Zircon oxygen and hafnium isotope decoupling during regional metamorphism: implications for the generation of low δ18O magmas

Abstract

Measurements of U–Th–Pb, Lu–Hf and oxygen isotopes as well as selected trace and rare earth elements were carried out on zircon grains from high-grade metasedimentary rocks from the Albany–Fraser Orogen in southwest Australia. Oxygen isotopes from detrital zircon domains yield δ18O (VSMOW) values ranging from 5.8 to 8.0‰ and exhibit coupled Hf- and O-isotope compositions. In contrast, metamorphic zircon domains show considerably less variability in O-isotope compositions with a median δ18O of 5.6 ± 0.5‰ and exhibit decoupled Hf- and O-isotope compositions. 176Hf/177Hf isotope ratios of metamorphic zircon grains are within the evolutionary trend defined by the detrital grains implying negligible input from external sources during their growth. In addition, the relatively low δ18O value of the metamorphic zircons implies crystallisation from a relatively 18O depleted crustal melt. Such δ18O values are lower than those typically observed in most metamorphic zircon, including those preserved in partial melts of metasediments (10–12‰) and suggest that the sedimentary protolith was hydrothermally altered by meteoric fluids prior to high-temperature metamorphism. We suggest that alteration of the sedimentary protolith is related to the proximity to the Fraser Shear Zone, a major crustal scale structure within the orogen, which may represent an old reactivated structure. Occurrences of low δ18O metamorphic zircon, and potentially also low δ18O igneous rocks, in ancient collisional settings may, therefore, delineate long-lived fluid pathways within the crust.