北京密云变质杂岩矿物学和结晶的P-T条件

MINERALOGY AND P-T CONDITIONS OF CRYSTALLIZATION OF THE MIYUN METAMORPHIC COMPLEX, BEIJING

密云变质杂岩分布于北京密云县半城子以南到沙厂一带,由上壳岩和侵入岩组成。对麻粒岩、透辉斜长角闪岩中的石榴子石、斜方辉石,单斜辉石、角闪石和斜长石进行了电子探针分析;研究了镁铁矿物中的Fe、Mg、Ca和Na的分配系数,认为达到了化学平衡。根据麻粒岩中变质基性岩脉片麻理方向与麻粒岩的不同,本区可区分出两次变质作用。 本文应用4种二辉石地质温度计和5种石榴子石-单斜辉石地质温度计公式以及一些可靠的地质压力计公式,计算了变质作用的P-T条件。第一次变质作用为760—870℃和1.1—1.3GPa;第二次为740—850℃和0.8—0.84GPa。

The Miyun Metamorphic Complex, occurring around the area from Banchengzi to Shachang of Miyun County, Beijing, is composed of suprocrustal rocks and intrusive rocks.The chemical compositions of garnet, orthopyroxene, clinopyroxene, amphibole and plagioclase in granulites and pyroxene amphibolites have been determined by means of electron microprobe. Studies of the optical properties of these minerals and the partitioning coefficients of Fe, Mg, Ca and Na in ferromagnesian minerals indicate that these minerals are characteristic of the granulite facies and in equilibrium with one another. Moreover, on the basis of differences in gneissosity trends in metabasic dykes and granulites, two phases of granulite facies metamorphism have probably been identified. Four formulae for two-pyroxene geothermometers,five formulae for garnetclinopyroxene geobarometers and some available formulae for geobarometers have been used to calculate the P-T conditions of metamorphism. They are as follows: the temperature and pressure of the first phase of metamorphism are 760—870℃ and 1.1—1.3 GPa, respectively; and those of the second phase of metamorphism are 740—850℃ and 0.8—0.84 GPa, respectively.