Zr-in-rutile thermometry of eclogite in the Dabie orogen: Constraints on rutile growth during continental subduction-zone metamorphism

The Zr content of rutile was analysed by both EMP and LA-ICPMS for low-T/UHP eclogite and enclosed kyanite–quartz veins in the Dabie orogen, China. Zr-in-rutile temperatures were calculated at different pressures and then compared with temperatures derived from Ti-in-zircon, mineral-reaction, quartz–mineral O-isotope and garnet–clinopyroxene Fe–Mg exchange thermometers. All thermometric data are interpreted within the framework of petrologically and geochronologically constrained P–T–t path. As a consequence, variable Zr-in-rutile temperatures for different occurrences of rutiles are used to indicate their growth in different stages during continental subduction-zone metamorphism. Thus, some rutiles would grow at the peak pressure, whereas other rutiles would either grow before the peak pressure during the subduction or grow at or after the peak temperature during exhumation. The mineral O-isotope and Fe–Mg exchange thermometers also yield variably lower temperatures for the eclogite. The similar results were obtained for mid-T/UHP eclogites in the Dabie-Sulu orogenic belt. This is ascribed to the characteristic P–T–t path of UHP metamorphic rocks, in which metamorphic temperatures at peak pressures are lower than the peak temperatures at decreased pressures during exhumation. Thus, there is a contest between thermodynamics and kinetics during metamorphic reactions in response to P–T changes. Therefore, the reasonable interpretation of thermometric data requires comprehensive understanding of thermometric methodology from physicochemical principles to geological applicabilities.

Research highlights
► Continental collision: deep subduction, UHP metamorphism, and “hot” exhumation.
► Applicability of petrological, trace element and isotope geothermometers to subduction-zone eclogite.
► Multistage growth of rock-forming and accessory minerals during subduction-zone metamorphism.
► The contest between thermodynamics and kinetics during metamorphic reactions in response to P–T changes.
► P–T records of the peak metamorphic conditions versus the last equilibrium state.