NEOPROTEROZOIC RIFT-MAGMATISM AND HYDROTHERMAL CIRCULATION ALONG THE NORTHERN MARGIN OF SOUTH CHINA BLOCK: RESPONSE TO THE RODINIA BREAKUP?

The Dabie-Sulu HP to UHP metamorphic belt represents the eastern part of the Qinling-Dabie orogenic belt that resulted from the Triassic collision between South and North China Blocks. Apart from radiometric ages of 215 to 245 Ma that date the collision, U-Pb zircon dating indicates that bimodal magmatism of 700 to 800 Ma with a cluster at about 750 Ma can be traced in various types of metaigneous rocks exposed not only in the HP to UHP belt but also in low-grade metamorphic zones in the Dabie-Sulu belt. More significant is that zircons of numerous metaigneous rocks of middle Neoproterozoic emplacement ages from the Dabie-Sulu belt are noticeably depleted in ¹⁸O isotope in zircon and other rock-forming mineral, implying meteoric-hydrothermal alternation of magma sources probably related to the “snowball Earth event” along the northern margin of South China Block during the mid-Neoproterozoic. The event of low d¹⁸O magmatism and hydrothermal circulation in the Dabie-Sulu belt is generally contemporary and is probably related to the Rodinia breakup and associated deglaciation, and the bimodal compositions of the low d¹⁸O metaigneous rocks provide evidence for an extension setting along the northern margin of South China Block in the mid-Neoproterozoic. Granitoids having U-Pb ages of 700 to 800 Ma with a mode of 750 to 755Ma were reported to occur in the Seychelles of Indian Ocean, which also show low d¹⁸O values. The coeval occurrence of the mid-Neoproterozoic granitoids in Indian Ocean (also low d¹⁸O) with those in the northern margin of South China Block may point to a correlation in the geotectonic setting of magmatic activity during the Rodinia breakup. This may have potential implications for localization of South China in reconstructing the Rodinia supercontinent, i.e. close to India in northwest rather than close to Australia in east. Together with zircon U-Pb ages for igneous complex in Southeast Africa, this would favor the Palaeopangaea reconstruction, in which the relative position of the mantle plume heads beneath South China, India and Southeast Africa can be assembled as the same superplume in the center of the Palaeopangaea supercontinent. This would not be possible in the Rodinia configuration since the respective areas would have been at almost opposite ends of the supercontinent.