The New England Batholith of eastern Australia: Evidence of silicic magma mixing from zircon 176Hf/177Hf ratios

Zircon Hf isotopic data from six granite supersuites recognised in the New England Batholith of eastern Australia indicate that magma mixing is present in all plutons. Apart from the S-type supersuites, inherited zircons are rare to absent, suggesting that initial melt temperatures were above zircon saturation for the M-, I- and A-type granites. In all supersuites, the range of zircon Hf model ages calculated relative to arc mantle varies greatly, with younger model ages somewhat less than the Permian and Triassic age of crystallisation, and older model ages indicating derivation from source rocks that may be as old as Neoproterozoic. The older model ages are consistent with late Precambrian rocks in the lower crust that may be elements of the Lachlan Fold Belt underlying the New England Fold Belt.Whole-rock δ18O and εNd values, 87Sr/86Sr initial ratios (Sri) and oxidation state (Fe2O3/FeO) are summarised for the five named supersuites, and for a previously unnamed group of Triassic plutons situated east of the main Batholith for which we propose the name Carrai supersuite. Supersuites with higher δ18O, higher Sri, lower δ7Li and lower Fe2O3/FeO ratios indicate a metasedimentary component input. The pattern of zircon Hf isotopic variation reflects the other isotopic and geochemical indicators closely, the additional value being that it preserves a magmatic crystallisation record of Hf isotopic variation, that is, a measure of magma mixing from magma generation to final crystallisation. Using the above criteria, at least three distinct crustal components are considered necessary to explain the compositional and isotopic diversity within the Batholith. The crustal components indicated are: 1) a metasedimentary component of Carboniferous and Devonian age for the Hillgrove, Bundarra and Uralla supersuites; 2) a lower crustal K-rich (and Sr-rich) I-type component for the Moonbi, Carrai and possibly Uralla supersuites that could be as old as Proterozoic; and 3) a lower crustal K-poor I-type component of uncertain age for the Clarence River supersuite. In all supersuites, there is strong evidence of an M-type component based on the presence of zircons with high + εHf values.

► Zircons from granites of the New England Batholith have a range of Hf isotopes.
► The range of Hf isotopes indicates mixing between crustal and mantle-derived magmas.
► We examine the relationships between zircon Hf isotopes and rock geochemistry.
► We argue that the crust and mantle derived magmas are silicic in composition.