Discovery of coesite in the North Qaidam Early Palaeozoic ultrahigh pressure (UHP) metamorphic belt, NW China

Abstract

Coesite and graphite were discovered as inclusions in zircon separates from pelitic gneiss associated with a large eclogite body in the North Qaidam UHP terrane. This finding suggests UHP metamorphism at pressures below the diamond stability field. This supports previous indirect UHP evidences, such as polycrystalline quartz inclusions in eclogitic garnet, quartz lamellae in omphacite and P–T estimates for both eclogite and garnet peridotite. The U/Pb and Sm/Nd isotopic ages from the North Qaidam eclogite indicated that continental subduction occurred in Early Palaeozoic, most probably in relation with the collision between the Sino-Korean and Yangtze plates.