Neoproterozoic ultramafic–mafic-carbonatite complex and granitoids in Quruqtagh of northeastern Tarim Block, western China: Geochronology, geochemistry and tectonic implications
Introduction
Igneous rocks related to Neoproterozoic mantle plumes (e.g., Park et al., 1995, Li et al., 1999a, Li et al., 1999b) or a mantle superplume (Frimmel et al., 2001, Li et al., 2001, Li et al., 2003a, Li et al., 2003b, Li et al., 2006a, Li et al., 2006b) during the breakup of the Neoproterozoic supercontinent Rodinia (e.g., Moores, 1991) have been well documented in many Rodinian continents (Li et al., 2003a, Li et al., 2003b; and references therein for discussions). The Tarim Block was possibly located on the periphery of the proposed Rodinian superplume (Li et al., 2003a, Li et al., 2003b), but little has been known about the Neoproterozoic igneous rocks in Tarim in response to this superplume activity. Precambrian granitoids and ultramafic–mafic intrusions are widespread in northern Tarim Block, but they have traditionally been regarded as magmatism related to a ca. 1000–800 Ma Jinning orogeny (Xinjiang BGMR, 1993, Feng et al., 1995, Jiang et al., 2005). Precise geochronology and systematic geochemical data are rare for these rocks.
We present here a comprehensive geochronological, geochemical and Nd isotopic analyses of the Qiganbulake mafic–ultramafic-carbonatite complex (QMC) and associated granitoids in the Quruqtagh region of northeastern Tarim Block (Fig. 1) with the aim of characterizing their petrogenesis and tectonic implications. Our data demonstrate that the QMC and the granitoids were roughly coeval, formed at ca. 820–800 Ma in an intracontinental rift setting, possibly related to the Neoproterozoic mantle plume activities under Rodinia.
Section snippets
General geology and petrography
The Quruqtagh area is located in northeastern Tarim Block of NW China (Fig. 1a), with an excellent geologic exposure of ca. 100–250 km wide and ca. 500 km long (Xinjiang BGMR, 1993). Precambrian intrusive rocks are in general poorly dated, including Archaean tonalite–trondjemite–granodiorite (TTG) series (Lu, 1992, Lu et al., 2002a, Hu et al., 1999, Hu et al., 2000, Feng et al., 1995), the Qiganbulake mafic–ultramafic-carbonatite complex (QMC) (Ying, 1992, Peng et al., 1996, Huang et al., 2001,
Analytical procedures
Four samples were collected for U–Pb dating, including samples KL08 (41°00′8.6″N, 89°15′45.9″E) from the Taiyangdao granite, sample KL010 (41°14′8.9″N, 87°55′25″E) from the Xingdi granodiorite, a feldspar-bearing pyroxenite sample KL011 (41°13′32.6″N, 87°34′39.2″E) from the QMC and a carbonatite sample QG09 (41°13′31″N, 87°34′40.7″E) from the QMC. Mineral separation was carried out first using conventional magnetic and density techniques to concentrate the non-magnetic, heavy fractions.
The QMC carbonatite (sample QG09)
Baddeleyites are mostly anhedral, ranging from 50 to 150 μm in lengths. They are semi-transparent, purple to brown in colour. Five baddelyite fractions with different crystal length and colour were analysed using the TIMS method. Uranium concentrations are between 426 and 1315 ppm, and Pb between 76 and 186 ppm. Common Pb is low, between 0.44 and 1.1 ng. All five analyses are concordant (Fig. 3), and the measured 206Pb/238U ratios agree within analytical errors. Their weighted mean yields an age of
Petrogenesis of the QMC rocks
Estimation of the primary magma compositions for the QMC rocks is difficult because their geochemistry is mostly controlled by cumulation of rock-forming minerals and accessory minerals, such as apatite, baddeleyite, titanite and/or rutile. Mineral accumulation and/or fractional crystallization appear to have played an important role in the magma evolution (Fig. 9). It is noted that carboniatites and mafic–ultramafic silicate rocks within the QMC have very homogeneous Nd isotopic compositions,
Conclusions
We draw the following conclusions based on our new results:
- (1)
SHRIMP U–Pb zircon and TIMS U–Pb baddeleyite results indicate that the Qieganbulake ultramafic–mafic-carbonatite complex (QMC), the Xingdi granodiorite and the Taiyangdao granite were all formed during 820–800 Ma. Petrography, geochemistry and mineral chemistry argue that the parental magma of the QMC was generated by partial melting of a homogeneously CO2-metasomatized mantle in a rift environment.
- (2)
Geochemical and Nd isotopic
Acknowledgements
We thank X.R. Luo for assistance in the field trips; H. Tao and L.C. Miao for SHRIMP zircon U–Pb analysis; Y. Liu, Xirong Liang and X. Yan for geochemical and Nd isotopic analyses. We also thank the constructive suggestions of the two anonymous reviewers. This work is supported by the National Natural Science Foundation of China (grants 40421303, 40303007 and 40373032), and is a contribution to IGCP 440. Tectonic Special Research Centre publication #393.
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