The garnet effect on hafnium isotope compositions of granitoids during crustal anatexis

Abstract

AbstractRadiogenic Hf isotope disequilibrium during crustal anatexis complicates petrogenetic studies that link sources to sinks in granitoid systems and hinders the applications of Hf isotopes to evaluating long-term crustal growth and evolution. Garnet can be a dominant host of radiogenic Hf in crustal rocks, and its behavior in granitoid sources may play a crucial role in isotopic decoupling between residue and melt. We document covariation between (Gd/Lu)N ratios and εHf(t) in post-collisional granitoids from the Dabie orogen (central China). This covariation reflects different garnet modal contents in the residue during anatexis. Quantitative modeling further confirms the dominant role of mixing between melts derived from garnet-rich and garnet-poor residua in producing the observed covariation patterns, but results are inconsistent with the entrainment of garnet rich in radiogenic Hf in the melt. Our results demonstrate that the garnet effect on Hf isotope ratios during crustal anatexis is a crucial factor in elucidating the granitoid source and complicates interpretations of crustal growth from the global zircon archive.