Decoupled whole-rock and zircon Hf isotopes in young evolved post-collisional lavas from Dayingshan (SE Tibet): Evidence for open-system magmatic processes

Abstract

Young zircons from volcanic rocks provide ideal samples for evaluating whole-rock and zircon Hf isotope equilibrium. We report zircon and whole-rock Hf isotopic compositions for Holocene post-collisional lavas (trachyandesites and trachytes) from Dayingshan volcano, SE Tibetan Plateau. Individual zircon εHf values differ from the host whole-rock value by up to 7.4ε units, and average zircon εHf values in individual samples are about 3ε units lower than the corresponding whole-rock εHf values, indicating discrepancy of zircon and whole-rock Hf isotopes. δ18O values for Dayingshan zircons vary from 6.1 to 7.8‰ and are negatively correlated with zircon εHf values. The decoupling of zircon/whole-rock Hf isotopic compositions is attributed to open-system magmatic processes rather than disequilibrium partial melting, as indicated by the negative correlation of zircon Hf-O isotopic compositions, as well as the correlations between whole-rock SiO2 content and Nd-Sr-Hf isotope ratios. The decoupling of zircon/whole-rock Hf isotopic compositions in Dayingshan lavas suggest that care must be taken when using individual zircon εHf values to study crust-mantle evolution. This Hf isotope decoupling might be more common than previously thought in continental post-collisional volcanic rocks where open-system magmatic processes are common.