Abstract: Isotopic compositions of carbon and oxygen in some carbonate-mylonites and carbonate-pseudotachylites distributed in the Shuiyu ductile shear zone and Beishicheng fault zone,Beijing area have been measured. The result shows that the δ¹⁸O and δ¹³C values in the carbonates all decreased to different degree during their ductile deformation. The data of carbon isotope geothermometer show that the temperature for formation of the carbonate-mylonite is about 260—270℃,and it was formed in a slowly cooling geological environment. The temperature for formation of the carbonate-pseudotachylite is about 420—450℃,and it was formed in a geological environment with higher temperature and rapid cooling. The formation of carbonate-pseudotachylite may be related to a rapid rupture process of the fault during earthquake. The study of stable isotope compositions of the carbonates distributed on the fault planes may be useful for demonstrating paleoseismic events.

Key words: Shuiyu ductile shear zone, Beijing, Carbonate-mylonite, Carbonate-pseudotachylite, Carbon and oxygen isotopes

摘要： 北京地区部分碳酸盐糜棱岩和碳酸盐假熔岩碳氧同位素组成的测试结果表明,碳酸盐岩在发生塑性变形时其δ¹³C、δ¹⁸O值均有不同程度的降低。同位素证据表明,碳酸盐糜棱岩的形成温度较低,并具有缓慢冷却的生成环境;碳酸盐假熔岩具有高温淬冷的同位素地球化学特征,它的形成可能与发震断层的迅速破裂过程有关。研究断层面附近产出的碳酸盐岩的稳定同位素组成对确认古地震事件是有意义的。

关键词: 北京水峪韧性剪切带, 碳酸盐糜棱岩, 碳酸盐假熔岩, 碳氧同位素