摘要
前人工作认为北大别榴辉岩在榴辉岩相变质后,经历了麻粒岩相退变质作用,因此获得的Sm-Nd矿物等时线年龄代表了麻粒岩相变质时代。本文对北大别安徽洪庙百丈岩榴辉岩相岩石(辉石石榴石岩)的研究表明,该岩石经历了三叠纪超高压变质作用,经历了角闪岩相退变质。所研究样品在峰期变质之后是否经历了麻粒岩相退变质尚不能明确界定。结合矿物氧同位素体系平衡判据,辉石石榴石岩在峰期变质时达到了Sm-Nd同位素体系的均一化和平衡,并且在后期角闪岩相退变质中该同位素体系未出现明显的扰动,因此其Sm-Nd矿物等时线年龄犤(225±14)Ma和(229±13)Ma犦代表了榴辉岩相变质时代。同时,样品中矿物的同位素组成表明,在俯冲板块折返和退变质过程中,岩石受到外来和内部流体的不均匀作用,造成退变的单斜辉石在同位素组成上的不均一。
Previous studies on ultrahigh-pressure (UHP) metamorphic rocks in the northern Dabie indicated that the eclogites experienced granulite-facies retrogression after eclogite-facies metamorphism, and the obtained mineral Sm-Nd isochron ages therefore represented the time of granulite-facies retrogr ession. However, our results presented in this paper suggest that the pyroxene-garnetite occurring near Hongmiao, North Dabie terrain, underwent amphibole-f acies retrogression after the Triassic UHP metamorphism, no evidence of granulit e-facies retrogression can be observed in our samples studied. Oxygen isotopic equilibrium between pure clinopyroxene-garnet pair was achieved during the UH P metamorphism and preserved during the retrograde metamorphism. This observatio n suggests that the Sm-Nd isotope equilibrium should be also achieved during t he peak metamorphism and survived in the later amphibole-facies retrogression. The Sm-Nd isochrons give ages of (225±14) Ma and (229±13) Ma that are int erpreted as representing the time of eclogite-facies metamorphism. Different p arts of the rock experienced fluid-rock interaction to different degree during exhumation and retrogression. The different degree affections of external and internal fluid for the rock made isotopic compositions of altered clinopyroxenes dis-equilibrated.
出处
《地球化学》
CAS
CSCD
北大核心
2004年第4期343-352,共10页
Geochimica
基金
国家自然科学基金重点项目(40033010)
中国科学院知识创新工程项目(KZCX2-107)
