Abstract: In the 1∶50000 regional geological survey, we discovered retrograded eclogites, occurring as tectonic lenses in the metamorphic basement of the Dayanglong area in the western part of the northern margin of the central Qilian block. Petrographic and mineralogic examinations revealed that garnets in eclogites preserved prograde metamorphic zonation, exhibiting annulus characteristics in both their chemical composition and mineral inclusions, and that Dayanglong retrograded eclogites belong to C-type eclogites. The prograde metamorphic assemblages (M1) are mineral inclusions (such as biotite, amphibole and plagioclase) preserved in the garnet core. The p-T metamorphic conditions were estimated to be 568580 ℃ and 0.800.82 GPa, with peak metamorphism (M2) taking place at 669±5 ℃ and 2.1±0.2 GPa, consistent with high-pressure (HP) metamorphism. The M3 stage is characterized by the amphibole+plagioclase+biotite corona around garnet porphyroblasts, indicating an isothermal decompression process with estimated p-T metamorphic conditions of 681705 ℃ and 0.680.71 GPa. The p-T conditions for the subsequent retrogression under amphibolite-facies environment were estimated by coarse-grained matrix amphiboles to be 500545 ℃ and 0.380.43 GPa in the M4 stage. The estimated metamorphic conditions revealed a clockwise p-T path, suggesting that the subducted slab has undergone a process of rapid subduction and exhumation. Most of the cathodoluminescence (CL) images of zircons from retrograded eclogites showed low luminous intensity with no zoning, weak zoning or spongy zoning with 5 μm wide strong CL belt at edges, indicating that most zircons are metamorphic or inherited zircons. Results from LA-ICP-MS zircon U-Pb dating showed that the 485±22 Ma zircon age may represent the top of the HP metamorphic time. According to geochemical analysis and tectonic discrimination, the protolith of retrograded eclogites is MORB, which likely came from the North Qilian oceanic crust. The above results, combined with arc magmatic rocks in the northern margin of the central Qilian block, confirm that the North Qilian oceanic plate had experienced south-north bi-dive subduction during the Late CambrianMiddle Ordovician.

Key words: retrograded eclogite, high-pressure metamorphism, p-T path, central Qilian block, North Qilian orogenic belt