Timing of Precambrian Basement from East Segment of Tiekelike Tectonic Belt, Southwestern Tarim, China: Constrains from Zircon U-Pb and Hf Isotopic

INTRODUCTION

Tarim craton is one of the largest cratons in China, but the Precambrian geological and tectonic features of Tarim are less constrained than that of other cratons (North China craton and Yangtze craton). However, many recent researches indicate that the Tarim craton features clear signs of Rodinia-related orogenic and break-up processes (Turner, 2010; Zhang et al., 2009, 2007a; Lu et al., 2008; Wang et al., 2006; Guo et al. 2005; Xu et al., 2005), although the reconstruction of the history of Tarim in Rodinia remains controversial (e.g., Evans, 2009; Li et al., 2008; Lu et al., 2008; Huang et al., 2005). The Tiekelike tectonic belt, located in the southwestern margin of the Tarim block, regarded as a junction part between the West Kunlun orogenic belt and the Tarim block (Fig. 1a). It is mainly composed of Precambrian sequences, which is the key area for understanding the formation and early crustal evolution of the southwestern part of Tarim craton.

Figure  1.  (a) Simplified tectonic map of China; (b) sketch geological map of West Kunlun; (c) distribution of Precambrian rocks of the Tiekeleke belt from Tarim; (d) section of Precambrian rocks from Yulongkashi River showing the locations of samples. ① Aoyitage-Kegang fault; ② Kudi-Qimanyute fault; ③ Kangxiwa fault; ④ Guozacuo fault; ⑤ Longmucuo-Shuanghu fault.

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The Tiekelike tectonic belt distributes in EW trend and narrow westward (Fig. 1b). Tiekelikeshan, the east segment of the Tiekelike belt, is mainly composed of Precambrian sedimentary strata of the Ailiankate and Sailajiazitege groups. These strata have been previously regarded as Paleoproterozoic-Mesoproterozoic, according to published isotopic data of keratophyre (e.g., 1 764 Ma Rb-Sr isochron age, Bureau of Geology and Mineral Resources of Xinjiang Uygur Autonomous Region, 1993). Also, LA-ICP-MS U-Pb dating on detrital zircons indicates that the Ailiankate Group contains three age groups: 2.3-2.45, 0.9-1.0, 0.8 Ga, and the last two were regarded as the time of metamorphism (Zhang et al., 2007b). Recently, Neoproterozoic intraplate magmatic rocks have been reported around Tarim lasting from 0.82 to 0.74 Ga (Zhang et al., 2009, 2007a; Xu et al., 2005; Chen et al., 2004). These data from Tiekelike belt have not shown the records related to the supercontinent Rodinia. Therefore, these results for the strata require us to revisit the age of the basement sedimentary rocks, and to obtain more detailed chronological data. In this article, we present new LA-ICP-MS detrital zircon U-Pb and Lu-Hf isotopic data for chlorite quartz schist from the Ailiankate Group and tuff from Sailejiazitage Group in Yulongkashi, eastern segment of the Tiekelike tectonic belt, which provide a refined view of the age of the Ailiankete Group and Sailejiazitage Group, and the Precambrian crustal formation and evolution of the southwestern portion of the Tarim block.

GEOLOGICAL SETTING AND SAMPLES

Tiekelike belt is separated by the northern margin fault belt in the north and Kegang fault in the south (Fig. 1b). It had been suggested to be a component of West Kunlun (Jiang et al., 2000), and outcrops as the dome-shaped thrusting slice in the core of North Kunlun anticlinorium (Cui et al., 2006). However, Tiekelike belt should be considered as the basement of the Tarim block reasonably (Ding et al., 1996). According to the new regional geological map (Li et al., 2009), we present that the study area is mainly composed of four units from north to south: Tiekelike belt, West Kunlun, Quanshuigou terrane and TaxkoganTianshuihai massif (Fig. 1b). Tiekelike belt extends in EW trend and narrows westward. The western segment comprises a series of tectonic slices, while the eastern segment takes an EW elongated diamondshaped block at Tiekeliteshan. Two segments display different kinetics characteristics (Cui et al., 2006). In addition, the Precambrian strata of these two parts have different constitutions. The west part consists of Paleoproterozoic Heluositan Group, namely Paleoproterozoic-Mesoproterozoic Ailiankate Group and Sailejiazitage Group, Mesoproterozoic-Neoproterozoic Bochatetage Formation, Sumalan Formation and Sukuluoke Formation to the south of Yecheng, whereas the east segment is mainly constituted by Ailiankate Group and Sailejiazitage Group (Fig. 1c). The strata of the east part are well outcropped at Yulongkashi Creek (Fig. 1d).

Ailiankate Group, also named "Kalakashi Group" in some literature, is mainly composed of schist, marble and phylite, which have undergone greenschist facies metamorphism (Bureau of Geology and Mineral Resources of Xinjiang Uygur Autonomous Region, 1993). Zhang et al. (2007b) suggested the stratum is most likely to be late Mesoproterozoic rather than Palaeoproterozoic on the basis of LA-ICP-MS zircon U-Pb dating on the quartz-schist. In the Tiekelikeshan, the eastern part of Tiekelike belt, Ailiankate Group tectonically contacted with the Sailajizitage Group. The representative profile locates in the east segment of Tiekelike belt, adjacent to the Yulongkashi Creek, and comprises mainly of intermediate-acidic rocks and clastic rock. A chlorite quartz schist sample (07HT-41) was collected for detailed zircon analysis at 33°55'21"N, 112°40'52"E (Fig. 1d).

Zhang (1958) had first defined the typical spilite keratophyre exposed at Yecheng County as the "Sailajiazitage Group", assigned to Cambrian-Sinian. A whole-rock Rb-Sr isochron age for potassic keratophyre from Aqike Creek yielded to 1 764 Ma (Wang, 1983). In addition, according to the field observation, this group overlain angular unconformity the Ailiankate Group as well as overlain unconformity by Jixianian Bochatetage Formation, therefore, the group had been previously defined to be Mesoproterozoic. It is noteworthy that these field relationships only distribute in the west section of Tiekelike belt. In the east section, the Sailajiazitage Group expose mostly with chlorite schist and contact ductile faultly with Ailiankate Group, extending from NEE to SWW. Samples (07HT-42) collected for U-Pb and Hf isotopic analysis in this article were collected from the meta-tuff at 36°40'13.3"N, 79°51'56.6"E.

ANALYTICAL METHODS

Zircons from chlorite quartz schist sample of Ailiankate Group (07HT-41) and tuff sample from Sailajiazitage Group (07HT-42) were selected using heavy liquids and a Frantz magnetic separator. Zircons from each sample were handpicked, mounted in epoxy resin and polished until the inner section was exposed. Cathodoluminescence (CL) images were obtained using a Mono CL3+ microprobe, to reveal internal structures and choose appropriate target sites for U-Pb and Hf analyses. The analyses were carried out in the State Key Laboratory of Continental Dynamics at Northwest University, China. This is because zircon grains from the chlorite quartz schist (07HT-41) are smaller and the U-Pb dating was only undertaken at 32 μm spot size. The U-Pb and Hf analyses were taken on the zircons which have larger grain size from tuff. The spot size is 44 μm.

U-Pb data were obtained using an Agilent 7500a ICP-MS. The ICP-MS equipped with unique Shield Torch brought about higher sensitivity. Zircon Hf isotope analysis was performed on a Nu PlasmaHR MC-ICP-MS (Nu Instruments Ltd., UK). The used GeoLas 200M laser ablation system consisted of a ComPex102 (193 nm ArF-excimer laser, Lambda Physik) and optical system (MicroLas). The in situ method for simultaneous measurement of U-Pb, Lu-Hf isotopes and trace element compositions of zircons using a quadrupole and multiple-collector inductively-coupled-plasma mass spectrometer (Q-ICP-MS and MC-ICP-MS, respectively) connected to a single excimer laser-ablation system.

Before analysis, ICP-MS operating conditions were generally optimized using continuous ablation of reference glass NIST SRM 610, to provide maximum sensitivity for the high masses while maintaining low oxide formation and low background. U, Th and Pb concentrations were calibrated by using ²⁹Si as the internal standard and NIST SRM 610 as the external standard. ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁶Pb/²³⁸U ratios were calculated using the GLITTER 4.0 program, and then corrected using Harvard zircon 91500 as the external standard. Apparent and discordia U-Pb ages were calculated by the ISOPLOT program (Ludwig, 2003). The detailed instrumental parameters and analytical procedures can be referred to Yuan et al. (2008).

For in stiu Lu-Hf isotope analyses, interference of ¹⁷⁶Lu on ¹⁷⁶Hf was corrected by measuring the intensity of interference-free ¹⁷⁵Lu isotope and the recommended ¹⁷⁶Lu/¹⁷⁵Lu ratio of 0.026 69 was applied (Bievre and Taylor, 1993). Similarly, the interference of ¹⁷⁶Yb on ¹⁷⁶Hf was corrected by measuring ¹⁷²Yb and using ¹⁷⁶Lu/¹⁷²Yb ratio of 0.588 6 (Chu et al., 2002). Standard zircon 91500 and GJ-1 were used as the reference standards for calibration and monitoring the condition of analytical instrumentation. A decay constant for ¹⁷⁶Lu of 1.865×10^-11 (Scherer et al., 2001), the present-day chondritic ratios of ¹⁷⁶Hf/¹⁷⁷Hf= 0.282 772 and ¹⁷⁶Lu/¹⁷⁷Hf=0.033 2 (Blichert-Toft and Albarede, 1997) were adopted to calculate ε_Hf(t) values. Single-stage Hf model ages (T_DM1) were calculated by reference to depleted mantle with a present-day ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.283 25 and ¹⁷⁶Lu/¹⁷⁷Hf ratio of 0.038 4 (Vervoort and Blichert-Toft, 1999). One-stage model Hf was calculated relative to the depleted mantle reservoir whereas two-stage model Hf age relative to average continental crust reservoir.

RESULTS

U-Pb Age Data: Chlorite Quartz Schist of Ailiankate Group (07HT-41)

Twenty-seven analyses of sample 07HT-41 (Table 1) defined 3 age groups (Figs. 3a, 3b). Most of the populations of zircon have an age between 736-810 Ma. The data are all near concordant and cluster at ca. 780 Ma (Fig. 3b). CL imaging display oscillatory zoning patterns for those zircons (Fig. 2a). U content ranges from 72 ppm to 1 746 ppm while Th ranges from 92 ppm to 1 464 ppm (Table 1). Th/U ratios vary between 0.26 and 1.52, typical range of magmatic zircon. Two grains gave a slightly discordant age of just less than 700 Ma. This is anomalously young, because the dominate grains analyzed are older (Fig. 2a, Table 1). Four older grains are identified and define a small population about 1.45 Ga. Two of the rim analyses (07HT-41-9 and 07HT-41-19) define ²⁰⁶Pb/²³⁸U age at 515-540 Ma with low Th and U contents and Th/U ratio (0.07-0.34), interpreted as the metamorphic age.

Table  1.  U-Pb isotopic data for zircons obtained from Sailajiazitage Group and Ailiankage Group in the southwestern margin of Tarim craton

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Figure  2.  CL images of respective zircons from chlorite quartz schist sample 07HT-41 (a) and tuff sample 07HT-42 (b).

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Figure  3.  Concordia diagrams of U-Pb analytical results for zircon from Tiekelikeshan of southwestern Tarim craton. (a), (b) Chlorite quartz schist sample 07HT-41; (c), (d) tuff sample 07HT-42. More discordance analyses are discarding in the age concordant diagram.

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U-Pb Age and Hf-Isotope Composition of Zircon from Tuff of Sailejiazitage Group (07HT-42)

Most analyzed zircons from sample 07HT-42 have oscillatory zoning (Fig. 2b). A group of 23 young grains give the concordant age range from 779 to 792 Ma, and define an age group with weighted mean ²⁰⁶Pb/²³⁸U age of 787±4 Ma (Fig. 3d), which is interpreted as the crystallization age of the zircons. Th/U ratios vary from 0.21 to 1.03, indicating their magmatic genesis. Besides, some zircons also contain cores without any zoning that are commonly discontinuous (Fig. 2b). Of them, 3 analyses define a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1 994±18 Ma. One near concordant analyses has an apparent ²⁰⁷Pb/²⁰⁶Pb age of 1 220±19 Ma. The youngest age of 704±8 Ma from the rim of grain represent the metamorphic age (Figs. 2b, 3c).

Thirty-nine detrital zircons from 07HT-42 have been analyzed for Hf isotope compositions (Table 2 and Fig. 3). Neoproterozoic zircons (~787 Ma) have dominantly negative ε_Hf(t) values (-13.3 to +1.6) and crustal model ages (1 507-2 179 Ma), with ¹⁷⁶Hf/¹⁷⁷Hf values varying between 0.282 007 and 0.282 298 (Fig. 4a). One Mesoproterozoic zircon (1 220 Ma) has ε_Hf(t)=+5.1 and crustal model age of 1 607 Ma. Whereas the older zircons (~1 994 Ma) have ε_Hf(t) values of -5.9 to +4.4 and crustal model ages of 2 272-2 784 Ma (Fig. 4b).

Table  2.  Hf isotopic compositions of zircons from Sailajiazitage Group in the southwestern margin of Tarim craton

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Figure  4.  (a) Temporal variations of ε_Hf(t) values of the studied detrital zircons and (b) T_DM2 model age histogram of zircons from Sailejiazitage Group of Tiekelikeshan of southwestern Tarim craton.

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DISCUSSION AND CONCLUSION

Formation Age of the Precambrian Strata of Tiekelikeshan and Its Implications

New obtained U-Pb ages indicate that the Ailiankate Group with one dominant mode around 780 Ma and yields the youngest concordant age of 736±8 Ma. This defines a maximum age for deposition of the Ailiankate Group in the eastern Tiekelike belt and indicates that the group is no older than Middle Neoproterozoic in age and older than 451 Ma (the oldest granite that intrudes into the Ailiankate Group, Wang et al. unpublished data).

The obtained zircon crystallization age of the tuff in the eastern Tiekelike belt is ~787 Ma, which we consider the best age estimate for the extrusion of the Sailejiazitage Group tuff. Geochemistry of the volcanics of Sailejiazitage Group indicates their characteristics of intraplate flooding basalt (Wang et al. unpublished data). The meta-volcanic rocks from the Ailiankate Group also record the intraplate geochemical signatures (Zhang et al., 2003).

Geochemistry and geochronology data of these igneous rocks indicate that the Sailejiazitage Group and Ailiankate Group are temporally equivalent and are likely parts of a single widespread succession, records a ~780 Ma extensional phases in the southwestern margin of Tarim craton. This is likely a response to an early rift phase of Rodinia at Neoproterozoic, rather than to Paleoproterozoic-Mesoproterozoic. The age cluster (~780 Ma) of Ailiankate Group corresponds well to the crystallization ages of Sailejiazitage Group tuff, suggesting that Ailiankate Group is probably derived from a local source. In Tiekelikeshan, there is no unconformity contact between Sailejiazitage Group and Ailiankate Group. The contact between these groups is likely a fault, as indicated by our independent field observations, which is consistent with our interpreted age distribution for the Sailejiazitage Group and Ailiankate Group.

The consistent new geochronology, composition characteristics and the geochemical characteristics of those rocks strongly support that the Sailejiazitage Group and Ailiankate Group were deposited at Neoproterozoic. The overall geological records of the region (unconformity interface, Precambrian sedimentary strata, kinetics characteristics, etc.) suggest that the Sailejiazitage Group and Ailiankate Group most likely needed to disassemble in the Tiekelike belt.

Neoproterozoic Crustal Growth in Southwest Tarim Block

Perhaps 20% of the Earth's continental crust formed or was intensely reworked during the Neoproterozoic. Both juvenile Neoproterozoic crust and older crust were melted, metamorphosed or otherwise reworked during this time (Stern, 2008). Combined in situ zircon U-Pb dating and Hf isotope analyses are able to constrain the growth and reworking of the continental crust and have been widely used (e.g., Wu et al., 2007; Zheng and Zhang, 2007). Zircon U-Pb ages are generally considered as the timing of igneous or metamorphic events, which is consistent with the time of crustal melting and reworking. The Hf model age represents the time of crustal growth, and used as a proxy for the formation age of the source rocks from which the igneous magmas were derived.

LA-ICPMS dating indicates that the magmatic zircons from the meta-tuff of the Sailejiazitage Group formed at about 787 Ma, whereas, the inherited zircon cores at about 1 994 Ma. Hf isotope analyses indicate that nearly all zircons of ~787 Ma have negative ε_Hf(t) values, with peak crustal model ages of ~2 000 Ma, indicating the sedimentary of rift melting of Paleoproterozoic crust. These older zircons are of detrital origin, and their provenance is the ancient wallrocks at the rift shoulder during the Middle Neoproterozoic rifting. They were weathered into the Sailejiazitage Group during its sedimentation in the rift basin. More Paleoproterozoic ages in the Ailiankate Group and the Heluositan Group of the west part of Tiekelike belt were founded too (Zhang et al., 2007b). These Paleoproterozoic zircons show positive and negative ε_Hf(t) values of -5.9-4.4 with their crustal model ages of 2 272-2 784 Ma, suggesting both growth of juvenile Paleoproterozoic crust and re-working of Archean crustal materials.