Oxygen and lead isotope characteristics of granitic rocks from the Nansha block (South China Sea): Implications for their petrogenesis and tectonic affinity

The pre-Cenozoic history of the South China Sea (SCS) region is the object of continued debate. To trace the evolution of the SCS, a better understanding of the petrogenesis and tectonic affiliation of the granitic rocks that comprise the microblocks within the region is necessary. In this study, we analyzed the whole-rock oxygen and lead isotope ratios of granitic samples dredged from two locations in the Nansha microblock, one of the microblocks in the SCS. Oxygen isotope data combined with previously published Sr isotope data show that group I rocks (delta 18O = 6.00-7.20 parts per thousand; average = 6.64 parts per thousand) originated from a mantle source contaminated by material and/or fluid input from a Mesozoic subduction zone in the southeastern side of the microblock. Group II rocks (delta 18O = 6.86 parts per thousand-9.13 parts per thousand; average = 7.75 parts per thousand) also came from the same source, but they were additionally affected by crustal contamination. The Nansha microblock has high radiogenic lead ratios (206Pb/204Pb(i) = 18.602-18.756, 207Pb/204Pb(i) = 15.660-15.713, 208Pb/204Pb(i) = 38.693-38.893), which indicate that the Nansha microblock is tectonically affiliated with the Nanling-Hainan or South China block. This notion is consistent with the results of a previous Nd isotope study. As a whole, results of our study suggest that some of the other microblocks dispersed in the SCS are also possible fragments of South China block, and thus further studies are needed to better constrain the pre-Cenozoic evolutionary history of the SCS region.