The Distribution, P-T-t Evolution and Formation Mechanism of HP-UHP Metamorphic Rocks in the North Qinling Orogenic Belt

The Early Paleozoic tectonic framework and evolution history of the North Qinling orogenic belt (NQOB) is in great dispute and has drawn wide concern. In this contribution,we systematically summarized our recent progresses on Early Paleozoic HP-UHP metamorphism in the NQOB and constrained the Early Paleozoic tectonic evolution of the NQOB from the perspectives of metamorphism. The discovery of coesite in the Danfeng amphibolite provides a conclusive evidence for the UHP metamorphism in the NQOB. The amphibolites,which are widely distributed in Qinling Complex in East Qinling,commonly experienced HP-UHP metamorphism and are products of continental (deep) subduction. HP-UHP eclogites and host gneisses have clockwise P-T-t paths and similar peak metamorphic ages of 500-490 Ma,and experienced two stages retrogression and anataxis at 470-450 Ma and 420-400 Ma,respectively. The times of the two stage retrogression and anataxis are coincident with the magmatism in the NQOB,which occurred at 460-450 Ma and~420 Ma,suggesting that the Early Paleozoic magmatism in the NQOB were formed under the tectonic setting of breaking-off of deeply subducted continental slab and post-orogenic extension-thinning,respectively. The HP-UHP rocks have protolith ages of ca. 800 Ma,and geochemical characteristics similar to Mid-Late Neoproterozoic magmatic rocks from South Qinling,indicating that continental materials from South Qinling were dragged down to mantle depth and experienced UHP metamorphism by the north dipping Shangdan oceanic lithosphere,and the Shangdan Ocean was already closed at 500 Ma. The Qinling Group is a tectonic complex rather than a uniform stratigraphic unit or a microcontinent and only part of it underwent continental deep subduction. The Early Paleozoic orogenisis in North Qinling was ended and the HP-UHP rocks were exhumated up to the surface in Mid-Devonian,and became the main provenance of sedimentary rocks in Liuling group in South Qinling. The Liuling basin is a post-orogenic extensional basin,rather than a fore-arc basin.