CONSTRAINING THE EXTENT OF METASOMATISM WITH OXYGEN ISOTOPE GEOCHEMISTRY OF ZIRCONS FROM UHP ORTHOGNEISS, NORTH QAIDAM, CHINA

The Luliang Shan locality of North Qaidam UHP terrane, China, contains eclogite-bearing leucogneiss. Previous work on this unit identified a hydrated selvage in the gneiss surrounding the eclogite blocks. Whole rock geochemistry, loss on ignition data and petrologic analysis have interpreted the selvage to be metasomatic. In order to prove metasomatic origins, Menold et al., in prep collected samples along a traverse between eclogite and the gneiss. Oxygen isotope analyses of metamorphic quartz and mica in the gneiss and were found to be heavier than typical granite levels (~12-16‰). In order to determine if externally-derived fluids caused the heavy isotope signature, it became important to determine the original igneous oxygen composition. Because oxygen diffusivity is generally sluggish in zircon at 600⁰C metamorphic conditions, zircon should retain the igneous oxygen isotopic composition. Zircon was extracted from four samples in the gneiss, both in and outside of the selvage zone. Cathodeluminescence imaging of sectioned and polished zircon revealed oscillatory zoning within grain interiors mantled by homogenous overgrowths. The UCLA ims1270 ion microprobe was used to measure O isotopes in the zircons. The δ¹⁸Oof the igneous zircon from the granite gneiss several meters from the selvage yielded 5-7‰ oxygen isotope values. These results are consistent with a whole rock composition of ~6-8 ‰ for the granitic protolith of gneiss and indicate that it was significantly enriched in ¹⁸O during UHP metamorphism. We hypothesize that an externally derived, high δ¹⁸O fluid infiltrated the gneiss to produce the hydrous selvages mantling the eclogite blocks.