The impact of zircon–garnet REE distribution data on the interpretation of zircon U–Pb ages in complex high-grade terrains: An example from the Rauer Islands, East Antarctica

Abstract

The responsiveness of zircon to granulite metamorphism and post-peak fluid infiltration in polymetamorphic terranes has been investigated utilising a case study from the Rauer Islands of East Antarctica. Imaging (BSE, CL), textural analysis, in-situ ion microprobe trace element analysis of zircon, garnet, orthopyroxene, plagioclase and apatite, and ion microprobe oxygen isotope analysis of zircon, have been integrated to establish the event significance of c. 510 Ma zircon age domains that occur within and on c. 2840 Ma protolith magmatic zircons. Initial magmatic zircon is characterised by moderate Th/U (0.4–0.6) and elevated, steep HREE (Yb N/Gd N = 20). C. 510 Ma zircon domains feature lobate, cuspate and planar BSE and CL zones that have lower Th/U (to 0.05) and either decreased MREE, or decreased MREE and HREE. Metamorphic garnet, initially characterised by flat HREE, shows extensive replacement by orthopyroxene and plagioclase, accompanied by near-rim increases in HREE to values that are consistent with equilibrium between garnet rims and product orthopyroxene. Zircon/garnet MREE–HREE distribution coefficients, D REE, calculated using compositions of modified zircon domains coupled with both initial metamorphic garnet cores and rims are far removed from presently accepted equilibrium values, with the zircon MREE being strongly depleted relative to garnet. These D REE indicate that the observed modification of zircon did not occur in response to the major high-grade metamorphic events recorded in the studied rock mineralogy, but instead occurred after the high-grade metamorphism. The zircon was ‘blind’ to these metamorphic events, but instead was responsive to a later post-peak fluid infiltration episode that shifted zircon δ 18O values downwards by some 3‰. The likelihood of ‘blind’ or unresponsive behaviour of zircon to high-grade metamorphism must be considered when reconstructing crustal histories and terrane scenarios based on U–Pb age data, and tested for using an integrated approach that includes in-situ REE and other trace element analysis of zircon and its potentially coexisting minerals.