Using hafnium and oxygen isotopes in zircons to unravel the record of crustal evolution

The development of improved techniques for the in situ analysis of U–Pb, Lu–Hf and O isotopes and of trace element contents has revolutionized approaches to modeling the origins and evolution of the continental crust, and the petrogenesis of granite. The key has been to obtain representative samples, and it is increasingly accepted that these are best provided by zircons. In magmatic rocks they may be inherited or have crystallized in the evolution of the host magma, and in sediments they may be detrital from broad areas in the continental crust. In situ Hf isotope ratios are now routinely measured with sub-epsilon unit precision by laser ablation ICP-MS. Trace element contents broadly constrain the tectonic setting of the magmas from which the zircons crystallized, Hf isotopes reflect when new crust was generated from the mantle, and O isotopes are sensitive to low temperature processes, as in erosion and sedimentation. The potential of an integrated approach is explored with new data from the Lachlan Fold Belt in south-east Australia. Inherited (pre-magmatic) zircons in the granites and detrital zircons in the surrounding Ordovician metasedimentary rocks yield very similar information. Their crystallisation age spectra are dominated by peaks at 450–600 Ma and 0.9–1.2 Ga, however the Hf model ages of zircons with broadly mantle-like δ18O values (< 6.5‰) fall into two relatively narrow age ranges, c 1.7–1.9 and 2.9–3.1 Ga. Since such crustal formation peaks do not match those registering the predominant zircon crystallisation ages, the 450–600 Ma and 0.9–1.2 Ga periods are essentially times of crustal differentiation, rather than growth, in this region. Furthermore, the Hf model ages of zircons with high δ18O values (greater than 6.5‰) peak at about 1.8–2.0 Ga, close to the Nd model ages of the Ordovician metasedimentary rocks. Variations in Hf, O isotopes and trace elements in zircons from two I-type granite suites from the Lachlan Fold Belt highlight the presence of mantle contributions in the generation of these granites, and when the crustal component is a partial melt of the local metasedimentary rocks. Little of such specific information is available from the whole rock compositions, and yet it is a prerequisite for more physically realistic models for granite petrogenesis, better constrained thermal budgets, and much greater insight into the evolution of particular segments of continental crust.