Thermodynamic modelling of the phengite geobarometry  

作　　者：ZHU WenPing  WEI ChunJing 

机构地区：[1]The MOE Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China

出　　处：《Science China Earth Sciences》2007年第7期1033-1039,共7页中国科学（地球科学英文版）

基　　金：Supported by the National Natural Science Foundation of China (Grant Nos. 40372032 and 40525006)

摘　　要：Using the updated models of activity-composition relations for solid solutions,P-T pseudosections and Si isopleths of phengite in the system KFMASH (K2O-FeO-MgO-Al2O3-SiO2-H2O) are calculated with the software THERMOCALC 323 for three bulk compositions of the average,Al-rich and K-rich metapelites. The Si isopleths of phengite in the calculated pseudosections are highly dependent on mineral as-semblages. In the chlorite-and biotite-bearing di-and trivariant fields,and in the garnet + kyanite-bearing trivariant fields,the phengite can be used as a good geobarometer. But in the carpho-lite-present and chlorite-absent fields,the Si contents of phengite mainly indicate variance of tem-perature. For a given Si value,the phengite in kyanite-bearing assemblages shows higher pressures than in the K-feldspar-bearing assemblages. Addition of Na2O into the KFMASH leads to reduction of stability of the AFM phases. The Si isopleths of phengite in the mineral assemblages containing only one Na-phase are similar to those in the corresponding KFMASH assemblages. But in the assemblages with two or more Na-phases,the phengite Si isopleths differ remarkably from those in the KFMASH.Using the updated models of activity-composition relations for solid solutions, P-Tpseudosections and Si isopleths of phengite in the system KFMASH (K_2O-FeO-MgO-Al_2O_3- SiO_2-H_2O) are calculated with the software THERMOCALC 323 for three bulk compositions of the average, Al-rich and K-rich metapelites. The Si isopleths of phengite in the calculated pseudosections are highly dependent on mineral assemblages. In the chlorite- and biotite-bearing di- and trivariant fields, and in the garnet + kyanite-bearing trivariant fields, the phengite can be used as a good geobarometer. But in the carpholite-present and chlorite-absent fields, the Si contents of phengite mainly indicate variance of temperature. For a given Si value, the phengite in kyanite-bearing assemblages shows higher pressures than in the K-feldspar-bearing assemblages. Addition of Na_2O into the KFMASH leads to reduction of stability of the AFM phases. The Si isopleths of phengite in the mineral assemblages containing only one Na-phase are similar to those in the corresponding KFMASH assemblages. But in the assemblages with two or more Na-phases, the phengite Si isopleths differ remarkably from those in the KFMASH.

关 键 词：P-T PSEUDOSECTION PHENGITE GEOBAROMETRY 

分 类 号：P584[天文地球—岩石学]