Integrated in situ U–Pb age and Hf–O analyses of zircon from Suixian Group in northern Yangtze: New insights into the Neoproterozoic low-δ18O magmas in the South China Block

Abstract

The mid- to late-Neoproterozoic magmatic rocks from the northern margin of the Yangtze Block are major protoliths of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks in the Dabie–Sulu orogenic belt along the northern margin of the South China Block. Oxygen isotopic compositions of these mid- to late-Neoproterozoic magmatic rocks hold a key to understanding the origin of large-scale 18O-depletion in the HP and UHP metamorphic rocks. We report here the integrated in situ U–Pb dating and Hf–O isotope analyses of zircon grains from sedimentary and volcanic rocks in the Neoproterozoic Suixian Group along the northern margin of the Yangtze Block, South China. This study shows that the Suixian Group was deposited at 740–720 Ma, corresponding to late stage of the second deposition cycle (820–720 Ma) of the Neoproterozoic deposition in the Yangtze Block. Detrital zircon grains from the Suixian Group display age peaks at 0.73–0.74 Ga, 0.79 Ga, and 2.0 Ga. Zircon U–Pb ages together with Hf–O isotopic compositions indicate provenance of the Suixian Group dominantly from the proximal Neoproterozoic igneous rocks with possible contribution from Paleoproterozoic rocks along the northern margin of the South China Block. Zircon δ18O values from the Suixian Group have a large range from 10.5‰ to 1.3‰.Zircon grains with negative δ18O values, typical index of magma-meteoric water interaction, were not identified in this study. The major phase of low-δ18O magmas (δ18Ozircon < 4.6‰) initiated at ca. 800 Ma, long before the first glaciation event arising at about 716 Ma in the South China Block, and lasted for over 100 m.y. The ɛHf(t) values of the low-δ18O zircon grains from the Suixian Group range between −15.5 and 10.7. About 74 percent of low-δ18O zircon grains have negative ɛHf(t) values varying from −15.5 to 0. This strongly argues against the possibility that the low-δ18O magma was generated dominantly by partial melting of high-T hydrothermally altered oceanic lower crust. This study emphasizes that high-T water-rock interaction and continental rifting tectonic setting are essential to produce the abundant low-δ18O magmas, and confirms that most of negative δ18O signature identified in zircon grains from HP and UHP metamorphic rocks may not have been inherited from their Neoproterozoic protoliths.