Neoproterozoic tectonic evolution of the Precambrian Aksu blueschist terrane, northwestern Tarim, China: Insights from LA-ICP-MS zircon U–Pb ages and geochemical data

Precambrian Research 185 (2011) 215–230

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Neoproterozoic tectonic evolution of the Precambrian Aksu blueschist terrane, northwestern Tarim, China: Insights from LA-ICP-MS zircon U–Pb ages and geochemical data Wenbin Zhu ∗ , Bihai Zheng, Liangshu Shu, Dongsheng Ma, Hailin Wu, Yongxiang Li, Wentao Huang, Junjie Yu State Key Laboratory for Mineral Deposits Research (Nanjing University), Department of Earth Sciences, Nanjing University, Nanjing 210093, PR China

a r t i c l e

i n f o

Article history: Received 13 August 2010 Received in revised form 11 December 2010 Accepted 7 January 2011 Available online 15 January 2011 Keywords: U–Pb zircon dating Geochemical analysis Aksu blueschist terrane Gondwana Tarim Craton

a b s t r a c t The Precambrian Aksu blueschist terrane (ABT) located in the northwestern Tarim Craton was formerly regarded as a Mesoproterozoic or an early Neoproterozoic complex. Yet, its tectonic significance remains poorly understood due to the lack of reliable age. We have conducted a detrital zircon U–Pb geochronological study of both the metasedimentary rocks from the ABT and the unmetamorphosed sandstones from the overlying Sinian succession to better constrain the age of the ABT. In addition, geochemical analyses were performed on the metasedimentary rocks to establish the broad tectonic setting of the source region of sediments. Our first U–Pb dating results suggest a maximum deposition age of ca. 730 Ma for the protolith of the metasedimentary rocks in the ABT and a maximum deposition age of 602 Ma for the unmetamorphased Sinian sandstones immediately overlying the ABT. Therefore, the blueschist-facies metamorphism in the ABT must have taken place after ca. 730 Ma, but prior to 602 Ma. This metamorphism may manifest the Pan-African orogeny (ca. 700–500 Ma), which is related to the assemblage of Gondwana, in the northern Tarim. Furthermore, the age range of 1.3–0.9 Ga was not recorded in the detrital zircons from both the metasedimentary rocks and the Sinian sandstones, suggesting that the northern Tarim Craton may not be significantly affected by the Grenville-age orogeny. A major age population at Paleopreoterozoic (ca. 2.0–1.8 Ga) was found in all samples, implying a Paleoproterozoic orogeny in the northern Tarim, which is coincident with the timing of the orogeny associated with the assembly of the Columbia supercontinent. Taking together the geochemical and chronological data, we propose that the northern margin of the Tarim Craton was probably a late Neoproterozoic active continental margin and a major source for the sedimentary rocks of the ABT, which provided a mixture of both old recycled sedimentary material from the basement rocks and juvenile material from the igneous rocks. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Blueschists are metamorphosed basaltic rocks with diagnostic mineral glaucophane. A typical mineral assemblage of blueschists include glaucophanic amphibole + lawsonite (or epidote) + chlorite + albite + quartz ± sodic (jadeitic) clinopyroxene ± aragonite (Ota and Kaneko, 2010). The protoliths of blueschists could be mid-ocean ridge basalt (MORB; e.g. Becker et al., 2000), ocean island basalt (OIB; e.g. Volkova and Budanov, 1999) or calc-alkaline basalt (CAB, e.g. Mahe’o et al., 2006). It is well known that blueschists are produced during high-pressure metamorphism and it is widely believed that the occurrence of blueschist-facies

∗ Corresponding author at: Department of Earth Sciences, Nanjing University, Nanjing 210093, PR China. Tel.: +86 25 83592921; fax: +86 25 83686016. E-mail address: [email protected] (W. Zhu). 0301-9268/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2011.01.012

metamorphism is indicative of subduction events (Ernst, 1988; Stern, 2005). Based on the nature of subduction, Maruyama et al. (1996) and Maruyama and Liou (1998) divided high-pressure (HP) and ultra-high-pressure (UHP) blueschist–eclogite belts into Atype (collision type) and B-type (Cordillera type), corresponding to A-type and B-type subduction, respectively. The A-type HP–UHP metamorphic belts result from continental collisions, usually consisting of passive continental margin materials and recording higher metamorphic pressure as well as temperature. The B-type HP blueschists correspond to subduction of oceanic plates and are commonly derived from oceanic basalt (MORB or OIB) or island arc basalt. Blueschists most commonly occur in Mesozoic and Cenozoic terranes (Molnar and Gray, 1979) and rarely appear in Precambrian terranes (Jahn et al., 2001). The Aksu blueschist terrane (ABT) is located in northwestern China (Fig. 1) and is regarded as one of the oldest well-substantiated Precambrian blueschist terranes in the world (Liou et al., 1989,

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Fig. 1. A. Main tectonic elements of China. NCB: North China block, SCB: South China block, SGT: Songpan-Ganzi terrane, QB: Qaidam basin, QT: Qiangtang terrane, LT: Lhasa terrane (modified after Li et al., 2007). B. Simplified tectonic map of the Tarim craton and its surrounding areas in northwest China (modified after Yin and Nie, 1996). C. Interpretation of remote sensing image of the Aksu blueschist terrane (Landsat-7 ETM+ image, composed of 5, 4, 3 bands). Pt: Proterozoic, Z: Sinian, Є: Cambrian, Q: Quaternary (after Zheng et al., 2010). D. Simplified stratigraphic column of the Neoproterozoic to Early Cambrian in the NW Tarim Basin (modified after Turner, 2010).

1996; Nakajima et al., 1990). The ABT was intruded by numerous unmetamorphosed mafic dykes and is overlain by the Sinian successions (Figs. 1 and 2). Despite of more than two decades of research, the tectonic significance of the ABT remains a matter of intense debate. The ABT has been interpreted either as (1) an accretionary complex formed 700 Ma ago along the northern margin of the proto-Tarim Craton, which presumably constituted the northernmost front of the Gondwana supercontinent (Liou et al., 1989, 1996; Nakajima et al., 1990); or as (2) a terrane experienced highpressure metamorphism associated with the Grenville Orogeny and the amalgamation of Rodinia around 1.0 Ga (Gao et al., 1993; Chen et al., 2004; Lu et al., 2008; Zhang et al., 2009a). Although several geochronological methods had been used to study the Aksu

blueschists and intruding mafic dykes in previous studies (Table 1), the conflicting geochronological results did not allow a satisfactory interpretation of the tectonic significance of the ABT. In order to obtain reliable age data we conducted detrital zircon U–Pb geochronological studies on both the metasedimentary rocks from the ABT and the unmetamorphosed sandstones from the overlying Sinian succession. The youngest zircon can define the maximum deposition ages of both the metasedimentary rock and the overlying Sinian succession. Logically, after the deposition of the protolith of metasedimentary rocks, but prior to the deposition of the overlying Sinian succession, the blueschist-facies metamorphism occurred. Moreover, single-grain U–Pb dating of detrital zircons is also a powerful tool for provenance studies because it

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Fig. 2. Field photographs of representative lithological units of the Aksu blueschist terrane (ABT). A. Unmetamorphosed mafic dykes cross-cutting the metamorphosed Aksu Group. B. An outcrop of glaucophane-rich blueschists in the ABT. C. Interbedded metasedimentary rocks and mafic schists in the ABT. Both of them are strongly deformed. Metasedimentary rocks here are mainly composed of pelitic schists, while greenschists and blueschists are interbedded in mafic schists. D. An outcrop of psammitic schists in the ABT. E. An outcrop of pelitic schists with minor folding in the ABT.

can provide information about source areas with distinctive zircon age populations (Gray and Zeitler, 1997; DeCelles et al., 1998, 2004; Gehrels et al., 1995, 1999, 2000; DeGraaff-Surpless et al., 2002; Dickinson and Gehrels, 2003; Link et al., 2005; Amidon et al., 2005a,b; Surpless et al., 2006; Wang et al., 2007; Duan et al., 2011; Fernández et al., 2010 and references therein; Kuznetsov et al., 2010

and references therein; Sun et al., 2009). Furthermore, geochemical analysis was performed on the metasedimentary rocks to infer the regional tectonic setting of the source region of sediments (Taylor et al., 1983; Bhatia, 1983; Taylor and McLennan, 1985; Bhatia and Crook, 1986; Roser and Korsch, 1986; Fralick and Kronberg, 1997; Patchett et al., 1999; Fralick et al., 2009). The age data, together

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Table 1 Metamorphosed ages of blueschist and intrusive ages of mafic dykes. Rock type

Age (Ma)

Dating method

References

Blueschist Blueschist Blueschist Blueschist Blueschist Blueschist Blueschist Mafic dyke Mafic dyke Mafic dyke

962 ± 12 Ma 944 ± 12 Ma 718 ± 22 Ma 698 ± 26 Ma 754 Ma 872 ± 2 Ma 862 ± 1 Ma 807 ± 12 Ma 785 ± 31 Ma 759 ± 7 Ma

Rb–Sr/whole-rock Rb–Sr/whole-rock K–Ar/phengitic mica Rb–Sr/whole-rock 40 Ar/39 Ar/sodic amphibole 40 Ar/39 Ar/crossite 40 Ar/39 Ar/glaucophane SHRIMP U–Pb/zircon SHRIMP U–Pb/zircon SHRIMP U–Pb/zircon

Gao et al. (1993) Gao et al. (1993) Nakajima et al. (1990) Nakajima et al. (1990) Liou et al. (1996) Chen et al. (2004) Chen et al. (2004) Chen et al. (2004) Zhan et al. (2007) Zhang et al. (2009a)

with the provenance and tectonic setting information, are taken to discuss the tectonic significance of the ABT in this study. 2. Geological setting China consists of three major cratonic blocks: the North China, South China and Tarim Cratons, which were amalgamated during Phanerozoic orogenic processes (Zhao et al., 2002, 2004; Lu et al., 2008). The Tarim Craton is located in northwestern China with an area of more than 600,000 km2 . Precambrian rocks in the Tarim Craton mainly occur at its northern and southern margins and recorded its early tectonic evolution (Lu et al., 2008; Zheng et al., 2008; Zhang et al., 2009c, 2011; Zhu et al., 2010, 2011). In the Aksu area, which is located in the northwestern part of the Tarim Craton (Fig. 1), a continuous outcrop of both the cratonic basement and the Upper Neoproterozoic successions of more than 28 km are well exposed (Zhang et al., 2009c; Turner, 2010). The stratigraphic successions contain the pre-Sinian Aksu Group of metamorphic rocks, the Sinian Sugetbrak and Chigebrak Formations, and the Lower Cambrian Yuertus Formation. The Aksu Group is composed of metasedimentary rocks and mafic schists (Fig. 2). They are complexly interbedded because of intense deformation (Fig. 2C), but mafic schists are absent in the southeastern part near the unconformity with the Sinian Formations. The protoliths of the blueschists are mafic rocks. Mineral parageneses and the composition of sodic amphibole in blueschists and phengite in metasedimentary rocks suggest that blueschistfacies recrystallization occurred at about 5.5–7 kbar and at about 350–450 ◦ C (Liou et al., 1996; Huang et al., 2009). Metasedimentary rocks of the ABT analyzed in this study consist primarily of pelitic schists and psammitic schists (Fig. 2D and E). Pelitic schist is mainly composed of fine-grained quartz, phengite, albite, minor graphite and rare tourmaline, apatite, zircon, stilpnomelane, and epidote (Fig. 3D), while psammitic schist is dominantly composed of quartz and feldspar and subordinate amounts of phengite, chlorite, and stilpnomelane (Fig. 3C). In mafic schists, greenschists predominate, with fewer blueschists. The blueschists consist primarily of sodic amphibole, epidote and chlorite (Fig. 3A), whereas the greenschists consist of epidote, chlorite and actinolite (Fig. 3B). The greenschists were probably produced by blueschist retrogressive metamorphism, and consequently all blueschists are interbedded with greenschists at millimeter scale. Besides mafic, pelitic and psammitic schists, thin layers of magnetite quartzite and meta-chert are common, and all types of metamorphic rocks were intruded by massive albite-quartz veins. The Aksu Group was intruded by a series of NW trending, unmetamorphosed mafic dykes (Figs. 1C and 2A), most of which are subparallel (Liou et al., 1996; Zhang et al., 2009a). The mafic dykes do not cut across the unconformity plane between the Aksu Group and the overlying Sugetbrak Formation, and pebbles of these dykes together with pebbles of the blueschist are found in the basal conglomerate layer of the Sugetbrak Formation.

The blueschist has been strongly deformed. Four phases of fold deformation have been identified by field observation. The first phase has produced numerous centimeter-to-meter-scale tight isoclinal folds throughout the ABT. The axial planes of these folds are parallel to the predominant foliations and lineations of the metamorphic rocks, indicating that the first phase of deformation and the blueschist-facies metamorphism were contemporaneous. The second folding event created many meter-scale broad folds. Frequently the axial planes of the first and second phase folds intersect at large angles and post-crystalline folding of minerals are common. The third phase deformation resulted in several mapscale synclines and anticlines, with the axes trending northeast. Finally the metamorphic rocks were deformed along with the overlying Sinian Formations to form a regional anticline. The Sugetbrak Formation is 400–450 m thick and is composed from bottom to top of red conglomerate (ca. 10 m thick), red fluvial sandstones (ca. 350 m thick), and gray lacustrine mudstones (ca. 50 m thick) (Turner, 2010). The 10 m thick red conglomerate, a base of the Sugetbrak Formation, is in sharp and angular contact with the underlying basement. Within the red fluvial sandstones there are three 15–20 m thick basalt layers, which were extruded as lava flows. No direct bio-stratigraphic age has been obtained from the Sugetbrak Formation. The overlaying Chigebrak Formation is mainly composed of layered gray stromatolitic dolomite (ca. 140 m thick). The age of the stromatolites has been constrained to the late Neoproterozoic (Gao et al., 1985). Based on the very coarse-grained limestone and stromatolite sequence, the Chigebrak Formation was interpreted to represent the development of a wide, extensive lake or a marine transgression by Turner (2010). The Lower Cambrian Yuertus Formation consists dominantly of siliceous rocks. The base of the Lower Cambrian, characterised by an 8–10 m succession of interbedded black shales and organic-rich carbonate beds, provides a useful marker to determine the paraconformity between the Upper Neoproterozoic and the Lower Cambrian. To summarize, the above field observations suggest the following sequence of events. The ABT first received deposition of sediments that formed the protolith of metasedimentary rocks. It then experienced blueschist-facies metamorphism producing the Aksu Group, which was subsequently intruded by mafic dykes. The continued evolution of the ABT involves the exhumation and erosion of the ABT that preceded the deposition of the Sinian Sugetbrak Formation.

3. Analytical procedures A total of 15 fresh, representative metasedimentary samples were collected from the ABT for geochemical analyses. All samples were prepared by crushing in an agate shatterbox and rock powder of less than 200-mesh size was used. Major elements were determined by ARL-9800 X-ray fluorescence (XRF) at the Center of Modern Analysis, Nanjing University, with precision better than 2%. Detailed analytical procedures and methods are described in Franzini et al. (1972). For trace element analyses, about 50 mg of powder for each sample were dissolved in screw-top Teflon beakers with an HF + HNO3 mixture, for 48 h at about 160 ◦ C. Trace element contents were determined at the State Key Laboratory for Mineral Deposits Research, Nanjing University with a Finnigan Element II inductively coupled plasma mass spectrometer (ICP-MS), with precision better than 5% for most and 10% for all of the elements analyzed. Detailed analytical procedure followed Gao et al. (2003). An international reference basalt material GSR-3 was also dissolved and analyzed following the same procedure and used for quality assurance. Three samples from the ABT metasedimentary rocks and two samples from the lower part of red fluvial sandstones of the Suget-

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Fig. 3. Photomicrographs of representative metamophosed rocks in the Aksu blueschist terrane. A. Foliated blueschist, consisting of sodic amphibole (Na Amp), epidotite (Ep) and minor chlorite, plane light. B. Foliated greenschist, consisting of Actinolite (Act) and epidotite (Ep), polarized light. C. Psammitic schist, consisting of elongated quartz (Q) and albite (Ab), with minor phengite (Phg), polarized light. D. Pelitic schist with quartz (Q) and phengite (Phg), polarized light.

brak Formation were selected for zircon U–Pb dating. Zircon grains of five samples were analyzed using the LA-ICP-MS method. Zircon concentrates were obtained using standard density and magnetic separation techniques. Around 600–800 zircons per sample were randomly extracted by hand-picking under a binocular microscope, but grains with visible fractures, inclusions, or compositional zoning and those smaller than beam size were avoided. The zircons were mounted in epoxy and polished to expose the cores of the grains for CL and LA-ICP-MS U–Pb analyses. Samples were analyzed at the State Key Laboratory for Mineral Deposits Research, NJU (Nanjing University), using an Agilent 7500s ICP-MS attached to a New Wave 213 nm laser ablation system with an in-house sample cell. Detailed analytical procedures are similar to those described by Griffin et al. (2004) and Wang et al. (2007). U–Pb fractionation was corrected using zircon standard GEMOC GJ-1 (207 Pb/206 Pb age of 608.5 ± 1.5 Ma, Jackson et al., 2004) and accuracy was controlled using zircon standard Mud Tank (intercept age of 732 ± 5 Ma, Black and Gulson, 1978). Samples were analyzed in runs of ca. 15 analyses which included 5 zircon standards and up to 10 sample points. Most analyses were carried out using a beam with a 20 ␮m diameter and a repetition rate of 5 Hz. U–Pb ages were calculated from the raw signal data using the on-line software package GLITTER (ver.

4.4) (http://www.mq.edu.au/GEMOC). Because 204 Pb could not be measured due to low signal and interference from 204 Hg in the gas supply, common lead correction was carried out using the EXCEL program ComPbCorr#3 15G (Andersen, 2002). 4. Results 4.1. Major and trace element geochemistry Whole rock geochemistry data and selected element abundances and element ratios are displayed in Supplementary Table 1. The Aksu rocks have variable Al2 O3 (8.05–18.05 wt%), Fe2 O3 (1.41–5.96 wt%), MgO (0.42–2.92 wt%), K2 O (0.76–4.83 wt%), Na2 O (0.08–4.60 wt%), CaO (0.23–4.16 wt%), MnO (0.04–0.43 wt%), P2 O5 (0.05–0.61 wt%) and moderate SiO2 (63.12–78.28 wt%) and relatively low TiO2 (0.31–0.69 wt%). In comparison with the standard composition of the average Post-Archean Australian Shale (PAAS) after McLennan (1989), these rocks have higher SiO2 and Na2 O, but clearly lower Al2 O3 , Fe2 O3 and TiO2 contents. The ABT protolith sediments were relatively immature. This is reflected in lower values of the Chemical Index of Alteration

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Fig. 4. A. Chondrite normalised rare earth element patterns for samples of the ABT. Normalised to chondritic values from Boynton (1984). B. Upper Continental Crust (UCC)normalised multielement diagrams for samples of the ABT. Normalised to UCC values from Taylor and McLennan (1995). The standard composition of average Post-Archean Australian Shale (PAAS) after McLennan (1989) is shown for comparison.

(CIA = [Al2 O3 /(Al2 O3 + CaO + Na2 O + K2 O)] × 100). CIA provides a useful measure of the maturity of the sedimentary protolith (Nesbitt and Young, 1982) and is frequently used to trace the source rocks and provenance of sediments (Gao et al., 1999; Cullers and Podkovyrov, 2000; Bhat and Ghosh, 2001; Joo et al., 2005). CIA values of Phanerozoic shales generally range from 70 to 75, indicating moderate chemical weathering, whereas intense chemical weathering results in the depletion of the alkali and alkali earth elements, which produces high CIA values close to 100. Most ABT samples have values as low as 37–72 (average 53), indicating a very low degree of chemical weathering (Supplementary Table 1). However, the CIA values of samples 07A-02 and 07A-18 (90, 97) are much higher than those of other samples and PAAS (69, Taylor and McLennan, 1985), indicating strong weathering of source areas. All samples examined in this study are enriched in light rare earth elements (LREE) with almost all La contents averaging more than 100 times chondrites (Boynton, 1984) and display

the flat heavy rare earth element (HREE) distributions with (Gd/Yb)n ratios ranging from 1.11 to 1.69. Although the absolute concentrations of REEs (107–247 ppm) are variable, overall, the chondrite-normalised patterns of the ABT rocks resemble that of the average PAAS, (McLennan, 1989) (Fig. 4A), with distinctive negative Eu anomalies (Eu/Eu* = 0.50–0.68). The multielement diagrams given in Fig. 4B display trace element concentrations of the ABT rocks normalised to the continental upper crust composition published by Taylor and McLennan (1995) on an element-byelement basis. Most samples exhibit uniform patterns in upper crust normalised spider diagrams, and show clear enrichments in Ba, La, Ce and variable depletions in Nb, Ta and U, relative to the neighboring elements (Fig. 4B). Compared to PAAS, the ABT samples have similar contents of high field strength element (HFSE, such as Zr: 104–286 ppm and Hf: 2.93–7.46 ppm) and lower contents of transition elements (Cr: 13.65–53.42 ppm and Ni: 0.9–116.3 ppm) (Supplementary Table 1). Sc generally shows geochemical behavior

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Fig. 5. Representative CL images for detrital zircons from samples of the ABT and the Sugetbrak Formation. Analytical spots and 206 Pb/238 U ages are marked.

similar to HFSE in sedimentation, but the Sc contents of most ABT samples are lower than that of PAAS (Fig. 4B). 4.2. U–Pb detrital zircon geochronology Three hundred and sixty zircons from five samples were chosen for U–Pb dating. Most grains required either no common Pb correction, or less than 1% common Pb correction. Zircon U–Pb compositions were analyzed with reference to their CL images (Fig. 5). The LA-ICP-MS data are presented in Supplementary Table 2 and all of the analyses are plotted on an inverse Concordia diagram (Fig. 6). Zircon grains with >10% discordance and >2% common Pb correction are excluded in the following discussion. 4.2.1. Sample 07A-20 (N41◦ 09.444 , E80◦ 02.049 ) This sample was collected from the northern part of the ABT. Two types of zircon grains are present in this sample. One type of zircon grains are dominantly rose to light brown, transparent, euhedral to subhedral and prismatic (Fig. 5 07A-20-07, 07A-20-19 and 07A-20-30), with clear oscillatory zoning and high Th/U ratios (>0.4,

Supplementary Table 2), showing characteristics of typical igneous origin. The other types of zircons are stubby or round in shape and exhibit no zoning patterns with weak CL brightness (Fig. 5 07A-20-06 and 07A-20-58), suggesting metamorphic origin. However, the metamorphic zircons have unusually high Th/U ratios (e.g. 07A-20-06: 0.88; 07A-20-58: 0.77, Supplementary Table 2), which may indicate that these detrital zircons originated from ultra-hightemperature rocks (Santosh et al., 2006, 2007, 2009a,b). In this sample, only a few zircons show narrow metamorphic rims (Fig. 5 07A-20-07 and 07A-20-43), which are too narrow to be analyzed. A total of 80 analyses were obtained for this sample. Seventythree of these are less than 10% discordant and are reported in Fig. 6B. One Neoarchean grain yields an age of 2501 ± 25 Ma, the oldest recorded from the sample. Thirty analyses of Paleoproterozoic zircon grains ranging from 2089 Ma to 1827 Ma in the sample appear as a major age peak at 1924 Ma in the histogram. Most Paleoproterozoic grains are homogeneous in texture, indicative of metamorphic origin. Two Neoproterozoic populations are present: a smaller population (n = 12: 877–800 Ma) with an age peak at 835 Ma and a larger one (n = 16: 791–736 Ma) with an age peak at 782 Ma. CL imaging reveals that all the Neoproterozoic grains

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Fig. 6. U–Pb zircon concordia diagrams and age histogram and relative probability plots for detrital zircon samples of the ABT and the Sugetbrak Formation.

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are magmatic origin. The youngest zircon is 8% discordant with a age of 721 ± 12 Ma.

206 Pb/238 U

4.2.2. Sample 07A-24 (N41◦ 08.511 , E80◦ 00.214 ) A total of 80 analyses were performed on 80 zircons from sample 07A-24 taken from the northern part of the ABT. Zircons from sample 07A-24 are dominantly rose, colorless and light brown. CL images display great variation in external morphology and internal structure, suggesting various origins of zircons (Fig. 5). Most grains occur as relatively dark and rounded crystals and are homogeneous in texture (Fig. 5 07A-24-98, 07A-24-102 and 07A-24-119), indicative of metamorphic origin. A few grains have high Th/U ratios (>0.4, Supplementary Table 2) and show oscillatory zoning in the CL images (Fig. 5 07A-24-87, 07A-24-105 and 07A-24-112), suggesting that they were dominantly derived from some igneous source rocks. The grains generally have not obvious metamorphic rims, indicating that they were not modified by HP metamorphism. Seventy out of 80 analytical spots plot on or around the Concordia curve in the 206 Pb/238 U–207 Pb/235 U diagram (Fig. 6C). The oldest grain in this sample gives a Paleoproterozoic age and the oldest single population has an age range of 2246–2144 Ma (n = 3). Fifty-two out of 80 analytical points cluster together and define a Paleoproterozoic age population (2052–1738 Ma) with a major age peak at 1929 Ma (Fig. 6D), likely reflecting zircon relicts of ancient crustal material. Most Paleoproterozoic grains are metamorphic origin. The 206 Pb/238 U ages of 14 Neoproterozoic zircon grains range between 850 Ma and 736 Ma. Seven of them yield an older population ranging in age from 850 Ma to 806 Ma with age peak at 829 Ma. The youngest population of Neoproterozoic grains (n = 7) defines a peak age at 765 Ma. All the Neoproterozoic grains are magmatic origin. The youngest zircon (07A-24-141) in this sample is 100% concordant with a 206 Pb/238 U age of 725 ± 10 Ma. 4.2.3. Sample 07A-35 (N41◦ 01.555 , E80◦ 02.809 ) Sample 07A-35 is from psammitic schist in the southern part of the ABT. The zircon grains in this sample are similar to those in samples 07A-20 and 07A-24. Some detrital zircons are generally transparent, euhedral and prismatic, with clear oscillatory zoning (Fig. 5 07A-35-194, 07A-35-207 and 07A-35-212) and high Th/U ratios (>0.4, Supplementary Table 2), indicating that they were magmatic in origin. Other zircons are stubby to round in shape, and do not have obvious internal structures (Fig. 5 07A35-177, 07A-35-192 and 07A-35-202), suggesting metamorphic origin. A total of 80 analyses were obtained for the sample, 67 of which plot on or near the Concordia (Fig. 6E). These detrital zircons have ages ranging from 2530 Ma to 732 Ma, suggesting a complex source. The oldest grain in this sample yields a Neoarchean age of 2530 ± 25 Ma. Two Paleoproterozoic populations have been recognized: a larger population at 2047–1900 Ma and a smaller one at 1874–1825 Ma. The 206 Pb/238 U ages of 42 nearly concordant points of Neoproterozoic zircon grains range from 880 Ma to 747 Ma. An older Neoproterozoic population ranges in age from 880 Ma to 800 Ma with age peak at 856 Ma. The youngest and largest population of the Neoproterozoic grains defines a peak age at 791 Ma. The youngest near-concordant grain (07A-24-169) is 10% discordant with a 206 Pb/238 U age of 732 ± 10 Ma and places a maximum limit for the depositional age of the protolith of psammitic schists. 4.2.4. Sample 07A-33 (N40◦ 59.370 , E79◦ 59.517 ) Sample 07A-33 collected for analyses of detrital zircons is from the lower part of layered red fluvial sandstone of the Sugetbrak Formation, immediately above the basal conglomerates. Zircons from sample 07A-33 are smaller in size than those in ABT metamorphic rocks. Most of them exhibit homogeneous internal structures (Fig. 5 07A-33-11, 07A-33-25 and 07A-33-37), some show oscillatory zoning in CL images (Fig. 5 07A-33-04, 07A-33-43 and 07A-33-48).

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These variable internal structures suggest several different origins, consistent with the sedimentary origin of the host protolith. A total of 60 analyses were performed on 58 zircons and are reported in the U–Pb Concordia diagram (Fig. 6G). 56 out of 60 analytical spots plot on or around the Concordia curve and are exhibited in the relative probability distribution diagram (Fig. 6H). Dated grains range in age from Archean to Neoproterozoic. One analysis, concordant at 2565 ± 29 Ma, documents the occurrence of Archean rocks in the source area. A large Paleoproterozoic concordant grains (n = 38, 2435–1773 Ma) were found in this sample. Two older Paleoproterozoic grains have ages of 2435 Ma and 2418 Ma. The slightly younger and largest population of Paleoproterozoic grains defines a peak age at 1826 Ma. These younger Paleoproterozoic grains are mostly metamorphic origin. Neoproterozoic zircons with ages broadly distributed between ca. 810–710 Ma and 640–580 Ma with age peaks at 746 Ma and 598 Ma, respectively. All the Neoproterozoic grains are magmatic origin. The youngest concordant detrital grain (07A33-54) was dated at 588 ± 12 Ma with 100% concordance. 4.2.5. Sample 07A-34 (N40◦ 59.237 , E79◦ 59.508 ) Sample 07A-34 is also from red fluvial sandstone, near the site of sample 07A-33. The zircon grains in this sample are similar to those in sample 07A-33. The CL images display great variation in external morphology and internal structure, suggesting a variety of origins. Most grains are homogeneous without obvious textural variations (Fig. 5 07A-34-01 and 07A-34-46) and some are euhedral to subhedral with obvious oscillatory zonation (Fig. 5 07A-34-03 and 07A-34-27). CL imaging also reveals that a few grains with the oldest ages have a clear core-to-rim structure (Fig. 5 07A-34-19 and 07A-34-23), suggesting a complex multi-stage evolution in the source regions. The cores have variable zoning patterns, whereas the rims are generally narrow, and exhibit no zoning patterns with either weak or strong CL brightness. A total of 60 analyses were obtained for the sample, all of which plot on or near the Concordia (Fig. 6I). U–Pb ages yield a broad age range extending from Neoproterozoic to Archean and the oldest value yields a maximum age of 3052 ± 25 Ma (Fig. 5 07A-34-19). Late Archean grain is also present and yields a concordant age of 2733 ± 31 Ma. Three Paleoproterozoic age populations have been identified. The oldest population has an age range from 2423 Ma to 2305 Ma (n = 3). Slightly younger Paleoproterozoic grains were found at 2190–1912 Ma (n = 20) and 1897–1747 Ma (n = 25) with two age peaks at 1940 Ma and 1816 Ma. Seven Neoproterozoic concordant grains yielded 206 Pb/238 U ages ranging from 796 Ma to 709 Ma, with a peak at 771 Ma. The remaining three analyses are slightly younger and have ages ranging from 689 Ma to 633 Ma. The youngest zircon gives an age of 633 ± 13 Ma with 100% concordance.

5. Discussion 5.1. Source composition and tectonic setting of the ABT metasedimentary rocks Geochemical data are traditionally used to reconstruct geodynamical settings of sedimentary basins (e.g. Bhatia, 1983; Roser and Korsch, 1986), since different plate tectonic configurations produce diverse magmatic suites (Dostal and Keppie, 2009; Fralick et al., 2009; Maruyama et al., 2009 and references therein; Williams et al., 2009) of different chemical characteristics. In this study, we apply geochemical data of the ABT metasedimentary rocks to identify tectonic settings of both passive and active margins by using existing tectonic discrimination diagrams. By convention, passive refers to rift-passive margins typical of the Atlantic Ocean, and active relates to subduction–collisional margins typical of the Pacific Ocean.

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Using major element ratios, Roser and Korsch (1988) proposed a discrimination function diagram for the provenance signatures of clastic sedimentary rocks. Within the diagram given in Fig. 7A, most metasedimentary rocks of the ABT spread into felsic to intermediate igneous sources whereas only a few samples can be attributed to quartzose sedimentary provenance field. A similar conclusion can also be obtained from the K2 O–Rb diagram of Floyd et al. (1989) (Fig. 7B). K and Rb are sensitive to sedimentary recycling and have been widely used as indicators for source composition (Floyd et al., 1989). Our samples have K/Rb ratios near 230 and plot in the field of acid + intermediate composition. Provenance signals can be identified much better when using trace elements since the chemical signature of these elements is commonly preserved in sedimentary rocks through weathering and diagenetic processes (McLennan et al., 1993). Felsic igneous source is also suggested by the La/Th versus Hf diagram of Floyd and Leveridge (1987) for the provenance signatures of clastic sediments, which shows that most rocks plot within the field of the acid arc source (Fig. 7C). The geochemical characteristics of sedimentary rocks have also been used to determine their tectonic settings (Bhatia, 1983; Bhatia and Crook, 1986). Geochemical compositions of sediments from four typical tectonic settings, i.e. oceanic island arc, continental island arc, active continental margins and passive margins, have been previously investigated (Bhatia, 1983; McLennan and Taylor, 1991). In the K2 O/Na2 O–SiO2 discrimination diagram (Roser and Korsch, 1986), most ABT metasedimentary rocks dominantly plot in the active continental margin field whereas a few samples can be attributed to a passive continental margin source (Fig. 8A). It is worth noting that tectonic setting results based on major elements, in general have to be treated with care, since some major elements can also be highly mobile through weathering and recycling and thus can vary original provenance signatures (Bahlburg, 1998). Unlike large ion lithophile elements, such as K, Na, Ca, and Sr, which can be easily fractionated during sedimentary processes, diagenesis, and later metamorphic overprints (Girty et al., 1993), REEs, Th, and Sc are relatively immobile during the secondary processes. Therefore, trace elements provide a much more satisfactory basis for discrimination. In the Th–Sc–Zr/10 and La–Th–Sc diagrams (Bhatia and Crook, 1986), most samples plot in the continental arc field (Fig. 8B), with only a few samples plotting near the active continental margin field. All above geochemical results suggest that the provenance of the ABT metasediments was dominated by materials most likely originated from felsic-intermediate igneous rocks (Bhatia and Crook, 1986) and reflect an active continental margin/continental arc tectonic setting. These suggest the ABT metasediments were probably deposited in a basin adjacent to a continental arc built on a well-developed continental basement. Although the outcrop of this ancient continental arc has not been found along the northern margin of the Tarim Craton, it is conceivable that it could have been eroded or destroyed by subsequent tectonic processes. 5.2. Depositional age 5.2.1. Depositional age of the ABT The ABT was formerly regarded as a Mesoproterozoic (Gao et al., 1985) or an early Neoproterozoic (Chen et al., 2004) complex and its absolute age is still unknown due to the lack of radiometric data and reliable bio-stratigraphic ages. Our first U–Pb dating results of the detrital zircons from the ABT metasedimentary rocks show ages clustered at the middle Neoproterozoic with significant age peaks at 765–791 Ma (Fig. 6B, D and F). Most of the detrital zircon grains show euhedral shapes (Fig. 5) with oscillatory zoning and Th/U values >0.4, suggesting a magmatic origin and showing no evidence of metamorphic resetting or recrystallization after

Fig. 7. Geochemical diagrams showing source composition for metasedimentary rocks from the ABT. A. Discrimination function diagram for the provenance signatures of clastic sedimentary rocks using major element ratios (after Roser and Korsch, 1988). The discriminant functions are: F1 = 30.638TiO2 /Al2 O3 − 12.541Fe2 O3 (total)/Al2 O3 + 7.329MgO/Al2 O3 + 12.031Na2 O/Al2 O3 + 35.402K2 O/Al2 O3 − 6.382; F2 = 56.500TiO2 /Al2 O3 − 10.879Fe2 O3 (total)/Al2 O3 + 30.875MgO/Al2 O3 − 5.404Na2 O /Al2 O3 + 11.112K2 O/Al2 O3 − 3.89. B. K2 O versus Rb diagram (after Floyd et al., 1989). C. La/Th versus Hf diagram (after Floyd and Leveridge, 1987). Circle: psammitic schist, square: pelitic schist.

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Fig. 8. Tectonic discrimination diagrams for metasedimentary rocks from the ABT. A. K2 O/Na2 O versus SiO2 diagram (after Roser and Korsch, 1986). ARC: arc, ACM: active continental margin, PM: passive continental margin. B. Th–La–Sc and Sc–Th–Zr/10 diagrams (after Bhatia and Crook, 1986). a: oceanic island arc, b: continental arc, c: active continental margin, d: passive continental margin. Circle: psammitic schist, square: pelitic schist.

their deposition. Sedimentary rocks of the ABT experienced transitional blueschist–greenschist facies metamorphism (Liou et al., 1989, 1996; Nakajima et al., 1990); but the U–Pb zircon system should remain closed under this condition. No evidence of recrystallization or overgrowth has been observed. Therefore, the age likely represents primary crystallization of these zircons. These factors meet the requirement proposed by Nelson (2001) in tightening age constraints of sedimentary sequences using detrital zircons. Therefore, the youngest concordant ages define the maximum depositional age of the protolith of the metasedimentary sequences in the area. Combining the youngest concordant ages (721 ± 12 Ma, 725 ± 10 Ma and 732 ± 10 Ma) of detrital zircons from the above-mentioned three samples, a weighted mean of 727 ± 12 Ma (MSWD = 0.27) is obtained. This age thus represents the best estimate of the maximum depositional age of the protolith of the metasedimentary sequences in the ABT. The maximum depositional age reported herein strongly indicates that the timing of both metamorphism of the blueschist and emplacement of the dyke should be younger than 727 ± 12 Ma. If our ages of detrital zircons are correct, the blueschist-facies metamorphism of the ABT occurred at approximately 700 Ma suggested by Nakajima et al. (1990) is reasonable, whereas other older metamorphosed ages of blueschist and intrusive ages of mafic dykes (Gao et al., 1993; Liou et al., 1996; Chen et al., 2004; Zhan et al., 2007; Zhang et al., 2009a) are conflict and should be reevaluated. It is a puzzle that the maximum depositional age of the metasedimentary sequences of the ABT is younger than the emplacement ages of mafic dykes penetrating the Aksu blueschists reported in previous studies (Chen et al., 2004; Zhan et al., 2007; Zhang et al., 2009a). One possibility is that zircon grains in the mafic dykes did not crystallize from the mafic magma, but are xenocrysts captured from the metasedimentary rocks of the ABT. Three ages of the Aksu mafic dykes have been obtained. Two of them are unpublished SHRIMP U–Pb ages (807 ± 12 Ma and 785 ± 31 Ma) quoted in Chen et al. (2004) and Zhan et al. (2007). Because no detailed information about these two age data is available from the publications, it is difficult to evaluate the quality of these data. A SHRIMP U–Pb zircon age of 759 ± 7 Ma with relatively detailed information was reported by Zhang et al. (2009a). Among 17 analyses, one yields the oldest Paleoproterozoic age of ca. 2.4 Ga and six analyses have slightly younger Paleoproterozoic ages of 1.7–1.9 Ga. Two other analyses yield discordant ages of 840 Ma and 916 Ma and the remaining eight analyses show Neoproterozoic ages of 766–746 Ma. In comparison with the samples from the ABT, zircons from the mafic dykes exhibit an age distribution similar to those of metasedimentary rocks. Moreover, the Neoproterozoic grains in the mafic rocks (see Fig. 3 of Zhang et al., 2009a) show abraded rims and even rounded shape and the features of morphology and internal structure are similar to those seen in zircons from the ABT metasedimentary

rocks, suggesting a sedimentary origin. These observations imply that zircons from mafic dykes are probably xenocrysts from the sedimentary rocks. Zircons with an age population at 795–735 Ma from the metasedimentary rocks in the ABT could obviously provide abundant xenocrystic zircons when the mafic dykes intruded the ABT. We therefore do not think that the age of the “basaltic” zircons reported by Zhang et al. (2009a) represents the best estimation of the crystallization age of the Aksu mafic dykes. 5.2.2. Depositional age of the Sugetbrak Formation The depositional age of the Sugetbrak Formation in the studied area is not well constrained. There are not direct bio-stratigraphic ages from the Sugetbrak Formation. Though a radiometric attempt has been made to date the basalt interlayer within the layered red sandstone of the Lower Sugetbrak Formation, no solid age constraint was obtained (Wang et al., 2010). However, a chemo-stratigraphic investigation on the section of the Sugetbrak Formation in Aksu has been carried out to estimate the formation age by Zhan et al. (2007). Chemo-stratigraphic comparison shows that the Sugetbrak Formation has a similar age as the Doushantuo Formation of South China, about 595 Ma, with the duration of ca. 80 m.y. (Zhan et al., 2007). We do not consider this approach can provide a robust basis for constraining the depositional age. In this study, the new dating results for the detrital zircons of the sedimentary rocks from the Lower Sugetbrak Formation show a large number of young ages, giving further constraints on the maximum time of deposition. Six ages (619 ± 8 Ma, 608 ± 15 Ma, 602 ± 9 Ma, 597 ± 10 Ma, 591 ± 9 Ma and 588 ± 12 Ma) of the youngest concordant detrital zircons in the two samples from red sandstones are equivalent within their uncertainties. Combining all the analyses with these young ages, a weighted average age of 602 ± 13 Ma (95% conf., MSWD = 1.6, n = 6) is obtained. All these young Neoproterozoic zircons show euhedral to subhedral shapes and some show clear oscillatory zoning with Th/U ranging from 2.16 to 0.93, suggesting magmatic origins. Furthermore, the Sinian sedimentary rocks in the Aksu area have not experienced metamorphism, so the metamorphic growth of new zircon is unlikely. We therefore regard the age of 602 ± 13 Ma as the best estimate for the maximum depositional age of the Sugetbrak Formation. 5.3. Sedimentary provenance The most striking feature of our age results is that all five samples show similar age distributions. Although the detrital zircons exhibit a wide age spectrum, their ages are mainly clustered at two age populations: one at Paleopreoterozoic (ca. 2.0–1.8 Ga) and another at Neoproterozoic (ca. 0.85–0.70 Ga). The slight difference in age distributions between the ABT and the Sugetbrak Formation is that the latter contains grains younger than 700 Ma.

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In comparison with the older one, Neoproterozoic detrital zircons generally show concentric zoning and high Th/U ratios, which are consistent with an igneous origin. Their euhedral shapes suggest that these detrital zircons experienced relatively short distance of transport and were probably related to proximal magmatism. This age population is identical to the emplacement ages of Neoproterozoic magmas at the northern margin of the Tarim Craton. Four types of Neoproterozoic magmatism with an age range of ca. 0.85–0.70 Ga have been identified in previous research (Guo et al., 2005; Huang et al., 2005; Xu et al., 2005, 2009; Zhang et al., 2007a, 2009a,b). The first type is dyke swarms. Three SHRIMP U–Pb zircon ages of 824 Ma, 777 Ma and 773 Ma for the mafic dykes have recently been obtained by Zhang et al. (2009a,b) in the southern part of the Kuruktag (also spelled as Quruqtagh or Kuruqtagh) uplift. The second type is volcanic rocks. Xu et al. (2005, 2009) reported two SHRIMP U–Pb ages of 755 Ma and 740 Ma for samples collected from volcanic rocks near the base of the Baiyisi Formation and a SHRIMP U–Pb age of 725 Ma for a sample near the top of the Baiyisi Formation in the Kuruktag uplift. The Baiyisi volcanic rocks in Xishankou area (western part of the Kuruktag uplift) that appear above the Baiyisi diamictite unit (Xiao et al., 2004) have also been geochronologically studied and a SHRIMP U–Pb zircon age of 727 Ma has been assigned to the top horizon of volcanic rocks (Huang et al., 2005). The third type is ultramafic–mafic intrusions. A TIMS U–Pb baddeleyite age of 810 Ma, a SHRIMP U–Pb zircon age of 818 Ma for a feldspar-bearing pyroxenite and a 40 Ar–39 Ar plateau age of 812 Ma for a phlogopite sample from the phlogopitelite, are reported for an ultramafic–mafic-carbonatite complex in the Kuruktag uplift (Zhang et al., 2007a). The forth type is granites. Guo et al. (2005) conducted geochemical and geochronologic analyses of diorites recovered from a deep well that reached a depth of 7000 m and drilled into the crystalline basement by 35 m beneath the Tarim basin. The minimum age of the diorite was determined by 40 Ar–39 Ar dating of hornblende, which yields three ages from three different samples: 790 Ma, 754 Ma, and 744 Ma, respectively. In the Kuruktag uplift, a granodiorite with SHRIMP U–Pb zircon age of 820 Ma and a granite with SHRIMP U–Pb zircon age of 795 Ma were presented by Zhang et al. (2007a) and four LA-ICP-MS U–Pb ages of 799 ± 24 Ma (gabbro, n = 39), 806 ± 8 Ma (granite, n = 20), 798 ± 7 Ma (granite, n = 20) and 778 ± 41 Ma (gneiss, n = 9) were obtained by Shu et al. (2010). Obviously, these igneous rocks could have supplied Neoproterozoic detritus to the basins in or near the Tarim Craton during that time. The age population ranging from 2.0 Ga to 1.8 Ga is similar to the ages of Paleopreoterozoic magmatism and metamorphism in the northern Tarim. Previous studies identified several Paleoproterozoic granitic plutons in the Kuruktag uplift (Gao, 1990; Hu et al., 2000). A meta-gabbro of 1916 ± 36 Ma was discovered in the northern Tarim (Deng et al., 2008). Recently, a large number of metamorphic zircons with ages of 2.0–1.8 Ga in various gneisses and schists from the Korla area have been firstly identified (our unpublished results). The ages of these zircons coincide with those of zircons of metamorphic origin in this study (Fig. 5, such as 07A20-06, 07A-35-202, 07A-33-11 and 07A-34-01). In addition, many inherited zircons with ages between 2.0 Ga and 1.8 Ga have been found in the 830–630 Ma volcanic rocks, mafic dykes and granites along the northern margin of the Tarim Craton (Xu et al., 2005, 2009; Zhang et al., 2007a, 2009a,b; Zhu et al., 2008, 2011). These data imply a Paleoproterozoic orogeny in the northern Tarim. This episode of Precambrian tectono-magmatic event is broadly coeval with those seen in many continents around the world such as Baltica, Laurentia, Northern Finland, Northern Fennoscandian Shield, Amazonia, North China and India, etc., which is coincident with the timing of the orogeny associated with the amalgamation of the Columbia supercontinent (Daly et al., 2001; Rogers and Santosh,

2002, 2009; Zhao et al., 2002, 2004, 2006; Santosh et al., 2006, 2009a,b). The youngest age population (<700 Ma) has also been found in magmatic rocks in the northern part of the Tarim Craton. Zhu et al. (2008) reported 650–630 Ma SHRIMP U–Pb zircon ages of the Korla mafic dykes, which documented the youngest known igneous activity along the northern margin of the Tarim Craton. Furthermore, a sample from volcanic bed between the Tereeken and Hankalchough diamictites in the Kuruktag area show a SHRIMP U–Pb age of 615 Ma (Xu et al., 2009). Recently, abundant zircons of <700 Ma have also been found in granites in the Korla area (our unpublished results). A very small population of 2.6–2.4 Ga zircons, which is close to the timing of the worldwide continental nuclei growth during late Neoarchean–early Paleoproterozoic (Rogers, 1996; Zhao et al., 2002, 2004; Rosa-Costa et al., 2006; Rogers and Santosh, 2009), may be related to granitic gneisses, tonalite, trondhjemite and potassic granites of this period in the northern Tarim Craton. The magmatic zircons derived from these rocks yielded concordant 207 Pb/206 Pb ages of 2534 ± 19 Ma (potassic granite) and 2602 ± 27 Ma (tonalite), indicating that the TTG rocks were mainly emplaced at 2.6–2.50 Ga (Zhang et al., 2007b). Geochronological studies of the orthogneisses exposed in the Kuruktag area (Long et al., 2010) suggest that the orthogneisses were predominantly emplaced in the late Neoarchean through early Paleoproterozoic (2.58–2.46 Ga). Two concordant 207 Pb/206 Pb ages of crystallization: 2470 ± 24 Ma (diorite, n = 17) and 2469 ± 12 Ma (gneissic granite, n = 17) were reported in the Kuruktag uplift by Shu et al. (2010). The 2.6–2.4 Ga detrital grains found in this study suggest that the currently exposed Archean to Paleoproterozoic basement rocks in the Kuruktag area were a possible source for sediments during the Neoproterozoic time. Early Paleoproterozoic ages of 2.3–2.1 Ga have not been found in any igneous rocks in the northern Tarim Craton. The failure to correlate the sources for some detrital material (2.3–2.1 Ga) in our sedimentary rocks with the exposed basement in the Tarim Craton may imply a more widespread source area in the basement for these zircons. Therefore, the northern part of the Tarim Craton may have been a major source region for the sedimentary rocks of the ABT and the Sugetbrak Formation, which probably provided a mixture of both old recycled sedimentary material from the basement rocks and juvenile material from the igneous rocks. 5.4. Tectonic implications The new chronological and geochemical data of metasedimentary rocks allow us to reevaluate the tectonic significance of the ABT. The maximum depositional age of ca. 730 Ma of the ABT samples implies that the high-pressure blueschist-facies metamorphism probably to have occurred at ca. 700 Ma as suggested by Nakajima et al. (1990), not at 960–860 Ma as previously suggested by other researchers (Gao et al., 1993; Chen et al., 2004; Lu et al., 2008). Thus, the ABT could be a manefistation of the Pan-African orogeny (ca. 700–500 Ma) related to the assemblage of Gondwana (Collins et al., 2005 and references therein; Meert and Lieberman, 2008 and references therein; Santosh et al., 2009a,b and references therein), rather than being the Grenville orogeny (ca. 1300–900 Ma) related to the assemblage of Rodinia. Although two granitic samples with ages of 933 Ma and 1048 Ma were reported in the Kuruktag area (Shu et al., 2010), petrogenesis and tectonic setting of these granites are not well constrained and their ages are clearly younger than typical Grenvillian ages (1300–1050 Ma) throughout southern Laurentia, Australia, Amazonia and the Maud – Namaqua – Natal Provinces of east Antarctica, and Africa (Fitzsimons and Hulscher, 2005). Therefore, it is unclear whether they are related to the

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Grenville orogeny. In addition, the high-grade metamorphic rocks with Grenville ages that could indicate a continental collision setting are absent in the studied area and the age range 1300–900 Ma is not recorded in the detrital zircons of this study (Fig. 6), which might suggest that Grenville-age orogeny was not significant in the northern part of the Tarim Craton. Lu et al. (2008) proposed that Mesoproterozoic tectonic settings include both passive and active margins in the Tarim Craton. Carbonates containing abundant stromatolites were deposited on the northern passive margin of the Tarim Craton, whereas calc-alkaline arc-volcanic rocks with Ar–Ar ages of 1050–1020 Ma were formed along its southern active margin, indicating the influence of tectono-thermal events related to the Grenville Orogeny. This conclusion supports our interpreta-

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tion that the Grenville-age orogeny may not have occurred along the northern margin of the Tarim Craton. It is widely accepted that there was an extensional tectonic setting from Neoproterozoic to Early Paleaozoic along the northern margin of the Tarim Craton. Neoproterozoic continental rifting has been evidenced by various 830–630 Ma magmatic events in the northern Tarim Craton (Chen et al., 2004; Huang et al., 2005; Xu et al., 2005, 2009; Zhan et al., 2007; Zhang et al., 2007a, 2009a,b; Zhu et al., 2008, 2011), which indicate that the Tarim Craton was involved in a mantle plume activity beneath western Rodinia that possibly resulted in the breakup of the supercontinent (Li et al., 1996, 2008). Neoproterozoic to Ordovician depositional facies along the northern margin of the Tarim Craton include basinal,

Fig. 9. Tectonic evolution of the northern Tarim Craton during the Mesoproterozoic to Neoproterozoic.

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slope, platform margin and restricted–open platform facies, which display a change in depositional environment from a continental rift to a passive continental margin (Duan et al., 2005; Huang et al., 2005). As a B-type blueschist, the ABT has been interpreted to represent a relic of an ocean crust around Tarim or a remnant suture between the Tarim Craton and the Kazakstan–Yili microcontinent, which has experienced subduction towards Tarim during the Neoproterozoic (Zheng et al., 2010; Shu et al., 2010). Accordingly, this late Precambrian subduction would result in the formation of an active continental margin along the northern margin of the Tarim Craton. As discussed above, the sediments of the ABT were mainly derived from felsic igneous rocks and their geochemistry also reveals an arc-related or an active continental margin setting. However, the subduction-related island-arc volcanic rocks have not been recognized in the northern Tarim Craton. A possible interpretation is that these rocks were eroded, reworked, or are deeply buried. Currently, we do not know when the tectonic environment of the northern Tarim had changed from an extensional passive continental margin or intraplate environment to an active continental margin during the Neoproterozoic. Based on the maximum depositional age of the ABT metasedimentary rocks and the high-pressure blueschist-facies metamorphic age, we suggest that the subduction had possibly started between ca. 730 Ma and 700 Ma. Combined with the available geochemical and chronological data from the ABT and the Tarim Craton, our new data support to establish Neoproetrozic tectonic framework for the northern Tarim Craton. Details of the subduction and accretion processes are summarized as follows (Fig. 9): (1) from Mesoproterozoic to middle Neoproterozoic, a continental rift or a passive continental margin occurred at the northern margin of Tarim (Fig. 9A). An ancient ocean basin was located to the north of proto-Tarim Craton, which was formed at least at 890 Ma (Zheng et al., 2010). (2) The ABT probably represents part of the arcuate subduction zone around the Tarim. S-directed subduction towards the proto-Tarim Craton resulted in the formation of a Neopoterozoic active continental margin along the northern margin of the proto-Tarim Craton. However, the timing of the initiation of subduction is not clear (Fig. 9B). (3) The subducted ocean crust and the overlying sediments were metamorphosed under blueschist-facies conditions at ca. 700 Ma and then rapidly exhumed towards the surface. As a subduction complex, the ABT accreted to the northern margin of the proto-Tarim Craton and constituted the northernmost margin of the Gondwana supercontinent at that time (Fig. 9C). (4) The mafic dykes intruded into the blueschist terrane imply a post-orogenic extensional environment (Fig. 9D). Liou et al. (1996) and Zhang et al. (2009a) have confirmed this by examining the geochemistry of these mafic dykes, which exhibit the source properties of metasomatized sub-continental lithospheric mantle. Thus by the time of dyke intrusion, the tectonic environment of the northern Tarim had changed from an active continental margin to an extensional passive continental margin or intraplate environment. The emplacement ages of the mafic dykes have not yet been resolved due to the lack of precise radiometric age data. But the maximum depositional age of the overlying Sugetbrak Formation suggests that the crystallization of the mafic dykes should occur no later than 602 Ma.

6. Conclusions Based on detrital zircon U–Pb geochronological studies on both the metasedimentary rocks from the ABT and the unmetamorphosed sandstones from the overlying Sinian succession and geochemical analysis on the metasedimentary rocks, we draw the following conclusions.

(1) U–Pb dating results of the detrital zircons from the metasedimentary rocks of the ABT show a maximum depositional age of ca. 730 Ma, confirming that the Aksu Group was deposited during the Middle Neoproterozoic, rather than the Mesoproterozoic or the Early Neoproterozoic. (2) U–Pb dating results of the detrital zircons from sandstones of the Lower Sugetbrak Formation define a maximum depositional age of ca. 602 Ma, providing the first precise radiometric age for this succession to our knowledge. (3) The maximum depositional age of the ABT sample provides an upper age limit of ca. 730 Ma for the high-pressure blueschistfacies metamorphism. This implies that the ABT could manifest the Pan-African orogeny (ca. 700–500 Ma) related to the assemblage of Gondwana, rather than being indicative of the Grenville orogeny (ca. 1300–900 Ma) related to the assemblage of Rodinia. (4) The age range of 1300–900 Ma was not recorded in the detrital zircons of this study, suggesting that the Grenville-age orogeny may not be very significant in the northern part of the Tarim Craton. (5) The major age population of 2.0–1.8 Ga implies a Paleoproterozoic orogeny in the northern Tarim, which is coincident with the timing of orogeny associated with the assembly of the supercontinent Columbia (Daly et al., 2001; Rogers and Santosh, 2002, 2009; Zhao et al., 2002, 2004, 2006; Santosh et al., 2006, 2009a,b; Santosh, 2010; Hou et al., 2008). (6) Provenance studies indicate the northern part of the Tarim Craton may have been a major source area for the sedimentary rocks of the ABT and the Sugetbrak Formation, which probably provided a mixture of old recycled sedimentary material from the basement rocks and juvenile material from the igneous rocks. Acknowledgments This research was financially supported by grants from the National Basic Research Program of China (No. 2007CB411301), the Natural Science Foundation of China (No. 40972133), the State Key Laboratory of Earthquake Dynamics (No. LED2010B06) and the State Key Laboratory for Mineral Deposits Research of Nanjing University (No. 2009-I-01). We thank Profs. P. Cawood, M. Santosh and Wenjiao Xiao for their critical reviews and constructive comments that significantly improved this manuscript. Authors are grateful to Mr. Bin Wu for his considerable help during the U–Pb zircon analyses. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.precamres.2011.01.012. References Amidon, W.H., Burbank, D.W., Gehrels, G.E., 2005a. Construction of detrital mineral populations: insights from mixing of U-Pb zircon ages in Himalayan rivers. Basin Res. 17, 463–485, doi:10.1111/j.1365-2117.2005.00279.x. Amidon, W.H., Burbank, D.W., Gehrels, G.E., 2005b. U-Pb zircon ages as a sediment mixing tracer in the Nepal Himalaya. Earth Planet. Sci. Lett. 235, 244–260. Andersen, T., 2002. Correction of common Pb in U–Pb analyses that do not report 204Pb. Chem. Geol. 192, 59–79. Bahlburg, H., 1998. The geochemistry and provenance of Ordovician turbidites in the Argentinian Puna. In: Pankhurst, R.J., Rapela, C.W. (Eds.), The Proto-Andean Margin of Gondwana, vol. 142. Spec. Publ. Geol. Soc. Lond., pp. 127–142. Becker, H., Jochum, K.P., Carlson, R.W., 2000. Trace element fractionation during dehydration of eclogites from high-pressure terranes and the implications for element fluxes in subduction zones. Chem. Geol. 163, 65–99. Bhatia, M.R., 1983. Plate tectonics and geochemical composition of sandstones. J. Geol. 91, 611–627.

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