江西足洞和关西花岗岩体的铀-铅、铷-锶体系同位素特征及其地质意义

CHARACTERISTICS OF ISOTOPIC SYSTEMATICS OF U-Pb AND Rb-Sr AND THEIR GEOLOGICAL IMPLICATIONS FOR ZUDONG AND GUANXI GRANITIC INTRUSIVES, LONGNAN COUNTY JIANGXI PROVINCE

江西龙南足洞重稀土和关西轻稀土花岗岩体锆石U-Pb不一致线下交点年龄分别为124和170Ma,分属燕山中期晚阶段和燕山早期岩浆活动产物。综合钾长石铅同位素组成和初始^(87)Sr/^(86)Sr值,可以排除两者作为同源岩浆(封闭或受混染体系)结晶分异演化的可能性。虽然两者的成岩物质均来自上地壳,但足洞岩体可能源于变质沉积岩,而关西岩体则可能由古老的中—基性火成岩派生。计算的μ_0值为8.35—8.38.源区的μ_1值和Th/U值,足洞分别为11.4和4.02,而关西分别为13.6和3.62。上述同位素新数据为南岭地区重、轻稀土花岗岩提供了对比资料。

Zircon U-Pb discordia curve give out lower intersect age of 124 Ma for Zudong (HREE) and 170 Ma for Guanxi (LREE) granitic intrusives, Longnan County, Jiangxi Province, South China (Fig.2 and 3) ,and they are magmatic products of the late stage of Middle-and Early-Yanshanian orogeny, respectively. Both intrusives have similar lead isotopic compositions with higher contents of radiogenetic lead for K-feldspars and whole-rock samples. Negative ages calculated by single-stage modle indicate that lead underwent an evolution history of more than one stage and their source materials were upper crust-de rived (Pig.4). Initial ^(87)Sr/^(88)Sr ratios which are 0.7188 for Zudong and 0.7056 for Guanxi granite show a remarkable distinction between them and exclude, by combining with uniform lead isotopic data, the possibility as the result from fractional crystallization of cogenetic magma neither in closed nor in contaminated system. Source iocks might be metasedimentary rocks for Zudong and older intermediatebasic igneous rocks for Guanxi granite. The μ_0 values range from 8.35 to 8.38 calculated by assuming that upper intersects of zircon U-Pb discordia represent the age (1843 and 1226 Ma) when source regions were forrmd. The μ_1 values show a significant difference between Zudong (11.4) and Guanxi (13.6) intrusires and the Th/U ratios are 4.02 and 3.62 respectively. These new isotopic data provide information for comparison among granites with HREE and LREE in Naling Mountain area.