Zoned Zircon from Eclogite Lenses in Marbles from the Dabie‐Sulu UHP Terrane, China: A Clear Record of Ultra‐deep Subduction and Fast Exhumation

Abstract: Eclogite lenses in marbles from the Dabie‐Sulu ultrahigh‐pressure (UHP) terrane are deeply subducted meta‐sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited (detrital) and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence (CL) imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones (domains) to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark‐luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high‐pressure (HP) metamorphic conditions of T = 571–668°C and P = 1.7‐2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white‐luminescent images. It contains coesite eclogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782–849°C and P > 5.5 GPa. Domain 3, with gray‐luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low‐pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600–710°C and P = 0.7‐1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245 ± 4 Ma for domain 1, 235 ± 3 Ma for domain 2 and 215 ± 6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244 ± 4 Ma, 233 ± 4 Ma and 214 ± 5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10–11 Ma and exhumation of the rocks occurred over a period of 19–20 Ma. Thus, subduction from ∼ 55 km to > 160 km deep mantle depth took place at rates of approximately 9.5–10.5 km/Ma and exhumation from depths >160 km to the base of the crust at −30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin Iithosphere followed by ultrafast exhumation driven by buoyancy forces after break‐off of the UHP slab deep within the mantle.