大别山榴辉岩退变质多硅白云母及地压计可用性讨论

COMPOSITION OF RETROGRESSIVE PHENGITES FROM ECLOGITES OF THE DABIE REGION: IMPLICATION FOR THE APPLICABILITY OF PHENGITE GEOBAROMETER

大别山榴辉岩中的多硅白云母可以分成三个世代 :形成于榴辉岩相进变质阶段的原生Phe1、榴辉岩相退变质晚阶段的Phe2和角闪岩相至绿片岩相退变质阶段的Phe3。电子探针测得 ,超高压下形成的Phe1 ,其Si和Fe2 + +Mg值出乎意料地低于低压下形成的Phe3 ,这与多硅白云母地压计的基本原理 ,即多硅白云母的Si和Fe2 + +Mg含量与其形成压力成正比这一原理产生了矛盾。产生这种矛盾的可能原因是 ,多硅白云母的成分除了取决于压力因素以外 ,还取决于主岩的铝 硅比值、Fe2 + +Mg含量、氧逸度的变化、流体的存在等因素。因此 ,利用多硅白云母的硅含量来探讨退变质过程压力时必须加倍小心。

Phengites in eclogites of the Dabieshan region can be divided into three generations: primary phengite formed during eclogite\|facies prograde metamorphism (Phe 1), phengite formed during the late stage of eclogite\|facies diaphthoresis (Phe 2) and phengite formed during amphibolite\| to greenschist\|facies diaphthoresis (Phe 3). Electron microprobe analyses show that the Si and Fe\+\{2+\}+Mg contents of Phe 1 which was formed under ultrahigh pressures are unexpectedly lower than those of Phe 3 which was formed under lower pressures. This fact is in consistent with the theoretical basis of phengite geobarometer that Si and Fe\+\{2+\}+Mg contents of phengite are proportional to the pressure for phengite formation. It could be the reason for such an inconsistency that the composition of phengite depends not only on pressure, but also on the Al\-2O\-3/SiO\-2 ratio and Fe\+\{2+\}+Mg content of the host rock, the variation of oxygen fugacity and the presence or absence of fluids. Therefore, much caution should be exercised when the Si content of phengite is used to estimate the pressure of a retrograde process.