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

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210±3Ma，石榴石+金红石Sm-Nd等时线给出了较高年龄237±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695±35℃，石英—金红石对为460±15℃，与根据金红石U—Pb内部等时线估计的Pb扩散封闭温度470±50℃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，且均比Nd快。

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

Sm—Nd isochron dating and O isotope an alysis 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 isochorn age of 210±3 M a．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 + futile with an age of 237±4 M a 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．A1l these data suggest that the rate of 0 diffusion in garnet is slower than，or close to that of Nd diffusion in garnet，wheress the rates of 0 and Pb diffusion in rutile are faster than those of Nd diffusion in rutile．0 isotope an alysis on different grains of rutile gave consistent results within an alytical uncertainty；the O isotope temperature defined by the quartz-rutile pair is in agrement with a closure temperature of 470±50℃ for Pb diffusion in futile as estimated from the futile U—Pb isochron dating ． These indicate that O an d Pb isotopic homogenization was achieved among different grains of rutile in the scale of han d specimen during the eclogite-facies metamorphism ，and that the rates of 0 an d Pb diffusion in rutile are similar to each other during eclogite cooling ．