Multiple crustal melting pulses and Hf systematics in zircons

Abstract

This paper focuses on the U–Pb ages and Lu–Hf isotopes in zircons in a long-lived anatectic terrane, to understand both the nature of melting events and controls on Hf-isotope ratios in the zircon record. We focus on rocks from the Wuyi-Yunkai orogeny, South China, that mainly comprises Proterozoic basement rocks that generated numerous granitoids during Early Paleozoic granulite facies metamorphism. We investigate zircons from three anatectic granites and one migmatite. Zircon U–Pb dates for individual samples vary between ~465 and ~410 Ma, covering the duration of the orogeny. U–Pb dates for two samples define three normal distributions with populations spaced by ~15–20 Myr, indicating protracted and pulsed melting events. The inherited pre-orogenic zircon cores yield a wide spread of εHf(435) values (−39.0 to −2.8), whilst all Early Paleozoic, magmatic syn-orogenic cores and rims have a narrower range shifted to higher values ranging between −14.3 and + 5.5 and no systematic variation with the date of the analytical spot. The data suggest that the magmatic syn-orogenic zircons grew from isotopically heterogeneous melts with ~9 to 16 ε units variation for individual samples. The narrower ranges compared to the inherited cores suggest that some magma homogenization must have occurred, and the upward shift of the εHf(435) values is likely due to the influence of non-zircon Hf rather than due to mixing with an external less-evolved magma. Thus, the samples record a ~55 Myr, multi-pulsed melting event, during which heterogeneous crustal magmas had their Hf isotopic composition controlled by inherited zircon dissolution modified by contributions from non-zircon phase.