Isotopic Constraints on Time and Duration of Fluid-Facilitated Eclogite-Facies Recrystallization During Exhumation of Deeply Subducted Continent

Fluid availability during HP and UHP metamorphism is a critical factor in dictating petrological, geochemical and isotopic reequilibration between metamorphic minerals, with fluid-absent metamorphism commonly resulting in neither zircon growth/recrystallization for U-Pb dating nor Sm-Nd isotopic resetting for isochron dating. While peak UHP metamorphism is characterized by fluid immobility, HP eclogite-facies recrystallization during exhumation is expected to take place in the presence of fluid. A multichronological study of UHP eclogite from CCSD-MH in the Sulu orogen indicates zircon growth at 224 to 216 Ma as well as mineral Sm-Nd and Rb-Sr reequilibration at 216±5 Ma, which are uniformly younger than UHP metamorphic ages of 231±4 to 227±2 Ma as dated by the SHRIMP U-Pb method for coesite-bearing domains of zircon. Metamorphic growth of zircon is evident from the CL images, which show that most of the zircons exhibit cloudy zoning, fir-tree zoning or planar zoning; some of the grains have not zoned or sector zoned cores of different sizes (10 to 100 μm). O isotope requilibration was achieved between the Sm-Nd and Rb-Sr isochron minerals, but Hf isotopes were not homogenized between different grains of zircon. Uniformly low 176Lu/177Hf ratios of 0.000007 to 0.000022 are obtained, which are typical for metamorphically grown zircon. Initial 176Hf/177Hf ratios are calculated at t = 220 Ma to yield negative ɛHf(t) values of -24.9 to -17.4. Two quartz-garnet pairs give the consistently highest O isotope temperatures of 650±20 and 685±20°C, indicating achievement and preservation of O isotope reequilibration during the eclogite-facies recrystallization. In contrast, two quartz-rutile pairs yield the consistently lowest temperatures of 465±15 and 475±15°C, implying continuous O isotope exchange during amphibolite-facies retrogression. The eclogite-facies recrystallization is also evident from petrography. Thus this process occurred during exhumation with fluid availability from decompression dehydration by the decomposition of hydrous minerals and the exsolution of hydroxyl from nominally anhydrous minerals. This provides significant amounts of internally derived fluid for extensive retrogression within the UHP metamorphosed slabs. The consistently valid Sm-Nd and Rb-Sr mineral isochron ages suggest that durations of Sm-Nd and Rb-Sr isotope reequilibration are not only similar to each other, but also fall within the timescales of O isotope reequilibration between the different minerals during the eclogite-facies recrystallization. This indicates that the mineral Sm-Nd, Rb-Sr and O isotope systems in the eclogites are apparently in the complete reequilibration with respect to the concerned isotopes and minerals. Because the isotope systems are subject to substantially different rates of volume diffusion, our study suggests that the reequilibration of different isotope systems in the minerals proceeds over geologically sizable scales of time and length. Based on available experimental diffusion data, the consistent reequilibration of U-Pb, Sm-Nd, Rb-Sr and O isotope systems in the eclogite minerals demonstrates that timescale for the eclogite-facies recrystallization is possibly in the magnitude of about 1.9 to 9.3 Ma or less. This provides a maximum estimate for duration of the fluid-facilitated process during the exhumation of deeply subducted continent.