Zircon Hf isotopic evidence for mixing of crustal and silicic mantle-derived magmas in a zoned granite pluton, Eastern Australia

Zircon Hf isotopic data from a zoned pluton of the Moonbi supersuite, New England batholith, eastern Australia, are consistent with magma mixing between two silicic melts, each derived from isotopically distinct sources. Although zircons from three zones within the Walcha Road pluton give a U-Pb crystallization age of 249 ± 3 Ma, zircon populations from each zone have a range in ε_Hf. Zircons from the mafic hornblende-biotite monzogranite pluton margin and intermediate zones have ε_Hf ∼ +5 to +11, whereas those from the more felsic centre of the pluton have ε_Hf ∼ +7 to +16, representing a total variation of 11 ε_Hf units. The Lu-Hf depleted mantle model ages range from ∼650 to 250 Ma, with the younger zircons present only in the felsic pluton centre. The variation in ε_Hf indicates the involvement of silicic melts from at least two sources, one a crustal component with a Neoproterozoic model age and the other a primitive mantle-derived component with model ages similar to the U-Pb crystallization age of the pluton. The zircons reflect the isotopic compositions of the different proportions of crustal-derived silicic melt, relative to mantle-derived silicic melt, between melt generation and final pluton construction. The Walcha Road pluton is considered to have formed by incremental assembly of progressively more felsic melt batches resulting from mixing, replenishment and crystal-melt separation, with final pluton construction involving mechanical concentration as zones of crystal mush. The zoned pluton and, more broadly, the Moonbi supersuite provide examples of magma mixing by which the more silicic units have more juvenile isotopic compositions as a result of increasing proportions of residual melt from basalt fractionation, relative to crustal partial melt.