Nd-Sr-Pb ISOTOPIC AND CHEMICAL EVIDENCE FOR THE VOLCANISM WITH MORB-OIB SOURCE CHARACTERISTICS IN THE LEIQIONG AREA, CHINA
Zhu Bingquan Wang Huifen(Institute of Geochemistry, Academia Sinica)
The Cenozoic volcanics in the Leiqiong area, South China, are mainly tholeiites, giving eruption ages lower than 16Ma. Their petrochemistry is intermediate between MORB and OIB. Nd, Sr and Pb isotopic compositions are in the ranges 0 51274-0.51304, 0.7031-0.7040 and 18.14-18.69, respectively. The Nd-Sr-Pb isotopic correlations also show MOR3-OIB features, which represent the most depleted mantle region beneath the East China Continent. A few samp-les have higher 87Sr/86Sr ratios( 0.7043), indicating the influence of the ancient mantle source like the Shanshui Basin. The plot of 143Nd/144Nd v.s. 147Sm/144Nd revels a history of three-stage evolution: depletion in Precambrian (570Ma), enrichment in Paleozoic (500-150Ma) and depletion again since Mesozoic. The Pb-Pb isochron yielded information on a mantle event of about 3.6 Ga. It is indicated from geochemical and geologic evidence that the volcanism in this area is associated with the junction between the tectonic lines of Sihui-Wuchuan and Jinsa-Red River, which serves as a channelway for magmas from the deep mantle.