Abstract
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 magmatic gap to the east of Hainan Island during this time, previously interpreted as reflecting a period of flat-slab subduction. Metaclastic samples mostly exhibit a dominant ca. 180–120 Ma zircon population, indicating extensive magmatism along the coastal areas. This finding, together with previously reported ca. 190–180 Ma I-type coastal granites and our newly dated ca. 115–98 Ma volcanic and granitic rocks under the western part of the South China Sea basin can best be interpreted by the former presence of an Andean-type continental arc during the Middle Jurassic – Early Cretaceous. Magmatism along this arc ceased soon after 90 Ma or before ca. 85 Ma, with its last pulse represented by a small group of ca. 100–85 Ma granitic intrusions in the eastern part of the northern South China Sea basin. The arc appears to have jumped toward the Western Pacific Ocean at or soon after 90 Ma, starting the development of the Western Pacific-type plate margin.
Published Version
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