北京西山中生代火山岩中单斜辉石矿物化学及成因意义

Mineral Chemistry and Genetic Significance of Clinopyroxenes from the Mesozoic Volcanic Rocks in Western Hills of Beijing

本文对北京西山中生代火山岩中单斜辉石的矿物化学特点进行了研究 ,确定了矿物种属 ,计算了晶胞体积、密度和平均折射率。对单斜辉石 -熔体平衡进行了判别 ,计算了平衡温度、压力和氧逸度 ,进而估算了岩浆房的深度。依据单斜辉石的成分和演化 ,结合岩石化学和地球化学特点 ,对火山岩的系列以及岩浆形成机制进行了讨论 ,探讨了岩浆活动的构造背景。

Outcrops of Mesozoic volcanic rocks are quite widespread in Western Hills of Beijing. Among them, basic_intermediate volcanic rocks have phenocrysts and microcrysts of clinopyroxens. Based on a study of mineral chemistry of clinopyroxene, the author has found that clinopyroxenes are mostly augites and subordinately diopsides. The unit cell volumes, densities and refractive indices of clinopyroxenes are calculated. The results show that augites have smaller unit cell volumes than diopsides. Equilibrium temperatures and pressures between clinopyroxene and melt, oxygen fugacity of magma and depth of magma chambers are computed. The depth of magma chambers of different volcanic rocks are as follows: basaltic magma of the first member of Nandaling Formation is 32km, magma of basaltic andesite and that of shoshonite_latite of the third member of Tiaojishan Formation are 24 km and 12～17 km respectively. The equilibrium temperatures between clinopyroxene and melt are as follows: basltic magma is 1229℃, magma of basaltic andesite is 1186℃, magma of shoshonite_latite is 1165～1181℃, and magma of latite is 1048～1128℃. From Early Jurassic to Early Cretaceous, magmatic activity shows the following common characteristics: chemical composition of magma varies from basic to acid, the depth of magma chambers tends to become shallow, the equilibrium temperatures between clinopyroxene and melt change from high to low. The viewpoints are advances on the basis of analyzing mineral chemistry, rock chemistry and trace elements. The volcanic rocks studied in this paper belong to the high_potash calc-alkaline series and shoshonite series, the magmatism is controlled chiefly by partial melting, the tectonic environment of magmatic activity is mainly the inside of continental plate and subordinately island arc.