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Integrated analyses of in situ zircon U-Pb ages and geochemistry were conducted on both granitic and metasedimentary borehole samples from immediately below the Cenozoic sedimentary cover of the northern South China Sea basin. Our work suggests that basement rocks of the South China Sea basin are not Proterozoic or Paleozoic rocks, as previously speculated based on gravity-magnetic inversion work. The analyzed metavolcaniclastic and metasedimentary rocks were likely deposited at or soon after ca. 145 Ma but prior to ca. 85 Ma. A nearby source in coastal South China is proposed based on the generally immature nature of the metasedimentary rocks and the lack of detritus from older basement rocks farther inland. The vast majority of the metavolcaniclastic samples have sparse ca. 250–190 Ma (particularly 200–190 Ma) zircons. This magmatic quiescence is consistent with the existence of a coastal …