Early Palaeozoic high-pressure granulites from the Dunhuang block, northeastern Tarim Craton: constraints on continental collision in the southern Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary collages in the world, and records a prolonged sequence of subduction-accretion and collision processes. The Tarim Craton is located at the southernmost margin of the CAOB. In this study, the discovery of early Palaeozoic high-pressure (HP) granulites from the Dunhuang block in the northeastern Tarim Craton is reported, and these rocks are characterized through detailed petrological and geochronological studies. The peak mineral assemblage of the HP mafic granulite is garnet + clinopyroxene + plagioclase + quartz + rutile, which is overprinted by amphibolite facies retrograde metamorphic assemblages. The calculated PT conditions of the peak metamorphism are similar to 1.41.7 GPa and similar to 800 degrees C. The retrograde PT conditions are similar to 0.7 GPa and similar to 700 degrees C. The metamorphic zircon grains from the HP mafic granulite show homogeneous CL-images, low Th/U ratios and flat HREE patterns and yield a weighted mean 206Pb/238U age of 444 +/- 5 Ma. The metamorphic zircon grains from the associated kyanite-bearing garnet gneiss and garnet-mica schist show a similar 206Pb/238U age of 429 +/- 3 and 435 +/- 4 Ma, respectively. The c. 440430 Ma age is interpreted to mark the timing of HP granulite facies metamorphism in the Dunhuang block. The results from this study suggest that the Dunhuang block experienced continental subduction prior to the early Palaeozoic collisional orogeny between the northeastern Tarim Craton and the southern CAOB, and the Dunhuang area could be considered as the southward extension of the CAOB. It is suggested that the continental collision in the eastern part involving the Dunhuang block of the southern CAOB may have occurred c. 120 Ma earlier than in the western part involving the Tianshan orogen.