CATHODOLUMINESCENCE GUIDED ZIRCON HF ISOTOPE DEPTH PROFILING: MOBILIZATION OF THE LU-HF SYSTEM DURING (U)HP ROCK EXHUMATION IN THE WOODLARK RIFT, PAPUA NEW GUINEA

This study uses cathodoluminescence image guided Hf isotope depth profiling by laser ablation of zircons from two quartzofeldspathic host gneisses to constrain the Lu-Hf system’s behavior during rapid (U)HP exhumation in the Woodlark Rift, Papua New Guinea. Depth profiling was first investigated in composite zircon standards, demonstrating that ~8 µm thick domains in which εHf_(present) differs by ≥4 units can be resolved. In a paragneiss, 2.89 ± 0.29 Ma zircon overgrowths on Cretaceous cores have radiogenic εHf_(present) indicating growth in a medium that was originally in equilibrium with garnet undergoing recrystallization (the ‘garnet effect’ of Zheng et al. 2005; Zircons acquired their εHf_(iniital) from metamorphic fluids with high 176Hf/177Hf but low Lu/Hf). In a separate exhumation related orthogneiss, 3.66 ± 0.13 Ma zircons lacking inheritance contain sub-domains that differ from each other by >15 εHf_(present). Some of these sub-domains are radiogenic and can be explained by the ‘garnet effect’, whereas others also contain highly elevated Lu and Yb in addition to their radiogenic Hf compositions, thus necessitating a medium derived from the complete breakdown of garnet. Zircons in this sample also contain non-radiogenic sub-domains that grew in the presence of Hf mobilized from the surrounding rocks of the subducted and metamorphosed remnants of the Australian continental margin. The results confirm that rapid exhumation of (U)HP rocks can result in the following: 1) transmission of radiogenic Hf (and sometimes Lu and the other HREE) from garnet bearing mafic lithologies into the quartzofeldspathic gneisses, and 2) mobilization and transport of unradiogenic Hf present within the quartzofeldspathic remnants of subducted continental crust. From a microanalytical perspective, results demonstrate that the Hf isotope depth profiling technique can identify and characterize zircon sub-domains that crystallized from liquids with different Hf isotope signatures. While the technique’s statistical limitations can result in imprecise data, the results demonstrate that spatially resolved data has the potential to provide geologic constraints on complexly zoned zircons that can be used to guide the interpretation of data acquired using higher precision techniques