Lu-Hf and Ar-Ar Chronometry Supports Extreme Rate of Subduction Zone Metamorphism Deduced from Geospeedometry

Abstract Recent diffusion modeling of eclogitic garnets from the Great Caucasus, Russia, and Yukon, Canada, have shown that the preservation of garnet growth zoning in rocks that have equilibrated at high temperature (680–700 °C) is possible only if rates of pressure and temperature change on the burial and/or exhumation paths are in the order of several cm/year and several hundreds of °C/Ma. In order to confirm this observation, we performed Lu–Hf and Sm–Nd dating of garnet and Ar–Ar dating of mica on the same samples that were used for geospeedometry measurements in an earlier study. In both localities, garnet grew during prograde metamorphism at 690±40 °C and >1.5 GPa (Yukon) and 680±40 °C and >1.6 GPa (Great Caucasus). In contrast, phengite formed soon after the main eclogitic foliation at 520±50 °C (Yukon) and 600±40 °C (Great Caucasus). Garnet of the Yukon samples yielded Lu–Hf ages of 252±7, 255±7, 257±6 and 264±6 Ma that fall within error of phengite Ar–Ar integrated ages of 261±2 (laser spot date) and 256±3 Ma (age of mineral separates). No Sm–Nd ages were measured on the Yukon samples. For Great Caucasus samples, all Sm–Nd ages with the exception of one garnet–whole rock pair yielding a Sm–Nd age of 311±22 Ma are poorly constrained. In contrast, the Lu–Hf garnet chronometer yields ages of 322±14, 316±5 and 296±11 Ma that again fall within error of the phengite Ar–Ar mean age of 303±5 Ma. Because the geospeedometry approach provides information on cooling rates, information on the closure temperature of a given isotopic system can be extracted from the analytical solution of Dodson [Contrib. Mineral. Petrol. 40 (1973) 259] using appropriate sets of experimentally determined diffusion data. The results of these calculations indicate that uncertainties of more than 200 °C are to be expected for the Sm–Nd and Lu–Hf closure temperatures for both the Great Caucasus (750±150 °C) and Yukon samples (710±120 °C). In all cases, calculated closure temperatures are equivalent to or in the upper range of peak metamorphic temperatures. With respect to Ar, calculated closure temperatures of 570 °C for the Yukon eclogites and 560–600 °C for the Great Caucasus eclogites are within error of the temperatures of the early stage of cooling and/or exhumation. These results indicate that the eclogitic rocks experienced a minimum cooling and exhumation of about 150 °C and 25 km in a time interval smaller than the errors on the ages. The fact that garnet and phengite yield indistinguishable Lu–Hf and Ar–Ar ages is in good agreement with the observation deduced from geospeedometry that the time elapsed at eclogitic conditions should be extremely short (of the order of 1 Ma). Considering the exceptional precision of the age information obtained on eclogitic garnet using the Lu–Hf technique and that Lu–Hf, Ar–Ar and geospeedometry approaches were carried out the same samples, these results suggest that the time-scale resolution required for unraveling rates of high-pressure metamorphism remains out of reach of current thermochronological methods.

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