Protolith control on fluid availability for zircon growth during continental subduction-zone metamorphism in the Dabie orogen

Abstract

Different episodes of zircon growth are recognized by a combined study of CL images, mineral inclusions, U–Pb ages, trace elements and Lu–Hf isotopes for ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks in the Dabie orogen. The results provide insights into the effect of protolith property on fluid liberation during continental collision. Fluid availability from premetamorphic protoliths of different origins is recognized as a key to the zircon growth. Zircon U–Pb dating for UHP metabasalt and metasediment (eclogite and its host paragneiss) yields two groups of ages at 244±3 and 225±2Ma, respectively. Mineral inclusion, trace element and Lu–Hf isotope analyses also suggest that these two groups of zircon grew from hydrous melt during the final subduction and supercritical fluid during the initial exhumation, respectively. In contrast, zircon U–Pb dating for UHP metaintrusive rocks (granitic orthogneiss and its hosted eclogite) gave only one group of age at 222±2Ma. Mineral inclusion, trace element and Lu–Hf isotope analyses suggest that the metamorphic zircon grew from aqueous fluid during the initial exhumation. The difference between the two groups of zircon U–Pb dates is attributed to the difference in their protolith origin. Volcanic and sedimentary rocks contain large amounts of water primarily in the form of molecular water (pore fluid), so that considerable amounts of aqueous fluid can be released from them during subduction. This episode of fluid action was recorded by the growth of anatectic zircon at ∼244Ma. In contrast, intrusive rock only contains small amounts of water primarily in the form of structural hydroxyl in crystalline minerals, so that little fluid can be released from them during subduction. Nevertheless, large amounts of retrograde fluid were released from UHP metamorphic rocks regardless of their protolith origin during decompression exhumation. This episode of fluid action was recorded by the growth of metamorphic and anatectic zircons at 220–225Ma. Therefore, the protolith property is a key to the liberation of aqueous fluid from metamorphosing rocks during subduction, which has great bearing on partial melting, element transport and mineral growth during continental subduction-zone metamorphism.