High-pressure metamorphic rocks from Tongbaishan, central China: U-Pb and 40Ar/39Ar age constraints on the provenance of protoliths and timing of metamorphism

The Tongbaishan area is situated at the junction between the Paleozoic North Qinling orogen and the Mesozoic South Qinling-Dabie-Sulu orogen. The area contains an important geological section for understanding the assembly of the Sino-Korean and Yangtze cratons. Two eclogite zones were recently recognized in this area. The peak metamorphic conditions are estimated to be 530-610 °C and 17-20 kbar for the northern eclogite zone and 460-560 °C and 13-19 kbar for the southern eclogite zone. SHRIMP U-Pb analyses on zircons of eclogite, metagabbro and orthogneiss indicate that their protoliths were formed during the Neoproterozoic, and experienced high-pressure (HP) recrystallization during the Permian-Triassic period. Therefore, the Tongbaishan area belongs to the western extension of the Dabieshan HP/UHP terrane. In addition, U-Pb ages of detrital zircons from quartzite, paragneiss and schist show three age populations of 2.49 Ga, 1.93 Ga and 1.85-1.82 Ga, suggesting that extensive Paleoproterozoic orogeny or tectonothermal events took place in the northern margin of the Yangtze craton. The SHRIMP dating and complementary 40Ar/39Ar dating of muscovite from quartzofeldspathic gneiss and quartzite reveal that the crustal rocks were subducted to great depths and underwent HP eclogite facies metamorphism at ca. 255 Ma, slightly earlier than the timing of deep subduction of the UHP slice. The HP rocks were later exhumed to mid- to upper-crustal levels and suffered from retrograde recrystallization at ca. 238 Ma. The dating also discloses a still later phase of deformation and recrystallization at ca. 215 Ma. The available age data throughout the Tongbai-Dabie-Sulu orogen reflect diachronous exhumation from ca. 238 Ma for the HP slice to ca. 213 Ma for the UHP slice. Such a phenomenon could be explained by successive subduction and exhumation of the broken slices of the subducting Yangtze slab.