Voluminous low-temperature S-type granite formation in the New England Orogen, eastern Australia during back arc evolution

Crustal melting leading to the formation of S-type granite is typically considered a characteristic of collisional regimes. However, the formation of S-type granites does not necessarily require overt crustal thickening. Instead, voluminous granite generation can occur where burial of fertile H₂O-rich material occurs in high heat flow environments. One obvious setting that has these attributes is rapidly developing back arc regimes. Settings such as these are ideal to generate large volumes of S-granite without necessarily requiring appreciable crustal thickening.

The Bundarra and Hillgrove granite intrusions in the southern New England Orogen, eastern Australia are part of an Early Permian garnet/cordierite-bearing S-type granite batholith (>1800km²) generated in a back-arc setting linked to the easterly (outboard) migration of a W-dipping subduction zone. This setting facilitated rapid and thick accumulation of compositionally immature detritus from the active arc and the inboard continent, creating a fertile system for granite genesis driven by back arc high heat flow. Deposition of the precursor sedimentary volume occurred between 360 and 300 Ma, with partial melting and batholithic-scale melting and granite accumulation occurring between ca. 298-288 Ma. Seismic reflection data suggests the currently preserved crustal column in places comprises ~30% granite, with the current level of denudation ranging between ~ 3-5kbar.

Ti in zircon and quartz, and zircon saturation temperatures suggest granite-formation temperatures may have been as low as 700-750 °C. Mineral equilibria modelling using protolith compositions considered presentative of the sedimentary protoliths suggest melting at the scale indicated by regional mapping and geophysics is only possible in a water rich environment, either from trapped water that was not expelled during burial in the back arc, or for an external source.

The geochemistry of the Bundarra and Hillgrove suites shows significant variability in their maficity. When comparing the modelled magma/melt compositions at different water contents the variation in maficity shows the granites are more mafic than the modelled melt compositions, suggesting much of the mafic content is entrained from the source. This is supported by the occurrence of hexagonal shaped biotite aggregates which contain occasional garnet relics.

We interpret that water-fluxed melting played an important role in the genesis of back arc hosted S-type granites in eastern Australia, and it may apply to the genesis of many other S-type granite batholiths. Variable entrainment of the residual mineral assemblage within a cool, hydrous silicic melt can probably explain the compositional range of New England and other S-type granite batholiths.