The occurrence of Neoproterozoic low δ18O igneous rocks in the northwestern margin of the South China Block: Implications for the Rodinia configuration

Low δ18O felsic igneous rocks are reported for the first time in the northwestern margin of the South China Block. Zircon U-Pb dating indicates that intermediate to felsic plutons were emplaced at 780 Ma and 750 Ma, respectively, and rhyolite was erupted at about 750 Ma. Compared with other rock-forming minerals that have variable δ18O values (−2.3‰ to 8.7‰ for quartz; −5.4‰ to 6.3‰ for plagioclase, −7.6‰ to 2.5‰ for K-feldspar), zircon δ18O values mainly range from 1.0‰ to 4.7‰ with a few negative values of −7.7‰ to −2.9‰. A two-stage 18O-depletion scenario was proposed, 1) high-temperature meteoric water–rock reaction in a continental rift setting, generating low δ18O melt from which precipitated zircons with significant lower δ18O values than normal mantle; 2) post-crystallization sub-solidus high-T meteoric-hydrothermal alteration, resulting in varying δ18O values for rock-forming minerals that leads to δ18O-disequilibrium between zircon and other minerals. In addition, zircons from these rocks show positive εHf(t) values of 4.6 to 11.2 with Hf model ages of ca. 1.0–1.4 Ga, suggesting a magma source of juvenile crust due to mantle melting during the Grenvillian subduction of oceanic lithosphere in response to accretion of the South China Block to Rodinia in the Late Mesoproterozoic. The occurrence of low δ18O felsic rocks in the northwestern margin of the South China Block indicates the development of a low δ18O magmatic belt during the rifting of the South China Block away from Rodinia at about 750 Ma. The coeval occurrence of Neoproterozoic low δ18O magmatism in NW India, South China, Madagascar and Seychelles suggests a link between these continental blocks in the Rodinia configuration.