Abstract:
The timing of ultra-high pressure (UHP) metamorphism has been difficult to determine because of a lack of age constraints on crucial events, especially those occurring on the prograde path. New Sensitive High-Resolution Ion Microprobe (SHRIMP) U-Pb age and rare-earth element (REE) data of zircon are presented for UHP metamorphic rocks (eclogite, garnet peridotite, garnet pyroxenite, jadeite quartzite and garnet gneiss) along the Dabie-Sulu UHP complex of China. With multiphase metamorphic textures and index mineral inclusions within zircon, the Dabie data define three episodes of eclogite-facies metamorphism, best estimated at 242.1 ± 0.4 Ma, 227.2 ± 0.8 Ma and 219.8 ± 0.8 Ma. Eclogite-facies zircons of the Sulu UHP complex grew during two major episodes at 242.7 ± 1.2 and 227.5 ± 1.3 Ma, which are indistinguishable from corresponding events in the Dabie UHP complex. A pre-eclogite metamorphic phase at 244.0 ± 2.6 Ma was obtained from two Sulu zircon samples which contain low pressure-temperature (plagioclase, stable below the quartz/Ab transformation) and hydrous (e.g., amphibole, stable below ~ 2.5 Gpa) mineral inclusions. In terms of Fe-Mg exchange of trapped garnet-clinopyroxene pairs within zircon domains, we are able to determine the Pressure-Temperature (PT) conditions for a specific episode of metamorphic zircon growth. We suggest that mineral phase transformations and associated dehydration led to episodic eclogite-facies zircon growth during UHP metamorphism (~ 2.7 Gpa) began at 242.2 ± 0.4 Ma (n = 74, pooling the Dabie-Sulu data), followed by peak UHP metamorphism (> ~ 4 Gpa) at 227.3 ± 0.7 Ma (n = 72), before exhumation (< ~ 220 Ma) to quartz stability (~ 1.8 Gpa). The Dabie-Sulu UHP metamorphism lasted for about 15 Ma, equivalent to a minimum subduction rate of 6 mm/year for the descending continental crust. © 2006 Elsevier B.V. All rights reserved.
