Geochemical characteristics and sedimentary-tectonic setting of the Cambrian sandstones and mudstones in Dayaoshan-Damingshan area,Guangxi

Abstract：The Daoyaoshan-Damingshan area is located in the southwestern section of the Qinzhou-Hangzhou juncture between the Yangtze Block and the Cathayian Block．The study of the early Paleaozoic sedimentary environment and tectonic setting of this area is a key to understanding the geotectonic problems in South China. Major and trace elements of 27 pieces of samples from Cambrian sandstones and mudstones in this area were analyzed and discussed. The samples generally have high ratios of Al2O3/TiO2 ( in the range of 11.95~36.26,20.81 on average) and lower ratios of Rb/Cs（in the range of 13.02~68.27, 32.21 on average） and Cr/Zr（ranging 0.14～1.15,averagely 0.59）. Geochemical characteristics, such as the plots of Ni-TiO2,Th/Sc-La/Sc and La/Th-Hf , indicate that the Cambrian sediments in the Daoyaoshan-Damingshan area were mainly sourced from the upper crustal felsic quartz rocks,with the addition of a small amount of igneous sources and ancient recirculation sediments．The distribution of trace and rare-earth elements and the data of La-Th-Sc,K2O/Na2O-SiO2, δCe,δEu,Tb/Yb,La/Sc,La/Th,Th/U as well as the comparison with different tectonic settings show that the study area belonged to a passive continent-marginal setting. In addition, such evidence as the marks of shallow-sea deposits,the data of sedimentary palaeogeography,regional geology and geochemistry suggests that during the Early Paleozoic there existed no geochemical evidence for the so called “ancient ocean in South China”．