Zircon U-Pb age and o isotope evidence for neoproterozoic low-18O magmatism during supercontinental rifting in South China: Implications for the snowball earth event

Abstract

Identification of Neoproterozoic low δ¹⁸O igneous rocks is a key to demonstrate that high-T water-rock interaction and low-¹⁸O magmatism in rift tectonic zones can serve as an efficient interface to transport heat and material from the Earth9s interior to exterior at the onset of the snowball Earth event. Low δ¹⁸O values of −10.9 to +4.4 permil characteristic of meteoric origin were observed, on an areal extent over 30,000 km², for mid-Neoproterozoic metaigneous zircons from the ultrahigh-pressure eclogite-facies metamorphic belt of Triassic ages along the Dabie-Sulu orogenic belt, China. Although the zircon O isotope system could be reset to acquire its low δ¹⁸O values by exchange with matrix minerals during “wet” eclogite-facies metamorphism, the abnormal ¹⁸O depletion is confirmed by occurrence of Neoproterozoic low δ¹⁸O zircons in low-grade metaigneous rocks in the same belt. Cathodoluminscence imaging of zircons from low-grade metagranitoid and metavolcanic rocks shows preservation of the internal structures of oscillatory zoning typical of magmatic origin. Hornblende from two granites of low δ¹⁸O zircons gave Ar-Ar plateau ages of 747 ± 6 and 776 ± 12 Ma, ruling out that Phanerozoic metamorphism effected mineral O isotope values. Two groups of U-Pb ages were measured at 756 ± 5 Ma and 782 ± 5 Ma, respectively, for the low δ¹⁸O zircons. This indicates that low δ¹⁸O zircons crystallized during two episodes of low-¹⁸O magmatism in response to tectonic evolution from supercontinental rifting at ∼780 Ma to breakup at ∼750 Ma. The second episode of low-¹⁸O magmatism is contemporaneous with the continental Kaigas iceage. A plate-rift model is advanced to account for the tectonic connection between the low-¹⁸O magmatism, the continental glaciation and the supercontinental rifting during the Cryogenian period. High-T meteoric water-rock interaction occurred prior to low-¹⁸O magmatism in rift tectonic zones, and caldera collapse was responsible for melting of hydrothermally altered low δ¹⁸O rocks during rifting of the South China Block from the Rodinia supercontinent.