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SHRIMP U-Pb Zircon Geochronology of Neoproterozoic Korla Mafic Dykes in the Northern Tarim Block, NW China: Implications for the Long- Lasting Breakup Process of Rodinia

September 19, 2008

By Zhu, Wenbin Zhang, Zhiyong; Shu, Liangshu; Lu, Huafu; Su, Jinbao; Yang, Wei

Mafic dykes are observed in the Korla region along the northern Tarim Block, NW China. Our sensitive high-resolution ion microprobe U-Pb zircon ages, the first determined for these dykes, indicate that the mafic dykes were mainly formed at 650-630 Ma, and thus document the youngest known igneous activity associated with rifting in the Tarim Block during the Neoproterozoic. Combined with previous geochronological data, at least three pulses of magmatic activity, from c. 830 to 800 Ma, from c. 790 to 740 Ma and from c. 650 to 630 Ma, are recognized, which reveal that multiple episodes of rifting occurred within the Tarim Block, implying that the breakup of the Rodinia supercontinent in the Tarim Block may have been a long- lasting process. It is accepted that Neoproterozoic widespread mafic dyke swarms, ultramafic-mafic intrusions, alkaline granites (resulting from crustal melt or magma differentiation), and bimodal volcanic rocks occurring in many old cratons are commonly related to the breakup of the Rodinia supercontinent (Li et al. 2003, 2008; Lu et al. 2008). The origin of the magmatism is not clear, but may be linked to mantle-plume activity or upwelling asthenosphere leading to continental rifting accompanying initial breakup of Rodinia (Ernst et al. 1995; Park et al. 1995). Precise dating of magmatic events plays an important role in understanding the process of continental fragmentation because they provide unique time markers that can sometimes be correlated between previously adjacent continental blocks. One of the many crucial questions for clarifying the breakup process of the supercontinent is the timing of their separation. Do they break up more or less simultaneously or do they disintegrate gradually over hundreds of millions of years (Frimmel et al. 2001)? The spread of separate rift-related Neoproterozic magmatic ages recorded in many continents suggests several alternative hypotheses: (1) rift events were diachronous within Rodinia; (2) there were multiple episodes of rifting during the breakup of Rodinia; (3) rifting may have been protracted from initial fragmentation to final drifting of Rodinia (Lund et al. 2003).

The Kuruktag Uplift is located to the north of the Tarim Block where the Precambrian crystalline basement rocks crop out widely (Fig. 1a). Neoproterozoic successions occur in the Kuruktag Uplift; they unconformably overlie Archaean to Mesoproterozoic gneisses, amphibolites and schists and are unconformably overlain by Early Palaeozoic rocks. Neoproterozoic continental rift setting has been evidenced by various 830-740 Ma magmatic events in the northern Tarim Block (Chen et al. 2004; Huang et al. 2005; Xu et al. 2005; Zhan et al. 2007; Zhang et al. 2007), which indicates that the Tarim Block was involved in mantle plume activity beneath west 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 Block were arranged as basinal, slope, platform margin and restrictedopen platform facies, which display the sedimentary environments from a continental rift to a passive continental margin (Duan et al. 2005; Huang et al. 2005).

A series of parallel mafic dykes occur in the Kuruktag Uplift, which intrude only the Archaean to Mesoproterozoic metamorphic and granitic rocks but do not penetrate the Neoproterozoic to Early Palaeozoic succession. Mafic dykes are abundant in the southern part of the Kuruktag Uplift, which strike about 330[degrees] and dip nearly vertically or slightly towards the SE. Mafic dykes also occur in the Korla region, at the western end of the Kuruktag Uplift (Fig. 1b), but have not been described in the literature. In addition, there are no robust isotopic ages or rigorous geochemical and mineralogical data for these dyke swarms. Taking into account the importance of mafic dykes as indicators of extensional processes in cratons (Halls 1982), especially in connection with reconstructions of Rodinia (e.g. Hoffman 1991; Park et al. 1995; Ernst et al. 2008), we have studied the dykes in the Korla region. Obviously, precise geochronological studies of the Neoproterozoic magmatism should play an important role in understanding of the response to the breakup of Rodinia during the Neoproterozoic in the Tarim Block. In this paper we present the first sensitive highresolution ion microprobe (SHRIMP) ages of the Korla mafic dykes, which document the youngest known igneous activity associated with rifting during the tectonic development along the northern margin of the Tarim Block, and discuss the relationships between the protracted rifting history of the Tarim Block and the late Neoproterozoic breakup of Rodinia.

Field occurrence and sample description. Mafic dykes in the Korla region are rare and have been documented in only a few localities. Dykes cut Archaean to Mesoproterozoic metamorphic and granitic rocks whose ages are uncertain because of the paucity of reliable isotopic data. Most dykes have NNW-SSE to NW-SE trends and are nearly vertical, the same as those in the southern part of the Kuruktag Uplift. Mapped lengths of single dykes range from several tens of metres to several hundreds of metres, and their thicknesses vary from several centimetres to several metres. Although these dykes are thinner than those with thickness of 10-30 m belonged to a major extensional-breakup event (e.g. Ernst 2008), they formed in an extensional setting mainly evidenced by regional sedimentation (Duan et al. 2005). Two types of dykes are observed in the Korla region: diabase and spessartite dykes. The diabase dykes usually have chilled margins composed of fine plagioclase and pyroxene grains. The main rock-forming minerals are plagioclase (50-55%), pyroxene (40-45%) and Fe-Ti oxides (c. 5%). Plagioclase is strongly saussuritized, with relict crystals being idiomorphic or semiidiomorphic. Idiomorphic to semi-idiomorphic pyroxene is interlocked with plagioclase, indicating its crystallization later than plagioclase. The spessartite contains simple assemblages of plagioclase (45-55%), hornblende (35-45%) and Fe-Ti oxides (5-10%), and minor amounts of biotite and quartz. Plagioclase is mostly altered by saussuritization. Three samples were collected for SHRIMP U-Pb zircon analyses. Sample T10 (c. 30cm thick) was collected from a spessartite dyke whereas samples T11 (c. 50cm thick) and T13 (c. 2m thick) were taken from diabase dykes. Sampling locations are shown in Figure 1b.

Analytical procedures and U-Pb results. Zircon concentrates were obtained using standard density and magnetic separation techniques. A representative selection of zircons was extracted by hand-picking under a binocular microscope. The zircons, together with several grains of TEMORA standard zircon, were mounted in epoxy and polished to expose the cores of the grains for cathodoluminescence (CL) and SHRIMP U-Pb analyses. The U-Pb analyses were performed on the Beijing SHRIMP II at the Beijing SHRIMP Centre of the Chinese Academy of Geological Sciences, following the method of Williams (1998). U-Th-Pb ratios were determined relative to the TEMORA standard zircon, and the U and Th absolute abundances relative to the SL13 standard zircon (Black et al. 2003). Sites for dating were selected on the basis of CL and microscope images. To maintain precision, one TEMORA analysis was performed after every three or four spots on the sample zircons during data collection. Software SQUID 1.02 and ISOPLOT (Ludwig 2001, 2003) were used for data processing. The weighted mean ages are quoted at 95% confidence level. Errors on all ages reported are given at the 1sigma level, unless otherwise stated. U-Pb zircon data are presented in Table 1.

The SHRIMP U-Pb analyses of zircons from sample T10 (41[degrees]49’4.8”N, 086[degrees]2’3.8”E), taken from a spessartite dyke, are plotted on the concordia diagram of Figure 2a. Zircon grains are 120-220 [mu]m in length, and the length/width ratios are about 2:1. Most of the zircons in TlO have CL dark cores and bright rims. The cores show oscillatory or linear zoning and are surrounded by nebulously zoned rims. The contacts between rims and cores are generally sharp. Previous studies suggested that the oscillatory-zoned core zircon, with older age, is primary zircon inherited from the parent igneous rock, whose age represents the magmatic crystallization age. The rim zircon, with younger age, suggests formation of new zircon overgrowth during a later stage metamorphism (Hoskin & Black 2000; Zhou et al. 2002). Out of a total of 15 grains analysed, 12 analyses on cores with oscillatory or linear zoning form a concordant group with a mean ^sup 206^Pb/^sup 238^U age of 628.7 +- 6.6 Ma (95% confidence, MSWD = 0.95). This age is interpreted as the best estimate of the crystallization age of sample T10. One rounded core (spot 13.1) gives an older discordant ^sup 206^Pb/ ^sup 238^U age of 1746 +- 35 Ma and is interpreted as inherited zircon, indicating that it is a xenocryst assimilated during the emplacement of the mafic magma. Spot 7.1 and spot 13.2 near the boundaries between cores and rims yielded significantly younger concordant ages, which may suggest the ages of new zircon overgrowth as a result of a later-stage metamorphic event, or probably reflect mixed ages between magmatic cores and metamorphic overgrowth. Sample T11 (41[degrees]49’4.8”N, 086[degrees]12’3.8”E), collected from a diabase dyke, is from the same location as sample T10. Field investigation revealed that the spessartite dyke cuts the diabase dyke, indicating that the spessartite dyke developed later than the diabase dyke. These zircon grains are small in size (100-150 [mu]m long) and the length/width ratios of about 1.5:1 (Fig. 2b). Zircons in T11 also have CL dark cores and bright rims. Ten analyses on cores with oscillatory or linear zoning form a simple concordant population with a mean rocks, ^sup 206^Pb/^sup 238^U age of 652.0 +- 7.4 Ma (95% confidence, MSWD = 0.61). For Neoproterozoic and younger rocks, ^sup 206^Pb/^sup 238^U ages are determined with higher precision than those based on ^sup 207^Pb/^sup 206^Pb ratios (Sircombe 1999), so the ^sup 206^Pb/ ^sup 238^U age of 652.0 +- 7.4 Ma is interpreted as the emplacement age of the diabase dyke. Two additional analyses (spots 3.1 and 5.1) between cores and rims yielded significantly younger concordant ^sup 206^Pb/^sup 238^U ages of c. 423 Ma and 611 Ma, which are metamorphosed or mixed ages.

Sample T13 (4A49’4.8”N, 086[degrees]12’3.8”E) is from another diabase dyke to the west of samples T10 and T11. The zircon crystals are euhedral with CL images showing clear oscillatory growth zonation. These zircon grains are large, ranging up to c. 300 [mu]m in length. Weighted mean ^sup 206^Pb/^sup 238^U ages are reported at 1sigma error and 95% confidence level. All analyses are concordant, with Th/U ratios 0.81-1.15, and MSWD 1.30. A mean ^sup 206^Pb/^sup 238^U age of 642.8 +- 6.88 Ma was calculated from a total of 14 spot analyses of single grains (Fig. 2c). This age is interpreted as the best estimate of the emplacement age of the diabase dyke.

In summary, our SHRIMP U-Pb zircon dating results indicate that the mafic dykes in the Korla region were mainly formed at 650-630 Ma.

Discussion and conclusion. Four types of Neoproterozoic magmatism (dyke swarms, bimodal volcanic rocks, ultramaficmafic intrusions and alkaline granites) have been reported in the northern Tarim Block and their ages of 830-740 Ma have been used to constrain the timing of the breakup of Rodinia (Chen et al. 2004; Huang et al. 2005; Xu et al. 2005; Zhang et al. 2007; Zhan et al. 2007).

However, our new SHRIMP U-Pb zircon dates for the Korla mafic dykes are much younger than ages previously reported for the Neoproterozoic igneous events in the Tarim Block, and thus imply that the Neoproterozoic breakup of the Tarim Block may have been a long-lasting process. The spread of potentially separate rift- related igneous ages, spanning c. 200 Ma (from 830 to 630Ma), suggests that this is not simply a rift event as commonly proposed (Li et al. 2003, 2008; Xu et al. 2005; Zhang et al. 2007) but may be a stepwise series of events as envisioned from data in Australia by Veevers et al. (1997) and Preiss (2000), in western Laurentia by Lund et al. (2003), in southern Africa by Frimmel et al. (2001) and in eastern Laurentia by Cawood et al. (2001). Combined with previous geochronological data as mentioned above, at least three pulses of magmatic activity, from c. 830 to 800 Ma, from c. 790 to 740 Ma and from c. 650 to 630 Ma, are recognized, which reveal that multiple episodes of rifting occurred within the Tarim Block during the Neoproterozoic.

In addition, our new geochronological data also support a hypothesis on the connection of the Yangtze-Tarim Blocks proposed by Lu et al. (2008), which is based on the similarities of the geological evidence and geochronological data between the Tarim Block and the Yangtze Block. Taking the error bars into consideration, our newly obtained U-Pb ages of the Korla 650630 Ma mafic magmatism are consistent with ages of magmatism in the Yangtze Block; for example, a U-Pb zircon age of 663 +- 4 Ma from a tuffaceous bed in the Datangpo Formation (Zhou et al. 2004) and a U- Pb zircon age of 621 +- 7 Ma from an ash bed (bentonite) above the Doushantuo cap carbonate (Zhang et al. 2005). Previous studies suggested that the Datangpo Formation and the Doushantuo Formation were deposited in basinal facies along a SE-facing (present orientation) passive margin on the Yangtze Block, which were developed as it broke away from Rodinia (Zhou et al. 2004, and references therein). It has also been confirmed that the 650-630 Ma magmatism in the Tarim Block was generated in a rift-related setting or a passive margin (Duan et al. 2005; Huang et al. 2005). Thus, we conclude that both the 650-630 Ma magmatism in the Tarim Block and the magmatism of 663 and 621 Ma in the Yangtze Block were formed in the same extensional setting. This is new evidence to support the connection of the Yangtze-Tarim Blocks. If the Yangtze-Tarim connection is creditable, then the Tarim Block should be located between Australia and Laurentia during the Neoproterozic as proposed by Lu et al. (2008, fig. 14), instead of being positioned adjacent to northwestern Australia (Li et al. 1996, 2003, 2008; Chen et al. 2004; Zhan et al. 2007). If this is the case, it seems that the Tarim Block was close to southern Australia, western Laurentia and the Yangtze Block, forming a wide landmass with other major continents. These continental blocks became even further aligned along the palaeo-equator and experienced a second widespread low- latitude glaciation at c. 650-630 Ma (the Marinoan glaciation, and another snowball-Earth event?; Hoffman & Schrag 2002) as depicted by Li et al. (2008). Thus, the glaciomarine strata of the Tereeken Formation in the Tarim Block (Gao & Qian 1985; Xu et al. 2005) may be interpreted as rift-glacial sediments that resulted from the 650- 630 Ma Korla rifting, which occurred at a low latitude, although direct geological evidence and geochronological data need to be obtained to constrain the correlations between the Tereeken glaciation and the 650-630 Ma Korla rifting.

This research was financially supported by grants from the National Basic Research Program of China (973 Program, No. 2007CB411301) and the Natural Science Foundation of China (No. 40573038). The authors are grateful to Y. Shi for his considerable help during the SHRIMP analyses. J. S. DaIy, R. E. Ernst, W. J. Xiao and subject editor K. McCaffrey are thanked for their constructive comments.

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Received 18 December 2007; revised typescript accepted 4 June 2008.

Scientific editing by Ken McCaffrey

WENBIN ZHU, ZHIYONG ZHANG, LIANGSHU SHU, HUAFU LU, JINBAO SU & WEI YANG

State Key Laboratory for Mineral Deposits Research

(Nanjing University), Department of Earth Sciences, Nanjing

University, Nanjing 210093, China

(e-mail: zwb@nju.edu.cn)

Copyright Geological Society Publishing House Sep 2008

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