Zircon growth in (U)HP quartzo-feldspathic host gneisses exhumed in the Woodlark Rift of Papua New Guinea

Abstract

To understand zircon behavior as a function of bulk composition and metamorphic grade in the world's youngest (U)HP terrane, we report U-Pb SIMS spot-mode and depth-profiling analyses for quartzo-feldspathic gneisses. Zircons from two gneisses contain Cretaceous inherited cores, with μm sized metamorphic rims requiring depth profiling for reliable dating. Linear regression of the common-Pb uncorrected data for rims yield 206Pb/238U ages of 2.89 ± 0.29 Ma and 2.77 ± 0.99 Ma (concordia intercept ages at 95% confidence). The older age is within two-sigma error of previously reported 206Pb/238U ages on zircons from mafic eclogite within the gneiss, indicating that rims formed on inherited cores within host gneisses during eclogite facies metamorphism. At the (U)HP locality zircons from host gneiss lack inheritance and yield a 206Pb/238U age of 3.66 ± 0.13 Ma. These results are younger than previously reported 206Pb/238U ages on zircons from coesite eclogite within the gneiss, but are within error of the youngest reported LA-ICP-MS 206Pb/238U zircon ages on retrogressed mafic eclogite. We also report intragrain geochemical heterogeneity, indicated by zircon Hf, Y, and Ti variations in depth profiles which suggest chemical disequilibrium over the interval of zircon growth. Collectively, these results indicate that zircon recrystallization and new growth of zircon rims on relict grains occurred during eclogite facies metamorphism and during subsequent retrogression, but not at (U)HP conditions. Comparison between results from PNG and other (U)HP terranes bolsters previous suggestions that the PNG (U)HP terrane evolved rapidly.