Artifact of protracted zircon ages after high-temperature granulite-facies metamorphism

Abstract

The duration of high-temperature (HT) granulite-facies metamorphism, as inferred from zircon geochronology, provides key constraints on the thermal state and regional tectonic setting. However, zircon U−Pb ages and rare earth element (REE) systematics can be disturbed by HT metamorphism due to diffusive resetting. This study presents decoupled zircon ages and trace element data for HT–ultra-HT (UHT) granulitized eclogites and pelitic granulites from the central Himalaya. Two cases were observed: (1) zircons with the same U−Pb ages, but different textures and geochemical signatures; and (2) zircons with similar geochemical compositions, yet different U−Pb ages. Zircon U−Pb diffusion modeling shows that (U)HT thermal overprinting can modify the pre-existing metamorphic zircon within 5 Myr at temperatures of >850 °C. In addition, the (U)HT granulite-facies overprinting can affect the REE patterns and result in the zircons having heavy REE-enriched patterns. We also quantitatively modeled the zircon geochemical variations in equilibrium with silicate melt and determined how these change with rock type. Europium anomalies in zircon associated with partial melting of pelitic granulite cannot be used as a proxy for the depth of crustal melting, whereas the zircon Eu anomalies related to eclogite melting are mainly affected by plagioclase content, which is a function of pressure. CI-chondrite-normalized Dy/Yb [(Dy/Yb)_N] ratios in zircons decrease and Lu/Hf ratios increase with garnet breakdown for both rock types. Zircon ages and geochemical features of granulitized eclogite and leucosomes constrain the transition time of eclogite-facies metamorphism to granulite-facies metamorphism occurred during 17−14 Ma. This period was previously recognized as the timing of peak pressure. Therefore, caution is required in dating UHT rocks, because the zircon ages possibly post-date the peak pressure stage. Furthermore, this study insists that the protracted zircon ages in HT–UHT granulite-facies rocks might be an artifact of disturbed zircon U–Pb and trace element systematics caused by zircon diffusive resetting.