Origin And Indicative Significance Of Pliocene "Background Dolomite" In Well CK-2,Xisha Islands,South China Sea

The Cenozoic island dolostones,which are geologocally young,have not been buried,and weakly modified by later diagenesis,are considered a natural laboratory for studying the classic“dolomite problem”.In the Cenozoic island reef environment,the early stage（late Miocene-early Pliocene）is dominated by the development of massive dolotones（tens to hundreds of meters thick,dolomite content>75%,plateform scale）,while the late stage（late Pliocene-Pleistocene）is dominated by the development of“background dolomite”,that refers to the dolomite content in the limestones is generally<50%.At present,main research focuses on the massive dolostones,whereas the research on“background dolomite”is relatively weak,which prevents the understanding of the overall appearance and genesis of island dolostones.Well CK-2（927.85 m）drilled on Chenhang Island in the Xisha Islands,South China Sea in 2013 revealed the complete stratigraphic sequence of island dolostones:the lower part（308.5～519m）is the late Miocene massive dolostones of Huangliu Formation,the upper part（180～308.5 m）is the“background dolomite”of the Pliocene Yinggehai Formation,which provides high-quality materials for studying the overall picture of the development and evolution of island dolostones.In this paper,the whole rocks and the pure“background dolomite”extracted from the whole rocks from the Pliocene Yinggehai Formation（128.5 m thick）in the CK-2 core were taken as the research objects.Based on the analysis of petrology,mineralogy and geochemistry(Sr,Mn,Na,REE+Y,δ¹³C,δ¹⁸O,⁸⁷Sr/⁸⁶Sr),we discussed the dolomitization time,fluid properties and diagenetic environment of the“background dolomite”in the Yinggehai Formation.Then,compared to underlysing Huangliu dolosontes and other typical island“background dolomite”,we further analyzed the development and evolution process and environmental control factors of the island“background dolomite”,which is of great significance to understand the origin of the Cenozoic island dolostones.The following main insights were obtained:（1）The original sedimentary environment of the Yinggehai Formation in Well CK-2 of the Xisha Islands is dominated by lagoon facies,and a variety of neritic biofossils（mainly red algae,coralline algae,and mollusks）are well preserved.The degree of dolomitization in this interval is relatively weak（dolomite content<50%）.Coralline algae and matrix show the characteristics of preferential dolomitization,in which the dolomite crystalline generally varies from 5-20μm.Mineralogical features show that these are mainly calcium-rich ordered dolomites.Dissolution pores especially moldic pores are developed in this interval,whereas there was no cements filling in the pores.（2）The geochemical characteristics show that the“background dolomite”of the Yinggehai Formation mainly formed in an open,oxidized and normal seawater environment.Sr content（232-494 ppm）and Mn content（9-51 ppm）indicate that these dolomites formed in an open and oxidized diagenetic environment,while Na content（394-710 ppm）,δ¹³C values（1.7-2.4‰）,and theδ¹⁸O values（3.2-5.0‰）indicate that the dolomitization fluid was mainly normal or slightly modified seawater.These background dolomites has a similar distribution stlye of rare earth elements（REE+Y）to seawater and marine carbonate rocks.The REE+Y shows moderate negative Ce anomalies（average 0.52）,significant negative Eu anomalies（average 0.74）,light rare earth（LREE）depletion（average Dy_N/Sm_N=1.5）,high Y/Ho ratio（average>40）,and positive La anomaly,suggesting that the general diagenetic environment should be in a low temperature seawater environment.The intersection points of Ce/Ce*and Pr/Pr*are all distributed in the range of modern seawater,which further confirmed the dolomitization fluid should be normal seawater.（3）The ⁸⁷Sr/⁸⁶Sr age shows that the“background dolomite”of the Yinggehai Formation formed from 4.5 to 0.5Ma（million years）ago,and the whole rock of the Yinggehai Formation was formed before 4.5 to 1.6Ma.At the same time,the selectively dolomitized of coralline algae and some matrixes and the extensive development of dissolution moldic pores reflect that the“background dolomite”of the Yinggehai Formation had been formed shortly after the depositional period and was the product of early dolomitization period.The significant difference in dolomitization time betweenthe Yinggehai Formation（4.5 to 0.5Ma）and the Huangliu Formation（10 to 4Ma）shows that the two parts of dolomites are the products of different dolomitization periods.On the one hand,the dolomitization time of Well CK-2 in the Xisha Islands（10 to 0.5Ma）almost covers the development time（10 to<0.5Ma）of the global island dolostones,which relatively completely records the whole process of development and evolution of island dolostones.On the other hand,the age of the dolomitization gradually becomes younger from the bottom to the top of the Yinggehai Formation,with insignificant discontinuities,are difficult to use with the traditional“event dolomitization”.This phenomenon can probably be explained by the process of（semi-）continuous“time-transgressive”dolomitization,and this process is mainly controlled by tectonic subsidence and global sea level changes.（4）Compared with the“background dolomite”of the Bahamas platform formed in a closed diagenetic environment where the Mg²⁺for dolomitization originated from the high-magnesium calcite minerals,the"background dolomite"of the Yinggehai Formation in the Xisha Islands was formed in a relatively open diagenetic environment.The duration of dolomitization had been for millions of years,indicating that the amount of Mg²⁺from seawater required for its dolomitization was sufficient.On the one hand,the transformation time of the massive dolostones of the Huangliu Formation into the“background dolomite”of the Yinggehai Formation was about～4Ma,which corresponded to the Pliocene cooling period（e.g.,expansion of the Arctic ice-sheet）,indicating that the low temperature diagenetic environment probably caused partial dolomitization of the Yinggehai Formation to form“background dolomite”.On the other hand,compared with the dolomitization pattern of seawater flowing from the edge of the platform to the center of the platform in Grand Cayman（centripetal dolomitization pattern）,since the original depositional environment of the Yinggehai Formation was the lagoonal environment in the center of the platform,it might be due to the seawater fluid for dolomitization flowed from the edge of the Xisha platform.During the seawater flowed to the center,with the gradual consumption of Mg²⁺during the dolomitization process,the lagoon sediments located in the center of the platform（location of well CK-2）had only partially been dolomitized to form“background dolomite”.Alternatively,the“background dolomite”of the Yinggehai Formation had been both affected by low temperature and"centripetal"dolomitization fluids.This paper is of great significance for understanding the genesis and controlling factors of Cenozoic reef dolostones or dolomites.