OXYGEN ISOTOPE EVIDENCE FOR SUBDUCTION ZONE FLUID METASOMATISM DURING UHP METAMORPHISM

Thick, garnet- and mica-rich metasomatic selvages developed between ubiquitous mafic eclogite blocks included within Si-rich gneiss are a characteristic feature within the Luliang Shan area of the North Qaidam ultrahigh-pressure (UHP) metamorphic terrane in northwestern China. Zircon δ¹⁸O data suggest that the gneiss protolith had δ¹⁸O values of ca.7.5‰. Quartz, white mica and garnet δ¹⁸O data from the gneiss and selvage, yield much higher values (up to 15‰). In constrast, garnet δ¹⁸O values from the eclogite core are 5‰ lower. This enrichment in the selvage can be explained either by substantial devolatilization and mobility of locally derived fluids within the host gneiss or regional infiltration of a high δ¹⁸O (ca. 13‰) fluid at near peak UHP temperatures. In the limiting case of an inherited heavy δ¹⁸O signature in the gneiss substantial fluid is required to explain the selvage values. Given the high-density of eclogite throughout the Luliang Shan, fluid would have to have been mobile throughout the entire >100 km² exposure to explain the extensive selvage development. Zircon δ¹⁸O results from the gneiss can be interpreted in terms of an igneous origin for the gneiss however. If true, the infiltrating high δ¹⁸O fluids were most likely far-sourced with the isotopic signature ultimately traceable to devolatilization of subducted, low-grade metasedimentary rocks. High δ¹⁸O isotope signatures broadly correlate with the concentration of unsupported radiogenic argon (⁴⁰Ar_E) in the selvage. Significant quantities of ⁴⁰Ar_E were incorporated into phengite in both the relatively impermeable and low-porosity mafic eclogite and the higher permeability schistose selvage. Phengite K-Ar ages are up to 150% higher and 180% higher than the 488 Ma zircon U-Pb age for UHP metamorphism in the mafic eclogite and selvage respectively. Alternatively, overprinting, retrograde muscovite produced geologically reasonable cooling ages indicating insignificant incorporation of ⁴⁰Ar_E subsequent to UHP metamorphism. In order to account for the combination of high fluid availability and high ⁴⁰Ar_E concentrations in phengite, we postulate that the phengite/fluid partition coefficient for argon must increase with pressure to values approaching 10^-2 under UHP conditions.