Diagenesis and Metamorphism of Triassic Flysch Along Profile Zoigê‐Lushan, Northwest Sichuan, China

AbstractWe used illite and chlorite crystallinities, index minerals, mineral assemblages, polytype and domain size of white mica, electron microprobe analysis (EMPA), b0 geobarometer and chlorite geothermometer to quantify the diagenetic and metamorphic overprint on the Triassic flysch formations in the Songpan‐Garzê orogen along profile Zoigê‐Lushan, northwest Sichuan. Two anchizones, two epizones, one diagenetic zone and a transition belt in‐between them are defined on the basis of the obtained data. Illite crystallinity correlates strongly with the best mean domain size of mica and moderately with chlorite crystallinity. 2M1 white mica polytype are observed within the epizone whilst 2M1 and 1M polytypes occur in the anchizone and diagenetic zone. Epizonal metamorphism reached maximum temperatures of 370°C±21°C at low‐to intermediate pressure conditions. Clay minerals underwent Ostwald ripening during metamorphism. Rocks from both sides of the Longmenshan fault reveal contrasting degrees of metamorphic overprint: on the northwest side of the Longmenshan fault, epimetamorphic conditions contrast with diagenetic rocks on the southeast side.