High temperature glacial meltwater–rock reaction in the Neoproterozoic: Evidence from zircon in-situ oxygen isotopes in granitic gneiss from the Sulu orogen

•First report of SIMS zircon δ18O values as low as −11.0‰ with concordant Neoproterozoic U–Pb ages in South China.•These extremely negative δ18O values represent the primary record of the Neoproterozoic magmatism.•The spatial variation of zircon O isotope compositions in the profiles is interpreted as a manifestation of a fossil hydrothermal system.•The low δ18O magma is an indicator of continental rifting and cold paleoclimates.