Age And Origin Of Neoproterozoic Mafic Magmatism In Northern Guangxi-western Hunan

During the middle Neoproterozoic (830-740Ma), the igneous rocks are extensively developed in the South China and they have been received a great deal of attention to many researchers of international geoscience community. The nature and tectonic environments of these magmatic rocks, whether in rifting setting caused by mantle plume activities or in arc setting of continental margin, hold the key to understanding the reconstruction and breakup of Rodinia supercontinent, as well as the location of the South China in the reconstructed model of Rodinia supercontinent. At the present time, the tectonic regime that accounted for the middle Neoproterozoic magmatism in South China has been an issue of hot debate. The contradicting tectonic interpretations ("plume" or "arc") arise mainly from different interpretations of the "arc-like" geochemical signatures. For example, the Nb-Ta depletion relative to LREE, which is commonly regarded as a diagnostic feature of arc igneous rock, is quite clear in some of the Neoproterozoic igneous rocks. On the basis of field investigations, the middle Neoproterozoic mafic and ultramafic intrusions in Northern Guangxi ("Guibei" in Chinese) and Western Hunan ("Xiangxi" in Chinese) provinces were selected for comprehensive studies on petrology, geochemistry, Sm-Nd isotope and SHRIMP zircon U-Pb dating in this paper, with the aim of revealing their petrogenesis and tectonic implications. These studies have significant implications in understanding the tectonic history of South China in Neoproterozoic and in establishing the tectono-magmatic affiliation related to the breakup of Rodinia Supercontinent. Some results are gained as following:1. SHRIMP U-Pb zircon results indicate that the ultramafic rocks from the Yuanbaoshan area of northern Guangxi were crystallized at 841±22 Ma (2σ), which is indistinguishable within errors with the age of~825Ma for the mafic intrusions from the Sanfang-Hejiawan area of northern Guangxi. Petrological characteristics show that these ultramafic rocks have remarked olivine peritectic texture with Fo values of 78-83 in olivine. The magmas, equilibrated to the olivine, have a maximum calculated MgO of 7.3%. So the ultramafic rocks are cumulus rocks. Trace element contents of these rocks are lower than NMORB. On the spidergram, the Nb, Ta, Zr and Hf are depleted but Ti is not. The spidergram has a flat pattern excepting the depletions of some HFSEs. The most incompatible element ratios have wide ranges, such as Nb/La (0.11-0.96) and Th/Nb (0.18-0.57). The εNd (T) of these rocks ranges from -1 to +6.5 and most have high εNd (T). So these characteristics indicate that the magmas of the ultramafic rocks undergone variable degrees of crustal contamination. Our study suggests that the mantle source of the ultramafic rocks was similar to that of the Neoproterozoic Gairdner maficdykes (GDS) of plume-origin from central Australia. For example, they have comparable εNd (T) and Th/Nb, and the lowest Th/Nb is close to that of primitive mantle rather than the continental arc basalt, indicating that the ultramafic rocks should have been generated within an intracontinental, rather than an arc, setting. So the petrogenesis of the ultramafic rocks in Yuanbaoshan area is possibly related to the -825 Ma mantle plume beneath South China of Rodinia.2. By comparison with other mafic intrusive rocks in the Sanfang area, the mafic intrusions in Baotan area were generated at -825 Ma. These rocks exhibit some "arc-like" geochemical features, such as Nb-Ta-Ti depletion relative to neighboring REEs on the spidergrams. The positive correlation between Th/La and La/Sm, Th/La and SiO2 and the variable negative εNd(T) show the crustal contamination of the mafic magma. These rocks were generated by crustal assimilation of mafic magmas derived from a metasomatised subcontinental lithospheric mantle source, which possesses of high Th/Nb(~0.8). These mafic rocks should share the same tectonic setting as the neighboring ultramafic rocks from the Yuanbaoshan.3.SHRIMP TJ-Pb zircon dating results demonstrate that the middle Neoproterozoic volcanic rocks and mafic intrusions in the Longsheng area of Guibei and Xiangxi along the southern margin of the Yangtze Block are generally synchronous, dated at -765 Ma. The least-contaminated dolerite samples from Xiangxi are characterized by high εNd(T) value of+3.3 to +5.3 and the OIB-type geochemical features, indicating that they were derived from an OIB-like mantle source in a continental rift setting. The spilites and gabbros in Guibei show transitional basaltic compositions from tholeiitic to calc-alkaline series. Despite depletion in Nb and Ta relative to La and Th, their other trace element ratios such as Zr/Sm and Ti/V are affinitive to the intraplate basalts. They have variable εNd(T) values, ranging from -1.2 to 3.2 for the spilites and from -1.7 to 2.9 for the gabbros. These spilites and gabbros were generated by crustal assimilation of mafic magmas derived from a metasomatised subcontinental lithospheric mantle source. The -765 Ma mafic magmatic rocks in Guibei and Xiangxi were generated in a same continental rift setting as part of the broadly concurrent -780-750 Ma rift-related magmatism over much of South China related to the plume activities during the breakup of Rodinia.4. The two stages of-825 Ma and -765 Ma mafic igneous show clear geochemical affinities to the intraplate-, or rift-, related basaltic suites, except some mafic rocks which were generated by crustal assimilation of mafic magmas derived from a metasomatised subcontinental lithospheric mantle source. These mafic rocks are interpreted as products generated in the Neroproterozic rift related to mantle superplume activities during 830-750 Ma. On the basis of the tectonomagmatic associations, the South China was more likely located in the central part of Rodinia (Li ZX et al., 1995, 1999, 2003a), but not in the periphery of the supercontinent (Zhou MF et al., 2002b, 2006;Wang XL et al., 2006).