EVIDENCE OF FLUID-MEDIATED METASOMATISM IN RECRYSTALLIZED ZIRCON DURING ULTRAHIGH-PRESSURE CONTINENTAL SUBDUCTION

Field, petrographic, compositional and isotopic evidence indicate that externally-derived, high d¹⁸O fluids supplied unsupported radiogenic argon (⁴⁰Ar_E) to chemically reactive gneiss-eclogite boundaries during UHP metamorphism within the Luliang Shan locality of the North Qaidam terrane in NW China. The well-exposed Luliang Shan rocks feature abundant eclogite blocks surrounded by thick garnet- and mica-rich selvages formed at the interface with granitic orthogneiss. Volatile enrichment of the selvage at eclogite-gneiss contacts correlates with significant ⁴⁰Ar_E concentrations in phengite, enrichment in δ¹⁸O, and presence of small (10-15 μm), anhedral, unzoned zircons. Gneiss beyond the selvage contains no ⁴⁰Ar_E and larger, oscillatory zoned, subhedral zircon. Muscovite present within discrete retrograde shear zones that cut the selvage has low ⁴⁰Ar_E concentrations and yields ⁴⁰Ar/³⁹Ar ages typical of regional cooling patterns (ca. 470 Ma). Previous thermobarometry and mica-quartz oxygen isotope thermometry results are best explained by influx of externally derived, high- δ¹⁸O and high-⁴⁰Ar_E fluid into the gneiss at or near peak pressure conditions. Petrochrology and oxygen isotope analysis of the two zircon populations supports this interpretation. The small, anhedral zircons are enriched in δ¹⁸O (+9.6‰), lack a Eu anomaly and yield U/Pb ages of ca. 550 Ma to 500 Ma. The other population with distinct zoning, subhedral crystal shapes contain a Eu anomaly several per mil lower δ¹⁸O levels (+5.9‰) with ages >1 Ga. These are interpreted to be remnant grains preserving the protolith isotopic signatures. Our results require that large-scale transport of volatiles is possible during UHP metamorphism of continental crust. This is contrary to typical UHP metamorphism of continental crust where dry conditions and limited fluid mobility prevail. The data suggests early-stage, accretionary-style collision event before the cessation of arc-magmatism.