珠江口盆地渐?中新世古气候及物源特征:以阳江东凹为例

珠江口盆地阳江东凹处于太平洋构造域与新特提斯构造域交汇处,地球动力学背景、构造演化历史复杂,对于盆地物源特征和物源区古气候演化方面研究比较薄弱。选取珠江口盆地阳江东凹A钻井岩心中的珠海组-韩江组砂泥岩样品,对其进行地球化学分析,结果显示,样品呈轻稀土元素富集、重稀土元素平坦的配分模式,有明显的负Eu异常特征,与花岗岩相似;Al_2O_3/TiO_2、F1-F2、La/Th-Hf和Co/Th-La/Sc判别图解表明源岩以长英质为主,并有镁铁质岩石的混入;Al_2O_3/TiO_2、Cr/Zr和Th/Sc值也体现出长英质成分为主的物源特征;(Fe_2O_3+MgO)-TiO_2、Th-Co-Zr/10和Th-Sc-Zr/10构造环境判别图解显示,大部分样品落在大陆岛弧和主动大陆边缘区域,综合研究表明源区岩石特征与中国东南沿海大面积发育的岩浆岩极为相似。化学蚀变指数(CIA)、化学风化指数(CIW)、斜长石蚀变指数(PIA)等指标以及Mg/Ca值,指示珠江口盆地阳江东凹晚渐新世至中中新世物源区经历了中等程度的化学风化,对应温暖湿润的古气候环境,且从早到晚气温降低、降水量减少。

Oligocene-Miocene Provenance and Paleoclimate of the Pearl River Mouth Basin: A Case Study of the Eastern Yangjiang Sag

The eastern Yangjiang Sag of the Pearl River Mouth Basin,which is located at the transition of the Pacific and the Neotethys tectonic domains,has a complicated geodynamic background and history of tectonic evolution.The provenance of sediments in the sag and paleoclimate of the provenance region remain elusive.The fresh sandstone and mudstone samples of the Zhuhai and Hanjiang Formations from a drilling core in the eastern Yangjiang Sag were selected for geochemical analysis to identify the material source and the tectonic background of the study area,and to determine the weathering degrees and paleoclimatic conditions of the provenance region.The chondrite normalized REE patterns of the samples are characterized by enriched LREE,flat HREE and obvious negative Eu anomalies,suggestive of mainly granitic or felsic source.Al2O3-TiO2,F1-F2,La/Th-Hf and Co/Th-La/Sc provenance discrimination diagrams showed that the source rocks are mainly felsic and less mafic rocks.The discrimination diagrams for tectonic setting of(Fe2O3+MgO)-TiO2,Th-Co-Zr/10 and Th-Sc-Zr/10 demonstrated that the Late Oligocene to Early Miocene sandstone and mudstone in the eastern Yangjiang Sag were derived from a continental island arc or active continental margin.The comprehensive study shows that the rock characteristics of the source in study area are very similar to those of magmatic rocks widely developed in southeast China.Combining the chemical weathering indexes such as chemical alteration index(CIA),chemical weathering index(CIW),plagioclase alteration index(PIA)and Mg/Ca ratios of the sediments in the eastern Yangjiang Sag of the Pearl River Mouth Basin,we propose that the paleoclimate of the provenance region was relative warm and humid,and moreover,it seems likely that the temperature dropped and precipitation decreased from Late Oligocene to Early Miocene.