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

作者：Hu Daqian^*; Zhan Naichen; Zhang Jingtong; Li Yang; Sun Guosheng

来源：Acta Geologica Sinica, 2018, 92(3): 1053-1062.

DOI：10.1111/1755-6724.13591

• illite
• chlorite
• very low-grade metamorphism
• geothermobarometer
• Upper Permian
• eastern Changchun
• Jilin
• China

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 2M(1) and 1M(d) polytypes, indicating a metamorphic temperature of >200 degrees 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 degrees C-204 degrees C. The b dimension of illite varies from 8.992 angstrom to 9.005 angstrom. 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.

• 出版日期2018-6
• 单位吉林大学