大别山高级变质岩云母Rb—Sr等时线年龄测定及其地质意义

应用云母Rb Sr等时线法对大别山高级变质岩的后期改造历史进行了初步的探讨。采用重液变温技术将云母按比重的区别 ,分成若干组分 ,然后用常规的Rb Sr方法测定。应用这种技术对大别山黄土岭太古代麻粒岩相岩石中的黑云母和产于超高压变质带内的石马含石榴石片麻岩的白云母进行了Rb Sr等时线年龄测定。黄土岭太古代麻粒岩相片麻岩中的黑云母 全岩Rb Sr等时线年龄为 12 7± 9Ma ,与该区片麻岩中的角闪石的K Ar和Ar Ar年龄 ,及燕山期花岗岩的Rb Sr年龄一致 ,说明这一太古代下地壳岩石受到了燕山期深熔 岩浆事件的影响 ;大别山东南部石马含石榴石片麻岩 (榴辉岩相 )中的白云母Rb Sr等时线年龄为 191± 10Ma ,反映了印支 早燕山期区域性低级变质事件的时代。研究表明 ,大别山区的不同构造单元中产出的不同类型的高级变质岩自中生代以来有不同的后期演化历史。

Rb-Sr Mica Dating of High-grade Metamorphic Rocks from the Dabie Mountains and Its Geological Significance

A preliminary study on late transformation events of high-grade metamorphic rocks was conducted by means of Rb-Sr da- ting of mica fractions. With heavy liquid separation technology of changing temperatures, the mica concentrate was divided into several fractions with respect to their gravity differences. The fractions were measured by conventional Rb-Sr method. Two samples from the Dabie Mountains were dated by this technique. Biotite samples from the Archean Huangtuling hyperthenegarnetbiotite gneiss were dated at 127±8Ma, which reveals an Yanshanian anatectic-magmatic event superimposed upon the granulite facies rock. The ages are in good agreement with the K-Ar and Ar-Ar ages of amphiboles from the gneisses as well as with the Rb-Sr ages of Yanshanian granites in the area. 8 muscovite fractions from the Shima eclogitefacies garnetbearing gneiss in the southeastern Dabie Mountains define an isochron age of 191±10 Ma. It reflects a low-grade (green schist facies) metamorphic event in the area. The present study demonstrates that there have existed different late-stage geological evolutions of high-grade metamorphic rocks in different tectonic units of the eastern Dabie Mountains since Mesozoic.