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SHRIMP U-Pb zircon dating from Sulu-Dabie dolomitic marble, eastern China constraints on prograde, ultrahigh-pressure and retrograde metamorphic ages [查看] F.L.LIU、A.GERDES、J.G.LIOU、H.M.XUE、F.H.LIANG

Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu-Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahighpressure(UHP) and retrograde metamorphic growths. The inherited zircon domains are darkluminescent in CL images and contain mineral inclusions of Qtz + Cal + Ap. The prograde metamorphic domains are white-luminescent in CL images and preserve a quartz eclogite facies assemblage of Qtz + Dol + Grt + Omp + Phe + Ap, formed at 542–693℃and 1.8–2.1 GPa. In contrast, the UHP metamorphic domains are grey-luminescent in CL images, retain the UHP assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP conditions of 739–866℃ and >5.5 GPa. The outermost retrograde rims have dark-luminescent CL images, and contain low-P minerals such as calcite, related to the regional amphibolite facies retrogression. Laser ablation ICP-MS trace-element data show striking difference between the inherited cores of mostly magmatic origin and zircon domains grown in response to prograde, UHP and retrograde metamorphism. SHRIMP U-Pb dating on these zoned zircon identified four discrete 206Pb/238U age groups: 1823–503 Ma is recorded in the inherited (detrital) zircon derived from various Proterozoic protoliths, the prograde domains record the quartz eclogite facies metamorphism at 254–239 Ma, the UHP growth domains occurred at 238–230 Ma, and the late amphibolite facies retrogressive overprint in the outermost rims was restricted to 218–206 Ma. Thus, Proterozoic continental materials of the Yangtze craton were subducted to 55–60 km depth during the Early Triassic and recrystallized at quartz eclogite facies conditions. Then these metamorphic rocks were further subducted to depths of 165–175 km in the Middle Triassic and experienced UHP metamorphism, and finally these UHP metamorphic rocks were exhumed to midcrustal levels (about 30 km) in the Late Triassic and overprinted by regional amphibolite facies metamorphism. The subduction and exhumation rates deduced from the SHRIMP data and metamorphic P–T conditions are 9–10 km Myr)1 and 6.4 km Myr)1, respectively, and these rapid subduction–exhumation rates may explain the obtained P–T–t path. Such a fast exhumation suggests that Sulu-Dabie UHP rocks that returned towards crustal depths were driven by buoyant forces, caused as a consequence of slab breakoff at mantle depth.