Diagenesis and Very Low‐grade Metamorphism of the Upper Permian Yangjiagou Formation in Eastern Changchun, China: Evidence from Clay Mineral Geothermobarometers. (16th July 2018)

• Record Type: Journal Article
• Authors: HU, Daqian; ZHAN, Naichen; ZHANG, Jingtong; LI, Yang; SUN, Guosheng
• Is Part Of: Acta geologica sinica. Volume 92:Number 3(2018)
• Page Start: 1053
• Page End: 1062
• Publication Date: 2018-07-16
• Subjects: illite -- chlorite -- very low‐grade metamorphism -- geothermobarometer -- Upper Permian -- eastern Changchun -- Jilin -- China; Geology -- Periodicals; Geology -- China -- Periodicals; 551
• DOI: 10.1111/1755-6724.13591
• Languages: English
• ISSNs: 1000-9515

Abstract

Abstract : 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.