NEOPROTEROZOIC METEORIC-HYDROTHERMAL ALTERATION AND LOW-18O MAGMATISM IN SOUTH CHINA

It is intriguing whether the snowball Earth event is genetically associated with the Neoproterozoic mantle superplume, supercontinent breakup and rift magmatism. In order to resolve this mystery it is critical to identify the signature of coeval low-¹⁸O water-rock interaction that incorporated the surface water into magma source to generate low-¹⁸O magmas in this period. We accomplish a combined study of zircon CL imaging, U-Pb dating and oxygen isotope analysis as well as hornblende Ar-Ar dating for greenschist-facies metavolcanics and metagranitoids in the Dabie region of China. CL imaging of zircons from the low-grade metaigneous rocks reveals internal structures of oscillatory zoning typical of magmatic origin. No metamorphic overgrowth or recrystallized domain was observed. LF oxygen isotope analysis of zircons shows low δ¹⁸O values of +1.3 to +3.1‰, which are significantly depleted in ¹⁸O in comparison with the normal mantle zircon δ¹⁸O values of +5.3±0.3‰. Even if zircon would be not refractory enough to survive oxygen isotope resetting by supersolidus hydrothermal alteration and “wet” eclogite-facies metamorphism, such low δ¹⁸O values for the least metamorphosed zircons clearly point to their crystallization from low-¹⁸O magmas. The low-¹⁸O zircons were dated by the TIMS U-Pb method, yielding concordant ages of 749±4 to 764±14 Ma. Hornblende from the metagranitoids of low-¹⁸O zircon gives Ar-Ar plateau ages of 747±6 to 776±12 Ma. An episodic growth of zircon from low-¹⁸O magmas is suggested to occur at about 740 to 760 Ma by repetitive rifting syntexis. The contemporary occurrence of hydrothermal alteration, rifting syntexis and low-¹⁸O magma emplacement during the middle Neoproterozoic provide insight into the mechanism of paleoclimate change at that time. The mantle superplume event and its derived large-scale rift-magmatic activity would significantly have advanced chemical weathering of global scale, and the meteoric water-rock interaction and low-¹⁸O magmatism would serve as an important interface for the exchange of energy and material between the Earth's interior and exterior. The over-consumption of atmospheric CO₂ by the chemical weathering may have triggered the Sturtian glaciation of continental scales.