Two Ultrahigh-Pressure Metamorphic Events Recognized in the Central Orogenic Belt of China: Evidence from the U-Pb Dating of Coesite-Bearing Zircons

摘要

A approx 4000 km long ultrahigh-pressure metamorphic (UHPM) belt in northern China has been documented on the basis of the discovery of coesite-bearing rocks in the Altun-North Qaidam terrane in the western Central Orogenic Belt (COB), and diamond-bearing rocks in Qinling in the central and Dabie-Sulu terrane in the east. New SIMS and SHRIMP U-Pb dates of zircons from coesile-bearing UHPM rocks indicate two UHPM events: one in the early Paleozoic and the other in the Triassic. Coesite-bearing zircons from a North Qaidam gneiss yielded UHP metamorphic ages of 452 +-13.8 Ma and retrograde ages of 419 +- 6.7 Ma. A diamond-bearing gneiss from Qinling gave a lower-intercept age of 502 +- 45 Ma, and an upper intercept age of 1545 +- 100 Ma, whereas a Qinling eclogite sample gave a lower intercept age of 493 +- 1 70 Ma and an upper intercept age of 1381 +- 82 Ma. The lower and upper intercept ages of the Qinling samples are interpreted as UHPM and protolith ages of the rocks, respectively. Coesite-bearing zircons from a QingJongshan eclogite in the south Sulu belt yielded early Paleozoic UHPM ages of 441 +- 9 Ma, 449 +- 9 Ma, and 442 +- 9 Ma, whereas the core of a zircon containing plagioclase and apatite inclusions gave a protolith age of 761 +- 13 Ma. These age data suggest that the early Paleozoic UHPM rocks extend from west to east for about 4000 km across the COB, whereas the Triassic UHPM belt extends across the Dabie-Sulu region for about 1000 km. Based on available geochronological and geochemical data, we suggest the following tectonic mode] for evolution of the COB. At about 1000 Ma, the area was amalgamated to form the Rodinian continent, which contained ophiolitic fragments of oceanic affinity. This part of Rodinia was then rifted at about 800-750 Ma to form an oceanic basin with a variety of MORB and intruded by granitic plutons. Closure of this ocean basin produced Neoproterozoic ophiolites and granitic gneisses. The UHPM rocks, along with subduction-related island-arc volcanics and granites of early Paleozoic age suggest a second cycle of rifting and subduction along the COB, whereas the Triassic UHPM rocks record a final subduction and collision event between the North China and South China blocks.