西沙群岛西科1井晚中新世-上新世生物礁沉积的磁性地层学初步结果

Preliminary results of rock magnetism and magnetostratigraphy for Late Miocene to Pliocene biogenetic reefs in the Xisha Islands,South China Sea

本文对西沙群岛西科1井钻孔岩心晚中新世-上新世生物礁沉积进行了详细的岩石磁学、磁性扫描与磁性地层学研究.结果显示,西科1井生物礁相沉积中的载磁矿物主要是磁铁矿.我们推测,这些磁铁矿的微小颗粒主要来自海水中含有的陆源物质,在生物生长过程中通过珊瑚体或其它寄生生物对海水的过滤与吸附作用保存在生物礁沉积中.进一步的磁性地层学研究及其与地磁极性年表的对比发现,在上新统莺歌海组、上中新统黄流组内部可获得多个年龄控制点,并对莺歌海组和黄流组的底界位置给出了初步制约.其中,莺歌海组记录了C2An.3n和C3n.4n,黄流组记录了C3An.2n至C5n.2n.虽然这一对比方案存在一定不确定性,但是在现阶段生物地层年代及其它年代学资料相对匮乏的情况下,我们认为本项研究的磁性地层学结果能为西沙群岛晚中新世以来的生物礁沉积提供更多可靠的年龄控制点,并为今后的区域地层对比提供磁性地层年代学依据.

The South China Sea（SCS）in West Pacific Ocean is the largest marginal sea in Asia.Due to the unique location,its Cenozoic evolution has been attracting great attentions during the past decades.As one of the most debated issues,geochronological frameworks of the SCS basin have been widely investigated mainly based on geophysical data or regional unconformities.As one of two major types of depositions（terrigenous and marine biogenic）in this critical region,numerous coral reefs have been developed on the Xisha-Zhongsha terrain since the early Miocene.Previous studies of the Xisha carbonate platforms have mainly focused on the analyses of sedimentology,tectonics,and modern ecology,but little geochronological work,which ought to be an important component of such researches,has been performed.This lack of geochronological information significantly hinders our understanding of the initiation and development of these biogenic reefs.Herein, multi-parameter rock magnetic measurements,including isothermal remanent magnetization（IRM）acquisition,hysteresis loops and magnetic scanning,were performed on typical samples collected from Borehole XK-1in the Xisha Islands.The results suggest the predominance of ferrimagnetic phases in the XK-1biogenic reefs,and the average sizes of magnetic mineral grains are relatively coarse,falling within the pseudo-singledomain to multidomain-like grainsize region.Further mathematical partitioning demonstrates the predominance of low-coercivity component（mainly magnetite）.Considering that the study area is far away from the East Asian continent and the growing process of the biogenic reefs,it is inferred that the main magnetic mineral in biogenetic reefs is magnetite,which is from terrestrial detrital materials and absorbed or filtered into the reef with its growth.Further magnetostratigraphic study shows that the identified normal and reverse magnetozones can be correlated with geomagnetic polarity time scale（GPTS）,providing several reliable age constraints for thosereefs.ItrecordsC2 An.3nandC3n.4nchrons in the Yinggehai Formation,and C3 An.2nto C5 n.2nchrons in the Huangliu Formation.Our new magnetostratigraphic findings have provided valuable geochronologic constraints for regional stratigraphic correlation and paleoenvironmental processes.