GEOCHEMISTRY OF CENOZOIC VOLCANIC ROCKS IN ZHU JIANGKOU BASIN AND ITS GEODYNAMIC SIGNIFICANCE
Zou Heping (Department of Geology, Zhongshan University, Guangzhou 510275) Li Pinglu; Rao Chuntao (Nanhai East Oil Corp. CNOOC, Guangzhou)
According to the major and trace element and isotope data of the Cenozoic volcanic rocks in the Zhujiangkou Basin of the South China Sea (SCS), it has been discovered that the Eogene volcanic rocks are complex in components. Not only basalts but also many intermediate and acid rocks formed, which are mainly the calc-alkali rocks. The ratios of K/P in the rocks are higher, but the abundances of high field strength elements (HFSE), and the ratios of Ti/Yb, Nb /La and Nb/ Y are lower. Their Sr and Nd isotopic compositions show that the rocks belong to a relatively enriched type. During the Neogene, however, more uniform rock association composed of alkali basalt and tholeiite formed. The ratios of K/P in the basalts are reduced, but HFSE and the ratios of Ti/Yb, Nb/La and Nb/Y are increased; and their Sr, Nd isotopic composition indicate that they belong to a relatively depleted type. This evolution tendancy of magmatic rocks is quite similar to that of volcanism in the Cenozoic rifting region of Western North American. In addition there exists a Dupal-like Pb isotopic anomaly in the basalts from the Zhujiangkou Basin. Such a tendancy of the magmatic evolution and geochemical characteristics reflect tha“tlower lithospheric delamination”would be an important dyn amic mechanism for the Cenozoic continental marginal spreading in the Northern South China Sea.
【CateGory Index】： P588.1