ELEMENT GEOCHEMISTRY OF WELL XIKE 1 ON THE XISHA ISLANDS AND ITS BEARING ON SEA LEVEL FLUCTUATION

This study focused on major and trace element geochemistry of the carbonate platforms of the Xisha Islands, northern South China Sea (SCS), based on the sedimentary characteristics of the succession since early-Miocene. A total of 1268 carbonate samples are collected and analyzed by ICP-MS. Both the major elements of Na₂O/K₂O, Na₂O/SiO₂, P₂O₅, and the trace elements of B/Ga, Zr/Al, Li, V show perfect consistence with petrographic, stratigraphic and paleontological variations caused by the changes in paleo-salinity, Eh, pH and sea level fluctuation, which can be well applied for the study of depositional sequences of the isolated carbonate platforms. However, Mg/Ca are hard to be used as a proxy to reveal sea level changes, owing to the strong dolomitization. Na₂O/K₂O, Na₂O/SiO₂, P₂O₅ B/Ga, Zr/Al, Li, V suggest that the bioherms started blooming with the sea water invasion since early-Miocene. During early middle-Miocene, bioherms with lagoonal and beach facies were well formed when the sea level reached its first peak. Then the sea level dropped in late middle-Miocene and early late-Miocene. In the late middle-Miocene, bioherms were strongly leached and eroded, as dominated by reef-beach facies. The sea level started to rise again during late late-Miocene and got to its second peak in Pliocene and then dropped for the second time during Pleistocene period.