Hf isotopic ratios in zircon reveal processes of anatexis and pluton construction

Abstract

Heterogenous isotopic ratios in magmatic rocks have been variously interpreted to be a result of magma mixing or inheritance from the source rock. Isotopic heterogeneities in zircons in the source may be homogenised in the melt by magma flow and chemical diffusion. This implies that the degree of homogenisation of the inherited Hf isotopic signal can be indicative of the degree of magma homogenisation, which depends on the nature and duration of magma flow before crystallisation. In order to understand the systematics of Hf isotope transfer from zircons in the source to those crystallised from the melt, we measured Hf isotopic ratios of magmatic and inherited detrital zircons in migmatites from turbidites of the Puncoviscana Formation in NW Argentina, as well as a granite domain within the sequence. Detrital zircons show ∼30 Eps units of variation in Hf isotope values. Anatectic zircons also show a large range of isotopic values but it is narrower than, and lies within, the range of the detrital zircons. Using analysis of variance (ANOVA), we found that the Hf isotope ratios of zircon rims are partly controlled by their cores and partly by the mean value of the magma. This demonstrates the effect of a boundary layer around dissolving detrital zircon grains that contains a high concentration of the core's isotopic signature (the “core effect”). Anatectic systems can be divided into four end-members on the basis of the extent of the core effect, the degree of magma homogenisation, and nature of the magma prior to zircon crystallisation. Recognising these end-members allows the nature of processes in anatectic rocks that homogenise and modify Hf isotopic ratios to be constrained.