Hafnium isotopic heterogeneity in zircons from granitic rocks: Geochemical evaluation and modeling of "zircon effect" in crustal anatexis

We carried out a geochemical evaluation and modeled the mechanism responsible for varied Hf isotopic ratios in magmatic zircons of a single granitic rock specimen. Five representative granitic samples were selected from southern China based on preliminary Hf isotopic data. Our new dataset of zircon Hf isotopes confirmed significant Hf-isotope variations (5-9 epsilon units) for each sample, and these zircons show roughly positive Th/U versus T (crystallizing temperature) correlations, while the Lu/Hf variation is independent from T. In addition, some zircons show significantly higher Ti concentrations in the rims compared to the interiors, implying reverse thermal zonation based on the Ti-in-zircon thermometry. These geochemical features in zircons suggest open-system processes which may have resulted from frequent replenishment. We modeled zircon dissolution during crustal anatexis to reveal the Hf isotopic evolution in the extracted melts. The model suggests that the extracted melts may have extremely variable Hf isotope compositions (>20 epsilon units) if the bulk Zr concentration in the source is initially above 100 ppm and the melting is rapid (>10^-4 yr^-1). The decoupled release of zircon Hf and non-zircon Hf from a single crust-derived magma source can lead to significant Hf-isotope variations in a solidified granitic body. This work provides an alternative explanation for the Hf isotopic heterogeneity in magmatic zircons of granitic rocks, which is in contrast to the common interpretation by mixing with mantle-derived magmas.