Diagenesis and Very Low‐grade Metamorphism of the Upper Permian Yangjiagou Formation in Eastern Changchun, China: Evidence from Clay Mineral Geothermobarometers

Abstract

The metamorphic conditions of the Upper Permian Yangjiagou Formation in eastern Changchun, China, were evaluated based on the mineral assemblage, illite crystallinity, illite polytypism, the b dimension of illite, and the chemical composition of chlorite. The pelitic rocks in the Yangjiagou Formation are characterized by illite + kaolinite + chlorite ± mixed‐layer chlorite/smectite and detrital quartz + plagioclase. Illite in the formation has a crystallinity of 0.38–0.55 and comprises mixed 2M1 and 1Md polytypes, indicating a metamorphic temperature of >200°C. Based on the chemical composition of chlorite and the chlorite geothermometer, we estimated diagenetic to very low‐grade metamorphic conditions with temperatures of 185°C–204°C. The b dimension of illite varies from 8.992Å to 9.005Å. We used a mathematical algorithm to extend Guidotti and Sassi's (1986) diagram relating illite b dimension with temperature and pressure, and used this diagram, together with illite crystallinity and chlorite chemical composition, to semi‐quantitatively estimate the formation pressure at<1.2 kbar. These reveal that the Yangjiagou Formation has experienced very low‐grade metamorphism.