Zircon U–Pb dating of water–rock interaction during Neoproterozoic rift magmatism in South China

A combined study of zircon U–Pb dating and mineral O isotope analysis was carried out for Neoproterozoic granite, gabbro and gabbrodiorite from the Beihuaiyang zone that marks a rifted margin between the South China Block and the supercontinent Rodinia. The results provide precise constraints on timing of water–rock interaction resulting in unusual 18O depletion in igneous minerals. CL imaging reveals that zircons mostly show clear oscillatory and/or sector zoning, with or without thin alteration rims. No metamorphic zircon occurs in the igneous intrusives, although they suffered greenschist-facies metamorphism during Triassic continental collision. LA-ICPMS zircon U–Pb dating yields consistent ages of 739 ± 8 to 754 ± 5 Ma, which are interpreted as timing of magma crystallization. Mineral separates have highly variable δ18O values of − 0.6 to 6.8‰ for zircon, 1.2 to 12.5‰ for quartz, and 0.3 to 8.4‰ for plagioclase. Oxygen isotope heterogeneities exist within single intrusions and between different intrusions. Magmatic zircons with low δ18O values and a weighted mean U–Pb age of 748 ± 6 Ma crystallized from low δ18O magmas that were derived from melting of hydrothermally altered low δ18O rocks. Both low and normal δ18O intrusives in the Beihuaiyang zone are shown to form at the same time, contemporaneous with the Rodinia breakup. Thus, tectonic evolution from supercontinental rift to breakup is proposed to create conditions for alternating processes of high-T hydrothermal alteration and low δ18O magmatism. Such processes are a characteristic feature of high-T water–rock interaction in volcanic rift margins associated with localized water-saturated melting. While the occurrence of low δ18O magmas demonstrates penetration of meteoric water into the rift tectonic zone of crustal melting, the existence of normal δ18O magmas indicates that their emplacement may serve as a thermal engine to drive the second stage of meteoric water–rock interaction resulting in the unusual 18O depletion in adjacent rocks. The concurrent occurrence of both low and normal δ18O magmas appears to indicate highly heterogeneous high-T water–rock interaction during rift magmatism.