Origin of the Tongbai-Dabie-Sulu Neoproterozoic low-δ 18O igneous province, east-central China

Zircons from 71 diverse rocks from the Qinling-Tongbai-Dabie-Sulu orogenic belt in east-central China and, for comparison, eight from adjoining areas in the South China and North China blocks, have been analyzed for in situ O-18/O-16 ratio and/or U-Pb age to further constrain the spatial distribution and genesis of Neoproterozoic low-delta O-18 magmas, that is, delta O-18(zircon) a parts per thousand currency sign4 aEuro degrees VSMOW. In many metaigneous rock samples from Tongbai-Dabie-Sulu, including high-pressure and ultrahigh-pressure eclogites and associated granitic orthogneisses, average delta O-18 values for Neoproterozoic igneous zircon cores (i.e., 800-600 Ma) vary from -0.9 to 6.9 aEuro degrees, and from -9.9 to 6.8 aEuro degrees for Triassic metamorphic rims (i.e., 245-200 Ma). The former extend to values lower than zircons in primitive magmas from the Earth's mantle (ca. 5-6 aEuro degrees). The average Delta O-18 (metamorphic zircon - igneous zircon) values vary from -11.6 to 0.9 aEuro degrees. The large volume of Neoproterozoic low-delta O-18 igneous protoliths at Tongbai-Dabie-Sulu is matched only by the felsic volcanic rocks of the Snake River Plain hotspot track, which terminates at the Yellowstone Plateau. Hence, the low-delta O-18 values at Tongbai-Dabie-Sulu are proposed to result from shallow subcaldera processes by comparison with Yellowstone, where repeated caldera-forming magmatism and hydrothermal alteration created similar low-delta O-18 magmas. However, the possibility of involvement of meltwaters from local continental glaciations, rather than global Neoproterozoic glaciations, cannot be precluded. Our data indicate that Neoproterozoic low-delta O-18 magmas that are either subduction- or rift-related are present locally along the western margin of the South China Block (e.g., Baoxing Complex). It appears that Neoproterozoic O-18-depletion events in the South China Block as the result of hydrothermal alteration and magmatism affected a much larger area than was previously recognized.