大别山金河桥榴辉岩矿物O-Nd-Pb同位素体系及其对扩散速率的制约  

O-Nd-Pb Isotopic Systems in Eclogite Minerals at Jinheqiao in Dabieshan and Constraints on Their Relative Diffusivity

作　　者：李秋立[1] 李曙光[1] 郑永飞[1] 龚冰[1] 洪吉安[1] 

机构地区：[1]中国科学技术大学地球和空间科学学院,合肥230026

出　　处：《高校地质学报》2003年第2期218-226,共9页Geological Journal of China Universities

基　　金：国家自然科学基金 (40 0 330 1 0 );国家重点基础研究发展规划项目 (G1 9990 7550 3)资助成果

摘　　要：对大别山太湖金河桥超高压榴辉岩作了矿物Sm Nd内部等时线定年研究和激光氧同位素分析。石榴石 +绿辉石Sm Nd等时线给出了较低年龄 2 10± 3Ma ,石榴石 +金红石Sm Nd等时线给出了较高年龄 2 37± 4Ma。岩相学观察发现 ,绿辉石具有角闪石退变质边。氧同位素分析表明 ,石榴石与绿辉石之间的氧同位素体系处于不平衡状态。据此 ,石榴石 +绿辉石Sm Nd同位素体系因退变质作用导致Nd同位素不平衡而给出不合理偏低年龄。较老的石榴石 +金红石Sm Nd年龄有可能指示了榴辉岩相前期阶段的时代 ,且在温度变质峰期没有使它们之间的Nd同位素再次均一化 ,它指示Nd在金红石中的扩散速率较慢 ,可能与石榴石相当。矿物对氧同位素测温得到 ,石英 -石榴石对温度为 6 95± 35℃ ,石英 -金红石对为 4 6 0± 15℃ ,与根据金红石U Pb内部等时线估计的Pb扩散封闭温度 4 70± 5 0℃一致。对比表明 ,O在石榴石中的扩散速率与Nd相当或略低 ,而O和Pb在金红石中的扩散速率相近 。Sm Nd isochron dating and O isotope analysis were carried out for minerals from eclogite at Jinheqiao in Dabieshan. Garnet+omphacite Sm Nd line gives an age of 210±3 Ma, and garnet+rutile, 237±4 Ma. Amphibole occurs as corona around omphacite, indicating significant retrograde reaction during exhumation. Correspondingly, O and Sm Nd isotopic disequilibria were observed between omphacite and garnet, resulting in an unreasonably low Sm Nd isochron age of 210±3 Ma. Mineral pair O isotope thermometers yielded a temperature of 695±35 ℃ for a quartz garnet pair and 460±15 ℃ for a quartz rutile pair. Sm Nd isochron defined by garnet + rutile with an age of 237±4 Ma records the eclogite facies prograde metamorphic time, which is considerably higher than rutile U Pb age of 218.2±1.2 Ma from the same sample. All these data suggest that the rate of O diffusion in garnet is slower than, or close to, that of Nd diffusion in garnet, whereas the rates of O and Pb diffusion in rutile are faster than those of Nd diffusion in rutile. O isotope analysis on different grains of rutile gave consistent results within analytical uncertainty; the O isotope temperature defined by the quartz rutile pair is in agreement with a closure temperature of 470±50 ℃ for Pb diffusion in rutile as estimated from the rutile U Pb isochron dating. These indicate that O and Pb isotopic homogenization was achieved among different grains of rutile in the scale of hand specimen during the eclogite facies metamorphism, and that the rates of O and Pb diffusion in rutile are similar to each other during eclogite cooling.

关 键 词：榴辉岩 金红石 O同位素 ND同位素 PB同位素 扩散 温度 

分 类 号：P597.2[天文地球—地球化学]