Formation and metamorphism of sedimentary rocks beneath the Dingxi Basin: Record of the evolution from subduction to collision in the North Qilian Orogen

The North Qilian Orogen (NQO) in northwest China underwent oceanic subduction and subsequent continental collision. Metasedimentary rocks from a deep borehole in the Dingxi Basin, NQO, contain garnet, biotite, plagioclase, quartz and minor cummingtonite and chlorite in the matrix, with inclusions of kyanite and staurolite in garnet. The mineral textures and compositions define clockwise pressure-temperature evolution with peak conditions of similar to 10.5 kbar and similar to 670 degrees C, followed by isothermal decompression down to similar to 6.5 kbar. Age and Hf isotope data of detrital zircon support the formation of the sedimentary protolith in an arc setting at similar to 460 Ma, and the age and rare earth element characteristics of metamorphic monazite reflect exhumation at similar to 425 Ma. These results indicate a complete cycle of deposition-burial-exhumation for the sedimentary rocks, and directly constrain the continental collision process in NQO to yield a geotherm of similar to 21 degrees C/km and to culminate before 425 Ma.