TWO ULTRAHIGH PRESSURE METAMORPHIC EVENTS RECOGNIZED IN THE CENTRAL OROGENIC BELT OF CHINA: EVIDENCE FROM THE U-PB DATING OF COESITE-BEARING ZIRCONS

A ~4000 km long ultrahigh pressure metamorphic (UHPM) belt has been set up based on the discovery of the coesite-bearing Altun-North Qaidam UHPM terrane in the west of the Central Orogenic Belt of China (COB), the finding of metamorphic diamond in Qinling in the middle, and the Dabie-Sulu UHPM terrane in the east. SIMS and SHRIMP U-Pb dating of zircons from coesite-bearing UHPM rocks indicate two UHPM events in the belt; one is occurred during the Early Paleozoic and the other during the Triassic. Coesite-bearing zircons from gneiss of North Qaidam yielded an UHP metamorphic age of 452±13.8 Ma and a retrograde age of 419±6.7 Ma. A diamond-bearing gneiss sample of Qinling gave a lower intercept age of 502±45Ma and an upper intercept age of 1545±100 Ma, while a Qinling eclogite sample gave a lower intercept age of 493±170Ma and an upper intercept age of 1381±82Ma. In both samples the lower and upper intercept ages are interpreted as UHPM ages and protolith ages of the rocks, respectively. Coesite-bearing zircons from the Qinglongshan eclogite in south Sulu yielded Early Paleozoic UHPM ages, 441±9Ma -449±9Ma and 442±9Ma, whereas the core of zircon with plagioclase and apatite inclusion in it gave a protolith age of 761±13Ma. The age data suggest that the Early Paleozoic UHPM rocks extend from west to east for about 4000 km across the COB and the Triassic UHPM belt extends within the Dabie-Sulu region for about 1000 km. Based on the available geochronological and geochemical data, we suggest the following tectonic model for the evolution of the COB. At about 1000 Ma the area was amalgamated to form the Rodinian continent, which contained some oceanic volcanic rocks, possibly as ophiolitic fragments. This part of Rodinia was then rifted at about 800-750 Ma to form an oceanic basin with a variety of MORB and ocean island basalts. Closure of this ocean basin produced the Neoproterozoic ophiolites and granitic gneisses. The UHPM rocks along with subduction related island arc volcanic rocks and granites of Early Paleozoic age suggest a second cycle from rifting to subduction along the COB; and the Triassic UHPM rocks recorded a final subduction and collision event between the North China and South China blocks.