In the northern South China Sea,the accumulation of enormous quantities of terrigenous sediment during Cenozoic rendered well-developed polymetallic nodules very rare.In this study,we analyzed a polymetallic nodule from the northwestern continental margin of the South China Sea using microscopic mineralogical observation,electron probes,X-ray diffraction(XRD),ICP-MS,and Be isotope dating.We found the nodule's shell layers rich in different types of microstructures,including columnar,laminar,stack-like,petal-like,and porphyritic structures.The major mineral components of the nodule are MnO2.Unlike nodules from the eastern Pacific basin,this nodule has high contents in Fe,Si,Al,and REEs but low contents in Mn,Cu,Co,and Ni.The Mn/Fe ratio is also low and the average REEs content is 1370.4 ppm.There is a strong positive anomaly of Ce;and the Be(beryllium) isotope dating shows the initial time of growth of the nodule to be about 3.29 Ma.The inner compact layer formed from 3.29 Ma to about 1.83 Ma.The laminar and stack-like structures and the low contents of the terrigenous elements such as Fe,Si,REE,and Al indicate the paleoceanographical environment with weak undersea currents and favorable oxidizing conditions.From 1.83 Ma to 0.73 Ma,the growth rate of the nodule increased by about 3%;the microstructures formed during this period are stack-like and columnar.The contents of Si and Al are increased by nearly 10%,indicating an increase of terrigenous sediment input in the northern South China Sea.The content of Ce is decreased by about 16%,indicating a significant weakening of the oxidizing conditions at the seabed.From 0.73 Ma to 0.69 Ma,the growth rate of the nodule rapidly rose up to 8.27 times that of the nodule's average growth rate,and the contents of Fe,Al,and REEs in the layer also increased,forming a loose layer characterized by oolitic,granular,porphyritic,and petal-like structures,indicating the paleoceanographical environment with a high sedimemtation rate and abundant supply of terrigenous sediment in the
The southwest basin is a key to study the origin and development of the South China Sea(SCS).We do not know much about its boundaries,geological history,and the formation of its sea floor because it has a complex and highly re-gional structural background,notable sediment activity,and yet few floor rocks.Here a grano-diorite sample was collected from the southern margin of the southwest basin of the South China Sea.The results indicated that the 40Ar-39Ar ages of biotites in the sample are 110.3±0.5 Ma,suggesting that they were prod-ucts of magmatic intrusion during the Early Cretaceous period.The sample's geochemistry showed it had high SiO2,K2O,and Al2O3 but low TiO2 levels.Tectonic discriminant diagrams suggested that the sample might represent extrusion-related magmatism,either in an arc or forearc setting in the SCS area and that the sample mainly belonged to the syncollision type,whose formation was related to orogenies.The sample may be part of the main rock that made up the boundaries of rift system.The process of tension cracking was similar to the development of the Red Sea,in which the rifting and sagging occurred in the continental crust.The southwest basin may not be an original ocean,but a rift developed through finite extension on continental crust basement.The oceanic crust came into being when the width and depth of the rift valley reached a certain scale.The granodiorite sample we collected provides a means of determining the boundary of the southwest basin and the clues that may help researchers expand relevant models.It constitutes an important datum regarding the analysis of the formation and development of the SCS.
