Margarite stability and compatibility relations in the system CaO-AlrOs-SiO2-H2O as a pressure-temperature indicator

Margarite is a common rock-forming dioctahedral mica, found in many low- to medium-grade metamorphic rocks. To faci l i tate petrogenetic interpretat ion of various observed assem-blages involving margarite, a complete elucidation of i ts compatibit i ty relat ions in the system CaO-AlrO.-SiOr-HrO is a necessary f irst step. Applying the methods of experimental and theoretical phase equil ibr ia, the phase relat ions of margarite in the above system have been deciphered. I t is found that the commonly observed assemblage margarite * quartz is stable to a maximum Pgr6 of 8.6 kbar, within a temperature range of 300 to 560'C. This explains why quartz * margarite is restricted in nature to low- and medium-grade crustal metamorphic rocks. At PH2o in excess of 7 to 8.6 kbar, the assemblage quartz * margarite gives way to the high-pressure assemblage zoisite * kyanite. This is in agreement with the repeatedly reported zoisite * kyanite assemblage from many greenschist to amphibolite facies eclogites. Further-more, if asr6 in the equilibrium fluid is reduced to 0.7 due to the presence of other inert gas species, zoisite * kyanite will be stabilized to as low z Ptotar as 6. I kbar. The assemblage quartz + margarite may, therefore, be regarded as an indicator of low- to moderate-pressure reqimes of rock metamorohism