Neoproterozoic subduction-related basaltic magmatism in the northern margin of the Tarim Craton: Implications for Rodinia reconstruction
Introduction
Several clusters of Neoproterozoic mafic-ultramafic intrusions in the margins of the Yangtze, Tarim and North-China Cratons (Fig. 1) have been studied extensively, mainly because the timing of their emplacements broadly coincided with the breakup of Rodinia (e.g., Li et al., 1999, Li et al., 2010), plus the occurrence of economically valuable Fe-Ni-Ni sulfide mineralization in some of these intrusions (e.g., Munteanu et al., 2010, Li and Ripley, 2011). The long-lasting debate on the geodynamic settings of these mafic-ultramafic intrusive clusters became really intriguing after some researchers (Li et al., 1999) suggested that the Neoproterozoic mafic-ultramafic intrusions in the southern margin of the Yangtze Craton (northern Guangxi, see Fig. 1) are the products of a hypothetical super mantle plume (referred to as the South China mantle plume by these authors) that are thought to be responsible for Rodinia breakup. Subsequently, the proponents of this hypothesis (e.g., Li et al., 2005, Li et al., 2006, Li et al., 2010, Zhang et al., 2007, Zhang et al., 2009) proposed that the Neoproterozoic mafic-ultramafic intrusions in the western margin of the Yangtze Craton (Hannan, Yanbian) and in the western part of the North China Craton (Jinchuan) are also the products of the so-called South China mantle plume, primarily based on their apparent temporal correlation with the northern Guangxi cluster (Fig. 1). The single mantle plume model for all of the above-mentioned clusters has been criticized by many other researchers. For examples, some researchers (Li and Ripley, 2011, Tang et al., 2014) argued against the notion that the Jinchuan and northern Guangxi clusters belong to the same super mantle plume based on contrasting Precambrian geological records for these two different regions; other researchers used holistic geological and petrological constraints to argue for an alternative, subduction-related origin for the circum-Yangtze clusters (Hannan, Yanbian, northern Guangxi) (e.g., Zhou et al., 2002, Zhou et al., 2006, Yao et al., 2014).
Similarly, the geodynamic processes for the Neoproterozoic mafic-ultramafic intrusions and dykes in the Kuluketage district, Tarim Craton (Fig. 1) has also been debated for some time. Some researchers suggested that the Neoproterozoic intrusive rocks in this region are the products of the so-called South China mantle plume (Zhang et al., 2007, Zhang et al., 2009, Zhang et al., 2011) or an independent mantle plume (Ye et al., 2013). Recently, Zhang et al. (2012) modified their original view and suggested that in addition to mantle plume activity, subduction-related basaltic magmatism also contributed to the formation of these mafic-ultramafic intrusive rocks. Since the various geodynamic models have different implications for Rodinia reconstruction, an independent study to evaluate the validity of the competing geodynamic models is needed.
In this study we use new and previous zircon U-Pb age data to determine whether the temporal-spatial distribution of the Neoproterozoic mafic-ultramafic intrusive rocks in the Kuluketage district is consistent with prolonged mantle plume activity or subduction-related magmatism, based on the fundamentals of mantle plume and plate tectonics. We then use whole-rock trace element and Sr-Nd isotope data as consistent arguments to support our new conclusion. The implications of the new results for Rodinia reconstruction is given at the end.
Section snippets
Geological background
Three major cratonic blocks (North China, Tarim and Yangtze) are present in mainland China (Fig. 1). The Yangtze Craton and the Precambrian Cathaysia continental block were amalgamated at some time between 800 Ma and 850 Ma to form the South China block (Yao et al., 2014). The Tarim Craton is bounded by the Paleozoic Kunlun Orogenic Belt to the south and by the Paleozoic Tianshan Orogenic Belt to the north (Fig. 2a). The Tianshan Orogenic Belt is the southernmost part of the Central Asian
Sample description
The rock samples used in this study were collected from the outcrops of three mafic-ultramafic intrusive complexes (I, II and IV) in the Kuluketage district (Fig. 2b), which is part of the Gobi desert. These include one harzburgite and three olivine gabbronorite samples from the No. I complex, one lherzolite and one olivine gabbronorite samples from the No. II complex, and two olivine gabbronorite samples from the No. IV complex. They are olivine, pyroxenes and plagioclase cumulates, with minor
Analytical methods
U-Pb dating of zircon crystals was carried out using a SHRIMP-II machine in the Beijing SHRIMP Center, following the procedures given in Song et al. (2002). The TEMORA zircon standard (206Pb/238U age = 416.75 ± 0.24 Ma, Black et al., 2003) was used as a reference. The measured 204Pb was used for common Pb correction. Plotting and age calculations were performed using SQUID and ISOPLOT/Ex 3.00 (Ludwig, 2001, Ludwig, 2003).
Whole-rock major and trace element compositions were determined by X-ray
Geochronology
The U-Pb isotope compositions of zircon crystals separated from two large gabbroic samples collected from two mafic-ultramafic complexes (I, IV) in the Kuluketage district are given as Supplementary materials (Table S1) and illustrated in Fig. 4. The weighted mean 206Pb/238U ages for these two samples from the No. I and No. IV complexes are 727.6 ± 5.9 Ma and 725.4 ± 3.5 Ma, respectively. The new results from this study and the previous age data from Zhang et al. (2012) for these two complexes are
No hotspot track at Kuluketage
It is well known that due to plate movement above a mantle plume (or hotspot) beneath the asthenosphere (Morgan, 1971), the eruption or emplacement of the mantle-derived magma in the crust will change its final destination with time, i.e., ∼1000 km for every 10 myr, as shown by two modern examples for this type of magmatism in both oceanic and continental settings, the Hawaii (Tilling et al., 2010) and Yellowstone (Branney et al., 2008) volcanic trains, respectively. At Hawaii, the erupted magma
Conclusions
Prolonged mantle plume activity cannot explain the trace element compositions and the temporal-spatial distribution of the Neoproterozoic protracted mafic-ultramafic intrusions and mafic dykes in the Kuluketage district, the northern rim of the Tarim Craton. In contrast, subduction-related basaltic magmatism in a continental arc setting can well explain the trace element and Sr-Nd isotope compositions of these intrusive rocks as well as the protracted nature of magma emplacement in the crust.
Acknowledgments
This study was financially supported by the Special Fund for the Land and Resources Scientific Research of Public Interest of China (201511020), the Natural Science Foundation of China (41472070, 41372095 and 41302052) and the Fundamental Research Funds for the central Universities of China (lzujbky-2015-64). We thank Jiangwei Zhang, Kan Li and Bing Qian for their assistance in fieldwork, sampling and analysis. Critical comments from the reviewers and editorial inputs from the editor are
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