北京密云群变质作用的温度、压力研究

GEOTHERMOMETRIC AND BAROMETRIC STUDIES OF METAMORPHISM OF THE MIYUN GROUP NEAR BEIJING

Metamorphic temperatures and pressures of the Archean Miyun Group have been determinad by using orthopyroxene-clinopyroxene garnet-clinopyroxene, garnet-biotite and δ^18OQ^2-δ^18OMt geothermometers and by using the pure mineral components of clinopyroxanes, The results show that the temperature in the first stage of metamorphism of the Miyun Group is 820°±50℃ and the pressure is about 10kb, providing an indication of medium pressure granulite fades with a geothermal gradient of 22°--25℃/km. The corresponding burial depth is about 35 kin. The temperature prevailing during the second metamorphic stage is in the range 650°---700℃, indicating an intermediate fades between granulite and high amphibolite facies. Oxygen isotope data also show that the temperature of later superimposad regression metamorphism of high green schist fades in this region may be within the range of 470°-560℃,

Metamorphic temperatures and pressures of the Archean Miyun Group have been determined by using orthopyroxene-clinopyroxene garnet-clinopyroxene, garnet-biotite andδ18OQz -δ18OMt geothermometers and by using the pure mineral components of clinopyroxenes. The results show that the temperature in the first stage of metamorphism of the Miyun Group is 820°±50℃ and the pressure is about 10kb, providing an indication of medium pressure granulite facies with a geothermal gradient of 22°-25℃/km. The corresponding burial depth is about 35 km. The temperature prevailing during the second metamorphic stage is in the range 650°-700℃, indicating an intermediate facies between granulite and high am-phibolite facies. Oxygen isoto'pe data also show that the temperature of later superimposed regression metamorphism of high green schist facies in this region may be within the range of 470°- 560℃.