Metamorphism records microcontinent accretion and subduction relocation: an example from early Paleozoic Qilian Orogenic Belt, NW China

Microcontinents are isolated fragments of continental crust surrounded by oceanic lithosphere. They commonly occur in modern ocean and are also recognized in orogenic system. They can be accreted onto continental margins through collision and subduction during ocean-continent subduction process, and lead to migration of subduction zone toward the oceanic side. However, it is not well understood whether and how this process can be recorded by metamorphism. In this study, a high grade metamorphic-magmatic terrane is recognized along the previously defined Qilian block. The Datong-Mengyuan terrane (DMT) is separated from the low-medium grade metamorphic basement of the Qilian block (QLB) by dextral strike-slip ductile shear zone and ophiolite mélange. The petrology and texturally-controlled U-Pb multi-mineral geochronology reveal that the mafic and felsic granulites from the DMT record two significance events of metamorphism. The earlier event experienced a pressure and temperature conditions of 11.4–13.7 kbar and 735–805°C at ca. 500 Ma, and later stage records a pressure and temperature conditions of 5.5–9.6 kbar and 790–840°C at ca. 460 Ma. We suggest that the earlier Cambrian high pressure granulite facies metamorphism is resulted from collision and thickening related to the accretion of the DMT to the Qilian block, and the later low-medium pressure granulite facies overprinting formed by decompression heating, which happened in continental arc setting and is associated with shift of subduction zone toward the ocean. These findings provide a critical example of metamorphic record on the microcontinent accretion and convergent plate boundary dynamics.