Composition of Phengites in Eclogites and Their Retrogressive Derivatives of the Dabieshan Region: Implication for the Applicability of Phengite Geobarometre

摘要

Phengites in ecologites of the Dabieshan region can be divided into three generations: primary phengite formed during ecologite-facies prograde metamorphism (Phe1), phengite formed during the late stage of eclogite-facies retrograde metamorphism (Phe2) and phengite formed during amphibolite- to greenschist-facies diaphthoresis (Phe3). Electron microprobe analyses show that the Si and Fe+{2+}+Mg contents of Phe1 are unexpectedly lower than those of Phe3. Therefore, much caution should be exercised while using the Si content of phengite to estimate the pressure of a retrograde process.