonic line, being 600 m long, and 30―70 m wide, and with an area of 0.03 km2. The porphyry intruded the Beitashan Formation, and its boundary is distinct. The big xenolith of the country rocks (up to 15 m×5 m) can be observed in the porphyry. The pet-rofacies zoning is obvious, consisting dominantly of monodiorite porphyry, and diorite porphyry and quartz diorite porphyry are located at the margin of the intru-sion. The latter is fine grained, with porphyritic texture. The phenocrystals contain pyroxene and amphibole. The quartz grains are fine, distributing in groundmass.

The porphyritic texture is not obvious in monodiorite porphyry, and displaying homogeneous compositions, and generally hosts some fine xenoliths of country rocks. Diorite porphyry can change gradually into quartz diorite porphyry, whereas there is distinct boundary with two other rock types. However, because of no good outcrop, their intrusive sequences cannot be confirmed. We infer that monodiorite porphyry might form at the earlier stage on the basis of their texture relationship. The alteration of the intrusion is not strong, with only slightly mineralized quartz, and epidote al-teration, and local copper mineralization is observed. The coordinate of the sample for SHRIMP dating is 46°20′24.3″ N and 90°16′55.4″E.

2 Analyzing method and results Representative zircon grains were picked under bin-

ocular microscope by using heavy liquid and magnetic techniques. They were handpicked and mounted in an epoxy resin disc, and then polished. Internal structure was examined using CL images and backscatter elec-tron (BSE) at the Electron Microprobe Lab of the In-stitute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS). The U-Pb analyses were performed using the Sensitive High-Resolution Ion Microprobe (SHRIMP- ) at CAGS in Beijing. OperⅡ a-tion and data processing procedures were referred to refs. [14, 15] in detail. The standard TEM zircons (age 417 Ma) of RSES were used in interelement fractiona-tion, and U, Th and Pb concentrations were determined based on the standard Sri Lankan gem zircon SL13, which has a U concentration of 238 µg/g corresponding to an age of 572 Ma. Data processing procedures were after PRAWN and isoplot, and the 204Pb-based method of common Pb correction was applied. Uncertainties of data points in Table 1 and Table 2 are given at ± 1σ. The ages quoted in the text are 206Pb/238U ages, which are the weighted mean at the 95% confidence level.

The U and Th contents of the Xileketehalasu and Kalasayi porphyries are 75―239 µg/g, 25―183 µg/g, 77―276 µg/g and 54―419 µg/g respectively (Table 1). Their Th/U ratios range from 0.33 to 0.88 and 0.56 to 1.52 respectively, characterizing magmatic zircon[16], which is also suggested by their shapes (Figs. 2 and 3). 13 spots of the Xileketehalasu granodiorite porphyry fall on the concordant U-Pb ages except for 3 spots (Fig. 4), and yield an average 206Pb/238U age of 381±6 Ma, with MSWD = 0.84. If all 16 spots of the zircon grains

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Fig. 2. CL images for the zircons of Sample HLS01.

Fig. 3. CL images for the zircons of Sample KL.

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Fig. 4. SHRIMP zircon U-Pb concordia diagram for the Xileketehalasu granodiorite porphyry (HLS01).

from the Kalasayi monodiorite porphyry were averaged, the MSDW value would be too large (3.8), suggesting that the error is too high. If we divided them into two groups, we will obtain the average 206Pb/238U age of 408±9 Ma for 6 spots (1.1, 2.1, 3.1, 10.1, 13.1, 14.1), and 376±10Ma for 10 spots with MSDW values of 0.78 and 1.9 respectively (Fig. 5), all are within the error range. 376±10 Ma is close to the age of the Xilekete-halasu granodiorite porphyry, possibly indicating the crystallization age of zircons whereas 408 ± 9 Ma is older than their country rocks, which is inconsistent with the geologic relationship, thus suggesting that this age might represent the age of the xenocryst zircon.

Fig. 5. SHRIMP zircon U-Pb concordia diagram for the Kalasayi monodiorite porphyry (KL).

3 Discussion Although the isotopic age of the Beitashan Forma-

tion has not been reported, it can be inferred to as Mid-dle Devonian on the light of the fossils hosted in the Beitashan Formation at the same belt (Qiaoxiahala re-gion in the west), such as Brachiopoda (Mucrospirifer mucronatus, Acrospirifer sp., Tridensills sp., Spina-

trypa sp.), bryozoan (Fenestella sp.) and plant fossils (Lepidosigillaria)[17]. The average 206Pb/238U age of zircons from the Xileketehalasu granodiorite porphyry is 381±6 Ma with low error, representing the formation age of the porphyry. The zircons of the Kalasayi mono-diorite porphyry have two groups of ages, and all of the zircons have oscillatory zoning and high Th/U ratios, suggesting magmatic origin. However, as the porphyry intruded the Beitashan Formation, 376±10 Ma should represent the crystallization age of zircons, because it is close to 206Pb/238U age of the Xileketehalasu granodio-rite (381±6 Ma). By contrast, 408±9 Ma is older than Middle Devonian, belonging to Early Devonian, thus suggesting that the zircons were captured from the ear-lier stage of the strata. In addition, most of them for this group are sub-rounded (e.g., 2.1, 3.1, 10.1), and can be interpreted as trapped zircons. According to the number of zircons, more than half zircons belong to the group of 376±10 Ma, thus it is more likely that they were de-rived from the porphyry. According to the regional ge-ology, the Beitashan Formation overlaid the Lower Devonian Tuoranggekuduke Formation, which consists mainly of lava and pyroclastic rocks, and it can there-fore be inferred that the 408±9 Ma can be interpreted as the age of the Lower Devonian Tuoranggekuduke For-mation, because it is consistent with the age of the Tuoranggekuduke Formation (Early Devonian).

Many studies have suggested that processes of oce-anic spreading, plate subduction, and subsequent colli-sion and postcollisional orogeny occurred in the Jung-gar region[1,2,7,9,10]. Recently, M-type of granites related to ocean spreading has been recognized, which formed at about 393.6 Ma[1], and those related to collision and postcollision formed at about 294 Ma[5,6]. However, the subducted granites as the most important stage of oro-genic processes have not been found, and the isotopic age of ca. 370―390 Ma of granites has not been re-ported[4]. In recent years, finding of boninite and picrite in the Beitashan Formation suggests that the volcanic rocks of the Beitashan Formation formed an island arc[18,19], and the latter resulted from the northeastward subduction of the Junggar ocean[19,20]. The geochemical characteristics of the two porphyries on the geochemi-cal-tectonic discrimination diagram fall in the field of arc-related granites (Fig. 6), and their primitive man-tle-normalized trace element patterns with negative Nb, Ta and Ti anomalies as well as slightly negative P anomalies (Fig. 7) are consistent with the geochemical characteristics of arc granitic rocks. Their A/NKC val-

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Fig. 6. Nb-Y and Rb-(Y+Nb) diagrams of the Xileketehalasu and Kalasayi porphyrites[24]. S-COLG: syncollision granite; WPG: within plate granite; ORG: oceanic ridge granite; VAG: volcanic arc granite.

Fig. 7 Primitive mantle-normalized incompatible element patterns of the Xileketehalasu and Kalasayi porphyrites (normalized values are from ref. [25]). ues range from 0.81―1.29, and most of samples have the values less than 1.1. The A/NKC values of the Kalasayi porphyry are lower than those of the Xileketehalasu porphyry, which can be attributed to strongly sericite alteration in the porphyry. Hence, the relatively low A/NKC values are inconsistent with typical S-type granites during collisional period[21]. In addition, their low initial 87Sr/86Sr ratios (0.70383－0.70410), high εNd(t) values (+7.3－+8.5) and mid δ18O (7.9‰－8.6 ‰) values also indicate an arc environment (Table 4). Especially, the relatively lower δ18O values preclude the possibility of crustally melting-granites. It is more likely that they were produced by melting of subducted oceanic basalts, which can also be supported by their relatively low SiO2 contents. Their (La/Yb)n ratios vary from 2.67 to 8.32, and δEu values range

from 1.00 to 1.27 (Table 3), which distinguish from that of anatectic granites that have pronounced Eu anomalies[22,23]. On the oceanic ridge granite (ORG)-normalized patterns (Fig. 8), they are similar to typical arc- island granites (e.g., Little Port), but different from col-lisional granites (e.g., Alps) or continental arc granites (e.g., Chile). In summary, their geochemical character-istics reflect that they formed in an arc environment, but not collisional setting or continental arc setting.

Fig. 8 Ocean-ridge-granite (ORG) normalized geochemical patterns for the Xileketehalasu and Kalasayi porphyrites (normalized values are from ref. [24], and therein)

Additionally, the previous work shows that the

SHRIMP U-Pb zircon dating on the gabbro and pla-giogranite of the Zhaheba-Aermantai ophiolites at the south of the two porphyries have yielded the ages of 481―489 Ma[26], thus forming at Early Paleozoic. However, they should only represent the ages of an-cient Junggar oceanic crust rather than subduction age[26]. If we consider the isotopic dates on the Kalasayi porphyry, it is very likely that 408±9 Ma of trapped zircons represents the ages of the beginning of the subduction of the Junggar ocean, or slightly earlier be-cause the Nb-enriched basalts and adakites of the Tuoranggekuduke Formation represent the early stage of the oceanic arc[18,27]. This age is very close to an-other age of the Zaheba ophiolite, which was inter-preted to be a melting event[26]. Zhang et al.[28] obtained the SHRIMP U-Pb zircon age of 372±19Ma for the Kuerti ophiolite at the northern part of the researched area, and they proposed that the ophiolite resulted from back-arc spreading following northward subduction of ocean plate[28,29]. This conclusion is completely consis-tent with our new data. As a result, we propose the fol-lowing processes to explain the tectonic evolution: the Zaheba-Aermantai ophiolite represents an ancient oce-anic crust, forming ca. 481―489 Ma, and started to subduct northeastward at ca. 408 Ma. Thus, the arc volcanic rocks of the Tuoranggekuduke Formation formed at this period. A mildly mature arc formed at ca.

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Table 3 Major elements (%) and trace elements (×10−6) analyses of Xileketehalasu and Kalasayi porphyries a) XH-3 XH-13 XH-18 HLS-4 HLS-5 KL-1 KL-8 KL-13

SiO2 63.41 65.45 62.97 63.30 63.78 61.48 64.33 53.36 TiO2 0.46 0.35 0.46 0.50 0.46 0.32 0.36 0.71 Al2O3 16.52 15.75 16.00 16.85 16.20 19.19 16.93 18.28 Fe2O3 3.14 2.15 3.39 3.41 2.85 1.59 2.59 4.65 FeO 2.39 1.83 3.23 1.56 2.24 1.45 1.45 3.14 MnO 0.03 0.06 0.08 0.02 0.03 0.04 0.02 0.09 MgO 1.72 1.61 1.61 1.66 1.68 1.31 1.30 2.87 CaO 0.83 2.11 0.61 0.94 1.92 4.08 3.38 6.89 Na2O 5.00 5.52 4.40 4.19 3.76 8.12 4.26 3.62 K2O 4.04 3.60 3.77 4.58 4.48 1.47 3.07 3.88 P2O5 0.09 0.17 0.21 0.24 0.21 0.15 0.15 0.39 CO2 0.48 0.22 0.30 0.26 0.78 0.17 0.21 0.35 H2O 1.58 0.92 1.86 1.64 1.48 0.68 1.38 1.62 Loss 1.62 2.16 0.69 1.29 1.68 Total 99.69 99.74 98.89 100.77 100.87 100.74 100.72 100.53

A/NKC 1.17 0.94 1.29 1.24 1.12 0.86 1.03 0.81 La 5.55 7.74 10.30 10.30 10.40 15.0 19.9 13.0 Ce 11.20 15.20 20.00 20.40 20.50 28.1 39.4 28.4 Pr 1.41 2.05 2.91 2.71 2.68 3.59 5.34 3.90 Nd 5.65 8.05 12.20 11.10 11.20 14.0 22.6 16.6 Sm 1.41 1.96 3.36 2.61 2.67 2.80 5.10 3.89 Eu 0.60 0.66 1.16 1.02 0.86 1.09 1.65 1.34 Gd 1.52 1.97 3.62 3.03 2.64 2.48 4.63 3.92 Tb 0.28 0.35 0.65 0.54 0.43 0.34 0.66 0.58 Dy 1.87 2.07 4.21 3.34 2.62 1.99 3.91 3.52 Ho 0.39 0.41 0.83 0.76 0.55 0.39 0.78 0.71 Er 1.20 1.21 2.37 2.32 1.68 1.14 2.16 2.00 Tm 0.19 0.19 0.37 0.37 0.27 0.17 0.31 0.29 Yb 1.37 1.29 2.23 2.54 1.81 1.18 2.04 1.87 Lu 0.23 0.21 0.43 0.42 0.29 0.19 0.31 0.29 Sc 8.65 7.58 8.38 8.07 6.82 4.84 13.4 3.85 V 104 95 86.9 124 91.4 74.4 175 48.5 Cr 9.65 24.1 12.7 19.0 9.45 9.92 6.72 9.03 Co 13.9 9.27 35.1 12.8 3.34 9.04 9.84 4.66 Ni 6.77 10.4 16.8 6.51 6.65 5.32 5.53 5.39 Cu 645 505 3017 1807 214 273 170 63.4 Zn 28.5 25.9 55 47.0 44.1 13.4 34.9 16.4 Ga 18.0 20.1 13.6 Rb 112 64.5 134 154 150 77.0 86.3 23.9 Sr 447 416 378 381 329 807 790 605 Zr 124 83.3 130 142 137 104 105 102 Nb 5.35 3.97 4.68 3.32 3.18 5.32 5.66 6.19 Ba 562 553 540 526 488 595 417 245 Hf 3.32 2.31 3.64 3.76 3.57 2.82 2.86 2.64 Ta 0.7 0.49 0.53 0.37 0.32 0.49 0.49 0.54 Pb 5.8 2.86 6.06 6.46 4.56 2.35 3.94 2.44 Th 1.37 2.03 3.24 3.24 2.63 2.11 2.95 1.55 U 0.49 0.45 0.64 0.93 0.54 0.62 0.75 0.46 Y 10.4 11.8 24.0 20.7 15.2 10.5 19.7 17.9

(La/Yb)n 2.67 3.95 3.04 2.67 3.78 8.37 6.43 4.58 δEu 1.27 1.03 1.03 1.12 1.00 1.26 1.04 1.06

a) Major elements were analyzed by XRF and trace elements were analyzed by ICP-MS at the National Research Center for Geoanalysis.

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Table 4 Sr, Nd and O isotope analyses of the Xileketehalasu porphyry a) Sample No. Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr 2σ (87Sr/86Sr)t δ18OV-SMOW (‰)

XH-3 115.94 447.2 0.750 0.707961 0.000019 0.703989 8.6 XH-13 73.25 416.1 0.509 0.706800 0.000020 0.704104 7.9 XH-18 127.96 377.5 0.981 0.709028 0.000020 0.703833 8.1

Sample No. Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd 2σ (εNd)0 (εNd)t

XH-3 1.30 5.73 0.1378 0.512920 0.000015 5.5 8.4 XH-13 1.76 8.15 0.1306 0.512844 0.000010 4.0 7.3 XH-18 2.95 11.80 0.1512 0.512957 0.000010 6.2 8.5

a) Sr and Nd isotopes were carried out by Institute of Geology and Geophysics, Chinese Academy of Sciences, and O isotopes by Institute of Min-eral Resources, Chinese Academy of Geological Sciences. 380 Ma was used for calculation. λ (Sr)=1.39×10−11, λ (Nd) = 6.54×10−12.

381 Ma, and the volcanic rocks of the Beitashan For-mation formed, and thereafter the Xileketehalasu and Kalasayi porphyrites intruded. At ca. 372 Ma, back-arc spreading, forming the Kuerti ophiolite, occurred at the north part of the researched area.

Consequently, it can also be inferred that the age of the subduction of the Junggar ocean can be in the lim-ited range of 408―376 Ma. Although the real age range of the subduction may be larger, it is certain that 408―376 Ma should be the subduction stage of the Junggar ocean.

In recent years, the discovery of the Xileketehalasu porphyry copper deposit[13] has received much attention. The SHRIMP U-Pb zircon dating further provides the evidence that the porphyry formed following the erup-tion of the Middle Devonian volcanic rocks, i.e. formed during the subduction of plate. Consequently, their tec-tonic setting is similar to the Andes Mountains hosting world-class porphyry copper deposits[30], and it can be inferred that it is a potential area for prospecting for large-scale of porphyry copper deposits.

On the other hand, a long debate surrounds the age of the basement of the Junggar Basin [31]. Some people proposed that the basement is a Precambrian continen-tal microblock or Precambrian basic-ultrabasic complex, and some other people argued that it might consist of Paleozoic limited ocean crust or Paleozoic continental block. No Precambrian ages of zircons from two por-phyries have been recognized (Table 1 and Table 2). In general, most intrusions would capture more or less zircons from the basement. From this view point, there have been no any signals of the Precambrian basements yet.

4 Conclusions (1) The Xileketehalasu granodiorite porphyry and

Kalasayi monodiorite porphyry formed at 381±6 Ma

and 376±10 Ma respectively. They were intruded following the formation of the volcanic rocks of the Mid- dle Devonian Beitashan Formation, and thus may be the representatives of subducted granites. Another SHRIMP U-Pb zircon age of 408±9 Ma may represent the formation age of the volcanic rocks of the underly- ing Lower Devonian Tuoranggekuduke Formation

(2) The time interval of the northeastward subduc-tion of the Junggar ocean from southwest may range from 408 to 376 Ma (the real interval might be larger), and the subduction occurred soon after the Junggar ocean formed.

(3) The target areas in this study has the similar tec-tonic setting to the Andes hosting world-class porphyry cooper deposits, the potential area of prospecting for large-scale porphyry cooper deposits.

Acknowledgements Helpful discussion from Profs. Han Baofu and Li Jingyi is much appreciated. This work was supported by the National Natural Science Foundation of China (Grant No. 40072061), National 305 Project (Grant No. 2001BA609A-07-02), Program for New Century Excellent Talents in University (Grant No. NCET-04-0728) and “973” Project (Grant No. 2001CB409807).

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(Received November 30, 2005; accepted January 18, 2006)

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ARTICLES · 2009-09-09 · ARTICLES 952 Chinese Science Bulletin Vol. 51 No. 8 April 2006 Chinese Science Bulletin 2006 Vol. 51 No. 8 952—962 DOI: 10.1007/s11434-008-0952-7 SHRIMP