Integrated in situ U-Pb Age and Hf-O Analyses of Zircon from the Northern Yangtze Block: New Insights into the Neoproterozoic Low-δ18O Magmas in the South China Block

Abstract:

The oxygen isotopic composition of Neoproterozoic magmas from the northern Yangtze Block holds a key for the origin of large-scale ¹⁸O depletion in the HP and UHP metamorphic rocks in the Dabie-Sulu orogenic belt, northern margin of the South China Block. We report here the integrated in situ U-Pb dating and O-Hf isotope analyses of zircon grains from sedimentary and volcanic rocks of the late Neoproterozoic Suixian Group (SG) from the northern Yangtze Block. Detrital zircon grains display age peaks of 0.73-0.74 Ga, 0.79 Ga, and 2.0 Ga. Zircon U-Pb ages together with Hf-O isotopic composition indicate provenance of SG dominantly from proximal Neoproterozoic igneous rock and likely hidden Paleoproterozoic basement along the northern margin of the Yangtze Block. The zircon δ¹⁸O values from SG range from 10.5‰ to 1.3‰. Zircon grains with negative δ¹⁸O value, typical result of magma-ice interaction, were not identified in this study. The major phase of low-δ¹⁸O (< 4‰) magmas initiated at ca. 780 Ma, long before the first glaciation event (< 715 Ma) in the South China Block. Thus caution should be taken when using low-δ¹⁸O zircon grains to infer cold climate. Low-δ¹⁸O zircon grains have large ranges of ε_Hf(t) values, varying from -15.5 to 10.7, concentrating on negative ε_Hf(t). This strongly argues against the possibility that the low-δ¹⁸O magma was produced by partial melting of high-temperature hydrothermally altered oceanic crust because this model predicted MORB-like Hf isotopes for the resultant low-δ¹⁸O magmas. This study emphasizes that high-T water-rock interaction and continental rifting tectonic setting are essential to generate abundant low-δ¹⁸O magmas. The important application of our study is to confirm that most of negative-δ¹⁸O zircons identified in HP and UHP metamorphic rocks may not have been inherited from their Neoproterozoic protoliths.