关键词: 岛礁白云岩, 早期白云石化, U-Pb定年, 南科1井, 南海南部

Abstract: Cenozoic dolomitization of reefal carbonates has been widely found on many tropical islands worldwide. However, conventional bulk-sample ages and geochemical data often fail to clarify the complex diagenetic history of these island dolostones, due to limited in situ constraints on age and fluid composition. In this study, a deep borehole drilled into Cenozoic carbonates on Meiji Island in the southern South China Sea encountered massive dolostones exceeding 400 m in thickness. We performed in situ U–Pb geochronology and elemental analysis on the lower Nanwan Formation (upper Miocene), focusing on three phases: undolomitized calcitic bioclast, replacive dolomite, and dolomite cement. Both strontium isotope ages and U–Pb dates indicate that penecontemporaneous replacive dolomitization occurred at 11.0–8.5 Ma, shortly after the deposition of precursor limestone. The dolomite cement followed, precipitating at 8.5–6.0 Ma. In situ elemental data suggest that seawater was the likely fluid source for both replacive dolomite and dolomite cement in the lower Nanwan Formation. Dolomite cements show higher Mg/Ca ratio and lower Mn and Sr contents, reflecting precipitation from more evaporated seawater. Dolomite content relates positively with porosity in the reefal limestones, highlighting that the importance of primary voids in facilitating fluid flow during dolomitization. Moreover, coralline algae and algal-rich lime mud appear to have promoted rapid dolomite nucleation. This study demonstrates that in situ laser ablation elemental analysis is a powerful tool for tracing the sources of multistage dolomitizing fluids and helps refine the existing dolomitization models of isolated atolls.

Key words: island dolostone, early dolomitization, U-Pb dating, Well NK-1, southern South China Sea