南海北部沉积物特征及其对珠江演变的响应

THE CHARACTERISTICS OF THE SEDIMENT IN NORTHERN SOUTH CHINA SEA AND ITS RESPONSE TO THE EVOLUTION OF THE PEARL RIVER

采用沉积地球化学、沉积物矿物成分分析等手段研究发现,自渐新世以来,南海北部沉积物成分具有阶段性改变的特征。在早渐新世,沉积物反映出其母岩以花岗岩和区域变质岩为主;到晚渐新世,母岩中区域变质岩类型消失,而以花岗岩、沉积岩和接触变质岩为特征;进入中新世,沉积物主要反映出碳酸盐岩等沉积岩为主的源区特点。这种沉积物成分演变特征与南海北部盆地基底及珠江流域不同区域物质组成特征极为一致,反映出珠江流域演化与南海北部沉积充填之间的对应关系。因此,通过这种研究方法,可以更好地理解32Ma以来南海北部沉积物成分发生改变的原因。同时,可以通过对比南海北部古珠江来源沉积物各时期的成分特点,建立珠江流域的侵蚀-搬运-沉积过程,探讨珠江流域演化历史。

The geochemical data of sediments from the South China Sea（SCS）provide constraints on both the composition of potential source rocks and the effects of sedimentary environments.In combination with knowledge of the regional geology,this data set allows us to decipher the tectonic implications since 32Ma on the basis of trends and major discontinuities observed in the stratigraphic succession of geochemical sediments compositions.This abrupt change coincided with a series of events including a seafloor spreading axis jump in the South China Sea and uplift of the western Yunnan Plateau and eastern Tibet Plateau,leading to obvious changes in sediment geochemical composition such as clay mineral,sand-mud ratio,major elements and sedimentary environments,suggesting great changes in the drainage areas of the paleo-Pearl River.Through this tectonic event,the sedimentary environment in the Baiyun Sag area transformed from continental shelf in the Late Oligocene to continental slope since the Early Miocene,and the sea level rose since the Early Miocene in the area.Heavy minerals in Pearl River sediments show excellent correlation with their source rocks,and obvious differences exist between heavy minerals assemblages and their representative minerals from different branches of the River.The source region of heavy minerals can be divided into three parts from west to east on the basis of assemblage types,index mineral（Ati,ZTi,and ZTR）values and their correlation.The western part includes the source region of the upper reaches with a zircon-leucoxene-apitite assemblage characterized by apitite and chromite,in an area largely covered by carbonate,partly by metamorphite and basic rocks.The middle part features a zircon-tourmaline-leucoxene-epidote assemblage,in which pyroxene,leucoxene,zircon and epidote are characteristic from a region predominated by clastic rocks and other sedimentary rocks,as a result of mixing between heavy minerals from local source rocks and alteration by river transportation from the upper reaches.The eastern part with a hornblende-tourmaline-epidote-garnet assemblage characterized by hornblende,tourmaline,garnet,titanite and sillimanite,indicating a source area mainly of acid rocks interbedded by some metamorphite and rare sedimentary rocks.The results demonstrate a probability to track the provenance evolution of the Pearl River by studying the characteristics of heavy minerals assemblages,and its significance in revealing erosion-transportation-sedimentation patterns.Samples both from the Pearl River branches and the Pearl River mouth basin were analyzed in this article,fulfilling the comparing between the two.Geochemistry analysis and heavy mineral analysis were applied.The results suggested：in Early Oligocene,the sediment in the northern South China Sea mainly came from the basement and adjacent areas of granite and metamorphic rocks; in Late Oligocene,the paleo-Pearl River dominated the northern South China Sea,with a smaller drainage at that time,including the lower reaches of the Pearl River,the Beijiang River and the Dongjiang River only,supplying silicate rocks like granite of the South China coastland; in Miocene,the middle reaches of the Xijiang River,the Guijiang River,the Liujiang River and the Youjiang River joined the paleo-Pearl River,bringing large area of Paleozoic sedimentary rocks and carbonate rocks into the drainage,which became the new dominant provenance of the northern South China Sea.In the meanwhile,some local parts of the southern deepwater area was not affected by the paleo-Pearl River as a result of the local volcanoes produced by strong tectonic events in Oligocene-Miocene.