Building the New England Batholith, eastern Australia—Linking granite petrogenesis with geodynamic setting using Hf isotopes in zircon

Abstract

U–Pb and Hf isotope analysis of zircons from granitoids of the Permian–Triassic New England Batholith (eastern Australia) was carried out to provide constraints on the evolution of an isotopically and compositionally diverse batholith. Incipient plutonism in the early Permian resulted in the formation of isotopically evolved, peraluminous granodiorites of the Hillgrove Suite. Following this, mixing between crustal-derived (+5–+8 εHf units) and depleted mantle-derived magmas (+13–+18 εHf units), was responsible for the formation of the c. 282Ma Bundarra Suite. The strongly metaluminous and isotopically depleted granites of the c. 268Ma Clarence River Suite (+11–+16 εHf units) signify an increased role of isotopically depleted magmas during the formation of plutonic rocks in the middle Permian. Interestingly, this isotopic and chemical transience coincides with orogenic extension that was associated with the relocation for the southern New England Orogen (NEO) from a continental margin accretionary setting to that of a back-arc basin. Following attenuation, the NEO was thickened by contraction during the Hunter Bowen Event (265–255Ma).Forming after the Hunter Bowen Event, the metaluminous rocks of the c. 256Ma Moonbi Suite were the product of mixing between magmas derived from evolved metaigneous rocks and enriched mantle (+3–+8 εHf units). We attribute the switch in isotopic character from highly depleted (i.e., Clarence River Suite) to evolved (i.e., Moonbi Suite) to crustal thickness before (i.e., thin) and after (i.e., thick) the Hunter Bowen Event. Evidence of renewed mixing between depleted and evolved magmas characterises the formation of the c. 249Ma Uralla Suite (+7–+16 εHf units), which interestingly, was coeval with renewed orogenic extension in the early Triassic. Finally, the melting of deep crustal basalts below the southern New England Orogen in the middle Triassic (c. 233Ma) resulted in the formation of moderately depleted leucomonzogranites and A-type magmas (+9–+12 εHf units).Through combining new U–Pb and Hf zircon isotope data with pre-existing whole rock Nd and geochemical data, a link between geodynamic setting and granite petrogenesis has been identified, where: (i) in thick orogens, granites formed via partial melting of the crust and lack a significant depleted component; (ii) in thin orogens, granites form by mixing of crustal-derived and depleted mantle-derived magmas; and (iii) in highly attenuated orogens, granites can be extremely depleted and dominantly derived from the melting of upper mantle sources. This relationship may be associated with the development or reactivation of lithosphere scale detachments during orogenic extension that could facilitate the transportation of depleted magmas into the crust.